Molecular Signatures of Neuroinflammation Induced by αSynuclein Aggregates in Microglial Cells

Alpha-synuclein (alpha Syn(Agg)) are pathological hallmarks of Parkinson's disease (PD) and other synucleinopathies that induce microglial activation and immune-mediated neurotoxicity, but the molecular mechanisms of alpha Syn(Agg)-induced immune activation are poorly defined. We performed quantitative proteomics by mass spectrometry coupled with PCR, immunohistochemical and functional validations studies to define the molecular characteristics of alpha synuclein mediated microglial activation. In mouse microglia, alpha Syn(Agg) induced robust pro-inflammatory activation (increased expression of 864 genes including Irg1, Ifit1, and Pyhin) and increased nuclear proteins involved in RNA synthesis, splicing, and anti-viral defense mechanisms. Conversely, alpha Syn(Agg) decreased expression several proteins (including Cdc123, Sod1, and Grn), which were predominantly cytosolic and involved in metabolic, proteasomal and lysosomal mechanisms. Pathway analyses and confirmatory in vitro studies suggested that alpha Syn(Agg) partly mediates its effects via Stat3 activation. As predicted by our proteomic findings, we verified that alpha Syn(Agg) induces mitochondrial dysfunction in microglia. Twenty-six proteins differentially expressed by alpha Syn(Agg) were also identified as PD risk genes in genome-wide association studies (upregulated: Brd2, Clk1, Siglec1; down-regulated: Memo1, Arhgap18, Fyn, and Pgrn/Grn). We validated progranulin (PGRN) as a lysosomal PD-associated protein that is downregulated by alpha Syn(Agg) in microglia in-vivo and is expressed by microglia in post-mortem PD brain, congruent with our in vitro findings. Conclusion: Together, proteomics approach both reveals novel molecular insights into alpha Syn-mediated neuroinflammation in PD and other synucleinopathies.