Defective fibrin deposition and thrombus stability in Bambi^-/- mice are mediated by elevated anticoagulant function.

BACKGROUND:Bone morphogenetic and activin membrane-bound inhibitor (BAMBI) is a transmembrane protein related to the type I transforming growth factor- β (TGF-β) receptor family that is present on both platelets and endothelial cells (ECs). Bambi-deficient mice exhibit reduced hemostatic function and thrombus stability characterized by an increased embolization. OBJECTIVE:We aimed to delineate how BAMBI influences endothelial function and thrombus stability. METHODS:Bambi-deficient mice were subjected to the laser-induced thrombosis model where platelet and fibrin accumulation was evaluated. Expression of thrombomodulin and tissue factor pathway inhibitor (TFPI) was also assessed in these mice. RESULTS:Thrombus instability in Bambi^-/- mice was associated with a profound defect in fibrin deposition. Injection of hirudin into Bambi^+/+ mice prior to thrombus formation recapitulated the Bambi^-/- thrombus instability phenotype. In contrast, hirudin had no additional effect upon thrombus formation in Bambi^-/- mice. Deletion of Bambi in ECs resulted in mice with defective thrombus stability caused by decreased fibrin accumulation. Increased levels of the anticoagulant proteins TFPI and thrombomodulin were detected in Bambi^-/- mouse lung homogenates. Endothelial cells isolated from Bambi^-/- mouse lungs exhibited enhanced ability to activate protein C due to elevated thrombomodulin levels. Blocking thrombomodulin and TFPI in vivo fully restored fibrin accumulation and thrombus stability in Bambi^-/- mice. CONCLUSIONS:We demonstrate that endothelial BAMBI influences fibrin generation and thrombus stability by modulating thrombomodulin and TFPI anticoagulant function of the endothelium; we also highlight the importance of these anticoagulant proteins in the laser-induced thrombosis model.

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