Contraction-induced endocardial plays a dual role in regulating myocardial contractility and valve formation.

Biomechanical cues play an essential role in sculpting organ formation. Comprehending how cardiac cells perceive and respond to biomechanical forces is a biological process with significant medical implications that remains poorly understood. Here, we show that biomechanical forces activate endocardial (inhibitor of DNA-binding 2b) expression, thereby promoting cardiac contractility and valve formation in zebrafish. Taking advantage of the unique strengths of zebrafish, particularly the viability of embryos lacking heartbeats, we systematically compared the transcriptomes of hearts with impaired contractility to those of control hearts. This comparison identified as a gene sensitive to blood flow. By generating a knock-in reporter line, our results unveiled the presence of in the endocardium, and its expression is sensitive to both pharmacological and genetic perturbations of contraction. Furthermore, loss-of-function resulted in progressive heart malformation and early lethality. Combining RNA-seq analysis, electrophysiology, calcium imaging, and echocardiography, we discovered profound impairment in atrioventricular (AV) valve formation and defective excitation-contraction coupling in mutants. Mechanistically, deletion of reduced AV endocardial cell proliferation and led to a progressive increase in retrograde blood flow. In the myocardium, directly interacted with the bHLH component (transcription factor 3b) to restrict its activity. Inactivating unleashed its inhibition on , resulting in enhanced repressor activity of , which subsequently suppressed the expression of (neuregulin 1), an essential mitogen for heart development. Overall, our findings identify as an endocardial cell-specific, biomechanical signaling-sensitive gene, which mediates intercellular communications between endocardium and myocardium to sculpt heart morphogenesis and function.