Host EPAC1 modulates rickettsial adhesion to vascular endothelial cells via regulation of ANXA2 Y23 phosphorylation

Recently we have identified that endothelial surface annexin A2 (ANAX2) functions as a receptor for spotted fever group rickettsial adhesin outer membrane protein B (OmpB), which binds to the endothelial cell (EC) surface. Moreover, we reported that intracellular cAMP receptor EPAC1 modulates ANXA2 tyrosine (Y) 23 phosphorylation, and inactivation of EPAC1 suppresses ANXA2 expression on the EC luminal surface by downregulating Y23 phosphorylation. Since we reported that EPAC1 plays a critical role in the initial step to successfully establish rickettsial infection of ECs, this work aims to answer the following: (a) What is the mechanism underlying the regulatory role of EPAC1 in ECs during the initial step of bacterial infection? (b) Is the EPAC1-ANXA2 signaling pathway involved in the regulation of rickettsial adhesion to ECs? In the present study, an established system that is anatomically-based and quantifies bacterial adhesion to ECs in vivo was combined with novel fluidic force microscopy (FluidFM) to dissect the functional role of the EPAC1-ANXA2 signaling pathway in rickettsiae-EC adhesion. We reveal that the deletion of the EPAC1 gene impedes rickettsial binding to endothelium in vivo. In addition, single living brain microvascular EC study that employs FluidFM and site-directed mutagenesis provides evidence that supports our finding that EPAC1 governs rickettsial adhesion to EC surfaces via regulation of ANXA2 Y23 phosphorylation.