Molecular biology of channel catfish gonadotropin receptors: 1. Cloning of a functional luteinizing hormone receptor and preovulatory induction of gene expression

Establishment of cell-lines stably expressing recombinant Japanese eel follicle-stimulating hormone and luteinizing hormone using CHO-DG44 cells: fully induced ovarian development at different modes

The gonadotropins (Gth), follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), play central roles in gametogenesis in vertebrates. However, available information on their differential actions in teleost, especially in vivo, is insufficient. In this study, we established stable CHO-DG44 cell lines expressing long-lasting recombinant Japanese eel Fsh and Lh with extra O-glycosylation sites (Fsh-hCTP and Lh-hCTP), which were produced in abundance. Immature female eels received weekly intraperitoneal injections of Gths. Fsh-hCTP induced the entire ovarian development by 8 weeks from the beginning of injection; thus, the ovaries of most fish were at the migratory nucleus stage while the same stage was observed in eels after 4 weeks in the Lh-hCTP-treated group. In contrast, all pretreated and saline-injected eels were in the pre-vitellogenic stage. Gonadosomatic indices in the Fsh-hCTP-treated group were significantly higher than those in the Lh-hCTP group at the migratory nucleus stage because of the significantly higher frequency of advanced ovarian follicles. Ovarian mRNA levels of genes related to E2 production (cyp11a1, cyp17a1, cyp19a1, hsd3b, fshr, and lhr) were measured using real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). All genes were induced by both Fsh-hCTP and Lh-hCTP, with a peak at either the mid- or late vitellogenic stages. Transcript abundance of cyp19a1 and fshr in the Lh-hCTP group were significantly higher than those in the Fsh-hCTP group, whereas no difference in the expression of other genes was observed between the groups. Fluctuations in serum levels of sex steroid hormones (estradiol-17β, 11-ketotestosterone, and testosterone) in female eels were comparable in the Fsh-hCTP and Lh-hCTP groups, thus increasing toward the maturational phase. Furthermore, the fecundity of the eels induced to mature by Fsh-hCTP was significantly higher than that induced by Lh-hCTP. These findings indicate that Fsh and Lh can induce ovarian development in distinctively different modes in the Japanese eel.

Expression Patterns and Gonadotropin Regulation of the TGF-β II Receptor (Bmpr2) during Ovarian Development in the Ricefield Eel Monopterus albus

Bmpr2 plays a central role in the regulation of reproductive development in mammals, but its role during ovarian development in fish is still unclear. To ascertain the function of bmpr2 in ovarian development in the ricefield eel, we isolated and characterized the bmpr2 cDNA sequence; the localization of Bmpr2 protein was determined by immunohistochemical staining; and the expression patterns of bmpr2 in ovarian tissue incubated with FSH and hCG in vitro were analyzed. The full-length bmpr2 cDNA was 3311 bp, with 1061 amino acids encoded. Compared to other tissues, bmpr2 was abundantly expressed in the ovary and highly expressed in the early yolk accumulation (EV) stages of the ovary. In addition, a positive signal for Bmpr2 was detected in the cytoplasm of oocytes in primary growth (PG) and EV stages. In vitro, the expression level of gdf9, the ligand of bmpr2, in the 10 ng/mL FSH treatment group was significantly higher after incubation for 4 h than after incubation for different durations. However, bmpr2 expression in the 10 ng/mL FSH treatment group at 2 h, 4 h and 10 h was significantly lower. Importantly, the expression level of bmpr2 and gdf9 in the 100 IU/mL hCG group had similar changes that were significantly decreased at 4 h and 10 h. In summary, Bmpr2 might play a pivotal role in ovarian growth in the ricefield eel, and these results provide a better understanding of the function of bmpr2 in ovarian development and the basic data for further exploration of the regulatory mechanism of gdf9 in oocyte development.

Advances in Reproductive Endocrinology and Neuroendocrine Research Using Catfish Models

Catfishes, belonging to the order siluriformes, represent one of the largest groups of freshwater fishes with more than 4000 species and almost 12% of teleostean population. Due to their worldwide distribution and diversity, catfishes are interesting models for ecologists and evolutionary biologists. Incidentally, catfish emerged as an excellent animal model for aquaculture research because of economic importance, availability, disease resistance, adaptability to artificial spawning, handling, culture, high fecundity, hatchability, hypoxia tolerance and their ability to acclimate to laboratory conditions. Reproductive system in catfish is orchestrated by complex network of nervous, endocrine system and environmental factors during gonadal growth as well as recrudescence. Lot of new information on the molecular mechanism of gonadal development have been obtained over several decades which are evident from significant number of scientific publications pertaining to reproductive biology and neuroendocrine research in catfish. This review aims to synthesize key findings and compile highly relevant aspects on how catfish can offer insight into fundamental mechanisms of all the areas of reproduction and its neuroendocrine regulation, from gametogenesis to spawning including seasonal reproductive cycle. In addition, the state-of-knowledge surrounding gonadal development and neuroendocrine control of gonadal sex differentiation in catfish are comprehensively summarized in comparison with other fish models.

Generation and use of recombinant gonadotropins in fish

Understanding the differential roles of the pituitary gonadotropins Fsh and Lh in gonad maturation is crucial for a successful manipulation of the reproductive process in fish, and requires species-specific tools and appropriate active hormones. With the increasing availability of fish cDNAs coding for gonadotropin subunits, the production of recombinant hormones in heterologous systems has gradually substituted the approach of isolating native hormones. These recombinant hormones can be continually produced without depending on the fish as starting material and no cross-contamination with other pituitary glycoproteins is assured. Recombinant gonadotropins should be produced in eukaryotic cells, which have glycosylation capacity, but this post-translational modification varies greatly depending on the cell system, influencing hormone activity and stability. The production of recombinant gonadotropin beta-subunits to be used as antigens for antibody production has allowed the development of immunoassays for quantification of gonadotropins in some fish species. The administration in vivo of dimeric homologous recombinant gonadotropins has been used in basic studies and as a biotechnological approach to induce gametogenesis. In addition, gene-based therapies using somatic transfer of the gonadotropin genes have been tested as an alternative for hormone delivery in vivo. In summary, the use of homologous hormonal treatments can open new strategies in aquaculture to solve reproductive problems or develop out-of-season breeding programs.

Molecular characterization of two Russian sturgeon gonadotropin receptors: Cloning, expression analysis, and functional activity

Sturgeons are being used in aquaculture because wild populations are now endangered due to overfishing for caviar. A challenge in working with sturgeon as an aquacultured species is its long and slow reproductive development. Reproduction is a hormonally regulated process that involves hierarchical signaling between the brain, pituitary gland, and gonads. In an effort to better understand the hormonal regulation of sturgeon reproduction, we have cloned the Russian sturgeon (st), Acipenser gueldenstaedtii, luteinizing hormone receptor (stLHR) and follicle stimulating hormone receptor (stFSHR) and measured their expression from previtellogenic to mature ovarian follicles. Sturgeon LHR and FSHR expression was elevated in early-vitellogenic and mature follicles compared with pre-vitellogenic and mid-vitellogenic follicles, and only LHR expression increased during late-vitellogenesis. Recombinant sturgeon FSH and LH both activated sturgeon LHR and FSHR in a cAMP reporter assay. Further molecular characterization of these receptors was accomplished by in silico modeling and cAMP reporter assays using heterologous recombinant gonadotropins from human and piscine species. There was no apparent trend in heterologous LH and/or FSH activation of the sturgeon LHR or FSHR. These data suggest that permissive activation of LHR and FSHR are a consequence of some yet undetermined biological characteristic(s) of different piscine species.

