Three distinct types of GnRH receptor characterized in the bullfrog

Multiple gonadotropin-releasing hormone systems in non-mammalian vertebrates: Ontogeny, anatomy, and physiology

Three paralogous genes for gonadotropin-releasing hormone (GnRH; gnrh1, gnrh2, and gnrh3) and GnRH receptors exist in non-mammalian vertebrates. However, there are some vertebrate species in which one or two of these paralogous genes have become non-functional during evolution. The developmental migration of GnRH neurons in the brain is evolutionarily conserved in mammals, reptiles, birds, amphibians, and jawed teleost fish. The three GnRH paralogs have specific expression patterns in the brain and originate from multiple sites. In acanthopterygian teleosts (medaka, cichlid, etc.), the preoptic area (POA)-GnRH1 and terminal nerve (TN)-GnRH3 neuronal types originate from the olfactory regions. In other fish species (zebrafish, goldfish and salmon) with only two GnRH paralogs (GnRH2 and GnRH3), the TN- and POA-GnRH3 neuronal types share the same olfactory origin. However, the developmental origin of midbrain (MB)-GnRH2 neurons is debatable between mesencephalic or neural crest site. Each GnRH system has distinctive anatomical and physiological characteristics, and functions differently. The POA-GnRH1 neurons are hypophysiotropic in nature and function in the neuroendocrine control of reproduction. The non-hypophysiotropic GnRH2/GnRH3 neurons probably play neuromodulatory roles in metabolism (MB-GnRH2) and the control of motivational state for sexual behavior (TN-GnRH3).

Amphibian reproductive technologies: approaches and welfare considerations

Captive breeding and reintroduction programs have been established for several threatened amphibian species globally, but with varied success. This reflects our relatively poor understanding of the hormonal control of amphibian reproduction and the stimuli required to initiate and complete reproductive events. While the amphibian hypothalamo-pituitary-gonadal (HPG) axis shares fundamental similarities with both teleosts and tetrapods, there are more species differences than previously assumed. As a result, many amphibian captive breeding programs fail to reliably initiate breeding behaviour, achieve high rates of fertilization or generate large numbers of healthy, genetically diverse offspring. Reproductive technologies have the potential to overcome these challenges but should be used in concert with traditional methods that manipulate environmental conditions (including temperature, nutrition and social environment). Species-dependent methods for handling, restraint and hormone administration (including route and frequency) are discussed to ensure optimal welfare of captive breeding stock. We summarize advances in hormone therapies and discuss two case studies that illustrate some of the challenges and successes with amphibian reproductive technologies: the mountain yellow-legged frog (Rana muscosa; USA) and the northern corroboree frog (Pseudophryne pengilleyi; Australia). Further research is required to develop hormone therapies for a greater number of species to boost global conservation efforts.

Functional Authentication of a Novel Gastropod Gonadotropin-Releasing Hormone Receptor Reveals Unusual Features and Evolutionary Insight

A gonadotropin-releasing hormone (GnRH)-like molecule was previously identified in a gastropod, Aplysia californica, and named ap-GnRH. In this study, we cloned the full-length cDNA of a putative ap-GnRH receptor (ap-GnRHR) and functionally authenticated this receptor as a bona fide ap-GnRHR. This receptor contains two potential translation start sites, each accompanied by a Kozak sequence, suggesting the translation of a long and a short form of the receptor is possible. The putative ap-GnRHR maintains the conserved structural motifs of GnRHR-like receptors and shares 45% sequence identity with the octopus GnRHR. The expression of the putative ap-GnRHR short form is ubiquitous in all tissues examined, whereas the long form is only expressed in parts of the central nervous system, osphradium, small hermaphroditic duct, and ovotestis. The cDNA encoding the long or the short receptor was transfected into the Drosophila S2 cell line and subject to a radioreceptor assay using ¹²⁵I-labeled ap-GnRH as the radioligand. Further, the transfected cells were treated with various concentrations of ap-GnRH and measured for the accumulation of cAMP and inositol monophosphate (IP1). Radioreceptor assay revealed that only the long receptor bound specifically to the radioligand. Further, only the long receptor responded to ap-GnRH with an increased accumulation of IP1, but not cAMP. Our studies show that despite the more prevalent expression of the short receptor, only the long receptor is the functional ap-GnRHR. Importantly, this is only the second report on the authentication of a protostome GnRHR, and based on the function and the phylogenetic grouping of ap-GnRHR, we suggest that this receptor is more similar to protostome corazonin receptors than chordate GnRHRs.

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.

Apoptotic death of prostate cancer cells by a gonadotropin-releasing hormone-II antagonist

Gonadotropin-releasing hormone-I (GnRH-I) has attracted strong attention as a hormonal therapeutic tool, particularly for androgen-dependent prostate cancer patients. However, the androgen-independency of the cancer in advanced stages has spurred researchers to look for new medical treatments. In previous reports, we developed the GnRH-II antagonist Trp-1 to inhibit proliferation and stimulate the autophagic death of various prostate cancer cells, including androgen-independent cells. We further screened many GnRH-II antagonists to identify molecules with higher efficiency. Here, we investigated the effect of SN09-2 on the growth of PC3 prostate cancer cells. SN09-2 reduced the growth of prostate cancer cells but had no effect on cells derived from other tissues. Compared with Trp-1, SN09-2 conspicuously inhibited prostate cancer cell growth, even at low concentrations. SN09-2-induced PC3 cell growth inhibition was associated with decreased membrane potential in mitochondria where the antagonist was accumulated, and increased mitochondrial and cytosolic reactive oxygen species. SN09-2 induced lactate dehydrogenase release into the media and annexin V-staining on the PC3 cell surface, suggesting that the antagonist stimulated prostate cancer cell death by activating apoptotic signaling pathways. Furthermore, cytochrome c release from mitochondria to the cytosol and caspase-3 activation occurred in a concentration- and time-dependent manner. SN09-2 also inhibited the growth of PC3 cells xenotransplanted into nude mice. These results demonstrate that SN09-2 directly induces mitochondrial dysfunction and the consequent ROS generation, leading to not only growth inhibition but also apoptosis of prostate cancer cells.

Local duplication of gonadotropin-releasing hormone (GnRH) receptor before two rounds of whole genome duplication and origin of the mammalian GnRH receptor

Gonadotropin-releasing hormone (GnRH) and the GnRH receptor (GnRHR) play an important role in vertebrate reproduction. Although many GnRHR genes have been identified in a large variety of vertebrate species, the evolutionary history of GnRHR in vertebrates is unclear. To trace the evolutionary origin of GnRHR we examined the conserved synteny of chromosomes harboring GnRHR genes and matched the genes to linkage groups of reconstructed vertebrate ancestor chromosomes. Consistent with the phylogenetic tree, three pairs of GnRHR subtypes were identified in three paralogous linkage groups, indicating that an ancestral pair emerged through local duplication before two rounds of whole genome duplication (2R). The 2R then led to the generation of six subtypes of GnRHR. Some subtypes were lost during vertebrate evolution after the divergence of teleosts and tetrapods. One subtype includes mammalian GnRHR and a coelacanth GnRHR that showed the greatest response to GnRH1 among the three types of GnRH. This study provides new insight into the evolutionary relationship of vertebrate GnRHRs.

