ASSAYS FOR GROWTH HORMONE SECRETAGOGUE RECEPTORS

Abstract: An assay for the detection of growth hormone secretagogue receptors and growth hormone secretagogue related receptors is described. As these receptors are a member of the G protein coupled receptors, a subunit of the G protein must be present in order for expression to be detected. A similar assay is described where the presence of growth hormone secretagogues are detected.

Description: TITLE OF THE INVENTION ASSAYS FOR GROWTH HORMONE SECRETAGOGUE RECEPTORS FIELD OF THE INVENTION This invention relates to an assay which involves identification of cell membrane receptors, specifically growth hormone secretagogoue receptors (GITSRs). By varying the protocol, receptor ligands can be identified, or the presence of a GHSR can be identified. BACKGROUND OF THE INVENTION Growth hormone (GH) is an anabolic hormone capable of promoting linear growth, weight gain and whole body nitrogen retention. Classically, GH is thought to be released primarily from the somatotroph cells of the anterior pituitary under the coordinate regulation of two hypothalamic hormones, growth hormone releasing factor fGHRF or GRF) and somatostatin. Both GHRF stimulation and somatostatin inhibition of the release of GH occurs by the specific engagement of receptors on the cell membrane of the somatotroph. Recent evidence has been mounting which suggests that GH release is also stimulated by a group of short peptides termed the growth hormone releasing peptides (GHRP; GHRP-6, GHRP-2 | hexarelin ] ) These peptides are described, for example, in U.S. Patent No. 4,4 1 1 ,890, PCT Patent Pub. No. WO 89/071 10, PCT Patent Pub. No. WO 89/07 1 1 1 , PCT Patent Pub. No. WO 93/040 1 , and J. Endo rinol Invest., / (Suppl 4), 45 ( 1992). These peptides function by selectively bind to a distinct somatotroph cell membrane receptor, the growth hormone secretagogue receptor (GHSR). A medicinal chemical approach has resulted in the design of several classes of orally-active, low molecular weight, non-peptidyl compounds which bind specifically to this receptor and result in the pulsatile release of GH. Such compounds possessing growth hormone secretagogue activity are disclosed, for example, in the following: U.S. Patent No. 3,239,345; U.S. Patent No. 4,036,979; U.S. Patent No. 4.41 1 .890; U.S. Patent No. 5,206,235; U.S. Patent No. 5,283,241 ; U.S. Patent No. 5,284,841 ; U.S. Patent No. 5.310,737; U.S. Patent No. 5,317,017; U.S. Patent No. 5,374,721 ; U.S. Patent No. 5,430, 144; U .S. Patent No. 5,434,261 ; U .S. Patent No. 5,438, 1 36; U.S. Patent No. 5,494,91 9; U.S. Patent No. 5,494,920; U .S. Patent No. 5,492,916; EPO Patent Pub. No. 0, 144.230; EPO Patent Pub. No. 0,513,974; PCT Patent Pub. No. WO 94/07486; PCT Patent Pub. No. WO 94/08583; PCT Patent Pub. No. WO 94/1 101 2; PCT Patent Pub. No. WO 94/1 3696; PCT Patent Pub. No. WO 94/1 9367; PCT Patent Pub. No. WO 95/03289; PCT Patent Pub. No. WO 95/03290; PCT Patent Pub. No. WO 95/09633; PCT Patent Pub. No. WO 95/1 1029; PCT Patent Pub. No. WO 95/1 2598; PCT Patent Pub. No. WO 95/13069; PCT Patent Pub. No. WO 95/14666: PCT Patent Pub. No. WO 95/16675; PCT Patent Pub. No. WO 95/1 6692; PCT Patent Pub. No. WO 95/17422; PCT Patent Pub. No. WO 95/ 17423; PCT Patent Pub. No. WO 95/343 1 1 ; PCT Patent Pub. No. WO 96/02530; Science, 260, 1640- 1643 (June 1 1 , 1993); Ann. Rep. Med. Chem., 2S, 177- 186 ( 1993 ); Biυorχ. Med. Chem. Ltr.s., 4(22), 2709-2714 ( 1994); and Proc. Natl. Acad. Sci. USA 92 , 700 J -7005 (July 1 995). The use of such orally-active agents which stimulate the pulsatile release of GH would be a significant advance in the treatment of growth hormone deficiency in children and adults as well as provide substantial benefit under circumstances where the anabol ic effects of GH might be exploited clinically (e.g. post-hip fracture rehabilitation, the frail elderly and in post-operative recovery patients ). Cell membrane receptors which are of low abundance on the cells can be difficult to isolate, clone and characterize. In the past, assays to identify a receptor in a mammalian cell or frog oocyte generally have depended on either: 1 ) directly detecting a receptor- ligand interaction, such as by binding of a radiolabeled ligand; or 2) indirectly detecting receptor-ligand binding by detecting either an intracellular event (such as calcium mobilization, or the identification of, for instance a calcium activated current) or an extracellular event (such as hormone secretion), that is the consequence of the ligand binding to its receptor. Most cloned receptors, which have been isolated using a functional expression assay have relied on immortalized cell lines or tumor derived tissues which are enriched for the receptor of interest. There are numerous receptors which cannot be readily identified using these types of assays, due to: 1 ) a paucity of biochemical information about the protein; 2) the low abundance of receptors present on the cell; and/or 3) the lack of a cell line or tumor material expressing the receptor. It would be desirable to develop an assay which can be used to identify and characterize cel l receptors not amenable lo study by conventional means. DETAILED DESCRIPTION OF THE INVENTION This invention relates to an assay method to determine the presence of a nucleic acid which encodes a G protein-linked cell membrane receptor comprising: a) introducing at least one nucleic acid suspected of encoding a CJ protein cell membrane receptor into a cell; b) introducing a G-protein subunit into the cell; c ) introducing a detector molecule or a nucleic acid encoding a detector molecule into the cell, wherein the detector molecule responds directly or indirectly to a G-protein receptor-ligand binding event; d) contacting the cell with a receptor ligand; and e) determining whether the oligonucleotide encoded a receptor by monitoring the detector molecule. In one preferred embodiment the cell does not naturally express the receptor on its cell membrane. In other preferred embodiments of the assay, the receptor is a member of the growth hormone secretagogue family of receptors, such as a growth homione secretagogue receptor (GHSR) or a growth hormone secretagogue related receptor (GHSRR). Thus, another aspect of this invention is an assay method to determine the presence of a nucleic acid which encodes a member of the growth hormone secretagogue receptor family comprising: a) introducing at least one nucleic acid suspected of encoding a GHSR or GHSRR into a cell which does not naturally express the receptor on its cel l membrane; b) introducing a G-protein subunit into the cell; c) introducing a detector molecule or a nucleic acid encoding a detector molecule into the cell, wherein the detector molecule is directly or indirectly responsive to a GHSR-ligand or GHSRR-ligand binding event; d) contacting the cell with a growth hormone secretagogue; and e) determining whether the nucleic acid encodes a receptor by monitoring the detector molecule. A further embodiment of this invention is an assay to determine the presence of a growth hormone secretagogue. Thus, this invention also comprises a method to deteπnine the presence of a growth homione secretagogue comprising: a) introducing a nucleic acid which encodes a growth hormone secretagogue receptor into a cell under conditions so that growth homione secretagogue receptor is expressed; b) introducing a G-protein subunit into the cell ; c) introducing a detector molecule or a nucleic acid encoding a detector molecule into the cell, wherein the detector molecule is directly or indirectly responsive to a GHSR-ligand binding event; d) contacting the cell with a compound suspected of being a growth hormone secretagogue; and e) determining whether the compound is a growth hoπnone secretagogue by monitoring the detector molecule. BRIEF DESCRIPTION OF THE FIGURES FIGURE 1 is the DNA of Swine GHSR (Type I) contained in Clone 7-3. FIGURE 2 is the amino acid sequence of swine GHSR encoded by the DNA of Figure 1 . FIGURE 3 is the entire open reading frame of the Type I clone, of Figure 1 . FIGURE 4 is the DNA of Swine GHSR (Type II) contained in Clone 1375. FIGURE 5 is the amino acid sequence of swine GHSR (Type II) encoded by the DNA of Figure 4. FIGURE 6 is the DNA for human GHSR (Type I) contained in Clone 1 146. FIGURE 7 is the amino acid sequence of human GHSR (Type 1 ) encoded by the DNA of Figure 6. FIGURE 8 is the entire open reading frame of Type I GHSR. encoded by DNA sequence of Figure 6. FIGURE 9 is the DNA for human GHSR (Type II) contained in Clone 1 141 . FIGURE 10 is the amino acid sequence of human GHSR (Type II ) encoded by Clone 1 141 . FIGURE 1 1 is the DNA for human GHSR (Type I) contained in Clone 1 143. FIGURE 12 is the amino acid sequence of human GHSR (Type I) encoded by Clone 1 143. FIGURE 13 compares the ORF of swine Type I (lacking the MET initiator of the full length GHSR and lacking 12 additional amino acids) to the homologous domain of swine Type II receptors. FIGURE 14 compares the homologous domain of human Type I and Type II receptors (the amino terminal sequence lacks the MET initiator and four additional amino acids). FIGURE 15 compares the ORFs of swine Type I and human Type I receptors (the amino temiinal sequence lacks the MET initiator and 12 additional amino acids). FIGURE 1 6 compares full length swine Type II and human Type II receptors. FIGURE 1 7 is a schematic diagram depicting the physical map of swine and human growth hormone secretagogue receptor cDNA clones. FIGURE 18 is a graph demonstrating the pharmacology of the expressed swine and human growth hormone secretagogue receptors in Xenopus oocytes using the aequorin bioluminescence assay. FIGURE 19 is a table demonstrating the pharmacology of the expressed swine and human growth hormone secretagogue receptors in Xenopus oocytes using the aequorin bioluminescence assay and various secretagogues. FIGURE 20 is a graph representing the pharmacology of the pure expressed swine growth hormone secretagogue receptor in COS-7 cells using the 3 -5 S-labeled Compound A binding assay. FIGURE 21 is a table representing the competition analysis with the pure expressed swine growth hormone secretagogue receptor in COS-7 cells using the ' 3 ~ 5 S-labeled Compound A binding assay and various secretagogues and other G-protein coupled- receptors (GPC- receptors) ligands in a competition assay. FIGURE 22 is the amino acid sequence of the full length human GHSR (Type I) encoded by clone 1 1 304. FIGURE 23A and 23B are graphs of measurement of [35S |- Compound A binding to swine anterior pituitary membranes. 23A shows results of saturation experiments using a fixed amount of membrane. 23 B shows saturation isothemis analyzed by Scatchard analysis. FIGURE 24 shows the inhibition of [^S i-Compound A binding to porcine anterior pituitary membranes by various compounds. FIGURE 25 shows the effect of GHRP-6 on specific | 35s j- Compound A binding to porcine anterior pituitary membranes at equi librium. FIGURE 26 shows the effects of GTP-γ-S and nucleotide on the specific [ 5S ]-Compound A binding to porcine anterior pituitary membranes. FIGURE 27 is the rat GHSR DNA sequence from the Met Initiation codon to the Stop codon. This sequence includes an intron. FIGURE 28 is the open reading frame only of the rat GHSR of Figure 27. FIGURE 29 is the deduced amino acid sequence of the ORF of Figure 28. FIGURE 30 shows expression of functional rat GHSR in transfected HEK-293 cells. As used throughout the specification and claims, the following definitions apply: "Ligands" are any molecule which binds to a GHSR of this invention. Ligands can have either agonist, partial agonist, partial antagonist or antagonist activity. "Growth hoπnone secretagogue" or "GHS" is any compound or agent that directly or indirectly stimulates or increases the release of growth hormone in an animal. "Compound A" is (N-[ 1 (R)-| 1 ,2-dιhydro- l -methane- sulfonlylspiro[ 3H-indole-3,4'-piperidin ]- r-yl)carbonyl]-2-(phenyl- methyloxy)-ethyl ]-2-amino-2-methyl-propanamide, described in Patchett et al, 1995 Proc. Natl. Acad. Sci 92: 7001 -7005. "Compound B" is (3-amino-3-methyl-N-(2, 3,4.5- tetrahydro-2-oxo- l { 2'-( 1 H-tetrazol-5-yl)( 1.1 '-biphenyl)-4-ylJ- methyl } l H- l -benzazepin-3(R)yl-butanamide, described in Patchett et al, 1995 Proc. Natl. Acad. Sci. 92: 7001 -7005. This invention relates to assays for members of the growth hormone secretagogue receptor family of proteins, which includes growth hormone secretagogue receptors and growth hormone secretagogue related receptors. The growth hormone secretagogue receptor proteins, growth homione receptor related proteins, nucleic acids encoding them and methods of making them using genetic engineering techniques are the subject of co-pending United States Provisional Patent Appl ication Nos. 60/008,582, filed December 13, 1995 and ( Attorney Docket No. 19589PV2), filed herewith. The proteins of this invention were found to have structural features which are typical of the 7-transmembrane domain (TM) containing G-protein linked receptor superfamily (GPC-R 's or 7-TM receptors ) receptors. Thus growth hormone secretagogue receptors make up new members of the GPC-R family of receptors. The intact receptors of this invention were found to have the general features of GPC-R's, including seven transmembrane regions, three intra- and extracellular loops, and the GPC-R protein signature sequence. The transmembrane domains and the GPC-receptor signature sequence are noted in the protein sequences of the Type I GHS receptor in Figures 3 and 8. Not all regions are required for functioning. The GHSRs share some sequence homology with previously cloned GPC-receptors including the rat and human neurotensin receptor (approximately 32% identity) and the rat and human TRH receptor (approximately 30% identity). The GHSRs were isolated and characterized using expression cloning techniques in Xenopus oocytes. The cloning was made difficult by three factors. First, prior to this invention. there was very l ittle information available about both the biochemical characteristics, and the intracellular signaling/effector pathways of the proteins. Thus, cloning approaches which depend on the use of protein sequence information for the design of degenerate oligonucleotides to screen cDNA libraries or utilize the PCR could not be effectively utilized. Therefore, receptor bioactivity needed to be determined. Secondly, the growth homione secretagogue receptor does not occur in abundance-- il is present on the cell membrane in about 10 fold less concentration than most other membrane receptors. In order to successfully clone the receptors, exhaustive precautions had be taken to ensure that the GHSR was represented in a cDNA library to be screened. This required: 1 ) isolation of intact, undegraded and pure poly (A )+ mRNA; 2) optimization of cDNA synthesis to maximize the production of full-length molecules; and 3 ) a library of larger size than normal needed to be screened (approximately 0.5 to 1 x 1 ' clones) to increase the probability that a functional cDNA clone may be obtained. Thirdly, no permanent cell line which expresses these receptors is known. Therefore, primary pituitary tissue had to be used as a source for mRNA or protein. This is an added difficulty because most primary tissues express lower amounts of a given receptor than an immortalized cell line or tumor tissues. Further, the surgical removal of a pig pituitary and extraction of biologically active intact mRNA for the construction of a cDNA expression library is considerably more difficult than the extraction of mRNA from a tissue culture cell line. Along with the need to obtain fresh tissue continuously, there are problems associated with its intrinsic inter-animal and inter-preparation variability. One aspect of this invention is directed to the development of an extremely sensitive, robust, reliable and high-throughput screening assay which could be used to identify portions of a cDNA library encoding the receptor. The ability to identify cDNAs which encode growth hormone secretagogue receptors depended upon two discoveries made in accordance with this invention: 1 ) that growth hormone secretagogue receptor-ligand binding events are transduced through G proteins; and 2) that a particular G protein subunit, such as God 1 > must be present in the cells in order to detect receptor activity. Only when these two discoveries were made could an assay be devised to detect the presence of GHSR encoding DNA sequences. Deteπnination that GHSR is distinct from the Growth Hormone Receptor A radioreceptor assay using high specific activity (700- 1 , 100 Ci/mmole ) [-^^S ]-labeled Compound A (a known GHS ) as ligand was developed. Saturable, high affinity binding was detected in porcine anterior pituitary membranes (FIGURE 2 A). Scatchard analysis (FIGURE 23B) indicated the presence of a single class of high affinity sites with an apparent dissociation constant (KD) of 1 61 + 1 I pM and a concentration (Bmax) of 6.3+0.6 fmol/mg of protein (n=4). A similar specific high affinity binding was detected in rat pituitary membranes indicating a KD value of 1 0+9 pM and B m ax o 2.3+ 1 . 