Na+-dependent high-affinity glutamate transport in macrophages

Remote Ischaemic Preconditioning Accelerates Brain to Blood Glutamate Efflux via EAATs-mediated Transport

Remote ischaemic conditioning (RIC) becomes an attractive strategy for the endogenous stimulation of mechanisms protecting neurons against ischaemia. Although the processes underlying the RIC are not clearly understood, the homeostasis of glutamate seems to play an important role. The present study is focused on the investigation of the brain to blood efflux of glutamate in a condition mimicking ischaemia-mediated excitotoxicity and remote ischaemic preconditioning (RIPC). The animals were pre-treated with a hind-limb tourniquet one hour before the intraventricular administration of glutamate and its release was monitored as the concentration of glutamate/glutathione in blood and liquor for up to 1 h. The transport mediated by excitatory amino acid transporters (EAATs) was verified by their inhibition with Evans Blue intraventricular co-administration. RIPC mediated the efflux of glutamate exceeding from CSF to blood in the very early stage of intoxication. As a consequence, the blood level of glutamate rose in a moment. EAATs inhibition confirmed the active role of glutamate transporters in this process. In the blood, elevated levels of glutamate served as a relevant source of antioxidant glutathione for circulating cells in RIPC-treated individuals. All of those RIPC-mediated recoveries in processes of glutamate homeostasis reflect the improvement of oxidative stress, suggesting glutamate-accelerated detoxication to be one of the key mechanisms of RIPC-mediated neuroprotection.

DNA methylation changes underlie the long-term association between periodontitis and atherosclerotic cardiovascular disease

Periodontitis, the leading cause of adult tooth loss, has been identified as an independent risk factor for cardiovascular disease (CVD). Studies suggest that periodontitis, like other CVD risk factors, shows the persistence of increased CVD risk even after mitigation. We hypothesized that periodontitis induces epigenetic changes in hematopoietic stem cells in the bone marrow (BM), and such changes persist after the clinical elimination of the disease and underlie the increased CVD risk. We used a BM transplant approach to simulate the clinical elimination of periodontitis and the persistence of the hypothesized epigenetic reprogramming. Using the low-density lipoprotein receptor knockout (LDLR^o ) atherosclerosis mouse model, BM donor mice were fed a high-fat diet to induce atherosclerosis and orally inoculated with Porphyromonas gingivalis (Pg), a keystone periodontal pathogen; the second group was sham-inoculated. Naïve LDLR ^o mice were irradiated and transplanted with BM from one of the two donor groups. Recipients of BM from Pg-inoculated donors developed significantly more atherosclerosis, accompanied by cytokine/chemokines that suggested BM progenitor cell mobilization and were associated with atherosclerosis and/or PD. Using whole-genome bisulfite sequencing, 375 differentially methylated regions (DMRs) and global hypomethylation in recipients of BM from Pg-inoculated donors were observed. Some DMRs pointed to the involvement of enzymes with major roles in DNA methylation and demethylation. In validation assays, we found a significant increase in the activity of ten-eleven translocase-2 and a decrease in the activity of DNA methyltransferases. Plasma S-adenosylhomocysteine levels were significantly higher, and the S-adenosylmethionine to S-adenosylhomocysteine ratio was decreased, both of which have been associated with CVD. These changes may be related to increased oxidative stress as a result of Pg infection. These data suggest a novel and paradigm-shifting mechanism in the long-term association between periodontitis and atherosclerotic CVD.

Accelerated capacity of glutamate uptake via blood elements as a possible tool of rapid remote conditioning mediated tissue protection

Recently, the function of blood cells in remote ischemic conditioning (RIC) mediated neuroprotection was undoubtedly confirmed. In the present paper, we have focused on the role of blood elements in glutamate homeostasis. The blood of remote conditioned (tolerant) animals was incubated ex vivo with 100 μM glutamate, and the quantitative and qualitative changes of excitatory amino acid transporters (EAAT 1, 2, and 3) were determined. We confirmed RIC mediated accelerated sequestration of extracellular glutamate via EAATs and altered distribution of that amino acid between plasma and cell elements compared to non-tolerant counterparts. The activity of EAATs was elevated in erythrocytes and monocytes, while the density of transporters was not affected. Quantitative changes of EAAT1 density were detected solely in platelets where the forced scavenging was independent of EAATs inhibition. Surprisingly, the trafficking of immunovisualised EAAT2 and 3 raised at tolerant erythrocytes and monocytes. We have found that protein synthesis underlined this process. On the other hand, depletion of protein synthesis did not significantly affect the scavenging capacity of those cell populations. Our work has demonstrated that the elevated blood scavenging of glutamate overdose could be one of the potential mechanisms underlying RIC mediated tissue protection.

Neuroimmune System as a Driving Force for Plasticity Following CNS Injury

Following an injury to the central nervous system (CNS), spontaneous plasticity is observed throughout the neuraxis and affects multiple key circuits. Much of this spontaneous plasticity can elicit beneficial and deleterious functional outcomes, depending on the context of plasticity and circuit affected. Injury-induced activation of the neuroimmune system has been proposed to be a major factor in driving this plasticity, as neuroimmune and inflammatory factors have been shown to influence cellular, synaptic, structural, and anatomical plasticity. Here, we will review the mechanisms through which the neuroimmune system mediates plasticity after CNS injury. Understanding the role of specific neuroimmune factors in driving adaptive and maladaptive plasticity may offer valuable therapeutic insight into how to promote adaptive plasticity and/or diminish maladaptive plasticity, respectively.

Solute carrier transporters: the metabolic gatekeepers of immune cells

Solute carrier (SLC) transporters meditate many essential physiological functions, including nutrient uptake, ion influx/efflux, and waste disposal. In its protective role against tumors and infections, the mammalian immune system coordinates complex signals to support the proliferation, differentiation, and effector function of individual cell subsets. Recent research in this area has yielded surprising findings on the roles of solute carrier transporters, which were discovered to regulate lymphocyte signaling and control their differentiation, function, and fate by modulating diverse metabolic pathways and balanced levels of different metabolites. In this review, we present current information mainly on glucose transporters, amino-acid transporters, and metal ion transporters, which are critically important for mediating immune cell homeostasis in many different pathological conditions.

Glutamate Transport and Preterm Brain Injury

Preterm birth complications are the leading cause of child death worldwide and a top global health priority. Among the survivors, the risk of life-long disabilities is high, including cerebral palsy and impairment of movement, cognition, and behavior. Understanding the molecular mechanisms of preterm brain injuries is at the core of future healthcare improvements. Glutamate excitotoxicity is a key mechanism in preterm brain injury, whereby the accumulation of extracellular glutamate damages the delicate immature oligodendrocytes and neurons, leading to the typical patterns of injury seen in the periventricular white matter. Glutamate excitotoxicity is thought to be induced by an interaction between environmental triggers of injury in the perinatal period, particularly cerebral hypoxia-ischemia and infection/inflammation, and developmental and genetic vulnerabilities. To avoid extracellular build-up of glutamate, the brain relies on rapid uptake by sodium-dependent glutamate transporters. Astrocytic excitatory amino acid transporter 2 (EAAT2) is responsible for up to 95% of glutamate clearance, and several lines of evidence suggest that it is essential for brain functioning. While in the adult EAAT2 is predominantly expressed by astrocytes, EAAT2 is transiently upregulated in the immature oligodendrocytes and selected neuronal populations during mid-late gestation, at the peak time for preterm brain injury. This developmental upregulation may interact with perinatal hypoxia-ischemia and infection/inflammation and contribute to the selective vulnerability of the immature oligodendrocytes and neurons in the preterm brain. Disruption of EAAT2 may involve not only altered expression but also impaired function with reversal of transport direction. Importantly, elevated EAAT2 levels have been found in the reactive astrocytes and macrophages of human infant post-mortem brains with severe white matter injury (cystic periventricular leukomalacia), potentially suggesting an adaptive mechanism against excitotoxicity. Interestingly, EAAT2 is suppressed in animal models of acute hypoxic-ischemic brain injury at term, pointing to an important and complex role in newborn brain injuries. Enhancement of EAAT2 expression and transport function is gathering attention as a potential therapeutic approach for a variety of adult disorders and awaits exploration in the context of the preterm brain injuries.

