The Epac-Rap1 signaling pathway controls cAMP-mediated exocytosis of Weibel-Palade bodies in endothelial cells.

Endothelial cells contain specialized storage organelles called Weibel-Palade bodies (WPBs) that release their content into the vascular lumen in response to specific agonists that raise intracellular Ca(2+) or cAMP. We have previously shown that cAMP-mediated WPB release is dependent on protein kinase A (PKA) and involves activation of the small GTPase RalA. Here, we have investigated a possible role for another PKA-independent cAMP-mediated signaling pathway in the regulation of WPB exocytosis, namely the guanine nucleotide exchange factor Epac1 and its substrate, the small GTPase Rap1. Epinephrine stimulation of endothelial cells leads to Rap1 activation in a PKA-independent fashion. siRNA-mediated knockdown of Epac1 abolished epinephrine-induced activation of Rap1 and resulted in decreased epinephrine-induced WPB exocytosis. Down-regulation of Rap1 expression and prevention of Rap1 activation through overexpression of Rap1GAP effectively reduced epinephrine- but not thrombin-induced WPB exocytosis. Taken together, these data uncover a new Epac-Rap1-dependent pathway by which endothelial cells can regulate WPB exocytosis in response to agonists that signal through cAMP.

Elevated plasma von Willebrand factor and propeptide levels in Malawian children with malaria.

BACKGROUND: In spite of the significant mortality associated with Plasmodium falciparum infection, the mechanisms underlying severe disease remain poorly understood. We have previously shown evidence of endothelial activation in Ghanaian children with malaria, indicated by elevated plasma levels of both von Willebrand factor (VWF) and its propeptide. In the current prospective study of children in Malawi with retinopathy confirmed cerebral malaria, we compared these markers with uncomplicated malaria, non malarial febrile illness and controls. METHODS AND FINDINGS: Children with cerebral malaria, mild malaria and controls without malaria were recruited into the study. All comatose patients were examined by direct and indirect ophthalmoscopy. Plasma VWF and propeptide levels were measured by ELISA. Median VWF and propeptide levels were significantly higher in patients with uncomplicated malaria than in children with non-malarial febrile illness of comparable severity, in whom levels were higher than in non-febrile controls. Median concentrations of both markers were higher in cerebral malaria than in uncomplicated malaria, and were similar in patients with and without retinopathy. Levels of both VWF and propeptide fell significantly 48 hours after commencing therapy and were normal one month later. CONCLUSIONS: In children with malaria plasma VWF and propeptide levels are markedly elevated in both cerebral and mild paediatric malaria, with levels matching disease severity, and these normalize upon recovery. High levels of both markers also occur in retinopathy-negative 'cerebral malaria' cases, many of whom are thought to be suffering from diseases other than malaria, indicating that further studies of these markers will be required to determine their sensitivity and specificity.

Rapid activation of endothelial cells enables Plasmodium falciparum adhesion to platelet-decorated von Willebrand factor strings.

During Plasmodium falciparum malaria infections, von Willebrand factor (VWF) levels are elevated, postmortem studies show platelets colocalized with sequestered infected erythrocytes (IEs) at brain microvascular sites, whereas in vitro studies have demonstrated platelet-mediated IE adhesion to tumor necrosis factor-activated brain endothelium via a bridging mechanism. This current study demonstrates how all these observations could be linked through a completely novel mechanism whereby IEs adhere via platelet decorated ultra-large VWF strings on activated endothelium. Using an in vitro laminar flow model, we have demonstrated tethering and firm adhesion of IEs to the endothelium specifically at sites of platelet accumulation. We also show that an IE pro-adhesive state, capable of supporting high levels of binding within minutes of induction, can be removed through the action of the VWF protease ADAMTS-13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). We propose that this new mechanism contributes to sequestration both independently of and in concert with current adhesion mechanisms.

A novel type 2A von Willebrand factor mutation (V1499E) associated with variable clinical expression.

We have identified a previously unreported mutation, V1499E, with a high penetrance in a family with type 2A von Willebrand disease. Affected family members were difficult to identify owing to variable von Willebrand factor (VWF) levels, variable expression of VWF multimers, and clinical symptoms. Recombinant V1499E-VWF was more readily cleaved by ADAMTS13 than the wild-type protein, suggesting that V1499E is the causative mutation. Surprisingly, this seemingly novel unique mutation was also found in other family members in 2 other hospitals displaying the same variable laboratory and clinical symptoms. The fact that this V1499E mutation was detected independently in 3 hospitals is strongly in favor of 1 central database, especially considering the variable laboratory and clinical picture.

