Pathogenetic Mechanisms of Oligodendrocytes in Multiple System Atrophy
Multiple system atrophy (MSA) is a progressive neurodegenerative disease characterized by parkinsonism and/or cerebellar abnormalities, and autonomic dysfunction. The pathological hallmark of MSA is the accumulation of misfolded α-synuclein protein primarily in oligodendrocytes. As α-synuclein is normally and primarily expressed by neurons, how α-synuclein aggregates form in oligodendrocytes and their contribution to disease pathogenesis remains elusive.
To address this issue, we have begun to obtain gene expression profiles for thousands of individual cells encompassing nearly all cell types using single-nucleus RNA sequencing (nucSeq) of postmortem human brain tissue. We have processed striatal tissue from seven MSA patients (three MSA-parkinsonian and four MSA-cerebellar subtypes) and five neurologically healthy individuals.
From these data, we have identified MSA subtype–specific gene expression differences in numerous cell types. Notably, we have identified “clusters” of oligodendrocytes that express high levels of α-synuclein from the MSA-P striatal brain tissue. These cells also express lower levels of myelin-associated genes compared to other clusters of oligodendrocytes.
We hypothesize that these oligodendrocytes contribute significantly to the pathogenesis of MSA and arising through process that is captured in the transcriptomic profile of other oligodendrocytes.
- Aim 1: MSA-P and MSA-C are characterized by primary pathology and cell loss in the striatum/substantia nigra and cerebellum, respectively. We hypothesize oligodendrocytes express the fully “pathological” profile only in the primarily affected brain region. Thus far, we have sequenced only striatal tissue of both MSA-P and MSA-C cases. We have matching cerebellar tissue for all cases and will run nucSeq on these samples. Additionally, we will add additional MSA-P and control striatal and cerebellar tissue to strengthen the results of our current dataset. In collaboration with the Liddelow Lab, we will also incorporate nucSeq datasets from published studies in multiple sclerosis, Alzheimer’s disease, and from a newly available preprint from MSA brains to compare cell type–specific changes across these diseases.
- Aim 2: We will validate the nucSeq findings using RNAScope for fluorescence in situ hybridization starting with verifying the increased expression of α-synuclein in oligodendrocytes. We will probe for the “pathological” signature in tissue sections derived from the tissues in the nucSeq studies and additional samples. This will validate the co-expression of genes unique to pathological cells and provide valuable data regarding the spatial distribution of these cells.
- Aim 3: We hypothesize that oligodendrocytes exist in a continuum from “normal” oligodendrocytes to fully pathologic cells expressing high levels of α-synuclein in MSA. We will estimate the trajectory of each cell in this process with single-cell transcriptome trajectory techniques, allowing us to identify cells in early points of this process. The transcriptomic profile of these cells will reveal key nodes of gene expression that regulate the progression towards the pathogenic phenotype.
In future studies, we will incorporate striatal cells from Parkinson’s disease brain tissue into our nucSeq analysis. Because Parkinson’s disease is primarily a neuronal synucleinopathy, this will allow us to probe the disease and cell type specificity for the transcriptional changes identified. Further, this will provide additional insight to the changes in striatal neurons, that though do not die, contribute significantly to symptomology of Parkinson’s disease.
From these studies, key identified genes of interest and transcriptional pathways will be manipulated in human induced pluripotent stem cell (iPSC)–derived neurons and oligodendrocytes, as well as in mice to understand how these changes contribute to disease pathogenesis, which is necessary for developing new therapeutic approaches. These reverse translational studies will overcome the limitations of current MSA models that force overexpression of α-synuclein in specific cell types, limiting scientific questions to downstream events rather than addressing the fundamental upstream stressors that lead to dysregulation of α-synuclein.