Variability with Astroglial Glutamate Transport Genetics Is Associated with Increased Risk for Post-Traumatic Seizures

Excitotoxicity contributes to epileptogenesis after severe traumatic brain injury (sTBI). Demographic and clinical risk factors for post-traumatic seizures (PTS) have been identified, but genetic risk remains largely unknown. Thus, we investigated whether genetic variation in astroglial glutamate transporter genes is associated with accelerated epileptogenesis and PTS risk after sTBI. Adults (n = 267) 18-75 years old were assessed over a three-year period post-TBI. Single nucleotide polymorphisms (SNPs) throughout the SLC1A2 and SLC1A3 genes were assayed. Kaplan-Meier estimates and log-rank statistics were used to compare seizure frequencies by genotype. Multivariate Cox proportional hazards regression was used to estimate hazard ratios (HRs) for genotypes significant in Kaplan-Meier analyses. Thirty-nine tagging SNPs were examined (SLC1A2: n = 21, SLC1A3: n = 18). PTS developed in 57 (21.4%) individuals. Of those with PTS, n = 20 (35.7%) had an immediate/early seizure within the first seven days, and n = 36 (64.3%) had a late seizure occurring between eight days and three years post-TBI. When adjusting for multiple comparisons, rs4869682 genotypes (SLC1A3, GG vs. T-carriers) were associated with time to first seizure (p = 0.003). Median time until first seizure was 20.4 days for individuals with a GG genotype and 44.8 days for T-carriers. After adjusting for covariates, rs4869682 GG-homozygotes had a 2.05 times increased PTS risk versus T-carriers (aHR = 2.08, 95% confidence interval: 1.20, 3.62, p = 0.009). Variation within SLC1A3 is associated with accelerated epileptogenesis and clinical PTS development after sTBI. Future studies should validate these findings and examine how genetic variation at rs4869682 may be a target for PTS prevention and treatment.

The Glutamatergic System in Primary Somatosensory Neurons and Its Involvement in Sensory Input-Dependent Plasticity

Glutamate is the most common neurotransmitter in both the central and the peripheral nervous system. Glutamate is present in all types of neurons in sensory ganglia, and is released not only from their peripheral and central axon terminals but also from their cell bodies. Consistently, these neurons express ionotropic and metabotropic receptors, as well as other molecules involved in the synthesis, transport and release of the neurotransmitter. Primary sensory neurons are the first neurons in the sensory channels, which receive information from the periphery, and are thus key players in the sensory transduction and in the transmission of this information to higher centers in the pathway. These neurons are tightly enclosed by satellite glial cells, which also express several ionotropic and metabotropic glutamate receptors, and display increases in intracellular calcium accompanying the release of glutamate. One of the main interests in our group has been the study of the implication of the peripheral nervous system in sensory-dependent plasticity. Recently, we have provided novel evidence in favor of morphological changes in first- and second-order neurons of the trigeminal system after sustained alterations of the sensory input. Moreover, these anatomical changes are paralleled by several molecular changes, among which those related to glutamatergic neurotransmission are particularly relevant. In this review, we will describe the state of the art of the glutamatergic system in sensory ganglia and its involvement in input-dependent plasticity, a fundamental ground for advancing our knowledge of the neural mechanisms of learning and adaptation, reaction to injury, and chronic pain.

The role of system Xc- in methamphetamine-induced dopaminergic neurotoxicity in mice

The cystine/glutamate antiporter (system Xc^-, Sxc) transports cystine into cell in exchange for glutamate. Since xCT is a specific subunit of Sxc, we employed xCT knockout mice and investigated whether this antiporter affected methamphetamine (MA)-induced dopaminergic neurotoxicity. MA treatment significantly increased striatal oxidative burdens in wild type mice. xCT inhibitor [i.e., S-4-carboxy-phenylglycine (CPG), sulfasalazine] or an xCT knockout significantly protected against these oxidative burdens. MA-induced increases in Iba-1 expression and Iba-1-labeled microglial immunoreactivity (Iba-1-IR) were significantly attenuated by CPG or sulfasalazine administration or xCT knockout. CPG or sulfasalazine significantly attenuated MA-induced TUNEL-positive cell populations in the striatum of Taconic ICR mice. The decrease in excitatory amino acid transporter-2 (or glutamate transporter-1) expression and increase in glutamate release were attenuated by CPG, sulfasalazine or xCT knockout. In addition, CPG, sulfasalazine or xCT knockout significantly protected against dopaminergic loss (i.e., decreases in tyrosine hydroxylase expression and immunoreactivity, and an increase in dopamine turnover rate) induced by MA. However, CPG, sulfasalazine or xCT knockout did not significantly affect the impaired glutathione system [i.e., decrease in reduced glutathione (GSH) and increase in oxidized glutathione (GSSG)] induced by MA. Our results suggest that Sxc mediates MA-induced neurotoxicity via facilitating oxidative stress, microgliosis, proapoptosis, and glutamate-related toxicity.

Metabotropic glutamate receptor 5 may be involved in macrophage plasticity

Background

Macrophages are a functionally heterogeneous cell population and depending on microenvironments they polarize in two main groups: M1 and M2. Glutamic acid and glutamate receptors may participate in the regulation of macrophage plasticity. To investigate the role of glutamatergic systems in macrophages physiology, we performed the transfection of mGluR5 cDNAs into RAW-264.7 cells.

Results

Comparative analysis of modified (RAW-mGluR5 macrophages) and non-modified macrophages (RAW-macrophages) has shown that the RAW-mGluR5 macrophages absorbed more glutamate than control cells and the amount of intracellular glutamate correlated with the expression of excitatory amino acid transporters -2 (EAAT-2). Besides, our results have shown that RAW-mGluR5 macrophages expressed a higher level of peroxisome proliferator-activated receptor γ (PPAR-γ) and secreted more IL-10, high mobility group box 1 proteins (HMGB1) and Galectin-3 than control RAW-macrophages.

Conclusions

We propose that elevation of intracellular glutamate and expression of mGluR5 may initiate the metabolic rearrangement in macrophages that could contribute to the formation of an immunosuppressive phenotype.

Neuroinflammation: the devil is in the details

There is significant interest in understanding inflammatory responses within the brain and spinal cord. Inflammatory responses that are centralized within the brain and spinal cord are generally referred to as 'neuroinflammatory'. Aspects of neuroinflammation vary within the context of disease, injury, infection, or stress. The context, course, and duration of these inflammatory responses are all critical aspects in the understanding of these processes and their corresponding physiological, biochemical, and behavioral consequences. Microglia, innate immune cells of the CNS, play key roles in mediating these neuroinflammatory responses. Because the connotation of neuroinflammation is inherently negative and maladaptive, the majority of research focus is on the pathological aspects of neuroinflammation. There are, however, several degrees of neuroinflammatory responses, some of which are positive. In many circumstances including CNS injury, there is a balance of inflammatory and intrinsic repair processes that influences functional recovery. In addition, there are several other examples where communication between the brain and immune system involves neuroinflammatory processes that are beneficial and adaptive. The purpose of this review is to distinguish different variations of neuroinflammation in a context-specific manner and detail both positive and negative aspects of neuroinflammatory processes. In this review, we will use brain and spinal cord injury, stress, aging, and other inflammatory events to illustrate the potential harm and benefits inherent to neuroinflammation. Context, course, and duration of the inflammation are highly important to the interpretation of these events, and we aim to provide insight into this by detailing several commonly studied insults. This article is part of the 60th anniversary supplemental issue.

Glioblastoma cells induce differential glutamatergic gene expressions in human tumor-associated microglia/macrophages and monocyte-derived macrophages

Glioblastoma cells produce and release high amounts of glutamate into the extracellular milieu and subsequently can trigger seizure in patients. Tumor-associated microglia/macrophages (TAMs), consisting of both parenchymal microglia and monocytes-derived macrophages (MDMs) recruited from the blood, are known to populate up to 1/3 of the glioblastoma tumor environment and exhibit an alternative, tumor-promoting and supporting phenotype. However, it is unknown how TAMs respond to the excess extracellular glutamate in the glioblastoma microenvironment. We investigated the expressions of genes related to glutamate transport and metabolism in human TAMs freshly isolated from glioblastoma resections. Quantitative real-time PCR analysis showed (i) significant increases in the expressions of GRIA2 (GluA2 or AMPA receptor 2), SLC1A2 (EAAT2), SLC1A3 (EAAT1), (ii) a near-significant decrease in the expression of SLC7A11 (cystine-glutamate antiporter xCT) and (iii) a remarkable increase in GLUL expression (glutamine synthetase) in these cells compared to adult primary human microglia. TAMs co-cultured with glioblastoma cells also exhibited a similar glutamatergic profile as freshly isolated TAMs except for a slight increase in SLC7A11 expression. We next analyzed these genes expressions in cultured human MDMs derived from peripheral blood monocytes for comparison. In contrast, MDMs co-cultured with glioblastoma cells compared to MDMs co-cultured with normal astrocytes exhibited decreased expressions in the tested genes except for GLUL. This is the first study to demonstrate transcriptional changes in glutamatergic signaling of TAMs in a glioblastoma microenvironment, and the findings here suggest that TAMs and MDMs might potentially elicit different cellular responses in the presence of excess extracellular glutamate.

