Isolation and characterization of two distinct gonadotropins from the pituitary gland of Mediterranean yellowtail, Seriola dumerilii (Risso, 1810)

Effects of different hormonal treatments on spermatogenesis advancement in hatchery-produced greater amberjack Seriola dumerili (Risso 1810)

In earlier studies, wild-caught greater amberjack Seriola dumerili (Risso, 1810) males reared in sea cages showed gametogenesis impairment and low sperm production and quality. Here, we (a) examined if F1 hatchery-produced males reared in sea cages also exhibit reproductive dysfunctions and (b) evaluated the effects of gonadotropin releasing hormone agonist (GnRHa) administration through injections (GnRHainj) or sustained-release implants (GnRHaimpl), and human chorionic gonadotropin (hGC) injections on spermatogenesis/spermiation enhancement. Fish were given a hormone treatment just prior to the spawning season, and were transferred to land-based tanks, according to an established spawning induction protocol. Blood samples (n = 6) were obtained on Days 0, 7 and 13 after treatment. Testis samples were obtained on Days 0 (n = 4) and 13 (n = 2 per treatment). The fish prior to their transfer from the sea cages to the land-based tanks, exhibited a low gonadosomatic index, altered sex steroid hormone profile and high density of testicular apoptotic cells. After transfer to tanks, there was a general depression of sex steroid plasma levels parallel to an increase in cortisol concentrations. Despite the negative effect on steroidogenesis by the transfer from the sea, the hormonal treatments increased the number of fish from where sperm could be obtained, as well as testis growth, and reduced testicular apoptosis. Treatment with hCG resulted in the most significant changes in spermatogenesis, while GnRHaimpl appeared to induce less intense, but likely longer-lasting effects. The study indicated that F1 hatchery-produced males also exhibited reproductive dysfunctions as wild-caught captive-reared greater amberjack, and that the observed positive effects of the hormone treatments on spermiation/spermatogenesis were likely mediated by factors other than sex steroid hormones.

Pituitary multi-hormone cells in mammals and fish: history, origin, and roles

The vertebrate pituitary is a dynamic organ, capable of adapting its hormone secretion to different physiological demands. In this context, endocrinologists have debated for the past 40 years if endocrine cells are mono- or multi-hormonal. Since its establishment, the dominant "one cell, one hormone" model has been continuously challenged. In mammals, the use of advanced multi-staining approaches, sensitive gene expression techniques, and the analysis of tumor tissues have helped to quickly demonstrate the existence of pituitary multi-hormone cells. In fishes however, only recent advances in imaging and transcriptomics have enabled the identification of such cells. In this review, we first describe the history of the discovery of cells producing multiple hormones in mammals and fishes. We discuss the technical limitations that have led to uncertainties and debates. Then, we present the current knowledge and hypotheses regarding their origin and biological role, which provides a comprehensive review of pituitary plasticity.

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.

Development of specific enzyme-linked immunosorbent assay for yellowtail kingfish (Seriola lalandi) follicle stimulating hormone using recombinant gonadotropins

We developed a specific competitive enzyme-linked immunosorbent assay (ELISA) for yellowtail kingfish (Seriola lalandi) follicle stimulating hormone (FSH). We previously produced a full-length single chain recombinant yellowtail kingfish FSH using the Pichia pastoris expression system. We used the same method to produce the β subunit of the hormone, against which polyclonal antibodies were raised in rabbits. We first confirmed immunoreactivity of the polyclonal antibodies with the recombinant full length FSH and FSHβ as well as plasma and pituitary FSH of sexually immature and mature yellowtail kingfish by Western blot analysis. We then developed a precise and reproducible ELISA for yellowtail kingfish FSH and validated the assay in plasma and pituitary extracts. The intra- and inter-assay coefficients of variation was <2.2% and 10.2%, respectively. The sensitivity of the assay was 78 pg/ml. For further validation of the assay, we measured the plasma FSH in immature yellowtail kingfish treated with increasing doses (blank, 50, 100 and 150 µg/kg) of kisseptin2-10 peptide from a previous study. The dose response observed in treated females was not significant, however the increased plasma FSH levels coincided with the significantly higher estradiol levels we previously reported in the treated groups. We assessed the applicability of the assay in measuring circulating FSH in other species. We observed parallelism between the linearized FSH standard curve and displacement curves of serially diluted plasma from Atlantic bluefin tuna (Thunnus thynnus) and tilapia (Oreochromis niloticus). We also observed similar parallelism with full length recombinant giant grouper (Epinephelus lanceolatus) FSH. The ELISA we developed for yellowtail kingfish FSH will be useful in understanding the reproductive biology of the species as well as enhancing its aquaculture.

Comparative Study of Reproductive Development in Wild and Captive-Reared Greater Amberjack Seriola dumerili (Risso, 1810)

The greater amberjack Seriola dumerili is a large teleost fish with rapid growth and excellent flesh quality, whose domestication represents an ambitious challenge for aquaculture. The occurrence of reproductive dysfunctions in greater amberjack reared in captivity was investigated by comparing reproductive development of wild and captive-reared individuals. Wild and captive-reared breeders were sampled in the Mediterranean Sea during three different phases of the reproductive cycle: early gametogenesis (EARLY, late April-early May), advanced gametogenesis (ADVANCED, late May-early June) and spawning (SPAWNING, late June-July). Fish reproductive state was evaluated using the gonado-somatic index (GSI), histological analysis of the gonads and determination of sex steroid levels in the plasma, and correlated with leptin expression in the liver and gonad biochemical composition. The GSI and sex steroid levels were lower in captive-reared than in wild fish. During the ADVANCED period, when the wild greater amberjack breeders were already in spawning condition, ovaries of captive-reared breeders showed extensive atresia of late vitellogenic oocytes and spermatogenic activity ceased in the testes of half of the examined males. During the SPAWNING period, all captive-reared fish had regressed gonads, while wild breeders still displayed reproductive activity. Liver leptin expression and gonad proximate composition of wild and captive greater amberjack were similar. However, the gonads of captive-reared fish showed different total polar lipid contents, as well as specific lipid classes and fatty acid profiles with respect to wild individuals. This study underlines the need for an improvement in rearing technology for this species, which should include minimum handling during the reproductive season and the formulation of a specific diet to overcome the observed gonadal decrements of phospholipids, DHA (22:6n-3) and ARA (20:4n-6), compared to wild breeders.

