Uncovering the Role of Glycogen in Brown Adipose Tissue.

The presence of glycogen in the brown adipose tissue (BAT) has been described 60 years ago. However, the role of this energetic storage in brown adipocytes has been long time underestimated. We have recently shown that during brown adipocyte differentiation in the embryo, glycogen accumulates and is degraded by glycophagy, a dynamic essential for lipid droplets biogenesis. Recent studies have shown that the storage and degradation of triglycerides in BAT are not essential for the activation of BAT in response to cold exposure in adults, and that glycogen can compensate for their absence. In this review, we report the recent advances related to the importance of glycogen in brown adipocytes.

Gap junctions coordinate the propagation of glycogenolysis induced by norepinephrine in the pineal gland.

Chemical and electrical synapses are the two major communication systems that permit cell-to-cell communication within the nervous system. Although most studies are focused on chemical synapses (glutamate, γ-aminobutyric acid, and other neurotransmitters), clearly both types of synapses interact and cooperate to allow the coordination of several cell functions within the nervous system. The pineal gland has limited independent axonal innervation and not every cell has access to nerve terminals. Thus, additional communication systems, such as gap junctions, have been postulated to coordinate metabolism and signaling. Using acutely isolated glands and dissociated cells, we found that gap junctions spread glycogenolytic signals from cells containing adrenoreceptors to the entire gland lacking these receptors. Our data using glycogen and lactate quantification, electrical stimulation, and high-performance liquid chromatography with electrochemical detection, demonstrate that gap junctional communication between cells of the rat pineal gland allows cell-to-cell propagation of norepinephrine-induced signal that promotes glycogenolysis throughout the entire gland. Thus, the interplay of both synapses is essential for coordinating glycogen metabolism and lactate production in the pineal gland.

Norepinephrine stimulates glycogenolysis in astrocytes to fuel neurons with lactate.

The mechanism of rapid energy supply to the brain, especially to accommodate the heightened metabolic activity of excited states, is not well-understood. We explored the role of glycogen as a fuel source for neuromodulation using the noradrenergic stimulation of glia in a computational model of the neural-glial-vasculature ensemble (NGV). The detection of norepinephrine (NE) by the astrocyte and the coupled cAMP signal are rapid and largely insensitive to the distance of the locus coeruleus projection release sites from the glia, implying a diminished impact for volume transmission in high affinity receptor transduction systems. Glucosyl-conjugated units liberated from glial glycogen by NE-elicited cAMP second messenger transduction winds sequentially through the glycolytic cascade, generating robust increases in NADH and ATP before pyruvate is finally transformed into lactate. This astrocytic lactate is rapidly exported by monocarboxylate transporters to the associated neuron, demonstrating that the astrocyte-to-neuron lactate shuttle activated by glycogenolysis is a likely fuel source for neuromodulation and enhanced neural activity. Altogether, the energy supply for both astrocytes and neurons can be supplied rapidly by glycogenolysis upon neuromodulatory stimulus.

The miR-379/miR-410 cluster at the imprinted Dlk1-Dio3 domain controls neonatal metabolic adaptation.

In mammals, birth entails complex metabolic adjustments essential for neonatal survival. Using a mouse knockout model, we identify crucial biological roles for the miR-379/miR-410 cluster within the imprinted Dlk1-Dio3 region during this metabolic transition. The miR-379/miR-410 locus, also named C14MC in humans, is the largest known placental mammal-specific miRNA cluster, whose 39 miRNA genes are expressed only from the maternal allele. We found that heterozygote pups with a maternal--but not paternal--deletion of the miRNA cluster display partially penetrant neonatal lethality with defects in the maintenance of energy homeostasis. This maladaptive metabolic response is caused, at least in part, by profound changes in the activation of the neonatal hepatic gene expression program, pointing to as yet unidentified regulatory pathways that govern this crucial metabolic transition in the newborn's liver. Not only does our study highlight the physiological importance of miRNA genes that recently evolved in placental mammal lineages but it also unveils additional layers of RNA-mediated gene regulation at the Dlk1-Dio3 domain that impose parent-of-origin effects on metabolic control at birth and have likely contributed to mammal evolution.

