Identification of three putative GnRH receptor subtypes in vertebrates

Gonadotropin-Releasing Hormone and GnRH Receptor: Structure, Function and Drug Development

BACKGROUND

Gonadotropin-Releasing Hormone (GnRH) is a key element in sexual maturation and regulation of the reproductive cycle in the human organism. GnRH interacts with the pituitary cells through the activation of the Gonadotropin Releasing Hormone Receptors (GnRHR). Any impairments/dysfunctions of the GnRH-GnRHR complex lead to the development of various cancer types and disorders. Furthermore, the identification of GnRHR as a potential drug target has led to the development of agonist and antagonist molecules implemented in various treatment protocols. The development of these drugs was based on the information derived from the functional studies of GnRH and GnRHR.

OBJECTIVE

This review aims at shedding light on the versatile function of GnRH and GnRH receptor and offers an apprehensive summary regarding the development of different agonists, antagonists and non-peptide GnRH analogues.

CONCLUSION

The information derived from these studies can enhance our understanding of the GnRH-GnRHR versatile nature and offer valuable insight into the design of new more potent molecules.

C-Src is Activated by the EGF Receptor in a Pathway that Mediates JNK and ERK Activation by Gonadotropin-Releasing Hormone in COS7 Cells

The key participants in G-protein-coupled receptor (GPCR) signaling are the mitogen-activated protein kinase (MAPK) signaling cascades. The mechanisms involved in the activation of the above cascades by GPCRs are not fully elucidated. The prototypical GPCR is the receptor for gonadotropin-releasing hormone (GnRHR), which serves as a key regulator of the reproductive system. Here, we expressed GnRHR in COS7 cells and found that GnRHR transmits its signals to MAPKs mainly via Gαi and the EGF receptor, without the involvement of Hb-EGF or PKCs. The main pathway that leads to JNK activation downstream of the EGF receptor involves a sequential activation of c-Src and PI3K. ERK activation by GnRHR is mediated by the EGF receptor, which activates Ras either directly or via c-Src. Beside the main pathway, the dissociated Gβγ and β-arrestin may initiate additional (albeit minor) pathways that lead to MAPK activation in the transfected COS7 cells. The pathways detected are significantly different from those in other GnRHR-bearing cells, indicating that GnRH can utilize various signaling mechanisms for MAPK activation. The unique pathway elucidated here, in which c-Src and PI3K are sequentially activated downstream of the EGF receptor, may serve as a prototype of signaling mechanisms by GnRHR and additional GPCRs in various cell types.

Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications

In humans, as in the other mammals, the neuroendocrine control of reproduction is ensured by the brain-pituitary gonadotropic axis. Multiple internal and environmental cues are integrated via brain neuronal networks, ultimately leading to the modulation of the activity of gonadotropin-releasing hormone (GnRH) neurons. The decapeptide GnRH is released into the hypothalamic-hypophysial portal blood system and stimulates the production of pituitary glycoprotein hormones, the two gonadotropins luteinizing hormone and follicle-stimulating hormone. A novel actor, the neuropeptide kisspeptin, acting upstream of GnRH, has attracted increasing attention in recent years. Other neuropeptides, such as gonadotropin-inhibiting hormone/RF-amide related peptide, and other members of the RF-amide peptide superfamily, as well as various nonpeptidic neuromediators such as dopamine and serotonin also provide a large panel of stimulatory or inhibitory regulators. This paper addresses the origin and evolution of the vertebrate gonadotropic axis. Brain-pituitary neuroendocrine axes are typical of vertebrates, the pituitary gland, mediator and amplifier of brain control on peripheral organs, being a vertebrate innovation. The paper reviews, from molecular and functional perspectives, the evolution across vertebrate radiation of some key actors of the vertebrate neuroendocrine control of reproduction and traces back their origin along the vertebrate lineage and in other metazoa before the emergence of vertebrates. A focus is given on how gene duplications, resulting from either local events or from whole genome duplication events, and followed by paralogous gene loss or conservation, might have shaped the evolutionary scenarios of current families of key actors of the gonadotropic axis.

Polymorphisms in GnRHI and GnRHII genes and their association with egg production and egg quality traits in chicken

1. Two candidate genes, namely, Gonadotropin releasing hormone I (GnRHI) and Gonadotropin releasing hormone II (GnRHII) play pivotal roles in ovulation and egg production in chicken. The objective of this study was to explore polymorphism in these genes and to estimate the effects of polymorphism of these two genes on egg production and egg quality traits in White Leghorn laying hens. 2. Single strand conformation polymorphism followed by sequencing was performed to detect polymorphism in these genes. 3. The coding regions of the GnRHI and GnRHII genes were found to be polymorphic. In the GnRH1 gene, 12 haplotypes were determined, of which the h1 haplotype was predominant and the h5, h9 and h11 haplotypes were the least frequent ones. In the GnRHII gene, eight haplotypes were found, of which the h1 haplotype was the most frequent and the h6 was the least frequent haplotype in the White Leghorn population. 4. The haplogroups of GnRHI had a significant effect on body weight and egg production up to 64 weeks of age, yolk content, Haugh units and egg shell parameters. The h1h2 haplogroup of the GnRHI gene showed the highest egg production, with 211.0 ± 24.3 eggs up to 64 weeks of age, while the highest yolk content and Haugh unit was found in h3h10 haplogrouped birds. The haplogroups of GnRHII had a significant effect on age at sexual maturity (ASM) where the shortest ASM was found in the h1h4 birds (147.3 ± 5.9 d) and the longest ASM was observed in the h1h3 birds (160.6 ± 23.4 d). 5. It was concluded that GnRHI and GnRHII genes are polymorphic and have a significant effect on body weight, egg production and egg quality traits in White Leghorn laying hens.

GnRH-A Key Regulator of FSH

The hypothalamic decapeptide, GnRH, is the gatekeeper of mammalian reproductive development and function. Activation of specific, high-affinity cell surface receptors (GnRH receptors) on gonadotropes by GnRH triggers signal transduction cascades to stimulate the coordinated synthesis and secretion of the pituitary gonadotropins FSH and LH. These hormones direct gonadal steroidogenesis and gametogenesis, making their tightly regulated production and secretion essential for normal sexual maturation and reproductive health. FSH and LH are glycoprotein heterodimers comprised of a common α-subunit and a unique β-subunit (FSHβ and LHβ, respectively), which determines the biological specificity of the gonadotropins. The unique β-subunit is the rate-limiting step for the production of the mature gonadotropins. Therefore, FSH synthesis is regulated at the transcriptional level by Fshb gene expression. The overarching goal of this review is to expand our understanding of the mechanisms and pathways underlying the carefully orchestrated control of FSH synthesis and secretion by GnRH, focusing on the transcriptional regulation of the Fshb gene. Identification of these regulatory mechanisms is not only fundamental to our understanding of normal reproductive function but will also provide a context for the elucidation of the pathophysiology of reproductive disorders and infertility to lead to potential new therapeutic approaches.

Molecular cloning and characterization of a gonadotropin-releasing hormone receptor homolog in the Chinese mitten crab, Eriocheir sinensis

As an essential mediator in the Gonadotropin-releasing hormone (GnRH) signaling pathway, GnRH receptor (GnRHR) coupled to GnRH, plays an important role in activating the downstream pathway after stimulating a series of cascades to regulate reproduction. To detect the existence of GnRHR and potential GnRH signaling pathway, we cloned and characterized GnRHR in the Chinese mitten crab, Eriocheir sinensis (named EsGnRHR). The full-length EsGnRHR cDNA is 2038 bp in length, including an open reading frame (ORF) of 1566 bp, a 57 bp 5'-untranslated region (5'-UTR) and a 415 bp 3'-UTR. Prediction of transmembrane domains in protein sequence revealed that the EsGnRHR protein contained seven hydrophobic transmembrane regions (TMs). Reverse transcription PCR revealed that EsGnRHR was mainly expressed in the thoracic nerve group and ovary, and weakly distributed in the testis and brain. In situ hybridization further demonstrated that EsGnRHR mRNA was localized at the protocerebrum and deutocerebrum. In the ovary and testis, the hybridization signal was dominantly at the earlier developmental stages. The signal was mainly localized in the cytoplasm cell in the ovary, and in the epithelium cell in the testis. During the different stages of gonadal development, EsGnRHR displayed increasing trends in both female and male when analyzed by quantitative real-time PCR, suggesting that EsGnRHR was involved in controlling gonadal development. Our study provides important information for further research on the molecular mechanisms underlying crab development.

