Multiple glycogen-binding sites in eukaryotic glycogen synthase are required for high catalytic efficiency toward glycogen

Atg45 is an autophagy receptor for glycogen, a non-preferred cargo of bulk autophagy in yeast

The mechanisms governing autophagy of proteins and organelles have been well studied, but how other cytoplasmic components such as RNA and polysaccharides are degraded remains largely unknown. In this study, we examine autophagy of glycogen, a storage form of glucose. We find that cells accumulate glycogen in the cytoplasm during nitrogen starvation and that this carbohydrate is rarely observed within autophagosomes and autophagic bodies. However, sequestration of glycogen by autophagy is observed following prolonged nitrogen starvation. We identify a yet-uncharacterized open reading frame, Yil024c (herein Atg45), as encoding a cytosolic receptor protein that mediates autophagy of glycogen (glycophagy). Furthermore, we show that, during sporulation, Atg45 is highly expressed and is associated with an increase in glycophagy. Our results suggest that cells regulate glycophagic activity by controlling the expression level of Atg45.

Proteomic profiling of polyglucosan bodies associated with glycogenin-1 deficiency in skeletal muscle

AIMS

Polyglucosan storage disorders represent an emerging field within neurodegenerative and neuromuscular conditions, including Lafora disease (EPM2A, EPM2B), adult polyglucosan body disease (APBD, GBE1), polyglucosan body myopathies associated with RBCK1 deficiency (PGBM1, RBCK1) or glycogenin-1 deficiency (PGBM2, GYG1). While the storage material primarily comprises glycans, this study aimed to gain deeper insights into the protein components by proteomic profiling of the storage material in glycogenin-1 deficiency.

METHODS

We employed molecular genetic analyses, quantitative mass spectrometry of laser micro-dissected polyglucosan bodies and muscle homogenate, immunohistochemistry and western blot analyses in muscle tissue from a 45-year-old patient with proximal muscle weakness from late teenage years due to polyglucosan storage myopathy.

RESULTS

The muscle tissue exhibited a complete absence of glycogenin-1 due to a novel homozygous deep intronic variant in GYG1 (c.7+992T>G), introducing a pseudo-exon causing frameshift and a premature stop codon. Accumulated proteins in the polyglucosan bodies constituted components of glycogen metabolism, protein quality control pathways and desmin. Muscle fibres containing polyglucosan bodies frequently exhibited depletion of normal glycogen.

CONCLUSIONS

The absence of glycogenin-1, a protein important for glycogen synthesis initiation, causes storage of polyglucosan that displays accumulation of several proteins, including those essential for glycogen synthesis, sequestosome 1/p62 and desmin, mirroring findings in RBCK1 deficiency. These results suggest shared pathogenic pathways across different diseases exhibiting polyglucosan storage. Such insights have implications for therapy in these rare yet devastating and presently untreatable disorders.

Architecture, Function, Regulation, and Evolution of α-Glucans Metabolic Enzymes in Prokaryotes

Bacteria have acquired sophisticated mechanisms for assembling and disassembling polysaccharides of different chemistry. α-d-Glucose homopolysaccharides, so-called α-glucans, are the most widespread polymers in nature being key components of microorganisms. Glycogen functions as an intracellular energy storage while some bacteria also produce extracellular assorted α-glucans. The classical bacterial glycogen metabolic pathway comprises the action of ADP-glucose pyrophosphorylase and glycogen synthase, whereas extracellular α-glucans are mostly related to peripheral enzymes dependent on sucrose. An alternative pathway of glycogen biosynthesis, operating via a maltose 1-phosphate polymerizing enzyme, displays an essential wiring with the trehalose metabolism to interconvert disaccharides into polysaccharides. Furthermore, some bacteria show a connection of intracellular glycogen metabolism with the genesis of extracellular capsular α-glucans, revealing a relationship between the storage and structural function of these compounds. Altogether, the current picture shows that bacteria have evolved an intricate α-glucan metabolism that ultimately relies on the evolution of a specific enzymatic machinery. The structural landscape of these enzymes exposes a limited number of core catalytic folds handling many different chemical reactions. In this Review, we present a rationale to explain how the chemical diversity of α-glucans emerged from these systems, highlighting the underlying structural evolution of the enzymes driving α-glucan bacterial metabolism.

