Myostatin

Myostatin (also known as growth differentiation factor 8, abbreviated GDF-8) is a protein that in humans is encoded by the MSTN gene.^[2] Myostatin is a secreted growth differentiation factor that is a member of the TGF beta protein family that inhibits muscle differentiation and growth in the process known as myogenesis. Myostatin is produced primarily in skeletal muscle cells, circulates in the blood and acts on muscle tissue, by binding a cell-bound receptor called the activin type II receptor.^[3][4]

Animals lacking myostatin or animals treated with substances that block the activity of myostatin have significantly larger muscles. This could be of economic benefit to the livestock industry. However these animals require special care and feeding which offsets the potential economic advantage.

Mutations in both copies of the human myostatin gene results in individuals that have significantly more muscle mass and hence are considerably stronger than normal. Furthermore blocking the activity of myostatin may have therapeutic application in treating muscle wasting diseases such as muscular dystrophy.

Discovery and sequencing

The gene encoding myostatin was discovered in 1997 by geneticists Dr. Se-Jin Lee and Alexandra McPherron who also produced a strain of mutant mice that lack the gene. These myostatin "knockout" mice have approximately twice as much muscle as normal mice.^[5] These mice were subsequently named "mighty mice".

Naturally occurring myostatin "nulls" have been identified in cattle, whippets, and humans; in each case the result is a dramatic increase in muscle mass. A mutation in the 3' UTR of the myostatin gene in Texel sheep creates target sites for the microRNAs miR-1 and miR-206. This is likely to cause the muscular phenotype of this breed of sheep.^[6]

Structure and mechanism of action

Myostatin is a member of the TGF beta superfamily of proteins.

Human myostatin consists of two identical subunits, each consisting of 109 (NCBI database claims human myostatin is 375 residues long) amino acid residues. Its total molecular weight is 25.0 kDa. The protein is made in an inactive form. For it to be activated, a protease cleaves the NH2-terminal, or "pro-domain" portion of the molecule, resulting in the now-active COOH-terminal dimer.

Myostatin binds to the activin type II receptor, resulting in a recruitment of a coreceptor called Alk-3 or Alk-4. This coreceptor then initiates a cell signaling cascade in the muscle, which includes the activation of transcription factors in the SMAD family - SMAD2 and SMAD3. These factors then induce myostatin-specific gene regulation. When applied to myoblasts, myostatin inhibits their differentiation into mature muscle fibers.

Recently, myostatin has also been shown to inhibit Akt, a kinase that is sufficient to cause muscle hypertrophy, in part through the activation of protein synthesis.

Therefore, myostatin acts in two ways, by inhibiting muscle differentiation and by inhibiting Akt-induced protein synthesis.

Effects in animals

Double muscled cattle

After that discovery, several laboratories cloned and established the nucleotide sequence of a myostatin gene in two breeds of cattle Belgian Blue and Piedmontese, and found that these animals have mutations in that myostatin gene (various mutations in each breed) which in one way or another lead to absence of functional myostatin.^[5][7][8] Unlike mice with a damaged myostatin gene, in these cattle breeds the muscle cells multiply rather than enlarge. People describe these cattle breeds as "double muscled", but the total increase in all muscles is no more than 40%.^[9][7][10]

Animals lacking myostatin or animals treated with substances such as follistatin that block the binding of myostatin to its receptor have significantly larger muscles. Thus, reduction of myostatin could potentially benefit the livestock industry, with even a 20 percent reduction in myostatin levels potentially having a large effect on the development of muscles.^[11][12]

However, the animal breeds developed as homozygous for myostatin deficiency have reproduction issues due to their unusually heavy and bulky offspring, and require special care and a more expensive diet to achieve a superior yield. This negatively affects economics of myostatin-deficient breeds to the point where they do not usually offer an obvious advantage. While e.g. Piedmontese beef has a place on the specialist market due to its unusual properties, at least for purebred myostatin-deficient strains the expenses and (especially in cattle) necessity of veterinary supervision place them at a disadvantage in the bulk market.^[13]

Performance enhancement in dogs

A "bully whippet" with a homozygous mutation in myostatin.^[1]

A 2007 NIH study in PLOS Genetics^[1] found a significant relationship in whippets between a myostatin mutation and racing performance. Whippets that were heterozygous for a 2 base pair deletion in the myostatin gene were significantly over-represented in the top racing classes. The mutation resulted in a truncated myostatin protein, likely resulting in an inactive form of myostatin.

