Dearth of glutamate transporters contributes to striatal excitotoxicity

The role of short-chain fatty acids in central nervous system diseases

Previous studies have found that intracorporal short-chain fatty acids (SCFAs), as the main metabolites of the gut microbiota, play important roles in the intestinal physiology and immune function. Along with the in-depth study of the brain-gut axis, the attention to the roles of SCFAs in central nervous system (CNS) has been raised. It has been found that SCFAs function in CNS diseases by regulating inflammatory response, neuronal apoptosis, oxidative stress, the integrity of the blood-brain barrier (BBB) and so on. Here, the changes, the effects and the mechanisms of different SCFA as individual or mixture in different CNS diseases were summarized. It is expected to lead to increased interest in SCFAs studies as an important regulator in CNS diseases and provide feasible suggestions based on SCFAs for the therapy of CNS diseases in the future.

Glutamic Acid Transporters: Targets for Neuroprotective Therapies in Parkinson's Disease

Parkinson's disease (PD) is a common neurodegenerative disease in middle-aged and elderly individuals. At present, no effective drug has been developed to treat PD. Although a variety of drugs exist for the symptomatic treatment of PD, they all have strong side effects. Most studies on PD mainly focus on dopaminergic neurons. This review highlights the function of glutamic acid transporters (GLTs), including excitatory amino acid transporters (EAATs) and vesicular glutamate transporters (VGLUTs), during the development of PD. In addition, using bioinformatics, we compared the expression of different types of glutamate transporter genes in the cingulate gyrus of PD patients and healthy controls. More importantly, we suggest that the functional roles of glutamate transporters may prove beneficial in the treatment of PD. In summary, VGLUTs and EAATs may be potential targets in the treatment of PD. VGLUTs and EAATs can be used as clinical drug targets to achieve better efficacy. Through this review article, we hope to enable future researchers to improve the condition of PD patients.

Sex differences in the glutamate signaling pathway in juvenile rats

Females have been found to be at lower risk for the development of neurodevelopmental disorders than males. The greater neuroprotection in females is mostly due to female sex hormones. Estrogen is hypothesized to provide neuroprotection by suppressing the neuro-excitotoxicity induced by glutamate (Glu). This study was conducted to understand the effect of sex in modulating Glu signaling in juvenile rats. Brain tissue homogenate of 15 Wistar albino rats (9 males, 6 females) weighing 60 to 80 g and aged approximately 28 days was used. Biochemical parameters related to Glu signaling, such as the absolute and relative concentrations of Glu, gamma aminobutyric acid (GABA), and glutamine, as well as glutamate transporter 1 (GLT1), glutamine synthetase (GS), glutaminase (GLN), and glutamate decarboxylase-67 (GAD-67), were measured by ELISA. The data obtained demonstrated that compared with the levels in males, female rats exhibited significantly lower levels of Glu (p = .001) and GLN/GS (p = .021). The Glu/GABA and Glu/GLT1 ratios as well as the levels of GAD-67 were also lower in female rats, although the difference was not significant. The GLN/GAD-67 ratio (p = .027) and levels of GS (p = .019) were significantly higher in female rats than in males. Multiple regression analysis confirmed the role of GLN/GS, together with the much higher affinity of GLT1 to Glu, in avoiding excitotoxicity in females. In conclusion, there was a significant difference in Glu signaling between female and male rats. The females exhibited a lower susceptibility to develop Glu-induced excitotoxicity, an etiological mechanism for multiple neurodevelopmental disorders.

