[U-13C]glutamate metabolism in astrocytes during hypoglycemia and hypoxia

Environmental toxicants in the brain: A review of astrocytic metabolic dysfunction

Exposure to environmental toxicants is linked to long-term adverse outcomes in the brain and involves the dysfunction of glial and neuronal cells. Astrocytes, the most numerous cell type, are increasingly implicated in the pathogenesis of many diseases of the central nervous system, including neurodegenerative diseases. Astrocytes are critical for proper brain function in part due to their robust antioxidant and unique metabolic capabilities. Additionally, astrocytes are positioned both at the blood-brain barrier, where they are the primary responders to xenobiotic penetrance of the CNS, and at synapses where they are in close contact with neurons and synaptic machinery. While exposure to several classes of environmental toxicants, including chlorinated and fluorinated compounds, and trace metals, have been implicated in neurodegenerative diseases, their impact on astrocytes represents an important and growing field of research. Here, we review existing literature focused on the impact of a range of synthetic compounds on astrocytic function. We focus specifically on perturbed metabolic processes in response to these compounds and consider how perturbation of these pathways impacts disease pathogenesis.

Metabolic and perfusion responses to acute hypoglycemia in the rat cortex: A non-invasive magnetic resonance approach

Hypoglycemia is critical condition during diabetic treatment that involves intensive insulin therapy, and it may impair brain function. We aimed to compare cortical responses of three hypoglycemic phases and the restoration of glycemia to control levels after a severe episode in rats using non-invasive perfusion magnetic resonance (MR) imaging and localized ¹ H MR spectroscopy. Under light α-chloralose anesthesia, cortical blood flow (cCBF) was 42 ± 3 ml/100 g/min at euglycemia (~ 5 mM plasma glucose), was not altered at mild hypoglycemia I (42 ± 4 ml/100 g/min, 2-3.5 mM), increased to 60 ± 8 ml/100 g/min under moderate hypoglycemia II (1-2 mM) and amplified to 190 ± 35 ml/100 g/min at severe hypoglycemia III (< 1 mM). ¹ H MRS revealed metabolic changes at hypoglycemia I without any perfusion alteration. At hypoglycemia III, glutamine and glutamate decreased, whereas aspartate increased. When animals subsequently regained glycemic control, not all metabolites returned to their control levels, for example, glutamine. Meanwhile, ascorbate was increased with amplified hypoglycemic severity, whereas glutathione was reduced; these compounds did not return to normal levels upon the restoration of glycemia. Our study is the first to report cCBF and neurochemical changes in cortex upon five glycemic stages. The cortical responses of different hypoglycemic phases would explain variable neuronal damages after hypoglycemia and might help identify the degrees of hypoglycemic insults and further improve alternative therapies.

AMP-activated protein kinase (AMPK) regulates astrocyte oxidative metabolism by balancing TCA cycle dynamics

AMP-activated protein kinase (AMPK) is an important energy sensor located in cells throughout the human body. From the periphery, AMPK is known to be a metabolic master switch controlling the use of energy fuels. The energy sensor is activated when the energy status of the cell is low, initiating energy-producing pathways and deactivating energy-consuming pathways. All brain cells are crucially dependent on energy production for survival, and the availability of energy substrates must be closely regulated. Intriguingly, the role of AMPK in the regulation of brain cell metabolism has been sparsely investigated, particularly in astrocytes. By investigating metabolism of ¹³ C-labeled energy substrates in acutely isolated hippocampal slices and cultured astrocytes, with subsequent mass spectrometry analysis, we here show that activation of AMPK increases glycolysis as well as the capacity of the TCA cycle, that is, anaplerosis, through the activity of pyruvate carboxylase (PC) in astrocytes. In addition, we demonstrate that AMPK activation leads to augmented astrocytic glutamate oxidation via pyruvate recycling (i.e., cataplerosis). This regulatory mechanism induced by AMPK activation is mediated via glutamate dehydrogenase (GDH) shown in a CNS-specific GDH knockout mouse. Collectively, these findings demonstrate that AMPK regulates TCA cycle dynamics in astrocytes via PC and GDH activity. AMPK functionality has been shown to be hampered in Alzheimer's and Parkinson's disease and our findings may therefore add to the toolbox for discovery of new metabolic drug targets.

Opposing effects of 2-deoxy-d-glucose on interictal- and ictal-like activity when K+ currents and GABAA receptors are blocked in rat hippocampus in vitro

The ketogenic diet (KD), a high-fat, carbohydrate-restricted diet, is used as an alternative treatment for drug-resistant epileptic patients. Evidence suggests that compromised glucose metabolism has a significant role in the anticonvulsant action of the KD; however, it is unclear what part of the glucose metabolism that is important. The present study investigates how selective alterations in glycolysis and oxidative phosphorylation influence epileptiform activity induced by blocking K+ currents and GABAA and NMDA receptors in the hippocampal slice preparation. Blocking glycolysis with the glucose derivative 2-deoxy-d-glucose (2-DG; 10 mM) gave a fast reduction of the frequency of interictal discharge (IED) consistent with findings in other in vitro models. However, this was followed by the induction of seizure-like discharges in area CA1 and CA3. Substituting glucose with sucrose (glucopenia) had effects similar to those of 2-DG, whereas substitution with l-lactate or pyruvate reduced the IED but had a less proconvulsant effect. Blockade of ATP-sensitive K+ channels, glycine or adenosine 1 receptors, or depletion of the endogenous anticonvulsant compound glutathione did not prevent the actions of 2-DG. Baclofen (2 μM) reproduced the effect of 2-DG on IED activity. The proconvulsant effect of 2-DG could be reproduced by blocking the oxidative phosphorylation with the complex I toxin rotenone (4 μM). The data suggest that inhibition of IED, induced by 2-DG and glucopenia, is a direct consequence of impairment of glycolysis, likely exerted via a decreased recurrent excitatory synaptic transmission in area CA3. The accompanying proconvulsant effect is caused by an excitatory mechanism, depending on impairment of oxidative phosphorylation.

