Examining Post-Transcriptional Regulation of Skeletal Muscle Satellite Cell Homeostasis, Activation and Fate Determination

Skeletal muscle is essential for respiration, mobility, reproduction and metabolism. Deficits in muscle function due to disease, injury or age reduce both quality of life and lifespan. Muscles are long-lived tissues that require maintenance to retain functional integrity throughout the life of an organism. Satellite cells are the adult stem cells responsible for muscle repair and maintenance. Upon myotrauma, satellite cells re-enter the cell cycle, proliferate, and terminally differentiate to repair the muscle. In uninjured tissue, satellite cells are quiescent and infrequently proceed through myogenesis for muscle maintenance. The molecular mechanisms that regulate satellite cell quiescence and activation are poorly defined. Additionally, no comprehensive studies have determined when satellite cells attain quiescence in the adult tissue or whether satellite cells in different muscle acquire quiescence at distinct times. Here, I investigate when satellite cells attain quiescence. Using Pax7ICreERT2; R26RtdTomato mice to fluorescently label satellite cells, I determine that hindlimb muscles establish quiescence by 12 weeks of age; whereas, extraocular muscles fail to establish quiescence by 27 weeks. Additionally, I find that satellite cell contribution to adult muscle is greater than was assumed based on the low number of cycling satellite cells in uninjured muscle. Furthermore, I show that satellite cells contribution varies by both age and muscle group. By comparing genome-wide expression profiles of quiescent and activating satellite cells, I identify RNA post-transcriptional regulation via RNA binding proteins as a regulatory mechanism of satellite cell activation. Specifically, I investigate the CELF family of RNA binding proteins. I find that CELF1/2/4 targets are enriched in genes downregulated during satellite cell activation. microRNAs that regulate satellite cell fate share targets with CELF1/2/4 during satellite cell activation. Additionally, CELF1/2/4 targets transcripts in key regulatory pathways of satellite cell activation, including p38 MAPK. I further examined gene expression profiles of aged satellite cells and identify elevated expression of genes downregulated during satellite cell activation, particularly RNA splicing and processing genes, that likely contribute to the impaired activation, self renewal and proliferation of aged satellite cells. Together, these data suggest the RNA post-transcriptional regulation of gene expression as a key mechanism mediating satellite cell activation