Glycogen structure and biogenesis

Crystallography of polysaccharides: Current state and challenges

Polysaccharides are the most abundant class of biopolymers, holding an important place in biological systems and sustainable material development. Their spatial organization and intra- and intermolecular interactions are thus of great interest. However, conventional single crystal crystallography is not applicable since polysaccharides crystallize only into tiny crystals. Several crystallographic methods have been developed to extract atomic-resolution structural information from polysaccharide crystals. Small-probe single crystal diffractometry, high-resolution fiber diffraction and powder diffraction combined with molecular modeling brought new insights from various types of polysaccharide crystals, and led to many high-resolution crystal structures over the past two decades. Current challenges lie in the analysis of disorder and defects by further integrating molecular modeling methods for low-resolution diffraction data.

Performance of novel GO-Gly/HNTs and GO-GG/HNTs nanocomposites for removal of Pb(II) from water: optimization based on the RSM-CCD model

For the first time, in this study, two novel glycogen-graphene oxide/halloysite nanotubes (GO-Gly/HNTs) and guar gum-graphene oxide/halloysite nanotubes (GO-GG/HNTs) nanocomposites were synthesized as the adsorbents for removal of Pb(II) from water, and the ionic liquid was used in the synthesis as a green solvent. According to the SEM, TEM, EDS, BET, zeta potential, FTIR, and XRD results, GO-Gly/HNTs and GO-GG/HNTs were synthesized successfully. Response surface methodology (RSM) was applied to optimize the experimental conditions. Nanocomposites followed the Langmuir equilibrium model and were best fitted to the pseudo-second-order model. According to the thermodynamic model, the adsorption process was endothermic. Due to several features, these two novel nanocomposites can be considered the proper candidate for Pb(II) removal from water and wastewater. First, these nanocomposites have good adsorption capacity for Pb(II) removal, which is 219 mg/g for GO-Gly/HNTs and 315 mg/g for GO-GG/HNTs. Moreover, nanocomposites can be recycled with proper adsorption capacity after four repeated cycles. These materials can be used to remove Pb(II) from water in the presence of other contaminants because nanocomposites have selective tendency toward Pb(II) in the presence of other pollutants such as Cd²⁺, Cu²⁺, Cr²⁺, and Co²⁺. In addition, the presence of Ca²⁺, Mg²⁺, Na⁺, and K⁺ improve Pb(II) removal. Finally, possible mechanisms for each nanocomposite were represented.

OUP accepted manuscript

Host carbohydrates, or glycans, have been implicated in the pathogenesis of many bacterial infections. Group A Streptococcus (GAS) is a Gram-positive bacterium that readily colonises the skin and oropharynx, and is a significant cause of mortality in humans. While the glycointeractions orchestrated by many other pathogens are increasingly well-described, the understanding of the role of human glycans in GAS disease remains incomplete. Although basic investigation into the mechanisms of GAS disease is ongoing, several glycointeractions have been identified and are examined herein. The majority of research in this context has focussed on bacterial adherence, however, glycointeractions have also been implicated in carbohydrate metabolism; evasion of host immunity; biofilm adaptations; and toxin-mediated haemolysis. The involvement of human glycans in these diverse avenues of pathogenesis highlights the clinical value of understanding glycointeractions in combatting GAS disease.

LUBAC: a new player in polyglucosan body disease

Altered protein ubiquitination is associated with the pathobiology of numerous diseases; however, its involvement in glycogen metabolism and associated polyglucosan body (PB) disease has not been investigated in depth. In PB disease, excessively long and less branched glycogen chains (polyglucosan bodies, PBs) are formed, which precipitate in different tissues causing myopathy, cardiomyopathy and/or neurodegeneration. Linear ubiquitin chain assembly complex (LUBAC) is a multi-protein complex composed of two E3 ubiquitin ligases HOIL-1L and HOIP and an adaptor protein SHARPIN. Together they are responsible for M1-linked ubiquitination of substrates primarily related to immune signaling and cell death pathways. Consequently, severe immunodeficiency is a hallmark of many LUBAC deficient patients. Remarkably, all HOIL-1L deficient patients exhibit accumulation of PBs in different organs especially skeletal and cardiac muscle resulting in myopathy and cardiomyopathy with heart failure. This emphasizes LUBAC's important role in glycogen metabolism. To date, neither a glycogen metabolism-related LUBAC substrate nor the molecular mechanism are known. Hence, current reviews on LUBAC's involvement in glycogen metabolism are lacking. Here, we aim to fill this gap by describing LUBAC's involvement in PB disease. We present a comprehensive review of LUBAC structure, its role in M1-linked and other types of atypical ubiquitination, PB pathology in human patients and findings in new mouse models to study the disease. We conclude the review with recent drug developments and near-future gene-based therapeutic approaches to treat LUBAC related PB disease.