[Refeeding syndrome in geriatric patients : A frequently overlooked complication]. / Refeeding-Syndrom bei geriatrischen Patienten : Eine oft übersehene Komplikation.

The refeeding syndrome is a life-threatening complication that can occur after initiation of a nutrition therapy in malnourished patients. If the risk factors and pathophysiology are known, the refeeding syndrome can effectively be prevented and treated, if recognized early. A slow increase of food intake and the close monitoring of serum electrolyte levels play an important role. Because the refeeding syndrome is not well known and the symptoms may vary extremely, this complication is poorly recognized, especially against the background of geriatric multimorbidity. This overview is intended to increase the awareness of the refeeding syndrome in the risk group of geriatric patients.

Diminished Phosphorylation of CREB Is a Key Event in the Dysregulation of Gluconeogenesis and Glycogenolysis in PCB126 Hepatotoxicity.

The dioxin-like PCB126 elicits toxicity in various target organs. In rat liver, an alteration in the transcript levels of several genes involved in glucose and fatty acid metabolism provides insights into the origin of its hepatotoxicity. To explore the mechanisms, male Sprague-Dawley rats, fed an AIN-93G diet, were injected with PCB126 (1 or 5 µmol/kg) or corn oil and euthanized after 2 weeks. PCB126 significantly decreased serum glucose levels and the transcript levels of genes of many gluconeogenic and glycogenolytic enzymes under the transcriptional control of a nuclear transcription factor, cAMP response element-binding protein (CREB). As a novel finding, we show that PCB126 significantly decreases CREB phosphorylation, which is important for regulating both gluconeogenesis and fatty acid oxidation in the liver and explains CREB's integrative effects on both carbohydrate and lipid metabolism in PCB126 toxicity.

Isoform-selective regulation of glycogen phosphorylase by energy deprivation and phosphorylation in astrocytes.

Glycogen phosphorylase (GP) is activated to degrade glycogen in response to different stimuli, to support both the astrocyte's own metabolic demand and the metabolic needs of neurons. The regulatory mechanism allowing such a glycogenolytic response to distinct triggers remains incompletely understood. In the present study, we used siRNA-mediated differential knockdown of the two isoforms of GP expressed in astrocytes, muscle isoform (GPMM), and brain isoform (GPBB), to analyze isoform-specific regulatory characteristics in a cellular setting. Subsequently, we tested the response of each isoform to phosphorylation, triggered by incubation with norepinephrine (NE), and to AMP, increased by glucose deprivation in cells in which expression of one GP isoform had been silenced. Successful knockdown was demonstrated on the protein level by Western blot, and on a functional level by determination of glycogen content showing an increase in glycogen levels following knockdown of either GPMM or GPBB. NE triggered glycogenolysis within 15 min in control cells and after GPBB knockdown. However, astrocytes in which expression of GPMM had been silenced showed a delay in response to NE, with glycogen levels significantly reduced only after 60 min. In contrast, allosteric activation of GP by AMP, induced by glucose deprivation, seemed to mainly affect GPBB, as only knockdown of GPBB, but not of GPMM, delayed the glycogenolytic response to glucose deprivation. Our results indicate that the two GP isoforms expressed in astrocytes respond to different physiological triggers, therefore conferring distinct metabolic functions of brain glycogen.

A low-protein diet combined with low-dose endotoxin leads to changes in glucose homeostasis in weanling rats.

