Biogenesis of Weibel–Palade bodies

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; HCMEC_D), or from nominally healthy donors (controls; HCMEC_C). Using fluorescence microscopy, WPBs in HCMEC_C (n = 3 donors) showed the typical rod-shaped morphology containing VWF, P-selectin and tPA. In contrast, WPBs in primary cultures of HCMEC_D (n = 6 donors) were predominantly rounded in shape and lacked tissue plasminogen activator (t-PA). Ultrastructural analysis of HCMEC_D revealed a disordered arrangement of VWF tubules in nascent WPBs emerging from the trans-Golgi network. HCMEC_D 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 HCMEC_D 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.

Overexpression of Plasmalemmal Vesicle-Associated Protein-1 Reflects Glomerular Endothelial Injury in Cases of Proliferative Glomerulonephritis with Monoclonal IgG Deposits

Introduction

Proliferative glomerulonephritis with monoclonal IgG deposits (PGNMID) occasionally presents refractory nephrotic syndrome resulting in poor renal prognosis, but its etiology is not fully elucidated. Given that glomerular endothelial cell (GEC) stress or damage may lead to podocytopathy and subsequent proteinuria, as in thrombotic microangiopathy (TMA), diabetic kidney disease, and focal segmental glomerulosclerosis, we investigated the evidence of glomerular endothelial injury by evaluating the expression of plasmalemmal vesicle-associated protein-1 (PV-1), a component of caveolae in the cases of PGNMID.

Methods

We measured the immunofluorescent PV-1 intensities of 23 PGNMID cases and compared with those of primary membranoproliferative glomerulonephritis (MPGN) (n = 5) and IgA nephropathy (IgAN) (n = 54) cases. PV-1 localization was evaluated with Caveolin-1, and CD31 staining, and the ultrastructural analysis was performed using a low-vacuum scanning electron microscope (LVSEM). To check the association of podocyte injury, we also conducted 8-oxoguanine and Wilms tumor 1 (WT1) double stain. We then evaluated PV-1 expression in other glomerulitis and glomerulopathy such as lupus nephritis and minimal change disease.

Results

The intensity of glomerular PV-1 expression in PGNMID is significantly higher than that in the other glomerular diseases, although the intensity is not associated with clinical outcomes such as urinary protein levels or renal prognosis. Immunostaining and LVSEM analysis revealed that glomerular PV-1 expression is localized in GECs in PGNMID. 8-oxoguanine accumulation was detected in WT1-positive podocytes but not in PV-1-expressing GECs, suggesting GEC-derived podocyte injury in PGNMID.

Conclusion

PV-1 overexpression reflects glomerular endothelial injury, which could be associated with podocyte oxidative stress in PGNMID cases.

PAM: diverse roles in neuroendocrine cells, cardiomyocytes, and green algae

Our understanding of the ways in which peptides are used for communication in the nervous and endocrine systems began with the identification of oxytocin, vasopressin, and insulin, each of which is stored in electron-dense granules, ready for release in response to an appropriate stimulus. For each of these peptides, entry of its newly synthesized precursor into the ER lumen is followed by transport through the secretory pathway, exposing the precursor to a sequence of environments and enzymes that produce the bioactive products stored in mature granules. A final step in the biosynthesis of many peptides is C-terminal amidation by peptidylglycine α-amidating monooxygenase (PAM), an ascorbate- and copper-dependent membrane enzyme that enters secretory granules along with its soluble substrates. Biochemical and cell biological studies elucidated the highly conserved mechanism for amidated peptide production and raised many questions about PAM trafficking and the effects of PAM on cytoskeletal organization and gene expression. Phylogenetic studies and the discovery of active PAM in the ciliary membranes of Chlamydomonas reinhardtii, a green alga lacking secretory granules, suggested that a PAM-like enzyme was present in the last eukaryotic common ancestor. While the catalytic features of human and C. reinhardtii PAM are strikingly similar, the trafficking of PAM in C. reinhardtii and neuroendocrine cells and secretion of its amidated products differ. A comparison of PAM function in neuroendocrine cells, atrial myocytes, and C. reinhardtii reveals multiple ways in which altered trafficking allows PAM to accomplish different tasks in different species and cell types.

Perinatal Derivatives: Where Do We Stand? A Roadmap of the Human Placenta and Consensus for Tissue and Cell Nomenclature

Progress in the understanding of the biology of perinatal tissues has contributed to the breakthrough revelation of the therapeutic effects of perinatal derivatives (PnD), namely birth-associated tissues, cells, and secreted factors. The significant knowledge acquired in the past two decades, along with the increasing interest in perinatal derivatives, fuels an urgent need for the precise identification of PnD and the establishment of updated consensus criteria policies for their characterization. The aim of this review is not to go into detail on preclinical or clinical trials, but rather we address specific issues that are relevant for the definition/characterization of perinatal cells, starting from an understanding of the development of the human placenta, its structure, and the different cell populations that can be isolated from the different perinatal tissues. We describe where the cells are located within the placenta and their cell morphology and phenotype. We also propose nomenclature for the cell populations and derivatives discussed herein. This review is a joint effort from the COST SPRINT Action (CA17116), which broadly aims at approaching consensus for different aspects of PnD research, such as providing inputs for future standards for the processing and in vitro characterization and clinical application of PnD.

The road to lysosome-related organelles: Insights from Hermansky-Pudlak syndrome and other rare diseases

Lysosome-related organelles (LROs) comprise a diverse group of cell type-specific, membrane-bound subcellular organelles that derive at least in part from the endolysosomal system but that have unique contents, morphologies and functions to support specific physiological roles. They include: melanosomes that provide pigment to our eyes and skin; alpha and dense granules in platelets, and lytic granules in cytotoxic T cells and natural killer cells, which release effectors to regulate hemostasis and immunity; and distinct classes of lamellar bodies in lung epithelial cells and keratinocytes that support lung plasticity and skin lubrication. The formation, maturation and/or secretion of subsets of LROs are dysfunctional or entirely absent in a number of hereditary syndromic disorders, including in particular the Hermansky-Pudlak syndromes. This review provides a comprehensive overview of LROs in humans and model organisms and presents our current understanding of how the products of genes that are defective in heritable diseases impact their formation, motility and ultimate secretion.

Von Willebrand Disease: From In Vivo to In Vitro Disease Models

Von Willebrand factor (VWF) plays an essential role in primary hemostasis and is exclusively synthesized and stored in endothelial cells and megakaryocytes. Upon vascular injury, VWF is released into the circulation where this multimeric protein is required for platelet adhesion. Defects of VWF lead to the most common inherited bleeding disorder von Willebrand disease (VWD). Three different types of VWD exist, presenting with varying degrees of bleeding tendencies. The pathophysiology of VWD can be investigated by examining the synthesis, storage and secretion in VWF producing cells. These cells can either be primary VWF producing cells or transfected heterologous cell models. For many years transfected heterologous cells have been used successfully to elucidate many aspects of VWF synthesis. However, those cells do not fully reflect the characteristics of primary cells. Obtaining primary endothelial cells or megakaryocytes with a VWD phenotype, requires invasive procedures, such as vessel collection or a bone marrow biopsy. A more recent and promising development is the isolation of endothelial colony forming cells (ECFCs) from peripheral blood as a true-to-nature cell model. Alternatively, various animal models are available but limiting, therefore, new approaches are needed to study VWD and other bleeding disorders. A potential versatile source of endothelial cells and megakaryocytes could be induced pluripotent stem cells (iPSCs). This review gives an overview of models that are available to study VWD and VWF and will discuss novel approaches that can be considered to improve the understanding of the structural and functional mechanisms underlying this disease.

