Regulation of hematopoiesis by Smad1

Abstract

The hematopoietic system develops in two distinct yet overlapping phases termed primitive (or embryonic) and definitive (or adult) hematopoiesis, which collectively supply the embryo and later the adult with a replenishing supply of mature blood cells. In contrast to our understanding of the definitive blood program, much less is known about the regulation of primitive hematopoiesis. An area of active research in this regard has been the interplay between primitive hematopoietic development and bone morphogenetic protein (BMP) signaling. BMPs are members of the transforming growth factor-beta super family of secreted cytokines that exert a wide range of biological activities during development, and are essential for normal hematopoiesis. However, the progenitor population that responds to active BMP signals is undefined and the relative role of downstream mediators, including Smad1 and Smad5, is unclear. This work focuses on examining the specificity of Smad signaling on early hematopoietic development. We sought to characterize the ability of Smad1 and Smad5 to affect primitive hematopoiesis, including hemangioblast formation. The hemangioblast represents the first committed hematopoietic progenitor specified from mesoderm. In order to assess the role of BMP-Smad signaling on hemangioblast development, we generated inducible ES cells for Smad1 expression and examined their hematopoietic potential using the embryonic stem (ES) cell/embryoid body (EB) model. Based largely on gene ablation studies and analysis of Smad1 expression during hematopoietic development, we hypothesized that Smad1 is important for the specification and development of the hemangioblast. We show that enforced expression of Smad1 during the "hemangioblast window" causes a significant increase in the number of primitive erythrocytes, macrophages, and megakaryocytes. This enhancement in primitive hematopoietic development results from Smad1 directly promoting hemangioblast specification. We also show Smad1-dependent increases in definitive macrophage, megakaryocyte, and mixed colony formation with forced Smad1 expression at later stages. Collectively, our findings suggest that Smad1 positively regulates both primitive and definitive hematopoietic development, probably through direct regulation of hemangioblast activity. This work has further refined our understanding of the role of BMP-Smad signaling at specific stages during specification of the hematopoietic system.