Fitusiran reduces bleeding in Factor X-deficient mice

Factor X (FX)-deficiency is a rare bleeding disorder manifesting a bleeding tendency caused by low FX activity levels. We aimed to explore the use of fitusiran (an investigational siRNA that silences antithrombin expression) to increase thrombin generation and the in vivo hemostatic potential under conditions of FX-deficiency. We therefore developed a novel model of inducible FX-deficiency, generating mice expressing <1% FX activity and antigen (f10low-mice). Compared to control f10WT-mice, f10low-mice had 6- and 4-fold prolonged clotting times in Prothrombin Time- and activated Partial Prothrombin Time-assays, respectively (p<0.001). Thrombin generation was severely reduced, irrespective whether tissue factor or factor XIa was used as initiator. In vivo analysis revealed near-absent thrombus formation in a laser-induced vessel injury-model. Furthermore, in two distinct bleeding models, f10low-mice displayed an increased bleeding tendency compared to f10WT-mice. In the tail-clip assay blood loss was increased from 12±16 microliter to 590±335 microliter (p<0.0001). In the saphenous vein puncture (SVP)-model, the number of clots generated was reduced from 19±5 clots/30 min for f10WT-mice to 2±2 clots/30 min (p<0.0001) for f10low-mice. In both models, bleeding was corrected upon infusion of purified FX. Treatment of f10low-mice with fitusiran (2x10 mg/kg at one-week interval) resulted in 17±6% residual antithrombin activity and increased thrombin generation (4-fold and 2-3-fold increase in endogenous thrombin potential and thrombin peak, respectively). In the SVP-model, the number of clots was increased to 8±6 clots/30 min (p=0.0029). Altogether, we demonstrate that reduction of antithrombin levels is associated with improved hemostatic activity under conditions of FX-deficiency.

A thrombopoietin receptor agonist to rescue an unusual platelet transfusion-induced reaction in a p.V1316M-associated von Willebrand disease type 2B patient

This report describes the first case of splenic injury in a patient with p.V1316M-associated von Willebrand disease type 2B (VWD2B) with chronic thrombocytopenia, successfully treated with nonoperative management including von Willebrand factor (VWF) replacement therapy, and platelet transfusions relayed by a thrombopoietin receptor agonist (TPO-RA, Eltrombopag). Eltrombopag was initially introduced to rescue an unusual post-platelet-transfusion reaction exacerbating the thrombocytopenia. In-depth analysis of the dramatic platelet count drop and VWF measurements timeline ruled out an allo-immune reaction and supported an alternative hypothesis of a sudden platelet clearance as a consequence of stress-induced release of abnormal VWF. One year later, a second life-threatening bleeding episode required urgent surgery successfully managed with VWF replacement therapy and platelet transfusions. Eltrombopag was further introduced in the post-surgery period to allow bleeding-free and platelet-transfusion-free successful recovery. Treatment decisions are particularly challenging in patients with VWD2B, and this case highlights how such decisions can benefit from understanding the molecular origin of platelet count fluctuations observed in these patients. Here, we successfully used a new therapeutic approach combining VWF-replacement therapy and initial platelet-transfusion relayed by TPO-RA to optimize patient management.

Single-domain antibodies targeting antithrombin reduce bleeding in hemophilic mice with or without inhibitors

Novel therapies for hemophilia, including non-factor replacement and in vivo gene therapy, are showing promising results in the clinic, including for patients having a history of inhibitor development. Here, we propose a novel therapeutic approach for hemophilia based on llama-derived single-domain antibody fragments (sdAbs) able to restore hemostasis by inhibiting the antithrombin (AT) anticoagulant pathway. We demonstrated that sdAbs engineered in multivalent conformations were able to block efficiently AT activity in vitro, restoring the thrombin generation potential in FVIII-deficient plasma. When delivered as a protein to hemophilia A mice, a selected bi-paratopic sdAb significantly reduced the blood loss in a model of acute bleeding injury. We then packaged this sdAb in a hepatotropic AAV8 vector and tested its safety and efficacy profile in hemophilic mouse models. We show that the long-term expression of the bi-paratopic sdAb in the liver is safe and poorly immunogenic, and results in sustained correction of the bleeding phenotype in hemophilia A and B mice, even in the presence of inhibitory antibodies to the therapeutic clotting factor.

von Willebrand factor and inflammation

Von Willebrand factor (VWF) is a plasma glycoprotein best known for its crucial hemostatic role in serving as a molecular bridge linking platelets to subendothelial components following vascular injury. In addition, VWF functions as chaperone for coagulation factor VIII. In pathological settings, VWF is recognized as a risk factor for both arterial and venous thrombosis, as well as a molecular player that directly promotes the thrombotic process. Recent years have seen the emergence of the concept of immuno-thrombosis by which inflammatory cells participate in thrombotic processes. In return, reports about the involvement of hemostatic proteins or cells (such as platelets) in inflammatory responses have become increasingly common, emphasizing the intricate link between hemostasis and inflammation. However, evidence of a link between VWF and inflammation arose much earlier than these recent developments. At first, VWF was considered only as a marker of inflammation in various pathologies, due to its acute release by the activated endothelium. Later on, a more complex role of VWF in inflammation was uncovered, owing to its capacity to direct the biogenesis of specific endothelial organelles, the Weibel-Palade bodies that contain known inflammation players such as P-selectin. Finally, a more direct link between VWF and inflammation has become apparent with the discovery that VWF is able to recruit leukocytes, either via direct leukocyte binding or by recruiting platelets which in turn will attract leukocytes. This review will focus on these different aspects of the connection between VWF and inflammation, with particular emphasis on VWF-leukocyte interactions.

