Altered Storage and Function of von Willebrand Factor in Human Cardiac Microvascular Endothelial Cells Isolated from Recipient Transplant Hearts.

The assembly of von Willebrand factor (VWF) into ordered helical tubules within endothelial Weibel-Palade bodies (WPBs) is required for the efficient deployment of the protein at sites of vascular injury. VWF trafficking and storage are sensitive to cellular and environmental stresses that are associated with heart disease and heart failure. Altered storage of VWF manifests as a change in WPB morphology from a rod shape to a rounded shape and is associated with impaired VWF deployment during secretion. In this study, we examined the morphology, ultrastructure, molecular composition and kinetics of exocytosis of WPBs in cardiac microvascular endothelial cells isolated from explanted hearts of patients with a common form of heart failure, dilated cardiomyopathy (DCM; HCMECD), or from nominally healthy donors (controls; HCMECC). Using fluorescence microscopy, WPBs in HCMECC (n = 3 donors) showed the typical rod-shaped morphology containing VWF, P-selectin and tPA. In contrast, WPBs in primary cultures of HCMECD (n = 6 donors) were predominantly rounded in shape and lacked tissue plasminogen activator (t-PA). Ultrastructural analysis of HCMECD revealed a disordered arrangement of VWF tubules in nascent WPBs emerging from the trans-Golgi network. HCMECD WPBs still recruited Rab27A, Rab3B, Myosin-Rab Interacting Protein (MyRIP) and Synaptotagmin-like protein 4a (Slp4-a) and underwent regulated exocytosis with kinetics similar to that seen in HCMECc. However, secreted extracellular VWF strings from HCMECD were significantly shorter than for endothelial cells with rod-shaped WPBs, although VWF platelet binding was similar. Our observations suggest that VWF trafficking, storage and haemostatic potential are perturbed in HCMEC from DCM hearts.

Platelet degranulation and bleeding phenotype in a large cohort of Von Willebrand disease patients.

Von Willebrand disease (VWD) is a bleeding disorder caused by quantitative (type 1 or 3) or qualitative (type 2A/2B/2M/2N) defects of circulating von Willebrand factor (VWF). Circulating VWF levels not always fully explain bleeding phenotypes, suggesting a role for alternative factors, like platelets. Here, we investigated platelet factor 4 (PF4) in a large cohort of patients with VWD. PF4 levels were lower in type 2B and current bleeding phenotype was significantly associated with higher PF4 levels, particularly in type 1 VWD. Based on our findings we speculate that platelet degranulation and cargo release may play a role across VWD subtypes.

Syntaxin 5 determines Weibel-Palade body size and von Willebrand factor secretion by controlling Golgi architecture.

Von Willebrand factor (VWF) is a multimeric hemostatic protein primarily synthesized in endothelial cells. VWF is stored in endothelial storage organelles, the Weibel-Palade bodies (WPB), whose biogenesis strongly depends on VWF anterograde trafficking and Golgi architecture. Elongated WPB morphology is correlated to longer VWF strings with better adhesive properties. We previously identified the SNARE SEC22B, which is involved in anterograde endoplasmic reticulum-to-Golgi transport, as a novel regulator of WPB elongation. To elucidate novel determinants of WPB morphology we explored endothelial SEC22B interaction partners in a mass spectrometry-based approach, identifying the Golgi SNARE Syntaxin 5 (STX5). We established STX5 knockdown in endothelial cells using shRNA-dependent silencing and analyzed WPB and Golgi morphology, using confocal and electron microscopy. STX5-depleted endothelial cells exhibited extensive Golgi fragmentation and decreased WPB length, which was associated with reduced intracellular VWF levels, and impaired stimulated VWF secretion. However, the secretion-incompetent organelles in shSTX5 cells maintained WPB markers such as Angiopoietin 2, P-selectin, Rab27A, and CD63. In brief, we identified SNARE protein STX5 as a novel regulator of WPB biogenesis.

Double-Hit-Induced Leukocyte Extravasation Driven by Endothelial Adherens Junction Destabilization.