Characterization of gonadotropin receptors Fshr and Lhr in Japanese medaka, Oryzias latipes

Reproduction in vertebrates is controlled by the brain-pituitary-gonad axis, where the two gonadotropins follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) play vital parts by activating their cognate receptors in the gonads. The main purpose of this work was to study intra- and interspecies ligand promiscuity of teleost gonadotropin receptors, since teleost receptor specificity is unclear, in contrast to mammalian receptors. Receptor activation was investigated by transfecting COS-7 cells with either Fsh receptor (mdFshr, tiFshr) or Lh receptor (mdLhr, tiLhr), and tested for activation by recombinant homologous and heterologous ligands (mdFshβα, mdLhβα, tiFshβα, tiLhβα) from two representative fish orders, Japanese medaka (Oryzias latipes, Beloniformes) and Nile tilapia (Oreochromis niloticus, Cichliformes). Results showed that each gonadotropin preferentially activates its own cognate receptor. Cross-reactivity was detected to some extent as mdFshβα was able to activate the mdLhr, and mdLhβα the mdFshr. Medaka pituitary extract (MPE) stimulated CRE-LUC activity in COS-7 cells expressing mdlhr, but could not stimulate cells expressing mdfshr. Recombinant tiLhβα, tiFshβα and tilapia pituitary extract (TPE) could activate the mdLhr, suggesting cross-species reactivity for mdLhr. Cross-species reactivity was also detected for mdFshr due to activation by tiFshβα, tiLhβα, and TPE, as well as for tiFshr and tiLhr due to stimulation by mdFshβα, mdLhβα, and MPE. Tissue distribution analysis of gene expression revealed that medaka receptors, fshr and lhr, are highly expressed in both ovary and testis. High expression levels were found for lhr also in brain, while fshr was expressed at low levels. Both fshr and lhr mRNA levels increased significantly during testis development. Amino acid sequence alignment and three-dimensional modelling of ligands and receptors highlighted conserved beta sheet domains of both Fsh and Lh between Japanese medaka and Nile tilapia. It also showed a higher structural homology and similarity of transmembrane regions of Lhr between both species, in contrast to Fshr, possibly related to the substitution of the conserved cysteine residue in the transmembrane domain 6 in medaka Fshr with glycine. Taken together, this is the first characterization of medaka Fshr and Lhr using homologous ligands, enabling to better understand teleost hormone-receptor interactions and specificities. The data suggest partial ligand promiscuity and cross-species reactivity between gonadotropins and their receptors in medaka and tilapia.

Identification and Characterization of a Luteinizing Hormone Receptor (LHR) Homolog from the Chinese Mitten Crab Eriocheir sinensis

Luteinizing hormone (LH), a pituitary gonadotropin, coupled with LH receptor (LHR) is essential for the regulation of the gonadal maturation in vertebrates. Although LH homolog has been detected by immunocytochemical analysis, and its possible role in ovarian maturation was revealed in decapod crustacean, so far there is no molecular evidence for the existence of LHR. In this study, we cloned a novel LHR homolog (named EsLHR) from the Chinese mitten crab Eriocheir sinensis. The complete sequence of the EsLHR cDNA was 2775bp, encoding a protein of 924 amino acids, sharing 71% amino acids identity with the ant Zootermopsis nevadensis LHR. EsLHR expression was found to be high in the ovary, while low in testis, gill, brain, and heart, and no expression in the thoracic ganglion, eye stalk, muscle, and hepatopancreas. Quantitative PCR revealed that the expression level of EsLHR mRNA was significantly higher in the ovaries in previtellogenic (Pvt), late vitellogenic (Lvt), and germinal vesicle breakdown (GVBD) stages than that in the vitellogenic (Mvt) and early vitellogenic (Evt) stages (P < 0.05), and, the highest and the lowest expression were in Lvt, and Evt, respectively. The strong signal was mainly localized in the ooplasm of Pvt oocyte as detected by in situ hybridization. The crab GnRH homolog can significantly induce the expression of EsLHR mRNA at 36 hours post injection in vivo (P < 0.01), suggesting that EsLHR may be involved in regulating ovarian development through GnRH signaling pathway in the mitten crab.

Cloning, expression and functional characterization of a novel luteinizing hormone receptor in the orange-spotted grouper, Epinephelus coioides

Luteinizing hormone receptor (LHR) plays a critical role in reproduction by mediating LH signaling in the gonad. In this study, we cloned a novel lhr gene from the orange-spotted grouper, named glhr2. The cloned complete open reading frame sequence of glhr2 was 2082 bp in length, encoding a protein of 693 amino acids, sharing approximately 50% amino acid identity with glhr1. glhr1 and glhr2 were primarily expressed in gonad, brain and hypothalamus with low expression in other tissues such as gill, spleen, etc. The expressions of both glhr1 and glhr2 increased during vitellogenesis, while decreased during natural female to male sex change. The two gLHRs both could be activated by equine LH or human chorionic gonadotropin, but not by human follicle stimulating hormone. Both gLHR1 and gLHR2 activation stimulated the expression of cAMP response element driven reporter gene in a dose-dependent manner, while gLHR2 but not gLHR1 also activated serum response element driven reporter gene expression. This was the first study to demonstrate that two active LHRs exist in fish with possible different functional roles.

Expression of two glycoprotein hormone receptors in larval, parasitic phase, and adult sea lampreys

All jawed vertebrates have three canonical glycoprotein hormones (GpHs: luteinizing hormone, LH; follicle stimulating hormone, FSH; and thyroid stimulating hormone, TSH) with three corresponding GpH receptors (GpH-Rs: LH-R, FSH-R, and TSH-R). In contrast, we propose that the jawless vertebrate, the sea lamprey (Petromyzon marinus), only has two pituitary glycoprotein hormones, lamprey (l)GpH and l-thyrostimulin, and two functional glycoprotein receptors, lGpH-R I and II. It is not known at this time whether there is a specific receptor for lGpH and l-thyrostimulin, or if both GpHs can differentially activate the lGpH-Rs. In this report, we determined the RNA expression of lGpH-R I and II in the gonads and thyroids of larval, parasitic phase, and adult lampreys. A highly sensitive dual-label fluorescent in situ hybridization technique (RNAScope™) showed lGpH-R I expression in the ovaries of larval lamprey, and co-localization and co-expression of lGpH-R I and II in the ovaries of parasitic phase and adult lampreys. Both receptors were also highly co-localized and co-expressed in the endostyle of larval lamprey and thyroid follicles of parasitic and adult lampreys. In addition, we performed in vivo studies to determine the actions of lamprey gonadotropin releasing hormones (lGnRHs) on lGpH-R I and II expression by real time PCR, and determined plasma concentrations of estradiol and thyroxine. Administration of lGnRH-III significantly (p ≤ 0.01) increased lGpHR II expression in the thyroid follicles of adult female lampreys but did not cause a significant increase in RNA expression of lGpH-R I and II in ovaries. Concomitantly, there was a significant increase (p ≤ 0.01) of plasma estradiol without any significant changes of plasma thyroxine concentrations in response to treatment to lGnRH-I, -II, or -III. In summary, our results provide supporting evidence that the lamprey pituitary glycoprotein hormones may differentially activate the lamprey GpH-Rs in regulating both thyroid and gonadal activities during each of the three life stages of the sea lamprey.

Gonadotropin receptors of Labeo rohita: Cloning and characterization of full-length cDNAs and their expression analysis during annual reproductive cycle

Follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), secreted from pituitary, stimulate gonadal function by binding to their cognate receptors FSH receptor (FSHR), and LH/choriogonadotropin receptor (LHCGR). Rohu (Labeo rohita) is a commercially important seasonal breeder freshwater fish species, but till date, the regulation of expression of gonadotropins and their receptors gene during different phases of annual reproductive cycle has not been investigated. We envisaged the critical role of these molecules during seasonal gonadal development in this carp species. We cloned full- length cDNAs of fshra and lhcgrba from rohu testis using RACE (Rapid amplification of cDNA ends) and analyzed their expression along with fsh and lh by quantitative real time PCR (qRT-PCR) assay at various gonadal developmental stages of the annual reproductive cycle. Full-length rohu fshra and lhcgrba cDNA encodes 670 and 716 amino acids respectively, and in adult fish, they were widely expressed in brain, pituitary, gonad, liver, kidney, head kidney, heart, muscle, gill, fin, eye and intestine. In male, both fsh and fshra transcripts showed high level of expression during spermatogenesis, however, in female, expression level was found to be higher in the fully grown oocyte stages. The expression of rohu lh and lhcgrba mRNA increased with increment of gonadosomatic index and showed highest level during spermiation stage in male and fully matured oocyte stage in female. These results together may suggest the involvement of fshra and lhcgrba in regulating function of seasonal gonadal development in rohu.

