OPTN

Researchers uncovered the mechanistic details behind how several proteins interact to help cells recognize and remove damaged mitochondria. Optineurin (OPTN) and its interactions are needed to provide a contact site for another protein, Tank-binding kinase 1 (TBK1), during this process. The OPTN-TBK1 relationship is necessary for these mitochondria to be recognized and eliminated from the cell. This mechanism may have relevance for developing drugs to treat Parkinson's disease. Autophagy is a process used by cells as a recycling system to transport and break down organelles and other cytosolic components, which become enveloped in a membrane called the autophagosome (Fig 1). When this involves the removal of damaged mitochondria, commonly called the "powerhouse" of the cell, it is known as mitophagy. In a recent article published in The EMBO Journal, a team led by researchers at Tokyo Medical and Dental University (TMDU) elucidated the molecular details of how an enzyme called Tank-binding kinase 1 (TBK1) participates in a disease-relevant mitophagy mechanism. Although autophagy has been characterized as a more general process meant to degrade and clear various cellular components, recent data have suggested that certain pathways are specifically involved in the autophagy of particular organelle types that are damaged or no longer needed. The researchers became interested in mitophagy mediated by molecules called PINK1 and Parkin, as they are proteins that have been pathologically linked to Parkinson's disease. "Mitophagy-related defects have been directly implicated in the neurodegeneration observed in Parkinson's disease patients," says Koji Yamano, lead author of the study. "Normally, PINK1 and Parkin work together to mark damaged mitochondria for removal by adding a chain of molecules called ubiquitin. This mark allows proteins called autophagy adaptors to associate with the mitochondria and bring in the autophagy machinery for autophagosome development." Although TBK1 is known to participate in PINK1/Parkin-mediated mitophagy, a detailed mechanism how it activates remained unclear. Using various molecular biology techniques, the team found that deleting the gene encoding TBK1 prevented association of an autophagy adaptor called optineurin (OPTN) during Parkin-mediated mitophagy (Fig 2). Additionally, deleting the OPTN gene prevented autophosphorylation of TBK1, which is necessary for it to function. Further work suggested that the interactions between OPTN and ubiquitin, as well as between OPTN and the developing autophagosome, were all needed for OPTN and TBK1 to come together at the contact site between damaged mitochondria and the pre-autophagosome membrane. Without this contact site, TBK1 autophosphorylation could not occur. The researchers also generated molecules called monobodies in their lab that could specifically bind OPTN and inhibit its physical interactions. The monobodies prevented OPTN accumulation at the mitophagy contact sites (Fig 3A). This subsequently blocked TBK1 activation (Fig 3B) and thereby mitochondrial degradation. These experiments further emphasized the importance of the OPTN-TBK1 relationship to support proper mitophagy. "Because PINK1 and Parkin are critical contributors to the molecular basis of Parkinson's disease, understanding the mechanistic details related to the mitophagy process mediated by these molecules is very important," explains Yamano. This study demonstrates a positive and reciprocal relationship between OPTN and TBK1 that is necessary for autophagosomes to begin forming on damaged mitochondria. The impactful finding may lead to the development of novel drugs to treat Parkinson's disease.

Researchers have solved a long-standing mystery about how a protein helps rid the body of damaged mitochondria, in findings that could help lead to potential new treatments for Parkinson's disease. While mitochondria play a crucial role in producing the energy our cells need to carry out their various functions, when damaged, they can have profound effects on cellular function and contribute to the development of various diseases. Broken-down mitochondria are usually removed and recycled through a garbage disposal process known as 'mitophagy'. PINK1 and Parkin are two proteins vital to this process, responsible for 'tagging' malfunctioning mitochondria for destruction. In Parkinson's disease, mutations in these proteins can result in the accumulation of damaged mitochondria in the brain, which can lead to motor symptoms such as tremors, stiffness and difficulty with movement. The new research, published in Molecular Cell, solves a mystery about how the protein Optineurin recognises unhealthy mitochondria 'tagged' by PINK1 and Parkin, enabling their delivery to our body's garbage disposal system. Associate Professor Michael Lazarou, a Laboratory Head in WEHI's Ubiquitin Signalling Division, said the discovery filled a vital knowledge gap that would transform our understanding of this cellular pathway. "Until this study, Optineurin's precise role in initiating our body's garbage disposal process was unknown," Assoc Prof Lazarou, who also holds a co-appointment with Monash University, said. "While there are many proteins that link damaged cellular materials to the garbage disposal machinery, we found that Optineurin does this in a highly unconventional way that is unlike anything else we've seen from similar proteins. "This finding is significant because the human brain relies on Optineurin to degrade its mitochondria through the garbage disposal system driven by PINK1 and Parkin. "Knowing how Optineurin does this provides us with a framework on how we might be able to target PINK1 and Parkin mitophagy in disease and prevent the build-up of damaged mitochondria in neurons as we age. "Achieving this would be instrumental to people with Parkinson's disease -- a condition that continues to impact more than 10 million people worldwide, including 80,000 Australians." Outlier discovery PINK1 acts as a 'watch-house' inside the mitochondria, responsible for monitoring their health. When it detects problems, it activates Parkin, which tags damaged mitochondria for removal. They work together to instruct our body to generate cellular 'garbage bags' around broken-down mitochondria and enlist the help of Optineurin to initiate this process. The new study revealed that Optineurin removes damaged mitochondria by binding to an enzyme known as TBK1. From there, they found that TBK1 goes on to activate a specific cellular machine that is key to generating these garbage bags around unhealthy mitochondria. First author Dr Thanh Nguyen said: "Other proteins don't need TBK1 to help them trigger this degradation process, making Optineurin a real outlier when it comes to how our bodies remove mitochondria. "This has allowed us to look at the features of this pathway involving TBK1 as a potential drug target, which is a significant step forward in our search for new Parkinson's disease treatments. "The ultimate goal would be to find a way to boost levels of PINK1/Parkin mitophagy in the body -- especially the brain -- so that damaged mitochondria can be removed more effectively. "We also hope to design a molecule that could mimic what Optineurin does, so damaged mitochondria could be removed even without PINK1 or Parkin." "Given Optineurin is critical in activating the garbage disposal system in our brains, this could then prevent the accumulation of damaged mitochondria in this region, which is a significant precursor to Parkinson's disease." Dr Nguyen said while clinical application of the research is years away, the discovery had laid the essential foundation needed to understand how Optineurin works and realise the pathway's potential as a future therapeutic target. "Our next step is to work with WEHI's Parkinson's Disease Centre to validate our findings in neuronal model systems to understand why Optineurin behaves this way, which will provide further insight into how we can target Optineurin and TBK1 to enhance treatment options for people with PINK1/Parkin mutations in the future." The research involves a collaboration with Professor Sascha Martens' lab at the Max Perutz Labs, University of Vienna and was supported by the National Health and Medical Research Council (NHMRC), the Australian Research Council (ARC), the Human Frontiers Science Program and Aligning Science Across Parkinson's (ASAP) through the Michael J. Fox Foundation for Parkinson's Research (MJFF).