Phosphorylation-dependent translocation of glycogen synthase to a novel structure during glycogen resynthesis

Impaired malin expression and interaction with partner proteins in Lafora disease

Lafora disease (LD) is an autosomal recessive myoclonus epilepsy with onset in the teenage years leading to death within a decade of onset. LD is characterized by the overaccumulation of hyperphosphorylated, poorly branched, insoluble, glycogen-like polymers called Lafora bodies. The disease is caused by mutations in either EPM2A, encoding laforin, a dual specificity phosphatase that dephosphorylates glycogen, or EMP2B, encoding malin, an E3-ubiquitin ligase. While glycogen is a widely accepted laforin substrate, substrates for malin have been difficult to identify partly due to the lack of malin antibodies able to detect malin in vivo. Here we describe a mouse model in which the malin gene is modified at the C-terminus to contain the c-myc tag sequence, making an expression of malin-myc readily detectable. Mass spectrometry analyses of immunoprecipitates using c-myc tag antibodies demonstrate that malin interacts with laforin and several glycogen-metabolizing enzymes. To investigate the role of laforin in these interactions we analyzed two additional mouse models: malin-myc/laforin knockout and malin-myc/LaforinCS, where laforin was either absent or the catalytic Cys was genomically mutated to Ser, respectively. The interaction of malin with partner proteins requires laforin but is not dependent on its catalytic activity or the presence of glycogen. Overall, the results demonstrate that laforin and malin form a complex in vivo, which stabilizes malin and enhances interaction with partner proteins to facilitate normal glycogen metabolism. They also provide insights into the development of LD and the rescue of the disease by the catalytically inactive phosphatase.

Roles of GFAT and PFK genes in energy metabolism of brown planthopper, Nilaparvata lugens

Glutamine:fructose-6-phosphate aminotransferases (GFATs) and phosphofructokinase (PFKs) are the principal rate-limiting enzymes involved in hexosamine biosynthesis pathway (HBP) and glycolysis pathway, respectively. In this study, the NlGFAT and NlPFK were knocked down through RNA interference (RNAi) in Nilaparvata lugens, the notorious brown planthopper (BPH), and the changes in energy metabolism were determined. Knockdown of either NlGFAT or NlPFK substantially reduced gene expression related to trehalose, glucose, and glycogen metabolism pathways. Moreover, trehalose content rose significantly at 72 h after dsGFAT injection, and glycogen content increased significantly at 48 h after injection. Glucose content remained unchanged throughout the experiment. Conversely, dsPFK injection did not significantly alter trehalose, but caused an extreme increase in glucose and glycogen content at 72 h after injection. The Knockdown of NlGFAT or NlPFK significantly downregulated the genes in the glycolytic pathway, as well as caused a considerable and significant decrease in pyruvate kinase (PK) activity after 48 h and 72 h of inhibition. After dsGFAT injection, most of genes in TCA cycle pathway were upregulated, but after dsNlPFK injection, they were downregulated. Correspondingly, ATP content substantially increased at 48 h after NlGFAT knockdown but decreased to an extreme extent by 72 h. In contrast, ATP content decreased significantly after NlPFK was knocked down and returned. The results have suggested the knockdown of either NlGFAT or NlPFK resulted in metabolism disorders in BPHs, highlighting the difference in the impact of those two enzyme genes on energy metabolism. Given their influence on BPHs energy metabolism, developing enzyme inhibitors or activators may provide a biological control for BPHs.

Effect of sample fractionation and normalization when immunoblotting for human muscle Na+/K+-ATPase subunits and glycogen synthase

Immunoblotting is widely used in muscle physiology to determine protein regulation and abundance. However, research groups use different protocols, which may result in differential outcomes. Herein, we investigated the effect of various homogenization procedures on determination of protein abundance in human m. vastus lateralis biopsies. Furthermore, we investigated differences in abundance between young healthy males (n = 12) and type-2 diabetics (n = 4), and the effect of data normalization. Fractionated lysates had the lowest variation in total protein determination as compared to non-fractionated homogenates. Abundance of NKAα₂, NKAβ₁, FXYD1, and glycogen synthase was higher (P < 0.05) in young healthy than in type-2 diabetics determined in both fractionated and non-fractionated samples for which normalization to the stain-free signal and/or standard curve did not affect outcomes. Precision and reliability of protein abundance determination between sample types showed a moderate to good reliability for these proteins, whereas the commonly used house-keeping protein, actin, showed poor reliability. In conclusion, fractionated and non-fractionated immunoblotting samples yield similar data for several sarcolemmal and cytosolic proteins, except for actin, which, therefore appears inappropriate for data normalization in immunoblotting of human skeletal muscle. Thus, fractionation does not seem to be a major source of bias when immunoblotting for NKA subunits and GS.

Catestatin induces glycogenesis by stimulating the phosphoinositide 3-kinase-AKT pathway

AIM

Defects in hepatic glycogen synthesis contribute to post-prandial hyperglycaemia in type 2 diabetic patients. Chromogranin A (CgA) peptide Catestatin (CST: hCgA352-372 ) improves glucose tolerance in insulin-resistant mice. Here, we seek to determine whether CST induces hepatic glycogen synthesis.

METHODS

We determined liver glycogen, glucose-6-phosphate (G6P), uridine diphosphate glucose (UDPG) and glycogen synthase (GYS2) activities; plasma insulin, glucagon, noradrenaline and adrenaline levels in wild-type (WT) as well as in CST knockout (CST-KO) mice; glycogen synthesis and glycogenolysis in primary hepatocytes. We also analysed phosphorylation signals of insulin receptor (IR), insulin receptor substrate-1 (IRS-1), phosphatidylinositol-dependent kinase-1 (PDK-1), GYS2, glycogen synthase kinase-3β (GSK-3β), AKT (a kinase in AKR mouse that produces Thymoma)/PKB (protein kinase B) and mammalian/mechanistic target of rapamycin (mTOR) by immunoblotting.

RESULTS

CST stimulated glycogen accumulation in fed and fasted liver and in primary hepatocytes. CST reduced plasma noradrenaline and adrenaline levels. CST also directly stimulated glycogenesis and inhibited noradrenaline and adrenaline-induced glycogenolysis in hepatocytes. In addition, CST elevated the levels of UDPG and increased GYS2 activity. CST-KO mice had decreased liver glycogen that was restored by treatment with CST, reinforcing the crucial role of CST in hepatic glycogenesis. CST improved insulin signals downstream of IR and IRS-1 by enhancing phospho-AKT signals through the stimulation of PDK-1 and mTORC2 (mTOR Complex 2, rapamycin-insensitive complex) activities.

CONCLUSIONS

CST directly promotes the glycogenic pathway by (a) reducing glucose production, (b) increasing glycogen synthesis from UDPG, (c) reducing glycogenolysis and (d) enhancing downstream insulin signalling.

Subcellular localization- and fibre type-dependent utilization of muscle glycogen during heavy resistance exercise in elite power and Olympic weightlifters

AIM

Glycogen particles are found in different subcellular localizations, which are utilized heterogeneously in different fibre types during endurance exercise. Although resistance exercise typically involves only a moderate use of mixed muscle glycogen, the hypothesis of the present study was that high-volume heavy-load resistance exercise would mediate a pattern of substantial glycogen depletion in specific subcellular localizations and fibre types.

