Regulation of netrin-1a Expression by Hedgehog Proteins James D. Lauderdale,* ,1 Sara K. Pasquali,* Reza Fazel,* Fredericus J. M. van Eeden, ² Heike E. Schauerte, ² Pascal Haffter, ² and John Y. Kuwada* *Department of Biology, University of Michigan, Ann Arbor, Michigan 48109-1048; and † Abteilung Genetik, Max-Planck-Institut fu ¨ r Entwicklungsbiologie, Spemannstrasse 35, 72076 Tu ¨ bingen, Germany Netrins, a family of growth cone guidance molecules, are expressed both in the ventral neural tube and in subsets of mesodermal cells. In an effort to better understand the regulation of netrins, we examined the expression of netrin-1a in mutant cyclops, no tail, and floating head zebrafish embryos, in which axial midline structures are perturbed. Netrin-1a expression requires signals present in notochord and floor plate cells. In the myotome, but not the neural tube, netrin-1a expression requires sonic hedge- hog. In embryos lacking sonic hedgehog, the sonic-you locus, netrin-1a expression is reduced or absent in the myotomes but present in the neural tube. Embryos lacking sonic hedgehog express tiggy-winkle hedgehog in the floor plate, suggesting that, in the neural tube, tiggy- winkle hedgehog can compensate for the lack of sonic hedgehog in inducing netrin-1a expression. Ectopic ex- pression of sonic hedgehog, tiggy-winkle hedgehog, or echidna hedgehog induces ectopic netrin-1a expression in the neural tube, and ectopic expression of sonic hedge- hog or tiggy-winkle hedgehog, but not echidna hedgehog, induces ectopic netrin-1a expression in somites. These data demonstrate that in vertebrates netrin expression is regulated by Hedgehog signaling. INTRODUCTION Growth cones, the motile tip at the leading edge of the axon, navigate to their target regions along highly stereotyped trajectories by interpreting guidance cues within their environment (reviewed in Goodman and Shatz, 1993; Goodman, 1996; Tessier-Lavigne and Good- man, 1996). Although the correct temporal and spatial distributions of guidance cues are likely to depend upon proper patterning of the embryo during development, the relationships between the genes involved in embry- onic patterning and genes involved in neuronal connec- tivity are mostly unknown. The netrins are a small, evolutionarily conserved family of genes involved in neuronal connectivity in worms, flies, and vertebrates (reviewed in Tessier- Lavigne and Goodman, 1996). Genetic, biochemical, and in vitro studies have revealed that netrins are involved in guiding growth cones both toward and away from the ventral midline in the central nervous system. Presum- ably guidance toward the midline is mediated by an attractive mechanism, whereas guidance away from the midline is mediated by a repulsive mechanism. In addition to this role in midline guidance, netrins are important for the guidance of retinal ganglion cell axons into the optic nerve head (de la Torre et al., 1997; Deiner et al., 1997) and have been implicated as being important for guidance in other regions of the nervous system (Kennedy et al., 1994; Serafini et al., 1996; Shirasaki et al., 1996; Lauderdale et al., 1997; Livesey and Hunt, 1997; Metin et al., 1997; Richards et al., 1997; Stra ¨hle et al., 1997). In vertebrates, the netrins are expressed in the ventral neural tube and in subsets of mesodermal cells. In mice, chicks, Xenopus, and zebrafish, netrin-1 is expressed in the ventral neural tube including the floor plate (Kennedy et al., 1994; Skarnes et al., 1995; Serafini et al., 1996; de la Torre et al., 1997; Lauderdale et al., 1997; Stra ¨hle et al., 1997). In chicks, netrin-2 is expressed in the 1 To whom correspondence should be addressed at current address: Howard Hughes Medical Institute Research Laboratories, The Univer- sity of Michigan Medical Center, MSRB I, Room 4510, 1150 West Medical Center Drive, Ann Arbor, MI 48109-0650. Fax: (734) 763-6640. E-mail: jdlauder@umich.edu. MCN Molecular and Cellular Neuroscience 11, 194–205 (1998) Article No. MCN980015 194 1044-7431/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.