Biosynthesis of human von Willebrand factor

Endothelium forms the inner lining of all blood vessels and, as a consequence, is in direct contact with the blood. Because of this and the synthesis and secretion of hemostatic components, the endothelium is able to modulate coagulation and fibrinolysis. An important hemostatic factor synthesized by endothelial cells is the von Willebrand factor (vWF). vWF is a large plasma glycoprotein which promotes the adhesion of platelets to the vessel wall after a vascular injury. vWF is initially synthesized as a pre-pro-polypeptide. During its transport to the outside of the cell, the single-chain polypeptides are assembled into multimers. The pro-polypeptide can be cleaved and also be secreted. Free pro-polypeptide is identified as von Willebrand antigen II, a plasma glycoprotein of unknown function. Plasma vWF consists of a heterogenous series of multimers, composed of an apparently single-type glycoprotein subunit, linked together by disulfide bonds. The hemostatic potency of vWF was shown to increase with increasing multimer size. Therefore, the multimeric assembly of vWF is a crucial aspect in vWF biosynthesis. Furthermore, vWF synthesized by endothelial cells can either be secreted constitutively or stored and released upon stimulation of the endothelial cell. In this review, data are presented which contribute to the understanding of the biosynthetic pathway and complex processing which vWF has to undergo before it is secreted by the endothelial cell. These data have allowed a prediction of the sequential events underlying vWF biosynthesis, processing, multimer assembly, and secretion.     

Proteolytic processing of von Willebrand factor subunit: Heterogeneity in type-IIA von Willebrand disease

Summary Type IIA von Willebrand disease (vWD) is a heterogeneous disorder for which two different pathogenetic mechanisms have been proposed: increased proteolytic susceptibility of von Willebrand factor (vWF), and/or interference of its post-translational processing. Subunit analysis of vWF in type-IIA vWD has revealed an increased relative proportion of the 176- and 140-kDa subunit-derived fragments, suggesting an augmented fragmentation of vWF, even in the resting state. We analyzed the subunit pattern of vWF in plasma from five previously described patients with type-IIA vWD. All of them showed the above-mentioned pattern. In addition, the presence of a new band with an apparent molecular mass of 200 kDa, not described in normal individuals or in patients with vWD, was repeatedly observed in one of these patients. This patient also exhibited an abnormal vWF multimeric structure in platelets and in plasma, before and after desmopressin administration, when the blood was collected either in the presence or in the absence of proteinase inhibitors. We believe that an abnormal primary structure of vWF could be responsible for this abnormal proteolytic fragmentation pattern, as well as for the abnormal multimerization of vWF. Moreover, an abnormal susceptibility to proteolysis appears to be present, as suggested by the increase in the relative proportion of the 176-kDa fragment observed in the same patient. Future sequencing studies and genetic analysis may clarify whether there are one or two different defects related to the vWF of that patient. Our results indicate that the subunit analysis of vWF may reveal additional defects present in type-IIA vWD that may help our understanding of the pathogenesis of such disease.Supported in part by grants 90/3229, 91-92/0372, 94/1509 (FIS, INSALUD, Spain).     

Insights into von Willebrand factor proteolysis: clinical implications

The proteolysis of von Willebrand factor (VWF) by the recently discovered metalloprotease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin repeats), is a normal processing step in VWF biochemistry. Emerging data indicate that this step may be influenced by a variety of factors, some of which favour increased proteolysis and some of which compromise proteolysis. The former may predispose to bleeding, whilst the latter appears to be the underlying mechanism for thrombotic thrombocytopenic purpura (TTP). The new insights support the concept of "risk" in bleeding, particularly in the case of type 1 von Willebrand disease (VWD), in much the same way that risk is considered in venous thrombosis. This review presents relevant current knowledge of VWF proteolysis by ADAMTS13, and a novel model of how this may be implicated in type 1 VWD is proposed, based on events at the vessel wall at a time of haemostatic challenge.     

