Cellular Biology Notch Signaling Regulates Smooth Muscle Differentiation of Epicardium-Derived Cells

Rationale: The embryonic epicardium plays a crucial role in the formation of the coronary vasculature and in myocardial development, yet the exact contribution of epicardium-derived cells (EPDCs) to the vascular and connective tissue of the heart, and the factors that regulate epicardial differentiation, are insufficiently understood. T he epicardium, the outermost layer of the heart, develops after cardiac looping from a cluster of mesothelial cells of the septum transversum region at the cardiac venous pole. From embryonic day (E)9.0 on in the mouse, cells of this proepicardial organ (PEO) float through the pericardial space, adhere to the myocardium, spread out and form a continuous epithelial layer around E10.5. A subset of these epicardial cells undergoes an epithelial–mesenchymal transition (EMT), migrates into the subepicardial space or invades the underlying myocardium. 1 Cell lineage tracings mainly done in the chick have shown that subepicardial mesenchyme further differentiates in interstitial and perivascular fibroblasts, in smooth muscle cells (SMCs) and coronary endothelial cells. 2,3 Genetic fate-mapping studies in the mouse suggested that a substantial fraction of cardiomyocytes may also derive from epicardial cells. 4,5 Besides its role as cell source for the coronary vasculature and the myocardium, the embryonic epicardium acts as a center of paracrine signals that promote the maturation of other cardiac components including the embryonic myocardium. The genetic pathways that regulate the different steps of epicardial development are insufficiently understood. 1 The Notch signaling pathway is an evolutionarily conserved regulator of local cell– cell interactions that mediate cell fate decisions, proliferation, apoptosis, boundary formation , and stem cell maintenance in a variety of tissues in development and homeostasis. 6 In mammals, 4 different transmembrane Notch receptors (Notch1 to-4) and 5 different transmembrane ligands (Delta-like [Dll]1,-3,-4; Jagged [Jag]1,-2) have been identified. Binding to a ligand on an adjacent cell induces 2 consecutive proteolytic cleavages of the Notch receptor to release the active Notch intracellular domain (NICD). NICD translocates to the nucleus, where its binding to the transcription factor recombination signal binding protein for immunoglobulin kappa J region (Rbpj) displaces corepressor complexes from this DNA-binding factor. Coactivators are recruited instead and transcription of target genes including members of the basic helix-loop-helix hairy/ enhancer-of-split related with YRPW motif gene family (Hey1, Hey2, HeyL) is initiated. 6 Notch signaling has been implicated in numerous processes in cardiovascular development including endocardial cushion formation, maturation of the ventricular myocardium, establishment of atrioventricular canal boundaries, arterial-venous fate decisions, …