Proglycogen and macroglycogen: artifacts of glycogen extraction?

Defect in degradation of glycogenin-exposed residual glycogen in lysosomes is the fundamental pathomechanism of Pompe disease

Pompe disease is a lysosomal storage disorder that preferentially affects muscles, and it is caused by GAA mutation coding acid alpha-glucosidase in lysosome and glycophagy deficiency. While the initial pathology of Pompe disease is glycogen accumulation in lysosomes, the special role of the lysosomal pathway in glycogen degradation is not fully understood. Hence, we investigated the characteristics of accumulated glycogen and the mechanism underlying glycophagy disturbance in Pompe disease. Skeletal muscle specimens were obtained from the affected sites of patients and mouse models with Pompe disease. Histological analysis, immunoblot analysis, immunofluorescence assay, and lysosome isolation were utilized to analyze the characteristics of accumulated glycogen. Cell culture, lentiviral infection, and the CRISPR/Cas9 approach were utilized to investigate the regulation of glycophagy accumulation. We demonstrated residual glycogen, which was distinguishable from mature glycogen by exposed glycogenin and more α-amylase resistance, accumulated in the skeletal muscle of Pompe disease. Lysosome isolation revealed glycogen-free glycogenin in wild type mouse lysosomes and variously sized glycogenin in Gaa-/- mouse lysosomes. Our study identified that a defect in the degradation of glycogenin-exposed residual glycogen in lysosomes was the fundamental pathological mechanism of Pompe disease. Meanwhile, glycogenin-exposed residual glycogen was absent in other glycogen storage diseases caused by cytoplasmic glycogenolysis deficiencies. In vitro, the generation of residual glycogen resulted from cytoplasmic glycogenolysis. Notably, the inhibition of glycogen phosphorylase led to a reduction in glycogenin-exposed residual glycogen and glycophagy accumulations in cellular models of Pompe disease. Therefore, the lysosomal hydrolysis pathway played a crucial role in the degradation of residual glycogen into glycogenin, which took place in tandem with cytoplasmic glycogenolysis. These findings may offer a novel substrate reduction therapeutic strategy for Pompe disease. © 2024 The Pathological Society of Great Britain and Ireland.

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 Structure of Glycogen Molecules in Cells

Glycogen is a strongly branched polymer of α-D-glucose, with glucose residues in the linear chains linked by 1→4-bonds (~93% of the total number of bonds) and with branching after every 4-8 residues formed by 1→6-glycosidic bonds (~7% of the total number of bonds). It is thought currently that a fully formed glycogen molecule (β-particle) with the self-glycosylating protein glycogenin in the center has a spherical shape with diameter of ~42 nm and contains ~ 55,000 glucose residues. The glycogen molecule also includes numerous proteins involved in its synthesis and degradation, as well as proteins performing a carcass function. However, the type and force of bonds connecting these proteins to the polysaccharide moiety of glycogen are significantly different. This review presents the available data on the spatial structure of the glycogen molecule and its changes under various physiological and pathological conditions.

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.

The dynamic life of the glycogen granule

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

Restoration of Muscle Glycogen and Functional Capacity: Role of Post-Exercise Carbohydrate and Protein Co-Ingestion

The importance of post-exercise recovery nutrition has been well described in recent years, leading to its incorporation as an integral part of training regimes in both athletes and active individuals. Muscle glycogen depletion during an initial prolonged exercise bout is a main factor in the onset of fatigue and so the replenishment of glycogen stores may be important for recovery of functional capacity. Nevertheless, nutritional considerations for optimal short-term (3–6 h) recovery remain incompletely elucidated, particularly surrounding the precise amount of specific types of nutrients required. Current nutritional guidelines to maximise muscle glycogen availability within limited recovery are provided under the assumption that similar fatigue mechanisms (i.e., muscle glycogen depletion) are involved during a repeated exercise bout. Indeed, recent data support the notion that muscle glycogen availability is a determinant of subsequent endurance capacity following limited recovery. Thus, carbohydrate ingestion can be utilised to influence the restoration of endurance capacity following exhaustive exercise. One strategy with the potential to accelerate muscle glycogen resynthesis and/or functional capacity beyond merely ingesting adequate carbohydrate is the co-ingestion of added protein. While numerous studies have been instigated, a consensus that is related to the influence of carbohydrate-protein ingestion in maximising muscle glycogen during short-term recovery and repeated exercise capacity has not been established. When considered collectively, carbohydrate intake during limited recovery appears to primarily determine muscle glycogen resynthesis and repeated exercise capacity. Thus, when the goal is to optimise repeated exercise capacity following short-term recovery, ingesting carbohydrate at an amount of ≥1.2 g kg body mass⁻¹·h⁻¹ can maximise muscle glycogen repletion. The addition of protein to carbohydrate during post-exercise recovery may be beneficial under circumstances when carbohydrate ingestion is sub-optimal (≤0.8 g kg body mass⁻¹·h⁻¹) for effective restoration of muscle glycogen and repeated exercise capacity.

