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.

UNC-6 gradients provide hierarchical netrin guidance cues for dorsoventral positioning of cells and nerves

William Wadsworth1, Xing-Cong Ren1, Eileen Ruppert1, Edward Hedgecock2

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.