Intracellular trafficking of factor VIII to von Willebrand factor storage granules

A new look at an old body: molecular determinants of Weibel-Palade body composition and von Willebrand factor exocytosis

Endothelial cells, forming a monolayer along blood vessels, intricately regulate vascular hemostasis, inflammatory responses, and angiogenesis. A key determinant of these functions is the controlled secretion of Weibel-Palade bodies (WPBs), which are specialized endothelial storage organelles housing a presynthesized pool of the hemostatic protein von Willebrand factor and various other hemostatic, inflammatory, angiogenic, and vasoactive mediators. This review delves into recent mechanistic insights into WPB biology, including the biogenesis that results in their unique morphology, the acquisition of intraluminal vesicles and other cargo, and the contribution of proton pumps to organelle acidification. Additionally, in light of a number of proteomic approaches to unravel the regulatory networks that control WPB formation and secretion, we provide a comprehensive overview of the WPB exocytotic machinery, including their molecular and cellular mechanisms.

Mechanisms of endothelial activation, hypercoagulation and thrombosis in COVID-19: a link with diabetes mellitus

Early since the onset of the COVID-19 pandemic, the medical and scientific community were aware of extra respiratory actions of SARS-CoV-2 infection. Endothelitis, hypercoagulation, and hypofibrinolysis were identified in COVID-19 patients as subsequent responses of endothelial dysfunction. Activation of the endothelial barrier may increase the severity of the disease and contribute to long-COVID syndrome and post-COVID sequelae. Besides, it may cause alterations in primary, secondary, and tertiary hemostasis. Importantly, these responses have been highly decisive in the evolution of infected patients also diagnosed with diabetes mellitus (DM), who showed previous endothelial dysfunction. In this review, we provide an overview of the potential triggers of endothelial activation related to COVID-19 and COVID-19 under diabetic milieu. Several mechanisms are induced by both the viral particle itself and by the subsequent immune-defensive response (i.e., NF-κB/NLRP3 inflammasome pathway, vasoactive peptides, cytokine storm, NETosis, activation of the complement system). Alterations in coagulation mediators such as factor VIII, fibrin, tissue factor, the von Willebrand factor: ADAMST-13 ratio, and the kallikrein-kinin or plasminogen-plasmin systems have been reported. Moreover, an imbalance of thrombotic and thrombolytic (tPA, PAI-I, fibrinogen) factors favors hypercoagulation and hypofibrinolysis. In the context of DM, these mechanisms can be exacerbated leading to higher loss of hemostasis. However, a series of therapeutic strategies targeting the activated endothelium such as specific antibodies or inhibitors against thrombin, key cytokines, factor X, complement system, the kallikrein-kinin system or NETosis, might represent new opportunities to address this hypercoagulable state present in COVID-19 and DM. Antidiabetics may also ameliorate endothelial dysfunction, inflammation, and platelet aggregation. By improving the microvascular pathology in COVID-19 and post-COVID subjects, the associated comorbidities and the risk of mortality could be reduced.

Shear Stress and Endothelial Mechanotransduction in Trauma Patients with Hemorrhagic Shock: Hidden Coagulopathy Pathways and Novel Therapeutic Strategies

Massive trauma remains a leading cause of death and a global public health burden. Post-traumatic coagulopathy may be present even before the onset of resuscitation, and correlates with severity of trauma. Several mechanisms have been proposed to explain the development of abnormal coagulation processes, but the heterogeneity in injuries and patient profiles makes it difficult to define a dominant mechanism. Regardless of the pattern of death, a significant role in the pathophysiology and pathogenesis of coagulopathy may be attributed to the exposure of endothelial cells to abnormal physical forces and mechanical stimuli in their local environment. In these conditions, the cellular responses are translated into biochemical signals that induce/aggravate oxidative stress, inflammation, and coagulopathy. Microvascular shear stress-induced alterations could be treated or prevented by the development and use of innovative pharmacologic strategies that effectively target shear-mediated endothelial dysfunction, including shear-responsive drug delivery systems and novel antioxidants, and by targeting the venous side of the circulation to exploit the beneficial antithrombogenic profile of venous endothelial cells.

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.

Comparison of different gene addition strategies to modify placental derived-mesenchymal stromal cells to produce FVIII

Introduction

Placenta-derived mesenchymal cells (PLCs) endogenously produce FVIII, which makes them ideally suited for cell-based fVIII gene delivery. We have previously reported that human PLCs can be efficiently modified with a lentiviral vector encoding a bioengineered, expression/secretion-optimized fVIII transgene (ET3) and durably produce clinically relevant levels of functionally active FVIII. The objective of the present study was to investigate whether CRISPR/Cas9 can be used to achieve location-specific insertion of a fVIII transgene into a genomic safe harbor, thereby eliminating the potential risks arising from the semi-random genomic integration inherent to lentiviral vectors. We hypothesized this approach would improve the safety of the PLC-based gene delivery platform and might also enhance the therapeutic effect by eliminating chromatin-related transgene silencing.

Methods

We used CRISPR/Cas9 to attempt to insert the bioengineered fVIII transgene "lcoET3" into the AAVS1 site of PLCs (CRISPR-lcoET3) and determined their subsequent levels of FVIII production, comparing results with this approach to those achieved using lentivector transduction (LV-lcoET3) and plasmid transfection (Plasmid-lcoET3). In addition, since liver-derived sinusoidal endothelial cells (LSECs) are the native site of FVIII production in the body, we also performed parallel studies in human (h)LSECs).

Results

PLCs and hLSECs can both be transduced (LV-lcoET3) with very high efficiency and produce high levels of biologically active FVIII. Surprisingly, both cell types were largely refractory to CRISPR/Cas9-mediated knockin of the lcoET3 fVIII transgene in the AAVS1 genome locus. However, successful insertion of an RFP reporter into this locus using an identical procedure suggests the failure to achieve knockin of the lcoET3 expression cassette at this site is likely a function of its large size. Importantly, using plasmids, alone or to introduce the CRISPR/Cas9 "machinery", resulted in dramatic upregulation of TLR 3, TLR 7, and BiP in PLCs, compromising their unique immune-inertness.

Discussion

Although we did not achieve our primary objective, our results validate the utility of both PLCs and hLSECs as cell-based delivery vehicles for a fVIII transgene, and they highlight the hurdles that remain to be overcome before primary human cells can be gene-edited with sufficient efficiency for use in cell-based gene therapy to treat HA.

The Heart as a Target of Vasopressin and Other Cardiovascular Peptides in Health and Cardiovascular Diseases

The automatism of cardiac pacemaker cells, which is tuned, is regulated by the autonomic nervous system (ANS) and multiple endocrine and paracrine factors, including cardiovascular peptides. The cardiovascular peptides (CPs) form a group of essential paracrine factors affecting the function of the heart and vessels. They may also be produced in other organs and penetrate to the heart via systemic circulation. The present review draws attention to the role of vasopressin (AVP) and some other cardiovascular peptides (angiotensins, oxytocin, cytokines) in the regulation of the cardiovascular system in health and cardiovascular diseases, especially in post-infarct heart failure, hypertension and cerebrovascular strokes. Vasopressin is synthesized mostly by the neuroendocrine cells of the hypothalamus. There is also evidence that it may be produced in the heart and lungs. The secretion of AVP and other CPs is markedly influenced by changes in blood volume and pressure, as well as by other disturbances, frequently occurring in cardiovascular diseases (hypoxia, pain, stress, inflammation). Myocardial infarction, hypertension and cardiovascular shock are associated with an increased secretion of AVP and altered responsiveness of the cardiovascular system to its action. The majority of experimental studies show that the administration of vasopressin during ventricular fibrillation and cardiac arrest improves resuscitation, however, the clinical studies do not present consisting results. Vasopressin cooperates with the autonomic nervous system (ANS), angiotensins, oxytocin and cytokines in the regulation of the cardiovascular system and its interaction with these regulators is altered during heart failure and hypertension. It is likely that the differences in interactions of AVP with ANS and other CPs have a significant impact on the responsiveness of the cardiovascular system to vasopressin in specific cardiovascular disorders.

Heterogeneity and reciprocity of FVIII and VWF expression, and the response to shear stress in cultured human endothelial cells

BACKGROUND

Substantial phenotypic heterogeneity exists in endothelial cells and while much of this heterogeneity results from local microenvironments, epigenetic modifications also contribute.

