US 5,849,869 AGrant
Delta Fragments and Derivatives and Methods Based Thereon
Issue Date:1998-12-15
•31 Claims
•34 Drawing Sheets
Abstract
The present invention relates to nucleotide sequences of the Delta genes, and amino acid sequence of the encoded protein, fragments and derivatives which retain binding activity are also provided.
Metadata
Assignees
- Yale University
- Indiana University Foundation
Inventors
- Spyridon Artavanis-Tsakonas
- Marc Alan-Telander Muskavitch
- Richard Grant Fehon
- Scott Brockwell Shephard
Application Information
Application Number:US 3461262
Filing Date:1994-11-28
Priority Date:1991-05-03
Art Unit:182
Classifications
IPC:
C07K 14705
Field of Search:
530930350;402;83910
Patent Drawings (34 sheets)
Description
1. INTRODUCTION
The present invention relates to the human Notch and Delta genes and their encoded products. The invention also relates to sequences (termed herein "adhesive sequences") within the proteins encoded by toporythmic genes which mediate homotypic or heterotypic binding to sequences within proteins encoded by toporythmic genes. Such genes include but are not limited to Notch, Delta, and Serrate.
2. BACKGROUND OF THE INVENTION
Genetic analyses in Drosophila have been extremely useful in dissecting the complexity of developmental pathways and identifying interacting loci. However, understanding the precise nature of the processes that underlie genetic interactions requires a knowledge of the biochemical properties of the protein products of the genes in question.
Null mutations in any one of the zygotic neurogenic loci--Notch (N), Delta (Dl), mastermind (mam), Enhancer of Split (E(spl), neuralized (neu), and big brain (bib)--in hypertrophy of the nervous system at the expense of ventral and lateral epidermal structures. This effect is due to the misrouting of epidermal precursor cells into a neuronal pathway, and implies that neurogenic gene function is necessary to divert cells within the neurogenic region from a neuronal fate to an epithelial fate. Studies that assessed the effects of laser ablation of specific embryonic neuroblasts in grasshoppers (Doe and Goodman 1985, Dev. Biol. 111, 206-219) have shown that cellular interactions between neuroblasts and the surrounding accessory cells serve to inhibit these accessory cells from adopting a neuroblast fate. Together, these genetic and developmental observations have led to the hypothesis that the protein products of the neurogenic loci function as components of a cellular interaction mechanism necessary for proper epidermal development (Artavanis-Tsakonas, 1988, Trends Genet. 4, 95-100).
Sequence analyses (Wharton et al., 1985, Cell 43, 567-581; Kidd et al., 1986, Mol. Cell. Biol. 6, 3094-3108; Vassin et al., 1987, EMBO J. 6, 3431-3440; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735) have shown that two of the neurogenic loci, Notch and Delta, appear to encode transmembrane proteins that span the membrane a single time. The Notch gene encodes a .about.300 kd protein (we use "Notch" to denote this protein) with a large N-terminal extracellular domain that includes 36 epidermal growth factor (EGF)-like tandem repeats followed by three other cysteine-rich repeats, designated Notch/lin-12 repeats (Wharton et al., 1985, Cell 43, 567-581; Kidd et al., 1986, Mol. Cell Biol. 6, 3094-3108; Yochem et al., 1988, Nature 335, 547-550). Delta encodes a .about.100 kd protein (we use "Delta" to denote DLZM, the protein product of the predominant zygotic and maternal transcripts; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735) that has nine EGF-like repeats within its extracellular domain (Vassin et al., 1987, EMBO J. 6, 3431-3440; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735). Although little is known about the functional significance of these repeats, the EGF-like motif has been found in a variety of proteins, including those involved in the blood clotting cascade (Furie and Furie, 1988, Cell 53, 505-518). In particular, this motif has been found in extracellular proteins such as the blood clotting factors IX and X (Rees et al., 1988, EMBO J. 7, 2053-2061; Furie and Furie, 1988, Cell 53, 505-518), in other Drosophila genes (Knust et al., 1987, EMBO J. 761-766; Rothberg et al., 1988, Cell 55, 1047-1059), and in some cell-surface receptor proteins, such as thrombomodulin (Suzuki et al., 1987, EMBO J. 6, 1891-1897) and LDL receptor (Sudhof et al., 1985, Science 228, 815-822). A protein binding site has been mapped to the EGF repeat domain in thrombomodulin and urokinase (Kurosawa et al., 1988, J. Biol. Chem 263, 5993-5996; Appella et al., 1987, J. Biol. Chem. 262, 4437-4440).
An intriguing array of interactions between Notch and Delta mutations has been described (Vassin, et al., 1985, J. Neurogenet. 2, 291-308; Shepard et al., 1989, Genetics 122, 429-438; Xu et al., 1990, Genes Dev., 4, 464-475). A number of genetic studies (summarized in Alton et al., 1989, Dev. Genet. 10, 261-272) has indicated that the gene dosages of Notch and Delta in relation to one another are crucial for normal development. A 50% reduction in the dose of Delta in a wild-type Notch background causes a broadening of the wing veins creating a "delta" at the base (Lindsley and Grell, 1968, Publication Number 627, Washington, D.C., Carnegie Institute of Washington). A similar phenotype is caused by a 50% increase in the dose of Notch in a wild-type Delta background (a "Confluens" phenotype; Welshons, 1965, Science 150, 1122-1129). This Delta phenotype is partially suppressed by a reduction in the Notch dosage. Recent work in our laboratories has shown that lethal interactions between alleles that correlate with alterations in the EGF-like repeats in Notch can be rescued by reducing the dose of Delta (Xu et al., 1990, Genes Dev. 4, 464-475). Xu et al. (1990, Genes Dev. 4, 464-475) found that null mutations at either Delta or mam suppress lethal interactions between heterozygous combinations of certain Notch alleles, known as the Abruptex (Ax) mutations. Ax alleles are associated with missense mutations within the EGF-like repeats of the Notch extracellular domain (Kelley et al., 1987, Cell 51, 539-548; Hartley et al., 1987, EMBO J. 6, 3407-3417).
Notch is expressed on axonal processes during the outgrowth of embryonic neurons (Johansen et al., 1989, J. Cell Biol. 109, 2427-2440; Kidd et al., 1989, Genes Dev. 3, 1113-1129).
A study has shown that certain Ax alleles of Notch can severely alter axon pathfinding during sensory neural outgrowth in the imaginal discs, although it is not yet known whether aberrant Notch expression in the axon itself or the epithelium along which it grows is responsible for this defect (Palka et al., 1990, Development 109, 167-175).
3. SUMMARY OF THE INVENTION
The present invention relates to nucleotide sequences of the human Notch and Delta genes, and amino acid sequences of their encoded proteins, as well as fragments thereof containing an antigenic determinant or which are functionally active. The invention is also directed to fragments (termed herein "adhesive fragments"), and the sequences thereof, of the proteins ("toporythmic proteins") encoded by toporythmic genes which mediate homotypic or heterotypic binding to toporythmic proteins. Toporythmic genes, as used herein, refers to the genes Notch, Delta, and Serrate, as well as other members of the Delta/Serrate family which may be identified, e.g., by the methods described in Section 5.3, infra. Analogs and derivatives of the adhesive fragments which retain binding activity are also provided. Antibodies to human Notch and to adhesive fragments are additionally provided.
In specific embodiments, the adhesive fragment of Notch is that fragment comprising the Notch sequence most homologous to Drosophila Notch EGF-like repeats 11 and 12; the adhesive fragment of Delta mediating heterotypic binding is that fragment comprising the sequence most homologous to Drosophila Delta amino acids 1-230; the adhesive fragment of Delta mediating homotypic binding is that fragment comprising the sequence most homologous to Drosophila Delta amino acids 32-230; and the adhesive fragment of Serrate is that fragment comprising the sequence most homologous to Drosophila Serrate amino acids 85-283 or 79-282.
3.1. Definitions
As used herein, the following terms shall have the meanings indicated:
AA=amino acid
EGF=epidermal growth factor
ELR=EGF-like (homologous) repeat
IC=intracellular
PCR=polymerase chain reaction
As used herein, underscoring the name of a gene shall indicate the gene, in contrast to its encoded protein product which is indicated by the name of the gene in the absence of any underscoring. For example, "Notch" shall mean the Notch gene, whereas "Notch" shall indicate the protein product of the Notch gene.
4. DESCRIPTION OF THE FIGURES
FIG. 1. Expression Constructs and Experimental Design for Examining Notch-Delta Interactions. S2 cells at log phase growth were transiently transfected with one of the three constructs shown. Notch encoded by the MGlla minigene (a cDNA/genomic chimeric construct: cDNA-derived sequences are represented by stippling, genomically derived sequences by diagonal-hatching (Ramos et al., 1989, Genetics 123, 337-348)) was expressed following insertion into the metallothionein promoter vector pRmHa-3 (Bunch et al., 1988, Nucl. Acids Res. 16, 1043-1061). Delta encoded by the Dl1 cDNA (Kopczynski et al., 1988, Genes Dev. 2, 1723-1735) was expressed after insertion into the same vector. The extracellular Notch (ECN1) variant was derived from a genomic cosmid containing the complete Notch locus (Ramos et al., 1989, Genetics 123, 337-348) by deleting the coding sequence for amino acids 1790-2625 from the intracellular domain (denoted by .delta.; Wharton et al., 1985, Cell 43, 567-581), leaving 25 membrane-proximal residues from the wild-type sequence fused to a novel 59 amino acid tail (see Experimental Procedures, Section 6.1, infra). This construct was expressed under control of the Notch promoter region. For constructs involving the metallothionein vector, expression was induced with CuSO.sub.4 following transfection. Cells were then mixed, incubated under aggregation conditions, and scored for their ability to aggregate using specific antisera and immunofluorescence microscopy to visualize expressing cells. MT, metallothionein promoter; ATG, translation start site; TM, transmembrane domain; 3' N, Notch gene polyadenylation signal; 3' Adh, polyadenylation signal from Adh gene; 5' N, Notch gene promoter region.
FIGS. 2A-2B. Expression of Notch and Delta in Cultured Cells. FIG. 2A: Lysates of nontransfected (S2) and Notch-transfected (N) cells induced with 0.7 mM CuSO.sub.4 for 12-16 hr were prepared for sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), run on 3%-15% gradient gels, and blotted to nitrocellulose. Notch was visualized using a monoclonal antibody (MAb C17.9C6) against the intracellular domain of Notch. Multiple bands below the major band at 300 kd may represent degradation products of Notch. FIG. 2B: Lysates of nontransfected (S2) and Delta-transfected (Dl) cells visualized with a monoclonal antibody (MAb 201) against Delta. A single band of .sup..about. 105 kd is detected. In both cases, there is no detectable endogenous Notch or Delta in the S2 cell line nor are there cross-reactive species. In each lane, 10 .mu.l of sample (prepared as described in Experimental Procedures) was loaded.
FIGS. 3A-3I. S2 Cells That Express Notch and Delta Form Aggregates. In all panels, Notch is shown in green and Delta in red.
FIG. 3A: A single Notch.sup.+ cell. Note the prominent intracellular stain, including vesicular structures as well as an obviously unstained nucleus.
FIG. 3A: Bright-field micrograph of same field, showing specificity of antibody staining.
FIG. 3B: A single Delta.sup.+ cell. Staining is primarily at the cell surface.
FIG. 3B: Bright-field micrograph of same field.
FIG. 3C: Aggregate of Delta.sup.+ cells from a 24 hr aggregation experiment. Note against that staining is primarily at the cell surface.
FIGS. 3D-3F: An aggregate of Notch.sup.+ and Delta.sup.+ cells formed from a 1:1 mixture of singly transfected cell populations that was allowed to aggregate overnight at room temperature. FIG. 3D shows Notch.sup.+ cells in this aggregate; FIG. 3E shows Delta.sup.+ cells; and FIG. 3F is a double exposure showing both cell types. Bands of Notch and Delta are prominent at points of contact between Notch.sup.+ and Delta.sup.+ cells (arrows). In FIG. 3F, these bands appear yellow because of the coincidence of green and red at these points. The apparently doubly stained single cell (*) is actually two cells (one on top of the other), one expressing Notch and the other Delta.
FIGS. 3G-3H: Pseudocolor confocal micrographs of Notch.sup.+ -Delta.sup.+ cell aggregates. Note that in FIG. 3G extensions (arrows) formed by at least two Delta.sup.+ cells completely encircle the Notch.sup.+ cell in the center of the aggregate. FIG. 3H shows an aggregate formed from a 2 hr aggregation experiment performed at 4.degree. C. Intense bands of Notch are apparent within regions of contact with Delta.sup.+ cells.
FIG. 3I: An aggregate composed of Delta.sup.+ cells and cells that express only the extracellular domain of Notch (ECN1 construct). Scale bar=10 .mu.m.
FIGS. 4A-4F. Notch and Delta are Associated in Cotransfected Cells. Staining for Notch is shown in the left column FIGS. 4A, 4C, and 4E and that for Delta is shown in the right column FIGS. 4B, 4D, and 4F.
FIGS. 4A and 4B: S2 cell cotransfected with both Notch and Delta constructs. In general, there was a good correlation between Notch and Delta localization at the cell surface (arrows).
FIGS. 4C and 4D: Cotransfected cells were exposed to polyclonal anti-Notch antiserum (a 1:250 dilution of each anti-extracellular domain antiserum) for 1 hr at room temperature before fixation and staining with specific antisera. Note punctate staining of Notch and Delta and the correlation of their respective staining (arrows).
FIGS. 4E and 4F: Cells cotransfected with the extracellular Notch (ECN1) and Delta constructs, induced, and then patched using anti-Notch polyclonal antisera. There was a close correlation between ECN1 and Delta staining at the surface as observed for full-length Notch. Scale bar=10 .mu.m.
FIGS. 5A-5B. Coimmunoprecipitation Shows that Delta and Notch are Associated in Lysates from Transfected S2 and Drosophila Embryonic Cells. In all experiments, Delta was precipitated from NP-40/deoxycholate lysates using a polyclonal anti-Delta rat antiserum precipitated with fixed Staph A cells, and proteins in the precipitated fraction were visualized on Western blots (for details, see Experimental Procedures). Lanes 1, 2, 3, and 5: Notch visualized with MAb C17.9C6; Lanes 4 and 6: Delta visualized using MAb 201.
In FIG. 5A, lanes 1 and 2 are controls for these experiments. Lane 1 shows a polyclonal anti-Delta immunoprecipitation from cells that express Notch alone visualized for Notch. No Notch was detectable in this sample, indicating that the polyclonal anti-Delta does not cross-react with Notch. Lane 2 shows Notch-Delta cotransfected cells immunoprecipitated with Staph A without initial treatment with anti-Delta antiserum and visualized for Notch, demonstrating that Notch is not precipitated nonspecifically by the Staph A or secondary antibody. Lane 3 shows protein precipitated with anti-Delta antiserum visualized for Delta (Dl), and lane 4 shows the same sample visualized for Notch (N). Lane 4 shows that Notch coprecipitates with immunoprecipitated Delta. Note that Notch appears as a doublet as is typical for Notch in immunoprecipitates.
FIG. 5B shows the same experiment using embryonic lysates rather than transfected cell lysates. Lane 5 shows protein precipitated with anti-Delta antiserum visualized for Delta (Dl), and lane 6 shows the same sample visualized for Notch (N). These lanes demonstrate that Notch and Delta are stably associated in embryo lysates. Bands (in all lanes) below the Delta band are from Staph A (SA) and the anti-Delta antiserum heavy (H) and light (L) chains.
FIGS. 6A-6B. Notch Expression Constructs and the Deletion Mapping of the Delta/Serrate Binding Domain. S2 cells in log phase growth were transiently transfected with the series of expression constructs shown; the drawings represent the predicted protein products of the various Notch deletion mutants created. All expression constructs were derived from construct #1 pMtNMg. Transiently transfected cells were mixed with Delta expressing cells from the stably transformed line L49-6-7 or with transiently transfected Serrate expressing cells, induced with CuSO.sub.4, incubated under aggregation conditions and then scored for their ability to aggregate using specific antisera and immunofluorescence microscopy. Aggregates were defined as clusters of four or more cells containing both Notch and Delta/Serrate expressing cells. The values given for % Aggregation refer to the percentage of all Notch expressing cells found in such clusters either with Delta (Dl) (left column) or with Serrate (Ser) (right column). The various Notch deletion constructs are represented diagrammatically with splice lines indicating the ligation junctions. Each EGF repeat is denoted as a stippled rectangular box and numbers of the EGF repeats on either side of a ligation junction are noted. At the ligation junctions, partial EGF repeats produced by the various deletions are denoted by open boxes and closed brackets (for example see #23 .DELTA.Cla+EGF(10-12)). Constructs #3-13 represent the ClaI deletion series. As diagrammed, four of the ClaI sites, in repeats 7, 9, 17 and 26, break the repeat in the middle, immediately after the third cysteine (denoted by open box repeats; see FIG. 7 for further clarification), while the fifth and most 3' site breaks neatly between EGF repeats 30 and 31 (denoted by closed box repeat 31; again see FIG. 7). In construct #15 split, EGF repeat 14 which carries the split point mutation, is drawn as a striped box. In construct #33 .DELTA.Cla+XEGF(10-13), the Xenopus Notch derived EGF repeats are distinguished from Drosophila repeats by a different pattern of shading. SP, signal peptide; EGF, epidermal growth factor repeat; N, Notch/lin-12 repeat; TM, transmembrane domain; cdc10, cdc10/ankyrin repeats; PA, putative nucleotide binding consensus sequence; opa, polyglutamine stretch termed opa; Dl, Delta; Ser, Serrate.
FIG. 7. Detailed Structure of Notch Deletion Constructs #19-24: Both EGF Repeats 11 and 12 are Required for Notch-Delta Aggregation. EGF repeats 10-13 are diagrammed at the top showing the regular spacing of the six cysteine residues (C). PCR products generated for these constructs (names and numbers as given in FIGS. 6A-6B are represented by the heavy black lines and the exact endpoints are noted relative to the various EGF repeats. Ability to aggregate with Delta is recorded as (+) or (-) for each construct. The PCR fragments either break the EGF repeats in the middle, just after the third cysteine in the same place as four out of the five ClaI sites, or exactly in between two repeats in the same place as the most C-terminal ClaI site.
FIG. 8. Comparison of Amino Acid Sequence of EGF Repeats 11 and 12 from Drosophila and Xenopus Notch. The amino acid sequence of EGF repeats 11 and 12 of Drosophila Notch (Wharton et al., 1985, Cell 43:567-581; Kidd et al., 1986, Mol. Cell Biol. 6:3094-3108) is aligned with that of the same two EGF repeats from Xenopus Notch (Coffman et al., 1990, Science 249:1438-1441). Identical amino acids are highlighted with a grey box. The six conserved cysteine residues of each EGF repeat and the Ca.sup.++ binding consensus residues (Rees et al., 1988, EMBO J. 7:2053-2061) are marked with an asterisk (*). The leucine to proline change found in the Xenopus PCR clone that failed to aggregate is noted underneath.
FIGS. 9A-9C. Constructs Employed in this Study. Schematic diagrams of the Delta variants defined in Table IV are shown. Extracellular, amino-proximal terminus is to the left in each case. S, signal peptide; "EGF", EGF-like motifs; M, membrane-spanning helix; H, stop-transfer sequence; solid lines, other Delta sequences; hatched lines, neuroglian sequences. Arrowheads indicate sites of translatable linker insertions. Sca, ScaI; Nae, NaeI; Bam, BamHI; Bgl, BglII; ELR, EGF-like repeat; Bst, BstEII; Dde, DdeI; Stu, StuI; NG1-NG5, Delta-neuroglian chimeras.
