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Also known as MSTN, GDF8, MSLHP, Miostatin
Myostatin, as research peptide, is a negative regulator of muscle cell growth, it is highly conserved across species. The loss of functional myostatin is known to cause the “double-muscled” phenotype in several cattle breeds, and similar phenotypes in other species. For nearly 200 years, double-muscled animals have captured the attention of livestock breeders and researchers, boasting enlarged musculature but beset by production difficulties. With the advent of transgenic technology, researchers have created a “knockout” mouse model with which to efficiently explore the biochemical pathways and influences of myostatin. Research involving this model has both agricultural and biomedical applications, and involves several cell growth and regulation mechanisms. Analysis of growth and development patterns in myostatin-null mice is necessary to link these findings with past research.
The discovery of the myostatin In spite of several investigations, the genetic cause of the muscular hypertrophy remained unknown for a long time. Although, the first written document about the ‘double muscled’ cattle dates back to 1807 (CULLEY, 1807, cit. MÉNISSIER, 1982), the consequent observation of the inheritance of the trait did not reveal more than a speculation: it is probably a monogenic, autosomal and partially recessive trait, with an incomplete penetrance (MÉNISSIER, 1982).
The members of the TGF-β (Transforming Growth Factor) family were studied in mouse as a model animal. Using gene-targeting (knock-out), the GDF-8, also known as myostatin, was removed from the genome of the mouse. The null-mutant mice showed muscular hypertrophy and hyperplasia i.e. their muscularity showed a considerable growth (MCPHERRON et a., 1997).
How does Myostatin Work?
The myostaten or GDF-8 (Growth/Differentiation Factor-8) functions as a negative regulator during muscle growth: it determines the skeletal muscle mass of a given species. Its mutation leads to the ‘double-muscled’ phenotype. The protein of myostatin bears all the characteristics of the TGF-β family: it has a secretion signal near to the N-terminus, a proteolitic processing site in the C-terminal region (RSRR, 263-266 amino acids) and nine cystein residues also in the C-terminal region of the protein, which is important in the dimerisation process (MCPHERRON et al., 1997). The propeptide of the myostatin is 376 amino acid long, which has becomes a 26 kDa active peptide after the proteolitic process (MCPHERRON et al., 1997; SHARMA et al., 1999).
Myostatin and Skeletal Muscles
In the mouse, the expression can be detected mainly in the skeletal muscle. First it can be observed at day 9.5 p.c. In adult animals, the expression also exists in skeletal muscles but with a lower expression level in the adipose tissue (MCPHERRON et al., 1997). In preadipocyte tissue cultures, the addition of myostatin protein inhibits the differentiation of these cells. During the treatment, the glycerol-3-phosphate dehydrogenase (GPDH), the CCAAT/enhancer binding protein alpha (C/EBP alpha) and the peroxisome proliferator-activated receptor gamma (PPAR gamma) activity was reduced (KIM et al., 2001). Although, the mechanism is not clear, this expression KOBOLÁK; GÓCZA: Role of myostatin protein in meat quality 162 pattern, which as a result explains the decrease in fat production of the miostatin mutation carrier cattle.
Other Myostatin Results
Recent publications revealed that the mRNA (or the protein) of the myostatin were detectable in other tissues. In pigs, the mRNA of the miostatin was detectable in the tissue of the mammary gland besides the skeletal muscle expression. In the end of the last year, THOMAS et al. (2000) demonstrated that the miostatin functions as an inhibitor factor for muscle cell proliferation.
The Role Of Myostatin
During the treatment with myostatin protein showed a low percentage of the cells were in phase S. It means that the myostatin blocks the cell cycle in G1 as well as the G2/M phase. MSTN regulates the cyclin-dependent kinase (Cdk) inhibitor p21 protein, but does not affect any other p21 or p16 family member. MSTN affects the Cdk2 protein level which down-regulates during the myostatin treatment.
Myostatin decreases the phosphorilation of the retinoblastoma protein (Rb), which is also known as a major substrate of the Cdks in the G1 phase. These results indicate that the myostatin blocks the cell cycle through these molecules (THOMAS et al., 2000). The hyperplasia observed in the Belgian White-Blue cattle or in the knock-out mice by MCPHERRON and colleagues is the result of a deregulated myoblast proliferation (1997).
Menissier, F. ” General survey of the effect of double muscling on cattle performance.” In: J. B. W. King and F. Menissier (eds.) Muscle hypertrophy of genetic origin and its use to improve beef production (1982): p 21-53. . Martinus Nijhoff, Hague.
CULLEY, G. “Observations on Livestock.” (1807). 4th ed., G. WOODFALL, London, U. K.
Kim, H S, et al. ” Inhibition of preadipocyte differentiation by myostatin treatment in 3t3-l1 cultures.” (2001): Biochem Biophys ResCommun 281: 902-906.
McPherron, A C and S j Lee. ” Double muscling in cattle due to mutations in the myostatin gene.” (1997): 94: 12457-12461. Proc Natl Acad Sci U S A .
McPherron, A C, A M Lawler and S J Lee. ” Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member.” (1997): Nature 387: 83-90.
Sharma, R M, et al. “a transforming growth factor-beta superfamily member… (1999): J Cell Physiol 180 1-9.
Smith, T P, N L Lopez-Corrales and S M Kappes,. ” Myostaten maps to the interval containing the bovine mh locus.” (1997): Mamm Genome 8: 742-744.
Thomas, et al. ” a negative regulator of muscle growth, functions by inhibiting myoblast proliferation.” (2000): J Biol Chem 275: 40235-40243.