Controlled processivity in glycosyltransferases: A way to expand the enzymatic toolbox

Glycosyltransferases (GT) catalyse the biosynthesis of complex carbohydrates which are the most abundant group of molecules in nature. They are involved in several key mechanisms such as cell signalling, biofilm formation, host immune system invasion or cell structure and this in both prokaryotic and eukaryotic cells. As a result, research towards complete enzyme mechanisms is valuable to understand and elucidate specific structure-function relationships in this group of molecules. In a next step this knowledge could be used in GT protein engineering, not only for rational drug design but also for multiple biotechnological production processes, such as the biosynthesis of hyaluronan, cellooligosaccharides or chitooligosaccharides. Generation of these poly- and/or oligosaccharides is possible due to a common feature of several of these GTs: processivity. Enzymatic processivity has the ability to hold on to the growing polymer chain and some of these GTs can even control the number of glycosyl transfers. In a first part, recent advances in understanding the mechanism of various processive enzymes are discussed. To this end, an overview is given of possible engineering strategies for the purpose of new industrial and fundamental applications. In the second part of this review, we focused on specific chain length-controlling mechanisms, i.e., key residues or conserved regions, and this for both eukaryotic and prokaryotic enzymes.

Molecular basis for the regulation of human glycogen synthase by phosphorylation and glucose-6-phosphate

Glycogen synthase (GYS1) is the central enzyme in muscle glycogen biosynthesis. GYS1 activity is inhibited by phosphorylation of its amino (N) and carboxyl (C) termini, which is relieved by allosteric activation of glucose-6-phosphate (Glc6P). We present cryo-EM structures at 3.0-4.0 Å resolution of phosphorylated human GYS1, in complex with a minimal interacting region of glycogenin, in the inhibited, activated and catalytically competent states. Phosphorylations of specific terminal residues are sensed by different arginine clusters, locking the GYS1 tetramer in an inhibited state via intersubunit interactions. The Glc6P activator promotes conformational change by disrupting these interactions and increases the flexibility of GYS1, such that it is poised to adopt a catalytically competent state when the sugar donor UDP-glucose (UDP-glc) binds. We also identify an inhibited-like conformation that has not transitioned into the activated state, in which the locking interaction of phosphorylation with the arginine cluster impedes subsequent conformational changes due to Glc6P binding. Our results address longstanding questions regarding the mechanism of human GYS1 regulation.

Mechanism of glycogen synthase inactivation and interaction with glycogenin

Glycogen is the major glucose reserve in eukaryotes, and defects in glycogen metabolism and structure lead to disease. Glycogenesis involves interaction of glycogenin (GN) with glycogen synthase (GS), where GS is activated by glucose-6-phosphate (G6P) and inactivated by phosphorylation. We describe the 2.6 Å resolution cryo-EM structure of phosphorylated human GS revealing an autoinhibited GS tetramer flanked by two GN dimers. Phosphorylated N- and C-termini from two GS protomers converge near the G6P-binding pocket and buttress against GS regulatory helices. This keeps GS in an inactive conformation mediated by phospho-Ser641 interactions with a composite “arginine cradle”. Structure-guided mutagenesis perturbing interactions with phosphorylated tails led to increased basal/unstimulated GS activity. We propose that multivalent phosphorylation supports GS autoinhibition through interactions from a dynamic “spike” region, allowing a tuneable rheostat for regulating GS activity. This work therefore provides insights into glycogen synthesis regulation and facilitates studies of glycogen-related diseases.

Glycogen is a major energy reserve in eukaryotes and is synthesised in part by glycogenin (GN) and glycogen synthase (GS). Here, authors describe the structural basis of GS regulation, specifically the mechanism of inactivation by phosphorylation.

Proteomic characterisation of polyglucosan bodies in skeletal muscle in RBCK1 deficiency

AIMS

Several neurodegenerative and neuromuscular disorders are characterised by storage of polyglucosan, consisting of proteins and amylopectin-like polysaccharides, which are less branched than in normal glycogen. Such diseases include Lafora disease, branching enzyme deficiency, glycogenin-1 deficiency, polyglucosan body myopathy type 1 (PGBM1) due to RBCK1 deficiency and others. The protein composition of polyglucosan bodies is largely unknown.

