Kinetic characteristics of the glutamate uptake into normal astrocytes in cultures

A tribute to Leif Hertz: The historical context of his pioneering studies of the roles of astrocytes in brain energy metabolism, neurotransmission, cognitive functions, and pharmacology identifies important, unresolved topics for future studies

Leif Hertz, M.D., D.Sc. (honōris causā) (1930-2018), was one of the original and noteworthy participants in the International Conference on Brain Energy Metabolism (ICBEM) series since its inception in 1993. The biennial ICBEM conferences are organized by neuroscientists interested in energetics and metabolism underlying neural functions; they have had a high impact on conceptual and experimental advances in these fields and on promoting collaborative interactions among neuroscientists. Leif made major contributions to ICBEM discussions and understanding of metabolic and signaling characteristics of astrocytes and their roles in brain function. His studies ranged from uptake of K+ from extracellular fluid and its stimulation of astrocytic respiration, identification, and regulation of enzymes specifically or preferentially expressed in astrocytes in the glutamate-glutamine cycle of excitatory neurotransmission, a requirement for astrocytic glycogenolysis for fueling K+ uptake, involvement of glycogen in memory consolidation in the chick, and pharmacology of astrocytes. This tribute to Leif Hertz highlights his major discoveries, the high impact of his work on astrocyte-neuron interactions, and his unparalleled influence on understanding the cellular basis of brain energy metabolism. His work over six decades has helped integrate the roles of astrocytes into neurotransmission where oxidative and glycogenolytic metabolism during neurotransmitter glutamate turnover are key aspects of astrocytic energetics. Leif recognized that brain astrocytic metabolism is greatly underestimated unless the volume fraction of astrocytes is taken into account. Adjustment for pathway rates expressed per gram tissue for volume fraction indicates that astrocytes have much higher oxidative rates than neurons and astrocytic glycogen concentrations and glycogenolytic rates during sensory stimulation in vivo are similar to those in resting and exercising muscle, respectively. These novel insights are typical of Leif's astute contributions to the energy metabolism field, and his publications have identified unresolved topics that provide the neuroscience community with challenges and opportunities for future research.

A common role for astrocytes in rhythmic behaviours?

Astrocytes are a functionally diverse form of glial cell involved in various aspects of nervous system infrastructure, from the metabolic and structural support of neurons to direct neuromodulation of synaptic activity. Investigating how astrocytes behave in functionally related circuits may help us understand whether there is any conserved logic to the role of astrocytes within neuronal networks. Astrocytes are implicated as key neuromodulatory cells within neural circuits that control a number of rhythmic behaviours such as breathing, locomotion and circadian sleep-wake cycles. In this review, we examine the evidence that astrocytes are directly involved in the regulation of the neural circuits underlying six different rhythmic behaviours: locomotion, breathing, chewing, gastrointestinal motility, circadian sleep-wake cycles and oscillatory feeding behaviour. We discuss how astrocytes are integrated into the neuronal networks that regulate these behaviours, and identify the potential gliotransmission signalling mechanisms involved. From reviewing the evidence of astrocytic involvement in a range of rhythmic behaviours, we reveal a heterogenous array of gliotransmission mechanisms, which help to regulate neuronal networks. However, we also observe an intriguing thread of commonality, in the form of purinergic gliotransmission, which is frequently utilised to facilitate feedback inhibition within rhythmic networks to constrain a given behaviour within its operational range.

Brain Energy and Oxygen Metabolism: Emerging Role in Normal Function and Disease

Dynamic metabolic changes occurring in neurons are critically important in directing brain plasticity and cognitive function. In other tissue types, disruptions to metabolism and the resultant changes in cellular oxidative state, such as increased reactive oxygen species (ROS) or induction of hypoxia, are associated with cellular stress. In the brain however, where drastic metabolic shifts occur to support physiological processes, subsequent changes to cellular oxidative state and induction of transcriptional sensors of oxidative stress likely play a significant role in regulating physiological neuronal function. Understanding the role of metabolism and metabolically-regulated genes in neuronal function will be critical in elucidating how cognitive functions are disrupted in pathological conditions where neuronal metabolism is affected. Here, we discuss known mechanisms regulating neuronal metabolism as well as the role of hypoxia and oxidative stress during normal and disrupted neuronal function. We also summarize recent studies implicating a role for metabolism in regulating neuronal plasticity as an emerging neuroscience paradigm.

Maxi-anion channels play a key role in glutamate-induced ATP release from mouse astrocytes in primary culture

Astrocytes are an abundant source of ATP, which might be released from the cytosol into extracellular spaces under various conditions and even affect cell fate under some circumstances. In the present study, we aimed to evaluate the pathway(s) contributing toward glutamate-induced ATP release from mouse astrocytes. Firstly, our study of cultured astrocytes showed marked ATP release in response to stimuli of glutamate at different concentrations (0.1-1 mM), with an interesting bimodal distribution in time course. Inhibitors or blockers of potential pathways for ATP release such as exocytotic vesicular release, gap junction hemichannels, P2X7 receptors, and volume-sensitive outwardly rectifying chloride channels had no significant effects on the observed ATP release. In contrast, glutamate-induced ATP release from astrocytes was significantly inhibited by gadolinium (50 µM), an inhibitor of a maxi-anion channel; meanwhile, the application of gadolinium can allay glutamate-induced cell injury significantly. Thus, we propose that the maxi-anion channel might play an important role in glutamate-induced ATP release from mouse astrocytes and inhibition of maxi-anion channel activities to reduce ATP release can produce protective effects in the case of glutamate stimuli.

Severe Convulsions and Dysmyelination in Both Jimpy and Cx32/47 -/- Mice may Associate Astrocytic L-Channel Function with Myelination and Oligodendrocytic Connexins with Internodal Kv Channels

The Jimpy mouse illustrates the importance of interactions between astrocytes and oligodendrocytes. It has a mutation in Plp coding for proteolipid protein and DM20. Its behavior is normal at birth but from the age of ~2 weeks it shows severe convulsions associated with oligodendrocyte/myelination deficits and early death. A normally occurring increase in oxygen consumption by highly elevated K+ concentrations is absent in Jimpy brain slices and cultured astrocytes, reflecting that Plp at early embryonic stages affects common precursors as also shown by the ability of conditioned medium from normal astrocytes to counteract histological abnormalities. This metabolic response is now known to reflect opening of L-channels for Ca2+. The resulting deficiency in Ca2+ entry has many consequences, including lack of K+-stimulated glycogenolysis and release of gliotransmitter ATP. Lack of purinergic stimulation compromises oligodendrocyte survival and myelination and affects connexins and K+ channels. Mice lacking the oligodendrocytic connexins Cx32 and 47 show similar neurological dysfunction as Jimpy. This possibly reflects that K+ released by intermodal axonal Kv channels is transported underneath a loosened myelin sheath instead of reaching the extracellular space via connexin-mediated transport to oligodendrocytes, followed by release and astrocytic Na+,K+-ATPase-driven uptake with subsequent Kir4.1-facilitated release and neuronal uptake.

Bi-phasic regulation of glycogen content in astrocytes via Cav-1/PTEN/PI3K/AKT/GSK-3β pathway by fluoxetine

OBJECTIVE

Here, we present the data indicating that chronic treatment with fluoxetine regulates Cav-1/PTEN/PI3K/AKT/GSK-3β signalling pathway and glycogen content in primary cultures of astrocytes with bi-phasic concentration dependence.