NEW & NOTEWORTHY This study reveals two opposing effects of 2-deoxy-d-glucose (2-DG) and glucopenia on in vitro epileptiform discharge observed during combined blockade of K+ currents and GABAA receptors. Interictal-like activity is inhibited by a mechanism that selectively depends on impairment of glycolysis and that results from a decrease in the strength of excitatory recurrent synaptic transmission in area CA3. In contrast, 2-DG and glucopenia facilitate ictal-like activity by an excitatory mechanism, depending on impairment of mitochondrial oxidative phosphorylation.

Mitochondrial function in Müller cells - Does it matter?

Growing evidence suggests that mitochondrial dysfunction might play a key role in the pathogenesis of age-related neurodegenerative inner retinal diseases such as diabetic retinopathy and glaucoma. Therefore, the present review provides a perspective on the impact of functional mitochondria in the most predominant glial cells of the retina, the Müller cells. Müller cells span the entire thickness of the neuroretina and are in close proximity to retinal cells including the retinal neurons that provides visual signaling to the brain. Among multiple functions, Müller cells are responsible for the removal of neurotransmitters, buffering potassium, and providing neurons with essential metabolites. Thus, Müller cells are responsible for a stable metabolic dialogue in the inner retina and their crucial role in supporting retinal neurons is indisputable. Müller cell functions require considerable energy production and previous literature has primarily emphasized glycolysis as the main energy provider. However, recent studies highlight the need of mitochondrial ATP production to upheld Müller cell functions. Therefore, the present review aims to provide an overview of the current evidence on the impact of mitochondrial functions in Müller cells.

Glutamate oxaloacetate transaminase enables anaplerotic refilling of TCA cycle intermediates in stroke-affected brain

Ischemic stroke results in excessive release of glutamate, which contributes to neuronal cell death. Here, we test the hypothesis that otherwise neurotoxic glutamate can be productively metabolized by glutamate oxaloacetate transaminase (GOT) to maintain cellular energetics and protect the brain from ischemic stroke injury. The GOT-dependent metabolism of glutamate was studied in primary neural cells and in stroke-affected C57-BL6 mice using magnetic resonance spectroscopy and GC-MS. Extracellular Glu sustained cell viability under hypoglycemic conditions and increased GOT-mediated metabolism in vitro Correction of stroke-induced hypoxia using supplemental oxygen in vivo lowered Glu levels as measured by ¹H magnetic resonance spectroscopy. GOT knockdown abrogated this effect and caused ATP loss in the stroke-affected brain. GOT overexpression increased anaplerotic refilling of tricarboxylic acid cycle intermediates in mouse brain during ischemic stroke. Furthermore, GOT overexpression not only reduced ischemic stroke lesion volume but also attenuated neurodegeneration and improved poststroke sensorimotor function. Taken together, our results support a new paradigm that GOT enables metabolism of otherwise neurotoxic extracellular Glu through a truncated tricarboxylic acid cycle under hypoglycemic conditions.-Rink, C., Gnyawali, S., Stewart, R., Teplitsky, S., Harris, H., Roy, S., Sen, C. K., Khanna, S. Glutamate oxaloacetate transaminase enables anaplerotic refilling of TCA cycle intermediates in stroke-affected brain.

The Role of Circulating Amino Acids in the Hypothalamic Regulation of Liver Glucose Metabolism

A pandemic of diabetes and obesity has been developing worldwide in close association with excessive nutrient intake and a sedentary lifestyle. Variations in the protein content of the diet have a direct impact on glucose homeostasis because amino acids (AAs) are powerful modulators of insulin action. In this work we review our recent findings on how elevations in the concentration of the circulating AAs leucine and proline activate a metabolic mechanism located in the mediobasal hypothalamus of the brain that sends a signal to the liver via the vagus nerve, which curtails glucose output. This neurogenic signal is strictly dependent on the metabolism of leucine and proline to acetyl-coenzyme A (CoA) and the subsequent production of malonyl-CoA; the signal also requires functional neuronal ATP-sensitive potassium channels. The liver then responds by lowering the rate of gluconeogenesis and glycogenolysis, ultimately leading to a net decrease in glucose production and in concentrations of circulating glucose. Furthermore, we review here how our work with proline suggests a new role of astrocytes in the central regulation of glycemia. Last, we outline how factors such as the consumption of fat-rich diets can interfere with glucoregulatory mechanisms and, in the long term, may contribute to the development of hyperglycemia, a hallmark of type 2 diabetes.

A Cytotoxic, Co-operative Interaction Between Energy Deprivation and Glutamate Release From System xc- Mediates Aglycemic Neuronal Cell Death

The astrocyte cystine/glutamate antiporter (system xc(-)) contributes substantially to the excitotoxic neuronal cell death facilitated by glucose deprivation. The purpose of this study was to determine the mechanism by which this occurred. Using pure astrocyte cultures, as well as, mixed cortical cell cultures containing both neurons and astrocytes, we found that neither an enhancement in system xc(-) expression nor activity underlies the excitotoxic effects of aglycemia. In addition, using three separate bioassays, we demonstrate no change in the ability of glucose-deprived astrocytes--either cultured alone or with neurons--to remove glutamate from the extracellular space. Instead, we demonstrate that glucose-deprived cultures are 2 to 3 times more sensitive to the killing effects of glutamate or N-methyl-D-aspartate when compared with their glucose-containing controls. Hence, our results are consistent with the weak excitotoxic hypothesis such that a bioenergetic deficiency, which is measureable in our mixed but not astrocyte cultures, allows normally innocuous concentrations of glutamate to become excitotoxic. Adding to the burgeoning literature detailing the contribution of astrocytes to neuronal injury, we conclude that under our experimental paradigm, a cytotoxic, co-operative interaction between energy deprivation and glutamate release from astrocyte system xc(-) mediates aglycemic neuronal cell death.

Hypoglycemia-induced alterations in hippocampal intrinsic rhythms: Decreased inhibition, increased excitation, seizures and spreading depression

Seizures are the most common clinical presentation of severe hypoglycemia, usually as a side effect of insulin treatment for juvenile onset type 1 diabetes mellitus and advanced type 2 diabetes. We used the mouse thick hippocampal slice preparation to study the pathophysiology of hypoglycemia-induced seizures and the effects of severe glucose depletion on the isolated hippocampal rhythms from the CA3 circuitry.

