Enhanced Local Disorder in a Clinically Elusive von Willebrand Factor Provokes High-Affinity Platelet Clumping

Nanobody activator improves sensitivity of the von Willebrand factor activity assay to multimer size

BACKGROUND

The activity of von Willebrand factor (VWF) in facilitating platelet adhesion and aggregation correlates with its multimer size. Traditional ristocetin-dependent functional assays lack sensitivity to multimer sizes. Recently, nanobodies targeting the autoinhibitory module and activating VWF were identified.

OBJECTIVES

To develop an assay that can differentiate the platelet-binding activity of VWF multimers.

METHODS

A novel enzyme-linked immunosorbent assay (nanobody-triggered glycoprotein Ib binding assay [VWF:GPIbNab]) utilizing a VWF-activating nanobody was developed. Recombinant VWF, plasma-derived VWF (pdVWF), and selected gel-filtrated fractions of pdVWF were evaluated for VWF antigen and activity levels. A linear regression model was developed to estimate the specific activity of VWF multimers.

RESULTS

Of the 3 activating nanobodies tested, 6C11 with the lowest activation effect exhibited the highest sensitivity for high-molecular-weight multimers (HMWMs) of VWF. VWF:GPIbNab utilizing 6C11 (VWF:GPIbNab6C11) produced significantly higher activity/antigen ratios for recombinant VWF (>2.0) and HMWM-enriched pdVWF fractions (>2.0) than for pdVWF (∼1.0) or fractions enriched with shorter multimers (<1.0). The differences were much larger than those produced by VWF:GPIbNab utilizing other nanobodies, VWF:GPIbM, VWF:GPIbR, or VWF:CB assays. Linear regression analysis of 5 pdVWF fractions of various multimer sizes produced an estimated specific activity of 2.7 for HMWMs. The analysis attributed >90% of the VWF activity measured by VWF:GPIbNab6C11 to that of HMWMs, which is significantly higher than all other activity assays tested.

CONCLUSION

The VWF:GPIbNab6C11 assay exhibits higher sensitivity to HMWMs than ristocetin-based and collagen-binding assays. Future studies examining the application of this assay in clinical settings and any associated therapeutic benefit of doing so are warranted.

Removal of the vicinal disulfide enhances the platelet-capturing function of von Willebrand factor

A redox autoinhibitory mechanism has previously been proposed, in which the reduced state of the vicinal disulfide bond in the von Willebrand factor (VWF) A2 domain allows A2 to bind to A1 and inhibit platelet adhesion to the A1 domain. The VWF A1A2A3 tridomain was expressed with and without the vicinal disulfide in A2 (C1669S/C1670S) via the atomic replacement of sulfur for oxygen to test the relevance of the vicinal disulfide to the physiological platelet function of VWF under shear flow. A comparative study of the shear-dependent platelet translocation dynamics on these tridomain variants reveals that the reduction of the vicinal disulfide moderately increases the platelet-capturing function of A1, an observation counter to the proposed hypothesis. Surface plasmon resonance spectroscopy confirms that C1669S/C1670S slightly increases the affinity of A1A2A3 binding to glycoprotein Ibα (GPIbα). Differential scanning calorimetry and hydrogen-deuterium exchange mass spectrometry demonstrate that reduction of the vicinal disulfide destabilizes the A2 domain, which consequently disrupts interactions between the A1, A2, and A3 domains and enhances the conformational dynamics of A1-domain secondary structures known to regulate the strength of platelet adhesion to VWF. This study clarifies that the reduced state of the A2 vicinal disulfide is not inhibitory but rather slightly activating.

Type 2B von Willebrand disease mutations differentially perturb autoinhibition of the A1 domain

