Follistatin (FST) is a glycosylated single-chain protein that is expressed in a wide variety of tissues.1 Activin stimulates pituitary FSH secretion whereas Inhibin and Follistatin are inhibitory.2 Follistatin has been shown to be a potent activin-binding protein which acts by neutralizing the actions of the Activins.3 The activin-follistatin binding complex is generally considered to be composed of one activin and two follistatin molecules, and the affinity of binding between follistatin and activin is similar to that of activin for its receptor. Several isoforms of follistatin of molecular weight 31 – 39 kDa have been identified. Molecular analysis of the isoforms indicate that follistatin is encoded by a single gene and that the variety of isoforms arise from alternative splicing, glycosylation and proteolytic cleavage. Alternative splicing occurs at the 3’-terminal of the gene resulting in a precursor form of 317 and 344 amino acids4, and then following subsequent cleavage of the 29 amino acid signal peptide, generates 2 mature follistatin isoforms of 288 and 315 amino acids (namely Follistatin-288 and Follistatin-315).
The ratio of Follistatin isoforms present in different tissues and bodily fluids varies, as does their relative binding affinities. Follistatin-288 is the predominant form present in human follicular fluid; whereas the main form in serum is Follistatin-315.5 Follistatin-288 has a greater capacity to bind and neutralize activin and is approximately 10-fold more potent than Follistatin-315 in suppressing FSH secretion from rat pituitary cells in vitro. In addition, Follistatin-288 also binds with much greater affinity to heparin sulfate proteoglycans located on the cell surface and extracellular matrix, indicating that the Follistatin-288 isoform is primarily a membrane-bound form of Follistatin, whereas Follistatin-315 is a circulating form.

References:
1. Lin SY et al. Reproduction. 126(2):133-148, 2003.
2. Robertson DM et al. Biochem Biophys Res Commun. 126(1):220-226, 1985.
3. Nakamura T et al. Science. 247(4944):836-838, 1990.
4. Shimasaki S et al. Biochem Biophys Res Commun. 152(2):717-723, 1988.
5. Schneyer AL et al. Endocrinology. 137(1):240-247, 1996.
6. HHS Publication, 5th ed., 2007. Biosafety in Microbiological and Biomedical Laboratories. Available http://www.cdc.gov/biosafety/publications/bmbl5/BMBL5
7. DHHS (NIOSH) Publication No. 78–127, August 1976. Current Intelligence Bulletin 13 – Explosive Azide Hazard. Available http:// www.cdc.gov/niosh.

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25. pii: S1525-7304(19)30287-6. doi: 10.1016/j.cllc.2019.10.013. [Epub ahead of print]

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Synolaki E, Papadopoulos V, Divolis G, Tsahouridou O, Gavriilidis E, Loli G, Gavriil A, Tsigalou C, Tziolos NR, Sertaridou E, Kalra B, Kumar A, Rafailidis P, Pasternack A, Boumpas DT, Germanidis G, Ritvos O, Metallidis S, Skendros P, Sideras P. The Activin/Follistatin-axis is severely deregulated in COVID-19 and independently associated
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