The fate of mitochondria during platelet activation

Blood platelets undergo several successive motor-driven reorganizations of the cytoskeleton when they are recruited to an injured part of a vessel. These reorganizations take place during the platelet activation phase, the spreading process on the injured vessel or between fibrin fibers of the forming clot, and during clot retraction. All these steps require a lot of energy, especially the retraction of the clot when platelets develop strong forces similar to those of muscle cells. Platelets can produce energy through glycolysis and mitochondrial respiration. However, although resting platelets have only 5 to 8 individual mitochondria, they produce adenosine triphosphate predominantly via oxidative phosphorylation. Activated, spread platelets show an increase in size compared with resting platelets, and the question arises as to where the few mitochondria are located in these larger platelets. Using expansion microscopy, we show that the number of mitochondria per platelet is increased in spread platelets. Live imaging and focused ion beam-scanning electron microscopy suggest that a mitochondrial fission event takes place during platelet activation. Fission is Drp1 dependent because Drp1-deficient platelets have fused mitochondria. In nucleated cells, mitochondrial fission is associated with a shift to a glycolytic phenotype, and using clot retraction assays, we show that platelets have a more glycolytic energy production during clot retraction and that Drp1-deficient platelets show a defect in clot retraction.

The Role of LIM Kinases during Development: A Lens to Get a Glimpse of Their Implication in Pathologies

The organization of cell populations within animal tissues is essential for the morphogenesis of organs during development. Cells recognize three-dimensional positions with respect to the whole organism and regulate their cell shape, motility, migration, polarization, growth, differentiation, gene expression and cell death according to extracellular signals. Remodeling of the actin filaments is essential to achieve these cell morphological changes. Cofilin is an important binding protein for these filaments; it increases their elasticity in terms of flexion and torsion and also severs them. The activity of cofilin is spatiotemporally inhibited via phosphorylation by the LIM domain kinases 1 and 2 (LIMK1 and LIMK2). Phylogenetic analysis indicates that the phospho-regulation of cofilin has evolved as a mechanism controlling the reorganization of the actin cytoskeleton during complex multicellular processes, such as those that occur during embryogenesis. In this context, the main objective of this review is to provide an update of the respective role of each of the LIM kinases during embryonic development.

Asymmetrical Forces Dictate the Distribution and Morphology of Platelets in Blood Clots

Primary hemostasis consists in the activation of platelets, which spread on the exposed extracellular matrix at the injured vessel surface. Secondary hemostasis, the coagulation cascade, generates a fibrin clot in which activated platelets and other blood cells get trapped. Active platelet-dependent clot retraction reduces the clot volume by extruding the serum. Thus, the clot architecture changes with time of contraction, which may have an important impact on the healing process and the dissolution of the clot, but the precise physiological role of clot retraction is still not completely understood. Since platelets are the only actors to develop force for the retraction of the clot, their distribution within the clot should influence the final clot architecture. We analyzed platelet distributions in intracoronary thrombi and observed that platelets and fibrin co-accumulate in the periphery of retracting clots in vivo. A computational mechanical model suggests that asymmetric forces are responsible for a different contractile behavior of platelets in the periphery versus the clot center, which in turn leads to an uneven distribution of platelets and fibrin fibers within the clot. We developed an in vitro clot retraction assay that reproduces the in vivo observations and follows the prediction of the computational model. Our findings suggest a new active role of platelet contraction in forming a tight fibrin- and platelet-rich boundary layer on the free surface of fibrin clots.

Marginal band microtubules are acetylated by αTAT1

The discoid shape of resting platelets is maintained by a peripheral, circular bundle of microtubules called marginal band. Marginal band microtubules are acetylated on lysine 40 of the alpha-tubulin subunits. We have previously shown that the deacetylase HDAC6 is responsible for tubulin deacetylation in platelets and that the hyperacetylated state of the microtubules in HDAC6KO platelets correlates with faster activation/spreading kinetics, pointing to a regulatory role of this modification. So far, the question about the reverse enzyme, responsible for tubulin acetylation in platelets, has remained unanswered. Several enzymes have been described as having tubulin acetylation activity. Here we identify αTAT1 as the enzyme responsible for the acetylation of marginal band microtubules. We show that αTAT1 deficiency has only minor consequences for platelet production and function. A residual tubulin acetylation level in αTAT1 deficient platelet lysates suggests the presence of an additional tubulin-acetylating enzyme that is unable to acetylate marginal band microtubules.

[On the road to deciphering the tubulin code: focus on acetylation and detyrosination]

Microtubules are cytoskeletal fibers formed by the assembly of α- and β-tubulin heterodimers. They contribute to cell morphology, mobility and polarity, as well as to cellular transport processes and cell division. The microtubular network constantly adapts to cellular needs and may be composed of very dynamic or more stable microtubules. To regulate their diverse functions in a spatio-temporal manner, microtubules are subjected to numerous reversible post-translational modifications, which generate the "tubulin code". This review focuses on two modifications characteristic of stable microtubules - acetylation and detyrosination of α-tubulin - and their deregulation in certain pathologies.

Epitope Mapping of Inhibitory Monoclonal Antibodies to Human von Willebrand Factor by Using Recombinant cDNA Libraries

Two recombinant expression libraries containing small (300-600 base pairs) cDNA fragments of von Willebrand Factor (vWF) were screened in order to map the epitope of monoclonal antibodies (MAbs) to vWF. Among eleven MAbs tested, seven were effectively mapped. The epitopes of MAbs 418 and 522, which inhibit the binding of vWF to Factor VIII (FVIII), were localized between Leu 2 and Arg 53 and between Glu 35 and He 81 of the vWF subunit respectively, within the N-terminal trypsin fragment called SpIII-T4 [amino acids (aa) 1-272] which contains a binding domain for FVIII. The epitope of MAb 710, which inhibits the binding of vWF to glycoprotein lb (GPIb), was identified between Ser 593 and Ser 678 on the tryptic 52/48 kDa fragment (aa 449-728) which contains binding domains for GPIb, collagen, heparin, sulfatides and subendothclium extracellular matrices. The epitope of MAb 723, which does not interfere with any known function of vWF, was localized between Ser 523 and Gly 588. The epitopes of MAb 505 and MAb 400, which inhibit the binding of v WF to collagen, were identified between Leu 927 and Arg 1114 within the SPI fragment (aa 911-1365) corresponding to the central part of the vWF subunit. The epitope of MAb 9, which inhibits the binding of vWF to GPIIb/IIIa, was identified in the C-terminal part of the vWF subunit between Gin 1704 and Asp 1746, the latter being the third aa of the RGD sequence common to adhesive proteins and serving as a recognition site for integrin receptors.

Characterization of Recombinant von Willebrand Factors Mutated on Cysteine 509 or 695

The interacting domain of vWF with platelet GPIb has been shown to overlap the large A1 loop formed by the intra-chain disulfide bond linking Cys 509 to Cys 695. In order to further investigate the role of the conformation of this region, we have expressed in COS-7 cells three mutated full-length recombinant vWFs (rvWFs) in which the substitutions Cys509Gly, Cys509Arg or Cys695Gly have been introduced by site-directed mutagenesis. SDS-agarose gel electrophoresis demonstrated an impaired multimerization of the mutants with undetectable high molecular weight multimers and a decrease of the relative amounts of the intermediate sized multimers. Binding analysis showed that rvWFC509G and rvWFC509R did not interact with botrocetin but spontaneously interacted with GPIb; the latter binding remained unchanged in the presence of ristocetin. This indicates that the substitution of Cys509 by Gly or Arg creates a conformation of vWF that increases its binding to GPIb. In contrast, rvWFC695G which did not react with botrocetin was also unable to interact with GPIb even in the presence of ristocetin, indicating that sequences interacting with GPIb are masked and/or disrupted. In conclusion, the substitution of each of the Cys509 and 695 results in mutant proteins which may be “locked” into active or inactive conformations in regard to the binding to platelet GPIb receptor.

