Metabolism of glucose into glutamate via the hexose monophosphate shunt and its inhibition by 6-aminonicotinamide in rat brain in vivo

Evidence for direct CO2 -mediated alterations in cerebral oxidative metabolism in humans

AIM

How the cerebral metabolic rates of oxygen and glucose utilization (CMRO2 and CMRGlc, respectively) are affected by alterations in arterial PCO2 (PaCO2) is equivocal and therefore was the primary question of this study.

METHODS

This retrospective analysis involved pooled data from four separate studies, involving 41 healthy adults (35 males/6 females). Participants completed stepwise steady-state alterations in PaCO2 ranging between 30 and 60 mmHg. The CMRO2 and CMRGlc were assessed via the Fick approach (CBF × arterial-internal jugular venous difference of oxygen or glucose content, respectively) utilizing duplex ultrasound of the internal carotid artery and vertebral artery to calculate cerebral blood flow (CBF).

RESULTS

The CMRO2 was altered by 0.5 mL × min-1 (95% CI: -0.6 to -0.3) per mmHg change in PaCO2 (p < 0.001) which corresponded to a 9.8% (95% CI: -13.2 to -6.5) change in CMRO2 with a 9 mmHg change in PaCO2 (inclusive of hypo- and hypercapnia). The CMRGlc was reduced by 7.7% (95% CI: -15.4 to -0.08, p = 0.045; i.e., reduction in net glucose uptake) and the oxidative glucose index (ratio of oxygen to glucose uptake) was reduced by 5.6% (95% CI: -11.2 to 0.06, p = 0.049) with a + 9 mmHg increase in PaCO2.

CONCLUSION

Collectively, the CMRO2 is altered by approximately 1% per mmHg change in PaCO2. Further, glucose is incompletely oxidized during hypercapnia, indicating reductions in CMRO2 are either met by compensatory increases in nonoxidative glucose metabolism or explained by a reduction in total energy production.

Brain Glucose Metabolism: Integration of Energetics with Function

Glucose is the long-established, obligatory fuel for brain that fulfills many critical functions, including ATP production, oxidative stress management, and synthesis of neurotransmitters, neuromodulators, and structural components. Neuronal glucose oxidation exceeds that in astrocytes, but both rates increase in direct proportion to excitatory neurotransmission; signaling and metabolism are closely coupled at the local level. Exact details of neuron-astrocyte glutamate-glutamine cycling remain to be established, and the specific roles of glucose and lactate in the cellular energetics of these processes are debated. Glycolysis is preferentially upregulated during brain activation even though oxygen availability is sufficient (aerobic glycolysis). Three major pathways, glycolysis, pentose phosphate shunt, and glycogen turnover, contribute to utilization of glucose in excess of oxygen, and adrenergic regulation of aerobic glycolysis draws attention to astrocytic metabolism, particularly glycogen turnover, which has a high impact on the oxygen-carbohydrate mismatch. Aerobic glycolysis is proposed to be predominant in young children and specific brain regions, but re-evaluation of data is necessary. Shuttling of glucose- and glycogen-derived lactate from astrocytes to neurons during activation, neurotransmission, and memory consolidation are controversial topics for which alternative mechanisms are proposed. Nutritional therapy and vagus nerve stimulation are translational bridges from metabolism to clinical treatment of diverse brain disorders.

Glucose, Lactate, β-Hydroxybutyrate, Acetate, GABA, and Succinate as Substrates for Synthesis of Glutamate and GABA in the Glutamine-Glutamate/GABA Cycle

The glutamine-glutamate/GABA cycle is an astrocytic-neuronal pathway transferring precursors for transmitter glutamate and GABA from astrocytes to neurons. In addition, the cycle carries released transmitter back to astrocytes, where a minor fraction (~25 %) is degraded (requiring a similar amount of resynthesis) and the remainder returned to the neurons for reuse. The flux in the cycle is intense, amounting to the same value as neuronal glucose utilization rate or 75-80 % of total cortical glucose consumption. This glucose:glutamate ratio is reduced when high amounts of β-hydroxybutyrate are present, but β-hydroxybutyrate can at most replace 60 % of glucose during awake brain function. The cycle is initiated by α-ketoglutarate production in astrocytes and its conversion via glutamate to glutamine which is released. A crucial reaction in the cycle is metabolism of glutamine after its accumulation in neurons. In glutamatergic neurons all generated glutamate enters the mitochondria and its exit to the cytosol occurs in a process resembling the malate-aspartate shuttle and therefore requiring concomitant pyruvate metabolism. In GABAergic neurons one half enters the mitochondria, whereas the other one half is released directly from the cytosol. A revised concept is proposed for the synthesis and metabolism of vesicular and nonvesicular GABA. It includes the well-established neuronal GABA reuptake, its metabolism, and use for resynthesis of vesicular GABA. In contrast, mitochondrial glutamate is by transamination to α-ketoglutarate and subsequent retransamination to releasable glutamate essential for the transaminations occurring during metabolism of accumulated GABA and subsequent resynthesis of vesicular GABA.

