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Abstract
Formation of the vertebrate body plan is controlled by discrete head and trunk organizers that establish the anteroposterior pattern of the body axis. The Goosecoid (Gsc) homeodomain protein is expressed in all vertebrate organizers and has been implicated in the activity of Spemann's organizer in Xenopus. The role of Gsc in organizer function was examined by fusing defined transcriptional regulatory domains to the Gsc homeodomain. Like native Gsc, ventral injection of an Engrailed repressor fusion (Eng-Gsc) induced a partial axis, while a VP16 activator fusion (VP16-Gsc) did not, indicating that Gsc functions as a transcriptional repressor in axis induction. Dorsal injection of VP16-Gsc resulted in loss of head structures anterior to the hindbrain, while axial structures were unaffected, suggesting a requirement for Gsc function in head formation. The anterior truncation caused by VP16-Gsc was fully rescued by Frzb, a secreted Wnt inhibitor, indicating that activation of ectopic Wnt signaling was responsible, at least in part, for the anterior defects. Supporting this idea, Xwnt8 expression was activated by VP16-Gsc in animal explants and the dorsal marginal zone, and repressed by Gsc in Activin-treated animal explants and the ventral marginal zone. Furthermore, expression of Gsc throughout the marginal zone inhibited trunk formation, identical to the effects of Frzb and other Xwnt8 inhibitors. A region of the Xwnt8 promoter containing four consensus homeodomain-binding sites was identified and this region mediated repression by Gsc and activation by VP16-Gsc, consistent with direct transcriptional regulation of Xwnt8 by Gsc. Therefore, Gsc promotes headorganizer activity by direct repression of Xwnt8 in Spemann's organizer and this activity is essential for anterior development.
Fig. 1. Transcriptional repression by Gsc regulates anterior development. (A) Schematic of the Gsc fusion constructs. A C-terminal region of Gsc (residues 128-244), containing the homeodomain (HD), was fused to the Engrailed repressor (residues 1-298) (Eng-Gsc) or the VP16 activator (residues 410-490) (VP16-Gsc). At the four-cell stage, one ventral (C,E,G) or two dorsal (B,D,F,J,K) blastomeres were injected with 150 pg of Gsc (B,C), 150 pg of Eng-Gsc (D,E) or 500 pg of VP16-Gsc (F,G,J,K) mRNA. See Table 1 for quantitation. In situ hybridization for Otx2 (H,J) or En2 (I,K) and immunocytochemistry with the muscle antibody 12/101 (H,J) or the notochord antibody Tor 70 (I,K) were performed to assess the axial and neural development of VP16-Gsc-injected or uninjected (Control) embryos. White arrowheads indicate partial secondary axis (C,E) or cement gland (B,D). Black arrowheads indicate muscle (H,J) or notochord staining (I,K). Black arrows indicate Otx2 (H) or En2 (I,K) staining. Scale bar: 0.5 mm.
Fig. 2. VP16-Gsc is a specific inhibitor of Gsc function. One ventral (A,B) or two dorsal (C,D) blastomeres were injected at the four-cell stage with 150 pg of Gsc (A), 500 pg of VP16-Gsc (C) or a combination of Gsc and VP16-Gsc (B,D). An uninjected embryo is also shown (E). Arrowheads indicate partial secondary axis (A) or cement gland (D). Scale bar: 0.5 mm.
Fig. 3. Ectopic activation of Wnt signaling by VP16-Gsc inhibits head formation. At the four-cell stage two dorsal blastomeres were injected with 500 pg of VP16-Gsc (B), 100 pg of Frzb (C), 150 pg of tBMPR (E), VP16-Gsc and Frzb (D) or VP16-Gsc and tBMPR (F). An uninjected embryo is also shown (A). Quantitation is shown in G. Each bar represents the percentage of embryos with anterior truncations resulting from five independent experiments. n, total number of injected embryos. Scale bar: 0.5 mm.
Fig. 4. VP16-Gsc activates Xwnt8 expression in animal explants. Animal explants were isolated from uninjected (A,B) or VP16-Gsc-injected embryos (C,D), and were cultured in the absence (A,C) or presence (B,D) of Activin protein. The morphology of explants was assessed at the tailbud stage. The expression of Xbra, Xwnt8 and BMP4 in response to Gsc or VP16-Gsc was examined by RT-PCR analysis at the gastrula stage (E). EF1α is a control for RNA recovery and loading. Intact embryos (embryo) served as a positive control and an identical reaction without reverse transcriptase controlled for PCR contamination (embryo-RT). Scale bar: 0.2 mm.
Fig. 5. Cell autonomous activation of Xwnt8 by VP16-Gsc. At the 16-cell stage, one dorsal marginal zone blastomere was injected with 750 pg of β-galactosidase (β-gal) mRNA (A) or a combination of β-gal and 500 pg of VP16-Gsc (B). Descendents of the injected cell were identified at the gastrula stage by the presence of β-gal, using the Rose-gal substrate. Activation of Xwnt8 by VP16-Gsc in the dorsal marginal zone and endogenous ventrolateral expression of Xwnt8 was detected by in situ hybridization with BM-purple substrate (A,B). Dorsal views of stage 10.25 embryos with endogenous Xwnt8 expression visible in lateral regions. Insets show higher magnification views of boxed regions. Black arrowheads indicate β-gal-positive nuclei (red) and the white arrowhead indicates cytoplasmic Xwnt8 in situ stain (purple). Scale bar: 0.25 mm; 80 μm in the insets.
Fig. 7. Complete axis induction by co-expression of Gsc and a truncated BMP receptor. At the four-cell stage, two ventral blastomeres were injected with 100 pg of Gsc (B), 250 pg of a truncated BMPR (tBMPR) (C) or a combination of both (D), and axis induction was assessed at the late tadpole stage. Arrowhead indicates partial axis (C) or ectopic eye in a complete secondary axis (D). Quantitation is shown in E. Each bar represents the percentage of embryos with ectopic axes resulting from four independent experiments. Dark bar, complete secondary axes; light bar, partial secondary axes; n, total number of injected embryos. Scale bar: 1.0 mm.
Fig. 8. VP16-Gsc alters gene expression at the gastrula and neurula stages. At the four-cell stage, two dorsal blastomeres were injected with 500 pg of VP16-Gsc. Embryos were fixed at stage 10.5 (early gastrula), 12.5 (late gastrula), 15 (mid-neurula) or 20 (late neurula). Gene expression was detected by in situ hybridization of uninjected (Control) and VP16-Gsc-injected embryos. VP16-Gsc caused an expansion of Xwnt8 (B) and MyoD (F) into the dorsal marginal zone at stage 10.5, and MyoD-positive cells were present at the midline (white arrow) at stage 15 (H). Frzb and Chordin expression in the organizer was not affected at stage 10.5 (D,J) and Chordin expression in the chordamesoderm was normal at stage 15 (L). Otx2 was reduced or absent at all stages examined (N,P,R,T). Opl expression in the prospective neural plate was not affected by VP16-Gsc at stage 10.5 (V), but was absent from the most anterior domain of the neural plate (bracket) at stage 12.5 (X). Black arrowheads indicate the dorsal blastopore lip. The black arrows indicate the primordia of the forebrain (fb), eyes (e) and cement gland (cg) (S). Vegetal views are shown in A-F,I,J,M,N, dorsal view is shown in G,H,K,L,U,V with anterior on the left in G,H,K,L and anterior view is shown in O-T,W,X. Scale bar: 0.5 mm.
