Dll4 and Jag1a signalling act sequentially and cooperatively to drive hematopoietic stem cell fate specification

Hematopoietic stem and progenitor cells (HSPCs) arise from a specialized subset of arterial endothelial cells, the hemogenic endothelium (HE), during embryonic development and sustain blood production throughout life. Notch signalling is a key regulator of this process: its ligand Jag1 promotes HSPC formation, whereas Dll4 promotes arterial identity. However, how the activities of these ligands are temporally coordinated during HSPC emergence remains unresolved. Here we demonstrate that Dll4 is required prior to circulation onset, acting by dampening MAPK signalling to drive the transition from pre-HE to HE fate and enabling HE differentiation towards HSPCs. Subsequently, after circulation starts, Jag1a acts to maintain gene expression in HE and support transition to HSPC fate. Jag1a activity depends on blood flow-induced shear stress and rescues HSPC loss caused by impaired flow. Thus, rather than playing opposing roles, Dll4 and Jag1a act sequentially and coordinately to drive the endothelial-to-hematopoietic transition and promote HSPC emergence.