Structure of the mouse gonadotropin-releasing hormone receptor gene: variant transcripts generated by alternative processing

Molecular cloning, sequencing, and distribution of feline GnRH receptor (GnRHR) and resequencing of canine GnRHR

GnRH receptors play vital roles in mammalian reproduction via regulation of gonadotropin secretion, which is essential for gametogenesis and production of gonadal steroids. GnRH receptors for more than 20 mammalian species have been sequenced, including human, mouse, and dog. This study reports the molecular cloning and sequencing of GnRH receptor (GnRHR) cDNA from the pituitary gland of the domestic cat, an important species in biomedical research. Feline GnRHR cDNA is composed of 981 nucleotides and encodes a 327 amino acid protein. Unlike the majority of mammalian species sequenced so far, but similar to canine GnRHR, feline GnRHR protein lacks asparagine in position three of the extracellular domain of the protein. At the amino acid level, feline GnRHR exhibits 95.1% identity with canine, 93.8% with human, and 88.9% with mouse GnRHR. Comparative sequence analysis of GnRHRs for multiple mammalian species led to resequencing of canine GnRHR, which differed from that previously published by a single base change that translates to a different amino acid in position 193. This single base change was confirmed in dogs of multiple breeds. Reverse transcriptase PCR analysis of GnRHR messenger RNA in different tissues from four normal cats indicated the presence of amplicons of varying lengths, including full-length as well as shortened GnRHR amplicons, pointing to the existence of truncated GnRHR transcripts in the domestic cat. This study is the first insight into molecular composition and expression of feline GnRHR and promotes better understanding of receptor organization, and distribution in various tissues of this species.

Gonadotropin-releasing hormone: GnRH receptor signaling in extrapituitary tissues

Gonadotropin-releasing hormone (GnRH) has historically been known as a pituitary hormone; however, in the past few years, interest has been raised in locally produced, extrapituitary GnRH. GnRH receptor (GnRHR) was found to be expressed in normal human reproductive tissues (e.g. breast, endometrium, ovary, and prostate) and tumors derived from these tissues. Numerous studies have provided evidence for a role of GnRH in cell proliferation. More recently, we and others have reported a novel role for GnRH in other aspects of tumor progression, such as metastasis and angiogenesis. The multiple actions of GnRH could be linked to the divergence of signaling pathways that are activated by GnRHR. Recent observations also demonstrate cross-talk between GnRHR and growth factor receptors. Intriguingly, the classical G(alphaq)-11-phospholipase C signal transduction pathway, known to function in pituitary gonadotropes, is not involved in GnRH actions at nonpituitary targets. Herein, we review the key findings on the role of GnRH in the control of tumor growth, progression, and dissemination. The emerging role of GnRHR in actin cytoskeleton remodeling (small Rho GTPases), expression and/or activity of adhesion molecules (integrins), proteolytic enzymes (matrix metalloproteinases) and angiogenic factors is explored. The signal transduction mechanisms of GnRHR in mediating these activities is described. Finally, we discuss how a common GnRHR may mediate different, even opposite, responses to GnRH in the same tissue/cell type and whether an additional receptor(s) for GnRH exists.

Human chorionic villous macrophages as a fetal biological shield from maternal chorionic gonadotropin

In addition to its most well characterized biological role in the rescue and maintenance of corpus luteum function, human chorionic gonadotropin (hCG) also stimulates the onset of fetal gonadal steroidogenesis. However, excess hCG is teratogenic to fetal gonadal tissues, and therefore hCG must be tightly regulated. Although there is an anatomical barrier between the fetal vessels and maternal blood, other mechanisms may regulate hCG levels. In the present study, we investigated whether human chorionic villous macrophages degraded maternal hCG. Isolated human macrophages incorporated and degraded hCG in a time-dependent manner. Human placental villous macrophages and phorbol myristate acetate (PMA)-treated THP-1 cells expressed the gene encoding an exon 9-deleted form of the luteinizing hormone/chorionic gonadotropin (LH/CG) receptor; expression of the full-length receptor was not determined. While both PMA-treated or untreated THP-1 cells could uptake hCG into their cytoplasms, hCG degradation and excretion of its byproducts only progressed in PMA-treated THP-1 cells. In conclusion, hCG internalization and degradation are different processes in macrophages that protect fetal gonadogenesis from excess hCG. The exon 9-deleted LH/CG receptor, but not the full-length receptor, is involved in the degradation of cytoplasmic hCG by organ-specific, dominant-negative interactions.

Identification of a novel pituitary-specific chicken gonadotropin-releasing hormone receptor and its splice variants

In all vertebrates, GnRH regulates gonadotropin secretion through binding to a specific receptor on the surface of pituitary gonadotropes. At least two forms of GnRH exist within a single species, and several corresponding GnRH receptors (GNRHRs) have been isolated with one form being pituitary specific. In chickens, only one type of widely expressed GNRHR has previously been identified. The objectives of this study were to isolate a chicken pituitary-specific GNRHR and to determine its expression pattern during a reproductive cycle. Using a combined strategy of PCR and rapid amplification of cDNA ends (RACE), a new GNRHR (chicken GNRHR2) and two splice variants were isolated in domestic fowl (Gallus gallus domesticus). Full-length GNRHR2 and one of its splice variant mRNAs were expressed exclusively in the pituitary, whereas mRNA of the other splice variant was expressed in most brain tissues examined. The deduced amino acid sequence of full-length chicken GNRHR2 reveals a seven transmembrane domain protein with 57%-65% homology to nonmammalian GNRHRs. Semiquantitative real-time PCR revealed that mRNA levels of full-length chicken GNRHR2 in the pituitary correlate with the reproductive status of birds, with maximum levels observed during the peak of lay and 4 wk postphotostimulation in females and males, respectively. Furthermore, GnRH stimulation of GH3 cells that were transiently transfected with cDNA that encodes chicken GNRHR2 resulted in a significant increase in inositol phosphate accumulation. In conclusion, we isolated a novel GNRHR and its splice variants in chickens, and spatial and temporal gene expression patterns suggest that this receptor plays an important role in the regulation of reproduction.

