Role of calcium in the control of steroidogenesis in preovulatory ovarian follicles of the goldfish

Expression Patterns and Gonadotropin Regulation of the TGF-β II Receptor (Bmpr2) during Ovarian Development in the Ricefield Eel Monopterus albus

Bmpr2 plays a central role in the regulation of reproductive development in mammals, but its role during ovarian development in fish is still unclear. To ascertain the function of bmpr2 in ovarian development in the ricefield eel, we isolated and characterized the bmpr2 cDNA sequence; the localization of Bmpr2 protein was determined by immunohistochemical staining; and the expression patterns of bmpr2 in ovarian tissue incubated with FSH and hCG in vitro were analyzed. The full-length bmpr2 cDNA was 3311 bp, with 1061 amino acids encoded. Compared to other tissues, bmpr2 was abundantly expressed in the ovary and highly expressed in the early yolk accumulation (EV) stages of the ovary. In addition, a positive signal for Bmpr2 was detected in the cytoplasm of oocytes in primary growth (PG) and EV stages. In vitro, the expression level of gdf9, the ligand of bmpr2, in the 10 ng/mL FSH treatment group was significantly higher after incubation for 4 h than after incubation for different durations. However, bmpr2 expression in the 10 ng/mL FSH treatment group at 2 h, 4 h and 10 h was significantly lower. Importantly, the expression level of bmpr2 and gdf9 in the 100 IU/mL hCG group had similar changes that were significantly decreased at 4 h and 10 h. In summary, Bmpr2 might play a pivotal role in ovarian growth in the ricefield eel, and these results provide a better understanding of the function of bmpr2 in ovarian development and the basic data for further exploration of the regulatory mechanism of gdf9 in oocyte development.

Membrane receptor cross talk in gonadotropin-, IGF-I-, and insulin-mediated steroidogenesis in fish ovary: An overview

Gonadal steroidogenesis is critical for survival and reproduction of all animals. The pathways that regulate gonadal steroidogenesis are therefore conserved among animals from the steroidogenic enzymes to the intracellular signaling molecules and G protein-coupled receptors (GPCRs) that mediate the activity of these enzymes. Regulation of fish ovarian steroidogenesis in vitro by gonadotropin (GtH) and GPCRs revealed interaction between adenylate cyclase and calcium/calmodulin-dependent protein kinases (CaMKs) and also MAP kinase pathway. Recent studies revealed another important pathway in GtH-induced fish ovarian steroidogenesis: cross talk between GPCRs and membrane receptor tyrosine kinases. Gonadotropin binding to Gαs-coupled membrane receptor in fish ovary leads to production of cAMP which in turn trans-activate the membrane-bound epidermal growth factor receptor (EGFR). This is followed by activation of ERK1/2 signaling that promotes steroid production. Interestingly, GtH-induced trans-activation of EGFR in the fish ovary uniquely requires matrix-metalloproteinase-mediated release of EGF. Inhibition of these proteases blocks GtH-induced steroidogenesis. Increased cAMP production in fish ovarian follicle upregulate follicular cyp19a1a mRNA expression and aromatase activity leading to increased biosynthesis of 17β-estradiol (E2). Evidence for involvement of SF-1 protein in inducing cyp19a1a mRNA and aromatase activity has also been demonstrated. In addition to GtH, insulin-like growth factor (IGF-I) and bovine insulin can alone induced steroidogenesis in fish ovary. In intact follicles and isolated theca cells, IGF-I and insulin had no effect on GtH-induced testosterone and 17a,hydroxysprogeaterone production. GtH-stimulated E2 and 17,20bdihydroxy-4-pregnane 3-one production in granulosa cells however, was significantly increased by IGF-I and insulin. Both IGF-I and insulin mediates their signaling via receptor tyrosine kinases leading to activation of PI3 kinase/Akt and MAP kinase. These kinase signals then activates steroidogenic enzymes which promotes steroid production. PI3 kinase, therefore considered to be an initial component of the signal transduction pathways which precedes MAP kinase in IGF-1 and insulininduced steroidogenesis in fish ovary. Thus, investigation on the mechanism of signal transduction regulating fish ovarian steroidogenesis have shown that multiple, apparently independent signal transduction pathways are needed to convey the message of single hormone or growth factor.