RESULTS

At lower concentrations, fluoxetine downregulates gene expression of Cav-1, decreases membrane content of PTEN, increases activity of PI3K/AKT, and elevates GSK-3β phosphorylation thus suppressing its activity. At higher concentrations, fluoxetine acts in an inverse fashion. As expected, fluoxetine at lower concentrations increased while at higher concentrations decreased glycogen content in astrocytes.

CONCLUSIONS

Our findings indicate that bi-phasic regulation of glycogen content via Cav-1/PTEN/PI3K/AKT/GSK-3β pathway by fluoxetine may be responsible for both therapeutic and side effects of the drug.

Glutamine-Glutamate Cycle Flux Is Similar in Cultured Astrocytes and Brain and Both Glutamate Production and Oxidation Are Mainly Catalyzed by Aspartate Aminotransferase

The glutamine-glutamate cycle provides neurons with astrocyte-generated glutamate/γ-aminobutyric acid (GABA) and oxidizes glutamate in astrocytes, and it returns released transmitter glutamate/GABA to neurons after astrocytic uptake. This review deals primarily with the glutamate/GABA generation/oxidation, although it also shows similarity between metabolic rates in cultured astrocytes and intact brain. A key point is identification of the enzyme(s) converting astrocytic α-ketoglutarate to glutamate and vice versa. Most experiments in cultured astrocytes, including those by one of us, suggest that glutamate formation is catalyzed by aspartate aminotransferase (AAT) and its degradation by glutamate dehydrogenase (GDH). Strongly supported by results shown in Table 1 we now propose that both reactions are primarily catalyzed by AAT. This is possible because the formation occurs in the cytosol and the degradation in mitochondria and they are temporally separate. High glutamate/glutamine concentrations abolish the need for glutamate production from α-ketoglutarate and due to metabolic coupling between glutamate synthesis and oxidation these high concentrations render AAT-mediated glutamate oxidation impossible. This necessitates the use of GDH under these conditions, shown by insensitivity of the oxidation to the transamination inhibitor aminooxyacetic acid (AOAA). Experiments using lower glutamate/glutamine concentration show inhibition of glutamate oxidation by AOAA, consistent with the coupled transamination reactions described here.

Astrocyte Cultures Mimicking Brain Astrocytes in Gene Expression, Signaling, Metabolism and K+ Uptake and Showing Astrocytic Gene Expression Overlooked by Immunohistochemistry and In Situ Hybridization

Based on differences in gene expression between cultured astrocytes and freshly isolated brain astrocytes it has been claimed that cultured astrocytes poorly reflect the characteristics of their in vivo counterparts. This paper shows that this is not the case with the cultures of mouse astrocytes we have used since 1978. The culture is prepared following guidelines provided by Drs. Monique Sensenbrenner and John Booher, with the difference that dibutyryl cyclic AMP is added to the culture medium from the beginning of the third week. This addition has only minor effects on glucose and glutamate metabolism, but it is crucial for effects by elevated K⁺ concentrations and for Ca²⁺ homeostasis, important aspects of astrocyte function. Work by Liang Peng and her colleagues has shown identity between not only gene expression but also drug-induced gene upregulations and editings in astrocytes cultured by this method and astrocytes freshly isolated from brains of drug-treated animals. Dr. Norenberg's laboratory has demonstrated identical upregulation of the cotransporter NKCC1 in ammonia-exposed astrocytes and rats with liver failure. Similarity between cultured and freshly isolated astrocytes has also been shown in metabolism, K⁺ uptake and several aspects of signaling. However, others have shown that the gene for the glutamate transporter GLT1 is not expressed, and rat cultures show some abnormalities in K⁺ effects. Nevertheless, the overall reliability of the cultured cells is important because immunohistochemistry and in situ hybridization poorly demonstrate many astrocytic genes, e.g., those of nucleoside transporters, and even microarray analysis of isolated cells can be misleading.

Ammonium increases Ca(2+) signalling and upregulates expression of Cav1.2 gene in astrocytes in primary cultures and in the in vivo brain

AIM

The primary aim of this study was to identify the effects of hyperammonaemia on functional expression of Cav1.2 L-type Ca(2+) channels in astroglia.

METHODS

Primary cultures of mouse astrocytes were used to study effects of chronic treatment (1-5 days) with ammonium chloride, at 1, 3 and 5 mm on depolarization-induced increases in free cytosolic Ca(2+) concentration ([Ca(2+)]i , measured with Fura-2 based microfluorimetry) in control conditions and following treatment with the L-type Ca(2+) channel inhibitor, nifedipine, or with ryanodine receptor inhibitor, ryanodine. Expression of Cav1.2 mRNA was identified with RT-PCR, whereas protein content was determined by Western blotting. Sustained hyperammonaemia in vivo was induced by daily injections of urease (33 units kg body weight(-1), i.p.) for 3 days.

RESULTS

Depolarization-induced [Ca(2+)]i transients sensitive to nifedipine (peak of the response) and to ryanodine (plateau phase) were significantly increased in astrocytes chronically exposed to ammonium. The ammonium-induced increase in Ca(2+) influx in astrocytes resulted from an upregulation of Cav1.2 channel's expression detected at mRNA and protein levels. Increase in Cav1.2 expression was prevented by ouabain antagonist canrenone. Similar upregulation of Cav1.2 gene expression was found in the brains of adult mice subjected to intraperitoneal injection of urease. In transgenic mice tagged with an astrocyte-specific or neurone-specific markers and treated with intraperitoneal injections of urease, the fluorescence-activated cell sorting of neurones and astrocytes demonstrated that Cav1.2 mRNA expression was upregulated in astrocytes, but not in neurones.

CONCLUSIONS

Ammonium-induced deregulation of astroglial Ca(2+) signalling, is, in part, associated with upregulation of Cav1.2 L-type calcium channels.

Glucose replaces glutamate as energy substrate to fuel glutamate uptake in glutamate dehydrogenase-deficient astrocytes

Cultured astrocytes treated with siRNA to knock down glutamate dehydrogenase (GDH) were used to investigate whether this enzyme is important for the utilization of glutamate as an energy substrate. By incubation of these cells in media containing different concentrations of glutamate (range 100-500 µM) in the presence or in the absence of glucose, the metabolism of these substrates was studied by using tritiated glutamate or 2-deoxyglucose as tracers. In addition, the cellular contents of glutamate and ATP were determined. The astrocytes were able to maintain physiological levels of ATP regardless of the expression level of GDH and the incubation condition, indicating a high degree of flexibility with regard to regulatory mechanisms involved in maintaining an adequate energy level in the cells. Glutamate uptake was found to be increased in these cells when exposed to increasing levels of extracellular glutamate independently of the GDH expression level. Moreover, increased intracellular glutamate content was observed in the GDH-deficient cells after a 2-hr incubation in the presence of 100 µM glutamate. It is significant that GDH-deficient cells exhibited an increased utilization of glucose in the presence of 250 and 500 µM glutamate, monitored as an increase in the accumulation of tritiated 2-deoxyglucose-6-phosphate. These findings underscore the importance of the expression level of GDH for the ability to utilize glutamate as an energy source fueling its own energy-requiring uptake.

