Retention of the Nix/BNIP3 Mitophagic Complex Within ER and Autophagosomes
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Mitophagy involves specific degradation of mitochondria and is vital to cellular health. Ineffective mitophagy is linked to Parkinson’s disease, and damaged mitochondria can lead to inflammation, aging, and programmed cell death. The purpose of this study was to track the location of Nix/BNIP3, an outer mitochondrial membrane (OMM) protein complex, throughout mitophagy. We treated HeLa cells with the mitophagy-inducing drugs antimycin and oligomycin. After mitophagy, we stained cells for Nix and for Opa1, an inner mitochondrial membrane (IMM) protein. In a separate time-course experiment, we measured the degradation rates of Nix/BNIP3 and the outer mitochondrial membrane (OMM) protein TOMM20. After complete mitophagy, Opa1 was degraded in the antimycin/oligomycin treated cells, but Nix remained present in experimental and control cells. TOMM20 was degraded within 12 hours, indicating that mitochondria were degraded. However, Nix was detectable at all time points, with a slow decline to 50% of initial values. During mitophagy, Nix colocalized with the endoplasmic reticulum (ER) and autophagosome organelles. Our results suggest that the presence of Nix/BNIP3 in the OMM protects mitochondria from degradation, but its removal allows the onset of mitophagy. Nix/BNIP3 retention and presence in the ER and autophagosomes could function in regulation of mitophagy. This project was completed with contributions from Hector Sandoval, Priyanka Chanana, Tonya Bauch, and Jin Wang from the Houston Methodist Research Institute, Immunobiology and Transplant Research Center.