Expression of the cystine-glutamate exchanger (x_c^-) in retinal ganglion cells and regulation by nitric oxide and oxidative stress

The cystine-glutamate exchanger, system x_c^-, mediates the Na⁺-independent exchange of cystine into cells, coupled to the efflux of intracellular glutamate. System x_c^- plays a critical role in glutathione homeostasis. Early studies of brain suggested that system x_c^- was present primarily in astrocytes but not neurons. More recent work indicates that certain brain neurons have an active system x_c^-. In the retina, system x_c ^- has been demonstrated in Müller and retinal pigment epithelial cells. We have recently suggested that two protein components of system x _c^-, xCT and 4F2hc, are present in ganglion cells of the intact retina. Here, we have used (1) molecular and immunohistochemical assays to determine whether system x_c^- is present in primary ganglion cells isolated from neonatal mouse retinas and (2) functional assays to determine whether its activity is regulated by oxidative stress in a retinal ganglion cell line (RGC-5). Primary mouse ganglion cells and RGC-5 cells express xCT and 4F2hc. RGC-5 cells take up [³H]glutamate in the absence of Na⁺, and this uptake is blocked by known substrates of system x _c^- (glutamate, cysteine, cystine, quisqualic acid). Treatment of RGC-5 cells with NO and reactive oxygen species donors leads to increased activity of system x_c^- associated with an increase in the maximal velocity of the transporter with no significant change in the substrate affinity. This is the first report of system x_c ^- in primary retinal ganglion cells and RGC-5 cells. Oxidative stress upregulates this transport system in RGC-5 cells, and the process is associated with an increase in xCT mRNA and protein but no change in 4F2hc mRNA or protein.