Molecular characterization and expression of FSHbeta, LHbeta, and common alpha-subunit in male Atlantic halibut (Hippoglossus hippoglossus)

Molecular characterization of fshb and lhb subunits and their expression profiles in captive white-edged rockfish, Sebastes taczanowskii

Fundamental knowledge on the regulation of reproduction by gonadotropins (Gths) is quite limited in viviparous fishes. In the present study, we performed molecular cloning and characterization of cDNAs encoding two Gth subunits (fshb and lhb) from the pituitaries of viviparous white-edged rockfish, Sebastes taczanowskii; expression profiles of both gene transcripts were elucidated in the pituitaries of reproductive males and females which were kept in a captive environment. The cloned fshb and lhb fragments exhibited high sequence identities with corresponding β-subunit sequences from black rockfish, S. schlegelii. Notably, the fshb of white-edged rockfish appeared to lack a putative N-glycosylation site, whereas lhb conserved it. Expression of fshb and lhb transcripts in the rockfish pituitaries largely changed in synchrony but for minor exceptions. In males, levels of both transcripts increased with progression of spermatogenesis, although the peak for fshb (October) appeared slightly earlier than that for lhb (November). In females, both gene transcripts exhibited synchronous bimodal changes. High expression of fshb and lhb transcripts in the female pituitary during the gestation period, followed by the drastic decrease at parturition, suggest their possible involvement in regulation of gestation of this species. The knowledge gained for Sebastes in this study superimposes fundamental information necessary for further physiological understanding of viviparity in teleost fish.

Hypothalamic- and pituitary-derived growth and reproductive hormones and the control of energy balance in fish

Most endocrine systems in the body are influenced by the hypothalamic-pituitary axis. Within this axis, the hypothalamus delivers precise signals to the pituitary gland, which in turn releases hormones that directly affect target tissues including the liver, thyroid gland, adrenal glands and gonads. This action modulates the release of additional hormones from the sites of action, regulating key physiological processes, including growth, metabolism, stress and reproduction. Pituitary hormones are released by five distinct hormone-producing cell types: somatotropes (which produce growth hormone), thyrotropes (thyrotropin), corticotropes (adrenocorticotropin), lactotropes (prolactin) and gonadotropes (follicle stimulating hormone and luteinizing hormone), each modulated by specific hypothalamic signals. This careful and distinct organization of the hypothalamo-pituitary axis has been classically associated with the existence of many lineal axes (e.g., the hypothalamic-pituitary-gonadal axis) in charge of the control of the different physiological processes. While this traditional concept is valid, it is becoming apparent that hormones produced by the hypothalamo-pituitary axis have diverse effects. For instance, gonadotropin-releasing hormone II has been associated with a suppressive effect on food intake in fish. Likewise, growth hormone has been shown to influence appetite, swimming activity and aggressive behavior in fish. This review will focus on the hypothalamic and pituitary hormones classically involved in regulating growth and reproduction, and will attempt to provide a general overview of the current knowledge on their actions on energy balance and appetite in fish. It will also give a brief perspective of the role of some of these peptides in integrating feeding, metabolism, growth and reproduction.

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.

Molecular cloning and characterization of pirarucu (Arapaima gigas) follicle-stimulating hormone and luteinizing hormone β-subunit cDNAs

The common gonadotrophic hormone α-subunit (GTHα) has been previously isolated by our research group from A. gigas pituitaries; in the present work the cDNA sequences encoding FSHβ and LHβ subunits have also been isolated from the same species of fish. The FSH β-subunit consists of 126 amino acids with a putative 18 amino acid signal peptide and a 108 amino acid mature peptide, while the LH β-subunit consists of 141 amino acids with a putative 24 amino acid amino acid signal peptide and a 117 amino acid mature peptide. The highest identity, based on the amino acid sequences, was found with the order of Anguilliformes (61%) for FSHβ and of Cypriniformes (76%) for LHβ, followed by Siluriformes, 53% for FSHβ and 75% for LHβ. Interestingly, the identity with the corresponding human amino acid sequences was still remarkable: 45.1% for FSHβ and 51.4% for LHβ. Three dimensional models of ag-FSH and ag-LH, generated by using the crystal structures of h-FSH and h-LH as the respective templates and carried out via comparative modeling and molecular dynamics simulations, suggested the presence of the so-called "seat-belt", favored by a disulfide bond formed between the 3rd and 12th cysteine in both β-subunits. The sequences found will be used for the biotechnological synthesis of A. gigas gonadotrophic hormones (ag-FSH and ag-LH). In a first approach, to ascertain that the cloned transcripts allow the expression of the heterodimeric hormones, ag-FSH has been synthesized in human embryonic kidney 293 (HEK293) cells, preliminarily purified and characterized.

Molecular characterization, modeling, in silico analysis of equine pituitary gonadotropin alpha subunit and docking interaction studies with ganirelix

Equine pituitary gonadotropins (eLH, eFSH, eCG) are heterodimeric glycoprotein hormones with alpha (α) and beta (β) subunits. It is responsible for maintenance of pregnancy in mares during early gestation and fairly valuable for inducing superovulation in animals other than equines. The alpha subunit is common, while beta subunit is species-specific in all glycoprotein hormones. In the present investigation, molecular cloning and in silico characterization including homology modeling and molecular docking analysis of the equine chorionic gonadotropin (eCG) alpha subunit was carried out for gaining structural and functional insights into the eCG alpha subunit and its possible interaction with ganirelix, a gonadotropin-releasing hormone (GnRH) antagonist. The equine chorionic gonadotropin (eCG) alpha subunit expressed in pituitary gland was selected, amplified from total RNA, cloned and sequenced. The in silico analyses were made for homology modelling, structural details, epitope identification and chromosomal localization. Molecular docking studies of eCG alpha were undertaken with a drug ganirelix which is used to control ovulation and has antagonistic activity against GnRH. The protein sequence corresponding to selected open reading frame (ORF) was 99-100% similar with domesticated horse, Przewalski's horse, and 92-93% with Burchell's zebra and donkey. Molecular docking studies revealed the possible interaction of eCG alpha with ganirelix. The possible drug-macromolecule interactions were visualized between eCG alpha and ganirelix. The study will provide structural insight into unique sites and an alternate route of gonadotropin suppression applicable to assisted reproductive technologies.

