title: Muscle-specific loss of Bmal1 leads to disrupted tissue glucose metabolism and systemic glucose homeostasis.
abstract: Diabetes is the seventh leading cause of death in the USA, and disruption of circadian rhythms is gaining recognition as a contributing factor to disease prevalence. This disease is characterized by hyperglycemia and glucose intolerance and symptoms caused by failure to produce and/or respond to insulin. The skeletal muscle is a key insulin-sensitive metabolic tissue, taking up ~80 % of postprandial glucose. To address the role of the skeletal muscle molecular clock to insulin sensitivity and glucose tolerance, we generated an inducible skeletal muscle-specific Bmal1 (-/-) mouse (iMSBmal1 (-/-)).
  
  
  
  Progressive changes in body composition (decreases in percent fat) were seen in the iMSBmal1 (-/-) mice from 3 to 12 weeks post-treatment as well as glucose intolerance and non-fasting hyperglycemia. Ex vivo analysis of glucose uptake revealed that the extensor digitorum longus (EDL) muscles did not respond to either insulin or 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) stimulation. RT-PCR and Western blot analyses demonstrated a significant decrease in mRNA expression and protein content of the muscle glucose transporter (Glut4). We also found that both mRNA expression and activity of two key rate-limiting enzymes of glycolysis, hexokinase 2 (Hk2) and phosphofructokinase 1 (Pfk1), were significantly reduced in the iMSBmal1 (-/-) muscle. Lastly, results from metabolomics analyses provided evidence of decreased glycolytic flux and uncovered decreases in some tricarboxylic acid (TCA) intermediates with increases in amino acid levels in the iMSBmal1 (-/-) muscle. These findings suggest that the muscle is relying predominantly on fat as a fuel with increased protein breakdown to support the TCA cycle.
  
  
  
  These data support a fundamental role for Bmal1, the endogenous circadian clock, in glucose metabolism in the skeletal muscle. Our findings have implicated altered molecular clock dictating significant changes in altered substrate metabolism in the absence of feeding or activity changes. The changes in body composition in our model also highlight the important role that changes in skeletal muscle carbohydrate, and fat metabolism can play in systemic metabolism.
authors.affiliation: Center for Muscle Biology, University of Kentucky, Lexington, KY USA ; Department of Physiology, College of Medicine, University of Kentucky, Lexington, KY USA.
authors.forename: Brianna D
authors.initials: BD
authors.lastname: Harfmann
authors.affiliation: Center for Muscle Biology, University of Kentucky, Lexington, KY USA ; Department of Physiology, College of Medicine, University of Kentucky, Lexington, KY USA.
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Schroder
authors.affiliation: Department of Internal Medicine, University of Michigan, Ann Arbor, MI USA.
authors.forename: Maureen T
authors.initials: MT
authors.lastname: Kachman
authors.affiliation: Center for Muscle Biology, University of Kentucky, Lexington, KY USA ; Department of Physiology, College of Medicine, University of Kentucky, Lexington, KY USA ; Present address: Department of Physiology and Functional Genomics, Myology Institute, University of Florida, Gainesville, FL 32610-0274 USA.
authors.forename: Brian A
authors.initials: BA
authors.lastname: Hodge
authors.affiliation: Center for Muscle Biology, University of Kentucky, Lexington, KY USA ; Department of Physiology, College of Medicine, University of Kentucky, Lexington, KY USA ; Present address: Department of Physiology and Functional Genomics, Myology Institute, University of Florida, Gainesville, FL 32610-0274 USA.
authors.forename: Xiping
authors.initials: X
authors.lastname: Zhang
authors.affiliation: Center for Muscle Biology, University of Kentucky, Lexington, KY USA ; Department of Physiology, College of Medicine, University of Kentucky, Lexington, KY USA ; Present address: Department of Physiology and Functional Genomics, Myology Institute, University of Florida, Gainesville, FL 32610-0274 USA.
authors.forename: Karyn A
authors.initials: KA
authors.lastname: Esser
mesh_terms: D056930:ARNTL Transcription Factors / Q000502:physiology*; D000273:Adipose Tissue / Q000378:metabolism; D000620:Aminoimidazole Carboxamide / Q000008:administration & dosage / Q000031:analogs & derivatives; D000818:Animals; D001786:Blood Glucose / Q000032:analysis / Q000378:metabolism*; D001835:Body Weight; D002940:Circadian Rhythm; D005260:Female; D051275:Glucose Transporter Type 4 / Q000378:metabolism; D006593:Hexokinase / Q000378:metabolism; D006706:Homeostasis; D007004:Hypoglycemic Agents / Q000008:administration & dosage; D007328:Insulin / Q000008:administration & dosage / Q000097:blood / Q000378:metabolism*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009043:Motor Activity; D018482:Muscle, Skeletal / Q000378:metabolism*; D025203:Phosphofructokinase-1, Muscle Type / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D012265:Ribonucleotides / Q000008:administration & dosage
chemical_list: D056930:ARNTL Transcription Factors; C538961:Bmal1 protein, mouse; D001786:Blood Glucose; D051275:Glucose Transporter Type 4; D007004:Hypoglycemic Agents; D007328:Insulin; D012333:RNA, Messenger; D012265:Ribonucleotides; C495693:Slc2a4 protein, mouse; D000620:Aminoimidazole Carboxamide; D025203:Phosphofructokinase-1, Muscle Type; D006593:Hexokinase; C000598049:hexokinase 2, mouse; C031143:AICA ribonucleotide
keywords: Circadian rhythm; Glucose metabolism; Skeletal muscle