Estradiol signaling in the regulation of reproduction and energy balance

Our knowledge of membrane estrogenic signaling mechanisms and their interactions that regulate physiology and behavior has grown rapidly over the past three decades. The discovery of novel membrane estrogen receptors and their signaling mechanisms has started to reveal the complex timing and interactions of these various signaling mechanisms with classical genomic steroid actions within the nervous system to regulate physiology and behavior. The activation of the various estrogenic signaling mechanisms is site specific and differs across the estrous cycle acting through both classical genomic mechanisms and rapid membrane-initiated signaling to coordinate reproductive behavior and physiology. This review focuses on our current understanding of estrogenic signaling mechanisms to promote: (1) sexual receptivity within the arcuate nucleus of the hypothalamus, (2) estrogen positive feedback that stimulates de novo neuroprogesterone synthesis to trigger the luteinizing hormone surge important for ovulation and estrous cyclicity, and (3) alterations in energy balance.

Ca(2+)-Calmodulin regulation of testicular androgen production in Mozambique tilapia (Oreochromis mossambicus)

The Ca(2+)-Calmodulin (CaM) signaling pathway has previously been shown to be involved in the regulation of teleost fish ovarian steroidogenesis. However, a putative role of CaM in testicular steroidogenesis and potential targets has not been examined. To examine whether basal steroidogenesis is modulated by Ca(2+) and CaM levels in the testis of Mozambique tilapia (Oreochromis mossambicus) we have incubated testicular fragments in vitro under different conditions and analyzed steroid output. Calcium-free medium with or without EGTA did not affect testicular basal 11-ketotestosterone (11-KT) and testosterone (T) secretion. However, addition of 80microM the CaM inhibitor W7 significantly reduced basal 11-KT, T and androstenedione secretion. Interestingly, the decreased androgen production by 80microM of W7 was accompanied by increased 11-desoxicortisol output and by the activation of cortisol synthesis in the testis, the latter undetected in untreated tissues. However, production of 17,20alpha-dihydroxy-4-pregnen-3-one was unaltered by W7. This suggests that C17,20 desmolase, 21-hydroxylase and possibly 11beta-hydroxysteroid dehydrogenase are targets for CaM. In addition, androgen production was also found to be regulated by the level of cAMP since incubations with forskolin (FK) significantly increased 11-KT and T output. A cross-talk between the cAMP and Ca(2+)-CaM signaling pathways was detected since W7 administration also decreased FK stimulated androgen production. Altogether, these data show that both basal and cAMP stimulated androgen levels were modulated by intracellular Ca(2+)-dependent CaM and that possibly Ca(2+)-CaM determines the shift in steroidogenesis from C21 steroids to androgens.

