Wnt signaling in hematopoiesis: crucial factors for self-renewal, proliferation, and cell fate decisions.

A large number of studies from many different laboratories have implicated the Wnt signaling pathway in regulation of hematopoiesis. However, different inducible gain- and loss-of-function approaches yielded controversial and some times contradictory results. In this prospect we will review the current ideas on Wnt signaling in hematopoiesis and early lymphopoiesis. Reviewing this large body of knowledge let us to hypothesize that different levels of activation of the pathway, dosages of Wnt signaling required and the interference by other signals in the context of Wnt activation collectively explain these controversies. Besides differences in dosage, differences in biological function of Wnt proteins in various blood cell types also is a major factor to take into account. Our own work has shown that while in the thymus Wnt signaling provides cytokine-like, proliferative stimuli to developing thymocytes, canonical Wnt signaling in HSC regulates cell fate decisions, in particular self-renewal versus differentiation. Copyright 2010 Wiley-Liss, Inc.