Myostatin

Myostatin (also known as growth differentiation factor 8, abbreviated GDF-8) is a is_associated_with::myokine, a is_associated_with::protein produced by muscle cells that acts on muscle cells (autocrine function) to inhibit is_associated_with::myogenesis: muscle cell growth and differentiation. In humans it is encoded by the MSTN is_associated_with::gene. Myostatin is a secreted is_associated_with::growth differentiation factor that is a member of the TGF beta protein family.

Animals either lacking myostatin or treated with substances that block the activity of myostatin have significantly more muscle mass. Furthermore, individuals who have mutations in both copies of the myostatin gene have significantly more muscle mass and are stronger than normal. Blocking the activity of myostatin may have therapeutic application in treating muscle wasting diseases such as is_associated_with::muscular dystrophy.

Discovery and sequencing
The is_associated_with::gene encoding myostatin was discovered in 1997 by geneticists 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. These mice were subsequently named "mighty mice".

Naturally occurring deficiencies of myostatin have been identified in cattle by Ravi Kambadur, is_associated_with::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 is_associated_with::microRNAs miR-1 and miR-206. This is likely to cause the muscular phenotype of this breed of sheep.

Structure and mechanism of action
Human myostatin consists of two identical subunits, each consisting of 109 (NCBI database claims human myostatin is 375 residues long) is_associated_with::amino acid residues. Its total molecular weight is 25.0 kDa. The protein is inactive until a is_associated_with::protease cleaves the NH2-terminal, or "pro-domain" portion of the molecule, resulting in the active COOH-terminal dimer.

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

Myostatin also inhibits Akt, a kinase that is sufficient to cause is_associated_with::muscle hypertrophy, in part through the activation of protein synthesis. However, Akt is not responsible for all of the observed muscle hyperthrophic effects which are mediated by myostatin inhibition Thus myostatin acts in two ways: by inhibiting is_associated_with::muscle differentiation, and by inhibiting Akt-induced protein synthesis.

Double muscled cattle
After that discovery, several laboratories cloned and established the is_associated_with::nucleotide sequence of a myostatin gene in two breeds of cattle is_associated_with::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. 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%.

Animals lacking myostatin or animals treated with substances such as is_associated_with::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.

However, the animal breeds developed as is_associated_with::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 hypertrophic meat (e.g. from 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.

Performance enhancement in dogs
A 2007 NIH study in PLOS Genetics found a significant relationship in is_associated_with::whippets between a myostatin mutation and racing performance. Whippets that were is_associated_with::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 is_associated_with::protein, likely resulting in an inactive form of myostatin.

is_associated_with::Whippets with a is_associated_with::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 is_associated_with::sighthounds such as is_associated_with::greyhounds, is_associated_with::Italian greyhounds, or is_associated_with::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).

Mutations
A technique for detecting mutations in myostatin variants has been developed. Mutations that reduce the production of functional myostatin lead to an overgrowth of muscle tissue. Myostatin-related muscle hypertrophy has an incomplete autosomal dominance pattern of inheritance. People with a mutation in both copies of the MSTN gene in each cell (is_associated_with::homozygotes) have significantly increased muscle mass and strength. People with a mutation in one copy of the MSTN gene in each cell (is_associated_with::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. An American boy born in 2005 was diagnosed with a clinically similar condition but with a somewhat different cause: his body produces a normal level of functional myostatin; but, because he is stronger and more muscular than most others his age, it is believed that a defect in his myostatin receptors prevents his muscle cells from responding normally to myostatin. He appeared on the television show World's Strongest Toddler.

Therapeutic potential
Further research into myostatin and the myostatin gene may lead to therapies for is_associated_with::muscular dystrophy. The idea is to introduce substances that block myostatin. A is_associated_with::monoclonal antibody specific to myostatin increases muscle mass in mice and monkeys.

A two-week treatment of normal mice with soluble is_associated_with::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%). 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. , no myostatin-inhibiting drugs for humans are on the market. An antibody genetically engineered to neutralize myostatin, is_associated_with::stamulumab, which was under development by pharmaceutical company is_associated_with::Wyeth., is no longer under development. Some athletes, eager to get their hands on such drugs and turn to the internet where fake "myostatin blockers" are being sold.

