Correlation between Cellular Uptake and Cytotoxicity of Fragmented α-Synuclein Amyloid Fibrils Suggests Intracellular Basis for Toxicity

Aggregation and intracellular deposition of the protein alpha-synuclein is an underlying characteristic of Parkinson's disease. alpha-Synuclein assemblies also undergo cell cell spreading, facilitating propagation of their cellular pathology. Understanding how cellular interactions and uptake of extracellular alpha-synuclein assemblies depend on their physical attributes is therefore important. We prepared fragmented fluorescently labeled alpha-synuclein amyloid fibrils of different average lengths (similar to 80 nm to >1 mu m) and compared their interactions with SH-SYSY cells. We report that fibrils of all lengths, but not monomers, bind avidly to the cell surface. Their uptake is inversely dependent on their average size, occurs via a heparan sulfate dependent endocytic route, and appears to have a size cutoff of similar to 400 nm. The uptake of a-synuclein fibrils, but not monomers, correlates with their cytotoxicity as measured by reduction in metabolic activity, strongly suggesting an intracellular basis for alpha-synuclein fibril toxicity, likely involving endolysosomes.