von Willebrand factor (VWF) propeptide binding to VWF D'D3 domain attenuates platelet activation and adhesion

von Willebrand factor antigen, von Willebrand factor propeptide and ADAMTS13 activity in TIA or ischaemic stroke patients changing antiplatelet therapy

Data are limited on the impact of commencing antiplatelet therapy on von Willebrand Factor Antigen (VWF:Ag) or von Willebrand Factor propeptide (VWFpp) levels and ADAMTS13 activity, and their relationship with platelet reactivity following TIA/ischaemic stroke. In this pilot, observational study, VWF:Ag and VWFpp levels and ADAMTS13 activity were quantified in 48 patients ≤4 weeks of TIA/ischaemic stroke (baseline), and 14 days (14d) and 90 days (90d) after commencing aspirin, clopidogrel or aspirin+dipyridamole. Platelet reactivity was assessed at moderately-high shear stress (PFA-100® Collagen-Epinephrine / Collagen-ADP / INNOVANCE PFA P2Y assays), and low shear stress (VerifyNow® Aspirin / P2Y12, and Multiplate® Aspirin / ADP assays). VWF:Ag levels decreased and VWFpp/VWF:Ag ratio increased between baseline and 14d and 90d in the overall population (P ≤ 0.03). In the clopidogrel subgroup, VWF:Ag levels decreased and VWFpp/VWF:Ag ratio increased between baseline and 14d and 90d (P ≤ 0.01), with an increase in ADAMTS13 activity between baseline vs. 90d (P ≤ 0.03). In the aspirin+dipyridamole subgroup, there was an inverse relationship between VWF:Ag and VWFpp levels with both PFA-100 C-ADP and INNOVANCE PFA P2Y closure times (CTs) at baseline (P ≤ 0.02), with PFA-100 C-ADP, INNOVANCE PFA P2Y and C-EPI CTs at 14d (P ≤ 0.05), and between VWF:Ag levels and PFA-100 INNOVANCE PFA P2Y CTs at 90d (P = 0.03). There was a positive relationship between ADAMTS13 activity and PFA-100 C-ADP CTs at baseline (R2 = 0.254; P = 0.04). Commencing/altering antiplatelet therapy, mainly attributed to commencing clopidogrel in this study, was associated with decreasing endothelial activation following TIA/ischaemic stroke. These data enhance our understanding of the impact of VWF:Ag and VWFpp especially on ex-vivo platelet reactivity status at high shear stress after TIA/ischaemic stroke.

Quantitative super-resolution imaging of platelet degranulation reveals differential release of von Willebrand factor and von Willebrand factor propeptide from alpha-granules

BACKGROUND

Von Willebrand factor (VWF) and VWF propeptide (VWFpp) are stored in eccentric nanodomains within platelet alpha-granules. VWF and VWFpp can undergo differential secretion following Weibel-Palade body exocytosis in endothelial cells; however, it is unclear if the same process occurs during platelet alpha-granule exocytosis. Using a high-throughput 3-dimensional super-resolution imaging workflow for quantification of individual platelet alpha-granule cargo, we studied alpha-granule cargo release in response to different physiological stimuli.

OBJECTIVES

To investigate how VWF and VWFpp are released from alpha-granules in response to physiological stimuli.

METHODS

Platelets were activated with protease-activated receptor 1 (PAR-1) activating peptide (PAR-1 ap) or collagen-related peptide (CRP-XL). Alpha-tubulin, VWF, VWFpp, secreted protein acidic and cysteine rich (SPARC), and fibrinogen were imaged using 3-dimensional structured illumination microscopy, followed by semiautomated analysis in FIJI. Uptake of anti-VWF nanobody during degranulation was used to identify alpha-granules that partially released content.

