Novel gain-of-function mutations of platelet glycoprotein IBalpha by valine mutagenesis in the Cys209-Cys248 disulfide loop. Functional analysis under statis and dynamic conditions

How I treat thrombocytopenia in pregnancy

ABSTRACT

Thrombocytopenia is a common hematologic abnormality in pregnancy, encountered in ∼10% of pregnancies. There are many possible causes, ranging from benign conditions that do not require intervention to life-threatening disorders necessitating urgent recognition and treatment. Although thrombocytopenia may be an inherited condition or predate pregnancy, most commonly it is a new diagnosis. Identifying the responsible mechanism and predicting its course is made challenging by the tremendous overlap of clinical features and laboratory data between normal pregnancy and the many potential causes of thrombocytopenia. Multidisciplinary collaboration between hematology, obstetrics, and anesthesia and shared decision-making with the involved patient is encouraged to enhance diagnostic clarity and develop an optimized treatment regimen, with careful consideration of management of labor and delivery and the potential fetal impact of maternal thrombocytopenia and any proposed therapeutic intervention. In this review, we outline a diagnostic approach to pregnant patients with thrombocytopenia, highlighting the subtle differences in presentation, physical examination, clinical course, and laboratory abnormalities that can be applied to focus the differential. Four clinical scenarios are presented to highlight the pathophysiology and treatment of the most common causes of thrombocytopenia in pregnancy: gestational thrombocytopenia, preeclampsia, and immune thrombocytopenia.

Physical Characteristics of von Willebrand Factor Binding with Platelet Glycoprotein Ibɑ Mutants at Residue 233 Causing Various Biological Functions

Glycoprotein (GP: HIS ¹ -PRO ²⁶⁵ ) Ibɑ is a receptor protein expressed on the surface of the platelet. Its N-terminus domain binds with the A1 domain (ASP ¹²⁶⁹ -PRO ¹⁴⁷² ) of its ligand protein von Willebrand factor (VWF) and plays a unique role in platelet adhesion under blood flow conditions. Single amino acid substitutions at residue 233 from glycine (G) to alanine (A), aspartic acid (D), or valine (V) are known to cause biochemically distinct functional alterations known as equal, loss, and gain of function, respectively. However, the underlying physical characteristics of VWF binding with GPIbɑ in wild-type and the three mutants exerting different biological functions are unclear. Here, we aimed to test the hypothesis: biological characteristics of macromolecules are influenced by small changes in physical parameters. The position coordinates and velocity vectors of all atoms and water molecules constructing the wild-type and the three mutants of GPIbɑ (G233A, G233D, and G233V) bound with VWF were calculated every 2 × 10 ^-15 seconds using the CHARMM (Chemistry at Harvard Macromolecular Mechanics) force field for 9 × 10 ^-10 seconds. Six salt bridges were detected for longer than 50% of the calculation period for the wild-type model generating noncovalent binding energy of -1096 ± 137.6 kcal/mol. In contrast, only four pairs of salt bridges were observed in G233D mutant with noncovalent binding energy of -865 ± 139 kcal/mol. For G233A and G233V, there were six and five pairs of salt bridges generating -929.8 ± 88.5 and -989.9 ± 94.0 kcal/mol of noncovalent binding energy, respectively. Our molecular dynamic simulation showing a lower probability of salt bridge formation with less noncovalent binding energy in VWF binding with the biologically loss of function G233D mutant of GPIbɑ as compared with wild-type, equal function, and gain of function mutant suggests that biological functions of macromolecules such as GPIbɑ are influenced by their small changes in physical characteristics.

Gestational thrombocytopenia: a case-control study of over 3,500 pregnancies

Gestational thrombocytopenia (GT) affects an estimated nine million women annually. Women with GT have recurrent episodes on subsequent pregnancies suggesting that GT is related to a maternal factor rather than a factor unique to the individual pregnancy. We performed a case-control study of over 3 500 pregnancies at a single hospital during 2017. We defined GT as any pregnancy with a platelet count <150 000/µl during the 100 days prior to delivery. We excluded women with platelet counts <50 000/µl or with conditions known to cause thrombocytopenia. GT was present in 12% of pregnancies. The median platelet count at delivery was 134 500/µl in cases versus 208 000/µl in controls, P < 0·0001. During the pregnancy, the platelet count declined 31·8% in cases compared with 18·3% in controls (P < 0·0001) in association with a significant increase in mean platelet volume during each trimester. Among women with GT, platelet counts rapidly increased during the first week postpartum, consistent with a mechanism directly related to high blood flow rates in the gravid uterus. GT, a recurrent condition of at-risk women, is a common haematological disorder of pregnancy. Future research may focus on genetic gain-of-function polymorphisms resulting in increased turnover of platelets uncovered only during periods of high-shear blood flow.

Atherosclerosis and Nanomedicine Potential: Current Advances and Future Opportunities

Atherosclerosis is the leading inducement of cardiovascular diseases, which ranks the first cause of global deaths. It is an arterial disease associated with dyslipidemia and changes in the composition of the vascular wall. Besides invasive surgical strategy, the current conservative clinical treatment for atherosclerosis falls into two categories, lipid regulating-based therapy and antiinflammatory therapy. However, the existing strategies based on conventional drug delivery systems have shown limited efficacy against disease development and plenty of side effects. Nanomedicine has great potential in the development of targeted therapy, controlled drug delivery and release, the design of novel specific drugs and diagnostic modalities, and biocompatible scaffolds with multifunctional characteristics, which has led to an evolution in the diagnosis and treatment of atherosclerosis. This paper will focus on the latest nanomedicine strategies for atherosclerosis diagnosis and treatment as well as discussing the potential therapeutic targets during atherosclerosis progress, which could form the basis of development of novel nanoplatform against atherosclerosis.

Platelet Protein Disulfide Isomerase Promotes Glycoprotein Ibα-Mediated Platelet-Neutrophil Interactions Under Thromboinflammatory Conditions

BACKGROUND

Platelet-neutrophil interactions contribute to vascular occlusion and tissue damage in thromboinflammatory disease. Platelet glycoprotein Ibα (GPIbα), a key receptor for the cell-cell interaction, is believed to be constitutively active for ligand binding. Here, we established the role of platelet-derived protein disulfide isomerase (PDI) in reducing the allosteric disulfide bonds in GPIbα and enhancing the ligand-binding activity under thromboinflammatory conditions.

