frazzled Encodes a Drosophila member of the DCC immunoglobulin subfamily and is required for CNS and motor axon guidance

We have identified a Drosophila member of the deleted in colorectal cancer (DCC) gene family. The frazzled gene encodes transmembrane proteins that contain four immunoglobulin C2 type domains, six fibronectin type III repeats, and a cytoplasmic domain of 278 amino acids. Like vertebrate members of the DCC family, Frazzled is expressed on axons in the embryonic central nervous system and on motor axons in the periphery. Frazzled is also expressed on epidermis and gut epithelium. Null mutants in frazzled are defective in axon guidance in the central nervous system and in motor axon guidance and targeting in the periphery. The phenotypes strongly resemble those of a deletion of the two Drosophila Netrin genes. We have rescued the frazzled CNS and motor axon defects by expressing Frazzled specifically in neurons; expression in target tissues does not rescue the phenotype. These data, together with vertebrate studies showing binding of DCC to netrin, suggest that Frazzled may function in vivo as a receptor or component of a receptor mediating Netrin-dependent axon guidance.