Sandra L. Haberichter

Identification of type 1 von Willebrand disease patients with reduced von Willebrand factor survival by assay of the VWF propeptide in the European study: Molecular and Clinical Markers for the Diagnosis and Management of Type 1 VWD (MCMDM-1VWD)

The decreased survival of von Willebrand factor (VWF) in plasma has been implicated as a mechanism in a subset of type 1 von Willebrand disease (VWD) patients. We have previously reported that the ratio of plasma levels of VWF and its propeptide (VWFpp) can be used to identify patients with reduced VWF survival. In this study, we report the assay of VWFpp and VWF:Ag in 19 individuals recruited from 6 European centers within the MCMDM-1VWD study. Eight individuals had a VWF:Ag level less than 30 IU/dL. Seven of these patients had a robust desmopressin response and significantly reduced VWF half-life that was predicted by a markedly increased steady-state plasma VWFpp/VWF:Ag ratio. VWF mutations previously associated with reduced VWF survival were identified in each of the 7 individuals. Thus, a substantially increased ratio of steady-state VWFpp/VWF:Ag predicted a reduced VWF half-life in patients with markedly decreased VWF:Ag levels. These data indicate that a reduced VWF survival is found in a subpopulation of patients with type 1 VWD. The systematic assay of both plasma VWF and the VWF propeptide in moderately severe type 1 VWD patients may identify patients with a reduced VWF survival phenotype.

Contribution of platelet vs. endothelial VWF to platelet adhesion and hemostasis

Summary.  Background:  von Willebrand factor (VWF) is a glycoprotein that plays an important role in primary hemostasis. VWF is synthesized and stored in endothelial cells (ECs) and megakaryocytes/platelets. Plasma VWF is primarily derived from ECs and is generally believed to be essential for hemostasis. VWF synthesized in megakaryocytes is stored in platelet α‐granules, from which it is released following platelet activation. The relative contribution of VWF stored in ECs or megakaryocytes/platelets or present in plasma to hemostasis is not clear.

VWF mutations and new sequence variations identified in healthy controls are more frequent in the African-American population

Diagnosis and classification of VWD is aided by molecular analysis of the VWF gene. Because VWF polymorphisms have not been fully characterized, we performed VWF laboratory testing and gene sequencing of 184 healthy controls with a negative bleeding history. The controls included 66 (35.9%) African Americans (AAs). We identified 21 new sequence variations, 13 (62%) of which occurred exclusively in AAs and 2 (G967D, T2666M) that were found in 10%-15% of the AA samples, suggesting they are polymorphisms. We identified 14 sequence variations reported previously as VWF mutations, the majority of which were type 1 mutations. These controls had VWF Ag levels within the normal range, suggesting that these sequence variations might not always reduce plasma VWF levels. Eleven mutations were found in AAs, and the frequency of M740I, H817Q, and R2185Q was 15%-18%. Ten AA controls had the 2N mutation H817Q; 1 was homozygous. The average factor VIII level in this group was 99 IU/dL, suggesting that this variation may confer little or no clinical symptoms. This study emphasizes the importance of sequencing healthy controls to understand ethnic-specific sequence variations so that asymptomatic sequence variations are not misidentified as mutations in other ethnic or racial groups.

A GPI-anchored co-receptor for tissue factor pathway inhibitor controls its intracellular trafficking and cell surface expression.

Summary.  Background: Tissue factor pathway inhibitor (TFPI) lacks a membrane attachment signal but it remains associated with the endothelial surface via its association with an, as yet, unidentified glycosyl phosphatidylinositol (GPI)‐anchored co‐receptor. Objectives/methods: Cellular trafficking of TFPI within aerolysin‐resistant ECV304 and EA.hy926 cells, which do not express GPI‐anchored proteins on their surface, was compared with their wild‐type counterparts. Results and conclusions: Although aerolysin‐resistant cells produce normal amounts of TFPI mRNA, TFPI is not expressed on the cell surface and total cellular TFPI is greatly decreased compared with wild‐type cells. Additionally, normal, not increased, amounts of TFPI are secreted into conditioned media indicating that TFPI is degraded within the aerolysin‐resistant cells. Confocal microscopy and studies using metabolic inhibitors demonstrate that aerolysin‐resistant cells produce TFPI and transport it into the Golgi with subsequent degradation in lysosomes. The experimental results provide no evidence that cell surface TFPI originates from secreted TFPI that binds back to a GPI‐anchored protein. Instead, the data suggest that TFPI tightly, but reversibly, binds to a GPI anchored co‐receptor in the ER/Golgi. The co‐receptor then acts as a molecular chaperone for TFPI by trafficking it to the cell surface of wild‐type cells or to lysosomes of aerolysin‐resistant cells. TFPI that escapes co‐receptor binding is secreted through the same pathway in both wild‐type and aerolysin‐resistant cells. The data provide a framework for understanding how TFPI is expressed on endothelium.

