The role of LRRK2 in the pathogenesis of Parkinson's disease

The G2019S mutation within the LRRK2 gene, is the most common genetic cause of Parkinson’s, being responsible for 20-40% of all familial PD cases, depending on the population under study. The actual function of the LRRK2 protein is not yet clear, although it has been implicated in several pathways including synaptic vesicle regulation, endocytosis and membrane trafficking. The gain of function G2019S mutation increases the kinase activity of the LRRK2 protein, contributing to the pathogenesis of PD. Several hypotheses exist on how G2019S contributes to PD, including regulation of dopamine metabolism and/or several Rab proteins, which have been identified as binding LRRK2, but the exact Rab is not consistent. Using the Drosophila visual system as an in vivo model, these hypotheses were addressed. HPLC analysis established that young flies expressing LRRK2-G2019S in their dopaminergic neurons (TH>G2019S flies) have lower levels of dopamine than control flies. In addition, inhibition of dopamine release by tetanus toxin showed an increase in visual sensitivity in control and old TH>G2019S flies, while young TH>G2019S flies showed a decrease in visual responses. Furthermore, new transgenic flies were generated, LexAop-LRRK2 and LexAop-G2019S, giving us the opportunity to use the Gal4 and LexA binary expression systems simultaneously at the same animal. Additionally, the expression protein levels of LRRK2 and G2019S were examined, indicating that LRRK2 is consistently expressing at higher levels than G2019S. That indicates that the kinase activity of the LRRK2 protein plays a vital role on the protein levels expression. Finally, the genetic screening that was performed in order to identify LRRK2-substrates in vivo identified six Rab proteins. Among those were Rab3, Rab5, Rab9, Rab10, Rab18 and Rab40, while Rab1 and Rab19 were identified interacting with the dopaminergic neurons of the flies. Overall, this study confirms the early hyperactivity in young TH>G2019S flies that could trigger the beginning of neurodegeneration, which is the hallmark of Parkinson’s.