Regulation Of Mitochondrial Dynamics And Quality Control In Mammalian Cells

Mitochondria are conserved eukaryotic organelles that carry out myriad cellular functions including energy generation, reactive oxygen species signaling, and lipid synthesis. In metazoans, mitochondria form dense, reticular networks that must be maintained through rigorous quality control mechanisms. Here, we describe three distinct aspects of mitochondria network homeostasis. In chapter 3, we investigate the spatiotemporal dynamics of mitophagy, tracking the association of autophagy machinery with individual damaged mitochondria. Using a range of damage paradigms, we dissect the kinetics of mitochondrial turnover and demonstrate that ALS-linked mutations in the proteins Optineurin and TBK1 interfere with efficient mitophagy. In chapter 5, we characterize a mechanism by which the actin cytoskeleton regulates mitochondrial network dynamics in interphase cells. Specifically, we investigate a traveling wave of actin filaments propagating through interphase mitochondria networks at a rate of ~5µm/min. This actin wave associates with 20% of mitochondria at a given time, where it transiently promotes mitochondrial fission. After 3-5 minutes, the actin wave travels to a neighboring region of the mitochondrial network, and the fragmented mitochondria fuse back together. We hypothesize that cycling actin waves function as constitutive regulators of mitochondrial length and prevent mitochondrial hyperfusion which is linked to cell senescence. Finally, in chapter 6, we identify a mechanism of mitochondrial network inheritance in somatic cells. Using spinning disk confocal and lattice light-sheet microscopy, we determine that mitochondria are highly dynamic through mitosis. We find that this motility is not dependent on microtubules, but rather on the actin cytoskeleton. Specifically, we identify a traveling wave of sub-cortical actin filaments propagating through metaphase mitochondria networks at ~15 µm/min and promoting bursts of mitochondrial motility. Inhibition of actin polymerization promotes aggregation of metaphase mitochondria networks and interferes with mixing of mtDNA nucleoids prior to division. Together these results provide a detailed picture of mitochondria network maintenance throughout the life cycle of mammalian cell lines.