Analysis of Platelet Receptor Expression in ITP,

Blood ◽  
2011 ◽  
Vol 118 (21) ◽  
pp. 3289-3289
Author(s):  
Jianlin Qiao ◽  
Huy Tran ◽  
Fi-Tjen Mu ◽  
Robert K Andrews ◽  
Elizabeth E Gardiner
Keyword(s):  
Platelet Count ◽  
Conflicts Of Interest ◽  
Surface Expression ◽  
Experimental Models ◽  
Receptor Expression ◽  
Response To Treatment ◽  
Platelet Surface ◽  
Healthy Donors ◽  
Platelet Receptor ◽  
Von Willebrand

Abstract Abstract 3289 In this study we assess platelet receptor expression and shedding in patients with immune thrombocytopenia (ITP) before and during treatment. The aim is to evaluate the potential value of quantitative measurement of platelet receptors for diagnosis and/or monitoring treatment in thrombocytopenia due to immune or other causes. The platelet-specific collagen receptor, glycoprotein (GP)VI, is associated with the Fc receptor γ-chain (FcRγ). GPVI/FcRγ is coassociated on platelet surface with the GPIb-IX-V complex; GPIbα of GPIb-IX-V binds von Willebrand factor and other ligands. Our previous studies showed engagement of platelet FcγRIIa by antiplatelet antibodies induced ectodomain shedding of GPVI, generating soluble ectodomain (sGPVI) in plasma. However, apart from one individual with an anti-GPVI antibody, whether anti-platelet antibodies associated with ITP affect GPVI/GPIb expression/shedding has not been addressed. In this study we used flow cytometry and a sGPVI ELISA to assess 1) whether patients with ITP had dysregulated expression/shedding of GPVI or GPIbα, and 2) whether platelet receptor expression changes prior to recovery of platelet count in individuals undergoing treatment for ITP. In 9 ITP patients (mean age=48.6, range 29–79; 6 female) with platelet count 61±9 × 109/L (range, 33–122 × 109/L), GPVI surface expression (GeoMean±SE, 137±17) was lower than healthy controls (274±26; n=17; platelet count 247±13), and sGPVI in patient plasma was significantly higher (39±4 ng/mL) compared to 17 healthy donors (19±3 ng/mL) (P=0.0006). In longitudinal samples analysed at weekly intervals during 2-month treatment with steroids, decreased GPVI surface expression and increased sGPVI in plasma remained essentially unchanged as the platelet count normalized, consistent with persistent anti-platelet antibody. However, while levels of intact platelet GPIbα were significantly reduced in ITP compared to healthy donors (P=0.0053), they approached healthy levels within 1 week of treatment, preceding improvement in platelet count or other measures. GPIbα expression/cleavage has been previously implicated in platelet clearance in experimental models, and our analysis suggests the proteolytic status of human GPIbα may be a novel early marker for evaluating response to treatment in ITP. Disclosures: No relevant conflicts of interest to declare.

Post-DDAVP Thrombocytopenia in Type 2B von Willebrand Disease Is not Associated with Platelet Consumption: Failure to Demonstrate Glycocalicin Increase or Platelet Activation

1999 ◽  
Vol 81 (02) ◽  
pp. 224-228 ◽  
Author(s):  
A. Steffan ◽  
E. Pontara ◽  
A. Zucchetto ◽  
C. Rossi ◽  
L. De Marco ◽  
...  
Keyword(s):  
Platelet Count ◽  
Platelet Activation ◽  
Surface Expression ◽  
Von Willebrand Disease ◽  
Platelet Surface ◽  
Platelet Counts ◽  
Normal Platelet ◽  
Thrombotic Complications ◽  
Von Willebrand ◽  
Willebrand Factor

