Polypeptides Having a Serotonin Receptor Activity, Nucleic Acids Coding for These Polypeptides and Uses

The present invention relates to new polypeptides and to the genetic material permitting their expression. More especially, it relates to new polypeptides having serotoninergic receptor activity.

Serotonin is a neuromodulator capable of inducing and modulating a wide variety of behavioural functions such as sleep, appetite, locomotion, sexual activity and vascular contraction. It is accepted that serotonin activity is mediated by its interaction with receptors, designated serotoninergic receptors or 5-HT (for 5-hydroxytryptamine) receptors. Molecular biology studies as well as pharmacological studies have revealed the existence of a large number of subtypes of 5-HT receptors. The 5-HT receptors which have been described to date belong either to the family of receptors associated with ion channels (5-HT3 receptors), or to the family of receptors which interact with G proteins and which possess seven transmembrane domains. Moreover, analysis of the amino acid sequences has shown that the 5-HT receptors which interact with G proteins may be subdivided into two distinct groups: 5HT1 receptors, comprising the mammalian subtypes 5HT1A, 5HT1B and 5HT1D, as well as three drosophila 5HT receptors; and 5HT2 receptors comprising the subtypes 5HT2 and 5HT1C.

These receptors are doubtless not the only 5HT receptors which exist, inasmuch as pharmacological studies have revealed other subtypes, such as 5HT4 receptors as well as some receptors related to the subtype 5HT1 ("5HT1-like" receptors). Furthermore, additional molecular biology studies have also revealed heterogeneities within the subtypes 5HT1B/1D.

Nowadays, contrary to the other subtypes, the 5HT2 receptors have not been widely studied. The 5HT2 receptors act via phospholipase C, and are responsible for numerous physiological activities of serotonin at the central and peripheral level. At the cardiovascular level, they are involved in the contraction of the blood vessels and in the morphological changes of the platelets; in the central nervous system, they act on the sensitization of the neurones to tactile stimuli and on the mediation of the hallucinogenic effects of diethylamidelysergic acid, and of the related phenylisopropylamines.

Numerous studies show that the 5HT2 receptors described up until now do not account for all the properties which are attributed to them. In particular, certain 5HT2-types effects of serotonin on the peripheral smooth muscles are classified as atypical effects, of which the effects are thought to be mediated by unknown receptors.

The present invention is the outcome of the demonstration of new polypeptides having serotoninergic receptor activity. While belonging to the family of receptors which interact with G proteins, these new polypeptides differ from the serotoninergic receptors already described (5HT1, 5HT2, 5HT3 and 5HT4) from both a structural standpoint and a pharmacological standpoint. More especially, the invention is the outcome of the isolation and characterization of these new polypeptides, designated 5HT2C, and also of the genetic material permitting their expression or identification. Pharmacological study of these receptors show that they could account for the atypical effects of serotonin which were previously observed.

A first subject of the invention hence lies in polypeptides comprising all or part of the peptide sequence SEQ ID No. 2 or of a derivative of the latter.

For the purposes of the present invention, the term derivative denotes any molecule obtained by modification, of a genetic and/or chemical nature, of the peptide sequence SEQ ID No. 2. Modification of a genetic and/or chemical nature may be understood to mean any mutation, substitution, deletion, addition and/or modification of one or more residues. Such derivatives may be generated for different purposes, such as, in particular, that of increasing the affinity of the peptide for its ligand(s), that of improving its levels of production, that of increasing its resistance to proteases, that of increasing and/or modifying its activity or that of endowing it with new pharmacokinetic and/or biological properties. Among derivatives resulting from an addition, chimeric polypeptides containing an additional heterologous portion attached at one end may, for example, be mentioned. The term derivative also comprises polypeptides homologous with the polypeptide SEQ ID No. 2, originating from other cell sources, and in particular from cells of human origin or of other organisms and possessing an activity of the same type. Such homologous polypeptides may be obtained by hybridization experiments, as described in the examples. In particular, Example 5 describes a derivative according to the invention (truncated form).

