Basal secretion of von Willebrand factor from human endothelial cells

von Willebrand factor antigen, von Willebrand factor propeptide and ADAMTS13 activity in TIA or ischaemic stroke patients changing antiplatelet therapy

Data are limited on the impact of commencing antiplatelet therapy on von Willebrand Factor Antigen (VWF:Ag) or von Willebrand Factor propeptide (VWFpp) levels and ADAMTS13 activity, and their relationship with platelet reactivity following TIA/ischaemic stroke. In this pilot, observational study, VWF:Ag and VWFpp levels and ADAMTS13 activity were quantified in 48 patients ≤4 weeks of TIA/ischaemic stroke (baseline), and 14 days (14d) and 90 days (90d) after commencing aspirin, clopidogrel or aspirin+dipyridamole. Platelet reactivity was assessed at moderately-high shear stress (PFA-100® Collagen-Epinephrine / Collagen-ADP / INNOVANCE PFA P2Y assays), and low shear stress (VerifyNow® Aspirin / P2Y12, and Multiplate® Aspirin / ADP assays). VWF:Ag levels decreased and VWFpp/VWF:Ag ratio increased between baseline and 14d and 90d in the overall population (P ≤ 0.03). In the clopidogrel subgroup, VWF:Ag levels decreased and VWFpp/VWF:Ag ratio increased between baseline and 14d and 90d (P ≤ 0.01), with an increase in ADAMTS13 activity between baseline vs. 90d (P ≤ 0.03). In the aspirin+dipyridamole subgroup, there was an inverse relationship between VWF:Ag and VWFpp levels with both PFA-100 C-ADP and INNOVANCE PFA P2Y closure times (CTs) at baseline (P ≤ 0.02), with PFA-100 C-ADP, INNOVANCE PFA P2Y and C-EPI CTs at 14d (P ≤ 0.05), and between VWF:Ag levels and PFA-100 INNOVANCE PFA P2Y CTs at 90d (P = 0.03). There was a positive relationship between ADAMTS13 activity and PFA-100 C-ADP CTs at baseline (R2 = 0.254; P = 0.04). Commencing/altering antiplatelet therapy, mainly attributed to commencing clopidogrel in this study, was associated with decreasing endothelial activation following TIA/ischaemic stroke. These data enhance our understanding of the impact of VWF:Ag and VWFpp especially on ex-vivo platelet reactivity status at high shear stress after TIA/ischaemic stroke.

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 defined clathrin-mediated trafficking pathway regulates sFLT1/VEGFR1 secretion from endothelial cells

FLT1/VEGFR1 negatively regulates VEGF-A signaling and is required for proper vessel morphogenesis during vascular development and vessel homeostasis. Although a soluble isoform, sFLT1, is often mis-regulated in disease and aging, how sFLT1 is trafficked and secreted from endothelial cells is not well understood. Here we define requirements for constitutive sFLT1 trafficking and secretion in endothelial cells from the Golgi to the plasma membrane, and we show that sFLT1 secretion requires clathrin at or near the Golgi. Perturbations that affect sFLT1 trafficking blunted endothelial cell secretion and promoted intracellular mis-localization in cells and zebrafish embryos. siRNA-mediated depletion of specific trafficking components revealed requirements for RAB27A, VAMP3, and STX3 for post-Golgi vesicle trafficking and sFLT1 secretion, while STX6, ARF1, and AP1 were required at the Golgi. Live-imaging of temporally controlled sFLT1 release from the endoplasmic reticulum showed clathrin-dependent sFLT1 trafficking at the Golgi into secretory vesicles that then trafficked to the plasma membrane. Depletion of STX6 altered vessel sprouting in 3D, suggesting that endothelial cell sFLT1 secretion influences proper vessel sprouting. Thus, specific trafficking components provide a secretory path from the Golgi to the plasma membrane for sFLT1 in endothelial cells that utilizes a specialized clathrin-dependent intermediate, suggesting novel therapeutic targets.

Kidney cell culture Macaca fascicularis as a candidate for vaccine development and in vitro model

BACKGROUND

Cell culture is the proliferation of a cell population in vitro by isolating from the original tissue or growing from existing ones. One essential source is the monkey kidney cell cultures which have an essential role in biomedical study. This is due to the significant homology between the human and macaque genomes making these useful for cultivating human viruses, especially enteroviruses, and growing vaccines.

METHODS

This study developed cell cultures derived from the kidney of Macaca fascicularis (Mf) and validated its gene expression.

RESULTS

The primary cultures were successfully subcultured up to six passages, grew as monolayers, and exhibited epithelial-like morphology. The cultured cells remained heterogeneous in phenotype and they expressed CD155 and CD46 as viral receptors, cell morphology (CD24, endosialin, and vWF), proliferation, also apoptosis markers (Ki67 and p53).

CONCLUSIONS

These results indicated that the cell cultures can be used as in vitro model cells for vaccine development and bioactive compound.

A defined clathrin-mediated trafficking pathway regulates sFLT1/VEGFR1 secretion from endothelial cells

FLT1/VEGFR1 negatively regulates VEGF-A signaling and is required for proper vessel morphogenesis during vascular development and vessel homeostasis. Although a soluble isoform, sFLT1, is often mis-regulated in disease and aging, how sFLT1 is trafficked and secreted from endothelial cells is not well understood. Here we define requirements for constitutive sFLT1 trafficking and secretion in endothelial cells from the Golgi to the plasma membrane, and we show that sFLT1 secretion requires clathrin at or near the Golgi. Perturbations that affect sFLT1 trafficking blunted endothelial cell secretion and promoted intracellular mis-localization in cells and zebrafish embryos. siRNA-mediated depletion of specific trafficking components revealed requirements for RAB27A, VAMP3, and STX3 for post-Golgi vesicle trafficking and sFLT1 secretion, while STX6, ARF1, and AP1 were required at the Golgi. Depletion of STX6 altered vessel sprouting in a 3D angiogenesis model, indicating that endothelial cell sFLT1 secretion is important for proper vessel sprouting. Thus, specific trafficking components provide a secretory path from the Golgi to the plasma membrane for sFLT1 in endothelial cells that utilizes a specialized clathrin-dependent intermediate, suggesting novel therapeutic targets.

Regulation of VWF (Von Willebrand Factor) in Inflammatory Thrombosis

Increasing evidence indicates that inflammation promotes thrombosis via a VWF (von Willebrand factor)-mediated mechanism. VWF plays an essential role in maintaining the balance between blood coagulation and bleeding, and inflammation can lead to aberrant regulation. VWF is regulated on a transcriptional and (post-)translational level, and its secretion into the circulation captures platelets upon endothelial activation. The significant progress that has been made in understanding transcriptional and translational regulation of VWF is described in this review. First, we describe how VWF is regulated at the transcriptional and post-translational level with a specific focus on the influence of inflammatory and immune responses. Next, we describe how changes in regulation are linked with various cardiovascular diseases. Recent insights from clinical diseases provide evidence for direct molecular links between inflammation and thrombosis, including atherosclerosis, chronic thromboembolic pulmonary hypertension, and COVID-19. Finally, we will briefly describe clinical implications for antithrombotic treatment.

Research progress on the mechanism of radiation enteritis

Radiation enteritis (Re) is one of the most common complications of radiation therapy for abdominal tumors. The efficacy of cancer treatment by radiation is often limited by the side effects of Re. Re can be acute or chronic. Treatment of acute Re is essentially symptomatic. However, chronic Re usually requires surgical procedures. The underlying mechanisms of Re are complex and have not yet been elucidated. The purpose of this review is to provide an overview of the pathogenesis of Re. We reviewed the role of intestinal epithelial cells, intestinal stem cells (ISCs), vascular endothelial cells (ECs), intestinal microflora, and other mediators of Re, noting that a better understanding of the pathogenesis of Re may lead to better treatment modalities.

Endothelial-derived von Willebrand factor accelerates fibrin clotting within engineered microvessels

BACKGROUND

Von Willebrand factor (VWF) is classically associated with primary hemostasis and platelet-rich arterial thromboses, but recently has also been implicated in fibrin clotting and venous thrombosis. Direct interaction between fibrin and VWF may mediate these processes, although prior reports are conflicting.

