Project Summary Mitochondria are centrally involved in the conversion of nutrients into energy, and thus, damage to this organelle results in altered nutrient catabolism. The skeletal muscle system is prominently affected in the setting of mitochondrial dysfunction, but little is known regarding the activity of metabolic pathways in diseased muscular tissue. Our preliminary work has discovered that central carbon metabolism is altered in cultured cells with pathogenic mitochondrial genome (mtDNA) mutations, and identified alterations in key members of the solute transport carrier (SLC) family which regulate these perturbations. Importantly, similar alterations in SLC family members, particularly the xc- transport system, occur in muscular tissues of subjects with mitochondrial myopathy. Using mouse models of mitochondrial disease, we propose three specific aims to characterize the role of xc- in altering metabolic fluxes in diseased skeletal muscle. In Aim 1, we will characterize metabolic pathways in diseased animals using stable isotope labeling techniques. These results will quantitate in vivo metabolism in mutant muscle tissue, providing a detailed mapping of the differences between normal and diseased states. In Aim 2, we will investigate the role of xc- in regulating skeletal muscle metabolism, making use of an available knockout allele. In Aim 3, we will test the hypothesis that altering xc- activity modulates disease progression in diseased mice, by following muscular physiology and function in live animals. Together, these aims will quantitate metabolic alterations in mitochondrial myopathies, and relate them to in vivo muscle function and health. The results have the potential to identify new therapies targeting carbon metabolism which may be beneficial for patients suffering from mitochondrial myopathies.