FIG. 9A. Dependence of Aggregation on Input DNA Amounts. Heterotypic aggregation observed using S2 cell populations transiently transfected, respectively, with varied amounts of pMTDl1 DNA (2, 4, 10 or 20 .mu.g/plate) that were subsequently incubated under aggregation conditions with S2 cell populations transiently transfected with a constant amount of pMtNMg DNA (20 .mu.g/plate). Data presented are mean fraction (%) of Delta cells in aggregates of four or more cells.+-.standard error for each input DNA amount (N=3 replicates, except 2 .mu.g and 10 .mu.g inputs for which N=2). A minimum of 100 Delta-expressing cells were counted for each replicate. FIG. 9B, Homotypic aggregation observed using S2 cell populations transiently transfected, respectively, with varied amounts of pMTDl1 DNA (2, 4, 10 or 20 .mu.g/plate) that were subsequently incubated under aggregation conditions. Data presented are mean fraction (%) of Delta cells in aggregates of four or more cells.+-.standard error for each input DNA amount (N=3 replicates). A minimum of 500 Delta-expressing cells were counted for each replicate.
FIG. 10. Delta-Serrate Amino-Terminal Sequence Alignment. Residues are numbered on the basis of conceptual translation of Delta (Dl, upper sequence (SEQ ID NO:3); beginning at amino acid 24, ending at amino acid 226) and Serrate (Ser, lower sequence (SEQ ID NO:4); beginning at amino acid 85, ending at amino acid 283) coding sequences. Vertical lines between the two sequences indicates residues that are identical within the Delta and Serrate sequences, as aligned. Dots represent gaps in the alignment. Boxes enclose cysteine residues within the aligned regions. N1, amino-proximal domain 1; N2, amino-proximal domain 2; N3, amino-proximal domain 3. Translatable insertions associated with STU B replacement of Delta amino acid 132 (A) with GKIFP! and NAE B insertion of RKIF between Delta amino acid 197 and amino acid 198! constructs, respectively, are depicted above the wild type Delta sequence.
FIGS. 11A-11B. Potential Geometries of Delta-Notch Interactions. FIG. 11A, Potential register of Delta (left) and Notch (right) molecules interacting between opposing plasma membranes. FIG. 11B, Potential register of Delta (left) and Notch (right) molecules interacting within the same plasma membranes. ELR, EGF-like repeat; open boxes, EGF-like repeats; dotted boxes, LNR repeats; solid boxes, membrane-spanning helices. Delta amino-terminal domain and Delta and Notch intracellular domains represented by ovals.
FIGS. 12A-12C. Potential Geometries of Delta--Delta Interactions. FIGS. 12A-12B, Potential register of Delta molecules interacting between opposing plasma membranes. FIG. 12C, Potential register of Delta molecules interacting within the same plasma membranes. Open boxes, EGF-like repeats; solid boxes, membrane-spanning helices. Delta amino-terminal extracellular and intracellular domains represented by ovals.
FIGS. 13A-13F. Primary Nucleotide Sequence of the Delta cDNA Dl1 (SEQ ID NO:5) and Delta amino acid sequence (SEQ ID NO:6) The DNA sequence of the 5'-3' strand of the Dl1 cDNA is shown, which contains a number of corrections in comparison to that presented in Kopczynksi et al. (1988, Genes Dev. 2, 1723-1735).
FIG. 14. Primary Nucleotide Sequence of the Neuroglian cDNA 1B7A-250 (SEQ ID NO:7). This is the DNA sequence of a portion of the 5'-3' strand of the 1B7A-250 cDNA (A. J. Bieber, pers. comm.; Hortsch et al., 1990, Neuron 4, 697-709). Nucleotide 2890 corresponds to the first nucleotide of an isoleucine codon that encodes amino acid 952 of the conceptually translated neuroglian-long form protein.
FIGS. 15A-15B. Nucleic Acid Sequence Homologies Between Serrate and Delta. A portion of the Drosophila Serrate nucleotide sequence (SEQ ID NO:8), with the encoded Serrate protein sequence (SEQ ID NO:9) written below, (Fleming et al., 1990, Genes & Dev. 4, 2188-2201 at 2193-94) is shown. The four regions showing high sequence homology with the Drosophila Delta sequence are numbered above the line and indicated by brackets. The total region of homology spans nucleotide numbers 627 through 1290 of the Serrate nucleotide sequence (numbering as in FIG. 4 of Fleming et al., 1990, Genes & Dev. 4, 2188-2201).
FIGS. 16A-16C. Primers used for PCR in the Cloning of Human Notch. The sequence of three primers used for PCR to amplify DNA in a human fetal brain cDNA library are shown. The three primers, cdc1 (SEQ ID NO:10), cdc2 (SEQ ID NO:11), and cdc3 (SEQ ID NO:12), were designed to amplify either a 200 bp or a 400 bp fragment as primer pairs cdc1/cdc2 or cdc1/cdc3, respectively. I: inosine.
FIG. 17. Schematic Diagram of Human Notch Clones. A schematic diagram of human Notch is shown. Heavy bold-face lines below the diagram show that portion of the Notch sequence contained in each of the four cDNA clones. The location of the primers used in PCR, and their orientation, are indicated by arrows.
FIG. 18. Human Notch Sequences Aligned with Drosophila Notch Sequence. Numbered vertical lines correspond to Drosophila Notch coordinates. Horizontal lines below each map show where clones lie relative to stretches of sequence (thick horizontal lines).
FIGS. 19A-19C. Nucleotide Sequences of Human Notch Contained in Plasmid cDNA Clone hN2k. FIG. 19A: The DNA sequence (SEQ ID NO:13) of a portion of the human Notch insert is shown, starting at the EcoRI site at the 3' end, and proceeding in the 3' to 5' direction. FIG. 19B: The DNA sequence (SEQ ID NO:14) of a portion of the human Notch insert is shown, starting at the EcoRI site at the 5' end, and proceeding in the 5' to 3' direction. FIG. 19C: The DNA sequence (SEQ ID NO:15) of a portion of the human Notch insert is shown, starting 3' of the sequence shown in FIG. 19B, and proceeding in the 5' to 3' direction. The sequences shown are tentative, subject to confirmation by determination of overlapping sequences.
FIGS. 20A-20D. Nucleotide Sequences of Human Notch Contained in Plasmid cDNA clone hN3k. FIG. 20A: The DNA sequence (SEQ ID NO:16) of a portion of the human Notch insert is shown, starting at the EcoRI site at the 3' end, and proceeding in the 3' to 5' direction. FIG. 20B: The DNA sequence (SEQ ID NO:17) of a portion of the human Notch insert is shown, starting at the EcoRI site at the 5' end, and proceeding in the 5' to 3' direction. FIG. 20C: The DNA sequence (SEQ ID NO:18) of a portion of the human Notch insert is shown, starting 3' of the sequence shown in FIG. 20B, and proceeding in the 5' to 3' direction. FIG. 20D: The DNA sequence (SEQ ID NO:19) of a portion of the human Notch insert is shown, starting 5' of the sequence shown in FIG. 20A, and proceeding in the 3' to 5' direction. The sequences shown are tentative, subject to confirmation by determination of overlapping sequences.
FIGS. 21A-21B. Nucleotide Sequences of Human Notch Contained in Plasmid cDNA clone hN4k. FIG. 21A: The DNA sequence (SEQ ID NO:20) of a portion of the human Notch insert is shown, starting at the EcoRI site at the 5' end, and proceeding in the 5' to 3' direction. FIG. 21B: The DNA sequence (SEQ ID NO:21) of a portion of the human Notch insert is shown, starting near the 3' end, and proceeding in the 3' to 5' direction. The sequences shown are tentative, subject to confirmation by determination of overlapping sequences.
FIGS. 22A-22D. Nucleotide Sequences of Human Notch Contained in Plasmid cDNA Clone hN5k. FIG. 22A: The DNA sequence (SEQ ID NO:22) of a portion of the human Notch insert is shown, starting at the EcoRI site at the 5' end, and proceeding in the 5' to 3' direction. FIG. 22B: The DNA sequence (SEQ ID NO:23) of a portion of the human Notch insert is shown, starting near the 3' end, and proceeding in the 3' to 5' direction. FIG. 22C: The DNA sequence (SEQ ID NO:24) of a portion of the human Notch insert is shown, starting 3' of the sequence shown in FIG. 22A, and proceeding in the 5' to 3' direction. FIG. 22D: The DNA sequence (SEQ ID NO:25) of a portion of the human Notch insert is shown, starting 5' of the sequence shown in FIG. 22B, and proceeding in the 3' to 5' direction. The sequences shown are tentative, subject to confirmation by determination of overlapping sequences.
5. DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to nucleotide sequences of the human Notch and Delta genes, and amino acid sequences of their encoded proteins. The invention further relates to fragments (termed herein "adhesive fragments") of the proteins encoded by toporythmic genes which mediate homotypic or heterotypic binding to toporythmic proteins or adhesive fragments thereof. Toporythmic genes, as used herein, shall mean the genes Notch, Delta, and Serrate, as well as other members of the Delta/Serrate family which may be identified, e.g. by the methods described in Section 5.3, infra.
The nucleic acid and amino acid sequences and antibodies thereto of the invention can be used for the detection and quantitation of mRNA for human Notch and Delta and adhesive molecules, to study expression thereof, to produce human Notch and Delta and adhesive sequences, in the study and manipulation of differentiation processes.
For clarity of disclosure, and not by way of limitation, the detailed description of the invention will be divided into the following sub-sections:
(i) Identification of and the sequences of toporythmic protein domains that mediate binding to toporythmic protein domains;
(ii) The cloning and sequencing of human Notch and Delta;
(iii) Identification of additional members of the Delta/Serrate family;
(iv) The expression of toporythmic genes;
(v) Identification and purification of the expressed gene product; and
(vi) Generation of antibodies to toporythmic proteins and adhesive sequences thereof.
5.1. Identification of and the Sequences of Toporythmic Protein Domains that Mediate Binding to Toporythmic Protein Domains
The invention provides for toporythmic protein fragments, and analogs or derivatives thereof, which mediate homotypic or heterotypic binding (and thus are termed herein "adhesive"), and nucleic acid sequences relating to the foregoing.
In a specific embodiment, the adhesive fragment of Notch is that comprising the portion of Notch most homologous to ELR 11 and 12, i.e., amino acid numbers 447 through 527 (SEQ ID NO:1) of the Drosophila Notch sequence (see FIG. 8). In another specific embodiment, the adhesive fragment of Delta mediating homotypic binding is that comprising the portion of Delta most homologous to about amino acid numbers 32-230 of the Drosophila Delta sequence (SEQ ID NO:6). In yet another specific embodiment, the adhesive fragment of Delta mediating binding to Notch is that comprising the portion of Delta most homologous to about amino acid numbers 1-230 of the Drosophila Delta sequence (SEQ ID NO:6). In a specific embodiment relating to an adhesive fragment of Serrate, such fragment is that comprising the portion of Serrate most homologous to about amino acid numbers 85-283 or 79-282 of the Drosophila Serrate sequence (see FIG. 10 (SEQ ID NO:4), and FIGS. 15A-15B (SEQ ID NO:9)).
The nucleic acid sequences encoding toporythmic adhesive domains can be isolated from porcine, bovine, feline, avian, equine, or canine, as well as primate sources and any other species in which homologues of known toporythmic genes including but not limited to the following genes (with the publication of sequences in parentheses): Notch (Wharton et al., 1985, Cell 43, 567-581), Delta (Vassin et al., 1987, EMBO J. 6, 3431-3440; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735; note corrections to the Kopczynski et al. sequence found in FIGS. 13A-13F hereof (SEQ ID NO:5 and SEQ ID NO:6)) and Serrate (Fleming et al., 1990, Genes & Dev. 4, 2188-2201)! can be identified. Such sequences can be altered by substitutions, additions or deletions that provide for functionally equivalent (adhesive) molecules. Due to the degeneracy of nucleotide coding sequences, other DNA sequences which encode substantially the same amino acid sequence as the adhesive sequences may be used in the practice of the present invention. These include but are not limited to nucleotide sequences comprising all or portions of the Notch, Delta, or Serrate genes which are altered by the substitution of different codons that encode a functionally equivalent amino acid residue within the sequence, thus producing a silent change. Likewise, the adhesive protein fragments or derivatives thereof, of the invention include, but are not limited to, those containing, as a primary amino acid sequence, all or part of the amino acid sequence of the adhesive domains including altered sequences in which functionally equivalent amino acid residues are substituted for residues within the sequence resulting in a silent change. For example, one or more amino acid residues within the sequence can be substituted by another amino acid of a similar polarity which acts as a functional equivalent, resulting in a silent alteration. Substitutes for an amino acid within the sequence may be selected from other members of the class to which the amino acid belongs. For example, the nonpolar (hydrophobic) amino acids include alanine, leucine, isoleucine, valine, proline, phenylalanine, tryptophan and methionine. The polar neutral amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine, and glutamine. The positively charged (basic) amino acids include arginine, lysine and histidine. The negatively charged (acidic) amino acids include aspartic acid and glutamic acid.
Adhesive fragments of toporythmic proteins and potential derivatives, analogs or peptides related to adhesive toporythmic protein sequences, can be tested for the desired binding activity e.g., by the in vitro aggregation assays described in the examples herein. Adhesive derivatives or adhesive analogs of adhesive fragments of toporythmic proteins include but are not limited to those peptides which are substantially homologous to the adhesive fragments, or whose encoding nucleic acid is capable of hybridizing to the nucleic acid sequence encoding the adhesive fragments, and which peptides and peptide analogs have positive binding activity e.g., as tested in vitro by an aggregation assay such as described in the examples sections infra. Such derivatives and analogs are envisioned and within the scope of the present invention.
The adhesive-protein related derivatives, analogs, and peptides of the invention can be produced by various methods known in the art. The manipulations which result in their production can occur at the gene or protein level. For example, the cloned adhesive protein-encoding gene sequence can be modified by any of numerous strategies known in the art (Maniatis, T., 1990, Molecular Cloning, A Laboratory Manual, 2d ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.). The sequence can be cleaved at appropriate sites with restriction endonuclease(s), followed by further enzymatic modification if desired, isolated, and ligated in vitro. In the production of the gene encoding a derivative, analog, or peptide related to an adhesive domain, care should be taken to ensure that the modified gene remains within the same translational reading frame as the adhesive protein, uninterrupted by translational stop signals, in the gene region where the desired adhesive activity is encoded.
Additionally, the adhesive-encoding nucleic acid sequence can be mutated in vitro or in vivo, to create and/or destroy translation, initiation, and/or termination sequences, or to create variations in coding regions and/or form new restriction endonuclease sites or destroy preexisting ones, to facilitate further in vitro modification. Any technique for mutagenesis known in the art can be used, including but not limited to, in vitro site-directed mutagenesis (Hutchinson, C., et al., 1978, J. Biol. Chem 253, 6551), use of TAB.RTM. linkers (Pharmacia), etc.
Manipulations of the adhesive sequence may also be made at the protein level. Included within the scope of the invention are toporythmic protein fragments, analogs or derivatives which are differentially modified during or after translation, e.g., by glycosylation, acetylation, phosphorylation, proteolytic cleavage, linkage to an antibody molecule or other cellular ligand, etc. Any of numerous chemical modifications may be carried out by known techniques, including but not limited to specific chemical cleavage by cyanogen bromide, trypsin, chymotrypsin, papain, V8 protease, NaBH.sub.4 ; acetylation, formulation, oxidation, reduction; metabolic synthesis in the presence of tunicamycin; etc.
In addition, analogs and peptides related to adhesive fragments can be chemically synthesized. For example, a peptide corresponding to a portion of a toporythmic protein which mediates the desired aggregation activity in vitro can be synthesized by use of a peptide synthesizer.
Another specific embodiment of the invention relates to fragments or derivatives of a Delta protein which have the ability to bind to a second Delta protein or fragment or derivative thereof, but do not bind to Notch. Such binding or lack thereof can be assayed in vitro as described in Section 8. By way of example, but not limitation, such a Delta derivative is that containing an insertion of the tetrapeptide Arg-Lys-Ile-Phe (SEQ ID NO:30) between Delta residues 197 and 198 of the Drosophila protein.
5.2. The Cloning and Sequencing of Human Notch and Delta
The invention further relates to the amino acid sequences of human Notch and human Delta and fragments and derivatives thereof which comprise an antigenic determinant (i.e., can be recognized by an antibody) or which are functionally active, as well as nucleic acid sequences encoding the foregoing. "Functionally active" material as used herein refers to that material displaying one or more known functional activities associated with the full-length (wild-type) protein product, e.g., in the case of Notch, binding to Delta, binding to Serrate, antigenicity (binding to an anti-Notch antibody), etc. The invention is further directed to the nucleotide sequences of human Notch and human Delta consisting of at least 8 nucleotides.
In a specific embodiment, the invention relates to the nucleic acid sequence of the human Notch gene, in particular, comprising those sequences depicted in FIGS. 19A-19C, 20A-20D, 21A-21B and/or 22A-22D (SEQ ID NO:13 through NO:25), and the encoded Notch protein sequences. Functionally active fragments and derivatives are also provided.
In a preferred, but not limiting, aspect of the invention, a human Notch DNA sequence can be cloned and sequenced by the method described in Section 9, infra.
A preferred embodiment for the cloning of human Delta, presented as a particular example but not by way of limitation follows:
A human expression library is constructed by methods known in the art. For example, human mRNA is isolated, cDNA is made and ligated into an expression vector (e.g., a bacteriophage derivative) such that it is capable of being expressed by the host cell into which it is then introduced. Various screening assays can then be used to select for the expressed human Delta product. In one embodiment, selection can be carried out on the basis of positive binding to the adhesive domain of human Notch, (i.e., that portion of human Notch most homologous to Drosophila ELR 11 and 12 (SEQ ID NO:1)). In an alternative embodiment, anti-Delta antibodies can be used for selection.
In another preferred aspect, PCR is used to amplify the desired sequence in the library, prior to selection. For example, oligonucleotide primers representing part of the adhesive domains encoded by a homologue of the desired gene can be used as primers in PCR.
The above-methods are not meant to limit the following general description of methods by which clones of human Notch and Delta may be obtained.
Any human cell can potentially serve as the nucleic acid source for the molecular cloning of the Notch and Delta gene. The DNA may be obtained by standard procedures known in the art from cloned DNA (e.g., a DNA "library"), by chemical synthesis, by cDNA cloning, or by the cloning of genomic DNA, or fragments thereof, purified from the desired human cell. (See, for example Maniatis et al., 1982, Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.; Glover, D. M. (ed.), 1985, DNA Cloning: A Practical Approach, MRL Press, Ltd., Oxford, U.K. Vol. I, II.) Clones derived from genomic DNA may contain regulatory and intron DNA regions in addition to coding regions; clones derived from cDNA will contain only exon sequences. Whatever the source, the gene should be molecularly cloned into a suitable vector for propagation of the gene.
In the molecular cloning of the gene from genomic DNA, DNA fragments are generated, some of which will encode the desired gene. The DNA may be cleaved at specific sites using various restriction enzymes. Alternatively, one may use DNAse in the presence of manganese to fragment the DNA, or the DNA can be physically sheared, as for example, by sonication. The linear DNA fragments can then be separated according to size by standard techniques, including but not limited to, agarose and polyacrylamide gel electrophoresis and column chromatography.
Once the DNA fragments are generated, identification of the specific DNA fragment containing the desired gene may be accomplished in a number of ways. For example, if an amount of a portion of a Notch or Delta (of any species) gene or its specific RNA, or a fragment thereof e.g., the adhesive domain, is available and can be purified and labeled, the generated DNA fragments may be screened by nucleic acid hybridization to the labeled probe (Benton, W. and Davis, R., 1977, Science 196, 180; Grunstein, M. And Hogness, D., 1975, Proc. Natl. Acad. Sci. U.S.A. 72, 3961). Those DNA fragments with substantial homology to the probe will hybridize. It is also possible to identify the appropriate fragment by restriction enzyme digestion(s) and comparison of fragment sizes with those expected according to a known restriction map if such is available. Further selection can be carried out on the basis of the properties of the gene. Alternatively, the presence of the gene may be detected by assays based on the physical, chemical, or immunological properties of its expressed product. For example, cDNA clones, or DNA clones which hybrid-select the proper mRNAs, can be selected which produce a protein that, e.g., has similar or identical electrophoretic migration, isolectric focusing behavior, proteolytic digestion maps, in vitro aggregation activity ("adhesiveness") or antigenic properties as known for Notch or Delta. If an antibody to Notch or Delta is available, the Notch or Delta protein may be identified by binding of labeled antibody to the putatively Notch or Delta synthesizing clones, in an ELISA (enzyme-linked immunosorbent assay)-type procedure.
