MADD-4 is a secreted cue required for midline-oriented guidance in Caenorhabditis elegans

The netrins and slits are two families of widely conserved cues that guide axons and cells along the dorsal-ventral (D-V) axis of animals. These cues typically emanate from the dorsal or ventral midlines and provide spatial information to migrating cells by forming gradients along the D-V axis. Some cell types, however, extend processes to both the dorsal and ventral midlines, suggesting the existence of additional guidance cues that are secreted from both midlines. Here, we report that a previously uncharacterized protein called MADD-4 is secreted by the dorsal and ventral nerve cords of the nematode C. elegans to attract sensory axons and muscle membrane extensions called muscle arms. MADD-4's activity is dependent on UNC-40/DCC, a netrin receptor, which functions cell-autonomously to direct membrane extension. The biological role of MADD-4 orthologs, including ADAMTSL1 and 3 in mammals, is unknown. MADD-4 may therefore represent the founding member of a family of guidance proteins.