Functional significance of the preconditioning-induced down-regulation of glutamate transporter GLT-1 in neuron/astrocyte co-cultures

N-Acetylcysteine and Ceftriaxone as Preconditioning Strategies in Focal Brain Ischemia: Influence on Glutamate Transporters Expression

Glutamate (Glu) plays a key role in excitotoxicity-related injury in cerebral ischemia. In the brain, Glu homeostasis depends on Glu transporters, including the excitatory amino acid transporters and the cysteine/Glu antiporter (xc-). We hypothesized that drugs acting on Glu transporters, such as ceftriaxone (CEF, 200 mg/kg, i.p.) and N-acetylcysteine (NAC, 150 mg/kg, i.p.), administered repeatedly for 5 days before focal cerebral ischemia in rats and induced by a 90-min middle cerebral artery occlusion (MCAO), may induce brain tolerance to ischemia. We compared the effects of these drugs on brain infarct volume, neurological deficits and the mRNA and protein expression of the Glu transporter-1 (GLT-1) and xc- with the effects of ischemic preconditioning and chemical preconditioning using 3-nitropropionic acid. Administration of CEF and NAC significantly reduced infarct size and neurological deficits caused by a 90-min MCAO. These beneficial effects were accompanied by changes in GLT-1 expression caused by a 90-min MCAO at both the mRNA and protein levels in the frontal cortex, hippocampus, and dorsal striatum. Thus, the results of this study suggest that the regulation of GLT-1 and xc- plays a role in the development of cerebral tolerance to ischemia and that this regulation may be a novel approach in the therapy of brain ischemia.

Glutamate transporters in brain ischemia: to modulate or not?

In this review, we briefly describe glutamate (Glu) metabolism and its specific transports and receptors in the central nervous system (CNS). Thereafter, we focus on excitatory amino acid transporters, cystine/glutamate antiporters (system xc-) and vesicular glutamate transporters, specifically addressing their location and roles in CNS and the molecular mechanisms underlying the regulation of Glu transporters. We provide evidence from in vitro or in vivo studies concerning alterations in Glu transporter expression in response to hypoxia or ischemia, including limited human data that supports the role of Glu transporters in stroke patients. Moreover, the potential to induce brain tolerance to ischemia through modulation of the expression and/or activities of Glu transporters is also discussed. Finally we present strategies involving the application of ischemic preconditioning and pharmacological agents, eg β-lactam antibiotics, amitriptyline, riluzole and N-acetylcysteine, which result in the significant protection of nervous tissues against ischemia.

Histone deacetylase inhibitors preserve function in aging axons

Aging increases the vulnerability of aging white matter to ischemic injury. Histone deacetylase (HDAC) inhibitors preserve young adult white matter structure and function during ischemia by conserving ATP and reducing excitotoxicity. In isolated optic nerve from 12-month-old mice, deprived of oxygen and glucose, we show that pan- and Class I-specific HDAC inhibitors promote functional recovery of axons. This protection correlates with preservation of axonal mitochondria. The cellular expression of HDAC 3 in the central nervous system (CNS), and HDAC 2 in optic nerve considerably changed with age, expanding to more cytoplasmic domains from nuclear compartments, suggesting that changes in glial cell protein acetylation may confer protection to aging axons. Our results indicate that manipulation of HDAC activities in glial cells may have a universal potential for stroke therapy across age groups.

Role of retinal glial cell glutamate transporters in retinal ganglion cell survival following stimulation of NMDA receptor

PURPOSE

To investigate the role of glutamate transporters (GLTs)in retinal glial cells that were treated with N-methyl-D-aspartate (NMDA), in retinal ganglion cell (RGC) survival.

METHODS

Primary cultures of retinal glial cells or RGCs from 3-day-old Sprague-Dawley rats were employed in the present study. Retinal glial cells were treated with NMDA and changes in GLT mRNA and protein expression were analyzed. The effects of pretreating retinal glial cells with the GLAST-specific inhibitor, rottlerin (ROT), and the GLT-1-specific inhibitor, dihydrokainic acid (DHK), on RGC survival were investigated under exposure to NMDA. The amount of glutamate in the culture medium of retinal glial cells was measured by high-performance liquid chromatography.

RESULTS

NMDA treatment increased GLAST and GLT-1 expression. GLAST and GLT-1 mRNA expression increased by 2.94-fold and 3.36-fold at 12 h after treatment with the highest concentration of NMDA (33 mM), and by 1.41-fold and 1.39-fold at 24 h, respectively. GLT-1 and GLAST protein expression also increased. MK801, an NMDA-receptor antagonist, inhibited the NMDA-induced upregulation of GLT mRNA expression. Co-culture with retinal glial cells increased the survival rate of RGCs. ROT decreased the survival rate of RGCs, whereas DHK significantly increased the survival rate of RGCs treated with 33 mM NMDA. NMDA treatment reduced the total amount of glutamate in the culture medium, particularly when 33 mM NMDA was added to the medium. ROT pretreatment increased the amount of glutamate in the culture medium, whereas DHK pretreatment decreased it.

