Contribution of <i>α</i>‐synuclein cytopathologies to distinct seeding of misfolded <i>α</i>‐synuclein

Synucleinopathies are a group of neurodegenerative diseases characterized by the deposition of misfolded α-synuclein (αSyn), predominantly in oligodendrocytes in multiple system atrophy (MSA) and in neurons in Lewy body diseases (LBD). The contribution of αSyn cytopathologies to the pathogenesis of these diseases is underappreciated. Seed amplification assays of MSA and LBD brains have revealed striking differences in αSyn seeding between regions and cases. Therefore, our aim was to evaluate whether different brain regions containing distinct αSyn cytopathologies contribute to different seeding characteristics. We collected 2-mm micro-punches of regions in MSA (n = 10) and LBD (n = 15) cases from formalin-fixed paraffin-embedded tissues. We performed double immuno-labeling for disease-associated αSyn and cellular markers on tissue microarrays, evaluated co-deposition of other neurodegenerative disease-related proteins and, from the same micro-punched samples, we analyzed αSyn seeding. Based on these variables, machine learning algorithms were used to reduce dimensionality of the dataset and cluster the regions in MSA and LBD cases, revealing that different compositions of αSyn cytopathologies influence αSyn seeding patterns. Our results support the notion of different cellular processing of αSyn and its contribution to the variability in seeding. This has implications for understanding disease progression, interpretation of seed amplification assays, and opens avenues for the development of cell type-specific antibodies against αSyn.