Evolution of the gonadotropin-releasing hormone (GnRH) gene family in relation to vertebrate tetraploidizations

Characterization of Kiss/Kissr system and expression profiling through developmental stages indicate kiss1 to be the active isotype in Clarias magur

Kisspeptin (Kiss) and kisspeptin receptor (Kissr) system is a key regulator of GnRH expression in several vertebrates. The Indian catfish, Clarias magur, is popular in the Indian sub-continent, and a neo-type of the Asian catfish, C. batrachus. Catfish breeding is constrained as males do not release milt captivity with/without stimulation. Magur Kiss/Kissr system comprising of kiss1, kiss2, kissr1, and kissr2 genes was characterized for the first time. Full-length mRNA was sequenced using RACE PCR. Neighbor-joining tree of predicted proteins shows one clade of teleost orthologs. Magur whole genome (NCBI GenBank) has single copies of each gene, though yet unannotated/misannotated. Anomalies in the nomenclature of earlier sequences in GenBank were noted. Relative gene expression was profiled during various ontogenic stages, in six tissues including brain and gonads at maturity, and also in brains and gonads of premature and spent fish. Expression of gnrh1, gnrhr1, and gnrhr2 was estimated concomitantly. The kiss1 was the first to be twofold upregulated (P < 0.05) at 12 h post fertilization. Kiss/Kissr genes expressed primarily in the brain, ovary, and testis. Though kiss2 was 10 times higher than kiss1, only kiss1 showed significant modulation across stages and appears to be the active isotype that regulates GnRH in magur.

Cellular localization and seasonal variation of GnRH and Bradykinin in the ovary of Heteropneustes fossilis (Bloch.) during its reproductive cycle

The present study investigates the distribution and dynamics of gonadotropin-releasing hormone I (GnRH I) and bradykinin in the air-breathing catfish, Heteropneustes fossilis, in relation to the reproductive cycle. Changes in bradykinin, bradykinin B2-receptor, and ovarian GnRH I regulation were demonstrated during the reproductive cycle. The localization of GnRH I, bradykinin, and their respective receptors in the ovaries was investigated by immunohistochemistry, while their levels were quantified by slot/western blot followed by densitometry. GnRH I and its receptor were mainly localized in the cytoplasm of oocytes during the early previtellogenic phase. However, as the follicles grew larger, immunoreactivity was observed in the granulosa and theca cells of the late previtellogenic follicles. The ovaries showed significantly higher expression of GnRH I protein and its receptor during the early to mid-previtellogenic phase, suggesting their involvement in follicular development. Bradykinin and bradykinin B2-receptor showed a distribution pattern similar to that of GnRH I and its receptor. This study further suggested the possibility that bradykinin regulates GnRH I synthesis in the ovary. Thus, we show that the catfish ovary has a GnRH-bradykinin system and plays a role in follicular development and oocyte maturation in H. fossilis.

Central regulation of reproduction in amphibians

This review article includes a literature review of synteny analysis of the amphibian gonadotropin-releasing hormone (GnRH) genes, the distribution of GnRH 1 and GnRH2 neurons in the central nervous system of amphibians, the function and regulation of hypophysiotropic GnRH1, and the function of GnRH1 in amphibian reproductive behaviors. It is generally accepted that GnRH is the key regulator of the hypothalamic-pituitary-gonadal axis. Three independent GnRH genes, GnRH1, GnRH2, and GnRH3, have been identified in vertebrates. Previous genome synteny analyses suggest that there are likely just two genes, gnrh1 and gnrh2, in amphibians. In three groups of amphibians: Anura, Urodela, and Gymnophiona, the distributions of GnRH1 and GnRH2 neurons in the central nervous system have also been previously reported. Moreover, these neuronal networks were determined to be structurally independent in all species examined. The somata of GnRH1 neurons are located in the terminal nerve, medial septum (MS), and preoptic area (POA), and some GnRH1 neurons in the MS and POA project into the median eminence. In contrast, the somata of GnRH2 neurons are located in the midbrain tegmentum. In amphibians, GnRH1 neurons originate from the embryonic olfactory placode, while GnRH2 neurons originate from the midbrain. The characterization and feedback regulation mechanisms of hypophysiotropic GnRH1 neurons in amphibians, the involvement of GnRH1 in amphibian reproductive behavior, and its possible mechanism of action should be elucidated in future.

Expression and localization of HPG axis-related genes in Carassius auratus with different ploidy

Introduction

In the Dongting water system, the Carassius auratus (Crucian carp) complex is characterized by the coexistence of diploid forms (2n=100, 2nCC) and polyploidy forms. The diploid (2nCC) and triploid C.auratus (3n=150, 3nCC) had the same fertility levels, reaching sexual maturity at one year.

Methods

The nucleotide sequence, gene expression, methylation, and immunofluorescence of the gonadotropin releasing hormone 2(Gnrh2), Gonadotropin hormone beta(Gthβ), and Gonadotropin-releasing hormone receptor(Gthr) genes pivotal genes of the hypothalamic-pituitary-gonadal (HPG) axis were analyzed.

Results

The analysis results indicated that Gnrh2, follicle-stimulating hormone receptor(Fshr), and Lethal hybrid rescue(Lhr) genes increased the copy number and distinct structural differentiation in 3nCC compared to that in 2nCC. The transcript levels of HPG axis genes in 3nCC were higher than 2nCC (P<0.05), which could promote the production and secretion of sex steroid hormones conducive to the gonadal development of 3nCC. Meanwhile, the DNA methylation levels in the promoter regions of the HPG axis genes were lower in 3nCC than in 2nCC. These results suggested that methylation of the promoter region had a potential regulatory effect on gene expression after triploidization. Immunofluorescence showed that the localization of the Fshβ, Lhβ, and Fshr genes between 3nCC and 2nCC remained unchanged, ensuring the normal expression of these genes at the corresponding sites after triploidization.

Discussion

Relevant research results provide cell and molecular biology evidence for normal reproductive activities such as gonad development and gamete maturation in triploid C. auratus, and contribute to further understanding of the genetic basis for fertility restoration in triploid C. auratus.

Functional Characterization of Three GnRH Isoforms in Small Yellow Croaker Larimichthys polyactis Maintained in Captivity: Special Emphasis on Reproductive Dysfunction

Fish reproduction is regulated by the brain–pituitary–gonad (BPG) axis where the gonadotropin-releasing hormone (GnRH) plays a central role. Seed production of small yellow croaker (Larimichthys polyactis) is performed using captive-reared broodstock known to undergo reproductive dysfunction, which is connected to endocrinological dysfunction. To determine the endocrinological mechanism of GnRHs in the BPG axis of small yellow croaker, full-length sequences of three GnRH isoforms encoding sbGnRH (GnRH1), cGnRH-II (GnRH2), and sGnRH (GnRH3) were cloned and characterized from brain tissue. qRT-PCR, in vivo, and in vitro experiments were performed for functional characterization. The mRNA expression of GnRH1 in the brain and gonadotropin subunits (GPα, FSHβ, and LHβ) in the pituitary were significantly higher at the ripen stage during gonadal development and GnRH1 at spawning stage during spawning events. Expression of both GnRH1 and GtH subunits was significantly lower in females than males. GtH subunits were induced at higher concentrations of GnRH1 in vivo and in vitro. Sex-steroids significantly inhibited the GnRH1 expression in vitro in a dose-dependent manner. Taken together, results indicated that GnRH1 plays a key role in gonadal maturation and sex-steroids induced negative feedback in the regulation of GnRH. A lower level of GnRH1 and GtHs might be responsible for reproductive dysfunction in a female small yellow croaker.

