Von Willebrand Factor
Von Willebrand Factor is a very important glycoprotein that is needed for Haemostasis in the body, without vWF patients can suffer from severe bleeding disorders, so it’s crucial to know about this important glycoprotein in order to be able to help patients who suffered from a vascular injury and also recovering from surgery.
Von Willebrand Factor (vWF) is an interesting glycoprotein which is restricted to blood vessels for injury restoration. It is made only by endothelial cells and by megakaryocytes, both are the precursors of platelets. (Wagner, 1990, p.217)
Sadler (1998) has noted that vWF is a blood glycoprotein that is essential for typical Haemostasis. When there isn’t an adequate amount of vWF in the body, this is an regular bleeding disorder passed down through genes. vWF brings together the binding of platelets to the areas of vascular disruption by binding certain types of membrane glycoproteins and to elements of exposed connective tissue. This action appears to be controlled by allosteric mechanisms and maybe by hydrodynamic forces. VWF has an interaction that is used for normal factor VIII survival in the circulation because vWF is also a carrier protein for blood factor VIII. (Sadler, 1998, p.395)
Sadler (1998) has also noted that vWF is large, multimeric glycoprotein that is found within subendothelial connective tissue, blood plasma and platelet a-granules. vWF is found subdivided in between two pathways in the Golgi of endothelial cells. Most of which is secreted at a steady rate, but some is still stored in the cytoplasmic granules which is named Weibel-Palade that are specifically for the endothelium. These covered membrane organelles are rod shaped and are 0.1-0.2 um wide and up to 4 um long. When in a cross section of the long striations of these they are closely fixed together and seem to be composed of vWF multimers. Some small clusters that contain vWF are found at the periphery of platelet a-granules. (Sadler, 1998, p.395)
Also in Sadler’s (1998) studies, a part of Haemostasis vWF has two crucial purposes; it regulates the binding of platelets to subendothelial connective tissue and it also helps bring together blood clotting factor VIII- which is the protein that is missing when someone has haemophilia. When there is a lack of vWF, blood clotting factor VIII is quickly taken from the circulation. This results in a severe bleeding disorder with people lacking vWF. This is because they have deep flaws in formation of platelet plugs at injury vascular sites and also in blood clotting. This is how vWF was discovered from bleeding patients who lacked these haemostatic proteins. (Sadler, 1998, p.395)
vWf is made in endothelial cells and megakaryocytes through a step by step process as a large polymer which is made up of identical disulphide-linked 250-kd sub units. Amongst the endothelial cells vWF does two things; it directs the creation of its own granules for storage (the Weibel-Palade bodies) and it also acts as a partner molecule to direct other proteins (such as P-selectin), into these granules. When the endothelium has been stimulated, the Weibel-Palade bodies are then re directed to the plasma membrane- along with their contents, will be secreted into the plasma milieu. vWF can be regulated at different levels by a number of factors which can be environmental or genetic which results in control of its activity. (Denis, 2002, p.3)
Reininger (2008) explains the steps involved in the protection against bleeding by platelet binding to the injury site and sealing of the disruption as follows; The first step comprises of the capture of platelets that have stick to the subendothelial structures, which is mainly collagen at the injury site. Under the conditions of low shear rates, platelet binding to the defected cell wall is facilitated by a few different protein of vWF is one of these. However, under high shear rates only present soluble vWF causes aggregation. In a solution, vWF becomes stagnate via its A3 domain on the fibrillar collagen of the vessel wall, which acts as a middle between the platelet receptor glycoprotein Iba (GPIba) and collagen. GPIba is the only platelet receptor that doesn’t require prior initiation for the forming of bonds. After GPIba binds to the A1 site of its main ligand vWF, more activation of the platelet (via intracellular signalling) occurs, which then allows other receptors to engage collagen and vWF which reinforces permanent adhesion or binding. On this first layer of platelets soluble vWF binds and folds which then attracts more platelets. This platelet interaction with stagnate and soluble vWF might also create platelet-derived microparticles that show pro-coagulant activity. Then with fully grown multi layered platelet combined, shows the adhesion of the platelet receptor integrin aIIbB3 to vWF and fibrinogen. The growing platelet thrombus is stabilized by the surface activated platelets which speeds up the coagulation cascade which its end product is fibrin. (Reininger, 2008, p.11)
Von Willebrand disease (vWD) is a decrease in plasma levels or a defect in vWF, which decreases the ability to blood clot which can lead to heavy bleeding and also continual bleeding after an injury. The knowledge of von Willebrand Disease has advanced over the past 25 years in regards to pathogenesis and subsequent application which has given benefit to vWD diagnosis and therapy. It has been established the distinction between vWf and factor VIII from the 1970’s, which the gene for vWF was cloned by 4 groups in 1985 which thus began an era of knowledge and understanding into the studies of disease pathogenesis, diagnosis and treatment. (Lillicrap, 2013, p.3735)
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