Decreased arginine methylation and myelin alterations in arsenic exposed rats

Methylation has an important role in the synthesis of myelin basic protein (MBP), an essential component that confers compactness to myelin, and the correct synthesis and assembling of myelin are fundamental in the development of the central nervous system. Since arsenic metabolism requires a high consumption of S-adenosylmethionine, the main donor of methyl groups in the organism, it has been proposed that arsenic exposure can lead to a demethylation status in the organism comprising DNA and protein hypomethylation. This study documents myelin alterations in brain and changes in levels of methylated arginines in brain and serum of adult female Wistar rats exposed to arsenic (3 and 36 ppm, drinking water) from gestation throughout lactation, development and until 1, 2, 3 and 4 months of age. Morphological characteristics were analyzed by means of light microscopy and methylated arginines were analyzed through HPLC. Arsenic intake resulted in myelin damage reflected as empty spaces in fiber tracts of the exposed animals. The low exposure group (approximately 0.4 mg/kg/day) did not present myelin damage during the first 2 months, only moderate alterations in the third and fourth months. By contrast, animals exposed to 36 ppm (approximately 4 mg/kg/day) showed moderate to severe damage to nerve tracts from the first month of age. These alterations were accompanied by significant lower levels of dimethyl arginine in both exposed groups, as compared with the controls, in the third and fourth months of age and exposure. These data demonstrate that myelin composition is a target of arsenic through interference with arginine methylation, and they suggest that disturbances in nervous transmission through myelinated fibers are an important component of arsenic neurotoxicity.