SummaryThrombocytopenia is frequently reported in type 2B von Willebrand disease (vWD), and thought to be related to the abnormally high affinity of 2B von Willebrand factor (vWF) for platelet GPIb-IX. To gain an insight into the nature of this thrombocytopenia, we measured plasma glycocalicin (GC) levels (as a marker of platelet turnover), and platelet surface expression of the alpha granule protein P-selectin (as a marker of platelet activation) in 9 patients with type 2B vWD before, and in 4 patients also following the infusion of 1-desamino-8-d-arginine vasopressin (DDAVP). Three patients presented a persistent decrease of platelet counts in the resting condition. GC levels were within the normal range, regardless of the platelet counts, in all but one patient who presented, on the other hand, a normal platelet count. Moreover, platelets expressed normal amounts of P-selectin on their surface, regardless of platelet counts. These findings suggest that the thrombocytopenia observed in type 2B vWD is not due to platelet activation and subsequent consumption in circulation.Despite a significant, albeit transient, decrease in platelet count, DDAVP did not induce an increase in plasma GC levels, nor enhance P-selectin expression. These observations indicate that the acute post-DDAVP thrombocytopenia in type 2B vWD is not related to platelet activation and consumption. We advance that the post-DDAVP 2B vWF is hemostatically more active, and able to induce agglutination but not aggregation of circulating platelets. This would explain both the prompt recovery of basal platelet counts after the post-DDAVP decrease, and the lack of reported thrombotic complications in this disorder.Therefore, even though 2B vWF is characterized by an enhanced affinity for the platelet surface, its binding to platelet GPIb-IX in the soluble phase is not able to induce true platelet aggregation; vWF thus appears to be mainly an adhesive protein, rather than an aggregating agent.

Ventricular Assist Devices and Extracorporeal Membrane Oxygenation Devices Are Associated with Loss of Platelet Receptors

Blood ◽  
2014 ◽  
Vol 124 (21) ◽  
pp. 5001-5001
Author(s):  
Pohan Lukito ◽  
Angus K.T Wong ◽  
Jing Jing ◽  
Jane F Arthur ◽  
Elizabeth E. Gardiner ◽  
...  
Keyword(s):  
Platelet Adhesion ◽  
Ventricular Assist Devices ◽  
P Value ◽  
Elevated Plasma ◽  
Ventricular Assist ◽  
Platelet Surface ◽  
Anti Platelet Therapy ◽  
Healthy Donors ◽  
Platelet Receptor ◽  
Von Willebrand

Abstract Aim Ventricular assist devices (VADs) and extracorporeal membrane oxygenation devices (ECMO) are associated with bleeding, not fully explained by anticoagulant or anti-platelet therapy. Acquired von Willebrand syndrome (AVWS) may contribute to bleeding in patients with these devices. We investigated the relationship between AVWS and platelet dysfunction, through the loss of von Willebrand factor (VWF) receptor, GPIb (of the GPIb-IX-V complex) and the major collagen receptor, GPVI. GPIb and GPVI are crucial for platelet function at arterial shear rates and we have shown that metalloproteolytic shedding of GPVI is triggered on exposure of platelets to high shear (Al-Tamimi et al. Blood, 2012). Methods A pilot analysis was performed in 21 VADs patients (0.5-30 months post-implant) and 13 ECMO patients (1-9 days post-implant). All patients were in receipt of at least one anticoagulant and/or one anti-platelet therapy. Platelet counts, coagulation tests and VWF analyses including VWF multimers were performed. Levels of platelet surface and shed receptors were measured by flow cytometry-based assays or ELISA developed in-house. The in-house assays are unaffected by routine therapeutics used in these patients. Data were analysed by non-parametric Wilcoxon-Mann-Whitney test. The National Cancer Institute (NCI) bleeding score was used and VADs patients were grouped into major (NCI ≥3) or non-major (NCI<3) bleeding. Results We demonstrated loss of high molecular weight VWF multimers in most VADs patients (19 of 21) and 60% of the ECMO patients (6 of 10, VWF multimer analysis was not performed in 3 patients), consistent with AVWS. In VADs patients, platelet receptor shedding was demonstrated by significantly elevated plasma soluble GPVI levels (p=0.025), and reduction in surface GPVI (p=0.0003) and GPIb levels (p=0.0008) compared to healthy donors (Table 1). Similarly, ECMO patients demonstrated significantly elevated plasma soluble GPVI levels (p=0.0002), and reduction in surface GPVI (p=0.001) and GPIb levels (p=0.001) compared to healthy donors (Table 1). Five patients with VADs (24%) had major bleeding however platelet receptor levels were not analysed prior to the bleeding event. Table 1. Platelet receptor levels in VADs or ECMO patients: p values reflect comparison to healthy donor Test Parameters Healthy Donors; n=40Median (range) VADs; n=21Median (range) p value ECMO; n=13Median (range) p value Surface GPIb 823 (406-1764) 546 (230-1069) 0.0008 452 (11-1030) 0.001 Surface aIIb 224 (60-581) 206 (65-482) NS 236 (11-331) NS Surface GPVI 183 (11-474) 100 (42-292) 0.0003 99 (7-200) 0.001 Soluble GPVI 26 (3.2-50) 30.6 (23.4-66.2) 0.025 41 (21-77) 0.0002 Conclusion We linked AVWS for the first time with loss of platelet adhesion receptors in patients with VADs or ECMO. Loss of platelet surface receptors GPIb or GPVI may impact platelet adhesion/activation and limit thrombus formation under pathologic shear conditions. Further investigation will elucidate mechanisms of platelet receptor loss and assess whether temporal changes in platelet receptor levels are associated with bleeding outcomes in these patients. Disclosures No relevant conflicts of interest to declare.

