Multiple signaling mechanisms of the UNC-6/netrin receptors UNC-5 and UNC-40/DCC in vivo

Cell and growth cone migrations along the dorsoventral axis of Caenorhabditis elegans are mediated by the UNC-5 and UNC-40 receptor subtypes for the secreted UNC-6 guidance cue. To characterize UNC-6 receptor function in vivo we have examined genetic interactions between unc-5 and unc-40 in the migrations of the hermaphrodite distal tip cells. We report that cell migration defects as severe as those associated with a null mutation in unc-6 are produced only by null mutations in both unc-5 and unc-40, indicating that either receptor retains some partial function in the absence of the other. We show that hypomorphic unc-5 alleles exhibit two distinct types of interallelic genetic interactions. In an unc-40 wild-type genetic background, some pairs of hypomorphic unc-5 alleles exhibit a partial allelic complementation. In an unc-40 null background, however, we observed that unc-5 hypomorphs exhibit dominant negative effects. We propose that the UNC-5 and UNC-40 netrin receptors can function to mediate chemorepulsion in DTC migrations either independently or together, and the observed genetic interactions suggest that this flexibility in modes of signaling results from the formation of a variety of oligomeric receptor complexes.