Brain aerobic glycolysis functions and Alzheimer's disease

Genetic, biochemical, pathological, and biomarker data demonstrate that Alzheimer's disease (AD) pathology, including the initiation and progressive buildup of insoluble forms of beta-amyloid (Aβ), appears to begin ~ 10-15 years prior to the onset of cognitive decline associated with AD. Metabolic dysfunction, a prominent feature of the evolving brain pathology, is reflected in a decline of total glucose utilization. Despite decades of interest in declining glucose use in AD no detailed consideration had been given to the possibility that this decline is not just a decline in energy consumption but rather in glycolysis alone. Glycolysis is a multi-step process that prepares the glucose molecule for oxidative phosphorylation and the generation of energy. In the normal brain, glycolysis exceeds that required for the needs of oxidative phosphorylation. Because it is occurring in a setting with adequate oxygen available for oxidative phosphorylation it is often referred to as aerobic glycolysis (AG). AG is a biomarker of a group of metabolic functions broadly supporting biosynthesis and neuroprotection. The distribution of AG in normal young adults correlates spatially with Aβ deposition in AD patients and cognitively normal individuals with elevated Aβ. In transgenic mice extracellular fluid Aβ and lactate, a marker of AG, vary in parallel regionally and with changes in activity. Reducing neuronal activity locally in transgenic mice attenuates plaque formation suggesting that plaque formation is an activity dependent process associated with aerobic glycolysis.

Sleep/wake dependent changes in cortical glucose concentrations

Most of the energy in the brain comes from glucose and supports glutamatergic activity. The firing rate of cortical glutamatergic neurons, as well as cortical extracellular glutamate levels, increase with time spent awake and decline throughout non rapid eye movement sleep, raising the question whether glucose levels reflect behavioral state and sleep/wake history. Here chronic (2-3 days) electroencephalographic recordings in the rat cerebral cortex were coupled with fixed-potential amperometry to monitor the extracellular concentration of glucose ([gluc]) on a second-by-second basis across the spontaneous sleep-wake cycle and in response to 3 h of sleep deprivation. [Gluc] progressively increased during non rapid eye movement sleep and declined during rapid eye movement sleep, while during wake an early decline in [gluc] was followed by an increase 8-15 min after awakening. There was a significant time of day effect during the dark phase, when rats are mostly awake, with [gluc] being significantly lower during the last 3-4 h of the night relative to the first 3-4 h. Moreover, the duration of the early phase of [gluc] decline during wake was longer after prolonged wake than after consolidated sleep. Thus, the sleep/wake history may affect the levels of glucose available to the brain upon awakening.

The uncoupling protein 2 -866G > a polymorphism is associated with the risk of ischemic stroke in Chinese type 2 diabetic patients

AIMS

To determine genetic predispsitions for diabetic cerebral ischemia, we investigated the relationship between the -866G>A polymorphism of uncoupling protein (UCP) 2 and the risk of ischemic stroke in two cohorts of type 2 diabetic patients.

METHODS

A total of 844 type 2 diabetic patients with 4-year prospective study were examined using a case-control methodology. And 404 cases with ischemical stroke, 440 cases without ischemical stroke. The -866G>A polymorphism in UCP2 was genotyped by TaqMan MGB probe method.

RESULTS

The -866G>A SNP in UCP2 was significantly associated with diabetic ischemical stroke (odds ratio [OR]= 1.94; 95% confidence interval [CI]= 0.68 to1.31; P < 0.037). Similar results were observed for baseline cases of IS. Stratification by sex confirmed an allelic association with IS in women, whereas no association was observed in men.

CONCLUSIONS

The A allele of the -866G>A variant of UCP2 was associated with increased risk of IS in Chinese diabetic women with type 2 diabetes in a 4-year prospective study. This association was independent of other common IS risk factors.

