Amino acid metabolism in glial cells: homeostatic regulation of intra- and extracellular milieu by C-6 glioma cells

Exploring the differences in metabolic behavior of astrocyte and glioblastoma: a flux balance analysis approach

Brain cancers demonstrate a complex metabolic behavior so as to adapt the external hypoxic environment and internal stress generated by reactive oxygen species. To survive in these stringent conditions, glioblastoma cells develop an antagonistic metabolic phenotype as compared to their predecessors, the astrocytes, thereby quenching the resources expected for nourishing the neurons. The complexity and cumulative effect of the large scale metabolic functioning of glioblastoma is mostly unexplored. In this study, we reconstruct a metabolic network comprising of pathways that are known to be deregulated in glioblastoma cells as compared to the astrocytes. The network, consisted of 147 genes encoding for enzymes performing 247 reactions distributed across five distinct model compartments, was then studied using constrained-based modeling approach by recreating the scenarios for astrocytes and glioblastoma, and validated with available experimental evidences. From our analysis, we predict that glycine requirement of the astrocytes are mostly fulfilled by the internal glycine-serine metabolism, whereas glioblastoma cells demand an external uptake of glycine to utilize it for glutathione production. Also, cystine and glucose were identified to be the major contributors to glioblastoma growth. We also proposed an extensive set of single and double lethal reaction knockouts, which were further perturbed to ascertain their role as probable chemotherapeutic targets. These simulation results suggested that, apart from targeting the reactions of central carbon metabolism, knockout of reactions belonging to the glycine-serine metabolism effectively reduce glioblastoma growth. The combinatorial targeting of glycine transporter with any other reaction belonging to glycine-serine metabolism proved lethal to glioblastoma growth.

1H MR spectroscopic imaging with short and long echo time to discriminate glycine in glial tumours

OBJECT

To investigate glycine (Gly) concentrations in low- and high-grade gliomas based on (1)H MR spectroscopic imaging (MRSI) with short and long echo time (TE). Myoinositol (MI) and Gly appear at the same resonance frequency of 3.56 ppm, but due to strong coupling the MI signal dephases more rapidly. Therefore, their contribution to the 3.56 ppm signal should be distinguishable comparing MRSI data acquired at short and long TE.

MATERIALS AND METHODS

(1)H MRSI (TE = 30 and 144 ms) was performed at 3 T in 29 patients with histopathological confirmed World Health Organization (WHO) grade II-IV gliomas and in FIVE healthy subjects. All spectra from the gliomas revealed increase of the 3.56 ppm resonance in the short TE spectra. Signal intensities of Gly and MI were differentiated either by analysing the short to long TE ratio of the resonance or by performing a weighted difference. Gly concentrations were compared between high-grade (WHO III-IV) and low-grade gliomas.

RESULTS

High-grade gliomas showed significantly higher Gly concentrations compared to low-grade gliomas.

CONCLUSION

Appropriate data processing of short and long TE (1)H MRSI provides a tool to distinguish and to quantify Gly and MI concentrations in gliomas. As Gly seems to be a marker of malignancy, more dedicated spectroscopic methods to differentiate these metabolites are justified.

Na(+)/H(+) exchanger inhibition modifies dopamine neurotransmission during normal and metabolic stress conditions

Na(+)/H(+) exchanger (NHE) proteins are involved in intracellular pH and volume regulation and may indirectly influence neurotransmission. The abundant NHE isoform 1 (NHE1) has also been linked to brain cell damage during metabolic stress. It is not known, however, whether NHE1 or other NHE isoforms play a role in striatal dopamine (DA) neurotransmission under normal or metabolic stress conditions. Our study tested the hypothesis that NHE inhibition with cariporide mesilate (HOE-642) modifies striatal DA overflow and DAergic terminal damage in mice caused by the mitochondrial inhibitor malonate. We also explored the expression of NHE1-5 in the striatum and substantia nigra. Reverse microdialysis of HOE-642 elicited a transient elevation followed by a reduction in DA overflow accompanied by a decline in striatal DA content. HOE-642 pre-treatment diminished the malonate-induced DA overflow without reducing the intensity of the metabolic stress or subsequent DAergic axonal damage. Although NHE isoforms 1-5 are expressed in the striatum and midbrain, NHE1 protein was not co-located on nigrostriatal DAergic neurons. The absence of NHE1 co-location on DAergic neurons suggests that the effects of HOE-642 on striatal DA overflow are either mediated via NHE1 located on other cell types or that HOE-642 is acting through multiple NHE isoforms.

Hydrogen peroxide removal and glutathione mixed disulfide formation during metabolic inhibition in mesencephalic cultures

Compromised mitochondrial energy metabolism and oxidative stress have been associated with the pathophysiology of Parkinson's disease. Our previous experiments exemplified the importance of GSH in the protection of neurons exposed to malonate, a reversible inhibitor of mitochondrial succinate dehydrogenase/complex II. This study further defines the role of oxidative stress during energy inhibition and begins to unravel the mechanisms by which GSH and other antioxidants may contribute to cell survival. Treatment of mesencephalic cultures with 10 microM buthionine sulfoximine for 24 h depleted total GSH by 60%, whereas 3 h exposure to 5 mM 3-amino-1,2,4-triazole irreversibly inactivated catalase activity by 90%. Treatment of GSH-depleted cells with malonate (40 mM) for 6, 12 or 24 h both potentiated and accelerated the time course of malonate toxicity, however, inhibition of catalase had no effect. In contrast, concomitant treatment with buthionine sulfoximine plus 3-amino-1,2,4-triazole in the presence of malonate significantly potentiated toxicity over that observed with malonate plus either inhibitor alone. Consistent with these findings, GSH depletion enhanced malonate-induced reactive oxygen species generation prior to the onset of toxicity. These findings demonstrate that early generation of reactive oxygen species during mitochondrial inhibition contributes to cell damage and that GSH serves as a first line of defense in its removal. Pre-treatment of cultures with 400 microM ascorbate protected completely against malonate toxicity (50 mM, 12 h), whereas treatment with 1 mM Trolox provided partial protection. Protein-GSH mixed disulfide formation during oxidative stress has been suggested to either protect vulnerable protein thiols or conversely to contribute to toxicity. Malonate exposure (50 mM) for 12 h resulted in a modest increase in mixed disulfide formation. However, exposure to the protective combination of ascorbate plus malonate increased membrane bound protein-GSH mixed disulfides three-fold. Mixed disulfide levels returned to baseline by 72 h of recovery indicating the reversible nature of this formation. These results demonstrate an early role for oxidative events during mitochondrial impairment and stress the importance of the glutathione system for removal of reactive oxygen species. Catalase may serve as a secondary defense as the glutathione system becomes limiting. These findings also suggest that protein-GSH mixed disulfide formation under these circumstances may play a protective role.

