Restraint of TGFβ family signaling by SMAD7 is necessary for hematopoietic stem cell maturation in the embryo.

Hematopoietic stem cells (HSCs), defined as cells that can engraft an adult when transplanted, mature from precursors (pre-HSCs) that differentiate from hemogenic endothelial cells (HECs) in the embryo. Many signaling pathways required to generate the first hematopoietic stem and progenitor cells in the embryo are well-characterized, but how HSCs mature from pre-HSCs is poorly understood. Here we show that "mothers against decapentaplegic homolog 7" (SMAD7), a negative regulator of transforming growth factor beta (TGFβ) and bone morphogenetic protein (BMP) signaling, is required for pre-HSC to HSC maturation. Deletion of Smad7 in endothelial cells allows the formation of pre-HSCs from HECs but impairs their maturation into HSCs. The data indicate that although TGFβ and BMP signaling are required for the generation of HECs and for HECs to undergo an endothelial-to-hematopoietic transition to generate pre-HSCs, one or both pathways must be subsequently down-regulated for effective pre-HSC to HSC maturation.