Glutamate 301 of the mouse gonadotropin-releasing hormone receptor confers specificity for arginine 8 of mammalian gonadotropin-releasing hormone

Lamprey GnRH-III acts on its putative receptor via nitric oxide to release follicle-stimulating hormone specifically

Lamprey gonadotropin-releasing hormone-III (l-GnRH-III), the putative follicle-stimulating hormone (FSH)-releasing factor (FSHRF), exerts a preferential FSH-releasing activity in rats both in vitro and in vivo. To test the hypothesis that l-GnRH-III acts on its own receptors to stimulate gonadotropin release, the functional activity of this peptide at mammalian (m) leutinizing hormone (LH)RH receptors transfected to COS cells was tested. l-GnRH-III activated m-LHRH receptors only at a minimal effective concentration (MEC) of 10(-6) M, whereas m-LHRH was active at a MEC of 10(-9) M, at least 1,000 times less than that required for l-GnRH-III. In 4-day monolayer cultured cells, l-GnRH-III was similarly extremely weak in releasing either LH or FSH, and, in fact, it released LH at a lower concentration (10(-7) M) than that required for FSH release (10(-6) M). In this assay, m-LHRH released both FSH and LH significantly at the lowest concentration tested (10(-10) M). On the other hand, l-GnRH-III had a high potency to selectively release FSH and not LH from hemipituitaries of male rats. The results suggest that the cultured cells were devoid of FSHRF receptors, thereby resulting in a pattern of FSH and LH release caused by the LHRH receptor. On the other hand, the putative FSH-releasing factor receptor accounts for the selective FSH release by l-GnRH-III when tested on hemipituitaries. Removal of calcium from the medium plus the addition of EGTA, a calcium chelator, suppressed the release of gonadotropins induced by either l-GnRH-III or LHRH, indicating that calcium is required for the action of either peptide. Previous results showed that sodium nitroprusside, a releaser of nitric oxide (NO), causes the release of both FSH and LH from hemipituitaries incubated in vitro. In the present experiments, a competitive inhibitor of NO synthase, L-NG-monomethyl-L-arginine (300 micro M) blocked the action of l-GnRH-III or partially purified FSHRF. The results indicate that l-GnRH-III and FSHRF act on putative FSHRF receptors by a calcium-dependent NO pathway.

Secondary structure of the third extracellular loop responsible for ligand selectivity of a mammalian gonadotropin-releasing hormone receptor

The extracellular loop 3 (ECL3) of the mammalian gonadotropin-releasing hormone receptor (GnRH-R) contains an acidic amino acid (Glu(301) in the mouse GnRH-R) that confers agonist selectivity for Arg(8) in mammalian GnRH. It is proposed that a specific conformation of ECL3 is necessary to orientate the carboxyl side chain of the acidic residue for interaction with Arg(8) of GnRH, which is supported by decreased affinity for Arg(8) GnRH but not Gln(8) GnRH when an adjacent Pro is mutated to Ala. To probe the structural contribution of the loop domain to the proposed presentation of the carboxyl side chain, we synthesized a model peptide (CGPEMLNRVSEPGC) representing residues 293-302 of mouse ECL3, where Cys and Gly residues are added symmetrically at the N and C termini, respectively, allowing the introduction of a disulfide bridge to simulate the distances at which the ECL3 is tethered to the transmembrane domains 6 and 7 of the receptor. The ability of the ECL3 peptide to bind GnRH with low affinity was demonstrated by its inhibition of GnRH stimulation of inositol phosphate production in cells expressing the GnRH-R. The CD bands of the ECL3 peptides exhibited a superposition of predominantly unordered structure and partial contributions from beta-sheet structure. Likewise, the analysis of the amide I and amide III bands from micro-Raman and FT Raman experiments revealed mainly unordered conformations of the cyclic and of the linear peptide. NMR data demonstrated the presence of a beta-hairpin among an ensemble of largely disordered structures in the cyclic peptide. The location of the turn linking the two strands of the hairpin was assigned to the three central residues L(296), N(297), and R(298). A small population of structured species among an ensemble of predominantly random coil conformation suggests that the unliganded receptor represents a variety of structural conformers, some of which have the potential to make contacts with the ligand. We propose a mechanism of receptor activation whereby binding of the agonist to the inactive receptor state induces and stabilizes a particular structural state of the loop domain, leading to further conformational rearrangements across the transmembrane domain and signal propagating interaction with G proteins. Interaction of the Glu(301) of the receptor with Arg(8) of GnRH induces a folded configuration of the ligand. Our proposal thus suggests that conformational changes of both ligand and receptor result from this interaction.

Impact of aromatic residues within transmembrane helix 6 of the human gonadotropin-releasing hormone receptor upon agonist and antagonist binding

To investigate the impact of aromatic residues within transmembrane helix 6 (TMH6) of the human gonadotropin-releasing hormone receptor (GnRH-R) on agonist and antagonist binding, residues Y(283), Y(284), W(289), Y(290), W(291), and F(292) were exchanged to alanine and analyzed comprehensively in functional reporter gene and ligand binding assays. Whereas receptor mutants Y(283)A, Y(284)A, and W(291)A were capable of neither ligand binding nor signal transduction, mutants W(289)A, Y(290)A, and F(292)A were functional: the F(292)A mutant behaved like wild-type receptor, while mutants W(289)A and Y(290)A differentiated between agonistic and antagonistic ligands. On the basis of the high-resolution X-ray structure of bovine rhodopsin as well as available data on GnRH-R mutants, models for ligand-receptor interactions are proposed. The model for D-Trp(6)-GnRH (Triptorelin) binding, representing a superagonistic ligand, is in full accordance to available data. Furthermore, new interactions are proposed: pGlu(1) interacts with N(212) in transmembrane helix 5, Tyr(5) with Y(290), and D-Trp(6) with W(289). The binding behavior of mutants W(289)A and Y(290)A corresponds to the proposed binding model for the antagonist Cetrorelix. In summary, our data as presented indicate that Y(290) plays a key function in agonist but not antagonist binding.

