Methylmercury inhibits the in vitro uptake of the glutathione precursor, cystine, in astrocytes, but not in neurons

Maintenance of adequate intracellular glutathione (GSH) levels is vital for intracellular defense against oxidative damage. The toxic effects of methylmercury (MeHg) are attributable, at least in part, to elevated levels of reactive oxygen species, and thus decreases in GSH synthesis may increase methylmercury toxicity. Astrocytes have recently been proposed to play an essential role in providing GSH precursors to neurons. Therefore, cystine transport, a prerequisite to GSH production, was characterized in cultured astrocytes and neurons, and the effects of methylmercury on this transport were assessed. Astrocytes and neurons both possessed temperature dependent transport systems for cystine. Astrocytes accumulated cystine by Na⁺-independent (X_C-) and -dependent (X_AG-) systems while neurons used exclusively Na⁺-independent systems. Inhibition of the X_AG- transport system decreased cystine transport in astrocytes to levels equivalent to those in sodium-depleted conditions, suggesting that cystine is carried by a glutamate/aspartate transporter in astrocytes. Inhibition of the multifunction ectoenzyme/amino acid transporter γ-glutamyltranspeptidase (GGT) decreased cystine transport in both neurons and astrocytes. Inhibition of System X_C- with quisqualate also decreased cystine uptake in both astrocytes and neurons. These data demonstrate that cultured astrocytes accumulate cystine via three independent mechanisms, System X_AG-, System X_C-, and GGT, while cultured hippocampal neurons use System X_C- and GGT exclusively. Inhibition of cystine uptake in astrocytes by methylmercury appears to be due to actions on the System X_AG- transporter.