Testicular responsiveness to gonadotropic hormonein vitro and Leydig and Sertoli cell ultrastructure during pubertal development of male African catfish (Clarias gariepinus)

Transcriptome Display During Testicular Differentiation of Channel Catfish (Ictalurus punctatus) as Revealed by RNA-Seq Analysis

Channel catfish (Ictalurus punctatus) has been recognized as a dominant freshwater aquaculture species in the United States. It is also a suitable model for studying the mechanisms of sex determination and differentiation because of its sexual plasticity and exhibition of both genetic and environmental sex determination. The testicular differentiation in male channel catfish normally starts between 90 and 102 days postfertilization (dpf), while the ovarian differentiation starts early from 19 dpf. As such, efforts to better understand the postponed testicular development at the molecular level are needed. Toward that end, we conducted transcriptomic comparison of gene expression of male and female gonads at 90, 100, and 110 dpf using high-throughput RNA-Seq. Transcriptomic profiles of male gonads on 90 and 100 dpf exhibited high similarities except for a small number of significantly up-regulated genes that were involved in development of germ cell-supporting somatic cells, while drastic changes were observed during 100-110 dpf, with a group of highly up-regulated genes that were involved in germ cells development, including nanog and pou5f1 Transcriptomic comparison between testes and ovaries identified male-preferential genes, such as gsdf, cxcl12, as well as other cytokines mediated the development of the gonad into a testis. Co-expression analysis revealed highly correlated genes and potential pathways underlying germ cell differentiation and spermatogonia stem cell development. The candidate genes and pathways identified in this study set the foundation for further studies on sex determination and differentiation in catfish as well as other teleosts.

Control of puberty in farmed fish

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

Leydig cells express follicle-stimulating hormone receptors in African catfish

This report aimed to establish, using African catfish, Clarias gariepinus, as model species, a basis for understanding a well-known, although not yet clarified, feature of male fish reproductive physiology: the strong steroidogenic activity of FSHs. Assays with gonadotropin receptor-expressing cell lines showed that FSH activated its cognate receptor (FSHR) with an at least 1000-fold lower EC50 than when challenging the LH receptor (LHR), whereas LH stimulated both receptors with similar EC50s. In androgen release bioassays, FSH elicited a significant response at lower concentrations than those required to cross-activate of the LHR, indicating that FSH stimulated steroid release via FSHR-dependent mechanisms. LHR/FSHR-mediated stimulation of androgen release was completely abolished by H-89, a specific protein kinase A inhibitor, pointing to the cAMP/protein kinase A pathway as the main route for both LH- and FSH-stimulated steroid release. Localization studies showed that intratubular Sertoli cells express FSHR mRNA, whereas, as reported for the first time in a vertebrate, catfish Leydig cells express both LHR and FSHR mRNA. Testicular FSHR and LHR mRNA expression increased gradually during pubertal development. FSHR, but not LHR, transcript levels continued to rise between completion of the first wave of spermatogenesis at about 7 months and full maturity at about 12 months of age, which was associated with a previously recorded approximately 3-fold increase in the steroid production capacity per unit testis weight. Taken together, our data strongly suggest that the steroidogenic potency of FSH can be explained by its direct trophic action on FSHR-expressing Leydig cells.

Gene expression profiling during spermatogenesis in early maturing male Atlantic salmon parr testes

The initiation of sexual maturation and spermatogenesis are complex processes that require the highly coordinated regulation of a number of key genes. The endocrine system plays crucial roles in these processes, but the precise mechanisms involved in sexual maturation of fish are poorly understood. We investigated the expression of genes encoding proteins involved in sex steroid biosynthesis (Ff1b (FTZ-F1 homolog), steroidogenic acute regulatory protein (StAR), cytochrome P450 cholesterol side-chain cleavage enzyme (P450scc), 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4)-isomerase (3beta-HSD), cytochrome P450 17alpha-hydroxylase/17,20-lyase (P450c17), cytochrome P450 11beta-hydroxylase (P45011beta) and 11beta-hydroxysteroid dehydrogenase (11beta-HSD)) and the anti-Müllerian hormone (AMH) homolog during early sexual maturation of one-summer-old male Atlantic salmon parr by RT-PCR. Genes encoding Ff1b, StAR, 3beta-HSD, P450c17 and 11beta-HSD were upregulated during spermatogonial proliferation. During the course of spermatogenesis expression profiles of Ff1b, StAR, 3beta-HSD, P450scc, P450c17, P45011beta, and 11beta-HSD were similar; transcript levels being low during early stages, then strongly increasing during spermiogenesis. These results indicate that coordinated de novo transcription of genes encoding StAR as well as 3beta-HSD, P450c17 and 11beta-HSD might be required for sex steroids production during the initiation of spermatogenesis in salmon. In contrast, transcription levels of AMH were comparatively high in immature testes, decreased when spermatogenesis was initiated, and were lowest during spermiogenesis, suggesting that AMH suppression plays a crucial role in the process of spermatogenesis in salmonids. Correlation analyses show that FSH and LH might be differentially involved in the regulation of several of these genes studied.