Transcriptomic information from Pacific white shrimp (Litopenaeus vannamei) ovary and eyestalk, and expression patterns for genes putatively involved in the reproductive process

The increased use of massive sequencing technologies has enabled the identification of several genes known to be involved in different mechanisms associated with reproduction that so far have only been studied in vertebrates and other model invertebrate species. In order to further investigate the genes involved in Litopenaeus vannamei reproduction, cDNA and SSH libraries derived from female eyestalk and gonad were produced, allowing the identification of expressed sequences tags (ESTs) that potentially have a role in the regulation of gonadal maturation. In the present study, different transcripts involved in reproduction were identified and a number of them were characterized as full-length. These transcripts were evaluated in males and females in order to establish their tissue expression profiles during developmental stages (juvenile, subadult and adult), and in the case of females, their possible association with gonad maturation was assessed through expression analysis of vitellogenin. The results indicated that the expression of vitellogenin receptor (vtgr) and minichromosome maintenance (mcm) family members in the female gonad suggest an important role during previtellogenesis. Additionally, the expression profiles of genes such as famet, igfbp and gpcr in brain tissues suggest an interaction between the insulin/insulin-like growth factor signaling pathway (IIS) and methyl farnesoate (MF) biosynthesis for control of reproduction. Furthermore, the specific expression pattern of farnesoic acid O-methyltransferase suggests that final synthesis of MF is carried out in different target tissues, where it is regulated by esterase enzymes under a tissue-specific hormonal control. Finally, the presence of a vertebrate type steroid receptor in hepatopancreas and intestine besides being highly expressed in female gonads, suggest a role of that receptor during sexual maturation.

Molecular characterization and quantification of the follicle-stimulating hormone receptor in turbot (Scophthalmus maximus)

Molecular cloning, characterization, and functional analysis of follicle-stimulating hormone receptor (FSHR) in female turbot (Scophthalmus maximus) were evaluated. Results showed that the full-length FSHR cDNA was 3824 bp long and contained a 2202 bp open reading frame that encoded a mature protein of 733 amino acids (aa) and a signal peptide of 18 aa. Multiple sequence analyses showed that turbot FSHR has high homology with the corresponding genes of other teleosts and significant homology with that of Hippoglossus hippoglossus. Turbot FSHR has the typical structural architecture of glycoprotein hormone receptors consisting of a large N-terminal extracellular domain, seven transmembrane domains and short C-terminal intracellular domain. FSHR mRNA was found to be abundant in the ovaries, but deficient in eyes, intestine, brain, muscle, gills, spleen, stomach, heart and kidney. Furthermore, FSHR mRNA was found to increase gradually from pre-vitellogenesis to migratory nucleus stages, with the highest values observed during the late vitellogenesis stage of the reproductive cycle. However, FSHR mRNA was found to decrease dramatically during the atresia stage. Meanwhile, functional analysis with HEK293T cells continual expressing FSHR demonstrated that FSHR was specifically stimulated by ovine FSH, but not ovine LH. These results indicate that turbot FSHR is mainly involved in the stimulation of vitellogenesis, regulation of oocyte maturation as well as promotion of ovarian development via specific ligand binding. These findings open doors to further investigation of physiological functions of FSHR, which will be valuable for fish reproduction and broodstock management.

Disruption of Zebrafish Follicle-Stimulating Hormone Receptor (fshr) But Not Luteinizing Hormone Receptor (lhcgr) Gene by TALEN Leads to Failed Follicle Activation in Females Followed by Sexual Reversal to Males

Gonadotropins are primary hormones that control vertebrate reproduction. In a recent study, we analyzed the impacts of FSH and LH on zebrafish reproduction by disrupting FSH and LH-β genes (fshb and lhb) using transcription activator-like effector nuclease (TALEN) technology. Using the same approach, we successfully deleted FSH and LH receptor genes (fshr and lhcgr) in the present study. In contrast to the deficiency of its cognate ligand FSH, the fshr-deficient females showed a complete failure of follicle activation with all ovarian follicles arrested at the primary growth-previtellogenic transition, which is the marker for puberty onset in females. Interestingly, after blockade at the primary growth stage for varying times, all females reversed to males, and all these males were fertile. In fshr-deficient males, spermatogenesis was normal in adults, but the initiation of spermatogenesis in juveniles was retarded. In contrast to fshr, the deletion of the lhcgr gene alone caused no obvious phenotypes in both males and females; however, double mutation of fshr and lhcgr resulted in infertile males. In summary, our results in the present study showed that Fshr was indispensable to folliculogenesis and the disruption of the fshr gene resulted in a complete failure of follicle activation followed by masculinization into males. In contrast, lhcgr does not seem to be essential to zebrafish reproduction in both males and females. Neither Fshr nor Lhcgr deficiency could phenocopy the deficiency of their cognate ligands FSH and LH, which is likely due to the fact that Fshr can be activated by both FSH and LH in the zebrafish.

Demonstration of the Coexistence of Duplicated LH Receptors in Teleosts, and Their Origin in Ancestral Actinopterygians

Pituitary gonadotropins, FSH and LH, control gonad activity in vertebrates, via binding to their respective receptors, FSHR and LHR, members of GPCR superfamily. Until recently, it was accepted that gnathostomes possess a single FSHR and a single LHR, encoded by fshr and lhcgr genes. We reinvestigated this question, focusing on vertebrate species of key-phylogenetical positions. Genome analyses supported the presence of a single fshr and a single lhcgr in chondrichthyans, and in sarcopterygians including mammals, birds, amphibians and coelacanth. In contrast, we identified a single fshr but two lhgcr in basal teleosts, the eels. We further showed the coexistence of duplicated lhgcr in other actinopterygians, including a non-teleost, the gar, and other teleosts, e.g. Mexican tetra, platyfish, or tilapia. Phylogeny and synteny analyses supported the existence in actinopterygians of two lhgcr paralogs (lhgcr1/ lhgcr2), which do not result from the teleost-specific whole-genome duplication (3R), but likely from a local gene duplication that occurred early in the actinopterygian lineage. Due to gene losses, there was no impact of 3R on the number of gonadotropin receptors in extant teleosts. Additional gene losses during teleost radiation, led to a single lhgcr (lhgcr1 or lhgcr2) in some species, e.g. medaka and zebrafish. Sequence comparison highlighted divergences in the extracellular and intracellular domains of the duplicated lhgcr, suggesting differential properties such as ligand binding and activation mechanisms. Comparison of tissue distribution in the European eel, revealed that fshr and both lhgcr transcripts are expressed in the ovary and testis, but are differentially expressed in non-gonadal tissues such as brain or eye. Differences in structure-activity relationships and tissue expression may have contributed as selective drives in the conservation of the duplicated lhgcr. This study revises the evolutionary scenario and nomenclature of gonadotropin receptors, and opens new research avenues on the roles of duplicated LHR in actinopterygians.

Catfish gonadotrophins: cellular origin, structural properties and physiology

Gonadotrophins (GTHs) play a central role in the regulation of gametogenesis and spawning. The structural duality of the GTHs [luteinising hormone (LH) and follicle-stimulating hormone (FSH)] is established in fishes with the exception of ancestral vertebrates. Most studies indicate that, in teleosts, the GTHs are secreted in separate cells. Phylogenetic analysis shows that the common α-subunit of the GTHs (and also of thyroid-stimulating hormone) and LHβ are highly conserved in fishes, as in tetrapods. However, FSHβ shows considerable divergence in teleosts. There may be 12 or 13 cysteine residues, with an additional one near the N-terminus. There may be one or two N-linked glycolsyation sites. In catfishes, there are 13 cysteine residues and one N-linked glycosylation site. In an extreme situation, a potential glycosylation site is lacking in some fishes. Both FSH and LH receptors are characterised in teleosts. The FSH receptor is promiscuous and can be cross-activated by LH. By contrast, the LH receptor is highly selective, being activated by its natural ligand or by heterologous ligands (e.g. human chorionic gonadotrophin). Consequently, teleosts show different patterns of LH and FSH secretion. In catfishes, in the absence of native FSH protein, LH controls all aspects of reproduction, from early gametogenesis to spawning.

