Contraction-induced endocardial id2b 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 id2b (inhibitor of DNA binding 2b) expression, thereby promoting cardiac contractility and valve formation. 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 id2b as a gene sensitive to blood flow. By generating a knockin reporter line, our results unveiled the presence of id2b in the endocardium, and its expression is sensitive to both pharmacological and genetic perturbations of contraction. Furthermore, id2b 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 id2b mutants. Deletion of id2b cell-autonomously reduced AV endocardial cell proliferation and led to a progressive increase in retrograde blood flow. Moreover, id2b also played a non-cell-autonomous role in maintaining cardiomyocyte function. Mechanistically, id2b directly interacted with the bHLH component tcf3b (transcription factor 3b) to restrict its activity. Inactivating id2b unleashed its inhibition on tcf3b, resulted in enhanced repressor activity of tcf3b, which subsequently suppressed the expression of nrg1 (neuregulin 1), an essential mitogen for heart development. Overall, our findings identify id2b as an endocardial cell-specific, biomechanical signaling-sensitive gene, which mediates intercellular communications between endocardium and myocardium to sculpt heart morphogenesis and function.