Crystal structure of the wild-type von Willebrand factor A1-glycoprotein Ibalpha complex reveals conformation differences with a complex bearing von Willebrand disease mutations

Surfing the Protein-Protein Interaction Surface Using Docking Methods: Application to the Design of PPI Inhibitors

Blocking protein-protein interactions (PPI) using small molecules or peptides modulates biochemical pathways and has therapeutic significance. PPI inhibition for designing drug-like molecules is a new area that has been explored extensively during the last decade. Considering the number of available PPI inhibitor databases and the limited number of 3D structures available for proteins, docking and scoring methods play a major role in designing PPI inhibitors as well as stabilizers. Docking methods are used in the design of PPI inhibitors at several stages of finding a lead compound, including modeling the protein complex, screening for hot spots on the protein-protein interaction interface and screening small molecules or peptides that bind to the PPI interface. There are three major challenges to the use of docking on the relatively flat surfaces of PPI. In this review we will provide some examples of the use of docking in PPI inhibitor design as well as its limitations. The combination of experimental and docking methods with improved scoring function has thus far resulted in few success stories of PPI inhibitors for therapeutic purposes. Docking algorithms used for PPI are in the early stages, however, and as more data are available docking will become a highly promising area in the design of PPI inhibitors or stabilizers.

Adhesive Forces between A1 Domain of von Willebrand Factor and N-terminus Domain of Glycoprotein Ibα Measured by Atomic Force Microscopy

AIM

von Willebrand factor (VWF) plays an important role in the regulation of hemostasis and thrombosis formation, particularly under a high shear rate. However, the adhesive force due to the molecular interaction between VWF and glycoprotein Ibα (GPIbα) has not been fully explored. Thus, we employed atomic force microscopy to directly measure the adhesive force between VWF and GPIbα.

METHODS

We measured the adhesive force between VWF and GPIbα at the molecular level using an atomic force microscope (AFM). An AFM cantilever was coated with recombinant N-terminus VWF binding site of GPIbα, whereas a cover glass was coated with native VWF.

RESULTS

The adhesive force at the molecular level was measured using an AFM. In the presence of 1 μg/mL VWF, the adhesion force was nearly 200 pN. As per the Gaussian fit analysis, the adhesive force of a single bond could have been 54 or 107 pN.

CONCLUSION

Our consideration with the Gaussian fit analysis proposed that the adhesive force of a single bond could be 54 pN, which is very close to that obtained by optical tweezers (50 pN).

Force-sensitive autoinhibition of the von Willebrand factor is mediated by interdomain interactions

Von Willebrand factor (VWF) plays a central role in hemostasis. Triggered by shear-stress, it adheres to platelets at sites of vascular injury. Inactivation of VWF has been associated to the shielding of its adhesion sites and proteolytic cleavage. However, the molecular nature of this shielding and its coupling to cleavage under shear-forces in flowing blood remain unknown. In this study, we describe, to our knowledge, a new force-sensory mechanism for VWF-platelet binding, which addresses these questions, based on a combination of molecular dynamics (MD) simulations, atomic force microscopy (AFM), and microfluidic experiments. Our MD simulations demonstrate that the VWF A2 domain targets a specific region at the VWF A1 domain, corresponding to the binding site of the platelet glycoprotein Ibα (GPIbα) receptor, thereby causing its blockage. This implies autoinhibition of the VWF for the binding of platelets mediated by the A1-A2 protein-protein interaction. During force-probe MD simulations, a stretching force dissociated the A1A2 complex, thereby unblocking the GPIbα binding site. Dissociation was found to be coupled to the unfolding of the A2 domain, with dissociation predominantly occurring before exposure of the cleavage site in A2, an observation that is supported by our AFM experiments. This suggests that the A2 domain prevents platelet binding in a force-dependent manner, ensuring that VWF initiates hemostasis before inactivation by proteolytic cleavage. Microfluidic experiments with an A2-deletion VWF mutant resulted in increased platelet binding, corroborating the key autoinhibitory role of the A2 domain within VWF multimers. Overall, autoinhibition of VWF mediated by force-dependent interdomain interactions offers the molecular basis for the shear-sensitive growth of VWF-platelet aggregates, and might be similarly involved in shear-induced VWF self-aggregation and other force-sensing functions in hemostasis.

Force-induced on-rate switching and modulation by mutations in gain-of-function von Willebrand diseases

Mutations in the ultralong vascular protein von Willebrand factor (VWF) cause the common human bleeding disorder, von Willebrand disease (VWD). The A1 domain in VWF binds to glycoprotein Ibα (GPIbα) on platelets, in a reaction triggered, in part, by alterations in flow during bleeding. Gain-of-function mutations in A1 and GPIbα in VWD suggest conformational regulation. We report that force application switches A1 and/or GPIbα to a second state with faster on-rate, providing a mechanism for activating VWF binding to platelets. Switching occurs near 10 pN, a force that also induces a state of the receptor-ligand complex with slower off-rate. Force greatly increases the effects of VWD mutations, explaining pathophysiology. Conversion of single molecule kon (s(-1)) to bulk phase kon (s(-1)M(-1)) and the kon and koff values extrapolated to zero force for the low-force pathways show remarkably good agreement with bulk-phase measurements.

Interaction of von Willebrand factor with platelets and the vessel wall

The initiation of thrombus formation at sites of vascular injury to secure haemostasis after tissue trauma requires the interaction of surface-exposed von Willebrand factor (VWF) with its primary platelet receptor, the glycoprotein (GP) Ib-IX-V complex. As an insoluble component of the extracellular matrix (ECM) of endothelial cells, VWF can directly initiate platelet adhesion. Circulating plasma VWF en-hances matrix VWF activity by binding to structures that become exposed to flowing blood, notably collagen type I and III in deeper layers of the vessel along with microfibrillar collagen type VI in the subendothelium. Moreover, plasma VWF is required to support platelet-to-platelet adhesion - i. e. aggregation - which promotes thrombus growth and consolidation. For these reasons, understanding how plasma VWF interaction with platelet receptors is regulated, particularly any distinctive features of GPIb binding to soluble as opposed to immobilized VWF, is of paramount importance in vascular biology. This brief review will highlight knowledge acquired and key problems that remain to be solved to elucidate fully the role of VWF in normal haemostasis and pathological thrombosis.

