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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. [ABSTRACT FROM AUTHOR]

Summary: Endothelial cell (EC) activation plays a key role in the pathogenesis of pulmonary microvascular occlusion, which is a hallmark of severe coronavirus disease 2019 (COVID‐19). Consistent with EC activation, increased plasma von Willebrand factor antigen (VWF:Ag) levels have been reported in COVID‐19. Importantly however, studies in other microangiopathies have shown that plasma VWF propeptide (VWFpp) is a more sensitive and specific measure of acute EC activation. In the present study, we further investigated the nature of EC activation in severe COVID‐19. Markedly increased plasma VWF:Ag [median (interquatile range, IQR) 608·8 (531–830)iu/dl] and pro‐coagulant factor VIII (FVIII) levels [median (IQR) 261·9 (170–315) iu/dl] were seen in patients with severe severe acute respiratory syndrome coronavirus‐2 (SARS‐CoV‐2) infection. Sequential testing showed that these elevated VWF–FVIII complex levels remained high for up to 3 weeks. Similarly, plasma VWFpp levels were also markedly elevated [median (IQR) 324·6 (267–524) iu/dl]. Interestingly however, the VWFpp/VWF:Ag ratio was reduced, demonstrating that decreased VWF clearance contributes to the elevated plasma VWF:Ag levels in severe COVID‐19. Importantly, plasma VWFpp levels also correlated with clinical severity indices including the Sequential Organ Failure Assessment (SOFA) score, Sepsis‐Induced Coagulopathy (SIC) score and the ratio of arterial oxygen partial pressure to fractional inspired oxygen (P/F ratio). Collectively, these findings support the hypothesis that sustained fulminant EC activation is occurring in severe COVID‐19, and further suggest that VWFpp may have a role as a biomarker in this setting. [ABSTRACT FROM AUTHOR]

Summary: The mechanisms involved in regulating von Willebrand factor (VWF) clearance remain poorly understood. However recent studies have shown that macrophages play a critical role in regulating the half‐life of VWF, and have identified specific lectin (including asialoglycoprotein, macrophage galactose‐type lectin, Sigec‐5 and C‐type lectin domain family 4 member M) and scavenger receptors (including low‐density lipoprotein receptor‐related protein‐1, scavenger receptor A1 and stabilin‐2) that are involved in VWF clearance. Further studies will be required to determine the relative importance of these individual receptors with respect to physiological and pathological VWF clearance. Nevertheless, recent clinical data have highlighted the importance of enhanced VWF clearance in the pathogenesis of type 1 von Willebrand disease (VWD). Moreover, increased clearance also contributes to reduced VWF levels in many patients with type 2 and type 3 VWD. Improved understanding regarding VWF clearance is not only of direct biological relevance, but may also have important implications for the development of novel therapeutic agents with extended plasma half‐lives for the treatment of both VWD and haemophilia A. [ABSTRACT FROM AUTHOR]

Previous studies have shown that loss of terminal sialic acid causes enhanced von Willebrand factor (VWF) clearance through the Ashwell-Morrell receptor (AMR). In this study, we investigated (1) the specific importance of N- vs O-linked sialic acid in protecting against VWF clearance and (2) whether additional receptors contribute to the reduced half-life of hyposialylated VWF. α2-3-linked sialic acid accounts for <20% of total sialic acid and is predominantly expressed on VWF O-glycans. Nevertheless, specific digestion with α2-3 neuraminidase (α2-3Neu-VWF) was sufficient to cause markedly enhanced VWF clearance. Interestingly, in vivo clearance experiments in dual VWF-/-/Asgr1-/- mice demonstrated enhanced clearance of α2-3Neu-VWF even in the absence of the AMR. The macrophage galactose-type lectin (MGL) is a C-type lectin that binds to glycoproteins expressing terminal N-acetylgalactosamine or galactose residues. Importantly, the markedly enhanced clearance of hyposialylated VWF in VWF-/-/Asgr1-/- mice was significantly attenuated in the presence of an anti-MGL inhibitory antibody. Furthermore, dose-dependent binding of human VWF to purified recombinant human MGL was confirmed using surface plasmon resonance. Additionally, plasma VWF:Ag levels were significantly elevated in MGL1-/- mice compared with controls. Collectively, these findings identify MGL as a novel macrophage receptor for VWF that significantly contributes to the clearance of both wild-type and hyposialylated VWF. [ABSTRACT FROM AUTHOR]

We read with interest the recent report from Li I et al i .[1] describing an association between ABO blood groups and risk of SARS-CoV-2 pneumonia. In an initial study of 265 patients with COVID, the authors observed that blood group O individuals were significantly underrepresented amongst patients who required hospitalization for severe COVID-19 infection ( I P i < 0-01). First, plasma VWF levels are 20-30% lower in normal blood group O individuals compared to non-O subjects.[5] These reduced VWF levels are due to the fact that group O VWF has a significantly reduced plasma half-life compared to non-O VWF (10-0 compared to 25-5 h).[14] Since FVIII circulates in high-affinity complex with VWF, plasma FVIII:C levels are also significantly reduced in blood group O individuals. [Extracted from the article]

• Plasma VWF antigen and FVIII:C levels are raised in patients with active multiple myeloma (MM), irrespective of therapy • High VWF propeptide and reduced propeptide/antigen ratios suggest endothelial activation and impaired VWF clearance in MM • Increased VWF propeptide is seen in precursor MM, strengthening the hypothesis of hypercoagulability in MGUS/smouldering MM • VWF multimer distribution is not altered in MM disease and accordingly, ADAMTS13 activity is not impaired [ABSTRACT FROM AUTHOR]

Enhanced von Willebrand factor (VWF) clearance is important in the etiology of von Willebrand disease. However, the molecular mechanisms underlying VWF clearance remain poorly understood. In this study, we investigated the role of VWF domains and specific glycan moieties in regulating in vivo clearance. Our findings demonstrate that the A1 domain of VWF contains a receptor-recognition site that plays a key role in regulating the interaction of VWF with macrophages. In A1-A2-A3 and full-length VWF, this macrophage-binding site is cryptic but becomes exposed following exposure to shear or ristocetin. Previous studies have demonstrated that the N-linked glycans within the A2 domain play an important role in modulating susceptibility to ADAMTS13 proteolysis.We further demonstrate that these glycans presented at N1515 and N1574 also play a critical role in protecting VWF against macrophage binding and clearance. Indeed, loss of the N-glycan at N1515 resulted in markedly enhanced VWF clearance that was significantly faster than that observed with any previously described VWF mutations. In addition, A1-A2-A3 fragments containing the N1515Q or N1574Q substitutions also demonstrated significantly enhanced clearance. Importantly, clodronate-induced macrophage depletion significantly attenuated the increased clearance observed with N1515Q and N1574Q in both full-length VWF and A1-A2-A3. Finally,wefurther demonstrate that loss of these N-linked glycans does not enhance clearance inVWFin the presence of a structurally constrained A2 domain. Collectively, these novel findings support the hypothesis that conformation of the VWF A domains plays a critical role in modulating macrophage-mediated clearance of VWF in vivo. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]