Testicular apoptosis in feral Clarias gariepinus using TUNEL and cleaved caspase-3 immunohistochemistry

This paper reports on the mechanistic basis of cellular death in the testis of Clarias gariepinus using the TUNEL and caspase-3 immunohistochemistry. It was also aimed to determine the testicular zone most suitable for the quantification of testicular apoptosis. The results showed that based on its immuno-expression patterns, activated caspase-3 has a clear and defined role in the progression of germ cell apoptosis in spermatogenically active catfish testis. Caspase-3 activation, and not TUNEL-detected DNA fragmentation, is associated with condensation of chromatin into a single mass. Testes of spermatogenically active catfish consist of spermatogenic, mature and spent tubules. Spermatogenic tubules were concluded to be the most suitable zone for the quantification of testicular cell death ratios as apoptotic events occurred predominantly in the secondary spermatocytes. The findings of this study will form the basis to link apoptotic events in the testes of C. gariepinus with the effects of EDC exposure in future studies.

Androgens modulate testicular androgen production in African catfish (Clarias gariepinus) depending on the stage of maturity and type of androgen

Previous work showed that androgen treatment suppressed testicular steroidogenesis in juvenile African catfish Clarias gariepinus. Similar to other vertebrates, however, circulating androgen levels increase during puberty in catfish. We therefore studied if androgen-induced inhibition of androgen production decreases during sexual maturation. As in other vertebrates, testosterone (T) is found in the circulation in fish but typically, 11-ketotestosterone (11-KT) is the quantitatively dominating androgen. In previous studies with juvenile catfish, these two androgens showed different biological activities as regards spermatogenesis or pituitary hormone production, but were equally effective in suppressing testicular steroidogenesis. Hence, the second question we studied was if the two types of androgens show distinct effects on the steroidogenic system in pubertal or adult males. The inhibitory effect of 11-KT on the testicular steroidogenic capacity waned with progressing sexual maturation, while T-mediated inhibition remained strong until adulthood reducing the in vitro steroid production 4- to 10-fold. However, the gonadotropin responsiveness of testicular tissue was not compromised and expression of testicular gonadotropin receptors did not respond differently to the two androgens. We conclude that the selective disappearance of the inhibitory effect of 11-KT contributes to allowing the pubertal increase of the plasma level of this androgen.

Sertoli cell proliferation in the adult testis--evidence from two fish species belonging to different orders

Germ cell survival and development critically depend on the cells' contact with Sertoli cells in the vertebrate testis. Fish and amphibians are different from mammals in that they show a cystic type of spermatogenesis in which a single germ cell clone is enclosed by and accompanied through the different stages of spermatogenesis by an accompanying group of Sertoli cells. We show that in maturing and adult testes from African catfish and Nile tilapia, Sertoli cell proliferation occurs primarily during spermatogonial proliferation, allowing the cyst-forming Sertoli cells to provide the increasing space required by the growing germ cell clone. In this regard, coincident with a dramatic increase in cyst volume and number of germ cells per cyst, in Nile tilapia, the number of Sertoli cells per cyst was strikingly increased from primary spermatogonia to spermatocyte cysts. In both African catfish and Nile tilapia, Sertoli cell proliferation is strongly reduced when germ cells have proceeded into meiosis, and stops in postmeiotic cysts. We conclude that Sertoli cell proliferation is the primary factor responsible for the increase in testis size and sperm production observed in teleost fish. In mammals, Sertoli cell proliferation in the adult testis is not observed under natural conditions. However, on the level of the individual spermatogenic cyst--similar to mammals--Sertoli cell proliferation ceases when germ cells have entered meiosis and when tight junctions are established between Sertoli cells. This suggests that fish are valid vertebrate models for studying Sertoli cell physiology.

