Dual mechanisms for glucose 6-phosphate inhibition of human brain hexokinase

Hexokinase-linked glycolytic overload and unscheduled glycolysis in hyperglycemia-induced pathogenesis of insulin resistance, beta-cell glucotoxicity, and diabetic vascular complications

Hyperglycemia is a risk factor for the development of insulin resistance, beta-cell glucotoxicity, and vascular complications of diabetes. We propose the hypothesis, hexokinase-linked glycolytic overload and unscheduled glycolysis, in explanation. Hexokinases (HKs) catalyze the first step of glucose metabolism. Increased flux of glucose metabolism through glycolysis gated by HKs, when occurring without concomitant increased activity of glycolytic enzymes-unscheduled glycolysis-produces increased levels of glycolytic intermediates with overspill into effector pathways of cell dysfunction and pathogenesis. HK1 is saturated with glucose in euglycemia and, where it is the major HK, provides for basal glycolytic flux without glycolytic overload. HK2 has similar saturation characteristics, except that, in persistent hyperglycemia, it is stabilized to proteolysis by high intracellular glucose concentration, increasing HK activity and initiating glycolytic overload and unscheduled glycolysis. This drives the development of vascular complications of diabetes. Similar HK2-linked unscheduled glycolysis in skeletal muscle and adipose tissue in impaired fasting glucose drives the development of peripheral insulin resistance. Glucokinase (GCK or HK4)-linked glycolytic overload and unscheduled glycolysis occurs in persistent hyperglycemia in hepatocytes and beta-cells, contributing to hepatic insulin resistance and beta-cell glucotoxicity, leading to the development of type 2 diabetes. Downstream effector pathways of HK-linked unscheduled glycolysis are mitochondrial dysfunction and increased reactive oxygen species (ROS) formation; activation of hexosamine, protein kinase c, and dicarbonyl stress pathways; and increased Mlx/Mondo A signaling. Mitochondrial dysfunction and increased ROS was proposed as the initiator of metabolic dysfunction in hyperglycemia, but it is rather one of the multiple downstream effector pathways. Correction of HK2 dysregulation is proposed as a novel therapeutic target. Pharmacotherapy addressing it corrected insulin resistance in overweight and obese subjects in clinical trial. Overall, the damaging effects of hyperglycemia are a consequence of HK-gated increased flux of glucose metabolism without increased glycolytic enzyme activities to accommodate it.

Optimization of energy production and central carbon metabolism in a non-respiring eukaryote

Most eukaryotes respire oxygen, using it to generate biomass and energy. However, a few organisms have lost the capacity to respire. Understanding how they manage biomass and energy production may illuminate the critical points at which respiration feeds into central carbon metabolism and explain possible routes to its optimization. Here, we use two related fission yeasts, Schizosaccharomyces pombe and Schizosaccharomyces japonicus, as a comparative model system. We show that although S. japonicus does not respire oxygen, unlike S. pombe, it is capable of efficient NADH oxidation, amino acid synthesis, and ATP generation. We probe possible optimization strategies through the use of stable isotope tracing metabolomics, mass isotopologue distribution analysis, genetics, and physiological experiments. S. japonicus appears to have optimized cytosolic NADH oxidation via glycerol-3-phosphate synthesis. It runs a fully bifurcated TCA pathway, sustaining amino acid production. Finally, we propose that it has optimized glycolysis to maintain high ATP/ADP ratio, in part by using the pentose phosphate pathway as a glycolytic shunt, reducing allosteric inhibition of glycolysis and supporting biomass generation. By comparing two related organisms with vastly different metabolic strategies, our work highlights the versatility and plasticity of central carbon metabolism in eukaryotes, illuminating critical adaptations supporting the preferential use of glycolysis over oxidative phosphorylation.

C. Pereira I, López-Montero I, Vélez M, Pita M, De Lacey AL. Electro-enzymatic ATP regeneration coupled to biocatalytic phosphorylation reactions

Adenosine-5-triphosphate (ATP) is the main energy vector in biological systems, thus its regeneration is an important issue for the application of many enzymes of interest in biocatalysis and synthetic biology. We have developed an electroenzymatic ATP regeneration system consisting in a gold electrode modified with a floating phospholipid bilayer that allows coupling the catalytic activity of two membrane-bound enzymes: NiFeSe hydrogenase from Desulfovibrio vulgaris and F₁F_o-ATP synthase from Escherichia coli. Thus, H₂ is used as a fuel for producing ATP. This electro-enzymatic assembly is studied as ATP regeneration system of phosphorylation reactions catalysed by kinases, such as hexokinase and NAD⁺-kinase for respectively producing glucose-6-phosphate and NADP⁺.

Metabolic Features of Brain Function with Relevance to Clinical Features of Alzheimer and Parkinson Diseases

Brain metabolism is comprised in Alzheimer’s disease (AD) and Parkinson’s disease (PD). Since the brain primarily relies on metabolism of glucose, ketone bodies, and amino acids, aspects of these metabolic processes in these disorders—and particularly how these altered metabolic processes are related to oxidative and/or nitrosative stress and the resulting damaged targets—are reviewed in this paper. Greater understanding of the decreased functions in brain metabolism in AD and PD is posited to lead to potentially important therapeutic strategies to address both of these disorders, which cause relatively long-lasting decreased quality of life in patients.

