title: A signalling relay involving Nodal and Delta ligands acts during secondary notochord induction in Ciona embryos.
abstract: The notochord is one of the defining features of chordates. The ascidian notochord is a rod like structure consisting of a single row of 40 cells. The anterior 32 ;primary' notochord cells arise from the A-line (anterior vegetal) blastomeres of the eight-cell stage embryo, whereas the posterior 8 ;secondary' notochord cells arise from the B-line (posterior vegetal) blastomeres of the eight-cell stage embryo. Specification of notochord precursors within these two lineages occurs in a spatially and temporally distinct manner. We show that specification of the secondary but not the primary notochord in Ciona intestinalis requires a relay mechanism involving two signalling pathways. First, we show evidence that acquisition of secondary notochord fate is dependent upon lateral Nodal signalling sources, situated in the adjacent b-line animal cells. Expression of the notochord specific gene Ci-Brachyury in the secondary notochord precursor was downregulated following selective inhibition of Nodal signal reception in B-line derivatives and also, strikingly, following selective inhibition of Nodal signal reception in A-line cell derivatives. Within the A-line, Nodal signals are required for localised expression of Delta2, which encodes a divergent form of Delta ligand. Using four distinct reagents to inhibit Delta2/Notch signals, we showed that Delta2 signalling from A-line cells, which activates the Notch/Su(H) pathway in adjacent B-line cells, is required for specification of the secondary notochord precursor. We propose a model whereby laterally produced Nodal acts to specify the secondary notochord precursor both directly in the B-line cells and via Delta2 induction in adjacent A-line cells.
authors.affiliation: Biologie du Développement, UMR 7009 CNRS/Universite Pierre et Marie Curie (Paris VI Villefranche-sur-Mer, France. clare.hudson@obs-vlfr.fr
authors.forename: Clare
authors.initials: C
authors.lastname: Hudson
authors.forename: Hitoyoshi
authors.initials: H
authors.lastname: Yasuo
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001757:Blastomeres / Q000502:physiology; D002938:Ciona intestinalis / Q000196:embryology*; D017124:Conserved Sequence; D004331:Drosophila melanogaster / Q000196:embryology; D004625:Embryo, Nonmammalian / Q000502:physiology*; D018507:Gene Expression Regulation, Developmental; D017403:In Situ Hybridization; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins / Q000235:genetics / Q000502:physiology*; D008969:Molecular Sequence Data; D055457:Nodal Protein; D009672:Notochord / Q000502:physiology*; D012333:RNA, Messenger / Q000235:genetics; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D013112:Spiders / Q000196:embryology; D016212:Transforming Growth Factor beta / Q000235:genetics / Q000502:physiology*
chemical_list: D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D055457:Nodal Protein; D012333:RNA, Messenger; D016212:Transforming Growth Factor beta; C064045:delta protein