The physical spacing between the von Willebrand factor D'D3 and A1 domains regulates platelet adhesion in vitro and in vivo

Conformational activation and inhibition of von Willebrand factor by targeting its autoinhibitory module

ABSTRACT

Activation of von Willebrand factor (VWF) is a tightly controlled process governed primarily by local elements around its A1 domain. Recent studies suggest that the O-glycosylated sequences flanking the A1 domain constitute a discontinuous and force-sensitive autoinhibitory module (AIM), although its extent and conformation remains controversial. Here, we used a targeted screening strategy to identify 2 groups of nanobodies. One group, represented by clone 6D12, is conformation insensitive and binds the N-terminal AIM (NAIM) sequence that is distal from A1; 6D12 activates human VWF and induces aggregation of platelet-rich plasma at submicromolar concentrations. The other group, represented by clones Nd4 and Nd6, is conformation sensitive and targets the C-terminal AIM (CAIM). Nd4 and Nd6 inhibit ristocetin-induced platelet aggregation and reduce VWF-mediated platelet adhesion under flow. A crystal structure of Nd6 in complex with AIM-A1 shows a novel conformation of both CAIM and NAIM that are primed to interact, providing a model of steric hindrance stabilized by the AIM as the mechanism for regulating GPIbα binding to VWF. Hydrogen-deuterium exchange mass spectrometry analysis shows that binding of 6D12 induces the exposure of the GPIbα-binding site in the A1 domain, but binding of inhibitory nanobodies reduces it. Overall, these results suggest that the distal portion of NAIM is involved in specific interactions with CAIM, and binding of nanobodies to the AIM could either disrupt its conformation to activate VWF or stabilize its conformation to upkeep VWF autoinhibition. These reported nanobodies could facilitate future studies of VWF functions and related pathologies.

The N-terminal autoinhibitory module of the A1 domain in von Willebrand factor stabilizes the mechanosensor catch bond

The N-AIM of VWF-A1 forms a Rotini-like structure, therefore partially autoinhibit VWF-A1–GPIbα interaction. The N-AIM acts as a defending sword to protect and stabilize the VWF-A1 structure under harsh environments.

Desialylation of O-glycans activates von Willebrand factor by destabilizing its autoinhibitory module

BACKGROUND

The binding of the A1 domain of von Willebrand factor (VWF) to platelet receptor glycoprotein (GP)Ibα defines the VWF activity in hemostasis. Recent studies suggest that sequences flanking A1 form cooperatively an autoinhibitory module (AIM) that reduces the accessibility of the GPIbα binding site on A1. Application of a tensile force induces unfolding of the AIM. Desialylation induces spontaneous binding of plasma VWF to platelets. Most O-glycans in VWF are located around the A1 domain. Removing certain O-glycans in the flanking sequences by site-directed mutagenesis enhances A1 binding to GPIbα and produces an effect similar to type 2B von Willebrand disease in animals.

OBJECTIVES

To understand if and how desialylation of O-glycans in the flanking sequences increases A1 activity.

METHODS

A recombinant AIM-A1 fragment encompassing VWF residues 1238-1493 and only O-glycans was treated with neuraminidase to produce desialylated protein. The glycan structure, dynamics, stability, and function of the desialylated protein was characterized by biochemical and biophysical methods and compared to the sialylated fragment.

RESULTS

Asialo-AIM-A1 exhibited increased binding activity and induced more apparent platelet aggregation than its sialylated counterpart. It exhibited a lower melting temperature, and increased hydrogen-deuterium exchange rates at residues near the secondary GPIbα binding site and the N-terminal flanking sequence. Asialo-AIM-A1 is less mechanically stable than sialo-AIM-A1, with its unstressed unfolding rate approximately 3-fold greater than the latter.

CONCLUSIONS

Desialylation of O-glycans around A1 increases its activity by destabilizing the AIM.

