Megakaryocyte-induced contraction of plasma clots: Cellular mechanisms and structural mechanobiology.

Non-muscle cell contractility is an essential feature underlying diverse cellular processes such as motility, morphogenesis, division and genome replication, intracellular transport, and secretion. Blood clot contraction is a well-studied process driven by contracting platelets. Megakaryocytes, which are the precursors to platelets, can be found in the bone marrow and in the lungs. Although they express many of the same proteins and structures found in platelets, little is known about their ability to engage with extracellular proteins such as fibrin and contract. Here we have measured the ability of megakaryocytes to compress plasma clots. Megakaryocytes derived from human induced pluripotent stem cells (iMKs) were suspended in human platelet-free blood plasma and stimulated with thrombin. Using real-time macroscale optical tracking, confocal microscopy, and biomechanical measurements, we found that activated iMKs caused macroscopic volumetric clot shrinkage, as well as densification and stiffening of the fibrin network via fibrin-attached plasma membrane protrusions undergoing extension-retraction cycles that cause shortening and bending of fibrin fibers. Contraction induced by iMKs involved two kinetic phases with distinct rates and durations. It was suppressed by inhibitors of non-muscle myosin IIA, actin polymerization, and integrin αIIbβ3-fibrin interactions, indicating that the molecular mechanisms of iMK contractility were similar or identical to those in activated platelets. Our findings provide new insights into megakaryocyte biomechanics and suggest that iMKs can be used as a model system to study platelet contractility. Physiologically, the ability of MKs to contract plasma clots may play a role in the mechanical remodeling of intravascular blood clots and thrombi.