Whole-cell biocatalytic synthesis of S-(4-chlorophenyl)-(pyridin-2-yl) methanol in a liquid-liquid biphasic microreaction system

This work aims to synthesize S-(4-chlorophenyl)-(pyridin-2-yl) methanol (S-CPMA) in a green, economic, and efficient way. In the water-cyclohexane liquid-liquid system, recombinant Escherichia coli (E. coli) was used as a whole-cell catalyst and retained > 60% of its catalytic activity after five reuse cycles. In situ accumulation of the substrate/product in the organic phase effectively improves substrate tolerance and reduces product inhibition and toxicity. Meanwhile, a microreaction system consisting of membrane dispersion and three-dimensional (3D) bending-microchannel was developed to successfully generate droplet swarms with an average diameter of 30 μm. Large specific surface area provided high mass transfer efficiency between phases. While the analogous reaction in a traditional stirred tank required > 270 min to achieve a yield of > 99%, in this biphasic microreaction system, the yield reached 99.6% with a high enantiomeric excess (ee) of > 99% in only 80 min. Efficient synthesis was achieved by reducing the time by 70%.

Virtual Screening for Potential Inhibitors of Human Hexokinase II for the Development of Anti-Dengue Therapeutics

Dengue fever, which is a disease caused by the dengue virus (DENV), is a major unsolved issue in many tropical and sub-tropical regions of the world. The absence of treatment that effectively prevent further viral propagation inside the human’s body resulted in a high number of deaths globally each year. Thus, novel anti-dengue therapies are required for effective treatment. Human hexokinase II (HKII), which is the first enzyme in the glycolytic pathway, is an important drug target due to its significant impact on viral replication and survival in host cells. In this study, 23.1 million compounds were computationally-screened against HKII using the Ultrafast Shape Recognition with a CREDO Atom Types (USRCAT) algorithm. In total, 300 compounds with the highest similarity scores relative to three reference molecules, known as Alpha-D-glucose (GLC), Beta-D-glucose-6-phosphate (BG6), and 2-deoxyglucose (2DG), were aligned. Of these 300 compounds, 165 were chosen for further structure-based screening, based on their similarity scores, ADME analysis, the Lipinski’s Rule of Five, and virtual toxicity test results. The selected analogues were subsequently docked against each domain of the HKII structure (PDB ID: 2NZT) using AutoDock Vina programme. The three top-ranked compounds for each query were then selected from the docking results based on their binding energy, the number of hydrogen bonds formed, and the specific catalytic residues. The best docking results for each analogue were observed for the C-terminus of Chain B. The top-ranked analogues of GLC, compound 10, compound 26, and compound 58, showed predicted binding energies of −7.2, −7.0, and −6.10 kcal/mol and 7, 5, and 2 hydrogen bonds, respectively. The analogues of BG6, compound 30, compound 36, and compound 38, showed predicted binding energies of −7.8, −7.4, and −7.0 kcal/mol and 11, 9, and 5 hydrogen bonds, while the top three analogues of 2DG, known as compound 1, compound 4, and compound 31, showed predicted binding energies of −6.8, −6.3, and −6.3 kcal/mol and 4, 3, and 1 hydrogen bonds, sequentially. The highest-ranked compounds in the docking analysis were then selected for molecular dynamics simulation, where compound 10, compound 30, and compound 1, which are the analogues of GLC, BG6, and 2DG, have shown strong protein-ligand stability with an RMSD value of ±5.0 A° with a 5 H bond, ±4.0 A° with an 8 H bond, and ±0.5 A° with a 2 H bond, respectively, compared to the reference molecules throughout the 20 ns simulation time. Therefore, by using the computational studies, we proposed novel compounds, which may act as potential drugs against DENV by inhibiting HKII’s activity.

Metabolic Dysregulation Contributes to the Progression of Alzheimer's Disease

Alzheimer's disease (AD) is an incurable neurodegenerative disease. Numerous studies have demonstrated a critical role for dysregulated glucose metabolism in its pathogenesis. In this review, we summarize metabolic alterations in aging brain and AD-related metabolic deficits associated with glucose metabolism dysregulation, glycolysis dysfunction, tricarboxylic acid (TCA) cycle, oxidative phosphorylation (OXPHOS) deficits, and pentose phosphate pathway impairment. Additionally, we discuss recent treatment strategies targeting metabolic defects in AD, including their limitations, in an effort to encourage the development of novel therapeutic strategies.

