The transcription factor FoxB mediates temporal loss of cellular competence for notochord induction in ascidian embryos

Abstract

In embryos of the ascidian Halocynthia roretzi, the competence of isolated presumptive notochord blastomeres to respond to fibroblast growth factor (FGF) for induction of the primary notochord decays by 1 hour after cleavage from the 32- to 64-cell stage. This study analyzes the molecular mechanisms responsible for this loss of competence and provides evidence for a novel mechanism. A forkhead family transcription factor, FoxB, plays a role in competence decay by preventing the induction of notochord-specific Brachyury (Bra) gene expression by the FGF/MAPK signaling pathway. Unlike the mechanisms reported previously in other animals, no component in the FGF signal transduction cascade appeared to be lost or inactivated at the time of competence loss. Knockdown of FoxB functions allowed the isolated cells to retain their competence for a longer period, and to respond to FGF with expression of Bra beyond the stage at which competence was normally lost. FoxB acts as a transcription repressor by directly binding to the cis-regulatory element of the Bra gene. Our results suggest that FoxB prevents ectopic induction of the notochord fate within the cells that assume a default nerve cord fate, after the stage when notochord induction has been completed. The merit of this system is that embryos can use the same FGF signaling cascade again for another purpose in the same cell lineage at later stages by keeping the signaling cascade itself available. Temporally and spatially regulated FoxB expression in nerve cord cells was promoted by the ZicN transcription factor and absence of FGF/MAPK signaling.

A time delay gene circuit is required for palp formation in the ascidian embryo

“…FGF9/16/20 is expressed as early as the 16-cell stage, and is used repeatedly to induce different genes in different cells at different times by functioning in combination with different transcription factors (Imai et al, 2002;Bertrand et al, 2003;Imai et al, 2003;Yagi et al, 2004;Imai et al, 2006;Kumano et al, 2006;Lamy et al, 2006;Hudson et al, 2007;Picco et al, 2007;Shi and Levine, 2008;Hashimoto et al, 2011). The present study shows that the same signaling pathway can evoke differential outputs (ZicL expression or not) in a single lineage of cells at different times by functioning in combination with Hes-a and BZ1/BZ2 repressors (supplementary material Fig.…”

Encoding anatomy: Developmental gene regulatory networks and morphogenesis

“…Each progenitor of a notochord founder cell divides asymmetrically to produce one presumptive neural cell and one presumptive notochord cell. Signals from animal blastomeres, acting through ephrin‐Eph, restrict MAPK signaling to the notochord founder cell, thereby preventing the expression of a direct repressor of bra , foxb , which is activated in the prospective neural cell (Picco etal ., ; Hashimoto etal ., ). These various findings demonstrate that the specification of notochord cells in the A‐line, as reflected by the restricted expression of bra , results from the integration of multiple, spatially restricted inputs (Kumano etal ., ).…”

Nodal and FGF coordinate ascidian neural tube morphogenesis

“…It will also be important to determine how this particular response to FGF signaling is achieved in contrast to the many other instances of MAPK-mediated FGF signaling during Ciona embryogenesis. FoxB is known to prevent notochord induction in response to FGF in the A-line neural lineage (Hashimoto et al, 2011), but it remains to be seen whether FoxB is sufficient to promote the development of floor plate cells in response to FGF.…”