The transcriptome of Legionella pneumophila-infected human monocyte-derived macrophages

Background

Legionella pneumophila is an intracellular bacterial pathogen that invades and replicates within alveolar macrophages through injection of ∼300 effector proteins by its Dot/Icm type IV translocation apparatus. The bona fide F-box protein, AnkB, is a nutritional virulence effector that triggers macrophages to generate a surplus of amino acids, which is essential for intravacuolar proliferation. Therefore, the ankB mutant represents a novel genetic tool to determine the transcriptional response of human monocyte-derived macrophages (hMDMs) to actively replicating L. pneumophila.

Methodology/Principal Findings

Here, we utilized total human gene microarrays to determine the global transcriptional response of hMDMs to infection by wild type or the ankB mutant of L. pneumophila. The transcriptomes of hMDMs infected with either actively proliferating wild type or non-replicative ankB mutant bacteria were remarkably similar. The transcriptome of infected hMDMs was predominated by up-regulation of inflammatory pathways (IL-10 anti-inflammatory, interferon signaling and amphoterin signaling), anti-apoptosis, and down-regulation of protein synthesis pathways. In addition, L. pneumophila modulated diverse metabolic pathways, particularly those associated with bio-active lipid metabolism, and SLC amino acid transporters expression.

Conclusion/Significance

Taken together, the hMDM transcriptional response to L. pneumophila is independent of intra-vacuolar replication of the bacteria and primarily involves modulation of the immune response and metabolic as well as nutritional pathways.

The therapeutic role of interleukin-10 after spinal cord injury

Spinal cord injury (SCI) is a devastating condition affecting 270,000 people in the United States. A potential treatment for decreasing the secondary inflammation, excitotoxic damage, and neuronal apoptosis associated with SCI, is the anti-inflammatory cytokine interleukin-10. The best characterized effects of IL-10 are anti-inflammatory-it downregulates pro-inflammatory species interleukin-1β (IL-1β), interleukin-2 (IL-2), interleukin-6 (IL-6), tumor necrosis factor-α, interferon-γ, matrix metalloproteinase-9, nitric oxide synthase, myeloperoxidase, and reactive oxygen species. Pro-apoptotic factors cytochrome c, caspase 3, and Bax are downregulated by IL-10, whereas anti-apoptotic factors B-cell lymphoma 2 (Bcl-2) and Bcl-2-associated X, B-cell lymphoma-extra large (Bcl-xl) are upregulated by IL-10. IL-10 also provides trophic support to neurons through the IL-10 receptor. Increased tissue sparing, functional recovery, and neuroprotection are seen with an immediate post-SCI systemic administration of IL-10. Treatment of SCI with IL-10 has been used successfully in combination with Schwann cell and olfactory glial cell grafts, as well as methylprednisolone. Minocycline, tetramethylpyrazine, and hyperbaric oxygen treatment all increase IL-10 levels in a SCI models and result in increased tissue sparing and functional recovery. A chronic systemic administration of IL-10 does not appear to be beneficial to SCI recovery and causes increased susceptibility to septicemia, pneumonia, and peripheral neuropathy. However, a localized upregulation of IL-10 has been shown to be beneficial and can be achieved by herpes simplex virus gene therapy, injection of poliovirus replicons, or surgical placement of a slow-release compound. IL-10 shows promise as a treatment for SCI, although research on local IL-10 delivery timeline and dosage needs to be expanded.

The cystine/glutamate antiporter system x(c)(-) in health and disease: from molecular mechanisms to novel therapeutic opportunities

The antiporter system x(c)(-) imports the amino acid cystine, the oxidized form of cysteine, into cells with a 1:1 counter-transport of glutamate. It is composed of a light chain, xCT, and a heavy chain, 4F2 heavy chain (4F2hc), and, thus, belongs to the family of heterodimeric amino acid transporters. Cysteine is the rate-limiting substrate for the important antioxidant glutathione (GSH) and, along with cystine, it also forms a key redox couple on its own. Glutamate is a major neurotransmitter in the central nervous system (CNS). By phylogenetic analysis, we show that system x(c)(-) is a rather evolutionarily new amino acid transport system. In addition, we summarize the current knowledge regarding the molecular mechanisms that regulate system x(c)(-), including the transcriptional regulation of the xCT light chain, posttranscriptional mechanisms, and pharmacological inhibitors of system x(c)(-). Moreover, the roles of system x(c)(-) in regulating GSH levels, the redox state of the extracellular cystine/cysteine redox couple, and extracellular glutamate levels are discussed. In vitro, glutamate-mediated system x(c)(-) inhibition leads to neuronal cell death, a paradigm called oxidative glutamate toxicity, which has successfully been used to identify neuroprotective compounds. In vivo, xCT has a rather restricted expression pattern with the highest levels in the CNS and parts of the immune system. System x(c)(-) is also present in the eye. Moreover, an elevated expression of xCT has been reported in cancer. We highlight the diverse roles of system x(c)(-) in the regulation of the immune response, in various aspects of cancer and in the eye and the CNS.

Functional expression of system x(c)- is upregulated by asbestos but not crystalline silica in murine macrophages

CONTEXT

Inhalation of asbestos or silica is associated with chronic and progressive diseases, including fibrosis, cancer, and increased risk of systemic autoimmunity. Because there is a need for treatment options for these diseases, a better understanding of their mechanistic etiologies is essential. While oxidative stress in macrophages is an early consequence of these exposures, it may also serve as a signaling mechanism involved in downstream immune dysregulation. The system x(c)(-) exchange protein is induced by oxidative stress, and exchanges equimolor levels of extracellular cystine for intracellular glutamate. Cystine is subsequently reduced to cysteine, the rate-limiting precursor for glutathione synthesis.

OBJECTIVE

As the primary transporter responsible for cystine/glutamate exchange on macrophages, system x(c)- was hypothesized to be inducible in response to asbestos and silica, and to increase viability through protection from oxidative stress.

RESULTS

When challenged with amphibole asbestos, but not crystalline silica, RAW 264.7 macrophages increased expression of xCT and the rate of cystine/glutamate exchange in sodium-free conditions. This upregulation was prevented with N-acetylcysteine, implicating oxidative stress. Cystine protected the macrophages from asbestos-induced oxidative stress and cell death, supporting the hypothesis that imported cystine was used for synthesis of cellular antioxidants. System x(c)(-) inhibitors, glutamate and S-4-carboxyphenylglycine ((S)-4-CPG), significantly increased oxidative stress and cell death of asbestos-treated macrophages.

CONCLUSION

System x(c)(-) plays a critical role in survival of macrophages exposed to asbestos, but not silica. These data demonstrate a very early difference in the cellular response to these silicates that may have important downstream implications in the pathologic outcome of exposure.

In vitro beneficial activation of microglial cells by mechanically-injured astrocytes enhances the synthesis and secretion of BDNF through p38MAPK

It has long been promulgated that microglial cells serve beneficial roles in the central nervous system (CNS). The beneficial role of microglial cells is considered to be linked with microglial activation and consequent up-regulation of various trophic factors. However, what triggers microglial activation and consequent elevated level of trophic factors, especially brain-derived neurotrophic factor (BDNF), following traumatic CNS injury has become a crucial but elusive issue. Furthermore, an effort still remains in understanding of the cellular and molecular mechanisms underlying the endogenous neuroprotection of activated microglial cells. In this study, we demonstrated that mechanically-injured astrocyte conditioned medium (ACM) could provoke beneficial activation of microglial cells and thus promote the transcription, synthesis and release of BDNF in cultured microglial cells. The microglia-derived BDNF can exerted a demonstrable biological role in promoting neurite outgrowth and intimate terminal contacts of dorsal root ganglion (DRG) neurons co-cultured with microglial cells. Moreover, ACM induced remarkable p38MAPK phosphorylation in cultured microglial cells that preceded the burst of BDNF. Activating p38-MAPK by anisomycin resulted in salutary effects similar to those seen with ACM, whereas specific inhibition of the p38MAPK by SB203580 abrogated all the positive effects of ACM, including BDNF promotion and subsequent neurite outgrowth of DRG neurite outgrowth of DRG neurons and their intimate terminal contacts with microglial cells. Together, our results indicated that the neuroprotection of the microglial source is mainly caused by micro-environmental soluble molecules released from injured astrocytes, and ACM-induced BDNF production and release from microglial cells may be mediated through p38-MAPK signaling pathway. Therefore, these findings may lay a foundation to further investigations on the microglial beneficial activation role in the repair of traumatic CNS injury and neurodegenerative diseases.

