Damaged mitochondria pose a lethal threat to cells that necessitates their prompt removal. The currently recognized mechanism for disposal of mitochondria is autophagy, where damaged organelles are marked for disposal via ubiquitylation by Parkin. Here we report a novel pathway for mitochondrial elimination, in which these organelles undergo Parkin-dependent sequestration into Rab5-positive early endosomes via the ESCRT machinery. Following maturation, these endosomes deliver mitochondria 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. The autophagy protein Beclin1 regulates activation of Rab5 and endosomal-mediated degradation of mitochondria, suggesting cross-talk between these two pathways. Abrogation of Rab5 function and the endosomal pathway results in the accumulation of stressed mitochondria and increases susceptibility to cell death in embryonic fibroblasts and cardiac myocytes. These data reveal a new mechanism for mitochondrial quality control mediated by Rab5 and early endosomes.

Cell based therapies represent a very promising strategy to repair and regenerate the injured heart to prevent progression to heart failure. To date, cell based therapies have had limited success due to a lack of survival and retention of the infused cells. Therefore, it is important to increase our understanding of the biology of these cells and utilize this information to enhance their survival and function in the unfavorable environment of the injured heart. Mitochondria are critical for progenitor cell function and survival. Here we demonstrate the importance of mitochondrial autophagy, or mitophagy, in the differentiation process in adult cardiac progenitor cells (CPCs). We found that mitophagy is rapidly induced upon initiation of differentiation in CPCs. We also found that mitophagy was mediated by the mitophagy receptors pathway, rather than the PINK1/Parkin pathway. Mitophagy receptors Nix and Fundc1, but not Bnip3, were upregulated during differentiation. Mitophagy mediated by Nix and Fundc1 was not involved in regulating progenitor cell fate determination, mitochondrial biogenesis, or reprogramming. Instead, mitophagy facilitated the CPCs to undergo proper mitochondrial network reorganization during differentiation. Abrogating Nix/Fundc1‐mediated mitophagy during differentiation led to mitochondrial fragmentation and failure to form an interconnected mitochondrial network. It also led to increased susceptibility to hydrogen peroxide mediated cell death during differentiation. Finally, aging is associated with accumulation of mtDNA mutations in cells and we found that acquiring mtDNA mutations selectively disrupted the differentiation‐activated mitophagy program in CPCs. These findings demonstrate the importance of Nix/Fundc1‐mediated mitophagy as a critical regulator of mitochondrial network formation in differentiating progenitor cells, as well as the consequences of accumulating mtDNA mutations. Support or Funding Information A.B. Gustafsson is supported by an AHA Established Investigator Award, and by NIH R21AG052280, R01HL087023, R01HL132300 and P01HL085577. M.A. Lampert is supported by the UCSD Graduate Training Program in Cellular and Molecular Pharmacology grant T32GM007752. A.M. Orogo is supported in part by the UCSD Graduate Training Program in Cellular and Molecular Pharmacology through an institutional training grant from the National Institute of General Medical Sciences T32GM007752, and National Institutes of Health NRSA Predoctoral Fellowship F31HL123309. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Myocyte function and survival relies on the maintenance of a healthy population of mitochondria. The PINK1/Parkin pathway plays an important role in clearing defective mitochondria via autophagy in cells. However, how the PINK1/Parkin pathway regulates mitochondrial quality control and whether it coordinates with other mitophagy pathways are still unclear. Therefore, the objective of this study was to investigate the effect of PINK1-deficiency on mitochondrial quality control in myocytes. Using PINK1-deficient (PINK1-/-) mice, we found that Parkin is recruited to damaged cardiac mitochondria in hearts after treatment with the mitochondrial uncoupler FCCP or after a myocardial infarction even in the absence of PINK1. Parkin recruitment to depolarized mitochondria correlates with increased ubiquitination of mitochondrial proteins and activation of mitophagy in PINK1-/- myocytes. In addition, induction of mitophagy by the atypical BH3-only protein BNIP3 is unaffected by lack of PINK1. Overall, these data suggest that Parkin recruitment to depolarized cardiac mitochondria and subsequent activation of mitophagy is independent of PINK1. Moreover, alternative mechanisms of Parkin activation and pathways of mitophagy remain functional in PINK1-/- myocytes and could compensate for the PINK1 deficiency.