Von Willebrand Factor Exocytosis Research Articles

Endothelial PTP1B Deletion Promotes VWF Exocytosis and Venous Thromboinflammation.

Endothelial activation promotes the release of procoagulant extracellular vesicles and inflammatory mediators from specialized storage granules. Endothelial membrane exocytosis is controlled by phosphorylation. We hypothesized that the absence of PTP1B (protein tyrosine phosphatase 1B) in endothelial cells promotes venous thromboinflammation by triggering endothelial membrane fusion and exocytosis. Mice with inducible endothelial deletion of PTP1B (End.PTP1B-KO) underwent inferior vena cava ligation to induce stenosis and venous thrombosis. Primary endothelial cells from transgenic mice and human umbilical vein endothelial cells were used for mechanistic studies. Vascular ultrasound and histology showed significantly larger venous thrombi containing higher numbers of Ly6G (lymphocyte antigen 6 family member G)-positive neutrophils in mice with endothelial PTP1B deletion, and intravital microscopy confirmed the more pronounced neutrophil recruitment following inferior vena cava ligation. RT2 PCR profiler array and immunocytochemistry analysis revealed increased endothelial activation and adhesion molecule expression in primary End.PTP1B-KO endothelial cells, including CD62P (P-selectin) and VWF (von Willebrand factor). Pretreatment with the NF-κB (nuclear factor kappa B) kinase inhibitor BAY11-7082, antibodies neutralizing CD162 (P-selectin glycoprotein ligand-1) or VWF, or arginylglycylaspartic acid integrin-blocking peptides abolished the neutrophil adhesion to End.PTP1B-KO endothelial cells in vitro. Circulating levels of annexin V+ procoagulant endothelial CD62E+ (E-selectin) and neutrophil (Ly6G+) extracellular vesicles were also elevated in End.PTP1B-KO mice after inferior vena cava ligation. Higher plasma MPO (myeloperoxidase) and Cit-H3 (citrullinated histone-3) levels and neutrophil elastase activity indicated neutrophil activation and extracellular trap formation. Infusion of End.PTP1B-KO extracellular vesicles into C57BL/6J wild-type mice most prominently enhanced the recruitment of endogenous neutrophils, and this response was blunted in VWF-deficient mice or by VWF-blocking antibodies. Reduced PTP1B binding and tyrosine dephosphorylation of SNAP23 (synaptosome-associated protein 23) resulting in increased VWF exocytosis and neutrophil adhesion were identified as mechanisms, all of which could be restored by NF-κB kinase inhibition using BAY11-7082. Our findings show that endothelial PTP1B deletion promotes venous thromboinflammation by enhancing SNAP23 phosphorylation, endothelial VWF exocytosis, and neutrophil recruitment.

C5a and C5aR1 are key drivers of microvascular platelet aggregation in clinical entities spanning from aHUS to COVID-19

AbstractUnrestrained activation of the complement system till the terminal products, C5a and C5b-9, plays a pathogenetic role in acute and chronic inflammatory diseases. In endothelial cells, complement hyperactivation may translate into cell dysfunction, favoring thrombus formation. The aim of this study was to investigate the role of the C5a/C5aR1 axis as opposed to C5b-9 in inducing endothelial dysfunction and loss of antithrombogenic properties. In vitro and ex vivo assays with serum from patients with atypical hemolytic uremic syndrome (aHUS), a prototype rare disease of complement-mediated microvascular thrombosis due to genetically determined alternative pathway dysregulation, and cultured microvascular endothelial cells, demonstrated that the C5a/C5aR1 axis is a key player in endothelial thromboresistance loss. C5a added to normal human serum fully recapitulated the prothrombotic effects of aHUS serum. Mechanistic studies showed that C5a caused RalA-mediated exocytosis of von Willebrand factor (vWF) and P-selectin from Weibel-Palade bodies, which favored further vWF binding on the endothelium and platelet adhesion and aggregation. In patients with severe COVID-19 who suffered from acute activation of complement triggered by severe acute respiratory syndrome coronavirus 2 infection, we found the same C5a-dependent pathogenic mechanisms. These results highlight C5a/C5aR1 as a common prothrombogenic effector spanning from genetic rare diseases to viral infections, and it may have clinical implications. Selective C5a/C5aR1 blockade could have advantages over C5 inhibition because the former preserves the formation of C5b-9, which is critical for controlling bacterial infections that often develop as comorbidities in severely ill patients. The ACCESS trial registered at www.clinicaltrials.gov as #NCT02464891 accounts for the results related to aHUS patients treated with CCX168.