Lafora disease: Current biology and therapeutic approaches

The ubiquitin system impacts most cellular processes and is altered in numerous neurodegenerative diseases. However, little is known about its role in neurodegenerative diseases due to disturbances of glycogen metabolism such as Lafora disease (LD). In LD, insufficiently branched and long-chained glycogen forms and precipitates into insoluble polyglucosan bodies (Lafora bodies), which drive neuroinflammation, neurodegeneration and epilepsy. LD is caused by mutations in the gene encoding the glycogen phosphatase laforin or the gene coding for the laforin interacting partner ubiquitin E3 ligase malin. The role of the malin-laforin complex in regulating glycogen structure remains with full of gaps. In this review we bring together the disparate body of data on these two proteins and propose a mechanistic hypothesis of the disease in which malin-laforin's role to monitor and prevent over-elongation of glycogen branch chains, which drive glycogen molecules to precipitate and accumulate into Lafora bodies. We also review proposed connections between Lafora bodies and the ensuing neuroinflammation, neurodegeneration and intractable epilepsy. Finally, we review the exciting activities in developing therapies for Lafora disease based on replacing the missing genes, slowing the enzyme - glycogen synthase - that over-elongates glycogen branches, and introducing enzymes that can digest Lafora bodies. Much more work is needed to fill the gaps in glycogen metabolism in which laforin and malin operate. However, knowledge appears already adequate to advance disease course altering therapies for this catastrophic fatal disease.

A thermodynamic function of glycogen in brain and muscle

Brain and muscle glycogen are generally thought to function as local glucose reserves, for use during transient mismatches between glucose supply and demand. However, quantitative measures show that glucose supply is likely never rate-limiting for energy metabolism in either brain or muscle under physiological conditions. These tissues nevertheless do utilize glycogen during increased energy demand, despite the availability of free glucose, and despite the ATP cost of cycling glucose through glycogen polymer. This seemingly wasteful process can be explained by considering the effect of glycogenolysis on the amount of energy obtained from ATP (ΔG'_ATP). The amount of energy obtained from ATP is reduced by elevations in inorganic phosphate (Pi). Glycogen utilization sequesters Pi in the glycogen phosphorylase reaction and in downstream phosphorylated glycolytic intermediates, thereby buffering Pi elevations and maximizing energy yield at sites of rapid ATP consumption. This thermodynamic effect of glycogen may be particularly important in the narrow, spatially constrained astrocyte processes that ensheath neuronal synapses and in cells such as astrocytes and myocytes that release Pi from phosphocreatine during energy demand. The thermodynamic effect may also explain glycolytic super-compensation in brain when glycogen is not available, and aspects of exercise physiology in muscle glycogen phosphorylase deficiency (McArdle disease).

Simple, fast and accurate method for the determination of glycogen in the model unicellular cyanobacterium Synechocystis sp. PCC 6803

Glycogen is a highly soluble branched polymer composed of glucose monomers linked by glycosidic bonds that represents, together with starch, one of the main energy storage compounds in living organisms. While starch is present in plant cells, glycogen is present in bacteria, protozoa, fungi and animal cells. Due to its essential function, it has been the subject of intense research for almost two centuries. Different procedures for the isolation and quantification of glycogen, according to the origin of the sample and/or the purpose of the study, have been reported in the literature. The objective of this study is to optimize the methodology for the determination of glycogen in cyanobacteria, as the interest in cyanobacterial glycogen has increased in recent years due to the biotechnological application of these microorganisms. In the present work, the methodology reported for the quantification of glycogen in cyanobacteria has been reviewed and an extensive empirical analysis has been performed showing how this methodology can be optimized significantly to reduce time and improve reliability and reproducibility. Based on these results, a simple and fast protocol for quantification of glycogen in the model unicellular cyanobacterium Synechocystis sp. PCC 6803 is presented, which could also be successfully adapted to other cyanobacteria.

Structure, bioactivity and applications of natural hyperbranched polysaccharides

In recent years, hyperbranched polymers, especially the natural hyperbranched polysaccharides (HBPSs), are receiving much attention due to their diverse biological activities and applications. With high degree of branching (DB), HBPSs mainly exist in the form of either a comb-brush shape, dendrimer-like particulate, or globular particle. HBPSs also possess some unique properties, such as high density, large spatial cavities, and numerous terminal functional groups, which distinguish them from other polymers. As a natural biopolymer, HBPS has excellent bioavailability, biocompatibility, and biodegradability, which have versatile applications in the fields of food, medicine, cosmetic, and nanomaterials. In this review, the source and structure of HBPSs from plant, animal, microbial and fungal origins as well as their biological functions and applications are covered, with the aim of further advancing the research of their structure and bioactivity.

Methodological considerations for studies of brain glycogen

Glycogen stores in the brain have been recognized for decades, but the underlying physiological function of this energy reserve remains elusive. This uncertainty stems in part from several technical challenges inherent in the study of brain glycogen metabolism. These include low glycogen content in the brain, non-homogeneous labeling of glycogen by radiotracers, rapid glycogenolysis during postmortem tissue handling, and effects of the stress response on brain glycogen turnover. Here we briefly review the aspects of the glycogen structure and metabolism that bear on these technical challenges and present ways they can be addressed.

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.

Glycogen-based self-healing hydrogels with ultra-stretchable, flexible, and enhanced mechanical properties via sacrificial bond interactions

The development of hydrogel materials with enhanced mechanical properties is the primary focus in designing autonomous self-healable hydrogel materials. Here, we present a facile and cost-effective method for the autonomous self-healing hydrogel based on Glycogen (Gly/PAA-Fe³⁺) with enhanced mechanical properties by simple insertion of ferric ions in the physically cross-linked network via metal-ligand interactions. This dual physically cross-linked hydrogel has an excellent elongation at break and self-healing properties due to the dynamic ionic cross-linking point. This work will encourage researchers to focus on this facile technique for the synthesis of self-healing hydrogel materials with enhanced mechanical properties.

