Increased glycogen accumulation in transgenic mice overexpressing glycogen synthase in skeletal muscle

The paradox of fatty-acid β-oxidation in muscle insulin resistance: Metabolic control and muscle heterogeneity

The skeletal muscle is a metabolically heterogeneous tissue that plays a key role in maintaining whole-body glucose homeostasis. It is well known that muscle insulin resistance (IR) precedes the development of type 2 diabetes. There is a consensus that the accumulation of specific lipid species in the tissue can drive IR. However, the role of the mitochondrial fatty-acid β-oxidation in IR and, consequently, in the control of glucose uptake remains paradoxical: interventions that either inhibit or activate fatty-acid β-oxidation have been shown to prevent IR. We here discuss the current theories and evidence for the interplay between β-oxidation and glucose uptake in IR. To address the underlying intricacies, we (1) dive into the control of glucose uptake fluxes into muscle tissues using the framework of Metabolic Control Analysis, and (2) disentangle concepts of flux and catalytic capacities taking into account skeletal muscle heterogeneity. Finally, we speculate about hitherto unexplored mechanisms that could bring contrasting evidence together. Elucidating how β-oxidation is connected to muscle IR and the underlying role of muscle heterogeneity enhances disease understanding and paves the way for new treatments for type 2 diabetes.

Myofiber-type-dependent 'boulder' or 'multitudinous pebble' formations across distinct amylopectinoses

At least five enzymes including three E3 ubiquitin ligases are dedicated to glycogen's spherical structure. Absence of any reverts glycogen to a structure resembling amylopectin of the plant kingdom. This amylopectinosis (polyglucosan body formation) causes fatal neurological diseases including adult polyglucosan body disease (APBD) due to glycogen branching enzyme deficiency, Lafora disease (LD) due to deficiencies of the laforin glycogen phosphatase or the malin E3 ubiquitin ligase and type 1 polyglucosan body myopathy (PGBM1) due to RBCK1 E3 ubiquitin ligase deficiency. Little is known about these enzymes' functions in glycogen structuring. Toward understanding these functions, we undertake a comparative murine study of the amylopectinoses of APBD, LD and PGBM1. We discover that in skeletal muscle, polyglucosan bodies form as two main types, small and multitudinous ('pebbles') or giant and single ('boulders'), and that this is primarily determined by the myofiber types in which they form, 'pebbles' in glycolytic and 'boulders' in oxidative fibers. This pattern recapitulates what is known in the brain in LD, innumerable dust-like in astrocytes and single giant sized in neurons. We also show that oxidative myofibers are relatively protected against amylopectinosis, in part through highly increased glycogen branching enzyme expression. We present evidence of polyglucosan body size-dependent cell necrosis. We show that sex influences amylopectinosis in genotype, brain region and myofiber-type-specific fashion. RBCK1 is a component of the linear ubiquitin chain assembly complex (LUBAC), the only known cellular machinery for head-to-tail linear ubiquitination critical to numerous cellular pathways. We show that the amylopectinosis of RBCK1 deficiency is not due to loss of linear ubiquitination, and that another function of RBCK1 or LUBAC must exist and operate in the shaping of glycogen. This work opens multiple new avenues toward understanding the structural determinants of the mammalian carbohydrate reservoir critical to neurologic and neuromuscular function and disease.

Lysosomal glycogen accumulation in Pompe disease results in disturbed cytoplasmic glycogen metabolism

Pompe disease is an inherited metabolic myopathy caused by deficiency of acid alpha-glucosidase (GAA), resulting in lysosomal glycogen accumulation. Residual GAA enzyme activity affects disease onset and severity, although other factors, including dysregulation of cytoplasmic glycogen metabolism, are suspected to modulate the disease course. In this study, performed in mice and patient biopsies, we found elevated protein levels of enzymes involved in glucose uptake and cytoplasmic glycogen synthesis in skeletal muscle from mice with Pompe disease, including glycogenin (GYG1), glycogen synthase (GYS1), glucose transporter 4 (GLUT4), glycogen branching enzyme 1 (GBE1), and UDP-glucose pyrophosphorylase (UGP2). Expression levels were elevated before the loss of muscle mass and function. For first time, quantitative mass spectrometry in skeletal muscle biopsies from five adult patients with Pompe disease showed increased expression of GBE1 protein relative to healthy controls at the group level. Paired analysis of individual patients who responded well to treatment with enzyme replacement therapy (ERT) showed reduction of GYS1, GYG1, and GBE1 in all patients after start of ERT compared to baseline. These results indicate that metabolic changes precede muscle wasting in Pompe disease, and imply a positive feedforward loop in Pompe disease, in which lysosomal glycogen accumulation promotes cytoplasmic glycogen synthesis and glucose uptake, resulting in aggravation of the disease phenotype.

The role of glycogen phosphorylase in glycogen biogenesis in skeletal muscle after exercise

Initially it was believed that phosphorylase was responsible for both glycogen breakdown and synthesis in the living cell. The discovery of glycogen synthase and McArdle's disease (lack of phosphorylase activity), together with the high P_i/glucose 1-P ratio in skeletal muscle, demonstrated that glycogen synthesis could not be attributed to reversal of the phosphorylase reaction. Rather, glycogen synthesis was attributable solely to the activity of glycogen synthase, subsequent to the transport of glucose into the cell. However, the well-established observation that phosphorylase was inactivated (i.e., dephosphorylated) during the initial recovery period after prior exercise, when the rate of glycogen accumulation is highest and independent of insulin, suggested that phosphorylase could play an active role in glycogen accumulation. But the quantitative contribution of phosphorylase inactivation was not established until recently, when studying isolated murine muscle preparations during recovery from repeated contractions at temperatures ranging from 25 to 35 °C. Thus, in both slow-twitch, oxidative and fast-twitch, glycolytic muscles, inactivation of phosphorylase accounted for 45%-75% of glycogen accumulation during the initial hours of recovery following repeated contractions. Such data indicate that phosphorylase inactivation may be the most important mechanism for glycogen accumulation under defined conditions. These results support the initial belief that phosphorylase plays a quantitative role in glycogen formation in the living cell. However, the mechanism is not via activation of phosphorylase, but rather via inactivation of the enzyme.

A century of exercise physiology: key concepts in regulation of glycogen metabolism in skeletal muscle

Glycogen is a branched, glucose polymer and the storage form of glucose in cells. Glycogen has traditionally been viewed as a key substrate for muscle ATP production during conditions of high energy demand and considered to be limiting for work capacity and force generation under defined conditions. Glycogenolysis is catalyzed by phosphorylase, while glycogenesis is catalyzed by glycogen synthase. For many years, it was believed that a primer was required for de novo glycogen synthesis and the protein considered responsible for this process was ultimately discovered and named glycogenin. However, the subsequent observation of glycogen storage in the absence of functional glycogenin raises questions about the true role of the protein. In resting muscle, phosphorylase is generally considered to be present in two forms: non-phosphorylated and inactive (phosphorylase b) and phosphorylated and constitutively active (phosphorylase a). Initially, it was believed that activation of phosphorylase during intense muscle contraction was primarily accounted for by phosphorylation of phosphorylase b (activated by increases in AMP) to a, and that glycogen synthesis during recovery from exercise occurred solely through mechanisms controlled by glucose transport and glycogen synthase. However, it now appears that these views require modifications. Moreover, the traditional roles of glycogen in muscle function have been extended in recent years and in some instances, the original concepts have undergone revision. Thus, despite the extensive amount of knowledge accrued during the past 100 years, several critical questions remain regarding the regulation of glycogen metabolism and its role in living muscle.

HM-chromanone, a component of Portulaca oleracea L., stimulates glucose uptake and glycogen synthesis in skeletal muscle cell

BACKGROUND

Diabetes mellitus is a chronic metabolic disease characterized by increased blood glucose levels. In order to lower blood glucose, it is important to stimulate glucose uptake and glycogen synthesis in the muscle. (E)-5-hydroxy-7-methoxy-3-(2'-hydroxybenzyl)-4-chromanone (HM-chromanone), a constituent isolated from Portulaca oleracea L., exhibits anti-diabetic effects; however, its mechanisms are not yet clearly understood on glucose uptake and glycogen synthesis in muscle cells.

