Von Willebrand Factor Multimers Research Articles

Coarse-grained simulations of von Willebrand factor adsorption to collagen with consequent platelet recruitment.

A multimeric glycoprotein of blood plasma-Von Willebrand factor (VWF)-mediates platelet adhesion to the fibrillar collagen of the subendothelial matrix if the blood vessel walls are damaged. The adsorption of VWF to collagen is thus essential for the initial stages of platelet hemostasis and thrombosis, as it plays a role of a molecular bridge between the injury and platelet adhesion receptors. Biomechanical complexity and sensitivity to the hydrodynamics are inherent in this system, therefore, modern computational methods supplement experimental studies of biophysical and molecular mechanisms that underlie platelet adhesion and aggregation in the blood flow. In the present paper, we propose a simulation framework for the VWF-mediated platelet adhesion to a plane wall with immobilized binding sites for VWF under the action of shear flow. VWF multimers and platelets are represented in the model by particles connected by elastic bonds and immersed in a viscous continuum fluid. This work complements the scientific field by taking into account the shape of a flattened platelet, but keeping a compromise between the detail of the description and the computational complexity of the model.

P14: Degradation of High Molecular Weight Multimers of von Willebrand Factor under Whole Blood Conditions in High Turbulent Flow: Implications for Continuous Flow Left Ventricular Assist Devices

Background: High turbulent flow often found in continuous flow left ventricular assist devices (cf-LVADs) is believed to cause acquired von Willebrand Syndrome (AvWS). Our group recently published a study on the kinetic and dynamic effects of various flows on the loss of high molecular weight multimers (HMWMs) of von Willebrand factor (vWF). In the study, we applied high shear to citrated human platelet-poor plasma (PPP) and characterized the flow by Reynolds number (Re). The present study aims to extend upon our previous findings by investigating the degradation of HMWM under whole blood conditions at higher flow conditions to be characterized by energy dissipation rate and clinically relevant exposure times previously employed. Methods: Diluted ovine blood with a hematocrit of 25%, dynamic viscosity of 3.0 cP and density of 1030 kg/m3 was flowed through a custom high shear rotary device. The device is designed to provide a fully controlled exposure time, texp and flow regime. The flow rate was set at 1.75 ml/sec, 0.76 ml/sec, or 0.38 ml/sec, resulting in an exposure time of 22 ms, 50 ms, or 100 ms, respectively. The dynamic flow condition was controlled by the speed of the rotor, ω, and characterized by the energy dissipation rate per unit mass, ε. The ω of 10k, 12k, 15k, 18k, 20k, and 22k RPM were applied, and the corresponding ε were estimated by running a series of large eddy simulations (LES). After each run, a 0.5 ml sample solution was collected, frozen in dry ice immediately, and shipped to the Blood Center of Wisconsin for multimer analysis. The amount of multimer degradation at each testing condition was assessed with respect to the mean of the initial concentration of HMWM of vWF in the controls. Results: The results show that the loss of HMWM at all given exposure times increases in a logarithmic manner within the extended flow conditions (Fig. 1A). The loss of HMWM versus energy dissipation rate at given texp is shown in Fig. 1B. It is noted that as the flow becomes more chaotic, the loss of HMWM appears to be primarily dominated by the energy dissipation rate while the exposure time to chaotic flow becomes less significant. It is not clear if the less significant effect of kinetics at relatively high flow conditions (>18k RPM) is because there are no more HMWM left to be cleaved by ADAMTS13, an enzyme that cleaves vWF. The results are consistent with our previous observations using human PPP, in that both flow conditions and exposure time play crucial roles in the destruction of HMWM. Conclusion: This study demonstrated the relative roles of flow and exposure time using whole blood to characterize the loss of HMWM in terms of energy dissipation rate and exposure time. The findings have the potential to assist in the design and optimization of blood pumps, ultimately leading to improved clinical outcomes for patients with AvWS.