Endocrine changes during onset of puberty in male spring Chinook salmon, Oncorhynchus tshawytscha

In male salmonids, the age of maturation varies from 1 to 6 years and is influenced by growth during critical periods of the life cycle. The endocrine mechanisms controlling spermatogenesis and how growth affects this process are poorly understood. Recent research has indicated that gonadotropins, 11-ketotestosterone, and insulin-like growth factor I play roles in spermatogenesis in fish. To expand our understanding of the roles of these endocrine factors in onset of puberty, male spring chinook salmon (Oncorhynchus tshawytscha) were sampled at monthly intervals 14 mo prior to spermiation. This sampling regime encompassed two hypothesized critical periods when growth influences the initiation and completion of puberty for this species. Approximately 80% of the males matured during the experimental period, at age 2 in September 1999. An initial decline in the ratio of primary A to transitional spermatogonia was observed from July to December 1998, and during this period plasma levels of 11-ketotestosterone and pituitary levels of FSH increased. From January 1999 onward, males with low plasma 11-ketotestosterone levels (<1 ng/ml) had low pituitary and plasma FSH levels and no advanced development of germ cells. Conversely, from January through September 1999, males with high plasma 11-ketotestosterone levels (>1 ng/ml) had testes with progressively more advanced germ cell stages along with elevated pituitary and plasma FSH. Plasma levels of insulin-like growth factor I increased during maturation. These data provide the first physiological evidence for activation of the pituitary-testis axis during the fall critical period when maturation is initiated for the following year.

The effects of long-term testosterone, gonadotropin-releasing hormone agonist and pimozide treatments on testicular development and luteinizing hormone levels in juvenile and early maturing striped bass, Morone saxatilis

The present study was conducted to test the responsiveness of the juvenile male reproductive axis to hormonal stimulation and to compare it to that of early maturing males. Long-term treatments with various combinations of T, GnRHa and pimozide did not result in an increased incidence of early maturing males, but did stimulate spermatogenesis slightly in juvenile fish. In early maturing males, the treatments appeared to be inhibitory since they resulted in a reduction of the GSI and a lower incidence of spermiating males. In early maturing males, pituitary LH content was elevated by GnRHa treatments alone while in juvenile males a combination of T and GnRHa was needed to increase the levels of LH in the pituitary. Thus, T may play an important role during puberty by potentiating the effects of GnRH on LH synthesis. In both juvenile and early maturing males, plasma LH levels could be increased only by high doses of GnRHa (in combination with T). Therefore, LH synthesis and release probably require different levels of GnRH stimulation. A GnRH challenge (single injection of 50 microg GnRHa/kg) at the end of the experiment resulted in a dramatic elevation of plasma LH levels in almost all animals. This finding demonstrates that pituitaries from juvenile and early maturing males were responsive to GnRHa stimulation, even after long-term hormonal treatments. The addition of pimozide did not affect the T- and GnRHa-induced increase in pituitary LH content but inhibited the release of LH in response to a GnRHa challenge. In conclusion, high doses of GnRHa in combination with T can increase plasma LH levels in juvenile males but do not induce complete testicular maturation. Factors other than T, GnRHa or LH are probably involved in the induction and completion of spermatogenesis.

Stress adaptation, cortisol and pubertal development in the male common carp, Cyprinus carpio

This paper reviews a series of recent studies on the effect of adaptation to chronic stress on pubertal development in the common carp. In pre-pubertal male common carp adaptation to temperature stress caused a retardation of testicular development. Stress-induced delay of the first wave of spermatogenesis could be prevented by treatment with a cortisol antagonist, indicating that the stress effect is mediated by cortisol. Chronically elevated cortisol levels affected all parts of the brain-pituitary-gonad (BPG)-axis. In the hypothalamus lower levels of sGnRH were observed, in the pituitary the steady state levels of FSHbeta-m RNA were decreased, while the testicular production of especially the 11-oxygenated androgens 11-ketoandrostenedione (OA) and 11keto-testosterone (11KT) was strongly diminished. OA and 11KT have been shown to promote testicular development in fish. The LH-induced androgen synthesis in vitro was strongly inhibited by cortisol and its agonist dexamethasone. Although cortisol was shown also to interfere with the synthesis of the 11-oxygenated androgens in vivo, the lower androgen levels induced by cortisol were mainly due to the reduced testicular mass. Restoration of the plasma concentrations of these androgens by implantation could not prevent the cortisol-induced retardation of testicular growth and the first wave of spermatogenesis. Therefore, it is suggested that cortisol acts directly on Sertoli cells and/or on germ cells, which is supported by the demonstration of GRs on germ cells. We have little indication that the cortisol-induced retardation of testicular development is mediated by a decreased secretion of LH, but a crucial role for FSH can not be excluded.