Presence of GnRH3 in sturgeon and the roles of GnRH1 and GnRH2 on the regulation of LH/FSH in mature female sterlet Acipenser ruthenus in vitro

Three forms of gonadotropin-releasing hormones (GnRHs), ArGnRH1, ArGnRH2, and ArGnRH3, were identified in sterlet. Compared with their orthologue, ArGnRH1 and ArGnRH2 have conserved core decapeptide but show low identity in the signal peptide and the rest of the sequences. The existence of the GnRH3 paralogue of sturgeon was predicted for the first time with TBLASTN by using the amino acid sequences of catshark and whale shark GnRH3 precursor as queries against the whole genome and transcript data of sterlet. The predicted ArGnRH3 cDNA sequence was composed of three exons containing all the elements of the GnRH family. The successful molecular cloning of GnRH3 from sterlets verified its expression in the brain of sturgeons. The analysis of the ArGnRH3 amino acid sequence revealed a completely conserved decapeptide sequence that shows 100% identity with the sequence of teleosts and differs in one amino acid with that of the cartilaginous fish (catshark and whale shark) at the 5th position. The structure of the phylogenetic tree showed that a total of 52 vertebrate GnRH sequences were clustered into three main clades corresponding to GnRH1, GnRH2, and GnRH3. The ArGnRH3 sequence is the oldest GnRH3 identified in teleosts. The tissue distribution analysis showed that ArGnRH1 was expressed in all the 13 examined tissues of females and in most of the tested tissues of male fish, with the highest expression in the pituitary and hypothalamus. ArGnRH2 is only expressed in the pituitary, hypothalamus, and gonads of both female and male sterlets. ArGnRH3 mRNA could be detected in the pituitary, hypothalamus, and gonad in both female and male fish. It is also present in the spleen, head kidney, and gill in female fish and in kidney and heart in male fish. However, the ArGnRH3 only showed weak expression in all the positive tissues. ArGnRH1 and ArGnRH2 active decapeptides were synthesized to investigate their roles on the regulation of LH/FSH using a mixed brain cell line from a sexually mature female sterlet. The results showed that ArGnRH1 and ArGnRH2 exerted different effects on the gene expression and release of gonadotropins. ArGnRH1 promoted the expression of fshβ significantly around 48 h, and the expression was suppressed when the treatment time was extended to 72 h. ArGnRH1 had no significant effects on the level of either mRNA or secreted lh in any of the tested treatment length or concentrations. Moreover, ArGnRH1 did not stimulate the activity of gonadotropins in the maturation stage of female sturgeons. ArGnRH2 promoted the expression of fshβ at 24 h and 48 h and increased mRNA level of lhβ at 6 h and 48 h, accompanied by the significant secretion of LH at 72 h, although the high mRNA level of fsh did not correlate with the secretion of FSH in ArGnRH2-treated groups. In conclusion, ArGnRH2 plays an important role in the maturation stage of female sterlets. Therefore, ArGnRH2 has the potential to induce ovulation and spermiation in sturgeons.

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).

Comparative insights of the neuroanatomical distribution of the gonadotropin-inhibitory hormone (GnIH) in fish and amphibians

This paper intends to apprise the reader regarding the existing knowledge on the neuroanatomical distribution of GnIH-like peptides in in fish and amphibians in both the adult stage and during ontogenesis. The neuroanatomical distribution of GnIH-like neuropeptides appears quite different in the studied species, irrespective of the evolutionary closeness. The topology of the olfactory bulbs can affect the distribution of neurons producing the GnIH-like peptides, with a tendency to show a more extended distribution into the brains with pedunculate olfactory bulbs. Therefore, the variability of the GnIH-like system could also reflect specific adaptations rather than evolutionary patterns. The onset of GnIH expression was detected very early during development suggesting its precocious roles, and the neuroanatomical distribution of GnIH-like elements showed a generally increasing trend. This review highlights some critical technical aspects and the need to increase the number of species to be studied to obtain a complete neuroanatomical picture of the GnIH-like system.

Novel pituitary actions of GnRH in teleost: The link between reproduction and feeding regulation

Gonadotropin-releasing hormone (GnRH), as a vital hypothalamic neuropeptide, was a key regulator for pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in the vertebrate. However, little is known about the other pituitary actions of GnRH in teleost. In the present study, two GnRH variants (namely, GnRH2 and GnRH3) and four GnRH receptors (namely, GnRHR1, GnRHR2, GnRHR3, and GnRHR4) had been isolated from grass carp. Tissue distribution displayed that GnRHR4 was more highly detected in the pituitary than the other three GnRHRs. Interestingly, ligand-receptor selectivity showed that GnRHR4 displayed a similar and high binding affinity for grass carp GnRH2 and GnRH3. Using primary culture grass carp pituitary cells as model, we found that both GnRH2 and GnRH3 could not only significantly induce pituitary reproductive hormone gene (GtHα, LHβ, FSHβ, INHBa, secretogranin-2) mRNA expression mediated by AC/PKA, PLC/IP₃/PKC, and Ca²⁺/CaM/CaMK-II pathways but also reduce dopamine receptor 2 (DRD2) mRNA expression via the Ca²⁺/CaM/CaMK-II pathway. Interestingly, GnRH2 and GnRH3 could also stimulate anorexigenic peptide (POMCb, CART2, UTS1, NMBa, and NMBb) mRNA expression via AC/PKA, PLC/IP₃/PKC, and Ca²⁺/CaM/CaMK-II pathways in grass carp pituitary cells. In addition, food intake could significantly induce brain GnRH2 mRNA expression. These results indicated that GnRH should be the coupling factor to integrate the feeding metabolism and reproduction in teleost.

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

Kisspeptin and GnRH interactions in the reproductive brain of teleosts

It is well known that gonadotropin-releasing hormone (Gnrh) has a key role in reproduction by regulating the synthesis and release of gonadotropins from the anterior pituitary gland of all vertebrates. About 25 years ago, another neuropeptide, kisspeptin (Kiss1) was discovered as a metastasis suppressor of melanoma cell lines and then found to be essential for mammalian reproduction as a stimulator of hypothalamic Gnrh and regulator of puberty onset. Soon after, a kisspeptin receptor (kissr) was found in the teleost brain. Nowadays, it is known that in most teleosts the kisspeptin system is composed of two ligands, kiss1 and kiss2, and two receptors, kiss2r and kiss3r. Even though both kisspeptin peptides, Kiss1 and Kiss2, have been demonstrated to stimulate gonadotropin synthesis and secretion in different fish species, their actions appear not to be mediated by Gnrh neurons as in mammalian models. In zebrafish and medaka, at least, hypophysiotropic Gnrh neurons do not express Kiss receptors. Furthermore, kisspeptinergic nerve terminals reach luteinizing hormone cells in some fish species, suggesting a direct pituitary action. Recent studies in zebrafish and medaka with targeted mutations of kiss and/or kissr genes reproduce relatively normally. In zebrafish, single gnrh mutants and additionally those having the triple gnrh3 plus 2 kiss mutations can reproduce reasonably well. In these fish, other neuropeptides known to affect gonadotropin secretion were up regulated, suggesting that they may be involved in compensatory responses to maintain reproductive processes. In this context, the present review explores and presents different possibilities of interactions between Kiss, Gnrh and other neuropeptides known to affect reproduction in teleost fish. Our intention is to stimulate a broad discussion on the relative roles of kisspeptin and Gnrh in the control of teleost reproduction.

