von Willebrand factor sialylation-A critical regulator of biological function

Variant mapping using mass spectrometry-based proteotyping as a diagnostic tool in von Willebrand disease

BACKGROUND

von Willebrand disease (VWD) is the most common inherited bleeding disorder, characterized by either partial or complete von Willebrand factor (VWF) deficiency or by the occurrence of VWF proteoforms of altered functionality. The gene encoding VWF is highly polymorphic, giving rise to a variety of proteoforms with varying plasma concentrations and clinical significance.

OBJECTIVES

To address this complexity, we translated genomic variation in VWF to corresponding VWF proteoforms circulating in blood.

METHODS

VWF was characterized in VWD patients (n = 64) participating in the Willebrand in the Netherlands study by conventional laboratory testing, DNA sequencing and complementary discovery, and targeted mass spectrometry-based plasma proteomic strategies.

RESULTS

Unbiased plasma profiling combined with immune enrichment of VWF verified VWF and its binding partner factor VIII as key determinants of VWD and revealed a remarkable heterogeneity in VWF amino acid sequence coverage among patients. Subsequent VWF proteotyping enabled identification of both polymorphisms (eg, p.Thr789Ala, p.Gln852Arg, and p.Thr1381Ala), as well as pathogenic variants (n = 16) along with their corresponding canonical sequences. Targeted proteomics using stable isotope-labeled peptides confirmed unbiased proteotyping for 5 selected variants and suggested differential proteoform quantities in plasma. The variant-to-wild-type peptide ratio was determined in 6 type 2B patients heterozygous for p.Arg1306Trp, confirming the relatively low proteoform concentration of the pathogenic variant. The elevated VWF propeptide/VWF ratio indicated increased clearance of specific VWF proteoforms.

CONCLUSION

This study highlights how VWF proteotyping from plasma could be the first step to bridge the gap between genotyping and functional testing in VWD.

Desialylation by neuraminidases in platelets, kiss of death or bittersweet?

PURPOSE OF REVIEW

Loss of surface sialic acid by neuraminidases is known as 'desialylation'. Platelets are desialylated in bacterial or viral infections, during storage, senescence, various mutations, platelet auto antibodies, hemostasis and shear stress. In this review the recent literature on the different sialic acid capped glycan structures will be covered as well as platelet desialylation in inherited glycan disorders and induced by external neuraminidases.

RECENT FINDINGS

Neuraminidases are released from platelet intracellular stores and translocated to the platelet surface. Apart from clearance, loss of surface sialic acid by neuraminidases ('desialylation') affects platelet signaling including ligand binding and their procoagulant function. Platelets are also desialylated in infections, various mutations, presence of platelet auto antibodies.

SUMMARY

Since platelet desialylation occurs in various healthy and pathological conditions, measuring desialylation might be a new diagnostic tool.

Clearance of VWF by hepatic macrophages is critical for the protective effect of ADAMTS13 in sickle cell anemia mice

ABSTRACT

Although it is caused by a single-nucleotide mutation in the β-globin gene, sickle cell anemia (SCA) is a systemic disease with complex, incompletely elucidated pathologies. The mononuclear phagocyte system plays critical roles in SCA pathophysiology. However, how heterogeneous populations of hepatic macrophages contribute to SCA remains unclear. Using a combination of single-cell RNA sequencing and spatial transcriptomics via multiplexed error-robust fluorescence in situ hybridization, we identified distinct macrophage populations with diversified origins and biological functions in SCA mouse liver. We previously found that administering the von Willebrand factor (VWF)-cleaving protease ADAMTS13 alleviated vaso-occlusive episode in mice with SCA. Here, we discovered that the ADAMTS13-cleaved VWF was cleared from the circulation by a Clec4f+Marcohigh macrophage subset in a desialylation-dependent manner in the liver. In addition, sickle erythrocytes were phagocytized predominantly by Clec4f+Marcohigh macrophages. Depletion of macrophages not only abolished the protective effect of ADAMTS13 but exacerbated vaso-occlusive episode in mice with SCA. Furthermore, promoting macrophage-mediated VWF clearance reduced vaso-occlusion in SCA mice. Our study demonstrates that hepatic macrophages are important in the pathogenesis of SCA, and efficient clearance of VWF by hepatic macrophages is critical for the protective effect of ADAMTS13 in SCA mice.