Contribution of glial metabolism to neuronal damage caused by partial inhibition of energy metabolism in retina

Glial cells are relatively resistant to energy impairment, although little is known of the extent to which glial metabolism is affected during partial energy impairment and how this influences neurons. Fluorocitrate has been shown to be a glial specific metabolic inhibitor. Its selective effect on chick retinal Müller cells was verified by measuring incorporation of radiolabel from 3H-acetate and U-14C-glucose into glutamate and glutamine following exposure of isolated embryonic day 15-18 chick retina to 20 microm fluorocitrate. Fluorocitrate significantly reduced the incorporation of radiolabel from acetate and glucose into glutamine, with less effect on incorporation of label from acetate into glutamate and no reduction of label from glucose into glutamate. The relative specific activity (RSA; ratio of glutamine to glutamate) increased between embryonic day 15 and 18 consistent with the increase in glutamine synthetase activity that occurs in Müller cells at this time in chick retinal development. As with previous findings, mild energy stress produced by inhibiting glycolysis with the general inhibitor iodoacetate (IOA) for up to 45 min, caused acute neuronal damage that was predominately NMDA receptor mediated and occurred in the absence of a net efflux of excitatory amino acids. Acute NMDA-mediated toxicity in this preparation is characterized by the selective damage to amacrine and ganglion cells and quantitatively, by GABA release into the medium. When IOA was combined with fluorocitrate, acute toxicity was potentiated and temporally accelerated. Acute damage was first noted at 15 min, occurred throughout all retinal layers and was accompanied by an overflow of excitatory amino acids at 30 and 45 min. Blocking NMDA receptors with MK-801 during IOA plus fluorocitrate exposure attenuated the rise in excitatory amino acids and prevented the swelling in neuronal, but not Müller cells. Following incorporation of radiolabel from acetate and glucose into glutamate and glutamine after different times of exposure to IOA showed that while the effects of incorporation of label from glucose were immediate, glutamine synthesis from acetate was preserved for a longer period of time. These findings suggest that during a partial energy impairment, neuronal metabolism is affected to a greater extent than is glial metabolism. Glial cells may play a protective role in this situation, and can delay the onset of acute neuronal damage.

Absolute concentrations of metabolites in human brain tumors using in vitro proton magnetic resonance spectroscopy

Water-soluble metabolites extracted from 60 surgically excised samples of various brain tumors and four nontumorous lobectomized brains were measured quantitatively using in vitro high-resolution magnetic resonance spectroscopy. A detailed MR spectrum-histology correlation study in a glioblastoma was made, to reveal MR spectral changes in accordance with the density of glioma cells. Furthermore, three cases that had difficult preoperative diagnoses are discussed. MR spectra from gliomas exhibited characteristic patterns according to malignancy, presumably reflecting its metabolic effects. Concentrations of choline-containing compounds, inositol, alanine, glycine and phosphorylethanolamine (PEA) increased according to the degree of malignancy, but it was noteworthy that in glioblastoma the choline-containing compounds, inositol, alanine, glycine and phosphorylethanolamine increased according to the degree of malignancy. In particular, the glycine concentration was very high in glioblastoma. We also detected a large amount of taurine in medulloblastoma. Although the total creatine concentrations decreased according to the malignancy, the concentration of total creatine was relatively preserved in neuroectodermal tumors but was low in nonneuroectodermal tumors. N-acetyl-aspartate was unequivocally demonstrated in normal tissues, but could not be detected in nonneuroectodermal brain tumors such as metastatic brain tumor, meningioma, neurinoma and chordoma. In meningioma, although a high peak of choline-containing compounds has been reported uniquely by in vitro and in vivo 1H-MRS, we demonstrated that its concentration was not increased in meningioma; instead, there was an increased alanine content. 1H-MRS of neurinoma demonstrated high inositol peaks, and a large amount of inositol. The reason for the high inositol content in neurinoma is unknown, but the prominent peak of inositol on MR spectra should be useful for the differential diagnosis of neurinoma from meningioma. PEA concentration was increased four to five times in pituitary adenoma, malignant lymphoma, and medulloblastoma as compared with normal brain. Thus 1H-MRS might provide clinically useful information on tumor malignancy and characteristic tumor metabolism. Although excellent anatomical information of tumors can be readily obtained by magnetic resonance imaging. MRS provides metabolic information. MRS may provide additional information in cases in which the differential diagnosis of tumors by neuroimaging is difficult.

Volume regulation in rat brain glial cells: lack of a substantial contribution of free amino acids

Volume regulation of C6 glioma cells was studied while the bath osmolality was reduced from 300 to 150 mosmol/kg. Exposure to a hyposmotic challenge elicited a typical regulatory volume decrease (RVD). No regulatory volume increase was observed upon restoration of isosmotic conditions. During a second subsequent hyposmotic challenge, the cells did not respond with RVD. High extracellular K+ concentration and the K+ channel blockers Ba2+ and quinine inhibited the RVD. RVD was abolished after Cl- was replaced by gluconate and by the Cl- channel blocker 5-nitro-2(3-phenylpropylamino)benzoic acid. Amino acid (AA) concentration in cell and perfusate was determined. In control, cell content was only 26 mmol/l. Hypotonicity increased the efflux of AA from 0.14 to 0.60 mmol/min. During the second hyposmotic challenge, the release was 0.32 mmol/min. The data show that C6 cells adjust their volume under hyposmotic conditions but lose the ability to restore their volume during a subsequent hyposmotic treatment. K+ and Cl- are the main osmolytes involved in volume adjustment through conductive pathways. AA do not contribute substantially to cell volume regulation.

