Two distinct actin filament populations have effects on mitochondria, with differences in stimuli and assembly factors

Abstract Recent studies show that mitochondria and actin filaments work together in two contexts: 1) increased cytoplasmic calcium induces cytoplasmic actin polymerization that stimulates mitochondrial fission, and 2) mitochondrial depolarization causes actin assembly around mitochondria, with roles in mitophagy. It is unclear whether these two processes utilize similar actin assembly mechanisms. Here, we show that these are distinct actin assembly mechanisms in the acute phase after treatment (<10 min). Calcium-induced actin assembly is INF2-dependent and Arp2/3 complex-independent, whereas depolarization-induced actin assembly is Arp2/3 complex-dependent and INF2-independent. The two types of actin polymerization are morphologically distinct, with calcium-induced filaments throughout the cytosol and depolarization-induced filaments as “clouds” around depolarized mitochondria. We have previously shown that calcium-induced actin stimulates increases in both mitochondrial calcium and recruitment of the dynamin GTPase Drp1. In contrast, depolarization-induced actin is temporally-associated with extensive mitochondrial dynamics that do not result in mitochondrial fission, but in circularization of the inner mitochondrial membrane (IMM). These dynamics are dependent upon the protease Oma1 and independent of Drp1. Actin cloud inhibition causes increased IMM circularization, suggesting that actin clouds limit these dynamics. Summary statement Mitochondrial depolarization induces Arp2/3 complex-dependent actin clouds that restrain mitochondrial shape changes induced by Oma1 on the inner mitochondrial membrane. A distinct actin network stimulates mitochondrial fission in response to calcium.