Follistatin (FS, Activin-binding protein, FST)

Follistatin (FST) was originally isolated from ovarian follicular fluid as a protein capable of inhibiting FSH release from pituitary cells in a manner similar to inhibin. Cloning and sequencing showed it to be a monomeric glycosylated protein unrelated to inhibin and its mode of action became clear with the demonstration of its ability to specifically bind activin. FST is encoded by a single gene which gives rise to various alternative splicing, glycosylation and proteolytic cleavage variants. Two main variants of FST are encoded through alternative splicing at the carboxyl-terminus. A 288-residue molecule (FST288) terminates after the third FST domain, whereas a 315-residue form is extended by an extra exon to include a highly acidic 27-residue “tail.” Both are capable of binding activin, although according to some reports the affinity of FST288 may be higher than that of FST315. FST is produced and plays a regulatory role in gonads, pituitary gland, pregnancy membranes, liver and kidney. FST also circulates in blood in the human and most likely in other mammals as well. The data obtained so far indicate that FST315 is the major FST isoform in the peripheral circulation, whereas FST in follicular fluid is predominately processed. Nearly all FST in circulation is bound to activin. FST315 is the major circulating FST isoform but is undetectable in follicular fluid samples aspirated from normal women or women with polycystic ovary syndrome. On the other hand, ovarian FST does not contribute appreciably to circulating FST levels during follicular development. The purification of FST from gonadal sources indicated that it also has biological functions within the gonad. Some studies on FST gene expression and protein production provided information suggesting possible function of this protein within the ovary. In almost all follicles, free-FST was always in excess of activin, suggesting that in follicular fluid, all activin is bioneutralized, consistent with the concept of activin activity being suppressed as follicles develop. FST level increases in the third trimester, with a dramatic increase in the final month, and disappears postpartum. FST has been identified also in seminal plasma, where it is still present after vasectomy. Immunoreactive FST, together with activin ßA were present in Leydig and Sertoli cells, and FST was demonstrable in epithelial cells of the seminal vesicle and prostate. Taken together, these findings suggest that activin-A is secreted by the testis, whereas the prostate makes a significant contribution to FST levels in seminal plasma. FST plays a role also in bone metabolism. During endochondral bone formation, activin:FST ratios fluctuate in correlation with chondrocyte maturation and bone cell development. Taken together with the identification of activin receptors on osteoblasts, these observations suggest that activin, and its regulation by FST, has a role in bone repair processes.