Social regulation of male reproductive plasticity in an African cichlid fish

Social interactions with the outcome of a position in a dominance hierarchy can have profound effects on reproductive behavior and physiology, requiring animals to integrate environmental information with their internal physiological state; but how is salient information from the animal's dynamic social environment transformed into adaptive behavioral, physiological, and molecular-level changes? The African cichlid fish, Astatotilapia burtoni, is ideally suited to understand socially controlled reproductive plasticity because activity of the male reproductive (brain-pituitary-gonad) axis is tightly linked to social status. Males form hierarchies in which a small percentage of brightly colored dominant individuals have an active reproductive axis, defend territories, and spawn with females, while the remaining males are subordinate, drably colored, do not hold a territory, and have a suppressed reproductive system with minimal opportunities for spawning. These social phenotypes are plastic and quickly reversible, meaning that individual males may switch between dominant and subordinate status multiple times within a lifetime. Here, we review the rapid and remarkable plasticity that occurs along the entire reproductive axis when males rise in social rank, a transition that has important implications for the operational sex ratio of the population. When males rise in rank, transformations occur in the brain, pituitary, circulation, and testes over short time-scales (minutes to days). Changes are evident in overt behavior, as well as modifications at the physiological, cellular, and molecular levels that regulate reproductive capacity. Widespread changes triggered by a switch in rank highlight the significance of external social information in shaping internal physiology and reproductive competence.

Molecular cloning and pharmacological characterization of two novel GnRH receptors in the lamprey (Petromyzon marinus)

This paper reports the identification, expression, binding kinetics, and functional studies of two novel type III lamprey GnRH receptors (lGnRH-R-2 and lGnRH-R-3) in the sea lamprey, a basal vertebrate. These novel GnRH receptors share the structural features and amino acid motifs common to other known gnathostome GnRH receptors. The ligand specificity and activation of intracellular signaling studies showed ligands lGnRH-II and -III induced an inositol phosphate (IP) response at lGnRH-R-2 and lGnRH-R-3, whereas the ligand lGnRH-I did not stimulate an IP response. lGnRH-II was a more potent activator of lGnRH-R-3 than lGnRH-III. Stimulation of lGnRH-R-2 and lGnRH-R-3 testing all three lGnRH ligands did not elicit a cAMP response. lGnRH-R-2 has a higher binding affinity in response to lGnRH-III than lGnRH-II, whereas lGnRH-R-3 has a higher binding affinity in response to lGnRH-II than IGnRH-III. lGnRH-R-2 precursor transcript was detected in a wide variety of tissues including the pituitary whereas lGnRH-R-3 precursor transcript was not as widely expressed and primarily expressed in the brain and eye of male and female lampreys. From our phylogenetic analysis, we propose that lGnRH-R-1 evolved from a common ancestor of all vertebrate GnRH receptors and lGnRH-R-2 and lGnRH-R-3 likely occurred due to a gene duplication within the lamprey lineage. In summary, we propose from our findings of receptor subtypes in the sea lamprey that the evolutionary recruitment of specific pituitary GnRH receptor subtypes for particular physiological functions seen in later evolved vertebrates was an ancestral character that first arose in a basal vertebrate.

Social control of the brain

In the course of evolution, social behavior has been a strikingly potent selective force in shaping brains to control action. Physiological, cellular, and molecular processes reflect this evolutionary force, particularly in the regulation of reproductive behavior and its neural circuitry. Typically, experimental analysis is directed at how the brain controls behavior, but the brain is also changed by behavior over evolution, during development, and through its ongoing function. Understanding how the brain is influenced by behavior offers unusual experimental challenges. General principles governing the social regulation of the brain are most evident in the control of reproductive behavior. This is most likely because reproduction is arguably the most important event in an animal's life and has been a powerful and essential selective force over evolution. Here I describe the mechanisms through which behavior changes the brain in the service of reproduction using a teleost fish model system.

Molecular Coevolution of Neuropeptides Gonadotropin-Releasing Hormone and Kisspeptin with their Cognate G Protein-Coupled Receptors

The neuropeptides gonadotropin-releasing hormone (GnRH) and kisspeptin (KiSS), and their receptors gonadotropin-releasing hormone receptor (GnRHR) and kisspeptin receptor (KiSSR) play key roles in vertebrate reproduction. Multiple paralogous isoforms of these genes have been identified in various vertebrate species. Two rounds of genome duplication in early vertebrates likely contributed to the generation of these paralogous genes. Genome synteny and phylogenetic analyses in a variety of vertebrate species have provided insights into the evolutionary origin of and relationship between paralogous genes. The paralogous forms of these neuropeptides and their receptors have coevolved to retain high selectivity of the ligand–receptor interaction. These paralogous forms have become subfunctionalized, neofunctionalized, or dysfunctionalized during evolution. This article reviews the evolutionary mechanism of GnRH/GnRHR and KiSS/KiSSR, and the fate of the duplicated paralogs in vertebrates.

Regulation of two forms of gonadotropin-releasing hormone receptor gene expression in the protandrous black porgy fish, Acanthopagrus schlegeli

Two GnRH receptors (GnRH-R I and GnRH-R II) were obtained in protandrous black porgy (Acanthopagrus schlegeli). We investigated their tissue distribution, developmental/seasonal changes and regulation of expression using in vivo and in vitro (primary cultures of dispersed pituitary cells) approaches. The relative expressions of GnRH-Rs in the pituitary and gonad were as follows: pituitary: GnRH-R I > GnRH-R II; testicular tissue: GnRH-R I > GnRH-R II; ovarian tissue: GnRH-R I = GnRH-R II. GnRH-R I but not GnRH-R II expression was higher in the pituitary during the spawning period as compared to the prespawning. The expression profiles of both forms of GnRH-R were variable in the gonads according to the gonadal stage and season. In vivo, hCG stimulated GnRH-R I and GnRH-R II expression in testis and ovary. The LHRH analog also up-regulated both receptors in testis and but increased only GnRH-R II in the ovary. Sex steroids (estradiol, E2 and testosterone, T) increased the expression of both receptors in the testis and ovary. In the pituitary, sex steroids (E2 and T) increased the expression of GnRH-R I, but not GnRH-II, both in vivo and in vitro. The LHRH analog also specifically up-regulated the expression of GnRH-R I, but not GnRH-R II, by pituitary cells in vitro. All these data suggest that GnRH-R I rather than GnRH-R II may play a major physiological role in the pituitary. In contrast, both GnRH-R I and GnRH-R II may participate in the regulation of gonadal functions, including a possible role during sex change.

A functional neuropeptide system in vertebrate gonads: Gonadotropin-inhibitory hormone and its receptor in testes of field-caught house sparrow (Passer domesticus)

Previously, the expression and action of GnIH (Gonadotropin-inhibitory hormone) has been characterized in the hypothalamus and pituitary, respectively. The action of this neurohormone is to inhibit the synthesis and secretion of luteinizing hormone and follicle-stimulating hormone. Several hormone systems identified in the vertebrate brain and classified as neurohormones are synthesized in and have a localized action on the gonads as well. Here we present several lines of evidence for the expression and action of GnIH and its receptor (GnIHR) in the testis of house sparrow (Passer domesticus). EXPRESSION: Transcripts for GnIH and GnIHR isolated from house sparrow testis were initially identified by PCR, then sequenced and found to be homologous to transcripts from European starling (96% to GnIH, 98% to GnIHR), Gambel's white-crowned sparrow (94% to GnIH) and Japanese quail (90% to GnIHR) brains. Further investigation using in situ hybridization confirmed the presence of GnIH precursor mRNA in the interstitium of the testis and GnIHR mRNA in the interstitium and on spermatocytes.

