MECHANISMS OF DORSAL-VENTRAL PATTERNING IN NOGGIN-INDUCED NEURAL TISSUE

We have investigated mechanisms of dorsal-ventral patterning of neural tissue, using Xenopus ectoderm neuralized by noggin protein. This tis sue appears to be patterned dorsoventrally; cpl-1, a gene expressed in the dorsal brain, and etr-l, a gene largely excluded from the dorsal brain, are expressed in separate territories in noggin-treated explant s (Knecht, A. K., Good, P. J., Dawid, I. B. and Harland, R. M. (1995) Development 121, 1927-1936). Here we show further evidence that this p attern represents a partial dorsal-ventral organization. Additionally, we test two mechanisms that could account for this pattern: a dose-de pendent response to a gradient of noggin protein within the explant, a nd regulative cell-cell interactions. We show that noggin exhibits con centration-dependent effects, inducing cpl-1 at low doses but repressi ng it at high doses. Since noggin acts by antagonizing Bone Morphogene tic Protein (BMP) signaling, this result suggests that BMPs also may a ct in a dose-dependent manner in vivo. However, in the absence of a no ggin gradient, regulative cell-cell interactions can also pattern the tissue. Such regulation is facilitated by increased motility of noggin -treated cells, Finally, the response of cells to both of these patter ning mechanisms is ultimately controlled by a third process, the chang ing competence of the responding tissue.