Filling the gaps The endothelium in regulating vascular leakage and leukocyte extravasation

In this thesis, we demonstrate the active role of the endothelium during the process of leukocyte transendothelial migration (TEM). In particular we focused on the role of the actin cytoskeleton, its regulation by the Rho family of small GTPases, and their spatiotemporal activation and deactivation by guanine nucleotide exchange factors (GEFs) and GTPase activing proteins (GAPs) respectively. Although many efforts have been made, we still know relatively little on the true contribution of Rho GTPases in the process of leukocyte TEM. The work presented in this thesis contributes to the elucidation and regulation of some of the most well-studied family members such as RhoA and Rac1, but also shows some more insight in the less investigated family members like RhoB, RhoC and Cdc42. We showed that leukocyte TEM and vascular permeability are two uncoupled events, and endothelial cells play a major role in actively limiting vascular permeability while leukocytes pass. Moreover, we showed how changes in the microenvironment such as increased tissue stiffness affects the endothelial cells and their behavior regarding the regulation of leukocyte TEM. However, future studies into the upstream activation and regulation of the involved GEFs is essential to completely understand the exact signaling pathways and mechanisms. Once the signaling pathways are elucidated, we can use this knowledge to interfere with these processes to develop potential therapies to target vascular diseases in which leukocyte extravasation is the underlying cause.