It has long been noted that the initiation of definitive hematopoiesis occurs just after blood circulation begins during embryonic development. In this issue, three companion papers demonstrate that this is not just a coincidence and describe a mechanism by which the shear stress of blood flow triggers a cascade of molecular events leading to the birth of hematopoietic stem cells (HSCs). In all vertebrate animals studied to date, HSCs arise from the transdifferentiation of hemogenic endothelial cells comprising the floor of the dorsal aorta. Although the complete set of signals regulating this unusual event remain to be described, an emerging theme is that flow shear stress acts as a key inducer of the endothelial-to-hematopoietic transition (EHT). The molecular mechanisms underlying shear stress sensing, however, have remained poorly defined.
The three papers published in this issue describe how blood flow sensing leads to the production of several factors that synergize towards HSC emergence from arterial endothelium. Interestingly, each factor serves to increase cyclic adenosine monophosphate (cAMP) production to up-regulate the activity of the cAMP response element–binding protein (CREB) via protein kinase A (PKA) intermediates. Each paper shows that activation of CREB, through the functions of different G protein–coupled cell surface receptors, leads to the transcription of a variety of genes that each play key roles in HSC emergence.
Previous studies have demonstrated that shear stress across endothelial surfaces leads to the production of extracellular ATP, which is rapidly converted to adenosine. Jing et al. show that binding of adenosine to the A2b adenylyl cyclase-stimulatory receptor on vascular endothelium leads to up-regulation of the cAMP–PKA–CREB pathway to activate the CXCL8 cytokine gene, which is in turn required for HSC development. Importantly, the authors show that this novel requirement for adenosine is conserved across vertebrate evolution through comparative experiments in the zebrafish and mouse embryo. Kim et al. show that flow-induced cAMP–PKA–CREB signaling also leads to secretion of bone morphogenetic proteins (BMPs) to promote HSC emergence. These authors suggest that CREB function leads to production and secretion of BMP2 and BMP4, which bind to type I BMP receptors on hemogenic endothelium to promote EHT. Finally, Diaz et al. demonstrate that blood flow promotes the synthesis and release of prostaglandin E2 (PGE2) by vascular endothelium. This also leads to stimulation of the cAMP–PKA–CREB signaling axis via the EP2/EP4 receptors to activate several master regulators of the hematopoietic program. Importantly, these authors show that provision of flow conditions to cultured E9.5 murine tissues that normally lack HSC activity confers long-term, multilineage engraftment potential.
That ectopic provision of flow conditions or PGE2 analogues can confer precocious HSC potential to cultured precursors suggests that this strategy could help in vitro efforts to instruct HSC fate from human pluripotent cells. Despite decades of efforts, this feat has not yet been achieved. Collectively, these studies suggest that exposing cultured cells to flow, or experimentally modulating the cAMP–PKA–CREB signaling pathway, may be a key step toward achieving this important milestone of regenerative medicine.