Caspase-3 Activation Is Involved in the Early Megakaryocyte Differentiation, but Not in Platelet Production.

Megakaryocytes (Mgk) differentiate from hematopoietic stem cells through the complex process of endomitosis and cytoplasmic development, and finally form proplatelets in the terminal stage of the differentiation. To elucidate the role of a proapoptotic effecter protein, caspase-3, in megakaryopoiesis and thrombopiesis, we analyzed the phenotype of Mgk in vav-Bcl-2 transgenic (Tg) mice, which overexpress Bcl-2 exclusively in hematopoietic cells by the regulation of vav promotor. As was expected, Bcl-2-overexressing Mgk were remarkably resistant to caspase-3 activation induced by the depletion of thrombopoietin (TPO) from the culture. Percentages of proplatelet formation (PPF) assayed by culturing primary Mgk were slightly increased in the Tg mice. Platelet production from cultured Mgk, which was assayed by flowcytometry, was also slightly increased, indicating normal ability for Plt production from Bcl-2-overexpressing mature Mgk. To further analyze the ability for thrombopoiesis, we assayed the trends of Plt recovery following experimental thrombocytopenia. As Bcl-2-overexpressing mice had giant splenomegaly, we used splenectomized mice, which showed normal Plt counts. After the injection of 5-FU, we observed significant delay of the Plt recovery in the Tg mice, as well as the lack of the overshoot of Plt counts during the recovery phase. To gain insights about the mechanism underlying the delayed Plt recovery, we analyzeded the ploidy of DNA content. In the steady state, peak ploidy of Mgk obtained from the bone marrow of the Tg mice shifted to 2–4N. In a sharp contrast, in Plt recovery phase after 5-FU injection, peak ploidy shifted to 16–32N, and the percentages of Mgk possessing less than 16N ploidy were dramatically decreased. Consistent with this finding, we observed the delay of PPF in Bcl-2-overexpressing cells when Mgk were derived from c-kit+/lineage- progenitor cells in the presence of TPO. These data indicate that differentiation of immature Mgk into mature Plt-producing stage is significantly delayed in the Tg mice, but once the maturation proceeds, Mgk normally produce Plt. Taken together, we conclude that caspase-3 activation is involved in the early stage of the differentiation of Mgk, but not in Plt production.