Preferably, the polypeptides of the invention are polypeptides possessing the capacity to bind serotonin. Still more preferably, they are polypeptides having serotoninergic receptor activity. Still according to a preferred embodiment, the polypeptides of the invention are capable of being recognized by antibodies which recognize the complete peptide sequence SEQ ID No. 2.

A particular embodiment of the invention is represented by the polypeptide 5HT2C comprising the whole peptide sequence SEQ ID No. 2. As shown in the examples, this polypeptide may be expressed in different cell types to form a functional serotoninergic receptor. This polypeptide comprises 479 amino acids and has a calculated molecular weight of 56,508 Daltons.

The polypeptides of the invention may be obtained by expression of a nucleotide sequence as described below in a cell host, by chemical synthesis on the basis of the sequence SEQ ID No. 2 using techniques known to a person skilled in the art, or by a combination of these techniques.

In what follows, the polypeptides of the invention as defined above are designated 5HT2C polypeptides.

The subject of the present invention is also any nucleotide sequence coding for a 5HT2C polypeptide. More preferably, such a sequence is chosen from:

(a) all or part of the nucleotide sequence SEQ ID No. 1 or of its complementary strand,

(b) any sequence hybridizing with a sequence (a) and coding for a polypeptide as defined above, and

(c) the sequences derived from the sequences (a) and (b) as a result of the degeneracy of the genetic code.

The different nucleotide sequences of the invention can be of artificial origin or otherwise. They can be genomic, cDNA or RNA sequences, hybrid sequences or synthetic or semi-synthetic sequences. These sequences may be obtained, for example, by screening DNA libraries (cDNA library, genomic DNA library) by means of probes devised on the basis of the sequence SEQ ID No. 1. Such libraries may be prepared from cells of different origins by standard techniques of molecular biology known to a person skilled in the art. The nucleotide sequences of the invention may also be prepared by chemical synthesis, in particular according to the phosphoramidite method, or alternatively by mixed methods including the chemical or enzymatic modification of sequences obtained by screening libraries.

The nucleotide sequences of the invention may be used for the production of 5HT2C polypeptides as defined above. In this case, the portion coding for the said polypeptide is generally placed under the control of signals permitting its expression in a cell host. The choice of these signals (promoters, terminators, and the like) can vary in accordance with the cell host used. To this end, the nucleotide sequences of the invention can form part of a vector, which can be an autonomously replicating or integrative vector. More especially, autonomously replicating vectors may be prepared using sequences which replicate autonomously in the chosen host. As regards integrative vectors, these may be prepared, for example, using sequences homologous with certain regions of the host's genome, permitting integration of the vector by homologous recombination. The cell hosts which are usable for the production of the 5HT2C polypeptides of the invention by the recombinant method are either eukaryotic or prokaryotic hosts. Among suitable eukaryotic hosts, animal cells, yeasts or fungi may be mentioned. In particular, as regards yeasts, yeasts of the genus Saccharomyces, Kluyveromyces, Pichia, Schwanniomyces or Hansenula may be mentioned. As regards animal cells, COS, CHO, Cl27 and NIH-3T3 cells, and the like, may be mentioned. Among fungi, Aspergillus ssp. or Trichoderma ssp. may be mentioned more especially. As prokaryotic hosts, it is preferable to use the following bacteria: E. coli, Bacillus or Streptomyces.

The nucleotide sequences of the present invention are also usable in the pharmaceutical field, either for the production of antisense sequences which may be used in the context of a gene therapy, or else for the production of probes permitting the detection, by hybridization experiments, of the expression of serotoninergic receptors in biological samples, and the demonstration of genetic abnormalities (polymorphism, mutations) or of aberrant expressions.

Inhibition of the expression of certain genes by antisense sequences has proved to be a promising strategy in controlling the activity of a gene. The antisense sequences are DNA sequences coding for complementary RNAs and, therefore, capable of hybridizing specifically with a given mRNA, inhibiting its translation into protein. A subject of the invention is thus the antisense sequences capable of at least partially inhibiting the production of 5HT2C polypeptides as defined above. Such sequences can consist of all or part of the nucleotide sequences defined above. They are generally sequences or fragments of sequences complementary to sequences coding for peptides of the invention. Such sequences may be obtained from the sequence SEQ ID No.1, by fragmentation, and the like, or by chemical synthesis.

