Exposure to an Environmental Neurotoxicant Hastens the Onset of Amyotrophic Lateral Sclerosis-Like Phenotype in Human Cu /Zn Superoxide Dismutase 1 G93A Mice: Glutamate-Mediated Excitotoxicity

Mice expressing the human Cu /Zn superoxide dismutase 1 (hSOD1) gene mutation (hSOD1; G93A) were exposed to methylmercury (MeHg) at concentrations that did not cause overt motor dysfunction. We hypothesized that low concentrations of MeHg could hasten development of the amyotrophic lateral sclerosis (ALS)-like phenotype in G93A mice. MeHg (1 or 3 ppm/day in drinking water) concentration-dependently accelerated the onset of rotarod failure in G93A, but not wild-type, mice. At the time of rotarod failure, MeHg increased Fluo-4 fluorescence (free intracellular calcium concentration [Ca ]i) in soma of brainstemhypoglossal nucleus. These motor neurons control intrinsic and some extrinsic tongue function and exhibit vulnerability in bulbaronset ALS. The -amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA)/kainic acid receptor antagonist 6-cyano-7nitroquinoxaline-2,3-dione reduced [Ca ]i in all G93A mice, irrespective of MeHg treatment. N-acetyl spermine, which antagonizes Ca -permeable AMPA receptors, further reduced [Ca ]i more effectively in MeHg-treated than untreated G93A mice, suggesting that MeHg-treated mice have a greater Ca -permeable AMPA receptor contribution. The non-Ca divalent cation chelator N,N,N ,N -tetrakis(pyridylmethyl)ethylenediamine reduced Fluo-4 fluorescence in all G93A mice; FluoZin-(Zn indicator) fluorescence was increased in all MeHg-treated mice. Thus in G93A mice Zn apparently contributed measurably to the MeHginduced effect. This is the initial demonstration of accelerated onset of ALS-like phenotype in a genetically susceptible organism by exposure to low concentrations of an environmental neurotoxicant. Increased [Ca ]i induced by the G93A-MeHg interaction apparently was associated with Ca -permeable AMPA receptors and may contribute to the hastened development of ALS-like phenotypes by subjecting motor neurons to excessive elevation of [Ca ]i, leading to excitotoxic cell death.