Identification of gonadotropin-releasing hormone and associated binding substances in the blood serum of a holocephalan (Hydrolagus colliei)

Characterization of Gonadotropin-Releasing Hormone (GnRH) Genes From Cartilaginous Fish: Evolutionary Perspectives

The neuropeptide gonadotropin-releasing hormone (GnRH) plays an important role in the control of reproductive functions. Vertebrates possess multiple GnRH forms that are classified into three main groups, namely GnRH1, GnRH2, and GnRH3. In order to gain more insights into the GnRH gene family in vertebrates, we sought to identify which paralogs of this family are present in cartilaginous fish. For this purpose, we searched the genomes and/or transcriptomes of three representative species of this group, the small-spotted catshark, Scyliorhinus canicula, the whale shark, Rhincodon typus and the elephant shark Callorhinchus milii. In each species, we report the identification of three GnRH genes. In catshark and whale shark, phylogenetic and synteny analysis showed that these three genes correspond to GnRH1, GnRH2, and GnRH3. In both species, GnRH1 was found to encode a novel form of GnRH whose primary structure was determined as follows: QHWSFDLRPG. In elephant shark, the three genes correspond to GnRH1a and GnRH1b, two copies of the GnRH1 gene, plus GnRH2. 3D structure prediction of the chondrichthyan GnRH-associated peptides (GAPs) revealed that catshark GAP1, GAP2, and elephant shark GAP2 peptides exhibit a helix-loop-helix (HLH) structure. This structure observed for many osteichthyan GAP1 and GAP2, may convey GAP biological activity. This HLH structure could not be observed for elephant shark GAP1a and GAP1b. As for all other GAP3 described so far, no typical 3D HLH structure was observed for catshark nor whale shark GAP3. RT-PCR analysis revealed that GnRH1, GnRH2, and GnRH3 genes are differentially expressed in the catshark brain. GnRH1 mRNA appeared predominant in the diencephalon while GnRH2 and GnRH3 mRNAs seemed to be most abundant in the mesencephalon and telencephalon, respectively. Taken together, our results show that the GnRH gene repertoire of the vertebrate ancestor was entirely conserved in the chondrichthyan lineage but that the GnRH3 gene was probably lost in holocephali. They also suggest that the three GnRH neuronal systems previously described in the brain of bony vertebrates are also present in cartilaginous fish.

Role of elasmobranchs and holocephalans in understanding peptide evolution in the vertebrates: Lessons learned from gonadotropin releasing hormone (GnRH) and corticotropin releasing factor (CRF) phylogenies

The cartilaginous fishes (Class Chondrichthyes) comprise two morphologically distinct subclasses; Elasmobranchii and Holocephali. Evidence indicates early divergence of these subclasses, suggesting monophyly of their lineage. However, such a phylogenetic understanding is not yet developed within two highly conserved peptide lineages, GnRH and CRF. Various GnRH forms exist across the Chondrichthyes. Although 4-7 immunoreactive forms have been described in Elasmobranchii, only one has been elucidated in Holocephali. In contrast, Chondrichthyan CRF phylogeny follows a pattern more consistent with vertebrate evolution. For example, three forms are expressed within the lamprey, with similar peptides present within the genome of the Callorhinchus milii, a holocephalan. Although these findings are consistent with recent evidence regarding the phylogenetic age of Chondrichthyan lineages, CRF evolution in vertebrates remains elusive. Assuming that the Elasmobranchii and Holocephali are part of a monocladistic clade within the Chondrichthyes, we interpret the findings of GnRH and CRF to be products of their respective lineages.