Searching for New Biomarkers to Assess COVID-19 Patients: A Pilot Study

During the initial diagnosis of urgent medical conditions, which include acute infectious diseases, it is important to assess the severity of the patient's clinical state as quickly as possible. Unlike individual biochemical or physiological indicators, derived indices make it possible to better characterize a complex syndrome as a set of symptoms, and therefore quickly take a set of adequate measures. Recently, we reported on novel diagnostic indices containing butyrylcholinesterase (BChE) activity, which is decreased in COVID-19 patients. Also, in these patients, the secretion of von Willebrand factor (vWF) increases, which leads to thrombosis in the microvascular bed. The objective of this study was the determination of the concentration and activity of vWF in patients with COVID-19, and the search for new diagnostic indices. One of the main objectives was to compare the prognostic values of some individual and newly derived indices. Patients with COVID-19 were retrospectively divided into two groups: survivors (n = 77) and deceased (n = 24). According to clinical symptoms and computed tomography (CT) results, the course of disease was predominantly moderate in severity. The first blood sample (first point) was taken upon admission to the hospital, the second sample (second point)-within 4-6 days after admission. Along with the standard spectrum of biochemical indicators, BChE activity (BChEa or BChEb for acetylthiocholin or butyrylthiocholin, respectively), malondialdehyde (MDA), and vWF analysis (its antigen level, AGFW, and its activity, ActWF) were determined and new diagnostic indices were derived. The pooled sensitivity, specificity, and area under the receiver operating curve (AUC), as well as Likelihood ratio (LR) and Odds ratio (OR) were calculated. The level of vWF antigen in the deceased group was 1.5-fold higher than the level in the group of survivors. Indices that include vWF antigen levels are superior to indices using vWF activity. It was found that the index [Urea] × [AGWF] × 1000/(BChEb × [ALB]) had the best discriminatory power to predict COVID-19 mortality (AUC = 0.91 [0.83, 1.00], p < 0.0001; OR = 72.0 [7.5, 689], p = 0.0002). In addition, [Urea] × 1000/(BChEb × [ALB]) was a good predictor of mortality (AUC = 0.95 [0.89, 1.00], p < 0.0001; OR = 31.5 [3.4, 293], p = 0.0024). The index [Urea] × [AGWF] × 1000/(BChEb × [ALB]) was the best predictor of mortality associated with COVID-19 infection, followed by [Urea] × 1000/(BChEb × [ALB]). After validation in a subsequent cohort, these two indices could be recommended for diagnostic laboratories.

Evaluating von Willebrand factor and ADAMTS13 levels in thalassemia major patients and assessing a possible association with Thrombospondin-1

INTRODUCTION

Alterations in the endothelium and endothelial adhesion proteins such as von Willebrand factor (vWF) play major roles in hypercoagulability in thalassemia. vWF protein release leads to platelet aggregation and thrombi formation at the site of vascular injury. It is then degraded by the proteolytic enzyme ADAMTS13. Thrombospondin-1 is a multifactorial glycoprotein, which was reported to compete with ADAMTS13 for sites of vWF proteolysis. In this study, levels of vWF, ADAMTS13, and TSP-1 proteins were determined in β-thalassemia major patients. A possible association between TSP-1 and vWF and ADAMTS-13 was also evaluated.

METHODS

The study was conducted on 80 β-thalassemia major patients and 80 age and sex matched healthy controls. The 80 patients were sub-divided into two groups; splenectomised and non-splenectomised. vWF, ADAMTS13 and TSP-1 plasma level were measured using ELISA technique.

RESULTS

There was no significant difference in vWF and TSP-1 levels between patients and controls (p > 0.05). However, ADAMTS13 levels and ADAMTS13 activity/vWF antigen ratio were significantly higher in patients compared to controls (p < 0.05). VWF antigen and TSP-1 level were significantly higher in splenectomised patients (p = 0.025 and p < 0.001, respectively). We also observed a significant decrease in ADAMTS13 activity/vWF antigen ratio among splenectomised compared to non- splenectomised patients (p = 0.019). Correlation analysis showed a significant negative correlation between TSP-1 and vWF Collagen Binding Activity (r = -0.394, p = 0.021) and a positive correlation with ADAMTS13 activity/vWF antigen ratio (r = 0.356, p = 0.039) in splenectomised compared to non- splenectomised patients.

CONCLUSION

Our findings highlight the adequacy of patient management protocols for β-TM in Kuwait as patients presented with comparable levels of platelets, vWF and TSP-1 compared to normal controls. The reported increase in ADAMTS13 in patients may be required to maintain normal levels of vWF. Although no active thrombotic episodes were reported at the time of the study, the significant rise in platelets, vWF:Ag and TSP-1 levels in splenectomised patients may indicate a tendency towards hypercoagulability. Monitoring of splenectomised patients is recommended.

Plasma Clearance of Coagulation Factor VIII and Extension of Its Half-Life for the Therapy of Hemophilia A: A Critical Review of the Current State of Research and Practice

Factor VIII (FVIII) is an important component of blood coagulation as its congenital deficiency results in life-threatening bleeding. Current prophylactic therapy of the disease (hemophilia A) is based on 3-4 intravenous infusions of therapeutic FVIII per week. This poses a burden on patients, demanding reduction of infusion frequency by using FVIII with extended plasma half-life (EHL). Development of these products requires understanding FVIII plasma clearance mechanisms. This paper overviews (i) an up-to-date state of the research in this field and (ii) current EHL FVIII products, including recently approved efanesoctocog alfa, for which the plasma half-life exceeds a biochemical barrier posed by von Willebrand factor, complexed with FVIII in plasma, which results in ~1 per week infusion frequency. We focus on the EHL FVIII products' structure and function, in particular related to the known discrepancy in results of one-stage clotting (OC) and chromogenic substrate (CS) assays used to assign the products' potency, dosing, and for clinical monitoring in plasma. We suggest a possible root cause of these assays' discrepancy that is also pertinent to EHL factor IX variants used to treat hemophilia B. Finally, we discuss approaches in designing future EHL FVIII variants, including those to be used for hemophilia A gene therapy.