RESULTS

VWFpp overlapped with VWF in eccentric alpha-granule subdomains in resting platelets and showed a higher degree of overlap with VWF than SPARC or fibrinogen. Activation of PAR-1 (0.6-20 μM PAR-1 ap) or glycoprotein VI (GPVI) (0.25-1 μg/mL CRP-XL) signaling pathways caused a dose-dependent increase in alpha-granule exocytosis. More than 80% of alpha-granules remained positive for VWF, even at the highest agonist concentrations. In contrast, the residual fraction of alpha-granules containing VWFpp decreased in a dose-dependent manner to 23%, whereas SPARC and fibrinogen were detected in 60% to 70% of alpha-granules when stimulated with 20 μM PAR-1 ap. Similar results were obtained using CRP-XL. Using an extracellular anti-VWF nanobody, we identified VWF in postexocytotic alpha-granules.

CONCLUSION

We provide evidence for differential secretion of VWF and VWFpp from individual alpha-granules.

The role of thrombin in haemostasis

Thrombin is a multifunctional serine protease generated in injured cells. The generation of thrombin in coagulation plays a central role in the functioning of haemostasis. The last enzyme in the coagulation cascade is thrombin, with the function of cleaving fibrinogen to fibrin, which forms the fibrin clot of a haemostatic plug. Although thrombin primarily converts fibrinogen to fibrin, it also has many other positive regulatory effects on coagulation. Thrombin has procoagulant, inflammatory, cellular proliferation and anticoagulant effects. In coagulation system, thrombin has two very distinct roles. Firstly, it acts as a procoagulant when it converts fibrinogen into an insoluble fibrin clot, activates factor (F) XIII, activates thrombin activatable fibrinolysis inhibitor (TAFI) and activates FV, FVIII and FXI. Thrombin also enhances platelet adhesion by inactivating a disintegrin and metalloprotease with thrombospondin type1 motif (ADAMTS13). However, when thrombin activates protein C, it acts as an anticoagulant. A natural anticoagulant pathway that supplies regulation of the blood coagulation system contains protein C, which is the key component. This is accomplished by the specific proteolytic inactivation of FV and FVIII. In this review, the multiple roles of thrombin in the haemostatic response to injury are studied in addition to the cofactors that determine thrombin activity and how thrombin activity is thought to be coordinated.

FVIII half-life extension by coadministration of a D'D3 albumin fusion protein in mice, rabbits, rats, and monkeys

A novel mechanism for extending the circulatory half-life of coagulation factor VIII (FVIII) has been established and evaluated preclinically. The FVIII binding domain of von Willebrand factor (D'D3) fused to human albumin (rD'D3-FP) dose dependently improved pharmacokinetics parameters of coadministered FVIII in all animal species tested, from mouse to cynomolgus monkey, after IV injection. At higher doses, the half-life of recombinant FVIII (rVIII-SingleChain) was calculated to be increased 2.6-fold to fivefold compared with rVIII-SingleChain administered alone in rats, rabbits, and cynomolgus monkeys, and it was increased 3.1-fold to 9.1-fold in mice. Sustained pharmacodynamics effects were observed (ie, activated partial thromboplastin time and thrombin generation measured ex vivo). No increased risk of thrombosis was observed with coadministration of rVIII-SingleChain and rD'D3-FP compared with rVIII-SingleChain alone. At concentrations beyond the anticipated therapeutic range, rD'D3-FP reduced the hemostatic efficacy of coadministered rVIII-SingleChain. This finding might be due to scavenging of activated FVIII by the excessive amount of rD'D3-FP which, in turn, might result in a reduced probability of the formation of the tenase complex. This observation underlines the importance of a fine-tuned balance between FVIII and its binding partner, von Willebrand factor, for hemostasis in general.

von Willebrand factor propeptide missense variants affect anterograde transport to Golgi resulting in ER retention