LDL receptor-related protein 1 contributes to the clearance of the activated factor VII-antithrombin complex

Essentials Factor VIIa is cleared principally as a complex with antithrombin. Enzyme/serpin complexes are preferred ligands for the scavenger-receptor LRP1. Factor VIIa/antithrombin but not factor VIIa alone is a ligand for LRP1. Macrophage-expressed LRP1 contributes to the clearance of factor VIIa/antithrombin.

SUMMARY

Background Recent findings point to activated factor VII (FVIIa) being cleared predominantly (± 65% of the injected protein) as part of a complex with the serpin antithrombin. FVIIa-antithrombin complexes are targeted to hepatocytes and liver macrophages. Both cells lines abundantly express LDL receptor-related protein 1 (LRP1), a scavenger receptor mediating the clearance of protease-serpin complexes. Objectives To investigate whether FVIIa-antithrombin is a ligand for LRP1. Methods Binding of FVIIa and pre-formed FVIIa-antithrombin to purified LRP1 Fc-tagged cluster IV (rLRP1-cIV/Fc) and to human and murine macrophages was analyzed. FVIIa clearance was determined in macrophage LRP1 (macLRP1)-deficient mice. Results Solid-phase binding assays showed that FVIIa-antithrombin bound in a specific, dose-dependent and saturable manner to rLRP1-cIV/Fc. Competition experiments with human THP1 macrophages indicated that binding of FVIIa but not of FVIIa-antithrombin was reduced in the presence of annexin-V or anti-tissue factor antibodies, whereas binding of FVIIa-antithrombin but not FVIIa was inhibited by the LRP1-antagonist GST-RAP. Additional experiments revealed binding of both FVIIa and FVIIa-antithrombin to murine control macrophages. In contrast, no binding of FVIIa-antithrombin to macrophages derived from macLRP1-deficient mice could be detected. Clearance of FVIIa-antithrombin but not of active site-blocked FVIIa was delayed 1.5-fold (mean residence time of 3.3 ± 0.1 h versus 2.4 ± 0.2 h) in macLRP1-deficient mice. The circulatory presence of FVIIa was prolonged to a similar extent in macLRP1-deficient mice and in control mice. Conclusions Our data show that FVIIa-antithrombin but not FVIIa is a ligand for LRP1, and that LRP1 contributes to the clearance of FVIIa-antithrombin in vivo.

Assessment of endothelial damage and cardiac injury in a mouse model mimicking thrombotic thrombocytopenic purpura

Essentials Endothelial injury is thought to be a key event in thrombotic thrombocytopenic purpura (TTP). Endothelial and cardiac damages were assessed in a model of TTP using ADAMTS-13 knockout mice. Damages of cardiac perfusion and function were associated with nitric oxide pathway alteration. Endothelial dysfunction constitutes a critical event in TTP development and cardiac injury.

SUMMARY

Background Cardiac alterations represent a major cause of mortality in patients with thrombotic thrombocytopenic purpura (TTP). Endothelial injury remains poorly defined, but seems to be a key initiating event leading to the formation of platelet-rich thrombi in TTP patients. Objectives To assess the changes in endothelial function and the induced cardiac damage in a mouse model of TTP. Patients/methods We used an animal model in which TTP-like symptoms are triggered by injection of 2000 units kg-1 of recombinant von Willebrand factor in ADAMTS-13 knockout mice. Results These mice developed TTP-like symptoms, i.e. severe thrombocytopenia, schistocytosis, and anemia. On day 2, magnetic resonance imaging demonstrated a decrease in left ventricular perfusion associated with alteration of left ventricular ejection fraction, fractional shortening, and cardiac output, suggesting early systolic dysfunction. This was associated with decrease in endothelium-mediated relaxation responses to acetylcholine in mesenteric and coronary arteries, demonstrating severe early endothelial dysfunction. In parallel, we showed decreased cardiac expression of endothelial nitric oxide (NO) synthase and increased expression of antioxidant enzymes, suggesting alteration of the NO pathway. At this time, cardiac immunohistochemistry revealed an increase in the expression of VCAM-1 and E-selectin. Conclusion This study provides evidence that the heart is a sensitive target organ in TTP, and shows, for the first time, strong mesenteric and coronary endothelial dysfunction in an induced-TTP model. The mechanisms incriminated are the occurrence of a pro-oxidant state, and proadhesive and proinflammatory phenotypes. This previously largely unrecognized vascular dysfunction may represent an important contributor to the systemic organ failure occurring in TTP.

Identification and characterization of the elusive mutation causing the historical von Willebrand Disease type IIC Miami

Essentials Von Willebrand disease IIC Miami features high von Willebrand factor (VWF) with reduced function. We aimed to identify and characterize the elusive underlying mutation in the original family. An inframe duplication of VWF exons 9-10 was identified and characterized. The mutation causes a defect in VWF multimerization and decreased VWF clearance from the circulation.