Reproductive endocrinology in chondrichthyans: the present and the future

The class Chondrichthyes, that includes Elasmobranchii and Holocephali, is a diverse group of fish occupying a key position at the base of vertebrate evolution. Their evolutionary success is greatly attributed to their wide range of reproductive strategies controlled by different endocrine mechanics. As in other vertebrates, hormonal control of reproduction in chondrichthyans is mediated by the neuropeptide gonadotropin-releasing hormone (GnRH) that regulates the brain control of gonadal activity via a hypothalamus-pituitary-gonadal (HPG) axis. Chondrichthyans lack of a direct vascular supply from the hypothalamus to the zone of the pituitary where the gonadotropic activity resides, thus transport between these two zones likely occurs via the general circulation. In the brain of elasmobranchs, two groups of GnRH, GnRH-I and GnRH-II were identified, and the presence of two immunoreactive gonadotropins similar to the luteinising (LH) and follicle stimulating (FSH) hormones was identified in the pituitary. In holocephalans, only GnRH-II has been confirmed, and while gonadotropin activity has been found in the buccal pituitary lobe, the presence of gonadotropin receptors in the gonads remains unknowns. The diversity of reproductive strategies display by chondrichthyans makes it difficult to generalize the control of gametogenesis and steroidogenesis; however, some general patterns emerge. In both sexes, androgens and estrogens are the main steroids during gonadal growth; while progestins have maturational activity. Androgens also form the precursors for estrogen steroid production. Estrogens stimulate the hepatic synthesis of yolk and stimulate the development of different part of the reproductive tract in females. The role of other gonadal steroids may play in chondrichthyan reproduction remains largely unknown. Future work should concentrate in filling the gaps into the current knowledge of the HPG axis regulation, and the use of reproductive endocrinology as a non-lethal technique for management of chondrichthyan populations.

Identification of members of the gonadotropin-releasing hormone (GnRH), corticotropin-releasing factor (CRF) families in the genome of the holocephalan, Callorhinchus milii (elephant shark)

The gonadotropin-releasing hormone (GnRH) and corticotropin-releasing family (CRF) are two neuropeptides families that are strongly conserved throughout evolution. Recently, the genome of the holocephalan, Callorhinchus milii (elephant shark) has been sequenced. The phylogenetic position of C. milii, along with the relatively slow evolution of the cartilaginous fish suggests that neuropeptides in this species may resemble the earliest gnathostome forms. The genome of the elephant shark was screened, in silico, using the various conserved motifs of both the vertebrate CRF paralogs and the insect diuretic hormone sequences to identify the structure of the C. milii CRF/DH-like peptides. A similar approach was taken to identify the GnRH peptides using conserved motifs in both vertebrate and invertebrate forms. Two CRF peptides, a urotensin-1 peptide and a urocortin 3 peptide were found in the genome. There was only about 50% sequence identity between the two CRF peptides suggesting an early divergence. In addition, the urocortin 2 peptide seems to have been lost and was identified as a pseudogene in C. milii. In contrast to the number of CRF family peptides, only a GnRH-II preprohormone with the conserved mature decapeptide was found. This confirms early studies about the identity of GnRH in the Holocephali, and suggests that the Holocephali and Elasmobranchii differ with respect to GnRH structure and function.

Testicular gonadotropin-releasing hormone activity, progression of spermatogenesis, and sperm transport in vertebrates

Since the end of the 1970s, studies have shown that, besides the endocrine route, a chemical mediator may also act through autocrine and/or paracrine mechanisms. This has opened new frontiers for research as a result of a redefinition of what endocrinology represents. Apart from androgens within the male gonad, testicular gonadotropin-releasing hormone, estrogens, molecular chaperones, proto-oncogenes, and, very recently, the endocannabinoid system have been shown to play important roles. Their activities to regulate spermatogenesis, including spermiogenesis and sperm maturation, will be discussed from the comparative viewpoint to describe adaptive phenomena and to speculate on evolution.

Gonadotropin‐Releasing Hormone: Gene Evolution, Expression, and Regulation

The gonadotropin-releasing hormone (GnRH) gene is a superb example of the diverse regulation that is required to maintain the function of an evolutionarily conserved and fundamental gene. Because reproductive capacity is critical to the survival of the species, physiological homeostasis dictates optimal conditions for reproductive success, and any perturbation from this balance may affect GnRH expression. These disturbances may include alterations in signals dictated by stress, nutritional imbalance, body weight, and neurological problems; therefore, changes in other neuroendocrine systems may directly influence the hypothalamic-pituitary-gonadal axis through direct regulation of GnRH. Thus, to maintain optimal reproductive capacity, the regulation of the GnRH gene is tightly constrained by a number of diverse signaling pathways and neuromodulators. In this review, we summarize what is currently known of GnRH gene structure, the location and function of the two isoforms of the GnRH gene, some of the many hormones and neuromodulators found to affect GnRH expression, and the molecular mechanisms responsible for the regulation of the GnRH gene. We also discuss the latest models used to study the transcriptional regulation of the GnRH gene, from cell models to evolving in vivo technologies. Although we have come a long way in the last two decades toward uncovering the intricacies behind the control of the GnRH neuron, there remain vast distances to cover before direct therapeutic manipulation of the GnRH gene to control reproductive competence is possible.

