Principal Investigator: Dick Rijken Ph.D.  

Introduction
Hemostasis can be distinguished in primary hemostasis (platelet plug formation), secondary hemostasis/coagulation (fibrin formation) and fibrinolysis (fibrin degradation). Dysfunction of one of these processes may lead to thrombosis, both in the venous system and in the arterial system. Fibrin is a central component in haemostasis. Molecular mechanisms of fibrin formation and fibrin degradation are studied by a biochemical (including proteomics) approach.

Goal
To study the role of fibrin formation and fibrin degradation in the pathogenesis of thrombosis.

Fibrin clot structure
Fibrin is formed by the action of thrombin on fibrinogen. The insoluble protein has a structural function in the maintenance of a blood clot. Fibrin has also a biological function because it binds a large number of proteins, which regulate fibrin formation and degradation as well as the interaction with cells and matrix.
The composition of fibrin clots is studied by using a proteomics approach. Mass spectrometry is performed at the Erasmsus Center for Biomics. These studies will provide information about known and still unknown fibrin-binding proteins as well as about fibrin(ogen) itself, in particular about its genetic and non-genetic heterogeneity. Potential mechanisms are studied in a variety of in vitro models and haemostatic tests. Pathophysiological relevance for thrombosis is evaluated by studies in different patient populations.

Fibrin degradation
Fibrin has only a temporary function and is enzymatically dissolved by the fibrinolytic system. This system is composed of proenzymes, enzymes and inhibitors (see Figure). The ultimate step is the proteolytic degradation of fibrin into soluble fibrin degradation products by plasmin.
Specific and global tests are being developed and applied in blood samples of patients to further elucidate the role of fibrinolysis in the pathogenesis of thrombosis.
Patients with thrombosis may be treated by stimulating the fibrinolytic system by infusion of plasminogen activators (thrombolytic therapy). Several in vitro models are used to further develop this kind of therapy. Ultrasound may be applied to accelerate fibrinolysis induced by plasminogen activators.
Thrombin-activatable fibrinolysis inhibitor (TAFI) is a relatively new plasma inhibitor of the fibrinolytic system. Specific assays for TAFI are developed and evaluated. Molecular mechanisms of TAFI are studied in a variety of in vitro models for clot lysis, using amongst others confocal microscopy. This will further elucidate TAFI’s role in arterial and venous thrombosis.
a2-Antiplasmin is the primary inhibitor of plasmin, the final enzyme of the fibrinolytic cascade. Recent studies have shown that a2-antiplasmin in plasma occurs in two molecular forms: native a2-antiplasmin with Met at the N-terminal end of the molecule and a slightly degraded form with Asn at the N-terminus. Future studies will indicate whether this heterogeneity is related to the occurrence of arterial and venous thrombosis.

Figure 1. The fibrinolytic system is triggered by tissue-type plasminogen activator (t-PA) or urokinase-type plasminogen activator (u-PA). The system is inhibited by plasminogen activator inhibitor-1 (PAI-1) as well as by a2-antiplasmin (a2-AP) and a2-macroglobulin (a2-M). Thrombin-activatable fibrinolysis inhibitor (TAFI) inhibits the system on the fibrin level and eventually decreases the conversion of fibrin into soluble fibrin degradation products (FDP).