PURPOSE

In the present study, we examined the effects of HM-chromanone on glucose uptake into L6 skeletal muscle cells and elucidated the underlying mechanisms.

METHODS

The effects of HM-chromanone on glucose uptake into L6 skeletal muscle cells were assessed by 2-Deoxyglucose uptake assay. Western blot analysis was carried out to elucidate the underlying molecular mechanisms.

RESULTS

We found that HM-chromanone promoted glucose uptake into L6 skeletal muscle cells in a dose-dependent manner. Moreover, HM-chromanone induced the phosphorylation of IRS-1^Tyr612 and AKT^Ser473, and the activation of PI3K. HM-chromanone also stimulated the phosphorylation of AMPK^Thr172, AS160^Thr642, TBC1D1^Ser237, and ACC via the CaMKKβ pathway. Furthermore, HM-chromanone increased glycogen synthesis through the inactivation of glycogen synthase kinase 3 α/β.

CONCLUSION

The results of this study indicate that HM-chromanone stimulates glucose uptake through the activation of the PI3K/AKT and CaMKKβ-AMPK pathways and glycogen synthesis via the GSK3 α/β pathway in L6 skeletal muscle cells.

Glucose transporters in adipose tissue, liver, and skeletal muscle in metabolic health and disease

A family of facilitative glucose transporters (GLUTs) is involved in regulating tissue-specific glucose uptake and metabolism in the liver, skeletal muscle, and adipose tissue to ensure homeostatic control of blood glucose levels. Reduced glucose transport activity results in aberrant use of energy substrates and is associated with insulin resistance and type 2 diabetes. It is well established that GLUT2, the main regulator of hepatic hexose flux, and GLUT4, the workhorse in insulin- and contraction-stimulated glucose uptake in skeletal muscle, are critical contributors in the control of whole-body glycemia. However, the molecular mechanism how insulin controls glucose transport across membranes and its relation to impaired glycemic control in type 2 diabetes remains not sufficiently understood. An array of circulating metabolites and hormone-like molecules and potential supplementary glucose transporters play roles in fine-tuning glucose flux between the different organs in response to an altered energy demand.

Chinese olive (Canarium album L.) fruit regulates glucose utilization by activating AMP-activated protein kinase

A growing body of evidence demonstrates obesity-induced insulin resistance is associated with the development of metabolic diseases. This study was designed to investigate ethyl acetate fraction of Chinese olive fruit extract (CO-EtOAc)-mediated attenuation of obesity and hyperglycemia in a mouse model. About 60% HFD-fed mice were treated intragastrically with CO-EtOAc for last 6 weeks, and body weight, blood biochemical parameters as well as hepatic inflammation response were investigated. Our results showed that CO-EtOAc treatment significantly reduced the formation of hepatic lipid droplets, body weight gain, blood glucose, and improved serum biochemical parameters in HFD-induced obese and insulin resistant mice. We further explored the molecular mechanism underlying the blood glucose modulating effect of CO-EtOAc using L6 myotubes model. We conclude that CO-EtOAc effectively increases the glycogen content and glucose uptake by stimulating the membrane translocation of glucose transporter 4. In addition, CO-EtOAc depolarizes the mitochondrial membrane and decreases the mitochondrial oxygen consumption, which may result in AMPK activation and the consequent mitochondrial fission. This study shows that CO-EtOAc prevents the development of obesity in mice fed with HFD and is also capable of stimulating glucose uptake. The possible mechanism might be due to the effects of CO-EtOAc on activation of AMPK and promotion of mitochondrial fission.

Corazonin signaling integrates energy homeostasis and lunar phase to regulate aspects of growth and sexual maturation in Platynereis

Gonadotropin Releasing Hormone (GnRH) acts as a key regulator of sexual maturation in vertebrates, and is required for the integration of environmental stimuli to orchestrate breeding cycles. Whether this integrative function is conserved across phyla remains unclear. We characterized GnRH-type signaling systems in the marine worm Platynereis dumerilii, in which both metabolic state and lunar cycle regulate reproduction. We find gnrh-like (gnrhl) genes upregulated in sexually mature animals, after feeding, and in specific lunar phases. Animals in which the corazonin1/gnrhl1 gene has been disabled exhibit delays in growth, regeneration, and maturation. Molecular analyses reveal glycoprotein turnover/energy homeostasis as targets of CRZ1/GnRHL1. These findings point at an ancestral role of GnRH superfamily signaling in coordinating energy demands dictated by environmental and developmental cues.

The molecular mechanisms by which animals integrate external stimuli with internal energy balance to regulate major developmental and reproductive events still remain enigmatic. We investigated this aspect in the marine bristleworm, Platynereis dumerilii, a species where sexual maturation is tightly regulated by both metabolic state and lunar cycle. Our specific focus was on ligands and receptors of the gonadotropin-releasing hormone (GnRH) superfamily. Members of this superfamily are key in triggering sexual maturation in vertebrates but also regulate reproductive processes and energy homeostasis in invertebrates. Here we show that 3 of the 4 gnrh-like (gnrhl) preprohormone genes are expressed in specific and distinct neuronal clusters in the Platynereis brain. Moreover, ligand–receptor interaction analyses reveal a single Platynereis corazonin receptor (CrzR) to be activated by CRZ1/GnRHL1, CRZ2/GnRHL2, and GnRHL3 (previously classified as AKH1), whereas 2 AKH-type hormone receptors (GnRHR1/AKHR1 and GnRHR2/AKHR2) respond only to a single ligand (GnRH2/GnRHL4). Crz1/gnrhl1 exhibits a particularly strong up-regulation in sexually mature animals, after feeding, and in specific lunar phases. Homozygous crz1/gnrhl1 knockout animals exhibit a significant delay in maturation, reduced growth, and attenuated regeneration. Through a combination of proteomics and gene expression analysis, we identify enzymes involved in carbohydrate metabolism as transcriptional targets of CRZ1/GnRHL1 signaling. Our data suggest that Platynereis CRZ1/GnRHL1 coordinates glycoprotein turnover and energy homeostasis with growth and sexual maturation, integrating both metabolic and developmental demands with the worm’s monthly cycle.

Regulation of Skeletal Muscle Glucose Transport and Glucose Metabolism by Exercise Training

Aerobic exercise training and resistance exercise training are both well-known for their ability to improve human health; especially in individuals with type 2 diabetes. However, there are critical differences between these two main forms of exercise training and the adaptations that they induce in the body that may account for their beneficial effects. This article reviews the literature and highlights key gaps in our current understanding of the effects of aerobic and resistance exercise training on the regulation of systemic glucose homeostasis, skeletal muscle glucose transport and skeletal muscle glucose metabolism.

Structure and Regulation of Glycogen Synthase in the Brain

Brain glycogen synthesis is a regulated, multi-step process that begins with glucose transport across the blood brain barrier and culminates with the actions of glycogen synthase and the glycogen branching enzyme to elongate glucose chains and introduce branch points in a growing glycogen molecule. This review focuses on the synthesis of glycogen in the brain, with an emphasis on glycogen synthase, but draws on salient studies in mammalian muscle and liver as well as baker's yeast, with the goal of providing a more comprehensive view of glycogen synthesis and highlighting potential areas for further study in the brain. In addition, deficiencies in the glycogen biosynthetic enzymes which lead to glycogen storage diseases in humans are discussed, highlighting effects on the brain and discussing findings in genetically modified animal models that recapitulate these diseases. Finally, implications of glycogen synthesis in neurodegenerative and other diseases that impact the brain are presented.