The gonadotropin-releasing hormones: Lessons from fish

Fish have been of paramount importance to our understanding of vertebrate comparative neuroendocrinology and the mechanisms underlying the physiology and evolution of gonadotropin-releasing hormones (GnRH) and their genes. This review integrates past and recent knowledge on the Gnrh system in the fish model. Multiple Gnrh isoforms (two or three forms) are present in all teleosts, as well as multiple Gnrh receptors (up to five types), which differ in neuroanatomical localization, pattern of projections, ontogeny and functions. The role of the different Gnrh forms in reproduction seems to also differ in teleost models possessing two versus three Gnrh forms, Gnrh3 being the main hypophysiotropic hormone in the former and Gnrh1 in the latter. Functions of the non-hypothalamic Gnrh isoforms are still unclear, although under suboptimal physiological conditions (e.g. fasting), Gnrh2 may increase in the pituitary to ensure the integrity of reproduction under these conditions. Recent developments in transgenesis and mutagenesis in fish models have permitted the generation of fish lines expressing fluorophores in Gnrh neurons and to elucidate the dynamics of the elaborate innervations of the different neuronal populations, thus enabling a more accurate delineation of their reproductive roles and regulations. Moreover, in combination with neuronal electrophysiology, these lines have clarified the Gnrh mode of actions in modulating Lh and Fsh activities. While loss of function and genome editing studies had the premise to elucidate the exact roles of the multiple Gnrhs in reproduction and other processes, they have instead evoked an ongoing debate about these roles and opened new avenues of research that will no doubt lead to new discoveries regarding the not-yet-fully-understood Gnrh system.

A 45-years journey within the reproductive brain of fish

This article summarizes the scientific carrier of Dr. Olivier Kah, currently emeritus research director at the National Center of Scientific Research (CNRS) in France. Olivier Kah partly grew up in Africa where he developed a strong interest for animals. He studied biology in Paris and Bordeaux. He next received his PhD at the University of Bordeaux en 1978 and his Doctor of Science degree in 1983. He joined the CNRS in 1979 until his retirement in 2016. Olivier Kah dedicated his carrier to the study of reproduction, in particular to the roles of brain neuropeptides and neurotransmitters in the control of the reproductive axis in vertebrates, mostly fish. More specifically, Olivier Kah was specialized in the use of morphofunctional techniques that he implemented to the study of the organization of the hypothalamo-pituitary complex. He was also interested in the steroid feedback and studied intensively the expression and regulation of estrogen and glucocorticoid receptors in the rainbow trout and the zebrafish. In the last 10 years, Olivier Kah's team focused on the expression and regulation of aromatase in the brain and established that aromatase expression is restricted to a unique brain cell type, the radial glial cells, which serve as progenitors during the entire life of fish. He is also interested in the impact of endocrine disruptors using the zebrafish as a model and recently his team has developed an exquisitely sensitive in vivo assay to screen estrogenic chemicals on zebrafish embryos.

Identification of gnrh2 and gnrh3 and their expression during brood pouch growth and short-term benzo(a)pyrene exposure in lined seahorse (Hippocampus erectus)

Gonadotropin-releasing hormones (GnRH) regulate gonadal growth of teleosts. Benzo(a)pyrene (BaP) functions as a reproductive endocrine disruptor. Furthermore, endocrine regulation on brood pouch growth of Syngnathidaes is elusive. To better understand the role of GnRH in brood pouch growth and effects of BaP on reproductive endocrine in lined seahorse (Hippocampus erectus), gnrh2 and gnrh3 genes were identified. Results showed that lined seahorse GnRH2 and GnRH3 precursors included the conservative tripartite structure and their transcripts highly expressed in brain as other teleosts. Expression profiles of gnrh2 and gnrh3 transcripts were detected during brood pouch growth. Results indicated that brain gnrh2 transcripts remarkably increased at the middle-stage and late-stage of brood pouch growth, while brain gnrh3 transcripts significantly raised at the early-stage and middle-stage. These suggested that GnRH2 and GnRH3 regulated brood pouch growth at different stages. Short-term BaP exposure in lined seahorse was performed. Transcripts of gnrh2 and gnrh3 remarkably increased in females and males exposed to BaP. Besides, plasma 17-beta estradiol (E2) levels presented a reduced trend during female fish exposed to BaP. This revealed that BaP functioned as anti-estrogenic effects and it may result in high expression of gnrh mRNA. However, plasma 11-ketone testosterone (11-KT) levels showed an increased trend during male fish exposed to BaP. Taken together, these indicated interesting results of BaP on reproduction in each sex of seahorse. These observations contribute to provide novel information of regulation on brood pouch growth and effects of BaP on reproductive endocrine in Syngnathidaes.

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

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

Neuroendocrinology of reproduction: Is gonadotropin-releasing hormone (GnRH) dispensable?

Gonadotropin releasing hormone (GnRH) is a highly conserved neuroendocrine decapeptide that is essential for the onset of puberty and the maintenance of the reproductive state. First identified in mammals, the GnRH signaling pathway is found in all classes of vertebrates; homologues of GnRH have also been identified in invertebrates. In addition to its role as a hypothalamic releasing hormone, GnRH has multiple functions including modulating neural activity within specific regions of the brain. These various functions are mediated by multiple isoforms, which are expressed at diverse locations within the central nervous system. Here we discuss the GnRH signaling pathways in light of new reports that reveal that some vertebrate genomes lack GnRH1. Not only do other isoforms of GnRH not compensate for this gene loss, but elements upstream of GnRH1, including kisspeptins, appear to also be dispensable. We discuss routes that may compensate for the loss of the GnRH1 pathway.

Morphological relationship between GnIH and GnRH neurons in the brain of the neotropical cichlid fish Cichlasoma dimerus

Reproduction is regulated by the hypothalamic-pituitary-gonadal axis. The first neuropeptide identified that regulates this function was the decapeptide gonadotropin-releasing hormone (GnRH). Nowadays, in gnatostomates, a number of GnRH variants have been identified and classified into three different types: GnRH1, GnRH2, and GnRH3. Almost 30 years later, a new peptide that inhibits gonadotropin synthesis and secretion was discovered and thus named as gonadotropin-inhibitory hormone (GnIH). In avians and mammals, the interaction and regulation between GnRH and GnIH neurons has been widely studied; however, in other vertebrate groups there is little information about the relationship between these neurons. In previous works, three GnRH variants and a GnIH propeptide were characterized in Cichlasoma dimerus, and it was demonstrated that GnIH inhibited gonadotropins release in this species. Because no innervation was detected at the pituitary level, we speculate that GnIH would inhibit gonadotropins via GnRH. Thus, the aim of the present study was to evaluate the anatomical relationship between neurons expressing GnIH and the three GnRH variants by double labelling confocal immunofluorescence in adults of C. dimerus. Our results showed no apparent contacts between GnIH and GnRH1, fiber to fiber interactions between GnIH and GnRH2, and co-localization of GnIH and GnRH3 variant in neurons of the nucleus olfacto-retinalis. In conclusion, whether GnIH regulates the expression or secretion of GnRH1 in this species, an indirect modulation seems more plausible. Moreover, the present results suggest an interaction between GnIH and GnRH2 systems. Finally, new clues were provided to investigate the role of nucleus olfacto-retinalis cells and putative GnIH and GnRH3 interactions in the modulation of the reproductive network in teleost fish.

