A novel mechanism of factor VIII protection by von Willebrand factor from activated protein C-catalyzed inactivation

von Willebrand factor binds to angiopoietin-2 within endothelial cells and after release from Weibel-Palade bodies

BACKGROUND

The von Willebrand factor (VWF) is a multimeric plasma glycoprotein essential for hemostasis, inflammation, and angiogenesis. The majority of VWF is synthesized by endothelial cells (ECs) and stored in Weibel-Palade bodies (WPB). Among the range of proteins shown to co-localize to WPB is angiopoietin-2 (Angpt-2), a ligand of the receptor tyrosine kinase Tie-2. We have previously shown that VWF itself regulates angiogenesis, raising the hypothesis that some of the angiogenic activity of VWF may be mediated by its interaction with Angpt-2.

METHODS

Static-binding assays were used to probe the interaction between Angpt-2 and VWF. Binding in media from cultured human umbilical vein ECs s and in plasma was determined by immunoprecipitation experiments. Immunofluorescence was used to detect the presence of Angpt-2 on VWF strings, and flow assays were used to investigate the effect on VWF function.

RESULTS

Static-binding assays revealed that Angpt-2 bound to VWF with high affinity (KD,app ∼3 nM) in a pH and calcium-dependent manner. The interaction was localized to the VWF A1 domain. Co-immunoprecipitation experiments demonstrated that the complex persisted following stimulated secretion from ECs and was present in plasma. Angpt-2 was also visible on VWF strings on stimulated ECs. The VWF-Angpt-2 complex did not inhibit the binding of Angpt-2 to Tie-2 and did not significantly interfere with VWF-platelet capture.

CONCLUSIONS

Together, these data demonstrate a direct binding interaction between Angpt-2 and VWF that persists after secretion. VWF may act to localize Angpt-2; further work is required to establish the functional consequences of this interaction.

circ_0001274 Competitively Binds miR-143-3p to Upregulate VWF Expression to Improve Acute Traumatic Coagulopathy

Accumulating evidence has noted the circRNA-microRNA- (circRNA-miRNA-) mRNA competing endogenous RNA (ceRNA) regulatory network in disease development and progression. The current study explored the ceRNA network in acute traumatic coagulopathy (ATC). Potential ATC-related genes were screened, and upstream miRNAs and circRNAs of VWF (the candidate target) were assayed through database searching and high-throughput sequencing technology. circ_0001274/miR-143-3p/VWF ceRNA regulatory network was constructed and validated. The expression of circ_0001274/miR-143-3p/VWF was determined in the peripheral blood samples from ATC patients and ATC mouse models. Online database and circRNA sequencing analysis results identified VWF as a key gene in ATC as supported by assays and that VWF was lowly expressed in ATC patients and mice. Further experiments demonstrated that miR-143-3p could target and inhibit VWF, and circ_0001274 could competitively sponge miR-143-3p. Functionally, circ_0001274 could competitively sequester miR-143-3p to upregulate VWF expression, potentially improving ATC. Our study highlights the critical role of circ_0001274/miR-143-3p/VWF axis in improving ATC.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Defective VWF secretion due to the expression of MYH9-RD E1841K mutant in endothelial cells disrupts hemostasis

Mutations in MYH9, the gene encoding the heavy chain of non-muscle myosin IIa (NMII-A), cause MYH9-related disease (MYH9-RD) that is an autosomal-dominant thrombocytopenia with bleeding tendency. Previously, we showed that NMII-A in endothelial cells (ECs) is critical for hemostasis via regulating von Willebrand factor (VWF) release from Weibel-Palade bodies (WPBs). The aim of this study was to determine the role of the expression of MYH9 mutants in ECs in the pathogenesis of the MYH9-RD bleeding symptom. First, we expressed the 5 most common NMII-A mutants in ECs, and found that E1841K mutant-expressing ECs secreted less VWF than the controls in response to a cAMP signaling agonist. Then, we generated 2 knockin mouse lines, one with Myh9 E1841K in ECs and the other in megakaryocytes. Endothelium-specific E1841K mice exhibited impaired cAMP-induced VWF release and a prolonged bleeding time with normal platelets, while megakaryocyte-specific E1841K mice exhibited macrothrombocytopenia and a prolonged bleeding time with normal VWF release. Finally, we present mechanistic findings that E1841K mutation not only interferes with S1943 phosphorylation and impairs the peripheral distribution of Rab27a positive WPBs in ECs under quiescent condition, but also interferes with S1916 phosphorylation by disrupting the interaction with zyxin and CKIIα, and reduces actin framework formation around WPBs and subsequent VWF secretion under the stimulation by a cAMP agonist. Altogether, our results suggest that impaired cAMP-induced endothelial VWF secretion by E1841K mutant expression may contribute to the MYH9-RD bleeding phenotype.

