br Introduction The coronary vasculature is required

Introduction The coronary vasculature is required for supplying oxygenated blood to the cardiac muscle. Proper coronary blood circulation is essential for embryonic and adult cardiac tissue homeostasis. Defects associated with the coronary function leads to myocardial ischemia, infarction, and heart failure. Therefore, identifying molecules and signaling pathways regulating coronary vessels morphogenesis, remodeling, and maturation is essential in understanding the etiology of coronary diseases. The incidents of coronary anomalies have been reported in up to 1% of the general population (Angelini, 2002). During embryogenesis, VX-702 sale from multiple sources, including the proepicardium and epicardium, contribute to the development of coronary vasculature (Chen et al., 2014, Red-Horse et al., 2010, Wu et al., 2012). The epicardium is a single layer of epithelial cells that covers the heart. It develops from the proepicardial organ (PEO), a transient structure that arises from the mesothelium of the septum transversum (Männer, 1993, Mikawa and Gourdie, 1996). The epicardium plays a significant role in heart development and gives rise to the majority of cells, including fibroblasts, smooth muscle cells, and endothelium, of the coronary vasculature (Männer, 1993, Mikawa and Gourdie, 1996, Singh and Epstein, 2012, Singh et al., 2011). Epicardium-deficient hearts exhibit impaired cardiac function due to a thin myocardium, suggesting that factors secreted from the epicardium are required not only for coronary vasculature development but also for the proliferation and differentiation of the underlying myocardial cells (Männer, 1993, Männer et al., 2005, Pennisi et al., 2003). The role of the epicardium in cardiac homeostasis was recently explored using an epicardial injury model. Developmental gene programs were re-activated following injury, which led to epicardial cell expansion and differentiation into cardiac fibroblasts and smooth muscle cells (Zhou et al., 2011). A better understanding of embryonic epicardial biology will help to understand the pathophysiology of coronary defects and it may suggest strategies to manipulate adult epicardial cells to facilitate myocardial regrowth and angiogenesis after cardiac injury. The Hippo signaling is an evolutionary conserved pathway that control organ size by regulating cell proliferation, cell survival, and stem cell self renewal (Zhao et al., 2011). Hippo signaling has been implicated in cardiac development as well as in cardiac repair and regeneration after myocardial injury. Genetic deletion, with a cardiac-specific Cre-recombinase, of Mst1/2, Lats2, or Salvador (Salv) leads to an expansion of ventricular myocardium due to increased cardiomyocyte proliferation (Heallen et al., 2011). Global deletion of Yap results in embryonic lethality around embryonic day 8.5 (E8.5) due to defects in yolk sac vasculogenesis, chorioallantonic fusion, and body axis elongation (Morin-Kensicki et al., 2006). However, Taz knockout mice are viable through adulthood, although some develop glomerulocystic kidney disease and pulmonary disease (Xin et al., 2013). Yap and Taz double-null embryos die prior to the morula stage, suggesting functional redundancy during early embryonic development (Nishioka et al., 2009). Expression of a constitutively active form of Yap in the heart results in increased cardiomyocyte proliferation and heart size (von Gise et al., 2012, Xin et al., 2011). Yap has been shown to regulate cardiomyocyte proliferation by interacting with the insulin-like growth factor (IGF) and Wnt signaling pathways (Heallen et al., 2011, Xin et al., 2011). In addition, recent work by Zhang et al. demonstrates that Yap can regulate epithelial-to-mesenchymal transition (EMT) of the atrioventricular cushion by modulating transforming growth factor β (TGF-β)/Smad signaling (Zhang et al., 2014). During cardiac development, Yap and Taz are functionally redundant, but tissue-specific deletion of both molecules leads to lethal cardiomyopathy in a gene-dose-dependent manner (Xin et al., 2013). Despite the studies described above, a role for Yap and Taz in the epicardium has not been explored.