ACTION

The effect of this system on the secretion of testosterone by the testis was investigated using gonadal culture. Testosterone secretion was significantly decreased by 1 microM and 10 microM GnIH in gonadotropin-stimulated testis cultures. In summary, these results indicate the GnIH/GnIHR system is expressed in the testis of house sparrows and its function is to reduce gonadotropin-stimulated testosterone production.

The chicken type III GnRH receptor homologue is predominantly expressed in the pituitary, and exhibits similar ligand selectivity to the type I receptor

Two GnRH isoforms (cGnRH-I and GnRH-II) and two GnRH receptor subtypes (cGnRH-R-I and cGnRH-R-III) occur in chickens. Differential roles for these molecules in regulating gonadotrophin secretion or other functions are unclear. To investigate this we cloned cGnRH-R-III from a broiler chicken and compared its structure, expression and pharmacological properties with cGnRH-R-I. The broiler cGnRH-R-III cDNA was 100% identical to the sequence reported in the red jungle fowl and white leghorn breed. Pituitary cGnRH-R-III mRNA was approximately 1400-fold more abundant than cGnRH-R-I mRNA. Northern analysis indicated a single cGnRH-R-III transcript. A pronounced sex and age difference existed, with higher pituitary transcript levels in sexually mature females versus juvenile females. In contrast, higher expression levels occurred in juvenile males versus sexually mature males. Functional studies in COS-7 cells indicated that cGnRH-R-III has a higher binding affinity for GnRH-II than cGnRH-I (K(d): 0.57 vs 19.8 nM) with more potent stimulation of inositol phosphate production (ED(50): 0.8 vs 4.38 nM). Similar results were found for cGnRH-R-I, (K(d): 0.51 vs 10.8 nM) and (ED(50): 0.7 vs 2.8 nM). The initial rate of internalisation was faster for cGnRH-R-III than cGnRH-R-I (26 vs 15.8%/min). Effects of GnRH antagonists were compared at the two receptors. Antagonist #27 distinguished between cGnRH-R-I and cGnRH-R-III (IC(50): 2.3 vs 351 nM). These results suggest that cGnRH-R-III is probably the major mediator of pituitary gonadotroph function, that antagonist #27 may allow delineation of receptor subtype function in vitro and in vivo and that tissue-specific recruitment of cGnRH-R isoforms has occurred during evolution.

Gonadotropin-releasing hormone II: a multi-purpose neuropeptide

Close to 30 forms of gonadotropin releasing hormone (GnRH) and at least five GnRH receptors have been identified in a wide variety of vertebrates and some invertebrates. One form, now called GnRH II, has the broadest distribution and the most ancient and conserved phylogeny. The distribution of the neurons that produce this peptide are completely nonoverlapping with any other GnRH forms. Fibers that project from these neurons overlap with GnRH I cells and/or fibers in a few regions, but are primarily divergent. The musk shrew (Suncus murinus) continues to be the most tractable mammalian species to use for studies of the function of GnRH II. The brain of the musk shrew has two GnRH genes (I and II), two GnRH receptors (types-1 and -2), and two different behaviors can be influenced by central infusion of GnRH II, but not by GnRH I; receptivity and feeding. Here, we summarize research on the musk shrew relative to the behavioral functions of GnRH II. First, female musk shrews are continually sexually receptive by virtue of their lack of an ovarian and/or behavioral estrus cycle. This feature of their reproductive ecology may be related to their semi-tropical distribution and their breeding season is highly dependent on changes in the availability of food. When food is not abundant, females stop mating, but brief bouts of feeding reinstate reproductive behavior. Likewise, intake of food is related to GnRH II mRNA and peptide content in the brain; after mild food restriction both decline. When GnRH II is infused centrally, at times when its content is low, it can both enhance receptivity and inhibit food intake. Simultaneous administration of a type-1 antagonist does not change the effect of GnRH II and use of an analog (135-18) that is a specific GnRH II agonist as well as a type-1 antagonist has the same effect as the endogenous GnRH II peptide. We propose that GnRH II plays a critical role by orchestrating the coordination of reproduction with the availability of nutritional support for these activities. Humans are bombarded with copious nutritional opportunities and at present obesity is a larger threat to health in many parts of the world than is under nutrition. It is our hope that understanding neuropeptides such as GnRH II that regulate food intake can ultimately lead to products that may curb appetite and thus decrease obesity and related risks to health.

Gonadotropin-inhibitory hormone and its receptor in the avian reproductive system

Many hormones that are classified as neuropeptides are synthesized in vertebrate gonads in addition to the brain. Receptors for these hormones are also expressed in gonadal tissue; thus there is potential for a highly localized autocrine or paracrine effect of these hormones on a variety of gonadal functions. In the present study we focused on gonadotropin-inhibitory hormone (GnIH), a neuropeptide that was first discovered in the hypothalamus of birds. We present different lines of evidence for the synthesis of GnIH and its receptor in the avian reproductive system including gonads and accessory reproductive organs by studies on two orders of birds: Passeriformes and Galliformes. Binding sites for GnIH were initially identified via in vivo and in vitro receptor fluorography, and were localized in ovarian granulosa cells along with the interstitial layer and seminiferous tubules of the testis. Furthermore, species-specific primers produced clear PCR products of GnIH and GnIH receptor (GnIH-R) in songbird and quail gonadal and other reproductive tissues, such as oviduct, epididymis and vas deferens. Sequencing of the PCR products confirmed their identities. Immunocytochemistry detected GnIH peptide in ovarian thecal and granulosa cells, testicular interstitial cells and germ cells and pseudostratified columnar epithelial cells in the epididymis. In situ hybridization of GnIH-R mRNA in testes produced a strong reaction product which was localized to the germ cells and interstitium. In the epididymis, the product was also localized in the pseudostratified columnar epithelial cells. In sum, these results indicate that the avian reproductive system has the capability to synthesize and bind GnIH in several tissues. The distribution of GnIH and its receptor suggest a potential for autocrine/paracrine regulation of gonadal steroid production and germ cell differentiation and maturation.

Diversity of actions of GnRHs mediated by ligand-induced selective signaling

Geoffrey Wingfield Harris' demonstration of hypothalamic hormones regulating pituitary function led to their structural identification and therapeutic utilization in a wide spectrum of diseases. Amongst these, Gonadotropin Releasing Hormone (GnRH) and its analogs are widely employed in modulating gonadotropin and sex steroid secretion to treat infertility, precocious puberty and many hormone-dependent diseases including endometriosis, uterine fibroids and prostatic cancer. While these effects are all mediated via modulation of the pituitary gonadotrope GnRH receptor and the G(q) signaling pathway, it has become increasingly apparent that GnRH regulates many extrapituitary cells in the nervous system and periphery. This review focuses on two such examples, namely GnRH analog effects on reproductive behaviors and GnRH analog effects on the inhibition of cancer cell growth. For both effects the relative activities of a range of GnRH analogs is distinctly different from their effects on the pituitary gonadotrope and different signaling pathways are utilized. As there is only a single functional GnRH receptor type in man we have proposed that the GnRH receptor can assume different conformations which have different selectivity for GnRH analogs and intracellular signaling proteins complexes. This ligand-induced selective-signaling recruits certain pathways while by-passing others and has implications in developing more selective GnRH analogs for highly specific therapeutic intervention.