Efficient inhibition of O-glycan biosynthesis using the hexosamine analog Ac5GalNTGc

There is a critical need to develop small-molecule inhibitors of mucin-type O-linked glycosylation. The best-known reagent currently is benzyl-GalNAc, but it is effective only at millimolar concentrations. This article demonstrates that Ac₅GalNTGc, a peracetylated C-2 sulfhydryl-substituted GalNAc, fulfills this unmet need. When added to cultured leukocytes, breast cells, and prostate cells, Ac₅GalNTGc increased cell-surface VVA binding by ∼10-fold, indicating truncation of O-glycan biosynthesis. Cytometry, mass spectrometry, and western blot analysis of HL-60 promyelocytes demonstrated that 50-80 μM Ac₅GalNTGc prevented elaboration of 30%-60% of the O-glycans beyond the Tn-antigen (GalNAcα1-Ser/Thr) stage. The effect of the compound on N-glycans and glycosphingolipids was small. Glycan inhibition induced by Ac₅GalNTGc resulted in 50%-80% reduction in leukocyte sialyl-Lewis X expression and L-/P-selectin-mediated rolling under flow conditions. Ac₅GalNTGc was pharmacologically active in mouse. It reduced neutrophil infiltration to sites of inflammation by ∼60%. Overall, Ac₅GalNTGc may find diverse applications as a potent inhibitor of O-glycosylation.

Intradimer forces and their implication for conformations of von Willebrand factor multimers

The largest blood glycoprotein von Willebrand factor (VWF) responds to hydrodynamic stresses in the bloodstream with abrupt conformation changes, thus increasing its adhesivity to platelets and collagen. Arterial and microvascular hemostasis relies on mechanical and physicochemical properties of this macromolecule. Recently, it was discovered that the mechanical properties of VWF are controlled by multiple pH-dependent interactions with opposite trends within dimeric subunits. In this work, computer simulations reveal the effect of these intradimer forces on the conformation of VWF multimers in various hydrodynamic conditions. A coarse-grained computer model of VWF has been proposed and parameterized to give a good agreement with experimental data. The simulations suggest that strong attraction between VWF D4 domains increases the resistance to elongation under shear stress, whereas even intermediate attraction between VWF C domains contributes to VWF compaction in nonsheared fluid. It is hypothesized that the detailed subdimer dynamics of VWF concatamers may be one of the biophysical regulators of initial hemostasis and arterial thrombosis.

von Willebrand factor self-association is regulated by the shear-dependent unfolding of the A2 domain

von Willebrand factor (VWF) self-association results in the homotypic binding of VWF upon exposure to fluid shear. The molecular mechanism of this process is not established. In this study, we demonstrate that the shear-dependent unfolding of the VWF A2 domain in the multimeric protein is a major regulator of protein self-association. This mechanism controls self-association on the platelet glycoprotein Ibα receptor, on collagen substrates, and during thrombus growth ex vivo. In support of this, A2-domain mutations that prevent domain unfolding due to disulfide bridging of N- and C-terminal residues ("Lock-VWF") reduce self-association and platelet activation under various experimental conditions. In contrast, reducing assay calcium concentrations, and 2 mutations that destabilize VWF-A2 conformation by preventing coordination with calcium (D1498A and R1597W VWD type 2A mutation), enhance self-association. Studies using a panel of recombinant proteins that lack the A1 domain ("ΔA1 proteins") suggest that besides pure homotypic A2 interactions, VWF-A2 may also engage other protein domains to control self-association. Addition of purified high-density lipoprotein and apolipoprotein-A1 partially blocked VWF self-association. Overall, similar conditions facilitate VWF self-association and ADAMTS13-mediated proteolysis, with low calcium and A2 disease mutations enhancing both processes, and locking-A2 blocking them simultaneously. Thus, VWF appears to have evolved 2 balancing molecular functions in a single A2 functional domain to dynamically regulate protein size in circulation: ADAMTS13-mediated proteolysis and VWF self-association. Modulating self-association rates by targeting VWF-A2 may provide novel methods to regulate the rates of thrombosis and hemostasis.

Microfluidic devices for studying coagulation biology

The ability to study the behavior of cells, proteins, and cell-cell or cell-protein interactions under dynamic forces such as shear stress under fluid flow, provides a more accurate understanding of the physiopathology of hemostasis. This review touches upon the traditional methods for studying blood coagulation and platelet aggregation and provides an overview on cellular and protein response to shear stress. We also elaborate on the biological aspects of how cells recognize mechanical forces and convert them into biochemical signals that can drive various signaling pathways. We give a detailed description of the various types of microfluidic devices that are employed to study the complex processes of platelet aggregation and blood coagulation under flow conditions as well as to investigate endothelial shear-response. We also highlight works mimicking artificial vessels as platforms to study the mechanisms of coagulation, and finish our review by describing anticipated clinical uses of microfluidics devices and their standardization.

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.