Estradiol stimulates progesterone synthesis in hypothalamic astrocyte cultures

The brain synthesizes steroids de novo, especially progesterone. Recently estradiol has been shown to stimulate progesterone synthesis in the hypothalamus and enriched astrocyte cultures derived from neonatal cortex. Estradiol-induced hypothalamic progesterone has been implicated in the control of the LH surge. The present studies were undertaken to determine whether hypothalamic astrocytes derived from female neonatal or female postpubertal rats increased production of progesterone in response to an estradiol challenge. Estradiol induced progesterone synthesis in postpubertal astrocytes but not neonatal astrocytes. This estradiol action was blocked by the estrogen receptor antagonist ICI 182,780. Previously we had demonstrated that estradiol stimulates a rapid increase in free cytosolic Ca(2+) ([Ca(2+)](i)) spikes in neonatal cortical astrocytes acting through a membrane estrogen receptor. We now report that estradiol also rapidly increased [Ca(2+)](i) spikes in hypothalamic astrocytes. The membrane-impermeable estradiol-BSA construct also induced [Ca(2+)](i) spikes. Both estradiol-BSA and estradiol were blocked by ICI 182,780. Depleting intracellular Ca(2+) stores prevented the estradiol-induced increased [Ca(2+)](i) spikes, whereas removing extracellular Ca(2+) did not prevent estradiol-induced [Ca(2+)](i) spikes. Together these results indicate that estradiol acts through a membrane-associated receptor to release intracellular stores of Ca(2+). Thapsigargin, used to mimicked the intracellular release of Ca(2+) by estradiol, increased progesterone synthesis, suggesting that estradiol-induced progesterone synthesis involves increases in [Ca(2+)](i). Estradiol treatment did not change levels of steroid acute regulatory protein, P450 side chain cleavage, 3beta-hydroxysteroid dehydrogenase, and sterol carrier protein-2 mRNAs as measured by quantitative RT-PCR, suggesting that in vitro, estradiol regulation of progesterone synthesis in astrocytes does not depend on transcription of new steroidogenic proteins. The present results are consistent with our hypothesis that estrogen-positive feedback regulating the LH surge involves stimulating local progesterone synthesis by hypothalamic astrocytes.

Gonadotropin regulation of testosterone production by primary cultured theca and granulosa cells of Atlantic croaker: I. Novel role of CaMKs and interactions between calcium- and adenylyl cyclase-dependent pathways

Theca and granulosa cells for in vitro primary culture were obtained by enzymatic digestion of mature ovarian tissue from Atlantic croaker (Micropogonias undulatus) and separation from the other cell types by Percoll density-gradient centrifugation. Histochemical staining and treatment with pregnenolone confirmed the presence in the cultured cells of enzymes involved in synthesizing the major sex steroids in croaker ovaries: testosterone, estradiol, and 17alpha,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S). Croaker theca and granulosa cells maintained their steroidogenic response to gonadotropin when cultured with serum-supplemented media and produced high levels of testosterone for up to 5 days, although estradiol production was low. Multiple signal transduction pathways mediating gonadotropin stimulation of androgen production were identified in Atlantic croaker ovarian theca and granulosa cells in primary co-culture. Inhibitors of voltage-sensitive calcium channels (VSCCs) and calmodulin decreased the steroidogenic response to gonadotropin, whereas activators of adenylyl cyclase and protein kinase A (PKA) increased testosterone production, indicating that both calcium and PKA-dependent signaling pathways are involved in the regulation of follicular steroid production. In addition, the first evidence in vertebrates for an involvement of calcium/calmodulin-dependent protein kinases (CaMKs) in gonadal steroidogenesis was obtained, since the stimulatory effects of gonadotropin on testosterone media accumulation were attenuated by specific inhibitors of CaMKs. Some interactions among the signaling pathways were observed as demonstrated by the positive effect of elevated intracellular calcium on adenylyl cyclase activity and the reduction of forskolin- and dbcAMP-induced testosterone production by inhibitors of VSCCs, calmodulin, and CaMKs.

Involvement of calcium and calmodulin in the regulation of ovarian steroidogenesis in Atlantic croaker (Micropogonias undulatus) and modulation by Aroclor 1254

