We may be one step closer to developing a treatment for Parkinson’s disease, thanks to new Australian research that has discovered how a protein called optineurin works in a distinct way to clear damaged mitochondria from the brain.
Mitochondria are the powerhouses of our cells. They combine oxygen with fuel molecules – sugars and fats – from food, breaking them down to produce energy. When mitochondria are defective, cells don’t have enough energy, and the unused oxygen and fuel molecules build up in the cells, causing damage. Broken-down mitochondria have been linked to various diseases, including Parkinson’s disease.
When they’ve broken down, mitochondria are normally removed and recycled by the body’s garbage disposal system in a process called mitophagy. Researchers from the Walter and Eliza Hall Institute (WEHI) examined the molecular mechanisms underlying the removal of damaged mitochondria, particularly in the setting of Parkinson’s disease, making an important discovery in the process.
The way the body removes damaged mitochondria is a cascading process. The protein PINK1 monitors mitochondrial health. When a problem is detected, it activates another protein, Parkin, to tag the damaged mitochondria for removal. The two proteins then enlist the help of a third protein, optineurin (OPTN), to create a cellular ‘garbage bag’ around the malfunctioning mitochondria.
All this the researchers already knew. What they discovered in their study, though, is that OPTN recognizes and removes damaged mitochondria by binding to an enzyme called TBK1. While the researchers were aware of the presence of OPTN and that it had a role in this process, its mode of action was unknown until now.
“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,” said Michael Lazarou, corresponding author of the study. “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.”
In Parkinson’s disease, mutations in PINK1 and Parkin can result in the accumulation of damaged mitochondria in the brain, leading to the tremors and stiffness that are the hallmarks of the neurodegenerative disease. The researchers say that discovering OPTN’s interaction with TBK1 may lead to new treatments.
“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,” said Thanh Nguyen, lead author of the study. “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 researchers say the discovery opens the door to developing treatments that exploit OPTN’s mechanism of action.
“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,” said Nguyen. “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.”
While the clinical application of the study’s findings is years away, more research is planned to better understand why OPTN does what it does.
“Our next step is to work with WEHI’s Parkinson’s Disease Center 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 mutation in the future,” Nguyen said.
The study was published in the journal Molecular Cell.
Source: WEHI