Functional Analysis of the Arg91GIn Substitution in the Factor VIII Binding Domain of von Willebrand Factor Demonstrates Variable Phenotypic Expression

An Arg91Gln substitution in the mature von Willebrand factor (vWF) has been associated with defective binding of vWF to factor VIII (FVIII). We studied four families with members initially classified as having type I von Willebrand disease (vWD) who were either homozygous or heterozygous for the Arg91Gln change. The first family was the original case described by Nishino et al. (1) where three members were homozygous for the Gln91 allele. They had a low FVIII coagulant activity:vWF antigen (VIIIC:vWFAg) ratio, from 0.29 to 0.44, and the ability of their plasma vWF to bind FVIII was markedly decreased. All the heterozygous members had normal vWF and FVIII levels but the capacity of their plasma vWF to bind FVIII was reduced and intermediate between the homozygous members and normals. The affected individual from the second family was heterozygous for the Gln91 allele and demonstrated a VIIIC:vWFAg ratio of 0.98. The FVIII binding assay confirmed the heterozygous status indicating that the moderately low levels of vWF were due to reduced expression of both alleles. The propositus from the third family was also heterozygous and had below normal levels of vWF as well as a low VIIIC:vWFAg ratio of 0.34; however, FVIII binding to her plasma vWF was similar to that of the homozygous individuals suggesting that Gln91-vWF was the major circulating form. Her daughter who has type I vWD inherited the allele without the Gln91 mutation indicating that the expression of this allele was indeed impaired. The heterozygous patient in the fourth family had a vWF level of 24 U/dl but an VIIIC:vWFAg ratio greater than 2. Her plasma vWF showed normal FVIII binding implying that the Gln91 mutation was linked to a defect that repressed the expression of the allele. The Arg91Gln substitution appears to decrease but not eradicate FVIII binding suggesting a critical concentration of vWF be attained before the defect becomes phenotypically important.

β1 integrins mediate the BMP2 dependent transcriptional control of osteoblast differentiation and osteogenesis

Osteoblast differentiation is a highly regulated process that requires coordinated information from both soluble factors and the extracellular matrix. Among these extracellular stimuli, chemical and physical properties of the matrix are sensed through cell surface receptors such as integrins and transmitted into the nucleus to drive specific gene expression. Here, we showed that the conditional deletion of β1 integrins in the osteo-precursor population severely impacts bone formation and homeostasis both in vivo and in vitro. Mutant mice displayed a severe bone deficit characterized by bone fragility and reduced bone mass. We showed that β1 integrins are required for proper BMP2 dependent signaling at the pre-osteoblastic stage, by positively modulating Smad1/5-dependent transcriptional activity at the nuclear level. The lack of β1 integrins results in a transcription modulation that relies on a cooperative defect with other transcription factors rather than a plain blunted BMP2 response. Our results point to a nuclear modulation of Smad1/5 transcriptional activity by β1 integrins, allowing a tight control of osteoblast differentiation.

Ser968Thr Mutation within the A3 Domain of Von Willebrand Factor (VWF) in Two Related Patients Leads to a Defective Binding of VWF to Collagen

We report the identification of a new mutation of von Willebrand Factor (VWF) gene within exon 30 occurring in two related patients (mother and daughter) with a hemorrhagic syndrome. A T A transvertion at nucleotide 5441 was found changing the serine 968 to threonine of the mature VWF subunit (S1731T of the preproVWF). The Ser968Thr mutation is located within the VWF A3 domain which interacts with type I and III collagens. Both patients were found to be heterozygous for the mutation. The propositus (daughter) exhibited a slightly prolonged bleeding time, levels of VWF:Ag and VWF:RCo at the lower limit of normal, contrasting with normal levels of VIII:C. Her mother exhibited borderline bleeding time and moderately decreased levels of VWF and VIII:C. In both patients multimeric structure of VWF and ristocetin- as well as botrocetin-induced binding of VWF to GPIb were normal; however both patients repeatedly showed decreased binding of VWF to collagen. The Ser968Thr substitution was reproduced by site-directed mutagenesis on the full-length cDNA of VWF. The mutated recombinant VWF (rVWF), T968rVWF, and the hybrid S/T968rVWF were transiently expressed by COS-7 cells. Both rVWF exhibited normal multimeric pattern and normal ristocetin- as well as botrocetin-induced binding to GPIb. T968rVWF showed significantly decreased binding to collagen while the hybrid S/T968rVWF bound to collagen in a similar way to that of the patients’ plasma VWF. Thus, our data demonstrate that the Ser968Thr mutation of the VWF A3 domain is clearly responsible for the abnormal binding of VWF to collagen observed in both patients. The Ser968Thr substitution of the VWF is the first mutation identified in two patients leading to a decreased affinity of VWF for collagen and a normal multimeric structure.

New Assay for Measuring Binding of Platelet Glycoprotein IIb/IIIa to Unpurified von Willebrand Factor

Among the numerous variants of vWD, no patient with an abnormal vWF binding to GPIIb/IIIa has been described to date. To search for such potential variants, we developed a two-site assay for measuring the binding of purified GPIIb/IIIa to vWF in biological fluids and we used it to study a large series of plasmas from various types of von Willebrand disease (vWD) and recombinant vWF (rvWF).

vWF in plasma or rvWF in culture medium was immobilized onto anti-vWF monoclonal antibodies (MoAb)-coated wells of microtiter plates. After incubation with either unlabeled GPIIb/IIIa and a 125I-anti-GPIIb/IIIa MoAb or 125I-GPIIb/IIIa, binding curves and binding isotherms were respectively established. Normal pool plasma and wild-type rvWF were used as reference samples. We tested plasmas from 85 normal subjects, 115 patients with different types of vWD (64 type 1, 2 type 3, 9 type 2A, 4 type 2M, 16 type 2B, 15 type 2N, 3 type IID and 2 acquired forms) and 50 patients with various bleeding disorders. Four mutated rvWF with 2A (Glu875Lys and Pro885Ser) or 2B (Dupl.Met540 and Val551Phe) substitutions and one rvWF mutated in the RGD domain of the C-terminal part of vWF-subunit (Asp1746Gly) were also studied.

Among the various samples tested, only rvWF Asp1746Gly had no affinity for GPIIb/IIIa. In contrast, GPIIb/IIIa similarly bound to the other vWF, independently of the proteic environment, the factor VIII level, the degree of multimerization or the mutation of vWF. Our results indicate that subjects with an abnormal vWF binding to GPIIb/IIIa are probably rare and difficult to target for a specific screening.

Impaired dimerization of von Willebrand factor subunit due to mutation A2801D in the CK domain results in a recessive type 2A subtype IID von Willebrand disease

The CK domain of von Willebrand factor (VWF) is involved in the dimerization of the protein.We identified the homozygous substitution A2801D of the CK domain in two siblings. Patients had low levels of VWF in plasma, abnormal ristocetin-induced binding to platelets and abnormal multimeric pattern witha lack of high molecular weight (HMW) forms and the presence of intervening bands between normal multimers. Accordingly, they were classified in type 2A, subtype IID, von Willebrand disease (VWD). Both asymptomatic parents carried the mutation at the heterozygous state.Their plasmaVWF exhibited the full range of multimers found in normal plasma.When analyzed by high resolution gel electrophoresis, very faint bands corresponding to the position of intervening bands of the propositus can be observed. The mutated recombinant (r)VWF-D2801, the hybrid rVWF-A/D2801 and the mutated C-terminal VWF fragment rSPII-D2801 were expressed in COS-7 cells. rVWF-D2801 showed an abnormal multimeric distribution similar to that of the propositus’VWF with intervening bands and a lack of HMW species. rVWF-A/D2801 exhibited the full range of multimers and the aberrant sized forms observed both in propositus’VWF and in rVWF-D2801. rSPII-WT assembled correctly into a dimer of 220 kDa. rSPII-D2801 appeared as a mixture of monomeric and dimeric forms which may be related to the abnormal multimeric pattern of the propositus and both mutated rVWF. We concluded that mutation A2801D disturbs the folding of the CK domain, which may result in a mixture of monomers and dimers of VWF. Multimers containing either an odd or even number of mature subunits are produced, and the presence of monomers appears to limit the degree of multimerization. In the heterozygousVWF,the presence of normal dimers improves the multimerization process. In conclusion, the mutation A2801D appears to be responsible fora recessive type 2A, subtype IID, VWD.