Regional aerobic glycolysis in the human brain

Aerobic glycolysis is defined as glucose utilization in excess of that used for oxidative phosphorylation despite sufficient oxygen to completely metabolize glucose to carbon dioxide and water. Aerobic glycolysis is present in the normal human brain at rest and increases locally during increased neuronal activity; yet its many biological functions have received scant attention because of a prevailing energy-centric focus on the role of glucose as substrate for oxidative phosphorylation. As an initial step in redressing this neglect, we measured the regional distribution of aerobic glycolysis with positron emission tomography in 33 neurologically normal young adults at rest. We show that the distribution of aerobic glycolysis in the brain is differentially present in previously well-described functional areas. In particular, aerobic glycolysis is significantly elevated in medial and lateral parietal and prefrontal cortices. In contrast, the cerebellum and medial temporal lobes have levels of aerobic glycolysis significantly below the brain mean. The levels of aerobic glycolysis are not strictly related to the levels of brain energy metabolism. For example, sensory cortices exhibit high metabolic rates for glucose and oxygen consumption but low rates of aerobic glycolysis. These striking regional variations in aerobic glycolysis in the normal human brain provide an opportunity to explore how brain systems differentially use the diverse cell biology of glucose in support of their functional specializations in health and disease.

Differential neurochemical responses of the canine striatum with pentobarbital or ketamine anesthesia: a 3T proton MRS study

Although anesthetic agents are known to affect cerebral metabolism, pentobarbital and ketamine have been widely used for animal imaging studies. The purpose of this study is to evaluate alterations in striatum metabolites in dogs between anesthetized with pentobarbital and with ketamine in proton magnetic resonance spectroscopy ((1)H-MRS). (1)H-MRS was performed to ten healthy adult beagle dogs (9-11 kg) at a field strength of 3 T in order to identify metabolic changes after pentobarbital or ketamine administration in the striatum in vivo. Ten dogs were divided into 2 groups as follows: 5 as the pentobarbital-administered group (P group) and 5 as the ketamine-administered group (K group). We found that levels of Glx of the P group was significantly lower than that of the K group (6.90 +/- 0.99 (SD) vs 9.77 +/- 1.14 in 5 dogs, p= 0.003). In addition, the P group also has lower levels of Cr (6.29 +/- 0.44 vs 7.89 +/- 0.91 in 5 dogs, p=0.009) and NAA (5.02 +/- 0.65 vs 6.45 +/- 1.13 in 5 dogs, p=0.041) compared to the K group. However, there were no significant difference between the P group and the K group in striatal levels of Cho and Ins (p>0.1). We demonstrated that MRS-measured metabolites in the specific regions of the brain can be influenced by anesthetic agents.

Neuroprotective role of mitochondrial uncoupling protein 2 in cerebral stroke

The uncoupling proteins (UCPs) are mitochondrial transporter proteins involved in proton conductance across inner mitochondrial membrane (IMM). UCP2, which is one of the members of this class of proteins, has a wide but restricted tissue distribution including brain. Its physiologic role according to emerging evidences, although still not clear, indicate that distribution of UCP2 may be related to regulation of mitochondria membrane potential (DeltaPsim), production of reactive oxygen species (ROS), preservation of calcium homeostasis, modulation of neuronal activity, and eventually inhibition of cellular damage. These factors are very important in determining the fate of neurons and damage progression in the brain during various neurodegenerative diseases including cerebral stroke. Recent evidence indicates that an increased expression and activity of UCP2 are well correlated with neuronal survival after stroke and trauma. This review briefly covers the present understanding of UCP2, which eventually may be beneficial to understand the precise role of UCP2 to develop strategy to identify its potential therapeutic application.

6-Aminonicotinamide inhibition of the pentose phosphate pathway in rat neocortex

6-Aminonicotinamide (6-AN) is thought to inhibit the pentose phosphate pathway (PPP) since large increases in 6-phosphogluconate are observed following its administration. Immediately following 45 min i.v. infusion of [2-(13)C]glucose to controls and 6-AN-treated (50 mg/kg i.p. given 4 h previously) Sprague-Dawley rats (n = 5 for both groups), metabolism was arrested using freeze-funnel fixation. Chloroform-methanol-water neocortical extracts from animals administered with 6-AN demonstrated elevated levels of 6-phosphogluconate and 6-phosphoglucono-delta-lactone, both of which demonstrated labeling through metabolism of [2-(13)C]glucose. Comparison of the C-2 and C-3 lactate positions using 1H NMR spectroscopy showed that the fraction of glucose metabolized through the PPP is unchanged by 6-AN (14+/-0.6% vs 14+/-0.3% in control animals). It is hypothesized that as the PPP is inhibited by metabolites of 6-AN in the neocortex, glycolysis is inhibited in a proportionate manner through an inhibitory effect on phosphoglucose isomerase by 6-phosphogluconate and/or 6-phosphoglucono-delta-lactone.

