Reduced Synaptophysin-like 2 (MG29/SYPL2) Levels Mimic Age-Related Alterations in Skeletal Muscle Calcium Homeostasis and Lipid Signaling
Sarcopenia is characterized by progressive loss of skeletal muscle mass and function and is a major contributor to frailty, disability, and mortality in older adults. Store-operated calcium entry (SOCE) is a crucial regulator of skeletal muscle calcium homeostasis, and impaired SOCE has been linked to age-related muscle weakness. Here, we identify the synaptophysin family member synaptophysin-like protein 2, also known as mitsugumin 29 (MG29; encoded by the human gene SYPL2 and the mouse ortholog Mg29), as a key organizer of triad membrane cholesterol and lipid signaling required for normal SOCE during aging. Using Mg29−/− mice as a model of accelerated sarcopenia, together with RNA interference against Mg29 in adult muscle and primary myotubes, we quantified changes in muscle morphology, contractile function, SOCE activity, and targeted lipidomic profiles. Reduced MG29 expression led to decreased muscle fiber cross-sectional area, reduced specific force, blunted SOCE, and marked alterations in membrane cholesterol content and fatty acid-derived lipid mediators. Cholesterol depletion by methyl-β-cyclodextrin in wild-type myotubes produced SOCE defects similar to those observed in aged wild-type and young Mg29−/− muscles, indicating that MG29-dependent maintenance of membrane cholesterol is required for normal SOCE. Acute Mg29 knockdown also altered myogenic differentiation, the expression of calcium-handling and stress-response genes, and the release and consumption of specific polyunsaturated fatty acid-derived lipid mediators. Together, these findings identify MG29 as a critical regulator of SOCE and lipid signaling in skeletal muscle and suggest that its age-related decline contributes to sarcopenia by disrupting triad membrane organization and excitation–contraction coupling.