Regulation of expression of mammalian gonadotrophin-releasing hormone receptor genes

Gonadotrophin-releasing hormone (GnRH), acting via its cognate GnRH receptor (GnRHR), is the primary regulator of mammalian reproductive function, and hence GnRH analogues are extensively used in the treatment of hormone-dependent diseases, as well as for assisted reproductive techniques. In addition to its established endocrine role in gonadotrophin regulation in the pituitary, evidence is rapidly accumulating to support the expression and functional roles for two forms of GnRHR (GnRHR I and GnRHR II) in multiple and diverse extra-pituitary mammalian tissues and cells. These findings, together with findings indicating that mutations of the GnRHR are linked to the disease hypogonadotrophic hypogonadism and that GnRHRs play a direct role in neuronal migration and reproductive cancers, have presented new therapeutic targets and intensified research into the structure, function and mechanisms of regulation of expression of GnRHR genes. The present review focuses on the current knowledge on tissue-specific and hormonal regulation of transcription of mammalian GnRH receptor genes. Emerging insights, such as the discovery of diverse regulatory mechanisms in pituitary and extra-pituitary cell types, nonclassical mechanisms of steroid regulation, the use of composite elements for cell-specific expression, the increasing profile of hormones involved in regulation, the complexity of kinase pathways that target the GnRHR I gene, as well as species-differences, are highlighted. Although further research is necessary to understand the mechanisms of regulation of expression of GnRHR I and GnRHR II genes, the GnRHR is emerging as a potential target gene for facilitating cross-talk between neuroendocrine, immune and stress-response systems in multiple tissues via autocrine, paracrine and endocrine signalling.

Sequence analysis, endocrine regulation, and signal transduction of GnRH receptors in teleost fish

Three gonadotropin-releasing hormones (GnRHs) and three cognate receptors have been identified in vertebrates, with distinct distributions and functions. According to their sequences, the receptors can be grouped into distinct classes: types I, II, and III. One branch contains all type-I GnRH receptors (GnRH-R-I) from mammals and fish; another branch clusters mainly amphibian and human type-II GnRH receptors; and a third branch includes evolved fish, mainly perciform species, type-III GnRH receptors. Taken tilapia GnRH receptors as a model, the present study summarizes the information regarding the amino-acid residues assumed to be involved in the receptors' structure, binding, activation, and intracellular signal transduction, including arrangement of the disulfide bonds, glycosylation sites, coupling to G proteins, and protein kinase A or protein kinase C phosphorylation sites.

Sheep exhibit novel variations in the organization of the mammalian type II gonadotropin-releasing hormone receptor gene

Species-specific differences in genes encoding type II GnRH receptor indicate that a functional hepta-helical receptor is produced in monkeys but not in rodents, cows, chimpanzees, or humans. To further investigate the extent of evolutionary differences, we sequenced the type II GnRH receptor gene from wild-type Soay sheep. The gene was isolated by long-distance PCR using primers to PEX11beta and RBM8A genes known to flank type II GnRH receptor gene homologues. The gene spans 5.7-kb DNA and was sequenced after shot-gun subcloning. Its novel features include absence of a Pit-1 transcription factor binding site, a premature stop codon (TAG) in exon 1, an in-frame deletion of 51 bp (17 codons) in exon 2, and several nonconservative codon changes. Sheep breed variation in the gene was assessed using genomic DNA in PCR-restriction digest assays for the premature stop codon and in a PCR assay for the deletion. Both characteristics were present in all 15 breeds tested. Receptor gene expression was investigated using poly-A(+) RNA Northern analysis, RT-PCR, and in situ hybridization. An oligonucleotide probe to exon 1 revealed an alternative transcript in testis but not in pituitary gland. No transcripts in testis or pituitary were detectable using an exon 2-3 probe. All tissues examined including multiple brain areas and gonadotrope-enriched cell cultures were negative for type II GnRH receptor in RT-PCR. Testis and pituitary sections were negative with exon 1 riboprobes and exon 1 or 2-3 oligonucleotide probes in in situ hybridization. A hepta-helical type II GnRH receptor is therefore not expressed from this sheep gene.