Low von Willebrand Disease: A Bleeding Disorder of Unknown Cause?

von Willebrand disease (VWD) represents the most common inherited bleeding disorder. The majority of VWD cases are characterized by partial quantitative reductions in plasma von Willebrand factor (VWF) levels. Management of patients with mild to moderate VWF reductions in the range of 30 to 50 IU/dL poses a common clinical challenge. Some of these low VWF patients present with significant bleeding problems. In particular, heavy menstrual bleeding and postpartum hemorrhage can cause significant morbidity. Conversely, however, many individuals with mild plasma VWF:Ag reductions do not have any bleeding sequelae. In contrast to type 1 VWD, most patients with low VWF do not have detectable pathogenic VWF sequence variants, and bleeding phenotype correlates poorly with residual VWF levels. These observations suggest that low VWF is a complex disorder caused by variants in other genes beyond VWF. With respect to low VWF pathobiology, recent studies have shown that reduced VWF biosynthesis within endothelial cells likely plays a key role. However, pathological enhanced VWF clearance from plasma has also been described in approximately 20% of low VWF cases. For low VWF patients who require hemostatic treatment prior to elective procedures, tranexamic acid and desmopressin have both been shown to be efficacious. In this article, we review the current state of the art regarding low VWF. In addition, we consider how low VWF represents an entity that appears to fall between type 1 VWD on the one hand and bleeding disorders of unknown cause on the other.

Heterogeneity in the half-life of factor VIII concentrate in patients with hemophilia A is due to variability in the clearance of endogenous von Willebrand factor

BACKGROUND

Previous studies have reported marked interindividual variation in factor VIII (FVIII) clearance in patients with hemophilia (PWH) and proposed a number of factors that influence this heterogeneity.

OBJECTIVES

To investigate the importance of the clearance rates of endogenous von Willebrand factor (VWF) compared with those of other FVIII half-life modifiers in adult PWH.

METHODS

The half-life of recombinant FVIII was determined in a cohort of 61 adult PWH. A range of reported modifiers of FVIII clearance was assessed (including plasma VWF:antigen and VWF propeptide levels; VWF-FVIII binding capacity; ABO blood group; and nonneutralizing anti-FVIII antibodies). The FVIII-binding region of the VWF gene was sequenced. Finally, the effects of variation in FVIII half-life on clinical phenotype were investigated.

RESULTS

We demonstrated that heterogeneity in the clearance of endogenous plasma VWF is a key determinant of variable FVIII half-life in PWH. Both ABO blood group and age significantly impact FVIII clearance. The effect of ABO blood group on FVIII half-life in PWH is modulated entirely through its effect on the clearance rates of endogenous VWF. In contrast, the age-related effect on FVIII clearance is, at least in part, VWF independent. In contrast to previous studies, no major effects of variation in VWF-FVIII binding affinity on FVIII clearance were observed. Although high-titer immunoglobulin G antibodies (≥1:80) were observed in 26% of PWH, these did not impact FVIII half-life. Importantly, the annual FVIII usage (IU/kg/y) was significantly (p = .0035) increased in patients with an FVIII half-life of <12 hours.

CONCLUSION

Our data demonstrate that heterogeneity in the half-life of FVIII concentrates in patients with hemophilia A is primarily attributable to variability in the clearance of endogenous VWF.

Dengue Infection - Recent Advances in Disease Pathogenesis in the Era of COVID-19

The dynamics of host-virus interactions, and impairment of the host’s immune surveillance by dengue virus (DENV) serotypes largely remain ambiguous. Several experimental and preclinical studies have demonstrated how the virus brings about severe disease by activating immune cells and other key elements of the inflammatory cascade. Plasmablasts are activated during primary and secondary infections, and play a determinative role in severe dengue. The cross-reactivity of DENV immune responses with other flaviviruses can have implications both for cross-protection and severity of disease. The consequences of a cross-reactivity between DENV and anti-SARS-CoV-2 responses are highly relevant in endemic areas. Here, we review the latest progress in the understanding of dengue immunopathogenesis and provide suggestions to the development of target strategies against dengue.