METHODS AND RESULTS

Dropping the glucose perfusate concentration from the standard 10 mM to 1 mM produced epileptiform activity in 14/16 of the slices. Seizure-like events (SLEs) originated in the CA3 region and then spread into the CA1 region. Following the SLE, a spreading-depression (SD)-like event occurred (12/16 slices) with irreversible synaptic failure in the CA1 region (8/12 slices). CA3 SD-like events followed ~30 s after the SD-like event in the CA1 region. Less commonly, SD-like events originated in the CA3 region (4/12). Additionally, prior to the onset of the SLE in the CA3 area, there was decreased GABA correlated baseline SPW activity (bSPW), while there was increased large-amplitude sharp wave (LASW) activity, thought to originate from synchronous pyramidal cell firing. CA3 pyramidal cells displayed progressive tonic depolarization prior to the seizure which was resistant to synaptic transmission blockade. The initiation of hypoglycemic seizures and SD was prevented by AMPA/kainate or NMDA receptor blockade.

CONCLUSIONS

Severe glucose depletion induces rapid changes initiated in the intrinsic CA3 rhythms of the hippocampus including depressed inhibition and enhanced excitation, which may underlie the mechanisms of seizure generation and delayed spreading depression.

Glutamate synthesis has to be matched by its degradation - where do all the carbons go?

The central process in energy production is the oxidation of acetyl-CoA to CO2 by the tricarboxylic acid (TCA, Krebs, citric acid) cycle. However, this cycle functions also as a biosynthetic pathway from which intermediates leave to be converted primarily to glutamate, GABA, glutamine and aspartate and to a smaller extent to glucose derivatives and fatty acids in the brain. When TCA cycle ketoacids are removed, they must be replaced to permit the continued function of this essential pathway, by a process termed anaplerosis. Since the TCA cycle cannot act as a carbon sink, anaplerosis must be coupled with cataplerosis; the exit of intermediates from the TCA cycle. The role of anaplerotic reactions for cellular metabolism in the brain has been studied extensively. However, the coupling of this process with cataplerosis and the roles that both pathways play in the regulation of amino acid, glucose, and fatty acid homeostasis have not been emphasized. The concept of a linkage between anaplerosis and cataplerosis should be underscored, because the balance between these two processes is essential. The hypothesis that cataplerosis in the brain is achieved by exporting the lactate generated from the TCA cycle intermediates into the blood and perivascular area is presented. This shifts the generally accepted paradigm of lactate generation as simply derived from glycolysis to that of oxidation and might present an alternative explanation for aerobic glycolysis. Intermediates leave the tricarboxylic acid cycle and must be replaced by a process termed anaplerosis that must be coupled to cataplerosis. We hypothesize that cataplerosis is achieved by exporting the lactate generated from the cycle into the blood and perivascular area. This shifts the paradigm of lactate generation as solely derived from glycolysis to that of oxidation and might present an alternative explanation for aerobic glycolysis.

Chromium modulates expressions of neuronal plasticity markers and glial fibrillary acidic proteins in hypoglycemia-induced brain injury

AIMS

This experiment investigated if chromium (Cr) as Cr-histidinate (CrHis) and Cr29 picolinate (CrPic) have a protective role in rats with hypoglycemia-induced brain injury, assessed by neuronal plasticity and regeneration potential.

MAIN METHODS

Male Sprague-Dawley rats were prospectively divided into 2 groups: control and hypoglycemic (induced by insulin administration, 15U/kg, i.p., n=56). Hypoglycemic rats were then received randomly 1) none, 2) dextrose (on the day of sampling), 3) CrHis, or 4) CrPic. Cr-chelates were delivered via drinking water (providing 8μg elemental Cr per day) for one week prior to the hypoglycemia induction. The expressions of neuroplasticity markers [neural cell adhesion molecule (NCAM), growth-associated protein-43 (GAP-43), glial fibrillary acidic protein (GFAP)], glucose transporters (GLUT), and nuclear transcription proteins [nuclear factor-kappa (NF-κB), nuclear factor (erythroid-derived 2)-like 2 (Nrf2), and 4-hydroxyl nonenal (HNE)] were determined using Western blot.

KEY FINDINGS

Hypoglycemia caused increases in the expressions of GLUT-1, GLUT-3, GFAP, NF-κB and HNE and decreases in the expression of NCAM's, GAP-43 and Nrf2 in the hippocampus, cerebellum, and cortex. Cr-chelates suppressed expressions of GLUTs, GFAP, NF-κB and HNE expressions and enhanced expressions of NCAM, GAP-43 and Nrf2, which were more notable for CrHis than for CrPic.

SIGNIFICANCE

In conclusion, hypoglycemia leads to cerebral injury and Cr-chelates, particularly CrHis have protective and regeneration potential in cerebral tissues through modulating neuroplasticity markers and nuclear transcription proteins as well as facilitating glucose transporters.

Evidence for a role of proline and hypothalamic astrocytes in the regulation of glucose metabolism in rats

The metabolism of lactate to pyruvate in the mediobasal hypothalamus (MBH) regulates hepatic glucose production. Because astrocytes and neurons are functionally linked by metabolic coupling through lactate transfer via the astrocyte-neuron lactate shuttle (ANLS), we reasoned that astrocytes might be involved in the hypothalamic regulation of glucose metabolism. To examine this possibility, we used the gluconeogenic amino acid proline, which is metabolized to pyruvate in astrocytes. Our results showed that increasing the availability of proline in rats either centrally (MBH) or systemically acutely lowered blood glucose. Pancreatic clamp studies revealed that this hypoglycemic effect was due to a decrease of hepatic glucose production secondary to an inhibition of glycogenolysis, gluconeogenesis, and glucose-6-phosphatase flux. The effect of proline was mimicked by glutamate, an intermediary of proline metabolism. Interestingly, proline's action was markedly blunted by pharmacological inhibition of hypothalamic lactate dehydrogenase (LDH) suggesting that metabolic flux through LDH was required. Furthermore, short hairpin RNA-mediated knockdown of hypothalamic LDH-A, an astrocytic component of the ANLS, also blunted the glucoregulatory action of proline. Thus our studies suggest not only a new role for proline in the regulation of hepatic glucose production but also indicate that hypothalamic astrocytes are involved in the regulatory mechanism as well.

