The C-terminal fragment of LRRK2 with the G2019S substitution increases the neurotoxicity of mutant A53T α-synuclein in dopaminergic neurons in vivo
Abstract Background: Alpha-synuclein (α-syn) and leucine-rich repeat kinase 2 (LRRK2) likely play crucial roles both in sporadic and familial forms of Parkinson’s disease (PD). The most prevalent mutation in LRRK2 is the G2019S substitution, which induces neurotoxicity through increased kinase activity. There is likely an interplay between LRRK2 and α-syn involved in the neurodegeneration of dopaminergic (DA) neurons in the substantia nigra (SNpc) in PD. However, the mechanisms underlying this interplay are ill-defined. Here, we investigated whether LRRK2 G2019S can increase the neurotoxicity induced by a mutant form of α-syn (A53T mutation) in DA neurons in vivo . Methods: We used a co-transduction approach with adeno-associated virus (AAV), AAV2/6 vectors encoding human α-syn A53T and the C-terminal portion of LRRK2 (ΔLRRK2), which contains the kinase domain, with either the G2019S mutation (ΔLRRK2 G2019S ) alone or the D1994A mutation (ΔLRRK2 G2019S/D1994A ), which inactivates the kinase activity of LRRK2. The AAVs were co-injected into the rat SNpc and histological evaluation was performed at 6- and 15-weeks post-injection (PI). Results: The majority of SNpc neurons co-expressed ΔLRRK2 and human α-syn A53T after transduction. ΔLRRK2 G2019S alone produced no cell loss at 15-weeks PI. Injection of AAV-α-syn A53T alone or mixed with a control AAV coding for GFP produced a significant loss of DA neurons. Co-injection of AAV-α-syn A53T with AAV-ΔLRRK2 G2019S instead of GFP slightly exacerbated that neuronal loss We also studied the inactive form, ΔLRRK2 G2019S/D1994A at 6 weeks PI. Injection of AAV-ΔLRRK2 G2019S mixed with AAV-α-syn A53T produced a neurotoxic effect that was stronger than that produced by the co-injection of AAV-DLRRK2 G2019S/D1994A and AAV-α-syn A53T . Conclusion: Thus, these results show that mutant LRRK2 may selectively facilitate α-syn toxicity in DA neurons through a cell-autonomous mechanism involving its kinase domain. However, considering that the effect of ΔLRRK2 G2019S upon human α-syn A53T is moderate in our paradigm where pathological proteins are overexpressed, the study supports the hypothesis that the interplay between LRRK2 and α-syn may also implicate non-cell-autonomous mechanisms such as those involved in neuroinflammation and spreading of α-syn aggregated species.