Severe Plasmodium falciparum malaria is associated with circulating ultra-large von Willebrand multimers and ADAMTS13 inhibition.

Plasmodium falciparum infection results in adhesion of infected erythrocytes to blood vessel endothelium, and acute endothelial cell activation, together with sequestration of platelets and leucocytes. We have previously shown that patients with severe infection or fulminant cerebral malaria have significantly increased circulatory levels of the adhesive glycoprotein von Willebrand factor (VWF) and its propeptide, both of which are indices of endothelial cell activation. In this prospective study of patients from Ghana with severe (n = 20) and cerebral (n = 13) P. falciparum malaria, we demonstrate that increased plasma VWF antigen (VWF:Ag) level is associated with disproportionately increased VWF function. VWF collagen binding (VWF:CB) was significantly increased in patients with cerebral malaria and severe malaria (medians 7.6 and 7.0 IU/ml versus 1.9 IU/ml; p<0.005). This increased VWF:CB correlated with the presence of abnormal ultra-large VWF multimers in patient rather than control plasmas. Concomitant with the increase in VWF:Ag and VWF:CB was a significant persistent reduction in the activity of the VWF-specific cleaving protease ADAMTS13 (approximately 55% of normal; p<0.005). Mixing studies were performed using P. falciparum patient plasma and normal pooled plasma, in the presence or absence of exogenous recombinant ADAMTS13. These studies demonstrated that in malarial plasma, ADAMTS13 function was persistently inhibited in a time-dependent manner. Furthermore, this inhibitory effect was not associated with the presence of known inhibitors of ADAMTS13 enzymatic function (interleukin-6, free haemoglobin, factor VIII or thrombospondin-1). These novel findings suggest that severe P. falciparum infection is associated with acute endothelial cell activation, abnormal circulating ULVWF multimers, and a significant reduction in plasma ADAMTS13 function which is mediated at least in part by an unidentified inhibitor.

Correlation between the potency of a beta2-glycoprotein I-dependent lupus anticoagulant and the level of resistance to activated protein C.

The antiphospholipid syndrome is characterized by the occurrence of vascular thrombosis combined with the presence of antiphospholipid antibodies in plasma of patients. It has been published that antibeta2-glycoprotein I (beta2-GPI) antibodies, with lupus anticoagulant activity (LAC), highly correlate with thrombosis. Resistance related to antiphospholipid antibodies against activated protein C (APC) is one of the proposed mechanisms responsible for thrombosis. We investigated a possible correlation between a beta2-GPI-dependent LAC (titration of cardiolipin into an activated partial thromboplastin time-based assay) and increased APC resistance in a population of 22 plasma samples with LAC activity. Eleven plasma samples that displayed a beta2-GPI-dependent LAC also showed increased APC resistance. In contrast, only one of the 11 plasma samples with a beta2-GPI-independent LAC displayed increased APC resistance. In addition, a monoclonal antibeta2-GPI antibody and patient-purified immunoglobulin G (both with LAC activity) were diluted in plasma with/without protein C. Both antibodies only displayed a beta2-GPI-dependent LAC in plasma in the presence of protein C. This indicates that the principle of the beta2-GPI LAC-assay was based on increased resistance against protein C. Surface plasmon resonance analysis was used to investigate binding between APC and beta2-GPI. We observed that beta2-GPI was able to bind APC directly, especially in the presence of a monoclonal antibeta2-GPI antibody. In conclusion, our observations show a direct correlation between a major clinical symptom of antiphospholipid syndrome (thrombosis), a diagnostic assay (beta2-GPI-dependent LAC) and a potential mechanism responsible for thrombosis in the antiphospholipid syndrome (increased APC resistance).

Survival of von Willebrand factor released following DDAVP in a type 1 von Willebrand disease cohort: influence of glycosylation, proteolysis and gene mutations.

