Spatiotemporal dynamics of autophagy receptors in selective mitophagy

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.

损伤的线粒体通过一种名为线粒体自噬（mitophagy）的选择性自噬过程被降解清除。在线粒体自噬过程中，功能异常的线粒体会被帕金森病相关蛋白PINK1和PARK2识别并发生泛素化修饰。随后招募的泛素结合型自噬受体，会将受损细胞器隔离包裹进LC3阳性吞噬泡——自噬体的前体结构。这类受体的确切分子身份及其调控机制，长期以来都是领域内广受关注的研究热点。我们的最新研究借助活细胞成像技术，探究了自噬受体招募的动态过程与调控机制。通过多种诱导线粒体损伤的实验范式，我们发现自噬受体OPTN、CALCOCO2/NDP52及TAX1BP1会以快速的两步模式被招募至受损线粒体。三类受体向受损线粒体的招募动力学特征相似，但仅OPTN对于高效形成将受损线粒体与胞质隔离的吞噬泡是必需的。OPTN会与其上游激酶TBK1共同被招募至损伤线粒体。敲低OPTN或TBK1的表达，或是在任一蛋白中引入肌萎缩侧索硬化（ALS）相关突变，都会阻碍去极化线粒体的高效自噬吞噬。上述研究结果表明，受损线粒体自噬功能不足可能参与神经退行性疾病的发生发展。