scholarly journals Effect of aspirin on platelet-von Willebrand factor surface expression on thrombin and ADP-stimulated platelets

Blood ◽  
1989 ◽  
Vol 74 (6) ◽  
pp. 2016-2021 ◽  
Author(s):  
RI Parker ◽  
HR Gralnick
Keyword(s):  
Von Willebrand Factor ◽  
Shape Change ◽  
Adenosine Diphosphate ◽  
Surface Expression ◽  
Platelet Surface ◽  
Granule Secretion ◽  
Von Willebrand ◽  
Willebrand Factor ◽  
Platelet Shape ◽  
Stimulation Of

Abstract Platelets contain a pool of endogenous platelet-von Willebrand factor (vWF) that becomes expressed on the platelet surface when platelets are stimulated by a variety of agonists. Maximal platelet-vWF expression occurs in concert with platelet alpha-granule secretion. Aspirin (ASA) is known to impair platelet activation and alpha-granule secretion by irreversible inhibition of platelet cyclo-oxygenase. We studied native and ASA-treated platelets for their ability to mobilize and to express platelet-vWF in response to adenosine diphosphate (ADP) or thrombin. We found that each agonist was effective in promoting increased platelet- vWF surface expression on native and ASA-treated platelets. ASA-treated platelets responded identically to native platelets to low (0.01 U/mL) and high (1.0 U/mL) concentrations of thrombin, while the ADP-induced increase in ASA-treated platelets was only 50% to 60% of that for control platelets. Measurement of secreted platelet-vWF and beta- thromboglobulin indicated that the increase seen with ADP was largely independent of alpha-granule secretion. Using monoclonal antibodies (MoAbs) against the platelet glycoproteins (GP) IIb/IIIa and Ib (MoAbs 10E5 and 6D1, respectively), we demonstrated that the ADP-induced increase in platelet-vWF expression on control platelets primarily involved the binding of secreted platelet-vWF to the platelet GPIIb/IIIa. In contrast, the increase in platelet-vWF that occurred following ADP stimulation of ASA-treated platelets was largely insensitive to GPIIb/IIIa blockade. No effect of GPIb blockade in platelet-vWf expression was noted for either control or ASA-treated platelets. When platelet shape change was prevented by the addition of cytochalasin D, ADP-induced platelet-vWf surface expression on ASA- treated platelets was reduced by more than 80%. Our data indicate that platelets in which the cyclooxygenase pathway is blocked by the action of aspirin can increase surface expression of platelet-vWf as a consequence of platelet shape change. We speculate that this process exposes platelet-vWf bound to GPIIb/IIIa, or possibly GPIb, within the surface connected canalicular system.

scholarly journals The activation-induced decrease in the platelet surface expression of the glycoprotein Ib-IX complex is reversible

