| The evolutionarily conserved Notch signaling pathway is a major contributor in the development of a variety of tissues. Moreover, research over the last decade has revealed a critical role of Notch signaling in vessel morphogenesis. Studies performed in zebrafish initially demonstrated that Notch signaling directs arterial specification. Subsequent work in mice, notably from gain and loss-of-function studies, confirmed that Notch activity is required for the expression of arterial-specific genes in the endothelium and smooth muscle. Nonetheless, much remains to be known to mechanistically determine the specific molecular events that underlie those effects.; Given that Notch had a well-known role in cell fate specification, we were intrigued by the possibility that it could mediate a similar function in vascular precursors. We explored the hypothesis that Notch1 signaling participates in this fate specification using embryonic stem cell-derived hemangioblasts able to give rise to both endothelial and smooth muscle cells. Our preliminary studies suggest that in hemangioblasts, Notch activity favors the differentiation of alpha smooth muscle positive cells at the expense of endothelial cells.; To investigate the contribution of Notch1 signaling in the endothelium, we engineered an endothelial-cell specific deletion of Notch1 in mice utilizing Cre-loxP technology. These mutants are embryonic lethal by E11.5, a day later than the global Notch1-knockout, and have multiple vascular defects. Lack of Notch1 in the endothelial compartment showed stenotic and irregular aortae, lack of remodeling and lumen reduction in yolk sac, and aortic arch vessels. In addition, mutants displayed increased vascular sprouting throughout the embryo. Although the contribution of Notch to vessel sprouting was previously suspected based on in vitro studies, the findings were controversial and not definitive. Our results unequivocally demonstrate that Notch is required to suppress the tip-cell phenotype during vessel sprouting.; The early lethality of Notch receptor and ligand deficient mice prevented an in vivo analysis of Notch signaling during later stages of embryogenesis. Using a tamoxifen-inducible VE-cadherin transgenic mouse developed by the laboratory, we evaluated the role of Notch1 during later embryonic stages. When the Notch1 deletion was induced after E10.5, multiple hemorrhages were detected in the head and neck areas of E14.5 embryos. Analyses of histological sections of these embryos revealed ruptured and occluded vessels. Similar results were found when mutants were evaluated during later stages of development. |