Role of aspartate7.32(302) of the human gonadotropin-releasing hormone receptor in stabilizing a high-affinity ligand conformation

Mammalian gonadotropin-releasing hormone (GnRH) receptors preferentially bind mammalian GnRH, which has Arg in position eight. The Glu(7.32(301)) residue, which determines selectivity of the mouse GnRH receptor for Arg(8)-containing GnRH, is Asp(7.32(302)) in the human GnRH receptor. We have confirmed that Asp(7.32(302)) confers selectivity of the human GnRH receptor for Arg(8) of GnRH and investigated the mechanism of this specificity using site-directed mutagenesis and ligand modification. We find that although Arg(8) and Asp(7.32(302)) are required for high-affinity binding of GnRH, conformationally constrained peptides, with D-amino acid substitutions in position six or with a 6,7 gamma-lactam, bind the human GnRH receptor with high affinity, which is independent of the presence of Asp(7.32(302)) in the receptor or Arg(8) in the ligand. The ability of the ligand constraints to compensate for the absence of both Arg(8) and Asp(7.32(302)) indicates that these residues both have roles in stabilizing a high affinity ligand conformation and that their roles are complementary. This suggests that the Arg(8) and Asp(7.32(302)) side chains interact to induce a high affinity conformation of native GnRH. Thus, Asp(7.32(302)) of the human GnRH receptor determines selectivity for mammalian GnRH by its ability to induce a high affinity conformation of its native ligand. However, this initial interaction seems not to contribute to the final ligand-receptor complex. We propose that Arg(8) interacts transiently with Asp(7.32(302)) to induce a high-affinity ligand conformation of GnRH, which then interacts with a binding pocket that is common for both constrained and unconstrained analogs of GnRH.

Identification and characterization of two distinct GnRH receptor subtypes in a teleost, the medaka Oryzias latipes

We report the identification and characterization of two distinct GnRH receptor (GnRH-R) subtypes, designated GnRH-R1 and GnRH-R2, in a model teleost, the medaka Oryzias latipes. These seven-transmembrane receptors of the medaka contain a cytoplasmic C-terminal tail, which has been found in all other nonmammalian GnRH-Rs cloned to date. The GnRH-R1 gene is composed of three exons separated by two introns, whereas the GnRH-R2 gene has an additional intron and therefore consists of four exons and three introns. The GnRH-R1 and GnRH-R2 genes, both of which exist as single-copy genes in the medaka genome, were mapped to linkage groups 3 and 16, respectively. Inositol phosphate assays using COS-7 cells transfected with GnRH-R1 and GnRH-R2 demonstrated that they had remarkably different ligand sensitivities, although both receptors showed highest preference for chicken-II-type GnRH. Phylogenetic analysis showed the presence of three paralogous lineages for vertebrate GnRH-Rs and indicated that neither GnRH-R1 nor GnRH-R2 is the medaka ortholog to mammalian GnRH-Rs that lack a cytoplasmic tail. This, together with an observation that medaka-type GnRH had low affinity for GnRH-R1 and GnRH-R2, suggests that a third GnRH-R may exist in the medaka.

Inhibitory activity of alternative splice variants of the bullfrog GnRH receptor-3 on wild-type receptor signaling

Recently we characterized three distinct GnRH receptors in the bullfrog (bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3). In the present study, we further investigated the expression and function of splice variants, generated from the primary bfGnRHR-3 transcript by exon skipping (splice variant 1), intron retention (splice variants 2 and 3), and/or transcriptional slippage (splice variant 4), apart from the constitutively spliced form (wild-type). Cellular expression and function of the splice variants were examined using a transient expression system. Immunoblot analysis revealed that the wild-type receptor and all splice variant proteins were expressed in transfected HeLa cells with no significant differences in expression levels. These splice variants showed a very low binding affinity to ligand and did not induce signal transduction in response to GnRH treatment. Interestingly, cotransfection of the wild-type with splice variants 2--4, but not with splice variant 1, significantly inhibited wild-type receptor-mediated signaling. Subcellular localization analysis of green fluorescent protein-tagged wild-type and splice variant proteins revealed that the wild-type receptor protein was mainly localized in the cell membrane, whereas the splice variant 1 protein was exclusively detected in the cytoplasm. The splice variant 2--4 proteins, however, were found in both the cell membrane and cytoplasm. The inhibition of wild-type receptor signaling by splice variants 2--4 and the subcellular localization of splice variants 2-4 suggest a possible physical interaction of splice variants 2--4 with the wild-type receptor protein. In addition, the ratio of mRNA levels of the wild-type to splice variants 2--4 significantly varied from hibernation (wild-type < splice variants 2--4) to the prebreeding season (wild-type > splice variants 2--4). Collectively, these results suggest that alternative splicing of the bfGnRHR-3 primary transcript plays a role in fine-tuning GnRH receptor function in amphibians.

A novel mammalian receptor for the evolutionarily conserved type II GnRH

Mammalian gonadotropin-releasing hormone (GnRH I: pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) stimulates pituitary gonadotropin secretion, which in turn stimulates the gonads. Whereas a hypothalamic form of GnRH of variable structure (designated type I) had been shown to regulate reproduction through a cognate type I receptor, it has recently become evident that most vertebrates have one or two other forms of GnRH. One of these, designated type II GnRH (GnRH II: pGlu-His-Ser-His-Gly-Trp-Tyr-Pro-Gly-NH2), is conserved from fish to man and is widely distributed in the brain, suggesting important neuromodulatory functions such as regulating K+ channels and stimulating sexual arousal. We now report the cloning of a type II GnRH receptor from marmoset cDNA. The receptor has only 41% identity with the type I receptor and, unlike the type I receptor, has a carboxyl-terminal tail. The receptor is highly selective for GnRH II. As with the type I receptor, it couples to G(alpha)q/11 and also activates extracellular signal-regulated kinase (ERK1/2) but differs in activating p38 mitogen activated protein (MAP) kinase. The type II receptor is more widely distributed than the type I receptor and is expressed throughout the brain, including areas associated with sexual arousal, and in diverse non-neural and reproductive tissues, suggesting a variety of functions. Surprisingly, the type II receptor is expressed in the majority of gonadotropes. The presence of two GnRH receptors in gonadotropes, together with the differences in their signaling, suggests different roles in gonadotrope functioning.

Critical implication of transmembrane Phe310, possibly in conjunction with Trp279, in the rat gonadotropin-releasing hormone receptor activation

The GnRH-R belongs to the superfamily of heptahelical GPCRs. A three-dimensional model of GnRH binding to its receptor predicted that Trp3 was the most deeply buried residue, potentially allowing it to interact with both Trp279, a highly conserved residue in the TMH 6 of GPCRs, and Phe310, present essentially in TMH 7 of GnRH-Rs. Replacement of Phe310 with Leu, the most common positional residue in GPCRs, induced a slightly decreased Bmax (1.6-fold) and affinity (3.8-fold); in addition, IP production was completely abolished. Similarly, replacement of Trp279 with Ser depressed the Bmax by 5.2-fold, the affinity by 2.3-fold, and totally abrogated IP production. The effect of the double mutation was not additive on binding, since the Bmax was reduced to the level of the Phe310Leu mutant, although the Kd was restored to a value not significantly different from that of the wild-type. The double mutant was also unable to induce IP production. Unexpectedly, no influence of any single or double substitution was noted on receptor internalization. These data provide evidence for the crucial role of Phe310, possibly in conjunction with Trp279, on GnRH transduction and suggest that the conformation for phospholipase C activation may not be required for GnRH-R internalization.

