Figure 3A Schematic drawing of the Myf6 locus showing insertion of Cre-ERT2 sequence rendering the Myf6 locus as a null allele (Southard et al., 2014). B Picture of 8 week-old Myf6-KO and WT mice. C Relative expression of Myf6 transcript in the hindlimb skeletal muscles of Myf6-KO and WT mice measured by Quantitative Real-Time PCR (RT-qPCR) (n=4 mice). Boxes represent 1st and 3rd quartile, central line represents the median, whiskers represent the range, two-tailed t-test. D Color map of genes that are up/down regulated in the whole muscle transcriptome (RNA-Seq) of injured and uninjured hindlimb muscles from Myf6-KO and WT counterparts. Differential expression is Log2 transformed and truncated at +/-3. Color scale goes from blue (-3=lower than mean), to yellow (+3=higher than mean). E Heat map showing the distribution of Myf6 peaks within 100kb of the TSS of the myokine genes ranging from zero (black) to six ChIP-Seq peaks (red) occupancy. F Expression of EGF protein in the blood serum from Myf6-KO and heterozygous counterparts as measured by Enzyme-Linked Immunosorbent Assay (ELISA). (n=5 heterozygous biological replicates, n=3 Myf6-KO biological replicates), two-tailed t-test, error bars = +/- SD G Expression of VEGFA protein in the blood serum from Myf6-KO and heterozygous counterparts as measured by Enzyme-Linked Immunosorbent Assay (ELISA). (n=5 heterozygous biological replicates, n=3 Myf6-KO biological replicates), two-tailed t-test, error bars = +/- SD. H Expression of VEGFA protein in the secretome of Myf6-KO and WT myotubes as measured by enzyme-linked immunosorbent assay (ELISA). Primary myotubes were derived from MuSCs isolated by Fluorescence Activated Cell Sorting (FACS, see the Extended Material and Methods) from Myf6-KO and WT mice. Myotubes were obtained by feeding >90% confluent primary myoblasts with differentiation media (DMEM supplemented with 5% horse serum) for seven days. Media from the last 48 hours of differentiation (days 5 to 7) was collected for ELISA secretome analysis. Protein readouts were normalized to the total number of nuclei in each plate. (n=4 WT biological replicates, n=3 Myf6-KO biological replicates), two-tailed t-test, error bars = +/- SD I Expression of EGF protein in the secretome of Myf6-KO and WT myotubes as measured by enzyme-linked immunosorbent assay (ELISA). Primary myotubes were derived from MuSCs isolated by Fluorescence Activated Cell Sorting (FACS, see the Extended Material and Methods) from Myf6-KO and WT mice. Myotubes were obtained by feeding >90% confluent primary myoblasts with differentiation media (DMEM supplemented with 5% horse serum) for seven days. Media from the last 48 hours of differentiation (days 5 to 7) was collected for ELISA secretome analysis. Protein readouts were normalized to the total number of nuclei in each plate. (n=4 WT biological replicates, n=3 Myf6-KO biological replicates), two-tailed t-test, error bars = +/- SD J Expression of EGF protein in the secretome of Myf6-HF expressing myotubes derived from Myf6-KO myoblasts (rescue) compared to Myf6-KO myotubes as measured by enzyme-linked immunosorbent assay (ELISA). Primary myotubes were derived from MuSCs isolated by Fluorescence Activated Cell Sorting (FACS, see the Extended Material and Methods) from Myf6-KO mice. Myotubes were obtained by feeding >90% confluent primary myoblasts with differentiation media (DMEM supplemented with 5% horse serum) for five days. Media from the last 48 hours of differentiation (days 3 to 5) was collected for ELISA secretome analysis. Protein readouts were normalized to the total number of nuclei in each plate. (n=3 Myf6-KO technical replicates, n=4 rescue technical replicates), two-tailed t-test, error bars = +/- SD. K RT-qPCR of Myf6 expression in Myf6-KO (n=2 technical replicates), WT (n=3 biological replicates) and Myf6-HF (rescue) myotubes (n=3 technical replicates) (5DM). "Rescue" myotubes were generated from Myf6-KO myoblasts infected with a retrovirus containing Myf6-HF. two-tailed t-test, error bars = +/- SD L-M RT-qPCR of EGF (L) and VEGFA (M) expression in Myf6-KO (n=5) and rescue (n=3-4) primary myotubes (5DM). two-tailed t-test, error bars = +/- SD Immunofluorescence staining of WT (N) and Myf6-KO EDL (O) myofibers stained with PAX7, EGFR, and phospho-EGFR antibodies. Live fibers were maintained in culture media for 44 hours, followed by 4 hours of serum starving in non-supplemented DMEM. For treated samples, 40ng/ml of recombinant EGF was added to the non-supplemented DMEM for 10 minutes before fixation. Activation status of EGFR was assessed using antibodies against phospho-EGFR (pEGFR-Y1068). Scale bar=30μm P Quantification of the percentage of all pEGFR+/PAX7+ cells (defined as PAX7+ cells showing low or high pEGFR signal) on WT (n=24 fibers from n=3 mice) , Myf6-KO (n=22 fibers from n=3 mice) and Myf6-KO EDL myofibers treated with EGF (n=14 fibers from n=3 mice). two-tailed t-test, error bars = +/- SD Q Quantification of the percentage of EGFRhigh/PAX7+ cells (defined as PAX7+ cells showing high pEGFR signal) on WT (n=37 fibers from n=3 mice), Myf6-KO (n=34 fibers from n=3 mice) and Myf6-KO EDL myofibers treated with EGF (n=11 fibers from n=3 mice). two-tailed t-test, error bars = +/- SD