The Intriguing Connections between von Willebrand Factor, ADAMTS13 and Cancer

von Willebrand factor (VWF) is a complex and large protein that is cleaved by ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motif, member 13), and together they serve important roles in normal hemostasis. Malignancy can result in both a deficiency or excess of VWF, leading to aberrant hemostasis with either increased bleeding or thrombotic complications, as respectively seen with acquired von Willebrand syndrome and cancer-associated venous thromboembolism. There is emerging evidence to suggest VWF also plays a role in inflammation, angiogenesis and tumor biology, and it is likely that VWF promotes tumor metastasis. High VWF levels have been documented in a number of malignancies and in some cases correlate with more advanced disease and poor prognosis. Tumor cells can induce endothelial cells to release VWF and certain tumor cells have the capacity for de novo expression of VWF, leading to a proinflammatory microenvironment that is likely conducive to tumor progression, metastasis and micro-thrombosis. VWF can facilitate tumor cell adhesion to endothelial cells and aids with the recruitment of platelets into the tumor microenvironment, where tumor/platelet aggregates are able to form and facilitate hematogenous spread of cancer. As ADAMTS13 moderates VWF level and activity, it too is potentially involved in the pathophysiology of these events. VWF and ADAMTS13 have been explored as tumor biomarkers for the detection and prognostication of certain malignancies; however, the results are underdeveloped and so currently not utilized for clinical use. Further studies addressing the basic science mechanisms and real word epidemiology are required to better appreciate the intriguing connections between VWF, ADAMTS13 and malignancy. A better understanding of the role VWF and ADAMTS13 play in the promotion and inhibition of cancer and its metastasis will help direct further translational studies to aid with the development of novel cancer prognostic tools and treatment modalities.

Sialylation on O-linked glycans protects von Willebrand factor from macrophage galactose lectin-mediated clearance

Terminal sialylation determines the plasma half-life of von Willebrand factor (VWF). A role for macrophage galactose lectin (MGL) in regulating hyposialylated VWF clearance has recently been proposed. In this study, we showed that MGL influences physiological plasma VWF clearance. MGL inhibition was associated with a significantly extended mean residence time and 3-fold increase in endogenous plasma VWF antigen levels (P<0.05). Using a series of VWF truncations, we further demonstrated that the A1 domain of VWF is predominantly responsible for enabling the MGL interaction. Binding of both full-length and VWF-A1-A2-A3 to MGL was significantly enhanced in the presence of ristocetin (P<0.05), suggesting that the MGL-binding site in A1 is not fully accessible in globular VWF. Additional studies using different VWF glycoforms demonstrated that VWF O-linked glycans, clustered at either end of the A1 domain, play a key role in protecting VWF against MGLmediated clearance. Reduced sialylation has been associated with pathological, increased clearance of VWF in patients with von Willebrand disease. Herein, we demonstrate that specific loss of α2-3 linked sialylation from O-glycans results in markedly increased MGL-binding in vitro, and markedly enhanced MGL-mediated clearance of VWF in vivo. Our data further show that the asialoglycoprotein receptor (ASGPR) does not have a significant role in mediating the increased clearance of VWF following loss of O-sialylation. Conversely however, we observed that loss of N-linked sialylation from VWF drives enhanced circulatory clearance predominantly via the ASGPR. Collectively, our data support the hypothesis that in addition to regulating physiological VWF clearance, the MGL receptor works in tandem with ASGPR to modulate enhanced clearance of aberrantly sialylated VWF in the pathogenesis of von Willebrand disease.

Obstetric outcomes in pregnant COVID-19 women: the imbalance of von Willebrand factor and ADAMTS13 axis

Background

Thrombotic microangiopathy has been invoked as one of the most important mechanisms of damage in COVID-19 patients. Protease ADAMTS13 is a marker of microangiopathy responsible for controlling von Willebrand multimers size. Von Willebrand factor/ADAMTS13 ratio has been found impaired in COVID-19 patients outside pregnancy.

Methods

We prospectively investigated 90 pregnant women admitted to two tertiary academic hospitals in Italy with a laboratory-confirmed diagnosis of SARS-CoV-2 infection. Demographic, clinical information and routine laboratory data were collected at the hospital admission and until discharge. We investigated whether vonWillebrand /ADAMTS13 axis imbalance is a predictor of adverse outcomes. Logistic regression analysis, which controlled for potential confounders, was performed to evaluate the association between laboratory parameters and clinical outcomes.

Results

Most women (55.6%) were parae, with median gestational age at admission of 39 weeks. At hospital admission, 63.3% were asymptomatic for COVID-19 and 24.4% showed more than one sign or symptom of infection. Nulliparae with group O showed Willebrand / ADA MTS-13 ratios significantly lower than non-O, whereas in multiparae this difference was not observed. Logistic regression showed that ratio von Willebrand to ADAMTS13 was significantly and independently associated with preterm delivery (OR 1.9, 95%CI 1.1–3.5).

Conclusion

This study shows an imbalance of vonWillebrand /ADAMTS13 axis in pregnant women with COVID-19, leading to a significantly higher and independent risk of preterm delivery. Monitoring these biomarkers might support decision making process to manage and follow-up pregnancies in this setting.

Desialylation of O-glycans activates von Willebrand factor by destabilizing its autoinhibitory module

BACKGROUND

The binding of the A1 domain of von Willebrand factor (VWF) to platelet receptor glycoprotein (GP)Ibα defines the VWF activity in hemostasis. Recent studies suggest that sequences flanking A1 form cooperatively an autoinhibitory module (AIM) that reduces the accessibility of the GPIbα binding site on A1. Application of a tensile force induces unfolding of the AIM. Desialylation induces spontaneous binding of plasma VWF to platelets. Most O-glycans in VWF are located around the A1 domain. Removing certain O-glycans in the flanking sequences by site-directed mutagenesis enhances A1 binding to GPIbα and produces an effect similar to type 2B von Willebrand disease in animals.