GnRH and GnRH receptors in metazoa: a historical, comparative, and evolutive perspective

About 50years after Harris's first demonstration of its existence, GnRH has strongly stimulated the interest and imagination of scientists, resulting in a high number of studies in an increasing number of species. For the endocrinologist, GnRH, via its actions on the synthesis and release of pituitary gonadotrophins, is first an essential hormone for the initiation and maintenance of the reproductive axis, but recent data suggest that GnRH emerged in animals lacking a pituitary. In this context, this review intends to explore the current status of knowledge on GnRH and GnRH receptors in metazoa in order to see if it is possible to draw an evolutive scenario according to which GnRH actions progressively evolved from the control of simple basic functions in early metazoa to an indirect mean of controlling gonadal activity in vertebrates through a sophisticated network of finely tuned neurons developing in a rather fascinating way. This review also intends to provide an evolutive scenario based on the recent advances of whole genome sequencing possibly explaining the number of GnRH and GnRH receptor variants according to the 2R and 3R theories accompanied by gene losses.

Molecular evolution of neuropeptide receptors with regard to maintaining high affinity to their authentic ligands

Recently, we cloned many of the bullfrog neuropeptide G protein-coupled receptors (GPCRs), including receptors for vasotocin (VT), mesotocin, gonadotropin-releasing hormone (GnRH), neurotensin, apelin, and metastin. Bullfrog GPCRs usually have high affinity for bullfrog ligands but relatively low affinity for mammalian ligands. Reciprocally, synthetic agonists and antagonists developed based upon mammalian ligands display lower affinity at bullfrog receptors. Studies using chimeric or domain-swapped receptors indicate that the motifs responsible for differential ligand selectivity usually reside within transmembrane domain 6 (TMD6)-extracellular loop 3 (ECL3)-transmembrane domain 7 (TMD7). Triple mutation of mammalian V1aR (Phe(6.51) to Tyr, Ile(6.53) to Thr, and Pro(7.33) to Thr) increases VT affinity but greatly reduces arginine vasopressin affinity. This binding profile is similar to that of bullfrog VT1R. Changing just three amino acids in the bullfrog GnRH receptor-1 (i.e. Ser-Gln-Ser in the ECL3) to those found in the type-I mammalian GnRH receptor (i.e. Ser-Glu-Pro) reverses GnRH selectivity. In conclusion, specific receptor motifs that govern ligand selectivity can be determined by comparative molecular analyses of GPCRs and their ligands. Such analysis provides clues for understanding how GPCRs maintain high affinity to their authentic ligands.

Involvement of the ser-glu-pro motif in ligand species-dependent desensitisation of the rat gonadotrophin-releasing hormone receptor

There are two forms of gonadotrophin-releasing hormone (GnRH), GnRH-I and GnRH-II, in the vertebrate brain. Both GnRH-I and GnRH-II are thought to interact with the type-I GnRH receptor (GnRHR). The present study attempted to demonstrate whether GnRH-I and GnRH-II induce differential desensitisation of GnRHR and to identify the motif involved. Time course inositol phosphate (IP) accumulation assay reveals that, in cells expressing the wild-type rat GnRHR, GnRH-I induced continuous increase in IP production, whereas GnRH-II-induced IP production rate at later time points (30-120 min after ligand treatment) became attenuated. However, in cells expressing the mutant receptor in which the Ser-Glu-Pro (SEP) motif in extracellular loop 3 was replaced by Pro-Glu-Val (PEV), IP accumulation rates at later time points were more decreased by GnRH-I than GnRH-II. Ca2+ responses to repetitive GnRH applications reveal that GnRH-II desensitised the wild-type receptor faster than GnRH-I, whereas the opposite situation was observed in the PEV mutant. In addition, cell surface loss of GFP-tagged wild-type receptor was more facilitated by GnRH-II than GnRH-I, whereas that of the GFP-tagged PEV mutant receptor was more enhanced by GnRH-I than GnRH-II. The present study indicates that the SEP motif is potentially responsible for ligand species-dependent receptor desensitisation. Together, these results suggest that GnRH-I and GnRH-II may have different effects on mammalian type-I GnRHR via modulation of desensitisation rates.

Development of GnRH cells: Setting the stage for puberty

Cells containing gonadotropin-releasing hormone (GnRH) are essential not only for reproduction but also for neuromodulatory functions in the adult animal. A variety of studies have hinted at multiple origins for GnRH-containing cells in the developing embryo. We have shown, using zebrafish as a model system, that GnRH cells originate from precursors lying outside the olfactory placode: the region of the anterior pituitary gives rise to hypothalamic GnRH cells and the cranial neural crest gives rise to the GnRH cells of the terminal nerve and midbrain. Cells of both the forming anterior pituitary and cranial neural crest are closely apposed to the precursors of the olfactory epithelium during early development. Disruption of kallmann gene function results in loss of the hypothalamic but not the terminal nerve GnRH cells during early development. The GnRH proteins are expressed early in development and this expression is mirrored by the onset of GnRH receptor (GnRH-R) expression during early development. Thus the signaling of the GnRH neuronal circuitry is set up early in development laying the foundation for the GnRH network that is activated at puberty leading to reproductive function in the mature animal.

Molecular and functional characterization of a novel gonadotropin-releasing-hormone receptor isolated from the common octopus (Octopus vulgaris)

GnRH (gonadotropin-releasing hormone) plays a pivotal role in the regulation of reproduction in vertebrates through interaction with a specific receptor. Previously, we isolated a GnRH homologue, oct-GnRH, from the common octopus (Octopus vulgaris). In the present study, we have identified a GnRH receptor (oct-GnRHR) specific for oct-GnRH from Octopus brain. Oct-GnRHR includes domains and motifs typical of vertebrate GnRH receptors. The intron-inserted positions are conserved between oct-GnRHR and the chordate GnRHR genes. The oct-GnRHR expressed in Xenopus (South African clawed frog) oocytes was responsive to oct-GnRH, but not to any other HPLC fractions of the Octopus brain extract. These results show that oct-GnRHR is an authentic receptor for oct-GnRH. Southern blotting of reverse-transcription PCR products revealed that the oct-GnRHR mRNA was widely distributed in the central and peripheral nervous systems and in several peripheral tissues. In situ hybridization showed that oct-GnRHR mRNA was expressed in some regions involved in autonomic functions, feeding, memory and movement. Oct-GnRH was shown to induce steroidogenesis of testosterone, progesterone and 17beta-oestradiol in Octopus ovary and testis, where oct-GnRHR was abundantly expressed. These results suggest that oct-GnRH, like its vertebrate counterparts, acts as a multifunctional neurotransmitter, neuromodulator and hormone-like factor, both in Octopus central nervous system and peripheral tissues, and that both structure and functions of the GnRH family are, at least partially, evolutionarily conserved between octopuses and chordates.

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.

