Investigating the roles of Mulan and Fis1 in Mitophagy

Abstract

Mitochondria are highly dynamic organelles that respond to physiological stimuli by moving, fusing together, and dividing. Because of this high degree of interaction, it is important for cells to sequester and degrade damaged mitochondria in order to maintain the health of the mitochondrial reticulum. Though mitochondrial autophagy, or mitophagy, has traditionally been thought to require a ubiquitin ligase called Parkin, recent studies have found stress conditions that promote Parkin‐independent mitophagy. These conditions may instead depend on Mulan, a ubiquitin ligase that works in parallel to Parkin in ubiquitinating mitofusin proteins, and/or Fis1, a protein that has been implicated in autophagosome recruitment during mammalian mitophagy. We investigated mitophagy rates in mouse embryonic fibroblasts under two stress conditions: the addition of the iron chelator deferiprone and post‐confluent cell growth. Our results suggest that deferiprone‐induced mitophagy is dependent on Parkin, Mulan, and Fis1, while mitophagy induced in post‐confluent conditions is independent of all three proteins. Both pathways rely on Atg3, a general autophagy factor. These data suggest that, in the latter condition, mitochondria are being degraded either as a by‐product of increased general autophagy or through a novel mitophagy pathway. Additionally, Fis1‐/‐ cells in deferiprone conditions show incomplete inactivation of a pH‐sensitive mitophagy marker, which may suggest a role for Fis1 in acidifying the lysosome where mitophagy is completed.

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