Homocysteine inhibits von Willebrand factor processing and secretion by preventing transport from the endoplasmic reticulum

Intracellular protein transport in endothelial cells is selectively inhibited by homocysteine, a thiol amino acid associated with both thrombosis and atherosclerosis. In a previous study, homocysteine decreased cell surface expression of the surface transmembrane glycoprotein thrombomodulin without decreasing secretion of another endothelial cell protein, plasminogen activator inhibitor-1. To define further the effects of homocysteine on protein transport, we examined the processing and secretion of the multimeric glycoprotein von Willebrand factor (vWF) in human umbilical vein endothelial cells. Incubation with 2 mmol/L homocysteine resulted in complete loss of vWF multimers and prevented asparagine-linked oligosaccharide maturation, propeptide cleavage, and secretion; these effects are consistent with impaired exit from the endoplasmic reticulum (ER). Dimerization was only partially inhibited, suggesting that homocysteine causes retention of provWF in the ER without preventing dimer formation. In pulse-chase incubations, intracellular provWF was degraded before exiting the ER in homocysteine-treated cells. Homocysteine also inhibited the processing and secretion of a carboxyl-terminal truncation mutant of human provWF expressed in rat insulinoma cells, indicating that retention in the endoplasmic reticulum can be mediated by regions of provWF apart from the carboxyl-terminal 20-Kd segment. These results suggest that retention of secretory proteins in the ER is regulated by redox mechanisms and imply that the intracellular transport of multiple endothelial cell proteins may be altered in patients with homocystinuria.     

Storage and secretion of von Willebrand factor by endothelial cells

Endothelial cells synthesize and store von Willebrand factor. We have studied the storage and secretion of von Willebrand factor in cultured human umbilical vein endothelial cells. In particular, we were interested in the nature of the storage compartment and the effects of perturbation on the storage and secretion processes. The storage compartment for von Willebrand factor was isolated from homogenates of endothelial cells. By an immunostaining technique the isolated vesicles stained for von Willebrand factor. The staining pattern was similar to that of Weibel-Palade bodies in intact endothelial cells. We concluded that the storage compartment containing von Willebrand factor is identical to the Weibel-Palade body. The von Willebrand factor of the isolated storage vesicles is predominantly constructed of polypeptide chains with a Mr of 220 kD. On the other hand, von Willebrand factor continuously secreted by endothelial cells is constructed of both a 220 kD and a larger precursor (apparent Mr of 275 kD) subunit. The storage vesicles contain von Willebrand factor that supports ristocetin-induced platelet aggregation. Thus, endothelial cells store fully processed, biologically active von Willebrand factor within Weibel-Palade bodies. Short-term (less than 1 h) treatment of endothelial cells with the perturbing phorbol ester 4 beta-phorbol-12-myristate-13-acetate (PMA) results in release of cellular stored von Willebrand factor. 24-48 h after exposure to PMA the endothelial cell distribution of von Willebrand factor is changed distinctly. While the contents of the von Willebrand factor storage sites in the cells are gradually restored within 48 h, enhanced amounts of von Willebrand factor are secreted into the medium. The number as well as the size of von Willebrand factor storage granules in the endothelial cells increase after exposure to phorbol ester, as determined by immunofluorescence microscopy. Phorbol ester treated cells release stored von Willebrand factor 48 h after they have been stimulated. PMA decreases the von Willebrand factor contents of the extracellular matrix; the deposition of von Willebrand factor in the subendothelium is blocked by PMA, whereas the degradation of matrix von Willebrand factor is not affected. Thus, perturbation of endothelial cells changes the cellular distribution of von Willebrand factor.     

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

Summary.  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.     