Expression and Role of Gonadotropin-Releasing Hormone 2 and Its Receptor in Mammals

Gonadotropin-releasing hormone 1 (GnRH1) and its receptor (GnRHR1) drive mammalian reproduction via regulation of the gonadotropins. Yet, a second form of GnRH (GnRH2) and its receptor (GnRHR2) also exist in mammals. GnRH2 has been completely conserved throughout 500 million years of evolution, signifying high selection pressure and a critical biological role. However, the GnRH2 gene is absent (e.g., rat) or inactivated (e.g., cow and sheep) in some species but retained in others (e.g., human, horse, and pig). Likewise, many species (e.g., human, chimpanzee, cow, and sheep) retain the GnRHR2 gene but lack the appropriate coding sequence to produce a full-length protein due to gene coding errors; although production of GnRHR2 in humans remains controversial. Certain mammals lack the GnRHR2 gene (e.g., mouse) or most exons entirely (e.g., rat). In contrast, old world monkeys, musk shrews, and pigs maintain the coding sequence required to produce a functional GnRHR2. Like GnRHR1, GnRHR2 is a 7-transmembrane, G protein-coupled receptor that interacts with G_αq/11 to mediate cell signaling. However, GnRHR2 retains a cytoplasmic tail and is only 40% homologous to GnRHR1. A role for GnRH2 and its receptor in mammals has been elusive, likely because common laboratory models lack both the ligand and receptor. Uniquely, both GnRH2 and GnRHR2 are ubiquitously expressed; transcript levels are abundant in peripheral tissues and scarcely found in regions of the brain associated with gonadotropin secretion, suggesting a divergent role from GnRH1/GnRHR1. Indeed, GnRH2 and its receptor are not physiological modulators of gonadotropin secretion in mammals. Instead, GnRH2 and GnRHR2 coordinate the interaction between nutritional status and sexual behavior in the female brain. Within peripheral tissues, GnRH2 and its receptor are novel regulators of reproductive organs. GnRH2 and GnRHR2 directly stimulate steroidogenesis within the porcine testis. In the female, GnRH2 and its receptor may help mediate placental function, implantation, and ovarian steroidogenesis. Furthermore, both the GnRH2 and GnRHR2 genes are expressed in human reproductive tumors and represent emerging targets for cancer treatment. Thus, GnRH2 and GnRHR2 have diverse functions in mammals which remain largely unexplored.

Ancient origins of metazoan gonadotropin-releasing hormone and their receptors revealed by phylogenomic analyses

The discovery of genes related to gonadotropin-releasing hormones (GnRH) and their receptors from diverse species has driven important advances in comparative endocrinology. However, our view of the evolutionary histories and nomenclature of these gene families has become inconsistent as several different iterations of GnRH and receptor relationships have been proposed. Whole genome sequence data are now available for most of the major lineages of animals, and an exhaustive view of the phylogenies of GnRH and their receptors is now possible. In this paper, we leverage data from publically available whole genome sequences to present a new phylogenomic analysis of GnRH and GnRH receptors and the distant relatives of each across metazoan phylogeny. Our approach utilizes a phylogenomics pipeline that searches data from 36 whole genome sequences and conducts phylogenetic analyses of gene trees. We provide a comprehensive analysis of the major groupings of GnRH peptides, related hormones and their receptors and provide some suggestions for a new nomenclature. Our study provides a framework for understanding the functional diversification of this family of neuromodulatory peptides and their receptors.

Dynamic evolution of the GnRH receptor gene family in vertebrates

BACKGROUND

Elucidating the mechanisms underlying coevolution of ligands and receptors is an important challenge in molecular evolutionary biology. Peptide hormones and their receptors are excellent models for such efforts, given the relative ease of examining evolutionary changes in genes encoding for both molecules. Most vertebrates possess multiple genes for both the decapeptide gonadotropin releasing hormone (GnRH) and for the GnRH receptor. The evolutionary history of the receptor family, including ancestral copy number and timing of duplications and deletions, has been the subject of controversy.

RESULTS

We report here for the first time sequences of three distinct GnRH receptor genes in salamanders (axolotls, Ambystoma mexicanum), which are orthologous to three GnRH receptors from ranid frogs. To understand the origin of these genes within the larger evolutionary context of the gene family, we performed phylogenetic analyses and probabilistic protein homology searches of GnRH receptor genes in vertebrates and their near relatives. Our analyses revealed four points that alter previous views about the evolution of the GnRH receptor gene family. First, the "mammalian" pituitary type GnRH receptor, which is the sole GnRH receptor in humans and previously presumed to be highly derived because it lacks the cytoplasmic C-terminal domain typical of most G-protein coupled receptors, is actually an ancient gene that originated in the common ancestor of jawed vertebrates (Gnathostomata). Second, unlike previous studies, we classify vertebrate GnRH receptors into five subfamilies. Third, the order of subfamily origins is the inverse of previous proposed models. Fourth, the number of GnRH receptor genes has been dynamic in vertebrates and their ancestors, with multiple duplications and losses.

CONCLUSION

Our results provide a novel evolutionary framework for generating hypotheses concerning the functional importance of structural characteristics of vertebrate GnRH receptors. We show that five subfamilies of vertebrate GnRH receptors evolved early in the vertebrate phylogeny, followed by several independent instances of gene loss. Chief among cases of gene loss are humans, best described as degenerate with respect to GnRH receptors because we retain only a single, ancient gene.

Functional significance of GnRH and kisspeptin, and their cognate receptors in teleost reproduction

Guanine nucleotide binding protein (G-protein)-coupled receptors (GPCRs) are eukaryotic transmembrane proteins found in all living organisms. Their versatility and roles in several physiological processes make them the single largest family of drug targets. Comparative genomic studies using various model organisms have provided useful information about target receptors. The similarity of the genetic makeup of teleosts to that of humans and other vertebrates aligns with the study of GPCRs. Gonadotropin-releasing hormone (GnRH) represents a critical step in the reproductive process through its cognate GnRH receptors (GnRHRs). Kisspeptin (Kiss1) and its cognate GPCR, GPR54 (=kisspeptin receptor, Kiss-R), have recently been identified as a critical signaling system in the control of reproduction. The Kiss1/Kiss-R system regulates GnRH release, which is vital to pubertal development and vertebrate reproduction. This review highlights the physiological role of kisspeptin-Kiss-R signaling in the reproductive neuroendocrine axis in teleosts through the modulation of GnRH release. Moreover, we also review the recent developments in GnRHR and Kiss-R with respect to their structural variants, signaling mechanisms, ligand interactions, and functional significance. Finally, we discuss the recent progress in identifying many teleost GnRH-GnRHR and kisspeptin-Kiss-R systems and consider their physiological significance in the control of reproduction.

The contribution of lower vertebrate animal models in human reproduction research

Many advances have been carried out on the estrogens, GnRH and endocannabinoid system that have impact in the reproductive field. Indeed, estrogens, the generally accepted female hormones, have performed an unsuspected role in male sexual functions thanks to studies on non-mammalian vertebrates. Similarly, these animal models have provided important contributions to the identification of several GnRH ligand and receptor variants and their possible involvement in sexual behavior and gonadal function regulation. Moreover, the use of non-mammalian animal models has contributed to a better comprehension about the endocannabinoid system action in several mammalian reproductive events. We wish to highlight here how non-mammalian vertebrate animal model research contributes to advancements with implications on human health as well as providing a phylogenetic perspective on the evolution of reproductive systems in vertebrates.