Biological sex and DNA repair deficiency drive Alzheimer's disease via systemic metabolic remodeling and brain mitochondrial dysfunction

Alzheimer's disease (AD) is an incurable neurodegenerative disease that is more prevalent in women. The increased risk of AD in women is not well understood. It is well established that there are sex differences in metabolism and that metabolic alterations are an early component of AD. We utilized a cross-species approach to evaluate conserved metabolic alterations in the serum and brain of human AD subjects, two AD mouse models, a human cell line, and two Caenorhabditis elegans AD strains. We found a mitochondrial complex I-specific impairment in cortical synaptic brain mitochondria in female, but not male, AD mice. In the hippocampus, Polβ haploinsufficiency caused synaptic complex I impairment in male and female mice, demonstrating the critical role of DNA repair in mitochondrial function. In non-synaptic, glial-enriched, mitochondria from the cortex and hippocampus, complex II-dependent respiration increased in female, but not male, AD mice. These results suggested a glial upregulation of fatty acid metabolism to compensate for neuronal glucose hypometabolism in AD. Using an unbiased metabolomics approach, we consistently observed evidence of systemic and brain metabolic remodeling with a shift from glucose to lipid metabolism in humans with AD, and in AD mice. We determined that this metabolic shift is necessary for cellular and organismal survival in C. elegans, and human cell culture AD models. We observed sex-specific, systemic, and brain metabolic alterations in humans with AD, and that these metabolite changes significantly correlate with amyloid and tau pathology. Among the most significant metabolite changes was the accumulation of glucose-6-phosphate in AD, an inhibitor of hexokinase and rate-limiting metabolite for the pentose phosphate pathway (PPP). Overall, we identified novel mechanisms of glycolysis inhibition, PPP, and tricarboxylic acid cycle impairment, and a neuroprotective augmentation of lipid metabolism in AD. These findings support a sex-targeted metabolism-modifying strategy to prevent and treat AD.

Galactose 1-phosphate accumulates to high levels in galactose-treated cells due to low GALT activity and absence of product inhibition of GALK

Classic Galactosaemia is a genetic disorder, characterised by galactose intolerance in newborns. It occurs due to recessive mutations in the galactose-1-phosphate uridylyltransferase (GALT) gene. One of the main alterations caused by GALT deficiency is the accumulation of galactose 1-phosphate (Gal-1P) in cells. Studies have suggested that Gal-1P exerts cellular toxicity, possibly by inhibiting cellular metabolism. However, the exact significance of Gal-1P in disease pathogenesis remains unclear. In this study, we tested the hypothesis that Gal-1P inhibits cellular glucose utilisation by competing with substrates in the glycolytic pathway. We also investigated the metabolism of both galactose and glucose in GALT-expressing HEK293T and 143B cells to identify critical reactions steps contributing to the metabolic toxicity of galactose. Notably, we found that galactose-treated HEK293T and 143B cells, which express endogenous GALT, accumulate markedly high intracellular Gal-1P concentrations. Despite very high intracellular Gal-1P concentrations, no inhibition of cellular glucose uptake and no significant changes in the intracellular concentrations of glycolytic metabolites were observed. This indicates that Gal-1P does not exert an inhibitory effect on glycolysis in cells and rules out one potential hypothesis for cellular Gal-1P toxicity. We also investigated the mechanism responsible for the observed Gal-1P accumulation. Our results suggest that Gal-1P accumulation is a result of both low GALT activity and the absence of product inhibition by Gal-1P on galactokinase (GALK1), the enzyme responsible for phosphorylating galactose to Gal-1P. These findings provide a better understanding of the disease mechanisms underlying Classic Galactoaemia.

How glycogen sustains brain function: A plausible allosteric signaling pathway mediated by glucose phosphates

Astrocytic glycogen is the sole glucose reserve of the brain. Both glycogen and glucose are necessary for basic neurophysiology and in turn for higher brain functions. In spite of low concentration, turnover and stimulation-induced degradation, any interference with normal glycogen metabolism in the brain severely affects neuronal excitability and disrupts memory formation. Here, I briefly discuss the glycogenolysis-induced glucose-sparing effect, which involves glucose phosphates as key allosteric effectors in the modulation of astrocytic and neuronal glucose uptake and phosphorylation. I further advance a novel and thus far unexplored effect of glycogenolysis that might be mediated by glucose phosphates.

Carbohydrate Kinases: A Conserved Mechanism Across Differing Folds

Carbohydrate kinases activate a wide variety of monosaccharides by adding a phosphate group, usually from ATP. This modification is fundamental to saccharide utilization, and it is likely a very ancient reaction. Modern organisms contain carbohydrate kinases from at least five main protein families. These range from the highly specialized inositol kinases, to the ribokinases and galactokinases, which belong to families that phosphorylate a wide range of substrates. The carbohydrate kinases utilize a common strategy to drive the reaction between the sugar hydroxyl and the donor phosphate. Each sugar is held in position by a network of hydrogen bonds to the non-reactive hydroxyls (and other functional groups). The reactive hydroxyl is deprotonated, usually by an aspartic acid side chain acting as a catalytic base. The deprotonated hydroxyl then attacks the donor phosphate. The resulting pentacoordinate transition state is stabilized by an adjacent divalent cation, and sometimes by a positively charged protein side chain or the presence of an anion hole. Many carbohydrate kinases are allosterically regulated using a wide variety of strategies, due to their roles at critical control points in carbohydrate metabolism. The evolution of a similar mechanism in several folds highlights the elegance and simplicity of the catalytic scheme.

