New Insights into the Role of F-actin in Regulation of Mitochondrial Fission

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Mitochondrial dynamics, including fusion and fission, are vital for supplying cellular energy as well as controlling other tasks including apoptosis, aging and cellular differentiation. Defects of mitochondrial fission pathway have been implicated in a wide spectrum of human diseases such as Parkinson’s disease and Alzheimer’s disease. Although recent findings point to a role of the actin cytoskeleton in regulating mitochondrial division, little is known about the mechanism. Here, I report that transient de novo polymerization of F-actin on the outer mitochondrial membrane contributes to Drp1-dependent mitochondrial division in mammalian cells. Transient de novo F-actin assembly on the mitochondria occurs upon induction of mitochondrial fission and F-actin accumulates on the mitochondria without forming detectable submitochondrial foci. Impairing mitochondrial division through Drp1 knockout or inhibition prolonged the time of mitochondrial accumulation of F-actin and also led to abnormal mitochondrial accumulation of the actin regulatory factors cortactin, cofilin, and Arp2/3 complex, suggesting that disassembly of mitochondrial F-actin depends on Drp1 activity. Furthermore, downregulation of actin regulatory proteins Arp2/3 complex, cortactin and cofilin led to abnormal elongation of mitochondria, associated with mitochondrial accumulation of Drp1. In addition, depletion of cortactin inhibited Mfn2 downregulation- or FCCP- induced mitochondrial fragmentation. These data indicate that the dynamic assembly and disassembly of F-actin on the mitochondria participates in Drp1-mediated mitochondrial fission. Moreover, I also discovered a novel F-actin involved mechanism of mitochondrial fission regulated by deubiquitinase Usp30. Overexpression of Usp30CS predicted to lack deubiquitinase activity induced abnormal elongation and thinning of mitochondrial tubules. Furthermore, expression of Usp30CS preferably binds to Drp1, inducing a dramatic redistribution of Drp1 from the cytosol to the mitochondria, and accumulation of high molecular weight Drp1 species. Importantly, FCCP induced a gradual tubulation of Drp1-containing structures, accompanied with mitochondrial associated F-actin in a similar timeframe in Usp30CS-expressing cells, suggesting that inhibition of Usp30 deubiquitnase activity stalls progression of Drp1-dependent mitochondrial division. In sum, here I report that mitochondrial F-actin polymerization is a required step of mitochondrial fission, regulated by actin-modifying proteins and deubiquitinase Usp30, providing in-depth vision and a novel mechanism of actin cytoskeleton participated mitochondrial fission.