Induction of glutamine synthetase in rat astrocytes by co-cultivation with embryonic chick neurons

Glial Glutamine Homeostasis in Health and Disease

Glutamine is an essential cerebral metabolite. Several critical brain processes are directly linked to glutamine, including ammonia homeostasis, energy metabolism and neurotransmitter recycling. Astrocytes synthesize and release large quantities of glutamine, which is taken up by neurons to replenish the glutamate and GABA neurotransmitter pools. Astrocyte glutamine hereby sustains the glutamate/GABA-glutamine cycle, synaptic transmission and general brain function. Cerebral glutamine homeostasis is linked to the metabolic coupling of neurons and astrocytes, and relies on multiple cellular processes, including TCA cycle function, synaptic transmission and neurotransmitter uptake. Dysregulations of processes related to glutamine homeostasis are associated with several neurological diseases and may mediate excitotoxicity and neurodegeneration. In particular, diminished astrocyte glutamine synthesis is a common neuropathological component, depriving neurons of an essential metabolic substrate and precursor for neurotransmitter synthesis, hereby leading to synaptic dysfunction. While astrocyte glutamine synthesis is quantitatively dominant in the brain, oligodendrocyte-derived glutamine may serve important functions in white matter structures. In this review, the crucial roles of glial glutamine homeostasis in the healthy and diseased brain are discussed. First, we provide an overview of cellular recycling, transport, synthesis and metabolism of glutamine in the brain. These cellular aspects are subsequently discussed in relation to pathological glutamine homeostasis of hepatic encephalopathy, epilepsy, Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. Further studies on the multifaceted roles of cerebral glutamine will not only increase our understanding of the metabolic collaboration between brain cells, but may also aid to reveal much needed therapeutic targets of several neurological pathologies.

Astrocyte energy and neurotransmitter metabolism in Alzheimer's disease: integration of the glutamate/GABA-glutamine cycle

Astrocytes contribute to the complex cellular pathology of Alzheimer's disease (AD). Neurons and astrocytes function in close collaboration through neurotransmitter recycling, collectively known as the glutamate/GABA-glutamine cycle, which is essential to sustain neurotransmission. Neurotransmitter recycling is intimately linked to astrocyte energy metabolism. In the course of AD, astrocytes undergo extensive metabolic remodeling, which may profoundly affect the glutamate/GABA-glutamine cycle. The consequences of altered astrocyte function and metabolism in relation to neurotransmitter recycling are yet to be comprehended. Metabolic alterations of astrocytes in AD deprive neurons of metabolic support, thereby contributing to synaptic dysfunction and neurodegeneration. In addition, several astrocyte-specific components of the glutamate/GABA-glutamine cycle, including glutamine synthesis and synaptic neurotransmitter uptake, are perturbed in AD. Integration of the complex astrocyte biology within the context of AD is essential for understanding the fundamental mechanisms of the disease, while restoring astrocyte metabolism may serve as an approach to arrest or even revert clinical progression of AD.

Acute Liver Toxicity Modifies Protein Expression of Glutamate Transporters in Liver and Cerebellar Tissue

Glutamate is the main excitatory amino acid acting at the level of pre and postsynaptic neurons, as well as in glial cells. It is involved in the coordinated modulation of energy metabolism, glutamine synthesis, and ammonia detoxification. The relationship between the functional status of liver and brain has been known for many years. The most widely recognized aspect of this relation is the brain dysfunction caused by acute liver injury that manifests a wide spectrum of neurologic and psychiatric abnormalities. Inflammation, circulating neurotoxins, and impaired neurotransmission have been reported in this pathophysiology. In the present contribution, we report the effect of a hepatotoxic compound like CCl₄ on the expression of key proteins involved in glutamate uptake and metabolism as glutamate transporters and glutamine synthetase in mice liver, brain, and cerebellum. Our findings highlight a differential expression pattern of glutamate transporters in cerebellum. A significant Purkinje cells loss, in parallel to an up-regulation of glutamine synthetase, and astrogliosis in the brain have also been noticed. In the intoxicated liver, glutamate transporter 1 expression is up-regulated, in contrast to glutamine synthetase which is reduced in a time-dependent manner. Taken together our results demonstrate that the exposure to an acute CCl₄ insult, leads to the disruption of glutamate transporters expression in the liver-brain axis and therefore a severe alteration in glutamate-mediated neurotransmission might be present in the central nervous system.

Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

Astrocytes are a major component of the resident non-neuronal glial cell population of the CNS. They are ubiquitously distributed throughout the brain and spinal cord, where they were initially thought to function in both structural and homeostatic capacities, providing the framework and environment in which neurons performed their parenchymal duties. However, this stroma-like view of astrocytes is no longer satisfactory. Mounting evidence particularly within the last decade indicates that astrocytes do not simply support neuronal activity but directly contribute to it. Congruent with this evolving view of astrocyte function in information processing is the emergent notion that these glial cells are not a homogeneous population of cells. Thus, astrocytes in various anatomically distinct regions of the normal CNS possess unique phenotypic characteristics that may directly influence the particular neuronal activities that define these regions. Remarkably, regional populations of astrocytes appear to exhibit local heterogeneity as well. Many phenotypic traits of the astrocyte lineage are responsive to local environmental cues (i.e., are adaptable), suggesting that plasticity contributes to this diversity. However, compelling evidence suggests that astrocytes arise from multiple distinct progenitor pools in the developing CNS, raising the intriguing possibility that some astrocyte heterogeneity may result from intrinsic differences between these progenitors. The purpose of this review is to explore the evidence for and mechanistic determinants of regional and local astrocyte diversity.

Energy metabolism in astrocytes and neurons treated with manganese: relation among cell-specific energy failure, glucose metabolism, and intercellular trafficking using multinuclear NMR-spectroscopic analysis

A central question in manganese neurotoxicity concerns mitochondrial dysfunction leading to cerebral energy failure. To obtain insight into the underlying mechanism(s), the authors investigated cell-specific pathways of [1-13C]glucose metabolism by high-resolution multinuclear NMR-spectroscopy. Five-day treatment of neurons with 100-micro mol/L MnCl(2) led to 50% and 70% decreases of ATP/ADP and phosphocreatine-creatine ratios, respectively. An impaired flux of [1-13C]glucose through pyruvate dehydrogenase, which was associated with Krebs cycle inhibition and hence depletion of [4-13C]glutamate, [2-13C]GABA, and [13C]glutathione, hindered the ability of neurons to compensate for mitochondrial dysfunction by oxidative glucose metabolism and further aggravated neuronal energy failure. Stimulated glycolysis and oxidative glucose metabolism protected astrocytes against energy failure and oxidative stress, leading to twofold increased de novo synthesis of [3-13C]lactate and fourfold elevated [4-13C]glutamate and [13C]glutathione levels. Manganese, however, inhibited the synthesis and release of glutamine. Comparative NMR data obtained from cocultures showed disturbed astrocytic function and a failure of astrocytes to provide neurons with substrates for energy and neurotransmitter metabolism, leading to deterioration of neuronal antioxidant capacity (decreased glutathione levels) and energy metabolism. The results suggest that, concomitant to impaired neuronal glucose oxidation, changes in astrocytic metabolism may cause a loss of intercellular homeostatic equilibrium, contributing to neuronal dysfunction in manganese neurotoxicity.

Glutamine synthetase in brain: effect of ammonia

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

Glutamine Synthetase is Expressed by Primary Sensory Neurons from Chick Embryos In Vitro but not In Vivo: Influence of Skeletal Muscle Extract

