Mechanosensitive Notch-Dll4 and Klf2-Wnt9 signaling pathways intersect in guiding valvulogenesis in zebrafish

In the zebrafish embryo, the onset of blood flow generates fluid shear stress on endocardial cells, which are specialized endothelial cells that line the interior of the heart. High levels of fluid shear stress activate both Notch and Klf2 signaling, which play crucial roles in atrioventricular valvulogenesis. However, it remains unclear why only individual endocardial cells ingress into the cardiac jelly and initiate valvulogenesis. Here, we show that lateral inhibition between endocardial cells, mediated by Notch, singles out Delta-like-4-positive endocardial cells. These cells ingress into the cardiac jelly, where they form an abluminal cell population. Delta-like-4-positive cells ingress in response to Wnt9a, which is produced in parallel through an Erk5-Klf2-Wnt9a signaling cascade also activated by blood flow. Hence, mechanical stimulation activates parallel mechanosensitive signaling pathways that produce binary effects by driving endocardial cells toward either luminal or abluminal fates. Ultimately, these cell fate decisions sculpt cardiac valve leaflets. [Display omitted] •Atrioventricular valvulogenesis in zebrafish is regulated by Notch and Klf2 pathways•Notch-mediated lateral inhibition singles out Delta-like-4-positive endocardial cells•These cells ingress into the cardiac jelly in response to Erk5-Klf2-Wnt9a signaling•Frizzled9b makes presumptive ingressing endocardial cells competent to respond to Wnt9a Paolini et al. show that lateral inhibition between endocardial cells, mediated by Notch, singles out Delta-like-4-positive endocardial cells. These Delta-like-4-positive cells ingress into the cardiac jelly during zebrafish cardiac valvulogenesis in response to Wnt9a, which is produced in parallel through an Erk5-Klf2-Wnt9a signaling cascade activated by blood flow.