Novel expression of gonadotropin subunit genes in oocytes of the gilthead seabream (Sparus aurata)

The late-evolving salmon and trout join the GnRH1 club

Although it is known that the whitefish, an ancient salmonid, expresses three distinct gonadotropin-releasing hormone (GnRH) forms in the brain, it has been thought that the later-evolving salmonids (salmon and trout) had only two types of GnRH: GnRH2 and GnRH3. We now provide evidence for the expression of GnRH1 in the gonads of Atlantic salmon by rapid amplification of cDNA ends, real-time quantitative PCR and immunohistochemistry. We examined six different salmonid genomes and found that each assembly has one gene that likely encodes a viable GnRH1 prepropeptide. In contrast to both functional GnRH2 and GnRH3 paralogs, the GnRH1 homeolog can no longer express the hormone. Furthermore, the viable salmonid GnRH1 mRNA is composed of only three exons, rather than the four exons that build the GnRH2 and GnRH3 mRNAs. Transcribed gnrh1 is broadly expressed (in 17/18 tissues examined), with relative abundance highest in the ovaries. Expression of the gnrh2 and gnrh3 mRNAs is more restricted, primarily to the brain, and not in the gonads. The GnRH1 proximal promoter presents composite binding elements that predict interactions with complexes that contain diverse cell fate and differentiation transcription factors. We provide immunological evidence for GnRH1 peptide in the nucleus of 1-year-old type A spermatogonia and cortical alveoli oocytes. GnRH1 peptide was not detected during other germ cell or reproductive stages. GnRH1 activity in the salmonid gonad may occur only during early stages of development and play a key role in a regulatory network that controls mitotic and/or meiotic processes within the germ cell.

Chronological Changes in Gonadotropin-Releasing Hormone 1, Gonadotropins, and Sex Steroid Hormones along the Brain-Pituitary-Gonadal Axis during Gonadal Sex Differentiation and Development in the Longtooth Grouper, Epinephelus bruneus

(1) Fshβ and Lhβ showed stronger signals and higher transcript levels from 590 to 1050 dph than at earlier stages, implying their active involvement during primary oocyte development. (2) Fshβ and Lhβ at lower levels were detected during the phases of ovarian differentiation and oogonial proliferation. (3) E2 concentrations increased significantly at 174, 333, and 1435 dph, while T concentrations exhibited significant increases at 174 and 333 dph. These findings suggest potential correlations between serum E2 concentrations and the phases of oogonial proliferation and pre-vitellogenesis.

Gonadotropins and Sex Steroid Hormones in Captive-Reared Small Yellow Croaker (Larimichthys polyactis) and Their Role in Female Reproductive Dysfunction

The seed production of small yellow croaker (SYC) is constrained by reproductive dysfunction in captive-reared females. Reproductive dysfunction is closely linked to endocrine reproductive mechanisms. To better understand the reproductive dysfunction in captive broodstock, functional characterization of gonadotropins (GtHs: follicle stimulating hormone β subunit, fshβ; luteinizing hormone β subunit, lhβ; and glycoprotein α subunit, gpα) and sex steroids (17β-estradiol, E2; testosterone, T; progesterone; P) was performed using qRT-PCR, ELISA, in vivo, and in-vitro assay. The pituitary GtHs and gonadal steroids levels were significantly higher in ripen fish of both sexes. However, changes in lhβ and E2 levels in females were not significant in the developing and ripen stages. Furthermore, GtHs and steroids levels were lower in females compared to males throughout the reproductive cycle. In vivo administration of gonadotropin releasing hormone analogue (GnRHa) significantly increased the expression of GtHs in both dose- and time-related manners. The lower and higher doses of GnRHa led to successful spawning in male and female SYC, respectively. Sex steroids in vitro significantly inhibited the expression of lhβ in female SYC. Overall, GtHs were shown to play a vital role in final gonadal maturation, while steroids promoted negative feedback in the regulation of pituitary GtHs. Lower levels of GtHs and steroids might be key components in the reproductive dysfunction of captive-reared female SYC.

A half century of fish gonadotropin-releasing hormones: Breaking paradigms

The field of fish gonadotropin-releasing hormones (GnRHs) is also celebrating its 50th anniversary this year. This review provides a chronological history of fish GnRH biology over the past five decades. It demonstrates how discoveries in fish regarding GnRH and GnRH receptor multiplicity, dynamic interactions between GnRH neurons, and additional neuroendocrine factors acting alongside GnRH, amongst others, have driven a paradigm shift in our understanding of GnRH systems and functions in vertebrates, including mammals. The role of technological innovations in enabling scientific discoveries is portrayed, as well as how fundamental research in fish GnRH led to translational outcomes in aquaculture. The interchange between fish and mammalian GnRH research is discussed, as is the value and utility of using fish models for advancing GnRH biology. Current challenges and future perspectives are presented, with the hope of expanding the dialogue and collaborations within the neuroendocrinology scientific community at large, capitalizing on diversifying model animals and the use of comparative strategies.

Molecular characterization of kisspeptin receptors and gene expression analysis during oogenesis in the Russian sturgeon (Acipenser gueldenstaedtii)

Sturgeons belong to a subclass of fishes that derived from ray-finned fish ancestors preceding the emergence of teleosts. The Russian sturgeon (Acipenser gueldenstaedtii) is a late-maturing fish with the females reaching puberty under aquaculture conditions at 6-10 years of age. Since kisspeptin has been shown to be a key hormone involved in regulation of major reproductive processes of many vertebrate species, this study was conducted to better understand the kisspeptin receptor (KissR) in sturgeon. In this study we have cloned Russian sturgeon KissR1 from brain mRNA and observed the ontogeny of rsKissR1 mRNA expression in ovarian follicles. Multiple sequence alignment of KissR1, KissR4, and their orthologs revealed that the Russian sturgeon (rs) KissR1 sequence shares 64%-77% identity with elephant shark, coelacanth, and gar and 44-58% identity with tetrapod and teleost KissR1 sequences, while KissR4 seemed to share <65% identity to eel KissR2 and ~57% identity to Perciformes and Cypriniformes. Further rsKissR4 showed <97% identity to reed fish KissR4, <63% with Squamata (Reptiles) and gar KissR4. A phylogenetic analysis revealed that rsKissR1 is more closely related to coelacanth and gar KissR1 than teleost, while rsKissR4 was part of the KissR4 clade and shared higher similarity with Actinopterygiian sequences. We have further predicted homology models for both rsKiss receptors and performed in-silico analyses of their binding to a kiss-10 peptide. Both sturgeon and zebrafish Kiss1 and Kiss2 activated rsKissR1 via both PKC/Ca2+ and PKA/cAMP signal-transduction pathways, while rsKissR2 was found to be less effective and was not activated by stKiss peptides. Ovarian rsKissR transcript levels for 10 fishes were determined by real-time PCR and significantly increased concomitantly with oogenesis, where the highest level of expression was evident in black follicles. These data suggest that extra-neuronal expression of the kisspeptin receptor may be involved in sturgeon reproduction in a manner dependent on reproductive development.

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.