Differential co-localization with choline acetyltransferase in nervus terminalis suggests functional differences for GnRH isoforms in bonnethead sharks (Sphyrna tiburo)

The nervus terminalis (NT) is a vertebrate cranial nerve whose function in adults is unknown. In bonnethead sharks, the nerve is anatomically independent of the olfactory system, with two major cell populations within one or more ganglia along its exposed length. Most cells are immunoreactive for either gonadotropin-releasing hormone (GnRH) or RF-amide-like peptides. To define further the cell populations and connectivity, we used double-label immunocytochemistry with antisera to different isoforms of GnRH and to choline acetyltransferase (ChAT). The labeling patterns of two GnRH antisera revealed different populations of GnRH-immunoreactive (ir) cell profiles in the NT ganglion. One antiserum labeled a large group of cells and fibers, which likely contain mammalian GnRH (GnRH-I) as described in previous studies and which were ChAT immunoreactive. The other antiserum labeled large club-like structures, which were anuclear, and a sparse number of fibers, but with no clear labeling of cell bodies in the ganglion. These club structures were choline acetyltrasferase (ChAT)-negative, and preabsorption control tests suggest they may contain chicken-GnRH-II (GnRH-II) or dogfish GnRH. The second major NT ganglion cell-type was immunoreactive for RF-amides, which regulate GnRH release in other vertebrates, and may provide an intraganglionic influence on GnRH release. The immunocytochemical and anatomical differences between the two GnRH-immunoreactive profile types indicate possible functional differences for these isoforms in the NT. The club-like structures may be sites of GnRH release into the general circulation since these structures were observed near blood vessels and resembled structures seen in the median eminence of rats.

Changes in brain mRNA levels of gonadotropin-releasing hormone, pituitary adenylate cyclase activating polypeptide, and somatostatin during ovulatory luteinizing hormone and growth hormone surges in goldfish

In goldfish, circulating LH and growth hormone (GH) levels surge at the time of ovulation. In the present study, changes in gene expression of salmon gonadotropin-releasing hormone (sGnRH), chicken GnRH-II (cGnRH-II), somatostatin (SS) and pituitary adenylate cyclase activating polypeptide (PACAP) were analyzed during temperature- and spawning substrate-induced ovulation in goldfish. The results demonstrated that increases in PACAP gene expression during ovulation are best correlated with the GH secretion profile. These results suggest that PACAP, instead of GnRH, is involved in the control of GH secretion during ovulation. Increases of two of the SS transcripts during ovulation are interpreted as the activation of a negative feedback mechanism triggered by high GH levels. The results showed a differential regulation of sGnRH and cGnRH-II gene expression during ovulation, suggesting that sGnRH controls LH secretion, whereas cGnRH-II correlates best with spawning behavior. This conclusion is further supported by the finding that nonovulated fish induced to perform spawning behavior by prostaglandin F2α treatment increased cGnRH-II expression in both forebrain and midbrain, but decreased sGnRH expression in the forebrain.

Inhibitory effect of chicken gonadotropin-releasing hormone II on food intake in the goldfish, Carassius auratus

Gonadotropin-releasing hormone (GnRH) is an evolutionarily conserved neuropeptide with 10 amino acid residues, which possesses some structural variants. A molecular form known as chicken GnRH II ([His(5) Trp(7) Tyr(8)] GnRH, cGnRH II) is widely distributed in vertebrates, and has recently been implicated in the regulation of sexual behavior and food intake in an insectivore, the musk shrew. However, the influence of cGnRH II on feeding behavior has not yet been studied in model animals such as rodents and teleost fish. In this study, therefore, we investigated the role of cGnRH II in the regulation of feeding behavior in the goldfish, and examined its involvement in food intake after intracerebroventricular (ICV) administration. ICV-injected cGnRH II at graded doses, from 0.1 to 10 pmol/g body weight (BW), induced a decrease of food consumption in a dose-dependent manner during 60 min after treatment. Cumulative food intake was significantly decreased by ICV injection of cGnRH II at doses of 1 and 10 pmol/g BW during the 60-min post-treatment observation period. ICV injection of salmon GnRH ([Trp(7) Leu(8)] GnRH, sGnRH) at doses of 0.1-10 pmol/g BW did not affect food intake. The anorexigenic action of cGnRH II was completely blocked by treatment with the GnRH type I receptor antagonist, Antide. However, the anorexigenic action of cGnRH II was not inhibited by treatment with the corticotropin-releasing hormone (CRH) 1/2 receptor antagonist, *-helical CRH((9-41)), and the melanocortin 4 receptor antagonist, HS024. These results suggest that, in the goldfish, cGnRH II, but not sGnRH, acts as an anorexigenic factor, as is the case in the musk shrew, and that the anorexigenic action of cGnRH II is independent of CRH- and melanocortin-signaling pathways.

Structural and functional evolution of gonadotropin-releasing hormone in vertebrates

The neuropeptide gonadotropin-releasing hormone (GnRH) has a central role in the neural control of vertebrate reproduction. This review describes an overview of what is currently known about GnRH in vertebrates in the context of its structural and functional evolution. A large body of evidence has demonstrated the existence of three paralogous genes for GnRH (GnRH1, GnRH2 and GnRH3) in the vertebrate lineage. They are most probably the products of whole-genome duplications that occurred early in vertebrate evolution. Although GnRH3 has been identified only in teleosts, comparative genomic analyses indicated that GnRH3 has not arisen from a teleost-specific genome duplication, but has been derived from an earlier genome duplication in an ancestral vertebrate, followed by its loss in the tetrapod lineage. A loss of other paralogous genes has also occurred independently in different vertebrate lineages, leading to species-specific differences in the organization of the GnRH system. In addition to the GnRH3 gene, the GnRH2 gene has been deleted or silenced in certain mammalian species, while some teleosts seem to have lost the GnRH1 or GnRH3 gene. The duplicated GnRH genes have undergone subfunctionalization during the evolution of vertebrates; GnRH1 has become the major stimulator of gonadotropins and probably other pituitary hormones as well, whereas GnRH2 and GnRH3 would have functioned as neuromodulators, affecting reproductive behaviour. Conversely, in cases where a paralogous gene for GnRH has been lost, one of the remaining paralogues appears to have adopted its role.

Expression, structure, function, and evolution of gonadotropin-releasing hormone (GnRH) receptors GnRH-R1SHS and GnRH-R2PEY in the teleost, Astatotilapia burtoni

Multiple GnRH receptors are known to exist in nonmammalian species, but it is uncertain which receptor type regulates reproduction via the hypothalamic-pituitary-gonadal axis. The teleost fish, Astatotilapia burtoni, is useful for identifying the GnRH receptor responsible for reproduction, because only territorial males reproduce. We have cloned a second GnRH receptor in A. burtoni, GnRH-R1(SHS) (SHS is a peptide motif in extracellular loop 3), which is up-regulated in pituitaries of territorial males. We have shown that GnRH-R1(SHS) is expressed in many tissues and specifically colocalizes with LH in the pituitary. In A. burtoni brain, mRNA levels of both GnRH-R1(SHS) and a previously identified receptor, GnRH-R2(PEY), are highly correlated with mRNA levels of all three GnRH ligands. Despite its likely role in reproduction, we found that GnRH-R1(SHS) has the highest affinity for GnRH2 in vitro and low responsivity to GnRH1. Our phylogenetic analysis shows that GnRH-R1(SHS) is less closely related to mammalian reproductive GnRH receptors than GnRH-R2(PEY). We correlated vertebrate GnRH receptor amino acid sequences with receptor function and tissue distribution in many species and found that GnRH receptor sequences predict ligand responsiveness but not colocalization with pituitary gonadotropes. Based on sequence analysis, tissue localization, and physiological response we propose that the GnRH-R1(SHS) receptor controls reproduction in teleosts, including A. burtoni. We propose a GnRH receptor classification based on gene sequence that correlates with ligand selectivity but not with reproductive control. Our results suggest that different duplicated GnRH receptor genes have been selected to regulate reproduction in different vertebrate lineages.

Homologous desensitization and visualization of the tilapia GnRH type 3 receptor

Two types of gonadotropin-releasing hormone (GnRH) receptors were found in the pituitary of tilapia (t), named GnRHR type 3 (tGnRHR3) and GnRHR type 1, according to phylogenetic analysis. tGnRHR3 is highly expressed in the posterior part of the pituitary which contains LH and FSH cells. We characterized tGnRHR3 in terms of both LH release rate and receptor internalization rate in response to continuous exposure to GnRH. Constant exposure of tilapia pituitary fragments to salmon GnRH analog (sGnRHa) resulted in an increased secretion rate for 3h, followed by a gradual decline, taking 17-19h, to the basal secretion rate. A chimera between tGnRHR3 and green fluorescent protein (GFP) was created and used to observe the changes in receptor distribution and translocation, activated by agonist with time. The results suggested that the receptor is initially localized at the plasma membrane and upon activation by a homologous ligand (e.g. sGnRHa) undergoes relatively rapid endocytosis. In summary, the present work demonstrates that tGnRHR3 has already undergone endocytosis after 30min, while desensitization of LH release occurs only after 17-19h. It is concluded that for tGnRHR3, internalization of the receptor is not exclusively responsible for the desensitization of LH release.