The Notch or Delta gene can also be identified by mRNA selection by nucleic acid hybridization followed by in vitro translation. In this procedure, fragments are used to isolate complementary mRNAs by hybridization. Such DNA fragments may represent available, purified Notch or Delta DNA of another species (e.g., Drosophila). Immunoprecipitation analysis or functional assays (e.g., aggregation ability in vitro; see examples infra) of the in vitro translation products of the isolated products of the isolated mRNAs identifies the mRNA and, therefore, the complementary DNA fragments that contain the desired sequences. In addition, specific mRNAs may be selected by adsorption of polysomes isolated from cells to immobilized antibodies specifically directed against Notch or Delta protein. A radiolabelled Notch or Delta cDNA can be synthesized using the selected mRNA (from the adsorbed polysomes) as a template. The radiolabelled mRNA or cDNA may then be used as a probe to identify the Notch or Delta DNA fragments from among other genomic DNA fragments.
Alternatives to isolating the Notch or Delta genomic DNA include, but are not limited to, chemically synthesizing the gene sequence itself from a known sequence or making cDNA to the mRNA which encodes the Notch or Delta gene. For example, RNA for cDNA cloning of the Notch or Delta gene can be isolated from cells which express Notch or Delta. Other methods are possible and within the scope of the invention.
The identified and isolated gene can then be inserted into an appropriate cloning vector. A large number of vector-host systems known in the art may be used. Possible vectors include, but are not limited to, plasmids or modified viruses, but the vector system must be compatible with the host cell used. Such vectors include, but are not limited to, bacteriophages such as lambda derivatives, or plasmids such as PBR322 or pUC plasmid derivatives. The insertion into a cloning vector can, for example, be accomplished by ligating the DNA fragment into a cloning vector which has complementary cohesive termini. However, if the complementary restriction sites used to fragment the DNA are not present in the cloning vector, the ends of the DNA molecules may be enzymatically modified. Alternatively, any site desired may be produced by ligating nucleotide sequences (linkers) onto the DNA termini; these ligated linkers may comprise specific chemically synthesized oligonucleotides encoding restriction endonuclease recognition sequences. In an alternative method, the cleaved vector and Notch or Delta gene may be modified by homopolymeric tailing. Recombinant molecules can be introduced into host cells via transformation, transfection, infection, electroporation, etc., so that many copies of the gene sequence are generated.
In an alternative method, the desired gene may be identified and isolated after insertion into a suitable cloning vector in a "shot gun" approach. Enrichment for the desired gene, for example, by size fractionization, can be done before insertion into the cloning vector.
In specific embodiments, transformation of host cells with recombinant DNA molecules that incorporate the isolated Notch or Delta gene, cDNA, or synthesized DNA sequence enables generation of multiple copies of the gene. Thus, the gene may be obtained in large quantities by growing transformants, isolating the recombinant DNA molecules from the transformants and, when necessary, retrieving the inserted gene from the isolated recombinant DNA.
The human Notch and Delta sequences provided by the instant invention include those nucleotide sequences encoding substantially the same amino acid sequences as found in human Notch and in human Delta, and those encoded amino acid sequences with functionally equivalent amino acids, all as described supra in Section 5.1 for adhesive portions of toporythmic proteins.
5.3. Identification of Additional Members of the Delta/Serrate Family
A rational search for additional members of the Delta/Serrate gene family may be carried out using an approach that takes advantage of the existence of the conserved segments of strong homology between Serrate and Delta (see FIG. 10, SEQ ID NO:3 and NO:4). For example, additional members of this gene family may be identified by selecting, from among a diversity of nucleic acid sequences, those sequences that are homologous to both Serrate and Delta (see FIGS. 13A-13F (SEQ ID NO:5), and FIGS. 15A-15B (SEQ ID NO:8)), and further identifying, from among the selected sequences, those that also contain nucleic acid sequences which are non-homologous to Serrate and Delta. The term "non-homologous" may be construed to mean a region which contains at least about 6 contiguous nucleotides in which at least about two nucleotides differ from Serrate and Delta sequence.
For example, a preferred specific embodiment of the invention provides the following method. Corresponding to two conserved segments between Delta and Serrate, Delta AA 63-73 and Delta AA 195-206 (see FIGS. 13A-13F, SEQ ID NO:6), sets of degenerate oligonucleotide probes of about 10-20 nucleotides may be synthesized, representing all of the possible coding sequences for the amino acids found in either Delta and Serrate for about three to seven contiguous codons. In another embodiment, oligonucleotides may be obtained corresponding to parts of the four highly conserved regions between Delta and Serrate shown in FIGS. 15A-15B (SEQ ID NO:8 and NO:9), i.e., that represented by Serrate AA 124-134, 149-158, 214-219, and 250-259. The synthetic oligonucleotides may be utilized as primers to amplify by PCR sequences from a source (RNA or DNA) of potential interest. (PCR can be carried out, e.g., by use of a Perkin-Elmer Cetus thermal cycler and Taq polymerase (Gene Amp.TM.)). This might include mRNA or cDNA or genomic DNA from any eukaryotic species that could express a polypeptide closely related to Serrate and Delta. By carrying out the PCR reactions, it may be possible to detect a gene or gene product sharing the above-noted segments of conserved sequence between Serrate and Delta. If one chooses to synthesize several different degenerate primers, it may still be possible to carry out a complete search with a reasonably small number of PCR reactions. It is also possible to vary the stringency of hybridization conditions used in priming the PCR reactions, to allow for greater or lesser degrees of nucleotide sequence similarity between the unknown gene and Serrate or Delta. If a segment of a previously unknown member of the Serrate/Delta gene family is amplified successfully, that segment may be molecularly cloned and sequenced, and utilized as a probe to isolate a complete cDNA or genomic clone. This, in turn, will permit the determination of the unknown gene's complete nucleotide sequence, the analysis of its expression, and the production of its protein product for functional analysis. In this fashion, additional genes encoding "adhesive" proteins may be identified.
In addition, the present invention provides for the use of the Serrate/Delta sequence homologies in the design of novel recombinant molecules which are members of the Serrate/Delta gene family but which may not occur in nature. For example, and not by way of limitation, a recombinant molecule can be constructed according to the invention, comprising portions of both Serrate and Delta genes. Such a molecule could exhibit properties associated with both Serrate and Delta and portray a novel profile of biological activities, including agonists as well as antagonists. The primary sequence of Serrate and Delta may also be used to predict tertiary structure of the molecules using computer simulation (Hopp and Woods, 1981, Proc. Natl. Acad. Sci. U.S.A. 78, 3824-3828); Serrate/Delta chimeric recombinant genes could be designed in light of correlations between tertiary structure and biological function. Likewise, chimeric genes comprising portions of any one or more members of the toporythmic gene family (e.g., Notch) may be constructed.
5.4. The Expression of Toporythmic Genes
The nucleotide sequence coding for an adhesive fragment of a toporythmic protein (preferably, Notch, Serrate, or Delta), or an adhesive analog or derivative thereof, or human Notch or Delta or a functionally active fragment or derivative thereof, can be inserted into an appropriate expression vector, i.e., a vector which contains the necessary elements for the transcription and translation of the inserted protein-coding sequence. The necessary transcriptional and translational signals can also be supplied by the native toporythmic gene and/or its flanking regions. A variety of host-vector systems may be utilized to express the protein-coding sequence. These include but are not limited to mammalian cell systems infected with virus (e.g., vaccinia virus, adenovirus, etc.); insect cell systems infected with virus (e.g., baculovirus); microorganisms such as yeast containing yeast vectors, or bacteria transformed with bacteriophage, DNA, plasmid DNA, or cosmid DNA. The expression elements of vectors vary in their strengths and specificities. Depending on the host-vector system utilized, any one of a number of suitable transcription and translation elements may be used. In a specific embodiment, the adhesive portion of the Notch gene, e.g., that encoding EGF-like repeats 11 and 12, is expressed. In another embodiment, the adhesive portion of the Delta gene, e.g., that encoding amino acids 1-230, is expressed. In other specific embodiments, the human Notch or human Delta gene is expressed, or a sequence encoding a functionally active portion of human Notch or Delta. In yet another embodiment, the adhesive portion of the Serrate gene is expressed.
Any of the methods previously described for the insertion of DNA fragments into a vector may be used to construct expression vectors containing a chimeric gene consisting of appropriate transcriptional/translational control signals and the protein coding sequences. These methods may include in vitro recombinant DNA and synthetic techniques and in vivo recombinants (genetic recombination). Expression of nucleic acid sequence encoding a toporythmic protein or peptide fragment may be regulated by a second nucleic acid sequence so that the toporythmic protein or peptide is expressed in a host transformed with the recombinant DNA molecule. For example, expression of a toporythmic protein may be controlled by any promoter/enhancer element known in the art. Promoters which may be used to control toporythmic gene expression include, but are not limited to, the SV40 early promoter region (Bernoist and Chambon, 1981, Nature 290, 304-310), the promoter contained in the 3' long terminal repeat of Rous sarcoma virus (Yamamoto, et al., 1980, Cell 22, 787-797), the herpes thymidine kinase promoter (Wagner et al., 1981, Proc. Natl. Acad. Sci. U.S.A. 78, 1441-1445), the regulatory sequences of the metallothionein gene (Brinster et al., 1982, Nature 296, 39-42); prokaryotic expression vectors such as the .beta.-lactamase promoter (Villa-Kamaroff, et al., 1978, Proc. Natl. Acad. Sci. U.S.A. 75, 3727-3731), or the tac promoter (DeBoer, et al., 1983, Proc. Natl. Acad. Sci. U.S.A. 80, 21-25); see also "Useful proteins from recombinant bacteria" in Scientific American, 1980, 242, 74-94; plant expression vectors comprising the nopaline synthetase promoter region (Herrera-Estrella et al., Nature 303, 209-213) or the cauliflower mosaic virus 35S RNA promoter (Gardner, et al., 1981, Nucl. Acids Res. 9, 2871), and the promoter of the photosynthetic enzyme ribulose biphosphate carboxylase (Herrera-Estrella et al., 1984, Nature 310, 115-120); promoter elements from yeast or other fungi such as the Gal 4 promoter, the ADC (alcohol dehydrogenase) promoter, PGK (phosphoglycerol kinase) promoter, alkaline phosphatase promoter, and the following animal transcriptional control regions, which exhibit tissue specificity and have been utilized in transgenic animals: elastase I gene control region which is active in pancreatic acinar cells (Swift et al., 1984, Cell 38, 639-646; Ornitz et al., 1986, Cold Spring Harbor Symp. Quant. Biol. 50, 399-409; MacDonald, 1987, Hepatology 7, 425-515); insulin gene control region which is active in pancreatic beta cells (Hanahan, 1985, Nature 315, 115-122), immunoglobulin gene control region which is active in lymphoid cells (Grosschedl et al., 1984, Cell 38, 647-658; Adames et al., 1985, Nature 318, 533-538; Alexander et al., 1987, Mol. Cell. Biol. 7, 1436-1444), mouse mammary tumor virus control region which is active in testicular, breast, lymphoid and mast cells (Leder et al., 1986, Cell 45, 485-495), albumin gene control region which is active in liver (Pinkert et al., 1987, Genes and Devel. 1, 268-276), alpha-fetoprotein gene control region which is active in liver (Krumlauf et al., 1985, Mol. Cell. Biol. 5, 1639-1648; Hammer et al., 1987, Science 235, 53-58; alpha 1-antitrypsin gene control region which is active in the liver (Kelsey et al., 1987, Genes and Devel. 1, 161-171), beta-globin gene control region which is active in myeloid cells (Mogram et al., 1985, Nature 315, 338-340; Kollias et al., 1986, Cell 46, 89-94; myelin basic protein gene control region which is active in oligodendrocyte cells in the brain (Readhead et al., 1987, Cell 48, 703-712); myosin light chain-2 gene control region which is active in skeletal muscle (Sani, 1985, Nature 314, 283-286), and gonadotropic releasing hormone gene control region which is active in the hypothalamus (Mason et al., 1986, Science 234, 1372-1378).
Expression vectors containing toporythmic gene inserts can be identified by three general approaches: (a) nucleic acid hybridization, (b) presence or absence of "marker" gene functions, and (c) expression of inserted sequences. In the first approach, the presence of a foreign gene inserted in an expression vector can be detected by nucleic acid hybridization using probes comprising sequences that are homologous to an inserted toporythmic gene. In the second approach, the recombinant vector/host system can be identified and selected based upon the presence or absence of certain "marker" gene functions (e.g., thymidine kinase activity, resistance to antibiotics, transformation phenotype, occlusion body formation in baculovirus, etc.) caused by the insertion of foreign genes in the vector. For example, if the toporythmic gene is inserted within the marker gene sequence of the vector, recombinants containing the toporythmic insert can be identified by the absence of the marker gene function. In the third approach, recombinant expression vectors can be identified by assaying the foreign gene product expressed by the recombinant. Such assays can be based, for example, on the physical or functional properties of the toporythmic gene product in vitro assay systems, e.g., aggregation (adhesive) ability (see Sections 6-8, infra).
Once a particular recombinant DNA molecule is identified and isolated, several methods known in the art may be used to propagate it. Once a suitable host system and growth conditions are established, recombinant expression vectors can be propagated and prepared in quantity. As previously explained, the expression vectors which can be used include, but are not limited to, the following vectors or their derivatives: human or animal viruses such as vaccinia virus or adenovirus; insect viruses such as baculovirus; yeast vectors; bacteriophage vectors (e.g., lambda), and plasmid and cosmid DNA vectors, to name but a few.
In addition, a host cell strain may be chosen which modulates the expression of the inserted sequences, or modifies and processes the gene product in the specific fashion desired. Expression from certain promoters can be elevated in the presence of certain inducers; thus, expression of the genetically engineered toporythmic protein may be controlled. Furthermore, different host cells have characteristic and specific mechanisms for the translational and post-translational processing and modification (e.g., glycosylation, cleavage) of proteins. Appropriate cell lines or host systems can be chosen to ensure the desired modification and processing of the foreign protein expressed. For example, expression in a bacterial system can be used to produce an unglycosylated core protein product. Expression in yeast will produce a glycosylated product. Expression in mammalian cells can be used to ensure "native" glycosylation of a heterologous mammalian toporythmic protein. Furthermore, different vector/host expression systems may effect processing reactions such as proteolytic cleavages to different extents.
In other specific embodiments, the adhesive toporythmic protein, fragment, analog, or derivative may be expressed as a fusion, or chimeric protein product (comprising the protein, fragment, analog, or derivative joined to a heterologous protein sequence). Such a chimeric product can be made by ligating the appropriate nucleic acid sequences encoding the desired amino acid sequences to each other by methods known in the art, in the proper coding frame, and expressing the chimeric product by methods commonly known in the art. Alternatively, such a chimeric product may be made by protein synthetic techniques, e.g., by use of a peptide synthesizer.
Both cDNA and genomic sequences can be cloned and expressed.
5.4.1. Identification and Purification of the Expressed Gene Product
Once a recombinant which expresses the toporythmic gene sequence is identified, the gene product may be analyzed. This can be achieved by assays based on the physical or functional properties of the product, including radioactive labelling of the product followed by analysis by gel electrophoresis.
Once the toporythmic protein is identified, it may be isolated and purified by standard methods including chromatography (e.g., ion exchange, affinity, and sizing column chromatography), centrifugation, differential solubility, or by any other standard technique for the purification of proteins. The functional properties may be evaluated using any suitable assay, including, but not limited to, aggregation assays (see Sections 6-8).
5.5. Generation of Antibodies to Toporythmic Proteins and Adhesive Sequences Thereof
According to the invention, toporythmic protein fragments or analogs or derivatives thereof which mediate homotypic or heterotypic binding, or human Notch or human Delta proteins or fragments thereof, may be used as an immunogen to generate anti-toporythmic protein antibodies. Such antibodies can be polyclonal or monoclonal. In a specific embodiment, antibodies specific to EGF-like repeats 11 and 12 of Notch may be prepared. In other embodiments, antibodies reactive with the "adhesive portion" of Delta can be generated. One example of such antibodies may prevent aggregation in an in vitro assay. In another embodiment, antibodies specific to human Notch are produced.
Various procedures known in the art may be used for the production of polyclonal antibodies to a toporythmic protein or peptide. In a particular embodiment, rabbit polyclonal antibodies to an epitope of the human Notch protein encoded by a sequence depicted in FIGS. 19A-19C, 20A-20D, 21A-21B or 22A-22D (SEQ ID NO:13 through NO:25), or a subsequence thereof, can be obtained. For the production of antibody, various host animals can be immunized by injection with the native toporythmic protein, or a synthetic version, or fragment thereof, including but not limited to rabbits, mice, rats, etc. Various adjuvants may be used to increase the immunological response, depending on the host species, and including but not limited to Freund's (complete and incomplete), mineral gels such as aluminum hydroxide, surface active substances such as lysolecithin, pluronic polyols, polyanions, peptides, oil emulsions, keyhold limpet hemocyanins, dinitrophenol, and potentially useful human adjuvants such as BCG (bacille Calmette-Guerin) and corynebacterium parvum.
For preparation of monoclonal antibodies directed toward a toporythmic protein sequence, any technique which provides for the production of antibody molecules by continuous cell lines in culture may be used. For example, the hybridoma technique originally developed by Kohler and Milstein (1975, Nature 256, 495-497), as well as the trioma technique, the human B-cell hybridoma technique (Kozbor et al., 1983, Immunology Today 4, 72), and the EBV-hybridoma technique to produce human monoclonal antibodies (Cole et al., 1985, in Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp. 77-96).
Antibody fragments which contain the idiotype of the molecule can be generated by known techniques. For example, such fragments include but are not limited to: the F(ab').sub.2 fragment which can be produced by pepsin digestion of the antibody molecule; the Fab' fragments which can be generated by reducing the disulfide bridges of the F(ab').sub.2 fragment, and the Fab fragments which can be generated by treating the antibody molecule with papain and a reducing agent.
In the production of antibodies, screening for the desired antibody can be accomplished by techniques known in the art, e.g. ELISA (enzyme-linked immunosorbent assay). For example, to select antibodies which recognize the adhesive domain of a toporythmic protein, one may assay generated hybridomas for a product which binds to a protein fragment containing such domain. For selection of an antibody specific to human Notch, one can select on the basis of positive binding to human Notch and a lack of binding to Drosophila Notch.
The foregoing antibodies can be used in methods known in the art relating to the localization and activity of the protein sequences of the invention. For example, various immunoassays known in the art can be used, including but not limited to competitive and non-competitive assay systems using techniques such as radioimmunoassays, ELISA (enzyme linked immunosorbent assay), "sandwich" immunoassays, precipitin reactions, gel diffusion precipitin reactions, immunodiffusion assays, agglutination assays, fluorescent immunoassays, protein A immunoassays, and immunoelectrophoresis assays, to name but a few.
5.6. Delivery of Agents into Notch-expressing Cells
The invention also provides methods for delivery of agents into Notch-expressing cells. As discussed in Section 8 infra, upon binding to a Notch protein on the surface of a Notch-expressing cell, Delta protein appears to be taken up into the Notch-expressing cell. The invention thus provides for delivery of agents into a Notch-expressing cell by conjugation of an agent to a Delta protein or an adhesive fragment or derivative thereof capable of binding to Notch, and exposing a Notch-expressing cell to the conjugate, such that the conjugate is taken up by the cell. The conjugated agent can be, but is not limited to, a label or a biologically active agent. The biologically active agent can be a therapeutic agent, a toxin, a chemotherapeutic, a growth factor, an enzyme, a hormone, a drug, a nucleic acid, (e.g., antisense DNA or RNA), etc. In one embodiment, the label can be an imaging agent, including but not limited to heavy metal contrast agents for x-ray imaging, magnetic resonance imaging agents, and radioactive nuclides (i.e., isotopes) for radioimaging. In a preferred aspect, the agent is conjugated to a site in the amino terminal half of the Delta molecule.
