Fibril fuzzy coat is important for α-synuclein pathological transmission activity.

α-synuclein transmission and propagation are hallmarks of synucleinopathies, yet the molecular mechanisms remain elusive. Using α-synuclein preformed fibrils as pathological seeds, we observed a gradual decline in neuronal transmission activity during serial propagation. Fibril polymorphisms were identified from the initial generation: mini-P, with higher neuronal seeding activity, and mini-S, which accelerated recombinant α-synuclein aggregation. Changes in their proportions during propagation explained the overall decline in transmission activity. Cryoelectron microscopy and solid-state nuclear magnetic resonance revealed that both fibrils shared similar core regions but differed in their fuzzy coat flexibilities. The interaction between the fuzzy coat and fibril core substantially influenced neuronal transmission, a model further supported by hydrogen/deuterium exchange mass spectrometry. A mini-P-selective antibody identified active fibril types in newly propagated brain regions in human synucleinopathies. This study highlights the fuzzy coat's pivotal role in pathological protein transmission and suggests it as a potential therapeutic target for synucleinopathies.