Severe malnutrition is a leading cause of global childhood mortality, and infection and hypoglycemia or hyperglycemia are commonly present. The etiology behind the changes in glucose homeostasis is poorly understood. Here, we generated an animal model of severe malnutrition with and without low-grade inflammation to investigate the effects on glucose homeostasis. Immediately after weaning, rats were fed diets containing 5 [low-protein diet (LP)] or 20% protein [control diet (CTRL)], with or without repeated low-dose intraperitoneal lipopolysaccharide (LPS; 2 mg/kg), to mimic inflammation resulting from infections. After 4 wk on the diets, hyperglycemic clamps or euglycemic hyperinsulinemic clamps were performed with infusion of [U-(13)C6]glucose and [2-(13)C]glycerol to assess insulin secretion, action, and hepatic glucose metabolism. In separate studies, pancreatic islets were isolated for further analyses of insulin secretion and islet morphometry. Glucose clearance was reduced significantly by LP feeding alone (16%) and by LP feeding with LPS administration (43.8%) compared with control during the hyperglycemic clamps. This was associated with a strongly reduced insulin secretion in LP-fed rats in vivo as well as ex vivo in islets but signficantly enhanced whole body insulin sensitivity. Gluconeogenesis rates were unaffected by LP feeding, but glycogenolysis was higher after LP feeding. A protein-deficient diet in young rats leads to a susceptibility to low-dose endotoxin-induced impairment in glucose clearance with a decrease in the islet insulin secretory pathway. A protein-deficient diet is associated with enhanced peripheral insulin sensitivity but impaired insulin-mediated suppression of hepatic glycogenolysis.

Roles of astrocytic Na(+),K(+)-ATPase and glycogenolysis for K(+) homeostasis in mammalian brain.

Neuronal excitation increases extracellular K(+) concentration ([K(+)]o) in vivo and in incubated brain tissue by stimulation of postsynaptic glutamatergic receptors and by channel-mediated K(+) release during action potentials. Convincing evidence exists that subsequent cellular K(+) reuptake occurs by active transport, normally mediated by Na(+),K(+)-ATPase. This enzyme is expressed both in neurons and in astrocytes but is stimulated by elevated [K(+)]o only in astrocytes. This might lead to an initial K(+) uptake in astrocytes, followed by Kir4.1-mediated release and neuronal reuptake. In cell culture experiments, K(+)-stimulated glycogenolysis is essential for operation of the astrocytic Na(+),K(+)-ATPase resulting from the requirement for glycogenolysis in a pathway leading to uptake of Na(+) for costimulation of its intracellular sodium-binding site. The astrocytic but not the neuronal Na(+),K(+)-ATPase is additionally stimulated by isoproterenol, a ß-adrenergic agonist, but only at nonelevated [K(+)]o. This effect is also glycogenolysis dependent and might play a role during poststimulatory undershoots. Attempts to replicate dependence on glycogenolysis for K(+) reuptake in glutamate-stimulated brain slices showed similar [K(+)]o recovery half-lives in the absence and presence of the glycogenolysis inhibitor 1,4-dideoxy-1,4-imino-D-arabinitol. The undershoot was decreased, but to the same extent as an unexpected reduction of peak [K(+)]o increase. A potential explanation for this difference from the cell culture experiments is that astrocytic glutamate uptake might supply the cells with sufficient Na(+). Inhibition of action potential generation by tetrodotoxin caused only a marginal, nonsignificant decrease in stimulated [K(+)]o in brain slices, hindering the evaluation if K(+) reaccumulation after action potential propagation requires glycogenolysis in this preparation.

Astrocyte glycogenolysis is triggered by store-operated calcium entry and provides metabolic energy for cellular calcium homeostasis.

Astrocytic glycogen, the only storage form of glucose in the brain, has been shown to play a fundamental role in supporting learning and memory, an effect achieved by providing metabolic support for neurons. We have examined the interplay between glycogenolysis and the bioenergetics of astrocytic Ca(2+) homeostasis, by analyzing interdependency of glycogen and store-operated Ca(2+) entry (SOCE), a mechanism in cellular signaling that maintains high endoplasmatic reticulum (ER) Ca(2+) concentration and thus provides the basis for store-dependent Ca(2+) signaling. We stimulated SOCE in primary cultures of murine cerebellar and cortical astrocytes, and determined glycogen content to investigate the effects of SOCE on glycogen metabolism. By blocking glycogenolysis, we tested energetic dependency of SOCE-related Ca(2+) dynamics on glycogenolytic ATP. Our results show that SOCE triggers astrocytic glycogenolysis. Upon inhibition of adenylate cyclase with 2',5'-dideoxyadenosine, glycogen content was no longer significantly different from that in unstimulated control cells, indicating that SOCE triggers astrocytic glycogenolysis in a cAMP-dependent manner. When glycogenolysis was inhibited in cortical astrocytes by 1,4-dideoxy-1,4-imino-D-arabinitol, the amount of Ca(2+) loaded into ER via sarco/endoplasmic reticulum Ca(2)-ATPase (SERCA) was reduced, which suggests that SERCA pumps preferentially metabolize glycogenolytic ATP. Our study demonstrates SOCE as a novel pathway in stimulating astrocytic glycogenolysis. We also provide first evidence for a new functional role of brain glycogen, in providing local ATP to SERCA, thus establishing the bioenergetic basis for astrocytic Ca(2+) signaling. This mechanism could offer a novel explanation for the impact of glycogen on learning and memory.