Identification of a juxtamembrane mechanosensitive domain in the platelet mechanosensor glycoprotein Ib-IX complex

How glycoprotein (GP)Ib-IX complex on the platelet surface senses the blood flow through its binding to the plasma protein von Willebrand factor (VWF) and transmits a signal into the platelet remains unclear. Here we show that optical tweezer-controlled pulling of the A1 domain of VWF (VWF-A1) on GPIb-IX captured by its cytoplasmic domain induced unfolding of a hitherto unidentified structural domain before the dissociation of VWF-A1 from GPIb-IX. Additional studies using recombinant proteins and mutant complexes confirmed its existence in GPIb-IX and enabled localization of this quasi-stable mechanosensitive domain of ∼60 residues between the macroglycopeptide region and the transmembrane helix of the GPIbα subunit. These results suggest that VWF-mediated pulling under fluid shear induces unfolding of the mechanosensitive domain in GPIb-IX, which may possibly contribute to platelet mechanosensing and/or shear resistance of VWF-platelet interaction. The identification of the mechanosensitive domain in GPIb-IX has significant implications for the pathogenesis and treatment of related blood diseases.

Platelet GpIba binding to von Willebrand Factor under fluid shear:contributions of the D′D3-domain, A1-domain flanking peptide and O-linked glycans

BACKGROUND

Von Willebrand Factor (VWF) A1-domain binding to platelet receptor GpIbα is an important fluid-shear dependent interaction that regulates both soluble VWF binding to platelets, and platelet tethering onto immobilized VWF. We evaluated the roles of different structural elements at the N-terminus of the A1-domain in regulating shear dependent platelet binding. Specifically, the focus was on the VWF D'D3-domain, A1-domain N-terminal flanking peptide (NFP), and O-glycans on this peptide.

METHODS AND RESULTS

Full-length dimeric VWF (ΔPro-VWF), dimeric VWF lacking the D'D3 domain (ΔD'D3-VWF), and ΔD'D3-VWF variants lacking either the NFP (ΔD'D3NFP(─)-VWF) or just O-glycans on this peptide (ΔD'D3OG(─)-VWF) were expressed. Monomeric VWF-A1 and D'D3-A1 were also produced. In ELISA, the apparent dissociation constant (KD) of soluble ΔPro-VWF binding to immobilized GpIbα (KD≈100 nmol/L) was 50- to 100-fold higher than other proteins lacking the D'D3 domain (KD~0.7 to 2.5 nmol/L). Additionally, in surface plasmon resonance studies, the on-rate of D'D3-A1 binding to immobilized GpIbα (kon=1.8±0.4×10(4) (mol/L)(-1)·s(-1); KD=1.7 μmol/L) was reduced compared with the single VWF-A1 domain (kon=5.1±0.4×10(4) (mol/L)(-1)·s(-1); KD=1.2 μmol/L). Thus, VWF-D'D3 primarily controls soluble VWF binding to GpIbα. In contrast, upon VWF immobilization, all molecular features regulated A1-GpIbα binding. Here, in ELISA, the number of apparent A1-domain sites available for binding GpIbα on ΔPro-VWF was ≈50% that of the ΔD'D3-VWF variants. In microfluidics based platelet adhesion measurements on immobilized VWF and thrombus formation assays on collagen, human platelet recruitment varied as ΔPro-VWF<ΔD'D3-VWF<ΔD'D3NFP(─)-VWF<ΔD'D3OG(─)-VWF.

CONCLUSIONS

Whereas VWF-D'D3 is the major regulator of soluble VWF binding to platelet GpIbα, both the D'D3-domain and N-terminal peptide regulate platelet translocation and thrombus formation.

Exploiting the kinetic interplay between GPIbα-VWF binding interfaces to regulate hemostasis and thrombosis

Platelet-von Willebrand factor (VWF) interactions must be tightly regulated in order to promote effective hemostasis and prevent occlusive thrombus formation. However, it is unclear what role the inherent properties of the bond formed between the platelet receptor glycoprotein Ibα and the A1 domain of VWF play in these processes. Using VWF-A1 knock-in mice with mutations that enhance (I1309V) or disrupt (R1326H) platelet receptor glycoprotein Ibα binding, we now demonstrate that the kinetic interplay between two distinct contact surfaces influences the site and extent to which platelets bind VWF. Incorporation of R1326H mutation into the major site shortened bond lifetime, yielding defects in hemostasis and thrombosis comparable to VWF-deficient animals. Similarly, disrupting this region of contact with an allosteric inhibitor impaired human platelet accrual in damaged arterioles. In contrast, the I1309V mutation near the minor site prolonged bond lifetime, which was essential for the development of a type 2B-like VWD phenotype. However, combining the R1326H and I1309V mutations normalized both bond kinetics and the hemostatic and thrombotic properties of VWF. These findings broaden our understanding of mechanisms governing platelet-VWF interactions in health and disease, and underscore the importance of combined biophysical and genetic approaches in identifying potential therapeutic avenues for treating bleeding and thrombotic disorders.

The structure of the TFIIH p34 subunit reveals a von Willebrand factor A like fold

RNA polymerase II dependent transcription and nucleotide excision repair are mediated by a multifaceted interplay of subunits within the general transcription factor II H (TFIIH). A better understanding of the molecular structure of TFIIH is the key to unravel the mechanism of action of this versatile protein complex within these vital cellular processes. The importance of this complex becomes further evident in the context of severe diseases like xeroderma pigmentosum, Cockayne's syndrome and trichothiodystrophy, that arise from single point mutations in TFIIH subunits. Here we describe the structure of the p34 subunit of the TFIIH complex from the eukaryotic thermophilic fungus Chaetomium thermophilum. The structure revealed that p34 contains a von Willebrand Factor A (vWA) like domain, a fold which is generally known to be involved in protein-protein interactions. Within TFIIH p34 strongly interacts with p44, a positive regulator of the helicase XPD. Putative protein-protein interfaces are analyzed and possible binding sites for the p34-p44 interaction suggested.

von Willebrand factor, Jedi knight of the bloodstream

When blood vessels are cut, the forces in the bloodstream increase and change character. The dark side of these forces causes hemorrhage and death. However, von Willebrand factor (VWF), with help from our circulatory system and platelets, harnesses the same forces to form a hemostatic plug. Force and VWF function are so closely intertwined that, like members of the Jedi Order in the movie Star Wars who learn to use "the Force" to do good, VWF may be considered the Jedi knight of the bloodstream. The long length of VWF enables responsiveness to flow. The shape of VWF is predicted to alter from irregularly coiled to extended thread-like in the transition from shear to elongational flow at sites of hemostasis and thrombosis. Elongational force propagated through the length of VWF in its thread-like shape exposes its monomers for multimeric binding to platelets and subendothelium and likely also increases affinity of the A1 domain for platelets. Specialized domains concatenate and compact VWF during biosynthesis. A2 domain unfolding by hydrodynamic force enables postsecretion regulation of VWF length. Mutations in VWF in von Willebrand disease contribute to and are illuminated by VWF biology. I attempt to integrate classic studies on the physiology of hemostatic plug formation into modern molecular understanding, and point out what remains to be learned.