Metformin normalizes the structural changes in glycogen preceding prediabetes in mice overexpressing neuropeptide Y in noradrenergic neurons

Hepatic insulin resistance and increased gluconeogenesis are known therapeutic targets of metformin, but the role of hepatic glycogen in the pathogenesis of diabetes is less clear. Mouse model of neuropeptide Y (NPY) overexpression in noradrenergic neurons (OE-NPY^DβH) with a phenotype of late onset obesity, hepatosteatosis, and prediabetes was used to study early changes in glycogen structure and metabolism preceding prediabetes. Furthermore, the effect of the anti-hyperglycemic agent, metformin (300 mg/kg/day/4 weeks in drinking water), was assessed on changes in glycogen metabolism, body weight, fat mass, and glucose tolerance. Glycogen structure was characterized by cytofluorometric analysis in isolated hepatocytes and mRNA expression of key enzymes by qPCR. OE-NPY^DβH mice displayed decreased labile glycogen fraction relative to stabile fraction (the intermediate form of glycogen) suggesting enhanced glycogen cycling. This was supported by decreased filling of glucose residues in the 10th outer tier of the glycogen molecule, which suggests accelerated glycogen phosphorylation. Metformin reduced fat mass gain in both genotypes, but glucose tolerance was improved mostly in wild-type mice. However, metformin inhibited glycogen accumulation and normalized the ratio between glycogen structures in OE-NPY^DβH mice indicating decreased glycogen synthesis. Furthermore, the presence of glucose residues in the 11th tier together with decreased glycogen phosphorylase expression suggested inhibition of glycogen degradation. In conclusion, structural changes in glycogen of OE-NPY^DβH mice point to increased glycogen metabolism, which may predispose them to prediabetes. Metformin treatment normalizes these changes and suppresses both glycogen synthesis and phosphorylation, which may contribute to its preventive effect on the onset of diabetes.

Enzymatic preparation of functional polysaccharide hydrogels by phosphorylase catalysis

This article reviews enzymatic preparation of functional polysaccharide hydrogels by means of phosphorylase-catalyzed enzymatic polymerization. A first topic of this review deals with the synthesis of amylose-grafted polymeric materials and their formation of hydrogels, composed of abundant natural polymeric main-chains, such as chitosan, cellulose, xantham gum, carboxymethyl cellulose, and poly(γ-glutamic acid). Such synthesis was achieved by combining the phosphorylase-catalyzed enzymatic polymerization forming amylose with the appropriate chemical reaction (chemoenzymatic method). An amylose-grafted chitin nanofiber hyrogel was also prepared by the chemoenzymatic approach. As a second topic, the preparation of glycogen hydrogels by the phosphorylase-catalyzed enzymatic reactions was described. When the phosphorylase-catalyzed enzymatic polymerization from glycogen as a polymeric primer was carried out, followed by standing the reaction mixture at room temperature, a hydrogel was obtained. pH-Responsive amphoteric glycogen hydrogels were also fabricated by means of the successive phosphorylase-catalyzed enzymatic reactions.

Precision Synthesis of Functional Polysaccharide Materials by Phosphorylase-Catalyzed Enzymatic Reactions

In this review article, the precise synthesis of functional polysaccharide materials using phosphorylase-catalyzed enzymatic reactions is presented. This particular enzymatic approach has been identified as a powerful tool in preparing well-defined polysaccharide materials. Phosphorylase is an enzyme that has been employed in the synthesis of pure amylose with a precisely controlled structure. Similarly, using a phosphorylase-catalyzed enzymatic polymerization, the chemoenzymatic synthesis of amylose-grafted heteropolysaccharides containing different main-chain polysaccharide structures (e.g., chitin/chitosan, cellulose, alginate, xanthan gum, and carboxymethyl cellulose) was achieved. Amylose-based block, star, and branched polymeric materials have also been prepared using this enzymatic polymerization. Since phosphorylase shows a loose specificity for the recognition of substrates, different sugar residues have been introduced to the non-reducing ends of maltooligosaccharides by phosphorylase-catalyzed glycosylations using analog substrates such as α-d-glucuronic acid and α-d-glucosamine 1-phosphates. By means of such reactions, an amphoteric glycogen and its corresponding hydrogel were successfully prepared. Thermostable phosphorylase was able to tolerate a greater variance in the substrate structures with respect to recognition than potato phosphorylase, and as a result, the enzymatic polymerization of α-d-glucosamine 1-phosphate to produce a chitosan stereoisomer was carried out using this enzyme catalyst, which was then subsequently converted to the chitin stereoisomer by N-acetylation. Amylose supramolecular inclusion complexes with polymeric guests were obtained when the phosphorylase-catalyzed enzymatic polymerization was conducted in the presence of the guest polymers. Since the structure of this polymeric system is similar to the way that a plant vine twines around a rod, this polymerization system has been named "vine-twining polymerization". Through this approach, amylose supramolecular network materials were fabricated using designed graft copolymers. Furthermore, supramolecular inclusion polymers were formed by vine-twining polymerization using primer⁻guest conjugates.