METHODS

We combined quantitative mass spectrometry, immunohistochemical and western blot analyses to identify the principal protein components of polyglucosan bodies in PGBM1. Histologically stained tissue sections of skeletal muscle from four patients were used to isolate polyglucosan deposits and control regions by laser microdissection. Prior to mass spectrometry, samples were labelled with tandem mass tags that enable quantitative comparison and multiplexed analysis of dissected samples. To study the distribution and expression of the accumulated proteins, immunohistochemical and western blot analyses were performed.

RESULTS

Accumulated proteins were mainly components of glycogen metabolism and protein quality control pathways. The majority of fibres showed depletion of glycogen and redistribution of key enzymes of glycogen metabolism to the polyglucosan bodies. The polyglucosan bodies also showed accumulation of proteins involved in the ubiquitin-proteasome and autophagocytosis systems and protein chaperones.

CONCLUSIONS

The sequestration of key enzymes of glycogen metabolism to the polyglucosan bodies may explain the glycogen depletion in the fibres and muscle function impairment. The accumulation of components of the protein quality control systems and other proteins frequently found in protein aggregate disorders indicates that protein aggregation may be an essential part of the pathobiology of polyglucosan storage.

Brain glycogen serves as a critical glucosamine cache required for protein glycosylation

Glycosylation defects are a hallmark of many nervous system diseases. However, the molecular and metabolic basis for this pathology is not fully understood. In this study, we found that N-linked protein glycosylation in the brain is metabolically channeled to glucosamine metabolism through glycogenolysis. We discovered that glucosamine is an abundant constituent of brain glycogen, which functions as a glucosamine reservoir for multiple glycoconjugates. We demonstrated the enzymatic incorporation of glucosamine into glycogen by glycogen synthase, and the release by glycogen phosphorylase by biochemical and structural methodologies, in primary astrocytes, and in vivo by isotopic tracing and mass spectrometry. Using two mouse models of glycogen storage diseases, we showed that disruption of brain glycogen metabolism causes global decreases in free pools of UDP-N-acetylglucosamine and N-linked protein glycosylation. These findings revealed fundamental biological roles of brain glycogen in protein glycosylation with direct relevance to multiple human diseases of the central nervous system.

Structural basis of glycogen metabolism in bacteria

The evolution of metabolic pathways is a major force behind natural selection. In the spotlight of such process lies the structural evolution of the enzymatic machinery responsible for the central energy metabolism. Specifically, glycogen metabolism has emerged to allow organisms to save available environmental surplus of carbon and energy, using dedicated glucose polymers as a storage compartment that can be mobilized at future demand. The origins of such adaptive advantage rely on the acquisition of an enzymatic system for the biosynthesis and degradation of glycogen, along with mechanisms to balance the assembly and disassembly rate of this polysaccharide, in order to store and recover glucose according to cell energy needs. The first step in the classical bacterial glycogen biosynthetic pathway is carried out by the adenosine 5'-diphosphate (ADP)-glucose pyrophosphorylase. This allosteric enzyme synthesizes ADP-glucose and acts as a point of regulation. The second step is carried out by the glycogen synthase, an enzyme that generates linear α-(1→4)-linked glucose chains, whereas the third step catalyzed by the branching enzyme produces α-(1→6)-linked glucan branches in the polymer. Two enzymes facilitate glycogen degradation: glycogen phosphorylase, which functions as an α-(1→4)-depolymerizing enzyme, and the debranching enzyme that catalyzes the removal of α-(1→6)-linked ramifications. In this work, we rationalize the structural basis of glycogen metabolism in bacteria to the light of the current knowledge. We describe and discuss the remarkable progress made in the understanding of the molecular mechanisms of substrate recognition and product release, allosteric regulation and catalysis of all those enzymes.

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.