An update on type 2B von Willebrand disease

Type 2B von Willebrand disease (VWD) accounts for fewer than 5% of all VWD patients. In this disease, mutations in the A1 domain result in increased von Willebrand factor (VWF) binding to platelet GPIbα receptors, causing increased platelet clearance and preferential loss of high molecular weight VWF multimers. Diagnosis is complicated because of significant clinical variations even among patients with identical mutations. Platelet transfusion often provides suboptimal results since transfused platelets may be aggregated by the patients' abnormal VWF. Desmopressin may cause a transient decrease in platelet count that could lead to an increased risk of bleeding. Replacement therapy with factor VIII/VWF concentrates is the most effective approach to prevention and treatment of bleeding in type 2B VWD.

The linker between the D3 and A1 domains of vWF suppresses A1-GPIbα catch bonds by site-specific binding to the A1 domain

Platelet attachment to von Willebrand factor (vWF) requires the interaction between the platelet GP1bα and exposed vWF-A1 domains. Structural insights into the mechanism of the A1-GP1bα interaction have been limited to an N-terminally truncated A1 domain that lacks residues Q1238  - E1260 that make up the linker between the D3 and A1 domains of vWF. We have demonstrated that removal of these residues destabilizes quaternary interactions in the A1A2A3 tridomain and contributes to platelet activation under high shear (Auton et al., J Biol Chem 2012;287:14579-14585). In this study, we demonstrate that removal of these residues from the single A1 domain enhances platelet pause times on immobilized A1 under rheological shear. A rigorous comparison between the truncated A1-1261 and full length A1-1238 domains demonstrates a kinetic stabilization of the A1 domain induced by these N-terminal residues as evident in the enthalpy of the unfolding transition. This stabilization occurs through site and sequence-specific binding of the N-terminal peptide to A1. Binding of free N-terminal peptide to A1-1261 has an affinity KD=46±6μM and this binding although free to dissociate is sufficient to suppress the platelet pause times to levels comparable to A1-1238 under shear stress. Our results support a dual-structure/function role for this linker region involving a conformational equilibria that maintains quaternary A domain associations in the inactive state of vWF at low shear and an intra-A1-domain conformation that regulates the strength of platelet GP1bα-vWF A1 domain associations in the active state of vWF at high shear.

Structural basis of regulation of von Willebrand factor binding to glycoprotein Ib

Activation by elongational flow of von Willebrand factor (VWF) is critical for primary hemostasis. Mutations causing type 2B von Willebrand disease (VWD), platelet-type VWD (PT-VWD), and tensile force each increase affinity of the VWF A1 domain and platelet glycoprotein Ibα (GPIbα) for one another; however, the structural basis for these observations remains elusive. Directed evolution was used to discover a further gain-of-function mutation in A1 that shifts the long range disulfide bond by one residue. We solved multiple crystal structures of this mutant A1 and A1 containing two VWD mutations complexed with GPIbα containing two PT-VWD mutations. We observed a gained interaction between A1 and the central leucine-rich repeats (LRRs) of GPIbα, previously shown to be important at high shear stress, and verified its importance mutationally. These findings suggest that structural changes, including central GPIbα LRR-A1 contact, contribute to VWF affinity regulation. Among the mutant complexes, variation in contacts and poor complementarity between the GPIbα β-finger and the region of A1 harboring VWD mutations lead us to hypothesize that the structures are on a pathway to, but have not yet reached, a force-induced super high affinity state.

Structure and function of platelet receptors initiating blood clotting

At the clinical level, recent studies reveal the link between coagulation and other pathophysiological processes, including platelet activation, inflammation, cancer, the immune response, and/or infectious diseases. These links are likely to underpin the coagulopathy associated with risk factors for venous thromboembolic (VTE) and deep vein thrombosis (DVT). At the molecular level, the interactions between platelet-specific receptors and coagulation factors could help explain coagulopathy associated with aberrant platelet function, as well as revealing new approaches targeting platelet receptors in diagnosis or treatment of VTE or DVT. Glycoprotein (GP)Ibα, the major ligand-binding subunit of the platelet GPIb-IX-V complex, that binds the adhesive ligand, von Willebrand factor (VWF), is co-associated with the platelet-specific collagen receptor, GPVI. The GPIb-IX-V/GPVI adheso-signaling complex not only initiates platelet activation and aggregation (thrombus formation) in response to vascular injury or disease but GPIbα also regulates coagulation through a specific interaction with thrombin and other coagulation factors. Here, we discuss the structure and function of key platelet receptors involved in thrombus formation and coagulation in health and disease, with a particular focus on platelet GPIbα.

GpIbα interacts exclusively with exosite II of thrombin

Activation of platelets by the serine protease thrombin is a critical event in haemostasis. This process involves the binding of thrombin to glycoprotein Ibα (GpIbα) and cleavage of protease-activated receptors (PARs). The N-terminal extracellular domain of GpIbα contains an acidic peptide stretch that has been identified as the main thrombin binding site, and both anion binding exosites of thrombin have been implicated in GpIbα binding, but it remains unclear how they are involved. This issue is of critical importance for the mechanism of platelet activation by thrombin. If both exosites bind to GpIbα, thrombin could potentially act as a platelet adhesion molecule or receptor dimerisation trigger. Alternatively, if only a single site is involved, GpIbα may serve as a cofactor for PAR-1 activation by thrombin. To determine the involvement of thrombin's two exosites in GpIbα binding, we employed the complementary methods of mutational analysis, binding studies, X-ray crystallography and NMR spectroscopy. Our results indicate that the peptide corresponding to the C-terminal portion of GpIbα and the entire extracellular domain bind exclusively to thrombin's exosite II. The interaction of thrombin with GpIbα thus serves to recruit thrombin activity to the platelet surface while leaving exosite I free for PAR-1 recognition.