1 fmol/mg protein ( n=3 ). The high affinity binding to the GFISR makes up yet another aspect of this invention. This invention is also directed to a method of identifying novel GHSR proteins comprising labeling a known ligand, exposing it to a putative GHSR protein and determining if binding occurs. The specificity of | ^S | -Compound A binding was established by determining the ability of GH secretagogues to compete with the radioligand for the binding sites (FIGURE 24 ). Unlabeled Compound A completely displaced [ ^^S |-Compound A from specific- binding sites with an inhibition constant. Kj, of 240 pM which is similar to the Ko value determined by Scatchard analysis. Other GHSs, GHRP- 6 (Kj 6.3 nM). and peptide antagonist Compound B (Kj 63 n ) had affinities of 3.8, 0.6 and 0.4%, respectively, of that of Compound A. Compound C, the biologically inactive stereoisomer of Compound B, competed poorly with [ S ]-Compound A binding. The saturation isotherm for | ^^S]-Compound A binding analyzed by double reciprocal plot showed that GHRP-6 inhibition was overcome by increasing concentration of | -^^S ]-Compound A (FIGURE 25). This result shows that GHRP-6 interacts competitively with Compound A in the same binding site. Similarly, Compound B was shown to be a competitor of [^ i-Compound A binding. The most potent agonists had the highest affinities for pituitary receptor sites. Compounds which did not compete with j-Compound A at 1 μM included GHRH, somatostatin, met-enkephalin, substance P, galanin, gonadotropin releasing hormone, thyrotropin releasing hormone, gastrin releasing peptide, PHM-27 , melanocyte stimulating hormone, pituitary adenylate cyclase activating polypeptide-38, phenoxybenzamine. dopamine, bromocriptine, methoxamine, benoxathian, isoproterenol, propanolol and clonidine. A GHSRR gene may be identified by hybridizing a cDNA encoding a GHSR to a genomic DNA, under relaxed post- hybridizational washing conditions (6 X SSC at 30°C) or moderate post- hybridizational washing conditions (6 X SSC at 45°C). The hybridized area can be identified, isolated and the GHSRR can be cloned and the receptor expressed using conventional techniques. Determination that GHSR is a G-Protein Receptor To study whether the 35$ (-Compound A specific binding site was G-protein linked, the effects of stable GTP analogs GTP-γ-S and GMP-PNP on f-^^S]-Compound A binding were studied. GTP-γ-S and GMP-PNP were found to be potent inhibitors of I ^SJ-Compound A binding with IC50 values of 30 and 1 10 nM, respectively (FIGURE 26 ). ATP-γ-S was ineffective. In addition, in the absence of Mg2+, only 15- 25% of specific binding of [ - S ]-Compound A binding was detected in comparison with control ( 10 mM Mg2+) suggesting that the specific binding of (-Compound A required the presence of Mg-+ regulate GH release in vivo) do not bind to the Compound A site. From these data, one can conclude that the receptor is G-protein linked. When the GHSR is bound by ligand (a growth homione secretagogue), the G-proteins present in the cell activate phosphatidylinositol-specific phospholipase C (Pl-PLC), an enzyme which releases intracellular signaling molecules (diacylglycerol and inositol tri-phosphate), which in turn start a cascade of biochemical events that promote calcium mobilization. In accordance with this invention, detection of this biochemical cascade can be used as the basis of an assay. Virtually any convenient eukaryotic cell may be used in the assay of this invention. These would include oocytes (preferred ones are from Xenopus sp. ) but cell lines may be used as well as Examples of preferred cell lines are mammalian cell lines, including COS, HEK-293 , CHO, HeLa, NS/0, CV- 1 , GC, GH3 and VERO. One important component of the assay is a detector molecule. Preferably, the detector molecule is responsive to an intracellular event which is part of the biochemical cascade initiated by GHS-GHSR binding. One class of preferred detector molecules can respond to changes in calcium concentrations. A preferred detector molecule which responds to calcium concentrations is aequorin ( a jel lyfish photoprotein) which acts on the substrate coelenterazine. Other detector molecules include calcium chelators with fluorescence capabilities, such as FURA-2 and indo- 1 . The detector molecule itself may be introduced into the cell, or nucleotides which encode the detector molecule may be introduced into the cell, under conditions which will allow the expression of the detector molecule. Generally, it is preferred to introduce nucleotides, such as DNA which encode the detector molecule into the cell, under conditions wherein the cell will express the detector molecule. Heterotrimeric G proteins, consisting of α, β and γ subunits, serve to relay information from cell surface receptors to intracellular effectors, such as phospholipase C and adenylate cyclase. The G-protein alpha subunit is an essential component of the intracellular signal transduction pathway activated by receptor-ligand interaction. In the process of l igand-induced GPCR activation, the Ga subunit of a trimeric Gαβγ complex wil l exchange its bound GDP for GTP and dissociate from the βγ heterodimer. The dissociated G - protein serves as the active signal transducer, often in concert with the βγ complex, thus starting the activation of the intracellular signal transduction pathway. G-alpha subunits are classified into sub-families based on sequence identity and the main type of effectors are coupled: G s , activate adenylate cyclase, CJi/o/t- inhibit adenylate cyclase, Gq/l l . activate PI-PLC, and G 12/1 3. effector unknown. The expression of several receptors in heterologous cells has been shown to be increased by the co-expression of certain Ga subunits. This observation formed the basis for the rationale to use Gα subunits of several sub-families in conjunction with a source of GHSR (swine poly A+ mRNA) to test if a GHS-induced functional response could be measured in the Xenopus oocyte system. GHS-induced responses were detected and were found to be strictly dependent on G x l 1 co-expression, a unprecedented finding outlining the specificity of the interaction. The finding that the expression of the GPCR could be fully dependent on the addition of a single G-protein subunit was unexpected, since in all previously published work the addition of a G- protein subunit modulated an already existing activity. Here a previously absent signal was fully restored. This finding indicated that the lack of a signal in Xenopus eggs was fully dependent on a G-protein subunit as the limiting factor. In conducting the assay, either the subunit itself or a nucleic acid encoding the subunit, or both may be added, and the addition events need not occur together. Next, a nucleic acid or pool of nucleic acids, wherein at least one nucleic acid is suspected of encoding a GHSR or GHSRR is introduced into the cell. When trying to identify a possible GHSR or GHSRR gene from a large library, it is often more efficient to use a pool of nucleic acids, each nucleic acid being different from the other nucleic acids in the pool. After the nucleic acid(s) suspected of encoding a GHSR or GHSRR is introduced into the cell, the cell is exposed to a known growth hormone secretagogue, such as Compound A (L- 163, 191 ). Any other growth hormone secretagogue may also be used. Preferred ones include: N-| 1 (R)-[( 1 ,2-dihydro- 1 -methanesulfonylspiro[3H-indole-3,4'- piperidin ]- r-yl)carbonyl]-2-(phenylmethyloxy)ethyl |-2-amino-2- methylpropanamide, or 3-amino-3-methyl-N -(2,3,4.5-tetrahydro-2-oxo- 1 - { [2'- 1 H-tetrazol-5-yI)( 1 .1 '-biphenyl)-4-y 1 ]methyl } - 1 H- 1 -benzazepin- 3(R)-yl-butanamide, or a compound disclosed, for example, in the following: U .S. Patent No. 3,239,345; U.S. Patent No. 4,036,979; U.S. Patent No. 4,41 1 ,890; U.S. Patent No. 5,206,235; U .S. Patent No. 5,283,241 ; U.S. Patent No. 5,284,841 ; U.S. Patent No. 5,310,737; U.S. 5 Patent No. 5,3 17.017; U.S. Patent No. 5,374,721 ; U.S. Patent No. 5.430, 144; U.S. Patent No. 5,434,261 ; U.S. Patent No. 5,438.136; U.S. Patent No. 5.494,91 9; U.S. Patent No. 5,494,920; U.S. Patent No. 5.492,916; EPO Patent Pub. No. 0, 144,230: EPO Patent Pub. No. 0.5 13,974; PCT Patent Pub. No. WO 94/07486; PCT Patent Pub. No. I o WO 94/08583; PCT Patent Pub. No. WO 94/1 1012; PCT Patent Pub. No. WO 94/1 3696; PCT Patent Pub. No. WO 94/ 19367; PCT Patent Pub. No. WO 95/03289; PCT Patent Pub. No. WO 95/03290; PCT Patent Pub. No. WO 95/09633; PCT Patent Pub. No. WO 95/1 1029; PCT Patent Pub. No. WO 95/12598; PCT Patent Pub. No. WO 1 5 95/ 1 3069; PCT Patent Pub. No. WO 95/14666; PCT Patent Pub. No. WO 95/1 6675; PCT Patent Pub. No. WO 95/16692; PCT Patent Pub. No. WO 95/17422; PCT Patent Pub. No. WO 95/17423; PCT Patent Pub. No. WO 95/343 1 1 ; PCT Patent Pub. No. WO 96/02530; Science. 260, 1640- 1 643 (June 1 1 , 1993 ); Ann. Rep. Med. Chem. , 2S, 177- 186 0 ( 1993): Bioorg. Med. Chem. Ltrs. , 4(22), 2709-2714 ( 1994); and Proc . Natl. Acad. Sci. USA 92 , 7001 -7005 (July 1995), or any other growth hormone secretagogue. If one or more of the nucleic acids does encode a GHSR, or 2 5 GHSRR, then the secretagogue ligand will bind the receptor, G-protein will be activated, the calcium level will fluctuate, and the detector molecule will change so that it can be monitored. For the system using aequorin and coelenterazine, receptor-GHS binding will produce measurable bioluminescence. 