Isolation and characterization of two distinct gonadotropins, GTHI and GTHII, from bonito (Katsuwonus plelamis) pituitary glands

Development of sandwich enzyme-linked immunosorbent assays for chub mackerel Scomber japonicus gonadotropins and regulation of their secretion in female reproduction

The pituitary gonadotropins (Gths), follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), play critical roles in regulating gonadal development and sexual maturation in vertebrates. We developed non-competitive enzyme-linked immunosorbent assays (ELISAs) to measure Fsh and Lh in chub mackerel Scomber japonicus, which is a commercially important scombrid species. Mouse monoclonal antibodies specific for Fsh and Lh, and a rabbit polyclonal antibody against both Gths were produced by immunization with hormones purified from chub mackerel pituitaries. These monoclonal and polyclonal antibodies were used as capture and detection antibodies in the developed sandwich ELISAs. The ELISAs were reproducible, sensitive, and specific for chub mackerel Fsh and Lh. Parallelism between the standard curve and serial dilutions of chub mackerel serum and pituitary extract was observed for both Fsh and Lh ELISAs. Comparison between vitellogenic and immature females revealed that Fsh is secreted during vitellogenesis and Lh is barely released during immaturity. After gonadotropin-releasing hormone analog (GnRHa) injection, vitellogenic females showed increases in serum Lh, whereas serum levels of Fsh did not vary. Moreover, the serum steroid profiles revealed that estradiol-17β was continuously produced after GnRHa treatment, whereas 17,20β-dihydroxy-4-pregnen-3-one secretion was transiently induced. These results indicate that, in vitellogenic females, GnRHa stimulates the release of Lh, but not Fsh, which results in acceleration of vitellogenesis and induction of oocyte maturation via steroid production.

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.

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.

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.

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.

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.

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.

Fundulus heteroclitus gonadotropins.5: Small scale chromatographic fractionation of pituitary extracts into components with different steroidogenic activities using homologous bioassays

Fractionation and characterization of gonadotropins (GtH) from Fundulus heteroclitus pituitary extracts were carried out using a biocompatible liquid chromatographic procedure (Pharmacia FPLC system). Chromatographic fractions were monitored for gonadotropic activities (induction of oocyte maturation and steroid production) using homologous follicle bioassays in vitro. Size-exclusion chromatography eluted gonadotropic activity in one major protein peak (Mr approximately 30,000). Anion-exchange and hydrophobic-interaction chromatography (HIC) yielded two distinct peaks of 17beta-estradiol (E2)- and 17alpha-hydroxy,20beta-dihydroprogesterone (DHP)-promoting activity with associated oocyte maturation. Two-dimensional chromatography (chromatofocusing followed by HIC) resolved pituitary extracts into two active fractions; both induced E2 synthesis, but one was relatively poor in eliciting DHP and testosterone production. Thus, using homologous bioassays, at least two quantitatively different gonadotropic (steroidogenic) activities: an E2-promoting gonadotropin (GtH I-like) and a DHP-promoting gonadotropin (GtH II-like), which has a lower isoelectric point but greater hydrophobicity than the former, can be distinguished from F. heteroclitus pituitaries by a variety of chromatographic procedures. This study complements previous biochemical and molecular data in F. heteroclitus and substantiates the duality of GtH function in a multiple-spawning teleost.

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.

Cloning and sequencing of the FSH-beta and LH beta-subunit in the three-spined stickleback, Gasterosteus aculeatus, and effects of photoperiod and temperature on LH-beta and FSH-beta mRNA expression