Blood ◽  
1994 ◽  
Vol 83 (12) ◽  
pp. 3562-3573 ◽  
Author(s):  
AD Michelson ◽  
SE Benoit ◽  
MH Kroll ◽  
JM Li ◽  
MJ Rohrer ◽  
...  
Keyword(s):  
Protein Kinase C ◽  
Protein Kinase ◽  
Light Chain ◽  
Myosin Light Chain Kinase ◽  
Myosin Light Chain ◽  
Surface Expression ◽  
Platelet Surface ◽  
Kinase C ◽  
Von Willebrand ◽  
Willebrand Factor

Abstract Thrombin decreases the platelet surface expression of the glycoprotein (GP) Ib-IX complex. To determine whether this effect is reversible, flow cytometric studies were performed with GPIb-IX-specific monoclonal antibodies. In both whole blood and washed platelet systems, incubation of platelets with thrombin or a combination of adenosine diphosphate and epinephrine resulted in a maximal decrease of the platelet surface expression of GPIb-IX within 5 minutes, after which there was a time- dependent return of the platelet surface GPIb-IX complex, which was maximal by 60 minutes. Exposure of the same platelets to additional exogenous thrombin resulted in a second decrease in platelet surface GPIb-IX, followed by a second reconstitution of platelet surface GPIb- IX. Throughout these experiments there was no measurable release from the platelets of glycocalicin (a proteolytic fragment of GPIb). Experiments in which platelets were preincubated with a biotinylated GPIb-specific MoAb showed that the GPIb molecules that returned to the platelet surface were the same molecules that had been translocated to the intraplatelet pool. The GPIb molecules that returned to the platelet surface were functionally competent to bind von Willebrand factor, as determined by ristocetin-induced platelet agglutination and ristocetin-induced binding of exogenous von Willebrand factor. Inhibitors of protein kinase C and myosin light-chain kinase enhanced the reexpression of platelet surface GPIb. In summary, the activation- induced decrease in the platelet surface expression of the GPIb-IX complex is reversible. Inactivation of protein kinase C and myosin light-chain kinase are important mechanisms in the reexpression of the platelet surface GPIb-IX complex.

scholarly journals von Willebrand factor bound to glycoprotein Ib is cleared from the platelet surface after platelet activation by thrombin

Blood ◽  
1992 ◽  
Vol 79 (8) ◽  
pp. 2011-2021 ◽  
Author(s):  
P Hourdille ◽  
HR Gralnick ◽  
E Heilmann ◽  
A Derlon ◽  
AM Ferrer ◽  
...  
Keyword(s):  
Flow Cytometry ◽  
Platelet Activation ◽  
Von Willebrand Factor ◽  
Platelet Rich Plasma ◽  
Surface Expression ◽  
Von Willebrand Disease ◽  
Platelet Surface ◽  
Immunogold Staining ◽  
Von Willebrand ◽  
Willebrand Factor

Abstract We recently reported that after activation of human platelets by thrombin, glycoprotein (GP) Ib-IX complexes are translocated to the surface-connected canalicular system (SCCS) (Blood 76:1503, 1990). As GPIb is a major receptor for von Willebrand factor (vWF) in platelet adhesion, we have now examined the consequences of thrombin activation on the organization of vWF bound to GPIb on the platelet surface. Studies were performed using monoclonal or polyclonal antibodies in either immunogold staining and electron microscopy (Au-EM) or in flow cytometry. When unstirred platelet-rich plasma was incubated with ristocetin, bound vWF was located by Au-EM as discrete masses regularly distributed over the cell surface. Platelets from a patient with Glanzmann's thrombasthenia, lacking GPIIb-IIIa complexes, gave a similar pattern, confirming that this represented binding to GPIb. That ristocetin was not precipitating vWF before their binding to the platelets was shown by the detection of similar masses on the surface of platelets of a patient with type IIB von Willebrand disease. Experiments were continued using washed normal platelets incubated in Tyrode-EDTA, the purpose of the EDTA being to limit the surface expression of endogenous vWF after platelet stimulation. Under these conditions, platelets were treated with ristocetin for 5 minutes at 37 degrees C in the presence of increasing amounts of purified vWF. This was followed by incubation with thrombin (0.5 U/mL) for periods of up to 10 minutes. Flow cytometry showed a time-dependent loss in the surface expression of vWF bound to GPIb and these changes were confirmed by Au-EM. In particular, immunogold staining performed on ultrathin sections showed that the bulk of the vWF was being cleared to internal membrane systems. Surface clearance of vWF during thrombin- induced platelet activation is a potential mechanism for regulating platelet adhesivity.