PEDS9: Bleeding in Pediatric Patients on Extracorporeal Membrane Oxygenation-Use of Procoagulant Factors

Background: Bleeding complications remain common in extracorporeal membrane oxygenation (ECMO) and can be associated with adverse outcomes. Effect of excessive shear stress on vasculature may increase bleeding risk associated with preferential loss of high molecular weight von Willebrand factor (high molecular weight (vWF) multimers; acquired vW syndrome, AVWS) and may also be associated with platelet dysfunction. Some pediatric patients may have a pre-existing increased risk of shear-related abnormalities which may further increase bleeding risk on ECMO. We aimed to review bleeding events requiring intervention in a pediatric ECMO cohort. Methods: Retrospective review of pediatric ECMO patients who underwent treatment of bleeding beyond simple transfusion in conjunction with ECMO anticoagulation consultation (EAC) between June 2020 and December 2022 was conducted. Results: Fifty patients required ECMO for 53 runs (52 VA, 2 VV, 1 VAV). Median weight was 4.3 kg (2.4-131 kg). Mean duration of ECMO was 11 days (2-97). 60% (30/50) of patients had congenital heart disease (CHD), and within these 30 patients, 27% of them had bleeding requiring procedural intervention. Ten patients with bleeding were screened for AVWS guided by EAC with 100% meeting criteria for AVWS. Platelet transfusions were administered as first line bleeding therapy with thrombocytopenia or platelet dysfunction and plasma or prothrombin complex concentrate (PCCC) in volume-sensitive patients for coagulopathy. With ongoing bleeding despite transfusion, antihemophilic factor/vWF complex (Humate P) was administered to 4 patients guided by EAC (figure 1). Of these 4 patients, 75% had CHD and 100% had valve abnormalities with 50% having new mechanical valves placed within 1 month prior to ECMO. Initial Humate P dose was given on mean day 14, with mean number of 8 doses. No circuit or mechanical valve thrombosis occurred. Platelet counts and function improved with a decrease in platelet transfusions and clinical bleeding in all patients following Humate P. Conclusion: Bleeding remains a common complication but is often multifactorial and may be difficult to determine specific etiologies. AVWS and thrombocytopenia commonly occur during pediatric ECMO. However, not all patients have clinical bleeding, such that additional risk factors may contribute to increased bleeding risk in these patients. Given impact of recurrent transfusions due to bleeding on morbidity and mortality, as well as cost consideration and potential thrombosis risk of vWF or PCCC supplementation, development of an expert-guided (EAC) algorithm for screening and treatment criteria for such agents appears to be safe, cost-effective and may be associated with decreased bleeding and transfusions. Further research into etiologies and consequences of ECMO-induced bleeding as well as treatment is needed.Figure 1. Algorithm for Assessment of Persistent Bleeding and Consideration of Humate P Administration (pilot).

P107: In Vitro, Benchtop Testing Indicates Improved Hemocompatibility with a New Magnetically Levitated VAD

Purpose: Ventricular assist devices (VADs) improve heart failure outcomes, yet the challenge of reducing bleeding, stroke, and infection persists. Biomarkers related to these adverse events were evaluated with a next-generation, totally magnetically suspended (TMS) VAD in comparison to other blood pumps. Method: Bovine whole blood or human plasma was circulated through benchtop flow loops with an Abbott CentriMag (CM) or HeartMate II (HMII), or a CH Biomedical USA BrioVAD (N = 5 tests). Flows were maintained at 4.5 L/min against an afterload of 75 mmHg. Samples were collected hourly for 6 hours. Plasma free hemoglobin was measured by spectroscopy. Normalized index of hemolysis (NIH) was calculated as described in ASTM F1841-19. VWF function (VWF:CB/Ag) was quantified using VWF:CB and VWF:Ag assay kits; VWF multimer distribution was determined using agarose-gel electrophoresis and in-gel immunofluorescent staining. Statistical significance was determined using an ANOVA (NIH) or regression analysis (VWF:CB/Ag) followed by a post-hoc Tukey test for significance between specific devices. Results: The CM and BrioVAD exhibited significantly lower hemolysis than the HMII (NIH: BrioVAD = 0.006 g/100L, CM = 0.002 g/100L, HMII = 0.029 g/100L; p < 0.01). The BrioVAD preserved VWF function after 6 hours compared to both HMII and CM (Fig. 1; VWF:CB/Ag, 6 h: BrioVAD = 94% retained, CM = 75%, HMII = 74%; p < 0.01). Similarly, the BrioVAD preserved a greater percentage of high molecular weight multimers (HMWM) than HMII and CM (BrioVAD = 96%, CM = 84%, HMII = 76%) with a higher ratio of HMWM to low molecular weight multimers (LMWM) (HMWM/LMWM: BrioVAD = 1.69, CM = 1.22, HMII = 1.09). Conclusion: Both the BrioVAD and CM achieved lower hemolysis than the HMII. However, the BrioVAD alone preserved VWF function and HMWM better than the CM and HMII. The inference that magnetically levitated centrifugal pumps are more hemocompatible than axial flow pumps with mechanical bearings is supported. The observation of improved In Vitro hemocompatibility with a newer magnetically levitated pump is encouraging and warrants further study.Figure 1. VWF function measured over the course of 6 hours.