Basic mechanism leading to stimulation of glycogenolysis by isoproterenol, EGF, elevated extracellular K+ concentrations, or GABA.

Glycogenolysis, in brain parenchyma an astrocyte-specific process, has changed from being envisaged as an emergency procedure to playing central roles during brain response to whisker stimulation, memory formation, astrocytic K(+) uptake and stimulated release of ATP. It is activated by several transmitters and by even very small increases in extracellular K(+) concentration, and to be critically dependent upon an increase in free cytosolic Ca(2+) concentration ([Ca(2+)]i), whereas cAMP plays only a facilitatory role together with increased [Ca(2+)]i. Detailed knowledge about the signaling pathways eliciting glycogenolysis is therefore of interest and was investigated in the present study in well differentiated cultures of mouse astrocytes. The ß-adrenergic agonist isoproterenol stimulated glycogenolysis by a ß1-adrenergic effect, which initiated a pathway in which cAMP/protein kinase A activated a Gi/Gs shift, leading to Ca(2+)-activated glycogenolysis. Inhibition of this pathway downstream of cAMP but upstream of the Gi/Gs shift abolished the glycogenolysis. However, inhibitors operating downstream of the Ca(2+)-sensitive step, but preventing transactivation-mediated epidermal growth factor (EGF) receptor stimulation, a later step in the activated pathway, also caused inhibition of glycogenolysis. For this reason the effect of EGF was investigated and it was found to be glycogenolytic. Large increases in extracellular K(+) activated glycogenolysis by a nifedipine-inhibited L-channel opening allowing influx of Ca(2+), known to be glycogenolysis-dependent. Small increases (addition of 5 mM KCl) caused a smaller effect by a similarly glycogenolysis-reliant opening of an IP3 receptor-dependent ouabain signaling pathway. The same pathway could be activated by GABA (also in brain slices) due to its depolarizing effect in astrocytes.

Uterine glycogen metabolism in mink during estrus, embryonic diapause and pregnancy.

We have determined uterine glycogen content, metabolizing enzyme expression and activity in the mink, a species that exhibits obligatory embryonic diapause, resulting in delayed implantation. Gross uterine glycogen concentrations were highest in estrus, decreased 50% by diapause and 90% in pregnancy (P ≤ 0.05). Endometrial glycogen deposits, which localized primarily to glandular and luminal epithelia, decreased 99% between estrus and diapause (P ≤ 0.05) and were nearly undetectable in pregnancy. Glycogen synthase and phosphorylase proteins were most abundant in the glandular epithelia. Glycogen phosphorylase activity (total) in uterine homogenates was higher during estrus and diapause, than pregnancy. While glycogen phosphorylase protein was detected during estrus and diapause, glycogen synthase was almost undetectable after estrus, which probably contributed to a higher glycogenolysis/glycogenesis ratio during diapause. Uterine glucose-6-phosphatase 3 gene expression was greater during diapause, when compared to estrus (P ≤ 0.05) and supports the hypothesis that glucose-6-phosphate resulting from phosphorylase activity was dephosphorylated in preparation for export into the uterine lumen. The relatively high amount of hexokinase-1 protein detected in the luminal epithelia during estrus and diapause may have contributed to glucose trapping after endometrial glycogen reserves were depleted. Collectively, our findings suggest to us that endometrial glycogen reserves may be an important source of energy, supporting uterine and conceptus metabolism up to the diapausing blastocyst stage. As a result, the size of uterine glycogen reserves accumulated prior to mating may in part, determine the number of embryos that survive to the blastocyst stage, and ultimately litter size.

