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Follistatin & Myostatin: Muscle Research Review

In 1997, geneticists Se-Jin Lee and Alexandra McPherron at Johns Hopkins knocked out a single gene in mice and watched them grow roughly twice the normal amount of muscle. The gene they silenced coded for a protein called myostatin — and its discovery opened an entirely new chapter in muscle biology.

In 1997, geneticists Se-Jin Lee and Alexandra McPherron at Johns Hopkins knocked out a single gene in mice and watched them grow roughly twice the normal amount of muscle. The gene they silenced coded for a protein called myostatin — and its discovery opened an entirely new chapter in muscle biology. Within months, researchers connected the dots to a phenomenon cattle breeders had observed for over a century: the extraordinary musculature of Belgian Blue and Piedmontese cattle, animals that carried natural loss-of-function mutations in the same gene.

On the other side of this equation sits follistatin, a protein that acts as myostatin's natural brake. Where myostatin tells muscles to stop growing, follistatin blocks that signal and lets growth continue. The relationship between these two proteins has become one of the most studied pathways in muscle physiology — and one of the most promising targets for treating muscle-wasting diseases.

This article reviews the major research on the follistatin-myostatin axis, from foundational animal studies to human gene therapy trials, and looks at where this field stands today.

Table of Contents

What Myostatin Does and Why It Matters {#what-myostatin-does}

Myostatin (also called Growth Differentiation Factor 8, or GDF-8) is a protein that belongs to the TGF-beta superfamily of signaling molecules. Its job is straightforward: it puts the brakes on muscle growth. Without it, muscles grow far beyond their normal size.

The signaling pathway works like this. Myostatin is produced by muscle cells as an inactive precursor called promyostatin. Through a series of cleavage steps involving furin convertase and BMP-1/tolloid metalloproteinases, the active mature myostatin is released. This active form binds to the activin type IIB receptor (ActRIIB) on muscle cell surfaces, which recruits type I receptors (ALK4 or ALK5). This triggers phosphorylation of Smad2 and Smad3, which pair with Smad4 and move into the cell nucleus to turn on genes that suppress muscle growth (PMC, 2009).

At the cellular level, myostatin does three things to limit muscle size:

  1. Arrests myoblast proliferation — It stops muscle precursor cells from dividing by locking them in the G1 phase of the cell cycle
  2. Blocks differentiation — It prevents myoblasts from maturing into muscle fibers by suppressing muscle-specific gene expression
  3. Keeps satellite cells dormant — It maintains the adult muscle stem cells in a quiescent state, preventing them from activating and contributing to muscle repair or growth

This makes myostatin a master regulator of muscle mass. Remove it, and all three growth-limiting mechanisms are released at once.

Follistatin: The Natural Myostatin Blocker {#follistatin-natural-blocker}

Follistatin is a glycoprotein produced throughout the body that functions as a potent inhibitor of several TGF-beta family members, including myostatin. It blocks myostatin through a direct binding mechanism — it physically grabs onto the myostatin molecule and prevents it from reaching its receptor.

The follistatin gene contains six exons and produces two main isoforms through alternative splicing:

  • FS344/FS315 — The longer isoform. FS344 is the mRNA transcript; after translation and removal of the signal peptide, it becomes the circulating form FS315. This isoform is primarily found in blood serum and has roughly 10-fold lower affinity for activin compared to FS288, making it more selective.
  • FS317/FS288 — The shorter isoform. FS317 is the mRNA transcript; FS288 is the processed protein. This form binds more tightly to cell surfaces through heparan sulfate proteoglycans and has broader binding activity.

What makes follistatin particularly interesting is that its muscle-building effects go beyond just blocking myostatin. A landmark experiment crossed myostatin-knockout mice with mice carrying a follistatin transgene. The offspring had quadruple the muscle mass of normal mice — double what you'd expect from eliminating myostatin alone (PNAS, 2007). This means follistatin also inhibits other TGF-beta family members — particularly activins — that independently restrict muscle growth.

The Belgian Blue Story: Natural Myostatin Mutations {#belgian-blue-story}

Before anyone knew what myostatin was, cattle breeders had been working with its effects for generations. Belgian Blue cattle are dramatically more muscular than typical breeds, a trait breeders call "double muscling."

In 1997, three research groups simultaneously identified the molecular basis: mutations in the myostatin gene. The Belgian Blue mutation is an 11-nucleotide deletion (nt821del11) in exon 3 that creates a frameshift and premature stop codon, eliminating virtually all of the mature, active region of the protein (Genome Research, 1997). Piedmontese cattle carry a different mutation — a missense change that swaps cysteine for tyrosine in the mature region, disrupting the protein's ability to form its normal disulfide-bonded structure.

