Project Summary Lung vascular endothelial barrier function is regulated primarily by adherens junctions (AJs) consisting of VE- cadherin, which mediates adhesion of endothelial cells through homotypic interaction, and associated catenins. Recently, another VE-cadherin partner Vascular Endothelial Protein Tyrosine Phosphatase, VE-PTP, has been identified in regulating endothelial barrier function. Disassembly of AJs through internalization and subsequent degradation of VE-cadherin disrupts the endothelial barrier resulting severe intractable protein-rich pulmonary edema, the central pathogenic feature of ARDS. Restoration of the endothelial barrier is essential for resolving edema, yet the mechanisms underlying re-assembly of AJs are poorly understood. Based on our Supporting Data, in Project 1 we posit that VE-PTP is a central regulator of re-assembly of AJs that plays a critical role in restoring endothelial barrier function through its ability to stabilize VE-cadherin at AJs. We also showed that signaling via Hypoxia-Inducible Factors (HIFs) was required for the synthesis of VE-PTP. Notably, in addition to VE-PTP binding to VE-cadherin, another pool of VE-PTP is associated with Tie2, the tyrosine kinase receptor present in the endothelial plasmalemma, and which functions to restrain Tie2 activity. On the basis of these observations, in Project 1 we will pursue the following Specific Aims: (i) we will determine the synergistic role of (a) HIF-dependent expression of VE-PTP as an adaptive mechanism promoting the stabilization of VE-cadherin at AJs, and (b) HIF-dependent activation of Rac1 and Cdc42 at AJs in sealing and strengthening the AJ barrier and thus restoring lung fluid balance; (ii) we will determine the role of VE-PTP interaction with Tie2 in regulating the repair of AJs through the activation of Tie2 signaling and the translocation of VE-PTP to VE-cadherin, and resulting in VE-cadherin stabilization; and (iii) we will determine the therapeutic value of (a) blocking prolyl hydroxylases (PHDs) to activate HIF-mediated transcription of VE-PTP, and (b) inhibiting Tie2 interaction with VE-PTP in restoring lung vascular barrier function and fluid balance in models of inflammatory lung injury. We will apply a rigorous multidisciplinary approach to define the signaling mechanisms activated by VE-PTP interaction with VE-cadherin in restoring the integrity of lung endothelial barrier, with the intent of identifying new targets to normalize lung fluid balance and the course of inflammatory lung injury.