HACE1 reduces oxidative stress and mutant Huntingtin toxicity by promoting the NRF2 response

Oxidative stress plays a key role in late onset diseases including cancer and neurodegenerative diseases such as Huntington disease. Therefore, uncovering regulators of the antioxidant stress responses is important for understanding the course of these diseases. Indeed, the nuclear factor erythroid 2-related factor 2 (NRF2), a master regulator of the cellular antioxidative stress response, is deregulated in both cancer and neurodegeneration. Similar to NRF2, the tumor suppressor Homologous to the E6-AP Carboxyl Terminus (HECT) domain and Ankyrin repeat containing E3 ubiquitin-protein ligase 1 (HACE1) plays a protective role against stress-induced tumorigenesis in mice, but its roles in the antioxidative stress response or its involvement in neurodegeneration have not been investigated. To this end we examined Hace1 WT and KO mice and found that Hace1 KO animals exhibited increased oxidative stress in brain and that the antioxidative stress response was impaired. Moreover, HACE1 was found to be essential for optimal NRF2 activation in cells challenged with oxidative stress, as HACE1 depletion resulted in reduced NRF2 activity, stability, and protein synthesis, leading to lower tolerance against oxidative stress triggers. Strikingly, we found a reduction of HACE1 levels in the striatum of Huntington disease patients, implicating HACE1 in the pathology of Huntington disease. Moreover, ectopic expression of HACE1 in striatal neuronal progenitor cells provided protection against mutant Huntingtin-induced redox imbalance and hypersensitivity to oxidative stress, by augmenting NRF2 functions. These findings reveal that the tumor suppressor HACE1 plays a role in the NRF2 antioxidative stress response pathway and in neurodegeneration.

Decreased expression of GLT-1 in the R6/2 model of Huntington's disease does not worsen disease progression

Excitotoxicity is thought to be important in the pathogenesis of Huntington's disease (HD). Glutamate is the predominant excitatory neurotransmitter in the brain, and excess activation of glutamate receptors can cause neuronal dysfunction and death. Glutamate transporters regulate the extracellular concentration of glutamate. GLT-1 is the most abundant glutamate transporter, and accounts for most of the glutamate transport in the brain. Administration of ceftriaxone, an antibiotic that increases the functional expression of GLT-1, can improve the behavioral phenotype of the R6/2 mouse model of HD. To test the hypothesis that GLT-1 expression critically affects the HD disease process, we generated a novel mouse model that is heterozygous for the null allele of GLT-1 and carries the R6/2 transgene (double mutation). We demonstrated that the protein expression of total GLT-1, as well as two of its isoforms, is decreased within the cortex and striatum of 12-week-old R6/2 mice, and that the expression of EAAC1 was decreased in the striatum. Protein expression of GLT-1 was further decreased in the cortex and striatum of the double mutation mice compared with the R6/2 mice at 11 weeks. However, the effects of the R6/2 transgene on weight loss, accelerating rotarod, climbing and paw-clasping were not exacerbated in these double mutants. Na(+) -dependent glutamate uptake into synapatosomes isolated from the striatum and cortex of 11-week-old R6/2 mice was unchanged compared with controls. These results suggest that changes in GLT-1 expression or function per se are unlikely to potentiate or ameliorate the progression of HD.

Ifenprodil, a NR2B-selective antagonist of NMDA receptor, inhibits reverse Na+/Ca2+ exchanger in neurons

Glutamate-induced delayed calcium dysregulation (DCD) is causally linked to excitotoxic neuronal death. The mechanisms of DCD are not completely understood, but it has been proposed that the excessive influx of external Ca(2+) is essential for DCD. The NMDA-subtype of glutamate receptor (NMDAR) and the plasmalemmal Na(+)/Ca(2+) exchanger operating in the reverse mode (NCX(rev)) have been implicated in DCD. In experiments with "younger" neurons, 6-8 days in vitro (6-8 DIV), in which the NR2A-containing NMDAR expression is low, ifenprodil, an inhibitor of NR2B-containing NMDAR, completely prevented DCD whereas PEAQX, another NMDAR antagonist that preferentially interacts with NR2A-NMDAR, was without effect. With "older" neurons (13-16 DIV), in which NR2A- and NR2B-NMDARs are expressed to a greater extent, both ifenprodil and PEAQX applied separately failed to prevent DCD. However, combined application of ifenprodil and PEAQX completely averted DCD. Ifenprodil and ifenprodil-like NR2B-NMDAR antagonists Ro 25-6981 and Co 101244 but not PEAQX or AP-5 inhibited gramicidin- and Na(+)/NMDG-replacement-induced increases in cytosolic Ca(2+) mediated predominantly by NCX(rev). This suggests that ifenprodil, Ro 25-6981, and Co 101244 inhibit NCX(rev). The ability of ifenprodil to inhibit NCX(rev) correlates with its efficacy in preventing DCD and emphasizes an important role of NCX(rev) in DCD. Overall our data suggest that both NR2A- and NR2B-NMDARs are involved in DCD in "older" neurons, and it is necessary to inhibit both NMDARs and NCX(rev) to prevent glutamate-induced DCD.