OBJECTIVES

We combined two complementary platforms to characterize VWF-fibrin(ogen) interactions and identify their potential physiologic significance.

METHODS

Engineered microvessels were lined with human endothelial cells, cultured under flow, and activated to release VWF and form transluminal VWF fibers. Fibrinogen, fibrin monomers, or polymerizing fibrin were then perfused, and interactions with VWF evaluated. Thrombin and fibrinogen were perfused into living versus paraformeldahyde-fixed microvessels and the pressure drop across microvessels monitored. Separately, protein binding to tethered VWF was assessed on a single-molecule level using total internal reflection fluorescence (TIRF) microscopy.

RESULTS

Within microvessels, VWF fibers colocalized with polymerizing fibrin, but not fibrinogen. TIRF microscopy showed no colocalization between VWF and fibrinogen or fibrin monomers in a microfluidic flow chamber across a range of shear rates and protein concentrations. Thrombin-mediated fibrin polymerization within living microvessels triggered endothelial VWF release, increasing the rate and amount of microvessel obstruction compared to fixed vessels with an inert endothelium.

CONCLUSIONS

We did not identify specific binding between fibrin(ogen) and VWF at a single-molecule level. Despite this, our results suggest that rapid release of endothelial VWF during clotting may provide a physical support for fibrin polymerization and accelerate thrombosis. This interaction may be of fundamental importance for the understanding and treatment of human thrombotic disease.

COVID-19 associated coagulopathy: Mechanisms and host-directed treatment

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) is associated with specific coagulopathy that frequently occurs during the different phases of coronavirus disease 2019 (COVID-19) and can result in thrombotic complications and/or death. This COVID-19-associated coagulopathy (CAC) exhibits some of the features associated with thrombotic microangiopathy, particularly complement-mediated hemolytic-uremic syndrome. In some cases, due to the anti-phospholipid antibodies, CAC resembles catastrophic anti-phospholipid syndrome. In other patients, it exhibits features of hemophagocytic syndrome. CAC is mainly identified by: increases in fibrinogen, D-dimers, and von Willebrand factor (released from activated endothelial cells), consumption of a disintegrin and metalloproteinase with thrombospondin type 1 motifs, member 13 (ADAMTS13), over activated and dysregulated complement, and elevated plasma cytokine levels. CAC manifests as both major cardiovascular and/or cerebrovascular events and dysfunctional microcirculation, which leads to multiple organ damage. It is not clear whether the mainstay of COVID-19 is complement overactivation, cytokine/chemokine activation, or a combination of these activities. Available data have suggested that non-critically ill hospitalized patients should be administered full-dose heparin. In critically ill, full dose heparin treatment is discouraged due to higher mortality rate. In addition to anti-coagulation, four different host-directed therapeutic pathways have recently emerged that influence CAC: (1) Anti-von Willebrand factor monoclonal antibodies; (2) activated complement C5a inhibitors; (3) recombinant ADAMTS13; and (4) Interleukin (IL)-1 and IL-6 antibodies. Moreover, neutralizing monoclonal antibodies against the virus surface protein have been tested. However, the role of antiplatelet treatment remains unclear for patients with COVID-19.

Novel cysteine substitution p.(Cys1084Tyr) causes variable expressivity of qualitative and quantitative VWF defects

von Willebrand factor (VWF) is an extremely cysteine-rich multimeric protein that is essential for maintaining normal hemostasis. The cysteine residues of VWF monomers form intra- and intermolecular disulfide bonds that regulate its structural conformation, multimer distribution, and ultimately its hemostatic activity. In this study, we investigated and characterized the molecular and pathogenic mechanisms through which a novel cysteine variant p.(Cys1084Tyr) causes an unusual, mixed phenotype form of von Willebrand disease (VWD). Phenotypic data including bleeding scores, laboratory values, VWF multimer distribution, and desmopressin response kinetics were investigated in 5 members (2 parents and 3 daughters) of a consanguineous family. VWF synthesis and secretion were also assessed in a heterologous expression system and in a transient transgenic mouse model. Heterozygosity for p.(Cys1084Tyr) was associated with variable expressivity of qualitative VWF defects. Heterozygous individuals had reduced VWF:GPIbM (<0.40 IU/mL) and VWF:CB (<0.35 IU/mL), as well as relative reductions in high-molecular-weight multimers, consistent with type 2A VWD. In addition to these qualitative defects, homozygous individuals also displayed reduced factor VIII (FVIII):C/VWF:Ag, leading to very low FVIII levels (0.03-0.1 IU/mL) and reduced VWF:Ag (<0.40 IU/mL) and VWF:GPIbM (<0.30 IU/mL). Accelerated VWF clearance and impaired VWF secretion contributed to the fully expressed homozygous phenotype with impaired secretion arising because of disordered disulfide connectivity.

Salmonella Enteritidis GalE Protein Inhibits LPS-Induced NLRP3 Inflammasome Activation

Microbial infection can trigger the assembly of inflammasomes and promote secretion of cytokines, such as IL-1β and IL-18. It is well-known that Salmonella modulates the activation of NLRC4 (NLR family CARD domain-containing protein 4) and NLRP3 (NLR family pyrin domain-containing 3) inflammasomes, however the mechanisms whereby Salmonella avoids or delays inflammasome activation remain largely unknown. Therefore, we used Salmonella Enteritidis C50336ΔfliC transposon library to screen for genes involved in modulating inflammasomes activation. The screen revealed the galactose metabolism-related gene galE to be essential for inflammasome activation. Here, we found that inflammasome activation was significantly increased in J774A.1 cells or wild-type bone marrow-derived macrophages (BMDMs) during infection by ΔfliCΔgalE compared to cells infected with ΔfliC. Importantly, we found that secretion of IL-1β was Caspase-1-dependent, consistent with canonical NLRP3 inflammasome activation. Furthermore, the virulence of ΔfliCΔgalE was significantly decreased compared to ΔfliC in a mouse model. Finally, RNA-seq analysis showed that multiple signaling pathways related to the inflammasome were subject to regulation by GalE. Taken together, our results suggest that GalE plays an important role in the regulatory network of Salmonella evasion of inflammasome activation.

Dynamic personalized risk prediction in chronic heart failure patients: a longitudinal, clinical investigation of 92 biomarkers (Bio-SHiFT study)

The aim of our observational study was to derive a small set out of 92 repeatedly measured biomarkers with optimal predictive capacity for adverse clinical events in heart failure, which could be used for dynamic, individual risk assessment in clinical practice. In 250 chronic HFrEF (CHF) patients, we collected trimonthly blood samples during a median of 2.2 years. We selected 537 samples for repeated measurement of 92 biomarkers with the Cardiovascular Panel III (Olink Proteomics AB). We applied Least Absolute Shrinkage and Selection Operator (LASSO) penalization to select the optimal set of predictors of the primary endpoint (PE). The association between repeatedly measured levels of selected biomarkers and the PE was evaluated by multivariable joint models (mvJM) with stratified fivefold cross validation of the area under the curve (cvAUC). The PE occurred in 66(27%) patients. The optimal set of biomarkers selected by LASSO included 9 proteins: NT-proBNP, ST2, vWF, FABP4, IGFBP-1, PAI-1, PON-3, transferrin receptor protein-1, and chitotriosidase-1, that yielded a cvAUC of 0.88, outperforming the discriminative ability of models consisting of standard biomarkers (NT-proBNP, hs-TnT, eGFR clinically adjusted) − 0.82 and performing equally well as an extended literature-based set of acknowledged biomarkers (NT-proBNP, hs-TnT, hs-CRP, GDF-15, ST2, PAI-1, Galectin 3) − 0.88. Nine out of 92 serially measured circulating proteins provided a multivariable model for adverse clinical events in CHF patients with high discriminative ability. These proteins reflect wall stress, remodelling, endothelial dysfunction, iron deficiency, haemostasis/fibrinolysis and innate immunity activation. A panel containing these proteins could contribute to dynamic, personalized risk assessment.

Clinical Trial Registration: 10/05/2013 https://clinicaltrials.gov/ct2/show/NCT01851538?term=nCT01851538&draw=2&rank=1.