scholarly journals Neutrophil cathepsin G modulates the platelet surface expression of the glycoprotein (GP) Ib-IX complex by proteolysis of the von Willebrand factor binding site on GPIb alpha and by a cytoskeletal-mediated redistribution of the remainder of the complex

Blood ◽  
1994 ◽  
Vol 84 (1) ◽  
pp. 158-168 ◽  
Author(s):  
CA LaRosa ◽  
MJ Rohrer ◽  
SE Benoit ◽  
MR Barnard ◽  
AD Michelson
Keyword(s):  
Binding Site ◽  
Von Willebrand Factor ◽  
Cytochalasin B ◽  
Surface Expression ◽  
Cathepsin G ◽  
Prostaglandin I2 ◽  
Platelet Surface ◽  
Von Willebrand ◽  
Willebrand Factor ◽  
Platelet Content

Abstract The effects of neutrophil cathepsin G on the glycoprotein (GP) Ib-IX complex of washed platelets were examined. Cathepsin G resulted in a concentration- and time-dependent decrease in the platelet surface GPIb- IX complex, as determined by flow cytometry, binding of exogenous von Willebrand factor (vWF) in the presence of ristocetin, and ristocetin- induced platelet agglutination. Cathepsin G resulted in proteolysis of the vWF binding site on GPIb alpha (defined by monoclonal antibody [MoAb] 6D1), as determined by increased supernatant glycocalicin fragment (a proteolytic product of GPIb alpha); decreased total platelet content of GPIb; and lack of effect of either cytochalasin B (an inhibitor of actin polymerization), prostaglandin I2 (an inhibitor of platelet activation), or prior fixation of the platelets. However, cathepsin G resulted in minimal decreases in the binding to fixed platelets of MoAbs TM60 (directed against the thrombin binding site on GPIb alpha) and WM23 (directed against the macroglycopeptide portion of GPIb alpha). In contrast to its proteolytic effect on GPIb alpha, the cathepsin G-induced decrease in platelet surface GPIX and the remnant of the GPIb-IX complex (defined by MoAbs FMC25 and AK1) was via a cytoskeletal-mediated redistribution, as determined by lack of change in the total platelet content of GPIX and the GPIb-IX complex; complete inhibition by cytochalasin B, prostaglandin I2, and prior fixation of platelets. Experiments with Serratia protease-treated and Bernard- Soulier platelets showed that neither platelet surface GPIb nor cathepsin G-induced proteolysis of GPIb were required for the cathepsin G-induced redistribution of the remnant of the GPIb-IX complex or the cathepsin G-induced increase in platelet surface P-selectin. In summary, neutrophil cathepsin G modulates the platelet surface expression of the GPIb-IX complex both by proteolysis of the vWF binding site on GPIb alpha and by a cytoskeletal-mediated redistribution of the remainder of the complex. Prior studies show that, although thrombospondin 1, antiserine proteases, and plasma are all inhibitors of cathepsin G, the effects of cathepsin G on platelets, including an increase in surface GPIIb-IIIa, occur during close contact between neutrophils and platelets in a protective microenvironment (eg, thrombosis and local inflammation).(ABSTRACT TRUNCATED AT 400 WORDS).

