The Hydroxyl Side Chain of a Highly Conserved Serine Residue Is Required for Cation Selectivity and Substrate Transport in the Glial Glutamate Transporter GLT-1/SLC1A2

Glutamate transporters maintain synaptic concentration of the excitatory neurotransmitter below neurotoxic levels. Their transport cycle consists of cotransport of glutamate with three sodium ions and one proton, followed by countertransport of potassium. Structural studies proposed that a highly conserved serine located in the binding pocket of the homologous GltPh coordinates L-aspartate as well as the sodium ion Na1. To experimentally validate these findings, we generated and characterized several mutants of the corresponding serine residue, Ser-364, of human glutamate transporter SLC1A2 (solute carrier family 1 member 2), also known as glutamate transporter GLT-1 and excitatory amino acid transporter EAAT2. S364T, S364A, S364C, S364N, and S364D were expressed in HEK cells and Xenopus laevis oocytes to measure radioactive substrate transport and transport currents, respectively. All mutants exhibited similar plasma membrane expression when compared with WT SLC1A2, but substitutions of serine by aspartate or asparagine completely abolished substrate transport. On the other hand, the threonine mutant, which is a more conservative mutation, exhibited similar substrate selectivity, substrate and sodium affinities as WT but a lower selectivity for Na(+) over Li(+). S364A and S364C exhibited drastically reduced affinities for each substrate and enhanced selectivity for L-aspartate over D-aspartate and L-glutamate, and lost their selectivity for Na(+) over Li(+). Furthermore, we extended the analysis of our experimental observations using molecular dynamics simulations. Altogether, our findings confirm a pivotal role of the serine 364, and more precisely its hydroxyl group, in coupling sodium and substrate fluxes.

The Potential Roles of Aquaporin 4 in Alzheimer's Disease

Aquaporin 4 (AQP4) is the major water channel expressed in the central nervous system (CNS), and it is primarily expressed in astrocytes. It has been studied in various brain pathological conditions. However, the potential for AQP4 to influence Alzheimer's disease (AD) is still unclear. Research regarding AQP4 functions related to AD can be traced back several years and has gradually progressed toward a better understanding of the potential mechanisms. Currently, it has been suggested that AQP4 influences synaptic plasticity, and AQP4 deficiency may impair learning and memory, in part, through glutamate transporter-1 (GLT-1). AQP4 may mediate the clearance of amyloid beta peptides (Aβ). In addition, AQP4 may influence potassium (K(+)) and calcium (Ca(2+)) ion transport, which could play decisive roles in the pathogenesis of AD. Furthermore, AQP4 knockout is involved in neuroinflammation and interferes with AD. To date, no specific therapeutic agents have been developed to inhibit or enhance AQP4. However, experimental results strongly emphasize the importance of this topic for future investigations.

Tumor necrosis factor-mediated downregulation of spinal astrocytic connexin43 leads to increased glutamatergic neurotransmission and neuropathic pain in mice

Spinal cord astrocytes are critical in the maintenance of neuropathic pain. Connexin 43 (Cx43) expressed on spinal dorsal horn astrocytes modulates synaptic neurotransmission, but its role in nociceptive transduction has yet to be fully elaborated. In mice, Cx43 is mainly expressed in astrocytes, not neurons or microglia, in the spinal dorsal horn. Hind paw mechanical hypersensitivity was observed beginning 3days after partial sciatic nerve ligation (PSNL), but a persistent downregulation of astrocytic Cx43 in ipsilateral lumbar spinal dorsal horn was not observed until 7days post-PSNL, suggesting that Cx43 downregulation mediates the maintenance and not the initiation of nerve injury-induced hypersensitivity. Downregulation of Cx43 expression by intrathecal treatment with Cx43 siRNA also induced mechanical hypersensitivity. Conversely, restoring Cx43 by an adenovirus vector expressing Cx43 (Ad-Cx43) ameliorated PSNL-induced mechanical hypersensitivity. The sensitized state following PSNL is likely maintained by dysfunctional glutamatergic neurotransmission, as Cx43 siRNA-induced mechanical hypersensitivity was attenuated with intrathecal treatment of glutamate receptor antagonists MK801 and CNQX, but not neurokinin-1 receptor antagonist CP96345 or the Ca(2+) channel subunit α2δ1 blocker gabapentin. The source of this dysfunctional glutamatergic neurotransmission is likely decreased clearance of glutamate from the synapse rather than increased glutamate release into the synapse. Astrocytic expression of glutamate transporter GLT-1, but not GLAST, and activity of glutamate transport were markedly decreased in mice intrathecally injected with Cx43-targeting siRNA but not non-targeting siRNA. Glutamate release from spinal synaptosomes prepared from mice treated with either Cx43-targeting siRNA or non-targeting siRNA was unchanged. Intrathecal injection of Ad-Cx43 in PSNL mice restored astrocytic GLT-1 expression. The cytokine tumor necrosis factor (TNF) has been implicated in the induction of central sensitization, particularly through its actions on astrocytes, in the spinal cord following peripheral injury. Intrathecal injection of TNF in naïve mice induced the downregulation of both Cx43 and GLT-1 in spinal dorsal horn, as well as hind paw mechanical hypersensitivity, as observed in PSNL mice. Conversely, intrathecal treatment of PSNL mice with the TNF inhibitor etanercept prevented not only mechanical hypersensitivity but also the downregulation of Cx43 and GLT-1 expression in astrocytes. The current findings indicate that spinal astrocytic Cx43 are essential for the maintenance of neuropathic pain following peripheral nerve injury and suggest modulation of Cx43 as a novel target for developing analgesics for neuropathic pain.

Effect of subarachnoid nerve block anesthesia on glutamate transporter GLAST and GLT-1 expressions in rabbits

OBJECTIVE

To observe the effect of subarachnoid nerve block anesthesia on glutamate transporter glutamate-aspartate transporter (GLAST) and GLT-1 expressions in rabbits, and to investigate the effect of peripheral nerve anesthesia on the morphology and function of the spinal cord.

METHODS

Twenty healthy New Zealand white rabbits were randomly divided into two groups: the experimental group and control group; with 10 rabbits in each group. For spinal nerve anesthesia, 5 g/L of bupivacaine was used in the experimental group, and sterile saline was used in the control group. After 30 min of cardiac perfusion, GLAST and GLT-1 protein expression in spinal neurons were detected by immunohistochemistry and immunofluorescence staining.

RESULTS

GLAST and GLT-1 protein-positive cells increased in neurons in the experimental group, compared with the control group (P < 0.05).

CONCLUSIONS

After subarachnoid nerve block anesthesia, rabbit glutamate transporter GLAST and GLT-1 expression is increased; and spinal cord nerve cell function is inhibited.

