Cloning of cDNA for goldfish activin beta B subunit, and the expression of its mRNA in gonadal and non-gonadal tissues

Genetic analysis of activin/inhibin β subunits in zebrafish development and reproduction

Activin and inhibin are both dimeric proteins sharing the same β subunits that belong to the TGF-β superfamily. They are well known for stimulating and inhibiting pituitary FSH secretion, respectively, in mammals. In addition, activin also acts as a mesoderm-inducing factor in frogs. However, their functions in development and reproduction of other species are poorly defined. In this study, we disrupted all three activin/inhibin β subunits (βAa, inhbaa; βAb, inhbab; and βB, inhbb) in zebrafish using CRISPR/Cas9. The loss of βAa/b but not βB led to a high mortality rate in the post-hatching stage. Surprisingly, the expression of fshb but not lhb in the pituitary increased in the female βA mutant together with aromatase (cyp19a1a) in the ovary. The single mutant of βAa/b showed normal folliculogenesis in young females; however, their double mutant (inhbaa-/-;inhbab-/-) showed delayed follicle activation, granulosa cell hypertrophy, stromal cell accumulation and tissue fibrosis. The ovary of inhbaa-/- deteriorated progressively after 180 dpf with reduced fecundity and the folliculogenesis ceased completely around 540 dpf. In addition, tumor- or cyst-like tissues started to appear in the inhbaa-/- ovary after about one year. In contrast to females, activin βAa/b mutant males showed normal spermatogenesis and fertility. As for activin βB subunit, the inhbb-/- mutant exhibited normal folliculogenesis, spermatogenesis and fertility in both sexes; however, the fecundity of mutant females decreased dramatically at 270 dpf with accumulation of early follicles. In summary, the activin-inhibin system plays an indispensable role in fish reproduction, in particular folliculogenesis and ovarian homeostasis.

Elevated expression of inhibin α gene in sterile allotriploid crucian carp

Inhibin and Activin, belong to the transforming growth factor β superfamily (TGF-β), which associate with the regulation of the reproductive process by the modulation of the hypothalamic-pituitary-gonad (HPG) axis. In this study, we reported the molecular cloning and tissue expression of inhibin α in allotriploid crucian carp and its parent- diploid red crucian carp. The full-length cDNA of inhibin α were respectively 1632 bp and 1642 bp in allotriploids and diploids, which both consisted of a 1044 bp open reading frame (ORF) encoding 347 amino acids. Real-time quantitative PCR (RT-qPCR) showed that allotriploids and diploids had significant expression of inhibin α in testis and ovary, and the expression of inhibin α in the gonads of allotriploids was higher than that of diploids. The immunohistochemistry indicated that the ovarian development of allotriploids was abnormal, and the expression of Inhibin α in the ovary of allotriploids was higher than that of diploids. Results of co-immunoprecitation (co-IP) demonstrated that the Inhibin α and Activin β_A, Inhibin α and Activin β_B can form dimers. These findings suggested that the elevated expression of inhibin α and the competitive binding of Inhibin α subunit with Activin β subunits in allotriploids may be releted to the sterility of allotriploids. Furthermore, these results will facilitate the investigation of reproduction characteristics in allotriploids and provide theoretical basis for the study of polyploid breeding in the future.

Identification and expression of the medaka inhibin βE subunit

Activin E, a member of the TGF-β super family, is a protein dimer of mature inhibin βE subunits. Recently, it is reported that hepatic activin E may act as a hepatokine that alter whole body energy/glucose metabolism in human. However, orthologues of the activin E gene have yet to be identified in lower vertebrates, including fish. Here, we cloned the medaka (Oryzias latipes) activin E cDNA from liver. Among all the mammalian inhibin β subunits, the mature medaka activin E amino acid sequence shares the highest homology with mammalian activin E. Recombinant expression studies suggest that medaka activin E, the disulfide-bound mature form of mature inhibin βE subunits, may exert its effects in a way similar to that in mammals. Although activin E mRNA is predominantly expressed in liver in mammals, it is ubiquitously expressed in medaka tissues. Since expression in the liver was enhanced after a high fat diet, medaka activin E may be associated with energy/glucose metabolism, as shown in mice and human.

