Regional brain distribution of metallothionein, zinc and copper in toxic milk mutant and transgenic mice

Abstract

The regional distribution of metallothionein (MT), zinc and copper was measured in brains of transgenic MT-I overexpressor (MT-I*) mice, MT-I/MT-II gene knockout (MT-I/MT-II null) mice, and in brains of control C57BL/6J mice with normal MT expression. Toxic milk ( tx) mutant mice with abnormally high MT and copper accumulation were also assessed. Although there were significant differences in MT levels (assessed by a cadmium-binding assay) in whole brain of MT-I/MT-II null and control mice (16.5±2.9 μg/g vs 25.6±7.4 μg/g), different regions of the brain (cerebral cortex, corpus striatum, hippocampus, thalamus plus hypothalamus, cerebellum, and brain stem) contained similar amounts of MT. Male MT-I* mice had significantly higher whole brain MT level than controls (35.5±8.1 μg/g vs 25.6±7.4 μg/g), and had a 2-fold higher MT level in cerebellum, but not in other brain regions. Female MT-I* mice had significantly increased MT levels in all brain regions, with the highest increase in cerebellum (3.5-fold), and the lowest increase in cortex (2-fold). MT level in whole brain of female MT-I* mice was also significantly higher than that of male MT-I* (75.2±8.0 μg/g vs 35.4±8.1 μg/g). Toxic milk mice had significantly higher MT levels in all brain regions compared to age-matched controls (51.8±10.8 μg/g vs 30.3±5.8 μg/g), while no specific region of tx mouse brain showed a preferential increase in MT. In MT-I* and MT-I/MT-II null mice, altered MT levels did not always result in altered zinc and/or copper concentrations. However, all mouse strains exhibited region-specific accumulation of zinc, with the highest level in hippocampus. In control, MT-I/MT-II null, and male MT-I* mice, the hippocampus accumulated the highest level of copper. However, MT-I/MT-II null and both male and female MT-I* mice had similar levels of copper, compared to control mice. Toxic milk mice, on the other hand, had significantly higher copper levels in cerebral cortex, corpus striatum, thalamus/hypothalamus, and brain stem, compared to control mice. Zinc levels in corpus striatum, hippocampus, and cerebellum were also significantly increased. These data indicate that, in normal control and MT-I/MT-II null mice, MT is expressed uniformly in different regions of the brain. MT-I* mice, on the other hand, exhibit regional and gender-associated change in brain MT, and tx mice have markedly increased MT, copper, and zinc levels in most brain regions. These mouse strains will be useful models in elucidating the role of MT in the pathological effects of altered zinc and copper in brain.