Reduced plasma survival of von Willebrand factor (VWF) may contribute towards the pathogenesis of type 1 von Willebrand disease (VWD). However, little is known about mechanism(s) of VWF clearance and factors that may affect it. The half-life of VWF-related parameters following the administration of DDAVP was measured in 26 patients with type 1 VWD and 10 haemophilia A controls. Binding of lectins Ricinus communis (RCA-I) and Erythina crystagalli (ECA) agglutinins to VWF and VWF susceptibility to ADAMTS-13-mediated proteolysis were investigated. Sequence analysis of targeted regions of the VWF gene was performed to inspect for mutations that have been associated with increased clearance. Post-DDAVP clearance of VWF was increased approximately three-fold in the type 1 VWD cohort overall. However this was not shown to consistently associate with steady-state VWF antigen (VWF:Ag) levels. Furthermore, increased VWF clearance was not consistently associated with increased ratios of VWF propeptide (VWFpp) to VWF:Ag indicating that a normal ratio does not necessarily reflect normal post-DDAVP survival in type 1 VWD patients. RCA-I and ECA binding to VWF were increased in type 1 VWD patients and, although inversely correlated with VWF levels, this was independent of VWF clearance. There was no association between VWF clearance and ADAMTS-13-mediated proteolysis. Three novel candidate mutations with an increased clearance phenotype were identified. The data are consistent with heterogeneity in pathogenic mechanisms in type 1 VWD and are consistent with type 1 VWD representing a complex genetic trait.

Guanine exchange factor RalGDS mediates exocytosis of Weibel-Palade bodies from endothelial cells.

The small GTP-binding protein Ral has been implicated in regulated exocytosis via its interaction with the mammalian exocyst complex. We have previously demonstrated that Ral is involved in exocytosis of Weibel-Palade bodies (WPBs). Little is known about intracellular signaling pathways that promote activation of Ral in response to ligand binding of G protein-coupled receptors. Here we show that RNAi-mediated knockdown of RalGDS, an exchange factor for Ral, results in inhibition of thrombin- and epinephrine-induced exocytosis of WPBs, while overexpression of RalGDS promotes exocytosis of WPBs. A RalGDS variant lacking its exchange domain behaves in a dominant negative manner by blocking release of WPBs. We also provide evidence that RalGDS binds calmodulin (CaM) via an amino-terminal CaM-binding domain. RalGDS association to CaM is required for Ral activation because a cell-permeable peptide comprising this RalGDS CaM-binding domain inhibits Ral activation and WPB exocytosis. Together our findings suggest that RalGDS plays a vital role in the regulation of Ral-dependent WPB exocytosis after stimulation with Ca(2+)- or cAMP-raising agonists.

Persistent factor VIII-dependent factor X activation on endothelial cells is independent of von Willebrand factor.

Endothelial cells are able to support the activation of coagulation factor X by activated factor IX in the presence of its cofactor, factor VIII. We have previously reported that this reaction is persistent on endothelial cells, but transient on activated platelets and phospholipid vesicles when activated factor X (Xa) is used as activator of factor VIII. Aim of the present study was to explore the influence of von Willebrand factor and that of the factor VIII activator, either factor Xa or thrombin, on the decay of factor X activation on the endothelial cell surface. Kinetics of factor X activation on human umbilical vein endothelial cells was compared with that on phospholipid vesicles employing purified coagulation factors from plasma as well as recombinant factor VIII variants. Employing factor Xa as factor VIII activator, rate constants for decay of membrane-bound factor X activation were consistently low on endothelial cells (0.02 min) as compared with phospholipid vesicles (0.2 min). Activation of factor VIII by thrombin resulted in two-fold increased decay rates. In the presence of excess of von Willebrand factor over factor VIII, decay rates were not significantly changed. Factor VIII variants with and without a Tyr to Phe substitution, which abolishes high-affinity binding to von Willebrand factor, displayed the same factor X activation decay kinetics. Although previous studies have shown that von Willebrand factor modulates factor VIII activation and stabilisation, this apparently does not affect the progression of factor X activation at the endothelium.

Cav3.1 (alpha1G) controls von Willebrand factor secretion in rat pulmonary microvascular endothelial cells.