The involvement of calcium-dependent signal transduction pathways in the regulation of ovarian steroidogenesis was investigated in Atlantic croaker. Treatment with the calcium ionophores A23187 and ionomycin caused a 2- to 5-fold increase in basal steroid accumulation by croaker ovarian tissue in vitro. A23187 potentiated human chorionic gonadotropin (hCG)-induced testosterone (T) accumulation, whereas it inhibited accumulation of estradiol-17beta (E(2)) and the conversion of T to E(2), suggesting that intracellular calcium modulates aromatase enzyme activity. Gonadotropin stimulation of ovarian steroidogenesis was decreased in the presence of EGTA and inhibitors of voltage-sensitive calcium channels (VSCCs) and inositol-1,4,5-triphosphate-receptors (IP(3)Rs), indicating that releases of calcium from both intracellular and extracellular stores are components of the signal transduction pathways initiated by gonadotropin. Calmodulin is also involved in the regulation of ovarian steroidogenesis in croaker, since the calmodulin inhibitors W-7 and trifluoperazine (TFP) attenuated hCG-stimulated T and E(2) accumulation. These results are broadly similar to those reported previously in goldfish and suggest that the major calcium-dependent signaling pathways involved in gonadotropin stimulation of ovarian steroidogenesis in tetrapods are also present in teleosts. In addition, the involvement of calcium in the regulation of aromatase activity was demonstrated for the first time in a vertebrate ovary. Finally, acute exposure to 0.001-1 ppm Aroclor 1254 induced up to a 5-fold increase in hCG-stimulated E(2) accumulation, and this effect was attenuated by co-treatment with inhibitors of VSCCs and calmodulin, suggesting the existence of a novel mechanism of endocrine disruption by an environmental contaminant involving alteration of calcium-dependent signaling pathways regulating steroidogenesis.

A comparative assessment of the adrenotoxic effects of cadmium in two teleost species, rainbow trout, Oncorhynchus mykiss, and yellow perch, Perca flavescens

Rainbow trout (Oncorhynchus mykiss) and yellow perch (Perca flavescens) have a different sensitivity to cadmium (Cd) in vivo (trout < LC50 < perch). Metals and particularly Cd impair cortisol secretion by adrenocortical cells in both species. The purpose of the present study was to assess in vitro the effect of Cd on cortisol secretion by adrenocortical cells of trout and perch, to compare the sensitivity of adrenal steroidogenesis in these two teleosts. Adrenocortical cells were exposed to Cd for 60 min, then stimulated with ACTH, dbcAMP or with pregnenolone, a cortisol precursor. Cd inhibited ACTH-stimulated cortisol secretion in a dose-dependent manner in both fish species, however, the EC50s (concentration resulting in 50% inhibition of cortisol secretion) was significantly lower in trout (EC50 = 0.09 mM) than perch (EC50 = 0.26 mM). To test the specificity of Cd to act as an endocrine disrupter, the LC50 (concentration that kills 50% of the cells) was also evaluated to determine the LC50/EC50 ratio ( LC50/EC50(O.mykiss) = 175.6 > LC50/EC50(P.flavescens) = 37.7). Adrenocortical cells of trout were more sensitive than those of perch and Cd had a higher endocrine-disrupting potential and specificity in trout than in perch. However, in both species, Cd had the same effect on ACTH, dbcAMP and pregnenolone-stimulated cortisol secretion, with pregnenolone maintaining cortisol secretion until cell viability was impaired. These results confirm that for both species, Cd interferes in the signalling pathway of cortisol synthesis in a step prior to the pregnenolone formation. Data provided by the present study revealed important differences in vulnerability of adrenal steroidogenesis between rainbow trout and yellow perch.

Role of Ca2+ stores in dopamine- and PACAP-evoked growth hormone release in goldfish

Growth hormone (GH) secretion, evoked by either pituitary adenylate cyclase-activating polypeptide (PACAP) or dopamine (DA), is dependent on both voltage-sensitive calcium channels (VSCC) and cAMP signaling in goldfish. We further characterized the involvement of Ca2+ in evoked release by PACAP and DA, by examining the sensitivity of evoked GH release to perturbations of Ca2+ signaling. Both VSCC and calmodulin/calmodulin-dependent kinase are involved in PACAP signaling as had been shown for DA. In spite of this apparent dependence on VSCC, blockade of TMB-8 but not ryanodine-sensitive intracellular Ca2+ stores inhibited both PACAP- and DA-evoked GH release. Using sarcoplasmic/endoplasmic reticulum Ca-ATPases (SERCA) inhibitors, we found BHQ blocked, whereas thapsigargin (Tg) enhanced stimulated GH release, suggesting that Tg-sensitive SERCA may counteract these cAMP-mobilizing neuroendocrine regulators by sequestering [Ca2+]i. As GH secretion stimulated by two endogenous gonadotropin-releasing hormones is not affected by Tg, it appears that distinct multiple Ca2+ stores mediate the hormone releasing response to different neuroendocrine regulators.