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We analysed interactions of the platelet receptor GpIbα with thrombin using three complementary methods.

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GpIbα exclusively binds to exosite II of thrombin.

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Exosite I remains available for binding to other ligands.

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GpIbα recruits thrombin to the platelet membrane as a cofactor for PAR-1 cleavage.

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.

Functional display of platelet-binding VWF fragments on filamentous bacteriophage

von Willebrand factor (VWF) tethers platelets to sites of vascular injury via interaction with the platelet surface receptor, GPIb. To further define the VWF sequences required for VWF-platelet interaction, a phage library displaying random VWF protein fragments was screened against formalin-fixed platelets. After 3 rounds of affinity selection, DNA sequencing of platelet-bound clones identified VWF peptides mapping exclusively to the A1 domain. Aligning these sequences defined a minimal, overlapping segment spanning P1254–A1461, which encompasses the C1272–C1458 cystine loop. Analysis of phage carrying a mutated A1 segment (C1272/1458A) confirmed the requirement of the cystine loop for optimal binding. Four rounds of affinity maturation of a randomly mutagenized A1 phage library identified 10 and 14 unique mutants associated with enhanced platelet binding in the presence and absence of botrocetin, respectively, with 2 mutants (S1370G and I1372V) common to both conditions. These results demonstrate the utility of filamentous phage for studying VWF protein structure-function and identify a minimal, contiguous peptide that bind to formalin-fixed platelets, confirming the importance of the VWF A1 domain with no evidence for another independently platelet-binding segment within VWF. These findings also point to key structural elements within the A1 domain that regulate VWF-platelet adhesion.

The type 2B p.R1306W natural mutation of von Willebrand factor dramatically enhances the multimer sensitivity to shear stress

BACKGROUND

Shear stress triggers conformational stretching of von Willebrand factor (VWF), which is responsible for its self-association and binding to the platelet receptor glycoprotein (GP)Ibα. This phenomenon supports primary hemostasis under flow. Type 2B VWF natural mutants are considered to have increased affinity for platelet GPIbα.

OBJECTIVES

To assess the mechanism responsible for the enhanced interaction of the p.R1306W VWF mutant with the platelet receptor.

METHODS

The interaction of GPIbα with wild-type (WT) and p.R1306W VWF multimers and A1-A2-A3 constructs was investigated with surface plasmon resonance spectroscopy. Analysis of the static VWF conformation in solution was performed with dynamic light scattering spectroscopy. The shear stress-induced self-association of VWF multimers was investigated with atomic force microscopy (AFM) over a 0-60 dyn cm(-2) range.

RESULTS

WT VWF did not interact with GPIbα under static conditions, whereas the mutant at ~ 2 μg mL(-1) already bound to the receptor. By contrast, the WT and p.R1306W-A1-A2-A3 constructs showed comparable affinities for GPIbα (Kd  ~ 20 nm). The hydrodynamic diameter of resting R1306W VWF multimers was significantly greater than that of the wild type (210 ± 60 nm vs. 87 ± 22 nm). At shear forces of < 14 dyn cm(-2) , the p.R1306W multimers rapidly changed conformation, entering a regime of self-aggregation, which, in contrast, was induced for WT VWF by shear forces of > 30 dyn cm(-2) . Mechanical stretching AFM experiments showed that p.R1306W multimers needed less energy per length unit (~ 10 pN) to be stretched than the WT protein.

CONCLUSIONS

The increased affinity of p.R1306W VWF for GPIbα arises mostly from higher sensitivity to shear stress, which facilitates exposure of GPIbα binding sites.

A mechanism for localized dynamics-driven affinity regulation of the binding of von Willebrand factor to platelet glycoprotein Ibα

Binding of the A1 domain of von Willebrand factor (vWF) to glycoprotein Ibα (GPIbα) results in platelet adhesion, activation, and aggregation that initiates primary hemostasis. Both the elevated shear stress and the mutations associated with type 2B von Willebrand disease enhance the interaction between A1 and GPIbα. Through molecular dynamics simulations for wild-type vWF-A1 and its eight gain of function mutants (R543Q, I546V, ΔSS, etc.), we found that the gain of function mutations destabilize the N-terminal arm, increase a clock pendulum-like movement of the α2-helix, and turn a closed A1 conformation into a partially open one favoring binding to GPIbα. The residue Arg(578) at the α2-helix behaves as a pivot in the destabilization of the N-terminal arm and a consequent dynamic change of the α2-helix. These results suggest a localized dynamics-driven affinity regulation mechanism for vWF-GPIbα interaction. Allosteric drugs controlling this intrinsic protein dynamics may be effective in blocking the GPIb-vWF interaction.

On the versatility of von Willebrand factor

Von Willebrand factor (VWF) is a large multimeric protein, the function of which has been demonstrated to be pivotal to the haemostatic system. Indeed, quantitative and/or qualitative abnormalities of VWF are associated with the bleeding disorder Von Willebrand disease (VWD). Moreover, increased plasma concentrations of VWF have been linked to an increased risk for thrombotic complications. In the previous decades, many studies have contributed to our understanding of how VWF is connected to the haemostatic system, particularly with regard to structure-function relationships. Interactive sites for important ligands of VWF (such as factor VIII, collagen, glycoprotein Ibα, integrin αIIbβ3 and protease ADAMTS13) have been identified, and mutagenesis studies have confirmed the physiological relevance of the interactions between VWF and these ligands. However, we have also become aware that VWF has a more versatile character than previously thought, given its potential role in various non-hemostatic processes, like intimal thickening, tumor cell apoptosis and inflammatory processes. In the presence review, a summary of our knowledge on VWF structure-function relationships is provided in the context of the “classical” haemostatic task of VWF and in perspective of pathological processes beyond haemostasis.