Effects of 11ß-hydroxysteroid dehydrogenase-1 inhibition on hepatic glycogenolysis and gluconeogenesis.

The aim of this study was to determine the effect of prolonged 11ß-hydroxysteroid dehydrogenase-1 (11ß-HSD1) inhibition on basal and hormone-stimulated glucose metabolism in fasted conscious dogs. For 7 days prior to study, either an 11ß-HSD1 inhibitor (HSD1-I; n = 6) or placebo (PBO; n = 6) was administered. After the basal period, a 4-h metabolic challenge followed, where glucagon (3×-basal), epinephrine (5×-basal), and insulin (2×-basal) concentrations were increased. Hepatic glucose fluxes did not differ between groups during the basal period. In response to the metabolic challenge, hepatic glucose production was stimulated in PBO, resulting in hyperglycemia such that exogenous glucose was required in HSD-I (P < 0.05) to match the glycemia between groups. Net hepatic glucose output and endogenous glucose production were decreased by 11ß-HSD1 inhibition (P < 0.05) due to a reduction in net hepatic glycogenolysis (P < 0.05), with no effect on gluconeogenic flux compared with PBO. In addition, glucose utilization (P < 0.05) and the suppression of lipolysis were increased (P < 0.05) in HSD-I compared with PBO. These data suggest that inhibition of 11ß-HSD1 may be of therapeutic value in the treatment of diseases characterized by insulin resistance and excessive hepatic glucose production.

Glycogen and its metabolism: some new developments and old themes.

Glycogen is a branched polymer of glucose that acts as a store of energy in times of nutritional sufficiency for utilization in times of need. Its metabolism has been the subject of extensive investigation and much is known about its regulation by hormones such as insulin, glucagon and adrenaline (epinephrine). There has been debate over the relative importance of allosteric compared with covalent control of the key biosynthetic enzyme, glycogen synthase, as well as the relative importance of glucose entry into cells compared with glycogen synthase regulation in determining glycogen accumulation. Significant new developments in eukaryotic glycogen metabolism over the last decade or so include: (i) three-dimensional structures of the biosynthetic enzymes glycogenin and glycogen synthase, with associated implications for mechanism and control; (ii) analyses of several genetically engineered mice with altered glycogen metabolism that shed light on the mechanism of control; (iii) greater appreciation of the spatial aspects of glycogen metabolism, including more focus on the lysosomal degradation of glycogen; and (iv) glycogen phosphorylation and advances in the study of Lafora disease, which is emerging as a glycogen storage disease.

Increase in skeletal-muscle glycogenolysis and perceived exertion with progressive dehydration during cycling in hydrated men.

This study investigated the effects of progressive mild dehydration during cycling on whole-body substrate oxidation and skeletal-muscle metabolism in recreationally active men. Subjects (N = 9) cycled for 120 min at ~65% peak oxygen uptake (VO2peak 22.7 °C, 32% relative humidity) with water to replace sweat losses (HYD) or without fluid (DEH). Blood samples were taken at rest and every 20 min, and muscle biopsies were taken at rest and at 40, 80, and 120 min of exercise. Subjects lost 0.8%, 1.8%, and 2.7% body mass (BM) after 40, 80, and 120 min of cycling in the DEH trial while sweat loss was not significantly different between trials. Heart rate was greater in the DEH trial from 60 to 120 min, and core temperature was greater from 75 to 120 min. Rating of perceived exertion was higher in the DEH trial from 30 to 120 min. There were no differences in VO2, respiratory-exchange ratio, total carbohydrate (CHO) oxidation (HYD 312 ± 9 vs. DEH 307 ± 10 g), or sweat rate between trials. Blood lactate was significantly greater in the DEH trial from 20 to 120 min with no difference in plasma free fatty acids or epinephrine. Glycogenolysis was significantly greater (24%) over the entire DEH vs. HYD trial (433 ± 44 vs. 349 ± 27 mmol · kg-1 · dm-1). In conclusion, dehydration of <2% BM elevated physiological parameters and perceived exertion, as well as muscle glycogenolysis, during exercise without affecting whole-body CHO oxidation.