High levels of Von Willebrand factor markers in COVID-19: a systematic review and meta-analysis

The SARS-CoV-2 virus has spread to all corners of the world. Thrombosis is the cause of organ failure and subsequent death in COVID-19. The pathophysiology of thrombosis in COVID-19 needs to be further explored to shed light on its downside. For this reason, this meta-analysis of Von Willebrand Factor profile (VWF: Ag, VWF: activity, VWF: RCo), ADAMTS-13, and factor VIII levels in COVID-19 was performed. To obtain data on the status of the aforementioned hemostatic factors, a systematic literature review and meta-analysis were performed on COVID-19. After reviewing the evaluation of 348 papers, 28 papers included in the meta-analysis, which was performed using STATA. The analysis showed an increase in VWF: Ag levels in COVID‐19 patients. VWF: Ac was higher in all COVID-19 patients, while it was lower in the COVID‐19 ICU patients. The pooled mean of VWF: RCO in all patients with COVID-19 was 307.94%. In subgroup analysis, VWF: RCO was significantly higher in ICU patients than in all COVID-19 patients. The pooled mean of ADAMTS-13 activity was 62.47%, and 58.42% in ICU patients. The pooled mean of factor VIII level was 275.8%, which was significantly higher in ICU patients with COVID-19 than all patients with COVID-19. Levels of VWF: Ag, VWF: activity, VWF: ristocetin, and factor VIII are increased in patients with COVID-19. The elevated levels in ICU patients with COVID-19 suggest that these markers may have prognostic value in determining the severity of COVID-19. New therapeutic programs can be developed as a result.

Blocking von Willebrand factor free thiols inhibits binding to collagen under high and pathological shear stress

BACKGROUND

Von Willebrand factor (VWF) contains a number of free thiols, the majority of which are located in its C-domains, and these have been shown to alter VWF function, However, the impact of free thiols on function following acute exposure of VWF to collagen under high and pathological shear stress has not been determined.

METHODS

VWF free thiols were blocked with N-ethylmaleimide and flow assays performed under high and pathological shear rates to determine the impact on platelet capture and collagen binding function. Atomic force microscopy (AFM) was used to probe the interaction of VWF with collagen and molecular simulations conducted to determine the effect of free thiols on the flexibility of the VWF-C4 domain.

RESULTS

Blockade of VWF free thiols reduced VWF-mediated platelet capture to collagen in a shear-dependent manner, with platelet capture virtually abolished above 5000 s-1 and in regions of stenosis in microfluidic channels. Direct visualization of VWF fibers formed under extreme pathological shear rates and analysis of collagen-bound VWF attributed the effect to altered binding of VWF to collagen. AFM measurements showed that thiol-blockade reduced the lifetime and strength of the VWF-collagen bond. Pulling simulations of the VWF-C4 domain demonstrated that with one or two reduced disulphide bonds the C4 domain has increased flexibility and the propensity to undergo free-thiol exchange.

CONCLUSIONS

We conclude that free thiols in the C-domains of VWF enhance the flexibility of the molecule and enable it to withstand high shear forces following collagen binding, demonstrating a previously unrecognized role for VWF free thiols.

Mechanisms of human neutrophil elastase-catalysed inactivation of factor VIII(a)

Mechanisms of inflammation and coagulation are linked through various pathways. Human neutrophil elastase (HNE), can bind to activated platelets, might be localised on platelet membranes that provide negatively-charged phospholipid essential for the optimum function of tenase complex. In this study, we examined the effect of HNE on factor (F)VIII. FVIII activity was rapidly diminished in the presence of HNE and was undetectable within 10 minutes. The inactivation rate waŝ8-fold greater than that of activated protein C (APC). This time-dependent inactivation was moderately affected by von Willebrand factor. HNE proteolysed the heavy chain (HCh) of FVIII into two terminal products, A11–358 and A2375–708, by limited proteolysis at Val358, Val374, and Val708. Cleavage at Val708 was much slower than that at Val358 in the >90-kDa A1-A2-B compared to the 90-kDa A1-A2. The 80-kDa light chain (LCh) was proteolysed to 75-kDa product by cleavage at Val1670. HNE-cata- lysed FVIIIa inactivation was markedly slower than that of native FVIII (by ~25-fold), due to delayed cleavage at Val708 in FVIIIa. The inactivation rate mediated by HNE was ~8-fold lower than that by APC. Cleavages at Val358 and Val708 were regulated by the presence of LCh and HCh, respectively. In conclusion, HNE-catalysed FVIII inactivation was associated with the limited-proteolysis that led to A11–358, A2375–708, and A3-C1-C21671–2332, and subsequently to critical cleavage at Val708. HNE-related FVIII(a) reaction might play a role in inactivation of HNE-induced coagulation process, and appeared to depend on the amounts of inactivated FVIII and active FVIIIa which is predominantly resistant to HNE inactivation.

Note: An account of this work was presented at the 51st annual meeting of the American Society of Hematology, December 10, 2009, New Orleans, LA, USA.

