Follistatin-315

also known as: FS-315, Follistatin 315

The dominant circulating follistatin isoform in human serum — binds heparan sulfate proteoglycans avidly, concentrating at tissue surfaces rather than circulating freely, which limits its systemic reach compared to FS-344.

The 315-residue splice variant of follistatin, distinguished from FS-344 by the absence of the C-terminal 29-residue acidic tail, which gives FS-315 higher affinity for cell-surface heparan sulfate proteoglycans and therefore a more tissue-bound distribution pattern.

Mechanism of action

Same ligand-binding specificity as FS-344 — neutralizes myostatin, activin A, and related TGF-β superfamily members. Functional differences between the two isoforms arise from distribution rather than intrinsic binding activity: FS-315's heparan sulfate affinity concentrates it at tissue surfaces, producing more local and less systemic effect when administered peripherally.

Primary uses

Muscle biology research
Activin/reproductive biology research
Community use (often sold interchangeably with FS-344, despite pharmacological differences)

Typical dosing

research-only varies (intramuscular or subcutaneous (community))

No validated human dosing. Community sourcing typically does not verify which isoform is actually supplied.

Regulatory status

Not approved. Less-studied clinically than FS-344 precisely because its tissue-binding profile limits systemic efficacy. Used primarily as a research tool and in some community preparations sold alongside or instead of FS-344.

References

[pubmed] Sugino K, et al. "Molecular heterogeneity of follistatin, an activin-binding protein: higher affinity of the carboxyl-terminal truncated forms for heparan sulfate proteoglycans on the ovarian granulosa cell." J Biol Chem, 1993;268:15579-15587.
[pubmed] Schneyer A, et al. "Differential distribution of follistatin isoforms: application of a new FS315-specific immunoassay." J Clin Endocrinol Metab, 2004;89:5067-5075.

Related peptides

Follistatin-344

The longer of the two principal follistatin splice variants — binds tissues less avidly than FS-315 and therefore has broader systemic distribution, making it the form used in most gene-therapy and myostatin-inhibition research.

ACE-031

Acceleron's first-generation myostatin trap — Phase 2 development in Duchenne muscular dystrophy was halted in 2013 after epistaxis and telangiectasia adverse events, but the same scaffold concept led to successor programs at Acceleron, Regeneron, and Lilly.

PEG-MGF

A pegylated fragment of a proposed muscle-specific IGF-1 splice variant — the underlying "MGF" biology remains scientifically contested, and PEG-MGF itself has no human clinical data.

Disclaimer

This entry is for educational purposes only and does not constitute medical advice. Dosing information reflects published regulatory or research data and is not a recommendation. Many compounds described here are not approved for human use in the United States. Consult a licensed medical professional before considering any peptide therapy.