Molecular cloning, characterization, and mRNA expression of gonadotropins during larval development in turbot (Scophthalmus maximus)

Gonadotropins (GtHs) play a pivotal role in regulating the reproductive axis and puberty. In this study, full-length sequences coding for common glycoprotein α subunit (CGα) and luteinizing hormone β (LHβ) were isolated from female turbot (Scophthalmus maximus) pituitary by homology cloning and a strategy based on rapid amplification of cDNA end-polymerase chain reaction. Results showed that the two cDNAs consisted of 669 and 660 nucleotides encoding 129 and 139 amino acids, respectively. CGα and LHβ manifested typical characteristics of glycoprotein hormones, high homologies with the corresponding sequences of available teleosts, and high homology with that of Hippoglossus hippoglossus. CGα, FSHβ, and LHβ mRNAs were abundant in the pituitary, but less expressed in extra-pituitary tissues. The cgα, fshβ, and lhβ were detected at 1-day post-hatching (dph) and peaked simultaneously at early-metamorphosis (22 dph). cgα and fshβ mRNA levels were significantly increased at pre-metamorphosis, peaked in early metamorphosis, and then gradually decreased until metamorphosis was completed. Conversely, lhβ mRNA levels gradually decreased at pre-metamorphosis, dramatically peaked at early metamorphosis, and then decreased during metamorphosis. In addition, the mRNA levels of cgα were significantly higher than those of fshβ and lhβ during turbot larval metamorphic development, whereas no significant difference was found between fshβ and lhβ. These results suggested (i) an early activation of the GtHs system after hatching, which was the highest expression at early metamorphosis, and (ii) FSHβ and LHβ were together involved in the establishment of the reproductive axis during larval development in turbot. These findings contribute to further understanding the potential roles of GtHs during fish larval development.

Characterization of the European Sea Bass (Dicentrarchus labrax) Gonadal Transcriptome During Sexual Development

The European sea bass is one of the most important cultured fish in Europe and has a marked sexual growth dimorphism in favor of females. It is a gonochoristic species with polygenic sex determination, where a combination between still undifferentiated genetic factors and environmental temperature determines sex ratios. The molecular mechanisms responsible for gonadal sex differentiation are still unknown. Here, we sampled fish during the gonadal developmental period (110 to 350 days post fertilization, dpf), and performed a comprehensive transcriptomic study by using a species-specific microarray. This analysis uncovered sex-specific gonadal transcriptomic profiles at each stage of development, identifying larger number of differentially expressed genes in ovaries when compared to testis. The expression patterns of 54 reproduction-related genes were analyzed. We found that hsd17β10 is a reliable marker of early ovarian differentiation. Further, three genes, pdgfb, snx1, and nfy, not previously related to fish sex differentiation, were tightly associated with testis development in the sea bass. Regarding signaling pathways, lysine degradation, bladder cancer, and NOD-like receptor signaling were enriched for ovarian development while eight pathways including basal transcription factors and steroid biosynthesis were enriched for testis development. Analysis of the transcription factor abundance showed an earlier increase in females than in males. Our results show that, although many players in the sex differentiation pathways are conserved among species, there are peculiarities in gene expression worth exploring. The genes identified in this study illustrate the diversity of players involved in fish sex differentiation and can become potential biomarkers for the management of sex ratios in the European sea bass and perhaps other cultured species.

Molecular characterization of the gonadal kisspeptin system: Cloning, tissue distribution, gene expression analysis and localization in sablefish (Anoplopoma fimbria)

The kisspeptin system plays pivotal roles in the regulation of vertebrate reproduction. Classically, kisspeptin produced in the brain stimulates brain gonadotropin-releasing hormone signaling, which in turn activates the pituitary-gonad axis. Expression of the kisspeptin system has also been documented in peripheral tissues, including gonads of mammals and fishes. However, the fish gonadal kisspeptin system remained uncharacterized. Herein we report identification and characterization of four kisspeptin system mRNAs (kisspeptin 1 (kiss1), kiss2, and G protein-coupled receptor 54-1 (gpr54-1) and gpr54-2) in sablefish, Anoplopoma fimbria. Sablefish predicted protein sequences were highly similar to those of other marine teleosts, but less so to freshwater teleosts. Tissue distribution analyses revealed that all four kisspeptin-system transcripts were expressed in both brain and gonad. However, kiss2 was the predominant transcript in the gonads and the only transcript detected in ovulated eggs. Ontogenetic analysis of kiss2 expression in juvenile sablefish gonads demonstrated that levels were low during sex differentiation but increased with fish size and gonadal development. Dramatic increases in kiss2 mRNA occurred during primary oocyte growth, while levels remained relatively low in testes. In situ hybridization revealed that kiss2 mRNA was localized to cytoplasm of perinucleolus stage oocytes, suggesting it could play a local role in oogenesis or could be synthesized and stored within oocytes as maternal mRNA. This represents the first study to focus on the gonadal kisspeptin system in fishes and provides important tools for further investigation of both the gonadal and brain kisspeptin systems in sablefish.

Molecular cloning and characterization of gonadotropin subunits (GTHα, FSHβ and LHβ) and their regulation by hCG and GnRHa in Japanese sea bass (Lateolabrax japonicas) in vivo

In this study, three cDNA sequences encoding common glycoprotein α subunit (GTHα), follicle-stimulating hormone β subunit (FSHβ) and luteinizing hormone β subunit (LHβ) were isolated from Japanese sea bass (Lateolabrax japonicas). Comparison of the deduced amino acid sequences with other gonadotropic hormones (GTHs) indicated that their cysteine residues and potential N-linked glycosylation sites were highly conserved, and high homology with those of other perciformes was showed in phylogenetic analysis. GTHs transcripts were present highly in the pituitary and brain and weakly in testis and other tissues. During testicular development, GTHs transcriptional levels in pituitary and brain (expect FSHβ subunit in brain) were significantly increased at spermiation period, stage V. Subsequently, the effects of hCG and GnRHa on the mRNA levels of GTHs subunits were examined. In brain, both hormones were detected to improve the expression of GTHα subunit mRNA. In pituitary, three GTHs subunits increased parallelly and abruptly in two hormone treatment groups. In testis, hCG was suggested to improve three GTHs subunits expression in Japanese sea bass for the first time. These results suggest that both gonadotropins are probably involved in the control of Japanese sea bass spermatogenesis and provide a framework for better understanding of the mechanisms of hormone-mediated reproduction control in Japanese sea bass and other teleosts.

Molecular characterization of three gonadotropin subunits and their expression patterns during ovarian maturation in Cynoglossus semilaevis

The endocrine regulation of reproduction in a multiple spawning flatfish with an ovary of asynchronous development remains largely unknown. The objectives of this study were to monitor changes in mRNA expression patterns of three gonadotropin hormone (GTH) subunits (FSHβ, LHβ and CGα) and plasma GTH levels during ovarian maturation of half-smooth tongue sole Cynoglossus semilaevis. Cloning and sequence analysis revealed that the cDNAs of FSHβ, LHβ and CGα were 541, 670 and 685 bp in length, and encode for peptides of 130, 158 and 127 amino acids, respectively. The number of cysteine residues and potential N-linked glycosylation sites of the flatfish GTHs were conserved among teleosts. However, the primary structure of GTHs in Pleuronectiformes appeared to be highly divergent. The FSHβ transcriptional level in the pituitary remained high during the vitellogenic stage while plasma levels of FSH peaked and oocyte development was stimulated. The LHβ expression in the pituitary and ovary reached the maximum level during oocyte maturation stages when the plasma levels of LH peaked. The brain GTHs were expressed at the different ovarian stages. These results suggested that FSH and LH may simultaneously regulate ovarian development and maturation through the brain-pituitary-ovary axis endocrine system in tongue sole.

