June 25, 2024
Damage Control: Targeting Mitochondria
Deficiency in mitophagy, a process by which damaged mitochondria are selectively degraded to maintain cellular health and homeostasis, is a hallmark of neurodegenerative diseases such as Parkinson’s. The molecular mechanisms that govern the initiation of mitochondrial degradation and subsequent autophagosome biogenesis, however, are not well understood. In their new publication in Nature Structural and Molecular Biology, first author Elias Adriaenssens, a post-doctoral researcher in the Martens lab (Max Perutz Labs), shows that the TBK1 kinase adaptors NAP1 and SINTBAD play crucial roles during mitochondrial degradation by controlling pathway initiation and driving its efficient progression.
Mitochondria, the powerhouses of the cell, are susceptible to damage caused by the reactive oxygen species that they generate. Damaged mitochondria are therefore selectively removed and recycled in a process called mitophagy. Under normal conditions, the signaling molecule PINK1 (PTEN-induced putative kinase 1) is subject to constant proteasome-mediated degradation, but this pathway is shut down upon the sensing of damaged mitochondria. PINK1, in turn, activates the E3 ubiquitin ligase Parkin, which then marks mitochondria with ubiquitin. The autophagy machinery recognizes the ubiquitin chains on the surface of the damaged mitochondrion through its cargo receptors OPTN (Optineurin) and NDP52 (Nuclear Dot Protein 52). Mutations in PINK1 and Parkin can lead to a failure in this process, causing the accumulation of damaged mitochondria and resulting in neurodegenerative diseases such as Parkinson’s disease.
“Mitophagy is one of the best studied autophagy pathways but there were clearly major pieces of the puzzle missing”, explains first author Elias Adriaenssens. “We know that the ‘master’ kinase TBK1 phosphorylates cargo receptors, thereby increasing their affinity for ubiquitin. And we know that NAP1 and SINTBAD are TBK1 adaptors – playing an important role during xenophagy, a selective autophagy pathway against intracellular bacterial pathogens.” However, the role of these TBK1 adaptors in mitophagy had so far remained enigmatic.
The researchers revealed that NAP1 and SINTBAD play inhibitory roles in the mitophagy pathway. “Both proteins act as rheostats for mitophagy, regulating its initiation by competing with OPTN for the TBK1 binding site,” Elias explains further. By occupying this site, they finely tune the onset of mitophagy, preventing OPTN from prematurely recruiting TBK1. However, once initiated, NAP1 and SINTBAD facilitate mitophagy progression: these rheostats stabilize the NDP52 complex, facilitating autophagosome formation.
In collaboration with Michael Lazarou at the WEHI institute in Melbourne, Australia, and in the framework of the Aligning Science Across Parkinson’s Initiative, the Martens lab combined their biochemical reconstitution approach with the Lazarou lab’s proficiency in producing CRISPR/Cas9 knockout cell lines. Additionally, the Perutz FACS facility, led by Kitti Dora Csalyi, provided invaluable expertise. “The FACS facility team was critical to optimize a key assay to monitor mitophagy in cells”, emphasizes Elias. This collaborative study provides valuable insights into mitophagy, with the ultimate goal of rationalizing disease pathogenesis at a molecular level and enabling the development of precision therapeutics for Parkinson’s disease.
Publication:
Control of mitophagy initiation and progression by the TBK1 adaptors NAP1 and SINTBAD.
Nature Structural and Molecular Biology. 2024 June 25.
DOI: 10.1038/s41594-024-01338-y
Authors:
Elias Adriaenssens, Thanh Ngoc Nguyen, Justyna Sawa-Makarska, Grace Khuu, Martina Schuschnig, Stephen Shoebridge, Marvin Skulsuppaisarn, Emily Maria Watts, Kitti Dora Csalyi, Benjamin Scott Padman, Michael Lazarou, Sascha Martens.
See the Max Perutz Labs News here.