Increased glutamate uptake and glutamine synthetase activity in neuronal cell cultures surviving chronic hypoxia

Metabolome responses of the sea cucumber Apostichopus japonicus to multiple environmental stresses: Heat and hypoxia

Economically important marine organisms face severe environmental challenges, such as high temperature and low dissolved oxygen, from global climate change. Adverse environmental factors impact the survival and growth of economically important marine organisms, thereby negatively influencing the aquaculture industry. However, little is known about the responses of sea cucumbers to combined environmental co-stressors till now. In this study, ultra-performance liquid chromatography (UPLC) was utilized to obtain metabolic profiles of sea cucumbers. Changes in the concentrations of 84, 68, and 417 metabolites related to the responses of sea cucumbers to heat (26 °C), hypoxia (2 mg/L) and the combined stress, respectively, were observed and analyzed. Representative biomarkers were discussed in detail, including deltaline, fusarin C, halichondrin B and rapanone. The concentration of metabolites involved in the regulation of energy metabolism, including amino acid, carbohydrate and lipid metabolism were significantly changed, and the tricarboxylic acid (TCA)-cycle was significantly altered under heat plus hypoxia. We interpreted these changes partly as an adaptation mechanism in response to environmental stress. Based on the decreased accumulation of glutamine, we hypothesized that heat stress is the main factor that interferes with the process of glutamic acid-glutamine metabolism. The present study showed that combined environmental stressors have a more extensive impact on the metabolites of the respiratory tree in sea cucumbers than single stress. These results would facilitate further development of the sea cucumber as an echinoderm model to study mechanisms of response to adverse environments, as well as to help advance knowledge of the adaptation of marine organisms to global climate change.

Neuronal vs glial glutamate uptake: Resolving the conundrum

Neither normal brain function nor the pathological processes involved in neurological diseases can be adequately understood without knowledge of the release, uptake and metabolism of glutamate. The reason for this is that glutamate (a) is the most abundant amino acid in the brain, (b) is at the cross-roads between several metabolic pathways, and (c) serves as the major excitatory neurotransmitter. In fact most brain cells express glutamate receptors and are thereby influenced by extracellular glutamate. In agreement, brain cells have powerful uptake systems that constantly remove glutamate from the extracellular fluid and thereby limit receptor activation. It has been clear since the 1970s that both astrocytes and neurons express glutamate transporters. However the relative contribution of neuronal and glial transporters to the total glutamate uptake activity, however, as well as their functional importance, has been hotly debated ever since. The present short review provides (a) an overview of what we know about neuronal glutamate uptake as well as an historical description of how we got there, and (b) a hypothesis reconciling apparently contradicting observations thereby possibly resolving the paradox.

Glutamate transporters in brain ischemia: to modulate or not?

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

Age-dependent astroglial vulnerability to hypoxia and glutamate: the role for erythropoietin

Extracellular accumulation of toxic concentrations of glutamate (Glu) is a hallmark of many neurodegenerative diseases, often accompanied by hypoxia and impaired metabolism of this neuromediator. To address the question whether the multifunctional neuroprotective action of erythropoietin (EPO) extends to the regulation of extracellular Glu-level and is age-related, young and culture-aged rat astroglial primary cells (APC) were simultaneously treated with 1mM Glu and/or human recombinant EPO under normoxic and hypoxic conditions (NC and HC). EPO increased the Glu uptake by astrocytes under both NC and especially upon HC in culture-aged APC (by 60%). Moreover, treatment with EPO up-regulated the activity of glutamine synthetase (GS), the expression of glutamate-aspartate transporter (GLAST) and the level of EPO mRNA. EPO alleviated the Glu- and hypoxia-induced LDH release from astrocytes. These protective EPO effects were concentration-dependent and they were strongly intensified with age in culture. More than a 4-fold increase in apoptosis and a 2-fold decrease in GS enzyme activity was observed in APC transfected with EPO receptor (EPOR)-siRNA. Our in vivo data show decreased expression of EPO and a strong increase of EPOR in brain homogenates of APP/PS1 mice and their wild type controls during aging. Comparison of APP/PS1 and age-matched WT control mice revealed a stronger expression of EPOR but a weaker one of EPO in the Alzheimer's disease (AD) model mice. Here we show for the first time the direct correlation between the extent of differentiation (age) of astrocytes and the efficacy of EPO in balancing extracellular glutamate clearance and metabolism in an in-vitro model of hypoxia and Glu-induced astroglial injury. The clinical relevance of EPO and EPOR as markers of brain cells vulnerability during aging and neurodegeneration is evidenced by remarkable changes in their expression levels in a transgenic model of AD and their WT controls.