Molecular characterization of four ADAMTS13 mutations responsible for congenital thrombotic thrombocytopenic purpura (Upshaw-Schulman syndrome)

ADAMTS13 mutations S203P, R268P, R507Q and A596V were previously identified in French patients with hereditary thrombotic thrombocytopenic purpura (TTP) (Upshaw-Schulman syndrome). Mutated recombinant (r) ADAMTS13 were transiently expressed in COS-7 cells and characterized in comparison with wild-type (WT) rADAMTS13.ADAMTS13 antigen was qualitatively and quantitatively estimated by electrophoretic analysis and ELISA. Enzymatic activity was qualitatively and quantitatively estimated using GST-VWF73,FRETS-VWF73 fragments and full-length rVWF-WT as substrates. The four mutants and rADAMTS13-WT were present within the cells. Secretion level of rADAMTS13-WT reached 1,200 ng/ml. The four mutations strongly altered the secretion and biological activity of rADAMTS13. The percentage secretion was 21, 38 and 17% for rADAMTS13-S203P, -R268P and -A596V compared with rADAMTS13- WT. rADAMTS13-R507Q concentration was under the detection limit of the assay. In the four cases, no enzymatic activity was detected. After concentration, we confirmed that mutations S203P and R268P totally abolished the proteolytic activity of ADAMTS13. Due to the very low protease concentration, activity of rADAMTS13-R507Q was below the threshold of the assays. rADAMTS13-A596V had no proteolytic activity towards the full-length rVWF-WT whereas it exhibited a decreased specific activity of about 30% of that of rADAMTS13- WT towards FRETS-VWF73 fragment. Binding study of mutated rADAMTS13-S203P, -R268P and -A596V showed that the three mutations strongly decreased the interaction of ADAMTS13 with VWF. In conclusion, the four mutations, which led to a secretion defect, a loss of enzymatic activity and a decreased binding to the substrate, are responsible for the hereditary TTP in patients.

β1 integrin-dependent Rac/group I PAK signaling mediates YAP activation of Yes-associated protein 1 (YAP1) via NF2/merlin

Cell adhesion to the extracellular matrix or to surrounding cells plays a key role in cell proliferation and differentiation and is critical for proper tissue homeostasis. An important pathway in adhesion-dependent cell proliferation is the Hippo signaling cascade, which is coregulated by the transcription factors Yes-associated protein 1 (YAP1) and transcriptional coactivator with PDZ-binding motif (TAZ). However, how cells integrate extracellular information at the molecular level to regulate YAP1's nuclear localization is still puzzling. Herein, we investigated the role of β1 integrins in regulating this process. We found that β1 integrin-dependent cell adhesion is critical for supporting cell proliferation in mesenchymal cells both in vivo and in vitro β1 integrin-dependent cell adhesion relied on the relocation of YAP1 to the nucleus after the down-regulation of its phosphorylated state mediated by large tumor suppressor gene 1 and 2 (LATS1/2). We also found that this phenotype relies on β1 integrin-dependent local activation of the small GTPase RAC1 at the plasma membrane to control the activity of P21 (RAC1)-activated kinase (PAK) of group 1. We further report that the regulatory protein merlin (neurofibromin 2, NF2) interacts with both YAP1 and LATS1/2 via its C-terminal moiety and FERM domain, respectively. PAK1-mediated merlin phosphorylation on Ser-518 reduced merlin's interactions with both LATS1/2 and YAP1, resulting in YAP1 dephosphorylation and nuclear shuttling. Our results highlight RAC/PAK1 as major players in YAP1 regulation triggered by cell adhesion.

ICAP-1 monoubiquitination coordinates matrix density and rigidity sensing for cell migration through ROCK2- MRCKα balance

Cell migration is a complex process requiring density and rigidity sensing of the microenvironment to adapt cell migratory speed through focal adhesion and actin cytoskeleton regulation. ICAP-1, a β1 integrin partner, is essential for ensuring integrin activation cycle and focal adhesion formation. We show that ICAP-1 is monoubiquitinated by Smurf1, preventing ICAP-1 binding to β1 integrin. The non-ubiquitinable form of ICAP-1 modifies β1 integrin focal adhesion organization and interferes with fibronectin density sensing. ICAP-1 is also required for adapting cell migration in response to substrate stiffness in a β1 integrin-independent manner. ICAP-1 monoubiquitination regulates rigidity sensing by increasing MRCKα-dependent cell contractility through myosin phosphorylation independently of substrate rigidity. We provide evidence that ICAP-1 monoubiquitination helps in switching from ROCK2-mediated to MRCKα-mediated cell contractility. ICAP-1 monoubiquitination serves as a molecular switch to coordinate extracellular matrix density and rigidity sensing thus acting as a critical modulator of cell migration and mechanosensing.

Calcium and calmodulin-dependent serine/threonine protein kinase type II (CaMKII)-mediated intramolecular opening of integrin cytoplasmic domain-associated protein-1 (ICAP-1α) negatively regulates β1 integrins

Focal adhesion turnover during cell migration is an integrated cyclic process requiring tight regulation of integrin function. Interaction of integrin with its ligand depends on its activation state, which is regulated by the direct recruitment of proteins onto the β integrin chain cytoplasmic domain. We previously reported that ICAP-1α, a specific cytoplasmic partner of β1A integrins, limits both talin and kindlin interaction with β1 integrin, thereby restraining focal adhesion assembly. Here we provide evidence that the calcium and calmodulin-dependent serine/threonine protein kinase type II (CaMKII) is an important regulator of ICAP-1α for controlling focal adhesion dynamics. CaMKII directly phosphorylates ICAP-1α and disrupts an intramolecular interaction between the N- and the C-terminal domains of ICAP-1α, unmasking the PTB domain, thereby permitting ICAP-1α binding onto the β1 integrin tail. ICAP-1α direct interaction with the β1 integrin tail and the modulation of β1 integrin affinity state are required for down-regulating focal adhesion assembly. Our results point to a molecular mechanism for the phosphorylation-dependent control of ICAP-1α function by CaMKII, allowing the dynamic control of β1 integrin activation and cell adhesion.

Expression of 14 von Willebrand factor mutations identified in patients with type 1 von Willebrand disease from the MCMDM-1VWD study

BACKGROUND

Candidate von Willebrand factor (VWF) mutations were identified in 70% of index cases in the European study 'Molecular and Clinical Markers for the Diagnosis and Management of type 1 von Willebrand Disease'. The majority of these were missense mutations.

OBJECTIVES

To assess whether 14 representative missense mutations are the cause of the phenotype observed in the patients and to examine their mode of pathogenicity.

METHODS

Transfection experiments were performed with full-length wild-type or mutant VWF cDNA for these 14 missense mutations. VWF antigen levels were measured, and VWF multimer analysis was performed on secreted and intracellular VWF.

RESULTS

For seven of the missense mutations (G160W, N166I, L2207P, C2257S, C2304Y, G2441C, and C2477Y), we found marked intracellular retention and impaired secretion of VWF, major loss of high molecular weight multimers in transfections of mutant constructs alone, and virtually normal multimers in cotransfections with wild-type VWF, establishing the pathogenicity of these mutations. Four of the mutations (R2287W, R2464C, G2518S, and Q2520P) were established as being very probably causative, on the basis of a mild reduction in the secreted VWF or on characteristic faster-running multimeric bands. For three candidate changes (G19R, P2063S, and R2313H), the transfection results were indistinguishable from wild-type recombinant VWF and we could not prove these changes to be pathogenic. Other mechanisms not explored using this in vitro expression system may be responsible for pathogenicity.

CONCLUSIONS

The pathogenic nature of 11 of 14 candidate missense mutations identified in patients with type 1 VWD was confirmed. Intracellular retention of mutant VWF is the predominant responsible mechanism.

Molecular characterization of four ADAMTS13 mutations responsible for congenital thrombotic thrombocytopenic purpura (Upshaw-Schulman syndrome)

ADAMTS13 mutations S203P, R268P, R507Q and A596V were previously identified in French patients with hereditary thrombotic thrombocytopenic purpura (TTP) (Upshaw-Schulman syndrome). Mutated recombinant (r) ADAMTS13 were transiently expressed in COS-7 cells and characterized in comparison with wild-type (WT) rADAMTS13. ADAMTS13 antigen was qualitatively and quantitatively estimated by electrophoretic analysis and ELISA. Enzymatic activity was qualitatively and quantitatively estimated using GST-VWF73, FRETS-VWF73 fragments and full-length rVWF-WT as substrates. The four mutants and rADAMTS13-WT were present within the cells. Secretion level of rADAMTS13-WT reached 1,200 ng/ml. The four mutations strongly altered the secretion and biological activity of rADAMTS13. The percentage secretion was 21, 38 and 17% for rADAMTS13-S203P, -R268P and -A596V compared with rADAMTS13-WT. rADAMTS13-R507Q concentration was under the detection limit of the assay. In the four cases, no enzymatic activity was detected. After concentration, we confirmed that mutations S203P and R268P totally abolished the proteolytic activity of ADAMTS13. Due to the very low protease concentration, activity of rADAMTS13-R507Q was below the threshold of the assays. rADAMTS13-A596V had no proteolytic activity towards the full-length rVWF-WT whereas it exhibited a decreased specific activity of about 30% of that of rADAMTS13-WT towards FRETS-VWF73 fragment. Binding study of mutated rADAMTS13-S203P, -R268P and -A596V showed that the three mutations strongly decreased the interaction of ADAMTS13 with VWF. In conclusion, the four mutations, which led to a secretion defect, a loss of enzymatic activity and a decreased binding to the substrate, are responsible for the hereditary TTP in patients.