Developmental expression and subcellular localization of wallaby gonadotropin-releasing hormone receptor and its splice variants

The developmental expression of gonadotropin-releasing hormone receptor (GnRH-R) and its splice variants was examined in the gonads of tammar wallaby pouch young in order to elucidate the functional role of GnRH-R in the developing testis and ovary. Wallaby GnRH-R, like eutherian GnRH-Rs, contains three exons and two introns. In the present study, the transcripts of two splice variants (GnRH-R Delta 1 and GnRH-R Delta 2) were cloned from the pituitary. GnRH-R Delta 1 contained a 291 bp deletion from nucleotide positions 232 to 522 within exon 1. This transcript appears to be distinctive in the wallaby and has not been reported in other species. GnRH-R Delta 2 contained a 220 bp deletion from nucleotide positions 523 to 742, corresponding to exon 2. We examined the subcellular localization of the wild type GnRH-R and its splice variants with confocal microscopy, showing that both the wild type receptor and the splice variants were membrane-associated molecules. The different pattern of expression of the wild type receptor and the variants transcripts found in adult and neonatal tissues suggests a specific developmental regulation of the GnRH-R Delta 2 transcript. In addition, the developmental expression of the GnRH-R and GnRH-R Delta 1 transcripts showed a possible association with key physiological events during gonadal development in the wallaby pouch young, suggesting that GnRH-R may be involved in the regulation of early development in the testis and ovary.

Direct binding of AP-1 (Fos/Jun) proteins to a SMAD binding element facilitates both gonadotropin-releasing hormone (GnRH)- and activin-mediated transcriptional activation of the mouse GnRH receptor gene

The response of pituitary gonadotropes to gonadotropin-releasing hormone (GnRH) correlates directly with the concentration of GnRH receptors (GnRHR) on the cell surface, which is mediated in part at the level of gene expression. Several factors are known to affect expression of the mouse GnRHR (mGnRHR) gene, including GnRH and activin. We have previously shown that activin augments GnRH-mediated transcriptional activation of mGnRHR gene, and that region -387/-308 appears to be necessary to mediate this effect. This region contains two overlapping cis-regulatory elements of interest: GnRHR activating sequence (GRAS) and a putative SMAD-binding element (SBE). This study investigates the role of these elements and their cognate transcription factors in transactivation of the mGnRHR gene. Transfection studies confirm the presence of GnRH- and activin-response elements within -387/-308 of mGnRHR gene promoter. Competition electrophoretic mobility shift assay experiments using -335/-312 as probe and alphaT3-1 nuclear extract or SMAD, Jun, and Fos proteins demonstrate direct binding of AP-1 (Fos/Jun) protein complexes to -327/-322 and SMAD proteins to -329/-328. Further transfection studies using mutant constructs of these cis-regulatory elements confirm that both are functionally important. These data define a novel cis-regulatory element comprised of an overlapping SBE and newly characterized non-consensus AP-1 binding sequence that integrates the stimulatory transcriptional effects of both GnRH and activin on the mGnRHR gene.

Two messenger RNA isoforms of the gonadotrophin-releasing hormone receptor, generated by alternative splicing and/or promoter usage, are differentially expressed in rainbow trout gonads during gametogenesis

The recent cloning of a gonadotrophin-releasing hormone receptor (GnRH-R) cDNA from rainbow trout showed that it contains several in-frame ATG codons, one of which, ATG2, corresponds to that found in other species. However, an upstream codon, ATG1, could give rise to a protein with a larger extracellular domain. Using S1 nuclease assay and a method combining primer extension and RACE-PCR, we characterized a second population of mRNA, termed mRNA-2, with a distinct 5'untranslated region and lacking ATG1. The genomic origin of the two mRNAs was determined by establishing the complete gene structure, which shows, for the first time in a vertebrate species that an alternative splicing and promoter usage generate two GnRH-R mRNA variants whose 5' extremities are encoded by two different exons. The analysis of the tissue distribution indicated that mRNA-2 presents a broader pattern of expression and is detected at higher levels than mRNA-1. Interestingly, it was found that those two mRNAs are differentially expressed in male and female gonads during gametogenesis. In particular, the variations of mRNA-1 levels parallel those of sGnRH expression during spermatogenesis, indicating that tissue-specific processing of the GnRH-R mRNA may underlie the effects of GnRH as a paracrine/autocrine regulator of gonadal functions.

Activin A augments GnRH-mediated transcriptional activation of the mouse GnRH receptor gene

The response of pituitary gonadotropes to GnRH correlates directly with the concentration of GnRH receptors (GnRHRs) on the cell surface, which is mediated in part at the level of GnRHR gene expression. We have previously localized GnRH responsiveness in the mouse GnRHR (mGnRHR) gene promoter to two elements: activating protein-1 and sequence underlying responsiveness to GnRH-1. This study was designed to investigate potential synergy between GnRH and activin A in transcriptional activation of the mGnRHR gene. In functional transfection studies using alphaT3-1 cells, GnRH agonist stimulation of the mGnRHR gene promoter (-765/+62) resulted in a 10.9-fold increase in activity, which was further increased 2-fold (to 21.3-fold) following activin pretreatment. Activin pretreatment alone had no effect. Deletion of region -387/-308 or mutation of a putative SMAD-binding element at -331/-324 (5'-GTCTAG[T]C-3') abrogated the augmented response to GnRH in the presence of activin but not the response to GnRH alone. Overexpression of SMAD2 and SMAD3 along with SMAD4 increased transcriptional activity of the mGnRHR gene, which was further increased by GnRH agonist stimulation. These data demonstrate that activin augments GnRH-mediated transcriptional activation of the mGnRHR gene and suggest that this effect may be mediated through SMAD transcription factors.

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.