The Delta-agent conjugate can be delivered to the Notch-expressing cell by exposing the Notch-expressing cell to cells expressing the Delta-agent conjugate or exposing the Notch-expressing cell to the Delta-agent conjugate in a solution, suspension, or other carrier. Where delivery is in vivo, the Delta-agent conjugate can be formulated in a pharmaceutically acceptable carrier or excipient, to comprise a pharmaceutical composition. The pharmaceutically acceptable carrier can comprise saline, phosphate buffered saline, etc. The Delta-agent conjugate can be formulated as a liquid, tablet, pill, powder, in a slow-release form, in a liposome, etc., and can be administered orally, intravenously, intramuscularly, subcutaneously, intraperitoneally, to name but a few routes, with the preferred choice readily made based on the knowledge of one skilled in the art.
6. MOLECULAR INTERACTIONS BETWEEN THE PROTEIN PRODUCTS OF THE NEUROGENIC LOCI NOTCH AND DELTA, TWO EGF-HOMOLOGOUS GENES IN DROSOPHILA
To examine the possibility of intermolecular association between the products of the Notch and Delta genes, we studied the effects of their expression on aggregation in Drosophila Schneider's 2 (S2) cells (Fehon et al., 1990, Cell 61, 523-534). We present herein direct evidence of intermolecular interactions between Notch and Delta, and describe an assay system that will be used in dissecting the components of this interaction. We show that normally nonadhesive Drosophila S2 cultured cells that express Notch bind specifically to cells that express Delta, and that this aggregation is calcium dependent. Furthermore, while cells that express Notch do not bind to one another, cells that express Delta do bind to one another, suggesting that Notch and Delta can compete for binding to Delta at the cell surface. We also present evidence indicating that Notch and Delta form detergent-soluble complexes both in cultured cells and embryonic cells, suggesting that Notch and Delta interact directly at the molecular level in vitro and in vivo. Our analyses suggest that Notch and Delta proteins interact at the cell surface via their extracellular domains.
6.1. Experimental Procedures
6.1.1. Expression Constructs
For the Notch expression construct, the 6 kb HpaI fragment from the 5' end of the Notch coding sequence in MgIIa (Ramos et al., 1989, Genetics 123, 337-348) was blunt-end ligated into the metallothionein promoter vector pRmHa-3 (Bunch, et al., 1988, Nucl. Acids Res. 16, 1043-1061) after the vector had been cut with EcoRI and the ends were filled with the Klenow fragment of DNA polymerase I (Maniatis et al., 1982, Molecular Cloning: A Laboratory Manual (Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory)). A single transformant, incorrectly oriented, was isolated. DNA from this transformant was then digested with SacI, and a resulting 3 kb fragment was isolated that contained the 5' end of the Notch coding sequence fused to the polylinker from pRmHa-3. This fragment was then ligated into the SacI site of pRmHa-3 in the correct orientation. DNA from this construct was digested with KpnI and XbaI to remove must of the Notch sequence and all of the Adh polyadenylation signal in pRmHa-3 and ligated to an 11 kb KpnI-XbaI fragment from MgIIa containing the rest of the Notch coding sequence and 3' sequences necessary for polyadenylation. In the resulting construct, designated pMtNMg, the metallothionein promoter in pRmHa-3 is fused to Notch sequences starting 20 nucleotides upstream of the translation start site.
For the extracellular Notch construct (ECN1), the CosP479BE Notch cosmid (Ramos et al., 1989, Genetics 123, 337-348), which contains all Notch genomic sequences necessary for normal Notch function in vivo, was partially digested with AatII. Fragment ends were made blunt using the exonuclease activity of T4 DNA polymerase (Maniatis et al., 1982, Molecular Cloning: A Laboratory Manual (Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory)), and the fragments were then redigested completely with StuI. The resulting fragments were separated in a low melting temperature agarose gel (SeaPlaque, FMC BioProducts), and the largest fragment was excised. This fragment was then blunt-end ligated to itself. This resulted in an internal deletion of the Notch coding sequences from amino acid 1790 to 2625 inclusive (Wharton et al., 1985, Cell 43, 567-581), and a predicted frameshift that produces a novel 59 amino acid carboxyl terminus. (The ligated junction of this construct has not been checked by sequencing.)
For the Delta expression construct, the Dl1 cDNA (Kopczynski et al., 1988, Genes Dev. 2, 1723-1735), which includes the complete coding capacity for Delta, was inserted into the EcoRI site of pRmHa-3. This construct was called pMTDl1.
6.1.2. Antibody Preparation
Hybridoma cell line C17.9C6 was obtained from a mouse immunized with a fusion protein based on a 2.1 kb SalI-HindIII fragment that includes coding sequences for most of the intracellular domain of Notch (amino acids 1791-2504; Wharton et al., 1985, Cell 43, 567-581). The fragment was subcloned into pUR289 (Ruther and Muller-Hill, 1983, EMBO J. 2, 1791-1794), and then transferred into the pATH 1 expression vector (Dieckmann and Tzagoloff, 1985, J. Biol. Chem. 260, 1513-1520) as a BglII-HindIII fragment. Soluble fusion protein was expressed, precipitated by 25% (NH.sub.4).sub.2 SO.sub.4, resuspended in 6M urea, and purified by preparative isoelectric focusing using a Rotofor (Bio-Rad) (for details, see Fehon, 1989, Rotofor Review No. 7, Bulletin 1518, Richmond, Calif.: Bio-Rad Laboratories).
Mouse polyclonal antisera were raised against the extracellular domain of Notch using four BstYl fragments of 0.8 kb (amino acids 237-501: Wharton et al., 1985, Cell 43, 567-581), 1.1 kb (amino acids 501-868), 0.99 kb (amino acids 868-1200), and 1.4 kb (amino acids 1465-1935) length, which spanned from the fifth EGF-like repeat across the transmembrane domain, singly inserted in-frame into the appropriate pGEX expression vector (Smith and Johnson, 1988, Gene 67, 31-40). Fusion proteins were purified on glutathione-agarose beads (SIGMA). Mouse and rat antisera were precipitated with 50% (NH.sub.4).sub.2 SO.sub.4 and resuspended in PBS (150 mM NaCl, 14 mM Na.sub.2 HPO.sub.4, 6 mM NaH.sub.2 PO.sub.4) with 0.02% NaN.sub.3.
Hybridoma cell line 201 was obtained from a mouse immunized with a fusion protein based on a 0.54 kb ClaI fragment that includes coding sequences from the extracellular domain of Delta (Kopczynski et al., 1988, Genes Dev. 2, 1723-1735) subcloned into the ClaI site within the lacZ gene of pUR 288 (Ruther and Muller-Hill, 1983, EMBO J. 2, 1791-1794). This fragment includes sequences extending from the fourth through the ninth EGF-like repeats in Delta (amino acids 350-529). Fusion protein was prepared by isolation of inclusion bodies (Gilmer et al., 1982, Proc. Natl. Acad. Sci. USA 79, 2152-2156); inclusion bodies were solubilized in urea (Carroll and Laughon, 1987, in DNA Cloning, Volume III, D. M. Glover, ed. (Oxford: IRL Press), pp. 89-111) before use in immunization.
Rat polyclonal antisera were obtained following immunization with antigen derived from the same fusion protein construct. In this case, fusion protein was prepared by lysis of IPTG-induced cells in SDS-Laemmli buffer (Carroll and Laughon, 1987, in DNA Cloning, Volume III, D. M. Glover, ed. (Oxford: IRL Press), pp. 89-111), separation of proteins by SDS-PAGE, excision of the appropriate band from the gel, and electroelution of antigen from the gel slice for use in immunization (Harlow and Lane, 1988, Antibodies: A Laboratory Manual (Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory)).
6.1.3. Cell Culture and Transfection
The S2 cell line (Schneider, 1972, J. Embryol. Exp. Morph. 27, 353-365) was grown in M3 medium (prepared by Hazleton Co.) supplemented with 2.5 mg/ml Bacto-Peptone (Difco), 1 mg/ml TC Yeastolate (Difco), 11% heat-inactivated fetal calf serum (FCS) (Hyclone), and 100 U/ml penicillin-100 .mu.g/ml streptomycin-0.25 .mu.g/ml fungizone (Hazleton). Cells growing in log phase at .about.2.times.10.sup.6 cells/ml were transfected with 20 .mu.g of DNA-calcium phosphate coprecipitate in 1 ml per 5 ml of culture as previously described (Wigler et al., 1979, Proc. Natl. Acad. Sci. USA 78, 1373-1376), with the exception that BES buffer (SIGMA) was used in place of HEPES buffer (Chen and Okayama, 1987, Mol. Cell. Biol. 7, 2745-2752). After 16-18 hr, cells were transferred to conical centrifuge tubes, pelleted in a clinical centrifuge at full speed for 30 seconds, rinsed once with 1/4 volume of fresh complete medium, resuspended in their original volume of complete medium, and returned to the original flask. Transfected cells were then allowed to recover for 24 hr before induction.
6.1.4. Aggregation Assays
Expression of the Notch and Delta metallothionein constructs was induced by the addition of CuSO.sub.4 to 0.7 mM. Cells transfected with the ECN1 construct were treated similarly. Two types of aggregation assays were used. In the first assay, a total of 3 ml of cells (5-10.times.10.sup.6 cells/ml) was placed in a 25 ml Erlenmeyer flask and rotated at 40-50 rpm on a rotary shaker for 24-48 hr at room temperature. For these experiments, cells were mixed 1-4 hr after induction began and induction was continued throughout the aggregation period. In the second assay, .about.0.6 ml of cells were placed in a 0.6 ml Eppendorf tube (leaving a small bubble) after an overnight induction (12-16 hr) at room temperature and rocked gently for 1-2 hr at 4.degree. C. The antibody inhibition and Ca.sup.2+ dependence experiments were performed using the latter assay. For Ca.sup.2+ dependence experiments, cells were first collected and rinsed in balanced saline solution (BSS) with 11% FCS (BSS-FCS; FCS was dialyzed against 0.9% NaCl, 5 mM Tris pH 7.5!) or in Ca.sup.2+ free BSS-FCS containing 10 mM EGTA (Snow et al., 1989, Cell 59, 313-323) and then resuspended in the same medium at the original volume. For the antibody inhibition experiments, Notch-transfected cells were collected and rinsed in M3 medium and then treated before aggregation in M3 medium for 1 hr at 4.degree. C. with a 1:250 dilution of immune or preimmune sera from each of the four mice immunized with fusion proteins containing segments from the extracellular domain of Notch (see Antibody Preparation above).
6.1.5. Immunofluorescence
Cells were collected by centrifugation (3000 rpm for 20 seconds in an Eppendorf microcentrifuge) and fixed in 0.6 ml Eppendorf tubes with 0.5 ml of freshly made 2% paraformaldehyde in PBS for 10 min at room temperature. After fixing, cells were collected by centrifugation, rinsed twice in PBS, and stained for 1 hr in primary antibody in PBS with 0.1% saponin (SIGMA) and 1% normal goat serum (Pocono Rabbit Farm, Canadensis, Pa.). Monoclonal antibody supernatants were diluted 1:10 and mouse or rat sera were diluted 1:1000 for this step. Cells were then rinsed once in PBS and stained for 1 hr in specific secondary antibodies (double-labeling grade goat anti-mouse and goat anti-rat, Jackson Immunoresearch) in PBS-saponin-normal goat serum. After this incubation, cells were rinsed twice in PBS and mounted on slides in 90% glycerol, 10% 1M Tris (pH 8.0), and 0.5% n-propyl gallate. Cells were viewed under epifluorescence on a Leitz Orthoplan 2 microscope.
Confocal micrographs were taken using the Bio-Rad MRC 500 system connected to a Zeiss Axiovert compound microscope. Images were collected using the BHS and GHS filter sets, aligned using the ALIGN program, and merged using MERGE. Fluorescent bleed-through from the green into the red channel was reduced using the BLEED program (all software provided by Bio-Rad). Photographs were obtained directly from the computer monitor using Kodak Ektar 125 film.
6.1.6. Cell Lysates, Immunoprecipitations, and Western Blots
Nondenaturing detergent lysates of tissue culture and wild-type Canton-S embryos were prepared on ice in .about.10 cell vol of lysis buffer (300 mM NaCl, 50 mM Tris pH 8.0!, 0.5% NP-40, 0.5% deoxycholate, 1 mM CaCl.sub.2, 1 mM MgCl.sub.2) with 1 mM phenylmethysulfonyl fluoride (PMSF) and diisopropyl fluorophosphate diluted 1:2500 as protease inhibitors. Lysates were sequentially triturated using 18G, 21G, and 25G needles attached to 1 cc tuberculin syringes and then centrifuged at full speed in a microfuge 10 min at 4.degree. C. to remove insoluble material. Immunoprecipitation was performed by adding .about.1 .mu.g of antibody (1-2 .mu.l of polyclonal antiserum) to 250-500 .mu.l of cell lysate and incubating for 1 hr at 4.degree. C. with agitation. To this mixture, 15 .mu.g of goat anti-mouse antibodies (Jackson Immunoresearch; these antibodies recognize both mouse and rat IgG) were added and allowed to incubate for 1 hr at 4.degree. C. with agitation. This was followed by the addition of 100 .mu.l of fixed Staphylococcus aureus (Staph A) bacteria (Zysorbin, Zymed; resuspended according to manufacturer's instructions), which had been collected, washed five times in lysis buffer, and incubated for another hour. Staph A-antibody complexes were then pelleted by centrifugation and washed three times in lysis buffer followed by two 15 min washes in lysis buffer. After being transferred to a new tube, precipitated material was suspended in 50 .mu.l of SDS-PAGE sample buffer, boiled immediately for 10 min, run on 3%-15% gradient gels, blotted to nitrocellulose, and detected using monoclonal antibodies and HRP-conjugated goat anti-mouse secondary antibodies as previously described (Johansen et al., 1989, J. Cell Biol. 109, 2427-2440). For total cellular protein samples used on Western blots (FIGS. 2A-2B), cells were collected by centrifugation, lysed in 10 cell vol of sample buffer that contained 1 mM PMSF, and boiled immediately.
6.2. Results
6.2.1. The Expression of Notch and Delta in Cultured Cells
To detect interactions between Notch and Delta, we examined the behavior of cells expressing these proteins on their surfaces using an aggregation assay. We chose the S2 cell line (Schneider, 1972, J. Embryol. Exp. Morph. 27, 353-365) for these studies for several reasons. First, these cells are relatively nonadhesive, grow in suspension, and have been used previously in a similar assay to study fasciclin III function (Snow et al., 1989, Cell 59, 313-323). Second, they are readily transfectable, and an inducible metallothionein promoter vector that has been designed for expression of exogenous genes in Drosophila cultured cells is available (Bunch et al., 1988, Nucl. Acids Res. 16, 1043-1061). Third, S2 cells express an aberrant Notch message and no detectable Notch due to a rearrangement of the 5' end of the Notch coding sequence (see below). These cells also express no detectable Delta (see below).
Schematic drawings of the constructs used are shown in FIG. 1 (see Experimental Procedures, Section 6.1, for details). To express Notch in cultured cells, the Notch minigene MGlla, described in Ramos et al. (1989, Genetics 123, 337-348) was inserted into the metallothionein promoter vector pRmHa-3 (Bunch et al., 1988, Nucl. Acids Res. 16, 1043-1061). The Delta expression construct was made by inserting Dl1 cDNA, which contains the entire coding sequence for Delta from the major embryonic Delta transcript (5.4Z; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735), into the same vector. A third construct, designated ECN1 for "extracellular Notch 1", contains the 5' Notch promoter region and 3' Notch polyadenylation signal together with coding capacity for the extracellular and transmembrane regions of the Notch gene from genomic sequences, but lacks coding sequences for 835 amino acids of the .about.1000 amino acid intracellular domain. In addition, due to a predicted frameshift, the remaining 78 carboxy-terminal amino acid residues are replaced by a novel 59 amino acid carboxyterminal tail (see Experimental Procedures).
For all of the experiments described in this paper, expression constructs were transfected into S2 cells and expressed transiently rather than in stable transformants. Expressing cells typically composed 1%-5% of the total cell population, as judged by immunofluorescent staining (data not shown). A Western blot of proteins expressed after transfection is shown in FIGS. 2A-2B. Nontransfected cells do not express detectable levels of Notch or Delta. However, after transfection, proteins of the predicted apparent molecular weights are readily detectable using monoclonal antibodies specific for each of these proteins, respectively. In the case of Notch, multiple bands were apparent in transfected cells below the .about.300 kd full-length product. We do not yet know whether these bands represent degradation of Notch during sample preparation or perhaps synthesis or processing intermediates of Notch that are present within cells, but we consistently detect them in samples from transfected cells and from embryos. In addition, we performed immunofluorescent staining of live transfected cells with antibodies specific for the extracellular domains of each protein to test for cell surface expression of these proteins. In each case we found surface staining as expected for a surface antigen. Taken together, these results clearly show that the Notch and Delta constructs support expression of proteins of the expected sizes and subcellular localization.
6.2.2. Cells that Express Notch and Delta Aggregate
To test the prediction that Notch and Delta interact, we designed a simple aggregation assay to detect these interactions between proteins expressed on the surface of S2 cells. We reasoned that if Notch and Delta are able to form stable heterotypic complexes at the cell surface, then cells that express these proteins might bind to one another and form aggregates under appropriate conditions. A similar assay system has recently been described for the fasciclin III protein (Snow et al., 1989, Cell 59, 313-323).
S2 cells in log phase growth were separately transfected with either the Notch or Delta metallothionein promoter construct. After induction with CuSO.sub.4, transfected cells were mixed in equal numbers and allowed to aggregate overnight at room temperature (for details, see Experimental Procedures, Section 6.1). Alternatively, in some experiments intended to reduce metabolic activity, cells were mixed gently at 4.degree. C. for 1-2 hr. To determine whether aggregates had formed, cells were processed for immunofluorescence microscopy using antibodies specific for each gene product and differently labeled fluorescent secondary antibodies. As previously mentioned, expressing cells usually constituted less than 5% of the total cell population because we used transient rather than stable transformants. The remaining cells either did not express a given protein or expressed at levels too low for detection by immunofluorescence microscopy. As controls, we performed aggregations with only a single type of transfected cell.
FIGS. 3A-3I shows representative photomicrographs from aggregation experiments, and Table I presents the results in numerical form. As is apparent from FIG. 3C and Table I, while Notch-expressing (Notch.sup.+) cells alone do not form aggregates in our assay, Delta-expressing (Delta.sup.+) cells do.
The tendency for Delta.sup.+ cells to aggregate was apparent even in nonaggregated control samples (Table I), where cell clusters of 4-8 cells that probably arose from adherence between mitotic sister cells commonly occurred. However, clusters were more common after incubation under aggregation conditions (e.g., 19% of Delta.sup.+ cells in aggregates before incubation vs. 37% of Delta.sup.+ cells in aggregates after incubation; Experiment 1 in Table I), indicating that Delta.sup.+ cells are able to form stable contacts with one another in this assay. It is important to note that while nonstaining cells constituted over 90% of the cells in our transient transfections, we never found them within aggregates. On rare occasions, nonstaining cells were found at the edge of an aggregate. Due to the common occurrence of weakly staining cells at the edges of aggregates, it is likely that these apparently nonexpressing cells were transfected but expressed levels of Delta insufficient to be detected by immunofluorescence.
In remarkable contrast to control experiments with Notch.sup.+ cells alone, aggregation of mixtures of Notch.sup.+ and Delta.sup.+ cells resulted in the formation of clusters of up to 20 or more cells (FIGS. 3D-3H, Table I). As Table I shows, the fraction of expressing cells found in clusters of four or more stained cells after 24 hr of aggregation ranged from 32%-54% in mixtures of Notch.sup.+ and Delta.sup.+ cells. This range was similar to that seen for Delta.sup.+ cells alone (37%-40%) but very different from that for Notch.sup.+ cells alone (only 0%-5%). Although a few clusters that consisted only of Delta.sup.+ cells were found, Notch.sup.+ cells were never found in clusters of greater than four to five cells unless Delta.sup.+ cells were also present. Again, all cells within these clusters expressed either Notch or Delta, even though transfected cells composed only a small fraction of the total cell population. At 48 hr (Table I, experiments 5 and 6), the degree of aggregation appeared higher (63%-71%), suggesting that aggregation had not yet reached a maximum after 24 hr under these conditions. Also, cells cotransfected with Notch and Delta constructs (so that all transfected cells express both proteins) aggregated in a similar fashion under the same experimental conditions.
