Shear stress-induced von Willebrand factor binding to platelet glycoprotein Ib initiates calcium influx associated with aggregation

Abstract

Platelets subjected to elevated levels of fluid shear stress in the absence of exogenous agonists will aggregate. Shear stress-induced aggregation requires von Willebrand factor (vWF) multimers, extracellular calcium (Ca2+), adenosine diphosphate (ADP), and platelet membrane glycoprotein (GP)Ib and GPIIb-IIIa. The sequence of interaction of vWF multimers with platelet surface receptors and the effect of these interactions on platelet activation have not been determined. To elucidate the mechanism of shear stress-induced platelet aggregation, suspensions of washed platelets were subjected to different levels of uniform shear stress (15 to 120 dyne/cm2) in an optically modified cone and plate viscometer. Cytoplasmic ionized calcium ([Ca2+]i) and aggregation of platelets were monitored simultaneously during the application of shear stress; [Ca2+]i was measured using indo-1 loaded platelets and aggregation was measured as changes in light transmission. Basal [Ca2+]i was approximately 60 to 100 nmol/L. An increase of [Ca2+]i (up to greater than 1,000 nmol/L) was accompanied by synchronous aggregation, and both responses were dependent on the shear force and the presence of vWF multimers. EGTA chelation of extracellular Ca2+ completely inhibited vWF-mediated [Ca2+]i and aggregation responses to shear stress. Aurin tricarboxylic acid, which blocks the GPIb recognition site on the vWF monomer, and 6D1, a monoclonal antibody to GPIb, also completely inhibited platelet responses to shear stress. The tetrapeptide RGDS and the monoclonal antibody 10E5, which inhibit vWF binding to GPIIb-IIIa, partially inhibited shear stress-induced [Ca2+]i and aggregation responses. The combination of creatine phosphate/creatine phosphokinase, which converts ADP to adenosine triphosphate and blocks the effect of ADP released from stimulated platelets, inhibited shear stress-induced platelet aggregation without affecting the increase of [Ca2+]i. Neither the [Ca2+]i nor aggregation response to shear stress was inhibited by blocking platelet cyclooxygenase metabolism with acetylsalicylic acid. These results indicate that GPIb and extracellular Ca2+ are absolutely required for vWF-mediated [Ca2+]i and aggregation responses to imposed shear stress, and that the interaction of vWF multimers with GPIIb-IIIa potentiates these responses. Shear stress-induced elevation of platelet [Ca2+]i, but not aggregation, is independent of the effects of release ADP, and both responses occur independently of platelet cyclooxygenase metabolism. These results suggest that shear stress induces the binding of vWF multimers to platelet GPIb and this vWF-GPIb interaction causes an increase of [Ca2+]i and platelet aggregation, both of which are potentiated by vWF binding to the platelet GPIIb-IIIa complex.

Distinct mechanisms of platelet aggregation as a consequence of different shearing flow conditions.

“…2) indicate that, depending on the mechanism of activation, either fibrinogen or vWf can become the ␣ IIb ␤ 3 -bound adhesive ligand, supporting aggregation at high shear rates. Binding of vWf may occur preferentially as a result of shear stress-induced activation because the latter process depends on an initial interaction of vWf itself with GP Ib␣ (9,10). Thus, the changes that render ␣ IIb ␤ 3 capable of interacting with soluble ligands may involve only the platelets onto which vWf is already immobilized, albeit perhaps transiently (16), effectively increasing the local concentration of this ligand and favoring its binding to ␣ IIb ␤ 3 over that of fibrinogen.…”

Thrombin-Induced Platelet Activation Is Inhibited by High- and Low-Molecular-Weight Heparin

“…The possibility of a direct signal transduction on Gp Ib binding by an agonist was raised by the evidence that vWF binding to Gp Ib is able to cause intraplatelet calcium elevation. 19 This finding has been attributed to the influx of extracellular calcium due to the activation of some membrane calcium channels by vWF ligation to Gp Ib. This mechanism was not possible in our experimental conditions because all the calcium experiments shown in the present study were carried out in the absence of external calcium.…”

Local Glycoprotein IIb/IIIa Receptor Inhibitor Delivery From the Pump Surface Attenuates Platelet Adhesion in Continuous Flow Ventricular Assist Devices

“…Over the past decade, the subcellular mechanisms of shear‐induced platelet aggregation and the involvement of the GP IIb/IIIa complex have been studied in detail (18–22). In contrast to this well‐understood mechanism, the interaction among platelet receptors and their interaction with surfaces are incompletely understood.…”