CONCLUSION

GLAST and GLT-1 may have different roles in the survival of RGCs mediated by retinal glial cells. These results suggest that the NMDA-associated induction of GLTs plays an important role in RGC survival.

Histone deacetylase inhibitors preserve white matter structure and function during ischemia by conserving ATP and reducing excitotoxicity

The importance of white matter (WM) injury to stroke pathology has been underestimated in experimental animal models and this may have contributed to the failure to translate potential therapeutics into the stroke clinic. Histone deacetylase (HDAC) inhibitors are neuroprotective and also promote neurogenesis. These properties make them ideal candidates for stroke therapy. In a pure WM tract (isolated mouse optic nerve), we show that pan- and class I-specific HDAC inhibitors, administered before or after a period of oxygen and glucose deprivation (OGD), promote functional recovery of axons and preserve WM cellular architecture. This protection correlates with the upregulation of an astrocyte glutamate transporter, delayed and reduced glutamate accumulation during OGD, preservation of axonal mitochondria and oligodendrocytes, and maintenance of ATP levels. Interestingly, the expression of HDACs 1, 2, and 3 is localized to astrocytes, suggesting that changes in glial cell gene transcription and/or protein acetylation may confer protection to axons. Our findings suggest that a therapeutic opportunity exists for the use of HDAC inhibitors, targeting mitochondrial energy regulation and excitotoxicity in ischemic WM injury.

Bromocriptine, an ergot alkaloid, inhibits excitatory amino acid release mediated by glutamate transporter reversal

Bromocriptine, a dopamine D(2) receptor agonist, has widely been used for patients with Parkinson's disease. The aim of the present study was to investigate the effect of bromocriptine on glutamate transporter. Since the astroglial glutamate transporter GLT-1 (EAAT2) is the predominant isoform in the forebrain, we generated EAAT2-expressing human embryonic kidney cells and immortalized mouse astrocytes. In the present studies, we observed a GLT-1-immunoreactive band and significant Na(+)-dependent d-[(3)H] aspartate uptake. Furthermore, the glutamate transporter inhibitors, dl-threo-beta-benzyloxyaspartic acid (TBOA) and dihydrokainate (DHK), displayed a dose-dependent reduction of d-[(3)H] aspartate uptake in both types of cells. In contrast, cells exposed to either chemical anoxia or high KCl elicited a marked release of d-[(3)H] aspartate, and the release was inhibited by TBOA and DHK, implying the contribution of glutamate transporter reversal. Interestingly, we found that bromocriptine dose-dependently inhibits d-[(3)H] aspartate release elicited by chemical anoxia or high KCl, while no changes occurred in the uptake. The inhibitory action of bromocriptine was not affected by sulpiride, a dopamine D(2) receptor antagonist. On the other hand, bromocriptine had no effect on swelling-induced d-[(3)H] aspartate release, which is mediated by volume-regulated anion channels. In vivo studies revealed that bromocriptine suppresses the excessive elevation of glutamate levels in gerbils subjected to transient forebrain ischemia in a manner similar to DHK. Taken together, these results provide evidence that bromocriptine inhibits excitatory amino acid release via reversed operation of GLT-1 without altering forward transport.

Molecular physiology of preconditioning-induced brain tolerance to ischemia

Ischemic tolerance describes the adaptive biological response of cells and organs that is initiated by preconditioning (i.e., exposure to stressor of mild severity) and the associated period during which their resistance to ischemia is markedly increased. This topic is attracting much attention because preconditioning-induced ischemic tolerance is an effective experimental probe to understand how the brain protects itself. This review is focused on the molecular and related functional changes that are associated with, and may contribute to, brain ischemic tolerance. When the tolerant brain is subjected to ischemia, the resulting insult severity (i.e., residual blood flow, disruption of cellular transmembrane gradients) appears to be the same as in the naive brain, but the ensuing lesion is substantially reduced. This suggests that the adaptive changes in the tolerant brain may be primarily directed against postischemic and delayed processes that contribute to ischemic damage, but adaptive changes that are beneficial during the subsequent test insult cannot be ruled out. It has become clear that multiple effectors contribute to ischemic tolerance, including: 1) activation of fundamental cellular defense mechanisms such as antioxidant systems, heat shock proteins, and cell death/survival determinants; 2) responses at tissue level, especially reduced inflammatory responsiveness; and 3) a shift of the neuronal excitatory/inhibitory balance toward inhibition. Accordingly, an improved knowledge of preconditioning/ischemic tolerance should help us to identify neuroprotective strategies that are similar in nature to combination therapy, hence potentially capable of suppressing the multiple, parallel pathophysiological events that cause ischemic brain damage.

