Blood platelets are the smallest cells in our body, with a size of only a few micrometers. When circulating in the blood stream together with red and white blood cells, they are essentially passive bystanders. This changes dramatically when biochemical signals after injury bring them into action to promote blood clotting. These small cells then spread out on the surrounding matrix and develop forces to contract the blood clot. But how does this contraction work on a subcellular level? Which mechanisms and cellular structures are involved? How large are the forces and how fast are they generated? We addressed these important questions in our study by combining traction-force measurements and high-resolution imaging.
The cover image of the August 22 issue of Biophysical Journal shows such a high-resolution image of a platelet that earlier had spread on a soft elastic substrate. We used a super-resolution technique, stimulated emission depletion microscopy, which was invented by Stefan Hell and earned him the 2014 Nobel Prize in Chemistry. The elastic substrate was made from polyacrylamide, and the whole system was chemically fixed after completing the force measurements. In cyan one sees the fluorescently-labeled actin cytoskeleton, in particular the so-called stress fibers, which span the whole cell from one edge to the other and end in so-called focal adhesions. One protein found in focal adhesions is vinculin, which is stained by using antibodies in our image and shown in magenta.
The image shows that despite their small size, platelets form very strong stress fibers that are firmly anchored in focal adhesions. The proof that this system leads to strong contractile forces in the environment comes from traction-force microscopy that we conducted before taking the image you see here. The combination of super-resolution imaging of protein structures in the cell and temporally and spatially resolved measurements of traction forces allowed us to correlate force generation with cell structure and to demonstrate that within minutes after activation, platelets generate very large and localized forces. In the process of blood clotting and wound healing, this process is important because many blood platelets stick together and form a blood clot. This clot then contracts as a first step to close the wound and enable normal blood flow. There are several diseases in which these mechanisms are out of control, and the patient must take medications to avoid excessive blood clotting.
—Anna Zelená, Johannes Blumberg, Dimitri Probst, Rūta Gerasimaitė, Gražvydas Lukinavicius, Ulrich S. Schwarz, and Sarah Köster