Molecular cloning and gene expression of the gonadotropin-releasing hormone receptor in the orange-spotted grouper, Epinephelus coioides

The objective of this study was to investigate the molecular mechanisms of gonadotropin-releasing hormone receptor (GnRH-R) involved in the endocrine regulation of reproduction in the orange-spotted grouper, Epinephelus coioides. The full-length cDNA encoding GnRH-R type I was successfully cloned from the pituitary by reverse-transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA end (RACE) methods in the grouper. The complete GnRH-R type I cDNA is 1607 bp, which includes an open reading frame of 1092 bp encoding a protein of 364 amino acids, a seven-alpha helix transmembrane domain, a N-terminal extracellular domain, and a C-terminal cytoplasmic domain. The expression of GnRH-R type I was found to be highest in the pituitary. An intramuscular injection of various GnRH types in vivo was attempted. The expression of GnRH-R type I was stimulated by a single injection of salmon GnRH, while in the case of chicken GnRH II treatment, the expression of GnRH-R type I was inhibited. This suggests that the action of chick GnRH II is probably enhanced through the GnRH receptor of different forms. Furthermore, none of them were expressed by an injection of seabream GnRH, and this is likely attributed to the injection dose being below the threshold level, and this remains to be further examined. In conclusion, GnRHs of various types are effective in stimulating the expression of gonadotropins through various forms of the GnRH-R, and multiple forms of the receptor gene likely exist in teleosts.

Role of gonadotropin-releasing hormone (GnRH) in the regulation of gonadal differentiation in the gilthead seabream (Sparus aurata)

It has been proposed that gonadotropin-releasing hormone (GnRH) plays an autocrine/paracrine regulatory role in mammalian and fish ovaries. The marine teleost gilthead seabream is an interesting model since, during the life span of the fish, gonadal tissues develop first as testes, which then regress allowing the development of ovarian follicles. Recent studies carried out in ovaries of the gilthead seabream have demonstrated that various GnRH transcripts as well as GnRH splicing variants are expressed. The mRNA level of several GnRH forms in the female and male areas of the switching gonad, and their possible role in this process, were further investigated. The results here reported show that sGnRH, cGnRH-II, and sbGnRH transcripts are locally expressed during gilthead seabream gonadal differentiation; the expression of the three GnRH forms was found to differ among the morphologically defined areas of the switching gonad, as demonstrated by applying reverse transcription-polymerase chain reaction (RT-PCR), together with in situ hybridization, and semiquantitative PCR analyses. Moreover, the hypothesis that GnRH forms may regulate testicular regression via an apoptotic mechanism was investigated by analyzing the different areas of switching gonads for caspase-3 activity as a measure of apoptosis. Our results showed a marked increase of caspase-3 activity in the area corresponding to the regressing testes in which a significant decrease of testosterone production was also found. The present findings demonstrate that the changes in the endogenous GnRH transcripts could be related with the gonadal differentiation in gilthead seabream, and that exogenous GnRH plays a role by stimulating apoptosis in the degenerating testis.

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.

Evolution of GnRH ligands and receptors in gnathostomata

Gonadotropin-releasing hormone (GnRH) is the final common signaling molecule used by the brain to regulate reproduction in all vertebrates. Until now, a total of 24 GnRH structural variants have been characterized from vertebrate, protochordate and invertebrate nervous tissue. Almost all vertebrates already investigated have at least two GnRH forms coexisting in the central nervous system. Furthermore, it is now well accepted that three GnRH forms are present both in early and late evolved teleostean fishes. The number and taxonomic distribution of the different GnRH variants also raise questions about the phylogenetic relationships between them. Most of the GnRH phylogenetic analyses are in agreement with the widely accepted idea that the GnRH family can be divided into three main groups. However, the examination of the gnathostome GnRH phylogenetic relationships clearly shows the existence of two main paralogous GnRH lineages: the ''midbrain GnRH" group and the "forebrain GnRH" group. The first one, represented by chicken GnRH-II forms, and the second one composed of two paralogous lineages, the salmon GnRH cluster (only represented in teleostean fish species) and the hypophysotropic GnRH cluster, also present in tetrapods. This analysis suggests that the two forebrain clades share a common precursor and reinforces the idea that the salmon GnRH branch has originated from a duplication of the hypophysotropic lineage. GnRH ligands exert their activity through G protein-coupled receptors of the rhodopsin-like family. As with the ligands, multiple GnRHRs are expressed in individual vertebrate species and phylogenetic analyses have revealed that all vertebrate GnRHRs cluster into three main receptor types. However, new data and a new phylogenetic analysis propose a two GnRHR type model, in which different rounds of gene duplications may have occurred in different groups within each lineage.

Gonadotropin-releasing hormone in apoptosis of prostate cancer cells

GnRH and its analogs (GnRH-a) are used extensively for the treatment of prostate cancer and other hormone-dependent diseases via the desensitization of pituitary gonadotropes, which consequently leads to the inhibition of gonadotropins, gonadal steroids and tumor growth. The actions of GnRH-a are mediated by the GnRH receptor (GnRHR) that is expressed in both the pituitary and extrapituitary sites, including normal tissues and tumors. Several studies have provided evidence that besides its pituitary effects, GnRH-a may exert direct anti-proliferative and apoptotic effects in tumor cells. These effects are mediated by the GnRHRs via signal transduction mechanisms that are distinct from the classical pituitary mechanisms. Here we describe the direct effects of GnRH-a on prostate cancer and other types of cancer. Interestingly, androgen ablation by GnRH-a is the main treatment for hormone-dependent prostate cancer. However, most of these tumors become eventually hormone-refractory, and are no longer sensitive to the GnRH-a-mediated reduction in androgen levels. Hence, the ability of GnRH-a to induce direct effects such as apoptosis may have large implications regarding the clinical use of GnRH-a. Therefore, an understanding of the cellular mechanisms involved in GnRH-a action may lead to better therapeutic modalities for the treatment of advanced prostate cancer and other malignancies.

Gonadotropin-releasing hormone II stimulates female sexual behavior in marmoset monkeys

GnRH II (pGlu-His-Trp-Ser-Try-Gly-Leu-Arg-Pro-GlyNH2), an evolutionarily conserved member of the GnRH family, stimulates reproductive behavior in a number of vertebrates. To explore a role for GnRH II in regulating primate sexual behavior, eight adult female common marmosets, each fitted with an indwelling intracerebroventricular (icv) cannula, were ovariectomized, implanted subcutaneously with empty (n = 4) or estradiol-filled (n = 4) SILASTIC brand capsules, and pair housed with an adult male mate. After icv infusion of vehicle or peptides, females were placed in an observation cage for 90 min, out of visual contact with other marmosets, before the 30-min behavioral test with their male partner. Compared with vehicle, GnRH II (1 and 10 microg) increased the total number of proceptive (sexual solicitation) behaviors (tongue flicking, proceptive stares, and frozen postures) exhibited by females toward their pair mates and specifically increased the frequency of freeze postures. Effects were maximal at 1 microg and not dependent upon estradiol supplementation. GnRH II agonists/GnRH I antagonists 135-18 (1 microg) and 132-25 (1 microg), which stimulate inositol phosphate production via the marmoset type II receptor, increased the frequency of total proceptive behavior but did not specifically stimulate freeze-posture behavior. In contrast, GnRH I, at 1 mug, did not alter the frequency of proceptive behaviors. Female receptivity (female compliance with male sexual behavior) was not altered by any of the peptides tested. These findings implicate a role for GnRH II and the cognate GnRH type II receptor in stimulating female marmoset sexual behavior.