Effect of von Willebrand disease type 2B and type 2M mutations on the susceptibility of von Willebrand factor to ADAMTS-13

BACKGROUND

von Willebrand disease (VWD) type 2 is associated with mutations in von Willebrand factor (VWF) that affect its secretion, multimeric pattern, affinity for platelet receptors and clearance of the protein. While increased proteolysis by a disintegrin-like and metalloprotease with thrombospondin type 1 motifs-13 (ADAMTS-13) has been clearly established for VWF type 2A, only little is known about VWF types 2B and 2M in this regard.

OBJECTIVES

Sensitivity of wild-type (WT) and mutated recombinant (r) VWF to proteolysis by ADAMTS-13 was investigated to better understand the role of this process in the pathophysiology of VWD.

METHODS

We used human rADAMTS-13-WT to digest 11 full-length recombinant forms of VWF carrying molecular abnormalities identified in patients with VWD type 2A (E1638K and P1648S), type 2B (InsM1303, R1306W, R1308P and V1314F) and type 2M (G1324A, E1359K, K1362T, R1374H and I1425F).

RESULTS

Using low ionic strength conditions, all mutations induced increased proteolysis of rVWF by rADAMTS-13 as compared with rVWF-WT. The susceptibility of mutants decreased in the following order: type 2A > type 2B > type 2M > rVWF-WT. At physiological salt concentration (150 mm NaCl) the sensitivity of all rVWF to rADAMTS-13 was significantly decreased. However, type 2A and type 2B mutants still exhibited a significantly higher susceptibility to rADAMTS-13 than rVWF-WT, whereas type 2M mutants normalized.

CONCLUSIONS

Type 2M mutants and rVWF-WT exhibit a similar sensitivity to rADAMTS-13-mediated proteolysis, in agreement with the normal multimeric pattern in vivo. In VWD type 2B, the spontaneous binding to platelets and excessive degradation by ADAMTS-13 of VWF high-molecular-weight multimers may account for their clearance from plasma.

Impaired dimerization of von Willebrand factor subunit due to mutation A2801D in the CK domain results in a recessive type 2A subtype IID von Willebrand disease

The CK domain of von Willebrand factor (VWF) is involved in the dimerization of the protein. We identified the homozygous substitution A2801D of the CK domain in two siblings. Patients had low levels of VWF in plasma, abnormal ristocetin-induced binding to platelets and abnormal multimeric pattern with a lack of high molecular weight (HMW) forms and the presence of intervening bands between normal multimers. Accordingly, they were classified in type 2A, subtype IID, von Willebrand disease (VWD). Both asymptomatic parents carried the mutation at the heterozygous state. Their plasmaVWF exhibited the full range of multimers found in normal plasma. When analyzed by high resolution gel electrophoresis, very faint bands corresponding to the position of intervening bands of the propositus can be observed. The mutated recombinant (r)VWF-D2801, the hybrid rVWF-A/D2801 and the mutated C-terminal VWF fragment rSPII-D2801 were expressed in COS-7 cells. rVWF-D2801 showed an abnormal multimeric distribution similar to that of the propositus'VWF with intervening bands and a lack of HMW species. rVWF-A/D2801 exhibited the full range of multimers and the aberrant sized forms observed both in propositus'VWF and in rVWF-D2801. rSPII-WT assembled correctly into a dimer of 220 kDa. rSPII-D2801 appeared as a mixture of monomeric and dimeric forms which may be related to the abnormal multimeric pattern of the propositus and both mutated rVWF. We concluded that mutation A2801D disturbs the folding of the CK domain, which may result in a mixture of monomers and dimers of VWF. Multimers containing either an odd or even number of mature subunits are produced, and the presence of monomers appears to limit the degree of multimerization. In the heterozygousVWF, the presence of normal dimers improves the multimerization process. In conclusion, the mutation A2801D appears to be responsible for a recessive type 2A, subtype IID, VWD.

Thrombotic Thrombocytopenic Purpura Associated with von Willebrand Factor-Cleaving Protease (ADAMTS13) Deficiency in Children

The physiopathology of thrombotic thrombocytopenic purpura (TTP) has been clarified since 1998, when it was shown that TTP in adults was most often associated with an acquired deficiency of von Willebrand factor-cleaving protease (ADAMTS13) due to autoantibodies, whereas TTP in children was most often associated with a hereditary autosomal recessive severe deficiency of ADAMTS13. The hereditary form of TPP (Upshaw-Schulman syndrome) is a very rare but life-threatening disease if adequate treatment (plasma therapy) is not administered. First manifestations occur before age 10 in two thirds of cases and as soon as birth in most cases. The subsequent course is characterized by recurrent hemolytic and thrombocytopenic crises, with intervals between relapses from every 3 to 4 weeks in two thirds of cases to several months or years in one third of cases. TTP crises are associated with cerebral vascular accidents in at least 30% of patients, with a risk of neurologic sequelae in approximately 20% of patients. Renal involvement includes frequent acute renal failure due to hemoglobinuria and/or thrombotic microangiopathy during hemolytic crisis and progressive renal deterioration in approximately 50% of cases, leading to chronic or end-stage renal failure in approximately 20% of patients. The clinical phenotype may vary from the typical congenital recurrent TTP. Some mild forms are limited to a fluctuating thrombocytopenia and may be misdiagnosed as idiopathic thrombocytopenic purpura. Phenotypic variability may be observed within a single family, which suggests a role of modifier genes. Fresh frozen plasma (FFP) replaces active ADAMTS13. Ten milliliters per kilogram FFP every 2 to 4 weeks suffices to maintain remission. FFP infusions are best used preventively, given that rescue infusions may not prevent central nervous system and renal involvement. It is hoped that plasmatic or recombinant purified ADAMTS13 will be available in the years to come.

Mutations C1157F and C1234W of von Willebrand factor cause intracellular retention with defective multimerization and secretion

The D3 domain of von Willebrand factor (VWF) is involved in the multimerization process of the protein through the formation of disulfide bridges. We identified heterozygous substitutions, C1157F and C1234W, in the VWF D3 domain in two unrelated families with unclassified and type 2A von Willebrand disease, respectively. VWF was characterized by a low plasmatic level, an abnormal binding to platelet GPIb and a high capacity of secretion from endothelial cells following DDAVP infusion. Using site-directed mutagenesis and expression in mammalian cells, we have investigated the impact of these mutations upon the multimerization, secretion and storage of VWF. Using COS-7 cells both mutated recombinant VWF (rVWF) displayed only lower molecular weight multimers. Pulse-chase analysis and endoglycosidase H digestion experiments showed the intracellular retention of mutated rVWF in pre-Golgi compartments. Study of hybrid rVWF obtained with a constant amount of wild-type (WT) DNA and increasing proportions of mutated plasmids established that both substitutions reduced the release of WT VWF in a dose-dependent manner and failed to form high molecular weight multimers. Using transfected AtT-20 stable cell lines, we observed similar granular storage of the two mutants and WT rVWF. Our data suggest that cysteines 1157 and 1234 play a crucial role in the early step of the folding of the molecule required for a normal transport pathway, maturation and constitutive secretion. In contrast, their substitution does not prevent the storage and inducible secretion of VWF.