Myostatin levels are effectively decreased by is_associated_with::creatine supplementation.

Gene doping
Inhibition of myostatin leads to muscle is_associated_with::hyperplasia and is_associated_with::hypertrophy. Myostatin inhibitors can improve athletic performance and therefore there is a concern these inhibitors might be abused in the field of sports. However, studies in mice suggest that myostatin inhibition does not directly increase the strength of individual muscle fibers.

Myostatin in the heart
Myostatin is expressed at very low levels in cardiac myocytes. Although its presence has been noted in is_associated_with::cardiomyocytes of both fetal and adult mice, its physiological function remains uncertain. However, it has been suggested that fetal cardiac myostatin may play a role in early heart development.

Myostatin is produced as promyostatin, a precursor protein kept inactive by the latent TGF-β binding protein 3 (LTBP3). Pathological cardiac stress promotes N-terminal cleavage by is_associated_with::furin convertase to create a biologically active C-terminal fragment. The mature myostatin is then segregated from the latent complex via proteolytic cleavage by is_associated_with::BMP-1 and tolloid metallopreoteinases. Free myostatin is able to bind its receptor, ActRIIB, and increase SMAD2/3 phosphorylation. The latter produces a heteromeric complex with is_associated_with::SMAD4, inducing myostatin translocation into the cardiomyocyte nucleus to modulate transcription factor activity. Manipulating the muscle creatinine kinase promoter can modulate myostatin expression, though it has only been observed in male mice thus far.

Myostatin may inhibit cardiomyocyte proliferation and differentiation by manipulating cell cycle progression. This argument is supported by the fact that myostatin mRNA is poorly expressed in proliferating fetal cardiomyocytes. In vitro studies indicate that myostatin promotes is_associated_with::SMAD2 phosphorylation to inhibit cardiomyocyte proliferation. Furthermore, myostatin has been shown to directly prevent cell cycle G1 to S phase transition by decreasing levels of cyclin-dependent kinase complex 2 (CDK2) and by increasing is_associated_with::p21 levels.

Growth of cardiomyocytes may also be hindered by myostatin-regulated inhibition of protein kinase p38 and the serine-threonine protein kinase is_associated_with::Akt, which typically promote cardiomyocyte is_associated_with::hypertrophy. However, increased myostatin activity only occurs in response to specific stimuli, such as in pressure stress models, in which cardiac myostatin induces whole-body muscular is_associated_with::atrophy.

Physiologically, minimal amounts of cardiac myostatin are secreted from the myocardium into serum, having a limited effect on muscle growth. However, increases in cardiac myostatin can increase its serum concentration, which may cause skeletal muscle atrophy. Pathological states that increase cardiac stress and promote is_associated_with::heart failure can induce a rise in both cardiac myostatin mRNA and protein levels within the heart. In ischemic or is_associated_with::dilated cardiomyopathy, increased levels of myostatin mRNA have been detected within the left ventricle. As a member of the TGF-β family, myostatin may play a role in post-infarct recovery. It has been hypothesized that hypertrophy of the heart induces an increase in myostatin as a negative feedback mechanism in an attempt to limit further myocyte growth. This process includes mitogen-activated protein kinases and binding of the MEF2 transcription factor within the promoter region of the myostatin gene. Increases in myostatin levels during is_associated_with::chronic heart failure have been shown to cause cardiac is_associated_with::cachexia. Systemic inhibition of cardiac myostatin with the JA-16 antibody maintains overall muscle weight in experimental models with pre-existing heart failure.

Myostatin also alters excitation-contraction (EC) coupling within the heart. A reduction in cardiac myostatin induces eccentric hypertrophy of the heart, and increases its sensitivity to beta-adrenergic stimuli by enhancing Ca2+ release from the SR during EC coupling. Also, is_associated_with::phospholamban phosphorylation is increased in myostatin-knockout mice, leading to an increase in Ca2+ release into the cytosol during systole. Therefore, minimizing cardiac myostatin may improve cardiac output.