Pituitary follistatin and activin gene expression, and the testicular regulation of FSH in the adult Rhesus monkey (Macaca mulatta)

In rats, FSHbeta gene expression and FSH secretion are increased and decreased, respectively, by pituitary activin and follistatin. Because little information is available on the paracrine control of FSH secretion in the primate, follistatin and activin/inhibin beta(B) messenger RNA (mRNA) levels were measured in pituitaries of adult male rhesus monkeys 6 weeks after castration or sham surgery (n = 5/group). Follistatin mRNA was determined by quantitative RT-PCR assay using oligonucleotide primers designed to span exons 3-5 of the human follistatin gene. Activin/inhibin beta(B) mRNA levels were measured by ribonuclease protection. Orchidectomy resulted in a 100-fold increase in plasma FSH concentrations and a 60-fold rise in those of LH. In castrated monkeys, levels of mRNA encoding FSHbeta, LHbeta, alpha- subunit, and GnRH receptor (GnRH-R) were increased 21-, 2.1-, 1.7-, and 1.7-fold, respectively (P < 0.01). Levels of pituitary follistatin and activin/inhibin beta(B) mRNAs, however, were similar in castrated and intact animals. These data suggest that the paracrine control of FSH secretion in the male differs substantially in primates and rodents. Specifically, the relatively greater postcastration rise in FSHbeta gene expression and FSH secretion in the adult male monkey may result because in this species pituitary follistatin gene expression does not increase after orchidectomy, as it does in the rat.

Inherited disorders of GnRH and gonadotropin receptors

Gonadotropin and GnRH receptors belong to the family of G protein coupled receptors. Gain of function mutations have been described, yielding constitutively active receptors. In the case of the LH receptor these dominant mutations determine familial male limited precocious puberty. Somatic mutations of this receptor may in some cases provoke Leydig-cell adenomas. The constitutive LH receptor is not associated with female precocious puberty. Inactivating mutations are recessive. Alterations in the GnRH receptor determine hypogonadotropic hypogonadism. The clinical diagnosis of this etiology of hypogonadism is extremely difficult, especially in sporadic cases. Mutations of gonadotropin receptors determine primary amenorrhea in girls, whereas in boys they are responsible for Leydig cell aplasia or hypoplasia (LH receptor) or of a variable alteration of spermatogenesis (FSH receptor). Mutations provoking only partial alterations of receptor functions are relatively more frequent, than those inducing complete receptor inactivity. They provide interesting insights into the physiology of GnRH and gonadotropin action.

Gonadotropin-releasing hormone receptor in the teleost Haplochromis burtoni: structure, location, and function

GnRH acts via GnRH receptors (GnRH-R) in the pituitary to cause the release of gonadotropins that regulate vertebrate reproduction. In the teleost fish, Haplochromis burtoni, reproduction is socially regulated through the hypothalamus-pituitary-gonadal axis, making the pituitary GnRH-R a likely site of action for this control. As a first step toward understanding the role of GnRH-R in the social control of reproduction, we cloned and sequenced candidate GnRH-R complementary DNAs from H. burtoni tissue. We isolated a complementary DNA that predicts a peptide encoding a G protein-coupled receptor that shows highest overall identity to other fish type I GnRH-R (goldfish IA and IB and African catfish). Functional testing of the expressed protein in vitro confirmed high affinity binding of multiple forms of GNRH: Localization of GnRH-R messenger RNA using RT-PCR revealed that it is widely distributed in the brain and retina as well as elsewhere in the body. Taken together, these data suggest that this H. burtoni GnRH receptor probably interacts in vivo with all three forms of GNRH:

Cloning, tissue distribution, and central expression of the gonadotropin-releasing hormone receptor in the rainbow trout (Oncorhynchus mykiss)

A full-length cDNA encoding a GnRH receptor (GnRH-R) has been obtained from the brain of rainbow trout. This cDNA encodes a protein of 386 amino acids (aa) exhibiting the typical arrangement of the G-protein-coupled receptors in seven transmembrane domains. However, a second ATG could give rise to a receptor with a 30-aa longer extracellular domain. As already shown in other fish and Xenopus, this protein possesses an intracellular domain, in contrast with its mammalian counterparts. In the case of rainbow trout, this intracellular carboxy-terminal tail consists of 58 residues. Northern blotting experiments carried out in the brain, the pituitary, and the liver only resulted in a single band of 1.9-2 kilobases in the pituitary, although reverse transcription-polymerase chain reaction amplification products were found in the brain, the pituitary, the retina, and the ovary. In situ hybridization using a probe corresponding to the full-length coding region of the receptor was performed on vitellogenic or ovulating females and allowed to detect a weak but specific signal in the proximal pars distalis of the pituitary, the preoptic region, the mediobasal hypothalamus, and the optic tectum. However, the strongest signal was consistently detected in a mesencephalic structure, the nucleus lateralis valvulae, the significance of which is presently open to speculation.