Type 2B von Willebrand disease (VWD) is an inherited bleeding disorder in which a subset of point mutations in the von Willebrand factor (VWF) A1 domain and recently identified autoinhibitory module (AIM) cause spontaneous binding to glycoprotein Ibα (GPIbα) on the platelet surface. All reported type 2B VWD mutations share this enhanced binding; however, type 2B VWD manifests as variable bleeding complications and platelet levels in patients, depending on the underlying mutation. Understanding how these mutations localizing to a similar region can result in such disparate patient outcomes is essential for detailing our understanding of VWF regulatory and activation mechanisms. In this study, we produced recombinant glycosylated AIM-A1 fragments bearing type 2B VWD mutations and examined how each mutation affects the A1 domain's thermodynamic stability, conformational dynamics, and biomechanical regulation of the AIM. We found that the A1 domain with mutations associated with severe bleeding occupy a higher affinity state correlating with enhanced flexibility in the secondary GPIbα-binding sites. Conversely, mutation P1266L, associated with normal platelet levels, has similar proportions of high-affinity molecules to wild-type (WT) but shares regions of solvent accessibility with both WT and other type 2B VWD mutations. V1316M exhibited exceptional instability and solvent exposure compared with all variants. Lastly, examination of the mechanical stability of each variant revealed variable AIM unfolding. Together, these studies illustrate that the heterogeneity among type 2B VWD mutations is evident in AIM-A1 fragments.

Von Willebrand factor A1 domain stability and affinity for GPIbα are differentially regulated by its O-glycosylated N- and C-linker

Hemostasis in the arterial circulation is mediated by binding of the A1 domain of the ultralong protein von Willebrand factor (VWF) to GPIbα on platelets to form a platelet plug. A1 is activated by tensile force on VWF concatemers imparted by hydrodynamic drag force. The A1 core is protected from force-induced unfolding by a long-range disulfide that links cysteines near its N- and C-termini. The O-glycosylated linkers between A1 and its neighboring domains, which transmit tensile force to A1, are reported to regulate A1 activation for binding to GPIb, but the mechanism is controversial and incompletely defined. Here, we study how these linkers, and their polypeptide and O-glycan moieties, regulate A1 affinity by measuring affinity, kinetics, thermodynamics, hydrogen deuterium exchange (HDX), and unfolding by temperature and urea. The N-linker lowers A1 affinity 40-fold with a stronger contribution from its O-glycan than polypeptide moiety. The N-linker also decreases HDX in specific regions of A1 and increases thermal stability and the energy gap between its native state and an intermediate state, which is observed in urea-induced unfolding. The C-linker also decreases affinity of A1 for GPIbα, but in contrast to the N-linker, has no significant effect on HDX or A1 stability. Among different models for A1 activation, our data are consistent with the model that the intermediate state has high affinity for GPIbα, which is induced by tensile force physiologically and regulated allosterically by the N-linker.

Development of antithrombotic peptides based on the molecular interactions between von Willebrand factor and GPIbα

Binding of platelets on vascular endothelia at the damaged site using von Willebrand factor (vWF) as a bridge is of great significance for platelet adhesion and subsequent arterial thrombosis. Molecular...

The effect of hyperoxia on the hemostasiological status of severely burned patients

The state of the hemostasis system was studied in 9 patients of the middle age group (44 ± 9.94 years) who received thermal trauma on an area of more than 32% (49.4 ± 18.3) of the body surface, accompanied by the development of burn shock. The standard therapy for burn injury was supplemented with HBO sessions. Treatment with hyperbaric oxygen was carried out in pressure chambers BLKS-307, BLKS-307/1. The state of the coagulation, anticoagulant and fibrinolytic links of the hemostasis system, as well as the viscoelastic properties of the blood, were assessed immediately before the HBO session and immediately after it. The total number of comparison pairs was 45. Under the influence of HBO therapy, there was an increase in the activity of antithrombin III (ATIII), protein C (PrS) and a decrease in the viscoelastic properties of blood (p <0.05). Positive deviations in the values of ATIII, Pr C, von Willebrand factor, APTT, prothrombin and thrombin time, fibrinogen, factor XIII, XIIa-dependent fibrinolysis, D-dimers and thromboelastography parameters were revealed. The maximum frequency of their occurrence was recorded for ATIII (95%), the minimum - for the D-dimer (62%). After HBO procedures, undesirable deviations of the hemostatic system parameters were also noted. They were chaotic, were compensated by an increase in the activity of physiological anticoagulants and were not accompanied by complications of a thrombogenic nature. Thus, conducting HBO therapy sessions in the acute period of burn disease increases the activity of physiological anticoagulants and stabilizes the viscoelastic properties of blood. There is a high frequency of occurrence of positive effects of hyperoxia on the components of the hemostasis system. The identification of its undesirable effects indicates the need to monitor the state of the hemostasis system during HBO procedures.