Glutamine synthetase (GS) catalyses the ATP-dependent formation of glutamine from glutamate and ammonia. To determine whether dorsal root ganglion (DRG) cells from chick embryos express the enzyme in vivo or in vitro, GS was detected by immunocytochemical reaction either in vibratome sections of DRG or in dissociated DRG cell cultures. The immunocytochemical detection of GS showed that in vivo the DRG taken from chick embryos at day 10 (E10), E14, E18 or from chickens after hatching were free of any GS-positive ganglion cells; in contrast, in neuron-enriched cultures of DRG cells grown in vitro at E10, virtually all the neuronal cells (98.6 +/- 1.0%) express GS at 3, 5 or 7 days of culture. In mixed DRG cell cultures, only 83.6+/-4.6% of the neurons displayed a GS-immunoreactivity. In both culture conditions, neither the presence of horse serum nor the age of the culture appeared to affect the percentage of neurons which displayed a GS-immunoreactivity. After [3H]glutamine uptake, radioautographs revealed that only 80% of the neurons were labelled in neuron-enriched DRG cell cultures while 96% of the neurons were radioactive in mixed DRG cell cultures. Furthermore the most heavily [3H]glutamine-labelled neurons were exclusively found in mixed DRG cell cultures. Combination of both immunocytochemical detection of GS and radioautography after [3H]glutamine uptake showed that strongly GS-immunostained neurons corresponded to poorly radioactive ones and vice versa. When skeletal muscle extract (ME) was added to DRG cell cultures, the number of GS-positive neurons was reduced to 77.5 +/- 2.5% in neuron-enriched cultures or to 43.6 +/- 3.8% in mixed DRG cell cultures; in both types of culture, the intensity of the neuronal immunostaining was depressed. Furthermore, combined action of ME and non-neuronal cells potentiates the enzyme repression exerted separately by ME or non-neuronal cells. Since GS-immunoreactivity is expressed in DRG cells grown in vitro, but not in vivo, it is suggested that microenvironmental factors influence the expression of GS. More specifically, the repression of GS by primary sensory neurons grown in vitro may be strongly induced by soluble factors present in skeletal muscle, and to a lesser extent in brain, and potentiated by non-neuronal cells.

NMR spectroscopic study on the metabolic fate of [3-(13)C]alanine in astrocytes, neurons, and cocultures: implications for glia-neuron interactions in neurotransmitter metabolism

Nuclear magnetic resonance (NMR) spectroscopy and biochemical assays were used to study the fate of [3-(13)C]alanine in astrocytes, neurons, and cocultures. (1)H- and (13)C-NMR analysis of the media demonstrated a high and comparable uptake of [3-(13)C]alanine by the cells. Thereafter, alanine is transaminated predominantly to [3-(13)C]pyruvate, from which the (13)C-label undergoes different metabolic pathways in astrocytes and neurons: Lactate is almost exclusively synthesized in astrocytes, while in neurons and cocultures labeled neurotransmitter amino acids are formed, i.e., glutamate and gamma-aminobutyric acid (GABA). A considerable contribution of the anaplerotic pathway is observed in cocultures, as concluded from the ratio (C-2-C-3)/C-4 of labeled glutamine. Analysis of the multiplet pattern of glutamate isotopomers indicates carbon scrambling through the TCA cycle and the use of alanine also as energy substrate in neurons. In cocultures, astrocyte-deduced lactate and unlabeled exogenous carbon substrates contribute to glutamate synthesis and dilute the [2-(13)C]acetyl-CoA pool by 30%. The coupling of neuronal activity with shuttling of tricarboxylic acid (TCA) cycle-derived metabolites between astrocytes and neurons is concluded from the use of [4-(13)C]-monolabeled glutamate leaving the first TCA cycle turn already for glutamine and GABA synthesis, as well as from the labeling pattern of extracellular glutamine. Further evidence of a metabolic interaction between astrocytes and neurons is obtained, as alanine serves as a carbon and nitrogen carrier through the synthesis and regulated release of lactate from astrocytes for use by neurons. Complementary to the glutamine-glutamate cycle in the brain, a lactate-alanine shuttle between astrocytes and neurons would account for the nitrogen exchange of the glutamatergic neurotransmitter cycle in mammalian brain.

Glucocorticoid receptor pathways are involved in the inhibition of astrocyte proliferation

In earlier studies, the neural cell adhesion molecule, N-CAM, was found to inhibit the proliferation of rat astrocytes both in vitro and in vivo. To identify the gene targets involved, we used subtractive hybridization to examine changes in gene expression that occur after astrocytes are exposed to N-CAM in vitro. While the mRNA levels for N-CAM decreased after such treatment, the levels of mRNAs for glutamine synthetase and calreticulin increased. Since glutamine synthetase and calreticulin are known to be involved in glucocorticoid receptor pathways, we tested a number of steroids for their effects on astrocyte proliferation. Dexamethasone, corticosterone, and aldosterone were all found to inhibit rat cortical astrocyte proliferation in culture in a dose-dependent manner. RU-486, a potent glucocorticoid antagonist, reversed the inhibitory effects of dexamethasone. These observations prompted the hypothesis that the inhibition of proliferation by N-CAM might be mediated through the glucocorticoid receptor pathway. Consistent with this hypothesis, the inhibition of astrocyte proliferation by N-CAM was reversed in part by a number of glucocorticoid antagonists, including RU-486, dehydroepiandrosterone, and progesterone. In transfection experiments with cultured astrocytes, N-CAM treatment increased the expression of a luciferase reporter gene under the control of a minimal promoter linked to a glucocorticoid response element. The enhanced activity of this construct stimulated by N-CAM was abolished in the presence of RU-486. The combined data suggest that astrocyte proliferation is in part regulated by alterations in glucocorticoid receptor pathways.

