title: Molecular mechanism by which AMP-activated protein kinase activation promotes glycogen accumulation in muscle.
abstract: OBJECTIVE
  During energy stress, AMP-activated protein kinase (AMPK) promotes glucose transport and glycolysis for ATP production, while it is thought to inhibit anabolic glycogen synthesis by suppressing the activity of glycogen synthase (GS) to maintain the energy balance in muscle. Paradoxically, chronic activation of AMPK causes an increase in glycogen accumulation in skeletal and cardiac muscles, which in some cases is associated with cardiac dysfunction. The aim of this study was to elucidate the molecular mechanism by which AMPK activation promotes muscle glycogen accumulation.
  
  
  METHODS
  We recently generated knock-in mice in which wild-type muscle GS was replaced by a mutant (Arg582Ala) that could not be activated by glucose-6-phosphate (G6P), but possessed full catalytic activity and could still be activated normally by dephosphorylation. Muscles from GS knock-in or transgenic mice overexpressing a kinase dead (KD) AMPK were incubated with glucose tracers and the AMPK-activating compound 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) ex vivo. GS activity and glucose uptake and utilization (glycolysis and glycogen synthesis) were assessed.
  
  
  RESULTS
  Even though AICAR caused a modest inactivation of GS, it stimulated muscle glycogen synthesis that was accompanied by increases in glucose transport and intracellular [G6P]. These effects of AICAR required the catalytic activity of AMPK. Strikingly, AICAR-induced glycogen synthesis was completely abolished in G6P-insensitive GS knock-in mice, although AICAR-stimulated AMPK activation, glucose transport, and total glucose utilization were normal.
  
  
  CONCLUSIONS
  We provide genetic evidence that AMPK activation promotes muscle glycogen accumulation by allosteric activation of GS through an increase in glucose uptake and subsequent rise in cellular [G6P].
authors.affiliation: MRC Protein Phosphorylation Unit, College of Life Sciences, University of Dundee, Dundee, UK. r.x.hunter@dundee.ac.uk
authors.forename: Roger W
authors.initials: RW
authors.lastname: Hunter
authors.forename: Jonas T
authors.initials: JT
authors.lastname: Treebak
authors.forename: Jørgen F P
authors.initials: JF
authors.lastname: Wojtaszewski
authors.forename: Kei
authors.initials: K
authors.lastname: Sakamoto
mesh_terms: D055372:AMP-Activated Protein Kinases / Q000235:genetics / Q000378:metabolism*; D000620:Aminoimidazole Carboxamide / Q000031:analogs & derivatives / Q000494:pharmacology; D000704:Analysis of Variance; D000818:Animals; D015153:Blotting, Western; D006003:Glycogen / Q000378:metabolism*; D006006:Glycogen Synthase / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects / Q000502:physiology; D012265:Ribonucleotides / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology
chemical_list: D012265:Ribonucleotides; D000620:Aminoimidazole Carboxamide; D006003:Glycogen; D006006:Glycogen Synthase; D019869:Phosphatidylinositol 3-Kinases; D055372:AMP-Activated Protein Kinases; C031143:AICA ribonucleotide