The first step in closing wounds is the aggregation of the von Willebrand factor glycoprotein with blood platelets, which plugs the hole in the blood vessel and stops blood from leaking out. Using computer simulations and artificial blood vessel experiments, researchers from Mannheim have shown that this aggregation is a reversible process that depends on the shear forces resulting from the flow of the blood. This also prevents the clogging of healthy blood vessels.
High shear forces occur when blood flows through the fine branches of the arteries and capillaries. These forces can potentially disrupt the soft walls that are lined with endothelial cells, resulting in minor bleeding that needs to be stopped immediately to prevent it from becoming life threatening. Von Willebrand factor (VWF) helps platelets stick to damaged blood vessels and is therefore a key protein in a force-sensing reaction cascade that triggers primary hemostasis, i.e. platelet plug formation.
VWF is a rather unusual protein and is named after the Finnish doctor Erik Adolf von Willebrand. It is synthesised by endothelial cells and stored in special organelles known as Weibel-Palade bodies. [VWF is also produced by megakaryocytes, which are bone marrow cells responsible for the production of blood thrombocytes (platelets), and released by the thrombocytes during secondary hemostasis, i.e. the formation of a stable fibrin clot.] VWF monomers, including the sugar side chains, have a molecular weight of around 360 kDa. VWF dimers, which are found in the Weibel-Palade bodies of the endothelial cells, can assemble into long three-dimensional chains with a molecular weight of around 20,000 kDa.
Chemical signals, e. g. those resulting from injured blood vessel walls, lead to the release of VWF multimers from the endothelial cells. VWFs are the largest proteins that occur in the bloodstream and have a shear-flow sensitive structure. Blood that seeps from a wound leads to high shear forces and the VWF multimer responds to this shear by expanding to an elongated form (several hundred micrometres long), thereby exposing particular binding sites for collagen and platelet receptors. VWF can thus adhere to the vessel wall and bind to thrombocytes. In the flowing blood, the shear forces lead to the aggregation of VWF multimers, which adhere to thrombocytes and form large aggregates which serve as a blood clot and close the wound.
Dr. Volker Huck and Professor Dr. Stefan W. Schneider from the Department of Experimental Dermatology at the Mannheim Medical Faculty of Heidelberg University focused on the question as to how this process enables the relatively quick closure of wounds without interfering with blood flow and without clogging healthy blood vessels. They simulated and measured the flow-driven self-assembly of polymer (VWF molecules) -colloid (thrombocytes) composites in the presence of shear flow. They found that the composite morphology was tunable and completely reversible: from a specific flow pressure onwards, the VWFs unfold, cross-link and adhere to the colloids. Decreasing shear flow leads to the disintegration of the aggregates.
The SHENC consortium also involves Dr. Frauke Gräter’s Molecular Biomechanics research group at the Heidelberg Institute for Theoretical Studies. Gräter had already previously worked with Schneider’s team at the Mannheim University Hospital. Using computer simulations, Gräter and her team succeeded in determining in the nanometre scale the mechanical forces that act on the VWF protein as a result of the shear flow of the blood. They were able to show that shear forces also play a role in the disintegration of the VWF aggregates: tensile forces cause the protein to partially unfold, thereby leading to the exposure of a binding site for ADAMTS 13, a metalloprotease enzyme that cleaves von Willebrand factor. The SHENC researchers from Heidelberg also simulate pathological VWF variants. The clarification of the basic biomechanical and molecular mechanisms of the control mechanisms in the VWF system is an important prerequisite for the identification and modulation of the crucial steps of primary hemostasis. Knowing how the highly-sensitive equilibrium of VWF activation, aggregation with thrombocytes and VWF degradation is maintained in primary hemostasis or disturbed in case of disease, might lead to new approaches for the diagnosis and therapy of pathological situations.
Publication:Hsieh Chen, Mohammad A. Fallah, Volker Huck, Jennifer I. Angerer, Armin J. Reininger, Stefan W. Schneider, Matthias F. Schneider, Alfredo Alexander-Katz : Blood-clotting-inspired reversible polymer-colloid composite assembly in flow. Nature Communications 2013. DOI: 10.1038/ncomms2326