Presence Of Botrocetin Research Articles

The TspanC8 Subgroup of Tetraspanins Interacts with A Disintegrin and Metalloprotease 10 (ADAM10) and Regulates Its Maturation and Cell Surface Expression

A disintegrin and metalloprotease 10 (ADAM10) is a ubiquitous transmembrane metalloprotease that cleaves the extracellular regions from over 40 different transmembrane target proteins, including Notch and amyloid precursor protein. ADAM10 is essential for embryonic development and is also important in inflammation, cancer, and Alzheimer disease. However, ADAM10 regulation remains poorly understood. ADAM10 is compartmentalized into membrane microdomains formed by tetraspanins, which are a superfamily of 33 transmembrane proteins in humans that regulate clustering and trafficking of certain other transmembrane "partner" proteins. This is achieved by specific tetraspanin-partner interactions, but it is not clear which tetraspanins specifically interact with ADAM10. The aims of this study were to identify which tetraspanins interact with ADAM10 and how they regulate this metalloprotease. Co-immunoprecipitation identified specific ADAM10 interactions with Tspan5, Tspan10, Tspan14, Tspan15, Tspan17, and Tspan33/Penumbra. These are members of the largely unstudied TspanC8 subgroup of tetraspanins, all six of which promoted ADAM10 maturation. Different cell types express distinct repertoires of TspanC8 tetraspanins. Human umbilical vein endothelial cells express relatively high levels of Tspan14, the knockdown of which reduced ADAM10 surface expression and activity. Mouse erythrocytes express predominantly Tspan33, and ADAM10 expression was substantially reduced in the absence of this tetraspanin. In contrast, ADAM10 expression was normal on Tspan33-deficient mouse platelets in which Tspan14 is the major TspanC8 tetraspanin. These results define TspanC8 tetraspanins as essential regulators of ADAM10 maturation and trafficking to the cell surface. This finding has therapeutic implications because focusing on specific TspanC8-ADAM10 complexes may allow cell type- and/or substrate-specific ADAM10 targeting.

Reconstitution of the Platelet Glycoprotein Ib-IX Complex in Phospholipid Bilayer Nanodiscs

The glycoprotein Ib-IX (GPIb-IX) complex expressed on platelet plasma membrane is involved in thrombosis and hemostasis via the initiation of adhesion of platelets to von Willebrand factor (VWF) exposed at the injured vessel wall. While most of the knowledge of the GPIb-IX complex was obtained from studies on platelets and transfected mammalian cells expressing the GPIb-IX complex, there is not an in vitro membrane system that allows systematic analysis of this receptor. The phospholipid bilayer Nanodisc composed of a patch of phospholipid surrounded by membrane scaffold protein is an attractive tool for membrane protein study. We show here that the GPIb-IX complex purified from human platelets has been reconstituted into the Nanodisc. The Nanodisc-reconstituted GPIb-IX complex was able to bind various conformation-sensitive monoclonal antibodies. Furthermore, it bound to VWF in the presence of botrocetin with an apparent K(d) of 0.73 ± 0.07 nM. The binding to VWF was inhibited by anti-GPIbα antibodies with epitopes overlapping with the VWF-binding site, but not by anti-GPIbβ monoclonal antibody RAM.1. Finally, the Nanodisc-reconstituted GPIb-IX complex exhibited ligand binding activity similar to that of the isolated extracellular domain of GPIbα. In conclusion, the GPIb-IX complex in Nanodiscs adopts a native-like conformation and possesses the ability to bind its natural ligands, thus making a Nanodisc a suitable in vitro platform for further investigation of this hemostatically important receptor complex.

A Modified Model of Type 2B von Willebrand Disease: Taking ADAMTS13-Mediated Cleavage out of the Equation

