Disturbance of lysosomal glycogen metabolism by liposomal anti-alpha-glucosidase and some anti-inflammatory drugs

Glycogen-autophagy: Molecular machinery and cellular mechanisms of glycophagy

Autophagy is an essential cellular process involving degradation of superfluous or defective macromolecules and organelles as a form of homeostatic recycling. Initially proposed to be a “bulk” degradation pathway, a more nuanced appreciation of selective autophagy pathways has developed in the literature in recent years. As a glycogen-selective autophagy process, “glycophagy” is emerging as a key metabolic route of transport and delivery of glycolytic fuel substrate. Study of glycophagy is at an early stage. Enhanced understanding of this major noncanonical pathway of glycogen flux will provide important opportunities for new insights into cellular energy metabolism. In addition, glycogen metabolic mishandling is centrally involved in the pathophysiology of several metabolic diseases in a wide range of tissues, including the liver, skeletal muscle, cardiac muscle, and brain. Thus, advances in this exciting new field are of broad multidisciplinary interest relevant to many cell types and metabolic states. Here, we review the current evidence of glycophagy involvement in homeostatic cellular metabolic processes and of molecular mediators participating in glycophagy flux. We integrate information from a variety of settings including cell lines, primary cell culture systems, ex vivo tissue preparations, genetic disease models, and clinical glycogen disease states.

Post mortem glycogenolysis is a combination of phosphorolysis and hydrolysis

1. Glycogen, glucose, lactate and glycogen phosphorylase concentrations and the activities of glycogen phosphorylase a and acid 1,4-alpha-glucosidase were measured at various times up to 120 min after death in the liver and skeletal muscle of Wistar and gsd/gsd (phosphorylase b kinase deficient) rats and Wistar rats treated with the acid alpha-glucosidase inhibitor acarbose. 2. In all tissues glycogen was degraded rapidly and was accompanied by an increase in tissue glucose and lactate concentrations and a lowering of tissue pH. In the liver of Wistar and acarbose-treated Wistar rats and in the skeletal muscle of all rats glycogen loss proceeded initially very rapidly before slowing. In the gsd/gsd rat liver glycogenolysis proceeded at a linear rate throughout the incubation period. Over 120 min 60, 20 and 50% of the hepatic glycogen store was degraded in the livers of Wistar, gsd/gsd and acarbose-treated Wistar rats, respectively. All 3 types of rat degraded skeletal muscle glycogen at the same rate and to the same extent (82% degraded over 2 hr). 3. In Wistar rat liver and skeletal muscle glycogen phosphorylase was activated soon after death and the activity of phosphorylase a remained well above the zero-time level at all later time points, even when the rate of glycogenolysis had slowed significantly. Liver and skeletal muscle acid alpha-glucosidase activities were unchanged after death. 4. The decreased rate and extent of hepatic glycogenolysis in both the gsd/gsd and acarbose-treated rats suggests that this process is a combination of phosphorolysis and hydrolysis. 5. Glycogen was purified from Wistar liver and skeletal muscle at various times post mortem and its structure investigated. Fine structural analysis revealed progressive shortening of the outer chains of the glycogen from both tissues, indicative of random, lysosomal hydrolysis. Analysis of molecular weight distributions showed inhomogeneity in the glycogen loss; in both tissues high molecular weight glycogen was preferentially degraded. This material is concentrated in lysosomes of both skeletal muscle and liver. These results are consistent with a role for lysosomal hydrolysis in glycogen degradation.

Acarbose is a competitive inhibitor of mammalian lysosomal acid alpha-D-glucosidases

Intraperitoneal injections (approximately 400 mg/kg of body weight) of acarbose, an inhibitor of acid (1----4)-alpha-D-glucosidase, perturb the metabolism of glycogen in the liver, resulting in excess storage of lysosomal glycogen. The metabolism of skeletal muscle glycogen was unaffected, suggesting that acarbose either does not enter the tissue or that the muscle alpha-D-glucosidase is not inhibited. The hydrolysis of maltose and glycogen by the acid alpha-D-glucosidases from rat liver, rat skeletal muscle, and human placenta was inhibited competitively by acarbose. Thus, the lack of effect of acarbose upon the metabolism of muscle glycogen is due to its inability to enter the tissue.