Activin/follistatin system in grass carp pituitary cells: - Regulation by local release of growth hormone and luteinizing hormone and its functional role in growth hormone synthesis and secretion

Gonadotrophin regulation by activin/follistatin system is well-documented, but the corresponding effect on growth hormone (GH) has not been fully characterized and with little information available in lower vertebrates, especially in fish models. In grass carp, local interactions of GH and luteinizing hormone (LH) can induce GH release and gene expression at pituitary level via autocrine/paracrine mechanisms. To shed light on the role of activin/follistatin system in GH regulation by local actions of GH and LH, grass carp activin βA and βB were cloned, shown to be single-copy genes expressed in the pituitary, and confirmed to encode activin proteins capable of transactivating promoter with activin-responsive elements. In grass carp pituitary cells, activin A and B were effective in reducing GH secretion and GH cell content with concurrent drop in GH mRNA level whereas the opposite was true for follistatin, the activin-binding protein known to neutralize the effects of endogenous activin. Treatment with activin A and B not only could suppress basal but also inhibit GH mRNA expression induced by GH and human chorionic gonadotropin (hCG), a functional analogue of LH in fish model. Apparently, down-regulation of GH mRNA by activin was mediated by reducing GH transcript stability with concurrent inhibition on GH promoter activity via the SMAD pathway. In reciprocal experiments, GH treatment was found to up-regulate activin βA, activin βB and follistatin mRNA levels in carp pituitary cells but the opposite was noted by removing endogenous GH with GH antiserum. Interestingly, parallel treatment with hCG could also inhibit basal as well as GH-induced activin βA, activin βB and follistatin gene expression. These results, as a whole, indicate that the pituitary activin/follistatin system can serve as a regulatory target for local interactions of GH and LH and contribute to GH regulation by autocrine/paracrine mechanisms in the carp pituitary.

Pituitary gonadotropins FSH and LH are oppositely regulated by the activin/follistatin system in a basal teleost, the eel

European eels are blocked at a prepubertal silver stage due to a deficient production of pituitary gonadotropins. We investigated the potential role of activin/follistatin system in the control of eel gonadotropins. Through the development of qPCR assays for European eel activin β(B) and follistatin, we first analyzed the tissue distribution of the expression of these two genes. Both activin β(B) and follistatin are expressed in the brain, pituitary and gonads. In addition, a striking expression of both transcripts was also found in the retina and in adipose tissue. The effects of recombinant human activins and follistatin on eel gonadotropin gene expression were studied using primary cultures of eel pituitary cells. Activins A and B strongly stimulated FSHβ subunit expression in a time- and dose-dependent manner. In contrast, activin reduced LHβ expression, an inhibitory effect which was highlighted in the presence of testosterone, a known activator of eel LHβ expression. No effect of activin was observed on other pituitary hormones. Follistatin antagonized both the stimulatory and inhibitory effects of activin on FSHβ and LHβ expression, respectively. Activin is the first major stimulator of FSH expression evidenced in the eel. These results in a basal teleost further support the ancient origin and strong conservation of the activin/follistatin system in the control of FSH in vertebrates. In contrast, the opposite regulation of FSH and LH may have emerged in the teleost lineage.

Characterization of inhibin alpha subunit (inha) in the zebrafish: evidence for a potential feedback loop between the pituitary and ovary