Delayed calcium dysregulation in neurons requires both the NMDA receptor and the reverse Na+/Ca2+ exchanger

Glutamate-induced delayed calcium dysregulation (DCD) is a causal factor leading to neuronal death. The mechanism of DCD is not clear but Ca2+ influx via N-methyl-d-aspartate receptors (NMDAR) and/or the reverse plasmalemmal Na+/Ca2+ exchanger (NCXrev) could be involved in DCD. However, the extent to which NMDAR and NCX(rev) contribute to glutamate-induced DCD is uncertain. Here, we show that both NMDAR and NCX(rev) are critical for DCD in neurons exposed to excitotoxic glutamate. In rat cultured hippocampal neurons, 25 μM glutamate produced DCD accompanied by sustained increase in cytosolic Na+ ([Na+]c) and plasma membrane depolarization. MK801 and memantine, noncompetitive NMDAR inhibitors, added shortly after glutamate, completely prevented DCD whereas AP-5, a competitive NMDAR inhibitor, failed to protect against DCD. None of the tested inhibitors lowered elevated [Na+]c or restored plasma membrane potential. In the experiments with NCX reversal by gramicidin, MK801 and memantine robustly inhibited NCXrev while AP-5 was much less efficacious. In electrophysiological patch-clamp experiments MK801 and memantine inhibited NCXrev-mediated ion currents whereas AP-5 failed. Thus, MK801 and memantine, in addition to NMDAR, inhibited NCXrev. Inhibition of NCXrev either with KB-R7943, or by collapsing Na+ gradient across the plasma membrane, or by inhibiting Na+/H+ exchanger with 5-(N-ethyl-N-isopropyl) amiloride (EIPA) and thus preventing the increase in [Na+]c failed to preclude DCD. However, NCXrev inhibition combined with NMDAR blockade by AP-5 completely prevented DCD. Overall, our data suggest that both NMDAR and NCXrev are essential for DCD in glutamate-exposed neurons and inhibition of individual mechanism is not sufficient to prevent calcium dysregulation.

Anti-superoxide and anti-peroxynitrite strategies in pain suppression

Superoxide (SO, O(2)·(-)) and its reaction product peroxynitrite (PN, ONOO(-)) have been shown to be important in the development of pain of several etiologies. While significant progress has been made in teasing out the relative contribution of SO and PN peripherally, spinally, and supraspinally during the development and maintenance of central sensitization and pain, there is still a considerable void in our understanding. Further research is required in order to develop improved therapeutic strategies for selectively eliminating SO and/or PN. Furthermore, it may be that PN is a more attractive target, in that unlike SO it has no currently known beneficial role. Our group has been at the forefront of research concerning the role of SO and PN in pain, and our current findings have led to the development of two new classes of orally active catalysts which are selective for PN decomposition while sparing SO. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.

