TXA2 Receptor and gene encoding the same

Abstract: A gene encoding a TXA₂ receptor (SEQ ID NO: 1) was isolated from a cDNA library from human placenta. A TXA₂ receptor was obtained by expressing this gene in a host. By using a probe specifically binding to this gene, a gene encoding a TXA₂ receptor was detected. The gene of this invention is useful for the production of a TXA₂ receptor, which is useful for the research of TXA₂-agonists and - antagonists. The probe of this invention is useful for the detection of a gene encoding a TXA₂ receptor.

Description: This invention relates to a gene encoding a TXA 2 receptor, a method for producing a TXA 2 receptor with the use of the gene, a TXA 2 receptor obtainable by the method, a host expressing the gene, a probe specifically hybridizing to the gene and a method for detecting the gene with the use of the probe. Thromboxane (TX) A 2 is a very unstable arachidonate metabolite, yet a potent stimulator of platelet aggregation and a constrictor of vascular and respiratory smooth muscles. It has been implicated as a mediator in diseases such as myocardial infarction, stroke and bronchial asthma. Using S-145, a stable analogue of this compound, the present inventors recently purified the human platelet TXA 2 receptor to apparent homogeneity (Ushikubi, F. et al, J. Biol. Chem. 264, 16496-16501 (1989)). Bioactive arachidonate metabolites, collectively called eicosanoids, comprise a large family of substances consisting of more than 30 members, and each member of the family acts on a specific receptor to exert a variety of activities. The structure of any member of this family has not yet been revealed. The cloning of one eicosanoid receptor will facilitate elucidation of receptor structures for other members of this family and make way for developing more selective drugs to control these pathophysiologically important mediators. A relatively large amount of TXA 2 receptor is required to research drugs specifically binding a TXA 2 receptor and to conduct a study of the TXA 2 receptor itself. However, there is no mean for providing a necessary amount of a TXA 2 receptor therefore. A cell expressing a TXA 2 receptor is indispensable to research for its agonist and antagonist and for elucidation of mechanism of signal-mediation from the receptor. There is no mean for specifically detecting the expression of a TXA 2 receptor gene in human cells. Using an oligonucleotide probe corresponding to the partial amino acid sequence of a human platelet TXA 2 receptor, the present inventors have obtained a cDNA clone encoding this receptor from human placenta and a partial clone from cultured human megakaryocytic leukemia cells. The placenta cDNA encodes a protein of 343 amino acids with seven putative transmembrane domains. The protein expressed in COS-7 cells binds drugs with an affinity identical to that of the platelet receptor, and that in Xenopus oocytes opens Ca 2+ -activated Cl - channel upon agonist stimulation. Northern blot analysis and the nucleotide sequences of the two clones suggest that an identical species of the TXA 2 receptor is present in platelets and vascular tissues. The elucidation of the amino acid and nucleotide sequences of the TXA 2 receptor by this invention facilitates elucidation of receptor structures for other members of the TXA 2 -belonging family. This invention provides a method for producing the TXA 2 receptor by expressing the gene in a host and the TXA 2 receptor obtainable thereby. This enables one to develop more selective drugs to control pathophysiologically important mediators. This invention further provides a host expressing the gene, which makes it possible to research agonists and antagonists of the TXA 2 receptor and study mechanism of signal-mediation from the receptor. This invention provides a probe specifically hybridizing to the gene, by which the gene can specifically be detected. Fig. 1 shows a restriction map of HPL and MEG. The first object of this invention is to provide a gene encoding a TXA 2 receptor, which desirably has the amino acid sequence of SEQ ID NO: 1. The TXA 2 