Skeletal muscle wasting with disuse atrophy is multi-dimensional: the response and interaction of myonuclei, satellite cells and signaling pathways
- 1Health and Exercise Science Research Group, School of Sport, University of Stirling, Stirling, UK
- 2Muscle Research Group, Department of Physiological Sciences, Stellenbosch University, Stellenbosch, South Africa
Maintenance of skeletal muscle is essential for health and survival. There are marked losses of skeletal muscle mass as well as strength and physiological function under conditions of low mechanical load, such as space flight, as well as ground based models such as bed rest, immobilization, disuse, and various animal models. Disuse atrophy is caused by mechanical unloading of muscle and this leads to reduced muscle mass without fiber attrition. Skeletal muscle stem cells (satellite cells) and myonuclei are integrally involved in skeletal muscle responses to environmental changes that induce atrophy. Myonuclear domain size is influenced differently in fast and slow twitch muscle, but also by different models of muscle wasting, a factor that is not yet understood. Although the myonuclear domain is 3-dimensional this is rarely considered. Apoptosis as a mechanism for myonuclear loss with atrophy is controversial, whereas cell death of satellite cells has not been considered. Molecular signals such as myostatin/SMAD pathway, MAFbx, and MuRF1 E3 ligases of the ubiquitin proteasome pathway and IGF1-AKT-mTOR pathway are 3 distinctly different contributors to skeletal muscle protein adaptation to disuse. Molecular signaling pathways activated in muscle fibers by disuse are rarely considered within satellite cells themselves despite similar exposure to unloading or low mechanical load. These molecular pathways interact with each other during atrophy and also when various interventions are applied that could alleviate atrophy. Re-applying mechanical load is an obvious method to restore muscle mass, however how nutrient supplementation (e.g., amino acids) may further enhance recovery (or reduce atrophy despite unloading or ageing) is currently of great interest. Satellite cells are particularly responsive to myostatin and to growth factors. Recently, the hibernating squirrel has been identified as an innovative model to study resistance to atrophy.
Keywords: skeletal muscle atrophy, muscle cell signaling, myostatin, MuRF1, MAFbx, IGF1-AKT-mTOR, unloading, resistance exercise
Citation: Brooks NE and Myburgh KH (2014) Skeletal muscle wasting with disuse atrophy is multi-dimensional: the response and interaction of myonuclei, satellite cells and signaling pathways. Front. Physiol. 5:99. doi: 10.3389/fphys.2014.00099
Received: 19 November 2013; Accepted: 27 February 2014;
Published online: 17 March 2014.
Edited by:Lucas Guimarães-Ferreira, Federal University of Espirito Santo, Brazil
Reviewed by:John Joseph McCarthy, University of Kentucky, USA
Lex Verdijk, Maastricht University, Netherlands
Copyright © 2014 Brooks and Myburgh. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Naomi E. Brooks, Health and Exercise Science Research Group, School of Sport, University of Stirling, Stirling FK9 4LA, UK e-mail: email@example.com