The aim of this study was to characterize the FSH-beta and LH-beta subunit in the three-spined stickleback, a fish used extensively in experimental studies, and to use the cloned cDNAs as probes for measuring FSH-beta and LH-beta mRNA expression in sticklebacks treated with different photoperiods and temperatures. A first strand cDNA was prepared from 10 pituitaries from male sticklebacks, and cDNA fragments were amplified by PCR using degenerated primers based on highly conserved regions of known teleost FSH-beta and LH-beta cDNA sequences. To obtain full-length cDNAs of FSH-beta and LH-beta, RACE amplifications were performed. The cDNA of FSH-beta was 540 bp long, encoding a protein of 122 amino acids and LH-beta cDNA was 568 bp long, encoding a protein of 148 amino acids. Of gonadotropin (GTH) beta-subunits published so far, those most similar to stickleback GTHs are found among other percomorph fishes, with amino acid similarities of 46-55% for FSH-beta and 75-77% for LH-beta subunits.The cloned cDNAs were used as probes to analyze LH and FSH mRNA expression in pituitaries from sticklebacks exposed to different photoperiods and temperatures. Two hundred males were divided into four groups and kept under different photoperiods and temperatures; light:dark (LD) 16:8, 20 degrees C; LD 16:8, 7 degrees C; LD 8:16, 20 degrees C, and LD 8:16, 7 degrees C. Red breeding colors and a marked kidney hypertrophy, androgen-dependent male secondary sexual characters in the stickleback, appeared in the group kept under LD 16:8 at 20 degrees C, but not in the other groups. The kidney epithelium height (KEH) was significantly lower in the LD 8:16, 20 degrees C group than in all other groups, and this was also the only group with active spermatogenesis. The LD 8:16 20 degrees C had significantly lower expression of both FSH-beta and LH-beta than all other groups. Thus, both GTHs displayed their lowest expression when spermatogenesis was active and when low KEH indicated that androgens levels were at their lowest.

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 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.

Preparation and use of specific antibodies to the beta-I and beta-II subunits of gonadotropic hormone from Fundulus heteroclitus pituitary

Fundulus heteroclitus naturally spawns with a semilunar periodicity throughout most of the year in its southernmost habitat, an activity that can be maintained in the laboratory. The alpha and two beta subunits comprising F. heteroclitus gonadotropic hormones (GtHs) I and II have been sequenced, and antibodies have been raised against unique peptides found in each of the two beta subunits. On immunoblots of pituitary proteins, each antibody recognizes a single band with a molecular mass of 16-17 kDa, somewhat larger than the deduced sizes (11-13 kDa) of the unglycosylated subunits. Each antibody also recognizes a different subset of pituitary cells in the central (GtH I) and peripheral (GtH II) proximal pars distalis, regions that display the typical tinctorial properties of gonadotrops. The distribution and distinct separation of cells containing GtH beta subunits I and II thus differ from those found for previously described teleost species, most of which are salmonids that engage in a single spawning episode during the year. The availability of these antibodies thus makes F. heteroclitus an inexpensive, easily manipulated model system for studies on the hormonal regulation of fractional spawning common to a large class of commercially important species other than salmonids.

Update: brain and pituitary hormones of lampreys

Lampreys and hagfish of the class Agnatha are of particular importance in understanding endocrinological relationships since they represent the oldest lineages of extant vertebrates which evolved over 550 million years ago. This review briefly summarizes the latest findings on the reproductive endocrinology of the sea lampreys. Since the First International Symposium of Fish Endocrinology in 1988, when virtually little was known of the hypothalamic-pituitary-gonadal axis, substantial new biochemical, molecular, physiological and immunological evidence has now clearly shown that lamprey reproduction is controlled by the neuroendocrine axis. In addition, five brain and six pituitary hormones of lampreys have been identified mainly by Sower and Kawauchi and colleagues between 1986 and 2000. We now hypothesize that lamprey reproduction is a highly synchronized process that is initiated or mediated by a coordination of complex integration of environmental cues and hormonal mechanisms which is broadly similar to that exhibited by gnathostome vertebrates.

cDNA cloning of two gonadotropin beta subunits (GTH-Ibeta and -IIbeta) and their expression profiles during gametogenesis in the Japanese flounder (Paralichthys olivaceus)

To clarify the profiles of two distinct gonadotropin (GTH-I and -II) mRNA levels during gametogenesis in a multiple spawner, the Japanese flounder (Paralichthys olivaceus), the cDNAs encoding GTH-Ibeta and -IIbeta from the pituitary gland have been cloned and sequenced. The nucleotide sequence of GTH-Ibeta was 542 bp long, encoding 120 amino acids, and that of GTH-IIbeta was 554 bp long, encoding 145 amino acids. In females, Northern blot analysis has revealed that relative mRNA levels of GTH-Ibeta and -IIbeta were low in immature fish, showed a gradual increase with ovarian development, and reached the highest level at the maturation stage. Both GTH-Ibeta and -IIbeta mRNA levels were highly correlated with gonadosomatic index (GSI) values and with circulating estradiol-17beta and testosterone (T) levels. In males, the mRNA levels of GTH-Ibeta increased with the increase in GSI values and in circulating 11-ketotestosterone and T levels, whereas the mRNA levels of GTH-IIbeta did not show any correlation with GSI values and with circulating steroid levels, suggesting a difference in regulatory mechanisms of GTH-I and -II synthesis in males. The similar changes in GTH-Ibeta and -IIbeta mRNA levels during oogenesis are considered to be characteristic of GTH synthesis in multiple spawners, differing from the differential changes reported in annual spawners such as salmonids.