N-Acetylcistein for thrombotic thrombocytopenic purpura: an observational case series study.

Acquired thrombotic thrombocytopenic purpura (TTP) is a life-threatening disorder. N-Acetylcysteine (NAC) rapidly degrades ultra-large von Willebrand factor multimers by disrupting the disulfide bonds. We report a series of twelve consecutive patients diagnosed with acquired TTP successfully treated with high-dose NAC (150 mg/kg/day) in combination with plasma exchange and steroids. Eight patients also received rituximab. Two patients presented refractory TTP. All patients achieved a quick clinical response in a median time of 5.5 days after starting NAC and are alive after a median follow-up of 29 months. The treatment was feasible and well tolerated. These data provide further evidence of the potential benefit and safety of adding NAC to the standard of care.

Elevated LDL Cholesterol Increases Microvascular Endothelial VWF and Thromboinflammation After Myocardial Infarction.

In reperfused myocardial infarction, VWF (von Willebrand factor)-mediated platelet adhesion contributes to impaired microvascular reflow and possibly also to postmyocardial infarction inflammation. We hypothesized that postischemic thromboinflammatory processes are worsened by elevated LDL (low-density lipoprotein) cholesterol. Myocardial ischemia-reperfusion or sham procedure was performed in wild-type mice and hyperlipidemic mice deficient for the LDL receptor and Apobec-1 (apolipoprotein-B mRNA editing enzyme catalytic polypeptide-1; DKO [double knockout]). DKO subgroups were treated with N-acetylcysteine, which inhibits pro-adhesive VWF multimers or with recombinant ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin motifs-13), which enzymatically cleaves endothelial surface-associated VWF. Myocardial contrast echocardiography perfusion imaging and molecular imaging for VWF, platelet glycoprotein Ibα, and leukocyte CD18 (cluster of differentiation) were performed 30 minutes post-reperfusion. Histology, infarct sizing, and echocardiography were performed at 1.5 or 72 hours; late echocardiography was performed at day 21. After ischemia-reperfusion, DKO compared with wild-type mice had ≈2-fold higher (P<0.05) risk area signal for microvascular platelet adhesion, VWF, and CD18; greater impairment in microvascular reflow, and 2-fold larger infarct size. Treatment of DKO mice with N-acetylcysteine and ADAMTS13 reduced molecular imaging signal for microvascular platelet adhesion, VWF, and CD18; improved early microvascular reflow; and reduced eventual infarct size. ADAMTS13 suppressed the postmyocardial infarction neutrophil and monocyte infiltration, enhanced the time-dependent recovery of left ventricular systolic function, and prevented late left ventricular remodeling. In reperfused myocardial infarction, elevated LDL cholesterol promotes thromboinflammation through excess microvascular endothelial VWF and platelet adhesion, resulting in less microvascular reflow and larger infarct size. In the presence of elevated LDL cholesterol, therapies that suppress endothelial-associated VWF can promote recovery of left ventricular function and protect against remodeling.

Immune and Hereditary Thrombotic Thrombocytopenic Purpura: Can ADAMTS13 Deficiency Alone Explain the Different Clinical Phenotypes?

