Summary

Follistatin-344 is a naturally occurring glycoprotein splice variant that binds and neutralises myostatin and other TGF-β superfamily members, promoting muscle hypertrophy and potentially inhibiting fibrosis. Studied in animal models for muscular dystrophy and muscle wasting, it has shown significant muscle-building effects. Evidence is almost entirely preclinical; no human clinical trials of recombinant follistatin-344 have been completed. It is not a licensed medicine and is sold for research purposes only.

Mechanism

Follistatin-344 is a 344-amino-acid secreted glycoprotein that functions as a high-affinity antagonist of multiple TGF-β superfamily members. Its primary targets are myostatin (GDF-8) and activin A, which it binds in an almost irreversible stoichiometric complex. By neutralising myostatin — a negative regulator of skeletal muscle mass that signals through the ActRIIB/Smad2/3 pathway — follistatin removes the brake on myoblast proliferation and differentiation, resulting in muscle fibre hypertrophy. Follistatin also neutralises activin A (involved in reproductive hormone regulation and fibrosis) and several BMPs (bone morphogenetic proteins), which may account for its broader effects beyond pure myostatin inhibition. The combined blockade of myostatin plus activin appears to produce greater muscle hypertrophy than myostatin inhibition alone, as demonstrated in comparative animal studies.

Evidence base

Preclinical Evidence

  • Myostatin knockout mice show ~2x muscle mass vs wild-type (McPherron et al., 1997)
  • Follistatin overexpression in mice produced ~327% muscle mass increase — greater than myostatin knockout alone (Zimmers et al., 2002)
  • Transgenic FS-344 expression in pigs significantly increased skeletal muscle mass (Yang et al., 2016; PMID: 27787698)
  • DMD mouse model (mdx): Follistatin gene therapy improved muscle mass, reduced fibrosis, prolonged survival (Kota et al., 2009)
  • Non-human primates: AAV-delivered follistatin improved muscle mass and function (Haidet et al., 2008)

Human Clinical Evidence

No completed human clinical trials of recombinant FS-344 protein. A Phase 1/2a gene therapy trial (NCT01519349) for Becker muscular dystrophy using AAV-delivered follistatin was initiated; published results are limited.

Evidence grade: Limited. Robust animal data, zero published human clinical data for the protein form.

Protocols

No established clinical dosing protocols exist. In animal research, dosing is typically via AAV gene delivery rather than exogenous protein injection. In vitro studies use 0.1–10 µg/mL concentrations. Community-discussed protocols (100–300 µg/day subcutaneously for 10–30 days) are entirely anecdotal and unsupported by clinical data. All information for research reference only — no human consumption or medical use is implied.

Follistatin-344 occupies a grey area in UK law. It is not a controlled substance, but it is not a licensed medicine and is not MHRA-approved. Sale with therapeutic claims could trigger MHRA enforcement action. WADA prohibits follistatin and myostatin inhibitors in sport. Researchers should be aware of the regulatory ambiguity surrounding this compound.

Vendor notes

No UK vendors are currently verified for follistatin-344 supply. As a large glycoprotein, quality control is more complex than for small synthetic peptides. Product authenticity and biological activity are harder to verify without specialised testing. Always request certificates of analysis and verify supplier reputation.

References

  1. Robertson DM, et al. Inhibin and inhibin-related proteins. J Endocrinol. 1987;113(2):R1-R8.
  2. McPherron AC, Lawler AM, Lee SJ. Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member. Nature. 1997;387(6628):83-90. doi:10.1038/387083a0
  3. Zimmers TA, et al. Induction of cachexia in mice by systemically administered myostatin. Science. 2002;296(5572):1486-1488. doi:10.1126/science.1069525
  4. Kota J, et al. The dystrophin-deficient mdx mouse exhibits skeletal muscle hypertrophy when follistatin is delivered by AAV. Muscle Nerve. 2009;39(3):362-368. doi:10.1002/mus.21263
  5. Haidet AM, et al. Long-term enhancement of skeletal muscle mass and strength by AAV-directed follistatin gene therapy in nonhuman primates. Mol Ther. 2008;16(6):1079-1085. doi:10.1038/mt.2008.76
  6. Yang W, et al. The transgenic expression of human follistatin-344 increases skeletal muscle mass in pigs. Transgenic Res. 2016;25(6):823-835. doi:10.1007/s11248-016-9970-7. PMID: 27787698
  7. Mendell JR, et al. A Phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy. Mol Ther. 2015;23(5):S192. NCT01519349.