Mitochondrial dynamics define muscle fiber type by modulating cellular metabolism pathways

Skeletal muscle is highly developed after birth, consisting of glycolytic fast- and oxidative slow-twitch fibers; however, the mechanisms of fiber type-specific differentiation are poorly understood. Here, we found an unexpected role for mitochondrial fission in the differentiation of fast-twitch oxidative fibers. Depletion of the mitochondrial fission factor dynamin-related protein 1 (Drp1) in mouse skeletal muscle and cultured myotubes resulted in the specific reduction of fast-twitch muscle fibers independently of respiratory function. Altered mitochondrial fission caused the activation of the Akt/mammalian target of rapamycin (mTOR) pathway via the mitochondrial accumulation of mTOR complex 2 (mTORC2), and rapamycin administration rescued the reduction of fast-twitch fibers in vivo and in vitro. Under Akt/mTOR activation, the mitochondria-related cytokine growth differentiation factor-15 was upregulated, which repressed fast-twitch fiber differentiation. Our findings reveal a novel role for mitochondrial dynamics in the activation of mTORC2 on mitochondria, resulting in the differentiation of muscle fibers. Gene expression in WT, Drp1-KO, and Drp1-KO exogenously expressing FLAG-Drp1 C2C12 cells at 4 days after differentiation.