Spatiotemporal dynamics of autophagy receptors in selective mitophagy

Abstract

Damaged mitochondria are turned over through a process of selective autophagy termed mitophagy. In mitophagy, unhealthy mitochondria are recognized and ubiquitinated by Parkinson disease-linked proteins PINK1 and PARK2. The subsequent recruitment of ubiquitin-binding autophagy receptors leads in turn to the sequestration of the damaged organelles into LC3-positive phagophores, precursors to autophagosomes. The precise identity of these receptors and how they are regulated has been the focus of considerable attention. Our recent work uses live-cell imaging to explore the dynamics and regulation of autophagy receptor recruitment. Utilizing multiple paradigms to induce mitochondrial damage, we identified the rapid, 2-step recruitment of autophagy receptors OPTN, CALCOCO2/NDP52, and TAX1BP1. All 3 receptors are recruited to damaged mitochondria with similar kinetics; however, only OPTN is necessary for efficient formation of a phagophore sequestering damaged mitochondria from the cytosol. OPTN is co-recruited to damaged mitochondria along with its upstream kinase TBK1. Depletion of OPTN or TBK1, or expression of amyotrophic lateral sclerosis (ALS)-linked mutations in either protein, interfere with efficient autophagic engulfment of depolarized mitochondria. These observations suggest that insufficient autophagy of damaged mitochondria may contribute to neurodegenerative disease.

Figure 1.

Mitophagy receptors are dynamically recruited to damaged mitochondria in response to PINK1- and PARK2-induced ubiquitination, leading to sequestration of the organelle by an LC3-positive phagophore. (1) An individual mitochondrion is acutely damaged. (2) The damaged mitochondrion is ubiquitinated through the action of Parkinson disease-linked proteins PINK1 and PARK2. (3) Within 15 min of PARK2-dependent ubiquitination, OPTN and its upstream kinase TBK1 are corecruited to the damaged mitochondrion. Simultaneously, autophagy receptors CALCOCO2 and TAX1BP1 translocate to the mitochondrial outer membrane. (4) Upon TBK1 phosphorylation, OPTN, associates with LC3, inducing autophagic engulfment of the damaged organelle. In the absence of TBK1 activity, CALCOCO2 can promote mitophagy. (5) Within 45 min of the initial injury, the damaged mitochondrion is sequestered within an autophagosome and functionally separated from the cytosol. (6) Over several hours, the mitochondrion is degraded within an autolysosome. Whereas Parkinson disease-linked mutations in PARK2 interfere with ubiquitination of damaged mitochondria, ALS-linked mutations in TBK1 and OPTN interfere with the subsequent recruitment of autophagy receptors and LC3-positive membranes.