Characterization of a pituitary GnRH-receptor from a perciform fish, Morone saxatilis: functional expression in a fish cell line

Gonadotropin-releasing hormones (GnRHs) bind to the specific receptor present on the gonadotrophs to activate the synthesis and release of gonadotropins (follicle stimulating hormone or FSH and luteinizing hormone or LH), which in turn control gonadal maturation, gametogenesis and gamete release. Perciform species have three endogenous GnRHs. The main objective of this study was to characterize the gonadotropin-releasing hormone receptor (GnRH-R) present in the pituitary of a perciform species, striped bass (Morone saxatilis) and demonstrate how it interacts with its potential ligand. In this study, a cDNA for GnRH-R from the pituitaries of striped bass was cloned. The cloned cDNA has an open reading frame (ORF) that codes for a 419 amino acids peptide. Like other G-protein coupled receptors including the non-mammalian GnRH-Rs, the peptide has seven putative transmembrane domains and a C-terminal tail. Comparative analysis of the amino acid sequence of striped bass (stb) GnRH-R shows 38-87% similarity with the known GnRH-Rs. A Northern blot analysis revealed a single GnRH-R transcript in the pituitary; however, its expression in various extrapituitary tissues was demonstrated by a reverse-transcription-PCR (RT-PCR). Functionally, upon induction by endogenous forms of GnRHs (seabream, chicken II and salmon GnRHs) and a mammalian GnRH-agonist, the recombinant stbGnRH-R mediated a reporter gene (luciferase) activity in a fish cell line (CHSE-214). A real-time relative quantitation method established that significantly higher (P<0.05) levels of stbGnRH-R mRNA were present in the pituitaries of striped bass with advanced stages of ovarian development, compared to the pituitaries of fish with less developed ovaries.

Molecular cloning and tissue-specific expression of a gonadotropin-releasing hormone receptor in the Japanese eel

Gonadotropin-releasing hormone (GnRH) is a key regulatory neuropeptide involved in the control of reproduction in vertebrates. In the Japanese eel, one of the most primitive teleost species, two molecular forms of GnRH, mammalian-type GnRH and chicken-II-type GnRH (cGnRH-II), have been identified. This study has isolated a full-length cDNA for a GnRH receptor from the pituitary of the eel. The 3233-bp cDNA encodes a 380-amino acid protein which contains seven hydrophobic transmembrane domains and N- and C-terminal regions. The exon/intron organization of the open reading frame of the eel GnRH receptor gene was also determined. The open reading frame consists of three exons and two introns. The exon-intron splice site is similar to that of the GnRH receptor genes of mammals reported so far. Expression of the eel GnRH receptor was detected in various parts of the brain, pituitary, eye, olfactory epithelium, and testis. This result suggests that GnRH has local functions in these tissues in addition to its actions on gonadotropin synthesis and release in the pituitary. This tissue-specific expression pattern is similar to that of the eel cGnRH-II. Furthermore, the present eel receptor shows very high amino acid identity with the catfish and goldfish GnRH receptors, which are highly selective for the cGnRH-II. These results suggest that the cGnRH-II acts through binding to the present receptor in the eel.

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.

Identification and characterization of the gonadotropin-releasing hormone response elements in the mouse gonadotropin-releasing hormone receptor gene

The response of the pituitary gonadotrope to gonadotropin-releasing hormone (GnRH) correlates directly with the concentration of GnRH receptors (GnRHR) on the cell surface, which is mediated in part at the level of GnRHR gene expression. Several hormones have been implicated in this regulation, most notably GnRH itself. Despite these observations and the central role that GnRH is known to play in reproductive development and function, the molecular mechanism(s) by which GnRH regulates transcription of the GnRHR gene has not been well elucidated. Previous studies in this laboratory have identified and partially characterized the promoter region of the mouse GnRHR gene and demonstrated that the regulatory elements for tissue-specific expression as well as for GnRH regulation are present within the 1.2-kilobase 5'-flanking sequence. By using deletion and mutational analysis as well as functional transfection studies in the murine gonadotrope-derived alphaT3-1 cell line, we have localized GnRH responsiveness of the mouse GnRHR gene to two DNA sequences at -276/-269 (designated Sequence Underlying Responsiveness to GnRH-2 (SURG-2), which contains the consensus sequence for the activating protein-1-binding site) and -292/-285 (a novel element designated SURG-1), and demonstrated that this response is mediated via protein kinase C. By using the electrophoretic mobility shift assay, we further demonstrate that a member(s) of the Fos/Jun heterodimer superfamily is responsible in part for the DNA-protein complexes formed on SURG-2, using alphaT3-1 nuclear extracts. These data define a bipartite GnRH response element in the mouse GnRHR 5'-flanking sequence and suggest that the activating protein-1 complex plays a central role in conferring GnRH responsiveness to the murine GnRHR gene.

Molecular cloning, genomic organization and developmental regulation of a novel receptor from Drosophila melanogaster structurally related to gonadotropin-releasing hormone receptors for vertebrates

After screening the data base of the Berkeley Drosophila Genome Project with a sequence coding for the transmembrane region of a G protein-coupled receptor, we found that Drosophila might contain a gene coding for a receptor that is structurally related to the Gonadotropin-Releasing Hormone (GnRH) receptors from vertebrates. Using the polymerase chain reaction, with Drosophila cDNA as a template, and oligonucleotide probes coding for the presumed exons of this gene, we were able to clone the cDNA coding for this receptor. The transmembrane region of the receptor shows 36% amino acid residue identity with the transmembrane region of the catfish and 31% amino acid residue identity with that of the rat GnRH receptor. The Drosophila receptor gene contains six introns, whereas the rat gene contains two: one intron in the Drosophila gene occurs at exactly the same position and has the same intron phasing as one intron in the rat gene, suggesting that the Drosophila and mammalian GnRH receptor genes are evolutionarily related. Northern blot analyses show that the Drosophila receptor gene is progressively expressed during larval development with a prominent maximum at the 3rd instar larval stage. Pupae contain low amounts of receptor mRNA, while adult flies contain higher levels, with males having about five times more receptor mRNA than females flies. Southern blot analyses show that Drosophila contains only one copy of the receptor gene, which is located at position 27A2-B1 of chromosome 2. This paper is the first report on the molecular cloning of a member of the GnRH receptor family from invertebrates.

