Neural induction in the absence of mesoderm: beta-catenin-dependent expression of secreted BMP antagonists at the blastula stage in Xenopus

Abstract

A growing body of work indicates that neural induction may be initiated prior to the establishment of the gastrula mesodermal organizer. Here, we examine neural induction in Xenopus embryos in which mesoderm induction has been blocked by Cerberus-short, a reagent that specifically inhibits Nodal-related (Xnr) signals. We find that extensive neural structures with cyclopic eyes and brain tissue are formed despite the absence of mesoderm. This neural induction correlates with the expression of chordin and other BMP inhibitors-such as noggin, follistatin, and Xnr3-at the blastula stage, and requires beta-Catenin signaling. Activation of the beta-Catenin pathway by mRNA microinjections or by treatment with LiCl leads to differentiation of neurons, as well as neural crest, in ectodermal explants. Xnr signals are required for the maintenance, but not for the initiation, of BMP antagonist expression. Recent work has demonstrated a role for beta-Catenin signaling in neural induction mediated by the transcriptional down-regulation of BMP-4 expression. The present results suggest an additional function for beta-Catenin, the early activation of expression of secreted BMP antagonists, such as Chordin, in a preorganizer region in the dorsal side of the Xenopus blastula.

FIG. 1

Inhibition of Nodal signaling does not prevent CNS formation. (A-D) External and histological views of embryos injected radially into the vegetal pole of each blastomere at the 4-cell stage with either 150 pg cer-S (n=167), 1.5 ng tAlk4 (n=21) or 1.5 ng antivin mRNA (n=89) at stage 32. The cyclopic eyes are indicated by arrowheads. (E) RT-PCR analysis of the same embryos showing expression of NCAM, but a decrease of the mesodermal markers α-actin and α-globin caused by the three anti-mesodermal agents. EF1-α serves as a loading control. (F-J) Whole-mount in situ hybridization analyses of control, cer-S and ΔN-XTcf-3 injected embryos with the neural plate marker Sox-2 at stage 12.5 (F-H; dorsal view) and BMP-4 at stage 10.5 (I, J; lateral view). D, dorsal; V, ventral.

FIG. 2

Neural induction is dependent on cortical rotation. (A) RT-PCR analysis of embryos that have been irradiated with UV light (lane 3–6) and injected radially in the marginal zone at the 4-cell stage with cer-S mRNA (lane 2, 4), cer-S and β-catenin (150 pg each) mRNA (lane 5), or cer-S and tBR (1.5 ng) mRNA (lane 6). Note that expression of the neural markers NCAM, Otx-2, Krox-20 and En-2 is inhibited by UV irradiation, but is restored after injection of either by β-catenin or a dominant-negative BMP receptor (3 embryos per sample, 3 independent analyses). (B, C) Histological analysis of embryos injected with cer-S mRNA at stage 42. UV treatment results in the loss of neural structures in cer-S injected embryos (n=81). fb, forebrain; hb, hindbrain.

FIG. 3

Pre-organizer expression of chordin requires the early β-Catenin pathway for its initiation, and Xnrs for its maintenance in the gastrula organizer. Whole-mount in situ hybridizations with chordin probe of embryos injected with cer-S (600pg), ΔN-XTcf-3 (600pg) or uninjected controls at stage 8 (A–C), 9 (D–F), 10 (G–I) and 10.5 (J–L). A patch of chordin expression is detectable at least two hours before the external dorsal lip is seen at stage 10 (three independent experiments). All embryos are shown in dorsal view; pigmented embryos with strong dorsalventral polarity were used in these experiments.

FIG. 4

Expression of organizer marker genes at the blastula and gastrula. (A, B) In situ hybridization on paraffin sections with a chordin probe at stages 9 and 10.5. Embryos were sectioned sagittally along the dorsal-ventral axis. Note the broad expression domain at stage 9, resembling the area of nuclear localization of β-Catenin (Schneider et al., 1996). (C) RT-PCR analysis of embryos at stage 9 and 10.5 in the presence or absence of microinjected cer-S mRNA. Many classical organizer genes can be detected as early as stage 9 (lane 1) and chordin, noggin, follistatin and cerberus continue to be expressed in the absence of Xnr signaling (lane 2). At stage 10.5 all organizer markers, as well as Xbra, are inhibited by the anti-Xnr reagent Cer-S (lanes 3, 4). The lack of Xbra expression at stage 9 indicates that at this early blastula stage mesoderm induction has not yet taken place. (D) RT-PCR analysis of embryos injected with ΔN-XTcf-3 (lane 2) and untreated controls (lane 1) at stage 9, showing that the induction of many organizer genes is dependent on an early β-Catenin signal. ODC serves as an RNA loading control.

FIG. 5

The β-Catenin pathway is sufficient to induce pre-organizer gene expression program at the blastula stage. (A–D). Embryos were injected into the animal pole at 4-cell stage with synthetic mRNA for β-catenin (150 pg per blastomere), dnGSK-3 (150 pg per blastomere) or were treated with LiCl and analyzed at stage 9 for chordin expression by in situ hybridization. The patch of expression of chordin in panel A marks the position of the pre-organizer on the dorsal margin of blastula embryos. All embryos are shown in animal view. (E,F) Ectodermal explants of the embryos treated the same way as above were isolated at stage 8 and the expression levels of (E) chordin, noggin, follistatin and Xnr3 or (F) cerberus, dkk-1, frzb-1 and gsc were determined by RT-PCR at stage 9. EF1α and ODC serve as control for equal loading. Note that organizer gene markers in E, but not those in F, were induced by the β-Catenin pathway in ectodermal explants at blastula.

FIG. 6

β-Catenin signaling promotes neural differentiation and neural crest formation. (A) RT-PCR analysis of ectodermal explants injected with dnGSK3 (lane 3; 150 pg/blastomere), β-catenin (lane 4; 150 pg/blastomere), treated with LiCl (lane 5; 120 mM in 0.1 × Barth for 30 min at 32-cell stage), or injected with a constitutive active BMP receptor (caBR; 1 ng/blastomere) and treated with LiCl (lane 6) and harvested at stage 24. Note that induction of the neural marker genes NCAM and Otx-2 is activated by the β-Catenin pathway and requires inhibition of BMP signaling. (B) Ectodermal explants of embryos treated with LiCl express the neural crest marker Slug by RT-PCR analysis. (C–K) Ectodermal explants microinjected with dnGSK3 or β–catenin mRNA and analyzed by in situ hybridization using β-neurotubulin as a marker for differentiated neurons at stage 24 (C–E) or by morphological (F–H) and histological (I–K) criteria at stage 42. The inset in panel C shows the expression of β-neurotubulin in a control embryo. Note that in explants of dnGSK3 and β-catenin injected embryos β-neurotubulin expression is patchy and that in histological sections neural crest-like tissues with melanocytes are observed. ae, atypical epidermis; ne, neural tissue; nc, neural crest; cg, cement gland.

FIG. 7

Model for organizer induction. The diagram indicates three steps in the establishment of a dorsal signaling center. At the blastula stage nuclear β-Catenin (dotted area) induces the early zygotic expression of organizer-specific genes (black) such as chordin, noggin, follistatin and Xnr3 in the pre-organizer region. These BMP antagonists may participate in the pre-determination of the neural plate. Later, Nodal signals originating from vegetal cells (hatched area) are required for the induction of mesoderm and for maintenance of organizer gene expression. At the gastrula stage, the same cocktail of factors secreted by the mature Spemann's organizer will pattern all three germ layers and is maintained by Nodal-related signals produced from within the mesoderm (hatched area).