Molecular cloning, characterization, and expression analysis of luteinizing hormone receptor gene in turbot (Scophthalmus maximus)

The luteinizing hormone receptor (LHR) plays a crucial role in female reproduction. In the present study, full-length sequence coding for the LHR was obtained from female turbot (Scophthalmus maximus) by homology cloning and a strategy based on rapid amplification of cDNA end-polymerase chain reaction. The full-length LHR cDNA was 3,184 bp long and contained a 2,058-bp open reading frame which encoded a protein of 685 amino acids. Multiple sequence alignments of the turbot LHR manifested high homologies with the corresponding sequences of available teleosts and representative vertebrates, and significant homology with that of Hippoglossus hippoglossus. In addition, the turbot LHR showed typical characteristics of glycoprotein receptors, including a long N-terminal extracellular domain, seven transmembrane domains, and a short C-terminal intracellular domain. LHR mRNA was abundant in the ovary, but was deficient in extra-ovarian tissues. Furthermore, LHR mRNA gradually developed from previtellogenesis to migratory nucleus stage, with the highest values observed in migratory nucleus stage during reproductive cycle. However, LHR mRNA sharply decreased in atresia stage. These results suggested that LHR is a typical G protein-coupled receptor that is involved in the promotion of turbot ovarian development and may be related to the final maturation and ovulation of oocyte. These findings contribute to the understanding of the potential roles of LHR in controlling the fish reproductive cycle.

Characterization of tilapia (Oreochromis niloticus) gonadotropins by modeling and immunoneutralization

In fish, both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) play important roles in reproduction. Here we explored the structure and differential specificity of tilapia (t) gonadotropins (GTHs) to delineate their physiological relevance and the nature of their regulation. We generated structural models of tGTHs and GTH receptors (R) that enabled us to better understand the hormone-receptor interacting region. In tilapia, FSH release is under the control of the hypothalamic decapeptide GnRH, an effect that was abolished by specific bioneutralizing antisera [anti-recombinant (r) tFSHβ]. These antisera also reduced the basal secretion and delayed GnRH-stimulated production of 11-ketotestosterone (11KT), and dramatically reduced LH levels. Immunoneutralization of tLH using anti-rtLHβ significantly reduced its GnRH-stimulated levels. Basal 11KT and FSH levels were also reduced. Taken together, these results suggest a feedback mechanism between FSH and LH release in tilapia.

Targeted gene disruption in zebrafish reveals noncanonical functions of LH signaling in reproduction

The pivotal role of gonadotropin signaling in regulating gonadal development and functions has attracted much research attention in the past 2 decades. However, the precise physiological role of gonadotropin signaling is still largely unknown in fish. In this study, we have established both LH β-subunit (lhb) and LH receptor (lhr) knockout zebrafish lines by transcription activator-like effector nucleases. Intriguingly, both homozygous lhb and lhr mutant male fish are fertile. The fertilization rate, sperm motility, and histological structure of the testis were not affected in either lhb or lhr mutant males. On the contrary, homozygous lhb mutant females are infertile, whereas homozygous lhr mutant females are fertile. Folliculogenesis was not affected in either lhb or lhr mutants, but oocyte maturation and ovulation were disrupted in lhb mutant, whereas only ovulation was affected in lhr mutant. Differential expression of genes in the ovary involved in steroidogenesis, oocyte maturation, and ovulation was found between the lhb and lhr mutants. These data demonstrate the essential role of LH signaling in oocyte maturation and ovulation, and support the notion that LH acts through the FSH receptor in the absence of LH receptor. Moreover, the defects of lhb mutant could be partially restored by administration of human chorionic gonadotropin. This in vivo evidence in the present study demonstrates, for the first time in any vertebrate species, that LH signaling is indispensable in female reproduction but not in male reproduction. LH signaling is demonstrated to control oocyte maturation and ovulation in the ovary.

Embryonic development of gonadotrope cells and gonadotropic hormones--lessons from model fish

Pituitary gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are key regulators of vertebrate reproduction. The differential regulation of these hormones, however, is poorly understood and little is known about gonadotrope embryonic development. The different cell types in the vertebrate pituitary develop from common progenitor cells just after gastrulation. Proper development and merging of the anterior and posterior pituitary is dependent upon carefully regulated cell-to-cell interactions, and a suite of signaling pathways with precisely organized temporal and spatial expression patterns, which include transcription factors and their co-activators and repressors. Among the pituitary endocrine cell types, the gonadotropes are the last to develop and become functional. Although much progress has been made during the last decade regarding details of gonadotrope development, the coordinated program for their maturation is not well described. FSH and LH form an integral part of the hypothalamo-pituitary-gonad axis, the main regulator of gonad development and reproduction. Besides regulating gonad development, pre- and early post-natal activity in this axis is thought to be essential for proper development, especially of the central nervous system in mammals. As a means to investigate early functions of FSH and LH in more detail, we have developed a stable transgenic line of medaka with the LH beta subunit gene (lhb) promoter driving green fluorescent protein (Gfp) expression to characterize development of lhb-expressing gonadotropes. The lhb gene is maternally expressed early during embryogenesis. lhb-Expressing cells are initially localized outside the primordial pituitary in the developing gut tube as early as 32 hpf. At hatching, lhb-Gfp is clearly detected in the gut epithelium and in the anterior digestive tract. lhb-Gfp expression later consolidates in the developing pituitary by 2 weeks post-fertilization. This review discusses status of knowledge regarding pituitary morphology and development, with emphasis on gonadotrope cells and gonadotropins during early development, comparing main model species like mouse, zebrafish and medaka, including possible developmental functions of the observed extra pituitary expression of lhb in medaka.

Pituitary gonadotropin and ovarian gonadotropin receptor transcript levels: seasonal and photoperiod-induced changes in the reproductive physiology of female Atlantic salmon (Salmo salar)

In female Atlantic salmon kept at normal light conditions, pituitary follicle-stimulating hormone beta (fshb) transcript levels were transiently elevated one year before spawning, re-increased in February, and remained high during spawning in November and in post-ovulatory fish in December. The first increase in plasma 17b-estradiol (E2), testosterone (T) and gonadosomatic index (GSI) was recorded in January; E2 rose up to one month prior to ovulation, while T and GSI kept increasing until ovulation. Pituitary luteinizing hormone beta (lhb) transcript levels peaked at the time of ovulation. Except for transient changes before and after ovulation, ovarian follicle stimulating hormone receptor (fshr) transcript amounts were relatively stable at a high level. By contrast, luteinizing hormone receptor (lhcgr) transcript levels started out low and increased in parallel to GSI and plasma E2 levels. Exposure to continuous light (LL) induced a bimodal response where maturation was accelerated or arrested. The LL-arrested females showed previtellogenic oil droplet stage follicles or primary yolk follicles only, and fshb and E2 plasma levels collapsed while fshr increased. The LL-accelerated females showed elevated lhb transcript levels and slightly elevated E2 levels during early vitellogenesis, and significantly elevated lhcgr E2 and GSI levels in late vitellogenesis. We conclude that Fsh-dependent signaling stimulates recruitment into and the sustained development through vitellogenesis. Up-regulation of lhcgr gene expression during vitellogenesis may reflect an estrogenic effect, while elevated fshr gene expression following ovulation or during LL-induced arrestment may be associated with ovarian tissue remodeling processes.

