Genetic And Molecular Characterization Of Two Maternal Effect Genes Required For Caenorhabditis Elegans Development PDF Download

Unc-40 is one of three genes that has major effects on cell and pioneer growth cone migrations along the dorsoventral axis in C. elegans. The goal of this project was to clone and molecularly characterize the unc-40 gene, in order to gain more insight into the molecules involved in directing neuronal growth cone migrations. The other two C. elegans genes with major effects on dorsoventral migrations have been cloned and characterized; unc-6 encodes a laminin-related path cue molecule required for guiding dorsal and ventral migrations on the epidermis and is expressed by the ventral epidermis; unc-5 encodes a transmembrane receptor of the IG superfamily required only for dorsal migrations and is expressed in cells and growth cones that migrate dorsally. unc-40 encodes a protein (UNC-40) that acts in concert with UNC-5 and UNC-6 to establish the dorsoventral directional polarity for cellular and pioneer growth cone migrations in C. elegans. This thesis presents the cloning and subsequent characterization of unc-40. The gene was cloned by rescue of the mutant phenotype through germline transformation. The data show that unc-40 encodes an integral membrane protein related in overall domain structure and amino acid sequence to the Drosophila Frazzled protein and the vertebrate DCC (deleted in colorectal cancer) and neogenin proteins. The expression pattern of UNC-40 was examined using a construct that expresses GFP (green fluorescent protein) under the control of the unc-40 promoter and a construct that expresses a GFP epitope-tagged UNC-40 protein. The cells expressing UNC-40 include all those whose migrations are affected by unc-40 mutations--including cells and growth cones that migrate ventrally (Hedgecock et al., 1990) and those that migrate dorsally. The data suggest that UNC-40 acts as a cell-autonomous transmembrane receptor that is involved in responding to the UNC-6 path cue. One attractive model is that UNC-40 guides cell and pioneer growth cone migrations on the epidermis of C. elegans toward ventral sources of UNC-6 by mediating chemoattractive responses to UNC-6. UNC-40 may also act in combination with the UNC-5 receptor for dorsal movements away from ventral sources of UNC-6 by mediating a chemorepulsive response to UNC-6. Most of the data presented in this thesis appears in Chan, S.S-Y., Zheng, H., Su, M.-W., Wilk, R., Killeen, M.T., Hedgecock, E.M., and Culotti, J.G., (1996). UNC-40, a C. elegans homolog of DCC (deleted in colorectal cancer), is required in motile cells responding to UNC-6 netrin cues. Cell 87, 187-195.