METHODS

10 male elite weightlifters performed resistance exercise consisting of four sets of five (4 × 5) repetitions at 75% of 1RM back squats, 4 × 5 at 75% of 1RM deadlifts and 4 × 12 at 65% of 1RM rear foot elevated split squats. Muscle biopsies (vastus lateralis) were obtained before and after the exercise session. The volumetric content of intermyofibrillar (between myofibrils), intramyofibrillar (within myofibrils) and subsarcolemmal glycogen was assessed by transmission electron microscopy.

RESULTS

After exercise, biochemically determined muscle glycogen decreased by 38 (31:45)%. Location-specific glycogen analyses revealed in type 1 fibres a large decrement in intermyofibrillar glycogen, but no or only minor changes in intramyofibrillar or subsarcolemmal glycogen. In type 2 fibres, large decrements in glycogen were observed in all subcellular localizations. Notably, a substantial fraction of the type 2 fibres demonstrated near-depleted levels of intramyofibrillar glycogen after the exercise session.

CONCLUSION

Heavy resistance exercise mediates a substantial utilization of glycogen from all three subcellular localization in type 2 fibres, while mostly taxing intermyofibrillar glycogen stores in type 1 fibres. Thus, a better understanding of the impact of resistance training on myocellular metabolism and performance requires a focus on compartmentalized glycogen utilization.

Beyond energy storage: roles of glycogen metabolism in health and disease

Beyond storing and supplying energy in the liver and muscles, glycogen also plays critical roles in cell differentiation, signaling, redox regulation, and stemness under various physiological and pathophysiological conditions. Such versatile functions have been revealed by various forms of glycogen storage diseases. Here, we outline the source of carbon flux in glycogen metabolism and discuss how glycogen metabolism guides CD8⁺ T-cell memory formation and maintenance. Likewise, we review how this affects macrophage polarization and inflammatory responses. Furthermore, we dissect how glycogen metabolism supports tumor development by promoting tumor-repopulating cell growth in hypoxic tumor microenvironments. This review highlights the essential role of the gluconeogenesis-glycogenesis-glycogenolysis-PPP metabolic chain in redox homeostasis, thus providing insights into potential therapeutic strategies against major chronic diseases including cancer.

Effect of glycogen synthase and glycogen phosphorylase knockdown on the expression of glycogen- and insulin-related genes in the rice brown planthopper Nilaparvata lugens

Nilaparvata lugens is a serious threat to rice growth. Glycogen metabolism is one of the important physiological processes of insects, which is mainly regulated by glycogen synthase (GS) and glycogen phosphorylase (GP). In the present study, trehalose content was significantly reduced at 72 h after NlGP and NlGS knockdown, whereas glucose content was significantly increased at both 48 h and 72 h after GS knockdown. RNAi combined with RNA-Seq was used to identify NlGP- and NlGS-related pathways and genes in N. lugens. A total of 593 genes were up-regulated and 5969 genes were down-regulated after NlGP and NlGS knockdown, respectively. Moreover, the NlGS-knockdown group was mapped to 10,967 pathways, whereas the NlGP-knockdown group was mapped to 7948 pathways, and the greatest differences between the groups were associated with carbohydrate, lipid, amino acid and energy metabolism. Meanwhile, 1800, 1217, and 1211 transcripts in the NlGP-knockdown group and 2511, 1666, and 1727 transcripts in the NlGS-knockdown group were involved in bioprocess, cellular ingredients and molecular function, respectively. Almost all these genes were down-regulated by either NlGP or NlGS knockdown, with significant down-regulation of the 6-trehalose phosphate synthase (TPS), trehalase (TRE), GS, GP, phosphoacetylglucosamine mutase (PGM, n = 2), Insulin receptors (InRs) and insulin-like peptides (Ilps) genes. These results have demonstrated that RNAi-mediated NlGP and NlGS knockdown could lead to content of trehalose and glucose out of balance, but have no obvious effect on glycogen content, and have suggested that GS plays more complex role in other metabolism pathway of N. lugens.

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.

Brain Glycogen Structure and Its Associated Proteins: Past, Present and Future

This chapter reviews the history of glycogen-related research and discusses in detail the structure, regulation, chemical properties and subcellular distribution of glycogen and its associated proteins, with particular focus on these aspects in brain tissue.

β-Actin shows limited mobility and is required only for supraphysiological insulin-stimulated glucose transport in young adult soleus muscle

Studies in skeletal muscle cell cultures suggest that the cortical actin cytoskeleton is a major requirement for insulin-stimulated glucose transport, implicating the β-actin isoform, which in many cell types is the main actin isoform. However, it is not clear that β-actin plays such a role in mature skeletal muscle. Neither dependency of glucose transport on β-actin nor actin reorganization upon glucose transport have been tested in mature muscle. To investigate the role of β-actin in fully differentiated muscle, we performed a detailed characterization of wild type and muscle-specific β-actin knockout (KO) mice. The effects of the β-actin KO were subtle; however, we confirmed the previously reported decline in running performance of β-actin KO mice compared with wild type during repeated maximal running tests. We also found insulin-stimulated glucose transport into incubated muscles reduced in soleus but not in extensor digitorum longus muscle of young adult mice. Contraction-stimulated glucose transport trended toward the same pattern, but the glucose transport phenotype disappeared in soleus muscles from mature adult mice. No genotype-related differences were found in body composition or glucose tolerance or by indirect calorimetry measurements. To evaluate β-actin mobility in mature muscle, we electroporated green fluorescent protein (GFP)-β-actin into flexor digitorum brevis muscle fibers and measured fluorescence recovery after photobleaching. GFP-β-actin showed limited unstimulated mobility and no changes after insulin stimulation. In conclusion, β-actin is not required for glucose transport regulation in mature mouse muscle under the majority of the tested conditions. Thus, our work reveals fundamental differences in the role of the cortical β-actin cytoskeleton in mature muscle compared with cell culture.

The dynamic life of the glycogen granule

Glycogen, the primary storage form of glucose, is a rapid and accessible form of energy that can be supplied to tissues on demand. Each glycogen granule, or "glycosome," is considered an independent metabolic unit composed of a highly branched polysaccharide and various proteins involved in its metabolism. In this Minireview, we review the literature to follow the dynamic life of a glycogen granule in a multicompartmentalized system, i.e. the cell, and how and where glycogen granules appear and the factors governing its degradation. A better understanding of the importance of cellular compartmentalization as a regulator of glycogen metabolism is needed to unravel its role in brain energetics.

A decrease of ATP production steered by PEDF in cardiomyocytes with oxygen-glucose deprivation is associated with an AMPK-dependent degradation pathway

AIMS

The activated AMP activated protein kinase (AMPK) serves as a transient protective cardiovascular kinase via preserving adenosine triphosphate (ATP) production under ischemic conditions. However, recent studies reveal that inhibition of AMPK in stroke is neuroprotection. Pigment epithelium derived factor (PEDF) is also known for the protection of ischemic cardiomyocytes. However, the relationship between PEDF and AMPK in cardiomyocytes is poorly understood.

METHODS AND RESULTS

Rat neonatal and adult left ventricular cardiomyocytes were isolated and subjected to oxygen-glucose deprivation (OGD). During OGD, PEDF significantly reduced AMPKα levels to decrease ATP production and reduced ATP expenditure both in neonatal and adult cardiomyocytes, which increased energy reserves and cell viability. Importantly, pharmacological AMPK inhibitor reduced ATP production but failed to decrease ATP expenditure, thus leading cells into death. Furthermore, AMPKα was degraded by a ubiquitin-dependent proteasomal degradation pathway, which is associated with a PEDF/PEDFR/peroxisome proliferator activated receptor γ (PPARγ) axis. Inhibition of PPARγ or proteasome disrupted the interaction of AMPKα and PPARγ, which abolished AMPKα degradation. Importantly, the decrease of AMPKα and ATP level was normalized after recovery of oxygen and glucose.

