Article In Brief
In two models of Parkinson's disease, researchers identified a molecular marker for the disease, Miro1, as well as a small molecule that significantly reduced Miro1 levels in fruit flies by facilitating its separation from mitochondria.
Researchers have identified a mitochondrial protein that they believe could serve as both a biomarker for Parkinson's disease (PD) and potentially as a therapeutic target for treating the disease.
The protein, Miro1, links healthy mitochondria to the cytoskeletal network and is removed and degraded when a mitochondrion becomes dysfunctional, allowing the organelle to be recycled through mitophagy. But according to a study published online September 23 in Cell Metabolism, Miro1 fails to disengage from damaged mitochondria in fibroblasts from almost all PD patients. The study also shows that treatment with a small molecule that targets Miro1 was beneficial in two PD models, suggesting that accumulation of dysfunctional mitochondria may be a final common pathway in PD pathogenesis, and preventing that accumulation may be broadly therapeutic.
“Miro1 appears to be a very strong signal for Parkinson's disease,” said David G. Standaert, MD, PhD, FAAN, chair and professor of neurology at the University of Alabama at Birmingham, who was not involved in the study.
Miro1 sits on the outer membrane of mitochondria and links to microtubules to facilitate mitochondrial motility. When a mitochondrion becomes damaged, it must be cleared and recycled via mitophagy, explained Xinnan Wang, MD, PhD, lead author on the new study and associate professor of neurosurgery at Stanford University. Damaged mitochondria that are not cleared leak toxins into the cell, causing chronic damage. Removing Miro1 is an early and necessary step in that clearance.
In 2011, Dr. Wang showed that Miro1 was a target for two key PD-related proteins, parkin and PINK1, which facilitate its removal. More recent work has shown that an additional PD protein, LRKK2, is also involved. All this led Dr. Wang to ask whether failure to remove Miro1 might be a general feature of PD.
Study Design, Findings
To explore this question, she examined fibroblasts from 83 PD patients from the National Institute of Neurological Disorders and Stroke human and cell repository, the Stanford Alzheimer's Disease Research Center, the Coriell Institute, Mayo Clinic, and the Parkinson's Progression Markers Initiative, including 28 with a positive family history, along with fibroblasts from five individuals at genetic risk for PD, 22 individuals with other neurodegenerative diseases, and 52 healthy controls.
She and her team treated fibroblasts with CCCP, a chemical that blocks mitochondrial oxidative phosphorylation, leading to depolarization of the mitochondrial membrane, a trigger for mitophagy in otherwise healthy cells. They then isolated mitochondria and tested for the presence of Miro1.
“We found that Miro1 was removed from mitochondria following depolarization in every control subject,” Dr. Wang said, including patients with Alzheimer's disease, Huntington's disease, frontotemporal dementia, corticobasal degeneration, progressive supranuclear palsy, and dementia with Lewy bodies. “In contrast, Miro1 was retained on the mitochondria in 94 percent of fibroblasts from PD patients and those at risk.”
There was no effect from gender, age of the patient at sampling time or disease onset, disease duration, or clinical status, she continued.There was also no dilution of the effect from the multiple times the various cell lines had been passaged, or subcultured before the experiment, which ranged from five to 19 times.
There was a difference among PD cell lines in the response of parkin and LRKK2 to depolarization. Dr. Wang found that some lines were impaired in recruitment of parkin to Miro1, some in recruitment of LRKK2, and some in both.
“Based on the patients in this study, it appears that this defect of Miro1 removal is a very common, convergent phenotype in Parkinson's disease,” Dr. Wang said.
Next, the team sought a small molecule that could bind to Miro1 and increase its dissociation from depolarized mitochondria and subsequent degradation. They used a machine-learning program to compare Miro1 binding sites to a library of small molecule structures and chose a handful with good pharmacokinetic properties for lab testing, first in fruit flies and then in patient fibroblasts. They found one compound, which they dubbed Miro1 Reducer, that caused a dose-dependent reduction in Miro1, possibly by promoting its degradation by the proteasome.
In dopaminergic neurons derived from two patient cell lines, the compound protected against induced mitochondrial stress and promoted clearance of damaged mitochondria. In three different PD fly models, carrying mutations in LRRK2, PINK1, or alpha-synuclein, the compound reduced age-dependent dopaminergic neurodegeneration.
The study reveals a “distinct molecular defect” that appears to be specific to PD, Dr. Wang said, and one that may be targetable. “This defect should be tested in other groups of patients, and we hope that others will follow our protocol to do so.”
“This is an extraordinarily important paper in that it that it identifies a potential biomarker for diagnosis, monitoring disease progression and evaluating the effects of therapeutic interventions in PD,” commented M. Flint Beal, MD, FAAN, professor of neurology and neuroscience at the Weill Medical College of Cornell University in New York. “Furthermore it provides further evidence that mitochondrial dysfunction plays a critical role in disease pathogenesis, and it establishes Miro1 as a target for developing a disease modifying therapy.”
“Perhaps the oldest story in Parkinson's disease is the abnormality of mitochondrial function, and this ties into that story,” said Dr. Standaert of the University of Alabama at Birmingham. “The authors have established that there is a mitochondrial defect across a very broad sample of PD patients.”
“What is really interesting here is that this is not in fresh material from biopsy, but in cells that have been repeatedly passaged,” he continued. “These cells are generations away from the original cells from the patients, and yet they display this defect. There is something, either genetic or epigenetic, that is persisting across generations of fibroblasts. The fact that this defect occurs in fibroblasts, not just neurons, also supports the idea that PD is a whole-body disease, not just a brain disease.”
It is unclear as yet what that persistent phenomenon might be. It is not any of the single-gene causes of PD, since the sample included many patients without those gene mutations.
“One conceivable explanation is that we are seeing a readout of the polygenic risk that leads to Parkinson's disease,” Dr. Standaert said, but that would require deeper genetic analysis than was done in this study.
“As a biomarker, Miro1 seems very compelling,” he said. “But a marker and a target are two very different things. It could be a target, but we would need a lot more evidence,” including from models that are closer to the human disease, in order to present a convincing case for Miro1's ability to have an effect on neurodegeneration. “But certainly as a biomarker it is very interesting.”
Drs. Wang, Standaert, and Beal disclosed no conflicts.