Metabolic concomitants in pure, pancreatic beta cells during glucose-stimulated insulin secretion

The role of the redox potential in insulin secretion by beta cells stimulated with high glucose was investigated using an in vitro pancreas perfusion system. To assess glycolytic flux the sum of fructose-1,6-P2 + triose-P was determined in pure beta cells microdissected from lyophilized sections of the isolated perfused pancreas quick frozen during the early insulin secretory response. L-Glycerol 3-phosphate and dihydroxyacetone phosphate were measured as indicators of the free cytosolic [NAD+]/[NADH] ratio and NADH and NADPH were also measured. Fructose-1,6-P2 + triose-P was increased in beta cells simultaneously with the onset of insulin secretion indicating an increase in glucose metabolism had occurred. The ratio of [dihydroxyacetone phosphate]/[L-glycerol 3-phosphate] increased simultaneously with the onset of insulin secretion. NADH content increased only after initiation of insulin secretion and NADPH levels remained unchanged during the early secretory response to high glucose. These data contradict the hypothesis that insulin secretion is triggered by a more reduced cytosolic redox state and instead indicate that insulin secretion is initiated by other metabolic coupling factor(s) generated in beta cells stimulated by high glucose.     

Perspective: emerging evidence for signaling roles of mitochondrial anaplerotic products in insulin secretion

The importance of mitochondrial biosynthesis in stimulus secretion coupling in the insulin-producing beta-cell probably equals that of ATP production. In glucose-induced insulin secretion, the rate of pyruvate carboxylation is very high and correlates more strongly with the glucose concentration the beta-cell is exposed to (and thus with insulin release) than does pyruvate decarboxylation, which produces acetyl-CoA for metabolism in the citric acid cycle to produce ATP. The carboxylation pathway can increase the levels of citric acid cycle intermediates, and this indicates that anaplerosis, the net synthesis of cycle intermediates, is important for insulin secretion. Increased cycle intermediates will alter mitochondrial processes, and, therefore, the synthesized intermediates must be exported from mitochondria to the cytosol (cataplerosis). This further suggests that these intermediates have roles in signaling insulin secretion. Although evidence is quite good that all physiological fuel secretagogues stimulate insulin secretion via anaplerosis, evidence is just emerging about the possible extramitochondrial roles of exported citric acid cycle intermediates. This article speculates on their potential roles as signaling molecules themselves and as exporters of equivalents of NADPH, acetyl-CoA and malonyl-CoA, as well as alpha-ketoglutarate as a substrate for hydroxylases. We also discuss the "succinate mechanism," which hypothesizes that insulin secretagogues produce both NADPH and mevalonate. Finally, we discuss the role of mitochondria in causing oscillations in beta-cell citrate levels. These parallel oscillations in ATP and NAD(P)H. Oscillations in beta-cell plasma membrane electrical potential, ATP/ADP and NAD(P)/NAD(P)H ratios, and glycolytic flux are known to correlate with pulsatile insulin release. Citrate oscillations might synchronize oscillations of individual mitochondria with one another and mitochondrial oscillations with oscillations in glycolysis and, therefore, with flux of pyruvate into mitochondria. Thus citrate oscillations may synchronize mitochondrial ATP production and anaplerosis with other cellular oscillations.     

What couples glycolysis to mitochondrial signal generation in glucose-stimulated insulin secretion?

Pancreatic islet beta-cells are poised to generate metabolic messengers in the mitochondria that link glucose metabolism to insulin exocytosis. This is accomplished through the tight coupling of glycolysis to mitochondrial activation. The messenger molecules ATP and glutamate are produced after the metabolism of glycolysis-derived pyruvate in the mitochondria. The entry of monocarboxylates such as pyruvate into the beta cell is limited, explaining why overexpression of monocarboxylate transporters unravels pyruvate-stimulated insulin secretion. NADH generated by glycolysis is efficiently reoxidized by highly active mitochondrial shuttles rather than by lactate dehydrogenase. Overexpression of this enzyme does not alter glucose-stimulated insulin secretion, suggesting that NADH availability restricts the conversion of pyruvate to lactate in the beta cell. These metabolic features permit the fuel function of glucose to be extended to the generation of signaling molecules, which increases cytosolic Ca2+ and promotes insulin exocytosis.     