Ammonium increases Ca(2+) signalling and up-regulates expression of TRPC1 gene in astrocytes in primary cultures and in the in vivo brain

Rapid rise in ammonium concentration in the brain is the major pathogenic factor in hepatic encephalopathy that is manifested by state of confusion, forgetfulness and irritability, psychotic symptoms, delusions, lethargy, somnolence and, in the terminal stages, coma. Primary cultures of mouse astrocytes were used to investigate effects of chronic treatment (3 days) with ammonium chloride (ammonium) at 3 mM, this being a relevant concentration for hepatic encephalopathy condition, on metabotropic receptor agonist-induced increases in free cytosolic Ca(2+) concentration [(Ca(2+))i], measured with fura-2 based microfluorimetry and on store-operated Ca(2+) entry (SOCE) activated following treatment with the SERCA inhibitor thapsigargin. The agonists used were the β-adrenergic agonist isoproterenol, the α2-adrenergic agonist dexmedetomidine, the InsP3 receptor (InsP3R) agonist adenophostin A and ryanodine receptor agonist 4-Chloro-m-cresol (4-CMC). Agonist-induced [Ca(2+)]i responses were significantly increased in astrocytes chronically exposed to ammonium. Similarly, the SOCE, meditated by the transient receptor potential channel 1 (TRPC1), was significantly augmented. The ammonium-induced increase in SOCE was a result of an up-regulation of mRNA and protein expression of TRPC1 in astrocytes. Increase in TRPC1 expression and in SOCE were both prevented by ouabain antagonist canrenone. Similar up-regulation of TRPC1 gene expression was found in the brain of adult mice subjected to intraperitoneal injection of urease for 3 days. In transgenic mice tagged with an astrocyte-specific or a neurone-specific markers and treated with intraperitoneal injections of urease for 3 days, the fluorescence-activated cell sorting of neurones and astrocytes demonstrated that TRPC1 mRNA expression was up-regulated in astrocytes, but not in neurones.

Methodological limitations in determining astrocytic gene expression

Traditionally, astrocytic mRNA and protein expression are studied by in situ hybridization (ISH) and immunohistochemically. This led to the concept that astrocytes lack aralar, a component of the malate-aspartate-shuttle. At least similar aralar mRNA and protein expression in astrocytes and neurons isolated by fluorescence-assisted cell sorting (FACS) reversed this opinion. Demonstration of expression of other astrocytic genes may also be erroneous. Literature data based on morphological methods were therefore compared with mRNA expression in cells obtained by recently developed methods for determination of cell-specific gene expression. All Na,K-ATPase-α subunits were demonstrated by immunohistochemistry (IHC), but there are problems with the cotransporter NKCC1. Glutamate and GABA transporter gene expression was well determined immunohistochemically. The same applies to expression of many genes of glucose metabolism, whereas a single study based on findings in bacterial artificial chromosome (BAC) transgenic animals showed very low astrocytic expression of hexokinase. Gene expression of the equilibrative nucleoside transporters ENT1 and ENT2 was recognized by ISH, but ENT3 was not. The same applies to the concentrative transporters CNT2 and CNT3. All were clearly expressed in FACS-isolated cells, followed by biochemical analysis. ENT3 was enriched in astrocytes. Expression of many nucleoside transporter genes were shown by microarray analysis, whereas other important genes were not. Results in cultured astrocytes resembled those obtained by FACS. These findings call for reappraisal of cellular nucleoside transporter expression. FACS cell yield is small. Further development of cell separation methods to render methods more easily available and less animal and cost consuming and parallel studies of astrocytic mRNA and protein expression by ISH/IHC and other methods are necessary, but new methods also need to be thoroughly checked.

Astrocytic and neuronal accumulation of elevated extracellular K(+) with a 2/3 K(+)/Na(+) flux ratio-consequences for energy metabolism, osmolarity and higher brain function

Brain excitation increases neuronal Na⁺ concentration by 2 major mechanisms: (i) Na⁺ influx caused by glutamatergic synaptic activity; and (ii) action-potential-mediated depolarization by Na⁺ influx followed by repolarizating K⁺ efflux, increasing extracellular K⁺ concentration. This review deals mainly with the latter and it concludes that clearance of extracellular K⁺ is initially mainly effectuated by Na⁺,K⁺-ATPase-mediated K⁺ uptake into astrocytes, at K⁺ concentrations above ~10 mM aided by uptake of Na⁺,K⁺ and 2 Cl⁻ by the cotransporter NKCC1. Since operation of the astrocytic Na⁺,K⁺-ATPase requires K⁺-dependent glycogenolysis for stimulation of the intracellular ATPase site, it ceases after normalization of extracellular K⁺ concentration. This allows K⁺ release via the inward rectifying K⁺ channel K_ir4.1, perhaps after trans-astrocytic connexin- and/or pannexin-mediated K⁺ transfer, which would be a key candidate for determination by synchronization-based computational analysis and may have signaling effects. Spatially dispersed K⁺ release would have little effect on extracellular K⁺ concentration and allow K⁺ accumulation by the less powerful neuronal Na⁺,K⁺-ATPase, which is not stimulated by increases in extracellular K⁺. Since the Na⁺,K⁺-ATPase exchanges 3 Na⁺ with 2 K⁺, it creates extracellular hypertonicity and cell shrinkage. Hypertonicity stimulates NKCC1, which, aided by β-adrenergic stimulation of the Na⁺,K⁺-ATPase, causes regulatory volume increase, furosemide-inhibited undershoot in [K⁺]_e and perhaps facilitation of the termination of slow neuronal hyperpolarization (sAHP), with behavioral consequences. The ion transport processes involved minimize ionic disequilibria caused by the asymmetric Na⁺,K⁺-ATPase fluxes.

Mechanisms for L-channel-mediated increase in [Ca(2+)]i and its reduction by anti-bipolar drugs in cultured astrocytes combined with its mRNA expression in freshly isolated cells support the importance of astrocytic L-channels

The importance of Ca(2+) signaling in astrocytes is undisputed but a potential role of Ca(2+) influx via L-channels in the brain in vivo is disputed, although expression of these channels in cultured astrocytes is recognized. This study shows that an increase in free cytosolic Ca(2+) concentration ([Ca(2+)]i) in astrocytes in primary cultures in response to an increased extracellular K(+) concentration (45mM) is inhibited not only by nifedipine (confirming previous observations) but also to a very large extent by ryanodine, inhibiting ryanodine receptor-mediated release of Ca(2+), known to occur in response to an elevation in [Ca(2+)]i. This means that the actual influx of Ca(2+) is modest, which may contribute to the difficulty in demonstrating L-channel-mediated Ca(2+) currents in astrocytes in intact brain tissue. Chronic treatment with any of the 3 conventional anti-bipolar drugs lithium, carbamazepine or valproic acid similarly causes a pronounced inhibition of K(+)-mediated increase in [Ca(2+)]i. This is shown to be due to an inhibition of capacitative Ca(2+) influx, reflected by decreased mRNA and protein expression of the 'transient receptor potential channel' (TRPC1), a constituent of store-operated channels (SOCEs). Literature data are cited (i) showing that depolarization-mediated Ca(2+) influx in response to an elevated extracellular K(+) concentration is important for generation of Ca(2+) oscillations and for the stimulatory effect of elevated K(+) concentrations in intact, non-cultured brain tissue, and (ii) that Ca(2+) channel activity is dependent upon availability of metabolic substrates, including glycogen. Finally, expression of mRNA for Cav1.3 is demonstrated in freshly separated astrocytes from normal brain.

