MG29/SYPL2 Contributes to Dysregulation of Lipid Composition and Store Operated Ca2+ Entry in Aging Skeletal Muscle

Abstract

We proposed over one decade ago that age-related muscle weakness and not the loss of muscle size is the principal contributor to disability. In searching for cellular and molecular explanations for this weakness that exceeds the loss of muscle mass, we 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 reduced content of mitsugumin 29 (MG29), a synaptophysin family protein (SYPL2) containing a MARVEL domain involved in cholesterol binding and formation of oligomers of these proteins. MG29 is essential for the formation of the transverse-tubule system and efficient SOCE, creating a link between reduced MG29 expression in aged muscle and the development of muscle dysfunction in sarcopenia as our previous data demonstrate that aspects of aged skeletal muscle are present in young mg29-/- muscle. Here we test how the loss of MG29 affects the phospholipid composition of skeletal muscle fibers and if producing similar changes in muscle fibers can influence SOCE activity. Lipidomic analysis of young mg29-/- skeletal muscle showed decreased cholesterol content and altered levels of various phospholipid species compared to young control muscle. To simulate the reduced cholesterol content observed in mg29-/- muscle we partially extracted it from mature C2C12 myotubes with methyl-β-cyclodextrin (MCD) and then measured SOCE in these myotubes. Reduced SOCE was observed following MCD extraction, similar to the limited SOCE we previously observed in young mg29-/- and wild-type aged muscle fibers. Further studies will determine if mutation of conserved residues in the MARVEL domain of MG29 modulate SOCE function in, and the lipid composition of, skeletal muscle cells.