Autophagy-dependent and -independent mechanisms of mitochondrial clearance during reticulocyte maturation.

Erythrocyte formation involves the elimination of mitochondria at the reticulocyte stage of development. Nix(-/-) reticulocytes fail to eliminate their mitochondria at this step due to a defect in the targeting of mitochondria to autophagosomes. To determine the role of autophagy in this process, we generated Atg7(-/-) transplant mice. Atg7(-/-) reticulocytes exhibit a partial defect in mitochondrial clearance, demonstrating that there are both autophagy-dependent and -independent mechanisms of mitochondrial clearance. We used Atg7(-/-) autophagy-defective reticulocytes to study temporal events in mitochondrial clearance. Mitochondrial depolarization precedes elimination, but in Atg7(-/-) reticulocytes the depolarization event is markedly delayed. Since Atg7 regulates autophagosome formation, we infer that mitochondrial depolarization occurs downstream of autophagosome formation in reticulocytes. We propose that there are two mechanisms of mitochondrial clearance: one that is triggered by mitochondrial depolarization, and a second NIX-dependent mechanism, which is not. The NIX-dependent mechanism remains to be elucidated.