Gonadotropin characterization, localization and expression in the European hake (Merluccius merluccius)

In vertebrates, the regulation of gametogenesis is under the control of gonadotropins (Gth), follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh). In fish, the physiological role of Gths is not fully understood, especially in species with asynchronous ovarian development. To elucidate the role of Gths in species with asynchronous ovary, we studied European hake (Merluccius merluccius) during the reproductive season. For this aim, we first cloned and sequenced both hormones. Then, we characterized their amino acid sequence and performed phylogenetic analyses to verify the relationship to their orthologues in other species. In addition, the quantification of gene expression during their natural reproductive season was analyzed in wild-caught female hake. Our results revealed that fshb peaked during the vitellogenic phase, remaining high until spawning. This is in contrast to the situation in species with synchronous ovary. lhb, on the other hand, peaked during maturation as it is also common in species with synchronous ovarian development. Finally, combining double-labeling fluorescent in situ hybridization (FISH) for Gth mRNAs with immunofluorescence for Lh protein, we evidenced the specific expression of fshb and lhb in different cells within the proximal pars distalis (PPD) of the pituitary. In addition to gonadotrope cells specific to expression of either fshb or lhb, some cells showed co-expression of both genes. This suggests either that gonadotropes with co-expression are not yet specified or they could have a plasticity that permits changes from one cell phenotype to another during certain life stages and in turn during different physiological states.

Identified lhb-expressing cells from medaka (Oryzias latipes) show similar Ca(2+)-response to all endogenous Gnrh forms, and reveal expression of a novel fourth Gnrh receptor

We have previously characterized the response to gonadotropin-releasing hormone (Gnrh) 2 in luteinizing hormone (lhb)-expressing cells from green fluorescent protein (Gfp)-transgenic medaka (Oryzias latipes), with regard to changes in the cytosolic Ca(2+) concentration. In the current study we present the corresponding responses to Gnrh1 and Gnrh3. Ca(2+) imaging revealed three response patterns to Gnrh1 and Gnrh3, one monophasic and two types of biphasic patterns. There were few significant differences in the shape of the response patterns between the three Gnrh forms, although the amplitude of the Ca(2+) signal was considerably lower for Gnrh1 and Gnrh3 than for Gnrh2, and the distribution between the two different biphasic patterns differed. The different putative Ca(2+) sources were examined by depleting intracellular Ca(2+) stores with thapsigargin, or preventing influx of extracellular Ca(2+) by either extracellular Ca(2+) depletion or the L-type Ca(2+)-channel blocker verapamil. Both Gnrh1 and 3 relied on Ca(2+) from both intracellular and extracellular sources, with some unexpected differences in the relative contribution. Furthermore, gene expression of Gnrh-receptors (gnrhr) in whole pituitaries was studied during development from juvenile to adult. Only two of the four identified medaka receptors were expressed in the pituitary, gnrhr1b and gnrhr2a, with the newly discovered gnrhr2a showing the highest expression level at all stages as analyzed by quantitative PCR. While both receptors differed in expression level according to developmental stage, only the expression of gnrhr2a showed a clear-cut increase with gonadal maturation. RNA sequencing analysis of FACS-sorted Gfp-positive lhb-cells revealed that both gnrhr1b and gnrhr2a were expressed in lhb-expressing cells, and confirmed the higher expression of gnrhr2a compared to gnrhr1b. These results show that although lhb-expressing gonadotropes in medaka show similar Ca(2+) response patterns to all three endogenous Gnrh forms through the activation of two different receptors, gnrhr1b and gnrhr2a, the differences observed between the Gnrh forms indicate activation of different Ca(2+) signaling pathways.

Influence of tidal cycles on the endocrine control of reproductive activity in common snook (Centropomus undecimalis)

The aim of our study was to confirm the role of tidal pattern on the coordination of oocyte maturation and spawning in common snook Centropomus undecimalis. To do so, we studied oocyte maturation during the spawning season in relation to the tidal pattern in both males and females by means of histology and hormonal profiling along the pituitary-gonadal axis. Plasma LH levels, as well as transcript levels of gonadotropin genes (fshβ and lhβ) from the pituitaries of sexually mature male and female common snook were analyzed using a heterologous ELISA and quantitative RT-PCR, respectively. The fshβ and lhβ cDNAs were isolated and phylogenetic analysis of the deduced amino acid sequences revealed strong identity with other teleosts (75-90%). A strong link was found between tide and follicular development irrespective of the time of the day: female snook sampled on the rising tide were all found to have oocytes in the Secondary Growth Stage whereas females sampled at high tide or on the falling tide had oocytes in the later stages of maturation and ovulation. In addition, LH plasma and mRNA levels of fshβ and lhβ increased during the later stages of vitellogenesis peaking at ovulation in females. Plasma estradiol and testosterone significantly increased in late vitellogenesis (Secondary Growth Stage) and oocyte maturation (Eccentric Germinal Vesicle Step) respectively. Among male common snook sampled, no correlation was identified between tide and gonadal development. In addition, lhβ mRNA expression in males peaked at the mid germinal epithelium stage as for testosterone and 11-KT in the blood while fshβ expression and plasma LH levels peaked at late germinal epithelium stage. This study confirms the role played by tidal cycle on the entrainment of the later stages of oogenesis of common snook and provides a better understanding of the link between environmental and endocrine control of reproduction in this species.

