Mechanisms and consequences of LRRK2 kinase activity in Parkinson’s disease

Pathogenic mutations in leucine-rich repeat kinase 2 (LRRK2) that affect its dynamics lead to an increase in kinase activity and are linked to increased risk of Parkinson’s disease (PD). The downstream consequences of sustained LRRK2 activity and what regulates wild-type (WT) LRRK2 kinase activation are not well characterized. In this dissertation, I set out to gain a better understanding of these aspects of LRRK2 biology. Overexpression of mutant LRRK2 has been associated with oxidative stress and antioxidants reportedly mitigate LRRK2 toxicity, but relevant mechanisms remain undefined. In Chapter 2, we showed that elevated LRRK2 kinase activity was associated with increased reactive oxygen species (ROS) production and lipid peroxidation, which were blocked by inhibitors of either LRRK2 or NADPH oxidase 2 (NOX2). Remarkably, rotenone-induced oxidative stress was blocked by kinase inhibition and was absent in LRRK2-null cells. In a rat model of PD, LRRK2 kinase inhibition prevented the lipid peroxidation normally seen in nigral dopamine neurons in this model. Mechanistically, LRRK2 kinase activity regulates phosphorylation of Ser345 of p47phox, the ‘organizing’ subunit of NOX2 which, in turn, leads to translocation of p47phox, resultant activation of NOX2 and production of ROS. To activate WT LRRK2 kinase, disparate stimuli that induce mitochondrial dysfunction, lysosomal dyshomeostasis or trafficking deficits have been used. In Chapter 3, I showed that a common feature of each of these stimuli is that they can lead to ROS production and accumulation of the lipid peroxidation end-product and reactive aldehyde, 4-hydroxyonenal (4HNE). In turn, 4HNE forms adducts with – and activates – endogenous WT LRRK2 kinase. Vicinal cysteine residues in the kinase activation loop of LRRK2 (Cys2024 and Cys2025) are essential for this oxidative activation, indicating that LRRK2 is a redox regulated kinase. The 4HNE generated by each of these stimuli is the enzymatic end-product of 15-lipoxygenase, inhibition of which prevents adduct formation with – and activation of – LRRK2 kinase. Thus, oxidative stress, long implicated in PD pathogenesis, may exert its effects, in part, by activating LRRK2 kinase. Collectively, our data have important therapeutic implications and revealed new strategies to target the LRRK2 pathway.