Project Summary: Mitochondria form interconnected reticula that dynamically break apart and re-fuse in response to multiple cellular cues. In mammals, mitochondrial fission and fusion are executed by large dynamin-like GTPases, including Opa1, Mitofusins 1 and 2, and Drp1. Mutations in the core fission/fusion machinery have been linked to several human disorders, including Dominant Optic Atrophy, Charcot-Marie-Tooth disease, and lethal encephalopathy. Though numerous investigations have examined the structure and function of the mitochondrial fission/fusion machinery, considerably less is known about the upstream cellular cues that specify mitochondrial dynamics at steady-state. Our recent work demonstrated that the actin cytoskeleton is a key regulator of mitochondrial network morphology. Specifically, we observed that F-actin assembles on locally hyperfused mitochondria where it inhibits fusion and promotes robust Drp1-dependent fission. Upon actin depolymerization, mitochondria quickly re-fuse, forming new connections and reintegrating into the larger network. Over time, actin transiently assembles on all mitochondrial subpopulations in a sequential, step-wise cycle. Here, I propose to investigate the precise mechanism of actin assembly on to and disassembly off of mitochondria. Next, I will investigate the process of mitochondrial actin-cycling during different stages of the cell cycle. Finally, I will elucidate the role of CDK1 in regulating actin cycling. The work proposed here will provide mechanistic insights into the steady-state regulation of mitochondria networks by the actin cytoskeleton.

Project Narrative: Mitochondrial dysfunction is a unifying feature of numerous human disorders, including neurodegenerative disease and cancer. Developing a deeper understanding of the mechanisms regulating mitochondrial network homeostasis could provide important insights into how these pathways are dysregulated in disease states. In this proposal we will elucidate the mechanism of actin assembly on mitochondria and investigate cell-cycle control of mitochondrial actin cycling.