Structural basis of a distinct a-synuclein strain that promotes tau inclusion in neurons

Amyloidoses are predominantly associated with the accumulation of persistent aggregates of a particular protein. For example, the protein cC-synuclein characteristically aggregates in Parkinson's disease (PD), while amyloid beta and tau deposits are associated with Alzheimer's disease (AD). However, cC-synuclein-positive inclusions have been reportedly found in some tauopathies, and vice versa; tau-positive inclusions can be found in synucleinopathies. This suggests that there may be coexistence or crosstalk between these proteinopathies. This coexistence suggests that the simultaneous presence of these misfolded proteins may amplify pathogenic mechanisms. However, the crosstalk between these two types of proteopathies remains poorly understood. We now determine the structure of cC-synuclein fibrils that directly promote tau aggregation by cryogenic electron microscopy. Helical reconstruction at 2.6 & Aring; resolution reveals a new cC-synuclein fibril polymorph we term "strain B"; its core is unique, incorporating both the N- and C-termini of cC-synuclein. The design of peptides meant to inhibit the formation of this structure demonstrates that the C-terminal domain fragment (D105-E115) of cCsynuclein is critical for the formation of "strain B" fibrils and may play a key role in its interaction with tau. We hypothesize that the unique structure of pathological cC-synuclein significantly contributes to tau co-aggregation and plays a role in the intricate interactions among Alzheimer's, Parkinson's, and other neurodegenerative diseases. These findings open new avenues for drug targeting, discovery, and improve our understanding of neurodegenerative pathology.