In situ amplification of α-synuclein amyloid fibril reveals a distinct polymorph related to Parkinson's disease and dementia with Lewy body.

High-resolution structure determination of ex vivo amyloid fibrils offers critical mechanistic insights into amyloid polymorphism and heterogeneity of neurodegenerative diseases. However, purifying amyloid fibrils from diseased brains may favor certain polymorphs over others. Here, instead of purifying fibrils, we used in situ amplification (ISA) of α-synuclein (α-syn) fibrils in brain homogenates. Cryoelectron microscopy (cryo-EM) structural analysis of the ISA fibrils from patients with Parkinson's disease (PD) or dementia with Lewy bodies (DLB) reveal polymorphic fibril ensembles of each case including fibrils with morphologies similar to previously reported Lewy fold, as well as many others. Remarkably, we determine the high-resolution structure of a distinct polymorph (ISA-P1) that differs from the Lewy fold but is similar to those of multiple system atrophy (MSA) and juvenile-onset synucleinopathy (JOS). Our work supports the existence of alternative α-syn fibril polymorphs in PD and DLB, some of which may share common prototypical folds across various synucleinopathies.