Whippets with a homozygous deletion were apparently less able runners although their overall appearance was significantly more muscular. Whippets with the homozygous deletion also had an unusual body shape, with a broader head, pronounced overbite, shorter legs, and thicker tails. These whippets have also been called "bully whippets" by the breeding community due to their size. Despite the name "bully", these dogs tend to have a friendly and positive demeanour towards people as usual for whippets.

This particular mutation was not found in other muscular dog breeds such as boxers and mastiffs, nor was it found in other sighthounds such as greyhounds, Italian greyhounds, or Afghan hounds. The authors of the study suggest that myostatin mutation may not be desirable in greyhounds, the whippets' nearest relative, because greyhound racing requires more significant endurance due to the longer races (900 meters for greyhounds vs. 300 meters for whippets).

Clinical significance

Mutations

Myostatin is active in muscles used for movement (skeletal muscles) both before and after birth. It normally restrains muscle growth, ensuring that muscles do not grow too large. Mutations that reduce the production of functional myostatin lead to an overgrowth of muscle tissue. Myostatin-related muscle hypertrophy has a pattern of inheritance known as incomplete autosomal dominance. People with a mutation in both copies of the MSTN gene in each cell (homozygotes) have significantly increased muscle mass and strength. People with a mutation in one copy of the MSTN gene in each cell (heterozygotes) also have increased muscle bulk, but to a lesser degree.

In 2004, a German boy was diagnosed with a mutation in both copies of the myostatin-producing gene, making him considerably stronger than his peers. His mother has a mutation in one copy of the gene.^{[14][15][16][17]}

An American boy born in 2005, Liam Hoekstra, was diagnosed with a clinically similar condition but with a somewhat different cause:^[18] his body produces a normal level of functional myostatin, but because he is stronger and more muscular than most others his age, his doctor believes that a defect in his myostatin receptors prevents his muscle cells from responding normally to myostatin. Liam appeared on the television show World's Strongest Toddler.

A technique for detecting mutations in myostatin variants has been developed.^[19]

Therapeutic potential

Further research into myostatin and the myostatin gene may lead to therapies for muscular dystrophy.^[20][21] The idea is to introduce substances that block myostatin. A monoclonal antibody specific to myostatin increases muscle mass in mice.^[22] Similar results in monkeys were obtained.^[12]

A two-week treatment of normal mice with soluble activin type IIB receptor, a molecule that is normally attached to cells and binds to myostatin, leads to a significantly increased muscle mass (up to 60%).^[23] It is thought that binding of myostatin to the soluble activin receptor prevents it from interacting with the cell-bound receptors.

It remains unclear as to whether long-term treatment of muscular dystrophy with myostatin inhibitors is beneficial, as the depletion of muscle stem cells could worsen the disease later on. As of 2012, no myostatin-inhibiting drugs for humans are on the market, but an antibody genetically engineered to neutralize myostatin was developed by New Jersey pharmaceutical company Wyeth.^[24] The inhibitor is called stamulumab, but, after an initial clinical trial, Wyeth says they will not be developing the drug.^[25] Some athletes, eager to get their hands on such drugs, turn to the internet, where fake "myostatin blockers" are being sold.^[12]

Myostatin levels are effectively decreased by creatine supplementation.^[26]

Gene doping

Inhibition of myostatin leads to muscle hyperplasia and hypertrophy. Myostatin inhibitors may improve athletic performance and therefore there is a concern these inhibitors might be abused in the field of sports.^[27] However, studies in mice suggest that myostatin inhibition does not directly increase the strength of individual muscle fibers.^[28]

See also

• Muscular dystrophy
• Muscle hypertrophy
• Superhuman strength

References

External links

• GeneReviews profile
• NPR.org: Myostatin Therapies Hold Hope for Muscle Diseases by Jon Hamilton
• Times Colonist Big Wendy the muscular whippet
• myostatin at the US National Library of Medicine Medical Subject Headings (MeSH)