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Abstract
Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most frequent genetic cause of dominantly inherited Parkinson’s disease (PD). However, how LRRK2 predisposes to PD remains largely unknown. LRRK2 mutations result in a loss of phosphorylation on LRRK2 at two serine sites (S910 and S935) that are important for LRRK2 protein interactions. LRRK2 phosphorylation is also reduced on these serines in PD brain tissue. Thus, loss of LRRK2 phosphorylation may contribute to PD pathogenesis. This is an important issue as LRRK2 kinase inhibitors, which are highly regarded as potential PD therapeutics, also cause a loss of LRRK2 phosphorylation.
To better understand LRRK2 expression and phosphorylation in PD brain, the levels of LRRK2 and phosphorylated LRRK2 were measured in post-mortem tissue from PD patients with and without LRRK2 mutations as well as controls. LRRK2 levels were reduced only in the PD cases with LRRK2 mutations. Two retromer proteins and glucocerebrosidase levels were also measured and were decreased in the LRRK2 mutation cohort. This data demonstrates a relationship between LRRK2 and retromer dysfunction in LRRK2-associated PD brain.
To more directly assess if/how LRRK2 phosphorylation contributes to PD pathogenesis, loss-of-phosphorylation knock-in mice were studied to determine their susceptibility to PD-related neurodegeneration following exposure to α-synuclein fibrils. There was no neurodegeneration or motor deficits in the knock-in mice, but they had a propensity for abnormal dopamine turnover, decreased astrocyte numbers and increased α-synuclein pathology in the striatum. These results suggest a potential link between loss of LRRK2 phosphorylation and prodromal PD pathological changes.
Finally, human induced pluripotent stem cell-derived neural cells with and without LRRK2 mutations were used to assess the cellular effects of reducing LRRK2 phosphorylation with LRRK2 kinase inhibitors. LRRK2 kinase inhibitors increased oxidative stress, but did not affect cell viability or growth. Moreover, chronic treatment with LRRK2 kinase inhibitors effectively reduced α-synuclein intensity, improved nuclear morphology and decreased autophagosome size in neural cells with LRRK2 mutations.
In conclusion, these results provide new information on LRRK2 phosphorylation that may be important for clinical translation of the LRRK2 inhibitors currently in development to treat PD.