Sarcopenia is the progressive loss of skeletal muscle mass during aging and is a leading factor contributing to frailty, debilitating injuries, loss of independence, and reduced quality of life in the elderly. Unfortunately, sarcopenia progresses despite interventions such as increased physical activity and improved diet. During sarcopenia, the age-related decreases in muscle strength result from a combination of loss of muscle mass (atrophy) and reduced muscle specific force (i.e., muscle force per unit of cross-sectional area). Increasingly, it is thought that it is the muscle weakness that accompanies sarcopenia, rather than the loss of muscle size per se, as the principal contributor to disability. We recently found that elements of contractile force generation in skeletal muscle are dependent on store-operated Ca2+ entry (SOCE) and that this capacity is lost in aging skeletal muscle. This altered SOCE is directly associated with a reduced content of mitsugumin 29 (MG29), a muscle-specific protein belonging to the synaptophysin (SYPL2) family which contains a MARVEL domain involved in cholesterol binding, lipid handling and formation of oligomers of these proteins. MG29 is essential for the proper formation of the transverse-tubule (TT) system and efficient SOCE. This leads to a link between reduced MG29 expression in aged muscle and the development of muscle dysfunction in sarcopenia. Skeletal muscles from young mg29-/- mice are similar to those from aged wild-type (WT) mice in that they demonstrate decreased specific contractile force, reduced SOCE, reduced Ca release from the SR, reduced content of MG29, and altered TT morphology. Therefore, our data demonstrate that key aspects of skeletal muscle aging are present in mg29-/- mice, making this model applicable for this line of research. In this proposal we specifically hypothesize that MG29 is required for proper TT formation and that SOCE functions in skeletal muscle through the action of specific protein domains. Decreased MG29 content in normal aged muscle leads to defective SOCE, which results in the decreased availability of Ca2+ for contractility and age-related loss of muscle strength not accounted for by muscle atrophy. We will test this hypothesis through two specific aims that will: a) examine the mechanistic basis for MG29 function in skeletal muscle, b) establish how MG29 fits into the broader context of the multi-modal aspects of aging and E-C coupling, and c) provide translational value for the treatment of sarcopenia.
Aim 1 will elucidate the contribution of MG29 to SOCE, SR-Ca release and contractility in aged skeletal muscle by using electroporation of MG29 cDNA or RNAi constructs to alter the expression of MG29 and concomitantly evaluate SOCE function, SR-Ca release, contractile force and gene expression levels.
In Aim 2 we use a novel mouse model supported by molecular biology and biochemical approaches to resolve the protein motifs responsible for the mechanism of MG29 function in lipid handling to establish the molecular mechanisms of SOCE regulation by MG29 in skeletal muscle and how these mechanisms are altered in aging.
-Relevance to Public Health Sarcopenia is the progressive loss of skeletal muscle mass that develops during aging that contributes to decreased muscle strength and is the leading factor contributing to frailty, debilitating injuries, loss of independence, and reduced quality of life in the elderly. It is estimated that over 30% of humans over 60 years of age suffer from sarcopenia, leading to more than 40 billion dollars per year in healthcare costs in the US alone. In our pre-clinical studies, we will examine the mechanisms that contribute to the loss of strength in muscle during aging and determine if modulating the expression of specific proteins can improve the function of aged skeletal muscle to address this unmet medical need.