GnRH-I and GnRH-II have differential modulatory effects on human peripheral blood mononuclear cell proliferation and interleukin-2 receptor gamma-chain mRNA expression in healthy males

GnRH-I and its receptor (GnRHR-I) have previously been demonstrated and shown to be biologically active in the immune system, notably within peripheral lymphocytes. Recently however, a second form of GnRH (GnRH-II) has been described in the human. The functions of both these neuropeptides in PMBCs have not been understood yet. The present study was therefore designed to investigate the effects of GnRH-I and/or GnRH-II on human PMBC proliferation in males. Secondly, the effects of GnRH-I and GnRH-II on IL-2 dependent lymphocyte proliferation were examined. Finally, we analysed the role of GnRH-I and GnRH-II in IL-2R gamma-chain expression. Peripheral venous blood samples were obtained from six male healthy volunteers (Mean age 27.75 +/- 1.5). Non-radioactive cell proliferation assay was used for proliferation studies and we used quantitative real-time RT-PCR to examine the role of GnRH-I and GnRH-II on IL-2R gamma-chain expression in PMBCs. Treatment of PMBCs with GnRH-I (10(-9) M and 10(-5) M) and with interleukin-2 (IL-2) (50 U/ml) resulted in a significant increase in cell proliferation compared with the untreated control. PMBCs cotreated with IL-2 and GnRH-I demonstrated higher proliferative responses than IL-2 treatment alone, the enhancement of GnRH-I on IL-2 response being significant only at GnRH-I concentration of 10(-5) M. Co-incubation of IL-2+ GnRH 10(-5) M with a GnRH antagonist (Cetrorelix; 10(-6) M) significantly decreased the proliferation. GnRH-II did not affect the proliferation of PMBCs alone, and did not alter the proliferative response to IL-2. The proliferative responses to GnRH-I (alone and with IL-2) were significantly attenuated by GnRH-II coincubation (each in equal molar concentrations; 10(-9) M to 10(-5) M). It was found that GnRH-I increased the expression of IL-2Rgamma mRNA in a dose dependent manner, with a significant increase of percentage 162.3 +/- 14 of control at 10(-5) M. In contrast, IL-2Rgamma expression was significantly decreased in all concentrations of GnRH-II (10(-9) M to10(-5) M), and the maximum decrease was detected at 10(-5) M, with percentage 37.7 +/- 6.6 of control. All these findings strongly suggest that regulation of IL-2R expression may therefore be an important target for GnRH-I and GnRH-II in PMBCs in males. In summary, present study clearly demonstrates the differential effects of GnRH-I and GnRH-II on PMBC proliferation, IL-2 proliferative response, and IL-2Rgamma expression in PMBCs in males. To our knowledge, our observations provide the first evidence for the interactions of these local neuropeptides at lymphocyte level. Further experimental data in human are warranted to explore the clinical implications of these data.

Dimerizations of the wallaby gonadotropin-releasing hormone receptor and its splice variants

Dimerization between wallaby GnRH-R and its splice variants was examined. A baculovirus-based fluorescence resonance energy transfer (Bv-FRET) assay was used to assess protein-protein interaction between wild type wallaby GnRH-R and splice variants (GnRH-RDelta1 and GnRH-RDelta2). FRET analysis demonstrated that GnRH-R, GnRH-RDelta1 or GnRH-RDelta2 are capable of assembling as homodimers. When GnRH-R is co-expressed with GnRH-RDelta1 or GnRH-RDelta2 splice variants, GnRH-R can form heterodimers with GnRH-RDelta1 and GnRH-RDelta2. GnRH agonist is not required for the initiation of dimerization. However, the addition of a GnRH agonist enhances the FRET signal in the GnRH-R homodimers, indicating that the GnRH agonist may be involved in modulating the extent of dimerization. In addition, this study reveals that dimerization of GnRH-R may be mediated by two or more protein interaction domains. One of them is probably located between amino acid residues 74 and 174, and the other one between residues 175 and 328. This is the first study to show dimerization between a wild type mammalian GnRH-R and its splice variants. It provides additional support for the potential involvement of splice variants in GnRH-R signaling.

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.

Seasonal changes in expression of genes encoding five types of gonadotropin-releasing hormone receptors and responses to GnRH analog in the pituitary of masu salmon

Five types of gonadotropin-releasing hormone receptor (GnRH-R) genes, designated as msGnRH-R1, R2, R3, R4, and R5, are expressed in the brain and pituitary of masu salmon (Oncorhynchus masou). In the present study, seasonal changes in the expression of these five genes were examined in the pituitary to elucidate their roles in GnRH action during growth and sexual maturation. In addition, the seasonal variation of these genes in response to GnRH was examined in a GnRH analog (GnRHa) implantation experiment. Pituitary samples were collected 1 week after the implantation every month from immaturity through spawning. The absolute amount of GnRH-R mRNA in single pituitaries was determined by real-time PCR assays. Among the five genes, R4 was predominantly expressed in the pituitaries. In the immature fish, the amount of GnRH-R mRNA varied with seasons and subtypes. In the pre-spawning period, R1 and R4 mRNAs in both sexes and R2 and R3 mRNAs in the females increased 4- to 20-fold and then decreased in the spawning season. The effects of GnRHa treatment were significantly different in both sexes. In the females, GnRHa tended to elevate the expression of all the subtypes of GnRH-R genes in various stages during the experimental period, whereas it had almost no apparent effects in the males. These results indicate that the expression of the five GnRH-R genes is seasonally variable and may be related to the responses of the pituitary hormone genes to GnRH, and the regulation of GnRH-R genes by GnRH is different in both sexes.

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.

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.

Extracellular Loop 3 (EL3) and EL3-Proximal Transmembrane Helix 7 of the Mammalian Type I and Type II Gonadotropin-Releasing Hormone (GnRH) Receptors Determine Differential Ligand Selectivity to GnRH-I and GnRH-II

Mammalian type I and II gonadotropin-releasing hormone (GnRH) receptors (GnRHRs) show differential ligand preference for GnRH-I and GnRH-II, respectively. Using a variety of chimeric receptors based on green monkey GnRHR-2 (gmGnRHR-2), a representative type II GnRHR, and rat GnRHR, a representative type I GnRHR, this study elucidated specific domains responsible for this ligand selectivity. A chimeric gmGnRHR-2 with the extracellular loop 3 (EL3) and EL3-proximal transmembrane helix 7 (TMH7) of rat GnRHR showed a great increase in ligand sensitivity to GnRH-I but not to GnRH-II. Point-mutation studies indicate that four amino acids, Leu/Phe(7.38), Leu/Phe(7.43), Ala/Pro(7.46), and Pro/Cys(7.47) in TMH7 are critical for ligand selectivity as well as receptor conformation. Furthermore, a combinatory mutation (Pro(7.31)-Pro(7.32)-Ser(7.33) motif to Ser-Glu-Pro in EL3 and Leu(7.38), Leu(7.43), Ala(7.46), and Pro(7.47) to those of rat GnRHR) in gmGnRH-2 exhibited an approximately 500-fold increased sensitivity to GnRH-I, indicating that these residues are critical for discriminating GnRH-II from GnRH-I. [Trp(7)]GnRH-I and [Trp(8)]GnRH-I but not [His(5)]GnRH-I exhibit a higher potency in activating wild-type gmGnRHR-2 than native GnRH-I, indicating that amino acids at positions 7 and 8 of GnRHs are more important than position 5 for differential recognition by type I and type II GnRHRs. As a whole, these data suggest a molecular coevolution of ligands and their receptors and facilitate the understanding of the molecular interaction between GnRHs and their cognate receptors.

