YAP Regulates Hematopoietic Stem Cell Formation in Response to the Biomechanical Forces of Blood Flow

Unmasking the mechanisms that control hematopoietic stem and progenitor cell (HSPC) renewal and maturation is critical towards utilizing these versatile cells in therapeutic applications; unfortunately, current approaches fail to generate multipotent long lived HSPCs.  Several recent studies highlighted the importance of the local embryonic environment, such as blood flow, in regulating HSPC production in vivo.  Lundin et al. sought to better understand the mechanisms by which mechanical forces control HSPC production, and whether manipulation of these signaling pathways can promote de novo HSPC formation in vitro. Utilizing a novel aorta-on-a-chip model allowed them to identify the YAP protein, a biophysical-sensing transcription factor, as a critical mediator of cyclic stretch-induced HSPC formation.  The group then utilized zebrafish embryo models to further implicate YAP as a critical facilitator of force induced HSPC promotion. Importantly, YAP overexpression was sufficient to rescue HSPC formation in a morpholinos-induced silent heart zebrafish model with disrupted blood flow. Because Rho GTPases have been identified as a mechanosensor that regulates YAP, the group sought to determine if these small G-proteins may be important in YAP-mediated HSPC production.  Activating Ras superfamily small G-proteins with CN02, CN03, and CN04, as well as inhibition with a potent inhibitor CT04, allowed investigators to determine that Rho GTPases play a critical role in HSPC formation.  Based on these findings the group subsequently tested whether manipulation of this force-sensing regulatory pathway could control HSPC production in vitro.  They utilized siRNA tools to determine that YAP drives hematopoietic specification in vitro; furthermore, RhoA-specific activation with CN03 promoted HSPC production de novo in a two-dimensional differentiation system in the absence of mechanical stimuli or genetic manipulation.  Cytoskeleton’s G-switch line of activators and inhibitors for Ras superfamily members Rac, Cdc42, and Rho were critical reagents in this study and could be essential for further defining the regulatory mechanisms that control HSPC formation.