calpain and methylmercuric-chloride

Overexposure to methylmercury (MeHg) has been known to induce neurotoxicity. The objective of this study is to explore mechanisms that contribute to MeHg-induced nerve cell apoptosis focusing on the alteration of intracellular Ca(2+) homeostasis and expression of N-methyl-D-aspartate receptors (NMDARs) subunits in rat cerebral cortex and whether MK801, a non-competitive NMDAR antagonist, could attenuate MeHg-induced neurotoxicity. Fifty rats were randomly divided into five groups of 10 animals in each group: control group, MK801 control group, MeHg-treated group (4 and 12 μmol/kg) and MK801 pre-treated group. Administration of MeHg at a dose of 12 μmol/kg for 4 weeks significantly increased in intracellular [Ca(2+)](i) and total Hg levels and that enhanced neurocyte apoptosis rate in cerebral cortex. In addition, the inhibitory effect of MeHg on Na(+)-K(+)-ATPase and Ca(2+)-ATPases might be one of the reasons that cause a significant increase of [Ca(2+)](i) in neurocyte. Over activated by increased cytosolic Ca(2+) loading, calpains degraded NMDAR subunits leading ultimately to nerve cell damage. However, pretreatment with MK801 at a dose of 0.3 μmol/kg could prevent Ca(2+) homeostasis dysregulation and alleviate the neurocyte apoptosis. In conclusion, the neuroprotective effects of MK801 appeared to be mediated not only via its NMDA receptor binding properties but also by maintaining intracellular calcium homeostasis.

Topics: Animals; Apoptosis; Calcium; Calcium-Transporting ATPases; Calpain; Cerebral Cortex; Cytoplasm; Dizocilpine Maleate; Homeostasis; Methylmercury Compounds; Neurons; Neuroprotective Agents; Rats; Rats, Wistar; Receptors, N-Methyl-D-Aspartate; Sodium-Potassium-Exchanging ATPase; Transcription, Genetic

Methylmercury, an environmental neurotoxicant, induces the apoptotic death of cerebellar granule cells in vitro at a low concentration. To further understand the mechanism of cell death, we used a rat cerebellar granule cell culture system to investigate whether the calpain/cyclin-dependent kinase 5 (cdk5)/p35 cascade, an important cascade for neuronal apoptosis, is involved in the methylmercury-induced death. A noteworthy finding was that the cerebellar granular cell death was increased at a very low concentration of methylmercury, 30 nM, which is lower than that previously reported. The high sensitivity to methylmercury indicates that this culture system is useful for studying methylmercury toxicity at very low concentrations. Using this system, we here found that the methylmercury-induced death was inhibited by the calpain inhibitor II. Furthermore, it was shown that, in methylmercury-exposed cells, alpha-fodrin and tau, calpain substrates, were cleaved to the fragments that disappeared by treatment with the calpain inhibitor II. We next assayed and showed that the intracellular Ca(2+) concentration in cerebellar granule cells increased after methylmercury exposure in a time- and dose-dependent manner, significantly even at 30 nM. These results indicated that a very low concentration of methylmercury causes the intracellular Ca(2+) concentration to increase, activates calpain in the cells, and then induces cell death. We further found that the p35 protein was also processed to p25 that forms the cdk5-p25 complex, a hyperactive kinase for tau. However, an immunoblot using the anti-phosphorylated tau antibody showed that there was no increase of phosphorylated tau in methylmercury-exposed cells. These results suggested that methylmercury-induced cell death via calpain activation should not involve the stimulation of tau phosphorylation activity.

Topics: Animals; Apoptosis; Calcium; Calpain; Carrier Proteins; Cell Survival; Cerebellar Cortex; Cysteine Proteinase Inhibitors; Enzyme Activation; Immunoblotting; Methylmercury Compounds; Microfilament Proteins; Nerve Tissue Proteins; Neurons; Oligopeptides; Rats; Rats, Wistar; Spectrometry, Fluorescence; tau Proteins