Evolution of GnRH ligand precursors and GnRH receptors in protochordate and vertebrate species

Primary structure relationships between GnRH precursors or GnRH receptors have received significant attention recently due to rapid DNA sequence determination of gene fragments and cDNAs from diverse species. Concepts concerning the evolutionary history of the GnRH system and its function in mammals, including humans, are likely to be modified as more complete sequence information becomes available. Current evidence suggests occurrence of fewer GnRH ligand and GnRH receptor genes in mammals compared to protochordates, fish and amphibians. Whilst several sequence-related GnRH decapeptide precursors and 2 or 3 separate GnRH receptors are encoded within the genomes of protochordates, fish and amphibians, only two types of GnRH (GnRH-I and GnRH-II) and two GnRH receptors occur in mammals. In addition, fish and mammalian genomes both retain inactive remnants of GnRH ligand or GnRH receptor genes. The number of distinct GnRH receptor genes in teleosts (at least five complete genes in pufferfish and three in zebrafish) partly reflects whole genome duplication during the evolution of this order of animals. Three GnRH receptor genes occur in certain frog species, consistent with the occurrence of up to three types of prepro-GnRH in amphibians. In contrast, only one functional GnRH receptor gene (the type I GnRH receptor) has been identified in humans and chimpanzees and a gene encoding a second receptor, homologous to a functional monkey receptor (the type II GnRH receptor), is either partially or completely silenced in a range of mammalian species (human, chimpanzee, sheep, cow, rat, and mouse). Further work is required to determine the significance of species-specific differences in the GnRH system to reproductive biology. For instance, recent data show that even species as closely related as humans and chimpanzees exhibit important organisational changes in the genes comprising the GnRH system.

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.

Hormonal regulation of human trophoblast differentiation: a possible role for 17beta-estradiol and GnRH

We have examined the role of 17beta-estradiol and gonadotropin releasing hormone (GnRH) in the regulation of functional differentiation in human trophoblasts. In contrast to its recognized functions as a proliferation-promoting hormone in a variety of cell types, we found that 17beta-estradiol induced terminal differentiation in human trophoblastic cells, and that this event was estrogen-receptor-mediated. This process involved a loss in expression of Cyclins A2 and E, and a coincident increase in p27(Kip1). The anti-proliferative effects of 17beta-estradiol were annulled by specific transforming growth factor-beta 1 (TGFbeta1)-neutralizing antibody, suggesting that 17beta-estradiol may mediate its growth-inhibitory actions, through TGFbeta1 activity. Following exposure to Buserelin, cultured human trophoblastic cells stopped proliferating and formed functionally mature syncytiotrophoblasts. This differentiation event, that involved a drastic loss in expression of proliferating-cell-nuclear-antigen, could be blocked by Cetrorelix, suggesting the involvement of functional GnRH receptors. Preliminary studies on the characterization of the human placental GnRH receptor, indicate the presence of multiple receptor isoforms across human gestation.

Changes in serum sex steroid levels throughout the reproductive cycle of Bufo arenarum females

The changes in the serum levels of the sexual steroids estradiol-17beta (E(2)), testosterone (T), dihydrotestosterone (DHT), and progesterone (P) in Bufo arenarum females were determined by radioimmunoassay (RIA) during 3 consecutive cycles (1999-2001). The serum concentrations of T and DHT, which showed a close parallelism during the annual reproductive cycle, exhibited the highest levels during the preovulatory period, when oogenesis is advanced, while lowest serum levels of these hormones were found during the ovulatory period. The data obtained for E(2) showed a pattern contrary to that determined for androgens. The maximum E(2) concentrations detected in the early postovulatory period might be associated with vitellogenesis and follicular growth. Lowest E(2) concentrations were reached during the period in which B. arenarum undergoes its final hibernation stage. Serum P showed a peak during the preovulatoy period, related to the induction of nuclear maturation in full grown oocytes. A strong decrease in the levels of the circulating hormones was observed after ovariectomy. Our results showed that, out of the four hormones examined, T and DHT were the best indicators of ovarian and oviductal stage, as shown by the strong positive correlation found between androgen levels and organ weight, while E(2) showed a weak negative correlation with ovarian and oviductal weight.

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.

Evolutionary aspects of GnRHs, GnRH neuronal systems and GnRH receptors in teleost fish

Gonadotrophin-releasing hormone (GnRH) was originally believed to be released by a unique set of hypophysiotrophic neurons to stimulate the release of gonadotrophins from the pituitary, therefore acting as a major initiator of the hormonal cascade controlling the reproductive axis. However, it now appears that each vertebrate species expresses two or three GnRH forms in multiple tissues and that GnRHs exert pleiotropic actions via several classes of receptors. This new vision of the GnRH systems arose progressively from numerous comparative studies in all vertebrate classes, but fish in general, and teleosts in particular, have often plaid a leading part in changing established concepts. To date fish still appear as attractive models to decipher the evolutionary mechanisms that led to the diversification of GnRH functions. Not only do teleosts exhibit the highest variety of GnRH variants, but recent data and whole genome analyses indicate that they may also possess multiple GnRH receptors. This paper intends to summarize the current situation with special emphasis on interspecies comparisons which provide insights into the possible evolutionary mechanisms leading to the diversification of GnRH functions.

Structure, expression, and cluster organization of genes encoding gonadotropin-releasing hormone receptors found in the neural complex of the ascidian Ciona intestinalis

Gonadotropin-releasing hormone (GnRH) controls reproductive functions in vertebrates. In the present study, two distinct homologues (Ci-GnRHR1 and Ci-GnRHR2) of the vertebrate GnRH receptor (GnRHR) were identified by cDNA cloning from the neural complex of the ascidian Ciona intestinalis. The Ci-GnRHR1 and Ci-GnRHR2 genes are closely linked to each other and form a cluster with another GnRHR-like gene in the genome. Ci-GnRHR1 and Ci-GnRHR2 are more closely related to vertebrate GnRHRs than to the GnRHR-related protein of Drosophila. The phylogenetic analysis indicates that vertebrates and urochordates independently acquired multiple GnRHRs by gene duplications that occurred during the evolution of each lineage. A voltage clamp of Xenopus oocytes injected with synthetic Ci-GnRHR1 mRNA revealed inward currents in response to an ascidian form of GnRH, suggesting that Ci-GnRHR1 is a bona-fide GnRHR. Expression patterns of Ci-GnRHR1 and Ci-GnRHR2 suggest that a GnRH signaling system is involved in regulation of neuronal and reproductive processes as well as in other physiological functions in ascidians.

The biology of gonadotropin hormone-releasing hormone: role in the control of tumor growth and progression in humans

It is now well known that different forms of GnRH coexist in the same vertebrate species. In humans, two forms of GnRH have been identified so far. The first form corresponds to the hypophysiotropic decapeptide, and is now called GnRH-I. The second form has been initially identified in the chicken brain, and it is referred to as GnRH-II. GnRH-I binds to and activates specific receptors, belonging to the 7 transmembrane (7TM) domain superfamily, present on pituitary gonadotropes. These receptors (type I GnRH receptors) are coupled to the Gq/11/PLC intracellular signalling pathway. A receptor specific for GnRH-II (type II GnRH receptor) has been identified in non-mammalian vertebrates as well as in primates, but not yet in humans. In the last 10-15 years experimental evidence has been accumulated indicating that GnRH-I is expressed, together with its receptors, in tumors of the reproductive tract (prostate, breast, ovary, and endometrium). In these hormone-related tumors, activation of type I GnRH receptors consistently decreases cell proliferation, mainly by interfering with the mitogenic activity of stimulatory growth factors (e.g., EGF, IGF). Recent data seem to suggest that GnRH-I might also reduce the migratory and invasive capacity of cancer cells, possibly by affecting the expression and/or activity of cell adhesion molecules and of enzymes involved in the remodelling of the extracellular matrix. These observations point to GnRH-I as an autocrine negative regulatory factor on tumor growth progression and metastatization. Extensive research has been performed to clarify the molecular mechanisms underlying the peculiar antitumor activity of GnRH-I. Type I GnRH receptors in hormone-related tumors correspond to those present at the pituitary level in terms of cDNA nucleotide sequence and protein molecular weight, but do not share the same pharmacological profile in terms of binding affinity for the different synthetic GnRH-I analogs. Moreover, the classical intracellular signalling pathway mediating the stimulatory activity of the decapeptide on gonadotropin synthesis and secretion is not involved in its inhibitory activity on hormone-related tumor growth. In these tumors, type I GnRH receptors are coupled to the Gi-cAMP, rather than the Gq/11-PLC, signal transduction pathway. Recently, we have reported that GnRH-I and type I GnRH receptors are expressed also in tumors not related to the reproductive system, such as melanoma. Also in melanoma cells, GnRH-I behaves as a negative regulator of tumor growth and progression. Interestingly, the biochemical and pharmacological profiles of type I GnRH receptors in melanoma seem to correspond to those of the receptors at pituitary level. The data so far reported on the expression and on the possible functions of GnRH-II in humans are still scanty. The decapeptide has been identified, together with a 'putative' type II GnRH receptor, both in the central nervous system and in peripheral structures, such as tissues of the reproductive tract (both normal and tumoral). The specific biological functions of GnRH-II in humans are presently under investigation.