Identification of extant vertebrate Myxine glutinosa VWF: evolutionary conservation of primary hemostasis

Hemostasis in vertebrates involves both a cellular and a protein component. Previous studies in jawless vertebrates (cyclostomes) suggest that the protein response, which involves thrombin-catalyzed conversion of a soluble plasma protein, fibrinogen, into a polymeric fibrin clot, is conserved in all vertebrates. However, similar data are lacking for the cellular response, which in gnathostomes is regulated by von Willebrand factor (VWF), a glycoprotein that mediates the adhesion of platelets to the subendothelial matrix of injured blood vessels. To gain evolutionary insights into the cellular phase of coagulation, we asked whether a functional vwf gene is present in the Atlantic hagfish, Myxine glutinosa We found a single vwf transcript that encodes a simpler protein compared with higher vertebrates, the most striking difference being the absence of an A3 domain, which otherwise binds collagen under high-flow conditions. Immunohistochemical analyses of hagfish tissues and blood revealed Vwf expression in endothelial cells and thrombocytes. Electron microscopic studies of hagfish tissues demonstrated the presence of Weibel-Palade bodies in the endothelium. Hagfish Vwf formed high-molecular-weight multimers in hagfish plasma and in stably transfected CHO cells. In functional assays, botrocetin promoted VWF-dependent thrombocyte aggregation. A search for vwf sequences in the genome of sea squirts, the closest invertebrate relatives of hagfish, failed to reveal evidence of an intact vwf gene. Together, our findings suggest that VWF evolved in the ancestral vertebrate following the divergence of the urochordates some 500 million years ago and that it acquired increasing complexity though sequential insertion of functional modules.

Site-specific analysis of von Willebrand factor O-glycosylation

BACKGROUND

O-glycosylation of von Willebrand factor (VWF) affects many of its functions; however, there is currently no information on the occupancy of the 10 putative O-glycosylation sites.

OBJECTIVES

The aim of this study was the site-specific analysis of VWF O-glycosylation.

METHODS

Tryptic VWF-O-glycopeptides were isolated by lectin affinity chromatography and/or by reverse-phase high-performance liquid chromatography. Subsequently, the purified glycopeptides were analyzed by glycosidase digestion and mass spectrometry.

RESULTS

We found that all 10 predicted O-glycosylation sites in VWF are occupied. The majority of the glycan structures on all glycosylation sites is represented by disialyl core 1 O-glycan. The presence of core 2 O-glycan was also confirmed; interestingly, this structure was not evenly distributed among all 10 glycosylation sites. Analysis of the glycopeptides flanking the A1 domain revealed that generally more core-2-type O-glycan was present on the C-terminal Cluster 2 glycopeptide (encompassing T(1468) , T(1477) , S(1486) and T(1487) ) compared with the N-terminal Cluster 1 glycopeptide (encompassing T(1248) , T(1255) , T(1256) and S(1263) ). Disialosyl motifs were present on both glycopeptides flanking the A1 domain and on the glycosylation site T(2298) in the C1 domain. In addition, we identify sulfation of core 2 O-glycans and the presence of the rare Tn antigen.

CONCLUSIONS

This is the first study to describe the qualitative and semi-quantitative distribution of O-glycan structures on all 10 O-glycosylation sites, which will provide a valuable starting point for further studies exploring the functional and structural implications of O-glycosylation in VWF.

The important role of von Willebrand factor in platelet-derived FVIII gene therapy for murine hemophilia A in the presence of inhibitory antibodies

BACKGROUND

Our previous studies have demonstrated that targeting FVIII expression to platelets results in FVIII storage together with von Willebrand factor (VWF) in platelet α-granules and that platelet-derived FVIII (2bF8) corrects the murine hemophilia A phenotype even in the presence of high-titer anti-FVIII inhibitory antibodies (inhibitors).

OBJECTIVE

To explore how VWF has an impact on platelet gene therapy for hemophilia A with inhibitors.

METHODS

2bF8 transgenic mice in the FVIII(-/-) background (2bF8(tg+/-) F8(-/-) ) with varying VWF phenotypes were used in this study. Animals were analyzed by VWF ELISA, FVIII activity assay, Bethesda assay and tail clip survival test.

RESULTS

Only 18% of 2bF8(tg+/-) F8(-/-) VWF(-/-) animals, in which VWF was deficient, survived the tail clip challenge with inhibitor titers of 3-8000 BU mL(-1) . In contrast, 82% of 2bF8(tg+/-) F8(-/-) VWF(+/+) mice, which had normal VWF levels, survived tail clipping with inhibitor titers of 10-50,000 BU mL(-1) . All 2bF8(tg+/-) F8(-/-) VWF(-/-) mice without inhibitors survived tail clipping and no VWF(-/-) F8(-/-) mice survived this challenge. Because VWF is synthesized by endothelial cells and megakaryocytes and is distributed in both plasma and platelets in peripheral blood, we further investigated the effect of each compartment of VWF on platelet-FVIII gene therapy for hemophilia A with inhibitors. In the presence of inhibitors, 42% of animals survived tail clipping in the group with plasma-VWF and 50% survived in the platelet-VWF group.

CONCLUSION

VWF is essential for platelet gene therapy for hemophilia A with inhibitors. Both platelet-VWF and plasma-VWF are required for optimal platelet-derived FVIII gene therapy for hemophilia A in the presence of inhibitors.

A new recombinant factor VIII: from genetics to clinical use

Advances in recombinant technology and knowledge about coagulation factor VIII (FVIII) are building a platform for new therapeutic options in patients with hemophilia A. The development of turoctocog alfa, a novel, high-purity, third-generation, B-domain truncated recombinant FVIII, has been produced and formulated without the use of animal-derived or human serum-derived components, in the wake of understanding of the new biochemical characteristics of FVIII, namely its protein structure, and glycosylation and sulfating patterns. Culture conditions and a five-step purification process have been developed to optimize the safety of turoctocog alfa. The results of two pilot clinical trials using turoctocog alfa confirmed high safety levels, with no patient developing inhibitors during the period of observation. The purpose of this review is to describe briefly the molecular and biological properties of turoctocog alfa, together with details of its clinical development, with emphasis on the needs of patients with hemophilia A.