Molecular characterization of the Chinese alligator follicle-stimulating hormone β subunit (FSHβ) and its expression during the female reproductive cycle

The Chinese alligator Alligator sinensis is an endangered species endemic to China, it has a highly specialized reproductive pattern with low fecundity. Up to date, little is known about the regulation of its female reproductive cycle. Follicle-stimulating hormone (FSH), a glycoprotein hormone, plays a key role in stimulating and regulating ovarian follicular development and egg production. In this study, the complete FSHβ cDNA from the ovary of the Chinese alligator was obtained for the first time, it consists of 843-bp nucleotides, including 120-bp nucleotides of the 5'-untranslated region (UTR), 396-bp of the open reading frame, and 3'-UTR of 327-bp nucleotides. It encodes a 131-amino acid precursor molecule of FSHβ with a signal peptide of 18 amino acids followed by a mature protein of 113 amino acids. Its deduced amino acid sequence shares high identities with the American alligator (100%) and birds (89-92%). Phylogenetic tree analysis of the FSHβ amino acid sequence indicated that alligators cluster into the bird branch. Tissue distribution analyses indicated that FSHβ mRNA is expressed in ovary, intestine and liver with the highest level in the ovary, while not in stomach, pancreas, heart, thymus and thyroid. Expression of FSHβ in ovary increases in May (breeding prophase) and peaks in July (breeding period), it is maintained at high levels through September, then decreases significantly in November (post-reproductive period) and remains relatively low from January to March (hibernating period). These temporal changes of FSHβ expression implicated that it might play an important role in promoting ovarian development during the female reproductive cycle.

Gene expression profiling of key genes in hypothalamus-pituitary-gonad axis of rare minnow Gobiocypris rarus in response to EE2

The 17α-ethinylestradiol (EE2), which could induce estrogenic effects, is found in different aquatic systems. The current study aimed to assess in vivo effects of short-term EE2 exposure on the transcriptional activity of genes in the brain and gonad tissues in order to characterize the mode of action of EE2 on the hypothalamus-pituitary-gonad axis in rare minnow (Gobiocypris rarus). The full length cDNAs of fshβ, lhβ, fshr and lhr were first characterized in G. rarus. The homology and phylogenetic analyses of the amino acid sequences revealed that these four genes share high identity in cyprinid fish. The tissue distribution analysis by qRT-PCR showed that fshβ and lhβ were mainly expressed in the brain and fshr and lhr were mainly expressed in gonads. Adult G. rarus was exposed to EE2 at 1, 5, 25 and 125 ng/L for 3 and 6 days and the expression of brain cyp19a1b, fshβ and lhβ, estrogen receptors (esr1, esr2a, and esr2b) and gonadal fshr, lhr and cyp19a1a were assessed. Cyp19a1b was significantly up-regulated in the brains of female exposed to EE2 at 1-125 ng/L for 6 days. The brain lhβ, but not fshβ was strongly suppressed in most EE2 exposure groups of both sexes. The brain esr2b was inhibited in both sexes exposed to EE2 at all of the four concentrations for 6 days. Esr2a was up-regulated in the females by 6-day EE2 treatment at 1 and 25 ng/L. The high responsiveness of brain lhβ and esr2s to EE2 and their significant correlation in both sexes suggested that the transcriptional activity of Esr2s could play key roles in modulation of lhβ expression via direct action on gonadotropic cells in response to EE2. In gonads, fshr was strongly inhibited by EE2 in males, while lhr was significantly stimulated by EE2 in females. Cyp19a1a was inhibited by EE2 in both sexes. The positive correlations of gene expressions of both fshr and lhr with cyp19a1a in testes suggest that the suppression of 17α-estradiol (E2) synthesis in testis by exogenous estrogen could mediate via both Fsh/Fshr and Lh/Lhr signaling in male G. rarus.

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.

Effects of ovaprim, a commercial spawning inducer, on vasotocin and steroid hormone profiles in the catfish Heteropneustes fossilis: in vivo and in vitro studies

Ovaprim (OVP) is used as an effective spawning inducer for artificial breeding of fishes and contains a salmon gonadotropin-releasing hormone analogue and a dopamine receptor-2 antagonist, domperidone. Previously, we have shown that vasotocin (VT) stimulates ovarian final oocyte maturation, hydration, and ovulation through a mechanism involving induction of a steroidogenic shift, favouring the production of a maturation-inducing hormone (MIH). In the present study, we demonstrated that OVP stimulated brain, plasma and ovarian VT levels, suggesting multiple sites of action, apart from its well established role in the induction of a preovulatory LH surge. An intraperitoneal injection of 0.5μL/g body weight of OVP for different time intervals (0, 4, 8, 12, 16 and 24h) induced ovulation as well as increased significantly brain and plasma VT levels in a time-dependent manner. Plasma steroids were differentially altered; the levels of estradiol-17β (E2) and testosterone (T) decreased, and the MIH (17, 20β-dihydroxy-4-pregnen-3-one; 17, 20β-DP) level increased time-dependently. In order to demonstrate whether OVP acts at the level of the ovary directly, in vitro experiments were conducted. The incubation of ovarian slices/follicles with OVP (1, 5 and 10μL/mL) for different time points (0, 4, 8, 12, 16 and 24h) induced germinal vesicle breakdown (GVBD) in a concentration- and time-dependent manner. Ovarian VT increased significantly in a concentration- and time-dependent manner with a maximal increment at 16h. Ovarian T and E2 levels decreased concurrently with the rise in the MIH level, dose- and duration-dependently. The results show that OVP stimulates VT at the brain and ovarian level. The direct OVP-VT cascade has the potential to stimulate FOM and ovulation, sidelining the pituitary glycoprotein hormone (LH) surge.

Melanin-concentrating hormone (MCH) and gonadotropin-releasing hormones (GnRH) in Atlantic cod, Gadus morhua: tissue distributions, early ontogeny and effects of fasting

Melanin-concentrating hormone (MCH) is classically known for its role in regulating teleost fish skin color change for environmental adaptation. Recent evidence suggests that MCH also has appetite-stimulating properties. The gonadotropin-releasing hormone (GnRH) peptide family has dual roles in endocrine control of reproduction and energy status in fish. Atlantic cod (Gadus morhua) are a commercially important aquaculture species inhabiting the shores of Atlantic Canada. In this study, we examine MCH and GnRH transcript expression profiles during early development as well as in central and peripheral tissues and quantify juvenile Atlantic cod MCH and GnRH hypothalamic mRNA expressions following food deprivation. MCH and GnRH3 cDNAs are maternally deposited into cod eggs, while MCH has variable expression throughout early development. GnRH2 and GnRH3 mRNAs "turn-on" during mid-segmentation once the brain is fully developed. For both MCH and GnRH, highest expression appears during the exogenous feeding stages, perhaps supporting their functions as appetite regulators during early development. MCH and GnRH transcripts are found in brain regions related to appetite regulation (telencephalon/preoptic area, optic tectum/thalamus, hypothalamus), as well as the pituitary gland and the stomach, suggesting a peripheral function in food intake regulation. Atlantic cod MCH mRNA is upregulated during fasting, while GnRH2 and GnRH3 transcripts do not appear to be influenced by food deprivation. In conclusion, MCH might be involved in stimulating food intake in juvenile Atlantic cod, while GnRHs may play a more significant role in appetite regulation during early development.

