Notch and Bmp signaling pathways act coordinately during the formation of the proepicardium.

The epicardium is the outer mesothelial layer of the heart. It encloses the myocardium and plays key roles in heart development and regeneration. It derives from the proepicardium (PE), cell clusters that appear in the dorsal pericardium (DP) close to the atrioventricular canal and the venous pole of the heart, and are released into the pericardial cavity. PE cells are advected around the beating heart until they attach to the myocardium. Bmp and Notch signaling influence PE formation, but it is unclear how both signaling pathways interact during this process in the zebrafish. Here, we show that the developing PE is influenced by Notch signaling derived from the endothelium. Overexpression of the intracellular receptor of notch in the endothelium enhances bmp expression, increases the number of pSmad1/5 positive cells in the DP and PE, and enhances PE formation. On the contrary, pharmacological inhibition of Notch1 impairs PE formation. bmp2b overexpression can rescue loss of PE formation in the presence of a Notch1 inhibitor, but Notch gain-of-function could not recover PE formation in the absence of Bmp signaling. Endothelial Notch signaling activates bmp expression in the heart tube, which in turn induces PE cluster formation from the DP layer.Nadia Mercader was funded by the Spanish Ministry of Economy and Competitiveness through grant BFU2014-56970-P (Plan Estatal de Investigación Científica y Técnica y de Innovación 2013-2016. Programa Estatal de I+D+i Orientada a los Retos de la Sociedad Retos Investigación: Proyectos I+D +i 2016, del Ministerio de Economía competitividad e Industria), and cofunding by Fondo Europeo de Desarrollo Regional (FEDER). Nadia Mercader is also supported by the European Industrial Doctorate Program EID 722427. Nadia Mercader and Julien Vermot are supported by the Swiss National Science Foundation grant ANR-SNF 310030L_182575. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 708312 (MP) and from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme: GA Nº682938. Laura Andrés-Delgado was funded (2014-16) through the postdoctoral fellowship Ayudas Postdoctorales 2013. José Luis de la Pompa was supported by grants SAF2016-78370-R, CB16/11/00399 (CIBER CV) and RD16/0011/0021 (TERCEL) from the Spanish Ministry of Science and Innovation. The CNIC is supported by the Ministry of Science and Innovation and the Pro CNIC Foundation, and is a Severo Ochoa Center of Excellence (SEV-2015-0505).S