GDF-8 (Myostatin)
Also known as: Myostatin, Growth/differentiation factor 8, MSTN
No approved myostatin-based product; myostatin inhibitors investigational and prohibited by WADA
5 notable studies · 6 references
TLDR
GDF-8 (myostatin) is a TGF-β family protein that limits skeletal muscle growth: losing it causes large muscles in mice, cattle and one reported child, while giving extra myostatin caused wasting in mice; research focuses on blocking it, early human inhibitor trials did not improve strength, and no myostatin product is approved.
At a Glance
Classification: Endogenous secreted signaling protein of the TGF-β superfamily; negative regulator of skeletal muscle mass; activin type II receptor (ActRIIB/ActRIIA) ligand
GDF-8, better known as myostatin, is a protein the body makes to limit muscle growth. It was discovered in 1997, when mice lacking the gene grew individual muscles two to three times heavier than normal. Natural loss of myostatin explains "double-muscled" cattle breeds such as Belgian Blue and Piedmontese, and a myostatin mutation was found in a child with unusually large muscles. Giving myostatin itself does the opposite of what bodybuilding marketing suggests. In adult mice, raising circulating myostatin caused severe loss of muscle and fat resembling cachexia (wasting). Most therapeutic research targets blocking myostatin, not supplying it. Myostatin inhibitors are investigational and prohibited in sport by WADA. No myostatin-based product is approved.
What Is GDF-8 (Myostatin)?
Myostatin is made as an inactive precursor. It is cut into a propeptide and a mature C-terminal dimer, which stay bound together in a latent (inactive) complex until released. Myostatin (GDF-8) → the muscle-limiting signal itself Follistatin / follistatin-344 → natural binding protein that blocks myostatin and activins ACE-031 → engineered soluble ActRIIB decoy receptor that traps myostatin and related ligands MYO-029 (stamulumab) → antibody that neutralizes myostatin Research-market "GDF-8" labeling is confusing: the protein itself is a muscle inhibitor.
What Are Researchers Studying?
Muscle mass regulation
GDF-8 knockout mice show widespread increases in muscle mass from both more muscle fibers (hyperplasia) and larger fibers (hypertrophy).
Natural mutations
Belgian Blue cattle carry an 11-base deletion that removes the active part of myostatin, and Piedmontese cattle carry a missense mutation, both producing double muscling. A child with a homozygous loss-of-function myostatin mutation had marked muscle hypertrophy, the first human case reported.
Myostatin and wasting
Myostatin circulates in an inactive form in adult mice. Raising myostatin levels systemically caused profound muscle and fat loss, supporting it as a target in wasting conditions.
Myostatin inhibition in muscular dystrophy
In a phase I/II trial of 116 adults with Becker, facioscapulohumeral or limb-girdle muscular dystrophy, the neutralizing antibody MYO-029 was generally tolerated (apart from skin hypersensitivity at high doses) but did not improve strength or function. The trial was not powered for efficacy.
How Does It Work?
Mature myostatin dimer (released from its latent complex) ↓ activin type II receptors (mainly ActRIIB) with type I receptors (ALK4/ALK5) ↓ SMAD2/3 signaling ↓ suppression of muscle growth and satellite-cell activity, increased protein breakdown ↓ limits skeletal muscle mass. Blockers include follistatin, the myostatin propeptide, decoy receptors and antibodies. Muscle-specific overexpression of each produced large muscle gains in mice.
What Does the Research Show?
Human research
LIMITED. Human data are genetic (a single reported child) and from trials of myostatin inhibitors, not of myostatin itself. No human study supports giving GDF-8 protein; animal data predict muscle wasting.
Animal research
HIGH: knockout mice, double-muscled cattle and systemic-overexpression mouse studies define its biology.
In-vitro research
MODERATE: receptor-binding studies show myostatin binds ActRIIB and that follistatin and the propeptide block it.
Notable Studies
Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member
animalJournal: Nature. Identified GDF-8, expressed specifically in skeletal muscle. GDF-8 knockout mice were larger, with individual muscles weighing 2-3 times more than normal from combined fiber hyperplasia and hypertrophy.
Limitations: Lifelong genetic deletion in mice; does not show effects of blocking myostatin in adults.
View publication →Double muscling in cattle due to mutations in the myostatin gene
animalJournal: Proceedings of the National Academy of Sciences of the USA. Belgian Blue cattle carry an 11-nucleotide deletion that eliminates mature myostatin; Piedmontese carry a cysteine-to-tyrosine substitution. Both match the knockout-mouse phenotype.
Limitations: Genetic association in livestock; naturally occurring lifelong loss.
View publication →Myostatin mutation associated with gross muscle hypertrophy in a child
humanJournal: New England Journal of Medicine. Case report of a child with marked muscle hypertrophy carrying a loss-of-function myostatin mutation, showing myostatin limits muscle mass in humans.
Limitations: Single case; genetic loss from birth; no information on effects of drugs that block or supply myostatin.
View publication →Induction of cachexia in mice by systemically administered myostatin
animalJournal: Science. Myostatin circulates in adult mouse blood in a latent form. Systemic overexpression in adult mice caused profound muscle and fat loss resembling human cachexia.
Limitations: Mouse model using overexpression; shows harm from excess myostatin, not therapeutic benefit.
View publication →A phase I/II trial of MYO-029 in adult subjects with muscular dystrophy
humanJournal: Annals of Neurology. Double-blind placebo-controlled dose-escalation trial in 116 adults with Becker, facioscapulohumeral or limb-girdle muscular dystrophy. MYO-029 was generally safe apart from cutaneous hypersensitivity at 10 and 30 mg/kg; no improvement in strength or function, with a trend toward larger muscles in a few subjects.
Limitations: Tests a myostatin-blocking antibody, not myostatin; not powered for efficacy.
View publication →What We Still Don't Know
• Whether myostatin inhibition produces meaningful functional benefit in human muscle diseases (early trials were disappointing) • Long-term safety of chronic myostatin blockade in adults • Why muscle gains seen in animals translate poorly to humans • Any legitimate rationale for administering GDF-8 protein itself (animal data predict wasting) • Identity and contents of research-market products labeled "GDF-8"
Related Compounds
Recent publications
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Sources & References
- McPherron AC et al. Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member. Nature. 1997.
- McPherron AC et al. Double muscling in cattle due to mutations in the myostatin gene. Proceedings of the National Academy of Sciences of the USA. 1997.
- Schuelke M et al. Myostatin mutation associated with gross muscle hypertrophy in a child. New England Journal of Medicine. 2004.
- Zimmers TA et al. Induction of cachexia in mice by systemically administered myostatin. Science. 2002.
- Wagner KR et al. A phase I/II trial of MYO-029 in adult subjects with muscular dystrophy. Annals of Neurology. 2008.
- Lee SJ, McPherron AC. Regulation of myostatin activity and muscle growth. Proc Natl Acad Sci USA. 2001.
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