Angiopoietin-like proteins stimulate HSPC development through interaction with notch receptor signaling.

Angiopoietin-like proteins (angptis) are capable of ex vivo expansion of mouse and human hematopoietic stem and progenitor cells (HSPCs). Despite this intriguing ability, their mechanism is unknown. Here, we show that angpti2 overexpression is sufficient to expand definitive HSPCs in zebrafish embryos. Angpti1/2 are required for definitive hematopoiesis and vascular specification of the hemogenic endothelium. The loss-of-function phenotype is reminiscent of the notch mutant mindbomb (mib) and a strong genetic interaction occurs between angptis and notch. Overexpressing angpti2 rescues mib while overexpressing notch rescues angpti1/2 morphants. Gene expression studies in AngptI2-stimulated CD34(+) cells showed a strong Myc activation signature and myc overexpression in angpti1/2 morphants or mib restored HSPCs formation. AngptI2 can increase Notch activation in cultured cells and Angptl receptor interacted with Notch to regulate Notch cleavage. Together our data provide insight to the angpti-mediated notch activation through receptor interaction and subsequent activation of myc targets.