Dysregulated Glucose Metabolism as a Therapeutic Target to Reduce Post-traumatic Epilepsy

Traumatic brain injury (TBI) is a significant cause of disability worldwide and can lead to post-traumatic epilepsy. Multiple molecular, cellular, and network pathologies occur following injury which may contribute to epileptogenesis. Efforts to identify mechanisms of disease progression and biomarkers which predict clinical outcomes have focused heavily on metabolic changes. Advances in imaging approaches, combined with well-established biochemical methodologies, have revealed a complex landscape of metabolic changes that occur acutely after TBI and then evolve in the days to weeks after. Based on this rich clinical and preclinical data, combined with the success of metabolic therapies like the ketogenic diet in treating epilepsy, interest has grown in determining whether manipulating metabolic activity following TBI may have therapeutic value to prevent post-traumatic epileptogenesis. Here, we focus on changes in glucose utilization and glycolytic activity in the brain following TBI and during seizures. We review relevant literature and outline potential paths forward to utilize glycolytic inhibitors as a disease-modifying therapy for post-traumatic epilepsy.

Supragranular Pyramidal Cells Exhibit Early Metabolic Alterations in the 3xTg-AD Mouse Model of Alzheimer's Disease

The impairment of cerebral glucose utilization is an early and predictive biomarker of Alzheimer’s disease (AD) that is likely to contribute to memory and cognition disorders during the progression of the pathology. Yet, the cellular and molecular mechanisms underlying these metabolic alterations remain poorly understood. Here we studied the glucose metabolism of supragranular pyramidal cells at an early presymptomatic developmental stage in non-transgenic (non-Tg) and 3xTg-AD mice, a mouse model of AD replicating numerous hallmarks of the disease. We performed both intracellular glucose imaging with a genetically encoded fluorescence resonance energy transfer (FRET)-based glucose biosensor and transcriptomic profiling of key molecular elements of glucose metabolism with single-cell multiplex RT-PCR (scRT-mPCR). We found that juvenile pyramidal cells exhibit active glycolysis and pentose phosphate pathway at rest that are respectively enhanced and impaired in 3xTg-AD mice without alteration of neuronal glucose uptake or transcriptional modification. Given the importance of glucose metabolism for neuronal survival, these early alterations could initiate or at least contribute to the later neuronal dysfunction of pyramidal cells in AD.

S100B impairs glycolysis via enhanced poly(ADP-ribosyl)ation of glyceraldehyde-3-phosphate dehydrogenase in rodent muscle cells

S100 calcium-binding protein B (S100B), a multifunctional macromolecule mainly expressed in nerve tissues and adipocytes, has been suggested to contribute to the pathogenesis of obesity. To clarify the role of S100B in insulin action and glucose metabolism in peripheral tissues, we investigated the effect of S100B on glycolysis in myoblast and myotube cells. Rat myoblast L6 cells were treated with recombinant mouse S100B to examine glucose consumption, lactate production, glycogen accumulation, glycolytic metabolites and enzyme activity, insulin signaling, and poly(ADP-ribosyl)ation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Glycolytic metabolites were investigated by enzyme assays or metabolome analysis, and insulin signaling was assessed by Western blot analysis. Enzyme activity and poly(ADP-ribosyl)ation of GAPDH was evaluated by an enzyme assay and immunoprecipitation followed by dot blot with an anti-poly(ADP-ribose) antibody, respectively. S100B significantly decreased glucose consumption, glucose analog uptake, and lactate production in L6 cells, in either the presence or absence of insulin. In contrast, S100B had no effect on glycogen accumulation and insulin signaling. Metabolome analysis revealed that S100B increased the concentration of glycolytic intermediates upstream of GAPDH. S100B impaired GAPDH activity and increased poly(ADP-ribosyl)ated GAPDH proteins. The effects of S100B on glucose metabolism were mostly canceled by a poly(ADP-ribose) polymerase inhibitor. Similar results were obtained in C₂C₁₂ myotube cells. We conclude that S100B as a humoral factor may impair glycolysis in muscle cells independent of insulin action, and the effect may be attributed to the inhibition of GAPDH activity from enhanced poly(ADP-ribosyl)ation of the enzyme.

Structure-function analysis of human sucrase-isomaltase identifies key residues required for catalytic activity

Sucrase-isomaltase (SI) is an intestinal membrane-associated α-glucosidase that breaks down di- and oligosaccharides to absorbable monosaccharides. SI has two homologous functional subunits (sucrase and isomaltase) that both belong to the glycoside hydrolase family 31 (GH31) and differ in substrate specificity. All GH31 enzymes share a consensus sequence harboring an aspartic acid residue as a catalytic nucleophile. Moreover, crystallographic structural analysis of isomaltase predicts that another aspartic acid residue functions as a proton donor in hydrolysis. Here, we mutagenized the predicted proton donor residues and the nucleophilic catalyst residues in each SI subunit. We expressed these SI variants in COS-1 cells and analyzed their structural, transport, and functional characteristics. All of the mutants revealed expression levels and maturation rates comparable with those of the wild-type species and the corresponding nonmutated subunits were functionally active. Thereby we determined rate and substrate specificity for each single subunit without influence from the other subunit. This approach provides a model for functional analysis of the single subunits within a multidomain protein, achieved without the necessity to express the individual subunits separately. Of note, we also found that glucose product inhibition regulates the activities of both SI subunits. We experimentally confirmed the catalytic function of the predicted proton donor residues, and sequence analysis suggested that these residues are located in a consensus region in many GH31 family members. In summary, these findings reveal the kinetic features specific for each human SI subunit and demonstrate that the activities of these subunits are regulated via product inhibition.