Greater amberjack Fsh, Lh, and their receptors: Plasma and mRNA profiles during ovarian development

To understand the endocrine regulation of ovarian development in a multiple spawning fish, the relationship between gonadotropins (Gths; follicle-stimulating hormone [Fsh] and luteinizing hormone [Lh]) and their receptors (Gthrs; Fshr and Lhr) were investigated in greater amberjack (Seriola dumerili). cDNAs encoding the Gth subunits (Fshβ, Lhβ, and glycoprotein α [Gpα]) and Gthrs were cloned. The in vitro reporter gene assay using recombinant hormones revealed that greater amberjack Fshr and Lhr responded strongly to their own ligands. Competitive enzyme-linked immunosorbent assays (ELISAs) were developed for measuring greater amberjack Fsh and Lh. Anti-Fsh and anti-Lh antibodies were raised against recombinant chimeric single-chain Gths consisting of greater amberjack Fshβ (or Lhβ) with rabbit GPα. The validation study showed that the ELISAs were precise (intra- and inter-assay coefficient of variation, <10%) and sensitive (detection limit of 0.2ng/ml for Fsh and 0.8ng/ml for Lh) with low cross-reactivity. A good parallelism between the standard curve and serial dilutions of greater amberjack plasma and pituitary extract were obtained. In female greater amberjack, pituitary fshb, ovarian fshr, and plasma E2 gradually increased during ovarian development, and plasma Fsh significantly increased during the post-spawning period. This suggests that Fsh plays a role throughout ovarian development and during the post-spawning period. Pituitary lhb, ovarian lhr, and plasma Lh were high during the spawning period, suggesting that the synthesis and secretion of Lh, and Lhr expression are upregulated to induce final oocyte maturation and ovulation.

Catfish gonadotrophins: cellular origin, structural properties and physiology

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

Expression and localization of gonadotropic hormone subunits (Gpa, Fshb, and Lhb) in the pituitary during gonadal differentiation in medaka

To clarify the appearance of and chronological changes in two different gonadotropic hormone (Gth) cells, we examined the dynamics of Gth cells in detail during gonadal differentiation and development in the d-rR strain of medaka (Oryzias latipes). Expression of the sex-determining gene Dmy was evident in gonadal somatic cells at 5 days post-fertilization (dpf). Glycoprotein-α (Gpa)-positive cells first appeared in the pituitary at 4 dpf, regardless of genetic sex, while follicle-stimulating hormone-β (Fshb)-positive cells was detected in XX and XY embryos at 5 and 6 dpf, respectively. In contrast, luteinizing hormone-β (Lhb)-positive cells were observed in both sexes of medaka after 70 days post-hatching (dph). The density of Fshb-positive cells in the pituitary was significantly and transiently higher in XX than in XY fry at 0 dph, and thereafter no significant differences were detected before sexual maturation. In this study, temporal expression of Fshb was observed, indicating that Fsh cells become differentiated before hatching and that sexual dimorphism in Fsh cells occurs transiently after sex determination in medaka.

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.

Development of non-competitive enzyme-linked immunosorbent assays for mummichog Fundulus heteroclitus gonadotropins - examining seasonal variations in plasma FSH and LH levels in both sexes

The mummichog Fundulus heteroclitus is an excellent experimental fish for reproductive physiology because of its adequate size, easiness for rearing, and controllable reproduction under laboratory conditions. Furthermore, it is the only species that the native GtHs and their subunits have been purified among small experimental fishes. In this study, homologous non-competitive enzyme-linked immunosorbent assays (ELISAs) for the mummichog FSH and LH were developed by raising monoclonal and polyclonal antibodies against the purified GtHs or their subunits, and the plasma hormone levels in various seasons were examined. The cross-reactivity of LH in the FSH ELISA and the cross-reactivity of FSH in the LH ELISA were low, 2.3% and 0.2% respectively, indicating high specificities of both GtH assays. The practical detection limits were 10 pg/well (0.125 ng/ml plasma) for the FSH ELISA and 8 pg/well (0.1 ng/ml plasma) for the LH ELISA. Plasma FSH levels in females indicated distinct correlations with ovarian stages: they were almost undetectable (<0.125 ng/ml) during the post-spawning immature phase (September), low values (0.3 ng/ml) during the cortical alveoli accumulation phase (December), considerably high (1.8 ng/ml) in the vitellogenic phase (February), and very high values (12 ng/ml) during the spawning season (June). The male FSH levels showed similar pattern of changes to that of females, also indicating distinct correlations with testicular activities. Plasma LH levels were considerably high during the spawning period in both sexes (3.3 ng/ml in females and 4.5 ng/ml in males). They were low or undetectable values in non-spawning seasons, and clear correlation with the gonadal stages was not observed. These results indicate the importance of FSH for various reproductive events in multiple spawning fishes, and are consistent with the general understanding that the LH is responsible for final gametes maturation in both sexes. Nonetheless, they further suggest that the role of LH for various reproductive events other than the final maturation may be limited.

Steroidogenic and maturation-inducing potency of native gonadotropic hormones in female chub mackerel, Scomber japonicus

BACKGROUND

The gonadotropins (GtHs), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are produced in the pituitary gland and regulates gametogenesis through production of gonadal steroids. However, respective roles of two GtHs in the teleosts are still incompletely characterized due to technical difficulties in the purification of native GtHs.

METHODS

Native FSH and LH were purified from the pituitaries of adult chub mackerel, Scomber japonicus by anion-exchange chromatography and immunoblotting using specific antisera. The steroidogenic potency of the intact chub mackerel FSH (cmFSH) and LH (cmLH) were evaluated in mid- and late-vitellogenic stage follicles by measuring the level of gonadal steroids, estradiol-17beta (Ε2) and 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P). In addition, we evaluated the maturation-inducing potency of the GtHs on same stage follicles.

RESULTS

Both cmFSH and cmLH significantly stimulated E2 production in mid-vitellogenic stage follicles. In contrast, only LH significantly stimulated the production of 17,20beta-P in late-vitellogenic stage follicles. Similarly, cmLH induced final oocyte maturation (FOM) in late-vitellogenic stage follicles.

CONCLUSIONS

Present results indicate that both FSH and LH may regulate vitellogenic processes, whereas only LH initiates FOM in chub mackerel.

