Scientists at Johns Hopkins Medicine say they have discovered how a group of proteins linked to Parkinson's disease and amyotrophic lateral sclerosis act as "guardians" of mitochondria, small organelles, or subunits, within a cell that make and store energy and are found in almost all plants and animals.
The findings, resulting from experiments with genetically engineered mice should advance understanding of the development of Parkinson's disease - a chronic and progressive neurodegenerative disorder whose causes are not clearly known - the scientists say. Most experts believe Parkinson's disease is a result of some combination of genetics and environmental factors.
A summary of the researchers' experiments appears in the March 20 issue of the journal Nature.
The new research, funded by the National Institutes of Health, has its roots in previous efforts to show how cells respond to stresses, including external pressures such as low oxygen levels and internal stresses such as an imbalance of nutrients.
According to Hiromi Sesaki, Ph.D., professor of cell biology at the Johns Hopkins University School of Medicine, who studies how mitochondria grow, divide and fuse, the organelle's size must be neither too big nor too small to work well.
Scientists have long known that when mitochondria are stressed too much or are damaged beyond repair, they stop fusing, become smaller and degrade. With damaged mitochondria, cells are not as well equipped to make energy. In the brain, stressed cells can cause neurodegeneration and neuroinflammation.
As a consequence, cells deploy various means to protect mitochondria and keep them right-sized. For example, cells may repair mitochondria by fusing them together to maintain their genomic material and energy output.
Mitochondria may also divide after growth to maintain their size and number or to separate damaged parts. However, if mitochondria become too large, they stop fusing, preventing the formation of harmful giant mitochondria, which in turn, hinder the efficient degradation of damaged mitochondria.
According to Miho Iijima, Ph.D., also a professor of cell biology at the Johns Hopkins University School of Medicine, one way that cells respond to mitochondria size control issues caused by stress or damage is by turning on the activities of several proteins. Two of the proteins, Parkin and PINK1, hang around the mitochondria's membrane and work as a pair to enable the mitochondria to fuse or degrade. Abnormalities in the genes for Parkin and PINK1 are associated with the onset of Parkinson's disease in humans.
Another protein linked to amyotrophic lateral sclerosis, OMA1, is also known to stop mitochondria from fusing upon stress.
Previous mouse studies have shown that, when conditions in cells are normal, removing, or "knocking out," any one of the genes that encode the Parkin, PINK1, and OMA1 proteins causes no abnormalities in mice or their mitochondria.
However, Iijima and Sesaki wondered what would happen to mice and their cells if two of the three genes - Parkin, PINK1, and OMA1 - were knocked out under normal physiological conditions.
By knocking out Parkin and OMA1 or PINK1 and OMA1, the scientists found that the double knockout mice were small and had movement problems, along with excessively fused, oversized mitochondria in neurons when compared with mice with normal versions of the genes.
However, if only one gene is knocked out, the other genes still regulate mitochondrial fusion, and mice show no signs of mitochondrial enlargement or dysfunction.
The Johns Hopkins Medicine scientists speculate that mitochondrial fusion is "double-locked." Because mitochondria have two membranes, turning off only one of the genes may disable one membrane but not both, and mitochondria can still fuse and remain somewhat healthy.
In total, the research team genetically engineered mice with 18 variations of normal and knockout combinations of the three genes, as well as other genes, to substantiate this conclusion.
"Working in tandem, Parkin-PINK1 and OMA1 act as guardians of mitochondria, ensuring that the organelles maintain their normal size and function," says Iijima.
The scientists also measured mitochondria's main product: energy in the form of adenosine triphosphate (ATP). In all the mice studied, they found no significant difference in ATP levels in samples of brain cells.
To sort out that finding, the scientists looked more closely at the immune system response in relation to the brain cells. They found that when mitochondria got too big, their mitochondrial DNA leaked out into the cytosol, the fluid that fills up the space inside cells. This triggered an increase in the release of interferons, proteins that spark an inflammatory response.
The scientists plan to advance their work by studying more precisely how mitochondrial DNA leaks out of the organelle when it gets too large. They also want to identify which cell types respond to neuronal mitochondrial DNA release to induce innate immune responses, with the goal of discovering new insights about the development of Parkinson's disease and possibly new therapeutic drug targets.
The research was supported by the National Institutes of Health (R35GM144103, R35GM131768 and P20GM104320), the Human Aging Project and the Adrienne Helis Malvin Medical Research Foundation. Sesaki is the Ethan and Karen Leder CIM/HAP Scholar.