Pituitary gonadotropin-releasing hormone (GnRH) receptor: structure, distribution and regulation of expression

Reproduction in mammals is controlled by interactions between the hypothalamus, anterior pituitary and gonads. Interaction of GnRH with its cognate receptor is essential to regulating reproduction. Characterization of the structure, distribution and expression of GnRH receptors (GnRH-R) has furthered our understanding of the physiological consequences of GnRH stimulation of pituitary gonadotropes. Based on the putative topology of the amino acid sequence of the GnRH-R and point mutation studies, key elements of the GnRH-R have been identified to play a role in ligand recognition and binding, G-protein activation and internalization. Normally, reproductive function is mediated by GnRH-R expressed only on the membranes of pituitary gonadotropes. The density of GnRH-R on gonadotropes determines their ability to respond to GnRH. This density is highest just prior to ovulation and likely is important for complete expression of the pre-ovulatory surge of LH. Therefore, knowledge regarding what regulates the density of GnRH-R is essential to understanding changes in pituitary sensitivity to GnRH and ultimately, to expression of the LH surge. Regulation of GnRH-R gene expression is influenced by a multitude of factors including gonadal steroid hormones, inhibin, activin and perhaps most importantly GnRH itself.

GnRH in non-hypothalamic reproductive tissues

Gonadotropin releasing hormone (GnRH) is a hypothalamic neuronal secretory decapeptide that plays a pivotal role in mammalian reproduction. GnRH and its analogues are used extensively in the treatment of hormone dependent diseases and assisted reproductive technology. Fourteen structural variants and three different forms of GnRH, named as hypothalamic GnRH or GnRH-I, mid brain GnRH or GnRH-II and GnRH-III across various species of protochordates and vertebrates have been recognised. The hormone acts by binding to cell surface transmembrane G protein coupled receptors (GPCRs) and activates Gq/11 subfamily of G proteins. Although hypothalamus and pituitary are the principal source and target sites for GnRH, several reports have recently suggested extra-hypothalamic GnRH and GnRH receptors in various reproductive tissues such as ovaries, placenta, endometrium, oviducts, testes, prostrate, and mammary glands. GnRH-II appears to be predominantly expressed in extra pituitary reproductive tissues where it produces its effect by PLC, PKA2, PLD, and AC cell signalling pathways. In these tissues, GnRH is considered to act by autocrine or paracrine manner and regulate ovarian steroidogenesis by having stimulatory as well as inhibitory effect on the production of steroid hormones and apoptosis in ovarian follicle and corpus luteum. In male gonads, GnRH has been shown to cause a direct stimulatory effect on basal steroidogenesis and an inhibitory effect on gonadotropin-stimulated androgen biosynthesis. Recent studies have shown that GnRH is more abundantly present in ovarian, endometrial and prostrate carcinomas. The presence of type-II GnRH receptors in reproductive tissues (e.g. gonads, prostrate, endometrium, oviduct, placenta, and mammary glands) suggests existence of distinct role(s) for type-II GnRH molecule in these tissues. The existence of different GnRH forms indicates the presence of distinctive cognate receptors types in vertebrates and is a productive area of research and may contribute to the development of new generation of GnRH analogues with highly selective and controlled action on different reproductive tissues and the target-specific GnRH analogues could be developed.

Identification and molecular characterization of three GnRH ligands and five GnRH receptors in the spotted green pufferfish

Gonadotropin-releasing hormone (GnRH) is thought to have diverse physiological functions. Understanding regulatory mechanisms of GnRH functions requires detailed knowledge of gene expressions of both GnRH ligands and receptors in a single species. This report concerns identification and molecular characterization of GnRH ligands and receptors in the spotted green pufferfish Tetraodon nigroviridis. It was identified that the pufferfish possessed three types of GnRH ligands and five types of GnRH receptors. All types of ligands and receptors showed different expression patterns, and were widely expressed both inside and outside the brain. Gonads expressed all the ligand and receptor subtypes. Two of five receptor subtypes could not be detected in the pituitary gland of reproductively active individuals, suggesting the existence of novel GnRH systems independent of hypothalamic-pituitary-gonadal axis. Alternative splicing was also observed for some receptor subtypes. The present results indicate that diversified gene expressions combined with molecular diversity contribute to the functional diversity of GnRH.

GnRHs and GnRH receptors

GnRH is the pivotal hypothalamic hormone regulating reproduction. Over 20 forms of the decapeptide have been identified in which the NH2- and COOH-terminal sequences, which are essential for receptor binding and activation, are conserved. In mammals, there are two forms, GnRH I which regulates gonadotropin and GnRH II which appears to be a neuromodulator and stimulates sexual behaviour. GnRHs also occur in reproductive tissues and tumours in which a paracrine/autocrine role is postulated. GnRH agonists and antagonists are now extensively used to treat hormone-dependent diseases, in assisted conception and have promise as novel contraceptives. Non-peptide orally-active GnRH antagonists have been recently developed and may increase the flexibility and range of utility. As with GnRH, GnRH receptors have undergone co-ordinated gene duplications such that cognate receptor subtypes for respective ligands exist in most vertebrates. Interestingly, in man and some other mammals (e.g. chimp, sheep and bovine) the Type II GnRH receptor has been silenced. However, GnRH I and GnRH II still appear to have distinct roles in signalling differentially through the Type I receptor (ligand-selective-signalling) to have different downstream effects. The ligand-receptor interactions and receptor conformational changes involved in receptor activation have been partly delineated. Together, these findings are setting the scene for generating novel selective GnRH analogues with potential for wider and more specific application.

Cloning and characterization of a functional type II gonadotropin-releasing hormone receptor with a lengthy carboxy-terminal tail from an ancestral vertebrate, the sea lamprey

A full-length transcript encoding a functional type II GnRH receptor was cloned from the pituitary of the sea lamprey, Petromyzon marinus. The current study is the first to identify a pituitary GnRH receptor transcript in an agnathan, which is the oldest vertebrate lineage. The cloned receptor retains the conserved structural features and amino acid motifs of other known GnRH receptors and notably includes a C-terminal intracellular tail of approximately 120 amino acids, the longest C-terminal tail of any vertebrate GnRH receptor identified to date. The lamprey GnRH receptor was shown to activate the inositol phosphate (IP) signaling system; stimulation with either lamprey GnRH-I or lamprey GnRH-III led to dose-dependent responses in transiently transfected COS7 cells. Furthermore, analyses of serially truncated lamprey GnRH receptor mutants indicate perturbations of the C-terminal tail disrupts IP accumulation, however, the tailless lamprey GnRH receptor was not only functional but was also capable of stimulating IP levels equal to wild type. Expression of the receptor transcript was demonstrated in the pituitary and testes using RT-PCR, whereas in situ hybridization showed expression and localization of the transcript in the proximal pars distalis of the pituitary. The phylogenetic placement and structural and functional features of this GnRH receptor suggest that it is representative of an ancestral GnRH receptor. In addition to having an important role in lamprey reproductive processes, the extensive C-terminal tail of this lamprey GnRH receptor may have great significance for understanding the evolutionary change of this vital structural feature within the GnRH receptor family.

Season- and age-related variation in neural cGnRH-I and cGnRH-II immunoreactivity in house sparrows (Passer domesticus)

Vertebrates exhibit several forms of gonadotropin-releasing hormone (GnRH). In birds, chicken GnRH-I (cGnRH-I) is the primary hypophysiotropic form that regulates the hypothalamo-pituitary-gonad (HPG) axis. In seasonally breeding songbirds, cGnRH-I immunoreactivity is primarily in the preoptic septo-infidibular regions and varies greatly between the breeding and non-breeding season. Chicken GnRH-II (cGnRH-II) is more widely distributed throughout the avian brain, but cGnRH-II immunoreactivity has been found in the median eminence of quail. Thus, cGnRH-II may function in addition to cGnRH-I in the regulation of the HPG axis. If so, we predicted that cGnRH-II immunoreactivity should exhibit seasonal and age related variation similar to that observed for cGnRH-I. In this study, we compared hypothalamic immunoreactivity of cGnRH-I and -II (ir-cGnRH-I and ir-cGnRH-II) between breeding and non-breeding adult and juvenile house sparrows (Passer domesticus). We used immunocytochemistry with highly specific antibodies developed for quail to examine brains of free-living house sparrows collected in the breeding and non-breeding season. Results showed a greater number of ir-cGnRH-I cell bodies and beaded fibres in breeding adult birds compared to non-breeding adult and juvenile birds. Moreover, there were significantly more ir-cGnRH-II fibres in the preoptic area of breeding adult birds compared to non-breeding adult and juvenile birds. These data suggest that cGnRH-II undergoes seasonal- and age-related changes similar to cGnRH-I and implicate a role in regulating reproduction.