Role of von Willebrand factor in venous thromboembolic disease

Objective

Evolving evidence of the shared risk factors and pathogenic mechanisms in arterial and venous thrombosis questions of the strict vascular dichotomy of arterial vs venous. The connection between arterial and venous thrombosis has been highlighted by common underlying inflammatory processes, a concept known as thromboinflammatory disease. Using this relationship, we can apply knowledge from arterial disease to better understand and potentially mitigate venous disease. A protein that has been extensively studied in atherothrombotic disease and inflammation is von Willebrand factor (VWF). Because many predisposing and provoking factors of venous thromboembolism (VTE) have been shown to directly modulate VWF levels, it is, perhaps, not surprising that VWF has been highlighted by several recent association studies of patients with VTE.

Methods

In the present narrative review, we investigated more deeply the effects of VWF in venous disease by synthesizing the data from clinical studies of deep vein thrombosis of the limbs, pulmonary embolism, portal and cerebral vein thrombosis, and the complications of thrombosis, including post-thrombotic syndrome, venous insufficiency, and chronic thromboembolic pulmonary hypertension. We have also discussed the findings from preclinical studies to highlight novel VWF biochemistry in thrombosis and therapeutics.

Results

Across the spectrum of venous thromboembolic disease, we consistently observed that elevated VWF levels conferred an increased risk of VTE and long-term venous complications. We have highlighted important findings from VWF molecular research and have proposed mechanisms by which VWF participates in venous disease. Emerging evidence from preclinical studies might reveal novel targets for thromboinflammatory disease, including specific VWF pathophysiology. Furthermore, we have highlighted the utility of measuring VWF to prognosticate and risk stratify for VTE and its complications.

Conclusions

As the prevalence of inflammatory processes, such as aging, obesity, and diabetes increases in our population, it is critical to understand the evolving role of VWF in venous disease to guide clinical decisions and therapeutics.

V-ATPase V0a1 promotes Weibel-Palade body biogenesis through the regulation of membrane fission

Membrane fission, the division of a membrane-bound structure into two discrete compartments, is essential for diverse cellular events, such as endocytosis and vesicle/granule biogenesis; however, the process remains unclear. The hemostatic protein von Willebrand factor is produced in vascular endothelial cells and packaged into specialized secretory granules, Weibel–Palade bodies (WPBs) at the trans-Golgi network (TGN). Here, we reported that V0a1, a V-ATPase component, is required for the membrane fission of WPBs. We identified two V0a isoforms in distinct populations of WPBs in cultured endothelial cells, V0a1 and V0a2, on mature and nascent WPBs, respectively. Although WPB buds were formed, WPBs could not separate from the TGN in the absence of V0a1. Screening using dominant–negative forms of known membrane fission regulators revealed protein kinase D (PKD) as an essential factor in biogenesis of WPBs. Further, we showed that the induction of wild-type PKDs in V0a1-depleted cells does not support the segregation of WPBs from the TGN; suggesting a primary role of V0a1 in the membrane fission of WPBs. The identification of V0a1 as a new membrane fission regulator should facilitate the understanding of molecular events that enable membrane fission.

GDP/GTP exchange factor MADD drives activation and recruitment of secretory Rab GTPases to Weibel-Palade bodies

von Willebrand factor (VWF) is an essential hemostatic protein that is synthesized and secreted by endothelial cells and stored in Weibel-Palade bodies (WPBs). The secretory Rab GTPases Rab27A, Rab3B, and Rab3D have been linked with WPB trafficking and secretion. How these Rabs are activated and recruited to WPBs remains elusive. In this study, we identified MAP kinase-activating death domain (MADD) as the guanine nucleotide exchange factor for Rab27A and both Rab3 isoforms in primary human endothelial cells. Rab activity assays revealed a reduction in Rab27A, Rab3B, and Rab3D activation upon MADD silencing. Rab activation, but not binding, was dependent on the differentially expressed in normal and neoplastic cells (DENN) domain of MADD, indicating the potential existence of 2 Rab interaction modules. Furthermore, immunofluorescent analysis showed that Rab27A, Rab3B, and Rab3D recruitment to WPBs was dramatically decreased upon MADD knockdown, revealing that MADD drives Rab membrane targeting. Artificial mistargeting of MADD using a TOMM70 tag abolished Rab27A localization to WPB membranes in a DENN domain-dependent manner, indicating that normal MADD localization in the cytosol is crucial. Activation of Rab3B and Rab3D was reduced upon Rab27A silencing, suggesting that activation of these Rabs is enhanced through previous activation of Rab27A by MADD. MADD silencing did not affect WPB morphology, but it did reduce VWF intracellular content. Furthermore, MADD-depleted cells exhibited decreased histamine-evoked VWF release, similar to Rab27A-depleted cells. In conclusion, MADD acts as a master regulator of VWF secretion by coordinating the activation and membrane targeting of secretory Rabs to WPBs.

Intracellular receptor EPAC regulates von Willebrand factor secretion from endothelial cells in a PI3K-/eNOS-dependent manner during inflammation

Coagulopathy is associated with both inflammation and infection, including infections with novel severe acute respiratory syndrome coronavirus-2, the causative agent Coagulopathy is associated with both inflammation and infection, including infection with novel severe acute respiratory syndrome coronavirus-2, the causative agent of COVID-19. Clot formation is promoted via cAMP-mediated secretion of von Willebrand factor (vWF), which fine-tunes the process of hemostasis. The exchange protein directly activated by cAMP (EPAC) is a ubiquitously expressed intracellular cAMP receptor that plays a regulatory role in suppressing inflammation. To assess whether EPAC could regulate vWF release during inflammation, we utilized our EPAC1-null mouse model and revealed increased secretion of vWF in endotoxemic mice in the absence of the EPAC1 gene. Pharmacological inhibition of EPAC1 in vitro mimicked the EPAC1-/- phenotype. In addition, EPAC1 regulated tumor necrosis factor-α-triggered vWF secretion from human umbilical vein endothelial cells in a manner dependent upon inflammatory effector molecules PI3K and endothelial nitric oxide synthase. Furthermore, EPAC1 activation reduced inflammation-triggered vWF release, both in vivo and in vitro. Our data delineate a novel regulatory role for EPAC1 in vWF secretion and shed light on the potential development of new strategies to control thrombosis during inflammation.

Secretome and Tunneling Nanotubes: A Multilevel Network for Long Range Intercellular Communication between Endothelial Cells and Distant Cells

As a cellular interface between the blood and tissues, the endothelial cell (EC) monolayer is involved in the control of key functions including vascular tone, permeability and homeostasis, leucocyte trafficking and hemostasis. EC regulatory functions require long-distance communications between ECs, circulating hematopoietic cells and other vascular cells for efficient adjusting thrombosis, angiogenesis, inflammation, infection and immunity. This intercellular crosstalk operates through the extracellular space and is orchestrated in part by the secretory pathway and the exocytosis of Weibel Palade Bodies (WPBs), secretory granules and extracellular vesicles (EVs). WPBs and secretory granules allow both immediate release and regulated exocytosis of messengers such as cytokines, chemokines, extracellular membrane proteins, coagulation or growth factors. The ectodomain shedding of transmembrane protein further provide the release of both receptor and ligands with key regulatory activities on target cells. Thin tubular membranous channels termed tunneling nanotubes (TNTs) may also connect EC with distant cells. EVs, in particular exosomes, and TNTs may contain and transfer different biomolecules (e.g., signaling mediators, proteins, lipids, and microRNAs) or pathogens and have emerged as a major triggers of horizontal intercellular transfer of information.

Validation of the relationship between coagulopathy and localization of hydroxyethyl starch on the vascular endothelium in a rat hemodilution model

Various anticoagulant properties have been associated with hydroxyethyl starch (HES). However, the mechanism remains unclear and it has not been fully considered whether these properties are beyond the dilutional effect itself. The aim of this study was to reproduce the coagulopathy induced by HES and to test the hypothesis that the coagulopathy is caused by endothelial or glycocalyx damage due to localization of HES on the endothelium, which is caused by the high shear viscosity of dilutional blood. Using a rat model, we compared blood coagulability measured by Sonoclot, levels of endothelial and glycocalyx damage markers and coagulation factors, and blood shear viscosity when hemodilution was performed with physiological saline (PS), 6% HES 130/0.4 in PS, and 10% HES 200/0.5 in PS. We also evaluated the localization rates of fluorescently labeled HES on endothelium in the isolated aorta. HES decreased the fibrin gel formation rate more than did PS. HES was shown to cover the endothelium, possibly due to its high shear viscosity, and this mechanism potentially acted to protect, rather than damage, the endothelium and glycocalyx. However, this covering effect may be the cause of coagulopathy due to inhibition of von Willebrand factor secretion from the endothelium.

