PROGRAM INTRODUCTION SUMMARY: This is the re-submission of a new Program Project (HL146373-01) that we have re-named “Studies of Physiologic and Pathologic Platelet Plug Formation” to more accurately reflect the topics the Program Project addresses. In addition to forming hemostatic plugs at sites of vascular injury, platelets make important contributions to processes such as inflammation, tissue regeneration, host defense, angiogenesis, lymphatic development, and tumor metastasis. Pathologic platelet thrombi are also responsible for much of the morbidity and mortality of arterial vascular disease. There remain large gaps in our understanding of physiologic and pathologic platelet function. Building upon our collective scientific accomplishments, we address these gaps using the cell-biologic, structural-biologic, and computational-biologic methods we have developed. The Program Project we propose consists of four projects and one administrative core unit. All of the projects are based at the Perelman School of Medicine of the University of Pennsylvania. Three projects are based in the Hematology-Oncology Division of the Department of Medicine; the fourth is based in the Division of Hematology of the Department of Pediatrics at The Children's Hospital of Philadelphia. Project 1, re-named “Novel Roles for Phosphoinositide Signaling in α-Granule Biogenesis”, is based on the hypotheses that phosphoinositide synthesis is an essential step in the loading of α-granules with components synthesized in the Golgi and that megakaryocyte phosphoinositides play a previously unrecognized role in the development of congenital megakaryocyte disorders. The objectives of Project 2, entitled “Platelet Integrin Structure and Function”, are to use novel computational and experimental techniques to compare the behavior of αIIbβ3 with that of the other integrins and to identify and quantify the protein-protein interactions responsible for αIIbβ3-mediated fibrin clot contraction. Project 3 is entitled “A Systems Approach to Hemostasis and Thrombosis“. The goals of the studies proposed in this Project are to extend past analyses of platelet thrombus formation and structure from the microvasculature to the macrovasculature, from mice to humans, and from hemostasis to thrombosis. Project 4, entitled “Platelet Factor 4 and Heparin in NETosis and Sepsis”, will test the hypothesis that NETs, neutrophil extracellular traps composed of chromatin released by neutrophils, require partial digestion and release of DNA and histones to be toxic during sepsis. Because infused platelet factor 4, as well as the monoclonal antibody KKO that binds to the complex of platelet factor 4 and heparin, block DNA digestion, both will be protective in sepsis. The four projects are supported by a single core unit that provides for the common administrative needs of the Program.