Neurodegenerative disorders including Lewy body Dementia (LBD) and Parkinson’s disease (PD) are characterized by aggregation of a-synuclein (a-syn), however the downstream toxic events that lead to cell death are not understood. Proteome dysfunction is a prominent feature of synucleinopathies, as indicated by genetics and pathology. To gain a comprehensive understanding of how the proteome changes in PD, we performed a quantitative proteomic study to identify proteins that aggregate in patient derived iPSC-neurons as a consequence of a-syn accumulation. By comparing iPSC neurons expressing A53T a-syn with isogenic corrected lines, we discovered a remarkable level of selectivity in the classes of proteins that aggregate. Specifically, we found that RNA binding proteins NONO and SFPQ undergo dramatic solubility shifts from detergent soluble into the insoluble state. NONO and SFPQ are multifunctional nuclear proteins that play critical roles in transcription regulation, RNA splicing, and RNA editing of genes that regulate axon guidance. They are core components of a membraneless sub-compartment in the nucleus called the paraspeckle, and contain prion- like low complexity domains that permit phase separation under physiological conditions. Paraspeckles occur in neuronal cultures and in vivo in the brain, and are thought to play key roles in regulating homeostatic stress by transiently sequestering transcription factors and RNAs to prevent translation. Once stress subsides, paraspeckles normally dissolve and gene expression returns to normal. However, we have found that NONO and SFPQ irreversibly form pathological aggregates in patient iPSC-neurons and LBD patient brain. This effect is specifically associated with a-syn accumulation, and does not occur with general cellular stress. Mechanistic studies in iPSC-neurons suggest that formation of NONO/SFPQ aggregates is associated with loss of their functions, resulting in neurite degeneration. We find that the SFPQ transcriptional target, ADAR3 that mediates RNA editing, is nearly completely depleted in patient neurons. Here, we propose to examine the mechanism of how a-syn accumulation leads to aberrant NONO/SFPQ aggregation in the nucleus and downstream pathophysiology. Given their role in RNA splicing and editing, we propose to employ both targeted and unbiased methods to identify changes in RNA editing including RNA-seq, exon-junction microarrays to examine RNA splicing, and ChIP-seq to detect changes in SFPQ transcriptional activity. These phenotypes will be correlated with distinct aggregated forms of a-syn and neurodegeneration. Finally, we will attempt to rescue established phenotypes in patient iPSC-neurons by promoting soluble, function NONO/SFPQ. Our preliminary studies have identified a novel pathogenic pathway in synucleinopathies, and we will extend these findings by examining the mechanisms of gene dysregulation and RNA processing. We will provide the first description of RNA editing and s...