Alpha-synuclein reduces the levels and impairs the function of ATP-dependent Clp protease (ClpP), an enzyme found in mitochondria — the cell compartments responsible for the production of energy — causing mitochondrial damage and oxidative stress, a study shows.
Parkinson’s disease mainly results from the gradual loss of dopaminergic neurons in the substantia nigra, a region of the brain responsible for movement control.
The disease also seems to be associated with overproduction of the protein alpha-synuclein in nerve cells of the brain. When this protein clumps together, it gives rise to small toxic deposits inside brain cells, inflicting damage and eventually killing them. Besides the accumulation of alpha-synuclein, Parkinson’s disease has also been linked to mitochondrial dysfunction.
Previous studies have shown that alpha-synuclein, in particular its A53T mutant form, accumulates in mitochondria, progressively causing mitochondrial damage. However, the mechanisms by which alpha-synuclein and mitochondrial proteins interact and regulate each other during this process are not fully understood.
ClpP, a mitochondrial enzyme that breaks down proteins (protease), is important for maintaining healthy mitochondria. In addition, ClpP dysfunctions have been associated with neurodegenerative diseases, suggesting that this enzyme might be one of the missing links that could explain the relationship between alpha-synuclein and mitochondria dysfunction in Parkinson’s disease.
To explore this idea, a group of researchers from the Case Western Reserve University School of Medicine and their collaborators set out to investigate if and how alpha-synuclein and its A53T mutant form affected the levels and function of ClpP in neurons isolated from patients with Parkinson’s and in animal models of disease.
Researchers showed that the levels of ClpP dropped significantly in the presence of high levels of alpha-synuclein. This was true for neurons derived from patients’ induced pluripotent stem cells (iPSCs), in dopamine-producing neurons isolated from mice genetically engineered to produce the A53T mutant form of alpha-synuclein and in postmortem brain samples from patients. iPSCs are fully matured cells that are reprogrammed back to a stem cell state, where they are able to grow into almost any type of cell.
On the other hand, a strong reduction in the levels of ClpP led to an overproduction of abnormal misfolded mitochondrial proteins, a reduction in mitochondrial activity, and increase in oxidative stress and cell death. Oxidative stress is an imbalance between the production of free radicals and the ability of cells to detoxify them, resulting in cellular damage as a consequence of high levels of oxidant molecules.
Interestingly, forcing the production of ClpP successfully reduced oxidative stress associated with alpha-synuclein and prevented the accumulation of phosphorylated alpha-synuclein in neurons derived from iPSCs of patients carrying the alpha-synuclein A53T mutant.
Phosphorylation is a chemical modification in which a phosphate group is added to the protein. Alpha-synuclein phosphorylation is known to occur in Parkinson’s disease, and is thought to be a critical step in disease progression as it enhances alpha-synuclein’s toxicity — possibly by increasing the formation of alpha-synuclein aggregates.
In addition, investigators found that both the normal and A53T mutant forms of alpha-synuclein physically interacted with ClpP, blocking its activity. Remarkably, when researchers induced the production of an artificial form of ClpP in the brains of mice that produced the A53T mutant form of alpha-synuclein, they managed to prevent mitochondrial damage and oxidative stress, and to slow disease progression and behavioral impairments.
“In this study, we have identified, for the first time, an important role of mitochondrial matrix protease ClpP in [alpha-synuclein]-associated neuropathology [in Parkinson’s disease]. Thus, our findings should stimulate the development of ClpP modulators as potential disease-modifying therapeutic agents in [Parkinson’s disease] and other synucleinopathies,” the researchers concluded.