Platelet-derived growth factor receptors of mouse central nervous system cells in vitro

This study evaluates the distribution of receptors for platelet-derived growth factor (PDGF) on central nervous cells maintained in vitro using colloidal gold-labeled immunocytochemical markers at the electron microscopic level. Platelet-derived growth factor receptors were found to be sparsely distributed over the surface of type 1 astrocytes, apparent type 2 astrocytes, and neurons. Receptors appeared to be preferentially associated with filopodia-like extensions of the cell membrane. The existence of functional receptors was confirmed using the impermeant, water-soluble affinity cross-linking agent bis(sulfosuccinimidyl)suberate to covalently link radiolabeled PDGF to its receptor. The PDGF/receptor complexes could also be immunoprecipitated with the same antibody used in immunocytochemical experiments. The improved resolution of these techniques allows definitive identification of PDGF receptors on cultured mammalian central nervous system cells other than oligodendrocytes. These data expand the range of possible roles of PDGF during nervous system development. Receptors for PDGF are likely to play a key role in the differentiation of cells in the central nervous system.

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

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

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

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

Involvement of a C/EBP-like protein in the acquisition of responsiveness to glucocorticoid hormones during chick neural retina development

The glucocorticoid receptor in chicken embryonic neural retina is expressed early in ontogeny, yet the tissue's response to the glucocorticoid hormone, i.e., induction of glutamine synthetase (GS), develops later, only during week 2 of ontogeny. Transient transfection of embryonic day 7 (E7) retinal cells, which are nonresponsive to glucocorticoids, with chimeric plasmids containing the chloramphenicol acetyltransferase reporter gene under the control of glucocorticoid-responsive promoters demonstrated that GR in E7 cells is a functional transactivating factor. We show that the limiting transcription factor that controls the developmental acquisition of responsiveness to glucocorticoids is similar to a CCAAT enhancer-binding protein (C/EBP). This protein recognizes a sequence in the promoter of the chick GS gene, which is required for eliciting the glucocorticoid response. Retinal C/EBP-like protein was not detected in the glucocorticoid-nonresponsive (E7) proliferating glioblasts but was found to be present in the glucocorticoid-responsive (E12) postmitotic cells. Premature expression of C/EBP in the nonresponsive E7 cells by transfection was shown to enhance the developmental acquisition of responsiveness to the glucocorticoid hormone, as deduced from the level of GS inducibility.

Involvement of a C/EBP-like protein in the acquisition of responsiveness to glucocorticoid hormones during chick neural retina development

The glucocorticoid receptor in chicken embryonic neural retina is expressed early in ontogeny, yet the tissue's response to the glucocorticoid hormone, i.e., induction of glutamine synthetase (GS), develops later, only during week 2 of ontogeny. Transient transfection of embryonic day 7 (E7) retinal cells, which are nonresponsive to glucocorticoids, with chimeric plasmids containing the chloramphenicol acetyltransferase reporter gene under the control of glucocorticoid-responsive promoters demonstrated that GR in E7 cells is a functional transactivating factor. We show that the limiting transcription factor that controls the developmental acquisition of responsiveness to glucocorticoids is similar to a CCAAT enhancer-binding protein (C/EBP). This protein recognizes a sequence in the promoter of the chick GS gene, which is required for eliciting the glucocorticoid response. Retinal C/EBP-like protein was not detected in the glucocorticoid-nonresponsive (E7) proliferating glioblasts but was found to be present in the glucocorticoid-responsive (E12) postmitotic cells. Premature expression of C/EBP in the nonresponsive E7 cells by transfection was shown to enhance the developmental acquisition of responsiveness to the glucocorticoid hormone, as deduced from the level of GS inducibility.