Preferential ligand selectivity of the monkey type-II gonadotropin-releasing hormone (GnRH) receptor for GnRH-2 and its analogs

Gonadotropin-releasing hormone (GnRH) regulates the reproductive system through the cognate GnRH receptor (GnRHR) in vertebrates. In this study, we cloned a cDNA encoding the full-length open reading frame sequence for green monkey type-II GnRHR (gmGnRHR-2) from the genomic DNA of CV-1 cells. Transient transfection study showed that gmGnRHR-2 was able to induce both c-fos promoter- and cAMP responsive element-driven transcriptional activities, indicating that gmGnRHR-2 couples to both Gs- and Gq/11-linked signaling pathways. gmGnRHR-2 responded better to GnRH-2 ([His5, Trp7, Tyr8]GnRH) than GnRH-1 ([Tyr5, Leu7, Arg8]GnRH). Substitutions of His5, Trp7, and/or Tyr8 in GnRH-1 increased the potency to activate gmGnRHR-2, suggesting that individual His5, Trp7, and Tyr8 in GnRH-2 contributed to differential ligand sensitivity of gmGnRHR-2. Substitution of D-Ala for Gly6 in GnRH-2 increased the potency to activate the receptor, suggesting that GnRH-2 has a constrained conformation when it binds to the receptor. GnRH-induced gmGnRHR-2 activation was specifically inhibited by GnRH-2 antagonists, Trptorelix-1 and -2, but not by a GnRH-1 antagonist, Cetrorelix. In conclusion, gmGnRHR-2 revealed preferential ligand selectivity for GnRH-2 and its analogs, suggesting that gmGnRHR-2 has a functional activity that is different from mammalian type-I GnRHRs but similar to non-mammalian GnRHRs.

A novel third gonadotropin-releasing hormone receptor in the medaka Oryzias latipes: evolutionary and functional implications

Gonadotropin-releasing hormone (GnRH) plays pivotal roles in the regulation of vertebrate reproduction through binding to its specific membrane receptor. Within the past few years, substantial evidence has accumulated that more than one GnRH receptor (GnRH-R) is expressed in individual vertebrate species. Two GnRH-Rs, termed GnRH-R1 and GnRH-R2, have been identified in a teleost, the medaka Oryzias latipes. Here we describe the identification and characterization of a novel third member of GnRH-R, designated GnRH-R3, in the medaka. GnRH-R3 share high sequence homology (77% amino acid identity in the transmembrane domain) with GnRH-R1. Phylogenetic analysis and genetic mapping demonstrated that both GnRH-R1 and GnRH-R3 were orthologous to the type 2 GnRH-R in primates and that these two medaka receptors were duplicates resulting from the genome-wide duplication within the teleost lineage. GnRH-R3, however, contained three introns, whereas GnRH-R1 had only two. Moreover, unlike GnRH-R1, GnRH-R3 exhibited an approximately equal selectivity for two of three native GnRH forms in the medaka, chicken-II-type GnRH (cGnRH-II) and salmon-type GnRH (sGnRH), and a less sensitivity for the other form, medaka-type GnRH. GnRH-R3 was found to be expressed throughout the brain, and thus appeared to mediate the neuromodulatory functions of both cGnRH-II and sGnRH. These data identify GnRH-R3 as a new member of GnRH-R that arose in a recent genome duplication but has distinctive genomic structure and functional characteristic.

Differential G protein coupling preference of mammalian and nonmammalian gonadotropin-releasing hormone receptors

Recently, we have identified three distinct types of gonadotropin-releasing hormone receptor (GnRHR) in the bullfrog (designated bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3). In the present study, we compared G protein coupling preference of mammalian and nonmammalian GnRHRs. In a transient expression system, stimulation of either bfGnRHRs or rat GnRHR by GnRH significantly increased both inositol phosphates (IP) and cAMP productions, but ratios of IP to cAMP induction levels were quite different among the receptors, indicating differential G protein coupling preference. Using cAMP-dependent protein kinase A (PKA)-specific (CRE-luc) or protein kinase C (PKC)-specific reporter (c-fos-luc) systems, we further examined G(s) and G(q/11) coupling preference of these GnRHRs. Since activities of CRE-luc and c-fos-luc were highly dependent on cell types, GnRH-induced CRE-luc or c-fos-luc activity was normalized by forskolin-induced CRE-luc or 12-O-tetradecanoylphenol-13-acetate (TPA)-induced c-fos-luc activity, respectively. This normalized result indicated that bfGnRHR-2 couples to G(s) more actively than G(q/11), while bfGnRHR-1 and -3 couple to G(s) and G(q/11) with similar strength. However, the rat GnRHR appeared to couple to G(q/11) more efficiently than G(s). This study was further confirmed by an experiment in which GnRH augmented CRE-driven luciferase activity in alphaT3-1 cells when CRE-luc was cotransfected with bfGnRHRs but not with vehicle or rat GnRHR. Collectively, these results indicate that mammalian and nonmammalian GnRHRs may induce diverse cellular and physiological responses through differential activation of PKA and PKC signaling pathways.

Structure of the GnRH receptor-stimulated signaling network: insights from genomics

The GnRH receptor influences gene expression in the gonadotrope through activating signaling cascades that modulate transcription factor expression and activity. A longstanding question in neuroendocrinology is how instructions received at the membrane in the form of the pattern of receptor stimulation are processed into specific biosynthetic changes at each gonadotropin promoter. Signal transduction from the membrane to preformed transcription factors relies on recognition of altered conformations. Signal transduction through the layers of the gene network also requires the biosynthesis of new transcription factors. The signal processing of this system depends on its molecular connectivity map and its feedback and feed-forward loops. Review of signal transduction, gene control, and genomic studies provide evidence of key loops that cross between cellular and nuclear compartments. Genomic studies suggest that the signal transduction and gene network form a continuum. We propose that information transfer in the gonadotrope depends on robust signaling modules that serve to integrate events at different time scales across cytoplasmic and nuclear compartments.

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.

