GENETIC CASCADE UNDERLYING NOTOCHORD FORMATION IN ASCIDIAN EMBRYOS

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Notochord is the most prominent feature of chordates. The present study aimed to explore the genetic cascade underlying notochord formation in ascidian embryos. Previous studies, mostly from my own laboratory, demonstrated that Brachyury gene of Ciona intestinalis (Ci-Bra) begins to be expressed at the 64-cell stage and plays-an essential role in the notochord differentiation. In the present study, we investigated (1) Ci-Bra upstream cascade which leads to the initiation of Ci-Bra expression and (2) Ci-Bra downstream cascade which leads to morphological formation of the notochord(1) Ci-Bra upstream cascade : In the present study, we make clear that (1) maternally supplied beta-catenin is translocated after fertilization from the cytoplasm into nuclei of vegetal blastomeres and specifies the endoderm developmental fate, (2) beta-catenin triggers the activation of various target genes including FoxD, ZicL, and Fgf9/16/20, (3) FoxD is a direct target of beta-catenin and is expressed in th … More e endoderm cells at the 16-and 32-cell stage, and the gene function is essential for Ci-Bra activation, (4) ZicL is downstream of FoxD and is expressed in notochord/nerve cord cells at the 32-cell stage, and this gene function is essential for Ci-Bra activation. Therefore, the present study has revealed that Ci-Bra upstream genetic cascade consists of beta-catenin, FoxD, ZicL and Ci-Bra in this order(2) Ci-Bra downstream cascade : We have succeeded in the isolation and characterization of Ci-Bra downstream genes including prickle, ERM, tyrosine phosphate, and cdc45. The 5' upstream sequences of these genes contain Ci-Bra protein binding sequences, suggesting that they are direct targets of Ci-Bra. When function of these Ci-Bra downstream genes is suppressed, convergent-extension movement of the notochord cells is prohibited. Therefore, most of the Ci-Bra downstream genes act as members involved in morphogenetic movementsThe present study is therefore an achievement of genetic cascade underlying the notochord formation in the ascidian embryo Less