The Glutamate-Glutamine (GABA) Cycle: Importance of Late Postnatal Development and Potential Reciprocal Interactions between Biosynthesis and Degradation

The gold standard for studies of glutamate-glutamine (GABA) cycling and its connections to brain biosynthesis from glucose of glutamate and GABA and their subsequent metabolism are the elegant in vivo studies by (13)C magnetic resonance spectroscopy (NMR), showing the large fluxes in the cycle. However, simpler experiments in intact brain tissue (e.g., immunohistochemistry), brain slices, cultured brain cells, and mitochondria have also made important contributions to the understanding of details, mechanisms, and functional consequences of glutamate/GABA biosynthesis and degradation. The purpose of this review is to attempt to integrate evidence from different sources regarding (i) the enzyme(s) responsible for the initial conversion of α-ketoglutarate to glutamate; (ii) the possibility that especially glutamate oxidation is essentially confined to astrocytes; and (iii) the ontogenetically very late onset and maturation of glutamine-glutamate (GABA) cycle function. Pathway models based on the functional importance of aspartate for glutamate synthesis suggest the possibility of interacting pathways for biosynthesis and degradation of glutamate and GABA and the use of transamination as the default mechanism for initiation of glutamate oxidation. The late development and maturation are related to the late cortical gliogenesis and convert brain cortical function from being purely neuronal to becoming neuronal-astrocytic. This conversion is associated with huge increases in energy demand and production, and the character of potentially incurred gains of function are discussed. These may include alterations in learning mechanisms, in mice indicated by lack of pairing of odor learning with aversive stimuli in newborn animals but the development of such an association 10-12 days later. The possibility is suggested that analogous maturational changes may contribute to differences in the way learning is accomplished in the newborn human brain and during later development.

Norepinephrine: a neuromodulator that boosts the function of multiple cell types to optimize CNS performance

Norepinephrine (NE) is a neuromodulator that in multiple ways regulates the activity of neuronal and non-neuronal cells. NE participates in the rapid modulation of cortical circuits and cellular energy metabolism, and on a slower time scale in neuroplasticity and inflammation. Of the multiple sources of NE in the brain, the locus coeruleus (LC) plays a major role in noradrenergic signaling. Processes from the LC primarily release NE over widespread brain regions via non-junctional varicosities. We here review the actions of NE in astrocytes, microglial cells, and neurons based on the idea that the overarching effect of signaling from the LC is to maximize brain power, which is accomplished via an orchestrated cellular response involving most, if not all cell types in CNS.

Inhibitory effects of ginseng total saponins on hypoxia-induced dysfunction and injuries of cultured astrocytes

The effects of ginseng total saponins (GTS) on hypoxic damage of primary cultures of astrocytes were studied. Hypoxia was created by placing cultures in an air tight chamber that was flushed with 95% N(2)/5% CO(2) for 15 min before being sealed. Cultures showed evidence of significant cell injury after 24 h of hypoxia (increased lactate dehydrogenase (LDH) content in the culture medium, cell swelling and decreased glutamate uptake and protein content). Addition of GTS (0.1, 0.3 mg/ml) to the cultures during the exposure to hypoxic conditions produced dose-dependent inhibition of the LDH efflux. GTS (0.1, 0.3 mg/ml) also produced significant inhibition of the increased cell volume of astrocytes measured by [(3)H]O-methyl-D-glucose uptake under the hypoxic conditions. Decreased glutamate uptake and protein content was inhibited by GTS. These data suggest that GTS prevents astrocytic cell injury induced by severe hypoxiain vitro.

Primary cultures of astrocytes: their value in understanding astrocytes in health and disease

During the past few decades of astrocyte research it has become increasingly clear that astrocytes have taken a central position in all central nervous system activities. Much of our new understanding of astrocytes has been derived from studies conducted with primary cultures of astrocytes. Such cultures have been an invaluable tool for studying roles of astrocytes in physiological and pathological states. Many central astrocytic functions in metabolism, amino acid neurotransmission and calcium signaling were discovered using this tissue culture preparation and most of these observations were subsequently found in vivo. Nevertheless, primary cultures of astrocytes are an in vitro model that does not fully mimic the complex events occurring in vivo. Here we present an overview of the numerous contributions generated by the use of primary astrocyte cultures to uncover the diverse functions of astrocytes. Many of these discoveries would not have been possible to achieve without the use of astrocyte cultures. Additionally, we address and discuss the concerns that have been raised regarding the use of primary cultures of astrocytes as an experimental model system.

Contributions in astrocytes of SMIT1/2 and HMIT to myo-inositol uptake at different concentrations and pH

myo-Inositol is important for cell signaling both in cytoplasm and in intracellular organelles. It is required in the plasma membrane and cytoplasm for maintained synthesis of the second messengers, inositoltrisphosphate (IP(3)) and diacylglycerol (DAG) from phosphatidylinositol bisphosphate (PIP(2)), and in organelles as precursor for synthesis of complex signaling phospholipids and inositolphosphates from IP(3) and PIP(2). myo-Inositol must be taken up into the cell where its is used, because neither neurons nor astrocytes synthesize it. It is also an osmolyte, taken up in response to surrounding hyperosmolarity and released during hypo-osmolarity. There are three myo-inositol transporters, the Na(+)-dependent SMIT1 and SMIT2, and HMIT, which co-transports myo-inositol with H(+). Their relative expressions in astrocytes and neurons are unknown. Uptake kinetics for myo-inositol in astrocytes has repeatedly been determined, but always on the assumption of only one component, leaving kinetics for the individual transporters unknown. This paper demonstrates that astrocytes obtained directly from the brain express SMIT1 and HMIT, but little SMIT2, and that all three transporters are expressed in neurons. Cultured mouse astrocytes show a high-affinity/low-capacity myo-inositol uptake (V(max): 60.0 ± 3.0 pmol/min per mg protein; K(m): 16.7 ± 2.6 μM), mediated by SMIT1 and perhaps partly by SMIT2. It was determined in cells pre-treated with HMIT-siRNA and confirmed by specific inhibition of SMIT. However at physiologically relevant myo-inositol concentrations most uptake is by a lower-affinity/higher-capacity uptake, mediated by HMIT (V(max): 358 ± 60 pmol/min per mg protein; K(m): 143 ± 36 μM) and determined by subtraction of SMIT-mediated from total uptake. At high myo-inositol concentrations, its uptake is inhibited by incubation in medium with increased pH, and increased during intracellular acidification with NH(4)Cl. This is in agreement with literature data for HMIT alone. At low concentration, where SMIT1/2 activity gains importance, myo-inositol uptake is reduced by ammonia-induced intracellular acidification, consistent with the transporter's pH sensitivity reported in the literature.

Isotope-based quantitation of uptake, release, and metabolism of glutamate and glucose in cultured astrocytes

Protocols are described for measurement in primary cultures of astrocytes of unidirectional fluxes of glutamate (influx and efflux), glutamate metabolism to glutamine or CO(2), glucose influx, glycolysis, pyruvate dehydrogenation, oxidative metabolism of glucose, pyruvate carboxylation, glycogen synthesis, and glycogenolysis. References are made to the in vivo situation, and the importance of using metabolically competent cultures is emphasized.