Stability of Ensemble Models Predicts Productivity of Enzymatic Systems

Stability in a metabolic system may not be obtained if incorrect amounts of enzymes are used. Without stability, some metabolites may accumulate or deplete leading to the irreversible loss of the desired operating point. Even if initial enzyme amounts achieve a stable steady state, changes in enzyme amount due to stochastic variations or environmental changes may move the system to the unstable region and lose the steady-state or quasi-steady-state flux. This situation is distinct from the phenomenon characterized by typical sensitivity analysis, which focuses on the smooth change before loss of stability. Here we show that metabolic networks differ significantly in their intrinsic ability to attain stability due to the network structure and kinetic forms, and that after achieving stability, some enzymes are prone to cause instability upon changes in enzyme amounts. We use Ensemble Modelling for Robustness Analysis (EMRA) to analyze stability in four cell-free enzymatic systems when enzyme amounts are changed. Loss of stability in continuous systems can lead to lower production even when the system is tested experimentally in batch experiments. The predictions of instability by EMRA are supported by the lower productivity in batch experimental tests. The EMRA method incorporates properties of network structure, including stoichiometry and kinetic form, but does not require specific parameter values of the enzymes.

Long term effects of neonatal hypoglycaemia on pancreatic function

OBJECTIVES

Low blood glucose in neonates predisposes to long term pancreatic damage. We focused on evaluating long term consequences of neonatal hypoglycaemia in pancreatic functions.

METHODS

Pancreatic function was analysed by measuring DNA/protein synthesis, glucose/ATP uptake in vitro. Gene expression of Pdx1, NeuroD1, Pax4, Bax, caspase 3, Beclin1 were done. Muscarinic receptors were analysed by radio receptor assay.

RESULT

Overall pancreatic efficiency was reduced in one-month-old rats exposed to neonatal hypoglycaemia as indicated by decreased DNA/protein synthesis and glucose/ATP uptake in vitro. Both Pdx1 and Neuro D1 expression were significantly down-regulated whereas Pax4 was up-regulated. Up-regulated Bax, caspase 3 and beclin1 along with reduced muscarinic receptors accounts for activation of cell death pathways.

CONCLUSION

The study revealed a drastic reduction in pancreatic functions along with activation of apoptotic factors in one month old rats exposed to neonatal hypoglycaemia.

Hexokinase and not glycogen synthase controls the flux through the glycogen synthesis pathway in frog oocytes

Here we set out to evaluate the role of hexokinase and glycogen synthase in the control of glycogen synthesis in vivo. We used metabolic control analysis (MCA) to determine the flux control coefficient for each of the enzymes involved in the pathway. Acute microinjection experiments in frog oocytes were specifically designed to change the endogenous activities of the enzymes, either by directly injecting increasing amounts of a given enzyme (HK, PGM and UGPase) or by microinjection of a positive allosteric effector (glc-6P for GS). Values of 0.61 ± 0.07, 0.19 ± 0.03, 0.13 ± 0.03, and -0.06 ± 0.08 were obtained for the flux control coefficients of hexokinase EC 2.7.1.1 (HK), phosphoglucomutase EC 5.4.2.1 (PGM), UDPglucose pyrophosphorylase EC 2.7.7.9 (UGPase) and glycogen synthase EC 2.4.1.11 (GS), respectively. These values satisfy the summation theorem since the sum of the control coefficients for all the enzymes of the pathway is 0.87. The results show that, in frog oocytes, glycogen synthesis through the direct pathway is under the control of hexokinase. Phosphoglucomutase and UDPG-pyrophosphorylase have a modest influence, while the control exerted by glycogen synthase is null.

Cooperativity in monomeric enzymes with single ligand-binding sites

Cooperativity is widespread in biology. It empowers a variety of regulatory mechanisms and impacts both the kinetic and thermodynamic properties of macromolecular systems. Traditionally, cooperativity is viewed as requiring the participation of multiple, spatially distinct binding sites that communicate via ligand-induced structural rearrangements; however, cooperativity requires neither multiple ligand binding events nor multimeric assemblies. An underappreciated manifestation of cooperativity has been observed in the non-Michaelis-Menten kinetic response of certain monomeric enzymes that possess only a single ligand-binding site. In this review, we present an overview of kinetic cooperativity in monomeric enzymes. We discuss the primary mechanisms postulated to give rise to monomeric cooperativity and highlight modern experimental methods that could offer new insights into the nature of this phenomenon. We conclude with an updated list of single subunit enzymes that are suspected of displaying cooperativity, and a discussion of the biological significance of this unique kinetic response.

