The endothelium barrier regulates the exchange of blood fluid, electrolytes and proteins across the vascular wall, and it is subjected to dynamic remodeling. The function of this barrier is affected by various conditions, including inflammation, diabetes and sepsis. Endothelial barrier dysfunction (EBD) is the hallmark of severe respiratory disease, such as Acute Lung Injury (ALI), Acute Respiratory Distress Syndrome (ARDS) and sepsis. The development of novel therapeutic strategies against these devastating pathologies is of the utmost need. Our endeavors to expose new therapeutic avenues towards EBD-related diseases revealed that Hsp90 inhibitors and GHRH antagonists protect against endothelial barrier dysfunction, and induce the Unfolded Protein Response (UPR). This is a highly conserved molecular machinery, able to initiate protective and repairing cellular responses in human tissues, including the lungs. It consists of three ER transmembrane proteins: IRE1α (inositol-requiring enzyme 1α), PERK (pancreatic endoplasmic reticulum kinase), and ATF6 (activating transcription factor 6). UPR induction due to heat shock protein 90 inhibition or growth hormone releasing hormone antagonists counteracted the Kifunensine (UPR suppressor) - induced endothelial hyperpermeability in vitro. Our proposal will focus on the effects of ATF6 in lung endothelial barrier function. Specific Aim 1 will associate ATF6 activation with endothelial barrier function, to demonstrate the supportive role of ATF6 against EBD in vitro. Specific Aim 2 will focus on an in vivo model of LPS-induced ALI, to substantiate our in vitro findings. To do so, we will generate ATF6 null ATF6 endothelial specific knock out mice. The outcomes of our study will enrich our current understanding on endothelial barrier regulation, to provide new targets for the management of diseases related to EBD.