Which way does the citric acid cycle turn during hypoxia? The critical role of α-ketoglutarate dehydrogenase complex

The citric acid cycle forms a major metabolic hub and as such it is involved in many disease states involving energetic imbalance. In spite of the fact that it is being branded as a "cycle", during hypoxia, when the electron transport chain does not oxidize reducing equivalents, segments of this metabolic pathway remain operational but exhibit opposing directionalities. This serves the purpose of harnessing high-energy phosphates through matrix substrate-level phosphorylation in the absence of oxidative phosphorylation. In this Mini-Review, these segments are appraised, pointing to the critical importance of the α-ketoglutarate dehydrogenase complex dictating their directionalities.

Non-cell autonomous influence of the astrocyte system xc- on hypoglycaemic neuronal cell death

Despite longstanding evidence that hypoglycaemic neuronal injury is mediated by glutamate excitotoxicity, the cellular and molecular mechanisms involved remain incompletely defined. Here, we demonstrate that the excitotoxic neuronal death that follows GD (glucose deprivation) is initiated by glutamate extruded from astrocytes via system xc---an amino acid transporter that imports L-cystine and exports L-glutamate. Specifically, we find that depriving mixed cortical cell cultures of glucose for up to 8 h injures neurons, but not astrocytes. Neuronal death is prevented by ionotropic glutamate receptor antagonism and is partially sensitive to tetanus toxin. Removal of amino acids during the deprivation period prevents--whereas addition of L-cystine restores--GD-induced neuronal death, implicating the cystine/glutamate antiporter, system xc-. Indeed, drugs known to inhibit system xc- ameliorate GD-induced neuronal death. Further, a dramatic reduction in neuronal death is observed in chimaeric cultures consisting of neurons derived from WT (wild-type) mice plated on top of astrocytes derived from sut mice, which harbour a naturally occurring null mutation in the gene (Slc7a11) that encodes the substrate-specific light chain of system xc- (xCT). Finally, enhancement of astrocytic system xc- expression and function via IL-1β (interleukin-1β) exposure potentiates hypoglycaemic neuronal death, the process of which is prevented by removal of l-cystine and/or addition of system xc- inhibitors. Thus, under the conditions of GD, our studies demonstrate that astrocytes, via system xc-, have a direct, non-cell autonomous effect on cortical neuron survival.

Metabolic fate of a high concentration of glutamine and glutamate in rat brain slices: a ¹³C NMR study

This study was performed to analyze the metabolic fate of a high concentration (5 mM) of glutamine and glutamate in rat brain slices and the participation of these amino acids in the glutamine-glutamate cycle. For this, brain slices were incubated for 60 min with [3-¹³C]glutamine or [3-¹³C]glutamate. Tissue plus medium extracts were analyzed by enzymatic and ¹³C NMR measurements and fluxes through pathways of glutamine and glutamate metabolism were calculated. We demonstrate that both substrates were utilized and oxidized at high rates by rat brain slices and served as precursors of neurotransmitters, tricarboxylic acid (TCA) cycle intermediates and alanine. In order to determine the participation of glutamine synthetase in the appearance of new glutamine molecules with glutamine as substrate, brain slices were incubated with [3-¹³C]glutamine in the presence of methionine sulfoximine, a specific inhibitor of glutamine synthetase. Our results indicate that 36.5% of the new glutamine appeared was glutamine synthetase-dependent and 63.5% was formed from endogenous substrates. Flux through glutamic acid decarboxylase was higher with glutamine than with glutamate as substrate whereas fluxes from α-ketoglutarate to glutamate and through glutamine synthetase, malic enzyme, pyruvate dehydrogenase, pyruvate carboxylase and citrate synthase were in the same range with both substrates.

Neurochemical changes in the developing rat hippocampus during prolonged hypoglycemia

Hypoglycemia is common during development and is associated with the risk of neurodevelopmental deficits in human infants. The effects of hypoglycemia on the developing hippocampus are poorly understood. The sequential changes in energy substrates, amino acids and phosphocreatine were measured from the hippocampus during 180 min of insulin-induced hypoglycemia (blood glucose < 2.5 mmol/L) in 14-day-old rats using in vivo(1)H NMR spectroscopy. Hypoglycemia resulted in neuroglycopenia (brain glucose < 0.5 micromol/g). However, the phosphocreatine/creatine (PCr/Cr) ratio was maintained in the physiological range until approximately 150 min of hypoglycemia, indicating that energy supply was sufficient to meet the energy demands. Lactate concentration decreased soon after the onset of neuroglycopenia. Beyond 60 min, glutamine and glutamate became the major energy substrates. A precipitous decrease in the PCr/Cr ratio, indicative of impending energy failure occurred only after significant depletion of these amino acids. Once glutamate and glutamine were significantly exhausted, aspartate became the final energy source. N-acetylaspartate concentration remained unaltered, suggesting preservation of neuronal/mitochondrial integrity during hypoglycemia. Correction of hypoglycemia normalized the PCr/Cr ratio and partially restored the amino acids to pre-hypoglycemia levels. Compensatory neurochemical changes maintain energy homeostasis during prolonged hypoglycemia in the developing hippocampus.

