Delta/Notch signaling level is decoded through transcriptional burst duration during muscle progenitor specification

Abstract

The ability of progenitor cells to interpret different levels of signaling pathway activity is fundamental to the generation of cellular diversity during development. Muscle progenitors, for example, are exposed to different levels of signals from their microenvironment. However, how they decode these quantitative differences to regulate their cellular behaviors during myogenesis remains poorly understood. Here, we addressed this question during the specification of two populations of Drosophila adult muscle progenitors (AMPs). We found that, despite being spatially adjacent, indirect flight muscle (IFM) and direct flight muscle (DFM) progenitors receive distinct levels of Notch signaling. We further show that the spatial distribution of epithelial Delta contributes to differential Notch activation. Higher Delta levels adjacent to IFM progenitors promote stronger Notch signaling. To investigate how these quantitative differences are translated into transcriptional responses, we developed a Notch-responsive MS2 reporter to monitor transcription in living AMPs. Our measurements reveal that Notch-dependent transcription occurs in stochastic bursts, with IFM progenitors displaying longer but less frequent bursts than DFM progenitors. Strikingly, equalizing epithelial Delta levels abolished differences between the two populations in both Notch-dependent transcriptional activity and the protein levels of Notch target genes. Together, our findings suggest that quantitative differences in signaling may be translated into distinct transcriptional outputs through modulation of transcriptional bursting. This may represent a mechanism by which a single signaling pathway can be deployed to regulate diverse cellular responses during development.