METHODS

Bioinformatic analysis identified 2 potential allosteric disulfide bonds in GPIbα. Agglutination assays, flow cytometry, surface plasmon resonance analysis, a protein-protein docking model, proximity ligation assays, and mass spectrometry were used to demonstrate a direct interaction between PDI and GPIbα and to determine a role for PDI in regulating GPIbα function and platelet-neutrophil interactions. Also, real-time microscopy and animal disease models were used to study the pathophysiological role of PDI-GPIbα signaling under thromboinflammatory conditions.

RESULTS

Deletion or inhibition of platelet PDI significantly reduced GPIbα-mediated platelet agglutination. Studies using PDI-null platelets and recombinant PDI or Anfibatide, a clinical-stage GPIbα inhibitor, revealed that the oxidoreductase activity of platelet surface-bound PDI was required for the ligand-binding function of GPIbα. PDI directly bound to the extracellular domain of GPIbα on the platelet surface and reduced the Cys4-Cys17 and Cys209-Cys248 disulfide bonds. Real-time microscopy with platelet-specific PDI conditional knockout and sickle cell disease mice demonstrated that PDI-regulated GPIbα function was essential for platelet-neutrophil interactions and vascular occlusion under thromboinflammatory conditions. Studies using a mouse model of ischemia/reperfusion-induced stroke indicated that PDI-GPIbα signaling played a crucial role in tissue damage.

CONCLUSIONS

Our results demonstrate that PDI-facilitated cleavage of the allosteric disulfide bonds tightly regulates GPIbα function, promoting platelet-neutrophil interactions, vascular occlusion, and tissue damage under thromboinflammatory conditions.

Platelet-type von Willebrand disease: Local disorder of the platelet GPIbα β-switch drives high-affinity binding to von Willebrand factor

BACKGROUND

Mutations in the β-switch of GPIbα cause gain-of-function in the platelet-type von Willebrand disease. Structures of free and A1-bound GPIbα suggest that the β-switch undergoes a conformational change from a coil to a β-hairpin.

OBJECTIVES

Platelet-type von Willebrand disease (VWD) mutations have been proposed to stabilize the β-switch by shifting the equilibrium in favor of the β-hairpin, a hypothesis predicated on the assumption that the complex crystal structure between A1 and GPIbα is the high-affinity state.

METHODS

Hydrogen-deuterium exchange mass spectrometry is employed to test this hypothesis using G233V, M239V, G233V/M239V, W230L, and D235Y disease variants of GPIbα. If true, the expectation is a decrease in hydrogen-deuterium exchange within the β-switch as a result of newly formed hydrogen bonds between the β-strands of the β-hairpin.

RESULTS

Hydrogen-exchange is enhanced, indicating that the β-switch favors the disordered loop conformation. Hydrogen-exchange is corroborated by differential scanning calorimetry, which confirms that these mutations destabilize GPIbα by allowing the β-switch to dissociate from the leucine-rich-repeat (LRR) domain. The stability of GPIbα and its A1 binding affinity, determined by surface plasmon resonance, are correlated to the extent of hydrogen exchange in the β-switch.

CONCLUSION

These studies demonstrate that GPIbα with a disordered loop is binding-competent and support a mechanism in which local disorder in the β-switch exposes the LRR-domain of GPIbα enabling high-affinity interactions with the A1 domain.

A novel platelet-type von Willebrand disease mutation (GP1BA p.Met255Ile) associated with type 2B “Malmö/New York” von Willebrand disease

Interaction between von Willebrand factor (VWF) and platelet GPIbα is required for primary haemostasis. Lack or loss-of-function in the ligand-receptor pair results in bleeding complications. Paradoxically, gain-of-function mutations in VWF or GPIbα also result in bleeding complications as observed in type 2B von Willebrand disease (VWD) and platelet-type- (PT-) VWD, respectively. A similar phenotype is observed with increased ristocetin-induced platelet agglutination and disappearance of the highest molecular weight multimers of VWF. We evaluated a patient with a bleeding disorder and a biological presentation compatible with type 2B VWD. VWF and platelet functional assays, sequencing of the VWF and GP1BA genes, and expression studies in HEK cells were performed. Sequencing of the VWF gene in the propositus revealed a heterozygous p.Pro1266Leu mutation previously found in type 2B VWD Malmö/New York. These variants are characterised by a mild phenotype and a normal VWF multimer composition suggesting the presence of a second mutation in our propositus. Sequencing of the GP1BA gene revealed a heterozygous c.765G>A substitution changing Met at position 255 of GPIbα to Ile. This new mutation is located in the β-switch domain where five other gain-of-function mutations have been reported in PT-VWD. Expression of GPIbα Ile255 in HEK GPIb-IX cells resulted in enhanced VWF binding compared to wild-type, similar to known PT-VWD mutations (p.Val249, p.Ser249 and p.Val255) indicating that it contributes to the propositus defects. This first report associating PT-with type 2B VWD illustrates the importance of combining biological assays with genetic testing to better understand the clinical phenotype.

A novel D235Y mutation in the GP1BA gene enhances platelet interaction with von Willebrand factor in an Iranian family with platelet-type von Willebrand disease

Platelet-type von Willebrand disease (PT-VWD) is a rare bleeding disorder with an intrinsic defect in platelets rather than von Willebrand factor (VWF), but has clinical and laboratory features similar to the more common type 2B VWD. The intriguing nature of the pathophysiology and molecular genetics of PT-VWD has created lengthy debate in literature regarding its discrimination from type 2B VWD, and essentially confirming DNA analysis as the gold standard in diagnosis and revealing pathologic mutations. In this report we identify a novel Asp235Tyr mutation in the GP1BA gene of two Iranian patients showing the PT-VWD phenotype who were originally misdiagnosed as type 2B VWD. By structural modelling of the mutant by introducing Tyr235 into the available crystal structure of the glycoprotein (GP)Ibα N-terminal domain, we observed the mutant Tyr235 generates a hydrophobic tip to the extended β-switch loop of GPIbα. Further modelling of the resulting complex with VWFA1 indicates this could result in an enhanced interface compared to wild-type Asp235. This data provides an update to the present knowledge about this rare disorder, and confirms the necessity of genetic testing for accurate diagnosis, and the importance of studying natural mutations to better understand molecular aspects of GPIbα-VWFA1 interaction.