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.

GnRH and gpcr: laser-captured single cell gene profiling

We have developed a novel single cell real-time quantitative PCR technique, which incorporates harvesting marker-identified single cells using laser-capture. Here, for the first time in a vertebrate species, using this innovative single cell gene profiling technique, we report the presence of G-protein coupled receptors in individual gonadotropin-releasing hormone (GnRH) neurons and endocrine cells of the pituitary of the tilapia Oreochromis niloticus. The differential expression of multiple combinations of three GnRH receptor types (R1, R2 and R3) in individual gonadotropic and nongonadotropic cells demonstrates cellular and functional heterogeneity. The differential use of GnRH receptors in corticotropes, melanotropes and thyrotropes during gonadal maturation and reproductive behaviors suggests new roles for these hormones. Further, we provide evidence of the structure of a novel nonmammalian G-protein coupled receptor (GPR54) for kisspeptins, encoded by Kiss-1 gene, which is highly conserved during evolution and expressed in GnRH1, GnRH2 and GnRH3 neurons. We hypothesize GPR54 stimulates GnRH secretion and is crucial for pubertal maturation. We speculate, the use of this method will allow the identification and quantification of known and unknown genes in single cells, which would greatly facilitate our understanding of the complex interactions that govern the physiology of individual cells in vertebrates species.

Molecular biology of gonadotropin-releasing hormone (GnRH)-I, GnRH-II, and their receptors in humans

In human beings, two forms of GnRH, termed GnRH-I and GnRH-II, encoded by separate genes have been identified. Although these hormones share comparable cDNA and genomic structures, their tissue distribution and regulation of gene expression are significantly dissimilar. The actions of GnRH are mediated by the GnRH receptor, which belongs to a member of the rhodopsin-like G protein-coupled receptor superfamily. However, to date, only one conventional GnRH receptor subtype (type I GnRH receptor) uniquely lacking a carboxyl-terminal tail has been found in the human body. Studies on the transcriptional regulation of the human GnRH receptor gene have indicated that tissue-specific gene expression is mediated by differential promoter usage in various cell types. Functionally, there is growing evidence showing that both GnRH-I and GnRH-II are potentially important autocrine and/or paracrine regulators in some extrapituitary compartments. Recent cloning of a second GnRH receptor subtype (type II GnRH receptor) in nonhuman primates revealed that it is structurally and functionally distinct from the mammalian type I receptor. However, the human type II receptor gene homolog carries a frameshift and a premature stop codon, suggesting that a full-length type II receptor does not exist in humans.

Localization of the three GnRH types and GnRH receptors in the brain of a cichlid fish: Insights into their neuroendocrine and neuromodulator functions

The cognate receptor for any of the known gonadotropin-releasing hormones (GnRHs) has not been directly demonstrated. In order to establish this and shed light on the functions of GnRH types, we analyzed the neuroanatomical location and time of initial expression of three distinct GnRH receptors (GnRH-Rs) and the three endogenous GnRHs in the brain of developing and sexually mature tilapia Oreochromis niloticus using immunocytochemistry. In all age groups, including males and females, GnRH-RIA was seen specifically in gonadotropes (Parhar et al. [2002] J Neuroendocrinol 14:657-665) but was undetectable in the brain. On day 8 after fertilization, GnRH-RIB was first seen in the periventricular hypothalamus (lateral recess nucleus, posterior recess nucleus, posterior tuberal nucleus) and GnRH-RIII in the olfactory epithelium, olfactory bulb, telencephalon, preoptic region, mediobasal hypothalamus, thalamus, mesencephalon, and in the hindbrain. Double-label immunocytochemistry showed GnRH1 (Ser(8) GnRH)-immunoreactive neuronal processes projecting mainly to the proximal pars distalis of the pituitary, while GnRH2 (His(5), Trp(7), Tyr(8) GnRH) and GnRH3 (Trp(7), Leu(8) GnRH) fibers were observed in close association with cells containing GnRH-RIB and GnRH-RIII in the brain. These results suggest that GnRH-RIA might be hypophysiotropic in nature, whereas GnRH-RIB and GnRH-RIII could have additional neuromodulatory functions. Further, evidence of close proximity of GnRH-R-containing cells and neuronal processes of multiple GnRH types suggests complex cross-talk between several GnRH ligands and GnRH-Rs.

Cloning and Expression of Gonadotropin-Releasing Hormone Receptor in the Brain and Pituitary of the European Sea Bass: An In Situ Hybridization Study1

A full-length cDNA encoding a GnRH receptor (GnRH-R) has been obtained from the pituitary of the European sea bass, Dicentrarchus labrax. The complete cDNA is 1814 base pairs (bp) in length and encodes a protein of 416 amino acids. The 5' UTR and 3' UTR are 239 bp and 324 bp in size, respectively. The expression sites of this GnRH-R were studied in the brain and pituitary of sea bass by means of in situ hybridization. A quantitative analysis of the expression of the GnRH-R gene along the reproductive cycle was also performed. The GnRH-R brain expression was especially relevant in the ventral telencephalon and rostral preoptic area. Some GnRH-R messenger-expressing cells were also evident in the dorsal telencephalon, caudal preoptic area, ventral thalamus, and periventricular hypothalamus. A conspicuous and specific GnRH-R expression was detected in the pineal gland. The highest expression of the GnRH-R gene was observed in the proximal pars distalis of the pituitary. This expression was evident in all LH cells and some FSH cells but not in somatotrophs. In the pituitary, the quantitative analysis revealed a higher expression of GnRH-R gene during late vitellogenesis in comparison with maturation, spawning, and postspawning/resting periods. However, in the brain, the highest GnRH-R expression was evident at spawning or postspawning/ resting periods. These results suggest that the expression of this GnRH-R is regulated in a different manner in the brain and the pituitary of sea bass.

Pro7.33(303) of the human GnRH receptor regulates selective binding of mammalian GnRH

Mammalian gonadotropin releasing hormone (GnRH) receptors have a conserved acidic residue (Glu7.32(301) or Asp7.32(302)) in extracellular loop (ECL) three that confers selectivity for mammalian GnRH, which has Arg8. Comparison of mammalian and non-mammalian GnRH receptors suggested that the acidic residue is not the only determinant of ligand selectivity in mammalian receptors. The acidic residue is followed by a conserved Pro7.33 in mammalian GnRH receptors, but not non-mammalian receptors. Unique structural constraints imposed by Pro residues suggested that Pro7.33 determines selective binding of Arg8-containing GnRH, by stabilising the conformation of the third extracellular loop of the receptor. Substituting Pro7.33(303) or introducing Pro to position 7.31 decreased affinity for GnRH, but not analogs lacking Arg8. Substituting Pro7.33(303) changed the predicted alpha-helix content of the loop-helix interface. These results show that Pro7.33(303) of the human GnRH receptor is required for selective high affinity binding of mammalian GnRH and supports the hypothesis that Pro7.33(303) stabilises a loop conformation that is necessary for selective ligand binding.

Gonadotropin-releasing hormone receptors

GnRH and its analogs are used extensively for the treatment of hormone-dependent diseases and assisted reproductive techniques. They also have potential as novel contraceptives in men and women. A thorough delineation of the molecular mechanisms involved in ligand binding, receptor activation, and intracellular signal transduction is kernel to understanding disease processes and the development of specific interventions. Twenty-three structural variants of GnRH have been identified in protochordates and vertebrates. In many vertebrates, three GnRHs and three cognate receptors have been identified with distinct distributions and functions. In man, the hypothalamic GnRH regulates gonadotropin secretion through the pituitary GnRH type I receptor via activation of G(q). In-depth studies have identified amino acid residues in both the ligand and receptor involved in binding, receptor activation, and translation into intracellular signal transduction. Although the predominant coupling of the type I GnRH receptor in the gonadotrope is through productive G(q) stimulation, signal transduction can occur via other G proteins and potentially by G protein-independent means. The eventual selection of intracellular signaling may be specifically directed by variations in ligand structure. A second form of GnRH, GnRH II, conserved in all higher vertebrates, including man, is present in extrahypothalamic brain and many reproductive tissues. Its cognate receptor has been cloned from various vertebrate species, including New and Old World primates. The human gene homolog of this receptor, however, has a frame-shift and stop codon, and it appears that GnRH II signaling occurs through the type I GnRH receptor. There has been considerable plasticity in the use of different GnRHs, receptors, and signaling pathways for diverse functions. Delineation of the structural elements in GnRH and the receptor, which facilitate differential signaling, will contribute to the development of novel interventive GnRH analogs.