Glycogenolysis in astrocytes supports blood-borne glucose channeling not glycogen-derived lactate shuttling to neurons: evidence from mathematical modeling

In this article, we examined theoretically the role of human cerebral glycogen in buffering the metabolic requirement of a 360-second brain stimulation, expanding our previous modeling study of neurometabolic coupling. We found that glycogen synthesis and degradation affects the relative amount of glucose taken up by neurons versus astrocytes. Under conditions of 175:115 mmol/L (∼1.5:1) neuronal versus astrocytic activation-induced Na(+) influx ratio, ∼12% of astrocytic glycogen is mobilized. This results in the rapid increase of intracellular glucose-6-phosphate level on stimulation and nearly 40% mean decrease of glucose flow through hexokinase (HK) in astrocytes via product inhibition. The suppression of astrocytic glucose phosphorylation, in turn, favors the channeling of glucose from interstitium to nearby activated neurons, without a critical effect on the concurrent intercellular lactate trafficking. Under conditions of increased neuronal versus astrocytic activation-induced Na(+) influx ratio to 190:65 mmol/L (∼3:1), glycogen is not significantly degraded and blood glucose is primarily taken up by neurons. These results support a role for astrocytic glycogen in preserving extracellular glucose for neuronal utilization, rather than providing lactate to neurons as is commonly accepted by the current 'thinking paradigm'. This might be critical in subcellular domains during functional conditions associated with fast energetic demands.

Hypoxia and succinate antagonize 2-deoxyglucose effects on glioblastoma

Glioblastoma multiforme (GBM) are highly proliferative brain tumors characterized by a hypoxic microenvironment which controls GBM stem cell maintenance. Tumor hypoxia promotes also elevated glycolytic rate; thus, limiting glucose metabolism is a potential approach to inhibit tumor growth. Here we investigate the effects mediated by 2-deoxyglucose (2-DG), a glucose analogue, on primary GBM-derived cells maintained under hypoxia. Our results indicate that hypoxia protects GBM cells from the apoptotic effect elicited by 2-DG, which raises succinate dehydrogenase activity thus promoting succinate level decrease. As a consequence hypoxia inducible factor-1α (HIF-1α) degradation occurs and this induces GBM cells to acquire a neuronal committed phenotype. By adding succinate these effects are reverted, as succinate stabilizes HIF-1α and increases GBM stem cell fraction particularly under hypoxia, thus preserving the tumor stem cell niche. 2-DG inhibits anaerobic glycolysis altering GBM cell phenotype by forcing tumor cells into mitochondrial metabolism and by inducing differentiation.

Hexokinase-mitochondrial interaction in cardiac tissue: implications for cardiac glucose uptake, the 18FDG lumped constant and cardiac protection

The hexokinases are fundamental regulators of cardiac glucose uptake; by phosphorylating free intracellular glucose, they maintain the concentration gradient driving myocardial extraction of glucose from the bloodstream. Hexokinases are highly regulated proteins, subject to activation by insulin, hypoxia or ischaemia, and inhibition by their enzymatic product glucose-6-phosphate. In vitro and in many non-cardiac cell types, hexokinases have been shown to bind to the mitochondria, both increasing their phosphorylative capacity, and having a putative role in the anti-apoptotic function of protein kinase B (PKB)/Akt. Whether hexokinase-mitochondrial interaction is a dynamic and responsive process in the heart has been difficult to prove, but there is growing evidence that this association does indeed increase in response to insulin stimulation or ischaemia. In this review I discuss the relevance of hexokinase-mitochondrial interaction to cardiac glycolytic control, our interpretation of (18)FDG cardiac PET scans, and its possible role in protecting the myocardium from ischaemic injury.

Under normoxia, 2-deoxy-D-glucose elicits cell death in select tumor types not by inhibition of glycolysis but by interfering with N-linked glycosylation

In tumor cells growing under hypoxia, inhibiting glycolysis with 2-deoxy-d-glucose (2-DG) leads to cell death, whereas under normoxic conditions cells similarly treated survive. Surprisingly, here we find that 2-DG is toxic in select tumor cell lines growing under normal oxygen tension. In contrast, a more potent glycolytic inhibitor, 2-fluorodeoxy-d-glucose, shows little or no toxicity in these cell types, indicating that a mechanism other than inhibition of glycolysis is responsible for their sensitivity to 2-DG under normoxia. A clue to this other mechanism comes from previous studies in which it was shown that 2-DG interferes with viral N-linked glycosylation and is reversible by exogenous addition of mannose. Similarly, we find that 2-DG interferes with N-linked glycosylation more potently in the tumor cell types that are sensitive to 2-DG under normoxia, which can be reversed by exogenous mannose. Additionally, 2-DG induces an unfolded protein response, including up-regulation of GADD153 (C/EBP-homologous protein), an unfolded protein response-specific mediator of apoptosis, more effectively in 2-DG-sensitive cells. We conclude that 2-DG seems to be toxic in select tumor cell types growing under normoxia by inhibition of N-linked glycosylation and not by glycolysis. Because in a phase I study 2-DG is used in combination with an anticancer agent to target hypoxic cells, our results raise the possibility that in certain cases, 2-DG could be used as a single agent to selectively kill both the aerobic (via interference with glycosylation) and hypoxic (via inhibition of glycolysis) cells of a solid tumor.

