Mitochondrial import stress and PINK1-mediated mitophagy: the role of the PINK1-TOMM-TIMM23 supercomplex.

Mutations in the PINK1 kinase cause Parkinson disease (PD) through physiological processes that are not yet fully elucidated. PINK1 kinase accumulates selectively on damaged mitochondria, where it recruits the E3 ubiquitin ligase PRKN/Parkin to mediate mitophagy. Upon mitochondrial import failure, PINK1 accumulates in association with the translocase of outer mitochondrial membrane (TOMM). However, the molecular basis of this PINK1 accumulation on the TOMM complex remain elusive. We recently demonstrated that TIMM23 (translocase of the inner mitochondrial membrane 23) is a component of the PINK1-supercomplex formed in response to mitochondrial stress. We also uncovered that PINK1 is required for the formation of this supercomplex and highlighted the biochemical regulation and significance of this supercomplex; expanding our understanding of mitochondrial quality control and PD pathogenesis.

Tom20 gates PINK1 activity and mediates its tethering of the TOM and TIM23 translocases upon mitochondrial stress.

Mutations in PTEN-induced putative kinase 1 (PINK1) cause autosomal recessive early-onset Parkinson's disease (PD). PINK1 is a Ser/Thr kinase that regulates mitochondrial quality control by triggering mitophagy mediated by the ubiquitin (Ub) ligase Parkin. Upon mitochondrial damage, PINK1 accumulates on the outer mitochondrial membrane forming a high-molecular-weight complex with the translocase of the outer membrane (TOM). PINK1 then phosphorylates Ub, which enables recruitment and activation of Parkin followed by autophagic clearance of the damaged mitochondrion. Thus, Parkin-dependent mitophagy hinges on the stable accumulation of PINK1 on the TOM complex. Yet, the mechanism linking mitochondrial stressors to PINK1 accumulation and whether the translocases of the inner membrane (TIMs) are also involved remain unclear. Herein, we demonstrate that mitochondrial stress induces the formation of a PINK1-TOM-TIM23 supercomplex in human cultured cell lines, dopamine neurons, and midbrain organoids. Moreover, we show that PINK1 is required to stably tether the TOM to TIM23 complexes in response to stress such that the supercomplex fails to accumulate in cells lacking PINK1. This tethering is dependent on an interaction between the PINK1 N-terminal-C-terminal extension module and the cytosolic domain of the Tom20 subunit of the TOM complex, the disruption of which, by either designer or PD-associated PINK1 mutations, inhibits downstream mitophagy. Together, the findings provide key insight into how PINK1 interfaces with the mitochondrial import machinery, with important implications for the mechanisms of mitochondrial quality control and PD pathogenesis.

Mitochondrial quality control in health and in Parkinson's disease.

As a central hub for cellular metabolism and intracellular signaling, the mitochondrion is a pivotal organelle, dysfunction of which has been linked to several human diseases including neurodegenerative disorders and in particular Parkinson's disease. An inherent challenge that mitochondria face is the continuous exposure to diverse stresses that increase their likelihood of dysregulation. In response, eukaryotic cells have evolved sophisticated quality control mechanisms to monitor, identify, repair, and/or eliminate abnormal or misfolded proteins within the mitochondrion and/or the dysfunctional mitochondrion itself. Chaperones identify unstable or otherwise abnormal conformations in mitochondrial proteins and can promote their refolding to recover their correct conformation and stability. However, if repair is not possible, the abnormal protein is selectively degraded to prevent potentially damaging interactions with other proteins or its oligomerization into toxic multimeric complexes. The autophagic-lysosomal system and the ubiquitin-proteasome system mediate the selective and targeted degradation of such abnormal or misfolded protein species. Mitophagy (a specific kind of autophagy) mediates the selective elimination of dysfunctional mitochondria, to prevent the deleterious effects of the dysfunctional organelles within the cell. Despite our increasing understanding of the molecular responses toward dysfunctional mitochondria, many key aspects remain relatively poorly understood. Here, we review the emerging mechanisms of mitochondrial quality control including quality control strategies coupled to mitochondrial import mechanisms. In addition, we review the molecular mechanisms regulating mitophagy, with an emphasis on the regulation of PINK1/Parkin-mediated mitophagy in cellular physiology and in the context of Parkinson's disease cell biology.

Multifaceted targeted protein degradation systems for different cellular compartments.

Selective protein degradation maintains cellular homeostasis, but this process is disrupted in many diseases. Targeted protein degradation (TPD) approaches, built upon existing cellular mechanisms, are promising methods for therapeutically regulating protein levels. Here, we review the diverse palette of tools that are now available for doing so throughout the gene expression pathway and in specific cellular compartments. These include methods for directly removing targeted proteins via the ubiquitin proteasome system with proteolysis targeting chimeras (PROTACs) or dephosphorylation targeting chimeras (DEPTACs). Similar effects can also be achieved through the lysosomal system with autophagy-targeting chimeras (AUTACs), autophagosome tethering compounds (ATTECs), and lysosome targeting chimeras (LYTACs). Other methods act upstream to degrade RNAs (ribonuclease targeting chimeras; RIBOTACs) or transcription factors (transcription factor targeting chimeras; TRAFTACs), offering control throughout the gene expression process. We highlight the evolution and function of these methods and discuss their clinical implications in diverse disease contexts.