Corticosteroids affect the testicular androgen production in male common carp (Cyprinus carpio L.)

Our previous experiments to study the effect of stress adaptation on pubertal development in carp showed that repeated temperature stress and prolonged feeding with cortisol-containing food pellets, which mimics the endocrine stress effects, retarded the first waves of spermatogenesis and decreased 11-ketotestosterone (11KT) plasma levels. The objective of the present study was to investigate whether the decrease in plasma 11KT is caused by a direct effect of cortisol on the steroid-producing capacity of the testis or by an indirect effect, such as a decrease in plasma LH. Pubertal and adolescent isogenic male common carp (Cyprinus carpio L.) were fed with either cortisol-containing food pellets or control food pellets over a prolonged period. Our results indicate that cortisol has a direct inhibitory effect on the testicular androgen secretion independent of the LH secretion. Furthermore, the pubertal period is critical to the influence of cortisol regarding testicular androgen secretion, because the effect is no longer observed at adolescence.

Testosterone inhibits 11-ketotestosterone-induced spermatogenesis in African catfish (Clarias gariepinus)

Male fish produce 11-ketotestosterone as a potent androgen in addition to testosterone. Previous experiments with juvenile African catfish (Clarias gariepinus) showed that 11-ketotestosterone, but not testosterone, stimulated spermatogenesis, whereas testosterone, but not 11-ketotestosterone, accelerated pituitary gonadotroph development. Here, we investigated the effects of combined treatment with these two types of androgens on pituitary gonadotroph and testis development. Immature fish were implanted for 2 wk with silastic pellets containing 11-ketotestosterone, testosterone, 5alpha-dihydrotestosterone, or estradiol-17beta; cotreatment groups received 11-ketotestosterone in combination with one of the other steroids. Testicular weight and pituitary LH content were higher (two- and fivefold, respectively) in the end control than in the start control group, reflecting the beginning of normal pubertal development. Treatment with testosterone or estradiol-17beta further increased the pituitary LH content four- to sixfold above the end control levels. This stimulatory effect on the pituitary LH content was not modulated by cotreatment with 11-ketotestosterone. However, the stimulatory effect of 11-ketotestosterone on testis growth and spermatogenesis was abolished by cotreatment with testosterone, but not by cotreatment with estradiol-17beta or 5alpha-dihydrotestosterone. Also, normal pubertal testis development was inhibited by prolonged (4 wk) treatment with testosterone. The inhibitory effect of testosterone may involve feedback effects on pituitary FSH and/or on FSH receptors in the testis. It appears that the balanced production of two types of androgens, and the control of their biological activities, are critical to the regulation of pubertal development in male African catfish.

Gonadal steroids and the maturation of the species-specific gonadotropin-releasing hormone system in brain and pituitary of the male African catfish (Clarias gariepinus)

The effect of testosterone (T), 11-ketotestosterone (KT) and estradiol (E(2)) on the development of the catfish gonadotropin-releasing hormone system (cfGnRH) of male African catfish (Clarias gariepinus), at the onset of puberty [between 10 and 12 weeks post hatching (ph)] was investigated. The cfGnRH neurons, located in the ventral forebrain, were visualized by immunofluorescence and their numbers were determined and the amounts of cfGnRH-associated peptide (cfGAP) in the pituitary were measured by RIA. Steroid treatments did not significantly alter the numbers of immunoreactive GnRH neurons. However, T and E(2) caused an increase in the amount of GnRH, demonstrated by the intensity of the immunostaining of GnRH neurons and fibers in the brain and the amount of cfGAP in the pituitary. Treatment with KT, the main circulating androgen in adult male catfish, neither changed the number of cfGnRH neurons, nor elevated the cfGnRH content in the pituitary. In previous experiments with younger, prepubertal fish (2-6 weeks ph), T caused an elevation of the number of cfGnRH neurons to the same level as present in pubertal fish of 12-14 weeks. We conclude that the onset of puberty in the male African catfish coincides with the completion of the steroid-dependent structural maturation of the cfGnRH system in the brain. T and/or E(2), however, are still able to exert a positive influence on the amounts of cfGnRH during the later stages of pubertal development, thus still playing a role in the control of the cfGnRH system.