Amoxicillin and amoxicillin/clavulanate reduce ethanol intake and increase GLT-1 expression as well as AKT phosphorylation in mesocorticolimbic regions

Studies have shown that administration of the β-lactam antibiotic ceftriaxone (CEF) attenuates ethanol consumption and cocaine seeking behavior as well as prevents ethanol-induced downregulation of glutamate transporter 1 (GLT-1) expression in central reward brain regions. However, it is not known if these effects are compound-specific. Therefore, the present study examined the effects of two other β-lactam antibiotics, amoxicillin (AMOX) and amoxicillin/clavulanate (Augmentin, AUG), on ethanol drinking, as well as GLT-1 and phosphorylated-AKT (pAKT) levels in the nucleus accumbens (Acb) and medial prefrontal cortex (mPFC) of alcohol-preferring (P) rats. P rats were exposed to free-choice of ethanol (15% and 30%) for five weeks and were given five consecutive daily i.p. injections of saline vehicle, 100 mg/kg AMOX or 100mg/kg AUG. Both compounds significantly decreased ethanol intake and significantly increased GLT-1 expression in the Acb. AUG also increased GLT-1 expression in the mPFC. Results for changes in pAKT levels matched those for GLT-1, indicating that β-lactam antibiotic-induced reductions in ethanol intake are negatively associated with increases in GLT-1 and pAKT levels within two critical brains regions mediating drug reward and reinforcement. These findings add to a growing literature that pharmacological increases in GLT-1 expression are associated with decreases in ethanol intake and suggest that one mechanism mediating this effect may be increased phosphorylation of AKT. Thus, GLT-1 and pAKT may serve as molecular targets for the treatment of alcohol and drug abuse/dependence.

Ameliorative potential of pioglitazone and ceftriaxone alone and in combination in rat model of neuropathic pain: Targeting PPARγ and GLT-1 pathways

BACKGROUND

The relation between glutamate homeostasis and PPAR gamma has got tremendous importance in nerve trauma and pain. Present study has been designed to elucidate the interaction between the GLT-1 activator (ceftriaxone) and PPAR gamma agonist (pioglitazone) in the spinal nerve ligation induced neuropathic pain.

METHODS

Male SD rats were subjected to spinal nerve ligation to induce neuropathic pain. Pioglitazone, ceftriaxone and their combination treatments were given for 28 days. Various behavioral, biochemical, neuroinflammatory and apoptotic mediators were assessed subsequently.

RESULTS

In the present study, ligation of L5 and L6 spinal nerves resulted in marked hyperalgesia and allodynia to different mechanical and thermal stimuli. In addition there is marked increase in oxidative-nitrosative stress parameters, inflammatory and apoptotic markers in spinal cord of spinal nerve ligated rats. Treatment with pioglitazone and ceftriaxone significantly prevented these behavioral, biochemical, mitochondrial and cellular alterations in rats. Further, combination of pioglitazone (10mg/kg, ip) with ceftriaxone (100mg/kg, ip) significantly potentiated the protective effects as compared to their effects per se.

CONCLUSION

Based on these results we propose that possible interplay between the neuroprotective effects of pioglitazone and ceftriaxone exists in suppressing the behavioral, biochemical, mitochondrial, neuroinflammatory and apoptotic cascades in spinal nerve ligation induced neuropathic pain in rats.

Effect of GLT-1 modulator and P2X7 antagonists alone and in combination in the kindling model of epilepsy in rats

INTRODUCTION

Multiple lines of investigation have explored the role of glutamatergic and purinergic systems in epilepsy, related cognitive impairment, and oxidative stress. Glutamate transporters, particularly GLT-1 expression, were found to be decreased, and purinergic receptor, P2X7 expression, was found to be increased in brain tissue associated with epilepsy. The present study was carried out to investigate the effect of ceftriaxone (GLT-1 upregulator) and Brilliant Blue G (P2X7 antagonist) against PTZ-induced kindling in rats. The study was further extended to elucidate the cross-link between glutamatergic and purinergic pathways in epilepsy.

MATERIAL AND METHODS

Systemic administration of subconvulsant dose of PTZ (30 mg/kg) every other day for 27days (14 injections) significantly increased the mean kindling, and developed generalized tonic-clonic seizures, and reduced motor co-ordination, cognitive skills, oxidative defense (increases lipid peroxidation, nitrite levels and decreases GSH level) and acetylcholinesterase enzyme activities in the cortex and subcortical region. Treatments with CEF (100 and 200mg/kg) and BBG (15 and 30 mg/kg) alone and in combination (CEF 100mg/kg and BBG 15 mg/kg) significantly decreased the mean kindling score and restored behavioral and oxidative defense activities compared with treatment with PTZ.

CONCLUSIONS

The combination of both the drugs was shown to have better effect in preventing kindled seizures and a significantly synergistic effect compared with their effect alone in PTZ-kindled rats. The present study elucidated the mechanistic role of GLT-1 modulator and selective P2X7 antagonist and their combination against PTZ-induced kindling. The study for the first time demonstrated the cross-link between glutamatergic and purinergic pathways in epilepsy treatment.

Ceftriaxone attenuates ethanol drinking and restores extracellular glutamate concentration through normalization of GLT-1 in nucleus accumbens of male alcohol-preferring rats

Alteration of glutamatergic-neurotransmission is a hallmark of alcohol dependence. We have previously reported that chronic ethanol-drinking downregulated glutamate transporter 1 (GLT-1) in nucleus accumbens (NAc) in male P rats in a manner that was reversed by ceftriaxone treatment. However, the effect of ceftriaxone on extracellular glutamate concentrations in NAc after chronic ethanol-drinking has not yet been studied. In the present study, male P rats were treated with ceftriaxone (100 mg/kg/day, i.p.) for five consecutive days following five-weeks of free choice ethanol (15% and 30%) drinking. In vivo microdialysis was performed to measure the extracellular glutamate concentrations in NAc and the effect of blockade of GLT-1 with dihydrokainic acid (DHK) on extracellular glutamate in NAc of ceftriaxone-treated rats was determined. Ceftriaxone treatment attenuated ethanol intake as well as ethanol preference. Extracellular glutamate was significantly higher in NAc after five-weeks of ethanol drinking in saline-treated compared to water control rats. Ceftriaxone treatment blocked the increase extracellular glutamate produced by ethanol intake. Blockade of GLT-1 by DHK reversed the effects of ceftriaxone on glutamate and implicated the role of GLT-1 in the normalization of extracellular glutamate by ceftriaxone. In addition, GLT-1 protein was decreased in ethanol exposed animals and ceftriaxone treatment reversed this deficit. Ceftriaxone treatment also increased glutamine synthetase activity in NAc but not in PFC as compared to ethanol drinking saline-treated rats. Our present study demonstrates that ceftriaxone treatment prevents ethanol drinking in part through normalization of extracellular glutamate concentrations in NAc of male P rats via GLT-1.

Astrocytic glutamate uptake is slow and does not limit neuronal NMDA receptor activation in the neonatal neocortex

Glutamate uptake by astrocytes controls the time course of glutamate in the extracellular space and affects neurotransmission, synaptogenesis, and circuit development. Astrocytic glutamate uptake has been shown to undergo post-natal maturation in the hippocampus, but has been largely unexplored in other brain regions. Notably, glutamate uptake has never been examined in the developing neocortex. In these studies, we investigated the development of astrocytic glutamate transport, intrinsic membrane properties, and control of neuronal NMDA receptor activation in the developing neocortex. Using astrocytic and neuronal electrophysiology, immunofluorescence, and Western blot analysis we show that: (1) glutamate uptake in the neonatal neocortex is slow relative to neonatal hippocampus; (2) astrocytes in the neonatal neocortex undergo a significant maturation of intrinsic membrane properties; (3) slow glutamate uptake is accompanied by lower expression of both GLT-1 and GLAST; (4) glutamate uptake is less dependent on GLT-1 in neonatal neocortex than in neonatal hippocampus; and (5) the slow glutamate uptake we report in the neonatal neocortex corresponds to minimal astrocytic control of neuronal NMDA receptor activation. Taken together, our results clearly show fundamental differences between astrocytic maturation in the developing neocortex and hippocampus, and corresponding changes in how astrocytes control glutamate signaling.

