Follistatin-344
Also known as: FST-344, FS344, Follistatin 344, FST344, follistatin precursor isoform 344
3 notable studies · 4 references
TLDR
Follistatin-344 is a human follistatin precursor isoform that antagonizes myostatin and activin-family signaling, with compelling muscle-growth evidence from animal studies and small human FS344 gene-therapy trials, but those studies should not be confused with evidence for administering research-market Follistatin-344 protein. https://pubmed.ncbi.nlm.nih.gov/20368179/ https://pubmed.ncbi.nlm.nih.gov/25322757/ https://pubmed.ncbi.nlm.nih.gov/28279643/
Research Supports
- Studied for inhibiting myostatin and activin signaling to promote skeletal-muscle growth
- Investigated as AAV-delivered gene therapy for muscular dystrophy and inclusion body myositis
- Studied for effects on muscle size and strength in nonhuman primates
- Investigated for broader activin-family and reproductive endocrine signaling effects
Observations Reported in Research
- AAV-FS344 gene therapy produced durable increases in muscle size and strength in cynomolgus macaques (Kota et al.)
- In a 6-person Becker muscular dystrophy trial, 4 of 6 improved six-minute walk distance after AAV1.CMV.FS344 gene therapy (Mendell et al., 2015)
- In sporadic inclusion body myositis, AAV-FS344 plus exercise improved annualized six-minute walk performance by about 56 m/year versus an untreated comparison group (2017)
- An analysis of 17 black-market products labeled "Follistatin-344" found only 9 actually contained follistatin
These are brief summaries of what the referenced research examined or reported — not conclusions about safety or effectiveness, and not medical advice. See Notable Studies and Sources & References below for the full context and citations.
At a Glance
Classification: Endogenous follistatin precursor isoform; activin-binding protein; myostatin-pathway antagonist
Follistatin is a naturally occurring human protein that binds and neutralizes several members of the TGF-β superfamily, including: Myostatin / GDF-8; Activin A; Activin B; and related ligands. Because myostatin normally restrains skeletal-muscle growth, blocking this pathway can produce substantial increases in muscle mass. Follistatin-344 is particularly important because it is the precursor associated with the circulating FS315 form of follistatin. NCBI identifies FST344 as the longer human follistatin precursor isoform. Human studies of AAV-delivered FS344 gene therapy are not studies of repeatedly administering research-market “Follistatin-344” protein. That distinction should be impossible to miss on Aminodex.
What Is Follistatin-344?
The human FST gene produces multiple follistatin isoforms through alternative RNA processing. Two forms frequently encountered in the literature are FS288 and FS315, which is generated from the FS344 precursor. The forms behave differently. FS288 interacts more strongly with cell-surface proteoglycans and tends to remain associated with tissues. FS315 is more commonly found circulating in serum. This matters because follistatin isn't simply a molecule that “turns off myostatin.” It binds multiple related signaling proteins. Researchers pursuing muscle therapies therefore have to balance: myostatin inhibition against unwanted activin-system effects. The FS344/FS315 strategy became attractive partly because it was expected to have less problematic interaction with the pituitary activin-inhibin axis than some other forms.
What Are Researchers Studying?
Skeletal muscle growth
Myostatin is one of the body's major negative regulators of skeletal-muscle growth. The simplified pathway is: Myostatin → ActRIIB → ALK4/ALK5 → SMAD2/3 → transcriptional signaling → restraint of skeletal-muscle growth. Follistatin binds myostatin before it can efficiently activate this pathway. Reducing myostatin signaling therefore removes part of the biological “brake” on muscle growth.
Muscular dystrophy
Follistatin gene therapy has been investigated as a strategy for muscular dystrophies. In a small Phase 1/2a study, six people with Becker muscular dystrophy received intramuscular AAV1 vectors carrying the FS344 gene. Four of six improved their six-minute walk distance, while two did not meaningfully improve. Muscle biopsies showed changes including hypertrophy and reduced fibrosis. No treatment-related adverse effects were reported in that small study. https://pubmed.ncbi.nlm.nih.gov/25322757/ This was encouraging proof-of-concept research. But it was: AAV gene transfer → sustained local production of follistatin, not injection of bottled recombinant Follistatin-344.
Inclusion body myositis
FS344 gene delivery has also been studied in sporadic inclusion body myositis (sIBM). Six treated subjects received bilateral quadriceps AAV-FS344 gene transfer along with an exercise program. Their annualized six-minute walk performance improved compared with an untreated comparison group. https://pubmed.ncbi.nlm.nih.gov/28279643/ The study was small and wasn't a large randomized placebo-controlled efficacy trial. Still, it provides additional direct human investigation of the FS344 biological strategy.
Muscle strength
Increasing muscle size doesn't necessarily mean increasing useful muscle strength. That's why the nonhuman-primate research was important. AAV-FS344 delivered to cynomolgus macaque quadriceps produced durable increases in both muscle size and strength, helping establish translational proof of concept before human gene-therapy studies. https://pubmed.ncbi.nlm.nih.gov/20368179/
Myostatin biology
Follistatin is frequently called a “myostatin inhibitor.” That's correct but incomplete. Follistatin also binds: Activins; Some BMP-family ligands; and Other TGF-β-family signaling molecules. Consequently: Follistatin ≠ selective myostatin antibody. Its broader ligand profile may contribute both to biological potency and to unwanted effects.