Arf GTPase-activating proteins SMAP1 and AGFG2 regulate the size of Weibel-Palade bodies and exocytosis of von Willebrand factor.

ABSTRACTArf GTPase-Activating proteins (ArfGAPs) mediate the hydrolysis of GTP bound to ADP-ribosylation factors (Arfs), which are critical to form transport intermediates. ArfGAPs have been thought to be negative regulators of Arfs; however, accumulating evidence indicates that ArfGAPs are important for cargo sorting and promote membrane traffic. Weibel-Palade bodies (WPBs) are cigar-shaped secretory granules in endothelial cells that contain von Willebrand factor (vWF) as their main cargo. WPB biogenesis at the Golgi was reported to be regulated by Arf and their regulators, but the role of ArfGAPs has been unknown. In this study, we performed siRNA screening of ArfGAPs to investigate the role of ArfGAPs in the biogenesis of WPBs. We found two ArfGAPs, SMAP1 and AGFG2, to be involved in WPB size and vWF exocytosis, respectively. SMAP1 depletion resulted in small-sized WPBs, and the lysosomal inhibitor leupeptin recovered the size of WPBs. The results indicate that SMAP1 functions in preventing the degradation of cigar-shaped WPBs. On the other hand, AGFG2 downregulation resulted in the inhibition of vWF secretion upon Phorbol 12-myristate 13-acetate (PMA) or histamine stimulation, suggesting that AGFG2 plays a role in vWF exocytosis. Our study revealed unexpected roles of ArfGAPs in vWF transport.

Intracellular targets: A multiple cargo transporting molecule.

The generation of cell-penetrating peptides as cargo-delivery systems has produced an immense number of studies owing to the importance of these systems as tools to deliver molecules into the cells, as well as due to the interest to shed light into a yet unclear mechanism of the entrance of these peptides into the cells. However, many cell-penetrating peptides might present drawbacks due to causing cellular toxicity, or due to being entrapped in endosomes, or as a result of their degradation before they meet their target. In this work, a cargo transporting molecule, the Cell Penetrating Sequential Oligopeptide Carrier (CPSOC), formed by the repetitive -Lys-Aib-Cys- moiety, was tested for its ability to penetrate the cell membrane and transport the conjugated peptides into the cells. The cysteine residue anchors bioactive molecules through a stable thioether bond. The lysine supplies the positive charge to the construct, whereas the α-amino isobutyric acid is well known to induce helicoid conformation to the peptide backbone and protects from enzymatic degradation. The present study demonstrates that CPSOC penetrates the membrane transporting the conjugated cargo into the cell. When we tested CPSOC-conjugated peptides carrying critical domains of Cdc42, a small GTPase implicated in exocytosis, the internalized peptides were found to be functional because they inhibited exocytosis of von Willebrand factor from endothelial Weibel-Palade bodies a trafficking event depending on the Cdc42 protein. The data suggest that the carrier can deliver efficiently functional peptides into the cells, and thus, it can be used as a multiple-cargo transporting molecule.

The Use of Fluorescently Labeled ARC1779 Aptamer for Assessing the Effect of H2O2 on von Willebrand Factor Exocytosis

Here, we propose a new approach for quantitative estimation of von Willebrand factor (vWF) exposed on the surface of endothelial cells (ECs) using the ARC1779 aptamer that interacts with the vWF A1 domain. To visualize complex formation between vWF and the aptamer, the latter was conjugated with the Cy5 fluorescent label. Cultured human umbilical vein endothelial cells (HUVEC) were stained with the ARC1779-Cy5 conjugate and imaged with a fluorescence microscope. The images were analyzed with the CellProfiler software. vWF released from the Weibel–Palade bodies was observed as bright dot-like structures of round and irregular shape, the number of which increased several times after HUVEC exposure to histamine or thrombin. Staining with ARC1779-Cy5 also revealed long filamentous vWF structures on the surface of activated HUVEC. vWF secretion by ECs is activated by the second messengers cAMP and Ca2+. There is evidence that hydrogen peroxide also acts as a second messenger in ECs. In addition, exogenous H2O2 formed in leukocytes can enter ECs. The aim of our study was to determine the effect of H2O2 on the vWF exposure at the surface of HUVEC using the proposed method. It was shown that hydrogen peroxide at concentration 100 µM, which is lower than the cytotoxicity threshold of H2O2 for cultured HUVEC, increased several times the number of dot-like structures and total amount of vWF exposed on plasma membrane of HUVEC, which suggest that H2O2 acts as a mediator that activates exocytosis of Weibel–Palade bodies and vWF secretion in the vascular endothelium during inflammation and upon elevated generation of endogenous reactive oxygen species in ECs.