During inflammation, endothelial cells are bombarded with cytokines and other stimuli from surrounding cells. Leukocyte extravasation and vascular leakage are both prominent but believed to be uncoupled as they occur in separate spatiotemporal patterns. In this study, we investigated a "double-hit" approach on primary human endothelial cells primed with LPS followed by histamine. Using neutrophil transendothelial migration (TEM) under physiological flow assays, we found that an LPS-primed endothelium synergistically enhanced neutrophil TEM when additionally treated with histamine, whereas the effects on neutrophil TEM of the individual stimuli were moderate to undetectable. Interestingly, the double-hit-induced TEM increase was not due to decreased endothelial barrier, increased adhesion molecule expression, or Weibel-Palade body release. Instead, we found that it was directly correlated with junctional remodeling. Compounds that increased junctional "linearity" (i.e., stability) counteracted the double-hit effect on neutrophil TEM. We conclude that a compound, in this case histamine (which has a short primary effect on vascular permeability), can have severe secondary effects on neutrophil TEM in combination with an inflammatory stimulus. This effect is due to synergic modifications of the endothelial cytoskeleton and junctional remodeling. Therefore, we hypothesize that junctional linearity is a better and more predictive readout than endothelial resistance for compounds aiming to attenuate inflammation.

Synaptotagmin 5 regulates Ca2+-dependent Weibel-Palade body exocytosis in human endothelial cells.

Elevations of intracellular free Ca2+ concentration ([Ca2+]i) are a potent trigger for Weibel-Palade body (WPB) exocytosis and secretion of von Willebrand factor (VWF) from endothelial cells; however, the identity of WPB-associated Ca2+-sensors involved in transducing acute increases in [Ca2+]i into granule exocytosis remains unknown. Here, we show that synaptotagmin 5 (SYT5) is expressed in human umbilical vein endothelial cells (HUVECs) and is recruited to WPBs to regulate Ca2+-driven WPB exocytosis. Western blot analysis of HUVECs identified SYT5 protein, and exogenously expressed SYT5-mEGFP localised almost exclusively to WPBs. shRNA-mediated knockdown of endogenous SYT5 (shSYT5) reduced the rate and extent of histamine-evoked WPB exocytosis and reduced secretion of the WPB cargo VWF-propeptide (VWFpp). The shSYT5-mediated reduction in histamine-evoked WPB exocytosis was prevented by expression of shRNA-resistant SYT5-mCherry. Overexpression of SYT5-EGFP increased the rate and extent of histamine-evoked WPB exocytosis, and increased secretion of VWFpp. Expression of a Ca2+-binding defective SYT5 mutant (SYT5-Asp197Ser-EGFP) mimicked depletion of endogenous SYT5. We identify SYT5 as a WPB-associated Ca2+ sensor regulating Ca2+-dependent secretion of stored mediators from vascular endothelial cells.

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, so-called Weibel-Palade bodies (WPBs). Hemostatic activity of VWF is strongly tied to WPB length, but how endothelial cells control the dimensions of their WPBs is unclear. In this study we used a targeted shRNA screen to identify the 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 along with disintegration of the Golgi and dilation of rough ER (rER) cisternae. This was accompanied by reduced proteolytic processing of VWF, accumulation of VWF in the dilated rER and reduced basal and stimulated VWF secretion. Our data demonstrate that the elongation of WPBs, and thus adhesive activity of its cargo VWF, is determined by the rate of anterograde transport between ER and Golgi, which depends on Sec22b-containing SNARE complexes.

Orchestration of Primary Hemostasis by Platelet and Endothelial Lysosome-Related Organelles.

Megakaryocyte-derived platelets and endothelial cells store their hemostatic cargo in α- and δ-granules and Weibel-Palade bodies, respectively. These storage granules belong to the lysosome-related organelles (LROs), a heterogeneous group of organelles that are rapidly released following agonist-induced triggering of intracellular signaling pathways. Following vascular injury, endothelial Weibel-Palade bodies release their content into the vascular lumen and promote the formation of long VWF (von Willebrand factor) strings that form an adhesive platform for platelets. Binding to VWF strings as well as exposed subendothelial collagen activates platelets resulting in the release of α- and δ-granules, which are crucial events in formation of a primary hemostatic plug. Biogenesis and secretion of these LROs are pivotal for the maintenance of proper hemostasis. Several bleeding disorders have been linked to abnormal generation of LROs in megakaryocytes and endothelial cells. Recent reviews have emphasized common pathways in the biogenesis and biological properties of LROs, focusing mainly on melanosomes. Despite many similarities, LROs in platelet and endothelial cells clearly possess distinct properties that allow them to provide a highly coordinated and synergistic contribution to primary hemostasis by sequentially releasing hemostatic cargo. In this brief review, we discuss in depth the known regulators of α- and δ-granules in megakaryocytes/platelets and Weibel-Palade bodies in endothelial cells, starting from transcription factors that have been associated with granule formation to protein complexes that promote granule maturation. In addition, we provide a detailed view on the interplay between platelet and endothelial LROs in controlling hemostasis as well as their dysfunction in LRO related bleeding disorders.