Analysis of intracellular storage and regulated secretion of 3 von Willebrand disease-causing variants of von Willebrand factor

The rapid exocytosis of von Willebrand factor (VWF) in response to vascular injury can be attributed to the fact that VWF is stored in the Weibel-Palade bodies (WPBs) of endothelial cells. We describe a system for examining the ability of VWF to drive both the formation of a storage compartment and the function of that compartment with respect to regulated secretion. Transient transfection of HEK293 cells with wild-type human VWF cDNA leads to the formation of numerous elongated organelles that resemble WPBs. These "pseudo-WPBs" exhibit the internal structure, as well as the ability to recruit membrane proteins including P-selectin, of bona fide WPBs. Finally, VWF was efficiently secreted upon stimulation by phorbol ester. We used this system to examine 3 VWF mutations leading to von Willebrand disease that affect VWF multimerization and constitutive secretion. Surprisingly we find that all 3 mutants can, to some extent, make pseudo-WPBs that recruit appropriate membrane proteins and that are responsive to secretagogues. The most striking defects are a delay in formation and a reduction in the length and number of pseudo-WPBs in proportion to the clinical severity of the mutation. Studies of pseudo-WPB formation in this system thus yield insights into the structure-function relationships underpinning the ability of VWF to form functional WPBs.     

von Willebrand factor in CHD and stroke: Relationships and therapeutic implications

Opinion statement Atherosclerotic cardiovascular disease (CVD), which includes coronary heart disease (CHD) and stroke, is now the most common cause of death in the middle aged and elderly in all parts of the world except subSaharan Africa. The direct cause of death is frequently an acute thrombotic arterial occlusion. Because atherosclerosis is a diffuse disease, patients with CHD also have a high risk of ischemic stroke. The hemostatic process is a needed defense mechanism to control hemorrhage after injury but at same time, if overactive, may have the potential to precipitate diseases such as myocardial infarction or stroke in the setting of atherosclerosis. In approximately 1% of all patients with ischemic stroke, and in up to 4% of young adults with stroke, the major precipitant of brain ischemia is a hematologic disorder or coagulopathy that predisposes to thrombosis. von Willebrand factor (vWF) plays an important role in platelet adhesion to subendothelial structures and in the intrinsic pathway of coagulation. It is regarded as an indirect measure of endothelial dysfunction. Deficiency of vWF in von Willebrand’s disease is well established. However, much less is known regarding the pathophysiologic implications of an elevated level of vWF, particularly in relation to CVD and cerebrovascular disease. The importance of vWF in the pathogenesis of this disease is poorly defined and information is limited and inconsistent. Elevated levels of vWF have been variably linked with risk of CHD; causal criteria are not fully met. Relationships with stroke risk are even less well established. Measurement of vWF adds little to risk prediction after considering the major risk factors—age, sex, smoking, raised blood cholesterol, and hypertension. vWF may have a greater role in predicting outcome in subjects with acute coronary syndromes (ACS), stroke, and perhaps atrial fibrillation. Investigation of the use of vWF level to guide treatment of ACS or stroke is ongoing; however, there is no compelling evidence to date.     

Two novel type 2N von Willebrand disease-causing mutations that result in defective factor VIII binding, multimerization, and secretion of von Willebrand factor

Two novel mutations, a T-to-C transition at nucleotide 2612 and a T-to-G transversion at nucleotide 3923 of the von Willebrand factor (vWF) complementary DNA, were detected by analysis of the vWF gene in DNA from members of 2 families with atypical von Willebrand disease. The T2612C transition predicts substitution of cysteine by arginine at amino acid position 788 (C788R), and the T3923G transversion predicts substitution of cysteine by glycine at position 1225 (C1225G) of pre-pro-vWF. The patients homozygous for the C788R and C1225G mutations both had a partial vWF deficiency (0. 18 IU/mL and 0.07 IU/mL vWF antigen, respectively); vWF in plasma from patients homozygous for either the C788R or the C1225G mutation failed to bind factor VIII and lacked high molecular weight multimers. Recombinant (r) vWF molecules having the C788R or C1225G mutation were expressed in COS-7 cells. Both rvWF C788R and rvWF C1225G exhibited significantly impaired secretion and failed to bind factor VIII. Recombinant vWF C788R in COS-7 culture medium showed a severe reduction in high molecular weight multimers, whereas rvWF C1225G showed a very mild reduction in high molecular weight multimers when compared with wild-type rvWF. (Blood. 2000;95:2000-2007)     

