Worm Breeder's Gazette 14(4): 76 (October 1, 1996)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Department of Pathology Robert Wood Johnson Medical School Piscataway, NJ 08854
During embryogenesis, the basic axon scaffold of the nervous system is formed by special axons that pioneer pathways between groups of cells. To find their way, the pioneer growth cones detect specific cues in their extracellular environment. One of these guidance cues is netrin. Observations and experimental manipulations in vertebrates and nematodes have shown that netrin is a bifunctional guidance cue that can simultaneously attract and repel axons (Hedgecock et al., 1990; Ishii et al., 1992; Kennedy et al., 1994; Serafini et al., 1994). During the formation of the basic axon scaffold in C. elegans, the netrin UNC-6 is expressed by neuroglia and pioneer neurons to provide hierarchical guidance cues throughout the animal (Wadsworth et al., 1996; Wadsworth and Hedgecock, 1996). Each cue has a characteristic role depending on the cell type, its position, and the developmental stage. These roles include activities as global, decussation, and labeled-pathway cues. This hierarchical model of UNC-6 netrin mediated guidance suggests a method by which guidance cues can direct formation of basic axon scaffolds in developing nervous systems. To further study development of the embryonic axon scaffold, we have ectopically expressed UNC-6, laser ablated specific UNC-6-expressing cells, and studied nerve development in unc-6 mutants. For the laser ablation experiments we removed the midline PVT neuron that expresses UNC-6 during embryogenesis and which is thought to direct pioneer axons for ventral nerve cord/ lumbar commissure development. In these animals, the commissures do not form, instead multiple axon tracts develop, often with misguided trajectories to the ventral nerve cord. We have also examined the migrations of GFP-labeled axons and cells in unc-6 null mutants. Many, but not all, longitudinally migrating axonal growth cones stray dorsally and ventrally in the animals. For the ectopic expression studies, heterologous promoter sequences from the mec-7 and unc-119 genes (provided by A. Fire and D. Pilgrim (Maduro and Pilgrim, 1995)) were linked to full-length unc-6 genomic DNA, integrated transgenic lines were established in various genetic backgrounds, and the migrations of axons were scored in each strain. In these animals the patterning of nerve tracts is altered near neurons that ectopically express UNC-6. We conclude from these experiments that a global UNC-6 netrin cue has a broad role in positioning cells and longitudinal nerves along the dorsoventral axis. Furthermore, that spatial UNC-6 gradients can be formed in vivo and that these UNC-6 gradients can direct axon migrations. The hierarchical model predicts that UNC-6 expression by special neurons alters a global UNC-6 pattern to direct migrations at specific locations and we believe the current experiments help to demonstrate this ability. Hedgecock, E. M., Culotti, J. G., and Hall, D. H. (1990). Neuron 2, 61-85. Ishii, N., Wadsworth, W. G., Stern, B. D., Culotti, J. G., and Hedgecock, E. M. (1992) Neuron 9, 873-881. Kennedy, T. E., Serafini, T., de la Torre, J., and Tessier-Lavigne, M. (1994). Cell 78, 425-435. Maduro, M., and Pilgrim, D. (1995). Genetics 141, 977-988. Serafini, T., Kennedy, T. E., Galko, M. J., Mirzayan, C., Jessell, T. M., and Tessier-Lavigne, M. (1994). Cell 78, 409-424. Wadsworth, W. G., Bhatt, H., and Hedgecock, E. M. (1996). Neuron 16, 35-46. Wadsworth, W. G., and Hedgecock, E. M. (1996). Bioessays 18, 355-362.