Actin dynamics during Ca2+-dependent exocytosis of endothelial Weibel-Palade bodies

Weibel-Palade bodies (WPBs) are specialized secretory organelles of endothelial cells that serve important functions in the response to inflammation and vascular injury. WPBs actively respond to different stimuli by regulated exocytosis leading to full or selective release of their contents. Cellular conditions and mechanisms that distinguish between these possibilities are only beginning to emerge. To address this we analyzed dynamic rearrangements of the actin cytoskeleton during histamine-stimulated, Ca²⁺-dependent WPB exocytosis. We show that most WPB fusion events are followed by a rapid release of von-Willebrand factor (VWF), the large WPB cargo, and that this occurs concomitant with a softening of the actin cortex by the recently described Ca²⁺-dependent actin reset (CaAR). However, a considerable fraction of WPB fusion events is characterized by a delayed release of VWF and observed after the CaAR reaction peak. These delayed VWF secretions are accompanied by an assembly of actin rings or coats around the WPB post-fusion structures and are also seen following direct elevation of intracellular Ca²⁺ by plasma membrane wounding. Actin ring/coat assembly at WPB post-fusion structures requires Rho GTPase activity and is significantly reduced upon expression of a dominant-active mutant of the formin INF2 that triggers a permanent CaAR peak-like sequestration of actin to the endoplasmic reticulum. These findings suggest that a rigid actin cortex correlates with a higher proportion of fused WPB which assemble actin rings/coats most likely required for efficient VWF expulsion and/or stabilization of a WPB post-fusion structure. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Tacrolimus prevents von Willebrand factor secretion by allostimulated human glomerular endothelium

Little is known about the endothelial injury caused directly by circulating donor-specific antibodies (DSAs) during antibody-mediated rejection. von Willebrand factor (vWF) is a highly thrombotic glycoprotein stored in Weibel-Palade bodies in endothelial cells. It has been shown that its secretion is triggered by allostimulation. Calcineurin-like phosphatases regulate pathways involved in vWF secretion. Therefore, we hypothesized that tacrolimus would prevent alloantibody-induced glomerular lesions, in part via inhibition of vWF secretion from endothelial cells. Here, we used a human in vitro model of glomerular endothelium expressing HLA class I and II antigens and demonstrated that anti-HLA class II antibodies elicit a higher endothelial release of vWF than do anti-HLA class I antibodies in cell supernatants. We observed that tacrolimus treatment decreased vWF secretion after stimulation with both classes of anti-HLA antibodies and decreased platelet adhesion on allostimulated endothelial cells in a microfluidic chamber. In kidney recipients, tacrolimus trough levels were negatively associated with vWF blood levels. These results indicate that direct disruption of hemostasis via vWF secretion is a potential mechanism of antibody-mediated injury in patients with DSAs. Our results further suggest that the targeting of microcirculation hemostasis may be beneficial to prevent the development of microangiopathic lesions in antibody-mediated rejection.

Targeting von Willebrand Factor in Ischaemic Stroke: Focus on Clinical Evidence

Despite great efforts in stroke research, disability and recurrence rates in ischaemic stroke remain unacceptably high. To address this issue, one potential target for novel therapeutics is the glycoprotein von Willebrand factor (vWF), which increases in thrombogenicity especially under high shear rates as it bridges between vascular sub-endothelial collagen and platelets. The rationale for vWF as a potential target in stroke comes from four bodies of evidence. (1) Animal models which recapitulate the pathogenesis of stroke and validate the concept of targeting vWF for stroke prevention and the use of the vWF cleavage enzyme ADAMTS13 in acute stroke treatment. (2) Extensive epidemiologic data establishing the prognostic role of vWF in the clinical setting showing that high vWF levels are associated with an increased risk of first stroke, stroke recurrence or stroke-associated mortality. As such, vWF levels may be a suitable marker for further risk stratification to potentially fine-tune current risk prediction models which are mainly based on clinical and imaging data. (3) Genetic studies showing an association between vWF levels and stroke risk on genomic levels. Finally, (4) studies of patients with primary disorders of excess or deficiency of function in the vWF axis (e.g. thrombotic thrombocytopenic purpura and von Willebrand disease, respectively) which demonstrate the crucial role of vWF in atherothrombosis. Therapeutic inhibition of VWF by novel agents appears particularly promising for secondary prevention of stroke recurrence in specific sub-groups of patients such as those suffering from large artery atherosclerosis, as designated according to the TOAST classification.

An endosomal syntaxin and the AP-3 complex are required for formation and maturation of candidate lysosome-related secretory organelles (mucocysts) in Tetrahymena thermophila

Lysosome-related organelles (LROs) are secretory organelles formed by convergence between secretory and endosomal trafficking pathways. In Tetrahymena, secretory vesicles that resemble dense core granules are a new class of LROs whose synthesis depends on a conserved syntaxin required for heterotypic fusion and AP-3 for maturation.

The ciliate Tetrahymena thermophila synthesizes large secretory vesicles called mucocysts. Mucocyst biosynthesis shares features with dense core granules (DCGs) in animal cells, including proteolytic processing of cargo proteins during maturation. However, other molecular features have suggested relatedness to lysosome-related organelles (LROs). LROs, which include diverse organelles in animals, are formed via convergence of secretory and endocytic trafficking. Here we analyzed Tetrahymena syntaxin 7-like 1 (Stx7l1p), a Qa-SNARE whose homologues in other lineages are linked with vacuoles/LROs. Stx7l1p is targeted to both immature and mature mucocysts and is essential in mucocyst formation. In STX7L1-knockout cells, the two major classes of mucocyst cargo proteins localize independently, accumulating in largely nonoverlapping vesicles. Thus initial formation of immature mucocysts involves heterotypic fusion, in which a subset of mucocyst proteins is delivered via an endolysosomal compartment. Further, we show that subsequent maturation requires AP-3, a complex widely implicated in LRO formation. Knockout of the µ-subunit gene does not impede delivery of any known mucocyst cargo but nonetheless arrests mucocyst maturation. Our data argue that secretory organelles in ciliates may represent a new class of LROs and reveal key roles of an endosomal syntaxin and AP-3 in the assembly of this complex compartment.

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.

Fipronil induces lung inflammation in vivo and cell death in vitro

Background

Fipronil is an insecticide that acts at the gamma-aminobutyric acid receptor and glutamate-gated chloride channels in the central nervous systems of target organisms. The use of fipronil is increasing across the globe. Presently, very little data exist on the potential impact of exposure to fipronil on the lungs.

Methods

We studied effects of intranasal (N = 8) and oral (N = 8) treatment with fipronil (10 mg/kg) on lungs of mice. Control mice were given groundnut oil orally (N = 7) or ethanol intranasally (N = 7) as these were the vehicles for respective treatments.

