Autosomal dominant mutations in LRRK2 are the most common genetic cause of Parkinson's disease (PD). Notably, LRRK2-associated PD is clinically indistinguishable from idiopathic PD, and generally is accompanied by the presence of the classic intracellular inclusions of ?-synuclein called Lewy bodies, the pathological hallmark of PD. However, identifying biochemical links between ?-synuclein aggregation and LRRK2 function has proven difficult. Prior work may have been thwarted by the assumption that the common pathways involving these two disease-linked proteins must occur within the same cell. Recent evidence demonstrates the expression and potent regulation of LRRK2 in non-neuronal cells including microglia, particularly during neuroinflammation. ?-synuclein, on the other hand, is exclusively expressed by neurons in the brain but is secreted by a yet unidentified mechanism. Recent data from multiple labs suggest that extracellular ?-synuclein can serve as a ligand to activate microglial cells, which express LRRK2. In this application we will screen primary cultured murine microglia for the effects of multiple conformations and assemblies of ?-synuclein, identifying those that activate microglial LRRK2, and confirm these effects in cultured human microglia. Next, using two distinct knock-in mouse models expressing endogenous levels of mutant LRRK2 (R1441C and G2019S), we will determine how pathogenic mutations in LRRK2 alter the ?-synuclein- dependent microglial responses, both in vitro and in vivo. This will involve an analysis of how pathogenic LRRK2 mutations influence microglial responses to pathogenic ?-synuclein exposure probing important biochemical properties of the LRRK2 protein and the effects on microglial behavior and function. This exploratory project will test a novel hypothesis regarding the functional interactions between LRRK2 and ?-synuclein, and may add valuable insight into potential pathways integrating what are arguably the two most important PD-linked gene products.
Alpha-synuclein is thought to play a major role in virtually all forms of Parkinson's disease, including idiopathic and most familial cases. Autosomal dominant mutations in LRRK2 are the most common genetic cause of Parkinson's disease, but the role of alpha-synuclein in LRRK2-mediated pathogenesis is unclear. The goal of this proposal is to examine how the alpha-synuclein protein may activate LRRK2 function in microglia and to develop a better understanding of how these two proteins are linked in the etiology of PD.