CONCLUSIONS

We demonstrate a novel mechanism for regulation of cardiac ATP production by PEDF involving AMPKα and PPARγ. PEDF promotes proteasomal degradation of AMPK and, subsequently, reduces ATP production. The reduction of ATP production associated with the decrease of ATP expenditure completed by PEDF increase energy reserves and reduces cell energy failure, prolonging the cell activity during OGD.

Glycogen Phosphorylase and Glycogen Synthase: Gene Cloning and Expression Analysis Reveal Their Role in Trehalose Metabolism in the Brown Planthopper, Nilaparvata lugens Stål (Hemiptera: Delphacidae)

RNA interference has been used to study insects' gene function and regulation. Glycogen synthase (GS) and glycogen phosphorylase (GP) are two key enzymes in carbohydrates' conversion in insects. Glycogen content and GP and GS gene expression in several tissues and developmental stages of the Brown planthopper Nilaparvata lugens Stål (Hemiptera: Delphacidae) were analyzed in the present study, using quantitative reverse-transcription polymerase chain reaction to determine their response to double-stranded trehalases (dsTREs), trehalose-6-phosphate synthases (dsTPSs), and validamycin injection. The highest expression of both genes was detected in the wing bud, followed by leg and head tissues, and different expression patterns were shown across the developmental stages analyzed. Glycogen content significantly decreased 48 and 72 h after dsTPSs injection and 48 h after dsTREs injection. GP expression increased 48 h after dsTREs and dsTPSs injection and significantly decreased 72 h after dsTPSs, dsTRE1-1, and dsTRE1-2 injection. GS expression significantly decreased 48 h after dsTPS2 and dsTRE2 injection and 72 h after dsTRE1-1 and dsTRE1-2 injection. GP and GS expression and glycogen content significantly decreased 48 h after validamycin injection. The GP activity significantly decreased 48 h after validamycin injection, while GS activities of dsTPS1 and dsTRE2 injection groups were significantly higher than that of double-stranded GFP (dsGFP) 48 h after injection, respectively. Thus, glycogen is synthesized, released, and degraded across several insect tissues according to the need to maintain stable trehalose levels.

A highly prevalent equine glycogen storage disease is explained by constitutive activation of a mutant glycogen synthase

BACKGROUND

Equine type 1 polysaccharide storage myopathy (PSSM1) is associated with a missense mutation (R309H) in the glycogen synthase (GYS1) gene, enhanced glycogen synthase (GS) activity and excessive glycogen and amylopectate inclusions in muscle.

METHODS

Equine muscle biochemical and recombinant enzyme kinetic assays in vitro and homology modelling in silico, were used to investigate the hypothesis that higher GS activity in affected horse muscle is caused by higher GS expression, dysregulation, or constitutive activation via a conformational change.

RESULTS

PSSM1-affected horse muscle had significantly higher glycogen content than control horse muscle despite no difference in GS expression. GS activity was significantly higher in muscle from homozygous mutants than from heterozygote and control horses, in the absence and presence of the allosteric regulator, glucose 6 phosphate (G6P). Muscle from homozygous mutant horses also had significantly increased GS phosphorylation at sites 2+2a and significantly higher AMPKα1 (an upstream kinase) expression than controls, likely reflecting a physiological attempt to reduce GS enzyme activity. Recombinant mutant GS was highly active with a considerably lower K_m for UDP-glucose, in the presence and absence of G6P, when compared to wild type GS, and despite its phosphorylation.

CONCLUSIONS

Elevated activity of the mutant enzyme is associated with ineffective regulation via phosphorylation rendering it constitutively active. Modelling suggested that the mutation disrupts a salt bridge that normally stabilises the basal state, shifting the equilibrium to the enzyme's active state.

GENERAL SIGNIFICANCE

This study explains the gain of function pathogenesis in this highly prevalent polyglucosan myopathy.

The interaction between AMPKβ2 and the PP1-targeting subunit R6 is dynamically regulated by intracellular glycogen content

Breakdown of intracellular glycogen enhances interaction of the AMPKβ2 subunit and the R6 glycogen-targeting subunit of protein phosphatase type 1 (PP1), which occurs in conjunction with increased β2-Thr-148 phosphorylation.

The carbohydrate-binding domain of overexpressed STBD1 is important for its stability and protein-protein interactions

STBD1 (starch-binding domain-containing protein 1) belongs to the CBM20 (family 20 carbohydrate binding module) group of proteins, and is implicated in glycogen metabolism and autophagy. However, very little is known about its regulation or interacting partners. Here, we show that the CBM20 of STBD1 is crucial for its stability and ability to interact with glycogen-associated proteins. Mutation of a conserved tryptophan residue (W293) in this domain abolished the ability of STBD1 to bind to the carbohydrate amylose. Compared with the WT (wild-type) protein, this mutant exhibited rapid degradation that was rescued upon inhibition of the proteasome. Furthermore, STBD1 undergoes ubiquitination when expressed in COS cells, and requires the N-terminus for this process. In contrast, inhibition of autophagy did not significantly affect protein stability. In overexpression experiments, we discovered that STBD1 interacts with several glycogen-associated proteins, such as GS (glycogen synthase), GDE (glycogen debranching enzyme) and Laforin. Importantly, the W293 mutant of STBD1 was unable to do so, suggesting an additional role for the CBM20 domain in protein–protein interactions. In HepG2 hepatoma cells, overexpressed STBD1 could associate with endogenous GS. This binding increased during glycogenolysis, suggesting that glycogen is not required to bridge this interaction. Taken together, our results have uncovered new insights into the regulation and binding partners of STBD1.

STBD1 is a protein with a carbohydrate-binding domain that is implicated in autophagy and glycogen metabolism. Here we show the carbohydrate-binding domain is crucial for its stability and ability to bind to several glycogen-associated proteins.

Localisation of AMPK γ subunits in cardiac and skeletal muscles

The trimeric protein AMP-activated protein kinase (AMPK) is an important sensor of energetic status and cellular stress, and mutations in genes encoding two of the regulatory γ subunits cause inherited disorders of either cardiac or skeletal muscle. AMPKγ2 mutations cause hypertrophic cardiomyopathy with glycogen deposition and conduction abnormalities; mutations in AMPKγ3 result in increased skeletal muscle glycogen. In order to gain further insight into the roles of the different γ subunits in muscle and into possible disease mechanisms, we localised the γ2 and γ3 subunits, along with the more abundant γ1 subunit, by immunofluorescence in cardiomyocytes and skeletal muscle fibres. The predominant cardiac γ2 variant, γ2-3B, gave a striated pattern in cardiomyocytes, aligning with the Z-disk but with punctate staining similar to T-tubule (L-type Ca²⁺ channel) and sarcoplasmic reticulum (SERCA2) markers. In skeletal muscle fibres AMPKγ3 localises to the I band, presenting a uniform staining that flanks the Z-disk, also coinciding with the position of Ca²⁺ influx in these muscles. The localisation of γ2-3B- and γ3-containing AMPK suggests that these trimers may have similar functions in the different muscles. AMPK containing γ2-3B was detected in oxidative skeletal muscles which had low expression of γ3, confirming that these two regulatory subunits may be co-ordinately regulated in response to metabolic requirements. Compartmentalisation of AMPK complexes is most likely dependent on the regulatory γ subunit and this differential localisation may direct substrate selection and specify particular functional roles.