Update on von Willebrand factor multimers: focus on high-molecular-weight multimers and their role in hemostasis

Normal hemostasis requires von Willebrand factor (VWF) to support platelet adhesion and aggregation at sites of vascular injury. VWF is a multimeric glycoprotein built from identical subunits that contain binding sites for both platelet glycoprotein receptors and collagen. The adhesive activity of VWF depends on the size of its multimers, which range from 500 to over 10 000 kDa. There is good evidence that the high-molecular-weight multimers (HMWM), which are 5000-10 000 kDa, are the most effective in supporting interaction with collagen and platelet receptors and in facilitating wound healing under conditions of shear stress. Thus, these HMWM of VWF are of particular clinical interest. The unusually large multimers of VWF are, under normal conditions, cleaved by the plasma metalloproteinase ADAMTS13 to smaller, less adhesive multimers. A reduction or lack of HMWM, owing to a multimerization defect of VWF or to an increased susceptibility of VWF for ADAMTS13, leads to a functionally impaired VWF and the particular type 2A of von Willebrand disease. This review considers the biology and function of VWF multimers with a particular focus on the characterization of HMWM - their production, storage, release, degradation, and role in normal physiology. Evidence from basic research and the study of clinical diseases and their management highlight a pivotal role for the HMWM of VWF in hemostasis.

Contribution of factor VIII light-chain residues 2007-2016 to an activated protein C-interactive site

Although factor (F) VIIIa is inactivated by activated protein C (APC) through cleavages in the FVIII heavy chain-derived A1 (Arg(336)) and A2 subunits (Arg(562), the FVIII light chain (LC) contributes to catalysis by binding the enzyme. ELISA-based binding assays showed that FVIII and FVIII LC bound to immobilised active site-modified APC (DEGR-APC) (apparent K(d) ~270 nM and 1.0 μM, respectively). Fluid-phase binding studies using fluorescence indicated an estimated K(d) of ~590 nM for acrylodan-labelled LC binding to DEGR-APC. Furthermore, FVIII LC effectively competed with FVIIIa in blocking APC-catalysed cleavage at Arg(336) (K(i) = 709 nM). A binding site previously identified near the C-terminal end of the A3 domain (residues 2007-2016) of FVIII LC was subjected to Ala-scanning mutagenesis. FXa generation assays and western and dot blotting were employed to assess the contribution of these residues to FVIIIa interactions with APC. Virtually all variants tested showed reductions in the rates of APC-catalysed inactivation of the cofactor and cleavage at the primary inactivation site (Arg(336)), with maximal reductions in inactivation rates (~3-fold relative to WT) and cleavage rates (~3 to ~9-fold relative to WT) observed for the Met2010Ala, Ser2011Ala, and Leu2013Ala variants. Titration of FVIIIa substrate concentration monitoring cleavage by a dot blot assay indicated that these variants also showed ~3-fold increases relative to WT while a double mutant (Met2010Ala/Ser2011Ala) showed a >4-fold increase in K(m). These results show a contribution of a number of residues within the 2007-2016 sequence, and in particular residues Met2010, Ser2011, and Leu2013 to an APC-interactive site.

Mechanisms of factor VIIa-catalyzed activation of factor VIII

BACKGROUND

Factor (F)VIIa, complexed with tissue factor (TF), is a primary trigger of blood coagulation, and has extremely restricted substrate specificity. The complex catalyzes limited proteolysis of FVIII, but these mechanisms are poorly understood.

OBJECTIVES

In the present study, we investigated the precise mechanisms of FVIIa/TF-catalyzed FVIII activation.

RESULTS

FVIII activity increased ~4-fold within 30 s in the presence of FVIIa/TF, and then decreased to initial levels within 20 min. FVIIa (0.1 nM), at concentrations present physiologically in plasma, activated FVIII in the presence of TF, and this activation was more rapid than that induced by thrombin. The heavy chain (HCh) of FVIII was proteolyzed at Arg(740) and Arg(372) more rapidly by FVIIa/TF than by thrombin, consistent with the enhanced activation of FVIII. Cleavage at Arg(336) was evident at ~1 min, whilst little cleavage of the light chain (LCh) was observed. Cleavage of the HCh by FVIIa/TF was governed by the presence of the LCh. FVIII bound to Glu-Gly-Arg-active-site-modified FVIIa (K(d), ~0.8 nM) with a higher affinity for the HCh than for the LCh (K(d), 5.9 and 18.9 nm). Binding to the A2 domain was particularly evident. Von Willebrand factor (VWF) modestly inhibited FVIIa/TF-catalyzed FVIII activation, in keeping with the concept that VWF could moderate FVIIa/TF-mediated reactions.

CONCLUSIONS

The results demonstrated that this activation mechanism was distinct from those mediated by thrombin, and indicated that FVIIa/TF functions through a 'priming' mechanism for the activation of FVIII in the initiation phase of coagulation.