Mouse lacking NAD+-linked glycerol phosphate dehydrogenase has normal pancreatic beta cell function but abnormal metabolite pattern in skeletal muscle

We surveyed the BALB/cHeA mouse, which lacks cytosolic glycerol phosphate dehydrogenase an enzyme that catalyzes a reaction in the glycerol phosphate shuttle. The other enzyme of this shuttle, mitochondrial glycerol phosphate dehydrogenase, is abundant in skeletal muscle and pancreatic islets suggesting that the shuttle's activity is high in these tissues. Levels of glycerol phosphate (low) and dihydroxyacetone phosphate (high) were very abnormal in nonislet tissue, especially in skeletal muscle. Intermediates situated before the triose phosphates in the glycolysis pathway were increased and those after the triose phosphates were generally low, depending on the tissue. The lactate/pyruvate ratio in muscle was low signifying a low cytosolic NAD/NADH ratio. This suggests that a nonfunctional glycerol phosphate shuttle caused a block in glycolysis at the step catalyzed by glyceraldehyde phosphate dehydrogenase. When exercised, mice were unable to maintain normal ATP levels in skeletal muscle. Blood glucose, serum insulin levels, and pancreatic islet mass were normal. In isolated pancreatic islets insulin release, glucose metabolism and ATP levels were normal, but lactate levels and lactate/pyruvate ratios with a glucose load were slightly abnormal. The BALB/cHeA mouse can maintain NAD/ NADH ratios sufficient to function normally under most conditions, but the redox state is not normal. Glycerol phosphate is apparently formed at a slow rate. Skeletal muscle is severely affected probably because it is dependent on the glycerol phosphate shuttle more than other tissues. It most likely utilizes glycerol phosphate rapidly and, due to the absence of glycerol kinase in muscle, is unable to rapidly form glycerol phosphate from glycerol. Glycerol kinase is also absent in the pancreatic insulin cell, but this cell's function is essentially normal probably because of redundancy of NAD(H) shuttles.     

Overcoming the metabolic burden of protein secretion in Schizosaccharomyces pombe – A quantitative approach using 13C-based metabolic flux analysis

Protein secretion in yeast is generally associated with a burden to cellular metabolism. To investigate this metabolic burden in Schizosaccharomyces pombe, we constructed a set of strains secreting the model protein maltase in different amounts. We quantified the influence of protein secretion on the metabolism applying ¹³C-based metabolic flux analysis in chemostat cultures. Analysis of the macromolecular biomass composition revealed an increase in cellular lipid content at elevated levels of protein secretion and we observed altered metabolic fluxes in the pentose phosphate pathway, the TCA cycle, and around the pyruvate node including mitochondrial NADPH supply. Supplementing acetate to glucose or glycerol minimal media was found to improve protein secretion, accompanied by an increased cellular lipid content and carbon flux through the TCA cycle as well as increased mitochondrial NADPH production. Thus, systematic metabolic analyses can assist in identifying factors limiting protein secretion and in deriving strategies to overcome these limitations.     

Impaired anaplerosis and insulin secretion in insulinoma cells caused by small interfering RNA-mediated suppression of pyruvate carboxylase

Anaplerosis, the synthesis of citric acid cycle intermediates, by pancreatic beta cell mitochondria has been proposed to be as important for insulin secretion as mitochondrial energy production. However, studies designed to lower the rate of anaplerosis in the beta cell have been inconclusive. To test the hypothesis that anaplerosis is important for insulin secretion, we lowered the activity of pyruvate carboxylase (PC), the major enzyme of anaplerosis in the beta cell. Stable transfection of short hairpin RNA was used to generate a number of INS-1 832/13-derived cell lines with various levels of PC enzyme activity that retained normal levels of control enzymes, insulin content, and glucose oxidation. Glucose-induced insulin release was decreased in proportion to the decrease in PC activity. Insulin release in response to pyruvate alone, 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) plus glutamine, or methyl succinate plus beta-hydroxybutyrate was also decreased in the PC knockdown cells. Consistent with a block at PC, the most PC-deficient cells showed a metabolic crossover point at PC with increased basal and/or glucose-stimulated pyruvate plus lactate and decreased malate and citrate. In addition, in BCH plus glutamine-stimulated PC knockdown cells, pyruvate plus lactate was increased, whereas citrate was severely decreased, and malate and aspartate were slightly decreased. The incorporation of 14C into lipid from [U-14C]glucose was decreased in the PC knockdown cells. The results confirm the central importance of PC and anaplerosis to generate metabolites from glucose that support insulin secretion and even suggest PC is important for insulin secretion stimulated by noncarbohydrate insulin secretagogues.     