Sec22b determines Weibel-Palade body length by controlling anterograde ER-Golgi transport

Von Willebrand factor (VWF) is a multimeric hemostatic protein that is synthesized in endothelial cells, where it is stored for secretion in elongated secretory organelles called Weibel-Palade bodies (WPB). The hemostatic activity of VWF is strongly related to the length of these bodies, but how endothelial cells control the dimensions of their WPB is unclear. In this study, using a targeted short hairpin RNA screen, we identified longin-SNARE Sec22b as a novel determinant of WPB size and VWF trafficking. We found that Sec22b depletion resulted in loss of the typically elongated WPB morphology together with disintegration of the Golgi and dilation of rough endoplasmic reticulum cisternae. This was accompanied by reduced proteolytic processing of VWF, accumulation of VWF in the dilated rough endoplasmic reticulum and reduced basal and stimulated VWF secretion. Our data demonstrate that the elongation of WPB, and thus adhesive activity of their cargo VWF, is determined by the rate of anterograde transport between endoplasmic reticulum and Golgi, which depends on Sec22b-containing SNARE complexes.

Emerging mechanisms to modulate VWF release from endothelial cells

Agonist-mediated exocytosis of Weibel-Palade bodies underpins the endothelium's ability to respond to injury or infection. Much of this important response is mediated by the major constituent of Weibel-Palade bodies: the ultra-large glycoprotein von Willebrand factor. Upon regulated WPB exocytosis, von Willebrand factor multimers unfurl into long, platelet-catching 'strings' which instigate the pro-haemostatic response. Accordingly, excessive levels of VWF are associated with thrombotic pathologies, including myocardial infarction and ischaemic stroke. Failure to appropriately cleave von Willebrand Factor strings results in thrombotic thrombocytopenic purpura, a life-threatening pathology characterised by tissue ischaemia and multiple microvascular occlusions. Historically, treatment of thrombotic thrombocytopenic purpura has relied heavily on plasma exchange therapy. However, the demonstrated efficacy of Rituximab and Caplacizumab in the treatment of acquired thrombotic thrombocytopenic purpura highlights how insights into pathophysiology can improve treatment options for von Willebrand factor-related disease. Directly limiting von Willebrand factor release from Weibel-Palade bodies has the potential as a therapeutic for cardiovascular disease. Cell biologists aim to map the WPB biogenesis and secretory pathways in order to find novel ways to control von Willebrand factor release. Emerging paradigms include the modulation of Weibel-Palade body size, trafficking and mechanism of fusion. This review focuses on the promise, progress and challenges of targeting Weibel-Palade bodies as a means to inhibit von Willebrand factor release from endothelial cells.

von Willebrand Factor Antigen, von Willebrand Factor Propeptide, and ADAMTS13 in Carotid Stenosis and Their Relationship with Cerebral Microemboli

BACKGROUND

 The relationship between von Willebrand factor antigen (VWF:Ag), VWF propeptide (VWFpp), VWFpp/VWF:Ag ratio, ADAMTS13 activity, and microembolic signal (MES) status in carotid stenosis is unknown.

METHODS

 This prospective, multicenter study simultaneously assessed plasma VWF:Ag levels, VWFpp levels and ADAMTS13 activity, and their relationship with MES in asymptomatic versus symptomatic moderate-to-severe (≥50-99%) carotid stenosis patients. One-hour transcranial Doppler ultrasound of the middle cerebral arteries classified patients as MES+ve or MES-ve.

RESULTS

 Data from 34 asymptomatic patients were compared with 43 symptomatic patients in the "early phase" (≤4 weeks) and 37 patients in the "late phase" (≥3 months) after transient ischemic attack (TIA)/ischemic stroke. VWF:Ag levels were higher (p = 0.049) and VWFpp/VWF:Ag ratios lower (p = 0.006) in early symptomatic than in asymptomatic patients overall, and in early symptomatic versus asymptomatic MES-ve subgroups (p ≤0.02). There were no intergroup differences in VWFpp expression or ADAMTS13 activity (p ≥0.05). VWF:Ag levels and ADAMTS13 activity decreased (p ≤ 0.048) and VWFpp/VWF:Ag ratios increased (p = 0.03) in symptomatic patients followed up from the early to late phases after TIA/stroke. Although there were no differences in the proportions of symptomatic and asymptomatic patients with blood group O, a combined analysis of early symptomatic and asymptomatic patients revealed lower median VWF:Ag levels in patients with blood group O versus those without blood group O (9.59 vs. 12.32 µg/mL, p = 0.035).

DISCUSSION

 VWF:Ag expression, a marker of endothelial ± platelet activation, is enhanced in recently symptomatic versus asymptomatic carotid stenosis patients, including in MES-ve patients, and decreases with ADAMTS13 activity over time following atherosclerotic TIA/ischemic stroke.

EPAC regulates von Willebrand factor secretion from endothelial cells in a PI3K/eNOS-dependent manner during inflammation

Coagulopathy is associated with both inflammation and infection, including infection with the novel SARS-CoV-2 (COVID-19). Endothelial cells (ECs) fine tune hemostasis via cAMP-mediated secretion of von Willebrand factor (vWF), which promote the process of clot formation. The e xchange p rotein directly a ctivated by c AMP (EPAC) is a ubiquitously expressed intracellular cAMP receptor that plays a key role in stabilizing ECs and suppressing inflammation. To assess whether EPAC could regulate vWF release during inflammation, we utilized our EPAC1 -null mouse model and revealed an increased secretion of vWF in endotoxemic mice in the absence of the EPAC1 gene. Pharmacological inhibition of EPAC1 in vitro mimicked the EPAC1 -/- phenotype. EPAC1 regulated TNFα-triggered vWF secretion from human umbilical vein endothelial cells (HUVECs) in a phosphoinositide 3-kinases (PI3K)/endothelial nitric oxide synthase (eNOS)-dependent manner. Furthermore, EPAC1 activation reduced inflammation-triggered vWF release, both in vivo and in vitro . Our data delineate a novel regulatory role of EPAC1 in vWF secretion and shed light on potential development of new strategies to controlling thrombosis during inflammation.

Key Point

PI3K/eNOS pathway-mediated, inflammation-triggered vWF secretion is the target of the pharmacological manipulation of the cAMP-EPAC system.

RGS4 controls secretion of von Willebrand factor to the subendothelial matrix

The haemostatic protein von Willebrand factor (VWF) exists in plasma and subendothelial pools. The plasma pools are secreted from endothelial storage granules, Weibel-Palade bodies (WPBs), by basal secretion with a contribution from agonist-stimulated secretion, and the subendothelial pool is secreted into the subendothelial matrix by a constitutive pathway not involving WPBs. We set out to determine whether the constitutive release of subendothelial VWF is actually regulated and, if so, what functional consequences this might have. Constitutive VWF secretion can be increased by a range of factors, including changes in VWF expression, levels of TNF and other environmental cues. An RNA-seq analysis revealed that expression of regulator of G protein signalling 4 (RGS4) was reduced in endothelial cells (HUVECs) grown under these conditions. siRNA RGS4 treatment of HUVECs increased constitutive basolateral secretion of VWF, probably by affecting the anterograde secretory pathway. In a simple model of endothelial damage, we show that RGS4-silenced cells increased platelet recruitment onto the subendothelial matrix under flow. These results show that changes in RGS4 expression alter levels of subendothelial VWF, affecting platelet recruitment. This introduces a novel control over VWF function.