Enzymes as Green Catalysts for Precision Macromolecular Synthesis

The present article comprehensively reviews the macromolecular synthesis using enzymes as catalysts. Among the six main classes of enzymes, the three classes, oxidoreductases, transferases, and hydrolases, have been employed as catalysts for the in vitro macromolecular synthesis and modification reactions. Appropriate design of reaction including monomer and enzyme catalyst produces macromolecules with precisely controlled structure, similarly as in vivo enzymatic reactions. The reaction controls the product structure with respect to substrate selectivity, chemo-selectivity, regio-selectivity, stereoselectivity, and choro-selectivity. Oxidoreductases catalyze various oxidation polymerizations of aromatic compounds as well as vinyl polymerizations. Transferases are effective catalysts for producing polysaccharide having a variety of structure and polyesters. Hydrolases catalyzing the bond-cleaving of macromolecules in vivo, catalyze the reverse reaction for bond forming in vitro to give various polysaccharides and functionalized polyesters. The enzymatic polymerizations allowed the first in vitro synthesis of natural polysaccharides having complicated structures like cellulose, amylose, xylan, chitin, hyaluronan, and chondroitin. These polymerizations are "green" with several respects; nontoxicity of enzyme, high catalyst efficiency, selective reactions under mild conditions using green solvents and renewable starting materials, and producing minimal byproducts. Thus, the enzymatic polymerization is desirable for the environment and contributes to "green polymer chemistry" for maintaining sustainable society.

Short sprints (30s) attenuate post-prandial blood glucose in young healthy males

AIMS

Low-volume, high-intensity exercise is a time-efficient method of inducing physiological responses and may also improve glucose homeostasis. Therefore, effects of two different volumes of sprint-interval cycling on post-prandial blood glucose were assessed.

METHODS

Twenty healthy young males undertook two Wingate anaerobic tests (2WAT), four Wingate anaerobic (4WAT) and without-exercise (CON) 90 min after eating a standard meal. Blood glucose was examined at 60, 90, 105, 120, 135 and 150 min post-prandially.

RESULTS

2WAT and 4WAT both accelerated the decrease of blood glucose compared with CON (P<0.05). There were significant reductions at 120 (4.45 ± 0.64 vs. 4.93 ± 0.9 vs. 5.68 ± 0.69), 135 (4.28 ± 0.50 vs. 4.48 ± 0.75 vs. 5.54 ± 0.6) and 150 min (4.64 ± 0.71 vs. 4.71 ± 0.73 vs. 5.36 ± 0.48, all P<0.05). Blood glucose at 120 min was lower after 2WAT than 4WAT (4.45 ± 0.64 vs. 4.93 ± 0.9, P<0.05), this producing a significant statistical interaction between groups and post-exercise time (P<0.005).

CONCLUSIONS

2WAT and 4WAT tests both accelerate the post-prandial decrease in blood glucose in young healthy males, 2WAT being superior to 4WAT in producing this response, even though 2WAT is easier to perform and less time consuming.

Protein–polymer therapeutics: a macromolecular perspective

The development of protein–polymer hybrids emerged several decades ago with the vision that their synergistic combination will offer macromolecular hybrids with manifold features to succeed as the next generation therapeutics.

Recombinant prion protein refolded with lipid and RNA has the biochemical hallmarks of a prion but lacks in vivo infectivity

During prion infection, the normal, protease-sensitive conformation of prion protein (PrP^C) is converted via seeded polymerization to an abnormal, infectious conformation with greatly increased protease-resistance (PrP^Sc). In vitro, protein misfolding cyclic amplification (PMCA) uses PrP^Sc in prion-infected brain homogenates as an initiating seed to convert PrP^C and trigger the self-propagation of PrP^Sc over many cycles of amplification. While PMCA reactions produce high levels of protease-resistant PrP, the infectious titer is often lower than that of brain-derived PrP^Sc. More recently, PMCA techniques using bacterially derived recombinant PrP (rPrP) in the presence of lipid and RNA but in the absence of any starting PrP^Sc seed have been used to generate infectious prions that cause disease in wild-type mice with relatively short incubation times. These data suggest that lipid and/or RNA act as cofactors to facilitate the de novo formation of high levels of prion infectivity. Using rPrP purified by two different techniques, we generated a self-propagating protease-resistant rPrP molecule that, regardless of the amount of RNA and lipid used, had a molecular mass, protease resistance and insolubility similar to that of PrP^Sc. However, we were unable to detect prion infectivity in any of our reactions using either cell-culture or animal bioassays. These results demonstrate that the ability to self-propagate into a protease-resistant insoluble conformer is not unique to infectious PrP molecules. They suggest that the presence of RNA and lipid cofactors may facilitate the spontaneous refolding of PrP into an infectious form while also allowing the de novo formation of self-propagating, but non-infectious, rPrP-res.

Synthesis of highly branched anionic α-glucans by thermostable phosphorylase-catalyzed α-glucuronylation

Highly branched anionic α-glucans were enzymatically synthesized by thermostable phosphorylase-catalyzed α-glucuronylation of highly branched cyclic dextrin using α-D-glucuronic acid 1-phosphate (GlcA-1-P) as a glycosyl donor. The resulting products were characterized by ¹H NMR measurement as well as high performance anion exchange chromatographic and MALDI-TOF MS analyses after treatments with several amylases. α-D-Glucose 1-phosphate was detected in the reaction mixtures, suggesting the occurrence of phosphorolysis in the α-glucuronylation. The glucuronylation ratios of glucuronic acid residues to non-reducing ends were evaluated from quantification of α-D-glucose 1-phosphate and inorganic phosphate in the reaction mixtures, which were relatively in good agreement with those determined by ¹H NMR analysis of the products. The glucuronylation ratios increased with increasing feed ratios of GlcA-1-P/non-reducing ends.