Fluoxetine affects GluK2 editing, glutamate-evoked Ca(2+) influx and extracellular signal-regulated kinase phosphorylation in mouse astrocytes

BACKGROUND

We sought to study the effects of chronic exposure to fluoxetine - a selective serotonin reuptake inhibitor (SSRI) and specific 5-HT(2B) receptor agonist in astrocytes - on the expression of kainate receptors (GluK1-5) in cultured astrocytes and in intact brains in mice and on GluK2 editing by adenosine deaminase acting on RNA (ADAR), as well as the ensuing effects of fluoxetine on glutamate-mediated Ca(2+) influx and extracellular signal-regulated kinase (ERK)(1/2) phosphorylation in astrocytes.

METHODS

We performed reverse transcription-polymerase chain reaction (PCR) to assess mRNA expression. We analyzed RNA editing with amplification refractory mutation system PCR and complementary DNA sequencing. Protein expression and ERK phosphorylation were assessed using Western blots. We studied gene silencing with specific small interfering RNAs (siRNA), and we studied intracellular Ca(2+) using fluorometry.

RESULTS

All GluK subunits were present in the brain in vivo, and GluK2-5 subunits were present in cultured astrocytes. Fluoxetine upregulated GluK2 and ADAR2. Enhanced GluK2 editing by fluoxetine abolished glutamate-mediated increases in intra cellular Ca(2+) and ERK(1/2) phosphorylation. Enhanced editing of GluK2 was prevented by siRNA against the 5-HT(2B) receptor or ADAR2.

LIMITATIONS

Limitations of our study include the use of an in vitro system, but our cultured cells in many respects behave like in vivo astrocytes.

CONCLUSION

Fluoxetine alters astrocytic glutamatergic function.

Astrocyte-derived thrombospondin-2 is critical for the repair of the blood-brain barrier

Thrombospondin (TSP)-2-null mice have an altered brain foreign body response (FBR) characterized by increases in inflammation, extracellular matrix deposition, and leakage of the blood-brain barrier (BBB). In the present study, we investigated the role of TSP-2 in BBB repair during the brain FBR to mixed cellulose ester filters implanted in the cortex of wild-type (WT) and TSP-2-null mice for 2 days to 8 weeks. Histological and immunohistochemical analysis revealed enhanced and prolonged neuroinflammation in TSP-2-null mice up to 8 weeks after implantation. In addition, recovery of the BBB was compromised and was associated with increased gelatinolytic activity and low levels of collagen type IV in the basement membranes of TSP-2-null blood vessels. An analysis of protein extracts from implantation sites revealed elevated levels of matrix metalloproteinase (MMP)-2 and MMP-9 in TSP-2-null brains. TSP-2-null astrocytes secreted higher levels of both MMPs in vitro compared with their WT counterparts. Furthermore, TSP-2-null astrocytes were deficient in supporting the recovery of barrier function in WT endothelial cells. Finally, Western blot analysis of astrocytes and brain endothelial cells revealed TSP-2 expression only in the former. Taken together, our observations suggest that astrocyte-derived TSP-2 is critical for the maintenance of physiological MMP-2 and MMP-9 levels during the FBR and contributes to the repair of the BBB.

Effects of chronic treatment with fluoxetine on receptor-stimulated increase of [Ca2+]i in astrocytes mimic those of acute inhibition of TRPC1 channel activity

Primary cultures of mouse astrocytes were used to investigate effects by chronic treatment (3-21 days) with fluoxetine (0.5-10 μM) on capacitative Ca(2+) influx after treatment with the SERCA inhibitor thapsigargin and on receptor agonist-induced increases in free cytosolic Ca(2+) concentration [Ca(2+)](i), determined with Fura-2. The agonists were the 5-HT(2B) agonist fluoxetine, the α(2)-adrenergic agonist dexmedetomidine, and ryanodine receptor (RyR) and IP(3) receptor (IP(3)R) agonists. In untreated sister cultures each agonist distinctly increased [Ca(2+)](i), but in cultures treated for sufficient length of time or with sufficiently high doses of fluoxetine, acute administration of fluoxetine, dexmedetomidine, or RyR or IP(3)R agonists elicited reduced, in some cases abolished, effects. Capacitative Ca(2+) entry, meditated by TRPC1 channels, was sufficiently inhibited to cause a depletion of Ca(2+) stores, which could explain the reduced agonist effects. All effects of chronic fluoxetine administration could be replicated by TRPC1 channel antibody or siRNA. Since increases in astrocytic [Ca(2+)](i) regulate release of gliotransmitters, these effects may have profound effects on brain function. They may be important for therapeutic effects of all 5 conventional 'serotonin-specific reuptake inhibitors' (SSRIs), which at concentrations used therapeutically (∼1 μM) share other of fluoxetine's chronic effects (Zhang et al., Neuron Glia Biol. 16 (2010) 1-13).

Characterization of primary and secondary cultures of astrocytes prepared from mouse cerebral cortex

Astrocyte cultures were prepared from cerebral cortex of new-born and 7-day-old mice and additionally, the cultures from new-born animals were passaged as secondary cultures. The cultures were characterized by immunostaining for the astrocyte markers glutamine synthetase (GS), glial fibrillary acidic protein, and the glutamate transporters EAAT1 and EAAT2. The cultures prepared from 7-day-old animals were additionally characterized metabolically using (13)C-labeled glucose and glutamate as well as (15)N-labeled glutamate as substrates. All types of cultures exhibited pronounced immunostaining of the astrocyte marker proteins. The metabolic pattern of the cultures from 7-day-old animals of the labeled substrates was comparable to that seen previously in astrocyte cultures prepared from new-born mouse brain showing pronounced glycolytic and oxidative metabolism of glucose. Glutamate was metabolized both via the GS pathway and oxidatively via the tricarboxylic acid cycle as expected. Additionally, glutamate underwent pronounced transamination to aspartate and alanine and the intracellular pools of alanine and pyruvate exhibited compartmentation. Altogether the results show that cultures prepared from cerebral cortex of 7-day-old mice have metabolic and functional properties indistinguishable from those of classical astrocyte cultures prepared from neocortex of new-born animals. This provides flexibility with regard to preparation and use of these cultures for a variety of purposes.