Glycoprotein Ib-IX-V Complex Transmits Cytoskeletal Forces That Enhance Platelet Adhesion

Platelets bind to exposed vascular matrix at a wound site through a highly specialized surface receptor, glycoprotein (GP) Ib-IX-V complex, which recognizes von Willebrand factor (VWF) in the matrix. GPIb-IX-V is a catch bond for it becomes more stable as force is applied to it. After attaching to the wound site, platelets generate cytoskeletal forces to compact and reinforce the hemostatic plug. Here, we evaluated the role of the GPIb-IX-V complex in the transmission of cytoskeletal forces. We used arrays of flexible, silicone nanoposts to measure the contractility of individual platelets on VWF. We found that a significant proportion of cytoskeletal forces were transmitted to VWF through GPIb-IX-V, an unexpected finding given the widely held notion that platelet forces are transmitted exclusively through its integrins. In particular, we found that the interaction between GPIbα and the A1 domain of VWF mediates this force transmission. We also demonstrate that the binding interaction between GPIbα and filamin A is involved in force transmission. Furthermore, our studies suggest that cytoskeletal forces acting through GPIbα are involved in maintaining platelet adhesion when external forces are absent. Thus, the GPIb-IX-V/VWF bond is able to transmit force, and uses this force to strengthen the bond through a catch-bond mechanism. This finding expands our understanding of how platelets attach to sites of vascular injury, describing a new, to the best of our knowledge, mechanism in which the catch bonds of GPIb-IX-V/VWF can be supported by internal forces produced by cytoskeletal tension.

Comparison of the gene encoding, and the predicted amino acid composition of, platelet membrane receptor subunit glycoprotein Ibα in members of the family Felidae

BACKGROUND

There is minimal information regarding platelet receptors in the family Felidae. Comparative studies assist with identifying amino acids critical for protein structure and function.

OBJECTIVE

The purpose of the study was to compare the gene encoding, and the predicted amino acid composition of, platelet membrane receptor subunit GPIbα in Felidae family members.

METHODS

Genomic DNA samples isolated from whole blood of 13 domestic cats and 50 big cats representing 8 different species were subjected to PCR using primers designed to flank the coding region of GPIbα in overlapping fashion. PCR products were separated via electrophoresis on agarose gels, and extracted products were submitted for sequencing. DNA sequences were used to predict the length and amino acid composition of the protein.

RESULTS

Varying protein lengths were predicted in Felidae family members which were primarily due to polymorphisms in the variable number of tandem repeats region encoding the macroglycopeptide region of GPIbα. Other areas of the gene and predicted amino acid compositions were fairly conserved when compared to human sequences and between Felidae family members.

CONCLUSION

Various polymorphisms within GPIbα, including length variants encoding the macroglycopeptide region, were identified in members of the family Felidae. More studies are needed to determine if a correlation exists between various polymorphisms and predisposition for hemorrhage or thrombosis as suggested in people.

The N-terminal flanking region of the A1 domain regulates the force-dependent binding of von Willebrand factor to platelet glycoprotein Ibα

Binding of platelet glycoprotein Ibα (GPIbα) to von Willebrand factor (VWF) initiates platelet adhesion to disrupted vascular surface under arterial blood flow. Flow exerts forces on the platelet that are transmitted to VWF-GPIbα bonds, which regulate their dissociation. Mutations in VWF and/or GPIbα may alter the mechanical regulation of platelet adhesion to cause hemostatic defects as found in patients with von Willebrand disease (VWD). Using a biomembrane force probe, we observed biphasic force-decelerated (catch) and force-accelerated (slip) dissociation of GPIbα from VWF. The VWF A1 domain that contains the N-terminal flanking sequence Gln(1238)-Glu(1260) (1238-A1) formed triphasic slip-catch-slip bonds with GPIbα. By comparison, using a short form of A1 that deletes this sequence (1261-A1) abolished the catch bond, destabilizing its binding to GPIbα at high forces. Importantly, shear-dependent platelet rolling velocities on these VWF ligands in a flow chamber system mirrored the force-dependent single-bond lifetimes. Adding the Gln(1238)-Glu(1260) peptide, which interacted with GPIbα and 1261-A1 but not 1238-A1, to whole blood decreased platelet attachment under shear stress. Soluble Gln(1238)-Glu(1260) reduced the lifetimes of GPIbα bonds with VWF and 1238-A1 but rescued the catch bond of GPIbα with 1261-A1. A type 2B VWD 1238-A1 mutation eliminated the catch bond by prolonging lifetimes at low forces, a type 2M VWD 1238-A1 mutation shifted the respective slip-catch and catch-slip transition points to higher forces, whereas a platelet type VWD GPIbα mutation enhanced the bond lifetime in the entire force regime. These data reveal the structural determinants of VWF activation by hemodynamic force of the circulation.

Biodegradable nanoparticles mimicking platelet binding as a targeted and controlled drug delivery system

This research aims to develop targeted nanoparticles as drug carriers to the injured arterial wall under fluid shear stress by mimicking the natural binding ability of platelets via interactions of glycoprotein Ib-alpha (GPIbα) of platelets with P-selectin of damaged endothelial cells (ECs) and/or with von Willebrand factor (vWF) of the subendothelium. Drug-loaded poly(d,l-lactic-co-glycolic acid) (PLGA) nanoparticles were formulated using a standard emulsion method and conjugated with glycocalicin, the external fraction of platelet GPIbα, via carbodiimide chemistry. Surface-coated and cellular uptake studies in ECs showed that conjugation of PLGA nanoparticles, with GPIb, significantly increased nanoparticle adhesion to P-selectin- and vWF-coated surfaces as well as nanoparticle uptake by activated ECs under fluid shear stresses. In addition, effects of nanoparticle size and shear stress on adhesion efficiency were characterized through parallel flow chamber studies. The observed decrease in bound nanoparticle density with increased particle sizes and shear stresses is also explained through a computational model. Our results demonstrate that the GPIb-conjugated PLGA nanoparticles can be used as a targeted and controlled drug delivery system under flow conditions at the site of vascular injury.

Gain-of-function GPIb ELISA assay for VWF activity in the Zimmerman Program for the Molecular and Clinical Biology of VWD

von Willebrand disease (VWD) is a common bleeding disorder, but diagnosis is sometimes challenging because of issues with the current von Willebrand factor (VWF) assays, VWF antigen (VWF:Ag) and VWF ristocetin cofactor activity (VWF:RCo), used for diagnosis. We evaluated 113 healthy controls and 164 VWD subjects enrolled in the T.S. Zimmerman Program for the Molecular and Clinical Biology of VWD for VWF:Ag, VWF:RCo, and a new enzyme-linked immunosorbent assay (ELISA)-based assay of VWF-glycoprotein Ib (GPIb) interactions using a gain-of-function GPIb construct (tGPIbα(235Y;239V)) as a receptor to bind its ligand VWF in an assay independent of ristocetin (VWF:IbCo ELISA). Healthy controls, type 1, 2A, 2M, and 2N subjects had VWF:RCo/VWF:Ag ratios similar to the ratio obtained with VWF:IbCo ELISA/VWF:Ag. Type 2B VWD subjects, however, had elevated VWF:IbCo ELISA/VWF:Ag ratios. Type 3 VWD subjects had undetectable (< 1.6 U/dL) VWF:IbCo ELISA values. As previously reported, VWF:RCo/VWF:Ag ratio was decreased with a common A1 domain polymorphism, D1472H, as was direct binding to ristocetin for a 1472H A1 loop construct. The VWF:IbCo ELISA, however, was not affected by D1472H. The VWF:IbCo ELISA may be useful in testing VWF binding to GPIb, discrimination of type 2 variants, and in the diagnosis of VWD as it avoids some of the pitfalls of VWF:RCo assays.

