Dual Regulation of Muscle Glycogen Synthase during Exercise by Activation and Compartmentalization

Glycogen synthase (GS) is considered the rate-limiting enzyme in glycogenesis but still today there is a lack of understanding on its regulation. We have previously shown phosphorylation-dependent GS intracellular redistribution at the start of glycogen re-synthesis in rabbit skeletal muscle (Prats, C., Cadefau, J. A., Cussó, R., Qvortrup, K., Nielsen, J. N., Wojtaszewki, J. F., Wojtaszewki, J. F., Hardie, D. G., Stewart, G., Hansen, B. F., and Ploug, T. (2005) J. Biol. Chem. 280, 23165–23172). In the present study we investigate the regulation of human muscle GS activity by glycogen, exercise, and insulin. Using immunocytochemistry we investigate the existence and relevance of GS intracellular compartmentalization during exercise and during glycogen re-synthesis. The results show that GS intrinsic activity is strongly dependent on glycogen levels and that such regulation involves associated dephosphorylation at sites 2+2a, 3a, and 3a + 3b. Furthermore, we report the existence of several glycogen metabolism regulatory mechanisms based on GS intracellular compartmentalization. After exhausting exercise, epinephrine-induced protein kinase A activation leads to GS site 1b phosphorylation targeting the enzyme to intramyofibrillar glycogen particles, which are preferentially used during muscle contraction. On the other hand, when phosphorylated at sites 2+2a, GS is preferentially associated with subsarcolemmal and intermyofibrillar glycogen particles. Finally, we verify the existence in human vastus lateralis muscle of the previously reported mechanism of glycogen metabolism regulation in rabbit tibialis anterior muscle. After overnight low muscle glycogen level and/or in response to exhausting exercise-induced glycogenolysis, GS is associated with spherical structures at the I-band of sarcomeres. Glycogen synthase (GS) is considered the rate-limiting enzyme in glycogenesis but still today there is a lack of understanding on its regulation. We have previously shown phosphorylation-dependent GS intracellular redistribution at the start of glycogen re-synthesis in rabbit skeletal muscle (Prats, C., Cadefau, J. A., Cussó, R., Qvortrup, K., Nielsen, J. N., Wojtaszewki, J. F., Wojtaszewki, J. F., Hardie, D. G., Stewart, G., Hansen, B. F., and Ploug, T. (2005) J. Biol. Chem. 280, 23165–23172). In the present study we investigate the regulation of human muscle GS activity by glycogen, exercise, and insulin. Using immunocytochemistry we investigate the existence and relevance of GS intracellular compartmentalization during exercise and during glycogen re-synthesis. The results show that GS intrinsic activity is strongly dependent on glycogen levels and that such regulation involves associated dephosphorylation at sites 2+2a, 3a, and 3a + 3b. Furthermore, we report the existence of several glycogen metabolism regulatory mechanisms based on GS intracellular compartmentalization. After exhausting exercise, epinephrine-induced protein kinase A activation leads to GS site 1b phosphorylation targeting the enzyme to intramyofibrillar glycogen particles, which are preferentially used during muscle contraction. On the other hand, when phosphorylated at sites 2+2a, GS is preferentially associated with subsarcolemmal and intermyofibrillar glycogen particles. Finally, we verify the existence in human vastus lateralis muscle of the previously reported mechanism of glycogen metabolism regulation in rabbit tibialis anterior muscle. After overnight low muscle glycogen level and/or in response to exhausting exercise-induced glycogenolysis, GS is associated with spherical structures at the I-band of sarcomeres. The desire to understand metabolism and its regulation dates back several centuries, but it has exponentially increased during the last decades in an effort to treat or prevent type 2 diabetes mellitus (T2DM). 