Reproductive endocrine signaling
Activins participate in regulation of FSH — follicle-stimulating hormone. Because follistatin can bind activin, systemic manipulation of follistatin biology potentially intersects with reproductive endocrine signaling. This was one reason investigators deliberately considered isoform selection when developing FS344 gene therapy.
How Does It Work?
The simplified muscle pathway is: Follistatin → binds myostatin / activins → reduces ligand interaction with ActRIIB → reduces downstream SMAD2/3 signaling → less inhibition of muscle-growth pathways → increased muscle growth potential. But this isn't simply: Follistatin → myostatin OFF. Several ligands overlap within the ActRIIB signaling network. That complexity becomes particularly important when comparing Follistatin-344 with ACE-031.
What Does the Research Show?
Human research
Human FS344 evidence exists. But the most important studies used AAV1.CMV.FS344 gene therapy. Researchers delivered DNA encoding human FS344 into skeletal muscle, causing treated tissue to produce follistatin. That's biologically and pharmacologically very different from administering a purified peptide/protein. In the six-person Becker muscular dystrophy trial, four participants improved six-minute walk distance while two did not. https://pubmed.ncbi.nlm.nih.gov/25322757/ Human sIBM gene-transfer research also reported encouraging functional signals. https://pubmed.ncbi.nlm.nih.gov/28279643/ Therefore: “Follistatin has never been studied in humans” = incorrect. But: “Injected research-market Follistatin-344 has demonstrated muscle-building efficacy in humans” = also incorrect.
Animal research
Strong. Rodent models demonstrated dramatic muscle effects from follistatin manipulation. More importantly, gene delivery was tested in nonhuman primates. AAV-FS344 produced pronounced and durable increases in muscle size and strength in cynomolgus macaques. https://pubmed.ncbi.nlm.nih.gov/20368179/ That work helped justify movement into human gene-therapy studies.
In-vitro research
Follistatin biology is extensively characterized. Research has examined: myostatin binding; activin binding; ActRIIB signaling; SMAD signaling; muscle-cell differentiation; protein isoforms; cell-surface binding; and endocrine signaling. The uncertainty is not whether follistatin participates in this pathway. It unquestionably does. The larger questions concern how best to manipulate the pathway safely and therapeutically.
Notable Studies
Follistatin Gene Delivery Enhances Muscle Growth and Strength in Nonhuman Primates
animalResearchers delivered AAV1 expressing FS344 into cynomolgus macaque skeletal muscle. Treatment produced pronounced, durable increases in muscle size and strength without the obvious generalized adverse effects researchers were specifically monitoring for. https://pubmed.ncbi.nlm.nih.gov/20368179/
Limitations: Nonhuman primate study using gene delivery, not administration of commercial recombinant FS344 protein.
View publication →A Phase 1/2a Follistatin Gene Therapy Trial for Becker Muscular Dystrophy
humanSix people with Becker muscular dystrophy received bilateral intramuscular AAV1.CMV.FS344. Four participants improved six-minute walk distance, while two showed little or no improvement. Histology showed changes consistent with muscle hypertrophy and reduced pathological features. No adverse effects were encountered in this small trial. https://pubmed.ncbi.nlm.nih.gov/25322757/
Limitations: Only six participants, no randomized placebo control, heterogeneous responses and gene delivery rather than exogenous follistatin administration.
View publication →Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional Outcomes
humanSix people with sporadic inclusion body myositis received bilateral quadriceps rAAV1.CMV.huFS344 alongside an exercise regimen. Annualized six-minute walking performance improved by approximately 56 m/year in treated subjects, compared with deterioration in an untreated comparison group. https://pubmed.ncbi.nlm.nih.gov/28279643/
Limitations: Very small study, nonrandomized comparison group, concurrent exercise intervention and gene-transfer methodology.
View publication →What We Still Don't Know
• Safety of chronic systemic FS344 exposure • Human pharmacokinetics of exogenous recombinant FS344 • Effective exposure when administered as purified protein • Whether systemic use meaningfully increases human strength • Long-term cardiovascular consequences • Effects on tendons and connective tissue • Reproductive/endocrine effects • Effects of broad activin-family inhibition • Whether increased muscle mass translates proportionally into function • Cancer-related implications of prolonged growth-pathway manipulation • Long-term effects of sustained gene expression • Whether research-market products actually contain authentic follistatin That last point isn't hypothetical. A published analysis of 17 black-market products labeled Follistatin-344 found that only nine actually contained follistatin, while some others contained different growth-related peptides. https://pubmed.ncbi.nlm.nih.gov/31758732/
Related Compounds
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Sources & References
- Mendell JR, et al. A Phase 1/2a Follistatin Gene Therapy Trial for Becker Muscular Dystrophy. Molecular Therapy. 2015.
- Follistatin Gene Delivery Enhances Muscle Growth and Strength in Nonhuman Primates. Science Translational Medicine.
- Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional Outcomes.
- Detection of Black Market Follistatin 344.
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