Direct demonstration by [13C]NMR spectroscopy that glutamine from astrocytes is a precursor for GABA synthesis in neurons

Primary cultures of cerebral cortical astrocytes and neurons, as well as neurons growing on top of the astrocytes (sandwich co-cultures), were incubated with 1-[13C]glucose or 2-[13C]acetate and in the presence or absence of the glutamine synthetase inhibitor methionine sulfoximine. [13C]NMR spectroscopy at 125 MHz was performed on perchloric acid extracts of the cells or on media collected from the cultures. In addition, the [13C/12C] ratios of the amino acids glutamine, glutamate and 4-aminobutyrate (GABA) were determined by gas chromatography/mass spectroscopy, showing a larger degree of labeling in GABA than in glutamate and glutamine from glucose. Glutamine and glutamate were predominantly labeled from acetate. A picture of cellular metabolism mainly regarding the tricarboxylic acid cycle and glycolysis was obtained. Due to the fact that acetate is not metabolized by neurons to any significant extent, it could be shown that precursors from astrocytes are incorporated into the GABA pool of neurons grown in co-culture with astrocytes. Spectra of media removed from these cultures revealed that likely precursor candidates for GABA were glutamine and citrate. The importance of glutamine is further substantiated by the finding that inhibition of glutamine synthetase, an enzyme present in astrocytes only, significantly decreased the labeling of GABA in co-cultures incubated with 2-[13C]acetate.

Light and electron microscopic immunocytochemical localization of glutamine synthetase in the superficial pineal gland of the rat

Glutamine synthetase (L-glutamate:ammonia ligase; EC 6.3.1.2), an enzyme catalysing the ATP-dependent formation of glutamine from glutamate and ammonia, was detected immunocytochemically only in glial (interstitial) cells of the superficial pineal gland of the rat. The results show the important role of pineal glial cells in the metabolism of the presumptive neurotransmitters, glutamate and gamma-aminobutyric acid (GABA) as well as in detoxification of ammonia.

Regulation of the glutamine content of astrocytes by cAMP and hydrocortisone: Effect of pH

It was reported recently that the glutamine content of astrocytes incubated with glutamate and ammonium is steeply dependent on the pH of the solution. The present study shows that pretreatment of astrocytes with dibutyryl cAMP or with hydrocortisone, conditions that induce glutamine synthetase activity, increased glutamine content 2.4-fold and 5.3-fold, respectively. Nevertheless, a shift of pH from 7.4 to 7.8 increased glutamine content further by 2.7-fold and 3.0-fold, respectively. The net rates of uptake of glutamate and export of glutamine varied narrowly compared to these very large changes in glutamine content.

Uptake and metabolism of malate in neurons and astrocytes in primary cultures

Uptake and oxidative metabolism of [14C]malate as well as its incorporation into aspartate, glutamate, glutamine, and GABA were studied in cultured cerebral cortical neurons (GABAergic), cerebellar granule neurons (glutamatergic), and cerebral cortical astrocytes. All cell types exhibited high affinity uptake of malate (Km 10-85 microM) with slightly higher Vmax values in neurons (0.1-0.2 nmol x min-1 x mg-1) than in astrocytes (0.06 nmol x min-1 x mg-1). Malate was oxidatively metabolized in all three cell types with nominal rates of 14CO2 production of 2-15 pmol x min-1 x mg-1. The oxidation of malate was only slightly inhibited by 5 mM aminooxyacetic acid (AOAA). In granule cell preparations [14C]malate was incorporated into aspartate and glutamate and, to a much less extent, into glutamine. This incorporation was blocked by 5 mM AOAA. Astrocytes exhibited slightly higher incorporation rates into aspartate and glutamate, but in these cells glutamine was labelled to a considerable extent. AOAA (5 mM) inhibited the incorporation by 60-70%. In cultures of cerebral cortical neurons, very low levels of radioactivity derived from [14C]malate were found in aspartate and glutamate, and GABA was not labelled at all. Glutamine had the same specific activity as glutamate, indicating that the low rates of incorporation of radioactivity into amino acids in this preparation is likely to exclusively represent metabolism of malate in the small population of astrocytes (5% of total cell number), contaminating the neuronal cultures. The findings suggest that exogenous malate to a quantitatively limited extent may serve as a precursor for transmitter glutamate in glutamatergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in neuronal and glial cell phenotypic expression in neuron-glia cocultures: Influence of glia-conditioned media and living glial cell substrata

