Human blood platelet adhesion to artery subendothelium is mediated by factor VIII-Von Willebrand factor bound to the subendothelium

A new look at an old body: molecular determinants of Weibel-Palade body composition and von Willebrand factor exocytosis

Endothelial cells, forming a monolayer along blood vessels, intricately regulate vascular hemostasis, inflammatory responses, and angiogenesis. A key determinant of these functions is the controlled secretion of Weibel-Palade bodies (WPBs), which are specialized endothelial storage organelles housing a presynthesized pool of the hemostatic protein von Willebrand factor and various other hemostatic, inflammatory, angiogenic, and vasoactive mediators. This review delves into recent mechanistic insights into WPB biology, including the biogenesis that results in their unique morphology, the acquisition of intraluminal vesicles and other cargo, and the contribution of proton pumps to organelle acidification. Additionally, in light of a number of proteomic approaches to unravel the regulatory networks that control WPB formation and secretion, we provide a comprehensive overview of the WPB exocytotic machinery, including their molecular and cellular mechanisms.

A novel bifunctional multilayered nanofibrous membrane combining polycaprolactone and poly (vinyl alcohol) enriched with platelet lysate for skin wound healing

Skin wound healing is a complex physiological process that involves various cell types, growth factors, cytokines, and other bioactive compounds. In this study, a novel dual-function multilayered nanofibrous membrane is developed for chronic wound application. The membrane is composed of five alternating layers of polycaprolactone (PCL) and poly (vinyl alcohol) (PVA) nanofibers (PCL-PVA) with a dual function: the PCL nanofibrous layers allow cell adhesion and growth, and the PVA layers enriched with incorporated platelet lysate (PCL-PVA + PL) serve as a drug delivery system for continuous release of bioactive compounds from PL into an aqueous environment. The material is produced using a needleless multi-jet electrospinning approach which can lead to homogeneous large-scale production. The bioactive PCL-PVA + PL membranes are cytocompatible and hemocompatible. A spatially compartmented co-culture of three cell types involved in wound healing - keratinocytes, fibroblasts and endothelial cells - is used for cytocompatibility studies. PCL-PVA + PL membranes enhance the proliferation of all cell types and increase the migration of both fibroblasts and endothelial cells. The membranes are also hemocompatible without any deleterious effect for thrombogenicity, hemolysis and coagulation. Thus, the beneficial effect of the PCL-PVA + PL membrane is demonstrated in vitro, making it a promising scaffold for the treatment of chronic wounds.

Von Willebrand factor hyperactivity affects the outcome of lower limb revascularization in subjects with type 2 diabetes mellitus complicated by diabetic foot vasculopathy: An observational pilot study

Aim of this study is to evaluate any differences in VWF antigen, VWF activity and ADAMTS-13 activity before and after successful and non-successful Percutaneous Transluminal Angioplasty (PTA) in subjects with type 2 diabetes (T2DM) complicated by Chronic limb-threatening ischemia (CLTI) in diabetic foot vasculopathy.

METHODS

In this prospective observational pilot study, we enrolled 35 T2DM subjects who underwent lower limb PTA. Transcutaneous oximetry was performed in all patients before and 6 weeks after PTA. The change in oxygen partial pressure (TcpO2) before and after PTA was expressed as TcpO2-delta (ΔTcpO2). VWF antigen, VWF activity and ADAMTS-13 activity were measured before and 6 weeks after PTA; changes were expressed as delta and ratio from baseline.

RESULTS

Subjects with ∆TcpO2 < 15 mmHg presented higher ΔVWF activity (p = 0.050) and lower ADAMTS-13 activity ratio (p = 0.080). Subjects with ∆TcpO2 < 30 mmHg showed lower ADAMTS-13 activity Δ and ratio (p = 0.028).

CONCLUSIONS

VWF antigen levels and VWF activity may potentially affect PTA outcome. Higher levels of VWF could derive from VWF release as consequence of PTA-induced mechanical endothelial damage and/or oxidative stress-induced modifications of VWF structure with impairment of VWF-ADAMTS13 interactions.

Thrombosis and cachexia in cancer: two partners in crime?

Among cancer patients, thrombosis and cachexia are major causes of morbidity and mortality. Although the two may occur together, little is known about their possible relationship. Thus, a literature review was conducted by screening the databases PubMed, Scopus, SciELO, Medline and Web of Science. To summarize, cancer-associated thrombosis (CAT) and cancer-associated cachexia (CAC) seem to share several patient-, tumour- and treatment-related risk factors. Inflammation alongside metabolic and endocrine derangement is the potential missing link between CAT, CAC and cancer. Many key players, including specific pro-inflammatory cytokines, immune cells and hormones, appear to be implicated in both thrombosis and cachexia, representing attractive predictive markers and potential therapeutic targets. Altogether, the current evidence suggests a link between CAT and CAC, however, epidemiological studies are required to explore this potential relationship. Given the high incidence and negative impact of both diseases, further studies are needed for the better management of cancer patients.

Neuroinflammation and neovascularization in diabetic eye diseases (DEDs): identification of potential pharmacotherapeutic targets

The goal of this review is to increase public knowledge of the etiopathogenesis of diabetic eye diseases (DEDs), such as diabetic retinopathy (DR) and ocular angiosarcoma (ASO), and the likelihood of blindness among elderly widows. A widow's life in North India, in general, is fraught with peril because of the economic and social isolation it brings, as well as the increased risk of death from heart disease, hypertension, diabetes, depression, and dementia. Neovascularization, neuroinflammation, and edema in the ocular tissue are hallmarks of the ASO, a rare form of malignant tumor. When diabetes, hypertension, and aging all contribute to increased oxidative stress, the DR can proceed to ASO. Microglia in the retina of the optic nerve head are responsible for causing inflammation, discomfort, and neurodegeneration. Those that come into contact with them will get blind as a result of this. Advanced glycation end products (AGE), vascular endothelial growth factor (VEGF), protein kinase C (PKC), poly-ADP-ribose polymerase (PARP), metalloproteinase9 (MMP9), nuclear factor kappaB (NFkB), program death ligand1 (PDL-1), factor VIII (FVIII), and von Willebrand factor (VWF) are potent agents for ocular neovascularisation (ONV), neuroinflammation and edema in the ocular tissue. AGE/VEGF, DAG/PKC, PARP/NFkB, RAS/VEGF, PDL-1/PD-1, VWF/FVIII/VEGF, and RAS/VEGF are all linked to the pathophysiology of DEDs. The interaction between ONV and ASO is mostly determined by the VWF/FVIII/VEGF and PDL-1/PD-1 axis. This study focused on retinoprotective medications that can pass the blood-retinal barrier and cure DEDs, as well as the factors that influence the etiology of neovascularization and neuroinflammation in the eye.