Reactive oxygen species from human astrocytes induced functional impairment and oxidative damage

Reactive oxygen species (ROS) have been shown to be a contributor to aging and disease. ROS also serve as a trigger switch for signaling cascades leading to corresponding cellular and molecular events. In the central nervous system (CNS), microglial cells are likely the main source of ROS production. However, activated astrocytes also appear to be capable of generating ROS. In this study we investigated ROS production in human astrocytes stimulated with interleukin (IL)-1β and interferon (IFN)-γ and its potential harmful effects. Although IFN-γ alone had no effect, it potentiated IL-1β-induced ROS production in a time-dependent manner. One of the sources of ROS in IL-1β-activated astrocytes was from increased superoxide production in mitochondria accompanied by enhanced manganese superoxide dismutase and inhibited catalase expression. NADPH oxidase (NOX) may also contribute to ROS production as astrocytes express NOX isoforms. Glutamate uptake, which represents one of the most important methods of astrocytes to prevent excitotoxicity, was down-regulated in IL-1β-activated astrocytes, and was further suppressed in the presence of IFN-γ; IFN-γ itself exerted minimal effect. Elevated levels of 8-isoprostane in IL-1β ± IFN-γ-activated human astrocytes indicate downstream lipid peroxidation. Pretreatment with diphenyleneiodonium abolished the IL-1β ± IFN-γ-induced ROS production, restored glutamate uptake function and reduced 8-isoprostane to near control levels suggesting that ROS contributes to the dysfunction of activated astrocytes. These results support the notion that dampening activated human astrocytes to maintain the redox homeostasis is vital to preserve their neuroprotective potential in the CNS.

Neuroprotective effects of ischemic postconditioning on global brain ischemia in rats through upregulation of hippocampal glutamine synthetase

Brain ischemic postconditioning is the induction of brief periods of ischemia-reperfusion during the early stages following ischemia, and it has been shown to produce neuroprotective effects. The mechanisms underlying these neuroprotective effects are poorly understood. Glutamate excitotoxicity is one cause of postischemic neuronal death. Glutamine synthetase (GS) is an enzyme that is expressed in glial cells and may affect glutamate excitotoxicity. We induced global ischemia in rats and performed postconditioning with 6 cycles of 10 seconds reperfusion and 10 seconds reocclusion before final reperfusion. Hematoxylin and eosin staining revealed extensive neuronal loss (44.0 ± 2.8% cell survival) in the hippocampal CA1 region. Ischemic postconditioning decreased neuronal death (82.0 ± 5.6% cell survival; p<0.05). Western blotting revealed significantly increased GS expression in the hippocampus for the ischemia-reperfusion group over time compared with the sham group (p<0.05). Ischemic postconditioning resulted in significantly increased (p<0.05) GS expression compared with both the sham and ischemia-reperfusion groups, suggesting that upregulation of GS expression after ischemia constitutes a neuroprotective mechanism.

Down-regulation of glutamine synthetase enhances migration of rat astrocytes after in vitro injury

Astrocytes undergo reactive transformation in response to physical injury (reactive gliosis) that may impede neural repair. Glutamine synthetase (GS) is highly expressed by astrocytes, and serves a neuroprotective function by converting cytotoxic glutamate and ammonia into glutamine. Glutamine synthetase was down-regulated in reactive astrocytes at the site of mechanical spinal cord injury (SCI) and in cultured astrocytes at the margins of a scratch wound, suggesting that GS may modulate reactive transformation and glial scar development. We evaluated this potential function of GS using siRNA-mediated GS knock-down. Suppression of astrocytic GS by GS siRNA increased cell migration into the scratch wound zone and decreased substrate adhesion as indicated by the number of focal adhesions expressing the adaptor protein paxillin. Migration was enhanced by glutamine and suppressed by glutamate, in contrast to the result expected if enhanced migration was due solely to changes in glutamine and glutamate concomitant with reduced GS activity. The membrane type 1-matrix metalloproteinase (MT1-MMP) was up-regulated in GS siRNA-treated astrocytes, while a broad-spectrum MMP antagonist inhibited migration in both wild type and GS knock-down astrocytes. In addition, GS siRNA inhibited expression of integrin β1, while antibody-mediated inhibition of integrin β1 impaired direction-specific protrusion and motility. Thus, GS may modulate motility and substrate adhesion through transmembrane integrin β1 signaling to the cytoskeleton and by MMT-mediated proteolysis of the extracellular matrix.

Glutamine synthetase down-regulation reduces astrocyte protection against glutamate excitotoxicity to neurons

Although the role of astrocyte glutamate transporters in glutamate clearance is well illustrated, the role of glutamine synthetase (GS) that influences this process remains to be elucidated. We examined whether GS affected the uptake of glutamate in astrocytes in vitro. The glutamate uptake was assessed by measuring the concentration of glutamate and glutamine in culture medium in the presence or absence of glutamate. We demonstrated that inhibition of GS in astrocytes by MSO significantly impaired glutamate uptake and glutamine release. Conversely, induction of GS expression in astrocytes by gene transfer significantly enhanced the glutamate uptake and glutamine release. When an inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) was applied to the cultures, it significantly reduced GS expression and inhibited glutamate-induced GS activation resulting in increased excitotoxicity to neurons. These results suggest that GS in astrocytes may represent a novel target for neuroprotection against neuronal dysfunction and death that occur in many neurological disorders.

