We found that fragments immediately next to the bend of the lip were the most potent inducers ( Fig. To further narrow down the list of candidates, we subdivided the donor blastopore lips into four minute fragments along the oral-aboral axis and tested whether a certain part of the lip is particularly efficient in inducing secondary axes. Five Wnt genes are expressed in concentric, partially overlapping ectodermal rings around the blastopore at mid-gastrula: Wnt1, Wnt2, Wnt3, Wnt4 and WntA ( Fig. Since only the blastopore lip is capable of inducing a second axis upon transplantation 2, our search for a candidate Wnt was restricted to Wnt genes, which are expressed in the blastopore lip of the Nematostella mid-gastrula. The Nematostella genome harbors 13 Wnt genes, which are expressed in staggered domains along the oral-aboral axis of the embryo and larva 15, 16. 1) suggesting that ectopic activation of Wnt/β-catenin signaling in discrete cell patches is sufficient for the induction of head structures. Primary polyps mosaic for the APC mutation formed multiple ectopic tentacles and oral openings in F0 ( Fig. We addressed the role of Wnt signaling in axis formation in Nematostella by performing CRISPR/Cas9-mediated knockout of the putative intracellular Wnt/β-catenin signaling inhibitor APC. Wnt1 and Wnt3 convey axial organizer capacity Our data suggest an ancient origin of the Wnt/β-catenin dependent blastopore-associated axial organizer, predating the cnidarian-bilaterian split. In this paper, using a combination of experimental embryology and molecular analyses, we show that two Nematostella Wnt ligands are capable of conveying axial organizing capacity to any region of the gastrula ectoderm, where they are co-expressed. We set out to test whether Wnt signaling is conveying inductive capacity to the blastoporal organizer of Nematostella. Moreover, it also results in the formation of double-headed polyps 14, making canonical Wnt signaling the prime candidate for the role of the early axial inducer in Nematostella, similar to the situation in frog. Pharmacological activation of Wnt/β-catenin signaling in Nematostella larvae affects the oral-aboral patterning, as detected by changes in the expression of Wnt2, Wnt4, brachyury ( Bra) and several other genes 11, 12, 13. In contrast, complete ectopic body axes can be induced by single blastomere injection of Wnt-1, Xwnt-8, dishevelled, β-catenin and dominant negative GSK3β mRNA 7, 8, 9, 10, showing that Wnt/β-catenin signaling is crucial for setting up the organizer. However, complete ectopic body axes, like the ones seen during transplantation of the dorsal blastopore lip 1 or the Nieuwkoop center cells 6, nearly never form. In the frog Xenopus, the Spemann organizer expresses Wnt and BMP antagonists 3, whose microinjection in single vegetal blastomeres can induce ectopic rostral or dorsal structures correspondingly 4, 5. Whether this similarity reflects the homologous or convergent origin of the cnidarian and vertebrate blastoporal axial organizers remained unclear, since the molecular nature of the signal conveying axial organizer properties to an embryonic tissue has not been determined outside deuterostomes. Although axis-forming blastoporal organizers were long thought to be a chordate-specific feature, transplantation of a fragment of the mid-gastrula blastopore lip in the sea anemone Nematostella vectensis, a member of the early branching non-bilaterian phylum Cnidaria (which, in addition to sea anemones, includes corals, hydroids and jellyfish), also resulted in the formation of an ectopic body axis 2, just like the Mangold-Spemann organizer in amphibians. The ability of the amphibian dorsal blastopore lip to induce ectopic body axes upon transplantation has fascinated biologists for nearly a century 1.
0 kommentar(er)