An experimental model to study the in vivo survival of von Willebrand factor. Basic aspects and application to the R1205H mutation

To explore the molecular basis of von Willebrand factor (VWF) clearance, an experimental model employing VWF-deficient mice was developed. Biodistribution was examined by the injection of radiolabeled VWF, which was primarily directed to the liver with minor amounts in other organs. Disappearance of VWF from plasma was characterized by a rapid initial phase (t((1/2))alpha = 13 min) and a slow secondary phase (t((1/2))beta = 3 h), with a mean residence time (MRT) of 2.8 h. A similar clearance was observed for VWF consisting of only high or low molecular weight multimers, indicating that, in our experimental model, clearance is independent of multimeric distribution. This allowed us to compare the survival of full-length VWF to truncated variants. Deletion of both the amino-terminal D'-D3 and carboxyl-terminal D4-CK domains resulted in a fragment with a similar clearance to wild-type VWF. Deletion of only the D'-D3 region was associated with an almost 2-fold lower recovery and increased clearance (MRT = 1.6 h), whereas deletion of only the D4-CK region resulted in a significantly reduced clearance (MRT = 4.5 h, p < 0.02). These results point to a role of the D'-D3 region in preventing clearance of VWF. Furthermore, replacement of D3 domain residue Arg-1205 by His resulted in a markedly increased clearance (MRT = 0.3 h; p = 0.004). Therefore, this mutation seems to abrogate the protective effect of the D'-D3 region. In vitro analysis of this mutant also revealed a 2-fold reduced affinity for VWF propeptide at low pH, showing that mutation of Arg-1205 results not only in an increased clearance rate but is also associated with an impaired pH-dependent interaction with VWF propeptide.

A new mutation, S1285F, within the A1 loop of von Willebrand factor induces a conformational change in A1 loop with abnormal binding to platelet GPIb and botrocetin causing type 2M von Willebrand disease

We report the identification of a new mutation in exon 28 of the von Willebrand factor (VWF) gene in two related patients with type 2M von Willebrand disease (VWD). The molecular abnormality changes the Ser 1285 to Phe within the A1 loop of VWF. The S1285F mutation was reproduced by site-directed mutagenesis on the full-length VWF cDNA. The mutated recombinant VWF (rVWF), F1285rVWF, and the hybrid, S/F1285rVWF, were expressed in COS-7 cells. F1285rVWF exhibited a slight decrease of high-molecular-weight multimers and markedly reduced ristocetin- or botrocetin-induced binding of VWF to platelets in association with a decreased binding to botrocetin. The hybrid S/F1285rVWF showed a normal multimeric profile and bound to platelets in a similar way to the patients' plasma VWF, in the presence of ristocetin or botrocetin. Thus, the new S1285F mutation within the A1 loop was responsible for the type 2M VWD observed in these patients, and was involved in the binding of VWF to botrocetin and to platelet glycoprotein Ib (GPIb). Three anti-VWF monoclonal antibodies, with conformational epitopes within the A1 loop but distinct GPIb binding inhibitory properties, showed a different interaction with F1285rVWF. These results indicate that the S1285F substitution alters the folding of the A1 loop and prevents the correct exposure of the VWF binding sites to botrocetin and GPIb.

Two clusters of charged residues located in the electropositive face of the von Willebrand factor A1 domain are essential for heparin binding

The VWF A1 domain seems to possess two heparin binding regions (residues 565-587 and 633-648) displaying positively charged amino acids, but the overall polyanion-A1 domain interaction scheme remains essentially elusive. To probe this molecular reaction as well as the role of electrostatic forces in VWF-heparin interaction, we performed mutagenesis and molecular modeling experiments. Fifteen mutated rVWFs were expressed [R571A, K572A, R573A, K585A, R571A/K572A/R573A, R578A/R579A, R578A/R579A/K585A, R571A/K572A/R573A/R578A/R579A/K585A (6A), K642G, K643G, K644G, K645G, K642G/K645G, K643G/K644G, and K642G/K643G/K644G/K645G (4G)]. Experimental results indicate that the multimeric structure of the mutants was similar to that of wild-type (WT) rVWF and that all rVWFs displayed normal binding to four conformation-dependent mAbs directed against the A1 domain. Three variants displayed significant reductions in the level of heparin binding. The 6A variant showed 39.2 +/- 1.3% of the WT rVWF level (p < 0.005), while mutants K643G/K644G and 4G showed 63.6 +/- 3.2 and 53.3 +/- 5% of the WT rVWF level, respectively (p < 0.005). Computational investigations showed that one face of the A1 domain is strongly electropositive, indicating that electrostatic forces should be essential in steering heparin onto its binding site. In agreement with our experimental data, the most striking alterations of the electrostatic potential contours were seen for mutants 4G, K643G/K644G, and 6A. Our data suggest that two clusters, one at positions 571-573, 578, 579, and 585 and the other at positions 642-645, act in concert for the recognition of heparin, forming a single extended binding surface across the electropositive face of the VWF A1 domain. A structural model of the VWF A1 domain-heparin complex is proposed, taking into account both experimental and computer modeling data.

Selective inactivation of Von Willebrand factor binding to glycoprotein IIb/IIIa and to inhibitor monoclonal antibody 9 by site-directed mutagenesis

INTRODUCTION

The purpose of this study was to assess the requirement for the RGD sequence of von Willebrand factor (VWF) for its binding to the beta3 chain of integrins and the structural basis for the specificity of monoclonal antibody (MoAb) 9 which specifically binds to VWF and inhibits this interaction.

MATERIAL AND METHODS

: Seven point mutations were introduced into VWF by site-directed mutagenesis. Mutated recombinant VWF were tested for their multimeric pattern and their ability to bind to purified GPIIb/IIIa, thrombin-activated platelets and MoAb 9.

RESULTS

All recombinant VWF were fully multimerized. The conservative Arg 1744 to Lys substitution into the RGD sequence resulted in an 80% loss of function whereas the Arg to Ala change led to a total loss of function. The two substitutions however did not impair the binding to MoAb 9. In contrast Arg 1715 to Ala substitution had no effect on the binding to GPIIb/IIIa but the binding of the corresponding mutated recombinant VWF to MoAb 9 was strikingly decreased.

CONCLUSION

Direct evidence of the role of the structure and the charge of Arg 1744 into the RGD sequence were established by changing Arg to Lys (KGD) and to Ala (AGD). Our results also demonstrate that Arg 1715 is not essential in the function but it is necessary for maintaining the conformation recognized by MoAb 9 specific for the GPIIb/IIIa-binding domain of VWF.

Identification of a new type 2M von Willebrand disease mutation also at position 1324 of von Willebrand factor

Type 2M von Willebrand disease (VWD) refers to variants with decreased platelet-dependent function that is not associated with the loss of high molecular weight (HMW) von Willebrand factor (VWF) multimers. This category includes the so-called "phenotype B" responsible for inexistent ristocetin-induced but normal botrocetin-induced binding of VWF to platelet glycoprotein lb. The missense mutation G1324S was identified in the first patient reported to display "phenotype B". We report here on the identification in four members of a French family of a missense mutation also affecting this glycine residue but changing it into an alanine residue. These individuals are heterozygous for this mutation and two of them display an additional quantitative VWF deficiency resulting from a stop codon at position 2470. After transient transfection in Cos-7 cells, the mutated recombinant protein harbouring the G1324A substitution was shown to exhibit normal multimers and inexistent ristocetin-induced but normal botrocetin-induced binding to GPIb, confirming the classification of this new mutation as a type 2M VWD mutation.

Defect of heparin binding in plasma and recombinant von Willebrand factor with type 2 von Willebrand disease mutations

The aim of our study was to characterise heparin-binding properties of mutated von Willebrand factor (VWF) in 24 patients plasmas with type 2 von Willebrand disease (VWD). and in 15 recombinant VWF (rVWF) with the corresponding mutations. Binding of mutated rVWF or plasma VWF was compared to that of WT-rVWF or normal pool plasma VWF. Four mutations, at positions C509, V551, R552 and R611 lead to significantly decreased binding to heparin in both plasma and rVWF. Interestingly, whereas these four residues are distant in the primary structure of VWF-A1domain, they are close to each other in its three-dimensional structure. Structural analysis suggested how folding problems and destabilisation due to these mutations could induce reorganisation of surface regions involved in heparin binding. In contrast, no heparin-binding defect was found associated with different type 2 VWF mutants, at positions G561, E596, I662, R543, R545, V553, R578 or L697.