Cloning and expression analysis of follicle-stimulating hormone and luteinizing hormone receptor during the reproductive cycle in Korean rockfish (Sebastes schlegeli)

Full-length cDNA sequences encoding the receptors for follicle-stimulating hormone (FSHR) and luteinizing hormone (LHR) were isolated from ovary of Korean rockfish (Sebastes schlegeli) using reverse transcription-polymerase chain reaction (PCR) and rapid amplification of cDNA ends procedures. The cDNA of the KrFSHR encodes a predicted protein of 703 amino acids that showed the greatest homology with European seabass (Dicentrarchus labrax) (78 %) and gilthead seabream (Sparus aurata) (73 %). The cDNA of the KrLHR encodes a predicted protein of 703 amino acids and exhibited the highest homology with European seabass (Dicentrarchus labrax) (79 %) and gilthead seabream (Sparus aurata) (76 %). Besides the gonads, expressions of GTHRs mRNA were also obtained in extra gonadal tissues. Seasonal changes in the gonads expression profiles of KrGTHRs mRNA were examined by quantitative real-time PCR, and the present results suggest that levels for KrFSHR mRNA increase during gonadal growth, whereas KrLHR shows high levels during the late gamete generation period. Our study provides molecular characterization of the GTHRs and expressions profile during reproductive cycles, reinforcing previous knowledge of GTHRs important role in the reproductive endocrine regulation of Korean rockfish.

Molecular cloning of two gonadotropin receptors in mummichog Fundulus heteroclitus and their gene expression during follicular development and maturation

Two cDNAs encoding gonadotropin receptors, follicle-stimulating hormone receptor (FSHR) and luteinizing hormone receptor (LHR) were cloned from mummichog (Fundulus heteroclitus) ovary. Deduced amino acid sequences of the mummichog FSHR (fhFSHR) and LHR (fhLHR) showed high homologies to teleost FSHRs (77-53%) and teleost LHRs (76-62%), respectively. Both the fhFSHR and fhLHR are composed of a typical structural architecture of glycoprotein hormone receptors consisting of the large N-terminal extracellular domain, the transmembrane domain containing seven cell surface membrane-spanning regions, and the intracellular domain. Functional analysis using HEK293 cells stably expressing the fhFSHR or fhLHR demonstrated that both the receptors are specifically activated by mummichog FSH or LH, respectively. Reverse transcription-polymerase chain reaction revealed that both the fhFSHR and fhLHR were expressed in the ovary, testis, and pituitary, and the fhLHR was also expressed in several extra-gonadal tissues. Real-time quantitative-PCR analysis revealed that the fhFSHR gene was abundantly expressed in developing follicles whereas expression of the fhLHR gene markedly increased in follicles of the final maturational stage. These results indicate that gonadotropin stimulation on follicles is regulated by the two distinct pathways via their cognate receptors.

Endocrine disruptive potential of endosulfan on the reproductive axis of Cichlasoma dimerus (Perciformes, Cichlidae)

Endosulfan (ES), a persistent organochlorine pesticide, is widely used despite its toxicity to non-target animals. Upon reaching water bodies, ES can cause negative effects on aquatic animals, including disruption of hormonal systems. However, the action of ES on fish reproductive axis has been hardly studied thus far. The aim of the present work was to assess the endocrine disruptive potential of endosulfan on the pituitary gonadotropins levels and on the testes function due to ES in the South American freshwater fish Cichlasoma dimerus, using in vitro and in vivo approaches. In vitro experiments showed that ES inhibited the LH-stimulated steroidogenesis in gonads; no change was observed in gonadotropins release from pituitaries in culture. Laboratory waterborne ES (0.1, 0.3 and 1 μg/L) exposure for two months caused decrease in βFSH pituitary content and γGT activity in the testes (Sertoli cell function marker). Testicular histology revealed pathologies such as scarce intermediate stages of spermatogenesis, release of immature germ cells into the lobular lumen, presence of foam cells and interstitial fibrosis. As FSH and FSH-mediated steroidogenesis regulate spermatogenesis and Sertoli cell function, the effect of ES on FSH could be responsible for the morphological alterations observed in testes. In vitro, ES disrupted steroidogenesis in gonads, therefore similar effects in vivo cannot be ruled out. Based on this evidence, ES exhibits an endocrine disruptive action on the reproductive axis of C. dimerus, causing disruption at the pituitary and/or at the gonad level. These effects could acquire ecological significance under prolonged exposure to the pesticide in nature.

Experimental and computational study of inter- and intra- species specificity of gonadotropins for various gonadotropin receptors

The gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and their receptors play critical roles in vertebrate reproduction. In order to study intra- and interspecies ligand promiscuity of gonadotropins, COS-7 cells were transiently transfected with one of the gonadotropin receptor genes, FSHR or LHR, and tested for activation by gonadotropins from representative fish orders: Aquilliformes (eel; e), Salmoniformes (trout; tr), and Perciformes (tilapia; ta), and of mammalian origin: porcine (p), bovine (b) and human (h). The study reveals complex relations between the gonadotropin hormones and their receptors. Each gonadotropin activated its own cognate receptor. However, taLHR was also activated by hCG and eLHR was activated by hFSH, hCG, and trFSH. For FSHR, the only cross-reactivity detected was for hFSHR, which was activated by pFSH and bFSH. These findings are of great interest and applicability in the context of activation of various GTHRs by their ligands and by ligands from other vertebrates. Analysis of the three-dimensional models of the structures highlights the importance of residues outside of the currently established hormone-receptor interface region. In addition, the interface residues in taFSHR and the effect of exon duplication, which causes an insert in the LRR domain, are suggested to affect the interaction and binding of taFSH.

Differential regulation of gonadotropin receptors (fshr and lhcgr) by estradiol in the zebrafish ovary involves nuclear estrogen receptors that are likely located on the plasma membrane

FSH and LH are gonadotropins (GTH) that control all major events of gonadal function. FSH and LH signal through their cognate receptors, FSH receptor and LH/choriogonadotropin receptor, respectively, across vertebrates. Compared with the information in mammals, very little is known about these receptors in fish, especially the regulation of their expression. In female zebrafish, fshr and lhcgr exhibit significant temporal difference in expression, with fshr increasing first when the follicles are activated to enter the vitellogenic growth phase and lhcgr lagging behind. This raises an interesting question on the differential regulation of these two GTH receptors (GTHR) during folliculogenesis. Using a primary follicle cell culture, the present study demonstrated that 17β-estradiol (E2), but not testosterone, was a potent endocrine hormone that differentially regulated the expression of fshr and lhcgr. Although E2 stimulated both receptors, its effect on the steady-state level of lhcgr mRNA was much higher (>8-fold up-regulation) than that of fshr (∼0.5-fold increase). E2 likely acted at the transcription level via its nuclear estrogen receptors (ERα and ERβ), because ICI 182,780 could abolish its effects. However, our evidence suggested that these receptors might be localized on the plasma membrane, because β-estradiol 6-(O-carboxy methyl)oxime:BSA could fully mimic the effects of E2. Demonstrating that E2 is likely one of the differentiating factors for the distinct expression of the two GTHR in the zebrafish ovary, this study sheds important light on the functions of the two GTH and their receptors in fish as well as the conservation and diverse aspects of GTHR regulation across vertebrates.

Disruption of the secretion and action of 17,20β-dihydroxy-4-pregnen-3-one in response to a rise in temperature in the Arctic charr, Salvelinus alpinus. Consequences on oocyte maturation and ovulation

Plasma levels of 17,20β-dihydroxy-4-pregnen-3-one (17,20βP), and the timing of ovulation were investigated in female Arctic charr (Salvelinus alpinus) reared at 5°C and at 10°C during the pre-spawning period. The effects of switching from 5 to 10°C, and from 10 to 5°C were also investigated. 17,20βP plasma levels were higher at 5°C than at 10°C. A switch from 10 to 5°C stimulated 17,20βP secretion, whereas a switch from 5 to 10°C had the opposite effect. Ovulation occurred spontaneously in the females kept at 5°C, and in those switched from 10 to 5°C. In contrast, ovulation was inhibited in females reared at 10°C, and in those switched from 5 to 10°C. Oocyte maturation at 5°C and at 10°C in the presence of LH or of 17,20βP was also investigated in vitro using donor females reared at 5 or 10°C. Both LH and 17,20βP stimulated oocyte maturation more effectively in oocytes incubated at 5°C than at 10°C. At both incubation temperatures, the rearing temperature of the donor females had a significant impact on their responsiveness to LH stimulation, but had no effect on their responsiveness to 17,20βP stimulation. In addition to the inhibition of LH secretion, which had already been reported, the results reported here show that in Arctic charr raising the temperature above the physiological range reduces both follicular responsiveness to LH stimulation and the sensitivity of oocytes to 17,20βP stimulation.