Roles of reactive oxygen and nitrogen species in pain

Peroxynitrite (PN; ONOO⁻) and its reactive oxygen precursor superoxide (SO; O₂•⁻) are critically important in the development of pain of several etiologies including pain associated with chronic use of opiates such as morphine (also known as opiate-induced hyperalgesia and antinociceptive tolerance). This is now an emerging field in which considerable progress has been made in terms of understanding the relative contributions of SO, PN, and nitroxidative stress in pain signaling at the molecular and biochemical levels. Aggressive research in this area is poised to provide the pharmacological basis for development of novel nonnarcotic analgesics that are based upon the unique ability to selectively eliminate SO and/or PN. As we have a better understanding of the roles of SO and PN in pathophysiological settings, targeting PN may be a better therapeutic strategy than targeting SO. This is because, unlike PN, which has no currently known beneficial role, SO may play a significant role in learning and memory. Thus, the best approach may be to spare SO while directly targeting its downstream product, PN. Over the past 15 years, our team has spearheaded research concerning the roles of SO and PN in pain and these results are currently leading to the development of solid therapeutic strategies in this important area.

Simulated whiplash modulates expression of the glutamatergic system in the spinal cord suggesting spinal plasticity is associated with painful dynamic cervical facet loading

The cervical facet joint and its capsule have been reported to be injured during whiplash scenarios and are a common source of chronic neck pain from whiplash. Both the metabotropic glutamate receptor 5 (mGluR5) and the excitatory amino acid carrier 1 (EAAC1) have pivotal roles in chronic pain. In this study, spinal mGluR5 and EAAC1 were quantified following painful facet joint distraction in a rat model of facet-mediated painful loading and were evaluated for their correlation with the severity of capsule loading. Rats underwent either a dynamic C6/C7 joint distraction simulating loading experienced during whiplash (distraction; n = 12) or no distraction (sham; n = 6) to serve as control. The severity of capsular loading was quantified using strain metrics, and mechanical allodynia was assessed after surgery. Spinal cord tissue was harvested at day 7 and the expression of mGluR5 and EAAC1 were quantified using Western blot analysis. Mechanical allodynia following distraction was significantly (p < 0.001) higher than sham. Spinal expression of mGluR5 was also significantly (p < 0.05) greater following distraction relative to sham. However, spinal EAAC1 was significantly (p = 0.0003) reduced compared to sham. Further, spinal mGluR5 expression was significantly positively correlated to capsule strain (p < 0.02) and mechanical allodynia (p < 0.02). Spinal EAAC1 expression was significantly negatively related to one of the strain metrics (p < 0.003) and mechanical allodynia at day 7 (p = 0.03). These results suggest that the spinal glutamatergic system may potentiate the persistent behavioral hypersensitivity that is produced following dynamic whiplash-like joint loading; chronic whiplash pain may be alleviated by blocking mGluR5 expression and/or enhancing glutamate transport through the neuronal transporter EAAC1.

Calpain activation and Na+/Ca2+ exchanger degradation occur downstream of calcium deregulation in hippocampal neurons exposed to excitotoxic glutamate

Delayed calcium deregulation (DCD) plays an essential role in glutamate excitotoxicity, a major detrimental factor in stroke, traumatic brain injury, and various neurodegenerations. In the present study, we examined the role of calpain activation and Na(+)/Ca(2+) exchanger (NCX) degradation in DCD and excitotoxic cell death in cultured hippocampal neurons. Exposure of neurons to glutamate caused DCD accompanied by secondary mitochondrial depolarization. Activation of calpain was evidenced by detecting NCX isoform 3 (NCX3) degradation products. Degradation of NCX isoform 1 (NCX1) was below the detection limit of Western blotting. Degradation of NCX3 was detected only after 1 hr of incubation with glutamate, whereas DCD occurred on average within 15 min after glutamate application. Calpeptin, an inhibitor of calpain, significantly attenuated NCX3 degradation but failed to inhibit DCD and excitotoxic neuronal death. Calpain inhibitors I, III, and VI also failed to influence DCD and glutamate-induced neuronal death. On the other hand, MK801, an inhibitor of the NMDA subtype of glutamate receptors, added shortly after the initial glutamate-induced jump in cytosolic Ca(2+), completely prevented DCD and activation of calpain and strongly protected neurons against excitotoxicity. Taken together, our results suggest that, in glutamate-treated hippocampal neurons, the initial increase in cytosolic Ca(2+) that precedes DCD is insufficient for sustained calpain activation, which most likely occurs downstream of DCD.