Spatial heterogeneity of glycogen and its metabolizing enzymes in Aspergillus nidulans hyphal tip cells

Glycogen is a homopolymer of glucose and a ubiquitous cellular-storage carbon. This study investigated which Aspergillus nidulans genes are involved in glycogen metabolism. Gene disruptants of predicted glycogen synthase (gsyA) and glycogenin (glgA) genes accumulated less cellular glycogen than the wild type strain, indicating that GsyA and GlgA synthesize glycogen similarly to other eukaryotes. Meanwhile, gene disruption of gphA encoding glycogen phosphorylase increased the amount of glycogen to a higher degree than wild type during the stationary phase that accompanies carbon-source limitation. GFP-tagged GsyA and GphA were distributed in the cytosol and formed punctate and filamentous structures, respectively. Carbon starvation resulted in elongated GphA-GFP filaments and increased numbers of filaments. These structures were more frequently located in the basal regions of tip cells and adjacent cells than in the apical regions of tip cells. Cellular glycogen visualized by incorporation of a fluorescent glucose analog accumulated in cytoplasmic puncta that were more prevalent in the basal regions, a pattern similar to that seen for GsyA. The colocalization of glycogen and GsyA at punctate structures in tip and sub-apical cells likely represents the cellular machinery for synthesizing glycogen. More frequent colocalization in the basal, rather than tip cell apical regions indicated that tip cells have differentiated subcellular regions for glycogen synthesis. Our findings regarding glycogen, GsyA and GphA distribution evoke the spatial heterogeneity of glycogen metabolism in fungal hyphae.

Glycogen synthase from the parabasalian parasite Trichomonas vaginalis: An unusual member of the starch/glycogen synthase family

Trichomonas vaginalis, a parasitic protist, is the causative agent of the common sexually-transmitted infection trichomoniasis. The organism has long been known to synthesize substantial glycogen as a storage polysaccharide, presumably mobilizing this compound during periods of carbohydrate limitation, such as might be encountered during transmission between hosts. However, little is known regarding the enzymes of glycogen metabolism in T. vaginalis. We had previously described the identification and characterization of two forms of glycogen phosphorylase in the organism. Here, we measure UDP-glucose-dependent glycogen synthase activity in cell-free extracts of T. vaginalis. We then demonstrate that the TVAG_258220 open reading frame encodes a glycosyltransferase that is presumably responsible for this synthetic activity. We show that expression of TVAG_258220 in a yeast strain lacking endogenous glycogen synthase activity is sufficient to restore glycogen accumulation. Furthermore, when TVAG_258220 is expressed in bacteria, the resulting recombinant protein has glycogen synthase activity in vitro, transferring glucose from either UDP-glucose or ADP-glucose to glycogen and using both substrates with similar affinity. This protein is also able to transfer glucose from UDP-glucose or ADP-glucose to maltose and longer oligomers of glucose but not to glucose itself. However, with these substrates, there is no evidence of processivity and sugar transfer is limited to between one and three glucose residues. Taken together with our earlier work on glycogen phosphorylase, we are now well positioned to define both how T. vaginalis synthesizes and utilizes glycogen, and how these processes are regulated.

Redox Switch for the Inhibited State of Yeast Glycogen Synthase Mimics Regulation by Phosphorylation

Glycogen synthase (GS) is the rate limiting enzyme in the synthesis of glycogen. Eukaryotic GS is negatively regulated by covalent phosphorylation and allosterically activated by glucose-6-phosphate (G-6-P). To gain structural insights into the inhibited state of the enzyme, we solved the crystal structure of yGsy2-R589A/R592A to a resolution of 3.3 Å. The double mutant has an activity ratio similar to the phosphorylated enzyme and also retains the ability to be activated by G-6-P. When compared to the 2.88 Å structure of the wild-type G-6-P activated enzyme, the crystal structure of the low-activity mutant showed that the N-terminal domain of the inhibited state is tightly held against the dimer-related interface thereby hindering acceptor access to the catalytic cleft. On the basis of these two structural observations, we developed a reversible redox regulatory feature in yeast GS by substituting cysteine residues for two highly conserved arginine residues. When oxidized, the cysteine mutant enzyme exhibits activity levels similar to the phosphorylated enzyme but cannot be activated by G-6-P. Upon reduction, the cysteine mutant enzyme regains normal activity levels and regulatory response to G-6-P activation.

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.