Low Bone Mineral Density in Hemophiliacs

Objective

To review the current knowledge on bone health in patients with hemophilia A and the underlying pathogenetic mechanisms.

Data Sources

Original research articles, meta-analyses, and scientific reviews.

Data Synthesis

Already in childhood, patients with hemophilia A are prone to low bone mineral density, leading to osteopenia and/or osteoporosis. Initially associated with the life style of hemophilia, today we are faced with accumulating evidence that coagulation factor VIII is involved directly or indirectly in bone physiology.

Conclusion

Understanding the role of factor VIII and the mechanisms of decreased bone mineral density in hemophilia A is critically important, especially as non-factor replacement therapies are available, and treatment decisions potentially impact bone health.

Type IV Pili of Streptococcus sanguinis Contribute to Pathogenesis in Experimental Infective Endocarditis

Streptococcus sanguinis is a common cause of infective endocarditis (IE). Efforts by research groups are aimed at identifying and characterizing virulence factors that contribute to the ability of this organism to cause IE. This Gram-positive pathogen causes heart infection by gaining access to the bloodstream, adhering to host extracellular matrix protein and/or platelets, colonizing the aortic endothelium, and incorporating itself into the aortic vegetation. While many virulence factors have been reported to contribute to the ability of S. sanguinis to cause IE, it is noteworthy that type IV pili (T4P) have not been described to be a virulence factor in this organism, although S. sanguinis strains typically encode these pili. Type IV pili are molecular machines that are capable of mediating diverse virulence functions and surface motility. T4P have been shown to mediate twitching motility in some strains of S. sanguinis, although in most strains it has been difficult to detect twitching motility. While we found that T4P are dispensable for direct in vitro platelet binding and aggregation phenotypes, we show that they are critical to the development of platelet-dependent biofilms representative of the cardiac vegetation. We also observed that T4P are required for in vitro invasion of S. sanguinis into human aortic endothelial cells, which indicates that S. sanguinis may use T4P to take advantage of an intracellular niche during infection. Importantly, we show that T4P of S. sanguinis are critical to disease progression (vegetation development) in a native valve IE rabbit model. The results presented here expand our understanding of IE caused by S. sanguinis and identify T4P as an important virulence factor for this pathogen.

IMPORTANCE This work provides evidence that type IV pili produced by Streptococcus sanguinis SK36 are critical to the ability of these bacteria to attach to and colonize the aortic heart valve (endocarditis). We found that an S. sanguinis type IV pili mutant strain was defective in causing platelet-dependent aggregation in a 24-h infection assay but not in a 1-h platelet aggregation assay, suggesting that the type IV pili act at later stages of vegetation development. In a rabbit model of disease, a T4P mutant strain does not develop mature vegetations that form on the heart, indicating that this virulence factor is critical to disease and could be a target for IE therapy.

Endothelial cells and coagulation

Endothelial cells form a monolayer, which lines blood vessels. They are crucially involved in maintaining blood fluidity and providing controlled vascular hemostasis at sites of injury. Thereby endothelial cells facilitate multiple mechanisms, including both procoagulant and anticoagulant, which must be kept in balance. Under physiological conditions, endothelial cells constitute a nonadhesive surface preventing activation of platelets and the coagulation cascade. Multiple fibrinolytic and antithrombotic properties act on their cell surface contributing to the maintenance of blood fluidity. These include platelet inhibition, the heparin-antithrombin III system, tissue factor pathway inhibition, thrombomodulin/protein C system, and fibrinolytic qualities. At sites of vascular damage, platelets react immediately by adhering to the exposed extracellular matrix, followed by platelet-platelet interactions to form a clot that effectively seals the injured vessel wall to prevent excessive blood loss. For solid thrombus formation, functional platelets are essential. In this process, endothelial cells serve as a support surface for formation of procoagulant complexes and clotting. This review gives an overview about the central role of the endothelium as a dynamic lining which controls the complex interplay of the coagulation system with the surrounding cells.

Loss of GPVI and GPIbα contributes to trauma-induced platelet dysfunction in severely injured patients

Trauma-induced coagulopathy (TIC) is a complex, multifactorial failure of hemostasis that occurs in 25% of severely injured patients and results in a fourfold higher mortality. However, the role of platelets in this state remains poorly understood. We set out to identify molecular changes that may underpin platelet dysfunction after major injury and to determine how they relate to coagulopathy and outcome. We performed a range of hemostatic and platelet-specific studies in blood samples obtained from critically injured patients within 2 hours of injury and collected prospective data on patient characteristics and clinical outcomes. We observed that, although platelet counts were preserved above critical levels, circulating platelets sampled from trauma patients exhibited a profoundly reduced response to both collagen and the selective glycoprotein VI (GPVI) agonist collagen-related peptide, compared with those from healthy volunteers. These responses correlated closely with overall clot strength and mortality. Surface expression of the collagen receptors GPIbα and GPVI was reduced on circulating platelets in trauma patients, with increased levels of the shed ectodomain fragment of GPVI detectable in plasma. Levels of shed GPVI were highest in patients with more severe injuries and TIC. Collectively, these observations demonstrate that platelets experience a loss of GPVI and GPIbα after severe injury and translate into a reduction in the responsiveness of platelets during active hemorrhage. In turn, they are associated with reduced hemostatic competence and increased mortality. Targeting proteolytic shedding of platelet receptors is a potential therapeutic strategy for maintaining hemostatic competence in bleeding and improving the efficacy of platelet transfusions.

The Role of Anesthetic Selection in Perioperative Bleeding

Perioperative bleeding is one of the major comorbidities associated with surgery. While anesthesia is a critical component to perform surgery, a number of clinical studies supported the contribution of anesthetic drugs to perioperative bleeding. Here, we reviewed the literature on this topic including the underlying mechanism and discussed the future direction on coagulation research in anesthesia.

Titanium implant alters the effect of zoledronic acid on the behaviour of endothelial cells

OBJECTIVES

To evaluate the effect of zoledronic acid (ZA) on human umbilical vein endothelial cells (HUVECs) attached to different surfaces.

MATERIALS AND METHODS

A total of three groups were evaluated in this study: sandblasting and acid etching (SLA) + HUVECs; mechanically polished (MP) + HUVECs; and plastic cell culture plates + HUVECs. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, surface roughness and water contact angle were tested for titanium surface characterisation. ZA was added at different concentrations (0, 1, 10, 50 and 100 μM). Cell adhesion, proliferation, viability, apoptosis and gene expression were evaluated.

RESULTS

Mechanically polished and SLA surfaces showed negative effects on cell adhesion and proliferation and promoted cell apoptosis with 100 μM ZA (p < .05). The highest expression of intercellular adhesion molecule-1 (ICAM-1) and angiopoietin-1 was found on SLA surfaces (p < .01). The lowest expression of platelet-endothelial cell adhesion molecule-1 and ICAM-1 was found on MP surfaces (p < .05). A significant decrease in von Willebrand factor was detected on MP and SLA surfaces (p < .001).

