Transcriptional translational modulators The discovery that

Transcriptional/translational modulators. The discovery that β-lactam antibiotics like ceftriaxone enhance the in vivo expression of EAAT2/GLT-1 [31] has pioneered a completely new approach to EAAT modulation, with several additional EAAT activators following in its trail (Figure 2b) [32, 33, 34•, 35]. As exemplified for ceftriaxone, valproic ic bupropion and LDN/OSU-0212320 in Figure 2b, these activators target a wide range of transcriptional and translational processes, and consequently they are structurally very diverse [34•, 35, 36]. Since this type of modulators, be it activators or inhibitors, may act through targets involved in the expression of a specific EAAT gene, several of them are bound to exhibit pronounced subtype-selectivity. On the other hand, many of the mechanisms targeted underlie the expression of numerous other genes, and thus transcriptional/translational modulators could potentially exert off-target effects outside the glutamatergic system. Interestingly, the kinetics and duration of the modulation exerted by at least some of these modulators are likely to be significantly different from the properties of those targeting the EAAT protein, which could constitute an advantage or a problem depending on the specific use.
Recent findings for EAATs in health and disease The availability of EAAT2/GLT-1 activators has enabled investigations into the therapeutic potential in augmentation of synaptic Glu clearance. A detailed review of the findings from these studies is beyond the scope of this update, but EAAT2 activators have exhibited efficacy in a wide range of animal models of neurotoxic/neurodegenerative and psychiatric disorders, pain, alcoholism and other forms of drug abuse and dependence [34•, 37, 38, 39, 40, 41, 42]. However, increasing the in vivo expression levels of EAAT2, the major physiological Glu uptake carrier, could have profound impact on glutamatergic neurotransmission throughout the brain with the inherent risk of inducing adverse effects [43]. Thus, it remains to be seen whether this therapeutic strategy will face the same challenges as others before it in the Glu field. A recent pathophysiological study assigns a potential role in regulating cellular excitability to EAAT1 anion channels. A heterozygous patient carrying a point mutation in the SLC1A3 gene encoding for the P290R mutation in EAAT1 has been found to suffer from episodic ataxia type 6 characterized by episodes of ataxia, epilepsy and hemiplegia [44], and subsequent studies in heterologous expression systems have revealed reduced Glu transport and increased anion channel activity of this EAAT1 mutant [20••, 45]. Since the lack of GLAST-mediated Glu transport in homozygous Glast−/− knockout mice does not result in significant cerebellar symptoms and since the affected patient was heterozygous for the SLC1A3 mutation [44], the disease-associated changes in EAAT1 anion currents and not in Glu transport appear to be the major pathological process in this disease. EAAT1 is predominantly expressed in Bergmann glial cells that exhibit a high intracellular chloride concentration with a chloride equilibrium potential significantly more positive than their resting potential. Increased EAAT1 anion currents might permit chloride efflux and decrease intracellular [Cl−] in these cells, which in turn could augment GABA uptake and thus inhibit inhibitory synaptic transmission []. Thus, a physiological role of EAAT anion channels might be to impair inhibitory synaptic transmission in response to Glu release and thereby mediate a crosstalk between excitatory and inhibitory transmission. The specific contributions of EAAT3/EAAC1, the predominant neuronal EAAT, to synaptic Glu uptake are still poorly understood. The substantially lower densities of EAAC1 compared to GLAST and GLT-1 in the rodent CNS reported from previous studies [1] have recently been supported by a quantitative immunocytochemistry study estimating the number of EAAC1 molecules per synapse in rat hippocampus to only ∼130 [46]. On the basis of these observations and the phenotypes exhibited by EAAC1−/− mice the principal role of EAAT3/EAAC1 has been proposed to be as mediator of the neuronal cysteine uptake important for synthesis of glutathione, metabolism of reactive oxygen species and maintenance of reduced thiol groups on proteins [47, 48]. However, other findings suggest that EAAC1 contributes significantly to glutamatergic neurotransmission [49], and the transporter has recently been proposed to act as a synaptic buffer and to be important for the dynamics of the clearance of Glu from active synapses, hereby regulating the signaling of parasynaptically localized NR2B-containing NMDA receptors and in turn synaptic AMPA receptor distribution and turnover [50, 51•]. The notion of an important role for the transporter in shaping glutamatergic synaptic transients is further supported by a recent study by Amara and coworkers, where influx of amphetamine through the dopamine transporter was demonstrated to trigger internalization of EAAC1 from the cell surface of midbrain dopaminergic neurons []. This in turn reduced synaptic Glu clearance and enhanced glutamatergic responses, and this fascinating mechanism implicates EAAT3/EAAC1 as an important mediator of the augmented glutamatergic signaling contributing to the behavioral effects of amphetamine []. Finally, in addition to such a role in the neurotransmission mediated by extracellular Glu, growing evidence suggests that EAAT3/EAAC1 acts a direct facilitator of intracellular Glu receptor signaling. Analogously to previous findings in striatal neurons [53], EAAC1 has recently been shown to be co-localized with metabotropic Glu 5 (mGlu5) receptors on both cell surface and intracellular membranes in rat CA1 hippocampal neurons, and the transporter was proposed to mediate the uptake of extracellular Glu into the dendrite and subsequently the endoplasmic reticulum, hereby enabling Glu to activate intracellular mGlu5 receptors []. Notably, this component of mGlu5 signaling was demonstrated to be important for hippocampal synaptic plasticity []. Although the physiological importance of nuclear Glu receptor signaling has yet to be fully elucidated, this role of EAAT3/EAAC1 as a purveyor of Glu to its intracellular targets is truly intriguing, and it will be interesting to see whether other EAATs hold similar functions in neurons or glia cells.