Genetic analysis of Omi/HtrA2 and MUL1 in the PINK1/Parkin pathway

Parkinson's disease (PD) is the second most common neurodegenerative disorder. Poor understanding of PD pathogenesis has limited the development of effective therapies. Recently, studies on PD associated genes have linked mitochondrial dysfunction to underlying causes of PD. Mutations in the mitochondrial Ser/Thr kinase PINK1 or the Ubiquitin E3 ligase Parkin cause early onset hereditary PD. Genetic studies indicate that PINK1 and parkin function in the same pathway to regulate mitochondrial dynamics through Mitofusin (MFN). This dissertation aims to identify other components in the PINK1/Parkin pathway. In chapter two, we investigate genetic interaction between PINK1 and Omi/HtrA2 in vivo. Previously, mutations in the mitochondrial protease Omi/HtrA2 were identified in PD patients, and in vitro studies show that phosphorylation of Omi/HtrA2 is PINK1-dependent, suggesting that Omi/HtrA2 acts downstream of PINK1. However, our work suggests that Omi/HtrA2 does not function in the same genetic pathway as PINK1. Omi/HtrA2 null mutants in Drosophila do not share any of PINK1 mutant phenotypes. Furthermore, Omi/HtrA2 and PINK1 fail to modify each other's mutant phenotypes. Based on our results, we do not favor a hypothesis in which Omi/HtrA2 plays an essential role in PINK1/parkin mediated PD pathogenesis. These data are also consistent with recent human genetics studies that show no association between Omi/HtrA2 and PD. In chapter three, we identify MUL1, a mitochondrial ubiquitin E3 ligase, as a novel suppressor of PINK1 mutants. MUL1 suppresses PINK1 and parkin phenotypes including mitochondrial morphology, muscle degeneration, and dopaminergic neuronal phenotypes. MUL1 suppresses PINK1 phenotypes by reducing MFN levels through ubiquitination. Further genetic epistasis studies indicate that MUL1 acts in parallel to the PINK1/parkin pathway to regulate mitochondrial integrity. In mammalian cells, the PINK1/Parkin pathway has been shown to mediate the selective degradation of damaged mitochondria called mitophagy. We found that although MUL1 regulates MFN levels, Parkin-mediated mitophagy is not affected. In consistent with our genetic studies in Drosophila, these data support that MUL1 acting in parallel to PINK1/Parkin is conserved. Our work suggests that reduction of MFN by MUL1 is sufficient to reverse PINK1/parkin deficiency phenotypes and proposes MUL1 as a potential therapeutic target to modulate PD pathology.