Dopamine regulates the expression of the glutamate transporter GLT1 but not GLAST in developing striatal astrocytes

Dopamine and L: -glutamate are important signals which guide the development of functional neural circuits within the striatal complex. Disequilibrium of these neurotransmitter systems is believed to be etiological for the genesis of neurological and psychiatric diseases. Since dopamine plays a crucial role for the early transmitter-regulated differentiation of striatal GABAergic neurons, we emphasized that dopaminergic transmission may also be involved in the fine tuning of intra-striatal glutamate action. In this study, we report that dopamine decreases the expression of the glutamate transporter GLT1 but not GLAST in striatal astrocytes by measuring gene and protein expression. Using glutamate-uptake approaches, we demonstrate an increase in glutamate clearance of externally added glutamate in dopamine-treated cultures compared to controls. Our findings imply that dopamine regulates the availability of L: -glutamate in the developing striatum. It is also suggested that the application of dopaminergic drugs can interfere with ontogenetic processes within the striatal complex.

Role of cyclophilin D-dependent mitochondrial permeability transition in glutamate-induced calcium deregulation and excitotoxic neuronal death

In the present study we tested the hypothesis that the cyclophilin D-dependent (CyD) mitochondrial permeability transition (CyD-mPT) plays an important role in glutamate-triggered delayed calcium deregulation (DCD) and excitotoxic neuronal death. We used cultured cortical neurons from wild-type C57BL/6 and cyclophilin D-knockout mice (Ppif(-/-)). Induction of the mPT was identified by following the rapid secondary acidification of mitochondrial matrices monitored with mitochondrially targeted pH-sensitive yellow fluorescent protein. Suppression of the CyD-mPT due to genetic CyD ablation deferred DCD and mitochondrial depolarization, and increased the survival rate after exposure of neurons to 10 microM glutamate, but not to 100 microM glutamate. Ca(2+) influx into Ppif(-/-) neurons was not diminished in comparison with WT neurons judging by (45)Ca accumulation. In both types of neurons, 100 microM glutamate produced greater Ca(2+) influx than 10 microM glutamate. We hypothesize that greater Ca(2+) influx produced by higher glutamate rapidly triggered the CyD-independent mPT in both WT and Ppif(-/-) neurons equalizing their responses to supra-physiologic excitotoxic insults. In neurons exposed to moderate but pathophysiologically-relevant glutamate concentrations, an induction of the CyD-mPT appears to play an important role in mitochondrial injury contributing to DCD and cell death.

Detergent-insoluble EAAC1/EAAT3 aberrantly accumulates in hippocampal neurons of Alzheimer's disease patients

Disturbed glutamate homeostasis may contribute to the pathological processes involved in Alzheimer's disease (AD). Once glutamate is released from synapses or from other intracellular sources, it is rapidly cleared by glutamate transporters. EAAC1 (also called EAAT3 or SLC1A1) is the primary glutamate transporter in forebrain neurons. In addition to transporting glutamate, EAAC1 plays other roles in regulating GABA synthesis, reducing oxidative stress in neurons, and is important in supporting neuron viability. Currently, little is known about EAAC1 in AD. To address whether EAAC1 is disturbed in AD, immunohistochemistry was performed on tissue from hippocampus and frontal cortex of AD and normal control subjects matched for age and gender. While EAAC1 immunostaining in cortex appeared comparable to controls, in the hippocampus, EAAC1 aberrantly accumulated in the cell bodies and proximal neuritic processes of CA2-CA3 pyramidal neurons in AD patients. Biochemical analyses showed that Triton X-100-insoluble EAAC1 was significantly increased in the hippocampus of AD patients compared to both controls and Parkinson's disease patients. These findings suggest that aberrant glutamate transporter expression is associated with AD-related neuropathology and that intracellular accumulation of detergent-insoluble EAAC1 is a feature of the complex biochemical lesions in AD that include altered protein solubility.