von Willebrand disease (VWD), the most prevalent congenital bleeding disorder, arises from a deficiency in von Willebrand factor (VWF), which has crucial roles in hemostasis. The present study investigated functional consequences and underlying pathomolecular mechanisms of several VWF propeptide (VWFpp) missense variants detected in our cohort of VWD patients for the first time. Transient expression experiments in HEK293T cells demonstrated that four out of the six investigated missense variants (p.Gly55Glu, p.Val86Glu, p.Trp191Arg, and p.Cys608Trp) severely impaired secretion. Their cotransfections with the wild-type partly corrected VWF secretion, displaying loss of large/intermediate multimers. Immunostaining of the transfected HEK293 cells illustrated the endoplasmic reticulum (ER) retention of the VWF variants. Docking of the COP I and COP II cargo recruitment proteins, ADP-ribosylation factor 1 and Sec24, onto the N-terminal VWF model (D1D2D'D3) revealed that these variants occur at VWFpp putative interfaces, which can hinder VWF loading at the ER exit quality control. Furthermore, quantitative and automated morphometric exploration of the three-dimensional immunofluorescence images showed changes in the number/size of the VWF storage organelles, Weibel-Palade body (WPB)-like vesicles. The result of this study highlighted the significance of the VWFpp variants on anterograde ER-Golgi trafficking of VWF as well as the biogenesis of WPB-like vesicles.

Functional Roles of the von Willebrand Factor Propeptide

The primary polypeptide sequence of von Willebrand factor (VWF) includes an N-terminal 741-amino acid VWF propeptide (VWFpp). In cells expressing VWF, the VWFpp performs two critical functions. In the Golgi, VWFpp mediates the intermolecular disulfide linkages that generate high-molecular-weight VWF multimers. Subsequently, the VWFpp, which is proteolytically cleaved from mature VWF by furin, functions to generate the endothelial storage organelles (Weibel-Palade bodies) in which VWF and a distinct collection of proteins are stored, and from where they undergo regulated secretion from the endothelium. The VWFpp is secreted from endothelial cells as dimers and circulates in plasma with at least some of the dimers associating with a noncovalent manner with the D'D3 domain of mature VWF. The VWFpp has a half-life of 2 to 3 hours in plasma, but to date no extracellular function has been determined for the molecule. Nevertheless, its large size and several biologically interesting structural features (two sets of vicinal cysteines and an RGD sequence) suggest that there may be roles that the VWFpp plays in hemostasis or associated physiological processes such as angiogenesis or wound repair.

Peptidome analysis of umbilical cord mesenchymal stem cell (hUC-MSC) conditioned medium from preterm and term infants

Background

The therapeutic role of mesenchymal stem cells (MSCs) has been widely confirmed in several animal models of premature infant diseases. Micromolecule peptides have shown promise for the treatment of premature infant diseases. However, the potential role of peptides secreted from MSCs has not been studied. The purpose of this study is to help to broaden the knowledge of the hUC-MSC secretome at the peptide level through peptidomic profile analysis.

Methods

We used tandem mass tag (TMT) labeling technology followed by tandem mass spectrometry to compare the peptidomic profile of preterm and term umbilical cord MSC (hUC-MSC) conditioned medium (CM). Gene Ontology (GO) enrichment analysis and ingenuity pathway analysis (IPA) were conducted to explore the differentially expressed peptides by predicting the functions of their precursor proteins. To evaluate the effect of candidate peptides on human lung epithelial cells stimulated by hydrogen peroxide (H₂O₂), quantitative real-time PCR (qRT-PCR), western blot analysis, and enzyme-linked immunosorbent assay (ELISA) were, respectively, adopted to detect inflammatory cytokines (TNF-α, IL-1β, and IL-6) expression levels at the mRNA and protein levels.

Results

A total of 131 peptides derived from 106 precursor proteins were differentially expressed in the preterm hUC-MSC CM compared with the term group, comprising 37 upregulated peptides and 94 downregulated peptides. Bioinformatics analysis showed that these differentially expressed peptides may be associated with developmental disorders, inflammatory response, and organismal injury. We also found that peptides ⁷¹¹⁸TGAKIKLVGT⁷¹²⁷ derived from MUC19 and ⁵⁰⁸AAAAGPANVH⁵¹⁷ derived from SIX5 reduced the expression levels of TNF-α, IL-1β, and IL-6 in H₂O₂-treated human lung epithelial cells.