Multiscale model of platelet translocation and collision

The tethering of platelets on the injured vessel surface mediated by glycoprotein Ibα (GPIbα) - Von Willebrand factor (vWF) bonds, as well as the interaction between flowing platelets and adherent platelets, are two key events that take place immediately following blood vessel injury. This early-stage platelet deposition and accumulation triggers the initiation of hemostasis, a self-defensive mechanism to prevent the body from excessive blood loss. To understand and predict this complex process, one must integrate experimentally determined information on the mechanics and biochemical kinetics of participating receptors over very small time frames (1-1000 µs) and length scales (10-100 nm), to collective phenomena occurring over seconds and tens of microns. In the present study, a unique three dimensional multiscale computational model, platelet adhesive dynamics (PAD), was applied to elucidate the unique physics of (i) a non-spherical, disk-shaped platelet interacting and tethering onto the damaged vessel wall followed by (ii) collisional interactions between a flowing platelet with a downstream adherent platelet. By analyzing numerous simulations under different physiological conditions, we conclude that the platelet's unique spheroid-shape provides heterogeneous, orientation-dependent translocation (rolling) behavior which enhances cell-wall interactions. We also conclude that platelet-platelet near field interactions are critical for cell-cell communication during the initiation of microthrombi. The PAD model described here helps to identify the physical factors that control the initial stages of platelet capture during this process.

Osteoprotegerin: multiple partners for multiple functions

Osteoprotegerin (OPG) is an essential secreted protein in bone turnover due to its role as a decoy receptor for the Receptor Activator of Nuclear Factor-kB ligand (RANKL) in the osteoclasts, thus inhibiting their differentiation. However, there are additional ligands of OPG that confer various biological functions. OPG can promote cell survival, cell proliferation and facilitates migration by binding TNF-related apoptosis inducing ligand (TRAIL), glycosaminoglycans or proteoglycans. A large number of in vitro, pre-clinical and clinical studies provide evidences of OPG involvement in vascular, bone, immune and tumor biology. This review describes an overview of the different OPG ligands regulating its biological functions.

Platelet interaction with von Willebrand factor is enhanced by shear-induced clustering of glycoprotein Ibα

Initial platelet arrest at the exposed arterial vessel wall is mediated through glycoprotein Ibα binding to the A1 domain of von Willebrand factor. This interaction occurs at sites of elevated shear force, and strengthens upon increasing hydrodynamic drag. The increased interaction requires shear-dependent exposure of the von Willebrand factor A1 domain, but the contribution of glycoprotein Ibα remains ill defined. We have previously found that glycoprotein Ibα forms clusters upon platelet cooling and hypothesized that such a property enhances the interaction with von Willebrand factor under physiological conditions. We analyzed the distribution of glycoprotein Ibα with Förster resonance energy transfer using time-gated fluorescence lifetime imaging microscopy. Perfusion at a shear rate of 1,600 s(-1) induced glycoprotein Ibα clusters on platelets adhered to von Willebrand factor, while clustering did not require von Willebrand factor contact at 10,000 s(-1). Shear-induced clustering was reversible, not accompanied by granule release or αIIbβ3 activation and improved glycoprotein Ibα-dependent platelet interaction with von Willebrand factor. Clustering required glycoprotein Ibα translocation to lipid rafts and critically depended on arachidonic acid-mediated binding of 14-3-3ζ to its cytoplasmic tail. This newly identified mechanism emphasizes the ability of platelets to respond to mechanical force and provides new insights into how changes in hemodynamics influence arterial thrombus formation.

The methionine-aromatic motif plays a unique role in stabilizing protein structure

Of the 20 amino acids, the precise function of methionine (Met) remains among the least well understood. To establish a determining characteristic of methionine that fundamentally differentiates it from purely hydrophobic residues, we have used in vitro cellular experiments, molecular simulations, quantum calculations, and a bioinformatics screen of the Protein Data Bank. We show that approximately one-third of all known protein structures contain an energetically stabilizing Met-aromatic motif and, remarkably, that greater than 10,000 structures contain this motif more than 10 times. Critically, we show that as compared with a purely hydrophobic interaction, the Met-aromatic motif yields an additional stabilization of 1-1.5 kcal/mol. To highlight its importance and to dissect the energetic underpinnings of this motif, we have studied two clinically relevant TNF ligand-receptor complexes, namely TRAIL-DR5 and LTα-TNFR1. In both cases, we show that the motif is necessary for high affinity ligand binding as well as function. Additionally, we highlight previously overlooked instances of the motif in several disease-related Met mutations. Our results strongly suggest that the Met-aromatic motif should be exploited in the rational design of therapeutics targeting a range of proteins.

Mapping paratope on antithrombotic antibody 6B4 to epitope on platelet glycoprotein Ibalpha via molecular dynamic simulations

Binding of platelet receptor glycoprotein Ibα (GPIbα) to the A1 domain of von Willebrand factor (vWF) is a critical step in both physiologic hemostasis and pathologic thrombosis, for initiating platelet adhesion to subendothelium of blood vessels at sites of vascular injury. Gain-of-function mutations in GPIbα contribute to an abnormally high-affinity binding of platelets to vWF and can lead to thrombosis, an accurate complication causing heart attack and stroke. Of various antithrombotic monoclonal antibodies (mAbs) targeting human GPIbα, 6B4 is a potent one to inhibit the interaction between GPIbα and vWF-A1 under static and flow conditions. Mapping paratope to epitope with mutagenesis experiments, a traditional route in researches of these antithrombotic mAbs, is usually expensive and time-consuming. Here, we suggested a novel computational procedure, which combines with homology modeling, rigid body docking, free and steered molecular dynamics (MD) simulations, to identify key paratope residues on 6B4 and their partners on GPIbα, with hypothesis that the stable hydrogen bonds and salt bridges are the important linkers between paratope and epitope residues. Based on a best constructed model of 6B4 bound with GPIbα, the survival ratios and rupture times of all detected hydrogen bonds and salt bridges in binding site were examined via free and steered MD simulations and regarded as indices of thermal and mechanical stabilizations of the bonds, respectively. Five principal paratope residues with their partners were predicted with their high survival ratios and/or long rupture times of involved hydrogen bonds, or with their hydrogen bond stabilization indices ranked in top 5. Exciting, the present results were in good agreement with previous mutagenesis experiment data, meaning a wide application prospect of our novel computational procedure on researches of molecular of basis of ligand-receptor interactions, various antithrombotic mAbs and other antibodies as well as theoretically design of biomolecular drugs.