Steroid hormones stimulate gonadotrophs in juvenile male African catfish (Clarias gariepinus)

In juvenile African catfish (Clarias gariepinus), the pituitary LH content strongly increased after the beginning of spermatogonial proliferation. We hypothesized that a signal of testicular origin is involved in stimulating the gonadotrophs. We investigated the effects of castration and sex steroid treatment on gonadotrophs in juvenile males by quantifying LH production and release and LH subunit transcript levels and by examining gonadotroph morphology and proliferation. Castration reduced but did not abolish the maturation-associated elevation in pituitary LH content. Treatment with testosterone but not with 11-ketotestosterone, an otherwise potent androgen in fish, reversed the castration-induced decrease of pituitary LH levels. An increased pituitary LH content was accompanied by an increased number of cytologically mature gonadotrophs. However, no evidence was found for gonadotroph proliferation, so that quiescent gonadotrophs may have become activated. Although 11-ketotestosterone treatments had no effect in castrated males, this androgen attenuated gonadotroph activation in intact males. Because androgen production in juvenile catfish is downregulated by treatment with 11-ketotestosterone, its inhibitory effects on gonadotrophs in gonad-intact males may be due to suppression of Leydig cell testosterone production, which appears to be a limiting factor for the activation of catfish gonadotrophs. Aromatizable androgens may have opposite effects on fish (stimulatory) and mammalian (inhibitory) gonadotrophs.

Sex steroids and the initiation of puberty in male African catfish (Clarias gariepinus)

The effects of sex steroids on spermatogenesis and testicular androgen secretion were studied in juvenile (spermatogonia present in testes) African catfish. Fish were implanted with Silastic pellets containing 11-ketotestosterone (KT), 11beta-hydroxyandrostenedione (OHA), androstenetrione (OA), androstenedione (A), testosterone (T), 5alpha-dihydrotestosterone (DHT), or estradiol-17beta (E2). Control groups received steroid-free pellets. Two weeks later, testis tissue fragments were incubated with African catfish luteinizing hormone (LH) and the amount of OHA secreted in vitro (the main androgen produced by African catfish testes) was quantified. Tissue fragments were then fixed for histological analysis of spermatogenesis. Treatment with KT, OHA, and OA stimulated testicular growth and spermatogenesis (spermatocytes and spermatids were found), whereas T, DHT, A, or E2 had no such effects. All steroids, except for DHT and E2, reduced OHA secretion in the absence and presence of LH to approximately 10% of the control values. Previous studies have shown that KT, OHA, and OA have little effect on circulating LH levels in juvenile male African catfish, so that these androgens probably had direct effects on the testis. Inasmuch as OHA, OA, and KT have largely similar effects and because OHA and OA are converted to KT in vivo, we suggest that KT is physiologically the most relevant androgen for the initiation of spermatogenesis in African catfish.

Fish as models for the neuroendocrine regulation of reproduction and growth

Models are essential for the full understanding of neuroendocrine control processes. In this regard fish offer a rich source of biological material. They have diverse growth and reproductive strategies, inhabiting most of the Earth's aquatic ecological niches. Fish possess many of the common vertebrate features but also offer several unique aspects to allow the biologist easy access to the study of hypothalamic and pituitary function. Several key examples of how teleosts, or the bony fish, can offer insight into fundamental mechanisms of vertebrate sex differentiation, growth and reproduction are reviewed.

Modulation of testicular androgen production in adolescent African catfish (Clarias gariepinus)

At 6 months of age the first spermatozoa appear in the testes of the African catfish considered to be adolescent, since the development to adulthood (12 months of age) is accompanied by further morphological and functional differentiation of Leydig cells. There are increasing plasma levels of 11-ketotestosterone (11-KT) and an increasing responsiveness to luteinizing hormone (LH) of testicular androgen secretion in vitro. Whether treatment of adolescent males with key hormones of the brain-pituitary-gonad axis [gonadotropin-releasing hormone (GnRH), LH, and 11-KT] affects the testicular steroidogenic response to a challenge with LH in vitro 7 days later has been investigated. Injection of GnRH (2.5 microg chicken GnRH-II per kilogram of body weight), LH (25 microg/kg), or a high dose of 11-KT (50 microg/kg) down-regulated basal and LH-stimulated testicular androgen secretion to a minimum of 35% of control values. Treatment with LH was, moreover, associated with changes in the ultrastructure of Leydig cell mitochondria which were either swollen and had a less electron-dense matrix or showed an elongated shape. Conversely, a moderate dose of 11-KT (20 microg/kg) enhanced LH-stimulated, but not basal, androgen secretion in vitro to a maximum of 190% of control values. In view of the generally low LH plasma levels and of the steadily increasing 11-KT plasma levels during puberty, 11-KT may be involved in the up-regulation of the testicular steroidogenic capacity observed during development to full maturity.