As stated above, the invention also makes it possible to produce nucleotide probes, synthetic or otherwise, capable of hybridizing-with the nucleotide sequences defined above which code for 5HT2C polypeptides of the invention, or with the corresponding mRNAs. Such probes may be used in vitro as a diagnostic tool for detecting the expression of a 5HT2C serotoninergic receptor, or alternatively for demonstrating genetic abnormalities (incorrect splicing, polymorphism, point mutations, and the like). In view of the numerous activities of serotonin, the probes of the invention may thus enable neurological, cardiovascular or psychiatric disorders to be identified as being associated with 5HT2C receptors. These probes may also be used for demonstrating and isolating homologous nucleic acid sequences coding for 5HT2C polypeptides as defined above, from other cell sources and preferably from cells of human origin, as illustrated in the examples. The probes of the invention generally contain at least 10 bases, and they can contain as much as the whole of the sequence SEQ ID No. 1 or of its complementary strand. Preferably, these probes are labelled prior to their use. For this purpose, different techniques known to a person skilled in the art may be employed (radioactive or enzymatic labelling, and the like). The hybridization conditions under which these probes may be used are mentioned in the general cloning techniques below, as well as in the examples.

Another subject of the invention relates to recombinant cells capable of expressing at their surface a 5HT2C polypeptide as defined above. These cells may be obtained by introducing a nucleotide sequence as defined above, coding for a polypeptide of the invention, and then culturing the said cells under conditions for expression of the said sequence.

The recombinant cells according to the invention can be either eukaryotic or prokaryotic cells. Among suitable eukaryotic cells, animal cells, yeasts or fungi may be mentioned. In particular, as regards yeasts, yeasts of the genus Saccharomyces, Kluyveromyces, Pichia, Schwanniomyces, or Hansenula may be mentioned. As regards animal cells, COS, CHO, Cl27 and NIH-3T3 cells, and the like, may be mentioned. Among fungi, Aspergillus ssp. or Trichoderma ssp. may be mentioned more especially. As prokaryotic cells, it is preferable to use the following bacteria: E. coli, Bacillus or Streptomyces. The cells thereby obtained may be used to measure the ability of different molecules to behave as a ligand or as a modulator of the activity of the polypeptides of the invention. More especially, they may thus be used in a method for demonstrating and isolating ligands or modulator of the activity of the polypeptides of the invention, and more preferably serotonin agonists and antagonists.

Another subject of the invention hence relates to a method for demonstrating and/or isolating ligands of the 5HT2C polypeptides of the invention, according to which the following steps are carried out:

a molecule or a mixture containing different molecules, which are possibly unidentified, is brought into contact with a recombinant cell as described above, expressing at its surface a polypeptide of the invention, under conditions permitting interaction between the said polypeptide of the invention and the said molecule should the latter possess an affinity for the said-polypeptide, and

the molecules bound to the said polypeptide of the invention are detected and/or isolated.

In a particular embodiment, this method of the invention is suitable for demonstrating and/or isolating serotonin agonists and antagonists for the 5HT2C polypeptides.

Another subject of the invention relates to a method for demonstrating and/or isolating modulators of the 5HT2C polypeptides of the invention, according to which the following steps are carried out:

a molecule or a mixture containing different molecules, which are possibly unidentified, is brought into contact with a recombinant cell as described above, expressing at its surface a polypeptide of the invention, in the presence of 5HT, under conditions permitting interaction between the said polypeptide of the invention and 5HT, and

the molecules capable of modulating the activity of 5HT with respect to the said polypeptide of the invention are detected and/or isolated.

Another subject of the invention relates to the use of a ligand or modulator identified and/or obtained according to the method described above, as a medicinal product. Such ligands or modulators can, in effect, enable certain neurological, cardiovascular or psychiatric disorders associated with 5HT2C receptors to be treated.