The T-type Ca2+ channel Cav3.1 subunit is present in pulmonary microvascular endothelial cells (PMVECs), but not in pulmonary artery endothelial cells (PAECs). The present study sought to assess the role of Cav3.1 in thrombin-induced Weibel-Palade body exocytosis and consequent von Willebrand factor (VWF) release. In PMVECs and PAECs transduced with a green fluorescent protein (GFP)-tagged VWF chimera, we examined the real-time dynamics and secretory process of VWF-GFP-containing vesicles in response to thrombin and the cAMP-elevating agent isoproterenol. Whereas thrombin stimulated a progressive decrease in the number of VWF-GFP-containing vesicles in both cell types, isoproterenol only decreased the number of VWF-GFP-containing vesicles in PAECs. In PMVECs, thrombin-induced decrease in the number of VWF-GFP-containing vesicles was nearly abolished by the T-type Ca2+ channel blocker mibefradil as well as by Cav3.1 gene silencing with small hairpin RNA. Expression of recombinant Cav3.1 subunit in PAECs resulted in pronounced increase in thrombin-stimulated Ca2+ entry, which is sensitive to mibefradil. Together, these data indicate that VWF secretion from lung endothelial cells is regulated by two distinct pathways involving Ca2+ or cAMP, and support the hypothesis that activation of Cav3.1 T-type Ca2+ channels in PMVECs provides a unique cytosolic Ca2+ source important for Gq-linked agonist-induced VWF release.

Amino acid regions 572-579 and 657-666 of the spacer domain of ADAMTS13 provide a common antigenic core required for binding of antibodies in patients with acquired TTP.

Antibodies directed against ADAMTS13 have been detected in the majority of patients with acquired thrombotic thrombocytopenic purpura (TTP). We have previously localized a major antigenic determinant within the spacer domain of ADAMTS13. To identify the amino acid residues of the spacer domain that are involved in binding of anti-ADAMTS13 antibodies, we constructed a series of fifteen hybrids (designated A-O) in which 5-10 amino acids of the spacer domain were exchanged for the corresponding region of ADAMTS1. Plasma from six patients with antibodies directed against the spacer domain was analyzed for reactivity with the ADAMTS13/ADAMTS1 chimeras. Exchange of amino acid residues 572-579 (hybrid C) and 657-666 (hybrid M) completely abolished the binding of antibodies from all six patients analyzed. Regions 580-587 (D), 602-620 (G, H), 629-638 (J), and 667-767 (N) contributed to binding of antibodies from patients 2, 4, and 5 (epitope present within regions CDGHJMN). Antibodies derived from patient 1 required region 602-620 (G, H) for binding (CGHM-epitope). For antibodies of patient 3, residues 564-571 (B), 580-587 (D), and 629-638 (J) were required (BCDJM-epitope), whereas replacement of residues 602-610 (G) and 629-638 (J) greatly diminished binding of antibodies from patient 6 (CGJM-epitope). Despite the presumably polyclonal origin of the antibodies present in plasma of patients, our results suggest that residues 572-579 (C) and 657-666 (M) comprise a common antigenic core region that is crucial for binding of anti-ADAMTS13 antibodies. Other regions that spatially surround this antigenic core further modulate binding of antibodies to the spacer domain.

von Willebrand factor propeptide in malaria: evidence of acute endothelial cell activation.

The pathogenicity of Plasmodium falciparum is thought to relate to the unique ability of infected erythrocytes to adhere to and subsequently activate the vascular endothelium. To study the state of endothelial activation during falciparum malaria, we measured plasma levels of both von Willebrand factor (VWF) and its propeptide, indices of chronic and acute endothelial cell perturbation, respectively. Results were correlated with clinical and biochemical markers of disease severity, including plasma lactate. Our data show that acute endothelial cell activation is a hallmark of malaria in children, indicated by a significant rise in VWF and VWF propeptide. The highest VWF and propeptide levels were seen in cerebral and non-cerebral severe malaria, and associations found between VWF propeptide level and lactate (P < 0.001). Mean VWF propeptide levels (nmol/l) were in cerebral malaria 33.4, non-cerebral severe malaria 26.3, mild malaria 22.1, non-malaria febrile illness 10.2, and controls 10.1. Differences between patient and control groups were highly significant (P < 0.005). Follow-up of 26 cerebral malaria cases showed that levels of VWF propeptide, but not VWF fell by 24 h, following the clinical course of disease and recovery. These novel findings potentially implicate acute, regulated exocytosis of endothelial cell Weibel-Palade bodies in the pathogenesis of Plasmodium falciparum malaria.

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.

Dynein-dynactin complex mediates protein kinase A-dependent clustering of Weibel-Palade bodies in endothelial cells.