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.

Characterization of kiss2 and kissr2 genes and the regulation of kisspeptin on the HPG axis in Cynoglossus semilaevis

Reproduction allows organisms to produce offspring. Animals shift from immature juveniles into mature adults and become capable of sexual reproduction during puberty, which culminates in the first spermiation and sperm hydration or ovulation. Reproduction is closely related to the precise control of the hypothalamic-pituitary-gonadal (HPG) axis. Kisspeptin peptides are considered as the important regulator of HPG axis in mammalian. However, the current understanding of kisspeptin in flatfish is not comprehensive. In this study, we cloned and analyzed the kiss2 and kissr2 genes in Cynoglossus semilaevis. Interesting alternative splicing in the 5'-untranslated regions (UTR) of the Cskissr2 gene was found. The expression profiles of Cskiss2 and Cskissr2 showed relative high messenger RNA (mRNA) levels at the late gastrula stage during embryonic development, at total length = 40 mm during early gonadal differentiation, and in the brains and gonads of all investigated tissues. These results suggested that the kisspeptin system participated in embryogenesis and in the regulation of gonadal differentiation and development. Considering that the control and regulatory mechanisms of kisspeptin in the central reproductive axis are still unclear, we documented that the intramuscular injection of kisspeptin caused different sGnRH and cGnRH mRNA levels in a dose- and tissue-dependent manner. The mRNA expressions of FSH and LH were stimulated in the ovary and were inhibited in the testis under the kisspeptin treatments. These results provided foundations for understanding the roles of kisspeptin in the neuroendocrine system in fish. The manipulation of the kisspeptin system may provide new opportunities to control the gonadal development and even reproduction in fish.

Genetic Evidence for Multifactorial Control of the Reproductive Axis in Zebrafish

It is generally believed that kisspeptin and gonadotropin-releasing hormone (GnRH) are required for reproduction in vertebrates. In this study, we generated gnrh3-null zebrafish and found that gnrh3 mutation did not impair gonad development and reproductive capacity. Moreover, zebrafish triple knockout mutant lacking gnrh3 and the 2 kiss1s genes undergo normal puberty and gonad maturation. The expression of follicle-stimulating hormone beta (fshβ) and luteinizing hormone beta (lhβ) was not significantly altered whereas the expression of neuropeptide Y (npy), tachykinin 3 (tac3), and secretogranin-II (sgII) was significantly increased in the triple knockout mutant, suggesting that compensation mechanisms exist to stimulate the reproductive axis in the absence of kiss and gnrh. Our results challenge the prevailing view that GnRH is indispensable for reproduction across species. These data provide genetic evidence that different mechanisms have evolved for the neuroendocrine control of reproduction between mammals and fish: pulsatile release of GnRH to the portal system is the final gateway to stimulate the reproductive axis in mammals, whereas multiple factors act in parallel with GnRH to stimulate the reproductive axis in certain fish species.

Genetic analysis of the reproductive axis in fish using genome-editing nucleases

Reproduction in fish is controlled by the brain-pituitary-gonad reproductive axis. Although genes of the reproductive axis are conserved from fish to humans, their in vivo functions are less clear in fish. Mutant lines of the reproductive axis have been systematically investigated in zebrafish and medaka using recently developed genome-editing nucleases. Here, we review recent progress in the genetic analysis of the reproductive axis in fish as well as the opportunities and challenges of applying genome-editing nucleases in fisheries.

Conservation of Three-Dimensional Helix-Loop-Helix Structure through the Vertebrate Lineage Reopens the Cold Case of Gonadotropin-Releasing Hormone-Associated Peptide

GnRH-associated peptide (GAP) is the C-terminal portion of the gonadotropin-releasing hormone (GnRH) preprohormone. Although it was reported in mammals that GAP may act as a prolactin-inhibiting factor and can be co-secreted with GnRH into the hypophyseal portal blood, GAP has been practically out of the research circuit for about 20 years. Comparative studies highlighted the low conservation of GAP primary amino acid sequences among vertebrates, contributing to consider that this peptide only participates in the folding or carrying process of GnRH. Considering that the three-dimensional (3D) structure of a protein may define its function, the aim of this study was to evaluate if GAP sequences and 3D structures are conserved in the vertebrate lineage. GAP sequences from various vertebrates were retrieved from databases. Analysis of primary amino acid sequence identity and similarity, molecular phylogeny, and prediction of 3D structures were performed. Amino acid sequence comparison and phylogeny analyses confirmed the large variation of GAP sequences throughout vertebrate radiation. In contrast, prediction of the 3D structure revealed a striking conservation of the 3D structure of GAP1 (GAP associated with the hypophysiotropic type 1 GnRH), despite low amino acid sequence conservation. This GAP1 peptide presented a typical helix-loop-helix (HLH) structure in all the vertebrate species analyzed. This HLH structure could also be predicted for GAP2 in some but not all vertebrate species and in none of the GAP3 analyzed. These results allowed us to infer that selective pressures have maintained GAP1 HLH structure throughout the vertebrate lineage. The conservation of the HLH motif, known to confer biological activity to various proteins, suggests that GAP1 peptides may exert some hypophysiotropic biological functions across vertebrate radiation.

Identification of prohormones and pituitary neuropeptides in the African cichlid, Astatotilapia burtoni

Background

Cichlid fishes have evolved remarkably diverse reproductive, social, and feeding behaviors. Cell-to-cell signaling molecules, notably neuropeptides and peptide hormones, are known to regulate these behaviors across vertebrates. This class of signaling molecules derives from prohormone genes that have undergone multiple duplications and losses in fishes. Whether and how subfunctionalization, neofunctionalization, or losses of neuropeptides and peptide hormones have contributed to fish behavioral diversity is largely unknown. Information on fish prohormones has been limited and is complicated by the whole genome duplication of the teleost ancestor. We combined bioinformatics, mass spectrometry-enabled peptidomics, and molecular techniques to identify the suite of neuropeptide prohormones and pituitary peptide products in Astatotilapia burtoni, a well-studied member of the diverse African cichlid clade.

Results

Utilizing the A. burtoni genome, we identified 148 prohormone genes, with 21 identified as a single copy and 39 with at least 2 duplicated copies. Retention of prohormone duplicates was therefore 41 %, which is markedly above previous reports for the genome-wide average in teleosts. Beyond the expected whole genome duplication, differences between cichlids and mammals can be attributed to gene loss in tetrapods and additional duplication after divergence. Mass spectrometric analysis of the pituitary identified 620 unique peptide sequences that were matched to 120 unique proteins. Finally, we used in situ hybridization to localize the expression of galanin, a prohormone with exceptional sequence divergence in cichlids, as well as the expression of a proopiomelanocortin, prohormone that has undergone an additional duplication in some bony fish lineages.