A p.Arg127Gln variant in GPIbα LRR5 allosterically enhances affinity for VWF: a novel form of platelet-type VWD

We identified a novel GP1BA variant in the LRR5 domain of GPIbα (p.Arg127Gln) in a patient with a mild PT-VWD phenotype.

GOF variants in the LRR of GPIbα alter the dynamics of the C-terminal disulfide loop generating a conformation with high affinity for VWF.

Gain-of-function (GOF) variants in GP1BA cause platelet-type von Willebrand disease (PT-VWD), a rare inherited autosomal dominant bleeding disorder characterized by enhanced platelet GPIbα to von Willebrand factor (VWF) interaction, and thrombocytopenia. To date, only 6 variants causing PT-VWD have been described, 5 in the C-terminal disulfide loop of the VWF-binding domain of GPIbα and 1 in the macroglycopeptide. GOF GP1BA variants generate a high-affinity conformation of the C-terminal disulfide loop with a consequent allosteric conformational change on another region of GPIbα, the leucine-rich-repeat (LRR) domain. We identified a novel GP1BA variant (p.Arg127Gln) affecting the LRR5 domain of GPIbα in a boy with easy bruising and laboratory test results suggestive of PT-VWD. We thus aimed to investigate the impact of the p.Arg127Gln variant on GPIbα affinity for VWF and GPIbα structure. Chinese hamster ovary cells expressing p.Arg127Gln GPIbα showed increased binding of VWF induced by ristocetin and enhanced tethering on immobilized VWF as compared with cells expressing wild-type GPIbα. Surface plasmon resonance confirmed that p.Arg127Gln enhances the binding affinity of GPIbα for VWF. Hydrogen‐deuterium exchange mass spectrometry showed that p.Arg127Gln of LRR, while having little effect on the dynamics of the LRR locally, enhances the conformational dynamics of the GPIbα C-terminal disulfide loop structure. Our data demonstrate for the first time that GOF variants outside the GPIbα C-terminal disulfide loop may be pathogenic and that aminoacidic changes in the LRR may cause allosterically conformational changes in the C-terminal disulfide loop of GPIbα, inducing a conformation with high affinity for VWF.

Impact of Von Willebrand Factor on Bacterial Pathogenesis

Von Willebrand factor (VWF) is a mechano-sensitive protein with crucial functions in normal hemostasis, which are strongly dependant on the shear-stress mediated defolding and multimerization of VWF in the blood stream. Apart from bleeding disorders, higher plasma levels of VWF are often associated with a higher risk of cardiovascular diseases. Herein, the disease symptoms are attributed to the inflammatory response of the activated endothelium and share high similarities to the reaction of the host vasculature to systemic infections caused by pathogenic bacteria such as Staphylococcus aureus and Streptococcus pneumoniae. The bacteria recruit circulating VWF, and by binding to immobilized VWF on activated endothelial cells in blood flow, they interfere with the physiological functions of VWF, including platelet recruitment and coagulation. Several bacterial VWF binding proteins have been identified and further characterized by biochemical analyses. Moreover, the development of a combination of sophisticated cell culture systems simulating shear stress levels of the blood flow with microscopic visualization also provided valuable insights into the interaction mechanism between bacteria and VWF-strings. In vivo studies using mouse models of bacterial infection and zebrafish larvae provided evidence that the interaction between bacteria and VWF promotes bacterial attachment, coagulation, and thrombus formation, and thereby contributes to the pathophysiology of severe infectious diseases such as infective endocarditis and bacterial sepsis. This mini-review summarizes the current knowledge of the interaction between bacteria and the mechano-responsive VWF, and corresponding pathophysiological disease symptoms.

Glycosylation sterically inhibits platelet adhesion to von Willebrand factor without altering intrinsic conformational dynamics

BACKGROUND

A molecular basis for von Willebrand factor (VWF) self-inhibition has been proposed by which the N-terminal and C-terminal flanking sequences of the globular A1 domain disulfide loop bind to and suppress the conformational dynamics of A1. These flanking sequences are rich in O-linked glycosylation (OLG), which is known to suppress platelet adhesion to VWF, presumably by steric hindrance. The inhibitory mechanism remains unresolved as to whether inhibition is due to steric exclusion by OLGs or a direct self-association interaction that stabilizes the domain.

