| The Notch pathway regulates cell fate decisions in numerous developmental processes. Recently, a role for Notch proteins has been implicated in various aspects of adult mammalian hematopoiesis. Less, however, is known about a possible role for Notch signaling during hematopoietic development in the embryo. In order to characterize the potential functions of Notch1 during developmental hematopoiesis, pharmacological and genetic approaches were used to manipulate the Notch1 signal pathway in primary cells in vitro and in vivo. First, pharmacological inhibition of Notch signaling with inhibitors of γ-secretase activity, required for intramembranous, proteolytic processing of all Notch receptors and thus for downstream signaling, was shown to repress early T lymphocyte development, consistent with previously reported loss of function experiments in Notch1, and also reduced CD8 T cell development. Second, using an embryonic stem (ES) cell model of hematopoietic differentiation in vitro, an expansion of the earliest hematopoietic compartment, primitive erythropoiesis, was demonstrated in cells deficient for Notch1 or by pharmacological inhibition of Notch signaling. This was complemented by an observed reduction in primitive erythroid progenitors in an ES line harboring a constitutively active form of Notch1 under the regulation of the endogenous Notch1 promoter. Interestingly, however, in mice deficient for Notch1 or in chimeric mice containing cells derived from Notch1-deficient ES cells, no evidence was found for the regulation of primitive erythropoiesis by Notch1. Combined with differences observed in expression patterns of Notch1 in vitro and in vivo, these results suggested distinct functions of Notch1 in the in vitro ES model of early hematopoiesis not observed in vivo. Finally, in contrast to primitive hematopoiesis, early definitive hematopoiesis was not altered in the absence of Notch1 in the ES differentiation model. This result was supported by analysis of early definitive progenitors in the yolk sac of Notch1-deficient embryos. Interestingly, however, a chimeric analysis of Notch1 in mice revealed an essential role for Notch1 in later definitive hematopoiesis in the embryo proper and yolk sac, potentially at the level of differentiation of an arterial endothelial precursor. Together, these results demonstrate novel and unique roles for Notch1 in developmental hematopoiesis in vitro and in vivo. |