Activin and transforming growth factor-beta as local regulators of ovarian steroidogenesis in the goldfish

This study explores the hypothesis that activin and TGFbeta(1) serve as local regulators of ovarian function in the goldfish. Initial studies demonstrated the presence of TGFbeta in the ovary through the use RT-PCR, which amplified a 225 bp product from early vitellogenic (EVIT) and prematurational full-grown (PFG) follicles. This transcript showed high homology to TGFbeta in other teleosts. Both goldfish recombinant activin B and human recombinant TGFbeta(1) suppressed basal testosterone production by EVIT follicles incubated in vitro. Activin B also inhibited hCG-stimulated testosterone production by EVIT follicles. Our experiments suggest that activin B mediates these effects through actions at sites upstream of cholesterol formation and/or mobilization in the steroidogenic pathway, and through mechanisms that were independent of effects on cAMP formation. In experiments with PFG follicles, TGFbeta(1) decreased basal testosterone production. Activin B did not affect T production by PFG follicles, suggesting that this hormone has differential effects on steroidogenesis in the goldfish ovary depending on the stage of ovarian maturity. In other tests with PFG follicles, TGFbeta(1) and activin B, to a limited extent, inhibited the conversion of 17 alpha-OHP to the maturation-inducing hormone, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one. In conclusion, this study shows that TGF is expressed in the goldfish ovary, and that both activin and TGFbeta affect steroid production, which provides evidence that these members of the TGFbeta superfamily may act as local regulators of ovarian function in a teleost.

Cloning of transforming growth factor-beta 1 (TGF-beta 1) and its type II receptor from zebrafish ovary and role of TGF-beta 1 in oocyte maturation

TGF-beta is a multifunctional factor involved in regulating a variety of cellular activities. In mammals, TGF-beta is known to regulate reproduction, including ovarian functions. The role of TGF-beta in lower vertebrates, such as fish, is poorly understood. To examine the role of TGF-beta in fish reproduction, cDNAs encoding TGF-beta 1 and the type II TGF-beta receptor (T beta RII) were cloned from the zebrafish ovary using PCR- based strategies. The mature peptide region of the zebrafish TGF-beta 1 shows 70-85% identity with TGF-beta 1 from other species. The zebrafish T beta RII cDNA sequence is the first to be reported from a fish species, and it shows a high level of conservation at the kinase domain. Using RT-PCR, we have detected mRNA expression of TGF-beta 1, T beta RII, as well as its downstream signaling molecules Smad2, 3, and 4 in ovarian follicles at different stages of development. In addition, we have examined the effect of TGF-beta 1 on oocyte maturation. TGF-beta 1 significantly inhibited both gonadotropin- and 17 alpha, 20 beta-dihydroxyprogesterone-induced oocyte maturation in a dose- and time-dependent manner. These findings demonstrate, for the first time, that TGF-beta 1 plays a role in regulating oocyte maturation in fish and suggest that a TGF-beta/Smad signaling pathway is present in the zebrafish ovary.

