Parkinson's Disease Mutation May Trigger Astrocyte-Driven Neural Alpha-Synucleinopathy
By Kurt Samson
February 21, 2019
Article In Brief
Investigators found that Parkinson's disease astrocytes—derived from induced pluripotent stem cell-derived astrocytes and neurons from patients with familial Parkinson's disease (harboring mutant LRRK2)—disrupt cellular degradation pathways, leading to the accumulation of toxic alpha-synuclein. Their work suggests an important role for glial cells in PD and offers potential new targets for developing therapies.
The most common genetic mutation in Parkinson's disease may cause a breakdown in the ability of astrocytes to clear out toxic alpha-synuclein and the transfer of the excess protein to neighboring ventral midbrain dopaminergic neurons (vmDANs), causing their death, researchers from Spain reported in the January 10 issue of Stem Cell Reports.
Using induced pluripotent stem cell-derived (iPSCs) astrocytes and neurons from patients with the LRRK2 mutation, the authors discovered the defect is a non-cell autonomous deleterious phenotype that, when co-cultured with healthy wild-type iPSC-derived neurons, reduced their number, suggesting some form of pathological ‘cross talk’ between the two cell types.
Astrocytes developed dysfunctional chaperone-mediated autophagy (CMA) and impaired macro-autophagy, two intracellular “housekeeping” processes whereby proteins are selectively targeted for degradation.
“Our findings show that astrocytes are altered and accumulate toxic proteins, which in turn are passed on to neurons, ultimately causing their death,” explained Angel Raya, MD, PhD, co-senior author and director of the Center of Regenerative Medicine, in Barcelona, Spain.
“We found that patients have disruption in several cellular degradation pathways, and intervention on these pathways might be a potential therapy, especially because astrocytes are connected to the vascular system,” he told Neurology Today.
Co-author Antonella Consiglio, PhD, an investigator at the University of Barcelona's Institute of Biomedicine, said more research is needed to determine whether additional factors might also contribute to neuronal cell death.
Potential molecular therapies targeting the pathways might prove beneficial, she said, noting that by chemically enhancing CMA, they were able to protect cells by promoting alpha-synuclein clearance.
“Some drugs that increase the activity of the degradation pathways that we found were altered in our studies are currently in pre-clinical development,” Dr. Consiglio told Neurology Today. “We tried one of them and found that the treatment was efficient at reverting signs of the disease, at least on cells in the laboratory.”
The researchers used iPSCs from three PD patients who carried mutations in the LRRK2 gene to grow astrocytes and neurons, which were then co-cultured with healthy midbrain neurons from two control patients. Structural changes associated with neurodegeneration and neuron loss developed in control neurons exposed to the mutated astrocytes after approximately one month.
Although it is also possible that PD astrocytes interfered with differentiation and/or maturation of vmDAN progenitors from the stem cells, the researchers noted that they used vmDAN differentiated cultures at 35 days, past the point when they might have matured into other types of cells. Moreover, there was a progressive decline and accumulation of alpha-synuclein over time only in healthy neurons co-cultured with PD astrocytes, and not in those co-cultured with control cells, they said.
That a CMA activator at least partially restored normal alpha-synuclein activity indicated that upregulation was still possible and sufficient to return affected neurons to near normal levels. However, because the researchers were unable to completely restore them suggests that other non-alpha synuclelin-related factors secreted by PD astrocytes might play a role, Dr. Raya said.
“Interestingly, the treatment with [CMA] activator not only cleared out alpha-synuclein in astrocytes, but also in vmDANs, partially restoring neuron survival and decreasing the number of TH-positive cells with a degenerative morphology.”
Dr. Raya said the team's next goal is to investigate whether the findings also apply to patients with sporadic forms of the disease. The investigators are already looking into other mechanisms by which PD astrocytes might pass the disease to neurons.
Neurology professor Serge Przedborski, MD, PhD, chief of the movement disorders division and co-director of the Center for Motor Neuron Biology and Disease at Columbia University Irving Medical Center, told Neurology Today that as in other neurodegenerative disorders such as amyotrophic lateral sclerosis, the study found that non-neuronal cells can contribute to the demise of neurons. However, he noted, much more work is needed to better clarify and confirm the reported findings.
“While the study provides an exciting proof-of principle, several points deserve clarification and call for additional studies,” he said. “For instance, further demonstration is necessary to establish whether or not the decline in the number of vmDANs truly reflects death, as the authors concluded.”
“It would have been more convincing to see data for actual cell death markers rather than vmDANs counts since phenotypic dopaminergic markers such as tyrosine hydroxylase can be readily downregulated in response to stress, giving rise to a false interpretation of neuron loss,” Dr. Przedborski told Neurology Today.
He said that he remains uncertain about whether the mutant astrocytes exerted a deleterious effect on their neighboring vmDANs by way of a toxic phenomenon, as the authors maintained, or by merely proviidiing beneficial factors necessary to the well-being and survival of vmDANs.
“While this question can be readily clarified with future studies, it would have been crucial to have settled this issue since, to accept the pathogenic scenario proposed by the authors, one must first agree with the notion that mutant astrocytes are toxic,” he said.
“Lastly, the type of molecular mechanism targeted, autophagy, is so fundamental and a hub for so many factors, that it is difficult to conclude with certainty whether the improvement the authors report is truly or solely related to changes in alpha-synuclein turnover.”
Commenting on the study, Alice Chen-Plotkin, MD, an associate professor of neurology at the University of Pennsylvania Perelman School of Medicine, said: “I think that the key thing about this study is that it builds on a body of literature that is emerging that suggests alpha-synuclein protein can spread from cell to cell, and may occur between neurons and other types of cells,” she said.
Dr. Chen-Plotkin directs the Molecular Integration in Neurological Diagnosis (MIND) Initiative at the School of Medicine, a cross-departmental program aimed at characterizing neurological disease patients at the DNA and molecular levels in order to improve clinical care and accelerate therapeutic development
“This [study] suggests the possibility of exploring therapeutic approaches aimed at interrupting this spread between various cell types that might have different functions; in the case of the astrocytes, it may be to degrade abnormal proteins in lysosomes.”