α-Synuclein Aggregation and Transmission Are Enhanced by Leucine-Rich Repeat Kinase 2 in Human Neuroblastoma SH-SY5Y Cells

Formation of alpha-synuclein aggregates is a key step in Parkinson's disease pathogenesis although the etiology remains elusive. alpha-synuclein is accumulated in degenerating neurons, leading to the production of filamentous inclusions such as Lewy bodies. However, the in vitro overexpression of alpha-synuclein alone failed to induce inclusion bodies consisting of phosphorylated alpha-synuclein. The seeded aggregates-initiated polymerization of alpha-synuclein and tau has been reported elsewhere. What molecule is an initiator of filamentous inclusions remains to be defined. Here, we report that leucine-rich repeat kinase 2 (LRRK2)-cotransfection together with alpha-synuclein enhance the aggregate formation, phosphorylation, release to extracellular media of alpha-synuclein, and the cell-to-cell transmission into neighboring cells in human neuroblastoma SH-SY5V cells. In cells transfected with alpha-synuclein alone, the proteins were distributed in the cytosol and did not form inclusions. On the other hand, the inclusions and phosphorylation of alpha-synuclein were formed in cells cotransfected with alpha-synuclein and LRRK2 G2019S mutant together. LRRK2 G2019S-cotransfected PC12 cells also induced the aggregates. Furthermore, the cell-to-cell transmission of alpha-synuclein and the cell toxicity were also enhanced by either LRRK2 wild type or G2019S mutant, whereas the cell viability was not decreased in cells transfected with alpha-synuclein alone. These results suggest that overexpression of LRRK2, especially G2019S mutant, whose functions remain unclear, initiate the aggregate formation, release and transmission of alpha-synuclein, resulting in the propagation of alpha-synuclein to neighboring cells and reduction of cell viability.