Results

Hematoxylin-eosin stained lung sections showed normal histology in the control lungs compared to the thickened alveolar septa, disruption of the airways epithelium and damage to vascular endothelium in the intranasal and the oral groups. Mice exposed to fipronil either orally or intranasally showed increased von Willebrand factor staining in the endothelium and septal capillaries. Compared to the control mice, TLR4 expression in airway epithelium was increased in mice treated intranasally but not orally with fipronil. Oral fipronil reduced TLR9 staining in the airway epithelium but intranasal exposure caused intense staining in the alveolar septa and airway epithelium. There were higher numbers of TLR4 positive cells in alveolar septa in lungs of mice treated intranasally (P = 0.010) compared to the respective control and orally treated mice but no significant differences between treatments for TLR9 positive stained cells (P = 0.226). The U937 macrophage cells exposed to fipronil at concentrations of 0.29 μm to 5.72 μm/ml over 3- or 24-hour showed significant increase in cell death at higher concentrations of fipronil (P < 0.0001). Western blots revealed no effect of fipronil on TLR4 (P = 0.49) or TLR9 (P = 0.94) expression on macrophage cell line.

Conclusion

While both oral or intranasal fipronil treatments induced signs of lung inflammation, the number TLR4-positive septal cells was increased only following intranasal treatment. Fipronil causes macrophage cell death without altering TLR4 and TLR9 expression in vitro.

Factor VIII Is Synthesized in Human Endothelial Cells, Packaged in Weibel-Palade Bodies and Secreted Bound to ULVWF Strings

The cellular synthesis site and ensuing storage location for human factor VIII (FVIII), the coagulation protein deficient in hemophilia A, has been elusive. FVIII stability and half-life is dependent on non-covalent complex formation with von Willebrand factor (VWF) to avoid proteolysis and clearance. VWF is synthesized in megakaryocytes and endothelial cells, and is stored and secreted from platelet alpha granules and Weibel-Palade bodies of endothelial cells. In this paper we provide direct evidence for FVIII synthesis in 2 types of primary human endothelial cells: glomerular microvascular endothelial cells (GMVECs) and umbilical vein endothelial cells (HUVECs). Gene expression quantified by real time PCR revealed that levels of F8 and VWF are similar in GMVECs and HUVECs. Previous clinical studies have shown that stimulation of vasopressin V2 receptors causes parallel secretion of both proteins. In this study, we found that both endothelial cell types express AVPR2 (vasopressin V2 receptor gene) and that AVPR2 mRNA levels are 5-fold higher in GMVECs than HUVECs. FVIII and VWF proteins were detected by fluorescent microscopy in Weibel-Palade bodies within GMVECs and HUVECs using antibodies proven to be target specific. Visual presence of FVIII and VWF in Weibel-Palade bodies was confirmed by correlation measurements. The high extent of correlation was compared with negative correlation values obtained from FVIII detection with cytoplasmic proteins, β-actin and Factor H. FVIII activity was positive in GMVEC and HUVEC cell lysates. Stimulated GMVECs and HUVECs were found to secrete cell-anchored ultra-large VWF strings covered with bound FVIII.

Industrial production of clotting factors: Challenges of expression, and choice of host cells

The development of recombinant forms of blood coagulation factors as safer alternatives to plasma derived factors marked a major advance in the treatment of common coagulation disorders. These are complex proteins, mostly enzymes or co-enzymes, involving multiple post-translational modifications, and therefore are difficult to express. This article reviews the nature of the expression challenges for the industrial production of these factors, vis-à-vis the translational and post-translational bottlenecks, as well as the choice of host cell lines for high-fidelity production. For achieving high productivities of vitamin K dependent proteins, which include factors II (prothrombin), VII, IX and X, and protein C, host cell limitation of γ-glutamyl carboxylation is a major bottleneck. Despite progress in addressing this, involvement of yet unidentified protein(s) impedes a complete cell engineering solution. Human factor VIII expresses at very low levels due to limitations at several steps in the protein secretion pathway. Protein and cell engineering, vector improvement and alternate host cells promise improvement in the productivity. Production of Von Willebrand factor is constrained by its large size, complex structure, and the need for extensive glycosylation and disulfide-bonded oligomerization. All the licensed therapeutic factors are produced in CHO, BHK or HEK293 cells. While HEK293 is a recent adoption, BHK cells appear to be disfavored.

G protein-coupled receptor kinase 2 moderates recruitment of THP-1 cells to the endothelium by limiting histamine-invoked Weibel-Palade body exocytosis

BACKGROUND

G protein-coupled receptors (GPCRs) are a major family of signaling molecules, central to the regulation of inflammatory responses. Their activation upon agonist binding is attenuated by GPCR kinases (GRKs), which desensitize the receptors through phosphorylation. G protein-coupled receptor kinase 2(GRK2) down-regulation in leukocytes has been closely linked to the progression of chronic inflammatory disorders such as rheumatoid arthritis and multiple sclerosis. Because leukocytes must interact with the endothelium to infiltrate inflamed tissues, we hypothesized that GRK2 down-regulation in endothelial cells would also be pro-inflammatory.

OBJECTIVES

To determine whether GRK2 down-regulation in endothelial cells is pro-inflammatory.

METHODS

siRNA-mediated ablation of GRK2 in human umbilical vein endothelial cells (HUVECs) was used in analyses of the role of this kinase. Microscopic and biochemical analyses of Weibel-Palade body (WPB) formation and functioning, live cell imaging of calcium concentrations and video analyses of adhesion of monocyte-like THP-1 cells provide clear evidence of GRK2 function in histamine activation of endothelial cells.

RESULTS

G protein-coupled receptor kinase 2 depletion in HUVECs increases WPB exocytosis and P-selectin-dependent adhesion of THP-1 cells to the endothelial surface upon histamine stimulation, relative to controls. Further, live imaging of intracellular calcium concentrations reveals amplified histamine receptor signaling in GRK2-depleted cells, suggesting GRK2 moderates WPB exocytosis through receptor desensitization.

CONCLUSIONS

G protein-coupled receptor kinase 2 deficiency in endothelial cells results in increased pro-inflammatory signaling and enhanced leukocyte recruitment to activated endothelial cells. The ability of GRK2 to modulate initiation of inflammatory responses in endothelial cells as well as leukocytes now places GRK2 at the apex of control of this finely balanced process.

Human endothelial colony-forming cells expanded with an improved protocol are a useful endothelial cell source for scaffold-based tissue engineering

One of the major challenges in tissue engineering is to supply larger three-dimensional (3D) bioengineered tissue transplants with sufficient amounts of nutrients and oxygen and to allow metabolite removal. Consequently, artificial vascularization strategies of such transplants are desired. One strategy focuses on endothelial cells capable of initiating new vessel formation, which are settled on scaffolds commonly used in tissue engineering. A bottleneck in this strategy is to obtain sufficient amounts of endothelial cells, as they can be harvested only in small quantities directly from human tissues. Thus, protocols are required to expand appropriate cells in sufficient amounts without interfering with their capability to settle on scaffold materials and to initiate vessel formation. Here, we analysed whether umbilical cord blood (CB)-derived endothelial colony-forming cells (ECFCs) fulfil these requirements. In a first set of experiments, we showed that marginally expanded ECFCs settle and survive on different scaffold biomaterials. Next, we improved ECFC culture conditions and developed a protocol for ECFC expansion compatible with 'Good Manufacturing Practice' (GMP) standards. We replaced animal sera with human platelet lysates and used a novel type of tissue-culture ware. ECFCs cultured under the new conditions revealed significantly lower apoptosis and increased proliferation rates. Simultaneously, their viability was increased. Since extensively expanded ECFCs could still settle on scaffold biomaterials and were able to form tubular structures in Matrigel assays, we conclude that these ex vivo-expanded ECFCs are a novel, very potent cell source for scaffold-based tissue engineering.