Hydrogen peroxide stimulates exocytosis of von Willebrand factor in human umbilical vein endothelial cells

The aim of our research was to study the influence of hydrogen peroxide on the exocytosis of von Willebrand factor (vWF) in human umbilical vein endothelial cells (HUVEC). We have found that H2O2 at a non-toxic concentration (100 μM) increases the amount of vWF secreted by HUVEC by 43 ± 14% over control (p < 0.03) and elevates total exposition of vWF on cell surface by 89 ± 5% (p < 0.01). Analysis of immunofluorescent images of HUVEC with CellProfiler program revealed that the average number of antigen positive structures on the single cell surface increases from 11.4 ± 0.16 in control up to 17.5 ± 0.21 after incubation with H2O2 (p < 0.01). vWF is exposed on the cell surface as dots with the average sizes around 1–3 μm. H2O2 causes an increase in the number of these dots and the appearence of the strings of vWF which are absent in control HUVEC. It is suggested that H2O2 may serve as a messenger which stimulates vWF exocytosis.

SNAP23 Regulates Endothelial Exocytosis of von Willebrand Factor.

Endothelial exocytosis regulates vascular thrombosis and inflammation. The trafficking and release of endothelial vesicles is mediated by SNARE (Soluble NSF Attachment protein REceptors) molecules, but the exact identity of endothelial SNAREs has been unclear. Three SNARE molecules form a ternary complex, including isoforms of the syntaxin (STX), vesicle-associated membrane protein (VAMP), and synaptosomal-associated protein (SNAP) families. We now identify SNAP23 as the predominant endothelial SNAP isoform that mediates endothelial exocytosis of von Willebrand Factor (VWF). SNAP23 was localized to the plasma membrane. Knockdown of SNAP23 decreased endothelial exocytosis, suggesting it is important for endothelial exocytosis. SNAP23 interacted with the endothelial exocytic machinery, and formed complexes with other known endothelial SNARE molecules. Taken together, these data suggest that SNAP23 is a key component of the endothelial SNARE machinery that mediates endothelial exocytosis.

Chapter 4 - Correlative Light Microscopy and Electron Tomography to Study Von Willebrand Factor Exocytosis from Vascular Endothelial Cells

Revealing the ultrastructure and function of fluorescently labeled cellular components by correlative light and electron microscopy (CLEM) facilitates the study of structure-function relationships in complex biological processes. Given the diversity of available fluorescent tags, light microscopy is ideal for monitoring dynamic cellular processes, while electron microscopy reveals the morphological context of structures at high resolution. Endothelial cells lining the blood vessel wall contain storage organelles called Weibel-Palade bodies (WPBs), which contain tubules of densely packed helical spirals of the blood coagulation protein Von Willebrand factor (VWF). Exocytosis of WPBs is triggered upon vascular damage and results in the transformation of stored tubular VWF into secreted extracellular VWF. Upon exocytosis, VWF rearranges into long filamentous strings to recruit platelets from the blood. During this secretion process, large intracellular VWF exocytosis structures are formed called secretory pods. Here, we describe a CLEM method used to study the relationship between the secretory pod and secreted VWF where confocal microscopy on whole cells was combined with serial electron tomography on chemically fixed, plastic-embedded sections. We show that the combination of these two well-established microscopy modalities provides a robust and generic CLEM method suitable for the characterization of VWF secretion sites.

Biologic Response and Adverse Events To Stimate In Patients With Von Willebrand Disease

