Abstract
Mice expressing the human Cu2+/Zn2+ superoxide dismutase 1 (hSOD1) gene mutation (hSOD1G93A; 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 [Ca2+]i) in soma of brainstem-hypoglossal nucleus. These motor neurons control intrinsic and some extrinsic tongue function and exhibit vulnerability in bulbar-onset ALS. The α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA)/kainic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione reduced [Ca2+]i in all G93A mice, irrespective of MeHg treatment. N-acetyl spermine, which antagonizes Ca2+-permeable AMPA receptors, further reduced [Ca2+]i more effectively in MeHg-treated than untreated G93A mice, suggesting that MeHg-treated mice have a greater Ca2+-permeable AMPA receptor contribution. The non-Ca2+ divalent cation chelator N,N,N′,N′-tetrakis(pyridylmethyl)ethylenediamine reduced Fluo-4 fluorescence in all G93A mice; FluoZin-(Zn2+ indicator) fluorescence was increased in all MeHg-treated mice. Thus in G93A mice Zn2+ apparently contributed measurably to the MeHg-induced 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 [Ca2+]i induced by the G93A-MeHg interaction apparently was associated with Ca2+-permeable AMPA receptors and may contribute to the hastened development of ALS-like phenotypes by subjecting motor neurons to excessive elevation of [Ca2+]i, leading to excitotoxic cell death.
Footnotes
This study was supported by the National Institutes of Health National Institute of Environmental Health Sciences [Grants 5T32-ES007255, R21-ES014357, R01-ES03299] and an American Recovery and Reinvestment Act supplement. A.C. was supported in part by a Summer Undergraduate Research Fellowship from the American Society of Pharmacology and Experimental Therapeutics and was the recipient of a Pfizer Undergraduate Research Award from the Society of Toxicology (2010).
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
doi:10.1124/jpet.110.174466.
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ABBREVIATIONS:
- ALS
- amyotrophic lateral sclerosis
- FALS
- familial ALS
- SALS
- sporadic ALS
- AMPA
- α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate
- CNQX
- 6-cyano-7-nitroquinoxaline-2,3-dione
- EAAT
- excitatory amino acid transporter
- KA
- kainic acid
- MeHg
- methylmercury
- NAS
- 1-naphthylacetyl spermine trihydrochloride
- NMDA
- N-methyl-d-aspartate
- NXII
- nucleus hypoglossus
- SOD1
- Cu2+/Zn2+ superoxide dismutase 1
- hSOD1
- human SOD1
- TPEN
- N,N,N′,N′-tetrakis(pyridylmethyl)ethylenediamine
- WT
- wild type
- [Ca2+]i
- free intracellular calcium concentration
- PND
- postnatal day
- ACSF
- artificial cerebrospinal fluid
- ANOVA
- analysis of variance
- GluR2
- glutamate receptor 2.
- Received September 15, 2010.
- Accepted May 9, 2011.
- Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics
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