The 3T3-L1 adipocyte glycogen proteome

Background

Glycogen is a branched polysaccharide of glucose residues, consisting of α-1-4 glycosidic linkages with α-1-6 branches that together form multi-layered particles ranging in size from 30 nm to 300 nm. Glycogen spatial conformation and intracellular organization are highly regulated processes. Glycogen particles interact with their metabolizing enzymes and are associated with a variety of proteins that intervene in its biology, controlling its structure, particle size and sub-cellular distribution. The function of glycogen in adipose tissue is not well understood but appears to have a pivotal role as a regulatory mechanism informing the cells on substrate availability for triacylglycerol synthesis. To provide new molecular insights into the role of adipocyte glycogen we analyzed the glycogen-associated proteome from differentiated 3T3-L1-adipocytes.

Results

Glycogen particles from 3T3-L1-adipocytes were purified using a series of centrifugation steps followed by specific elution of glycogen bound proteins using α-1,4 glucose oligosaccharides, or maltodextrins, and tandem mass spectrometry. We identified regulatory proteins, 14-3-3 proteins, RACK1 and protein phosphatase 1 glycogen targeting subunit 3D. Evidence was also obtained for a regulated subcellular distribution of the glycogen particle: metabolic and mitochondrial proteins were abundant. Unlike the recently analyzed hepatic glycogen proteome, no endoplasmic proteins were detected, along with the recently described starch-binding domain protein 1. Other regulatory proteins which have previously been described as glycogen-associated proteins were not detected, including laforin, the AMPK beta-subunit and protein targeting to glycogen (PTG).

Conclusions

These data provide new molecular insights into the regulation of glycogen-bound proteins that are associated with the maintenance, organization and localization of the adipocyte glycogen particle.

Physiological aspects of the subcellular localization of glycogen in skeletal muscle

Glucose is stored in skeletal muscle fibers as glycogen, a branched-chain polymer observed in electron microscopy images as roughly spherical particles (known as β-particles of 10–45 nm in diameter), which are distributed in distinct localizations within the myofibers and are physically associated with metabolic and scaffolding proteins. Although the subcellular localization of glycogen has been recognized for more than 40 years, the physiological role of the distinct localizations has received sparse attention. Recently, however, studies involving stereological, unbiased, quantitative methods have investigated the role and regulation of these distinct deposits of glycogen. In this report, we review the available literature regarding the subcellular localization of glycogen in skeletal muscle as investigated by electron microscopy studies and put this into perspective in terms of the architectural, topological, and dynamic organization of skeletal muscle fibers. In summary, the distribution of glycogen within skeletal muscle fibers has been shown to depend on the fiber phenotype, individual training status, short-term immobilization, and exercise and to influence both muscle contractility and fatigability. Based on all these data, the available literature strongly indicates that the subcellular localization of glycogen has to be taken into consideration to fully understand and appreciate the role and regulation of glycogen metabolism and signaling in skeletal muscle. A full understanding of these phenomena may prove vital in elucidating the mechanisms that integrate basic cellular events with changing glycogen content.

Regulation of glycogen synthase from mammalian skeletal muscle--a unifying view of allosteric and covalent regulation

It is widely accepted that insufficient insulin-stimulated activation of muscle glycogen synthesis is one of the major components of non-insulin-dependent (type 2) diabetes mellitus. Glycogen synthase, a key enzyme in muscle glycogen synthesis, is extensively regulated, both allosterically (by glucose-6-phosphate, ATP, and others) and covalently (by phosphorylation). Although glycogen synthase has been a topic of intense study for more than 50 years, its kinetic characterization has been confounded by its large number of phosphorylation states. Questions remain regarding the function of glycogen synthase regulation and the relative importance of allosteric and covalent modification in fulfilling this function. In this review, we consider both earlier kinetic studies and more recent site-directed mutagenesis and crystal structure studies in a detailed qualitative discussion of the effects of regulation on the kinetics of glycogen synthase. We propose that both allosteric and covalent modification of glycogen synthase may be described by a Monod-Wyman-Changeux model in terms of apparent changes to L, the equilibrium constant for transition between the T and R conformers. As, with the exception of L, all parameters of this model are independent of the glycogen synthase phosphorylation state, the need to determine kinetic parameters for all possible states is eliminated; only the relationship between a particular state and L must be established. We conclude by suggesting that renewed efforts to characterize the relationship between phosphorylation and the kinetics of glycogen synthase are essential in order to obtain a better quantitative understanding of the function of glycogen synthesis regulation. The model we propose may prove useful in this regard.

Single fiber analyses of glycogen-related proteins reveal their differential association with glycogen in rat skeletal muscle

To understand how glycogen affects skeletal muscle physiology, we examined enzymes essential for muscle glycogen synthesis and degradation using single fibers from quiescent and stimulated rat skeletal muscle. Presenting a shift in paradigm, we show these proteins are differentially associated with glycogen granules. Protein diffusibility and/or abundance of glycogenin, glycogen branching enzyme (GBE), debranching enzyme (GDE), phosphorylase (GP), and synthase (GS) were examined in fibers isolated from rat fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscle. GDE and GP proteins were more abundant (∼10- to 100-fold) in fibers from EDL compared with SOL muscle. GS and glycogenin proteins were similar between muscles while GBE had an approximately fourfold greater abundance in SOL muscle. Mechanically skinned fibers exposed to physiological buffer for 10 min showed ∼70% total pools of GBE and GP were diffusible (nonbound), whereas GDE and GS were considerably less diffusible. Intense in vitro stimulation, sufficient to elicit a ∼50% decrease in intracellular glycogen, increased diffusibility of GDE, GP, and GS (∼15–60%) and decreased GBE diffusibility (∼20%). Amylase treatment, which breaks α-1,4 linkages of glycogen, indicated differential diffusibilities and hence glycogen associations of GDE and GS. Membrane solubilization (1% Triton-X-100) allowed a small additional amount of GDE and GS to diffuse from fibers, suggesting the majority of nonglycogen-associated GDE/GS is associated with myofibrillar/contractile network of muscle rather than membranes. Given differences in enzymes required for glycogen metabolism, the current findings suggest glycogen particles have fiber-type-dependent structures. The greater catabolic potential of glycogen breakdown in fast-twitch fibers may account for different contraction induced rates of glycogen utilization.

Hexokinase 2, glycogen synthase and phosphorylase play a key role in muscle glycogen supercompensation

Background

Glycogen-depleting exercise can lead to supercompensation of muscle glycogen stores, but the biochemical mechanisms of this phenomenon are still not completely understood.

Methods

Using chronic low-frequency stimulation (CLFS) as an exercise model, the tibialis anterior muscle of rabbits was stimulated for either 1 or 24 hours, inducing a reduction in glycogen of 90% and 50% respectively. Glycogen recovery was subsequently monitored during 24 hours of rest.