von Willebrand factor (VWF) is a large multimeric glycoprotein secreted from platelet α-granules and Weibel-Palade bodies of endothelial cells. VWF mediates the initial adhesion of platelets at sites of vascular injury and binds to and stabilizes blood coagulation factor VIII in the circulation to protect it from inactivation and clearance. von Willebrand disease (VWD) is the most common hereditary bleeding disorder and results from the deficiency or dysfunction of VWF leading to mucocutaneous bleeding, including epistaxis, menorrhagia, and excessive bleeding after trauma or surgery. Bleeding in patients with VWD is treated with infusion of plasma-derived VWF containing FVIII concentrates or 1-desamino-8-D-arginine vasopressin (DDAVP, desmopression). DDAVP is an analog of vasopressin antidiuretic hormone which can stimulate the exocytosis of VWF from storage sites and increase VWF and FVIII levels. DDAVP can be administrated by intravenous and intranasal routes. There are several reports of the safety and efficacy of intravenous DDAVP, but few with concentrated intranasal DDAVP (Stimate®). Here we report on the efficacy and safety of Stimate® in patients with VWD MethodsHospital records for 72 patients with VWD who received Stimate® from 1998 to 2012 were reviewed. The primary endpoint was the patients' biological response to Stimate®, defined as at least a 3 fold increase compared to baseline of both ristocetin cofactor activity (VWF:RCo), factor VIII procoagulant activity (FVIII:C), or both VWF:RCo and FVIII:C are over 100% after Stimate®. Individuals not meeting the response criteria were deemed to be nonresponse. The adverse events were analyzed between different VWD types, response, age, gender, and race. ResultOf those 72 VWD patients, 45 have type 1 (62%), 7 have type 2 (10%), and in 20 cases the subtype was unclear (28%). Responsive to Stimate® was observed in 43 (95.6%) of type 1, 4 (57.1%) of type 2, and 19 (95%) in which the diagnosis was unclear. In total, 16 patients (22.2%) had adverse events: 1 – allegoric response, 8 - mild headache, 4 - mild hyponatremia, and 4 - blood pressure (BP) reduced more than 20 mmHg. ConclusionPatients with VWD 1 have higher response rates to Stimate® than patients with VWD 2 in our test. The Stimate® is effective and safe for treatment of VWD. Disclosures:Valentino:Baxter, Bayer, Biogen Idec, GTC Biotherapeutics, Inspiration Biopharmaceuticals, Novo Nordisk: Consultancy, Membership on an entity's Board of Directors or advisory committees.

Discs Large 1 (Dlg1) Scaffolding Protein Participates with Clathrin and Adaptator Protein Complex 1 (AP-1) in Forming Weibel-Palade Bodies of Endothelial Cells

Weibel-Palade bodies (WPBs) are specific cigar-shaped granules that store von Willebrand factor (VWF) for its regulated secretion by endothelial cells. The first steps of the formation of these granules at the trans-Golgi network specifically require VWF aggregation and an external scaffolding complex that contains the adaptator protein complex 1 (AP-1) and clathrin. Discs large 1 (Dlg1) is generally considered to be a modular scaffolding protein implicated in the control of cell polarity in a large variety of cells by specific recruiting of receptors, channels, or signaling proteins to specialized zones of the plasma membrane. We propose here that in endothelial cells, Dlg1, in a complex with AP-1 and clathrin, participates in the biogenesis of WPBs. Supporting data show that Dlg1 colocalizes with microtubules, intermediate filaments, and Golgi markers. Tandem mass spectrometry experiments led to the identification of clathrin as an Dlg1-interacting partner. Interaction was confirmed by in situ proximity ligation assays. Furthermore, AP-1 and VWF immunoprecipitate and colocalize with Dlg1 in the juxtanuclear zone. Finally, Dlg1 depletion by siRNA duplexes disrupts trans-Golgi network morphology and WPB formation. Our results provide the first evidence for an unexpected role of Dlg1 in controlling the formation of specific secretory granules involved in VWF exocytosis in endothelial cells.

Imaging of Von Willebrand Factor Remodeling Upon Secretion From Vascular Endothelial Cells