Increased glutamine synthetase but normal EAAT2 expression in platelets of ALS patients

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease and glutamate excitotoxicity has been implicated in its pathogenesis. Platelets contain a glutamate uptake system and express components of the glutamate-glutamine cycle, such as the predominant glial excitatory amino acid transporter 2 (EAAT2). In several neurological diseases platelets have proven to be systemic markers for the disease. We compared properties of key components of the glutamate-glutamine cycle in blood platelets of ALS patients and healthy controls. Platelets were analyzed for (3)H-glutamate uptake in the presence or absence of thrombin and for EAAT2 and glutamine synthetase protein expression by Western blotting. Platelets of ALS patients showed a 37% increase in expression of glutamine synthetase, but normal expression of glutamate transporter EAAT2. Glutamate uptake in resting or thrombin-stimulated platelets did not differ significantly between platelets from ALS patients and controls. Thrombin-stimulation resulted in about a seven-fold increase in glutamate uptake. Our data suggest that glutamine synthetase may be a peripheral marker of ALS and encourage further investigation into the role of this enzyme in ALS.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Vasoactive intestinal peptide prevents activated microglia-induced neurodegeneration under inflammatory conditions: potential therapeutic role in brain trauma

In most neurodegenerative disorders, including multiple sclerosis, Parkinson's disease, and Alzheimer's disease, a massive neuronal cell death occurs as a consequence of an uncontrolled inflammatory response, where activated microglia and its cytotoxic agents play a crucial pathologic role. Because current treatments for these diseases are not effective, several regulatory molecules termed "microglia-deactivating factors" recently have been the focus of considerable research. Vasoactive intestinal peptide (VIP) is a neuropeptide with a potent anti-inflammatory effect, which has been found to protect from other inflammatory disorders, such as endotoxic shock and rheumatoid arthritis. In the present study, we investigate the effect of VIP on inflammation-mediated neurodegeneration in vitro and in vivo as well as on the putative neuroprotective effect of VIP on experimental pathological conditions in which central nervous system (CNS) inflammation is involved, such as brain trauma. The involvement of activated microglia and their derived cytotoxic products is also studied. VIP has a clear neuroprotective effect on inflammatory conditions by inhibiting the production of microglia-derived proinflammatory factors (tumor necrosis factor alpha, interleukin-1beta, nitric oxide). In this sense, VIP prevents neuronal cell death following brain trauma by reducing the inflammatory response of neighboring microglia. Therefore, VIP emerges as a valuable neuroprotective agent for the treatment of pathologic conditions of the CNS where inflammation-induced neurodegeneration occurs.

Glutamine synthetase in brain: effect of ammonia

Glutamine synthetase (GS) in brain is located mainly in astrocytes. One of the primary roles of astrocytes is to protect neurons against excitotoxicity by taking up excess ammonia and glutamate and converting it into glutamine via the enzyme GS. Changes in GS expression may reflect changes in astroglial function, which can affect neuronal functions. Hyperammonemia is an important factor responsible of hepatic encephalopathy (HE) and causes astroglial swelling. Hyperammonemia can be experimentally induced and an adaptive astroglial response to high levels of ammonia and glutamate seems to occur in long-term studies. In hyperammonemic states, astroglial cells can experience morphological changes that may alter different astrocyte functions, such as protein synthesis or neurotransmitters uptake. One of the observed changes is the increase in the GS expression in astrocytes located in glutamatergic areas. The induction of GS expression in these specific areas would balance the increased ammonia and glutamate uptake and protect against neuronal degeneration, whereas, decrease of GS expression in non-glutamatergic areas could disrupt the neuron-glial metabolic interactions as a consequence of hyperammonemia. Induction of GS has been described in astrocytes in response to the action of glutamate on active glutamate receptors. The over-stimulation of glutamate receptors may also favour nitric oxide (NO) formation by activation of NO synthase (NOS), and NO has been implicated in the pathogenesis of several CNS diseases. Hyperammonemia could induce the formation of inducible NOS in astroglial cells, with the consequent NO formation, deactivation of GS and dawn-regulation of glutamate uptake. However, in glutamatergic areas, the distribution of both glial glutamate receptors and glial glutamate transporters parallels the GS location, suggesting a functional coupling between glutamate uptake and degradation by glutamate transporters and GS to attenuate brain injury in these areas. In hyperammonemia, the astroglial cells located in proximity to blood-vessels in glutamatergic areas show increased GS protein content in their perivascular processes. Since ammonia freely crosses the blood-brain barrier (BBB) and astrocytes are responsible for maintaining the BBB, the presence of GS in the perivascular processes could produce a rapid glutamine synthesis to be released into blood. It could, therefore, prevent the entry of high amounts of ammonia from circulation to attenuate neurotoxicity. The changes in the distribution of this critical enzyme suggests that the glutamate-glutamine cycle may be differentially impaired in hyperammonemic states.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Hypoxia regulates glutamate metabolism and membrane transport in rat PC12 cells

We investigated the effect of hypoxia on glutamate metabolism and uptake in rat pheochromocytoma (PC12) cells. Various key enzymes relevant to glutamate production, metabolism and transport were coordinately regulated by hypoxia. PC12 cells express two glutamate-metabolizing enzymes, glutamine synthetase (GS) and glutamate decarboxylase (GAD), as well as the glutamate-producing enzyme, phosphate-activated glutaminase (PAG). Exposure to hypoxia (1% O(2)) for 6 h or longer increased expression of GS mRNA and protein and enhanced GS enzymatic activity. In contrast, hypoxia caused a significant decrease in expression of PAG mRNA and protein, and also decreased PAG activity. In addition, hypoxia led to an increase in GAD65 and GAD67 protein levels and GAD enzymatic activity. PC12 cells express three Na(+)-dependent glutamate transporters; EAAC1, GLT-1 and GLAST. Hypoxia increased EAAC1 and GLT-1 protein levels, but had no effect on GLAST. Chronic hypoxia significantly enhanced the Na(+)-dependent component of glutamate transport. Furthermore, chronic hypoxia decreased cellular content of glutamate, but increased that of glutamine. Taken together, the hypoxia-induced changes in enzymes related to glutamate metabolism and transport are consistent with a decrease in the extracellular concentration of glutamate. This may have a role in protecting PC12 cells from the cytotoxic effects of glutamate during chronic hypoxia.