Seasonal changes and endocrine regulation of pejerrey (Odontesthes bonariensis) spermatogenesis in the wild

The endocrine mechanisms that regulate spermatogenesis and their interaction with environmental cues have been poorly studied compared with oogenesis in fish. The aim of this work was to study the spermatogenesis in pejerrey under the influence of photoperiod and water temperature fluctuation in the wild, evaluating the transcript levels of brain Gnrh variants and cyp19a1b, pituitary Gth subunits, gonadal Gth receptors, 11β-hsd, and 11-KT plasma levels. Males at spermiogenic stage were observed during spring and autumn, under a photoperiod above 11h of light and a water temperature below 23 °C. Most arrested males were observed in summer when water temperatures increased above 23 °C. Males at spermatogonial stage were mainly observed in autumn, while most males at spermatocytary stage were caught in winter. An increase of gnrh-I, cyp19a1b, fshb, gpha and 11β-hsd transcripts and 11-KT plasma levels was observed during spermatogonial and/or spermatocytary stage (early spermatogenesis). The spermiogenic stage was associated to the maximum gnrh-I gene expression level and a significant increase of Gth receptors transcripts, being this fact more evident for lhcgr. During this last gonadal stage, cyp19a1b transcript level remained high, while fshb mRNA and 11-KT plasma levels showed a significant decreased compared to that occurred at the spermatocytary stage. Also, gphα and 11β-hsd gene expression levels fell during spermiation up to similar values to those observed in arrested males. A significant correlation between 11-KT and gnrh-I, cyp19a1b, gphα, fshb, 11β-hsd transcripts, and the number of spermatocytes was observed during spermatogenesis. All these findings suggested that in pejerrey, the spermatocyte proliferation occurs mainly during winter under the stimulation of 11-KT induced by FSH through the stimulation of specific enzymes, including the 11β-hsd while spermiation occurs after photoperiod increase and with temperatures of the water below 23 °C, through the stimulation of gnrh-I, cyp19a1b and lhcgr.

Development of a flatfish-specific enzyme-linked immunosorbent assay for Fsh using a recombinant chimeric gonadotropin

In flatfishes with asynchronous and semicystic spermatogenesis, such as the Senegalese sole (Solea senegalensis), the specific roles of the pituitary gonadotropins during germ cell development, particularly of the follicle-stimulating hormone (Fsh), are still largely unknown in part due to the lack of homologous immunoassays for this hormone. In this study, an enzyme-linked immunosorbent assay (ELISA) for Senegalese sole Fsh was developed by generating a rabbit antiserum against a recombinant chimeric single-chain Fsh molecule (rFsh-C) produced by the yeast Pichia pastoris. The rFsh-C N- and C-termini were formed by the mature sole Fsh β subunit (Fshβ) and the chicken glycoprotein hormone common α subunit (CGA), respectively. Depletion of the antiserum to remove anti-CGA antibodies further enriched the sole Fshβ-specific antibodies, which were used to develop the ELISA using the rFsh-C for the standard curve. The sensitivity of the assay was 10 and 50 pg/ml for Fsh measurement in plasma and pituitary, respectively, and the cross-reactivity with a homologous recombinant single-chain luteinizing hormone was 1%. The standard curve for rFsh-C paralleled those of serially diluted plasma and pituitary extracts of other flatfishes, such as the Atlantic halibut, common sole and turbot. In Senegalese sole males, the highest plasma Fsh levels were found during early spermatogenesis but declined during enhanced spermiation, as found in teleosts with cystic spermatogenesis. In pubertal males, however, the circulating Fsh levels were as high as in adult spermiating fish, but interestingly the Fsh receptor in the developing testis containing only spermatogonia was expressed in Leydig cells but not in the primordial Sertoli cells. These results indicate that a recombinant chimeric Fsh can be used to generate specific antibodies against the Fshβ subunit and to develop a highly sensitive ELISA for Fsh measurements in diverse flatfishes.

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.

Intra-pituitary relationship of follicle stimulating hormone and luteinizing hormone during pubertal development in Atlantic bluefin tuna (Thunnus thynnus)

As part of the endeavor aiming at the domestication of Atlantic bluefin tuna (BFT; Thunnus thynnus), first sexual maturity in captivity was studied by documenting its occurrence and by characterizing the key hormones of the reproductive axis: follicle stimulating hormone (FSH) and luteinizing hormone (LH). The full length sequence encoding for the related hormone β-subunits, bftFSHβ and bftLHβ, were determined, revealing two bftFSHβ mRNA variants, differing in their 5' untranslated region. A quantitative immuno-dot-blot assay to measure pituitary FSH content in BFT was developed and validated enabling, for the first time in this species, data sets for both LH and FSH to be compared. The expression and accumulation patterns of LH in the pituitary showed a steady increase of this hormone, concomitant with fish age, reaching higher levels in adult females compared to males of the same age class. Conversely, the pituitary FSH levels were elevated only in 2Y and adult fish. The pituitary FSH to LH ratio was consistently higher (>1) in immature than in maturing or pubertal fish, resembling the situation in mammals. Nevertheless, the results suggest that a rise in the LH storage level above a minimum threshold may be an indicator of the onset of puberty in BFT females. The higher pituitary LH levels in adult females over males may further support this notion. In contrast three year-old (3Y) males were pubertal while cognate females were still immature. However, it is not yet clear whether the advanced puberty in the 3Y males was a general feature typifying wild BFT populations or was induced by the culture conditions. Future studies testing the effects of captivity and hormonal treatments on precocious maturity may allow for improved handling of this species in a controlled environment which would lead to more cost-efficient farming.

