Regular Article PLATELETS AND THROMBOPOIESIS Shear enhances thrombopoiesis and formation of microparticles that induce megakaryocytic differentiation of stem cells

• Physiological shear stress promotes megakaryocytic maturation, DNA synthesis, phosphatidylserine exposure and caspase-3 activation. • Shear enhances the production and function of PLPs and Mk-derived microparticles possessing a novel function. In vivo visualization of thrombopoiesis suggests an important role for shear flow in platelet biogenesis. In vitro, shear stress was shown to accelerate proplatelet formation frommature megakaryocytes (Mks). Yet, the role of biomechanical forces on Mk biology and platelet biogenesis remains largely unexplored. In this study, we investigated the impact of shear stress on Mk maturation and formation of platelet-like particles (PLPs), pro/preplatelets (PPTs), and Mk microparticles (MkMPs), and furthermore, we explored a physiological role for MkMPs. We found that shear accelerated DNA synthesis of immature Mks in an exposure time– and shear stress level–dependent manner. Both phosphatidylserine exposure and caspase-3 activation were enhanced by shear stress. Exposure to physiological shear dramatically increased generation of PLPs/PPTs and MkMPsbyup to10.8 and47-fold, respectively. Caspase-3 inhibition reducedshear-induced PLP/PPT and MkMP formation. PLPs generated under shear flow displayed improved functionality as assessed by CD62P exposure and fibrinogen binding. Significantly, coculture ofMkMPswith hematopoietic stemandprogenitor cells promotedhematopoietic stem and progenitor cell differentiation to mature Mks synthesizing aand dense-granules, and forming PPTs without exogenous thrombopoietin, thus identifying a novel and unexplored potential physiological role for MkMPs. (Blood. 2014;124(13):2094-2103)