Mechanism of GnRH receptor signaling on gonadotropin release and gene expression in pituitary gonadotrophs

Gonadotropin releasing hormone (GnRH), the first key hormone of reproduction, is synthesized and secreted from the hypothalamus in a pulsatile manner and stimulates pituitary gonadotrophs (5-10% of the pituitary cells) to synthesize and release gonadotropin luteinizing hormone (LH) and follicle stimulating hormone (FSH). Gonadotrophs consist of 60% multihormonal cells (LH+FSH) and 18% LH- and 22% FSH-containing cells. LH and FSH, members of the glycoprotein hormone family, stimulate spermatogenesis, folliculogenesis, and ovulation. Although GnRH plays a pivotal role in gonadotropin synthesis and release, other factors such as gonadal steroids and gonadal peptides exert positive and negative feedback mechanisms, which affect GnRH actions. GnRH actions include activation of phosphoinositide turnover as well as phospholipase D and A2, mobilization and influx of Ca2+, activation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). A complex crosstalk between the above messenger molecules mediates the diverse actions of GnRH. Understanding the signaling mechanisms involved in GnRH actions is the basis for our understanding of basic reproductive functions in general and gonadotropin synthesis and release in particular.

A chicken gonadotropin-releasing hormone receptor that confers agonist activity to mammalian antagonists. Identification of D-Lys(6) in the ligand and extracellular loop two of the receptor as determinants

Mammalian receptors for gonadotropin-releasing hormone (GnRH) have over 85% sequence homology and similar ligand selectivity. Biological studies indicated that the chicken GnRH receptor has a distinct pharmacology, and certain antagonists of mammalian GnRH receptors function as agonists. To explore the structural determinants of this, we have cloned a chicken pituitary GnRH receptor and demonstrated that it has marked differences in primary amino acid sequence (59% homology) and in its interactions with GnRH analogs. The chicken GnRH receptor had high affinity for mammalian GnRH (K(i) 4.1 +/- 1.2 nM), similar to the human receptor (K(i) 4.8 +/- 1.2 nM). But, in contrast to the human receptor, it also had high affinity for chicken GnRH ([Gln(8)]GnRH) and GnRH II ([His(5),Trp(7),Tyr(8)]GnRH) (K(i) 5.3 +/- 0.5 and 0.6 +/- 0.01 nM). Three mammalian receptor antagonists were also pure antagonists in the chicken GnRH receptor. Another three, characterized by D-Lys(6) or D-isopropyl-Lys(6) moieties, functioned as pure antagonists in the human receptor but were full or partial agonists in the chicken receptor. This suggests that the Lys side chain interacts with functional groups of the chicken GnRH receptor to stabilize it in the active conformation and that these groups are not available in the activated human GnRH receptor. Substitution of the human receptor extracellular loop two with the chicken extracellular loop two identified this domain as capable of conferring agonist activity to mammalian antagonists. Although functioning of antagonists as agonists has been shown to be species-dependent for several GPCRs, the dependence of this on an extracellular domain has not been described.

Three distinct types of GnRH receptor characterized in the bullfrog

It has been proposed recently that two types of GnRH receptors (GnRHR) exist in a particular species. Here we present data demonstrating that at least three types of GnRHR are expressed in a single diploid species, the bullfrog. Three different cDNAs, encoding distinct types of bullfrog GnRHR (bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3), were isolated from pituitary and hindbrain of the bullfrog. BfGnRHR-1 mRNA was expressed predominantly in pituitary, whereas bfGnRHR-2 and -3 mRNAs were expressed in brain. The bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3 proteins have an amino acid identity of approximately 30% to approximately 35% with mammalian GnRHRs and approximately 40% to approximately 50% with nonmammalian GnRHRs. Interestingly, bfGnRHR-2 has an 85% amino acid homology with Xenopus GnRHR. Less than 53% amino acid identity was observed among the three bfGnRHRs. All isolated cDNAs encode functional receptors because their transient expression in COS-7 cells resulted in a ligand-dependent increase in inositol phosphate production. Notably, all three receptors exhibited a differential ligand selectivity. For all receptors, cGnRH-II has a higher potency than mGnRH. In addition, salmon GnRH also has a strikingly high potency to stimulate all three receptors. In conclusion, we demonstrated the presence of three GnRHRs in the bullfrog. Their expression in pituitary and brain suggests that bfGnRHRs play an important role in the regulation of reproductive functions in the bullfrog.

Multiple interactions of the Asp(2.61(98)) side chain of the gonadotropin-releasing hormone receptor contribute differentially to ligand interaction

Mutation of Asp(2.61(98)) at the extracellular boundary of transmembrane helix 2 of the gonadotropin-releasing hormone (GnRH) receptor decreased the affinity for GnRH. Using site-directed mutagenesis, ligand modification, and computational modeling, different side chain interactions of Asp(2.61(98)) that contribute to high-affinity binding were investigated. The conservative Asp(2. 61(98))Glu mutation markedly decreased the affinity for a series of GnRH analogues containing the native His(2) residue. This mutant showed smaller decreases in affinity for His(2)-substituted ligands. The loss of preference for His(2)-containing ligands in the mutant receptor shows that Asp(2.61(98)) determines the specificity for His(2). Analysis of the affinities of a series of position 2-substituted ligands suggests that a hydrogen bond forms between Asp(2.61(98)) and the delta NH group of His(2) and that Asp(2. 61(98)) forms a second hydrogen bond with the ligand. Substitution of Asp(2.61(98)) with an uncharged residue further decreased the affinity for all ligands and also decreased receptor expression. Computational modeling indicates an intramolecular ionic interaction of Asp(2.61(98)) with Lys(3.32(121)) in transmembrane helix 3. The uncharged, Lys(3.32(121))Gln mutation also markedly decreased agonist affinity. The modeling and the similar phenotypes of mutants with uncharged substitutions for Asp(2.61(98)) or Lys(3.32(121)) are consistent with the presence of this helix 2-helix 3 interaction. These studies support a dual role for Asp(2.61(98)): formation of an interhelical interaction with Lys(3.32(121)) that contributes to the structure of the agonist binding pocket and an interaction with His(2) of GnRH that helps stabilize agonist complexing.