Results

In muscles stimulated for 1 hour, glycogen recovered basal levels during the rest period. However, in those stimulated for 24 hours, glycogen was supercompensated and its levels remained 50% higher than basal levels after 6 hours of rest, although the newly synthesized glycogen had fewer branches. This increase in glycogen correlated with an increase in hexokinase-2 expression and activity, a reduction in the glycogen phosphorylase activity ratio and an increase in the glycogen synthase activity ratio, due to dephosphorylation of site 3a, even in the presence of elevated glycogen stores. During supercompensation there was also an increase in 5′-AMP-activated protein kinase phosphorylation, correlating with a stable reduction in ATP and total purine nucleotide levels.

Conclusions

Glycogen supercompensation requires a coordinated chain of events at two levels in the context of decreased cell energy balance: First, an increase in the glucose phosphorylation capacity of the muscle and secondly, control of the enzymes directly involved in the synthesis and degradation of the glycogen molecule. However, supercompensated glycogen has fewer branches.

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.

Skeletal muscle glycogen content and particle size of distinct subcellular localizations in the recovery period after a high-level soccer match

Whole muscle glycogen levels remain low for a prolonged period following a soccer match. The present study was conducted to investigate how this relates to glycogen content and particle size in distinct subcellular localizations. Seven high-level male soccer players had a vastus lateralis muscle biopsy collected immediately after and 24, 48, 72 and 120 h after a competitive soccer match. Transmission electron microscopy was used to estimate the subcellular distribution of glycogen and individual particle size. During the first day of recovery, glycogen content increased by ~60% in all subcellular localizations, but during the subsequent second day of recovery glycogen content located within the myofibrils (Intramyofibrillar glycogen, a minor deposition constituting 10-15% of total glycogen) did not increase further compared with an increase in subsarcolemmal glycogen (-7 vs. +25%, respectively, P = 0.047). Conversely, from the second to the fifth day of recovery, glycogen content increased (53%) within the myofibrils compared to no change in subsarcolemmal or intermyofibrillar glycogen (P < 0.005). Independent of location, increment in particle size preceded increment in number of particles. Intriguingly, average particle size decreased; however, in the period from 3 to 5 days after the match. These findings suggest that glycogen storage in skeletal muscle is influenced by subcellular localization-specific mechanisms, which account for an increase in number of glycogen particles located within the myofibrils in the period from 2 to 5 days after the soccer match.

Regulation of glucose and glycogen metabolism during and after exercise

Utilization of carbohydrate in the form of intramuscular glycogen stores and glucose delivered from plasma becomes an increasingly important energy substrate to the working muscle with increasing exercise intensity. This review gives an update on the molecular signals by which glucose transport is increased in the contracting muscle followed by a discussion of glycogen mobilization and synthesis by the action of glycogen phosphorylase and glycogen synthase, respectively. Finally, this review deals with the signalling relaying the well-described increased sensitivity of glucose transport to insulin in the post-exercise period which can result in an overshoot of intramuscular glycogen resynthesis post exercise (glycogen supercompensation).

Differential pattern of glycogen accumulation after protein phosphatase 1 glycogen-targeting subunit PPP1R6 overexpression, compared to PPP1R3C and PPP1R3A, in skeletal muscle cells

BACKGROUND

PPP1R6 is a protein phosphatase 1 glycogen-targeting subunit (PP1-GTS) abundant in skeletal muscle with an undefined metabolic control role. Here PPP1R6 effects on myotube glycogen metabolism, particle size and subcellular distribution are examined and compared with PPP1R3C/PTG and PPP1R3A/G(M).

RESULTS

PPP1R6 overexpression activates glycogen synthase (GS), reduces its phosphorylation at Ser-641/0 and increases the extracted and cytochemically-stained glycogen content, less than PTG but more than G(M). PPP1R6 does not change glycogen phosphorylase activity. All tested PP1-GTS-cells have more glycogen particles than controls as found by electron microscopy of myotube sections. Glycogen particle size is distributed for all cell-types in a continuous range, but PPP1R6 forms smaller particles (mean diameter 14.4 nm) than PTG (36.9 nm) and G(M) (28.3 nm) or those in control cells (29.2 nm). Both PPP1R6- and G(M)-derived glycogen particles are in cytosol associated with cellular structures; PTG-derived glycogen is found in membrane- and organelle-devoid cytosolic glycogen-rich areas; and glycogen particles are dispersed in the cytosol in control cells. A tagged PPP1R6 protein at the C-terminus with EGFP shows a diffuse cytosol pattern in glucose-replete and -depleted cells and a punctuate pattern surrounding the nucleus in glucose-depleted cells, which colocates with RFP tagged with the Golgi targeting domain of β-1,4-galactosyltransferase, according to a computational prediction for PPP1R6 Golgi location.

CONCLUSIONS

PPP1R6 exerts a powerful glycogenic effect in cultured muscle cells, more than G(M) and less than PTG. PPP1R6 protein translocates from a Golgi to cytosolic location in response to glucose. The molecular size and subcellular location of myotube glycogen particles is determined by the PPP1R6, PTG and G(M) scaffolding.

Human skeletal muscle glycogen utilization in exhaustive exercise: role of subcellular localization and fibre type

Although glycogen is known to be heterogeneously distributed within skeletal muscle cells, there is presently little information available about the role of fibre types, utilization and resynthesis during and after exercise with respect to glycogen localization. Here, we tested the hypothesis that utilization of glycogen with different subcellular localizations during exhaustive arm and leg exercise differs and examined the influence of fibre type and carbohydrate availability on its subsequent resynthesis. When 10 elite endurance athletes (22 ± 1 years, VO2 max = 68 ± 5 ml kg-1 min-1, mean ± SD) performed one hour of exhaustive arm and leg exercise, transmission electron microscopy revealed more pronounced depletion of intramyofibrillar than of intermyofibrillar and subsarcolemmal glycogen. This phenomenon was the same for type I and II fibres, although at rest prior to exercise, the former contained more intramyofibrillar and subsarcolemmal glycogen than the latter. In highly glycogen-depleted fibres, the remaining small intermyofibrillar and subsarcolemmal glycogen particles were often found to cluster in groupings. In the recovery period, when the athletes received either a carbohydrate-rich meal or only water the impaired resynthesis of glycogen with water alone was associated primarily with intramyofibrillar glycogen. In conclusion, after prolonged high-intensity exercise the depletion of glycogen is dependent on subcellular localization. In addition, the localization of glycogen appears to be influenced by fibre type prior to exercise, as well as carbohydrate availability during the subsequent period of recovery. These findings provide insight into the significance of fibre type-specific compartmentalization of glycogen metabolism in skeletal muscle during exercise and subsequent recovery. .

Regulation of glycogen metabolism in yeast and bacteria

Microorganisms have the capacity to utilize a variety of nutrients and adapt to continuously changing environmental conditions. Many microorganisms, including yeast and bacteria, accumulate carbon and energy reserves to cope with the starvation conditions temporarily present in the environment. Glycogen biosynthesis is a main strategy for such metabolic storage, and a variety of sensing and signaling mechanisms have evolved in evolutionarily distant species to ensure the production of this homopolysaccharide. At the most fundamental level, the processes of glycogen synthesis and degradation in yeast and bacteria share certain broad similarities. However, the regulation of these processes is sometimes quite distinct, indicating that they have evolved separately to respond optimally to the habitat conditions of each species. This review aims to highlight the mechanisms, both at the transcriptional and at the post-transcriptional level, that regulate glycogen metabolism in yeast and bacteria, focusing on selected areas where the greatest increase in knowledge has occurred during the last few years. In the yeast system, we focus particularly on the various signaling pathways that control the activity of the enzymes of glycogen storage. We also discuss our recent understanding of the important role played by the vacuole in glycogen metabolism. In the case of bacterial glycogen, special emphasis is placed on aspects related to the genetic regulation of glycogen metabolism and its connection with other biological processes.