AbstractAbstract 23 Introduction:Type 2B von Willebrand disease (2B VWD), a qualitative variant of VWD, is characterized by thrombocytopenia and loss of high molecular weight multimers. 2B VWF has both an increased affinity for platelet glycoprotein Ibα (GPIbα) and increased susceptibility to ADAMTS13-mediated cleavage. In order to determine whether the loss of high molecular weight (HMW) multimers associated with 2B VWD occurs due to the increased ADAMTS13 susceptibility or due to the increased binding to platelet GPIbα, we created a modified type 2B mouse model, in which the mice expressed ADAMTS13-insensitive 2B VWF. Methods:Three common type 2B VWD mutations (R1306W, V1316M, and R1341Q) were independently introduced into the mouse Vwf (mVWF) cDNA sequence containing the ADAMTS13 cleavage site knockout, Y1605A/M1606A (CSKO). Recombinant mutant mVWF was produced using transient transfection of HEK293T cells. ADAMTS13 digestion of the recombinant mVWF was performed using a Tris-Urea system. Platelet binding affinity was assessed using a botrocetin-induced platelet-binding ELISA assay. 8–9 week old C57Bl6 VWF KO mice were hydrodynamically injected with 100 μ g of naked plasmid DNA containing the liver specific ET promoter and wildtype (WT) or mutant mouse Vwf DNA. The mice were sampled weekly; complete blood counts, VWF:Ag levels, VWFpp levels, and VWF multimer structure were examined. Results:Following transient transfections, mutant and wildtype mVWF were secreted at similar levels, with a full range of VWF multimer sizes. Recombinant 2B/CSKO mVWF is markedly ADAMTS13 cleavage insensitive, with >80 U mADAMTS13 required for 50% cleavage (WT mVWF requires 1 U mADAMTS13). Similar to 2B VWF, 2B/CSKO mVWF showed enhanced platelet binding affinity in the presence of botrocetin. Hydrodynamic injections of the naked expression plasmids did not result in adverse events in the mice. Mean 2B/CSKO platelet counts at day 7 were significantly reduced compared to WT platelet counts (149 and 370, respectively). At day 14, only the mice expressing V1316M/CSKO remained thrombocytopenic, with platelet counts similar to those seen with the V1316M mutation alone (170 and 171, respectively). By day 14, the mice expressing 2B/CSKO VWF had significantly lower mean VWF:Ag levels than WT mice or mice expressing the CSKO alone (see table below). The mean VWFpp/VWF:Ag ratios were significantly increased in the 2B/CSKO mice when compared to the WT mice (1.53 and 1.00, respectively). There was no loss of HMW multimers in the mice expressing R1306W/CSKO and R1341Q/CSKO and, indeed, these mice showed significantly more multimer bands than WT mice (days 7 to 21; 14.5±0.6 and 13.9±1.2 multimer bands versus 10.2±0.5 bands for WT). In marked contrast, the mice expressing V1316M/CSKO VWF showed complete loss of high molecular weight multimers, similar to mice expressing the same 2B VWF mutation alone (8.3±1.8 and 7.7±0.9 multimer bands, respectively). Conclusions:Mice expressing R1306W/CSKO and R1341Q/CSKO VWF have an absence of thrombocytopenia and show the presence of supranormal high molecular weight VWF multimer bands. These results suggest that enhanced ADAMTS13-mediated cleavage plays an important role in the type 2B phenotype associated with these mutations. In contrast, low V1316M/CSKO VWF:Ag was associated with both thrombocytopenia and loss of HMW multimers, demonstrating that enhanced GPIbα binding affinity is the predominant mechanism associated with this mutation. Thus, the importance of ADAMTS13 susceptibility in 2B VWD is mutation-dependent, and ADAMTS13-mediated cleavage plays an important role in enhancing the severity of the type 2B VWD phenotype associated with the R1306W and R1341Q mutations. Disclosures:No relevant conflicts of interest to declare.

Leucine-rich Repeats 2-4 (Leu60-Glu128) of Platelet Glycoprotein Ibα Regulate Shear-dependent Cell Adhesion to von Willebrand Factor

Glycoprotein (GP) Ib-IX-V binds von Willebrand factor (VWF), initiating thrombosis at high shear stress. The VWF-A1 domain binds the N-terminal domain of GPIbalpha (His1-Glu282); this region contains seven leucine-rich repeats (LRR) plus N- and C-terminal flanking sequences and an anionic sequence containing three sulfated tyrosines. Our previous analysis of canine/human and human/canine chimeras of GPIbalpha expressed on Chinese hamster ovary (CHO) cells demonstrated that LRR2-4 (Leu60-Glu128) were crucial for GPIbalpha-dependent adhesion to VWF. Paradoxically, co-crystal structures of the GPIbalpha N-terminal domain and GPIbalpha-binding VWF-A1 under static conditions revealed that the LRR2-4 sequence made minimal contact with VWF-A1. To resolve the specific functional role of LRR2-4, we compared wild-type human GPIbalpha with human GPIbalpha containing a homology domain swap of canine for human sequence within Leu60-Glu128 and a reverse swap (canine GPIbalpha with human Leu60-Glu128) for the ability to support adhesion to VWF under flow. Binding of conformation-specific anti-GPIbalpha antibodies and VWF binding in the presence of botrocetin (which does not discriminate between species) confirmed equivalent expression of wild-type and mutant receptors in a functional form competent to bind ligand. Compared with CHO cells expressing wild-type GPIbalpha, cells expressing GPIbalpha, where human Leu60-Glu128 sequence was replaced by canine sequence, supported adhesion to VWF at low shear rates but became increasingly ineffective as shear increased from 50 to 2000 s(-1). Together, these data demonstrate that LRR2-4, encompassing a pronounced negative charge patch on human GPIbalpha, is essential for GPIbalpha.VWF-dependent adhesion as hydrodynamic shear increases.

