Direct Demonstration of Seed Size-Dependent alpha-Synuclein Amyloid Amplification

The size of amyloid seeds is known to modulate their autocatalytic amplification and cellular toxicity. However, the seed size-dependent secondary nudeation mechanism, toxicity, and disease-associated biological processes mediated by alpha-synudein (alpha-Syn) fibrils are largely unknown. Using the cellular model and in vitro reconstitution, we showed that the size of alpha-Syn fibril seeds dictates not only their cellular internalization and associated cell death but also the distinct mechanisms of fibril amplification pathways involved in the pathological conformational change of alpha-Syn. Specifically, small fibril seeds showed elongation possibly through monomer addition at the fibril termini, whereas longer fibrils template the fibril amplification by surface-mediated nudeation as demonstrated by super-resolution microscopy. The distinct mechanism of fibril amplification and cellular uptake along with toxicity suggest that breakage of fibrils into seeds of different sizes determines the underlying pathological outcome of synucleinopathies.