Metabolic Network for the Biosynthesis of Intra- and Extracellular α-Glucans Required for Virulence of Mycobacterium tuberculosis

Mycobacterium tuberculosis synthesizes intra- and extracellular α-glucans that were believed to originate from separate pathways. The extracellular glucose polymer is the main constituent of the mycobacterial capsule that is thought to be involved in immune evasion and virulence. However, the role of the α-glucan capsule in pathogenesis has remained enigmatic due to an incomplete understanding of α-glucan biosynthetic pathways preventing the generation of capsule-deficient mutants. Three separate and potentially redundant pathways had been implicated in α-glucan biosynthesis in mycobacteria: the GlgC-GlgA, the Rv3032 and the TreS-Pep2-GlgE pathways. We now show that α-glucan in mycobacteria is exclusively assembled intracellularly utilizing the building block α-maltose-1-phosphate as the substrate for the maltosyltransferase GlgE, with subsequent branching of the polymer by the branching enzyme GlgB. Some α-glucan is exported to form the α-glucan capsule. There is an unexpected convergence of the TreS-Pep2 and GlgC-GlgA pathways that both generate α-maltose-1-phosphate. While the TreS-Pep2 route from trehalose was already known, we have now established that GlgA forms this phosphosugar from ADP-glucose and glucose 1-phosphate 1000-fold more efficiently than its hitherto described glycogen synthase activity. The two routes are connected by the common precursor ADP-glucose, allowing compensatory flux from one route to the other. Having elucidated this unexpected configuration of the metabolic pathways underlying α-glucan biosynthesis in mycobacteria, an M. tuberculosis double mutant devoid of α-glucan could be constructed, showing a direct link between the GlgE pathway, α-glucan biosynthesis and virulence in a mouse infection model.

Capsule formation is critical for the virulence of many bacterial and fungal pathogens. Mycobacterium tuberculosis cells are known to be surrounded by a capsule layer that is mainly composed of an α-glucan glucose polymer that resembles glycogen. Progress in understanding its role in the virulence of this important human pathogen has been held back by a lack of knowledge of its biosynthesis, preventing the generation of α-glucan-deficient mutants that could be tested in animal infection models. In this work, we unraveled an unexpected metabolic network configuration revealing the exclusive production of both intracellular and capsular α-glucans by the maltosyltransferase GlgE in mycobacteria. GlgE polymerizes an α-maltose 1-phosphate building block, which is generated by two alternative pathways that are connected by a common intermediate allowing rechanneling of flux from one route to the other. Elucidation of this unexpected configuration of the metabolic pathways underlying α-glucan biosynthesis allowed the rational construction of an M. tuberculosis mutant strain devoid of α-glucan, showing a direct link between the GlgE pathway, α-glucan biosynthesis and virulence in a mouse infection model.

Crystal structure of glycogen debranching enzyme and insights into its catalysis and disease-causing mutations

Glycogen is a branched glucose polymer and serves as an important energy store. Its debranching is a critical step in its mobilization. In animals and fungi, the 170 kDa glycogen debranching enzyme (GDE) catalyses this reaction. GDE deficiencies in humans are associated with severe diseases collectively termed glycogen storage disease type III (GSDIII). We report crystal structures of GDE and its complex with oligosaccharides, and structure-guided mutagenesis and biochemical studies to assess the structural observations. These studies reveal that distinct domains in GDE catalyse sequential reactions in glycogen debranching, the mechanism of their catalysis and highly specific substrate recognition. The unique tertiary structure of GDE provides additional contacts to glycogen besides its active sites, and our biochemical experiments indicate that they mediate its recruitment to glycogen and regulate its activity. Combining the understanding of the GDE catalysis and functional characterizations of its disease-causing mutations provides molecular insights into GSDIII.

Debranching of glycogen is an important step in its use as an energy source. Here, the authors describe the crystal structures of glycogen debranching enzyme alone and in complex with oligosaccharides and provide molecular insights into the function, and into associated diseases.

Role of autophagy in glycogen breakdown and its relevance to chloroquine myopathy

Several myopathies are associated with defects in autophagic and lysosomal degradation of glycogen, but it remains unclear how glycogen is targeted to the lysosome and what significance this process has for muscle cells. We have established a Drosophila melanogaster model to study glycogen autophagy in skeletal muscles, using chloroquine (CQ) to simulate a vacuolar myopathy that is completely dependent on the core autophagy genes. We show that autophagy is required for the most efficient degradation of glycogen in response to starvation. Furthermore, we show that CQ-induced myopathy can be improved by reduction of either autophagy or glycogen synthesis, the latter possibly due to a direct role of Glycogen Synthase in regulating autophagy through its interaction with Atg8.

