Accelerated uptake of VWF/platelet complexes in macrophages contributes to VWD type 2B-associated thrombocytopenia

The contribution of von Willebrand factor-GPIbα interactions to persistent aggregate formation in apheresis platelet concentrates

BACKGROUND AND OBJECTIVES

Apheresis platelet concentrates sometimes contain persistent aggregates (PA). Because apheresis involves extracorporeal circulation, we hypothesized that interactions between GPIbα and von Willebrand factor (VWF) underlie their origin.

MATERIALS AND METHODS

Platelets in donations with PA were compared to aggregate-free (AF) controls. Flow cytometry was used to determine platelet bound VWF. Degranulation was measured using P-selectin expression in flow cytometry and cytokine release using immunosorbent assays. Platelet adhesion to VWF was assessed in hydrodynamic flow and real-time video microscopy.

RESULTS

Platelets in PA concentrates had significantly more (P = 0·009, n ≥ 8) bound VWF compared to AF platelets, but differences in VWF concentration, VWF collagen binding, activated VWF or GPIbα expression were not found. Degranulation was higher (P = 0·030, n = 7) in PA than AF concentrates on day 1 of storage, but adhesion to immobilized VWF under hydrodynamic flow conditions was normal at that moment. On day 6, however, significantly less VWF adhesion (P = 0·009, n ≥ 6) was found for PA platelets compared to AF, indicating accelerated storage lesion in PA products. In a model that mimicks PA formation by chemically induced binding of VWF to platelets, we found that degranulation, phosphatidylserine expression and metabolism did not differ with paired controls at any time during subsequent storage.

CONCLUSION

Accelerated storage lesion is found in concentrates with PA, but this cannot be explained solely by increased platelet bound VWF following apheresis. Therefore, additional stressors are probably responsible for the increases observed in platelet degranulation and storage lesion in products with PA.

Meta-Analysis of Non-O Blood Group as an Independent Risk Factor for Coronary Artery Disease

To determine whether non-O blood group is an independent risk factor for coronary artery disease (CAD), we performed a meta-analysis of contemporary studies reporting adjusted relative risk estimates using multivariable logistic regression and multivariable Cox proportional hazards regression. MEDLINE and EMBASE were searched from January 2001 to March 2015 using Web-based search engines (PubMed and OVID). Search terms included blood group/type and coronary (artery/heart) disease, ischemic/ischaemic heart disease, acute coronary syndrome, myocardial infarction, or angina. Eligible were case-control, cross-sectional, or cohort studies reporting adjusted odds ratios (ORs) and hazard ratios (HRs) for CAD in subjects with non-O versus O blood group using multivariable logistic regression/multivariable Cox proportional hazards regression. Study-specific estimates were combined using inverse variance-weighted averages of logarithmic ORs/HRs in the random-effects model. Of 117 potentially relevant articles screened initially, 12 relative risk estimates in 10 eligible studies were identified and included. Pooled analysis of all the 12 ORs/HRs demonstrated that non-O blood group was associated with a statistically significant 14% increase in CAD incidence relative to O blood group (OR/HR 1.14, 95% confidence interval 1.04 to 1.25, p = 0.006). There was no evidence of significant publication bias. Combining 8 studies reporting data regarding (acute) myocardial infarction also generated a statistically significant result unfavoring non-O blood group (OR/HR 1.16, 95% confidence interval 1.02 to 1.31, p = 0.02). In conclusion, we found that based on a meta-analysis of 10 studies enrolling a total of 174,945 participants, non-O blood group appears to be an independent risk factor for CAD and myocardial infarction.

von Willebrand factor propeptide and the phenotypic classification of von Willebrand disease

VWFpp discriminates between type 3 VWD patients and severe type 1 VWD patients with very low VWF levels. The pathophysiological mechanisms of all types of VWD can be defined by the combined ratios of VWFpp/VWF:Ag and FVIII:C/VWF:Ag.