OBJECTIVES

The platelet adhesive function, thermodynamic stability, and conformational dynamics of the wild-type and type 2M G1324S A1 domain lacking glycosylation (Escherichia coli) are compared with the wild-type glycosylated A1 domain (HEK293 cell culture) to decipher the self-inhibitory mechanism.

METHODS

Surface plasmon resonance and analytical rheology are utilized to assess Glycoprotein Ibα (GPIbα) binding at equilibrium and platelet adhesion under shear flow. The conformational stability is assessed through a combination of protein unfolding thermodynamics and hydrogen-deuterium exchange mass spectrometry (HXMS).

RESULTS

A1 glycosylation inhibits both GPIbα binding and platelet adhesion. Glycosylation increases the hydrodynamic size of A1 and stabilizes the thermal unfolding of A1 without changing its equilibrium stability. Glycosylation does not alter the intrinsic conformational dynamics of the A1 domain.

CONCLUSIONS

These studies invalidate the proposed inhibition through conformational suppression since glycosylation within these flanking sequences does not alter the native state stability or the conformational dynamics of A1. Rather, they confirm a mechanism by which glycosylation sterically hinders platelet adhesion to the A1 domain at equilibrium and under rheological shear stress.

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.

Evidence for the Misfolding of the A1 Domain within Multimeric von Willebrand Factor in Type 2 von Willebrand Disease

Von Willebrand factor (VWF), an exceptionally large multimeric plasma glycoprotein, functions to initiate coagulation by agglutinating platelets in the blood stream to sites of vascular injury. This primary hemostatic function is perturbed in type 2 dysfunctional subtypes of von Willebrand disease (VWD) by mutations that alter the structure and function of the platelet GPIbα adhesive VWF A1 domains. The resulting amino acid substitutions cause local disorder and misfold the native structure of the isolated platelet GPIbα-adhesive A1 domain of VWF in both gain-of-function (type 2B) and loss-of-function (type 2M) phenotypes. These structural effects have not been explicitly observed in A1 domains of VWF multimers native to blood plasma. New mass spectrometry strategies are applied to resolve the structural effects of 2B and 2M mutations in VWF to verify the presence of A1 domain structural disorder in multimeric VWF harboring type 2 VWD mutations. Limited trypsinolysis mass spectrometry (LTMS) and hydrogen-deuterium exchange mass spectrometry (HXMS) are applied to wild-type and VWD variants of the single A1, A2, and A3 domains, an A1A2A3 tridomain fragment of VWF, plasmin-cleaved dimers of VWF, multimeric recombinant VWF, and normal VWF plasma concentrates. Comparatively, these methods show that mutations known to misfold the isolated A1 domain increase the rate of trypsinolysis and the extent of hydrogen-deuterium exchange in local secondary structures of A1 within multimeric VWF. VWD mutation effects are localized to the A1 domain without appreciably affecting the structure and dynamics of other VWF domains. The intrinsic dynamics of A1 observed in recombinant fragments of VWF are conserved in plasma-derived VWF. These studies reveal that structural disorder does occur in VWD variants of the A1 domain within multimeric VWF and provides strong support for VWF misfolding as a result of some, but not all, type 2 VWD variants.

Arabinose Alters Both Local and Distal H-D Exchange Rates in the Escherichia coli AraC Transcriptional Regulator

In the absence of arabinose, the dimeric Escherichia coli regulatory protein of the l-arabinose operon, AraC, represses expression by looping the DNA between distant half-sites. Binding of arabinose to the dimerization domains forces AraC to preferentially bind two adjacent DNA half-sites, which stimulates RNA polymerase transcription of the araBAD catabolism genes. Prior genetic and biochemical studies hypothesized that arabinose allosterically induces a helix-coil transition of a linker between the dimerization and DNA binding domains that switches the AraC conformation to an inducing state [Brown, M. J., and Schleif, R. F. (2019) Biochemistry, preceding paper in this issue (DOI: 10.1021/acs.biochem.9b00234)]. To test this hypothesis, hydrogen-deuterium exchange mass spectrometry was utilized to identify structural regions involved in the conformational activation of AraC by arabinose. Comparison of the hydrogen-deuterium exchange kinetics of individual dimeric dimerization domains and the full-length dimeric AraC protein in the presence and absence of arabinose reveals a prominent arabinose-induced destabilization of the amide hydrogen-bonded structure of linker residues (I₁₆₇ and N₁₆₈). This destabilization is demonstrated to result from an increased probability to form a helix capping motif at the C-terminal end of the dimerizing α-helix of the dimerization domain that preceeds the interdomain linker. These conformational changes could allow for quaternary repositioning of the DNA binding domains required for induction of the araBAD promoter through rotation of peptide backbone dihedral angles of just a couple of residues. Subtle changes in exchange rates are also visible around the arabinose binding pocket and in the DNA binding domain.