Thrombotic thrombocytopenic purpura (TTP) is a thrombotic microangiopathy caused by a hereditary or immune-mediated deficiency of the enzyme ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). TTPs are caused by the following pathophysiological mechanisms: (1) the presence of inhibitory autoantibodies against ADAMTS13; and (2) hereditary mutations of the ADAMTS13 gene, which is present on chromosome 9. In both syndromes, TTP results from a severe deficiency of ADAMTS13, which is responsible for the impaired proteolytic processing of high-molecular-weight von Willebrand factor (HMW-VWF) multimers, which avidly interact with platelets and subendothelial collagen and promote tissue and multiorgan ischemia. Although the acute presentation of the occurring symptoms in acquired and hereditary TTPs is similar (microangiopathic hemolytic anemia, thrombocytopenia, and variable ischemic end-organ injury), their intensity, incidence, and precipitating factors are different, although, in both forms, a severe ADAMTS13 deficiency characterizes their physiopathology. This review is aimed at exploring the possible factors responsible for the different clinical and pathological features occurring in hereditary and immune-mediated TTPs.

Use of n-acetylcysteine therapy in patients with relapsed refractory thrombotic thrombocytopenic purpura

There is limited data on the use of NAC in the literature. We would like to present the satisfactory results we obtained in our resistant and relapsed patients as a case series.Thrombotic thrombocytopenic purpura (TTP) is a life-threatening thrombotic microangiopathy caused by ADAMTS13 (a disintegrin with thrombospondin type 1 motif and metalloprotease activity, member13) deficiency. Von Willebrand factor (vWF) initiates platelet aggregation and thus thrombus formation. The multimers of vWF are cleaved by ADAMTS13. Because of the decreased activity of ADAMTS13, ultra-large multimers accumulate and end-organ damage occurs. TTP is characterized by microangiopathic hemolytic anemia (MAHA), severe thrombocytopenia, and organ ischemia resulting from vascular occlusion caused by thrombi. Plasma exchange therapy (PEX) remains the mainstay of TTP therapy. Patients who do not respond to PEX and corticosteroids require additional treatments such as rituximab and caplacizumab. NAC reduces disulfide bonds in mucin polymers through its free sulfhydryl group. Thus, the size and viscosity of the mucins are reduced. VWF is structurally similar to mucin. Based on this similarity, Chen and colleagues showed that NAC can reduce the size and reactivity of ultralarge multimers of vWF, such as ADAMTS13. Currently, there is not much information to suggest that NAC has any clinical value in the treatment of TTP. In this case series of 4 refractory patients, we would like to present the responses we obtained with the addition of NAC therapy. NAC can be added to PEX and glucocorticoid therapy as supportive therapy, especially in unresponsive patients.

Rhomb-I, a P–I metalloproteinase from Lachesis muta rhombeata venom degrades vessel extra cellular matrix components and impairs platelet aggregation

Rhomb-I, a 23-kDa metalloproteinase was isolated from L. m. rhombeata venom. Its dimethylcasein proteolysis was abolished by metal chelators, and slightly enhanced by Ca2+ and Mg2+ ions, but inhibited by Co2+, Zn2+ and α2-macroglobulin. In aqueous solution, rhomb-I autoproteolyzed to a 20- and 11-kDa fragments at 37 °C. The amino acid sequence showed high homology with other snake venom metalloproteinases. Rhomb-I causes hemorrhage that may be ascribed to hydrolysis of essential basement membrane, extracellular matrix and plasma proteins. It preferentially cleaves the α-chains of fibrin (ogen). Rhomb-I inhibited convulxin- and von Willebrand factor (vWF)-induced aggregation on human platelets without significant effect on collagen-stimulated aggregation or other effectors. It digests vWF into a low-molecular-mass multimers of vWF and a rvWF-A1 domain to a 27-kDa fragment as revealed by western blotting with mouse anti-rvWF A1-domain IgG. Incubation of platelets with rhomb-I resulted in adhesion to and cleavage of platelet receptors glycoprotein (GP)Ibα and GPVI to release a 55-kDa soluble form. Both membrane glycoproteins GPIbα that binds vWF, together with GPVI which binds collagen, play a key role in mediating platelet adhesion/activation and can initiate (patho)physiological thrombus formation. Conclusions: rhomb-I is implicated in the pathophysiology of Lachesis envenoming by disrupting vasculature, hemostasis and platelet aggregation through impairing vWF-GPIb axis and blocking GPVI-collagen binding.