Molecular insights into glycogen α-particle formation

Glycogen, a hyperbranched complex glucose polymer, is an intracellular glucose store that provides energy for cellular functions, with liver glycogen involved in blood-glucose regulation. Liver glycogen comprises complex α particles made up of smaller β particles. The recent discovery that these α particles are smaller and fewer in diabetic, compared with healthy, mice highlights the need to elucidate the nature of α-particle formation; this paper tests various possibilities for binding within α particles. Acid hydrolysis effects, examined using dynamic light scattering and size exclusion chromatography, showed that the binding is not simple α-(1→4) or α-(1→6) glycosidic linkages. There was no significant change in α particle size after the addition of various reagents, which disrupt disulfide, protein, and hydrogen bonds and hydrophobic interactions. The results are consistent with proteinaceous binding between β particles in α particles, with the inability of protease to break apart particles being attributed to steric hindrance.

Cryoprotection in dampwood termites (Termopsidae, Isoptera)

In contrast to the majority of the Order, the dampwood termites of the family Termopsidae found in colder regions can experience frost and snow, either in cool temperate areas at high latitudes (45 degrees ), or alpine areas at high elevations (>1000m). This suggests that dampwood termites are adapted to cold climates. We investigated this hypothesis in two dampwood termites, Porotermes adamsoni Froggatt and Stolotermes victoriensis Hill. We measured nest temperatures and atmospheric temperatures of their alpine habitat during winter, and measured survival and recovery at subzero temperatures. We also determined the minimum temperature at which these species remain active and the LT50 values. We used a novel gas chromatographic strategy to examine eight metabolites from individuals of both species collected in winter and summer to identify possible cryoprotectants. Both P. adamsoni and S. victoriensis had significantly higher levels of trehalose, a known cryoprotectant, in winter than in summer; in addition S. victoriensis also had higher levels of unsaturated fatty acid ligands in winter than in summer, consistent with patterns observed for cold adaptation in other organisms. These results are the first to reveal that dampwood termites are adapted to cold climates and use trehalose and unsaturated lipids as cryoprotectants.

Extremely short duration high intensity interval training substantially improves insulin action in young healthy males

BACKGROUND

Traditional high volume aerobic exercise training reduces cardiovascular and metabolic disease risk but involves a substantial time commitment. Extremely low volume high-intensity interval training (HIT) has recently been demonstrated to produce improvements to aerobic function, but it is unknown whether HIT has the capacity to improve insulin action and hence glycemic control.

METHODS

Sixteen young men (age: 21 +/- 2 y; BMI: 23.7 +/- 3.1 kg x m-2; VO2peak: 48 +/- 9 ml x kg-1 x min-1) performed 2 weeks of supervised HIT comprising of a total of 15 min of exercise (6 sessions; 4-6 x 30-s cycle sprints per session). Aerobic performance (250-kJ self-paced cycling time trial), and glucose, insulin and NEFA responses to a 75-g oral glucose load (oral glucose tolerance test; OGTT) were determined before and after training.

RESULTS

Following 2 weeks of HIT, the area under the plasma glucose, insulin and NEFA concentration-time curves were all reduced (12%, 37%, 26% respectively, all P < 0.001). Fasting plasma insulin and glucose concentrations remained unchanged, but there was a tendency for reduced fasting plasma NEFA concentrations post-training (pre: 350 +/- 36 v post: 290 +/- 39 micromol x l-1, P = 0.058). Insulin sensitivity, as measured by the Cederholm index, was improved by 23% (P < 0.01), while aerobic cycling performance improved by approximately 6% (P < 0.01).

CONCLUSION

The efficacy of a high intensity exercise protocol, involving only ~250 kcal of work each week, to substantially improve insulin action in young sedentary subjects is remarkable. This novel time-efficient training paradigm can be used as a strategy to reduce metabolic risk factors in young and middle aged sedentary populations who otherwise would not adhere to time consuming traditional aerobic exercise regimes.

The polysaccharide scaffold of PrP 27-30 is a common compound of natural prions and consists of alpha-linked polyglucose

An inert polysaccharide scaffold identified as a 5-15% component of prion rods (PrP 27-30) is unambiguously distinguishable from the N-glycosyl groups and the GPI anchor of PrP, and consists predominantly of 1,4-linked glucose with some branching via 1,4,6-linked glucose. We show that this polysaccharide scaffold is a common secondary component of prions found in hamster full-length PrP(Sc), prion rods and in mouse ScN2a prions from cell culture. The preparation from prion rods was improved, resulting in a polysaccharide scaffold free of remaining infectivity. Furthermore, we determined the stereochemistry of the glycoside linkages as pre-dominantly if not entirely alpha-glycosidic. The origin of the polysaccharide, its interaction with PrP and its potential relation to glycogen and corpora amylacea are discussed.

