Endothelial topography, phosphatidylserine, and procoagulant activity

点击翻译按钮获取中文摘要

Blood coagulation enzymes function efficiently only on membranes containing phosphatidylserine. However, phosphatidylserine is not ordinarily available because blood cells sequester it on the interior of cell membranes. Blood platelets respond to collagen in injured tissue, exposing abundant phosphatidylserine on bleb-like protrusions of the membrane and these platelets have been called “procoagulant platelets” and are thought to be essential for prevention of bleeding. However, recent clinical information from patients with platelet phosphatidylserine-exposing defects indicate that these patients have only mild-moderate bleeding from mucous membranes. This raises the question as to whether platelet phosphatidylserine exposure is, indeed, a critical component of blood coagulation. Preliminary studies from our laboratory have identified several factors that may help to explain the apparent contradiction. First, blood coagulation complexes recognize convex membrane curvature in addition to phosphatidylserine content. A membrane with protrusions and invaginations may have coagulation complexes highly localized to the convex protrusions. Second, platelets and endothelial cells have modes of limited membrane phosphatidylserine exposure. In these modes, phosphatidylserine exposure is below the threshold of detection for most phosphatidylserine assays. Thus, low level phosphatidylserine exposure is present and can support coagulation complexes, yet goes undetected. Third, phosphatidylserine-rich membranes also support anticoagulant proteins to a degree that can suppress or eliminate the procoagulant potential. The net anticoagulant effect, like procoagulant support, is dependent on both phosphatidylserine content and on membrane curvature. These insights, and the methods used to gain them, give us the unique opportunity to study the manner in which platelet and endothelial cell phosphatidylserine exposure localizes procoagulant enzyme activity. We have hypothesized that blood anticoagulants ordinarily suppress the procoagulant potential of stimulated platelets with phosphatidylserine-rich blebs. The platelets gain true procoagulant activity in environments where anticoagulants are attenuated or bypassed by proteins in the micro-environment. This proposal will focus on gaining insight into the anticoagulant proteins that ordinarily suppress blood coagulation reactions on procoagulant complexes, particularly in regard to phosphatidylserine exposure and membrane curvature. We will next probe the extent to which the anticoagulants effect is attenuated or bypassed by the effects of plasmin in the context of mucous membranes. In addition, we will study the manner in which conditioned endothelial support limited blood coagulation reactions on focal, highly complex membrane projections. We will also evaluate the extent to which the endothelial procoagulant activity may be amplified by adherent platelets and whether the endothelial generated factor Xa bypasses anticoagulant activity on platelet blebs. The platelet studies will provide insights relevant to treating surgical and traumatic bleeding, relevant to hemostasis for hemophilia patients. The endothelial studies are relevant to the coagulopathy known as disseminated intravascular coagulation and relevant to the vascular injury and inflammation of COVID-19.