Conclusions

In summary, this study provides further secretomics information on hUC-MSCs and provides a series of peptides that might have antiinflammatory effects on pulmonary epithelial cells and contribute to the prevention and treatment of respiratory diseases in premature infants.

von Willebrand factor self-association is regulated by the shear-dependent unfolding of the A2 domain

von Willebrand factor (VWF) self-association results in the homotypic binding of VWF upon exposure to fluid shear. The molecular mechanism of this process is not established. In this study, we demonstrate that the shear-dependent unfolding of the VWF A2 domain in the multimeric protein is a major regulator of protein self-association. This mechanism controls self-association on the platelet glycoprotein Ibα receptor, on collagen substrates, and during thrombus growth ex vivo. In support of this, A2-domain mutations that prevent domain unfolding due to disulfide bridging of N- and C-terminal residues ("Lock-VWF") reduce self-association and platelet activation under various experimental conditions. In contrast, reducing assay calcium concentrations, and 2 mutations that destabilize VWF-A2 conformation by preventing coordination with calcium (D1498A and R1597W VWD type 2A mutation), enhance self-association. Studies using a panel of recombinant proteins that lack the A1 domain ("ΔA1 proteins") suggest that besides pure homotypic A2 interactions, VWF-A2 may also engage other protein domains to control self-association. Addition of purified high-density lipoprotein and apolipoprotein-A1 partially blocked VWF self-association. Overall, similar conditions facilitate VWF self-association and ADAMTS13-mediated proteolysis, with low calcium and A2 disease mutations enhancing both processes, and locking-A2 blocking them simultaneously. Thus, VWF appears to have evolved 2 balancing molecular functions in a single A2 functional domain to dynamically regulate protein size in circulation: ADAMTS13-mediated proteolysis and VWF self-association. Modulating self-association rates by targeting VWF-A2 may provide novel methods to regulate the rates of thrombosis and hemostasis.

The physical spacing between the von Willebrand factor D'D3 and A1 domains regulates platelet adhesion in vitro and in vivo

Essentials The role of von Willebrand factor (VWF) domains in regulating platelet adhesion was studied in vivo. Multimeric VWF with spacers at the N- and C-terminus of VWF-A1 were systematically tested. N-terminal modified VWF avidly bound platelet GpIbα, causing VWD Type2B like phenotype in mice. Novel anti-D'D3 mAbs suggest that changes at the D'D3-A1 interface may be biologically relevant.

SUMMARY

Background Previous ex vivo studies using truncated VWF (von Willebrand factor) suggest that domain-level molecular architecture may control platelet-GpIbα binding function. Objective We determined if this is the case with multimeric VWF in vivo. Methods Full-length human VWF ('hV') was modified with a 22-amino acid mucinous stretch at either the N-terminus of VWF-A1 to create 'hNV' or C-terminus to yield 'hCV'. This extends the physical distance between VWF-A1 and the adjacent domains by ~6 nm. Similar mucin inserts were also introduced into a human-murine chimera ('h[mA1]V') where murine-A1 replaced human-A1 in hV. This yielded 'h[mA1]NV' and 'h[mA1]CV', with N- and C-terminal inserts. The constructs were tested ex vivo and in vivo. Results Mucin insertion at the N-terminus, but not C-terminus, in both types of constructs resulted in >50-fold increase in binding to immobilized GpIbα. N-terminal insertion also resulted in greater shear-induced platelet activation, more thrombus formation on collagen, enhanced platelet accumulation and slower platelet translocation on immobilized VWF in microfluidics assays. Hydrodynamic injection-based expression of h[mA1]NV, but not h[mA1]V or h[mA1]CV, in VWF-/- mice caused profound thrombocytopenia, reduced plasma VWF concentrations, lower multimer distribution, and incessant tail bleeding that is reminiscent of von Willebrand disease type 2B. Platelet plugs were noted in the portal veins and hepatic arteries. An anti-D'D3 mAb DD3.3 that displays enhanced binding to VWF containing the N-terminal mucin insert also exhibited increased binding to wild-type VWF under shear and upon ristocetin addition. Conclusion Conformation changes at the VWF D'D3-A1 interface may be a key regulator of thrombosis in vivo. Structural features at the A1-A2 interface are likely of less significance.