Molecular cloning, distribution and pharmacological characterization of a novel gonadotropin-releasing hormone ([Trp8] GnRH) in frog brain

To date nine structural variants of GnRH have been identified in vertebrates and two additional forms have been isolated from a tunicate. In amphibians only mammalian GnRH ([Arg8] GnRH) and type II GnRH (chicken GnRH II, [His5, Trp7, Tyr8] GnRH) have been identified. In the present study, a full-length cDNA encoding a novel type of GnRH was isolated from pituitary of Rana dybowskii. The GnRH gene encodes a GnRH peptide ([Trp8] GnRH) in which tryptophan is substituted for arginine of mammalian GnRH Northern blot analysis revealed the presence of a single 500 bp transcript for the [Trp8] GnRH precursor in forebrain but its absence in testis, ovary, kidney and liver. Restriction digests of genomic DNA demonstrated a single copy of the gene. The [Trp8] GnRH immunoreactive cells were identified in the preoptic area of the frog brain. Synthetic [Trp8] GnRH was tested for its ability to stimulate inositol phosphate production by COS-1 cells transfected with the cloned Xenopus pituitary GnRH receptor and the cloned human GnRH receptor. [Trp8] GnRH had a potency of about 60% compared with mammalian GnRH ([Arg8] GnRH) for the Xenopus receptor, whereas the potency of [Trp8] GnRH was approximately 5% compared with mammalian GnRH for the human receptor. Both mammalian GnRH and [Trp8] GnRH were 1000-fold less potent than type II GnRH for the Xenopus GnRH receptor. The similar potency of [Arg8] GnRH and the novel [Trp8] GnRH for the Xenopus pituitary receptor indicates that, unlike the human receptor, the Xenopus receptor does not discriminate between these amino acids in position eight thereby allowing substitution of the arginine in the mammalian GnRH.

Identification of Phe313 of the gonadotropin-releasing hormone (GnRH) receptor as a site critical for the binding of nonpeptide GnRH antagonists

The dog GnRH receptor was cloned to facilitate the identification and characterization of selective nonpeptide GnRH antagonists. The dog receptor is 92% identical to the human GnRH receptor. Despite such high conservation, the quinolone-based nonpeptide GnRH antagonists were clearly differentiated by each receptor species. By contrast, peptide antagonist binding and functional activity were not differentiated by the two receptors. The basis of the differences was investigated by preparing chimeric receptors followed by site-directed mutagenesis. Remarkably, a single substitution of Phe313 to Leu313 in the dog receptor explained the major differences in binding affinities and functional activities. The single amino acid replacement of Phe313 of the human receptor with Leu313 resulted in a 160-fold decrease of binding affinity of the nonpeptide antagonist compound 1. Conversely, the replacement of Leu313 of the dog receptor with Phe313 resulted in a 360-fold increase of affinity for this compound. These results show that Phe313 of the GnRH receptor is critical for the binding of this structural class of GnRH antagonists and that the dog receptor can be "humanized" by substituting Leu for Phe. This study provides the first identification of a critical residue in the binding pocket occupied by nonpeptide GnRH antagonists and reinforces cautious extrapolation of ligand activity across highly conserved receptors.

Complementary deoxyribonucleic acid cloning, gene expression, and ligand selectivity of a novel gonadotropin-releasing hormone receptor expressed in the pituitary and midbrain of Xenopus laevis

We have cloned the full-length complementary DNA (cDNA) for a GnRH receptor from Xenopus laevis pituitary cDNA and determined its gene structure. The cDNA encodes a 368-amino acid protein that has a 46% amino acid identity to the human GnRH receptor. The X laevis GnRH receptor has all of the amino acids identified in the mammalian GnRH receptors as sites of interaction with the GnRH ligand. However, this receptor cDNA shares the same distinguishing structural features of the GnRH receptor that have been characterized from other nonmammalian vertebrates. These include the pair of aspartate residues in the transmembrane domains II and VII compared with the aspartate/asparagine arrangement in mammalian receptors, the amino acid PEY motif in extracellular loop III (SEP in mammals), and the presence of a carboxyl-terminal tail. Previous studies have reported that mammalian GnRH was equipotent to other naturally occurring GnRH subtypes in stimulating LH release from the amphibian pituitary. However, in this study we show that the X. laevis GnRH receptor has ligand selectivity for the naturally occurring GnRHs similar to other nonmammalian GnRH receptors. The order of potency of the GnRHs in stimulating inositol phosphate production in COS-1 cells transiently transfected with the X. laevis GnRH receptor cDNA was chicken GnRH II>salmon GnRH>mammalian GnRH. Transcripts of this GnRH receptor are expressed in the pituitary and midbrain of X. laevis.

Consensus bioactive conformation of cyclic GnRH antagonists defined by NMR and molecular modeling

Little is known of the conformation of peptide hormones as they interact with their receptors for a number of reasons: peptide hormones are notoriously flexible in solution, their receptors are particularly complex, and there is strong evidence that receptor-ligand interaction leading to activation is a dynamic process. Insights into the active conformation of the decapeptide gonadotropin releasing hormone (GnRH) have been obtained previously from the solution structures of four constrained GnRH antagonists ¿cyclo(1-10)[Ac-Delta(3)-Pro(1),DCpa(2),DTrp(3,6),NMeLeu+ ++(7), betaAla(10)]GnRH (1), cyclo(4-10)[Ac-Delta(3)Pro(1),DFpa(2),DTrp(3), Asp(4),DNal(6),Dpr(10)]GnRH (2), dicyclo(4-10/5-8)[Ac-DNal(1), DCpa(2),DTrp(3),Asp(4),Glu(5),DArg(6),Lys(8),Dpr (10)]GnRH (3), and dicyclo(4-10/5-5'-8)[Ac-DNal(1),DCpa(2),DPal(3), Asp(4),Glu(5)(Gly), DArg(6),Dbu(8),Dpr(10)]GnRH (4)¿. However, the precise location of the N-terminal tripeptide in the highly potent (K(i) < 0.4 nM) 2-4 remained unclear due to the lack of constraints in this region. The NMR structure of the newly discovered and potent (K(i) = 0.24 nM) dicyclo(1-1'-5/4-10)[Ac-Glu(1)(Gly),DCpa(2),DTrp(3),As p(4),Dbu(5), DNal(6),Dpr(10)]GnRH (5) now allows the definition of the conformation of this region. A combined computational analysis (consensus forcing) of compounds 2-5, designed to explore the common conformations available to them that are simultaneously consistent with the NMR data corresponding to each compound, leads to a consensus structural model for the GnRH pharmacophore. This model shares some common features with the structure of the nonpeptidic GnRH mimetic T-98475. In the course of that comparative study, two additional contact points to those proposed by the authors are identified, suggesting that this model has predictive value.