Influence of platelet count on the expression of platelet collagen receptor glycoprotein VI (GPVI) in patients with acute coronary syndrome

2009 ◽  
Vol 101 (05) ◽  
pp. 911-915 ◽  
Author(s):  
Boris Bigalke ◽  
Konstantinos Stellos ◽  
Dimitrios Stakos ◽  
Thomas Joos ◽  
Oliver Pötz ◽  
...  
Keyword(s):  
Acute Coronary Syndrome ◽  
Platelet Count ◽  
Thrombus Formation ◽  
Surface Expression ◽  
Platelet Surface ◽  
Glycoprotein Vi ◽  
Coronary Syndrome ◽  
Symptomatic Coronary Artery Disease ◽  
Artery Disease ◽  
Collagen Receptor

SummaryPlatelets play a key role in the development of an acute coronary syndrome (ACS) and contribute to cardiovascular events. Platelet collagen receptor glycoprotein VI (GPVI) contributes significantly to platelet adhesion and thrombus formation in ACS. We consecutively investigated both the platelet count and the platelet surface expression of GPVI in 843 patients with a symptomatic coronary artery disease verified by coronary angiography. Four hundred fourteen patients presented with stable angina pectoris and 429 patients with ACS. Platelet surface expression of GPVI and CD62P was determined by flow cytometry and platelet count with a coulter counter, plasmatic soluble GPVI was measured by ELISA. Platelet GPVI expression in patients with ACS was compared to platelet count. Patients with ACS showed significantly elevated GPVI expression levels in the first and second quartiles of platelet count compared to patients with higher platelet count [mean fluorescence intensity (MFI) ± standard deviation): 1st vs. 4th: 20.44 ± 6.1 vs. 18.62 ± 3.7; p=0.012; 2ndvs.3rd:21.2±8.5vs.18.76±3.7;P=0.03; 2ndvs.4th: 21.2±8.5vs.18.62±3.7;P=0.004], which was paralleled in trend for the CD62P expression [MFI: 1st vs. 4th: 11.2 ± 6.8 vs. 12.3 ± 9; p=0.057; 2nd vs. 3rd: 16.3 ± 16 vs.12.7 ± 5.3; p=0.138; 2nd vs. 4th: 16.3 ± 16 vs.11 ± 4.4; p=0.043]. In a subgroup of 48 patients with ACS, determination of soluble GPVI showed similar results [plasma GPVI (ng/ml): 1stvs.4th: 1.6 ± 0.6 vs. 1.2 ± 0.4; p=0.046; 1st vs. 3rd: 1.6 ± 0.6 vs. 1.1 ± 0.5; p=0.038; 2nd vs. 3rd: 1.9 ± 0.8 vs. 1.1 ± 0.5; p=0.04; 2nd vs. 4th: 1.9 ± 0.8 vs. 1.2 ± 0.4; p=0.056]. Thus, a lower platelet count comes along with a higher GPVI surface expression and plasma concentration in patients with ACS, which potentially reflects increased activation and enhanced recruitment of platelets to the site of vascular injury.

scholarly journals Thrombin-induced GPIb-IX centralization on the platelet surface requires actin assembly and myosin II activation

Blood ◽  
1996 ◽  
Vol 87 (2) ◽  
pp. 618-629 ◽  
Author(s):  
TJ Kovacsovics ◽  
JH Hartwig
Keyword(s):  
Actin Filaments ◽  
Myosin Ii ◽  
Surface Expression ◽  
Membrane Skeleton ◽  
Cytosolic Calcium ◽  
Human Platelets ◽  
Actin Binding ◽  
Actin Assembly ◽  
Platelet Surface ◽  
Von Willebrand