Stimulatory effects of selected PAHs on testosterone production in goldfish and rainbow trout and possible mechanisms of action

This study investigates the effects of selected polycyclic aromatic hydrocarbons (PAHs) on testicular function in fish. In vitro incubations of goldfish testis tissue exposed to three selected PAHs [naphthalene, beta-naphthoflavone (betaNF), or retene] resulted in biphasic potentiation of gonadotropin (GtH)-stimulated testosterone (T) production, with no effect on basal levels. In vivo exposure of goldfish to betaNF also resulted in biphasic increases in plasma T levels. Experiments with rainbow trout revealed increased levels of in vitro T production when exposed to naphthalene, albeit only at higher concentrations than the responses observed in goldfish. Studies of possible sites of action at the level of the goldfish testis showed that betaNF exerted its most potent action when T production was initiated by a ligand such as GtH or prostaglandin E(2). Within the steroidogenic pathway, betaNF potentiation of GtH-stimulated T production may be partially mediated by increases in cAMP production, but appear to be independent of cytosolic calcium content. These studies demonstrate that PAHs may function as disruptors of reproductive endocrine function in teleosts through the unexpected effect of enhancing testicular steroidogenesis.

Intracellular calcium in prothoracic glands of Manduca sexta

Cytosolic free calcium was measured in individual prothoracic gland cells of Manduca larvae with Fura-2. During the last larval instar there was no correlation between intracellular calcium concentration and ecdysteroid secretion by the glands. The addition of prothoracicotropic hormone (PTTH) from brains of Manduca larvae to prothoracic glands in vitro resulted in a significant increase in the calcium concentration of the gland cells. The effect of PTTH was inhibited by the inorganic calcium channel antagonists, cadmium, lanthanum and nickel, and by the antagonist of T-type calcium channels, amiloride, whereas all the other antagonists tested failed to block the action of PTTH. TMB-8, an inhibitor of intracellular calcium mobilization, did not reduce the PTTH-induced rise in calcium, which suggests that IP(3)-dependent intracellular calcium stores are not involved in the calcium-mediated stimulation of ecdysteroid synthesis. Moreover, PTTH is thought to increase intracellular calcium in prothoracic glands of Manduca by influencing calcium channels in the plasma membrane.

Inhibition of gonadotropin-stimulated ovarian steroid production by polyunsaturated fatty acids in teleost fish

The effects of the polyunsaturated fatty acids (PUFAs)--eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and arachidonic acid (AA)--on in vitro steroid production by full-grown prematurational ovarian follicles from goldfish and rainbow trout were investigated. EPA and DHA inhibited gonadotropin-stimulated testosterone production in a dose-related manner, but AA was inhibitory only at the highest dose tested (400 microM). AA alone stimulated testosterone production by increasing cAMP production, but the effects of other PUFAs alone were marginal. The inhibitory actions by PUFAs were not restricted to long-chain PUFAs, as linoleic and linolenic acids had similar actions in the goldfish. The inhibitory action of EPA on testosterone production was reversible upon removal of the PUFA from medium. Testosterone production stimulated by the addition of the cAMP analogues, dibutyryl cAMP, and 8-bromo cAMP, was attenuated by PUFAs, suggesting that they act at a site distal to cAMP formation. A post-cAMP site regulating cholesterol availability may be involved as testosterone production induced by addition of 25OH-cholesterol was not affected by the PUFAs in either fish species. Together, these findings underscore the importance of lipids in ovarian physiology and suggest that PUFAs may participate in the regulation of ovarian steroidogenesis in teleost fish.

