Incorporation of isotopic carbon into cerebral glycogen from non-glucose substrates

Cerebral Gluconeogenesis and Diseases

The gluconeogenesis pathway, which has been known to normally present in the liver, kidney, intestine, or muscle, has four irreversible steps catalyzed by the enzymes: pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose 1,6-bisphosphatase, and glucose 6-phosphatase. Studies have also demonstrated evidence that gluconeogenesis exists in brain astrocytes but no convincing data have yet been found in neurons. Astrocytes exhibit significant 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3 activity, a key mechanism for regulating glycolysis and gluconeogenesis. Astrocytes are unique in that they use glycolysis to produce lactate, which is then shuttled into neurons and used as gluconeogenic precursors for reduction. This gluconeogenesis pathway found in astrocytes is becoming more recognized as an important alternative glucose source for neurons, specifically in ischemic stroke and brain tumor. Further studies are needed to discover how the gluconeogenesis pathway is controlled in the brain, which may lead to the development of therapeutic targets to control energy levels and cellular survival in ischemic stroke patients, or inhibit gluconeogenesis in brain tumors to promote malignant cell death and tumor regression. While there are extensive studies on the mechanisms of cerebral glycolysis in ischemic stroke and brain tumors, studies on cerebral gluconeogenesis are limited. Here, we review studies done to date regarding gluconeogenesis to evaluate whether this metabolic pathway is beneficial or detrimental to the brain under these pathological conditions.

Why does the brain (not) have glycogen?

In the present paper we formulate the hypothesis that brain glycogen is a critical determinant in the modulation of carbohydrate supply at the cellular level. Specifically, we propose that mobilization of astrocytic glycogen after an increase in AMP levels during enhanced neuronal activity controls the concentration of glucose phosphates in astrocytes. This would result in modulation of glucose phosphorylation by hexokinase and upstream cell glucose uptake. This mechanism would favor glucose channeling to activated neurons, supplementing the already rich neuron-astrocyte metabolic and functional partnership with important implications for the energy compounds used to sustain neuronal activity. The hypothesis is based on recent modeling evidence suggesting that rapid glycogen breakdown can profoundly alter the short-term kinetics of glucose delivery to neurons and astrocytes. It is also based on review of the literature relevant to glycogen metabolism during physiological brain activity, with an emphasis on the metabolic pathways identifying both the origin and the fate of this glucose reserve.

Characterization of the mouse liver fructose-1,6-bisphosphatase gene

A cDNA encoding fructose-1,6-bisphosphatase (FBPase) was isolated from mouse liver RNA. The cDNA encodes a polypeptide of 338 amino acids (36.9 kDa). The liver and muscle FBPase isoenzymes of the mouse show positional identities of 69% at the cDNA level and 72% at the protein primary structure level. Starting from genomic YAC libraries and based upon the cDNA sequence all functional parts of the mouse liver FBPase gene (including exon-intron boundaries) were PCR-amplified and sequenced. The 5'-flanking regions of the liver and muscle FBPase genes were compared and showed no sequence similarity. Both genes are co-localized at chromosome 13B3-C1. The transcriptional start site was assigned to a guanine 118 bases before the start codon in the liver FBPase gene. An analysis of the steady state mRNA levels of liver and muscle FBPase in various mouse tissues was performed by Northern blotting and RT/PCR.

Various fructose-1,6-bisphosphatase mRNAs in mouse brain, liver, kidney and heart

The mouse fructose-1,6-bisphosphatase (FBPase) cDNA was previously cloned from testicular teratocarcinoma cultured cells (F9 cells). Using this published nucleotide sequence four primer sets were defined and used to amplify FBPase transcript from cerebral cortex, heart, kidney, liver and testis of male C57B1/6 mice. Only one primer set was efficient in all total RNA prepared from the various tissues. The restriction maps of these RNA amplification products suggested the existence of three different FBPase transcripts; this was confirmed by the nucleotide sequences of the FBPase transcripts and by the deduced amino acid sequences. These data are consistent with the existence of three different FBPase genes. This may be relevant in neurological disease in which abnormalities of brain glucose metabolism are involved.