Postpartum von Willebrand factor levels in women with and without von Willebrand disease and implications for prophylaxis

The aim of this study was to elucidate the fall in von Willebrand factor (VWF) and factor VIII activity (FVIII) after childbirth in women with and without von Willebrand disease (VWD). VWF:RCo, VWF:Ag, and FVIII were obtained in the third trimester of pregnancy, on admission for childbirth, and 10 times postpartum. Specimens were processed within 4 h and analysed centrally. Means were calculated at each time point. Forty women (40 pregnancies) without VWD and 32 women (35 pregnancies) with VWD were enrolled. 15/32 with VWD were treated (30% of those with type 1 and all of those with type 2) in 17 pregnancies. Treatments prior to delivery consisted of desmopressin (2/17), VWF concentrate (15/17) and after delivery VWF concentrate (16/17). Duration of treatment was 0–21 days (median 6). VWF levels peaked at 250% of baseline – 4 h postpartum in women with VWD and 12 h postpartum in women without VWD. Thereafter, VWF levels fell rapidly, approached baseline at 1 week and reached baseline at 3 weeks. Except immediately postpartum, when the levels among treated cases were higher, levels among women with VWD appeared to parallel, but were lower than those among women without VWD. Levels were lowest among those who received treatment. VWF levels fall rapidly after childbirth. Except immediately postpartum, current treatment strategies do not raise VWF levels to the levels of women without VWD or even to the levels of women with milder, untreated VWD. Consequently, women with VWD may be at risk of postpartum haemorrhage despite treatment.     

Divergent fates of von Willebrand factor and its propolypeptide (von Willebrand antigen II) after secretion from endothelial cells.

The intracellular site of cleavage of pro-von Willebrand factor subunit and the subsequent fate of the propolypeptide (von Willebrand antigen II) and of the mature von Willebrand factor (vWf) were investigated. Both the propolypeptide, which was found to be a homodimer of noncovalently linked subunits, and mature vWf were released from Weibel-Palade bodies of endothelial cells following stimulation with secretagogues. The stoichiometry of the two released proteins was essentially equimolar. This indicates that vWf and the propolypeptide were packaged into the Weibel-Palade bodies as one unit, pro-vWf, and that the proteolytic cleavage of pro-vWf is likely to be a post-Golgi event. The association of prosequences into dimers supports their hypothetical role in the multimerization process. After secretion, the two proteins were distributed differently, as based on the following observations. The propolypeptide did not associate with vWf in the culture medium, did not codistribute with vWf in the extracellular "patches of release" on stimulated endothelial cells, and was not detected in the endothelial cell extracellular matrix, which did contain vWf. Additionally, in contrast to vWf, the propolypeptide did not bind to the matrix of human foreskin fibroblasts. Since the propolypeptide does not associate with vWf and does not interact with extracellular matrices in vitro, it is highly unlikely that it would promote platelet adhesion to subendothelium in vivo.     

Increased factor VIII/von Willebrand factor antigen and von Willebrand factor activity in renal failure.

Factor VIII/von Willebrand factor antigen and von Willebrand factor activity (ristocetin assay) were studied in 12 patients in renal failure. A dramatic increase in both activities was observed (antigen 315 +/- 30 per cent in patients verus 104 +/- 9 per cent in control subjects; activity 402 +/- 48 per cent in patients versus 111 +/- 5 per cent in control subjects; p less than 0.001 for both). Since von Willebrand factor is thought to play at least a facilitative role in the development of arteriosclerosis, these increased activities may contribute to the premature arteriosclerosis reported in patients with chronic renal failure undergoing dialysis.     

