LRRK2 and Parkinson disease: Reduced synaptic vesicle density has also been observed in other animal models of PD, in particular the synaptojanin SJ1RQ KI mouse and LRRK2 G2019S transgenic mouse.21,42 In both mouse models, synaptic vesicle recycling dysfunction was accompanied by axon degeneration and selective dopaminergic neurodegeneration.