The invention also relates to any medicinal product comprising as active principle at least one molecule which acts on a 5HT2C polypeptide of the invention. Preferably, the molecule is a ligand or a modulator identified and/or isolated according to the method described above.

Other advantages of the present invention will become apparent on reading the examples which follow, which are to be considered as illustrative and nonlimiting.

FIG. 1 Scatchard curve of the results of experiments for the binding, at saturation, of ›.sup.125 I!DOI to the membranes of Cos-7 cells expressing the 5HT2C receptor.

FIG. 2 Demonstration of homologous sequences by PCR on total RNAs (10 .mu.g) of different mouse tissues: line 1: RNA from the LMTK cell line, line 2: RNA from 10-day old embryos, line 3: RNA from testicles, line 4: RNA from the 3T6 cell line, line 5: RNA from the MBK cell line, line 6: RNA from the liver, line 7: RNA from the brain, line 8: RNA from the heart, line 9: RNA from the intestine, line 10: RNA from the spleen, line 11: RNA from the kidney, line 12: total RNA from the brain.

Table 1 Pharmacological profile of the whole (N) or truncated (T) 5HT2C receptor. The results correspond to competition experiments for the binding of ›.sup.125 I!DOI to the membranes of Cos-7 cells expressing the 5HT2C receptor. The values, expressed as pKD (-log mol/1) are the result of at least three separate experiments, carried out in triplicate.

The methods traditionally used in molecular biology,-such as preparative extractions of plasmid DNA, centrifugation of plasmid DNA in a caesium chloride gradient, agarose or acrylamide gel electrophoresis, purification of DNA fragments by electroelution, protein extractions with phenol or phenol/chloroform, ethanol or isopropanol precipitation of DNA in a saline medium, transformation in Escherichia coli, and the like, are well known to a person skilled in the art and are amply described in the literature ›Maniatis T. et al., "Molecular Cloning, a Laboratory Manual", Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., 1982; Ausubel F. M. et al. (eds), "Current Protocols in Molecular Biology", John Wiley & Sons, New York 1987!.

The restriction enzymes, AMV reverse transcriptase, DNA polymerase I, T4 polynucleotide kinase, T4 DNA ligase, and T3 or T7 RNA polymerases were supplied by New England Biolabs (Biolabs), Bethesda Research Laboratories (BRL), Boehringer-Mannheim or Stratagene and are used according to the suppliers' recommendations.

For ligation, the DNA fragments are separated according to size by agarose or acrylamide gel electrophoresis, extracted with phenol or with a phenol/chloroform mixture, precipitated with ethanol and then incubated in the presence of phage T4 DNA ligase according to the supplier's recommendations.

The filling-in of 5' protruding ends is performed with the Klenow fragment of E. coli DNA polymerase I according to the supplier's specifications. The destruction of 3' protruding ends is performed in the presence of phage T4 DNA polymerase used according to the manufacturer's recommendations. The destruction of 5' protruding ends is performed by a controlled treatment with S1 nuclease.

Mutagenesis directed In vitro by synthetic oligodeoxynucleotides is performed according to the method developed by Taylor et al. ›Nucleic Acids Res. 13 (1985) 8749-8764!.

The enzymatic amplification of DNA fragments by the so-called PCR ›Polymerase-catalyzed Chain Reaction, Saiki R. K. et al., Science 230 (1985) 1350-1354; Mullis K. B. and Faloona F. A., Meth. Enzym. 155 (1987) 335-350! technique is performed using a "DNA thermal cycler" (Perkin Elmer Cetus) according to the manufacturer's specifications.

Verification of the nucleotide sequences is performed by the method developed by Sanger et al. ›Proc. Natl. Acad. Sci. USA, 74 (1977) 5463-54673!.

For the hybridization experiments, the normal conditions of stringency are generally as follows: hybridization: 5.times. SCC in the presence of 5.times. Denhart's at 65.degree. C.; washing: 0.5.times. SSC at 65.degree. C.