METHODS

Cultured human umbilical vein endothelial cells, human pulmonary microvascular endothelial cells, human hepatic sinusoidal endothelial cells, human lymphatic endothelial cells (hLECs), and two different isolations of endothelial colony forming cells (ECFCs) were assessed for levels of factor VIII (FVIII) and von Willebrand factor (VWF) RNA and protein. The intracellular location and co-localization of both proteins was evaluated with immunofluorescence microscopy and stimulated release toof FVIII and VWF from Weibel-Palade bodies (WPBs) was evaluated. Changes in expression of FVIII and VWF RNA after hLECs and ECFCs were exposed to 2 or 15 dynes/cm2 of laminar shear stress were also assessed.

RESULTS

We observed considerable heterogeneity in FVIII and VWF expression among the endothelial cells. With the exception of hLECs, FVIII RNA and protein were barely detectable in any of the endothelial cells and a reciprocal relationship between levels of FVIII and VWF appears to exist. When FVIII and VWF are co-expressed, they do not consistently co-localize in the cytoplasm. However, in hLECs where significantly higher levels of FVIII are expressed, FVIII and VWF co-localize in WPBs and are released together when stimulated. Expression of both FVIII and VWF is markedly reduced when hLECs are exposed to higher or lower levels of laminar shear stress, while in ECFCs there is a minimal response for both proteins.

CONCLUSIONS

Variable levels of FVIII and VWF RNA and protein exist in a subset of cultured human endothelial cells. Higher levels of FVIII present in hLECs co-localize with VWF and are released together when exposed to a secretagogue.

Molecular therapeutics of Hemophilia A and B

INTRODUCTION

Hemophilia A (HA) or B (HB) is an X-linked recessive disorder caused by a defect in the factor VIII (FVIII) or factor IX (FIX) gene which leads to the dysfunction of blood coagulation. Protein replacement therapy (PRT) uses recombinant proteins and plasma-derived products, which incurs high cost and inconvenience requiring routine intravenous infusions and life-time treatment. Understanding of detailed molecular mechanisms on FVIII gene function could provide innovative solutions to amend this disorder. In recent decades, gene therapeutics have advanced rapidly and a one-time cure solution has been proposed.

AREAS COVERED

This review summarizes current understanding of molecular pathways involved in blood coagulation, with emphasis on FVIII's functional role. The existing knowledge and challenges on FVIII gene expression, from transcription, translation, post-translational modification including glycosylation to protein processing and secretion, and co-factor interactions are deciphered and potential molecular interventions discussed.

EXPERT OPINION

This article reviews the potential treatment targets for HA and HB, including antibodies, small molecules and gene therapeutics, based on molecular mechanisms of FVIII biosynthesis, and further, assessing the pros and cons of these various treatment strategies. Understanding detailed FVIII protein synthesis and secretory pathways could provide exciting opportunities in identifying novel therapeutics to ameliorate hemophilia state.

High levels of Von Willebrand factor markers in COVID-19: a systematic review and meta-analysis

The SARS-CoV-2 virus has spread to all corners of the world. Thrombosis is the cause of organ failure and subsequent death in COVID-19. The pathophysiology of thrombosis in COVID-19 needs to be further explored to shed light on its downside. For this reason, this meta-analysis of Von Willebrand Factor profile (VWF: Ag, VWF: activity, VWF: RCo), ADAMTS-13, and factor VIII levels in COVID-19 was performed. To obtain data on the status of the aforementioned hemostatic factors, a systematic literature review and meta-analysis were performed on COVID-19. After reviewing the evaluation of 348 papers, 28 papers included in the meta-analysis, which was performed using STATA. The analysis showed an increase in VWF: Ag levels in COVID‐19 patients. VWF: Ac was higher in all COVID-19 patients, while it was lower in the COVID‐19 ICU patients. The pooled mean of VWF: RCO in all patients with COVID-19 was 307.94%. In subgroup analysis, VWF: RCO was significantly higher in ICU patients than in all COVID-19 patients. The pooled mean of ADAMTS-13 activity was 62.47%, and 58.42% in ICU patients. The pooled mean of factor VIII level was 275.8%, which was significantly higher in ICU patients with COVID-19 than all patients with COVID-19. Levels of VWF: Ag, VWF: activity, VWF: ristocetin, and factor VIII are increased in patients with COVID-19. The elevated levels in ICU patients with COVID-19 suggest that these markers may have prognostic value in determining the severity of COVID-19. New therapeutic programs can be developed as a result.

Investigating Optimal Autologous Cellular Platforms for Prenatal or Perinatal Factor VIII Delivery to Treat Hemophilia A

Patients with the severe form of hemophilia A (HA) present with a severe phenotype, and can suffer from life-threatening, spontaneous hemorrhaging. While prophylactic FVIII infusions have revolutionized the clinical management of HA, this treatment is short-lived, expensive, and it is not available to many A patients worldwide. In the present study, we evaluated a panel of readily available cell types for their suitability as cellular vehicles to deliver long-lasting FVIII replacement following transduction with a retroviral vector encoding a B domain-deleted human F8 transgene. Given the immune hurdles that currently plague factor replacement therapy, we focused our investigation on cell types that we deemed to be most relevant to either prenatal or very early postnatal treatment and that could, ideally, be autologously derived. Our findings identify several promising candidates for use as cell-based FVIII delivery vehicles and lay the groundwork for future mechanistic studies to delineate bottlenecks to efficient production and secretion of FVIII following genetic-modification.

Bioengineering hemophilia A-specific microvascular grafts for delivery of full-length factor VIII into the bloodstream

Hemophilia A (HA) is a bleeding disorder caused by mutations in the F8 gene encoding coagulation factor VIII (FVIII). Current treatments are based on regular infusions of FVIII concentrates throughout a patient's life. Alternatively, viral gene therapies that directly deliver F8 in vivo have shown preliminary successes. However, hurdles remain, including lack of infection specificity and the inability to deliver the full-length version of F8 due to restricted viral cargo sizes. Here, we developed an alternative nonviral ex vivo gene-therapy approach that enables the overexpression of full-length F8 in patients' endothelial cells (ECs). We first generated HA patient-specific induced pluripotent stem cells (HA-iPSCs) from urine epithelial cells and genetically modified them using a piggyBac DNA transposon system to insert multiple copies of full-length F8. We subsequently differentiated the modified HA-iPSCs into competent ECs with high efficiency, and demonstrated that the cells (termed HA-FLF8-iECs) were capable of producing high levels of FVIII. Importantly, following subcutaneous implantation into immunodeficient hemophilic (SCID-f8ko) mice, we demonstrated that HA-FLF8-iECs were able to self-assemble into vascular networks, and that the newly formed microvessels had the capacity to deliver functional FVIII directly into the bloodstream of the mice, effectively correcting the clotting deficiency. Moreover, our implant maintains cellular confinement, which reduces potential safety concerns and allows effective monitoring and reversibility. We envision that this proof-of-concept study could become the basis for a novel autologous ex vivo gene-therapy approach to treat HA.

Physiological Roles of the von Willebrand Factor-Factor VIII Interaction

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

A rat model of severe VWD by elimination of the VWF gene using CRISPR/Cas9

BACKGROUND

Von Willebrand Disease (VWD) is the most common inherited bleeding disorder, caused by quantitative and qualitative changes in von Willebrand factor (VWF). The biology of VWD, studied in canine, porcine, and murine models, differ in species-specific biology of VWF and the amenability to experimental manipulations such as phlebotomy. The factor VIII (FVIII) levels in these models are higher than in humans with type 3 VWD, suggesting functional differences between FVIII and VWF.ObjectivesTo develop a VWF knock out (VWF-/-) rat by excision of all 52 exons of the VWF locus.

METHODS

The entire VWF gene was eliminated in Sprague-Dawley (Crl:SD) rats via CRISPR/Cas9-mediated gene editing. VWF antigen (VWF:Ag), VWF propeptide, and VWF collagen IV binding (VWF:CB4) levels were determined by ELISA assays and FVIII chromogenic activity (FVIII:C) levels by chromogenic FVIII assays. Lateral tail veins were transected to measure bleeding time. VWF-/- rats were infused with FVIII-/- rat platelet poor plasma (PPP) to determine response of plasma FVIII.

RESULTS

Breeding of VWF ± rats yielded VWF-/- offspring at normal Mendelian ratios. VWF:Ag, VWF propeptide, VWF:CB4, and FVIII:C plasma levels were undetectable in VWF-/- rats. VWF-/- rats bled longer and more than VWF+/- and VWF+/+ rats when challenged. Transfusion of FVIII-deficient platelet-poor plasma induced a rapid rise in endogenous FVIII:C in VWF-/- rats.