Effects of global warming on fish reproductive endocrine axis, with special emphasis in pejerrey Odontesthes bonariensis

The ongoing of global warming trend has led to an increase in temperature of several water bodies. Reproduction in fish, compared with other physiological processes, only occurs in a bounded temperature range; therefore, small changes in water temperature could significantly affect this process. This review provides evidence that fish reproduction may be directly affected by further global warming and that abnormal high water temperature impairs the expression of important genes throughout the brain-pituitary-gonad axis. In all fishes studied, gonads seem to be the organ more readily damaged by heat treatments through the inhibition of the gene expression and subsequent synthesis of different gonadal steroidogenic enzymes. In view of the feedback role of sex steroids upon the synthesis and release of GnRH and GtHs in fish, it is possible that the inhibition observed at brain and pituitary levels in treated fish is consequence of the sharp decrease in plasma steroids levels. Results of in vitro studies on the inhibition of pejerrey gonad aromatase expression by high temperature corroborate that ovary functions are directly disrupted by high temperature independently of the brain-pituitary axis. For the reproductive responses obtained in laboratory fish studies, it is plausible to predict changes in the timing and magnitude of reproductive activity or even the total failure of spawning season may occur in warm years, reducing annual reproductive output and affecting future populations.

Dorsomorphin promotes survival and germline competence of zebrafish spermatogonial stem cells in culture

Zebrafish spermatogonial cell cultures were established from Tg(piwil1:neo);Tg(piwil1:DsRed) transgenic fish using a zebrafish ovarian feeder cell line (OFC3) that was engineered to express zebrafish Lif, Fgf2 and Gdnf. Primary cultures, initiated from testes, were treated with G418 to eliminate the somatic cells and select for the piwil1:neo expressing spermatogonia. Addition of dorsomorphin, a Bmp type I receptor inhibitor, prolonged spermatogonial stem cell (SSC) survival in culture and enhanced germline transmission of the SSCs following transplantation into recipient larvae. In contrast, dorsomorphin inhibited the growth and survival of zebrafish female germline stem cells (FGSCs) in culture. In the presence of dorsomorphin, the spermatogonia continued to express the germ-cell markers dazl, dnd, nanos3, vasa and piwil1 and the spermatogonial markers plzf and sox17 for at least six weeks in culture. Transplantation experiments revealed that 6 week-old spermatogonial cell cultures maintained in the presence of dorsomorphin were able to successfully colonize the gonad in 18% of recipient larvae and produce functional gametes in the resulting adult chimeric fish. Germline transmission was not successful when the spermatogonia were cultured 6 weeks in the absence of dorsomorphin before transplantation. The results indicate that Bmp signaling is detrimental to SSCs but required for the survival of zebrafish FGSCs in culture. Manipulation of Bmp signaling could provide a strategy to optimize culture conditions of germline stem cells from other species.

Production of zebrafish offspring from cultured female germline stem cells

Zebrafish female germline stem cell (FGSC) cultures were generated from a transgenic line of fish that expresses Neo and DsRed under the control of the germ cell specific promoter, ziwi [Tg(ziwi:neo);Tg(ziwi:DsRed)]. Homogeneous FGSC cultures were established by G418 selection and continued to express ziwi for more than 6 weeks along with the germ cell markers nanos3, dnd, dazl and vasa. A key component of the cell culture system was the use of a feeder cell line that was initiated from ovaries of a transgenic line of fish [Tg(gsdf:neo)] that expresses Neo controlled by the zebrafish gonadal soma derived factor (gsdf) promoter. The feeder cell line was selected in G418 and engineered to express zebrafish leukemia inhibitory factor (Lif), basic fibroblast growth factor (Fgf2) and glial-cell-line derived neurotrophic factor (Gdnf). These factors were shown to significantly enhance FGSC growth, survival and germline competency in culture. Results from cell transplantation experiments revealed that the cultured FGSCs were able to successfully colonize the gonad of sterile recipient fish and generate functional gametes. Up to 20% of surviving recipient fish that were injected with the cultured FGSCs were fertile and generated multiple batches of normal offspring for at least 6 months. The FGSC cultures will provide an in vitro system for studies of zebrafish germ cell growth and differentiation and their high frequency of germline transmission following transplantation could form the basis of a stem cell-mediated strategy for gene transfer and manipulation of the zebrafish genome.

Characterization of luteinizing hormone and luteinizing hormone receptor and their indispensable role in the ovulatory process of the medaka

The molecular properties and roles of luteinizing hormone (Lh) and its receptor (Lhcgrbb) have not been studied for the medaka (Oryzias latipes), which is an excellent animal model for ovulation studies. Here, we characterized the medaka Lh/Lhcgrbb system, with attention to its involvement in the ovulatory process of this teleost fish. In the medaka ovary, follicle-stimulating hormone receptor mRNA was expressed in small and medium-sized follicles, while lhcgrbb mRNA was expressed in the follicle layers of all growing follicles. Experiments using HEK 293T cells expressing medaka Lhcgrbb in vitro revealed that gonadotropin from pregnant mare's serum and medaka recombinant Lh (rLh) bound to the fish Lhcgrbb. The fish gonadotropin subunits Gtha, Fshb, and Lhb were essentially expressed at fairly constant levels in the pituitary of the fish during a 24-h spawning cycle. Using medaka rLh, we developed a follicle culture system that allowed us to follow the whole process of oocyte maturation and ovulation in vitro. This follicle culture method enabled us to determine that the Lh surge for the preovulatory follicle occurred in vivo between 19 and 15 h before ovulation. The present study also showed that oocyte maturation and ovulation were delayed several hours in vitro compared with in vivo. Treatment of large follicles with medaka rLh in vitro significantly increased the expression of Mmp15, which was previously demonstrated to be crucial for ovulation in the fish. These findings demonstrate that Lh/Lhcgrbb is critically involved in the induction of oocyte maturation and ovulation.

The Kiss2 receptor (Kiss2r) gene in Southern Bluefin Tuna, Thunnus maccoyii and in Yellowtail Kingfish, Seriola lalandi - functional analysis and isolation of transcript variants

The kisspeptin system plays an essential role in reproductive function in vertebrates, particularly in the onset of puberty. We investigated the kisspeptin system in two Perciform teleosts, the Southern Bluefin Tuna (SBT; Thunnus maccoyii), and the Yellowtail Kingfish (YTK; Seriola lalandi), by characterising their kisspeptin 2 receptor (Kiss2r) genes. In addition to the full length Kiss2r cDNA sequences, we have isolated from SBT and YTK a transcript variant that retained an intron. We have further obtained three ytkKiss2r transcript variants that contained deletions. In vitro functional analysis of the full length SBT and YTK Kiss2r showed higher response to Kiss2-10 than to Kiss1-10, with stronger transduction via PKC than PKA. The full length ytkKiss2r and two deletion variants were differentially expressed in the brain of male, but not in female, juvenile YTK treated with increasing doses of Kiss2-10 peptide. In the gonads, the expression level of the ytkKiss2r transcripts did not vary significantly either in the male or female fish. This is the first time that transcript variants of the Kiss2r gene that contain deletions and show responsiveness to treatments with kisspeptin have been reported in any teleost.