Physiology of transformed glial cells

Much of our present knowledge of glial cell function stems from studies of glioma cell lines, both rodent (C6, C6 polyploid, and TR33B) and human (1321N1, 138MG, D384, R-111, T67, Tp-276MG, Tp-301MG, Tp-483MG, Tp-387MG, U-118MG, U-251MG, U-373MG, U-787MG, U-1242MG, and UC-11MG). New methods such as patch clamp and Ca2+ imaging have lead to rapid progress the last few years in our knowledge about glial cells, where an unexpected presence and diversity of receptors and ion channels have emerged. Basic mechanisms related to membrane potential and K+ transport and the presence of voltage gated ion channels (Na+, inwardly rectifying K+, Ca(2+)-activated K+, Ca2+, and Cl- channels) have been identified. Receptor function and intracellular signaling for glutamate, acetylcholine, histamine, serotonin, cathecolamines, and a large number of neuropeptides (bradykinin, cholecystokinin, endothelin, opioids, and tachykinins) have been characterized. Such studies are facilitated in cell lines which offer a more homogenous material than primary cultures. Although the expression of ion channels and receptors vary considerably between different cell lines and comparative studies are rare, a few differences (compared to astrocytes in primary culture) have been identified which may turn out to be characteristic for glioma cells. Future identification of specific markers for receptors on glial and glioma cells related to cell type and growth properties may have great potential in clinical diagnosis and therapy.

Proton magnetic resonance spectroscopy of brain tumors: an in vitro study

The ability of proton magnetic resonance spectroscopy (1H MRS) to diagnose brain tumors was investigated using in vitro high-resolution spectra. Fifty-eight surgically excised samples of brain tumors (12 glioblastomas, 4 anaplastic astrocytomas, 6 astrocytomas, 12 meningiomas, 6 neurinomas, 4 chordomas, 3 craniopharyngiomas, 2 pituitary adenomas, 2 malignant lymphomas, 1 ependymoma, 1 medulloblastoma, and metastatic brain tumors including 3 pulmonary adenocarcinomas, a hepatocellular carcinoma, and a renal cell carcinoma) and 4 nontumorous lobectomized brains were examined by in vitro 1H MRS. N-Acetyl-aspartate was demonstrated in normal tissues but could not be detected in nonneuroectodermal tumors. Total creatine was decreased in all brain tumors in comparison with normal brain tissues, but was relatively higher in neuroectodermal tumors than in other brain tumors. Choline-containing compounds were present in all tumors except craniopharyngioma, and their concentrations were particularly high in a metastatic brain tumor from hepatocellular carcinoma. The concentration of glycine was high in neuroectodermal tumors, whereas that of taurine was high in medulloblastoma, pituitary adenoma, and renal cell carcinoma. Alanine was increased in meningioma, glioma, and pituitary adenoma. Neurinoma had the largest inositol content among the tumors examined. Thus each type of brain tumor exhibited a characteristic MR spectrum. These data suggested that in vivo 1H MRS might provide clinically useful information about tumor metabolism and aid in the differential diagnosis of tumors. Although excellent anatomical localization of tumors can be readily obtained by MR imaging, MRS may provide additional information in cases in which the differential diagnosis of tumors by MR imaging is difficult.

Evidence that the loss of the voltage-dependent Mg2+ block at the N-methyl-D-aspartate receptor underlies receptor activation during inhibition of neuronal metabolism

In this study, the importance of the Mg2+ blockade of the N-methyl-D-aspartate (NMDA) receptor during metabolic stress was examined in embryonic day 13 chick retina. Retina exposed to mild conditions of metabolic stress (i.e., blockade of glycolysis with 1 mM iodoacetate for 30 min) underwent acute histological somal and neuritic swelling and an increase in gamma-aminobutyric acid (GABA) release into the medium. These acute signs of metabolic stress were eliminated by NMDA antagonists present during pharmacological blockade of glycolysis, occurred in the absence of a net increase in extracellular glutamate or aspartate, and were not affected by the presence or absence of Ca2+ in the incubation medium. One possible explanation for the activation of NMDA receptors in the absence of an increase in extracellular ligand is that NMDA sensitivity during metabolic stress may be governed at the receptor level. Depolarization of membrane potential during metabolic stress may result in the loss of the Mg2+ blockade from the NMDA receptor channel, resulting in an increased potency for glutamate. To test this, the dose-response characteristics for NMDA, glutamate, and kainate in the presence or absence of extracellular Mg2+ and the effects of Mg2+ on metabolic inhibition were examined. The potency for NMDA- or glutamate-mediated acute toxicity was enhanced two- to fivefold in the absence of Mg2+. Omission of Mg2+ greatly decreased the minimal concentration of agonist needed to produce acute excitotoxicity; 25 versus 5 microM for NMDA and 300 versus 10 microM for glutamate in 1.2 or zero Mg2+, respectively. Elevating external Mg2+ to 20 mM completely protected against NMDA-mediated acute toxic effects. In contrast, varying external Mg2+ had no effect on kainate-induced toxicity. Acute toxicity caused by inhibition of metabolism was not potentiated in the absence of Mg2+ but was attenuated by elevating extracellular Mg2+. The protective effect of Mg2+ during metabolic inhibition was not additive with NMDA antagonists, suggesting that the action of Mg2+ was at the level of the NMDA receptor. These findings are consistent with the hypothesis that the Mg2+ block is lifted during metabolic inhibition and may be the primary event resulting in NMDA receptor activation.