Regulation of oocyte growth and maturation in fish

This chapter has briefly reviewed the current status of investigations on the hormonal regulation of oocyte growth and maturation in fish (see Figs. 4 and 9). Pituitary gonadotropins are of primary importance in triggering these processes in fish oocytes. In both cases, however, the actions of gonadotropins are not direct, but are mediated by the follicular production of steroidal mediators, estradiol-17 beta (oocyte growth) and 17 alpha,20 beta-DP or 20 beta-S (oocyte maturation). Investigators have established that both estradiol-17 beta and 17 alpha,20 beta-DP are biosynthesized by salmonid ovarian follicles via an interaction of two cell layers, the thecal and granulosa cell layers (two-cell-type model). The granulosa cell layers are the site of production of these two steroidal mediators, but their production depends on the provision of precursor steroids by the thecal cell layers. A distinct steroidogenic shift from estradiol-17 beta to 17 alpha,20 beta-DP, occurring in salmonid ovarian follicles immediately prior to oocyte maturation, is a prerequisite for the growing oocytes to enter the maturation stage, and requires a complex and integrated network of gene regulation involving cell specificity, hormonal regulation, and developmental patterning. The cDNAs for most of the steroidogenic enzymes responsible for estradiol-17 beta and 17 alpha,20 beta-DP biosynthesis have been cloned from rainbow trout ovaries. Our next task is to determine how gonadotropin and other factors act on ovarian follicle cells to turn the expression of these specific genes on and off at specific times during oocyte growth and maturation. Increasing evidence now suggests that a variety of neuromodulatory, autocrine, and paracrine factors may also be involved in the regulation of steroidogenesis in fish ovarian follicles. Molecular biological technologies should be applied to identify these substances. Of considerable interest is the finding that MIH, unlike most steroid hormones, acts on its receptors at the surface of oocytes. Further studies of the association of the MIH-MIH receptor complex with a Gi protein, probably resulting in the inactivation of adenylate cyclase, should lead to a discovery of a new mechanism of steroid hormone action. The early steps following MIH action involve the formation of the major cytoplasmic mediator of MIH, MPF. Fish MPF, like that of Xenopus and starfish, consists of two components: cdc2 kinase and cyclin B. Nevertheless, the mechanism of MIH-induced MPF activation in fish oocytes differs from that in Xenopus and starfish because the appearance of cyclin B protein is a crucial step for 17 alpha,20 beta-DP-induced oocyte maturation in fish.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular endocrinology of oocyte growth and maturation in fish

Pituitary gonadotropins (GTHs) are of primary importance in triggering oocyte growth and maturation. However, the actions of GTHs are not direct, but are mediated by the ovarian production of steroidal mediators of oocyte growth (estradiol-17β) and maturation (maturation-inducing hormone, MIH; 17α,20β-dihydroxy-4-pregnen-3-one, 17α,20β-DP in salmonid fishes; 17α,20β,21-trihydroxy-4-pregnen-3-one, 20β-S in sciaenid fishes). It is established that production of estradiol-17β and 17α,20β-DP by salmonid ovarian follicles occurs via the interaction of two cell layers, the thecal and granulosa cell layers (two-cell type model). A distinct shift in the salmonid steroidogenesis from estradiol-17β to 17α,20β-DP occurs in the ovarian follicle layer immediately prior to oocyte maturation. It is possible that this shift is a consequence of dramatic changes in the expression of the genes encoding various steroidogenic enzymes. As an initial step to address this question, we have isolated and characterized the cDNAs encoding a number of ovarian steroidogenic enzymes including the rainbow trout cholesterol side-chain cleavage cytochrome P-450, 3β-hydroxysteroid dehydrogenase (HSD), 17α-hydroxylase/17,20 lyase cytochrome P-450, aromatase cytochrome P-450 cDNAS as well as the pig 20β-HSD cDNA.Estradiol-17β stimulates the hepatic synthesis and secretion of a yolk precursor, vitellogenin. Vitellogenin is then transported to the ovary where it is selectively taken up into the oocyte by a receptor-mediated process involving specific cell-surface receptors. Estradiol-17β was also shown to induce the synthesis of egg membrane proteins in the liver. The maturation-inducing action of 17α,20β-DP and 20β-S is through the binding to the oocyte plasma membrane. This initial MIH-surface interaction is followed by the formation of the major mediator of MIH, maturation-promoting factor (MPF). We have purified MPF from mature oocytes of carp. Carp MPF consists of two components: the homolog of the cdc2(+) gene product of fission yeast (p34(cdc2)) and cyclin B. The cdc2 kinase protein is present in immature oocytes as well as in oocytes induced to mature by 17α,20β-DP treatment, while cyclin B proteins can be detected only in mature oocytes. Addition of bacterially expressed goldfish cyclin B to the extracts of immature goldfish oocytes induced MPF activation. These results suggest that the appearance of cyclin B protein is a crucial step for 17α,20β-DP-induced oocyte maturation in fish.