receptor-encoding gene of this invention includes all genes encoding receptor proteins which show TXA 2 receptor activity and, especially, specifically bind to a compound S-145 as described below, not limited to one gene encoding the amino acid sequence of SEQ ID NO: 1. The gene may contain a sequence useful for its expression, for example, a promoter/operator sequence, an SD sequence, a terminator, a poly-adenylation signal or the like, upstream or downstream of the receptor-coding region. The gene may contain any sequences which do not inhibit its expression. Therefore, this invention comprehends plasmid or viral genes containing the gene and expression vectors for the gene. The gene means DNA and/or RNA, not limited to only one of them. The second object of this invention is to provide a method for producing a TXA 2 receptor by expressing the above-mentioned gene in a host and a TXA 2 receptor obtainable by the method. The host is preferably eukaryotic. In the method of this invention, not only DNA but also mRNA can be used. The TXA 2 receptor of this invention includes all receptor proteins which show TXA 2 receptor activity and, especially, specifically bind to a compound S-145 as described below, not limited to one protein having the amino acid sequence of SEQ ID NO: 1. The third object of this invention is to provide a host expressing the above-mentioned gene. The host is preferably eukaryotic and more preferably Xenopus oocytes or COS-7 cells. The gene of this invention can be expressed in any conventional host including prokaryotes such as Escherichia coli and Bacillus subtilis and eukaryotes such as yeast and CHO cells with the use of host/vector systems well known in this field. The last object of this invention is to provide a probe capable of specifically hybridizing to the above-mentioned gene. The probe includes any probes which specifically hybridize to the gene and is capable of specifically detecting the gene, for example, the Hin cII fragment (586 bp) of the MEG clone. The probe is not limited to DNA and may be RNA. The probe may preferably be labelled by appropriate labels including radioisotopes such as 32 P. By hybridizing this probe with the gene in a sample according to the usual Northern blot analysis or the like, the gene can specifically be detected. This invention is further detailed below. Purified human platelet TXA 2 receptor was subjected to proteolysis, and four partial amino acid sequences were determined. A part of the sequences was used to design a 41-mer oligonucleotide probe. With this probe a cDNA library of MEG-01 cultured human megakaryocytic leukemia cells (Nakajima, M. et al., Biochem. Biophys. Res. Commun. 158 , 958-965 (1988)) was screened. One positive clone (MEG) with a 1.4-kilobase (kb) insert was isolated. The nucleotide sequence of MEG revealed that it was a partial clone. Therefore, a 586 base pair (bp) Hin cII MEG fragment was used as a probe to screen a human placenta cDNA library. Placenta was chosen as a cDNA source because it showed most intense signals on Northern blot analysis. One positive clone, HPL, was isolated that had a 2.9 kb insert containing an entire coding region. When this invention is reproduced, clones encoding a TXA 2 receptor may easily be isolated by screening with the use of probes synthesized according to the DNA sequence of SEQ ID NO: 1. Fig. 1 and Sequence Listing show the restriction map and nucleotide (2,932 bp) and deduced amino acid sequences derived from HPL. The open reading frame (1,029 bp) codes for a 343-amino-acid protein (relative molecular mass, 37,429). MEG covers the nucleotide sequence from 1,572 to 2,923 of HPL shown in SEQ ID NO: 1 with two substitutions, and the deduced amino acid sequence in the overlapping region is identical. The N-terminal sequence of HPL is identical to that of one of the fragment peptides and there are two putative N-glycosylation sites where no signal was found on peptide analysis. Hydrophobicity analysis of the deduced amino acid sequence revealed seven hydrophobic stretches that could represent transmembrane domains (in SEQ ID NO: 