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Role of the thrombopoietin (TPO)/Mpl system: c-Mpl-like molecule/TPO signaling enhances early hematopoiesis in Xenopus laevis
Authors:Kakeda Minoru  Kyuno Jun-ichi  Kato Takashi  Nishikawa Mitsuo  Asashima Makoto
Institution:Department of Biological Science, Graduate School of Science, University of Tokyo, Tokyo 153-8902, Japan.
Abstract:Multiple organs are induced in the primitive embryonic ectoderm excised from blastula stage Xenopus laevis embryos, under the strict control of mesoderm inducing factors. This in vitro system is useful for exploring the mechanisms of development. In this study, the function of thrombopoietin (TPO)/c-Mpl signaling in the development of hematopoietic cells was investigated. An optimal hematopoietic cell induction system was established to evaluate the influence of growth factors on hematopoiesis. It was found that exogenous TPO enhanced hematopoiesis in explants induced by activin and bone morphogenetic protein (BMP)-4 and increased the number of both erythrocytes and leukocytes in a dose-dependent manner. Addition of anti-c-Mpl antibody completely inhibited the expansion of hematopoietic cells stimulated by TPO, and the antibody specifically recognized blood-like cells. These results demonstrate that TPO acts on hematopoietic progenitors induced in explants and the c-Mpl-like molecule in Xenopus mediates the cellular function of TPO. We also found that forced expression of TPO in embryos promoted hematopoiesis in the ventral blood island and the dorsal-- lateral plate mesoderm. These results suggest that hematopoietic stem and progenitor cells are regulated by TPO/c-Mpl signaling from when they appear in their ontogeny. They also suggest that TPO/c-Mpl signaling play a crucial role in the formation of hematopoietic cells in Xenopus.
Keywords:activin  BMP-4  c-Mpl  hematopoiesis  thrombopoietin              Xenopus
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