Isolation, characterization and expression analyses of FSH receptor in protogynous grouper

Follicle-stimulating hormone (FSH) and its receptor (FSHR) play important roles in spermatogenesis. We cloned and characterized the honeycomb grouper Epinephelus merra FSHR (EmFSHR) to elucidate its role in the protogynous sex change in groupers. Reverse transcription-polymerase chain reaction (RT-PCR) analysis suggested that EmFSHR was expressed exclusively in the gonads. In situ hybridization showed the distribution of EmFSHR in the granulosa cells of previtellogenic oocytes and Leydig cells in the testis. Quantitative reverse transcription PCR (RT-qPCR) analysis of gonadal EmFSHR transcripts during the process of sex change indicated that the lowest levels were found in the female phase before sex change. EmFSHR transcripts increased during the early transitional phase, when oocytes began to degenerate in parallel with the initiation of gonial germ cell differentiation into spermatogonia. A dramatic increase in EmFSHR transcription occurred during the late transitional phase, when the gonad contained numerous proliferating male germ cells and many degenerated oocytes. EmFSHR expression remained high until the transformation from ovary to testis was complete. The data reveal that female to male sex change is associated with the upregulation of EmFSHR transcripts, and that this upregulation may be responsible for the development of testicular tissue and the progression of spermatogenesis. Furthermore, how the upregulation of EmFSHR is controlled in the initiation of sex change remains to be elucidated.

Perspectives on fish gonadotropins and their receptors

Teleosts lack a hypophyseal portal system and hence neurohormones are carried by nerve fibers from the preoptic region to the pituitary. The various cell types in the teleost pituitary are organized in discrete domains. Fish possess two gonadotropins (GtH) similar to FSH and LH in other vertebrates; they are heterodimeric hormones that consist of a common alpha subunit non-covalently associated with a hormone-specific beta subunit. In recent years the availability of molecular cloning techniques allowed the isolation of the genes coding for the GtH subunits in 56 fish species representing at least 14 teleost orders. Advanced molecular engineering provides the technology to produce recombinant GtHs from isolated cDNAs. Various expression systems have been used for the production of recombinant proteins. Recombinant fish GtHs were produced for carp, seabream, channel and African catfish, goldfish, eel, tilapia, zebrafish, Manchurian trout and Orange-spotted grouper. The hypothalamus in fishes exerts its regulation on the release of the GtHs via several neurohormones such as GnRH, dopamine, GABA, PACAP, IGF-I, norepinephrine, NPY, kisspeptin, leptin and ghrelin. In addition, gonadal steroids and peptides exert their effects on the gonadotropins either directly or via the hypothalamus. All these are discussed in detail in this review. In mammals, the biological activities of FSH and LH are directed to different gonadal target cells through the cell-specific expression of the FSH receptor (FSHR) and LH receptor (LHR), respectively, and the interaction between each gonadotropin-receptor couple is highly selective. In contrast, the bioactivity of fish gonadotropins seems to be less specific as a result of promiscuous hormone-receptor interactions, while FSHR expression in Leydig cells explains the strong steroidogenic activity of FSH in certain fish species.

Spermatogenesis in fish

Spermatogenesis is a developmental process during which a small number of diploid spermatogonial stem cells produce a large number of highly differentiated spermatozoa carrying a haploid, recombined genome. We characterise morphologically the different germ cell stages with particular attention for the spermatogonial generations, including the stem cells and their specific capacity to colonise a recipient's testis after transplantation. We propose a nomenclature for fish germ cells to improve the comparability among different teleost fish but also to higher vertebrates. Survival and development of germ cells depends on their continuous and close contact to Sertoli cells, and we review their multiple roles in the cystic mode of spermatogenesis seen in fish. We then discuss gene expression patterns associated with testis maturation. The endocrine system of vertebrates has evolved as master control system over spermatogenesis. In fish, both pituitary gonadotropins LH and FSH stimulate gonadal sex steroid hormone production directly by activating Leydig cells. Information is reviewed on the effects of progestin, androgens, and estrogens on global testicular gene expression patterns (microarray analysis), and on the molecular mechanisms by which steroids regulate specific candidate genes (identified by subtractive hybridization approaches) during early stages of testis maturation. Moreover, progestin and androgen effects on spermiation and milt hydration are discussed. Sex steroids mainly act via receptors expressed by Sertoli cells. One type of response is that Sertoli cells change growth factor expression, which subsequently modulates germ cell proliferation/differentiation via mechanisms yet to be characterised. Finally, we review data on germ cell autonomous processes, mainly derived from loss-of-function mutant fish lines, before identifying a number of focus areas for future research activities.

Control of puberty in farmed fish

Puberty comprises the transition from an immature juvenile to a mature adult state of the reproductive system, i.e. the individual becomes capable of reproducing sexually for the first time, which implies functional competence of the brain-pituitary-gonad (BPG) axis. Early puberty is a major problem in many farmed fish species due to negative effects on growth performance, flesh composition, external appearance, behaviour, health, welfare and survival, as well as possible genetic impact on wild populations. Late puberty can also be a problem for broodstock management in some species, while some species completely fail to enter puberty under farming conditions. Age and size at puberty varies between and within species and strains, and are modulated by genetic and environmental factors. Puberty onset is controlled by activation of the BPG axis, and a range of internal and external factors are hypothesised to stimulate and/or modulate this activation such as growth, adiposity, feed intake, photoperiod, temperature and social factors. For example, there is a positive correlation between rapid growth and early puberty in fish. Age at puberty can be controlled by selective breeding or control of photoperiod, feeding or temperature. Monosex stocks can exploit sex dimorphic growth patterns and sterility can be achieved by triploidisation. However, all these techniques have limitations under commercial farming conditions. Further knowledge is needed on both basic and applied aspects of puberty control to refine existing methods and to develop new methods that are efficient in terms of production and acceptable in terms of fish welfare and sustainability.

Pharmacological characterization, localization and quantification of expression of gonadotropin receptors in Atlantic salmon (Salmo salar L.) ovaries

The gonadotropins Fsh and Lh interact with their receptors (Fshr and Lhr, respectively) in a highly specific manner in mammals with little overlap in biological activities. In fish, the biological activities seem less clearly separated considering, for example, the steroidogenic potency of both Fsh and Lh. Important determinants of the biological activity are the specificity of hormone-receptor interaction and the cellular site of receptor expression. Here, we report the pharmacological characterization of Atlantic salmon Fshr and Lhr, identify receptor-expressing cells in the ovary, and validate receptor mRNA quantification systems. For the pharmacological studies, we used highly purified coho salmon gonadotropins and found that the Fshr preferentially responded to Fsh, but was also activated by approximately 6-fold higher levels of Lh. The Lhr was specific for Lh and did not respond to Fsh. Photoperiod manipulation was used to generate ovarian tissue samples with largely differing stages of maturation. Specific real-time, quantitative (rtq) PCR assays revealed up to 40-fold (fshr) and up to 350-fold (lhr) changes in ovarian expression levels, which correlated well with the differences in ovarian weight, histology, and circulating oestrogen levels recorded in January and June, respectively. Vitellogenic ovaries were used to localise receptor-expressing cells by in situ hybridization. Granulosa cells of small and large vitellogenic follicles were positive for both receptors. Also theca cells of small and large vitellogenic follicles expressed fshr mRNA, while only in large vitellogenic follicles theca cells were (weakly) positive for lhr mRNA. While only ovulatory Lh levels seem high enough to cross-activate the Fshr, expression by both receptors by granulosa and theca cells suggests that homologous ligand receptor interaction will prevail.

