UNRAVELING QUALITY CONTROL PATHWAYS: THE ROLE AND FUNCTION OF PARKIN-INDEPENDENT MITOPHAGY

Abstract

Mitochondrial dysfunction is attributed to several human diseases, including neurological, immunological, and cardiological. Given the underlying mitochondrial dysfunction in these disorders and lack of therapeutic options, it is important to understand the mechanisms by which mitochondrial homeostasis is maintained. Characterization of mitochondrial quality control pathways, such as mitophagy, have greatly aided in understanding the mechanisms that control proper mitochondrial function. While there is a robust understanding of the molecular machinery and mechanisms of PINK1/Parkin-mediated mitophagy, Parkin-independent mitophagy mechanisms are less understood. The aim of this thesis is to investigate the requirements for iron chelation-induced mitophagy and to investigate the requirements for the mitochondrial-anchored E3 ubiquitin ligases, MUL1, MARCH5, and MARCH9 in mitophagy. Iron chelation is a potent activator of mitophagy and iron chelation-induced mitophagy is thought to be independent of both Parkin and PINK1. We show that iron homeostatic pathways may also govern mitophagy caused by iron loss. We also found that iron chelation-induced mitophagy utilized canonical autophagy machinery for autophagosome formation and fusion. However, our data indicates that mitophagy stimulated by iron chelation does not use the canonical ATG16L1 protein, but rather ATG16L2, a protein that has not been implicated in autophagy. Finally, we show that the autophagy receptor protein, NDP52, may mediate mitophagy in a ubiquitin-dependent manner. The second aim of this dissertation is to understand the role of the mitochondrial-anchored E3 ubiquitin ligases, MUL1, MARCH5, and the unreported MARCH9, in mitochondrial quality control systems. Mitochondrial E3 ligases are thought to help maintain mitochondrial integrity by mediating basal turnover of integral mitochondrial membrane proteins as well as by participating in quality control pathways. This dissertation shows that knockout of any of the mitochondrial-anchored E3 ubiquitin ligases alone does not affect either PINK1/Parkin-dependent mitophagy nor Parkin-independent mitophagy. However, we found that combinatorial knockout of the mitochondrial-anchored E3 ubiquitin ligases did not affect PINK1/Parkin-dependent mitophagy but did cause a defect in Parkin-independent mitophagy. This indicates a role for the mitochondrial-anchored E3 ubiquitin ligases in Parkin-independent mitophagy. In sum, this dissertation begins unraveling the role and function of Parkin-independent mitophagy mechanisms.