Specific N-linked glycosylation sites modulate synthesis and secretion of von Willebrand factor

We examined the role that N-linked glycans play in the synthesis and expression of von Willebrand Factor (VWF). Blocking the addition of N-linked glycans (NLGs) or inhibiting initial glycan processing prevented secretion of VWF. To determine whether specific glycosylation sites were important, the 16 VWF N-linked glycosylation sites were mutated followed by expression in HEK293T cells. Four NLG mutants affected VWF expression: N99Q (D1 domain), N857Q (D' domain), N2400Q (B1 domain), and N2790Q (CK domain) either abolished or reduced secretion of VWF and this was confirmed by metabolic labeling. Multimer analysis of mutant N2790Q cell lysate revealed an increase in VWF monomers, which was also observed when the isolated CK domain was expressed with N2790 mutated. Immunofluorescence microscopy showed that mutants N99Q, N857Q, and N2790Q were primarily retained within the ER, producing only few pseudo Weibel-Palade bodies over longer time periods compared with wtVWF. All the variants also showed an increase in free thiol reactivity. This was greatest with N857Q and D4-C2 NLG mutants, which had approximately 6-fold and 3- to 4-fold more free thiol reactivity than wtVWF. These data provide further evidence of the critical role that individual N-linked glycans play in determining VWF synthesis and expression.

Factor VIII and von Willebrand factor interaction: biological, clinical and therapeutic importance

The interaction of factor VIII (FVIII) with von Willebrand Factor (VWF) is of direct clinical significance in the diagnosis and treatment of patients with haemophilia A and von Willebrand disease (VWD). A normal haemostatic response to vascular injury requires both FVIII and VWF. It is well-established that in addition to its role in mediating platelet to platelet and platelet to matrix binding, VWF has a direct role in thrombin and fibrin generation by acting as a carrier molecule for the cofactor FVIII. Recent studies show that the interaction affects not only the biology of both FVIII and VWF, and the pathology of haemophilia and VWD, but also presents opportunities in the treatment of haemophilia. This review details the mechanisms and the molecular determinants of FVIII interaction with VWF, and the role of FVIII-VWF interaction in modulating FVIII interactions with other proteases, cell types and cellular receptors. The effect of defective interaction of FVIII with VWF as a result of mutations in either protein is discussed.

Selective factor VIII and V inactivation by iminodiacetate ion exchange resin through metal ion adsorption

The procoagulant activity of factors VIII and V depends on the presence of metal ion(s). We examined the effect of cation-exchange resins with different functional groups on both factors, of which only reaction with iminodiacetate resin resulted in the complete loss of their activity levels in plasma. However, the antigen level of factor VIII was preserved by >95%. This resin reduced divalent cations content present in factor VIII preparations, indicating that it inactivated this factor by direct deprivation of predominant Ca(2+) (>Mn(2+)>>Cu(2+)), rather than adsorption of the factor itself. The antigen level of recombinant factor VIII alone was decreased by >95% by reaction with resin, whilst that complexed with von Willebrand factor was preserved by >95%. Iminodiacetate resin-treated plasma was evaluated by measuring factor VIII and V activity in plasma with various levels of either activity. These were significantly correlated to the values obtained using factor VIII- or V-deficient plasma prepared commercially by immunodepletion. We demonstrated that iminodiacetate resin-induced factors VIII and V inactivation is because of direct deprivation of metal ions, predominantly Ca(2+), which is more essential for the functional structure of their molecules. Furthermore, iminodiacetate resin-treated plasma would be useful as a substrate for measuring the activity of these factors.

Increasing the humoral immunogenic properties of the HIV-1 Tat protein using a ligand-stabilizing strategy

Tat is regarded as an attractive target for the development of an AIDS vaccine. However, works suggest that Tat is a poorly immunogenic protein and therefore we attempted to increase its immunogenic potency. As we observed that Tat is highly sensitive to enzymatic degradation in vitro we tried to make it less susceptible to proteolysis using ligands. We complexed Tat101 with various sulfated sugars and observed that some of these ligands made the protein more resistant to proteolysis and more immunogenic. In a more thorough study, we observed that a low-molecular-weight heparin fragment, called Hep6000, altered both the cell-binding capacity and transactivating activity of Tat101, suggesting that this sulfated polysaccharide can make the protein less toxic. Sera raised against Tat101 and Tat101/Hep6000 similarly bound mainly to the N-terminal region of the protein, indicating that formation of the complex does not alter the B-cell immunodominant region. Anti-Tat101/Hep6000 antisera neutralized the transactivating activity of Tat101 more efficiently than anti-Tat101 antisera. Altogether, these results indicate that stabilization of Tat101 using sulfated sugars increases its immunogenicity and might be of value in increasing its vaccine efficacy.