Analysis of the storage and secretion of von Willebrand factor in blood outgrowth endothelial cells derived from patients with von Willebrand disease

Patients with von Willebrand disease (VWD) are often heterozygous for a missense mutation in the von Willebrand factor (VWF) gene. Investigating the pathogenic features of VWF mutations in cells directly derived from patients has been challenging. Here, we have used blood outgrowth endothelial cells (BOECs) isolated from human peripheral blood to analyze the storage and secretion of VWF. BOECs showed full endothelial characteristics and responded to Weibel-Palade body (WPB) secretagogues except desmopressin. We examined BOECs derived from a single subject heterozygous for a type 2N mutation (p.Arg854Gln) and from 4 patients with type 1 VWD who were, respectively, heterozygous for p.Ser1285Pro, p.Leu1307Pro, p.Tyr1584Cys, and p.Cys2693Tyr. Compared with normal BOECs, BOECs heterozygous for p.Ser1285Pro, p.Leu1307Pro, or p.Cys2693Tyr showed morphologically abnormal WPB and retention of VWF in the endoplasmic reticulum, whereas BOECs heterozygous for p.Arg854Gln or p.Tyr1584Cys showed normal WPB. The agonist-induced exocytosis of WPB from BOECs and formation of VWF strings on BOECs heterozygous for p.Ser1285Pro, p.Leu1307Pro, or p.Cys2693Tyr, but not for p.Arg854Gln or p.Tyr1584Cys, were reduced. In conclusion, VWD phenotype can be recapitulated in BOECs, and thus BOECs provide a feasible bona fide cell model to study the pathogenic effects of VWF mutations.

Streptococcus pneumoniae induces exocytosis of Weibel-Palade bodies in pulmonary endothelial cells

Invasive pneumococcal infections due to Streptococcus pneumoniae lead to inflammatory infiltration of leucocytes into lung alveolus, meninges and to septic dissemination within the vascular system. The lung microvasculature is covered by pulmonary endothelial cells containing Weibel-Palade bodies (WPB) releasing procoagulant von Willebrand factor (vWF) and other proteins in response to inflammatory stimuli. The influence of pathogenic pneumococci on secretion of WPB proteins is unknown. Here, we report that adherence of S. pneumoniae to primary human pulmonary microvascular endothelial cells (HPMEC) stimulates the WPB exocytosis and the secretion of vWF and interleukin 8 (IL-8). Moreover, infection analyses performed with pneumococcal mutants deficient in the expression of cytotoxic pneumolysin demonstrated that, in addition to direct bacterial adherence, sublytic concentrations of pneumolysin stimulated vWF secretion. The release of vWF was induced after infection with pneumococci from both the apical and the basal cell surfaces, which implies a stimulation of WPB exocytosis in both directions: from inside the vasculature and also following invasive pneumococcal transmigration from pulmonary tissue into the bloodstream. In conclusion, this study demonstrates that the most relevant pulmonary pathogen S. pneumoniae induces release of proinflammatory and procoagulative components directly contributing to pathophysiological processes leading to fatal tissue injury during course of infection.

CD63 is an essential cofactor to leukocyte recruitment by endothelial P-selectin

The activation of endothelial cells is critical to initiating an inflammatory response. Activation induces the fusion of Weibel-Palade Bodies (WPB) with the plasma membrane, thus transferring P-selectin and VWF to the cell surface, where they act in the recruitment of leukocytes and platelets, respectively. CD63 has long been an established component of WPB, but the functional significance of its presence within an organelle that acts in inflammation and hemostasis was unknown. We find that ablating CD63 expression leads to a loss of P-selectin-dependent function: CD63-deficient HUVECs fail to recruit leukocytes, CD63-deficient mice exhibit a significant reduction in both leukocyte rolling and recruitment and we show a failure of leukocyte extravasation in a peritonitis model. Loss of CD63 has a similar phenotype to loss of P-selectin itself, thus CD63 is an essential cofactor to P-selectin.

Protein mobilities and P-selectin storage in Weibel-Palade bodies

Using fluorescence recovery after photobleaching (FRAP) we measured the mobilities of EGFP-tagged soluble secretory proteins in the endoplasmic reticulum (ER) and in individual Weibel-Palade bodies (WPBs) at early (immature) and late (mature) stages in their biogenesis. Membrane proteins (P-selectin, CD63, Rab27a) were also studied in individual WPBs. In the ER, soluble secretory proteins were mobile; however, following insertion into immature WPBs larger molecules (VWF, Proregion, tPA) and P-selectin became immobilised, whereas small proteins (ssEGFP, eotaxin-3) became less mobile. WPB maturation led to further decreases in mobility of small proteins and CD63. Acute alkalinisation of mature WPBs selectively increased the mobilities of small soluble proteins without affecting larger molecules and the membrane proteins. Disruption of the Proregion-VWF paracrystalline core by prolonged incubation with NH(4)Cl rendered P-selectin mobile while VWF remained immobile. FRAP of P-selectin mutants revealed that immobilisation most probably involves steric entrapment of the P-selectin extracellular domain by the Proregion-VWF paracrystal. Significantly, immobilisation contributed to the enrichment of P-selectin in WPBs; a mutation of P-selectin preventing immobilisation led to a failure of enrichment. Together these data shed new light on the transitions that occur for soluble and membrane proteins following their entry and storage into post-Golgi-regulated secretory organelles.

[Formation and function of Weibel-Palade bodies]

Weibel-Palade bodies (WPB) are specialized cigar-shaped secretory organelles in endothelial cells, which contain a variety of biologically active molecules. These contents can be released rapidly by stimulation and involved in hemostasis, inflammation and angiogenesis. The main component of WPB is von Willebrand factor (vWF), whose expression and tubulation are necessary for the formation of the unique rod-like WPBs. Different molecules such as vWF, P-selectin, CD63, Rab27A and Rab3D are recruited into WPB mediated by the AP-1, AP-3 or other transport machinery. The underlying mechanism of the formation of WPB remains further investigation, which will gain insights into its function. The molecular mechanism of WPB formation and its function were discussed in this review.