Duality of gonadotropin in a primitive teleost, Japanese eel (Anguilla japonica)

The duality of gonadotropin (GTH) is well-established in relatively modern teleosts. In primitive teleosts such as eel and catfish, however, only a single GTH (GTH-II) has been isolated and characterized. Therefore, the objective of this study was to clarify the duality of GTH, particularly the presence of GTH-I in primitive teleosts. We attempted to obtain a cDNA encoding the beta subunit of GTH-I from Japanese eel, Anguilla japonica, as a representative primitive teleost species. Rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR) was used to amplify GTH-Ibeta cDNA prepared from immature male Japanese eel pituitaries, and the obtained PCR products were subcloned and sequenced. A degenerate PCR primer was designed based on a highly conserved region between teleost GTH-Ibeta and mammalian FSHbeta. DNA sequence analysis of the cloned PCR products confirmed the presence of the predicted complete coding region as well as the 5' and 3' untranslated regions. The deduced amino acid sequence from these clones showed high homology to goldfish GTH-Ibeta (60%), whereas the identity between Japanese eel GTH-Ibeta and GTH-IIbeta was lower (42%). Phylogenetic analysis confirmed that Japanese eel GTH-Ibeta belonged to the teleost GTH-Ibeta group. These results provide a definitive proof of the presence of two types of GTHs (GTH-I and GTH-II) in Japanese eel, as has been shown in other teleosts. The duality of GTHs is applicable for teleosts in general. Northern blot analysis showed the transcripts of Japanese eel GTH-Ibeta and GTH-IIbeta to be 1200 and 750 bases, respectively. GTH-Ibeta gene was expressed in immature fish, while GTH-IIbeta gene was expressed in spermiating males and ovulated females, suggesting that two GTHs are differentially expressed at different sexual stages and may play separate roles in the reproductive process in Japanese eel.

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.

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.

Cloning of cDNA encoding the common alpha subunit precursor molecule of pituitary glycoprotein hormones in the Australian lungfish, Neoceratodus forsteri

We have isolated cDNA clones encoding a putative glycoprotein hormone alpha subunit precursor molecule from a pituitary cDNA library of the Australian lungfish (Neoceratodus forsteri) by a two-step plaque hybridization technique initially using cDNA encoding the toad glycoprotein hormone alpha subunit precursor molecule as the hybridization probe. The inserts (799 bp) of two of the isolated cDNA clones contained sequences of 5' and 3' untranslated regions, including a poly(A) stretch, and the entire coding sequence of the alpha subunit precursor molecule. The deduced amino acid sequence showed that its signal peptide consists of 24 residues and its mature protein 97 residues. In the mature protein, there is an insert of one residue (Ala) just after the 9th residue. This insert is unique to the Australian lungfish among vertebrate species studied. The amino acid sequence of the mature protein shares the common, or the same-group, amino acid residues at 9 positions with tetrapod and not actinopterygian vertebrates, while only one residue is common to some teleosts and the lungfish to the exclusion of the tetrapods. The overall sequence of the mature protein of the Australian lungfish also shares more similarity with those of tetrapods (69 to 84%) than it does with teleosts (57 to 74%). These results on the relation of the alpha subunit precursor molecule between the lungfish and other vertebrates are consistent with the recent molecular phylogenetic studies suggesting a closer relationship between lungfish and tetrapods than between lungfish and teleosts. We also found that the primary structure of the lungfish alpha subunit is slightly but significantly more similar to that of homeothermic vertebrates than to that of amphibians. This may be due to specialization of the amphibian alpha subunit molecules.

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.

Purification of gonadotropin II from a teleost fish, the hybrid striped bass, and development of a specific enzyme-linked immunosorbent assay

