Abstract 243: The E3 Ubiquitin Ligase Parkin Mediates Clearance of Damaged Mitochondria via Two Distinct Pathways

Damaged mitochondria release reactive oxygen species and pro-death factors which can lead to loss of cardiac myocytes. To protect against such damage, myocytes have developed several mechanisms of quality control that act both on the protein and organelle levels. We have previously identified the E3 ubiquitin ligase Parkin as an important regulator of mitochondrial clearance via autophagy in the myocardium. Here, we report that Parkin can also mediate clearance of mitochondria via the endosomal-lysosomal pathway. We found that Parkin promotes clearance of damaged mitochondria in both wild type (WT) and autophagy-deficient Atg5 knockout mouse embryonic fibroblasts (MEFs) treated with the mitochondria uncoupler FCCP. Mitochondrial damage leads to rapid activation of the endosomal-lysosomal pathway in both WT and Atg5-/- MEFs. We further observed increased activation of Rab5, a protein involved in early endosome formation, in both WT and Atg5-/- MEFs after treatment with FCCP. In addition, we observed sequestration of damaged mitochondria in Rab5+ and Rab7+ early and late endosomes, respectively. Mitochondria also colocalized with Lamp2+ vesicles in Atg5-/- MEFs indicating that the mitochondria are ultimately being delivered to the lysosomes for degradation. Overexpression of Rab5S34N, a dominant negative of Rab5, reduces FCCP-mediated clearance and increases cell death in Atg5-/- MEFs. Pharmacological inhibition of the endosomal-lysosomal pathway also results in increased FCCP-mediated cell death. Furthermore, we confirmed that FCCP treatment or simulated ischemia reperfusion exposure induces Rab5 activation with subsequent mitochondrial sequestration in early endosomes in neonatal myocytes. Interestingly, the activation of Rab5 is abrogated in the presence of the mitochondrial targeted antioxidant Mito-Tempo, suggesting that mitochondrial ROS is involved in the activation the endosomal pathway. Mitochondrial clearance via this pathway is also dependent on Parkin, as FCCP treatment fails to activate Rab5 and induce mitochondrial clearance in both WT and Atg5-/- MEFS in the absence of Parkin. Thus, our data suggest that Parkin can mediate clearance of damaged mitochondria via two distinct pathways in cells.