Endothelial cell confluence regulates Weibel-Palade body formation

Secretory granules called Weibel-Palade bodies (WPBs) containing Von Willebrand factor (VWF) are characteristic of the mammalian endothelium. We hypothesized that vascular-specific antigens such as VWF are linked to endothelial identity and proliferation in vitro. To test this idea, the cellular accumulation of VWF in WPBs was monitored as a function of cell proliferation, confluence and passage number in human umbilical vein endothelial cells (HUVECs). We found that as passage number increased the percentage of cells containing VWF in WPBs was reduced significantly, whilst the protein was still detected within the secretory pathway at all times. However, the endothelial-specific marker protein, PECAM-1, is present on all cells even when WPBs are absent, indicating partial maintenance of endothelial identity. Biochemical studies show that a significant pool of immature pro-VWF can be detected in sub-confluent HUVECs; however, a larger pool of mature, processed VWF is detected in confluent cells. Newly synthesized VWF must thus be differentially sorted and packaged along the secretory pathway in semi-confluent versus confluent endothelial cells. Our studies thus show that WPB formation is linked to the formation of a confluent endothelial monolayer.

von Willebrand factor to the rescue

von Willebrand factor (VWF) is a large multimeric adhesive glycoprotein with complex roles in thrombosis and hemostasis. Abnormalities in VWF give rise to a variety of bleeding complications, known as von Willebrand disease (VWD), the most common inherited bleeding disorder in humans. Current treatment of VWD is based on the replacement of the deficient or dysfunctional protein either by endogenous release from endothelial Weibel-Palade bodies or by administration of plasma-derived VWF concentrates. During the last years, several efforts have been made to optimize existing therapies for VWD, but also to devise new approaches, such as inducing endogenous expression with interleukin-11, administering exogenous recombinant VWF, or introducing the protein via gene delivery. Clearly, the efficacy of any strategy will depend on several factors, including, for example, the quantity, activity, and stability of the delivered VWF. The inherent complexity of VWF biosynthesis, which involves extensive posttranslational processing, may be limiting in terms of producing active VWF outside of its native cellular sources. This review summarizes recent progress in the development of different treatment strategies for VWD, including those that are established and those that are at the experimental stage. Potential pitfalls and benefits of each strategy are discussed.

IL-4 alters expression patterns of storage components of vascular endothelial cell-specific granules through STAT6- and SOCS-1-dependent mechanisms

IL-4 develops Th2-biased immunity or allergic inflammation through activation of STAT6-dependent signaling. In vascular endothelial cells (ECs), IL-4 elicits regulatory effects on chemokine production and adhesion molecule expression to recruit T cells and eosinophils. In this study, we examined how IL-4 affects Weibel-Palade bodies (WPBs), EC-specific storage granules capable to store multiple protein components, including von Willebrand factor (vWF), P-selectin, eotaxin-3, IL-8 and angiopoietin-2 (Ang-2). Among 11 WPB component genes that we examined, IL-4 potently upregulated the expression levels of P-selectin and eotaxin-3, whereas it downregulated the expression levels of IL-8 and Ang-2. Both regulatory effects were dependent on STAT6. In addition, the IL-4-induced downregulatory effect on WPB component genes depended on the negative feedback regulation by SOCS-1 induced by STAT6 signaling. Furthermore, IL-4-regulated gene expression through STAT6 and SOCS-1 was consistent with WPB compositional changes in cultivated ECs and capillary-like tube networks. Since WPBs enable ECs to rapidly regulate multiple critical functions of vasculatures, IL-4-induced alteration of expression patterns of WPB storage components may convert the physiological functions of WPBs into Th2-biased immune functions or allergic functions.

Tetraspanins and vascular functions

Tetraspanins are multiple membrane-spanning proteins that likely function as the organizers of membrane microdomains. Tetraspanins associate with other membrane-bound molecules such as cell-adhesion proteins, growth factor receptors, and Ig superfamily members and regulate key cellular processes such as adhesion, migration, and fusion. Tetraspanins are widely expressed in vascular and haematopoietic cells and are involved in both physiological and pathological processes related to angiogenesis, vascular injury, thrombosis, and haemostasis. A wide body of evidence suggests that tetraspanins directly regulate the development and functions of the vascular system and the pathogenesis of vascular diseases. This article reviews current understanding of the roles of tetraspanins in vascular functions.

Biogenesis of Dense-Core Secretory Granules

Dense core granules (DCGs) are vesicular organelles derived from outbound traffic through the eukaryotic secretory pathway. As DCGs are formed, the secretory pathway can also give rise to other types of vesicles, such as those bound for endosomes, lysosomes, and the cell surface. DCGs differ from these other vesicular carriers in both content and function, storing highly concentrated cores’ of condensed cargo in vesicles that are stably maintained within the cell until a specific extracellular stimulus causes their fusion with the plasma membrane. These unique features are imparted by the activities of membrane and lumenal proteins that are specifically delivered to the vesicles during synthesis. This chapter will describe the DCG biogenesis pathway, beginning with the sorting of DCG proteins from proteins that are destined for other types of vesicle carriers. In the trans-Golgi network (TGN), sorting occurs as DCG proteins aggregate, causing physical separation from non-DCG proteins. Recent work addresses the nature of interactions that produce these aggregates, as well as potentially important interactions with membranes and membrane proteins. DCG proteins are released from the TGN in vesicles called immature secretory granules (ISGs). The mechanism of ISG formation is largely unclear but is not believed to rely on the assembly of vesicle coats like those observed in other secretory pathways. The required cytosolic factors are now beginning to be identified using in vitro systems with purified cellular components. ISG transformation into a mature fusion-competent, stimulus-dependent DCG occurs as endoproteolytic processing of many DCG proteins causes continued condensation of the lumenal contents. At the same time, proteins that fail to be incorporated into the condensing core are removed by a coat-mediated budding mechanism, which also serves to remove excess membrane and membrane proteins from the maturing vesicle. This chapter will summarize the work leading to our current view of granule synthesis, and will discuss questions that need to be addressed in order to gain a more complete understanding of the pathway.

Disorders of lysosome-related organelle biogenesis: clinical and molecular genetics

Lysosome-related organelles (LROs) are a heterogeneous group of vesicles that share various features with lysosomes, but are distinct in function, morphology, and composition. The biogenesis of LROs employs a common machinery, and genetic defects in this machinery can affect all LROs or only an individual LRO, resulting in a variety of clinical features. In this review, we discuss the main components of LRO biogenesis. We also summarize the function, composition, and resident cell types of the major LROs. Finally, we describe the clinical characteristics of the major human LRO disorders.

Von Willebrand factor propeptide makes it easy to identify the shorter Von Willebrand factor survival in patients with type 1 and type Vicenza von Willebrand disease

Reduced von Willebrand factor (VWF) half-life has been suggested as a new pathogenic mechanism in von Willebrand disease (VWD). The usefulness of VWF propeptide (VWFpp) in exploring VWF half-life was assessed in 22 type 1 and 14 type Vicenza VWD patients, and in 30 normal subjects, by comparing the findings on post-Desmopressin (DDAVP) VWF t(1/2) elimination (t(1/2el)). The VWFpp/VWF antigen ratio (VWFpp ratio) was dramatically increased in type Vicenza VWD (13.02 +/- 0.49) when compared to normal subjects (1.45 +/- 0.06), whereas it appeared to be normal in all type 1 VWD patients (1.56 +/- 0.7), except for the four carrying the C1130F mutation (4.69 +/- 0.67). A very short VWF t(1/2el) was found in type Vicenza VWD (1.3 +/- 0.2 h), while all type 1 VWD patients had a t(1/2el) similar to that of the controls (11.6 +/- 1.4 and 15.4 +/- 2.5 h respectively), except for the four patients carrying the C1130F mutation, who had a significantly shorter VWF survival (4.1 +/- 0.2 h). A significant inverse correlation emerged between VWFpp ratio and VWF t(1/2el) in both VWD patients and normal subjects. The VWFpp ratio thus seemed very useful for distinguishing between type 1 VWD cases with a normal and a reduced VWF survival, as well as for identifying type Vicenza VWD.