SUMMARY

Background A variant of von Willebrand disease (VWD) type 2A, phenotype IIC (VWD2AIIC), is characterized by recessive inheritance, low von Willebrand factor antigen (VWF:Ag), lack of VWF high-molecular-weight multimers, absence of VWF proteolytic fragments and mutations in the VWF propeptide. A family with dominantly inherited VWD2AIIC but markedly elevated VWF:Ag of > 2 U L(-1) was described as VWD type IIC Miami (VWD2AIIC-Miami) in 1993; however, the molecular defect remained elusive. Objectives To identify the molecular mechanism underlying the phenotype of the original VWD2AIIC-Miami. Patients and Methods We studied the original family with VWD2AIIC-Miami phenotypically and by genotyping. The identified mutation was recombinantly expressed and characterized by standard techniques, confocal imaging and in a mouse model, respectively. Results By Multiplex ligation-dependent probe amplification we identified an in-frame duplication of VWF exons 9-10 (c.998_1156dup; p.Glu333_385dup) in all patients. Recombinant mutant (rm)VWF only presented as a dimer. Co-expressed with wild-type VWF, the multimer pattern was indistinguishable from patients' plasma VWF. Immunofluorescence studies indicated retention of rmVWF in unusually large intracellular granules in the endoplasmic reticulum. ADAMTS-13 proteolysis of rmVWF under denaturing conditions was normal; however, an aberrant proteolytic fragment was apparent. A decreased ratio of VWF propeptide to VWF:Ag and a 1-desamino-8-d-arginine vasopressin (DDAVP) test in one patient indicated delayed VWF clearance, which was supported by clearance data after infusion of rmVWF into VWF(-/-) mice. Conclusion The unique phenotype of VWD2 type IIC-Miami results from dominant impairment of multimer assembly, an aberrant structure of mutant mature VWF and reduced clearance in vivo.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Neutrophils mediate edema formation but not mechanical allodynia during zymosan-induced inflammation

Inflammatory pain is based on stimulation and sensitization of peripheral endings of sensory neurons (nociceptors) by pronociceptive mediators. These mediators can be released by resident cells, as well as invading immune cells. Although neutrophils are known to release various mediators, which can stimulate or sensitize nociceptors, the extent of their contribution to nociceptive responses is unclear. Here, we studied the contribution of neutrophils to zymosan-induced inflammatory pain, which is characterized by an early recruitment of high numbers of neutrophils. Surprisingly, antibody-mediated neutrophil depletion caused a complete loss of edema formation but had no effect on mechanical pain thresholds. Blockage of the interaction between neutrophils and platelets or endothelial cells using antibodies directed against CD11b and CD162 reduced neutrophil recruitment to the site of inflammation. Again, the treatment decreased zymosan-induced edemas without altering mechanical pain thresholds. Also, HLB-219 mice, which have five to 10 times less platelets than WT mice, showed reduced neutrophil recruitment to the site of inflammation and decreased edema sizes, whereas, again, mechanical thresholds were unaltered. The effects observed in HLB-219 mice were relatively small and not reproduced in vWF-deficient mice or after antibody-mediated blockage of GPIbα. Flow chamber and transmigration assays showed that platelets were not necessary for neutrophil adhesion to endothelial cells but increased their transmigration. Taken together, zymosan-induced mechanical allodynia is, in contrast to edema formation, independent of neutrophils, and recruitment of neutrophils is only partly influenced by interactions with platelets.

Clearance of von Willebrand factor

Quantitative deficiencies in von Willebrand factor (VWF) are associated with abnormal hemostasis that can manifest in bleeding or thrombotic complications. Consequently, many studies have endeavored to elucidate the mechanisms underlying the regulation of VWF plasma levels. This review focuses on the role of VWF clearance pathways. A summary of recent developments are provided, including results from genetic studies, the relationship between glycosylation and VWF clearance, the contribution of increased VWF clearance to the pathogenesis of von Willebrand disease and the identification of VWF clearance receptors. These different studies converge in their conclusion that VWF clearance is a complex phenomenon that involves multiple mechanisms. Deciphering how such different mechanisms coordinate their role in this process is but one of the remaining challenges. Nevertheless, a better insight into the complex clearance pathways of VWF may help us to better understand the clinical implications of aberrant clearance in the pathogenesis of von Willebrand disease and perhaps other disorders as well as aid in developing alternative therapeutic approaches.

Terminal platelet production is regulated by von Willebrand factor

It is established that proplatelets are formed from mature megakaryocytes (MK) as intermediates before platelet production. Recently, the presence of proplatelets was described in blood incubated in static conditions. We have previously demonstrated that platelet and proplatelet formation is upregulated by MK exposure to high shear rates (1800 s⁻¹) on immobilized von Willebrand factor (VWF). The purpose of the present study was to investigate whether VWF is involved in the regulation of terminal platelet production in blood. To this end, Vwf ^−/− mice, a model of severe von Willebrand disease, were used to create a situation in which blood cells circulate in a vascular tree that is completely devoid of VWF. Murine platelets were isolated from Vwf ^−/− and Vwf ^+/+ blood, exposed to VWF at 1800 s⁻¹ in a microfluidic platform, and examined by means of videomicroscopy, as well as fluorescence and activation studies. Proplatelets became visible within 5 minutes, representing 38% of all platelets after 12 minutes and 46% after 28 min. The proportion of proplatelets was 1.8-fold higher in blood from Vwf^−/− mice than from Vwf^+/+ mice, suggesting a role of VWF in vivo. Fragmentation of these proplatelets into smaller discoid platelets was also observed in real-time. Platelets remained fully activatable by thrombin. Compensation of plasmatic VWF following hydrodynamic gene transfer in Vwf^−/− mice reduced the percentage of proplatelets to wild-type levels. A thrombocytopenic mouse model was studied in the flow system, 7 days after a single 5-FU injection. Compared to untreated mouse blood, a 2-fold increase in the percentage of proplatelets was detected following exposure to 1800 s⁻¹ on VWF of samples from mice treated with 5-FU. In conclusion, VWF and shear stress together appear to upregulate proplatelet reorganization and platelet formation. This suggests a new function for VWF in vivo as regulator of bloodstream thrombopoiesis.