Defective AP-3-dependent VAMP8 trafficking impairs Weibel-Palade body exocytosis in Hermansky-Pudlak Syndrome type 2 blood outgrowth endothelial cells.

Weibel-Palade bodies are endothelial secretory organelles that contain von Willebrand factor, P-selectin and CD63. Release of von Willebrand factor from Weibel-Palade bodies is crucial for platelet adhesion during primary hemostasis. Endosomal trafficking of proteins like CD63 to Weibel-Palade bodies during maturation is dependent on the adaptor protein complex 3 complex. Mutations in the AP3B1 gene, which encodes the adaptor protein complex 3 ß1 subunit, result in Hermansky-Pudlak syndrome 2, a rare genetic disorder that leads to neutropenia and a mild bleeding diathesis. This is caused by abnormal granule formation in neutrophils and platelets due to defects in trafficking of cargo to secretory organelles. The impact of these defects on the secretory pathway of the endothelium is largely unknown. In this study, we investigated the role of adaptor protein complex 3-dependent mechanisms in trafficking of proteins during Weibel-Palade body maturation in endothelial cells. An ex vivo patient-derived endothelial model of Hermansky-Pudlak syndrome type 2 was established using blood outgrowth endothelial cells that were isolated from a patient with compound heterozygous mutations in AP3B1 Hermansky-Pudlak syndrome type 2 endothelial cells and CRISPR-Cas9-engineered AP3B1-/- endothelial cells contain Weibel-Palade bodies that are entirely devoid of CD63, indicative of disrupted endosomal trafficking. Hermansky-Pudlak syndrome type 2 endothelial cells have impaired Ca2+-mediated and cAMP-mediated exocytosis. Whole proteome analysis revealed that, apart from adaptor protein complex 3 ß1, also the µ1 subunit and the v-SNARE VAMP8 were depleted. Stimulus-induced von Willebrand factor secretion was impaired in CRISPR-Cas9-engineered VAMP8-/-endothelial cells. Our data show that defects in adaptor protein complex 3-dependent maturation of Weibel-Palade bodies impairs exocytosis by affecting the recruitment of VAMP8.

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

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.

Interaction between MyRIP and the actin cytoskeleton regulates Weibel-Palade body trafficking and exocytosis.

Weibel-Palade body (WPB)-actin interactions are essential for the trafficking and secretion of von Willebrand factor; however, the molecular basis for this interaction remains poorly defined. Myosin Va (MyoVa or MYO5A) is recruited to WPBs by a Rab27A-MyRIP complex and is thought to be the prime mediator of actin binding, but direct MyRIP-actin interactions can also occur. To evaluate the specific contribution of MyRIP-actin and MyRIP-MyoVa binding in WPB trafficking and Ca(2+)-driven exocytosis, we used EGFP-MyRIP point mutants with disrupted MyoVa and/or actin binding and high-speed live-cell fluorescence microscopy. We now show that the ability of MyRIP to restrict WPB movement depends upon its actin-binding rather than its MyoVa-binding properties. We also show that, although the role of MyRIP in Ca(2+)-driven exocytosis requires both MyoVa- and actin-binding potential, it is the latter that plays a dominant role. In view of these results and together with the analysis of actin disruption or stabilisation experiments, we propose that the role of MyRIP in regulating WPB trafficking and exocytosis is mediated largely through its interaction with actin rather than with MyoVa.

A subset of anti-HLA antibodies induces FcγRIIa-dependent platelet activation.

HLA antibodies are associated with refractoriness to platelet transfusion, leading to rapid platelet clearance, sometimes coinciding with clinical side effects such as fever and chills. The presence of HLA antibodies is not always manifested by clinical symptoms. It is currently unclear why refractoriness to platelet transfusion is only observed in a subset of patients. Here, we utilized the availability of a unique panel of human monoclonal antibodies to study whether these were capable of activating platelets. Three out of eight human HLA-specific monoclonal antibodies induced activation of HLA-matched platelets from healthy donors as evidenced by enhanced α-granule release, aggregation, and αIIbb3 activation. The propensity of HLA monoclonal antibodies to activate platelets was independent of the HLA subtype to which they were directed, but was dependent on the recognized epitope. Activation was fully inhibited either by blocking FcγRIIa, or by blocking FcγRIIa-dependent signaling with Syk inhibitor IV. Furthermore, activation required the presence of the IgG-Fc part, as F(ab')2 fragments of HLA monoclonal antibodies were unable to induce platelet activation. Mixing experiments revealed that activation of platelets occurred in an intra-platelet dependent manner. Accordingly, a proportion of sera from refractory patients with HLA antibodies induced FcγRIIa-dependent platelet activation. Our data show that a subset of HLA antibodies is capable of crosslinking HLA and FcγRIIa thereby promoting platelet activation and enhancing these cells' phagocytosis by macrophages. Based on these findings we suggest that FcγRIIa-dependent platelet activation may contribute to the decreased platelet survival in platelet-transfusion-dependent patients with HLA antibodies.

