Cyclin C's Role in Myoblast Differentiation‐Induced Mitochondrial Fragmentation

One of the largest and most dynamic tissues in the body, skeletal muscle, requires constant regeneration and upkeep. Dysregulation of this regeneration process has been implicated in many neuromuscular diseases and myotonic dystrophies. Regeneration requires the differentiation of myogenic lineages including exiting the cell cycle, gene expression changes, and fusing of myoblasts into multinucleate myotubes. Part of this reconstruction requires the breakdown and repopulation of mitochondrial networks. At the early onset of myoblast differentiation, there is an upregulation of dynamin-related protein, Drp1, and an increase in mitophagy mediated by sequestosome (SQSTM1) removal of mitochondria. Previously, our lab has shown that mitochondrial fragmentation following stress requires the transcriptional regulator cyclin C, the regulatory subunit for cyclin-dependent kinase 8 (Cdk8). Preliminary data indicate that cyclin C is required for mitochondrial fragmentation during myoblast differentiation. At the early onset, cyclin C co-localizes with the mitochondria, as visualized with indirect immunofluorescence. Cells were additionally treated with PFTμ, a cytosolic chaperone inhibitor that blocks translocation of cyclin C to the mitochondria, and in turn inhibits cyclin C-mediated mitochondrial fragmentation. This treatment resulted in lack of mitochondrial fragmentation typically seen during the differentiation process. In addition, efficiency of differentiation was quantified using gene expression of myogenic regulatory factors (MRFs) MyoD and Myosin Heavy Chain (MyHC), which are normally expressed in a temporal manner throughout differentiation. Treatment of cells with PFTμ significantly decreased expression of MyoD by 1.2 fold compared to endogenous controls, as well as a 1.6 fold decrease in MyHC expression. Future studies aim to elucidate the role of cyclin C-mediated mitochondrial fragmentation during myoblast differentiation and how this relates to the homeostasis of muscle regeneration processes.