Protease- activated receptors (PARs) comprise a family of four G-protein coupled receptors that mediate cellular responses to proteases. PARs were identified in a search for the platelet receptor for the coagulation protease thrombin. Mouse deficiency models later established critical roles for PARs in hemostasis and thrombosis and in vascular development, yet unexplained functions ascribed to proteases hinted at a broader functional repertoire. To uncover new roles for PARs, we addressed redundancy between PARs and found that combined loss of all PARs in mice resulted in embryonic lethality not accounted for by bleeding or lack of endothelial signaling, suggesting unappreciated roles for PARs in development. Addressing redundancy with other signaling systems, we found that loss of plasma sphingosine-1-phosphate (S1P), a lipid agonist for a related family of G-protein coupled receptors, had profound effects on endothelial barrier homeostasis including sensitization to barrier modulation by PARs. This proposal aims to explore a novel role for PARs in development of the cardiovascular system and to study the role of PARs and S1P receptors and their interaction in endothelial barrier homeostasis. Better understanding of PARs, their agonist proteases and redundant signaling systems in embryonic development and adult disease may refine the use of existing and emerging drugs targeting this system and, potentially, expand their application beyond current indications.