1. Isolation of the 5HT2C Receptor

Sequence comparisons between the different known serotoninergic receptors bring out some degree of conservation. With the aim of demonstrating and isolating a new receptor, three degenerate oligonucleotides corresponding to the VI and VII transmembrane regions were prepared and then used in a series of PCR reactions on a mouse genomic DNA preparation. The sequence of the degenerate oligonucleotides is the following:

Oligonucleotide (i) (SEQ ID No. 4 ) TACCTCGAGGTCGACGGTIATGTGGTG(C,T)CCITT(C,T)TT(C,T)AT

Oligonucleotide (ii) (SEQ ID No. 5 ) AGAACTAGTGGTACCCA(G,A)IGT(G,A)TAIACIA(G,A)IGG(G,A)TT

Oligonucleotide (iii) (SEQ ID No. 6 ) AGAACTAGTGGTACCC(G,C) (A,T) (G,A) (G,A)TAICC(G,A,T)ATCCA

A multiple cloning site was added to the 5' end of the oligonucleotides to allow subsequent subcloning of the amplified fragment. The PCR reactions were performed in the following manner: 1 .mu.g of mouse genomic DNA was denatured for 1 minute at, 94.degree. C., maintained for 2 minutes at 55.degree. C. and then subjected to 20 amplification cycles at 72.degree. C. in the presence of 10 Taq polymerase units (Cetus), 3 mM MgCl.sub.2 and 1 .mu.g of each of the oligonucleotides (i) and (ii). 1/10 th of the product of this reaction was then subjected to 20 additional amplification cycles under the same conditions, in the presence of 1 .mu.g of each of the oligonucleotides (i) and (iii). The products thus obtained were then digested with the SpeI and XhoI enzymes, inserted into the corresponding sites of the plasmid Bluescript (Strategene), and sequenced. One of the fragments thus obtained, having some homology with the 5HT2 serotoninergic receptors, was synthesized in vitro and labelled at its 5' end by polynucleotide kinase, and then used as a probe to screen a mouse brain cDNA library. A positive clone was selected. This clone was designated NP75. The insert carried by the NP75 clone was sequenced on the 2 strands using the dideoxynucleotide technique with the aid of synthetic oligonucleotides. Analysis of the sequence obtained reveals the presence of an open reading frame coding for a protein of 479 amino acids (SEQ ID No. 1 and 2). Moreover, analysis of hydrophobicity shows that this protein carries seven hydrophobic domains, a characteristic encountered in members of the family of receptors coupled to G proteins. The N-terminal end contains, moreover, 1 potential site for N-glycosylation, and the presumed cytoplasmic domain contains the consensus sites for phosphorylation by the C and A protein kinases. In addition the presence of 19 serine and threonine residues in the 121 C-terminal amino acids seems to indicate that this region is involved in the desensitization of the receptor by kinases.

2. Sequence Homology Study

The sequence of the 5HT2C receptor isolated above was compared with the sequences of the following receptors coupled to G proteins: 5HT1A, 5HT1B, 5HT1C, and 5HT2. These experiments revealed homology in the I to VII transmembrane domains with the 5HT1C and 5HT2 receptors greater than the homology encountered with the 5HT1A and 5HT1B receptors. These results suggest, that the receptors of the invention belong to the 5HT2 subtype. This is in fact in agreement with the presence of 2 introns in the gene coding for the 5HT2C receptor (not shown in the sequence).

3. Expression of the 5HT2C Receptor in Cos-7 Cells and Pharmacological Characterization

The cDNA fragment isolated in Example 1 was inserted into an eukaryotic expression vector, which was used to transfect Cos-7 cells. The membranes of the transfected cells obtained were then prepared and tested for their capacity to bind certain labelled serotoninergic ligands.

The cDNA encoding the 5HT2C receptor was isolated in the form of a 1372 pb XbaI-EcoRI fragment and then inserted at the corresponding sites of the vector p513. The vector p513 is derived from the vector pSG5 ›Green et al., Nucl. Acids Res. 16 (1988) 369! by addition of a multiple cloning site. The recombinant vector thus obtained, designated p513NP75, was then used (10 .mu.g per 10 cm dish) to transfect the Cos-7 cells in the presence of calcium phosphate.