Molecular characterization and quantification of sablefish (Anoplopoma fimbria) gonadotropins and their receptors: reproductive dysfunction in female captive broodstock

Efforts to establish an aquaculture industry for sablefish (Anoplopoma fimbria) are constrained by reproductive dysfunction in wild-caught fish and by lack of reproduction of F1 females. Toward a better understanding of the reproductive dysfunction of captive broodstock, full-length cDNAs encoding the sablefish gonadotropin subunits (fshb, lhb and cga) and their receptors (fshr and lhcgr) were cloned, sequenced and quantitative real-time PCR assays developed. Sablefish gonadotropin subunits display some unique features, such as two additional Cys residues in the N-terminal region of Fshb and a lack of potential N-glycosylation sites in Fshb and Lhb, whereas Fshr and Lhcgr possess conserved structural characteristics described in other vertebrates. Wild females captured in fall completed gametogenesis in captivity the next spawning season, whereas females captured three months earlier, during summer, failed to mature. Interestingly, these wild non-maturing females exhibited similar reproductive features as prepubertal F1 females, including low levels of pituitary gonadotropin and ovarian receptor mRNAs and plasma sex steroids, and ovarian follicles arrested at the perinucleolus stage. In conclusion, this study described the cloning, molecular characterization and development of qPCRs for sablefish gonadotropins and their receptors. Rearing conditions may impair vitellogenic growth of ovarian follicles in sablefish, compromising the reproductive success of broodstock.

The pituitary gland of the European eel reveals massive expression of genes involved in the melanocortin system

Hormones secreted from the pituitary gland regulate important processes such as development, growth and metabolism, reproduction, water balance, and body pigmentation. Synthesis and secretion of pituitary hormones are regulated by different factors from the hypothalamus, but also through feedback mechanisms from peripheral organs, and from the pituitary itself. In the European eel extensive attention has been directed towards understanding the different components of the brain-pituitary-gonad axis, but little is known about the regulation of upstream processes in the pituitary gland. In order to gain a broader mechanistic understanding of the eel pituitary gland, we have performed RNA-seq transcriptome profiling of the pituitary of prepubertal female silver eels. RNA-seq reads generated on the Illumina platform were mapped to the recently assembled European eel genome. The most abundant transcript in the eel pituitary codes for pro-opiomelanocortin, the precursor for hormones of the melanocortin system. Several genes putatively involved in downstream processing of pro-opiomelanocortin were manually annotated, and were found to be highly expressed, both by RNA-seq and by qPCR. The melanocortin system, which affects skin color, energy homeostasis and in other teleosts interacts with the reproductive system, has so far received limited attention in eels. However, since up to one third of the silver eel pituitary's mRNA pool encodes pro-opiomelanocortin, our results indicate that control of the melanocortin system is a major function of the eel pituitary.

Signal transduction involved in GnRH2-stimulation of identified LH-producing gonadotropes from lhb-GFP transgenic medaka (Oryzias latipes)

We have characterized the response to gonadotropin-releasing hormone 2 (GnRH2) in luteinizing hormone producing cells from gfp-transgenic medaka. Teleosts have separate cells producing the two types of gonadotropins, enabling us for the first time to study the intracellular signaling that controls secretion of each gonadotropin separately. Pituitary cell cultures were prepared, and lhb-producing cells were selected by their GFP expression. Cytosolic Ca(2+) imaging revealed three response patterns to GnRH2, one monophasic and two types of biphasic patterns. The Ca(2+) sources were examined by depleting intracellular Ca(2+) stores and preventing influx of extracellular Ca(2+). Both treatments reduced response amplitude, and affected latency and time to peak. Blocking L-type Ca(2+) channels reduced amplitude and time to peak, but did not remove extracellular Ca(2+) contribution. Patch-clamp recordings showed spontaneous action potentials in several cells, and GnRH2 increased the firing frequency. Presence of Ca(2+)-activated K(+) channels was revealed, BK channels being the most prominent.

The importance of the olfactory rosettes in maintaining pituitary prolactin and prolactin-releasing peptide levels during hyposmotic acclimation in silver sea bream (Sparus sarba)

A potential role of the olfactory rosettes in maintaining prolactin (PRL) and prolactin-releasing peptide (PrRP) levels was examined in the euryhaline silver sea bream (Sparus sarba). The olfactory rosettes were surgically removed in silver sea bream adapted to hypo- (6 ppt) and hyper-osmotic (33 ppt) salinities and the mRNA expression of the two previously identified freshwater-adapting factors, prolactin (PRL) and prolactin-releasing peptide (PrRP), in silver sea bream was measured. The elevation of pituitary PRL and PrRP mRNA expression levels as seen in 6 ppt-adapted fish was abolished by surgical removal of the olfactory rosettes. The PRL and PrRP expression levels in fish adapted to 6 ppt were significantly lowered following olfactory rosette removal. On the other hand, hypothalamic PrRP mRNA expression in 6 ppt-adapted fish did not change. Specific signals for Na(+)-K(+)-ATPase but not CFTR mRNA expression were detected in the surface layers of olfactory epithelial cells by in situ hybridization. The mRNA abundance of CFTR and Na(+)-K(+)-ATPase α and β subunits remained unchanged in the olfactory rosette of silver sea bream adapted to 0, 6, 12, 33 and 50 ppt for 4 weeks and in fish abruptly transferred from 33 ppt to 6 ppt. Data obtained from the olfactory rosette removal experiments suggest a possible role of the olfactory system for maintaining PRL and PrRP expression during hyposmotic acclimation in sea bream.