The role of pituitary gonadotropins in gonadal sex differentiation in the protogynous Malabar grouper, Epinephelus malabaricus

The aim of this study was to clarify the roles of 2 gonadotropins (GTHs), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), on sex differentiation in the protogynous Malabar grouper, Epinephelus malabaricus. To do this, the mRNA expression patterns of GTH subunits (cga, fshb, and lhb) in the fish pituitary throughout gonadal sex differentiation were investigated. Real-time reverse transcriptase (RT)-PCR showed that cga and fshb were present in the undifferentiated and ovarian differentiation stages, and that the expression levels significantly increased after ovarian differentiation (AOD). However, lhb was not expressed before ovarian differentiation (BOD) and was first detected AOD. Next, to investigate the differentiation and distribution of Fshb and Lhb-producing cells in the pituitary of fish throughout gonadal sex differentiation, immunohistochemical analysis was used to detect teleost GTH subunits. Positive immunoreactivity against Fshb and Lhb was not detected in the pituitary BOD; Fshb and Lhb-positive cells first appeared in the pituitary AOD. It therefore seems unlikely that pituitary gonadotropins play a major role in the control of gonadal sex differentiation in the Malabar grouper.

Extrahypophyseal expression of gonadotropin subunits in pejerrey Odontesthes bonariensis and effects of high water temperatures on their expression

It has been traditionally accepted that the gonadotropins (GtHs), follicle stimulating hormone (FSH) and luteinizing hormone (LH), are synthesized and secreted only by the pituitary. However, the presence of theses hormones in extrapituitary tissues has been demonstrated in mammals, and more recently also in fish. In this study, we cloned the cDNAs and characterized the expression of FSH-β, LH-β, and glycoprotein hormone α (GPH-α) subunits from brain and gonads of male and female pejerrey Odontesthes bonariensis at different stages of gonadal maturation. In situ hybridization revealed that, in addition to their classical location in pituitary cells, the three GtH transcripts were also located in the gonads. FSH-β and GPH-α subunits were found in the cytoplasm of oogonia, previtellogenic and vitellogenic oocytes in ovaries. LH-β expression was detected in previtellogenic and vitellogenic oocytes but not in oogonia. In males, the three subunits were expressed in spermatogonia and to a lesser extent in spermatocytes. Exposure of fish to high water temperatures that impair pejerrey reproduction also induced a decrease of extrahypophyseal expression of GtH subunits.

A graphical systems model and tissue-specific functional gene sets to aid transcriptomic analysis of chemical impacts on the female teleost reproductive axis

Oligonucleotide microarrays and other 'omics' approaches are powerful tools for unsupervised analysis of chemical impacts on biological systems. However, the lack of well annotated biological pathways for many aquatic organisms, including fish, and the limited power of microarray-based analyses to detect low level differential expression of individual genes can hinder the ability to infer and understand chemical effects based on transcriptomic data. Here we report on the supervised assembly of a series of tissue-specific functional gene sets intended to aid transcriptomic analysis of chemical impacts on the female teleost reproductive axis. Gene sets were defined based on an updated graphical systems model of the teleost brain-pituitary-gonadal-hepatic axis. Features depicted in the model were organized into gene sets and mapped to specific probes on three zebrafish (Danio rerio) and two fathead minnow (Pimephales promelas) microarray platforms. Coverage of target genes on the microarrays ranged from 48% for the fathead minnow arrays to 88% for the most current zebrafish platform. Additionally, extended fathead minnow gene sets, incorporating first degree neighbors identified from a Spearman correlation network derived from a large compendium of fathead minnow microarray data, were constructed. Overall, only 14% of the 78 genes queried were connected in the network. Among those, over half had less than five neighbors, while two genes, cyclin b1 and zona pellucida glycoprotein 3, had over 100 first degree neighbors, and were neighbors to one another. Gene set enrichment analyses were conducted using microarray data from a zebrafish hypoxia experiment and fathead minnow time-course experiments conducted with three different endocrine-active chemicals. Results of these analyses demonstrate the utility of the approach for supporting biological inference from ecotoxicogenomic data and comparisons across multiple toxicogenomic experiments. The graphical model, gene mapping, and gene sets described are now available to the scientific community as tools to support ecotoxicogenomic research.

Discovery of a new reproductive hormone in teleosts: pituitary adenylate cyclase-activating polypeptide-related peptide (PRP)

Pituitary adenylate cyclase-activating polypeptide (PACAP)-related peptide (PRP) is a peptide encoded with PACAP in the same precursor protein. Non-mammalian PRPs were previously termed growth hormone-releasing hormone (GHRH)-like peptide, and was regarded as the mammalian GHRH homologue in non-mammalian vertebrates until the discovery of authentic GHRH genes in teleosts and amphibians. Although a highly specific receptor for PRP, which is lost in mammals, is present in non-mammals, a clear function of PRP in vertebrates remains unknown. Using goldfish as a model, here we show the expression of PRP and its cognate receptor in the brain-pituitary-gonadal (BPG) axis, thus suggesting a function of goldfish (gf) PRP in regulating reproduction. We found that gfPRP controls the expression of reproductive hormones in the brain, pituitary and ovary. Goldfish PRP exerts stimulatory effects on the expression of salmon gonadotropin-releasing hormone (sGnRH) in the brain, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in pituitary primary culture cells, but inhibits the expression of LH in the ovary. Using the same technique, we showed that gfPRP did not alter the mRNA level of growth hormone in the pituitary primary culture. In summary, we have discovered the first function of vertebrate PRP in regulating reproduction, which provides a new research direction in studying the neuroendocrine control of reproduction not only in teleosts, but also in other non-mammalian vertebrates.

Relationships between the transcriptome and physiological indicators of reproduction in female rainbow trout over an annual cycle

Normal transcriptomic patterns along the brain-pituitary-gonad-liver (BPGL) axis should be better characterized if endocrine-disrupting compound-induced changes in gene expression are to be understood. Female rainbow trout were studied over a complete year-long reproductive cycle. Tissue samples from pituitary, ovary, and liver were collected for microarray analysis using the 16K Genomic Research on Atlantic Salmon Project (GRASP) microarray and for quantitative polymerase chain reaction measures of estrogen receptor (ER) isoform messenger RNA (mRNA) levels. Plasma was collected to determine levels of circulating estradiol-17β (E2), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). As an a priori hypothesis, changes in gene expression were correlated to either circulating levels of E2, FSH, and LH, or ER mRNAs quantified by quantitative polymerase chain reaction. In the liver, most transcriptomic patterns correlated to levels of either E2, LH, or ERs. Fewer ovarian transcripts could be correlated to levels of E2, ERα, or FSH. No significant associations were obvious in the pituitary. As a post hoc hypothesis, changes in transcript abundance were compared with microarray features with known roles in gonadal maturation. Many altered transcripts in the ovary correlated to transcript levels of estradiol 17-beta-dehydrogenase 8 or 17 B HSD12, or to glycoprotein alpha chain 1 or 2. In the pituitary, genes involved with the growth axis (e.g., growth hormone, insulin-related growth factor binding protein) correlated with the most transcripts. These results suggest that transcriptional networks along the BPGL axis may be regulated by factors other than circulating steroid hormones.