Steroidogenesis in Fundulus heteroclitus. IV. Dichotomous effects of a phorbol ester on ovarian steroid production and oocyte maturation

The possible role of protein kinase C (PKC) activation in mediating the stimulatory actions of a Fundulus pituitary extract (FPE) on ovarian steroidogenesis and oocyte maturation was investigated. The phorbol ester, phorbol 12-myristate 13-acetate (PMA), alone slightly increased basal 17 alpha-hydroxy,20 beta-dihydroprogesterone (DHP) and 17 beta-estradiol (E2) synthesis and significantly stimulated germinal vesicle breakdown (GVBD). Addition of FPE promoted synthesis of DHP, testosterone (T), and E2, and initiated GVBD. Phorbol ester inhibited FPE-induced steroidogenesis but increased the number of oocytes that underwent GVBD. Phorbol ester also markedly impeded induction of steroidogenesis by dibutyryl cAMP and differentially affected the conversion of 25-hydroxycholesterol, pregnenolone, or progesterone to DHP, T, and E2: DHP production was not affected; T production diminished; and E2 synthesis increased (T aromatization also increased). These results suggest an inhibitory role for the PKC pathway on FPE-induced ovarian steroid production, with PMA appearing to affect various steroidogenic steps. The stimulatory action of PMA on oocyte maturation seems to be independent of follicular steroid production since aminoglutethimide, an inhibitor of steroidogenesis, did not block PMA-induced GVBD. Moreover, PMA had a marked stimulatory effect on GVBD in denuded oocytes. Thus, in contrast to the inhibitory role found for the PKC pathway on ovarian follicular steroidogenesis, activation of PKC in the oocyte may serve as a signal-transducing mechanism leading to GVBD.

The control of testicular androgen production in the goldfish: effects of activators of different intracellular signalling pathways

The putative roles of different signal transduction pathways in the regulation of testicular androgen production in goldfish were investigated. In addition to the role of the gonadotropin-adenylate cyclase pathway, which was studied using human chorionic gonadotropin and forskolin, we determined the effects of changes in intracellular calcium content and protein kinase C activation on androgen production using calcium ionophore A23187 and phorbol 12-myristate 13-acetate (PMA), respectively. Testis fragments incubated in vitro respond to hCG in a time- and dose-dependent manner with a resultant increase in the secretion of testosterone (T) and 11-ketotestosterone (11-KT). Although ineffective alone, PMA (400 nM) and A23187 (4000 nM) stimulate a small but significant increase (3-fold above basal) in T production. This response is minor compared to the up to 200-fold increase in T secretion observed in response to either hCG or forskolin. PMA (25-400 nM) alone and A23187 (250-4000 nM) alone inhibit the stimulatory actions of hCG on T production. Unlike PMA, the inactive phorbol 4 alpha-phorbol didecanoate, which does not activate PKC, had no effect on hCG-stimulated T production. PMA and A23187 did not influence the effects of forskolin on T production, suggesting that the compounds exert their effects prior to adenylate cyclase activation. In summary, the present studies suggest that in addition to the stimulatory actions of the adenylate cyclase second messenger system, changes in intracellular calcium content and protein kinase C activation may modulate testicular androgen production in the goldfish.

Biotechnology and aquaculture: The role of cell cultures

Cell culturing complements recombinant DNA technology in the application of biotechnology to aquaculture. Cell cultures can be prepared from the three main groups of multicellular organisms in aquaculture: fish, shellfish, and seaweeds. These cultures can contribute indirectly to the successful farming of these organisms by providing basic insights into how their growth, reproduction, and health can be understood and manipulated. Finally, they can be a direct source of diverse biochemical products for use in aquaculture, medicine and the food industry.