Neuron-glia cocultures were prepared using, as a source for glial cells, either C6 glia (2B clone) of early (2B23) or late (2B111) passages or advanced passages of glial cells derived from primary cultures prepared from aged mouse cerebral hemispheres (MACH). Six-day-old chick embryo cerebral hemispheres (E6CH) were the source of neuron-enriched cultures. Glutamine synthetase (GS) activity was used as a marker for astrocytes and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) activity was used as a marker for oligodendrocytes. GS activity was markedly enhanced in cocultures of E6CH neurons and 2B23 glioblastic cells, whereas GS activity was reduced in cocultures of E6CH neurons and 2B111 astrocytic glia. In contrast, CNP activity was enhanced in cocultures of C6 glial cells with E6CH neurons. Glial cells from aged mouse brain did not respond to coculturing with E6CH neurons. It appears from these findings that neuronal input enhances the differentiation of glioblastic cells to either astrocytic or oligodendrocytic expression, whereas it decreases the activity of committed astrocytes. In contrast, glial cells from aged mouse brain do not respond to neuronal input. Choline acetyltransferase (ChAT) activity, a marker for cholinergic neurons, was enhanced only when E6CH cultures were grown in conditioned medium (CM) from 2B23 glioblastic cells. In contrast, ChAT activity was markedly diminished when E6CH neurons were cocultured with MACH glial cells but not when grown in CM from MACH glial cells. Thus, humoral factors from immature glial cells appear to enhance cholinergic neuronal phenotypic expression whereas cell-cell membrane contacts with aged glial cells diminish cholinergic phenotypic expression. The findings present supportive evidence that neuron-glia interrelationships are age dependent.

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

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

Neurons are replenished in cultures of embryonic chick optic tectum after immunomagnetic depletion

The effect of intercellular interactions on the determination and differentiation of early embryonic brain cells was tested by immunomagnetic cell separation techniques. Using the A2B5 monoclonal antibody, which in chick brain reacts with a neuron-specific surface ganglioside, we produced initially pure populations of optic tectum cells devoid of the antigen. A coincident depletion of neurofilament(+) cells (95%) and nonneuronal growth characteristics of the separated A2B5(-) cells indicated that the vast majority of neurons had been removed initially. Surprisingly, A2B5(+) cells rapidly appeared in separated A2B5(-) cell cultures. After 1 day, the percentage of A2B5(+) cells in separated cell cultures equalled those in unseparated cultures (approximately 50%). By a week in culture, A2B5(+) cells developed neuronal morphology and contained neurofilaments. A2B5(-) to (+) conversion was a regulated phenomenon in that removal of different proportions of the (+) cells resulted in different numbers of (-) to (+) conversions. New DNA synthesis was not required for the acquisition of cell surface A2B5 antigen or for differentiation of cells into definitive A2B5(+) neurons. Our results demonstrate that postmitotic embryonic brain contains cells which are capable of replacing depleted neurons in vitro.

Metabolism and release of glutamate in cerebellar granule cells cocultured with astrocytes from cerebellum or cerebral cortex

Cerebellar granule cells were cocultured with astrocytes from either cerebral cortex or cerebellum in two different systems. In one system the cells were plated next to each other only sharing the culture medium (separated cocultures) and in the other system the granule cells were plated on top of a preformed layer of astrocytes (sandwich cocultures). Using astrocytes from cerebellum, granule cells developed morphologically and functionally showing a characteristic high activity of the glutamate synthesizing enzyme aspartate aminotransferase (AAT) as well as a high stimulus-coupled transmitter release regardless of the culture system, i.e., granule cells could grow on top of cerebellar astrocytes as well as next to these cells. In the case of cerebral cortex astrocytes it was found that cerebellar granule cells did not develop (11% survival) when seeded on top of these astrocytes. This was indicated by the morphological appearance of the cultures as well as by a negligible difference between the AAT activity in sandwich cocultures and astrocytes cultured alone. On the other hand, granule cells in separated cocultures with cerebral cortex astrocytes exhibited a normal morphology and a high activity of AAT as well as a large stimulus-coupled transmitter release. Cerebellar and cortical astrocytes expressed the astrocyte specific enzyme glutamine synthetase in a glucocorticoid-inducible form regardless of the culture system. The results show that under conditions of direct contact between granule cells and astrocytes, regional specificity exists with regard to neuron-glia contacts. This specificity does not seem to involve soluble factors present in the culture medium because in separated cocultures the cerebellar granule cells developed normally regardless of the regional origin of the astrocytes.