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

Glycogen metabolism has been the subject of extensive research, but the mechanisms by which it is regulated are still not fully understood. It is well accepted that the rate-limiting enzymes in glycogenesis and glycogenolysis are glycogen synthase (GS) and glycogen phosphorylase (GPh), respectively. Both enzymes are regulated by reversible phosphorylation and by allosteric effectors. However, evidence in the literature indicates that changes in muscle GS and GPh intracellular distribution may constitute a new regulatory mechanism of glycogen metabolism. Already in the 1960s, it was proposed that glycogen was present in dynamic cellular organelles that were termed glycosomas but no such cellular entities have ever been demonstrated. The aim of this study was to characterize muscle GS and GPh intracellular distribution and to identify possible translocation processes of both enzymes. Using in situ stimulation of rabbit tibialis anterior muscle, we show GS and GPh intracellular redistribution at the beginning of glycogen resynthesis after contraction-induced glycogen depletion. We identify a new "player," a new intracellular compartment involved in skeletal muscle glycogen metabolism. They are spherical structures that were not present in basal muscle, and we present evidence that indicate that they are products of actin cytoskeleton remodeling. Furthermore, for the first time, we show a phosphorylation-dependent intracellular distribution of GS. Here, we present evidence of a new regulatory mechanism of skeletal muscle glycogen metabolism based on glycogen enzyme intracellular compartmentalization.

Trehalose metabolism genes in Caenorhabditis elegans and filarial nematodes

The sugar trehalose is claimed to be important in the physiology of nematodes where it may function in sugar transport, energy storage and protection against environmental stresses. In this study we investigated the role of trehalose metabolism in nematodes, using Caenorhabditis elegans as a model, and also identified complementary DNA clones putatively encoding genes involved in trehalose pathways in filarial nematodes. In C. elegans two putative trehalose-6-phosphate synthase (tps) genes encode the enzymes that catalyse trehalose synthesis and five putative trehalase (tre) genes encode enzymes catalysing hydrolysis of the sugar. We showed by RT-PCR or Northern analysis that each of these genes is expressed as mRNA at all stages of the C. elegans life cycle. Database searches and sequencing of expressed sequence tag clones revealed that at least one tps gene and two tre genes are expressed in the filarial nematode Brugia malayi, while one tps gene and at least one tre gene were identified for Onchocerca volvulus. We used the feeding method of RNA interference in C. elegans to knock down temporarily the expression of each of the tps and tre genes. Semiquantitative RT-PCR analysis confirmed that expression of each gene was silenced by RNA interference. We did not observe an obvious phenotype for any of the genes silenced individually but gas-chromatographic analysis showed >90% decline in trehalose levels when both tps genes were targeted simultaneously. This decline in trehalose content did not affect viability or development of the nematodes.

Characterization of cyanobacterial glycogen isolated from the wild type and from a mutant lacking of branching enzyme

Cyanobacteria produce glycogen as their primary form of carbohydrate storage. The genomic DNA sequence of Synechocystis sp. PCC6803 indicates that this strain encodes one glycogen-branching enzyme (GBE) and two isoforms of glycogen synthase (GS). To confirm the putative GBE and to demonstrate the presence of only one GBE gene, we generated a mutant lacking the putative GBE gene, sll0158, by replacing it with a kanamycin resistance gene through homologous recombination. GBE in sll0158(-) mutant was eliminated; the mutant strain produced less glucan, equivalent to 48% of that produced by the wild type. In contrast to the wild-type strain that had 74% of the glucan being water-soluble, the mutant had only 14% of the glucan water-soluble. Molecular structures of glucans produced by the mutant and the wild type were characterized by using high-performance size-exclusion and anion-exchange chromatography. The glycogen produced by the wild type displayed a molecular mass of 6.6 x 10(7) daltons (degree of polymerization (DP) 40700) and 10% branch linkages, and the alpha-D-glucan produced by the mutant displayed a molecular mass of 4.7-5.6 x 10(3) daltons (DP 29-35) with slight branch linkages. The results indicated that sll0158 was the major functional GBE gene in Synechocystis sp. PCC6803.

Zonal distribution of glycogen synthesis in isolated rat hepatocytes

Incubation of hepatocytes isolated from fasted rats with [14C]glucose for short periods of time showed that the initial stages of glycogen synthesis occur near the plasma membrane. Incubation with [14C]glucose followed by cold glucose demonstrated that glycogen synthesis is always active at the hepatocyte periphery and that previously synthesised glycogen moves towards the centre of the cell, while its place is filled by newly synthesised molecules. However, the reverse experiment, incubation with cold glucose before addition of [14C]glucose, showed that, as glycogen synthesis progresses, it also becomes gradually active in more internal sites of the hepatocyte. These results indicate a spatial order in the synthesis of hepatic glycogen.

Glycogen synthase: towards a minimum catalytic unit?

Classically, alpha-1,4-glucan synthases have been divided into two families, animal/fungal glycogen synthases (GS) and bacterial/plant starch synthases (G(S)S), according to differences in sequence, sugar donor specificity and regulatory mechanisms. Detailed sequence analysis, predicted secondary structure comparison and threading analysis show that these two families are structurally related and that some domains of GSs were acquired to meet regulatory requirements. Archaeal G(S)S present structural and functional features that are conserved in one, the other or both families. Therefore, they are the link between GS and G(S)S and harbor the minimal sequence and structural features that constitute the minimum catalytic unit of the alpha-1,4-glucan synthase superfamily.

