Contraction and AICAR Stimulate IL-6 Vesicle Depletion from Skeletal Muscle Fibers in vivo

  1. Laurie J. Goodyear1
  1. 1Research Division, Joslin Diabetes Center and Harvard Medical School, Boston, MA 02215, USA.
  2. 2Department of Health Sciences, Gettysburg College, Gettysburg, PA 17325, USA.
  3. 3Institute of Sports Medicine, Dept. of Orthopedic Surgery M, Bispebjerg Hospital and Center for Healthy Aging, Faculty of Health Sciences, University of Copenhagen, Denmark.
  4. 4Department of Rheumatology and Institute of Inflammation Research, Rigshospitalet, Copenhagen University Hospital, Denmark.
  5. 5Department of The Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark.
  1. Corresponding Author: Laurie J. Goodyear, laurie.goodyear{at}joslin.harvard.edu
  1. * These authors contributed equally.

Abstract

Recent studies suggest that Interleukin 6 (IL-6) is released from contracting skeletal muscles; however the cellular origin, the secretion kinetics, and signaling mechanisms regulating IL-6 secretion are unknown. To address these questions, we developed imaging methodology to study IL-6 in fixed mouse muscle fibers and in live animals in vivo. Using confocal imaging to visualize endogenous IL-6 protein in fixed muscle fibers, we found IL-6 in small vesicle structures distributed throughout the fibers under basal (resting) conditions. To determine the kinetics of IL-6 secretion, intact quadriceps muscles were transfected with EGFP-tagged IL-6 and five days later anesthetized mice were imaged before and after muscle contractions in situ. Contractions decreased IL-6-EGFP containing vesicles and protein by 62% (p<0.05), occurring rapidly and progressively over 25 minutes of contraction. However, contraction-mediated IL-6-EGFP reduction was normal in muscle-specific AMPKα2-inactive transgenic mice. In contrast, the AMPK activator AICAR decreased IL-6-EGFP vesicles, an effect that was inhibited in the transgenic mice. In conclusion, resting skeletal muscles contain IL-6-positive vesicles which are expressed throughout myofibers. Contractions stimulate the rapid reduction of IL-6 in myofibers, occurring through an AMPKα2-independent mechanism. This novel imaging methodology clearly establishes IL-6 as a contraction-stimulated myokine, and can be used to characterize the secretion kinetics of other putative myokines.

  • Received September 12, 2012.
  • Accepted June 8, 2013.

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