Basal secretion of von Willebrand factor from human endothelial cells

Endothelial cells store the adhesive glycoprotein von Willebrand factor (VWF) in Weibel-Palade bodies (WPBs), distinctively shaped regulated secretory organelles that undergo exocytosis in response to secretagogue. A significant proportion of newly synthesized VWF is also secreted spontaneously from nonstimulated cells, through what is thought to be the constitutive secretory pathway. To learn more about VWF trafficking, we performed kinetic analyses of the storage and nonstimulated secretion of VWF in cultured human endothelial cells. We found that most VWF was secreted through a route that was significantly delayed compared with constitutive secretion, although this pathway was responsible for secretion of a small amount of uncleaved VWF precursor. Disruption of pH-dependent sorting processes with ammonium chloride converted the secretion kinetics of mature VWF to that of its precursor. Conversely, preventing constitutive secretion of nascent protein with brefeldin A had only a modest effect on the spontaneous release of VWF, showing that most VWF secreted by nonstimulated cells was not constitutive secretion but basal release of a post-Golgi storage organelle, presumably the WPB. These data suggest that VWF is sorted to the regulated secretory pathway in endothelial cells much more efficiently than previously reported.

Selective release of molecules from Weibel-Palade bodies during a lingering kiss

Exocytosis of specialized endothelial cell secretory organelles, Weibel-Palade bodies (WPBs), is thought to play an important role in regulating hemostasis and intravascular inflammation. The major WPB core proteins are Von Willebrand factor (VWF) and its propolypeptide (Proregion), constituting more than 95% of the content. Although the composition of the WPBs can be fine-tuned to include cytokines and chemokines (eg, interleukin-8 [IL-8] and eotaxin-3), it is generally assumed that WPB exocytosis is inextricably associated with secretion of VWF. Here we show that WPBs can undergo a form of exocytosis during which VWF and Proregion are retained while smaller molecules, such as IL-8, are released. Imaging individual WPBs containing fluorescent cargo molecules revealed that during weak stimulation approximately 25% of fusion events result in a failure to release VWF or Proregion. The WPB membrane protein P-selectin was also retained; however, the membrane tetraspannin CD63 was released. Accumulation or exclusion of extracellular fluorescent dextran molecules ranging from 3 kDa to 2 mDa show that these events arise due to the formation of a fusion pore approximately 12 nm in diameter. The pore behaves as a molecular filter, allowing selective release of WPB core and membrane proteins. WPB exocytosis is not inextricably associated with secretion of VWF.

Characterization of the interaction between von Willebrand factor and osteoprotegerin

BACKGROUND AND OBJECTIVE

Osteoprotegerin (OPG), a member of the tumor necrosis-factor receptor superfamily, plays an important role in bone remodeling and is also involved in vascular diseases. OPG is physically associated with von Willebrand factor (VWF), a glycoprotein involved in primary hemostasis, within the Weibel-Palade bodies (WPBs) of endothelial cells and in plasma. The present study aimed to elucidate the molecular mechanisms underlying the interaction between OPG and VWF.

METHODS AND RESULTS

In a solid-phase binding assay, VWF was able to bind specifically to OPG in a calcium-dependent manner. This interaction displayed strong pH dependence with optimal binding occurring at pH 6.5 and was severely impaired by chloride-ion concentrations above 40 mm. Using a series of purified VWF derivatives the functional site that supports VWF interaction with OPG was localized on its Al domain. Fluorescence microscopy on human umbilical vein endothelial cells showed co-localization of VWF and OPG in WPBs. When secretion was induced, OPG remained associated with VWF in extracellular patches of release under biochemical conditions found in blood plasma.

CONCLUSIONS

Our observations demonstrate the existence of an interactive site for OPG within the VWF A1-domain. This study established that the optimal biochemical parameters allowing a complex formation between VWF and OPG are those thought to prevail in the trans-Golgi network. These conditions would allow VWF to act as a cargo targeting OPG to WPBs. Finally, blood environments appear suitable to preserve the complex, which may participate in vascular injury, arterial calcification and inflammation.

A new look at Weibel-Palade body structure in endothelial cells using electron tomography

Multimers of von Willebrand Factor (vWF), a protein mediating blood clotting in response to vascular injury, are stored as tubular structures by endothelial cells in specific organelles, the Weibel-Palade Bodies (WPBs). To date very little is known about the 3D structure of WPBs in relation to the organization of the tubules. Therefore, we have initiated a thorough electron microscopic study in human umbilical vein endothelial cells (HUVECs) using electron tomography to gain further understanding of the ultrastructure of WPBs. We found that in addition to the well-documented cigar-shape, WPBs adopt irregular forms, which appeared to result from homotypic fusion. In transverse views of WPBs the tubular striations appear evenly spaced, which indicates a high level of organization that is likely to involve an underlying scaffold of structural proteins. Additionally, we found that the tubular striations twisted in an orderly fashion, suggesting that they are stored within the WPBs by a spring-loading mechanism. Altogether these data suggest that WPBs undergo a relatively complex maturation process involving homotypic fusion. Although the mechanism of assembly of vWF multimers into tubules is still unknown, the curled arrangement of the tubules within WPBs suggests a high degree of folding of the protein inside the organelle.

Rate, extent and concentration dependence of histamine-evoked Weibel-Palade body exocytosis determined from individual fusion events in human endothelial cells

The rate, concentration dependence and extent of histamine-evoked Weibel-Palade body (WPB) exocytosis were investigated with time-resolved fluorescence microscopy in cultured human umbilical vein endothelial cells expressing WPB-targeted chimeras of enhanced green fluorescent protein (EGFP). Exocytosis of single WPBs was characterized by an increase in EGFP fluorescence, morphological changes and release of WPB contents. The fluorescence increase was due to a rise of intra-WPB pH from resting levels, estimated as pH 5.45+/-0.26 (s.d., n=144), to pH 7.40. It coincided with uptake of extracellular Alexa-647, indicating the formation of a fusion pore, prior to loss of fluorescent contents. Delays between the increase in intracellular free calcium ion concentration evoked by histamine and the first fusion event were 10.0+/-4.42 s (n=9 cells) at 0.3 microM histamine and 1.57+/-0.21 s (n=15 cells) at 100 microM histamine, indicating the existence of a slow process or processes in histamine-evoked WPB exocytosis. The maximum rates of exocytosis were 1.20+/-0.16 WPB s(-1) (n=9) at 0.3 microM and 3.66+/-0.45 WPB s(-1) at 100 microM histamine (n=15). These occurred 2-5 s after histamine addition and declined to lower rates with continued stimulation. The initial delays and maximal rate of exocytosis were unaffected by removal of external Ca2+ indicating that the initial burst of secretion is driven by Ca2+ release from internal stores, but sustained exocytosis required external Ca2+. Data were compared to exocytosis evoked by a maximal concentration of the strong secretagogue ionomycin (1 microM), for which there was a delay between calcium elevation and secretion of 1.67+/-0.24 s (n=6), and a peak fusion rate of approximately 10 WPB s(-1).