CONCLUSION

This rat model of severe VWD due to elimination of the entire VWF gene recapitulates the severe secondary deficiency of FVIII seen in human type 3 VWD and facilitates the study of VWF and FVIII and their interactions.

A decreased and less sustained desmopressin response in hemophilia A carriers contributes to bleeding

The cause of hemophilia A carrier bleeding is not well established. Desmopressin (DDAVP), used clinically to treat or prevent bleeding, can also be used as a medical stress surrogate. This study's objective was to compare the response to DDAVP in hemophilia A carriers with that in normal control patients. Bleeding was assessed by the International Society on Thrombosis and Hemostasis Bleeding Assessment Tool (ISTH-BAT). DDAVP (0.3 μg/kg) was administered either IV or subcutaneously (SC), and blood was drawn at baseline and 1, 2, and 4 hours postadministration. Blood was assessed for factor VIII (FVIII) level, von Willebrand factor (VWF) antigen (VWF:Ag), VWF activity (VWF:RCo or VWF:GPIbM), thromboelastography (TEG), and thrombin generation assay (TGA) at all points, and for VWF propeptide (VWFpp):Ag ratio and ABO blood type at baseline. Carriers were older than control patients (median age, 34 and 21 years, respectively; P = .003) and had higher ISTH-BAT bleeding scores (median bleeding score, 8 and 0, respectively; P = .001). Carriers had a significantly reduced FVIII response to DDAVP compared with control patients (P ≤ .0001). When only carriers with normal baseline FVIII levels (n = 10) were included, carriers maintained a reduced FVIII response (P ≤ .0001). Furthermore, participants with abnormal bleeding scores had a significantly lower FVIII response to DDAVP compared with those with normal bleeding scores (P = .036). Hemophilia A carriers have a lower and less sustained FVIII response to DDAVP, suggesting an impaired ability to respond to hemostatic stress, which contributes to bleeding.

Abnormal von Willebrand factor secretion, factor VIII stabilization and thrombus dynamics in type 2N von Willebrand disease mice

Essentials Type 2N von Willebrand disease involves impaired von Willebrand factor to factor VIII binding. Type 2N von Willebrand disease mutations exhibit qualitative and mild quantitative deficiencies. Type 2N von Willebrand disease mice exhibit unstable venous hemostatic thrombi. The factor VIII-binding ability of von Willebrand factor regulates arteriole thrombosis dynamics.

SUMMARY

Background von Willebrand factor (VWF) and factor VIII (FVIII) circulate as a non-covalent complex, with VWF serving as the carrier for FVIII. VWF indirectly influences secondary hemostasis by stabilizing FVIII and transporting it to the site of primary hemostasis. Type 2N von Willebrand disease involves impaired binding of VWF to FVIII, resulting in decreased plasma levels of FVIII. Objectives In these studies, we characterize the impact of three type 2N VWD variants (R763A, R854Q, R816W) on VWF secretion, FVIII stabilization and thrombus formation in a murine model. Methods Type 2N VWD mice were generated by hydrodynamic injections of mutant murine VWF cDNAs and the influence of these variants on VWF secretion and FVIII binding was evaluated. In vivo hemostasis and the dynamics of thrombus formation and embolization were assessed using a murine tail vein transection hemostasis model and an intravital thrombosis model in the cremaster arterioles. Results Type 2N VWD variants were associated with decreased VWF secretion using cell and animal-based models. FVIII-binding to type 2N variants was impaired in vitro and was variably stabilized in vivo by expressed or infused 2N variant VWF protein. Both transgenic type 2N VWD and FVIII knockout (KO) mice demonstrated impaired thrombus formation associated with decreased thrombus stability. Conclusions The type 2N VWD phenotype can be recapitulated in a murine model and is associated with both quantitative and qualitative VWF deficiencies and impaired thrombus formation. Patients with type 2N VWD may have normal primary hemostasis formation but decreased thrombus stability related to ineffective secondary hemostasis.

Factor VIII/V C-domain swaps reveal discrete C-domain roles in factor VIII function and intracellular trafficking

Factor VIII C-domains are believed to have specific functions in cofactor activity and in interactions with von Willebrand factor. We have previously shown that factor VIII is co-targeted with von Willebrand factor to the Weibel-Palade bodies in blood outgrowth endothelial cells, even when factor VIII carries mutations in the light chain that are associated with defective von Willebrand factor binding. In this study, we addressed the contribution of individual factor VIII C-domains in intracellular targeting, von Willebrand factor binding and cofactor activity by factor VIII/V C-domain swapping. Blood outgrowth endothelial cells were transduced with lentivirus encoding factor V, factor VIII or YFP-tagged C-domain chimeras, and examined by confocal microscopy. The same chimeras were produced in HEK293-cells for in vitro characterization and chemical foot-printing by mass spectrometry. In contrast to factor VIII, factor V did not target to Weibel-Palade bodies. The chimeras showed reduced Weibel-Palade body targeting, suggesting that this requires the factor VIII C1–C2 region. The factor VIII/V-C1 chimera did not bind von Willebrand factor and had reduced affinity for activated factor IX, whereas the factor VIII/V-C2 chimera showed a minor reduction in von Willebrand factor binding and normal interaction with activated factor IX. This suggests that mainly the C1-domain carries factor VIII-specific features in assembly with von Willebrand factor and activated factor IX. Foot-printing analysis of the chimeras revealed increased exposure of lysine residues in the A1/C2- and C1/C2-domain interface, suggesting increased C2-domain mobility and disruption of the natural C-domain tandem pair orientation. Apparently, this affects intracellular trafficking, but not extracellular function.

Life in the shadow of a dominant partner: the FVIII-VWF association and its clinical implications for hemophilia A

A normal hemostatic response to vascular injury requires both factor VIII (FVIII) and von Willebrand factor (VWF). In plasma, VWF and FVIII normally circulate as a noncovalent complex, and each has a critical function in the maintenance of hemostasis. Furthermore, the interaction between VWF and FVIII plays a crucial role in FVIII function, immunogenicity, and clearance, with VWF essentially serving as a chaperone for FVIII. Several novel recombinant FVIII (rFVIII) therapies for hemophilia A have been in clinical development, which aim to increase the half-life of FVIII (∼12 hours) and reduce dosing frequency by utilizing bioengineering techniques including PEGylation, Fc fusion, and single-chain design. However, these approaches have achieved only moderate increases in half-life of 1.5- to 2-fold compared with marketed FVIII products. Clearance of PEGylated rFVIII, rFVIIIFc, and rVIII-SingleChain is still regulated to a large extent by interaction with VWF. Therefore, the half-life of VWF (∼15 hours) appears to be the limiting factor that has confounded attempts to extend the half-life of rFVIII. A greater understanding of the interaction between FVIII and VWF is required to drive novel bioengineering strategies for products that either prolong the survival of VWF or limit VWF-mediated clearance of FVIII.

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.

New approaches to gene and cell therapy for hemophilia

Hemophilia is considered suitable for gene therapy because it is caused by a single gene abnormality, and therapeutic coagulation factor levels may vary across a broad range. Recent success of hemophilia B gene therapy with an adeno-associated virus (AAV) vector in a clinical trial showed the real prospect that, through gene therapy, a cure for hemophilia may become a reality. However, AAV-mediated gene therapy is not applicable to patients with hemophilia A at present, and neutralizing antibodies against AAV reduce the efficacy of AAV-mediated strategies. Because patients that benefit from AAV treatment (hemophilia B without neutralizing antibodies) are estimated to represent only 15% of total patients with hemophilia, the development of basic technologies for hemophilia A and those that result in higher therapeutic effects are critical. In this review, we present an outline of gene therapy methods for hemophilia, including the transition of technical developments thus far and our novel techniques.