These results indicate that the aggregation observed in these experiments requires the expression of Notch and Delta and is not due to the fortuitous expression of another interacting protein in nontransfected S2 cells. We further tested the specificity of this interaction by diluting Notch.sup.+ and Delta.sup.+ cells 10-fold with nontransfected S2 cells and allowing them to aggregate for 24 hr at room temperature. In this experiment, 39% of the expressing cells were found in aggregates with other expressing cells, although they composed less than 0.1% of the total cell population. Not surprisingly, however, these aggregates were smaller on average than those found in standard aggregation experiments. In addition, to control for the possibility that Notch.sup.+ cells are nonspecifically recruited into the Delta.sup.+ aggregates because they overexpress a single type of protein on the cell surface, we mixed Delta.sup.+ cells with cells that expressed neuroglian, a transmembrane cell-surface protein (Bieber et al., 1989, Cell 59, 447-460), under the control of the metallothionein promoter (this metallothionein-neuroglian construct was kindly provided by A. Bieber and C. Goodman). We observed no tendency for neuroglian-cells to adhere to Delta.sup.+ aggregates, indicating that Notch-Delta aggregation is not merely the result of high levels of protein expression on the cell surface.
We also tested directly for Notch involvement in the aggregation process by examining the effect of a mixture of polyclonal antisera directed against fusion proteins that spanned almost the entire extracellular domain of Notch on aggregation (see Experimental Procedures, Section 6.1). To minimize artifacts that might arise due to a metabolic response to patching of surface antigens, antibody treatment and the aggregation assay were performed at 4.degree. C. in these experiments. Notch.sup.+ cells were incubated with either preimmune or immune mouse sera for 1 hr, Delta.sup.+ cells were added, and aggregation was performed for 1-2 hr. While Notch.sup.+ cells pretreated with preimmune sera aggregated with Delta.sup.+ cells (in one of three experiments, 23% of the Notch.sup.+ cells were in Notch.sup.+ -Delta.sup.+ cell aggregates), those treated with immune sera did not (only 2% of Notch.sup.+ cells were in aggregates). This result suggests that the extracellular domain of Notch is required for Notch.sup.+ -Delta.sup.+ cell aggregation, although we cannot rule out the possibility that the reduced aggregation was due to inhibitory steric or membrane structure effects resulting from exposure of Notch.sup.+ cells to the antiserum.
Three other observations worth noting are apparent in FIGS. 3A-3I. First, while Delta was almost always apparent only at the cell surface (FIGS. 3B and 3C), Notch staining was always apparent both at the cell surface and intracellularly, frequently associated with vesicular structures (FIG. 3A). Second, we consistently noted a morphological difference between Delta.sup.+ and Notch.sup.+ cells in mixed aggregates that were incubated overnight. Delta.sup.+ cells often had long extensions that completely surrounded adjacent Notch.sup.+ cells, while Notch.sup.+ cells were almost always rounded in appearance without noticeable cytoplasmic extensions (FIG. 3G). Third, Notch and Delta often appeared to gather within regions of contact between Notch.sup.+ and Delta.sup.+ cells, producing a sharp band of immunofluorescent staining (FIGS. 3D-3F). These bands were readily visible in optical sections viewed on the confocal microscope (FIG. 3H), indicating that they were not merely due to a whole-mount artifact. We also observed that these bands formed rapidly (within 2 hr of mixing cells) and at 4.degree. C., indicating that their formation probably did not depend upon cellular metabolism. These observations would be expected if, within regions of cell contact, Notch and Delta bind to one another and therefore become immobilized. This pattern of expression is also consistent with that observed for other proteins that mediate cell aggregation (Takeichi, 1988, Development 102, 639-655; Snow et al., 1989, Cell 59, 313-323).
6.2.3. Notch-Delta-Mediated Aggregation is Calcium Dependent
Previous studies have suggested that EGF-like repeats that contain a particular consensus sequence may serve as calcium (Ca.sup.2+) binding domains (Morita et al., 1984, J. Biol. Chem. 259, 5698-5704; Sugo et al., 1984, J. Biol. Chem. 259, 5705-5710; Rees et al., 1988, EMBO J. 7, 2053-2061; Handford et al., 1990, EMBO J. 9, 475-480). For at least two of these proteins, C and Cl, Ca.sup.2+ binding has further been shown to be a necessary component of their interactions with other proteins (Villiers et al., 1980, FEBS Lett. 117, 289-294; Esmon et al., 1983, J. Biol. Chem. 258, 5548-5553; Johnson, et al., 1983, J. Biol. Chem. 258, 5554-5560). Many of the EGF-homologous repeats within Notch and most of those within Delta contain the necessary consensus sequence for Ca.sup.2+ binding (Rees et al., 1988, EMBO J. 7, 2053-2061; Stenflo et al., 1987, Proc. Natl. Acad. Sci. USA 84, 368-372; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735; Handford et al., 1990, EMBO J. 9, 475-480), although it has not yet been determined whether or not these proteins do bind calcium. We therefore tested the ability of expressing cells to aggregate in the presence or absence of Ca.sup.2+ ions to determine whether there is a Ca.sup.2+ ion requirement for Notch-Delta aggregation. To minimize possible nonspecific effects due to metabolic responses to the removal of Ca.sup.2+, these experiments were performed at 4.degree. C. Control mixtures of Notch.sup.+ and Delta.sup.+ cells incubated under aggregation conditions in Ca.sup.2+ -containing medium at 4.degree. C. readily formed aggregates (an average of 34%.+-.13%, mean.+-.SD, n=3; Table II). In contrast, cells mixed in medium that lacked Ca.sup.2+ ions and contained EGTA formed few aggregates (5%.+-.5%). These results clearly demonstrate a dependence of Notch-Delta-mediated aggregation on exogenous Ca.sup.2+ and are in marked contrast to those recently published for the Drosophila fasciclin III and fasciclin I proteins in S2 cells (Snow et al., 1989, Cell 59, 313-323; Elkins et al., 1990, J. Cell Biol. 110, 1825-1832), which detected no effect of Ca.sup.2+ ion removal on aggregation mediated by either protein.
6.2.4. Notch and Delta Interact within a Single Cell
We asked whether Notch and Delta are associated within the membrane of one cell that expresses both proteins by examining the distributions of Notch and Delta in cotransfected cells. As shown in FIGS. 4A and 4B, these two proteins often show very similar distributions at the surface of cotransfected cells. To test whether the observed colocalization was coincidental or represented a stable interaction between Notch and Delta, we treated live cells with an excess of polyclonal anti-Notch antiserum. This treatment resulted in "patching" of Notch on the surface of expressing cells into discrete patches as detected by immunofluorescence. There was a distinct correlation between the distributions of Notch and Delta on the surfaces of these cells after this treatment (FIGS. 4C and 4D), indicating that these proteins are associated within the membrane. It is important to note that these experiments do not address the question of whether this association is direct or mediated by other components, such as the cytoskeleton. To control for the possibility that Delta is nonspecifically patched in this experiment, we cotransfected cells with Notch and with the previously mentioned neuroglian construct (A. Bieber and C. Goodman, unpublished data) and patched with anti-Notch antisera. In this case there was no apparent correlation between Notch and neuroglian.
6.2.5. Interactions with Delta do not Require the Intracellular Domain of Notch
In addition to a large extracellular domain that contains EGF-like repeats, Notch has a sizeable intracellular (IC) domain of .about.940 amino acids. The IC domain includes a phosphorylation site (Kidd et al., 1989, Genes Dev. 3, 1113-1129), a putative nucleotide binding domain, a polyglutamine stretch (Wharton et al., 1985, Cell 43, 567-581; Kidd, et al., 1986, Mol. Cell. Biol. 6, 3094-3108), and sequences homologous to the yeast cdc10 gene, which is involved in cell cycle control in yeast (Breeden and Nasmyth, 1987, Nature 329, 651-654). Given the size and structural complexity of this domain, we wondered whether it is required for Notch-Delta interactions. We therefore used a variant Notch construct from which coding sequences for .sup..about. 835 amino acids of the IC domain, including all of the structural features noted above, had been deleted (leaving 25 membrane-proximal amino acids and a novel 59 amino acid carboxyl terminus; see Experimental Procedures and FIG. 1 for details). This construct, designated ECN1, was expressed constitutively under control of the normal Notch promoter in transfected cells at a level lower than that observed for the metallothionein promoter constructs, but still readily detectable by immunofluorescence.
In aggregation assays, cells that expressed the ECN1 construct consistently formed aggregates with Delta.sup.+ cells (31% of ECN1-expressing cells were in aggregates in one of three experiments; see also FIG. 3I), but not with themselves (only 4% in aggregates), just as we observed for cells that expressed intact Notch. We also observed sharp bands of ECN1 staining within regions of contact with Delta.sup.+ cells, again indicating a localization of ECN1 within regions of contact between cells. To test for interactions within the membrane, we repeated the surface antigen co-patching experiments using cells cotransfected with the ECN1 and Delta constructs. As observed for intact Notch, we found that when ECN1 was patched using polyclonal antisera against the extracellular domain of Notch, ECN1 and Delta colocalized at the cell surface (FIGS. 4E and 4F). These results demonstrate that the observed interactions between Notch and Delta within the membrane do not require the deleted portion of the IC domain of Notch and are therefore probably mediated by the extracellular domain. However, it is possible that the remaining transmembrane or IC domain sequences in ECN1 are sufficient to mediate interactions within a single cell.
6.2.6. Notch and Delta Form Detergent-soluble Intermolecular Complexes
Together, we take the preceding results to indicate molecular interactions between Notch and Delta present within the same membrane and between these proteins expressed on different cells. As a further test for such interactions, we asked whether these proteins would coprecipitate from nondenaturing detergent extracts of cells that express Notch and Delta. If Notch and Delta form a stable intermolecular complex either between or within cells, then it should be possible to precipitate both proteins from cell extracts using specific antisera directed against one of these proteins. We performed this analysis by immunoprecipitating Delta with polyclonal antisera from NP-40/deoxycholate lysates (see Experimental Procedures) of cells cotransfected with the Notch and Delta constructs that had been allowed to aggregate overnight or of 0-24 hr wild-type embryos. We were unable to perform the converse immunoprecipitates because it was not possible to discern unambiguously a faint Delta band among background Staph A bands. It is important to note that we tested this polyclonal anti-Delta antiserum for cross-reactivity against Notch in cell lysates (FIG. 5A, lane 1) and by immunofluorescence (e.g., compare FIGS. 3D and 3E) and found none. After repeated washing to remove nonspecifically adhering proteins, we assayed for coprecipitation of Notch using a monoclonal antibody (MAb C17.9C6) against Notch on Western blots.
As FIGS. 5A-5B shows, we did detect coprecipitation of Notch in Delta immunoprecipitates from cotransfected cells and embryos. However, coprecipitating Notch appeared to be present in much smaller quantities than Delta and was therefore difficult to detect. This disparity is most likely due to the disruption of Notch-Delta complexes during the lysis and washing steps of the procedure. However, it is also possible that this disparity reflects a nonequimolar interaction between Notch and Delta or greatly different affinities of the antisera used to detect these proteins. The fact that immunoprecipitation of Delta results in the coprecipitation of Notch constitutes direct evidence that these two proteins form stable intermolecular complexes in transfected S2 cells and in embryonic cells.
6.3. Discussion
We have studied interactions between the protein products of two of the neurogenic loci, Notch and Delta, in order to understand their cellular functions better. Using an in vitro aggregation assay that employs normally nonadhesive S2 cells, we showed that cells that express Notch and Delta adhere specifically to one another. The specificity of this interaction is apparent from the observation that Notch.sup.+ -Delta.sup.+ cell aggregates rarely contained nonexpressing cells, even though nonexpressing cells composed the vast majority of the total cell population in these experiments. We propose that this aggregation is mediated by heterotypic binding between the extracellular domains of Notch and Delta present on the surfaces of expressing cells. Consistent with this proposal, we find that antisera directed against the extracellular domain of Notch inhibit Notch-Delta-mediated aggregation, and that the ECN1 Notch variant, which lacks almost all of the Notch intracellular domain, can mediate aggregation with cells that express Delta. We also found that cells that express only Delta aggregate with one another, while those that express only Notch do not. These findings suggest that Delta can participate in a homotypic interaction when present on apposed cell surfaces but that Notch cannot under our assay conditions.
The proposal that Notch and Delta interact at the cell surface is further supported by three lines of evidence. First, we find an intense localization of both proteins within regions of contact which Notch.sup.+ and Delta.sup.+ cells, implying that Notch and Delta interact directly, even when expressed in different cells. Second, Notch and Delta colocalize on the surface of cells that express both proteins, suggesting that these proteins can interact within the cell membrane. Third, Notch and Delta can be coprecipitated from nondenaturing detergent extracts of cultured cells that express both proteins as well as from extracts of embryonic cells. Together, these results strongly support the hypothesis that Notch and Delta can interact heterotypically when expressed on the surfaces of either the same or different cells.
The underlying basis for the observed genetic interactions between Notch and Delta and between Notch and mam (Xu et al., 1990, Genes Dev. 4, 464-475) may be a dose-sensitive interaction between the proteins encoded by these genes.
Two lines of evidence suggest that the Notch and Delta proteins function similarly in vitro and in vivo. First, the genetic analyses have indicated that the stoichiometry of Notch and Delta is crucial for their function in development. Our observations that both Notch-Delta and Delta--Delta associations may occur in vitro imply that Notch and Delta may compete for binding to Delta. Thus, dose-sensitive genetic interactions between Notch and Delta may be the result of competitive binding interactions between their protein products. Second, we were able to detect Notch-Delta association in lysates of cultured cells and in lysates of Drosophila embryos using immunoprecipitation. Taken together, these genetic and biochemical analyses suggest that Notch and Delta do associate in vivo in a manner similar to that which we propose on the basis of our aggregation assays.
Genetic and molecular analyses of Notch have also raised the possibility that there may be interactions between individual Notch proteins (Portin, 1975, Genetics 81, 121-133; Kelley et al., 1987, Cell 51, 539-548; Artavanis-Tsakonas, 1988, Trends Genet. 4, 95-100). Indeed, Kidd et al. (1989, Genes Dev. 3, 1113-1129) have proposed that this protein forms disulfide cross-linked dimers, although this point has not yet been rigorously proven. With or without the formation of covalent cross-links, such interactions could presumably occur either within a single cell or between cells. However, our find that Notch.sup.+ cells do not aggregate homotypically suggests that Notch--Notch associations are likely to occur within a single cell and not between cells. Alternatively, it is possible that homotypic Notch interactions require gene products that are not expressed in S2 cells.
The Notch-Delta interactions indicated by our analysis are probably mediated by the extracellular domains of these proteins. Aggregation experiments using the ECN1 construct, from which almost the entire intracellular domain of Notch has been removed or altered by in vitro mutagenesis, confirmed this conclusion. Further experiments that demonstrate ECN1-Delta associations within the membrane on the basis of their ability to co-patch indicated that these interactions are also likely to be mediated by the extracellular domains of Notch and Delta, although in this case we cannot exclude possible involvement of the transmembrane domain or the remaining portion of the Notch intracellular domain. These results are especially interesting in light of the fact that both Notch and Delta have EGF-like repeats within their extracellular domains (Wharton et al., 1985, Cell 43, 567-581; Kidd et al., 1986, Mol. Cell Biol. 6, 3094-3108; Vassin et al., 1987, EMBO J. 6, 3431-3440; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735).
A second issue of interest regarding EGF domains is the proposal that they can serve as Ca.sup.2+ binding domains when they contain a consensus sequence consisting of Asp, Asp/Asn, Asp/Asn, and Tyr/Phe residues at conserved positions within EGF-like repeats (Rees et al., 1988, EMBO J. 7, 2053-2061; Handford et al., 1990, EMBO J. 9, 475-480). Comparisons with a proposed consensus sequence for Ca.sup.2+ binding have revealed that similar sequences are found within many of the EGF-like repeats of Notch (Rees et al., 1988, EMBO J. 7, 2053-2061) and within most of the EGF-like repeats of Delta (Kopczynski et al., 1988, Genes Dev. 2, 1723-1735). Furthermore, sequence analyses of Notch mutations have shown that certain Ax alleles are associated with changes in amino acids within this putative Ca.sup.2+ binding domain (Kelley et al., 1987, Cell 51, 539-548; Hartley et al., 1987, EMBO J. 6, 3407-3417; Rees et al., 1988, EMBO J. 7, 2053-2061). For example, the Ax.sup.E2 mutation, which correlates with a His to Tyr change in the 29th EGF-like repeat, appears to change this repeat toward the consensus for Ca.sup.2+ binding. Conversely, the Ax.sup.9B2 mutation appears to change the 24th EGF-like repeat away from this consensus as a result of an Asp to Val change. Thus, the genetic interactions between Ax alleles and Delta mutations (Xu et al., 1990, Genes Dev., 4, 464-475) raise the possibility that Ca.sup.2+ ions play a role in Notch-Delta interactions. Our finding that exogenous Ca.sup.2+ is necessary for Notch-Delta-mediated aggregation of transfected S2 cells supports this contention.
As we have argued (Johansen et al., 1989, J. Cell Biol. 109, 2427-2440; Alton et al., 1989, Dev. Genet. 10, 261-272), on the basis of previous molecular and genetic analyses one could not predict with any certainty the cellular function of either Notch or Delta beyond their involvement in cell--cell interactions. However, given the results presented here, it now seems reasonable to suggest that Notch and Delta may function in vivo to mediate adhesive interactions between cells. At the same time, it is quite possible that the observed Notch-Delta interactions may not reflect a solely adhesive function and may in addition reflect receptor-ligand binding interactions that occur in vivo. Indeed, the presence of a structurally complex 1000 amino acid intracellular domain within Notch may be more consistent with a role in signal transduction than with purely adhesive interactions. Given that Notch may have an adhesive function in concert with Delta, axonal expression of Notch may play some role in axon guidance.
7. EGF REPEATS 11 AND 12 OF NOTCH ARE REQUIRED AND SUFFICIENT FOR NOTCH-DELTA-MEDIATED AGGREGATION
In this study, we use the same aggregation assay as described in Section 6, together with deletion mutants of Notch to identify regions within the extracellular domain of Notch necessary for interactions with Delta. We present evidence that the EGF repeats of Notch are directly involved in this interaction and that only two of the 36 EGF repeats appear necessary. We demonstrate that these two EGF repeats are sufficient for binding to Delta and that the calcium dependence of Notch-Delta mediated aggregation also associates with these two repeats. Finally, the two corresponding EGF repeats from the Xenopus homolog of Notch also mediate aggregation with Delta, implying that not only has the structure of Notch been evolutionarily conserved, but also its function. These results suggest that the extracellular domain of Notch is surprisingly modular, and could potentially bind a variety of proteins in addition to Delta.
7.1. Experimental Procedures
7.1.1. Expression Constructs
The constructs described are all derivatives of the full length Notch expression construct #1 pMtNMg (see Section 6, supra). All ligations were performed using DNA fragments cut from low melting temperature agarose gels (Sea Plaque, FMC BioProducts). The 6 kb EcoRI-XhoI fragment from pMtNMg containing the entire extracellular domain of Notch was ligated into the EcoRI-XhoI sites of the Bluescript vector (Stratagene), and named RI/XBS. All subsequent deletions and insertions of EGF repeats were performed in this subclone. The Notch sequence containing the EcoRI-XhoI fragment of these RI/XBS derivatives was then mixed with the 5.5 kb XhoI-XbaI fragment from pMtNMg containing the intracellular domain and 3' sequences needed for polyadenylation, and then inserted into the EcoRI-XbaI site of pRMHa-3 (Bunch et al., 1988, Nucl. Acids Res. 16, 1043-1061) in a three piece ligation. All subsequent numbers refer to nucleotide coordinates of the Notch sequence according to Wharton et al. (1985, Cell 43, 567-581).