The prion protein inhibits monocytic cell migration by stimulating ß1 integrin adhesion and uropod formation.

The broad tissue distribution and evolutionary conservation of the glycosylphosphatidylinositol (GPI)-anchored prion protein (PrP, also known as PRNP) suggests that it plays a role in cellular homeostasis. Given that integrin adhesion determines cell behavior, the proposed role of PrP in cell adhesion might underlie the various in vitro and in vivo effects associated with PrP loss-of-function, including the immune phenotypes described in PrP(-/-) mice. Here, we investigated the role of PrP in the adhesion and (transendothelial) migration of human (pro)monocytes. We found that PrP regulates ß1-integrin-mediated adhesion of monocytes. Additionally, PrP controls the cell morphology and migratory behavior of monocytes: PrP-silenced cells show deficient uropod formation on immobilized VCAM and display bleb-like protrusions on the endothelium. Our data further show that PrP regulates ligand-induced integrin activation. Finally, we found that PrP controls the activation of several proteins involved in cell adhesion and migration, including RhoA and its effector cofilin, as well as proteins of the ERM family. We propose that PrP modulates ß1 integrin adhesion and migration of monocytes through RhoA-induced actin remodeling mediated by cofilin, and through the regulation of ERM-mediated membrane-cytoskeleton linkage.

STXBP1 promotes Weibel-Palade body exocytosis through its interaction with the Rab27A effector Slp4-a.

Vascular endothelial cells contain unique rod-shaped secretory organelles, called Weibel-Palade bodies (WPBs), which contain the hemostatic protein von Willebrand factor (VWF) and a cocktail of angiogenic and inflammatory mediators. We have shown that the Rab27A effector synaptotagmin-like protein 4-a (Slp4-a) plays a critical role in regulating hormone-evoked WPB exocytosis. Using a nonbiased proteomic screen for targets for Slp4-a, we now identify syntaxin-binding protein 1 (STXBP1) and syntaxin-2 and -3 as endogenous Slp4-a binding partners in endothelial cells. Coimmunoprecipitations showed that STXBP1 interacts with syntaxin-2 and -3, but not with syntaxin-4. Small interfering RNA-mediated silencing of STXBP1 expression impaired histamine- and forskolin-induced VWF secretion. To further substantiate the role of STXBP1, we isolated blood outgrowth endothelial cells (BOECs) from an early infantile epileptic encephalopathy type 4 (EIEE4) patient carrying a de novo mutation in STXBP1. STXBP1-haploinsufficient EIEE4 BOECs contained similar numbers of morphologically normal WPBs compared with control BOECs of healthy donors; however, EIEE4 BOECs displayed significantly impaired histamine- and forskolin-stimulated VWF secretion. Based on these findings, we propose that the Rab27A-Slp4-a complex on WPB promotes exocytosis through an interaction with STXBP1, thereby controlling the release of vaso-active substances in the vasculature.

The interplay between the Rab27A effectors Slp4-a and MyRIP controls hormone-evoked Weibel-Palade body exocytosis.

Weibel-Palade body (WPB) exocytosis underlies hormone-evoked VWF secretion from endothelial cells (ECs). We identify new endogenous components of the WPB: Rab3B, Rab3D, and the Rab27A/Rab3 effector Slp4-a (granuphilin), and determine their role in WPB exocytosis. We show that Rab3B, Rab3D, and Rab27A contribute to Slp4-a localization to WPBs. siRNA knockdown of Slp4-a, MyRIP, Rab3B, Rab3D, Rab27A, or Rab3B/Rab27A, or overexpression of EGFP-Slp4-a or EGFP-MyRIP showed that Slp4-a is a positive and MyRIP a negative regulator of WPB exocytosis and that Rab27A alone mediates these effects. We found that ECs maintain a constant amount of cellular Rab27A irrespective of the WPB pool size and that Rab27A (and Rab3s) cycle between WPBs and a cytosolic pool. The dynamic redistribution of Rab proteins markedly decreased the Rab27A concentration on individual WPBs with increasing WPB number per cell. Despite this, the probability of WPB release was independent of WPB pool size showing that WPB exocytosis is not determined simply by the absolute amount of Rab27A and its effectors on WPBs. Instead, we propose that the probability of release is determined by the fractional occupancy of WPB-Rab27A by Slp4-a and MyRIP, with the balance favoring exocytosis.

