Characterisation of the cross-seeding of α-synuclein by Amyloid-β fibril polymorphs

Alzheimer’s disease is the most common neurodegenerative disease whose main histopathological hallmark is the presence of extracellular plaques, primarily composed of Amyloid β (Aβ) fibrils. Recent finding suggest Aβ fibril polymorphism is associated with distinct clinical presentations of Alzheimer’s disease. Moreover, intracellular inclusions of α-synuclein fibrillar aggregates known as Lewy bodies, classically associated with Parkinson’s disease, are found in up to 60% of Alzheimer’s disease individuals. Using a set of in vitro biochemical and cellular approaches, the involvement of Aβ fibril polymorphism in α-synuclein aggregation and Lewy body-like formation is investigated here. Structurally defined Aβ40 and Aβ42 fibril polymorphs formed in vitro were used as models to study the effect of polymorphism on the ability to induce α-synuclein fibrillar aggregation. It was found that in vitro, there is a pH dependency on the cross-seeding of α-synuclein by Aβ fibril polymorphs. Fluorescence polarization and thioflavin T fluorescence kinetics showed that although binding between α-synuclein and Aβ fibrils can occur at pH 7.5, it is only at pH 4.5 that the fibrillar aggregation of α-synuclein can be accelerated by Aβ fibrils presence. Notably, the molecular mechanism of cross-seeding was found to be governed by surface-catalysed secondary nucleation. Further characterisation of the α-synuclein cross-seeded fibrils revealed differences in their curcumin fluorescence, denoting distinct structural arrangements. Cellular analysis revealed that whilst not causing cell death, incubation with the Aβ fibril polymorphs differentially induced the formation of insoluble GFP-α-synuclein puncta. Furthermore, Aβ fibrils colocalized to acidic compartments, and increased resistance to lysosomal degradation was observed for α-synuclein monomeric samples in the presence of Aβ fibrils. Taken together these results pose lysosomes as a site of α-synuclein cross-seeding by Aβ fibrils and suggest polymorphism may play a role in the extent of cellular cross-seeding, highlighting the importance of specific interactions between Aβ and α-synuclein in disease.