Adrenoceptors in brain: cellular gene expression and effects on astrocytic metabolism and [Ca(2+)]i

Recent in vivo studies have established astrocytes as a major target for locus coeruleus activation (Bekar et al., 2008), renewing interest in cell culture studies on noradrenergic effects on astrocytes in primary cultures and calling for additional information about the expression of adrenoceptor subtypes on different types of brain cells. In the present communication, mRNA expression of alpha(1)-, alpha(2)- and beta-adrenergic receptors and their subtypes was determined in freshly isolated, cell marker-defined populations of astrocytes, NG2-positive cells, microglia, endothelial cells, and Thy1-positive neurons (mainly glutamatergic projection neurons) in murine cerebral cortex. Immediately after dissection of frontal, parietal and occipital cortex of 10-12-week-old transgenic mice, which combined each cell-type marker with a specific fluorescent signal, the tissue was digested, triturated and centrifuged, yielding a solution of dissociated cells of all types, which were separated by fluorescence-activated cell sorting (FACS). mRNA expression in each cell fraction was determined by microarray analysis. alpha(1A)-Receptors were unequivocally expressed in astrocytes and NG2-positive cells, but absent in other cell types, and alpha(1B)-receptors were not expressed in any cell population. Among alpha(2)-receptors only alpha(2A)-receptors were expressed, unequivocally in astrocytes and NG-positive cells, tentatively in microglia and questionably in Thy1-positive neurons and endothelial cells. beta(1)-Receptors were unequivocally expressed in astrocytes, tentatively in microglia, and questionably in neurons and endothelial cells, whereas beta(2)-adrenergic receptors showed tentative expression in neurons and astrocytes and unequivocal expression in other cell types. This distribution was supported by immunochemical data and its relevance established by previous studies in well-differentiated primary cultures of mouse astrocytes, showing that stimulation of alpha(2)-adrenoceptors increases glycogen formation and oxidative metabolism, the latter by a mechanism depending on intramitochondrial Ca(2+), whereas alpha(1)-adrenoceptor stimulation enhances glutamate uptake, and beta-adrenoceptor activation causes glycogenolysis and increased Na(+), K(+)-ATPase activity. The Ca(2+)- and cAMP-mediated association between energy-consuming and energy-yielding processes is emphasized.

Signaling pathways of isoproterenol-induced ERK1/2 phosphorylation in primary cultures of astrocytes are concentration-dependent

Stimulation of β-adrenoceptors activates the canonical adenylate cyclase pathway (via G(s) protein) but can also evoke phosphorylation of extracellular-regulated kinases 1 and 2 (ERK(1/2) ) via G(s)/G(i) switching or β-arrestin-mediated recruitment of Src. In primary cultures of mouse astrocytes, activation of the former of these pathways required micromolar concentrations of the β(1)/β(2) -adrenergic agonist isoproterenol, that acted on β(1)-adrenoceptors, whereas the latter was activated already by nanomolar concentrations, acting on β(2) receptors. Protein kinase A activity was required for G(s)/G(i) switching, which was followed by Ca(2+) release from intracellular stores and G(iα)- and metalloproteinase-dependent transactivation of the epidermal growth factor receptor (EGFR; at its Y1173 phophorylation site), via its receptor-tyrosine kinase, β-arrestin 1/2 recruitment, and MAPK/ERK kinase-dependent ERK(1/2) phosphorylation. ERK(1/2) phosphorylation by Src activation depended on β-arrestin 2, but not β-arrestin 1, was accompanied by Src/EGFR co-precipitation and phosphorylation of the EGFR at the Src-phosphorylated Y845 site and the Y1045 autophosphorylation site; it was independent of transactivation but dependent on MAPK/ERK kinase activity, suggesting EGFR phosphorylation independently of the receptor-tyrosine kinase or activation of Ras or Raf directly from Src. Most astrocytic consequences of activating either pathway (or both) are unknown, but morphological differentiation and increase in glial fibrillary acidic protein in response to dibutyryl cAMP-mediated increase in cAMP depend on G(s)/G(i) switching and transactivation.

5-HT2B receptors are expressed on astrocytes from brain and in culture and are a chronic target for all five conventional ‘serotonin-specific reuptake inhibitors’

In well-differentiated primary cultures of mouse astrocytes, which express no serotonin transporter (SERT), the ‘serotonin-specific reuptake inhibitor’ (SSRI) fluoxetine leads acutely to 5-HT2B receptor-mediated, transactivation-dependent phosphorylation of extracellular regulated kinases 1/2 (ERK1/2) with an EC50 of ~5 μM, and chronically to ERK1/2 phosphorylation-dependent upregulation of mRNA and protein expression of calcium-dependent phospholipase A2 (cPLA2) with ten-fold higher affinity. This affinity is high enough that fluoxetine given therapeutically may activate astrocytic 5-HT2B receptors (Li et al., 2008, 2009). We now confirm the expression of 5-HT2B receptors in astrocytes freshly dissociated from mouse brain and isolated by fluorescence-activated cell sorting (FACS) and investigate in cultured cells if the effects of fluoxetine are shared by all five conventional SSRIs with sufficiently high affinity to be relevant for mechanism(s) of action of SSRIs. Phosphorylated and total ERK1/2 and mRNA and protein expression of cPLA2a were determined by Western blot and reverse transcription polymerase chain reaction (RT-PCR). Paroxetine, which differs widely from fluoxetine in affinity for SERT and for another 5-HT2 receptor, the 5-HT2C receptor, acted acutely and chronically like fluoxetine. One micromolar of paroxetine, fluvoxamine or sertraline increased cPLA2a expression during chronic treatment; citalopram had a similar effect at 0.1–0.5 μM; these are therapeutically relevant concentrations.

Astrocytic alkalinization by therapeutically relevant lithium concentrations: implications for myo-inositol depletion

RATIONALE

One theory for therapeutic effects of the lithium ion (Li+) in bipolar disorder is that myo-inositol, needed for phospholipase C-mediated signaling, is depleted by Li(+)-induced inhibition of inositolphosphate hydrolysis or of myo-inositol uptake, an effect demonstrated in cultured mouse astrocytes at high myo-inositol concentrations. In contrast, myo-inositol uptake is inhibited at low concentrations, reflecting that it occurs both by the high-affinity Na(+)-dependent myo-inositol transporter (SMIT) and the lower-affinity H(+)-dependent inositol transporter (HMIT). Increased intracellular pH (pHi) stimulates SMIT but inhibits HMIT, suggesting that the effect of Li+ could be caused by intracellular alkalinization. In this study, we therefore investigated Li+ effects on intracellular pH in astrocytes, measured by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) fluorescence.

RESULTS

Chronic treatment with the therapeutically relevant Li+ concentration of 1 mM for 2 or 3 weeks increased pHi by approximately 0.10, whereas 0.5 mM was ineffective, and 2 mM caused a larger increase. The alkalinization resulted from acute stimulation of the Na+/H+ exchanger (NHE) by extracellular Li+, demonstrated after acid load with NH4Cl. In response to continuous stimulation, NHE1 mRNA was down-regulated, but protein was not.

CONCLUSIONS

Chronic treatment with pharmacologically relevant Li+ concentrations increases pHi in astrocytes, creating conditions for decreased uptake of high myo-inositol concentrations and increased uptake of low concentrations. The pharmacological relevance of this effect is supported by literature data suggesting brain acidosis in bipolar patients and by preliminary observations that carbamazepine and valproate also increase pHi in astrocytes. Stimulation of NHE1-stimulated sodium ion uptake might also trigger uptake of chloride ions and osmotically obliged water.

Astrocyte and neuron intone through glutamate

The unexpected finding of astrocytes to release glutamate as gliotransmitter challenges the traditional concepts on astrocyte being "passive" in CNS communications. Glutamate is the major excitatory transmitter in transferring information between neurons, but is now also known to activate astrocyte through transporters and receptors. Together with the sensitive swelling response, astrocytes could respond directly to glutamate and neuronal activity. Other new functions of astrocytes include modulation of synaptic plasticity and cerebral blood flow (CBF). The classic glutamate deplenishment through glutamine synthesis and CO(2) production does not account for the total glutamate internalized into astrocytes. This leads us to speculate there are many hidden functions of glutamate in neurons and astrocytes waiting to be discovered. In this review, we attempted to reexamine some of these new and older functions of glutamate and to reevaluate the roles of glutamate intoning these two cell types.