Multi-factorial role of GnRH-I and GnRH-II in the human ovary

Normal ovarian functions are regulated by a wide variety of endocrine hormones, local paracrine and autocrine factors, which functionally interact with each other in a highly coordinated fashion. Recent findings have demonstrated that both forms of gonadotropin-releasing hormone (GnRH-I and GnRH-II) are expressed in various compartments of the human ovary including the granulosa-luteal cells, ovarian surface epithelial cells and ovarian tumors, and their expressions have been shown to be tightly regulated by gonadal steroids and gonadotropins. Functionally, these neuropeptides exert diverse biological effects in the ovary via binding to their cognate receptors, supporting the notion that these peptides act as paracrine and autocrine factors in modulating local ovarian functions. In this review, we will summarize recent literatures regarding the regulation of GnRH-I and GnRH-II gene expressions in the human ovary, and discuss the possible signal transduction mechanisms by which these hormones exert their actions in the gonad. Recent cloning of the second form of the GnRH receptor (GnRH-II receptor) in primates and other vertebrates demonstrated that it was structurally, and thus, functionally distinct from the GnRH-I receptor. Cell proliferation studies showed that GnRH-II inhibited the growth of human ovarian cancer cells that express GnRH-II but not GnRH-I receptor, indicating that the GnRH-II binding sites are functional in these cells. However, it remains unknown if GnRH-II receptor is expressed as a full-length, properly processed and functional gene transcript in humans, and its potential physiological roles such as differential regulation of gonadotropin secretion, neuroendocrine modulation and female sexual behavior await further investigation.

GnRH II and type II GnRH receptors

Hypothalamic gonadotrophin-releasing hormone (GnRH I), which is of a variable structure in vertebrates, is the central regulator of the reproductive system through its stimulation of gonadotrophin release from the pituitary. A second form of GnRH (GnRH II) is ubiquitous and conserved in structure from fish to humans, suggesting that it has important functions and a discriminating receptor that selects against structural change. GnRH II is distributed in discrete regions of the central and peripheral nervous systems and in nonneural tissues. The cognate receptor for GnRH II has recently been cloned from amphibians and mammals. It is highly selective for GnRH II, has a similar distribution to GnRH II in the nervous system and, notably, in areas associated with sexual behaviour. It is also found in reproductive tissues. An established function of GnRH II is in the inhibition of M currents (K(+) channels) through the GnRH II receptor in the amphibian sympathetic ganglion, and it might act through this mechanism as a neuromodulator in the central nervous system. The conservation of structure over 500 million years and the wide tissue distribution of GnRH II suggest that it has a variety of reproductive and nonreproductive functions and will be a productive area of research.

Gonadotropin-releasing hormone (GnRH): from fish to mammalian brains

This work deals with a family of neuropeptides, gonadotropin-releasing hormone (GnRH), that play a key role in the development and maintenance of reproductive function in vertebrates. 2. Until now, a total of 16 GnRH structural variants have been isolated and characterized from vertebrate and protochordate nervous tissue. All vertebrate species already investigated have at least two GnRH forms coexisting in the central nervous system. However, it is now well accepted that three forms of GnRH in early and late evolved bony fishes are present. 3. In these cases, cGnRH-II is expressed by midbrain neurons, a species-specific GnRH is present mainly in the preoptic area and the hypothalamus, and sGnRH is localized in the terminal nerve ganglion (TNG). In this context it is possible to think that three GnRH forms and three GnRH receptor (GnRH-R) subtypes are expressed in the central nervous system of a given species. 4. Then it is possible to propose three different GnRH lineages expressed by distinct brain areas in vertebrates: (1) the conserved cGnRH-II or mesencephalic lineage; or (2) the hypothalamic or "releasing" lineage whose primary structure has diverged by point mutations (mGnRH and its orthologous forms: hrGnRH, wfGnRH, cfGnRH, sbGnRH, and pjGnRH); and (3) the telencephalic sGnRH form. Also different GnRH nomenclatures are discussed.

Effects of mGnRH on testicular steroidogenesis in the toad Bufo arenarum

GnRH controls vertebrate reproduction in several ways. This hormone not only affects the secretion of gonadotropins from the pituitary gland but also has a direct influence on several gonadal functions such as steroidogenesis, spermatogenesis, and spermiation. In the present paper we have studied the in vitro effects of GnRH on the testicular steroidogenesis of Bufo arenarum to ascertain the role of this peptide in the control of the steroidogenic pathway previously described in this species. It was found that GnRH is able to reduce basal as well as hCG-stimulated testosterone release, having an inhibitory effect on P450(c17) activity. Thus, GnRH could be involved in the mechanism that regulates the metabolic change in the testicular steroidogenesis. Additionally, testicular GnRH binding site has been characterised, showing a K(d) of 34 nM and a maximum binding of 4.7 pmol/mg protein.

Control of gonadotropin secretion by follicle-stimulating hormone-releasing factor, luteinizing hormone-releasing hormone, and leptin

Fractionation of hypothalamic extracts on a Sephadex G-25 column separates follicle-stimulating hormone-releasing factor (FSHRF) from luteinizing hormone-releasing hormone (LHRH). The FSH-releasing peak contained immunoreactive lamprey gonadotropin-releasing hormone (lGnRH) by radioimmunoassay, and its activity was inactivated by an antiserum specific to lGnRH. The identity of lGnRH-III with FSHRF is supported by studies with over 40 GnRH analogs that revealed that this is the sole analog with preferential FSH-releasing activity. Selective activity appears to require amino acids 5-8 of lGnRH-III. Chicken GnRH-II has slight selective FSH-releasing activity. Using a specific lGnRH-III antiserum, a population of lGnRH-III neurons was visualized in the dorsal and ventral preoptic area with axons projecting to the median eminence in areas shown previously to control FSH secretion based on lesion and stimulation studies. Some lGnRH-III neurons contained only this peptide, others also contained LHRH, and still others contained only LHRH. The differential pulsatile release of FSH and LH and their differential secretion at different times of the estrous cycle may be caused by differential secretion of FSHRF and LHRH. Both FSH and LHRH act by nitric oxide (NO) that generates cyclic guanosine monophosphate. lGnRH-III has very low affinity to the LHRH receptor. Biotinylated lGnRH-III (10(-9) M) labels 80% of FSH gonadotropes and is not displaced by LHRH, providing evidence for the existence of an FSHRF receptor. Leptin has equal potency as LHRH to release gonadotropins by NO. lGnRH-III specifically releases FSH, not only in rats but also in cows.

Intracellular signaling pathways mediated by the gonadotropin-releasing hormone (GnRH) receptor

The hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator of the reproductive system, triggering the synthesis and release of LH and FSH in the pituitary. GnRH transmits its signal via two specific serpentine receptors that belong to the large group of G-protein coupled receptors (GPCRs). Here we review the intracellular signaling pathways mediated by the GnRH receptor (GnRHR). In pituitary-derived alpha T3-1 cells, a widely used model for GnRH action, GnRHR signaling includes activation of mitogen-activated protein kinase (MAPK) cascades, which provide an important link for the transmission of signals from the cell surface to the nucleus and play a role in the regulation of gonadotropin transcription. Activation of ERK--one of the MAPK cascades--by GnRH in these cells depends mainly on the phosphorylation of Raf1 by PKC, supported by a pathway involving c-Src, dynamin, and Ras. On the other hand, the activation of JNK, another MAPK cascade, involves PKC, c-Src, CDC42/Rac1, and probably MEKK1. The GnRHR is also expressed in non-pituitary cells and was found to be involved in the inhibition of cell proliferation in certain cells. Therefore, GnRHR represents a potential target for GnRH-analogs used for cancer treatment. Interestingly, the signaling mechanism of the GnRHR in other cell types significantly differs from that in pituitary cells. Studies conducted in GnRHR-expressing COS7 cells have shown that GnRHR transmits its signals mainly via Gi, EGF receptor, c-Src, and is not dependent on PKC. Understanding the signaling mechanisms elicited by GnRHR can shed light on the mechanism of action of GnRH in pituitary and extra-pituitary tissues.