Evidence for the existence of a local activin–follistatin negative feedback loop in the goldfish pituitary and its regulation by activin and gonadal steroids

Abstract

Activin is an important regulator of gonadotropin expression and production in the vertebrate pituitary, and its activity is fine-tuned by its binding protein follistatin. In the present study, a full-length cDNA for follistatin was cloned in the goldfish, which shows 74% amino acid sequence identity with that of mammals. Recombinant goldfish follistatin expressed in the Chinese hamster ovary cells significantly blocked activin-induced F5-5 cell differentiation. Goldfish follistatin is expressed in a wide range of tissues including the brain, pituitary, ovary, and testis. The expression of follistatin mRNA in the pituitary is regulated by both activin and gonadal steroids in vitro. Treatment with goldfish activin B for 48 h significantly up-regulated follistatin expression in cultured pituitary cells, suggesting a closed activin-follistatin feedback loop in the pituitary. In agreement with this, both human and goldfish follistatin down-regulated the expression of follistatin itself, probably due to the neutralization of endogenous activin. Examination of FSHb and LHb expression in the same samples supports the role of activin and follistatin in the differential regulation of FSH and LH as demonstrated previously. Since the expression level of activin b B in the pituitary is rather stable both in vitro and in vivo, it is conceivable that follistatin may play a pivotal regulatory role in the intra-pituitary activin system. Both estradiol and testosterone up-regulated follistatin expression in vitro, suggesting a mediating role for follistatin in steroid feedback on pituitary hormones. These results provide clues to the potential physiological roles of activin-follistatin system in the regulation of gonadotropin biosynthesis in teleosts.

Grass Carp Follisatin: Molecular Cloning, Functional Characterization, Dopamine D1 Regulation at Pituitary Level, and Implication in Growth Hormone Regulation

“…In grass carp, high levels of follistatin expression could be located in the gonad and intestine and to a lower extent in various brain areas and pituitary. In general, the pattern of follistatin expression in grass carp is comparable with that reported in other fish species, e.g., in goldfish (34). Despite a lack of information for testicular function of follistatin in fish models, high level of gonadal expression of follistatin in carp species is in agreement with the role of activin/follistatin system in folliculogenesis and oocyte maturation reported in fish ovary, e.g., in zebrafish (30, 62).…”

“…In pituitary cells from fish species, e.g., in goldfish (31), zebrafish (32), and more recently in European eel (33), activin treatment can exert differential effects on the two gonadotropins, with stimulation on FSH but inhibition on LH expression. Similar to mammals (e.g., in rat pituitary cells) (26), follistatin expression in fish pituitary cells can be up-regulated by activin (34), and the differential effects of activin on the two gonadotropins can be blocked by follistatin, e.g., in goldfish (35), suggesting that a local feedback of activin/follistatin system is present in the fish pituitary and contributes to gonadotropin regulation. In grass carp, together with activin βA and βB mRNA detected in our cell culture experiments, follistatin was found to be expressed at the pituitary level with a single transcript of 2.2 kb in size.…”

“…In lower vertebrates, e.g., in fish models, the activin/follistatin system in the pituitary and its involvement in gonadotropin regulation appear to be well-conserved (30) and have been documented in the goldfish (31), zebrafish (32), and more recently in European eel (33). In fish species, interestingly, activin was found to have opposite effects on the two gonadotropins, with stimulation on FSH but inhibition on LH expression (31–33), and these differential effects could be blocked by follistatin at pituitary level, e.g., in zebrafish (32) and goldfish (34, 35). Of note, unlike the studies in mammals, cell-type-specific expression of follistatin has not been characterized in fish pituitary.…”

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

“…However, the opposite was true with parallel rises of these gene targets after removing endogenous LH with immuno-neutralization using LH antiserum, indicating that local release of LH by activating LH receptor but not FSH receptor can serve as a negative regulator of the activin/follistatin system in the carp pituitary. In mammals, activin-induced follistatin expression to nullify activin’s actions is well documented at tissue level [58] and a similar local feedback for activin/follistatin system appears to be well conserved in fish models, e.g., in zebrafish [74] and goldfish [31]. In our studies, follistatin mRNA expression could be notably enhanced by activin A and B in carp pituitary cells and these stimulatory actions were negated by hCG co-treatment.…”

“…At the pituitary level, activin was shown to have differential effects on LH and FSH expression, with stimulation on FSHβ but inhibition on LHβ gene expression, e.g., in zebrafish [28], goldfish [29] and more recently in European eel [30]. In goldfish, pituitary expression of follistatin has been confirmed [31] and the differential effects on LH and FSH by activin could be blocked/reverted by co-treatment with follistatin [32], suggesting the presence of an activin/follistatin system in the fish pituitary similar to that of mammals. Despite the inhibitory effect of activin on LHβ gene expression, activin and its functional antagonist inhibin were both effective in inducing LH release in goldfish pituitary fragments [33].…”