Fetal alcohol delays the developmental expression of myelin basic protein and transferrin in rat primary oligodendrocyte cultures

This study has examined the development of immunoreactive myelin basic protein and transferrin in primary glial cell cultures. Cultures were initiated from control and experimental Sprague-Dawley rats 1-2 days postnatally. Experimental treatment involved exposure to 5% (w/v) ethanol in a liquid diet during the last two weeks of gestation. Prenatal alcohol administration delayed the expression of myelin basic protein and transferrin during the first three weeks postnatally. Other oligodendroglial and astroglial markers were little affected, if at all, by fetal alcohol exposure.

Thrombin and its inhibitors regulate morphological and biochemical differentiation of astrocytes in vitro

Flat, amorphous astroblasts in culture differentiate into rounded process-bearing cells after removal of serum from the media or following addition of dibutyryl cyclic-AMP (dbcAMP). We report here that addition of thrombin (10 nM) to rat primary astroglial cultures reversed both the spontaneous morphological differentiation of astroblasts caused by serum removal, and the more extensive morphological differentiation caused by pre-treatment with dbcAMP. The astroblasts retained the ability to differentiate upon removal of thrombin from the medium. Proteolytic activity of thrombin was required for the reversal of differentiation. Moreover, addition of serine protease inhibitors active against thrombin elicited a prolonged morphological differentiation rivaling that induced by dbcAMP, suggesting that inactivation of cell-associated thrombin might be sufficient for morphological differentiation to occur. Two other serine proteases with a cleavage specificity similar to thrombin were ineffective in reversing differentiation. Both the induction of morphological differentiation by dbcAMP and its reversal by thrombin were rapid, being essentially complete by 1 h. With more prolonged treatments, thrombin also reduced the dbcAMP-mediated increase in glutamine synthetase, a biochemical marker for astroglial differentiation. Thrombin also inhibited morphological differentiation in C6 glioma and altered the morphology of microglial cells; however, thrombin did not prevent neurite outgrowth in primary central neuronal cultures in contrast to its previously reported effects on the neuroblastoma 2a cell line. These findings indicate that a proteolytic mechanism mediated by thrombin and its inhibitors may underlie the regulation of astroglial differentiation.

Neuronal modulation of calcium channel activity in cultured rat astrocytes

The patch-clamp technique was used to study whether cocultivation of neurons and astrocytes modulates the expression of calcium channel activity in astrocytes. Whole-cell patch-clamp recordings from rat brain astrocytes cocultured with rat embryonic neurons revealed two types of voltage-dependent inward currents carried by Ca2+ and blocked by either Cd2+ or Co2+ that otherwise were not detected in purified astrocytes. This expression of calcium channel activity in astrocytes was neuron dependent and was not observed when astrocytes were cocultured with purified oligodendrocytes.

Glutamine synthetase modulation in astrocyte cultures of different mouse brain areas

Astroglial cells from mouse cerebral hemispheres, cerebellum, olfactory bulbs, and medulla oblongata were grown in the presence of either hormones (hydrocortisone, insulin) or cell second messengers (dBcAMP, dBcGMP). Glutamine synthetase (GS) specific activity, GS protein level, and GS translation were investigated under the effect of these factors. Hydrocortisone produced a simultaneous increase in GS translation, GS level, and activity. This increase was observed in the astrocytes cultured from the four brain areas but at a variable magnitude depending on the area. The hydrocortisone effect appeared at the transcriptional level. Inversely, insulin decreased both the GS activity and the in vitro translated GS. This effect was seen only in the olfactory bulbs and the medulla. DBcAMP increased the GS biological activity only in the cerebral hemisphere cultures. It raised, however, the level of translated GS and GS protein in astrocytes from all the areas, suggesting a post-translational effect for intracellular cAMP. DBcGMP only affected GS in the astrocytes from cerebral hemispheres and the medulla modulating either the GS transcription or the messenger RNA stability. These results suggest specific regulation for GS expression, depending on the brain area from which the cells were dissociated or on the astroglial cell population present in these cultures affecting either the transcription, the mRNA stability, or the biological activity of the protein.