Dephosphorylation of 2-deoxyglucose 6-phosphate and 2-deoxyglucose export from cultured astrocytes

Neurotransmitter-stimulated mobilization of astrocyte glycogen has been proposed as a basis for local energy homeostasis in brain. However, uncertainty remains over the fate of astrocyte glycogen. Upon transfer of cultured astrocytes pre-loaded with [2-3H]2-deoxyglucose 6-phosphate at non-tracer concentrations to a glucose-free, 2-deoxyglucose-free medium, rapid dephosphorylation of a proportion of the intracellular 2-deoxyglucose 6-phosphate pool and export of 2-deoxyglucose to the extracellular fluid occurs. Astrocytes show very low, basal rates of gluconeogenesis from pyruvate (approx. 1 nmol mg protein-1 h-1). Astrocytes in vivo may be capable of physiologically significant glucose export from glucose-6-phosphate. The low gluconeogenic activity in astrocytes suggests that the most likely source of glucose-6-phosphate may be glycogen. These findings support the hypothesis that export, as glucose, to adjacent neurons may be one of the possible fate(s) of astrocytic glycogen. Such export of glycogen as glucose occurring in response to increases in neuronal activity could contribute to energy homeostasis on a paracrine scale within brain.

Regulation of fructose-1,6-bisphosphatase activity in primary cultured astrocytes

In the gluconeogenic pathway, fructose-1,6-bisphosphatase (EC 3. 1. 3. 11) is the last key-enzyme before the synthesis of glucose-6-phosphate. The extreme diversity of cells present in the whole brain does not facilitate in vivo study of this enzyme and makes it difficult to understand the regulatory mechanisms of the related carbohydrate metabolism. It is for instance difficult to grasp the actual effect of ions like potassium, magnesium and manganese on the metabolic process just as it is difficult to grasp the effect of different pH values and the influence of glycogenic compounds such as methionine sulfoximine. The present investigation attempts to study the expression and regulation of fructose-1,6-bisphosphatase in cultured astrocytes. Cerebral cortex of new-born rats was dissociated into single cells that were then plated. The cultured cells were flat and roughly polygonal and were positively immunostained by anti-glial fibrillary acidic protein antibodies. Cultured astrocytes are able to display the activity of fructose-1,6-bisphosphatase. This activity was much higher than that in brain tissue in vivo. Fructose-1,6-bisphosphatase in cultured astrocytes did not require magnesium ions for its activity. The initial velocity observed when the activity was measured in standard conditions was largely increased when the enzyme was incubated with Mn2+. This increase was however followed by a decrease in absorbance resulting in the induction, by the manganese ions, of a singular kinetics in the enzyme activity. Potassium ions also stimulated fructose-1,6-bisphosphatase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Significant amounts of glycogen are synthesized from 3-carbon compounds in astroglial primary cultures from mice with participation of the mitochondrial phosphoenolpyruvate carboxykinase isoenzyme

The incorporation was studied of the gluconeogenic substrates lactate, alanine, aspartate and glutamate into glycogen of astroglial primary cultures derived from mouse brain. The incorporation was inhibited by 3-mercaptopicolinate, an inhibitor of one of the characteristic gluconeogenic enzymes, phosphoenolpyruvate carboxykinase. Only the mitochondrial isoenzyme of phosphoenolpyruvate carboxykinase was detectable in the astroglial primary cultures. After the incubation of glucose-starved cells with medium containing a mixture of [6-3H]glucose and [U-14C]glucose, the newly synthesized glycogen showed a 3H/14C ratio which was approximately 15% less than the isotope ratio for the medium. The decrease of the isotope ratio was not significantly inhibited by 3-mercaptopicolinate, indicating a cycling of approximately 15% of the glucose to the level of the triose phosphates before its incorporation into astroglial glycogen. During the initial phase of glycogen resynthesis, the contribution of the gluconeogenic substrates appeared to be higher. This was in agreement with the accumulation of fructose 2,6-bisphosphate during refeeding. A participation of gluconeogenic substrates in glycogen metabolism was also detectable when the glycogen content was not changing significantly.