The similarity between the double-muscled cattle phenotype and myostatin-knockout mice confirmed that myostatin performs the same biological function across species. In both cases, loss of myostatin results in muscle fiber hyperplasia (more fibers) rather than just hypertrophy (bigger fibers) — the animals are born with a higher number of muscle fibers that then grow to normal or larger-than-normal size (PNAS, 1997).

There's a practical trade-off, though. Belgian Blue cattle often need caesarean sections for delivery because the calves are so heavily muscled, and the breed requires more expensive feed to maintain. Biology doesn't give away something for nothing.

Natural myostatin mutations have since been documented in other species, including whippet dogs (where heterozygous "bully" whippets are faster runners, but homozygous animals are excessively muscled) and rare cases in humans. A German boy documented in 2004 carried a myostatin mutation and displayed extraordinary musculature from birth.

Key Animal Studies {#key-animal-studies}

Myostatin Knockout Mice (McPherron et al., 1997)

The original myostatin knockout study remains foundational. Mice lacking functional myostatin showed a 200-300% increase in skeletal muscle mass compared to wild-type controls. The increase came from both hyperplasia (more muscle fibers) and hypertrophy (larger individual fibers). These animals were otherwise healthy and fertile.

Follistatin Overexpression (Lee, 2007)

Transgenic mice overexpressing follistatin showed an even more dramatic response — a 194-327% increase in muscle mass compared to controls (PNAS, 2007). This exceeded the muscle gains seen in myostatin-null animals, confirming that follistatin targets multiple growth-inhibiting pathways beyond myostatin alone.

The double intervention — follistatin overexpression in myostatin-knockout mice — produced quadrupled muscle mass. This experiment was the definitive proof that follistatin's effects are not limited to myostatin inhibition.

Follistatin-Deficient Mice

On the other end of the spectrum, mice lacking follistatin have reduced muscle mass, skeletal defects, retarded growth, and die within hours of birth. This lethality shows that follistatin isn't just a muscle regulator — it plays roles throughout development by keeping TGF-beta family signaling in check.

Systemic Follistatin-288 Administration (Winbanks et al., 2012)

A study published in Scientific Reports tested what happens when you inject the FS288 isoform into normal mice. Daily injections produced dose-dependent increases in lean body mass over 13 weeks, with significant muscle mass gains at higher doses. The researchers also observed muscle fiber type switching through myosin heavy chain remodeling and a corresponding decrease in body fat mass (Scientific Reports, 2012).

Engineered Follistatin for Muscular Dystrophy (Pearsall et al., 2019)

Researchers designed a long-acting follistatin variant called FS-EEE-hFc — an engineered molecule with three glutamate mutations that reduce heparin binding (allowing greater systemic exposure) while retaining potent binding to both myostatin and activin A. When tested in the mdx mouse model of Duchenne muscular dystrophy, this engineered follistatin outperformed anti-myostatin antibody alone, showing greater improvements in muscle function and reduced dystrophic pathology (Skeletal Muscle, 2019).

This finding reinforced a recurring theme: blocking both myostatin and activin produces better outcomes than targeting myostatin alone.

Human Clinical Trials: ACE-031 {#ace-031-trials}

ACE-031 was the first myostatin pathway inhibitor to reach human clinical trials. Developed by Acceleron Pharma, it is a fusion protein combining the extracellular domain of the activin type IIB receptor with an IgG1 antibody tail. By acting as a decoy receptor, ACE-031 binds myostatin (and related ligands) before they can reach the real receptors on muscle cells.

Phase 1: Healthy Volunteers

The first-in-human study tested ACE-031 in 48 healthy postmenopausal women at escalating doses (0.02-3 mg/kg subcutaneously). At the highest dose, total body lean mass increased by 3.3% and thigh muscle volume increased by 5.1% at day 29 — from a single injection. The half-life was 10-15 days, and the drug was generally well-tolerated (PubMed, 2012).

Phase 2: Duchenne Muscular Dystrophy

The more ambitious trial enrolled 24 ambulatory boys with Duchenne muscular dystrophy (DMD) in a randomized, double-blind, placebo-controlled design. Eighteen patients received ACE-031 and six received placebo over 12 weeks (Muscle & Nerve, 2017).

