von Willebrand Factor
A hereditary bleeding disorder, which is now known as von Willebrand disease (VWD), was first discovered in 1924 by Dr. Erik von Willebrand, who noted that this disorder was different to hemophilia and exhibited prolonged bleeding time, normal clotting time, and was an autosomal inherited condition. It was further discovered that blood transfusions were able to prevent this bleeding condition, suggesting that there may be a plasma factor that is deficient within the circulation (Von Willebrand 1999). The reason for the prolonged bleeding time was first revealed to be an abnormality or lack of von Willebrand factor (vWF) (Soulier and Larrieu 1954). vWF is a large glycoprotein made up of numerous subunits, each with an approximate molecular weight of 220 kDa. These subunits undergo dimerization and multimerization to form larger vWF molecules consisting of a mature peptide with molecular weights ranging from 450 kDa up to 20,000 kDa, essential for hemostatic functions (Ginsburg et al. 1985).
Genetic analysis revealed that the vWF locus is situated on human chromosome 12p13.3 (Ginsburg et al. 1985) and contains 52 exons, spanning 178 kb of DNA (Mancuso et al. 1989). The signal peptide and propeptide consist of 17 exons and 80 kb of DNA, whereas the mature vWF peptide contains 35 exons and 100 kb of DNA. Upon transcription, the vWF mRNA is 9 kb in length (Mancuso et al. 1989). In situ hybridization analysis has revealed that a vWF pseudogene is located on human chromosome 22q11.22–22q11.23. This pseudogene is a partial form and contains exons 23–24, spanning 25 kb of DNA (Patracchini et al. 1989).
Following injury to the blood vessel wall, subendothelial collagen is exposed and high levels of shear stress results in a conformational change in vWF, exposing a binding site for the platelet receptor GPIbα at the A1 domain. vWF binds simultaneously to the exposed subendothelial collagen and the platelet receptor complex GPIbα-V-IX. The exposed collagen causes platelet rolling and the binding of collagen to platelet receptors α2β1, and GPVI strengthens platelet adhesion to the vessel wall and induces platelet activation, respectively. vWF multimers are secreted from activated platelet and endothelial cells to aid platelet adhesion and thrombus formation during hemostasis (Nuyttens et al. 2011).
Interestingly, this process is limited to static conditions since vWF requires high shear stress to induce the conformational change, triggered by shear rates of >5,000 s−1 (equivalent to a shear stress of 40–50 dyn/cm2). Moreover, this conformational change exposes the Tyr1605-Met1606 scissile bond at the A2 domain of vWF, which is subsequently cleaved by a disintegrin and metalloprotease with thrombospondin type 1 repeats 13 (ADAMTS13) to lower the hemostatic potential of vWF multimers. The change from closed to open confirmation is a fast process and essential during hemostasis (Di Stasio and De Cristofaro 2010). Under high shear stress, ADAMTS13 is responsible for binding to up to two vWF multimers at the exposed A2 domain and cleaving the Tyr1605-Met1606 scissile bonds. ADAMTS13 cleaves vWF to generate smaller molecular weight multimers with less activity; however, excess cleavage of vWF hinders its hemostatic functions and may result in conditions such as VWD (Majerus et al. 2005). vWF release is enhanced up to 15-fold upon endothelial cell stimulation; however, ADAMTS13 release is unaltered. Following secretion, long vWF multimers anchor to the surface of endothelial cells as hyperadhesive strings. Endothelial cells constitutively secrete ADAMTS13, which is able to bind to and cleave the long vWF strings, releasing multimers into the circulation in soluble form (Turner et al. 2009). A lack of ADAMTS13, either quantitatively or qualitatively, leads to an accumulation of ultra-HMW vWF multimers and can result in thrombotic thrombocytopenic purpura (TTP), a disorder with symptoms such as increased platelet aggregation and subsequent thrombocytopenia (Majerus et al. 2005). vWF has also been implicated in regulating its own degradation via the allosteric activation of the metalloprotease ADAMTS13. ADAMTS13 is able to bind to the vWF D4 domain and induce allosteric activation, in turn, enhancing proteolytic activity and positioning ADAMTS13 at the Tyr1605-Met1606 scissile bond for cleavage. Thus, vWF is able to act as a cofactor and substrate while regulating hemostatic functions (Muia et al. 2014).