LIM protein KyoT2 negatively regulates transcription by association with the RBP-J DNA-binding protein

The RBP-J/Su(H) DNA-binding protein plays a key role in transcriptional regulation by targeting Epstein-Barr virus nuclear antigen 2 (EBNA2) and the intracellular portions of Notch receptors to specific promoters. Using the yeast two-hybrid system, we isolated a LIM-only protein, KyoT, which physically interacts with RBP-J. Differential splicing gave rise to two transcripts of the KyoT gene, KyoT1 and KyoT2, that encoded proteins with four and two LIM domains, respectively. With differential splicing resulting in deletion of an exon, KyoT2 lacked two LIM domains from the C terminus and had a frameshift in the last exon, creating the RBP-J-binding region in the C terminus. KyoT1 had a negligible level of interaction with RBP-J. Strong expression of KyoT mRNAs was detected in skeletal muscle and lung, with a predominance of KyoT1 mRNA. When expressed in F9 embryonal carcinoma cells, KyoT1 and KyoT2 were localized in the cytoplasm and the nucleus, respectively. The binding site of KyoT2 on RBP-J overlaps those of EBNA2 and Notchl but is distinct from that of Hairless, the negative regulator of RBP-J-mediated transcription in Drosophila. KyoT2 but not KyoT1 repressed the RBP-J-mediated transcriptional activation by EBNA2 and Notch1 by competing with them for binding to RBP-J and by dislocating RBP-J from DNA. KyoT2 is a novel negative regulatory molecule for RBP-J-mediated transcription in mammalian systems.

Mutation analysis of the gonadotropin-releasing hormone receptor gene in idiopathic hypogonadotropic hypogonadism

OBJECTIVE

To determine if GnRH receptor mutations occur in patients with idiopathic hypogonadotropic hypogonadism.

DESIGN

Patients and controls were studied by molecular genetic analysis.

SETTING

A tertiary medical center setting.

PATIENT(S)

Twenty-four patients with idiopathic hypogonadotropic hypogonadism and 20 controls.

INTERVENTION(S)

Deoxyribonucleic acid from all individuals was analyzed by Southern blot analysis and denaturing gradient gel electrophoresis. Genomic DNA was digested with restriction enzymes, and Southern blots and denaturing gradient gel blots were constructed. Blots were hybridized with the GnRH receptor complementary DNA probe. The DNA sequencing was performed on samples from two representative patients.

MAIN OUTCOME MEASURE(S)

Gonadotropin-releasing hormone receptor gene structure was ascertained by comparing fragments from autoradiographs in patients and controls. Individual nucleotides were ascertained from DNA sequencing gels.

RESULT(S)

No GnRH receptor gene deletions or polymorphisms were identified by Southern blot analysis. New restriction-fragment melting polymorphisms using the enzymes DpnII, RsaI, and HaeIII were identified by denaturing gradient gel blots in patients and controls.

CONCLUSION(S)

Gonadotropin-releasing hormone receptor gene deletions or rearrangements were not observed in our idiopathic hypogonadotropic hypogonadism patients. Denaturing gradient gel electrophoresis failed to identify single-base differences unique to patients with idiopathic hypogonadotropic hypogonadism, dramatically reducing the likelihood that point mutations of the GnRH receptor gene are present in idiopathic hypogonadotropic hypogonadism.

Inhibition of gonadotropin-releasing hormone receptor signaling by expression of a splice variant of the human receptor

GnRH binds to a specific G protein-coupled receptor in the pituitary to regulate synthesis and secretion of gonadotropins. Using RT-PCR and human pituitary poly(A)+ RNA as a template, the full-length GnRH receptor (wild type) and a second truncated cDNA characterized by a 128-bp deletion between nucleotide positions 522 and 651 were cloned. The deletion causes a frame shift in the open reading frame, thus generating new coding sequence for further 75 amino acids. The truncated cDNA arises from alternative splicing by accepting a cryptic splicing acceptor site in exon 2. Distinct translation products of approximately 45-50 and 42 kDa were immunoprecipitated from COS-7 cells transfected with cDNA coding for wild type GnRH receptor and the truncated splice variant, respectively. Immunocytochemical and enzyme-linked immunosorbent assay studies revealed a membranous expression pattern for both receptor isoforms. Expression of the splice variant, however, occurred at a significantly lower cell surface receptor density. In terms of ligand binding and phospholipase C activation, the wild type receptor showed characteristics of a typical GnRH receptor, whereas the splice variant was incapable of ligand binding and signal transduction. Coexpression of wild type and truncated proteins in transiently or stably transfected cells, however, resulted in impaired signaling via the wild type receptor by reducing maximal agonist-induced inositol phosphate accumulation. The inhibitory effect depended on the amount of splice variant cDNA cotransfected and was specific for the GnRH receptor because signaling via other G(q/11)-coupled receptors, such as the thromboxane A2, M5 muscarinic, and V1 vasopressin receptors, was not affected. Immunological studies revealed that coexpression of the wild type receptor and the truncated splice variant resulted in impaired insertion of the wild type receptor into the plasma membrane. Thus, expression of truncated receptor proteins may highlight a novel principle of specific functional inhibition of G protein-coupled receptors.