Shear-regulated uptake of nanoparticles by endothelial cells and development of endothelial-targeting nanoparticles

The purpose of this research project was to develop nanoparticles with improved targeting, adhesion, and cellular uptake to activated or inflamed endothelial cells (ECs) under physiological flow conditions. Our hypothesis is that by mimicking platelet binding to activated ECs through the interaction between platelet glycoprotein Ibalpha (GP Ibalpha) and P-selectin on activated endothelial cells, GP Ibalpha-conjugated nanoparticles could exhibit increased targeting and higher cellular uptake in injured or activated endothelial cells under physiological flow conditions. To test this hypothesis, fluorescent-carboxylated polystyrene nanoparticles were selected for the study as a model particle because of its narrow size distribution as a "proof-of-concept." Using confocal microscopy, fluorescent measurements, and protein assays, cellular uptake properties were characterized for these polystyrene nanoparticles. The study also found that conjugation of 100-nm polystyrene nanoparticles with glycocalicin (the extracellular segment of GP Ibalpha) significantly increased the particle adhesion on P-selectin-coated surfaces and cellular uptake of nanoparticles by activated endothelial cells under physiological flow conditions. The results demonstrate that these novel endothelial-targeting nanoparticles could be the first step toward developing a targeted and sustained drug delivery system that can improve shear-regulated particle adhesion and cellular uptake.

Flow induces loop-to-beta-hairpin transition on the beta-switch of platelet glycoprotein Ib alpha

Interaction of glycoprotein Ib alpha (GPIb alpha) with von Willebrand factor (VWF) initiates platelet adhesion to injured vascular wall to stop bleeding. A major contact between GPIb alpha and VWF involves the beta-switch region, which is a loop in the unliganded GPIb alpha but switches to a beta-hairpin in the complex structure. Paradoxically, flow enhances rather than impedes GPIb alpha-VWF binding. Gain-of-function mutations (e.g., M239V) in the beta-switch reduce the flow requirement for VWF binding, whereas loss-of-function mutations (e.g., A238V) increase the flow requirement. These phenomena cannot be explained by crystal structures or energy calculations. Herein we demonstrate that the beta-hairpin is unstable without contacting VWF, in that it switches to a loop in free molecular dynamics simulations. Simulations with a novel flow molecular dynamics algorithm show that the loop conformation is unstable in the presence of flow, as it switches to beta-hairpin even without contacting VWF. Compared with the wild-type, it is easier for the M239V mutant but harder for the A238V mutant to switch to beta-hairpin in the presence of flow. These results elucidate the structural basis for the two mutants and suggest a regulatory mechanism by which flow activates GPIb alpha via inducing a loop-to-beta-hairpin conformational transition on the beta-switch, thereby promoting VWF binding.

Trp207Gly in platelet glycoprotein Ibalpha is a novel mutation that disrupts the connection between the leucine-rich repeat domain and the disulfide loop structure and causes Bernard-Soulier syndrome

BACKGROUND

Bernard-Soulier syndrome (BSS) is a severe inherited bleeding disorder that is caused by a defect in glycoprotein (GP)Ib-IX-V complex, the platelet membrane receptor for von Willebrand factor.

PATIENTS

The diagnosis of BSS was made in two members of a Bukharian Jewish family who had life-long thrombocytopenia associated with mucocutaneous bleeding manifestations.

METHODS AND RESULTS

Flow cytometry and Western blot analyses showed only trace amounts of GPIb and GPIX on the patients' platelets. Sequence analysis of the GPIbalpha gene revealed a homozygous T > G transversion at nucleotide 709 predicting Trp207Gly substitution in the mature protein. Introduction of the mutation into a mammalian expression construct abolished the surface expression of GPIbalpha in transfected baby hamster kidney cells. The crystal structure of the N-terminus of GPIbalpha (PDB: 1SQ0) indicates that Trp207 is completely buried and located in a disulfide loop structure that interacts with the leucine-rich repeat (LRR) domain.

CONCLUSION

A novel mutation, Trp207Gly, causes BSS and predicts disruption of the interaction between a disulfide loop and the LRR domain that is essential for the integrity of GPIbalpha structure.

Effect of von Willebrand Factor on the Pharmacokinetics of Recombinant Human Platelet Glycoprotein Ibα-Immunoglobulin G1 Chimeric Proteins

PURPOSE

Recombinant human platelet glycoprotein Ibalpha-immunoglobulin G1 chimeric proteins (GPIbalpha-Ig) have varying levels of anti-thrombotic activities based on their ability to compete for platelet mediated adhesion to von Willebrand Factor (vWF). Valine substituted GPIbalpha-Ig chimeras, at certain position, increase the binding affinity to vWF over its "wild-type" GPIbalpha-Ig analog. The purpose of this study was to determine the pharmacokinetics of two valine substituted GPIbalpha-Ig chimeras, GPIbalpha-Ig/1V (valine substitution at 239 position) and GPIbalpha-Ig/2V (double valine substitution at 233 and 239 position), in mice, rats and dogs.

METHODS

Head-to-head comparisons of pharmacokinetics of GPIbalpha-Ig/1V and GPIbalpha-Ig/2V were investigated in rats and dogs after intravenous administration. Since vWF precipitates in the serum but not in plasma preparation, the concentration-time profiles of GPIbalpha-Ig/2V in rats were examined from the same blood samples for determination of matrix effect. The disposition of GPIbalpha-Ig/2V was also compared in vWF-deficient versus wild-type mice.

RESULTS

For GPIbalpha-Ig/2V, the serum clearances were 2.62+/-0.27 ml/hr/kg in rats and 1.97+/-0.24 ml/hr/kg in dogs. The serum clearances of less potent GPIbalpha-Ig/1V were 1.08+/-0.08 and 0.97+/-0.19 ml/hr/kg in rats and dogs, respectively. In addition, the serum clearance of GPlbalpha-Ig/2V of 1.53 ml/hr/kg in vWF-deficient mice was lower than that in wild-type mice of 2.79 ml/hr/kg.

