First, I would like to thank Dr. Timmons for initiating a discussion on the hypothesis that the MyomiR network has a role in regulating adult-stage skeletal muscle plasticity. Second, I would like to extend my sincere apology to Dr. Timmons for overlooking his recent studies on microRNAs as the data provided from these studies would have improved the overall balance of the review (1,2). Although Dr. Timmons makes a good argument against a role for the MyomiR network in human skeletal muscle plasticity, I think, as he suggested, the discrepancy in microRNA expression in response to exercise may be an issue of acute, short-term training versus more prolonged training. Moreover, a change in gene expression observed after a single bout of exercise may be absent following a prolonged training program. The study by Nielsen and colleagues (3) reported a significant increase in miR-1 and miR-133a expression 1 h after a bout of endurance exercise that was not observed following 12 wk of training (3). Given the complexity of skeletal muscle plasticity, there are certain to be other microRNAs, in addition to themyomiRs, that likely have an important role in muscle adaptation to exercise, as Dr. Timmon's own work clearly suggests. My hope is that discussions like this will help promote future research aimed at better defining the role of microRNAs in skeletal muscle plasticity.
John J. McCarthy, Ph.D.
Department of Physiology
Center for Muscle Biology
University of Kentucky
1. Davidsen PK, Gallagher IJ, Hartman JW, et al. High responders to resistance exercise training demonstrate differential regulation of skeletal muscle microRNA expression. J. Appl. Physiol.
2. Keller P, Vollaard NB, Gustafsson T, et al. A transcriptional map of the impact of endurance exercise training on skeletal muscle phenotype. J. Appl. Physiol.
©2011 The American College of Sports Medicine
3. Nielsen S, Scheele C, Yfanti C, et al. Muscle specific microRNAs are regulated by endurance exercise in human skeletal muscle. J. Physiol.