CONCLUSIONS

Zoledronic acid has an anti-angiogenic effect on HUVECs attached to titanium implants, while the SLA surface might stimulate HUVECs to express angiogenic and adhesive factor genes despite ZA treatment.

The Effect of a Polyester Nanofibrous Membrane with a Fibrin-Platelet Lysate Coating on Keratinocytes and Endothelial Cells in a Co-Culture System

Chronic wounds affect millions of patients worldwide, and it is estimated that this number will increase steadily in the future due to population ageing. The research of new therapeutic approaches to wound healing includes the development of nanofibrous meshes and the use of platelet lysate (PL) to stimulate skin regeneration. This study considers a combination of a degradable electrospun nanofibrous blend of poly(L-lactide-co-ε-caprolactone) and poly(ε-caprolactone) (PLCL/PCL) membranes (NF) and fibrin loaded with various concentrations of PL aimed at the development of bioactive skin wound healing dressings. The cytocompatibility of the NF membranes, as well as the effect of PL, was evaluated in both monocultures and co-cultures of human keratinocytes and human endothelial cells. We determined that the keratinocytes were able to adhere on all the membranes, and their increased proliferation and differentiation was observed on the membranes that contained fibrin with at least 50% of PL (Fbg + PL) after 14 days. With respect to the co-culture experiments, the membranes with fibrin with 20% of PL were observed to enhance the metabolic activity of endothelial cells and their migration, and the proliferation and differentiation of keratinocytes. The results suggest that the newly developed NF combined with fibrin and PL, described in the study, provides a promising dressing for chronic wound healing purposes.

Development of a dual hybrid AAV vector for endothelial-targeted expression of von Willebrand factor

Von Willebrand disease (VWD), the most common inherited bleeding disorder in humans, is caused by quantitative or qualitative defects in von Willebrand factor (VWF). VWD represents a potential target for gene therapy applications, as a single treatment could potentially result in a long-term correction of the disease. In recent years, several liver-directed gene therapy approaches have been exploited for VWD, but their efficacy was generally limited by the large size of the VWF transgene and the reduced hemostatic activity of the protein produced from hepatocytes. In this context, we aimed at developing a gene therapy strategy for gene delivery into endothelial cells, the natural site of biosynthesis of VWF. We optimized an endothelial-specific dual hybrid AAV vector, in which the large VWF cDNA was put under the control of an endothelial promoter and correctly reconstituted upon cell transduction by a combination of trans-splicing and homologous recombination mechanisms. In addition, we modified the AAV vector capsid by introducing an endothelial-targeting peptide to improve the efficiency for endothelial-directed gene transfer. This vector platform allowed the reconstitution of full-length VWF transgene both in vitro in human umbilical vein endothelial cells and in vivo in VWD mice, resulting in long-term expression of VWF.

von Willebrand factor promotes platelet-induced metastasis of osteosarcoma through activation of the VWF-GPIb axis

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Immunohistochemistry results directly show VWF is increased during tumor progression.

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VWF is expressed as low molecular weight multimer in OS cell line SAOS2.

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VWF promotes platelet-induced metastasis of OS through VWF-GPIb pathway.

von Willebrand factor (VWF) is exclusively expressed in endothelial cells (ECs) and megakaryocytes, which plays a crucial role in the initiation of arterial thrombosis. Recent studies have shown that VWF is also expressed in osteosarcoma (OS) cells and participates in adhesion of cancer cells to platelets, thus promoting metastasis of OS cells. However, it is unclear how OS cell-derived VWF-platelet interaction contributes to the metastasis of OS. We hypothesized that the interaction is mediated by the binding between VWF A1 and GPIbα of platelets, a molecular mechanism similar to that of thrombosis. The increased expression of VWF in SAOS2 cells may contribute to the enhancement of platelet adhesion through the VWF-GPIb pathway, which could promote the migration and invasion capacities of SAOS2 cells in vitro. Antibodies that block the pathway could significantly inhibit the platelet-induced metastasis of OS cells. Our results suggest a theoretical basis for the development of new anti-OS metastasis drugs, and further enrich the mechanism of OS metastasis.

Pre-procedural abnormal function of von Willebrand Factor is predictive of bleeding after surgical but not transcatheter aortic valve replacement

Both transcatheter aortic valve implantation (TAVI) and surgical aortic valve replacement (SAVR) have been proven to effectively correct von Willebrand Factor (vWF) pathologies, however there is limited data simultaneously comparing outcomes of both approaches. We prospectively enrolled patients with severe aortic stenosis referred for TAVI (n = 52) or SAVR (n = 48). In each case, vWF antigen (vWF:Ag), vWF activity (vWF:Ac) and activity-to-antigen (vWF:Ac/Ag) ratio were assessed at baseline, 24 h and 72 h after procedure. VWF abnormalities were defined as reduced vWF:Ac/Ag ratio (< 0.8). Bleeding events in both arms were classified according to Valve Academic Research Consortium (VARC-2) definitions. Overall, there was no difference between patients referred for TAVI and SAVR in vWF:Ac (1.62 ± 0.52 vs 1.71 ± 0.64; p = 0.593), vWF:Ag (1.99 ± 0.81 vs 2.04 ± 0.81; p = 0.942) or vWF:Ac/Ag ratio (0.84 ± 0.16 vs 0.85 ± 0.12; p = 0.950). Pathological vWF:Ac/Ag ratio was found in 20 (38%) TAVI and 15 (31%) SAVR patients (p = 0.407). Normalization of vWF:Ac/Ag ratio at day 3 after procedure was achieved in 19 (95%) TAVI and 13 (87%) SAVR patients (p = 0.439). Similar prevalence of major or life-threatening bleedings (MLTB) after TAVI and SAVR in entire groups was observed (19% vs. 23%, p = 0.652). VWF abnormalities were associated with higher incidence of MLTB in SAVR (53% vs 9%, p < 0.001), but not TAVI (15% vs. 22%, p = 0.132). Accordingly, in receiver-operating characteristic curve analysis vWF:Ac/Ag ratio < 0.8 showed significant sensitivity and specificity for predicting MLTB in SAVR group (AUC 0.735, 95% CI 0.538–0.931, p = 0.019). We proved that abnormal function of vWF is corrected successfully after both TAVI and SAVR, but vWF abnormalities are predictive of MLTB only in surgical patients.

