Mitochondrial Fusion Is Essential For Regulation Of Adult Skeletal Muscle Mass And Protein Synthesis

Abstract

PURPOSE: Mitochondrial dynamics and the function and health of skeletal muscle are inextricably linked. To preserve proper function, muscle mitochondria undergo constant remodeling through fission and fusion events. Mitochondrial fission is regulated by fission 1 (Fis1) and dynamin-related protein 1 (Drp1), while fusion is regulated by three GTPases; mitofusin 1 and 2 (Mfn1, Mfn2) and optic atrophy 1 (Opa1). However, the role of mitochondrial fusion in adult skeletal muscle mass regulation is not fully understood. We hypothesized that genetic disruption of mitochondrial fusion in adult skeletal muscle will impair muscle function and growth. METHODS: We therefore developed and characterized adult inducible skeletal muscle specific Mfn1/2 double knockout mice (MFNDKO). Genetic deletion was induced in adult mice and confirmed by qPCR and western blot. Body/muscle size and composition was analyzed gravimetrically and by QMR. Muscle function was assessed by grip test. Interrogation of pathways regulating muscle mass, including atrophy and autophagy were performed by qPCR and western blotting. In vivo and in vitro protein synthesis rates were evaluated using a puromycin incorporation assay. RESULTS: MFNDKO mice exhibited a progressive decrease in body weight (~20% lower than CON, respectively; p < 0.05). This reduction in body weight was associated with a decrease in lean mass, confirmed by QMR, gastrocnemius wet weight and cross sectional area (50% and 36% reduction, respectively; p < 0.05). Gene profiling of pathways that regulate muscle homeostasis revealed upregulation of FBXO30, FBXO32, MT1 and CTSL (p < 0.05, all) suggesting an activation of muscle atrophy. Furthermore, muscles from MFNDKO mice revealed increased markers of autophagy with increased p62 mRNA and increased LC3II/I and p62 protein levels. Protein synthesis rates were decreased in vitro and in vivo (14% and 75%; p < 0.05, respectively) in MFNDKO muscle. CONCLUSIONS: Taken together, these observations suggest that normal mitochondrial fusion is required for maintaining normal adult skeletal muscle mass.