A molecular technique for detecting the liberation of intracellular zinc in cultured neurons.

We have previously reported that oxidative stimuli liberate Zn(2+) from metalloproteins, a phenomenon that can trigger neuronal cell death. Excessive intracellular Zn(2+) in many cell types triggers the expression of genes that encode metal binding proteins, such as metallothionein, via the activation and nuclear translocation of metal response element (MRE)-binding transcription factor-1 (MTF-1). Cd(2+) strongly induces nuclear translocation of MTF-1 in non-neuronal cells, but it does so by displacing Zn(2+) from its metal binding sites within the cell and increasing the intracellular concentration of this ion. Here, we describe the use of MRE-driven expression of a luciferase reporter gene as a sensitive molecular assay for detecting increases in intracellular zinc concentrations. MRE transactivation was induced in primary cortical neurons upon brief exposure to Zn(2+) or Cd(2+). Enhanced MRE transactivation was observed upon co-exposure of neurons to Cd(2+) together with NMDA, as this metal can permeate through the receptor channel. Luciferase expression was observed regardless of whether or not neurons had been co-transfected with an MTF-1-containing plasmid, suggesting the presence of an endogenous MTF-1-like protein. Indeed, RT-PCR revealed that MTF-1 mRNA is present in neurons. In contrast, MTF-1 deficient dko7 cells were only observed to have MRE transactivation when co-transfected with MTF-1. Our results indicate that Cd(2+) can effectively induce transactivation of MRE in neurons by liberating Zn(2+) from its intracellular binding sites.