1, I:30-52, II:67-86, III:107-128, IV:150-172, V:194-219, VI:247-270, and VII:290-311). Comparison of the sequence with those of the rhodopsin-type receptors revealed a significant extent of homology between this and other receptors, particularly in their putative transmembrane regions, suggesting that it belongs to this receptor family. As in the rhodopsin and substance P receptor, there is no Asp in the third transmembrane domain of the TXA 2 receptor which is presumed to serve for binding of the amino group of the ligand in adrenergic receptors. On the other hand, the TXA 2 receptor of this invention has Arg 295 which is located at the position analogous to Lys 296 of bovine rhodopsin in the seventh transmembrane domain. The latter amino acid residue was assigned for retinal attachment in the rhodopsin molecule. These structural features of the TXA 2 receptor may reflect the acidic nature of the ligand for this receptor. The third cytoplasmic loop of the TXA 2 receptor consists of less than 30 residues, and the N-terminal end of this loop shows significant homology to rhodopsin. However, the C-terminal end of this loop lacks the basic amino acid cluster which is present in the other receptors. These regions are involved in binding and determining specificity of a coupling G-protein. The G-protein coupling to the TXA 2 receptor is insensitive to pertussis and cholera toxins and remains to be identified. The short carboxy tail contains six serine and threonine residues and shows some homology to the adrenergic receptors, especially the β 1 receptor. This structure may be related to agonist-induced desensitization of the signalling pathway. Like the β 1 receptors, the TXA 2 receptor undergoes desensitization upon agonist stimulation. To establish that HPL encodes the TXA 2 receptor, the present inventors subcloned a 2.0 kb fragment of HPL into an eukaryotic expression vector, CDM8 (Seed, B., Nature 329 , 840-842 (1987)), and transfected the plasmid into COS-7 cells. Ligand binding was examined with the selective TXA 2 receptor antagonist, [ 3 H]S-145 (Ushikubi, F. et al, Eicosanoids 2 , 21-27 (1989)). The membranes of transfected COS cells bound [ 3 H]S-145 in a saturable manner with the dissociation constant, Kd, of 1.2 nM, which is comparable to that observed with membranes from fresh platelets. Specificity of this binding was analysed with several TXA 2 analogues as well as various prostanoids. The TXA 2 agonist, STA 2 , and antagonists, S-145 and ONO-3708, competed for the [ 3 H]S-145 binding with an order of potency identical to that observed with the TXA 2 receptor in other cells. Other prostaglandins (PGs) and an inactive TXA 2 metabolite, TXB 2 , were at least two orders of magnitude less active in the competition. The TXA 2 receptor stimulation results in PI turnover and Ca 2+ mobilization. The present inventors, therefore, expressed the clone in Xenopus oocyte where agonist stimulation linked to PI turnover opens endogenous Ca 2+ -dependent Cl - channel. Application of 1 µM STA 2 to the oocytes injected with the in vitro transcribed mRNA of the clone evoked a fast inward current of a few hundred nA followed by a slow phase of current of low amplitude. This response was seen at as low as 1 nM STA 2 concentration. PGD 2 and PDF 2 α did not evoke response at 1 µM. Other PGs such as PDE 1 , E 2 and I 2 and TXB 2 did not evoke response, either, under these experimental conditions. Thus, the protein encoded by HPL possesses the ligand binding properties characteristics of the TXA 2 receptor and evokes a response expected for this receptor. Northern blot analysis of the TXA 2 receptor mRNA expression in various tissues is of interest because of a controversy over the presence of the receptor subtypes. As extensive analysis using poly(A) + RNA from various rat tissues yielded little signals probably due to the species difference of the receptor, the present inventors performed the analysis only on poly(A) + RNA from human placenta and lung and cultured MEG-01 cells. Placenta and lung are representative tissues rich in the TXA 2 receptor. A major hybridization