What is Follistatin 315?
Follistatin 315 (also written FST-315) is a naturally occurring protein found circulating in the bloodstream. It is one of two main versions — called isoforms — of follistatin, the other being Follistatin 288. Think of them as two slightly different editions of the same book. The number refers to how many amino acids (the building blocks of proteins) make up each version.
Follistatin 315 is the form that floats freely in the blood, while Follistatin 288 tends to stick to cell surfaces and tissues. Both versions are produced naturally in the body and are being actively studied by researchers around the world.[1]
Follistatin belongs to a larger family of signaling proteins. Its best-known job is to latch onto a molecule called activin and neutralize it. Because activin plays roles in muscle, reproduction, liver function, and bone, follistatin's ability to block it has made FST-315 a compound of serious scientific interest.[1]
How Follistatin 315 Works
Here is a simple way to picture it. Activin is like a brake pedal for certain kinds of tissue growth — including muscle. Follistatin 315 acts like a hand that grabs the brake pedal and holds it still, so the brakes don't engage as strongly. The result, in theory, is that growth signals can operate with less interference.
More precisely, FST-315 binds tightly and essentially irreversibly to activin A, preventing activin from docking with its receptor on cell surfaces.[4] By doing this, FST-315 can alter downstream signaling in muscle cells, liver cells, bone cells, and reproductive tissues — depending on where it is active.
Because FST-315 circulates in the blood rather than staying stuck to local tissues (unlike FST-288), researchers believe it can have broader, body-wide (systemic) effects.[1] That systemic quality is one reason it is so often highlighted in muscle and anabolic research.
FST-315 is also part of the larger TGF-beta superfamily network — a group of signaling molecules that regulate everything from cell growth to inflammation. Follistatin can interact with other members of this family beyond activin, including bone morphogenetic proteins (BMPs).[1]
What the Research Shows
Liver regeneration: One rat study looked at what happens after a 70% partial hepatectomy — that is, surgically removing most of the liver. Researchers found that activin receptors peaked between 48 and 72 hours after surgery, which coincided with a slowdown in new cell growth. When FST-315 was given intravenously at that window, it produced a more potent stimulation of DNA synthesis and a greater increase in body weight compared to control animals. The authors suggested FST-315 may have anabolic actions and could play an endocrine (hormone-like) role in liver regeneration.[2]
Bone repair: Scientists at Erasmus MC tested both FST-315 and FST-288 in bone tissue engineering experiments. In cell culture (in vitro), follistatin promoted migration of mesenchymal stem cells and endothelial cells — both important for healing — and supported vascular tube formation. A key finding specific to FST-315 was that it was released more readily from a biomaterial scaffold over four weeks, whereas FST-288 stayed mostly trapped inside. Neither variant improved bone healing in living rats versus controls in that particular model, but researchers noted that finding the right delivery method remains an open question.[4]
Reproductive biology: FST-315 is the dominant follistatin transcript found in human ovarian granulosa cells, with roughly three FST-315 messages for every one FST-288 message. However, its levels did not change significantly as follicles grew, and they were not statistically different in women with polycystic ovary syndrome (PCOS) compared to healthy controls.[6] Separate mouse studies confirmed that follistatin is critical for normal postnatal development of the uterus and oviduct, with animals that only express FST-315 (lacking FST-288) showing reproductive abnormalities including inflammation and structural defects.[3]
Intestinal cells: Research in rat intestinal epithelial cells confirmed that these cells naturally express FST-315 mRNA and use it to counteract activin A's effects on cell growth — showing follistatin's role extends well beyond muscle and reproduction.[5]
What Follistatin 315 Is Being Studied For
- Muscle biology — FST-315's ability to block activin's brake-like effect on muscle tissue makes it a focus of anabolic and muscle-wasting research.[2]
- Liver regeneration — Early animal data suggests potential roles in supporting hepatocyte (liver cell) regrowth after injury.[2]
- Bone and tissue engineering — Researchers are exploring FST-315 as a component in scaffolds designed to encourage bone and vascular repair.[4]
- Reproductive physiology — Its role in ovarian and uterine function continues to be characterized in animal models.[3]
How Follistatin 315 Is Dosed in Research
Dosing protocols for FST-315 vary considerably across published animal studies and have not been established in human clinical trials. Because this is a research-use-only compound, there is no standardized human dosage. For a full breakdown of the amounts used in preclinical studies, please refer to the dosage chart on this page. You can also use our calculator to convert and cross-reference research quantities. Always consult the primary literature before designing any research protocol.
Mixing and Storing Follistatin 315
FST-315 is a peptide supplied as a lyophilized (freeze-dried) powder. To reconstitute it, researchers typically add bacteriostatic water slowly down the inside wall of the vial — not directly onto the powder — and gently swirl rather than shake, to avoid damaging the fragile protein structure. Once mixed, the solution should be stored in a refrigerator (around 2–8 °C) and used within a few weeks for best stability, or kept frozen for longer-term storage. Avoid repeated freeze-thaw cycles, as these can degrade the protein. Always use sterile technique and keep reconstituted peptide away from direct light.
Sources
- Regulation of ovarian function by the TGF-beta superfamily and follistatin. — Reproduction (Cambridge, England), 2003. PMID 12887271.
- Possible endocrine control by follistatin 315 during liver regeneration based on changes in the activin receptor after a partial hepatectomy in rats. — Hepato-gastroenterology, 2005. PMID 15782995.
- Follistatin is essential for normal postnatal development and function of mouse oviduct and uterus. — Reproduction, fertility, and development, 2015. PMID 24630125.
- Follistatin Effects in Migration, Vascularization, and Osteogenesis in vitro and Bone Repair in vivo. — Frontiers in bioengineering and biotechnology, 2019. PMID 30881954.
- Peroxisome proliferator-activated receptor gamma down-regulates follistatin in intestinal epithelial cells through SP1. — The Journal of biological chemistry, 2008. PMID 18768463.
- Dynamics of inhibin subunit and follistatin mRNA during development of normal and polycystic ovary syndrome follicles. — The Journal of clinical endocrinology and metabolism, 2001. PMID 11549651.
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