Molecular characterization of Chinese sturgeon gonadotropins and cellular distribution in pituitaries of mature and immature individuals

Chinese sturgeon (Acipenser sinensis) is a rare and endangered species, and also an important resource for the sturgeon aquaculture industry. To understand molecular characterization of Chinese sturgeon gonadotropins (GTHs), we cloned the full-length cDNAs of gonadotropin subunits common alpha (GTH-alpha), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from a pituitary cDNA library of mature female. Two subtypes of GTH-alpha were identified. The nucleotide sequences of A. sinensis common alpha I (AsGTH-alpha I), common alpha II (AsGTH-alpha II), FSHbeta (AsFSHbeta) and LHbeta (AsLHbeta) subunit cDNAs are 345, 363, 387 and 414bp in length, and encode mature peptides of 115, 121, 129 and 138aa, respectively. Then, three polyclonal antibodies were prepared from the in vitro expressed AsGTH-alpha I, AsFSHbeta and AsLHbeta mature proteins, respectively. Significant expression differences were revealed between immature and mature sturgeon pituitaries. Western blot detection and immunofluoresence localization revealed the existence of three-gonadotropin subunits (AsGTH-alpha, AsFSHbeta and AsLHbeta) in mature sturgeon pituitaries, but only AsFSHbeta was detected in immature individual pituitaries during early stages in the sturgeon life, and obvious difference was observed between males and females. In males, AsFSHbeta was expressed in 4-year-old individuals, whereas in females, AsFSHbeta was just expressed in 5-year-old individuals.

Purification and characterization of follicle-stimulating hormone from pituitary glands of sea bass (Dicentrarchus labrax)

Follicle-stimulating hormone (FSH) was purified from pituitaries of sea bass (Dicentrarchus labrax), and its biochemical and biological properties were studied. Sea bass FSH (sbsFSH) was purified by ethanol extraction-precipitation (40-85%), followed by anion-exchange chromatography on a LKB Ultropac TSK-DEAE column using a linear gradient of ammonium bicarbonate (50-1000 mM) and reverse phase chromatography on a RESOURCE 15RPC column with a linear gradient of acetonitrile (0-50%), using a FPLC system. The molecular mass of the purified sbsFSH, estimated by mass spectrometry, was of 28.5 kDa for the dimer, 12.6 kDa for the glycoprotein alpha (GPalpha) and 13.6 kDa for FSHbeta subunits. After separation by SDS-PAGE under reducing condition, the intact sbsFSH was dissociated in the respective subunits (GPalpha and FSHbeta). Subunit identity was confirmed by immunological detection and N-terminal amino acid sequencing. Deglycosylation treatment with N-glycosidase F, decreased the molecular mass of both subunits. Intact sbsFSH activated the sea bass FSH receptor stably expressed in the cell line HEK 293, in a dose dependent manner. Purified sbsFSH showed gonadotropic activity, by stimulating the release of estradiol-17beta (E2) from sea bass ovary and testosterone (T) and 11-ketotestosterone (11KT) from testicular tissue cultured in vitro, in a dose and time dependent manner. These results showed that the purified sbsFSH is a heterodimeric hormone, composed of two distinct glycoprotein subunits (GPalpha and FSHbeta), and has biological activity judged by its ability to stimulate its receptor in a specific manner and to promote steroid release from gonadal tissue fragments.

Development of specific enzyme-linked immunosorbent assay for determining LH and FSH levels in tilapia, using recombinant gonadotropins

We recently produced Oreochromis niloticus recombinant LH and FSH as single-chain polypeptides in the methylotrophic yeast Pichia pastoris. Glycoprotein subunit alpha was joined with tilapia (t) LHbeta or tFSHbeta mature protein-coding sequences to form a fusion gene that encodes a ;;tethered" polypeptide, in which the gonadotropin beta-subunit forms the N-terminal part and the alpha-subunit forms the C-terminal part. Recombinant (r) gonadotropins were used to develop specific and homologous competitive ELISAs for measurements of FSH and LH in the plasma and pituitary of tilapia, using primary antibodies against rtLHbeta or rtFSHbeta, respectively, and rtLHbetaalpha or rtFSHbetaalpha for the standard curves. The wells were coated with either rtLHbeta (2ng/ml) or rtFSHbeta (0.5ng/well), and the final concentrations of the antisera were 1:5000 (for tLH) or 1:50,000 (for tFSH). The sensitivity of the assay was 15.84pg/ml for tLH and 0.24pg/ml for tFSH measurements in the plasma, whereas for the measurements in the pituitary, the sensitivity was 2.43ng/ml and 1.52ng/ml for tLH and tFSH, respectively. The standard curves for tFSH and tLH paralleled those of serially diluted pituitary extracts of other cichlids, as well as of serially diluted pituitary extract of seabream, European seabass and hybrid bass. We examined plasma tFSH and tLH levels in the course of one reproductive cycle, between two successive spawnings, in three individual tilapia females. Plasma levels of both FSH and LH increased during the second day after the eggs had been removed, probably related to the vitellogenic phase. LH levels increased toward spawning, which occurred on the 11th day. FSH levels also increased on day of cycle, probably due to recruitment of a new generation of follicles for the successive spawning. The development of specific ELISAs using recombinant gonadotropins is expected to advance the study of the distinct functions of each of these important hormones.

Tilapia Follicle-Stimulating Hormone (FSH): Immunochemistry, Stimulation by Gonadotropin-Releasing Hormone, and Effect of Biologically Active Recombinant FSH on Steroid Secretion1

In fish, FSH is generally important for early gonadal development and vitellogenesis. As in mammals, FSH is a heterodimer composed of an alpha subunit that is noncovalently associated with the hormone-specific beta subunit. The objective of the present study was to express glycosylated, properly folded, and biologically active tilapia FSH (tFSH) using the Pichia pastoris expression system. Using this material, we aimed to develop a specific ELISA and to enable the study of FSH response to GnRH. The methylotrophic yeast P. pastoris was used to coexpress recombinant genes formed by fusion of mating factor alpha leader and tilapia fshb and cga coding sequences. Western blot analysis of tilapia pituitary FSH, resolved by SDS-PAGE, yielded a band of 15 kDa, while recombinant tFSH beta (rtFSH beta) and rtFSH beta alpha had molecular masses of 17-18 kDa and 26-30 kDa, respectively. Recombinant tFSH beta alpha was found to bear only N-linked carbohydrates. Recombinant tFSH beta alpha significantly enhanced 11-ketotestosterone (11-KT) and estradiol secretion from tilapia testes and ovaries, respectively, in a dose-dependent manner (similar to tilapia pituitary extract, affinity-purified pituitary FSH, and porcine FSH). Using antibodies raised against rtFSH beta, FSH-containing cells were localized adjacent to hypothalamic nerve fibers ramifying in the proximal pars distalis (PPD), while LH cells were localized in a more peripheral region of the PPD. Moreover, FSH is under the control of hypothalamic decapeptide GnRH, an effect that was abolished through the use of specific bioneutralizing antisera, anti-rtFSH beta. It also reduced basal secretion of 11-KT.