Simulation of thrombosis in a stenotic microchannel: The effects of vWF-enhanced shear activation of platelets

This study was undertaken to develop a numerical/computational simulation of von Willebrand Factor (vWF) - mediated platelet shear activation and deposition in an idealized stenosis. Blood is treated as a multi-constituent mixture comprised of a linear fluid component and a porous solid component (thrombus). Chemical and biological species involved in coagulation are modeled using a system of coupled convection-reaction-diffusion (CRD) equations. This study considers the cumulative effect of shear stress (history) on platelet activation. The vWF activity is modeled as an enhancement function for the shear stress accumulation and is related to the experimentally-observed unfolding rate of vWF. A series of simulations were performed in an idealized stenosis in which the predicted platelets deposition agreed well with previous experimental observations spatially and temporally, including the reduction of platelet deposition with decreasing expansion angle. Further simulation indicated a direct relationship between vWF-mediated platelet deposition and degree of stenosis. Based on the success with these benchmark simulations, it is hoped that the model presented here may provide additional insight into vWF-mediated thrombosis and prove useful for the development of more hemo-compatible blood-wetted devices in the future.

Hemophilia A and B mice, but not VWF-/-mice, display bone defects in congenital development and remodeling after injury

While joint damage is the primary co-morbidity of hemophilia, osteoporosis and osteopenia are also observed. Coagulation factor VIII deficient (FVIII^−/−) mice develop an osteoporotic phenotype in the absence of induced hemarthrosis that is exacerbated two weeks after an induced joint injury. Here we have compared comprehensively the bone health of clotting factor VIII, factor IX, and Von Willebrand Factor knockout (FVIII^−/−, FIX^−/−, and VWF^−/− respectively) mice both in the absence of joint hemorrhage and following induced joint injury. We found FVIII^−/− and FIX^−/− mice, but not VWF^−/− mice, developmentally have an osteoporotic phenotype. Unilateral induced hemarthrosis causes further bone damage in both FVIII^−/− and FIX^−/− mice, but has little effect on VWF^−/− bone health, indicating that the FVIII.VWF complex is not required for normal bone remodeling in vivo. To further investigate the bone healing following hemarthrosis in hemophilia we examined a two week time course using microCT, serum chemistry, and histological analysis. Elevated ratio of osteoprotegerin (OPG)/receptor activator of nuclear factor-kappa B ligand (RANKL), increased osterix⁺ osteoblastic cells, and decreased smoothness of the cortical bone surface were evident within several days of injury, indicative of acute heterotopic mineralization along the cortical surface. This was closely followed by increased interleukin-6 (IL-6) levels, increased osteoclast numbers, and significant trabecular bone loss. Uncoupled and disorganized bone formation and resorption continued for the duration of the study resulting in significant deterioration of the joint. Further elucidation of the shared mechanisms underlying abnormal bone homeostasis in the absence of FVIII or FIX is needed to guide evidence-based approaches to the screening and treatment of the prevalent bone defects in hemophilia A and B.

Evaluation of von Willebrand factor concentrates by platelet adhesion to collagen using an in vitro flow assay

BACKGROUND

Von Willebrand disease (VWD) results from quantitative or qualitative deficiency of von Willebrand factor (VWF) and is treated using VWF-containing concentrates. Several studies have compared the function of various VWF containing concentrates however this has not been performed using shear based assays.

OBJECTIVES

To compare the platelet-capture potential of 10 commercially available, plasma-derived VWF concentrates under shear conditions.

METHODS

VWF containing concentrates were assessed for VWF:Ag, VWF:CB, VWF:RCo, factor VIII:C ADAMTS13 content, VWF multimeric profile and glycan content using lectin binding assays. Free-thiol content of each concentrate was investigated using MPB binding assays. An in vitro flow assay was used to determine the ability of each concentrate to mediate platelet capture to collagen.

RESULTS

VWF multimeric analysis revealed reduction of high molecular weight (HMW) forms in four of the concentrates (Alphante, Octanate and Haemoctin, and 8Y). The high MW multimer distribution of the remaining six concentrates (Optivate, Wilate, Fandhi, Wilfactin, Haemate P, and Voncento) was similar to the plasma control. Lectin analysis demonstrated that 8Y had increased amount of T-antigen. Although platelet capture after 5 minutes perfusion was similar for all concentrates; Alphante, Octanate, and Haemoctin, demonstrated the lowest levels of platelet capture after 60 seconds of perfusion. Free-thiol content and ADAMTS13 levels varied widely between the concentrates but was not correlated with function.

CONCLUSION

Alphanate, Octanate, and Haemoctin, lacked HMW multimers and had the lowest initial platelet capture levels suggesting that the presence of VWF HMW multimers are required for initial platelet deposition.

Thrombosis in Cerebral Aneurysms and the Computational Modeling Thereof: A Review

Thrombosis is a condition closely related to cerebral aneurysms and controlled thrombosis is the main purpose of endovascular embolization treatment. The mechanisms governing thrombus initiation and evolution in cerebral aneurysms have not been fully elucidated and this presents challenges for interventional planning. Significant effort has been directed towards developing computational methods aimed at streamlining the interventional planning process for unruptured cerebral aneurysm treatment. Included in these methods are computational models of thrombus development following endovascular device placement. The main challenge with developing computational models for thrombosis in disease cases is that there exists a wide body of literature that addresses various aspects of the clotting process, but it may not be obvious what information is of direct consequence for what modeling purpose (e.g., for understanding the effect of endovascular therapies). The aim of this review is to present the information so it will be of benefit to the community attempting to model cerebral aneurysm thrombosis for interventional planning purposes, in a simplified yet appropriate manner. The paper begins by explaining current understanding of physiological coagulation and highlights the documented distinctions between the physiological process and cerebral aneurysm thrombosis. Clinical observations of thrombosis following endovascular device placement are then presented. This is followed by a section detailing the demands placed on computational models developed for interventional planning. Finally, existing computational models of thrombosis are presented. This last section begins with description and discussion of physiological computational clotting models, as they are of immense value in understanding how to construct a general computational model of clotting. This is then followed by a review of computational models of clotting in cerebral aneurysms, specifically. Even though some progress has been made towards computational predictions of thrombosis following device placement in cerebral aneurysms, many gaps still remain. Answering the key questions will require the combined efforts of the clinical, experimental and computational communities.

