Cell autonomous role of leucine-rich repeat kinase in protection of dopaminergic neuron survival

Abstract

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson’s disease (PD), which is the leading neurodegenerative movement disorder characterized by the progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). However, whether LRRK2 mutations cause PD and degeneration of DA neuronsviaa toxic gain-of-function or a loss-of-function mechanism is unresolved and has pivotal implications in LRRK2 based PD therapy. In this study, we investigate whether LRRK2 and its functional homologue LRRK1 play an essential, intrinsic role in DA neuron survival through the development of DA neuron-specificLRRKconditional double knockout (cDKO) mice. We first generated and characterized floxedLRRK1andLRRK2mice and then confirmed that germline deletions of the floxedLRRK1andLRRK2alleles result in null alleles, as evidenced by the absence ofLRRK1andLRRK2mRNA and protein in the respective homozygous deleted mutant mice. We further examined the specificity of Cre-mediated recombination driven by thedopamine transporter-Cre(DAT-Cre) knockin (KI) allele using a GFP reporter line and confirmed thatDAT-Cre-mediated recombination is restricted to DA neurons in the SNpc. Crossing these validated floxedLRRK1andLRRK2mice withDAT-CreKI mice, we then generated DA neuron-restrictedLRRKcDKO mice and further showed reduced levels of LRRK1 and LRRK2 in dissected ventral midbrains ofLRRKcDKO mice. While DA neuron-restrictedLRRKcDKO mice of both sexes exhibit normal mortality and body weight, they develop age-dependent loss of DA neurons in the SNpc, as demonstrated by the progressive reduction of DA neurons in the SNpc of cDKO mice at 20 and 24 months of age. Moreover, DA neurodegeneration is accompanied with increases of apoptosis and elevated microgliosis in the SNpc ofLRRKcDKO mice. These findings provide unequivocal evidence for the importance of LRRK in DA neurons and raise the possibility that LRRK2 mutations may impair its protection of DA neurons, leading to the loss of DA neurons in Parkinson’s disease.