Synergistic action of HNF-3 and Brachyury in the notochord differentiation of ascidian embryos.

In vertebrate embryos, the class I subtype forkhead domain gene HNF-3 is essential for the formation of the endoderm, notochord and overlying ventral neural tube. In ascidian embryos, Brachyury is involved in the formation of the notochord. Although the results of previous studies imply a role of HNF-3 in notochord differentiation in ascidian embryos, no experiments have been carried out to address this issue directly. Therefore the present study examined the developmental role of HNF-3 in ascidian notochord differentiation. When embryos were injected with a low dose of HNF-3 mRNA, their tails were shortened and when embryos were injected with a high dose of HNF-3 mRNA, which was enough to inhibit differentiation of epidermis and muscle, no obvious ectopic differentiation of endoderm or notochord cells was observed. However, co-injection of HNF-3 mRNA along with Brachyury mRNA resulted in ectopic differentiation of notochord cells in the animal hemisphere, suggesting that HNF-3 acts synergistically with Brachyury in ascidian notochord differentiation. Notochord differentiation of the A-line precursor cells depends on inducing signal(s) from endodermal cells, which can be mimicked by bFGF treatment. Treatment of notochord precursor cells isolated from the 32-cell stage embryoswith bFGF resulted in upregulation of both the HNF-3 and Brachyury genes.