The upregulation of glial glutamate transporter-1 participates in the induction of brain ischemic tolerance in rats

Glial glutamate transporter-1 (GLT-1) plays an essential role in removing glutamate from the extracellular space and maintaining the glutamate below neurotoxic level in the brain. To explore whether GLT-1 plays a role in the acquisition of brain ischemic tolerance (BIT) induced by cerebral ischemic preconditioning (CIP), the present study was undertaken to observe in vivo changes in the expression of GLT-1 and glial fibrillary acidic protein (GFAP) in the CA1 hippocampus during the induction of BIT, and the effect of dihydrokainate (DHK), an inhibitor of GLT-1, on the acquisition of BIT in rats. Immunohistochemistry for GFAP showed that the processes of astrocytes were prolonged after a CIP 2 days before the lethal ischemic insult, which could protect pyramidal neurons in the CA1 hippocampus against delayed neuronal death induced normally by lethal ischemic insult. The prolonged processes extended into the area between the pyramidal neurons and tightly surrounded them. These changes made the pyramidal layer look like a 'shape grid'. Simultaneously, the prolonged and extended processes showed a great deal of GLT-1. Western blotting analysis showed significant upregulation of GLT-1 expression after the CIP, especially when it was administered 2 days before the subsequent lethal ischemic insult. Neuropathological evaluation by thionin staining showed that DHK dose-dependently blocked the protective role of CIP against delayed neuronal death induced normally by lethal brain ischemia. It might be concluded that the surrounding of pyramidal neurons by astrocytes and upregulation of GLT-1 induced by CIP played an important role in the acquisition of the BIT induced by CIP.

Changes in the spontaneous calcium oscillations for the development of the preconditioning-induced ischemic tolerance in neuron/astrocyte co-culture

Spontaneous Ca(2+) oscillations are believed to contribute to the regulation of gene expression. Here we investigated whether and how the dynamics of Ca(2+) oscillations changed after sublethal preconditioning (PC) for PC-induced ischemic tolerance in neuron/astrocyte co-cultures. The frequency of spontaneous Ca(2+) oscillations significantly decreased between 4 and 8 h after the end of PC in both neurons and astrocytes. Treatment with 2-APB, an inhibitor of IP3 receptors, decreased the oscillatory frequency, induced ischemic tolerance and a down-regulation of glutamate transporter GLT-1 contributing to the increase in the extracellular glutamate during ischemia. The expression of GLT-1 is known to be up-regulated by PACAP. Treatment with PACAP38 increased the oscillatory frequency, and antagonized both the PC-induced down-regulation of GLT-1 and ischemic tolerance. These results suggested that the PC suppressed the spontaneous Ca(2+) oscillations regulating the gene expressions of various proteins, especially of astrocytic GLT-1, for the development of the PC-induced ischemic tolerance.

Glia mechanisms in mood regulation: a novel model of mood disorders

INTRODUCTION

Recent evidence in clinical and preclinical studies has implicated glutamate neurotransmissions in pathophysiology of mood disorders. The regulation of amino acid neurotransmission, i.e., glutamate and gamma-aminobutyric acid (GABA) involves coordinated mechanisms of uptake and transport within a tripartite synaptic system that includes neurons and glia. Newly appreciated role of the glia, more specifically astrocytes on neuronal functions combined with reported postmortem abnormalities of glia in patients with mood disorders further supports the role of glia in mood disorders.

MATERIALS AND METHODS

This report presents some of our preliminary results utilizing glia-selective toxins and other pharmacological tools to suppress glial function within the limbic system to study the resulting behavioral abnormalities, and thus, elucidate glial involvement in the development of mood disorders.

RESULTS AND DISCUSSION

We demonstrate that chronic blockade of glutamate uptake by a glial/neuronal transporter antagonist L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) within the amygdala, a key area implicated in mood regulation, results in dose-dependent reduction in social exploratory behavior and disrupts circadian activity patterns consistent with symptoms of mood disorders. Similarly, the selective astrocytic glutamate transporter type 1 (GLT-1) blocker dihydrokainic acid (DHK) injected into the amygdala also results in reduced social interaction that is blocked by selective glutamate N-methyl-D-aspartate (NMDA) type receptor antagonist AP5. The results are discussed in the context of glial and glutamate mechanisms in mood disorders and potential therapeutic avenues to address these mechanisms.

Nitric oxide produced during sublethal ischemia is crucial for the preconditioning-induced down-regulation of glutamate transporter GLT-1 in neuron/astrocyte co-cultures

In the brain, prior sublethal ischemia (preconditioning, PC) produces tolerance of neurons to subsequent lethal ischemia. This study aims at elucidating whether and how nitric oxide (NO) produced during PC is involved in the PC-induced ischemic tolerance of neurons in neuron/astrocyte co-cultures. The rise in the extracellular concentration of glutamate during ischemia caused by the reversed uptake of glutamate (Glu) by the astrocytic Glu transporter GLT-1 was markedly suppressed by the prior PC treatment, but the suppression was reversed by treatment with an inhibitor of nitric oxide synthase (NOS) during PC. Immunocytochemical and Western blot analyses demonstrated that the expression of GLT-1 was down-regulated after the PC insult, and this down-regulation was also antagonized by treatment with NOS inhibitors during PC. Here we show that nNOS-derived NO produced during PC was crucial for the down-regulation of astrocytic GLT-1, and this down-regulation coincided with an increased survival rate of neurons.