MR relaxometry in Huntington's disease: Correlation between imaging, genetic and clinical parameters

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disease. It has been hypothesized that changes of iron content in the brain may be involved in the pathogenesis of HD. To ascertain the hypothesis, we investigated the relationship between T2 relaxation time (T2), the number of cytosine-adenine-guanine triplet repeats (CAG) and clinical status in patients suffering from HD. 34 HD patients (mean age 50.1+/-11.8 standard deviation (SD) years) and 34 control subjects (49.6+/-13.3) were scanned using a 1.5 tesla magnetic resonance (MR) scanner and the patients underwent clinical and genetic testing. A multiple echo sequence was employed for T2 measurements. T2 from healthy volunteers matched previous studies. A T2 shortening was found in the pallidum of HD patients compared to controls (65.4+/-6.4 ms vs. 71.8+/-3.6 ms, P<0.00001). A correlation between the number of CAG and T2 was found for the left pallidum (decrease in T2, P<0.05) and an inverse correlation for the left caudate (increase in T2, P<0.05). In HD patients, alterations in iron levels may be caused by an alteration in its axonal transport. The observed T2/CAG covariations may reflect changes in levels and forms of iron: this suggests that HD patients with a higher genetic load have more ferritin-bound ("safe form") iron in the pallidum and/or more low-molecular ("toxic") iron in the caudate. An increase in "toxic" iron in the caudate may enable oxidative stress and thus underlie progression of the disease.

The role of glutamate transporters in neurodegenerative diseases and potential opportunities for intervention

Extracellular concentrations of the predominant excitatory neurotransmitter, glutamate, and related excitatory amino acids are maintained at relatively low levels to ensure an appropriate signal-to-noise ratio and to prevent excessive activation of glutamate receptors that can result in cell death. The latter phenomenon is known as 'excitotoxicity' and has been associated with a wide range of acute and chronic neurodegenerative disorders, as well as disorders that result in the loss of non-neural cells such as oligodendroglia in multiple sclerosis. Unfortunately clinical trials with glutamate receptor antagonists that would logically seem to prevent the effects of excessive receptor activation have been associated with untoward side effects or little clinical benefit. In the mammalian CNS, the extracellular concentrations of glutamate are controlled by two types of transporters; these include a family of Na(+)-dependent transporters and a cystine-glutamate exchange process, referred to as system X(c)(-). In this review, we will focus primarily on the Na(+)-dependent transporters. A brief introduction to glutamate as a neurotransmitter will be followed by an overview of the properties of these transporters, including a summary of the presumed physiologic mechanisms that regulate these transporters. Many studies have provided compelling evidence that impairing the function of these transporters can increase the sensitivity of tissue to deleterious effects of aberrant activation of glutamate receptors. Over the last decade, it has become clear that many neurodegenerative disorders are associated with a change in localization and/or expression of some of the subtypes of these transporters. This would suggest that therapies directed toward enhancing transporter expression might be beneficial. However, there is also evidence that glutamate transporters might increase the susceptibility of tissue to the consequences of insults that result in a collapse of the electrochemical gradients required for normal function such as stroke. In spite of the potential adverse effects of upregulation of glutamate transporters, there is recent evidence that upregulation of one of the glutamate transporters, GLT-1 (also called EAAT2), with beta-lactam antibiotics attenuates the damage observed in models of both acute and chronic neurodegenerative disorders. While it seems somewhat unlikely that antibiotics specifically target GLT-1 expression, these studies identify a potential strategy to limit excitotoxicity. If successful, this type of approach could have widespread utility given the large number of neurodegenerative diseases associated with decreases in transporter expression and excitotoxicity. However, given the massive effort directed at developing glutamate receptor agents during the 1990s and the relatively modest advances to date, one wonders if we will maintain the patience needed to carefully understand the glutamatergic system so that it will be successfully targeted in the future.