Novel independent and synergistic regulation of gonadotropin-alpha subunit gene by luteinizing hormone/human choriogonadotropin and gonadotropin releasing hormone in the alphaT3-1 gonadotrope cells

The alphaT3-1 cells are immortalized anterior pituitary gonadotropes which express gonadotropin-alpha subunit gene. These cells contain receptors for gonadotropin releasing hormone (GnRH) as well as for luteinizing hormone (LH) which can also bind human choriogonadotropin (hCG). Like GnRH, LH and hCG can upregulate the expression of gonadotropin-alpha subunit gene. While 0.1-1.0 ng/ml hCG can upregulate, higher concentrations have no effect. However, these higher hCG concentrations can act in a synergistic manner with GnRH to increase the steady state mRNA and protein levels of gonadotropin-alpha subunit. The synergism between hCG and GnRH was mimicked by LH but not by other hormones in the glycoprotein hormone family or alpha or beta subunits of hCG, suggesting that the synergism is hormone specific and requires the conformation of native hormone. The hCG induced increase in gonadotropin-alpha subunit mRNA levels was due to a significant increase in the half-life of gonadotropin-alpha subunit transcripts from 7.8 +/- 1.0 h in the controls to 16.5 +/- 3.8 h after treatment with hCG. The GnRH induced increase in gonadotropin-alpha subunit mRNA levels was due to both a significant increase in the half-life to 26.2 +/- 3.0 h as well as a significant increase in the transcription rate of the gene (159.0 +/- 7.7% of the control). A greater increase in gonadotropin-alpha subunit mRNA levels following a combined treatment with GnRH and hCG was due to a further increase in half-life to 37.6 +/- 3.1 h as well as a greater increase in the transcription rate of the gene (295.1 +/- 24.2% of the control) as compared to the treatment with GnRH alone. In summary, we conclude that LH and hCG can independently and synergistically act with GnRH to increase the expression of gonadotropin-alpha subunit gene by transcriptional as well as by post-transcriptional mechanisms in alphaT3-1 cells. These effects may be important for the increase of LH levels during the preovulatory surge.

Structural organization and characterization of the promoter region of the rat gonadotropin-releasing hormone receptor gene

The gene encoding the rat gonadotropin-releasing hormone (GnRH) receptor was isolated, and its structural organization and promoter region were characterized. The gene was found to consist of three exons that encode the receptor protein, and spanned about 20 kb. Of two genomic clones analyzed, one contained the 5'-untranslated region and the first exon, and the other contained the second and third exons. The sizes of the first, second, and third exons are 625, 217, and 1476 nt, respectively. The first intron is at least 12 kb in length and is located between nucleotides 522 and 523 of the cDNA reading frame, in the middle of the fourth transmembrane domain. The second intron is about 2.5 kb and is also located in the reading frame between nucleotides 739 and 740, separating the fifth and sixth transmembrane domains. Genomic blots in combination with cloning and sequencing suggested that a single GnRH receptor gene is present in the rat genome. Primer extension indicated that the transcription start site is located 103 nt upstream of the translational start codon. A putative TATA box is positioned 23 nt in front of the transcription initiation site. The 1.8 kb 5' flanking sequence contains an SF-1 site, an AP-1 site, CCAAT sequences, a Pit-1 binding site, and a potential CRE-like sequence. To evaluate promoter activity, the 1.8 kb and two 5' deleted fragments of 1.2 and 0.6 kb were fused to the luciferase reporter gene and transiently expressed in immortalized pituitary gonadotrophs (alphaT3-1 cells) and hypothalamic neurons (GT1-7 cells), and in nonpituitary (COS-7) cells. Luciferase gene expression was significantly increased by all three fragments in pituitary and hypothalamic cells, but not in COS-7 cells. The promoter activity of the 1.2 kb fragment was higher than that of the other fragments. Forskolin and cAMP analogs increased luciferase gene expression in both alphaT3-1 and GT1-7 cells, but activation of protein kinase C by phorbol myristate acetate had no effect. These studies indicate that positive and negative regulatory elements are present within the 1.8 kb 5' flanking sequence of the GnRH receptor. Knowledge of the genomic organization and analysis of the promoter region of the rat GnRH receptor gene will facilitate the elucidation of its transcriptional control in pituitary gonadotrophs and hypothalamic neurons.

Epitope-tagged gonadotropin-releasing hormone receptors heterologously-expressed in mammalian (COS-1) and insect (Sf9) cells