Bioenergetics of cerebral ischemia: a cellular perspective

In cerebral ischemia survival of neurons, astrocytes, oligodendrocytes and endothelial cells is threatened during energy deprivation and/or following re-supply of oxygen and glucose. After a brief summary of characteristics of different cells types, emphasizing the dependence of all on oxidative metabolism, the bioenergetics of focal and global ischemia is discussed, distinguishing between events during energy deprivation and subsequent recovery attempt after re-circulation. Gray and white matter ischemia are described separately, and distinctions are made between mature and immature brains. Next comes a description of bioenergetics in individual cell types in culture during oxygen/glucose deprivation or exposure to metabolic inhibitors and following re-establishment of normal aerated conditions. Due to their expression of NMDA and non-NMDA receptors neurons and oligodendrocytes are exquisitely sensitive to excitotoxicity by glutamate, which reaches high extracellular concentrations in ischemic brain for several reasons, including failing astrocytic uptake. Excitotoxicity kills brain cells by energetic exhaustion (due to Na(+) extrusion after channel-mediated entry) combined with mitochondrial Ca(2+)-mediated injury and formation of reactive oxygen species. Many (but not all) astrocytes survive energy deprivation for extended periods, but after return to aerated conditions they are vulnerable to mitochondrial damage by cytoplasmic/mitochondrial Ca(2+) overload and to NAD(+) deficiency. Ca(2+) overload is established by reversal of Na(+)/Ca(2+) exchangers following Na(+) accumulation during Na(+)-K(+)-Cl(-) cotransporter stimulation or pH regulation, compensating for excessive acid production. NAD(+) deficiency inhibits glycolysis and eventually oxidative metabolism, secondary to poly(ADP-ribose)polymerase (PARP) activity following DNA damage. Hyperglycemia can be beneficial for neurons but increases astrocytic death due to enhanced acidosis.

Up-regulation of cPLA(2) gene expression in astrocytes by all three conventional anti-bipolar drugs is drug-specific and enzyme-specific

RATIONALE

Common biological effects by all three conventional anti-bipolar drugs, the lithium ion (Li(+)), carbamazepine, and valproic acid, are important because identical effects may provide information about the pathophysiology of affective disorders. It has been reported that chronic treatment with either drug in vivo down-regulates the turnover of arachidonic acid in brain. This reaction is catalyzed by Ca(2+)-dependent phospholipase A(2) (cPLA(2)), the expression of which was down-regulated by Li(+) or carbamazepine but not by valproic acid; expression of two other PLA subtypes, iPLA(2) and sPLA(2) was unaffected. cPLA(2) is amply expressed in astrocytes, and in the present study, effects of 1-4 weeks of treatment with clinically relevant concentrations of each of the three anti-bipolar drugs on cPLA(2), iPLA(2), and sPLA(2) mRNA and protein expression were determined in primary cultures of mouse astrocytes by reverse transcription polymerase chain reaction (RT-PCR) and immunoblotting.

RESULTS

Two or more weeks treatment with Li(+) concentrations below 2 mM, carbamazepine or valproic acid up-regulated mRNA and protein expression of cPLA(2), but had no effect on iPLA(2) and sPLA(2), showing enzyme specificity. The effect occurred more rapidly at higher than lower concentrations but also tended to end after 4 weeks at the higher concentrations. Two millimolar Li(+) caused an initial increase of cPLA(2) followed by a decrease after 3 and 4 weeks. Topiramate had no effect, indicating specificity for anti-bipolar drugs.

CONCLUSIONS

Both up- and down-regulation of cPLA(2) gene expression are involved in the mechanisms of action of anti-bipolar drugs; astrocytes are a target for these drugs.

Effect of topiramate and dBcAMP on expression of the glutamate transporters GLAST and GLT-1 in astrocytes cultured separately, or together with neurons

The mechanism of the antiepileptic drug topiramate is not fully understood, but interaction with the excitatory neurotransmission, e.g. glutamate receptors, is believed to be part of its anticonvulsant effect. The glutamate transporters GLAST and GLT-1 are responsible for the inactivation of glutamate as a neurotransmitter and it was therefore investigated if topiramate might affect the expression of GLAST and GLT-1 in astrocytes cultured separately or together with neurons. Since expression and membrane trafficking of glutamate transporters are affected by the protein kinase C system as well as by dBcAMP it was also investigated if these signalling pathways might play a role. In astrocyte cultures expressing mainly GLAST treatment with dBcAMP (0.25 mM) led to an increased expression of the total amount of GLAST as well as of its membrane association. The enhanced expression in the membrane was particularly pronounced for the oligomeric form of GLAST. No detectable effect on the expression of GLAST in astrocytes treated with topiramate in the presence and absence of protein kinase C activators or inhibitors was observed. Astrocytes co-cultured with neurons expressed both GLAST and GLT-1. In these cultures prolonged exposure to 30 muM topiramate (10 days) led to a statistically significant increase (P<0.025) in the membrane expression of GLAST. In case of GLT-1, culture in the presence of 30 microM topiramate for 1 and 10 days led to alterations in the total, cytoplamic and membrane expression of the oligomeric form of the transporter.

Role of astrocytes in glutamate homeostasis: implications for excitotoxicity

Glutamate homeostasis in the brain is maintained by its well balanced release, uptake and metabolism. It appears that astrocytes play a prominent role in this context since they possess a very powerful battery of glutamate transporters. Thus, malfunction of astrocytic glutamate transporters will lead to an excessively high extracellular glutamate concentration which may result in neurodegeneration caused by the excitotoxic action of glutamate.

Astrocytic control of glutamatergic activity: astrocytes as stars of the show

It is a major recent finding that astrocytes can influence synaptic activity by release of glutamate, but many other glutamate-mediated activities are also controlled by astrocytes. Even the most obvious neuronal function of glutamate - its release as a transmitter - is regulated by astrocytes; these cells are needed for formation of precursors for glutamate synthesis, for reuptake of released transmitter, and for disposal of excess glutamate. Without astrocytic involvement, normal function of glutamatergic neurons is not possible, as exemplified by almost instantaneous abrogation of normal vision and learning upon inhibition of astrocyte-specific metabolic pathways. In addition, astrocytes are essential for production of the neuroprotectant glutathione, yet they can also contribute to neuronal death during ischemia by maintaining glutamine synthesis, enabling neuronal formation of neurotoxic glutamate.