N-methyl-D-aspartate receptor-dependent regulation of the glutamate transporter excitatory amino acid carrier 1

The neuronal transporter excitatory amino acid carrier 1 (EAAC1) is enriched in perisynaptic regions, where it may regulate synaptic spillover of glutamate. In this study we examined potential interactions between EAAC1 and ionotropic glutamate receptors. N-Methyl-D-aspartate (NMDA) receptor subunits NR1, NR2A, and NR2B, but not the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor subunit GluR2, were co-immunoprecipitated with EAAC1 from neuron-enriched hippocampal cultures. A similar interaction was observed in C6 glioma and human embryonic kidney cells after co-transfection with Myc epitope-tagged EAAC1 and NMDA receptor subunits. Co-transfection of C6 glioma with the combination of NR1 and NR2 subunits dramatically increased (approximately 3-fold) the amount of Myc-EAAC1 that can be labeled with a membrane-impermeable biotinylating reagent. In hippocampal cultures, brief (5 min), robust (100 microM NMDA, 10 microM glycine) activation of the NMDA receptor decreased biotinylated EAAC1 to approximately 50% of control levels. This effect was inhibited by an NMDA receptor antagonist, intracellular or extracellular calcium chelators, or hypertonic sucrose. Glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid with cyclothiazide, and thapsigargin mimicked the effects of NMDA. These studies suggest that NMDA receptors interact with EAAC1, facilitate cell surface expression of EAAC1 under basal conditions, and control internalization of EAAC1 upon activation. This NMDA receptor-dependent regulation of EAAC1 provides a novel mechanism that may shape excitatory signaling during synaptic plasticity and/or excitotoxicity.

Neuroprotective potential of ceftriaxone in in vitro models of stroke

Astrocytic glutamate transporters are considered an important target for neuroprotective therapies as the function of these transporters is abnormal in stroke and other neurological disorders associated with excitotoxicity. Recently, Rothstein et al., [Rothstein JD, Patel S, Regan MR, Haenggeli C, Huang YH, Bergles DE, Jin L, Dykes Hoberg M, Vidensky S, Chung DS, Toan SV, Bruijn LI, Su ZZ, Gupta P, Fisher PB (2005) Beta-lactam antibiotics offer neuroprotection by increasing glutamate transporter expression. Nature 433:73-77] reported that beta-lactam antibiotics (including ceftriaxone, which easily crosses the blood-brain barrier) increase glutamate transporter 1 (GLT-1) expression and reduce cell death resulting from oxygen-glucose deprivation (OGD) in dissociated embryonic cortical cultures. To determine whether a similar neuroprotective mechanism operates in more mature neurons, which show a different pattern of response to ischemia than primary cultures, we exposed acute hippocampal slices obtained from rats treated with ceftriaxone for 5 days (200 mg/kg; i.p.) to OGD. Whole-cell patch clamp recording of glutamate-induced N-methyl-d-aspartate (NMDA) currents from CA1 pyramidal neurons showed a larger potentiation of these currents after application of 15 microM dl-threo-beta-benzyloxyaspartic acid (TBOA; a potent blocker of glutamate transporters) in ceftriaxone-injected animals than in untreated animals, indicating increased glutamate transporter activity. Western blot analysis did not reveal GLT-1 upregulation in the hippocampus. Delay to OGD-induced hypoxic spreading depression (HSD) recorded in slices obtained from ceftriaxone-treated rats was longer (6.3+/-0.2 vs. 5.2+/-0.2 min; P<0.001) than that in the control group, demonstrating a neuroprotective action of the antibiotic in this model. The effect of ceftriaxone was also tested in organotypic hippocampal slices obtained from P7-9 rats (>14 days in vitro). OGD or glutamate (3.5-5.0 mM) damaged CA1 pyramidal neurons as assessed by propidium iodide (PI) fluorescence. Similar damage was observed after pre-treatment with ceftriaxone (10-200 microM; 5 days) and ceftriaxone exposure did not result in GLT-1 upregulation as assayed by Western blot. Treatment of slice cultures with dibutyryl cAMP (100-250 microM; 5 days) increased GLT-1 expression but did not reduce cell damage induced by OGD or glutamate. Thus we confirm the neuroprotective effect of antibiotic exposure on OGD-induced injury, but suggest that this action is related to independent modulation of transporter activity rather than to the level of GLT-1 protein expression. In addition, our results indicate that the protective effects of beta-lactam antibiotics are highly dependent on the experimental model.