Ser968Thr mutation within the A3 domain of von Willebrand factor (VWF) in two related patients leads to a defective binding of VWF to collagen

We report the identification of a new mutation of von Willebrand Factor (VWF) gene within exon 30 occurring in two related patients (mother and daughter) with a hemorrhagic syndrome. A T-->A transvertion at nucleotide 5441 was found changing the serine 968 to threonine of the mature VWF subunit (S1731T of the preproVWF). The Ser968Thr mutation is located within the VWF A3 domain which interacts with type I and III collagens. Both patients were found to be heterozygous for the mutation. The propositus (daughter) exhibited a slightly prolonged bleeding time, levels of VWF:Ag and VWF:RCo at the lower limit of normal, contrasting with normal levels of VIII:C. Her mother exhibited borderline bleeding time and moderately decreased levels of VWF and VIII:C. In both patients multimeric structure of VWF and ristocetin- as well as botrocetin-induced binding of VWF to GPIb were normal; however both patients repeatedly showed decreased binding of VWF to collagen. The Ser968Thr substitution was reproduced by site-directed mutagenesis on the full-length cDNA of VWF. The mutated recombinant VWF (rVWF), T968rVWF, and the hybrid S/T968rVWF were transiently expressed by COS-7 cells. Both rVWF exhibited normal multimeric pattern and normal ristocetin- as well as botrocetin-induced binding to GPIb. T968rVWF showed significantly decreased binding to collagen while the hybrid S/T968rVWF bound to collagen in a similar way to that of the patients' plasma VWF. Thus, our data demonstrate that the Ser968Thr mutation of the VWF A3 domain is clearly responsible for the abnormal binding of VWF to collagen observed in both patients. The Ser968Thr substitution of the VWF is the first mutation identified in two patients leading to a decreased affinity of VWF for collagen and a normal multimeric structure.

Type 2 von Willebrand disease causing defective von Willebrand factor-dependent platelet function

Type 2 von Willebrand disease causing defective von Willebrand factor-dependent platelet function comprises mainly subtypes 2A, 2B and 2M. The diagnosis of type 2 von Willebrand disease may be guided by the observation of a disproportionately low level of ristocetin cofactor activity or collagen-binding activity relative to the von Willebrand factor antigen level. The decreased platelet-dependent function is often associated with an absence of high molecular weight multimers (types 2A and 2B), but the high molecular weight multimers may also be present (type 2M and some type 2B), and supranormal multimers may exist (as in the Vicenza variant). Today, the identification of mutations in particular domains of the pre-provon Willebrand factor is helpful to classify these variants and to provide further insight into the structure-function relationship and the biosynthesis of von Willebrand factor. Thus, mutations in the D2 domain, involved in the multimerization process, are found in patients with type 2A, formerly named IIC von Willebrand disease. Mutations in the D3 domain characterize the Vicenza variant, or type IIE patients. Mutations in the A1 domain may modify the binding of von Willebrand factor multimers to platelets, either increasing (type 2B) or decreasing (types 2M and 2A/2M) the affinity of von Willebrand factor for platelets. In type 2A disease, molecular abnormalities identified in the A2 domain, which contains a specific proteolytic site, are associated with alterations in folding that impair the secretion of von Willebrand factor or increase its susceptibility to proteolysis. Finally, a mutation localized in the C terminus cysteine knot domain, which is crucial for the dimerization of von Willebrand factor subunit, has been identified in a rare subtype 2A, formerly named IID.

Defect of Heparin Binding in Plasma and Recombinant von Willebrand Factor with Type 2 von Willebrand Disease Mutations

The aim of our study was to characterise heparin-binding properties of mutated von Willebrand factor (VWF) in 24 patients plasmas with type 2 von Willebrand disease (VWD), and in 15 recombinant VWF (rVWF) with the corresponding mutations. Binding of mutated rVWF or plasma VWF was compared to that of WT-rVWF or normal pool plasma VWF. Four mutations, at positions C509, V551, R552 and R611 lead to significantly decreased binding to heparin in both plasma and rVWF. Interestingly, whereas these four residues are distant in the primary structure of VWF-A1 domain, they are close to each other in its three-dimensional structure. Structural analysis suggested how folding problems and destabilisation due to these mutations could induce reorganisation of surface regions involved in heparin binding. In contrast, no heparin-binding defect was found associated with different type 2 VWF mutants, at positions G561, E596, I662, R543, R545, V553, R578 or L697.

Effect of recombinant von Willebrand factor reproducing type 2B or type 2M mutations on shear-induced platelet aggregation

The aim was to better understand the function of von Willebrand factor (vWF) A1 domain in shear-induced platelet aggregation (SIPA), at low (200) and high shear rate (4000 seconds(-1)) generated by a Couette viscometer. We report on 9 fully multimerized recombinant vWFs (rvWFs) expressing type 2M or type 2B von Willebrand disease (vWD) mutations, characterized respectively by a decreased or increased binding of vWF to GPIb in the presence of ristocetin. We expressed 4 type 2M (-G561A, -E596K, -R611H, and -I662F) and 5 type 2B (rvWF-M540MM, -V551F, -V553M, -R578Q, and -L697V). SIPA was strongly impaired in all type 2M rvWFs at 200 and 4000 seconds(-1). Decreased aggregation was correlated with ristocetin binding to platelets. In contrast, a distinct effect of botrocetin was observed, since type 2M rvWFs (-G561A, -E596K, and -I662F) were able to bind to platelets to the same extent as wild type rvWF (rvWF-WT). Interestingly, SIPA at 200 and 4000 seconds(-1) confirmed the gain-of-function phenotype of the 5 type 2B rvWFs. Our data indicated a consistent increase of SIPA at both low and high shear rates, reaching 95% of total platelets, whereas SIPA did not exceed 40% in the presence of rvWF-WT. Aggregation was completely inhibited by monoclonal antibody 6D1 directed to GPIb, underlining the importance of vWF-GPIb interaction in type 2B rvWF. Impaired SIPA of type 2M rvWF could account for the hemorrhagic syndrome observed in type 2M vWD. Increased SIPA of type 2B rvWF could be responsible for unstable aggregates and explain the fluctuant thrombocytopenia of type 2B vWD. (Blood. 2000;95:3796-3803)

Regulated von Willebrand factor (vWf) secretion is restored by pro-vWf expression in a transfectable endothelial cell line

Von Willebrand factor (vWf) is a glycoprotein involved in primary hemostasis and synthesized in endothelial cells (EC). vWf is stored in secretory granules specific for EC called Weibel-Palade bodies (WPb). Studies on the molecular mechanisms of vWf storage and acute release are hampered by the limitations of the available endothelial cell culture models. We created a suitable model by stable transfection of the vWf-negative ECV304 endothelial cell line with pro-vWf cDNA. Pro-vWf was normally cleaved to mature vWf and stored in WPb. Acute vWf release occurred in response to the calcium ionophore A23187. Thus, vWf expression is sufficient to restore functional secretory granules in ECV304 cells. We used this model to study the role of WPb in the storage of tissue-type plasminogen activator (t-PA), a key fibrinolytic enzyme that is acutely released by EC, but whose intracellular storage compartment is still a matter of debate. We observed that restoration of WPb in ECV304 cells results in the targeting of t-PA to these storage granules.

New assay for measuring binding of platelet glycoprotein IIb/IIIa to unpurified von Willebrand factor

Among the numerous variants of vWD, no patient with an abnormal vWF binding to GPIIb/IIIa has been described to date. To search for such potential variants, we developed a two-site assay for measuring the binding of purified GPIIb/IIIa to vWF in biological fluids and we used it to study a large series of plasmas from various types of von Willebrand disease (vWD) and recombinant vWF (rvWF). vWF in plasma or rvWF in culture medium was immobilized onto anti-vWF monoclonal antibodies (MoAb)-coated wells of microtiter plates. After incubation with either unlabeled GPIIb/IIIa and a 125I-anti-GPIIb/IIIa MoAb or 125I-GPIIb/IIIa, binding curves and binding isotherms were respectively established. Normal pool plasma and wild-type rvWF were used as reference samples. We tested plasmas from 85 normal subjects, 115 patients with different types of vWD (64 type 1, 2 type 3, 9 type 2A, 4 type 2M, 16 type 2B, 15 type 2N, 3 type IID and 2 acquired forms) and 50 patients with various bleeding disorders. Four mutated rvWF with 2A (Glu875Lys and Pro885Ser) or 2B (Dupl.Met540 and Val551Phe) substitutions and one rvWF mutated in the RGD domain of the C-terminal part of vWF-subunit (Asp1746Gly) were also studied. Among the various samples tested, only rvWF Asp1746Gly had no affinity for GPIIb/IIIa. In contrast, GPIIb/IIIa similarly bound to the other vWF, independently of the proteic environment, the factor VIII level, the degree of multimerization or the mutation of vWF. Our results indicate that subjects with an abnormal vWF binding to GPIIb/IIIa are probably rare and difficult to target for a specific screening.

