NFATc1 in cardiac valve development and EPDC invasion

Congenital malformations are the most common cause of death in infancy in the United States. Of these birth defects, most are malformations of cardiac valvuloseptal structures and a significant number are coronary vessel malformations. Therefore, identifying molecular mechanisms that regulate cardiac valve and coronary artery development is of great clinical importance. Cardiac valve morphogenesis begins with growth of endocardial cushions in the atrioventricular canal and outflow tract regions of the looping heart. After growth, endocardial cushions are remodeled into thin leaflets, characteristic of mature heart valves. Nuclear Factor of Activated T-cells cytoplasmic 1 (NFATc1) is a transcription factor necessary for heart valve development. The studies detailed here demonstrate that Vascular Endothelial Growth Factor A (VEGF)/ NFATc1 pathway function promotes endocardial cushion growth, while Receptor Activator of NFκB (RANKL)/ NFATc1 pathway function is associated with valve remodeling. These studies further demonstrate that NFATc1 serves as a nodal point in the transition from endocardial cushion growth to valve remodeling via ERK1/2 or JNK1/2 copathway activation. In the course of these studies NFATc1 expression by PE, epicardium, and EPDCs was discovered. During heart looping, PE cells migrate onto the myocardium and proliferate to form the epicardium. A subset of epicardial cells undergo epithelial-tomesenchymal transformation (EMT) and invade the subepicardium and myocardium as epicardium-derived cells (EPDCs). EPDCs differentiate into coronary endothelial and smooth muscle cells, as well as adventitial fibroblasts that produce the fibrous matrix. Studies detailed in this dissertation demonstrate that conditional loss of NFATc1 expression in EPDCs in mice causes embryonic death by E18.5 with reduced coronary vessel and fibrous matrix penetration into myocardium. These studies further demonstrate that RANKL/NFATc1 pathway components are expressed in EPDCs and loss of NFATc1 in EPDCs causes loss of CtsK expression in the myocardial interstitium in vivo. Likewise, RANKL treatment induces CtsK expression in PE-derived cell cultures via a calcineurin-dependent mechanism. In chicken embryo hearts, RANKL treatment increases the distance of EPDC invasion into myocardium, and this response is calcineurin-dependent. Together, these data demonstrate a critical role for the RANKL/ NFATc1 signaling pathway in promoting invasion of EPDCs into myocardium by induction of extracellular matrix-degrading enzyme gene expression