Leucine-rich Repeat Kinase 2 (lrrk2) Knockin Mouse Model For Pathophysiological Studies Of Parkinson's Disease (pd)

Abstract

Introduction: Parkinson's disease (PD) is a neurodegenerative disorder characterized by degeneration of dopaminergic neurons in the brain, with motor symptoms including tremor, rigidity, bradykinesia, and postural instability. The disease pathogenesis is unclear with no cure. There is a lack of appropriate in-vivo models which accurately mimic human PD. Most existing models rely on short-term lethal intoxication to induce acute neuronal damages and do not reflect the natural lifelong progression of PD. Knockout (removal of a gene) and transgenic (overexpression of a gene) models do not mimic the human condition due to non-physiological level of gene expression. Developing an appropriate disease model which incorporates the effects of aging, genetics and environmental factors and reproduces the pathogenic processes in human PD will facilitate the identification of biomarkers and therapeutic targets. Mutations in leucine-rich repeat kinase 2 (LRRK2) represents the commonest genetic risk in both familial and sporadic PD. Patients with LRRK2-associated PD demonstrate indistinguishable clinical features from typical idiopathic cases, suggesting similar pathogenic mechanisms. Hence, LRRK2-based genetic models can best represent the wide clinical spectrum of PD. Project Objectives and Methods: We generated homozygous LRRK2R1441G knockin mutant mice to study their susceptibility to nigrostriatal dopaminergic cell death, and examined for pathological changes in nigrostriatal network, dopamine uptake and locomotor activity. Our LRRK2R1441G knockin mice express mutant LRRK2 at physiological levels, similar to human PD. In contrast to the LRRK2 knockout or transgenic mice, these knockin mice do not show overt motor phenotype but have subtle abnormalities and increased susceptibility tostress-induced locomotor deficits and dopamine depletion. Results: Striatal synaptosomes from young knockin mice demonstrated lower levels of dopamine uptake with impaired locomotor activity after injection with reserpine (an inhibitor of dopamine uptake into synaptic vesicles) (Liu et al., Ann Clin Transl Neurol. 2014;1:199-208). Mutant mice chronically exposed to low sub-lethal doses of oral rotenone (a natural pesticide) for 50 weeks (~half the mouse's lifespan) resulted in more severe locomotor deficit, compared to their wildtype littermates (Liu et al., Sci Rep. 2017;7:40887). Conclusions: The combined effects of aging, genetic mutation and environmental stress contribute to disease development. Presymptomatic humans with LRRK2 mutation may already harbour subtle abnormalities in the brain, and show greater susceptibility to developing PD as they age and upon chronic exposure to environmental toxins. Our novel experimental model more closely mimics human PD, and will be invaluable in future studies to elucidate early pathogenic processes and to develop novel therapies. Project Number: 01120976