UC San Diego

Mitochondria are critical for multiple cellular functions. However, damaged mitochondria pose a lethal threat to cells that necessitates their prompt removal, and failure to do so contributes to neurodegenerative and cardiovascular diseases. Degradation of mitochondria is an important cellular quality control mechanism and is mediated by two distinct pathways: one involving Parkin-mediated autophagy and the other dependent on mitophagy receptors. Mitochondrial disposal via autophagy is

dependent on damaged organelles being marked using several mechanisms including ubiquitylation by Parkin. Here I report a novel pathway for mitochondrial elimination that is dependent on functional Parkin but not on Drp1-mediated mitochondrial fission. This pathway is also distinct from Rab9- and Ulk1/2-mediated alternative autophagy. I show that mitochondria are sequestered by Rab5-positive early endosomes via the ESCRT machinery. These vesicles mature into late endosomes before delivery to lysosomes for degradation. Although this endosomal pathway is activated by stressors that also activate mitochondrial autophagy, endosomal-mediated mitochondrial clearance is initiated before autophagy. Moreover, in cells defective for autophagy, mitochondria are eliminated as promptly and completely through this endosomal pathway as in wild type cells. The autophagy protein Beclin1 regulates activation of Rab5 and endosomal-mediated degradation of mitochondria, suggesting crosstalk between these two pathways. Abrogation of Rab5 and the endosomal pathway results in the accumulation of damaged mitochondria and increases susceptibility to cell death in embryonic fibroblasts and cardiac myocytes. I also demonstrate that the mitophagy receptor BNIP3 can utilize the Rab5-endosomal pathway to clear mitochondria in cells, independent of Parkin. These data reveal a new mechanism for mitochondrial quality control mediated by Rab5 and early endosomes. Defects in this pathway could be involved in or contribute to a variety of disease states, and it may be possible to therapeutically target it in order to compensate for impaired autophagy that is a result of disease or aging.