Functional importance of transmembrane helix 6 Trp(279) and exoloop 3 Val(299) of rat gonadotropin-releasing hormone receptor

Previous studies have established that the interaction of gonadotropin-releasing hormone (GnRH) with its receptor (GnRHR) would require partial entry of the N- and C-terminal regions of ligand into the transmembrane core. The functional significance of the conserved aromatic residue Trp(279) present in the transmembrane helix 6, and Val(299) located in exoloop 3 of the rat GnRHR was investigated by mutagenesis followed by expression in Chinese hamster ovary-K1 cells. Compared with wild-type, substitution of Trp(279) with Ser or Arg resulted in a marked reduction or total abolition, respectively, of ligand binding and, in both cases, abrogation of GnRH-induced inositol phosphate production. A total absence of functionality was observed when Val(299) was simply replaced with Ala. Mention should be made that an expression of all mutated and wild-type receptor proteins was observed. Interestingly, the double mutant [Trp(279)Arg/Val(299)Ala]GnRHR restored B(max) to wild type (504 +/- 43 versus 541 +/- 41 fmol/mg protein), but with a diminished affinity (4.95 +/- 1.05 versus 0.94 +/- 0.35 nM), and GnRH failed to induce inositol phosphate. No influence of the mutations was seen on internalization of the receptor. The three-dimensional model of GnRH binding to the rat GnRHR was built predicting that Trp(279) is buried at 20 A in the transmembrane core of the receptor, directly in contact with Trp(3) of GnRH. In contrast, Val(299) is located in a region that cannot be precisely defined at the extracellular end of transmembrane helix 7. Although models cannot provide any clue concerning the observed interactivity between the two distal residues, altogether these data reveal the functional importance of both GnRHR Trp(279) and Val(299) and suggest that Trp(279), interacting with GnRH Trp(3), represents the bottom of the binding pocket.

A new photoreactive antagonist cross-links to the N-terminal domain of the gonadotropin-releasing hormone receptor

A new photoreactive gonadotropin-releasing hormone (GnRH) antagonist [Ac-(4-azidobenzoyl)-D-Lys1, D-4-Cl-Phe2, D-Trp3, D-Arg6, D-Ala10]GnRH (PAnt-1) was synthesized and shown to bind covalently to mouse and human GnRH receptors after ultraviolet irradiation. PAnt-1 exhibited high binding affinity (Ki = 3.1 +/- 0.8 nM), and high crosslinking efficiency as shown by loss of 78% of binding sites following crosslinking at saturating concentration. Crosslinking resulted in irreversible receptor blockade as shown by inhibition of GnRH-stimulated inositol phosphate production. PAnt-1 has a photoreactive group at residue 1 of the peptide, a region believed to be critical in determining antagonist versus agonist properties of GnRH analogues. The attachment site of PAnt- to the receptor was localized between residues 11 and 19 of the extracellular N-terminal domain of the receptor by peptide mapping studies using natural sequence differences between human, mouse and sheep GnRH receptors, as well as a panel of GnRH receptor constructs with a series of engineered protease cleavage sites. A disulphide bridge between Cys14 and Cys200 was cleaved during crosslinking, suggesting that Cys14 is the crosslinked residue. These results suggest that peptide GnRH antagonists bind to the receptor with the N-terminal end of the peptide positioned in a site comprising the constrained regions of the N-terminal domain and second extracellular loop in the vicinity of the Cys14-Cys200 disulphide bridge.

ASP147 in the third transmembrane helix of the rat mu opioid receptor forms ion-pairing with morphine and naltrexone

We tested the hypotheses that the carboxylate side chain of Asp147 of the mu opioid receptor interacts with the protonated nitrogen of naltrexone and morphine and that this interaction is important for pharmacological properties of the two compounds. Mutation of Asp147 to Ala or Asn substantially reduced the affinity of naltrexone and the affinity, potency and efficacy of morphine, while the Glu mutant had similar properties as the wildtype, indicating the significant role of the carboxylate group of Asp147 in receptor binding and activation. This role could be due to its direct interaction with ligands or involvement in interhelical interactions. The unprotonated analogs of naltrexone and morphine, cyclopropylcarbonyl noroxymorphone (CPCNOM) and N-formylnormorphine (NFNM), respectively, were used to discriminate between these mechanisms. CPCNOM was much less potent as an antagonist and had substantially lower affinity for the mu receptor than naltrexone. Similarly, NFNM was unable to activate the mu receptor and had much lower affinity than morphine. These results indicate the importance of the protonated nitrogen. Notably, the D147A and D147N mutations did not appreciably affect the binding affinities of CPCNOM and NFNM. In addition, the D147E mutant had similar affinities for CPCNOM and NFNM as the D147A and D147N mu receptors. Thus, the carboxylate group of Asp147 is not important for binding of the two unprotonated compounds. These results indicate that the carboxylate group of Asp147 of the mu receptor interacts directly with the protonated nitrogen of naltrexone and morphine and this interaction is important for binding and receptor activation.

Somatostatin and its receptor family

Somatostatin (SST), a regulatory peptide, is produced by neuroendocrine, inflammatory, and immune cells in response to ions, nutrients, neuropeptides, neurotransmitters, thyroid and steroid hormones, growth factors, and cytokines. The peptide is released in large amounts from storage pools of secretory cells, or in small amounts from activated immune and inflammatory cells, and acts as an endogenous inhibitory regulator of the secretory and proliferative responses of target cells that are widely distributed in the brain and periphery. These actions are mediated by a family of seven transmembrane (TM) domain G-protein-coupled receptors that comprise five distinct subtypes (termed SSTR1-5) that are endoded by separate genes segregated on different chromosomes. The five receptor subtypes bind the natural SST peptides, SST-14 and SST-28, with low nanomolar affinity. Short synthetic octapeptide and hexapeptide analogs bind well to only three of the subtypes, 2, 3, and 5. Selective nonpeptide agonists with nanomolar affinity have been developed for four of the subtypes (SSTR1, 2, 3, and 4) and putative peptide antagonists for SSTR2 and SSTR5 have been identified. The ligand binding domain for SST ligands is made up of residues in TMs III-VII with a potential contribution by the second extracellular loop. SSTRs are widely expressed in many tissues, frequently as multiple subtypes that coexist in the same cell. The five receptors share common signaling pathways such as the inhibition of adenylyl cyclase, activation of phosphotyrosine phosphatase (PTP), and modulation of mitogen-activated protein kinase (MAPK) through G-protein-dependent mechanisms. Some of the subtypes are also coupled to inward rectifying K(+) channels (SSTR2, 3, 4, 5), to voltage-dependent Ca(2+) channels (SSTR1, 2), a Na(+)/H(+) exchanger (SSTR1), AMPA/kainate glutamate channels (SSTR1, 2), phospholipase C (SSTR2, 5), and phospholipase A(2) (SSTR4). SSTRs block cell secretion by inhibiting intracellular cAMP and Ca(2+) and by a receptor-linked distal effect on exocytosis. Four of the receptors (SSTR1, 2, 4, and 5) induce cell cycle arrest via PTP-dependent modulation of MAPK, associated with induction of the retinoblastoma tumor suppressor protein and p21. In contrast, SSTR3 uniquely triggers PTP-dependent apoptosis accompanied by activation of p53 and the pro-apoptotic protein Bax. SSTR1, 2, 3, and 5 display acute desensitization of adenylyl cyclase coupling. Four of the subtypes (SSTR2, 3, 4, and 5) undergo rapid agonist-dependent endocytosis. SSTR1 fails to be internalized but is instead upregulated at the membrane in response to continued agonist exposure. Among the wide spectrum of SST effects, several biological responses have been identified that display absolute or relative subtype selectivity. These include GH secretion (SSTR2 and 5), insulin secretion (SSTR5), glucagon secretion (SSTR2), and immune responses (SSTR2).