Correction of a murine model of von Willebrand disease by gene transfer

Abstractvon Willebrand disease (VWD), the most common inherited bleeding disorder in the U.S. population, is caused by defects in the expression and processing of von Willebrand factor (VWF), a blood glycoprotein required for normal hemostasis that mediates the adhesion of platelets to sites of vascular damage by binding to specific platelet glycoproteins and to constituents of exposed connective tissue. To assess whether VWF deficiency can be corrected by gene transfer, a plasmid expressing the intact 8.4-kb murine VWF coding sequence, directed by the cyto-megalovirus immediate/early promoter/enhancer, was delivered through hydrodynamic tail vein injection into VWF knockout mice (VWF–/–) that exhibit defects in hemostasis, including highly prolonged bleeding time and spontaneous bleeding events, closely mimicking severe human VWD. VWF antigen levels in plasma from animals receiving VWF cDNA, but not control animals, revealed normalized levels of circulating VWF that persisted for at least 1 week after injection. Western blot analysis of plasma from animals receiving VWF cDNA, but not control animals, revealed high molecular–weight multimers with patterns similar to those observed in wild-type mice. Reverse transcription–polymerase chain reaction (RT-PCR) on RNA isolated from the livers of animals receiving VWF cDNA, but not control animals, demonstrated that VWF was expressed in the liver, and immunohistochemical analysis of the livers of treated VWF–/– mice revealed VWF-specific staining throughout the liver parenchyma but not in endothelial cells. Plasma from treated VWF–/– mice, but not control VWF–/– mice, supported the hypothesis that murine platelets aggregate in the presence of botrocetin. Although levels of circulating factor VIII in untreated VWF–/– mice were less than 10% those in wild-type mice, levels of factor VIII in VWF–/– animals treated with VWF cDNA, but not in control animals, were normalized to values in wild-type mice, indicating the restoration of factor VIII carrier function for VWF in treated mice that persisted for at least 1 week at higher doses of VWF cDNA. Most important, bleeding time was normalized by 48 hours after the delivery of VWF cDNA, but not by the control plasmid. These data suggest that with the use of gene transfer of VWF cDNA, VWF protein can be expressed, processed, and secreted in a physiologically active form; thus, it may be possible to correct VWD using gene transfer.

Von Willebrand factor mediates platelet spreading through glycoprotein Ib and alpha(IIb)beta3 in the presence of botrocetin and ristocetin, respectively.

von Willebrand factor (VWF) plays a critical role in the process of hemostasis by mediating flow-dependent adhesion and spreading of platelets on exposed extracellular matrix proteins following vascular injury. To accomplish this, VWF binds to two distinct platelet receptors: glycoprotein (GP)Ib-IX-V and integrin alpha(IIb)beta3. To evaluate the ability of GPIb and alpha(IIb)beta3 to mediate platelet adhesion and lamellipodia formation on immobilized VWF in the presence of the biochemical modulators, ristocetin and botrocetin. In the presence of botrocetin and inhibitors of adenosine diphosphate (ADP) and thromboxane A2 (TxA2), VWF is able to support formation of lamellipodia through a GPIb-dependent mechanism that is independent of alpha(IIb)beta3 and PI3-kinase. Lamellipodia formation under these conditions is incomplete. In marked contrast, in the presence of ristocetin, VWF stimulates formation of fully spread lamellipodia through a pathway that is dependent upon alpha(IIb)beta3 and PI3-kinase. Furthermore, alpha(IIb)beta3 also supports platelet spreading on VWF alone, but only in the absence of inhibitors of ADP and TxA2. The localization of filamentous actin and the Arp2/3 complex in platelets on VWF in the presence of botrocetin and ristocetin are distinct, yielding disparate lamellipodium kinetic signatures. Interestingly, botrocetin significantly enhances platelet adhesion to VWF under flow in whole blood in an alpha(IIb)beta3-independent manner, while ristocetin augments washed platelet adhesion and spreading to VWF under flow in an alpha(IIb)beta3-dependent manner. These observations demonstrate that VWF is able to induce lamellipodia formation through distinct receptors, and has important consequences for investigation of the role of VWF-GPIb interactions in the context of platelet regulation.

Site-directed mutagenesis of platelet glycoprotein Ibα demonstrating residues involved in the sulfation of tyrosines 276, 278, and 279

The interaction between platelet glycoprotein (GP) Ib alpha and von Willebrand factor (VWF) is essential for initiation of hemostasis. The sulfation of the 3 tyrosine residues 276, 278, and 279 in GPIb alpha is an important posttranslational modification that seems to promote the interaction with VWF. The environment where sulfation of tyrosines occurs has been proposed to contain highly acidic residues. This investigation has examined the highly acidic region from Asp249 to Asp287 in the mature GPIb alpha protein. Changes to most of the carboxylic acids in this region resulted in decreased reactivity to VWF. Only 3 mutants (Glu270Gln, Asp283Asn, Asp283Asn/Glu285Gln/Asp287Asn) resulted in the abolition of sulfation. Two novel mutations were also created. First, a deletion of the 7 amino acids from Tyr276 to Glu282 led to a loss of sulfation and totally abolished VWF binding in the presence of botrocetin. This confirms that it is these 3 tyrosines that undergo sulfation and that this region is crucial for botrocetin-mediated VWF binding. The second mutation involves changing the lysine residues at 253, 258, and 262 to alanine. This also led to distinct changes in VWF binding and abolition of sulfation.