AbstractAbstract 263In response to vascular injury, endothelial cells rapidly secrete high molecular weight multimers of the coagulation protein Von Willebrand factor (VWF). Once expelled from the cells, VWF unfurls in long strings that bind platelets from the bloodstream to induce primary hemostasis. VWF secreted upon stimulation is released from specialized storage compartments called Weibel Palade bodies (WPB) which have a typical rod or cigar shape. They emerge from the Trans Golgi network in a process driven by the formation of helical tubules consisting of VWF multimers and the VWF propeptide. When WPBs undergo exocytosis and release VWF, rapid structural changes occur which eventually result in platelet capturing VWF strings. It has been postulated that the tubular storage of VWF in WPBs is required for sufficient unfolding of the protein during string formation as agents disrupting the VWF tubules were shown to result in less strings. Recently we described a novel structure involved in VWF exocytosis which is formed only upon stimulation. We refer to this structure as a “secretory pod” as it seemed to derive from multiple WPBs and was identified as a VWF release site where strings seemed to be formed. By transmission electron microscopy (TEM) we identified this structure to be a membrane-delimited organelle containing filamentous material resembling unfurled VWF. The VWF tubules as seen in WPBs are absent in secretory pods suggesting that tubular packaging of VWF is not essential for sufficient release and string formation.To study the formation of secretory pods and the subsequent release and remodeling of VWF, several imaging techniques were used such as live-cell imaging and correlative light and electron microscopy. We expressed propeptide-EGFP in endothelial cells to label the WPBs and stimulated them with PMA. By live-cell imaging we visualized the exocytotic events. We observed, apart from single WPB exocytosis, the formation of secretory pods which occurred by the coalescence of several WPBs. In some cases the individual WPBs rounded up first, before they joined into one round structure while in other cases the coalescence event seemed to happen at once. After coalescence, fusion with the plasma membrane occurred to release the pooled VWF which resulted in the disappearance of the fluorescent signal as the propeptide rapidly diffused into the extracellular medium. How the secreted VWF is remodeled after secretion into VWF strings was studied by correlative light and electron microscopy. We correlated confocal pictures of stimulated endothelial cells, which were stained with VWF specific fluorescent antibodies, to consecutive TEM sections. We found that fluorescently labeled VWF dots that were connected to strings, correlated to secretory pods but also to globular mass of secreted VWF. Interestingly, when we analyzed consecutive EM sections, the globular masses were found to originate from the secretory pods. From the globular masses we also observed deriving strings indicating that once VWF is expelled, remodeling occurs independently from secretion. We hypothesize that fluid flow remodels the secreted globular VWF mass into strings. To study this we stimulated endothelial cells under flow. The intracellular VWF pool in the WPBs was labeled green by transient expression of propeptide-EGFP and the secreted VWF was labeled red with strongly diluted red fluorescent VWF specific antibodies in the perfusate. Using live-cell imaging we observed that upon fusion of EGFP labeled WPBs, the green signal transformed into a red signal revealing dots of labeled secreted VWF. These dots rolled, in the direction of the flow, to the edge of the cell where they aggregated and only then formed strings. In non-transfected cells we performed similar experiments and there we observed the same pattern, confirming even more the VWF aggregation and string formation at the edges of the cell.In conclusion, we demonstrated that several WPBs can fuse with each other to form secretory pods and that VWF is secreted as a globular mass of protein. From these globular masses strings originated indicating that string formation occurs independently from the mechanism of secretion in which the tubular packaging of VWF in WPBs does not seem to be of importance. Disclosures:No relevant conflicts of interest to declare.

Alloantibody induced platelet responses in transplants: potent mediators in small packages.

The early histological studies of organ allografts noted platelets attached to vascular endothelium. Platelets adhere to vessels before any morphological evidence of endothelial injury. Subsequently, in vitro and in vivo experiments have demonstrated that alloantibodies can induce exocytosis of von Willebrand factor and P-selectin from endothelial cells and attachment of platelets within minutes. Platelets also adhere to and stimulate leukocytes. These interactions are increased by complement activation. After attachment platelets degranulate, releasing preformed mediators. Some chemokines stored together in platelet granules can form heteromers with synergistic functions. Heteromers containing platelet factor 4 (PF4; CXCL4) are specific to platelets and provide insights to unique platelet functions and opportunities for therapeutic intervention.

Highly Invasive Melanoma Cells Activate the Vascular Endothelium via an MMP-2/Integrin αvβ5–Induced Secretion of VEGF-A

Tumor cell extravasation is a critical step in the metastatic cascade and requires interaction between the tumor cell and the endothelium. Although cancer progression depends on a complex network of mechanisms, including inflammation and coagulation, the involvement of tumor-induced endothelium activation and the subsequent release of procoagulatory factors in this process are not well understood. Using tissue sections from patients with malignant melanoma, immunofluorescence studies for the presence of von Willebrand factor (VWF) clearly demonstrated endothelium activation and the formation of ultra-large VWF fibers in these patients. In vitro analyses revealed that supernatants from highly invasive melanoma cells induced an acute endothelium activation measured by VWF, P-selectin, and angiopoietin-2 release. Proteome profiling identified vascular endothelial growth factor A (VEGF-A) as the main mediator of endothelium activation. Inhibition and knock-down of VEGF-A in melanoma cells led to a rigorous decrease in VWF exocytosis. Selective small-interfering RNA to matrix metalloproteinase-2 (MMP-2) inhibited endothelium activation, and this effect correlated with reduced VEGF-A content in the supernatants of melanoma cells. Further experiments showed that active MMP-2 regulates VEGF-A in melanoma cells on a transcriptional level via an integrin αvβ5/phosphoinositide-3-kinase-dependent pathway. In conclusion, these results indicate an important role of VEGF-A in acute endothelium activation and provide clear evidence that MMP-2 plays a pivotal role in the autocrine regulation of VEGF-A expression in melanoma cells.