Pituitary gonadotropin and testicular gonadotropin receptor expression in Atlantic cod (Gadusmorhua L.) during the first reproductive season: Effects of photoperiod modulation

Pituitary mRNA levels of gonadotropin β-subunits and of their cognate receptors in the testis were studied during puberty in Atlantic cod under normal and experimental photoperiod conditions that suppressed, delayed or accelerated testis maturation. Results are discussed in context with changes in testicular histology and plasma androgen levels, considered as end points of gonadotropic regulation. Up-regulation of fshb was closely associated with the onset of puberty, decreased when spermatogenesis was completed and reached minimum levels after spawning. These results demonstrate, for the first time using an experimental approach, that activation of Fsh-dependent signaling is associated with spermatogonial proliferation and formation of spermatogenic cysts. Changes in fshr expression were less prominent and could be explained by changes in the cellular composition and RNA content of cod testis tissue. At more advanced stages of development (spermiogenesis, spermiation and spawning), lhb and, one month later, lhcgr transcript levels increased and reached peak values in spawning fish, in a positive feedback loop involving plasma androgens and Lh/Lhcgr-dependent signaling. This loop was broken by a loss of lhb expression at the end of the spawning season. Continuous light (LL) from summer solstice, ~8 months prior to spawning, suppressed the start of testis maturation and the changes in gonadotropin and receptor mRNA levels, while LL from winter solstice initially up-regulated lhb and lhcgr expression, before resulting in a precocious termination of the spawning season and low expression of all four genes. Our studies provide experimental evidence for a clear functional discrimination of cod gonadotropins.

Single-cell qPCR on dispersed primary pituitary cells -an optimized protocol

Background

The incidence of false positives is a potential problem in single-cell PCR experiments. This paper describes an optimized protocol for single-cell qPCR measurements in primary pituitary cell cultures following patch-clamp recordings. Two different cell harvesting methods were assessed using both the GH₄ prolactin producing cell line from rat, and primary cell culture from fish pituitaries.

Results

Harvesting whole cells followed by cell lysis and qPCR performed satisfactory on the GH₄ cell line. However, harvesting of whole cells from primary pituitary cultures regularly produced false positives, probably due to RNA leakage from cells ruptured during the dispersion of the pituitary cells. To reduce RNA contamination affecting the results, we optimized the conditions by harvesting only the cytosol through a patch pipette, subsequent to electrophysiological experiments. Two important factors proved crucial for reliable harvesting. First, silanizing the patch pipette glass prevented foreign extracellular RNA from attaching to charged residues on the glass surface. Second, substituting the commonly used perforating antibiotic amphotericin B with β-escin allowed efficient cytosol harvest without loosing the giga seal. Importantly, the two harvesting protocols revealed no difference in RNA isolation efficiency.

Conclusion

Depending on the cell type and preparation, validation of the harvesting technique is extremely important as contaminations may give false positives. Here we present an optimized protocol allowing secure harvesting of RNA from single cells in primary pituitary cell culture following perforated whole cell patch clamp experiments.

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.

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.

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.

Photoperiod-modulated testis maturation in Atlantic cod (Gadus morhua, L.)

Precocious male puberty is a significant problem in Atlantic cod aquaculture. While photoperiod manipulation can inhibit testis growth, a detailed analysis of effects on spermatogenesis is missing. Starting July 1, 2004, prepubertal fish were exposed to different photoperiod regimens in indoor tanks for 17 mo. Testis histology, germ cell dynamics (proliferation and apoptosis), and plasma androgen levels were analyzed. In the natural light (NL) group, testis growth started in September 2004 and was completed in February 2005, when a 2-mo spawning period started. In the constant light (LL) group, none or very few spermatogenic cysts were recruited into spermatogenesis, and apoptotic germ cell loss was high. A change of photoperiod from NL to LL at winter solstice (December 21, 2004) resulted in premature (2 mo) completion of the reproductive cycle, while changing from LL to NL at winter solstice triggered faster than normal testis development. Plasma testosterone levels increased in the NL group from spermatogonial proliferation toward meiosis, while those of 11-ketotestosterone increased toward spermiogenesis and spermiation. Plasma androgen levels did not rise under LL conditions. Comparing fish with developing testes from all groups indicated that low androgen levels were associated with a high incidence of spermatogonial apoptosis; we also found that androgen receptor mRNA expression was most prominent in Sertoli cells in contact with growing spermatogonial clones. Our data show that an inhibitory photoperiod (LL) reduced or blocked differentiation of spermatogonia, increased apoptosis (particularly among proliferating spermatogonia), and was associated with reduced androgen levels, a situation possibly reflecting insufficient gonadotropic stimulation.

Functional characterization and expression analysis of the androgen receptor in zebrafish (Danio rerio) testis

The biological activity of androgens, important for male sexual differentiation and development, is mediated by the androgen receptor (AR) that binds to specific DNA recognition sites regulating the transcription of androgen target genes. We investigated androgen production by adult zebrafish testis tissue, and identified 11β-hydroxyandrostenedione, 11-ketoandrostenedione (OA), and 11-ketotestosterone (11-KT) as main products, and hence potential ligands, for the zebrafish Ar. These androgens were then included in the pharmacological characterization of the zebrafish Ar. The zebrafish Ar responded well in terms of binding and transactivation to synthetic androgens as well as to testosterone and 11-KT, and reasonably well to OA and androstenedione. In situ hybridization analysis of zebrafish testis revealed that ar mRNA expression was detected in the subpopulation of Sertoli cells contacting early spermatogonia.

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.

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.

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.