Measurement of the binding forces between von Willebrand factor and variants of platelet glycoprotein Ibalpha using optical tweezers.

Thrombus formation is initiated by adhesion of the platelet receptor, glycoprotein (GP) Ib-IX-V complex, to its adhesive ligand, von Willebrand factor (vWf), in the subendothelium or plasma. The vWf-binding domain of GP Ib-IX-V is in the GP Ibalpha subunit of the complex and contains a leucine-rich repeat region. The adhesion of different leucine-rich repeats was studied using optical tweezers in order to determine which ones were critical for the vWf/GP Ibalpha interaction. Canine GP Ibalpha does not normally bind to human vWf, and thus canine-human GP Ibalpha chimeras were constructed by sequentially replacing human GP Ibalpha structural regions with their canine counterparts. Chinese hamster ovary (CHO) cells, which are frequently used to express platelet GP complexes, were transfected with the chimeric proteins. Optical tweezers (lambda = 830 nm) were used to investigate bond strengths between vWf and different GP Ibalpha canine-human chimeras. Since vWf does not bind GP Ibalpha without high shear stress, the compounds botrocetin and ristocetin were used to induce binding between human vWf and the chimeras. All human-canine GP Ibalpha chimeras bound to vWf in the presence of botrocetin. Replacement of the N-terminal flanking sequence and the first leucine-rich repeat resulted in lower GP Ibalpha/vWf bond strengths than the wild-type human GP Ibalpha/vWf bond strength (P < 0.05). Chimeras lacking the second leucine-rich repeat did not adhere to vWf with ristocetin acting as modulator. The N-terminal flanking sequence and the first leucine-rich repeat of GP Ibalpha were found to be important but not necessary for GP Ibalpha to adhere to vWf. The second leucine-rich repeat was found to be critical for GP Ibalpha to bind vWf and could potentially be used in the development of a novel recombinant anti-thrombotic drugs.

Novel treatment modalities: new platelet preparations and substitutes.

Keywords: platelets; platelet substitutes; platelet haemostatic function; blood substitutes; platelet function

Role of the Leucine-Rich Domain of Platelet GPIbα in Correct Post-translational Processing – The Nancy I Bernard-Soulier Mutation Expressed on CHO Cells

The mechanisms governing the biosynthesis and surface expression of platelet adhesive receptors on parent megakaryocytes are as yet poorly understood. In particular, the assembly and processing of the multisubunit glycoprotein (GP) Ib-IX-V complex, a receptor for von Willebrand factor (vWf) is not fully understood. In the present work, these questions were addressed by reproducing a natural mutation of GPIbalpha found in a variant case of Bernard-Soulier syndrome (Nancy I), due to the deletion of leucine 179 in the seventh leucine-rich repeat of the polypeptide. Wild type and mutated GPIbalpha were transfected into CHO cells expressing GPlbbeta and GPIX. Flow cytometry showed surface expression of the three subunits of both GPIb-IX complexes, but GPlbalphadeltaLeu was present at lower levels (20-40%) and was recognized only by a sub class of monoclonal antibodies which epitopes were not modified by the mutation. These properties reproduce the defect found in the patient's platelets, demonstrating the causative nature of the mutation and validate the use of the CHO cells model. Biochemical studies were performed in an attempt to elucidate the mechanism of the conformational change of GPIbalphadeltaLeu. They unexpectedly revealed a major glycosylation deficiency of the mutated GPIbalpha leading to a 40% decrease in molecular weight. The other two subunits of the complex were however normal and present at the plasma membrane. The deletion led to complete functional deficiency with lack of vWf binding of CHOalphadeltaLeu transfected cells in the presence of botrocetin and defective adhesion to a vWf coated surface under static conditions. Finally, in contrast to normal CHOalphabetaIX cells, which displayed rolling and deceleration when perfused over a vWf surface, CHOalphadeltaLeubetaIX cells were unable to roll over or attach to a vWf substratum. These results show that the integrity of the leucine-rich region of GPIbalpha is essential for normal processing and function of the GPIb-IX complex. In addition, these results obtained in a cellular system supported the suspected role of the macroglycopeptide region of GPIbalpha in maintaining a suitable conformation of this multisubunit receptor to perform its adhesive function.