Inhibin and activin are closely related disulphide-linked dimers that belong to the transforming growth factor beta superfamily. Although inhibin has been extensively studied in mammals, the information about its existence and function in lower vertebrates is very scarce. Using zebrafish as a model, the present study demonstrated that the inhibin-specific alpha subunit (inha) was predominantly expressed in the gonads and no transcript could be detected in other tissues including the pituitary and brain. In the ovary, the expression of inha was restricted to the somatic follicle cells surrounding the oocyte, together with the beta subunits (inhbaa and inhbb). This was further supported by the absence of its expression in the ovulated unfertilized eggs. During folliculogenesis, inha expression in the follicles slightly but steadily increased from primary growth to the mid-vitellogenic stage; however, its expression surged dramatically at the full-grown stage. Interestingly, the expression level of inha decreased significantly in the follicles whose oocytes were undergoing spontaneous maturation or germinal vesicle breakdown. When tested on cultured ovarian fragments, both goldfish pituitary extract and forskolin significantly stimulated inha expression. Further experiments showed that recombinant zebrafish FSH but not LH significantly increased inha expression in the same assay system. When tested in vitro, human inhibin A exhibited a slight but significant inhibitory effect on 17alpha, 20beta-dihydroxyprogesterone-induced oocyte maturation after 4 h incubation. The stimulation of inha expression by FSH and the potential inhibition of FSH by inhibin suggest a possible existence of a negative feedback loop between the pituitary and ovary in the zebrafish.

Development of real-time RT-PCR assays for eel gonadotropins and their application to the comparison of in vivo and in vitro effects of sex steroids

Gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), are key factors in the brain-pituitary-gonad axis and understanding their regulation remains essential for future management of eel reproduction. In this regard, we developed quantitative real-time RT-PCR (qrtRT-PCR) assays for the expression of European eel LHbeta, FSHbeta and GPalpha subunits, using the Light Cycler system. The qrtRT-PCR was adapted to permit detection of the three gonadotropin subunit mRNAs in individual pituitaries and in dispersed pituitary cells. The validated assays were applied to investigate the effects of sex steroids (estrogens and androgens) on gonadotropin subunit expression, in vivo in steroid-injected eels, and in vitro by steroid treatments of primary cultures of eel pituitary cells. In vivo, a stimulation of LHbeta mRNA was observed after estradiol (E2) treatments, while testosterone (T) or the non-aromatizable androgen dihydrotestosterone (DHT) had no effect. Concerning FSHbeta expression, slight but non-significant decreases were observed after both E2 and androgen treatments. Different results were obtained in vitro: E2 induced an increase in FSHbeta mRNA levels but had no effect on LHbeta expression. In contrast, androgens (T and DHT) stimulated LHbeta expression while no significant variation was observed on FSHbeta mRNA levels following androgen treatment. Concerning the GPalpha mRNA, no significant effect of sexual steroids was observed in vivo or in vitro. This demonstrated specific direct actions of steroids on gonadotropin subunit expression. The differences observed between in vivo and in vitro experiments may be explained by the involvement of cerebral control, including GnRH and dopamine neurons, and their specific regulation by sex steroids. The data indicate that sex steroid feedbacks on gonadotropins are exerted via multiple pathways, indirectly at the brain level and directly on pituitary gonadotrope cells.

Role of inhibin and activin in the modulation of gonadotropin- and steroid-induced oocyte maturation in the teleost Fundulus heteroclitus

BACKGROUND

Activin and inhibin are glycoproteins structurally related to the transforming growth factor-beta superfamily. These peptides were first described as factors that regulate the follicle-stimulating hormone (FSH) at the pituitary level. The possible role of inhibin and activin, at the ovarian level, in mediating the stimulatory actions of a Fundulus pituitary extract (FPE) and 17alpha,20beta-dihydroprogesterone (DHP) on oocyte maturation was investigated in this study.

METHODS

In vitro culture of ovarian follicles and induction of oocyte maturation were carried out in 75% Leibovitz L-15 medium. Follicles or denuded oocytes were exposed to FPE, inhibin, activin, ethanol vehicle (control group), or DHP. The competence of the follicles or denuded oocytes to respond to the hormones was assessed by scoring germinal vesicle breakdown (GVBD) used as an indication of the reinitiation of meiosis or oocyte maturation. DHP level was measured by radioimmunoassay.

