title: A Twist-like bHLH gene is a downstream factor of an endogenous FGF and determines mesenchymal fate in the ascidian embryos.
abstract: Ascidian larvae develop mesenchyme cells in their trunk. A fibroblast growth factor (FGF9/16/20) is essential and sufficient for induction of the mesenchyme in Ciona savignyi. We have identified two basic helix-loop-helix (bHLH) genes named Twist-like1 and Twist-like2 as downstream factors of this FGF. These two genes are phylogenetically closely related to each other, and were expressed specifically in the mesenchymal cells after the 110-cell stage. Gene-knockdown experiments using a specific morpholino oligonucleotide demonstrated that Twist-like1 plays an essential role in determination of the mesenchyme and that Twist-like2 is a downstream factor of Twist-like1. In addition, both overexpression and misexpression of Twist-like1 converts non-mesenchymal cells to mesenchymal cells. We also demonstrate that the upstream regulatory mechanisms of Twist-like1 are different between B-line mesenchymal cells and the A-line mesenchymal cells called 'trunk lateral cells'. FGF9/16/20 is required for the expression of Twist-like1 in B-line mesenchymal precursor cells, whereas FGF, FoxD and another novel bHLH factor called NoTrlc are required for Twist-like1 to be expressed in the A-line mesenchymal precursor cells. Therefore, two different but partially overlapping mechanisms are required for the expression of Twist-like1 in the mesenchymal precursors, which triggers the differentiation of the mesenchyme in Ciona embryos.
authors.affiliation: Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Kaoru S
authors.initials: KS
authors.lastname: Imai
authors.forename: Nori
authors.initials: N
authors.lastname: Satoh
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Satou
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002938:Ciona intestinalis / Q000166:cytology / Q000196:embryology / Q000235:genetics; D004627:Embryonic Induction; D005346:Fibroblast Growth Factors / Q000235:genetics / Q000378:metabolism*; D018507:Gene Expression Regulation, Developmental; D018257:Helix-Loop-Helix Motifs; D017403:In Situ Hybridization; D008648:Mesoderm / Q000166:cytology / Q000502:physiology*; D008969:Molecular Sequence Data; D009024:Morphogenesis; D009687:Nuclear Proteins / Q000737:chemistry / Q000145:classification / Q000235:genetics* / Q000378:metabolism; D016376:Oligonucleotides, Antisense / Q000378:metabolism; D010802:Phylogeny; D016415:Sequence Alignment; D014157:Transcription Factors / Q000378:metabolism; D051879:Twist-Related Protein 1; D014561:Urochordata / Q000166:cytology / Q000196:embryology* / Q000235:genetics
chemical_list: D009687:Nuclear Proteins; D016376:Oligonucleotides, Antisense; D014157:Transcription Factors; D051879:Twist-Related Protein 1; D005346:Fibroblast Growth Factors