Binding of plasma factor XIII to thrombin-receptor activated human platelets

Abstract

Platelet-bound coagulation factor XIII (FXIII) is targeted and concentrated at the site where platelet-rich thrombi are formed. Previous studies were in disagreement about the nature of FXIII binding to platelets. In this study, thrombin-receptor activating peptide (TRAP)-stimulated human whole blood and washed platelets were analysed by flow cytometry for the binding of FXIII using a monoclonal antibody against the A subunit of FXIII (FXIII-A). Here, we demonstrate that FXIII-A positivity significantly increased on activated platelets in whole blood compared to unstimulated sample, but not in washed platelets. GPIIb/IIIa receptor plays an essential role in FXIII binding, as fibrinogen receptor antagonist eptifibatide and fibrinogen binding inhibitor RGDS tetrapeptide significantly prevented the binding of FXIII. Furthermore, stimulated platelets from a patient with severe type I Glanzmann thrombasthenia showed insignificant FXIII-A positivity versus healthy controls. In addition, basal negligible amount of FXIII on washed platelets was only slightly increased when highly purified plasma FXIII (FXIII-A(2)B(2)) was added upon platelet activation by TRAP. Similarly, no remarkable FXIII-A positivity was observed when we used FXIII-A(2)B(2) with gammaA/gammaA fibrinogen. However, gammaA/gamma' fibrinogen significantly augmented FXIII binding on TRAP-stimulated platelets in the presence of non-activated FXIII-A(2)B(2). We conclude that FXIII-A(2)B(2) of plasma origin binds to thrombin-receptor activated platelets via GPIIb/IIIa receptor-bound fibrinogen with gamma'-chain and is not capable of direct platelet binding.

Functional factor XIII-A is exposed on the stimulated platelet surface

“…46 Nagy et al 47 reported that less than 5% of platelets expose FXIII-A after TRAP stimulation, values that differ markedly from this study. Differences in experimental set-up, including activation time and the sole use of TRAP-6, may account for these discrepancies.…”

The pleiotropic role of the fibrinogen γ′ chain in hemostasis

“…61 In addition, a very recent study showed that FXIII binding to activated platelets is mediated via the ␥Ј chain of aIIb␤3-bound fibrinogen, suggesting a bridging function of the ␥Ј chain between FXIII and activated platelets. 62 However, binding of FXIII to the ␥Ј chain has been put into question by a report from Gersh and Lord who used an enzymelinked immunosorbent assay-based system with recombinant fibrinogens and found no difference in K d for the binding of FXIII to ␥A/␥A, ␥A/␥Ј, or ␥Ј/␥Ј fibrinogen. 63 Further studies on FXIII and ␥A/␥Ј fibrinogen will be required to clarify the interaction and whether it may modulate clot stability.…”

Platelet factor XIIIa release during platelet aggregation and plasma clot strength measured by thrombelastography in patients with coronary artery disease treated with clopidogrel

“…FXIIIa has been demonstrated on the surface of activated platelets and has been shown to enhance platelet adhesion under flow conditions [13,22]. Alternatively, subjects with increased levels of platelet derived FXIIIa may concomitantly deposit other coagulation factors that could in part influence maximal clot formation phenotype, such as Factor V and Factor VII which are also found in platelet granules [9].…”