OLIVEIRA L, DE VICENTE LG, FERREIRA GN, DE FREITAS EC, DA SILVA ASR. Uphill Running Excessive Training Increases Gastrocnemius Glycogen Content in C57BL/6 Mice

The main aim of the present investigation was to verify the effects of three overtraining (OT) protocols performed in downhill (OTR/down), uphill (OTR/up) and without inclination (OTR) on the protein levels of Akt (Ser473), AMPKα (Thr172), PGC-1α, plasma membrane GLUT-1 and GLUT-4 as well as on the glycogen contents in mice gastrocnemius. A trained (TR) protocol was used as positive control. Rodents were divided into naïve (N, sedentary mice), control (CT, sedentary mice submitted to the performance evaluations), TR, OTR/down, OTR/up and OTR groups. At the end of the experimental protocols, gastrocnemius samples were removed and used for immunoblotting analysis as well as for glycogen measurements. There was no significant difference between the experimental groups for the protein levels of pAkt (Ser473), pAMPKα (Thr172), PGC-1α, plasma membrane GLUT-1 and GLUT-4. However, the OTR/up protocol exhibited higher contents of glycogen compared to the CT and TR groups. In summary, the OTR/up group increased the gastrocnemius glycogen content without significant changes of pAkt (Ser473), pAMPKα (Thr172), PGC-1α, plasma membrane GLUT-1 and GLUT-4.

Islet Transplantation under the Kidney Capsule Corrects the Defects in Glycogen Metabolism in Both Liver and Muscle of Streptozocin-Diabetic Rats

Insulin-deficient rats are characterized by multiple defects in the pathway of glycogen synthesis and breakdown in both liver and skeletal muscle. The aim of this study was to clarify whether islet transplantation under the kidney capsule, which is associated with delivery of insulin into the peripheral circulation, is able to normalize glycogen metabolism in liver and muscle of streptozotocin-diabetic rats. Three groups of male Lewis rats were studied under fasting condition: controls, untreated diabetics, and islet transplanted diabetics. Glycogen content, glucose-6-phosphate concentration, and glycogen synthase activity were measured in both liver and skeletal muscle. Untreated diabetic rats were characterized by an increase in glycogen content of 178% and a reduction of glucose-6-phosphate level of 50%. Both glycogen and glucose-6-phosphate contents were restored to normal in transplanted diabetic rats. Active glycogen synthase (0.35 ± 0.1 nmol/min/mg) and activity ratio (0.22 ± 0.04) were significantly impaired compared with controls (0.99 ± 0.2 nmol/min/mg and 0.43 ± 0.06, respectively) and were normalized by islet transplantation. In the skeletal muscle, glycogen content was similar in the three groups of animals, whereas muscle glucose-6-phosphate level was reduced by 28% and glycogen synthase was in a less active state in the untreated diabetic rats. Both the glucose-6-phosphate concentration and the kinetic profile of glycogen synthase were normalized by islet transplantation. In conclusion, islet transplantation under the kidney capsule corrects the diabetes-induced abnormalities in glycogen and glucose-6-phosphate content and glycogen synthase activity in both liver and skeletal muscle.

Gain of function AMP-activated protein kinase γ3 mutation (AMPKγ3R200Q) in pig muscle increases glycogen storage regardless of AMPK activation

Chronic activation of AMP‐activated protein kinase (AMPK) increases glycogen content in skeletal muscle. Previously, we demonstrated that a mutation in the ryanodine receptor (RyR1^R615C) blunts AMPK phosphorylation in longissimus muscle of pigs with a gain of function mutation in the AMPKγ3 subunit (AMPKγ3^R200Q); this may decrease the glycogen storage capacity of AMPKγ3^R200Q + RyR1^R615C muscle. Therefore, our aim in this study was to utilize our pig model to understand how AMPKγ3^R200Q and AMPK activation contribute to glycogen storage and metabolism in muscle. We selected and bred pigs in order to generate offspring with naturally occurring AMPKγ3^R200Q, RyR1^R615C, and AMPKγ3^R200Q + RyR1^R615C mutations, and also retained wild‐type littermates (control). We assessed glycogen content and parameters of glycogen metabolism in longissimus muscle. Regardless of RyR1^R615C, AMPKγ3^R200Q increased the glycogen content by approximately 70%. Activity of glycogen synthase (GS) without the allosteric activator glucose 6‐phosphate (G6P) was decreased in AMPKγ3^R200Q relative to all other genotypes, whereas both AMPKγ3^R200Q and AMPKγ3^R200Q + RyR1^R615C muscle exhibited increased GS activity with G6P. Increased activity of GS with G6P was not associated with increased abundance of GS or hexokinase 2. However, AMPKγ3^R200Q enhanced UDP‐glucose pyrophosphorylase 2 (UGP2) expression approximately threefold. Although UGP2 is not generally considered a rate‐limiting enzyme for glycogen synthesis, our model suggests that UGP2 plays an important role in increasing flux to glycogen synthase. Moreover, we have shown that the capacity for glycogen storage is more closely related to the AMPKγ3^R200Q mutation than activity.

Nrf2-driven TERT regulates pentose phosphate pathway in glioblastoma

Given the involvement of telomerase activation and dysregulated metabolism in glioma progression, the connection between these two critical players was investigated. Pharmacological inhibition of human Telomerase reverse transcriptase (hTERT) by Costunolide induced glioma cell apoptosis in a reactive oxygen species (ROS)-dependent manner. Costunolide induced an ROS-dependent increase in p53 abrogated telomerase activity. Costunolide decreased Nrf2 level; and ectopic Nrf2 expression decreased Costunolide-induced ROS generation. While TERT knock-down abrogated Nrf2 levels, overexpression of Nrf2 increased TERT expression. Inhibition of hTERT either by Costunolide, or by siRNA or dominant-negative hTERT (DN-hTERT) abrogated (i) expression of Glucose-6-phosphate dehydrogenase (G6PD) and Transketolase (TKT) – two major nodes in the pentose phosphate (PPP) pathway; and (ii) phosphorylation of glycogen synthase (GS). hTERT knock-down decreased TKT activity and increased glycogen accumulation. Interestingly, siRNA-mediated knock-down of TKT elevated glycogen accumulation. Coherent with the in vitro findings, Costunolide reduced tumor burden in heterotypic xenograft glioma mouse model. Costunolide-treated tumors exhibited diminished TKT activity, heightened glycogen accumulation, and increased senescence. Importantly, glioblastoma multiforme (GBM) patient tumors bearing TERT promoter mutations (C228T and C250T) known to be associated with increased telomerase activity; exhibited elevated Nrf2 and TKT expression and decreased glycogen accumulation. Taken together, our findings highlight the previously unknown (i) role of telomerase in the regulation of PPP and glycogen accumulation and (ii) the involvement of Nrf2-TERT loop in maintaining oxidative defense responses in glioma cells.

Role of reactive oxygen species in regulation of glucose transport in skeletal muscle during exercise

Glucose derived from extracellular sources serves as an energy source in virtually all eukaryotic cells, including skeletal muscle. Its contribution to energy turnover increases with exercise intensity up to moderately heavy workloads. However, at very high workloads, the contribution of extracellular glucose to energy turnover is negligible, despite the high rate of glucose transport. Reactive oxygen species (ROS) are involved in the stimulation of glucose transport in isolated skeletal muscle preparations during intense repeated contractions. Consistent with this observation, heavy exercise is associated with significant production of ROS. However, during more mild to moderate stimulation or exercise conditions (in vitro, in situ and in vivo) antioxidants do not affect glucose transport. It is noteworthy that the production of ROS is limited or not observed under these conditions and that the concentration of the antioxidant used was extremely low. The results to date suggest that ROS involvement in activation of glucose transport occurs primarily during intense short-term exercise and that other mechanisms are involved during mild to moderate exercise. What remains puzzling is why ROS-mediated activation of glucose transport would occur under conditions where glucose transport is highest and utilization (i.e. phosphorylation of glucose by hexokinase) is low. Possibly ROS production is involved in priming glucose transport during heavy exercise to accelerate glycogen biogenesis during the initial recovery period after exercise, as well as altering other aspects of intracellular metabolism.

Impaired glucose metabolism and exercise capacity with muscle-specific glycogen synthase 1 (gys1) deletion in adult mice

Objective

Muscle glucose storage and muscle glycogen synthase (gys1) defects have been associated with insulin resistance. As there are multiple mechanisms for insulin resistance, the specific role of glucose storage defects is not clear. The aim of this study was to examine the effects of muscle-specific gys1 deletion on glucose metabolism and exercise capacity.

Methods

Tamoxifen inducible and muscle specific gys-1 KO mice were generated using the Cre/loxP system. Mice were subjected to glucose tolerance tests, euglycemic/hyperinsulinemic clamps and exercise tests.

