A revised model of platelet aggregation

Abstract

IntroductionPlatelet aggregation at sites of vascular injury is essential for the formation of the primary hemostatic plug and also for the development of pathological thrombi at sites of atherosclerotic plaque rupture. The initial contact of platelets with the injured vessel wall (platelet adhesion) is a complex process involving multiple adhesive substrates (von Willebrand factor [vWf], collagen) and receptors on the platelet surface (GPIb/V/IX, integrins α IIb β 3 and α 2 β 1 ) (1). The interaction between matrix-bound vWf and GPIbα on the platelet surface serves primarily to tether platelets to the area of vascular injury (2, 3), particularly under conditions of high shear stress, as a prerequisite step for integrin-mediated cell arrest (4). Whereas the molecular events underlying platelet adhesion under different shear conditions have been well delineated, the mechanism(s) by which platelets in freeflowing blood subsequently adhere to the initial layer of adherent platelets (platelet cohesion or aggregation) under flow have been less clearly defined.The traditional model of platelet aggregation, in which integrin α IIb β 3 was thought to have an exclusive role in mediating platelet-platelet adhesion contacts, has been largely determined from studies using a platelet aggregometer (5). With this method, the addition of a soluble agonist to a stirred platelet suspension induces activation of integrin α IIb β 3, converting it from a low-to a high-affinity receptor capable of binding soluble fibrinogen. The dimeric nature of fibrinogen enables it to cross-link adjacent activated platelets leading to stable platelet aggregation. Studies of platelet aggregation under high shear conditions, using a coneplate viscometer, have demonstrated that plasma vWf becomes the relevant ligand responsible for platelet activation (6). Shear-induced binding of soluble vWf to GPIbα initiates platelet activation independent of the addition of exogenous stimuli. Whereas the vWf-GPIbα interaction is indispensable for the initiation of platelet-platelet adhesion contacts under high shear, irreversible platelet aggregation requires a second adhesive interaction between vWf and integrin α IIb β 3 (7).The molecular events governing the formation of stable adhesion contacts between platelets in suspension have been well delineated; however, the mechanism by In this study we have examined the mechanism of platelet aggregation under physiological flow conditions using an in vitro flow-based platelet aggregation assay and an in vivo rat thrombosis model. Our studies demonstrate an unexpected complexity to the platelet aggregation process in which platelets in flowing blood continuously tether, translocate, and/or detach from the luminal surface of a growing platelet thrombus at both arterial and venous shear rates. Studies of platelets congenitally deficient in von Willebrand factor (vWf) or integrin α IIb β 3 demonstrated a key role for platelet vWf in mediating platelet tethering and translocation, whereas integrin α IIb β 3 mediated cell ...

von Willebrand factor and factor VIII are independently required to form stable occlusive thrombi in injured veins

“…13,14,16 Our results now point to an important role of VWF in platelet adhesion, aggregation, and occlusive thrombus formation in an experimental injury model of thrombosis under venous shear conditions. We have demonstrated earlier an important role for VWF in promoting platelet endothelial interaction in vivo under venous conditions 23 that was accentuated in mice lacking ADAMTS13, the enzyme cleaving VWF multimers.…”

“…by guest www.bloodjournal.org From shear was unclear with conflicting conclusions from various in vitro models. 13,14,21,25 In VWF-deficient mice we found that under low shear VWF was also required for optimal platelet adhesion to the injured vessel wall (Figure 1). This finding is consistent with 2 previously published reports where it was shown in vitro that VWF-GPIb␣ interaction participates in the adhesion process at venous shear rate.…”

“…10 Under high shear, the VWF-GPIb␣ interaction is necessary for initial platelet contact with the subendothelium, while irreversible platelet aggregation also requires interaction between the VWF and integrin ␣IIb␤3. 8,[11][12][13] Although an important role for VWF in platelet-platelet cohesion and thrombus formation at arterial shear rates was demonstrated both in vitro 13,14 and in vivo, 15,16 its role in thrombus formation in vivo under venous shear conditions is unclear. Under flow using type I collagen-coated coverslips, inhibition of the A3 domain or the A1 domain of VWF had no effect on thrombus volume at shear rates of 500s Ϫ1 and 100s Ϫ1 .…”

“…13 On the other hand, using glass microcapillary tubes coated with a VWF-deficient platelet monolayer (prepared from individuals with type III von Willebrand disease [VWD]) and perfusing whole blood from VWD patients, it was shown that VWF was required in platelet tethering at a shear rate of 150 s Ϫ1 . 14 In vivo the formation of a thrombus can be different than in in vitro conditions; for example, fibrinogen is required for platelet aggregation in vitro, whereas in vivo thrombi form readily in fibrinogen-deficient mice. 16 Therefore, in order to understand the in vivo role of VWF and FVIII in experimental thrombosis under venous flow conditions, we decided to evaluate thrombosis in VWF Ϫ/Ϫ , FVIII Ϫ/Ϫ , and transgenic mice lacking the GPIb␣ extracellular domain that was replaced by human interleukin-4 receptor (IL4R␣/GPIb␣-tg).…”

Characterization of the effects of isoprostanes on platelet aggregation in human whole blood

“…Furthermore, in our study 8-iso PGF 2a did not potentiate the proaggregatory e ects of either U46619 or collagen. This apparent discrepancy between platelet stimulation in isolated preparations and whole blood may be attributable to proaggregatory alterations in platelets and plasma during separation (Nicholson et al, 1998;Cox, 1998;Kulkarni et al, 2000) and the removal of components of whole blood that inhibit platelet aggregation (Zatta et al, 1990).…”