CONCLUSION

The difference in disposition for valine substituted forms of GPIbalpha-Ig in laboratory animals are likely affected by their enhanced binding affinity for circulating vWF.

Gain of von Willebrand factor-binding function by mutagenesis of a species-conserved residue within the leucine-rich repeat region of platelet glycoprotein Ibalpha

Glycoprotein (GP) Ibalpha, a member of the leucine-rich repeat (LRR) protein family, mediates platelet adhesion to immobilized von Willebrand factor (VWF). We investigated the role in VWF binding of charged residues in the LRR region of GP Ibalpha that are conserved in human, canine, and murine proteins. Substitution of His86 with either Ala or Glu resulted in a gain of VWF-binding function as judged by increased VWF binding in the presence of the modulators ristocetin and botrocetin and by enhanced adhesion of Chinese hamster ovary (CHO) cells expressing the mutant GP Ibalpha to immobilized VWF under conditions of flow. This is the first report of a gain-of-function phenotype resulting from mutations in the LRR region of GP Ibalpha. Because His86 is 2 nm away from the region of GP Ibalpha with the largest surface of contact with VWF, the data suggest that the LRRs regulate GP Ibalpha affinity for VWF allosterically.

Dynamic force spectroscopy of glycoprotein Ib-IX and von Willebrand factor

The first stage in hemostasis is the binding of the platelet membrane receptor glycoprotein (GP) Ib-IX complex to the A1 domain of von Willebrand factor in the subendothelium. A bleeding disorder associated with this interaction is platelet-type von Willebrand disease, which results from gain-of-function (GOF) mutations in amino acid residues 233 or 239 of the GP Ibalpha subunit of GP Ib-IX. Using optical tweezers and a quadrant photodetector, we investigated the binding of A1 to GOF and loss-of-function mutants of GP Ibalpha with mutations in the region containing the two known naturally occurring mutations. By dynamically measuring unbinding force profiles at loading rates ranging from 200-20,000 pN/s, we found that the bond strengths between A1 and GP Ibalpha GOF mutants (233, 235, 237, and 239) were significantly greater than the A1/wild-type GP Ib-IX bond at all loading rates examined (p < 0.05). In addition, mutants 231 and 232 exhibited significantly lower bond strengths with A1 than the wild-type receptors (p < 0.05). We computed unloaded dissociation rate constant (k(off)(0)) values for interactions involving mutant and wild-type GP Ib-IX receptors with A1 and found the A1/wild-type GP Ib-IX k(off)(0) value of 5.47 +/- 0.25 s(-1) to be significantly greater than the GOF k(off)(0) values and significantly less than the loss-of-function k(off)(0) values. Our data illustrate the importance of the bond kinetics associated with the VWF/GP Ib-IX interaction in hemostasis and also demonstrate the drastic changes in binding that can occur when only a single amino acid of GP Ibalpha is altered.

Platelets adhered to endothelial cell-bound ultra-large von Willebrand factor strings support leukocyte tethering and rolling under high shear stress

Leukocyte rolling on vascular endothelium is mediated by an interaction between P-selectin expressed on endothelial cells and P-selectin glycoprotein ligand-1 on leukocytes. This interaction reduces the velocity of leukocyte movements to allow subsequent firm adhesion and transmigration. However, the interaction has so far been observed only under low venous shear stress and cannot explain the accumulation of monocytes in atherosclerotic plaques found in arteries, where shear stress is much higher. We have previously shown that newly released ultra-large von Willebrand factor (ULVWF) forms extremely long string-like structures to which platelets tether. Here, we investigated whether platelets adhered to ULVWF strings are activated and form aggregates. We also determined whether activated platelets on ULVWF strings can support leukocyte tethering and rolling under high shear stresses. We found that platelets adhered to ULVWF expressed P-selectin and bound PAC-1, suggesting their rapid activation. We also found that leukocytes tethered to and rolled on these platelet-decorated ULVWF strings, but not directly on endothelial cells, under high shear stresses of 20 and 40 dyn/cm(2) in a P-selectin dependent manner. These results suggest that the endothelial cell-bound ULVWF provide an ideal matrix to aggregate platelets and recruit leukocytes to endothelial cells under high shear stress. The observed phenomenon delineates a mechanism for leukocytes to be tethered to arterial endothelial cells under high shear, providing a potential link between inflammation and thrombosis.

The alpha1 helix-beta13 strand spanning Leu214 to Val229 of platelet glycoprotein Ibalpha facilitates the interaction with von Willebrand factor: evidence from characterization of the epitope of monoclonal antibody AP1

The glycoprotein Ib-IX-V (GP Ib-IX-V) complex mediates platelet binding to von Willebrand factor (VWF) through its largest polypeptide, GP Ibalpha. Of the many GP Ibalpha monoclonal antibodies described, AP1 is of particular interest because it blocks static VWF binding induced by 2 modulators, ristocetin and botrocetin, and platelet adhesion to VWF surfaces under flow. We mapped the AP1 binding site to a region encompassing Arg218 to Tyr228, comprising the alpha1 helix and beta13 strand defined by the GP Ibalpha crystal structure. AP1 binding absolutely required Arg218, Asp222, and Glu225. We evaluated the ability of cells expressing mutants of this region to bind VWF under static conditions in the presence of modulators, and to attach to and roll on a VWF matrix under flow. These data indicate that 2 regions within the sequence Arg218 to Tyr228 have important roles in VWF binding: the alpha1 helix has a regulatory role and the beta turn and beta13 strand bind VWF directly. Despite this, the only effect of a synthetic peptide corresponding to Leu214 to Val229 was to slightly increase the rolling velocity of GP Ibalpha-expressing Chinese hamster ovary (CHO) cells on VWF. This region thus appears to be more important for maintaining the regional conformation of GP Ibalpha, thereby facilitating the interaction with VWF.

