RAB14 Mediates Mitochondrial Dynamics and Parkin-Mediated Mitophagy

Abstract

Mitochondrial dynamics and mitophagy are essential to maintain cell metabolism, differentiation, and viability. Different modes of metabolism are essential for the growth and differentiation of the intestinal epithelium - epithelial cells migrating along the crypt-villus axis and differentiated enterocytes use oxidative phosphorylation (OXPHOS) whereas Paneth cells use glycolysis. In contrast, intestinal stem cells require high activities of both OXPHOS and glycolytic pathways. However, how overall cell metabolism is regulated to maintain these cell populations is incompletely understood. Mitochondrial dynamics include both mitochondrial fusion and fission, whereas mitophagy is a specialized autophagic mechanism that requires fission at the periphery of the mitochondria and targets damaged mitochondria for lysosomal degradation. Dysfunctional mitophagy leads to the accumulation of damaged mitochondria and reactive oxygen species (ROS), leading to increased inflammation. Defective mitophagy is associated with diseases such as inflammatory bowel disease, chronic kidney disease, and colorectal cancer. While the Rab family of small GTPase proteins are primarily involved with vesicular trafficking in the endocytic and biosynthetic pathways, recent studies have found that endosomal Rab-GTPases have a role in mitochondrial integrity. However, a molecular role for Rab14, a trans-Golgi network (TGN)-endosomal Rab-GTPase, in mitochondrial dynamics and mitophagy has not been described. In cells that have been knocked down for Rab14, mitochondria acquire an elongated morphology, increased oxidative phosphorylation, and increased levels of the mitochondrial proteins COX IV and mitofusin (Mfn), whereas overexpression of Rab14 increased mitochondrial fission and decreased these proteins. Consistent with the degradation of mitochondrial proteins, the mito-Keima assay that measures the mitophagic flux show an increase in mitophagy upon overexpression of Rab14. High-resolution imaging shows an association of Rab14 with mitochondrial dynamics machinery, including Mfn and Fis1, as well as the mitophagy receptor Optineurin. PIK-93 treatment that prevents the generation of Golgi/TGN derived [PI(4)P] containing vesicles prevents the degradation of mitochondrial proteins upon Rab14 overexpression suggesting that TGN-associated Rab14 vesicles mediate mitophagy. These results suggest that Rab14 promotes both mitochondrial fission and mitophagy and plays an essential role in modulating cellular metabolism.