1-Methyl-4-(2'-ethylphenyl)-1,2,3,6-tetrahydropyridine-induced toxicity in PC12 cells is enhanced by preventing glycolysis

The effects of 1-methyl-4-(2'-ethylphenyl)-1,2,3,6-tetrahydropyridine (2'Et-MPTP), 1-methyl-4-(2'-ethylphenyl)pyridinium (2'Et-MPP+), and the classic complex 1 inhibitor, rotenone, on toxicity as well as on rates of glucose use and lactate production were studied using the pheochromocytoma PC12 cell line. PC12 cells are neoplastic in nature and have a high rate of glycolysis accompanied by a large production of lactate and a low use of glucose carbon through the Krebs cycle. 1-Methyl-4-phenylpyridinium (MPP+) and analogues such as 2'Et-MPP+ are actively accumulated by mitochondrial preparations in vitro and block NADH dehydrogenase of complex 1. This blockade results in biochemical sequelae that are ultimately cytotoxic. In this study, untreated PC12 cells used glucose and concomitantly accumulated lactate in a time-dependent manner at all concentrations of glucose studied. Treatment with 50 microM 2'Et-MPP+ or 50 nM rotenone increased both rates significantly, indicating a shift toward increased glycolysis. Cell death caused by the neurotoxins was also time and concentration dependent and markedly enhanced by glucose depletion in the medium. The increase in 2'Et-MPTP-induced toxicity in low glucose-supplemented cells was not due to an increase in pyridinium formation from the tetrahydropyridine, but rather to the lack of glucose for glycolysis. Moreover, inhibition of glycolysis with 2-deoxyglucose or iodoacetic acid also enhanced the lethality of the neurotoxins to the cells. The data in this study provide additional support to the hypothesis that 2'Et-MPP+ or related analogues act to kill cells by inhibiting mitochondrial respiration.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamine synthetase in oligodendrocytes and astrocytes: new biochemical and immunocytochemical evidence

The results of recent immunocytochemical experiments suggest that glutamine synthetase (GS) in the rat CNS may not be confined to astrocytes. In the present study, GS activity was assayed in oligodendrocytes isolated from bovine brain and in oligodendrocytes, astrocytes, and neurons isolated from rat forebrain, and the results were compared with new immunochemical data. Among the cells isolated from rat brain, astrocytes had the highest specific activities of GS, followed by oligodendrocytes. Oligodendrocytes isolated from white matter of bovine brain had GS specific activities almost fivefold higher than those in white matter homogenates. Immunocytochemical staining also showed the presence of GS in both oligodendrocytes and astrocytes in bovine forebrain, in three white-matter regions of rat brain, and in Vibratome sections as well as paraffin sections.

Developmental profile of glutamine synthetase in lines of mice bred for ethanol sensitivity

Glutamine synthetase (GS) activity was used as a marker to examine differences in astrocyte development in mice selectively bred for ethanol sensitivity: long sleep (LS), short sleep (SS), mild ethanol withdrawal (MEW), severe ethanol withdrawal (SEW) and control ethanol withdrawal (CEW). We found that 1) GS activity in MEW and SEW was higher than in LS and SS during the first 2 weeks of postnatal development, in the forebrain but not in the cerebellum; 2) lower GS activity was observed consistently in all areas examined with the SS mice as compared to the LS; 3) glutamine synthetase activity in MEW and SEW differed significantly from their controls (CEW) during the early developmental period regardless of the brain region examined; however, after 30 days of maturation, GS activity in SEW was higher than that in MEW and CEW in the forebrain. Astrocytes are known to contribute in the regulation of the neuronal microenvironment. Therefore, we interpret the differences we found in astrocytic function during early brain development among these lines of mice to account in part for the neuronal predisposition to ethanol sensitivity.

Mesencephalic dopamine neurons become less sensitive to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity during development in vitro

The in vitro development of monoamine oxidase (MAO) activity and [3H]dopamine (DA) uptake capacity of dissociated cell cultures from rat embryo mesencephalon were correlated with the potency of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridine (MPP+) neurotoxicity. Specific activities of both MAO-A and MAO-B increased during in vitro development of the cultures, with MAO-B activity increasing 20-fold between the first and fourth week. Similarly, [3H]DA accumulation increased 2.6-fold between the first and third week in vitro, when it reached a plateau. Unexpectedly, the toxicities of MPTP and MPP+ were substantially decreased in the older cultures. Exposure to MPTP reduced [3H]DA accumulation per culture by 77% in 1-week-old cultures and by 36% in 4-week-old cultures. Similarly, damage caused by MPPT was reduced from 84% of control in the first week to 34% of control in the fourth week. The attenuation of neurotoxicity was not due to an increase in storage of MPP+ in the synaptic vesicles of DA neurons, nor to a change in the distribution of MPP+ between dopaminergic and other cellular components of the cultures. The damage to DA neurons caused by the mitochondrial toxin, rotenone, also showed a similar reduction in the older cultures. These observations coupled with an increase in lactate formation and glucose consumption during the in vitro development of the cultures suggest a shift toward increased glycolysis and decreased dependence on aerobic metabolism. This would render the cells more resistant to the inhibition of mitochondrial function by MPP+.

Differential effects of ammonia and beta-methylene-DL-aspartate on metabolism of glutamate and related amino acids by astrocytes and neurons in primary culture

The effects of ammonium chloride (3 mM) and beta-methylene-DL-aspartate (BMA; 5 mM) (an inhibitor of aspartate aminotransferase, a key enzyme of the malate-aspartate shuttle (MAS] on the metabolism of glutamate and related amino acids were studied in primary cultures of astrocytes and neurons. Both ammonia and BMA inhibited 14CO2 production from [U-14C]- and [1-14C]glutamate by astrocytes and neurons and their effects were partially additive. Acute treatment of astrocytes with ammonia (but not BMA) increased astrocytic glutamine. Acute treatment of astrocytes with ammonia or BMA decreased astrocytic glutamate and aspartate (both are key components of the MAS). Acute treatment of neurons with ammonia decreased neuronal aspartate and glutamine and did not apparently affect the efflux of aspartate from neurons. However, acute BMA treatment of neurons led to decreased neuronal glutamate and glutamine and apparently reduced the efflux of aspartate and glutamine from neurons. The data are consistent with the notion that both ammonia and BMA may inhibit the MAS although BMA may also directly inhibit cellular glutamate uptake. Additionally, these results also suggest that ammonia and BMA exert differential effects on astroglial and neuronal glutamate metabolism.