Ceftriaxone ameliorates tau pathology and cognitive decline via restoration of glial glutamate transporter in a mouse model of Alzheimer's disease

Glial glutamate transporter, GLT-1, is the major Na(+)-driven glutamate transporter to control glutamate levels in synapses and prevent glutamate-induced excitotoxicity implicated in neurodegenerative disorders including Alzheimer's disease (AD). Significant functional loss of GLT-1 has been reported to correlate well with synaptic degeneration and severity of cognitive impairment among AD patients, yet the underlying molecular mechanism and its pathological consequence in AD are not well understood. Here, we find the temporal decrease in GLT-1 levels in the hippocampus of the 3xTg-AD mouse model and that the pharmacological upregulation of GLT-1 significantly ameliorates the age-dependent pathological tau accumulation, restores synaptic proteins, and rescues cognitive decline with minimal effects on Aβ pathology. In primary neuron and astrocyte coculture, naturally secreted Aβ species significantly downregulate GLT-1 steady-state and expression levels. Taken together, our data strongly suggest that GLT-1 restoration is neuroprotective and Aβ-induced astrocyte dysfunction represented by a functional loss of GLT-1 may serve as one of the major pathological links between Aβ and tau pathology.

Adaptive changes of extracellular amino acid concentrations in mouse dorsal striatum by 4-AP-induced cortical seizures

The striatum is a major target of cerebral cortical output. The cortico-striatal projection has been well described, however, the neurochemical changes that occur in the striatum after prolonged cortical hyperactivation remain to be investigated. In this study, extracellular levels of glutamate, GABA, and alanine levels were measured in the dorsal striatum using microdialysis in anesthetized mice at resting condition and during 4-aminopyridine (4-AP)-induced cortical seizures. After topical application of 4-AP on the primary motor cortex that induced cortical seizures, the extracellular level of striatal GABA increased by 40% in 60 min. By contrast, the extracellular level of striatal glutamate decreased by 20%. Moreover, the surface amounts of striatal glutamate/aspartate transporter (GLAST) and glutamate transporter 1 (GLT-1), the major astrocytic high-affinity glutamate transporters, tended to increase by cortical seizures in 60 min, suggesting a recruitment of the glutamate transporters from internal stores. 4-AP also resulted in a steady increase of alanine levels which are thought to reflect glutamate and pyruvate metabolism in neurons and astrocytes. These observations possibly delineate adaptive changes of striatal metabolism by severe cortical seizures.

Effects of (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline on glutamate transporter 1 and cysteine/glutamate exchanger as well as ethanol drinking behavior in male, alcohol-preferring rats

Alcohol consumption is largely associated with alterations in the extracellular glutamate concentrations in several brain reward regions. We recently showed that glutamate transporter 1 (GLT-1) is downregulated following chronic exposure to ethanol for 5 weeks in alcohol-preferring (P) rats and that upregulation of the GLT-1 levels in nucleus accumbens and prefrontal cortex results, in part, in attenuating ethanol consumption. Cystine glutamate antiporter (xCT) is also downregulated after chronic ethanol exposure in P rats, and its upregulation could be valuable in attenuating ethanol drinking. This study examines the effect of a synthetic compound, (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153), on ethanol drinking and expressions of GLT-1 and xCT in the amygdala and the hippocampus of P rats. P rats were exposed to continuous free-choice access to water, 15% and 30% ethanol, and food for 5 weeks, after which they received treatments of MS-153 or vehicle for 5 days. The results show that MS-153 treatment significantly reduces ethanol consumption. It was revealed that GLT-1 and xCT expressions were downregulated in both the amygdala and the hippocampus of ethanol-vehicle-treated rats (ethanol-vehicle group) compared with water-control animals. MS-153 treatment upregulated GLT-1 and xCT expressions in these brain regions. These findings demonstrate an important role for MS-153 in these glutamate transporters for the attenuation of ethanol-drinking behavior.

Functional changes in glutamate transporters and astrocyte biophysical properties in a rodent model of focal cortical dysplasia

Cortical dysplasia is associated with intractable epilepsy and developmental delay in young children. Recent work with the rat freeze-induced focal cortical dysplasia (FCD) model has demonstrated that hyperexcitability in the dysplastic cortex is due in part to higher levels of extracellular glutamate. Astrocyte glutamate transporters play a pivotal role in cortical maintaining extracellular glutamate concentrations. Here we examined the function of astrocytic glutamate transporters in a FCD model in rats. Neocortical freeze lesions were made in postnatal day (PN) 1 rat pups and whole cell electrophysiological recordings and biochemical studies were performed at PN 21–28. Synaptically evoked glutamate transporter currents in astrocytes showed a near 10-fold reduction in amplitude compared to sham operated controls. Astrocyte glutamate transporter currents from lesioned animals were also significantly reduced when challenged exogenously applied glutamate. Reduced astrocytic glutamate transport clearance contributed to increased NMDA receptor-mediated current decay kinetics in lesioned animals. The electrophysiological profile of astrocytes in the lesion group was also markedly changed compared to sham operated animals. Control astrocytes demonstrate large-amplitude linear leak currents in response to voltage-steps whereas astrocytes in lesioned animals demonstrated significantly smaller voltage-activated inward and outward currents. Significant decreases in astrocyte resting membrane potential and increases in input resistance were observed in lesioned animals. However, Western blotting, immunohistochemistry and quantitative PCR demonstrated no differences in the expression of the astrocytic glutamate transporter GLT-1 in lesioned animals relative to controls. These data suggest that, in the absence of changes in protein or mRNA expression levels, functional changes in astrocytic glutamate transporters contribute to neuronal hyperexcitability in the FCD model.

Astrocytic dysfunction and addiction: consequences of impaired glutamate homeostasis

Addiction is characterized as a chronic relapsing disorder whereby addicted individuals persistently engage in drug seeking and use despite profound negative consequences. The results of studies using animal models of addiction and relapse indicate that drug seeking is mediated by alterations in cortico-accumbal plasticity induced by chronic drug exposure. Among the maladaptive responses to drug exposure are long-lasting alterations in the expression of proteins localized to accumbal astrocytes, which are responsible for maintaining glutamate homeostasis. These alterations engender an aberrant potentiation of glutamate transmission in the cortico-accumbens circuit that is linked to the reinstatement of drug seeking. Accordingly, pharmacological restoration of glutamate homeostasis functions as an efficient method of reversing drug-induced plasticity and inhibiting drug seeking in both rodents and humans.

Spinal astrocyte gap junction and glutamate transporter expression contributes to a rat model of bortezomib-induced peripheral neuropathy

There is increasing evidence implicating astrocytes in multiple forms of chronic pain, as well as in the specific context of chemotherapy-induced peripheral neuropathy (CIPN). However, it is still unclear what the exact role of astrocytes may be in the context of CIPN. Findings in oxaliplatin and paclitaxel models have displayed altered expression of astrocytic gap junctions and glutamate transporters as means by which astrocytes may contribute to observed behavioral changes. The current study investigated whether these changes were also generalizable to the bortezomib CIPN. Changes in mechanical sensitivity were verified in bortezomib-treated animals, and these changes were prevented by co-treatment with a glial activation inhibitor (minocycline), a gap junction decoupler (carbenoxolone), and by a glutamate transporter upregulator (ceftriaxone). Immunohistochemistry data at day 30 in bortezomib-treated animals showed increases in expression of glial fibrillary acidic protein (GFAP) and connexin 43 but a decrease in GLAST expression. These changes were prevented by co-treatment with minocycline. Follow-up Western blotting data showed a shift in connexin 43 from a non-phosphorylated state to a phosphorylated state, indicating increased trafficking of expressed connexin 43 to the cell membrane. These data suggest that increases in behavioral sensitivity to cutaneous stimuli may be tied to persistent synaptic glutamate resulting from increased calcium flow between spinal astrocytes.

