Biomedical subjects
Publications and source records attributed to K Tanegashima.
The Ran gene family encodes small GTP binding proteins that are associated with a variety of nuclear processes. We isolated a Xenopus Ran cDNA and analyzed the pattern of expression of this gene during embryogenesis. Ran is expressed maternally and later in the CNS, neural crest, mesenchyme, eyes, and otic vesicles. However, expression is not detected in the somites or the notochord.
In a screening for activin-responsive genes, we isolated a Xenopus lefty/antivin-related gene, called Xantivin (Xatv). In the animal cap assay, the expression of Xatv was induced by activin signaling, and in the embryo, by nodal-related genes. Overexpression of Xatv in the marginal zone caused suppression of mesoderm formation and gastrulation defects, and inhibited the secondary axis formation induced by Xnr1 and Xactivin, suggesting that Xatv acted as a feedback inhibitor of activin signaling. However, in the animal cap, Xatv failed to antagonize Xnr1 and Xactivin. This result suggested that Xatv has different responses in the marginal zone and in the animal region, and antagonizes to a higher degree activin signaling in the marginal zone.
In vertebrates, Nodal-related protein plays crucial roles in mesoderm and endoderm induction. Here we describe two novel Xenopus nodal-related genes, Xnr5 and Xnr6, which are first zygotically expressed at the mid-blastula transition, in the dorsal-vegetal region including the Nieuwkoop center. Xnr5 and Xnr6 were isolated by expression screening of a library enriched with immediate-early-type transcripts, and are strong inducers of both mesoderm and endoderm. They also induce the other nodal-related genes in the animal cap. In embryos, cerberus-short (nodal-specific inhibitor) can inhibit Xnr1 and Xnr2 express to the same extent goosecoid, but not Xnr5 and Xnr6 transcription. Xnr5 and Xnr6 are regulated completely cell autonomously, differently from other Xnrs in the cell-dissociated embryos. The expression of Xnr5 and Xnr6 is regulated by maternal VegT and (beta)-catenin, but does not require TGF-(beta) signaling. Therefore, expression of Xnr5 and Xnr6 is controlled by different mechanisms from other Xnr family genes.
When presumptive ectoderm is treated with high concentrations of activin A, it mainly differentiates into axial mesoderm (notochord, muscle) in Xenopus and into yolk-rich endodermal cells in newt (Cynops pyrrhogaster). Xenopus ectoderm consists of multiple layers, different from the single layer of Cynops ectoderm. This multilayer structure of Xenopus ectoderm may prevent complete treatment of activin A and subsequent whole differentiation into endoderm. In the present study, therefore, Xenopus ectoderm was separated into an outer layer and an inner layer, which were individually treated with a high concentration of activin A (100 ng/mL). Then the differentiation and inductive activity of these ectodermal cells were examined in explantation and transplantation experiments. In isolation culture, ectoderm treated with activin A formed endoderm. Ectodermal and mesodermal tissues were seldom found in these explants. The activin-treated ectoderm induced axial mesoderm and neural tissues, and differentiated into endoderm when it was sandwiched between two sheets of ectoderm or was transplanted into the ventral marginal zone of other blastulae. These findings suggest that Xenopus ectoderm treated with a high concentration of activin A forms endoderm and mimics the properties of the organizer as in Cynops.
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