48 Hours after transfection, the recombinant cells are harvested and the membranes are prepared according to the technique described by Amlaiky and Caron ›J. Biol. Chem. 260 (1985) 1983!.

Saturation binding and competition experiments were then carried out on these membranes in the presence of ›.sup.125 I!DOI (2200 Ci/mmol; New England Nuclear) and various test compounds.

The ›.sup.125 I!DOI (dimethoxyphenyl isopropylamine) was first used alone, in order to detect the expression of the gene encoding the receptors of the invention in the membranes of the recombinant cells. For this purpose, 100 .mu.l of membrane suspension (50 .mu.l) of proteins) were incubated for 30 minutes at 30.degree. C. in the presence of the radioactive ligand (50 .mu.l) and 50 .mu.l of 50 mM Tris-HCl buffer (pH 7.4), 150 mM NaCl, 10 mm MgCl.sub.2, final volume: 200 .mu.l. The reaction is then stopped by addition of an isoosmotic ice-cold solution, then by filtration on Whatman GF/B filters, followed by 4 rinses in 5 ml of the above ice-cold buffer. The filters are then dried and the radioactivity is measured by counting (.gamma. counter). Non-specific binding, determined in the presence of 10 .mu.M of cold DOI, represented about 30% of the total binding.

The results obtained show that DOI binds specifically to the membranes of the transfected cells, and not to the membranes of the control cells (transfected with plasmid pSG5). These cells therefore indeed express a functional serotonin receptor. Furthermore, the affinity of DOI for the transfected cells is high and the binding is saturable (FIG. 1). The apparent KD is 25.8.+-.0.54 nM, and the apparent Bmax varies between 14.8 and 21.0 pmol/mg of protein, depending on the transformation efficiency.

To determine the pharmacological profile of this receptor, the ›.sup.125 I!DOI bound to the membranes was displaced in the presence of different serotoninergic drugs (Table 1). For this purpose, the cellular membranes are incubated under the conditions described above, with, in place of the 50 .mu.l of buffer, increasing concentrations of the compounds to be tested. These different drugs show the following order of efficacy of displacement: ritanserin>N-acetyl 5-HT>methysergide>setoperone=cyproheptadine>spiperone>ketanserin>tryptamin e>5-HT >8-OH-DPAT (Table 1). The low affinity (relative) of this receptor for serotonin confirms that it belongs to the 5HT2 subtype. Furthermore, the absence of competition by the compounds ICS 205-930, MDL 72222 and quipazine, which are specific for the 5HT3 receptors, excludes the possibility that the receptors of the invention belong to this subtype.

4. Search for Homologous Sequences in Other Tissues

The nucleotide sequence SEQ ID No. 1 was then used to demonstrate homologous sequences on other tissues by PCR.

The tissues used to search for homologous sequences are the following ones of murine origin: intestine, heart, brain, kidney, spleen and liver.

The following probes were used:

Probe (iv): 5'-GATCCTGACTAACCGTTCTGGA-3'(SEQ ID No. 7 )

Probe (v) : 5'-TGCCTATTGAAATTAACCATACCA-3(SEQ ID No. 8 )

Probe (iv) corresponds to position 85 on SEQ ID No. 1 and probe (v) to position 547 (complementary strand).

Total RNAs were prepared from the tissues indicated above according to the technique described by Cathala et al. (DNA 2(4) (1983). 10 .mu.g of these RNAs were subjected to reverse transcription in the presence of 13 units of AMV reverse transcriptase, 5 units of Taq polymerase and 1 .mu.g of probes (iv) and (v), for 15 minutes at 50.degree. C., and then amplified (20, 25 and 30 cycles) (Maroteaux et al., Proc. Natl. Acad. Sci. USA 89 (1992) 3020). The primers corresponding to the mRNA of the ribosomal elongation factor EF1A were used as internal standard, under the same conditions as probes (iv) and (v).