Molecular genetics of von Willebrand disease

Von Willebrand disease (vWD), the most common congenital bleeding disorder in man, is related to quantitative and/or qualitative abnormalities of von Willebrand factor (vWF). This multimeric glycoprotein serves as carrier protein of factor VIII, an essential cofactor of coagulation in plasma, and promotes platelet adhesion to the damaged vessel and platelet aggregation. Distinct abnormalities of vWF are responsible for the three types of vWD. Types 1 and 3 are characterized by a quantitative defect of vWF whereas type 2, comprising subtypes 2A, 2B, 2M and 2N, refers to molecular variants with a qualitative defect of vWF. The knowledge of the structure of the vWF gene and the use of Polymerase Chain Reaction (PCR) have led to the identification of the molecular basis of vWD in a significant number of patients. Type 2A is characterized by a decreased platelet-dependent function of vWF associated with the absence of high molecular weight (HMW) multimers of vWF. Most of the type 2A mutations have been identified in the A2 domain of vWF which contains a proteolytic site, while a few others have been found within the propeptide and the C-terminal part of vWF which are involved in its multimerization and dimerization, respectively. In type 2B, defined by an increased affinity of vWF to platelet glycoprotein Ib (GPIb), various amino-acid (aa) substitutions or insertion have been localized within the A1 domain containing the GPIb binding site. In the latter domain have been also identified the few molecular abnormalities described in type 2M which is defined by a decreased platelet-dependent function not caused by the absence of HMW multimers. In type 2N, characterized by a defective binding of vWF to factor VIII, several aa substitutions have been identified within the factor VIII-binding domain in the N-terminal part of vWF. The identification of gene defects remains difficult in types 1 and 3. Whereas various abnormalities (total, partial or point deletions, point insertions, nonsense mutations) have already been identified in type 3, the molecular basis of type 1 is still unresolved in most cases. The characterization of the molecular basis of vWD is of fundamental interest in providing further insight into the structure-function relationship and the biosynthesis of vWF.

Gene defects in 150 unrelated French cases with type 2 von Willebrand disease: from the patient to the gene. INSERM Network on Molecular Abnormalities in von Willebrand Disease

Type 2 vWD is defined by qualitative defects of vWF and is subdivided into four subtypes: 2N, 2B, 2A and 2M. The characterization of 150 unrelated French cases with type 2 vWD emphasizes the heterogeneity of this group. In 51 cases of type 2N vWD, new mutations were found not only in the D' domain (Cys25Tyr and Cys95Phe) but also in the D3 domain (Asp116Asn and Cys297Arg). In 42 cases of type 2B vWD, no new mutation was detected. In 45 cases with type 2A phenotype, three new candidate mutations were found in the A2 domain: Gln793Arg, Val841Phe and Leu876Pro. In addition, four new candidate mutations were detected in the A1 domain: Cys509Gly, Arg545His, Arg552Cys and Cys695Tyr. Finally, five new candidate mutations were identified in 12 patients with 2M (or unclassified) phenotype: Leu513Pro, Gly561A1a, Glu596Lys, Arg611Leu and IIe662Phe. For all candidate mutations, expression studies are in progress. This study of a large number of French variants of vWD brings further insight into the relationship between phenotype and genotype.

Primary structure of the propeptide and factor VIII-binding domain of bovine von Willebrand factor

A 2811 base-pair cDNA, encoding the amino-terminal part of the bovine pre-pro-von Willebrand factor, was characterized and sequenced. The deduced amino acid sequence shares significant homology with the human von Willebrand antigen II and the amino-terminal part of the factor VIII-binding domain of von Willebrand factor. In contrast to human, there is no RGD motif in the bovine von Willebrand antigen II. High levels of Cys, characteristic of D domains, are also found in bovine and the Cys position is markedly conserved between the two species.

Characterization of recombinant von Willebrand factors mutated on cysteine 509 or 695

The interacting domain of vWF with platelet GPIb has been shown to overlap the large A1 loop formed by the intra-chain disulfide bond linking Cys 509 to Cys 695. In order to further investigate the role of the conformation of this region, we have expressed in COS-7 cells three mutated full-length recombinant vWFs (rvWFs) in which the substitutions Cys509Gly, Cys509Arg or Cys695Gly have been introduced by site-directed mutagenesis. SDS-agarose gel electrophoresis demonstrated an impaired multimerization of the mutants with undetectable high molecular weight multimers and a decrease of the relative amounts of the intermediate sized multimers. Binding analysis showed that rvWFC509G and rvWFC509R did not interact with botrocetin but spontaneously interacted with GPIb; the latter binding remained unchanged in the presence of ristocetin. This indicates that the substitution of Cys509 by Gly or Arg creates a conformation of vWF that increases its binding to GPIb. In contrast, rvWFC695G which did not react with botrocetin was also unable to interact with GPIb even in the presence of ristocetin, indicating that sequences interacting with GPIb are masked and/or disrupted. In conclusion, the substitution of each of the Cys509 and 695 results in mutant proteins which may be "locked" into active or inactive conformations in regard to the binding to platelet GPIb receptor.

Structure-function relationship of the A1 domain of von Willebrand factor

von Willebrand factor (vWF) is a multimeric glycoprotein composed of multiple homologous domains. The A1 domain contains a remarkably large disulfide loop of 185 amino acids (Cys 509-695) which plays a key role in promoting platelet adhesion to the subendothelium. Following the initial binding of the A1 domain to the subendothelium, a conformational change occurs which allows its binding to platelet GPIb. In an attempt to further understand the structure-function relationship of the A1 domain, we analyzed 1) the functional properties of recombinant vWF mutated on either Cys 509 or 695 and 2) the reactivity of a monoclonal antibody (MoAb B724) with vWF conformations interacting with GPIb. Our data underline the crucial role of the 509-695 disulfide bond in the binding of vWF to GPIb and discriminate the specificity of each Cys in this binding. They also indicate that two different conformations of the A1 loop, with high or low affinity for MoAb B724, allow the exposure of its GPIb-binding site. Since the low affinity conformation is observed in type 2B vWF, MoAb B724 appears as a useful tool to probe this type of von Willebrand disease (vWD).

Influence of mutations and size of multimers in type II von Willebrand disease upon the function of von Willebrand factor

We compared the properties of plasma von Willebrand factor (vWF) from normal individuals and from two patients with type IIA (Glu875Lys) and type IIB (duplication of Met 540) von Willebrand disease (vWD) with the corresponding fully multimerized recombinant proteins. We included cryosupernatant from normal human plasma and type IIA plasma (Cys509Arg). Functions of vWF were analyzed by binding assays to platelets in the presence of ristocetin or botrocetin. Parameters of binding (number of binding sites per vWF subunit, and dissociation constant Kd) were quantitatively estimated from the binding isotherms of 125I-botrocetin or glycocalicin to vWF, independently of the size of the multimers. We found that ristocetin- or botrocetin-induced binding to platelets was correlated in all cases with the size of vWF multimers. In the absence of inducer, only type IIB rvWF Met-Met540 spontaneously bound to platelets. No significant difference of binding of purified botrocetin to vWF was found between normal and patients' plasma, or between wild-type rvWF (rvWF-WT) and rvWF-Lys875. In contrast, affinity of botrocetin for type IIB rvWF Met-Met540 was decreased. Botrocetin-induced binding of glycocalicin to vWF from all plasma and cryosupernatant was similar. Compared with rvWF-WT, binding of glycocalicin to rvWF-Lys875 was normal. In contrast, the affinity for type IIB rvWF Met-Met540 was 10-fold greater. Thus, our data suggest that, in the patients tested, the abnormal IIA phenotype results from the lack of large-sized multimers and is independent of the point mutations. In contrast, the type IIB mutation is directly involved by providing a conformation to the vWF subunits that allows the high molecular weight multimers to spontaneously interact with platelet glycoprotein Ib.