Differential effects of iodoacetamide and iodoacetate on glycolysis and glutathione metabolism of cultured astrocytes

Iodoacetamide (IAA) and iodoacetate (IA) have frequently been used to inhibit glycolysis, since these compounds are known for their ability to irreversibly inhibit the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). However, the consequences of a treatment with such thiol reagents on the glutathione (GSH) metabolism of brain cells have not been explored. Exposure of astroglia-rich primary cultures to IAA or IA in concentrations of up to 1 mM deprived the cells of GSH, inhibited cellular GAPDH activity, lowered cellular lactate production and caused a delayed cell death that was detectable after 90 min of incubation. However, the two thiol reagents differed substantially in their potential to deprive cellular GSH and to inhibit astrocytic glycolysis. IAA depleted the cellular GSH content more efficiently than IA as demonstrated by half-maximal effects for IAA and IA that were observed at concentrations of about 10 and 100 μM, respectively. In contrast, IA was highly efficient in inactivating GAPDH and lactate production with half-maximal effects observed already at a concentration below 100 μM, whereas IAA had to be applied in 10 times higher concentration to inhibit lactate production by 50%. These substantial differences of IAA and IA to affect GSH content and glycolysis of cultured astrocytes suggest that in order to inhibit astrocytic glycolysis without substantially compromising the cellular GSH metabolism, IA – and not IAA – should be used in low concentrations and/or for short incubation periods.

Pyruvate recycling in cultured neurons from cerebellum

Pyruvate recycling is a pathway for complete oxidation of glutamate. The cellular location and the physiological significance of such recycling has been debated during the last decade. The present study was aimed at elucidating whether recycling takes place in neuron-enriched cultures of dissociated cerebella, consisting mainly of glutamatergic granule cells, some GABAergic neurons, and few astrocytes. These cultures and cultures of astrocytes from cerebellum were incubated in medium containing [U-(13)C]glutamate, and cell extracts were analyzed by gas chromatography and mass spectrometry. Additionally, in the case of the neuron-enriched cultures, a magnetic resonance (MR) spectrum was obtained. It could be shown that the atom percentage excess of the isotopomer representing pyruvate recycling in glutamate (M + 4) was similar for astrocytes and neuron-enriched cultures. However, the latter showed more recycling in glutamine (synthesized in the small fraction of astrocytes) than the pure astrocyte cultures, whereas the reverse was the case for aspartate. In fact, the atom percentage excess of the isotopomer representing pyruvate recycling in glutamine was slightly but significantly higher than that in glutamate in the neuron-enriched cultures. It can be concluded that pyruvate recycling is clearly present in neurons, and this was verified by MR spectroscopy.

Glutamine as an energy substrate in cultured neurons during glucose deprivation

During glucose deprivation an increase in aspartate formation from glutamine has been observed in different brain preparations, including synaptosomes and cultured astrocytes. To what extent this reaction, which provides a substantial amount of energy, occurs in different types of neurons is unknown. The present study shows that (14)CO(2) formation from [U-(14)C]glutamine in cerebellar granule neurons, a glutamatergic preparation, increased by 60% during glucose deprivation, indicating enhanced aspartate formation or increased complete oxidative degradation of glutamine. In primary cultures of cerebrocortical interneurons, a GABAergic preparation, the rate of (14)CO(2) production from [U-(14) C] glutamine was four times lower and not stimulated by glucose deprivation. During incubation with glutamine (0.8 mM) as the only metabolic substrate, cerebellar granule cells maintained an oxygen consumption rate of 12 nmol/min/mg protein, corresponding to an aspartate formation of 8 nmol/min/mg protein (three oxidations occur between glutamine and aspartate) or to a total oxidative degradation of 3 nmol/min/mg protein. During glucose deprivation, the rate of aspartate formation increased, and during a 20-min incubation in phosphate-buffered saline it amounted to 3.3 nmol/min/mg protein at 0.2 mM glutamine, which might have been more if measured at 0.8 mM glutamine. These values are consistent with the rate of glutamine utilization calculated based on oxygen consumption and leaves open the possibility that some glutamine is completely degraded oxidatively, as has been shown by other authors based on pyruvate recycling and labeling of lactate from aspartate in cerebellar granule neurons.

Complex Glutamate Labeling from [U-13C]glucose or [U-13C]lactate in Co-cultures of Cerebellar Neurons and Astrocytes

Glutamate metabolism was studied in co-cultures of mouse cerebellar neurons (predominantly glutamatergic) and astrocytes. One set of cultures was superfused (90 min) in the presence of either [U-(13)C]glucose (2.5 mM) and lactate (1 mM) or [U-(13)C]lactate (1 mM) and glucose (2.5 mM). Other sets of cultures were incubated in medium containing [U-(13)C]lactate (1 mM) and glucose (2.5 mM) for 4 h. Regardless of the experimental conditions cell extracts were analyzed using mass spectrometry and nuclear magnetic resonance spectroscopy. (13)C labeling of glutamate was much higher than that of glutamine under all experimental conditions indicating that acetyl-CoA from both lactate and glucose was preferentially metabolized in the neurons. Aspartate labeling was similar to that of glutamate, especially when [U-(13)C]glucose was the substrate. Labeling of glutamate, aspartate and glutamine was lower in the cells incubated with [U-(13)C]lactate. The first part of the pyruvate recycling pathway, pyruvate formation, was detected in singlet and doublet labeling of alanine under all experimental conditions. However, full recycling, detectable in singlet labeling of glutamate in the C-4 position was only quantifiable in the superfused cells both from [U-(13)C]glucose and [U-(13)C]lactate. Lactate and alanine were mostly uniformly labeled and labeling of alanine was the same regardless of the labeled substrate present and higher than that of lactate when superfused in the presence of [U-(13)C]glucose. These results show that metabolism of pyruvate, the precursor for lactate, alanine and acetyl-CoA is highly compartmentalized.

Expression of neuronal plasticity markers in hypoglycemia induced brain injury

The expression of neuroplasticity markers was analyzed in four brain regions, namely cerebral hemispheres (CH), cerebellum (CB), brain stem (BS) and diencephalon (DC) from insulin-induced hypoglycemic young adult rats. Significant decrease in neural cell adhesion molecule (NCAM) isoforms and growth-associated protein-43 (GAP-43) was observed following hypoglycemic injury from majority of brain regions studied. The glial fibrillary acidic protein (GFAP) level increased significantly in cerebral hemispheres and diencephalon regions, whereas, synaptophysin level increased in cerebellum, brain stem and diencephalon regions. The selective downregulation of the neuronal plasticity marker proteins (GAP-43 and NCAM), and enhanced expression of GFAP and synaptophysin suggests that in acute hypoglycemia, mechanisms other than energy failure may also contribute to neuronal cell damage in the brain.