Hyperglycemia enhances extracellular glutamate accumulation in rats subjected to forebrain ischemia

BACKGROUND AND PURPOSE

An increase in serum glucose at the time of acute ischemia has been shown to adversely affect prognosis. The mechanisms for the hyperglycemia-exacerbated damage are not fully understood. The objective of this study was to determine whether hyperglycemia leads to enhanced accumulation of extracellular concentrations of excitatory amino acids and whether such increases correlate with the histopathological outcome.

METHODS

Rats fasted overnight were infused with either glucose or saline 45 minutes before the induction of 15 minutes of forebrain ischemia. Extracellular glutamate, glutamine, glycine, taurine, alanine, and serine concentrations were measured before, during, and after ischemia in both the hippocampus and the neocortex in both control and hyperglycemic animals. The histopathological outcome was evaluated by light microscopy.

RESULTS

There was a significant increase in extracellular glutamate levels in the hippocampus and cerebral cortex in normoglycemic ischemic animals. The increase in glutamate levels in the cerebral cortex, but not in the hippocampus, was significantly higher in hyperglycemic animals than in controls. Correspondingly, exaggerated neuronal damage was observed in neocortical regions in hyperglycemic animals.

CONCLUSIONS

The present results demonstrate that, at least in the neocortex, preischemic hyperglycemia enhances the accumulation of extracellular glutamate during ischemia, providing a tentative explanation for why neuronal damage is exaggerated.

Hypothermia ameliorates ischemic brain damage and suppresses the release of extracellular amino acids in both normo- and hyperglycemic subjects

It has previously been shown that hypothermia markedly reduces cellular release of the excitatory amino acid glutamate and ameliorates ischemic damage. Based on extensive data showing that preischemic hyperglycemia exaggerates brain damage due to transient forebrain ischemia we posed the question whether glutamate release during ischemia in hyperglycemic rats is attenuated or prevented by induced hypothermia, and if such attenuation/prevention correlates with amelioration of the characteristic brain damage observed in hyperglycemic subjects. The experiments were performed in rats subjected to a 15-min period of forebrain ischemia, plasma glucose concentration being maintained at approximately 5 mM (control) or approximately 20 mM (hyperglycemia) prior to ischemia. Extracellular amino acid concentrations were measured by HPLC techniques on microdialysis samples which were collected from left dorsal hippocampus and right neocortex, and tissue damage was assessed by histopathology. Hypothermia (30 degrees C), which was induced 45 min prior to ischemia, reduced the neuronal damage not only in the ischemia-vulnerable regions but also in the normally ischemia-resistant areas that are recruited in the damage process in hyperglycemic subjects. The extracellular glutamate concentration was markedly increased in response to the ischemic insult in normothermic-normoglycemic animals. The concentration of glutamate was further increased in normothermic-hyperglycemic animals. Hypothermia inhibited the rise in glutamate concentrations, as well as in the concentrations of other excitatory and inhibitory amino acids. It is discussed whether hypothermia reduces the hyperglycemia-mediated damage by inhibiting extracellular glutamate release during an ischemic transient.

Positron emission tomographic measurements of cerebral glucose utilization using [1-11C]D-glucose

Regional CMRglc was measured in seven healthy volunteers with positron emission tomography using [1-11C]D-glucose. Regional CBF was measured using [11C]fluoromethane. The arteriovenous differences of unlabeled glucose and oxygen together with 11C metabolites were also measured. In addition to the loss of [11C]CO2, a loss of acidic 11C metabolites was also detected. A three-compartment model was applied to the tracer data in the time interval 0-24 min. After correction for the loss of 11C metabolites, the tracer method gave an average CMRglc of 26.4 +/- 1.9 (SD) mumol/100 g/min, close to the value obtained with the Fick principle. After correction for the loss of [11C]CO2 only, the tracer method gave 23.6 +/- 2.1 mumol/100 g/min, compatible with (1/6) CMRO2, obtained with the Fick principle. These results and the time course of the loss of acidic 11C metabolites are consistent with the presence of nonoxidative metabolism of glucose that causes an early loss of mainly [11C]lactate after a bolus injection of the tracer. This implies that [1-11C]D-glucose measures the rate of glucose oxidation rather than the total CMRglc. The experiments using [1-11C]D-glucose were compared to five analogous experiments using [U-11C]D-glucose together with [15O]H2O as a flow tracer. After correction for the loss of [11C]CO2, the two glucose tracers gave similar global values of CMRglc and other parameters associated with glucose utilization, but with labeling in the carbon-1 position, the loss of [11C]CO2 was substantially delayed and the contrast between gray and white matter was improved.(ABSTRACT TRUNCATED AT 250 WORDS)