Physicochemical Characteristics of Rat Muscle Glycogen Fractions

INTRODUCTION

Homogenization of animal tissues with cold Perchloric Acid (PCA) produces two fractions of glycogen, Acid Soluble Glycogen (ASG) and Acid Insoluble Glycogen (AIG).

AIM

To determine some physicochemical characteristics of muscle glycogen fractions in two groups of rat.

MATERIALS AND METHODS

An experimental study was conducted on two groups of five male rats. Rats in control group were kept at rest and in case group on 30 minutes physical activity. The content of carbohydrate, protein, phosphate, index and relative Molecular Weights (MWs) were determined for glycogen fractions.

RESULTS

Total glycogen decreased following muscular activity (1.40±0.08, mg/g wet muscle vs. 0.97±0.11, p<0.05) and the change occurred totally in ASG (1.02±0.07 vs. 0.57±0.07, p=0.017), whereas, AIG changed insignificantly (0.39±0.05 vs. 0.36±0.02, p=0.5). The protein content of AIG was about 5.5 times of ASG fraction. The ratio of carbohydrate to protein was 0.33±0.01 (mg/mg) in ASG and decreased to 0.19±0.02, p=0.01 after 30 minute activity. This ratio in AIG was about 6% of ASG fraction and did not change significantly during physical activity. The ratio of phosphate to protein was three times in ASG relative to AIG at rest and did not change following activity. The index of molecular weight was calculated for each fraction of glycogen as the ratio of concentration per osmolality (mg/mmol). The index was 1.82±0.02 for ASG at rest and decreased significantly to 1.07±0.12, p<0.05 following 30 minutes activity. The index did not change significantly for AIG fraction (0.56±0.05 vs. 0.48±0.10, p=0.4). The relative MW of the fractions of ASG to AIG was 3.3±0.3 at rest and decreased significantly to 2.2±0.6, p<0.05 following 30 minutes activity.

CONCLUSION

Two fractions of muscle glycogen, ASG and AIG, differ in the relative carbohydrate: protein content and ASG have a higher mean of MW and is more metabolic active form.

Comparison of Methods to Assay Liver Glycogen Fractions: The Effects of Starvation

INTRODUCTION

There are several methods to extract and measure glycogen in animal tissues. Glycogen is extracted with or without homogenization by using cold Perchloric Acid (PCA).

AIM

Three procedures were compared to determine glycogen fractions in rat liver at different physiological states.

MATERIALS AND METHODS

The present study was conducted on two groups of rats, one group of five rats were fed standard rodent laboratory food and were marked as controls, and another five rats were starved overnight (15 hour) as cases. The glycogen fractions were extracted and measured by using three methods: classical homogenization, total-glycogen-fractionation and homogenization-free protocols.

RESULTS

The data of homogenization methods showed that following 15 hour starvation, total glycogen decreased (36.4±1.9 vs. 27.7±2.5, p=0.01) and the change occurred entirely in Acid Soluble Glycogen (ASG) (32.0±1.1 vs. 22.7±2.5, p=0.01), while Acid Insoluble Glycogen (AIG) did not change significantly (4.9±0.9 vs. 4.6±0.3, p=0.7). Similar results were achieved by using the method of total-glycogen-fractionation. Homogenization-free procedure indicated that ASG and AIG fractions compromise about 2/3 and 1/3 of total glycogen and the changes occurred in both ASG (24.4±2.6 vs. 16.7±0.4, p<0.05) and AIG fraction (8.7±0.8 vs. 7.1±0.3, p=0.05).