A highly purified gonadotropin II (GtH II), referred to as striped bass GtH II (stbGtH II), and its alpha and beta subunits were prepared from pituitaries of sexually mature hybrid striped bass. Pituitary glycoproteins were extracted with ethanol and intact stbGtH II purified by gel-filtration chromatography on Sephadex G-100, ion-exchange chromatography (IEC) on DE-52, and fast-performance liquid chromatography (FPLC) on Superdex 75. The presence of GtHs during the purification procedure was monitored by characteristic elution on reversed-phase high-performance liquid chromatography (rpHPLC) and in vitro steroidogenic activity. The stbGtH II alpha and beta subunits were purified from the pituitary ethanol extract by gel-filtration, IEC, and rpHPLC, and their identities assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), rpHPLC, and N-terminal amino acid sequencing. Molecular weights of intact stbGtH II and its alpha and beta subunits, determined by SDS-PAGE, were 34.5, 14.8, and 20.4 kDa, respectively. The stbGtH II beta subunit was used to produce specific antibodies, and a competitive enzyme-linked immunosorbent assay was developed using intact stbGtH II for the standard curve. The sensitivity of the assay was 156 pg/ml (15.6 pg/well) and the intra- and interassay coefficients of variation (at 50% binding) were 7.7% (n = 16) and 8.7% (n = 10), respectively. Physiological validation of the assay was performed by measuring changes of plasma GtH II levels in mature striped bass females, after injection of GnRHa ([d-Ala6,Pro9-NEt]-mGnRH, 100 microg/kg BW). A maximum surge of GtH II in plasma was observed at 12 hr postinjection (22.5 +/- 3. 01 ng/ml), whereas GtH II levels in control fish (around 4 ng/ml) remained unchanged. Displacement curves obtained with serial dilutions of plasma and pituitaries from a number of perciform species were parallel to the standard curve, indicating that this assay can be used for GtH II measurements in a variety of fish species.

Use of immobilized metal ion affinity chromatography and dye-ligand chromatography for the separation and purification of rainbow trout pituitary gonadotropins, GTH I and GTH II

A new procedure is described for the purification of gonadotropic hormones (GTHs) from the pituitary glands of vitellogenic rainbow trout. The procedure utilizes immobilized metal ion affinity chromatography (IMAC) on a column containing immobilized iminodiacetic acid (Toyopearl AF Chelate) charged with Cu2+ ions as a critical step for the efficient separation of GTH I and GTH II. Further purification of both GTH fractions on Cibacron Blue F3GA immobilized on Toyopearl was followed by HPLC size-exclusion for GTH II. The resulting electrophoretically homogeneous preparations possessed a characteristic range of biological activity in the stimulation of steroidogenesis in vitro. N-Terminal sequences of the GTH I and GTH II subunits purified using reversed-phase HPLC revealed a high level of homology with those of GTH subunits found in three other salmonid fish species. In contrast to salmon GTH II, two different alpha-subunits were observed in trout GTH II. The cross-reactivity between GTH I and GTH II was studied in radioimmunoassay using antibodies against Chinook salmon GTH II beta-subunit and rainbow trout GTH I dimer.

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

Two gonadotropins, GTH I and GTH II, were isolated and chemically characterized from the pituitary of Mediterranean yellowtail. They were extracted with 35% ethanol-10% ammonium acetate, separated by ion-exchange chromatography on a DE-52 column, and purified by reversed-phase high-performance liquid chromatography on Asahipak C4P-50 and subsequently by gel filtration chromatography on Superdex 75. The molecular weights were estimated at 47 kDa for GTH I and 29 kDa for GTH II by SDS-PAGE and at 49 kDa for GTH I and 42 kDa for GTH II by gel filtration. GTH II was completely dissociated, while GTH I was partially dissociated into alpha- and beta-subunits by treatment with 0.1% trifluoroacetic acid. The complete amino acid sequences of GTH alpha-, GTH I beta-, and GTH II beta-subunits were determined. The GTH alpha-subunit consisted of 91 amino acid residues. The GTH I beta and GTH II beta consisted of 105 and 115 amino acid residues, respectively, and had a 28% sequence identity to each other. They had the highest sequence identity with the respective gonadotropin subunits of bonito, tuna, and striped bass: 81-83% for GTH alpha, 67-71% for GTH I beta, and 91-93% for GTH II beta. The sequence identity of the GTH alpha-subunit with those of other teleosts and human and bovine LH and FSH was 57-67%. The GTH I beta-subunit showed a low sequence identity with other known fish GTH I beta s (36-51%) and was more similar to human and bovine FSH beta s (34% identity) than to human and bovine LH beta s (29% identity). The sequence identity of the GTH II beta-subunit with those of other teleosts was higher (60-73%), being more similar to LH beta s (43% identity) than FSH beta s (38% identity). Thus, two distinct gonadotropins, GTH I and GTH II, homologous to mammalian FSH and LH, respectively, are synthetized by M. yellowtail pituitary glands.