Results

gys1-KO mice showed ≥85% reduction in muscle gys1 mRNA and protein concentrations, 70% reduction in muscle glycogen levels, postprandial hyperglycaemia and hyperinsulinaemia and impaired glucose tolerance. Under insulin-stimulated conditions, gys1-KO mice displayed reduced glucose turnover and muscle glucose uptake, indicative of peripheral insulin resistance, as well as increased plasma and muscle lactate levels and reductions in muscle hexokinase II levels. gys1-KO mice also exhibited markedly reduced exercise and endurance capacity.

Conclusions

Thus, muscle-specific gys1 deletion in adult mice results in glucose intolerance due to insulin resistance and reduced muscle glucose uptake as well as impaired exercise and endurance capacity.

In brief

This study demonstrates why the body prioritises muscle glycogen storage over liver glycogen storage despite the critical role of the liver in supplying glucose to the brain in the fasting state and shows that glycogen deficiency results in impaired glucose metabolism and reduced exercise capacity.

•

Muscle-specific gys1 knockdown in adult mice results in 70% reduction in skeletal muscle glycogen levels.

•

Muscle-specific gys1 knockdown leads to glucose intolerance and peripheral insulin resistance.

•

Muscle glycogen depletion caused impaired performance, as well as fatigue development during exercise.

Dietary milk fat globule membrane improves endurance capacity in mice

Milk fat globule membrane (MFGM) comprises carbohydrates, membrane-specific proteins, glycoproteins, phospholipids, and sphingolipids. We evaluated the effects of MFGM consumption over a 12-wk period on endurance capacity and energy metabolism in BALB/c mice. Long-term MFGM intake combined with regular exercise improved endurance capacity, as evidenced by swimming time until fatigue, in a dose-dependent manner. The effect of dietary MFGM plus exercise was accompanied by higher oxygen consumption and lower respiratory quotient, as determined by indirect calorimetry. MFGM intake combined with exercise increased plasma levels of free fatty acids after swimming. After chronic intake of MFGM combined with exercise, the triglyceride content in the gastrocnemius muscle increased significantly. Mice given MFGM combined with exercise had higher mRNA levels of peroxisome proliferator-activated receptor-γ coactivator 1α (Pgc1α) and CPT-1b in the soleus muscle at rest, suggesting that increased lipid metabolism in skeletal muscle contributes, in part, to improved endurance capacity. MFGM treatment with cyclic equibiaxial stretch consisting of 10% elongation at 0.5 Hz with 1 h on and 5 h off increased the Pgc1α mRNA expression of differentiating C2C12 myoblasts in a dose-dependent manner. Supplementation with sphingomyelin increased endurance capacity in mice and Pgc1α mRNA expression in the soleus muscle in vivo and in differentiating myoblasts in vitro. These results indicate that dietary MFGM combined with exercise improves endurance performance via increased lipid metabolism and that sphingomyelin may be one of the components responsible for the beneficial effects of dietary MFGM.

Anti-obesity effects of 3-hydroxychromone derivative, a novel small-molecule inhibitor of glycogen synthase kinase-3

Glycogen synthase kinase 3 (GSK-3) plays a central role in cellular energy metabolism, and dysregulation of GSK-3 activity is implicated in a variety of metabolic disorders, including obesity, type 2 diabetes, and cancer. Hence, GSK-3 has emerged as an attractive target molecule for the treatment of metabolic disorders. Therefore, this research focused on identification and characterization of a novel small-molecule GSK-3 inhibitor. Compound 1a, a structure based on 3-hydroxychromone bearing isothiazolidine-1,1-dione, was identified from chemical library as a highly potent GSK-3 inhibitor. An in vitro kinase assay utilizing a panel of kinases demonstrated that compound 1a strongly inhibits GSK-3β. The potential effects of compound 1a on the inactivation of GSK-3 were confirmed in human liver HepG2 and human embryonic kidney HEK293 cells. Stabilization of glycogen synthase and β-catenin, which are direct targets of GSK-3, by compound 1a was assessed in comparison with two other GSK-3 inhibitors: LiCl and SB-415286. In mouse 3T3-L1 preadipocytes, compound 1a markedly blocked adipocyte differentiation. Consistently, intraperitoneal administration of compound 1a to diet-induced obese mice significantly ameliorated their key symptoms such as body weight gain, increased adiposity, dyslipidemia, and hepatic steatosis due to the marked reduction of whole-body lipid level. In vitro and in vivo effects were accompanied by upregulation of β-catenin stability and downregulation of the expression of several critical genes related to lipid metabolism. From these results, it can be concluded that compound 1a, a novel small-molecule inhibitor of GSK-3, has potential as a new class of therapeutic agent for obesity treatment.

The role of glycogen synthase in the development of hyperglycemia in type 2 diabetes: 'To store or not to store glucose, that's the question'

This review deals with the role of glycogen storage in skeletal muscle for the development of insulin resistance and type 2 diabetes. Specifically, the role of the enzyme glycogen synthase, which seems to be locked in its hyperphosphorylated and inactivated state, is discussed. This defect seems to be secondary to ectopic lipid disposition in the muscle cells. These molecular defects are discussed in the context of the overall pathophysiology of hyperglycemia in type 2 diabetic subjects.

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.

Molecular and functional characterization of glycogen synthase in the porcine satellite cells under insulin treatment

Glycogen synthase (GS) catalyzes the key step of glycogen synthesis and plays an important role in glycogen metabolism in liver and muscle. In this study, we cloned the cDNA and promoter sequences of porcine glycogen synthesis genes (GYS1 and GYS2). Expression analysis revealed that porcine GYS1 was highly expressed in the skeletal muscle and heart. GYS2 was expressed specifically in liver and subcutaneous adipose tissue. The expression level of GYS1 was up-regulated from proliferation to differentiation in the porcine satellite cells, and insulin did not significantly affect the transcription of GYS1. Insulin stimulated 72-h-differentiated satellite cells as indicated by decrease in phosphorylation of GS, but did not affect GYS1 transcription and total GS protein level, suggesting that the effect of insulin is primarily mediated via posttranscriptional control rather than regulated at the transcriptional level. Four single-nucleotide polymorphisms (SNPs) were detected in the promoter and cDNA sequences of porcine GYS1. Association analyses revealed that the GYS1 Hin6I and MvaI polymorphisms both had significant associations (P < 0.05) with pH of M. longissimus dorsi (pHLD), M. biceps femoris (pHBF) and M. semipinalis capitis (pHSC) at 45 min postmortem. These results provide useful information for further investigation on the function of glycogen synthase in porcine skeletal muscle.

Identification of differentially expressed genes in chickens differing in muscle glycogen content and meat quality

Background

The processing ability of poultry meat is highly related to its ultimate pH, the latter being mainly determined by the amount of glycogen in the muscle at death. The genetic determinism of glycogen and related meat quality traits has been established in the chicken but the molecular mechanisms involved in variations in these traits remain to be fully described. In this study, Chicken Genome Arrays (20 K) were used to compare muscle gene expression profiles of chickens from Fat (F) and Lean (L) lines that exhibited high and low muscle glycogen content, respectively, and of individuals exhibiting extremely high (G+) or low (G-) muscle glycogen content originating from the F₂ cross between the Fat and Lean lines. Real-time RT-PCR was subsequently performed to validate the differential expression of genes either selected from the microarray analysis or whose function in regulating glycogen metabolism was well known.

Results

Among the genes found to be expressed in chicken P. major muscle, 197 and 254 transcripts appeared to be differentially expressed on microarrays for the F vs. L and the G+ vs. G- comparisons, respectively. Some involved particularly in lipid and carbohydrate metabolism were selected for further validation studies by real-time RT-PCR. We confirmed that, as in mammals, the down-regulation of CEBPB and RGS2 coincides with a decrease in peripheral adiposity in the chicken, but these genes are also suggested to affect muscle glycogen turnover through their role in the cAMP-dependent signalling pathway. Several other genes were suggested to have roles in the regulation of glycogen storage in chicken muscle. PDK4 may act as a glycogen sensor in muscle, UGDH may compete for glycogen synthesis by using UDP-glucose for glucoronidation, and PRKAB1, PRKAG2, and PHKD may impact on glycogen turnover in muscle, through AMP-activated signalling pathways.