Laforin-malin complex degrades polyglucosan bodies in concert with glycogen debranching enzyme and brain isoform glycogen phosphorylase

In Lafora disease (LD), the deficiency of either EPM2A or NHLRC1, the genes encoding the phosphatase laforin and E3 ligase, respectively, causes massive accumulation of less-branched glycogen inclusions, known as Lafora bodies, also called polyglucosan bodies (PBs), in several types of cells including neurons. The biochemical mechanism underlying the PB accumulation, however, remains undefined. We recently demonstrated that laforin is a phosphatase of muscle glycogen synthase (GS1) in PBs, and that laforin recruits malin, together reducing PBs. We show here that accomplishment of PB degradation requires a protein assembly consisting of at least four key enzymes: laforin and malin in a complex, and the glycogenolytic enzymes, glycogen debranching enzyme 1 (AGL1) and brain isoform glycogen phosphorylase (GPBB). Once GS1-synthesized polyglucosan accumulates into PBs, laforin recruits malin to the PBs where laforin dephosphorylates, and malin degrades the GS1 in concert with GPBB and AGL1, resulting in a breakdown of polyglucosan. Without fountional laforin-malin complex assembled on PBs, GPBB and AGL1 together are unable to efficiently breakdown polyglucosan. All these events take place on PBs and in cytoplasm. Deficiency of each of the four enzymes causes PB accumulation in the cytoplasm of affected cells. Demonstration of the molecular mechanisms underlying PB degradation lays a substantial biochemical foundation that may lead to understanding how PB metabolizes and why mutations of either EPM2A or NHLRC1 in humans cause LD. Mutations in AGL1 or GPBB may cause diseases related to PB accumulation.

Polyglucosan neurotoxicity caused by glycogen branching enzyme deficiency can be reversed by inhibition of glycogen synthase

Uncontrolled elongation of glycogen chains, not adequately balanced by their branching, leads to the formation of an insoluble, presumably neurotoxic, form of glycogen called polyglucosan. To test the suspected pathogenicity of polyglucosans in neurological glycogenoses, we have modeled the typical glycogenosis Adult Polyglucosan Body Disease (APBD) by suppressing glycogen branching enzyme 1 (GBE1, EC 2.4.1.18) expression using lentiviruses harboring short hairpin RNA (shRNA). GBE1 suppression in embryonic cortical neurons led to polyglucosan accumulation and associated apoptosis, which were reversible by rapamycin or starvation treatments. Further analysis revealed that rapamycin and starvation led to phosphorylation and inactivation of glycogen synthase (GS, EC 2.4.1.11), dephosphorylated and activated in the GBE1-suppressed neurons. These protective effects of rapamycin and starvation were reversed by overexpression of phosphorylation site mutant GS only if its glycogen binding site was intact. While rapamycin and starvation induce autophagy, autophagic maturation was not required for their corrective effects, which prevailed even if autophagic flux was inhibited by vinblastine. Furthermore, polyglucosans were not observed in any compartment along the autophagic pathway. Our data suggest that glycogen branching enzyme repression in glycogenoses can cause pathogenic polyglucosan buildup, which might be corrected by GS inhibition.

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.

Interdomain disulfide bridge in the rice granule bound starch synthase I catalytic domain as elucidated by X-ray structure analysis

The catalytic domain of rice (Oryza sativa japonica) granule bound starch synthase I (OsGBSSI-CD) was overexpressed and the three-dimensional structures of the ligand-free and ADP-bound forms were determined. The structures were similar to those reported for bacterial and archaeal glycogen synthases, which belong to glycosyltransferase family 5. They had Rossmann fold N- and C-domains connected by canonical two-hinge peptides, and an interdomain disulfide bond that appears to be conserved in the Poaceae plant family. The presence of three covalent linkages might explain why both OsGBSSI-CD structures adopted only the closed domain arrangement.

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.