For construct #2 DSph, RI/XBS was digested to completion with SphI and then recircularized, resulting in a 3.5 kb in-frame deletion from SphI(996) to SphI(4545).
For construct #3 .DELTA.Cla, RI/XBS was digested to completion with ClaI and then religated, producing a 2.7 kb in-frame deletion from ClaI(1668) to ClaI(4407). The ligation junction was checked by double strand sequencing (as described by Xu et al., 1990, Genes Dev. 4, 464-475) using the Sequenase Kit (U.S. Biochemical Corp., Cleveland). We found that although the ClaI site at position 4566 exists according to the sequence, it was not recognized under our conditions by the ClaI restriction enzyme.
For constructs #4-12, RI/XBS was partially digested with ClaI and then religated to produce all possible combinations of in-frame deletions: construct #4 .DELTA.EGF7-17 removed the sequence between ClaI(1668) and ClaI(2820); Construct #5 .DELTA.EGF9-26 removed the sequence between ClaI(1905) and ClaI(3855); construct #6 .DELTA.EGF17-31 removed the sequence between ClaI(2820) and ClaI(4407); construct #7 .DELTA.EGF7-9 removed the sequence between ClaI(1668) and ClaI(1905); construct #8 .DELTA.EGF9-17 removed the sequence between ClaI(1905) and ClaI(2820); construct #9 .DELTA.EGF17-26 removed the sequence between ClaI(2820) and ClaI(3855); construct #10 .DELTA.EGF 26-30 removed the sequence between ClaI(3855) and ClaI(4407); construct #11 .DELTA.EGF9-30 removed the sequence between ClaI(1905) and ClaI(4407); construct #12 .DELTA.EGF 7-26 removed the sequence between ClaI(1668) and ClaI(3855).
For constructs #13 .DELTA.Cla+EGF9-17 and #14 .DELTA.Cla+EGF17-26, the .about.0.9 kb fragment between ClaI(1905) and ClaI(2820), and the .about.1.0 kb fragment between ClaI(2820) and ClaI(3855), respectively, were inserted into the unique ClaI site of construct #3 .DELTA.Cla.
For construct #16 split, the 11 kb KpnI/XbaI fragment of pMtNMg was replaced with the corresponding KpnI/XbaI fragment from a Notch minigene construct containing the split mutation in EGF repeat 14.
For constructs #17-25, synthetic primers for polymerase chain reaction (PCR) were designed to amplify stretches of EGF repeats while breaking the EGF repeats at the ends of the amplified piece in the same place as the common ClaI sites just after the third cysteine of the repeat (see FIG. 7). The PCR products were gel purified as usual and ligated into the ClaI site of construct #3 .DELTA.Cla which was made blunt by filling with the Klenow fragment of DNA Polymerase I (Maniatis et al., 1990, Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.). The correct orientation of the inserts was determined by PCR using a sense strand primer within the insert together with an antisense strand primer in EGF repeat 35. All primers were 20-mers, and were named with the number of the nucleotide at their 5' end, according to the nucleotide coordinates of the Notch sequence in Wharton et al. (1985, Cell 43, 567-581), and S refers to a sense strand primer while A refers to an antisense strand primer. Construct #16 .DELTA.Cla+EGF(9-13) used primers S1917 and A2367. Construct #17 .DELTA.Cla+EGF(11-15) used primers S2141 and A2591. Construct #18 .DELTA.Cla+EGF(13-17) used primers S2375 and A2819. Construct #19 .DELTA.Cla+EGF(10-13) used primers S2018 and A2367. Construct #20 .DELTA.Cla+EGF(11-13) used primers S2141 and A2367. Construct #21 .DELTA.Cla+EGF(10-12) used primers S2018 and A2015. Construct #22 .DELTA.Cla+EGF(10-11) used primers S2018 and A2322. Construct #23 .DELTA.Cla+EGF(10-12) used primers S2018 and A2322. Construct #24 .DELTA.Cla+EGF(11-12) used primers S2081 and A2322.
For construct #25 .DELTA.EGF, construct R1/XBS was digested to completion with SphI(996) and partially digested with BamHI(5135). The resulting incompatible ends were joined using a synthetic linker designed to create a unique ClaI site. This produced an in frame deletion which removed all 36 EGF repeats with the exception of the first half of repeat 1. For constructs #26-29, the EGF fragments were inserted into this ClaI site as previously described for the corresponding constructs #13, 16, 19, and 23.
For construct #30 .DELTA.ECN, construct R1/XBS was digested to completion with BglI, EcoRI and XhoI. The .about.0.2 kb EcoRI-BglI fragment (722-948) and the .about.0.7 kb BglI-XhoI (5873-6627) fragments were ligated with EcoRI-XhoI cut Bluescript vector and a synthetic linker designed to create a unique ClaI site, resulting in an in-frame deletion from BglI(941) to BglI(5873) that removed all 36 EGF repeats except for the first third of repeat 1 as well as the 3 Notch/lin-12 repeats. For constructs #31 and 32, the EGF fragments were inserted into the unique ClaI site as previously described for constructs #19 and 23.
For constructs #33 and 34, PCR primers S1508 and A1859 based on the Xenopus Notch sequence (Coffman et al., 1990, Science 249, 1438-1441; numbers refer to nucleotide coordinates used in this paper), were used to amplify EGF repeats 11 and 12 out of a Xenopus stage 17 cDNA library (library was made by D. Melton and kindly provided by M. Danilchek). The fragment was ligated into construct #3 DCla and sequenced.
7.1.2. Cell Culture and Transfection
The Drosphila S2 cell line was grown and transfected as described in Section 6, supra. The Delta-expressing stably transformed S2 cell line L-49-6-7 (kindly established by L. Cherbas) was grown in M3 medium (prepared by Hazleton Co.) supplemented with 11% heat inactivated fetal calf serum (FCS) (Hyclone), 100 U/ml penicillin-100 .mu.g/ml streptomycin-0.25 .mu.g/ml fungizone (Hazleton), 2.times.10.sup.-7 M methotrexate, 0.1 mM hypoxanthine, and 0.016 mM thymidine.
7.1.3. Aggregation Assays and Immunofluorescence
Aggregation assays and Ca.sup.++ dependence experiments were as described supra, Section 6. Cells were stained with the anti-Notch monoclonal antibody 9C6.C17 and anti-Delta rat polyclonal antisera (details described in Section 6, supra). Surface expression of Notch constructs in unpermeabilized cells was assayed using rat polyclonal antisera raised against the 0.8 kb (amino acids 237-501; Wharton et al., 1985, Cell 43, 567-581) BstYI fragment from the extracellular domain of Notch. Cells were viewed under epifluorescence on a Leitz Orthoplan 2 microscope.
7.2. Results
7.2.1. EGF Repeats 11 and 12 of Notch are Required for Notch-Delta Mediated Aggregation
We have undertaken an extensive deletion analysis of the extracellular domain of the Notch protein, which we have shown (supra, Section 6) to be involved in Notch-Delta interactions, to identify the precise domain of Notch mediating these interactions. We tested the ability of cells transfected with the various deletion constructs to interact with Delta using the aggregation assay described in Section 6. Briefly, Notch deletion constructs were transiently transfected into Drosophila S2 cells, induced with CuSO.sub.4, and then aggregated overnight at room temperature with a small amount of cells from the stably transformed Delta expressing cell line L49-6-7(Cherbas), yielding a population typically composed of .about.1% Notch expressing cells and .about.5% Delta expressing cells, with the remaining cells expressing neither protein. To assay the degree of aggregation, cells were stained with antisera specific to each gene product and examined with immunofluorescent microscopy (see experimental procedures for details). Aggregates were defined as clusters of four or more cells containing both Notch and Delta expressing cells, and the values shown in FIGS. 6A-6B represent the percentage of all Notch expressing cells found in such clusters. All numbers reflect the average result from at least two separate transfection experiments in which at least 100 Notch expressing cell units (either single cells or clusters) were scored.
Schematic drawings of the constructs tested and results of the aggregation experiments are shown in FIGS. 6A-6B (see Experimental Procedures for details). All expression constructs were derivatives of the full length Notch expression construct #1 pMtNMg (described in Section 6, supra).
The initial constructs (#2 DSph and #3 .DELTA.Cla) deleted large portions of the EGF repeats. Their inability to promote Notch-Delta aggregation suggested that the EGF repeats of Notch were involved in the interaction with Delta. We took advantage of a series of six in-frame ClaI restriction sites to further dissect the region between EGF repeats 7 and 30. Due to sequence homology between repeats, five of the ClaI sites occur in the same relative place within the EGF repeat, just after the third cysteine, while the sixth site occurs just before the first cysteine of EGF repeat 31 (FIG. 7). Thus, by performing a partial ClaI digestion and then religating, we obtained deletions that not only preserved the open reading frame of the Notch protein but in addition frequently maintained the structural integrity and conserved spacing, at least theoretically, of the three disulfide bonds in the chimeric EGF repeats produced by the religation (FIGS. 6A-6B, constructs #4-14). Unfortunately, the most 3' ClaI site was resistant to digestion while the next most 3' ClaI site broke between EGF repeats 30 and 31. Therefore, when various ClaI digestion fragments were reinserted into the framework of the complete ClaI digest (construct #3 .DELTA.Cla), the overall structure of the EGF repeats was apparently interrupted at the 3' junction.
Several points about this series of constructs are worth noting. First, removal of the ClaI restriction fragment breaking in EGF repeats 9 and 17 (construct #8 .DELTA.EGF9-17) abolished aggregation with Delta, while reinsertion of this piece into construct #3 .DELTA.Cla, which lacks EGF repeats 7-30, restored aggregation to roughly wild type levels (construct #13 .DELTA.Cla+EGF9-17), suggesting that EGF repeats 9 through 17 contain sequences important for binding to Delta. Second, all constructs in this series (#4-14) were consistent with the binding site mapping to EGF repeats 9 through 17. Expression constructs containing these repeats (#6, 7, 9, 10, 13) promoted Notch-Delta interactions while constructs lacking these repeats (#4, 5, 8, 11, 12, 14) did not. To confirm that inability to aggregate with Delta cells was not simply due to failure of the mutagenized Notch protein to reach the cell surface, but actually reflected the deletion of the necessary binding site, we tested for cell surface expression of all constructs by immunofluorescently staining live transfected cells with antibodies specific to the extracellular domain of Notch. All constructs failing to mediate Notch-Delta interactions produced a protein that appeared to be expressed normally at the cell surface. Third, although the aggregation assay is not quantitative, two constructs which contained EGF repeats 9-17, #9 .DELTA.EGF17-26 or most noticeably #10 .DELTA.EGF26-30, aggregated at a seemingly lower level. Cells transfected with constructs #9 .DELTA.EGF17-26 and 10 .DELTA.EGF26-30 showed considerably less surface staining than normal, although fixed and permeabilized cells reacted with the same antibody stained normally, indicating we had not simply deleted the epitopes recognized by the antisera. By comparing the percentage of transfected cells in either permeabilized or live cell populations, we found that roughly 50% of transfected cells for construct #9 .DELTA.EGF17-26 and 10% for construct #10 .DELTA.EGF26-30 produced detectable protein at the cell surface. Thus these two constructs produced proteins which often failed to reach the cell surface, perhaps because of misfolding, thereby reducing, but not abolishing, the ability of transfected cells to aggregate with Delta-expressing cells.
Having mapped the binding site to EGF repeats 9 through 17, we checked whether any Notch mutations whose molecular lesion has been determined mapped to this region. The only such mutation was split, a semidominant Notch allele that correlates with a point mutation in EGF repeat 14 (Hartley et al., 1987, EMBO J. 6, 3407-3417; Kelley et al., 1987, Mol. Cell. Biol. 6, 3094-3108). In fact, a genetic screen for second site modifiers of split revealed several alleles of Delta, suggesting a special relationship between the split allele of Notch, and Delta (Brand and Campus-Ortega, 1990, Roux's Arch. Dev. Biol. 198(5), 275-285). To test for possible effects of the split mutation on Notch-Delta mediated aggregation, an 11 kb fragment containing the missense mutation associated with split was cloned into the Notch expression construct (#15 split). However, aggregation with Delta-expressing cells was unaffected in this construct suggesting, as was confirmed by subsequent constructs, that EGF repeat 14 of Notch was not involved in the interactions with Delta modelled by our tissue culture assay.
Thus, to further map the Delta binding domain within EGF repeats 9-17, we used specific oligonucleotide primers and the PCR technique to generate several subfragments of this region. To be consistent with constructs #4-14 which produced proteins that were able to interact with Delta, we designed the primers to splice the EGF repeats just after the third cysteine, in the same place as the common ClaI site (FIG. 7). The resulting PCR products were ligated into the ClaI site of construct #3 .DELTA.Cla. Three overlapping constructs, #16, 17 and 18 were produced, only one of which, #16 .DELTA.Cla+EGF9-13, when transfected into S2 cells, allowed aggregation with Delta cells. Construct #19 .DELTA.Cla+EGF(10-13), which lacks EGF repeat 9, further defined EGF repeats 10-13 as the region necessary for Notch-Delta interactions.
Constructs #20-24 represented attempts to break this domain down even further using the same PCR strategy (see FIG. 7). We asked first whether both EGF repeats 11 and 12 were necessary, and second, whether the flanking sequences from EGF repeats 10 and 13 were directly involved in binding to Delta. Constructs #20 .DELTA.Cla+EGF(11-13), in which EGF repeat 12 is the only entire repeat added, and #21 .DELTA.Cla+EGF(10-12), in which EGF repeat 11 is the only entire repeat added, failed to mediate aggregation, suggesting that the presence of either EGF repeat 11 or 12 alone was not sufficient for Notch-Delta interactions. However, since the 3' ligation juncture of these constructs interrupted the overall structure of the EGF repeats, it was possible that a short "buffer" zone was needed to allow the crucial repeat to function normally. Thus for example in construct #19 .DELTA.Cla+EGF(10-13), EGF repeat 12 might not be directly involved in binding to Delta but instead might contribute the minimum amount of buffer sequence needed to protect the structure of EGF repeat 11, thereby allowing interactions with Delta. Constructs #22-24 addressed this issue. We designed PCR primers that broke at the end of the EGF repeat and therefore were less likely to disrupt the EGF disulfide formation at the 3' ligation juncture. Constructs #22 .DELTA.Cla+EGF(10-11), which did not mediate aggregation, and #23 .DELTA.Cla+EGF(10-12), which did, again suggested that both repeats 11 and 12 are required while the flanking sequence from repeat 13 clearly is not. Finally, construct #24 .DELTA.Cla+EGF(11-12), although now potentially structurally disrupted at the 5' junction, convincingly demonstrated that the sequences from EGF repeat 10 are not crucial. Thus based on entirely consistent data from 24 constructs, we propose that EGF repeats 11 and 12 of Notch together define the smallest functional unit obtainable from this analysis that contains the necessary sites for binding to Delta in transfected S2 cells.
7.2.2. EGF Repeats 11 and 12 of Notch are Sufficient for Notch-Delta Mediated Aggregation
The large ClaI deletion into which PCR fragments were inserted (#3 .DELTA.Cla) retains roughly 1/3 of the original 36 EGF repeats as well as the three Notch/lin-12 repeats. While these are clearly not sufficient to promote aggregation, it is possible that they form a necessary framework within which specific EGF repeats can interact with Delta. To test whether only a few EGF repeats were in fact sufficient to promote aggregation, we designed two constructs, #25 .DELTA.EGF which deleted all 36 EGF repeats except for the first two-thirds of repeat 1, and #30 .DELTA.ECN which deleted the entire extracellular portion of Notch except for the first third of EGF repeat 1 and .sup..about. 35 amino acids just before the transmembrane domain. Fragments which had mediated Notch-Delta aggregation in the background of construct #3 .DELTA.Cla, when inserted into construct #25 .DELTA.EGF, were again able to promote interactions with Delta (constructs #26-30). Analogous constructs (#31,32) in which the Notch/lin-12 repeats were also absent, again successfully mediated Notch-Delta aggregation. Thus EGF repeats 11 and 12 appear to function as independent modular units which are sufficient to mediate Notch-Delta interactions in S2 cells, even in the absence of most of the extracellular domain of Notch.
7.2.3. EGF Repeats 11 and 12 of Notch Maintain the Calcium Dependence of Notch-Delta Mediated Aggregation
As described in Section 6, supra (Fehon et al., 1990, Cell 61, 523-534), we showed that Notch-Delta-mediated S2 cell aggregation is calcium dependent. We therefore examined the ability of cells expressing certain deletion constructs to aggregate with Delta expressing cells in the presence or absence of Ca.sup.++ ions. We tested constructs #1 pMtNMg as a control, and #13, 16, 19, 23, 24, 26, 27 and 28, and found that cells mixed in Ca.sup.++ containing medium at 4.degree. C. readily formed aggregates while cells mixed in Ca.sup.++ free medium containing EGTA failed to aggregate (Table III).
Clearly, the calcium dependence of the interaction has been preserved in even the smallest construct, consistent with the notion that the minimal constructs containing EGF repeats 11 and 12 bind to Delta in a manner similar to that of full length Notch. This result is also interesting in light of recent studies suggesting EGF-like repeats with a particular consensus sequence may act as Ca.sup.++ binding domains (Morita et al., 1984, J. Biol. Chem. 259, 5698-5704; Sugo et al., 1984, J. Biol. Chem. 259, 5705-5710; Rees et al., 1988, EMBO J. 7, 2053-2061; Handford et al., 1990, EMBO J. 9, 475-480). Over half of the EGF repeats in Notch, including repeats 11 and 12, conform to this consensus, further strengthening the argument that EGF repeats 11 and 12 are responsible for promoting Notch-Delta interactions.
7.2.4. The Delta Binding Function of EGF Repeats 11 and 12 of Notch is Conserved in the Xenopus Homolog of Notch
Having mapped the Delta binding site to EGF repeats 11 and 12 of Notch, we were interested in asking whether this function was conserved in the Notch homolog that has been identified in Xenopus (Coffman et al., 1990, Science 249, 1438-1441). This protein shows a striking similarity to Drosophila Notch in overall structure and organization. For example, within the EGF repeat region both the number and linear organization of the repeats has been preserved, suggesting a possible functional conservation as well. To test this, we made PCR primers based on the Xenopus Notch sequence (Coffman et al., 1990, Science 249, 1438-1441) and used these to obtain an .about.350 bp fragment from a Xenopus Stage 17 cDNA library that includes EGF repeats 11 and 12 flanked by half of repeats 10 and 13 on either side. This fragment was cloned into construct #3 .DELTA.Cla, and three independent clones were tested for ability to interact with Delta in the cell culture aggregation assay. Two of the clones, #33a&b.DELTA.Cla+XEGF(10-13), when transfected into S2 cells were able to mediate Notch-Delta interactions at a level roughly equivalent to the analogous Drosophila Notch construct #19.DELTA.Cla+EGF(10-13), and again in a calcium dependent manner (Table III). However, the third clone #33c.DELTA.Cla+XEGF(10-13) failed to mediate Notch-Delta interactions although the protein was expressed normally at the cell surface as judged by staining live unpermeabilized cells. Sequence comparison of the Xenopus PCR product in constructs #33a and 33c revealed a missense mutation resulting in a leucine to proline change (amino acid #453, Coffman, et al., 1990, Science 249, 1438-1441) in EGF repeat 11 of construct #33c. Although this residue is not conserved between Drosophila and Xenopus Notch (FIG. 8), the introduction of a proline residue might easily disrupt the structure of the EGF repeat, and thus prevent it from interacting properly with Delta.
