Antagonizing Activity of Chick Grg4 against Tectum-Organizing Activity

Alar plate of chick mesencephalon differentiates into the optic tectum. It has been shown that factors expressed in the mes–metencephalic boundary induce the tectum and give positional specificity. Chick Grg4 is expressed at first in the anterior neural fold. The expression localizes from the posterior diencephalon to the mesencephalon by stage 10. To investigate the function of Grg4 in mesencephalic development, Grg4 overexpression was carried out by in ovo electroporation. After Grg4 overexpression, expression of En-2, Pax5, Fgf8, and EphrinA2 was repressed, and Pax6 was upregulated in the mesencephalic region. Grg4 overexpression caused the morphological change; mesencephalic swelling became smaller and the di–mesencephalic boundary shifted posteriorly, that is, the anterior limit of tectum shifted posteriorly. Importantly, cotransfection of Grg4 with Pax5 canceled the tectum-inducing activity of Pax5. These results suggest that Grg4 works as an antagonist against tectum-organizing activity. It was also shown that transfected N-terminal domains of Grg4 induced En-2 expression. Since N-terminal domains were transported to the nucleus in the neuroepithelium, they could act as dominant negative for endogenous Grg4. These results indicate that Grg4 has repressing activity against the organizing molecules and suggest that Grg4 plays important roles in formation of anterior tectal boundary and polarity.