The molecular cloning and nucleotide sequencing of the gonadotropin-releasing hormone (GnRH) receptor represented an enhanced step in the experimental effort to understand this key molecule in the reproductive process at a cell and molecular level. A subsequent step in this broad effort is heterologous expression of the receptor in model cell systems for studies of signal transduction and desensitization, processes that may require immunologic detection of the receptor. Therefore, the GnRH receptor was tagged at its N-terminus using recombinant DNA procedures with the HA-1 epitope that is bound by a monoclonal antibody (12CA5). COS-1 cells expressing this receptor bound [(125)I]D-Ala6-desGly10-GnRH ethylamide (GnRH-A) with the expected high affinity (IC(50) = 0.47 nM), and were immunocytochemically stained by the 12CA5 antibody. Signal transduction was demonstrated by GnRH-induced [(3)H]inositol phosphate accumulation in receptor-expressing COS-1 cells. Western blotting of COS-1 cell membranes expressing the receptor revealed protein bands at 67, 57, and 32 kDa. Immunoprecipitation occurred when the solubilized receptor from COS-1 cell membranes was reacted with 12CA5 antibody and anti-mouse IgG Sepharose, and the presence of the receptor demonstrated either by its binding of [(125)I]GnRH-A or by its detection on Western blots. Desensitization of inositol 1,4,5-trisphosphate (IP(3)) production by N-epitope-tagged GnRH receptor expressing COS-1 cells was evoked by a five min GnRH pretreatment; [(32)P]i labeling of such cells during desensitization followed by immunoprecipitation of the N-epitope-tagged receptor was not associated with receptor phosphorylation. Finally, the epitope tagged receptor was expressed in the high-yield baculovirus/insect Sf9 cell system: the membrane receptor bound [(125)I]GnRH-A with slightly lowered affinity (IC(50) = 1.4 nM), and in Western blots yielded protein bands of 32, 56/57, 69, and 120/140 kDa. The development and validation of these heterologous systems will permit the study of several GnRH receptor-mediated processes that are poorly understood.

The gene encoding the ovine gonadotropin-releasing hormone (GnRH) receptor: cloning and initial characterization

We have isolated four lambda clones, which, in their aggregate, contain the entire coding sequence of the ovine gene encoding the gonadotropin-releasing hormone (GnRH) receptor (GnRHR). Like its human and murine counterparts, ovine GnRHR exists as a single-copy gene and is comprised of three exons and two introns. Furthermore, the locations of all exon-intron boundaries are perfectly conserved among the human, ovine and murine genes. The most striking difference among these genes is the location of the transcription start points (tsp) and, thus, the length of 5' untranslated region (UTR). This variation in size of the 5' UTR between the murine, human and ovine genes raises the possibility that different mechanisms have evolved for cell-specific expression of this gene. Isolation of the ovine GnRHR and its associated 5' flanking region is the essential first step in defining the molecular mechanisms underlying cell-specific and hormonal regulation of its expression in ruminants.

Cell-specific expression of the mouse gonadotropin-releasing hormone (GnRH) receptor gene is conferred by elements residing within 500 bp of proximal 5' flanking region

Gonadotropin-releasing hormone (GnRH) is a decapeptide produced by the hypothalamus. Upon binding to specific high-affinity receptors on gonadotrope cells of the anterior pituitary gland, GnRH stimulates the synthesis and secretion of LH. In light of the critical role of GnRH in reproduction much effort has been directed toward understanding the regulation of this hormone and its cognate receptor. The recent availability of genomic clones for the GnRH receptor has facilitated research to address the molecular mechanisms underlying regulation of GnRH receptor gene expression. We have expanded the analysis of the promoter for the mouse GnRH receptor gene and report that in addition to transcriptional start sites located within 100 bp of the translation start codon there is a more distal transcriptional start site approximately 200 bp 5' of the initiation codon. The initiation of transcription from this more distal site was sufficient to confer cell-specific expression on luciferase. Further, transient expression assays of constructs containing progressive 5' deletions in the GnRH receptor gene promoter reveal the presence of one or morecis-acting elements located between -500 and -400 (relative to ATG) necessary for transcriptional activity in the gonadotrope-derived αT3 cell line. Finally, αT3 but not COS-7 cell nuclear extract contained protein(s) that bind to at least two separate motifs contained within the -500 to -400 region. We suggest that activation of GnRH receptor gene expression in the αT3 cell line requires the binding of at least two transcriptional regulatory proteins to basal enhancer elements located within a 100 bp region between -500 to -400 relative to the translation start codon in the mouse GnRH receptor gene.

The human gonadotropin-releasing hormone receptor gene: complete structure including multiple promoters, transcription initiation sites, and polyadenylation signals

The interaction of gonadotropin-releasing hormone and its receptor is a critical event in the endocrine regulation of reproduction. We have recently cloned the gene encoding for the human gonadotropin-releasing hormone receptor (hGnRHR). Partial sequence analysis revealed a structural organization consisting of three exons and two introns. Exon II contains only 219 bp and the remainder of the approximately 5 kb transcript is distributed between exons I and III. The complete coding region for the hGnRHR represented only 987 bp leaving an extensive 5' and 3' non-translated region and potentially additional exons unaccounted for. This report provides the complete sequence of exon I and III and demonstrates that further exons are unlikely to be contained within this gene. Sequencing of the 5' end of the gene revealed the presence of five consensus TATA sequences distributed within a 700 nucleotide region. Primer extension analysis detected multiple transcription initiation sites associated with this cluster of TATA sequences. Transcription of this region up to the most 5' initiation site was demonstrated by the reverse transcription-polymerase chain reaction (RT-PCR) method. The 5' non-translated region stretches between 703 and 1393 bp, depending on which initiation site is used. Several consensus cis-acting regulatory sequences were identified within the 5' end. These include, among others, sites for PEA-3, AP-1, and Pit-1. In addition, cAMP response element (CRE)-like and glucocorticoid/progesterone response element (GRE/PRE)-like sequences were found.(ABSTRACT TRUNCATED AT 250 WORDS)

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.