The efficacy signals were encouraging: dose-dependent increases in lean body mass up to 4.1% in the high-dose group (versus 2.5% in placebo), with trends toward increased six-minute walk distance.

But the trial was halted after the second dosing cohort due to safety concerns. Some patients developed nosebleeds (epistaxis) and telangiectasias — dilated blood vessels visible on the skin. The likely culprit was ACE-031's non-selective binding: in addition to myostatin, it blocks BMP-9 and BMP-8, proteins involved in blood vessel maintenance (Current Opinion in Neurology, 2020).

Acceleron and partner Shire halted the program in 2011 and permanently discontinued it in 2013. ACE-031 proved that myostatin pathway inhibition works in humans, but also demonstrated the risks of non-selective approaches.

Follistatin Gene Therapy: From Lab to Clinic {#follistatin-gene-therapy}

While ACE-031 took the drug approach, a team at Nationwide Children's Hospital led by Dr. Jerry Mendell pursued a different strategy: delivering the follistatin gene directly to muscle using an adeno-associated virus (AAV) vector.

Phase 1/2a Trial in Becker Muscular Dystrophy

The trial (NCT01519349) enrolled six patients with Becker muscular dystrophy (BMD) — a milder form of dystrophin-deficient muscle disease. Researchers used AAV1 carrying the FS344 isoform gene, injected directly into the quadriceps muscles of both legs (Molecular Therapy, 2015).

Results at six months:

  • The primary endpoint — distance walked on the six-minute walk test (6MWT) — improved by an average of 11.5% across all patients (p = 0.02)
  • Individual responses varied: two patients improved dramatically (58 and 125 meters), while one showed no change in the low-dose cohort. In the high-dose cohort, improvements were 108 and 29 meters, with one non-responder.
  • Muscle biopsies showed reduced endomysial fibrosis (scarring), decreased central nucleation (a marker of muscle damage), and muscle fiber hypertrophy — particularly at the higher dose
  • The high-dose cohort showed fibrosis reduced to 35-43% of baseline values (p < 0.017)
  • No adverse effects were reported

Why some patients responded better than others:

MRI analysis revealed that the non-responders had significantly more pre-existing muscle deterioration and fat infiltration. Patients with less advanced disease responded more robustly, suggesting a window of opportunity for gene therapy before muscle tissue is replaced by fat and scar tissue (Journal of Neuromuscular Diseases, 2015).

This was the first human clinical trial delivering the follistatin gene, and while the sample size was small, the results demonstrated safety and provided clear biological evidence of muscle improvement.

Bimagrumab: The Antibody Approach {#bimagrumab}

After ACE-031's termination, the field shifted toward more selective approaches. Bimagrumab (developed by Novartis) is a monoclonal antibody that targets activin type II receptors (both ActRIIA and ActRIIB), blocking myostatin, activins, and GDF-11 from binding.

Obesity and Type 2 Diabetes

Perhaps the most striking results came from a Phase 2 trial in 75 patients with obesity and type 2 diabetes. Over 48 weeks, bimagrumab treatment produced (JAMA, 2021):

  • A 20.5% reduction in total body fat mass
  • A 9.5 cm decrease in waist circumference
  • A 4.4% increase in lean body mass
  • A 0.76 percentage-point reduction in HbA1c

These are remarkable numbers. No other single agent has simultaneously increased muscle mass while decreasing fat mass to this degree in a clinical trial.

Sarcopenia

In older adults with sarcopenia, bimagrumab showed significant increases in muscle mass and improvements in physical performance measures. A 2024 systematic review and meta-analysis confirmed the body composition benefits across multiple studies (Aging Clinical and Experimental Research, 2024).

Ongoing Research

Bimagrumab is currently being evaluated in the Phase 2b BELIEVE study, both as monotherapy and in combination with semaglutide for overweight and obese adults. The combination is particularly interesting because GLP-1 agonists like semaglutide cause significant fat loss but also some muscle loss — adding bimagrumab could potentially preserve lean mass during weight reduction (ClinicalTrials.gov, NCT05616013).

Beyond Muscle: Metabolic Effects {#beyond-muscle}

The follistatin-myostatin axis doesn't just control muscle size — it has significant metabolic implications.

Fat Loss and Adipose Browning

A 2021 review in Frontiers in Endocrinology explored how follistatin and myostatin regulate adipose tissue biology. TGF-beta signaling promotes fat storage and white adipose tissue accumulation. Blocking this pathway — whether through follistatin overexpression or myostatin inhibition — promotes white adipose browning: the conversion of energy-storing white fat cells into energy-burning brown-like (beige) fat cells through activation of uncoupling protein-1 (UCP1) (Frontiers in Endocrinology, 2021).

