Manganese Health Research Program: Phase 2, Core 13
Our objectives are 2-fold: 1) We will determine whether Mn2+-induced DA neuron degeneration in the nematode C. elegans is dependent on DA or DA neuron-specific proteins, and the neurodegeneration can be amplified by exposure to 6-OHDA, or expression of human a-synuclein. 2) We will establish and evaluate C. elegans transgenic lines overexpressing endogenous parkin and normal and mutant human parkin and determine whether these genes play a role in Mn2+- induced neurodegeneration both in WT and cell-death pathway deficient mutants
Manganese (Mn2+) neurotoxicity resembles a number of aspects of the dopamine (DA) neuron degenerating disorder Parkinson's disease (PD). Both PD and Mn2+ toxicity are 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 disorders. Furthermore, expression of the pre-synaptic protein - 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, and that prior exposure to Mn2+ amplifies the 6-OHDA-induced DA neurodegeneration. 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 genome-wide screen to identify mediators and suppressors of Mn2+-induced toxicity that will facilitate the identification of novel genes and molecular pathways involved in this highly relevant health and environmental concern.
Key research accomplishments:
Summary: Parkinson disease (PD) and manganism are characterized by motor deficits and a loss of dopamine (DA) neurons in the substantia nigra pars compacta. Epidemiological studies indicate significant correlations between manganese exposure and the propensity to develop PD. The vertebrate divalent metal transporter-1 (DMT-1) contributes to maintaining cellular Mn2! homeostasis and has recently been implicated in Fe2!-mediated neurodegeneration in PD. In this study we describe a novel model for manganism that incorporates the genetically tractable nematode Caenorhabditis elegans. We show that a brief exposure to Mn2! increases reactive oxygen species and glutathione production, decreases oxygen consumption and head mitochondria membrane potential, and confers DA neuronal death. DA neurodegeneration is partially dependent on a putative homologue to DMT-1, SMF-1, as genetic knockdown or deletion partially inhibits the neuronal death. Mn2! also amplifies the DA neurotoxicity of the PD-associated protein "-synuclein. Furthermore, both SMF-1 and SMF-2 are expressed in DA neurons and contribute to PD-associated neurotoxicant-induced DA neuron death. These studies describe a C. elegans model for manganism and show that DMT-1 homologues contribute to Mn2!- and PD-associated DA neuron vulnerability.
1) R. Settivari et al, (2009). The Divalent Metal Transporter Homologues SMF-1/2 Mediate Dopamine Neuron Sensitivity in Caenorhabditis elegans Models of Manganism and Parkinson Disease. Journal of Biological Chemistry, Vol 284 No. 51
2) R. Nass and R. Settivari. (2008) CAENORHABDITIS ELEGANS MODELS OF PARKINSONíS DISEASE: A ROBUST GENETIC SYSTEM TO IDENTIFY AND CHARACTERIZE ENDOGENOUS AND ENVIRONMENTAL COMPONENTS INVOLVED IN DOPAMINE NEURON DEGENERATION. Chapter 26, Parkinsonís Disease: molecular and therapeutic insights from model systems
Last updated: July 2011