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 methy lmercury (MeHg) are attributable, at least in part, to elevated levels of r eactive oxygen species, and thus decreases in GSH synthesis may increase me thylmercury toxicity. Astrocytes have recently been proposed to play an ess ential role in providing GSH precursors to neurons. Therefore, cystine tran sport, a prerequisite to GSH production, was characterized in cultured astr ocytes and neurons, and the effects of methylmercury on this transport were assessed. Astrocytes and neurons both possessed temperature dependent tran sport systems for cystine. Astrocytes accumulated cystine by Na (+)-indepen dent (X-c-) and -dependent (X-AG-) systems while neurons used exclusively N a+-independent systems. Inhibition of the X-AG- transport system decreased cystine transport in astrocytes to levels equivalent to those in sodium-dep leted conditions, suggesting that cystine is carried by a glutamate/asparta te transporter in astrocytes. Inhibition of the multifunction ectoenzyme/am ino acid transporter gamma -gluramyltranspeptidase (GGT) decreased cystine transport in both neurons and astrocytes. Inhibition of System X-c- with qu isqualate also decreased cystine uptake in both astrocytes and neurons. The se data demonstrate that cultured astrocytes accumulate cystine via three i ndependent mechanisms. System X-AG -, System X-C-, and GGT, while cultured hippocampal neurons use System X-C- and GGT exclusively. Inhibition of cyst ine uptake in astrocytes by methylmercury appears to be due to actions on t he System X-AG- transporter. (C) 2001 Elsevier Science B.V. All rights rese rved.