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

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

In vivo analysis of the role of O-glycosylations of von Willebrand factor

The objective of this project was to study the function of O-glycosylations in von Willebrand factor (VWF) life cycle. In total, 14 different murine Vwf cDNAs mutated on one or several O-glycosylations sites were generated: 9 individual mutants, 2 doublets, 2 clusters and 1 mutant with all 9 murine glycosylation sites mutated (Del-O-Gly). We expressed each mutated cDNA in VWF deficient-mice by hydrodynamic injection. An immunosorbent assay with Peanut Agglutinin (PNA) was used to verify the O-glycosylation status. Wild-type (WT) VWF expressed by hepatocytes after hydrodynamic injection was able to bind PNA with slightly higher affinity than endothelial-derived VWF. In contrast, the Del-O-Gly VWF mutant did not bind PNA, demonstrating removal of O-linked glycans. All mutants displayed a normal multimeric pattern. Two mutants, Del-O-Gly and T1255A/T1256A, led to expression levels 50% lower than those induced by WT VWF and their half-life in vivo was significantly reduced. When testing the capacity of each mutant to correct the bleeding time of VWF-deficient mice, we found that S1486A, T1255A, T1256A and the doublet T1255A/T1256A were unable to do so. In conclusion we have shown that O-glycosylations are dispensable for normal VWF multimerization and biosynthesis. It also appears that some O-glycosylation sites, particularly the T1255 and T1256 residues, are involved in the maintenance of VWF plasma levels and are essential for normal haemostasis. As for the S1486 residue, it seems to be important for platelet binding as demonstrated in vitro using perfusion experiments.

Factor VIII and von Willebrand factor--too sweet for their own good

SUMMARY

Although factor VIII (FVIII) and von Willebrand factor (VWF) are products of two distinct genes, they circulate in plasma as a tight non-covalent complex. Moreover, they both play a critical role in the haemostatic process, a fact that is illustrated by the severe bleeding tendency associated with the functional absence of either protein. FVIII is an essential cofactor for coagulation factor IX, while VWF is pertinent to the recruitment of platelets to the injured vessel wall under conditions of rapid flow. FVIII and VWF have in common that they are heavily glycosylated: full-length FVIII contains 20 N-linked and at least seven O-linked glycans, while VWF contains 12 N-linked and 10 O-linked glycans. Three decades of research have revealed that the carbohydrate structures of FVIII and VWF contribute to many of the steps that can be distinguished in the life-cycle of these proteins, including biosynthesis/secretion, function and clearance. In this review, several of these aspects will be discussed. In addition, the interaction of the FVIII/VWF complex with two families of carbohydrate-binding proteins, i.e. Galectins and Siglecs, and their potential physiological relevance will be discussed.

Arterial thrombosis: relevance of a model with two levels of severity assessed by histologic, ultrastructural and functional characterization

BACKGROUND

We previously described a model of laser-induced thrombosis in mesenteric arterioles with superficial and deep levels of injury producing a transient thrombus resolving within 2 min and a larger almost occlusive thrombus, respectively. Both types of lesion were sensitive to platelet GPIIb-IIIa and P2Y(12) inhibition, whereas only deep injuries were sensitive to thrombin blockade.

OBJECTIVE

The aim of the present study was to use histologic methods and electron and intravital microscopy to characterize the lesions and thrombi and to extend our knowledge of the sensitivity of this model to genetic and pharmacologic inhibition.

RESULTS

A superficial injury was found to detach the endothelial cells and expose a collagen III- and IV-rich subendothelium where platelets could adhere. Tissue factor and fibrin were not detected. Deeper penetration of the external elastic lamina occurred in deep injuries, with exposure of collagen I, III and IV. Here the thrombus was composed of platelets exhibiting a decreasing gradient of degranulation from the deepest lesion area to the surface. Fibrin was found close to the most activated platelets. Consistently, glycoprotein VI (GPVI)-collagen and GPIb-von Willebrand factor (VWF) interactions were found to be critical in superficial injuries. After deep lesion, thrombus formation was modestly reduced in GPVI-immunodepleted mice and still strongly inhibited in VWF(-/-) mice. Combined hirudin infusion and GPVI depletion further inhibited thrombosis after deep injury.

CONCLUSIONS

This study confirms the feasibility of inducing arterial thrombosis with distinct levels of severity and establishes the central roles of collagen and VWF in thrombus formation after superficial injury. Collagen, VWF and thrombin all appear to contribute to thrombosis after deep arterial lesion.

Mouse models of von Willebrand disease

von Willebrand disease (VWD), caused by quantitative or qualitative abnormalities in von Willebrand factor (VWF) is considered the most common inherited bleeding disorder in humans. Mild and severe quantitative defects in VWF cause VWD types 1 and 3 respectively, whereas qualitative abnormalities induce VWD type 2. VWD has also been diagnosed in a number of animal species such as dogs, pigs, cats and horses, as a result of naturally occurring mutations. More recently, murine models have drawn a great deal of attention. Their small size along with their well-defined genetic background makes them ideal tools to study the in vivo function of VWF. The most commonly used model is the VWF-deficient mouse engineered through homologous recombination. However, models resulting from changes in modifier genes indirectly affecting VWF have also been described. These various models have proven very useful in elucidating some aspects of VWF biology not easily addressed through in vitro approaches.

Molecular characterization of Iranian patients with type 3 von Willebrand disease

von Willebrand's disease (VWD) type 3 is a rare but severe autosomal-recessive inherited bleeding disorder with a prevalence higher in certain locations where consanguineous marriages are relatively frequent. The genetic defects causing recessive type 3 VWD in 10 unrelated families from Iran have been investigated and the genetic heterogeneity among these patients was evaluated. All exons and their flanking regions of von Willebrand factor gene were amplified by PCR and sequenced using specific primers. Eight patients were fully characterized at the molecular level. Six different gene alterations were identified. All the mutations caused null alleles, three being nonsense mutations (Q104X, Q793X and E1981X), two possible splice site mutations (2443-1G>C and 1110-1G>A) and one small deletion (3237delA). Three of them have not been described previously. Most patients were born from consanguineous marriages and all were homozygous for their mutations. The results confirm that molecular defects in type 3 VWD are heterogeneous with mutations arising randomly within the entire gene.

von Willebrand factor is a major determinant of ADAMTS-13 decrease during mouse sepsis induced by cecum ligation and puncture

SUMMARY BACKGROUND

During sepsis, von Willebrand factor (VWF) is abundantly secreted; the main mechanism regulating its size involves specific proteolysis by the metalloprotease ADAMTS-13.