Studying the variations in differently expressed serum proteins of Hainan black goat during the breeding cycle using isobaric tags for relative and absolute quantitation (iTRAQ) technology

The Hainan black goat is a high-quality local goat breed in Hainan Province of China. It is resistant to high temperatures, humidity, and disease. Although the meat of this breed is tender and delicious, its reproductive performance and milk yield are low. In this study, isobaric tags for relative and absolute quantitation (iTRAQ) technology was used to analyze the differentially expressed proteins in the serum of female Hainan black goats during the reproductive cycle (empty pregnant, estrus, gestation, and lactation). The pathway enrichment analysis results showed that most of the differentially expressed proteins between each period belonged to the complement and coagulation cascades. Analysis of the differential protein expression and function revealed seven proteins that were directly associated with reproduction, namely pre-SAA21, ANTXR2, vWF, SFRP3, β4GalT1, pre-IGFBP2 and Ran. This study revealed the changing patterns of differentially expressed proteins in the reproductive cycle of the Hainan black goat. pre-SAA21, ANTXR2, vWF, SFRP3, β4GalT1, pre-IGFBP2, and Ran were identified as candidate proteins for mediating the physiological state of Hainan black goats and regulating their fertility. This study elucidated the changes in expression levels of differentially expressed proteins during the reproductive cycle of Hainan black goats and also provides details about its breeding pattern.

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.

Microclot array elastometry for integrated measurement of thrombus formation and clot biomechanics under fluid shear

Blood clotting at the vascular injury site is a complex process that involves platelet adhesion and clot stiffening/contraction in the milieu of fluid flow. An integrated understanding of the hemodynamics and tissue mechanics regulating this process is currently lacking due to the absence of an experimental system that can simultaneously model clot formation and measure clot mechanics under shear flow. Here we develop a microfluidic-integrated microclot-array-elastometry system (clotMAT) that recapitulates dynamic changes in clot mechanics under physiological shear. Treatments with procoagulants and platelet antagonists and studies with diseased patient plasma demonstrate the ability of the system to assay clot biomechanics associated with common antiplatelet treatments and bleeding disorders. The changes of clot mechanics under biochemical treatments and shear flow demonstrate independent yet equally strong effects of these two stimulants on clot stiffening. This microtissue force sensing system may have future research and diagnostic potential for various bleeding disorders.

Delimiting the autoinhibitory module of von Willebrand factor

Essentials The self-inhibitory mechanism of von Willebrand factor (VWF) remains unclear. Residues flanking the A1 domain of VWF form a discontinuous autoinhibitory module (AIM). rVWF1238-1493 exhibited greater thermostability and inactivity than its shorter counterparts. The cooperative coupling between the N- and C- AIM regions are required for inhibiting A1.

SUMMARY

Background The hierarchical hemostasis response involves a self-inhibitory feature of von Willebrand factor (VWF) that has not been fully characterized. The residues flanking the A1 domain of VWF are important in this self-inhibition by forming an autoinhibitory module (AIM) that masks the A1 domain. Objectives To delimit the AIM sequence and to evaluate the cooperative interplay between the discontinuous AIM regions. Methods ELISA, flow cytometry, a thermal stability assay and hydrogen-deuterium exchange (HDX) mass spectrometry were used to characterize recombinant VWF A1 fragments varying in length. Results The longest A1 fragment (rVWF1238-1493 ) showed higher inactivity in binding the platelet receptor glycoprotein (GP) Ibα and greater thermostability than its shorter counterparts. The HDX results showed that most of the N-terminal residues and residues 1459-1478 at the C-terminus of rVWF1238-1493 have slower deuterium uptake than the residues in its denatured counterpart, implying that these residues may interact with the A1 domain. In contrast, residues 1479-1493 showed less difference from the denatured form, indicating that these residues are unlikely to be involved in binding the A1 domain. The A1 fragment that lacks either the entire C-terminal flanking region of the AIM (C-AIM), i.e. rVWF1238-1461 , or the entire N-terminal flanking region of the AIM (N-AIM), i.e. rVWF1271-1493 , showed high GPIbα-binding affinity and low thermostability, suggesting that removal of either N-terminal or C-terminal residues resulted in loss of AIM inhibition of the A1 domain. Conclusion The AIM is probably composed of residues 1238-1271 (N-AIM) and 1459-1478 (C-AIM). Neither the N-AIM nor the C-AIM alone could fully inhibit binding of the A1 domain to GPIbα.