Neuron-glial interactions involved in the regulation of glutamine synthetase

Cocultures of rat cortical astrocytes with cerebellar granule cell neurons, but not a variety of other cell types tested, resulted in an induction of glutamine synthetase (GS) mRNA over the basal levels expressed in pure astrocyte cultures. This induction involved both contact- and noncontact-mediated events and may be a result of astroglial differentiation promoted by interactions with the primary neurons. Astrocytes grown in the presence of the granule neurons (but not the other cell types tested) exhibited a more complex, process-bearing morphology typical of more differentiated cells. In addition, glial cell proliferation was inhibited not only by the presence of live granule cells, but also by fixed neurons and neuronal membranes. Under the same experimental conditions, GS mRNA was increased (two- to threefold) compared with the expression observed in pure astrocyte cultures. Because of the role of GS in glutamate metabolism and the influence of the glutamatergic granule neurons on glial GS mRNA levels, the effect of exogenous glutamate was examined. The addition of 100 microM glutamate to the culture medium resulted in an increase in GS mRNA in the astrocyte cultures similar to that observed in the cocultures, where the addition of glutamate did not further increase GS mRNA levels. These results provide further evidence for the importance of neuron-glial interactions in the regulation of glial gene expression.

Reduction of glutamine synthetase specific activity in cultured astroglia by ferrous chloride

Immature and mature rat astroglia in culture were assayed for glutamine synthetase (GS) activity after a single exposure to the epileptogen FeCl2. Cells were cultured with both standard and elevated extracellular potassium or glutamate (Glu) concentrations. FeCl2 reduced GS activity below control levels, whereas high Glu increased GS activity. However, stimulation by high Glu was significantly attenuated in cultures given both FeCl2 and high Glu, indicating that cells treated with FeCl2 were not able to respond as effectively to increased extracellular glutamate by increasing their GS activity. The significance of these findings is that glial regulation of the neuronal environment may be impaired, based on the proposed importance of GS in ammonia detoxification in the brain.

Early and late passage C-6 glial cell growth: similarities with primary glial cells in culture

Earlier studies in our laboratory have shown that C-6 glial cells in culture exhibit astrocytic properties with increasing cell passage. In this study, we tested the responsiveness of early and late passage C-6 glial cells to various cultures conditions: culture substrata (collagen, poly-L-lysine, plastic), or supplements for the culture medium, DMEM, [fetal calf, or heat inactivated (HI) serum, or media conditioned from mouse neuroblastoma cells (NBCM) or primary chick embryo cultured neurons (NCM)]. Glutamine synthetase (GS) and cyclic nucleotide phosphohydrolase (CNP), astrocytic and oligodendrocytic glial markers, were used. Cell number and protein content increased exponentially with days in culture regardless of the type of the substratum or cell passage. Differences in cell morphology among the three types of substratum were also reflected on GS activity, which rose by three-fold on culture day 3 for cells grown on collagen; thereafter, GS profiles were similar for all substrata. This early rise in GS is interpreted to reflect differential cell adhesion processes on the substrata; specifically, cell adhesion on the collagen stimulated differentiation into "astrocytic phenotype". Analogous to immature glia cells in primary cultures, early passage C-6 glial cells responded to neuronal factors supplied either from NCM or NBCM by expressing reduced GS activity, the astrocytic marker and enhanced CNP activity, the oligodendrocytic marker. Thus, early passage cells can be induced to express either astrocytic or oligodendrocytic phenotype. In accordance with our previous reports on primary glial cells, late passage C-6 cells exhibit their usual astrocytic behavior, responding to serum factors with GS activity. Moreover, whereas NCM or NBCM alone markedly lowered GS activity, a combination with serum restored activity. The present findings confirm our previous observations and further establish the C-6 glial cells as a reliable model to study immature glia.