Editor’s Choice Articles for April

Editor’s Choice Articles for April

Clinical significance of assessing ADAMTS-13 and von Willebrand factor level in COVID-19 convalescent pregnant women

Introduction. Coronavirus infection is associated with severe endotheliopathy, thromboinflammation and immunothrombosis leading to excessive release of von Willebrand factor (vWF) multimers from Weibel–Palade bodies, which can affect activity of ADAMTS-13 metalloproteinase (a disintegrin and metalloproteinase with thrombospondin type 1 motif, member 13) and the ADAMTS-13/vWF axis previously shown by us to be altered in non-pregnant women with severe COVID-19. Aim: to study a clinical role of hemostasis activation particularly ADAMTS-13/vWF axis in pregnant women after COVID-19. Materials and Methods. A prospective case–control study was conducted with pregnant women (n = 135) divided into 3 groups: group 1 included 45 women with prior COVID-19 during pregnancy, group 2 – 45 women in the acute phase of the infection during pregnancy, group 3 – 45 healthy pregnant women. The level of vWF and ADAMTS-13 was assessed in all patients. Results. The concentration of vWF antigen (vWF:Ag) in the acute period of the disease in pregnant women with COVID-19 was significantly higher compared to the control group (p &lt; 0.001). ADAMTS-13 level in pregnant women after COVID-19 did not differ from that of in control group, while vWF level was significantly higher in 66.7 % (30/45). The ADAMTS-13/vWF ratio was increased and significantly differed both in pregnant patients during the acute period of the disease (p &lt; 0.001) and pregnant women after infection (p = 0.0002) compared with the control group. Conclusion. Our results show that endotheliopathy was prominently manifested in pregnant women with COVID-19 and persisted for several months after disease. The ADAMTS-13/vWF ratio determines the pathway functioning, the risk of microcirculation disorders and clinical complications.

Functioning of the ADAMTS-13/vWF axis and its clinical significance

Advances in biology have allowed us to substantially deepen our knowledge about hemostasis functioning both in health and disease. ADAMTS-13 (a disintegrin and metalloproteinase with thrombospondin type 1 motif, member 13) and von Willebrand factor (vWF) are components of the hemostasis system, which physiological interaction holds an important place in maintaining homeostasis. ADAMTS-13 is a metalloproteinase mainly acting to release vWF fragments into the blood plasma, as well as regulating its activity by cleaving ultra-large vWF multimers (UL-vWF) into smaller and less active forms. The study of such factors is of great clinical importance, since a decrease in ADAMTS-13 activity and an increase in vWF level can be predictors of microcirculatory disorders that play an important role in developing multiple organ failure. However, very few and fully contradictory studies devoted to the physiological aspects of the ADAMTS-13/vWF axis functioning in the mother–fetus system are available, therefore requiring to be further investigated.

An evaluation of von Willebrand factor (recombinant) therapy for adult patients living with severe type 3 von Willebrand disease

ABSTRACT Introduction Von Willebrand Factor (VWF) containing concentrates have been used for the treatment of von Willebrand Disease (VWD) for many years. Recently, however, a novel recombinant VWF (rVWF or vonicog alpha, VONVENDI [US], VEYVONDI [Europe]) has arrived to the market for the treatment of VWD. Initially, rVWF was approved by the U.S. Food and Drug Administration (FDA) for the on-demand treatment and control of bleeding episodes and for the perioperative management of bleeding for patients with VWD. More recently, however, the FDA has approved rVWF for routine prophylaxis to prevent bleeding episodes for those patients with severe type 3 VWD receiving on-demand therapy. Areas Covered This review will focus on recent phase III trial results from NCT02973087 regarding the use of long-term routine twice weekly prophylaxis with rVWF for the prevention of bleed events in patients with severe type 3 VWD. Expert Opinion A novel rVWF concentrate may have greater hemostatic potential over prior plasma-derived VWF concentrates and is now FDA approved for use in routine prophylaxis for patients with severe type 3 VWD in the United States. This greater hemostatic potential may be due to the presence of ultra-large VWF multimers and a more favorable high-molecular-weight multimer pattern compared to prior pdVWF concentrates.