RESULTS

Addition of FPE promoted the synthesis of DHP by the granulose cells of fully grown ovarian follicles and thus stimulated GVBD in the oocyte. Presence of porcine inhibin did not hinder the synthesis of DHP stimulated by FPE, although it did inhibit the subsequent GVBD in a dose-dependent manner, suggesting that the action of inhibin was at the oocyte level. Similarly to the findings with FPE, inhibin also blocked the DHP-induced GVBD in intact follicles, as well as the spontaneous and steroid-induced GVBD of denuded oocyte. Inhibin straightforwardly blocked the response to a low dose of DHP throughout the culture period, while higher doses of the steroid appeared to overcome the inhibitory effect especially at later times. In contrast to inhibin, recombinant human activin A significantly enhanced DHP-induced GVBD in a dose-dependent manner after 48 hr, although activin alone was not able to induce GVBD without the presence of the steroid.

CONCLUSION

Taking together with our previous studies that demonstrate the presence of activin/inhibin subunits in the ovary of F. heteroclitus, these in vitro findings indicate that inhibin and activin are local regulators in the teleost ovary and have opposing effects in modulating oocyte maturation.

Cloning of a second form of activin-betaA cDNA and regulation of activin-betaA subunits and activin type II receptor mRNA expression by gonadotropin in the zebrafish ovary

Activins are dimeric proteins consisting of two inhibin beta subunits. Homo- and hetero-dimerizations of two isoforms of beta subunits, betaA and betaB, produce three forms of activins, activin-A, -B, and -AB. Recent studies have suggested that activin-A mediates gonadotropin-induced oocyte maturation in the zebrafish. To further understand the physiological role of activin-A in the zebrafish ovary, we have cloned cDNAs for a second isoform of the activin-betaA subunit and the activin type IIA (ActRIIA) receptor and determined their regulation by gonadotropin. Two sequences were obtained during the cloning of activin-betaA subunit, both of which showed high identity to betaA subunits of other species, and were therefore designated as isoform 1 and 2. Real-time PCR quantification was used to measure mRNA levels of activin-betaA1 and -betaA2, as well as two type II receptors, ActRIIA and ActRIIB, in the zebrafish ovary. Activin-betaA1 mRNA levels in stages III and IV follicles were similar and higher than those in stage II while high activin-betaA2 mRNA levels were only found in stage IV follicles. Highest levels of mRNA expression were detected in small and large stage III follicles for ActRIIA and ActRIIB, respectively. Treatment with human chorionic gonadotropin induced dose- and time-dependent increases in mRNA levels of activin-betaA1 and -betaA, as well as ActRIIA and ActRIIB. These findings further support the involvement of the activin signaling cascade in gonadotropin-regulated gonadal activities.

Cloning of Smad2, Smad3, Smad4, and Smad7 from the goldfish pituitary and evidence for their involvement in activin regulation of goldfish FSHbeta promoter activity

Follicle-stimulating hormone (FSH), a glycoprotein consisting of an alpha subunit and a unique beta subunit, is essential for gonadal development and function in vertebrates including teleosts. FSH is regulated by a variety of neuroendocrine and endocrine factors, and its biosynthesis is primarily determined by the expression of the beta subunit. Although the regulation of FSH biosynthesis has been well documented in mammals, the molecular mechanisms underlying the regulation are poorly understood. Our previous studies demonstrated that activin stimulated goldfish FSHbeta expression in the primary pituitary cell culture and enhanced its promoter activity in the mouse gonadotrope cell line LbetaT-2 cells. However, little is known about the signal transduction pathway involved in the transcriptional activation of this gene by activin. To assess the involvement of intracellular signaling protein Smads in regulating goldfish FSHbeta promoter, we first cloned full-length cDNAs for goldfish Smad2, Smad3, Smad4, and Smad7 from the pituitary. All Smads cloned show high sequence conservation with their mammalian counterparts. The spatial expression of these Smads overlapped with that of activin subunits and its receptors in various tissues examined. In addition, we demonstrated that activin induced Smad3 and Smad7 expression, but not Smad2 and Smad4. Co-transfection of Smad2 or Smad3 cDNA into the LbetaT-2 cells with the reporter construct of goldfish FSHbeta promoter significantly enhanced basal and activin-stimulated reporter (SEAP, secreted alkaline phosphatase) expression, while Smad7 completely blocked basal and Smad2/3-stimulated FSHbeta activity. Interestingly, the effect of Smad3 was much higher than that of Smad2, suggesting that Smad3 is likely the principal signal transducing molecule involved in activin stimulation of FSHbeta expression in the goldfish. This work lays a foundation for further analysis of goldfish FSHbeta promoter for the cis-regulatory elements involved in activin signaling.

