3039 – YAP REGULATES HEMATOPOIETIC STEM CELL FORMATION IN RESPONSE TO THE BIOMECHANICAL FORCES OF BLOOD FLOW

Hematopoietic stem cells (HSCs) are born from cells in the dorsal aorta that adopt hematopoietic potential and commitment during development. Physical forces, namely wall shear stress (WSS) and cyclic stretch (CS), produced by hemodynamic blood flow are required to generate HSCs from arterial endothelium, but the mechanisms by which these forces are transduced into a “stemness” regulatory program remain unknown. Using a novel “dorsal-aorta-on-a-chip” microfluidic platform, we demonstrate that exposure to WSS and CS increases expression of the hematopoietic transcription factor RUNX1 in human CD34+ hemogenic endothelial cells derived from induced pluripotent stem cells. Notably, CS specifically induced activity of the YAP transcription factor, a mechanically-activated regulator of organ size and pluripotency. We corroborate these findings in a zebrafish model, and demonstrate that loss or gain of YAP function in vivo can blunt or augment the production of HSCs during definitive hematopoiesis. Importantly, we find that YAP is responsible for the maintenance, not initiation, of the hematopoietic program in newly specified hemogenic endothelial cells. Molecularly, we identify a stretch-induced, RhoGTPase-dependent mechanotransduction network controlling YAP nuclear availability to regulate RUNX1 expression, both in zebrafish and human cells. Moreover, we show that small molecule stimulation of RhoGTPases can rescue HSC production in zebrafish embryos with no flow, and enhance the hematopoietic potential of human iPSC-derived CD34+ endothelial cells grown in static culture. Together these findings uncover YAP as a transcriptional regulator of stem cell fate commitment during tissue morphogenesis, and suggest a pharmacologically amenable mechanotransduction pathway that could be exploited to improve the in vitro derivation of HSCs from human cells for therapeutic purposes.