Select Rab GTPases Regulate the Pulmonary Endothelium via Endosomal Trafficking of VE-cadherin

Pulmonary edema occurs in settings of acute lung injury, in diseases such as pneumonia, and acute respiratory distress syndrome (ARDS). The lung interendothelial junctions (IEJ) are maintained in part by VE-cadherin, an adherens junction (AJ) protein, and its surface expression is regulated by endocytic trafficking. The Rab family of small GTPases are regulators of endocytic trafficking. The key trafficking pathways are regulated by Rab4, 7 and 9. Rab4 regulates the recycling of endosomes to the cell surface through a rapid-shuttle process whilst Rab7 and 9 regulate trafficking to the late endosome/lysosome for degradation or from the trans-Golgi network to the late endosome, respectively. We recently demonstrated a role for the endosomal adaptor protein, p18, in regulation of the pulmonary endothelium through enhanced recycling of VE-cadherin to AJ. Thus, we hypothesized that Rab4, 7 and 9 regulate pulmonary endothelial barrier function through modulating trafficking of VE-cadherin-positive endosomes. We used Rab mutants with varying activities and associations to the endosome, to study endothelial barrier function in vitro and in vivo. Our study demonstrates a key role for Rab4 activation and Rab9 inhibition in regulation of vascular permeability through enhanced VE-cadherin expression at the IEJ. We further showed that endothelial barrier function mediated through Rab4 is dependent on ERK phosphorylation and activity. Thus we demonstrate that Rab4 and 9 regulate VE-cadherin levels at the cell surface to modulate the pulmonary endothelium through ERK-dependent and independent pathways, respectively. We propose that regulating select Rab GTPases represents novel therapeutic strategies for patients suffering with ARDS.