The Ratio of von Willebrand Factor Antigen to ADAMTS13 Activity: Usefulness as a Prognostic Biomarker in Acute-on-Chronic Liver Failure

Acute-on-chronic liver failure (ACLF) has a high risk of short-term mortality. A disintegrin-like and metalloproteinase with thrombospondin type-1 motifs 13 (ADAMTS13) is a metalloproteinase that specifically cleaves multimeric von Willebrand factor (VWF). Imbalance between ADAMTS13 and VWF is associated with portal hypertension, which induces ACLF development. A previous study reported that ADAMTS13 activity (ADAMTS13:AC) and VWF antigen (VWF:Ag) are predictive biomarkers of ACLF development in patients with cirrhosis. This study investigated the changes in ADAMTS13:AC and VWF:Ag levels from before to after the development of ACLF to determine their usefulness as a prognostic biomarker in patients with ACLF. In total, 101 patients with cirrhosis were enrolled in this study. The level of ADAMTS13:AC and VWF:Ag was determined by an enzyme-linked immunosorbent assay. Cox proportional hazard regression analysis was conducted to determine independent prognostic factors for patients with liver cirrhosis in the post-ACLF group. ADAMTS13:AC levels gradually decreased in the order of non-ACLF group, pre-ACLF group, and finally post-ACLF group. VWF:Ag and the ratio of VWF:Ag to ADAMTS13:AC (VWF:Ag/ADAMTS13:AC) levels gradually increased in the order of non-ACLF group, pre-ACLF group, followed by post-ACLF group. VWF:Ag/ADAMTS13:AC and CLIF-C ACLF scores were associated with prognosis in the post-ACLF group in multivariate analysis. The cumulative survival of the post-ACLF group was significantly lower for patients with high VWF:Ag/ADAMTS13:AC (>9) compared with those with low VWF:Ag/ADAMTS13:AC (≤9) (HR: 10.72, 95% confidence interval: 1.39-82.78, p < 0.05). The VWF:Ag/ADAMTS13:AC increased according to the progression of ACLF in patients with cirrhosis and predicted prognosis in patients with cirrhosis with ACLF.

The role of von Willebrand factor in hemostasis pathology

Von Willebrand factor (VWF) is a multimeric plasma glycoprotein present in endothelial cells, megakaryocytes, platelets, and connective tissue. It mediates platelet adhesion in small arteries. VWF also binds and protects coagulation factor VIII from degradation. Moreover, VWF is involved in inflammatory response, linking hemostasis and inflammation. VWF multimers and platelets attached to damaged or activated endothelium mediate leukocyte recruitment, facilitating local inflammatory response. At shear rates above 5000 s–1, VWF molecules are capable of hydrodynamic activation that changes their conformation from globular to fibrillar. Therefore, VWF plays a key role in cellular hemostasis at high shear rates. Acquired and inherited disfunction, defective synthesis or increased proteolysis of VWF multimers lead to bleeding, as in von Willebrand disease or Heyde syndrome. Pathological activation of VWF may lead to the development of thrombotic complications of coronary artery disease. COVID-19, especially severe form, is characterized by prothrombotic shift in pulmonary vascular bed. Following endothelial damage, VWF plasma level rises and ADAMTS-13 activity decreases. In patients with COVID-19, a change in the VWF/ADAMTS-13 ratio is associated with an increase in the risk of thromboembolic complications. Therefore, assessment of hydrodynamic activation of VWF under flow conditions may be valuable in fundamental research and laboratory diagnostics.

Hidden behind thromboinflammation: revealing the roles of von Willebrand factor in sickle cell disease pathophysiology.

This review provides an update on the pathophysiology of sickle cell disease (SCD) with a particular focus on the dysregulation of the von Willebrand factor (VWF) - ADAMTS13 axis that contributes to its pathogenesis. In discussing recent developments, we hope to encourage new and ongoing discussions surrounding therapeutic targets for SCD. Within the last 5 years, the role of VWF in the pathophysiology of SCD has been further elucidated and is now a target of study in ongoing clinical trials. The pathophysiology of SCD is multifaceted, as it involves systemwide vascular activation, altered blood rheology, and the activation of immune responses and coagulative pathways. The presence of VWF in excess in SCD, particularly in its largest multimeric form, greatly contributes to its pathogenesis. Understanding the molecular mechanisms that underly the presence of large VWF multimers in SCD will provide further insight into the pathogenesis of SCD and provide specific targets for therapy.

Biomechanical activation of blood platelets via adhesion to von Willebrand factor studied with mesoscopic simulations.