OBJECTIVES

To determine whether ADAMTS-13 consumption due to its binding to, and/or cleavage, of VWF contributes to its decrease during sepsis and whether abrogating or enhancing ADAMTS-13 activity influences sepsis outcome.

METHODS

ADAMTS-13 activity was evaluated in a model of sepsis induced by cecum ligature and puncture (CLP) in wild-type and Vwf(-/-) mice. Sepsis outcome was studied in those mice and in Adamts-13(-/-) mice. Finally, survival was studied in wild-type mice injected hydrodynamically with the human ADAMTS-13 gene.

RESULTS

In wild-type mice, CLP-induced sepsis elicited a significant ADAMTS-13 decrease, and a strong negative correlation existed between VWF and ADAMTS-13. In Vwf(-/-) mice, CLP also induced severe sepsis, but ADAMTS-13 was not significantly diminished. Notably, Vwf(-/-) mice lived significantly longer than wild-type mice. In contrast, Adamts-13(-/-) mice and wild-type mice were comparable with regard to thrombocytopenia, VWF concentrations, absence of thrombi, and survival. Hydrodynamic hADAMTS-13 gene transfer with the pLIVE expression vector resulted in high and stable ADAMTS13 activity in CLP mice; however, no impact on survival was observed.

CONCLUSIONS

VWF secretion is a major determinant of ADAMTS-13 decrease in the CLP model, and plays an important role in sepsis-induced mortality, but the complete absence of its regulating protease, ADAMTS-13, had no detectable impact in this sepsis model. Furthermore, increasing ADAMTS-13 activity had no impact on survival.

Mouse models to study von Willebrand factor structure-function relationships in vivo

Von Willebrand factor (VWF) structure-function relationship has been studied only through in vitro approaches. The VWF-deficient mouse model has been extremely useful to examine the in vivo function of VWF but does not allow a more subtle analysis of the relative importance of its different domains. However, considering the large size of VWF and its capacity to interact with various ligands in order to support platelet adhesion and aggregation, the necessity to evaluate independently these interactions appeared increasingly crucial. A recently developed technique, known as hydrodynamic injection, which allows transient expression of a transgene by mouse hepatocytes, proved very useful in this regard. Indeed, transient expression of various VWF mutants in VWF-deficient mice contributed to improve our knowledge about the role of VWF interaction with subendothelial collagens and with platelets receptors in VWF roles in haemostasis and thrombosis. These findings can provide new leads in the development of anti-thrombotic therapies.

Characterization of the interaction between von Willebrand factor and osteoprotegerin

BACKGROUND AND OBJECTIVE

Osteoprotegerin (OPG), a member of the tumor necrosis-factor receptor superfamily, plays an important role in bone remodeling and is also involved in vascular diseases. OPG is physically associated with von Willebrand factor (VWF), a glycoprotein involved in primary hemostasis, within the Weibel-Palade bodies (WPBs) of endothelial cells and in plasma. The present study aimed to elucidate the molecular mechanisms underlying the interaction between OPG and VWF.

METHODS AND RESULTS

In a solid-phase binding assay, VWF was able to bind specifically to OPG in a calcium-dependent manner. This interaction displayed strong pH dependence with optimal binding occurring at pH 6.5 and was severely impaired by chloride-ion concentrations above 40 mm. Using a series of purified VWF derivatives the functional site that supports VWF interaction with OPG was localized on its Al domain. Fluorescence microscopy on human umbilical vein endothelial cells showed co-localization of VWF and OPG in WPBs. When secretion was induced, OPG remained associated with VWF in extracellular patches of release under biochemical conditions found in blood plasma.

CONCLUSIONS

Our observations demonstrate the existence of an interactive site for OPG within the VWF A1-domain. This study established that the optimal biochemical parameters allowing a complex formation between VWF and OPG are those thought to prevail in the trans-Golgi network. These conditions would allow VWF to act as a cargo targeting OPG to WPBs. Finally, blood environments appear suitable to preserve the complex, which may participate in vascular injury, arterial calcification and inflammation.

Staphylococcal superantigen-like 5 binds PSGL-1 and inhibits P-selectin–mediated neutrophil rolling

Staphylococcus aureus secretes several virulence factors interfering with host-cell functions. Staphylococcal superantigen-like (SSL) proteins are a family of 11 exotoxins with structural homology to superantigens but with generally unknown functions. Recently, we described that chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS31-121), a potent inhibitor of C5a-induced responses, is structurally homologous to the C-terminal domain of SSL5. Here, we identify P-selectin glycoprotein ligand-1 (PSGL-1), involved in the initial rolling of neutrophils along the endothelium, as a target for SSL5. SSL5 specifically bound to Chinese hamster ovary cells stably expressing PSGL-1 (CHO–PSGL-1), which was dependent of sulfation and sialylation. Furthermore, SSL5 bound to PSGL-1/Ig fusion protein immobilized on a biosensor chip. SSL5 affected binding of soluble P-selectin/Fc chimera, the principle ligand of PSGL-1, to CHO–PSGL-1 cells and inhibited adhesion of neutrophils to immobilized P-selectin under static conditions. Under flow conditions SSL5 strongly decreased neutrophil rolling on immobilized P-selectin/Fc and activated human endothelial cells. In conclusion, SSL5 interferes with the interaction between PSGL-1 and P-selectin, suggesting that S aureus uses SSL5 to prevent neutrophil extravasation toward the site of infection. This makes SSL5 a potential lead for the development of new anti-inflammatory compounds for disorders characterized by excessive recruitment of leukocytes.

