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

Abstract

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 'overall 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.

The splicing factor PQBP1 regulates mesodermal and neural development through FGF signaling

“…Specifically, induction of cdx4 and bra by FGF4 was diminished in PQBP1 morphants, despite normal induction of cdx4 by Wnt8 or bra by BMP4. Furthermore, induction of fgf4 by FGF4, induced by FGF4/Brachyury-positive feedback loops (Isaacs et al, 1994;Fujii et al, 2008), was also blocked by PQBP1 knockdown.…”

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

“…), they do not form dorsal structures or express dorsal marker genes (Fujii et al . , ). When Wnt8 and VegT mRNAs were co‐injected into a PBE blastomere, it forms dorsal structures and expresses the organizer marker gene chordin .…”

“…Wnt/b-catenin signaling is likely involved in this cellautonomous process (Moon 2005). Wnt8 is not present in cleavage stage embryos, and its function in normal development is the posteriorization, not the dorsalization, of the embryo (Christian & Moon 1993;Fujii et al 2008). Interestingly, however, Wnt8 mRNA induces a complete secondary body when injected into a ventral blastomere of early Xenopus embryos (Christian et al 1991;Smith & Harland 1991).…”

“…Permanent blastula-type embryos (PBEs) are obtained when >60% of the vegetal region (including both the surface and cytoplasm) is ablated from fertilized Xenopus embryos. Because PBEs neither have dorsal nor subequatorial determinants, the molecular nature of the latter is VegT mRNA (Zhang et al 1998), they do not form dorsal structures or express dorsal marker genes (Fujii et al 2002(Fujii et al , 2008. When Wnt8 and VegT mRNAs were co-injected into a PBE blastomere, it forms dorsal structures and expresses the organizer marker gene chordin.…”