The extracellular matrix protein agrin is essential for epicardial epithelial-to-mesenchymal transition during heart development

During embryonic heart development, epicardial cells residing within the outer layer of the heart undergo epithelial-mesenchymal transition (EMT) and migrate into the myocardium to support and stimulate organ growth and morphogenesis. Disruption of epicardial EMT results in aberrant heart formation and embryonic lethality. Despite being an essential process during development, the regulation of epicardial EMT is poorly understood. Here we report EMT on the epicardial surface of the embryonic heart at subcellular resolution using scanning electron microscopy (SEM). We identified high- and low-EMT regions within the mesothelial layer of the epicardium and an association with key components of the extracellular matrix (ECM). The ECM basement membrane-associated proteoglycan agrin was found to localize in the epicardium in regions actively undergoing EMT. Deletion of agrin resulted in impaired EMT and compromised development of the epicardium, accompanied by down-regulation of the epicardial EMT regulator WT1. Agrin enhanced EMT in human embryonic stem cell-derived epicardial-like cells by decreasing β-catenin and promoting pFAK localization at focal adhesions. In addition, agrin promoted the aggregation of its receptor dystroglycan to the Golgi apparatus in murine epicardial cells and loss of agrin resulted in dispersal of dystroglycan throughout the epicardial cells in embryos, disrupting basement membrane integrity and impairing EMT. Our results provide new insights into the role of the ECM in heart development, and implicate agrin as a critical regulator of EMT, functioning to ensure dystroglycan connects signals between the ECM and activated epicardial cells.