Analysis of the roles of 14-3-3 in the platelet glycoprotein Ib-IX-mediated activation of integrin alpha(IIb)beta(3) using a reconstituted mammalian cell expression model.

We have reconstituted the platelet glycoprotein (GP) Ib-IX-mediated activation of the integrin alpha(IIb)beta(3) in a recombinant DNA expression model, and show that 14-3-3 is important in GPIb-IX signaling. CHO cells expressing alpha(IIb)beta(3) adhere poorly to vWF. Cells expressing GPIb-IX adhere to vWF in the presence of botrocetin but spread poorly. Cells coexpressing integrin alpha(IIb)beta(3) and GPIb-IX adhere and spread on vWF, which is inhibited by RGDS peptides and antibodies against alpha(IIb)beta(3). vWF binding to GPIb-IX also activates soluble fibrinogen binding to alpha(IIb)beta(3) indicating that GPIb-IX mediates a cellular signal leading to alpha(IIb)beta(3) activation. Deletion of the 14-3-3-binding site in GPIbalpha inhibited GPIb-IX-mediated fibrinogen binding to alpha(IIb)beta(3) and cell spreading on vWF. Thus, 14-3-3 binding to GPIb-IX is important in GPIb-IX signaling. Expression of a dominant negative 14-3-3 mutant inhibited cell spreading on vWF, suggesting an important role for 14-3-3. Deleting both the 14-3-3 and filamin-binding sites of GPIbalpha induced an endogenous integrin-dependent cell spreading on vWF without requiring alpha(IIb)beta(3), but inhibited vWF-induced fibrinogen binding to alpha(IIb)beta(3). Thus, while different activation mechanisms may be responsible for vWF interaction with different integrins, GPIb-IX-mediated activation of alpha(IIb)beta(3) requires 14-3-3 interaction with GPIbalpha.

Identification and Characterization of Endothelial Glycoprotein Ib Using Viper Venom Proteins Modulating Cell Adhesion

AbstractThe expression and function of a glycoprotein Ib (GPIb) complex on human umbilical vein endothelial cells (HUVECs) is still a matter of controversy. We characterized HUVEC GPIb using viper venom proteins: alboaggregins A and B, echicetin, botrocetin, and echistatin. Echicetin is an antagonist, and alboaggregins act as agonists of the platelet GPIb complex. Botrocetin is a venom protein that alters von Willebrand factor (vWF) conformation and increases its binding affinity for the GPIb complex. Echistatin is a disintegrin that blocks αvβ3. Echistatin, but not echicetin, inhibited the adhesion to vWF of Chinese hamster ovary (CHO) cells transfected with αvβ3. We found the following: (1) Binding of monoclonal antibodies against GPIbα to HUVECs was moderately increased after stimulation with cytokines and phorbol ester. Echicetin demonstrated an inhibitory effect. (2) Both echicetin and echistatin, an αvβ3 antagonist, inhibited the adhesion of HUVECs to immobilized vWF in a dose-dependent manner. The inhibitory effect was additive when both proteins were used together. (3) Botrocetin potentiated the adhesion of HUVECs to vWF, and this effect was completely abolished by echicetin, but not by echistatin. (4) CHO cells expressing GPIbαβ/IX adhered to vWF (in the presence of botrocetin) and to alboaggregins; GPIbα was required for this reaction. Echicetin, but not echistatin, inhibited the adhesion of cells transfected with GPIbαβ/IX to immobilized vWF. (5) HUVECs adhered strongly to immobilized vWF and alboaggregins with extensive spreading, which was inhibited by LJ1b1, a monoclonal antibody against GPIb. The purified αvβ3 receptor did not interact with the alboaggregins, thereby excluding the contribution of αvβ3 in inducing HUVEC spreading on alboaggregins. In conclusion, our data confirm the presence of a functional GPIb complex expressed on HUVECs in low density. This complex may mediate HUVEC adhesion and spreading on immobilized vWF and alboaggregins.

Identification and Characterization of Endothelial Glycoprotein Ib Using Viper Venom Proteins Modulating Cell Adhesion