Paroxysmal atrial fibrillation: changes in factor VIII and von Willebrand factor impose early hypercoagulability

Introduction

Paroxysmal atrial fibrillation (PAF) is a well-documented prothrombotic state that carries significant embolic risk. However, precise hemostatic changes in the very early stage of the disease are not completely studied. The aim of the study was to study von Willebrand factor (vWF) and coagulation factor VIII (FVIII) plasma levels and activity in the first hours (up to 24 h) of PAF clinical manifestation.

Material and methods

We selected consecutively 51 non-anticoagulated patients (26 men, 25 women, mean age: 59.84 ±1.60) with PAF and 52 controls (26 men, 26 women, mean age: 59.50 ±1.46 years) corresponding in gender, accompanying diseases and conducted treatment. The indicators were examined using enzyme-linked immunoassays and photometric tests.

Results

All patients were hospitalized between the 2^nd and 24^th h after the onset of arrhythmia (mean: 8.14 ±0.74 h). Higher FVIII levels (107.52 ±3.48% vs. 93.85 ±2.93%, p < 0.05) and activity (200.03 ±11.11% vs. 109.73 ±4.90%, p < 0.001) were found in the PAF group. vWF levels (178.40 ±12.95% vs. 119.53 ±6.12%, p < 0.001) and activity (200.92 ±12.45% vs. 110.80 ±5.14%, p < 0.001) were also higher. These changes did not depend on age, sex, body mass index or CHA₂DS₂-VASc score in the PAF group (p > 0.05). PAF duration was a significant predictor of increased FVIII levels and activity. Increased PAF duration was followed by increased values of the factors (r = 0.85, p < 0.001; r = 0.83, p < 0.001).

Conclusions

The results presented an activated coagulation cascade and endothelial injury, suggesting hypercoagulability still in the early hours of PAF. These changes in PAF did not correlate with CHA₂DS₂-VASc score risk factors, outlining PAF as a possible independent embolic risk factor.

Immune Consequences of Endothelial Cells' Activation and Dysfunction During Sepsis

The vascular endothelium provides a direct interface between circulating blood cells and parenchymal cells. Thus, it has a key role in vasomotor tone regulation, primary hemostasis, vascular barrier, and immunity. In the case of systemic inflammation, endothelial cell (EC) activation initiates a powerful innate immune response to eliminate the pathogen. In some specific conditions, ECs may also contribute to the activation of adaptive immunity and the recruitment of antigen-specific lymphocytes. However, the loss of EC functions or an exaggerated activation of ECs during sepsis can lead to multiorgan failure.

Unfractionated heparin attenuates histone-mediated cytotoxicity in vitro and prevents intestinal microcirculatory dysfunction in histone-infused rats

BACKGROUND

Extracellular histones are major mediators of organ dysfunction and death in sepsis, and they may cause microcirculatory dysfunction. Heparins have beneficial effects in sepsis and have been reported to bind to histones and neutralize their cytotoxicity. The aim of this study was to investigate the impact of histones on intestinal microcirculation and the intestinal endothelium and to discuss the protective effect of unfractionated heparin (UFH) on the endothelial cytotoxicity and microcirculatory dysfunction induced by histones.

METHODS

Anesthetized rats were infused with 30 mg/kg calf thymus histones, and UFH was administered intravenously at a concentration of 100 IU/kg per hour. The intestinal microcirculation was visualized and measured with incident dark field microscope. Plasma von Willebrand factor (vWF) and soluble thrombomodulin were detected, and structural changes in the rat intestinal microvascular endothelium were examined. The effects of histones and UFH on cell survival rates, vWF release and calcium influx were investigated in human intestinal microvascular endothelial cells (HIMECs).

RESULTS

Histone infusion caused severe intestinal microcirculatory dysfunction in the absence of obvious hemodynamic changes, and UFH protected intestinal microcirculation in histone-infused rats. Concentrations of the plasma endothelial injury markers vWF and soluble thrombomodulin were elevated, and structural abnormalities were found in the intestinal microvascular endothelium in the histone-infused rats. These events were attenuated by UFH. In vitro, UFH significantly reduced the histone-induced cytotoxicity of HIMECs, reduced the release of vWF from the cytoplasm into the culture medium, and inhibited calcium influx into HIMECs.

CONCLUSION

Histones induce intestinal microcirculatory dysfunction followed by direct injury to the endothelial cells; UFH protects the intestinal microcirculation partly by antagonizing the endothelial toxicity of histones.

Physiological Roles of the von Willebrand Factor-Factor VIII Interaction

Von Willebrand factor (VWF) and coagulation factor VIII (FVIII) circulate as a complex in plasma and have a major role in the hemostatic system. VWF has a dual role in hemostasis. It promotes platelet adhesion by anchoring the platelets to the subendothelial matrix of damaged vessels and it protects FVIII from proteolytic degradation. Moreover, VWF is an acute phase protein that has multiple roles in vascular inflammation and is massively secreted from Weibel-Palade bodies upon endothelial cell activation. Activated FVIII on the other hand, together with coagulation factor IX forms the tenase complex, an essential feature of the propagation phase of coagulation on the surface of activated platelets. VWF deficiency, either quantitative or qualitative, results in von Willebrand disease (VWD), the most common bleeding disorder. The deficiency of FVIII is responsible for Hemophilia A, an X-linked bleeding disorder. Here, we provide an overview on the role of the VWF-FVIII interaction in vascular physiology.

[Prognostic value of endothelial dysfunction and von Willebrand factor in acute and chronic hemispheric intracerebral hemorrhage]

AIM

To study the changes in endothelial dysfunction and von Willebrand factor activity in acute and chronic stages of hemispheric intracerebral hemorrhage (ICH) and their influence on clinical severity and functional recovery.

MATERIAL AND METHODS

Fifty patients with hemispheric ICH, aged 61.6±11.2 years, and 30 patients with AH, aged 59.6±6.2 years, (comparison group) were examined. Patients with ICH were examined on admission, 6-8^th, 13-15^th days, and 11.1±0.9 months after stroke onset. Patients with arterial hypertension (AH) were examined on admission. Changes in NIHSS, Glasgow coma scale, and modified Rankin scale were studied. Restocetin induced platelet aggregation (RIPA) was assessed by optical aggregometry (BIOLA LA230-2 AGGRWB) in modification by G. Born and Z. Gabbasov. von Willebrand factor (vWF) activity was examined as described by J. Olson.

RESULTS

RIPA was significantly higher in acute ICH compared to chronic ICH, AH and reference values. RIPA values were negatively correlated with hematoma volume and midline shift (r≥ -0.308, p≤0.035). vWF activity was significantly higher in ICH patients than in AH and reference values. Patients with AH also had significantly higher vWF activity than reference values. In acute ICH, vWF activity steadily increased reaching maximal values by 13-15^th day. In chronic ICH, vWF activity decreased compared to the acute phase, but still remained higher than in AH patients or reference values. In acute phase, 1% increment in vWF values resulted in 0.5% increase in the risk of death during the follow-up period (95% CI 1.001-1.008, p=0.007).

CONCLUSION

Endothelial dysfunction assessed by vWF activity increases during the acute hemispheric ICH and remains elevated in the chronic stage. vWF activity may be used as a marker in assessing stroke outcome and prognosis.

Interaction networks of Weibel-Palade body regulators syntaxin-3 and syntaxin binding protein 5 in endothelial cells

The endothelium stores the hemostatic protein Von Willebrand factor (VWF) in endothelial storage organelles called Weibel-Palade bodies (WPBs). During maturation, WPBs recruit a complex of Rab GTPases and effectors that associate with components of the SNARE machinery that control WPB exocytosis. Recent genome wide association studies have found links between genetic variations in the SNAREs syntaxin-2 (STX2) and syntaxin binding protein 5 (STXBP5) and VWF plasma levels, suggesting a role for SNARE proteins in regulating VWF release. Moreover, we have previously identified the SNARE proteins syntaxin-3 and STXBP1 as regulators of WPB release. In this study we used an unbiased iterative interactomic approach to identify new components of the WPB exocytotic machinery. An interactome screen of syntaxin-3 identifies a number of SNAREs and SNARE associated proteins (STXBP2, STXBP5, SNAP23, NAPA and NSF). We show that the VAMP-like domain (VLD) of STXBP5 is indispensable for the interaction with SNARE proteins and this capacity of the VLD could be exploited to identify an extended set of novel endothelial SNARE interactors of STXBP5. In addition, an STXBP5 variant with an N436S substitution, which is linked to lower VWF plasma levels, does not show a difference in interactome when compared with WT STXBP5. SIGNIFICANCE: The hemostatic protein Von Willebrand factor plays a pivotal role in vascular health: quantitative or qualitative deficiencies of VWF can lead to bleeding, while elevated levels of VWF are associated with increased risk of thrombosis. Tight regulation of VWF secretion from WPBs is therefore essential to maintain vascular homeostasis. We used an unbiased proteomic screen to identify new components of the regulatory machinery that controls WPB exocytosis. Our data expand the endothelial SNARE protein network and provide a set of novel candidate WPB regulators that may contribute to regulation of VWF plasma levels and vascular health.