Conclusions

This study is the first stage in the understanding of molecular mechanisms underlying variations in poultry meat quality. Large scale analyses are now required to validate the role of the genes identified and ultimately to find molecular markers that can be used for selection or to optimize rearing practices.

Animal models of glycogen storage disorders

Glycogen is a polymer of glucose needed to provide for a continuous source of glucose during fasting. Glycogen synthesis and degradation are tightly controlled by complex regulatory mechanisms and any disturbance in this regulation can lead to an inadequate reservoir of glycogen or an accumulation of excess or abnormal glycogen stored either in the cytosol or in the lysosomes. Problems in the degradation or synthesis of glycogen are referred to as glycogen storage disorders (GSDs), which individually are rare diseases, yet collectively are a major category of inborn errors of metabolism in humans. To date, 11 distinct forms of GSDs are represented in animal models. These models provide a means to understand the mechanisms that regulate and execute the synthesis and degradation of glycogen. In this review, we summarize animal models that have arisen spontaneously in nature or have been engineered in the laboratory by recombinant DNA techniques, and categorize the disorders of glycogen metabolism as disorders of either synthesis or degradation.

JNK deficiency enhances fatty acid utilization and diverts glucose from oxidation to glycogen storage in cultured myotubes

Although germ-line deletion of c-Jun NH(2)-terminal kinase (JNK) improves overall insulin sensitivity in mice, those studies could not reveal the underlying molecular mechanism and the tissue site(s) in which reduced JNK activity elicits the observed phenotype. Given its importance in nonesterified fatty acids (NEFA) and glucose utilization, we hypothesized that the insulin-sensitive phenotype associated with Jnk deletion originates from loss of JNK function in skeletal muscle. Short hairpin RNA (shRNA)-mediated gene silencing was used to identify the functions of JNK subtypes in regulating energy metabolism and metabolic responses to elevated concentrations of NEFA in C2C12 myotubes, a cellular model of skeletal muscle. We show for the first time that cellular JNK2- and JNK1/JNK2-deficiency divert glucose from oxidation to glycogenesis due to increased glycogen synthase (GS) activity and induction of Pdk4. We further show that JNK2- and JNK1/JNK2-deficiency profoundly increase cellular NEFA oxidation, and their conversion to phospholipids and triglyceride. The increased NEFA utilization was coupled to increased expressions of selective NEFA handling genes including Cd36, Acsl4, and Chka, and enhanced palmitic acid (PA)-dependent suppression of acetyl-CoA carboxylase (Acc). In JNK-intact cells, PA inhibited insulin signaling and glycogenesis. Although silencing Jnk1 and/or Jnk2 prevented PA-induced inhibition of insulin signaling, it did not completely block decreased insulin-mediated glycogenesis, thus indicating JNK-independent pathways in the suppression of glycogenesis by PA. Muscle-specific inhibition of JNK2 (or total JNK) improves the capacity of NEFA utilization and glycogenesis, and is a potential therapeutic target for improving systemic insulin sensitivity in type 2 diabetes (T2D).

Characterization of mouse UDP-glucose pyrophosphatase, a Nudix hydrolase encoded by the Nudt14 gene

Recombinant mouse UDP-glucose pyrophosphatase (UGPPase), encoded by the Nudt14 gene, was produced in Escherichia coli and purified close to homogeneity. The enzyme catalyzed the conversion of [beta-(32)P]UDP-glucose to [(32)P]glucose-1-P and UMP, confirming that it hydrolyzed the pyrophosphate of the nucleoside diphosphate sugar to generate glucose-1-P and UMP. The enzyme was also active toward ADP-ribose. Activity is dependent on the presence of Mg(2+) and was greatest at alkaline pH above 8. Kinetic analysis indicated a K(m) of approximately 4mM for UDP-glucose and approximately 0.3mM for ADP-ribose. Based on V(max)/K(m) values, the enzyme was approximately 20-fold more active toward ADP-ribose. UGPPase behaves as a dimer in solution and can be cross-linked to generate a species of M(r) 54,000 from a monomer of 30,000 as judged by SDS-PAGE. The dimerization was not affected by the presence of glucose-1-P or UDP-glucose. Using antibodies raised against the recombinant protein, Western analysis indicated that UGPPase was widely expressed in mouse tissues, including skeletal muscle, liver, kidney, heart, lung, fat, heart and pancreas with a lower level in brain. It was generally present as a doublet when analyzed by SDS-PAGE, suggesting the occurrence of some form of post-translational modification. Efforts to interconvert the species by adding or inhibiting phosphatase activity were unsuccessful, leaving the nature of the modification unknown. Sequence alignments and database searches revealed related proteins in species as distant as Drosophila melanogaster and Caenorhabditis elegans.

Restoration of muscle functionality by genetic suppression of glycogen synthesis in a murine model of Pompe disease

Glycogen storage disease type II (GSDII) or Pompe disease is an autosomal recessive disorder caused by acid alpha-glucosidase (GAA) deficiency, leading to lysosomal glycogen accumulation. Affected individuals store glycogen mainly in cardiac and skeletal muscle tissues resulting in fatal hypertrophic cardiomyopathy and respiratory failure in the most severe infantile form. Enzyme replacement therapy has already proved some efficacy, but results remain variable especially in skeletal muscle. Substrate reduction therapy was successfully used to improve the phenotype in several lysosomal storage disorders. We have recently demonstrated that shRNA-mediated reduction of glycogen synthesis led to a significant reduction of glycogen accumulation in skeletal muscle of GSDII mice. In this paper, we analyzed the effect of a complete genetic elimination of glycogen synthesis in the same GSDII model. GAA and glycogen synthase 1 (GYS1) KO mice were inter-crossed to generate a new double-KO model. GAA/GYS1-KO mice exhibited a profound reduction of the amount of glycogen in the heart and skeletal muscles, a significant decrease in lysosomal swelling and autophagic build-up as well as a complete correction of cardiomegaly. In addition, the abnormalities in glucose metabolism and insulin tolerance observed in the GSDII model were corrected in double-KO mice. Muscle atrophy observed in 11-month-old GSDII mice was less pronounced in GAA/GYS1-KO mice, resulting in improved exercise capacity. These data demonstrate that long-term elimination of muscle glycogen synthesis leads to a significant improvement of structural, metabolic and functional defects in GSDII mice and offers a new perspective for the treatment of Pompe disease.

Effects of aging and calorie restriction on rat skeletal muscle glycogen synthase and glycogen phosphorylase

Calorie restriction's (CR) effects on age-associated changes in glycogen-metabolizing enzymes were studied in rat soleus (SOL) and tibialis anterior (TA) muscles. Old (24 months) compared to young (6 months) rats maintained ad libitum on a standard diet had reduced glycogen synthase (GS) activity, lower muscle GS protein levels, increased phosphorylation of GS at site 3a with less activation in SOL. Age-associated impairments in GS protein and activation-phosphorylation were also shown in TA. There was an age-associated reduction in glycogen phosphorylase (GP) activity level in SOL, while brain/muscle isoforms (B/M) of GP protein levels were higher. GP activity and protein levels were preserved, but GP was inactivated in TA with age. Glycogen content was unchanged in both muscles. CR did not alter GS or GP activity/protein levels in young rats. CR hindered age-related decreases in GS activity/protein, unrelated to GS mRNA levels, and GS inactivation-phosphorylation; not on GP. In older rats, CR enhanced glycogen accumulation in SOL. Short-term fasting did not recapitulate CR effects in old rats. Thus, the predominant age-associated impairments on skeletal muscle GS and GP activities occur in the oxidative SOL muscle of rats, and CR can attenuate the loss of GS activity/activation and stimulate glycogen accumulation.

