Sarcopenia, the loss of muscle mass during aging, is an inevitable consequence of aging that takes a tremendous toll on the quality of life of the elderly, and predisposes the aging population to injuries that can be further debilitating. Societal cost of sarcopenia and associated injuries and complications is huge, and will grow rapidly as the United States demographic rapidly shifts toward a more senior population. However, the shortage of mechanistic knowledge concerning the causes of sarcopenia has prevented more specific interventions to slow the process of muscle wasting. Our findings show that aging muscle undergoes a dramatic reduction in neuronal nitric oxide synthase (nNOS) and that restoring nNOS to aging muscle prevents sarcopenia in mice. Our goal in this investigation is to identify the mechanisms through which nNOS protects against sarcopenia. Based upon our preliminary studies and the findings of others, we have generated the hypothesis that nNOS-derived NO slows sarcopenia through two processes: 1) inhibition of muscle proteases calpain-1, calpain-2 and caspase-3, to reduce proteolysis of myofibrillar proteins, and 2) inhibition in the production of inflammatory cytokines that are able to promote muscle wasting, especially tumor necrosis factor-alpha (TNFa). We will address this hypothesis in aging mouse models by testing whether muscle protease inhibition by NO-mediated S-nitrosylation occurs during muscle aging. Whether calpain inhibition is sufficient to reduce sarcopenia will be tested in mice that have been genetically-modified to produce elevated levels of the calpain inhibitor, calpastatin, in muscle. We will also identify the specific proteolytic modifications of major muscle proteins in myofibrils that are caused by calpain-2 or caspase-3. In other experiments, we will test whether manipulations of NO production by muscle can reduce the production of inflammatory cytokines, or muscle invasion by inflammatory cells. In addition, we will determine whether reducing the production of pro-inflammatory cytokines is sufficient to slow sarcopenia by testing whether muscle wasting is reduced in aging, TNFa null mutant on interferon-gamma null mice. We anticipate that the results of this investigation can provide new insights into potential therapeutic strategies to reduce sarcopenia.