N-linked glycosylation of VWF modulates its interaction with ADAMTS13

We examined the role of N-linked glycan structures of VWF on its interaction with ADAMTS13. PNGase F digestion followed by lectin analysis demonstrated that more than 90% of VWF N-linked glycan chains could be removed from the molecule (PNG-VWF) without disruption of its multimeric structure or its ability to bind to collagen. PNG-VWF had an approximately 4-fold increased affinity for ADAMTS13 compared with control VWF. PNG-VWF was cleaved by ADAMTS13 faster than control VWF and was also proteolysed in the absence of urea. Occupancy of the N-linked glycan sites at N1515 and N1574 and their presentation of ABO(H) blood group sugars were confirmed with an isolated tryptic fragment. Recombinant VWF was mutated to prevent glycosylation at these sites. Mutation of N1515 did not alter ADAMTS13 binding or increase rate of ADAMTS13 proteolysis. Mutation of N1574 increased the susceptibility of VWF to ADAMTS13 proteolysis and allowed cleavage in the absence of urea. Mutation of N1574 in the isolated recombinant VWF-A2 domain also increased binding and ADAMTS13 proteolysis. These data demonstrate that the N-linked glycans of VWF have a modulatory effect on the interaction with ADAMTS13. At least part of this effect is conformational, but steric hindrance may also be important.

Relationship between the binding sites for von Willebrand factor, phospholipid, and human factor VIII C2 inhibitor alloantibodies within the factor VIII C2 domain

Some factor VIII (FVIII) inhibitor alloantibodies block FVIII binding to von Willebrand factor (VWF) and phospholipid (PL) and recognize a C2 domain epitope that overlaps both binding sites. We previously showed that FVIII peptide 2315-2330 neutralized FVIII inhibitors and that Cys2326 and Glu2327 contributed to the maximum neutralizing effect. In the present study, we investigated the relationship between the essential binding sites for VWF, PL, and anti-C2 inhibitors by means of competitive-inhibition assays with overlapping synthetic peptides that span the C terminus of the C2 domain (residues 2288-2332). We identified 2 peptides (residues 2303-2317 and 2315-2330) that specifically blocked FVIII binding to VWF or PL by approximately 80% (50%-inhibitory concentration [IC50], 9.0 microM) and 95% (IC50, 0.12 microM), respectively. To examine in detail the residues responsible for PL binding, we prepared mutants of peptide 2315-2330 in which we sequentially substituted each residue with Gly. Two residues, Ile2317 and Met2321, were shown to be essential for PL binding. Their substitution with Gly reduced the inhibitory effect by >90%. The data suggest that the binding sites for VWF, PL, and anti-C2 inhibitors in the C2 domain are in very close proximity but are not identical.

A role for exposed mannosylations in presentation of human therapeutic self-proteins to CD4+ T lymphocytes

Several therapeutic self-proteins elicit immune responses when administered to patients. Such adverse immune responses reduce drug efficacy. To induce an immune response, a protein must interact with different immune cells, including antigen-presenting cells, T cells, and B cells. Each cell type recognizes distinct immunogenic patterns on antigens. Mannose-terminating glycans have been identified as pathogen-associated molecular patterns that are essential for internalization of microbes by antigen-presenting cells, leading to presentation. Here, we have investigated the importance of exposed mannosylation on an immunogenic therapeutic self-protein, procoagulant human factor VIII (FVIII). Administration of therapeutic FVIII to hemophilia A patients induces inhibitory anti-FVIII antibodies in up to 30% of the cases. We demonstrate that entry of FVIII into human dendritic cells (DC) leading to T cell activation, is mediated by mannose-terminating glycans on FVIII. Further, we identified macrophage mannose receptor (CD206) as a candidate endocytic receptor for FVIII on DC. Saturation of mannose receptors on DC with mannan, and enzymatic removal of mannosylated glycans from FVIII lead to reduced T cell activation. The interaction between FVIII and CD206 was blocked by VWF, suggesting that, under physiological conditions, the intrinsic mannose-dependent immunogenicity of FVIII is quenched by endogenous immunochaperones. These data provide a link between the mannosylation of therapeutic self-proteins and their iatrogenic immunogenicity. Such a link would be of special relevance in the context of replacement therapy where mechanisms of central and peripheral tolerance have not been established during ontogeny because of the absence of the antigen.

Mechanisms of Plasmin-catalyzed Inactivation of Factor VIII

Plasmin not only functions as a key enzyme in the fibrinolytic system but also directly inactivates factor VIII and other clotting factors such as factor V. However, the mechanisms of plasmin-catalyzed factor VIII inactivation are poorly understood. In this study, levels of factor VIII activity increased approximately 2-fold within 3 min in the presence of plasmin, and subsequently decreased to undetectable levels within 45 min. This time-dependent reaction was not affected by von Willebrand factor and phospholipid. The rate constant of plasmin-catalyzed factor VIIIa inactivation was approximately 12- and approximately 3.7-fold greater than those mediated by factor Xa and activated protein C, respectively. SDS-PAGE analysis showed that plasmin cleaved the heavy chain of factor VIII into two terminal products, A1(37-336) and A2 subunits, by limited proteolysis at Lys(36), Arg(336), Arg(372), and Arg(740). The 80-kDa light chain was converted into a 67-kDa subunit by cleavage at Arg(1689) and Arg(1721), identical to the pattern induced by factor Xa. Plasmin-catalyzed cleavage at Arg(336) proceeded faster than that at Arg(372), in contrast to proteolysis by factor Xa. Furthermore, breakdown was faster than that in the presence of activated protein C, consistent with rapid inactivation of factor VIII. The cleavages at Arg(336) and Lys(36) occurred rapidly in the presence of A2 and A3-C1-C2 subunits, respectively. These results strongly indicated that cleavage at Arg(336) was a central mechanism of plasmin-catalyzed factor VIII inactivation. Furthermore, the cleavages at Arg(336) and Lys(36) appeared to be selectively regulated by the A2 and A3-C1-C2 domains, respectively, interacting with plasmin.

