PB0882 - Contractility of Megakaryocytes and Platelets: Similarities and Distinctions

Background: Blood clot contraction driven by activated platelets is important for hemostasis and thrombosis. A growing body of evidence suggests that megakaryocytes derived from induced pluripotent stem cells (iMKs) can be used as a model system to study platelet function. We have recently shown that MKs also shrink plasma clots, but it remains unclear whether the mechanisms of MK- and platelet-driven clot contraction are similar.

Aims: To elucidate and compare the mechanisms of the iMK- and platelet-driven contraction of plasma clots.

Methods: iMKs were suspended in human platelet-free citrated plasma followed by thrombin-induced cell activation and plasma clotting. In parallel, clot formation and contraction was induced in thrombin-activated platelet-rich plasma and both clots were followed using the optical tracking of clot shrinkage and live fluorescent confocal microscopy of the activated cells and densifying fibrin network.

Results: Either platelet- or iMKs-mediated clot contraction was suppressed by blebbistatin, an inhibitor of non-muscle myosin II, and latrunculin A, which disrupts actin microfilament organization. Abciximab, an antagonist of the platelet integrin αIIbβ3, also inhibits the clot shrinkage induced by thrombin-activated iMKs. Platelet-driven clot contraction involves formation of thin filopodia that attach to adjacent fibrin fibers and retract, pulling fibers toward the platelet body. Activated MKs also interact with fibrin via membrane protrusions, but, in contrast to platelets, iMKs generate, in addition to filopodia, larger membranous projections (blebs) to pull on fibrin fibers. The quantitative structural dynamics of platelet filopodia versus transient blebs and filopodia formed by iMKs have various kinematics and ability to remodel fibrin clots.

Conclusion(s): The intracellular mechanisms of MK contractility are similar or identical to those of activated platelets and involve the actomyosin machinery associated with mechanotransduction to fibrin mediated via the integrin αIIbβ3. However, the microscopic biomechanical mechanisms have important distinctions related to various dynamics of cellular protrusions and their interactions with the fibrin network.