Human immunodeficiency virus viral protein R as an extracellular protein in neuropathogenesis

Numerous studies published in the past two decades have identified the viral protein R (Vpr) as one of the most versatile proteins in the life cycle of human immunodeficiency virus type 1 (HIV-1). In this regard, more than a thousand Vpr molecules are present in extracellular viral particles. Subsequent to viral entry, Vpr participates in early replicative events by assisting in viral genome nuclear import and, during the viral life cycle, by shuttling between the nucleus and the cytoplasm to accomplish its functions within the context of other replicative functions. Additionally, several studies have implicated Vpr as a proapoptotic protein because it promotes formation of permeability transition pores in mitochondria, which in turn affects transmembrane potential and adenosine triphosphate synthesis. Recent studies have identified Vpr as a virion-free protein in the serum and cerebrospinal fluid of patients infected with HIV-1 whose plasma viremia directly correlates with the extracellular concentration of Vpr. These observations pointed to a new role for Vpr as an additional weapon in the HIV-1 arsenal, involving the use of an extracellular protein to target and possibly inhibit HIV-1-uninfected bystander cells to enable them to escape immune surveillance. In addition, extracellular Vpr decreases adenosine triphosphate levels and affects the intracellular redox balance in neurons, ultimately causing their apoptosis. Herein, we review the role of Vpr as an extracellular protein and its downstream effects on cellular metabolism, functionality, and survival, with particular emphasis on how extracellular Vpr-induced oxidative stress might aggravate HIV-1-induced symptoms, thus affecting pathogenesis and disease progression.

Refractory epilepsy associated with microglial activation

BACKGROUND

Experimental and clinical studies support a pathogenic role of microglial activation and proliferation (MAP) in epileptogenesis.

METHODS

From a consecutive series of 319 surgically treated epilepsy cases, we retrospectively reviewed the histopathological sections of 92 cases to define the prevalence and severity of MAP after excluding the other 227 because of coexisting disorders that might contribute to MAP. Severity of MAP was compared with underlying abnormalities. We assessed the response to intravenous immunoglobulin and plasmapheresis in one patient with severe MAP who had failed multiple antiepileptic drugs and epilepsy surgery.

RESULTS

MAP was detected with routine (hematoxylin and eosin) stain in 46 of 92 cases (50%). MAP was mild in 32 cases (69.6%), moderate in 12 (26.1%), and severe in 2 (4.3%). The prevalence and severity of MAP were independent of underlying abnormalities. Immunomodulatory therapy was followed by a greater than 90% reduction in seizure activity in the treated patient.

CONCLUSION

MAP is prevalent in resected human epilepsy tissue. Failure to down-regulate MAP contributes to chronic neuronal hyperexcitability. We hypothesize that MAP initiates a cycle of inflammation-induced seizures and seizure-induced inflammation. Microglia-driven epilepsy may be a primary pathogenic process in a small number of cases, as suggested by the pathology and therapeutic response in our patient, but may contribute to epileptogenesis in many more.

Microglial self-defence mediated through GLT-1 and glutathione

Glutamate is stored in synaptic vesicles in presynaptic neurons. It is released into the synaptic cleft to provide signalling to postsynaptic neurons. Normally, the astroglial glutamate transporters GLT-1 and GLAST take up glutamate to mediate a high signal-to-noise ratio in the synaptic signalling, and also to prevent excitotoxic effects by glutamate. In astrocytes, glutamate is transformed into glutamine, which is safely transported back to neurons. However, in pathological conditions, such as an ischemia or virus infection, astroglial transporters are down-regulated which could lead to excitotoxicity. Lately, it was shown that even microglia can express glutamate transporters during pathological events. Microglia have two systems for glutamate transport: GLT-1 for transport into the cells and the x (c) (-) system for transport out of the cells. We here review results from our work and others, which demonstrate that microglia in culture express GLT-1, but not GLAST, and transport glutamate from the extracellular space. We also show that TNF-α can induce increased microglial GLT-1 expression, possibly associating the expression with inflammatory systems. Furthermore, glutamate taken up through GLT-1 may be used for direct incorporation into glutathione and to fuel the intracellular glutamate pool to allow cystine uptake through the x (c) (-) system. This can lead to a defence against oxidative stress and have an antiviral function.

EAAT expression by macrophages and microglia: still more questions than answers

Glutamate is the main excitatory amino acid, but its presence in the extracellular milieu has deleterious consequences. It may induce excitotoxicity and also compete with cystine for the use of the cystine-glutamate exchanger, blocking glutathione neosynthesis and inducing an oxidative stress-induced cell death. Both mechanisms are critical in the brain where up to 20% of total body oxygen consumption occurs. In normal conditions, the astrocytes ensure that extracellular concentration of glutamate is kept in the micromolar range, thanks to their coexpression of high-affinity glutamate transporters (EAATs) and glutamine synthetase (GS). Their protective function is nevertheless sensitive to situations such as oxidative stress or inflammatory processes. On the other hand, macrophages and microglia do not express EAATs and GS in physiological conditions and are the principal effector cells of brain inflammation. Since the late 1990s, a number of studies have now shown that both microglia and macrophages display inducible EAAT and GS expression, but the precise significance of this still remains poorly understood. Brain macrophages and microglia are sister cells but yet display differences. Both are highly sensitive to their microenvironment and can perform a variety of functions that may oppose each other. However, in the very particular environment of the healthy brain, they are maintained in a repressed state. The aim of this review is to present the current state of knowledge on brain macrophages and microglial cells activation, in order to help clarify their role in the regulation of glutamate under pathological conditions as well as its outcome.

The cystine/glutamate antiporter regulates dendritic cell differentiation and antigen presentation

The major cellular antioxidant glutathione is depleted during HIV infection and in obesity. Although the consequence of glutathione depletion on immune function is starting to emerge, it is currently not known whether glutathione dysregulation influences the differentiation and maturation of dendritic cells (DCs). Moreover, the effect of glutathione depletion on DC effector functions, such as Ag presentation, is poorly understood. Glutathione synthesis depends on the cystine/glutamate antiporter, which transports the rate-limiting precursor cystine into the cell in exchange for glutamate. In this paper, we present a detailed study of antiporter function in DCs and demonstrate a role for the antiporter in DC differentiation and cross-presentation. We show that the antiporter is the major mechanism for transport of cystine and glutamate and modulates the intracellular glutathione content and glutathione efflux from DCs. Blocking antiporter-dependent cystine transport decreases intracellular glutathione levels, and these effects correlate with reduced transcription of the functional subunit of the antiporter. We further demonstrate that blocking antiporter activity interferes with DC differentiation from monocyte precursors, but antiporter activity is not required for LPS-induced phenotypic maturation. Finally, we show that inhibiting antiporter uptake of cystine interferes with presentation of exogenous Ag to class II MHC-restricted T cells and blocks cross-presentation on MHC class I. We conclude that aberrant antiporter function disrupts glutathione homeostasis in DCs and may contribute to impaired immunity in the diseased host.

Microglial ablation and lipopolysaccharide preconditioning affects pilocarpine-induced seizures in mice

Activated microglia have been associated with neurodegeneration in patients and in animal models of Temporal Lobe Epilepsy (TLE), however their precise functions as neurotoxic or neuroprotective is a topic of significant investigation. To explore this, we examined the effects of pilocarpine-induced seizures in transgenic mice where microglia/macrophages were conditionally ablated. We found that unilateral ablation of microglia from the dorsal hippocampus did not alter acute seizure sensitivity. However, when this procedure was coupled with lipopolysaccharide (LPS) preconditioning (1 mg/kg given 24 h prior to acute seizure), we observed a significant pro-convulsant phenomenon. This effect was associated with lower metabolic activation in the ipsilateral hippocampus during acute seizures, and could be attributed to activity in the mossy fiber pathway. These findings reveal that preconditioning with LPS 24 h prior to seizure induction may have a protective effect which is abolished by unilateral hippocampal microglia/macrophage ablation.

FTY720 reduces inflammation and promotes functional recovery after spinal cord injury

A robust and complex inflammatory cascade is known to be a prominent component of secondary injury following spinal cord injury (SCI). Specifically, the concept of trauma-induced autoimmunity has linked the lymphocyte population with neural tissue injury and neurologic deficit. FTY720, a sphingosine receptor modulator that sequesters lymphocytes in secondary lymphoid organs, has been shown to be effective in the treatment of a variety of experimental autoimmune disorders. Accordingly, by reducing lymphocyte infiltration into the spinal cord following SCI, this novel immunomodulator may enhance tissue preservation and functional recovery. In the present study, a moderate to severe contusion SCI was simulated in adult Long-Evans hooded rats. Using flow cytometry we showed that daily FTY720 treatment dramatically reduced T-cell infiltration into the SCI lesion site at 4 and 7 days post-injury, while other inflammatory cell populations were relatively unaltered. To assess functional recovery, three groups of injured animals (treated, vehicle, and injury only) were evaluated weekly for hindlimb recovery. Animals in the treated group consistently exhibited higher functional scores than animals in the control groups after 2 weeks post-injury. This finding was associated with a greater degree of white matter sparing at the lesion epicenter when cords were later sectioned and stained. Furthermore, treated animals were found to exhibit improved bladder function and a reduced incidence of hemorrhagic cystitis compared to control counterparts. Collectively these results demonstrate the neuroprotective potential of FTY720 treatment after experimental SCI.

