A Field of Myocardial-Endocardial NFAT Signaling Underlies Heart Valve Morphogenesis

Abstract

cells. Once transformed, the mesenchymal cells prolifer-1 Departments of Pathology and Developmental ate and invade the cardiac jelly, a basement membrane-Biology like substance elaborated by the myocardial cells. A Howard Hughes Medical Institute second process, which is even less well understood, 2 Cardiovascular Division begins after EMT. The endocardial cushion area elon-Department of Medicine gates and undergoes continuous remodeling that even-Stanford University Medical School tually refines the primitive cushion into thin elongated Stanford, California 94305 valve leaflets. Several signaling pathways have been implicated in heart valve morphogenesis. Studies with neutralizing Summary antibodies and antisense RNA indicate that TGF-␤ signaling is essential for EMT in the chick (Eisenberg and The delicate leaflets that make up vertebrate heart Markwald, 1995; Nakajima et al., 2000). However, probavalves are essential for our moment-to-moment exisbly due to functional redundancy in the mammalian systence. Abnormalities of valve formation are the most tem, mice with single mutations in the TGF-␤ family common serious human congenital defect. Despite members or their receptors do not have impaired EMT. their importance, relatively little is known about valve The Wnt/␤-catenin pathway is essential for EMT in zedevelopment. We show that the initiation of heart valve brafish (Hurlstone et al., 2003). In mice, however, Wnt morphogenesis in mice requires calcineurin/NFAT to plays an essential early role in cardiac myogenesis repress VEGF expression in the myocardium underly-(Olson and Schneider, 2003), thus obscuring any later ing the site of prospective valve formation. This represroles in heart valve formation. Also, Notch signaling has sion of VEGF at E9 is essential for endocardial cells recently been shown to promote EMT in mice (Timmerto transform into mesenchymal cells. Later, at E11, a man et al., 2004). EGF-related growth factors and their second wave of calcineurin/NFAT signaling is required receptors are important in regulating heart valve formain the endocardium, adjacent to the earlier myocardial tion in mice. Loss of endocardial HB-EGF or EGFR resite of NFAT action, to direct valvular elongation and sults in abnormal hyperplasia of cushion mesenchymal refinement. Thus, NFAT signaling functions sequencells (Chen et al., 2000; Iwamoto et al., 2003; Jackson tially from myocardium to endocardium within a valvuet al., 2003). Lack of endocardial neuregulin-1 or cushion lar morphogenetic field to initiate and perpetuate mesenchymal ErbB3 results in severe mesenchymal hyembryonic valve formation. This mechanism also oppoplasia in the cushion area (Camenisch et al., 2002; erates in zebrafish, indicating a conserved role for Erickson et al., 1997; Meyer and Birchmeier, 1995). Ras calcineurin/NFAT signaling in vertebrate heart valve signaling is also crucial for cushion mesenchymal develmorphogenesis. opment. Loss of endothelial neurofibromin-1, which inactivates Ras, results in hyperplasia of cushion mesen-

Vascular endothelial growth factor receptor signaling is required for cardiac valve formation in zebrafish

“…This speculation is consistent with recent findings reported by Chang and colleagues (Chang et al, 2004). In their study of murine valvulogenesis, they showed that NFAT signaling in the myocardium at E9 suppresses VEGF expression, producing conditions that initiate EMT.…”

Illuminating cardiac development: Advances in imaging add new dimensions to the utility of zebrafish genetics

“…In silent heart mutants, AV endocardium remains squamous and does not express Dm-grasp, indicating the importance of cardiac function for the initiation of endocardial differentiation [24]. In contrast, Calcineurin signaling is not required for initial differentiation of the AV canal endocardium, but it is important for the subsequent epithelial to mesenchymal transformation that creates ECs [24,40]. Notch signaling, on the other hand, seems to be responsible for the spatial restriction of AV canal differentiation [24].…”

“…Perhaps Ugdh is important for the control of Wnt signaling during EC formation: constitutive activation of Wnt signaling via mutation of the tumor suppressor gene apc leads to excessive EC formation beyond the boundaries of the AV canal, and overexpression of apc inhibits EC formation [39]. Other studies employing pharmacological inhibitors have suggested that Calcineurin/NFAT signaling promotes EC formation, while Notch signaling inhibits EC formation [24,40]. Additionally, a number of lines of evidence demonstrate that cardiac function plays a key role in the induction of ECs.…”

Zyxin Mediates Actin Fiber Reorganization in Epithelial–Mesenchymal Transition and Contributes to Endocardial Morphogenesis

“…An example of developmentally regulated EMT occurs during the initial stages of cardiac morphogenesis. At embryonic day 9.5 (E9.5), endocardial cells undergo EMT ("endocardial EMT"); delaminating from the endothelial sheet, invading the matrix tissue called cardiac jelly, and engaging in endocardial cushion cellularization required for valve and septum formation (Chang et al, 2004). Because a number of congenital heart diseases are caused by abnormal atrioventricular canal (AVC) development (Bruneau, 2008), understanding of the molecular basis of AVC morphogenesis has been long sought, but still not fully achieved.…”