Valvulogenesis begins with acellular matrix filled cardiac cushions that are subsequently invaded by collagen producing mesenchymal cells that ultimately give rise to fibroblasts populated valve leaflets. Defects arising from malformations of the valves are the most prevalent of all congenital heart defects underscoring the need to investigate molecules that may regulate proper valve development. Central to the process of valve formation is the deposition and remodeling of collagen within the cushions and leaflets. Recently, a novel class of receptor tyrosine kinases, the discoidin domain receptors (DDR), have been demonstrated to specifically bind collagen and increase expression of enzymes involved in collagen remodeling. While there are two members of the protein family, DDR1 and DDR2, DDR2 has been detected on mesenchymal cells and fibroblast, the collagen producing cells within the heart. Preliminary data presented herein demonstrates a role for DDR2 in the formation of the migrating mesenchymal cells required for cellular invasion of the cushions and the production of collagen within the valves. The nature of the interaction of DDR2 with collagen and the ability of this receptor to up-regulate proteins involved in collagen remodeling has led to the hypothesis that interactions between DDR2 and collagen are critical determinants in valve mesenchymal cell differentiation and ECM signaling during valve formation. Furthermore, that the interaction of DDR2 with collagen in different organizational states regulates receptor distribution and modulates mesenchymal cell function. The following specific aims have been designed to test this hypothesis: 1) to determine the role of DDR2 in mediating cellular invasion and ECM remodeling during EMT and early valve formation; 2) to determine the effect of collagen density and organization on DDR2 activation in developing valves; and 3) to characterize the molecular interactions occurring between DDR2 and collagen. The characterization of the physical and molecular parameters that govern the interaction of DDR2 with collagen will aid in our understanding of how this receptor perceives changes occurring in collagen organization and distribution within the developing valves and how these changes are ultimately translated into cellular functions.