Zebrafish germline chimeras produced by transplantation of ovarian germ cells into sterile host larvae

High frequency production of zebrafish germline chimeras was achieved by transplanting ovarian germ cells into sterile Danio hybrid recipients. Ovarian germ cells were obtained from 3-mo-old adult Tg(vasa:DsRed2-vasa);Tg(bactin:EGFP) double transgenic zebrafish by discontinuous Percoll gradient centrifugation. An average of 755 ± 108 DsRed-positive germ cells was recovered from each female. For transplantations, a total of approximately 620 ± 242 EGFP-positive cells of which 12 ± 4.7 were DsRed-positive germ cells were introduced into the abdominal cavity under the swim bladder of 2-wk-old sterile hybrid larvae. Six weeks after transplantation, a total of 10 recipients, obtained from 2 different transplantations, were examined, and 2 individuals (20%) were identified that possessed a large number of DsRed- and EGFP-positive cells in the gonadal region. The transplanted ovarian germ cells successfully colonized the gonads and differentiated into sperm in the male hybrid recipients. Of 67 adult recipients, 12 (18%) male chimeric fish reproduced and generated normal offspring when paired with wild-type zebrafish females. The fertilization efficiency ranged from 23% to 56%. Although the fertile male chimeras were generated by transplantation of ovarian germ cells, the F1 generation produced by the male chimeras contained both male and female progeny, indicating that male sex determination in zebrafish is not controlled by sex chromosome heterogamy. Our findings indicate that a population of ovarian germ cells that are present in the ovary of adult zebrafish can function as germline stem cells, able to proliferate and differentiate into testicular germ cells and functional sperm in male recipients. The high frequency of germline chimera formation achieved with the ovarian germ cells and the convenience of identifying the chimeras in the sterile host background should make this transplantation system useful for performing genetic manipulations in zebrafish.

Intraovarian expression of GnRH-1 and gonadotropin mRNA and protein levels in Siberian hamsters during the estrus cycle and photoperiod induced regression/recrudescence

The hypothalamic-pituitary-gonadal (HPG) axis is the key reproductive regulator in vertebrates. While gonadotropin releasing hormone (GnRH), follicle stimulating (FSH), and luteinizing (LH) hormones are primarily produced in the hypothalamus and pituitary, they can be synthesized in the gonads, suggesting an intraovarian GnRH-gonadotropin axis. Because these hormones are critical for follicle maturation and steroidogenesis, we hypothesized that this intraovarian axis may be important in photoperiod-induced ovarian regression/recrudescence in seasonal breeders. Thus, we investigated GnRH-1 and gonadotropin mRNA and protein expression in Siberian hamster ovaries during (1) the estrous cycle; where ovaries from cycling long day hamsters (LD;16L:8D) were collected at proestrus, estrus, diestrus I, and diestrus II and (2) during photoperiod induced regression/recrudescence; where ovaries were collected from hamsters exposed to 14 weeks of LD, short days (SD;8L:16D), or 8 weeks post-transfer to LD after 14 weeks SD (PT). GnRH-1, LHβ, FSHβ, and common α subunit mRNA expression was observed in cycling ovaries. GnRH-1 expression peaked at diestrus I compared to other stages (p < 0.05). FSHβ and LHβ mRNA levels peaked at proestrus and diestrus I (p < 0.05), with no change in the α subunit across the cycle (p > 0.05). SD exposure decreased ovarian mass and plasma estradiol concentrations (p<0.05) and increased GnRH-1, LHβ, FSHβ, and α subunit mRNA expression as compared to LD and, except for LH, compared to PT (p < 0.05). GnRH and gonadotropin protein was also dynamically expressed across the estrous cycle and photoperiod exposure. The presence of cycling intraovarian GnRH-1 and gonadotropin mRNA suggests that these hormones may be locally involved in ovarian maintenance during SD regression and/or could potentially serve to prime ovaries for rapid recrudescence.

Molecular cloning and regulation of mRNA expression of the thyrotropin β and glycoprotein hormone α subunits in red drum, Sciaenops ocellatus

Full-length cDNAs for thyrotropin β (TSHβ) and glycoprotein hormone α (GSUα) subunits were cloned and sequenced from the red drum (Sciaenops ocellatus). The cDNAs for TSHβ (877 bp) and GSUα (661 bp) yielded predicted coding regions of 126 and 94 amino acid proteins, respectively. Both sequences contain all invariant cysteine and putative glycosylated asparagines characteristic of each as deduced by comparison with other GSUα and TSHβ sequences from representative vertebrate species. Multiple protein sequence alignments show that each subunit shares highest identity (79% for the TSHβ and 86% for the GSUα) with perciform fish. Furthermore, in a single joint phylogenetic analysis, each subunit segregates most closely with corresponding GSUα and TSHβ subunit sequences from closely related fish. Tissue-specific expression assays using RT-PCR showed expression of the TSHβ subunit limited to the pituitary. GSUα mRNA was predominantly expressed in the pituitary but was also detected in the testis and ovary of adult animals. Northern hybridization revealed the presence of a single transcript for both TSHβ and GSUα, each close in size to mRNA transcripts from other species. Dot blot assays from total RNA isolated from S. ocellatus pituitaries showed that in vivo T3 administration significantly diminished mRNA expression of both the TSHβ and GSUα subunits and that goitrogen treatment caused a significant induction of TSHβ mRNA only. Both TSHβ and GSUα mRNA expression in the pituitary varied significantly in vivo over a 24-h period. Maximal expression for both TSHβ and GSUα occurred during the early scotophase in relation to a peak in T4 blood levels previously documented. These results suggest the production of TSH in this species which may serve to drive daily cycles of thyroid activity. Readily quantifiable, variable, and thyroid hormone-responsive pituitary TSH expression, coupled with previously described dynamic daily cycles of circulating T4 and extensive background on the growth, nutrition, and laboratory culture of red drum, suggests that this species will serve as a useful model for experimental studies of the physiological regulation of TSH production.

Bisphenol A modulates expression of gonadotropin subunit genes in the hermaphroditic fish, Kryptolebias marmoratus

Pituitary gonadotropins (GTHs), follicle stimulating hormone beta (FSH-beta), and luteinizing hormone beta (LH-beta) are the key hormones in the hypothalamus-pituitary-gonad (HPG) axis, and form the heterodimers between a common alpha subunit (gonadotropin-alpha) and FSH-beta and/or LH-beta. To obtain a better understanding on the modulation of gonadotropin subunit genes expression upon bisphenol A (BPA) exposure in hermaphroditic fish, we studied differential regulation of gonadotropin subunit genes from Kryptolebias marmoratus after the exposure of several EDCs. Expression profiles of these three genes when using quantitative real-time RT-PCR revealed that brain/pituitary tissues were highly expressed in these genes compared to other tissues. At different developmental stages, expression of those genes dramatically increased over the course of development but showed a decrease in expression at the secondary male (showing atresia) stage. When adult fish were exposed to BPA (600 microg/L for 96 h), a significant upregulation of these three genes was observed in the brain/pituitary. A time course study also revealed the increased expression of gonadotropin subunit genes over 12 h with a more pronounced effect on the expression of FSH-beta and LH-beta genes, indicating that both genes were associated with the BPA exposure on the transcriptional regulation. This is the first report of gonadotropin subunit genes from K. marmoratus, with particular emphasis on the modulation of their expressions by EDCs. In addition, these findings suggest that EDCs modulate the expression of gonadotropin subunit genes and would act as potential biomarkers upon EDCs exposure.