Modulation of AMPA receptor subunits GluR1 and GluR2/3 in the rat cerebellum in an experimental hepatic encephalopathy model

The immunohistochemical expression and distribution of the AMPA-selective receptor subunits GluR1 and GluR2/3 were investigated in the rat cerebellum following portocaval anastomosis (PCA) at 1 and 6 months. With respect to controls, GluR1 and GluR2/3 immunoreactivities increased over 1 to 6 months following PCA, although immunolabelling patterns for both antibodies were different at the two analysed times. GluR1 immunoreactivity was expressed by Bergmann glial cells, which showed immunoreactive glial processes crossing the molecular layer at 6 months following PCA. The GluR2/3 subunit was expressed by Purkinje neurons and moderately expressed by neurons of the granule cell layer. Immunoreactivity for GluR2/3 was detectable in cell bodies and dendrites of Purkinje cells in young control cerebella, whereas GluR2/3 immunoreactivity was scarce 1 month post PCA. However, despite a lack of immunoreactivity in the Purkinje somata and main processes of adult control rats, GluR2/3 immunoreactivity was strongly enhanced in Purkinje neurons following long-term PCA. These findings suggest that the localization of the GluR2/3 subunit in Purkinje cells undergoes an alteration and/or reorganization as a consequence of long-term PCA. The combination of enhanced GluR immunoreactivity in long-term PCA, both in Bergmann glial cells and in Purkinje neurons, suggests some degree of neuro-glial interaction, possibly through glutamate receptors, in this type of encephalopathy.

Expression of glutamate uptake transporters after dibutyryl cyclic AMP differentiation and traumatic injury in cultured astrocytes

Our findings indicate that differentiation of primary astrocytes by dibutyryl cyclic adenosine monophosphate (dBcAMP) and scratch injury together resulted in increased glutamate transporter gene expression. Confluent primary cultures were prepared from cerebral cortex of normal new born rat pups. The primary cultures were then divided into four groups each: control and scratch-injured, and dBcAMP-treated control and scratch-injured cultures. Total RNA was extracted at 0, 1, 2, 4, and 7 days after injury. Expression of the electrogenic glutamate transporters, GLAST, GLT-1, and EAAC-1, was quantitated by the reverse transcriptase-polymerase chain reaction method (RT-PCR) and slot blot hybridization followed by densitometric scanning. Triplicate cultures were analyzed for each time-point. Our studies indicate that all these astrocyte cultures expressed the two glial transporters, GLAST and GLT-1, while none of the cultures expressed the neuronal transporter, EAAC-1. The expression of the two transporters in the dBcAMP-treated primary cultures were markedly increased from the non-treated cultures. The dBcAMP-treated cultures had 2- to 4-times increase in levels of GLAST and GLT-1-mRNA expression both before and after scratch injury, as compared to untreated non-injured and injured primary cultures. All of the cultures expressed GLAST in greater proportion than GLT-1.

Regional differences in glial cell modulation of synaptic transmission

Metabolic integrity of glial cells in field CA1 of the guinea pig hippocampus is critical to maintenance of synaptic transmission (Keyser and Pellmar [1994] Glia 10:237-243). To determine if this tight glial-neuronal coupling is equally important in other brain regions, we compared the effect of fluoroacetate (FAC), a glial specific metabolic blocker, on synaptic transmission in field CA1 to synaptic transmission in area dentata (DG). FAC was significantly more effective in decreasing synaptic potentials in CA1 than in DG. A similar regional disparity in the FAC-induced decrease in ATP levels was evident. Isocitrate, a glial specific metabolic substrate, prevented the FAC-induced synaptic depression in both CA1 and DG. The results suggest that glia of CA1 and dentate respond differently to metabolic challenge. Modulation of this glial-neuronal coupling could provide a regionally specific mechanism for synaptic plasticity. Additionally, site-specific glial-neuronal interactions can impact on a variety of physiological and pathophysiological conditions.

Hormonal and non-hormonal regulation of glutamine synthetase in the developing neural retina

Two isoforms of the glucocorticoid receptor, with apparent molecular mass of 90 and 95 kDa, are expressed in embryonic chicken neural retina. The 95-kDa receptor represents a hyperphosphorylated form of the 90-kDa receptor. Activation of the glucocorticoid receptor by cortisol results in a dose-dependent increase in receptor phosphorylation, translocation of receptor molecules into the nucleus and a decline in the total amount of the receptor. Activation of the glucocorticoid receptor can also be observed in the developing retinal tissue in ovo. At late embryonic ages, when the systemic level of glucocorticoids increases, a substantial quantity of receptor molecules becomes translocated into the nucleus, the relative level of the 95-kDa isoform increases, and the total amount of receptor declines. Activation of the receptor molecules in ovo correlates directly with an increase in transcription of the glucocorticoid-inducible gene, glutamine synthetase. The close correlation between the increase in systemic glucocorticoids, activation of glucocorticoid receptor molecules and induction of glutamine synthetase gene transcription suggests that glucocorticoids are directly involved in the developmental control of glutamine synthetase expression. Long-term organ culturing of embryonic retinal tissue in the absence of hormone results in an increase in glutamine synthetase expression. This increase, which is only 5 to 10% of that observed in ovo, is not mediated by activated receptor molecules and represents a mechanism for non-hormonal regulation of glutamine synthetase.

