Endocardial to Myocardial Notch-Wnt-Bmp Axis Regulates Early Heart Valve Development

<div><p>Endocardial to mesenchymal transformation (EMT) is a fundamental cellular process required for heart valve formation. Notch, Wnt and Bmp pathways are known to regulate this process. To further address how these pathways coordinate in the process, we specifically disrupted <i>Notch1</i> or <i>Jagged1</i> in the endocardium of mouse embryonic hearts and showed that Jagged1-Notch1 signaling in the endocardium is essential for EMT and early valvular cushion formation. qPCR and RNA <i>in situ</i> hybridization assays reveal that endocardial Jagged1-Notch1 signaling regulates <i>Wnt4</i> expression in the atrioventricular canal (AVC) endocardium and <i>Bmp2</i> in the AVC myocardium. Whole embryo cultures treated with Wnt4 or Wnt inhibitory factor 1 (Wif1) show that <i>Bmp2</i> expression in the AVC myocardium is dependent on Wnt activity; Wnt4 also reinstates <i>Bmp2</i> expression in the AVC myocardium of endocardial <i>Notch1</i> null embryos. Furthermore, while both Wnt4 and Bmp2 rescue the defective EMT resulting from Notch inhibition, Wnt4 requires Bmp for its action. These results demonstrate that Jagged1-Notch1 signaling in endocardial cells induces the expression of Wnt4, which subsequently acts as a paracrine factor to upregulate Bmp2 expression in the adjacent AVC myocardium to signal EMT.</p>