The role of the fourth extracellular domain of the rat corticotropin-releasing factor receptor type 1 in ligand binding

The role of the fourth extracellular loop (e4) of rat corticotropin-releasing factor (CRF) receptor, type 1, in ligand binding was investigated using chimeric receptor molecules. e4 of CRF receptor, type 1, was replaced by the corresponding domains of two other G protein-coupled receptors, the rat glucagon receptor or the human pituitary adenylate cyclase activating polypeptide (PACAP) receptor. Both chimeras were transported properly to the cell membranes of transfected chinese hamster ovary cells as indicated by immunocytochemical analysis. Ovine CRF (oCRF) was bound specifically, but with low affinity (Kd = 2-5 microm). Cyclic AMP was not accumulated intracellularly in response to increasing concentrations of oCRF. Based on these data, it is concluded that e4 of rat CRF receptor, type 1, is involved in ligand binding. To confirm the importance of e4 in binding CRF, three negatively charged amino acids of e4, Glu336, Asp337 and Glu338, were replaced by Gln, Asn and Gln, respectively. No effect on ligand binding and cyclic AMP accumulation was observed (Kd = 5 nm; EC50 = 1.5 nm). However, when Tyr346, Phe347 and Asn348 of e4 were changed to three alanine residues, ligand binding affinity as well as efficacy in cyclic AMP accumulation were significantly decreased (Kd = 64 nm; EC50 = 32 nm).

Identification of three putative GnRH receptor subtypes in vertebrates

The majority of vertebrates have two or three forms of gonadotropin-releasing hormone (GnRH), which appear to have arisen by successive gene duplication events. This suggests the possibility of concordant gene duplications of the GnRH receptor to produce two or more cognate receptors. Since the extracellular loop 3 (EC3) domain of mammalian GnRH receptors plays a role in distinguishing the different forms of GnRH, we have contemplated that the sequence of this domain will differ significantly in the putative cognate receptors. Degenerate oliognucleotides encoding the sequences of the transmembrane domains preceding and following EC3 were used for PCR amplification of genomic DNA from zebrafish (Brachydanio rerio), goldfish (Carassius auratus), African clawed frog (Xenopus laevis), chicken (Gallus domesticus), and lizard (Agama atra). Isolation and sequencing of specific clones revealed that they fell into three groups. Two of these were most similar to the mammalian pituitary GnRH receptor and were therefore designated Type IA and Type IB. The third form (designated Type II) was most different from the others and was identified in Xenopus, lizard, and human DNA. These findings support the concept of the existence of three distinct GnRH receptors, which have evolved in conjunction with three distinct GnRH ligand classes present in many vertebrates.

Functional analysis of GnRH receptor ligand binding using biotinylated GnRH derivatives

The objective of this study was to determine whether the gonadotrophin-releasing hormone (GnRH) ligand binds to the GnRH receptor (GnRH-R) with either the N- and C-termini or the beta-II turn pointing towards the cell. The functionality of GnRH and two biotinylated GnRH derivatives, biotin [D-Lys6]GnRH and biotin [Gln1]GnRH biotinylated at positions 6 and 1, respectively was assessed. Streptavidin was also used in combination with these peptides to investigate the effects of the steric hindrance caused by this molecule on ligand binding when bound to the biotin molecules at the two positions. GnRH bound to the receptor with high affinity, which was not affected by the addition of streptavidin. Both the biotinylated derivatives bound to the receptor though with lower affinities than GnRH. The biotin [D-Lys6]GnRH-streptavidin complex bound to the receptor albeit with lower affinity compared to biotin [D-Lys6]GnRH only, although it maintained its ability to cause receptor internalisation. The ability of the biotin [Gln1]GnRH to bind to the receptor was abolished in the presence of excess streptavidin. Both GnRH and biotin [D-Lys6]GnRH stimulated total inositol phosphate production whereas biotin [Gln1]GnRH exhibited GnRH antagonist activity. It appears that the small biotin molecule can be accommodated within the binding pore when attached to position 1 of the ligand but not when complexed to streptavidin. The fact that biotin [D-Lys6]GnRH maintains functionality when complexed to streptavidin while biotin [Gln1]GnRH does not, suggests that the N- and possibly the C-termini are required for receptor binding. Thus the most likely binding orientation for the ligand is with the N- and C-termini pointing inwards with the residue at position 6 pointing away from the binding site.

Ligand binding pocket of the human somatostatin receptor 5: mutational analysis of the extracellular domains

The ligand binding domain of G protein-coupled receptors for peptide ligands consists of a pocket formed by extracellular and transmembrane domain (TM) residues. In the case of somatostatin (SRIF), however, previous studies have suggested that the binding cavity of the octapeptide analog SMS201-995 (SMS) is lined by residues in TMs III-VII. The additional involvement of the extracellular domains for binding SMS or the natural SRIF ligands (SRIF-14, SRIF-28) has not been clarified. Using a cassette construct cDNA for the human somatostatin 5 receptor (sst5R), we systematically examined the role of exofacial structures in ligand binding by creating a series of mutants in which the extracellular portions have been altered by conservative segment exchange (CSE) mutagenesis for the extracellular loops (ECLs) and by deletion (for the NH2-terminal segment) or truncation analysis (ECL3). CHO-K1 cells were stably transfected with wild type or mutant human sst5R constructs, and agonist binding was assessed using membrane binding assays with 125I-LTT SRIF-28 ligand. Deletion of the NH2 terminus or CSE mutagenesis of ECL1 and ECL3 produced minor 2-8-fold decreases in affinity for SRIF-14, SRIF-28, and SMS ligands. Truncation of ECL3 to mimic the size of this loop in sst1R and sst4R (the two subtypes that do not bind SMS) did not interfere with the binding of SMS, SRIF-14, or SRIF-28. In contrast, both ECL2 mutants failed to bind 125I-LTT SRIF-28. Immunocytochemical analysis of nonpermeabilized cells with a human sst5R antibody revealed that the mutant receptors were targeted to the plasma membrane. Labeled SMS (125I-Tyr3 SMS) also failed to bind to the mutant ECL2 receptors. These results suggest a potential contribution of ECL2 (in addition to the previously identified residues in TMs III-VII) to the SRIF ligand binding pocket.

