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Browsing by Author "Tagliabracci, Vincent S."
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Item Glycogen and its metabolism: some new developments and old themes(Portland Press Ltd., 2012-02-01) Roach, Peter J.; Depaoli-Roach, Anna A.; Hurley, Thomas D.; Tagliabracci, Vincent S.; Department of Biochemistry & Molecular Biology, IU School of MedicineGlycogen 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.Item Metabolism of the covalent phosphate in glycogen(2010-07) Tagliabracci, Vincent S.; Roach, Peter J.; DePaoli-Roach, Anna A.; Hurley, Thomas D., 1961-; Morral, NuriaGlycogen is a highly branched polymer of glucose that functions to store glucose residues for future metabolic use. Skeletal muscle and liver comprise the largest glycogen reserves and play critical roles in maintaining whole body glucose homeostasis. In addition to glucose, glycogen contains small amounts of covalent phosphate of unknown function, origin and structure. Evidence to support the involvement of glycogen associated phosphate in glycogen metabolism comes from patients with Lafora Disease. Lafora disease is an autosomal recessive, fatal form of progressive myoclonus epilepsy. Approximately 90% of cases of Lafora disease are caused by mutations in either the EPM2A or EPM2B genes that encode, respectively, a dual specificity phosphatase called laforin and an E3 ubiquitin ligase called malin. Lafora patients accumulate intracellular inclusion bodies, known as Lafora bodies that are primarily composed of poorly branched, insoluble glycogen-like polymers. We have shown that laforin is a glycogen phosphatase capable of releasing phosphate from glycogen in vitro and that this activity is dependent on a functional carbohydrate binding domain. In studies of laforin knockout mice, we observed a progressive change in the properties and structure of glycogen that paralleled the formation of Lafora bodies. Glycogen isolated from these mice showed increased glycogen phosphate, up to 6-fold (p< 0.001) compared to WT, providing strong evidence that laforin acts as a glycogen phosphatase in vivo. Furthermore we have demonstrated that glycogen synthase introduces phosphate into glycogen during synthesis by transferring the beta-phosphate of UDP-glucose into the polymer and that laforin is capable of releasing the phosphate incorporated by glycogen synthase. Analysis of mammalian glycogen revealed the presence of covalently linked phosphate at the 2 hydroxyl and the 3 hydroxyl of glucose residues in the polysaccharide, providing the first direct evidence of the chemical nature of the phosphate linkage. We envision a glycogen damage/repair process, analogous to errors during DNA synthesis that are subsequently repaired. We propose that laforin action parallels that of DNA repair enzymes and Lafora disease results from the inability of the phosphatase to repair damaged glycogen, adding another biological polymer to the list of those prone to errors by their respective polymerizing enzymes.Item Muscle glycogen remodeling and glycogen phosphate metabolism following exhaustive exercise of wild type and laforin knockout mice(American Society for Biochemistry & Molecular Biology, 2015-09-11) Irimia, Jose M.; Tagliabracci, Vincent S.; Meyer, Catalina M.; Segvich, Dyann M.; DePaoli-Roach, Anna A.; Roach, Peter J.; Department of Biochemistry & Molecular Biology, IU School of MedicineGlycogen, the repository of glucose in many cell types, contains small amounts of covalent phosphate, of uncertain function and poorly understood metabolism. Loss-of-function mutations in the laforin gene cause the fatal neurodegenerative disorder, Lafora disease, characterized by increased glycogen phosphorylation and the formation of abnormal deposits of glycogen-like material called Lafora bodies. It is generally accepted that the phosphate is removed by the laforin phosphatase. To study the dynamics of skeletal muscle glycogen phosphorylation in vivo under physiological conditions, mice were subjected to glycogen-depleting exercise and then monitored while they resynthesized glycogen. Depletion of glycogen by exercise was associated with a substantial reduction in total glycogen phosphate and the newly resynthesized glycogen was less branched and less phosphorylated. Branching returned to normal on a time frame of days, whereas phosphorylation remained suppressed over a longer period of time. We observed no change in markers of autophagy. Exercise of 3-month-old laforin knock-out mice caused a similar depletion of glycogen but no loss of glycogen phosphate. Furthermore, remodeling of glycogen to restore the basal branching pattern was delayed in the knock-out animals. From these results, we infer that 1) laforin is responsible for glycogen dephosphorylation during exercise and acts during the cytosolic degradation of glycogen, 2) excess glycogen phosphorylation in the absence of laforin delays the normal remodeling of the branching structure, and 3) the accumulation of glycogen phosphate is a relatively slow process involving multiple cycles of glycogen synthesis-degradation, consistent with the slow onset of the symptoms of Lafora disease.Item Sterol regulatory element-binding protein-1 (SREBP-1) is required to regulate glycogen synthesis and gluconeogenic gene expression in mouse liver(ASBMB, 2014-01-07) Ruiz, Rafaela; Jideonwo, Victoria; Ahn, Miwon; Surendran, Sneha; Tagliabracci, Vincent S.; Hou, Yongyong; Gamble, Aisha; Kerner, Janos; Irimia-Dominguez, Jose M.; Puchowicz, Michelle A.; Hoppel, Charles; Roach, Peter; Morral, Nuria; Department of Medical & Molecular Genetics, IU School of MedicineSterol regulatory element-binding protein-1 (SREBP-1) is a key transcription factor that regulates genes in the de novo lipogenesis and glycolysis pathways. The levels of SREBP-1 are significantly elevated in obese patients and in animal models of obesity and type 2 diabetes, and a vast number of studies have implicated this transcription factor as a contributor to hepatic lipid accumulation and insulin resistance. However, its role in regulating carbohydrate metabolism is poorly understood. Here we have addressed whether SREBP-1 is needed for regulating glucose homeostasis. Using RNAi and a new generation of adenoviral vector, we have silenced hepatic SREBP-1 in normal and obese mice. In normal animals, SREBP-1 deficiency increased Pck1 and reduced glycogen deposition during fed conditions, providing evidence that SREBP-1 is necessary to regulate carbohydrate metabolism during the fed state. Knocking SREBP-1 down in db/db mice resulted in a significant reduction in triglyceride accumulation, as anticipated. However, mice remained hyperglycemic, which was associated with up-regulation of gluconeogenesis gene expression as well as decreased glycolysis and glycogen synthesis gene expression. Furthermore, glycogen synthase activity and glycogen accumulation were significantly reduced. In conclusion, silencing both isoforms of SREBP-1 leads to significant changes in carbohydrate metabolism and does not improve insulin resistance despite reducing steatosis in an animal model of obesity and type 2 diabetes.