Follistatin-315 (FST-315)
/ Native follistatin short isoform (315 residues; predominant circulating form)ALIAS · FST-315 isoform · Follistatin short isoform
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Tier 3 by gene-therapy proxy. Native follistatin is a well-characterised activin/myostatin antagonist; recombinant FST-315 protein has rodent literature but no Phase 1 trials of the synthetic protein. AAV-FST344 gene therapy (Mendell 2015) is the closest clinical analog and produced human dosing data, but the gene-delivered isoform was FST-344, not FST-315.
Follistatin is a secreted glycoprotein that binds activin A, myostatin, GDF11, and several BMPs with high affinity, neutralising their TGF-β-superfamily receptor signalling. The 315-residue short isoform is the predominant circulating form in humans; the longer 344-residue form has higher heparan sulfate binding and tissue retention. Sequestration of myostatin permits muscle hypertrophy.
Tier 3 (animal). Rodent overexpression and recombinant administration studies show muscle hypertrophy. AAV-mediated FST-344 gene therapy in humans (Becker muscular dystrophy and inclusion body myositis) is the closest clinical evidence — see Mendell 2015 in the FST-344 page.
No formal Phase 1 safety database for recombinant FST-315 protein. Theoretical concerns from broad-spectrum TGF-β-family ligand sequestration parallel those that ended ACE-031 development.
Regulatory status
- FDA status:
- Not FDA-approved
Vendor-sold 'follistatin' material is rarely characterised as to isoform (FST-315 vs FST-344) or glycosylation state. Activity in cell-based assays varies by source and lot; the published preclinical literature uses recombinant proteins of defined specification that vendor product is unlikely to match.