Astrocytic amino acid metabolism under control conditions and during oxygen and/or glucose deprivation

Amino acid contents were measured in 1- and 3-week-old primary cultures of astrocytes and in their incubation media, an amino acid-free salt solution with or without glucose, during 3-h incubation under normoxic or anoxic conditions. Most essential amino acids were rapidly released to the medium during the beginning of the incubation. A subsequent slow medium increase reflected proteolysis. Glutamate and aspartate were absent from the media during all conditions, indicating fueling of their uptake by either glycolytically or oxidatively derived energy. The total content of glutamine increased, except during incubation in glucose-deprived media, when it declined or remained constant. Changes in aspartate were negligible, suggesting oxidative degradation of aspartate-derived oxaloacetate during normoxia and its reduction to succinate during anoxia, driving regeneration of NAD+ from NADH. An increase of alanine was reduced in glucose-free media, whereas serine showed especially large increase during isolated glucose deprivation, suggesting its production from glutamine via 3-phosphoglycerate.

Increased astrocyte reactivity in the hippocampus of murine models of type 1 diabetes: the nonobese diabetic (NOD) and streptozotocin-treated mice

Diabetes can be associated with cerebral dysfunction in humans and animal models of the disease. Moreover, brain anomalies and alterations of the neuroendocrine system are present in type 1 diabetes (T1D) animals, such as the spontaneous nonobese diabetic (NOD) mouse model and/or the pharmacological streptozotocin (STZ)-induced model. Because of the prevalent role of astrocytes in cerebral glucose metabolism and their intimate connection with neurones, we investigated hippocampal astrocyte alterations in prediabetic and diabetic NOD mice and STZ-treated diabetic mice. The number and cell area related to the glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes were quantified in the stratum radiatum region of the hippocampus by computerized image analysis in prediabetic (2, 4 and 8 weeks of age) and diabetic (16-week-old) NOD female mice, age and sex-matched lymphocyte-deficient NODscid and C57BL/6 control mice and, finally, STZ-induced diabetic and vehicle-treated nondiabetic 16-week-old C57BL/6 female mice. Astrocyte number was higher early in life in prediabetic NOD and NODscid mice than in controls, when transient hyperinsulinemia and low glycemia were found in these strains. The number and cell area of GFAP(+) cells further increased after the onset of diabetes in NOD mice. Similarly, in STZ-treated diabetic mice, the number of GFAP(+) cells and cell area were higher than in vehicle-treated mice. In conclusion, astrocyte changes present in genetic and pharmacological models of T1D appear to reflect an adaptive process to alterations of glucose homeostasis.

Demonstration of pyruvate recycling in primary cultures of neocortical astrocytes but not in neurons

Pyruvate recycling was studied in primary cultures of mouse cerebrocortical astrocytes, GABAergic cerebrocortical interneurons, and co-cultures consisting of both cell types by measuring production of [4-(13)C]glutamate from [3-(13)C]glutamate by aid of nuclear magnetic resonance spectroscopy. This change in the position of the label can only occur by entry of [3-(13)C]glutamate into the tricarboxylic acid (TCA) cycle, conversion of labeled alpha-ketoglutarate to malate or oxaloacetate, malic enzyme-mediated decarboxylation of malate to pyruvate or phosphoenolpyruvate carboxykinase-mediated conversion of oxaloacetate to phosphoenolpyruvate and subsequent hydrolysis of the latter to pyruvate, and introduction of the labeled pyruvate into the TCA cycle, i.e., after exit of the carbon skeleton of pyruvate from the TCA cycle followed by re-entry of the same pyruvate molecules via acetyl CoA. In agreement with earlier observations, pyruvate recycling was demonstrated in astrocytes, indicating the ability of these cells to undertake complete oxidative degradation of glutamate. The recycled [4-(13)C]glutamate was not further converted to glutamine, showing compartmentation of astrocytic metabolism. Thus, absence of recycling into glutamine in the brain in vivo cannot be taken as indication that pyruvate recycling is absent in astrocytes. No recycling could be demonstrated in the cerebrocortical neurons. This is consistent with a previously demonstrated lack of incorporation of label from glutamate into lactate, and it also indicates that mitochondrial malic enzyme is not operational. Nor was there any indication of pyruvate recycling in the co-cultures. Although this may partly be due to more rapid depletion of glutamate in the co-cultures, this observation at the very least indicates that pyruvate recycling is not up-regulated in the neuronal-astrocytic co-cultures.

Metabolic compartmentation in cortical synaptosomes: influence of glucose and preferential incorporation of endogenous glutamate into GABA

Metabolism of glutamine was determined under a variety of conditions to study compartmentation in cortical synaptosomes. The combined intracellular and extracellular amounts of [U-13C] GABA, [U-13C]glutamate and [U-13C]glutamine were the same in synaptosomes incubated with U-13C]glutamine in the presence and absence of glucose. However, the concentration of these amino acids was decreased in the latter group, demonstrating the requirement for glucose to maintain the size of neurotransmitter pools. In hypoglycemic synaptosomes more [U-13C]glutamine was converted to [U-13C]aspartate, and less glutamate was re-synthesized from the tricarboxylic acid (TCA) cycle, suggesting use of the partial TCA cycle from alpha-ketoglutarate to oxaloacetate for energy. Compartmentation was studied in synaptosomes incubated with glucose plus labeled and unlabeled glutamine and glutamate. Incubation with [U-13C]glutamine plus unlabeled glutamate gave rise to [U-13C]GABA but not labeled aspartate; however, incubation with [U-13C]glutamate plus unlabeled glutamine gave rise to [U-13C]aspartate, but not labeled GABA. Thus the endogenous glutamate formed via glutaminase in synaptic terminals is preferentially used for GABA synthesis, and is metabolized differently than glutamate taken up from the extracellular milieu.

Citrate, beneficial or deleterious in the CNS?