Regulation of lysosomal glycogen metabolism: studies of the actions of mammalian acid alpha-glucosidases

1. Acid alpha-glucosidases were purified to homogeneity from rat liver, rat skeletal muscle and human placenta. The properties of these enzymes were investigated. 2. Their pH optima for activity toward various substrates were in the range 4-5. 3. Time course and pH dependence experiments revealed that all glycogen substrates were not hydrolysed at the same rate; the rate of hydrolysis was inversely related to the molecular size of the substrate. The most rapidly hydrolysed glycogen substrate was the smallest (commercial oyster) while the least rapidly hydrolysed was the largest (native rat or rabbit liver). Intermediate sized glycogens were hydrolysed at intermediate rates. 4. Glycogen hydrolysis was stimulated by added sodium ions; this stimulation was pH dependent. 5. It is suggested that lysosomal glycogen metabolism may be controlled by pH, salt concentration and the size of the glycogen substrate. 6. Since the high molecular weight glycogen associated with lysosomes is formed by disulphide bridges between lower molecular weight material it is proposed that an important step of lysosomal glycogen degradation is disulphide bond reduction.

Rat skeletal muscle lysosomes contain glycogen

1. A lysosome- and mitochondria-enriched fraction was obtained from rat skeletal muscle using a differential centrifugation procedure. This fraction contained a proportion (3-4%) of the tissue glycogen content. 2. Lysosomes and mitochondria were separated from one another by centrifugation of a cell-free extract upon discontinuous Ficoll-sucrose gradients. Little glycogen was associated with the resulting mitochondrial fraction. The lysosomal fraction, however, contained a significant amount of glycogen, accounting for 5% of the skeletal muscle glycogen. 3. The lysosomal glycogen was purified and found to be enriched in high molecular weight material. 4. The compartmentation of skeletal muscle glycogen metabolism is suggested.

The structure of placental glycogen

Glycogen was purified from human term placenta and its structural features investigated. The beta-amylolysis limit and average chain lengths indicated that some degradation of the glycogen had occurred prior to its extraction. The sedimentation coefficient distribution of the purified glycogen showed that it contained a significant proportion of aggregated material. Diffusion coefficient measurements allowed calculation of the molecular weight distribution. The placental glycogen contained a significant proportion of high molecular weight material, although not as much as liver or skeletal muscle glycogens. Because the high molecular weight glycogen of liver and skeletal muscle is associated with the lysosome it is likely that this is also true of the large placental glycogen. Lysosomal glycogen is degraded hydrolytically to glucose and so placental glycogen may be involved in fetal glucose homeostasis.

Compartmentation of glycogen metabolism in the liver

The incorporation of radioactivity into liver glycogen has been shown not only to be a metabolically inhomogeneous process but also to depend critically on the nature of the precursor. D-Galactose is incorporated into glycogen by a mechanism which is separate from that associated with the incorporation of D-glucose. D-Galactose is favoured for incorporation into high-molecular-weight glycogen and consequently is affected more by treatment of the animal with the antibiotic tunicamycin, since high-molecular-weight glycogen is preferentially found in the lysosomal compartment.

Lysosomal glycogen storage induced by Acarbose, a 1,4-alpha-glucosidase inhibitor

The 1,4-alpha-glucosidase inhibitor. Acarbose, when injected intraperitoneally disturbs liver lysosome metabolism, causing distinct and persistent inhibition of the enzymes and acute disturbances of lysosomal glycogen metabolism. A feedback control mechanism appears to operate, affecting cytosolic carbohydrate metabolism. A model is suggested for the adult form of lysosomal storage disease. The biochemical effects closely resemble those occurring in glycogenosis type II (Pompe's disease), and these have been confirmed by electron microscopy.

Factors affecting the metabolic control of cytosolic and lysosomal glycogen levels in the liver

Rats with a gentic deficiency of phosphorylase kinase have been treated with the 1,4-alpha-glucosidase inhibitor, Acarbose. Lysosomal glycogen metabolism has been markedly altered and the results support the concept of a feedback control mechanism operating on the uptake mechanism into the lysosomal compartment.

Glycogen of high molecular weight from mammalian muscle

Glycogen of high molecular weight has been isolated from mammalian muscle, in contrast to the material of low molecular weight commonly described. The large polysaccharide is similar to liver glycogen in the structure of its individual beta-particles and also, partially, in the mode of assembly into the gross alpha-particles. The large particles may be disrupted by 2-mercaptoethanol, but not to the same extent as their liver counterparts.

Ordered synthesis and degradation of liver glycogen involving 2-amino-2-deoxy-D-glucose

The incorporation of 2-amino-2-deoxy-D-glucose from precursor 2-amino-2-deoxy-D-galactose into liver glycogen has been shown to be a metabolically inhomogeneous process after starvation. The protein-to-polysaccharide ratio is also heterogeneous with respect to molecular size, and enhanced overall as compared to normal glycogen. The results are discussed from the viewpoint of a molecular order in the synthesis and degradation of liver glycogen.