In normal hematopoiesis, proliferation is tightly linked to differentiation in ways that involve cell-cell interaction with stromal elements in the bone marrow stem cell niche. Numerous in vitro and in vivo studies strongly support a role for Notch signaling in the regulation of stem cell renewal and hematopoiesis. Not surprisingly, mutations in the Notch gene have been linked to a number of types of malignancies. To better define the function of Notch in both normal and neoplastic hematopoiesis, a tetracycline-inducible system regulating expression of a ligand-independent, constitutively active form of Notch1 was introduced into murine E14Tg2a embryonic stem cells. During coculture, OP9 stromal cells induce the embryonic stem cells to differentiate first to hemangioblasts and subsequently to hematopoietic stem cells. Our studies indicate that activation of Notch signaling in flk+ hemangioblasts dramatically reduces their survival and proliferative capacity and lowers the levels of hematopoietic stem cell markers CD34 and c-Kit and the myeloid marker CD11b. Global gene expression profiling of day 8 hematopoietic progenitors in the absence and presence of activated Notch yield candidate genes required for normal hematopoietic differentiation, as well as putative downstream targets of oncogenic forms of Notch including the noncanonical Wnts Wnt4 and 5A.
Disclosure of potential conflicts of interest is found at the end of this article.