The dawn and evolution of hormones in the adenohypophysis

The adenohypophysial hormones have been believed to have evolved from several ancestral genes by duplication followed by evolutionary divergence. To understand the origin and evolution of the endocrine systems in vertebrates, we have characterized adenohypophysial hormones in an agnathan, the sea lamprey Petromyzon marinus. In gnathostomes, adrenocorticotropin (ACTH) and melanotropin (MSH) together with beta-endorphins (beta-END) are encoded in a single gene, designated as proopiomelanocortin (POMC), however in sea lamprey, ACTH and MSH are encoded in two distinct genes, proopoicortin (POC) gene and proopiomelanotropin (POM) gene, respectively. The POC and POM genes are expressed specifically in the rostral pars distalis (RPD) and the pars intermedia (PI), respectively. Consequently, the final products from both tissues are the same in all vertebrates, i.e., ACTH from the PD and MSH from the PI. The POMC gene might have been established in the early stages of invertebrate evolution by internal gene duplication of the MSH domains. The ancestral gene might be then inherited in lobe-finned fish and tetrapods, while internal duplication and deletion of MSH domains as well as duplication of whole POMC gene took place in lamprey and gnathostome fish. Sea lamprey growth hormone (GH) is expressed in the cells of the dorsal half of the proximal pars distalis (PPD) and stimulates the expression of an insulin-like growth factor (IGF) gene in the liver as in other vertebrates. Its gene consists of 5 exons and 4 introns spanning 13.6 kb, which is the largest gene among known GH genes. GH appears to be the only member of the GH family in the sea lamprey, which suggests that GH is the ancestral hormone of the GH family that originated first in the molecular evolution of the GH family in vertebrates and later, probably during the early evolution of gnathostomes. The other member of the gene family, PRL and SL, appeared by gene duplication. A beta-chain cDNA belonging to the gonadotropin (GTH) and thyrotropin (TSH) family was cloned. It is expressed in cells of the ventral half of PPD. Since the expression of this gene is stimulated by lamprey gonadotropin-releasing hormone, it was assigned to be a GTHbeta. This GTHbeta is far removed from beta-subunits of LH, FSH, and TSH in an unrooted tree derived from phylogenetic analysis, and takes a position as an out group, suggesting that lampreys have a single GTH gene, which duplicated after the agnathans and prior to the evolution of gnathostomes to give rise to LH and FSH.

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.

Glycoprotein Hormone Assembly in the Endoplasmic Reticulum

Vertebrate glycoprotein hormone heterodimers are stabilized by a strand of their beta-subunits known as the "seatbelt" that is wrapped around loop 2 of their alpha-subunits (alpha2). The cysteine that terminates the seatbelt is "latched" by a disulfide to a cysteine in beta-subunit loop 1 (beta1) of all vertebrate hormones except some teleost follitropins (teFSH), wherein it is latched to a cysteine in the beta-subunit NH(2) terminus. As reported here, teFSH analogs of human choriogonadotropin (hCG) are assembled by a pathway in which the subunits dock before the seatbelt is latched; assembly is completed by wrapping the seatbelt around loop alpha2 and latching it to the NH(2) terminus. This differs from hCG assembly, which occurs by threading the glycosylated end of loop alpha2 beneath the latched seatbelt through a hole in the beta-subunit. The seatbelt is the part of the beta-subunit that has the greatest influence on biological function. Changes in its sequence during the divergence of lutropins, follitropins, and thyrotropins and the speciation of teleost fish may have impeded heterodimer assembly by a threading mechanism, as observed when the hCG seatbelt was replaced with its salmon FSH counterpart. Whereas wrapping is less efficient than threading, it may have facilitated natural experimentation with the composition of the seatbelt during the co-evolution of glycoprotein hormones and their receptors. Migration of the seatbelt latch site to the NH(2)-terminal end of the beta-subunit would have facilitated teFSH assembly by a wraparound mechanism and may have contributed also to its ability to distinguish lutropin and follitropin receptors.

Temporal profile of beta follicle-stimulating hormone, beta luteinizing hormone, and growth hormone gene expression in the protandrous hermaphrodite, gilthead seabream, Sparus aurata

The temporal profile of betaFSH, betaLH, and growth hormone (GH) gene expression was measured throughout the periods of gonadal development, spawning, and post-spawning in the protandrous hermaphrodite gilthead seabream (sb), Sparus aurata (L.). Sampling was carried out monthly, covering a 8-31 month fish age. Pituitary RNA was extracted individually. The levels of betaFSH, betaLH, and GH mRNA were measured by dot blot hybridization using sb betaFSH, betaLH, and GH cDNA as probes and analyzed by computing densitometer (values standardized using individuals' beta-actin pituitary mRNA levels). All three genes, betaFSH, betaLH, and GH were expressed throughout the year, with seasonal variations. However, transcript levels of betaLH were consistently higher than those of betaFSH. During the spawning season (which lasts for about 4 months), the mRNA levels of both betaFSH and betaLH subunits increased dramatically. betaFSH peaked at the start of the spawning season for both males and females, and was significantly higher in males. As for betaLH transcripts, a statistical interaction between sex and date was observed. No significant differences between males and females were found for GH. The pattern of GH expression levels was found to be correlated to that of betaLH.

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.

