Exogenous alpha-synuclein fibrils seed the formation of Lewy body-like intracellular inclusions in cultured cells.

Cytoplasmic inclusions containing alpha-synuclein (alpha-Syn) fibrils, referred to as Lewy bodies (LBs), are the signature neuropathological hallmarks of Parkinson's disease (PD). Although alpha-Syn fibrils can be generated from recombinant alpha-Syn protein in vitro, the production of fibrillar alpha-Syn inclusions similar to authentic LBs in cultured cells has not been achieved. We show here that intracellular alpha-Syn aggregation can be triggered by the introduction of exogenously produced recombinant alpha-Syn fibrils into cultured cells engineered to overexpress alpha-Syn. Unlike unassembled alpha-Syn, these alpha-Syn fibrils "seeded" recruitment of endogenous soluble alpha-Syn protein and their conversion into insoluble, hyperphosphorylated, and ubiquitinated pathological species. Thus, this cell model recapitulates key features of LBs in human PD brains. Also, these findings support the concept that intracellular alpha-Syn aggregation is normally limited by the number of active nucleation sites present in the cytoplasm and that small quantities of alpha-Syn fibrils can alter this balance by acting as seeds for aggregation.