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Follistatin Isoforms: FST-288 vs FST-315
Follistatin exists in two principal isoforms: FST-288 (288 amino acids, ~31 kDa) and FST-315 (315 amino acids, ~35 kDa), generated by alternative splicing. FST-288 binds heparan sulphate proteoglycans (HSPGs) on the cell surface and in the extracellular matrix
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- Follistatin exists in two principal isoforms: FST-288 (288 amino acids, ~31 kDa) and FST-315 (315 amino acids, ~35 kDa), generated by alternative splicing. FST-288 binds heparan sulphate proteoglycans (HSPGs) on the cell surface and in the extracellular matrix, resulting in local tissue retention and concentration at the sarcolemma. FST-315 lacks the C-terminal domain required for HSPG binding and circulates freely, providing systemic coverage.
- In skeletal muscle-specific research, FST-288 is the preferred isoform for local paracrine myostatin inhibition — AAV-mediated skeletal muscle FST-288 overexpression produces 60-80% greater muscle mass increases than FST-315 in the same delivery system, attributed to higher local myostatin sequestration efficiency at the muscle fibre surface. In contrast, FST-315 is the isoform of choice for systemic models where coordinated multi-organ myostatin inhibition is the experimental goal.
- Recombinant FST-315 at 10mg/kg s.c. 2×/week in C57BL/6J mice produces gastrocnemius mass increases of 28-34% over 4 weeks with corresponding fibre CSA increases of 32-38% (Type IIA fibres +38-44%, Type IIX +28-34%). Myofibrillar protein synthesis rates increase by 34-42% (SUnSET puromycin incorporation assay), with mTORC1 activity (pS6K1 Thr389) elevated 1.4-fold and 4E-BP1 phosphorylation +1.3-fold. SMAD2/3 phosphorylation in muscle is reduced by 38-44% relative to vehicle, confirming target engagement. ActRIIB-Fc (ACE-031) at equivalent dose produces SMAD2/3 reduction of 54-62% — indicating broader ligand neutralisation consistent with the wider ligand spectrum of the decoy receptor approach.