The regulation of muscle glycogen: the granule and its proteins

Despite decades of studying muscle glycogen in many metabolic situations, surprisingly little is known regarding its regulation. Glycogen is a dynamic and vital metabolic fuel that has very limited energetic capacity. Thus its regulation is highly complex and multifaceted. The stores in muscle are not homogeneous and there appear to be various metabolic pools. Each granule is capable of independent regulation and fundamental aspects of the regulation appear to be associated with a complex set of proteins (some are enzymes and others serve scaffolding roles) that associate both with the granule and with each other in a dynamic fashion. The regulation includes altered phosphorylation status and often translocation as well. The understanding of the roles and the regulation of glycogenin, protein phosphatase 1, glycogen targeting proteins, laforin and malin are in their infancy. These various processes appear to be the mechanisms that give the glycogen granule precise, yet dynamic regulation.

Analysis of hepatic glycogen-associated proteins

Glycogen particles are associated with a population of proteins that mediate its biological functions, including: management of glucose flux into and out of the glycogen particle, maintenance of glycogen structure and regulation of particle size, number, and cellular location. A survey of the glycogen-associated proteome would be predicted to identify the relative representation of known members of this population, and associations with unexpected proteins that have the potential to mediate other functions of the glycogen particle. We therefore purified glycogen particles from both mouse and rat liver, using different techniques, and analyzed the resulting tryptic peptides by MS. We also specifically eluted glycogen-binding proteins from the pellet using malto-oligosaccharides. Comparison of the rat and mouse populations, and analysis of specifically eluted proteins allow some conclusions to be made about the hepatic glycogen sub-proteome. With the exception of glycogen branching enzyme all glycogen metabolic proteins were detected. Novel associations were identified, including ferritin and starch-binding domain protein 1, a protein that contains both a transmembrane endoplasmic reticulum signal peptide and a carbohydrate-binding module. This study therefore provides insight into the organization of the glycogen proteome, identifies other associated proteins and provides a starting point to explore the dynamic nature and cellular distribution of this metabolically important protein population.

The subcellular localization of yeast glycogen synthase is dependent upon glycogen content

The budding yeast, Saccharomyces cerevisiae, accumulates the storage polysaccharide glycogen in response to nutrient limitation. Glycogen synthase, the major form of which is encoded by the GSY2 gene, catalyzes the key regulated step in glycogen storage. Here, we utilized Gsy2p fusions to green fluorescent protein (GFP) to determine where glycogen synthase was located within cells. We demonstrated that the localization pattern of Gsy2-GFP depended upon the glycogen content of the cell. When glycogen was abundant, Gsy2-GFP was found uniformly throughout the cytoplasm, but under low glycogen conditions, Gsy2-GFP localized to discrete spots within cells. Gsy2p is known to bind to glycogen, and we propose that the subcellular distribution of Gsy2-GFP reflects the distribution of glycogen particles. In the absence of glycogen, Gsy2p translocates into the nucleus. We hypothesize that Gsy2p is normally retained in the cytoplasm through its interaction with glycogen particles. When glycogen levels are reduced, Gsy2p loses this anchor and can traffic into the nucleus.

Homogenization-dependent responses of acid-soluble and acid-insoluble glycogen to exercise and refeeding in human muscles

Muscle glycogen exists as acid-insoluble (AIG) and acid-soluble (ASG) forms, with AIG levels reported in most recent studies in humans to be the most responsive to exercise and refeeding. Because the muscle samples in these studies were not homogenized to extract glycogen, such homogenization-free protocols might have resulted in a suboptimal yield of ASG. Our goal, therefore, was to determine whether similar findings can be achieved using homogenized muscle samples by comparing the effect of exercise and refeeding on ASG and AIG levels. Eight male participants cycled for 60 minutes at 70% Vo(2peak) before ingesting 10.9 +/- 0.6 g carbohydrate per kilogram body mass over 24 hours. Muscle biopsies were taken before exercise and after 0, 2, and 24 hours of recovery. Using a homogenization-dependent protocol to extract glycogen, 77% to 91% of it was extracted as ASG, compared with 11% to 24% with a homogenization-free protocol. In response to exercise, muscle glycogen levels fell from 366 +/- 24 to 184 +/- 46 mmol/kg dry weight and returned to 232 +/- 32 and 503 +/- 59 mmol/kg dry weight after 2 and 24 hours, respectively. Acid-soluble glycogen but not AIG accounted for all the changes in total glycogen during exercise and refeeding when extracted using a homogenization-dependent protocol, but AIG was the most responsive fraction when extracted using a homogenization-free protocol. In conclusion, the patterns of response of ASG and AIG levels to changes in glycogen concentrations in human muscles are highly dependent on the protocol used to acid-extract glycogen, with the physiologic significance of the many previous studies on AIG and ASG being in need of revision.

Dysregulation of glycogen synthase COOH- and NH2-terminal phosphorylation by insulin in obesity and type 2 diabetes mellitus

CONTEXT

Insulin-stimulated glucose disposal is impaired in obesity and type 2 diabetes mellitus (T2DM) and is tightly linked to impaired skeletal muscle glucose uptake and storage. Impaired activation of glycogen synthase (GS) by insulin is a well-established defect in both obesity and T2DM, but the underlying mechanisms remain unclear.

DESIGN AND PARTICIPANTS

Insulin action was investigated in a matched cohort of lean healthy, obese nondiabetic, and obese type 2 diabetic subjects by the euglycemic-hyperinsulinemic clamp technique combined with muscle biopsies. Activity, site-specific phosphorylation, and upstream signaling of GS were evaluated in skeletal muscle.

RESULTS

GS activity correlated inversely with phosphorylation of GS site 2+2a and 3a. Insulin significantly decreased 2+2a phosphorylation in lean subjects only and induced a larger dephosphorylation at site 3 in lean compared with obese subjects. The exaggerated insulin resistance in T2DM compared with obese subjects was not reflected by differences in site 3 phosphorylation but was accompanied by a significantly higher site 1b phosphorylation during insulin stimulation. Hyperphosphorylation of another Ca(2+)/calmodulin-dependent kinase-II target, phospholamban-Thr17, was also evident in T2DM. Dephosphorylation of GS by phosphatase treatment fully restored GS activity in all groups.

CONCLUSIONS

Dysregulation of GS phosphorylation plays a major role in impaired insulin regulation of GS in obesity and T2DM. In obesity, independent of T2DM, this is associated with impaired regulation of site 2+2a and likely site 3, whereas the exaggerated insulin resistance to activate GS in T2DM is linked to hyperphosphorylation of at least site 1b. Thus, T2DM per se seems unrelated to defects in the glycogen synthase kinase-3 regulation of GS.

Glycogen: an overview of possible regulatory roles of the proteins associated with the granule

While scientists have routinely measured muscle glycogen in many metabolic situations for over 4 decades, there is surprisingly little known regarding its regulation. In the past decade, considerable evidence has illustrated that the carbohydrate stores in muscle are not homogeneous, and it is very likely that metabolic pools exist or that each granule has independent regulation. The fundamental aspects appear to be associated with a complex set of proteins that associate with both the granule and each other in a dynamic fashion. Some of the proteins are enzymes and others play scaffolding roles. A number of the proteins can translocate, depending on the metabolic stimulus. These various processes appear to be the mechanisms that give the glycogen granule precise yet dynamic regulation. This may also allow the stores to serve as an important metabolic regulator of other metabolic events.

