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Background Ventricular assist devices (VADs) and extracorporeal membrane oxygenation (ECMO) are associated with bleeding that is not fully explained by anticoagulant or antiplatelet use. Exposure of platelets to elevated shear in vitro leads to increased shedding. Objectives To investigate whether loss of platelet receptors occurs in vivo, and the relationship with acquired von Willebrand syndrome (AVWS). Methods Platelet counts, coagulation tests and von Willebrand factor (VWF) analyses were performed on samples from 21 continuous flow VAD (CF-VAD), 20 ECMO, 12 heart failure and seven aortic stenosis patients. Levels of platelet receptors were measured by flow cytometry or ELISA. Results The loss of high molecular weight VWF multimers was observed in 18 of 19 CF-VAD and 14 of 20 ECMO patients, consistent with AVWS. Platelet receptor shedding was demonstrated by elevated soluble glycoprotein (GP) VI levels in plasma and significantly reduced surface GPIbα and GPVI levels in CF-VAD and ECMO patients as compared with healthy donors. Platelet receptor levels were also significantly reduced in heart failure patients. Conclusions These data link AVWS and increased platelet receptor shedding in patients with CF-VADs or ECMO for the first time. Loss of the platelet surface receptors GPIbα and GPVI in heart failure, CF-VAD and ECMO patients may contribute to ablated platelet adhesion/activation, and limit thrombus formation under high/pathologic shear conditions.

Ligand-induced ectodomain shedding of glycoprotein VI (GPVI) is a metalloproteinase-dependent event. We examined whether shear force, in the absence of GPVI ligand, was sufficient to induce shedding of GPVI. Human-citrated platelet-rich plasma or washed platelets were subjected to increasing shear rates in a cone-plate viscometer, and levels of intact and cleaved GPVI were examined by Western blot and ELISA. Pathophysiologic shear rates (3000-10 000 seconds ؊1 ) induced platelet aggregation and metalloproteinase-dependent appearance of soluble GPVI ectodomain, and GPVI platelet remnant. Shedding of GPVI continued after transient exposure to shear. Blockade of ␣ IIb ␤ 3 , GPIb␣, or intracellular signaling inhibited shear-induced platelet aggregation but minimally affected shearinduced shedding of GPVI. Shearinduced GPVI shedding also occurred in platelet-rich plasma or washed platelets isolated from a von Willebrand disease type 3 patient with no detectable VWF, implying that shear-induced activation of platelet metalloproteinases can occur in the absence of GPVI and GPIb␣ ligands. Significantly elevated levels of sGPVI were observed in 10 patients with stable angina pectoris, with well-defined single vessel coronary artery disease and mean intracoronary shear estimates at 2935 seconds ؊1 (peak shear, 19 224 seconds ؊1 ). Loss of GPVI in platelets exposed to shear has potential implications for the stability of a forming thrombus at arterial shear rates. IntroductionPlatelet activation and accumulation at sites of vascular injury play a critical role in thrombus formation. This complex process is mainly initiated by 3 different but overlapping pathways: (1) exposure of subendothelial matrix proteins, including collagen and VWF, which activate the platelet adhesion-signaling receptors glycoprotein VI (GPVI) and GPIb␣ of the GPIb-IX-V complex, respectively; (2) exposure of tissue factor, which activates the coagulation cascade resulting in formation of active thrombin facilitating fibrin deposition as well as enhancing platelet activation; and (3) disturbed blood flow because of narrowing of the vascular lumen, which modulates the adhesive function of platelets and accelerates platelet activation and thrombus growth. 1 Indeed, changes in blood flow rates and hydrodynamic force are now recognized to play a more critical role in thrombus formation, especially at sites of vascular occlusion, as indicated by the ability of elevated (pathologic) shear stress to induce stable platelet aggregate formation without the requirement for platelet activation and adhesion 2 or for soluble agonists. 3 Human platelets normally circulate in a resting state and are exposed to shear rates within a physiologic range (ϳ 20-2000 seconds Ϫ1 ). Platelets may encounter shear rates beyond 10 000 seconds Ϫ1 under pathologic conditions, for example, in a stenosed atherosclerotic artery, and become activated and begin to aggregate. [3][4][5] Shear-dependent platelet activation is initiated by binding of plasma VWF to platelets primarily throu...

We demonstrate mechanochemical regulation of platelet adhesion to von Willebrand factor in thrombosis and hemostasis.

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