In resting platelets, the GPIb-IX complex, the receptor for the von Willebrand factor (vWF), is linked to underlying actin filaments by actin-binding protein (ABP-280). Thrombin stimulation of human platelets leads to a decrease in the surface expression of the GPIb-IX complex, which is redistributed from the platelet surface into the open canalicular system (OCS). Because the centralization of GPIb-IX is inhibited by cytochalasin, it is believed to be linked to actin cytoskeletal rearrangements that take place during platelet activation. We have further characterized the mechanism of GPIb-IX centralization in platelets in suspension. Following thrombin stimulation, GPIb-IX shifts from the membrane skeleton of the resting cell to the cytoskeleton of the activated cell in a reaction sensitive to cytochalasin B. The cytoskeletal association of GPIb-IX involves ABP- 280, as it correlates with the incorporation of ABP-280 into the activated cytoskeleton and because no dissociation of the ABP-280/GPIb- IX complexes is detected after thrombin activation. However, the incorporation of GPIb-IX into the cytoskeleton is complete within 1 minute, whereas GPIb-IX centralization requires 5 to 10 minutes for completion. The movement of GPIb-IX to the cytoskeleton of activated platelets is therefore necessary, but not sufficient for GPIb-IX centralization. Blockage of cytosolic calcium increases induced by thrombin by loading with the cell permeant calcium chelator Quin-2 AM inhibited GPIb-IX centralization by 70%, but did not prevent its association with the activated cytoskeleton. Quin-2 loading did, however, decrease the incorporation of myosin II into the activated cytoskeleton. The role of myosin II was further probed using the myosin light chain kinase (MLCK) inhibitor wortmannin. Wortmannin prevents myosin II association to the activated cytoskeleton and inhibits GPIb- IX centralization by 50%, without affecting actin assembly or the association of GPIb-IX to the cytoskeleton. Only micromolar concentrations of wortmannin, high enough to inhibit MLCK, prevent GPIb- IX centralization. These results indicate that thrombin-induced GPIb-IX centralization requires a minimum of two steps, one associating GPIb-IX to the activated cytoskeleton and the second requiring myosin II activation. The involvement of myosin II implies that GPIb-IX/ABP-280 complexes, linked to actin filaments, are pulled into the cell center, and that platelets may exert contractile tension on vWF bound to its receptor.

Mapping of the Binding Site within Glycoprotein Ibα for the Leukocyte Integrin Mac-1 (αMβ2).

Blood ◽  
2009 ◽  
Vol 114 (22) ◽  
pp. 472-472
Author(s):  
Adam D Munday ◽  
Yuandong Peng ◽  
Yunmei Wang ◽  
Daniel I Simon ◽  
Jose A. Lopez
Keyword(s):  
Binding Site ◽  
Platelet Adhesion ◽  
Conflicts Of Interest ◽  
Peptide Binding ◽  
Von Willebrand Disease ◽  
Targeted Mutagenesis ◽  
Dimensional Structure ◽  
Monocytic Cell ◽  
Platelet Surface ◽  
Von Willebrand

Abstract Abstract 472 To maintain hemostasis, the human body uses more that 7,000 platelets/μl of blood a day; 3.5 billion platelets a day in a 70 kg adult male. Lack of fully functional platelets results in bleeding disorders such as Bernard-Soulier syndrome, Glanzmann thrombasthenia and platelet-type von Willebrand disease. It is becoming increasingly well appreciated that in addition to their hemostatic role, platelets play important roles in inflammation and wound healing. The initial step of platelet adhesion is mediated by the glycoprotein (GP) Ib-IX-V complex on the platelet surface, which binds von Willebrand factor (VWF). This interaction leads to activation of the integrin αIIbβ3, platelet arrest, and spreading and aggregation. The GPIb-IX-V complex also has a key role in inflammation, mediating a key interaction of platelets with leukocytes by binding the integrin Mac-1 (αMβ2, CD11b/CD18). This interaction mediates the firm adhesion of leukocytes on platelet thrombi, enabling their migration through the thrombus into the vessel wall. Interestingly, the insert domain (I-domain) of the αM subunit of Mac-1 has a similar 3-dimensional structure to the A1 domain of VWF. Our previous studies showed that the I-domain of Mac-1 binds the C-terminal flanking sequence of GPIbα (Phe201-Gly268), demonstrated by the ability of the anti-GPIbα monoclonal antibody AP1 to inhibit the interaction. The epitope of AP1 has been mapped to a 10-amino acid sequence spanning Arg218 to Tyr228. In the current investigation, we constructed a series of cell lines expressing mutants of human GPIbα, either by replacement of the human sequence with the corresponding dog sequence (dog GPIbα does not bind human Mac-1) or by targeted mutagenesis, and tested their ability to bind the recombinant αM I domain. TheGPIbα region Phe201–Asn223 was crucial for Mac-1 binding, with residues Arg218, Asp222 and Asn223 playing vital roles. In addition, a peptide containing the AP1 epitope (Leu214–Val229) bound αM I-domain specifically and saturably. Peptide binding was blocked by LPM19c, a monoclonal anti-αM I-domain antibody, and soluble GPIbα, and by the M2 peptide, which corresponds to the GPIbα–binding site in the αM I domain (Phe201–Lys217). Peptide binding was also blocked by an antibody against the M2 sequence. The AP1 peptide inhibited the attachment of GPIb-IX complex–expressing CHO cells to immobilized αM I domain, and the adhesion of THP-1 cells—a monocytic cell line expressing Mac-1—to immobilized GPIbα. In summary, we have defined the GPIbα sequence Arg218 to Ala224 as a critical binding site for Mac-1. Because a peptide corresponding to this region inhibits GPIbα binding to Mac-1 but blocks neither platelet adhesion to immobilized VWF nor thrombin-induced platelet aggregation, it has potential to guide the development of agents that will specifically inhibit leukocyte-platelet complexes that promote vascular inflammation. Disclosures: No relevant conflicts of interest to declare.