Different contribution of muscle and liver lipid metabolism to endurance capacity and obesity susceptibility of mice

We investigated strain differences in whole body energy metabolism, peripheral lipid metabolism, and energy metabolism-related gene expression and protein levels in BALB/c, C57BL/6J, and A/J mice to evaluate the relationship between endurance capacity, susceptibility to diet-induced obesity, and differences in lipid metabolism in muscle and liver. A high-fat diet significantly increased body weight and fat weight in C57BL/6J mice, but not in BALB/c and A/J mice. The endurance capacity of BALB/c mice was 52% greater than that of C57BL/6J mice and 217% greater than that of A/J mice. The respiratory exchange ratio was lowest in BALB/c mice, higher in C57BL/6J mice, and highest in A/J mice, which inversely correlated with the endurance capacity and fatty acid β-oxidation activity in the muscle. Plasma lactate levels measured immediately after exercise were lowest in BALB/c mice and highest in A/J mice, although there was no difference under resting conditions, suggesting that carbohydrate breakdown is suppressed by enhanced fat utilization during exercise in BALB/c mice. On the other hand, the body weight increase induced by high-fat feeding was related to a reduced whole body energy expenditure, higher respiratory quotient, and lower fatty acid β-oxidation activity in the liver. In addition, β-oxidation activity in the muscle and liver roughly paralleled the mRNA and protein levels of lipid metabolism-related molecules, such as peroxisome proliferator-activated receptor and medium-chain acyl-CoA dehydrogenase, in each tissue. These findings indicate that genetically determined basal muscle and liver lipid metabolism and responsiveness to exercise influence physical performance and obesity susceptibility.

Deletion of hexose-6-phosphate dehydrogenase activates the unfolded protein response pathway and induces skeletal myopathy

Hexose-6-phosphate dehydrogenase (H6PD) is the initial component of a pentose phosphate pathway inside the endoplasmic reticulum (ER) that generates NADPH for ER enzymes. In liver H6PD is required for the 11-oxoreductase activity of 11beta-hydroxysteroid dehydrogenase type 1, which converts inactive 11-oxo-glucocorticoids to their active 11-hydroxyl counterparts; consequently, H6PD null mice are relatively insensitive to glucocorticoids, exhibiting fasting hypoglycemia, increased insulin sensitivity despite elevated circulating levels of corticosterone, and increased basal and insulin-stimulated glucose uptake in muscles normally enriched in type II (fast) fibers, which have increased glycogen content. Here, we show that H6PD null mice develop a severe skeletal myopathy characterized by switching of type II to type I (slow) fibers. Running wheel activity and electrically stimulated force generation in isolated skeletal muscle are both markedly reduced. Affected muscles have normal sarcomeric structure at the electron microscopy level but contain large intrafibrillar membranous vacuoles and abnormal triads indicative of defects in structure and function of the sarcoplasmic reticulum (SR). SR proteins involved in calcium metabolism, including the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA), calreticulin, and calsequestrin, show dysregulated expression. Microarray analysis and real-time PCR demonstrate overexpression of genes encoding proteins in the unfolded protein response pathway. We propose that the absence of H6PD induces a progressive myopathy by altering the SR redox state, thereby impairing protein folding and activating the unfolded protein response pathway. These studies thus define a novel metabolic pathway that links ER stress to skeletal muscle integrity and function.

Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis

Polysaccharide storage myopathy (PSSM) is a novel glycogenosis in horses characterized by abnormal glycogen accumulation in skeletal muscle and muscle damage with exertion. It is unlike glycogen storage diseases resulting from known defects in glycogenolysis, glycolysis, and glycogen synthesis that have been described in humans and domestic animals. A genome-wide association identified GYS1, encoding skeletal muscle glycogen synthase (GS), as a candidate gene for PSSM. DNA sequence analysis revealed a mutation resulting in an arginine-to-histidine substitution in a highly conserved region of GS. Functional analysis demonstrated an elevated GS activity in PSSM horses, and haplotype analysis and allele age estimation demonstrated that this mutation is identical by descent among horse breeds. This is the first report of a gain-of-function mutation in GYS1 resulting in a glycogenosis.

Insulin promotes glycogen synthesis in the absence of GSK3 phosphorylation in skeletal muscle

Insulin promotes dephosphorylation and activation of glycogen synthase (GS) by inactivating glycogen synthase kinase (GSK) 3 through phosphorylation. Insulin also promotes glucose uptake and glucose 6-phosphate (G-6-P) production, which allosterically activates GS. The relative importance of these two regulatory mechanisms in the activation of GS in vivo is unknown. The aim of this study was to investigate if dephosphorylation of GS mediated via GSK3 is required for normal glycogen synthesis in skeletal muscle with insulin. We employed GSK3 knockin mice in which wild-type GSK3 alpha and -beta genes are replaced with mutant forms (GSK3 alpha/beta S21A/S21A/S9A/S9A), which are nonresponsive to insulin. Although insulin failed to promote dephosphorylation and activation of GS in GSK3 alpha/beta S21A/S21A/S9A/S9A mice, glycogen content in different muscles from these mice was similar compared with wild-type mice. Basal and epinephrine-stimulated activity of muscle glycogen phosphorylase was comparable between wild-type and GSK3 knockin mice. Incubation of isolated soleus muscle in Krebs buffer containing 5.5 mM glucose in the presence or absence of insulin revealed that the levels of G-6-P, the rate of [14C]glucose incorporation into glycogen, and an increase in total glycogen content were similar between wild-type and GSK3 knockin mice. Injection of glucose containing 2-deoxy-[3H]glucose and [14C]glucose also resulted in similar rates of muscle glucose uptake and glycogen synthesis in vivo between wild-type and GSK3 knockin mice. These results suggest that insulin-mediated inhibition of GSK3 is not a rate-limiting step in muscle glycogen synthesis in mice. This suggests that allosteric regulation of GS by G-6-P may play a key role in insulin-stimulated muscle glycogen synthesis in vivo.

Muscle-specific deletion of rictor impairs insulin-stimulated glucose transport and enhances Basal glycogen synthase activity

Rictor is an essential component of mTOR (mammalian target of rapamycin) complex 2 (mTORC2), a kinase complex that phosphorylates Akt at Ser473 upon activation of phosphatidylinositol 3-kinase (PI-3 kinase). Since little is known about the role of either rictor or mTORC2 in PI-3 kinase-mediated physiological processes in adult animals, we generated muscle-specific rictor knockout mice. Muscle from male rictor knockout mice exhibited decreased insulin-stimulated glucose uptake, and the mice showed glucose intolerance. In muscle lacking rictor, the phosphorylation of Akt at Ser473 was reduced dramatically in response to insulin. Furthermore, insulin-stimulated phosphorylation of the Akt substrate AS160 at Thr642 was reduced in rictor knockout muscle, indicating a defect in insulin signaling to stimulate glucose transport. However, the phosphorylation of Akt at Thr308 was normal and sufficient to mediate the phosphorylation of glycogen synthase kinase 3 (GSK-3). Basal glycogen synthase activity in muscle lacking rictor was increased to that of insulin-stimulated controls. Consistent with this, we observed a decrease in basal levels of phosphorylated glycogen synthase at a GSK-3/protein phosphatase 1 (PP1)-regulated site in rictor knockout muscle. This change in glycogen synthase phosphorylation was associated with an increase in the catalytic activity of glycogen-associated PP1 but not increased GSK-3 inactivation. Thus, rictor in muscle tissue contributes to glucose homeostasis by positively regulating insulin-stimulated glucose uptake and negatively regulating basal glycogen synthase activity.

Relationship between glycogen accumulation and the laforin dual specificity phosphatase

Laforin, encoded by the EPM2A gene, is a dual specificity protein phosphatase that has a functional glycogen-binding domain. Mutations in the EPM2A gene account for around half of the cases of Lafora disease, an autosomal recessive neurodegenerative disorder, characterized by progressive myoclonus epilepsy. The hallmark of the disease is the presence of Lafora bodies, which contain polyglucosan, a poorly branched form of glycogen, in neurons and other tissues. We examined the level of laforin protein in several mouse models in which muscle glycogen accumulation has been altered genetically. Mice with elevated muscle glycogen have increased laforin as judged by Western analysis. Mice completely lacking muscle glycogen or with 10% normal muscle glycogen had reduced laforin. Mice defective in the GAA gene encoding lysosomal alpha-glucosidase (acid maltase) overaccumulate glycogen in the lysosome but did not have elevated laforin. We propose, therefore, that laforin senses cytosolic glycogen accumulation which in turn determines the level of laforin protein.