Molecular cloning, characterization and expression of thyroid-stimulating hormone receptor in channel catfish

Thyroid-stimulating hormone receptors (TSHRs) are primarily expressed in the thyroid of vertebrates, however recently, transcripts encoding TSHR have been found abundantly in the gonads in a variety of fish species. The purpose of this study is to characterize the channel catfish TSHR and to examine whether the transcript are translated into protein in the gonad or store the transcript as maternal RNA for later use. The cDNA encoding the TSHR was isolated from the channel catfish thyroid but the transcript was determined to be expressed in a number of tissues, including the gonads. In fact, the ovarian expression of TSHR changed significantly during the reproductive season and peaked after the vitellogenic growth phase. Furthermore, the TSHR transcript was also detected in unfertilized eggs but not in fertilized egg of catfish. LM-PAT analysis demonstrated that catfish TSHR transcripts were fully polyadenylated in thyroidal follicles, gonads and unfertilized eggs suggesting that they were translated into protein opposed to being "stored mRNA". Western blot analysis using polyclonal antibodies against the catfish TSHR verified this assumption by visualizing immunoreactive protein in the thyroid, testis, and the post-vitellogenic ovary in abundance. A functional assay clearly showed that the recombinant catfish TSHR was specifically activated by bovine TSH but not by recombinant catfish follicle-stimulating hormone (FSH) and luteinizing hormone (LH). As in other species, the heterologous gonadotropin, hCG, partially activated the receptor. These results suggested that TSHR plays important roles for gametogenesis rather than embryogenesis.

Quantification of gonadotropin subunits GPalpha, FSHbeta, and LHbeta mRNA expression from Atlantic cod (Gadus morhua) throughout a reproductive cycle

To elucidate the role of the gonadotropins in Atlantic cod (Gadus morhua), complete coding sequences with partially or fully un-translated regions for the three subunits GPalpha, FSHbeta, and LHbeta were determined. The sequences of the corresponding genomic loci were also determined, allowing the design of mRNA-targeting quantitative PCR assays. Relative expression was analyzed during a complete seasonal sexual maturation cycle in Atlantic cod females. Increasing levels of lhbeta mRNA were observed during gonadal growth, peaking at spawning in February-March which corresponds to maximum gonadosomatic index. In contrast, both gpalpha and fshbeta gradually increased to a peak in December, two months before spawning started, and decreased in January just prior to spawning. Both mRNAs increased again and remained high during the spawning season, with a decline at the end of the spawning period, a further decrease in spent females, followed by a new gradual increase concurrent with the start of the next reproductive cycle. In addition to its role in vitellogenesis prior to spawning, FSH seems to have additional functions during the spawning period, possibly related to vitellogenesis that runs in parallel with final oocyte maturation and ovulation of the multiple batch spawner Atlantic cod.

Sex change in the Gobiid fish is mediated through rapid switching of gonadotropin receptors from ovarian to testicular portion or vice versa

Sex-changing fish Trimma okinawae can change its sex back and forth from male to female and then to male serially, depending on the social status in the harem. T. okinawae is well equipped to respond to its social status by possessing both ovarian and testicular tissues even though only one gonad remains active at one time. Here we investigated the involvement of gonadotropins in sex change by determining the changes in gonadotropin receptor (GtHR) gene expression during the onset of sex change from female to male and male to female. The expression of the GtHR was found to be confined to the active gonad of the corresponding sexual phase. During the sex-change from female to male, initially the ovary had high levels of FSHR and LHR, which eventually went up in the testicular tissue if the fish was bigger. Changing of the gonads started with switching of GtHR expression discernible within 8-12 h of the visual cue. Further in vitro culture of the transitional gonads with a supply of exogenous gonadotropin (human chorionic gonadotropin) revealed that the to-be-active gonad acquired the ability to produce the corresponding sex hormone within 1 d of the activation of GtHR. Conversely, the to-be-regressed gonad did not respond to the exogenous gonadotropin. Our findings show that the gonads of successive sex-changing fish possess the intrinsic mechanism to respond to the social cue differentially. Additionally, this location switching of GtHR expression also could substantiate the importance of the hypothalamo-pituitary-gonadotropic axis.

Molecular characterization and quantification of the gonadotropin receptors FSH-R and LH-R from Atlantic cod (Gadus morhua)

In order to elucidate regulatory mechanisms during puberty final oocyte maturation and spawning, full-length sequences coding for the receptors for follicle-stimulating hormone (FSH-R) and luteinizing hormone (LH-R) were isolated from female Atlantic cod (Gadus morhua) by a RACE-PCR based strategy. The nucleotide and amino acid sequences showed high homologies with the corresponding sequences of other fish species but contained some distinct differences. Conserved features important for functionality, such as a long N-terminal extracellular domain (ECD), seven transmembrane domains and a short C-terminal intracellular domain, were identified in both predicted proteins. Partial genomic sequences for these genes were also determined, allowing the design of mRNA-specific quantitative PCR assays. Due to suspected alternative splicing during expression of these genes, additional real-time PCR assays detecting variants containing the membrane-anchoring domain were established. Besides the expected expression of FSH-R and LH-R mRNA in the gonads similarly strong signals for LH-R were also obtained in male gill, and in female and male brain. When relative expression was analysed at different stages of sexual maturation, levels for FSH-R increased moderately during gonadal growth whereas those of LH-R showed a high peak at spawning.

Ovarian function of the trout preovulatory ovary: new insights from recent gene expression studies

During the preovulatory period the follicle-enclosed oocyte progressively acquires maturational and developmental competence. In addition, the follicle is also preparing for the release of the oocyte from the follicle at ovulation. Using real-time PCR and cDNA microarrays we have investigated the molecular mechanisms of oocyte competence acquisition and ovulation in rainbow trout (Oncorhynchus mykiss) by monitoring gene expression in the preovulatory ovary. These studies have demonstrated that many molecular events related to maturational competence and developmental competence acquisition, and ovulation occur concomitantly in the preovulatory ovarian follicle. Oocyte maturational competence acquisition is associated with a decrease of estrogen synthesis and signaling capacities. We also observed a differential expression of genes encoding for igfs and related binding protein, members of the TGF beta superfamily, proteins involved in ion and water transport, bone morphogenetic proteins, and cathepsins. In addition, our observation of a strong up-regulation, prior to ovulation, of genes encoding for proteins putatively involved in proteolysis, inflammation, coagulation, vasodilatation, and angiogenesis further supports the hypothesis comparing ovulation with an inflammatory-like reaction. Together, our results suggest that a finely tuned cross-talk exists between oocyte and follicular layers and between the ovulatory process and the oocyte maturational and developmental competence acquisition processes.

Japanese eel follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh): production of biologically active recombinant Fsh and Lh by Drosophila S2 cells and their differential actions on the reproductive biology

Two gonadotropins (Gths), follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), control gonadal steroidogenesis and gametogenesis in vertebrates, including teleost fish. Here, we report on the production of biologically active recombinant Fsh (rec-Fsh) and Lh (rec-Lh) in Japanese eel using Drosophila S2 cells. The three subunits composing Gths, i.e., glycoprotein hormone, alpha polypeptide (Cga), follicle-stimulating hormone, beta polypeptide (Fshb), and luteinizing hormone, beta polypeptide (Lhb), were at first independently produced and were proven to be glycosylated and secreted as the mature peptides. Each beta subunit, along with its Cga, was simultaneously coexpressed to produce heterodimeric rec-Fsh and rec-Lh that were subsequently highly purified. The biological activity of rec-Gths was demonstrated in various in vitro assays. The rec-Gths differentially activated their receptors, which resulted in an increase in 11-ketotestosterone (11KT) secretion, a differential alteration of gene expression of steroidogenic enzymes in immature testis, and the induction of the complete process of spermatogenesis in vitro. The data strongly suggest that Fsh and Lh differentially play important roles in the reproductive physiology of the Japanese eel. By contrast, these rec-Gths exhibited little activity in the gonad when administered in vivo. This difference between in vitro and in vivo bioactivity is probably due to the qualitative nature of glycosylation in S2 cells, which resulted in degradation of the recombinant protein in vivo. These differences in the carbohydrate moieties need to be elucidated and ameliorated.

