Contraction-regulated mTORC1 and protein synthesis: Influence of AMPK and glycogen

Abstract

Key points: AMP-activated protein kinase (AMPK)-dependent Raptor Ser792 phosphorylation does not influence mechanistic target of rapamycin complex 1 (mTORC1)-S6K1 activation by intense muscle contraction. α₂ -AMPK activity-deficient mice have lower contraction-stimulated protein synthesis. Increasing glycogen activates mTORC1-S6K1. Normalizing muscle glycogen content rescues reduced protein synthesis in AMPK-deficient mice.

Abstract: The mechansitic target of rapamycin complex 1 (mTORC1)-S6K1 signalling pathway regulates muscle growth-related protein synthesis and is antagonized by AMP-activated protein kinase (AMPK) in multiple cell types. Resistance exercise stimulates skeletal muscle mTORC1-S6K1 and AMPK signalling and post-contraction protein synthesis. Glycogen inhibits AMPK and has been proposed as a pro-anabolic stimulus. The present study aimed to investigate how muscle mTORC1-S6K1 signalling and protein synthesis respond to resistance exercise-mimicking contraction in the absence of AMPK and with glycogen manipulation. Resistance exercise-mimicking unilateral in situ contraction of musculus quadriceps femoris in anaesthetized wild-type and dominant negative α₂ AMPK kinase dead transgenic (KD-AMPK) mice, measuring muscle mTORC1 and AMPK signalling immediately (0 h) and 4 h post-contraction, and protein-synthesis at 4 h. Muscle glycogen manipulation by 5 day oral gavage of the glycogen phosphorylase inhibitor CP316819 and sucrose (80 g L^-1 ) in the drinking water prior to in situ contraction. The mTORC1-S6K1 and AMPK signalling axes were coactivated immediately post-contraction, despite potent AMPK-dependent Ser792 phosphorylation on the mTORC1 subunit raptor. KD-AMPK muscles displayed normal mTORC1-S6K1 activation at 0 h and 4 h post-exercise, although there was impaired contraction-stimulated protein synthesis 4 h post-contraction. Pharmacological/dietary elevation of muscle glycogen content augmented contraction-stimulated mTORC1-S6K1-S6 signalling and rescued the reduced protein synthesis-response in KD-AMPK to wild-type levels. mTORC-S6K1 signalling is not influenced by α₂ -AMPK during or after intense muscle contraction. Elevated glycogen augments mTORC1-S6K1 signalling. α₂ -AMPK-deficient KD-AMPK mice display impaired contraction-induced muscle protein synthesis, which can be rescued by normalizing muscle glycogen content.

Keywords: AMPK; glycogen; mTORC1; protein synthesis; resistance exercise; skeletal muscle.