3The abbreviations used are:T2DMtype 2 diabetes mellitusGSglycogen synthaseG6Pglucose 6-phosphatePKAprotein kinase AVLvastus lateralisFVfractional velocityglcglucoseCHOcarbohydrateGSIglycogen synthase glucose 6-phosphate-independent form. 3The abbreviations used are:T2DMtype 2 diabetes mellitusGSglycogen synthaseG6Pglucose 6-phosphatePKAprotein kinase AVLvastus lateralisFVfractional velocityglcglucoseCHOcarbohydrateGSIglycogen synthase glucose 6-phosphate-independent form. Defective muscle glycogen synthesis has been repeatedly reported in patients with T2DM (1.Bogardus C. Lillioja S. Stone K. Mott D. J. Clin. Invest. 1984; 73: 1185-1190Crossref PubMed Scopus (264) Google Scholar, 2.Roden M. Petersen K.F. Shulman G.I. Recent Prog. Horm. Res. 2001; 56: 219-237Crossref PubMed Scopus (93) Google Scholar, 3.Shulman G.I. Rothman D.L. Jue T. Stein P. DeFronzo R.A. Shulman R.G. N. Engl. J. Med. 1990; 322: 223-228Crossref PubMed Scopus (1045) Google Scholar). Several studies have shown impairments of insulin-induced glycogen synthase (GS) activation in skeletal muscle from T2DM patients and in healthy subjects at increased risk for T2DM, such as healthy obese and first-degree relatives of patients with T2DM (4.Damsbo P. Vaag A. Hother-Nielsen O. Beck-Nielsen H. Diabetologia. 1991; 34: 239-245Crossref PubMed Scopus (194) Google Scholar, 5.Højlund K. Staehr P. Hansen B.F. Green K.A. Hardie D.G. Richter E.A. Beck-Nielsen H. Wojtaszewski J.F. Diabetes. 2003; 52: 1393-1402Crossref PubMed Scopus (116) Google Scholar, 6.Højlund K. Beck-Nielsen H. Curr. Diabetes Rev. 2006; 2: 375-395Crossref PubMed Scopus (49) Google Scholar, 7.Schalin-Jäntti C. Härkonen M. Groop L.C. Diabetes. 1992; 41: 598-604Crossref PubMed Google Scholar, 8.Eriksson J. Franssila-Kallunki A. Ekstrand A. Saloranta C. Widén E. Schalin C. Groop L. N. Engl. J. Med. 1989; 321: 337-343Crossref PubMed Scopus (799) Google Scholar, 9.Vaag A. Henriksen J.E. Beck-Nielsen H. J. Clin. Invest. 1992; 89: 782-788Crossref PubMed Scopus (300) Google Scholar). type 2 diabetes mellitus glycogen synthase glucose 6-phosphate protein kinase A vastus lateralis fractional velocity glucose carbohydrate glycogen synthase glucose 6-phosphate-independent form. type 2 diabetes mellitus glycogen synthase glucose 6-phosphate protein kinase A vastus lateralis fractional velocity glucose carbohydrate glycogen synthase glucose 6-phosphate-independent form. The first scientific studies on GS date from the 1960s, but still today there is a lack of understanding on its regulation. GS is the rate-limiting enzyme in glycogenesis and is classically used as an example of an allosterically and covalently regulated enzyme. It is well accepted that GS is complexly regulated by sequences of hierarchal phosphorylations (10.Roach P.J. J. Biol. Chem. 1991; 266: 14139-14142Abstract Full Text PDF PubMed Google Scholar) in at least nine sites and by its allosteric activator, glucose 6-phosphate (G6P) (11.Rothman-Denes L.B. Cabib E. Biochemistry. 1971; 10: 1236-1242Crossref PubMed Scopus (39) Google Scholar, 12.Villar-Palasi C. Biochim. Biophys. Acta. 1991; 1095: 261-267Crossref PubMed Scopus (45) Google Scholar). However, the exact effects of GS phosphorylation at different sites on its regulation are still not clear. GS phosphorylation sites are distributed between the NH2- and the COOH-terminal domains. The NH2 terminus domain contains two sites, 2 (Ser7) and 2a (Ser10), that are phosphorylated in a hierarchal mode. Phosphorylation of site 2 is needed as a recognition motif for casein kinase 1 to phosphorylate site 2a (13.Flotow H. Roach P.J. J. Biol. Chem. 1989; 264: 9126-9128Abstract Full Text PDF PubMed Google Scholar, 14.Flotow H. Graves P.R. Wang A.Q. Fiol C.J. Roeske R.W. Roach P.J. J. Biol. Chem. 1990; 265: 14264-14269Abstract Full Text PDF PubMed Google Scholar). Several protein kinases have been reported to phosphorylate site 2 in vitro, among them PKA, CaMKII, PKC, AMPK, GPhK, and MAPKAPKII (15.Carling D. Hardie D.G. Biochim. Biophys. Acta. 1989; 1012: 81-86Crossref PubMed Scopus (255) Google Scholar, 16.Huang T.S. Krebs E.G. Biochem. Biophys. Res. Commun. 