Molecular cloning and localization of the luteinizing hormone beta subunit and glycoprotein hormone alpha subunit from Japanese anchovy Engraulis japonicus

Although Clupeiformes contain many economically important species, there is limited information on their reproductive physiology. To obtain more insight into reproductive mechanisms in clupeiform fishes, molecular cloning of the Japanese anchovy Engraulis japonicus luteinizing hormone beta (LHbeta) and glycoprotein hormone alpha (GPHalpha) subunits, and immunocytochemistry of gonadotrophs in the pituitary using antisera raised against the synthetic peptides for both subunits were carried out. The cDNAs for LHbeta and GPHalpha subunits consisted of 963 and 535 nucleotides encoding 141 and 122 amino acids, respectively. The deduced amino acid sequences of the E. japonicus LHbeta subunit showed a 60% similarity to the Pacific herring Clupea pallasii LHbeta subunit and 24-31% similarities to FSHbeta subunits of other fish species. The E. japonicus GPHalpha subunit showed 52-57% similarities to anguilliform and cypriniform GPHalpha subunits. Both the subunits have typical structural characteristics of each subunit such as N-linked glycosylation sites, conserved cysteine residues and highly conserved short amino acid sequences. These results indicate that cDNAs cloned in this study encode the E. japonicus LHbeta and GPHalpha subunits. Reverse-transcription polymerase chain reaction (RT-PCR) revealed that both the LHbeta and GPHalpha subunit genes were abundantly expressed in the pituitary, and the GPHalpha subunit was observed to be weakly expressed in the extrapituitary tissues. Immunocytochemistry of the E. japonicus pituitary showed that cells that immunoreacted with antiserum against the LHbeta subunit were distributed in the peripheral regions of proximal pars distalis, and these cells were also immunoreactive to antiserum against the GPHalpha subunit. An abundant number of both LHbeta and GPHalpha cells in the pituitary of matured fish were observed, in comparison with immature fish. These results indicate that the E. japonicus LH is involved in the final reproductive maturation as well as those of other teleosts.

Expression of recombinant zebrafish follicle-stimulating hormone (FSH) in methylotropic yeast Pichia pastoris

Pituitary gonadotropin follicle-stimulating hormone (FSH) was identified in fish two decades ago, but its functional importance in fish reproduction remains poorly defined, especially in non-salmonid species. This gap in our knowledge is partially due to the lack of the hormone in pure form in most of the species studied. We describe here the production of two different forms of biologically active recombinant zebrafish FSH (zfFSH and zfFSH(HIS)) using methylotrophic yeast, Pichia pastoris, as the bioreactor. One form (zfFSH) was produced as the molecule closer to the native form, with the two subunits (Cga and Fshb) expressed separately under different promoters. The other form (zfFSH(HIS)) was produced as a single polypeptide, with the cDNAs for the two subunits joined to form a fusion gene that contained a 6X His tag as part of the linker between the two subunits. The culture conditions were optimized for pH and incubation time for maximal production of the proteins. Using a zebrafish FSH receptor (Fshr)-based reporter gene assay, we tested and compared the biological activities of the two forms of recombinant zebrafish FSH. Our results provide useful information for the future production of recombinant gonadotropins in other fish species.

Studies on feminization, sex determination, and differentiation of the Southern catfish, Silurus meridionalis--a review

The sex ratio of the feral Southern catfish was reported to be about 1:1, while the fish obtained by artificial fertilization were always female. Hence, we examined the possible influence of the micro-environment during artificial insemination (pH of the ovarian fluid and concentration of the semen) and early development (feed, hatching temperature, and water) on the sex ratio of Southern catfish fry. In order to examine the possibility of the occurrence of gynogenesis during artificial propagation, cytological observations on the insemination processes and the artificial induction of gynogenesis were also performed. However, no male fish were obtained even in these experiments, excluding the possibilities of these micro-environmental changes on catfish sex ratio and the occurrence of gynogenesis during artificial propagation. Female-to-male sex reversal was achieved by treatment with fadrozole (an aromatase inhibitor) and tamoxifen (an estrogen receptor antagonist). Histological analyses on the gonadal development of both female and induced male fish were subsequently performed. Moreover, several genes involved in sex differentiation, such as dmrt1, foxl2, and cyp19, and three subunits of gonadotropin (gth), i.e., gthalpha, lhbeta, and fshbeta, were isolated. Their expression patterns were studied under normal gonadal development and sex reversal conditions. The results revealed that dmrt1, foxl2, and cyp19a were closely related to catfish sex differentiation, and the gth subunits were possibly related to ovarian differentiation and oocyte development. Taken together, we hypothesized that estrogen was highly responsible for the ovarian differentiation and feminization of catfish fry under artificial propagation, although the mechanism involved remains elusive.

Regulation, expression and characterization of aromatase (cyp19b1) transcripts in ovary and testis of rainbow trout (Oncorhynchus mykiss)

Cytochrome P450 aromatase is the key enzyme in the pathway that converts androgens to estrogens. The enzyme functions in the smooth endoplasmic reticulum in a complex with NADPH-cytochrome P450 reductase. In teleost fish, at least two separate loci, cyp19a and cyp19b, encode distinct aromatase isoforms. The activity of cyp19a and cyp19b are predominantly associated with the ovary and brain, respectively, although their expression is not confined solely to these tissues. We found that at least five cyp19b1 transcripts with different 5'-UTRs are generated in the ovary and testis of rainbow trout. Regulation for selection of these variants may be through signals present in exon 2 that recruit alternative splicing factors. Also, binding elements for FOXL2 and SF-1 located within the cyp19b1 intron 1 may influence formation of transcripts that contain the 3'-end of the intron. Another transcript devoid of the exon 2 methionine initiator codon may utilize other downstream in-frame start codons. Less developed stages of ovarian and testicular tissues express only the intron-containing transcripts whereas precocious and more mature gonads express all five cyp19b1 messages. The function of these different 5'-UTRs may be for regulation of cyp19b1 at particular developmental stages or to specify control in distinct gonadal cell-types.

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.