Plasma Peptidylarginine Deiminase IV Promotes VWF-Platelet String Formation and Accelerates Thrombosis After Vessel Injury

RATIONALE

PAD4 (peptidylarginine deiminase type IV), an enzyme essential for neutrophil extracellular trap formation (NETosis), is released together with neutrophil extracellular traps into the extracellular milieu. It citrullinates histones and holds the potential to citrullinate other protein targets. While NETosis is implicated in thrombosis, the impact of the released PAD4 is unknown.

OBJECTIVE

This study tests the hypothesis that extracellular PAD4, released during inflammatory responses, citrullinates plasma proteins, thus affecting thrombus formation.

METHODS AND RESULTS

Here, we show that injection of r-huPAD4 in vivo induces the formation of VWF (von Willebrand factor)-platelet strings in mesenteric venules and that this is dependent on PAD4 enzymatic activity. VWF-platelet strings are naturally cleaved by ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type-1 motif-13). We detected a reduction of endogenous ADAMTS13 activity in the plasma of wild-type mice injected with r-huPAD4. Using mass spectrometry and in vitro studies, we found that r-huPAD4 citrullinates ADAMTS13 on specific arginine residues and that this modification dramatically inhibits ADAMTS13 enzymatic activity. Elevated citrullination of ADAMTS13 was observed in plasma samples of patients with sepsis or noninfected patients who were elderly (eg, age >65 years) and had underlying comorbidities (eg, diabetes mellitus and hypertension) as compared with healthy donors. This shows that ADAMTS13 is citrullinated in vivo. VWF-platelet strings that form on venules of Adamts13^-/- mice were immediately cleared after injection of r-huADAMTS13, while they persisted in vessels of mice injected with citrullinated r-huADAMTS13. Next, we assessed the effect of extracellular PAD4 on platelet-plug formation after ferric chloride-induced injury of mesenteric venules. Administration of r-huPAD4 decreased time to vessel occlusion and significantly reduced thrombus embolization.

CONCLUSIONS

Our data indicate that PAD4 in circulation reduces VWF-platelet string clearance and accelerates the formation of a stable platelet plug after vessel injury. We propose that this effect is, at least in part, due to ADAMTS13 inhibition.

Advancing multimer analysis of von Willebrand factor by single-molecule AFM imaging

The formation of hemostatic plugs at sites of vascular injury crucially involves the multimeric glycoprotein von Willebrand factor (VWF). VWF multimers are linear chains of N-terminally linked dimers. The latter are formed from monomers via formation of the C-terminal disulfide bonds Cys2771-Cys2773', Cys2773-Cys2771', and Cys2811-Cys2811'. Mutations in VWF that impair multimerization can lead to subtype 2A of the bleeding disorder von Willebrand Disease (VWD). Commonly, the multimer size distribution of VWF is assessed by electrophoretic multimer analysis. Here, we present atomic force microscopy (AFM) imaging as a method to determine the size distribution of VWF variants by direct visualization at the single-molecule level. We first validated our approach by investigating recombinant wildtype VWF and a previously studied mutant (p.Cys1099Tyr) that impairs N-terminal multimerization. We obtained excellent quantitative agreement with results from earlier studies and with electrophoretic multimer analysis. We then imaged specific mutants that are known to exhibit disturbed C-terminal dimerization. For the mutants p.Cys2771Arg and p.Cys2773Arg, we found the majority of monomers (87 ± 5% and 73 ± 4%, respectively) not to be C-terminally dimerized. While these results confirm that Cys2771 and Cys2773 are crucial for dimerization, they additionally provide quantitative information on the mutants' different abilities to form alternative C-terminal disulfides for residual dimerization. We further mutated Cys2811 to Ala and found that only 23 ± 3% of monomers are not C-terminally dimerized, indicating that Cys2811 is structurally less important for dimerization. Furthermore, for mutants p.Cys2771Arg, p.Cys2773Arg, and p.Cys2811Ala we found 'even-numbered' non-native multimers, i.e. multimers with monomers attached on both termini; a multimer species that cannot be distinguished from native multimers by conventional multimer analysis. Summarizing, we demonstrate that AFM imaging can provide unique insights into VWF processing defects at the single-molecule level that cannot be gained from established methods of multimer analysis.

Exocytosis of Weibel-Palade bodies: how to unpack a vascular emergency kit

The blood vessel wall has a number of self-healing properties, enabling it to minimize blood loss and prevent or overcome infections in the event of vascular trauma. Endothelial cells prepackage a cocktail of hemostatic, inflammatory and angiogenic mediators in their unique secretory organelles, the Weibel-Palade bodies (WPBs), which can be immediately released on demand. Secretion of their contents into the vascular lumen through a process called exocytosis enables the endothelium to actively participate in the arrest of bleeding and to slow down and direct leukocytes to areas of inflammation. Owing to their remarkable elongated morphology and their secretory contents, which span the entire size spectrum of small chemokines all the way up to ultralarge von Willebrand factor multimers, WPBs constitute an ideal model system for studying the molecular mechanisms of secretory organelle biogenesis, exocytosis, and content expulsion. Recent studies have now shown that, during exocytosis, WPBs can undergo several distinct modes of fusion, and can utilize fundamentally different mechanisms to expel their contents. In this article, we discuss recent advances in our understanding of the composition of the WPB exocytotic machinery and how, because of its configuration, it is able to support WPB release in its various forms.

3D human bone marrow stromal and endothelial cell spheres promote bone healing in an osteogenic niche

The current study used an ex vivo [embryonic day (E)18] chick femur defect model to examine the bone regenerative capacity of implanted 3-dimensional (3D) skeletal–endothelial cell constructs. Human bone marrow stromal cell (HBMSC) and HUVEC spheroids were implanted within a bone defect site to determine the osteogenic potential of the skeletal–endothelial cell unit. Cells were pelleted as co- or monocell spheroids and placed within 1-mm-drill defects in the mid-diaphysis of E18 chick femurs and cultured organotypically for 10 d. Micro-computed tomography analysis revealed significantly (P = 0.0001) increased levels of bone volume (BV) and BV/tissue volume ratio in all cell-pellet groups compared with the sham defect group. The highest increase was seen in BV in femurs containing the HUVEC and HBMSC monocell constructs. Type II collagen expression was particularly pronounced within the cell spheres containing HBMSCs and HUVECs, and CD31-positive cell clusters were prominent within HUVEC-implanted defects. These studies demonstrate the importance of the 3D osteogenic-endothelial niche interaction in bone regeneration. Elucidating the component cell interactions in the osteogenic-vascular niche and the role of exogenous factors in driving these osteogenic processes will aid the development of better bone reparative strategies.—Inglis, S., Kanczler, J. M., Oreffo, R. O. C. 3D human bone marrow stromal and endothelial cell spheres promote bone healing in an osteogenic niche.

Prognostic value of von Willebrand factor in patients with atrial fibrillation: A meta-analysis

BACKGROUND

Studies on the prognostic role of von Willebrand factor (vWF) in patients with atrial fibrillation (AF) are conflicting. This meta-analysis aimed to evaluate the association of elevated circulating vWF level with adverse outcomes in patients with AF.

METHODS

PubMed and Embase were used to search literature through August 2017. Prospective observational studies that evaluated the association of elevated vWF level with major adverse cardiac events (MACEs) and all-cause mortality in patients with AF were deemed eligible. The MACEs included death, stroke/transient ischemic attack, heart failure, myocardial infarction, and systemic/peripheral embolism.

