Cell-autonomous and inductive processes among three embryonic domains control dorsal-ventral and anterior-posterior development of Xenopus laevis

Cell-autonomous signal transduction in the Xenopus egg Wnt/β-catenin pathway

Wnt proteins are thought to bind to their receptors on the cell surfaces of neighboring cells. Wnt8 likely substitutes for the dorsal determinants in Xenopus embryos to dorsalize early embryos via the Wnt/β-catenin pathway. Here, we show that Wnt8 can dorsalize Xenopus embryos working cell autonomously. Wnt8 mRNA was injected into a cleavage-stage blastomere, and the subcellular distribution of Wnt8 protein was analyzed. Wnt8 protein was predominantly found in the endoplasmic reticulum (ER) and resided at the periphery of the cells; however, this protein was restricted to the mRNA-injected cellular region as shown by lineage tracing. A mutant Wnt8 that contained an ER retention signal (Wnt8-KDEL) could dorsalize Xenopus embryos. Finally, Wnt8-induced dorsalization occurred only in cells injected with Wnt8 mRNA. These experiments suggest that the Wnt8 protein acts within the cell, likely in the ER or on the cell surface in an autocrine manner for dorsalization.

Growth differentiation factor 11 is an encephalic regionalizing factor in neural differentiated mouse embryonic stem cells

Background

The central nervous system has a complex structural organization and consists of different subdomains along the antero-posterior axis. However, questions remain about the molecular mechanisms leading to the regionalization of this organ. We used a previously developed methodology to identify the novel patterning role of GDF11, a TGF-β signaling factor.

Findings

Using an assay based on neural differentiated mouse embryonic stem cells, GDF11 is shown to induce diencephalic (posterior forebrain), mesencephalic (midbrain) and metencephalic (anterior hindbrain) fates at the expense of telencephalic (anterior forebrain) specification. GDF11 has not previously been implicated in the early patterning of the nervous system. In addition, inhibition of the TGF-β type I receptors Alk4, Alk5 and Alk7 by the pharmacological inhibitor SB431542 caused a strong anteriorization of the cells.

Conclusions

Our findings suggest that GDF11 is involved in the earliest steps of the brain patterning during neurogenesis in the vertebrate embryo and is shown to be a regionalizing factor of the regional fate in the developing brain. This regionalization is not a typical posteriorizing signal as seen with retinoic acid, FGF or BMP molecules. To our knowledge, this is the first time that GDF11 is implicated in the earliest steps of the patterning of the neural plate.

Anterior-posterior patterning of neural differentiated embryonic stem cells by canonical Wnts, Fgfs, Bmp4 and their respective antagonists

Embryonic stem (ES) cells are pluripotent and can differentiate into every cell type of the body. Next to their potential in regenerative medicine, they are excellent tools to study embryonic development. In this work the processes of neural induction and neural patterning along the antero-posterior (A/P) body axis are studied and evidence suggests a two step mechanism for these events. First, neural induction occurs by default in the primitive ectoderm, forming anterior neural tissue and thereafter, a series of factors can posteriorize this anterior neurectoderm. In a gain-of-function/loss-of-function approach using mouse ES cells, we show that Fgf2 has the strongest caudalizing potential of all Fgfs tested. Furthermore, Bmp4 and Wnt3a, but not Wnt1, can caudalize the neurectodermal cells. The effect of the antagonists of these factors was also examined and though Dkk1 and Noggin clearly have an effect that opposes that of Wnt3a and Bmp4 respectively, they fail to anteriorize the neurectoderm. The patterning effect of SU5402, an Fgf receptor inhibitor, was rather limited. These data confirm that in the mouse, two steps are involved in neural patterning and we show that while Fgf4, Fgf8 and Wnt1 have no strong patterning effect, Fgf2, Wnt3a and Bmp4 are strong posteriorizing factors.

VegT, eFGF and Xbra cause overall posteriorization while Xwnt8 causes eye-level restricted posteriorization in synergy with chordin in early Xenopus development

We examined several candidate posterior/mesodermal inducing molecules using permanent blastula-type embryos (PBEs) as an assay system. Candidate molecules were injected individually or in combination with the organizer factor chordin mRNA. Injection of chordin alone resulted in a white hemispherical neural tissue surrounded by a large circular cement gland, together with anterior neural gene expression and thus the development of the anterior-most parts of the embryo, without mesodermal tissues. When VegT, eFGF or Xbra mRNAs were injected into a different blastomere of the chordin-injected PBEs, the embryos elongated and formed eye, muscle and pigment cells, and expressed mesodermal and posterior neural genes. These embryos formed the full spectrum of the anteroposterior embryonic axis. In contrast, injection of CSKA-Xwnt8 DNA into PBEs injected with chordin resulted in eye formation and expression of En2, a midbrain/hindbrain marker, and Xnot, a notochord marker, but neither elongation, muscle formation nor more posterior gene expression. Injection of chordin and posteriorizing molecules into the same cell did not result in elongation of the embryo. Thus, by using PBEs as the host test system we show that (i) overall anteroposterior neural development, mesoderm (muscle) formation, together with embryo elongation can occur through the synergistic effect(s) of the organizer molecule chordin, and each of the 'verall posteriorizing molecules'eFGF, VegT and Xbra; (ii) Xwnt8-mediated posteriorization is restricted to the eye level and is independent of mesoderm formation; and (iii) proper anteroposterior patterning requires a separation of the dorsalizing and posteriorizing gene expression domains.