Basal secretion of von Willebrand factor from human endothelial cells

Endothelial cells store the adhesive glycoprotein von Willebrand factor (VWF) in Weibel-Palade bodies (WPBs), distinctively shaped regulated secretory organelles that undergo exocytosis in response to secretagogue. A significant proportion of newly synthesized VWF is also secreted spontaneously from nonstimulated cells, through what is thought to be the constitutive secretory pathway. To learn more about VWF trafficking, we performed kinetic analyses of the storage and nonstimulated secretion of VWF in cultured human endothelial cells. We found that most VWF was secreted through a route that was significantly delayed compared with constitutive secretion, although this pathway was responsible for secretion of a small amount of uncleaved VWF precursor. Disruption of pH-dependent sorting processes with ammonium chloride converted the secretion kinetics of mature VWF to that of its precursor. Conversely, preventing constitutive secretion of nascent protein with brefeldin A had only a modest effect on the spontaneous release of VWF, showing that most VWF secreted by nonstimulated cells was not constitutive secretion but basal release of a post-Golgi storage organelle, presumably the WPB. These data suggest that VWF is sorted to the regulated secretory pathway in endothelial cells much more efficiently than previously reported.     

Platelet von Willebrand factor – structure,function and biological importance

Besides circulating in normal plasma, von Willebrand factor (VWF) is also stored at relatively high concentration within the α‐granules of platelets. This pool of platelet VWF exists distinct from plasma VWF, and is enriched in haemostatically‐active high molecular weight multimers. Interestingly, the glycosylation profile of platelet VWF differs significantly from that of plasma VWF. Total sialic acid and galactose expression are reduced twofold on platelet VWF, and ABO blood group carbohydrate determinants are not present on the N‐linked glycans of platelet VWF. Consequently, in view of the critical role played by VWF glycans in modulating its activity, it is not surprising that the functional properties of platelet VWF differ markedly compared to those of plasma VWF. Nevertheless, animal model studies suggest that both plasma and platelet VWF play important roles in securing primary haemostasis. In addition, platelet VWF antigen and activity levels vary markedly between patients with different types of von Willebrand disease (VWD). Future studies to define the biochemical mechanisms responsible for these differences between plasma and platelet VWF are thus not only of basic scientific interest, but also of direct translational importance.     

Control of von Willebrand factor multimer size and implications for disease

Plasma von Willebrand factor (vWF) is a multimeric protein that mediates adhesion of platelets to sites of vascular injury, however only the very large vWF multimers are haemostatically competent. Plasma vWF derives predominantly from the vascular endothelium and is of a smaller average multimer size to that found in the sub-endothelial matrix. The existence of plasma factors that control the size of vWF multimers and account for this difference has long been suspected. vWF cleaving protease (vWFCP), a metalloproteinase that regulates vWF multimer size by proteolytic cleavage, and thrombospondin-1 (TSP-1), a trimeric glycoprotein that uncouples multimers by reducing the disulfide-bonds which link individual subunits, are two such factors that have recently been identified. The large and ultra large vWF multimers play a central role in arterial thrombosis and agents that regulate their size hold promise as novel anti-thrombotic drugs.     

Reduced von Willebrand factor survival in type Vicenza von Willebrand disease.

Type Vicenza variant of von Willebrand disease (VWD) is characterized by a low plasma von Willebrand factor (VWF) level and supranormal VWF multimers. Two candidate mutations, G2470A and G3864A at exons 17 and 27, respectively, of the VWF gene were recently reported to be present in this disorder. Four additional families, originating from northeast Italy, with both mutations of type Vicenza VWD are now described. Like the original type Vicenza subjects, they showed a mild bleeding tendency and a significant decrease in plasma VWF antigen level and ristocetin cofactor activity but normal platelet VWF content. Unlike the original patients, ristocetin-induced platelet aggregation was found to be normal. Larger than normal VWF multimers were also demonstrated in the plasma. Desmopressin (DDAVP) administration increased factor VIII (FVIII) and VWF plasma levels, with the appearance of even larger multimers. However, these forms, and all VWF oligomers, disappeared rapidly from the circulation. The half-life of VWF antigen release and of elimination was significantly shorter than that in healthy counterparts, so that at 4 hours after DDAVP administration, VWF antigen levels were close to baseline. Similar behavior was demonstrated by VWF ristocetin cofactor activity and FVIII. According to these findings, it is presumed that the low plasma VWF levels of type Vicenza VWD are mainly attributed to reduced survival of the VWF molecule, which, on the other hand, is normally synthesized. In addition, because normal VWF-platelet GPIb interaction was observed before or after DDAVP administration, it is proposed that type Vicenza VWD not be considered a 2M subtype.     