3 0 If the procedure used a complex pool of nucleic acids, one or more of which may encode the receptor, then further screening wil l be necessary to determine which nucleic acid is responsible for encoding GHSR or GHSRR. Once a positive result is found, the procedure can be repeated with a sub-division of the nucleic acid pool (for example. starting with approximately 10,000 nucleic acids, then using approximately 1 ,000, then approximately 500, then approximately 50, and then pure). In this procedure, RNA pools are preferred. Using this general protocol in Xenopus oocytes with a swine cDNA expression library, Clone 7-3 was identified as containing nucleic acid encoding a swine growth hormone secretagogue receptor. The clone is approximately 1.5 kb in size, and downstream from the presumed initiator methionine (MET), contains an open reading frame ( ORF) encoding 302 amino acids (M r = 34,51 6). The DNA and deduced amino acid sequence is given in FIGURES 1 and 2. When hydropathy ana lysis (e.g. Kyte-Doolittle; Eisenberg, Schwartz, Komaron and Wall ) is performed on the protein sequence of clone 7-3 , only 6 predicted transmembrane domains are present downstream of the presumed MET initiator. However, translation of the longest ORF encoded in clone 7-3 encodes a protein of 353 amino acids (M|-= 39,787), but is devoid of an apparent initiator MET (FIGURE 3 ). Seven transmembrane segments are encoded in the longer, 353 amino acid protein in which a MET translation initiation codon located upstream of TM I is absent (FIGURE 3 ). Thus, clone 7-3 appears truncated at its amino terminus, but is fully functional, demonstrating that clone 7-3 is a functional equivalent of a native GHSR. The resultant cDNA clone (or shorter portions of for instance only 15 nucleotides long) may be used to probe libraries under hybridization conditions to find other receptors which are similar enough so that the nucleic acids can hybridize, and is particularly useful for screening libraries from other species. Using this procedure, additional human, swine and rat GHSR cDNAs have been cloned and their nucleotide sequence detem ined. In this step, one of ordinary skil l in the art will appreciate that the hybridization conditions can vary from very stringent to relaxed. Proper temperature, salt concentrations, and buffers are well known. As used herein, "standard post hybridizational washing" conditions mean 6 x SSC at 55°C. "Relaxed post hybridizational washing" conditions means 6 x SSC at 30°C. A swine pituitary library, a human pituitary library, and a rat pituitary library were hybridized with a radiolabeled cDNA derived from the open reading frame of the swine GHSR clone 7-3. Twenty one positive human GHSR cDNA clones were isolated and five swine library pools yielded a strong hybridization signal and contained clones with inserts larger than clone 7-3, as judged from their insert size on Southern blots. A single rat cDNA clone was also isolated. Nucleotide sequence analysis revealed two types of cDNAs for both the human and swine GHSR cDNAs. The first (Type I ) encodes a protein represented by clone 7-3 , encoding 7-TM domains (the amino acid sequence of a full length human clone 1 1304 is shown in FIGURE 22). The ful l length open reading frame extends 13 amino acids beyond the largest predicted open reading frame of clone 7-3, (353 amino acids). The second (type II ) diverges in its nucleotide sequence from the type I cDNA at its 3 ' -end, at the second predicted amino acid of TM-6. In the type II cDNAs, TM-6 is truncated and fused to a short contiguous reading frame of only 24 amino acids, followed by a translation stop codon. Swine clone 1375 is an example of a Type II cDN A (FIGURES 4 and 5). These 24 amino acids beyond TM-6 are highly conserved when compared between human and swine cDNAs. The DNA and amino acid sequences of the human GHSR Type I and II are given in FIGURES 6- 12 and 22. A predicted ful l length cDNA encoding the human Type I receptor, that is, a molecule encoding 7-TM domains with an initiator MET in a favorable context preceded by an inframe termination codon is isolated, and termed clone 1 1 304. The predicted ORF of clone 1 1304 for the full length Type I GHSR measures 366 amino acids ( M r = 41 , 198; FIGURE 22). A full length human Type II cDN A encodes a polypeptide of 289 amino acids (M r =32, 156; FIGURES 9 and 10). Sequence alignments perfomied at both the nucleic acid and protein levels show that Type I and II GHSR's are highly related to each other and across species ( FIGURES 1 3- 1 ). The human and swine GHSR sequences are 93% identical and 98 < similar at the amino acid level. The nucleotide sequence encoding the missing amino terminal extension of swine T ype I clone 7-3 is derived from the ful l length human Type I clone as well as the human and swine Type II cDNAs. The reading frame of the full length clones extended 1 3 amino acids beyond the amino terminal sequence of clone 7-3 and this sequence was conserved in 12/1 3 amino acid residues when compared between human and swine. The amino terminal extension includes a translation initiator methionine in a favorable context according to Kosak ' s rule, with the reading frame further upstream being interrupted by a stop codon. A schematic physical map of Type I and II swine and human cDNA clones is given in FIGURE 17. The rat clone was also further investigated. Sequence analysis revealed the presence of a non-coding intronic sequence at nt 790 corresponding to a splice-donor site (see FIGURES 27, 28, and 29. ) The G/GT splice-donor site occurs two amino acids after the completion of the predicted transmembrane domain 5 (leucine 263), thus dividing the rat GHSR into an amino-terminal segment (containing the extra cellular domain, TM- 1 through TM-5, and the first two intra- and extra- cellular loops) and a carboxy-terminal segment (containing TM- 6, TM-7, the third intra- and extra- cellular loops, and the intra-cellular domain). The point of insertion and flanking DNA sequences are highly conserved, and also present in both human and swine Type I and II cDNAs. Comparison of the complete open reading frame encoding the rat GHSR protein to human and swine homologs reveals a high degree of sequence identity (rat vs. human, 95. 1 %; rat vs. swine 93.4%). Human and swine Type I cRNAs expressed in oocytes were functional and responded to concentrations Compound A ranging from 1 μM to as low as 0.1 nM in the aequorin bioluminescence assay. Human or swine Type Il-derived cRNAs that are truncated in TM-6 failed to give a response when injected into oocytes and these represent a receptor subtype which may bind the GHS, but cannot effectively activate the intracellular signal transduction pathway. In addition the Type II receptor may interact with other proteins and thus reconstitute a functional GFISR. Proteins such as these which may have ligand-binding activity, but are not active in signal transduction are particularly useful for ligand-binding assays. In these cases, one may also over-express a mutant protein υn the cell membrane and test the binding abilities of putative labeled ligands. By using a non-signaling mutant which is constitutively in a high affinity state, binding can be measured, but no adverse metabolic consequences would result. Thus use of non-signaling mutants are an important aspect of this invention. The pharmacological characterization of human Type I , swine Type I and rat receptors in the aequorin bioluminescence assay in oocytes is summarized in FIGURES 1 , 19 and 30. Peptidyl and non- peptidyl bioactive GHS's were active in a similar rank order of potency as observed for the native pituitary receptor. Independent confirmatory evidence that the Type I GHSR (shown for swine clone 7-3) encodes a fully-functional GHSR is given by the finding that when clone 7-3 is expressed transiently in mammalian COS -7 cells, high affinity (KD ~ 0.2 nM), saturable (B m ax~N0 fmol/mg protein) and specific binding (> 90 % displaced by 50 nM unlabeled Compound A) is observed for -^S- Compound A (FIGURES 20-21 ). By varying the parameters of the above assays, one can search for other unknowns. For example, in the assay which detects whether a nucleic acid which encodes a GHSR or GHSRR is present, one can modify the assay so that it detects whether a GHS is present, ln this embodiment, a nucleic acid encoding GHSR or GHSRR is introduced into the cell, as well as a nucleic acid encoding a detector molecule, and a G protein subunit. The cell is contacted with at least one compound which is a putative GHS. If the compound is a GHS, then the GHS will bind the GHSR or GHSRR, and the resultant intracellular events can be detected by monitoring the detector molecule. If the compound is not a GHS, then no such activity will be detected. This GHS assay