Kinetics of GPIbalpha-vWF-A1 tether bond under flow: effect of GPIbalpha mutations on the association and dissociation rates

The interaction between platelet glycoprotein (GP) Ib-IX-V complex and von Willebrand factor (vWF) is the first step of the hemostatic response to vessel injury. In platelet-type von Willebrand disease, two mutations, G233V and M239V, have been described within the Cys209-Cys248 disulfide loop of GPIbalpha that compromise hemostasis by increasing the affinity for vWF. We have earlier shown that converting other residues in this region to valine alters the affinity of GPIbalpha for vWF, with mutations K237V and Q232V, respectively, showing the greatest increase and decrease in affinity. Here, we investigated further the effect of these two mutations on the kinetics of the GPIbalpha interaction with the vWF-A1 domain under dynamic flow conditions. We measured the cellular on- and off-rate constants of Chinese hamster ovary cells expressing GPIb-IX complexes containing wild-type or mutant GPIbalpha interacting with vWF-A1-coated surfaces at different shear stresses. We found that the gain-of-function mutant, K237V, rolled very slowly and continuously on vWF-A1 surface while the loss-of-function mutant, Q232V, showed fast, saltatory movement compared to the wild-type (WT). The off-rate constants, calculated based on the analysis of lifetimes of transient tethers formed on surfaces coated with limiting densities of vWF-A1, revealed that the Q232V and K237V dissociated 1.25-fold faster and 2.2-fold slower than the WT. The cellular on-rate constant of WT, measured in terms of tethering frequency, was threefold more and threefold less than Q232V and K237V, respectively. Thus, the gain- and loss-of-function mutations in GPIbalpha affect both the association and dissociation kinetics of the GPIbalpha-vWF-A1 bond. These findings are in contrast to the functionally similar selectin bonds where some of the mutations have been reported to affect only the dissociation rate.

Von Willebrand factor present in fibrillar collagen enhances platelet adhesion to collagen and collagen-induced platelet aggregation

We examined the basis of the differences observed between different collagen preparations in their ability to aggregate platelets and support their adhesion under flow. As in previous studies, we found fibrillar collagen to be 10-fold more potent than acid-soluble collagen in inducing platelet aggregation and found that acid-soluble collagen did not support the adhesion of washed platelets under flow. Further, platelets in whole blood adhered to surfaces coated with either fibrillar or acid-soluble collagen, but thrombi formed faster and grew larger on fibrillar collagen. As a possible basis for this difference, we found that fibrillar collagen, but not acid-soluble collagen, contains a substantial quantity of von Willebrand factor (VWF), as demonstrated by enzyme-linked immunosorbent assay and by the ability of fibrillar collagen to support the adhesion of VWF antibody-coated beads and to agglutinate GPIb-IX-V complex-expressing Chinese hamster ovary cells. Supporting a role for VWF in collagen-induced platelet aggregation, aggregation induced by acid-soluble collagen was greatly enhanced by added VWF. Further, platelet aggregation by fibrillar collagen was partially blocked by a GPIbalpha antibody that inhibits the GPIb-VWF interaction. Taken together, these results suggest that much of the difference in prothrombotic potency of different collagens is directly related to their differences in VWF content. This probably accounts for the different conclusions made regarding the relative importance of different direct and indirect collagen receptors in collagen-dependent platelet functions and further emphasizes the close synergistic roles of the GPIb-IX-V complex and the collagen receptors GPVI and alpha2beta1 in supporting platelet adhesion.

Identification of a novel point mutation in platelet glycoprotein Ibalpha, Gly to Ser at residue 233, in a Japanese family with platelet-type von Willebrand disease

BACKGROUND

Interaction between platelet glycoprotein (GP)Ibalpha and von Willebrand factor (VWF) has critical roles in both physiological hemostasis and thrombosis. Platelet-type von Willebrand disease (plt-VWD) is a congenital bleeding disorder characterized by gain-of-function mutations of GPIbalpha. To date, two mutations in GPIbalpha, G233V and M239V, have been reported in four unrelated families with plt-VWD.

OBJECTIVE

The present study aimed to determine whether G233S of GPIbalpha, a new mutation observed in plt-VWD patients, causes the plt-VWD phenotype and to examine whether conversions to other residues at this position affect VWF binding.

PATIENTS AND METHODS

The propositus was a 3-year-old Japanese male. He displayed bleeding symptoms and moderate thrombocytopenia. His brother was similarly affected. Platelets from both patients were analyzed by ristocetin- or shear-induced platelet aggregation. DNA sequencing was performed to analyze the GPIbalpha sequence. We examined the 125I-labeled VWF binding using a series of recombinant GPIbalpha fragments with different residues at position 233 (G233S, G233A, G233K, and G233D) together with naturally occurring mutations previously reported in patients (G233V and M239V).

RESULTS

Platelet function analysis indicated that platelets from both patients had a typical plt-VWD phenotype. DNA sequencing analysis showed a heterozygous mutation of Gly to Ser at residue 233 of GPIbalpha in both patients. The 125I-labeled VWF binding to mutant compared with the wild type displayed three patterns, gain-of-function (G233S, G233V, and M239V), equivalent function (G233A), and loss-of-function (G233K and G233D).

CONCLUSIONS

The G233S is a molecular basis of plt-VWD, and residue 233 plays critical roles in regulating VWF binding.

ADAMTS-13 metalloprotease interacts with the endothelial cell-derived ultra-large von Willebrand factor

Thrombotic thrombocytopenic purpura is caused by congenital or acquired deficiency of ADAMTS-13, a metalloprotease that cleaves the endothelium-derived ultra-large multimers of von Willebrand factor (ULVWF). The proteolysis converts hyper-reactive and thrombogenic ULVWF into smaller and less adhesive plasma forms. Activity of ADAMTS-13 is usually measured in a static system under non-physiological conditions that require protein denaturation and prolonged incubation. We have demonstrated previously that ULVWF multimers, upon release from endothelial cells, form platelet-decorated string-like structures that are rapidly cleaved by ADAMTS-13. Here we report the direct interaction between ADAMTS-13 and VWF under both static and flowing conditions. ADAMTS-13-coated beads adhered to both immobilized VWF and ULVWF strings presented by stimulated endothelial cells. These beads adhered to VWF under both venous (2.5 dynes/cm2) and arterial (30 dynes/cm2) shear stresses. We then demonstrated that ADAMTS-13 beads adhered to immobilized recombinant VWF-A1 and -A3 domains, but soluble metalloprotease bound preferentially to the A3 domain, suggesting that the VWF A3 domain may be the primary docking site for the metalloprotease. We suggest that tensile stresses imposed by fluid shear stretch endothelial bound ULVWF multimers to expose binding sites within the A domains for circulating ADAMTS-13. The bound enzyme then cleaves within the A2 domain that lies in close proximity and releases smaller VWF multimers into the plasma. Once released, these cleaved VWF fragments become inaccessible for the metalloprotease to prevent further cleavage.

