XB-ART-8311Development 2001 Oct 01;12819:3783-93. doi: 10.1242/dev.128.19.3783.
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Role of Goosecoid, Xnot and Wnt antagonists in the maintenance of the notochord genetic programme in Xenopus gastrulae.
The Xenopus trunk organiser recruits neighbouring tissues into secondary trunk axial and paraxial structures and itself differentiates into notochord. The inductive properties of the trunk organiser are thought to be mediated by the secretion of bone morphogenetic protein (BMP) antagonists. Ectopic repression of BMP signals on the ventral side is sufficient to mimic the inductive properties of the trunk organiser. Resultant secondary trunks contain somite and neural tube, but no notochord. We show that inhibition of BMP signalling is sufficient for the initiation of the trunk organiser genetic programme at the onset of gastrulation. During late gastrulation, however, this programme is lost, due to an invasion of secreted Wnts from neighbouring tissues. Maintenance of this programme requires co-repression of BMP and Wnt signalling within the presumptive notochord region. To shed light on the molecular cascade that leads to the repression of the Wnt pathway, we looked for individual organiser genes whose overexpression could complement the inhibition of BMP signalling to promote notochord formation in the secondary trunks. Two genes, gsc and Xnot, were thus identified and shown to act in different ways. Xnot acts as a transcriptional repressor within the mesodermal region. Gsc acts in deeper vegetal cells, where it regulates Frzb expression to maintain Xnot expression in the neighbouring notochord territory. These results suggest that, during gastrulation, the necessary repression of Wnt/beta-catenin signalling in notochord precursors is achieved by the action of secreted inhibitors, such as Frzb, emitted by gsc-expressing dorsal vegetal cells.
PubMed ID: 11585804
Article link: Development
Species referenced: Xenopus laevis
Genes referenced: chrd.1 foxa4 frzb gsc lhx1 not
Antibodies: Notochord Ab2
Morpholinos: ctnnb1 MO1
Article Images: [+] show captions
|Fig. 3. Expression of organiser genes is initiated on the ventral side when BMP signals are ectopically blocked, but is not maintained by the late gastrula stage. (A,E,I,M,Q,U) Vegetal view of uninjected mid-gastrulae (stage 11). (B,F,J,N,R,V) Vegetal view of mid-gastrulae injected ventrally with tBR RNA and lacZ RNA as lineage tracer. (C,G,K,O,S,W) Dorsal-vegetal view of uninjected late gastrulae (stage 13). (D,H,L,P,T,X) Dorsal-vegetal view of late gastrulae injected ventrally with tBR RNA. The identity of the probes used is indicated on the left of each row of panels. Note that the expression of all the genes tested is lost at the late gastrula stage, with the exception of chordin (D). β-galactosidase activity is shown in red, gene expression in purple.|
|Fig. 6. Comparative analysis of the expression of Xnot, gsc and frzb during the course of gastrulation. Embryos at the indicated stages were bisected along the dorsoventral axis, and processed for whole-mount in situ hybridisation analyses with the indicated probes. Dorsal is towards the right.|
|Fig. 8. Regulatory interactions between Xnot, gsc and frzb during notochord formation. Embryos at the indicated stages (left) were injected with the indicated combination of factors (lower right corner of each panel) and processed for in situ hybridisation with the indicated probes (upper right corner of each panel). (A-H) Vegetal views. (I-L) Dorsal or dorso-lateral views. (M-T) Embryos were bisected along the dorsoventral axis before being processed for whole-mount in situ hybridisation. Injected cells are revealed by their β-galactosidase activity (red). Gene expression is shown in purple.|