Ironically another series of studies examining

Ironically, another series of studies examining the role of EAATs in CNS injury and disease suggest that under some pathological conditions, the operation of the EAATs is not protective, but actually contributes to the excitotoxic process (Rossi et al., 2000). The transporters do so by serving as the sites of efflux through which glutamate reaches the extracellular environment and, subsequently, over-activates EAA receptors. Thus, under conditions that disrupt the ability of glia or neurons to maintain large concentrations gradients of glutamate (e.g., Triptolide depletion, failed ion gradients), the transporters end up operating in reverse and facilitate the efflux of glutamate down its concentration gradient and out of the cells. Interestingly, the substrates and non-substrate inhibitors described above have been shown to shape this injury-induced efflux through the EAATs (Longuemare & Swanson, 1995, Koch et al., 1999a, Anderson et al., 2001, Bonde et al., 2003). For example, DHK and l-trans-2,3-PDC, non-substrate inhibitors of EAAT2, markedly reduced the efflux of substrate through this transporter in synaptosomes that had been treated with the metabolic inhibitors cyanide and iodoacetate, presumably by binding to and trapping the substrate-binding domain on the extracellular surface (Koch et al., 1999a; see Fig. 1). It must be remembered, however, that the insults used in many of these model systems are much greater than that what might be encountered in vivo and that considerable research is still needed to delineate the potential contribution of reversed transport under conditions truly representative of those present in CNS insults such as stroke. From a therapeutic perspective, these results suggest that there might be some clinical value in drugs that enhance uptake (i.e., to limit excitotoxic injury), as well as reduce activity (i.e., to attenuate injury-mediated efflux). While there are several good lead compounds to facilitate the development of potent inhibitors, including those that might exhibit a subtype-specific action, their potential utility will be questioned because it remains unclear under what conditions and to what extent transporters might act in reverse in vivo. Furthermore, current data would indicate that the therapeutic window for such inhibitors to block injury-induced efflux, yet not enhance the accumulation of glutamate by inhibiting normal uptake, would be very difficult to delineate. On the other hand, accumulating data indicates that excitotoxicity contributes to neuronal injury in a number of chronic neurodegenerative diseases. This has recently been exemplified by the clinical use of NMDA receptor blockers to reduce neurodegeneration in late stage Alzheimer's disease (Reisberg et al., 2003). Even if this excitotoxicity is a secondary mechanisms, facilitating EAAT activity may represent a plausible therapeutic approach to reduce extracellular glutamate levels and prevent neuronal damage. Potential strategies include allosteric modulators, as well as molecular approaches. The challenge with “drug-based” EAAT activators is that our current structure–activity models would provide little insight into their discovery, as the agents will undoubtedly act at as yet unknown regulatory site(s). One potential lead, however, may be found with the discovery that arachidonic acid stimulates EAAT activity (Zerangue et al., 1995, Tzingounis et al., 1998). A few laboratories have also begun to explore the potential use of viral vectors to deliver EAATs to CNS cells, although these studies are very much in their early stages (Selkirk et al., 2003, Ozawa et al., 2004). Other approaches on the horizon that lie somewhere between a pharmaceutical and molecular strategies are to identify drugs that increase expression and activity of EAATs by potentially targeting (i) the signaling pathways that regulate EAAT expression (Danbolt, 2001, Robinson, 2002, Gonzalez & Robinson, 2004b); (ii) the recently identified proteins that participate in the trafficking and membrane localization of the EAATs (i.e., GTRAPS; Jackson et al., 2001, Lin et al., 2001); or (iii) the newly identified EAAT promoters (Jin et al., 2002, Kim et al., 2003). In addition to their future clinical applicability, these lines of investigation will also provide considerable insight into the regulation and function of the EAATs.