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 ind uce the tectum and give positional specificity. Chick Grg4 is expressed at first in the anterior neural fold. The expression localizes from the poster ior diencephalon to the mesencephalon by stage 10. To investigate the funct ion of Grg4 in mesencephalic development, Grg4 overexpression was carried o ut by in ovo electroporation. After Grg4 overexpression, expression of En-2 , Pax5, Fgf8, and EphrinA2 was repressed, and Pax6 was upregulated in the m esencephalic region. Grg4 overexpression caused the morphological change; m esencephalic swelling became smaller and the di-mesencephalic boundary shif ted 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 a gainst tectum-organizing activity. It was also shown that transfected N-ter minal domains of Grg4 induced En-2 expression. Since N-terminal domains wer e transported to the nucleus in the neuroepithelium, they could act as domi nant negative for endogenous Grg4. These results indicate that Grg4 has rep ressing activity against the organizing molecules and suggest that Grg4 pla ys important roles in formation of anterior tectal boundary and polarity. ( C) 2000 Academic Press.