GLT-1 transporter: an effective pharmacological target for various neurological disorders

L-Glutamate is the predominant excitatory neurotransmitter in the central nervous system (CNS) and is directly and indirectly involved in a variety of brain functions. Glutamate is released in the synaptic cleft at a particular concentration that further activates the various glutaminergic receptors. This concentration of glutamate in the synapse is maintained by either glutamine synthetase or excitatory amino acid proteins which reuptake the excessive glutamate from the synapse and named as excitatory amino acid transporters (EAATs). Out of all the subtypes GLT-1 (glutamate transporter 1) is abundantly distributed in the CNS. Down-regulation of GLT-1 is reported in various neurological diseases such as, epilepsy, stroke, Alzheimer's disease and movement disorders. Therefore, positive modulators of GLT-1 which up-regulate the GLT-1 expression can serve as a potential target for the treatment of neurological disorders. GLT-1 translational activators such as ceftriaxone are found to have significant protective effects in ALS and epilepsy animal models, suggesting that this translational activation approach works well in rodents and that these compounds are worth further pursuit for various neurological disorders. This drug is currently in human clinical trials for ALS. In addition, a thorough understanding of the mechanisms underlying translational regulation of GLT-1, such as identifying the molecular targets of the compounds, signaling pathways involved in the regulation, and translational activation processes, is very important for this novel drug-development effort. This review mainly emphasizes the role of glutamate and its transporter, GLT-1 subtype in excitotoxicity. Further, recent reports on GLT-1 transporters for the treatment of various neurological diseases, including a summary of the presumed physiologic mechanisms behind the pharmacology of these disorders are also explained.

Loss of SIRT4 decreases GLT-1-dependent glutamate uptake and increases sensitivity to kainic acid

Glutamate transport is a critical process in the brain that maintains low extracellular levels of glutamate to allow for efficient neurotransmission and prevent excitotoxicity. Loss of glutamate transport function is implicated in epilepsy, traumatic brain injury, and amyotrophic lateral sclerosis. It remains unclear whether or not glutamate transport can be modulated in these disease conditions to improve outcome. Here, we show that sirtuin (SIRT)4, a mitochondrial sirtuin, is up-regulated in response to treatment with the potent excitotoxin kainic acid. Loss of SIRT4 leads to a more severe reaction to kainic acid and decreased glutamate transporter expression and function in the brain. Together, these results indicate a critical and novel stress response role for SIRT4 in promoting proper glutamate transport capacity and protecting against excitotoxicity.

Laser-scanning astrocyte mapping reveals increased glutamate-responsive domain size and disrupted maturation of glutamate uptake following neonatal cortical freeze-lesion

Astrocytic uptake of glutamate shapes extracellular neurotransmitter dynamics, receptor activation, and synaptogenesis. During development, glutamate transport becomes more robust. How neonatal brain insult affects the functional maturation of glutamate transport remains unanswered. Neonatal brain insult can lead to developmental delays, cognitive losses, and epilepsy; the disruption of glutamate transport is known to cause changes in synaptogenesis, receptor activation, and seizure. Using the neonatal freeze-lesion (FL) model, we have investigated how insult affects the maturation of astrocytic glutamate transport. As lesioning occurs on the day of birth, a time when astrocytes are still functionally immature, this model is ideal for identifying changes in astrocyte maturation following insult. Reactive astrocytosis, astrocyte proliferation, and in vitro hyperexcitability are known to occur in this model. To probe astrocyte glutamate transport with better spatial precision we have developed a novel technique, Laser Scanning Astrocyte Mapping (LSAM), which combines glutamate transport current (TC) recording from astrocytes with laser scanning glutamate photolysis. LSAM allows us to identify the area from which a single astrocyte can transport glutamate and to quantify spatial heterogeneity in the rate of glutamate clearance kinetics within that domain. Using LSAM, we report that cortical astrocytes have an increased glutamate-responsive area following FL and that TCs have faster decay times in distal, as compared to proximal processes. Furthermore, the developmental shift from GLAST- to GLT-1-dominated clearance is disrupted following FL. These findings introduce a novel method to probe astrocyte glutamate uptake and show that neonatal cortical FL disrupts the functional maturation of cortical astrocytes.

Role of transcription factor yin yang 1 in manganese-induced reduction of astrocytic glutamate transporters: Putative mechanism for manganese-induced neurotoxicity

Astrocytes are the most abundant non-neuronal glial cells in the brain. Once relegated to a mere supportive role for neurons, contemporary dogmas ascribe multiple active roles for these cells in central nervous system (CNS) function, including maintenance of optimal glutamate levels in synapses. Regulation of glutamate levels in the synaptic cleft is crucial for preventing excitotoxic neuronal injury. Glutamate levels are regulated predominantly by two astrocytic glutamate transporters, glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST). Indeed, the dysregulation of these transporters has been linked to several neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD) and Parkinson's disease (PD), as well as manganism, which is caused by overexposure to the trace metal, manganese (Mn). Although Mn is an essential trace element, its excessive accumulation in the brain as a result of chronic occupational or environmental exposures induces a neurological disorder referred to as manganism, which shares common pathological features with Parkinsonism. Mn decreases the expression and function of both GLAST and GLT-1. Astrocytes are commonly targeted by Mn, and thus reduction in astrocytic glutamate transporter function represents a critical mechanism of Mn-induced neurotoxicity. In this review, we will discuss the role of astrocytic glutamate transporters in neurodegenerative diseases and Mn-induced neurotoxicity.

Astrocyte-derived growth factors and estrogen neuroprotection: role of transforming growth factor-α in estrogen-induced upregulation of glutamate transporters in astrocytes

Extensive studies from the past decade have completely revolutionized our understanding about the role of astrocytes in the brain from merely supportive cells to an active role in various physiological functions including synaptic transmission via cross-talk with neurons and neuroprotection via releasing neurotrophic factors. Particularly, numerous studies have reported that astrocytes mediate the neuroprotective effects of 17β-estradiol (E2) and selective estrogen receptor modulators (SERMs) in various clinical and experimental models of neuronal injury. Astrocytes contain two main glutamate transporters, glutamate aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1), that play a key role in preventing excitotoxic neuronal death, a process associated with most neurodegenerative diseases. E2 has been shown to increase expression of both GLAST and GLT-1 mRNA and protein and glutamate uptake in astrocytes. Growth factors such as transforming growth factor-α (TGF-α) appear to mediate E2-induced enhancement of these transporters. These findings suggest that E2 exerts neuroprotection against excitotoxic neuronal injuries, at least in part, by enhancing astrocytic glutamate transporter levels and function. Therefore, the present review will discuss proposed mechanisms involved in astrocyte-mediated E2 neuroprotection, with a focus on glutamate transporters.

