A common feature of many neurodegenerative diseases is the deposition of filamentous protein aggregates in the central nervous system (CNS), including neurofibrillary tangles (NFTs) composed of tau, and Lewy bodies (LBs) consisting of α-synuclein (α-syn), which are the hallmark lesions of Alzheimer's disease (AD) and Parkinson's disease (PD) respectively. What causes the conversion of normally soluble proteins into insoluble fibrils has always been enigmatic, and cell models that recapitulate the abnormal accumulation of tau into NFT-like aggregates were lacking due to high solubility of tau. Enlightened by in vitro studies showing nucleation-dependent fibrillization of tau, we tested the hypothesis that preformed tau fibrils (tau pffs) assembled from recombinant protein may act as seeds to nucleate the fibrillization of soluble tau in cultured cells. Indeed, we found that minute quantities of tau pffs internalized into cells over-expressing tau can rapidly recruit large amounts of endogenous tau into detergent-insoluble filamentous inclusions with properties very similar to NFTs. Moreover, the spontaneous uptake of tau pffs was shown to be mediated by endocytosis. Together with similar studies on tau and other disease-associated proteins, our study implicates cell-to-cell transmission of misfolded proteins through templated recruitment as a plausible mechanism for the onset and progression of CNS amyloidosis. Another mysterious phenomenon of neurodegenerative diseases is the frequent co-occurrence of different protein aggregates, such as NFTs and LBs. To test whether fibrillar α-syn can directly cross-seed tau into pathological aggregates, we utilized our recently developed synucleinopathy models in primary neurons and transgenic mice involving delivery of α-syn pffs. Intriguingly, we discovered two distinct strains of α-syn fibrils demonstrating striking difference in the efficiency of cross-seeding tau pathology both in neurons and transgenic mice. Biochemical analyses indicated conformational differences between the two strains, thereby revealing the ability of a single molecule to assemble into more than one misfolded conformers. We speculate that the existence of conformationally diverse strains may be another shared feature of amyloid aggregates, accounting for the tremendous heterogeneity of neurodegenerative diseases with differential extent of concomitant pathologies and highly variable but sometimes overlapping clinical symptoms.