band was observed at 2.8 kb in all three lanes under stringent conditions, which was most intense with the placenta RNA. In this lane, a minor band was present at 3.5 kb. This band became apparent also in the lane of the MEG-01 poly(A) + RNA on extended exposure. These results together with the above findings that MEG and HPL encode the identical amino acid sequence suggest that the same species of mRNA are expressed in platelet precursor cells and vascular rich tissues. Bioactive arachidonate metabolites, collectively called eicosanoids, comprise a large family of substances consisting of more than 30 members, and each member of the family acts on the specific receptor to exert a variety of activities. This cloning of an eicosanoid receptor will facilitate elucidation of receptor structures for other members of this family and make way for developing more selective drugs to control these pathophysiologically important mediators. Example Human platelet TXA 2 receptor was purified to apparent homogeniety (Ushikubi, F. et al, J. Biol. Chem. 264, 16496-16501 (1989)) and subjected to proteolysis with cyanogen bromide, lysyl endopeptidase and trypsin. Fragment peptides were isolated by reverse phased HPLC and sequenced by automated pulse-liquid phase protein sequencer. Four partial amino acid sequences were obtained. A part of these sequences (amino acid sequence from 296 to 309 of SEQ ID NO: 1) was used to design a 41-mer oligonucleotide probe. MEG-01 cDNA library was screened with this probe radiolabelled at the 5'-end. One positive clone, MEG, was isolated. The sequence of a 1.4 kb insert in this clone revealed an open reading frame which contained two of the partial amino acid sequences of the purified protein. A 586 bp fragment of MEG was then used to screen a placenta cDNA library (T. Maniatis et al, Molecular Cloning (1989), Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY). One positive clone, HPL, was obtained and analysed. DNA sequencing was carried out on double stranded templates using dideoxy chain termination method. Fig. 1 shows a restriction map of TXA 2 receptor clones from human placenta (HPL) and cultured human megakaryocytic leukemia cells (MEG). Stippled boxes indicate coding region. SEQ ID NO: 1 in Sequence Listing shows nucleotide and deduced amino acid sequences of HPL. The most 5' ATG triplet found in frame with all of the partial amino acid sequences of the purified protein was assigned as the site of translation initiation. There are 18 other ATGs in 5'-noncoding region, all of which terminate after short stretch of reading frames. The putative poly-adenylation signal is from 2,908 to 2,913 of SEQ ID NO: 1. Sequences obtained from proteolytic peptides are those of 1-131-141, 277-284 and 289-322 of amino acid sequence shown in SEQ ID NO: 1. The putative N-glycosylation sites are two Asn residues at positions 4 and 16 of the amino acid sequence of SEQ ID NO: 1. MEG covers the nucleotide sequence from 1,572 to 2,923 of HPL shown in SEQ ID NO: 1 with two substitutions; both T→C at the positions of 1,786 and 1,915 of HPL shown in SEQ ID NO: 1. These substitutions do not alter the amino acids encoded by these codons. Expression HPL was digested with Bam HI to remove most of the 5'-untranslated region, and its 2.0 kb fragment was inserted between the Hin dIII- Xba I site of eukaryotic expression vector CDM8 (Seed, B., Nature 239 , 840-842 (1987)). A modified DEAE-dextran method (Ausbel, F. M. et al, Current Protocols in Molecular Biology. 