Novel investigations on the protective role of the FVIII/VWF complex in inhibitor development

Development of inhibitory antibodies to infused factor VIII (FVIII) concentrates is the most serious unresolved complication of haemophilia A treatment. Systematic reviews suggest a twofold higher incidence of inhibitors with recombinant (rFVIII) vs. plasma-derived (pdFVIII) FVIII products, but study methodologies vary widely. The lower immunogenicity of pdFVIII concentrates is believed to derive from the presence of von Willebrand factor (VWF) which acts as protector and chaperone for FVIII. Several novel investigations reinforce the protective role of the VWF/FVIII complex in inhibitor development. At the basic science level, numerous in vitro and in vivo experiments have demonstrated that VWF-containing pdFVIII concentrates (pdFVIII/VWF) provide better protection against inhibitor neutralization than rFVIII products. Conformational aspects of the binding between VWF and FVIII are thought to prevent the 'attack' on FVIII by inhibitory antibodies. VWF/FVIII binding is 100% in pdFVIII products but only 80% in recombinant products and this 'free' FVIII may be a target for inhibitory antibodies. At the clinical level, newer strategies to prevent inhibitor development in previously untreated patients with severe haemophilia are under investigation. The concept of early prophylaxis (before the onset of a bleed) is convincing from a theoretical point of view but requires further evaluation. The Study on Inhibitors in Plasma-Product Exposed Toddlers is specifically addressing the issue of relative immunogenicity between classes of FVIII product (recombinant vs. plasma-derived). Currently nearing its target enrolment of 300 patients, this international randomized controlled trial is expected to provide some definitive answers about this ever-present clinical dilemma.

Platelet-targeted gene therapy with human factor VIII establishes haemostasis in dogs with haemophilia A

It is essential to improve therapies for controlling excessive bleeding in patients with haemorrhagic disorders. As activated blood platelets mediate the primary response to vascular injury, we hypothesize that storage of coagulation Factor VIII within platelets may provide a locally inducible treatment to maintain haemostasis for haemophilia A. Here we show that haematopoietic stem cell gene therapy can prevent the occurrence of severe bleeding episodes in dogs with haemophilia A for at least 2.5 years after transplantation. We employ a clinically relevant strategy based on a lentiviral vector encoding the ITGA2B gene promoter, which drives platelet-specific expression of human FVIII permitting storage and release of FVIII from activated platelets. One animal receives a hybrid molecule of FVIII fused to the von Willebrand Factor propeptide-D2 domain that traffics FVIII more effectively into α-granules. The absence of inhibitory antibodies to platelet-derived FVIII indicates that this approach may have benefit in patients who reject FVIII replacement therapies. Thus, platelet FVIII may provide effective long-term control of bleeding in patients with haemophilia A.

Update on von Willebrand factor multimers: focus on high-molecular-weight multimers and their role in hemostasis

Normal hemostasis requires von Willebrand factor (VWF) to support platelet adhesion and aggregation at sites of vascular injury. VWF is a multimeric glycoprotein built from identical subunits that contain binding sites for both platelet glycoprotein receptors and collagen. The adhesive activity of VWF depends on the size of its multimers, which range from 500 to over 10 000 kDa. There is good evidence that the high-molecular-weight multimers (HMWM), which are 5000-10 000 kDa, are the most effective in supporting interaction with collagen and platelet receptors and in facilitating wound healing under conditions of shear stress. Thus, these HMWM of VWF are of particular clinical interest. The unusually large multimers of VWF are, under normal conditions, cleaved by the plasma metalloproteinase ADAMTS13 to smaller, less adhesive multimers. A reduction or lack of HMWM, owing to a multimerization defect of VWF or to an increased susceptibility of VWF for ADAMTS13, leads to a functionally impaired VWF and the particular type 2A of von Willebrand disease. This review considers the biology and function of VWF multimers with a particular focus on the characterization of HMWM - their production, storage, release, degradation, and role in normal physiology. Evidence from basic research and the study of clinical diseases and their management highlight a pivotal role for the HMWM of VWF in hemostasis.

Mesenchymal stem cells contribute to endogenous FVIII:c production

Besides the liver, it has been difficult to identify which organ(s) and/or cellular component(s) contribute significantly to the production of human FVIII:c (FVIII). Thus far, only endothelial cells have been shown to constitute a robust extrahepatic source of FVIII, possibly explaining both the diverse presence of FVIII mRNA in the body, and the observed increase in FVIII levels during liver failure. Here, we investigate whether human mesenchymal stem cells (MSC), ubiquitously present in different organs, could also contribute to FVIII production. MSC isolated from human lung, liver, brain, and bone marrow expressed FVIII message as determined by quantitative-RT-PCR. Using an antibody specific for FVIII, confocal microscopy, and umbilical cord-derived endothelial cells (HUVEC) as a negative control, we demonstrated that, in MSC, FVIII protein was not stored in granules; rather, it localized to the perinuclear region. Furthermore, functional FVIII was detected in MSC supernatants and cell lysates by aPTT and chromogenic assays. These results demonstrate that MSC can contribute at low levels to the functional FVIII pool, and advance the understanding of the physiology of FVIII production and secretion.

Platelet and endothelial expression of clotting factors for the treatment of hemophilia

Hemostasis is achieved by the coordinate interaction of plasma, platelets, and vascular endothelium. Coagulation factors circulate in plasma with synthesis in liver and in endothelium. Interaction between Factor VIII (FVIII) and von Willebrand factor (VWF) in plasma is critically important, but there remains some question about whether this relationship is first established within the endothelial cell or in plasma. When FVIII is expressed with VWF in a cell that stores VWF, FVIII will also be stored and released. The manuscript will summarize some studies in which gene therapy exploits this relationship between VWF and FVIII to achieve hemostasis even in the presence of circulating inhibitory antibodies to FVIII. VWF is critical to this efficacy in the presence of inhibitors. Since FIX expression in platelets is effective for hemophilia B, efficacy in the presence of inhibitory antibodies to FIX was not achieved and emphasized the importance of VWF to the efficacy of platelet FVIII expression. These approaches have been studied in murine models but will need further study before this approach can be attempted clinically.

Ectopic platelet-delivered factor (F) VIII for the treatment of Hemophilia A: Plasma and platelet FVIII, is it all the same?

Hemophilia A is the most common inherited bleeding diathesis and is due to a deficiency of functional coagulation factor (F) VIII. Most patients have a severe deficiency and require a program of prophylactic plus acute infusions of recombinant FVIII to prevent significant joint and other target organ damage. One of the greatest challenges remaining in the care of these patients is that one fifth to third of the patients develop inhibitors to the infused proteins. While a significant portion of such inhibitors can be either overcome or the inhibitors eliminated, some patients with persistent and significant titers of inhibitors need to rely on second tier therapies that are not as effective at preventing significant bleeding morbidity or mortality. A number of groups have been developing therapeutic strategies for FVIII gene therapy for this disorder. Virtually all of these therapies have in common a rise in the plasma level of FVIII, and interpretation of their efficacy is straightforward related to levels achieved. However, several groups have also shown that FVIII can be ectopically expressed in developing megakaryocytes, where although plasma FVIII levels remain undetectable, this FVIII can be released and be effective at sites of platelet activation. Moreover, it is clear that this platelet (p) FVIII is protected to a degree from inhibitors, making pFVIII a particularly attractive strategy for gene therapy for hemophilia A. Yet at the same time, we have shown that pFVIII has a different availability and distribution in a growing thrombus than plasma FVIII. The clinical implications and challenges of these findings as murine and canine hemophilia A preclinical studies go forward with pFVIII are discussed.

Factor VIII alters tubular organization and functional properties of von Willebrand factor stored in Weibel-Palade bodies

In endothelial cells, von Willebrand factor (VWF) multimers are packaged into tubules that direct biogenesis of elongated Weibel-Palade bodies (WPBs). WPB release results in unfurling of VWF tubules and assembly into strings that serve to recruit platelets. By confocal microscopy, we have previously observed a rounded morphology of WPBs in blood outgrowth endothelial cells transduced to express factor VIII (FVIII). Using correlative light-electron microscopy and tomography, we now demonstrate that FVIII-containing WPBs have disorganized, short VWF tubules. Whereas normal FVIII and FVIII Y1680F interfered with formation of ultra-large VWF multimers, release of the WPBs resulted in VWF strings of equal length as those from nontransduced blood outgrowth endothelial cells. After release, both WPB-derived FVIII and FVIII Y1680F remained bound to VWF strings, which however had largely lost their ability to recruit platelets. Strings from nontransduced cells, however, were capable of simultaneously recruiting exogenous FVIII and platelets. These findings suggest that the interaction of FVIII with VWF during WPB formation is independent of Y1680, is maintained after WPB release in FVIII-covered VWF strings, and impairs recruitment of platelets. Apparently, intra-cellular and extracellular assembly of FVIII-VWF complex involves distinct mechanisms, which differ with regard to their implications for platelet binding to released VWF strings.