Von Willebrand Factor Mediates Pneumococcal Aggregation and Adhesion in Blood Flow

Streptococcus pneumoniae is a major cause of community acquired pneumonia and septicaemia in humans. These diseases are frequently associated with thromboembolic cardiovascular complications. Pneumococci induce the exocytosis of endothelial Weibel-Palade Bodies and thereby actively stimulate the release of von Willebrand factor (VWF), which is an essential glycoprotein of the vascular hemostasis. Both, the pneumococcus induced pulmonary inflammation and the thromboembolytic complications are characterized by a dysbalanced hemostasis including a marked increase in VWF plasma concentrations. Here, we describe for the first time VWF as a novel interaction partner of capsulated and non-encapsulated pneumococci. Moreover, cell culture infection analyses with primary endothelial cells characterized VWF as bridging molecule that mediates bacterial adherence to endothelial cells in a heparin-sensitive manner. Due to the mechanoresponsive changes of the VWF protein conformation and multimerization status, which occur in the blood stream, we used a microfluidic pump system to generate shear flow-induced multimeric VWF strings on endothelial cell surfaces and analyzed attachment of RFP-expressing pneumococci in flow. By applying immunofluorescence visualization and additional electron microscopy, we detected a frequent and enduring bacterial attachment to the VWF strings. Bacterial attachment to the endothelium was confirmed in vivo using a zebrafish infection model, which is described in many reports and acknowledged as suitable model to study hemostasis mechanisms and protein interactions of coagulation factors. Notably, we visualized the recruitment of zebrafish-derived VWF to the surface of pneumococci circulating in the blood stream and detected a VWF-dependent formation of bacterial aggregates within the vasculature of infected zebrafish larvae. Furthermore, we identified the surface-exposed bacterial enolase as pneumococcal VWF binding protein, which interacts with the VWF domain A1 and determined the binding kinetics by surface plasmon resonance. Subsequent epitope mapping using an enolase peptide array indicates that the peptide ¹⁸¹YGAEIFHALKKILKS¹⁹⁵ might serve as a possible core sequence of the VWF interaction site. In conclusion, we describe a VWF-mediated mechanism for pneumococcal anchoring within the bloodstream via surface-displayed enolase, which promotes intravascular bacterial aggregation.

The Chaperonin GroEL: A Versatile Tool for Applied Biotechnology Platforms

The nucleotide-free chaperonin GroEL is capable of capturing transient unfolded or partially unfolded states that flicker in and out of existence due to large-scale protein dynamic vibrational modes. In this work, three short vignettes are presented to highlight our continuing advances in the application of GroEL biosensor biolayer interferometry (BLI) technologies and includes expanded uses of GroEL as a molecular scaffold for electron microscopy determination. The first example presents an extension of the ability to detect dynamic pre-aggregate transients in therapeutic protein solutions where the assessment of the kinetic stability of any folded protein or, as shown herein, quantitative detection of mutant-type protein when mixed with wild-type native counterparts. Secondly, using a BLI denaturation pulse assay with GroEL, the comparison of kinetically controlled denaturation isotherms of various von Willebrand factor (vWF) triple A domain mutant-types is shown. These mutant-types are single point mutations that locally disorder the A1 platelet binding domain resulting in one gain of function and one loss of function phenotype. Clear, separate, and reproducible kinetic deviations in the mutant-type isotherms exist when compared with the wild-type curve. Finally, expanding on previous electron microscopy (EM) advances using GroEL as both a protein scaffold surface and a release platform, examples are presented where GroEL-protein complexes can be imaged using electron microscopy tilt series and the low-resolution structures of aggregation-prone proteins that have interacted with GroEL. The ability of GroEL to bind hydrophobic regions and transient partially folded states allows one to employ this unique molecular chaperone both as a versatile structural scaffold and as a sensor of a protein's folded states.