Irreversible activation of the gonadotropin-releasing hormone receptor by photoaffinity cross-linking: localization of attachment site to Cys residue in N-terminal segment

Photoaffinity cross-linking with [azidobenzoyl-d-Lys6]GnRH leads to irreversible activation of the gonadotropin-releasing hormone (GnRH) receptor. In order to localize the cross-linking site, the disulfide bridge structure was initially probed by mutagenesis. A consistent pattern of changes in the ability of GnRH to stimulate signal transduction after Ser substitutions of extracellularly located Cys residues indicated that Cys14 in the N-terminal domain is connected to Cys200 in the second extracellular loop, while Cys196 in this loop is connected to the highly conserved Cys114 at the extracellular end of transmembrane helix 3. Protein chemical analysis of radioactive fragments of cross-linked GnRH receptor following deglycosylation and enzymatic digest with endoproteinase Glu-C and trypsin before and after introduction or elimination of potential protease cleavage sites indicated that 125I[azidobenzoyl-d-Lys6]GnRH cross-links to a segment comprising residues 12-18 of the N-terminal domain. The existence of the Cys114-Cys196 bridge was directly confirmed as a labeled fragment, including that Cys114 was resolvable only under reducing conditions. The observation that the cross-linked N-terminal enzymatic fragments had identical apparent size under non-reducing conditions shows that the cross-linking reaction disconnected the disulfide bridge between Cys14 and Cys200 and indicates that Cys14 is probably the residue involved in cross-linking of the ligand. It is concluded that covalent tethering of GnRH through a photoreactive side chain located at position 6 in the middle of the peptide leads to continued activation of the receptor presumably through covalent binding to Cys14 in the N-terminal domain of the receptor.

Distinct efficacies for two endogenous ligands on a single cognate gonadoliberin receptor

A cDNA encoding a putative gonadoliberin receptor was cloned from the pituitary of the African catfish. Conceptual translation predicts a protein of 379 amino acids which shows typical characteristics of GTP-binding-protein-coupled receptors. The isolated cDNA was stable expressed in human embryonic kidney (HEK) 293 cells which were used for studies on gonadoliberin-activated second messenger systems (inositol phosphate production; increase in cAMP and/or intracellular Ca2+). The isolated cDNA encoded a functional receptor, designated catfish gonadoliberin receptor (cfGnRH-R), which had an amino acid sequence similarity of 38% with mammalian gonadoliberin receptors. In contrast to its mammalian counterparts which lack an intracellular carboxy-terminal domain, the cfGnRH-R contains an additional 49 amino acid residues. From the two endogenous gonadoliberins in African catfish, chicken gonadoliberin-II had a several hundredfold higher potency than catfish gonadoliberin to activate cfGnRH-R-associated second messenger systems in transfected HEK 293 cells. This is in line with the previously determined higher gonadotropin-release capacity of chicken gonadoliberin-II in catfish. Stimulation of second messenger systems with chicken gonadoliberin-II, but not with catfish gonadoliberin, resulted in a biphasic effect and chicken gonadoliberin-II led to a higher maximum stimulation than catfish gonadoliberin. Challenging cfGnRH-R simultaneously with chicken gonadoliberin-II and catfish gonadoliberin did not lead to additive effects. In contrast, two types of mutual inhibitory effects were recorded. These data indicate that a single cognate cfGnRH-R couples with distinct efficacies to signal transduction systems upon stimulation by the two endogenous gonadoliberins which, in addition, may interact negatively.

Asn102 of the gonadotropin-releasing hormone receptor is a critical determinant of potency for agonists containing C-terminal glycinamide

We demonstrate a critical role for Asn102 of the human gonadotropin-releasing hormone (GnRH) receptor in the binding of GnRH. Mutation of Asn102, located at the top of the second transmembrane helix, to Ala resulted in a 225-fold loss of potency for GnRH. Eight GnRH analogs, all containing glycinamide C termini like GnRH, showed similar losses of potency between 95- and 750-fold for the [Ala102]GnRHR, compared with wild-type receptor. In contrast, four GnRH analogs that had ethylamide in place of the C-terminal glycinamide residue, showed much smaller decreases in potency between 2.4- and 11-fold. In comparisons of three agonist pairs, differing only at the C terminus, glycinamide derivatives showed an 11-20-fold greater loss of potency for the mutant receptor than their respective ethylamide derivatives. Thus Asn102 is a critical determinant of potency specifically for ligands with C-terminal glycinamide, while ligands with C-terminal ethylamide are less dependent on Asn102. These findings indicate a role for Asn102 in the docking of the glycinamide C terminus and are consistent with hydrogen bonding of the Asn102 side chain with the C-terminal amide moiety. Taken with previous data, they suggest a region of the GnRH receptor formed by the top of helices 2 and 7 as a binding pocket for the C-terminal part of the ligand.

A refined model of the thyrotropin-releasing hormone (TRH) receptor binding pocket. Experimental analysis and energy minimization of the complex between TRH and TRH receptor

Seven transmembrane (TM) spanning, G protein-coupled receptors (GPCRs) appear to bind large glycoprotein hormones predominantly within their extracellular domains, small nonpeptidic ligands within the TM helical bundle, and peptide ligands within the extracellular domains and TM bundle. The tripeptide thyrotropin-releasing hormone (TRH, pyroGlu-His-ProNH2) may bind entirely within the TM bundle of the TRH receptor (TRH-R). We have previously demonstrated direct binding contacts between the pyroGlu of TRH and two residues in TM helix 3 (TM-3) of TRH-R and proposed a model of the binding pocket of TRH-R [Perlman, J. H., Laakkonen, L., Osman, R., & Gershengorn, M. C. (1994) J. Biol. Chem. 269, 23383-23386]. Here, we provide evidence for two additional direct interactions between TRH and TRH-R. One interaction is between the aromatic ring of Tyr 282 of TM-6 and His of TRH. This is based on a large increase in the half-maximally effective concentration (EC50) of TRH for stimulation of inositol phosphate formation by Y282A TRH-R and a loss of selectivity of this mutant receptor for TRH analogs substituted at His. We provide evidence for another interaction between Arg 306 of TM-7 and the terminal carboxamide of TRH. Using four direct interactions as anchors, a refined model of the TRH-R binding pocket was constructed using geometry optimization through energy minimization. A novel method for modeling GPCRs based on Monte Carlo and stochastic dynamics simulations is presented in the accompanying paper [Laakkonen, L. J., Guarnieri, F., Perlman, J. H., Gershengorn, M. C., & Osman, R. (1996) Biochemistry 35, 7651-7663].