Regulation of fish gonadotropins

Neurohormones similar to those of mammals are carried in fish by hypothalamic nerve fibers to regulate directly follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin-releasing hormone (GnRH) stimulates the secretion of FSH and LH and the expression of the glycoprotein hormone alpha (GPalpha), FSHbeta, and LHbeta, as well as their secretion. Its signal transduction leading to LH release is similar to that in mammals although the involvement of cyclic AMP-protein kinase A (cAMP-PKA) cannot be ruled out. Dopamine (DA) acting through DA D2 type receptors may inhibit LH release, but not that of FSH, at sites distal to activation of protein kinase C (PKC) and PKA. GnRH increases the steady-state levels of GPalpha, LHbeta, and FSHbeta mRNAs. Pituitary adenylate cyclase-activating polypeptide (PACAP) 38 and neuropeptide Y (NPY) potentiate GnRH effect on gonadotropic cells, and also act directly on the pituitary cells. Whereas PACAP increases all three subunit mRNAs, NPY has no effect on that of FSHbeta. The effect of these peptides on the expression of the gonadotropin subunit genes is transduced differentially; GnRH regulates GPalpha and LHbeta via PKC-ERK and PKA-ERK cascades, while affecting the FSHbeta transcript through a PKA-dependent but ERK-independent cascade. The signals of both NPY and PACAP are transduced via PKC and PKA, each converging at the ERK level. NPY regulates only GPalpha- and LHbeta-subunit genes whereas PACAP regulates the FSHbeta subunit as well. Like those of the mammalian counterparts, the coho salmon LHbeta gene promoter is driven by a strong proximal tripartite element to which three different transcription factors bind. These include Sf-1 and Pitx-1 as in mammals, but the function of the Egr-1 appears to have been replaced by the estrogen receptor (ER). The GnRH responsive region in tilapia FSHbeta 5' flanking region spans the canonical AP1 and CRE motifs implicating both elements in conferring GnRH responsiveness. Generally, high levels of gonadal steroids are associated with high LHbeta transcript levels whereas those of FSHbeta are reduced when pituitary cells are exposed to high steroid levels. Gonadal or hypophyseal activin also participate in the regulation of FSHbeta and LHbeta mRNA levels. However, gonadal effects are dependent on the gender and stage of maturity of the fish.

Expression of a biologically active recombinant follicle stimulating hormone of Japanese Eel Anguilla japonica using methylotropic yeast, Pichia pastoris

In the Japanese eel Anguilla japonica, the administration of exogenous GTH is necessary for the artificial induction and completion of gonadal maturation due to its GTH deficiency under captive conditions. The isolation of native eel GTH has not been accomplished, which has made it difficult to fully elucidate the biological functioning of the two GTHs (FSH and LH) in eel. In this study, we attempted to produce a recombinant Japanese eel GTH (rjeFSH) having biological activity using methylotrophic yeast, Pichia pastoris in order to gain more understanding of the functioning of GTH in this species. An expression vector in which jeFSHbeta and GTHalpha subunit cDNAs were tandemly connected was constructed. P. pastoris was transformed with the vector, and rjeFSH was expressed. The rjeFSH thus expressed was detected by Western blot analysis. The glycoprotein fraction of the yeast culture supernatant was separated by native PAGE, and a band showed positive reaction with anti-GTHalpha and FSHbeta antisera similarly, suggesting that both subunits are associated. After deglycosylation, both subunits were decreased in molecular mass, indicating that rjeFSH was glycosylated. In in vitro assay, rjeFSH stimulated the release of testosterone and 11-ketotestosterone from immature eel testis, whereas release was not stimulated in maturing eel testis. This is the first report investigating the biological activity of eel GTH using the recombinant eel FSH.

Immunocytochemical characterization of adenohypophyseal cells in the greater weever fish (Trachinus draco)

The adenohypophysis of the greater weever fish (Trachinus draco) was studied using histochemical and immunocytochemical methods. The adenohypophysis comprised the rostral pars distalis (RPD), the proximal pars distalis (PPD), and the pars intermedia (PI). Neurohypophysis showed a patent hypophyseal stalk which was divided into several branches intermingled with the adenohypophysis. Salmon prolactin (PRL)-immunoreactive (ir) cells, arranged in follicles, resided in the RPD and the most rostral part of the ventral PPD. Human adrenocorticotropin (ACTH)-ir cells were located in the RPD between PRL-ir cells and the neurohypophyseal processes. Salmon and seabream somatotropin (GH)-ir cells were located in both the dorsal and the ventral PPD. Some GH-ir cells were seen in surrounding and in contact with neurohypophyseal branches, whereas other isolated or clustered GH-ir cells were embedded in adenohypophyseal cells of the PPD. In addition, isolated or clustered GH-ir cells were also detected in the tissue of the PPD covering the most rostral part of PI. Only one class of salmon and carp gonadotropin (GTH)-ir cells was detected. Isolated or clustered GTH-ir cells resided in both the dorsal and the ventral PPD and were seen surrounding the PI and in the tissue of the PPD covering the most rostral part of PI. In addition, a few scattered GTH-ir cells were observed in the ventral RPD. Scattered groups of thyrotropin (TSH)-ir cells were present in the anteroventral PPD. Salmon and seabream somatolactin (SL)-ir and bovine melanotropin (MSH)-ir cells were intermingled surrounding the neurohypophyseal tissue. SL-ir cells were negative to periodic acid-Schiff technique. MSH-ir cells showed a very weak immunoreactivity to anti-human ACTH((1-24)) serum. In addition to the PI location, few isolated or clustered SL- and MSH-ir cells were observed in the dorsal PPD.

Immunocytochemical applications of specific antisera raised against synthetic fragment peptides of mummichog GtH subunits: examining seasonal variations of gonadotrophs (FSH cells and LH cells) in the mummichog and applications to other acanthopterygian fishes

Two distinct types of gonadotrophs, FSH (GtH I) cells and LH (GtH II) cells, were immunocytochemically identified from mummichog (Fundulus heteroclitus; Cyprinodontiformes, Acanthopterygii) pituitary using antisera raised against synthetic fragment peptides of FSHbeta and LHbeta. Both cell types were abundant during the spawning period (spring and early summer) and decreased in number during the post-spawning immature period. The number of FSH cells increased again during the early phases of gonadal development (cortical alveoli accumulation in the oocytes and basal spermatogenesis) in early winter, whereas the number of LH cells did not. Only FSH cells were abundant during the latter phases of gonadal development (vitellogenesis and active spermatogenesis) in early spring. These observations suggest that both GtHs have important yet different roles for reproduction in this species. Antisera against the conservative region of the FSHbeta and the LHbeta subunits immunostained FSH cells and LH cells, respectively, also in red seabream (Pagrus major; Perciformes, Acanthopterygii) and small mouth bass (Micropterus dolomieu; Perciformes, Acanthopterygii), suggesting the possibility of their use for other acanthopterygian fishes.

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.