Normal and abnormal calcium homeostasis in neurons: a basis for the pathophysiology of traumatic and ischemic central nervous system injury

Clinical recovery after central nervous system (CNS) trauma or ischemia may be limited by a neural injury process that is triggered and perpetuated at the cellular level, rather than by a lesion amenable to surgical repair. It is widely thought that one such process, a fundamental pathological mechanism initiated by CNS injury, is a disruption of cellular Ca2+ homeostasis. Because of the critical role of Ca2+ ions in regulating innumerable cellular functions, this major homeostatic disturbance is thought to trigger neuronal and axonal degeneration and produce clinical disability. We review those aspects of normal and pathological Ca2+ homeostasis in neurons that relate to neurodegeneration and to the application of neuroprotective strategies for the treatment of CNS injury. In particular, we examine the contribution of Ca(2+)-permeable ionic channels, Ca2+ pumps, intracellular Ca2+ stores, intracellular Ca2+ buffering systems, and the roles of secondary, Ca(2+)-dependent processes in neurodegeneration. A number of hypotheses linking Ca2+ ions and Ca2+ permeable channels to neurotoxicity are discussed with an emphasis on strategies for lessening Ca(2+)-related damage. A number of these strategies may have a future role in the treatment of traumatic and ischemic CNS injury.

Glutamate, glutamine and glutamine synthetase in the neonatal rat brain following hypoxia

Exposing 7-day-old rat pups to hypoxia, 8% oxygen/92% nitrogen, for 3 h alters glutamate (GLU), glutamine and glutamine synthetase (GS) activity in the striatum, frontal cortex and hippocampus. Immediately following the hypoxic insult there is a rapid transient elevation of GLU followed by a fall and then recovery to control values within 6 h. Glutamine content initially decreased after the termination of the insult, rose thereafter and approached control values within 6 h. GS activity was depressed after hypoxia and gradually returned to normal levels within 6 h. GS mRNA was increased in the three brain regions studied after hypoxia and returned to control values within 24 h. These results suggest that hypoxia alters GLU metabolism in the immature brain.

Potentiation of excito-toxicity by glutamate uptake inhibitor rather than glutamine synthetase inhibitor

The neuroprotective functions of glia cells in the presence of excessive amounts of extracellular glutamate (Glu) were examined using glia-rich and glia-poor cultured cerebellar granule cells that contained the same number of neurons. In order to focus on the metabolic enzyme glutamine synthetase (GS) and the uptake system in glia cells, selective inhibitors such as L-methionine sulfoximine (MSO) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) were used as pharmacological tools. The increased amount of lactate dehydrogenase (LDH) leakage induced by 50 microM Glu and SITS was equivalent to that of 1 mM Glu. However, the simultaneous treatment with 50 microM Glu and 5 microM MSO did not increase the LDH leakage. The larger quantities of extracellular Glu were sustained in both glia-rich and glia-poor cultures. After the administration of Glu and MSO, however, the larger quantities of Glu were not sustained. Taking these results into consideration, the Glu uptake system in glia cells seems to be more important than the Glu metabolic enzyme system in the regulation of neuronal protection from Glu toxicity.

Effect of fructose-1,6-bisphosphate on glutamate uptake and glutamine synthetase activity in hypoxic astrocyte cultures

Astrocytes are important in regulating the microenvironment of neurons both by catabolic and synthetic pathways. The glutamine synthetase (GS) activity observed in astrocytes affects neurons by removing toxic substances, NH3 and glutamate; and by providing an important neuronal substrate, glutamine. This glutamate cycle might play a critical role during periods of hypoxia and ischemia, when an increase in extracellular excitatory amino acids is observed. It was previously shown in our laboratory that fructose-1,6-bisphosphate (FBP) protected cortical astrocyte cultures from hypoxic insult and reduced ATP loss following a prolonged (18-30 hrs) hypoxia. In the present study we established the effects of FBP on the level of glutamate uptake and GS activity under normoxic and hypoxic conditions. Under normoxic conditions, [U-14C]glutamate uptake and glutamine production were independent of FBP treatment; whereas under hypoxic conditions, the initial increase in glutamate uptake and an overall increase in glutamine production in astrocytes were FBP-dependent. Glutamine synthetase activity was dependent on FBP added during the 22 hours of either normoxic- or hypoxic-treatment, hence significant increases in activity were observed due to FBP regardless of the oxygen/ATP levels in situ. These studies suggest that activation of GS by FBP may provide astrocytic protection against hypoxic injury.