Shear stress-independent binding of von Willebrand factor-type 2B mutants p.R1306Q & p.V1316M to LRP1 explains their increased clearance

BACKGROUND

von Willebrand factor (VWF) is cleared in a shear stress- and macrophage-dependent manner by LRP1. von Willebrand disease (VWD)-type 2B mutants are endocytosed more efficiently than wild-type (wt)-VWF by macrophages.

OBJECTIVE

To investigate if VWD-type 2B mutations in the VWF A1-domain affect LRP1 binding and LRP1-dependent clearance.

METHODS

Recombinant Fc-tagged A1 domain (A1-Fc, A2-Fc, A3-Fc) and full-length VWF (wt or mutants thereof) were tested for binding to LRP1 or a recombinant fragment thereof in a static immunosorbent assay. Mutant and wt-VWF were also compared for clearance in mice lacking macrophage LRP1 (macLRP1(-) ) and control mice (macLRP1(+) ).

RESULTS

We found that A1-Fc but not A2-Fc or A3-Fc binds dose-dependently to LRP1. Binding of A1-Fc to LRP1 was markedly enhanced by the VWD-type 2B mutation p.V1316M. As expected, full-length wt-VWF was unable to bind LRP1 under static conditions unless ristocetin was added. In contrast, the presence of the p.V1316M or p.R1306Q mutation induced spontaneous binding to LRP1 without the need for ristocetin or shear stress. Both mutants were cleared more rapidly than wt-VWF in control macLRP1(+) mice. Surprisingly, deletion of macrophage LRP1 abrogated the increased clearance of the VWF/p.R1306Q and VWF/p.V1316M mutant.

CONCLUSION

The VWF A1-domain contains a binding site for LRP1. Certain VWD-type 2B mutations relieve the need for shear stress to induce LRP1 binding. Enhanced LRP1 binding coincides with a reduced survival of VWF/p.R1306Q and VWF/p.V1316M. Our data provide a rationale for reduced VWF levels in at least some VWD-type 2B patients.

von Willebrand factor biosynthesis, secretion, and clearance: connecting the far ends

To understand the placement of a certain protein in a physiological system and the pathogenesis of related disorders, it is not only of interest to determine its function but also important to describe the sequential steps in its life cycle, from synthesis to secretion and ultimately its clearance. von Willebrand factor (VWF) is a particularly intriguing case in this regard because of its important auxiliary roles (both intra- and extracellular) that implicate a wide range of other proteins: its presence is required for the formation and regulated release of endothelial storage organelles, the Weibel-Palade bodies (WPBs), whereas VWF is also a key determinant in the clearance of coagulation factor VIII. Thus, understanding the molecular and cellular basis of the VWF life cycle will help us gain insight into the pathogenesis of von Willebrand disease, design alternative treatment options to prolong the factor VIII half-life, and delineate the role of VWF and coresidents of the WPBs in the prothrombotic and proinflammatory response of endothelial cells. In this review, an update on our current knowledge on VWF biosynthesis, secretion, and clearance is provided and we will discuss how they can be affected by the presence of protein defects.

Of von Willebrand factor and platelets

Hemostasis and pathological thrombus formation are dynamic processes that require multiple adhesive receptor-ligand interactions, with blood platelets at the heart of such events. Many studies have contributed to shed light on the importance of von Willebrand factor (VWF) interaction with its platelet receptors, glycoprotein (GP) Ib-IX-V and αIIbβ3 integrin, in promoting primary platelet adhesion and aggregation following vessel injury. This review will recapitulate our current knowledge on the subject from the rheological aspect to the spatio-temporal development of thrombus formation. We will also discuss the signaling events generated by VWF/GPIb-IX-V interaction, leading to platelet activation. Additionally, we will review the growing body of evidence gathered from the recent development of pathological mouse models suggesting that VWF binding to GPIb-IX-V is a promising target in arterial and venous pathological thrombosis. Finally, the pathological aspects of VWF and its impact on platelets will be addressed.

