[Real-time Quaking-induced Conversion Analysis of Prion-like Seeding Activity of Pathological α-Synuclein]

Pathological α-synuclein (αSyn) has been shown to retain the ability to propagate as prions in humans and animals. However, the molecular basis underlying the prion-like properties of αSyn remains poorly understood. We examined whether brain tissues from cases of dementia with Lewy bodies (DLB), which contain serine 129 (Ser129)-phosphorylated insoluble aggregates of αSyn, exhibit prion-like seeding activity in vitro using the real-time quaking-induced conversion (RT-QuIC) seeding assay. Brain tissues from cases of diffuse neocortical DLB yielded a 50% seeding dose of 10^7.3-10^9.8/g brain. The RT-QuIC assay could discriminate between DLB and other neurological and neurodegenerative disorders, suggesting its potential applicability for differential diagnosis. Insoluble aggregates of αSyn>250 kDa detected only in DLB brain tissues by Western blotting analysis were specifically phosphorylated at Ser129. Therefore, we postulated that Ser129-phosphorylated insoluble aggregates of αSyn have prion-like seeding activity. However, insoluble aggregates of recombinant human αSyn (rSyn) with increased β-sheet structures showed little seeding activity in either the phosphorylated or nonphosphorylated state. In contrast, prefibrillar oligomers of rSyn showed seeding activity both with and without phosphorylation. The findings of the present study suggested that soluble oligomeric αSyn, but not the fully fibrillary form, is a seeding species in vitro.