forms yet another aspect of this invention. A further aspect of this invention are novel ligands which are identified using the above assay. Expression of several receptors in heterologous cells has been shown to be increased by the co-expression of certain G« subunits. This observation formed the basis for the rationale to the use of Gα subunits of several sub-families in conjunction with a source of GHSR (swine polyl A+ | mRNA) to test if a CfHS-induced functional response could be measured in the Xenopus oocyte system. GHS-induced responses were detected and were found to be strictly dependent on Ga l 1 co-expression, an unprecedented finding outlining the specificity of the interaction. Thus another aspect of this invention is a method of detecting a GHS response comprising co-expressing a Gα l 1 protein subunit in a cell also expressing a GHSR, exposing the cell to a GHS, and detecting the response. The presence of Gocl 1 was essential in using poly A+ RNA or complex cRNA pools (i.e. 10.000 cRNAs). However, once a pure clone was obtained the requirement for the G-protein addition was no longer essential. This indicates that the need for G-protein addition depended on the purity of the nucleic acid, the most sensitive assay requiring Gα subunit addition. Thus another aspect of this invention is a method of determining the presence of an nucleic acid which encodes a growth hormone secretagogue receptor or growth hormone secretagogue related receptor comprising: a) introducing a nucleic acid suspected of encoding a GHSR or GHSRR into a cell which does not naturally express the receptor on its cell membrane; b) introducing a detector molecule or a nucleic acid encoding a detector molecule into the cell, wherein the detector molecule is directly or indirectly responsive to a receptor-ligand binding event; c) contacting the cell with a growth homione secretagogue; and d) determining whether the nucleic acid encodes a receptor by monitoring the detector molecule. Simi larly, another aspect of this invention is an assay method to detemiine the presence of a growth homione secretagogue comprising: a) introducing a nucleic acid which encodes a growth hormone secretagogue receptor into a cell under conditions so that growth hormone secretagogue receptor is expressed; b) introducing a detector molecule or a nucleic acid encoding a detector molecule into the cell , wherein the detector molecule is directly or indirectly responsive to a GHSR-ligand binding event; c) contacting the cell with a compound suspected of being a growth hormone secretagogue; and d) determining whether the compound is a growth hormone secretagogue by monitoring the detector molecule. Ligands detected using assays described herein may be used in the treatment of conditions which occur when there is a shortage of growth hormone, such as observed in growth hoπnone deficient children, elderly patients with musculoskeletal impairment and recovering from hip fracture, patients with neurodegenerative diseases, and patients recovering from coronary by-pass surgery, and osteoporosis. A GHS receptor, preferably imυbilized on a solid support, may be used diagnostically for the determination of the concentration of growth homione secretagogues, or metabolites thereof, in physiological fluids, e.g.. body fluids, including serum, and tissue extracts, as for example in patients who are undergoing therapy with a growth hormone secretagogue. The administration of a GHS receptor to a patient may also be employed for purposes of: amplifying the net effect of a growth homione secretagogue by providing increased downstream signal following administration of the growth hoπnone secretagogue thereby diminishing the required dosage of growth homione secretagogue; or diminishing the effect of an overdosage of a growth hormone secretagogue during therapy. The following non-limiting Examples are presented to better illustrate the invention. EXAMPLE 1 Preparation of High Specific Activity Radioligand j -ΪS l -Compound A- [ -^S i-Compound A was prepared from an appropriate precursor, N-| l (R)-[ ( l ,2-dihydrospiro[ 3H-indole-3,4 ' -pipeι din ]- r-yl )- carbonyl ]-2-(phenyl-methyloxy)ethyl ]-2-amino-t-butoxycarbonyl-2- methylpropanamide, using methane [ " 5 Sj sulfonyl chloride as described in Dean DC, et al., 1995, In: Allen J, Voges R (eds) Synthesis and Applications of Isotopically Labelled Compounds, John Wiley & Sons, New York, pp. 795-801 , Purification by semi-preparative HPLC (Zorbax SB-phenyl column, 68% MeOH/water, 0. 1 % TFA, 5 ml/min) was followed by N-t-BOC cleavage using \ 5% trifluroacetic acid in dichloromethane (25°C, 3 hr) to give [ methylsulfonyl- - s]Compound A in near quantitative yield. HPLC purification (Hamilton PRP- 1 4.6x250 mm column, linear gradient of 50-75% methanol-water with 1 mM HCI over 30 min, 1 .3 ml/min) provided the ligand in >99% radiochemical purity. The structure was established by HPLC coelution with unlabeled Compound A and by mass spectral analysis. The latter method also indicated a specific activ ity of - 1000 Ci/mmol. EXAMPLE 2 Preparation of Pituitary Membranes Frozen anterior pituitary glands from male swine ( 50-80 Kg) or from the Wistar male rats ( 150-200 g) were homogenized in ;ι tissue homogenizer in ice-cold buffer (50 mM Tris-HCl buffer, pH 7.4, 5 mM MgCl2, 2.5 M EDTA, 0.1 % bovine serum albumin and 30 μg/ml bacitracin ). The homogenates were centrifuged for 5 min at 1 ,400 xg and the resulting supematants were then centrifuged at 34,000 xg for 20 min. The pellets were resuspended in same buffer to a 1 ,500 μg protein/ml and stored at -80°C. Protein was deteπnined by a Bio¬ Rad method (Bio-Rad Laboratories, Richmond, CA). EXAMPLE 3 Receptor Binding Assay The standard binding solution contained: 400 m of 25 mM Tris-HCl buffer, pH 7.4. 10 mM MgCD, 2.5 mM EDTA. and 100 pM l-Compound A. Pituitary membranes ( 100 μl, 150 μg protein ) were added to initiate the binding reaction. Aliquots were incubated at 20°C for 60 min and bound radio ligand was separated from free by filtration through GF/C filters pretreated with 0.5% of polyethyl- enimine in a Brandel cell harvester. The filters were washed three times with 3-ml of ice-cold buffer (50 mM Tris-HCl, pH 7.4, 10 mM MgCl2, 2.5 mM EDTA and 0.015% Triton X- 1 0) and the radioactivity on the filters were counted in Aquasol 2. Specific binding was defined as the difference between total binding and nonspecific binding assayed in 500 nM unlabeled Compound A. Specific bindings were 65-85 and 45-60% of total binding, in porcine and rat membranes, respectively. Assays were carried out in triplicate and experiments repeated at least three times. EXAMPLE 4 Oocyte Preparation and Selection Xenopus laevis oocytes were isolated and injected using standard methods previously described by Arena, et. al. 1991 , Mol. Pharmacol. 40, 368-374, which is hereby incorporated by reference. Adult female Xenopus laevis frogs (purchased from Xenopus One, Ann Arbor, MI) were anesthetized with 0.17% tricaine methanesulfonate and the ovaries were surgically removed and placed in a 60 mm culture dish (Falcon) containing OR-2 medium without calcium (82.5 mM NaCl, 2 mM KCl, 2.5 mM sodium pyruvate, 1 mM MgC , 100 μ/ml penici llin. 1 mg/ml streptomycin, 5 mM HEPES, pH=7.5; ND-96 from Specialty Media, NJ ). Ovarian lobes were broken open, rinsed several times, and oocytes were released from their sacs by collagenase A digestion (Boehringer-Mannheim; 0.2% for 2-3 hours at 1 8°C) in calcium-free OR-2. When approximately 50% of the follicular layers were removed. Stage V and VI oocytes were selected and placed in ND-86 with calcium (86 mM NaCl, 2 mM KCl, 1 mM MgCl2, LS mM CaCh, 2.5 mM sodium pyruvate, 0.5 mM theopylline, 0.1 mM gentamycin, 5 mM HEPES | pH=7.5 | ). For each round of injection, typically 3-5 frogs were pre-tested for their ability to express a control G-protein l inked receptor (human gonadotropin-releasing hormone receptor) and show a robust phospholipase C intracellular signaling pathway (incubation with 1 % chicken serum which promotes calcium mobilization by activation of phospholipase C). Based on these results, 1 -2 frogs were chosen for library pool injection (50 nl of cRNA at a concentration of 25 ng (complex pools) to 0.5 ng (pure clone) per oocyte usually 24 to 48 hours following oocyte isolation. EXAMPLE 5 mRNA Isolation Total RNA from swine (50-80 kg, Yorkshire strain) pituitaries (snap-frozen in liquid nitrogen within 1 -2 minutes of animal sacrifice) was prepared by a modified phenohguanidinium thiocyanate procedure (Chomczynski, et al, 1987 Anal. Biochem. 