Molecular cloning of Senegalese sole (Solea senegalensis) follicle-stimulating hormone and luteinizing hormone subunits and expression pattern during spermatogenesis

Pituitary gonadotropins (GTHs), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are key regulators of vertebrate reproduction. However, in teleosts with testis of semi-cystic type and asynchronous spermatogenesis, as the flatfish Senegalese sole (Solea senegalensis), the physiological roles of FSH and LH are still not well understood. To gain insight into this mechanism, full-length complementary DNAs (cDNAs) encoding Senegalese sole FSH beta and LH beta subunits, and the common glycoprotein alpha subunit (CG alpha), were cloned and sequenced. The three cDNAs consisted of 550, 582 and 744 nucleotides encoding peptides of 120, 148 and 132 amino acids, respectively. Comparison of the deduced amino acid sequences of sole FSH beta, LH beta and CG alpha with those from other teleosts indicated that cysteine residues and potential N-linked glycosylation sites were fully conserved with respect to other percomorphs and salmonids. However, the primary structure of FSH beta and LH beta in pleuronectiforms appeared to be highly divergent. In situ hybridization of mature male pituitaries showed that fshb, lhb and cga mRNAs were localized in the proximal pars distalis and in the periphery of pars intermedia. Real-time quantitative reverse transcription-polymerase chain reaction indicated that the levels of all three transcripts in the pituitary of males increased during winter and spring, at the time when plasma levels of androgens raised and testicular germ cell development and spermatozoa production were stimulated. These results suggest that FSH and LH may regulate spermatogenesis in Senegalese sole similarly to that described for other teleosts with testis of cystic type and synchronous germ cell development.

Molecular cloning and histological localization of LH-like substances in a bottlenose dolphin (Tursiops truncatus) placenta

All mammals exhibit pituitary-specific expression of LH and FSH, whereas placental expression of gonadotropins has been reported only in primates and equids. Some cetaceans, such as dolphins, have a long gestational period and a sexual cycle of about 27 days almost comparable with that of humans. Histologically, dolphins have an epitheliochorial placentae that resembles placentas of Perissodactyla including horses. In the present study, we cloned cDNAs encoding gonadotropins and observed their immunohistochemical localization in the placenta of bottlenose dolphin. The cDNAs obtained encoded 120 amino acids for the alpha-subunit (including 96 amino acids of mature proteins), and 141 amino acids for the beta-subunit (including 121 amino acids of mature proteins). The sequence of the alpha-subunit was similar to that in the pig (Artiodactyla) pituitary glycoprotein hormone [96.7% homology at amino acids (aa) level], and the sequence of the beta-subunit was similar to that of luteinizing hormone (LH) in the pig [94.3% homology at aa level] and white rhinoceros (Perissodactyla) [93.3% homology at aa level]. Of interest, dolphin LHbeta lacks carboxyl-terminal-peptides (CTP). This fact suggests that CTP are not essential for placental expression of gonadotropin in dolphins. Immunohistochemical observations employing anti-ovine LHbeta antibody revealed positive staining in the villositycal tissue. Our observations suggest placental expression of gonadotropin homologues in cetaceans and possible evolutionary conservation of placentae-derived hormonal control of ovarian functions during pregnancy.

Electrophysiological properties of pituitary cells in primary culture from Atlantic cod (Gadus morhua)

The aim of the present study was to explore the electrophysiological properties of pituitary cells from Atlantic cod (Gadus morhua), as a basis for future studies of the signaling pathways involved in the control of pituitary secretion in this species. Primary cultures of pituitary cells from maturing Atlantic cod were prepared by trypsin treatment and mechanical dispersion. Electrophysiological recordings were performed using the perforated patch clamp method. A subpopulation of large cells were selected for recordings. Spontaneous action potentials were observed in about 30% of the cells. The action potentials displayed a fast initial spike followed by a prolonged plateau. Correspondingly, the inward current elicited by depolarizing steps consisted of both a transient, tetrodotoxin-sensitive Na(+) component and a nifedipine-sensitive Ca(2+) component that was sustained when Ba(2+) replaced Ca(2+) as current carrier. The outward current was partially blocked both by 5 mM tetraethylammonium and 10 mM 4-aminopyridine. The voltage-activated ion channels present in these cells largely correspond to the ion channels of pituitary cells in other teleosts (goldfish, Carassius auratus, and tilapia, Oreochromis mossambicus) and mammals, although differences exist regarding the shape and duration of action potentials.

Isolation of Atlantic halibut pituitary hormones by continuous-elution electrophoresis followed by fingerprint identification, and assessment of growth hormone content during larval development

Continuous-elution electrophoresis (CEE) has been applied to separate putative hormones from adult Atlantic halibut pituitaries. Soluble proteins were separated by size and charge on Model 491 Prep Cell (Bio-Rad), where the homogenate runs through a cylindrical gel, and protein fractions are collected as they elute from the matrix. Protein fractions were assessed by SDS-PAGE and found to contain purified proteins of molecular size from 10 to 33 kDa. Fractions containing proteins with molecular weights of approximately 21, 24, 28 and 32 kDa, were identified as putative growth hormone (GH), prolactin, somatolactin and gonadotropins, respectively. These were analyzed further by mass spectrometry and identified with peptide mass protein fingerprinting. The CEE technique was used successfully for purification of halibut GH with a 5% yield, and appears generally well suited to purify species-specific proteins often needed for research in comparative endocrinology, including immunoassay work. Thus, the GH obtained was subsequently used as standards and iodination label in a homologous radioimmunoassay, applied to analyze GH content through larval development in normally and abnormally metamorphosing larvae. As GH is mainly found in the pituitary, GH contents were analyzed in tissue extracts from the heads only. The pituitary GH content increases proportionally to increased larval weight from first feeding to metamorphic climax. No difference in relative GH content was found between normal and abnormal larvae and it still remains to be established if GH has a direct role in metamorphosis.