Comparison of the amino acid sequence of EGF repeats 11 and 12 of Drosophila and Xenopus Notch reveals a high degree of amino acid identity, including the calcium binding consensus sequence (FIG. 8, SEQ ID NO:1 and NO:2). However the level of homology is not strikingly different from that shared between most of the other EGF repeats, which overall exhibit about 50% identity at the amino acid level. This one to one correspondence between individual EGF repeats suggests that perhaps they too may comprise conserved functional units. Delta interactions, again in a calcium ion-dependent manner.
7.3. Discussion
We have continued our study of interactions between the protein products of the genes Notch and Delta, using the in vitro S2 cell aggregation assay described in Section 6, supra. Based on an extensive deletion analysis of the extracellular domain of Notch, we show that the regions of Notch containing EGF-homologous repeats 11 and 12 are both necessary and sufficient for Notch-Delta-mediated aggregation, and that this Delta binding capability has been conserved in the same two EGF repeats of Xenopus Notch. Our finding that the aggregation mapped to EGF repeats 11 and 12 of Notch demonstrates that the EGF repeats of Notch also function as specific protein binding domains.
Recent studies have demonstrated that EGF domains containing a specific consensus sequence can bind Ca.sup.++ ions (Morita et al., 1984, J. Biol. Chem. 259, 5698-5704; Sugo et al., 1984, J. Biol. Chem. 259, 5705-5710; Rees et al., 1988, EMBO J. 7, 2053-2061; Handford et al., 1990, EMBO J. 9, 475-480). In fact, about one half of the EGF repeats in Notch, including repeats 11 and 12, conform to this consensus. We have shown that exogenous Ca.sup.++ was necessary for Notch-Delta mediated aggregation of transfected S2 cells (see Section 6; Fehon et al., 1990, Cell 61, 523-534). We tested a subset of our deletion constructs and found that EGF repeats 11 and 12 alone (#32.DELTA.ECN+EGF(11-12)) were sufficient to maintain the Ca.sup.++ dependence of Notch-Delta interactions.
A number of studies have suggested that the genetic interactions between Notch and Delta may reflect a dose sensitive interaction between their protein products. Genetic studies have indicated that the relative gene dosages of Notch and Delta are crucial for normal development. For example, Xu et al. (1990, Genes Dev. 4, 464-475) found that null mutations at Delta could suppress lethal interactions between heterozygous combinations of Abruptex (Ax) alleles, a class of Notch mutations that correlate with missense mutations within the EGF repeats (Hartley et al., 1987, EMBO J. 6, 3407-3417; Kelley et al., 1987, Mol. Cell Biol. 6, 3094-3108). The in vitro interactions we have described in which we observe both Notch-Delta and Delta--Delta associations (see Section 6) imply that a competitive interaction between Notch and Delta for binding to Delta may reflect the underlying basis for the observed genetic interactions. Furthermore, we were able to coimmunoprecipitate Notch and Delta from both tissue culture and embryonic cell extracts (see Section 6), indicating a possible in vivo association of the two proteins. In addition, mRNA in situ analyses of Notch and Delta expression patterns in the embryo suggest that expression of the two is overlapping but not identical (Kopczynski and Muskavitch, 1989, Development 107, 623-636; Hartley et al., 1987, EMBO J. 6, 3407-3417). Detailed antibody analysis of Notch protein expression during development have recently revealed Notch expression to be more restricted at the tissue and subcellular levels than previous studies had indicated (Johansen et al., 1989, J. Cell Biol. 109, 2427-2440; Kidd et al., 1989, Genes Dev. 3, 1113-1129).
Our finding that the same two EGF repeats from the Xenopus Notch homolog are also able to mediate interactions with Delta in tissue culture cells argues strongly that a similar function will have been conserved in vivo. Although these two EGF repeats are sufficient in vitro, it is of course possible that in vivo more of the Notch molecule may be necessary to facilitate Notch-Delta interactions. In fact, we were somewhat surprised for two reasons to find that the Delta binding site did not map to EGF repeats where several of the Ax mutations have been shown to fall, first, because of the genetic screen (Xu et al., 1990, Genes Dev. 4, 464-475) demonstrating interactions between Ax alleles and Delta mutations, and second, because of sequence analyses that have shown certain Ax alleles are associated with single amino acid changes within the putative Ca.sup.++ binding consensus of the EGF repeats. For example, the AX.sup.E2 mutation changes EGF repeat 29 toward the Ca.sup.++ binding consensus sequence while the AX.sup.9B2 mutation moves EGF repeat 24 away from the consensus. It is possible that in vivo these regions of the Notch protein may be involved in interactions, either with Delta and/or other proteins, that may not be accurately modelled by our cell culture assay.
Our in vitro mapping of the Delta binding domain to EGF repeats 11 and 12 of Notch represents the first assignment of function to a structural domain of Notch. In fact, the various deletion constructs suggest that these two EGF repeats function as a modular unit, independent of the immediate context into which they are placed. Thus, neither the remaining 34 EGF repeats nor the three Notch/lin-12 repeats appear necessary to establish a structural framework required for EGF repeats 11 and 12 to function. Interestingly, almost the opposite effect was observed: although our aggregation assay does not measure the strength of the interaction, as we narrowed down the binding site to smaller and smaller fragments, we observed an increase in the ability of the transfected cells to aggregate with Delta expressing cells, suggesting that the normal flanking EGF sequences actually impede association between the proteins. For two separate series of constructs, either in the background of construct #3 .DELTA.Cla (compare #9, 16, 19, 23) or in the background of construct #25 .DELTA.EGF (compare #26, 27, 28), we observed an increase in ability to aggregate such that the smallest constructs (#19, 23, 28, 29) consistently aggregated above wild type levels (#1 pMtNMg). These results imply that the surrounding EGF repeats may serve to limit the ability of EGF repeats 11 and 12 to access Delta, thereby perhaps modulating Notch-Delta interactions in vivo.
Notch encodes a structurally complex transmembrane protein that has been proposed to play a pleotropic role throughout Drosophila development. The fact that EGF repeats 11 and 12 appear to function as an independent modular unit that is sufficient, at least in cell culture, for interactions with Delta, immediately presents the question of the role of the hypothesis is that these may also form modular binding domains for other proteins interacting with Notch at various times during development.
In addition to Xenopus Notch, lin-12 and glp-1, two genes thought to function in cell--cell interactions involved in the specification of certain cell fates during C. elegans development, encode EGF homologous transmembrane proteins which are structurally quite similar to Drosophila and Xenopus Notch. All four proteins contain EGF homologous repeats followed by three other cysteine rich repeats (Notch/lin-12 repeats) in the extracellular domain, a single transmembrane domain, and six cdc10/ankyrin repeats in the intracellular region. Unlike Xenopus Notch, which, based on both sequence comparison as well as the results of our Delta binding assay, seems likely to encode the direct functional counterpart of Drosophila Notch, lin-12 and glp-1 probably encode distinct members of the same gene family. Comparison of the predicted protein products of lin-12 and glp-1 with Notch reveal specific differences despite an overall similar organization of structural motifs. The most obvious difference is that lin-12 and glp-1 proteins contain only 13 and 10 EGF repeats, respectively, as compared to the 36 for both Xenopus and Drosophila Notch. In addition, in the nematode genes the array of EGF repeats is interrupted after the first EGF repeat by a distinct stretch of sequence absent from Notch. Furthermore, with respect to the Delta binding domain we have defined as EGF repeats 11 and 12 of Notch, there are no two contiguous EGF repeats in the lin-12 or glp-1 proteins exhibiting the Ca.sup.++ binding consensus sequence, nor any two contiguous repeats exhibiting striking similarity to EGF repeats 11 and 12 of Notch, again suggesting that the lin-12 and glp-1 gene products are probably functionally distinct from Notch.
Our finding that EGF repeats 11 and 12 of Notch form a discrete Delta binding unit represents the first concrete evidence supporting the idea that each EGF repeat or small subset of repeats may play a unique role during development, possibly through direct interactions with other proteins. The homologies seen between the adhesive domain of Delta and Serrate (see Section 8.3.4, infra) suggest that the homologous portion of Serrate is "adhesive" in that it mediates binding to other toporythmic proteins. In addition, the gene scabrous, which encodes a secreted protein with similarity to fibrinogen, may interact with Notch.
In addition to the EGF repeat, multiple copies of other structural motifs commonly occur in a variety of proteins. One relevant example is the cdc10/ankyrin motif, six copies of which are found in the intracellular domain of Notch. Ankyrin contains 22 of these repeats. Perhaps repeated arrays of structural motifs may in general represent a linear assembly of a series of modular protein binding units. Given these results together with the known structural, genetic and developmental complexity of Notch, Notch may interact with a number of different ligands in a precisely regulated temporal and spacial pattern throughout development. Such context specific interactions with extracellular proteins could be mediated by the EGF and Notch/lin-12 repeats, while interactions with cytoskeletal and cytoplasmic proteins could be mediated by the intracellular cdc10/ankyrin motifs.
8. THE AMINO-TERMINUS OF DELTA IS AN EGF-BINDING DOMAIN THAT INTERACTS WITH NOTCH AND DELTA
Aggregation of cultured cells programmed to express wild type and variant Delta proteins has been employed to delineate Delta sequences required for heterotypic interaction with Notch and homotypic Delta interaction. We have found that the amino terminus of the Delta extracellular domain is necessary and sufficient for the participation of Delta in heterotypic (Delta-Notch) and homotypic (Delta--Delta) interactions. We infer that the amino terminus of Delta is an EGF motif-binding domain (EBD), given that Notch EGF-like sequences are sufficient to mediate heterotypic interaction with Delta. The Delta EBD apparently possesses two activities: the ability to bind EGF-related sequences and the ability to self-associate. We also find that Delta is taken up by cultured cells that express Notch, which may be a reflection of a mechanism by which these proteins interact in vivo.
8.1. Materials and Methods
8.1.1. Cell Lines
The S2 Drosophila cell line (Schneider, 1972, J. Embryol. Exp. Morph. 27, 353-365)) used in these experiments was grown as described in Section 6.
8.1.2. Immunological Probes
Immunohistochemistry was performed as described in Section 6, supra, or sometimes with minor modifications of this procedure. Antisera and antibodies employed included mouse polyclonal anti-Delta sera raised against a Delta ELR array segment that extends from the fourth through ninth ELRs (see Section 6); rat polyclonal anti-Delta sera raised against the same Delta segment (see Section 6); rat polyclonal anti-Notch sera raised against a Notch ELR array segment that extends from the fifth through thirteenth ELRs; mouse monoclonal antibody C17.9C6 (see Section 6), which recognizes the Notch intracellular domain; and mouse monoclonal antibody BP-104 (Hortsch et al., 1990, Neuron 4, 697-709), which recognizes the long form of Drosophila neuroglian.
8.1.3. Expression Vector Constructs
Constructs employed to program expression of wild type Delta (pMTDl1) and wild type Notch (pMTNMg) are described in Section 6, supra. Constructs that direct expression of variant Delta proteins were generated using pMTDl1, the Dl1 cDNA cloned into Bluescript+ (pBSDll; Kopczynski et al., 1988, Genes Dev. 2, 1723-1735), and pRmHa3-104 (A. J. Bieber, pers. comm.), which consists of the insertion of the 1B7A-250 cDNA into the metallothionein promoter vector pRmHa-3 (Bunch et al., 1988, Nucl. Acids Res. 16, 1043-1061) and supports inducible expression of the long form of Drosophila neuroglian (Hortsch et al., 1990, Neuron 4, 697-709).
Briefly, constructs were made as follows:
Del(Sca-Nae)--Cut pBSDl1 with SalI (complete digest) and ScaI (partial), isolate vector-containing fragment. Cut pBSDl1 with NaeI (partial) and SalI (complete), isolate Delta carboxyl-terminal coding fragment. Ligate fragments, transform, and isolate clones. Transfer EcoRI insert into pRmHa-3.
Del(Bam-Bgl)--Cut pBSDl1 with BglII (complete) and BamHI (partial), fill ends with Klenow DNA polymerase, ligate, transform, and isolate clones. Transfer EcoRI insert into pRmHa-3.
Del(ELR1-ELR3)--PCR-amplify basepairs 236-830 of the Dl1 cDNA using 5-ACTTCAGCAACGATCACGGG-3' (SEQ ID NO:26) and 5'-TTGGGTATGTGACAGTAATCG-3' (SEQ ID NO:27), treat with T4 DNA polymerase, ligate into pBSDl1 cut with ScaI (partial) and BglII (complete) and end-filled with Klenow DNA polymerase, transform, and isolate clones. Transfer BamHI-SalI Delta carboxyl-terminal coding fragment into pRmHa-3.
Del(ELR4-ELR5)--pBSDl1 was digested to completion with BglII and partially with PstI. The 5.6 kb vector-containing fragment was isolated, circularized using T4 DNA ligase in the presence of a 100.times. molar excess of the oligonucleotide 5'-GATCTGCA-3', and transformed and clones were isolated. The resulting EcoRI insert was then transferred into pRmHa-3.
Ter(Dde)--Cut pBSDl1 with DdeI (partial), end-fill with Klenow DNA polymerase, ligate with 100.times. molar excess of 5'-TTAAGTTAACTTAA-3' (SEQ ID NO:28), transform, and isolate clones. Transfer EcoRI insert into pRmHa-3.
Ins(Nae)A--Cut pMTDl1 with NaeI (partial), isolate vector-containing fragment, ligate with 100.times. molar excess of 5'-GGAAGATCTTCC-3' (SEQ ID NO:29), transform, and isolate clones.
NAE B--pMTDl1 was digested partially with NaeI, and the population of tentatively linearized circles approximately 5.8 kb in length was isolated. The fragments were recircularized using T4 DNA ligase in the presence of a 100.times. molar excess of the oligonucleotide 5'-GGAAGATCTTCC-3' (SEQ ID NO:29) and transformed, and a clone (NAE A) that contained multiple inserts of the linker was isolated. NAE A was digested to completion with BglII, and the resulting 0.4 kb and 5.4 kb fragments were isolated, ligated and transformed, and clones were isolated.
Ins(Stu)--Cut pMTDl1 with StuI (complete), isolate vector-containing fragment, ligate with 100.times. molar excess of 5'-GGAAGATCTTCC-3' (SEQ ID NO:29), transform and isolate clones.
STU B--pMTDl1 was digested completely with StuI, and the resulting 5.8 kb fragment was isolated. The fragment was recircularized using T4 DNA ligase in the presence of a 100.times. molar excess of the oligonucleotide 5'-GGAAGATCTTCC-3' (SEQ ID NO:29) and transformed, and a clone (STU A) that contained multiple inserts of the linker was isolated. STU B was digested to completion with BglII, and the resulting 0.6 kb and 5.2 kb fragments were isolated, ligated and transformed, and clones were isolated.
NG1--Cut pRmHa3-104 with BglII (complete) and EcoRI (complete), isolate vector-containing fragment. Cut Ins(Nae)A with EcoRI (complete) and BglII (complete), isolate Delta amino-terminal coding fragment. Ligate fragments, transform and isolate clones.
NG2--Cut pRmHa3-104 with BglII (complete) and EcoRI (complete), isolate vector-containing fragment. Cut Del(ELR1-ELR3) with EcoRI (complete) and BglII (complete), isolate Delta amino-terminal coding fragment. Ligate fragments, transform and isolate clones.
NG3--Cut pRmHa3-104 with BglII (complete) and EcoRI (complete), isolate vector-containing fragment. Cut pMTDl1 with EcoRI (complete) and BglII (complete), isolate Delta amino-terminal coding fragment. Ligate fragments, transform and isolate clones.
NG4--Cut pRmHa3-104 with BglII (complete) and EcoRI (complete), isolate vector containing fragment. Cut Del(Sca-Nae) with EcoRI (complete) and BglII (complete), isolate Delta amino-terminal coding fragment. Ligate fragments, transform and isolate clones.
NG5--Generate Del(Sca-Stu) as follows: cut pMTDl1 with ScaI (complete) and StuI (complete), isolate ScaI-ScaI amino-terminal coding fragment and StuI-ScaI carboxyl-terminal coding fragment, ligate, transform and isolate clones. Cut Del(Sca-Stu) with EcoRI (complete) and BglII (complete), isolate Delta amino terminal coding fragment. Cut pRmHa3-104 with BglII (complete) and EcoRI (complete), isolate vector-containing fragment. Ligate fragments, transform and isolate clones.
The sequence contents of the various Delta variants are shown in Table IV. Schematic diagrams of the Delta variants defined in Table IV are shown in FIG. 9C.
8.1.4. Aggregation Protocols
Cell transfection and aggregation were performed as described in Section 6, supra, or with minor modifications thereof.
8.2. Results
8.2.1. Amino-terminal Sequences within the Delta Extracellular Domain are Necessary and Sufficient for the Heterotypic Interaction with Notch
Because we anticipated that some Delta variants might not be efficiently localized on the cell surface, we investigated the relationship between the level of expression of wild type Delta and the extent of aggregation with Notch-expressing cells by varying the input amount of Delta expression construct in different transfections. We found that the heterotypic Delta-Notch interaction exhibits only slight dependence on the Delta input level over a 10-fold range in this assay (FIGS. 9A-9B). Given the robustness of the heterotypic interaction over the range tested and our observations that each of the Delta variants we employed exhibited substantial surface accumulation in transfected cells, we infer that the inability of a given Delta variant to support heterotypic aggregation most probably reflects a functional deficit exhibited by that variant, as opposed to the impact of reduced levels of surface expression on heterotypic agregation.
The results of the heterotypic aggregation experiments mediated by Delta variants and wild-type Notch are shown in Table V.
Delta amino acids (AA) 1-230 is the current minimum sequence interval defined as being sufficient for interaction with Notch. This is based on the success of NG2-Notch aggregation. Within this interval, Delta AA198-230 are critical because their deletion in the NG1 construct inactivated the Notch-binding activity observed for the NG2 construct. Also within this interval, Delta AA32-198 are critical because their deletion in the NG4 construct also inactivated the Notch-binding activity observed for the NG3 construct. The importance of Delta AA192-230 is also supported by the observation that the Del(ELR1-ELR3) variant, which contains all Delta amino acids except AA231-331, possessed Notch-binding activity, while the Del(Bam-Bgl) variant, which contains all Delta amino acids except AA192-331, was apparently inactivated for Notch-binding activity.
Conformation and/or primary sequence in the vicinity of Delta AA197/198 is apparently critical because a multimeric insertion of the tetrapeptide--Arg-Lys-Ile-Phe in one letter code (see e.g. Lehninger et al., 1975, Biochemistry, 2d ed., p. 72), RKIF! (SEQ ID NO:30)--between these two residues, as in the Ins(Nae)A construct, inactivated the Notch-binding activity observed with wild type Delta.
In addition, the observation that the Del(ELR1-ELR3) construct supported aggregation implies that ELR1-ELR3 are not required for Delta-Notch interaction; the observation that the Del(ELR4-ELR5) construct supported aggregation implies that ELR4 and ELR5 are not required for Delta-Notch interaction, and the observation that the Ter(Dde) construct supported aggregation implies that the Delta intracellular domain is not required for Delta-Notch interaction.
8.2.2. Amino-terminal Sequences within the Delta Extracellular Domain are Necessary and Sufficient for Homotypic Interaction
The results of the homotypic aggregation experiments mediated by Delta variants is shown in Table VI.
Deletion of Delta AA32-198 Del(Sca-Nae)! or Delta AA192-331 Del(Bam-Bgl)! from the full-length Delta protein eliminated the Delta--Delta interaction. Deletion of Delta AA231-331 Del(ELR1-ELR3)! did not eliminate the Delta--Delta interaction. Therefore, sequences within the Delta AA32-230 are required for the Delta--Delta interaction.
Conformation and/or primary sequence in the vicinity of Delta AA197/198 is apparently critical for the Delta--Delta interaction because a multimeric insertion of the tetrapeptide -Arg-Lys-Ile-Phe- (SEQ ID NO:30) between these two residues, as in the Ins(Nae)A construct, inactivated Delta--Delta interaction.
In addition, the observation that the Del(ELR1-ELR3) construct could support aggregation implies that ELR1-ELR3 are not required for Delta--Delta interaction; the observation that the Del(ELR-ELR5) construct supported aggregation implies that ELR4 and ELR5 are not required for Delta--Delta interaction, and the observation that the Ter(Dde) construct supported aggregation implies that the Delta intracellular domain is not required for Delta--Delta interaction.