OBJECTIVES

To understand if and how desialylation of O-glycans in the flanking sequences increases A1 activity.

METHODS

A recombinant AIM-A1 fragment encompassing VWF residues 1238-1493 and only O-glycans was treated with neuraminidase to produce desialylated protein. The glycan structure, dynamics, stability, and function of the desialylated protein was characterized by biochemical and biophysical methods and compared to the sialylated fragment.

RESULTS

Asialo-AIM-A1 exhibited increased binding activity and induced more apparent platelet aggregation than its sialylated counterpart. It exhibited a lower melting temperature, and increased hydrogen-deuterium exchange rates at residues near the secondary GPIbα binding site and the N-terminal flanking sequence. Asialo-AIM-A1 is less mechanically stable than sialo-AIM-A1, with its unstressed unfolding rate approximately 3-fold greater than the latter.

CONCLUSIONS

Desialylation of O-glycans around A1 increases its activity by destabilizing the AIM.

ADAMTS13 regulation of VWF multimer distribution in severe COVID-19

BACKGROUND

Consistent with fulminant endothelial cell activation, elevated plasma von Willebrand factor (VWF) antigen levels have been reported in patients with COVID-19. The multimeric size and function of VWF are normally regulated through A Disintegrin And Metalloprotease with ThrombSpondin Motif type 1 motif, member 13 (ADAMTS-13)--mediated proteolysis.

OBJECTIVES

This study investigated the hypothesis that ADAMTS-13 regulation of VWF multimer distribution may be impaired in severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) infection contributing to the observed microvascular thrombosis.

PATIENTS AND METHODS

Patients with COVID-19 (n = 23) were recruited from the Beaumont Hospital Intensive Care Unit (ICU) in Dublin. Plasma VWF antigen, multimer distribution, ADAMTS-13 activity, and known inhibitors thereof were assessed.

RESULTS

We observed markedly increased VWF collagen-binding activity in patients with severe COVID-19 compared to controls (median 509.1 versus 94.3 IU/dl). Conversely, plasma ADAMTS-13 activity was significantly reduced (median 68.2 IU/dl). In keeping with an increase in VWF:ADAMTS-13 ratio, abnormalities in VWF multimer distribution were common in patients with COVID-19, with reductions in high molecular weight VWF multimers. Terminal sialylation regulates VWF susceptibility to proteolysis by ADAMTS-13 and other proteases. We observed that both N- and O-linked sialylation were altered in severe COVID-19. Furthermore, plasma levels of the ADAMTS-13 inhibitors interleukin-6, thrombospondin-1, and platelet factor 4 were significantly elevated.

CONCLUSIONS

These findings support the hypothesis that SARS-CoV-2 is associated with profound quantitative and qualitative increases in plasma VWF levels, and a multifactorial down-regulation in ADAMTS-13 function. Further studies will be required to determine whether therapeutic interventions to correct ADAMTS-13-VWF multimer dysfunction may be useful in COVID-microvascular thrombosis and angiopathy.

Von Willebrand Factor in Health and Disease

Abstract—

Von Willebrand factor (vWF), the key component of hemostasis, is synthesized in endothelial cells and megakaryocytes and released into the blood as high molecular weight multimeric glycoproteins weighing up to 20 million Daltons. Blood plasma metalloprotease ADAMTS13 cleaves ultra-large vWF multimers to smaller multimeric and oligomeric molecules. The vWF molecules attach to the sites of damage at the surface of arterioles and capillaries and unfold under conditions of shear stress. On the unfolded vWF molecule, the regions interacting with receptors on the platelet membrane are exposed. After binding to the vWF filaments, platelets are activated; platelets circulating in the vessels are additionally attached to them, leading to thrombus formation, blocking of microvessels, and cessation of bleeding. This review describes the history of the discovery of vWF, presents data on the mechanisms of vWF secretion and its structure, and characterizes the processes of vWF metabolism in the body under normal and pathological conditions.

The Biological Significance of von Willebrand Factor O-Linked Glycosylation

Glycosylation is a key posttranslational modification, known to occur on more than half of all secreted proteins in man. As such, the role of N- and O-linked glycan structures in modulating various aspects of protein biology is an area of much research. Given their prevalence, it is perhaps unsurprising that variations in glycan structures have been demonstrated to play critical roles in modulating protein function and have been implicated in the pathophysiology of human diseases. von Willebrand factor (VWF), a plasma glycoprotein that is essential for normal hemostasis, is heavily glycosylated, containing 13 N-linked and 10 O-linked glycans. Together, these carbohydrate chains account for 20% of VWF monomeric mass, and have been shown to modulate VWF structure, function, and half-life. In this review, we focus on the specific role played by O-linked glycans in modulating VWF biology. Specifically, VWF O-linked glycans have been shown to modulate tertiary protein structure, susceptibility to ADAMTS13 proteolysis, platelet tethering, and VWF circulatory half-life.

