fibrin and Bernard-Soulier-Syndrome

Bernard-Soulier syndrome (BSS) is a rare autosomal bleeding disorder characterized clinically by prolonged skin bleeding time, normal clot retraction and thrombocytopenia with large and morphologically abnormal platelets, and biochemically by the absence of platelet membrane glycoproteins (GP) Ib, V and IX. GP Ib and GP IX exist in the platelet membrane as a heterodimer complex which acts as the major receptor mediating platelet adhesion to blood vessel subendothelium. Studies with BSS platelets have proved particularly rewarding in the investigation of the GP Ib-IX complex as a multifunctional receptor protein. The transmembrane complex contains binding domains for von Willebrand factor, thrombin, fibrin and quinine/quinidine drug-dependent antibodies as well as an attachment site on the cytoplasmic side of the membrane for a platelet cytoskeleton. In addition, the internal segment of the beta-chain of GP Ib contains a cyclic AMP-dependent protein kinase-associated phosphorylation site which appears to regulate platelet reactivity. Limited proteolytic cleavage of the complex, in particular the GP Ib alpha-chain, has allowed immunological and functional characterization of three distinct domains; a 45 kDa segment at the N-terminal end of the alpha-chain of GP Ib, which contains binding sites for von Willebrand factor and thrombin, a 90 kDa highly glycosylated region of GP Ib alpha and a membrane-associated region consisting of the remnant of GP Ib alpha disulphide-linked to GP Ib beta and non-covalently-complexed with GP IX. This membrane-associated region contains the antigenic epitope(s) for quinine/quinidine drug-dependent antibodies. It is highly probable that the future study of platelets from patients with the Bernard-Soulier syndrome will further clarify the role of the GP Ib-IX complex in platelet physiology.

Topics: Bernard-Soulier Syndrome; Blood Platelet Disorders; Blood Platelets; Cell Membrane; Fibrin; Humans; Platelet Adhesiveness; Platelet Membrane Glycoproteins; Receptors, Cell Surface; Receptors, Thrombin

Defective prothrombin consumption has been reported in the proband case of Bernard-Soulier syndrome (BSS). There is no consensus, however, on whether the formation of platelet procoagulant activity (PPA) is impaired in BSS and, if so, whether this is due to the lack of GPIb-V-IX-dependent binding of thrombin or of von Willebrand factor (VWF). We show thrombin generation (TG) in platelet-rich plasma of BSS (BSS-PRP) to be defective provided that fibrin remains present in the reaction mixture and that the giant platelets are not damaged by frequent subsampling. In BSS-PRP addition of (thrombin-free) fibrin did not increase TG as in normal PRP, supporting our previous hypothesis that the interaction of fibrin, VWF and GPIb triggers PPA development. Fibrin formed during the lag phase of TG by a snake venom enzyme which only removed fibrinopeptide A induced an immediate burst of TG, that was inhibited by a monoclonal antibody against GPIb (6D1) that abolishes ristocetin-induced binding of VWF to platelets. Inversely, inhibition of polymerization decreased TG and the residual activity was insensitive to 6D1. We conclude that polymerizing fibrin interacts with VWF so as to activate GPIb.

Topics: Bernard-Soulier Syndrome; Biopolymers; Blood Platelets; Female; Fibrin; Humans; In Vitro Techniques; Middle Aged; Models, Biological; Platelet Glycoprotein GPIb-IX Complex; Thrombin; von Willebrand Factor

Topics: Bernard-Soulier Syndrome; Factor VIIa; Fibrin; Hemostatics; Humans; Platelet Activation; Recombinant Proteins

Platelet adhesion to fibrin at high shear rates depends on both the glycoprotein (GP) IIb:IIIa complex and a secondary interaction between GPIb and von Willebrand factor (vWF). This alternative link between platelets and vWF in promoting platelet adhesion to fibrin has been examined in flowing whole blood with a rectangular perfusion chamber. Optimal adhesion required both platelets and vWF, as shown by the following observations. No binding of vWF could be detected when plasma was perfused over a fibrin surface or when coated fibrinogen was incubated with control plasma in an enzyme-linked immunosorbent assay. However, when platelets were present during perfusion, interactions between vWF and fibrin could be visualized with immunoelectron microscopy. Exposure of fibrin surfaces to normal plasma before perfusion with severe von Willebrand's disease blood did not compensate for the presence of plasma vWF necessary for adhesion. vWF mutants in which the GPIIb:IIIa binding site was mutated or the GPIb binding site was deleted showed that vWF only interacts with GPIb on platelets in supporting adhesion to fibrin and not with GPIIb:IIIa. Complementary results were obtained with specific monoclonal antibodies against vWF. Thus, vWF must first bind to platelets before it can interact with fibrin and promote platelet adhesion. Furthermore, only GPIb, but not GPIIb:IIIa is directly involved in this interaction of vWF with platelets.

Topics: Bernard-Soulier Syndrome; Binding Sites; Blood Flow Velocity; Blood Platelets; Fibrin; Humans; In Vitro Techniques; Microscopy, Immunoelectron; Mutation; Perfusion; Platelet Adhesiveness; Platelet Glycoprotein GPIIb-IIIa Complex; Thrombasthenia; von Willebrand Diseases; von Willebrand Factor

We have found that glycoprotein IIb:IIIa (GPIIb:IIIa) expressed on nonstimulated platelets is the primary receptor for platelet adhesion to immobilized fibrinogen or fibrin. At low shear rates of the blood the interaction between GPIIb:IIIa and fibrin(ogen) is strong enough to resist the shear forces exerted on the platelet as was shown with experiments with antibodies against platelet membrane glycoproteins and perfusion studies with blood from patients lacking platelet membrane receptors. Impaired platelet adhesion to fibrin(ogen) was found with blood from a patient with Glanzmann's thrombasthenia (lacking GPIIb:IIIa), blood from patients with the Bernard-Soulier syndrome (lacking GPIb) and blood from patients with severe von Willebrand's disease. This indicates that at higher shear rates additional interactions via GPIb on the platelet and von Willebrand factor originating from plasma or platelets are necessary to increase the affinity of the platelet for fibrin(ogen).

Topics: Bernard-Soulier Syndrome; Fibrin; Fibrinogen; Humans; Platelet Adhesiveness; Platelet Membrane Glycoproteins; von Willebrand Diseases