Identification of a small molecule that modulates platelet glycoprotein Ib-von Willebrand factor interaction

The von Willebrand factor (VWF) A1-glycoprotein (GP) Ibα interaction is of major importance during thrombosis mainly at sites of high shear stress. Inhibitors of this interaction prevent platelet-dependent thrombus formation in vivo, without major bleeding complications. However, the size and/or protein nature of the inhibitors currently in development limit oral bioavailability and clinical development. We therefore aimed to search for a small molecule protein-protein interaction inhibitor interfering with the VWF-GPIbα binding. After determination of putative small molecule binding pockets on the surface of VWF-A1 and GPIbα using site-finding algorithms and molecular dynamics, high throughput molecular docking was performed on both binding partners. A selection of compounds showing good in silico docking scores into the predicted pockets was retained for testing their in vitro effect on VWF-GPIbα complex formation, by which we identified a compound that surprisingly stimulated the VWF-GPIbα binding in a ristocetin cofactor ELISA and increased platelet adhesion in whole blood to collagen under arterial shear rate but in contrast inhibited ristocetin-induced platelet aggregation. The selected compound adhering to the predicted binding partner GPIbα could be confirmed by saturation transfer difference NMR spectroscopy. We thus clearly identified a small molecule that modulates VWF-GPIbα binding and that will now serve as a starting point for further studies and chemical modifications to fully characterize the interaction and to manipulate specific activity of the compound.

Extracellular matrix proteins in hemostasis and thrombosis

The adhesion and aggregation of platelets during hemostasis and thrombosis represents one of the best-understood examples of cell-matrix adhesion. Platelets are exposed to a wide variety of extracellular matrix (ECM) proteins once blood vessels are damaged and basement membranes and interstitial ECM are exposed. Platelet adhesion to these ECM proteins involves ECM receptors familiar in other contexts, such as integrins. The major platelet-specific integrin, αIIbβ3, is the best-understood ECM receptor and exhibits the most tightly regulated switch between inactive and active states. Once activated, αIIbβ3 binds many different ECM proteins, including fibrinogen, its major ligand. In addition to αIIbβ3, there are other integrins expressed at lower levels on platelets and responsible for adhesion to additional ECM proteins. There are also some important nonintegrin ECM receptors, GPIb-V-IX and GPVI, which are specific to platelets. These receptors play major roles in platelet adhesion and in the activation of the integrins and of other platelet responses, such as cytoskeletal organization and exocytosis of additional ECM ligands and autoactivators of the platelets.

Historical perspective and future directions in platelet research

Platelets are a remarkable mammalian adaptation that are required for human survival by virtue of their ability to prevent and arrest bleeding. Ironically, however, in the past century, the platelets' hemostatic activity became maladaptive for the increasingly large percentage of individuals who develop age-dependent progressive atherosclerosis. As a result, platelets also make a major contribution to ischemic thrombotic vascular disease, the leading cause of death worldwide. In this brief review, I provide historical descriptions of a highly selected group of topics to provide a framework for understanding our current knowledge and the trends that are likely to continue into the future of platelet research. For convenience, I separate the eras of platelet research into the "Descriptive Period" extending from ~1880-1960 and the "Mechanistic Period" encompassing the past ~50 years since 1960. We currently are reaching yet another inflection point, as there is a major shift from a focus on traditional biochemistry and cell and molecular biology to an era of single molecule biophysics, single cell biology, single cell molecular biology, structural biology, computational simulations, and the high-throughput, data-dense techniques collectively named with the "omics postfix". Given the progress made in understanding, diagnosing, and treating many rare and common platelet disorders during the past 50 years, I think it appropriate to consider it a Golden Age of Platelet Research and to recognize all of the investigators who have made important contributions to this remarkable achievement..

Biology and physics of von Willebrand factor concatamers

Structural specialisations enable von Willebrand factor (VWF) to assemble during biosynthesis into helical tubules in Weibel-Palade bodies (WPB). Specialisations include a pH-regulated dimeric bouquet formed by the C-terminal half of VWF and helical assembly guided by the N-terminal half that templates inter-dimer disulphide bridges. Orderly assembly and storage of ultra-long concatamers in helical tubules, without crosslinking of neighboring tubules, enables unfurling during secretion without entanglement. Length regulation occurs post-secretion, by hydrodynamic force-regulated unfolding of the VWF A2 domain, and its cleavage by the plasma protease ADAMTS13 (a disintegrin and metalloprotease with a thrombospondin type 1 motif, member 13). VWF is longest at its site of secretion, where its haemostatic function is most important. Moreover, elongational hydrodynamic forces on VWF are strongest just where needed, when bound to the vessel wall, or in elongational flow in the circulation at sites of vessel rupture or vasoconstriction in haemostasis. Elongational forces regulate haemostasis by activating binding of the A1 domain to platelet GPIbα, and over longer time periods, regulate VWF length by unfolding of the A2 domain for cleavage by ADAMTS13. Recent structures of A2 and single molecule measurements of A2 unfolding and cleavage by ADAMTS13 illuminate the mechanisms of VWF length regulation. Single molecule studies on the A1-GPIb receptor-ligand bond demonstrate a specialised flex-bond that enhances resistance to the strong hydrodynamic forces experienced at sites of haemorrhage.

Binding of alpha-thrombin to surface-anchored platelet glycoprotein Ib(alpha) sulfotyrosines through a two-site mechanism involving exosite I

The involvement of exosite I in α-thrombin (FIIa) binding to platelet glycoprotein Ibα (GPIbα), which could influence interactions with other substrates, remains undefined. To address the problem, we generated the GPIbα amino terminal domain (GPIbα-N) fully sulfated on three tyrosine residues and solved the structure of its complex with FIIa. We found that sulfotyrosine (Tys) 278 enhances the interaction mainly by establishing contacts with exosite I. We then evaluated how substituting tyrosine with phenylalanine, which cannot be sulfated, affects FIIa binding to soluble or surface-immobilized GPIbα-N. Mutating Tyr(276), which mostly contacts exosite II residues, markedly reduced FIIa interaction with both soluble and immobilized GPIbα-N; mutating Tyr(278) or Tyr(279), which mostly contact exosite I residues, reduced FIIa complexing in solution by 0-20% but affinity for immobilized GPIbα-N 2 to 6-fold, respectively. Moreover, three exosite I ligands--aptamer HD1, hirugen, and lepirudin--did not interfere with soluble FIIa complexing to GPIbα-N, excluding that their binding caused allosteric effects influencing the interaction; nonetheless, all impaired FIIa binding to immobilized GPIbα-N and platelet GPIb nearly as much as aptamer HD22 and heparin, both exosite II ligands. Bound HD1 and hirugen alter Trp(148) orientation in a loop near exosite I preventing contacts with the sulfate oxygen atoms of Tys(279). These results support a mechanism in which binding occurs when the two exosites of one FIIa molecule independently interact with two immobilized GPIbα molecules. Through exosite engagement, GPIbα may influence FIIa-dependent processes relevant to hemostasis and thrombosis.