Dynamic modulation of intracellular glucose imaged in single cells using a FRET-based glucose nanosensor

To study intracellular glucose homeostasis, the glucose nanosensor FLIPglu-600 microM, which undergoes changes in fluorescence resonance energy transfer (FRET) upon interaction with glucose, was expressed in four mammalian cell lines: COS-7, CHO, HEK293, and C2C12. Upon addition of extracellular glucose, the intracellular FRET ratio decreased rapidly as intracellular glucose increased. The kinetics were fast (tau=5 to 15 s) in COS and C2C12 cells and slow (tau=20 to 40 s) in HEK and CHO cells. Upon removal of extracellular glucose, the FRET ratio returned to its initial value at similar rates (tau=15 to 40 s) in all cell types. In all cell types, the glucose uptake FRET signal was blocked by the glucose transporter (GLUTx) inhibitor cytochalasin B and was not affected by the Na/glucose transporter inhibitor phlorizin. Glucose clearance was inhibited by the glycolytic inhibitor iodoacetate. Using beta-escin to permeabilize the cell, we found that the glucose gradient across the membrane was strongly dependent on the rates of glucose uptake versus glucose clearance. With 10 mM extracellular glucose and a high rate of glucose clearance, intracellular glucose level fell below 100 muM when glucose uptake rate was low, whereas it exceeded 0.5 mM when glucose uptake was high. Cells cultured in high glucose maintained lower basal intracellular glucose levels than cells cultured in low glucose, attributed to "reciprocal regulation" of glycolysis and gluconeogenesis. Basal glucose level also increased with elevated temperatures. Experiments performed with C2C12 cells demonstrated a shift from fast glucose uptake to slow glucose uptake in the absence of insulin during differentiation.

Efficacy of 2-halogen substituted D-glucose analogs in blocking glycolysis and killing "hypoxic tumor cells"

PURPOSE

Since 2-deoxy-D-glucose (2-DG) is currently in phase I clinical trials to selectively target slow-growing hypoxic tumor cells, 2-halogenated D-glucose analogs were synthesized for improved activity. Given the fact that 2-DG competes with D-glucose for binding to hexokinase, in silico modeling of molecular interactions between hexokinase I and these new analogs was used to determine whether binding energies correlate with biological effects, i.e. inhibition of glycolysis and subsequent toxicity in hypoxic tumor cells.

METHODS AND RESULTS

Using a QSAR-like approach along with a flexible docking strategy, it was determined that the binding affinities of the analogs to hexokinase I decrease as a function of increasing halogen size as follows: 2-fluoro-2-deoxy-D-glucose (2-FG) > 2-chloro-2-deoxy-D-glucose (2-CG) > 2-bromo-2-deoxy-D-glucose (2-BG). Furthermore, D-glucose was found to have the highest affinity followed by 2-FG and 2-DG, respectively. Similarly, flow cytometry and trypan blue exclusion assays showed that the efficacy of the halogenated analogs in preferentially inhibiting growth and killing hypoxic vs. aerobic cells increases as a function of their relative binding affinities. These results correlate with the inhibition of glycolysis as measured by lactate inhibition, i.e. ID50 1 mM for 2-FG, 6 mM for 2-CG and > 6 mM for 2-BG. Moreover, 2-FG was found to be more potent than 2-DG for both glycolytic inhibition and cytotoxicity.

CONCLUSIONS

Overall, our in vitro results suggest that 2-FG is more potent than 2-DG in killing hypoxic tumor cells, and therefore may be more clinically effective when combined with standard chemotherapeutic protocols.

Glucose 6-phosphate release of wild-type and mutant human brain hexokinases from mitochondria

One molecule of glucose 6-phosphate inhibits brain hexokinase (HKI) with high affinity by binding to either one of two sites located in distinct halves of the enzyme. In addition to potent inhibition, glucose 6-phosphate releases HKI from the outer leaflet of mitochondria; however, the site of glucose 6-phosphate association responsible for the release of HKI is unclear. The incorporation of a C-terminal polyhistidine tag on HKI facilitates the rapid purification of recombinant enzyme from Escherichia coli. The tagged construct has N-formyl methionine as its first residue and has mitochondrial association properties comparable with native brain hexokinases. Release of wild-type and mutant hexokinases from mitochondria by glucose 6-phosphate follow equilibrium models, which explain the release phenomenon as the repartitioning of ligand-bound HKI between solution and the membrane. Mutations that block the binding of glucose 6-phosphate to the C-terminal half of HKI have little or no effect on the glucose 6-phosphate release. In contrast, mutations that block glucose 6-phosphate binding to the N-terminal half require approximately 7-fold higher concentrations of glucose 6-phosphate for the release of HKI. Results here implicate a primary role for the glucose 6-phosphate binding site at the N-terminal half of HKI in the release mechanism.