162, 1 56- 159, which is hereby incorporated by reference), using the TRI-Reagent LS as per the manufacturer's instructions (Molecular Research Center. Cincinnati, OH). Typically, 5 mg of total RNA was obtained from 3.5 g wet weight of pituitary tissue. Poly (A) RNA was isolated from total RNA by column chromatography (two passes) on oligo (dT) cellulose (Pharmacia, Piscataway, NJ). The yield of poly (A ) + mRNA from total RNA was usually 0.5%. RNA from other tissues was isolated similarly. EXAMPLE 6 cDNA Library Construction First-strand cDNA was synthesized from poly ( A ) + mRNA using M-MLV RNAse (-) reverse transcriptase (Superscript, GIBCO- BRL, Gaithersberg, MD) as per the manufacturer's instructions with an oligo (dT)/Not I primer-adapter. Following second-strand cDNA synthesis, double-stranded cDNA was subjected to the following steps: 1 ) ligation to EcoR I adapters, 2) Not I digestion, and 3) enrichment for large cDNAs and removal of excess adapters by gel filtration chromatography on a Sephacryl S-500 column (Pharmacia ). Fractions corresponding to high molecular weight cDNA were ligated to EcoR I/Not I digested pSV-7, a eucaryotic expression vector capable of expressing cloned cDNA in mammalian cells by transfection (driven by SV-40 promoter) and in oocytes using in vitro transcripts ( initiated from the T7 RNA polymerase promoter). pSV-7 was constructed by replacing the multiple cloning site in pSG-5 (Stratagene, La Jolla, CA; Green, S. et al, 1988 Nucleic Acids Res. 16:369, which is hereby incorporated by reference) with an expanded multiple cloning site. Ligated vectoπcDNA was transformed into E.coli strain DH 10B (GIBCO-BRL) by electroporation with a transfoπnation efficiency of I x 10 pfu/10 ng double-stranded cDNA. The library contained approximately 3 x 10 ° independent clones with greater than 95% having inserts with an average size approximating 1 .65 kb (range 0.8-2.8 kb). Unamplified library stocks were frozen in glycerol at -70°C until needed. Aliquots of the library were amplified once prior to screening by a modification of a solid-state method (Kriegler, M. in Gene Transfer and Expression: A Laboratory Manual Stockton Press, NY 1990). Library stocks were titered on LB plates and then the equivalent of 500- 1000 colonies was added to 13 ml of 2 x YT media containing 0.3% agarose and 100 μg/ml carbenicillin in a 14 ml round-bottom polypropylene tube (Falcon). The bacterial suspension was chilled in a wet ice bath for 1 hour to solidify the suspension, and then grown upright at 37°C for 24 hrs. The resultant bacterial colonies were harvested by centrifugation at 2000 x g at RT for 10 min, resuspended in 3 ml 2X YT/ carbenicillin. Aliquots were taken for frozen stocks (5%) and plasmid DNA preparation. EXAMPLE 7 Plasmid DNA Preparation and cRNA Transcription Plasmid DNA was purified from pellets of solid-state grown bacteria ( 1000 pools of 500 independent clones each) using the Wizard Miniprep kit according to the manufacturer's instruct ions (Promega Biotech, Madison, WI). The yield of plasmid DNA from a 14 ml solid-state amplification was 5- 10 μg. In preparation for cRNA synthesis, 4 μg of DNA was digested with Not I, and the subsequent linearized DNA was made protein and RNase-free by proteinase K treatment ( 10 μg for 1 hour at 37°C), followed by two phenol, two chloroform/isoamyl alcohol extractions, and two ethanol precipitations. The DNA was resuspended in approximately 15 μl of RNase-free water and stored at -70 (, C until needed. cRNA was synthesized using a kit from Promega Biotech with modifications. Each 50 μl reaction contained: 5 μl of linearized plasmid (approximately 1 μg), 40 mM Tris-HCl (pH=7.5), 6 mM MgCl2, 2 mM spermidine, 10 mM NaCl, 10 mM DTT, 0.05 mg/ml bovine serum albumin, 2 units/ml RNasin, 800 μ M each of ATP, CTP and UTP, 200 μM GTP, 800 μM m7G(5 ' )ppp(5 ')G, 80 units of T7 RNA polymerase, and approximately 20,000 cpm of 32p_CTP as a trace for quantitation of synthesized RNA by TCA precipitation. The reaction was incubated for 3 hrs. at 30°C; 20 units of RNase-free DNase was added, and the incubation was allowed to proceed for an additional 15 min. at 37°C. cRNA was purified by two phenol, chloroform/isoamyl alcohol extractions, two ethanol precipitations, and resuspended at a concentration of 500 ng/ml in RNase-free water immediately before use. EXAMPLE 8 Aequorin Bioluminescence Assay (ABA) and Clone Identification The ABA requires injection of library pool cRNA (25 ng/egg for pool sizes of 500 to 10,000) with aequorin cRNA (2 ng/egg ) supplemented with the G-protein alpha subunit Gcx l I (2 ng/egg). To facilitate stabilization of synthetic transcripts from aequorin and Gςx l 1 plasmids, the expression vector pCDNA-3 was modified (termed pcDNA-3v2) by insertion ( in the Apa I restriction enzyme site of the polyl inker) of a cassette to append a poly (A ) tract on all cRNA ' s which initiate from the T7 RNA polymerase promoter. This cassette includes (5 ' to 3 ' ): a Bgl II site, pA (20) and a Sfi I site which can be used for plasmid linearization. Polymerase chain reaction (PCR) was utilized to generate a DNA fragment corresponding to the open reading frame (ORF) of the aequorin cDNA with an optimized Kosak translational initiation sequence (Inouye, S. et. al., 1985. Proc. Natl. Acad. Sci. USA 82:3154-31 58). This DNA was ligated into pCDNA-3v2 linearized with EcoR I and Kpn I in the EcoR I/Kpn I site of pCDNA-3v2. G l 1 cDNA was excised as a Cla I/Not I fragment from the pCMV-5 vector (Woon, C. et. al., 1989 J. Biol. Chem. 264: 5687-93). made blunt with Klenow DNA polymerase and inserted into the EcoR V site of pcDNA- 3v2. cRNA was injected into oocytes using the motorized "Nanoject" injector (Drummond Sci . Co., Broomall, PA.) in a volume of 50 nl. Injection needles were pulled in a single step using a Flaming/Brown micropipette puller, Model P-87 (Sutter Instrument Co) and the tips were broken using 53X magnification such that an acute angle was generated with the outside diameter of the needle being <3 μm. Following injection, oocytes were incubated in ND-96 medium, with gentle orbital shaking at 1 °C in the dark. Oocytes were incubated for 24 to 48 hours (depending on the experiment and the time required for expression of the heterologous RNA) before "charging" the expressed aequorin with the essential chromophore coelenterazine. Oocytes were "charged" with coelenterazine by transferring them into 35 mm dishes containing 3 ml charging medium and incubating for 2-3 hours with gentle orbital shaking in the dark at 18 Ω C. The charging medium contained 10 μM coelenterazine (Molecular Probes, Inc., Eugene, OR. ) and 30 μ M reduced glutathione in OR-2 media (no calcium). Oocytes were then returned to ND-86 medium with calcium medium described above and incubation continued in the dark with orbital shaking until bioluminescence measurements were initiated. Measurement of GHSR expression in oocytes was performed using a Berthold Luminometer LB953 (Wallac Inc., Gaithersburg, MD) connected to a PC running the Λutolumat-PC Control software (Wallac Inc., Gaithersburg. MD). Oocytes (singly or in pairs) were transferred to plastic tubes (75 x 12 mm. Sarstedt) containing 2.9 ml Ca ++ -free OR-2 medium. Each cRNA pool was tested using a minimum of 3 tubes containing oocytes. Bioluminescence measurements were triggered by the injection of 0. 1 ml of 30 μM Compound A ( 1 μM final concentation) and recordings were followed for 2 min. to observe kinetic responses consistent with an IP3-mediated response. Pool S 10-20 was prepared from the unfractionated swine pituitary cDNA library and was composed of 10 pools each of 1000 clones. S 10-20 gave a positive signal on two luminometer instruments and the component pools were then individually tested for activity. From the 10 pools of 1000 clones, only pool S271 gave a positive response. This pool was made from two pools of 500 clones designated P541 and P542. Again, only one of the pools, P541 , gave a positive bioluminescent signal in the presence of 1 μM Compound A. At this point, the bacterial titer was determined in the glycerol stock of P541 such that dilutions could be plated onto LB agar plates containing 100 μg/ml carbenicillin to yield approximately 50 colonies per plate. A total of 1527 colonies were picked and replicated from 34 plates. The colonies on the original plates were then washed off, plasmids isolated, cRNA synthesized and injected into oocytes. cRNA prepared from 8 of the 34 plates gave positive signals in oocytes. Two plates were selected and the individual colonies from these plates were grown up, plasmid isolated, cRNA prepared and injected into oocytes. A single clonal isolate from each plate (designated as clones 7-3 and 28- 18) gave a positive bioluminescence response to 1 μM Compound A. Clone 7-3 was further characterized. EXAMPLE 9 Receptor Characterization DNA sequencing was performed on both strands using an automated Applied Biosystems instrument (ABI model 373 ) and manually by the dideoxy chain termination method using Sequenase II (US Biochemical, Cleveland, OH). Database searches (Genbank 88, EMBL 42, Swiss-Prot 