RESULTS

A total of 6 studies were included this meta-analysis. Patients with AF with the highest vWF level were independently associated with greater risk of MACEs (risk ratio [RR] 2.20; 95% confidence intervals [CI] 1.61-3.01) and all-cause mortality (RR 1.63; 95% CI 1.39-1.91). Subgroup analysis showed that the prognostic role of higher vWF level was consistently observed in each defined subgroups.

CONCLUSION

Patients with AF with elevated vWF level are independently associated with a higher risk of MACEs and all-cause mortality. However, more well-designed prospective studies are needed to confirm these findings.

Weibel-Palade Body Localized Syntaxin-3 Modulates Von Willebrand Factor Secretion From Endothelial Cells

Supplemental Digital Content is available in the text.

Objective—

Endothelial cells store VWF (von Willebrand factor) in rod-shaped secretory organelles, called Weibel-Palade bodies (WPBs). WPB exocytosis is coordinated by a complex network of Rab GTPases, Rab effectors, and SNARE (soluble NSF attachment protein receptor) proteins. We have previously identified STXBP1 as the link between the Rab27A-Slp4-a complex on WPBs and the SNARE proteins syntaxin-2 and -3. In this study, we investigate the function of syntaxin-3 in VWF secretion.

Approach and Results—

In human umbilical vein endothelial cells and in blood outgrowth endothelial cells (BOECs) from healthy controls, endogenous syntaxin-3 immunolocalized to WPBs. A detailed analysis of BOECs isolated from a patient with variant microvillus inclusion disease, carrying a homozygous mutation in STX3(STX3^−/−), showed a loss of syntaxin-3 protein and absence of WPB-associated syntaxin-3 immunoreactivity. Ultrastructural analysis revealed no detectable differences in morphology or prevalence of immature or mature WPBs in control versus STX3^−/− BOECs. VWF multimer analysis showed normal patterns in plasma of the microvillus inclusion disease patient, and media from STX3^−/− BOECs, together indicating WPB formation and maturation are unaffected by absence of syntaxin-3. However, a defect in basal as well as Ca²⁺- and cAMP-mediated VWF secretion was found in the STX3^−/− BOECs. We also show that syntaxin-3 interacts with the WPB-associated SNARE protein VAMP8 (vesicle-associated membrane protein-8).

Conclusions—

Our data reveal syntaxin-3 as a novel WPB-associated SNARE protein that controls WPB exocytosis.

Weibel-Palade bodies at a glance

The vascular environment can rapidly alter, and the speed with which responses to both physiological and pathological changes are required necessitates the existence of a highly responsive system. The endothelium can quickly deliver bioactive molecules by regulated exocytosis of its secretory granules, the Weibel-Palade bodies (WPBs). WPBs include proteins that initiate both haemostasis and inflammation, as well those that modulate blood pressure and angiogenesis. WPB formation is driven by von Willebrand factor, their most abundant protein, which controls both shape and size of WPBs. WPB are generated in a range of sizes, with the largest granules over ten times the size of the smallest. In this Cell Science at a Glance and the accompanying poster, we discuss the emerging mechanisms by which WPB size is controlled and how this affects the ability of this organelle to modulate haemostasis. We will also outline the different modes of exocytosis and their polarity that are currently being explored, and illustrate that these large secretory organelles provide a model for how elements of secretory granule biogenesis and exocytosis cooperate to support a complex and diverse set of functions.

Thrombotic Thrombocytopenic Purpura: Etiopathogenesis, Diagnostics and Basic Principles of Treatment

Thrombotic thrombocytopenic purpura (TTP) is a clinical syndrome that manifests with thrombocytopenia, microangiopathic haemolytic anaemia and symptoms and signs of kidney and brain damage, but it rarely involves other organs. The main pathophysiological cause of TTP is diminished metalloproteinase ADAMTS13 activity; the main function of ADAMTS13 is to degrade large multimers of the von Willebrand factor. Diminished activity of ADAMTS13 is caused either by a genetic mutation in the gene that codes ADAMTS13 (congenital TTP) or by antibodies that block ADAMTS13 enzyme activity or accelerate the degradation of ADAMTS13 (acquired TTP). Clinically, TTP presents most frequently with signs and symptoms of brain and kidney damage with concomitant haemorrhagic syndrome. TTP is suspected when a patient presents with a low platelet count, microangiopathic haemolytic anaemia (negative Coombs tests, low haptoglobine concentration, increased serum concentration of indirect bilirubin and lactate dehydrogenase, increased number of schysocytes in peripheral blood) and the typical clinical presentation. A definitive diagnose can be made only by measuring the ADAMTS13 activity. The differential diagnosis in such cases includes both typical and atypical haemolytic uremic syndrome, disseminated intravascular coagulation, HELLP syndrome in pregnant women and other thrombotic microangiopathies. The first line therapy for TTP is plasma exchange. In patients with acquired TTP, in addition to plasma exchange, immunosuppressive medications are used (corticosteroids and rituximab). In patients with hereditary TTP, the administration of fresh frozen plasma is sometimes required.

Profile of von Willebrand factor antigen and von Willebrand factor propeptide in an overall TIA and ischaemic stroke population and amongst subtypes

INTRODUCTION

Von Willebrand factor propeptide (VWF:Ag II) is proposed to be a more sensitive marker of acute endothelial activation than von Willebrand factor antigen (VWF:Ag). Simultaneous data on VWF:Ag and VWF:Ag II profiles are very limited following TIA and ischaemic stroke.

METHODS

In this prospective, observational, case-control study, plasma VWF:Ag and VWF:Ag II levels were quantified in 164 patients≤4weeks of TIA or ischaemic stroke (baseline), and then ≥14days (14d) and ≥90days (90d) later, and compared with those from 27 healthy controls. TIA and stroke subtyping was performed according to the TOAST classification. The relationship between VWF:Ag and VWF:Ag II levels and platelet activation status was assessed.

RESULTS

'Unadjusted' VWF:Ag and VWF:Ag II levels were higher in patients at baseline, 14d and 90d than in controls (p≤0.03). VWF:Ag levels remained higher in patients than controls at baseline (p≤0.03), but not at 14d or 90d after controlling for differences in age or hypertension, and were higher in patients at baseline and 90d after controlling for smoking status (p≤0.04). 'Adjusted' VWF:Ag II levels were not higher in patients than controls after controlling for age, hypertension or smoking (p≥0.1). Patients with symptomatic carotid stenosis (N=46) had higher VWF:Ag and VWF:Ag II levels than controls at all time-points (p≤0.002). There was no significant correlation between platelet activation status and VWF:Ag or VWF:Ag II levels.

CONCLUSIONS

VWF:Ag and VWF:Ag II levels are increased in an overall TIA and ischaemic stroke population, especially in patients with recently symptomatic carotid stenosis. VWF:Ag II was not superior to VWF:Ag at detecting acute endothelial activation in this cohort and might reflect timing of blood sampling in our study.

Lifecycle of Weibel-Palade bodies

Weibel-Palade bodies (WPBs) are rod or cigar-shaped secretory organelles that are formed by the vascular endothelium. They contain a diverse set of proteins that either function in haemostasis, inflammation, or angiogenesis. Biogenesis of the WPB occurs at the Golgi apparatus in a process that is dependent on the main component of the WPB, the haemostatic protein von Willebrand Factor (VWF). During this process the organelle is directed towards the regulated secretion pathway by recruiting the machinery that responds to exocytosis stimulating agonists. Upon maturation in the periphery of the cell the WPB recruits Rab27A which regulates WPB secretion. To date several signaling pathways have been found to stimulate WPB release. These signaling pathways can trigger several secretion modes including single WPB release and multigranular exocytosis. In this review we will give an overview of the WPB lifecycle from biogenesis to secretion and we will discuss several deficiencies that affect the WPB lifecycle.

von Willebrand Disease: A Concise Review and Update for the Practicing Physician

von Willebrand disease (vWD) is the most common inherited disorder of hemostasis and comprises a spectrum of heterogeneous subtypes. Significant advances have been made in understanding von Willebrand factor ( vWF) gene mutations, resultant physiologic deficits in the vWF peptide, and their correlation to clinical presentation. Diagnostic tests for this disorder are complex, and interpretation requires a thorough understanding of the underlying pathophysiology by the practicing physician. The objective of this review is to summarize our current understanding of pathophysiology, laboratory investigations, and evolving treatment paradigm of vWD with the availability of recombinant von Willebrand factor.