P-selectin glycoprotein ligand 1 and beta2-integrins cooperate in the adhesion of leukocytes to von Willebrand factor

Von Willebrand factor (VWF) is an essential component of hemostasis. However, animal studies using VWF-deficient mice suggest that VWF may also contribute to inflammation. In the present study, we demonstrate that VWF was able to interact with polymorphonuclear leukocytes (PMNs) and monocytes under static and flow conditions. Adhesion under flow was dominated by short-lasting contact with resting PMNs, whereas adhesion of phorbol-12-myristate-13-acetate (PMA)-stimulated PMNs was characterized by firm adhesion. Transient binding of PMNs to VWF appeared to be mediated by P-selectin glycoprotein ligand-1 (PSGL-1). Moreover, recombinant PSGL-1 protein and cell surface-expressed PSGL-1 directly interacted with VWF. As for stable adhesion by PMA-stimulated PMNs, we observed that static adhesion and adhesion under flow were strongly inhibited (greater than 75%) by neutrophil-inhibitory factor, an inhibitor of beta2-integrin function. In addition, the isolated I-domain of alphaMbeta2 bound to VWF, and cell lines expressing alphaLbeta2 or alphaXbeta2 adhered efficiently to VWF. Taken together, our data showed that VWF can function as an adhesive surface for various leukocyte subsets (monocytes, PMNs). Analogous to VWF-platelet interaction, VWF provided binding sites for leukocyte receptors involved in rolling (PSGL-1) and stable (beta2-integrins) adhesion. VWF is unique in its intrinsic capacity to combine the rolling and the stable adhesion step in the interaction with leukocytes.

Increased metastatic potential of tumor cells in von Willebrand factor-deficient mice

BACKGROUND

The key role played by von Willebrand factor (VWF) in platelet adhesion suggests a potential implication in various pathologies, where this process is involved. In cancer metastasis development, tumor cells interact with platelets and the vessel wall to extravasate from the circulation. As a potential mediator of platelet-tumor cell interactions, VWF could influence this early step of tumor spread and therefore play a role in cancer metastasis.

OBJECTIVES

To investigate whether VWF is involved in metastasis development.

METHODS

In a first step, we characterized the interaction between murine melanoma cells B16-BL6 and VWF in vitro. In a second step, an experimental metastasis model was used to compare the formation of pulmonary metastatic foci in C57BL/6 wild-type and VWF-null mice following the injection of B16-BL6 cells or Lewis lung carcinoma cells.

RESULTS

In vitro adhesion assays revealed that VWF is able to promote a dose-dependent adhesion of B16-BL6 cells via its Arg-Gly-Asp (RGD) sequence. In the experimental metastasis model, we found a significant increase in the number of pulmonary metastatic foci in VWF-null mice compared with the wild-type mice, a phenotype that could be corrected by restoring VWF plasma levels. We also showed that increased survival of the tumor cells in the lungs during the first 24 h in the absence of VWF was the cause of this increased metastasis.

CONCLUSION

These findings suggest that VWF plays a protective role against tumor cell dissemination in vivo. Underlying mechanisms remain to be investigated.

Cysteine-mutations in von Willebrand factor associated with increased clearance

BACKGROUND

von Willebrand disease (VWD) is a bleeding disorder caused by the decrease of functional von Willebrand factor (VWF). Low levels of VWF can result from decreased synthesis, impaired secretion, increased clearance or combinations thereof. Several mutations lead to impaired synthesis or secretion of VWF, however, little is known about the survival of VWF in the circulation.

OBJECTIVES

To evaluate the effect of several VWF mutations on VWF clearance.

PATIENTS/METHODS

The effect of three cysteine-mutations (C1130F, C1149R or C2671Y) on the in vivo survival of VWF was studied in patients carrying these mutations and in a VWF-deficient mice model.

RESULTS

In patients carrying these mutations, we observed increased propeptide/mature VWF ratios and rapid disappearance of VWF from the circulation after desmopressin treatment. Detailed analysis of in vivo clearance of recombinant VWF in a VWF-deficient mice model revealed a fourfold increased clearance rate of the mutants. The mutations C1130F, C1149R and C2671Y are each associated with reduced survival of VWF in the circulation. Detailed analysis of the recombinant mutant VWF demonstrated that increased clearance was not due to increased proteolysis by ADAMTS-13. We did not identify functional or structural characteristics that the mutant proteins have in common and could be associated with the phenomenon of increased clearance.

CONCLUSIONS

Cysteine-mutations in VWF may result in reduced in vivo survival. The observation that various mutations are associated with increased in vivo clearance may have major implications for the therapeutic strategies that rely on the rise of endogenous VWF after desmopressin administration.

The alpha -chains of C4b-binding protein mediate complex formation with low density lipoprotein receptor-related protein

C4b-binding protein (C4BP) is a heparin-binding protein that participates in both the complement and hemostatic system. We investigated the interaction between C4BP and low density lipoprotein receptor-related protein (LRP), an endocytic receptor involved in the catabolism of various heparin-binding proteins. Both plasma-derived C4BP and recombinant C4BP consisting of only its alpha-chains (rC4BPalpha) bound efficiently to immobilized LRP, as determined by surface plasmon resonance analysis. Complementary, two distinct fragments of LRP, i.e. clusters II and IV, both associated to immobilized rC4BPalpha, and binding could be inhibited by the LRP antagonist receptor-associated protein. Further analysis showed that association of rC4BPalpha to LRP was inhibited by heparin or by anti-C4BP antibody RU-3B9, which recognizes the heparin-binding region of the C4BP alpha-chains. In cellular degradation experiments, LRP-expressing fibroblasts effectively degraded (125)I-labeled rC4BPalpha, whereas their LRP-deficient counterparts displayed a 4-fold diminished capacity of degrading (125)I-rC4BPalpha. Finally, initial clearance of C4BP in mice was significantly delayed upon co-injection with receptor-associated protein. In conclusion, our data demonstrate that the alpha-chains of C4BP comprise a binding site for LRP. We propose that LRP mediates at least in part the catabolism of C4BP and, as such, may regulate C4BP participation in complement and hemostatic processes.

