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Alpha-synuclein (α-syn) and LRRK2 are two proteins that play crucial roles in Parkinson’s disease (PD) and recent data suggest the existence of an interplay between these two proteins (1,2). We hypothesized that the level of kinase activity of LRRK2 may modulate α-syn toxicity. To address this, using adeno-associated virus (AAV), we overexpressed in the rat substantia nigra pars compacta (SNc), both mutated (A53T) α-syn and 3 different LRRK2 constructs: the C-terminal part of LRRK2 (ΔLRRK2), a ΔLRRK2 with the G2019S pathogenic mutation, and the dead kinase double mutant form of LRRK2 (G2019S/A1994E). First we established that AAV-ΔLRRK2G2019S alone or mixed with AAV-GFP was not toxic at different time points up to 25 weeks post-infection (p.i.) Then we examined the effects of the ΔLRRK2 forms at two different titers of AAV-α-synA53T: low (leading to ~30% loss of DA neurons at 15 weeks) or high (leading to ~15% loss of DA neurons within 8 weeks). Our results showed that injection of a mixture of AAV-ΔLRRK2G2019S and low titer AAV-α-synA53T produced significantly more degeneration than AAV- α-synA53T alone or AAV- α-synA53T mixed with AAV-GFP. Moreover, the mild toxicity of low titer AAV-α-synA53T seen at 6 weeks p.i. was reduced when mixed with AAV-ΔLRRK2G2019S/A1994E. In a set up where AAV- α-synA53T was injected at higher (5x1010 vg/site) titer , AAV-ΔLRRK2G2019S, ΔLRRK2G2019S/A1994E and AAV-GFP had no impact on the resulting neuronal loss in SNpc. These results suggest that, when neurodegeneration of DA neurons results from relatively low levels of α-synA53T overexpression, increasing the levels of the active C-terminal domain of LRRK2G2019S can be sufficient to exacerbate α-synA53T toxicity., Work funded by Fondation de France and Association France Parkinson, and European Union Horizon 2020 Programme (H2020-MSCA-ITN-2015) under the Marie Skłodowska-Curie Innovative Training Networks and Grant Agreement No. 676408. We thank G. Auregan, M. Guillermier, F. Petit, and M. Gaudin for their technical assistance.