Shear stress-induced Piezo1 activates CD99L2 to facilitate the initiation of blood circulation

The onset of blood circulation is a pivotal developmental event, yet the molecular mechanisms that enable erythrocytes to disengage from the endothelium and enter the bloodstream remain unclear. Here, we identify CD99L2 as a mechanoresponsive adhesion regulator, transiently induced in primitive erythrocytes by shear stress-activated Piezo1 signaling. Using zebrafish and mouse models, we show that CD99L2 is essential for erythrocyte de-adhesion and circulation entry. Loss of CD99L2 leads to aberrant nuclear translocation of β-catenin, activation of Rap1 signaling, and persistent expression of adhesion molecules, culminating in erythrocyte retention, impaired maturation, and hemolytic anemia. Mechanistically, CD99L2 binds and anchors β-catenin at the membrane, and shear-induced Piezo1 activation promotes its expression during a narrow developmental window. This pathway is conserved in mice and modulated by biomechanical forces, unveiling a mechanism that couples hemodynamic force to erythrocyte adhesion control during the initiation of blood flow.