A Study in PINK: How a Kinase May Protect the Brain from Parkinson’s Disease

Aslow and steady death of neurons that release the neurotransmitter dopamine causes one of the most common neurodegenerative disorders, Parkinson’s disease. One reason that the neurons die, scientists think, is due to the accumulation of stressed and damaged mitochondriathat power these active neurons.¹ PTEN-induced kinase 1 (PINK1) is an enzyme that responds to damaged mitochondria. Mutations in its gene lead to early onset Parkinson’s disease, making it a vital enzyme to study in the context of therapeutic interventions.² But how damaged mitochondria trigger PINK1 into action was still unclear to scientists.

Recently, Miratul Muqit, a neurologist at the University of Dundee, and his team found an answer. Their results, published in Science Advances, give insight into how cells respond to damaged mitochondria, with implications for what could be going wrong in dopaminergic neurons and Parkinson’s disease as well.³

PINK1 triggers a downstream cascade of events that leads to mitophagy—a process by which the cell clears away any mitochondria it doesn’t need.⁴ In response to the damage, PINK1 is stabilized at the outer mitochondrial membrane in association with a protein called the translocase of outer membrane (TOM) complex.

To understand how PINK1 gets activated and associates with the TOM complex, Muqit and his team wanted to study human PINK1 and TOM complex interactions in mammalian and insect cells. But some factor in the cells they were using seemed to prevent the two proteins from teaming up. A random chat with his colleague Karim Labib, who studies genomics at the same university, gave him an idea. “He mentioned to me, ‘you know, why not try doing this in yeast?’ Because in his field, people were expressing human genes in yeast and studying them and effectively using yeast as an expression system,” explained Muqit.

PINK1 is highly conserved across different species ranging from mice to men, but not in yeast—they had their own method of removing damaged mitochondria. Muqit and his team then generated yeast strains expressing human PINK1 and human TOM complex to study their interaction in detail.

“What was really striking is that in all of the work up until our yeast work, in order to activate PINK1, you have to stress the mitochondria,” said Muqit.

“But what's really interesting, is just expressing PINK1 and the TOM subunits in the unstressed yeast was enough to stabilize PINK1, presumably in a conformation that allowed it to become active.”

The TOM complex is made up of seven subunits, and Muqit wanted to figure out which ones play a role in stabilizing and activating PINK1. So, the team used seven different strains of yeast, with each one lacking a unique subunit of the TOM complex. Some subunits were already known to be important for the TOM complex to form in the outer mitochondrial membrane, but the group discovered something new about two subunits.⁵

TOM 20 and TOM 70 were previously only thought to play a role in importing PINK1 into healthy mitochondria, a mechanism by which PINK1 is eventually destroyed in the absence of any damage.⁶ However, deleting these two subunits prevented PINK1 from getting activated in the yeast, suggesting that they also played a role in stabilizing and activating PINK1 in damaged mitochondria.

“My overall impression is that this is an amazing paper,” said Richard Youle, a neurobiologist at the National Institutes of Health. One of the pioneers in the field of protein biochemistry studying PINK1 and its interactions, Youle thinks the work opens a lot of opportunities. “I'm really surprised they're able to reconstitute this complex system in yeast.”

Muqit and his team then wanted to delve deeper into the interactions between PINK1 and TOM 20 and 70. Using AlphaFold2, the group was able to predict where exactly human PINK1 binds to the two TOM subunits. The group then mutated PINK1 in these binding regions and expressed it along with the TOM complex in mammalian cells. They found that PINK1 failed to activate and tether to the TOM complex in this case.

Muqit plans to further test the physiological relevance of this interaction by using mutated PINK1 in mice and checking to see if the PINK1-TOM complex interaction is prevented in neurons as well.

Youle is also curious about how TOM 7—a subunit that he discovered is crucial for PINK1 activation—interacts with and activates PINK1. “There's a lot more to understand and they've got a really beautiful system to probe it in more detail.”