Rap1a activation by CalDAG-GEFI and p38 MAPK is involved in E-selectin-dependent slow leukocyte rolling

Rolling leukocytes are exposed to different adhesion molecules and chemokines. Neutrophils rolling on E-selectin induce integrin α_Lβ₂-mediated slow rolling on ICAM-1 by activating a phospholipase C (PLC)γ2-dependent and a separate PI3Kγ-dependent pathway. E-selectin-signaling cooperates with chemokine signaling to recruit neutrophils into inflamed tissues. However, the distal signaling pathway linking PLCγ2 (Plcg2) to α_Lβ₂-activation is unknown. To identify this pathway, we used different Tat-fusion-mutants and gene-deficient mice in intravital microscopy, autoperfused flow chamber, peritonitis, and biochemical studies. We found that the small GTPase Rap1 is activated following E-selectin engagement and that blocking Rap1a in Pik3cg^-/- mice by a dominant-negative Tat-fusion mutant completely abolished E-selectin-mediated slow rolling. We identified CalDAG-GEFI (Rasgrp2) and p38 MAPK as key signaling intermediates between PLCγ2 and Rap1a. Gα_i-independent leukocyte adhesion to and transmigration through endothelial cells in inflamed postcapillary venules of the cremaster muscle were completely abolished in Rasgrp2^-/- mice. The physiological importance of CalDAG-GEFI in E-selectin-dependent integrin activation is shown by complete inhibition of neutrophil recruitment into the inflamed peritoneal cavity of Rasgrp2^-/- leukocytes treated with pertussis toxin to block Gα_i-signaling. Our data demonstrate that Rap1a activation by p38 MAPK and CalDAG-GEFI is involved in E-selectin-dependent slow rolling and leukocyte recruitment.