Role of calcium in regulating the intra- and extracellular cleavage of von Willebrand factor by the protease ADAMTS13

von Willebrand factor (VWF) and the metalloprotease a disintegrin and metalloprotease with thrombospondin type 1 motif 13 (ADAMTS13) are present both within endothelial cells (ECs) and in peripheral blood. Calcium concentrations are lower in intracellular compartments (80-400 μM) compared with the extracellular milieu (∼1.25 mM). Because low calcium favors VWF A2-domain proteolysis by ADAMTS13, the dependence of proteolysis rates on calcium was assayed both within ECs and in blood. Confocal microscopy studies demonstrate partial perinuclear colocalization of VWF with ADAMTS13 in human umbilical vein ECs (HUVECs). Consequently, low levels (5%-10%) of VWF cleavage products were detected in HUVEC lysates and also culture-supernatant following EC stimulation. This proteolysis occurred before disulfide bond formation. Compared with wild-type VWF A2-domain, calcium-binding mutants including the common von Willebrand disease (VWD) type 2A R1597W mutant were expressed in an open conformation in ECs and were highly susceptible to intracellular proteolysis. Fluorescence resonance energy transfer measurements demonstrate strong calcium-dependent VWF-A2 conformation changes at concentrations <500 μM, with unfolding rates being fourfold higher for monomeric VWF A2-domain compared with multimeric, full-length VWF. Under shear, physiological levels of ADAMTS13 did not cleave VWF strings on HUVECs, unless platelets were attached to stretch these strings under flow. Further, VWF-platelet string cleavage under shear proceeded with equal efficiency in the absence and presence of calcium at shear stress ≥1 dyn/cm². Overall, low calcium levels may promote intracellular VWF proteolysis particularly during VWD type 2A disease. Calcium has a negligible effect on VWF-platelet string proteolysis under physiologically relevant fluid shear.

Application of microfluidic devices in studies of thrombosis and hemostasis

Due to the importance of fluid flow during thrombotic episodes, it is quite appropriate to study clotting and bleeding processes in devices that have well-defined fluid shear environments. Two common devices for applying these defined shear stresses include the cone-and-plate viscometer and parallel-plate flow chamber. While such tools have many salient features, they require large amounts of blood or other protein components. With growth in the area of microfluidics over the last two decades, it has become feasible to miniaturize such flow devices. Such miniaturization not only enables saving of precious samples but also increases the throughput of fluid shear devices, thus enabling the design of combinatorial experiments and making the technique more accessible to the larger scientific community. In addition to simple flows that are common in traditional flow apparatus, more complex geometries that mimic stenosed arteries and the human microvasculature can also be generated. The composition of the microfluidics cell substrate can also be varied for diverse basic science investigations, and clinical investigations that aim to assay either individual patient coagulopathy or response to anti-coagulation treatment. This review summarizes the current state of the art for such microfluidic devices and their applications in the field of thrombosis and hemostasis.

Segmental outflow of aqueous humor in mouse and human

The main and only modifiable risk factor in glaucoma, the group of usually late onset progressive and irreversible blinding optic neuropathies, is elevated intraocular pressure (IOP). The increase in IOP is due to impeded aqueous humor (AH) outflow through the conventional pathway. The aberrant increased resistance at the trabecular meshwork (TM), the filter-like region in the anterior eye chamber is the major contributory factor in causing the impeded outflow. In normal as well as in glaucoma eyes the regions of the TM are divided into areas of high and low flow. The collector channels and distal outflow regions are now increasingly being recognized as potential players in contributing to impede AH outflow. Structural and molecular make-up contributing to the segmental blockage to outflow is likely to provide greater insight. Establishing segmental blockage to outflow in model systems of glaucoma such as the mouse in parallel to human eyes will expand our repertoire of tools for investigation. Further study into this area of interest has the potential to ultimately lead to the development of new therapeutics focused on lowering IOP by targeting the various components of segmental blockage of outflow in the TM and in the distal outflow region.

