Lafora disease is a progressive myoclonus epilepsy with onset in the teenage years followed by neurodegeneration and death within 10 years. A characteristic is the widespread formation of poorly branched, insoluble glycogen-like polymers (polyglucosan) known as Lafora bodies, which accumulate in neurons, muscle, liver, and other tissues. Approximately half of the cases of Lafora disease result from mutations in the EPM2A gene, which encodes laforin, a member of the dual specificity protein phosphatase family that is able to release the small amount of covalent phosphate normally present in glycogen. In studies of Epm2a ؊/؊ mice that lack laforin, we observed a progressive change in the properties and structure of glycogen that paralleled the formation of Lafora bodies. At three months, glycogen metabolism remained essentially normal, even though the phosphorylation of glycogen has increased 4-fold and causes altered physical properties of the polysaccharide. By 9 months, the glycogen has overaccumulated by 3-fold, has become somewhat more phosphorylated, but, more notably, is now poorly branched, is insoluble in water, and has acquired an abnormal morphology visible by electron microscopy. These glycogen molecules have a tendency to aggregate and can be recovered in the pellet after low speed centrifugation of tissue extracts. The aggregation requires the phosphorylation of glycogen. The aggregrated glycogen sequesters glycogen synthase but not other glycogen metabolizing enzymes. We propose that laforin functions to suppress excessive glycogen phosphorylation and is an essential component of the metabolism of normally structured glycogen.Glycogen is a branched polymer of glucose that acts as a repository of energy used in times of need (1). Liver and skeletal muscle contain the two largest deposits in mammals, but heart, brain, adipose, as well as many other tissues synthesize the polymer. Polymerization occurs via ␣-1,4-glycosidic linkages between glucose residues, with branch points introduced by ␣-1,6-glycosidic linkages. The frequency of branching determines the topology of glycogen and distinguishes it from the carbohydrate moiety of starch (2). A unique three-dimensional structure of glycogen cannot be determined experimentally because of the polydispersity of the molecule, but a widely accepted model of its structure has been proposed (3). Glycogen is composed of successive layers, or tiers, of glucose residues; a full size molecule consists of 12 tiers, with M r ϭ ϳ10 7 and a diameter of ϳ40 nm (4). Glycogen has been reported to contain small amounts of covalently linked phosphate, on the order of 0.064% by weight or 0.121% mol/mol (5-7). The mechanism by which the phosphate is introduced remains unclear. One suggestion (5) has been the existence of a glucose-1-phosphate transferase that would form C1-C6 bridging phosphodiesters, but this enzyme has not been further defined at the molecular level. In plant amylopectin, C1 and C3 phosphomonoesters have been shown to be formed by dikinase enzymes (8), but...

Approximately 90% of cases of Lafora disease, a fatal teenage-onset progressive myoclonus epilepsy, are caused by mutations in either the EPM2A or the EPM2B genes that encode, respectively, a glycogen phosphatase called laforin and an E3 ubiquitin ligase called malin. Lafora disease is characterized by the formation of Lafora bodies, insoluble deposits containing poorly branched glycogen or polyglucosan, in many tissues including skeletal muscle, liver, and brain. Disruption of the Epm2b gene in mice resulted in viable animals that, by 3 months of age, accumulated Lafora bodies in the brain and to a lesser extent in heart and skeletal muscle. Analysis of muscle and brain of the Epm2b ؊/؊ mice by Western blotting indicated no effect on the levels of glycogen synthase, PTG (type 1 phosphatase-targeting subunit), or debranching enzyme, making it unlikely that these proteins are targeted for destruction by malin, as has been proposed. Total laforin protein was increased in the brain of Epm2b ؊/؊ mice and, most notably, was redistributed from the soluble, low speed supernatant to the insoluble low speed pellet, which now contained 90% of the total laforin. This result correlated with elevated insolubility of glycogen and glycogen synthase. Because up-regulation of laforin cannot explain Lafora body formation, we conclude that malin functions to maintain laforin associated with soluble glycogen and that its absence causes sequestration of laforin to an insoluble polysaccharide fraction where it is functionally inert.

