Nuclearization of β-catenin in ectodermal precursors confers organizer-like ability to induce endomesoderm and pattern a pluteus larva.

Christine A Byrum,Athula H Wikramanayake

doi:10.1186/2041-9139-4-31

Christine A Byrum, Athula H Wikramanayake

Open Access

https://doi.org/10.1186/2041-9139-4-31

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Abstract

BackgroundIn many bilaterians, asymmetric activation of canonical Wnt (cWnt) signaling at the posterior pole is critical for anterior-posterior (AP) body axis formation. In 16-cell stage sea urchins, nuclearization of β-catenin in micromeres activates a gene regulatory network that defines body axes and induces endomesoderm. Transplanting micromeres to the animal pole of a host embryo induces ectopic endomesoderm in the mesomeres (ectoderm precursors) whereas inhibiting cWnt signaling blocks their endomesoderm-inducing activity and the micromeres become ectoderm-like. We have tested whether ectopic activation of cWnt signaling in mesomeres is sufficient to impart the cells with organizer-like abilities, allowing them to pattern normal embryonic body axes when recombined with a field of mesomeres.ResultsFertilized eggs were microinjected with constitutively active Xenopus β-catenin (actβ-cat) mRNA and allowed to develop until the 16-cell stage. Two mesomeres from injected embryos were then recombined with isolated animal halves (AH) from uninjected 16-cell stage embryos. Control chimeras produced animalized phenotypes (hollow balls of ectoderm) and rarely formed skeletogenic mesoderm (SM)-derived spicules, endoderm or pigment cells, a type of non-skeletogenic mesoderm (NSM). In contrast, over half of the 0.5 pg/pL actβ-cat mesomere/AH chimeras formed a partial or complete gut (exhibiting AP polarity), contained mesenchyme-like cells similar to SM, and produced pigment cells. At three days, chimeras formed plutei with normal embryonic body axes. When fates of the actβ-cat mRNA-injected mesomeres were tracked, we found that injected mesomeres formed mesenchyme-like and pigment cells, but endoderm was induced. Higher concentrations of actβ-cat mRNA were less likely to induce endoderm or pigment cells, but had similar mesenchyme-like cell production to 0.5 pg/pL actβ-cat mesomere/AH chimeras.ConclusionsOur results show that nuclear β-catenin is sufficient to endow naïve cells with the ability to act as an organizing center and that β-catenin has both cell-autonomous and non-autonomous effects on cell fate specification in a concentration-dependent manner. These results are consistent with the hypothesis that a shift in the site of early cWnt signaling in cleaving embryos could have modified polarity of the main body axes during metazoan evolution.

Highlights

In many bilaterians, asymmetric activation of canonical Wnt signaling at the posterior pole is critical for anterior-posterior (AP) body axis formation
By the late blastula stage, cell-cell signaling establishes distinct territories arranged as follows from the animal to vegetal pole: anterior neuroectoderm (ANE) and posterior neuroectoderm (PNE) both arise from the mesomeres; a small portion of the ectoderm, most of the endoderm, and non-skeletogenic mesoderm (NSM) form from the macromeres; and skeletogenic mesoderm (SM) and germline cells originate from the micromeres [4,5,6,7,8,9,10]
Mesomere descendents expressing nuclear β-catenin are micromere-like, and form NSM cells At the 16-cell stage, canonical Wnt (cWnt) signaling is activated at the vegetal pole in the sea urchin embryo when β-catenin translocates from the cytoplasm to the nucleus in the micromeres [24]

Summary

Introduction

Asymmetric activation of canonical Wnt (cWnt) signaling at the posterior pole is critical for anterior-posterior (AP) body axis formation. The distribution of critical maternal determinants required for AP axis patterning of the sea urchin embryo was revealed in classical embryological experiments where the unfertilized egg was bisected equatorially, and each half was fertilized In these experiments, the vegetal halves produced embryos that were almost normal, but the animal halves (AHs) developed into hollow ciliated balls of ectoderm (a dauerblastula) lacking endomesoderm and ectoderm-derived structures such as the stomodeum and ciliary band [2,3], demonstrating that cues for global patterning of the embryo exist in vegetal portions of the unfertilized egg. AP polarity is established early, the presence of this axis is not morphologically evident until the 16-cell stage. By the late blastula stage, cell-cell signaling establishes distinct territories arranged as follows from the animal to vegetal pole: anterior neuroectoderm (ANE) and posterior neuroectoderm (PNE) both arise from the mesomeres; a small portion of the ectoderm, most of the endoderm, and non-skeletogenic mesoderm (NSM) form from the macromeres; and skeletogenic mesoderm (SM) and germline cells originate from the micromeres [4,5,6,7,8,9,10]

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