Treatment of von Willebrand disease with a high-purity factor VIII/von Willebrand factor concentrate: a prospective, multicenter study.

Among patients with von Willebrand disease (VWD) who are unresponsive to desmopressin therapy, replacement with plasma-derived concentrates is the treatment of choice. Because prospective studies are lacking, such treatment has been largely empirical. A multicenter, prospective study has been conducted in 81 patients with VWD (15 patients with type 1, 34 with type 2, and 32 with type 3 disease) to investigate the efficacy of a high-purity factor VIII/von Willebrand factor (FVIII/VWF) concentrate for treatment of bleeding and surgical prophylaxis. Two preparations of the concentrate-one virally inactivated with solvent detergent, the other with an additional heat-treatment step--were evaluated. Pharmacokinetic parameters were similar for both preparations. Using pre-established dosages based on the results of pharmacokinetic studies, 53 patients were administered either preparation for the treatment of 87 bleeding episodes, and 39 patients were treated prophylactically for 71 surgical or invasive procedures. Sixty-five (74.7%) and 10 (11.5%) of the bleeding episodes were controlled with 1 or 2 infusions, respectively. Patients with severe type 3 VWD typically required more infusions and higher doses, at shorter time intervals, than did patients with generally milder types 1 and 2. Among patients undergoing surgical procedures, blood loss was lower than that predicted prospectively, and losses exceeding the predicted value did not correlate with the postinfusion skin bleeding time. In conclusion, the concentrate effectively stopped active bleeding and provided adequate hemostasis for surgical or invasive procedures, even in the absence of bleeding time correction.     

Correlation between von Willebrand factor antigen,von Willebrand factor ristocetin cofactor activity and factor VIII activity in plasma

Background The laboratory diagnosis of von Willebrand Factor (VWF) deficiencies includes qualitative and quantitative measurements of VWF and clotting factor VIII (FVIII). Since the FVIII activity is frequently normal in patients with mild type 1 or 2 von Willebrand disease (VWD), there is controversy whether FVIII testing should accompany VWF Antigen (VWF:Ag) assay. Methods The aim of this study was to explore the correlation between VWF:Ag, VWF ristocetin cofactor activity (VWF:RCo) and FVIII in 213 consecutive patients undergoing screening for VWD. Results Forty-six patients were identified with VWF:Ag levels lower than the diagnostic threshold (54 IU/dl). A significant correlation was observed between VWF:Ag and VWF:RCo (r = 0.892; p < 0.001), VWF:Ag and FVIII (r = 0.834; p < 0.001), VWF:RCo and FVIII (r = 0.758; p < 0.001). Receiver operating characteristic curve analysis of the VWF:Ag assay revealed an area under the curve of 0.978 and 0.957 for detecting life-threatening values of FVIII (<30 IU/dl) and VWF:RCo (<40 IU/dl), respectively. The negative and positive predictive values at the VWF:Ag threshold value of 54 IU/dl were 100% and 33% for detecting life-threatening FVIII deficiencies, 94% and 80% for identifying abnormal values of VWF:RCo. Conclusions Due to the excellent correlation between VWF:Ag and FVIII and to the diagnostic efficiency of VWF:Ag for identifying abnormal FVIII levels in patients with VWF deficiency, routine measurement of FVIII may not be necessary in the initial screening of patients with suspected VWD. However, the limited negative predictive value of VWF:Ag for identifying type 2 VWD does not allow to eliminate VWF:RCo or VWF:FVIIIB assays from the diagnostic workout.