Molecular cloning of the cDNAs encoding two gonadotropin beta subunits (GTH-I beta and -II beta) from the goldfish, Carassius auratus

Two types of cDNAs (GTH-I beta and -II beta) that encode the beta subunit of goldfish gonadotropin were cloned from a goldfish pituitary cDNA library. The nucleotide sequence of GTH-I beta cDNA was 616 bp long, encoding 130 amino acids, and that of GTH-II beta was 552 bp long, encoding 140 amino acids. These two types of goldfish GTH beta subunits showed an identity of 52% in the nucleotide sequence and 38% in the amino acid sequence. When compared to mammalian GTH beta subunits, goldfish GTH-I beta showed higher homology to FSH-beta s than LH-beta s, and goldfish-II beta to LH-beta s. Genomic Southern blot analysis revealed that cyprinid species harbored genes which were homologous to goldfish GTH-I beta and -II beta, demonstrating the duality of GTH in cyprinid fish as has been shown in other teleost fish. Northern blot analysis of GTH-I beta and -II beta mRNAs in goldfish at differing stages of ovarian maturity (immature, maturing, mature, and regressed) showed that both GTH-I beta and -II beta mRNA levels increased with the progression of maturity and declined in regressed stage. These results suggested that gonadal development in the goldfish was regulated by the two GTHs.

Molecular cloning of cDNA encoding two types of pituitary gonadotropin alpha subunit from the goldfish, Carassius auratus

Two types of cDNA encoding the alpha subunit of pituitary gonadotropin (GTH) were cloned by polymerase chain reaction (PCR) for goldfish pituitary cDNA library. The goldfish GTH-alpha subunit cDNAs, designated as alpha 1 and alpha 2, encode 117 and 118 amino acids, respectively, including a 23-amino-acid signal peptide. These two types of cDNAs showed high homology in nucleotide and amino acid sequence, but deletion of a triplet nucleotides was present in alpha 1. Deduced amino acid sequences represented differences in four residues between alpha 1 and alpha 2 subunits. Analyses of goldfish genomic DNA revealed that each individual goldfish possesses two distinct genes relevant to GTH-alpha 1 and -alpha 2 subunits. Results of reverse transcriptase-PCR analysis suggest that these two GTH-alpha subunit genes are expressed at different levels without relation to sex and gonadal maturity.

The adenohypophysis of Mediterranean yellowtail, Seriola dumerilii (Risso, 1810): an immunocytochemical study

The adenohypophysis (ADH) of the Mediterranean yellowtail was studied using the peroxidase-antiperoxidase technique. Human corticotropin (ACTH) (1-24)-immunoreactive (ir) cells were found bordering the neurohypophysis (NH) and salmon prolactin (PRL)-ir cells were arranged in thick cords, both in the rostral pars distalis (RPD). Gonadotropin (GTH)-, thyrotropin (TSH)- and growth hormone (GH)-ir cells were observed in the proximal pars distalis (PPD). Anti-chum salmon GTH I and anti-chum salmon GTH II immunostained the same cells in the outermost part of the ADH at the level of the PPD and the PI. In addition to these cells, some cells grouped in the inner areas of the posterior PPD were revealed by catfish alpha, beta-GTH antiserum. Human beta-TSH-ir cells formed small groups and discontinuous strands in the PPD often in contact with the NH. Tilapia GH-ir cells formed cords mainly surrounding the NH in the central PPD, while cod somatolactin- and alpha MSH-ir cells mainly surrounded the NH branches in the PI.

cDNA cloning of the beta subunit of teleost thyrotropin

cDNA clones encoding the beta subunit of thyrotropin (thyroid-stimulating hormone; TSH) were isolated from a cDNA library made from the pituitaries of immature rainbow trout and sequenced. The precursor of rainbow trout TSH beta consists of 147 aa, which can be cleaved into a signal peptide (20 aa) and a mature protein (127 aa) containing one potential N-glycosylation site and 12 cysteine residues. The protein showed highest homology with human TSH beta (51%) and lesser homology with human follitropin (42%), human lutropin (32%), and salmon gonadotropin (31-33%) beta subunits. The identification of TSH in addition to two gonadotropins (gonadotropins I and II) in the teleost fish suggests that the divergence of three kinds of glycoprotein hormones from an ancestral molecule took place earlier than the time of divergence of teleosts from the main line of evolution leading to tetrapods. Northern blot analysis showed that the expression of the rainbow trout TSH beta gene is specific to the pituitary gland and is significantly higher in immature fish than in mature fish, suggesting that TSH plays some role in the biological processes of immature fish.