Phosphate Activated Glutaminase Activity and Glutamine Uptake in Primary Cultures of Astrocytes

Schousboe, Arne; Hertz, Leif; Svenneby, Gerd; Kvamme, Elling

doi:10.1111/j.1471-4159.1979.tb04579.x

Cited by 143 publications

(74 citation statements)

References 42 publications

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“…23 Because our study was conducted with elevated and saturating blood glutamine concentrations, effectively reducing the effects of other competing amino acids, our estimate of V Gln(in) for physiologic blood glutamine level likely reflects an upper limit. Compared with the in vivo estimates, reported maximum rates of glutamine transport in cultured neurons and astrocytes are much higher (e.g., V max of 0.7 to 5.0 mmol/g per minute assuming 100 mg protein/g), [27][28][29] suggesting that the lower rates measured in vivo reflect mainly transport through the blood-brain barrier. Such comparisons, however, must be made with ample caution due to the incomplete understanding of transporter types involved, their physical properties (e.g., whether electrogenic or electroneutral, ionic dependencies, and potential for exchange), and the consequence of averaging over multiple transporter types and compartments reflected in the in vivo measurement.…”

Section: Comparison Of Glutamine Influx With Previously Reported Valuesmentioning

confidence: 65%

Characterization of Cerebral Glutamine Uptake from Blood in the Mouse Brain: Implications for Metabolic Modeling of¹³C NMR Data

Bagga

Behar

Mason

et al. 2014

J Cereb Blood Flow Metab

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13 C Nuclear Magnetic Resonance (NMR) studies of rodent and human brain using [1-13 C]/[1,6-13 C 2 ]glucose as labeled substrate have consistently found a lower enrichment (B25% to 30%) of glutamine-C4 compared with glutamate-C4 at isotopic steady state. The source of this isotope dilution has not been established experimentally but may potentially arise either from blood/brain exchange of glutamine or from metabolism of unlabeled substrates in astrocytes, where glutamine synthesis occurs. In this study, the contribution of the former was evaluated ex vivo using 1 H-[ 13 C]-NMR spectroscopy together with intravenous infusion of [U-13 C 5 ]glutamine for 3, 15, 30, and 60 minutes in mice. 13 C labeling of brain glutamine was found to be saturated at plasma glutamine levels 41.0 mmol/L. Fitting a blood-astrocyte-neuron metabolic model to the 13 C enrichment time courses of glutamate and glutamine yielded the value of glutamine influx, V Gln(in) , 0.036±0.002 mmol/g per minute for plasma glutamine of 1.8 mmol/L. For physiologic plasma glutamine level (B0.6 mmol/L), V Gln(in) would be B0.010 mmol/g per minute, which corresponds to B6% of the glutamine synthesis rate and rises to B11% for saturating blood glutamine concentrations. Thus, glutamine influx from blood contributes at most B20% to the dilution of astroglial glutamine-C4 consistently seen in metabolic studies using [1-13 C]glucose. Keywords: glutamate; neurotransmitter; nuclear magnetic resonance spectroscopy INTRODUCTION Glucose, the primary source of energy in the mature brain, is mainly oxidized in neurons to support neuronal firing, and the release and recycling of neurotransmitters, glutamate, and GABA, through the neuron-astrocyte glutamate-glutamine and GABAglutamine cycles. The metabolic pathways comprising these cycles have been revealed in vivo by 13 C Nuclear Magnetic Resonance (NMR) spectroscopy combined with infusions of 13 C-labeled substrates, 1,2 permitting detailed investigation of the metabolism of glucose and alternative substrates in the brain.A consistent finding in NMR studies of rodent and human brain metabolism using [1-13 C] or [1,6-13 C 2 ]glucose as precursor is the lower fractional 13 C enrichment of brain glutamate and glutamine at isotopic steady state compared with blood glucose, 3-5 revealing the presence of constant 'dilutional' flows of unlabeled substrates into the respective tricarboxylic acid (TCA) cycles of neurons and astroglia. For glutamate, this dilution amounts to 25% to 30% in rodents 5,6 (B14% in human brain 7 ) of the theoretical maximum expected from the 13 C-labeled glucose, and is thought to arise within neurons mainly from metabolism of blood-borne substrates such as lactate or ketone bodies and within the brain from glutamine produced in astrocytes through the glutamate-glutamine cycle. [8][9][10] In addition to the glutamate-C4 dilution (relative to glucose-C1), 13 C enrichment of glutamine-C4 is further B20% to 30% less than that of glutamate-C4 at isotopic steady state. 4,5,[11][12][13] As the majority of...

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Section: Comparison Of Glutamine Influx With Previously Reported Valuesmentioning

confidence: 65%

Characterization of Cerebral Glutamine Uptake from Blood in the Mouse Brain: Implications for Metabolic Modeling of¹³C NMR Data

Bagga

Behar

Mason

et al. 2014

J Cereb Blood Flow Metab

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“…33 Likewise, DON significantly inhibited cell swelling caused by ammonia in cultured astrocytes. 33 It should be emphasized that part of the glutamine synthesized in astrocytes is transported into mitochondria where it is then hydrolyzed to glutamate and ammonia by PAG, 81,82 potentially yielding high levels of ammonia in mitochondria that could trigger OS and the mPT. Such OS and the mPT generated in mitochondria would likely contribute to the astrocyte swelling/brain edema observed in hyperammonemic states.…”