Mechanism of raloxifene-induced upregulation of glutamate transporters in rat primary astrocytes

Raloxifene (RX), a selective estrogen receptor modulator (SERM), exerts neuroprotection in multiple clinical and experimental settings. Astrocytic glutamate transporters GLT-1 (EAAT2) and GLAST (EAAT1) are the main glutamate transporters in the central nervous system, taking up most of excess glutamate from the synaptic cleft to prevent excitotoxic neuronal death. Since drugs targeting astrocytic glutamate transporters to enhance their expression and function represent potential therapeutics for neurodegenerative disorders associated with excitotoxicity, we tested if RX modulates the expression and function of GLT-1 and GLAST in rat primary astrocytes. The results showed that RX significantly increased glutamate uptake and expression of GLT-1 mRNA and protein levels. RX enhanced GLT-1 expression by the activation of multiple signaling pathways including ERK, EGFR, and CREB mediated by estrogen receptors (ERs) ER-α, ER-β, and GPR30. At the transcriptional level, NF-κB played a critical role in RX-induced GLT-1 expression as RX increased NF-κB reporter activity and induced binding of NF-κB p65 and p50 to the GLT-1 promoter. RX attenuated the reduction of GLT-1 expression and glutamate uptake induced by manganese (Mn) whose chronic high levels of exposure cause manganism. RX also upregulated GLAST by increasing its promoter activity and protein levels via the NF-κB pathway and ERs. Our findings provide new insight into the mechanism of RX-induced enhancement of GLT-1 and GLAST expression, as well as the attenuation of Mn-reduced expression of these transporters. These findings will be highly valuable for developing therapeutics of neurodegenerative diseases associated with impaired astrocytic glutamate transporters.

Sumoylation of the astroglial glutamate transporter EAAT2 governs its intracellular compartmentalization

EAAT2 is a predominantly astroglial glutamate transporter responsible for the majority of synaptic glutamate clearance in the mammalian central nervous system (CNS). Its dysfunction has been linked with many neurological disorders, including amyotrophic lateral sclerosis (ALS). Decreases in EAAT2 expression and function have been implicated in causing motor neuron excitotoxic death in ALS. Nevertheless, increasing EAAT2 expression does not significantly improve ALS phenotype in mouse models or in clinical trials. In the SOD1-G93A mouse model of inherited ALS, the cytosolic carboxy-terminal domain is cleaved from EAAT2, conjugated to SUMO1, and accumulated in astrocytes where it triggers astrocyte-mediated neurotoxic effects as disease progresses. However, it is not known whether this fragment is sumoylated after cleavage or if full-length EAAT2 is already sumoylated prior to cleavage as part of physiological regulation. In this study, we show that a fraction of full-length EAAT2 is constitutively sumoylated in primary cultures of astrocytes in vitro and in the CNS in vivo. Furthermore, the extent of sumoylation of EAAT2 does not change during the course of ALS in the SOD1-G93A mouse and is not affected by the expression of ALS-causative mutant SOD1 proteins in astrocytes in vitro, indicating that EAAT2 sumoylation is not driven by pathogenic mechanisms. Most interestingly, sumoylated EAAT2 localizes to intracellular compartments, whereas non-sumoylated EAAT2 resides on the plasma membrane. In agreement, promoting desumoylation in primary astrocytes causes increased EAAT2-mediated glutamate uptake. These findings could have implications for optimizing therapeutic approaches aimed at increasing EAAT2 activity in the dysfunctional or diseased CNS.

Contrasting changes in extracellular dopamine and glutamate along the rostrocaudal axis of the anterior cingulate cortex of the rat following an acute d-amphetamine or dopamine challenge

There is evidence for functional specificity of subregions along the rostrocaudal axis of the anterior cingulate cortex (ACC). The subregion-specific distribution of dopaminergic afferents and glutamatergic efferents along the ACC make these obvious candidates for coding such regional responses. We investigated this possibility using microdialysis in freely-moving rats to compare changes in extracellular dopamine and glutamate in the rostral (‘rACC': Cg1 and Cg3 (prelimbic area)) and caudal (‘cACC’: Cg1 and Cg2) ACC induced by systemic or local administration of d-amphetamine. Systemic administration of d-amphetamine (3 mg/kg, i.p.) caused a transient increase in extracellular dopamine in the rACC, but an apparent increase in the cACC of the same animals was less clearly defined. Local infusion of d-amphetamine increased dopamine efflux in the rACC, only. Glutamate efflux in the rACC was increased by local infusion of dopamine (5–50 μM), which had negligible effect in the cACC, but only systemic administration of d-amphetamine increased glutamate efflux and only in the cACC. The asymmetry in the neurochemical responses within the rACC and cACC, to the same experimental challenges, could help explain why different subregions are recruited in the response to specific environmental and somatosensory stimuli and should be taken into account when studying the regulation of neurotransmission in the ACC.

This article is part of the Special Issue entitled ‘CNS Stimulants’.

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Dopamine and glutamate efflux in two anterior cingulate subregions were compared.

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Responses to d-amphetamine depended on subregion and route of drug administration.

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These findings could help explain the disparate roles of the two subregions.

Thiamine deficiency results in release of soluble factors that disrupt mitochondrial membrane potential and downregulate the glutamate transporter splice-variant GLT-1b in cultured astrocytes

Loss of astrocytic glutamate transporters is a major feature of both thiamine deficiency (TD) and Wernicke's encephalopathy. However, the underlying basis of this process is not well understood. In the present study we have investigated the possibility of release of astrocytic soluble factors that might be involved in the regulation of the glutamate transporter GLT-1b in these cells. Treatment of naïve astrocytes with conditioned media from astrocytes exposed to TD conditions resulted in a progressive decrease in glutamate uptake over 24 h. Immunoblotting and flow cytometry measurements indicated this was accompanied by a 20-40% loss of GLT-1b. Astrocytes exposed to either TD or TD conditioned media showed increased disruption of mitochondrial membrane potential compared to control cells, and treatment of astrocytes with TD resulted in an increase in the pro-inflammatory cytokine TNF-α and elevated levels of phospho-IκB fragment, indicative of increased activation of NF-κB. Inhibition of TNF-α activity with the use of a neutralizing antibody blocked the increased NF-κB activation, while inhibition of NF-κB ameliorated the decrease in GLT-1b and reversed the decrease in glutamate uptake occurring with TD treatment. Together, these findings indicate that astrocytes exposed to TD conditions show responses suggesting that soluble factors released by these cells under conditions of TD play a regulatory role in terms of glutamate transport function and mitochondrial integrity.

Neuronal activity and glutamate uptake decrease mitochondrial mobility in astrocytes and position mitochondria near glutamate transporters

Within neurons, mitochondria are nonuniformly distributed and are retained at sites of high activity and metabolic demand. Glutamate transport and the concomitant activation of the Na(+)/K(+)-ATPase represent a substantial energetic demand on astrocytes. We hypothesized that mitochondrial mobility within astrocytic processes might be regulated by neuronal activity and glutamate transport. We imaged organotypic hippocampal slice cultures of rat, in which astrocytes maintain their highly branched morphologies and express glutamate transporters. Using time-lapse confocal microscopy, the mobility of mitochondria within individual astrocytic processes and neuronal dendrites was tracked. Within neurons, a greater percentage of mitochondria were mobile than in astrocytes. Furthermore, they moved faster and farther than in astrocytes. Inhibiting neuronal activity with tetrodotoxin (TTX) increased the percentage of mobile mitochondria in astrocytes. Mitochondrial movement in astrocytes was inhibited by vinblastine and cytochalasin D, demonstrating that this mobility depends on both the microtubule and actin cytoskeletons. Inhibition of glutamate transport tripled the percentage of mobile mitochondria in astrocytes. Conversely, application of the transporter substrate d-aspartate reversed the TTX-induced increase in the percentage of mobile mitochondria. Inhibition of reversed Na(+)/Ca(2+) exchange also increased the percentage of mitochondria that were mobile. Last, we demonstrated that neuronal activity increases the probability that mitochondria appose GLT-1 particles within astrocyte processes, without changing the proximity of GLT-1 particles to VGLUT1. These results imply that neuronal activity and the resulting clearance of glutamate by astrocytes regulate the movement of astrocytic mitochondria and suggest a mechanism by which glutamate transporters might retain mitochondria at sites of glutamate uptake.

