Transgenic Model for Intraneuronal Oxidative Stress

One of the major questions we have often encountered in the Parkinson's disease (PD) study is why dopamine neurons in the substantia nigra of patient brains are selectively undergone degeneration. The discovery of neurotoxin MPTP and its use in making animal models of PD have contributed a great deal in our understanding of the pathogenesis of PD. However, little progress has been made in identifying any MPTP-like substance in our daily life that links to the onset of sporadic PD. On the other hand, dopamine (DA) is not only an important neurotransmitter for normal brain function but also a toxin through auto-oxidation and formation of the toxic compound quinones that induce neuronal death. To prevent such toxicity, DA neurons normally have several specific mechanisms including an efficient package of DA into the secretory vesicles for its physiologic function, which keeps DA away from other organelles inside of neurons. This package is mediated by a group of proteins called vesicular monoamine transporters (VMATs), which in turn can serve as very important neuroprotective machinery. Our previous studies have shown that VMATs indeed protect against MPTP toxicity in DA cells. We hypothesize that dopamine might play a role as an intrinsic toxin to predispose DA neuron to injury during PD. Interference with or elimination of the protective mechanism by VMATs may also enable us to directly test the role of DA toxicity in neural degeneration. Although inactivation or 'knockout' of VMAT2 (brain isoform of VMATs) in mice held promise as an animal model to test the hypothesis, the complete inactivation (homozygous) in mice is lethal shortly after birth, preventing their use in the study of aging related neuropathogenesis. The goal of this project is to generate an animal model that conditionally inactivates VMAT2 in adult or aging mice. This model would allow us to test how the elimination of the protective sequestration of DA produces endogenous DA toxicity in DA neurons and how this predisposed condition contributes to the pathogenesis of PD. Specifically, we will establish a transgenic animal model in that we can achieve a spatial and temporal inactivation of VMAT2 gene in the brain. We will then examine how the impaired DA sequestration induces oxidative stress inside of the cells that are therefore selectively vulnerable to degeneration. These studies aim at advancing our current knowledge on the protective machinery and endogenous detoxification of DA by VMAT2 as well as on a potential role of DA toxicity in the pathogenesis of DA neurons in PD. Using conditional inactivation of VMAT2, we will also develop a unique genetic model to study molecular mechanisms underlying DA function and its clinical application in PD.