Platelet glycoprotein Ib beta/IX mediates glycoprotein Ib alpha localization to membrane lipid domain critical for von Willebrand factor interaction at high shear

The localization of the platelet glycoprotein GP Ib-IX complex (GP Ibα, GP Ibβ, and GP IX) to membrane lipid domain, also known as glycosphingolipid-enriched membranes (GEMs or raft) lipid domain, is essential for the GP Ib-IX complex mediated platelet adhesion to von Willebrand factor (vWf) and subsequent platelet activation. To date, the mechanism for the complex association with the GEMs remains unclear. Although the palmitate modifications of GP Ibβ and GP IX were thought to be critical for the complex presence in the GEMs, we found that the removal of the putative palmitoylation sites of GP Ibβ and GP IX had no effects on the localization of the GP Ib-IX complex to the GEMs. Instead, the disruption of GP Ibα disulfide linkage with GP Ibβ markedly decreased the amount of the GEM-associated GP Ibα without altering the GEM association of GP Ibβ and GP IX. Furthermore, partial dissociation with the GEMs greatly inhibited GP Ibα interaction with vWf at high shear instead of in static condition or under low shear stress. Thus, for the first time, we demonstrated that GP Ibβ/GP IX mediates the disulfide-linked GP Ibα localization to the GEMs, which is critical for vWf interaction at high shear.

Platelet adhesion to collagen

Platelets play a central role in maintaining hemostasis mainly by binding to subendothelial collagen exposed upon vascular injury, thereby initiating thrombus formation. Platelets can bind directly to the exposed collagen through two major receptors i.e. the integrin a2b1 and glycoprotein (GP) VI. However, under high shear conditions the GPIb-V-IX receptor complex and its main ligand von Willebrand Factor are additionally needed for firm platelet adhesion to the vessel wall. In this review, we summarize the current knowledge on the individual roles and structure-function relationships of these main platelet adhesion receptors.

Thrombotic thrombocytopenic purpura: reducing the risk?

von Willebrand factor (vWF) has a key role in initiating platelet aggregation, and thereby thrombus formation, that is dependent on its ability to form appropriately sized multimers. Ultralarge multimers promote the formation of the microvascular thrombi that are hallmarks of the life-threatening condition thrombotic thrombocytopenic purpura (TTP). In this issue of the JCI, Chen et al. show that the drug N-acetylcysteine (NAC) can decrease the size of vWF multimers in vitro and in vivo, resolving thrombi in mice. These data suggest that NAC could potentially be used to treat thrombotic conditions such as TTP.

The mechanism of VWF-mediated platelet GPIbalpha binding

The binding of Von Willebrand Factor to platelets is dependent on the conformation of the A1 domain which binds to platelet GPIbalpha. This interaction initiates the adherence of platelets to the subendothelial vasculature under the high shear that occurs in pathological thrombosis. We have developed a thermodynamic strategy that defines the A1:GPIbalpha interaction in terms of the free energies (DeltaG values) of A1 unfolding from the native to intermediate state and the binding of these conformational states to GPIbalpha. We have isolated the intermediate conformation of A1 under nondenaturing conditions by reduction and carboxyamidation of the disulfide bond. The circular dichroism spectrum of reduction and carboxyamidation A1 indicates that the intermediate has approximately 10% less alpha-helical structure that the native conformation. The loss of alpha-helical secondary structure increases the GPIbalpha binding affinity of the A1 domain approximately 20-fold relative to the native conformation. Knowledge of these DeltaG values illustrates that the A1:GPIbalpha complex exists in equilibrium between these two thermodynamically distinct conformations. Using this thermodynamic foundation, we have developed a quantitative allosteric model of the force-dependent catch-to-slip bonding that occurs between Von Willebrand Factor and platelets under elevated shear stress. Forced dissociation of GPIbalpha from A1 shifts the equilibrium from the low affinity native conformation to the high affinity intermediate conformation. Our results demonstrate that A1 binding to GPIbalpha is thermodynamically coupled to A1 unfolding and catch-to-slip bonding is a manifestation of this coupling. Our analysis unites thermodynamics of protein unfolding and conformation-specific binding with the force dependence of biological catch bonds and it encompasses the effects of two subtypes of mutations that cause Von Willebrand Disease.

Flow-induced beta-hairpin folding of the glycoprotein Ibalpha beta-switch

Flow-induced shear has been identified as a regulatory driving force in blood clotting. Shear induces beta-hairpin folding of the glycoprotein Ibalpha beta-switch which increases affinity for binding to the von Willebrand factor, a key step in blood clot formation and wound healing. Through 2.1-micros molecular dynamics simulations, we investigate the kinetics of flow-induced beta-hairpin folding. Simulations sampling different flow velocities reveal that under flow, beta-hairpin folding is initiated by hydrophobic collapse, followed by interstrand hydrogen-bond formation and turn formation. Adaptive biasing force simulations are employed to determine the free energy required for extending the unfolded beta-switch from a loop to an elongated state. Lattice and freely jointed chain models illustrate how the folding rate depends on the entropic and enthalpic energy, the latter controlled by flow. The results reveal that the free energy landscape of the beta-switch has two stable conformations imprinted on it, namely, loop and hairpin--with flow inducing a transition between the two.

Platelet physiology and antiplatelet agents

Apart from the central beneficial role platelets play in hemostasis, they are also involved in atherothrombotic diseases. Here, we review the current knowledge of platelet intracellular signal transduction pathways involved in platelet adhesion, activation, amplification of the activation signal and aggregation, as well as pathways limiting platelet aggregation. A thorough understanding of these pathways allows explanation of the mechanism of action of existing antiplatelet agents, but also helps to identify targets for novel drug development.

Rolling cell adhesion

Rolling adhesion on vascular surfaces is the first step in recruiting circulating leukocytes, hematopoietic progenitors, or platelets to specific organs or to sites of infection or injury. Rolling requires the rapid yet balanced formation and dissociation of adhesive bonds in the challenging environment of blood flow. This review explores how structurally distinct adhesion receptors interact through mechanically regulated kinetics with their ligands to meet these challenges. Remarkably, increasing force applied to adhesive bonds first prolongs their lifetimes (catch bonds) and then shortens their lifetimes (slip bonds). Catch bonds mediate the counterintuitive phenomenon of flow-enhanced rolling adhesion. Force-regulated disruptions of receptor interdomain or intradomain interactions remote from the ligand-binding surface generate catch bonds. Adhesion receptor dimerization, clustering in membrane domains, and interactions with the cytoskeleton modulate the forces applied to bonds. Both inside-out and outside-in cell signals regulate these processes.

