Investigating selective regional and neuronal disease vulnerability in Spinocerebellar Ataxia Type 2

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PROJECT SUMMARY: Neurodegenerative disorders manifest selectively in particular neuronal populations and brain regions, through mechanisms we do not yet understand. Spinocerebellar Ataxia Type 2 (SCA2) is a fatal, hereditary neurodegenerative disorder caused by expansions in the CAG nucleotide repeat region of the ATXN2 gene, causing a gain-of-function ATXN2 protein with neurotoxic properties. While ATXN2 is expressed throughout the CNS, only a subset of brain structures and neurons are vulnerable to gross atrophy and cell death. The cerebellum is highly affected, resulting in the loss of voluntary movement, motor coordination, and speech difficulties in SCA2 patients. Across disease progression, there is pronounced cerebellar degeneration and selective loss of cerebellar Purkinje cells. Cerebellar degeneration occurs anterior to posterior, with the most posterior regions resistant to neurodegeneration. Multiple genetic and environmental insults also cause anterior to posterior cerebellar atrophy, suggesting that a conserved mechanism underlies the neurodegeneration gradient. To date, the cellular and molecular mechanisms underlying the variable cerebellar and Purkinje cell- specific vulnerability in SCA2 have not been investigated. I hypothesize that regional cerebellar vulnerability in SCA2 is modified by differential expression of neuroprotective pathways in response to cellular stress and disease. I will address this hypothesis in the following specific aims. In Aim 1, I will test whether Hspb1, a gene enriched in the posterior cerebellum, and previously identified as neuroprotective in other cerebellar degenerative disorders, can contribute to the protection of Purkinje cell loss in a mouse model of SCA2. If so, this will further support a conserved mechanism underlying the degenerative pattern and suggest common therapeutic avenues. In Aim 2A, I will identify novel neuroprotective pathways in a SCA2 model with the use of high throughput single-cell RNA-sequencing of the cerebellar transcriptome at fine temporal and spatial resolution. In Aim 2B, I will specifically address regional Purkinje cell vulnerability in SCA2 through the use of cell-type-specific ribosomal profiling to assess differential gene expression between disease-vulnerable and - resilient Purkinje cell populations. By comparing differential gene expression between the anterior and posterior cerebellum in both Aim 2A and Aim 2B, I will identify novel genetic modifiers of disease vulnerability for future validation. This study will be the first to characterize SCA2 degeneration and disease progression at a single- cell spatiotemporal resolution. As there is no effective treatment available for SCA2 patients, understanding the factors that protect against neurodegeneration and cell-type-specific contributions to disease pathology will help to better inform future therapeutic interventions.