Date of Award

Summer 8-24-2022

Document Type

Thesis (Ph.D.)

Department or Program

Biochemistry and Cell Biology

First Advisor

Prof Henry N. Higgs

Abstract

Possessing the ability to efficiently generate ATP required to sustain cellular functions, mitochondria are often considered the ‘powerhouses of the cell’. However, our understanding of mitochondria in cell biology was further expanded when we recognized that communication between this unique organelle and the rest of the cell regulates cellular bioenergetics, metabolism and signaling processes such as mitophagy and apoptosis. Here, I investigate signaling between mitochondria and the actin cytoskeleton, and how this signaling regulates mitochondrial dynamics and cellular function. Specifically, I find that, upon mitochondrial dysfunction, actin polymerizes rapidly around the dysfunctional organelle, which we term ‘acute damage-induced actin’ (ADA). Hitherto, neither the mechanism of ADA activation nor the cellular role of ADA are well understood.

In this dissertation, I show that that two parallel signaling pathways are required for ADA: driven by calcium signaling and ATP depletion, respectively. In the first pathway, mitochondrial calcium efflux through the sodium/calcium exchanger NCLX leads to elevation of cytosolic calcium, activating protein kinase C (PKC)-β. PKC-β activation in turn activates the Rac-GEF Trio, leading to activation of the Rho family GTPase Rac, the WAVE complex, and finally the actin-polymerizing Arp2/3 complex. Simultaneously, ATP depletion caused by mitochondrial dysfunction activates the energy sensor AMPK. AMPK activates in turn the Cdc42 GEF Fgd1, the Rho-family GTPase Cdc42, and finally a specific family of actin-polymerizing formin proteins, FMNL formins. Both FMNL formins and Arp2/3 s are to assemble mitochondrially-associated actin filaments.

Next, I elucidated roles for ADA in mitochondrial and cellular dynamics, finding three distinct ADA effects. First, ADA stimulates glycolysis in multiple cell types, including mouse embryonic fibroblasts (MEFs) and cytotoxic T lymphocytes (CTLs). Second, ADA inhibits a specific form of mitochondrial dynamics we term ‘circularization’. Third, ADA delays mitochondrial recruitment of the E3 ubiquitin ligase, Parkin, which delays mitophagy. Taken together, I propose that ADA is an acute mechanism for sensing and responding to mitochondrial damage, by promoting re-establishment of ATP production and giving the cell a ‘pause’ for recovery before the damaged organelle is permanently cleared.

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