Abstract 1063: Essential Functions of Alk3 during Atrioventricular Cushion Morphogenesis in Mouse Embryonic Hearts
Congenital heart diseases (CHDs) are the leading cause of infant morbidity and mortality, occurring in as many as 1% of newborns. The most common CHDs are caused by maldevelopment of septation and valves. Epithelial-Mesenchymal-Transformation (EMT) in the atrioventricular canal (AVC) and outflow tract (OFT) regions is a critical process regulating initial valve formation where a subpopulation of endocardial cells invade the extracellular matrix as the result of regional interaction between the myocardium and endocardium. These mesenchymalized cushions serve as the primordia of valves and septa and are developed into mature structures through complicated remodeling processes. Accumulated evidence has suggested that BMP pathways play critical roles during septa formation and valvulogenesis, and impairment of BMP signaling may contribute to CHDs. Alk3 encodes a BMP specific type I receptor expressed in mouse embryonic hearts. To reveal functions of Alk3 during AV cushion morphogenesis and to overcome the early lethality of Alk3−/− embryos, we applied a Cre/loxp approach to specifically inactivate Alk3 in the endothelium/endocardium using Tie1-Cre mice. Our studies showed that endocardial depletion of Alk3 severely impairs EMT in the AVC region; the number of mesenchymal cells formed in Tie1-Cre; Alk3loxp/loxp embryos was reduced to only ~20% of the normal level from in vivo section studies. The defective EMT in AV cushions of mutant embryos was further confirmed with in vitro collagen gel analysis. In addition to its functions on mesenchyme formation, we showed in this study for the first time, that Alk3 can also promote growth/survival of AV cushion mesenchymal cells. Functions of Alk3 are accomplished through regulating expression/activation/subcellular localization of multiple downstream genes including Smads and cell-cycle regulators. We are currently performing experiments to test the hypothesis that BMP ligands can directly act on AV cushion mesenchymal cells using a newly developed cell culture system. Taken together, our study supports the notion that Alk3 mediated BMP signaling in AV endocardial/mesenchymal cells plays a central role during cushion morphogenesis.