Flow chamber and microfluidic approaches for measuring thrombus formation in genetic bleeding disorders

Platelet adhesion and aggregation, coagulation, fibrin formation, and fibrinolysis are regulated by the forces and flows imposed by blood at the site of a vascular injury. Flow chambers designed to observe these events are an indispensable part of doing hemostasis and thrombosis research, especially with human blood. Microfluidic methods have provided the flexibility to design flow chambers with complex geometries and features that more closely mimic the anatomy and physiology of blood vessels. Additionally, microfluidic systems with integrated optics and/or pressure sensors and on-board signal processing could transform what have been primarily research tools into clinical assays. Here, we describe a historical review of how flow-based approaches have informed biophysical mechanisms in genetic bleeding disorders, challenges and potential solutions for developing models of bleeding in vitro, and outstanding issues that need to be addressed prior to their use in clinical settings.

von Willebrand factor disruption and continuous-flow circulatory devices

Bleeding events remain a significant and frequent complication of continuous-flow left ventricular assist devices (VADs). von Willebrand factor (VWF) is critical to hemostasis by acting as a bridging molecule at sites of vascular injury for normal platelet adhesion as well as promoting platelet aggregation under conditions of high shear. Clinical and experimental data support a role for acquired von Willebrand disease in VAD bleeding episodes caused by shear-induced qualitative defects in VWF. Pathologic shear induces VWF unfolding and proteolysis of large multimers into smaller less hemostatic multimers via ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). This review outlines the pathobiology of VWF disruption in the context of VADs as well as current diagnostic and management strategies of the associated acquired von Willebrand disease.

Utility of microfluidic devices to study the platelet-endothelium interface

The integration of biomaterials and understanding of vascular biology has led to the development of perfusable endothelialized flow models, which have been used as valuable tools to study the platelet-endothelium interface under shear. In these models, the parameters of geometry, compliance, biorheology, and cellular complexity are varied to recapitulate the physical biology of platelet recruitment and activation under physiologically relevant conditions of blood flow. In this review, we summarize the mechanistic insights learned from perfusable microvessel models and discuss the potential utility as well as challenges of endothelialized microfluidic devices to study platelet function in the bloodstream in vitro.

A Microfluidic Flow Chamber Model for Platelet Transfusion and Hemostasis Measures Platelet Deposition and Fibrin Formation in Real-time

Microfluidic models of hemostasis assess platelet function under conditions of hydrodynamic shear, but in the presence of anticoagulants, this analysis is restricted to platelet deposition only. The intricate relationship between Ca²⁺-dependent coagulation and platelet function requires careful and controlled recalcification of blood prior to analysis. Our setup uses a Y-shaped mixing channel, which supplies concentrated Ca²⁺/Mg²⁺ buffer to flowing blood just prior to perfusion, enabling rapid recalcification without sample stasis. A ten-fold difference in flow velocity between both reservoirs minimizes dilution. The recalcified blood is then perfused in a collagen-coated analysis chamber, and differential labeling permits real-time imaging of both platelet and fibrin deposition using fluorescence video microscopy. The system uses only commercially available tools, increasing the chances of standardization. Reconstitution of thrombocytopenic blood with platelets from banked concentrates furthermore models platelet transfusion, proving its use in this research domain. Exemplary data demonstrated that coagulation onset and fibrin deposition were linearly dependent on the platelet concentration, confirming the relationship between primary and secondary hemostasis in our model. In a timeframe of 16 perfusion min, contact activation did not take place, despite recalcification to normal Ca²⁺ and Mg²⁺ levels. When coagulation factor XIIa was inhibited by corn trypsin inhibitor, this time frame was even longer, indicating a considerable dynamic range in which the changes in the procoagulant nature of the platelets can be assessed. Co-immobilization of tissue factor with collagen significantly reduced the time to onset of coagulation, but not its rate. The option to study the tissue factor and/or the contact pathway increases the versatility and utility of the assay.

Endothelial cell markers from clinician's perspective

Endothelial cell markers are membrane-bound or cytoplasmic molecules expressed by endothelial cells, which help their easier identification and discrimination from other cell types. During vasculogenesis, endothelial cells differentiate from hemangioblasts to form new blood vessels. With the discovery of endothelial progenitor cells (EPC) and their ability to form new blood vessels, the term vasculogenesis is not only reserved for the embryonic development. Possibility of de novo blood vessel formation from EPC is now widely explored in different ischemic conditions, especially in cardiovascular medicine. Numerous clinical trials have tested enhancing tissue vascularization by delivering hematopoietic cells that expressed endothelial markers. This therapeutic approach proved to be challenging and promising, particularly for patients who have exhausted all conventional therapeutic modalities. Angiogenesis, which refers to the formation of new blood vessels from existing vasculature, is indispensable process during tumor progression and metastasis. Blockage of tumor angiogenesis by targeting and inhibiting endothelial cell has emerged as novel safe and efficacious method to control many advanced malignant diseases. Numerous clinical studies are currently testing new antiangiogenic drugs which target and inhibit endothelial cell markers, receptors or molecules which transmit receptor-mediated signals, therefore inhibiting endothelial cell proliferation, migration and vascular tube formation. Many of these drugs are now widely used in clinical settings as first- or second-line chemotherapy in advanced malignant conditions. So far, these therapeutic approaches gave modest, yet encouraging clinical improvements, prolonging survival and improving functional capacity and quality of life for many terminally ill patients. Here we present the most commonly used endothelial cell markers along with their applicability in contemporary clinical practice.

Advances of blood cell-based drug delivery systems

Blood cells, including erythrocytes, leukocytes and platelets are used as drug carriers in a wide range of applications. They have many unique advantages such as long life-span in circulation (especially erythrocytes), target release capacities (especially platelets), and natural adhesive properties (leukocytes and platelets). These properties make blood cell based delivery systems, as well as their membrane-derived carriers, far superior to other drug delivery systems. Despite the advantages, the further development of blood cell-based delivery systems was hindered by limitations in the source, storage, and mass production. To overcome these problems, synthetic biomaterials that mimic blood cell and nanocrystallization of blood cells have been developed and may represent the future direction for blood cell membrane-based delivery systems. In this paper, we review recent progress of the rising blood cell-based drug delivery systems, and also discuss their challenges and future tendency of development.

von Willebrand factor multimerization and the polarity of secretory pathways in endothelial cells

The von Willebrand factor (VWF) synthesized and secreted by endothelial cells is central to hemostasis and thrombosis, providing a multifunctional adhesive platform that brings together components needed for these processes. VWF secretion can occur from both apical and basolateral sides of endothelial cells, and from constitutive, basal, and regulated secretory pathways, the latter two via Weibel-Palade bodies (WPB). Although the amount and structure of VWF is crucial to its function, the extent of VWF release, multimerization, and polarity of the 3 secretory pathways have only been addressed separately, and with conflicting results. We set out to clarify these relationships using polarized human umbilical vein endothelial cells (HUVECs) grown on Transwell membranes. We found that regulated secretion of ultra-large (UL)-molecular-weight VWF predominantly occurred apically, consistent with a role in localized platelet capture in the vessel lumen. We found that constitutive secretion of low-molecular-weight (LMW) VWF is targeted basolaterally, toward the subendothelial matrix, using the adaptor protein complex 1 (AP-1), where it may provide the bulk of collagen-bound subendothelial VWF. We also found that basally-secreted VWF is composed of UL-VWF, released continuously from WPBs in the absence of stimuli, and occurs predominantly apically, suggesting this could be the main source of circulating plasma VWF. Together, we provide a unified dataset reporting the amount and multimeric state of VWF secreted from the constitutive, basal, and regulated pathways in polarized HUVECs, and have established a new role for AP-1 in the basolateral constitutive secretion of VWF.

