Summary Inclusions comprised of α-synuclein (α-syn), i.e. Lewy bodies (LBs) and Lewy neurites (LNs), define synucleinopathies including Parkinson’s Disease (PD) and dementia with Lewy Bodies (DLB). Here, we demonstrate that pre-formed fibrils generated from full length and truncated recombinant α-syn enter primary neurons, likely by adsorptive-mediated endocytosis and promote recruitment of soluble endogenous α-syn into insoluble PD-like LBs and LNs. Remarkably, endogenous α-syn was sufficient for formation of these aggregates, and overexpression of wild type or mutant α-syn was not required. LN-like pathology first developed in axons and propagated to form LB-like inclusions in perikarya. Accumulation of pathologic α-syn led to selective decreases in synaptic proteins, progressive impairments in neuronal excitability and connectivity, and eventually, neuron death. Thus, our data contribute important insights into the etiology and pathogenesis of PD-like α-syn inclusions, their impact on neuronal functions, and provide a model for discovering therapeutics targeting pathologic α-syn- mediated neurodegeneration.

Cytoplasmic inclusions containing ␣-synuclein (␣-Syn) fibrils, referred to as Lewy bodies (LBs), are the signature neuropathological hallmarks of Parkinson's disease (PD). Although ␣-Syn fibrils can be generated from recombinant ␣-Syn protein in vitro, the production of fibrillar ␣-Syn inclusions similar to authentic LBs in cultured cells has not been achieved. We show here that intracellular ␣-Syn aggregation can be triggered by the introduction of exogenously produced recombinant ␣-Syn fibrils into cultured cells engineered to overexpress ␣-Syn. Unlike unassembled ␣-Syn, these ␣-Syn fibrils ''seeded'' recruitment of endogenous soluble ␣-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 ␣-Syn aggregation is normally limited by the number of active nucleation sites present in the cytoplasm and that small quantities of ␣-Syn fibrils can alter this balance by acting as seeds for aggregation.Parkinson's disease ͉ pathology ͉ protein misfolding

SUMMARY Many neurodegenerative diseases are characterized by the accumulation of insoluble protein aggregates, including neurofibrillary tangles comprised of tau in Alzheimer’s disease and Lewy bodies composed of α-synuclein in Parkinson’s disease. Moreover, different pathological proteins frequently codeposit in disease brains. To test whether aggregated α-synuclein can directly cross-seed tau fibrillization, we administered preformed α-synuclein fibrils assembled from recombinant protein to primary neurons and transgenic mice. Remarkably, we discovered two distinct strains of synthetic α-synuclein fibrils that demonstrated striking differences in the efficiency of cross-seeding tau aggregation, both in neuron cultures and in vivo. Proteinase K digestion revealed conformational differences between the two synthetic α-synuclein strains and also between sarkosyl-insoluble α-synuclein extracted from two subgroups of Parkinson’s disease brains. We speculate that distinct strains of pathological α-synuclein likely exist in neurodegenerative disease brains and may underlie the tremendous heterogeneity of synucleinopathies.