Select Rab GTPases Mediate Pulmonary Endothelial Barrier

Abstract

Vascular permeability, a major hallmark of acute lung Injury (ALI), is regulated in part by VE‐cadherin (VE‐cad) forming adherens junctions at the interendothelial junctions (IEJ). We have recently demonstrated that the endosomal protein, p18, binds to the early endosome to enhance recycling of VE‐cad to IEJ, improve endothelial (EC) barrier function and attenuate pulmonary edema formation. The endosome system is a dynamic process which sorts cargo and directs it either back to the plasma membrane, through Rab4‐mediated recycling or to the lysosome through Rab7 and Rab9. We thus hypothesize that Rab GTPases regulates EC barrier function through modulation of VE‐cad trafficking to and from the IEJ. Lung EC were transfected with wild type Rab4 (Rab4wt), GTP‐locked endosome‐anchored Rab4 (Rab4Q67L), GDP‐locked non‐endosome binding Rabs, Rab4S22N, Rab7T22N and Rab9S21N or GFP as control. The effects of Rab4, Rab7 and Rab9 on EC endocytosis of VE‐cad, in vitro EC barrier function and in vivo pulmonary edema formation were examined. We show that activation of pro‐recycling Rab4wt and Rab4Q67L or inhibition of pro‐degradative Rab9S21N protects EC barrier function in settings of ALI in vitro and in vivo in settings of LPS‐induced injury. In contrast, Rab4S22N or pro‐lysosome forming Rab7T22N play no protective role on EC barrier integrity. We further show enhanced levels of VE‐cad at the plasma membrane concomitant with improved EC barrier function. We conclude that Rab4 and Rab9 regulate pulmonary EC barrier function by enhancing recycling of VE‐cad+ endosomes to the IEJ. We propose that targeting Rab GTPase‐mediated endocytic trafficking represents a novel therapy in the treatment of ALI.