In the systemic FS288 injection study, mice showed not just increased muscle but also significant decreases in body fat mass over 13 weeks — consistent with the dual muscle-building, fat-reducing effect seen across multiple models.

Insulin Sensitivity and Metabolic Health

More muscle mass means more tissue for glucose disposal, which improves insulin sensitivity. Myostatin inhibition is associated with improved glucose metabolism in both animal models and the human bimagrumab trials. The connection between muscle mass, metabolic health, and myostatin signaling has positioned this pathway as a potential target for metabolic syndrome and type 2 diabetes treatment.

These metabolic effects explain why myostatin pathway research has expanded beyond muscular dystrophy into obesity, sarcopenia, and age-related metabolic decline.

Resistance Training and the Follistatin-Myostatin Response {#resistance-training}

You don't need drugs or gene therapy to modulate the follistatin-myostatin axis. Resistance training does it naturally.

A 2023 systematic review and meta-analysis examined 26 randomized studies involving 768 participants (ages 18-82) and found that resistance training consistently:

  • Decreased myostatin levels with a standardized mean difference of -1.31 (95% CI: -1.74 to -0.88, p = 0.001)
  • Increased follistatin levels with a standardized mean difference of 2.04 (95% CI: 1.51 to 2.52, p = 0.001)

(PubMed, 2023)

In other words, lifting weights shifts the follistatin-myostatin balance in the same direction as the pharmacological interventions — suppressing the growth inhibitor while boosting the growth promoter. The magnitude is obviously smaller than genetic knockout or drug intervention, but the direction is the same, and it occurs reliably across age groups.

This has practical implications for managing sarcopenia (age-related muscle loss). Exercise programs combined with adequate protein intake represent the current first-line approach, and understanding the follistatin-myostatin mechanism helps explain why resistance training is more effective than aerobic exercise alone for preserving muscle mass in older adults.

Limitations and Open Questions {#limitations}

Despite decades of research, several important questions remain unanswered.

The selectivity problem. ACE-031's failure highlighted a core challenge: myostatin shares its receptor with other TGF-beta family members that have different functions elsewhere in the body. Blocking the receptor non-selectively can cause off-target effects. Newer approaches like selective myostatin antibodies and engineered follistatin variants aim to solve this, but the selectivity-potency trade-off remains a design challenge.

Functional gains vs. mass gains. Increasing muscle mass doesn't always translate to proportional increases in strength or function. Some myostatin inhibitor trials have shown significant lean mass gains without corresponding improvements in physical performance measures. Understanding why — and how to ensure mass gains translate to functional improvement — is an active area of investigation.

Long-term effects are unknown. Most clinical trials have lasted weeks to months. The long-term consequences of chronic myostatin pathway inhibition — on tendon strength, cardiac muscle, hormonal axes, and other systems — remain unstudied.

Cardiac considerations. Myostatin is expressed in heart tissue, and the effects of systemic myostatin inhibition on cardiac function are not fully understood. Some preclinical data suggest potential concerns about cardiac hypertrophy with prolonged treatment.

Timing matters. The follistatin gene therapy trial showed that patients with advanced muscle deterioration responded poorly. This suggests that myostatin inhibition may need to be applied before irreversible tissue damage occurs — an important consideration for progressive diseases like Duchenne muscular dystrophy.

Frequently Asked Questions {#faq}

What is the difference between follistatin and myostatin?

Myostatin is a protein that limits muscle growth. Follistatin is a protein that blocks myostatin (and other related growth inhibitors). They work as a natural check-and-balance system: myostatin keeps muscle size under control, and follistatin prevents myostatin from being too restrictive. When this balance shifts toward less myostatin or more follistatin, muscle mass increases.

Can you increase follistatin naturally?

Yes. Resistance training consistently increases follistatin levels and decreases myostatin levels in human studies. A 2023 meta-analysis of 26 studies confirmed this effect across age groups. The magnitude is smaller than pharmacological intervention, but the direction is consistent and reliable.

What happened to ACE-031?

ACE-031 showed promising increases in lean body mass in both healthy volunteers and boys with Duchenne muscular dystrophy, but the program was halted in 2011 due to vascular side effects (nosebleeds and telangiectasias). These were caused by the drug's non-selective blocking of BMP-9, a protein involved in blood vessel maintenance. The program was permanently discontinued in 2013.

