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The Von Willebrand Factor (VWF) is a carrier protein of blood coagulation factor VIII and captures platelets at locations with vascular damage. It undergoes conformational changes within the A1 domain and through the development of the A2 domain. Its size and function are regulated by the metalloproteinase ADAMTS13. ADAMTS13 deficiency can, therefore, lead to the life-threatening disease thrombotic thrombocytopenic purpura (TTP).
Prof. Dr. Anne Goodeve from the Department of Infection, Immunity & Cardiovascular Disease at the University of Sheffield emphasized that vWF is more than just a carrier of Factor VIII in a state-of-the-art symposium at the GTH in Vienna. In her presentation, Dr. Goodeve identified three essential structural elements that control the development of the A2 domain.
The heterogeneous or oversized vWF stored in the Weibel-Palade bodies are released constitutively or when required. During its interaction with vWF, ADAMTS13 undergoes a conformational change, which increases its activity with the substrate. ADAMTS13 then reduces the size and hemostatic function of vWF by controlled separation.
vWF recognizes and binds collagen exposed to vascular lesions via its A3 domain. This causes it to adhere to the wound and the shear stress generated by the blood flow then lead it to unfold in the A1 domain, revealing a platelet capture site, therefore allowing primary hemostasis and the formation of a first platelet plug.
Because the A2 domain has no domain-encompassing, stabilizing disulfide bonds, it is also exposed to shear stress. In principle, three structural elements were identified which control the A2 domain development.
In its folded form, the fissile site of the A2 domain remains hidden. The conformational change depends on three specialized structural elements:
The shear stress of the flowing blood gradually causes the stability of the A2 domain to be lost. A2 deployment events were observed at forces of 20 piconewtons (pN) & another between 50 pN and 120 pN. At the end of the day, the vWF will be fully developed.
When the vWF dimer is opened, the cation-dependent bonds between the D4 domains are dissociated. ADAMTS13 is also most active in its full length (but not in its shortened MDTCS variant) and at a pH value of 6. The D4 domains of vWF, which dock to the two CUB domains and to the TSP-8 domain of ADAMTS13, help to open the enzyme and thus expose its otherwise cryptic exosite 3.
The unfolding of vWF A2 also reveals cryptic exosites, which gradually increase the binding affinity between ADAMTS13 and VWF:
The crystal structure of the vWF A2 domain shows a disulfide bond between neighboring Cys1669 & Cys1670 at the C-terminus of the alpha 6 helix. An 8-membered ring is formed, forcing the backbone of the protein into an unusual conformation. This disulfide bond interacts directly with the hydrophobic core of the domain, suggesting that it stabilizes conformation.
The A2 domain unfolds progressively from its C-terminus: from the alpha6 helix via the beta 6 to the alpha5 helix. The vicinal disulfide binding primarily influences the initial decoupling of the Alpha6 helix and directly modulates the exposure of the high-affinity ADAMTS13 binding site. Without these vicinal cysteines, the vWF A2 domain would develop much more easily and proteolysis would take place much faster.
Prof. Dr. Christoph Reinhardt, University of Mainz, reported that the microbiota on the synthesis of the Von Willebrand factor and the vWF plasma levels apparently influences the formation of thrombi. The role of intestinal microbiota in the physiology and development of cardiovascular diseases is well known. How they can also influence thrombosis has hardly been defined to date.
To investigate these patterns, Reinhardt et al. established a colony of germ-free mice. In gnotobiotic rodent models at the Thrombosis Center in Mainz, germ-free animals that were born without contact with microorganisms are colonized with known mixtures or intestinal bacteria "cocktails".
Carriers of LDLR deficiency have LDL cholesterol levels of up to 1200 mg/dl from birth. The LDLR-deficient animals showed reduced leukocyte deposition and thrombogenicity in carotid artery plaques. In another series, the researchers were able to show that germ-free mice with a Toll-like receptor 2 (TLR-2) deficiency have reduced thrombus growth compared to control mice without this TLR-2 deficiency when the carotid artery is injured. Germ-free TLR2/- and wild-type mice could not be distinguished in thrombus growth, but colonization with microbiota restored a significant difference between the genotypes.
The intestinal microbiota regulated two critical factors that determine thrombus growth in TLR2-/- mice and that Reinhardt et al. were able to identify: a reduced Von Willebrand factor plasma level and a reduced vWF synthesis especially in the endothelial cells of the liver. The result: reduced platelet deposition in the injured carotid artery. vWF plasma levels reduced by 50% were already sufficient for the effect. Thus, the intestinal microbiota apparently affects the hepatic vWF synthesis, the vWF plasma levels, and influences the platelet deposition at the arterial site of injury.
State-of-the-Art: Revised by Willebrand factor, 21 February 2018, GTH Vienna.