Novel expression of gonadotropin subunit genes in oocytes of the gilthead seabream (Sparus aurata)

It is widely believed that FSH and LH, which are known to play key roles in controlling the production of functional oocytes in vertebrates, are synthesized and secreted exclusively by the anterior pituitary. Here we present evidence for the novel expression of FSHbeta, LHbeta, and the common glycoprotein-alpha (Cgalpha) in the gilthead seabream ovary. Using in situ hybridization and immunocytochemistry, FSHbeta was detected in primary-growth and secondary-growth-I oocytes, LHbeta was found in secondary-growth oocytes, and Cgalpha was observed in both primary and secondary-growth oocytes. Northern blot analyses demonstrated that Fshbeta transcript is 0.6 kb in both pituitary and ovary, whereas the ovarian Lhbeta transcript (1.1 kb), unexpectedly, is longer than the known pituitary Lhbeta transcript (0.6 kb). Sequence analyses revealed that ovarian Lhbeta is driven by a different promoter than pituitary Lhbeta, which generates an additional 459 bases at the distal portion of the 5'-untranslated region of the ovarian Lhbeta. Furthermore, using in vitro ovarian fragment incubation, we demonstrated that mammalian GnRH analog agonist enhanced the expression of ovarian Fshbeta (up to 2.7-fold), Lhbeta (up to 1.4-fold), Cgalpha (up to 1.8-fold), and the secretion of ovarian LH (up to 2.2-fold). In contrast, GnRH antagonist, analog E, suppressed the secretion of ovarian LH. Our findings suggest that a GnRH-gonadotropin axis is present in the gilthead seabream ovary and that FSH and LH, the well-characterized pituitary hormones, may have prominent novel roles in teleost intraovarian communication between oocytes and ovarian follicle cells.

Follistatin suppresses FSHbeta but increases LHbeta expression in the goldfish - evidence for an activin-mediated autocrine/paracrine system in fish pituitary

Our previous study demonstrated that recombinant goldfish activin B stimulated goldfish FSHbeta but inhibited LHbeta expression. Similar to activin B, activin A also exhibited the inverse effects on the expression of the two gonadotropins. The novel dual effects of activins on FSH and LH in the goldfish raise an interesting question as to where the activin comes from in vivo. In the present study, we first demonstrated the expression of activin, its receptors and binding protein follistatin in the goldfish pituitary, leading to a suggestion that an autocrine/paracrine regulatory system involving activin is operative in fish pituitary. To investigate the functionality of the pituitary-derived activin system in the regulation of gonadotropin biosynthesis, we further examined the effects of follistatin, an activin-binding protein, on goldfish FSHbeta and LHbeta expression. Follistatin not only reversed the effects of exogenous activin on FSHbeta and LHbeta expression but also had inverse effects on the basal expression of the genes; and its effects were opposite to those of activin. This suggests that the endogenous activin plays roles in controlling the expression of both FSHbeta and LHbeta genes. It is conceivable that any factors that influence the intrapituitary activin system in vivo will likely affect the biosynthesis of the two gonadotropins in the goldfish.

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.

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.