Expression of gonadotropin and gonadotropin receptor genes during early sexual maturation in male Atlantic salmon parr

Atlantic salmon males may mature already as small parr in freshwater. Sexual maturation in teleosts as in vertebrates is characterized by the activation of the brain-pituitary-gonad axis. The endocrine regulation of early puberty is still not well understood. In the present study, one-summer-old male Atlantic salmon parr were sampled regularly from December several months prior to the beginning of spermatogenesis until spawning in October. Pituitary expression levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) beta subunit genes were analyzed in parallel with testis expression of FSH receptor (FSHR) and LH receptor (LHR) genes by RT-PCR and plasma 11-ketostestosterone (11-KT) was measured. Expression levels of FSHbeta, low during winter and spring started to increase prior to the onset of gonadal growth at the end of May while LHbeta mRNA levels were hardly detectable. Both gonadotropin receptor genes were expressed in immature testis with FSHR transcripts being more abundant (8-fold). FSHR transcript levels increased in parallel to FSHbeta levels from early spermatogenesis onwards, while LHR mRNA started to increase prior to any large changes in LHbeta expression. Both transcript levels of LHbeta and LHR were highest during spermiation. Plasma 11-KT increased at the beginning of spermatogenesis reaching highest levels at spermiogenesis suggesting a possible role of FSH in inducing 11-KT production during early spermatogenesis while LH stimulates via its specific receptor 11-KT production at spermiogenesis. The commitment into sexual maturation appears to be dependant on both the presence of FSHR in immature testis and the increase of FSH expression.

The gonadotropin receptors FSH-R and LH-R of Atlantic halibut (Hippoglossus hippoglossus), 1: isolation of multiple transcripts encoding full-length and truncated variants of FSH-R

As a first step towards understanding the regulatory mechanisms underlying the asynchronous oogenesis in repetitive spawning fish, full-length cDNAs encoding the receptors for follicle stimulating hormone (FSH-R) and luteinizing hormone (LH-R) were isolated from the gonads of the flatfish Atlantic halibut (Hippoglossus hippoglossus). The predicted halibut FSH-R and LH-R of 664 and 698 amino acids, respectively, both contain the characteristic features of a large extracellular (EC) domain, a hepta-helical transmembrane (TM) domain, and a short cytoplasmic C-terminal tail. Halibut FSH-R and LH-R share only 42% overall sequence identity mostly due to low homology in the ligand-binding EC domain. Both receptors show high sequence identity to their orthologs of Nile tilapia, but seem to be more remotely related to the receptors in catfish, zebrafish and salmonids. In contrast to the intron-less TM domain of almost all vertebrate gonadotropin receptors, three introns were identified in this domain of halibut FSH-R, thus resembling the gene structure of Drosophila glycoprotein hormone receptor type I. The FSH-R pre-mRNA was shown to be processed in alternative ways by isolating two different transcripts encoding the complete receptor and four alternative spliced transcripts encoding different truncated receptor variants. Based on the DNA sequence variation and chromosomal organization of the gonadotropin receptors in several teleosts, we propose that the encoding genes have been duplicated in the fish lineage.

Gonadotropin hormone and receptor sequences from model teleost species

Fish offer some advantages for the study of vertebrate reproductive physiology. Only a few of the genes encoding the components of the hypothalamic-pituitary-gonadal axis have been identified from model teleosts. This study describes a combination of database searching and molecular approaches to identify the FSH and LH gonadotropin beta-subunits (fshb and lhb, respectively), and the LH receptor (lhr) from two model teleost species: zebrafish (Danio rerio) and Fugu (Takifugu rubripes). Sequence and phylogenetic analyses were used to examine the relationships that exist between gonadotropins and their receptors from species representing several piscine orders. The gonadotropin alpha-subunit (Cga) is highly conserved among teleosts and tetrapods. The presence of a genomic pseudogene (cgap) was also noted in zebrafish. Generally, teleostean FSHbeta protein sequences share less identity with each other than do LHbeta protein sequences, supporting the hypothesis that FSHbeta diverged more rapidly during teleost evolution. Interestingly, and uniquely, zebrafish Fshb lacked two highly conserved cysteine residues in the "determinant loop" which is thought to contribute towards receptor binding and specificity. Teleost gonadotropin receptor sequences clearly diverged into two distinct groups, FSHR and LHR. As has been seen with mammalian gonadotropin receptor transcripts, splice variants of zebrafish lhr were also observed.

Human placental development is impaired by abnormal human chorionic gonadotropin signaling in trisomy 21 pregnancies

Placental development is markedly abnormal in women bearing a fetus with trisomy 21, with defective syncytiotrophoblast (ST) formation and function. The ST occurs from cytotrophoblast (CT) fusion and plays an essential role by secreting human chorionic gonadotropin (hCG), which is essential to placental development. In trisomy of chromosome 21 (T21) pregnancies, CTs do not fuse and differentiate properly into STs, leading to the secretion of an abnormal and weakly bioactive hCG. In this study we report for the first time, a marked decrease in the number of mature hCG receptor (LH/CG-R) molecules expressed at the surface of T21-affected CTs. The LH/CG-R seems to be functional based on sequencing that revealed no mutations or deletions and binding of recombinant hCG as well as endogenous hCG. We hypothesize that weakly bioactive hCG and lower LH/CG-R expression may be involved in the defect of ST formation. Interestingly, the defective ST formation is mimicked in normal CT cultures by using LH/CG-R small interfering RNA, which result in a lower hCG secretion. Furthermore, treatment of T21-affected CTs with recombinant hCG overcomes in vitro the T21 phenotype, allowing CTs to fuse and form a large ST. These results illustrate for the first time in trisomy 21 pathology, how abnormal endogenous hCG signaling impairs human placental development.

Differential expression and regulation of gonadotropins and their receptors in the Japanese eel, Anguilla japonica

Eel species have a striking life cycle with a blockade of puberty until the oceanic migration. We report the first molecular data on eel gonadotropin receptors. The partial sequences cloned covered two-third of the open reading frame and included most of the extracellular and transmembrane domains. Phylogenetic analysis partitioned the two eel gonadotropin receptors into the two teleost FSHR and LHR clusters, respectively. Real-time quantitative RT-PCR was used to quantify the expression of eel gonadotropins and their receptors. Similar levels of pituitary FSH-beta and LH-beta transcripts were found in the immature previtellogenic female eels. In contrast, ovarian FSHR mRNA level was at 100- to 185-fold higher than that of LHR. This revealed that FSHR rather LHR would mediate gonadotropin stimulation of the early stages of ovarian growth. Chronic treatment with fish pituitary homogenates, applied to induce eel sexual maturation, stimulated pituitary LH-beta but suppressed FSH-beta transcripts. In the ovaries, both FSHR and LHR mRNA were significantly increased in experimentally matured eels. Treatments with sexual steroids showed a stimulatory effect of estradiol-17beta (E(2)) on pituitary LH-beta mRNA levels, while FSH-beta transcripts were suppressed by E(2) or testosterone (T). In contrast, neither E(2) nor T-treatment had any significant effect on ovarian FSHR nor LHR transcripts. This suggests that steroid feedbacks may be responsible for the opposite regulation of pituitary gonadotropins in experimentally matured eels, but are not involved in the regulation of gonadotropin receptors. In conclusion, these are the first data on the sequence, expression and regulation of gonadotropin receptors in the eel. They provide new foundation for basic and applied research on eel reproduction.