Oogenesis in teleosts: how eggs are formed

One of the major objectives of the aquaculture industry is the production of a large number of viable eggs with high survival. Major achievements have been made in recent years in improving protocols for higher efficiency of egg production and viability of progeny. Main gaps remain, however, in understanding the dynamic processes associated with oogenesis, the formation of an egg, from the time that germ cells turn into oogonia, until the release of ova during spawning in teleosts. Recent studies on primordial germ-cells, yolk protein precursors and their processing within the developing oocyte, the deposition of vitamins in eggs, structure and function of egg envelopes and oocyte maturation processes, further reveal the complexity of oogenesis. Moreover, numerous circulating endocrine and locally-acting paracrine and autocrine factors regulate the various stages of oocyte development and maturation. Though it is clear that the major regulators during vitellogenesis and oocyte maturation are the pituitary gonadotropins (LH and FSH) and sex steroids, the picture emerging from recent studies is of complex hormonal cross-talk at all stages between the developing oocyte and its surrounding follicle layers to ensure coordination of the various processes that are involved in the production of a fertilizable egg. In this review we aim at highlighting recent advances on teleost fish oocyte differentiation, maturation and ovulation, including those involved in the degeneration and reabsorption of ovarian follicles (atresia). The role of blood-borne and local ovarian factors in the regulation of the key steps of development reveal new aspects associated with egg formation.

Molecular characterization of marbled eel (Anguilla marmorata) gonadotropin subunits and their mRNA expression profiles during artificially induced gonadal development

Three cDNA sequences encoding the gonadotropin subunits, common glycoprotein alpha subunit (GTHalpha), FSHbeta and LHbeta subunits were isolated from marbled eel. The cDNA of GTHalpha encodes 116 amino acids with a signal peptide of 24 amino acids and a mature peptide of 92 amino acids. The FSHbeta subunit consists of 127 amino acids with a 22 amino acid signal peptide and a 105 amino acid mature peptide, while the LHbeta subunit consists of 140 amino acids with a 24 amino acid signal peptide and a 116 amino acid mature peptide. Comparison of the deduced amino acid sequences of marbled eel GTHalpha, FSHbeta, and LHbeta with that of other fishes shows a high degree of conservation in the number of cysteine residues and potential N-linked glycosylation sites. The mRNA of GTHalpha, FSHbeta and LHbeta were not only detected in pituitary, but also in ovary and testes by RT-PCR. Quantitative realtime PCR analysis revealed that the GTHalpha and LHbeta transcriptional levels in pituitaries of female and male eels gradually increased during the artificially inducing gonadal development, and peaked at late vitellogenic stage and spermiation stage, respectively. FSHbeta mRNA in the pituitaries of female eels maintained a high level at previtellogenic stage, early vitellogenic stage as well as mid-vitellogenic stage but declined sharply at late vitellogenic stage and migratory nucleus stage. In male eels, the mRNA levels of FSHbeta in the pituitaries were higher at early spermatogenesis stage than at both late spermatogenesis stage and spermiation stage. These results suggested that FSH would be in control of initiation and maintenance of gonadal growth and gametogenesis, whereas LH would be involved in the final gonadal maturation and spermiation/ovulation in the tropic eel Anguilla marmorata.

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.

Thyrotropin in teleost fish

Thyrotropin (TSH), a pituitary glycoprotein hormone that stimulates the thyroid gland, has been cloned and sequenced from over a dozen teleost fish species. Although TSH is established as a primary driver of systemic thyroid status in mammals, its importance in the regulation of fish thyroid function is still uncertain. We review recent studies indicating that TSH structure is highly conserved across species representing six teleost families. These studies have found TSH messenger RNA consistently expressed in teleost pituitary tissue, although ectopic expression, particularly in gonads, has also been observed. They have also provided evidence for negative feedback inhibition of TSH expression by thyroid hormones, as well as stimulation by hypothalamic peptides. Descriptive studies have found increased TSHbeta expression associated with life history events thought to be promoted by thyroid hormones. These results, coupled with the discovery of a G-protein coupled TSH receptor in several teleost species, supports an active and conserved role for TSH in the regulation of teleost thyroid function. The relative importance of central pathways in regulating thyroid hormone provision to targets and the identity of a proposed thyrotropin-inhibiting factor in teleost fish are still unanswered questions whose resolution will be facilitated by development of methods to measure circulating TSH and its secretion from the pituitary gland.

Molecular identification and functional characterization of the kisspeptin/kisspeptin receptor system in lower vertebrates

The KISS1 gene encodes the kisspeptin neuropeptide, which activates the KISS1 receptor (KISS1R; G protein-coupled receptor 54; GPR54) and participates in neuroendocrine regulation of GnRH secretion. To study the physiological function(s) and evolutionary conservation of KISS1, we cloned opossum, Xenopus, and zebrafish kiss1 cDNAs. Processing zebrafish, Xenopus, or opossum KISS proteins would liberate a carboxy-terminal amidated peptide with 52, 54, or 53 amino acid residues, respectively. Phylogenetic analysis of all known vertebrate KISS1 peptides showed clear clustering of the sequences according to canonical vertebrate classes. The zebrafish kiss1 gene consists of two exons and one intron. Real-time PCR analysis of two kiss1R cloned from zebrafish brain found expression of kiss1, kiss1ra, and kiss1rb, with kiss1ra-more similar to other piscine Kiss1 receptors-highly expressed in the gonads and kiss1rb in other nonbrain tissues. In females kiss1 mRNA levels gradually increased during the first few weeks of life to peak in fish with ovaries containing mature oocytes, while in males kiss1 mRNA levels peaked after 6 wk postfertilization when the testes exhibited initial stages of spermatogenesis and decreased after puberty. Zebrafish kiss1ra and kiss1rb were expressed differentially with similar patterns in both genders. These results indicate that the Kiss1/Kiss1r system may participate in puberty initiation in fish as well. Like human KISS1R, Kiss1ra transduces its activity via the PKC pathway, whereas Kiss1rb does so via both PKC and PKA pathways. The human KISS1R was highly activated by both huKISS10amide and zfKISS10amide, whereas both zebrafish Kiss1 receptor types were less sensitive to amidation.

The gonadotropin receptors FSH-R and LH-R of Atlantic halibut (Hippoglossus hippoglossus)--2. Differential follicle expression and asynchronous oogenesis

The biological activity and spatio-temporal expression patterns of the gonadotropin receptors FSH-R and LH-R were examined in the repetitive spawner Atlantic halibut to elucidate the gonadotropic regulation of the asynchronous follicle development. The cloned receptors were expressed in mammalian COS-7 cells, and stimulation with sea bass FSH and LH increased the cAMP production. The halibut FSH-R and LH-R genes were shown to be highly expressed in the gonads of sexually mature fish, but the transcripts were also found in extra-gonadal tissues such as pituitary and brain. Different expression patterns of FSH-R and LH-R in the developing follicles were documented by semi-quantitative RT-PCR. Abundant FSH-R mRNA was found in the small follicles during primary growth and vitellogenesis, and the signals were localized to the granulosa cells by in situ hybridization. In contrast, follicular LH-R mRNA was hardly detectable during the early stages. Conversely, in follicles during final maturation FSH-R mRNA levels tended to decrease, while the expression of LH-R was highly upregulated. Whereas the pituitary FSH and LH are asynchronously expressed in annual spawners, both gonadotropins were expressed in the female halibut pituitary throughout the reproductive cycle, except in the prespawning females. Hence, the sequential gonadotropic activation of ovarian follicle growth and maturation in repetitive spawners is probably regulated by modulating the temporal expression of FSH-R and LH-R in the follicle membrane.