Motor Impairments and Dopaminergic Defects Caused by Loss of Leucine-Rich Repeat Kinase Function in Mice

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease (PD), but the pathogenic mechanism underlying LRRK2 mutations remains unresolved. In this study, we investigate the consequence of inactivation of LRRK2 and its functional homolog LRRK1 in male and female mice up to 25 months of age using behavioral, neurochemical, neuropathological, and ultrastructural analyses. We report that LRRK1 and LRRK2 double knock-out (LRRK DKO) mice exhibit impaired motor coordination at 12 months of age before the onset of dopaminergic neuron loss in the substantia nigra (SNpc). Moreover, LRRK DKO mice develop age-dependent, progressive loss of dopaminergic terminals in the striatum. Evoked dopamine (DA) release measured by fast-scan cyclic voltammetry in the dorsal striatum is also reduced in the absence of LRRK. Furthermore, LRRK DKO mice at 20-25 months of age show substantial loss of dopaminergic neurons in the SNpc. The surviving SNpc neurons in LRRK DKO mice at 25 months of age accumulate large numbers of autophagic and autolysosomal vacuoles and are accompanied with microgliosis. Surprisingly, the cerebral cortex is unaffected, as shown by normal cortical volume and neuron number as well as unchanged number of apoptotic cells and microglia in LRRK DKO mice at 25 months. These findings show that loss of LRRK function causes impairments in motor coordination, degeneration of dopaminergic terminals, reduction of evoked DA release, and selective loss of dopaminergic neurons in the SNpc, indicating that LRRK DKO mice are unique models for better understanding dopaminergic neurodegeneration in PD.SIGNIFICANCE STATEMENT Our current study employs a genetic approach to uncover the normal function of the LRRK family in the brain during mouse life span. Our multidisciplinary analysis demonstrates a critical normal physiological role of LRRK in maintaining the integrity and function of dopaminergic terminals and neurons in the aging brain, and show that LRRK DKO mice recapitulate several key features of PD and provide unique mouse models for elucidating molecular mechanisms underlying dopaminergic neurodegeneration in PD.