Low VWF: insights into pathogenesis, diagnosis, and clinical management

von Willebrand disease (VWD) constitutes the most common inherited human bleeding disorder. Partial quantitative von Willebrand factor (VWF) deficiency is responsible for the majority of VWD cases. International guidelines recommend that patients with mild to moderate reductions in plasma VWF antigen (VWF:Ag) levels (typically in the range of 30-50 IU/dL) should be diagnosed with low VWF. Over the past decade, a series of large cohort studies have provided significant insights into the biological mechanisms involved in type 1 VWD (plasma VWF:Ag levels <30 IU/dL). In striking contrast, however, the pathogenesis underpinning low VWF has remained poorly understood. Consequently, low VWF patients continue to present significant clinical challenges with respect to genetic counseling, diagnosis, and management. For example, there is limited information regarding the relationship between plasma VWF:Ag levels and bleeding phenotype in subjects with low VWF. In addition, it is not clear whether patients with low VWF need treatment. For those patients with low VWF in whom treatment is deemed necessary, the optimal choice of therapy remains unknown. However, a number of recent studies have provided important novel insights into these clinical conundrums and the molecular mechanisms responsible for the reduced levels observed in low VWF patients. These emerging clinical and scientific findings are considered in this review, with particular focus on pathogenesis, diagnosis, and clinical management of low VWF.

The natural sequence of events in larval settlement and metamorphosis of Hydroides elegans (Polychaeta; Serpulidae)

The broadly distributed serpulid worm Hydroides elegans has become a model organism for studies of marine biofouling, development and the processes of larval settlement and metamorphosis induced by surface microbial films. Contrasting descriptions of the initial events of these recruitment processes, whether settlement is induced by (1) natural multi-species biofilms, (2) biofilms composed of single bacterial species known to induce settlement, or (3) a bacterial extract stimulated the research described here. We found that settlement induced by natural biofilms or biofilms formed by the bacterium Pseudoalteromonas luteoviolacea is invariably initiated by attachment and secretion of an adherent and larva-enveloping primary tube, followed by loss of motile cilia and ciliated cells and morphogenesis. The bacterial extract containing complex tailocin arrays derived from an assemblage of phage genes incorporated into the bacterial genome appears to induce settlement events by destruction of larval cilia and ciliated cells, followed by attachment and primary-tube formation. Similar destruction occurred when precompetent larvae of H. elegans or larvae of a nudibranch gastropod were exposed to the extract, although neither of them metamorphosed. We argue that larvae that lose their cilia before attachment would be swept away from the sites that stimulated settlement by the turbulent flow characteristic of most marine habitats.

Perspective - The case for zero bleeds and drug bioequivalence in the treatment of congenital hemophilia A in 2021

Not all patients with severe hemophilia A (HA) respond optimally to a given dose of a given product. Within-individual variance in cross-over studies makes each patient unique in the response to each standard half-life (SHL) factor VIII (FVIII) product in pharmacokinetic (PK) terms. This hampers the prediction of efficacy when a SHL FVIII product is employed. PK data showing that half-lives of SHL rFVIII are unsatisfactory to achieve zero bleeding in individual HA patients provide the rationale for switching from SHL to extended half-life (EHL) products. However, not all subjects receiving prophylaxis with EHL products achieve zero bleeding, the most cogent objective of personalized prophylaxis. Known determinants of FVIII half-life (age, von Willebrand factor [VWF] levels, blood group) cumulatively account for one third of the total inter-individual variation in FVIII clearance in subjects with severe HA. Investigations into precision, and accuracy of laboratory measurement to be employed; newer pathways for the clearance of both free-FVIII and VWF-bound FVIII, and adequately powered studies on omics and phenotypic heterogeneity, are likely to provide additional information on the remaining two thirds of inter-individual variation in FVIII clearance in HA. Variability in the clinical response has also been documented in patients when FVIII activity is mimicked by fixed subcutaneous doses of the bispecific antibody emicizumab. National registries that collect PK data of available FVIII products and ad hoc information on the individual response to emicizumab should be encouraged, to establish newer standards of care and ease personalized clinical decisions to achieve zero bleeding.

The relationship between ABO blood group, von Willebrand factor, and primary hemostasis

Numerous studies have reported significant associations between ABO blood group and risk of cardiovascular disease. These studies have consistently demonstrated that thrombotic risk is significantly reduced in individuals in blood group O. Nevertheless, the biological mechanisms through which ABO influences hemostasis have remained poorly understood. Exciting recent data have provided novel insights into how these ABO effects are modulated and have highlighted that ABO group significantly influences platelet plug formation at sites of vascular injury (primary hemostasis). In particular, ABO affects multiple aspects of von Willebrand factor (VWF) biology. In keeping with their reduced thrombotic risk, plasma VWF levels are ∼25% lower in healthy group O compared with healthy group non-O individuals. In addition, blood group O VWF demonstrates enhanced susceptibility to ADAMTS13 proteolysis. Finally, preliminary findings suggest that the interaction of group O VWF with platelets may also be reduced. Although the molecular mechanisms underlying these ABO effects on VWF have not been fully elucidated, it seems likely that they are mediated in large part by the ABO(H) carbohydrate structures that are carried on both the N- and O-linked glycans of VWF. Interestingly, ABO(H) determinants are also expressed on several different platelet surface glycoprotein receptors. Recent studies support the hypothesis that ABO group not only exerts major quantitative and qualitative effects on VWF, but also affect specific aspects of platelet function. Given the severe morbidity and the mortality associated with thrombotic disorders, defining the mechanisms underlying these ABO effects is not only of scientific interest, but also of direct clinical importance.