vWF also acts as a carrier for blood coagulation factor VIII (FVIII), which acts as a cofactor for the factor IX in the coagulation cascade, and which is usually cleared by pinocytosis or proteolyzed unless bound. In the circulation, vWF and FVIII travel as a noncovalent complex to increase the half-life and stability of FVIII. vWF and FVIII synthesis occurs simultaneously in endothelial cells, and FVIII gene expression occurs at similar levels to vWF gene expression. Fluorescent microscopy analysis further revealed that both vWF and FVIII are stored in WPBs, and ultra-HMW vWF strings secreted from endothelial cells are found anchored to the cell surface with FVIII simultaneously bound (Turner and Moake 2015). FVIII is able to bind to vWF with high affinity, mediated by the N-terminal acidic region of the FVIII light chain as a site of interaction. Upon activation by thrombin, two sites in the heavy chain and the N-terminal acidic region of the light chain of FVIII are cleaved to produce the active cofactor FVIIIa. This cleavage reduces the affinity of FVIII for vWF, so FVIIIa is released into the circulation (Eaton et al. 1986).
Plasma free hemoglobin (pfHb), at levels of more than 50 mg/dL, enhances vWF-mediated platelet adhesion to fibrinogen and collagen, which increases microthrombi stability. The A1 domain of vWF has been found to interact directly with hemoglobin, leading to enhanced interactions between platelet receptor GPIbα and vWF. This may be a result of a pfHb-induced conformational change in vWF that enhances GPIbα binding (Da et al. 2015).
Interestingly, vWF has been implicated in further roles beyond hemostasis and may be involved in vascular processes. vWF has been observed to regulate angiogenesis, which may explain the cases of increased vascular endothelial growth factor (VEGF), angiodysplasia, and gastrointestinal bleeding in VWD patients. Low vWF antigen levels have been suggested to upregulate angiopoietin-2 expression in endothelial cells and promote vascular endothelial growth factor receptor-2 (VEGFR2) signaling to activate angiogenesis (Randi et al. 2013).
von Willebrand Disease
VWD is characterized by a quantitative or qualitative reduction in vWF, resulting in an inherited bleeding disorder. There are various types of VWD, consisting of types: 1, 2A, 2B, 2M, 2N, and 3. The development of VWD is not restricted to mutations in the vWF gene. An acquired form of VWD also exists and is classified as acquired von Willebrand syndrome (aVWS). Various types of VWD, such as 1 and 2A, can be easily tested in laboratory settings by measuring vWF antigen levels or risotocetin-induced platelet aggregometry (Sadler et al. 2006). VWD type 1 is characterized by a partial loss of vWF antigen levels but not limited to the loss of HMW multimers only. This may be a result of reduced secretion of vWF from endothelial cells or the enhanced clearance of vWF from the circulation (Eikenboom et al. 2006). VWD type 2 is characterized by defects in vWF function that impairs FVIII binding or platelet adhesion and is divided into four subtypes. Type 2A includes reduced vWF-dependent platelet adhesion and a selective decrease in HMW vWF multimers. This may be the result of defected multimerization or enhanced ADAMTS13-induced cleavage of normal vWF multimers, leading to reduced binding. Defected multimerization can result from vWF gene mutations that produce mutant vWF molecules that are unable to form multimers in the Golgi complex, resulting in the secretion of smaller vWF multimers. Enhanced cleavage by ADAMTS13 can also result from gene mutations in the A2 domain of vWF, which impairs vWF folding and exposes the Tyr1605-Met1606 scissile bond (Sutherland et al. 2004). Type 2B is characterized by vWF with enhanced affinity for platelet receptor GPIbα and can be measured using ristocetin-induced platelet aggregometry. A majority of type 2B patients have enhanced vWF cleavage and subsequent reduction in HMW vWF multimers. vWF gene mutations can result in enhanced vWF binding to platelets and enhanced ADAMTS13-induced cleavage, which may inhibit platelet interactions. vWF gene mutations occur near the A1 domain, which stabilizes vWF binding to GPIbα (Ruggeri et al. 1980). Type 2M is characterized by reduced vWF-dependent platelet adhesion but without a selective decrease in HMW vWF multimers. This reduction in platelet adhesion is the result of a vWF gene mutation that reduces ADAMTS13-induced cleavage and thereby maintains HMW vWF levels (Ciavarella et al. 1985). Type 2N is characterized by vWF multimers with significantly reduced FVIII-binding capacity. vWF gene