Researchers at the University of Queensland Brain Institute have for the first time shown that a proteotoxic species can increase mitochondrial DNA mutations in neurons.

The finding creates a direct link between two big hallmarks of neurodegeneration – proteotoxic species and mitochondrial dysfunction – that are associated with neurodegeneration, and the study links them together at the level of the mitochondrial genome.

The powerhouse of the cell, the mitochondria generate most of the energy for our cells. It is the only organelle in our cells that has its own genome. Around 2 billion years ago, the mitochondria used to be a separate species – a bacterial-like species – that our cells engulfed and became endosymbiont, lead researcher and study author Steven Zuryn told BioWorld Science.

"Around 2 billion years ago when we first engulfed these mitochondria, that's when we had this big explosion in evolution. That's when we had the energy to evolve and become complex. Before that we were all single-cell little creatures, but as soon as the mitochondria were used as an energy source, we had a lot of excess energy and could develop into multicellular plants and animals," said Zuryn, who leads the lab at the Queensland Brain Institute.

Mitochondrial dysfunction and age-related disease

There is renewed interest in the role mitochondrial DNA plays in aging, Zuryn said, noting that his lab is looking at what drives the differences between different cells and understanding that certain mutations and certain pathogenic threats like proteotoxicity can influence mitochondrial mutation levels and influence certain types of diseases, including age-related neurodegenerative diseases.

Published in the June 1, 2021, issue of Cell Reports, the Zuryn lab discovered that certain cell types in the body are prone to propagating mitochondria DNA mutations more than others.

Mitochondrial dysfunction is a major contributor to the pathogenesis of Parkinson's disease. PINK1 and Parkin are proteins mutated in familial Parkinson's disease, which function to maintain mitochondrial health by culling damaged mitochondria through mitophagy. PINK1 and Parkin drive mitophagy by selectively tagging damaged mitochondria to trigger their degradation.

"What we're suggesting in this paper is that if you have mutations in PINK1 and Parkin, which are actually nuclear genes, that leads to an increase in mitochondrial DNA mutations. It points towards a possible cause for a loss of neural function in diseases like Parkinson's disease, possibly other diseases as well.

"When we overexpress certain proteotoxic species, such as tau, which is associated with Alzheimer's disease, or we overexpress a polyglutamate repeat protein, which is similar to the Huntington's protein that causes Huntington's disease, we see a similar thing. We see an increase in mitochondrial DNA mutation levels in neurons."

But in understanding mitochondrial mutations in Parkinson's disease, researchers first had to work out what causes the disease and where the pathogenesis originated from, he said.

"We know there are certain mutations in genes that can cause Parkinson's disease," he said. When humans have mutations in either of the Parkin and PINK1 genes, they can develop early-onset Parkinson's disease."

By analyzing mitochondria from different cell types, the researchers were able to create transgenic animals by expressing human proteins in the nervous system of the Caenorhabditis elegans nematode worm.

C. elegans was fist pioneered as a model organism in the 1960s by geneticist Sydney Brenner, who won the Nobel Prize for his work with the species in understanding neurobiology and how organisms developed.

"C. elegans is the first animal to have its genome sequenced. It's probably the best characterized animal genetically speaking," Zuryn said, noting that every single gene has been at least partially characterized.

"What we found is that if you have mutations in those two genes that cause Parkinson's disease, in the C. elegans you see a consequential buildup of mitochondrial DNA mutations in the nervous system. If you have mutations in that blueprint, the mitochondria don't function properly.

By taking the human tau and the human model of polyglutamate repeats and expressing them in the nervous system of the C. elegans worm, that causes an increase of mitochondrial DNA mutations.

"What it points to is that several different types of mutations or proteotoxic proteins that are associated with human neurodegenerative diseases are linked to an increase in these mitochondrial DNA mutations.

"Once you form some sort of hypothesis as to maybe that is contributing to this disease, then you can start to look at ways of fixing that problem or reducing that problem," he said.

Earlier studies revealed that different types of cells – neuronal, skin, muscle, intestinal – have differences in the level of mitochondrial DNA mutations between cells.

Cell differences in mitochondria mutations

Some cells accumulate more mitochondrial DNA mutations than other cells naturally, Zuryn said, and that finding led the group to explore not only why that is the case but what was the mechanism driving that difference between cell types.

"There must be some molecular mechanisms happening within those cells that allow mutations to accumulate in the mitochondria, and then the other cells that prevent that from happening.

"What we found is that the genes PINK1 and Parkin seem to be the important difference between the different cell types."

For example, in neurons there are less mutations in a wild type of animal compared to muscle cells where there are more mutations.

"But if you mutate (inactivate) PINK1 and Parkin, the levels become equalized between the two cell types. That means those genes normally create the difference between those two cell types."

He said the two genes reduce mitochondrial mutations. Neurons particularly require PINK1 and Parkin to keep mitochondrial DNA mutations low, he explained.

"This was one of the first studies to show that a proteotoxic species can increase mitochondrial DNA mutations in neurons. That's something that's never been shown before," he said, "and that creates a direct link between two big hallmarks of neurodegeneration – proteotoxic species and mitochondrial dysfunction – they seem to always be associated with neurodegeneration, and this links them together at the level of the mitochondrial genome, because proteotoxicity causes an increase in mitochondrial DNA mutations, which affects mitochondrial function."

The lab is now looking at whether an increase in mitochondrial mutations affects inheritance and whether cells harboring mutations affect other cells in the body and increase mutations in those cells