Thrombin activatable fibrinolysis inhibitor (TAFI) at the interface between coagulation and fibrinolysis

Ultrastructural comparison of the morphology of three different platelet and fibrin fiber preparations

The aim of the current study was to investigate the ultrastructural morphology of three different sources of fibrin networks and platelets, namely, lyophilized human platelet-rich plasma (LPRP), freshly prepared human platelet-rich plasma (FPRP), and human platelet concentrate (HPC). The ultrastructural morphology of the three different fibrin networks was studied using the scanning electron microscope (SEM). Turbidity curves were drawn at 405 nm at room temperature and fibrinogen concentrations were measured. Scanning electron micrographs showed that all clots produced thick major fibrin fibers as well as a well-defined fine fibrin network, which appeared to be a superimposed process that occurred after the major fibrin network was established. These features were decidedly more pronounced in the HPC specimens. Turbidity curves of the three types of plasma showed differences in LPRP and FPRP. Fibrinogen concentrations of all three preparations were in the normal ranges. Because of the great similarity between LPRP, HPC, and FPRP, we suggest that LPRP could be used successfully to study morphological changes in fibrin fibers and platelets, which may occur after exposure to certain therapeutic agents. However, functionality studies such as turbidity curves should concurrently be included. We therefore conclude that from a basic science point of view, LPRP is a valuable research tool and that such results may add information that could be valuable for clinical application.

Curiouser and curiouser: recent advances in measurement of thrombin-activatable fibrinolysis inhibitor (TAFI) and in understanding its molecular genetics, gene regulation, and biological roles

The thrombin-activatable fibrinolysis inhibitor (TAFI) pathway defines a novel molecular connection between blood coagulation and both fibrinolysis and inflammation. TAFI is a plasma zymogen that can be activated by thrombin, the thrombin-thrombomodulin complex, or plasmin. The activated form of TAFI (TAFIa) attenuates fibrinolysis by removing the carboxyl-terminal lysine residues from partially degraded fibrin that mediate positive feedback in the fibrinolytic cascade. A role for TAFIa in modulating inflammation is suggested by the ability of this enzyme to down-regulate pericellular plasminogen activation and to inactivate the inflammatory peptides bradykinin and the anaphylatoxins C3a and C5a. The focus of this review is on recent advances in the clinical measurement of the TAFI pathway in human subjects and what this has revealed in terms of the molecular genetics of TAFI, the biological variation in plasma TAFI antigen levels, potential regulators of expression of the gene encoding TAFI, and the TAFI pathway as a risk factor for the development of vascular diseases. Although this field is in its infancy, much recent progress has been made and the available data suggest that the TAFI pathway is an intriguing new player in a variety of physiological and pathophysiological contexts.

Regulation of fibrinolysis by thrombin activatable fibrinolysis inhibitor, an unstable carboxypeptidase B that unites the pathways of coagulation and fibrinolysis

The coagulation and fibrinolytic systems safeguard the patency of the vasculature and surrounding tissue. Cross regulation of coagulation and fibrinolysis plays an important role in preserving a balanced hemostatic process. Identification of Thrombin Activatable Fibrinolysis Inhibitor (TAFI) as an inhibitor of fibrinolysis and one of the main intermediates between coagulation and fibrinolysis, greatly improved our understanding of cross regulation of coagulation and fibrinolysis. As TAFI is an enzyme that is activated by thrombin generated by the coagulation system, its activation is sensitive to the dynamics of the coagulation system. Defects in coagulation, such as in thrombosis or hemophilia, resonate in TAFI-mediated regulation of fibrinolysis and imply that clinical symptoms of coagulation defects are amplified by unbalanced fibrinolysis. Thrombomodulin promotes the generation of both antithrombotic activated protein C (APC) and prothrombotic (antifibrinolytic) activated TAFI, illustrating the paradoxical effects of thrombomodulin on the regulation of coagulation and fibrinolysis. This review will discuss the role of TAFI in the regulation of fibrinolysis and detail its regulation of activation and its potential therapeutic applications in thrombotic disease and bleeding disorders.

From normal to pathological hemostasis

PURPOSE

To review the evolution of knowledge on physiological hemostasis and the main abnormalities that may interfere with hemostasis in the perioperative period.

METHODS

Narrative review of the literature, including relevant papers published in English.

PRINCIPAL FINDINGS

Physiological hemostasis controls blood fluidity and rapidly induces hemostatic plug formation in order to stop or limit bleeding. The three distinct phases of the hemostatic process, primary hemostasis, coagulation and fibrinolysis are closely linked to each other and precisely regulated in order to efficiently close vessel wounds, promote vascular healing and maintain vessel patency. Primary hemostasis is the result of complex interactions between the vascular wall, platelets and adhesive proteins. Initiation of the coagulation pathway in vivo is secondary to the exposure of tissue factor (TF) and the formation of TF/VIIa complex which can activate both FIX and FX. This initiation phase is followed by a propagation phase with amplification of thrombin generation. Several control mechanisms exist for localizing fibrin formation to the site of injury including tissue factor pathway inhibitor, protein C system, antithrombin, and glycosaminoglycans on the vessel wall. Fibrinolysis is also a highly regulated system that controls fibrin dissolution. Both constitutive and acquired hemostasic defects exist. The consequences of these abnormalities are highly variable according to the type of defect, and to the genetic and environmental background.

CONCLUSION

Hemostasis is one of the most complex physiological self-defence systems, not only involved in control of blood fluidity but also interfering in major physiopathological processes. The evolution of our knowledge of the physiology of hemostasis has numerous implications for therapy.

Coagulation and atherothrombotic disease

Atherothrombotic disease arises secondary to a complex gene-environment interaction. In the initial stages, the condition is clinically silent but with more advanced disease, an occlusive thrombus is formed resulting in the classical clinical manifestations. Both environmental factors and genetic variations in elements of the clotting cascade influence thrombosis risk by inducing quantitative and qualitative changes in the mature protein, which may affect the final structure of the clot and determine its resistance to lysis. Understanding the fine details of gene-environment interactions in relation to thrombus formation will help to shed more light on disease pathogenesis. Consequently, this will allow the development of more efficacious treatment strategies and will also help to identify subjects at risk, thereby enabling the introduction of early preventative measures.