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.

Endothelial colony-forming cells in the spotlight: insights into the pathophysiology of von Willebrand disease and rare bleeding disorders.

Endothelial cells deliver a vital contribution to the maintenance of hemostasis by constituting an anatomical as well as functional barrier between the blood and the rest of the body. Apart from the physical barrier function, endothelial cells maintain the hemostatic equilibrium by their pro- and anticoagulant functions. An important part of their procoagulant contribution is the production of von Willebrand factor (VWF), which is a carrier protein for coagulation factor VIII and facilitates the formation of a platelet plug. Thus, VWF is indispensable for both primary and secondary hemostasis, which is exemplified by the bleeding disorder von Willebrand disease that results from qualitative or quantitative deficiencies in VWF. A cellular model that was found to accurately reflect the endothelium and its secretory organelles are endothelial colony-forming cells, which can be readily isolated from peripheral blood and constitute a robust ex vivo model to investigate the donor's endothelial cell function. This review summarizes some of the valuable insights on biology of VWF and pathogenic mechanisms of von Willebrand disease that have been made possible using studies with endothelial colony-forming cells derived from patients with bleeding disorders.

Transcriptional and functional profiling identifies inflammation and endothelial-to-mesenchymal transition as potential drivers for phenotypic heterogeneity within a cohort of endothelial colony forming cells.

BACKGROUND: Endothelial colony-forming cells (ECFCs) derived from patients can be used to investigate pathogenic mechanisms of vascular diseases like von Willebrand disease. Considerable phenotypic heterogeneity has been observed between ECFC clones derived from healthy donors. This heterogeneity needs to be well understood in order to use ECFCs as endothelial models for disease. OBJECTIVES: Therefore, we aimed to determine phenotypic and gene expression differences between control ECFCs. METHODS: A total of 34 ECFC clones derived from 16 healthy controls were analyzed. The transcriptome of a selection of ECFC clones (n = 15) was analyzed by bulk RNA sequencing and gene set enrichment analysis. Gene expression was measured in all ECFC clones by quantitative polymerase chain reaction. Phenotypic profiling was performed and migration speed of the ECFCs was measured using confocal microscopy, followed by automated quantification of cell morphometrics and migration speed. RESULTS: Through hierarchical clustering of RNA expression profiles, we could distinguish 2 major clusters within the ECFC cohort. Major differences were associated with proliferation and migration in cluster 1 and inflammation and endothelial-to-mesenchymal transition in cluster 2. Phenotypic profiling showed significantly more and smaller ECFCs in cluster 1, which contained more and longer Weibel-Palade bodies. Migration speed in cluster 1 was also significantly higher. CONCLUSION: We observed a range of different RNA expression patterns between ECFC clones, mostly associated with inflammation and clear differences in Weibel-Palade body count and structure. We developed a quantitative polymerase chain reaction panel that can be used for the characterization of ECFC clones, which is essential for the correct analysis of pathogenic mechanisms in vascular disorders.

O-glycan determinants regulate VWF trafficking to Weibel-Palade bodies.

ABSTRACT: von Willebrand factor (VWF) undergoes complex posttranslational modification within endothelial cells (ECs) before secretion. This includes significant N- and O-linked glycosylation. Previous studies have demonstrated that changes in N-linked glycan structures significantly influence VWF biosynthesis. In contrast, although abnormalities in VWF O-linked glycans (OLGs) have been associated with enhanced VWF clearance, their effect on VWF biosynthesis remains poorly explored. Herein, we report a novel role for OLG determinants in regulating VWF biosynthesis and trafficking within ECs. We demonstrate that alterations in OLGs (notably reduced terminal sialylation) lead to activation of the A1 domain of VWF within EC. In the presence of altered OLG, VWF multimerization is reduced and Weibel-Palade body (WPB) formation significantly impaired. Consistently, the amount of VWF secreted from WPB after EC activation was significantly reduced in the context of O-glycosylation inhibition. Finally, altered OLG on VWF not only reduced the amount of VWF secreted after EC activation but also affected its hemostatic efficacy. Notably, VWF secreted after WPB exocytosis consisted predominantly of low molecular weight multimers, and the length of tethered VWF string formation on the surface of activated ECs was significantly reduced. In conclusion, our data therefore support the hypothesis that alterations in O-glycosylation pathways directly affect VWF trafficking within human EC. These findings are interesting given that previous studies have reported altered OLG on plasma VWF (notably increased T-antigen expression) in patients with von Willebrand disease.