Storage of factor VIII variants with impaired von Willebrand factor binding in Weibel-Palade bodies in endothelial cells

BACKGROUND

Point mutations resulting in reduced factor VIII (FVIII) binding to von Willebrand factor (VWF) are an important cause of mild/moderate hemophilia A. Treatment includes desmopressin infusion, which concomitantly increases VWF and FVIII plasma levels, apparently from storage pools containing both proteins. The source of these VWF/FVIII co-storage pools and the mechanism of granule biogenesis are not fully understood.

METHODOLOGY/PRINCIPAL FINDINGS

We studied intracellular trafficking of FVIII variants implicated in mild/moderate hemophilia A together with VWF in HEK293 cells and primary endothelial cells. The role of VWF binding was addressed using FVIII variants displaying reduced VWF interaction. Binding studies using purified FVIII proteins revealed moderate (Arg2150His, Del2201, Pro2300Ser) to severe (Tyr1680Phe, Ser2119Tyr) VWF binding defects. Expression studies in HEK293 cells and primary endothelial cells revealed that all FVIII variants were present within VWF-containing organelles. Quantitative studies showed that the relative amount of FVIII storage was independent of various mutations. Substantial amounts of FVIII variants are co-stored in VWF-containing storage organelles, presumably by virtue of their ability to interact with VWF at low pH.

CONCLUSIONS

Our data suggest that the potential of FVIII co-storage with VWF is not affected in mild/moderate hemophilia A caused by reduced FVIII/VWF interaction in the circulation. These data support the hypothesis that Weibel-Palade bodies comprise the desmopressin-releasable FVIII storage pool in vivo.

Functional architecture of Weibel-Palade bodies

Weibel-Palade bodies (WPBs) are elongated secretory organelles specific to endothelial cells that contain von Willebrand factor (VWF) and a variety of other proteins that contribute to inflammation, angiogenesis, and tissue repair. The remarkable architecture of WPBs is because of the unique properties of their major constituent VWF. VWF is stored inside WPBs as tubules, but on its release, forms strikingly long strings that arrest bleeding by recruiting blood platelets to sites of vascular injury. In recent years considerable progress has been made regarding the molecular events that underlie the packaging of VWF multimers into tubules and the processes leading to the formation of elongated WPBs. Mechanisms directing the conversion of tightly packaged VWF tubules into VWF strings on the surface of endothelial cells are starting to be unraveled. Several modes of exocytosis have now been described for WPBs, emphasizing the plasticity of these organelles. WPB exocytosis plays a role in the pathophysiology and treatment of von Willebrand disease and may have impact on common hematologic and cardiovascular disorders. This review summarizes the major advances made on the biogenesis and exocytosis of WPBs and places these recent discoveries in the context of von Willebrand disease.

Targeting FVIII expression to endothelial cells regenerates a releasable pool of FVIII and restores hemostasis in a mouse model of hemophilia A

The natural cell type(s) that synthesize and release factor VIII (FVIII) into the circulation are still not known with certainty. In vitro studies indicate that artificial expression of FVIII in endothelial cells produces an intracellular pool of FVIII that can be mobilized together with its carrier protein, von Willebrand factor (VWF), by agonists. Here, we show that expression of human B-domain deleted FVIII (hFVIII) in the vascular endothelium of otherwise FVIII-deficient mice results in costorage of FVIII and VWF in endothelial Weibel-Palade bodies and restores normal levels and activity of FVIII in plasma. Stored FVIII was mobilized into the circulation by subcutaneous administration of epinephrine. Human FVIII activity in plasma was strictly dependent on the presence of VWF. Endothelial-specific expression of hFVIII rescued the bleeding diathesis of hemophilic mice lacking endogenous FVIII. This hemostatic function of endothelial cell-derived hFVIII was suppressed in the presence of anti-FVIII inhibitory antibodies. These results suggest that targeting FVIII expression to endothelial cells may establish a releasable pool of FVIII and normalize plasma FVIII level and activity in hemophilia A, but does not prevent the inhibitory effect of anti-FVIII antibodies on the hemostatic function of transgene-derived hFVIII as is seen with platelet-derived FVIII expression.

A revised model for the secretion of tPA and cytokines from cultured endothelial cells

Endothelial cells are reported to contain several distinct populations of regulated secretory organelles, including Weibel-Palade bodies (WPBs), the tissue plasminogen activator (tPA) organelle, and the type-2 chemokine-containing organelle. We show that the tPA and type-2 organelles in human endothelial cells represent a single compartment primarily responsible for unstimulated secretion of tPA or, in cells exposed to interleukin-1β (IL-1β), the cytokines IL-8, IL-6, monocyte chemoattractant protein-1 (MCP-1), and growth-regulated oncogene-α (GRO-α). This compartment was distinct from WPBs in that it lacked detectable von Willebrand factor, P-selectin, Rab27a, or CD63 immunoreactivity, displayed no time-dependent decrease in intragranule pH, underwent detectable unstimulated exocytosis, and was very poorly responsive to Ca(2+)-elevating secretagogues. WPBs could also contain tPA, and in IL-1β-treated cells, IL-8, IL-6, MCP-1, and GRO-α, and were the primary source for histamine or ionomycin-stimulated secretion of these molecules. However, analysis of the storage efficiency of cytokines and tPA revealed that all were very poorly stored compared with von Willebrand factor. The nonmammalian, nonsecretory protein EGFP, when expressed in the secretory pathway, also entered WPBs and had a storage efficiency similar to tPA and the cytokines tested. Based on these data, we proposed a revised model for storage and secretion of cytokines and tPA.

Activation of human endothelial cells from specific vascular beds induces the release of a FVIII storage pool

Although the liver is known to be the main site of factor VIII (FVIII) production, other organs are probably also important for the regulation of FVIII secretion. However, the study of the regulation of extrahepatic FVIII production has been hampered by the lack of definitive identification of human tissues able to secrete FVIII. Recent studies have shown that lung endothelial cells can synthesize FVIII. We therefore studied the production of FVIII by endothelial cells purified from other vascular beds. Because physiologic stress results in a rapid elevation of FVIII, we also investigated whether endothelial cells can store FVIII and secrete it after treatment with agonists. Microvascular endothelial cells from lung, heart, intestine, and skin as well as endothelial cells from pulmonary artery constitutively secreted FVIII and released it after treatment with phorbol-myristate acetate and epinephrine. By contrast, endothelial cells from the aorta, umbilical artery and umbilical vein did not constitutively secrete FVIII or release it after treatment with agonists, probably because of a lack of FVIII synthesis. Extrahepatic endothelial cells from certain vascular beds therefore appear to be an important FVIII production and storage site with the potential to regulate FVIII secretion in chronic and acute conditions.

Alternative strategies for gene therapy of hemophilia

Hemophilia A and B are monogenic disorders that were felt to be ideal targets for initiation of gene therapy. Although the first hemophilia gene therapy trial has been over 10 years ago, few trials are currently actively recruiting. Although preclinical studies in animals were promising, levels achieved in humans did not achieve long-term expression at adequate levels to achieve cures. Transplantation as a source of cellular replacement therapy for both hemophilia A and B have been successful following liver transplantation in which the recipient produces normal levels of either factor VIII (FVIII) or factor IX (FIX). Most of these transplants have been conducted for the treatment of liver failure rather than for "curing" hemophilia. There are a variety of new strategies for delivering the missing clotting factor through ectopic expression of the deficient protein. One approach uses hematopoietic stem cells using either a nonspecific promoter or using a lineage-specific promoter. An alternative strategy includes enhanced expression in endothelial cells or blood-outgrowth endothelial cells. An additional approach includes the expression of FVIII or FIX intraarticularly to mitigate the intraarticular bleeding that causes much of the disability for hemophilia patients. Because activated factor VII (FVIIa) can be used to treat patients with inhibitory antibodies to replacement clotting factors, preclinical gene therapy has been performed using platelet- or liver-targeted FVIIa expression. All of these newer approaches are just beginning to be explored in large animal models. Whereas improved recombinant replacement products continue to be the hallmark of hemophilia therapy, the frequency of replacement therapy is beginning to be addressed through longer-acting replacement products. A safe cure of hemophilia is still the desired goal, but many barriers must still be overcome.

