PROJECT SUMMARY/ABSTRACT Valves are present within hearts, veins and lymphatic vessels to regulate the flow of blood and lymph. Defects in the development or functioning of valves could lead to diseases such as lymphedema and degenerative heart valve disease. Venous and lymphatic (vascular) valves are composed of two layers of endothelial cells with a thin layer of collagen-rich extra cellular matrix (ECM). Few if any interstitial cells are observed within vascular valves. In contrast, cardiac valves are made of two major cell types: numerous ECM producing valvular interstitial cells (VICs), which are surrounded by a single layer of valvular endothelial cells (VECs). Despite significant differences in their structure and the mechanical force that they experience, cardiac and vascular valves share interesting similarities. The homeobox transcription factor PROX1 is necessary for the development of vascular valves. PROX1 is also expressed in a subset of VECs on the downstream side of heart valves. We have now determined that the deletion of PROX1 from VECs results in the accumulation of proteoglycans and the consequent thickening of heart valves. Thus, PROX1 in VECs regulates the development and functioning of VICs through yet unknown mechanisms. Building on our preliminary data we will test our Central Hypothesis that a previously unknown PROX1àFOXC2àPDGF-B signaling pathway from downstream VECs regulates VIC identity and, in turn, the ECM composition of valves. Abnormally high synthesis of proteoglycans is observed in several valve disorders. Valve defects are observed in ~8% in individuals >65 years old and ~13% in those who are >75. Pharmacological approaches to treat these valve defects do not exist. Hence, the outcomes of our proposed research are expected to be significant because they will substantially advance our insight into the molecular and cellular mechanisms of VEC-VIC crosstalk during cardiac valve development and shed light on potential therapeutic strategies.