Methylmercury (MeHg) is a potent neurotoxin affecting both the developing and mature central nervous systems (CNS), indiscriminately disrupting multiple homeostatic pathways. MeHg and Parkinson's disease (PD) share common biochemical and molecular mechanisms of dopaminergic (DAergic) neurodegeneration, including oxidative damage and mitochondrial dysfunction. Understanding gene-environment interactions in human disease has been hindered by the complexity of the vertebrate brain. Complexities associated with vertebrate models of MeHg have also limited the understanding of molecular mechanisms associated with its neurotoxicity. The nematode, Caenorhabditis elegans (C. elegans) and mammals share a highly conserved genetic code;that is, mammalian genes responsible for MeHg uptake (large amino acid transporter 1;Lat1) and stress and antioxidant responses [nuclear factor erythroid 2-related factor 2 (Nrf2) and Foxo] are conserved (aat, skn-1 and daf-16, respectively) and functional in the worm. Our overall hypothesis is that modulation of C. elegans genes (aat, skn-1, daf-16) that are homologous to mammalian regulators of MeHg uptake and cellular resistance will modify DAergic neurodegeneration in response to MeHg exposure. It will be tested in the following Specific Aims: (1) Knockdown of aat-1, aat-2, aat-3 or aat-1;aat- 2;aat-3 renders C. elegans less susceptible to MeHg-induced DAergic neurodegeneration;(2) Knockdown of skn-1 or daf-16 renders C. elegans more susceptible to MeHg-induced DAergic neurodegeneration;and (3) Knockdown of aat-1, aat-2, aat-3 or aat-1;aat-2;aat-3 in SKN-1 or DAF-16 overexpressing worms renders C. elegans less susceptible to MeHg-induced DAergic neurodegeneration. Using RNAi and overexpression technology we will generate multiple worm combinations and assess DAergic neurodegeneration by green fluorescent protein (GFP) fluorescence and 1-synuclein aggregation assays. Oxidative injury will be assayed by measuring F2- and F3-isoprostanes, ATP and mitochondrial membrane potential. The multi-gene approach proposed herein will generate models of combinatorial genetic variation to study the influence of multigenic polymorphisms in environmental risk and vulnerability as it relates to MeHg-induced DAergic neurodegeneration.
Understanding the contribution of genes that regulate methylmercury brain accumulation and stress responses is essential for understanding the mechanisms by which this metal affects brain function. Results from studies in the worm on the sensitivity of distinct populations of brain cells to this ubiquitous environmental pollutant will also offer new targets for therapeutic interventions.
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