Gene expression profiling of mice with genetically modified muscle glycogen content

Glycogen, a branched polymer of glucose, forms an energy re-serve in numerous organisms. In mammals, the two largest glyco-gen stores are in skeletal muscle and liver, which express tissue-specific glycogen synthase isoforms. MGSKO mice, in which mGys1 (mouse glycogen synthase) is disrupted, are devoid of muscle glycogen [Pederson, Chen, Schroeder, Shou, DePaoli-Roach and Roach (2004) Mol. Cell. Biol. 24, 7179-7187]. The GSL30 mouse line hyper-accumulates glycogen in muscle [Manchester, Skurat, Roach, Hauschka and Lawrence (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 10707-10711]. We performed a microarray analysis of mRNA from the anterior tibialis, medial gastrocnemius and liver of MGSKO mice, and from the gastroc-nemius of GSL30 mice. In MGSKO mice, transcripts of 79 genes varied in their expression in the same direction in both the anterior tibialis and gastrocnemius. These included several genes encoding proteins proximally involved in glycogen metabolism. The Ppp1r1a [protein phosphatase 1 regulatory (inhibitor) sub-unit 1A] gene underwent the greatest amount of downregulation. In muscle, the downregulation of Pfkfb1 and Pfkfb3, encoding isoforms of 6-phosphofructo-2-kinase/fructose-2,6-bisphospha-tase, is consistent with decreased glycolysis. Pathways for branched-chain amino acid, and ketone body utilization appear to be downregulated, as is the capacity to form the gluconeogenic precursors alanine, lactate and glutamine. Expression changes among several members of the Wnt signalling pathway were identified, suggesting an as yet unexplained role in glycogen meta-bolism. In liver, the upregulation of Pfkfb1 and Pfkfb3 expression is consistent with increased glycolysis, perhaps as an adaptation to altered muscle metabolism. By comparing changes in muscle expression between MGSKO and GSL30 mice, we found a subset of 44 genes, the expression of which varied as a function of muscle glycogen content. These genes are candidates for regulation by glycogen levels. Particularly interesting is the observation that 11 of these genes encode cardiac or slow-twitch isoforms of muscle contractile proteins, and are upregulated in muscle that has a greater oxidative capacity in MGSKO mice.

Muscle-specific deletion of the Glut4 glucose transporter alters multiple regulatory steps in glycogen metabolism

Mice with muscle-specific knockout of the Glut4 glucose transporter (muscle-G4KO) are insulin resistant and mildly diabetic. Here we show that despite markedly reduced glucose transport in muscle, muscle glycogen content in the fasted state is increased. We sought to determine the mechanism(s). Basal glycogen synthase activity is increased by 34% and glycogen phosphorylase activity is decreased by 17% (P < 0.05) in muscle of muscle-G4KO mice. Contraction-induced glycogen breakdown is normal. The increased glycogen synthase activity occurs in spite of decreased signaling through the insulin receptor substrate 1 (IRS-1)-phosphoinositide (PI) 3-kinase-Akt pathway and increased glycogen synthase kinase 3beta (GSK3beta) activity in the basal state. Hexokinase II is increased, leading to an approximately twofold increase in glucose-6-phosphate levels. In addition, the levels of two scaffolding proteins that are glycogen-targeting subunits of protein phosphatase 1 (PP1), the muscle-specific regulatory subunit (RGL) and the protein targeting to glycogen (PTG), are strikingly increased by 3.2- to 4.2-fold in muscle of muscle-G4KO mice compared to wild-type mice. The catalytic activity of PP1, which dephosphorylates and activates glycogen synthase, is also increased. This dominates over the GSK3 effects, since glycogen synthase phosphorylation on the GSK3-regulated site is decreased. Thus, the markedly reduced glucose transport in muscle results in increased glycogen synthase activity due to increased hexokinase II, glucose-6-phosphate, and RGL and PTG levels and enhanced PP1 activity. This, combined with decreased glycogen phosphorylase activity, results in increased glycogen content in muscle in the fasted state when glucose transport is reduced.

Changes in exercise-induced gene expression in 5'-AMP-activated protein kinase gamma3-null and gamma3 R225Q transgenic mice

5'-AMP-activated protein kinase (AMPK) is important for metabolic sensing. We used AMPKgamma3 mutant-overexpressing Tg-Prkag3(225Q) and AMPKgamma3-knockout Prkag3-/- mice to determine the role of the AMPKgamma3 isoform in exercise-induced metabolic and gene regulatory responses in skeletal muscle. Mice were studied after 2 h swimming or 2.5 h recovery. Exercise increased basal and insulin-stimulated glucose transport, with similar responses among genotypes. In Tg-Prkag3(225Q) mice, acetyl-CoA carboxylase (ACC) phosphorylation was increased and triglyceride content was reduced after exercise, suggesting that this mutation promotes greater reliance on lipid oxidation. In contrast, ACC phosphorylation and triglyceride content was similar between wild-type and Prkag3-/- mice. Expression of genes involved in lipid and glucose metabolism was altered by genetic modification of AMPKgamma3. Expression of lipoprotein lipase 1, carnitine palmitoyl transferase 1b, and 3-hydroxyacyl-CoA dehydrogenase was increased in Tg-Prkag3(225Q) mice, with opposing effects in Prkag3-/- mice after exercise. GLUT4, hexokinase II (HKII), and glycogen synthase mRNA expression was increased in Tg-Prkag3(225Q) mice after exercise. GLUT4 and HKII mRNA expression was increased in wild-type mice and blunted in Prkag3-/- mice after recovery. In conclusion, the Prkag3(225Q) mutation, rather than presence of a functional AMPKgamma3 isoform, directly promotes metabolic and gene regulatory responses along lipid oxidative pathways in skeletal muscle after endurance exercise.

Is non-insulin dependent glucose uptake a therapeutic alternative? Part 1: physiology, mechanisms and role of non insulin-dependent glucose uptake in type 2 diabetes

Several decades of research for treating type 2 diabetes have yielded new drugs but the actual experience with the available oral antidiabetic compounds clearly shows that therapeutic needs are not matched. This highlights the urgent need for exploring other pathways. All cell types have the capacity to take up glucose independently of insulin, whereby basal but also hyperglycaemia-promoted glucose supply is ensured. Although poorly explored, insulin-independent glucose uptake might nevertheless represent a therapeutic target, as an alternative to the clear limits of actual drug treatments. This review not only critically examines some major pathways not requiring insulin (although they may be influenced by the hormone) but importantly, this analysis extends to the clinical applicability of these potential therapeutic principles by also considering their predictable tolerability for long-term therapy. In particular vascular safety (the ultimate problem linked with diabetes) will be envisaged because of the ubiquitous distribution of glucose transporters and some linked mechanisms. Several mechanisms can be identified which do not require insulin for their functioning. The first part of this review deals with the description, the regulation and the limits of some mechanisms representing potential pharmacological targets capable of having a highly significant impact on glucose uptake. These selected topics are: a) unmasking and/or activation of glucose transporters in cell plasma membranes, b) insulin mimetics acting at postreceptor level, c) activation of AMPK, d) increasing nitric oxide and e) increasing glucose-6P and glycogen stores.

Mice with elevated muscle glycogen stores do not have improved exercise performance

Skeletal muscle glycogen is considered to be an important source of energy for contraction and increasing the level of the glucose polymer is generally thought to improve exercise performance in humans. A genetically modified mouse model (GSL30), which overaccumulates glycogen due to overexpression of a hyperactive form of glycogen synthase, was used to examine whether increasing the level of the polysaccharide enhances the ability of mice to run on a treadmill. The skeletal muscle of the GSL30 mice had large deposits of glycogen. There were no significant increases in the work performed by GSL30 mice as compared to their respective wild type littermates when exercised to exhaustion. The amount of muscle glycogen utilized by GSL30 mice, however, was greater, while the amount of liver glycogen consumed during exhaustive exercise was less than wild type animals. This result suggests that increased muscle glycogen stores do not necessarily improve exercise performance in mice.