Microfluidic Flow Chambers Using Reconstituted Blood to Model Hemostasis and Platelet Transfusion In Vitro

Blood platelets prepared for transfusion gradually lose hemostatic function during storage. Platelet function can be investigated using a variety of (indirect) in vitro experiments, but none of these is as comprehensive as microfluidic flow chambers. In this protocol, the reconstitution of thrombocytopenic fresh blood with stored blood bank platelets is used to simulate platelet transfusion. Next, the reconstituted sample is perfused in microfluidic flow chambers which mimic hemostasis on exposed subendothelial matrix proteins. Effects of blood donation, transport, component separation, storage and pathogen inactivation can be measured in paired experimental designs. This allows reliable comparison of the impact every manipulation in blood component preparation has on hemostasis. Our results demonstrate the impact of temperature cycling, shear rates, platelet concentration and storage duration on platelet function. In conclusion, this protocol analyzes the function of blood bank platelets and this ultimately aids in optimization of the processing chain including phlebotomy, transport, component preparation, storage and transfusion.

Factoring in Factor VIII With Acute Ischemic Stroke

There is growing research interest into the etiologies of cryptogenic stroke, in particular as it relates to hypercoagulable states. An elevation in serum levels of the procoagulant factor VIII is recognized as one such culprit of occult cerebral infarctions. It is the objective of the present review to summarize the molecular role of factor VIII in thrombogenesis and its clinical use in the diagnosis and prognosis of acute ischemic stroke. We also discuss the utility of screening for serum factor VIII levels among patients at risk for, or those who have experienced, ischemic stroke.

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.

Evaluation of prothrombin time and activated partial thromboplastin time in hypertensive patients attending a tertiary hospital in calabar, Nigeria

Introduction. Several biomedical findings have established the effects of hypertension on haemostasis and roles of blood coagulation products in the clinical course of hypertension. Methods. This cross-sectional study aimed at determining effects of hypertension on prothrombin time (PT) and activated partial thromboplastin time (APTT) in hypertensive patients in comparison with normotensive subjects attending a tertiary hospital in Calabar. Forty-two (42) hypertensive patients and thirty-nine (39) normotensive control subjects were investigated for PT and APTT using Quick one-stage methods. Results. Systolic blood pressure (SBP) and diastolic blood pressure (DBP) correlated positively with APTT (r = 0.3072, r = 0.4988; P < 0.05) in hypertensive patients. DBP, SBP, PT, and APTT were significantly higher in hypertensive patients when compared to normotensive subjects (P < 0.05). DBP correlated negatively with duration of illness (r = −0.3097; P < 0.05) in hypertensive patients and positively with age of normotensive subjects (r = 0.3523; P < 0.05). Conclusion. The results obtained indicated that measurements of PT and APTT may serve as indices for evaluating hemostatic abnormalities in hypertensive patients and guide for antihypertensive therapy. However, to have better understanding of hemostatic activities in hypertension, it is recommended to conduct D-dimer, platelet factors, and protein assays.

Effects of increased von Willebrand factor levels on primary hemostasis in thrombocytopenic patients with liver cirrhosis

In patients with liver cirrhosis procoagulant and anticoagulant changes occur simultaneously. During primary hemostasis, platelets adhere to subendothelial structures, via von Willebrand factor (vWF). We aimed to investigate the influence of vWF on primary hemostasis in patients with liver cirrhosis. Therefore we assessed in-vitro bleeding time as marker of primary hemostasis in cirrhotic patients, measuring the Platelet Function Analyzer (PFA-100) closure times with collagen and epinephrine (Col-Epi, upper limit of normal ≤ 165 s) or collagen and ADP (Col-ADP, upper limit of normal ≤ 118 s). If Col-Epi and Col-ADP were prolonged, the PFA-100 was considered to be pathological. Effects of vWF on primary hemostasis in thrombocytopenic patients were analyzed and plasma vWF levels were modified by adding recombinant vWF or anti-vWF antibody. Of the 72 included cirrhotic patients, 32 (44.4%) showed a pathological result for the PFA-100. They had mean closure times (± SD) of 180 ± 62 s with Col-Epi and 160 ± 70 s with Col-ADP. Multivariate analysis revealed that hematocrit (P = 0.027) and vWF-antigen levels (P = 0.010) are the predictors of a pathological PFA-100 test in cirrhotic patients. In 21.4% of cirrhotic patients with platelet count ≥ 150/nL and hematocrit ≥ 27.0%, pathological PFA-100 results were found. In thrombocytopenic (< 150/nL) patients with cirrhosis, normal PFA-100 results were associated with higher vWF-antigen levels (462.3 ± 235.9% vs. 338.7 ± 151.6%, P = 0.021). These results were confirmed by multivariate analysis in these patients as well as by adding recombinant vWF or polyclonal anti-vWF antibody that significantly shortened or prolonged closure times, respectively. In conclusion, primary hemostasis is impaired in cirrhotic patients. The effect of reduced platelet count in cirrhotic patients can at least be partly compensated by increased vWF levels. Recombinant vWF could be an alternative to platelet transfusions in the future.

Comparative endothelial cell response on topographically patterned titanium and silicon substrates with micrometer to sub-micrometer feature sizes

In this work, we evaluate the in vitro response of endothelial cells (EC) to variation in precisely-defined, micrometer to sub-micrometer scale topography on two different substrate materials, titanium (Ti) and silicon (Si). Both substrates possess identically-patterned surfaces composed of microfabricated, groove-based gratings with groove widths ranging from 0.5 to 50 µm, grating pitch twice the groove width, and groove depth of 1.3 µm. These specific materials are chosen due to their relevance for implantable microdevice applications, while grating-based patterns are chosen for the potential they afford for inducing elongated and aligned cellular morphologies reminiscent of the native endothelium. Using EA926 cells, a human EC variant, we show significant improvement in cellular adhesion, proliferation, morphology, and function with decreasing feature size on patterned Ti substrates. Moreover, we show similar trending on patterned Si substrates, albeit to a lesser extent than on comparably patterned Ti substrates. Collectively, these results suggest promise for sub-micrometer topographic patterning in general, and sub-micrometer patterning of Ti specifically, as a means for enhancing endothelialization and neovascularisation for novel implantable microdevice applications.

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.

Von Willebrand Factor Drives the Association Between Elevated Factor VIII and Poor Outcomes in Patients With Ischemic Stroke

Background and Purpose—

Despite clear roles of factor VIII (FVIII) and von Willebrand factor (vWF) in thrombosis, few studies have examined the relationship of these factors with acute ischemic stroke (AIS). We sought to determine whether concurrent elevation in FVIII and vWF was associated with adverse events and outcomes.