Alternative splicing generates a novel non-secretable cytosolic isoform of NELL2

NELL2 is as a neuron-specific secreted glycoprotein. The present study provides evidence of an alternatively spliced variant of the rat NELL2 gene that yields cytosolic NELL2 (cNELL2). cNELL2 was initially detected in the thymus and subsequently found to be ubiquitously expressed in many other tissues. The absence of the sequences corresponding to the third exon, which contains the terminal portion of the signal peptide, accounts for the uniform distribution of cNELL2 throughout the cytoplasm. This is in contrast to NELL2, which is preferentially located at distinct subcellular structures involved in the secretary process, such as endoplasmic reticulum and Golgi apparatus. Western blot analysis showed that cNELL2 was not present in the medium but only in lysates, while NELL2 was detected as a glycosylated larger form in both lysates and media. Immunoprecipitation analysis revealed that cNELL2 interacts with PKCbeta1. These results suggest that cNELL2 is involved in PKCbeta1-mediated intracellular signaling.

Factor V C2 domain contains a major thrombin-binding site responsible for thrombin-catalyzed factor V activation

Factor (F)V is converted into its active form, FVa, by limited proteolysis. Thrombin-catalyzed activation of FV is essential for its full cofactor activation. Previously, we reported that thrombin was bound to the C2 domain in the light chain of FVIII. As FV has a similar domain structure to FVIII, we focused on the FV C2 domain as a possible binding region for thrombin. Kinetic parameters, measured by surface plasmon resonance, revealed that the K(d) values of anhydro-thrombin for FV, FVa, and the FV C2 domain were 66, 240, and 670 nmol L(-1), respectively. FV activation was increased by approximately 9-fold by the addition of thrombin. In the presence of the FV C2 domain, this increase of the FV activation was inhibited. However, FV activation was not inhibited by the addition of the FVIII C2 domain. FV was cleaved into a 105-kDa heavy chain and a 71/74-kDa light chain by thrombin-catalyzed proteolysis at Arg709, Arg1018 and Arg1545. In the presence of the FV C2 domain, the cleavage was inhibited at all sites. Proteolysis was not affected by the addition of the FVIII C2 domain. These results indicated that the FV C2 domain contains a major binding site for thrombin and that this domain is necessary for the proteolysis at all cleavage sites. Furthermore, the present results also suggested that thrombin has an independent binding site for FV different from that for FVIII.

Factor VIII Structure and Function

Factor VIII, a non-covalent heterodimer comprised of a heavy chain (A1-A2-B domains) and light chain (A3-C1-C2 domains), circulates as an inactive procofactor in complex with von Willebrand factor. Metal ions are critical to the integrity of factor VIII, with Cu and Ca ions stabilizing the heterodimer and generating the active conformation, respectively. Activation of factor VIII catalyzed by thrombin appears dependent upon interactions with both anion-binding exosites I and II, and converts the heterodimer to the active cofactor, factor VIIIa. This protein, comprised of A1, A2, and A3-C1-C2 subunits, is labile due to weak affinity of the A2 subunit. Association of factor VIIIa with factor IXa to form the intrinsic factor Xase complex is membrane-dependent and involves multiple inter-protein contacts that remain poorly characterized. This complex catalyzes the conversion of factor X to factor Xa, a reaction that is essential for the propagation phase of coagulation. The role of factor VIIIa in this complex is to increase the catalytic efficiency for factor Xa generation by several orders of magnitude. Mechanisms for the down-regulation of factor Xase focus upon inactivation of the cofactor and include dissociation of the A2 subunit as well as activated protein C-catalyzed proteolysis.

Highly Conserved Antigenic Structure of the Factor VIII C2 Domain in Some Mammals

To elucidate differences in the antigenic structure of factor VIII (FVIII) among mammals, we evaluated cross-reactivities of well-defined antihuman FVIII antibodies with canine and other mammalian FVIII proteins. Monoclonal antibodies against human FVIII recognizing the A1 domain in the heavy chain and the A3 domain in the light chain showed 2.7% and 0% cross-reactivities, respectively, with canine FVIII. The cross-reactivities of 2 alloantibodies and a monoclonal antibody that recognized the A2 domain in the heavy chain were 10%, 38%, and 0%, respectively. On the other hand, 2 kinds of alloantibodies and a monoclonal antibody recognizing the C2 domain in the light chain showed 160%, 390%, and 130% cross-reactivity, respectively, with canine FVIII. The anti-C2 monoclonal antibody (NMC-VIII/5) showed a type 2 inactivating property when tested with canine and human plasma. Moreover, cross-reactivities of NMC-VIII/5 with simian and feline FVIII were 54.5% and 82.8%, respectively, while the cross-reactivities of the anti-A2 monoclonal antibody (JR8) with simian and feline FVIII were 1.3% and 0%, respectively. These findings suggest that the antigenic structure of the C2 epitope has remained relatively conserved throughout mammalian evolution in contrast to the A2 epitope and that a canine model of hemophilia A is useful for FVIII inhibitor experiments.