Elevated NEFA levels impair glucose effectiveness by increasing net hepatic glycogenolysis.

AIMS/HYPOTHESIS: Acute hyperglycaemia rapidly suppresses endogenous glucose production (EGP) in non-diabetic individuals, mainly by inhibiting glycogenolysis. Loss of this 'glucose effectiveness' contributes to fasting hyperglycaemia in type 2 diabetes. Elevated NEFA levels characteristic of type 2 diabetes impair glucose effectiveness, although the mechanism is not fully understood. Therefore we examined the impact of increasing NEFA levels on the ability of hyperglycaemia to regulate pathways of EGP. METHODS: We performed 4 h 'pancreatic clamp' studies (somatostatin; basal glucagon/growth hormone/insulin) in seven non-diabetic individuals. Glucose fluxes (D-[6,6-(2)H(2)]glucose) and hepatic glycogen concentrations ((13)C magnetic resonance spectroscopy) were quantified under three conditions: euglycaemia, hyperglycaemia and hyperglycaemia with elevated NEFA (HY-NEFA). RESULTS: EGP was suppressed by hyperglycaemia, but not by HY-NEFA. Hepatic glycogen concentration decreased ~14% with prolonged fasting during euglycaemia and increased by ~12% with hyperglycaemia. In contrast, raising NEFA levels in HY-NEFA caused a substantial ~23% reduction in hepatic glycogen concentration. Moreover, rates of gluconeogenesis were decreased with hyperglycaemia, but increased with HY-NEFA. CONCLUSIONS/INTERPRETATION: Increased NEFA appear to profoundly blunt the ability of hyperglycaemia to inhibit net glycogenolysis under basal hormonal conditions.

Effect of ghrelin on glucose regulation in mice.

Improvement of glucose metabolism after bariatric surgery appears to be from the composite effect of the alterations in multiple circulating gut hormone concentrations. However, their individual effect on glucose metabolism during different conditions is not clear. The objective of this study was to determine whether ghrelin has an impact on glycogenolysis, gluconeogenesis, and insulin sensitivity (using a mice model). Rate of appearance of glucose, glycogenolysis, and gluconeogenesis were measured in wild-type (WT), ghrelin knockout (ghrelin(-/-)), and growth hormone secretagogue receptor knockout (Ghsr(-/-)) mice in the postabsorptive state. The physiological nature of the fasting condition was ascertained by a short-term fast commenced immediately at the end of the dark cycle. Concentrations of glucose and insulin were measured, and insulin resistance and hepatic insulin sensitivity were calculated. Glucose concentrations were not different among the groups during the food-deprived period. However, plasma insulin concentrations were lower in the ghrelin(-/-) and Ghsr(-/-) than WT mice. The rates of gluconeogenesis, glycogenolysis, and indexes of insulin sensitivity were higher in the ghrelin(-/-) and Ghsr(-/-) than WT mice during the postabsorptive state. Insulin receptor substrate 1 and glucose transporter 2 gene expressions in hepatic tissues of the ghrelin(-/-) and Ghsr(-/-) were higher compared with that in WT mice. This study demonstrates that gluconeogenesis and glycogenolysis are increased and insulin sensitivity is improved by the ablation of the ghrelin or growth hormone secretagogue receptor in mice.

Differential glycolytic and glycogenogenic transduction pathways in male and female bovine embryos produced in vitro.