Molecular cloning and characterization of a gonadotropin-releasing hormone receptor in the guinea pig, Cavia porcellus

Guinea pig gonadotropin-releasing hormone (gpGnRH) is predicted to have a unique structure among all known forms of GnRH molecule [Endocrinology 138 (1997) 4123] and it is of great interest to determine whether the unique structure of gpGnRH is manifested in the characteristics of the guinea pig GnRH receptor. In the present study, we isolated a full-length cDNA for a GnRH receptor from the pituitary gland of the guinea pig. The putative guinea pig GnRH receptor protein has an amino acid identity of 79-87% with mammalian type I GnRH receptors. The amino acid residues which have been demonstrated to be important for ligand binding and signal transduction were conserved in the guinea pig GnRH receptor. However, there are several specific amino acid substitutions among mammalian type I GnRH receptors. Moreover, though the guinea pig has generally been classified as a rodent, the putative GnRH receptor protein did not have some rodent-specific characteristics. Total IP assays demonstrated that the cloned guinea pig GnRH receptor is a functional GnRH receptor and that it shows different preference of ligand sensitivities from the rat GnRH receptor.

The brain–pituitary–gonad axis in male teleosts, with special emphasis on flatfish (Pleuronectiformes)

The key component regulating vertebrate puberty and sexual maturation is the endocrine system primarily effectuated along the brain-pituitary-gonad (BPG) axis. By far most investigations on the teleost BPG axis have been performed on salmonids, carps, catfish and eels. Accordingly, earlier reviews on the BPG axis in teleosts have focused on these species, and mainly on females (e.g. 'Fish Physiology, vol. IXA. Reproduction (1983) pp. 97'; 'Proceedings of the Fourth International Symposium on the Reproductive Physiology of Fish. FishSymp91, Sheffield, UK, 1991, pp. 2'; 'Curr. Top. Dev. Biol. 30 (1995) pp. 103'; 'Rev. Fish Biol. Fish. 7 (1997) pp. 173'; 'Proceedings of the Sixth International Symposium on the Reproductive Physiology of Fish. John Grieg A/S, Bergen, Norway, 2000, pp. 211'). However, in recent years new data have emerged on the BPG axis in flatfish, especially at the level of the brain and pituitary. The evolutionarily advanced flatfishes are important model species both from an evolutionary point of view and also because many are candidates for aquaculture. The scope of this paper is to review the present status on the male teleost BPG axis, with an emphasis on flatfish. In doing so, we will first discuss the present understanding of the individual constituents of the axis in the best studied teleost models, and thereafter discuss available data on flatfish. Of the three constituents of the BPG axis, we will focus especially on the pituitary and gonadotropins. In addition to reviewing recent information on flatfish, we present some entirely new information on the phylogeny and molecular structure of teleost gonadotropins.

Identification and characterization of a reptilian GnRH receptor from the leopard gecko

Gonadotropin-releasing hormone (GnRH) plays a pivotal role in the regulation of reproductive functions through interactions with its specific receptor. We describe the first molecular cloning and characterization of a full-length GnRH receptor (GnRHR) from the leopard gecko Eublepharis macularius. It has a distinct genomic structure consisting of five exons and four introns, compared with all the other reported GnRHR genes. A native GnRH form, cGnRH-II, stimulated inositol phosphate (IP) production in COS-7 cells transiently transfected with the GnRHR, in a dose dependent manner. The mRNA was expressed in all the tissues and organs examined. Molecular phylogenetic analysis revealed that the cloned GnRHR belongs to the type 2/nonmammalian I GnRHR. Low-expression levels were observed from the pituitary glands of reproductively active leopard geckos, indicating the possibility that there is at least one more type of GnRHR highly expressed in the pituitary gland for the gonadotropin secretion in this reptile.

Regulation of gonadotropin-releasing hormone (GnRH)-receptor gene expression in tilapia: effect of GnRH and dopamine

The present work was designed to study certain aspects of the endocrine regulation of gonadotropin-releasing hormone receptor (GnRH-R) in the pituitary of the teleost fish tilapia. A GnRH-R was cloned from the pituitary of hybrid tilapia (taGnRH-R) and was identified as a typical seven-transmembrane receptor. Northern blot analysis revealed a single GnRH-R transcript in the pituitary of approximately 2.3 kilobases. The taGnRH-R mRNA levels were significantly higher in females than in males. Injection of the salmon GnRH analog (sGnRHa; 5-50 microg/kg) increased the steady-state levels of taGnRH-R mRNA, with the highest response recorded at 25 microg/kg and at 36 h. At the higher dose of sGnRHa (50 microg/kg), taGnRH-R transcript appeared to be down-regulated. Exposure of tilapia pituitary cells in culture to graded doses (0.1-100 nM) of seabream (sbGnRH = GnRH I), chicken II (cGnRH II), or salmon GnRH (sGnRH = GnRH III) resulted in a significant increase in taGnRH-R mRNA levels. The highest levels of both LH release and taGnRH-R mRNA levels were recorded after exposure to cGnRH II and the lowest after exposure to sbGnRH. The dopamine-agonist quinpirole suppressed LH release and mRNA levels of taGnRH-R, indicating an inhibitory effect on GnRH-R synthesis. Collectively, these data provide evidence that GnRH in tilapia can up- regulate, whereas dopamine down-regulates, taGnRH-R mRNA levels.

c-Src is activated by the epidermal growth factor receptor in a pathway that mediates JNK and ERK activation by gonadotropin-releasing hormone in COS7 cells

Key participants in G protein-coupled receptor (GPCR) signaling are the mitogen-activated protein kinase (MAPK) signaling cascades. The mechanisms involved in the activation of the above cascades by GPCRs are not fully elucidated. A prototypic GPCR that has been widely used to study these signaling mechanisms is the receptor for gonadotropin-releasing hormone (GnRHR), which serves as a key regulator of the reproductive system. Here we expressed GnRHR in COS7 cells and found that GnRHR transmits its signals to MAPKs mainly via G alpha i, EGF receptor without the involvement of Hb-EGF, and c-Src, but independently of PKCs. The main pathway that leads to JNK activation downstream of the EGF receptor involves a sequential activation of c-Src and phosphatidylinositol 3-kinase (PI3K). ERK activation by GnRHR is mediated by the EGF receptor, which activates Ras either directly or via c-Src. Besides the main pathway, the dissociated G beta gamma and beta-arrestin may initiate additional, albeit minor, pathways that lead to MAPK activation in the transfected COS7 cells. The pathways detected are significantly different from those in other cell lines bearing GnRHR, indicating that GnRH can utilize various signaling mechanisms for the activation of MAPK cascades. The unique pathway elucidated here in which c-Src and PI3K are sequentially activated downstream of the EGF receptor may serve as a prototype of signaling mechanisms by GnRHR and by additional GPCRs in various cell types.

Expression of gonadotropin-releasing hormone and gonadotropin-releasing hormone receptor in sheep spinal cord

Consistent with its neuroendocrine role, gonadotropin-releasing hormone (GnRH) is located principally within the hypothalamus, although extra-hypothalamic expression has been reported. The present study characterized the expression of GnRH and GnRH receptor (GnRH-R) in sheep spinal cord using real-time PCR and immunocytochemistry. Both GnRH and GnRH-R mRNA were detected in sheep spinal cord. Expression of GnRH peptide was localized to discrete locations in the spinal cord, including lamina X (the area surrounding the central canal) and motoneurons in the ventral horn. Although there is no known functional role for GnRH in spinal cord, a role as a potential neurotransmitter/neuromodulator is supported by the expression of both GnRH and GnRH-R in this tissue.