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

To elucidate the role of the gonadotropins in the multiple spawner Atlantic halibut (Hippoglossus hippoglossus) full length cDNAs encoding FSHbeta, LHbeta, and the common alpha-subunit were cloned from pituitary glands by RACE-PCR. The three cDNAs consisted of 614, 595, and 666 nucleotides encoding peptides of 131, 146, and 124 amino acids, respectively. Halibut FSHbeta and LHbeta showed unique structural features among the vertebrate glycoprotein hormones. First, in contrast to all known FSHbeta, which contain either one or two conserved N-glycosylation sites, no potential binding site was found in Atlantic halibut FSHbeta. Second, the conserved glycosylation site in the N-terminus of all vertebrate LHbeta has been substituted with a unique C-terminal binding site in Atlantic halibut LHbeta. Furthermore, a specific cysteine residue of importance for the folding and heterodimerization of mammalian FSH is lacking in the FSHbeta from Atlantic halibut as well as many other teleosts. However, teleost FSHbeta is characterized by an additional N-terminal cysteine, which has likely replaced the missing residue, implicating a modified folding pattern of this subunit. In situ hybridization of mature male pituitaries revealed that FSHbeta and LHbeta mRNA were expressed in distinct cell types throughout the proximal pars distalis of the adenohypophysis, while alpha-subunit mRNA was identified in all parts of the proximal pars distalis, and also along the periphery of pars intermedia. Consistently, Northern blot analysis of pituitary RNA from mature males showed that FSHbeta, LHbeta, and alpha-subunit mRNAs were highly expressed. In juvenile male pituitaries very few cells containing FSHbeta, LHbeta, and alpha-subunit mRNA were identified by in situ hybridization. Low mRNA levels encoding LHbeta and the alpha-subunit were also demonstrated by Northern blot analysis of the juvenile pituitaries, while no FSHbeta mRNA was detected using this less sensitive technique. The results suggest that both FSH and LH play a role during both the very early and the final reproductive stages in Atlantic halibut males.

Investigation into the duality of gonadotropic cells of Mediterranean yellowtail (Seriola dumerilii, Risso 1810): immunocytochemical and ultrastructural studies

Two antisera against the follicle-stimulating hormone-like gonadotropin (FSH) of Mediterranean (M.) yellowtail, anti-My FSHa and anti-My FSHb, were obtained. Anti-My FSHa serum specifically recognized FSH cells and did not react with any other pituitary cell type, while anti-My FSHb serum recognized the alpha-subunit of the pituitary glycoprotein hormones and immunostained FSH, luteinizing hormone-like gonadotropin (LH), and thyrotropin (TSH) cells. Anti-My FSHa serum, together with a previously obtained anti-My LHbeta serum, were used to further investigate the duality of gonadotropic cells in M. yellowtail by light and electron microscopic immunocytochemistry; three immunologically different gonadotropic cell populations expressing FSH, LH, or both hormones, were revealed. The three cell populations had the same regional distribution in the pituitary gland: the proximal pars distalis, including the thin ring surrounding the pars intermedia. However, while FSH cells were found isolated or forming small clusters, LH cells formed strands or compact groups, and were more numerous than FSH cells. FSH/LH cells were scarce. At the ultrastructural level, vesicular, granular, and intermediate FSH, LH, and FSH/LH cells were found; secretory granules and globules, on the one hand, or conspicuous dilated cisternae of rough endoplasmic reticulum (or both) predominated, respectively, in these cell types. The production of either FSH or LH, or both hormones, was not reflected in the ultrastructural features of gonadotropic cells. Thus, a single morphological cell type of varying ultrastructure depending on the functional stage seemed to encompass all gonadotropic cells in M. yellowtail. All forms of FSH, LH, and FSH/LH cells were found in involution.

Purification of mummichog (Fundulus heteroclitus) gonadotropins and their subunits, using an immunochemical assay with antisera raised against synthetic peptides

To detect mummichog gonadotropins (GtHs) and their subunits immunochemically, fragment peptides with amino acid sequences corresponding to cDNA data were synthesized, and antisera were raised against them. In the case of GtH-IIbeta, large loops such as the second loop and the "seat belt" structure (deduced from the hCG 3D structural data) were considered to be favorable regions for antigen, although further examination is needed to determine if this is the case of GtH-Ibeta and GtH-alpha. In the purification process, glycoprotein was extracted from acetone-dried mummichog pituitary and separated by various liquid chromatography procedures. Each fraction was assayed by immunoblotting with the appropriate antisera against synthetic peptides. Subunits (GtH-alpha, GtH-Ibeta, and GtH-IIbeta) were obtained through gel filtration, anion-exchange chromatography, and reverse-phase HPLC. Intact bioactive GtH-I and GtH-II were obtained through gel filtration, anion-exchange chromatography, and hydrophobic chromatography. Both GtH-I and GtH-II dissociated into subunits under acidic conditions. Nominal MW of each subunit was estimated from SDS-PAGE as 23,000 for GtH-alpha from GtH-I, 22,000 for GtH-alpha from GtH-II, 18,000 for GtH-Ibeta, and 21,000 for GtH-IIbeta.

Immunocytochemical identification of GtH1 and GtH2 cells during the temperature-sensitive period for sex determination in pejerrey, Odontesthes bonariensis

The ontogeny of gonadotropin 1 (GtH1) and 2 (GtH2) cells and its possible link to gonadal sex differentiation were studied in pejerrey, Odontesthes bonariensis, by immunocytochemistry using anti-chum salmon beta-GtH1 and beta-GtH2 antisera. In adults, GtH1 cells were found in the proximalis pars distalls (PPD) close to the neurohypophysis, whereas GtH2 cells were identified surrounding GtH1 cells, at the external layer of the PPD and in the pars intermedia. Essentially the same distribution was observed in larvae. In pejerrey, the phenotypic sex is governed by the temperature during the critical period of sex determination (temperature-dependent sex determination, TSD). Female proportions vary gradually from 100% at 15-19 degrees to 0% at 29 degrees, and the critical time of TSD has been estimated to be 3-5, 2-4, and 1-4 weeks after hatching at 17, 19, and 27 degrees, respectively. Thus, the expression of both GtHs was examined weekly in larvae reared from hatching to week 11, at 17, 24, and 29 degrees. The proportion of females at 17, 24, and 29 degrees was, 100%, 78%, and 0%, respectively. Histological ovarian differentiation was noticed at 7 and 4 weeks at 17 and 24 degrees, whereas testicular differentiation occurred at 7 and 4 weeks at 24 and 29 degrees. GtH1 cells were first observed at week 3 at 29 degrees and at week 4 at 17 and 24 degrees. These cells increased in number until week 4 and then decreased, disappearing after week 6 at all temperature regimes. GtH2 cells appeared at week 2 at 24 and 29 degrees and at week 3 at 17 degrees. GtH2 cell number increased until week 3 at 29 degrees and until week 4 at 17 and 24 degrees and then temporarily decreased, thereafter increasing again. These results strongly suggest that GtH1 and GtH2 are expressed by different cells. The fact that GtH1 and GtH2 cells appear just before histological gonadal differentiation at all temperatures, together with the peak of GtH1 and GtH2 cell number during the temperature-sensitive period, suggests that GtHs are related to sex differentiation or TSD in O. bonariensis.