Transcriptional response in the peripheral blood of patients infected with Salmonella enterica serovar Typhi

We used microarrays and transcriptional profiling of peripheral blood to investigate the host response of 29 individuals who contracted typhoid fever in the Mekong Delta region of Vietnam. Samples were taken over a nine month period encompassing acute disease, convalescence, and recovery. We found that typhoid fever induced a distinct and highly reproducible signature in the peripheral blood that changed during treatment and convalescence, returning in the majority of cases to the "normal" profile as measured in healthy uninfected controls. Unexpectedly, there was a strong, distinct signature of convalescence present at day 9 after infection that remained virtually unchanged one month after acute infection and in some cases persisted as long as nine months despite a complete clinical recovery in all patients. Patients who retain the convalescent signature may be genetically or temporarily incapable of developing an effective immune response and may be more susceptible to reinfection, relapse, or the establishment of a carrier state.

Neuroprotective immunity: T cell-derived glutamate endows astrocytes with a neuroprotective phenotype

A well-controlled T cell response to CNS injury may result in increased neuronal survival. However, the precise mechanism of T cell-induced neuroprotection is unknown. In this study, we report the unexpected finding that during culture of T cells, high levels of glutamate accumulate, which are efficiently cleared if T cells are cocultured with astrocytes. The T cell-derived glutamate elicits in turn, the release of neuroprotective thiols (cysteine, glutathione, and cysteinyl-glycine) and lactate from astrocytes. Media obtained from astrocytes conditioned in the presence of T cells reduce neuronal apoptosis induced by oxidative stress in primary neuronal cultures from 48 +/- 14 to 9 +/- 4% (p < 0.001). Inhibition of glutamate-dependent signaling during astrocyte-T cell cocultivation by a glutamate uptake inhibitor, l-aspartic acid beta-hydroxamate, abolishes this neuroprotective effect. The ability of astrocytes to clear extracellular glutamate is impaired under conditions of oxidative stress. We demonstrate that T cells, via secreted cytokines, restore glutamate clearance capacity of astrocytes under oxidative conditions. Furthermore, under normoxic conditions, glutamate-buffering capacity of astrocytes is increased upon cocultivation with T cells. It is known that, following CNS injury, astrocytes can respond with beneficial or destructive effects on neurons. However, the context and signaling mechanisms for this dual astrocytic response are unknown. Our results implicate T cells as potential determinants of the context that elicits a protective role for astrocytes in the damaged CNS.

Glutamate transporter EAAT2 expression is up-regulated in reactive astrocytes in human periventricular leukomalacia

The major neuropathological correlate of cerebral palsy in premature infants is periventricular leukomalacia (PVL), a disorder of the immature cerebral white matter. Cerebral ischemia leading to excitotoxicity is thought to be important in the pathogenesis of this disorder, implying a critical role for glutamate transporters, the major determinants of extracellular glutamate concentration. Previously, we found that EAAT2 expression is limited primarily to premyelinating oligodendrocytes early in development and is rarely observed in astrocytes until >40 weeks. In this study, we analyzed the expression of EAAT2 in cerebral white matter from PVL and control cases. Western blot analysis suggested an up-regulation of EAAT2 in PVL compared with control cases. Single- and double-label immunocytochemistry showed a significantly higher percentage of EAAT2-immunopositive astrocytes in PVL (51.8% +/- 5.6%) compared with control white matter (21.4% +/- 5.6%; P = 0.004). Macrophages in the necrotic foci in PVL also expressed EAAT2. Premyelinating oligodendrocytes in both PVL and control cases expressed EAAT2, without qualitative difference in expression. The previously unrecognized up-regulation of EAAT2 in reactive astrocytes and its presence in macrophages in PVL reported here may reflect a response to either hypoxic-ischemic injury or inflammation.

Serotonin decreases HIV-1 replication in primary cultures of human macrophages through 5-HT(1A) receptors

BACKGROUND AND PURPOSE

5-HT (serotonin) is known to be involved in neuroinflammation and immunoregulation. The human immunodeficiency virus (HIV) targets cells such as monocytes/macrophages, which colocalize with 5-HT-releasing cell types, mostly platelets. In this study, we investigated the effects of 5-HT on HIV-1-infected macrophages in vitro.

EXPERIMENTAL APPROACH

Human macrophages cultured in serum-free medium were treated over 7 days with 5-HT at three concentrations (0.01, 1 and 100 microM) with or without agonists and antagonists of 5-HT(1A) and 5-HT(2) receptors. After 7 days of treatment, macrophages were infected with HIV-1/Ba-L and virus replication was monitored over 16 days and expression of proviral HIV DNA was investigated by PCR after 24 h of infection. Cell surface expression of HIV-1/Ba-L receptor (CD4) and coreceptor (CCR5) was investigated by flow cytometry. The CCR5 ligand, macrophage inflammatory protein-1alpha (MIP-1alpha), was quantified by ELISA in cell culture supernatants and MIP-1alpha mRNA expression was assessed by reverse transcriptase-PCR.

KEY RESULTS

In vitro, 5-HT downregulated the membranous expression of CCR5 and led to a decrease of HIV-1 infection, probably through its action on 5-HT(1A) receptors. 5-HT (100 microM) was also able to induce overexpression of MIP-1alpha mRNA leading to an increase of MIP-1alpha secretion by human macrophages.

CONCLUSIONS AND IMPLICATIONS

The effects of 5-HT on HIV infection could be a consequence of the increase in MIP-1alpha concentrations and/or CCR5 receptor downregulation. These results suggest that 5-HT can inhibit the replication of HIV-1 in primary culture of human macrophages through its action on 5-HT(1A) receptors.

Changes in neurotransmitter levels and proinflammatory cytokine mRNA expressions in the mice olfactory bulb following nanoparticle exposure

Recently, there have been increasing reports that nano-sized component of particulate matter can reach the brain and may be associated with neurodegenerative diseases. Previously, our laboratory has studied the effect of intranasal instillation of nano-sized carbon black (CB) (14 nm and 95 nm) on brain cytokine and chemokine mRNA expressions and found that 14-nm CB increased IL-1 beta, TNF-alpha, CCL2 and CCL3 mRNA expressions in the olfactory bulb, not in the hippocampus of mice. To investigate the effect of a single administration of nanoparticles on neurotransmitters and proinflammatory cytokines in a mouse olfactory bulb, we performed in vivo microdialysis and real-time PCR methods. Ten-week-old male BALB/c mice were implanted with guide cannula in the right olfactory bulb and, 1 week later, were instilled vehicle or CB (14 nm, 250 microg) intranasally. Six hours after the nanoparticle instillation, the mice were intraperitoneally injected with normal saline or 50 mug of bacteria cell wall component lipoteichoic acid (LTA), which may potentiate CB-induced neurologic effect. Extracellular glutamate and glycine levels were significantly increased in the olfactory bulb of CB-instilled mice when compared with vehicle-instilled control mice. Moreover, we found that LTA further increased glutamate and glycine levels. However, no alteration of taurine and GABA levels was observed in the olfactory bulb of the same mice. We also detected immunological changes in the olfactory bulb 11 h after vehicle or CB instillation and found that IL-1 beta mRNA expression was significantly increased in CB- and LTA-treated mice when compared with control group. However, TNF-alpha mRNA expression was increased significantly in CB- and saline-treated mice when compared with control group. These findings suggest that nanoparticle CB may modulate the extracellular amino acid neurotransmitter levels and proinflammatory cytokine IL-1 beta mRNA expressions synergistically with LTA in the mice olfactory bulb.

Inflammation and its role in neuroprotection, axonal regeneration and functional recovery after spinal cord injury

Trauma to the central nervous system (CNS) triggers intraparenchymal inflammation and activation of systemic immunity with the capacity to exacerbate neuropathology and stimulate mechanisms of tissue repair. Despite our incomplete understanding of the mechanisms that control these divergent functions, immune-based therapies are becoming a therapeutic focus. This review will address the complexities and controversies of post-traumatic neuroinflammation, particularly in spinal cord. In addition, current therapies designed to target neuroinflammatory cascades will be discussed.