A systematic immunohistochemical survey of the distribution patterns of GH, prolactin, somatolactin, beta-TSH, beta-FSH, beta-LH, ACTH, and alpha-MSH in the adenohypophysis of Oreochromis niloticus, the Nile tilapia

Fish pituitary plays a central role in the control of growth, development, reproduction and adaptation to the environment. Several types of hormone-secreting adenohypophyseal cells have been characterised and localised in diverse teleost species. The results suggest a similar distribution pattern among the species investigated. However, most studies deal with a single hormone or hormone family. Thus, we studied adjacent sections of the pituitary of Oreochromis niloticus, the tilapia, by conventional staining and immunohistochemistry with specific antisera directed against growth hormone (GH), prolactin (PRL), somatolactin (SL), thyrotropin (beta-TSH), follicle-stimulating hormone (beta-FSH), luteinising hormone (beta-LH), adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (alpha-MSH). The pituitary was characterised by a close interdigitating neighbourhood of neurohypophysis (PN) and adenohypophysis. PRL-immunoreactive and ACTH-immunoreactive cells were detected in the rostral pars distalis. GH-immunoreactive cells were present in the proximal pars distalis (PPD). A small region of the PPD contained beta-TSH-immunoreactive cells, and beta-LH-immunoreactive cells covered approximately the remaining parts. Centrally, beta-FSH-immunoreactive cells were detected in the vicinity of the GH-containing cells. Some of these cells also displayed beta-LH immunoreactivity. The pars intermedia was characterised by branches of the PN surrounded by SL-containing and alpha-MSH-immunoreactive cells. The ACTH and alpha-MSH antisera were observed to cross-react with the respective antigens. This cross-reactivity was abolished by pre-absorption. We present a complete map of the distinct localisation sites for the classical pituitary hormones, thereby providing a solid basis for future research on teleost pituitary.

Molecular cloning of cDNAs and structural model analysis of two gonadotropin beta-subunits of snakehead fish (Channa maculata)

The cDNAs encoding beta-subunits of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) have been cloned from the pituitary of snakehead fish, Channa maculata, and the three-dimensional structural models of the encoded FSH and LH were investigated. The cloned cDNAs, including 5'-untranslated region (UTR), open-reading frame, and 3'-UTR followed by a poly(A) tail, were obtained by reverse transcription-polymerase chain reaction and rapid amplification of cDNA end methods. The open-reading frames of FSH-beta cDNA encodes a 120-amino acid protein with a signal peptide of 18 amino acids and a mature protein of 102 amino acids; while LH-beta cDNA encodes a 140-amino acid protein with a signal peptide of 33 amino acids and a mature protein of 115 amino acids. The amino acid sequence identities of snakehead fish FSH-beta and LH-beta in comparison with other fish are 27.8-81.9% and 45.2-88.8%, respectively; while in comparison with tetrapods are 26.2-28.9% and 37.5-51.2%, respectively. Both FSH-beta and LH-beta of snakehead fish resemble most to those of Perciformes, implying their closer phylogenetic relationship. All 12 cysteine residues are conserved in snakehead fish LH-beta; while 11 cysteine residues are conserved in its FSH-beta. The third cysteine is absent in snakehead fish FSH-beta; instead, a positionally shifted cysteine residue is present at the N-terminus, as found in some phylogenetic related fish. The structure models of snakehead fish FSH and LH, constructed by using the crystal structures of human FSH and human chorionic gonadotropin as respective template, showed that the positionally shifted N-terminal cysteine residue of snakehead fish FSH-beta likely can substitute the third cysteine to form a disulfide bond with the 12th cysteine.

Purification of follicle-stimulating hormone from immature Japanese eel, Anguilla japonica, and its biochemical properties and steroidogenic activities

Follicle-stimulating hormone (FSH) was purified, for the first time, from immature Japanese eel, Anguilla japonica, and its biochemical properties were investigated. FSH was extracted from immature eel pituitaries and purified by gel-filtration on Sephadex G-100, and two step anion-exchange chromatography: stepwise elution on DE-52, followed by gradient elution on TSK-gel Super-Q using HPLC. Purification was performed using its molecular mass and the positive reaction with anti-Japanese eel (je) FSHbeta antiserum. Purified eel FSH was detected as a single band after separation by SDS-PAGE under a non-reducing condition, showing positive reaction with both anti-je glycoprotein (GP) alpha and anti-jeFSHbeta antisera. The molecular mass of purified eel FSH was estimated to be approximately 33 kDa. After separation by SDS-PAGE under reducing condition, the intact molecule was detected as distinct proteins, whose N-terminus amino acid sequences coincided with those predicted from cDNA sequences for jeGPalpha and jeFSHbeta mature peptides. Deglycosylation of these subunits led to a decrease in their molecular mass. These results suggest that eel FSH is a heterodimeric molecule which consists of distinct glycoprotein subunits, GPalpha and FSHbeta. Cells reacting with anti-jeFSHbeta antiserum were observed in the proximal pars distalis of an immature eel pituitary, while jeLHbeta-immunoreactive cells were not detected. Gonadotropic activities of eel FSH were demonstrated in vitro by stimulating testosterone and 11-ketotestosterone secretions in immature eel testes. Purified eel FSH stimulated the secretion of both androgens from the immature eel testis in a dose-dependent manner, similar to immature eel pituitary homogenate and recombinant eel FSH produced by yeast. These results show that endogenous and recombinant FSH in this species possess similar activities, presumably stimulating the gametogenesis through the sex steroid secretion during the early stages of gonadal development.