3 1 , PIR 40, dEST, Prosite, dbGPCR ), sequence alignments and analysis of the GHSR nucleotide and protein sequences were carried out using the GCG Sequence Analysis Software Package ( Madison, WI; pileup, peptide structure and motif programs), FASTA and BLAST search programs, and the PC/Gene software suite from Intelligenetics (San Francisco, CA; protein analysis programs). Northern blot analysis was conducted using total (20 μg/lane) or poly ( A)+ mRNA (5- 10 μg/lane) prepared as described above. RNA was fractionated on a 1 % agarose gel containing 2.2 M formaldehyde and blotted to a nitrocellulose membrane. Blots were hybridized with a PCR generated probe encompassing the majority of the ORF predicted by clone 7-3 (nt 291 to 1 1 32). The probe was radiolabeled by random- priming with | cx] ~ ~P-dCTP to a specific activity of greater than 10^ dpm/μg. Blots were pre-hybridized at 42°C for 4 hrs. in 5 X SSC, 5 X Denhardt's solution, 250 μg/ml tRNA, 1 % glycine, 0.075% SDS, 50 mM NaPθ4 (pH 6) and 50% formamide. Hybridizations were carried out at 42°C for 20 hrs. in 5 X SSC, 1 X Denhardt 's solution, 0.1 % SDS, 50 mM NaPθ , and 50% formamide. RNA blots were washed in 2 X SSC, 0.2% SDS at 42°C and at -70 U C. RNA size markers were 28S and 18S rRNA and in vitro transcribed RNA markers (Novagen). Nylon membranes containing EcoR I and Hind III digested genomic DNA from several species (Clontech; 10 mg/lane) were hybridized for 24 hrs. at 30°C in 6 X SSPE, 10 X Denhardt's, 1 % SDS, and 50% formamide. Genomic blots were washed twice with room temperature 6 X SSPE, twice with 55°C 6 X SSPE, and twice with 55°C 4 X SSPE. Additional swine GHSR clones from the swine cDNA library (described above) were identified by hybridization to plasmid DNA (in pools of 500 clones each) immobilized to nylon membranes in a slot-blot apparatus ( Scheicher and Schuell). Pure clonal isolates were subsequently identified by colony hybridization. Swine GHSR clones that extend further in a 5 ' direction were identified using 5' RACE procedures (Frohman, M. A., 1993 Methods Enzymol. 218:340-358, which is incorporated by reference) using swine pituitary poly (A) mRNA as template. EXAMPLE 10 Human GHSR Human pituitary homologues of the swine GHSR were obtained by screening a commercially available cDNA library constructed in the vector lambda ZAP II (Stratagene) as per the manufacturer's instructions. Approximately 1.86 x 10° phages were initially plated and screened using a random-primer labeled portion of swine clone 7-3 (described above) as hybridization probe. Twenty one positive clones were plaque purified. The inserts from these clones were excised from the bacteriophage into the phagemid pBluescript II SK- by co-infection with helper phage as described by the manufacturer (Stratagene). Human clones were characterized as has been described above for the swine clone. EXAMPLE 1 1 DNA Encoding a Rat Growth Hormone Secretagogue Receptor (GHSR) Type Ia Cross-hybridization under reduced stringency was the strategy utilized to isolate the rat GHSR type Ia. Approximately 10^ phage plaques of a once-amplified rat pituitary cDNA library in lambda gt l 1 (RL 1 51 b; Clontech, Palo Alto, CA) were plated on E. coli strain Y 1090r. The plaques were transferred to maximum-strength Nytran (Schleicher & Schuell, Keene, NH) denatured, neutralized and screened with a 1 .6 kb EcoRI/NotI fragment containing the entire coding and untranslated regions of the swine GHSR, clone 7-3. The membranes were incubated at 30°C in prehybridization solution (50% formamide, 2 X Denhardts, 5 X SSPE, 0. 1 % SDS, 1 0 μg/ml salmon speπn DNA) for 3 hours followed by overnight incubation in hybridization solution (50% formamide, 2 X Denhardts, 5 X SSPE, 0.1 % SDS, 10% dextran sulfate, 100 μg/ml salmon sperm DNA) with 1 x 10 6 cpm/ml of 2 P]- labeled probe. The probe was labeled with [ 2p dcτ using a random priming kit (Gibco BRL, Gaithersburg, ND). After hybridization the blots were washed two times each with 2 X SSC. 0.1 % SDS (at 24"C, then 37°C, and final ly 55°C). A single positive clone was isolated following three rounds of plaque purification. Phage containing the GHSR was eluted from plate plaques with l x lambda buffer (0.1 M NaCl, 0.01 M MgSθ4«7H2θ, 35mM Tris-HCl, pH 7.5) following overnight growth of approximately 200 pfu/150mm dish. After a ten minute centrifugation at 10,000 x/g to remove debris, the phage solution was treated with 1 μg/ml RNAse A and DNAse I for thirty minutes at 24°C, followed by precipitation with 20% PEG (8000)/2M NaCl for two hours on ice, and collection by centrifugation at 10,000 x/g for twenty minutes. Phage DNA was isolated by incubation in 0.1 % SDS, 30mM EDTA, 50 μg/ml proteinase K for one hour at 68°C, with subsequent phenol (three times) and chloroform (twice) extraction before isopropanol precipitation ovemight. The GHSR DNA insert (-6.4 kb) was sub-cloned from lambda gt l 1 into the plasmid vector Litmus 28 (New England Biolabs, Beverly, MA). 2 μg of phage DNA was heated to 65°C for ten minutes, then digested with 100 units BsiWI (New England Biolab, Bevely, MA) at 37°C ovemight. A 6.5 kb fragment was gel purified, electroeluted and phenol/chloroform extracted prior to ligation to BsiWI-digested Litmus 28 vector. Double-stranded DNA was sequenced on both strands on a ABI 373 automated sequencer using the ABI PRISM dye termination cycle sequencing ready reaction kit (Perkin Elmer; Foster City, CA ). For sequence comparisons and functional expression studies, a contiguous DNA fragment encoding the complete ORF (devoid of intervening sequence) for the rat GHSR type Ia was generated. The PCR was utilized to synthesize a amino-terminal fragment from Met-1 to Val-260 with EcoRI (5') and Hpal (3') restriction sites appended, while a carboxyl-terminal fragment was generated from Lys-261 to Thr-364 with Dra I (5') and Not I (3') restriction sites appended. The ORF construct was assembled into the mammalian expression vector pSV7 via a three-way ligation with EcoRI/Not I-digested pSV7, EcoRI/Hpa I-digested NH2-terminal fragment, and Dra I/Not I-digested C-terminal fragment. Functional activity of the ORF construct was assessed by transfecting (using lipofectamine; GIBCO/BRL) 5 μg of plasmid DNA into the aequorin expressing reporter cell line (293-AEQ 17) cultured in 60 mm dishes. Following approximately 40 hours of expression the aequorin in the cells was charged for 2 hours with coelenterazine, the cells were harvested, washed and pelleted by low speed centrifugation into luminometer tubes. Functional activity was determined by measuring Compound A dependent mobilization of intracellular calcium and concommitant calcium induced aequorin bioluminescence. Shown in Fig. 26 are three replicate samples exhibiting Compound A induced luminescent responses. EXAMPLE 12 Assays Mammalian cells (COS-7) were transfected with GHSR expression plasmids using Lipofectamine (GIBCO-BRL; Hawley-Nelson, 1993, Eocus 15:73). Transfections were perfomied in 60 mm dishes on 80% confluent cells (approximately 4 x 10 ~ cells ) with 8 μg of Lipofectamine and 32 μg of GHSR plasmid DNA. ^ 35 Binding of [ ~ ' S ]-Compound A to swine pituitary membranes and crude membranes prepared from COS-7 cells transfected with GHSR expression plasmids was conducted. Crude cell membranes from COS-7 transfectants were prepared on ice, 48 hrs. post-transfection. Each 60 mm dish was washed twice with 3 ml of PBS, once with 1 ml homogenization buffer (50 mM Tris-HCl [ pH 7.4 ], 5 mM MgCl2, 2.5 mM EDTA, 30 μg/ml bacitracin). 0.5 ml of homogenization buffer was added to each dish, cells were removed by scraping and then homogenized using a Polytron device (Brinkmann, Syosset, NY : 3 bursts of 10 sec. at setting 4). The homogenate was then centrifuged for 20 min. at 1 1 ,000 x g at 0°C and the resulting crude membrane pellet (chiefly containing cell membranes and nuclei ) was resuspended in homogenization buffer supplemented with 0.06% BSA (0.1 ml/60 mm dish) and kept on ice. Binding reactions were performed at 20°C for 1 hr. in a total volume of 0.5 ml containing: 0.1 ml of membrane suspension, 10 μl of | ^^S |-Compound A (0.05 to 1 nM; specific activity approximately 900 Ci/mmol), 10 μl of competing drug and 380-390 μl of homogenization buffer. Bound radioligand was separated by rapid vacuum filtration (Brandel 48-well cell harvester) through GF/C filters pretreated for 1 hr. with 0.5% polyethylenimine. After application of the membrane suspension to the filter, the filters were washed 3 times with 3 ml each of ice cold 50 mM Tris-HCl [pH 7.4 ], 10 mM MgCD, 2.5 mM EDTA and 0.015%. Triton X- 100. and the bound radioactivity on the filers was quantitated by scintillation counting. Specific binding (> 90% of total) is defined as the difference between total binding and non-specific binding conducted in the presence of 50 nM unlabeled Compound A.