The expression and function of a glycoprotein Ib (GPIb) complex on human umbilical vein endothelial cells (HUVECs) is still a matter of controversy. We characterized HUVEC GPIb using viper venom proteins: alboaggregins A and B, echicetin, botrocetin, and echistatin. Echicetin is an antagonist, and alboaggregins act as agonists of the platelet GPIb complex. Botrocetin is a venom protein that alters von Willebrand factor (vWF) conformation and increases its binding affinity for the GPIb complex. Echistatin is a disintegrin that blocks vβ3. Echistatin, but not echicetin, inhibited the adhesion to vWF of Chinese hamster ovary (CHO) cells transfected with vβ3. We found the following: (1) Binding of monoclonal antibodies against GPIb to HUVECs was moderately increased after stimulation with cytokines and phorbol ester. Echicetin demonstrated an inhibitory effect. (2) Both echicetin and echistatin, an vβ3 antagonist, inhibited the adhesion of HUVECs to immobilized vWF in a dose-dependent manner. The inhibitory effect was additive when both proteins were used together. (3) Botrocetin potentiated the adhesion of HUVECs to vWF, and this effect was completely abolished by echicetin, but not by echistatin. (4) CHO cells expressing GPIbβ/IX adhered to vWF (in the presence of botrocetin) and to alboaggregins; GPIb was required for this reaction. Echicetin, but not echistatin, inhibited the adhesion of cells transfected with GPIbβ/IX to immobilized vWF. (5) HUVECs adhered strongly to immobilized vWF and alboaggregins with extensive spreading, which was inhibited by LJ1b1, a monoclonal antibody against GPIb. The purified vβ3 receptor did not interact with the alboaggregins, thereby excluding the contribution of vβ3 in inducing HUVEC spreading on alboaggregins. In conclusion, our data confirm the presence of a functional GPIb complex expressed on HUVECs in low density. This complex may mediate HUVEC adhesion and spreading on immobilized vWF and alboaggregins.

Cloning and Expression of Canine Glycoprotein Ibα

The interaction of the glycoprotein (GP) Ib-IX-V complex with von Willebrand factor (vWF) is critical in initiation of haemostasis and thrombosis through platelet adhesion to damaged endothelium. The binding site for vWF resides within the GPIbalpha subunit of the complex. To further define the physiological function of platelet GPIbalpha we cloned and expressed the canine GPIbalpha cDNA. A canine platelet cDNA library was constructed and screened with a randomly primed 32P-labeled 1041-base-pair restriction fragment of the human GPIbalpha cDNA. Analysis of 23 clones demonstrated that the canine GPIbalpha cDNA is 2530 nucleotides in length and includes a short 5' untranslated segment of 42 nucleotides followed by a signal peptide of 16 amino acids, a mature peptide of 645 amino acids and a 3' noncoding region of 455 nucleotides. A single intron of 142 nucleotides, 6 nucleotides upstream from the ATG translation initiation codon was identified in the canine gene in a similar location to that present in the human gene. Chinese hamster ovary cells that stably express human GPIbbeta and GPIX were transfected with the canine GPIbalpha cDNA. Canine GPIbalpha was expressed on the surface of these cells and bound vWF in the presence of botrocetin. The binding of vWF was inhibited by an anti-vWF human monoclonal antibody known to inhibit vWF binding to GPIbalpha. The results of this investigation will allow the development of reagents to study the physiological function of GPIbalpha in an animal model.

Platelet Aggregation Induced by a Monoclonal Antibody to the A1 Domain of von Willebrand Factor

Shear-induced platelet aggregation (SIPA) involves von Willebrand Factor (vWF) binding to platelet glycoprotein (GP)Ib at high shear stress, followed by the activation of αIIbβ3. The purpose of this study was to determine the vWF sequences involved in SIPA by using monoclonal antibodies (MoAbs) to vWF known to interfere with its binding to GPIb and to αIIbβ3. Washed platelets were exposed to shear rates between 100 and 4,000 seconds−1 in a rotational viscometer. SIPA was quantitated by flow cytometry as the disappearance of single platelets (DSP) in the sheared sample in the presence of vWF, relative to a control in the absence of shear and vWF. At a shear rate of 4,000 seconds−1, DSP was increased from 5.9% ± 3.5% in the absence of vWF to 32.7% ± 6.3% in the presence of vWF. This increase in SIPA was not associated with an elevation of P-selectin expression. vWF-dependent SIPA was completely abolished by MoAb 6D1 to GPIb and partially inhibited by MoAb 10E5 to αIIbβ3. Three MoAbs to vWF were compared for their effect on SIPA at 4,000 seconds−1 in the presence of vWF: MoAb 328, known to block vWF binding to GPIb in the presence of ristocetin, MoAb 724 blocking vWF binding to GPIb in the presence of botrocetin, and MoAb 9, an inhibitor of vWF binding to αIIbβ3. Similar to the effect of MoAb 6D1, MoAb 328 completely inhibited the effect of vWF, whereas MoAb 9 had a partial inhibitory effect, as MoAb 10E5 did. In contrast, MoAb 724, as well as its F(ab′)2 fragments, promoted shear-dependent platelet aggregation (165% of the DSP value obtained in the absence of MoAb 724), indicating that MoAb 724 was responsible for an enhanced aggregation, which was independent of binding to the platelet Fcγ receptor. In addition, the enhancement of aggregation induced by MoAb 724 was abrogated by MoAb 6D1 or 10E5 to the level of SIPA obtained in the presence of vWF incubated with a control MoAb to vWF. Finally, the activating effect of MoAb 724 was also found under static conditions at ristocetin concentrations too low to induce platelet aggregation. Our results suggested that on binding to a botrocetin-binding site on vWF, MoAb 724 mimics the effect of botrocetin by inducing an active conformation of vWF that is more sensitive to shear stress or to low ristocetin concentration.