Von Willebrand factor processing

Von Willebrand factor (VWF) is a multimeric glycoprotein essential for primary haemostasis that is produced only in endothelial cells and megakaryocytes. Key to VWF's function in recruitment of platelets to the site of vascular injury is its multimeric structure. The individual steps of VWF multimer biosynthesis rely on distinct posttranslational modifications at specific pH conditions, which are realized by spatial separation of the involved processes to different cell organelles. Production of multimers starts with translocation and modification of the VWF prepropolypeptide in the endoplasmic reticulum to produce dimers primed for glycosylation. In the Golgi apparatus they are further processed to multimers that carry more than 300 complex glycan structures functionalized by sialylation, sulfation and blood group determinants. Of special importance is the sequential formation of disulfide bonds with different functions in structural support of VWF multimers, which are packaged, stored and further processed after secretion. Here, all these processes are being reviewed in detail including background information on the occurring biochemical reactions.

Regulation of angiopoietin-2 secretion from human pulmonary microvascular endothelial cells

INTRODUCTION

Sepsis is characterized by dysregulated systemic inflammation and cytokine storm. Angiopoietin-2 (Ang-2) is known to closely correlate with severity of sepsis-related acute lung injury and mortality. The aim of this study was to clarify the mechanisms involved in Ang-2 secretion to better understand the pathophysiology of sepsis.

MATERIALS AND METHODS

The concentration of Ang-2 was assessed in culture medium of pulmonary microvascular endothelial cells in the presence or absence of Gram-positive bacteria cell wall components [lipoteichoic acid (LTA) and peptidoglycan (PGN)] stimulation at different time points ranging from 15 minutes to 24 hours. Constitutive and LTA-PGN-stimulated Ang-2 level changes were also assessed after cells were pretreated with different pathway inhibitors for 1 hour.

RESULTS

Two distinctive mechanisms of Ang-2 secretion, constitutive and stimulated secretion, were identified. Constitutive secretion resulted in slow but continuous increase in Ang-2 in culture medium over time. It was regulated by 3'5'-cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA)-Ca²⁺ and nitric oxide (NO)-3'5'-cyclic guanosine monophosphate (cGMP)-protein kinase G (PKG)-Ca²⁺ pathways and partially regulated by N-ethyl-maleimide-sensitive factor-Ca²⁺ pathways. LTA-PGN stimulation caused rapid and potent increase followed by gradual decrease of Ang-2. It was partially regulated by both Ral A-phospholipase D and NSF-Ca²⁺ pathways.

CONCLUSIONS

We demonstrated characteristics and involved pathways for two distinctive secretory mechanisms, constitutive and stimulated, of Ang-2 in pulmonary microvascular endothelial cells. Considering the close correlation of Ang-2 with sepsis outcomes, our findings provide a better understanding of an important mechanism associated with sepsis pathophysiology and identify possible therapeutic targets to improve outcomes in the potentially lethal disease.

Mutations in the D'D3 region of VWF traditionally associated with type 1 VWD lead to quantitative and qualitative deficiencies of VWF

Type 1 von Willebrand disease (VWD) is characterized by low plasma levels of von Willebrand factor (VWF) and clinical bleeding. Several mechanisms have been described that cause a decrease in plasma VWF levels in VWD, and the goal of this study was to elucidate the pathogenic origins of VWD for a group of mutations in the VWF D'D3 region traditionally associated with type 1 VWD. Varying ratios of mutant-to-wild-type VWF were expressed in two cell lines in order to study the intracellular location, multimer assembly, secretion and function of VWF. We identified four mutants (M771I, Y1146C, T1156M, R782Q) that caused defective intracellular packaging and markedly reduced VWF secretion. Consistent with previous reports, Y1146C and T1156M VWF led to a loss of high molecular weight multimers. In a functional analysis, Y1146C demonstrated a novel FVIII binding defect. Mutations R924W and I1094T were processed normally and did not show abnormal FVIII binding suggesting that other mechanisms such as plasma clearance or platelet binding defects may contribute to the pathogenicity of these mutants.

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.

Endothelial cell control of thrombosis

Hemostasis encompasses a set of tightly regulated processes that govern blood clotting, platelet activation, and vascular repair. Upon vascular injury, the hemostatic system initiates a series of vascular events and activates extravascular receptors that act in concert to seal off the damage. Blood clotting is subsequently attenuated by a plethora of inhibitors that prevent excessive clot formation and eventual thrombosis. The endothelium which resides at the interface between the blood and surrounding tissues, serves an integral role in the hemostatic system. Depending on specific tissue needs and local stresses, endothelial cells are capable of evoking either antithrombotic or prothrombotic events. Healthy endothelial cells express antiplatelet and anticoagulant agents that prevent platelet aggregation and fibrin formation, respectively. In the face of endothelial dysfunction, endothelial cells trigger fibrin formation, as well as platelet adhesion and aggregation. Finally, endothelial cells release pro-fibrinolytic agents that initiate fibrinolysis to degrade the clot. Taken together, a functional endothelium is essential to maintain hemostasis and prevent thrombosis. Thus, a greater understanding into the role of the endothelium can provide new avenues for exploration and novel therapies for the management of thromboembolisms.

The potential of mesenchymal stem cells in the management of radiation enteropathy

Although radiotherapy is effective in managing abdominal and pelvic malignant tumors, radiation enteropathy is still unavoidable. This disease severely affects the quality of life of cancer patients due to some refractory lesions, such as intestinal ischemia, mucositis, ulcer, necrosis or even perforation. Current drugs or prevailing therapies are committed to alleviating the symptoms induced by above lesions. But the efficacies achieved by these interventions are still not satisfactory, because the milieus for tissue regeneration are not distinctly improved. In recent years, regenerative therapy for radiation enteropathy by using mesenchymal stem cells is of public interests. Relevant results of preclinical and clinical studies suggest that this regenerative therapy will become an attractive tool in managing radiation enteropathy, because mesenchymal stem cells exhibit their pro-regenerative potentials for healing the injuries in both epithelium and endothelium, minimizing inflammation and protecting irradiated intestine against fibrogenesis through activating intrinsic repair actions. In spite of these encouraging results, whether mesenchymal stem cells promote tumor growth is still an issue of debate. On this basis, we will discuss the advances in anticancer therapy by using mesenchymal stem cells in this review after analyzing the pathogenesis of radiation enteropathy, introducing the advances in managing radiation enteropathy using regenerative therapy and exploring the putative actions by which mesenchymal stem cells repair intestinal injuries. At last, insights gained from the potential risks of mesenchymal stem cell-based therapy for radiation enteropathy patients may provide clinicians with an improved awareness in carrying out their studies.

Flow-driven assembly of VWF fibres and webs in in vitro microvessels

Several systemic diseases, including thrombotic thrombocytopenic purpura, manifest much of their pathology through activation of endothelium and thrombotic occlusion of small blood vessels, often leading to multi-organ failure and death. Modelling these diseases is hampered by the complex three-dimensional architecture and flow patterns of the microvasculature. Here, we employ engineered microvessels of complex geometry to examine the pathological responses to endothelial activation. Our most striking finding is the capacity of endothelial-secreted von Willebrand factor (VWF) to assemble into thick bundles or complex meshes, depending on the vessel geometry and flow characteristics. Assembly is greatest in vessels of diameter ≤300 μm, with high shear stress or strong flow acceleration, and with sharp turns. VWF bundles and webs bind platelets, leukocytes and erythrocytes, obstructing blood flow and sometimes shearing passing erythrocytes. Our findings uncover the biophysical requirements for initiating microvascular thrombosis and suggest mechanisms for the onset and progression of microvascular diseases.

3D microvessels with complex geometries and intact endothelium can be built in vitro. Using these engineered microvessels, here the authors show that the generation of the pathologic meshwork of the blood protein von Willebrand factor is affected by vessel architecture, flow and the proteolytic activity of ADAMTS13.