Binding of Coagulation Factor XIII Zymogen to Activated Platelet Subpopulations: Roles of Integrin αIIbβ3 and Fibrinogen

Factor XIIIa (fXIIIa) is a transglutaminase that plays a crucial role in fibrin clot stabilization and regulation of fibrinolysis. It is known to bind to procoagulant platelets. In contrast, the zymogen fXIII interaction with platelets is not well characterized. We investigated the interaction of zymogen fXIII with activated platelet subpopulations. Confocal microscopy and flow cytometry using fluorescently labelled factors and antibodies. Phosphatidylserine (PS)-positive activated platelets bound 700 to 800 molecules/cell of fXIII at 100 nM, while both PS-negative activated platelets and resting platelets bound 200 to 400 molecules/cell. The binding was reversible, calcium-independent and linear within the fXIII concentration range of up to 1,000 nM. fXIII predominantly bound to the caps of procoagulant platelets and co-localized with fibrinogen. Exogenous fibrinogen promoted fXIII binding by activated PS-negative platelets; this effect was abolished by the integrin αIIbβ3 antagonist monafram. The fXIII binding was 1.5- to 3-fold decreased for platelets from four patients with grey platelet syndrome, and was variable for platelets from six patients with Glanzmann's thrombasthenia. Strong platelet stimulation, fibrinogen and αIIbβ3 play essential roles in fXIII binding, without any of them fXIII does not bind to platelets. The preferential binding in the cap-like structures might be important for increasing local fXIII concentration in platelet thrombi.

[The effect of "aging" of fibrinogen molecule on the structure and properties of fibrin gel]

The effect of molecular "aging" of fibrinogen stimulated by preincubation in solution on the fibrin three-dimensional architecture, its ability to crosslink fibrin-stabilizing factor, and the sensitivity of fibrin gel to plasmin hydrolysis have been studied. The method of elastic light scattering was used to demonstrate that fibrin generated from "defective" fibrinogen had a coarser structure with a higher mean mass-length ratio of polymeric fibers compared to native fibrinogen (2.24 x 10(9) and 1.46 x 10(9) g/(mol x cm), respectively). Crosslinking had no effect on the architecture of both control and experimental fibrin samples. Spectrophotometric and electrophoretic analysis has shown a higher sensitivity of coarse fibrin gels to plasmin. A close correlation between spontaneous local conformational reconstructions in fibrinogen molecule and its functional activity is concluded.

[Influence of end products of plasmin-mediated hydrolysis of fibrinogen and fibrin (EF and Ef fragments) on fibrinogen cross-linking]

We studied the influence of the end products of plasmin-mediated hydrolysis of fibrinogen and nonstabilized fibrin (EF and Ef fragments) on covalent cross-linking of fibrinogen molecules catalyzed by a fibrin-stabilizing factor (factor XIIIa). The data on elastic and dynamic light scattering reveal no difference in the spatial structure of covalently linked fibrinogen molecules in the presence of the hydrolysis end products EF and Ef. In contrast to the inactive fragment EF, fragment Ef significantly accelerates the enzymatic reaction. This is also confirmed by electrophoresis of the reduced samples indicating a relatively fast accumulation of gamma-dimers and A alpha-polymers as compared to the control samples. Possible molecular mechanisms of this effect are discussed.