Differential brain distribution of gonadotropin-releasing hormone receptors in the goldfish

The present study describes the differential distributions in the brain of the two goldfish gonadotropin-releasing hormone (GnRH) receptors, using both immunohistochemistry and in situ hybridization approaches. The goldfish GnRH GfA and GfB receptors are variant forms of the same receptor subtype, although with distinct differences in ligand binding characteristics, and differential distributions in the pituitary and body tissues [Proc. Natl. Acad. Sci. USA 96 (1999) 2526]. The goldfish GnRH GfA receptor was found to be widespread throughout the brain, with neurons showing immunoreactivity in the olfactory bulbs, telencephalon, preoptic region, ventro-basal hypothalamus, thalamus, midbrain, motor neurons of the fifth, seventh, and tenth cranial nerves, reticular formation, cerebellum, and motor zone of the vagal lobes. The tracts in the posterior commissure, optic tectum, and motor zone of the vagal lobes also demonstrated immunoreactivity. While the brain was not systematically surveyed for in situ hybridization, hybridization was found in similar locations in the telencephalon, preoptic region, ventro-basal hypothalamus, cerebellum, and optic tectum. Hybridization was additionally found in the medial hypothalamus. The goldfish GnRH GfB receptor was found to have a more restricted distribution in the brain, with neurons showing immunoreactivity in the telencephalon, preoptic region, and ventro-basal hypothalamus. In situ hybridization demonstrated a somewhat wider distribution of expression of the receptor, with hybridization occurring in the preoptic region, ventro-basal and medial hypothalamus, as well as in the thalamus, epithalamus, and optic tectum. The widespread distribution of GnRH GfA receptor, and in particular its localization in the midbrain tegmentum in the region of the GnRH-II neurons, suggests that this receptor may be involved in the behavioral actions of GnRH peptides in the goldfish.

Evolutionary aspects of cellular communication in the vertebrate hypothalamo-hypophysio-gonadal axis

This review emphasizes the comparative approach for developing insight into knowledge related to cellular communications occurring in the hypothalamus-pituitary-gonadal axis. Indeed, research on adaptive phenomena leads to evolutionary tracks. Thus, going through recent results, we suggest that pheromonal communication precedes local communication which, in turn, precedes communication via the blood stream. Furthermore, the use of different routes of communication by a certain mediator leads to a conceptual change related to what hormones are. Nevertheless, endocrine communication should leave out of consideration the source (glandular or not) of mediator. Finally, we point out that the use of lower vertebrate animal models is fundamental to understanding general physiological mechanisms. In fact, different anatomical organization permits access to tissues not readily approachable in mammals.

Gonadotropin-releasing hormone receptor: cloning, expression and transcriptional regulation

In summary, isolation of GnRH receptor cDNA, its gene, and identification of regulatory elements in the flanking region of the gene have added to our knowledge regarding the tissue-specific expression of the GnRH receptor gene, and the mechanisms that mediate and influence its transcriptional regulation. However, the interactions of the different regulatory factors (nuclear factors) and the effects of these interactions on the regulation of the GnRH receptor gene remain unclear. Due to existence of multiple promoters and transcriptional start sites in human GnRH receptor gene and the lack of a human gonadotrope cell line, the precise promoter and transcriptional start sites in human pituitary, extra-pituitary tissues and tumors have not yet been identified.

Cell migration and evolutionary significance of GnRH subtypes

Hypothetically it can be assumed that in advanced teleost fishes, GnRH-III and GnRH-IV neurons migrate along the 'telencephalonic' (anterior) and 'diencephalonic' (posterior) migratory route, which perhaps fuses in primitive teleost fishes and land vertebrates to form the 'ancient migratory route' (in all probability = nervus terminalis; see Von Bartheld et al., 1988) of GnRH-I neurons. The difference in distribution pattern of GnRH forms in the vertebrate brain is due to distinct embryonic origins: (1) Cells of olfactory origin, which give rise to GnRH-I (salmon, catfish, chicken I, mammalian GnRH) are distributed along the olfactory system and the basal forebrain in primitive fishes and in land vertebrates; GnRH-I might be pivotal for LH/FSH synthesis-release, olfaction and metamorphosis in lower vertebrates. In advanced teleost fishes, neurons synthesizing GnRH-III ('salmon' GnRH) originate from the olfactory system; they are distributed along the basal olfactory bulbs, with distinct ganglia (NOR) at the caudalmost part of the olfactory bulbs and few scattered cells in the basal telencephalon. The NOR might function as a neuromodulator, hypophysiotropic hormone and regulate visual associated reproductive behaviors. (2) Cells of mesencephalonic origin, which give rise to GnRH-II (chicken-II GnRH) are evolutionarily conserved; might function as a neuromodulator involved in motor-associated reproductive behaviors and acid-base balance. (3) Cells of diencephalonic origin, which give rise to GnRH-IV (seabream, medaka GnRH); they are localized in the anterior-basal OVLT-POA area and present only in advanced teleost fishes. GnRH-IV has been implicated in gonadal sex differentiation, gonadal maturation, LH/FSH secretion and territorial behavior. Advance teleost fishes for yet unknown functions might have acquired GnRH-IV. Although all GnRH subtypes participate in some aspect of reproduction; the precise function of each GnRH form still remains unclear.

Multiplicity of gonadotropin-releasing hormone signaling: a comparative perspective

GnRH regulation of GtH synthesis and release involves PKC- and Ca(2+)-dependent pathways. There are differential signaling mechanisms in different cells, tissues and species. Signaling mechanisms involved in GnRH-mediated GtH release appear to be more conserved compared to that of GnRH-induced GtH gene expression. This may in part be due to different 5' regulatory regions on the GtH-subunit genes. Cell type specific expression of various signaling and/or exocytotic components may also be responsible for the observed differences in signaling between gonadotropes and somatotropes in the goldfish and tilapia pituitaries. However, this can not explain the observed differences in post receptor mechanisms for sGnRH and cGnRH-II in gonadotropes which is more likely to result from the existence of GnRH receptor subtypes. Support for this hypothesis is also provided by observations on mechanisms of autocrine/paracrine regulation of ovarian function by sGnRH and cGnRH-II in the goldfish ovary in which GnRH antagonists only block GnRH stimulation of oocyte meiosis and do not affect inhibitory effects of sGnRH. It should be easier to explain observed variations concerning GnRH-induced responses as more information becomes available on different types of GnRH receptors, and their distribution and function in mammals and non-mammalian vertebrates.

Ala/Thr(201) in extracellular loop 2 and Leu/Phe(290) in transmembrane domain 6 of type 1 frog gonadotropin-releasing hormone receptor confer differential ligand sensitivity and signal transduction

Recently, we have identified three distinct types of bullfrog GnRH receptor (designated bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3). In the present study, we have isolated three GnRHR clones in Rana dybowskii (dyGnRHR-1, dyGnRHR-2, and dyGnRHR-3). Despite high homology of dyGnRHRs with the corresponding bfGnRHRs, dyGnRHRs revealed different signaling pathways and ligand sensitivity compared with the bfGnRHR counterparts. Activation of dyGnRHRs with GnRH stimulated cAMP-mediated gene expression. However, dyGnRHR-3 but not dyGnRHR-1 and -2 induced c-fos promoter-driven gene expression. Consistently, dyGnRHR-1 and dyGnRHR-2 were not able to increase GnRH-induced inositol phosphate accumulation, whereas all bfGnRHRs and dyGnRHR-3 were, indicating that dyGnRHR-1 and dyGnRHR-2 are coupled to solely G(s), whereas all bfGnRHRs and dyGnRHR-3 are coupled to both G(s) and G(q/11). Moreover, dyGnRHR-1 and dyGnRHR-2 showed about 10-fold less sensitivity to each ligand than that of the bfGnRHR counterparts. Using type 1 chimeric and point-mutated receptors, we further elucidated that specific amino acids, Ala/Thr(201) in extracellular loop 2 and Leu/Phe(290) in transmembrane domain 6 of the type 1 receptor, are responsible for ligand sensitivity and signal transduction pathway. Particularly, substitution of Leu(290) to Phe in dyGnRHR-1 increased GnRH-induced inositol phosphate production as well as c-fos promoter-driven gene expression whereas substitution of Phe(290) to Leu in bfGnRHR-1 decreased those activities. Collectively, these results demonstrate the presence of three types of GnRHR in amphibians, and suggest species- and type-specific ligand recognition and different signaling pathways in frog GnRHRs.