Methods—

From our prospective stroke registry, patients consecutively admitted with AIS between July 2008 and October 2013 were included if both FVIII and vWF were measured during admission. The primary outcome was the modified Rankin Scale score on discharge.

Results—

Among 1453 cases in our stroke registry, 148 patients with AIS met inclusion criteria; 62 patients (41.9%) had FVIII−/vWF−, 16 patients (10.8%) had FVIII+/vWF−, and 51 patients (34.5%) had FVIII+/vWF+. In the fully adjusted model, patients with FVIII+/vWF+ had increased odds of inpatient complications (odds ratio, 8.6; 95% confidence interval, 1.58–46.85; P =0.013) and neuroworsening (odds ratio, 3.2; 95% confidence interval, 1.18–8.73; P =0.022) than patients with FVIII−/vWF−. Adjusted for age, baseline stroke severity, and glucose, patients with FVIII+/vWF+ had increased odds of poor functional outcome (modified Rankin Scale>2; odds ratio, 2.87; 95% confidence interval, 1.16–7.06; P =0.021) than patients with FVIII−/vWF−.

Conclusions—

Concurrent FVIII/vWF elevation predicts higher odds of inpatient complications, neuroworsening, and worse functional outcomes for patients with AIS compared with patients with normal levels. Our findings suggest that FVIII and vWF levels may serve as clinically useful stroke biomarkers by providing risk profiles for patients with AIS.

Perils, problems, and progress in laboratory diagnosis of von Willebrand disease

Diagnosis of von Willebrand disease (VWD) merits consideration of personal and family history of bleeding symptoms along with confirmatory laboratory testing. As the latter yields quantifiable results, overreliance on a laboratory diagnosis may occur. However, existing tests for VWD contain potential sources for error. Both intrinsic and extrinsic factors affecting these assays can contribute to either falsely normal or falsely abnormal results. This article will discuss the present available assays as well as new developments in diagnostic testing. A clear understanding of the limitations of VWD testing is helpful for ensuring the correct diagnosis of affected patients.

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.

C1galt1-deficient mice exhibit thrombocytopenia due to abnormal terminal differentiation of megakaryocytes

C1galt1 is essential for synthesis of the core 1 structure of mucin-type O-glycans. To clarify the physiological role of O-glycans in adult hematopoiesis, we exploited the interferon-inducible Mx1-Cre transgene to conditionally ablate the C1galt(flox) allele (Mx1-C1). Mx1-C1 mice exhibit severe thrombocytopenia, giant platelets, and prolonged bleeding times. Both the number and DNA ploidy of megakaryocytes in Mx1-C1 bone marrow were similar to those in wild-type (WT) mice. However, there were few proplatelets in Mx1-C1 primary megakaryocytes. Conversely, bone marrow transplanted from Mx1-C1 to WT and splenectomized Mx1-C1 mice gave rise to observations similar to those described above. The expression of GPIbα messenger RNA was unchanged in Mx1-C1 bone marrow, whereas flow cytometric and western blot analyses using megakaryocytes and platelets revealed that the expression of GPIbα protein was significantly reduced in Mx1-C1 mice. Moreover, circulating Mx1-C1 platelets exhibited an increase in the number of microtubule coils, despite normal levels of α- and β-tubulin. Our observations suggest that O-glycan is required for terminal megakaryocyte differentiation and platelet production and that the decrease in GPIbα in cells lacking O-glycan might be caused by increased proteolysis.

Side view thrombosis microfluidic device with controllable wall shear rate and transthrombus pressure gradient

Hemodynamic conditions vary throughout the vasculature, creating diverse environments in which platelets must respond. To stop bleeding, a growing platelet deposit must be assembled in the presence of fluid wall shear stress (τw) and a transthrombus pressure gradient (ΔP) that drives bleeding. We designed a microfluidic device capable of pulsing a fluorescent solute through a developing thrombus forming on collagen ± tissue factor (TF), while independently controlling ΔP and τw. Computer control allowed step changes in ΔP with a rapid response time of 0.26 mm Hg s(-1) at either venous (5.2 dynes cm(-2)) or arterial (33.9 dynes cm(-2)) wall shear stresses. Side view visualization of thrombosis with transthrombus permeation allowed for quantification of clot structure, height, and composition at various ΔP. Clot height was reduced 20% on collagen/TF and 28% on collagen alone when ΔP was increased from 20.8 to 23.4 mm Hg at constant arterial shear stress. When visualized with a platelet-targeting thrombin sensor, intrathrombus thrombin levels decreased by 62% as ΔP was increased from 0 to 23.4 mm Hg across the thrombus-collagen/TF barrier, consistent with convective removal of thrombogenic solutes due to pressure-driven permeation. Independent of ΔP, the platelet deposit on collagen had a permeability of 5.45 × 10(-14) cm(2), while the platelet/fibrin thrombus on collagen/TF had a permeability of 2.71 × 10(-14) cm(2) (comparable to that of an intact endothelium). This microfluidic design allows investigation of the coupled processes of platelet deposition and thrombin/fibrin generation in the presence of controlled transthrombus permeation and wall shear stress.

Progressive endothelial damage revealed by multilevel von Willebrand factor plasma concentrations in atrial fibrillation patients

AIMS

Abnormal plasma concentrations of von Willebrand factor (vWF), a marker of prothrombotic risk, have been found in atrial fibrillation (AF) patients, but the extent of this variation is not clear. This study aimed to investigate the effect of different clinical forms of AF on plasma concentrations of vWF at different levels of the circulatory tree, both intracardiac and extracardiac.

METHODS AND RESULTS

Peripheral (Pf), left atrial (LA), and coronary sinus (CS) blood samples were obtained during cardiac catheterization from 52 patients with paroxysmal AF (PAF), 36 with persistent AF (PsAF), and 17 control subjects (Ct) with left-sided accessory pathway Wolff-Parkinson-White syndrome. Plasma concentrations of vWF were determined by immunoturbidimetry. Compared with Ct, patients with PAF had higher LA plasma levels of vWF (P = 0.004), but similar Pf and CS levels (both P > 0.30). In contrast, patients with PsAF had higher plasma concentrations of vWF in Pf (P = 0.04), LA (P < 0.001), and CS (P = 0.04) samples compared with Ct. Left atrial plasma concentrations of vWF in patients with PsAF were also higher than in the PAF group (P = 0.04).

CONCLUSION

Regardless of the clinical form of the arrhythmia, AF patients presented significantly higher plasma concentrations of vWF compared with sinus rhythm controls. Multilevel vWF plasma concentration assessment suggests an association between the clinical evolution of AF and the progression of endothelial dysfunction. Further studies will have to establish the exact mechanisms that link endothelial dysfunction and stroke in the context of AF.

