| Parkinson's disease (PD) is a neurodegenerative disorder particularly characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta. Its etiology is unknown but likely includes both genetic and environmental factors. Since pesticide use has been associated with PD occurrence, we conducted a screen to identify pesticides that impair the ubiquitin-proteasome system (UPS), a degradative process implicated in PD pathogenesis. Benomyl was identified as a UPS inhibitor in this screen and became the focus of this dissertation.;In an epidemiologic study, we used state-mandated Pesticide Use Reports to estimate chronic exposures and observed 65-92% increased PD risk in a population exposed to the fungicide benomyl. In experimental models, we found that benomyl induced selective dopaminergic neuronal loss in vitro with primary mesencephalic cultures and selective aminergic neuronal loss in a novel in vivo zebrafish system. Benomyl is readily metabolized to S-methyl N-butylthiocarbamate and its sulfoxide, a potent aldehyde dehydrogenase (ALDH) inhibitor. These thiocarbamates inhibited ALDH activity in primary neurons at submicromolar concentrations leading to accumulation of the dopamine metabolite 3,4-dihydroxyphenylacetaldehyde, the proposed neurotoxicant. This model for PD etiology via ALDH inhibition may help explain the selective vulnerability of dopaminergic neurons in PD.;To follow up on this finding, we developed a novel ex vivo neuronal assay to screen other pesticides for ALDH inhibitory activity. All dithiocarbamates tested (e.g., mancozeb, maneb, ziram), two dicarboxymides (captan, folpet), and two imidazoles (benomyl, triflumizole) inhibited ALDH activity, potentially via metabolic byproducts (e.g., carbon disulfide, thiophosgene). Exposures to ALDH-inhibiting pesticides (i.e., positive hits) applied in our study area were associated with dose-dependent twofold to fourfold increases in PD risk. Genetic variation in the ALDH2 gene potentiated this risk considerably (up to sixfold) for people working where these pesticides were sprayed liberally.;This is the first report of a pesticide that damages dopaminergic neurons by inhibiting ALDH activity. The identification of multiple ALDH-inhibiting pesticides associated with increased PD risk supports the potential of ALDH as a novel therapeutic target and a subject for consideration by regulators and policymakers. |