Follicle-stimulating hormone and its alpha and beta subunits in rainbow trout (Oncorhynchus mykiss): purification, characterization, development of specific radioimmunoassays, and their seasonal plasma and pituitary concentrations in females

Gonad development in fish, as in mammals, is regulated by two gonadotropins (GTHs), FSH and LH. The function of LH in fish has been clearly established; however, the function(s) of FSH is less certain. The lack of specific and sensitive assays to quantify FSH and its alpha and beta subunits has hindered studies to assess physiological function. In this study, gel filtration chromatography, ion exchange chromatography, and HPLC were employed to purify FSH and its subunits from pituitary glands of rainbow trout (Oncorhynchus mykiss), and the identities of the isolates were confirmed by amino acid analysis. Polyclonal antibodies were raised against the free GTHalpha2 and free FSHbeta subunits to develop specific RIAs. The sensitivities of the intact FSH, GTHalpha2, and FSHbeta assays were 1 ng/ml, 0.2 ng/ml, and 0.1 ng/ml, respectively, and the cross-reaction of these molecules with each other and with intact LH in the heterologous assays was <10.4% throughout. Pituitary and plasma samples diluted in parallel with the standards in all three assays and spiked sample recoveries were >90% throughout. Measurement of plasma and pituitary concentrations of intact FSH in female rainbow trout confirmed the established seasonal profiles. Concentrations of free GTHalpha2 subunit were elevated both in the plasma and in the pituitary in females at ovulation (maximum concentrations: 34.93 +/- 6.3 ng/ml in plasma; 37.63 +/- 5.79 microg/pituitary). In both the plasma and the pituitary, free FSHbeta subunit was present throughout the reproductive cycle but at very low concentrations when compared with both free GTHalpha2 and intact FSH. The presence of free GTHalpha2 subunit in the plasma similarly occurs in mammals, but its functional significance in fish has yet to be established.

GTH II but not GTH I induces final maturation and the development of maturational competence of oocytes of red seabream in vitro

The effects of red seabream gonadotropins (PmGTH I and PmGTH II) on the induction in vitro of germinal vesicle breakdown (GVBD) and the development of maturational competence (responsiveness to maturation-inducing steroid) were examined in the oocytes of red seabream. PmGTH II was highly effective in inducing GVBD in both maturationally incompetent (45.6 +/- 3.2% GVBD at a concentration of 300 ng/ml) and competent oocytes (42.3 +/- 0.4% GVBD at a concentration of 300 ng/ml). 17,20beta-Dihydroxy-4-pregnen-3-one (DHP, 10 ng/ml) increased the frequency of GVBD induced by PmGTH II. PmGTH I (33, 100, 300, and 900 ng/ml) was unable to induce GVBD at any tested concentration in the presence or absence of DHP. Actinomycin D (1 microgram/ml) and cycloheximide (1 microgram/ml) totally inhibited the PmGTH II-induced GVBD in the presence and in the absence of DHP. Both PmGTH I and PmGTH II stimulated in vitro production of 11-ketotestosterone in sliced testes of red seabream in a similar potency. These results indicate that PmGTH II, but not PmGTH I, induces the final maturation of oocytes, as well as the development of the maturational competence of oocytes, in red seabream.

Immunological cross-reactivity between rainbow trout GTH I and GTH II and their alpha and beta subunits: application to the development of specific radioimmunoassays

Immunological cross-reactivities between rainbow trout GTH I and GTH II and their alpha and beta have been studied using highly purified rainbow trout gonadotropins and subunits and antibodies raised against beta subunits. From these observations radioimmunoassays have been developed for rainbow trout GTH I and GTH II. The GTH II RIA was highly specific and cross-reacted only with GTH II and its beta 1 subunits, with beta 2 being less potent than beta 1 in competing GTH II binding. There was no cross-reactivity with GTH I. Its sensitivity varied between 0.1 and 0.2 ng/ml, allowing GTH II measurement early in the reproductive cycle. Variations between and within assays were less than 10%. There was a lack of specificity of GTH I RIA (44% cross-reactivity with GTH II, when using labelled native GTH I). Reasons for this lack of sensitivity were studied. It cannot be attributed to beta subunits (less than 1.2% cross-reactivity). However, the cross-reactivity of alpha subunits was very important. This suggests that the presence of free alpha subunits in the medium can be responsible for the lack of specificity. Labelling native GTH I resulted in conformational change in molecular weight and dissociation of the hormone into subunits, whereas iodination did not induce GTH II dissociation. This dissociation can be avoided by labelling the stable form of GTH I. Using this radio-tracer, the specificity and the sensitivity of the assay were greatly improved (GTH II cross-reactivity was decreased to 3.7, mean sensitivity 0.87 +/- 0.072 ng/ml). The sensitivity of the assay diminished with ageing of labelled GTH I. The assay variation was 4.6% within an assay and 9.8% between assays. The use of labelled beta GTH I still increases the specificity (2.3% GTH II cross-reactivity), but with a 2.4-fold loss of sensitivity. In both GTH I and GTH II RIA plasma and spiked plasma with purified GTHs gave displacement curves parallel to standard. These assays were used to study pituitary responsiveness to a GnRH analogue in female rainbow trout prior to oocyte maturation. The effects of GnRH on GTH II secretion were confirmed. The peptide did not significantly stimulate GTH I secretion.