The cellular fate of glucose and its relevance in type 2 diabetes

Type 2 diabetes is a complex disorder with diminished insulin secretion and insulin action contributing to the hyperglycemia and wide range of metabolic defects that underlie the disease. The contribution of glucose metabolic pathways per se in the pathogenesis of the disease remains unclear. The cellular fate of glucose begins with glucose transport and phosphorylation. Subsequent pathways of glucose utilization include aerobic and anaerobic glycolysis, glycogen formation, and conversion to other intermediates in the hexose phosphate or hexosamine biosynthesis pathways. Abnormalities in each pathway may occur in diabetic subjects; however, it is unclear whether perturbations in these may lead to diabetes or are a consequence of the multiple metabolic abnormalities found in the disease. This review is focused on the cellular fate of glucose and relevance to human type 2 diabetes.

Photoaffinity labeling on magnetic microspheres (PALMm) methodology for topographic mapping: preparation of PALMm reagents and demonstration of biochemical relevance

Photoaffinity labeling (PAL) is a technique widely used for identifying the binding-site within proteins. Although the classic method is both versatile and powerful, it suffers significant disadvantages, such as the need to radiolabel the PAL ligand, and the need to conduct highly complicated separations of both the labeled protein and the labeled peptides derived from it. Here, we propose a novel and universal methodology--Photo-Affinity Labeling on Magnetic microspheres (PALMm) designed to simplify and shorten the PAL protocol. In this context, we describe the preparation of PALMm reagents and the evaluation of their biochemical relevance regarding two ATP-binding enzymes: hexokinase and apyrase.

Kinetic and regulatory properties of HK I(+), a modified form of the type I isozyme of mammalian hexokinase in which interactions between the N- and C-terminal halves have been disrupted

A modified form (HK I(+)) of rat Type I hexokinase (HK I) has been expressed. HK I(+) contains a centrally located polyalanine insert which, along with the known helical propensity of adjacent sequence, was expected to lead to alpha-helix formation, with resulting distension of the molecule and disruption of interactions between the N- and C-terminal halves. The properties of HK I(+) are consistent with this expectation and with previous proposals that (1) inhibition of HK I by Glc-6-P or its analogs and antagonism of this inhibition by P(i) result from competition of these ligands for a binding site in the N-terminal half of HK I, with resulting conformational changes propagated through interactions with the catalytic C-terminal half, and (2) binding of Glc-6-P to a site in the C-terminal half of HK I is obstructed by interactions between the halves, present in HK I but not HK I(+).

Glucose sensing in pancreatic beta-cells: a model for the study of other glucose-regulated cells in gut, pancreas, and hypothalamus

Nutrient homeostasis is known to be regulated by pancreatic islet tissue. The function of islet beta-cells is controlled by a glucose sensor that operates at physiological glucose concentrations and acts in synergy with signals that integrate messages originating from hypothalamic neurons and endocrine cells in gut and pancreas. Evidence exists that the extrapancreatic cells producing and secreting these (neuro)endocrine signals also exhibit a glucose sensor and an ability to integrate nutrient and (neuro)hormonal messages. Similarities in these cellular and molecular pathways provide a basis for a network of coordinated functions between distant cell groups, which is necessary for an appropriate control of nutrient homeostasis. The glucose sensor seems to be a fundamental component of these control mechanisms. Its molecular characterization is most advanced in pancreatic beta-cells, with important roles for glucokinase and mitochondrial oxidative fluxes in the regulation of ATP-sensitive K+ channels. Other glucose-sensitive cells in the endocrine pancreas, hypothalamus, and gut were found to share some of these molecular characteristics. We propose that similar metabolic signaling pathways influence the function of pancreatic alpha-cells, hypothalamic neurons, and gastrointestinal endocrine and neural cells.

Crystal structures of mutant monomeric hexokinase I reveal multiple ADP binding sites and conformational changes relevant to allosteric regulation

Hexokinase I, the pacemaker of glycolysis in brain tissue, is composed of two structurally similar halves connected by an alpha-helix. The enzyme dimerizes at elevated protein concentrations in solution and in crystal structures; however, almost all published data reflect the properties of a hexokinase I monomer in solution. Crystal structures of mutant forms of recombinant human hexokinase I, presented here, reveal the enzyme monomer for the first time. The mutant hexokinases bind both glucose 6-phosphate and glucose with high affinity to their N and C-terminal halves, and ADP, also with high affinity, to a site near the N terminus of the polypeptide chain. Exposure of the monomer crystals to ADP in the complete absence of glucose 6-phosphate reveals a second binding site for adenine nucleotides at the putative active site (C-half), with conformational changes extending 15 A to the contact interface between the N and C-halves. The structures reveal distinct conformational states for the C-half and a rigid-body rotation of the N-half, as possible elements of a structure-based mechanism for allosteric regulation of catalysis.