1977; 75: 643-650Crossref PubMed Scopus (34) Google Scholar, 17.Roach P.J. DePaoli-Roach A.A. Larner J. J. Cyclic Nucleotide. Res. 1978; 4: 245-257PubMed Google Scholar). At the COOH terminus of muscle GS, there are at least seven phosphorylation sites; sites 3a (Ser640), 3b (Ser644), 3c (Ser648), 4 (Ser652), 5 (Ser656), 1a (Ser697), and 1b (Ser710). Sites 3, 4, and 5 are phosphorylated in a hierarchal mode. Casein kinase II phosphorylates site 5, establishing a recognition motif for GSK-3 to phosphorylate sequentially sites 4, 3c, 3b, and 3a (18.DePaoli-Roach A.A. Ahmad Z. Camici M. Lawrence Jr., J.C. Roach P.J. J. Biol. Chem. 1983; 258: 10702-10709Abstract Full Text PDF PubMed Google Scholar, 19.Fiol C.J. Mahrenholz A.M. Wang Y. Roeske R.W. Roach P.J. J. Biol. Chem. 1987; 262: 14042-14048Abstract Full Text PDF PubMed Google Scholar, 20.Picton C. Woodgett J. Hemmings B. Cohen P. FEBS Lett. 1982; 150: 191-196Crossref PubMed Scopus (117) Google Scholar, 21.Roach P.J. FASEB J. 1990; 4: 2961-2968Crossref PubMed Scopus (194) Google Scholar). Dephosphorylation of sites 2 and 3 increase GS intrinsic activity much more than dephosphorylation of the remaining sites, which have little or no effect on the enzyme activity (22.Skurat A.V. Wang Y. Roach P.J. J. Biol. Chem. 1994; 269: 25534-25542Abstract Full Text PDF PubMed Google Scholar). The effect of GS phosphorylation at sites 1a and 1b remains elusive. G6P binding reverses covalent inactivation of GS by phosphorylation (11.Rothman-Denes L.B. Cabib E. Biochemistry. 1971; 10: 1236-1242Crossref PubMed Scopus (39) Google Scholar) and increases susceptibility of the enzyme to be activated by the action of protein phosphatases (23.Villar-Palasi C. Biochim. Biophys. Acta. 1995; 1244: 203-208Crossref PubMed Scopus (19) Google Scholar), mainly by glycogen-targeted protein phosphatase 1. Intracellular compartmentalization of GS has been reported in several studies. In isolated hepatocytes, incubation with glucose induces GS activation and intracellular translocation to the cell periphery (24.Fernández-Novell J.M. Bellido D. Vilaró S. Guinovart J.J. Biochem. J. 1997; 321: 227-231Crossref PubMed Scopus (66) Google Scholar). In contrast, in the absence of glucose, GS has been shown to be mainly located inside the nucleus of both cultured liver and muscle cells; however, following addition of glucose GS translocates to the cytosol (25.Ferrer J.C. Baqué S. Guinovart J.J. FEBS Lett. 1997; 415: 249-252Crossref PubMed Scopus (49) Google Scholar). In a previous study, we reported a novel regulatory mechanism of skeletal muscle glycogen metabolism (26.Prats C. Cadefau J.A. Cussó R. Qvortrup K. Nielsen J.N. Wojtaszewki J.F. Wojtaszewki J.F. Hardie D.G. Stewart G. Hansen B.F. Ploug T. J. Biol. Chem. 2005; 280: 23165-23172Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). We showed that severe glycogen depletion induced by muscle contraction leads to rearrangement of cytoskeleton actin filaments to form dynamic intracellular compartments. Both GS and phosphorylase associate with such compartments to start glycogen re-synthesis. Furthermore, we showed that GS phosphorylated at site 1b (P-GS1b) was located at the cross-striations, the I-band of sarcomeres, whereas when phosphorylated at sites 2+2a (P-GS2+2a), GS formed distributed muscle In the present study we