Incorporation of an additional glycosylation site enhances expression of functional human gonadotropin-releasing hormone receptor

Mutation ofN-glycosylation sites in the mouse gonadotropin-releasing hormone receptor was previously shown to impair its expression in COS-1 cells. We therefore investigated the effects of adding an extra glycosylation site to the human gonadotropin-releasing hormone receptor, as a means for increasing its expression. Covalent labeling of the mutant receptor expressed in COS-1 cells with a gonadotropin-releasing hormone (GnRH) photoreactive analog demonstrated a shift in apparent molecular weight, indicating that the new site was in fact glycosylated. The receptor with extra glycosylation site displayed normal binding affinities for agonists buserelin and [D: -Ala(6)-Pro(9)-NHEt]-GnRH, and the antagonist antide, and a slightly increased affinity for GnRH. Receptor number was increased by 1.7-fold in membrane preparations from cells expressing the mutant receptor, compared with wild-type. Photoaffinity labeling of cell-surface receptors in intact cells demonstrated a 1.8-fold increase in binding sites on the cell surface. The GnRH receptor (GnRHR) with extra glycosylation site conferred a markedly enhanced signaling response to agonist. Dose-response curves for GnRH-stimulated inositol phosphate production were left-shifted by an average of 4.4-fold, and maximal inositol phosphate responses were increased by 1.2 fold, in cells transfected with mutant compared with wild-type receptor, indicating that the increase in binding sites represented functional receptors. These results demonstrate that addition of an extra glycosylation site enhances expression of the human GnRHR, a strategy that may be applicable to other cell-surface receptors.

Deletions of portions of the extracellular loops of the lutropin/choriogonadotropin receptor decrease the binding affinity for ovine luteinizing hormone, but not human choriogonadotropin, by preventing the formation of mature cell surface receptor

The rat lutropin/choriogonadotropin receptor (rLHR) is a G protein-coupled receptor which binds either human choriogonadotropin (hCG) or lutropin (luteinizing hormone, LH) and, therefore, plays a central role in reproductive physiology. In addition to the seven transmembrane helices, three extracellular loops, three intracellular loops, and a cytoplasmic tail characteristic of all G protein-coupled receptors, the rLHR also contains a relatively large N-terminal extracellular domain. Since high affinity hormone binding occurs to this N-terminal extracellular domain and since G proteins are activated by intracellular regions of the receptor, it has been hypothesized that upon hormone binding a portion of the hormone or the receptor's extracellular domain might interact with the receptor's extracellular loops and/or transmembrane helices, thus evoking an intracellular conformational change. To explore this possibility, we prepared and characterized several mutants of the rLHR in which portions of the extracellular loops were deleted. Ultimately, it was not possible to examine the signal transduction properties of the mutants because all but one mutant were retained intracellularly. Although the intracellularly retained mutants must be somewhat misfolded, all were found to bind hCG with high affinity if the cells were first solubilized in detergent. However, the binding of oLH to the detergent solubilized mutants was altered. Thus, whereas the wild-type rLHR bound oLH with two apparent affinities, the solubilized deletion mutants bound oLH with only one apparent affinity. Although these data could be interpreted to suggest that an ovine LH (oLH) binding site on the extracellular loops of the rLHR was deleted, data shown argue against this hypothesis. Rather, the results presented suggest that the two apparent affinities of the wild-type rLHR for oLH represent the binding affinities of two populations of rLHR where the mature, cell surface form binds oLH with a higher affinity than the immature, intracellular form. Furthermore, we show that mutations of the rLHR which cause intracellular retention of the receptor result in a decrease from two to one apparent binding sites for oLH due to the absence of the high affinity oLH binding component contributed by the mature cell surface receptor. Therefore, whereas hCG cannot discriminate between the mature cell surface wild-type receptor and an intracellularly retained rLHR mutant, oLH can make this discrimination, thus suggesting a conformational difference between the two forms of the receptor.

Signal transduction of the gonadotropin releasing hormone (GnRH) receptor: cross-talk of calcium, protein kinase C (PKC), and arachidonic acid

1. The decapeptide neurohormone gonadotropin releasing hormone (GnRH) is the first key hormone of the reproductive system. Produced in the hypothalamus, GnRH is released in a pulsatile manner into the hypophysial portal system to reach the anterior pituitary and stimulates the release and synthesis of the gonadotropin hormones LH and FSH. GnRH, a Ca2+ mobilizing ligand, binds to its respective binding protein, which is a member of the seven transmembrane domain receptor family and activates a G-protein (Gq). 2. The alpha subunit of Gq triggers enhanced phosphoinositide turnover and the elevation of multiple second messengers required for gonadotropin release and biosynthesis. 3. The messenger molecules IP3, diacylglycerol, Ca2+, protein kinase C, arachidonic acid and leukotriene C4 cross-talk in a complex networks of signaling, culminating in gonadotropin release and gene expression.

A locus of the gonadotropin-releasing hormone receptor that differentiates agonist and antagonist binding sites

The decapeptide gonadotropin-releasing hormone controls reproductive function via interaction with a heptahelical G protein-coupled receptor. Because of molecular model of the receptor predicts that Lys121 in the third transmembrane helix contributes to the binding pocket, the function of this side chain was studied by site-directed mutagenesis. Substitution of Arg at this position preserved high affinity agonist binding, whereas Gln at this position reduced binding below the limits of detection. Leu and Asp at this locus abolished both binding and detectable signal transduction. The EC50 of concentration-response curves for coupling to phosphatidyl inositol hydrolysis obtained with the Gln121 receptor was more than 3 orders of magnitude higher than that obtained for the wild-type receptor. In order to determine whether the increased EC50 obtained with this mutant reflects an altered receptor affinity, the effect of decreases in wild-type receptor density on concentration-response curves was determined by irreversible antagonism. Progressively decreasing the concentration of the wild-type receptor increased the EC50 values obtained to a maximal level of 2.4 +/- 0.2 nM. Comparison of this value with the EC50 of 282 +/- 52 nM observed with the Gln121 receptor mutant indicates that the agonist affinity for this mutant is reduced more than 100-fold. In contrast, antagonist had comparable high affinities for the wild-type, Arg121, and Gln121 mutants. The results indicate that a charge-strengthened hydrogen bond donor is required at this locus for high affinity agonist binding but not for high affinity antagonist binding.

Activation of membrane receptors

Many extracellular messengers interact with discriminate receptors on the cell surface. Some of bound ligands activate receptors whereas others fail to do so. Only activated receptors are capable of generating and transferring signal through the membrane. Recent advances in our understanding of agonist-induced and constitutive receptor activation suggest several molecular mechanisms for receptor activation, signal generation and transmembrane signal transfer.

Functional domains of the gonadotropin-releasing hormone receptor

1. The cloning of the mammalian gonadotropin-releasing hormone receptor sets the stage for rapid progress in understanding the structure of the receptor, its interaction with ligand, and its mechanisms of activation. 2. The receptor is a 327 to 328-amino acid seven-transmembrane domain G protein-coupled receptor. 3. Recent site-direct mutagenesis studies have provided considerable insight into glycosylation of the receptor, the arrangement of the helices, and the ligand binding domains.