Cytotoxicity of silica nanoparticles through exocytosis of von Willebrand factor and necrotic cell death in primary human endothelial cells

Nanoparticle-induced endothelial cell (EC) dysfunction, due to the induction of inflammation and/or the activation of the coagulation system, is associated with pulmonary and ischemic cardiovascular diseases. Although it is contigent on several mechanisms, involving formation of reactive oxygen species and inflammatory cytokines such as interleukin (IL)-6 and 8, the involvement of the coagulation system is not well understood. The results of toxicity assays using the tetrazolium reduction (MTT) and lactate dehydrogenase (LDH) release showed that silica NP-induced cytotoxicity depends on the size and the dose of applied NP. Moreover, propidium iodide (PI) stainings and caspase 3/7 assays identified increased necrosis in ECs. Exposing human umbilical vein endothelial cells (HUVECs) to SiO 2 NP with diameters of 304 nm and 310 nm led to significant increase of Weibel-Palade body (WPB) exocytosis, associated with the release of von Willebrand factor (VWF) and the formation of ultralarge fibers (ULVWF). High resolution microscopy techniques revealed that internalization and perinuclear localization of perylene-labeled NP with a size of 310 nm affect not only viability, but also cell migration and proliferation. In conclusion, our data indicate that NP-induced activation and dysfunction of ECs is reflected by release of VWF and necrotic cell death.

The use of desmopressin in acquired haemophilia A: a systematic review.

Desmopressin (1-deamino-8-D-arginine vasopressin, DDAVP) is a synthetic analogue of the antidiuretic hormone vasopressin, which was originally developed for the treatment of diabetes insipidus1,2. Following its original clinical use in 1977, DDAVP rapidly emerged as the treatment of choice for patients with type 1 von Willebrand’s disease (VWD) and mild haemophilia A (factor VIII coagulant activity [FVIII:C] >5%)3–7. The widespread clinical use of this synthetic compound over the past 30 years has been mainly due to the fact that it is relatively inexpensive and safe, being associated with a very low rate of adverse reactions and no risk of transmission of blood-borne viral infections8. This latter aspect saved numerous patients with VWD and mild haemophilia A from the catastrophic consequences of human immunodeficiency virus (HIV) transmission with non-virus inactivated clotting factor concentrates prior to 19859. The mechanisms of action of DDAVP are still incompletely understood, despite its extensive clinical use. Desmopressin induces a remarkable increase in the levels of plasma von Willebrand factor (VWF), FVIII:C, and tissue plasminogen activator (t-PA), and also exerts a vasodilatory effect. Desmopressin shortens the prolonged activated partial thromboplastin time (APTT) and the bleeding time. These effects probably result from increases in FVIII:C and VWF, both of which play a rate-accelerating role in these global tests of intrinsic coagulation and primary haemostasis10. The effects of desmopressin on VWF and t-PA, as well as its vasodilatory action, are explained by a direct effect on the endothelium, via activation of the endothelial vasopressin V2 receptor (V2R) and c-AMP-mediated signalling. This leads to exocytosis of VWF and t-PA from endothelial cell Weibel-Palade bodies where VWF and t-PA are stored11. The cellular mechanisms leading to the DDVAP-induced release of FVIII are, however, less certain. One hypothesis is that the close correlation between FVIII:C and VWF plasma levels could be explained by co-secretion, but a cell type that stores and secretes both proteins has not yet been identified as yet. Another reasonable hypothesis is that the effect of DDAVP on FVIII is mediated by indirect mechanisms, via VWF secretion, thereby making more binding sites for FVIII available within the VWF molecules11,12. Desmopressin can be administered by intravenously (0.3 μg/kg diluted in 50–100 mL of isotonic saline and infused over 30 minutes), subcutaneously (0.3 μg/kg) or via the intranasal route (300 μg). The FVIII levels usually increase by 2- to 6-fold over baseline values but the response is not always linked to the basal FVIII:C levels13–18. For this reason and also considering that there is a high consistency of responses to separate DDAVP treatments, a test infusion/injection should be carried out in every patient to assess his/her level of response19. The level of the FVIII:C elicited after DDAVP administration peaks in the first hour while its half-life ranges between 5 and 8 hours, with inter-individual heterogeneity2. Decreased responsiveness after repeated administrations of DDAVP at 12–24 hour intervals has been reported in some patients. This phenomenon, known as tachyphylaxis, is thought to be due to FVIII:C depletion from cellular stores19,20. Desmopressin has also been used successfully for the treatment of patients with autoantibodies against coagulation FVIII, a therapeutic use analysed in this article by systematically reviewing the published literature.