Modification of hypoxia-induced injury in cultured rat astrocytes by high levels of glucose

BACKGROUND AND PURPOSE

Preexisting hyperglycemia exacerbates central nervous system injury after transient global and focal cerebral ischemia. Increased anaerobic metabolism with resultant lactic acidosis has been shown to cause the hyperglycemic, neuronal injury. The contribution of astrocytes in producing lactic acidosis under hyperglycemic/ischemic conditions is unclear, whereas the protective role of astrocytes in ischemic-induced neuronal injury has been documented. The ability of astrocytes to maintain energy status and ion homeostasis under hyperglycemic conditions could ultimately reduce neuronal injury. Therefore, we determined the effects of increased glucose concentrations on glucose utilization, lactate production, extracellular pH, and adenosine triphosphate concentrations in hypoxia-treated astrocyte cultures.

METHODS

Primary astrocytes were prepared from neonatal rat cerebral cortices. After 35 days in vitro, cultures were incubated with 0-60 mmol/L glucose and subjected to hypoxic conditions at 95% N2/5% CO2 for 24 hours. In addition, under high-glucose conditions (30 mmol/L), astrocytes were exposed to up to 72 hours of hypoxia. Determination of lactate dehydrogenase efflux, adenosine triphosphate concentrations, and extracellular lactate concentrations defined astrocyte status. Equiosmolar levels of mannitol were added in place of high glucose concentrations to distinguish hyperosmotic effect.

RESULTS

When physiological concentrations of glucose (7.5 mmol/L) or lower concentrations were used, significant cell damage occurred with 24 hours of hypoxia, as determined by increased efflux of lactate dehydrogenase and loss of cell protein. When higher glucose concentrations (15-60 mmol/L) were used, efflux of lactate dehydrogenase was similar to that observed in normoxic cultures, despite an increased utilization of glucose. Lactate concentrations in the media at low or normal glucose concentrations exceeded normoxic levels, but higher glucose concentrations (15-30 mmol/L) failed to increase lactate levels further. Values of adenosine triphosphate for hypoxic astrocytes treated with high glucose concentrations were significantly higher than those of astrocytes with zero or low glucose levels. In cultures exposed to hypoxia and high glucose levels (30 mmol/L), no cellular injury was observed before 48 hours of hypoxia. Lactate concentrations in the media increased during the first 24 hours of hypoxia and reached steady state. The pH of the media decreased to 6.4 after 24 hours and 5.5 at 48 hours. The latter pH was concomitant with a marked increase in extracellular lactate dehydrogenase activity. Hyperosmotic mannitol failed to protect cultured astrocytes against hypoxia.

CONCLUSIONS

Hypoxic injury to mature astrocytes was reduced by the presence of 15-60 mmol/L glucose in the medium during 24-30 hours of hypoxia. Injury occurred when the pH of the medium was < 5.5. This protection was not afforded by the hyperosmotic effect of high glucose concentrations, nor was the hypoxic injury at later time periods with 30 mmol/L glucose mediated solely by lactate accumulation.

Induction of astrocyte glutamine synthetase activity by the Lathyrus toxin beta-N-oxalyl-L-alpha,beta-diaminopropionic acid (beta-L-ODAP)

beta-N-Oxalyl-L-alpha,beta-diaminopropionic acid (beta-L-ODAP) is thought to be the causative agent in lathyrism due to its neuroexcitatory and neurotoxic properties. We have recently reported that beta-L-ODAP is also gliotoxic at high concentrations (Bridges et al.: Brain Res 561:262, 1991). Evidence is now presented that low, subgliotoxic concentrations of beta-L-ODAP may alter the ability of astrocytes to regulate glutamate concentrations in the CNS by increasing astrocyte glutamine synthetase activity. When astrocytes cultured from rat cortex were exposed to 100 microM beta-L-ODAP for 24 h, the resulting glutamine synthetase activity was 155% of control levels. This effect was enantiomer- and isomer-specific, dose-dependent, and required protein translation as the induction was blocked with cycloheximide. The effect of beta-L-ODAP on glutamine synthetase was not mimicked by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) or kainate, suggesting that the induction was not transduced solely through activation of cell surface non-N-methyl-D-aspartate (NMDA) glutamate receptors. An intracellular site of action of beta-L-ODAP is proposed because its effect on glutamine synthetase activity could be blocked by the amino acid uptake blocker dihydrokainate.

Does ischemia with reperfusion lead to oxidative damage to proteins in the brain?

Recent results suggest that even relatively brief periods of ischemia in gerbils (10 min) lead to oxidative damage to brain proteins, reflected in an increased carbonyl content in the soluble protein fraction and a decreased glutamine synthetase (GS) activity. Since we failed to reproduce these findings in rats subjected to 15 min of transient ischemia, we explored whether oxidative damage to proteins could be observed after longer ischemic periods. To that end, one middle cerebral artery was occluded in rats for either 1 or 3 h, with recirculation periods of 0 min, 15 min, 1 h, and 6 h. Protein carbonyl content and GS activity were determined in focal and perifocal tissues and compared with values obtained in the same areas on the contralateral side. Ischemia, particularly of 3-h duration, followed by various reperfusion periods was accompanied by a significant (16-35%) decrease in the concentration of proteins of the soluble protein fraction. However, in no group was there an increased carbonyl content of the remaining proteins in this fraction. When expressed per milligram of protein, GS activity remained unchanged or rose somewhat. An inconsistent (and moderate) decrease in GS activity was present only if GS activity was expressed per milligram of wet tissue. The present findings, which fail to document oxidative damage to proteins following focal ischemia of 1- or 3-h duration, are thus radically different from those obtained in gerbils. The results suggest that appreciable species differences exist and raise the question of whether free radical-mediated oxidation of proteins is an invariable component of ischemic brain damage.