Dual regulation of muscle glycogen synthase during exercise by activation and compartmentalization

Glycogen synthase (GS) is considered the rate-limiting enzyme in glycogenesis but still today there is a lack of understanding on its regulation. We have previously shown phosphorylation-dependent GS intracellular redistribution at the start of glycogen re-synthesis in rabbit skeletal muscle (Prats, C., Cadefau, J. A., Cussó, R., Qvortrup, K., Nielsen, J. N., Wojtaszewki, J. F., Wojtaszewki, J. F., Hardie, D. G., Stewart, G., Hansen, B. F., and Ploug, T. (2005) J. Biol. Chem. 280, 23165-23172). In the present study we investigate the regulation of human muscle GS activity by glycogen, exercise, and insulin. Using immunocytochemistry we investigate the existence and relevance of GS intracellular compartmentalization during exercise and during glycogen re-synthesis. The results show that GS intrinsic activity is strongly dependent on glycogen levels and that such regulation involves associated dephosphorylation at sites 2+2a, 3a, and 3a + 3b. Furthermore, we report the existence of several glycogen metabolism regulatory mechanisms based on GS intracellular compartmentalization. After exhausting exercise, epinephrine-induced protein kinase A activation leads to GS site 1b phosphorylation targeting the enzyme to intramyofibrillar glycogen particles, which are preferentially used during muscle contraction. On the other hand, when phosphorylated at sites 2+2a, GS is preferentially associated with subsarcolemmal and intermyofibrillar glycogen particles. Finally, we verify the existence in human vastus lateralis muscle of the previously reported mechanism of glycogen metabolism regulation in rabbit tibialis anterior muscle. After overnight low muscle glycogen level and/or in response to exhausting exercise-induced glycogenolysis, GS is associated with spherical structures at the I-band of sarcomeres.

Regulation of muscle glycogen synthase phosphorylation and kinetic properties by insulin, exercise, adrenaline and role in insulin resistance

In mammals, excess carbohydrate is stored as glycogen and glycogen synthase is the enzyme that incorporates glucose units into the glycogen particle. Glycogen synthase activity is regulated by phosphorylation and allosterically activated by glucose 6-phosphate. Phosphorylation of nine serines by different kinases regulates glycogen synthase affinity for glucose 6-phosphate and its substrate UDP-glucose. Glucose 6-phosphate increases both enzyme activity and substrate affinity. Insulin and exercise increase glycogen synthase affinity for glucose 6-phosphate and activity whereas high glycogen content and adrenaline decrease affinity for glucose 6-phosphate and activity. However, insulin, exercise and adrenaline also regulate intracellular concentration of glucose 6-phosphate which will influence in vivo glycogen synthase activity. Importantly, type 2 diabetes is associated with reduced insulin-stimulated glycogen synthase activation. The nine phosphorylation sites theoretically allow 512 combinations of phosphorylation configurations of glycogen synthase with different kinetic properties. However, due to hierarchal phosphorylation, the number of configurations in vivo is most likely much lower. Unfortunately, many studies only report data on glycogen synthase activity measured with high concentration of UDP-glucose which holds back information about changes in substrate affinity. In this paper we discuss the physiological regulation of glycogen synthase phosphorylation and how the phosphorylation pattern regulates glycogen synthase kinetic properties.

Fiber-specific responses of muscle glycogen repletion in fasted rats physically active during recovery from high-intensity physical exertion

Mild physical activity performed immediately after a bout of intense exercise in fasting humans results in net glycogen breakdown in their slow oxidative (SO) muscle fibers and glycogen repletion in their fast twitch (FT) fibers. Because several animal species carry a low proportion of SO fibers, it is unclear whether they can also replenish glycogen in their FT fibers under these conditions. Given that most skeletal muscles in rats are poor in SO fibers (<5%), this issue was examined using groups of 24-h fasted Wistar rats (n=10) that swam for 3 min at high intensity with a 10% weight followed by either a 60-min rest (passive recovery, PR) or a 30-min swim with a 0.5% weight (active recovery, AR) preceding a 30-min rest. The 3-min sprint caused 61-79% glycogen fall across the muscles examined, but not in the soleus (SOL). Glycogen repletion during AR without food was similar to PR in the white gastrocnemius (WG), where glycogen increased by 71%, and less than PR in both the red and mixed gastrocnemius (RG, MG). Glycogen fell by 26% during AR in the SOL. Following AR, glycogen increased by 36%, 87%, and 37% in the SOL, RG, and MG, respectively, and this was accompanied by the sustained activation of glycogen synthase and inhibition of glycogen phosphorylase in the RG and MG. These results suggest that mammals with a low proportion of SO fibers can also replenish the glycogen stores of their FT fibers under extreme conditions combining physical activity and fasting.

Glycogenin protein and mRNA expression in response to changing glycogen concentration in exercise and recovery

Glycogenin (GN-1) is essential for the formation of a glycogen granule; however, rarely has it been studied when glycogen concentration changes in exercise and recovery. It is unclear whether GN-1 is degraded or is liberated and exists as apoprotein (apo)-GN-1 (unglycosylated). To examine this, we measured GN-1 protein and mRNA level at rest, at exhaustion (EXH), and during 5 h of recovery in which the rate of glycogen restoration was influenced by carbohydrate (CHO) provision. Ten males cycled (65% VO2 max) to volitional EXH (117.8 +/- 4.2 min) on two separate occasions. Subjects were administered carbohydrate (CHO; 1 g.kg(-1).h(-1) Gatorlode) or water [placebo (PL)] during 5 h of recovery. Muscle biopsies were taken at rest, at EXH, and following 30, 60, 120, and 300 min of recovery. At EXH, total glycogen concentration was reduced (P < 0.05). However, GN-1 protein and mRNA content did not change. By 5 h of recovery, glycogen was resynthesized to approximately 60% of rest in the CHO trial and remained unchanged in the PL trial. GN-1 protein and mRNA level did not increase during recovery in either trial. We observed modest amounts of apo-GN-1 at EXH, suggesting complete degradation of some granules. These data suggest that GN-1 is conserved, possibly as very small, or nascent, granules when glycogen concentration is low. This would provide the ability to rapidly restore glycogen during early recovery.

Role of glycogen content in insulin resistance in human muscle cells

We have used primary human muscle cell cultures to investigate the role of glycogen loading in cellular insulin resistance. Insulin pre-treatment for 2 h markedly impaired insulin signaling, as assessed by protein kinase B (PKB) phosphorylation. In contrast, insulin-dependent glycogen synthesis, glycogen synthase (GS) activation, and GS sites 3 de-phosphorylation were impaired only after 5 h of insulin pre-treatment, whereas 2-deoxyglucose transport was only decreased after 18 h pre-treatment. Insulin-resistant glycogen synthesis was associated closely with maximal glycogen loading. Both glucose limitation and 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR) treatment during insulin pre-treatment curtailed glycogen accumulation, and concomitantly restored insulin-sensitive glycogen synthesis and GS activation, although GS de-phosphorylation and PKB phosphorylation remained impaired. Conversely, glycogen super-compensation diminished insulin-sensitive glycogen synthesis and GS activity. Insulin acutely promoted GS translocation to particulate subcellular fractions; this was abolished by insulin pre-treatment, as was GS dephosphorylation therein. Limiting glycogen accumulation during insulin pre-treatment re-instated GS dephosphorylation in particulate fractions, whereas glycogen super-compensation prevented insulin-stimulated GS translocation and dephosphorylation. Our data suggest that diminished insulin signaling alone is insufficient to impair glucose disposal, and indicate a role for glycogen accumulation in inducing insulin resistance in human muscle cells.