Conclusion

We characterized the A. burtoni prohormone complement. Two thirds of prohormone families contain duplications either from the teleost whole genome duplication or a more recent duplication. Our bioinformatic and mass spectrometric findings provide information on a major vertebrate clade that will further our understanding of the functional ramifications of these prohormone losses, duplications, and sequence changes across vertebrate evolution. In the context of the cichlid radiation, these findings will also facilitate the exploration of neuropeptide and peptide hormone function in behavioral diversity both within A. burtoni and across cichlid and other fish species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2914-9) contains supplementary material, which is available to authorized users.

Neuronal Gonadotrophin-Releasing Hormone (GnRH) and Astrocytic Gonadotrophin Inhibitory Hormone (GnIH) Immunoreactivity in the Adult Rat Hippocampus

Gonadotrophin-releasing hormone (GnRH) and gonadotrophin inhibitory hormone (GnIH) are neuropeptides secreted by the hypothalamus that regulate reproduction. GnRH receptors are not only present in the anterior pituitary, but also are abundantly expressed in the hippocampus of rats, suggesting that GnRH regulates hippocampal function. GnIH inhibits pituitary gonadotrophin secretion and is also expressed in the hippocampus of a songbird; its role outside of the reproductive axis is not well established. In the present study, we employed immunohistochemistry to examine three forms of GnRH [mammalian GnRH-I (mGnRH-I), chicken GnRH-II (cGnRH-II) and lamprey GnRH-III (lGnRH-III)] and GnIH in the adult rat hippocampus. No mGnRH-I and cGnRH-II+ cell bodies were present in the hippocampus. Sparse mGnRH-I and cGnRH-II+ fibres were present within the CA1 and CA3 fields of the hippocampus, along the hippocampal fissure, and within the hilus of the dentate gyrus. No lGnRH-III was present in the rodent hippocampus. GnIH-immunoreactivity was present in the hippocampus in cell bodies that resembled astrocytes. Males had more GnIH+ cells in the hilus of the dentate gyrus than females. To confirm the GnIH+ cell body phenotype, we performed double-label immunofluorescence against GnIH, glial fibrillary acidic protein (GFAP) and NeuN. Immunofluorescence revealed that all GnIH+ cell bodies in the hippocampus also contained GFAP, a marker of astrocytes. Taken together, these data suggest that GnRH does not reach GnRH receptors in the rat hippocampus primarily via synaptic release. By contrast, GnIH might be synthesised locally in the rat hippocampus by astrocytes. These data shed light on the sites of action and possible functions of GnRH and GnIH outside of the hypothalamic-pituitary-gonadal axis.

A journey through the pituitary gland: Development, structure and function, with emphasis on embryo-foetal and later development

The pituitary gland and the hypothalamus are morphologically and functionally associated in the endocrine and neuroendocrine control of other endocrine glands. They therefore play a key role in a number of regulatory feedback processes that co-ordinate the whole endocrine system. Here we review the neuroendocrine system, from the discoveries that led to its identification to some recently clarified embryological, functional, and morphological aspects. In particular we review the pituitary gland and the main notions related to its development, organization, cell differentiation, and vascularization. Given the crucial importance of the factors controlling neuroendocrine system development to understand parvocellular neuron function and the aetiology of the congenital disorders related to hypothalamic-pituitary axis dysfunction, we also provide an overview of the molecular and genetic studies that have advanced our knowledge in the field. Through the action of the hypothalamus, the pituitary gland is involved in the control of a broad range of key aspects of our lives: the review focuses on the hypothalamic-pituitary-gonadal axis, particularly GnRH, whose abnormal secretion is associated with clinical conditions involving delayed or absent puberty and reproductive dysfunction.

Identification and expression of GnRH2 and GnRH3 in the black sea bass (Centropristis striata), a hermaphroditic teleost

We cloned two cDNAs for two gonadotropin-releasing hormones, GnRH2 (chicken GnRH-II) and GnRH3 (salmon GnRH), respectively, from the black sea bass (Centropristis striata). Black sea bass are protogynous hermaphroditic teleosts that change from females to males between 2 and 5 years of age. Similar to other GnRH precursors, the precursors of black sea bass GnRH2 and GnRH23 consisted of a signal peptide, decapeptide, a downstream processing site, and a GnRH-associated peptide. Our analyses failed to identify GnRH1. GnRH3 precursor transcript was more widely distributed in a variety of tissues compared with GnRH2. Further examination of GnRH expression and gonadal histology was done in black sea bass from three different size groups: small (11.4-44.1 g), medium (179.4-352.2 g) and large (393.8-607.3 g). Interestingly, GnRH3 expression occurred only in the pituitaries of males in the small and medium groups compared with expression of GnRH2. Future functional studies of the sea bass GnRHs will be valuable in elucidating the potential underlying neuroendocrine mechanisms of black sea bass reproduction and may ultimately contribute to management advances in this commercially important fish.

GnRH, anosmia and hypogonadotropic hypogonadism--where are we?

Gonadotropin releasing hormone (GnRH) neurons originate the nasal placode and migrate into the brain during prenatal development. Once within the brain, these cells become integral components of the hypothalamic-pituitary-gonadal axis, essential for reproductive function. Disruption of this system causes hypogonadotropic hypogonadism (HH). HH associated with anosmia is clinically defined as Kallman syndrome (KS). Recent work examining the developing nasal region has shed new light on cellular composition, cell interactions and molecular cues responsible for the development of this system in different species. This review discusses some developmental aspects, animal models and current advancements in our understanding of pathologies affecting GnRH. In addition we discuss how development of neural crest derivatives such as the glia of the olfactory system and craniofacial structures control GnRH development and reproductive function.

GnRH receptors and peptides: skating backward

Gonadotropin-releasing hormone (GnRH) and its receptor are essential for reproduction in vertebrates. Although there are three major types of GnRH peptides and two major types of receptors in vertebrates, the pattern of distribution is unusual. Evidence is presented from genome mining that type I GnRHRs are not restricted to mammals, but can be found in the lobe-finned and cartilaginous fishes. This implies that this tail-less GnRH receptor emerged early in vertebrate evolution, followed by several independent losses in different lineages. Also, we have identified representatives from the three major GnRH peptide types (mammalian GnRH1, vertebrate GnRH2 and dogfish GnRH3) in a single cartilaginous fish, the little skate. Skate and coelacanth are the only examples of animals with both type I and II GnRH receptors and all three peptide types, suggesting this was the ancestral condition in vertebrates. Our analysis of receptor synteny in combination with phylogeny suggests that there were three GnRH receptor types present before the two rounds of whole genome duplication in early vertebrates. To further understand the origin of the GnRH peptide-receptor system, the relationship of vertebrate and invertebrate homologs was examined. Our evidence supports the hypothesis of a GnRH superfamily with a common ancestor for the vertebrate GnRHs, invertebrate (inv)GnRHs, corazonins and adipokinetic hormones. The invertebrate deuterostomes (echinoderms, hemichordates and amphioxus) have derived GnRH-like peptides, although one amphioxus GnRH with a syntenic relationship to human GnRHs has been shown to be functional. Phylogenetic analysis suggests that gene duplications in the ancestral bilaterian produced two receptor types, one of which became adipokinetic hormone receptor/GnRHR and the other corazonin receptor/invGnRHR. It appears that the ancestral deuterostome had both a GnRHR and invGnRHR, and this is still the case in amphioxus. During the transition to vertebrates both the invertebrate-type peptide and receptor were lost, leaving only the vertebrate-type system that presently exists.

