Manganese (Mn2+) neurotoxicity resembles a number of aspects of the dopamine (DA) neuron degenerating disorder Parkinson's disease (PD). Both PD and Mn2+ toxicity is characterized by motor deficits and damage to substantia nigra and other basal ganglia nuclei, and dopamine or its metabolites are believed to contribute to the disorder. Furthermore, expression of the pre-synaptic protein alpha-synuclein, and the oxidative stress- induced protein parkin have been proposed to contribute to the pathogenesis of both disorders, and occupational exposure to Mn2+ has been invoked to predispose individuals to PD. Despite the initial characterization of the disorder over 150 years ago, and intensive research within the past several decades, the origin of the pathogenesis and the molecular determinants involved in Mn2+ neurotoxicity have yet to be fully elucidated. A significant hindrance in dissecting the molecular components of Mn2+-induced neurotoxicity is the high complexity of the vertebrate brain and lack of facile in vivo genetic models to determine and explore the mechanisms involved in the cell death. We have developed a novel pharmacogenetic model using the genetically tractable nematode C. elegans to dissect and characterize the molecular components involved in DA neuron degeneration (see Nass et al, PNAS, 2002; Nass and Blakely, Ann. Rev. Toxicol. Pharmacol., 2003). At the molecular level, the C. elegans nervous system is highly conserved both genetically and functionally with mammals, and all the genes responsible for DA biosynthesis, packaging, and reuptake are present and functional in the worm. We have shown that the nematode C. elegans DA neurons can be selectively damaged by exposure of whole animals to the parkinsonian-inducing neurotoxin 6- hydroxydopamine (6-OHDA) (see Nass et al, PNAS, 2002)2. We have also recently shown that a brief exposure to Mn2+ causes DA neuron cell death in the worm, that prior exposure to Mn2+ amplifies the 6- OHDA-induced DA neurodegeneration, and that RNA knockdown of the divalent metal transporter-1 (DMT- 1), a putative Mn transporter, partially protects against Mn toxicity in the worm. In our model system, the expression of the green fluorescent protein (GFP) in DA neurons will allow us a facile and powerful test to examine the role that DA, its metabolites, endogenous proteins, and neurotoxins play in Mn2+ - induced degeneration of DA neurons in vivo. These studies will also include a novel genome-wide screen to identify mediators and suppressors of Mn2+-induced toxicity.