The products thus obtained were then transferred onto nitrocellulose filters and hybridized under the following conditions: the probe used for the hybridization had the following sequence:

Probe (vi): 5'-TGCCTGGTTATTCCTCGATGTTC-3'(SEQ ID No. 9 )

corresponding to position 398 on the sequence SEQ ID No. 1.

The hybridization was carried out under high stringency conditions: 50.degree. C., in a 20 mM sodium phosphate buffer (pH 6.5) containing 5.times. SSC, 1.times. Denhardt's 0.1% SDS and 100 .mu.g/ml of tRNA. The washes were performed at 50.degree. C. in 1.times. SSC, 0.1% SDS buffer.

This study enabled homologous specific DNA fragments to be demonstrated in the intestine, the heart, the kidneys and the brain (FIG. 2).

It is understood that the same experiments can be repeated using other tissues and especially tissues of human origin, other probes, and other techniques (in situ hybridization, Northern blotting and the like). Moreover, the homologous sequences demonstrated during these experiments can obviously then be isolated and/or amplified by conventional molecular biology techniques.

5. Isolation and Characterization of a Truncated Receptor SEO ID No. 2.sub.(1-177) -SEO ID No. 3

During the cloning of the cDNA encoding the 5-HT2C receptor (Example 1), numerous forms of truncated mRNA were identified. One of them, after characterization, turned out to encode a receptor having only the first three transmembrane domains ›residues 1 to 177 SEQ ID No. 2, to which the following 14 C-terminal amino acids can be added: Ala Ser Pro Ser Gln Ser Leu Leu Lys Glu Ser Arg Leu Met (SEQ ID No. 3)! Analysis of the organization of the genomic DNA shows that this RNA is obtained from the use of an alternative donor for splicing at the end of exon 2 resulting in the removal of bases 550-568 from the nucleic sequence SEQ ID No. 1. This causes a change of phase and introduces a stop codon into the RNA, producing a truncated protein after the third transmembrane domain. This alternative form of mRNA, first isolated from brain RNA, is differentially expressed depending on the tissues. The relative level of expression of the truncated RNA compared with the complete RNA is maximum in mouse heart, where it represents more than 50%, whereas it is only 15% in the intestine.

Unexpectedly, analysis of the pharmacological profile of this truncated protein by transient expression in COS cells according to the method described in Example 3 shows that it binds serotonin with an affinity very close to that observed for the complete receptor (Table 1). Furthermore, its affinity for agonists remains, although slightly reduced, whereas the binding for antagonists is completely abolished, as well as the translation of the intracellular signal. The physiological role of this form of the receptor remains unknown, but it seems likely that it serves to protect certain sensitive targets, such as the vascular endothelium, from harmful stimulations by locally "diluting" the effect of the ligand, in this case serotonin.

Finally, more precise analysis, by in situ hybridization, of the sites of expression of the 5-HT2C receptor in mouse heart situates them in the endocardial and vascular endothelium. This expression is also detected in the inner layer (endothelium) of cardiac anlages in mouse early embryos (8.5 days). Knowing that serotonin is involved in the phenomena of development of the cardiovascular system in chicken embryo (Heuther et al., Biog Amine 8 (1992) 421), it is possible that the 5-HT2C receptor participates in these phenomena. Likewise, recent observations implicate serotonin in the pathophysiology of coronary diseases: human coronary atherosclerosis is associated with an increased sensitivity to serotonin of the contraction of the coronary arteries. This phenomenon can be reduced by ketanserin (Golino et al., New Engl. J. Med. 324 (1991) 641) and results from lesions of the vascular endothelium where serotonin is released by the platelets. It is thus likely that the truncated 5-HT2C receptor-expressed in the vascular endothelium participates in the regulation of the effect of serotonin on the cardiovascular system both during its differentiation and its pathological deregulation.

Consequently, the development of compounds specific for the whole or truncated 5-HT2C receptor is of therapeutic value in certain human cardiac pathologies.