Role of fluid shear stress in regulating VWF structure, function and related blood disorders

Von Willebrand factor (VWF) is the largest glycoprotein in blood. It plays a crucial role in primary hemostasis via its binding interaction with platelet and endothelial cell surface receptors, other blood proteins and extra-cellular matrix components. This protein is found as a series of repeat units that are disulfide bonded to form multimeric structures. Once in blood, the protein multimer distribution is dynamically regulated by fluid shear stress which has two opposing effects: it promotes the aggregation or self-association of multiple VWF units, and it simultaneously reduces multimer size by facilitating the force-dependent cleavage of the protein by various proteases, most notably ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type repeats, motif 1 type 13). In addition to these effects, fluid shear also controls the solution and substrate-immobilized structure of VWF, the nature of contact between blood platelets and substrates, and the biomechanics of the GpIbα–VWF bond. These features together regulate different physiological and pathological processes including normal hemostasis, arterial and venous thrombosis, von Willebrand disease, thrombotic thrombocytopenic purpura and acquired von Willebrand syndrome. This article discusses current knowledge of VWF structure–function relationships with emphasis on the effects of hydrodynamic shear, including rapid methods to estimate the nature and magnitude of these forces in selected conditions. It shows that observations made by many investigators using solution and substrate-based shearing devices can be reconciled upon considering the physical size of VWF and the applied mechanical force in these different geometries.

A systematic analysis of acceptor specificity and reaction kinetics of five human α(2,3)sialyltransferases: Product inhibition studies illustrate reaction mechanism for ST3Gal-I

Sialyltransferases (STs) catalyze the addition of sialic acids to the non-reducing ends of glycoproteins and glycolipids. In this work, we examined the acceptor specificity of five human α(2,3)sialyltransferases, namely ST3Gal -I, -II, -III, -IV and -VI. KM values for each of these enzymes is presented using radioactivity for acceptors containing Type-I (Galβ1,3GlcNAc), Type-II (Galβ1,4GlcNAc), Type-III (Galβ1,3GalNAc) and Core-2 (Galβ1,3(GlcNAcβ1,6)GalNAc) reactive groups. Several variants of acceptors inhibited ST3Gal activity emphasizing structural role of acceptor in enzyme-catalyzed reactions. In some cases, mass spectrometry was performed for structural verification. The results demonstrate human ST3Gal-I catalysis towards Type-III and Core-2 acceptors with KM = 5-50 μM and high VMax values. The KM for ST3Gal-I and ST3Gal-II was 100 and 30-fold lower, respectively, for Type-III compared to Type-I acceptors. Variants of Type-I and Type-II structures characterized ST3Gal-III, -IV and -VI for their catalytic specificity. This manuscript also estimates KM for human ST3Gal-VI using Type-I and Type-II substrates. Together, these findings built a platform for designing inhibitors of STs having therapeutic potential.

Pathological von Willebrand factor fibers resist tissue plasminogen activator and ADAMTS13 while promoting the contact pathway and shear-induced platelet activation

BACKGROUND

Under severe stenotic conditions, von Willebrand factor (VWF) multimerizes into large insoluble fibers at pathological shear rates.

OBJECTIVE

Evaluate the mechanics and biology of VWF fibers without the confounding effects of endothelium or collagen.

METHODS

Within a micropost-impingement microfluidic device, > 100-μm long VWF fibers multimerized on the post within 10 min using EDTA-treated platelet-free plasma (PFP) perfused at wall shear rates > 5000 s(-1) .

RESULTS

von Willebrand factor fiber thickness increased to > 10 μm as a result of increasing the shear rate to 10,000 s(-1) . In a stress-strain test, fibrous VWF had an elastic modulus of ~50 MPa. The insoluble VWF fibers were non-amyloid because they rapidly dissolved in trypsin, plasmin or 2% SDS, but were resistant to 50 nm ADAMTS13 or 100 nm tissue plasminogen activator in plasma. Following fiber formation, perfusion of low corn trypsin inhibitor (CTI)-treated (4 μg mL(-1) ), recalcified citrated plasma at 1500 s(-1) caused fibrin formation on the VWF fibers, a result not observed with purified type 1 collagen or a naked micropost. During VWF fiber formation, contact pathway factors accumulated on VWF because the use of EDTA/D-Phe-Pro-Arg chloromethylketone (PPACK)/apixaban/high CTI-treated PFP during VWF fiber formation prevented the subsequent fibrin production from low-CTI, recalcified citrated PFP. VWF fibers displayed FXIIa-immunostaining. When PPACK-inhibited whole blood was perfused over VWF fibers, platelets rolled and arrested on the surface of VWF, but only displayed P-selectin if prevailing shear rates were pathological. Platelet arrest on VWF fibers was blocked with αIIb β3 antagonist GR144053.

