Mercuric Chloride Inhibits the In Vitro Uptake of Glutamate in GLAST- and GLT-1-Transfected Mutant CHO-K1 Cells

Abstract

Glutamate is removed mainly by astrocytes from the extracellular fluid via high-affinity astroglial Na+ -dependent excitatory amino acid transporters, glutamate/aspartate transporter (GLAST), and glutamate transporter-1 (GLT-1). Mercuric chloride (HgCl2) is a highly toxic compound that inhibits glutamate uptake in astrocytes, resulting in excessive extracellular glutamate accumulation, leading to excitotoxicity and neuronal cell death. The mechanisms associated with the inhibitory effects of HgCl2 on glutamate uptake are unknown. This study examines the effects of HgCl2 on the transport of 3H-D-aspartate, a nonmetabolizable glutamate analog, using Chinese hamster ovary cells (CHO) transfected with two glutamate transporter subtypes, GLAST (EAAT1) and GLT-1 (EAAT2), as a model system. Additionally, studies were undertaken to determine the effects of HgCl2 on mRNA and protein levels of these transporters. The results indicate that (1) HgCl2 leads to significant (p < 0.001) inhibition of glutamate uptake via both transporters, but is a more potent inhibitor of glutamate transport via GLAST and (2) the effect of HgCl2 on inhibition of glutamate uptake in transfected CHO cells is not associated with changes in transporter protein levels despite a significant decrease in mRNA expression; thus, (3) HgCl2 inhibition is most likely related to its direct binding to the functional thiol groups of the transporters and interference with their uptake function.