The arterial wall is the target of plasma-borne components like LDL and proteolytic enzymes known to be involved in the evolution of the atherothrombotic disease. Vascular cells must be able to protect themselves from proteolytic injuries by producing antiproteases. Serine protease inhibitors (serpins) form a large family of structurally related proteins present in the plasma or in tissues and play a central role in the regulation of protease activity. Among them, serpinE2 or protease nexin-1 (PN-1) which is produced by most cell types, including vascular and inflammatory cells, is often found overexpressed at sites of tissue injury. PN-1 emerged as an important actor in the regulation of tissue proteolytic degradation since it is a powerful inhibitor of several serine proteases including thrombin, plasminogen activators (uPA, tPA) and plasmin. Many of these target serine proteases are known to be involved in thrombus formation and degradation, in matrix degradation and in cell loss. We have indeed demonstrated that PN-1 constitutes a key factor in the responses of vessels to injury, via its antithrombotic and antifibrinolytic properties. The present project is aimed at demonstrating the protective role of PN-1 in vascular tissue, in particular the arterial wall and the “neo-tissue” that is the thrombus. We want to demonstrate that the presence of PN-1 in either the arterial wall or the thrombus represents a mechanism of tissue defense against aggression by blood-borne proteases. PN-1 avidly binds glycosaminoglycans (GAGs) such as heparan sulfates which potentiate its activity, target it to the pericellular space, and impede its diffusion, proposing PN-1 as a model of tissue serpin, able to bind serine proteases and form complexes which can be, in situ, removed and degraded by endocytosis via the LRP1 scavenger receptor. In tissue, the pericellular activated serine proteases bind to serpin, forming complexes that are cleared by LRP1. This mechanism of protease-antiprotease clearance and its consequences have not yet been extensively explored in human arterial wall in vivo and particularly in VSMCs. The potential close relationships between PN-1/protease complexes & LRP1 in VSMCs underline a possible function of these two proteins in the control of protease activities in the vascular wall. We hypothesize that PN-1/protease complexes and LRP1 interact in VSMCs to eliminate deleterious proteases, participating in maintaining the homeostatic function of the vascular wall, and that this physiological clearance function of VSMCs could be overwhelmed in vascular pathology. When thrombosis affects the arterial cerebral bed, it is the most frequent cause of stroke. Interestingly, PN-1 is not only present in platelet, but has also been identified in the central nervous system as a regulator of thrombin effects on nervous cells. We demonstrated that platelet PN-1 present in blood clot contributes to clot resistance to fibrinolysis by its ability to inhibit plasminergic enzymes. However this latter property does not seem to be the only reason of such an effect of PN-1. Indeed, we observed a direct influence of PN-1 on blood clot structure and retraction. Our objective will consist to decipher at the molecular level by which mechanism PN-1 interferes on clot structure and retraction. Moreover, the characterization of PN-1 impact in blood clots opens new therapeutic possibilities for the thrombolytic treatment of stroke. Only few teams in the world are working on PN-1 and LRP1 in physiology and pathology. The Inserm U698 is leader in the field of PN-1 in the vascular system and has provided clear-cut evidence for a relevant role of PN-1 in vascular biology. In parallel, the team of Philippe Boucher is leader in the field of LRP1 functions in VSMC. The novelty of our project is to highlight new aspects of PN-1-dependent processes in vascular wall physiological protection and the role of PN-1 in the thrombus resistance to proteolysis.