9.2.5 (Wiley, New York, 1989)) was used for the transfection of COS-7 cells. At 60 h after the transfection, cells were collected and membranes were prepared by homogenizing the cells according to the method of Ausbel, F. M. et al., as described above in the presence of 1 mM benzamidine and 0.3 mM PMSF. The membranes were finally suspended in 20 mM Na-Mes, pH 6.4, and 5 mM EDTA, and binding experiments with [ 3 H]S-145 (24 Ci/mmol) were carried out as described in Ushikubi, F. et al., Eicosanoids 2 , 21-27 (1989). Competition experiments were carried out with 1.25 nM [ 3 H]S-145 and various concentrations of ligands. The membranes of transfected COS cells bound [ 3 H]S-145 in a saturable manner with the dissociation constant, Kd, of 1.2 nM, which is comparable to that observed with membranes from fresh platelets. The estimated maximal binding, Bmax, was 2.2 pmol/mg protein. The TXA 2 agonist, STA 2 , and antagonists, S-145 and ONO-3708, competed for the [ 3 H]S-145 binding with an order of potency identical to that observed with the TXA 2 receptor in other cells. Other prostaglandins (PGs) and an inactive TXA 2 metabolite, TXB 2 , were at least two orders of magnitude less active in the competition. Response Reaction The mRNA synthesized in vitro from the 2.0 kb Bam HI subclone of HPL was injected into Xenopus oocytes and electrophysiological responses were measured after 48 hours. A TXA 2 agonist, STA 2 , and other PGs dissolved in ethanol at 100 µM were added to the bath at 1 µM concentration, and responses of oocytes were recorded by the two-micropipette voltage clamp method as described in Masu, Y. et al., Nature 329 , 836-838 (1987). Application of 1 µM STA 2 evoked a fast inward current of a few hundred nA followed by a slow phase of current of low amplitude. This response was seen at as low as 1 nM STA 2 concentration. PGD 2 and PGF 2 α did not evoke response at 1 µM. Other PGs such as PGE 1 , E 2 and I 2 and TXB 2 did not evoke response, either, under these experimental conditions. Thus, the protein encoded by HPL possesses the ligand binding properties characteristics of the TXA 2 receptor and evokes a response expected for this receptor. Northern Blot Analysis Total RNA was prepared from each of cultured MEG-01 cells and human placenta and lung tissues using guanidinium thiocyanate-cesium chloride method, and poly(A) + RNA was isolated using a oligo(dT) cellulose column. Twenty µg of each poly(A) + RNA were subjected to formaldehyde agarose gel electrophoresis, transferred to Biodyne membrane and UV-cross-linked. The filter was prehybridised in 5 × SSC buffer, 5 × Denhardt's solution, SO mM sodium phosphate, pH 6.5, 200 µg/ml heat denatured salmon testis DNA, 50 % formamide and 0.1 % SDS at 42°C for 4 h, and hybridised in the same solution at 42°C overnight with a 586 bp Hin cII fragment of MEG clone labelled with 32 P-dCTP by random-priming (4 × 10 6 c.p.m.ml -1 ). The filter was washed twice in 0.1 × SSC, 0.1 % SDS at 65°C for 1 h and exposed to an X-ray film for 12 days in the presence of an intensifying screen at -80°C. The present inventors performed the analysis only on poly(A) + RNA from human placenta and lung and cultured MEG-01 cells. Placenta and lung are representative tissues rich in the TXA 2 receptor. A major hybridization band was observed at 2.8 kb in all three lanes under stringent conditions,which was most intense with the placenta RNA. In this lane, a minor band was present at 3.5 kb. This band became apparent also in the lane of the MEG-01 poly(A) + RNA on extended exposure. These results together with the above findings that MEG and HPL encode the identical amino acid sequence suggest that the same species of mRNA are expressed in platelet precursor cells and vascular rich tissues. Sequence Listing SEQ ID NO: 1 SEQUENCE LENGTH: 2,932 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULAR TYPE: cDNA to mRNA HYPOTHETICAL: No ANTI-SENSE: No ORIGINAL SOURCE ORGANISM: human TISSUE TYPE: placenta CLONE: HPL FEATURE KEY: polyA signal LOCATION: 2908..2913 IDENTIFICATION METHOD: S

Claims: An isolated gene encoding a thromboxane A 2 (TXA 2 ) receptor wherein said TXA 2 receptor contains the amino acid sequence of SEQ ID NO: 1. A method for producing a thromboxane A 2 (TXA 2 ) receptor which comprises expressing said gene of claim 1 in a host. The method of claim 2 wherein said host is eukaryotic. A host expressing said gene of claim 1. The host of claim 4 which is eukaryotic.

Inventors: NAKANISHI SHIGETADA, NARUMIYA SHUH, NAKANISHI, SHIGETADA, NARUMIYA, SHUH
Applicants: SHIONOGI & CO, SHIONOGI SEIYAKU KABUSHIKI KAISHA
Application year: 1991
Main classification: C12N15
IPC codes: C12P21, C12R1, C12N15, C12N5, C12Q1, C07K14