Localized reduction of atherosclerosis in von Willebrand factor-deficient mice

To examine the role of the platelet adhesion molecule von Willebrand factor (vWf) in atherogenesis, vWf-deficient mice (vWf-/-) were bred with mice lacking the low-density lipoprotein receptor (LDLR-/-) on a C57BL/6J background. LDLR-/-vWf+/+ and LDLR-/-vWf-/- mice were placed on a diet rich in saturated fat and cholesterol for different lengths of time. The atherogenic diet stimulated leukocyte rolling in the mesenteric venules in both genotypes, indicating an increase in P-selectin-mediated adhesion to the endothelium. After 8 weeks on the atherogenic diet, the fatty streaks formed in the aortic sinus of LDLR-/-vWf-/- mice of either sex were 40% smaller and contained fewer monocytes than those in LDLR-/-vWf+/+ mice. After 22 weeks on the atherogenic diet (early fibrous plaque stage), the difference in lesion size in the aortic sinus persisted. Interestingly, the lesion distribution in the aortas of LDLR-/-vWf-/- animals was different from that of LDLR-/- vWf+/+ animals. In vWf-positive mice, half of all lesions were located at the branch points of the renal and mesenteric arteries, whereas lesions in this area were not as prominent in the vWf-negative mice. These results indicate that the absence of vWf primarily affects the regions of the aorta with disturbed flow that are prone to atherosclerosis. Thus, vWf may recruit platelets/leukocytes to the lesion in a flow-dependent manner or may be part of the mechano-transduction pathway regulating endothelial response to shear stress.

Defect in regulated secretion of P-selectin affects leukocyte recruitment in von Willebrand factor-deficient mice

Stimulation of endothelial cells by various inflammatory mediators leads to release of Weibel--Palade bodies and therefore to exocytosis of both P-selectin (adhesion receptor for leukocytes) and von Willebrand factor (vWf) (platelet ligand). The potential role of vWf in leukocyte recruitment was investigated with the use of vWf-deficient mice. We report a strong reduction of leukocyte rolling in venules of vWf-deficient mice. Similarly, vWf deficiency led to a decrease in neutrophil recruitment in a cytokine-induced meningitis model as well as in early skin wounds. In all instances with an antibody that preferentially recognizes plasma membrane P-selectin, we observed a dramatic reduction in P-selectin expression at the cell surface of vWf-deficient endothelium. With confocal microscopy, we found that the typical rodlike shape of the Weibel--Palade body is missing in vWf -/- endothelial cells and that part of the P-selectin content in the vWf -/- cells colocalized with LAMP-1, a lysosomal marker. However, intracellular P-selectin levels were similar in tumor necrosis factor alpha- and lipopolysaccharide-activated cells of both genotypes. We conclude that the absence of vWf, as found in severe von Willebrand disease, leads to a defect in Weibel--Palade body formation. This defect results in decreased P-selectin translocation to the cell surface and reduced leukocyte recruitment in early phases of inflammation.

Interleukin 11 significantly increases plasma von Willebrand factor and factor VIII in wild type and von Willebrand disease mouse models

Interleukin (IL)-11 is a cytokine with thrombopoietic activity that has been shown to increase plasma von Willebrand factor (vWf) in preliminary clinical studies. This led to further evaluation of the effect of recombinant human (rh)IL-11 on vWf and factor VIII (FVIII) secretion. In vitro, rhIL-11 did not increase vWf production by cultured endothelial cells, which suggests an indirect mechanism. Also, in vivo, plasma vWf was not elevated in mice shortly after a single intravenous (IV) bolus injection of 250 or 1000 microg/kg rhIL-11. The effect of continuous exposure to rhIL-11 was accessed by treating wild type mice for 7 consecutive days with subcutaneous 250 microg/kg/d rhIL-11. Platelet counts increased by 25% and 40% after 4 and 7 days, respectively. Plasma vWf and FVIII levels increased 2-fold after 4 and 7 days. Surprisingly, no effect of rhIL-11 on vWf or FVIII messenger RNA was observed, which suggests that the regulation by rhIL-11 occurs after transcription. No increase in soluble P-selectin was observed after rhIL-11 treatment, indicating that platelet activation is not the source of elevated vWf. Similarly to wild type mice, vWf heterozygous mice responded to rhIL-11 treatment by a significant increase in platelet counts and vWf and FVIII levels. Importantly, in vWf-deficient mice, rhIL-11 also induced a significant increase in FVIII independent of vWf and was able to reduce skin bleeding time. These results suggest that a clinical evaluation of the effects of rhIL-11-induced vWf/FVIII elevation in maintaining hemostasis in mild hemophilia A or von Willebrand disease would be worthwhile.

