Anti-Neural-Inhibition: A Conserved Mechanism for Neural Induction

Ethan Bier of the embryo, but as a repressor of genes expressed in Department of Biology dorsal cells. In ventral cells, the mesoderm-determining and Center for Molecular Genetics genes twist and snail are activated by peak levels of University of California, San Diego Dorsal. Lower levels of Dorsal activate expression of 9500 Gilman Drive rhomboid (rho) in lateral presumptive neuroectodermal La Jolla, California 92093-0349 cells. Similarly, lateral expression of short gastrulation (sog) is dependent on Dorsal, although it remains to be determined whether this effect is due to direct activa-Ever since Spemann and Mangold showed that the dor-tion. A cluster of related bHLH-encoding genes compris-sal lip of the Xenopus embryo could organize sur-ing the achaete-scute complex (AS-C) are also ex-rounding cells to form a complete embryonic axis, devel-pressed in lateral cells. Dorsal, however, is not an opmental biologists have been in search of endogenous essential activator of these genes. Cells of the dorsal embryonic-inducing molecules. Among the most sought nonneural ectoderm have undetectable levels of nuclear after molecules were the putative neural inducers sup-Dorsal, which permits the expression of several genes plied by mesodermal cells of the Spemann organizer, including decapentaplegic (dpp), zerknü llt (zen), and tol-which could induce overlying ectodermal cells to form loid (tld). In ventral and lateral cells, Dorsal represses neural structures. Most investigators thought of neural expression of these target genes. inducers as substances actively promoting neural devel-Sog Prevents Dpp Signaling from Invading opment. However, recent work in both vertebrates and the Neuroectoderm invertebrates favors the more convoluted hypothesis While the mother is responsible for determining the posi-that the default state of ectoderm is neural and that tion of lateral neuroectoderm versus dorsal nonneural this developmental program is actively blocked in naive ectoderm in Drosophila, zygotic genes expressed in ectoderm by a neural inhibitor. Consistent with this view, each of these domains are required to maintain that several recently identified endogenous neural inducers initial subdivision. Two genes that play a pivotal role in function by neutralizing a neural inhibitor. Thus, neural the neural versus nonneural subdivision of the ectoderm inducers provide a permissive condition for neurogen-are sog and dpp. dpp is expressed in dorsal nonneural esis through a double negative mechanism. Experi-cells and encodes a secreted TGF-␤ family member ments favoring this model in vertebrates have been re-(Dpp) most related to the vertebrate bone morphogenetic viewed recently (Hemmati-Brivanlou and Melton, 1997). The purpose of the current …