Section: Discussionmentioning

confidence: 99%

Brain Edema in Acute Liver Failure

Rao

Reddy

Tong

et al. 2010

The American Journal of Pathology

115

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Brain edema and the associated increase in intracranial pressure are potentially lethal complications of acute liver failure (ALF). Astrocyte swelling (cytotoxic edema) represents a significant component of the brain edema in ALF , and elevated blood and brain ammonia levels have been strongly implicated in its formation. We earlier showed in cultured astrocytes that oxidative stress (OS) and the mitochondrial permeability transition (mPT) play major roles in the mechanism of ammonia-induced astrocyte swelling. Glutamine , a byproduct of ammonia metabolism, has also been shown to induce OS , the mPT , and astrocyte swelling. Such effects of glutamine were suggested to be mediated by its hydrolysis in mitochondria , potentially yielding high levels of ammonia in this organelle and leading to OS and the mPT. L-histidine, an inhibitor of mitochondrial glutamine transport, was recently shown to mitigate OS , mPT , and cell swelling in cultured astrocytes treated with ammonia. The present study examined whether L-histidine similarly abolishes OS , the mPT , and brain edema in a rat model of ALF. Treatment of rats with thioacetamide caused a significant degree of brain edema , which was associated with induction of OS and the mPT. These changes were completely abolished by L-histidine , supporting a key role of mitochondrial glutamine transport and hydrolysis in the mechanism of the brain edema associated with ALF. Acute liver failure (ALF) is a life-threatening condition accounting for approximately 80% mortality in these patients.1,2 Such high mortality is in part attributable to the development of severe brain edema, leading to increased intracranial pressure and brain herniation. Currently, there is no effective treatment for the brain edema in ALF other than an emergency liver transplantation. 2,3Of several factors implicated in the development of ALF, ammonia is generally considered an important one as its levels in blood and brain correlate well with the degree of encephalopathy and brain edema. 4 -7

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“…56 Preliminary studies have also shown a similar attenuation of the MPT, as well as reduction in ROS formation by histidine in cultured astrocytes treated with ammonia. 62 While PAG is clearly present in cultured astrocytes, [63][64][65] its localization in astrocytes in vivo is a matter of controversy. Some immunohistochemical studies have not shown PAG in glia 60,66 ; yet, other immunohistochemical, 67 enzyme histochemical studies, 68 as well as studies using bulk-isolated astrocytes, 69 have all shown a glial localization of PAG.…”

Section: The Trojan Horsementioning

confidence: 99%

Glutamine

2006

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Mechanisms involved in hepatic encephalopathy still remain to be defined. Nonetheless, it is well recognized that ammonia is a major factor in its pathogenesis, and that the astrocyte represents a major target of its CNS toxicity. In vivo and in vitro studies have shown that ammonia evokes oxidative/nitrosative stress, mitochondrial abnormalities (the mitochondrial permeability transition, MPT) and astrocyte swelling, a major component of the brain edema associated with fulminant hepatic failure. How ammonia brings about these changes in astrocytes is not well understood. It has long been accepted that the conversion of glutamate to glutamine, catalyzed by glutamine synthetase, a cytoplasmic enzyme largely localized to astrocytes in brain, represented the principal means of cerebral ammonia detoxification. Yet, the "benign" aspect of glutamine synthesis has been questioned. This article highlights evidence that, at elevated levels, glutamine is indeed a noxious agent. We also propose a mechanism by which glutamine executes its toxic effects in astrocytes, the "Trojan horse" hypothesis. Much of the newly synthesized glutamine is subsequently metabolized in mitochondria by phosphate-activated glutaminase, yielding glutamate and ammonia. In this manner, glutamine (the Trojan horse) is transported in excess from the cytoplasm to mitochondria serving as a carrier of ammonia. We propose that it is the glutamine-derived ammonia within mitochondria that interferes with mitochondrial function giving rise to excessive production of free radicals and induction of the MPT, two phenomena known to bring about astrocyte dysfunction, including cell swelling. Future therapeutic approaches might include controlling excessive transport of newly synthesized glutamine to mitochondria and its subsequent hydrolysis. (HEPATOLOGY 2006;44:788-794.)

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Phosphate Activated Glutaminase Activity and Glutamine Uptake in Primary Cultures of Astrocytes

Cited by 143 publications

References 42 publications

Characterization of Cerebral Glutamine Uptake from Blood in the Mouse Brain: Implications for Metabolic Modeling of¹³C NMR Data

Characterization of Cerebral Glutamine Uptake from Blood in the Mouse Brain: Implications for Metabolic Modeling of¹³C NMR Data

Brain Edema in Acute Liver Failure

Glutamine

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Phosphate Activated Glutaminase Activity and Glutamine Uptake in Primary Cultures of Astrocytes

Cited by 143 publications

References 42 publications

Characterization of Cerebral Glutamine Uptake from Blood in the Mouse Brain: Implications for Metabolic Modeling of13C NMR Data

Characterization of Cerebral Glutamine Uptake from Blood in the Mouse Brain: Implications for Metabolic Modeling of13C NMR Data

Brain Edema in Acute Liver Failure

Glutamine

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Characterization of Cerebral Glutamine Uptake from Blood in the Mouse Brain: Implications for Metabolic Modeling of¹³C NMR Data

Characterization of Cerebral Glutamine Uptake from Blood in the Mouse Brain: Implications for Metabolic Modeling of¹³C NMR Data