Mouse monocytes (RAW CELLS) and the handling of cysteine and homocysteine S-conjugates of inorganic mercury and methylmercury

Although there is evidence indicating that mononuclear phagocytes can take up mercury by some forms of endocytosis, very little is known about the potential for the uptake of mercuric species by carrier-mediated processes. Thus, we hypothesized that monocytes also possess mechanisms allowing these cells to take up inorganic mercury (Hg2+) and/or methylmercury (CH3Hg+) as cysteine (Cys) and/or homocysteine (Hcy) S-conjugates by certain membrane transport proteins. The specific thiol S-conjugates were chosen for study because our laboratory and those of some other investigators have demonstrated that these species of mercury are indeed transportable substrates for several membrane transport proteins in certain types of epithelial cells. We chose to use RAW 264.7 cells for our experiments. These cells represent an adherent line of mouse monocytes. Kinetic analyses for the uptake of Cys-Hg-Cys, CH3Hg-Cys, Hcy-Hg-Hcy, and CH3Hg-Hcy revealed that uptake occurred by a saturable, concentration-dependent mechanism, displaying Michaelis-Menten properties. Interestingly, in the cells exposed to the Cys or Hcy S-conjugate of Hg2+, significantly more Hg2+ was taken up in the presence of 140 mM sodium chloride (NaCl) than in the presence of 140 mM N-methyl-D-glucamine (NMDG), indicating that Na-dependent processes play more of a role in the uptake of these species of Hg2+ than sodium-independent ones. With respect to the uptake of CH3Hg+, rates of uptake of the Cys and Hcy S-conjugates of CH3Hg+ were similar under both Na-dependent and Na-independent conditions, although the levels of uptake of these mercuric species far exceeded the levels of uptake of the corresponding S-conjugate of Hg2+. Uptake of Hg2+ and CH3Hg+, as the Cys or Hcy S-conjugates, was also time-dependent. We also showed that when the temperature in the bathing medium was reduced to 4 degrees C, uptake of the Cys S-conjugates Hg2+ or CH3Hg+ was for the most part reduced to negligible levels in the RAW cells; indicating that the preponderance of uptake at 37 degrees C was not due primarily to simple diffusion and/or non-specific binding. Overall, the present findings strongly suggest that the uptake of the Cys and Hcy S-conjugates of Hg2+ and/or CH3Hg+ occurs in monocytes by one or more mechanisms involving carrier proteins.

Microglial GLT-1 is upregulated in response to herpes simplex virus infection to provide an antiviral defence via glutathione

Herpes simplex virus (HSV) can enter the central nervous system and cause encephalitis (HSV-1) or meningitis (HSV-2). Microglia, the immunocompetent cells of the central nervous system, are potentially able to detect viral infections. Microglia have been shown to express the glutamate transporter GLT-1 during pathological events, leading to increased microglial glutamate uptake and glutathione synthesis. This study aims to address the role of GLT-1 and glutathione, a major antioxidant with antiviral properties, during HSV infections. Using neuron-enriched mixed primary cultures from rat, it was found that microglia have higher resistance to HSV infections than neurons or astrocytes after 24 h incubation with HSV. Purified microglia in culture were used to further address this. It was found that microglia were able to detect HSV and responded by releasing tumor necrosis factor-alpha (TNF-alpha) and upregulating GLT-1 after 24 h incubation with 1 PFU/cell HSV-1 or HSV-2. Furthermore, the microglial glutathione levels were not significantly diminished after 24 h. Inhibition of the microglial glutathione synthesis with 200 microM buthionine sulfoximide (BSO) led to significantly more infected cells after 24 h incubation with 1 PFU/cell HSV-1 or HSV-2. These data indicate that the higher resistance in microglia against HSV infections may be due to the expression of GLT-1, which can maintain the glutathione levels and provide a mechanism for microglial self-defense against HSV.

The glutamate-glutamine cycle as an inducible, protective face of macrophage activation

Neuronal damage in HIV infection results mainly from chronic activation of brain tissue and involves inflammation, oxidative stress, and glutamate-related neurotoxicity. Glutamate toxicity acts via two distinct pathways: an excitotoxic one, in which glutamate receptors are hyperactivated, and an oxidative one, in which cystine uptake is inhibited, resulting in glutathione depletion, oxidative stress, and cell degeneration. A number of studies have shown that astrocytes normally take up glutamate, keeping extracellular glutamate concentration low in the brain and preventing excitotoxicity. They, in turn, provide the trophic amino acid glutamine via their expression of glutamine synthetase. These protective and trophic actions are inhibited in HIV infection, probably as a result of the effects of inflammatory mediators and viral proteins. In vitro and in vivo studies have demonstrated that activated microglia and brain macrophages (AMM) express the transporters and enzymes of the glutamate cycle. This suggests that in addition to their recognized neurotoxic properties in HIV infection, these cells exhibit some neuroprotective properties, which may partly compensate for the inhibited astrocytic function. This hypothesis might explain the discrepancy between microglial activation, which occurs early in the disease, and neuronal apoptosis and neuronal loss, which are late events. In this review, we discuss the possible neuroprotective and neurotrophic roles of AMM and their relationships with inflammation and oxidative stress.

Microglial glutamate uptake is coupled to glutathione synthesis and glutamate release

The physiological function of microglial glutamate uptake has been debated as it is about 10% of that measured for astrocytes. This study addresses how glutamate, taken up from the extracellular space, is utilized by microglia. It was found that purified rat microglia incubated for 60 min with (3)H-glutamate had an increased intracellular accumulation of (3)H-glutamate after 12 h incubation with tumour necrosis factor alpha (TNF-alpha) but not after incubation with lipopolysaccharide (LPS). Furthermore, LPS- but not TNF-alpha-treated cells showed an increased efflux of (3)H-labelled compounds, presumably glutamate through the X(C) (-) system and treatment with LPS or TNF-alpha increased the microglial glutathione concentrations and led to an increased incorporation of (3)H-glutamate into glutathione. Depending on the stimuli, 3-6% of the total labelled contents were found in the form of glutathione and 25-35% in the form of glutamate. These results show that microglial glutamate uptake is directly coupled to glutathione synthesis and release of glutamate and/or glutamate metabolites. Additionally, the increased glutathione contents after LPS or TNF-alpha treatment were able to reduce microglial cell death after H(2)O(2) challenge, showing a potential (self)-protective function for microglial glutamate transporter expression and glutathione synthesis.

Distribution of the glutamate transporters GLT-1 (SLC1A2) and GLAST (SLC1A3) in peripheral organs

The glutamate transporters GLT-1 and GLAST are widely expressed in astrocytes in the brain where they fulfill important functions during glutamatergic neurotransmission. The present study examines their distribution in peripheral organs using in situ hybridization (ISH) and immunocytochemistry. GLAST was found to be more widely distributed than GLT-1. GLAST was expressed primarily in epithelial cells, cells of the macrophage-lineage, lymphocytes, fat cells, interstitial cells, and salivary gland acini. GLT-1 was primarily expressed in glandular tissue, including mammary gland, lacrimal gland, and ducts and acini in salivary glands, but also by perivenous hepatocytes and follicular dendritic cells in spleen and lymph nodes. The findings demonstrate that, although expressed by the same cells in the brain, these two glutamate transporters have different distribution patterns in peripheral tissues and that they fulfill glutamate transport functions apart from glutamatergic neurotransmission in these areas.

Group I mGlu receptor stimulation inhibits activation-induced cell death of human T lymphocytes

1. The effects of L-glutamate on activation-induced cell death (AICD) of human activated (1 microg ml(-1) phytohemagglutinin plus 2 U ml(-1) interleukin-2; 8 days) T lymphocytes were studied by measuring anti-CD3 monoclonal antibody (10 microg ml(-1); 18 h)-induced cell apoptosis (Annexin V and propidium iodide staining). 2. L-Glutamate (1 x 10(-8)-1 x 10(-4) M) significantly (P < or = 0.01) inhibited AICD in a concentration-dependent manner (EC50=6.3 x 10(-8) M; maximum inhibition 54.8+/-6.3% at 1 x 10(-6) M). 3. The L-glutamate inhibitory effect was pharmacologically characterized as mediated by group I mGlu receptors, since mGlu receptor agonists reproduced this effect. The EC50 values were: 3.2 x 10(-7) M for (1S,3R)-ACPD; 4.5 x 10(-8) M for quisqualate; 1.0 x 10(-6) M for (S)-3,5-DHPG; 2.0 x 10(-5) M for CHPG. 4. Group I mGlu receptor antagonists inhibited the effects of quisqualate 1.0 x 10(-6) M. The IC50 values calculated were: 8.7 x 10(-5), 4.3 x 10(-6) and 6.3 x 10(-7) M for AIDA, LY 367385 and MPEP, respectively. 5. L-Glutamate (1 x 10(-6) M; 18 h) significantly (P < or = 0.05) inhibited FasL expression (40.8+/-11.3%) (cytofluorimetric analysis), whereas it did not affect Fas signalling. 6. Expression of both mGlu1 and mGlu5 receptor mRNA by T lymphocytes and T-cell lines, as demonstrated by reverse transcriptase-PCR analysis, suggests that L-glutamate-mediated inhibition of AICD was exerted on T cells. 7. These data depict a novel role for L-glutamate in the regulation of the immune response through group I mGlu receptor-mediated mechanisms.