Cerebellar granule neurons were incubated with or without glucose (3 mM) in the presence or absence of citrate (20 mM) using normoxic and/or hypoxic incubation conditions. During 4 h of hypoglycemia and also during hypoxia plus hypoglycemia, citrate increased lactate dehydrogenase (LDH) leakage from the cells and decreased mitochondrial activity, the latter was also the case in the presence of glucose. After 24 h of hypoglycemia, however, citrate decreased LDH leakage slightly, possibly due to its metabolism in the tricarboxylic acid cycle under these conditions. It should be noted that during mild hypoxia plus hypoglycemia a reduced LDH leakage was observed when compared to hypoglycemia alone. The 4 h low oxygen period did protect the neurons also during the 20 h re-oxygenation period. The present study might indicate that incubation of brain cell cultures in an atmosphere of air (30% oxygen) and 5% CO2, which is used in most laboratories, can be toxic and that oxygen concentration should be lowered considerably to mimic conditions in the brain.

The post-mortem concentration of glutamate in the structures of rat brain as an exponent of short aversive sensory stimulation preceding death

The problem of post-mortem assessment of short central fear reaction preceding death has been obscure till now. The preliminary results obtained by authors have encouraged them to further research on this topic. Time of aversive sensory stimulation in the presented study was considerably decreased in comparison to the preliminary investigations. The concentration of glutamate was assayed in the selected structures of limbic system and in the cerebellum. The rats were subjected to different simultaneous modalities of sensory stimulation. The involvement of the hippocampi and the cerebellum was revealed as measured by glutamate concentration increase in the whole structures homogenates. The dominant role of mechanical stimulus has been suggested based on the measured increase of glutamate concentration in the whole cerebellum homogenate. By using the presented experimental paradigm a possible application of the biochemical assessment of human brain tissue might be developed in the future for implementing in the field of forensic pathology. The biochemical evaluation of "frozen frames" of neurotransmission can possibly help reconstruct the events which had happened just before sudden and violent death.

Glucose and lactate metabolism during brain activation

The dependence of brain function on blood glucose as a fuel does not exclude the possibility that lactate within the brain might be transferred between different cell types and serve as an energy source. It has been recently suggested that 1) about 85% of glucose consumption during brain activation is initiated by aerobic glycolysis in astrocytes, triggered by demand for glycolytically derived energy for Na+ -dependent accumulation of transmitter glutamate and its amidation to glutamine, and 2) the generated lactate is quantitatively transferred to neurons for oxidative degradation. However, astrocytic glutamate uptake can be fueled by either glycolytically or oxidatively derived energy, and the extent to which "metabolic trafficking" of lactate might occur during brain function is unknown. In this review, the potential for an astrocytic-neuronal lactate flux has been estimated by comparing rates of glucose utilization in brain and in cultured neurons and astrocytes with those for lactate release and uptake. Working brain tissue and isolated brain cells release large amounts of lactate. Cellular lactate uptake occurs by carrier-mediated facilitated diffusion and is normally limited by its dependence on metabolism of accumulated lactate to maintain a concentration gradient. The rate of this process is similar in cultured astrocytes and glutamatergic neurons, and, at physiologically occurring lactate concentrations, lactate uptake corresponds at most to 25% of the rate of glucose oxidation, which accordingly is the upper limit for "metabolic trafficking" of lactate. Because of a larger local release than uptake of lactate and the necessity for rapid lactate clearance to maintain the intracellular redox state to support lactate production in the presence of normal oxygen levels, brain activation in vivo is probably, in many cases, accompanied by a substantial overflow of glycolytically generated lactate, both to different brain areas and under some conditions (spreading depression, hyperammonemia) to circulating blood.

Role of astrocytes in pathogenesis of ischemic brain injury

Astrocytes play an important role in the homeostasis of the CNS both in normal conditions and after ischemic injury. The swelling of astrocytes is observed during and several seconds after brain ischemia. Then ischemia stimulates sequential morphological and biochemical changes in glia and induces its proliferation. Reactive astrocytes demonstrate stellate morphology, increased glial fibrillary acidic protein (GFAP) immunoreactivity, increased number of mitochondria as well as elevated enzymatic and non-enzymatic antioxidant activities. Astrocytes can re-uptake and metabolize glutamate and in this way they control its extracellular concentration. The ability of astrocytes to protect neurons against the toxic action of free radicals depends on their specific energy metabolism, high glutathione level, increased antioxidant enzyme activity (catalase, superoxide dismutase, glutathione peroxidase) and overexpression of antiapoptotic bcl-2 gene. Astrocytes produce cytokines (TNF-alpha, IL-1, IL-6) involved in the initiation and maintaining of immunological response in the CNS. In astrocytes, like in neurones, ischemia induces the expression of immediate early genes: c-fos, c-jun, fos B, jun B, jun D, Krox-24, NGFI-B and others. The protein products of these genes modulate the expression of different proteins, both destructive ones and those involved in the neuroprotective processes.

Effects of L-glutamate, D-aspartate, and monensin on glycolytic and oxidative glucose metabolism in mouse astrocyte cultures: further evidence that glutamate uptake is metabolically driven by oxidative metabolism

The hypothesis was tested that oxidative metabolism, mainly fueled by glutamate itself, provides the energy for active, Na(+),K(+)-ATPase-catalyzed Na(+) extrusion following glutamate uptake in conjunction with Na(+). This hypothesis was supported by the following observations: (i) glutamate had either no effect or caused a slight reduction in glycolytic rate, measured as deoxyglucose phosphorylation; (ii) D-aspartate, which is accumulated by the L-glutamate carrier, but cannot be metabolized by the cells, caused an increase in glycolytic rate; (iii) monensin which, like D-aspartate, stimulates the intracellular, Na(+)-activated site of the Na, K-ATPase and thus energy metabolism, but provides no metabolic substrate, stimulated both glycolysis and glucose oxidation; and (iv) oxidation of glucose was potently inhibited by glutamate, although glutamate is known to stimulate oxygen consumption in primary cultures of astrocytes, a combination showing that oxidation of a non-glucose substrate is increased in the presence of glutamate. These findings should be considered in attempts to understand metabolic interactions between neurons and astrocytes and regulation of energy metabolism in brain.