Phospholipase D1 Is Required for Regulated Secretion of VWF Induced by the B-Subunits of Shiga-Like Toxins 1 and 2

AbstractAbstract 2113Shiga toxin (Stx) causes diarrhea-associated hemolytic uremic syndrome (D+HUS) by damaging renal microvascular endothelium. Stx is composed of an active (A) N-glycosidase subunit that is responsible for cytotoxicity, and 5 binding (B) subunits that interact with cell surface Gb3 and promote Stx endocytosis. We have demonstrated that catalytically inactive pentameric B5 subunits of Shiga-like toxin type 1 and 2 (Stx B5) are sufficient to stimulate the acute secretion of von Willebrand factor (VWF) from human endothelial cells and can cause thrombotic microangiopathy in Adamts13–/– mice. Because our previous observations indicated that Stx1 B5 and Stx2 B5 exert distinct effects on Ca2+ and cAMP signaling pathways, we investigated the role of alternative signaling components in Stx B5-induced VWF exocytosis. Incubation of human umbilical vein endothelial cells (HUVECs) with Stx1 B5 (5 nM), Stx2 B5 (5 nM) or histamine (100 μM) caused a time-dependent increase in phospholipase D (PLD) activity that was maximal at 10 minutes after exposure to agonists. For HUVECs under static conditions, inhibition of PLD with n-butanol, or shRNA mediated PLD1 knock down, abolished Stx1 B5- or Stx2 B5-induced acute VWF secretion assayed by ELISA of conditioned media. To assess the stimulated secretion of cell-associated VWF strings under fluid shear stress, HUVECs were perfused in a flow chamber with fluorescently labeled anti-VWF antibody. When Stx1 B5 or Stx2 B5 (5 nM) was added to the perfusate, maximal induction of VWF strings was observed within 5 minutes by immunofluorescence video microscopy. However, tert-butanol, a structural analog of n-butanol that does not inhibit PLD activity, had no effect on VWF secretion or VWF string formation. In addition, treatment of HUVECs with Stx1 B5 or Stx2 B5 triggered actin stress fiber formation (assayed by fluorescence microscopy of Alexa Fluor 488-phalloidin-treated cells), enhanced monolayer permeability (assayed by the penetration of FITC-dextran in transwell chambers), and increased the level of GTP-bound RhoA (assayed by pull-down with Rhotekin-RBD protein coupled to agarose beads). These data indicate that B5 subunits of both Stx1 and Stx2 activate RhoA. Interestingly, Stx1 B5-induced PLD activation and VWF secretion were significantly reduced by pretreatment of HUVECs with the PKC-α inhibitor Go6976 (2 μM) or by shRNA mediated PKC-α knock down, but not by pretreatment with the Rho inhibitor exoenzyme C3 (1 μM) or Rho kinase inhibitor Y27632 (20 μM). Conversely, Stx2 B5-induced PLD activation and VWF secretion were reduced by these Rho/Rho kinase inhibitors, but not by inhibition of PKC-α with Go6976 or by PKC-α shRNA knock down. In addition, transfection of HUVECs with a plasmid encoding dominant negative RhoA (T19N) reduced Weibel-Palade body exocytosis induced by both Stx1 B5 and Stx2 B5. These data indicate that the B5 subunits of both Stx1 and Stx2 activate RhoA and induce acute VWF secretion in a PLD1 dependent manner. However, Stx1 B5 activates PKC-α mediated, Ca2+-dependent signaling, whereas Stx2 B5 preferentially activates RhoA mediated, Ca2+-independent signaling. Disclosures:No relevant conflicts of interest to declare.

Multigranular exocytosis of Weibel-Palade bodies in vascular endothelial cells

AbstractRegulated exocytosis of Weibel-Palade bodies (WPBs) is a pivotal mechanism via which vascular endothelial cells initiate repair in response to injury and inflammation. Several pathways have been proposed to enable differential release of bioactive molecules from WPBs under different pathophysiologic conditions. Due to the complexity, many aspects of WPB biogenesis and exocytosis are still poorly understood. Herein, we have investigated the regulated exocytosis of the major WPB constituent, von Willebrand Factor (VWF), which upon its release forms strings of up to several millimeters long that capture circulating platelets and thereby initiate the formation of a haemostatic plug. Using correlative, fluorescence, and electron microscopic imaging techniques, we provide evidence that multigranular exocytosis is an important pathway for VWF release in secretagogue-challenged human umbilical vein endothelial cells. A novel membrane-delimited structure (secretory pod) was identified as the site of WPB coalescence and VWF exocytosis. Clathrin-coated profiles present on the secretory pods suggested remodeling via compensatory membrane retrieval. Small, 30- to 40-nm cytoplasmic vesicles (nanovesicles) mediated the fusion of WPBs with secretory pods. Multigranular exocytosis may facilitate VWF string formation by pooling the content of multiple WPBs. In addition, it may provide a novel mechanism for the differential release of WPB cargo.