Previous studies have shown that developmental kinetic rates following IVF are lower in female than in male blastocysts and that this may be related to differences in glucose metabolism. In addition, an inhibition of phosphatidylinositol 3-kinase (PI3-K) inhibits glucose uptake in murine blastocysts. Therefore, the aim of this study was to identify and compare the expression of proteins involved in glucose metabolism (hexokinase-I, HK-I; phosphofructokinase-1, PFK-1; pyruvate kinase 1/2, PK1/2; glyceraldehyde-3-phosphate dehydrogenase, GAPDH; glucose transporter-1, GLUT-1; and glycogen synthase kinase-3, GSK-3) in male and female bovine blastocysts to determine whether PI3-K has a role in the regulation of the expression of these proteins. Hexokinase-I, PFK-1, PK1/2, GAPDH and GLUT-1 were present in bovine embryos. Protein expression of these proteins and GSK-3 was significantly higher in male compared with female blastocysts. Inhibition of PI3-K with LY294002 significantly decreased the expression of HK-I, PFK-1, GAPDH, GSK-3A/B and GLUT-1. Results showed that the expression of glycolytic proteins HK-I, PFK-1, GAPDH and PK1/2, and the transporters GLUT-1 and GSK-3 is regulated by PI3-K in bovine blastocysts. Moreover, the differential protein expression observed between male and female blastocysts might explain the faster developmental kinetics seen in males, as the expression of main proteins involved in glycolysis and glycogenogenesis was significantly higher in male than female bovine embryos and also could explain the sensitivity of male embryos to a high concentration of glucose, as a positive correlation between GLUT-1 expression and glucose uptake in embryos has been demonstrated.

Evaluation of hepatic glucose metabolism via gluconeogenesis and glycogenolysis after oral administration of insulin nanoparticles.

Nanoparticles were designed to promote insulin intestinal absorption via the oral route, to increase portal insulin levels to better mimic the physiological pathway, providing enhanced glucose control through glycogenolysis and gluconeogenesis. Nanoparticles were formulated with insulin encapsulated in the core material consisting of alginate and dextran sulfate, associated with poloxamer and subsequently coated with chitosan then albumin. A spherical and slightly rough core was observed in electron micrographs with the appearance of a concentration gradient of the polysaccharide structure toward the periphery of the nanoparticle. Atomic force microscopy showed that the fully formed nanoparticles are about 200 nm in diameter with smooth and spherical morphology. Histopathological analysis of organs and tissues of diabetic rats dosed daily for 15 days with insulin nanoparticles was used to evaluate toxicological issues. No morphological or pathological alterations were observed in rat liver, spleen, pancreas, kidney or intestinal sections. Following, the effect of nanoencapsulated insulin on inhibiting hepatic gluconeogenesis was evaluated after a single insulin administration and oral glucose tolerance test, which represents a significant metabolic challenge to the liver. Alterations in the hepatic glucose metabolism of fasted streptozotocin-diabetic rats were determined by the percent contribution of glycogenolysis and gluconeogenesis, measured by using metabolic tracers, however similar gluconeogenesis contribution to the hepatic metabolism was observed between diabetic rats receiving nanoencapsulated insulin or insulin solution. The metabolic results may be explained by the inability of a single dose in shifting the gluconeogenesis/glycogenolysis contributions, sampling time, fasting period or by influence of the kidney enzymes and impairment in insulin signaling observed in stz-diabetic rats.

Glucose sparing by glycogenolysis (GSG) determines the relationship between brain metabolism and neurotransmission.

Over the last two decades, it has been established that glucose metabolic fluxes in neurons and astrocytes are proportional to the rates of the glutamate/GABA-glutamine neurotransmitter cycles in close to 1:1 stoichiometries across a wide range of functional energy demands. However, there is presently no mechanistic explanation for these relationships. We present here a theoretical meta-analysis that tests whether the brain's unique compartmentation of glycogen metabolism in the astrocyte and the requirement for neuronal glucose homeostasis lead to the observed stoichiometries. We found that blood-brain barrier glucose transport can be limiting during activation and that the energy demand could only be met if glycogenolysis supports neuronal glucose metabolism by replacing the glucose consumed by astrocytes, a mechanism we call Glucose Sparing by Glycogenolysis (GSG). The predictions of the GSG model are in excellent agreement with a wide range of experimental results from rats, mice, tree shrews, and humans, which were previously unexplained. Glycogenolysis and glucose sparing dictate the energy available to support neuronal activity, thus playing a fundamental role in brain function in health and disease.