Genetic analysis of the GRIK2 modifier effect in Huntington's disease

BACKGROUND

In Huntington's disease (HD), age at neurological onset is inversely correlated with the length of the CAG trinucleotide repeat mutation, but can be modified by genetic factors beyond the HD gene. Association of a relatively infrequent 16 TAA allele of a trinucleotide repeat polymorphism in the GRIK2 3'UTR with earlier than expected age at neurological onset has been suggested to reflect linkage disequilibrium with a functional polymorphism in GRIK2 or an adjacent gene.

RESULTS

We have tested this hypothesis by sequencing all GRIK2 exons, the exon-flanking sequences and 3'UTR in several individuals who were crucial to demonstrating the modifier effect, as they showed much earlier age at neurological onset than would be expected from the length of their HD CAG mutation. Though ten known SNPs were detected, no sequence variants were found in coding or adjacent sequence that could explain the modifier effect by linkage disequilibrium with the 16 TAA allele. Haplotype analysis using microsatellites, known SNPs and new variants discovered in the 3'UTR argues against a common ancestral origin for the 16 TAA repeat alleles in these individuals.

CONCLUSION

These data suggest that the modifier effect is actually due to the TAA repeat itself, possibly via a functional consequence on the GRIK2 mRNA.

Downregulation of spinal glutamate transporter EAAC1 followingnerve injury is regulated by central glucocorticoid receptors in rats

Previous studies have shown that glucocorticoid receptors (GR) were upregulated, whereas glutamate transporters were downregulated, within the spinal cord dorsal horn after peripheral nerve injury. However, the relationship between the expression of spinal GR and glutamate transporter after nerve injury remains unknown. In the present study, we examined the hypothesis that central GR would regulate the expression of spinal glutamate transporter EAAC1 following chronic constriction nerve injury (CCI) in rats. CCI induced a significant downregulation of EAAC1 expression primarily within the ipsilateral spinal cord dorsal horn when examined on postoperative day 7 using both Western blot and immunohistochemistry. The downregulation of EAAC1 was significantly diminished after either the GR antagonist RU38486 (4 > 2 = 0.5 microg = vehicle) or a GR antisense oligonucleotide was administered intrathecally twice daily for postoperative day 1-6. Moreover, CCI induced a significant downregulation of nuclear factor kappaB (NF-kappaB) within the ipsilateral spinal cord dorsal horn, which also was attenuated by either RU38486 (4 > 2 = 0.5 microg = vehicle) or a GR antisense oligonucleotide. The immunohistochemical data indicated a pattern of colocalization between GR and EAAC1 as well as GR and NF-kappaB within the spinal cord dorsal horn. Since, NF-kappaB has been shown to regulate the expression of those cellular elements linked to inflammation and tissue injury and its activity can be negatively regulated by GR activation, these results suggest that spinal GR through NF-kappaB may play a significant role in the regulation of EAAC1 expression after peripheral nerve injury, a cellular pathway that may contribute to the development of neuropathic pain behaviors in rats.