Inhibition of the glycolytic pathway by methylglyoxal in human platelets

The incubation of human platelets with methylglyoxal and glucose produces a rapid transformation of the ketoaldehyde to D-lactate by the glyoxalase system and a partial reduction in GSH. Glucose utilization is affected at the level of the glycolytic pathway. No effect of the ketoaldehyde on glycogenolysis and glucose oxidation through the hexose monophosphate shunt was demonstrated. Phosphofructokinase, fructose 1,6 diphosphate (F1, 6DP) aldolase, glyceraldehyde 3-phosphate dehydrogenase and 3-phosphoglycerate mutase were mostly inhibited by methylglyoxal. A decrease in lactate and pyruvate formation and an accumulation of some glycolytic intermediates (fructose 1,6 diphosphate, dihydroxyacetone phosphate, 3-phosphoglycerate) was observed. Moreover methylglyoxal induced a fall in the metabolic ATP concentration. Since methylglyoxal is an intermediate of the glycolytic bypass system from dihydroxyacetone phosphate to D-lactate, it may be assumed that ketoaldehyde exerts a regulating effect on triose metabolism.     

Metabolomic analyses reveal profound differences in glycolytic and tricarboxylic acid cycle metabolism in glucose-responsive and -unresponsive clonal β-cell lines

Insulin secretion from pancreatic β-cells is controlled by complex metabolic and energetic changes provoked by exposure to metabolic fuels. Perturbations in these processes lead to impaired insulin secretion, the ultimate cause of T2D (Type 2 diabetes). To increase our understanding of stimulus-secretion coupling and metabolic processes potentially involved in the pathogenesis of T2D, a comprehensive investigation of the metabolic response in the glucose-responsive INS-1 832/13 and glucose-unresponsive INS-1 832/2 β-cell lines was performed. For this metabolomics analysis, we used GC/MS (gas chromatography/mass spectrometry) combined with multivariate statistics. We found that perturbed secretion in the 832/2 line was characterized by disturbed coupling of glycolytic and TCA (tricarboxylic acid)-cycle metabolism. The importance of this metabolic coupling was reinforced by our observation that insulin secretion partially could be reinstated by stimulation of the cells with mitochondrial fuels which bypass glycolytic metabolism. Furthermore, metabolic and functional profiling of additional β-cell lines (INS-1, INS-1 832/1) confirmed the important role of coupled glycolytic and TCA-cycle metabolism in stimulus-secretion coupling. Dependence of the unresponsive clones on glycolytic metabolism was paralleled by increased stabilization of HIF-1α (hypoxia-inducible factor 1α). The relevance of a similar perturbation for human T2D was suggested by increased expression of HIF-1α target genes in islets from T2D patients.     

Increased Glucose Metabolism and Glycerolipid Formation by Fatty Acids and GPR40 Receptor Signaling Underlies the Fatty Acid Potentiation of Insulin Secretion

Acute fatty acid (FA) exposure potentiates glucose-stimulated insulin secretion in β cells through metabolic and receptor-mediated effects. We assessed the effect of fatty acids on the dynamics of the metabolome in INS-1 cells following exposure to [U-¹³C]glucose to assess flux through metabolic pathways. Metabolite profiling showed a fatty acid-induced increase in long chain acyl-CoAs that were rapidly esterified with glucose-derived glycerol-3-phosphate to form lysophosphatidic acid, mono- and diacylglycerols, and other glycerolipids, some implicated in augmenting insulin secretion. Glucose utilization and glycolytic flux increased, along with a reduction in the NADH/NAD⁺ ratio, presumably by an increase in conversion of dihydroxyacetone phosphate to glycerol-3-phosphate. The fatty acid-induced increase in glycolysis also resulted in increases in tricarboxylic cycle flux and oxygen consumption. Inhibition of fatty acid activation of FFAR1/GPR40 by an antagonist decreased glycerolipid formation, attenuated fatty acid increases in glucose oxidation, and increased mitochondrial FA flux, as evidenced by increased acylcarnitine levels. Conversely, FFAR1/GPR40 activation in the presence of low FA increased flux into glycerolipids and enhanced glucose oxidation. These results suggest that, by remodeling glucose and lipid metabolism, fatty acid significantly increases the formation of both lipid- and TCA cycle-derived intermediates that augment insulin secretion, increasing our understanding of mechanisms underlying β cell insulin secretion.     