Functional interactions between heterologously expressed starch-branching enzymes of maize and the glycogen synthases of Brewer's yeast

Starch-branching enzymes (SBEs) catalyze the formation of alpha(1-->6) glycoside bonds in glucan polymers, thus, affecting the structure of amylopectin and starch granules. Two distinct classes of SBE are generally conserved in higher plants, although the specific role(s) of each isoform in determination of starch structure is not clearly understood. This study used a heterologous in vivo system to isolate the function of each of the three known SBE isoforms of maize (Zea mays) away from the other plant enzymes involved in starch biosynthesis. The ascomycete Brewer's yeast (Saccharomyces cerevisiae) was employed as the host species. All possible combinations of maize SBEs were expressed in the absence of the endogenous glucan-branching enzyme. Each maize SBE was functional in yeast cells, although SBEI had a significant effect only if SBEIIa and SBEIIb also were present. SBEI by itself did not support glucan accumulation, whereas SBEIIa and SBEIIb both functioned along with the native glycogen synthases (GSs) to produce significant quantities of alpha-glucan polymers. SBEIIa was phenotypically dominant to SBEIIb in terms of glucan structure. The specific branching enzyme present had a significant effect on the molecular weight of the product. From these data we suggest that SBEs and GSs work in a cyclically interdependent fashion, such that SBE action is needed for optimal GS activity; and GS, in turn, influences the further effects of SBE. Also, SBEIIa and SBEIIb appear to act before SBEI during polymer assembly in this heterologous system.

In vitro cytocompatibility of porcine type I atelocollagen crosslinked by oxidized glycogen

Oxidized glycogen is used as collagen crosslinker to obtain materials with defined crosslinking degrees. These materials are characterized by their swelling ratio. calorimetric properties and the crosslinking level. Direct and indirect cytotoxicities of the materials obtained as sheets, are evaluated in vitro in cultures of human fibroblasts. The crosslinking degree depends on the ratio CHO glycogen/NH2 glycogen, but whatever this ratio (4.0, 2.0 or 0.4), an important percentage of the introduced CHO groups remains free and these groups are responsible for the cytotoxicity observed with the strongly crosslinked materials. This cytotoxicity appears in cell shape modification and in significant reduction of cell growth. Whatever the crosslinking degree, this toxicity can be suppressed by a single treatment with sodium borohydride, which reduced the remaining free CHO groups in OH functions and stabilizes the materials by a concomitant reduction of the crosslinking imine bonds. After reduction, all materials allow cellular adhesion and proliferation. This new crosslinking method of the collagen by the oxidized glycogen could be promising in the preparation of matrix for in vitro and in vivo tissue regeneration.

Glycoconjugate distribution in early human notochord and axial mesenchyme

Glycosylation patterns of cells and tissues give insights into spatially and temporally regulated developmental processes and can be detected histochemically using plant lectins with specific affinities for sugar moieties. The early development of the vertebral column in man is a process which has never been investigated by lectin histochemistry. Therefore, we studied binding of several lectins (AIA, Con A, GSA II, LFA, LTA, PNA, RCA I, SBA, SNA, WGA) in formaldehyde-fixed sections of the axial mesenchyme of 5 human embryos in Carnegie stages 12-15. During these developmental stages, an unsegmented mesenchyme covers the notochord. Staining patterns did not show striking temporal variations except for SBA which stained the cranial axial mesenchyme only in the early stage 12 embryo and for PNA, of which the staining intensity in the mesenchyme decreased with age. The notochord appeared as a highly glycosylated tissue. Carbohydrates detected may correspond to adhesion molecules or to secreted substances like proteoglycans or proteins which could play an inductive role, for example, for the neural tube. The axial perinotochordal unsegmented mesenchyme showed strong PNA binding. Therefore, its function as a PNA-positive "barrier" tissue is discussed. The endoderm of the primitive gut showed a lectin-binding pattern that was similar to that of the notochord, which may correlate with interactions between these tissues during earlier developmental stages.

Microscopic and histochemical manifestations of hyaline cartilage dynamics

Structure and function of hyaline cartilages has been the focus of many correlative studies for over a hundred years. Much of what is known regarding dynamics and function of cartilage constituents has been derived or inferred from biochemical and electron microscopic investigations. Here we show that in conjunction with ultrastructural, and high-magnification transmission light and polarization microscopy, the well-developed histochemical methods are indispensable for the analysis of cartilage dynamics. Microscopically demonstrable aspects of cartilage dynamics include, but are not limited to, formation of the intracellular liquid crystals, phase transitions of the extracellular matrix and tubular connections between chondrocytes. The role of the interchondrocytic liquid crystals is considered in terms of the tensegrity hypothesis and non-apoptotic cell death. Phase transitions of the extracellular matrix are discussed in terms of self-alignment of chondrons, matrix guidance pathways and cartilage growth in the absence of mitosis. The possible role of nonenzymatic glycation reactions in cartilage dynamics is also reviewed.

Ultrastructural aspects of phytoglycogen from cryo-transmission electron microscopy and quasi-elastic light scattering data

Phytoglycogen particles extracted from the sugary maize mutant su 1 and dispersed in water were studied using transmission electron microscopy (TEM) and light scattering. Dried specimens were either negatively stained with uranyl acetate or shadowed with W/Ta. Frozen-hydrated unstained particles embedded in a thin film of vitreous ice were also observed using cryo-TEM. The particles exhibited a spheroidal shape, with a diameter ranging from 30 to 100 nm. Some of them presented a multilobular morphology and appeared to be formed by smaller subunits, 20-30 nm in diameter, resembling the described beta-particles for animal glycogen. The diameter of stained and ice-embedded particles was measured from electron micrographs. The corresponding size distribution histograms showed that the average weight diameter of ice-embedded particles was higher than that of stained ones. In the latter case, a shrinkage of the particle was believed to occur during the drying process. Light scattering experiments confirmed the diameter of ice-embedded particles and indicated that they could be considered as uniformly dense spheroidal objects.