Interleukin-1 beta enhances endocytosis of glial glutamate transporters in the spinal dorsal horn through activating protein kinase C

Excessive activation of glutamate receptors in spinal dorsal horn neurons is a key mechanism leading to abnormal neuronal activation in pathological pain conditions. Previous studies have shown that activation of glutamate receptors in the spinal dorsal horn is enhanced by impaired glial glutamate transporter functions and proinflammatory cytokines including interleukin-1 beta (IL-1β). In this study, we for the first time revealed that spinal glial glutamate transporter activities in the neuropathic animals are attenuated by endogenous IL-1β. Specifically, we demonstrated that nerve injury results in an increased expression of IL-1β and activation of PKC in the spinal dorsal horn as well as suppression of glial glutamate uptake activities. We provided evidence that the nerve-injury induced suppression of glial glutamate uptake is at least in part ascribed to endogenous IL-1β and activation of PKC in the spinal dorsal horn. IL-1β reduces glial glutamate transporter activities through enhancing the endocytosis of both GLT-1 and GLAST glial glutamate transporters. The IL-1β induced trafficking of glial glutamate transporters is through the calcium/PKC signaling pathway, and the dynamin-dependent endocytosis, which is dependent on the integrity of actin filaments. The signaling pathway regulating glial glutamate transporters revealed in this study provides novel targets to attenuate aberrant activation of glutamate receptors in the spinal dorsal horn, which could ultimately help the development of analgesics.

Ceftriaxone, a GLT-1 transporter activator, disrupts hippocampal learning in rats

Glutamate transporters (GluTs) are important for maintaining optimal glutamate concentrations at the synapse. This allows proper synaptic response, plasticity and prevents neurotoxicity. It has been shown that the β-lactam antibiotic ceftriaxone (Rocephin) induces an up-regulation of the glutamate transporter GLT-1. This GLT-1 up-regulation blocks the metabotropic glutamate receptor (mGluR) dependent long-term depression (LTD) at the mossy fiber (MF)-CA3 hippocampal synapse. It also has negative effects on long-term potentiation (LTP). However, the effects of GLT-1 up-regulation on hippocampal learning in rats are not known. In this study, we examine the role of chronic administration of ceftriaxone on novel object recognition, which is a hippocampal-dependent spatial learning task. Male Sprague Dawley rats (2-3 months old) were administered ceftriaxone (via i.p. injections, 200 mg/kg) for 8 consecutive days prior to training and testing on a standard novel object recognition task. We found that rats administered with ceftriaxone display memory impairments in novel object recognition, when compared to control rats (p<0.05). Our findings show that a potential up-regulation of GLT-1 via ceftriaxone administration has detrimental effects on spatial learning and memory in rats. Our results further support the notion that glutamate transporters provide an essential regulatory role in hippocampal learning and memory.

Characterization of Gcf2/Lrrfip1 in experimental cerebral ischemia and its role as a modulator of Akt, mTOR and β-catenin signaling pathways

Leucine-rich repeat in Flightless-1 interaction protein 1 (Lrrfip1) is an up-regulated protein after cerebral ischemia whose precise role in the brain both in healthy and ischemic conditions is unclear. Different Lrrfip1 isoforms with distinct roles have been reported in human and mouse species. The present study aimed to analyze the Lrrfip1 transcriptional variants expressed in rat cortex, to characterize their expression patterns and subcellular location after ischemia, and to define their putative role in the brain. Five transcripts were identified and three of them (Lrrfip1, CRA_g and CRA_a' (Fli-I leucine-rich repeat associated protein 1 - Flap-1)) were analyzed by quantitative real-time polymerase chain reaction (qPCR). All the transcripts were up-regulated and showed differential expression patterns after in vivo and in vitro ischemia models. The main isoform, Lrrfip1, was found to be up-regulated from the acute to the late phases of ischemia in the cytoplasm of neurons and astrocytes of the peri-infarct area. This study demonstrates that Lrrfip1 activates β-catenin, Akt, and mammalian target of rapamycin (mTOR) proteins in astrocytes and positively regulates the expression of the excitatory amino acid transporter subtype 2 (GLT-1). Our findings point to Lrrfip1 as a key brain protein that regulates pro-survival pathways and proteins and encourages further studies to elucidate its role in cerebral ischemia as a potential target to prevent brain damage and promote functional recovery after stroke.

Ginsenoside Rd promotes glutamate clearance by up-regulating glial glutamate transporter GLT-1 via PI3K/AKT and ERK1/2 pathways

Ginsenoside Rd (Rd), one of the main active ingredients in Panax ginseng, has been showed to protect against ischemic cerebral damage both in vitro and in vivo. However, the underlying mechanism of Rd is largely unknown. Excessive extracellular glutamate causes excitatory toxicity, leading to cell death, and neurodegenerative processes after brain ischemia. The clearance of extracellular glutamate by astrocytic glutamate transporter GLT-1 is essential for neuronal survival after stroke. Here we investigated the effects of Rd on the levels of extracellular glutamate and the expression of GLT-1 in vivo and in vitro. After rat middle cerebral artery occlusion, Rd significantly increased the mRNA and protein expression levels of GLT-1, and reduced the burst of glutamate as revealed by microdialysis. Consistently, specific glutamate uptake by cultured astrocytes was elevated after Rd exposure. Furthermore, we showed that Rd increased the levels of phosphorylated protein kinase B (PKB/Akt) and phospho-ERK1/2 (p-ERK1/2) in astrocyte culture after oxygen-glucose deprivation. Moreover, the effect of Rd on GLT-1 expression and glutamate uptake can be abolished by PI3K/AKT agonist LY294002 or ERK1/2 inhibitor PD98059. Taken together, our findings provide the first evidence that Rd can promote glutamate clearance by up-regulating GLT-1 expression through PI3K/AKT and ERK1/2 pathways.

Primary cultures of rat cortical microglia treated with nicotine increases in the expression of excitatory amino acid transporter 1 (GLAST) via the activation of the α7 nicotinic acetylcholine receptor

Although the clearance of glutamate from the synapse under physiological conditions is performed by astrocytic glutamate transporters, their expression might be diminished under pathological conditions. Microglia glutamate transporters, however, might serve as a back-up system when astrocytic glutamate uptake is impaired, and could have a prominent neuroprotective function under pathological conditions. In the current study, the effect of nicotine, well known as a neuroprotective molecule, on the function of glutamate transporters in cultured rat cortical microglia was examined. Reverse transcription polymerase chain reaction and pharmacological approaches demonstrated that, glutamate/aspartate transporter (GLAST), not glutamate transporter 1 (GLT-1), is the major functional glutamate transporter in cultured cortical microglia. Furthermore, the α7 subunit was demonstrated to be the key subunit comprising nicotinic acetylcholine (nACh) receptors in these cells. Treatment of cortical microglia with nicotine led to a significant increase of GLAST mRNA expression and (14)C-glutamate uptake in a concentration- and time-dependent manner, which were markedly inhibited by pretreatment with methyllycaconitine, a selective α7 nACh receptor antagonist. The nicotine-induced expression of GLAST mRNA and protein is mediated through an inositol trisphosphate (IP3) and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) depend intracellular pathway, since pretreatment with either xestospongin C, an IP3 receptor antagonist, or KN-93, a CaMKII inhibitor, blocked GLAST expression. Together, these findings indicate that activation of nACh receptors, specifically those expressing the α7 subunit, on cortical microglia could be a key mechanism of the neuroprotective effect of nACh receptor ligands such as nicotine.