Microenvironmental regulation of the sinusoidal endothelial cell phenotype in vitro

Liver sinusoidal endothelial cells (LSECs) differ, both structurally and functionally, from endothelial cells (ECs) lining blood vessels of other tissues. For example, in contrast to other ECs, LSECs possess fenestrations, have low detectable levels of platelet endothelial cell adhesion molecule 1 expression, and in rat tissue, they distinctively express a cell surface marker recognized by the SE-1 antibody. These unique phenotypic characteristics seen in hepatic tissue are lost over time upon culture in vitro; therefore, this study sought to systematically examine the effects of microenvironmental stimuli--namely, extracellular matrix and neighboring cells, on the LSEC phenotype in vitro. In probing the role of the underlying extracellular matrix, we identified collagen I and collagen III as well as mixtures of collagen I/collagen IV/fibronectin as having a positive effect on LSEC survival. Furthermore, using a stable hepatocellular model (hepatocyte-fibroblast) we were able to prolong the expression of both SE-1 and phenotypic functions of LSEC such as factor VIII activity and AcLOL uptake in cocultured LSECs through the production of short-range paracrine signals. In the course of these experiments, we identified the antigen recognized by SE-1 as CD32b.

CONCLUSION

Collectively, this study has identified several microenvironmental regulators of liver sinusoidal endothelial cells that prolong their phenotypic functions for up to 2 weeks in culture, enabling the development of better in vitro models of liver physiology and disease.

Functional and ultrastructural analysis of endothelial-like cells derived from bone marrow stromal cells

BACKGROUND

Recent studies have suggested that bone marrow stromal cells (BMSC) have the potential to differentiate into endothelial cells. However, the physiologic functions of the endothelial-like cells derived from BMSC have not been well studied.

METHODS

Human BMSC were induced to differentiate into endothelial-like cells with a combination of cytokines. Morphologic, phenotypic, ultrastructural and functional characterizations of the endothelial-like cells were made.

RESULTS

Human BMSC were successfully differentiated into cells with endothelial-like morphology and phenotype in vitro. These cells expressed various endothelial cell functions in vitro, such as release of von Willebrand factor (vWF) mediated by histamine, acetylated low-density lipoprotein (acLDL) uptake, binding of Ulex europaeus agglutinin-1 (UEA-1) and in vitro capillary formation. The cells also acquired important ultrastructural and physiologic properties of endothelial cells as they contained Weibel-Palade bodies, abundant mitochondria with a homogeneous mitochondrial matrix, diluted rough endoplasmic reticula, enlarged Golgi complexes, a regular arrangement of microfilaments and many surface cytoplasmic processes and plasmalemmal vesicles, as well as intercellular tight junctions and desmosome-like structures. Subcutaneous implantation of the endothelial-like cells in Matrigel plugs in immunodeficient mice resulted in the formation of functional blood vessels that contained erythrocytes. Moreover, these cells contributed to in vivo neovascularization during wound healing in rabbit ischemic hindlimb models.

DISCUSSION

Physiologic features of the endothelial-like cells derived from BMSC suggest the potential use of these cells as a functional cell source for therapeutic applications.

Factor VIII and von Willebrand factor interaction: biological, clinical and therapeutic importance

The interaction of factor VIII (FVIII) with von Willebrand Factor (VWF) is of direct clinical significance in the diagnosis and treatment of patients with haemophilia A and von Willebrand disease (VWD). A normal haemostatic response to vascular injury requires both FVIII and VWF. It is well-established that in addition to its role in mediating platelet to platelet and platelet to matrix binding, VWF has a direct role in thrombin and fibrin generation by acting as a carrier molecule for the cofactor FVIII. Recent studies show that the interaction affects not only the biology of both FVIII and VWF, and the pathology of haemophilia and VWD, but also presents opportunities in the treatment of haemophilia. This review details the mechanisms and the molecular determinants of FVIII interaction with VWF, and the role of FVIII-VWF interaction in modulating FVIII interactions with other proteases, cell types and cellular receptors. The effect of defective interaction of FVIII with VWF as a result of mutations in either protein is discussed.

Storage and regulated secretion of factor VIII in blood outgrowth endothelial cells

BACKGROUND

Gene therapy provides an attractive alternative for protein replacement therapy in hemophilia A patients. Recent studies have shown the potential benefit of directing factor (F)VIII gene delivery to cells that also express its natural carrier protein von Willebrand factor (VWF). In this study, we explored the feasibility of blood outgrowth endothelial cells as a cellular FVIII delivery device with particular reference to long-term production levels, intracellular storage in Weibel-Palade bodies and agonist-induced regulated secretion.

DESIGN AND METHODS

Human blood outgrowth endothelial cells were isolated from peripheral blood collected from healthy donors, transduced at passage 5 using a lentiviral vector encoding human B-domain deleted FVIII-GFP and characterized by flow cytometry and confocal microscopy.

RESULTS

Blood outgrowth endothelial cells displayed typical endothelial morphology and expressed the endothelial-specific marker VWF. Following transduction with a lentivirus encoding FVIII-GFP, 80% of transduced blood outgrowth endothelial cells expressed FVIII-GFP. Levels of FVIII-GFP positive cells declined slowly upon prolonged culturing. Transduced blood outgrowth endothelial cells expressed 1.6+/-1.0 pmol/1 x 10(6) cells/24h FVIII. Morphological analysis demonstrated that FVIII-GFP was stored in Weibel-Palade bodies together with VWF and P-selectin. FVIII levels were only slightly increased following agonist-induced stimulation, whereas a 6- to 8-fold increase of VWF levels was observed. Subcellular fractionation revealed that 15-22% of FVIII antigen was present within the dense fraction containing Weibel-Palade bodies.

CONCLUSIONS

We conclude that blood outgrowth endothelial cells, by virtue of their ability to store a significant portion of synthesized FVIII-GFP in Weibel-Palade bodies, provide an attractive cellular on-demand delivery device for gene therapy of hemophilia A.

Intracellular cotrafficking of factor VIII and von Willebrand factor type 2N variants to storage organelles

Weibel-Palade bodies (WPBs) are the endothelial storage organelles that are formed upon von Willebrand factor (VWF) expression. Apart from VWF, WPBs contain a variety of hemostatic and inflammatory proteins. Some of these are thought to be targeted to WPBs by directly interacting with VWF in the secretory pathway. Previous studies have demonstrated that coexpression of factor VIII (FVIII) with VWF results in costorage of both proteins. However, whether cotrafficking is driven by intracellular FVIII-VWF assembly has remained unclear. We now have addressed this issue using recombinant VWF type 2N variants that are known to display reduced FVIII binding in the circulation. Binding studies using purified fluorescent FVIII and VWF type 2N variants revealed FVIII binding defects varying from moderate (Arg854Gln, Cys1060Arg) to severe (Arg763Gly, Thr791Met, Arg816Trp). Upon expression in HEK293 cells, all VWF variants induced formation of WPB-like organelles that were able to recruit P-selectin, as well as FVIII. WPBs containing FVIII did not display their typical elongated shape, suggesting that FVIII affects the organization of VWF tubules therein. The finding that VWF type 2N variants are still capable of cotargeting FVIII to storage granules implies that trafficking of WPB cargo proteins does not necessarily require high-affinity assembly with VWF.

The use of desmopressin in von Willebrand disease: the experience of the first 30 years (1977-2007)

The aim of the treatment for von Willebrand disease (VWD) is to correct the dual defect of haemostasis, i.e. the abnormal platelet adhesion as a result of reduced and/or dysfunctional von Willebrand factor (VWF) and the abnormal coagulation expressed by low levels of factor VIII (FVIII). Correction of both deficiencies can be achieved by administering the synthetic peptide desmopressin (DDAVP) or, in cases unresponsive to this agent, the plasma concentrates containing VWF and FVIII (VWF/FVIII). DDAVP is the treatment of choice for type 1 VWD because it can induce release of normal VWF from cellular compartments, but the drug can be clinically useful also in other VWD types, including acquired von Willebrand syndrome (AVWS). A test dose of DDAVP at the time of diagnosis is recommended to establish the individual patterns of biological response and to predict clinical efficacy during bleeding and surgery. DDAVP is not effective in VWD type 3 and in severe forms of VWD 1 and 2. It can induce transient thrombocytopenia in patients with VWD type 2B. The results of several retrospective studies on the use of DDAVP in VWD management have been reported by many authors in different countries for the last 30 years. However, despite the widespread use of DDAVP in the treatment of VWD, there are only a few prospective clinical trials in a large number of cases on DDAVP efficacy and safety aimed at determining benefits and limits of this therapeutic approach. An investigator-driven observational prospective study on clinical efficacy of DDAVP in 200 patients with VWD types 1 and 2 has been recently organized: the effectiveness and safety of DDAVP will be evaluated prospectively for 24 months during bleeding episodes and minor or major surgeries in the VWD patients who were exposed to an infusion trial at enrollment.

