Worm Breeder's Gazette 14(3): 59 (June 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.
|1||Department of Pathology, Robert Wood Johnson Medical School, Piscataway, NJ 08854|
|2||Department of Biology, Johns Hopkins University, Baltimore, MD 21218|
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. During the formation of this 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. 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. Two requirements of this model, the ability of neuronal UNC-6 expression to locally direct migrations and the ability of cells and pioneer axons to detect and follow UNC-6 gradients in vivo have been tested. First, by laser ablation, we removed the midline PVT neuron which 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. Second, we have 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. We conclude that a global netrin UNC-6 cue has a broad role in positioning cells and longitudinal nerves along the dorsoventral axis. Finally, we created transgenic animals that generate ectopic UNC-6 in neurons along the body wall. Heterologous promoter sequences from the mec-7 and unc-119 genes 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. These experiments show that spatial UNC-6 gradients can be formed in vivo and that these UNC-6 gradients can direct axon migrations. The migrating growth cones detect the extracellular gradients to find their dorsoventral positions. The hierarchical model suggests that UNC-6 expression by special neurons alters a global UNC-6 pattern to direct migrations at specific locations. The current experiments demonstrate this ability.