Glutamate metabolism in HIV-infected macrophages: implications for the CNS

Central nervous system disorders are still a common complication of human immunodeficiency virus (HIV) infection and can lead to dementia and death. They are mostly the consequences of an inflammatory macrophagic activation and relate to glutamate-mediated excitotoxicity. However, recent studies also suggest neuroprotective aspects of macrophage activation through the expression of glutamate transporters and glutamine synthetase. We thus aimed to study whether HIV infection or activation of macrophages could modulate glutamate metabolism in these cells. We assessed the effect of HIV infection on glutamate transporter expression as well as on glutamate uptake by macrophages and showed that glutamate transport was partially decreased in the course of virus replication, whereas excitatory amino acid transporter-2 (EAAT-2) gene expression was dramatically increased. The consequences of HIV infection on glutamine synthetase were also measured and for the first time we show the functional expression of this key enzyme in macrophages. This expression was repressed during virus production. We then quantified EAAT-1 and EAAT-2 gene expression as well as glutamate uptake in differentially activated macrophages and show that the effects of HIV are not directly related to pro- or anti-inflammatory mediators. Finally, this study shows that glutamate transport by macrophages is less affected than what has been described in astrocytes. Macrophages may thus play a role in neuroprotection against glutamate in the infected brain, through their expression of both EAATs and glutamine synthetase. Because glutamate metabolism by activated macrophages is sensitive to both HIV infection and inflammation, it may thus be of potential interest as a therapeutic target in HIV encephalitis.

Regulated expression of sodium-dependent glutamate transporters and synthetase: a neuroprotective role for activated microglia and macrophages in HIV infection?

It is now widely accepted that neuronal damage in HIV infection results mainly from microglial activation and involves apoptosis, oxidative stress and glutamate-mediated neurotoxicity. Glutamate toxicity acts via 2 distinct pathways: an excitotoxic one in which glutamate receptors are hyperactivated, and an oxidative one in which cystine uptake is inhibited, resulting in glutathione depletion and oxidative stress. A number of studies show that astrocytes normally take up glutamate, keeping extracellular glutamate concentration low in the brain and preventing excitotoxicity. This action is inhibited in HIV infection, probably due to the effects of inflammatory mediators and viral proteins. Other in vitro studies as well as in vivo experiments in rodents following mechanical stimulation, show that activated microglia and brain macrophages express high affinity glutamate transporters. These data have been confirmed in chronic inflammation of the brain, particularly in SIV infection, where activated microglia and brain macrophages also express glutamine synthetase. Recent studies in humans with HIV infection show that activated microglia and brain macrophages express the glutamate transporter EAAT-1 and that expression varies according to the disease stage. This suggests that, besides their recognized neurotoxic properties in HIV infection, these cells also have a neuroprotective function, and may partly make up for the inhibited astrocytic function, at least temporarily. This hypothesis might explain the discrepancy between microglial activation which occurs early in the disease, and neuronal apoptosis and neuronal loss which is a late event. In this review article, we discuss the possible neuroprotective and neurotrophic roles of activated microglia and macrophages that may be generated by the expression of high affinity glutamate transporters and glutamine synthetase, 2 major effectors of glial glutamate metabolism, and the implications for HIV-induced neuronal dysfunction, the underlying cause of HIV dementia.

Beta-amyloid-stimulated microglia induce neuron death via synergistic stimulation of tumor necrosis factor alpha and NMDA receptors

Although abundant reactive microglia are found associated with beta-amyloid (Abeta) plaques in Alzheimer's disease (AD) brains, their contribution to cell loss remains speculative. A variety of studies have documented the ability of Abeta fibrils to directly stimulate microglia in vitro to assume a neurotoxic phenotype characterized by secretion of a plethora of proinflammatory molecules. Collectively, these data suggest that activated microglia play a direct role in contributing to neuron death in AD rather than simply a role in clearance after plaque deposition. Although it is clear the Abeta-stimulated microglia acutely secrete toxic oxidizing species, the identity of longer-lived neurotoxic agents remains less defined. We used Abeta-stimulated conditioned media from primary mouse microglia to identify more stable neurotoxic secretions. The NMDA receptor antagonists memantine and 2-amino-5-phosphopetanoic acid as well as soluble tumor necrosis factor alpha (TNFalpha) receptor protect neurons from microglial-conditioned media-dependent death, implicating the excitatory neurotransmitter glutamate and the proinflammatory cytokine TNFalpha as effectors of microglial-stimulated death. Neuron death occurs in an oxidative damage-dependent manner, requiring activity of inducible nitric oxide synthase. Toxicity results from coincident stimulation of the TNFalpha and NMDA receptors, because stimulations of either alone are insufficient to initiate cell death. These findings suggest the hypothesis that AD brains provide the appropriate microglial-mediated inflammatory environment for TNFalpha and glutamate to synergistically stimulate toxic activation of their respective signaling pathways in neurons as a contributing mechanism of cell death.

Corticosterone inhibits expression of the microglial glutamate transporter GLT-1 in vitro

The present study investigates the effect of the glucocorticoid corticosterone on microglial glutamate transporters in vitro. Microglial cultures obtained from rat cerebral cortex were found to express the excitatory amino acid transporter GLT-1, but not GLAST, and this expression was increased by 1 ng/ml lipopolysaccharide after 12 h of stimulation. This increase has previously been shown to be mediated by tumor necrosis factor-alpha, a cytokine released by microglia during pathological conditions. Furthermore, lipopolysaccharide increased the microglial release of tumor necrosis factor-alpha and 1 microM corticosterone inhibited this effect. Corticosterone also inhibited the lipopolysaccharide-induced increase of the GLT-1 expression as well as the expression in non-activated cells. The effect of corticosterone on the GLT-1 expression was dose dependent and accompanied by similar effects on the microglial glutamate uptake capacity. Additionally, exogenous tumor necrosis factor-alpha was found to counteract the effect of corticosterone on microglial GLT-1 expression. The effect of corticosterone appeared to be glucocorticoid receptor specific since 10 microM of the glucocorticoid receptor antagonist mifepristone inhibited the effect. Thus, corticosterone decreased the microglial uptake of glutamate by decreasing the expression of glutamate transporters, probably due to the inhibited microglial tumor necrosis factor-alpha release. These results provide insights into the mechanisms behind microglial glutamate transporter expression during pathological conditions, and contribute to the debate about the beneficial or harmful effects of glucocorticoids.

Stimulation of group I metabotropic glutamate receptors evokes calcium signals and c-jun and c-fos gene expression in human T cells

To study if the activation of group I mGlu receptors in human T cells modifies intracellular Ca2+ concentration ([Ca2+](i)) and cell function, we measured [Ca2+](i) on cell suspensions (spectrofluorimetric method) or single cell (digital Ca2+ imaging system) using fura-2 as indicator. Early-inducible gene (c-jun and c-fos) expression was studied by reverse transcriptase-polymerase chain reaction assay as representative of Ca(2+)-sensitive gene expression. (1S,3R)-ACPD (100 microM), the selective mGlu receptor agonist, evoked a significant increase (34.1+/-4.9%) of [Ca2+](i), pharmacologically characterized as mediated by group I mGlu receptors, since both (S)-3,5-DHPG (100 microM), a selective group I mGlu receptor agonist and CHPG (1mM), the specific mGlu5 receptor agonist, reproduced the effects, that were abolished by AIDA (1mM), a selective group I mGlu receptor antagonist. (S)-3,5-DHPG-induced a rapid [Ca2+](i) rise (initial phase) followed by a slow decrease (second phase) to the baseline. Both extracellular Ca2+ and Ca2+ released from intracellular stores contribute to the [Ca2+](i) increase which depend on PLC activation. In a Ca(2+)-free buffer, the second phase rapidly return to the baseline; LaCl3 (1-10 microM), an inhibitor of extracellular Ca2+ influx, significantly reduced the second phase only; thapsigargin (1microM), by discharging intracellular Ca2+ stores, U 73122 (10 microM) and D609 (300 microM), by inhibiting PLC activity, prevented both phases. In our system, PTX pre-treatment increased (S)-3,5-DHPG effects, demonstrating that PXT-sensitive G(i/o) proteins are involved. Finally, specific stimulation of these receptors in Jurkat cells upregulates c-jun and c-fos gene expression, thus activating multiple downstream signalling regulating important T cell functions.