Syngeneic transplantation of hematopoietic stem cells that are genetically modified to express factor VIII in platelets restores hemostasis to hemophilia A mice with preexisting FVIII immunity

Although genetic induction of factor VIII (FVIII) expression in platelets can restore hemostasis in hemophilia A mice, this approach has not been studied in the clinical setting of preexisting FVIII inhibitory antibodies to determine whether such antibodies would affect therapeutic engraftment. We generated a line of transgenic mice (2bF8) that express FVIII only in platelets using the platelet-specific alphaIIb promoter and bred this 2bF8 transgene into a FVIII(null) background. Bone marrow (BM) from heterozygous 2bF8 transgenic (2bF8(tg+/-)) mice was transplanted into immunized FVIII(null) mice after lethal or sublethal irradiation. After BM reconstitution, 85% of recipients survived tail clipping when the 1100-cGy (myeloablative) regimen was used, 85.7% of recipients survived when 660-cGy (nonmyeloablative) regimens were used, and 60% of recipients survived when the recipients were conditioned with 440 cGy. Our further studies showed that transplantation with 1% to 5% 2bF8(tg+/-) BM cells still improved hemostasis in hemophilia A mice with inhibitors. These results demonstrate that the presence of FVIII-specific immunity in recipients does not negate engraftment of 2bF8 genetically modified hematopoietic stem cells, and transplantation of these hematopoietic stem cells can efficiently restore hemostasis to hemophilic mice with preexisting inhibitory antibodies under either myeloablative or nonmyeloablative regimens.

Requirements for cellular co-trafficking of factor VIII and von Willebrand factor to Weibel-Palade bodies

BACKGROUND

von Willebrand factor (VWF) serves a critical role as a carrier of factor (F)VIII in circulation. While it is generally believed that FVIII and VWF assemble in circulation after secretion from different cells, an alternative view is that cells should exist that co-express FVIII and VWF.

OBJECTIVES

In this study, intracellular co-expression of FVIII and VWF was studied, with particular reference to complex assembly and high-affinity interaction.

METHODS

Using yellow fluorescent protein-tagged FVIII (FVIII-YFP) and cyan fluorescent protein-tagged VWF (VWF-CFP), we studied intracellular trafficking in human embryonic kidney (HEK293) cells and human umbilical vein endothelial cells (HUVEC). The role of the high-affinity interaction between FVIII and VWF was assessed using a FVIII-YFP variant carrying a Tyr1680Phe substitution, which abolishes high-affinity binding to VWF. Cellular trafficking studies were complemented by binding studies employing purified proteins.

RESULTS

Solid phase binding assays employing FVIII-YFP demonstrated that the presence of the fluorescent moiety did not compromise high-affinity binding (K(d) = 0.065 +/- 0.008 nm) whereas the binding of the Tyr1680Phe FVIII-YFP variant was significantly reduced. Co-expression studies in HEK293 cells revealed intracellular co-storage of both FVIII-YFP and Tyr1680Phe FVIII-YFP within VWF-containing storage organelles. In addition, expression of FVIII-YFP and Tyr1680Phe FVIII-YFP in HUVEC demonstrated co-trafficking with endogenous VWF to authentic Weibel-Palade bodies (WPBs).

CONCLUSIONS

Our findings demonstrate that FVIII trafficking to WPBs is independent of Tyr1680 and high-affinity binding to VWF. We therefore conclude that the structural requirements that determine intracellular co-trafficking differ from those that determine complex assembly in circulation.

Lentivirus-mediated platelet-derived factor VIII gene therapy in murine haemophilia A

BACKGROUND

Previous studies from our laboratory have demonstrated that lineage-targeted synthesis of factor VIII (FVIII) under the direction of the platelet-specific integrin alphaIIb gene promoter (2bF8) can correct the murine haemophilia A phenotype even in the presence of high titer inhibitory antibodies in a transgenic mouse model.

OBJECTIVE

In this study, we assessed the efficacy of using a genetic therapy approach to correct haemophilia A in FVIII-deficient (FVIII(null)) mice by transplantation of bone marrow (BM) transduced with a lentivirus (LV)-based gene transfer cassette encoding 2bF8.

RESULTS

Functional FVIII activity (FVIII:C) was detected in platelet lysates from treated mice and the levels were similar to 2bF8 heterozygous transgenic mice. Mice transplanted with 2bF8 LV-transduced BM survived tail clipping and we did not detected inhibitory or non-inhibitory FVIII antibodies over the period of this study (11 months). Furthermore, BM transferred from the primary transplant recipients into FVIII(null) secondary recipients demonstrated sustained platelet-FVIII expression leading to correction of the haemophilia A phenotype showing that gene transfer occurred within long-term repopulating haematopoietic stem cells.

CONCLUSIONS

These results demonstrate that ectopic expression of FVIII in platelets by lentivirus-mediated bone marrow transduction/transplantation may be a promising strategy for gene therapy of haemophilia A in humans.

Endothelial Differentiation Potential of Human Monocyte-Derived Multipotential Cells

We previously reported a unique CD14(+)CD45(+)CD34(+) type I collagen(+) cell fraction derived from human circulating CD14(+) monocytes, named monocyte-derived multipotential cells (MOMCs). This primitive cell population contains progenitors capable of differentiating along the mesenchymal and neuronal lineages. Here, we investigated whether MOMCs can also differentiate along the endothelial lineage. MOMCs treated with angiogenic growth factors for 7 days changed morphologically and adopted a caudate appearance with rod-shaped microtubulated structures resembling Weibel-Palade bodies. Almost every cell expressed CD31, CD144, vascular endothelial growth factor (VEGF) type 1 and 2 receptors, Tie-2, von Willebrand factor (vWF), endothelial nitric-oxide synthase, and CD146, but CD14/CD45 expression was markedly downregulated. Under these culture conditions, the MOMCs continued to proliferate for up to 7 days. Functional characteristics, including vWF release upon histamine stimulation and upregulated expression of VEGF and VEGF type 1 receptor in response to hypoxia, were indistinguishable between the MOMC-derived endothelial-like cells and cultured mature endothelial cells. The MOMCs responded to angiogenic stimuli and promoted the formation of mature endothelial cell tubules in Matrigel cultures. Finally, in xenogenic transplantation studies using a severe combined immunodeficient mouse model, syngeneic colon carcinoma cells were injected subcutaneously with or without human MOMCs. Cotransplantation of the MOMCs promoted the formation of blood vessels, and more than 40% of the tumor vessel sections incorporated human endothelial cells derived from MOMCs. These findings indicate that human MOMCs can proliferate and differentiate along the endothelial lineage in a specific permissive environment and thus represent an autologous transplantable cell source for therapeutic neovasculogenesis.

Factor VIII ectopically targeted to platelets is therapeutic in hemophilia A with high-titer inhibitory antibodies

Inhibitory immune response to exogenously infused factor VIII (FVIII) is a major complication in the treatment of hemophilia A. Generation of such inhibitors has the potential to disrupt gene therapy for hemophilia A. We explore what we believe to be a novel approach to overcome this shortcoming. Human B-domain-deleted FVIII (hBDDFVIII) was expressed under the control of the platelet-specific alphaIIb promoter in platelets of hemophilic (FVIIInull) mice to create 2bF8trans mice. The FVIII transgene product was stored in platelets and released at the site of platelet activation. In spite of the lack of FVIII in the plasma of 2bF8trans mice, the bleeding phenotype of FVIIInull mice was corrected. More importantly, the bleeding phenotype was corrected in the presence of high inhibitory antibody titers introduced into the mice by infusion or by spleen cell transfer from recombinant hBDDFVIII-immunized mice. Our results demonstrate that this approach to the targeted expression of FVIII in platelets has the potential to correct hemophilia A, even in the presence of inhibitory immune responses to infused FVIII.

