The assembly of the C. elegans nervous system requires the directed migration of cells and growth cones along the anteroposterior body axis. To understand the mechanisms that guide these migrations, we initiated a genetic and molecular approach to identify molecules required for longitudinal cell migration and axon outgrowth. Transgenic strains expressing the green fluorescent protein (GFP) under cell-type-specific promoters were generated to observe particular neurons and their axonal processes in living animals. For example, AVA, AVB, AVD and PVC were examined using a glr-1::GFP strain and CAN was visualized using a ceh-23::GFP strain. In collaboration with B. Wightman and G. Garriga, we found that vab-8 is needed to guide most posteriorly-directed cell and growth cone migrations. To find additional guidance genes, we sought mutants with phenotypes similar to those of vab-8 animals. From screening the progeny of 7,000 mutagenized glr-1::GFP or ceh-23::GFP F1 animals, we recovered 19 uncoordinated and withered tail mutants. These mutations represent 12 genes, including epi-1, mig-2, mig-10, mig-11, unc-33, unc-34, unc-35, unc-44, unc-73 and vab-8. Some of these genes were known to act in cell migration and axon outgrowth; others were less well understood. For example, in addition to blocking the growth of the AVA, AVB and AVD axons, we found that mutations in mig-2, unc-34 and unc(ky190) caused these axons to extend inappropriately along the dorsal or lateral nerve cords, indicating that these genes are required for axon outgrowth and pathfinding. Mutations in epi-1, which encodes a laminin A subunit (K. Joh & E. Hedgecock, pers. comm.) caused the CAN axons to branch inappropriately and stray from their normal trajectory, suggesting a role for laminin in maintaining normal axon morphology, fasciculation and directed growth. The unc-35 mutant isolated in our screen, ky196, has an uncoordinated phenotype like that of the canonical unc-35 allele, e259. However, unlike e259 mutants, many ky196 animals have notched heads and protruding vulvae, are sterile or die as larvae. ky196 mutants have defects in the organization of dense bodies and other phenotypes characteristic of Pat mutants (B. Barstead, pers. comm.; our observations). These phenotypes are similar to those conferred by mutations in an alpha integrin gene (P. Baum, D. Perry & G. Garriga, pers. comm.), suggesting that unc-35 might act in an integrin-mediated process. We have begun the molecular characterization of unc-35 and mig-11, which, like vab-8, appears to act in the guidance of posteriorly-directed cell and growth cone migrations.

Affiliation:
- HHMI, Department of Anatomy, University of California, San Francisco, CA 94143 USA