Gain-of-function GPIb ELISA assay for VWF activity in the Zimmerman Program for the Molecular and Clinical Biology of VWD

von Willebrand disease (VWD) is a common bleeding disorder, but diagnosis is sometimes challenging because of issues with the current von Willebrand factor (VWF) assays, VWF antigen (VWF:Ag) and VWF ristocetin cofactor activity (VWF:RCo), used for diagnosis. We evaluated 113 healthy controls and 164 VWD subjects enrolled in the T.S. Zimmerman Program for the Molecular and Clinical Biology of VWD for VWF:Ag, VWF:RCo, and a new enzyme-linked immunosorbent assay (ELISA)-based assay of VWF-glycoprotein Ib (GPIb) interactions using a gain-of-function GPIb construct (tGPIbα(235Y;239V)) as a receptor to bind its ligand VWF in an assay independent of ristocetin (VWF:IbCo ELISA). Healthy controls, type 1, 2A, 2M, and 2N subjects had VWF:RCo/VWF:Ag ratios similar to the ratio obtained with VWF:IbCo ELISA/VWF:Ag. Type 2B VWD subjects, however, had elevated VWF:IbCo ELISA/VWF:Ag ratios. Type 3 VWD subjects had undetectable (< 1.6 U/dL) VWF:IbCo ELISA values. As previously reported, VWF:RCo/VWF:Ag ratio was decreased with a common A1 domain polymorphism, D1472H, as was direct binding to ristocetin for a 1472H A1 loop construct. The VWF:IbCo ELISA, however, was not affected by D1472H. The VWF:IbCo ELISA may be useful in testing VWF binding to GPIb, discrimination of type 2 variants, and in the diagnosis of VWD as it avoids some of the pitfalls of VWF:RCo assays.

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.

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.

Induction of megakaryocytes to synthesize and store a releasable pool of human factor VIII

Summary.  von Willebrand factor (VWF) is a complex plasma glycoprotein that modulates platelet adhesion at the site of a vascular injury, and it also serves as a carrier protein for factor (F)VIII. As megakaryocytes are the only hematopoietic lineage to naturally synthesize and store VWF within α‐granules, this study was performed to determine if expression of a FVIII transgene in megakaryocytes could lead to trafficking and storage of FVIII with VWF in platelet α‐granules. Isolex® selected CD34+ cells from human G‐CSF mobilized peripheral blood cells (PBC) and murine bone marrow were transduced with a retrovirus encoding the B‐domain deleted form of human FVIII (BDD‐FVIII). Cells were then induced with cytokines to form a population of multiple lineages including megakaryocytes. Chromogenic analysis of culture supernatant from FVIII‐transduced human cells demonstrated synthesis of functional FVIII. Treatment of cells with agonists of platelet activation (ADP, epinephrine, and thrombin receptor‐activating peptide) resulted in the release of VWF antigen and active FVIII into the supernatant from transduced cells. Immunofluorescence analysis of cultured human and murine megakaryocytes revealed a punctate pattern of staining for FVIII that was consistent with staining for VWF. Electron microscopy of transduced megakaryocytes using immunogold‐conjugated antibodies colocalized FVIII and VWF within the α‐granules. FVIII retained its association with VWF in human platelets isolated from the peripheral blood of NOD/SCID mice at 2–6 weeks post‐transplant of transduced human PBC. These results suggest feasibility for the development of a locally inducible secretory pool of FVIII in platelets of patients with hemophilia A.

Allosteric activation of ADAMTS13 by von Willebrand factor

Significance The blood protein von Willebrand factor (VWF) is required for platelets to stop bleeding at sites of injury, and the metalloprotease ADAMTS13 limits platelet adhesion by cleaving VWF only when flowing blood stretches it, especially within a growing thrombus. This feedback inhibition is essential because ADAMTS13 deficiency causes fatal microvascular thrombosis. How ADAMTS13 recognizes VWF so specifically is not understood. We now find that ADAMTS13 is folded roughly in half so that its distal domains inhibit the metalloprotease domain. VWF relieves this autoinhibition and promotes its own destruction by allosterically activating ADAMTS13. Thus, VWF is both a substrate and a cofactor in this critical regulatory process. The metalloprotease ADAMTS13 cleaves von Willebrand factor (VWF) within endovascular platelet aggregates, and ADAMTS13 deficiency causes fatal microvascular thrombosis. The proximal metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C), and spacer (S) domains of ADAMTS13 recognize a cryptic site in VWF that is exposed by tensile force. Another seven T and two complement C1r/C1s, sea urchin epidermal growth factor, and bone morphogenetic protein (CUB) domains of uncertain function are C-terminal to the MDTCS domains. We find that the distal T8-CUB2 domains markedly inhibit substrate cleavage, and binding of VWF or monoclonal antibodies to distal ADAMTS13 domains relieves this autoinhibition. Small angle X-ray scattering data indicate that distal T-CUB domains interact with proximal MDTCS domains. Thus, ADAMTS13 is regulated by substrate-induced allosteric activation, which may optimize VWF cleavage under fluid shear stress in vivo. Distal domains of other ADAMTS proteases may have similar allosteric properties.

The Von Willebrand Factor Propeptide (VWFpp) Traffics an Unrelated Protein to Storage

The von Willebrand factor (VWF) propeptide (VWFpp) is critical for the targeting of VWF multimers to storage granules. VWFpp alone efficiently navigates the storage pathway in AtT-20 and endothelial cells and chaperones mature VWF multimers to storage granules when the two proteins are expressed in cis or in trans. To further define the role of VWFpp in granular sorting, we examined its ability to sort an unrelated protein, C3&agr; into the regulated secretory pathway. Chimeric constructs of VWFpp and the &agr;-chain of C3 were developed. The C3&agr; protein expressed alone did not sort to granules in AtT-20 cells. The trans expression of C3&agr; and VWFpp resulted in granular storage of VWFpp but no corresponding storage of C3&agr;. When C3&agr; is expressed as a single chain molecule with VWFpp that was rendered uncleavable by furin, C3&agr; is re-routed to storage and is colocalized with VWFpp. The uncleavable protein was expressed in bovine aortic endothelial cells where it sorted to Weibel-Palade bodies, colocalized with bovine VWF, and was released when agonist stimulated. We now demonstrate that VWFpp re-routes a constitutively secreted protein to the regulated storage pathway. Furthermore, our studies suggest that the VWFpp storage signal is contained within amino acids 201 to 741.