Molecular evolution of GPCRs: Somatostatin/urotensin II receptors

Somatostatin (SS) and urotensin II (UII) are members of two families of structurally related neuropeptides present in all vertebrates. They exert a large array of biological activities that are mediated by two families of G-protein-coupled receptors called SSTR and UTS2R respectively. It is proposed that the two families of peptides as well as those of their receptors probably derive from a single ancestral ligand-receptor pair. This pair had already been duplicated before the emergence of vertebrates to generate one SS peptide with two receptors and one UII peptide with one receptor. Thereafter, each family expanded in the three whole-genome duplications (1R, 2R, and 3R) that occurred during the evolution of vertebrates, whereupon some local duplications and gene losses occurred. Following the 2R event, the vertebrate ancestor is deduced to have possessed three SS (SS1, SS2, and SS5) and six SSTR (SSTR1-6) genes, on the one hand, and four UII (UII, URP, URP1, and URP2) and five UTS2R (UTS2R1-5) genes, on the other hand. In the teleost lineage, all these have been preserved with the exception of SSTR4. Moreover, several additional genes have been gained through the 3R event, such as SS4 and a second copy of the UII, SSTR2, SSTR3, and SSTR5 genes, and through local duplications, such as SS3. In mammals, all the genes of the SSTR family have been preserved, with the exception of SSTR6. In contrast, for the other families, extensive gene losses occurred, as only the SS1, SS2, UII, and URP genes and one UTS2R gene are still present.

Comprehensive analysis of GnRH2 neuronal projections in zebrafish

The presence and conservation of GnRH2 across vertebrate species suggest important biological roles. However, the function of GnRH2 remains unclear. A good research model for GnRH2 functional studies is still lacking largely due to the absence of GnRH2 in the widely used mouse model. Hence, we used the zebrafish, for which powerful genetic tools are available, and developed a transgenic (Tg) line expressing enhanced green fluorescence protein (eGFP). The high sensitivity of eGFP, which can diffuse throughout the neuron, enables us to document the complete projectome of GnRH2 neurons at different developmental stages. Fine projection structures were observed without sacrificing the fish. Crossed with the GnRH3:tdTomato Tg line, the GnRH2:eGFP Tg line provides us with an opportunity to visualize the entire GnRH system simultaneously in one organism. This work will provide a framework to understand the function of the highly-conserved GnRH2 system.

Evolution of the gastrin-cholecystokinin gene family revealed by synteny analysis

Gastrin (GAST) and cholecystokinin (CCK) are two structurally and functionally related peptide hormones that exert many functions, including regulation of gastric and pancreatic secretion, feeding behaviour and energy homeostasis. GAST and CCK genes are assumed to have diverged from a common ancestral gene, over 500 million years ago in the vertebrate lineage. However, although a large number of GAST and CCK-related sequences have been identified both in vertebrate and non-vertebrate species, the evolutionary history of the GAST/CCK family remains little understood. To address this issue, we used extensive genome synteny comparisons of vertebrate chromosomes, in particular to evaluate the impact of whole-genome duplications. In the present study, we confirm that the GAST/CCK family in vertebrates is composed of two paralogous genes, namely GAST and CCK, and even three in teleosts, namely GAST, CCK1 and CCK2. We also show that the GAST and CCK genes arose by duplications of a single ancestral gene through the 2R and that the two copies of the CCK gene found in teleosts have probably been generated through the 3R. Finally, our results suggest that the vertebrate ancestor possessed four members of the GAST/CCK family, of which two have likely been lost during evolution.

At the transition from invertebrates to vertebrates, a novel GnRH-like peptide emerges in amphioxus

Gonadotropin-releasing hormone (GnRH) is a critical reproductive regulator in vertebrates. Homologous peptides are also found in invertebrates, with a variety of characterized functions. In the amphioxus, an invertebrate that provides the best model for the transition to vertebrates, four GnRH receptors (GnRHRs) were previously described, but their native ligands were not identified. Using a more sensitive search methodology with hidden Markov models, we identified the first GnRH-like peptide confirmed in the amphioxus Branchiostoma floridae. This peptide specifically activated one of the four GnRHRs. Although the primary structure of this peptide was divergent from any previously isolated GnRH peptide, the minimal conserved residues found in all other GnRH superfamily members were retained. The peptide was immunolocalized in proximity of the central canal of the anterior nerve cord, a region where other neuropeptides and receptors have been found. Additionally, the amphioxus GnRH-like gene was positioned in a locus surrounded by syntenic homologs of the human GnRH paralogon. The amphioxus GnRH-like peptide, with its distinct primary structure, activated a receptor with equal potency to multiple ligands that span the GnRH superfamily.

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.

Insight from the lamprey genome: glimpsing early vertebrate development via neuroendocrine-associated genes and shared synteny of gonadotropin-releasing hormone (GnRH)

Study of the ancient lineage of jawless vertebrates is key to understanding the origins of vertebrate biology. The establishment of the neuroendocrine system with the hypothalamic-pituitary axis at its crux is of particular interest. Key neuroendocrine hormones in this system include the pivotal gonadotropin-releasing hormones (GnRHs) responsible for controlling reproduction via the pituitary. Previous data incorporating several lines of evidence showed all known vertebrate GnRHs were grouped into four paralogous lineages: GnRH1, 2, 3 and 4; with proposed evolutionary paths. Using the currently available lamprey genome assembly, we searched genes of the neuroendocrine system and summarize here the details representing the state of the current lamprey genome assembly. Additionally, we have analyzed in greater detail the evolutionary history of the GnRHs based on the information of the genomic neighborhood of the paralogs in lamprey as compared to other gnathostomes. Significantly, the current evidence suggests that two genome duplication events (both 1R and 2R) that generated the different fish and tetrapod paralogs took place before the divergence of the ancestral agnathans and gnathostome lineages. Syntenic analysis supports this evidence in that the previously-classified type IV GnRHs in lamprey (lGnRH-I and -III) share a common ancestry with GnRH2 and 3, and thus are no longer considered type IV GnRHs. Given the single amino acid difference between lGnRH-II and GnRH2 we propose that a GnRH2-like gene existed before the lamprey/gnathostome split giving rise to lGnRH-II and GnRH2. Furthermore, paralogous type 3 genes (lGnRH-I/III and GnRH3) evolved divergent structure/function in lamprey and gnathostome lineages.