Glial swelling during extracellular acidosis in vitro

Intracellular and extracellular acidosis may determine the ultimate outcome for brain tissue in cerebral ischemia. An extracellular acidosis that occurs in the penumbra zone was investigated in vitro as to its role in the formation of cytotoxic cell swelling. For that purpose, C6 glioma cells or primary cultured astrocytes were suspended in normal isotonic medium in normoxia during acidification to a final pH of 6.2. The cell volume response was determined by flow cytometry using hydrodynamic focusing, which allows one to recognize changes in cell size of less than 1%. A threshold pH of 6.8 was found that had to be crossed to induce cell swelling by acidosis. Once pH fell below this threshold, the increase in cell size appeared to be an all-or-nothing phenomenon. The cells rapidly assumed a final cell size of 115% of normal in the case of C6 glioma or of 118% in the case of primary cultured astrocytes independent of the actual level of acidosis or the duration of exposure. Acidosis-induced glial swelling could be significantly attenuated by 1) addition of amiloride, 2) administration of acetazolamide, or 3) replacement of bicarbonate buffer against N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES). Replacement of extracellular Na+ by choline chloride led to complete prevention of the acidosis-induced cell swelling. Taken together, the findings strongly indicate a central involvement of Na+/H+ and Cl-/HCO3- exchange mechanisms in the development of cell swelling under these conditions. Activation of the Na+/H+ antiporter can be considered an attempt to maintain a normal intracellular pH at the expense of an abnormal cell volume.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative study of GABA-T from glial cells, neuronal perikarya cells and synaptosomes

1. This paper presents a fast method of brain cell separation and a comparative study of GABA-T from different cellular compartments (glial cells, neuronal perikarya cells and synaptosomes). 2. The method of cellular separation offers the advantages of rapidity, ease and reproducibility. 3. The GABA-T from the three studied compartments had similar kinetic characteristics in respect of their Kms and Vmaxs. 4. The GABA-T needs PLP to reach its maximum activity; this dependence is related to the enzyme localization.

Control of glial cell volume in anoxia. In vitro studies on ischemic cell swelling

Volume regulation of C6 glial cells was studied in anoxia in vitro to improve the understanding of ischemic cell swelling in the brain. Contrary to in vivo conditions, anoxia or anoxia plus iodoacetate for additional inhibition of anaerobic energy metabolism did not induce glial swelling. However, intracellular K+ was markedly decreased while intracellular Na+ increased. Induction of energy failure by anoxia plus iodoacetate was found to prevent the regulatory volume decrease on hyposmotic exposure of the cells, which is regularly observed in normoxic control conditions. Hyposmotic exposure in anoxia plus iodoacetate led only to an initial tendency of cell volume normalization followed by secondary cell swelling. This was associated with a net increase of intracellular Na+ that may explain the failure of volume regulation under these circumstances. Maintenance of a normal glial cell size during complete energy deprivation by anoxia plus iodoacetate in isotonic medium strongly indicates that energy failure per se does not suffice to induce cell swelling. Cell swelling in cerebral ischemia in vivo thus is likely to require additional mechanisms, most likely an increase of membrane permeability to Na+, which may be caused by release and accumulation of excitotoxins such as glutamate or by an extracellular release of K+. Such a mechanism would hardly influence the extracellular homeostasis in vitro due to the large medium-to-cell volume ratio. The findings demonstrate, nonetheless, the significance of a competent energy metabolism to support cell volume regulation. This is concluded from the failure of regulatory volume decrease of hypotonically suspended glial cells in anoxia plus iodoacetate.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine: inhibition of NAD-linked substrate oxidation by its metabolite, 1-methyl-4-phenylpyridinium

The effects of the parkinsonism-inducing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its 4-electron oxidation product 1-methyl-4-phenylpyridinium (MPP+) were studied in isolated mitochondria and in mouse brain striatal slices. ADP-stimulated oxidation of NAD-linked substrates was inhibited in a time-dependent manner by MPP+ (0.1-0.5 mM), but not MPTP, in mitochondria prepared from rat brain, mouse brain, or rat liver. Under identical conditions, succinate oxidation was relatively unaffected. In neostriatal slices prepared from the mouse, a species susceptible to the dopaminergic neurotoxicity of MPTP, incubation with either MPP+ or MPTP caused metabolic changes consistent with inhibition of mitochondrial oxidation, i.e., an increase in the formation of lactate and accumulation of the amino acids glutamate and alanine with concomitant decreases in glutamine and aspartate levels. The changes resulting from incubation with MPTP were prevented by the monoamine oxidase inhibitor pargyline, which blocks formation of MPP+ from MPTP. The results suggest that compromise of mitochondrial function and its metabolic sequelae within dopaminergic neurons could be an important factor in the neurotoxicity observed after MPTP administration.

The role of glial cells in regulation of neurotransmitter amino acids in the external environment. II. Mechanism of aspartate transport

Active, high-affinity (Km = 4.4 microM) D-aspartate transport in C6 astrocytoma cells has been investigated. Uptake of radioactive D-aspartate was competitively inhibited by L-aspartate (Ki = 8.5 microM) and L-glutamate (Ki = 0.95 mM) and was essentially independent of pH between 6.2 and 7.8. The rate of uptake of labeled D-aspartate and its maximum accumulation ratio, [Asp]i/[Asp]e increased as the second power of the transmembrane electrical potential (measured by the potassium concentration gradient, [K+]i/[K+]e) which indicates that aspartate is transported with a net charge of +2. Aspartate transport rate and gradient also increased as the second power of the sodium concentration gradient, [Na+]e/[Na+]i, indicating that two Na+ are transported inward with each aspartate. The maximum gradient measured from total intra- and extracellular concentrations of aspartate showed the same dependence on electrical potential and sodium concentration gradient as that determined from the distribution of [3H]D-aspartate. This indicates that energy for aspartate uptake is provided by a combination of transmembrane electrical potential and sodium concentration gradient. At physiological [Na+]e (140 mM) and [K+]e (3.5-5 mM) the energy available for aspartate uptake substantially exceeded the maximum aspartate gradient. It is suggested that aspartate uptake by C6 cells is kinetically prevented from attaining high concentration differences and that the excess of driving forces over accumulation ratio ensures that glial high-affinity transport systems for amino acid neurotransmitters function in vivo predominantly in the direction of net uptake.