OBJECTIVE: Perinuclear clustering is observed for several different organelles and illustrates dynamic regulation of the secretory pathway and organelle distribution. Previously, we observed that a subset of Weibel-Palade bodies (WPBs), endothelial cell-specific storage organelles, undergo centralization when endothelial cells are stimulated with cAMP-raising agonists of von Willebrand factor (vWF) secretion. In this study, we investigated this phenomenon of WPB clustering in more detail. METHODS AND RESULTS: Our results demonstrate that the clustered WPBs are localized at the microtubule organizing center and that cluster formation depends on an intact microtubule network. Disruption of the microtubules by nocodazole completely abolished clustering, whereas treatment with the actin depolymerizing compound cytochalasin B had no effect on WPB clustering. Interfering with the dynein-dynactin interaction by overexpression of the p50 dynamitin subunit or the CC1 domain of the p150glued subunit of the dynactin complex completely inhibited perinuclear clustering of WPBs, suggesting that dynein activity mediates this process. Furthermore, we found that inhibition of dephosphorylation resulted in an increase in clustering, whereas inhibition of protein kinase A (PKA) markedly reduced WPB clustering. CONCLUSIONS: These results suggest that perinuclear clustering of WPBs involves PKA-dependent regulation of the dynein-dynactin complex. Endothelial cell stimulation with epinephrine results in retrograde movement of a subset of WPBs to the microtubule organizing center. This minus-end directed transport requires an intact microtubular network and is mediated by the motor protein dynein. Together, our results suggest that epinephrine-induced clustering of WPBs involves PKA-dependent regulation of the dynein-dynactin complex.

The spacer domain of ADAMTS13 contains a major binding site for antibodies in patients with thrombotic thrombocytopenic purpura.

Thrombotic thrombocytopenic purpura (TTP) is a microangiopathy often associated with a severely decreased activity of ADAMTS13. In plasma of the majority of patients with TTP, antibodies are present that inhibit the von Willebrand factor (VWF) processing activity of ADAMTS13. We describe a sensitive assay that monitors binding of recombinant ADAMTS13 to immobilized IgG derived from patient plasma. Analysis of fifteen patients with TTP and severely reduced ADAMTS13 activity revealed that in all patients antibodies directed to ADAMTS13 were present. Levels of anti-ADAMTS13 antibodies varied considerably among patients, specific antibody levels in plasma range from less than 100 ng/ml to over 1 microg/ml. Longitudinal analysis in three patients revealed that anti-ADAMTS13 antibody levels declined with different kinetics. For further characterization of anti-ADAMTS13 antibodies, we prepared a series of recombinant fragments corresponding to the various ADAMTS13 domains. All seven TTP plasma samples tested, showed reactivity of antibodies towards a fragment consisting of the disintegrin/TSR1/cysteine-rich/spacer domains. In one patient, we also observed reactivity towards the TSR2-8 repeats. No binding of antibodies to propeptide, metalloprotease and CUB domains was detected. To further delineate the binding site in the disintegrin/TSR1/cysteine-rich/spacer region, we prepared additional ADAMTS13 fragments. Antibodies directed towards the cysteine-rich/spacer fragment were found in all plasma samples analyzed. No antibodies reacting with the disintegrin/TSR1 domains were detected. A recombinant fragment comprising the spacer domain was recognized by all patients samples analyzed, suggesting that the 130-amino-acid spacer domain harbors a major binding site for anti-ADAMTS-13 antibodies.

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.

Factor VIII expression in liver disease.

Liver disease is associated with markedly elevated plasma factor VIII (FVIII) levels, whereas the synthesis of many other coagulation factors and proteins is reduced. In order to define the mechanism of FVIII increase, we have determined the expression levels of FVIII, both at mRNA and protein level, in patients with liver disease who underwent partial liver resection. In addition, the expression of von Willebrand factor (VWF) and low density lipoprotein receptor-related protein (LRP), proteins known for their ability to modulate FVIII plasma levels, were examined. Tissue samples for RNA extraction were obtained from 4 patients with cirrhosis, 9 patients with liver failure without cirrhosis and 6 patients with liver metastasis of a colon or rectum carcinoma (control group). In patients with liver cirrhosis hepatic FVIII and LRP mRNA levels were significantly lower than controls (p < or = 0.010), while VWF mRNA was significantly higher (p < or = 0.050). Immunohistochemical analysis revealed that cellular VWF protein distribution was also increased in cirrhotic livers compared to liver tissue from patients with non-cirrhotic liver disease. In cirrhotic tissue enlarged portal veins appeared to overgrow FVIII producing sinusoidal endothelial cells. Similarly, the number of LRP-producing cells appeared to be lower in cirrhotic tissue than in controls. The plasma concentration of both FVIII and VWF was significantly higher in patients with cirrhosis than control subjects (p = 0.038 and 0.010 respectively). These results demonstrate that elevated plasma FVIII levels in liver cirrhosis are associated with increased hepatic biosynthesis of VWF and decreased expression of LRP, rather than increased FVIII synthesis.

