Dll1 is a ligand that mediates Notch signals and is provided by sender cells, such as muscle stem cells. The experiments performed in this study revealed an important role of Dll1 expression dynamics in muscle development and regeneration. Dll1 protein oscillates and its oscillations are important for maintaining the balance between self-renewal and differentiation of muscle stem and progenitor cells. The function of Dll1 oscillation cannot be replaced by sustained Dll1 expression.
I found that Dll1 was expressed in activated and differentiating muscle stem cells, but that Dll1 is not present in quiescent stem cells or myofibers. To analyze the function of Dll1, I introduced a conditional Dll1 mutation with tamoxifen dependent Pax7creERT2 line. When Dll1 was ablated in adult muscle stem cells, changes in differentiation were only observed in regenerating muscle, i.e. when muscle stem cells were activated and contacted each other.
To observe Dll1 expression dynamics, I used cultures of Dll1-luc muscle progenitor and stem cells. This demonstrated that the expression of Dll1 protein oscillated. Previous data that had shown that also Hes1 and MyoD expression oscillate in activated muscle stem cells. Therefore I investigated whether Hes1 and MyoD regulated Dll1 and Dll1 oscillations. I demonstrated that Hes1 suppressed Dll1 transcription and served as an oscillatory pacemaker for Dll1. In the absence of Hes1, Dll1 no longer oscillated. In contrast, MyoD induced Dll1 transcription, but did not affect Dll1 oscillation.
Finally, I investigated the role of Dll1 expression dynamics. A mutation named Dll1-type2 increased the transcription time because it lengthened the Dll1 gene, but didn’t affect the overall Dll1 mRNA levels. Dll1 oscillations were abolished when Dll1-type2 muscle stem cells contacted each other, and instead Dll1 was expressed in a sustained manner. This resulted in premature differentiation of muscle progenitor and stem cells. Thus, oscillations of Dll1 are important for a correct balance between self-renewal and differentiation of muscle stem cells.