investigate the existence and relevance of such regulatory mechanism in human muscle healthy well subjects 1 of and of 2 of in the subjects and to The study was by the of and and to the The was on two On 1 a was as the glycogen and a glycogen depletion exercise a P. G. A. B. J. PubMed Scopus Google Scholar). The of a exercise by a of of at subjects to for 2 by at to the the was subjects 1 of at of an The to and liver glycogen subjects and to of a 5 and the On the subjects overnight muscle from vastus lateralis as previously J. J. Clin. Invest. PubMed Google Scholar). subjects an exhausting exercise which as a exercise at of and at After of exercise the was increased to to to and to At of the first exercise was and a muscle was that the on when a from the muscle was subjects but the the to a 1 of glucose of in of After of a muscle was from the subjects a of glucose of After of a muscle was the subjects to and to for of glucose on an and by a by a 2 After the the was and a was in whereas a was in for 5 and for or M. C. L. PubMed Scopus Google Scholar, T. B. H. E. J. Biol. PubMed Scopus Google Scholar). glycogen was as previously C. C. Cadefau J.A. J. M. R. Biochim. Biophys. Acta. PubMed Scopus Google Scholar). GS activity was in a Jr., J. Biol. Chem. Full Text PDF PubMed Google Scholar). GS activity was in the of or G6P and reported as or as and to K. D. C. Beck-Nielsen H. Wojtaszewski J.F. Diabetes. PubMed Scopus Google Scholar). used for was a rabbit K. D. C. Beck-Nielsen H. Wojtaszewski J.F. Diabetes. PubMed Scopus Google Scholar). of and different as (26.Prats C. Cadefau J.A. Cussó R. Qvortrup K. Nielsen J.N. Wojtaszewki J.F. Wojtaszewki J.F. Hardie D.G. Stewart G. Hansen B.F. Ploug T. J. Biol. Chem. 2005; 280: 23165-23172Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar, K. Staehr P. Hansen B.F. Green K.A. Hardie D.G. Richter E.A. Beck-Nielsen H. Wojtaszewski Diabetes. 2003; 52: PubMed Scopus Google Scholar). The of has previously been reported K. Staehr P. Hansen B.F. Green K.A. Hardie D.G. Richter E.A. Beck-Nielsen H. Wojtaszewski Diabetes. 2003; 52: PubMed Scopus Google Scholar). The was from was as previously (26.Prats C. Cadefau J.A. Cussó R. Qvortrup K. Nielsen J.N. Wojtaszewki J.F. Wojtaszewki J.F. Hardie D.G. Stewart G. Hansen B.F. Ploug T. J. Biol. Chem. 2005; 280: 23165-23172Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). GS was a rabbit or a rabbit a the last nine from human muscle from J. GS phosphorylated at sites 2+2a, or the as for with or in and in incubation muscle with the but the last was with for of the by or and in and with a a at that no was in the and a low of showed from the to the of muscle in the of and of or muscle and was as to After the muscle with for 1 3 5 with J. Biol. PubMed Google Scholar) and 3 5 with was and muscle with in for in a of to and in to with a and on with with and and with a at an of a 2 and with the The are as A of was used to of the between The between different of was by the healthy subjects a glycogen depletion exercise with the and muscle glycogen by an exercise and to a low carbohydrate subjects to an exhausting exercise with both a glucose at the of the exhausting exercise and a and exhausting exercise and and of and exhausting exercise, increased from to and to levels levels during exhausting from to during exhausting the of glucose glucose and levels increased to and At subjects to a glucose levels increased to increased to remaining both increased of muscle from both and exhausting exercise, the of the glucose and the of the from the muscle. the exhausting exercise, glycogen in muscle was in the with the and After exhausting exercise, glycogen levels and in and Glycogen in muscle was than in After of glycogen levels increased from to but a glycogen re-synthesis was first GS