Two novel gonadotropin-releasing hormones (GnRHs) from the urochordate ascidian, Halocynthia roretzi: implications for the origin of vertebrate GnRH isoforms

Three forms of gonadotropin-releasing hormone (GnRH) are found in vertebrates; these differ in amino acid sequence, localization, distribution, and embryological origin. We used northern blot analysis, and in situ hybridization to detect GnRH transcripts in various tissues in the large ascidian Halocynthia roretzi. We cloned a cDNA encoding two novel GnRHs, termed tGnRH-10 and tGnRH-11, from H. roretzi, with deduced amino acid sequences of QHWSYGFSPG and QHWSYGFLPG, respectively. Both GnRHs are highly similar to those of teleosts and tetrapods. For example, the tGnRH-10 sequence is 90% identical to seabream GnRH1, and tGnRH-11 is 90% identical to salmon GnRH3. The primary structure of the deduced preprotein is similar to that of chordate GnRHs and consists of a signal peptide, two decapeptides, up- and downstream processing sequences (containing lysine and arginine), and a GnRH-associated peptide. The transcripts of the H. roretzi GnRH gene were expressed in all tissues examined. Comparison of the signal peptide of the lamprey GnRH-II precursor with those of three forms from representative vertebrates revealed homology to GnRH2 precursors. These novel ascidian GnRHs offer a new perspective on the origin of vertebrate GnRH subtypes. We hypothesize that gnathostome GnRH2 was derived only from lamprey GnRH-II and that ancestral gnathostome GnRH, which produces neurons that originate in peripheral organs, gave rise to vertebrate GnRH1 and GnRH3 through whole-genome duplication.

Impact of gene/genome duplications on the evolution of the urotensin II and somatostatin families

The present review describes the molecular evolution of two phylogenetically related families of neuropeptides, the urotensin II (UII) and somatatostatin (SS) families. The UII family consists of four paralogous genes called UII, URP, URP1 and URP2 and the SS family is composed of six paralogous genes named SS1, SS2, SS3, SS4, SS5 and SS6. All these paralogs are present in teleosts, while only four of them, UII, URP, SS1 and SS2 are detected in tetrapods. Comparative genomics showed that most of these genes, namely UII, URP, URP1 and URP2 on the one hand and SS1, SS2 and SS5 on the other hand arose through the 2R. In contrast, the teleost-specific 3R had a much more moderate impact since it only concerned the UII and SS1 genes, which once duplicated, generated a second UII copy and SS4, respectively. The two remaining genes, SS3 and SS6, arose through tandem duplications of the SS1 and SS2 genes respectively, probably in the stem lineage of actinopterygians, before the emergence of teleosts. The history of the UII and SS families has also been marked by massive gene lost, both in tetrapods and in teleosts, but only after the 3R in this latter lineage. Finally, ancestral UII and SS genes are thought to have arisen through tandem duplication of a single ancestral gene, largely before the 1R. An important challenge for the future will be to understand the physiological significance of the molecular diversity of these two families.

Neuroendocrine regulation of gonadotropin secretion in seasonally breeding birds

Seasonally breeding birds detect environmental signals, such as light, temperature, food availability, and presence of mates to time reproduction. Hypothalamic neurons integrate external and internal signals, and regulate reproduction by releasing neurohormones to the pituitary gland. The pituitary gland synthesizes and releases gonadotropins which in turn act on the gonads to stimulate gametogenesis and sex steroid secretion. Accordingly, how gonadotropin secretion is controlled by the hypothalamus is key to our understanding of the mechanisms of seasonal reproduction. A hypothalamic neuropeptide, gonadotropin-releasing hormone (GnRH), activates reproduction by stimulating gonadotropin synthesis and release. Another hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH), inhibits gonadotropin synthesis and release directly by acting on the pituitary gland or indirectly by decreasing the activity of GnRH neurons. Therefore, the next step to understand seasonal reproduction is to investigate how the activities of GnRH and GnIH neurons in the hypothalamus and their receptors in the pituitary gland are regulated by external and internal signals. It is possible that locally-produced triiodothyronine resulting from the action of type 2 iodothyronine deiodinase on thyroxine stimulates the release of gonadotropins, perhaps by action on GnRH neurons. The function of GnRH neurons is also regulated by transcription of the GnRH gene. Melatonin, a nocturnal hormone, stimulates the synthesis and release of GnIH and GnIH may therefore regulate a daily rhythm of gonadotropin secretion. GnIH may also temporally suppress gonadotropin secretion when environmental conditions are unfavorable. Environmental and social milieus fluctuate seasonally in the wild. Accordingly, complex interactions of various neuronal and hormonal systems need to be considered if we are to understand the mechanisms underlying seasonal reproduction.

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

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

Early development of the gonadotropin-releasing hormone neuronal network in transgenic zebrafish

Understanding development of gonadotropin-releasing hormone (GnRH) neuronal circuits is fundamental to our understanding of reproduction, but not yet well understood. Most studies have been focused on GnRH neurons located in the hypothalamus and preoptic area (POA), which directly regulate the pituitary-gonadal axis. In zebrafish (Danio rerio), two forms of GnRH have been identified: GnRH2 and GnRH3. GnRH3 neurons in this species plays two roles: hypophysiotropic and neuromodulatory, depending on their location. GnRH3 neurons in the ventral telencephalon, POA, and hypothalamus control pituitary-gonadal function; in other areas (e.g., terminal nerve), they are neuromodulatory and without direct action on reproduction. To investigate the biology of GnRH neurons, a stable line of transgenic zebrafish was generated in which the GnRH3 promoter drives expression of a bright variant of green fluorescent protein (Emerald GFP, or EMD). This provides unprecedented sensitivity in detecting and imaging GnRH3 neurons during early embryogenesis in the transparent embryo. Using timelapse confocal imaging to monitor the time course of GnRH3:EMD expression in the live embryo, we describe the emergence and development of GnRH3 neurons in the olfactory region, hypothalamus, POA, and trigeminal ganglion. By 50 h post fertilization, these diverse groups of GnRH3 neurons project broadly in the central and peripheral nervous systems and make anatomical connections with each other. Immunohistochemistry of synaptic vesicle protein 2 (a marker of synaptic transmission) in this transgenic model suggests synaptic formation is occurring during early development of the GnRH3 neural network. Electrophysiology reveals early emergence of responsiveness to the stimulatory effects of kisspeptin in terminal nerve GnRH3 neurons. Overall, our findings reveal that the GnRH3 neuronal system is comprised of multiple populations of neurons as a complicated network.

Characterization of the true ortholog of the urotensin II-related peptide (URP) gene in teleosts

It has been recently established that the urotensin II (UII) family consists of four distinct paralogs in bony vertebrates, namely UII, and the three UII-related peptides (URPs) called URP, URP1 and URP2. These four peptides are encoded by genes which arose from the two rounds of tetraploidization (2R) which took place early during vertebrate evolution. Up to now, three of them, UII, URP1 and URP2, have been identified in teleosts, while only two, UII and URP, have been reported in tetrapods. The fact that fish URP has not been found in previous studies led to the suggestion that the corresponding gene had been lost in the teleost lineage. In the present study, we show that this view is not correct. A search of the most recent release of the Ensembl genome database led us to identify a novel UII/URP-like gene in teleosts. Using synteny analysis, we demonstrate that this gene corresponds to the true ortholog of the tetrapod URP gene. Molecular cloning of the corresponding cDNA in medaka revealed that URP gene encodes a putative peptide, with the primary structure GEPCFWKYCV. In stickleback, tilapia and takifugu, URP exhibited the same sequence while, in tetraodon, it differed by only one amino acid substitution Gly ↔ Ser. In zebrafish, URP appeared totally divergent at its N-terminus with the structure DDTCFWKYCV. In conclusion, the occurrence of a true URP in teleosts shows that the quartet of UII-related genes which arose from 2R has been integrally preserved in this lineage.