Differential regulation of tyrosine hydroxylase and estradiol receptor expression in the rainbow trout brain

In numerous fish species, dopamine has been found to strongly inhibit gonadotropin release. Among the enzymes that regulate dopamine turnover, tyrosine hydroxylase (TH), the rate-limiting anabolic enzyme, could be a target for endocrine feedback regulation. Since dopamine turnover is stimulated by estradiol in rainbow trout, we have investigated the effect of estradiol on TH and estradiol receptor expression. In situ hybridization was used to quantify mRNA levels in the brain of ovariectomized female rainbow trout implanted or not with estradiol pellets. We demonstrated that preoptic TH and estradiol receptor mRNA levels are greatly decreased by gonadectomy during vitellogenesis. For TH expression, this effect was reversed in part by estradiol supplementation. We have also confirmed the existence of an inhibitory gonadal feedback on FSH secretion, mediated by estradiol. The stimulating effect of estradiol on TH expression found in this study could be a pathway involved in gonadal feedback on gonadotropin release.

Gonadotropin and activin enhance maturational competence of oocytes in the zebrafish (Danio rerio)

In most teleosts, 17alpha,20beta-dihydroxy-4-pregnen-3-one (DHP) serves as the most potent maturation-inducing steroid (MIS) to initiate final oocyte maturation. The maturational competence or the responsiveness of oocytes to DHP increases when the ovarian follicles approach the final stage of growth. In the zebrafish, we demonstrated in the present study that full-grown oocytes (approximately 0.7 mm) exhibited the highest maturational competence, which diminished progressively with decreasing size of the follicles. Using midvitellogenic follicles (0.49-0.56 mm), which had little response to DHP, as the material, the present study aimed at investigating the endocrine and paracrine mechanisms that regulate maturational competence of the oocytes. In agreement with the results of studies in other teleost fish, pretreatment of follicles with gonadotropin (hCG) significantly enhanced the responsiveness of midvitellogenic oocytes to DHP in a clear time- and dose-dependent manner. Interestingly, activin, an ovarian growth factor, also had a potent stimulatory effect on the acquisition of oocyte maturational competence. Pretreatment with either recombinant human activin A or goldfish activin B significantly increased the rate of DHP-induced oocyte maturation from approximately 3% to approximately 70%, also in a clear dose-dependent manner. Similar to the results with hCG, pretreatment with activin alone had no effect in inducing maturation of midvitellogenic oocytes without subsequent DHP treatment, although both exhibited a strong effect in promoting maturation of full-grown oocytes. The effect of activin on maturational competence of oocytes could be reduced by cotreatment with follistatin, a potent activin-binding protein. Interestingly, follistatin treatment also significantly reduced the effect of hCG on maturational competence of oocytes, suggesting a mediating role for endogenous activin or activin-related molecules in the action of gonadotropin. The effects of hCG and activin on maturational competence of oocytes could be significantly inhibited by actinomycin D (1 microg/ml) and completely blocked by cycloheximide (1 microg/ml), suggesting that the hCG and activin-induced acquisition of oocyte maturational competence involves de novo protein synthesis at both the transcriptional and translational levels.

Roles of the activin regulatory system in fish reproduction

Activin (βAβA, βAβB, and βBβb) is a dimeric growth factor with diverse biological activities in vertebrate reproduction. Activin exerts its actions by binding to its specific type II and type I receptors. The activity of activin is regulated by follistatin, its binding protein, and the antagonists inhibin and antivin. All major components of the activin-inhibin-follistatin system have been identified in fish except the α subunit of inhibin. Using goldfish as a model, we have demonstrated that activin is expressed in the pituitary and the recombinant goldfish activin B has novel inverse effects on the expression of GTH β subunits. Activin increases the mRNA level of GTH-Iβ while significantly suppressing the expression of GTH-IIβ. We have also demonstrated the expression of activin and its receptors in the goldfish and zebrafish ovary. Using an in vitro ovarian follicle incubation as the system, we have investigated the involvement of the activin system in the process of final oocyte maturation. Our evidence clearly indicates that activin has potent effect of promoting final oocyte maturation, and that it may play a role in mediating the stimulatory effect of pituitary gonadotropin in the event of oocyte maturation. Key words: activin, inhibin, follistatin, fish, reproduction.

