Segregation of brain and organizer precursors is differentially regulated by Nodal signaling at blastula stage

Abstract

The Blastula Chordin- and Noggin Expressing Center (BCNE) 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.

Nodal andchurchill1position the expression of a notch ligand duringXenopusgerm layer segregation

“…Since the spatial expression pattern of churc1 in Xenopus was not previously described, we performed in situ hybridization (ISH) from mid-blastula to early neurula stages. At NF8, expression was restricted to one-half of the animal hemisphere ( Fig 1A ), persisting there at late blastula ( Fig 1B, B’, and D ), when chordin.1 ( chrd.1 ), which encodes a BMP antagonist and neural inducer, is readily expressed at the BCNE center ( Kuroda et al, 2004 ; Castro Colabianchi et al, 2021 ) ( Fig 1C ). The BCNE comprises animal and marginal cells at the blastula’s dorsal region and contains precursors of the GO, forebrain, and most of the midbrain and hindbrain ( Kuroda et al, 2004 ).…”

“…A truncated form, Cerberus-short (Cer-S), only retains the ability to bind Nodal ( Piccolo et al, 1999 ). Thus, we injected cer-S mRNA, which has been successfully employed as a general Nodal antagonist to inhibit mesoderm and endoderm induction in plenty of works ( Piccolo et al, 1999 ; Engleka et al, 2001 ; Kofron et al, 2004 ; Castro Colabianchi et al, 2021 ). On the injected side, cer-S suppressed dll1 in the IMZ ( Fig 8B, B’, and H ; also compare with uninjected control in Fig 8D’ ) and hes4 in the NIMZ ( Fig 8F, F’, and I ), demonstrating that Nodal is necessary for establishing their MZ domains.…”