Regulated release of VWF and FVIII and the biologic implications

von Willebrand factor (VWF) performs a critical function in platelet binding at the site of vascular injury and also serves as the carrier protein for coagulation factor FVIII (FVIII), protecting it from proteolytic degradation in plasma. Both proteins undergo rapid, regulated release in response to DDAVP administration in patients with mild hemophilia A or von Wille-brand disease. Here, we attempt to summarize our current understanding of the establishment of the regulated storage pool of VWF and FVIII. The data presented indicate that regulated secretion of both proteins occurs only if there is endogenous synthesis of FVIII together with VWF.

Correction of a murine model of von Willebrand disease by gene transfer

von Willebrand disease (VWD), the most common inherited bleeding disorder in the U.S. population, is caused by defects in the expression and processing of von Willebrand factor (VWF), a blood glycoprotein required for normal hemostasis that mediates the adhesion of platelets to sites of vascular damage by binding to specific platelet glycoproteins and to constituents of exposed connective tissue. To assess whether VWF deficiency can be corrected by gene transfer, a plasmid expressing the intact 8.4-kb murine VWF coding sequence, directed by the cyto-megalovirus immediate/early promoter/enhancer, was delivered through hydrodynamic tail vein injection into VWF knockout mice (VWF(-/-)) that exhibit defects in hemostasis, including highly prolonged bleeding time and spontaneous bleeding events, closely mimicking severe human VWD. VWF antigen levels in plasma from animals receiving VWF cDNA, but not control animals, revealed normalized levels of circulating VWF that persisted for at least 1 week after injection. Western blot analysis of plasma from animals receiving VWF cDNA, but not control animals, revealed high molecular-weight multimers with patterns similar to those observed in wild-type mice. Reverse transcription-polymerase chain reaction (RT-PCR) on RNA isolated from the livers of animals receiving VWF cDNA, but not control animals, demonstrated that VWF was expressed in the liver, and immunohistochemical analysis of the livers of treated VWF(-/-) mice revealed VWF-specific staining throughout the liver parenchyma but not in endothelial cells. Plasma from treated VWF(-/-) mice, but not control VWF(-/-) mice, supported the hypothesis that murine platelets aggregate in the presence of botrocetin. Although levels of circulating factor VIII in untreated VWF(-/-) mice were less than 10% those in wild-type mice, levels of factor VIII in VWF(-/-) animals treated with VWF cDNA, but not in control animals, were normalized to values in wild-type mice, indicating the restoration of factor VIII carrier function for VWF in treated mice that persisted for at least 1 week at higher doses of VWF cDNA. Most important, bleeding time was normalized by 48 hours after the delivery of VWF cDNA, but not by the control plasmid. These data suggest that with the use of gene transfer of VWF cDNA, VWF protein can be expressed, processed, and secreted in a physiologically active form; thus, it may be possible to correct VWD using gene transfer.

Lack of desmopressin (DDAVP) response in men with hemophilia A following liver transplantation

Although hemophilia A, a congenital disorder caused by defective or deficient factor VIII:C (FVIII), is cured by liver transplantation, the exact site of hepatic FVIII production is unknown. Further, while intracellular co-localization of FVIII and von Willebrand factor (VWF) is required for in vitro FVIII secretion, whether it is required for in vivo FVIII secretion is not known. An ideal setting to study this problem is in individuals with hemophilia A following liver transplantation, as their FVIII is synthesized primarily in hepatic, but not extrahepatic endothelial cells, while VWF is synthesized primarily in extrahepatic vascular endothelium. Following liver transplantation for end-stage liver disease, three hemophilic men showed VWF, but no FVIII response to (DDAVP) infusion. By contrast, both VWF and FVIII increased in a non-hemophilic transplant recipient after DDAVP. These findings support a model in which intracellular co-localization of FVIII and VWF is necessary for in vivo FVIII secretion after DDAVP.

Transplantation of endothelial cells corrects the phenotype in hemophilia A mice

BACKGROUND

The deficiency of factor VIII, a co-factor in the intrinsic coagulation pathway results in hemophilia A. Although FVIII is synthesized largely in the liver, the specific liver cell type(s) responsible for FVIII production is controversial.

OBJECTIVE

This study aimed to determine the cellular origin of FVIII synthesis and release in mouse models.

METHODS

We transplanted cells into the peritoneal cavity of hemophilia A knockout mice. Plasma FVIII activity was measured using a Chromogenix assay 2-7 days after cell transplantation, and phenotypic correction was determined with tail-clip challenge 7 days following cell transplantation. Transplanted cells were identified by histologic and molecular assays.

RESULTS

Untreated hemophilia A mice, as well as mice treated with the hepatocyte-enriched fraction, showed extensive mortality following tail-clip challenge. In contrast, recipients of unfractionated liver cells (mixture of hepatocytes, liver sinusoidal endothelial cells (LSEC), Kupffer cells, and hepatic stellate cells) or of the cell fraction enriched in LSECs survived tail-clip challenge (P < 0.001). FVIII was secreted in the blood stream in recipients of unfractionated liver cells, LSECs and pancreatic islet-derived MILE SVEN 1 (MS1) endothelial cells. Although transplanted hepatocytes maintained functional integrity in the peritoneal cavity, these cells did not produce detectable plasma FVIII activity.

CONCLUSIONS

The assay of cell transplantation in the peritoneal cavity showed that endothelial cells but not hepatocytes produced phenotypic correction in hemophilia A mice. Therefore, endothelial cells should be suitable additional targets for cell and gene therapy in hemophilia A.

A Novel FcγR-Defined, IgG-Containing Organelle in Placental Endothelium

Placental transfer of IgG from maternal circulation to that of the fetus is crucial for fetal and newborn immunity. This process requires that IgG broach two cellular layers of the placenta. IgG transport across the first layer, the syncytiotrophoblast, is almost certainly mediated by the MHC-related FcR for IgG, FcRn. The second layer, the villus endothelium, was until recently thought to allow IgG movement nonspecifically by constitutive transcytosis in caveolae. However, we recently showed that villus endothelium expressed a separate FcR for IgG, the inhibitory motif-bearing Fc gammaRIIb2 seen most notably on macrophages and as a minor fraction of the Fc gammaRIIb expressed on B cells. Now, by quantitative microscopy, we find Fc gammaRIIb2 to be expressed abundantly in an unidentifiable and likely novel organelle of the villus endothelium, unassociated with caveolae. About half of these Fc gammaRIIb2 organelles contain IgG; the remainder lack IgG. The majority fraction (approximately 80%) of IgG-containing organelles is associated with Fc gammaRIIb. No IgG-containing organelles are associated with caveolin. These findings are compatible with Fc gammaRIIb-mediated transfer of IgG across the villus endothelium, independent of caveolae.

Evidence for a granule targeting sequence within platelet factor 4

Platelets achieve bleeding arrest at sites of vascular injury via secretion of secretory proteins from their storage granules, termed alpha-granules. We have recently analyzed granule targeting of platelet factor 4 (PF4), a secretory alpha-granule chemokine, and demonstrated that PF4 alpha-granule storage relied upon determinants within PF4 mature sequence. To define these determinants, PF4 mutants fused to the fluorescent reporter protein green fluorescent protein were generated by progressive deletions and site-directed mutagenesis. They were then transfected in AtT20 cells and assessed for granule targeting by colocalization with ACTH-containing granules, using laser scanning confocal microscopy. This strategy identified the amino acid 41-50 (LIATLKNGRK) sequence as most critical for PF4 granule targeting and/or storage; its deletion from PF4 induced a marked decrease in granule storage (from 81 +/- 2% to 17 +/- 3%, p < or = 0.0001). Ala-scanning mutagenesis of LIATLKNGRK narrowed down the targeting motif to LKNG. A direct role for LKNG in alpha-granule targeting was confirmed in the thrombopoietin-induced human megakaryocytic Dami cells, in which the LKNG-green fluorescent protein chimera exhibited an 82.5 +/- 1.8% colocalization with the alpha-granule proteins von Willebrand factor and P-selectin. LKNG is poorly conserved within the chemokine family. However three-dimensional alignments of the human alpha-granule chemokines Nap-2 (neutrophil-activating peptide) and RANTES (Regulated upon Activation Normal T Cell Expressed and Secreted) with PF4 revealed that LKNG, a surface-exposed hydrophilic turn/loop, matched Nap-2 (LKDG) and RANTES (TRKN) peptides with similar features. Moreover Nap-2 and RANTES peptides exhibited the same alpha-granule targeting efficiency than LKNG. We therefore postulate that the three-dimensional and physicochemical characteristics of PF4 LKNG are of general relevance to alpha-granule targeting of chemokines and possibly of other alpha-granule proteins.