Figure 5Immunofluorescent analysis of TA muscle cross-sections from Myf6-KO and WT mice with PAX7 and KI67 antibodies at postnatal day 7. Scale bar=100μm, arrowheads highlight select MuSC B Quantification of the percentage of KI67+ over PAX7+ MuSCs between Myf6-KO and WT mice at postnatal day 7 (n=15 fields of view from n=3 animals) Two-tailed t-test, error bars = +/- SD. C Immunofluorescent analysis of TA muscle cross-sections from Myf6-KO and WT mice with PAX7 and KI67 antibodies at postnatal day 21. Scale bar=100μm, arrowheads highlight select MuSC D Quantification of the percentage of KI67+ over PAX7+ MuSCs between Myf6-KO (n=28 fields of view from n=3 animals) and WT mice (n=31 fields of view from n=3 animals) at postnatal day 21. Two-tailed t-test, error bars = +/- SD E Immunofluorescent analysis of TA muscle cross-sections from Myf6-KO and WT mice with PAX7 and KI67 antibodies at 3 months of age. Scale bar=100μm, arrowheads highlight select MuSC F Quantification of the percentage of KI67+ over PAX7+ MuSCs between Myf6-KO and WT mice at 3 months of age (n=30 fields of view from n=5 mice). Two-tailed t-test, error bars = +/- SD G Schematic drawing of the strategy for FACS isolation of Myf6-KO and WT satellite cells for RNA-Seq library preparation or for culture and differentiation. H (Left) Heat Map of up and down-regulated genes in the p38 MAPK pathway (WP400) in Myf6-KO and WT satellite cells. (Right) Gene set enrichment analysis of p38 MAPK pathway genes. In each plot, the blue curve represents the gene set enrichment score profile (Subramanian et al., 2005) across p38 MAPK pathway genes that are ranked by their differential expression. The grey curve represents the null expectation. P-values are based on permutation test (10,000 permutations). Immunofluorescent analysis of WT and Myf6-KO primary myotubes cultured in differentiation media for 7 days and stained with MF20 and DAPI. Scale bar=200μm J Quantification of the fusion index between Myf6-KO (n=54 fields of view from n=3 mice) and WT (n=81 fields of view from n=4 mice) 7DM myotubes. Fusion index is calculated as the percentage of myonuclei out of the total number of nuclei per field of view. Two-tailed t-test, error bars = +/- SD EDL myofibers isolated from WT and Myf6-KO mice fixed at time T=0 and stained with PAX7, MYOD and DAPI. Scale bar=30μm L Quantification of the percentage of MYOD+/PAX7+ satellite cells per fiber in Myf6-KO (n= 88 myofibers) vs WT (n= 86 myofibers) myofibers. Two-tailed t-test, error bars = +/- SD M Quantification of the average percentage of MYOD+/PAX7+ satellite cells per mouse in Myf6-KO and WT myofibers (n=3 mice). Two-tailed t-test, error bars = +/- SD Staining for phospho-p38 MAPK on EDL myofibers cultured for 48 hours from Myf6-KO and WT mice. Scale bar=30μm O Quantification of the percentage of phospho-p38+/PAX7+ satellite cells per fiber in Myf6-KO (n=56 myofibers) vs WT (n=67 myofibers) myofibers. Two-tailed t-test, error bars = +/- SD P Quantification of the average percentage of phospho-p38+/PAX7+ satellite cells per mouse in Myf6-KO vs WT myofibers (n=3 mice). Two-tailed t-test, error bars = +/- SD Western blot analysis of C2C12 myoblasts treated with EGF, LIF, or EGF+LIF while in differentiation media and stained with phosphor-p38, total p38, myogenin, MF20 and GAPDH. Cells were treated with EGF (50ng/ml), LIF (1000U/ml) or EGF+LIF after 48 hours in differentiation media, and every 6 hours following initial treatment, followed by cell lysis at 72 hours. R Phospho-p38 western blot signal intensity normalized to total p38 signal intensity in control, EGF-treated, LIF-treated and EGF+LIF-treated C2C12s. Signal intensity was measured by placing ROIs over western blot bands using LI-COR Image Studio. (n=3 technical replicates) Two-tailed t-test, error bars = +/- SD S Myogenin western blot signal intensity normalized to GAPDH signal intensity in control, EGF-treated, LIF-treated and EGF+LIF-treated C2C12s. Signal intensity was measured by placing ROIs over western blot bands using LI-COR Image Studio. (n=3 technical replicates) Two-tailed t-test, error bars = +/- SD T MF20 western blot signal intensity normalized to GAPDH signal intensity in control, EGF-treated, LIF-treated and EGF+LIF-treated C2C12s. Signal intensity was measured by placing ROIs over western blot bands using LI-COR Image Studio. (n=3 technical replicates) Two-tailed t-test, error bars = +/- SD EDL myofibers treated repeatedly with recombinant EGF protein (100ng/ml) at 0h, 24h, 32h, 40h, and 48h in culture, and fixed at 48 hours, compared to untreated controls. V Quantification of the number of PAX7+ cells per fiber in EGF-treated (n=19 fibers from n=1 mouse) and untreated control EDL myofibers (n=16 fibers from n=1 mouse) after 48 hours in culture. Two-tailed t-test, error bars = +/- SD