Abstract
ABSTRACTThe blastula Chordin- and Noggin-expressing (BCNE) center comprises animal-dorsal and marginal-dorsal cells of the amphibian blastula and contains the precursors of the brain and the gastrula organizer. Previous findings suggested that the BCNE behaves as a homogeneous cell population that only depends on nuclear β-catenin activity but does not require Nodal and later segregates into its descendants during gastrulation. In contrast to previous findings, in this work, we show that the BCNE does not behave as a homogeneous cell population in response to Nodal antagonists. In fact, we found that chordin.1 expression in a marginal subpopulation of notochordal precursors indeed requires Nodal input. We also establish that an animal BCNE subpopulation of cells that express both, chordin.1 and sox2 (a marker of pluripotent neuroectodermal cells), and gives rise to most of the brain, persisted at blastula stage after blocking Nodal. Therefore, Nodal signaling is required to define a population of chordin.1+ cells and to restrict the recruitment of brain precursors within the BCNE as early as at blastula stage. We discuss our findings in Xenopus in comparison to other vertebrate models, uncovering similitudes in early brain induction and delimitation through Nodal signaling.This article has an associated First Person interview with the first author of the paper.
Highlights
When the dorsal lip of an amphibian early gastrula is grafted into the ventral side of a host, a secondary embryo develops with complete anterior-posterior and dorsal-ventral axis (Spemann and Mangold, 1924; De Robertis and Kuroda, 2004)
blastula Chordin- and Noggin-expressing (BCNE) cells do not respond uniformly to Nodal blockade We blocked Nodal by injecting cer-S and analyzed at s9 by in situ hybridization (ISH) if this could result in spatial changes of the BCNE marker chrd.1 and its upstream regulator, the direct Wnt/ nβ-cat target gene sia1 (Ishibashi et al, 2008; Reid et al, 2012)
At blastula stages, tbxt expression was suppressed in the dorsal region but persisted in the remaining presumptive mesodermal ring, while gsc expression was suppressed in these mutants (Feldman et al, 1998; Gritsman et al, 1999). These observations indicate that the population of chrd+ cells in the zebrafish blastula is heterogeneous in Nodal requirements, as we found in Xenopus. They show that the chordal mesoderm (CM) precursors as well as the prechordal mesoderm (PM) precursors require Nodal signaling for their initial specification (Fig. 7D)
Summary
When the dorsal lip of an amphibian early gastrula is grafted into the ventral side of a host, a secondary embryo develops with complete anterior-posterior and dorsal-ventral axis (Spemann and Mangold, 1924; De Robertis and Kuroda, 2004). The siamois-related homeobox genes (sia) are directly activated by the dorsal Wnt/nβ-cat cascade They encode the first transcription factors expressed in the BCNE and activate chrd. expression by directly binding to its promoter (Ishibashi et al, 2008; Reid et al, 2012). It appeared that only dorsal nβ-cat signaling initiates brain and GO development through the establishment of the BCNE, while Nodal would be required later for the maintenance of the GO and its descendants. These findings suggested that the BCNE behaves as a homogeneous cell population induced by dorsal nβ-cat and segregates later, during gastrulation, into brain and GO
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