Some aspects of carbohydrate metabolism in the brain

A convenient physiology of the nervous system closely depends on the availability of glucose, the lack of which quickly results in syncope and death. Carbohydrate metabolism in the brain was long thought of as being specific and different from liver carbohydrate metabolism. The present report tries to summarize current data and advances in our knowledge about carbohydrate metabolism. Glucose is brought to the brain by blood flowing through a special network of arteries and is quickly catabolized by the glycolytic and tricarboxylic acid cycle pathways to synthesize energy. It is also used in the synthesis of numerous amino acids, nucleotides and NADPH. Glucose can be polymerized into glycogen in the brain. The nerve tissue is capable of synthesizing glucose-6-phosphate in the gluconeogenic pathway since the fructose-1,6-bisphosphatase, the key enzyme believed to be absent, is actually active and has been purified up to electrophoretic homogeneity. Moreover, the possibility of free glucose synthesis by astrocytes exists. Although the exact role of glycogen in the brain is not totally clear, it is known that the polysaccharide content generally decreases when the functioning of the brain is stimulated and increases in sedative state. This carbohydrate can therefore serve as an indicator for the level of brain activity. Through the administration of methionine sulfoximine, it is possible to increase the amount of glycogen in the brain massively and obtain particles similar to those found in the liver. These in vivo findings have been confirmed by studies based on cultured astrocytes. It has been shown with cultured astrocytes that glutamate increases glycogen synthesis in a pathway which still remains to be elucidated. Brain carbohydrate metabolism is thus in many ways similar to liver carbohydrate metabolism. The astrocyte constitutes the main cell implicated in this metabolism. Improvement in our knowledge about brain carbohydrate metabolism should spread the use of brain glucose metabolism in the diagnosis of certain diseases.

Is brain a gluconeogenic organ?

Glucagon increased the activities of alanine amino transferase (AAT), fructose-1:6-bisphosphatase (fru-P2ase) and glucose-6-phosphatase (G-6-Pase) in goat brain tissue by about 100%, 150% and 50% respectively. These increase in activities were reversed by beta-antagonists propranolol. Well known alpha-agonist and antagonist like phenylephrine and phenoxybenzamine also increased AAT and G-6-Pase activities and these increased activities were reversed by propranolol. Phenylephrine and phenoxybenzamine however did not increase brain Fru-P2ase activity. However the most interesting finding is that cerebral cortical slices could produce glucose from alanine and this glucose production was enhanced by glucagon, phenylephrine and phenoxybenzamine. Propranolol reversed the effects of these agonists and antagonist to a great extent. From all these experiments we suggest brain to be a gluconeogenic organ although much less efficient than liver.

Glutamate increases glycogen content and reduces glucose utilization in primary astrocyte culture

The glycogen content of primary cultured astrocytes was approximately doubled by incubation with 1 mM L-glutamate or L-aspartate. Other amino acids and excitatory neurotransmitters were without effect. The increase in glycogen level was not blocked by the glutamate receptor antagonist kynurenic acid but was completely blocked by the glutamate uptake inhibitor threo-3-hydroxy-D,L-aspartate and by removal of Na+ from the medium. Incubation with radiolabeled glucose and glutamate revealed that the increased glycogen content was derived almost entirely from glucose. Glutamate at 1 mM was also found to cause a 53 +/- 12% decrease in glucose utilization and a 112 +/- 69% increase in glucose-6-phosphate levels. These results suggest that the glycogen content of astrocytes is linked to the rate of glucose utilization and that glucose utilization can, in turn, be affected by the availability of alternative metabolic substrates. These relationships suggest a mechanism by which brain glycogen accumulation occurs during decreased neuronal activity.

Purification and characterization of fructose-1,6-bisphosphatase from bovine brain

Fructose-1,6-bisphosphatase from bovine brain tissue has been purified to near homogeneity. This enzyme is similar to other mammalian fructose-1,6-bisphosphatases in many respects, and its properties are distinctly different from those reported for the enzyme from rat brain [A. L. Majumder and F. Eisenberg (1977) Proc. Natl. Acad. Sci. USA 74, 3222-3225; S. Chattoraj and A. L. Majumder (1986) Biochem. Biophys. Res. Commun. 139, 571-580]. The bovine enzyme (sp act 4, pH ratio (7.5/9.6) = 3.6) has a pH optimum of 7.5. The Km is 2 microM. Divalent metal ion is required for activity, and Vmax is obtained at either 4 mM Mg2+ or 0.3 mM Mn2+. Fructose 2,6-bisphosphate is a competitive inhibitor (Ki = 0.07 microM), and AMP a noncompetitive inhibitor (kis = 24 microM, Kii = 10 microM) of bovine brain fructose-1,6-bisphosphatase. The enzyme activity is enhanced by small amounts of EDTA relative to metal, and AMP inhibits fructose-1,6-bisphosphatase in either the presence or absence of the metal chelator; however, AMP is more effective in the absence of EDTA.