Unravelling the mechanism and significance of thrombin binding to platelet glycoprotein Ib

The main question concerning the mechanism of a-thrombin binding to platelet membrane glycoprotein (GP)Ib is whether it involves both thrombin exosite I and exosite II. The solution of two independent crystal structures suggests alternative explanations that may actually reflect different modes of binding with distinct pathophysiological significance. With respect to function, it is still unclear whether thrombin binding to GPIb promotes procoagulant and prothrombotic pathways of response to vascular injury or limits such responses by sequestering, at least temporarily, the active enzyme. We review here published information on these topics and touch upon ongoing studies aimed at finding definitive answers to outstanding questions relevant for a better understanding of thrombosis and haemostasis.

The catch bond mechanism between von Willebrand factor and platelet surface receptors investigated by molecular dynamics simulations

The multi-domain protein von Willebrand factor is crucial in the blood coagulation process at high shear. The A1 domain binds to the platelet surface receptor glycoprotein Ibalpha (GpIb alpha) and this interaction is known to be strengthened by tensile force. The molecular mechanism behind this observation was investigated here by molecular dynamics simulations. The results suggest that the proteins unbind through two distinct pathways depending whether a high-tensile force is applied or whether unbinding happens through thermal fluctuations. In the high-force unbinding pathway the A1 domain was observed to rotate away from the C-terminus of GpIb alpha. In contrast, during thermal unbinding the A1 domain rotated in the opposite direction as in the high-force pathway and the distance between the terminii of A1 and the GpIb alpha C-terminus shortened. This shortening was reduced and the lifetime of the bond extended if a moderate tensile force was applied across the complex. This suggests that the thermal unbinding pathway is inhibited by a moderate tensile force which is in agreement with the catch bond property shown previously in single molecule experiments. A designed mutant of GpIb alpha is suggested here in order to test in vitro the thermal unbinding pathway observed in silico.

A mechanically stabilized receptor-ligand flex-bond important in the vasculature

Haemostasis in the arteriolar circulation mediated by von Willebrand factor (VWF) binding to platelets is an example of an adhesive interaction that must withstand strong hydrodynamic forces acting on cells. VWF is a concatenated, multifunctional protein that has binding sites for platelets as well as subendothelial collagen. Binding of the A1 domain in VWF to the glycoprotein Ib alpha subunit (GPIbalpha) on the surface of platelets mediates crosslinking of platelets to one another and the formation of a platelet plug for arterioles. The importance of VWF is illustrated by its mutation in von Willebrand disease, a bleeding diathesis. Here, we describe a novel mechanochemical specialization of the A1-GPIbalpha bond for force-resistance. We have developed a method that enables, for the first time, repeated measurements of the binding and unbinding of a receptor and ligand in a single molecule (ReaLiSM). We demonstrate two states of the receptor-ligand bond, that is, a flex-bond. One state is seen at low force; a second state begins to engage at 10 pN with a approximately 20-fold longer lifetime and greater force resistance. The lifetimes of the two states, how force exponentiates lifetime, and the kinetics of switching between the two states are all measured. For the first time, single-molecule measurements on this system are in agreement with bulk phase measurements. The results have important implications not only for how platelets bound to VWF are able to resist force to plug arterioles, but also how increased flow activates platelet plug formation.

Molecular insight into human platelet antigens: structural and evolutionary conservation analyses offer new perspective to immunogenic disorders

BACKGROUND

Human platelet antigens (HPAs) are polymorphisms in platelet membrane glycoproteins (GPs) that can stimulate production of alloantibodies once exposed to foreign platelets (PLTs) with different HPAs. These antibodies can cause neonatal alloimmune thrombocytopenia, posttransfusion purpura, and PLT transfusion refractoriness. Most HPAs are localized on the main PLT receptors: 1) integrin αIIbβ3, known as the fibrinogen receptor; 2) the GPIb-IX-V complex that functions as the receptor for von Willebrand factor; and 3) integrin α2β1, which functions as the collagen receptor.

STUDY DESIGN AND METHODS

We analyzed the structural location and the evolutionary conservation of the residues associated with the HPAs to characterize the features that induce immunologic responses but do not cause inherited diseases.

RESULTS

We found that all HPAs reside in positions located on the protein surface, apart from the ligand-binding site, and are evolutionary variable.

CONCLUSION

Disease-causing mutations often reside in highly conserved and buried positions. In contrast, the HPAs affect residues on the protein surface that were not conserved throughout evolution; this explains their naive effect on the protein function. Nonetheless, the HPAs involve substitutions of solvent-exposed positions that lead to altered interfaces on the surface of the protein and might present epitopes foreign to the immune system.

A central role of GPIb-IX in the procoagulant function of platelets that is independent of the 45-kDa GPIbα N-terminal extracellular domain

Activated platelets become procoagulant and efficiently promote the conversion of prothrombin to thrombin. A role of the GPIb-V-IX complex has long been postulated in view of the decreased prothrombin consumption in Bernard-Soulier patients. We evaluated the impact of GPIb-V-IX deficiency and the requirement for the GPIbα extracellular domain. In GPIbβ−/− mice, thrombin generation was profoundly decreased in tissue factor– or collagen-related peptide (CRP)–activated platelet-rich plasma and in washed platelets supplemented with normal plasma or with FVa, FXa, and prothrombin. Phosphatidylserine (PS) exposure was similarly decreased in response to thrombin, CRP, or CRP + PAR4 peptide despite a normal platelet phospholipid composition. The hypothesis that these defects originate from lack of the GPIbα N-terminal domain was evaluated after its removal from normal mouse and human platelets with Nk protease or O-sialoglycoprotein endopeptidase. Unexpectedly, the treated platelets exhibited normal thrombin generation and PS exposure, indicating that GPIb-V-IX regulates procoagulant activity independently of its GPIbα-binding region. These results suggested a more general structuring role through intracellular cytoskeleton-anchoring portions regulating responses leading to PS exposure. This hypothesis was supported by the decreased calcium mobilization observed in GPIbβ−/− platelets in response to several agonists, some acting independently of GPIb, in contrast to the normal calcium responses in Nk protease–treated platelets.