CONCLUSIONS

We report VWF fiber-contact pathway crosstalk and mechanisms of thrombolytic resistance in hemodynamic settings of myocardial infarction.

von Willebrand factor propeptide: biology and clinical utility

von Willebrand factor (VWF) is a large multimeric glycoprotein that mediates the attachment of platelets to damaged endothelium and also serves as the carrier protein for coagulation factor VIII (FVIII), protecting it from proteolytic degradation. Quantitative or qualitative defects in VWF result in von Willebrand disease (VWD), a common inherited bleeding disorder. VWF is synthesized with a very large propeptide (VWFpp) that is critical for intracellular processing of VWF. VWFpp actively participates in the process of VWF multimerization and is essential for trafficking of VWF to the regulated storage pathway. Mutations identified within VWFpp in VWD patients are associated with altered VWF structure and function. The assay of plasma VWFpp has clinical utility in assessing acute and chronic vascular perturbation associated with diseases such as thrombotic thrombocytopenic purpura, sepsis, and diabetes among others. VWFpp assay also has clear utility in the diagnosis of VWD subtypes, particularly in discriminating true type 3 subjects from type 1C (reduced plasma survival of VWF), which is clinically important and has implications for therapeutic treatment.

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.

Platelet GpIba binding to von Willebrand Factor under fluid shear:contributions of the D′D3-domain, A1-domain flanking peptide and O-linked glycans

BACKGROUND

Von Willebrand Factor (VWF) A1-domain binding to platelet receptor GpIbα is an important fluid-shear dependent interaction that regulates both soluble VWF binding to platelets, and platelet tethering onto immobilized VWF. We evaluated the roles of different structural elements at the N-terminus of the A1-domain in regulating shear dependent platelet binding. Specifically, the focus was on the VWF D'D3-domain, A1-domain N-terminal flanking peptide (NFP), and O-glycans on this peptide.

METHODS AND RESULTS

Full-length dimeric VWF (ΔPro-VWF), dimeric VWF lacking the D'D3 domain (ΔD'D3-VWF), and ΔD'D3-VWF variants lacking either the NFP (ΔD'D3NFP(─)-VWF) or just O-glycans on this peptide (ΔD'D3OG(─)-VWF) were expressed. Monomeric VWF-A1 and D'D3-A1 were also produced. In ELISA, the apparent dissociation constant (KD) of soluble ΔPro-VWF binding to immobilized GpIbα (KD≈100 nmol/L) was 50- to 100-fold higher than other proteins lacking the D'D3 domain (KD~0.7 to 2.5 nmol/L). Additionally, in surface plasmon resonance studies, the on-rate of D'D3-A1 binding to immobilized GpIbα (kon=1.8±0.4×10(4) (mol/L)(-1)·s(-1); KD=1.7 μmol/L) was reduced compared with the single VWF-A1 domain (kon=5.1±0.4×10(4) (mol/L)(-1)·s(-1); KD=1.2 μmol/L). Thus, VWF-D'D3 primarily controls soluble VWF binding to GpIbα. In contrast, upon VWF immobilization, all molecular features regulated A1-GpIbα binding. Here, in ELISA, the number of apparent A1-domain sites available for binding GpIbα on ΔPro-VWF was ≈50% that of the ΔD'D3-VWF variants. In microfluidics based platelet adhesion measurements on immobilized VWF and thrombus formation assays on collagen, human platelet recruitment varied as ΔPro-VWF<ΔD'D3-VWF<ΔD'D3NFP(─)-VWF<ΔD'D3OG(─)-VWF.