New perspectives on von Willebrand factor functions in hemostasis and thrombosis

The adhesive protein, von Willebrand factor (VWF), mediates the initiation and progression of thrombus formation at sites of vascular injury by means of specific interactions with extracellular matrix components and platelet receptors. The essential biologic properties of VWF have been elucidated, with progress particularly in the areas of genetic regulation, biosynthesis, and specific bimolecular interactions. The three-dimensional structure of selected domains has been solved, but our understanding of detailed structure-function relationships is still fragmented, partly because of the complexity and size of the VWF molecule. The biomechanical properties of the interaction between the VWF A1 domain and the platelet receptor glycoprotein (GP) Ibalpha also are better known, but we can still only hypothesize how this adhesive bond can oppose the fluid dynamic effects of rapidly flowing blood to initiate thrombus formation and contribute to platelet activation. Elucidating the details of VWF and GPIbalpha function will lead to a more satisfactory definition of the role of platelets in atherothrombosis, since hemodynamic forces greatly influence responses to vascular injury in stenosed and partially occluded arteries. Progress in this direction is also aided by rapidly expanding novel information on the mechanisms that regulate VWF multimer size in the circulation, a topic of relevance to explain microvascular thrombosis and, perhaps, arterial thrombosis in general. Developments in these areas of research will refine our understanding of the role played by VWF in vascular biology and pathology.

Activation of factor VIII and mechanisms of cofactor action

The factor VIII procofactor circulates as a metal ion-dependent heterodimer of a heavy chain and light chain. Activation of factor VIII results from limited proteolysis catalyzed by thrombin or factor Xa, which binds the factor VIII substrate over extended interactive surfaces. The proteases efficiently cleave factor VIII at three sites, two within the heavy and one within the light chain resulting in alteration of its covalent structure and conformation and yielding the active cofactor, factor VIIIa. The role of factor VIIIa is to markedly increase the catalytic efficiency of factor IXa in the activation of factor X. This effect is manifested in a dramatic increase in the catalytic rate constant, k(cat), by mechanisms that remain poorly understood.

Drosophila hemolectin gene is expressed in embryonic and larval hemocytes and its knock down causes bleeding defects

We have previously identified and characterized Drosophila hemolectin (Hml) in the conditioned medium of a Drosophila cell line. The deduced amino acids sequence contains a number of domains conserved in human von Willebrand factor, coagulation factor V/VIII, and complement factors. To characterize Hml localization and function, we have established two transgenic lines (hml-GAL4 and UAS-hmlRNAi). By crossing hml-GAL4 with UAS-eGFP, we observed that Hml is specifically expressed in embryonic and in larval hemocytes. We determined that Hml is expressed in a subpopulation of plasmatocytes and crystal cells but not in lamellocytes. hml RNAi larvae are viable and do not display obvious developmental defects under normal conditions. However, they show a bleeding defect upon injury. We confirmed that the failure to seal a wound is not due to a defect in melanin production or in phenoloxidase activity. The expression of antimicrobial peptides was not significantly affected on hml RNAi adults. Altogether, our data strongly suggest that Hml is involved in hemostasis and/or coagulation in Drosophila larvae.

Von Willebrand factor, platelets and endothelial cell interactions

The adhesive protein von Willebrand factor (VWF) contributes to platelet function by mediating the initiation and progression of thrombus formation at sites of vascular injury. In recent years there has been considerable progress in explaining the biological properties of VWF, including the structural and functional characteristics of specific domains. The mechanism of interaction between the VWF A1 domain and glycoprotein Ibalpha has been elucidated in detail, bringing us closer to understanding how this adhesive bond can oppose the fluid dynamic effects of rapidly flowing blood contributing to platelet adhesion and activation. Moreover, novel findings have been obtained on the link between regulation of VWF multimer size and microvascular thrombosis. This progress in basic research has provided critical information to define with greater precision the role of VWF in vascular biology and pathology, including its possible involvement in the onset of atherosclerosis and its acute thrombotic complications.

Von Willebrand factor

The adhesive protein von Willebrand factor contributes to platelet function by mediating the initiation and progression of thrombus formation at sites of vascular injury. In the last 2 years, there has been considerable progress in explaining the biologic properties of von Willebrand factor. The three-dimensional structure of specific domains has been explained, with the demonstration of distinct conformational changes in the A1 domain caused by single amino acid substitutions associated with enhanced binding to platelets. The structural and functional properties of the interaction between the von Willebrand factor A1 domain and glycoprotein Ibalpha have also been elucidated in greater detail, bringing researchers closer to understanding how this adhesive bond can oppose the fluid dynamic effects of rapidly flowing blood to initiate thrombus formation and, concurrently, contribute to platelet activation. Because hemodynamic forces greatly influence platelet responses to vascular injury in stenosed and partially occluded arteries, a detailed description of how von Willebrand factor interacts with tissues and platelets may help in the design of more specific therapeutic inhibitors of arterial thrombosis. Moreover, enlightening findings have been obtained on the link between regulation of von Willebrand factor multimer size and microvascular thrombosis. This progress in basic research has provided critical information to define with greater precision the role of von Willebrand factor in vascular biology and pathology.