The role of the GEF Trio in ICAM-1-mediated leukocyte transendothelial migration

During inflammation, leukocytes transmigrate across the endothelium towards the site of injury. This process of extravasation requires integrin-mediated adhesion to endothelial adhesion receptors such as ICAM-1. Upon binding and clustering of ICAM-1, the small GTPases RhoA and Rac1 become activated, which induce endothelial cell shape changes to allow passage of transmigrating leukocytes. In addition, we demonstrated that another Rho family GTPase, RhoG, becomes activated after ICAM-1 engagement, controlling the cytoskeletal remodelling that is triggered downstream of ICAM-1 engagement. However, the signals leading to the activation of these small GTPases remain largely unclear.

The guanine nucleotide exchange factor (GEF) Trio is able to activate RhoG, Rac1 and RhoA. We show here that Trio is expressed in endothelial cells and localized to F-actin stress fibers and cell borders. Moreover, the inflammatory stimuli TNFα, IL1β and LPS but not INFγ increase the mRNA and protein expression of Trio (~3-fold) in endothelial cells, but not in epithelial cells. A GFP-fusion protein of full-length Trio expressed in HeLa cells co-localized with ICAM-1-mCherry in lateral and apical membrane ruffles and is recruited to sites of ICAM-1 engagement.
In addition, we show that full-length Trio is able to interact with a peptide encoding the intracellular tail of ICAM-1. Finally, using a GST-tagged nucleotide-free mutant of RhoG (G15A), we demonstrate that ICAM-1 clustering activates endogenous Trio in endothelial cells.
Current studies are focused on the role of endothelial Trio in the adhesion to and migration across the endothelium of leukocytes. Thus, ICAM-1-mediated activation the GEF Trio may a key event in ICAM-1-mediated endothelial signalling through a set of Rho-like GTPases, which is critical for the engagement of ICAM-1 as well as for efficient leukocyte transendothelial migration.

No relevant conflicts of interest declared.