Effects of insulin secretagogues on the secretion of insulin during thiamine deficiency

The mechanism by which glucose and other nutrient secretagogues induce the insulin secretion, is still controversial. Thiamine deficient rats, having a block in the glucose and branched chain amino acid metabolism at pyruvate and branched chain keto acids dehydrogenases respectively, were used to study the effects of insulin secretagogues. The levels of fasting blood glucose and serum insulin were estimated. Also, the serum insulin was assayed after intravenous administration of leucine, arginine and tolbutamide. The fasting blood glucose was increased and the serum insulin was decreased in thiamine deficiency. Leucine and arginine did not enhance insulin secretion in thiamine deficient animals. Tolbutamide induces the insulin secretion minimally in thiamine deficient rats. These results suggest that the nutrient secretagogues require an unimpaired glucose metabolism to induce insulin secretion.     

Nutritional regulation of insulin secretion: implications for diabetes

Pancreatic β-cells are exquisitely organised to continually monitor and respond to dietary nutrients, under the modulation of additional neurohormonal signals, in order to secrete insulin to best meet the needs of the organism. β-cell nutrient sensing requires complex mechanisms of metabolic activation, resulting in production of stimulus-secretion coupling signals that promote insulin biosynthesis and release. The primary stimulus for insulin secretion is an elevation in blood glucose concentration and β-cells are particularly responsive to this important nutrient secretagogue via the tight regulation of glycolytic and mitochondrial pathways at steps such as glucokinase, pyruvate dehydrogenase, pyruvate carboxylase, glutamate dehydrogenase and mitochondrial redoxshuttles. With respect to development of type-2 diabetes (T2DM), it is important to consider individual effects of different classes of nutrient or other physiological or pharmacological agents on metabolism and insulin secretion and to also acknowledge and examine the interplay between glucose metabolism and that of the two other primary nutrient classes, amino acids (such as arginine and glutamine) and fatty acids. It is the mixed nutrient sensing and outputs of glucose, amino and fatty acid metabolism that generate the metabolic coupling factors (MCFs) essential for signalling for insulin exocytosis. Primary MCFs in the β-cell include ATP, NADPH, glutamate, long chain acyl coenzyme A and diacylglycerol. It is the failure to generate MCFs in a coordinated manner and at sufficient levels that underlies the failure of β-cell secretion during the pathogenesis of T2DM.     

Differential Acute and Long Term Actions of Succinic Acid Monomethyl Ester Exposure on Insulin-Secreting BRIN-BD11 Cells

Esters of succinic acid are potent insulin secretagogues, and have been proposed as novel antidiabetic agents for type 2 diabetes. This study examines the effects of acute and chronic exposure to succinic acid monomethyl ester (SAM) on insulin secretion, glucose metabolism and pancreatic beta cell function using the BRIN-BD11 cell line. SAM stimulated insulin release in a dose-dependent manner at both non-stimulatory (1.1mM) and stimulatory (16.7mM) glucose. The depolarizing actions of arginine also stimulated a significant increase in SAM-induced insulin release but 2-ketoisocaproic acid (KIC) inhibited SAM induced insulin secretion indicating a possible competition between the preferential oxidative metabolism of these two agents. Prolonged (18hour) exposure to SAM revealed decreases in the insulin-secretory responses to glucose, KIC, glyceraldehyde and alanine. Furthermore, SAM diminished the effects of nonmetabolized secretagogues arginine and 3-isobutyl-1-methylxanthine (IBMX). While the ability of BRIN-BD11 cells to oxidise glucose was unaffected by SAM culture, glucose utilization was substantially reduced. Collectively, these data suggest that while SAM may enhance the secretory potential of non-metabolized secretagogues, it may also serve as a preferential metabolic fuel in preference to other important physiological nutrients and compromise pancreatic beta cell function following prolonged exposure.     

Metabolic effects of D-glyceraldehyde in isolated hepatocytes.

