The antiphospholipid syndrome (APS) is an autoimmune disorder characterized by the presence of circulating antiphospholipid antibodies (aPL) and recurrent thrombosis. A link between APS and a greater risk of cardiovascular diseases such as myocardial infarction and strokes has also been reported. A pathogenetically important subset of aPL is directed against β2-glycoprotein I (β2GPI). Binding of aPL to phospholipid-bound β2GPI causes its dimerization, which further increases its affinity for negatively charged phospholipids and cell surfaces. The endothelium is a primary target of aPL, and binding of the autoantibodies to β2GPI on the surface of the endothelial cells causes the upregulation of adhesion molecule expression and a proinflammatory and prothrombotic endothelial cell phenotype. Mechanisms by which aPL binding to β2GPI on the endothelial cell surface induces a transmembrane signal to modify endothelial cell behavior are not well understood. We have recently found that some of these phenotypes are caused by the inhibition of endothelial nitric oxide synthase (eNOS). Nitric oxide is a key determinant of vascular health that regulates several physiological processes including leukocyte adhesion, thrombosis, endothelial cell migration and proliferation. In both cultured endothelial cells and in mouse models, we showed that aPL antagonize eNOS and promote thrombosis and monocyte/leukocyte adhesion to the endothelium.
The lecture will focus on our recent findings that the endothelial dysfunction induced by aPL is mediated by the endothelial cell surface apolipoprotein E receptor 2 (apoER2). ApoER2 (also known as LRP8) is a member of LDL receptor family and it plays a critical role in brain development as a signal transducer for the glycoprotein Reelin. Previous in vitro studies demonstrated that dimerized β2GPI binds to multiple members of the LDL receptor family in purified form. Recognizing that apoER2 is an abundant plasma membrane receptor in endothelial cells and that it interacts with dimerized β2GPI, we tested the requirement for apoER2 in aPL-induced eNOS antagonism. In cultured endothelial cells, knockdown of apoER2 expression by small interference RNA fully prevented both eNOS antagonism and the enhancement of adhesion caused by aPL. Studies with a soluble peptide based on the sequence of the first LDL-binding domain of apoER2’ (BD1) that binds to β2GPI further demonstrated that the eNOS antagonism caused by antibody recognition of β2GPI requires interaction between domain V of β2GPI and BD1 of the receptor. Based on these observations in cell culture, we evaluated the role of apoER2 in aPL actions in vivo in a series of experiments in apoER2+/+ and apoER2-/- mice. ApoER2+/+ or apoER2-/- mice were treated with normal human IgG or aPL, and 24 hours later endothelial cell-leukocyte adhesion or thrombus formation was evaluated using intravital microscopy. In apoER2+/+ mice, aPL increased leukocyte adhesion to endothelium resulting in a decrease in leukocyte velocity. In contrast, aPL had no effect on leukocyte-endothelial cell adhesion in apoER2-/- mice. Similarly, in apoER2+/+ mice, aPL enhanced thrombus formation and thereby decreased the time to total vessel occlusion. In contrast, aPL did not alter thrombus formation in apoER2-/- mice. These cumulative observations indicate that aPL recognition of β2GPI causes eNOS antagonism, that this process is mediated by apoER2, and that it underlies the vascular disease phenotypes of APS.
Disclosure of Interest None Declared