Disruption of the gene encoding the liver isoform of glycogen synthase generates a mouse strain (LGSKO) that almost completely lacks hepatic glycogen, has impaired glucose disposal, and is pre-disposed to entering the fasted state. This study investigated how the lack of liver glycogen increases fat accumulation and the development of liver insulin resistance. Insulin signaling in LGSKO mice was reduced in liver, but not muscle, suggesting an organ-specific defect. Phosphorylation of components of the hepatic insulin-signaling pathway, namely IRS1, Akt, and GSK3, was decreased in LGSKO mice. Moreover, insulin stimulation of their phosphorylation was significantly suppressed, both temporally and in an insulin dose response. Phosphorylation of the insulin-regulated transcription factor FoxO1 was somewhat reduced and insulin treatment did not elicit normal translocation of FoxO1 out of the nucleus. Fat overaccumulated in LGSKO livers, showing an aberrant distribution in the acinus, an increase not explained by a reduction in hepatic triglyceride export. Rather, when administered orally to fasted mice, glucose was directed toward hepatic lipogenesis as judged by the activity, protein levels, and expression of several fatty acid synthesis genes, namely, acetyl-CoA carboxylase, fatty acid synthase, SREBP1c, chREBP, glucokinase, and pyruvate kinase. Furthermore, using cultured primary hepatocytes, we found that lipogenesis was increased by 40% in LGSKO cells compared with controls. Of note, the hepatic insulin resistance was not associated with increased levels of pro-inflammatory markers. Our results suggest that loss of liver glycogen synthesis diverts glucose toward fat synthesis, correlating with impaired hepatic insulin signaling and glucose disposal.

Targeting Pathogenic Lafora Bodies in Lafora Disease Using an Antibody-Enzyme Fusion Notes/Citation Information To be published in Cell Metabolism.

Background: Lafora disease is characterized by abnormal, hyperphosphorylated glycogen. Results: 20% of the total phosphate is present as a C6 phosphomonoester of glucose residues; this proportion is unchanged in glycogen from mouse models of Lafora disease. Conclusion: C6 phosphate is not the dominant phosphomonoester. Significance: C2, C3, or C6 phosphate could all contribute to aberrant glycogen structure.

No abstract

The overaccumulation of glycogen appears as a hallmark in various glycogen storage diseases (GSDs), including Pompe, Cori, Andersen, and Lafora disease. Accumulating evidence suggests that suppression of glycogen accumulation represents a potential therapeutic approach for treating these GSDs. Using a fluorescence polarization assay designed to screen for inhibitors of the key glycogen synthetic enzyme, glycogen synthase (GS), we identified a substituted imidazole, (rac)-2-methoxy-4-(1-(2-(1-methylpyrrolidin-2-yl)­ethyl)-4-phenyl-1H-imidazol-5-yl)­phenol (H23), as a first-in-class inhibitor for yeast GS 2 (yGsy2p). Data from X-ray crystallography at 2.85 Å, as well as kinetic data, revealed that H23 bound within the uridine diphosphate glucose binding pocket of yGsy2p. The high conservation of residues between human and yeast GS in direct contact with H23 informed the development of around 500 H23 analogs. These analogs produced a structure–activity relationship profile that led to the identification of a substituted pyrazole, 4-(4-(4-hydroxyphenyl)-3-(trifluoromethyl)-1H-pyrazol-5-yl)­pyrogallol, with a 300-fold improved potency against human GS. These substituted pyrazoles possess a promising scaffold for drug development efforts targeting GS activity in GSDs associated with excess glycogen accumulation.

BackgroundStored glycogen is an important source of energy for skeletal muscle. Human genetic disorders primarily affecting skeletal muscle glycogen turnover are well-recognised, but rare. We previously reported that a frameshift/premature stop mutation in PPP1R3A, the gene encoding RGL, a key regulator of muscle glycogen metabolism, was present in 1.36% of participants from a population of white individuals in the UK. However, the functional implications of the mutation were not known. The objective of this study was to characterise the molecular and physiological consequences of this genetic variant.Methods and FindingsIn this study we found a similar prevalence of the variant in an independent UK white population of 744 participants (1.46%) and, using in vivo 13C magnetic resonance spectroscopy studies, demonstrate that human carriers (n = 6) of the variant have low basal (65% lower, p = 0.002) and postprandial muscle glycogen levels. Mice engineered to express the equivalent mutation had similarly decreased muscle glycogen levels (40% lower in heterozygous knock-in mice, p < 0.05). In muscle tissue from these mice, failure of the truncated mutant to bind glycogen and colocalize with glycogen synthase (GS) decreased GS and increased glycogen phosphorylase activity states, which account for the decreased glycogen content.ConclusionsThus, PPP1R3A C1984ΔAG (stop codon 668) is, to our knowledge, the first prevalent mutation described that directly impairs glycogen synthesis and decreases glycogen levels in human skeletal muscle. The fact that it is present in ∼1 in 70 UK whites increases the potential biomedical relevance of these observations.