Glutamic acid decarboxylase-expressing astrocytes exhibit enhanced energetic metabolism and increase PC12 cell survival under glucose deprivation

Astrocytes play a key role by catabolizing glutamate from extracellular space into glutamine and tricarboxylic acid components. We previously produced an astrocytic cell line that constitutively expressed glutamic acid decarboxylase (GAD67), which converts glutamate into GABA to increase the capacity of astrocytes to metabolize glutamate. In this study, GAD-expressing astrocytes in the presence of glutamate were shown to have increased energy metabolism, as determined by a moderate increase of 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, by an increased ATP level, and by enhanced lactate release. These changes were due to GAD transgene expression because transient expression of a GAD antisense plasmid resulted in partial suppression of the ATP level increase. These astrocytes had an increased survival in response to glucose deprivation in the presence of glutamate compared with the parental astrocytes, and they were also able to enhance survival of a neuronal-like cell line (PC12) under glucose deprivation. This protection may be partially due to the increased lactate release by GAD-expressing astrocytes because PC12 cell survival was enhanced by lactate and pyruvate under glucose deprivation. These results suggest that the establishment of GAD expression in astrocytes enhancing glutamate catabolism could be an interesting strategy to increase neuronal survival under hypoglycemia conditions.

Glutamate in synaptic terminals is reduced by lack of glucose but not hypoxia in rat hippocampal slices

Although excessive release of the neurotransmitter glutamate contributes to ischemic neuronal damage, immunocytochemical studies have not found a loss of glutamate from ischemic axon terminals. We examined the effects of two components of ischemia, hypoxia and hypoglycemia, on glutamate loss from rat hippocampal slices. In vitro hypoglycemia induced by incubation for 1 h without glucose depleted 50% of glutamate from slices when ATP levels were about 5 nmol/mg protein. Hypoxic slices aerated with N2 reached similar ATP levels without significant glutamate depletion. To induce 50% glutamate losses with chemical hypoxia, ATP had to be depleted to < 1 nmol/mg protein. Immunocytochemical staining indicated that glutamate-like immunoreactivity was reduced throughout slices by hypoglycemia. Hypoxia decreased glutamate-like immunoreactivity in neuronal perikarya and dendrites of pyramidal cells and granule cells. However, in contrast to hypoglycemia, hypoxia maintained or increased glutamate-like immunoreactivity in many terminals. Hypoxia and hypoglycemia induced similar, ATP-dependent releases of glutamate into supernatants, which could account for only part of the hypoglycemic losses. The additional hypoglycemic losses were consistent with increased catabolism of glutamate. Glutamate losses from hypoglycemic terminals were reduced by blockade of aspartate aminotransferase or the tricarboxylic acid cycle. Exogenous glutamate increased glutamate in hypoglycemic slices to hypoxic levels and returned glutamate-like immunoreactivity to terminals, suggesting that terminals maintained glutamate uptake during metabolic insults. Hypoglycemia induces a large loss of glutamate that does not occur during hypoxia. The greater loss of glutamate from terminals during hypoglycemia is consistent with increased metabolism of glutamate via aspartate aminotransferase and not increased release of glutamate. Continued uptake of glutamate by hypoxic terminals may help to maintain their levels of glutamate. Hypoglycemic metabolism of glutamate may decrease pathologic glutamate release and contribute to the prolonged neurologic abnormalities associated with recovery from hypoglycemia.

Decreased glutamate metabolism in cultured astrocytes in the presence of thiopental

The effect of thiopental on glutamate metabolism was studied by 13C magnetic resonance spectroscopy. Cerebral cortical astrocytes were incubated with 0.5 mM [U-13C]glutamate for 2 hr in the presence of 0.5 or 1 mM thiopental. Labeled glutamate, glutamine, aspartate, and glutathione were observed in cell extracts, and glutamine, aspartate, and lactate in the medium. Not only present in the medium was uniformly labeled glutamate, but also glutamate derived from the tricarboxylic acid (TCA) cycle, and thus glutamate release could be detected. The amounts of [U-13C]glutamate and unlabeled glucose taken up by astrocytes were unchanged in the presence of 0.5 mM thiopental and decreased to about 50% and 80%, respectively when the concentration was increased to 1 mM. The amounts of most metabolites synthesized from [U-13C]glutamate were unchanged in the presence of 0.5 mM thiopental, but decreased [U-13C]glutamine, [U-13C]aspartate, and [U-13C]lactate were observed in the 1 mM group. Surprisingly, the amounts of [1,2,3-13C]glutamate, [2,3-13C]aspartate, and [3,4-13C]aspartate (2nd turn via the TCA cycle) were unchanged. However, this was not the case for [1,2-13C]lactate and [2,3-13C]lactate. Such variations indicate cellular compartmentation, possibly caused by a heterogeneous glutamate concentration within the cells affecting TCA cycle turnover rates differently.

[U-13C] aspartate metabolism in cultured cortical astrocytes and cerebellar granule neurons studied by NMR spectroscopy

The metabolism of [U-13C]aspartate was studied in cultured cortical astrocytes and cerebellar granule neurons in the presence of glucose and during inhibition of glycolysis. Redissolved, lyophilized cell extracts and incubation media were analyzed by 13C nuclear magnetic resonance spectroscopy for the determination of metabolites labeled from aspartate. Uniformly labeled lactate was prominent in control media of astrocytes and cerebellar granule neurons. In both cell types, aspartate entered the tricarboxylic acid (TCA) cycle, as shown by labeling patterns in glutamate and, in astrocytes, in glutamine. From the complex labeling patterns in aspartate in astrocytic perchloric acid extracts it was clear that acetylcoenzyme A (acetyl-CoA) derived from aspartate via oxaloacetate and pyruvate could enter the TCA cycle. Such "recycling," however, could not be detected in cerebellar granule neurons. Inhibition of glycolysis reduced aspartate uptake and metabolism in both cell types. Most notably, lactate derived from aspartate showed a large reduction, and in astrocytes, incorporation of labeled acetyl-CoA into the TCA cycle was significantly reduced. Thus, astrocytes and cerebellar granule neurons differ in their handling of aspartate. Furthermore, inhibition of glycolysis clearly affected aspartate metabolism by such cells.