Vertebrate homologues of C. elegans UNC-5 are candidate netrin receptors

In the developing nervous system, migrating cells and axons are guided to their targets by cues in the extracellular environment. The netrins are a family of phylogenetically conserved guidance cues that can function as diffusible attractants and repellents for different classes of cells and axons 1–10 . In vertebrates, insects and nematodes, members of the DCC subfamily of the immunoglobulin superfamily have been implicated as receptors that are involved in migration towards netrin sources 6,11–13,15 . The mechanisms that direct migration away from netrin sources (presumed repulsions) are less well understood. In Caenorhabditis elegans , the transmembrane protein UNC-5 (ref. 14) has been implicated in these responses, as loss of unc-5 function causes migration defects 16,17 and ectopic expression of unc-5 in some neurons can redirect their axons away from a netrin source 18 . Whether UNC-5 is a netrin receptor or simply an accessory to such a receptor has not, however, been defined. We now report the identification of two vertebrate homologues of UNC-5 which, with UNC-5 and the product of the mouse rostral cerebellar malformation gene ( rcm ) 19 , define a new subfamily of the immunoglobulin superfamily, and whose messenger RNAs show prominent expression in various classes of differentiating neurons. We provide evidence that these two UNC-5 homologues, as well as the rcm gene product, are netrin-binding proteins, supporting the hypothesis that UNC-5 and its relatives are netrin receptors.