Intercellular metabolic compartmentation in the brain: past, present and future

The first indication of 'metabolic compartmentation' in brain was the demonstration that glutamine after intracisternal [14C]glutamate administration is formed from a compartment of the glutamate pool that comprises at most one-fifth of the total glutamate content in the brain. This pool, which was designated 'the small compartment,' is now known to be made up predominantly or exclusively of astrocytes, which accumulate glutamate avidly and express glutamine synthetase activity, whereas this enzyme is absent from neurons, which eventually were established to constitute 'the large compartment.' During the following decades, the metabolic compartment concept was refined, aided by emerging studies of energy metabolism and glutamate uptake in cellularly homogenous preparations and by the histochemical observations that the two key enzymes glutamine synthetase and pyruvate carboxylase are active in astrocytes but absent in neurons. It is, however, only during the last few years that nuclear magnetic resonance (NMR) spectroscopy, assisted by previously obtained knowledge of metabolic pathways, has allowed accurate determination in the human brain in situ of actual metabolic fluxes through the neuronal tricarboxylic acid (TCA) cycle, the glial, presumably mainly astrocytic, TCA cycle, pyruvate carboxylation, and the 'glutamate-glutamine cycle,' connecting neuronal and astrocytic metabolism. Astrocytes account for 20% of oxidative metabolism of glucose in the human brain cortex and accumulate the bulk of neuronally released transmitter glutamate, part of which is rapidly converted to glutamine and returned to neurons in the glutamate-glutamine cycle. However, one-third of released transmitter glutamate is replaced by de novo synthesis of glutamate from glucose in astrocytes, suggesting that at steady state a corresponding amount of glutamate is oxidatively degraded. Net degradation of glutamate may not always equal its net production from glucose and enhanced glutamatergic activity, occurring during different types of cerebral stimulation, including the establishment of memory, may be associated with increased de novo synthesis of glutamate. This process may contribute to a larger increase in glucose utilization rate than in rate of oxygen consumption during brain activation. The energy yield in astrocytes from glutamate formation is strongly dependent upon the fate of the generated glutamate.

Inhibition of short-term memory formation in the chick by blockade of extracellular glutamate uptake

Injection of monosodium glutamate (40nmol/hemisphere) into the intermediate hyperstriatum ventrale of the day-old chick inhibits the formation of short-term memory for a single trial learning that discriminates between colours of beads. These experiments showed that an excess of glutamate close to learning could be damaging to memory. In the present experiments we have blocked the normal reuptake of glutamate and suggest that glutamate release plays a role in normal learning. Removal of glutamate, released from presynaptic neurones during learning, is achieved by various neuronal and astrocytic glutamate transporters. By blocking the primarily astrocytic removal of glutamate by the injection of L-aspartic acid beta-hydroxamate, we effectively increased extrasynaptic levels of glutamate and inhibited short-term memory in a similar manner to central injection of 40nmol glutamate per hemisphere. These experiments suggest that glutamate release within 2.5min of the learning experience is an important feature of short-term memory formation.

Binding and transport of [3H](2S,4R)- 4-methylglutamate, a new ligand for glutamate transporters, demonstrate labeling of EAAT1 in cultured murine astrocytes

Transporters for L-glutamate (excitatory amino acid transporters; EAATs), localized to astrocytes, are involved intimately in intermediary metabolism within the brain. Because (2S,4R)-4-methylglutamate (4MG) has affinity for glial EAATs, we employed [(3)H]4MG to define the characteristics of EAATs in cultured murine astrocytes and describe new approaches to analyze EAAT function. Specific binding of [(3)H]4MG in astrocytic membranes at 4 degrees C represented 90% of total binding. Binding was rapid (apparent t(1/2) approximately 7 min) and saturable. Saturation and Scatchard analyses indicated a single binding site (n(H) = 0.8) with a K(d) of 6.0 +/- 1.5 microM and B(max) = 9.7 +/- 2.9 pmol/mg protein. Binding of [(3)H]4MG to astrocytic homogenates was Na(+)-dependent and inhibited by K(+). Compounds acting at EAATs, such as L-glutamate (Glu), D-aspartate (D-Asp), L-(2S,3S,4R)-2-(carboxycyclopropyl)glycine and L-trans-pyrrolidine-2,4-dicarboxylate displaced binding to nonspecific levels. L-Serine-O-sulphate, an EAAT1-preferring ligand, fully displaced binding of [(3)H]4MG. In contrast, inhibitors having preferential affinity for EAAT2, L-threo-3-methylglutamate, dihydrokainate, and kainate, were relatively ineffective binding displacers. Agonists and antagonists for Glu receptors failed to significantly inhibit [(3)H]4MG binding. Studies with [(3)H]D-Asp reinforced evidence that [(3)H]4MG was binding to EAATs. These data were consistent with Western blot analyses, which indicated abundant expression of EAAT1 but not EAAT2. [(3)H]4MG was also accumulated rapidly (apparent t(1/2) approximately 4 min) into whole astrocytes by a sodium- and temperature-sensitive process (K(m) of 146 +/- 24 microM, V(max) = 336 +/- 27 nmol/mg protein/min), which possessed an EAAT1-like pharmacologic profile. These findings confirm that 4MG is a substrate for EAAT1 and that the binding assay developed using [(3)H]4MG can be utilized in various preparations including cultured astrocytes.

Reciprocal changes in forebrain contents of glycogen and of glutamate/glutamine during early memory consolidation in the day-old chick

Passive avoidance learning is with advantage studied in day-old chicks trained to distinguish between beads of two different colors, of which one at training was associated with aversive taste. During the first 30-min post-training, two periods of glutamate release occur in the forebrain. One period is immediately after the aversive experience, when glutamate release is confined to the left hemisphere. A second release, 30 min later, may be bilateral, perhaps with preponderance of the right hemisphere. The present study showed increased pool sizes of glutamate and glutamine, specifically in the left hemisphere, at the time when the first glutamate release occurs, indicating de novo synthesis of glutamate/glutamine from glucose or glycogen, which are the only possible substrates. Behavioral evidence that memory is extinguished by intracranial administration at this time of iodoacetate, an inhibitor of glycolysis and glycogenolysis, and that the extinction of memory is counteracted by injection of glutamine, supports this concept. A decrease in forebrain glycogen of similar magnitude and coinciding with the increase in glutamate and glutamine suggests that glycogen rather than glucose is the main source of newly synthesized glutamate/glutamine. The second activation of glutamatergic activity 30 min after training, when memory is consolidated into stable, long-term memory, is associated with a bilateral increase in pool size of glutamate/glutamine. No glycogenolysis was observed at this time, but again there is a temporal correlation with sensitivity to inhibition by iodoacetate and rescue by glutamine, indicating the importance of de novo synthesis of glutamate/glutamine from glucose or glycogen.

Cataplerotic TCA cycle flux determined as glutamate-sustained oxygen consumption in primary cultures of astrocytes

Utilization of glucose by adult brain as its metabolic substrate does not mean that glutamate cannot be synthesized from glucose and subsequently oxidatively degraded. Between 10 and 20% of total pyruvate metabolism in brain occurs as formation of oxaloacetate (OAA), a tricarboxylic acid (TCA) cycle intermediate, from pyruvate plus CO(2). This anaplerotic ('pool-filling') process occurs in astrocytes, which in contrast to neurons express pyruvate carboxylase (PC) activity. Equivalent amounts of pyruvate are converted to acetylcoenzyme A and condensed with oxaloacetate to form citrate (Cit), which is metabolized to alpha-ketoglutarate (generating oxidatively-derived energy), glutamate and glutamine and transferred to neurons in the glutamate-glutamine cycle and used as precursor for transmitter glutamate. Since the blood-brain barrier is poorly permeable to glutamate and its metabolites, net synthesis of glutamate must be followed by degradation of equivalent amounts of glutamate, a cataplerotic ('pool-emptying') process, in which glutamate is converted in the TCA cycle to malate or oxaloacetate (generating additional energy), which exit the cycle to form one molecule pyruvate. To obtain an estimate of the rate of astrocytic oxidation of glutamate the rate of oxygen consumption was measured in primary cultures of mouse astrocytes metabolizing glutamate in the absence of other metabolic substrates. The observed rate is compatible with complete oxidative degradation of glutamate.

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.