mutations at the FVIII-binding site, spanning domains D′ and D3, are the main cause of impaired binding affinity. As such, FVIII levels are reduced disproportionate to vWF antigen levels (Ginsburg and Sadler 1993). Further, research shows that all vWF mutants from type 2A and 2B VWD exhibit significant degradation compared to wild-type vWF, whereas, type 2M vWF mutants exhibit similar degradation to wild-type vWF. These results confirm in vivo observations and account for enhanced vWF degradation in VWD types 2A and 2B as well as the normal vWF profile in VWD type 2M (Rayes et al. 2007). Type 3 VWD is a recessive inherited trait, characterized by a virtual deficiency in vWF antigen levels. Frameshift or nonsense mutations in the vWF gene are often the main cause. As a result, FVIII levels are also reduced (Ginsburg and Sadler 1993). Furthermore, aVWS is a condition that is characterized by a deficiency in HMW vWF multimers. aVWS can occur due to autoimmune conditions where patients develop antibodies against vWF, resulting in greater clearance from the circulation. Further, aVWS can develop in patients implanted with left ventricular assist devices (LVADs). LVADs are heart pumps that contain rotating elements to help circulate blood around the body. However, these rotating elements induce high levels of nonphysiological shear stress. Such shear stress can induce a conformational change in vWF, exposing the Tyr1605-Met1606 scissile bond. This results in enhanced ADAMTS13 cleavage and reduced/absent HMW vWF activity and lower functional activity for platelet aggregation, which may contribute to the development of bleeding disorders in LVAD patients (Tiede et al. 2011; Chan et al. 2014).
Various anti-vWF antibodies are commercially available. A rabbit unconjugated polyclonal antihuman vWF antibody is available from Novus Biologicals (NB600-586). This antibody is suitable for use in immunocytochemistry (ICC) and immunohistochemistry (IHC) of frozen or paraffin embedded samples. Similarly, a mouse unconjugated monoclonal antihuman vWF antibody is available (NBP1-39497) from the same company for use in Western blotting and ELISA. A rat biotin-conjugated monoclonal antibody to human vWF is available from MyBioSource (MBS2007408) and is suitable for Western blot, IHC, and ELISA. A rabbit unconjugated polyclonal anti-vWF antibody, reactive against human, mouse, and rat, is available from Proteintech Group (11778-1-AP) and is suitable for use in ELISA, Western blot, immunoprecipitation, and IHC. A further mouse unconjugated monoclonal anti-vWF antibody is also available from Biorbyt (orb305844) that is reactive with human and suitable for use in ELISA, Western blot, flow cytometry, and IHC of paraffin embedded samples. A rabbit unconjugated polyclonal anti-vWF is available from Abcam, reactive against human, cow, pig, dog, and rat. This antibody is suitable for use in ICC, IHC (paraffin or frozen), Western blot, and flow cytometry (ab6994). Life Span Biosciences, Inc. offer a wide range of anti-vWF antibodies, including a mouse unconjugated monoclonal antibody to human vWF (N-terminus) for use in IHC-paraffin, Western blot, and ELISA (LS-C83336), as well as a sheep FITC-conjugated polyclonal antibody to human vWF for use in IHC (paraffin and frozen) and immunofluorescence (LS-B5692).
vWF is a multimeric glycoprotein that is involved in hemostasis by acting as a carrier for FVIII and regulating platelet adhesion. The vWF gene is located on chromosome 12p13.3 and contains 52 exons. vWF is first synthesized as a precursor pre-pro-vWF protein, consisting of domains D1, D2, D′, D3, A1, A2, A3, D4, B1, B2, B3, C1, C2, and CK. The pre-pro-vWF molecule is 2,813 aa in size, containing a 22 aa signal peptide, 741 aa propeptide, and 2,050 aa mature subunit molecule. Following injury to the blood vessel wall, subendothelial collagen is exposed and high levels of shear stress leads to a conformation change in vWF, exposing a binding site for the platelet receptor GPIbα. This conformational change also exposes the Tyr1605-Met1606 scissile bond on the A2 domain of vWF, which is subsequently cleaved by the metalloprotease ADAMTS13 to lower the hemostatic potential of vWF multimers. A deficiency, either qualitative or quantitative, in vWF results in VWD, a bleeding disorder categorized into types 1, 2A, 2B, 2M, 2N, 3, and aVWS. Additionally, vWF levels were found to regulate angiogenesis by controlling angiopoietin-2 expression and VEGFR2 signaling in endothelial cells, thus implicating vWF in further roles beyond hemostatic function.
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