Mechanisms of Oxidative Glutamate Toxicity: The Glutamate/Cystine Antiporter System xc¯ as a Neuroprotective Drug Target

Abstract

The glutamate/cystine antiporter system x(c)- transports cystine into cells in exchange for the important neurotransmitter glutamate at a ratio of 1:1. It is composed of a specific light chain, xCT, and a heavy chain, 4F2, linked by a disulfide bridge. Both subunits are localized prominently in the mouse and human brain especially in border areas between the brain and periphery including vascular endothelial cells, ependymal cells, choroid plexus, and leptomeninges. Glutamate exported by system x(c)- is largely responsible for the extracellular glutamate concentration in the brain, whereas the imported cystine is required for the synthesis of the major endogenous antioxidant, glutathione. System x(c)- thus connects the antioxidant defense with neurotransmission and behavior. Disturbances in the function of system x(c)- have been implicated in nerve cell death due to increased extracellular glutamate and reduced intracellular glutathione. In vitro, inhibition of cystine import through system x(c)- leads to cell death by a mechanism called oxidative glutamate toxicity or oxytosis, which includes depletion of intracellular glutathione, activation of 12-lipoxygenase, accumulation of intracellular peroxides, and the activation of a cyclic guanosine monophosphate (cGMP)-dependent calcium channel towards the end of the death cascade. Cell death caused by oxytosis is distinct from classical apoptosis. In this contribution, we discuss the function of system x(c)- in vitro and in vivo, the role of xCT as an important but due to its dual role probably ambivalent drug target, and the relevance of oxytosis as an in vitro assay for the identification of novel neuroprotective proteins and signaling pathways.

Mitochondrial Small Conductance SK2 Channels Prevent Glutamate-induced Oxytosis and Mitochondrial Dysfunction

“…This is further supported by the fact that in the present model system using immortalized HT-22 neurons cell membrane NMDA receptors are not expressed (41,42), and SK2 channel activation is neuroprotective even in the absence of extracellular Ca 2ϩ . Recent findings on the role of SK channel activation by CyPPA in spinocerebellar ataxia type 2 point to an emerging new therapeutic strategy for neurodegenerative diseases (43)(44)(45).…”

“…Our data now show functional expression of SK2 channels at the inner mitochondrial membrane of neuronal cells, providing protective roles under conditions of cellular stress. In particular, we found that inhibition of Ca 2ϩ influx through the plasma membrane only provided partial protection in models of glutamate-induced oxytosis and tBid toxicity (41,42,51) (Figs. 4, 5, and 6).…”

Neuroprotective Effect of Steamed and Fermented Codonopsis lanceolata

“…Since glutathione (GSH) is important antioxidant in CNS and GSH reductase (GR) is a critical enzyme for the production of GSH. High concentration of glutamate leads to deprivation of GSH by inhibiting of cysteine uptake into cells (Albrecht et al ., 2010). Depletion of GSH or antioxidant enzyme, such as GR causes neuronal cell death (Kane et al ., 1993).…”

Neuroprotection requires the functions of the RNA-binding protein HuR

“…These cells originate from the mouse CA1 region and are a suitable model for glutamate-induced toxicity rather than excitotoxicity, as they lack ionotropic GluRs; instead, glutamate blocks the cystine/glutamate antiporter system xc − . 26 Figure 5e). On the other hand, the increase in intracellular ROS correlated to the enhanced dysfunction of mitochondria in HuR lo neurons; this was demonstrated by significant drop in 3,3′-dihexyloxacarbocyanine iodide (DiOC6(3)) marking their membrane potential; and an enhancement in the incorporation of MitoSox marking the augmented release of superoxide from damaged organelles (Figure 3d).…”