CONCLUSIONS

Whereas VWF-D'D3 is the major regulator of soluble VWF binding to platelet GpIbα, both the D'D3-domain and N-terminal peptide regulate platelet translocation and thrombus formation.

Glycoproteomic analysis of the secretome of human endothelial cells

Previous proteomics studies have partially unraveled the complexity of endothelial protein secretion but have not investigated glycosylation, a key modification of secreted and membrane proteins for cell communication. In this study, human umbilical vein endothelial cells were kept in serum-free medium before activation by phorbol-12-myristate-13 acetate, a commonly used secretagogue that induces exocytosis of endothelial vesicles. In addition to 123 secreted proteins, the secretome was particularly rich in membrane proteins. Glycopeptides were enriched by zwitterionic hydrophilic interaction liquid chromatography resins and were either treated with PNGase F and H₂¹⁸O or directly analyzed using a recently developed workflow combining higher-energy C-trap dissociation (HCD) with electron-transfer dissociation (ETD) for a hybrid linear ion trap–orbitrap mass spectrometer. After deglycosylation with PNGase F in the presence of H₂¹⁸O, 123 unique peptides displayed ¹⁸O-deamidation of asparagine, corresponding to 86 proteins with a total of 121 glycosylation sites. Direct glycopeptide analysis via HCD-ETD identified 131 glycopeptides from 59 proteins and 118 glycosylation sites, of which 41 were known, 51 were predicted, and 26 were novel. Two methods were compared: alternating HCD-ETD and HCD-product-dependent ETD. The former detected predominantly high-intensity, multiply charged glycopeptides, whereas the latter preferentially selected precursors with complex/hybrid glycans for fragmentation. Validation was performed by means of glycoprotein enrichment and analysis of the input, the flow-through, and the bound fraction. This study represents the most comprehensive characterization of endothelial protein secretion to date and demonstrates the potential of new HCD-ETD workflows for determining the glycosylation status of complex biological samples.

von Willebrand factor: the old, the new and the unknown

von Willebrand factor (VWF) is a protein best known from its critical role in hemostasis. Indeed, any dysfunction of VWF is associated with a severe bleeding tendency known as von Willebrand disease (VWD). Since the first description of the disease by Erich von Willebrand in 1926, remarkable progress has been made with regard to our understanding of the pathogenesis of this disease. The cloning of the gene encoding VWF has allowed numerous breakthroughs, and our knowledge of the epidemiology, genetics and molecular basis of VWD has been rapidly expanding since then. These studies have taught us that VWF is rather unique in terms of its multimeric structure and the unusual mechanisms regulating its participation in the hemostatic process. Moreover, it has become increasingly clear that VWF is a more all-round protein than originally thought, given its involvement in several pathologic processes beyond hemostasis. These include angiogenesis, cell proliferation, inflammation, and tumor cell survival. In the present article, an overview of advances concerning the various structural and functional aspects of VWF will be provided.

The unfolded von Willebrand factor response in bloodstream: the self-association perspective

von Willebrand factor (vWF) is a multimeric glycoprotein essential for hemostasis after vascular injury, which modulates platelet-surface and platelet–platelet interactions by linking platelet receptors to the extracellular matrix and to each other. The crucial role of vWF in platelet function is particularly apparent when hemodynamic conditions create blood flow with high shear stress. Through multiple functional domains, vWF mediates the attachment of platelets to exposed tissues, where immobilized vWF is able to support a homotypic and/or heterotypic self-association. The self-association of vWF is also supported by a rapidly expanding reservoir of novel evidences that the thiol/disulfide exchange regulates vWF multimer size in the blood circulation. Moreover, in addition to proteolysis and reduction of ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), the regulation of vWF multimer size and self-association may depend on a disulfide bond reductase activity ascribed to thrombospondin-1 (TSP-1). Along with the classical signaling pathways in activated platelets, evidence is emerging that lipid rafts also play important roles in various phases of hemostasis and thrombosis and facilitate the interaction between the key signaling molecules. Developments in these areas will refine our understanding of the role played by vWF self-association in physiological hemostasis and pathological thrombosis.