Is follistatin gene therapy available?

Not as a standard treatment. A Phase 1/2a trial at Nationwide Children's Hospital showed encouraging results in six patients with Becker muscular dystrophy, with improved walking ability and reduced muscle fibrosis. However, the therapy is still experimental and limited to clinical trial settings.

What is bimagrumab and is it approved?

Bimagrumab is a monoclonal antibody that blocks activin type II receptors, preventing myostatin and related proteins from signaling. It has shown strong results in clinical trials — particularly a 20% reduction in body fat mass in obese patients with type 2 diabetes. It is not yet approved by the FDA and is currently in Phase 2b trials, including a study combining it with semaglutide.

This is one of the most promising potential applications. Sarcopenia affects an estimated 10-16% of older adults worldwide and increases the risk of falls, fractures, and loss of independence. Bimagrumab has shown muscle mass increases in sarcopenic older adults, and several newer myostatin inhibitors (including taldefgrobep alfa and SRK-439) are in development for this indication.

The Bottom Line {#the-bottom-line}

The follistatin-myostatin axis represents one of the best-characterized growth regulatory systems in biology. From the Belgian Blue cattle that revealed myostatin's role to the first human gene therapy trials delivering follistatin, the research journey has been both scientifically rich and clinically promising.

The core biology is clear: myostatin limits muscle growth through the TGF-beta/Smad signaling pathway, and follistatin blocks that signal while also inhibiting other growth-restricting factors like activins. Removing or blocking myostatin produces dramatic muscle gains in every species tested.

Translating this biology into safe, effective therapies has proven harder. ACE-031 showed that the pathway works in humans but also demonstrated the risks of non-selective approaches. Follistatin gene therapy showed proof-of-concept in muscular dystrophy patients but remains early-stage. Bimagrumab may be the closest to a practical therapy, with particularly strong data in obesity and metabolic disease.

Meanwhile, resistance training remains the most accessible way to shift the follistatin-myostatin balance in a favorable direction — a reminder that the best-studied molecular pathway for muscle growth is the same one activated by picking up heavy things and putting them down.

References {#references}

  1. McPherron AC, Lawler AM, Lee SJ. "Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member." Nature, 1997

  2. Grobet L, et al. "Mutations in myostatin (GDF8) in double-muscled Belgian Blue and Piedmontese cattle." Genome Research, 1997

  3. Kambadur R, et al. "Mutations in myostatin (GDF8) in double-muscled Belgian Blue and Piedmontese cattle." Genome Research, 1997

  4. Lee SJ. "Quadrupling muscle mass in mice by targeting TGF-beta signaling pathways." PNAS, 2007

  5. Gilson H, et al. "Follistatin induces muscle hypertrophy through satellite cell proliferation and inhibition of both myostatin and activin." American Journal of Physiology, 2009

  6. Rodino-Klapac LR, et al. "Inhibition of myostatin with emphasis on follistatin as a therapy for muscle disease." PMC, 2009

  7. Winbanks CE, et al. "Systemic administration of Follistatin288 increases muscle mass and reduces fat accumulation in mice." Scientific Reports, 2012

  8. Attie KM, et al. "A single ascending-dose study of muscle regulator ACE-031 in healthy volunteers." PubMed, 2012

  9. Mendell JR, et al. "A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy." Molecular Therapy, 2015

  10. Al-Zaidy SA, et al. "Follistatin gene therapy improves ambulation in Becker muscular dystrophy." Journal of Neuromuscular Diseases, 2015

  11. Campbell C, et al. "Myostatin inhibitor ACE-031 treatment of ambulatory boys with Duchenne muscular dystrophy: Results of a randomized, placebo-controlled clinical trial." Muscle & Nerve, 2017

  12. Pearsall RS, et al. "Myostatin and activin blockade by engineered follistatin results in hypertrophy and improves dystrophic pathology in mdx mouse more than myostatin blockade alone." Skeletal Muscle, 2019

  13. Heymsfield SB, et al. "Effect of bimagrumab vs placebo on body fat mass among adults with type 2 diabetes and obesity." JAMA, 2021

  14. Singh R, et al. "Novel roles of follistatin/myostatin in TGF-beta signaling and adipose browning." Frontiers in Endocrinology, 2021

  15. Sepulveda D, et al. "The effects of resistance training on myostatin and follistatin in adults: A systematic review and meta-analysis." PubMed, 2023

  16. "Therapeutic applications and challenges in myostatin inhibition for enhanced skeletal muscle mass and functions." PMC, 2025