Molecular characterization of rockfish (Sebastes schlegeli) gonadotropin subunits and their mRNA expression profiles during oogenesis

In an attempt to understand the reproductive regulation in viviparous teleosts, gonadotropin (GTH) subunit cDNAs were characterized and the expression levels of GTH subunit mRNAs in the pituitary glands of the rockfish, Sebastes schlegeli (Scorpaeniformes, Scorpaenidae), were examined by Northern blot analysis. The complete sequences of rockfish GTH subunits (GTHalpha, FSHbeta, and LHbeta) were determined by rapid amplification of cDNA ends (RACE) and nucleotide sequencing. Based on the RACE analysis, the cDNAs of GTHalpha, FSHbeta, and LHbeta consisted of 655, 540, and 529 nucleotides encoding peptides of 132, 130, and 143 amino acids, respectively. The mature peptides of rockfish FSHbeta, LHbeta, and common GTHalpha showed high sequence identities (FSHbeta, 58-62%; LHbeta, 86-94%; and GTHalpha, 87-88%) to those of other perciforme (e.g., orange spotted grouper, red seabream, stripped bass, and sea bass). In a sequence alignment of the mature peptides, rockfish FSHbeta exhibited a unique feature, the lack of a conserved N-glycosylation site. This is in contrast to both LHbeta and GTHalpha which contain one and two N-glycosylation sites, respectively, and is consistent with those of other teleosts. The mRNA levels of the GTHalpha subunit increased at the vitellogenic stage and remained steady from ovulation to post-parturition. FSHbeta mRNA levels increased abruptly during the vitellogenic stages and then decreased during ovulation, embryonic development, and post-parturition. LHbeta mRNA levels were observed to rapidly increase during the vitellogenic stage, reached its highest levels during ovulation and was then followed by a decrease at post-parturition. These results suggest that in the female rockfish FSH and LH syntheses are predominant during vitellogenesis and oocyte maturation, respectively.

The brain–pituitary–gonad axis in male teleosts, with special emphasis on flatfish (Pleuronectiformes)

The key component regulating vertebrate puberty and sexual maturation is the endocrine system primarily effectuated along the brain-pituitary-gonad (BPG) axis. By far most investigations on the teleost BPG axis have been performed on salmonids, carps, catfish and eels. Accordingly, earlier reviews on the BPG axis in teleosts have focused on these species, and mainly on females (e.g. 'Fish Physiology, vol. IXA. Reproduction (1983) pp. 97'; 'Proceedings of the Fourth International Symposium on the Reproductive Physiology of Fish. FishSymp91, Sheffield, UK, 1991, pp. 2'; 'Curr. Top. Dev. Biol. 30 (1995) pp. 103'; 'Rev. Fish Biol. Fish. 7 (1997) pp. 173'; 'Proceedings of the Sixth International Symposium on the Reproductive Physiology of Fish. John Grieg A/S, Bergen, Norway, 2000, pp. 211'). However, in recent years new data have emerged on the BPG axis in flatfish, especially at the level of the brain and pituitary. The evolutionarily advanced flatfishes are important model species both from an evolutionary point of view and also because many are candidates for aquaculture. The scope of this paper is to review the present status on the male teleost BPG axis, with an emphasis on flatfish. In doing so, we will first discuss the present understanding of the individual constituents of the axis in the best studied teleost models, and thereafter discuss available data on flatfish. Of the three constituents of the BPG axis, we will focus especially on the pituitary and gonadotropins. In addition to reviewing recent information on flatfish, we present some entirely new information on the phylogeny and molecular structure of teleost gonadotropins.

Isolation and characterization of FSH and LH from pituitary glands of Atlantic halibut (Hippoglossus hippoglossus L.)

Two gonadotropins (GtH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), were isolated and characterized from pituitary glands of Atlantic halibut (Hippoglossus hippoglossus L.). Glycoproteins were extracted in 40% ethanol followed by precipitation in 85% ethanol. Subsequently, glycoproteins were fractionated by ion-exchange chromatography on a Whatman DE-52 column using a stepwise gradient of ammonium bicarbonate (50-1000 mM). Intact FSH and LH were finally purified on rpHPLC using an AsahiPak C4P-50 column with an acetonitrile gradient (10-60%). SDS-PAGE showed a molecular mass of 33 and 32 kDa for intact FSH and LH, respectively. Final purification of subunits was performed by a subsequent purification step on rpHPLC using a Phenomenex Jupiter C18 column with an acetonitrile gradient (10-60%). FSHbeta, LHbeta, and the common alpha subunit showed molecular masses of 25, 24, and 19 kDa, respectively. Subunit identity was confirmed by N-terminal amino acid sequencing. Intact FSH and LH showed gonadotropic activity by stimulating release of 11-ketotestosterone from turbot (Scophthalmus maximus L.) testicular tissue in vitro. This provides the first purification of two distinct GtHs from an evolutionary advanced pleuronectiform teleost.

Identification and localization of eight distinct hormone-producing cell types in the pituitary of male Atlantic halibut (Hippoglossus hippoglossus L.)

The eight distinct hormone-producing cell types in the adenohypophysis of male Atlantic halibut (Hippoglossus hippoglossus L.) were identified and localized using immunohistochemistry and in situ hybridization. Lactotropes either occupied most of the rostral pars distalis (RPD) or they were arranged in follicular structures located along the periphery of the RPD. Corticotropes were confined to a thin layer of RPD cells bordering the pars nervosa (PN). The somatotropes were arranged in multicellular layers bordering the highly convoluted PN penetrating the proximal pars distalis (PPD), while thyrotropes, scattered in small islets in between the somatotropes, were located in the centro-dorsal part of the PPD. Gonadotropes were found throughout the PPD. Immunoreactivity to glycoprotein-alpha and luteinizing hormone beta-subunit was also observed along the periphery of the pars intermedia (PI), indicating that a thin extension of the PPD surrounded the PI. In situ hybridization showed that follicle-stimulating hormone and luteinizing hormone were produced in distinct cells of the PPD. PI contained somatolactotropes bordering the highly convoluted PN, and melanotropes that showed positive immunostaining against both anti-alpha-melanocyte-stimulating hormone and anti-beta-endorphin. The general cellular organization was similar to that of other teleost fish. These results lay the basis for future investigations on Atlantic halibut pituitary physiology.