Platelet Aggregation Induced by a Monoclonal Antibody to the A1 Domain of von Willebrand Factor

AbstractShear-induced platelet aggregation (SIPA) involves von Willebrand Factor (vWF) binding to platelet glycoprotein (GP)Ib at high shear stress, followed by the activation of αIIbβ3. The purpose of this study was to determine the vWF sequences involved in SIPA by using monoclonal antibodies (MoAbs) to vWF known to interfere with its binding to GPIb and to αIIbβ3. Washed platelets were exposed to shear rates between 100 and 4,000 seconds−1 in a rotational viscometer. SIPA was quantitated by flow cytometry as the disappearance of single platelets (DSP) in the sheared sample in the presence of vWF, relative to a control in the absence of shear and vWF. At a shear rate of 4,000 seconds−1, DSP was increased from 5.9% ± 3.5% in the absence of vWF to 32.7% ± 6.3% in the presence of vWF. This increase in SIPA was not associated with an elevation of P-selectin expression. vWF-dependent SIPA was completely abolished by MoAb 6D1 to GPIb and partially inhibited by MoAb 10E5 to αIIbβ3. Three MoAbs to vWF were compared for their effect on SIPA at 4,000 seconds−1 in the presence of vWF: MoAb 328, known to block vWF binding to GPIb in the presence of ristocetin, MoAb 724 blocking vWF binding to GPIb in the presence of botrocetin, and MoAb 9, an inhibitor of vWF binding to αIIbβ3. Similar to the effect of MoAb 6D1, MoAb 328 completely inhibited the effect of vWF, whereas MoAb 9 had a partial inhibitory effect, as MoAb 10E5 did. In contrast, MoAb 724, as well as its F(ab′)2 fragments, promoted shear-dependent platelet aggregation (165% of the DSP value obtained in the absence of MoAb 724), indicating that MoAb 724 was responsible for an enhanced aggregation, which was independent of binding to the platelet Fcγ receptor. In addition, the enhancement of aggregation induced by MoAb 724 was abrogated by MoAb 6D1 or 10E5 to the level of SIPA obtained in the presence of vWF incubated with a control MoAb to vWF. Finally, the activating effect of MoAb 724 was also found under static conditions at ristocetin concentrations too low to induce platelet aggregation. Our results suggested that on binding to a botrocetin-binding site on vWF, MoAb 724 mimics the effect of botrocetin by inducing an active conformation of vWF that is more sensitive to shear stress or to low ristocetin concentration.

Glycoprotein Ib-von Willebrand Factor Interactions Activate Tyrosine Kinases in Human Platelets

von Willebrand factor (vWF ) in the presence of botrocetin induces p72(syk) activation, assessed as its autophosphorylated level and in vitro kinase assays, the transient association of p72(syk) with p60(c-src), and the translocation of p60(c-src) and p54/58(lyn) to cytoskeletal fractions. Jararaca glycoprotein Ib-binding protein (GPIb-BP), which specifically binds to GPIb, abolished these phenomena, suggesting that they are mediated by the vWF-GPIb interaction. These tyrosine kinase-related events were not inhibited by GRGDS peptide (plus EGTA), indicating that GPIIb/IIIa is not involved in the observed responses. Shc, an adaptor protein, was also tyrosine phosphorylated by the botrocetin-vWF activation. When GPIb was immunoprecipitated with nonfunctional monoclonal antibodies (MoAbs) directed against GPIb, a kinase activity was found to associate with GPIb upon botrocetin-vWF activation. On the other hand, anti-GPIb MoAbs that inhibit the vWF-GPIb interaction did not coprecipitate a kinase activity. Because the recovery of GPIb did not differ significantly, it is suggested that the excessive presence of inhibitory anti-GPIb MoAb dissociated a kinase activity from GPIb. Phosphoamino acid analysis showed that the kinase activity was that of a tyrosine kinase. The identity of the tyrosine kinase and the mode of interaction with the cytoplasmic region of GPIb await to be determined. Our findings suggest that the tyrosine kinase associated with GPIb serves at a most proximal step in the signal transduction pathway involved in the vWF-GPIb-induced platelet activation, which leads to other tyrosine kinase-related intracellular signals.

Interaction of von Willebrand factor with the extracellular matrix and glycocalicin under static conditions.