Inherited platelet-based bleeding disorders

Inherited platelet-based bleeding disorders include abnormalities of platelet number and function, and are generally classified based on the abnormal functions or responses. However, a clear distinction is problematic, and in this review, the classification has been based on abnormalities of platelet components that share common characteristics. Inherited thrombocytopenias are rare, but probably underdiagnosed. They are usually classified according to both platelet size and the presence or absence of clinical features other than those deriving from the platelet defect. Hereditary disorders of platelet function can be classified as resulting from: (i) abnormalities of the platelet receptors for adhesive proteins; (ii) abnormalities of the platelet receptors for soluble agonists; (iii) abnormalities of the platelet granules; (iv) abnormalities of the signal-transduction pathways; (v) abnormalities of the membrane phospholipids; and (vi) miscellaneous abnormalities of platelet function. The literature on these disorders is reviewed, and the underlying defects discussed.

Alterations in the intrinsic properties of the GPIbalpha-VWF tether bond define the kinetics of the platelet-type von Willebrand disease mutation, Gly233Val

Platelet-type von Willebrand disease (PTVWD) is a bleeding disorder in which an increase of function mutation in glycoprotein Ibalpha (GPIbalpha), with respect to binding of von Willebrand factor (VWF), results in a loss of circulating high molecular weight VWF multimers together with a mild-moderate thrombocytopenia. To better ascertain the specific perturbations in adhesion associated with this disease state, we performed a detailed analysis of the kinetic and mechanical properties of tether bonds formed between PT-VWD platelets and the A1-domain of VWF. Results indicate that the GPIbalpha mutation, Gly233Val, promotes and stabilizes platelet adhesion to VWF at shear rates that do not support binding between the native receptor-ligand pair due to enhanced formation and increased longevity of the mutant tether bond (k0 off values for mutant versus native complex of 0.67 +/- 0.11 s-1 and 3.45 +/- 0.37 s-1, respectively). By contrast, the sensitivity of this interaction to an applied force, a measure of bond strength, was similar to the wild-type (WT) receptor. Although the observed alterations in the intrinsic properties of the GPIbalpha-VWF tether bond are comparable to those reported for the type 2B VWD, distinct molecular mechanisms may be responsible for these function-enhancing bleeding disorders, as interactions between the mutant receptor and mutant ligand resulted in a greater stability in platelet adhesion. We speculate that the enhanced cellular on-rate together with the prolongation in the lifetime of the mutant receptor-ligand bond contributes to platelet aggregation in circulating blood by permitting the formation of multiple GPIbalpha-VWF-A1 interactions.

ADAMTS-13 rapidly cleaves newly secreted ultralarge von Willebrand factor multimers on the endothelial surface under flowing conditions

Thrombotic thrombocytopenic purpura (TTP) is a devastating thrombotic disorder caused by widespread microvascular thrombi composed of platelets and von Willebrand factor (VWF). The disorder is associated with a deficiency of the VWF-cleaving metalloprotease, ADAMTS-13, with consequent accumulation of ultralarge (UL) VWF multimers in the plasma. ULVWF multimers, unlike plasma forms of VWF, attach spontaneously to platelet GP Ibalpha, a component of the GP Ib-IX-V complex. We have found that ULVWF multimers secreted from stimulated endothelial cells (ECs) remained anchored to the endothelial surface where platelets and Chinese hamster ovary cells expressing the GP Ib-IX-V complex attached to form long beads-on-a-string structures in the presence of fluid shear stresses in both the venous (2.5 dyne/cm(2)) and arterial (20 and 50 dyne/cm(2)) ranges. Although measurement of the activity of the ADAMTS-13 VWF-cleaving metalloprotease in vitro requires prolonged incubation of the enzyme with VWF under nonphysiologic conditions, EC-derived ULVWF strings with attached platelets were cleaved within seconds to minutes in the presence of normal plasma (containing approximately 100% ADAMTS-13 activity) or in the presence of partially purified ADAMTS-13. By contrast, the strings persisted for the entire period of perfusion (10 minutes) in the presence of plasma from patients with TTP containing 0% to 10% ADAMTS-13 activity. These results suggest that cleavage of EC-derived ULVWF multimers by ADAMTS-13 is a rapid physiologic process that occurs on endothelial cell surfaces.

Crystal structure of the platelet glycoprotein Ib(alpha) N-terminal domain reveals an unmasking mechanism for receptor activation

Glycoprotein Ib (GPIb) is a platelet receptor with a critical role in mediating the arrest of platelets at sites of vascular damage. GPIb binds to the A1 domain of von Willebrand factor (vWF-A1) at high blood shear, initiating platelet adhesion and contributing to the formation of a thrombus. To investigate the molecular basis of GPIb regulation and ligand binding, we have determined the structure of the N-terminal domain of the GPIb(alpha) chain (residues 1-279). This structure is the first determined from the cell adhesion/signaling class of leucine-rich repeat (LRR) proteins and reveals the topology of the characteristic disulfide-bonded flanking regions. The fold consists of an N-terminal beta-hairpin, eight leucine-rich repeats, a disulfide-bonded loop, and a C-terminal anionic region. The structure also demonstrates a novel LRR motif in the form of an M-shaped arrangement of three tandem beta-turns. Negatively charged binding surfaces on the LRR concave face and anionic region indicate two-step binding kinetics to vWF-A1, which can be regulated by an unmasking mechanism involving conformational change of a key loop. Using molecular docking of the GPIb and vWF-A1 crystal structures, we were also able to model the GPIb.vWF-A1 complex.

Structures of glycoprotein Ibalpha and its complex with von Willebrand factor A1 domain

Transient interactions of platelet-receptor glycoprotein Ibalpha (GpIbalpha) and the plasma protein von Willebrand factor (VWF) reduce platelet velocity at sites of vascular damage and play a role in haemostasis and thrombosis. Here we present structures of the GpIbalpha amino-terminal domain and its complex with the VWF domain A1. In the complex, GpIbalpha wraps around one side of A1, providing two contact areas bridged by an area of solvated charge interaction. The structures explain the effects of gain-of-function mutations related to bleeding disorders and provide a model for shear-induced activation. These detailed insights into the initial interactions in platelet adhesion are relevant to the development of antithrombotic drugs.

