The aggregation of high affinity IgE receptors (Fcɛ receptor I [FcɛRI]) on mast cells is potent stimulus for the release of inflammatory and allergic mediators from cytoplasmic granules. However, the molecular mechanism of degranulation has not yet been established. It is still unclear how FcɛRI-mediated signal transduction ultimately regulates the reorganization of the cytoskeleton and how these events lead to degranulation. Here, we show that FcɛRI stimulation triggers the formation of microtubules in a manner independent of calcium. Drugs affecting microtubule dynamics effectively suppressed the FcɛRI-mediated translocation of granules to the plasma membrane and degranulation. Furthermore, the translocation of granules to the plasma membrane occurred in a calcium-independent manner, but the release of mediators and granule–plasma membrane fusion were completely dependent on calcium. Thus, the degranulation process can be dissected into two events: the calcium-independent microtubule-dependent translocation of granules to the plasma membrane and calcium-dependent membrane fusion and exocytosis. Finally, we show that the Fyn/Gab2/RhoA (but not Lyn/SLP-76) signaling pathway plays a critical role in the calcium-independent microtubule-dependent pathway.

The ex vivo generation of platelets from human-induced pluripotent cells (hiPSCs) is expected to compensate donor-dependent transfusion systems. However, manufacturing the clinically required number of platelets remains unachieved due to the low platelet release from hiPSC-derived megakaryocytes (hiPSC-MKs). Here, we report turbulence as a physical regulator in thrombopoiesis in vivo and its application to turbulence-controllable bioreactors. The identification of turbulent energy as a determinant parameter allowed scale-up to 8 L for the generation of 100 billion-order platelets from hiPSC-MKs, which satisfies clinical requirements. Turbulent flow promoted the release from megakaryocytes of IGFBP2, MIF, and Nardilysin to facilitate platelet shedding. hiPSC-platelets showed properties of bona fide human platelets, including circulation and hemostasis capacities upon transfusion in two animal models. This study provides a concept in which a coordinated physico-chemical mechanism promotes platelet biogenesis and an innovative strategy for ex vivo platelet manufacturing.

Zinc (Zn) is an essential nutrient, and its deficiency causes growth retardation, immunodeficiency, and neuronal degeneration. However, the precise roles and molecular mechanism(s) of Zn function in immune response have not been clarified. Mast cells (MCs) are granulated cells that play a pivotal role in allergic reactions and inflammation. The granules of MCs contain various chemical mediators and inflammatory cytokines that are released upon FcεRI cross-linking. In this study, we report that Zn is essential for MC activation both in vitro and in vivo. We showed that a Zn chelator, N,N,N,N-tetrakis (2-pyridylmethyl) ethylenediamine, inhibited in vivo allergic reactions such as PCA and PSA. Consistent with this, N,N,N,N-tetrakis (2-pyridylmethyl) ethylenediamine significantly inhibited the FcεRI-induced degranulation and cytokine production. We found that Zn was required for FcεRI-induced translocation of granules to the plasma membrane, a process that we have shown to be important for MC degranulation. In addition, we showed that Zn was essential for plasma membrane translocation of protein kinase C and subsequent nuclear translocation of NF-κB, leading to cytokine production, such as IL-6 and TNF-α. These results revealed that Zn was involved in multiple steps of FcεRI-induced MC activation and required for degranulation and cytokine production.

Each transfusion requires 200-300 billion platelets in patients with thrombocytopenia. To continuously supply such a huge number of platelets by ex vivo generation, two distinct steps, megakaryopoiesis and platelet shedding, must be both considered. For the former, one approach is to increase the number of source cell, megakaryocytes. For example, the immortalized megakaryocyte cell line (imMKCL) system uses self-renewing megakaryocyte (MK) cell lines derived from induced pluripotent stem cells (iPSCs) (Nakamura et al., Cell Stem Cell, 2014). For the latter, there have been an idea of bioreactors whereby shedding of platelets from proplatelets could be promoted by flow-dependent shear force within the bone marrow in vivo (Junt et al., Science, 2007; Zhang et al., J Exp Med, 2012). Based upon this idea, we constructed a flow chamber type bioreactor recapitulating in vivo blood flow shear rate. However, this bioreactor failed to efficiently yield platelets, and moreover, the produced platelets had poor quality as indicated by high Annexin V levels (Exp Hematol, 2011 and unpublished result). Recently, we demonstrated two different kinetics of platelet biogenesis from bone marrow MKs, whereby either thrombopoietin (TPO) mostly regulates steady-state shedding of platelets from proplatelets, or interleukin-a (IL-1a) triggers inflammation-dependent rupture of MK cytoplasm contributing to a quick increase of platelet count at higher rate (Nishimura et al., J Cell Biol, 2015). However, the rupture type platelets revealed shorter half-life with relatively higher Annexin V levels. Therefore, to gain insights from platelet biogenesis in vivo, we focused on biophysical analysis of steady-state platelet biogenesis via proplatelets in bone marrow. Our observations strongly indicated that the presence of 'vorticity' defined by vortex turbulence in addition to shear-dependent 'stress' and 'strain' correlates with the efficient shedding of competent platelets. From this new finding, we developed an alternative bioreactor system, which enabled generation of 100 billion platelets from imMKCL in a 16L-scale liquid culture condition without any adherent machinery using two 10L-bioreactors. Furthermore, platelets generated via new bioreactors showed low Annexin V levels (<10-15%) and shortened bleeding time post transfusion into NOG mice and rabbits with thrombocytopenia, comparable to human blood product platelets. Regarding the platelet production using WAVE bag system (GE healthcare, UK), the system is already clinically available for cord blood cell expansion in most countries, but lacks adequate levels of vorticity and shear strain/stress. Accordingly, the produced platelets had high Annexin V levels (i.e., 50-65%) as well as diminished yield efficiency (P<0.001). In conclusion, our study has uncovered the novel biophysical aspect of platelet biogenesis. The application of the new set of physical parameters in constructing large sized bioreactors shall facilitate the industrialization of platelet production. Disclosures Eto: Megakaryon Co. Ltd.: Research Funding.