β-Catenin-Driven Binary Fate Specification Segregates Germ Layers in Ascidian Embryos

• Published in: Current biology 2013-03, Vol.23 (6), p.491-495
• Main Authors: Hudson, Clare, Kawai, Narudo, Negishi, Takefumi, Yasuo, Hitoyoshi
• Format: Article
• Language: English
• Subjects: Animals; Ascidiacea; beta Catenin - genetics; beta Catenin - metabolism; Ciona intestinalis - embryology; Ciona intestinalis - genetics; Ciona intestinalis - metabolism; Cnidaria; Development Biology; Echinodermata; Embryo, Nonmammalian - embryology; Embryo, Nonmammalian - metabolism; Gene Expression Regulation, Developmental; genes; Germ Layers - embryology; Germ Layers - metabolism; invertebrates; Life Sciences; Polymerase Chain Reaction; signal transduction; Species Specificity; transcription (genetics); transcription factors; Urochordata - embryology; Urochordata - genetics; Urochordata - metabolism
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2013.02.005

β-catenin is a transcriptional cofactor mediating the “canonical” Wnt signaling pathway, which activates target genes in a complex with TCF (LEF) transcription factors [1]. In many metazoans, embryos are first subdivided during early cleavage stages into nuclear β-catenin-positive and -negative domains, with β-catenin specifying endoderm or mesendoderm fate. This process has been demonstrated in a wide range of phyla including cnidarians, nemerteans, and invertebrate deuterostomes (echinoderms, hemichordates, and ascidians), implying that β-catenin-dependent (mes)endoderm specification is evolutionarily ancient [2–10]. However, the mechanisms leading to the segregation of mesoderm and endoderm fates from a transient mesendodermal state are less well defined. We show that subdivision of the ascidian embryo into the three germ layers involves differential nuclear β-catenin activity coupled with the first two animal-vegetal (A-V)-oriented cell divisions. We reveal that each of these A-V divisions operates as a binary fate choice: the first between ectoderm and mesendoderm and the second between margin (notochord and neural) and endoderm, such that a β-catenin activation sequence of ON-to-ON specifies endoderm, OFF-to-OFF ectoderm, and ON-to-OFF margin. [Display omitted] ► β-catenin nuclear asymmetry after animal-vegetal-oriented cell divisions ► β-catenin nuclear asymmetry drives binary cell fate choices ► Combinatorial codes of nuclear β-catenin activation segregate ascidian germ layers ► Nuclear β-catenin ON-to-OFF activity is required for marginal mesoderm formation