The binding of human von Willebrand factor (vWF) to a variety of extracellular matrix components immobilized on plates and the binding of vWF to platelet glycoprotein Ib (GPIb) after interacting with these matrix components were examined by means of an enzyme-linked immunosorbent assay. vWF preferably bound to type III collagen, whereas it did not significantly bind to type I, IV, V, or VI collagen, fibronectin, laminin, elastin, or proteoglycans. Soluble type III collagen did not bind to vWF coated on plates and showed a little effect on the vWF binding to the immobilized collagen, suggesting that solid-phase collagen is important for the interaction with vWF. When glycocalicin, the N-terminal carbohydrate-rich extracellular domain of GPIb alpha exhibiting the vWF-binding activity, was added to vWF bound to collagen type III, no significant binding of glycocalicin was observed, but it bound to vWF in the presence of botrocetin, a vWF modulator protein isolated from Bothrops jararaca snake venom. These results indicate that vWF immobilized on collagen can interact with GPIb but that binding of vWF to the collagen matrix alone is insufficient for modulating vWF so that it interacts with GPIb under static conditions. Another unknown physiological modulator functionally mimicking botrocetin or high-shear stress may be involved in the platelet adhesion to extracellular matrix in the early stage of hemostasis.

Frequency and functional relevance of genetic threonine145/methionine145 dimorphism in platelet glycoprotein Ib alpha in an Italian population.

Threonine145/methionine145 dimorphism in platelet glycoprotein (GP) Ib alpha defines the human platelet antigen (HPA)-2 system that has been implicated in refractoriness to HLA-matched platelet transfusion and in neonatal immune thrombocytopenic purpura. The occurrence of this amino acid dimorphism was investigated in 379 Italian blood donors by studying their genomic DNA. Two oligonucleotide primers, Ib alpha-3 (5'-GGACGTCTCCTTCAACCGGC-3') and Ib alpha-4 (5'-GCTTTGGTGGGGAACTTGAC-3'), were used in a polymerase chain reaction to generate a 591-base pair fragment that was digested with the restriction enzyme Acy I. To investigate whether this dimorphism is involved in the binding of von Willebrand factor (vWF) to GPlb, the binding of vWF to the GPlb/IX complex was measured in two Met145/Met145 and two Thr145/Thr145 subjects. The genotypic frequencies are 78.9% for Thr/Thr, 19.8% for Thr/Met), and 1.3% for Met/Met; the allelic frequencies are 88.8% for Thr145 and 11.2% for Met145. Estimates for binding of subunit molecules per platelet at saturation and inhibition constant in mol per L, respectively, follow. In the presence of ristocetin (0.5 mg/mL), they are 11,460 +/- 2,040 and 1.26 +/- 0.44 x 10(-8) for normals and 11,230 +/- 2,330 and 1.29 +/- 0.48 x 10(-8) for patients. In the presence of botrocetin (2.5 micrograms/mL), they are 64,260 +/- 7,760 and 2.99 +/- 0.96 x 10(-8) for normals and 65,770 +/- 11,570 and 2.47 +/- 0.22 x 10(-8) for patients. Platelet aggregation responses obtained using platelet-rich plasma from donors with Met145/Met145 or Thr145/Thr145 genotype were within normal limits. Genotypic and phenotypic frequencies in the HPA-2 system in this population are consistent with those reported among the white population. Furthermore, the HPA-2 system is not involved in the binding of vWF to GPlb.

Analysis of the structure and function of the von Willebrand factor A1 domain using targeted deletions and alanine-scanning mutagenesis.

von Willebrand factor (vWF) mediates the primary adhesion of platelets to sites of vascular damage through interaction with glycoprotein Ib (GPIb) of the platelet GPIb/IX complex. To investigate the vWF/GPIb interaction we introduced both in-frame deletions and substitutions into the vWF A1 domain. The introduction of nine sequential 20-amino acid deletions within the Cys509-Cys695 loop of the A1 domain caused the defective secretion of vWF from mammalian cells, and resulted in multimeric vWF without platelet-binding activity. In other experiments we substituted alanine for charged amino acids (residues 524, 534, 549, 552, 569-573, and 642-645) in proposed functional domains within the Cys509-Cys695 loop. All six substitution mutants showed normal secretion from transfected mammalian cells and bound to fixed platelets in the presence of botrocetin. In contrast, only mutants vWF-R524A and vWF-K549A showed significant binding to platelets in the presence of ristocetin. Mutant vWF-K549A showed increased platelet-binding at suboptimal concentrations of both botrocetin and ristocetin. These results suggest that the substituted amino acids do not play a critical role in the activation of vWF by botrocetin or in the direct interaction of vWF with the GPIb/IX complex. However, the charged amino acids at positions 534, 552, 569-573, and 642-645 do play an important role in the ristocetin-induced binding of vWF to platelets. The interaction of vWF with heparin was significantly reduced by substitution of Lys residues 642-645, indicating that these residues may form part of a heparin-binding domain in the carboxy-terminal half of the Cys509-Cys695 loop.