Toward Personalized Treatment for Patients with Low von Willebrand Factor and Quantitative von Willebrand Disease

The biological mechanisms involved in the pathogenesis of type 2 and type 3 von Willebrand disease (VWD) have been studied extensively. In contrast, although accounting for the majority of VWD cases, the pathobiology underlying partial quantitative VWD has remained somewhat elusive. However, important insights have been attained following several recent cohort studies that have investigated mechanisms in patients with type 1 VWD and low von Willebrand factor (VWF), respectively. These studies have demonstrated that reduced plasma VWF levels may result from either (1) decreased VWF biosynthesis and/or secretion in endothelial cells and (2) pathological increased VWF clearance. In addition, it has become clear that some patients with only mild to moderate reductions in plasma VWF levels in the 30 to 50 IU/dL range may have significant bleeding phenotypes. Importantly in these low VWF patients, bleeding risk fails to correlate with plasma VWF levels and inheritance is typically independent of the VWF gene. Although plasma VWF levels may increase to > 50 IU/dL with progressive aging or pregnancy in these subjects, emerging data suggest that this apparent normalization in VWF levels does not necessarily equate to a complete correction in bleeding phenotype in patients with partial quantitative VWD. In this review, these recent advances in our understanding of quantitative VWD pathogenesis are discussed. Furthermore, the translational implications of these emerging findings are considered, particularly with respect to designing personalized treatment plans for VWD patients undergoing elective procedures.

Circulating microRNAs May Serve as Biomarkers for Hypertensive Emergency End-Organ Injuries and Address Underlying Pathways in an Animal Model

There is an incomplete understanding of the underlying pathophysiology in hypertensive emergencies, where severely elevated blood pressure causes acute end-organ injuries, as opposed to the long-term manifestations of chronic hypertension. Furthermore, current biomarkers are unable to detect early end-organ injuries like hypertensive encephalopathy and renal thrombotic microangiopathy. We hypothesized that circulating microRNAs (c-miRs) could identify acute and chronic complications of severe hypertension, and that combinations of c-miRs could elucidate important pathways involved. We studied the diagnostic accuracy of 145 c-miRs in Dahl salt-sensitive rats fed either a low-salt (N = 20: 0.3% NaCl) or a high-salt (N = 60: 8% NaCl) diet. Subclinical hypertensive encephalopathy and thrombotic microangiopathy were diagnosed by histopathology. In addition, heart failure with preserved ejection fraction was evaluated with echocardiography and N-terminal pro-brain natriuretic peptide; and endothelial dysfunction was studied using acetylcholine-induced aorta ring relaxation. Systolic blood pressure increased severely in animals on a high-salt diet (high-salt 205 ± 20 mm Hg vs. low-salt 152 ± 18 mm Hg, p < 0.001). Partial least squares discriminant analysis revealed 68 c-miRs discriminating between animals with and without hypertensive emergency complications. Twenty-nine c-miRs were strongly associated with hypertensive encephalopathy, 24 c-miRs with thrombotic microangiopathy, 30 c-miRs with heart failure with preserved ejection fraction, and 28 c-miRs with endothelial dysfunction. Hypertensive encephalopathy, thrombotic microangiopathy and heart failure with preserved ejection fraction were associated with deviations in many of the same c-miRs, whereas endothelial dysfunction was associated with a different set of c-miRs. Several of these c-miRs demonstrated fair to good diagnostic accuracy for a composite outcome of hypertensive encephalopathy, thrombotic microangiopathy and heart failure with preserved ejection fraction in receiver-operating-curve analyses (area-under-curve 0.75–0.88). Target prediction revealed an enrichment of genes related to several pathways relevant for cardiovascular disease (e.g., mucin type O-glycan biosynthesis, MAPK, Wnt, Hippo, and TGF-beta signaling). C-miRs could potentially serve as biomarkers of severe hypertensive end-organ injuries and elucidate important pathways involved.

Glycosylation of Therapeutic Proteins: A Critical Quality Attribute

Glycosylation is a common posttranslational modification of therapeutic proteins. The glycosylation pattern is dependent on many parameters such as the host cell line or the culture conditions. N- and O-linked glycans usually play a great role on the stability, safety, and efficacy of the drug. For this reason, glycosylation is considered as a critical quality attribute of therapeutic glycoproteins, and a thorough characterization should be performed, as well as a systematic control for each batch produced. This chapter gives a short presentation of the structure of glycans commonly found on recombinant therapeutic proteins, and their role on the properties of the drug, in terms of stability, pharmacokinetics, safety, and efficacy. Lastly, the use of mass spectrometry for the analysis of glycoproteins is briefly described.