Glycogen synthase binds to sarcoplasmic reticulum and is phosphorylated by CaMKII in fast-twitch skeletal muscle

We investigated the subcellular localization of glycogen synthase (GS) in the adductor muscle of anesthetized rabbits injected intravenously with propranolol. Under these experimental conditions, glycogen content was about 10 mmol/kg of fresh tissue. Immunofluorescent and fractionation studies showed that GS associated with sarcoplasmic reticulum (SR) membranes. Glycogen and GS always co-sedimented, suggesting a predominant role of glycogen in targeting of GS to SR. SR-associated GS was phosphorylated in vitro by SR-bound Ca2+-calmodulin dependent protein kinase (CaMKII) and dephosphorylated by endogenous protein phosphatase 1 (PP1c). Based on measurements of GS activity ratio, in vitro phosphorylation of GS by CaMKII did not significantly affect GS activity per se. However, GS activity ratio was slightly reduced, when SR membranes were further incubated with ATP after prior phosphorylation by CaMKII, suggesting that CaMKII might act sinergistically with other protein kinases. We propose that SR-bound CaMKII plays a role in regulation of glycogen metabolism in skeletal muscle, when intracellular Ca2+ is raised.

Quantitative assessment of human muscle glycogen granules size and number in subcellular locations during recovery from prolonged exercise

Although data relating to muscle glycogen are interpreted as showing it is homogenous when quantified biochemically, it is actually in granules in specific subcellular locations. We hypothesized that postexercise restoration of muscle glycogen would occur initially by an increase in granule number followed by an increase in size, and also that restoration would differ in various subcellular locations. Five men performed prolonged exercise and had muscle biopsies taken at 0, 4, 24 and 48 h of recovery. We quantified granule number and size as well as the total volume of glycogen in the subsarcolemmal and the intra- and intermyofibrillar regions, using transmission electron microscopy. Muscle glycogen was reduced to 36 +/- 8.3 mmol glucosyl units (kg dry weight)(-1) at exhaustion, and was preferentially depleted and subsequently repleted in the intramyofibrillar space. The repletion rate was greatest in the first 4 h; this was associated with a 186% increase in number (P < or = 0.05) and no change in particle size (P > or = 0.05). From 4 h to 48 h, there was an increase in particle size (P < or = 0.05) but not number (P > or = 0.05). Net rate of G volume synthesis per unit area was 50% greater (P < or = 0.05) in the subsarcolemmal than the myofibrillar compartment. Conversely, the net rate of single-particle volume synthesis was greater (P < or = 0.05) in the myofibrillar than the subsarcolemmal compartment. Glycogen granules varied in size and number depending on location, and in all compartments resynthesis of glycogen was characterized initially by an increase in granule number and later by an increase in size.

Glycogen branches out: new perspectives on the role of glycogen metabolism in the integration of metabolic pathways

Glycogen is the storage form of carbohydrate for virtually every organism from yeast to primates. Most mammalian tissues store glucose as glycogen, with the major depots located in muscle and liver. The French physiologist Claude Bernard first identified a starch-like substance in liver and muscle and coined the term glycogen, or "sugar former," in the 1850s. During the 150 years since its identification, researchers in the field of glycogen metabolism have made numerous discoveries that are now recognized as significant milestones in biochemistry and cell signaling. Even so, more questions remain, and studies continue to demonstrate the complexity of the regulation of glycogen metabolism. Under classical definitions, the functions of glycogen seem clear: muscle glycogen is degraded to generate ATP during increased energy demand, whereas hepatic glycogen is broken down for release of glucose into the bloodstream to supply other tissues. However, recent findings demonstrate that the roles of glycogen metabolism in energy sensing, integration of metabolic pathways, and coordination of cellular responses to hormonal stimuli are far more complex.

Leishmania beta-1,2-mannan is assembled on a mannose-cyclic phosphate primer

Infective stages of the protozoan parasite Leishmania spp. accumulate a class of beta-1,2-mannan oligosaccharides as their major carbohydrate reserve material. Here, we describe the biosynthesis of Leishmania mannan. Mannan precursors were identified by metabolic labeling of Leishmania mexicana promastigotes with [(3)H]mannose. Label was initially incorporated into a phosphomannose primer and short phosphorylated beta-1,2-mannan oligomers that were two to five residues long. Analysis of the mannan primer by Fourier transform ion-cyclotron resonance MS and various enzymatic and chemical treatments and comparison with authentic mannose (Man) phosphates indicated the presence of Man-alpha-1,4-cyclic phosphate. This primer was synthesized from Man-6-phosphate by means of Man-1-phosphate in a cell-free system. Short mannan chains containing the primer were subsequently dephosphorylated and then further elongated by GDP-Man-dependent transferases in vivo and in the cell-free system. The synthesis of this glycan primer likely constitutes a key regulatory step in mannan biosynthesis and is a potential target for antileishmanial drugs.

Skeletal muscle insulin signaling defects downstream of phosphatidylinositol 3-kinase at the level of Akt are associated with impaired nonoxidative glucose disposal in HIV lipodystrophy

More than 40% of HIV-infected patients on highly active antiretroviral therapy (HAART) experience fat redistribution (lipodystrophy), a syndrome associated with insulin resistance primarily affecting insulin-stimulated nonoxidative glucose metabolism (NOGM(ins)). Skeletal muscle biopsies, obtained from 18 lipodystrophic nondiabetic patients (LIPO) and 18 nondiabetic patients without lipodystrophy (NONLIPO) before and during hyperinsulinemic (40 mU.m(-2).min(-1))-euglycemic clamps, were analyzed for insulin signaling effectors. All patients were on HAART. Both LIPO and NONLIPO patients were normoglycemic (4.9 +/- 0.1 and 4.8 +/- 0.1 mmol/l, respectively); however, NOGM(ins) was reduced by 49% in LIPO patients (P < 0.001). NOGM(ins) correlated positively with insulin-stimulated glycogen synthase activity (I-form, P < 0.001, n = 36). Glycogen synthase activity (I-form) correlated inversely with phosphorylation of glycogen synthase sites 2+2a (P < 0.001, n = 36) and sites 3a+b (P < 0.001, n = 36) during clamp. Incremental glycogen synthase-kinase-3alpha and -3beta phosphorylation was attenuated in LIPO patients (Ps < 0.05). Insulin-stimulated Akt Ser473 and Akt Thr308 phosphorylation was decreased in LIPO patients (P < 0.05), whereas insulin receptor substrate-1-associated phosphatidylinositol (PI) 3-kinase activity increased significantly (P < 0.001) and similarly (NS) in both groups during clamp. Thus, low glycogen synthase activity explained impaired NOGM(ins) in HIV lipodystrophy, and insulin signaling defects were downstream of PI 3-kinase at the level of Akt. These results suggest mechanisms for the insulin resistance greatly enhancing the risk of type 2 diabetes in HIV lipodystrophy.