November 1, 2004;
Neural induction requires BMP inhibition only as a late step, and involves signals other than FGF and Wnt antagonists.
A dominant molecular explanation for neural induction is the ''default model'', which proposes that the ectoderm
is pre-programmed towards a neural fate, but is normally inhibited by endogenous BMPs. Although there is strong evidence favouring this in Xenopus, data from other organisms suggest more complexity, including an involvement of FGF and modulation of Wnt. However, it is generally believed that these additional signals also act by inhibiting BMPs. We have investigated whether BMP inhibition is necessary and/or sufficient for neural induction. In the chick, misexpression of BMP4
in the prospective neural plate
inhibits the expression of definitive neural markers (Sox2
and late Sox3
), but does not affect the early expression of Sox3
, suggesting that BMP inhibition is required only as a late step during neural induction. Inhibition of BMP signalling by the potent antagonist Smad6
, either alone or together with a dominant-negative BMP receptor, Chordin
in competent epiblast is not sufficient to induce expression of Sox2
directly, even in combination with FGF2
and/or antagonists of Wnt signalling. These results strongly suggest that BMP inhibition is not sufficient for neural induction in the chick embryo
. To test this in Xenopus, Smad6
mRNA was injected into the A4 blastomere
(which reliably contributes to epidermis
but not to neural plate
or its border) at the 32-cell stage
: expression of neural markers (Sox3
) is not induced. We propose that neural induction involves additional signalling events that remain to be identified.
[+] show captions
Fig. 6. BMP inhibition by Smad6 is not sufficient for neural induction in Xenopus ventral epidermis. (A-I) cSmad6 was targeted to the ventral epidermis by injection into the A4 blastomere of 32-cell stage embryos (A). Smad6 does not activate Sox3 expression (B-E), compare with GFP-injected control embryos (F-I). B,C,F,G are dorsal views; D,E,H,I are ventral views of the embryos in B,C,F,G. C,E,G,I show the embryos in B,D,F,H after staining with anti-fluorescein to reveal the lineage tracer FDX which was co-injected with the mRNA. (J-L) To test that the progeny of the A4 blastomere is competent to respond to neural inducing signals from the organizer, a lineage tracer (FDX) was injected into the A4 blastomere. At late blastula/early gastrula stage, the labelled cells are transplanted into the dorsal side of an unlabelled host embryo, which is grown to early neurula stage (J). (K,L) The transplanted cells (FDX in brown in L) contribute to the neural plate and express Sox3 (purple).
Fig. 1. Smad6 does not induce neural markers in the chick. (A-G) Experiments to test the activity of Smad6. Smad6-IRES-GFP was electroporated into one half of the chick neural tube (A,B). Staining against phospho-Smad1 (A) reveals that activation of Smad1 has been inhibited in the electroporated cells (green in B), while in control GFP electroporated embryos (C,D) phospho-Smad1 is not altered (C). cSmad6 injection into the marginal zone in Xenopus (E) induces a secondary axis (F), while GFP-injected controls appear normal (G). (H-N) Smad6 does not induce neural markers. Electroporation of Smad6 into competent area opaca epiblast at stage 3+ (H) does not induce Brachyury (I,L; light blue), Sox2 (J) or Sox3 (M) (purple). In this and subsequent figures, electroporated cells were visualized by staining with anti-GFP antibody (K, N; brown). (O-Q) Positive controls. cSmad6 or GFP was injected at the two-cell stage and animal caps isolated at early gastrula (O). Smad6 (P) can neuralize animal caps, while GFP cannot (Q), as assessed by Sox3 expression.
Fig. 7. Effects of BMP inhibition in combination with FGF in Xenopus. Embryos were injected into the A4 blastomere at the 32-cell stage. FGF4 in combination with cSmad6 does not induce brachyury in prospective ventral ectoderm cells at stage 10+ (A-C). Neither FGF8 nor FGF4 is able to induce Sox3 in the ventral epidermis cells of early neurula stage embryos (D-I), while misexpression of FGF4 together with Smad6 induces a patch of Sox3 (J-L). (A,D,G,J) Dorsal views; (B,E,H,K) ventral views of same embryos, which are shown again in C,F,I,L after anti-fluorescein staining.