Hedgehog signaling induces arterial endothelial cell formation by repressing venous cell fate.

In vertebrate embryos, the dorsal aorta and the posterior cardinal vein form in the trunk to comprise the original circulatory loop. Previous studies implicate Hedgehog (Hh) signaling in the development of the dorsal aorta. However, the mechanism controlling specification of artery versus vein remains unclear. Here, we investigated the cell-autonomous mechanism of Hh signaling in angioblasts (endothelial progenitor cells) during arterial-venous specification utilizing zebrafish mutations in Smoothened (Smo), a G protein-coupled receptor essential for Hh signaling. smo mutants exhibit an absence of the dorsal aorta accompanied by a reciprocal expansion of the posterior cardinal vein. The increased number of venous cells is equivalent to the loss of arterial cells in embryos with loss of Smo function. Activation of Hh signaling expands the arterial cell population at the expense of venous cell fate. Time-lapse imaging reveals two sequential waves of migrating progenitor cells that contribute to the dorsal aorta and the posterior cardinal vein, respectively. Angioblasts deficient in Hh signaling fail to contribute to the arterial wave; instead, they all migrate medially as a single population to form the venous wave. Cell transplantation analyses demonstrate that Smo plays a cell-autonomous role in specifying angioblasts to become arterial cells, and Hh signaling-depleted angioblasts differentiate into venous cells instead. Collectively, these studies suggest that arterial endothelial cells are specified and formed via repressing venous cell fate at the lateral plate mesoderm by Hh signaling during vasculogenesis.

Down syndrome critical region-1 is a transcriptional target of nuclear factor of activated T cells-c1 within the endocardium during heart development.

Patients with Down syndrome have characteristic heart valve lesions resulting from endocardial cushion defects. The Down syndrome critical region 1 (DSCR1) gene, identified at the conserved trisomic 21 region in those patients, encodes a calcineurin inhibitor that inactivates nuclear factor of activated T cells (NFATc) activity. Here, we identify a regulatory sequence in the promoter region of human DSCR1 that dictates specific expression of a reporter gene in the endocardium, defined by the temporal and spatial expression of Nfatc1 during heart valve development. Activation of this evolutionally conserved DSCR1 regulatory sequence requires calcineurin and NFATc1 signaling in the endocardium. NFATc1 proteins bind to the regulatory sequence and trigger its enhancer activity. NFATc1 is sufficient to induce the expression of Dscr1 in cells that normally have undetectable or minimal NFATc1 or DSCR1. Pharmacologic inhibition of calcineurin or genetic Nfatc1 null mutation in mice abolishes the endocardial activity of this DSCR1 enhancer. Furthermore, in mice lacking endocardial NFATc1, the endogenous Dscr1 expression is specifically inhibited in the endocardium but not in the myocardium. Thus, our studies indicate that the DSCR1 gene is a direct transcriptional target of NFATc1 proteins within the endocardium during a critical window of heart valve formation.