Endothelial cell activation in thrombotic thrombocytopenic purpura (TTP): a prospective analysis.

It is generally assumed that endothelial cell injury is the primary event in the pathogenesis of thrombotic thrombocytopenic purpura (TTP). In this study, we have determined the extent of vascular perturbation during acute episodes of the disease. We performed a prospective, serial study of nine patients with relapsing TTP during hospitalization and treatment, and assessed the degree of endothelial cell involvement at admission, exacerbation and remission by measurement of von Willebrand factor (VWF) and VWF-propeptide levels. Measurement of both VWF and its propeptide enabled discrimination between acute and chronic perturbation of the endothelium. Elevated levels of both VWF and propeptide were found at admission. These levels decreased immediately upon plasma exchange therapy. However, plasma VWF and propeptide concentrations did not change, even at the time of acute exacerbation. These observations suggest that endothelial cell activation is not the primary event leading to TTP. Vascular perturbation seems to be a consequence, rather than a cause, of the disease.

Von Willebrand factor targets IL-8 to Weibel-Palade bodies in an endothelial cell line.

Vascular endothelial cells are able to store the chemotactic cytokine interleukin-8 (IL-8) in specialized storage vesicles, Weibel-Palade bodies, together with von Willebrand factor (VWF) and P-selectin. We investigated whether VWF plays a role in the sorting of IL-8 into these organelles. We examined the effect of VWF expression on IL-8 targeting in an endothelial cell line (EC-RF24). This cell line has retained the typical phenotypic characteristics of primary endothelial cells but has lost the capacity to produce VWF in appreciable amounts. EC-RF24 cells were retrovirally transduced with a vector encoding a VWF-green fluorescent protein chimera (VWF-GFP). This approach enables direct visualization of the cellular distribution and secretory behavior of the VWF-GFP hybrid. Expression of VWF-GFP resulted in the generation of Weibel-Palade body-like organelles as shown by the colocalization of VWF-GFP and P-selectin. VWF-GFP expressing EC-RF24 cells also showed significant colocalization of VWF-GFP with IL-8 in these storage vesicles. Live cell imaging revealed that the number of VWF-GFP-containing granules decreased upon cell stimulation. These observations indicate that VWF plays an active role in sequestering IL-8 into Weibel-Palade bodies.

Real-time imaging of the dynamics and secretory behavior of Weibel-Palade bodies.

OBJECTIVE: Weibel-Palade bodies (WPBs) are specialized secretory granules found in endothelial cells. These vesicles store hormones, enzymes, and receptors and exhibit regulated exocytosis on cellular stimulation. Here we have directly visualized intracellular trafficking and the secretory behavior of WPBs in living cells by using a hybrid protein consisting of von Willebrand factor (vWF), a prominent WPB constituent, and green fluorescent protein (GFP). METHODS AND RESULTS: Immunofluorescence microscopy demonstrated that this chimera was targeted into WPBs. In resting cells, some WPBs seemed motionless, whereas others moved at low speed in a stochastic manner. On stimulation of cells with [Ca2+]i- or cAMP-raising secretagogues, membrane-apposed patches were formed, suggesting fusion of WPBs with the plasma membrane. Patches remained visible for >20 minutes. This sustained, membrane-associated retention of vWF might play a role in focal adhesion of blood constituents to the endothelium after vascular injury. In addition, stimulation with cAMP-raising agonists resulted in clustering of a subset of WPBs in the perinuclear region of the cell. Apparently, these WPBs escaped secretion. This feature might provide a mechanism to control regulated exocytosis. CONCLUSIONS: In conclusion, the fusion protein vWF-GFP provides a powerful tool to study, in real time, signal-mediated trafficking of WPBs.