(OFT) development. Atrioventricular (AV) canal development AV boundary formation Notch signaling can regulate the atrioventricular boundary formation between the AV canal and the chamber myocardium. Studies have revealed that both loss- and gain-of-function of the Notch pathway results in defects in AV canal development. In addition, the Notch target genes HEY1 and HEY2 are involved in restricting the expression of two critical developmental regulator proteins, BMP2 and Tbx2, to the AV canal. AV epithelial-mesenchymal transition (EMT) Notch signaling is also important for the process of AV EMT, which is required for AV canal maturation. After the AV canal boundary formation, a subset of endocardial cells lining the AV canal are activated by signals emanating from the myocardium and by interendocardial signaling pathways to undergo EMT. Notch1 deficiency results in defective induction of EMT. Very few migrating cells are seen and these lack mesenchymal morphology. Notch may regulate this process by activating matrix metalloproteinase2 (MMP2) expression, or by inhibiting vascular endothelial (VE)-cadherin expression in the AV canal endocardium while suppressing the VEGF pathway via VEGFR2. In RBPJk/CBF1-targeted mutants, the heart valve development is severely disrupted, presumably because of defective endocardial maturation and signaling. Ventricular development Some studies in Xenopus and in mouse embryonic stem cells indicate that cardiomyogenic commitment and differentiation require Notch signaling inhibition. Active Notch signaling is required in the ventricular endocardium for proper trabeculae development subsequent to myocardial specification by regulating BMP10, NRG1, and EphrinB2 expression. Notch signaling sustains immature cardiomyocyte proliferation in mammals and zebrafish. A regulatory correspondence likely exists between Notch signaling and Wnt signaling, whereby upregulated Wnt expression downregulates Notch signaling, and a subsequent inhibition of ventricular cardiomyocyte proliferation results. This proliferative arrest can be rescued using Wnt inhibitors. The downstream effector of Notch signaling, HEY2, was also demonstrated to be important in regulating ventricular development by its expression in the interventricular septum and the endocardial cells of the cardiac cushions. Cardiomyocyte and smooth muscle cell-specific deletion of HEY2 results in impaired cardiac contractility, malformed right ventricle, and ventricular septal defects. Ventricular outflow tract development During development of the aortic arch and the aortic arch arteries, the Notch receptors, ligands, and target genes display a unique expression pattern. When the Notch pathway was blocked, the induction of vascular smooth muscle cell marker expression failed to occur, suggesting that Notch is involved in the differentiation of cardiac neural crest cells into vascular cells during outflow tract