Dissecting PINK1/parkin Mitophagy Dynamics

Abstract

Mitochondria homeostasis is crucial for cellular integrity. Several quality control mechanisms can revert mitochondrial damage. However, in situations of acute and irreversible mitochondrial stress, specific pathways are activated to promote the selective degradation of mitochondria: mitophagy. One of the pathways regulating this process is orchestrated by the mitochondrial serine/threonine kinase PINK1 and the E3 ubiquitin ligase parkin. Importantly, while the early mechanisms leading to PINK1/parkin mitophagy activation and progression have been well characterized and are known to involve phosphorylation and ubiquitination post-translational modifications (PTMs), the later steps of the pathway that culminate in complete mitochondrial elimination remained understudied. In addition, the possibility that cross-talk events between phosphorylation and ubiquitination occur during the pathway was unexplored. Thus, the main objectives of this work are: to first gain a better knowledge of the overall pathway dynamics, especially focusing on the later stages and secondly, to explore the role of individual PTMs and their potential cross-talk during parkin-dependent mitophagy. This study demonstrates that CCCP-induced parkin-dependent mitochondrial degradation does not occur at once but involves the consecutive ubiquitination and autophagic targeting of distinct mitochondrial sub-compartments. In addition, while early stages of PINK1/parkin mitophagy involve proteasomal degradation of several outer mitochondrial membrane (OMM) proteins, this step could be confirmed as a pre-requisite for mitophagy progression. Importantly, at later stages of PINK1/parkin mitophagy, a second proteasomal-dependent step was found to complete the degradation of inner mitochondrial structures. The E3 ligase parkin was ubiquitinated and degraded during mitophagy before pathway culmination. In addition, this study evidences an existing cross-talk between phosphorylation and ubiquitination of the parkin substrate VDAC2; phosphorylation of S115 negatively influences VDAC2 ubiquitination and impairs its parkin-dependent turnover. In addition, this work highlights the involvement of TAX1BP1 as a secondary autophagy receptor involved in PINK1/parkin mitophagy and raises the question of how ubiquitination of the autophagy receptor could modulate its function. Overall, this study sheds some light on the dynamics of PINK1/parkin mitophagy and proposes an updated model of the pathway. The findings described in this work may serve as a resource of knowledge for the design of future studies in the context of mitophagy in PD or in other pathophysiological scenarios where defects in mitochondrial quality control are at the limelight of disease pathophysiology.