Determinants of the nucleocytoplasmic shuttling of muscle glycogen synthase

Muscle glycogen synthase (MGS) presents a nuclear speckled pattern in primary cultured human muscle and in 3T3-L1 cells deprived of glucose and with depleted glycogen reserves. Nuclear accumulation of the enzyme correlates inversely with cellular glycogen content. Although the glucose-induced export of MGS from the nucleus to the cytoplasm is blocked by leptomycin B, and therefore mediated by CRM1, no nuclear export signal was identified in the sequence of the protein. Deletion analysis shows that the region comprising amino acids 555-633 of human MGS, which encompasses an Arg-rich cluster involved in the allosteric activation of the enzyme by Glc6P, is crucial for its nuclear concentration and aggregation. Mutation of these Arg residues, which desensitizes the enzyme towards Glc6P, interferes with its nuclear accumulation. In contrast, the known phosphorylation sites of MGS that regulate its activity are not involved in the control of its subcellular distribution. Nuclear human MGS co-localizes with the promyelocytic leukaemia oncoprotein and p80-coilin, a marker of Cajal bodies. The subnuclear distribution of MGS is altered by incubation with transcription inhibitors. These observations suggest that, in addition to its metabolic function, MGS may participate in nuclear processes.

Yeast glycogenin (Glg2p) produced in Escherichia coli is simultaneously glucosylated at two vicinal tyrosine residues but results in a reduced bacterial glycogen accumulation

Saccharomyces cerevisiae possesses two glycogenin isoforms (designated as Glg1p and Glg2p) that both contain a conserved tyrosine residue, Tyr232. However, Glg2p possesses an additional tyrosine residue, Tyr230 and therefore two potential autoglucosylation sites. Glucosylation of Glg2p was studied using both matrix-assisted laser desorption ionization and electrospray quadrupole time of flight mass spectrometry. Glg2p, carrying a C-terminal (His6) tag, was produced in Escherichia coli and purified. By tryptic digestion and reversed phase chromatography a peptide (residues 219-246 of the complete Glg2p sequence) was isolated that contained 4-25 glucosyl residues. Following incubation of Glg2p with UDPglucose, more than 36 glucosyl residues were covalently bound to this peptide. Using a combination of cyanogen bromide cleavage of the protein backbone, enzymatic hydrolysis of glycosidic bonds and reversed phase chromatography, mono- and diglucosylated peptides having the sequence PNYGYQSSPAM were generated. MS/MS spectra revealed that glucosyl residues were attached to both Tyr232 and Tyr230 within the same peptide. The formation of the highly glucosylated eukaryotic Glg2p did not favour the bacterial glycogen accumulation. Under various experimental conditions Glg2p-producing cells accumulated approximately 30% less glycogen than a control transformed with a Glg2p lacking plasmid. The size distribution of the glycogen and extractable activities of several glycogen-related enzymes were essentially unchanged. As revealed by high performance anion exchange chromatography, the intracellular maltooligosaccharide pattern of the bacterial cells expressing the functional eukaryotic transgene was significantly altered. Thus, the eukaryotic glycogenin appears to be incompatible with the bacterial initiation of glycogen biosynthesis.

Lipin, a lipodystrophy and obesity gene

Lipodystrophy and obesity represent extreme and opposite ends of the adiposity spectrum and have typically been attributed to alterations in the expression or function of distinct sets of genes. We previously demonstrated that lipin deficiency impairs adipocyte differentiation and causes lipodystrophy in the mouse. Using two different tissue-specific lipin transgenic mouse strains, we now demonstrate that enhanced lipin expression in either adipose tissue or skeletal muscle promotes obesity. This occurs through diverse mechanisms in the two tissues, with lipin levels in adipose tissue influencing the fat storage capacity of the adipocyte, and lipin levels in skeletal muscle acting as a determinant of whole-body energy expenditure and fat utilization. Thus, variations in lipin levels alone are sufficient to induce extreme states of adiposity and may represent a mechanism by which adipose tissue and skeletal muscle modulate fat mass and energy balance.

Effects of insulin and transgenic overexpression of UDP-glucose pyrophosphorylase on UDP-glucose and glycogen accumulation in skeletal muscle fibers

UDP-glucose (UDP-Glc) and glycogen levels in skeletal muscle fibers of defined fiber type were measured using microanalytical methods. Infusing rats with insulin increased glycogen in both Type I and Type II fibers. Insulin was without effect on UDP-Glc in Type I fibers but decreased UDP-Glc by 35-40% in Type IIA/D and Type IIB fibers. The reduction in UDP-Glc suggested that UDP-Glc pyrophosphorylase (PPL) activity might limit glycogen synthesis in response to insulin. To explore this possibility, we generated mice overexpressing a UDP-Glc PPL transgene in skeletal muscle. The transgene increased both UDP-Glc PPL activity and levels of UDP-Glc in skeletal muscles by approximately 3-fold. However, overexpression of UDP-Glc PPL was without effect on either the levels of skeletal muscle glycogen or glucose tolerance in vivo. The transgene was also without effect on either control or insulin-stimulated rates of (14)C-glucose incorporation into glycogen in muscles incubated in vitro. The results indicate that UDP-Glc PPL activity is not limiting for glycogen synthesis.

Development of glucose intolerance in male transgenic mice overexpressing human glycogen synthase kinase-3beta on a muscle-specific promoter

Glycogen synthase kinase-3 (GSK-3) protein levels and activity are elevated in skeletal muscle in type 2 diabetes, and inversely correlated with both glycogen synthase activity and insulin-stimulated glucose disposal. To explore this relationship, we have produced transgenic mice that overexpress human GSK-3beta in skeletal muscle. GSK-3beta transgenic mice were heavier, by up to 20% (P < .001), than their age-matched controls due to an increase in fat mass. The male GSK-3beta transgenic mice had significantly raised plasma insulin levels and by 24 weeks of age became glucose-intolerant as determined by a 50% increase in the area under their oral glucose tolerance curve (P < .001). They were also hyperlipidemic with significantly raised serum cholesterol (+90%), nonesterified fatty acids (NEFAs) (+55%), and triglycerides (+170%). At 29 weeks of age, GSK-3beta protein levels were 5-fold higher, and glycogen synthase activation (-27%), glycogen levels (-58%) and insulin receptor substrate-1 (IRS-1) protein levels (-67%) were significantly reduced in skeletal muscle. Hepatic glycogen levels were significantly increased 4-fold. Female GSK-3beta transgenic mice did not develop glucose intolerance despite 7-fold overexpression of GSK-3beta protein and a 20% reduction in glycogen synthase activation in skeletal muscle. However, plasma NEFAs and muscle IRS-1 protein levels were unchanged in females. We conclude that overexpression of human GSK-3beta in skeletal muscle of male mice resulted in impaired glucose tolerance despite raised insulin levels, consistent with the possibility that elevated levels of GSK-3 in type 2 diabetes are partly responsible for insulin resistance.

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.

Abnormal cardiac development in the absence of heart glycogen

Glycogen serves as a repository of glucose in many mammalian tissues. Mice lacking this glucose reserve in muscle, heart, and several other tissues were generated by disruption of the GYS1 gene, which encodes an isoform of glycogen synthase. Crossing mice heterozygous for the GYS1 disruption resulted in a significant underrepresentation of GYS1-null mice in the offspring. Timed matings established that Mendelian inheritance was followed for up to 18.5 days postcoitum (dpc) and that approximately 90% of GYS1-null animals died soon after birth due to impaired cardiac function. Defects in cardiac development began between 11.5 and 14.5 dpc. At 18.5 dpc, the hearts were significantly smaller, with reduced ventricular chamber size and enlarged atria. Consistent with impaired cardiac function, edema, pooling of blood, and hemorrhagic liver were seen. Glycogen synthase and glycogen were undetectable in cardiac muscle and skeletal muscle from the surviving null mice, and the hearts showed normal morphology and function. Congenital heart disease is one of the most common birth defects in humans, at up to 1 in 50 live births. The results provide the first direct evidence that the ability to synthesize glycogen in cardiac muscle is critical for normal heart development and hence that its impairment could be a significant contributor to congenital heart defects.