Effects of transforming growth factor-beta on murine astrocyte glutamine synthetase activity. Implications in neuronal injury

Cytokines have been implicated in the pathogenesis of a number of brain diseases in which neurological dysfunction has been attributed to a change in amino acid neurotransmitter metabolism. In the present in vitro study, we investigated the effects of cytokines on astrocyte glutamine synthetase (GS) activity and subsequently on N-methyl-D-aspartate (NMDA) receptor-mediated neurotoxicity. Proinflammatory cytokines IL-1 alpha, IL-1 beta, and IL-6 at a concentration of 20 ng/ml did not affect GS activity; however, tumor necrosis factor-alpha inhibited this activity by 20% in mixed neuronal/astrocyte cultures. Treatment for 24 h with transforming growth factor (TGF)-beta 1 or -beta 2 inhibited up to 60% GS activity. TGF-beta 2 also inhibited GS in enriched astrocyte cultures with an ED50 of 10 pg/ml. Antibodies specific to TGF-beta 2 blocked this effect. Treatment of astrocytes with TGF-beta 2 (250 pg/ml) resulted in markedly dilated rough endoplasmic reticulum. Since astrocyte GS may play a protective role in NMDA receptor-mediated neurotoxicity, we treated mixed neuronal/astrocyte cultures with TGF-beta 2 (250 pg/ml) and found a threefold potentiation of NMDA receptor-mediated neurotoxicity. These data suggest that TGF-beta impairs astrocyte GS function and enhances neurotoxicity, thus providing insight into understanding one mechanism of cytokine-mediated central nervous system disease.

Neuroprotective effects of graded reoxygenation following chronic hypoxia in neuronal cell cultures

The present study was undertaken to investigate the comparative effects of rapid vs graded correction of chronic hypoxia in vitro. Cerebral cortical cell cultures obtained from fetal mice were exposed to 5% O2 for 24 h and returned immediately to room air for the following 24 h (Group I); comparable cultures were exposed to 5% O2 for 24 h followed by 10% O2 for an additional 24 h before return to room air (Group II). At the conclusion of the experimental protocol (time 0), partial pressure of oxygen in the bathing medium of Group I cultures was significantly higher than that of Group II and non-hypoxic controls (151 mmHg vs 124 and 132 mmHg, respectively; P less than 0.05). Throughout the recovery period, Group II cultures evidenced improved neuronal survival (e.g. 35,800 vs 17,700 neurons/culture well at time 0, P less than 0.01), decreased lactate dehydrogenase efflux into the bathing medium, relative preservation of neuronal morphology, as well as higher specific and clonazepam-displaceable benzodiazepine binding and GABA uptake. Glutamate binding was not differentially affected and glutamine synthetase activity, a predominantly glial marker, was only modestly increased after graded reoxygenation. These results demonstrate that gradual reoxygenation after prolonged hypoxia in vitro (i) improves neuronal survival compared to rapid reoxygenation and (ii) delays the manifestations of metabolic dysfunction even though the length of hypoxic exposure is increased. The findings are also consistent with the concept that a period of relative hyperoxia may contribute to hypoxia-induced neuronal injury.

1-Aminocyclopentane-trans-1,3-Dicarboxylic Acid Induces Glutamine Synthetase Activity in Cultured Astrocytes

In this study we have investigated the effect of excitatory amino acids on the activity of glutamine synthetase, a glial-specific enzyme that plays a key role in the regulation of glutamate concentration in the CNS. We found that of L-glutamate, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, kainate, and 1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), only the metabotropic glutamate receptor agonist trans-ACPD had an effect on glutamine synthetase specific activity in cultures of rat type I cortical astrocytes. Exposure of astrocytes to 1.0 mM trans-ACPD for 24 h resulted in an increase in glutamine synthetase activity to 149 +/- 11% of that in control cultures. This effect was concentration dependent, stereoselective, and blocked by cycloheximide. In addition, the increase in glutamine synthetase activity occurred at lower concentrations of trans-ACPD that did not produce morphological alterations or lysis of the astrocytes as measured by the lactate dehydrogenase content. These findings are consistent with the hypothesis that activation of the metabotropic excitatory amino acid receptor in astrocytes is coupled to the regulation of an enzyme essential to the metabolism and recycling of the excitatory transmitter L-glutamate.