Epitope mapping of inhibitory monoclonal antibodies to human von Willebrand factor by using recombinant cDNA libraries

Two recombinant expression libraries containing small (300-600 base pairs) cDNA fragments of von Willebrand Factor (vWF) were screened in order to map the epitope of monoclonal antibodies (MAbs) to vWF. Among eleven MAbs tested, seven were effectively mapped. The epitopes of MAbs 418 and 522, which inhibit the binding of vWF to Factor VIII (FVIII), were localized between Leu 2 and Arg 53 and between Glu 35 and Ile 81 of the vWF subunit respectively, within the N-terminal trypsin fragment called SpIII-T4 [amino acids (aa) 1-272] which contains a binding domain for FVIII. The epitope of MAb 710, which inhibits the binding of vWF to glycoprotein Ib (GPIb), was identified between Ser 593 and Ser 678 on the tryptic 52/48 kDa fragment (aa 449-728) which contains binding domains for GPIb, collagen, heparin, sulfatides and subendothelium extracellular matrices. The epitope of MAb 723, which does not interfere with any known function of vWF, was localized between Ser 523 and Gly 588. The epitopes of MAb 505 and MAb 400, which inhibit the binding of vWF to collagen, were identified between Leu 927 and Arg 1114 within the SPI fragment (aa 911-1365) corresponding to the central part of the vWF subunit. The epitope of MAb 9, which inhibits the binding of vWF to GPIIb/IIIa, was identified in the C-terminal part of the vWF subunit between Gln 1704 and Asp 1746, the latter being the third aa of the RGD sequence common to adhesive proteins and serving as a recognition site for integrin receptors.

A novel case of compound heterozygosity with "Normandy"/type I von Willebrand disease (vWD). Direct demonstration of the segregation of one allele with a defective expression at the mRNA level causing type I vWD

We report the case of a family with type I von Willebrand disease (vWD), characterized by a quantitative defect in von Willebrand factor (vWF), associated with a defective binding of vWF to factor VIII (FVIII) also called the "Normandy" variant of vWD. PCR products from genomic DNA of the family members were analysed in the region coding for the binding domain of vWF to FVIII. It showed that the proposita and one of her sons were heterozygous for the Arg91Gln missense mutation, abolishing an MspI restriction enzyme site located in exon 20. The transcription of the normal and mutated alleles was tested by the amplification of cDNA after reverse transcription of platelet mRNA in this region. A total lack of expression of the normal allele was observed in the proposita, who appeared as a compound heterozygous with one allele mutated at Arg91 and a "silent" expression of the other one. The segregation of the "silent" allele was studied in the family with the exonic BstEII RFLP both at the DNA and mRNA levels. The proposita has transmitted her "silent" allele to her daughter and to another son. As this son was informative for this RFLP, the absence of expression of the allele could be demonstrated at the mRNA level, providing evidence that this defect was responsible for his type I vWD.

Discrepancy between IIA phenotype and IIB genotype in a patient with a variant of von Willebrand disease

Type IIA and IIB von Willebrand disease (vWD) result from qualitative abnormalities of von Willebrand factor (vWF) characterized by an absence in plasma of high molecular weight vWF multimers and an abnormal reactivity of vWF towards platelet glycoprotein (GP) Ib, which is decreased in type IIA and increased in type IIB. In this report, we describe the case of a patient having a IIA vWD phenotype associated with an intermittent thrombocytopenia atypical in this subtype but observed in type IIB vWD. The patient plasma vWF showed an absence of high molecular weight and intermediate multimers and had a decreased binding capacity to GPIb. The affinity of botrocetin was normal for plasma vWF from the propositus. Analysis of the propositus vWF gene showed the presence of a substitution Val 551 to Phe of the mature vWF subunit. This mutation is localized within a 509-695 disulphide loop of the vWF that plays an important role in the binding to GPIb and is where most of the molecular defects described so far were associated with type-IIB vWD. We have reproduced the Val 551 Phe substitution onto the vWF cDNA, expressed it in COS-7 cells, and performed structural and functional analysis of the mutant recombinant protein (rvWFPhe 551). The rvWFPhe 551 had a normal multimeric structure and showed the capacity to spontaneously interact with GPIb. Botrocetin had a decreased affinity for rvWFPhe 551. In conclusion, the Val 551 Phe mutation modifies the affinity of vWF for platelet GPIb, as does a type IIB mutation, and may be responsible for the thrombocytopenia of the patient and the clearance of the high molecular weight and intermediate-sized multimers of vWF from the plasma. The study of the rvWFPhe 551 has confirmed the discrepancy between the IIA phenotype and the IIB genotype of the patient.

Functional analysis of the Arg91Gln substitution in the factor VIII binding domain of von Willebrand factor demonstrates variable phenotypic expression

An Arg91Gln substitution in the mature von Willebrand factor (vWF) has been associated with defective binding of vWF to factor VIII (FVIII). We studied four families with members initially classified as having type I von Willebrand disease (vWD) who were either homozygous or heterozygous for the Arg91Gln change. The first family was the original case described by Nishino et al. (1) where three members were homozygous for the Gln91 allele. They had a low FVIII coagulant activity:vWF antigen (VIIIC:vWFAg) ratio, from 0.29 to 0.44, and the ability of their plasma vWF to bind FVIII was markedly decreased. All the heterozygous members had normal vWF and FVIII levels but the capacity of their plasma vWF to bind FVIII was reduced and intermediate between the homozygous members and normals. The affected individual from the second family was heterozygous for the Gln91 allele and demonstrated a VIIIC:vWFAg ratio of 0.98. The FVIII binding assay confirmed the heterozygous status indicating that the moderately low levels of vWF were due to reduced expression of both alleles. The propositus from the third family was also heterozygous and had below normal levels of vWF as well as a low VIIIC:vWFAg ratio of 0.34; however, FVIII binding to her plasma vWF was similar to that of the homozygous individuals suggesting that Gln91-vWF was the major circulating form. Her daughter who has type I vWD inherited the allele without the Gln91 mutation indicating that the expression of this allele was indeed impaired. The heterozygous patient in the fourth family had a vWF level of 24 U/dl but an VIIIC:vWFAg ratio greater than 2.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of recombinant von Willebrand factor corresponding to mutations in type IIA and type IIB von Willebrand disease

Type IIA and IIB von Willebrand disease (vWD) result from defects in von Willebrand factor (vWF). Although both type IIA and IIB vWD are characterized by the absence of high molecular weight multimers in plasma, vWF from patients with type IIA vWD demonstrates a decreased affinity for the platelet receptor glycoprotein Ib (GPIb), whereas vWF from patients with type IIB vWD show an increased affinity for GPIb. To investigate how structural alterations in vWF affect its interaction with GPIb, we reproduced the reported potential mutations in type IIA and IIB vWD in vWF cDNA and expressed the recombinant proteins in COS-1 cells. The type IIA vWF potential mutation was represented by a G-->A transversion which results in the substitution of Lys for Glu at position 875 in the mature vWF subunit (rvWFLys875). The type IIB vWF mutation corresponds to a duplicated ATG codon, resulting in three contiguous methionines starting at position 540-541 in the normal vWF sequence (rv-WFduplMet540-541). The subunit composition and multimeric structure of both mutant proteins were similar to the wild-type rvWF. The rvWFLys875 bound to fixed platelets in the presence of ristocetin similar to wild-type rvWF. The rvWFduplMet540-541 bound to fixed platelets in the absence of agonist. The rvWFLys875 appears to interact normally with GPIb, and the decreased affinity for the platelet receptor observed in plasma is probably a consequence of prior reduction in multimeric size resulting from the defect. In contrast, the duplication of Met540-541 increases the reactivity of vWF for its platelet receptor.

Defects in type IIA von Willebrand disease: a cysteine 509 to arginine substitution in the mature von Willebrand factor disrupts a disulphide loop involved in the interaction with platelet glycoprotein Ib-IX

Type IIA von Willebrand disease (vWD) is characterized by the loss of high and intermediate weight multimers of von Willebrand factor (vWF) from plasma. The 3' end of exon 28 in the vWF gene from four type IIA vWD patients was amplified by the polymerase chain reaction, cloned and sequenced. Sequencing identified two potential missense mutations resulting in the amino acid substitutions Arg 834-->Gln and Glu 875-->Lys in the mature vWF subunit within an area of vWF where mutations in type IIA vWD have been reported. Neither of these amino acid substitutions was found in over 100 normal alleles tested by allele specific oligonucleotide hybridization. A polymorphism (Val 802-->Leu) was identified in another patient. Other areas of exon 28 were analysed by denaturing gradient gel electrophoresis (DGGE) and DNA from one patient demonstrated an irregular DGGE pattern on the 5' end of the exon. Sequencing demonstrated an amino acid substitution of an arginine for cysteine at position 509 adjacent to an area of vWF where defects associated with type IIB vWD have been found. This substitution was not found in 100 normal chromosomes tested by restriction enzyme digestion. The Cys 509-->Arg substitution eliminates an intramolecular disulphide bridge formed by Cys 509 and Cys 695 which is important to maintain the configuration of vWF functional domains that interact with platelet glycoprotein Ib-IX.