Platelet adhesion and activation are essential initial processes of arterial and microvascular hemostasis, where high hydrodynamic forces from the bloodflow impede coagulation. The process relies on von Willebrand factor (VWF)-a linear multimeric protein of blood plasma plays a pivotal role in mechanochemical regulation of shear-induced platelet aggregation (SIPA). Adhesive interactions between VWF and glycoprotein receptors GPIb are crucial for platelet recruitment under high shear stress in fluid. Recent advances in experimental studies revealed that mechanical tension on the extracellular part of GPIb may trigger a cascade of biochemical reactions in platelets leading to activation of integrins [Formula: see text] (also known as GPIIb/IIIa) and strengthening of the adhesion. The present paper is aimed at investigation of this process by three-dimensional computer simulations of platelet adhesion to surface-grafted VWF multimers in pressure-driven flow of platelet-rich plasma. The simulations demonstrate that GPIb-mediated mechanotransduction is a feasible way of platelet activation and stabilization of platelet aggregates under high shear stress. Quantitative understanding of mechanochemical processes involved in SIPA would potentially promote the discovery of new anti-platelet medication and the development of multiscale numerical models of platelet thrombosis and hemostasis.

Structural hierarchy of mechanical extensibility in human von Willebrand factor multimers.

The von Willebrand factor (VWF) is a multimeric glycoprotein composed of 80- to 120-nm-long protomeric units and plays a fundamental role in mediating platelet function at high shear. The exact nature of the shear-induced structural transitions have remained elusive; uncovering them requires the high-resolution quantitative analysis of gradually extended VWF. Here, we stretched human blood-plasma-derived VWF with molecular combing and analyzed the axial structure of the elongated multimers with atomic force microscopy. Protomers extended through structural intermediates that could be grouped into seven distinct topographical classes. Protomer extension thus progresses through the uncoiling of the C1-6 domain segment, rearrangements among the N-terminal VWF domains, and unfolding and elastic extension of the A2 domain. The least and most extended protomer conformations were localized at the ends and the middle of the multimer, respectively, revealing an apparent necking phenomenon characteristic of plastic-material behavior. The structural hierarchy uncovered here is likely to provide a spatial control mechanism to the complex functions of VWF.

Transcatheter aortic valve implantation in patient with Heyde’s syndrome

Clinical case of an 85-year-old patient with Heid’s syndrome is being reported: recurrent gastrointestinal bleeding due to angiodysplasia of the stomach and intestines in combination with acquired destruction of large von Willebrand factor multimers in severe aortic stenosis. Actually it remains difficult to make a diagnosis in these patients and perform timely surgical intervention. Considering the age of the patient, severity of manifestations of heart failure and high risk of surgical complications, transcatheter aortic valve implantation (TAVI) was the method of choice. Surgical correction of heart disease in this case is aimed not only at treating its clinical manifestations, but also at eliminating the cause of recurrent gastrointestinal bleeding in Hyde’s syndrome.

Proximity proteomics identifies septins and PAK2 as decisive regulators of actomyosin-mediated expulsion of von Willebrand factor

AbstractIn response to tissue injury, within seconds the ultra-large glycoprotein von Willebrand factor (VWF) is released from endothelial storage organelles (Weibel−Palade bodies) into the lumen of the blood vasculature, where it leads to the recruitment of platelets. The marked size of VWF multimers represents an unprecedented burden on the secretory machinery of endothelial cells (ECs). ECs have evolved mechanisms to overcome this, most notably an actomyosin ring that forms, contracts, and squeezes out its unwieldy cargo. Inhibiting the formation or function of these structures represents a novel therapeutic target for thrombotic pathologies, although characterizing proteins associated with such a dynamic process has been challenging. We have combined APEX2 proximity labeling with an innovative dual loss-of-function screen to identify proteins associated with actomyosin ring function. We show that p21 activated kinase 2 (PAK2) recruits septin hetero-oligomers, a molecular interaction that forms a ring around exocytic sites. This cascade of events controls actomyosin ring function, aiding efficient exocytic release. Genetic or pharmacological inhibition of PAK2 or septins led to inefficient release of VWF and a failure to form platelet-catching strings. This new molecular mechanism offers additional therapeutic targets for the control of thrombotic disease and is highly relevant to other secretory systems that employ exocytic actomyosin machinery.