Year 5

The goal of this grant was to define mechanisms that govern blood stem cell specification and self-renewal. We have completed the studies on hematopoietic fate specification by defining how Scl/tal1 establishes hemogenic endothelium. We documented that, in addition to Scl’s critical function in activating blood cell regulators, Scl also has to repress heart factors to prevent the misspecification of blood precursors to heart muscle. We documented that Scl controls blood and heart regulators through enhancers that have been primed for activation prior to Scl action (Aim 1). We identified a new surface marker that is expressed in hemogenic endothelium and blood forming cells in the yolk sac (Lyve1), which provides new tools to investigate the origin of blood stem and progenitor cells during development (Aim 2). We identified GPI-80 as a novel marker for transplantable blood stem cells during human fetal development (Aim 2, 3). Taking advantage of this new marker for blood stem cells, we narrowed down the critical defects in the dysfunctional blood precursors that are generated from human ES cells, or expanded in culture from fetal liver blood stem cells (Aim 3). We showed that the inability to induce HOXA cluster genes and other novel blood stem cell regulators that cannot be sustained in culture hinder the generation of blood stem cells from pluripotent cells, and further validated these novel regulators using lentiviral knockdown and overexpression. These findings will now be used to develop novel strategies to generate blood stem cells in culture.