Expresser phenotype determines ABO(H) blood group antigen loading on platelets and von Willebrand factor

ABO blood group is associated with cardiovascular disease, with significantly lower risk in blood group O individuals. ABO(H) blood group determinants are expressed on different glycoproteins on platelet surfaces. In addition, ABO(H) structures are also present on VWF glycans. These ABO(H) carbohydrates influence both platelet and VWF function. Previous studies have reported that approximately 5-10% of normal blood donors express abnormally high or low levels of A or B blood group antigens on their platelet surfaces (high expresser phenotype, HXP or low expresser phenotype, LXP respectively). In this study, the biological effects of the ABO Expresser phenotype were investigated. ABO(H) expression on platelets and plasma VWF was studied in a series of 541 healthy blood donors. Overall, 5.6% of our study cohort were classified as HXP, whilst 4.4% satisfied criteria for LXP. We demonstrate that genotype at the ABO blood group locus plays a critical role in modulating the platelet HXP phenotype. In particular, A1A1 genotype is a major determinant of ABO high-expresser trait. Our data further show that ABH loading on VWF is also affected by ABO expresser phenotype. Consequently, A antigen expression on VWF was significantly elevated in HXP individuals and moderately reduced in LXP subjects (P < 0.05). Collectively, these findings suggest that ABO expresser phenotype influences primary hemostasis though several different pathways. Further studies will be required to define whether inter-individual variations in ABO(H) expression on platelets and/or VWF (particularly HXP and LXP) impact upon risk for cardiovascular disease.

Biological mechanisms underlying inter-individual variation in factor VIII clearance in haemophilia

Previous studies have highlighted marked inter‐individual variations in factor VIII (FVIII) clearance between patients with haemophilia (PWH). The half‐life of infused FVIII has been reported to vary from as little as 5.3 hours in some adult PWH, up to as long as 28.8 hours in other individuals. These differences in clearance kinetics have been consistently observed using a number of different plasma‐derived and recombinant FVIII products. Furthermore, recent studies have demonstrated that half‐life for extended half‐life (EHL‐) FVIII products also demonstrates significant inter‐patient variation. Since time spent with FVIII trough levels <1% has been shown to be associated with increased bleeding risk in PWH on prophylaxis therapy, this variability in FVIII clearance clearly has major clinical significance. Recent studies have provided significant novel insights into the cellular basis underlying FVIII clearance pathways. In addition, accumulating data have shown that endogenous plasma VWF levels, ABO blood group and age, all play important roles in regulating FVIII half‐life in PWH. Indeed, multiple regression analysis suggests that together these factors account for approximately 34% of the total inter‐individual variation in FVIII clearance observed between subjects with severe haemophilia A. In this review, we consider these and other putative modulators of FVIII half‐life, and discuss the biological mechanisms through which these factors impact upon FVIII clearance in vivo.

New developments in von Willebrand disease

Von Willebrand disease (VWD) constitutes the most common inherited human bleeding disorder. It is associated with a mucocutaneous bleeding phenotype that can significantly impact upon quality of life. Despite its prevalence and associated morbidity, the diagnosis and subclassification of VWD continue to pose significant clinical challenges. This is in part attributable to the fact that plasma von Willebrand factor (VWF) levels vary over a wide range in the normal population, together with the multiple different physiological functions played by VWF in vivo. Over recent years, substantial progress has been achieved in elucidating the biological roles of VWF. Significant advances have also been made into defining the pathophysiological mechanisms underpinning both quantitative and qualitative VWD. In particular, several new laboratory assays have been developed that enable more precise assessment of specific aspects of VWF activity. In the present review, we discuss these recent developments in the field of VWD diagnosis, and consider how these advances can impact upon clinical diagnostic algorithms for use in routine clinical practice. In addition, we review some important recent advances pertaining to the various treatment options available for managing patients with VWD.

ABO blood group is a determinant of von Willebrand factor protein levels in human pulmonary endothelial cells

ABO blood group antigens are expressed on von Willebrand factor (VWF) and glycosylation patterns influence circulating VWF levels. The aim of this study was to examine the effect of ABO blood type on tissue-associated VWF protein levels. We selected 35 formalin-fixed paraffin-embedded pulmonary tissue blocks obtained at autopsy from decedents who died from pulmonary embolism with known ABO blood groups (O, A, B and AB phenotypes), prepared tissue microarrays (TMAs) and stained TMAs with antibodies to VWF and platelet/endothelial cell adhesion marker-1 (PECAM-1) as a marker of endothelial cells. A pixel count scoring algorithm was used to quantify VWF and PECAM-1 staining intensity in pulmonary arterioles in digitised images. Compared with type O, non-O individuals have a significantly higher amount of endothelial cell-associated VWF protein expression. VWF protein levels associated with pulmonary vascular endothelial cells is influenced by ABO antigenic determinants.