Claims: WHAT IS CLAIMED IS 1. A method of determining the presence of an nucleic acid which encodes a growth hoπnone secretagogue receptor (GHSR ) or 5 a growth hormone related receptor (GHSRR) comprising: a) introducing a nucleic acid suspected of encoding a GHSR or GHSRR into a cell which does not naturally express the receptor on its cell membrane; I 0 b) introducing a detector molecule or a nucleic acid encoding a detector molecule into the cell, wherein the detector molecule is directly or indirectly responsive to a receptor-ligand binding event; 1 5 c) contacting the cell with a growth hormone secretagogue; and d) deteπnining whether the nucleic acid encodes a receptor by monitoring the detector molecule. 0 2. A method to determine the presence of a growth hoπnone secretagogue comprising: a) introducing a nucleic acid which encodes a growth homione secretagogue receptor into a cell under conditions so that growth hoπnone 5 secretagogue receptor is expressed; b) introducing a detector molecule or a nucleic acid encoding a detector molecule into the cell, wherein the detector molecule is directly or indirectly responsive to a GHSR-ligand binding 0 event; c) contacting the cell with a compound suspected of being a growth hormone secretagogue; and d) determining whether the compound is a growth hormone secretagogue by monitoring the detector molecule. 3. A method to determine the presence of a nucleic acid which encodes a G protein cell membrane receptor comprising: a) introducing at least one nucleic acid suspected of encoding a G protein cel l membrane receptor into a cell; b) introducing a G-protein subunit into the cell; c) introducing a detector molecule or a nucleic acid encoding a detector molecule into the cell, wherein the detector molecule responds directly or indirectly to a G-protein receptor- ligand binding event; d) contacting the cell with a receptor ligand; and e) deteπnining whether the nucleic acid encodes a receptor by monitoring the detector molecule. 4. A method according to Claim 3 wherein the cell does not natural ly express the receptor on its cel l membrane. 5. A method according to Claim 4 wherein the receptor is a growth homione secretagogue receptor (GHSR). 6. A method according to Claim 4 wherein the receptor is a growth hormone secretagogue related receptor (GHSRR). 7. An assay to detemiine the presence of an nucleic acid which encodes a growth hormone secretagogue receptor (GHSR) or a growth hormone secretagogue related receptor (GHSRR) comprising: a) introducing at least one nucleic acid suspected of encoding a GHSR or GI ISRR into a cell which does not naturally express the receptor on its cell membrane; 5 b) introducing a G-protein subunit into the cell; c) introducing a detector molecule or a nucleic acid encoding a detector molecule into the cell, wherein the detector molecule is directly or indirectly responsive to a receptor-l igand I 0 binding event; d) contacting the cell with a growth homione secretagogue; and e) determining whether the nucleic acid encodes a receptor by monitoring the detector molecule. 1 5 8. A method according to Claim 7 wherein a pool comprising at least 500 different nucleic acid molecules arc introduced into the cell in step a). 0 9. A method according to Claim 8 wherein the pool comprises RNA molecules. 10. A method according to Claim 8 wherein upon determining that the pool comprises a nucleic acid encoding a growth 5 hormone secretagogoue recptor is present, steps a) to e) are repeated, except that the pool comprises a smaller number of different nucleic acid molecules. 1 1 . A method according to Claim 10 wherein only one 0 type of oligonucleotide is introduced into the cell in step a). 12. A method according to Claim 7 wherein the G protein subunit is a G-alpha protein subunit. 13. A method according to Claim 12 wherein the G- protein subunit is the G(χ l ] subunit. 14. A method according to Claim 7 wherein the detector molecule is aequorin. 15. An assay method to determine the presence of a growth hormone secretagogue comprising: a) introducing a nucleic acid which encodes a growth hormone secretagogue receptor into a cel l under conditions so that growth hormone secretagogue receptor is expressed; b) introducing a G-protein subunit into the ceil ; c ) introducing a detector molecule or a nucleic acid encoding a detector molecule into the cell, wherein the detector molecule is directly or indirectly responsive to a GHSR-ligand binding event; d) contacting the cell with a compound suspected of being a growth hormone secretagogue; and e) determining whether the compound is a growth hormone secretagogue by monitoring the detector molecule. 16. A method according to Claim 1 5 wherein the G protein subunit is an G-alpha protein subunit. 17. A method according to Claim 16 wherein the G- protein subunit is the Gcx l 1 subunit. 1 8. A method according to Claim 15 wherein the result of step e) is compared to that obtained using a known growth hoπnone secretagogue. 19. An assay for identifying a ligand which binds to a human growth honnone secretagogue receptor comprising contacting a putative ligand with a human growth hoπnone secretagogue receptor in the presence of G protein subunit cxl 1 nd deteπnining whether binding 5 has occurred. 20. An assay according to Claim 19 wherein the human growth homione secretagogue receptor is expressed in a host cell which does not naturally express human growth hormone secretagogue 1 0 receptor. 21. An assay according to Claim 20 wherein binding is detected by measuring the activity of a detector molecule. I 5 22. An assay according to Claim 21 wherein the detector molecule is aequorin. 23. A ligand identified by the assay of any of Claims 1 5-22. 0 24. A method of identifying a novel GHSR protein comprising exposing a labeled ligand to a putative GHSR protein and determining if binding occurs.

Inventors: CHAUNG LEE-YUH, FEIGHNER SCOTT D, HOWARD ANDREW D, PONG SHENG S, VAN DER PLOEG LEONARDUS, CHAUNG, LEE-YUH, FEIGHNER, SCOTT, D., HOWARD, ANDREW, D., PONG, SHENG, S., VAN DER PLOEG, LEONARDUS
Applicants: CHAUNG LEE YUH, FEIGHNER SCOTT D, HOWARD ANDREW D, MERCK & CO INC, PLOEG LEONARDUS V D, PONG SHENG S, CHAUNG, LEE-YUH, FEIGHNER, SCOTT, D., HOWARD, ANDREW, D., MERCK & CO., INC., PONG, SHENG, S., VAN DER PLOEG, LEONARDUS
Application year: 1996
Main classification: G01N33
IPC codes: C07K14, C12N15, C12Q1, C12R1, G01N33