activity was in the of and the of fractional velocity and of form In GS was activated by exercise both as increased to and to exhausting exercise, muscle GS activity with with and in GS protein or phosphorylation at sites 1a and 2 at of the After exhausting exercise of GS phosphorylation at site 1b increased phosphorylation at site 3a the start of the exhausting exercise, GS phosphorylation at sites 2+2a, 3a, and was in the muscle with the in the exhausting exercise of muscle to increased GS phosphorylation at site 1b the glycogen level in muscle was than in in both exhausting exercise induced a increase in GS phosphorylation at site In exhausting exercise of muscle induced a in GS phosphorylation at GS site of the between GS activation and glycogen a between and glycogen and between and glycogen GS activity in the of G6P is considered the enzyme whereas in the of phosphorylated and of GS to In the of G6P of GS The results show that GS activity in the of or G6P was dependent on the phosphorylation at sites 2+2a, 3a, and Furthermore, of the between GS phosphorylation at the different sites and muscle glycogen a between glycogen levels and GS phosphorylation at sites 2+2a, 3a, and A glycogen phosphorylation at sites 2+2a and the and between the GS sites 2+2a, 3a, and used to investigate intracellular in human muscle. phosphorylated at sites 3a, GS was in several intracellular as the GS a and a of and associated with at the 4, In contrast, is at the muscle a of structures muscle 4, are in with previous study in rabbit skeletal the intracellular of and was (26.Prats C. Cadefau J.A. Cussó R. Qvortrup K. Nielsen J.N. Wojtaszewki J.F. Wojtaszewki J.F. Hardie D.G. Stewart G. Hansen B.F. Ploug T. J. Biol. Chem. 2005; 280: 23165-23172Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). muscle or The was with and for of and are shown in The results show that both of GS are associated with glycogen particles, the of which is 5, and with previously reported K. M. S. J. J. J. PubMed Scopus Google Scholar, Jr., J. Biol. Chem. Full Text PDF PubMed Google Scholar). The GS intracellular to binding of GS with two of glycogen on its phosphorylation is mainly associated with intramyofibrillar glycogen 5, A and is mainly associated with glycogen in the between the intermyofibrillar 5, that several in glycogen GS intracellular was immunocytochemistry in muscle In GS a of in the and the After exhausting exercise of GS from a to a more GS translocation to in response to exhausting exercise In muscle GS was both in the as spherical and the exhausting exercise in muscle from which been of glycogen the and been glucose GS was associated with spherical with a of After exhausting exercise glycogen re-synthesis was in the of exhausting exercise and the of a glucose GS was mainly associated with spherical at the of results show that GS intrinsic activity is strongly associated with glycogen levels the that GS is regulated by glycogen J. Biol. Chem. Full Text PDF PubMed Google Scholar, J.N. S. E. Ploug T. Richter E.A. J. 2001; PubMed Scopus Google Scholar). Furthermore, we show that GS activity in the of or G6P is strongly dependent on its phosphorylation at sites 2+2a, 3a, and and such phosphorylation are in strongly dependent on muscle glycogen 3, and in in which glycogen levels have been low overnight and are of the level in GS phosphorylation at sites 2+2a, 3a, and is than in is in with a previous study, which reported that GS phosphorylation at sites 3a and 2 is in muscle with low glycogen levels Nielsen J.N. B. P. S. Hardie D.G. Hansen B.F. Richter E.A. Wojtaszewski J.F. Diabetes. PubMed Scopus Google Scholar, J. J. PubMed Scopus Google Scholar). results that GS phosphorylation is strongly dependent on glycogen levels and that such regulation involves dephosphorylation of sites 2+2a, 3a, and The regulatory mechanism by which glycogen GS and intrinsic activity remains elusive. A study of glycogen on activity on its A. S. A. Hardie D.G. Full Text Full Text PDF PubMed Scopus Google Scholar), as a of the of glycogen on GS The phosphatase has been shown to an for muscle GS activation Y. K. C. K. M. DePaoli-Roach A.A. J. Biol. Chem. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar), but it is not preferentially on GS phosphorylation of GS at sites 2+2a is strongly with the enzyme activity in the of and of GS at sites have a of activity In contrast, when at sites GS is in the of and results that sites 3a and 3b are of GS intrinsic activity than sites 2+2a, GS at sites 2+2a but low Glycogen in exhausting exercise and in but GS phosphorylation at sites 2+2a and overnight low glycogen levels that the effect of on GS phosphorylation is not but it has been that of GS the enzyme J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar) by its susceptibility to GS phosphatases J.F. Y. Hardie D.G. Richter E.A. Diabetes. PubMed Scopus Google Scholar). that overnight low glycogen levels to of of GS phosphatases to sites 2+2a and is by a previous study J.F. Y. Hardie D.G. Richter E.A. Diabetes. PubMed Scopus Google Scholar), GS from muscle with glycogen levels was activated in the absence of the phosphatase that more than phosphorylation is GS activity in muscle with glycogen but not in muscle with low glycogen be that exhausting exercise-induced activation of GS such as AMPK, PKA, kinase or protein kinase GS the that with low glycogen levels a in GS phosphorylation at sites and 2+2a is Furthermore, overnight low glycogen a in GS phosphorylation at sites 2+2a was It has been shown that leads to GS dephosphorylation at sites 2+2a and that such response is in patients with T2DM and in with K. Staehr P. Hansen B.F. Green K.A. Hardie D.G. Richter E.A. Beck-Nielsen H. Wojtaszewski J.F. Diabetes. 2003; 52: 1393-1402Crossref PubMed Scopus (116) Google Scholar, D. K. Hansen B.F. Beck-Nielsen H. Wojtaszewski J.F. J. Clin. PubMed Scopus Google Scholar). that subjects the glycogen depletion on 1 and a low carbohydrate dephosphorylation of GS at sites 2+2a in muscle to during the glucose was for GS to start glycogen GS activated on 2 in muscle. It is well that GS phosphorylation sites mainly in the regulation of its intrinsic activity are sites 2+2a and (22.Skurat A.V. Wang Y. Roach P.J. J. Biol. Chem. 1994; 269: 25534-25542Abstract Full Text PDF PubMed Google Scholar) and that sites 4 and 5 are to phosphorylation of site 3 P.J. FASEB J. 1990; 4: 2961-2968Crossref PubMed Scopus (194) Google Scholar). The as to the of GS phosphorylation at sites 1a and of between phosphorylation sites and or was and a between and and levels It is to that and levels a is mainly to levels increased during exhausting exercise and back to levels whereas levels increased in response to the glucose and that is to phosphorylate GS site 1b P. FEBS Lett. 1997; PubMed Scopus Google Scholar), and that exhausting exercise an increase in site 1b phosphorylation was of glycogen levels which with the level of in results in response to exercise, epinephrine-induced activation leads to muscle GS site 1b Furthermore, we show that is mainly to the muscle cross-striations, to the I-band of the a of and previous in rabbit skeletal muscle (26.Prats C. Cadefau J.A. Cussó R. Qvortrup K. Nielsen J.N. Wojtaszewki J.F. Wojtaszewki J.F. Hardie D.G. Stewart G. Hansen B.F. Ploug T. J. Biol. Chem. 2005; 280: 23165-23172Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). the exact structures with which and of immunocytochemistry by was The results show that is mainly associated with glycogen located at the the intramyofibrillar glycogen muscle glycogen have been based on intracellular