Activated Notch1 Target Genes during Embryonic Cell Differentiation Depend on the Cellular Context and Include Lineage Determinants and Inhibitors

Background: Notch receptor signaling controls developmental cell fates in a cell-context dependent manner. Although Notch signaling directly regulates transcription via the RBP-J/CSL DNA binding protein, little is known about the target genes that are directly activated by Notch in the respective tissues. Methodology/Principal Findings: To analyze how Notch signaling mediates its context dependent function(s), we utilized a Tamoxifen-inducible system to activate Notch1 in murine embryonic stem cells at different stages of mesodermal differentiation and performed global transcriptional analyses. We find that the majority of genes regulated by Notch1 are unique for the cell type and vary widely dependent on other signals. We further show that Notch1 signaling regulates expression of genes playing key roles in cell differentiation, cell cycle control and apoptosis in a context dependent manner. In addition to the known Notch1 targets of the Hes and Hey families of transcriptional repressors, Notch1 activates the expression of regulatory transcription factors such as Sox9, Pax6, Runx1, Myf5 and Id proteins that are critically involved in lineage decisions in the absence of protein synthesis. Conclusion/Significance: We suggest that Notch signaling determines lineage decisions and expansion of stem cells by directly activating both key lineage specific transcription factors and their repressors (Id and Hes/Hey proteins) and propose a model by which Notch signaling regulates cell fate commitment and self renewal in dependence of the intrinsic and extrinsic cellular context.