Microfluidics and coagulation biology

The study of blood ex vivo can occur in closed or open systems, with or without flow. Microfluidic devices, which constrain fluids to a small (typically submillimeter) scale, facilitate analysis of platelet function, coagulation biology, cellular biorheology, adhesion dynamics, and pharmacology and, as a result, can be an invaluable tool for clinical diagnostics. An experimental session can accommodate hundreds to thousands of unique clotting, or thrombotic, events. Using microfluidics, thrombotic events can be studied on defined surfaces of biopolymers, matrix proteins, and tissue factor, under constant flow rate or constant pressure drop conditions. Distinct shear rates can be generated on a device using a single perfusion pump. Microfluidics facilitated both the determination of intraluminal thrombus permeability and the discovery that platelet contractility can be activated by a sudden decrease in flow. Microfluidic devices are ideal for multicolor imaging of platelets, fibrin, and phosphatidylserine and provide a human blood analog to mouse injury models. Overall, microfluidic advances offer many opportunities for research, drug testing under relevant hemodynamic conditions, and clinical diagnostics.

Novel antiplatelet agents: ALX-0081, a Nanobody directed towards von Willebrand factor

This manuscript reviews the studies performed with ALX-0081 (INN: caplacizumab), a Nanobody targeting von Willebrand factor, in the context of current antithrombotic therapy in coronary artery disease. ALX-0081 specifically inhibits platelet adhesion to the vessel wall, and may control platelet aggregation and subsequent clot formation without increasing bleeding risk. A substantial number of antithrombotics are aimed at this cascade; however, their generally indiscriminative mode of action can result in a narrow therapeutic window, defined by the risk for bleeding complications, and thrombotic events. Nonclinically, ALX-0081 compared favorably to several antithrombotics. In Phase I studies in healthy subjects and stable angina patients undergoing percutaneous coronary intervention (PCI), ALX-0081 was well tolerated, and effectively inhibited pharmacodynamic markers. Following these results, a phase II study was initiated in high-risk acute coronary syndrome patients undergoing PCI. Based on its mechanism of action, ALX-0081 is also being developed for acquired thrombotic thrombocytopenic purpura.

Circulating but not immobilized N-deglycosylated von Willebrand factor increases platelet adhesion under flow conditions

The role of von Willebrand factor (VWF) as a shear stress activated platelet adhesive has been related to a coiled-elongated shape conformation. The forces dominating this transition have been suggested to be controlled by the proteins polymeric architecture. However, the fact that 20% of VWF molecular weight originates from glycan moieties has so far been neglected in these calculations. In this study, we present a systematic experimental investigation on the role of N-glycosylation for VWF mediated platelet adhesion under flow. A microfluidic flow chamber with a stenotic compartment that allows one to mimic various physiological flow conditions was designed for the efficient analysis of the adhesion spectrum. Surprisingly, we found an increase in platelet adhesion with elevated shear rate, both qualitatively and quantitatively fully conserved when N-deglycosylated VWF (N-deg-VWF) instead of VWF was immobilized in the microfluidic channel. This has been demonstrated consistently over four orders of magnitude in shear rate. In contrast, when N-deg-VWF was added to the supernatant, an increase in adhesion rate by a factor of two was detected compared to the addition of wild-type VWF. It appears that once immobilized, the role of glycans is at least modified if not-as found here for the case of adhesion-negated. These findings strengthen the physical impact of the circulating polymer on shear dependent platelet adhesion events. At present, there is no theoretical explanation for an increase in platelet adhesion to VWF in the absence of its N-glycans. However, our data indicate that the effective solubility of the protein and hence its shape or conformation may be altered by the degree of glycosylation and is therefore a good candidate for modifying the forces required to uncoil this biopolymer.

Mapping the N-glycome of human von Willebrand factor

vWF (von Willebrand factor) is a key component for maintenance of normal haemostasis, acting as the carrier protein of the coagulant Factor VIII and mediating platelet adhesion at sites of vascular injury. There is ample evidence that vWF glycan moieties are crucial determinants of its expression and function. Of particular clinical interest, ABH antigens influence vWF plasma levels according to the blood group of individuals, although the molecular mechanism underlying this phenomenon remains incompletely understood. The present paper reports analyses of the human plasma vWF N-glycan population using advanced MS. Glycomics analyses revealed approximately 100 distinct N-glycan compositions and identified a variety of structural features, including lactosaminic extensions, ABH antigens and sulfated antennae, as well as bisecting and terminal GlcNAc residues. We estimate that some 300 N-glycan structures are carried by human vWF. Glycoproteomics analyses mapped ten of the consensus sites known to carry N-glycans. Glycan populations were found to be distinct, although many structural features were shared across all sites. Notably, the H antigen is not restricted to particular N-glycosylation sites. Also, the Asn(2635) site, previously designated as unoccupied, was found to be highly glycosylated. The delineation of such varied glycan populations in conjunction with current models explaining vWF activity will facilitate research aimed at providing a better understanding of the influence of glycosylation on vWF function.

The collagen-binding integrin α2β1 is a novel interaction partner of the Trimeresurus flavoviridis venom protein flavocetin-A

Many snake venoms are known for their antithrombotic activity. They contain components that specifically target different platelet-activating receptors such as the collagen-binding integrin α2β1 and the von Willebrand factor receptor GPIb. In a search for an α2β1 integrin-blocking component from the venom of the habu snake (Trimeresurus flavoviridis), we employed two independent purification protocols. First, we used the integrin α2A domain, a major collagen-binding domain, as bait for affinity purification of an α2β1 integrin-binding toxin from the crude venom. Second, in parallel, we used classical protein separation protocols and tested for α2β1 integrin-inhibiting capabilities by ELISA. Using both approaches, we identified flavocetin-A as an inhibitor of α2β1 integrin. Hitherto, flavocetin-A has been reported as a GPIb inhibitor. However, flavocetin-A inhibited collagen-induced platelet aggregation even after GPIb was blocked with other inhibitors. Moreover, flavocetin-A antagonized α2β1 integrin-mediated adhesion and migration of HT1080 human fibrosarcoma cells, which lack any GPIb, on collagen. Protein chemical analyses proved that flavocetin-A binds to α2β1 integrin and its α2A domain with high affinity and in a cooperative manner, which most likely is due to its quaternary structure. Kinetic measurements confirmed the formation of a strong complex between integrin and flavocetin-A, which dissociates very slowly. This study proves that flavocetin-A, which has long been known as a GPIb inhibitor, efficiently targets α2β1 integrin and thus blocks collagen-induced platelet activation. Moreover, our findings suggest that the separation of GPIb- and α2β1 integrin-blocking members within the C-type lectin-related protein family is less strict than previously assumed.