Ceftriaxone attenuates locomotor activity induced by acute and repeated cocaine exposure in mice

Ceftriaxone (CTX) decreases locomotor activation produced by initial cocaine exposure and attenuates development of behavioral sensitization produced by repeated cocaine exposure. An important question that has not yet been answered is whether or not CTX reduces behavioral sensitization to cocaine in cases in which the antibiotic is administered only during the period of cocaine absence that follows repeated cocaine exposure and precedes reintroduction to cocaine. We investigated this question using C57BL/6 mice. Mice pretreated with cocaine (15mg/kg×14 days) and then challenged with cocaine (15mg/kg) after 30 days of cocaine absence displayed sensitization of locomotor activity. For combination experiments, CTX injected during the 30 days of cocaine absence attenuated behavioral sensitization produced by cocaine challenge. In the case in which CTX was injected together with cocaine for 14 days, development of behavioral sensitization to cocaine challenge was also reduced. CTX attenuated the increase in locomotor activity produced by acute cocaine exposure; however, its efficacy was dependent on the dose of cocaine as inhibition was detected against 30mg/kg, but not 15mg/kg, of cocaine. These results from mice indicate that CTX attenuates locomotor activity produced by acute and repeated cocaine exposure and counters cocaine's locomotor activating properties in a paradigm in which the antibiotic is injected during the period of forced cocaine absence that follows repeated cocaine exposure.

Upregulation of glutamate transporter GLT-1 by mTOR-Akt-NF-кB cascade in astrocytic oxygen-glucose deprivation

Excessive extracellular glutamate leads to neuronal death in central nervous system. Excitatory glutamate transporter subtype 2 (GLT-1) carries bulk of glutamate reuptake in cerebral ischemia. Although GLT-1 expression fluctuates during the period of ischemia, little is known about its regulatory mechanism. Here we show an up-regulation of GLT-1 via mammalian target of rapamycin (mTOR)-Akt-nuclear factor-кB (NF-кB) signaling cascade in oxygen glucose deprivation (OGD). We found that brief rapamycin treatment significantly increased GLT-1 expression in cultured astrocytes. Rapamycin increased phosphorylation of raptor at Ser792 and decreased phosphorylation of rictor at Thr1135, suggesting that both mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) are involved in GLT-1 expression. This conclusion was further confirmed by raptor and rictor disruption experiments. Akt was activated by mTORC1 inhibition and required for GLT-1 expression because triciribine, a specific inhibitor of Akt, blocked the increase of GLT-1 expression. mTOR-Akt cascade then activated NF-кB and increased кB-motif-binding phosphoprotein (KBBP) expression and GLT-1 transcription. We next demonstrated that mTOR-Akt-NF-кB cascade was activated in OGD and subsequently caused the upregulation of GLT-1. Supporting evidence included: (1) inhibition of Akt or NF-кB occluded OGD-induced GLT-1 upregulation; (2) Raptor knock-down plus OGD did not add to the increase of GLT-1 expression; (3) Intact mTORC2 was required for GLT-1 enhancement. In summary, our data first showed that mTOR-Akt-NF-кB cascade played critical roles to up-regulate GLT-1 in OGD. This signaling cascade may work to promote glutamate uptake in brain ischemia and neurodegenerative diseases.

Inhibition of glycogen synthase kinase 3β activity with lithium prevents and attenuates paclitaxel-induced neuropathic pain

Paclitaxel (taxol) is a first-line chemotherapy-drug used to treat many types of cancers. Neuropathic pain and sensory dysfunction are the major toxicities, which are dose-limiting and significantly reduce the quality of life in patients. Two known critical spinal mechanisms underlying taxol-induced neuropathic pain are an increased production of pro-inflammatory cytokines including interleukin-1β (IL-1β) and suppressed glial glutamate transporter activities. In this study, we uncovered that increased activation of glycogen synthase kinase 3beta (GSK3β) in the spinal dorsal horn was concurrently associated with increased protein expressions of GFAP, IL-1β and a decreased protein expression of glial glutamate transporter 1 (GLT-1), as well as the development and maintenance of taxol-induced neuropathic pain. The enhanced GSK3β activities were supported by the concurrently decreased AKT and mTOR activities. The changes of all these biomarkers were basically prevented when animals received pre-emptive lithium (a GSK3β inhibitor) treatment, which also prevented the development of taxol-induced neuropathic pain. Further, chronic lithium treatment, which began on day 11 after the first taxol injection, reversed the existing mechanical and thermal allodynia induced by taxol. The taxol-induced increased GSK3β activities and decreased AKT and mTOR activities in the spinal dorsal horn were also reversed by lithium. Meanwhile, protein expressions of GLT-1, GFAP and IL-1β in the spinal dorsal horn were improved. Hence, suppression of spinal GSK3β activities is a key mechanism used by lithium to reduce taxol-induced neuropathic pain, and targeting spinal GSK3β is an effective approach to ameliorate GLT-1 expression and suppress the activation of astrocytes and IL-1β over-production in the spinal dorsal horn.

Regulation of system x(c)- in the SOD1-G93A mouse model of ALS

The cystine/glutamate antiporter (system xc-) is critical for the generation of the antioxidant glutathione by transporting cystine into the cell. At the same time, system xc- also releases glutamate, which can potentially lead to excitotoxicity. The dual actions of system xc- make it of great interest in any disease, like amyotrophic lateral sclerosis (ALS), in which there is evidence of the involvement of both oxidative stress and excitotoxicity. The present study investigated the regulation of system xc- in the spinal cord of the SOD1-G93A transgenic mouse model of ALS. In acute spinal cord slices of 70day old SOD1-G93A transgenic mice cystine uptake by system xc- was significantly increased compared to age matched non-transgenic mice; but it was not significantly different at 55, 100, or 130days. The 70day old SOD1-G93A transgenic mice also showed significantly increased glutamate release in the presence of cystine. d-Aspartate uptake through excitatory amino acid transporters (EAATs), the main mechanism by which glutamate is cleared from the extracellular space, was also examined. In spinal cord slices of 70day old SOD1-G93A mice no change in d-aspartate uptake was found. Together, these findings suggest that at 70days of age, SOD1-G93A transgenic mice have increased system xc- activity, but no change in EAAT function. These results raise the possibility that excitotoxicity in the SOD1-G93A transgenic mouse, at least at early time points, may be due to increased system xc- activity and not decreased EAAT function.

Neuroprotection by gonadal steroid hormones in acute brain damage requires cooperation with astroglia and microglia

The neuroactive steroids 17β-estradiol and progesterone control a broad spectrum of neural functions. Besides their roles in the regulation of classical neuroendocrine loops, they strongly influence motor and cognitive systems, behavior, and modulate brain performance at almost every level. Such a statement is underpinned by the widespread and lifelong expression pattern of all types of classical and non-classical estrogen and progesterone receptors in the CNS. The life-sustaining power of neurosteroids for tattered or seriously damaged neurons aroused interest in the scientific community in the past years to study their ability for therapeutic use under neuropathological challenges. Documented by excellent studies either performed in vitro or in adequate animal models mimicking acute toxic or chronic neurodegenerative brain disorders, both hormones revealed a high potency to protect neurons from damage and saved neural systems from collapse. Unfortunately, neurons, astroglia, microglia, and oligodendrocytes are comparably target cells for both steroid hormones. This hampers the precise assignment and understanding of neuroprotective cellular mechanisms activated by both steroids. In this article, we strive for a better comprehension of the mutual reaction between these steroid hormones and the two major glial cell types involved in the maintenance of brain homeostasis, astroglia and microglia, during acute traumatic brain injuries such as stroke and hypoxia. In particular, we attempt to summarize steroid-activated cellular signaling pathways and molecular responses in these cells and their contribution to dampening neuroinflammation and neural destruction. This article is part of a Special Issue entitled 'CSR 2013'.