High-pressure freezing provides insights into Weibel-Palade body biogenesis

The Weibel-Palade bodies (WPBs) of endothelial cells play an important role in haemostasis and the initiation of inflammation, yet their biogenesis is poorly understood. Tubulation of their major content protein, von Willebrand factor (VWF), is crucial to WPB function, and so we investigated further the relationship between VWF tubule formation and WPB formation in human umbilical vein endothelial cells (HUVECs). By using high-pressure freezing and freeze substitution before electron microscopy, we visualised VWF tubules in the trans-Golgi network (TGN), as well as VWF subunits in vesicular structures. Tubules were also seen in WPBs that were connected to the TGN by membranous stalks. Tubules are disorganised in the immature WPBs but during maturation we found a dramatic increase in the spatial organisation of the tubules and in organelle electron density. We also found coated budding profiles suggestive of the removal of missorted material after initial formation of these granules. Finally, we discovered that these large, seemingly rigid, organelles flex at hinge points and that the VWF tubules are interrupted at these hinges, facilitating organelle movement around the cell. The use of high-pressure freezing was vital in this study and it suggests that this technique might prove essential to any detailed characterisation of organelle biogenesis.

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.

P-Selectin and CD63 Use Different Mechanisms for Delivery to Weibel-Palade Bodies

The biogenesis of endothelial-specific Weibel-Palade bodies (WPB) is poorly understood, despite their key role in both haemostasis and inflammation. Biogenesis of specialized organelles of haemopoietic cells is often adaptor protein complex 3-dependent (AP-3-dependent), and AP-3 has previously been shown to play a role in the trafficking of both WPB membrane proteins, P-selectin and CD63. However, WPB are thought to form at the trans Golgi network (TGN), which is inconsistent with a role for AP-3, which operates in post-Golgi trafficking. We have therefore investigated in detail the mechanisms of delivery of these two membrane proteins to WPB. We find that P-selectin is recruited to forming WPB in the trans-Golgi by AP-3-independent mechanisms that use sorting information within both the cytoplasmic tail and the lumenal domain of the receptor. In contrast, CD63 is recruited to already-budded WPB by an AP-3-dependent route. These different mechanisms of recruitment lead to the presence of distinct immature and mature populations of WPB in human umbilical vein endothelial cells (HUVEC).

Rho GTPases and Leukocyte Adhesion Receptor Expression and Function in Endothelial Cells

Rho family GTPases are key signal transducers that regulate cell adhesion and migration and a variety of other cellular responses, including changes in gene expression. In this review, we discuss how Rho GTPases regulate signaling by endothelial cell receptors involved in leukocyte extravasation. First, Rho GTPases affect the expression of some leukocyte adhesion molecules on endothelial cells, such as intracellular adhesion molecule-1 and E-selectin, that can be induced by proinflammatory mediators, hypoxia, or shear stress. Second, Rho GTPases are activated by engagement of several leukocyte adhesion receptors and contribute to both early morphological changes and subsequent alterations in gene expression. Rho GTPases are therefore candidate targets for inhibiting leukocyte transendothelial migration in heart disease and chronic inflammatory disorders.

Dynamics and plasticity of Weibel-Palade bodies in endothelial cells

Agonist-induced release of endothelial cell specific storage granules, designated Weibel-Palade bodies (WPBs), provides the endothelium with the ability to rapidly respond to changes in its micro-environment. Originally being defined as an intracellular storage pool for von Willebrand factor (VWF), it has recently been shown that an increasing number of other components, including P-selectin, interleukin (IL)-8, eotaxin-3, endothelin-1, and angiopoietin-2, is present within this subcellular organelle, implicating a role for WPB exocytosis in inflammation, hemostasis, regulation of vascular tone and angiogenesis. Recent studies emphasize that WPBs provide a dynamic storage compartment whose contents can be regulated depending on the presence of inflammatory mediators in the vascular micro-environment. Additionally, release of WPBs is tightly regulated and feedback mechanisms have been identified that prevent excessive release of bioactive components from this subcellular organelle. The ability to regulate both contents and exocytosis of WPBs endows these endothelial cell specific organelles with a remarkable plasticity. This is most likely needed to allow for controlled delivery of bioactive components into the circulation on vascular perturbation.

Differential regulation of endothelial exocytosis of P-selectin and von Willebrand factor by protease-activated receptors and cAMP

Thrombin-mediated endothelial-cell release of von Willebrand factor (VWF) and P-selectin functionally links protease-activated receptors (PARs) to thrombosis and inflammation. VWF release can be stimulated by both Ca2+ and cAMP, and, although both VWF and P-selectin are found in Weibel-Palade bodies (WPBs), we found that their release could be differentially regulated. In these studies, human umbilical vein endothelial cells stimulated with cAMP or PAR2-AP led to a delayed release of VWF and significantly less P-selectin release compared with histamine, thrombin, or PAR1-AP. Dose-response studies revealed that PAR2-AP was significantly less efficacious in promoting the release of P-selectin compared with VWF. PAR2-AP-induced robust stimulation of intracellular Ca2+ coupled with a significantly greater inhibitory effect of calcium chelation on release of VWF compared with cell-surface expression of P-selectin, suggests an additional Ca2+-independent pathway involved in release of P-selectin. PAR2-AP failed to increase global cAMP levels; however, inhibition of protein kinase A led to a significant attenuation of PAR2-AP-mediated release of VWF. Confocal microscopy studies revealed that PAR2 and forskolin caused preferential release of a population of Weibel-Palade bodies (WPBs) consisting of only VWF. Thus, WPBs are pharmacologically and morphologically heterogeneous, and distinct granule populations are susceptible to differential regulation.

An AP-1/clathrin coat plays a novel and essential role in forming the Weibel-Palade bodies of endothelial cells

Clathrin provides an external scaffold to form small 50-100-nm transport vesicles. In contrast, formation of much larger dense-cored secretory granules is driven by selective aggregation of internal cargo at the trans-Golgi network; the only known role of clathrin in dense-cored secretory granules formation is to remove missorted proteins by small, coated vesicles during maturation of these spherical organelles. The formation of Weibel-Palade bodies (WPBs) is also cargo driven, but these are cigar-shaped organelles up to 5 mum long. We hypothesized that a cytoplasmic coat might be required to make these very different structures, and we found that new and forming WPBs are extensively, sometimes completely, coated. Overexpression of an AP-180 truncation mutant that prevents clathrin coat formation or reduced AP-1 expression by small interfering RNA both block WPB formation. We propose that, in contrast to other secretory granules, cargo aggregation alone is not sufficient to form immature WPBs and that an external scaffold that contains AP-1 and clathrin is essential.

Differential Kinetics of Cell Surface Loss of von Willebrand Factor and Its Propolypeptide after Secretion from Weibel-Palade Bodies in Living Human Endothelial Cells

The time course for cell surface loss of von Willebrand factor (VWF) and the propolypeptide of VWF (proregion) following exocytosis of individual Weibel-Palade bodies (WPBs) from single human endothelial cells was analyzed. Chimeras of enhanced green fluorescent protein (EGFP) and full-length pre-pro-VWF (VWF-EGFP) or the VWF propolypeptide (proregion-EGFP) were made and expressed in human umbilical vein endothelial cells. Expression of VWF-EGFP or proregion-EGFP resulted in fluorescent rod-shaped organelles that recruited the WPB membrane markers P-selectin and CD63. The WPB secretagogue histamine evoked exocytosis of these fluorescent WPBs and extracellular release of VWF-EGFP or proregion-EGFP. Secreted VWF-EGFP formed distinctive extracellular patches of fluorescence that were labeled with an extracellular antibody to VWF. The half-time for dispersal of VWF-EGFP from extracellular patches was 323.5 +/- 146.2 s (+/-S.D., n = 20 WPBs). In contrast, secreted proregion-EGFP did not form extracellular patches but dispersed rapidly from its site of release. The half-time for dispersal of proregion-EGFP following WPB exocytosis was 2.98 +/- 1.88 s (+/-S.D., n = 32 WPBs). The slow rate of loss of VWF-EGFP is consistent with the adhesive nature of this protein for the endothelial membrane. The much faster rate of loss of proregion-EGFP indicates that this protein does not interact strongly with extracellular VWF or the endothelial membrane and consequently may not play an adhesive role at the endothelial cell surface.