Abnormal platelet glutamate dehydrogenase activity and activation in dominant and nondominant olivopontocerebellar atrophy

Glutamate dehydrogenase (GDH) activity and its allosteric modulation by purine nucleotides were studied in platelet preparations from 4 patients with a nondominant form of adult-onset olivopontocerebellar atrophy (OPCA) and in affected and nonaffected members of two families with a dominant form of OPCA. A partial deficiency of GDH activity (40 to 50% of control values) was present in 3 patients with nondominant OPCA and in 2 patients, father and son, with a dominant form of OPCA. Platelet GDH from these patients and controls was regularly inactivated by 2 mM guanosine-5'-triphosphate (GTP) and simulated one- to twofold by 2 mM adenosine-5'-diphosphate (ADP). In the presence of 0.2% Triton X-100, the activating effect of ADP was enhanced four- to sixfold. The partial deficiency in maximum catalytic activity observed in these patients persisted under all conditions used for enzyme assay. In affected members, but not in one unaffected member of another family with a dominant type of OPCA, GDH activity was in the control range but was not activated by ADP in either the presence or absence of Triton. These results suggest that there may be at least two possible alterations of GDH in patients with OPCA: one which decreases the maximum catalytic activity and one which impairs the regulatory properties of the enzyme. Furthermore, this study suggests that platelet GDH determination in patients with OPCA may provide a simple and useful tool to classify these disorders and to understand the basic pathophysiological mechanisms involved.

Free amino acids of rat astrocytes in primary culture: changes during cell maturation

The concentrations of free amino acids were analysed in cultured primary astrocytes during cell maturation and in the starting material, i.e. the cerebral hemispheres of newborn rats. Taurine was the most abundant amino acid in all samples, the content of glutamine being comparable only in immature astrocytes (7 days in culture). The intracellular levels of most amino acids significantly decreased during the first 2 weeks in culture, remaining fairly stable during the third week.

The hormonal regulation of gene expression of glial markers: glutamine synthetase and glycerol phosphate dehydrogenase in primary cultures of rat brain and in C6 cell line

Increases in the mRNA levels of two neuroglial markers, glutamine synthetase (EC 6.3.1.2; GS) and glycerolphosphate dehydrogenase (EC 1.1.1.8; GPDH), were observed in hydrocortisone-treated cultures of astrocytes and oligodendrocytes, respectively, and in C6 cells by Northern blot analysis and in situ hybridization. In vitro transcription assays demonstrated increased GS transcription in isolated nuclei from hydrocortisone (HC)-treated primary cultures of astrocytes and C6 cells, relative to untreated cells. This increased transcription is reflected in increased GS mRNA levels in the cytoplasm and increased levels of GS protein synthesis. Sodium butyrate (NaB) blocked the glucocorticoid-mediated increase in GS transcription in the primary cultures of astrocytes but not in C6 cells. From our earlier observations (Kumar et al: J Neurochem 43:1455-1463, 1984) we found NaB in combination with HC to increase the levels of GS mRNA and GS protein synthesis (Weingarten et al: FEBS Lett 126:289-291, 1981). We now report that NaB, alone or in combination with HC, does not increase the rate of transcription, suggesting that NaB plays a role in post-transcriptional regulation of GS in C6. In addition, we report the presence of two distinct sizes of GS mRNA, 2.9 and 1.8 kb, in the primary cultures of astrocytes and C6 cells.

Concentration of free amino acids in primary cultures of neurones and astrocytes

The cellular distribution of free amino acids was estimated in primary cultures (14 days in vitro) composed principally of cerebellar interneurones or cerebellar and forebrain astrocytes. In cultured neural cells, the overall concentration of amino acids resembled that found in brain at the corresponding age in vivo. In the two neural cell types, there were marked differences in the distribution of amino acids, in particular, those associated with the metabolic compartmentation of glutamate. In neuronal cell cultures, the concentrations of glutamate, aspartate, and gamma-aminobutyric acid were, respectively, about three, four, and seven times greater than in astrocytes. By contrast, the amount of glutamine was approximately 65% greater in astroglial cell cultures than in interneurone cultures. An unexpected finding was a very high concentration of glycine in astrocytes derived from 8-day-old cerebellum, but the concentrations of both serine and glycine were greater in nerve cell cultures than in forebrain astrocytes. The essential amino acids threonine, valine, isoleucine, leucine, tyrosine, phenylalanine, histidine, lysine, and arginine were all present in the growth medium, and small cellular changes in the contents of some of these amino acids may relate to differences in their influx and efflux during culturing and washing procedures. The present results, together with our previous findings, provide further support for the model assigning the "small" compartment of glutamate to glial cells and the "large" compartment to neurones, and also underline the metabolic interaction between these two cell types in the brain.

Competition among oxidizable substrates in brains of young and adult rats. Whole homogenates

The rates of conversion into 14CO2 of D-(-)-3-hydroxy[3-14C]butyrate, [3-14C]acetoacetate, [6-14C]glucose and [U-14C]glutamine were measured in the presence and absence of unlabelled alternative oxidizable substrates in whole homogenates from the brains of young and adult rats. The addition of unlabelled glutamine resulted in decreased 14CO2 production from [6-14C]glucose in brain homogenates from both young and adult rats. In contrast, glucose had no effect on [U-14C]glutamine oxidation. In suckling animals, both 3-hydroxybutyrate and acetoacetate decreased the rate of oxidation of [6-14C]glucose, but in adults only 3-hydroxybutyrate had an effect, and to a lesser degree. The addition of unlabelled glucose markedly enhanced the rates of oxidation of both ketone bodies in adult brain tissue and had little or no effect in the young. The rate of production of 14CO2 from [U-14C]glutamine was increased by the addition of unlabelled ketone bodies in brain homogenates from young, but not from adult rats. In the converse situation, unlabelled glutamine added to 14C-labelled ketone bodies diminished 14CO2 production in young rats, but had no effect in adult animals. These results revealed a complex age-dependent pattern of interaction in which certain substrates apparently competed with each other, whereas an enhanced rate of 14CO2 production was found with others.