A summary of the results of assays for heterotypic and homotypic aggregation with various constructs are shown in Table VI A.
8.2.3. Delta Sequences Involved in Heterotypic and Homotypic Interactions are Qualitatively Distinct
The respective characteristics of Delta sequences repaired for heterotypic and homotypic interaction were further defined using Delta variants in which short, in-frame, translatable linker insertions were introduced into the Delta amino terminus (i.e., NAE B and STU B; FIG. 9C, Table VI A). Replacement of Delta residue 132 (A) with the pentapeptide GKIFP (STU B variant) leads to the inactivation of heterotypic and homotypic interaction activites of the Delta amino terminus. This suggests that some Delta sequences required for these two distinct interactions are coincident and reside in proximity to residue 132. On the other hand, insertion of the tetrapeptide RKIF between Delta residues 198 and 199 (NAE B variant) eliminates the ability of the Delta amino terminus to mediate heterotypic interaction with Notch, but has no apparent effect on the ability of the altered amino terminus to mediate homotypic interaction. The finding that the NAE B insertion affects only one of the two activities of the Delta amino terminus implies that the Delta sequences that mediate heterotypic and homotypic interactions, while coincident, are qualitatively distinct.
8.2.4. Delta is Taken Up by Cells that Express Notch
During the course of many heterotypic aggregation experiments, we have noted that Delta protein can sometimes be found within cells that have been programmed to express Notch, but not Delta. We conduct heterotypic aggregation assays by mixing initially separate populations of S2 cells that have been independently transfected with expression constructs that program expression of either Delta or Notch. Yet, we often detect punctate staining of Delta within Notch-expressing cells found in heterotypic aggregates using Delta-specific antisera. Our observations are consistent with Delta binding directly to Notch at the cell surface and subsequent clearance of this Delta-Notch complex from the cell surface via endocytosis.
8.3. Discussion
8.3.1. Amino-terminal Sequences Unrelated to EGF are Involved in the Interaction between Delta and Notch
We have employed cell aggregation assays to define a region within the amino-proximal region of the Delta extracellular domain that is necessary and sufficient to mediate the Delta-Notch interaction. Functional analyses of a combination of deletion and sufficiency constructs revealed that this region extends, maximally, from AA1 through AA230. It is striking that this region does not include any of the EGF-like sequences that reside within the Delta extracellular domain. It is probable that the particular Delta sequences within the sufficient interval required for interaction with Notch include AA198-230 because deletion of these residues eliminates Notch-binding activity. The fact that deletion of AA32-198 also inactivates Notch-binding activity suggests that sequences amino-proximal to AA198 are also required, although the deleterious impact of this deletion could result from the removal of additional amino acids in the immediate vicinity of AA198.
Sequences within Delta sufficient for interaction with Notch can be grouped into three subdomains--N1, N2, and N3--that differ in their respective contents of cysteine residues (FIG. 10, SEQ ID NO:3). The N1 and N3 domains each contain six cysteine residues, while the N2 domain contains none. The even number of cysteines present in N1 and N3, respectively, allows for the possibility that the respective structures of these subdomains are dictated, in part, by the formation of particular disulfide bonds. The broad organizational pattern of the Delta amino-terminus is also generally analogous to that of the extracellular domain of the vertebrate EGF receptor (Lax et al., 1988, Mol. Cell. Biol. 8, 1970-1978), in which sequences believed to interact with EGF are bounded by two cysteine-rich subdomains.
8.3.2. Delta Sequences Required for Homotypic and for Homotypic Heterotypic Interactions Appear to be Coincident
Our results also indicate that sequences essential for homotypic Delta interaction reside within the interval AA32-230. Deletion of sequences or insertion of additional amino acids within this amino-proximal domain eliminate the ability of such Delta variants to singly promote cell aggregation. Thus, sequences required for Delta--Delta interaction map within the same domain of the protein as those required for Delta-Notch interaction.
8.3.3. The Delta Amino Terminus Constitutes an EGF-binding Motif
The work described in examples supra has revealed that Notch sequences required for Delta-Notch interaction in the cell aggregation assay map within the EGF-like repeat array of the Notch extracellular domain. This finding implies that Delta and Notch interact by virtue of the binding of the Delta amino-terminus to EGF-like sequences within Notch and, therefore, that the amino-terminus of the Delta extracellular domain constitutes an EGF-binding domain (FIGS. 11A-11B).
These results also raise the possibility that homotypic Delta interaction involves the binding of the Delta amino-terminus to EGF-like sequences within the Delta extracellular domain (FIGS. 12A-12C). However, none of the EGF-like repeats within the Delta extracellular domain are identical to any of the EGF-like repeats within the Notch extracellular domain (FIGS. 13A-13F, SEQ ID NO:6; Wharton et al., 1985, Cell 43, 567-581). Given this fact, if Delta homotypic interactions are indeed mediated by interaction between the Delta amino-terminus and Delta EGF-like repeats, then the Delta EGF-binding domain has the capacity to interact with at least two distinct EGF-like sequences.
8.3.4. Delta Sequences Involved in the Delta-Notch Interaction are Conserved in the Serrate Protein
Alignment of amino acid sequences from the amino termini of the Delta (FIGS. 13A-13F, SEQ ID NO:6, and FIGS. 15A-15B, SEQ ID NO:9) and Serrate (Fleming et al., 1990, Genes & Dev. 4, 2188-2201; Thomas et al., 1991, Devel. 111, 749-761) reveals a striking conservation of structural character and sequence composition. The general N1-N2-N3 subdomain structure of the Delta amino terminus is also observed within the Serrate amino terminus, as is the specific occurrence of six cysteine residues within the Delta N1- and Delta N3-homologous domains of the Serrate protein. Two notable blocks of conservation correspond to Delta AA63-73 (8/11 residues identical) and Delta AA195-206 (10/11 residues identical). The latter block is of particular interest because insertion of additional amino acids in this interval can eliminate the ability of Delta to bind to Notch or Delta.
8.3.5. Cis and Trans Interactions between Delta and Notch May Involve Different Sequences within Notch
Inspection of the overall structures of Delta and Notch suggests that Delta-Notch interaction could involve contacts between the Delta EGF-binding domain with either of two regions within Notch, depending on whether the interaction were between molecules that reside on opposing membranes or within the same membrane (FIGS. 11A-11B). The cell aggregation assays, which presumably detect the interaction of molecules in opposing membranes, imply that the Delta EGF-binding domain interacts with Notch EGF-like repeats 11 and 12 (see examples supra). If tandem arrays of EGF-like motifs do form rod-like structures (Engel, 1989, FEBS Lett. 251, 1-7) within the Delta and Notch proteins, then the estimated displacement of the Delta EGF-binding domain from the cell surface would presumably be sufficient to accommodate the rigid array of Notch EGF-like repeats 1-10. It is also intriguing to note that the displacement of the Delta EGF-binding domain from the cell surface could place this domain in the vicinity of the Notch EGF-like repeats (25-29) that are affected by Abruptex mutations (Hartley et al., 1987, EMBO J. 6, 3407-3417; Kelley et al., 1987, Mol. Cell. Biol. 6, 3094-3108) and could allow for interaction of Delta and Notch proteins present within the same membrane.
8.3.6. Interactions Analogous to the Delta-Notch Interaction in Vertebrates
Given the interaction between Delta and Notch in Drosophila, it is quite probable that a Delta homologue (Helta?) exists in vertebrates and that the qualitative and molecular aspects of the Delta-Notch and Delta--Delta interactions that we have defined in Drosophila will be highly conserved in vertebrates, including humans. Such homologues can be cloned and sequenced as described supra, Section 5.2.
9. SEQUENCES WHICH MEDIATE NOTCH-SERRATE INTERACTIONS
We report a novel molecular interaction between Notch and Serrate, and show that the two EGF repeats of Notch which mediate interactions with Delta, namely EGF repeats 11 and 12, also constitute a Serrate binding domain.
To test whether Notch and Serrate directly interact, S2 cells were transfected with a Serrate expression construct and mixed with Notch expressing cells in our aggregation assay. For the Serrate expression construct, a synthetic primer containing an artificial BamHI site immediately 5' to the initiator AUG at position 442 (all sequence numbers are according to Fleming et al., 1990, Genes & Dev. 4:2188-2201) and homologous through position 464, was used in conjunction with a second primer from position 681-698 to generate a DNA fragment of .about.260 base pairs. This fragment was cut with BamHI and KpnI (position 571) and ligated into Bluescript KS+ (Stratagene). This construct, BTSer5'PCR, was checked by sequencing, then cut with KpnI. The Serrate KpnI fragment (571-2981) was inserted and the proper orientation selected, to generate BTSer5'PCR-Kpn. The 5' SacII fragment of BTSer5'PCR-Kpn (SacII sites in Bluescript polylinker and in Serrate (1199)) was isolated and used to replace the 5' SacII fragment of cDNA C1 (Fleming et al., 1990, Genes & Dev. 4:2188-2201), thus regenerating the full length Serrate cDNA minus the 5' untranslated regions. This insert was isolated by a SalI and partial BamHI digestion and shuttled into the BamHI and SalI sites of pRmHa-3 to generate the final expression construct, Ser-mtn.
We found that Serrate expressing cells adhere to Notch expressing cells in a calcium dependent manner (FIGS. 6A-6B and Table VII). However, unlike Delta, under the experimental conditions tested, Serrate does not appear to interact homotypically. In addition, we detect no interactions between Serrate and Delta.
We have tested a subset of our Notch deletion constructs to map the Serrate-binding domain and have found that EGF repeats 11 and 12, in addition to binding to Delta, also mediate interactions with Serrate (FIGS. 6A-6B; Constructs #1, 7-10, 13, 16, 17, 19, 28, and 32). In addition, the Serrate-binding function of these repeats also appears to have been conserved in the corresponding two EGF repeats of Xenopus Notch (#33.DELTA.Cla+XEGF(10-13)). These results unambiguously show that Notch interacts with both Delta and Serrate, and that the same two EGF repeats of Notch mediate both interactions. We were also able to define the Serrate region which is essential for the Notch/Serrate aggregation. Deleting nucleotides 676-1287 (i.e. amino acids 79-282) (See FIGS. 15A-15B) eliminates the ability of the Serrate protein to aggregate with Notch.
Notch and Serrate appear to aggregate less efficiently than Notch and Delta, perhaps because the Notch-Serrate interaction is weaker. For example, when scoring Notch-Delta aggregates, we detect .about.40% of all Notch expressing cells in clusters with Delta expressing cells (FIGS. 6A-6B, #1 pMtNMg) and .about.40% of all Delta expressing cells in contact with Notch expressing cells. For Notch-Serrate, we find only .about.20% of all Notch expressing cells (FIGS. 6A-6B; pMtNMg) and .about.15% of all Serrate expressing cells in aggregates. For the various Notch deletion constructs tested, we consistently detect a reduction in the amount of aggregation between Notch and Serrate as compared to the corresponding Notch-Delta levels (FIGS. 6A-6B), with the possible exception of constructs #9 and 10 which exhibit severely reduced levels of aggregation even with Delta. One trivial explanation for this reduced amount of aggregation could be that our Serrate construct simply does not express as much protein at the cell surface as the Delta construct, thereby diminishing the strength of the interaction. Alternatively, the difference in strength of interaction may indicate a fundamental functional difference between Notch-Delta and Notch-Serrate interactions that may be significant in vivo.
10. THE CLONING, SEQUENCING, AND EXPRESSION OF HUMAN NOTCH
Clones for the human Notch sequence were originally obtained using the polymerase chain reaction (PCR) to amplify DNA from a 17-18 week human fetal brain cDNA library in the Lambda Zap II vector (Stratagene). Degenerate primers to be used in this reaction were designed by comparing the amino acid sequences of the Xenopus homolog of Notch with Drosophila Notch. Three primers (cdc1 (SEQ ID NO:10), cdc2 (SEQ ID NO:11), and cdc3 (SEQ ID NO:12); FIGS. 16A-16C) were designed to amplify either a 200 bp or a 400 bp fragment as primer pairs cdc1/cdc2 or cdc1/cdc3, respectively.
The 400 bp fragment obtained in this manner was then used as a probe with which to screen the same library for human Notch clones. The original screen yielded three unique clones, hN3k, hN2K, and hN5k, all of which were shown by subsequent sequence analysis to fall in the 3' end of human Notch (FIG. 17). A second screen using the 5' end of hN3k as probe was undertaken to search for clones encompassing the 5' end of human Notch. One unique clone, hN4k, was obtained from this screen, and preliminary sequencing data indicate that it contains most of the 5' end of the gene (FIG. 17). Together, clones hN4k, hN3k and hN5k encompass about 10 kb of the human Notch homolog, beginning early in the EGF-repeats and extending into the 3' untranslated region of the gene. All three clones are cDNA inserts in the EcoRI site of pBluescript SK.sup.- (Stratagene). The host E. coli strain is XL1-Blue (see Maniatis, T., 1990, Molecular Cloning, A Laboratory Manual, 2d ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., p. A12).
The sequence of various portions of Notch contained in the cDNA clones was determined (by use of Sequenase.RTM., U.S. Biochemical Corp.) and is shown in FIGS. 19A-22D (SEQ ID NO:13 through NO:25).
Expression constructs were made using the clones discussed above. In the cases of hN3k and hN2k, the entire clone was excised from its vector as an EcoRI restriction fragment and subcloned into the EcoRI restriction site of each of the three pGEX vectors (Glutathione S-Transferase expression vectors; Smith and Johnson, 1988, Gene 7, 31-40). This allows for the expression of the Notch protein product from the subclone in the correct reading frame. In the case of hN5k, the clone contains two internal EcoRI restriction sites, producing 2.6, 1.5 and 0.6 kb fragments. Both the 2.6 and the 1.5 kb fragments have also been subcloned into each of the pGEX vectors.
11. DEPOSIT OF MICROORGANISMS
The following recombinant bacteria, each carrying a plasmid encoding a portion of human Notch, were deposited on May 2, 1991 with the American Type Culture Collection, 1201 Parklawn Drive, Rockville, Md. 20852, under the provisions of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedures.
The present invention is not to be limited in scope by the microorganisms deposited or the specific embodiments described herein. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description and accompanying figures. Such modifications are intended to fall within the scope of the appended claims.
Various publications are cited herein, the disclosures of which are incorporated by reference in their entireties.
Claims
What is claimed is:
1. A substantially pure fragment of a toporythmic Delta protein, which fragment is characterized by the ability in vitro, when expressed on the surface of a first cell, to bind to a Notch protein expressed on the surface of a second cell, which fragment lacks one or more amino acids of the extracellular domain of the Delta protein.
2. The fragment of claim 1 which consists of at least the portion of the Delta protein with the greatest homology over a sequence of 230 amino acids to amino acid numbers 1-230 as depicted in FIGS. 13A-13F (SEQ ID NO:6).
3. A chimeric protein comprising the fragment of claim 1 joined to a second protein sequence, in which said second protein is not said Delta protein.
4. A substantially pure fragment of a toporythmic Delta protein, which fragment is characterized by the ability in vitro, when expressed on the surface of a first cell, to bind to a second toporythmic Delta protein expressed on the surface of a second cell, which fragment lacks one or more amino acids of the extracellular domain of the Delta protein.
5. The substantially pure fragment of claim 4 which consists of at least the portion of the Delta protein with the greatest homology over a sequence of 199 amino acids, to about amino acid numbers 32-230 as depicted in FIGS. 13A-13F (SEQ ID NO:6).
6. A chimeric protein comprising the fragment of claim 4 joined to a protein sequence of a third protein, in which said third protein is not said Delta protein.
7. A derivative of the fragment of claim 1 having an insertion, substitution, or deletion of one or more amino acids relative to the fragment, which derivative is characterized by the ability in vitro, when expressed on the surface of a first cell, to bind to a Notch protein expressed on the surface of a second cell, said derivative lacking one or more amino acids of the extracellular domain of the Delta protein.
8. A derivative of the fragment of claim 4 having an insertion, substitution, or deletion of one or more amino acids relative to the fragment, which derivative is characterized by the ability in vitro, when expressed on the surface of a first cell, to bind to a second toporythmic Delta protein expressed on the surface of a second cell, said derivative lacking one or more amino acids of the extracellular domain of the Delta protein.
9. A substantially pure derivative of a toporythmic Delta protein, which derivative is characterized by (a) the ability in vitro, when expressed on the surface of a first cell to bind to a second toporythmic Delta protein or fragment or derivative expressed on the surface of a second cell; and (b) the inability, in vitro, when expressed on the surface of a third cell, to bind to a Notch protein expressed on the surface of a fourth cell; the derivative having an insertion, substitution, or deletion of one or more amino acids in at least the extracellular domain of the Delta protein.
10. A method of delivering an agent into a cell expressing a Notch protein comprising exposing a Notch-expressing cell to a molecule such that the molecule is delivered into the cell, in which the molecule comprises a toporythmic Delta protein or toporythmic Delta derivative bound to an agent, in which the Delta protein or derivative is characterized by the ability, in vitro, when expressed on the surface of a first cell, to bind to a Notch protein expressed on the surface of a second cell, the derivative having an insertion, substitution, or deletion of one or more amino acids relative to the Delta protein.
11. The fragment of claim 1 consisting of at least amino acid numbers 1-230 as depicted in FIGS. 13A-13F (SEQ ID NO:6).
12. The fragment of claim 4 consisting of at least amino acid numbers 32-230 as depicted in FIGS. 13A-13F (SEQ ID NO:6).
13. A chimeric protein comprising the fragment of claim 11 joined to a second protein sequence, in which said second protein is not said Delta protein.
14. A chimeric protein comprising the fragment of claim 12 joined to a second protein sequence, in which said second protein is not said Delta protein.
15. The derivative of claim 9 which comprises an insertion of amino acids relative to the wild-type amino acid sequence of said Delta protein.
16. The derivative of claim 9 which has said insertion, and in which said Delta protein has an amino acid sequence encoded by the nucleotide sequence shown in FIGS. 13A-13F (SEQ ID NO:5), and in which said insertion is an insertion of the amino acid sequence Arg-Lys-Ile-Phe (SEQ ID NO:30) between amino acid residues 197 and 198 (SEQ ID NO:6) of said Delta protein.
17. The derivative of claim 16 in which said Delta protein has the sequence depicted in FIGS. 13A-13F (SEQ ID NO:6).
18. The method according to claim 10 in which said Delta protein or Delta derivative consists of at least the extracellular and transmembrane domains of a toporythmic Delta protein.
19. The method according to claim 10 in which the molecule comprises a toporythmic Delta protein bound to the agent.
20. The method according to claim 10 in which the agent is a label.
21. The method according to claim 10 in which the agent is a biologically active agent.
22. The method according to claim 21 in which the biologically active agent is a therapeutic agent.
23. The fragment of claim 1, 2, 4 or 5 which lacks the intracellular domain of the Delta protein.
24. The fragment of claim 23 which also lacks the EGF-homologous and transmembrane regions of the Delta protein.
25. The protein of claim 3 or 6 in which said fragment lacks the intracellular domain of the Delta protein.
26. The protein of claim 25 in which said fragment also lacks the EGF-homologous and transmembrane regions of the Delta protein.
27. The derivative of claim 7 or 8 which also lacks the intracellular domain of the Delta protein.
28. The derivative of claim 27 which also lacks the EGF-homologous and transmembrane regions of the Delta protein.
29. A substantially pure derivative of a toporythmic Delta protein, which derivative is characterized by the ability in vitro, when expressed on the surface of a first cell, to bind to a Notch protein expressed on the surface of a second cell, the derivative having an insertion, substitution, or deletion of one or more amino acids in at least the extracellular domain of the Delta protein.
30. The derivative of claim 25 in which the insertion, substitution, or deletion is in at least a portion of an EGF-homologous repeat.
31. A chimeric protein comprising the derivative of claim 7, 8, 9, 29 or 30 joined to a protein sequence of a second protein, in which said second protein is not said Delta protein.
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