Activins and their receptors in female reproduction

Activins are growth and differentiation factors belonging to the transforming growth factor-β superfamily. They are dimeric proteins consisting of two inhibin β subunits. The structure of activins is highly conserved during vertebrate evolution. Activins signal through type I and type II receptor proteins, both of which are serine/threonine kinases. Subsequently, downstream signals such as Smad proteins are phosphorylated. Activins and their receptors are present in many tissues of mammals and lower vertebrates where they function as autocrine and (or) paracrine regulators of a variety of physiological processes, including reproduction. In the hypothalamus, activins are thought to stimulate the release of gonadotropin-releasing hormone. In the pituitary, activins increase follicle-stimulating hormone secretion and up-regulate gonadotropin-releasing hormone receptor expression. In the ovaries of vertebrates, activins are expressed predominantly in the follicular layer of the oocyte where they regulate processes such as folliculogenesis, steroid hormone production, and oocyte maturation. During pregnancy, activin-A is also involved in the regulation of placental functions. This review provides a brief overview of activins and their receptors, including their structures, expression, and functions in the female reproductive axis as well as in the placenta. Special effort is made to compare activins and their receptors in different vertebrates.

Key words: activins, activin receptors, reproductive axis, placenta.

Occurrence of immunoreactive Activin/Inhibin beta(B) in thyrotropes and gonadotropes in the bullfrog pituitary: possible Paracrine/Autocrine effects of activin B on gonadotropin secretion

Occurrence of immunoreactive activin/inhibin beta(B) in the bullfrog (Rana catesbeiana) pituitary was investigated immunocytochemically by use of antibody against Xenopus activin/inhibin beta(B) subunit. Thyrotropes were demonstrated to contain activin/inhibin beta(B)-immunoreactive substances. Moreover, immunoelectron microscopy revealed that in the secretory granules of thyrotropes and, to a lesser extent, in those of gonadotropes, activin/inhibin beta(B)-immunoreactive substances were present. Based on this observation, we investigated the effect of activin B on the release of gonadotropins from dispersed anterior pituitary cells of the bullfrog. Activin B stimulated the release of not only follicle-stimulating hormone (FSH) but also luteinizing hormone (LH) dose dependently. Under the culture conditions used in this experiment, inhibin B, as well as follistatin, did not affect the basal levels of LH and FSH, but they suppressed the activin-induced release of these hormones. This is the first study on the effect of activin on pituitary hormone secretion in lower tetrapods.

Activin stimulation of zebrafish oocyte maturation in vitro and its potential role in mediating gonadotropin-induced oocyte maturation

Activin plays important roles in the regulation of vertebrate reproduction. Using zebrafish, Danio rerio, as a model, the present study aimed at investigating the role of activin in the regulation of final oocyte maturation. Administration of recombinant goldfish activin B significantly increased the rate of oocyte maturation in vitro in a dose- and time-dependent manner. The effect of activin seemed to be additive to the effects of gonadotropin (hCG) and 17alpha,20beta-dihydroxyprogesterone, a potent maturation-inducing hormone in teleosts. The specificity of the activin action was confirmed by coincubation with recombinant human follistatin, which completely abolished the stimulatory effect of activin B. Interestingly, follistatin also significantly inhibited hCG-induced oocyte maturation, suggesting that endogenous activin may be a downstream mediator of gonadotropin actions. No effect of activin B was observed in the presence of actinomycin D, indicating that the action of activin may involve changes in transcriptional activity. These results, together with the demonstration that activin and its type II receptor are expressed in the zebrafish ovary, strongly suggest a paracrine/autocrine role for activin in the controlling of final oocyte maturation.

