Platelets in Cardiovascular Biology and Pathology

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It is becoming evident that the importance of platelets in animal physiology far exceeds their role in vascular injury. In this proposal, we will address platelet function in angiogenesis, inflammation and heart disease. In addition, we will investigate the role of the metalloproteinase ADAMTS-13 and the effect of elevated procoagulant activity on vascular function, in particular in the brain. The project will be divided into three independent but thematically related Specific Aims. The first specific aim will develop our preliminary observations that platelets play an important role in angiogenesis by preventing hemorrhage. In collaboration with Project V (Hynes), we will test the hypothesis that platelet adhesion through adhesion receptors is required for angiogenesis. We will visualize platelet interactions with angiogenic vessels by multiphoton microscopy and study angiogenesis in mice mutant in various adhesion molecules. The role of platelet secretion in vessel stabilization will also be addressed. The second specific aim will study the pro-inflammatory functions of platelets and, with Project I (Krieger), the role of platelets in severe heart disease and atherosclerosis. We have observed that the presence of activated platelets in circulation, such as may occur in surgical patients, systemically activates endothelium and increases P-selectin expression. This is reflected in enhanced leukocyte rolling on the vessel wall. The molecular mechanisms of this new inflammatory platelet function will be explored. Knowing the role of platelets in inflammation, we were surprised to observe that in advanced heart disease, such as seen in the mouse models developed in Project I, platelet function had a protective effect on the animals. We hypothesize that platelets are crucial to prevent hemorrhage in the diseased heart, thus preventing excessive fibrosis of the organ. Fibrin deposition in the lesions was drastically reduced in the absence of platelets, indicating that platelets may be required for localized coagulation to occur. We will address this experimentally. The last aim will examine the consequences of pro-coagulant and pro-thrombotic conditions on blood vessel function. We have preliminary results that indicate that high levels of pro-coagulant microparticles in an animal's plasma lead to a breakdown of the endothelial blood-brain barrier. We hypothesize that this involves excessive thrombin generation/fibrin deposition resulting in signaling leading to vascular permeability. The proposed molecular mechanism will be studied. We have also begun analysis of mice lacking ADAMTS-13, a mouse model of thrombotic thrombocytopenic purpura (TTP). We will evaluate the role of this enzyme in platelet adhesion to endothelium and in thrombotic models. Project II will further collaborate with Project III (Lodish) on the role of adiponectin and T-cadherin in thrombosis and atherosclerosis.