as or intermyofibrillar J. J. B. 1989; PubMed Scopus Google Scholar). we report a mechanism of GS regulation based on intracellular compartmentalization. In response to exhausting exercise, leads to muscle which phosphorylates GS at site 1b targeting the enzyme to the intramyofibrillar glycogen particles. Furthermore, that is mainly associated with intermyofibrillar glycogen which form of glycogen in between the intracellular by immunocytochemistry The GS phosphorylation at different sites increase its for a of glycogen In is the between glycogen located at the I-band of and glycogen between the in Jr., J. Biol. Chem. Full Text PDF PubMed Google Scholar) that different associated with glycogen and glycogen with PubMed Scopus Google Scholar) glycogen as with it is well that several associate to glycogen and that glycogen is not but on P.J. C. D. A. J. A.V. L. J. Clin. PubMed Scopus Google for of glycogen not to exercise leads to glycogen depletion of the subsarcolemmal and intermyofibrillar of glycogen M. J. B. 1982; PubMed Scopus Google Scholar), whereas glycogen associated with are more to muscle Jr., J. Biol. Chem. Full Text PDF PubMed Google Scholar, D.L. J. Res. PubMed Scopus Google Scholar, J.C. P. J. Biol. PubMed Scopus Google Scholar, J.C. P. J. Biol. PubMed Scopus (93) Google Scholar). glycogen metabolism is complexly regulated by intracellular and different glycogen are used for different results show that GS with different glycogen is phosphorylation-dependent and that exhausting exercise leads to an increase in which is associated with the intramyofibrillar glycogen 4, and that GS associated with the intramyofibrillar glycogen is by the that it is at such sites and is more a glycogen re-synthesis exhausting exercise in the intramyofibrillar particles, which are preferentially used during muscle contraction M. Petersen K.F. Shulman G.I. Recent Prog. Horm. Res. 2001; 56: 219-237Crossref PubMed Scopus (93) Google Scholar, 3.Shulman G.I. Rothman D.L. Jue T. Stein P. DeFronzo R.A. Shulman R.G. N. Engl. J. Med. 1990; 322: 223-228Crossref PubMed Scopus (1045) Google Scholar, P. Vaag A. Hother-Nielsen O. Beck-Nielsen H. Diabetologia. 1991; 34: 239-245Crossref PubMed Scopus (194) Google Scholar, 5.Højlund K. Staehr P. Hansen B.F. Green K.A. Hardie D.G. Richter E.A. Beck-Nielsen H. Wojtaszewski J.F. Diabetes. 2003; 52: 1393-1402Crossref PubMed Scopus (116) Google Scholar). studies are needed to understand the binding of the different phosphorylation of GS to glycogen and the on muscle glycogen in glycogen as a of the between GS and phosphorylase understand glycogen metabolism in response to exhausting exercise, phosphorylase activity and its intracellular be of muscle and show several associated with glycogen has been and still is an with to the of GS that be associated with of results show that more than GS be associated with glycogen 5, and In a previous study (26.Prats C. Cadefau J.A. Cussó R. Qvortrup K. Nielsen J.N. Wojtaszewki J.F. Wojtaszewki J.F. Hardie D.G. Stewart G. Hansen B.F. Ploug T. J. Biol. Chem. 2005; 280: 23165-23172Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar), of rabbit tibialis anterior we reported a novel in glycogen re-synthesis. After glycogen actin cytoskeleton rearrangement spherical structures at the of the to which glycogen at the start of glycogen re-synthesis. of the of the present study was to investigate the existence of such novel compartments in human skeletal muscle during results show the of such spherical structures in the and more and and in muscle results an in compartments are formed in response to to the needed and and increase the of the studies are to investigate and are structures are the that are structures We D. Hardie of of Diabetes and J. Guinovart of and of for to study, and and for