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.

Sexually dimorphic effects of NMDA receptor antagonism on brain-pituitary-gonad axis development in the platyfish

The N-methyl-D-aspartate glutamate receptor (NMDAR) is found in hypothalamic nuclei involved in the regulation of reproduction in several species of mammals and fishes. NMDAR is believed to affect reproductive development and function by regulating gonadotropin releasing hormone (GnRH)-producing cells. These pathways are likely to be sexually dimorphic, as are several other neurotransmitter systems involved in reproductive function. In this report, male and female platyfish received intraperitoneal injections of 0, 5, 10, 20, 40 or 60 microg/g body wt. of the non-competitive NMDAR antagonist MK-801. Injections began at 6 weeks of age and continued thrice weekly until control animals reached puberty, as evidenced by anal fin maturation. The percent of pubescent animals was significantly affected by sex and treatment, with fewer MK-801-injected females in puberty than control females at each dose (P<0.001), and fewer pubescent females than males at 10, 20 and 40 microg/g (P<0.05). There were no MK-801-related effects in males. Histological analyses revealed typical immature gonads and pituitary glands in treated females, and typical mature morphology in control females and all males. Immunocytochemical distribution of the R1 subunit of the NMDAR within the brain-pituitary-gonad (BPG) axis was limited to GnRH-containing brain cells in all animals; however, NMDAR1 distribution was in an immature pattern in treated females and a mature pattern in all others. Neural concentrations of GnRH were unaffected by MK-801 treatment in both sexes. These data suggest that in the platyfish, NMDAR influence on reproductive development is sexually dimorphic and occurs at, or above, the level of GnRH-containing cells of the BPG axis.

Chromatographic and immunological identification of GnRH (gonadotropin-releasing hormone) variants. Occurrence of mammalian and a salmon-like GnRH in the forebrain of an eutherian mammal: Hydrochaeris hydrochaeris (Mammalia, Rodentia)

The molecular variants of Gonadotropin releasing hormone (GnRH) in brain extracts of the eutherian mammal Hydrochaeris hydrochaeris (Mammalia, Rodentia) were characterized. An indirect method combining reverse-phase high-performance liquid chromatography (RP-HPLC) and radioimmunoassay (RIA) with different antisera was used. Two different forebrain regions (olfactory bulbs and preoptic-hypothalamic region) were analyzed. Characterization of RP-HPLC fractions from preoptic-hypothalamic extracts with three different RIA systems revealed two immunoreactive GnRH (ir-GnRH) peaks coeluting with mammalian GnRH (mGnRH) and salmon GnRH (sGnRH) synthetic standards. These results were additionally supported by serial dilution studies with specific antisera. Similar results were obtained from olfactory bulb extracts with the same methodology. However, a third ir-GnRH peak in a similar position to that of chicken GnRH II (cIIGnRH) synthetic standard was revealed. As far as we know, this is the first report showing chromatographic and immunological evidences for the presence of a second GnRH variant in the forebrain of an eutherian mammal.

