Abstract
Despite their importance as signaling hubs, the function of mitochondria-ER contact sites in mitochondrial quality control pathways remains unexplored. Here we describe a mechanism by which Mfn2, a mitochondria-ER tether, gates the autophagic turnover of mitochondria by PINK1 and parkin. Mitochondria-ER appositions are destroyed during mitophagy, and reducing mitochondria-ER contacts increases the rate of mitochondrial degradation. Mechanistically, parkin/PINK1 catalyze a rapid burst of Mfn2 phosphoubiquitination to trigger p97-dependent disassembly of Mfn2 complexes from the outer mitochondrial membrane, dissociating mitochondria from the ER. We additionally demonstrate that a major portion of the facilitatory effect of p97 on mitophagy is epistatic to Mfn2 and promotes the availability of other parkin substrates such as VDAC1. Finally, we reconstitute the action of these factors on Mfn2 and VDAC1 ubiquitination in a cell-free assay. We show that mitochondria-ER tethering suppresses mitophagy and describe a parkin-/PINK1-dependent mechanism that regulates the destruction of mitochondria-ER contact sites.
Highlights
Loss of PRKN or PINK1 results in an early-onset form of hereditary Parkinson’s disease (PD), a neurological disorder that is linked to mitochondrial dysfunction (Kitada et al, 1998; Ryan et al, 2015; Valente et al, 2004)
When CCCP-induced, parkin-independent mitochondrial fragmentation was taken into account (Figure 1C), parkin had a specific effect on reducing the percentage of the outer mitochondrial membrane (OMM) that remained in contact with the endoplasmic reticulum (ER) in depolarized cells (Figure 1D), as well as the percentage of total mitochondria that were still connected to the ER (Figure 1E)
Given that the mitochondria observed in our electron microscopy (EM) analyses were still intact organelles and not yet engulfed by the isolation membrane (IM) of the autophagosome (Figure 1A, right panel), we concluded that parkin ablates contact between mitochondria and the ER as an early step during depolarization-induced mitophagy in cells
Summary
Loss of PRKN or PINK1 results in an early-onset form of hereditary Parkinson’s disease (PD), a neurological disorder that is linked to mitochondrial dysfunction (Kitada et al, 1998; Ryan et al, 2015; Valente et al, 2004). As steady-state mitophagy in yeast requires mitochondria-ER contacts (Bockler and Westermann, 2014), it has been assumed that parkin-dependent mitophagy follows a similar mechanism (Yoshii and Mizushima, 2015). This model directly conflicts with the observation that mitofusin-2 (Mfn2) – a mitochondria-ER tether required for starvation-induced autophagosome formation in mammals (de Brito and Scorrano, 2008; Hamasaki et al, 2013; Naon et al, 2016) – is ubiquitinated by parkin and rapidly turned over by the proteasome (Tanaka et al, 2010). How mitophagy is regulated by contacts between mitochondria and the ER (if at all), and the location from which the mitophagic membrane originates, remain open questions in the field
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