Mitochondrial dynamics are important for cellular function, and it has been shown that actin polymerisation in response to cytosolic calcium increase participates in mitochondrial fission. In addition, there are reports of the formation of actin ‘clouds’ surrounding mitochondria that have been depolarised using an uncoupler. This raises the question of whether there are different means by which actin can interact with mitochondria. In this work (Fung et al., 2019), Henry Higgs, Rajarshi Chakrabarti and co-workers set out to investigate in detail the mechanisms of actin polymerisation in U2OS cells, following either a calcium stimulus or depolarisation. They show that although both processes are rapid, calcium-induced actin polymerisation is faster; it depends on the formin INF2, but not on Arp2/3, and filaments extend throughout the cytosol. By contrast, depolarisation induces the formation of transient, mitochondria-localised actin clouds that require Arp2/3, but not INF2. Similar actin clouds are also observed upon spontaneous mitochondrial depolarisation. Interestingly, depolarisation does not result in mitochondrial fission, but in circularisation of the inner mitochondrial membrane. The transient actin clouds restrain these shape changes. These findings suggest that there are distinct types of actin filaments interacting with mitochondria. Further work will be needed to elucidate their respective roles in mitochondrial dynamics and function.