Brain glutamine synthetase increases following cerebral ischemia in the rat

Changes in astrocyte glutamine synthetase (GS) in postischemic rat brain were evaluated and correlated with regional neuronal vulnerability or resistance to ischemia. Rats subjected to 20 or 30 min of cerebral ischemia were allowed to survive for 3 or 24 h after ischemia; normal animals served as controls. Resultant neuronal necrosis was severe in the striatum by 24 h and in the CA1 region of the hippocampus at 72 h; neurons in paramedian cortex and CA3 region of the hippocampus were not permanently damaged. Glutamine synthetase (GS) immunocytochemistry was performed on vibratome sections of paraformaldehyde-fixed brains and enzyme activity was assayed in frozen samples of cerebral cortex, striatum and hippocampus. At 3 and 24 h after ischemia, GS immunoreactivity increased and was secondary to enlargement of GS-positive cell bodies and processes as well as to increased numbers of GS-positive astrocytes. Enzyme activity also increased in cortex, striatum and hippocampus at 3 and 24 h (P less than or equal to 0.03). This study shows that increase in astrocyte GS occurs rapidly after ischemia, and prior studies indicate that this increase occurs in parallel with proliferative changes in astrocyte organelles. The results also suggest that astrocyte metabolism of glutamate increases after ischemia. The increased capacity for glutamine synthetase may be important in normalizing extracellular glutamate following ischemia and protecting brain from the neurotoxic effects of this excitatory amino acid.

2-APV and DGAMS are superior to MK-801 in preventing hypoxia-induced injury to developing neurons in vitro

The relative efficacy of competitive and noncompetitive excitatory amino acid antagonists in preventing hypoxic neuronal injury recently was examined in vitro. Immature (26 days post-conception) fetal mouse cerebral cortical cell cultures were exposed 10 days after plating to 5% oxygen for 24 hrs and returned to normoxia. After hypoxic insult, cultures were either not treated or the medium was supplemented with the competitive excitatory amino acid antagonists 2-amino-5-phosphonovalerate (2-APV), gamma-D-glutamylaminomethylsulphonate (DGAMS), or the noncompetitive antagonist methyl-10,11-dihydro-5-H-dibenzocyclohepten-5,10-imine maleate (MK-801). By 48 hrs after restitution of normoxia, untreated hypoxic cultures evidenced severe neuronal deterioration, elevated LDH concentrations in the medium, depressed benzodiazepine receptor binding, and reduced GABA and glutamate uptake. Enhanced glial cell activity was reflected by modestly elevated glutamine synthetase activity. In hypoxic cultures treated with 2-APV (10 microM) or DGAMS (30 microM), neuronal morphology and biochemical profiles were both improved significantly when compared to both untreated hypoxic cultures and also to those treated with MK-801; 2-APV provided greater, although incomplete, protection. MK-801, at the highest nonneurotoxic concentration (25 nM), did not improve neuronal viability when compared to untreated controls. These results suggest that competitive excitatory amino acid antagonists are superior to noncompetitive antagonists in preventing hypoxic neuronal injury to developing neurons in vitro. MK-801, at low concentrations, produced significant neurotoxicity without improving cell survival.

Effects of phenobarbital and phenytoin on cortical glial cells in culture

Studies were undertaken to determine the effects of 7-day phenobarbital and phenytoin exposure on 14-day-old glial cell cultures of fetal murine cortex. Biochemical markers monitored were Ro5-4684-displaceable 3H-flunitrazepam binding, 3H-beta-alanine uptake, glutamine synthetase activity, and protein content. Phenobarbital concentrations were 30, 60, and 120 micrograms/ml and phenytoin concentrations 15, 30, 60 micrograms/ml. There were no discernible phase microscopic changes at any concentration of either drug. Phenobarbital produced no significant changes in the biochemical measures monitored. Exposure to phenytoin produced no biochemical changes at 15 micrograms/ml, but did produce significant changes at 30 and 60 micrograms/ml. There was an increase in Ro5-4684-displaceable 3H-flunitrazepam binding signifying increased binding or an increase in the number of binding sites and perhaps an increased population of glial cells although, the unchanged protein content suggests that the number of glial cells was not increased. There was a decrease with 30 and 60 micrograms/ml phenytoin of 3H-beta-alanine uptake suggesting interference with normal membrane transport of this compound. The latter effect may well mirror changes in GABA uptake in glial cells in the presence of phenytoin.

GABA accumulating neurons are relatively resistant to chronic hypoxia in vitro: an autoradiographic study

Whether there is preferential loss of certain types of nerve cells or specific cellular functions after hypoxic or ischemic insults remains unclear. To evaluate this phenomenon in vitro, the vulnerability of GABAergic neurons to hypoxia was investigated both quantitatively and with autoradiography. Immature neuronal cortical cultures obtained from fetal mice were subjected to chronic hypoxia (5% O2) for 24 h or 48 h and then returned to the normoxic condition for 48 h. The shorter hypoxic exposure resulted in significantly reduced numbers of neurons in comparison to the longer exposure and also to controls (29% and 26%, respectively; p less than 0.001). LDH efflux, a reliable indicator of cell damage, also was higher after the shorter exposure insult. Nevertheless, in these same 24 h hypoxic cultures there was prominent sparing of those neurons which accumulate GABA: by 48 h of recovery GABAergic neurons constituted 29.3 +/- 2.0% of the remaining neuronal population in comparison to 11.6 +/- 0.6 and 14.4 +/- 0.8% for controls and 48 h hypoxia, respectively; (p less than 0.001). Although total GABA uptake per neuron was significantly decreased after both types of insult, there was a concomitant increase in glial GABA uptake (i.e., that which could be displaced by beta-alanine). These observations suggest that certain GABAergic cortical neurons are relatively more resistant to chronic hypoxia than the general neuronal population and that depression of overall neuronal GABA uptake may be associated with enhanced glial GABA uptake.