Selectin-like kinetics and biomechanics promote rapid platelet adhesion in flow: the GPIb(alpha)-vWF tether bond

The ability of platelets to tether to and translocate on injured vascular endothelium relies on the interaction between the platelet glycoprotein receptor Ib alpha (GPIb(alpha)) and the A1 domain of von Willebrand factor (vWF-A1). To date, limited information exists on the kinetics that govern platelet interactions with vWF in hemodynamic flow. We now report that the GPIb(alpha)-vWF-A1 tether bond displays similar kinetic attributes as the selectins including: 1) the requirement for a critical level of hydrodynamic flow to initiate adhesion, 2) short-lived tethering events at sites of vascular injury in vivo, and 3) a fast intrinsic dissociation rate constant, k(0)(off) (3.45 +/- 0.37 s(-1)). Values for k(off), as determined by pause time analysis of transient capture/release events, were also found to vary exponentially (4.2 +/- 0.8 s(-1) to 7.3 +/- 0.4 s(-1)) as a function of the force applied to the bond (from 36 to 217 pN). The biological importance of rapid bond dissociation in platelet adhesion is demonstrated by kinetic characterization of the A1 domain mutation, I546V that is associated with type 2B von Willebrand disease (vWD), a bleeding disorder that is due to the spontaneous binding of plasma vWF to circulating platelets. This mutation resulted in a loss of the shear threshold phenomenon, a approximately sixfold reduction in k(off), but no significant alteration in the ability of the tether bond to resist shear-induced forces. Thus, flow dependent adhesion and rapid and force-dependent kinetic properties are the predominant features of the GPIb(alpha)-vWF-A1 tether bond that in part may explain the preferential binding of platelets to vWF at sites of vascular injury, the lack of spontaneous platelet aggregation in circulating blood, and a mechanism to limit thrombus formation.

Site-directed mutagenesis of platelet glycoprotein Ib alpha 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.

Functional analysis of the C-terminal flanking sequence of platelet glycoprotein Ib alpha using canine-human chimeras

Platelet glycoprotein Ib-IX-V (GPIb-IX-V) mediates adhesion to von Willebrand factor (vWF) in (patho)physiological thrombus formation. vWF binds the N-terminal 282 residues of GPIb alpha, consisting of an N-terminal flank (His1-Ile35), 7 leucine-rich repeats (Leu36-Ala200), a C-terminal flank (Phe201-Gly268), and a sulfated tyrosine sequence (Asp269-Glu282). By expressing canine-human chimeras of GPIb alpha on Chinese hamster ovary cells, binding sites for functional anti-GPIb alpha antibodies to individual domains were previously mapped, and it was shown that leucine-rich repeats 2 to 4 were required for optimal vWF recognition under static or flow conditions. Using novel canine-human chimeras dissecting the C-terminal flank, it is now demonstrated that (1) Phe201-Glu225 contains the epitope for AP1, an anti-GPIb alpha monoclonal antibody that inhibits both ristocetin- and botrocetin-dependent vWF binding; (2) VM16d, an antibody that preferentially inhibits botrocetin-dependent vWF binding, recognizes the sequence Val226-Gly268, surrounding Cys248, which forms a disulfide-bond with Cys209; (3) vWF binding to chimeric GPIb alpha is comparable to wild-type in 2 chimeras in which the sixth leucine-rich repeat was of the same species as the first disulfide loop (Phe201-Cys248) of the C-terminal flank, suggesting an interaction between these domains may be important for optimal vWF binding; and (4) replacing the C-terminal flank second disulfide loop (Asp249-Gly268) in human GPIb alpha with the corresponding canine sequence enhanced vWF binding under static and flow conditions, providing the first evidence for a gain-of-function phenotype associated with the second loop of the C-terminal flank.

A point mutation in the cysteine-rich domain of glycoprotein (GP) IIIa results in the expression of a GPIIb-IIIa (alphaIIbbeta3) integrin receptor locked in a high-affinity state and a Glanzmann thrombasthenia-like phenotype

This article reports a Glanzmann thrombasthenia (GT) patient, N.M., with a point mutation in the third cysteine-rich repeat of beta3-integrin or platelet glycoprotein (GP) IIIa, leading to the expression of a constitutively activated fibrinogen receptor. The diagnosis of GT was based on a severely reduced platelet-aggregation response to a series of agonists and approximately 20% of surface-expressed GPIIb-IIIa. The patient's GPIIb-IIIa constitutively expressed epitopes recognized by antibodies to ligand-induced binding sites (LIBS) and also spontaneously bound the fibrinogen-mimetic antibody, PAC-1. Furthermore, significant amounts of bound fibrinogen were detected on his platelets ex vivo. No signs of platelet activation were observed on sections of unstimulated platelets from N.M. by electron microscopy. Immunogold labeling highlighted the presence of surface-bound fibrinogen but revealed platelet heterogeneity with regard to the surface density. When the patient's platelets were stimulated by thrombin-receptor activating peptide, amounts of surface-expressed GPIIb-IIIa increased and the aggregation response improved, although it failed to normalize. Platelets from N.M. were able to adhere and spread on immobilized fibrinogen. Sequence analysis of genomic DNA from N.M. revealed a homozygous g1776T>C mutation in GPIIIa, leading to a Cys560Arg amino acid substitution. A stable Chinese hamster ovary (CHO) cell line was prepared expressing surface GPIIb-Arg560IIIa. Like platelets from the patient, GPIIb-Arg560IIIa-transfected CHO cells constitutively bound LIBS antibodies and PAC-1. They also showed an enhanced ability to adhere on surface-bound fibrinogen. Overall, these data demonstrate that a gain-of-function mutation can still be associated with a thrombasthenic phenotype even though platelets show spontaneous fibrinogen binding.

Phenotype changes resulting in high-affinity binding of von Willebrand factor to recombinant glycoprotein Ib-IX: analysis of the platelet-type von Willebrand disease mutations

To maintain hemostasis under shear conditions, there must be an interaction between the platelet glycoprotein (GP) Ib-IX receptor and the plasma ligand von Willebrand factor (vWf). In platelet-type von Willebrand disease (Pt-vWD), hemostasis is compromised. Two mutations in the GPIbalpha polypeptide chain have been identified in these patients-a glycine-233 to valine change and a methionine-239 to valine change. For this investigation, these mutant proteins have been expressed in a Chinese hamster ovary cell model system. Ligand-binding studies were performed at various concentrations of ristocetin, and adhesion assays were performed under flow conditions. The Pt-vWD mutations resulted in a gain-of-function receptor. vWf binding was increased at all concentrations of ristocetin examined, and adhesion on a vWf matrix was enhanced in terms of cell tethering, slower rolling velocity, and decreased detachment with increasing shear rate. Two other mutations were also introduced into the GPIbalpha chain. One mutation, encompassing both the Pt-vWD mutations, created an increase in the hydrophobicity of this region. The second mutation, involving a valine-234 to glycine change, decreased the hydrophobicity of this region. Both mutations also resulted in a gain-of-function receptor, with the double mutation producing a hyperreactive receptor for vWf. These data further support the hypothesis that ligand binding is regulated by conformational changes in the amino-terminal region of GPIbalpha, thereby influencing the stability of the GPIbalpha-vWf interaction.