CONCLUSION

The findings of 'homogenization assay method' indicate that ASG is the major portion of liver glycogen and is more metabolically active form. The same results were obtained by using 'total-glycogen-fractionation method'. 'Homogenization-free method' gave different results, because AIG has been contaminated with ASG fraction. In both 'homogenization' and 'homogenization-free' methods ASG must be extracted at least twice to prevent contamination of AIG with ASG.

Changes in glycogen structure over feeding cycle sheds new light on blood-glucose control

Liver glycogen, a highly branched polymer of glucose, is important for maintaining blood-glucose homeostasis. It was recently shown that db/db mice, a model for Type 2 diabetes, are unable to form the large composite glycogen α particles present in normal, healthy mice. In this study, the structure of healthy mouse-liver glycogen over the diurnal cycle was characterized using size exclusion chromatography and transmission electron microscopy. Glycogen was found to be formed as smaller β particles, and then only assembled into large α particles, with a broad size distribution, significantly after the time when glycogen content had reached a maximum. This pathway, missing in diabetic animals, is likely to give optimal blood-glucose control during the daily feeding cycle. Lack of this control may contribute to, or result from, diabetes. This discovery suggests novel approaches to diabetes management.

Regulation of post-mortem glycolysis in ruminant muscle

As a tissue, muscle has the unique ability to switch its metabolic source of ATP, the energy currency underpinning muscle function. During oxygen debt, such as that occurring immediately following the death of animals, anaerobic metabolism is initiated in an attempt to restore homeostasis within the muscle. The cascade of biochemical events that are initiated is paramount in the context of meat quality. This review revisits this reasonably well-known subject but takes a new perspective by drawing on the understanding outside the traditional discipline of meat science. Our understanding of the intrinsic regulators of glycolytic flux has improved but knowledge gaps remain. Further efforts to understand how the glycolytic enzyme kinetics are influenced by both pre- and post-slaughter factors will be beneficial in the ongoing quest to maximise fresh meat quality.

Aerobic training as an adjunctive therapy to enzyme replacement in Pompe disease

BACKGROUND

Aerobic exercise may be used in conjunction with enzyme replacement therapy (ERT) to attenuate cardiovascular deconditioning, skeletal muscle wasting, and loss of motor function in Pompe disease (glycogen storage disease type II; GSDII), but the effects on lysosomal glycogen content and macroautophagy have not been defined to date.

PURPOSE

The main objectives of this study were to determine if acute aerobic exercise enhances 24-h uptake of recombinant human enzyme (rhGAA; Myozyme® [aim 1]) and if endurance training improves disease pathology when combined with ERT [aim 2] in Pompe mice.

METHODS

For the first aim in our study, Pompe mutant mice (6(neo)/6(neo)) were grouped into ERT (Myozyme® injection only [40 mg/kg]) and ERT+EX (Myozyme® injection followed by 90 min treadmill exercise) cohorts, and enzyme uptake was assessed in the heart and quadriceps 24h post injection. For the second aim of our study, mutant mice were randomized into control, endurance-trained, enzyme-treated, or combination therapy groups. Exercised animals underwent 14 weeks of progressive treadmill training with or without biweekly Myozyme® injections (40 mg/kg) and tissues were harvested 1 week post last treatment.

RESULTS

Myozyme® uptake (GAA activity) was not improved in ERT+EX over ERT alone at 24-h post injection. Endurance exercise training, with or without ERT, improved aerobic capacity and normalized grip strength, motor function, and lean mass (P<0.05), but did not reduce glycogen content or normalize macroautophagy beyond traditional enzyme replacement therapy.

CONCLUSIONS

Endurance training is beneficial as an adjunctive therapy to ERT in Pompe disease, although it works by mechanisms independent of a reduction in glycogen content.

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.