Insulin delivery governed by covalently modified lectin-glycogen gels sensitive to glucose

A glucose-sensitive gel formulation containing concanavalin A and glycogen has been reported previously. Precipitation resulting from the addition of concanavalin A to glycogen has been documented, but the formation of glucose-sensitive gels based on lectin-glycogen interactions is novel and used here in our studies. An improved in-vitro self-regulating drug-delivery system, using covalently modified glucose-sensitive gels based on concanavalin A and a polysaccharide displacement mechanism, is described. The successful use of the covalently modified gels addresses a problem identified previously where significant leaching of the mitogenic lectin from the gel membranes of non-coupled gels was encountered. Concanavalin A was covalently coupled to glycogen by use of derivatives of Schiff's bases. The resulting gels, like the non-coupled gels, were shown to undergo a gel-sol transformation in response to glucose. Insulin delivery was demonstrated using this covalently modified system in conditions of repeated glucose triggering at 20 degrees C and 37 degrees C. The magnitude of the response was less variable than for the dextran-based gels studied previously. The performance of this system has been improved in terms of concanavalin A leaching. This could, therefore, be used as the basis of the design of a self-regulating drug-delivery device for therapeutic agents used to treat diabetes mellitus.

Mechanisms of insulin resistance in non-oxidative glucose metabolism: the role of glycogen synthase

Insulin-mediated non-oxidative glucose metabolism is more or less identical to glycogen synthesis in skeletal muscle and that is why this pathway is specifically discussed in this paper. All three major steps in non-oxidative glucose processing--glucose transport, phosphorylation and glycogen synthesis--are found to be reduced in response to insulin in insulin-resistant type 2 diabetic subjects compared with controls. The insulin-signalling cascade from the insulin receptor to PI-3-K was also found to be abnormal, resulting in a severely reduced phosphorylation degree of the IRS-1 (IRS-2?)-PI-3-K complex, which can explain both reduced glucose transport and glycogen synthesis. The most pronounced finding in our studies is reduced glycogen synthase activation by insulin which is found in prediabetic subjects with normal glucose tolerance as well as in type 2 diabetics, but more severely. This defect was not reversible after treatment (normalization of blood glucose) and is therefore a candidate for the primary defect which is likely to be of genetic origin, but also could be caused by genetic imprinting, intrauterine malnutrition and social inheritance (obesity). Most of the abnormalities in non-oxidative glucose metabolism may be of secondary origin due to hyperglycemia itself or obesity. Both events may stimulate production of glucosamine, malonyl CoA and intramuscular triglyceride accumulation. These metabolites can theoretically induce most of the defects in glucose processing and furthermore impair insulin signalling. Whether the primary defect in activation of glycogen synthase is due to an abnormality in the enzyme complex itself or in the insulin signalling cascade still has to be investigated.

Physical constraints in the synthesis of glycogen that influence its structural homogeneity: a two-dimensional approach

Several aspects of glycogen optimization as an efficient fuel storage molecule have been studied in previous works: the chain length and the branching degree. These results demonstrated that the values of these variables in the cellular molecule are those that optimize the structure-function relationship. In the present work we show that structural homogeneity of the glycogen molecule is also an optimized variable that plays an important role in its metabolic function. This problem was studied by means of a two-dimensional approach, which allowed us to simplify the very complicated structure of glycogen. Our results demonstrate that there is a molecular size limit that guarantees the structural homogeneity, beyond which the structure of the molecule degenerates, as many chains do not grow. This strongly suggests that such a size limit is precisely what the molecule possesses in the cell.

1H NMR spectroscopy study of the dynamic properties of glycogen in solution by steady-state magnetisation measurement with off-resonance irradiation

The dynamics of size-selected fractions of glycogen in solution have been investigated by proton NMR spectroscopy, using a recently described relaxation study method which relies on strong offresonance irradiation. The dependence of the steady-state magnetisation on angle and intensity of the effective radio-frequency field was measured and compared to theoretical curves derived from different models of motion. Absence or presence of contributions to relaxation from molecular motions on the microsecond time scale can be tested with this method, without having to resort to models. We found that glycogen dipolar relaxation did not result from isotropic Brownian rotation, and despite some contribution from slow motion (> 1 microsecond) to relaxation in glycogen alpha-particles extracted from rat liver, bulk movement of the molecules did not appear to participate in averaging the dipolar term to zero. Whereas hepatic glycogen rat beta-particles and commercial oyster glycogen displayed very similar relaxation properties, alpha-particles showed significantly different behaviour. However, all results were compatible with a diversity of movements within the molecule, ranging from freely rotating pyranoside rings through collective chain motion and possibly to bulk movement of the beta sub-units within the alpha-particle.

Muscle glycogen synthase translocates from the cell nucleus to the cystosol in response to glucose

We have studied the intracellular localization of muscular glycogen synthase by fusing the green fluorescent protein (GFP) of the jelly-fish Aequorea victoria to the N-terminus of human muscle glycogen synthase (HMGS), and expressing the chimeric protein in C2C12, COS-1 cells, and primary cultured rat hepatocytes. In contrast to what we have recently found for the hepatic glycogen synthase (Fernandez-Novell et al. (1997) Biochem. J. 321, 227-231), the GFP/HMGS fusion protein is localized to the nucleus of the cell in the absence of glucose, and in the presence of the sugar it is essentially found in the cytosol. Insulin is not required for the translocation of the enzyme.