Molecular cloning, tissue distribution, and ontogeny of gonadotropin-releasing hormone III gene (GnRH-III) in half-smooth tongue sole (Cynoglossus semilaevis)

Gonadotropin-releasing hormone (GnRH) is a neuropeptide that plays a vital role in hypothalamus-pituitary-gonad (HPG) axis. In the present study, the GnRH-III gene was isolated from half-smooth tongue sole (Cynoglossus semilaevis). In the 1160 bp genomic sequence, four exons, three introns, and 5'-/3'-flanking sequences were identified. The putative peptide was 92 residues long, including a putative signal peptide containing 23 amino acids, the GnRH decapeptide, a proteolytic cleavage site of three amino acids and a GnRH associated peptide of 56 amino acids. The overall amino acid sequence of C. semilaevis GnRH-III (csGnRH-III) was highly conserved with other teleost GnRH-III genes. Phylogenetic analysis showed the evolutionary relationships of csGnRH-III with other known GnRH genes. A 320 bp promoter sequence of the csGnRH-III was also analyzed, and several potential regulatory motifs were identified which were conserved in the GnRH promoters of other teleosts. Quantitative real-time PCR analysis indicated csGnRH-III was expressed only in brain and gonads. In C. semilaevis, the csGnRH-III transcript was maternally deposited and appeared to be developmentally regulated during embryogenesis and early larval development. Comparing sequence and expression patterns of csGnRH-III with other teleosts GnRH-IIIs suggested that the main function of GnRH-III might be conserved in teleosts.

Schistocephalus solidus infections increase gonadotropins and gonadotropin releasing hormone (GnRH3) mRNA levels in the three-spined stickleback, Gasterosteus aculeatus

Parasites often impair the reproduction of their hosts, one well known case being the cestode Schistocephalus solidus which is a common parasite in three-spined sticklebacks, Gasterosteus aculeatus. One of the possible ways that this could be exerted is by suppression on the brain-pituitary-gonadal (BPG) axis. In this study, mRNA levels of FSH-β and LH-β and of GnRH2 (cGnRH II) and GnRH3 (sGnRH) were measured via Q-PCR in infected and uninfected fish sampled from the field a few weeks before the onset of breeding. The pituitary mRNA levels of both FSH-β and LH-β were higher in infected males than in uninfected males. Also in females, FSH-β mRNA levels were higher in infected individuals than in others, whereas there was no significant difference found in LH-β expression. Brain mRNA levels of GnRH3 were higher in infected fish than in uninfected fish in both sexes, but no difference was found in GnRH2 mRNA levels. Thus, infection by S. solidus was able to alter the expressions not only of gonadotropins (GtHs), but also of GnRH which has not been observed previously. However, the effects are opposite to what should be expected if the parasite suppressed reproduction via actions on the brain-pituitary level. The gonads are perhaps more likely to be impaired by the parasites in other ways, and changed feedbacks on the BPG axis could then lead to the increases in GtHs and GnRH.

Involvement of GnRH, PACAP and PRP in the reproduction of blue gourami females (Trichogaster trichopterus)

In vertebrates, gonadotropin-releasing hormone (GnRH) and pituitary adenylate cyclase-activating polypeptide (PACAP) are key hormones regulating growth and reproduction in the brain-pituitary axis. The regulating hormonal interactions are of great interest, therefore, the aim of this study is to provide novel insights into the involvement of brain GnRH and PACAP in oogensis and spermatogenesis in a fish model, the blue gourami (Trichogaster trichopterus). cDNA cloning of two GnRH forms combined with phylogenetic analysis revealed that three paralogous GnRH forms exist in blue gourami and evolve as a result of genome duplication. GnRH1 mRNA levels are related to final oocyte maturation (FOM), and this peptide stimulated β follicle-stimulating hormone (βFSH) and growth hormone (GH) gene expression; GnRH2 stimulated β gonadotropins (GtH) gene expression and GnRH analog combined with PACAP-38 synergistically upregulate GH and βFSH gene expression. The data presented, together with previous studies in our lab, enable suggesting mechanisms explaining the physiological relevance of these peptides in the regulation of gametogenesis and steroidogenesis in blue gourami females. These findings support the biological importance of the GnRH and PACAP hormones family, enabling them to stimulate differential biological functions in the regulation of growth and reproduction.

Occurrence of two distinct urotensin II-related peptides in zebrafish provides new insight into the evolutionary history of the urotensin II gene family

The urotensin II (UII) family is currently known to consist of two paralogous peptides, namely UII and UII-related peptide (URP). In contrast to UII, which has been identified in all vertebrate classes so far, URP has only been characterized in tetrapods. We report here the occurrence of two distinct URP genes in teleosts, which we have named URP1 and URP2. Synteny analysis revealed that teleost URP1 and URP2 genes and tetrapod URP genes represent three distinct paralog genes that, together with the UII gene, probably arose from the two rounds of tetraploidization, which took place early in vertebrate evolution. The absence of URP in fish indicates that the corresponding gene has been lost in the teleost lineage, whereas it is likely that both the URP1 and URP2 genes have been lost in the tetrapod lineage. Quantitative RT-PCR analysis revealed that the URP2 gene is mainly expressed in the spinal cord and the brain in adult zebrafish. In situ hybridization experiments showed that in zebrafish embryos, URP2 mRNA-containing cells are located in the floor plate of the neural tube. In adult, URP2-expressing cells occur in close contact with the ventral side of the ependymal canal along the whole spinal cord, whereas in the brain, they are located below the fourth ventricle. These URP-expressing cells may correspond to cerebrospinal fluid-contacting neurons. In conclusion, our study reveals the occurrence of four distinct UII paralogous systems in vertebrates that may exert distinct functions, both in tetrapods and teleosts.

Gonadotropin-releasing hormone and adipokinetic hormone signaling systems share a common evolutionary origin

Gonadotropin-releasing hormone (GnRH) is a critical and central hormone that regulates vertebrate reproduction. The high conservation of GnRH signaling within the chordates (deuterostomians) raises the important question as to whether its appearance might date back prior to the divergence of protostomian and deuterostomian lineages, about 700 million years ago. This leads to several important questions regarding the evolution of the GnRH family. Has GnRH been retained in most protostomian lineages? And was regulation of reproduction already a function of ancestral GnRH? The first question can undoubtedly be answered affirmatively since several GnRH-like sequences have been found in wide variety of protostomian and deuterostomian phyla. However, based on their different primary functions in different phyla - which implies a less unanimous answer on the second question - consistency in the nomenclature of this peptide family has been lost. A comparative and phylogenetic approach shows that the ecdysozoan adipokinetic hormones (AKHs), lophotrochozoan GnRHs and chordate GnRHs are structurally related and suggests that they all originate from a common ancestor. This review supports the view that the AKH-GnRH signaling system probably arose very early in metazoan evolution, prior to the divergence of protostomians and deuterostomians.