Myostatin

Myostatin (formerly known as Growth differentiation factor 8) is a growth factor that limits muscle tissue growth, i.e. higher concentrations of myostatin in the body may cause the individual to have less developed muscles. The myostatin protein is produced primarily in skeletal muscle cells, circulates in the blood and acts on muscle tissue, apparently by slowing down the development of muscle stem cells. The precise mechanism remains unknown. Its functions in non-mammalian vertebrates appear to be somewhat conserved as muscle-specific actions have been demonstrated in birds. However, it is produced in many different fish tissues, suggesting that it may regulate more than just muscle mass in these vertebrates.

Discovery and sequencing
Myostatin and the associated gene were discovered in 1997 by geneticists Alexandra McPherron and Se-Jin Lee, who also produced a strain of mutant mice that lack the gene and have about twice as much muscle as normal mice. These mice were subsequently named "mighty mice". The gene has been sequenced in humans, mice, several other mammals and many different fish species, most of which are current or potential cultivars. The primary coding sequence is highly conserved among all vertebrates as is the genomic organization. These and other recent studies also indicate that the myostatin gene in fish has been duplicated as most fish species possess two distinct myostatin genes (MSTN-1 & -2) while salmonids have four (MSTN-1a, -1b, -2a & -2b)   This further suggests that the physiological and developmental aspects of myostatin biology may be quite different from that in mammals.

Effects of inactivated myostatin
Lee and others found in 1997 that the double muscled cattle breeds Belgian Blue and Piedmontese have defective myostatin genes; these strains have been produced through breeding.

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, a former sprinter, has a mutation in one copy of the gene. More recently, an American boy born 2005 was diagnosed with the same condition.

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

Human Myostatin consists of two identical subunits, each consisting of 110 amino acid residues. Its total molecular weight is 25.0 kDa. It can be produced in genetically engineered E. coli or eukaryotic cells and the recombinant protein from both sources is commercially available. However, due to the unique manner by which the mature protein is processed, there is considerable doubt as to the effectiveness of myostatin generated in E. coli.

Performance Enhancement in Dogs
A 2007 NIH study in PLOS Genetics found a significant relationship in whippets between a myostatin mutation and racing performance. Whippets that were heterozygous for a 2 base pair deletion in myostatin were significantly over-represented in the top racing classes. Whippets with a homozygous deletion were apparently less able runners although their overall appearance was significantly more muscular. The 2 base pair mutation resulted in a truncated myostatin mRNA, likely resulting in an inactive form of myostatin.

Interestingly, whippets with the homozygous deletion also had an unusual body shape, with a broader head, pronounced overbite, and shorter legs. These whippets have also been called "bully whippets" by the breeding community due to their size, but not their temperament.

This particular mutation was not found in other muscular dog breeds such as boxers and mastiffs, nor was it found in other slight hounds 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).

Potential clinical significance
Further research into myostatin and the myostatin gene may lead to therapies for muscular dystrophy. The idea is to introduce substances that block myostatin. In 2002, researchers at the University of Pennsylvania showed that monoclonal antibody specific to myostatin improves the condition of mice with muscular dystrophy, presumably by blocking myostatin's action.

In 2005, Lee showed that 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%). It is thought that binding of myostatin to the soluble activin receptor prevents it from interacting with the cell-bound receptors.

It remains unclear whether long term treatment of muscular dystropy with myostatin inhibitors is beneficial: the depletion of muscle stem cells could worsen the disease later on.

As of 2005, 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. The inhibor is called MYO-029 and recently underwent testing however the results have not yet been made public. Some athletes, eager to get their hands on such drugs, turn to the internet, where fake "myostatin blockers" are being sold.

Johns Hopkins University owns the patents on myostatin.