Metabolic fate of 14C-labeled glutamate in astrocytes in primary cultures

The metabolic fate of L-[U-14C]- and L-[1-14C]glutamate was studied in primary cultures of mouse astrocytes. Conversion of the uniformly labeled compound to glutamine and aspartate was followed by determination of specific activities after dansylation with [3H]dansyl chloride and subsequent thin layer chromatography of the dansylated amino acids. Metabolic fluxes were calculated from the alterations of specific activities and the pool sizes, which were likewise measured by a dansylation method. Formation of 14CO2 from [1-14C]glutamate was determined by the trapping of CO2 in hyamine hydroxide in a gas-tight chamber, which is, in the known absence of glutamate decarboxylase activity in the cultured astrocytes, an unequivocal expression of the metabolic flux via alpha-ketoglutarate to CO2 and succinyl-CoA. The metabolic fluxes determined by these procedures amounted to 2.4 nmol/min/mg protein for glutamine synthesis, 1.1 nmol/min/mg protein for aspartate production, and 4.1 nmol/min/mg protein for formation and subsequent decarboxylation of alpha-ketoglutarate. The latter process was unaffected by virtually complete inhibition of glutamate-oxaloacetic transaminase with aminooxyacetic acid, indicating that the formation of alpha-ketoglutarate occurs as an oxidative deamination rather than as a transamination. This suggests that the formation of alpha-ketoglutarate from glutamate represents a net degradation, not an isotopic exchange.

Induction of glutamine synthetase by dibutyryl cyclic AMP in C-6 glioma cells

Glutamine synthetase was found to be increased in C-6 glioma cells as a result of increasing culture passage and N-6,2'-O-dibutyryl cyclic AMP (dbcAMP) treatment. At low passage dbcAMP produced a 2.5-fold increase in glutamine synthetase activity per unit of cellular protein. At high passage control glutamine synthetase was approximately double that seen at low passage, but dbcAMP produced an additional 65% increase. Lactate dehydrogenase activity was also increased by dbcAMP treatment at both low and high passage, but culture passage produced no change in the lactate dehydrogenase. With increasing culture passage, the ratio of cellular protein to DNA doubled. Therefore, expression of data per unit of protein tended to minimize the apparent changes in activity. The maximum increase in glutamine synthetase activity produced by both dbcAMP and increasing culture passage and expressed on a DNA basis was 5.6-fold. The increase in glutamine synthetase activity was generally linear during the first 20 h of drug treatment, after which enzyme activity remained nearly constant up to 72 h. Ninety percent or more of the dbcAMP remained in the medium at the end of 48-h exposure of cells to dbcAMP. 8-br-Cyclic AMP also increased glutamine synthetase activity of C-6-cels, but n-butyrate did not. Isoproterenol, which increases cyclic AMP in C-6-cells, increased glutamine synthetase activity. The effect of isoproterenol on glutamine synthetase was inhibited by the beta-adrenergic blocking agent sotalol. Cycloheximide (10 micrograms/ml) inhibited the dbcAMP effect on glutamine synthetase activity and also decreased the control enzyme activity by 60%.

Cellular localization and regulation of glutamine synthetase in primary cultures of brain cells from newborn mice

The cellular distribution of glutamine synthetase was determined by indirect immunofluorescence in cultures of dissociated brain cells from newborn mice. The enzyme could be detected in about 40% of all cells, among which cells with astrocytic morphology were clearly identified. Treatment with the glucocorticoid dexamethasone led to a strong increase in the number of positivity stained cells. Enzyme induction by dexamethasone was maximal after 36 h and at a concentration of 0.1 micrometer. Under these conditions glutamine synthetase specific activity was elevated about six fold. Steroid hormones other than corticosteroids had no effects. The basal activity in these cultures was near that found in brains of newborn mice, but far below the activity in adult brains, showing that in culture the normal development of these cells is disturbed. A comparison of glial and neuronal cell lines showed that glutamine synthetase is present in both types of cell lines at a very low specific activity. Inducibility of this enzyme by dexamethasone was found in glial but not in neuronal cell lines.

Glucocorticoid receptor properties and glucocorticoid regulation of glutamine synthetase activity in sensitive C6 and resistant C6H glial cells

The relationship between induction of glutamine synthetase activity by dexamethasone and binding of the steroid to cytosolic glucocorticoid receptors was examined in sensitive C6 and resistant C6H glial cell cultures. Glutamine synthetase activity increased 3-4-fold when C6 cultures were exposed to 7.6 x 10(-6) M dexamethasone. This inductive response was reversible, dose-dependent (ED50 approximately 2 x 10(-8) M), required de novo protein and RNA synthesis, and was elicited only by glucocorticoid steroids. Progesterone, but not epicortisol, antagonized the dexamethasone-induced enzyme increase. In contrast, only a slight inductive effect was observed in dexamethasone-treated C6H cells. Competitive binding assays demonstrated that specific binding of [3H]-dexamethasone to cytosolic receptors was also dose-dependent. The ED50 was approximately 10(-8) M for both C6 and C6H cells. Scatchard analysis revealed that each C6 cell contained approximately 10,800 receptor sites and that the equilibrium dissociation constant (Kd) was 4.5 x 10(-9) M. Each C6H cell possessed approximately 12,200 sites, and the Kd was 6.7 x 10(-9) M. Unlabeled dexamethasone and cortisol (but not epicortisol) competed effectively with [3H]-dexamethasone for binding to cytosolic receptor sites and nuclear sites of both cell types. These results suggest that induction of glutamine synthetase activity in dexamethasone-treated C6 cells is a glucocorticoid-directed response. Since C6H cells are refractory in this regard but contain functional cytosolic receptors which interact with cell nuclei, the basis for their resistance appears to involve some step beyond these cellular processes.

