Roundabout receptor 4 and its role in endothelial barrier dysfunction following cardiac surgery
Creators: name: Svermova, Tatiana
Publisher: name: Imperial College London
Year: 2018

description: Endothelial barrier dysfunction contributes to the systemic inflammatory response in patients undergoing cardiac surgery with surgery necessitating cardiopulmonary bypass. The bypass enables to perform complicated operations on static heart, but it comes with risks such as a damage to the endothelium which causes inflammation. Activation of endothelial cells by inflammatory mediators leads to changes in endothelial function. These changes include up-regulation of adhesion molecules, production and secretion of inflammatory mediators and chemo-attractant agents, as well as vasoactive compounds. These changes are central to the pathological processes leading to sepsis syndrome. The exaggerated release of pro- and anti-inflammatory cytokines leads to development of septic and vasoplegic syndromes which complicate recovery and can even cause a death. Currently, there are no specific therapies that target the endothelium, but greater insight into the pathogenesis of endothelial barrier dysfunction could lead to new biomarkers and therapies. Roundabout receptor 4 could be a novel biomarker and target for specific pharmacological intervention. Roundabout receptor 4 was originally identified as an axon guidance molecule but is now known to mediate various endothelial processes including cell migration and proliferation. At the outset of this thesis a role for Roundabout receptor 4 and its Slit 2 glycoprotein ligand as regulators of endothelial barrier function were also beginning to emerge; but there was little evidence of how Roundabout receptor 4 expression profiles and function might differ between endothelial cells; or of implications of Roundabout receptor 4 detected in human plasma. In patients (n=48) undergoing cardiac surgery, standard biomarkers of endothelial activation significantly increased 24h after surgery; by contrast, Roundabout receptor 4 increased immediately post-operatively, as did another novel biomarker of endothelial dysfunction - Syndecan-1. Roundabout receptor 4 could be a useful early marker of endothelial dysfunction in this patient group. There were significant associations between Roundabout receptor 4 plasma levels and prolonged length of stay in intensive care and hospital. Higher pre-operative plasma levels of Roundabout receptor 4 were also associated with the development of vasoplegia, a post-operative complication. Vasoplegia is caused by an imbalance of vasodilator and vasopressor mechanisms in the body, but the exact pathophysiology is not known. Vasoplegic patients are treated with high and prolonged doses of vasoconstrictor medications. This treatment can cause peripheral vasoconstriction and ischemia of limbs that can lead to metabolic acidosis and tissue necrosis. In vitro, Robo4 external expression decreased in endothelial cells treated with inflammatory cytokines. Robo4 plasma levels increased after cardiac surgery with snCPB. These results suggest that Robo4 might be cleaved or secreted by endothelial cells exposed to inflammation and released into the bloodstream. Roundabout receptor 4 expression was greater on human umbilical vein endothelial cells than on human pulmonary artery endothelial cells; and decreased following stimulation with tumour necrosis factor-alpha. In parallel, Slit2 increased barrier integrity in human umbilical vein endothelial cells but not in human pulmonary artery endothelial cells; difference could relate to differences in Roundabout receptor 4 expression levels, as Slit2 effects were inhibited with Roundabout receptor 4 short interfering RNA. The finding from these investigations inform greater understanding of the important role of Robo4 as an endothelial biomarker and its role in barrier function.