Endocrine regulation of gonadotropin and growth hormone gene transcription in fish

The pituitary of a number of teleosts contains two gonadotropins (GtHs) which are produced in distinct populations of cells; the beta subunit of the GtH I being found in close proximity to the somatotrophs, while the II beta cells are more peripheral. In several species the GtH beta subunits are expressed at varying levels throughout the reproductive cycle, the I beta dominating in early maturing fish, after which the II beta becomes predominant. This suggests differential control of the beta subunit synthesis which may be regulated by both hypothalamic hormones and gonadal steroids. At ovulation and spawning, changes also occur in the somatotrophs, which become markedly more active, while plasma growth hormone (GH) levels increase. In a number of species, GnRH elevates either the I beta or the II beta mRNA levels, depending on the reproductive state of the fish. In tilapia, the GnRH effect on the II beta appears to be mediated through both cAMP-PKA and PKC pathways. GnRH also stimulates GH release in both goldfish and tilapia, but it increases the GH transcript levels only in goldfish; both GnRH and direct activation of PKC are ineffective in altering GH mRNA in tilapia pituitary cells. Dopamine (DA) does not alter II beta transcript levels in cultured tilapia pituitary cells, but increases GH mRNA levels in both rainbow trout and tilapia, in a PKA-dependent manner. This effect appears to be through interactions with Pit-1 and also by stabilizing the mRNA. Somatostatin (SRIF) does not alter GH transcript levels in either tilapia or rainbow trout, although it may alter GH synthesis by modulation of translation. Gonadal steroids appear to have differential effects on the transcription of the beta subunits. In tilapia, testosterone (T) elevates I beta mRNA levels in cells from immature or early maturing fish (in low doses), but depresses them in cells from late maturing fish and is ineffective in cells from regressed fish. Similar results were seen in early recrudescing male coho salmon injected with T or E2. T or E2 administered in vivo has dramatic stimulatory effects on the II beta transcript levels in immature fish of a number of species, while less powerful effects are seen in vitro. A response is also seen in cells from early maturing rainbow trout or tilapia, or regressed tilapia, but not in cells from late maturing or spawning fish. These results are substantiated by the finding that the promoter of the salmon II beta gene contains several estrogen responsive elements (EREs) which react and interact differently when exposed to varying levels of E2. In addition, activator protein-1 (AP-1) and steroidogenic factor-1 (SF-1) response elements are also found in the salmon II beta promoter; the AP-1 site is located close to a half ERE, while the SF-1 acts synergistically with the E2 receptor. The mRNA levels of both AP-1 and SP-1 are elevated, at least in mammals, by GnRH, suggesting possible sites for cross-talk between GnRH and steroid activated pathways. Reports of the effects of T or E2 on GH transcription differ. No effect is seen in vitro in pituitaries of tilapia, juvenile rainbow trout or common carp, but T does increase the transcript levels in pituitaries of both immature and mature goldfish. Reasons for these discrepancies are unclear, but other systemic hormones may be more instrumental than the gonadal steroids in regulating GH transcription. These include T3 which increases both GH mRNA levels and de novo synthesis (in tilapia and common carp) and insulin-like growth factor-I (IGF-I) which reduces GH transcript levels as well as inhibiting GH release.

Immunocytochemical identification of two distinct gonadotropic cells (GTH I and GTH II) in the pituitary of bluefin tuna, Thunnus thynnus

Immunocytochemical identification of GTH I and GTH II cells in the pituitary of the bluefin tuna (Thunnus thynnus) was performed using antisera specific for the common alpha-subunit and the two distinct beta-subunits of tuna (Thunnus obesus) GTH I and GTH II. Cells of the dorsal part of the proximal pars distalis (PPD), in close association with somatotrophs, displayed immunoreactivity of GTHIbeta. GTH IIbeta immunoreactivity was present in cells of the central part of the PPD and the external border of the pars intermedia. Anti-GTHalpha immunostained both GTH Ibeta- and GTH IIbeta-immunoreactive cells and also thyrotrophs. Both GTH Ibeta- and GTH IIbeta-immunoreactive cells were observed in immature bluefin tuna, although there were greater numbers of GTH IIbeta immunoreactive cells. These results suggest that GTH I and GTH II are synthesized in separate cells in the pituitary of the bluefin tuna. The localization and appearance of the two distinct gonadotropic cells of the tuna are compared with the salmonid arrangement.

Gonadotropic and thyrotropic cells from the Mediterranean yellowtail (Seriola dumerilii; Risso, 1810): immunocytochemical and ultrastructural characterization

BACKGROUND

Gonadotropins GTH I and GTH II from the pituitary of Mediterranean (M.) yellowtail (Seriola dumerilii) were isolated and characterized, and antisera to the whole GTH II molecule (anti-My alpha,betaGTH II) and to its beta-subunit (anti-My betaGTH II) were obtained. At the light microscopic level, anti-My alpha,betaGTH II reacted with My betaGTH II-immunoreactive cells (GTH II cells), thyroid-stimulating hormone (TSH) cells, and a third cell population, which could have been GTH I cells. The aim of this study was the ultrastructural characterization of GTH and TSH cells in M. yellowtail using the immunogold method in order to provide a basis for future research into reproduction of this species.

METHODS

Pituitaries from mature male and female specimens reared in captivity were dissected out and processed for electron microscopy. The immunogold method was carried out by using anti-My alpha,betaGTH II, anti-My alpha,betaGTH II preabsorbed with the alpha subunit of the M. yellowtail GTH (My alphaGTH-subunit), anti-My betaGTH II, anti-human (h) alpha,betaTSH, and anti-h betaTSH sera to reveal gonadotropic and thyrotropic cells.

RESULTS

M. yellowtail gonadotropic cells were very heterogeneous with regard to their size, shape, and ultrastructural features. Cells were found with numerous, round, variably electron-dense, secretory granules and globules; others were found with their cytoplasm occupied mostly by dilated cisternae of rough endoplasmic reticulum (RER) and scarce secretory granules; and other intermediate cell forms were found that showed varying proportions of secretory granules and dilated RER. The secretory granules and globules were immunogold labeled with anti-My alpha,betaGTH II, and the reaction was weaker in the latter. A similar immunogold-labeling pattern was found with anti-My betaGTH II and with anti-My alpha,betaGTH II preabsorbed with the My alphaGTH-subunit, although some cells that showed the same ultrastructural features described above were not immunogold labeled and could have been GTH I cells. Thyrotropic cells had small, round, secretory granules of medium or high electron density that were immunogold labeled with anti-My alpha,betaGTH II, anti-h alpha,betaTSH, and anti-h betaTSH sera, but not with anti-My betaGTH II or anti-My alpha,betaGTH II serum preabsorbed with the My alphaGTH-subunit. All of the cell forms described for gonadotropes and thyrotropes were also found in a state of involution.

CONCLUSIONS

Gonadotropes that are of a single morphological type but that vary in ultrastructure are present in the pituitary of captive M. yellowtail. GTH II- and putative GTH I-producing cells were distinguishable from one another and from TSH cells by their different reactions to anti-My alpha,betaGTH II, anti-My betaGTH II, and anti-My alpha,betaGTH II preabsorbed with the My alphaGTH-subunit.