The effects of D-glyceraldehyde on the hepatocyte contents of various metabolites were examined and compared with the effects of fructose, glycerol and dihydroxyacetone, which all enter the glycolytic/gluconeogenic pathways at the triose phosphate level. D-Glyceraldehyde (10 MM) caused a substantial depletion of hepatocyte ATP, as did equimolar concentrations of fructose and glycerol. D-Glyceraldehyde and fructose each caused a 2-fold increase in fructose 1,6-bisphosphate and the accumulation of millimolar quantities of fructose 1-phosphate in the cells. D-Glyceraldehyde caused an increase in the glycerol 3-phosphate content and a decrease in the dihydroxyacetone phosphate content, whereas dihydroxyacetone increased the content of both metabolites. The increase in the [glycerol 3-phosphate]/[dihydroxyacetone phosphate] ratio caused by D-glyceraldehyde was not accompanied by a change in the cytoplasmic [NAD+]/[NADH] ratio, as indicated by the unchanged [lactate]/[pyruvate] ratio. The accumulation of fructose 1-phosphate from D-glyceraldehyde and dihydroxyacetone phosphate in the hepatocyte can account for the depletion of the intracellular content of the latter. Presumably ATP is depleted as the result of the accumulation of millimolar amounts of a phosphorylated intermediate, as is the case with fructose and glycerol. It is suggested that the accumulation of fructose 1-phosphate during hepatic fructose metabolism is the result of a temporary increase in the D-glyceraldehyde concentration because of the high rate of fructose phosphorylation compared with triokinase activity. The equilibrium constant of aldolase favours the formation and thus the accumulation of fructose 1-phosphate.     

ATP-ADP-dependent phosphorylations of glycolysis metabolites, creatine and glycerol: their compartition and thermodynamic relationship in gastrocnemius muscle cell of exercised guinea pigs

The concentrations of following metabolites were determined in freeze-clamped gastrocnemius muscle samples: glucose 1-phosphate, glucose 6-phosphate, glucose, fructose 1,6-diphosphate, fructose 6-phosphate, D-glyceraldehyde 3-phosphate. dihydroxyacetone phosphate, phosphoenolpyruvate, pyruvate, glycerol 3-phosphate, glycerol, creatine phosphate, creatine, glycerate 3-phosphate, glycerate 2-phosphate, adenosine monophosphate, adenosine diphosphate, adenosine triphosphate, inorganic phosphate. The results showed that within the limits of experimental error, concentration homeostasis for this metabolites is founded at least in some cases on equilibria between enzymic transformations. Discrepancies between constant mass ratios measured in this study and equilibrium constants allow the free energy variation delta G to keep creatine phosphate at high concentration to be calculated. For the phosphoglycerate mutase system, the equilibrium constant in controls and trained animals is unchanged and corresponds to that in vitro. Training hindered glycolysis and favoured phosphorylation of creatine by glycerol 3-phosphate. Metabolites of the pyruvate kinase and hexokinase system cannot be homogeneously distributed in one space. The creatine kinase system is also separated from the hexokinase und pyruvate kinase system. A compartition of glycolytic process in gastrocnemius muscle seems to be inferred from these results.     

NADH shuttle system regulates K(ATP) channel-dependent pathway and steps distal to cytosolic Ca(2+) concentration elevation in glucose-induced insulin secretion.

The NADH shuttle system is composed of the glycerol phosphate and malate-aspartate shuttles. We generated mice that lack mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH), a rate-limiting enzyme of the glycerol phosphate shuttle. Application of aminooxyacetate, an inhibitor of the malate-aspartate shuttle, to mGPDH-deficient islets demonstrated that the NADH shuttle system was essential for coupling glycolysis with activation of mitochondrial ATP generation to trigger glucose-induced insulin secretion. The present study revealed that blocking the NADH shuttle system severely suppressed closure of the ATP-sensitive potassium (K(ATP)) channel and depolarization of the plasma membrane in response to glucose in beta cells, although properties of the K(ATP) channel on the excised beta cell membrane were unaffected. In mGPDH-deficient islets treated with aminooxyacetate, Ca(2+) influx through the plasma membrane induced by a depolarizing concentration of KCl in the presence of the K(ATP) channel opener diazoxide restored insulin secretion. However, the level of the secretion was only approximately 40% of wild-type controls. Thus, glucose metabolism through the NADH shuttle system leading to efficient ATP generation is pivotal to activation of both the K(ATP) channel-dependent pathway and steps distal to an elevation of cytosolic Ca(2+) concentration in glucose-induced insulin secretion.     

Uncoupling protein-2 negatively regulates insulin secretion and is a major link between obesity, beta cell dysfunction, and type 2 diabetes.

beta cells sense glucose through its metabolism and the resulting increase in ATP, which subsequently stimulates insulin secretion. Uncoupling protein-2 (UCP2) mediates mitochondrial proton leak, decreasing ATP production. In the present study, we assessed UCP2's role in regulating insulin secretion. UCP2-deficient mice had higher islet ATP levels and increased glucose-stimulated insulin secretion, establishing that UCP2 negatively regulates insulin secretion. Of pathophysiologic significance, UCP2 was markedly upregulated in islets of ob/ob mice, a model of obesity-induced diabetes. Importantly, ob/ob mice lacking UCP2 had restored first-phase insulin secretion, increased serum insulin levels, and greatly decreased levels of glycemia. These results establish UCP2 as a key component of beta cell glucose sensing, and as a critical link between obesity, beta cell dysfunction, and type 2 diabetes.     