First Case of Platelet-Type Von Willebrand Disease (PT-VWD) Associated with Type 2B Von Willebrand Disease (2B VWD)

Blood ◽  
2011 ◽  
Vol 118 (21) ◽  
pp. 5313-5313
Author(s):  
Marie Dreyfus ◽  
Celine Desconclois ◽  
Corinne Guitton ◽  
Marie-Jeanne Baas ◽  
Helene Mandard ◽  
...  
Keyword(s):  
Platelet Count ◽  
Von Willebrand Disease ◽  
Biological Study ◽  
High Dose ◽  
Severe Thrombocytopenia ◽  
First Case ◽  
Platelet Receptor ◽  
Biological Phenotype ◽  
Neonatal Thrombocytopenia ◽  
Von Willebrand

Abstract Abstract 5313 Introduction VWD 2B and PT-VWD are rare diseases, due to mutations inducing a gain of function respectively of von Willebrand factor (VWF) and of its platelet receptor, Glycoprotein (GP)1bα Case history We report here the case of a young girl, born with an extensive purpura and a severe thrombocytopenia: platelet count: 16G/L. There was no associated biological nor clinical abnormality. A high dose of 1g/kg of immunoglobulin G infused on day 1 was unsuccessful, and a HPA-1a (−) platelet concentrate infusion led to a partial and transient increase of the platelet count up to 60G/L. Thrombocytopenia then resolved spontaneously. Biological study showed no sign of materno-fetal allo- or auto-immunity, parents were not consanguineous. The diagnosis of type 2B VWD was performed when she was 5 months old: VWF:RCo < 13 IU/dl, VWF:Ag 60 IU/dl, positive ristocetin induced platelet aggregation (RIPA) at a low ristocetin concentration (0.5 mg/ml). RIPA mixing studies were unconclusive. The same biological abnormalities were found in the father, whereas the mother had normal hemostasis tests. The biological phenotype also included a study of the multimeric VWF structure, showing a marked decrease in percentage of VWF high and intermediate molecular multimers. Genetic analysis performed on VWF gene showed the heterozygous p.Pro1266Leu missense mutation in the VWF A1 domain. This mutation ( o ) is only slightly deleterious, and induces usually a mild disease, without thrombocytopenia, even in stress situations, with normal VWF multimeric distribution; therefore, it could not explain the biological phenotype severity in this family. GPIBA was then analysed, and a candidate point mutation p.Met239Ile was evidenced. This mutation had not been described yet, but p.Met255Val had already been found in diagnosed cases of PT-VWD. Conclusion This case underlines the utmost importance to characterize precisely neonatal thrombocytopenia mechanism. Furthermore, it points out the difficulties to performing PT-VWD diagnosis, which incidence is most probably underestimated. In our case, it was the systematic and extensive biological workout performed in this case of isolated neonatal thrombocytopenia, without any obvious cause, which led to the diagnosis of a PT-VWD, inducing a severe biological phenotype, associated with type 2B VWD characterized by a mild expression. It is, to our knowledge, the first case described to date of such a morbid association. Disclosures: No relevant conflicts of interest to declare.

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