Endothelial cells (ECs) line all blood vessels and dynamically regulate the endothelial barrier function, which is critical for proper functioning of the vasculature and human health. ECs continuously sense and respond to various stimuli that can either strengthen or disrupt the endothelium, depending on the output of large and complex signaling networks that become activated. Activation of these respective networks often include regulation via GPCRs, one of the major groups of cell surface receptors that respond to a variety of stimuli and of which ECs express many different subtypes. In general, GPCRs initiate Rho GTPase signaling cascades, thereby acting on the actin cytoskeleton and subsequently regulating EC-cell contacts. Since the latter is a direct measure of permeability and endothelial barrier function, these signaling mechanisms are indispensable for a healthy endothelium and vasculature. Despite the interest regarding GPCR-Rho GTPase-mediated signaling networks in the endothelium, critical, molecualr details are lacking. Therefore, this thesis aims to reveal novel molecular players in GPCR-Rho GTPase signaling networks that control the endothelial barrier function. In short, we revealed novel regulators and dynamic signaling networks that control the endothelial barrier function via GPCR-Rho GTPase signaling. We specified major regulators at the level of receptors (e.g. PAR-1 and S1PRs), GEFs (e.g. pRex1) and Rho GTPases (e.g. Rac1, Cdc42 and Rho) and furthermore highlighted the dynamics of the endothelium. Thereby, our research has provided new, molecular insights into the physiology of the endothelium and endothelial barrier control, and furthermore identifies novel therapeutic targets in vascular, endothelialbased pathologies