"Transdifferentiation" of C6 glial cells in culture

The activities of cyclic nucleotide phosphohydrolase, an enzyme marker for oligodendrocytes, and glutamine synthetase, an enzyme marker for astrocytes, were studied at early (21 to 26) and late (82 to 88) cell passages. The activity of cyclic nucleotide phosphohydrolase was markedly high and that of glutamine synthetase was low in the early passages, but this relation was reversed in the late passages. These findings suggest a "transdifferentiation" of C6 glial cells with passage in culture.

Oxidative metabolism of 4-aminobutyrate by rat brain mitochondria: inhibition by branched-chain fatty acid

The oxidation of 4-aminobutyric acid (GABA) by nonsynaptosomal mitochondria isolated from rat forebrain and the inhibition of this metabolism by the branched-chain fatty acids 2-methyl-2-ethyl caproate (MEC) and 2.2-dimethyl valerate (DMV) were studied. The rate of GABA oxidation, as measured by O2 uptake, was determined in medium containing either 5 or 100 mM-[K+]. The apparent Km for GABA was 1.16 +/- 0.19 mM and the Vmax in state 3 was 23.8 +/- 5.5 ng-atoms O2 x min-1 x mg protein-1 in 5 m M-[K+]. In a medium with 100 mM-[K+] the apparent Km was 1.11 +/- 0.17 mM and Vmax was 47.4 +/- 5.7 ng-atoms O2 x min-1 x mg protein-1. The Ki for MEC was determined to be 0.58 +/- 0.24 or 0.32 +/- 0.08 mM, in 5 or 100 mM-[K+], respectively. For DMV, the Ki was 0.28 +/- 0.05 or 0.34 +/- 0.06 mM, in 5 or 100 mM-[K+] medium, respectively. The O2 uptake of the mitochondria in the presence of GABA was coupled to the formation of glutamate and aspartate; the ratio of oxygen uptake to the rate of amino acid formation was close to the theoretical value of 3. Neither the [K+] nor any of the above inhibitors had any effect on this ratio. The metabolism of exogenous succinic semialdehyde (SSA) by these same mitochondria was also examined. The Vmax for utilization of oxygen in the presence of SSA was much greater than that found with exogenously added GABA, indicating that the capacity for GABA oxidation by these mitochondria is not limited by SSA dehydrogenase. In addition, the branched-chain fatty acids did not inhibit the metabolism of exogenously added SSA. Thus, the inhibitors examined apparently act by competitively inhibiting the GABA transaminase system of the mitochondria.

Effect of methionine, glycine and serine on serine hydroxymethyltransferase activity in rat glioma and human neuroblastoma cells

Human neuroblastoma SK-N-SH-SY5Y (5Y) and rat glioma (C6) cells were cultured with supplemental methionine, glycine, or serine for three to six days. Serine hydroxmethyltransferase (SHMT: L-serine: tetrahydrofolate 5, 10-hydroxymethyltransferase, EC 2.12.1) was assayed radiometrically in whole cell homogenates, crude supernatant fractions and crude particulate fractions. No significant changes in specific activity or cellular morphology were noted at methionine, glycine, or serine concentrations up to 16 mM. Serine concentrations of 20 and 40 mM led to significantly lower gliomal enzyme specific activities. This activity was unevenly distributed between soluble and particulate fractions, with 190 and 398 nmoles of HCHO formed per mg of protein per hour, respectively. Growth stage and time of incubation were major determinants of enzyme specific activity. C6 cells' specific activity rose slowly with increasing time in culture until cellular confluence. At this time there was a pronounced elevation in specific activity, occurring more rapidly in cells grown in 1.2 mM methionine. Intracellular amino acid analysis of C6 cells demonstrated a significant rise in methionine after four days in media containing 0.2 mM methionine. No appreciable diminution in the intracellular levels of glycine or serine occurred following incubation in excess methionine. It is concluded that SHMT-specific activity in C6 and 5Y cells is not regulated by glycine, serine, or methionine levels and that high concentrations of these amino acids (> 30 mM) are not detrimental to these cells derived from the CNS.

Amino acids in rat neostriatum: alteration by kainic acid lesion

Unilateral stereotaxic injection of 10 nmol of the glutamomimetic substance, kainic acid, into the rat striatum caused permanent, significant decreases in the levels of glutamate (40--50%), aspartate (35--40%), taurine (20--30%) and GABA (60--70%). There were initial, transient decreases in serine, glycine and alanine which returned to normal values within 16--32 days after injection. Glutamine levels were not altered in lesioned striatum. This coincided with a 55% increase in glutamine synthetase activity in the lesioned striatum compared either to the non-injected striatum or controls injected with saline. The high affinity uptake of choline by synaptosomal preparations of lesioned striatum was decreased by 70% compared to controls whereas that of glutamate/aspartate was either unchanged or somewhat on a per mg protein basis. This latter point may be illusory in that, because of widespread neuronal destruction, the total 'synaptosomal' protein obtained from the lesioned striata was only about 50% that from control tissue. The biochemical data are consistent with the histological and behavioral effects of kainic acid administration. The unchaning glutamine levels and increase in glutamine synthetase activity are consistent with the widespread gliosis and the lack of change in glutamate/aspartate high affinity uptake is consistent with a sparing of afferent terminals. The large decrease in glutamate and aspartate is consistent with hypotheses concerning the intraneuronal localization of a major pool of these amino acids, especially in GABAergic neurons. The decrease in taurine suggests that a portion of this amino acid in striatum is probably associated with neurons destroyed by kainic acid. The bulk of the taurine, however, is therefore associated either with glial cells or the afferents to the striatum.