Delay of acute intracellular pH recovery after acidosis decreases endothelial cell activation

Reperfusion after ischemic conditions induces massive endothelial cell (EC) activation, an initial step of reperfusion injury. Reperfusion is characterized by reoxygenation, realkalinization and a localized increase of inflammatory stimuli. In this study, we focused on the influence of extracellular realkalinization on human umbilical vein endothelial cell (HUVEC) activation. We examined intracellular pH (pH(in)) and intracellular free calcium concentration ([Ca(2+)](in)), a second messenger known to mediate von Willebrand factor (VWF) exocytosis in endothelium, upon realkalinization. Furthermore, we measured the agonist-stimulated exocytosis of VWF, Interleukin-8 and soluble P-selectin (sP-Selectin) as markers of EC activation. To verify a morphological correlate of EC activation, we finally observed platelet-endothelial adherence during realkalinization using shear flow. Realkalinization of HUVEC was simulated by switching from bicarbonate buffered Ringer solution of an acidotic pH(ex) of 6.4 to a physiologic pH(ex) of 7.4. Extracellular realkalinization was accompanied by pH(in) recovery from 6.5 to 7.2 within 10 min. Application of cariporide, an inhibitor of the Na(+)/H(+) exchanger subtype 1 (NHE), during extracellular realkalinization significantly delayed the early kinetics of intracellular realkalinization. Histamine stimulated [Ca(2+)](in) was significantly increased upon realkalinization compared to control cells. Also agonist-stimulated release of VWF, Interleukin-8 and sP-Selectin was massively enhanced during pH(in) recovery in comparison to control. Furthermore, we observed an increased platelet binding to endothelium. Interestingly, each of these realkalinization-induced effects were significantly reduced by early application of cariporide. Therefore, delay of acute NHE-dependent pH(in) recovery may represent a promising mechanism for inhibition of EC activation upon reperfusion.

A Novel Class of Fusion Polypeptides Inhibits Exocytosis

N-Ethyl-maleimide-sensitive factor (NSF) plays a critical role in the regulation of exocytosis. NSF regulates exocytosis by interacting with a complex containing soluble NSF attachment protein receptor (SNARE) molecules, hydrolyzing ATP, and disassembling the SNARE complex. We hypothesized that peptide inhibitors of NSF would decrease exocytosis. We now report the development of a novel set of peptides that block exocytosis by inhibiting NSF activity. These NSF inhibitors are fusion polypeptides composed of an 11 amino acid human immunodeficiency virus transactivating regulatory protein (TAT) domain fused to a 22 amino acid NSF domain. These TAT-NSF fusion polypeptides cross endothelial cell membranes, inhibit NSF hydrolysis of ATP, decrease NSF disassembly of SNARE molecules, and block exocytosis of von Willebrand factor. Control peptides have no effect on exocytosis. TAT-NSF inhibitors administered to mice prolong the bleeding time. Blood concentrations of these TAT-NSF peptides rapidly decrease within 5 min after injection and then remain constant from 10 to 60 min after injection. These TAT-NSF compounds may be useful in the treatment of a variety of diseases in which exocytosis plays a prominent role, including myocardial infarction, stroke, thrombosis, and autoimmune disorders.

Analysis of intracellular storage and regulated secretion of 3 von Willebrand disease–causing variants of von Willebrand factor

AbstractThe rapid exocytosis of von Willebrand factor (VWF) in response to vascular injury can be attributed to the fact that VWF is stored in the Weibel-Palade bodies (WPBs) of endothelial cells. We describe a system for examining the ability of VWF to drive both the formation of a storage compartment and the function of that compartment with respect to regulated secretion. Transient transfection of HEK293 cells with wild-type human VWF cDNA leads to the formation of numerous elongated organelles that resemble WPBs. These “pseudo-WPBs” exhibit the internal structure, as well as the ability to recruit membrane proteins including P-selectin, of bona fide WPBs. Finally, VWF was efficiently secreted upon stimulation by phorbol ester. We used this system to examine 3 VWF mutations leading to von Willebrand disease that affect VWF multimerization and constitutive secretion. Surprisingly we find that all 3 mutants can, to some extent, make pseudo-WPBs that recruit appropriate membrane proteins and that are responsive to secretagogues. The most striking defects are a delay in formation and a reduction in the length and number of pseudo-WPBs in proportion to the clinical severity of the mutation. Studies of pseudo-WPB formation in this system thus yield insights into the structure-function relationships underpinning the ability of VWF to form functional WPBs. (Blood. 2003;102:2452-2458)