|dc.description.abstract||Myostatin, a TGF-β superfamily member, is a key negative regulator of embryonic and postnatal muscle growth. In order to further elucidate the role of myostatin during postnatal growth, several lines of investigation were undertaken in mice. Analysis of myostatin downstream target genes identified several known and unknown genes. From these, the regulation of an androgen receptor binding co-factor, ARA70, was selected for further investigation. Reverse Northern analysis on the differentially expressed cDNA library indicated an increased expression of ARA70 in myostatin-null muscles, which was later confirmed by Northern blot and semi-quantitative PCR analysis. In corroboration, treatment of myoblast cultures with exogenous myostatin resulted in the down-regulation of ARA70, confirming that myostatin is a negative regulator of ARA70 gene expression. The role of myostatin during sarcopenia, a progressive age-related loss of skeletal muscle mass and strength, was also investigated. The atrophy associated with sarcopenia is frequently correlated with insufficient muscle regeneration, resulting from an impaired propensity of satellite cells to activate and a subsequent decline in myogenesis. Myostatin is a known inhibitor of postnatal satellite cell activation and muscle regeneration, thus muscle mass and regeneration, and satellite cell behaviour were examined in young and aged myostatin-null mice. Myostatin-null mice had increased individual muscle weights, as a consequence of massive fibre hypertrophy and hyperplasia, and an increased proportion of type IIB fibres. Aging induced oxidative fibre type changes and atrophy in the wild-type muscle while no fibre type switching was observed in the myostatin-null muscle and atrophy was minimal. No decrease in satellite cell numbers was observed with aging in both genotypes; though a gradual decline in the number of activated satellite cells was noted during aging. However, both young and aged myostatin-null mice displayed increased satellite cells and activation compared to wild-type mice, suggesting a greater myogenic potential in the myostatin-null satellite cells. Consistent with this, aged myostatin-null myoblasts proliferated faster and displayed a higher fusion index during differentiation than the aged wild-type myoblasts, confirming that the reduced sarcopenia in the myostatin-null mice was due to a preserved increase in the myoblast myogenic activity. An increase in a Pax7-only myoblast population from myostatin-null muscle indicated an enhanced satellite cell self-renewal process, consistent with the increased satellite cell number observed on the myostatin-null muscle fibres. Additionally, muscle regeneration of aged myostatin-null muscle following notexin injury was accelerated, and fibre hypertrophy and type were recovered with regeneration, unlike the aged wild-type muscle. Testing the therapeutic value of a myostatin antagonist, Mstn-ant1, indicated that a short term blockade of myostatin by the antagonist significantly enhanced muscle regeneration in aged mice after injury and during sarcopenia. Antagonism of myostatin led to satellite cell activation, increased Pax7 and MyoD protein levels, and greater myoblast and macrophage cell migration culminating in enhanced muscle regeneration in the aged mice.
In conclusion, the hypertrophic phenotype associated with myostatin-null mice may in part result from increased androgen receptor (AR) activity due to the up-regulation of ARA70, given that increased expression of the AR leads to hypertrophy. Additionally, the increased muscle mass in myostatin-null mice is likely to result from an augmented myogenic potential and self-renewal process. Overall, a prolonged absence of myostatin reduced sarcopenia and the associated loss of muscle regenerative capacity. Furthermore, the antagonism of myostatin displayed significant therapeutic potential in the alleviation of sarcopenia, through the restoration of the myogenic and inflammatory responses in the aged environment. Thus, the research work clearly demonstrates the role of myostatin in sarcopenia, and documents for the first time a valid therapeutic for alleviating sarcopenia.||en_NZ