Metallothionein-III prevents glutamate and nitric oxide neurotoxicity in primary cultures of cerebellar neurons

Metallothionein (MT)-III, a member of the MT family of metal-binding protei ns, is mainly expressed in the CNS and is abundant in glutamatergic neurons . Results in genetically altered mice indicate that MT-III may play neuropr otective roles in the brain, but the mechanisms through which this protein functions have not been elucidated. The aim of this work was to assess whet her MT-III is able to prevent glutamate neurotoxicity and to identify the s tep of the neurotoxic process interfered with by MT-Ill. Glutamate neurotox icity in cerebellar neurons in culture is mediated by excessive activation of glutamate receptors, increased intracellular calcium, and increased nitr ic oxide. it is shown that MT-III prevented glutamate- and nitric oxide-ind uced neurotoxicity in a dose-dependent manner, with nearly complete protect ion at 0.3-1 mu g/ml. MT-III did not prevent the glutamate-induced rise of intracellular calcium level but reduced significantly the nitric oxide-indu ced formation of cyclic GMP. Circular dichroism analysis revealed that nitr ic oxide triggers the release of the metals coordinated to the cysteine res idues of MT-ill, indicative of the S-Cys-nitrosylation of the protein. Ther efore, the present results indicate that MT-ill can quench pathological lev els of nitric oxide, thus preventing glutamate and nitric oxide neurotoxici ty.