| Retinoic acid is a derivative of vitamin A. Retinoic acid regulates the pattern formation in the dorsal neural tube, development of neural crest cells, and morphogenesis of limbs. Since these features are characteristic to the chordates, we think that acquisition of retinoic acid signaling pathway was a key event during the evolution of chordates. | all-trans retinoic acid |
 | Then, does retinoic acid play important role during ascidian embryogenesis? Excess amount of retinoic acid affects the morphogenesis of the dorsal neural tube in the ascidian tadpole larva. |
 | The retinoic acid synthase, Raldh2,
is expressed in the anterior-most three muscle cells in the tail.
The retinoic acid-degrading enzyme, Cyp26,
is expressed in the anterior central nervous system. This
expression pattern is similar to that in vertebrates. Local
production of retinoic acid may regulate the morphogenesis of ascidian
embryos. The larvacean Oikopleura dioica is a close relative of ascidians (both larvaceans and ascidians belong to the sub-phylum Tunicata) and develops into the adult that looks like ascidian larvae. The genome of O. dioica does not contain Raldh2, Cyp26 and the retinoic acid receptor (RAR). This raised the possibility that retinoic acid was not required for the evolution and development of chordate-like body structure in tunicates. |
| Our microarray analyses revealed that many genes were activated or repressed by retinoic acid in the Ciona embryo. We demonstrated that retinoic acid and RAR are required for the activation of the Hox1 gene in normal Ciona embryos. The enhancer of Hox1 contains an RAR-binding sequence. This sequence is necessary for activation of the enhancer. In addition, knockdown of RAR-encoding gene by a morpholino oligo diminished the expression of Hox1 (the figure shown below). These results indicate that retinoic acid does have its roles within the Ciona embryo.  | Link to our microarray results Ishibashi et al. (2003） Dev. Growth Differ. 45: 249-259. |
A retinoic acid target gene |
| * | Kanda, M., Wada, H. & Fujiwara, S. (2009) Epidermal expression of Hox1 is directly activated by retinoic acid in the Ciona intestinalis embryo. Dev. Biol. 335: 454-463. |
| * | Fujiwara, S. (2006) Retinoids and non-vertebrate chordate development. J. Neurobiol. 66: 645-652. |
| * | Ishibashi, T., Usami, T., Fujie, M., Azumi, K., Satoh, N. & Fujiwara, S. (2005) Oligonucleotide-based microarray analysis of retinoic acid target genes in the protochordate, Ciona intestinalis. Dev. Dyn. 233: 1571-1578. |
| * | Fujiwara, S. & Kawamura, K. (2003) Acquisition of retinoic acid signaling pathway and innovation of the chordate body plan. Zool. Sci. 20: 809-818. |
| * | Ishibashi, T., Nakazawa, M., Ono, H., Satoh, N., Gojobori, T. & Fujiwara, S. (2003) Microarray analysis of embryonic retinoic acid target genes in the ascidian Ciona intestinalis. Dev. Growth Differ. 45: 249-259. |
| * | Nagatomo, K., Ishibashi, T., Satou, Y., Satoh, N. & Fujiwara, S. (2003) Retinoic acid affects gene expression and morphogenesis without upregulating the retinoic acid receptor in the ascidian Ciona intestinalis. Mech. Dev. 120: 363-372. |
| * | Nagatomo, K. & Fujiwara, S. (2003) Expression of Raldh2, Cyp26 and Hox-1 in normal and retinoic acid-treated Ciona intestinalis embryos. Gene Exp. Patt. 3: 273-277. |