Reviewer #2 (Public Review):
The work by Eliazer et al investigates the role of Dll4 spatial heterogeneity on myofibers in maintaining MuSC diversity. The authors show on isolated myofibers that individual MuSC exhibit different intensities, by immunofluorescence analysis, of Pax7 and Ddx6, expressed in quiescent MuSC, and that there is a positive correlation between the intensities of the two quiescence markers. They further isolated MuSC high, medium and low Pax7 from the Pax7-nGFP transgenic mouse and validated in vitro that that the Pax7 high are slower in entering the cell cycle and expressing myogenin. To understand whether diversity of factor on myofibers could regulate this spatial diversity, the authors focused on Notch signaling. By comparing by microarray data Notch ligands during postnatal muscle growth, they show that Dll4 showed the most enrichment as cells transition to quiescence. By immunofluorescence on isolated myofibers, the authors show heterogeneity of Dll4 localization across the myofiber, with enriched clusters around MuSC. The authors monitored along individual myofibers the distribution of Dll4 and found no correlation with the distance from the NMJ. Upon myofiber specific deletion of Dll4, the authors show that MuSC exhibit downregulation of Pax7 and Ddx6, as well as a reduced number of MuSC in tissues and increased expression of MyoD and myogenin. Upon injury, mice in which Dll4 was deleted in myofibers exhibited reduced myofiber cross-sectional area, indicating a defect in the repair process. By using mice in which Mib1, an activator of Notch signaling, is deleted in myofibers, the authors show reduced Dll4 intensity and reduced diversity of Pax7 expression in MuSC as well as impaired regeneration. Understanding how the microenvironment regulate MuSC diversity is relevant to dissect their heterogeneity. The findings are interesting and novel and the manuscript is well written. However, while the authors report diversity of Dll4 and Mib1 in myofibers, the approach of genetic deletion complete ablates gene expression, and it does not necessarily modulate spatial distribution. Thus, additional experiments are required in order to fully support the authors' interpretation.