Experimental study of fibrin/fibrin-specific molecular interactions using a sphere/plane adhesion model

Fibrin, the biopolymer produced in the final step of the coagulation cascade, is involved in the resistance of arterial thrombi to fragmentation under shearflow. However, the nature and strength of specific interactions between fibrin monomers are unknown. Thus, the shear-induced detachment of spherical monodispersed fibrincoated latex particles in adhesive contact with a plane fibrin-coated glass surface has been experimentally studied, using an especially designed shear stress flow chamber. A complete series of experiments for measuring the shear stress necessary to release individual particles under various conditions (various number of fibrin layers involved in the adhesive contact, absence or presence of plasmin, the main physiological fibrinolytic enzyme) has been performed. The nonspecific DLVO interactions have been shown to be negligible compared to the interactions between fibrin monomers.Asimple adhesion model based on the balance of forces and torque on particles, assuming an elastic behavior of the fibrin polymer bonds, to analyze the experimental data in terms of elastic force at rupture of an elementary intermonomeric fibrin bond has been used. The results suggested that this force (of order 400 pN) is an intrinsic quantity, independent of the number of fibrin layers involved in the adhesive contact

Structural basis for the pathophysiology of lipoprotein(a) in the athero-thrombotic process

Lipoprotein Lp(a) is a major and independent genetic risk factor for atherosclerosis and cardiovascular disease. The essential difference between Lp(a) and low density lipoproteins (LDL) is apolipoprotein apo(a), a glycoprotein structurally similar to plasminogen, the precursor of plasmin, the fibrinolytic enzyme. This structural homology endows Lp(a) with the capacity to bind to fibrin and to membrane proteins of endothelial cells and monocytes, and thereby to inhibit plasminogen binding and plasmin generation. The inhibition of plasmin generation and the accumulation of Lp(a) on the surface of fibrin and cell membranes favor fibrin and cholesterol deposition at sites of vascular injury. Moreover, insufficient activation of TGF-ß due to low plasmin activity may result in migration and proliferation of smooth muscle cells into the vascular intima. These mechanisms may constitute the basis of the athero-thrombogenic mode of action of Lp(a). It is currently accepted that this effect of Lp(a) is linked to its concentration in plasma. An inverse relationship between Lp(a) concentration and apo(a) isoform size, which is under genetic control, has been documented. Recently, it has been shown that inhibition of plasminogen binding to fibrin by apo(a) is also inversely associated with isoform size. Specific point mutations may also affect the lysine-binding function of apo(a). These results support the existence of functional heterogeneity in apolipoprotein(a) isoforms and suggest that the predictive value of Lp(a) as a risk factor for vascular occlusive disease would depend on the relative concentration of the isoform with the highest affinity for fibri

Ischémie cérébrale : les microparticules neurovasculaires, une alternative aux marqueurs biologiques sanguins

L’intérêt clinique des biomarqueurs est de pouvoir identifier des individus à risque de développer une maladie afin d’établir des mesures préventives, diagnostiques ou thérapeutiques. Or, nous manquons actuellement d’un test rapide, sensible et pratique pour le diagnostic de l’ictus ischémique aigu. Un nombre important de molécules circulantes ont été associées aux accidents vasculaires cérébraux (AVC) ischémiques, sans qu’aucun consensus n’établisse leur utilité. En effet, ces molécules ne sont pas spécifiques du complexe neurovasculaire altéré dans l’AVC et pourraient également témoigner d’autres pathologies vasculaires. Même l’association de certains de ces marqueurs biologiques (métalloprotéinase matricielle 9, peptide natriurétique de type B, D-dimères, protéine S100B…) n’a pas permis d’obtenir des informations diagnostiques et pronostiques supplémentaires. Récemment, un nouveau type de biomarqueur, les microparticules, fragments cellulaires émis par des cellules en souffrance, s’avère être potentiellement intéressant et suffisamment robuste. Les différents déterminants antigéniques et les effecteurs moléculaires portés par ces microparticules qui, par leur origine, sont spécifiques de la cellule activée ou lésée, pourraient permettre l’identification précoce du tissu affecté. Ces microparticules pourraient être détectées non seulement dans le liquide céphalorachidien, mais aussi dans les larmes ou dans la circulation sanguine en cas de dysfonctionnement de la barrière hématoencéphalique. Il est donc essentiel d’évaluer leur rôle de marqueur biologique dans la prévention, le diagnostic et le suivi thérapeutique des accidents vasculaires cérébraux ischémiques

Membrane microvesicles: a circulating source for fibrinolysis, new antithrombotic messengers.

International audienceThrombus lysis is the consequence of a restricted number of reactions localised on the surface of fibrin and cell membranes. A functional defect or an insufficient fibrinolytic response may result in thrombosis with severe or fatal clinical consequences. Despite this clinical exigency and a real progress in the knowledge of the different components of this system (plasminogen activators, inhibitors and receptors) including structure-function relationship unveiled by the crystal structure of plasminogen, the functional evaluation of fibrinolysis still remains a challenge in haemostasis. Interestingly, we recently discovered that circulating membrane microvesicles might be indicators of the fibrinolytic response to an inflammatory or prothrombotic process via their participation in a new mechanism of plasmin formation requiring a cross-talk between two different surfaces. We propose that the fibrinolytic activity conveyed by microvesicles could be the real source of fibrinolysis in circulating blood

Fibrinolysis, new concepts: fibrinolytic microvesicles and cross-talk Fibrinolyse, nouveaux concepts : microvésicules et cross-talk fibrinolytiques

International audienceThrombus lysis is the consequence of a restricted number of reactions localised to the surface of fibrin. A functional defect or an insufficient fibrinolytic response may lead to thrombosis with severe or fatal clinical consequences, e.g. myocardial infarction and ischemic stroke. Despite this clinical exigency and a real progress in the knowledge of the different components of this system (plasminogen and its activators, inhibitors and receptors), its functional evaluation still remains a challenge in haemostasis. The absolute requirement of a template for molecular assembly of plasminogen and its activators (tissue- and urokinase-type plasminogen activators: tPA and uPA) restricts the formation of plasmin and protection of its activity to the surface of, respectively, fibrin and cells. In contrast, plasmin and tPA released from the clot during its lysis are immediately neutralised by their respective inhibitors α2-antiplasmin and plasminogen activator inhibitor 1, PAI-1). It seems therefore almost impossible to detect fibrinolytic activity in plasma with methods currently in use. Because of its unavailability, it is also impossible to measure the degree of fibrinolysis directly on the clot. Notwithstanding, it was recently discovered that circulating membrane microvesicles might be indicators of the fibrinolytic response to an inflammatory or prothrombotic process. These cell-derived fibrinolytic microvesicles bear at their membrane the plasminogen activators expressed by the parent cell: tPA from endothelial cells and uPA from leukocytes. These molecules are localised at the membrane surface and have the capacity to activate plasminogen into plasmin in situ. Moreover, it was recently discovered that these microvesicles might participate in a new mechanism of plasmin formation requiring a cross-talk between two different surfaces. In this fibrinolytic cross-talk one of the surfaces bear plasminogen (fibrin, extracellular matrix or platelets) whereas the other surface carry the plasminogen activator, typically leukocyte-derived microvesicles bearing uPA. These new actors and concepts in plasminogen activation represent hitherto unknown pathways in our comprehension of fibrinolysis and potential novel biomarkers in clinical practice