Aβ42-Driven α-synuclein Fibril Polymorphism and Distinct Intracellular Aggregation

The frequent coexistence of α-synuclein (α-syn) and amyloid-β (Aβ) aggregates in neurodegenerative diseases suggests that heterotypic interactions between these amyloidogenic proteins may influence disease progression, yet their molecular consequences remain poorly understood. Here, we investigated how distinct aggregation states of Aβ42, monomers and preformed fibrils (PFFs), modulate α-syn fibril formation, structure, and downstream neuronal pathology. Thioflavin T kinetics showed that Aβ42 monomers delayed α-syn fibril formation, whereas Aβ42 PFFs exhibited a trend toward accelerated aggregation, indicating aggregation state-dependent effects on α-syn aggregation. Negative-stain TEM, proteinase K digestion, and solid-state NMR spectroscopy further demonstrated that both Aβ42 monomers and PFFs altered α-syn fibril structure, generating distinct fibril conformations depending on the Aβ42 concentration and aggregation state. To determine whether these conformational differences influence pathological activity, α-syn PFF variants generated in the presence of different concentrations of Aβ42 monomers or PFFs were applied to dopaminergic neuronal cells. α-Syn fibrils formed in the presence of Aβ42 PFFs showed greater capacity to induce intraneuronal α-syn aggregation than α-syn PFFs, whereas fibrils formed in the presence of Aβ42 monomers exhibited similar or reduced seeding capacity relative to α-syn PFFs. Together, our findings demonstrate that heterotypic interactions with Aβ42 reshape α-syn aggregation pathways and fibril conformations, generating structurally distinct α-syn fibril populations with different neuronal seeding activities. These results provide a molecular framework for understanding how cross-talk between amyloidogenic proteins may contribute to structural and pathological heterogeneity in mixed neurodegenerative diseases.