Involvement of IL-6 in the anti-human immunodeficiency virus activity of IFN-tau in human macrophages

IFN-tau is a non-cytotoxic type I IFN responsible for maternal recognition of the foetus in ruminants. IFN-tau has been found to inhibit HIV replication more strongly than human IFN-alpha, particularly in human monocyte-derived macrophages, without associated toxicity. Ovine IFN-tau uses the same anti-viral cellular pathways as human IFN-alpha in human macrophages, principally inhibiting the early steps of the biological cycle of HIV, preventing the integration of HIV DNA into the host-cell genome. In this study, we investigated the immunomodulatory properties of IFN-tau in human macrophages. We found that IFN-tau increased the production of IL-10 and IL-6, but not of IL-1beta or tumour necrosis factor alpha, in unstimulated, LPS-stimulated and HIV-1/Ba-L-infected macrophages. We also found that treatment with IL-6 inhibited HIV replication. Moreover, the neutralization of IL-6 activity in the cell culture supernatants of IFN-tau-treated macrophages led to a decrease in the anti-retroviral effects of IFN-tau, suggesting that IL-6 was involved in the anti-viral activity induced by IFN-tau. By focusing on the very early steps of the biological cycle of HIV, we showed that IL-6 co-operated with IFN-tau to decrease intracellular HIV RNA levels 2 h after infection.

Protective autoimmunity: interferon-gamma enables microglia to remove glutamate without evoking inflammatory mediators

Glutamate in excessive amounts is a major contributor to neuronal degeneration, and its removal is attributed mainly to astrocytes. Traumatic injury to the central nervous system (CNS) is often accompanied by disappearance of astrocytes from the lesion site and failure of the remaining cells to withstand the ensuing toxicity. Microglia that repopulate the lesion site are the usual suspects for causing redox imbalance and inflammation and thus further exacerbating the neurotoxicity. However, our group recently demonstrated that early post-injury activation of microglia as antigen-presenting cells correlates with an ability to withstand injurious conditions. Moreover, we found that T cells reactive to CNS-specific self-antigens protected neurons against glutamate toxicity. Here, we show that antigen-specific autoimmune T cells, by tailoring the microglial phenotype, can increase the ability of microglia-enriched cultures to remove glutamate. This T-cell-mediated effect could not be achieved by the potent microglia-activating agent lipopolysaccharide (LPS), but was dose-dependently reproduced by the Th1 cytokine interferon (IFN)-gamma and significantly reduced by neutralizing anti-IFN-gamma antibodies. Under the same conditions, IFN-gamma had no effect on cultured astrocytes. Up-regulation of glutamate uptake induced by IFN-gamma activation was not accompanied by the acute inflammatory response seen in LPS-activated cultures. These findings suggest that T cells or their cytokines can cause microglia to adopt a phenotype that facilitates rather than impairs glutamate clearance, possibly contributing to restoration of homeostasis.

Macrophages and HIV-1: dangerous liaisons

HIV-1, like the other lentiviruses, has evolved the ability to infect nondividing cells including macrophages. HIV-1 replication in monocytes/macrophages entails peculiar features and differs in many respects from that in CD4 T lymphocytes. HIV-1 exhibits different tropism for CD4 T cells and macrophages. The virus can enter macrophages via several routes. Mitosis is not required for nuclear import of viral DNA or for its integration into the host cell genome. Specific cellular factors are required for HIV-1 transcription in macrophages. The assembly and budding of viral particles in macrophages take place in late endosomal compartments. Viral particles can use the exosome pathway to exit cells. Given their functions in host defence against pathogens and the regulation of the immune response plus their permissivity to HIV-1 infection, monocytes/macrophages exert a dual role in HIV infection. They contribute to the establishment and persistence of HIV-1 infection, and may activate surrounding T cells favouring their infection. Furthermore, monocytes/macrophages act as a Trojan horse to transmit HIV-1 to the central nervous system. They also exhibit antiviral activity and express many molecules that inhibit HIV-1 replication. Activated microglia and macrophages may also exert a neurotrophic and neuroprotective effect on infected brain regulating glutamate metabolism or by secretion of neurotrophins. This review will discuss specific aspects of viral replication in monocytes/macrophages and the role of their interactions with the cellular environment in HIV-1 infection swinging between protection and pathogenesis.

Lipopolysaccharide increases microglial GLT-1 expression and glutamate uptake capacity in vitro by a mechanism dependent on TNF-α

This study investigates the effect of microglial activation on microglial glutamate transporters in vitro. Stimuli known to activate microglia and/or to be associated with pathological conditions with an impaired astroglial glutamate uptake were compared. Morphological changes and marked increases in ED1 antigen expression were found after 8-h incubation of rat microglia in 56 mM KCl, 1 ng/ml lipopolysaccharide (LPS), and 100 microM glutamate, as well as in acidic and basic conditions, showing that the cells were activated. Of the stimuli used, only LPS induced a significant release of the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6), and was the only stimulus that increased microglial GLT-1 expression and glutamate uptake capacity after 12-h incubation. This effect was probably mediated by TNF-alpha, since this cytokine mimicked the effect of LPS. Furthermore, the effect of LPS was blocked by thalidomide, an inhibitor of TNF-alpha synthesis. Additionally, neutralizing antibodies against TNF-alpha also blocked the increase, indicating TNF-alpha as an inducer of GLT-1 expression in microglia. It was also found that preincubation with glutamate dose-dependently inhibited the microglial glutamate uptake. This could suggest different physiological functions for microglial and astroglial glutamate uptake and might indicate a reciprocal control of GLT-1 expression between microglia and astrocytes.

Expression of Excitatory Amino Acid Transporter-1 (EAAT-1) in Brain Macrophages and Microglia of Patients with Prion Diseases

The mechanisms of neuronal apoptosis in prion diseases are unclear. Experimental studies suggest that it may result from 2 associated mechanisms: glutamate-mediated excitotoxicity and oxidative stress. Recent studies showed that activated macrophages/microglia (AMM) express excitatory amino acid transporters (EAATs) in HIV infection, suggesting that they may play a neuroprotective role by clearing extra-cellular glutamate and producing anti-oxidant glutathione. In order to test this hypothesis in prion diseases, samples from cerebral cortex, striatum, thalamus, and cerebellum from 14 patients with Creutzfeldt-Jakob disease (8 sporadic, 2 familial, 2 iatrogenic, and 2 variant), and 4 with fatal familial insomnia (3 homozygous Met/Met at codon 129 of the PRNP gene, 1 heterozygous Met/Val), and 3 controls were immunostained for EAAT-1, GFAP, HLA-DR, CD68, IL-1, caspase 3, and PrP. In prion diseases, EAAT-1 immunopositivity was found in affected areas. Only AMM, interstitial, perivascular, perineuronal (sometimes around apoptotic neurons), or close to reactive astrocytes, expressed EAAT-1. Astrocyte EAAT-1 expression was scarcely detectable in controls and was not detected in prion disease cases. The proportion of AMM expressing EAAT-1 did not correlate with the severity of neuronal apoptosis, spongiosis, astrocytosis, microgliosis, or PrP deposition, but only with disease duration. Occasional EAAT-1 expressing AMM were found in patients with short survival, whereas diffuse EAAT-1 expression by AMM was observed in cases with long survival (24 to 33 months) that most often were heterozygous for Met/Val at codon 129 of the PRNP gene. Our findings suggest that AMM may develop a partial neuroprotective function in long-lasting prion diseases, although it does not seem to efficiently prevent neurological and neuropathological deterioration. Whether this neuroprotective function of microglia is the cause or the effect of longer survival needs to be clarified.

A role for l-glutamate ionotropic receptors in the development of rat neurogenic pulmonary edema

The present study was undertaken to evaluate possible roles of L-glutamate ionotropic receptors in neurogenic pulmonary edema. Perfusion of L-glutamate into the fourth ventricles of rats increased nitric oxide (NO) signals in the efflux solution concentration-dependently, significantly reducing both the occurrence and severity of neurogenic pulmonary edema. This effect was completely reversed by prior intracisternal injection of an NO synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME), or an N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine maleate (MK-801), and partially by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a 2-amino-3-hydroxy-5-methyl-4-isoxazol propionic acid (AMPA)/kainic acid receptor antagonist. Administration of MK-801 or CNQX alone, without L-glutamate, almost completely prevented neurogenic pulmonary edema development. These results suggest that endogenous L-glutamate may facilitate underlining disease process, whereas L-glutamate exogenously applied into the fourth ventricle may have an inhibitory action via release of NO, through ionotropic receptors.