Detection of GnRH molecular forms in brains and gonads of the crested newt, Triturus carnifex

Gonadotrophin-releasing hormone (GnRH) immunoreactivity is detectable in the brain, ovary, and testis of the newt, Triturus carnifex, collected during February (reproductive phase), May, and July (nonreproductive phase). In the brain of May animals, chicken GnRH-II positive cell bodies are located within the terminal nerve, the anterior preoptic area, and the preoptic nucleus, which appears to be devoid of immunoreactive mammalian GnRH cell bodies. During February and July, both chicken GnRH-II and mammalian GnRH are detected only within the terminal nerve and anterior preoptic area. Generally, in the reproductive as well as the nonreproductive periods, chicken GnRH-II fibers are widely distributed in the brain; however, the distribution of fibers of both molecular forms suggests that they exert hypophysiotropic activity. High-pressure liquid chromatography (HPLC) coupled with radioimmunoassay indicates the presence of an early-eluting GnRH peak in brains and gonads but not in plasma. Using chicken GnRH-II antiserum, immunoreactivity is observed in spermatocytes, spermatozoa, and the external theca layer. Seasonal changes of the GnRH-like material are observed in both sexes, and its high concentration detectable during February is in good correlation with the timing of reproduction.

Peptide hormone evolution: functional heterogeneity within GnRH and CRF families

Recent investigations indicate that the gonadotropin-releasing hormone (GnRH) and corticotropin-releasing factor (CRF) family of peptides are each composed of at least two functionally discrete paralogous lineages. [His5Trp7Tyr8]GnRH (chicken GnRH-II) is associated with brain neuromodulatory and possibly peripheral endocrine activity, whereas [Arg8]GnRH (mammal GnRH) and its orthologues play major roles as hypothalamic releasing factors. Similarly, CRF appears to be the primary vertebrate ACTH-releasing peptide, whereas the paralogous lineage of urotensin-I-sauvagine has been associated with a variety of diverse peripheral activities. In phylogenetically older species, representatives of both GnRH and CRF family lineages have been characterized. Structural and functional conservation of these peptide systems in vertebrates suggest that additional GnRH-like and CRF-like peptides will be found in the mammal brain.

Evolutionary aspects of gonadotropin-releasing hormone and its receptor

1. Gonadotropin-releasing hormone (GnRH) was originally isolated as a hypothalamic peptide hormone that regulates the reproductive system by stimulating the release of gonadotropins from the anterior pituitary. However, during evolution the peptide was subject to gene duplication and structural changes, and multiple molecular forms have evolved. 2. Eight variants of GnRH are known, and at least two different forms are expressed in species from all vertebrate classes: chicken GnRH II and a second, unique, GnRH isoform. 3. The peptide has been recruited during evolution for diverse regulatory functions: as a neurotransmitter in the central and sympathetic nervous systems, as a paracrine regulator in the gonads and placenta, and as an autocrine regulator in tumor cells. 4. Evidence suggests that in most species the early-evolved and highly conserved chicken GnRH II has a neurotransmitter function, while the second form, which varies across classes, has a physiologic role in regulating gonadotropin release. 5. We review here evolutionary aspects of the family of GnRH peptides and their receptors.

Identification of chicken GnRH II in brains of metatherian and early-evolved eutherian species of mammals

Two molecular forms of GnRH (chicken GnRH II and a second variant) are present in the brains of species from all the major vertebrate groups. In mammals, two forms are present in metatherian species and early-evolved eutherian species, but chicken GnRH II has not been identified in more advanced eutherian species. We investigated the nature of GnRH molecular forms in several early-evolved mammalian species, using high performance liquid chromatography and radioimmunoassay with specific GnRH antisera. These chromatographic and immunological data indicate that in the brains of a metatherian species (possum, Trichosurus vulpecula) and in two early-evolved eutherian species (order Insectivora: musk shrew, Suncus murinus and mole, Chrysochloris asiatica), both mammalian and chicken II GnRHs are present, while in another relatively early-evolved eutherian species (order Chiroptera: bat, Miniopterus schreibersii) only mammalian GnRH is present. In the adult possum and mole brains the proportion of chicken GnRH II was lower than that of mammalian GnRH, while in the musk shrew brain chicken GnRH II predominated. A peptide likely to be mammalian proGnRH was detected in the brains of the three eutherian species (musk shrew, mole, and bat). These findings suggest that metatherian and primitive eutherian species of mammals continue to express chicken GnRH II as in the vast majority of nonmammalian vertebrates, while the peptide is apparently not expressed in modern placental mammalian species. The functional significance of chicken GnRH II is not yet clear, but there are indications that it has a neurotransmitter or neuromodulator role in addition to that of regulating pituitary hormone release in certain vertebrate species.