Recombinant goldfish activin B stimulates gonadotropin-Ibeta but inhibits gonadotropin-IIbeta expression in the goldfish, Carassius auratus

It is well documented that the pituitary in teleosts produces two gonadotropins, namely gonadotropin-I (GTH-I) and gonadotropin-II (GTH-II), which may regulate different phases of the reproductive cycle. However, unlike in mammals, very little is known about the differential regulation of the two GTHs in fish. Using goldfish as a model, the present study demonstrates, for the first time, that activin, a protein factor that plays a critical role in the differential regulation of mammalian FSH and LH, has opposite effects on GTH-Ibeta and GTH-IIbeta mRNA expression. Recombinant goldfish activin B stimulates GTH-Ibeta but significantly suppresses GTH-IIbeta mRNA levels in a dose-dependent manner in cultured goldfish pituitary cells. Administration of recombinant human follistatin completely abolished the effects of activin, thus demonstrating the specificity of the activin activities. The novel opposite effects of activin on the two goldfish GTHs make goldfish a very unique vertebrate model for activin studies. The present study not only contributes to our understanding of the mechanisms that control the temporal expression patterns of the two GTHs during the fish reproductive cycle, but also provides important information on the evolution of gonadotropin regulation in vertebrates.

Cloning and characterization of goldfish activin betaA subunit

We have cloned a full-length cDNA coding for activin betaA subunit from a goldfish brain and pituitary cDNA library, which represents the first for activin betaA in fish. Sequence analysis of goldfish activin betaA shows that this peptide is highly conserved across vertebrates. The mature region of goldfish activin betaA shares 81% amino acid identity with that of humans. Messenger RNA of goldfish activin betaA is expressed in a variety of tissues including ovary, testis, brain and liver, suggesting a wide range of physiological roles for activin A in the goldfish. The identity of the cloned goldfish activin betaA was confirmed by expressing the protein in the Chinese hamster ovary (CHO) cells followed by detection of the specific activin activity in the culture medium using erythroid differentiation factor (EDF) assay with F5-5 cells. Stable CHO cell lines producing high level of recombinant goldfish activin A were established and characterized.

Cloning of zebrafish activin type IIB receptor (ActRIIB) cDNA and mRNA expression of ActRIIB in embryos and adult tissues

A full-length cDNA encoding for activin type IIB receptor (ActRIIB) was cloned from zebrafish embryos. It encodes a protein with 509 amino acids consisting of a signal peptide, an extracellular ligand binding domain, a single transmembrane region, and an intracellular kinase domain with predicted serine/threonine specificity. The extracellular domain shows 74-91% sequence identity to human, bovine, mouse, rat, chicken, Xenopus and goldfish activin type IIB receptors, while the transmembrane region and the kinase domain show 67-78% and 82-88% identity to these known activin IIB receptors, respectively. In adult zebrafish, ActRIIB mRNA was detected by RT-PCR in the gonads, as well as in non-reproductive tissues, including the brain, heart and muscle. In situ hybridization on ovarian sections further localized ActRIIB mRNA to cytoplasm of oocytes at different stages of development. Using whole-mount in situ hybridization, ActRIIB mRNA was found to be expressed at all stages of embryogenesis examined, including the sphere, shield, tail bud, and 6-7 somite. These results provide the first evidence that ActRIIB mRNA is widely distributed in fish embryonic and adult tissues. Cloning of zebrafish ActRIIB demonstrates that this receptor is highly conserved during vertebrate evolution and provides a basis for further studies on the role of activin in reproduction and development in lower vertebrates.

Identification of a new member of transforming growth factor-beta superfamily in Drosophila: the first invertebrate activin gene

Activins, a subgroup of the transforming growth factor-beta (TGF-beta) superfamily, have been extensively studied in vertebrates for their roles in growth and development. However, activins are not thought to be expressed in invertebrates. The identification of the first invertebrate activin gene is reported here. A genomic clone representing 102 F region of the Drosophila chromosome 4 is found to encode a putative activin beta. The predicted protein sequence has a multibasic protease site that would generate a mature C-terminal peptide containing 113 amino acids showing > 60% similarity to the vertebrate activin beta B (inhibin beta B) sequences. A TGF-beta family signature as well as all 9 cysteine residues conserved in the vertebrate activins are also present in this mature peptide sequence. Northern blot and RT-PCR analyses indicated that the activin beta gene is expressed in embryo, larva and adult stages of Drosophila.