LRRK2 modifies α-syn pathology and spread in mouse models and human neurons

Progressive aggregation of the protein α-synuclein (α-syn) and loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) are key histopathological hallmarks of Parkinson disease (PD). Accruing evidence suggests that α-syn pathology can propagate through neuronal circuits in the brain, contributing to the progressive nature of the disease. Thus, it is therapeutically pertinent to identify modifiers of α-syn transmission and aggregation as potential targets to slow down disease progression. A growing number of genetic mutations and risk factors have been identified in studies of both familial and sporadic forms of PD. However, how these genes affect α-syn aggregation and pathological spread, and whether they can be targeted for therapeutic interventions, remains unclear. We performed a targeted genetic screen of risk genes associated with PD and parkinsonism for modifiers of α-syn aggregation, using an α-syn preformed-fibril (PFF) induction assay. We found that decreased expression of Lrrk2 and Gba modulated α-syn aggregation in mouse primary neurons. Conversely, α-syn aggregation increased in primary neurons from transgenic mice expressing the PD-linked LRRK2 G2019S mutation. In vivo, we investigated α-syn spreading in a LRRK2 G2019S transgenic mouse model and observed acceleration of α-syn aggregation and degeneration of dopaminergic neurons in the SNpc. Furthermore, LRRK2 G2019S transgenic mice displayed more pronounced degeneration-associated neuroinflammation and exacerbated behavioral deficits. To validate our findings in a human context, we established a human α-syn transmission model using induced pluripotent stem cell (iPS)-derived neurons (iNs). Recombinant human α-syn PFFs triggered aggregation of endogenous α-syn in a time-dependent manner in human iNs. In isogenic lines of human iNs, the G2019S mutation enhanced α-syn aggregation, whereas loss of LRRK2 decreased aggregation. Collectively, these findings establish a strong interaction between the PD risk gene LRRK2 and α-syn transmission across mouse and human models. Since clinical trials of LRRK2 inhibitors in PD are currently underway, our findings raise the possibility that these may be effective in PD broadly, not just in the cases caused by LRRK2 mutations.