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...

Summary. Background: Experimental animal studies have shown that the intimal hyperplasia (IH) responsible for occlusion after successful revascularization procedures may be partially caused by a bone marrow-derived cell that migrates to the site of vascular injury. Concurrent studies have demonstrated an extensive role in wound healing for the circulating fibrocyte. Objectives: We aimed to trace the path of the circulating cell that contributes to IH and determine if it is the fibrocyte. Methods and results: We established an in vitro model whereby purified monocytes from six healthy human volunteers were cultured into fibrocytes. These cells were morphometrically similar to the vascular smooth muscle cell (VSMC) found in IH and expressed alpha-smooth muscle actin (a-SMA) as well as CD34, CD45 and Collagen I (Col I), markers indicative of the fibrocyte. In an in vivo ovine carotid artery synthetic patch graft model, carboxyfluorescein diacetate, succinimidyl ester (CFSE) labeled circulating leukocytes were observed throughout the graft as well as in the neointima in 18 sheep. These cells were shown to produce collagen and a-SMA at 1, 2 and 4 weeks. These cells then underwent immunohistochemical analysis and were found to express a set of markers unique to the fibrocyte (CD34, CD45, Vimentin and a-SMA) and also to double stain for CD34 and a-SMA. Conclusions: IH in an ovine carotid artery patch graft model is partially derived from a hematopoietic circulating progenitor cell that acquires mesenchymal features as it matures at the site of injury.

Background Procoagulant platelets are a subset of highly activated platelets with a critical role in thrombin generation. Evaluation of their clinical utility in thrombotic disorders, such as coronary artery disease (CAD), has been thwarted by the lack of a sensitive and specific whole blood assay. Objectives We developed a novel assay, utilizing the cell death marker, GSAO [(4-(N-(S-glutathionylacetyl)amino)phenylarsonous acid], and the platelet activation marker, P-selectin, to identify procoagulant platelets in whole blood by flow cytometry. Patients/Methods Using this assay, we characterized the procoagulant platelet population in healthy controls and a cohort of patients undergoing elective coronary angiography. Results In patients with CAD, compared with patients without CAD, there was a heightened procoagulant platelet response to thrombin (25.2% vs. 12.2%), adenosine diphosphate (ADP) (7.8% vs. 2.7%) and thrombin plus collagen (27.2% vs. 18.3%). The heightened procoagulant platelet potential in CAD patients was not associated with other markers of platelet function, including aggregation, dense granule release and activation of α β integrin. Although dual antiplatelet therapy (DAPT) was associated with partial suppression of procoagulant platelets, this inhibitory effect on a patient level could not be predicted by aggregation response to ADP and was not fully suppressed by clopidogrel. Conclusions We report for the first time that procoagulant platelets can be efficiently detected in a few microliters of whole blood using the cell death marker, GSAO, and the platelet activation marker, P-selectin. A heightened procoagulant platelet response may provide insight into the thrombotic risk of CAD and help identify a novel target for antiplatelet therapies in CAD.

Recent in vitro studies have shown that shear stress can cause platelet activation by agonist-independent pathways. However, no studies have assessed the extent of shear-induced platelet activation within human coronary arteries. We sampled blood from the coronary arteries proximal and distal to coronary lesions and from the coronary sinus in humans with stable coronary disease who were taking both aspirin and clopidogrel. A novel, computationally based technique for estimating shear stress from 3-dimensional coronary angiographic images of these arteries was developed, and the effect of stenosis severity and calculated shear stress on in vivo platelet and related leukocyte activation pathways were determined. We provide evidence of intracoronary upregulation of platelet P-selectin, plateletmonocyte aggregation, and monocyte CD11b without platelet glycoprotein IIb-IIIa activation or soluble P-selectin up-regulation. This correlates with intracoronary stenosis severity and calculated shear stress and occurs despite the concurrent use of aspirin and clopidogrel. Our results show for the first time shear-related platelet and monocyte activation in human coronary arteries and suggest this as a potential therapeutic target that is resistant to conventional antiplatelet agents. (Blood. 2011;117(1): 11-20) IntroductionPlatelet activation and aggregation are critical to the pathogenesis of atherothrombosis. 1,2 Platelets are also known to play a main role in inflammation, partly by the interaction between platelets and leukocytes, 2-4 and it is known that shear stress can cause platelet activation, 5-12 platelet-leukocyte aggregation, 8,9 and leukocyte activation. 8,9 Procedures such as coronary angioplasty can alleviate focal coronary stenosis. However, many patients with diffuse, severe coronary disease are unsuitable for such treatment and may therefore be exposed to long-term platelet and leukocyte activation secondary to increased intracoronary shear stress.Although in vitro and animal studies have shown shear stressinduced platelet activation and aggregation, a direct relationship between shear stress and in situ platelet activation has not been shown within human coronary arteries. 13 Previous investigators have found evidence of transcardiac platelet activation with increased P-selectin expression as platelets travel from the aortic root to the coronary sinus (CS), the main vein draining blood from the heart in patients with coronary disease. 14 However, it is not known whether platelets are indeed activated as they cross coronary lesions, and no studies to date have investigated the direct effect of stenosis severity and shear stress on in situ platelet activation within human coronary arteries.Current antiplatelet agents such as aspirin, thienopyridines, and glycoprotein (GP) IIb-IIIa inhibitors attenuate agonist-induced platelet activation, [15][16][17] and GPIIb-IIIa inhibitors reduce shearinduced platelet aggregation. 10,18,19 However, shear stress overcomes aspirin inhibition of platelet aggregation. 20 Fu...

Platelet CD147 expression is evident in vivo and correlates moderately with traditional platelet activation markers and leukocyte CD147 expression. Platelet CD147 expression shows a stronger association with age, and leukocyte CD147 expression a stronger association with clinical CAD, suggesting differences in the regulation of platelet and leukocyte CD147 expression in vivo.

Colchicine inhibits coronary and cerebrovascular events in patients with coronary artery disease (CAD), and although known to have anti-inflammatory properties, its mechanisms of action are incompletely understood. In this study, we investigated the effects of colchicine on platelet activation with a particular focus on its effects on activation via the collagen glycoprotein (GP)VI receptor, P2Y12 receptor, and procoagulant platelet formation. Therapeutic concentrations of colchicine in vitro (equivalent to plasma levels) significantly decreased platelet aggregation in whole blood and in platelet rich plasma in response to collagen (multiplate aggregometry) and reduced reactive oxygen species (ROS) generation (H2DCF-DA, flow cytometry) in response to GPVI stimulation with collagen related peptide-XL (CRP-XL, GPVI specific agonist). Other platelet activation pathways including P-selectin expression, GPIIb/IIIa conformational change and procoagulant platelet formation (GSAO+/CD62P+) (flow cytometry) were inhibited with higher concentrations of colchicine known to inhibit microtubule depolymerization. Pathway specific mechanisms of action of colchicine on platelets, including modulation of the GPVI receptor pathway at low concentrations, may contribute to its protective role in CAD.

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