November 1, 2007;
The Xenopus POU class V transcription factor XOct-25 inhibits ectodermal competence to respond to bone morphogenetic protein-mediated embryonic induction.
morphogenetic proteins (BMPs) have been shown to play a key role in controlling ectodermal cell fates by inducing epidermis
at the expense of neural tissue
during gastrulation. Here, we present evidence that the Xenopus POU class V transcription factor XOct-25 regulates ectodermal cell fate decisions by inhibiting the competence of ectodermal cells to respond to BMP during Xenopus embryogenesis. When overexpressed in the ectoderm
after the blastula
stage, XOct-25 suppressed early BMP responses of ectodermal cells downstream of BMP receptor activation and promoted neural induction while suppressing epidermal differentiation. In contrast, inhibition of XOct-25 function in the prospective neuroectoderm
resulted in expansion of epidermal ectoderm
at the expense of neuroectoderm
. The reduction of neural tissue
by inhibition of XOct-25 function could be rescued by decreasing endogenous BMP signaling, suggesting that XOct-25 plays a role in the formation of neural tissue
at least in part by inhibiting BMP-mediated epidermal induction (neural inhibition). This hypothesis is supported by the observation that ectodermal cells from XOct-25 morphants were more sensitive to BMP signaling than cells from controls in inducing both immediate early BMP target genes and epidermis
at the expense of neural tissue
, while cells overexpressing XOct-25 are less competent to respond to BMP-mediated induction. These results document an essential role for XOct-25 in commitment to neural or epidermal cell fates in the ectoderm
and highlight the importance of a regulatory mechanism that limits competence to respond to BMP-mediated embryonic induction.
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Fig. 2. XOct-25 promotes commitment to neural fate at the expense of epidermal fate. The phenotypes of neurula stage embryos coinjected with GR-XOct-25 mRNA (500 pg) together with β-galactosidase (β-gal) mRNA into either one of the dorsal animal blastomeres at the 8-cell stage (A, B, E, F and I–P) or one of the ventral animal blastomeres at the 16-cell stage (G and H). In C and D, mRNA was injected unilaterally into both dorsal and ventral animal blastomeres of 8-cell stage embryos in order to determine the effect on the epidermal region. Except for controls, injected embryos were treated with DEX from the blastula stage to activate GR-XOct-25 (Oct). The overexpression of GR-XOct-25 expanded the neural plate marker Sox2 at the expense of the epidermal markers epidermal keratin and Dlx3 (compare B with D and F). In the neuroectoderm, GR-XOct-25 moderately suppressed the expression of the neuronal markers N-tubulin and Xngnr-1 (J and L). There was no effect on the expression of the mesodermal markers chordin and MyoD by XOct-25 overexpression (N and P). The expression of marker genes is shown in purple. β-gal was stained in red and the injected side of the embryo is indicated by brackets. A–F and M: frontal view; N: dorso-frontal view; G and H: ventral view; I–L, O and P: dorsal view.
Fig. 4. Perturbation of XOct-25 function in the ectoderm during embryogenesis. The phenotypes of neurula stage embryos coinjected with either Oct MO or 6-mismatch control MO together with β-gal mRNA into one of the dorsal animal blastomeres at the 8-cell stage. The amounts of injected antisense oligonucleotides per embryo were 5.6 ng (A–C) and 4.3 ng (G–P) for moderate amounts, and 8.6 ng (D–F) as a large amount. The reduction of Sox2 expression by Oct MO injection was rescued by coinjection of δBMPR mRNA (500 pg, C) or GR-mutXOct-25 mRNA (250 pg, F). Note that cells receiving Oct MO expressed the epidermal markers epidermal keratin and Dlx3 at the expense of the neural marker Sox2 (B, H, I, K and L). There was no effect on the expression of the mesodermal markers chordin and MyoD by XOct-25 knockdown (N and P). The expression of marker genes is shown in purple. β-gal was stained in red and the injected side of the embryo is indicated by brackets. I and L are magnifications of H and K, respectively. The edges of ectopic epidermal marker expression are indicated by dashed lines (I and L). A–F, O and P: dorsal view; G–L and N: frontal view; M: dorso-frontal view.