Exercise training does not correct abnormal cardiac glycogen accumulation in the db/db mouse model of type 2 diabetes

Substrate imbalance is a well-recognized feature of diabetic cardiomyopathy. Insulin resistance effectively limits carbohydrate oxidation, resulting in abnormal cardiac glycogen accumulation. Aims of the present study were to 1) characterize the role of glycogen-associated proteins involved in excessive glycogen accumulation in type 2 diabetic hearts and 2) determine if exercise training can attenuate abnormal cardiac glycogen accumulation. Control (db(+)) and genetically diabetic (db/db) C57BL/KsJ-lepr(db)/lepr(db) mice were subjected to sedentary or treadmill exercise regimens. Exercise training consisted of high-intensity/short-duration (10 days) and low-intensity/long-duration (6 wk) protocols. Glycogen levels were elevated by 35-50% in db/db hearts. Exercise training further increased (2- to 3-fold) glycogen levels in db/db hearts. Analysis of soluble and insoluble glycogen pools revealed no differential accumulation of one glycogen subspecies. Phosphorylation (Ser(640)) of glycogen synthase, an indicator of enzymatic fractional activity, was greater in db/db mice subjected to sedentary and exercise regimens. Elevated glycogen levels were accompanied by decreased phosphorylation (Thr(172)) of 5'-AMP-activated kinase and phosphorylation (Ser(79)) of its downstream substrate acetyl-CoA carboxylase. Glycogen concentration was not associated with increases in other glycogen-associated proteins, including malin and laforin. Novel observations show that exercise training does not correct diabetes-induced elevations in cardiac glycogen but, rather, precipitates further accumulation.

Carbohydrate energy reserves and effects of food deprivation in male and female rainbow trout

We investigated the effects of nutritional state on carbohydrate, lipid, and protein stores in the heart, liver, and white skeletal muscle of male and female rainbow trout. For fed animals we also partitioned glycogen into fractions based on acid solubility. Fish (10-14 months-old, ~400-500 g) were held at 14 °C and either fed (1% of body weight, every other day) or deprived of food for 14 days. Under fed conditions, glycogen was increased 54% in ventricles from males compared with females, and elevated in the liver (87%) and white muscle (70%) in sexually-maturing versus immature males. Acid soluble glycogen predominated over the acid insoluble fraction in all tissues and was similar between sexes. Food deprivation 1) selectively reduced glycogen and free glucose in male ventricles by ~30%, and 2) did not change glycogen in the liver or white muscle, or triglyceride, protein or water levels in any tissues for both sexes. These data highlight sex differences in teleost cardiac stores and the metabolism of carbohydrates, and contrast with mammals where cardiac glycogen increases during fasting and acid insoluble glycogen is a significant fraction. Increased glycogen in the hearts of male rainbow trout appears to pre-empt sex-specific cardiac growth while storage of acid soluble glycogen may reflect a novel strategy for efficient synthesis and mobilization of glycogen in fishes.

The regulation of muscle glycogen: the granule and its proteins

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

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

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

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

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

AMP-activated protein kinase--a sensor of glycogen as well as AMP and ATP?

The classical role of the AMP-activated protein kinase (AMPK) is to act as a sensor of the immediate availability of cellular energy, by monitoring the concentrations of AMP and ATP. However, the beta subunits of AMPK contain a glycogen-binding domain, and in this review we develop the hypothesis that this is a regulatory domain that allows AMPK to act as a sensor of the status of cellular reserves of energy in the form of glycogen. We argue that the pool of AMPK that is bound to the glycogen particle is in an active state when glycogen particles are fully synthesized, causing phosphorylation of glycogen synthase at site 2 and providing a feedback inhibition of further extension of the outer chains of glycogen. However, when glycogen becomes depleted, the glycogen-bound pool of AMPK becomes inhibited due to binding to alpha1-->6-linked branch points exposed by the action of phosphorylase and/or debranching enzyme. This allows dephosphorylation of site 2 on glycogen synthase by the glycogen-bound form of protein phosphatase-1, promoting rapid resynthesis of glycogen and replenishment of glycogen stores. This is an extension of the classical role of AMPK as a 'guardian of cellular energy', in which it ensures that cellular energy reserves are adequate for medium-term requirements. The literature concerning AMPK, glycogen structure and glycogen-binding proteins that led us to this concept is reviewed.