microM Methylmercury Research Articles

Expanded CTG repeats in myotonin protein kinase increase susceptibility to oxidative stress.

The effect of oxidative stress on myogenic cells with expanded CTG repeats in the myotonin protein kinase (MtPK) gene was investigated using MtPK cDNA-transformants in order to investigate the disease process underlying myotonic dystrophy. We employed methylmercury as a model for reagents that produce reactive oxygen species (ROS). Mutant MtPK cDNA transformants containing 46 CTG repeats treated with 1 microM methylmercury for 24 h underwent cell death showing the characteristics of apoptosis. In contrast, methylmercury-induced cytotoxicity was weaker in wild type MtPK cDNA transformants. Antioxidants such as N-acetyl-L-cysteine and trolox suppressed methylmercury-induced apoptosis, indicating that the intracellular generation of ROS plays an important role. These studies suggest that expanded CTG repeats in MtPK increase the susceptibility of cells to oxidative stress.

Effects of methylmercury on the spontaneous and potassium-evoked release of endogenous amino acids from mouse cerebellar slices.

The effects of methylmercury on the spontaneous and potassium-evoked release of endogenous amino acids from mouse cerebellar slices have been examined. Methylmercury induced a concentration-dependent increase in the spontaneous release of glutamate, aspartate, gamma-aminobutyric acid, and taurine from mouse cerebellar slices. Glycine release was slightly increased, but not in a concentration-dependent manner. The spontaneous release of glutamine from mouse cerebellar slices was not altered by any concentration of methylmercury examined (10, 20, and 50 microM). The tissue content of glutamate, gamma-aminobutyric acid, glutamine, and taurine decreased after exposure to methylmercury. Exposure of cerebellar slices to 20 microM methylmercury resulted in a significant enhancement in glutamate release during stimulation with 35 mM K+. This increase could be accounted for by the methylmercury-induced increase in spontaneous glutamate release. The increase in spontaneous release of glutamate and gamma-aminobutyric acid was independent of the availability of extracellular calcium. These results suggest that methylmercury increases the release of neurotransmitter amino acids, particularly gamma-aminobutyric acid and glutamate, by acting at intracellular sites to increase release from a neurotransmitter pool. The increase in the potassium-stimulated release of glutamate may reflect an increased sensitivity of the cerebellar granule cell to the effects of methylmercury. It is suggested that alterations in amino acid neurotransmitter function in the cerebellum may contribute to some of the neurological symptoms of methylmercury intoxication.

Studies of the in vitro effect of methylmercury chloride on rat brain neurotransmitter enzymes.

The in vitro effect of methylmercury (MM) on the enzymatic activities of brain cell specific marker enzymes, choline acetyltransferase (CAT), glutamic acid decarboxylase (GAD), 2',3'-cyclic nucleotide phosphohydrolase (CNP), glutamine synthetase (GS) and enolase was examined. The results demonstrate that at 100 microM MM, GS activity was not affected whereas a small decrease in the activity of both GAD (20%) and enolase (10%) was observed. CNP and CAT activity appeared to be more sensitive toward MM with 100 microM MM producing inhibition of 50% and 30%, respectively. The addition of sulfhydryl protecting reagents such as DTT or sodium thioglycolate can restore the enzyme activities to normal control levels despite prior exposure of the enzymes to MM.

Inhibition of RNA and protein synthesis in isolated cerebellar cells by in vitro and in vivo methyl mercury

Cerebellar perikarya isolated from neonatal rats were exposed to 0-20 microM methyl mercury to simultaneously compare the effect on RNA and protein synthesis. Although 50% inhibition was found at approximately 8 microM for both [3H]uridine and [3H]phenylalanine incorporation, lower concentrations of methyl mercury produced 10-15% greater inhibition of RNA than protein synthesis. In vivo methyl mercury experiments also indicated a greater sensitivity of RNA synthesis in isolated cerebellar perikarya. The observed inhibition of RNA synthesis was not caused by a defect in cellular [3H]uridine uptake or by increased degradation of RNA. Both of these activities were altered by less than 10% at concentrations of methyl mercury that produced greater than 60% inhibition of RNA synthesis. Experiments showing that the specific activity of cerebellar cell RNA synthesis peaks and remains high between 4 and 10 d of age, whereas the specific activity of protein synthesis declines rapidly emphasize the potential importance of transcriptional perturbation in neonatal rats.