Functional study of a novel protein kinase, leucine-rich repeat kinase 2 (LRRK2), associated with Parkinson's disease

For a long time, Parkinson's disease was considered a purely an environmental disease caused by certain neurotoxins or virus through the environment. The association of genetics was actually discounted because of high discordance rate in the monozygotic twin studies. However, much of the paradigm shifted due to the improvement of genetic techniques and discovery of several genes linked to familial PD in the last decade. The discovery of genes include mutations in autosomal dominant alpha-synuclein gene, recessive mutations in the DJ-1, Parkin and PINK-1 genes, and most recently in 2004, Leucine-rich repeat kinase 2 (LRRK2) is the most important gene associated with late onset PD that is clinically indistinguishable from typical, idiopathic PD exploded the field of genetics in PD. Although it is important to note that a majority of PD occurs sporadically, ~5% to 10% of cases have been linked to genetics and there are significant benefits that can be obtained from understanding the cause(s) of rare, familial PD phenotypes. Studies on several genetic mutations in familial PD identified thus far have helped us to understand the molecular basis of the disease.;LRRK2 is a multidomain structural protein; mutations occurring within different domains of LRRK2 could affect the functional properties of the protein. Since high prevalence rate of LRRK2 mutations is associated with PD, it is likely that a more intensive biological study of LRRK2 could provide valuable insight into understanding disease mechanisms underlying the pathogenesis of PD.;Here, in this thesis, it is demonstrated that LRRK2 is an authentic and functional GTPase. We found that LRRK2 is able to bind GTP and undergo intrinsic GTP hydrolysis. Furthermore, the Roc domain is sufficient for this native GTPase activity and binds and hydrolyzes GTP indistinguishably from the Ras-related small GTPase, Rac1 Additionally, LRRK2 mediated phosphorylation is stimulated upon binding of non-hydrolyzable GTP analogs, suggesting that LRRK2 is a MAPKKK activated intramolecularly by its own GTPase. Since GTPases and MAPKKKs are upstream regulators of multiple signal transduction cascades, LRRK2 may play a central role in integrating pathways involved in neuronal cell signaling and the pathogenesis of PD.;Further, to understand the biological function of LRRK2, I performed protein and protein interacting study of LRRK2 and found LRRK2 interacts with synaptic proteins, SYDE1 and SAD. Both SAD-1 and SYDE1 are known mediators of axonal dendritic polarity. Interestingly, I discovered that overexprssion of WT or mutant LRRK2 in cultured primary hippocampal neurons result in defects in neuronal polarity development compared to the vector control and LRRK2 kinase dead mutant. My findings suggest that LRRK2 is likely to play an important role in regulating neuronal polarity through its interaction with key synaptic proteins.