ABO blood group and thrombotic vascular disease

ABO blood group antigens are complex carbohydrate molecules expressed on red blood cells and a variety of tissues. The ABO blood type is implicated in the development of a number of human diseases and there is increasing evidence regarding its involvement in the pathogenesis of cardiovascular disorders, mainly through its effect on von Willebrand factor levels. In this review, after a brief analysis of the potential molecular mechanisms by which the blood group influences haemostasis, we focus on the clinical implications of such interaction. Overall, the literature data document the close relationship between venous thromboembolism (VTE) and non-O blood type, which is associated with an approximately two-fold increased risk of venous thrombosis. A supra-additive effect on VTE risk is observed when an inherited thrombophilic condition is associated with non-O blood group. A weaker association exists between non-O blood type and arterial thrombosis, which needs to be further investigated.

von Willebrand factor, Jedi knight of the bloodstream

When blood vessels are cut, the forces in the bloodstream increase and change character. The dark side of these forces causes hemorrhage and death. However, von Willebrand factor (VWF), with help from our circulatory system and platelets, harnesses the same forces to form a hemostatic plug. Force and VWF function are so closely intertwined that, like members of the Jedi Order in the movie Star Wars who learn to use "the Force" to do good, VWF may be considered the Jedi knight of the bloodstream. The long length of VWF enables responsiveness to flow. The shape of VWF is predicted to alter from irregularly coiled to extended thread-like in the transition from shear to elongational flow at sites of hemostasis and thrombosis. Elongational force propagated through the length of VWF in its thread-like shape exposes its monomers for multimeric binding to platelets and subendothelium and likely also increases affinity of the A1 domain for platelets. Specialized domains concatenate and compact VWF during biosynthesis. A2 domain unfolding by hydrodynamic force enables postsecretion regulation of VWF length. Mutations in VWF in von Willebrand disease contribute to and are illuminated by VWF biology. I attempt to integrate classic studies on the physiology of hemostatic plug formation into modern molecular understanding, and point out what remains to be learned.

Expression of a structurally constrained von Willebrand factor variant triggers acute thrombotic thrombocytopenic purpura in mice

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disease that presents with thrombocytopenia, disseminated thrombosis, hemolytic anemia, and organ dysfunction. The etiology of TTP has revealed that patients share a deficiency in plasma protease a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13), the enzyme responsible for cleaving ultra-large von Willebrand factor (VWF) multimers into nonthrombogenic fragments. Therefore, existing TTP mouse models were developed by targeted disruption of the ADAMTS13 gene. ADAMTS13(-/-) mice are mostly asymptomatic in the absence of a trigger, as redundant proteases appear to take on VWF processing. As an alternative approach to creating one such model, we devised a strategy based on the expression of a cleavage-resistant VWF mutant in mice. The creation of a disulfide bond within the A2 domain of VWF was found to render VWF multimers resistant to proteolysis by plasma proteases under flow. Furthermore, mice expressing the murine VWF/p.S1494C-p.A1534C mutant present with symptoms characteristics of acute TTP such as thrombocytopenia, red cell shredding, accumulation of VWF-rich thrombi in the microvasculature, and advanced TTP symptoms such as renal dysfunction and splenomegaly. Because this model appears to faithfully emulate the pathophysiology of TTP, it should prove most useful in the study of microangiopathic diseases and their treatment.

An update on type 2B von Willebrand disease

Type 2B von Willebrand disease (VWD) accounts for fewer than 5% of all VWD patients. In this disease, mutations in the A1 domain result in increased von Willebrand factor (VWF) binding to platelet GPIbα receptors, causing increased platelet clearance and preferential loss of high molecular weight VWF multimers. Diagnosis is complicated because of significant clinical variations even among patients with identical mutations. Platelet transfusion often provides suboptimal results since transfused platelets may be aggregated by the patients' abnormal VWF. Desmopressin may cause a transient decrease in platelet count that could lead to an increased risk of bleeding. Replacement therapy with factor VIII/VWF concentrates is the most effective approach to prevention and treatment of bleeding in type 2B VWD.