Substrate effects on oscillations in metabolism, calcium and secretion in single mouse islets of Langerhans

Glucose induces complex patterns of oscillations in intracellular Ca2+ concentration ([Ca2+]i), metabolism and secretion in islets of Langerhans including "slow" and "fast" pulses with period of 2-5 min and 10-20 s respectively. In an effort to elucidate the origin of slow oscillations, individual mouse islets were exposed to different fuels including glyceraldehyde, pyruvate, methyl pyruvate and alpha-ketoisocaproate (KIC), all of which bypass key steps of glycolytic metabolism, while monitoring [Ca2+]i, oxygen consumption and secretion. Glyceraldehyde gave rise to slow oscillations only when substimulatory glucose was also added to the media. Glucosamine, an inhibitor of glucokinase, blocked these slow oscillations. KIC, pyruvate, and methyl pyruvate did not give rise to slow oscillations alone or with glucose present. The addition of glucose to islets bathed in nutrient-rich cell culture media accelerated metabolism and initiated slow oscillations while glyceraldehyde did not. It is concluded that glucose has a special role in accelerating metabolism and generating slow oscillations in isolated islets of Langerhans from mice. Combined with previous observations of Ca2+ dependency for all oscillations in islets, we propose that interactions between Ca2+ influx and glycolysis are responsible for the slow oscillations. In contrast, fast oscillations can occur independent of glycolytic flux.     

Does high-sucrose diet alter skeletal muscle and liver mitochondrial respiration?

A diet high in sucrose or fructose progressively impairs glucose and lipid metabolism, which leads to insulin resistance. As mitochondria are the sites of the oxidation and utilization of these substrates, we hypothesized that a high sucrose diet would alter mitochondrial respiration. Male Wistar rats were fed high-sucrose (SU) or control (CTL) diet for one week; mitochondrial respiration was investigated in mitochondria isolated from liver and both glycolytic and oxidative muscles, with pyruvate and palmitate as substrates. To test for metabolic disturbances, we measured not only glycogen content in muscles and liver, but also lactate, glucose and triglyceride blood concentrations. After one week of high-sucrose intake, we found no change in blood concentration of these variables, but glycogen content was significantly increased in liver (17.28 +/- 2.98 mg/g tissue SU vs 6.47 +/- 1.67 mg/g tissue CTL), oxidative muscle (1.59 +/- 0.21 mg/g tissue SU vs 0.70 +/- 0.24 mg/g tissue CTL) though not in glycolytic muscle (1.72 +/- 0.44 mg/g tissue SU vs 1.52 +/- 0.20 mg/g tissue CTL). State 3 mitochondrial respiration was significantly decreased in SU rats compared with CTL (p < 0.05) with pyruvate, while no change was observed with palmitate. This study shows that 1-week of high-sucrose diet altered mitochondrial pyruvate oxidation in rats and suggests that, in the context of a high-sucrose diet, impaired mitochondrial respiration could contributed to the development of insulin resistance.     

Thematic Review Series: Calorie Restriction and Ketogenic Diets: The collective therapeutic potential of cerebral ketone metabolism in traumatic brain injury

The postinjury period of glucose metabolic depression is accompanied by adenosine triphosphate decreases, increased flux of glucose through the pentose phosphate pathway, free radical production, activation of poly-ADP ribose polymerase via DNA damage, and inhibition of glyceraldehyde dehydrogenase (a key glycolytic enzyme) via depletion of the cytosolic NAD pool. Under these post-brain injury conditions of impaired glycolytic metabolism, glucose becomes a less favorable energy substrate. Ketone bodies are the only known natural alternative substrate to glucose for cerebral energy metabolism. While it has been demonstrated that other fuels (pyruvate, lactate, and acetyl-L-carnitine) can be metabolized by the brain, ketones are the only endogenous fuel that can contribute significantly to cerebral metabolism. Preclinical studies employing both pre- and postinjury implementation of the ketogenic diet have demonstrated improved structural and functional outcome in traumatic brain injury (TBI) models, mild TBI/concussion models, and spinal cord injury. Further clinical studies are required to determine the optimal method to induce cerebral ketone metabolism in the postinjury brain, and to validate the neuroprotective benefits of ketogenic therapy in humans.