Chronic ceftriaxone treatment rescues hippocampal memory deficit in AQP4 knockout mice via activation of GLT-1

Aquaporin-4 (AQP4) is the predominant water channel protein in the mammalian brain, and is mainly expressed in astrocytes. Besides its important role in water transport across the blood-brain barrier, our present study demonstrated that AQP4 deficiency impaired hippocampal long-term potentiation (LTP) and hippocampus-dependent memory formation, accompanied by the increase in extracellular glutamate concentration and N-methyl-d-aspartate (NMDA) receptor-mediated currents in hippocampal dentate gyrus (DG) region. The impairment of LTP and memory formation of AQP4 knockout (KO) mice was mediated by the downregulation of glutamate transporter-1 (GLT-1) expression/function, since it can be rescued by β-lactam antibiotic ceftriaxone (Cef), a potent GLT-1 stimulator. These results suggest that AQP4 functions as the modulator of synaptic plasticity and memory, and chronic Cef treatment rescues hippocampal memory deficit induced by AQP4 knockout.

CX3CL1 protects neurons against excitotoxicity enhancing GLT-1 activity on astrocytes

In this paper we show for the first time that: i) astrocytes are required for the neuroprotective activity of CX3CL1 against excitotoxicity; ii) inhibition of the glutamate transporter 1 (GLT-1) prejudices CX3CL1-mediated neuroprotection; iii) CX3CL1 increases GLT-1 activity on astrocytes. The modulation of GLT-1 activity induced by CX3CL1 on astrocytes requires the presence and the activity of A1 adenosine receptor (A1R), being blocked by the specific antagonist DPCPX and absent in A1R(-/-) astrocytes. These data introduce the astrocytes as active players in CX3CL1-mediated signaling between microglia and neurons, identifying GLT-1 as a key mediator of the neuroprotective activity of CX3CL1.

Histamine up-regulates astrocytic glutamate transporter 1 and protects neurons against ischemic injury

Astrocytic glutamate transporter 1 (GLT-1) is responsible for the majority of extracellular glutamate clearance and is essential for preventing excitotoxicity in the brain. Up-regulation of GLT-1 shows benefit effect on ischemia-induced neuronal damage. In present study, we examined the effect of histamine, a neurotransmitter or neuromodulator, on GLT-1 expression and function. In acute hippocampal slices, histamine selectively increased GLT-1 expression independent of neuronal activities. Similar up-regulation of GLT-1 was also observed after histamine treatment in pure cultured astrocytes, which was abolished by H1 receptor antagonist or PKC inhibitor. Cell surface biotinylation and whole-cell patch recordings of glutamate transporter current confirmed the up-regulation of functional GLT-1 following histamine exposure. Histamine treatment decreased the extracellular glutamate content and alleviated neuronal cell death induced by exogenous glutamate challenge. Moreover, we found a significant neuroprotective effect of histamine in brain slices after oxygen-glucose deprivation (OGD). In addition, histidine, the precursor of histamine, also showed neuroprotection against ischemic injury, which was accompanied by reversion of declined expression of GLT-1 in adult rats subjected to middle cerebral artery occlusion (MCAO). These neuroprotective effects of histamine/histidine were blocked by GLT-1 specific inhibitor dihydrokainate or H1 receptor antagonist. In summary, our results suggest that histamine up-regulates GLT-1 expression and function via astrocytic H1 receptors, thus resulting in neuroprotection against excitotoxicity and ischemic injury.

Astrocytic group I mGluR-dependent potentiation of astrocytic glutamate and potassium uptake

One of the most important functions of astrocytes is removal of glutamate released during synaptic transmission. Surprisingly, the mechanisms by which astrocyte glutamate uptake is acutely modulated remain to be clarified. Astrocytes express metabotropic glutamate receptors (mGluRs) and other G protein-coupled receptors (GPCRs), which are activated during neuronal activity. Here, we test the hypothesis that astrocytic group I mGluRs acutely regulate glutamate uptake by astrocytes in situ. This hypothesis was tested in acute mouse hippocampal slices. Activation of astrocytic mGluRs, using a tetanic high-frequency stimulus (HFS) applied to Schaffer collaterals, led to potentiation of the amplitude of the synaptically evoked glutamate transporter currents (STCs) and associated charge transfer without changes in kinetics. Similar potentiation of STCs was not observed in the presence of group I mGluR antagonists or the PKC inhibitor, PKC 19-36, suggesting that HFS-induced potentiation of astrocyte glutamate uptake is astrocytic group I mGluR and PKC dependent. Pharmacological stimulation of a transgenic GPCR (MrgA1R), expressed exclusively in astrocytes, also potentiated STC amplitude and charge transfer, albeit quicker and shorter lasting compared with HFS-induced potentiation. The amplitude of the slow, inward astrocytic current due to potassium (K(+)) influx was also enhanced following activation of the endogenous mGluRs or the astrocyte-specific MrgA1 Gq GPCRs. Taken together, these findings suggest that astrocytic group I mGluR activation has a synergistic, modulatory effect on the uptake of glutamate and K(+).

Inhibitors of glutamate dehydrogenase block sodium-dependent glutamate uptake in rat brain membranes

We recently found evidence for anatomic and physical linkages between the astroglial Na(+)-dependent glutamate transporters (GLT-1/EAAT2 and GLAST/EAAT1) and mitochondria. In these same studies, we found that the glutamate dehydrogenase (GDH) inhibitor, epigallocatechin-monogallate (EGCG), inhibits both glutamate oxidation and Na(+)-dependent glutamate uptake in astrocytes. In the present study, we extend this finding by exploring the effects of EGCG on Na(+)-dependent l-[(3)H]-glutamate (Glu) uptake in crude membranes (P2) prepared from rat brain cortex. In this preparation, uptake is almost exclusively mediated by GLT-1. EGCG inhibited l-[(3)H]-Glu uptake in cortical membranes with an IC50 value of 230 μM. We also studied the effects of two additional inhibitors of GDH, hexachlorophene (HCP) and bithionol (BTH). Both of these compounds also caused concentration-dependent inhibition of glutamate uptake in cortical membranes. Pre-incubating with HCP for up to 15 min had no greater effect than that observed with no pre-incubation, showing that the effects occur rapidly. HCP decreased the V max for glutamate uptake without changing the K m, consistent with a non-competitive mechanism of action. EGCG, HCP, and BTH also inhibited Na(+)-dependent transport of d-[(3)H]-aspartate (Asp), a non-metabolizable transporter substrate, and [(3)H]-γ-aminobutyric acid (GABA). In contrast to the forebrain, glutamate uptake in crude cerebellar membranes (P2) is likely mediated by GLAST (EAAT1). Therefore, the effects of these compounds were examined in cerebellar membranes. In this region, none of these compounds had any effect on uptake of either l-[(3)H]-Glu or d-[(3)H]-Asp, but they all inhibited [(3)H]-GABA uptake. Together these studies suggest that GDH is preferentially required for glutamate uptake in forebrain as compared to cerebellum, and GDH may be required for GABA uptake as well. They also provide further evidence for a functional linkage between glutamate transport and mitochondria.