Platelets adhere to and translocate on von Willebrand factor presented by endothelium in stimulated veins

With the use of intravital microscopy, a new type of platelet-endothelial interaction in mouse mesenteric venules at low shear (80-100 seconds(-1)) is described. Stimulation of these vessels with calcium ionophore A23187, a known secretagogue of Weibel-Palade bodies, induced immediate platelet adhesion (within 15 seconds) and translocation without the formation of aggregates. This stop-and-go process reached a maximum in approximately 1 minute, when approximately 25 000 platelets adhered/mm(2).s, and then adhesion progressively decreased. This adhesion process was dependent on von Willebrand factor (vWF) and independent of P-selectin. Immunohistologic analysis showed that the venules were not denuded with A23187 treatment, suggesting that platelets adhered to vWF secreted on the luminal face of the endothelial cells. Histamine treatment induced a similar adhesion phenomenon. Platelet adhesion was not abolished in beta3-deficient mice or when the platelets were treated with inhibitory antibodies to PECAM-1 or PSGL-1, indicating that these molecules are not required for platelet-endothelium interaction at low shear. The adhesion was mediated by platelet glycoprotein Ibalpha (GPIbalpha) because the adhesion of murine platelets expressing exclusively the human GPIbalpha could be prevented by a pretreatment with mocarhagin, a snake venom protease that cleaves human GPIbalpha. The results indicate that vWF released from Weibel-Palade bodies can dramatically increase the concentration of platelets along the vessel wall through an interaction with GPIbalpha. It is proposed that this process may rapidly recruit platelets to sites of injury or inflammation in veins.

Persistence of platelet thrombus formation in arterioles of mice lacking both von Willebrand factor and fibrinogen

We used intravital microscopy to observe the formation of platelet plugs in ferric chloride-injured arterioles of live mice. With this model, we evaluated thrombus growth in mice lacking von Willebrand factor (vWF) and fibrinogen (Fg), the two key ligands known to mediate platelet adhesion and aggregation. In vWF(-/-) mice, despite the presence of arterial shear, delayed platelet adhesion occurred and stable thrombi formed. In many mice, a persisting high-shear channel never occluded. Abundant thrombi formed in Fg(-/-) mice, but they detached from the subendothelium, which ultimately caused downstream occlusion in all cases. Surprisingly, mice deficient in both vWF and Fg successfully formed thrombi with properties characteristic of both mutations, leading to vessel occlusion in the majority of vessels. Platelets of these doubly deficient mice specifically accumulated fibronectin in their alpha-granules, suggesting that fibronectin could be the ligand supporting the platelet aggregation.

P-Selectin glycoprotein ligand 1 (PSGL-1) is expressed on platelets and can mediate platelet-endothelial interactions in vivo

The platelet plays a pivotal role in maintaining vascular integrity. In a manner similar to leukocytes, platelets interact with selectins expressed on activated endothelium. P-selectin glycoprotein ligand 1 (PSGL-1) is the main P-selectin ligand expressed on leukocytes. Searching for platelet ligand(s), we used a P-selectin-immunoglobulin G (IgG) chimera to affinity purify surface-biotinylated proteins from platelet lysates. P-selectin-bound ligands were eluted with ethylenediaminetetraacetic acid. An approximately 210-kD biotinylated protein was isolated from both human neutrophil and platelet preparations. A band of the same size was also immunopurified from human platelets using a monoclonal anti-human PSGL-1 antibody and could be blotted with P-selectin-IgG. Under reducing conditions, both the predicted PSGL-1 approximately 210-kD dimer and the approximately 120-kD monomer were isolated from platelets. Comparative immunoelectron microscopy and Western blotting experiments suggested that platelet PSGL-1 expression is 25-100-fold lower than that of leukocytes. However, patients with chronic idiopathic thrombocytopenic purpura who harbor predominantly young platelets displayed greater expression, indicating that PSGL-1 expression may be decreased during platelet aging. By flow cytometry, thrombin-activated platelets from normal individuals exhibited greater expression than those unstimulated. An inhibitory anti-PSGL-1 antibody significantly reduced platelet rolling in mesenteric venules, as observed by intravital microscopy. Our results indicate that functional PSGL-1 is expressed on platelets, and suggest an additional mechanism by which selectins and their ligands participate in inflammatory and/or hemostatic responses.

Carboxypeptidase E does not mediate von Willebrand factor targeting to storage granules

Sorting of von Willebrand factor precursor (pro-vWf) from the trans-Golgi network to secretory granules (Weibel-Palade bodies) is critical for its conversion to the biologically active highly multimeric form, as well as for regulated secretion by the endothelial cells. When expressed in hormone-secretory cells, vWf is also recognized as a stored protein and is directed to storage granules. Recently, carboxypeptidase E (CPE) was proposed as a granular sorting receptor for prohormones (Cool et al., Cell 88: 73, 1997). To explore whether CPE is also involved in pro-vWf sorting, we initially examined its expression in human umbilical vein endothelial cells. A specific message for CPE and the protein itself were detected making it a plausible candidate as a targeting receptor for vWf in endothelium. To investigate this possibility, we used mice lacking CPE. The highly multimeric forms, subunit composition and plasma levels of vWf in CPE-deficient mice were similar to those of their wild-type littermates. vWf was also found in alpha-granules of platelets and in Weibel-Palade bodies of endothelial cells obtained from the CPE-deficient mice. Furthermore, vWf was released from the cultured CPE-deficient endothelial cells after stimulation with a secretagogue. We conclude that CPE is not essential for sorting vWf to the regulated secretory pathway. Thus, a CPE-independent mechanism must exist for protein sorting to storage granules.

Insights from von Willebrand disease animal models

von Willebrand disease is a genetic bleeding disorder that arises from abnormalities in von Willebrand factor, an adhesive glycoprotein involved in both primary hemostasis and coagulation. It is the most common inherited bleeding disorder in humans, and over the years several animal species have also been described as suffering from this disease whether through a spontaneous mutation (pigs, dogs) or a genetically engineered one (mouse). These different animal models are extremely useful in exploring the characteristics of von Willebrand disease and in testing new treatments. This review provides an update of the various von Willebrand disease models and the contribution that these models can make to a better understanding of human von Willebrand disease.