Glutamine synthetase and fructose-1, 6-diphosphatase activity in the putamen of control and Huntington's disease brain post mortem

There is a linear negative correlation between the activities of glutamine synthetase and fructose-1, 6-diphosphatase in normal Human putamen autopsy samples, and also in the Huntington's disease putamen. However, glutamine synthetase activity is reduced in choreic brain samples, while fructose-1, 6-diphosphatase activity is normal. The ratio of fructose-1, 6-diphosphatase to glutamine synthetase is therefore increased in Huntington's disease. The products of the two reactions, glutamine and fructose-6-phosphate, are the starting substrates for glycolipid and glycoprotein biosynthesis, via the glutamine:fructose-6-phosphate aminotransferase catalysed formation of glucoseamine-6-phosphate. The alternative metabolic route of fructose-6-phosphate leads to glycogen. The availability of glutamine, and the activity of glutamine synthetase may control fructose-6-phosphate metabolism, and the increased ratio of fructose-1,6-diphosphatase to glutamine synthetase in Huntington's disease may explain the accumulation of glycogen, and the reduction in ganglioside levels reported in this state.

Stimulation of fructose-1,6-biphosphatase activity and synthesis in the cerebral cortex of rats submitted to the convulsant methionine sulfoximine

Purification of rat cerebral cortex fructose-1,6-biphosphatase (FBPase) was performed by substrate elution from phosphocellulose, followed by Sephadex G-200 column filtration. The purified enzyme exhibited an optimum at pH 7.5, and its catalytic properties were very similar to those of the purified whole-brain enzyme previously prepared by Majumder and Eisenberg in 1977. The isolated preparation was electrophoretically homogeneous. The molecular weight of the enzyme subunit was 40,000; the hydrophobic amino acids predominated with 592 residues, and tryptophan was not detected. Expressed as mumol fructose-1,6-biphosphate hydrolysed per g brain tissue wet weight per min, FBPase activity increased twofold 24 h after an intraperitoneal injection of 100 mg per kg body weight of the convulsant methionine sulfoximine (MSO); the increase of the rate of incorporation of [1-14C]valine into brain FBPase was 2.8-fold under the same experimental conditions. A rabbit specific antiserum against rat cerebral cortex FBPase was prepared, and immunotitration studies confirmed both an increase in the number of molecules and the activation of brain FBPase, 24 h after administration of MSO. The increase of the number of brain FBPase molecules, induced by MSO, was due to an increase in synthesis of the enzyme, as shown by a double-label valine incorporation study.

Unequivocal demonstration of fructose-1,6-bisphosphatase in mammalian brain

Fructose-1,6-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrolase; EC 3.1.3.11) has been found in rat brain and identified unequivocally. The enzyme has been purified to 95% homogeneity by standard procedures, including adsorption to a phosphocellulose column followed by elution with substrate. The purified enzyme exhibits a broad optimum above pH 7.6. Both fructose 1,6-bisphosphate and sedoheptulose 1,7-bisphosphate are substrates of this enzyme; the hydrolysis of the latter occurs at about 20% of the rate of the former, and the Km for fructose 1,6-bisphosphate is approximately 1.32 X 10(-4) M. 5'-AMP, an inhibitor of other mammalian-fructose-1,6-bisphosphatases, is without effect, and in further contrast with the other enzymes there is no metal requirement for activity. Purified brain enzyme fails to crossreact with the antibody prepared against the purified liver fructose-1,6-bisphosphatase. On the other hand, antiserum produced against the brain fructose-1,6-bisphosphatase quantitatively precipitates the enzyme activity and forms precipitin bands with preparations of brain fructose-1,6-bisphosphatase.