How to roll an endothelial cigar: the biogenesis of Weibel-Palade bodies

Weibel-Palade bodies (WPB) are the regulated secretory organelles of endothelial cells. These cigar-shaped membrane-bound structures function in both hemostasis and inflammation but their biogenesis is poorly understood. Here, we review what is currently known about their formation. The content of WPBs is dominated by the hemostatic factor von Willebrand factor (VWF), whose complex biogenesis ends in the formation of high molecular weight multimers. VWF is also organized into proteinaceous tubules which underlie the striated interior of WPBs as seen in the EM. VWF expression is necessary for formation of WPBs, and its heterologous expression can even lead to the specific recruitment of WPB membrane proteins, including the leukocyte receptor P-selectin, the tetraspanin CD63, and Rab27a. Unusually, the VWF propeptide is implicated in the biogenesis of WPBs, being essential for formation of the storage compartment. The elongation of the cigars and the formation of the tubules are determined by non-covalent interactions between pro- and mature VWF proteins. Surprisingly, high molecular weight multimers seem neither necessary nor sufficient to trigger formation of a storage compartment, and do not seem to have any role in WPB biogenesis. Von Willebrand's disease, usually caused by mutations within VWF, has provided many of the insights into the way in which VWF drives the formation of these organelles.

Re-establishment of VWF-dependent Weibel-Palade bodies in VWD endothelial cells

Type 3 von Willebrand disease (VWD) is a severe hemorrhagic defect in humans. We now identify the homozygous mutation in the Chapel Hill strain of canine type 3 VWD that results in premature termination of von Willebrand factor (VWF) protein synthesis. We cultured endothelium from VWD and normal dogs to study intracellular VWF trafficking and Weibel-Palade body formation. Weibel-Palade bodies could not be identified in the canine VWD aortic endothelial cells (VWD-AECs) by P-selectin, VWFpp, or VWF immunostaining and confocal microscopy. We demonstrate the reestablishment of Weibel-Palade bodies that recruit endogenous P-selectin by expressing wild-type VWF in VWD-AECs. Expression of mutant VWF proteins confirmed that VWF multimerization is not necessary for Weibel-Palade body creation. Although the VWF propeptide is required for the formation of Weibel-Palade bodies, it cannot independently induce the formation of the granule. These VWF-null endothelial cells provide a unique opportunity to examine the biogenesis of Weibel-Palade bodies in endothelium from a canine model of type 3 VWD.

Liver sinusoidal endothelial cells are insufficient to activate T cells

Liver sinusoidal endothelial cells (LSEC) have been reported to express MHC class II, CD80, CD86, and CD11c and effectively stimulate naive T cells. Because dendritic cells (DC) are known to possess these characteristics, we sought to directly compare the phenotype and function of murine LSEC and DC. Nonparenchymal cells from C57BL/6 mice were obtained by collagenase digestion of the liver followed by density gradient centrifugation. From the enriched nonparenchymal cell fraction, LSEC (CD45(-)) were then isolated to 99% purity using immunomagnetic beads. Flow cytometric analysis of LSEC demonstrated high expression of CD31, von Willebrand factor, and FcgammaRs. However, unlike DC, LSEC had low or absent expression of MHC class II, CD86, and CD11c. LSEC demonstrated a high capacity for Ag uptake in vitro and in vivo. Although acetylated low-density lipoprotein uptake has been purported to be a specific function of LSEC, we found DC captured acetylated low-density lipoprotein to a similar extent in vivo. Consistent with their phenotype, LSEC were poor stimulators of allogeneic T cells. Furthermore, in the absence of exogenous costimulation, LSEC induced negligible proliferation of CD4(+) or CD8(+) TCR-transgenic T cells. Thus, contrary to previous reports, our data indicate that LSEC alone are insufficient to activate naive T cells.

Small GTP-binding protein Ral is involved in cAMP-mediated release of von Willebrand factor from endothelial cells

OBJECTIVE

von Willebrand factor (vWF) is synthesized by endothelial cells and stored in specialized vesicles called Weibel-Palade bodies (WPBs). Recently, we have shown that the small GTP-binding protein Ral is involved in thrombin-induced exocytosis of WPBs. In addition to Ca2+-elevating secretagogues such as histamine and thrombin, release of WPB is also observed after administration of cAMP-raising substances such as epinephrine and vasopressin. In the present study, we investigated whether Ral is also involved in cAMP-mediated vWF release.

METHODS AND RESULTS

Activation of Ral was observed 15 to 20 minutes after stimulation of endothelial cells with epinephrine, forskolin, or dibutyryl-cAMP. A cell-permeable peptide comprising the carboxy-terminal part of the Ral protein reduced both thrombin-induced and epinephrine-induced vWF secretion supporting a crucial role for Ral in this process. Furthermore, inhibition of protein kinase A by H-89 resulted in a marked reduction of vWF release and greatly diminished levels of GTP-Ral on stimulation with epinephrine. Activation of Ral was independent of the activation of Epac, a cAMP-regulated exchange factor for the small GTPases Rap1 and Rap2.

CONCLUSIONS

These results suggest that protein kinase A-dependent activation of Ral regulates cAMP-mediated exocytosis of WPB in endothelial cells.

The Tie-2 ligand angiopoietin-2 is stored in and rapidly released upon stimulation from endothelial cell Weibel-Palade bodies

The angiopoietins Ang-1 and Ang-2 have been identified as ligands with opposing functions of the receptor tyrosine kinase Tie-2 regulating endothelial cell survival and vascular maturation. Ang-1 acts in a paracrine agonistic manner, whereas Ang-2 appears to act primarily as an autocrine antagonistic regulator. To shed further light on the complexity of autocrine/paracrine agonistic/antagonistic functions of the angiopoietin/Tie-2 system, we have studied Ang-2 synthesis and secretion in different populations of wild-type and retrovirally Ang-2-transduced endothelial cells. Endogenous and overexpressed endothelial cell Ang-2 is expressed in a characteristic granular pattern indicative of a cytoplasmic storage granule. Light and electron microscopic double staining revealed Ang-2 colocalization with von Willebrand factor, identifying Ang-2 as a Weibel-Palade body molecule. Costaining with P-selectin showed that storage of Ang-2 and P-selectin in Weibel-Palade bodies is mutually exclusive. Stored Ang-2 has a long half-life of more than 18 hours and can be secreted within minutes of stimulation (eg, by phorbol 12-myristate 13-acetate [PMA], thrombin, and histamine). Collectively, the identification of Ang-2 as a stored, rapidly available molecule in endothelial cells strongly suggests functions of the angiopoietin/Tie-2 system beyond the established roles during angiogenesis likely to be involved in rapid vascular homeostatic reactions such as inflammation and coagulation.