The nervous system is made up of neurons that communicate through synapses formed between cellular extensions called dendrites and axons. Critical processes such as perception, movement, and thought rely on correct communication between neurons. Dendrites acquire information from the environment or other neurons, while axons relay information to other neurons. Injury to axons is difficult to treat because most axons in the central nervous system extend only during development. It is thought that molecular stop signals instruct axons when to stop extending during development and may maintain this state afterwards. Identifying these stop signals and their receptors may be an important step in designing therapies for nervous system injuries. The simple, well-characterized nervous system of C. elegans, make it an ideal model organism for such studies. We have labeled PHB sensory neurons with a cytosolic mCherry fluorophore to assay axon length. Analysis of loss-of-function mutants indicates that the transmembrane receptor
sax-3/Robo, previously isolated for its role in directing axons and cell bodies away from certain body regions, is also required for correct termination of axon outgrowth. Our project aims to investigate the role of
sax-3/Robo in axon outgrowth termination and to elucidate the pathway by which it transduces the termination signal. Timecourse experiments implicated
sax-3/Robo in terminating initial axon outgrowth rather than in the maintenance of PHB axon length. Furthermore, preliminary genetic analysis indicates that
slt-1, a
sax-3/Robo ligand, has an antagonistic effect on the pathway. We have also identified three cytosolic genes that may function downstream of
sax-3/Robo:
unc-73, which encodes guanine nucleotide exchange factor (GNEF) similar to the Trio protein,
unc-34, which encodes an EVH1 domain-containing protein orthologous to Enabled/VASP, and
unc-69, which encodes a coiled-coil protein orthologous to human SCOC. We are currently testing a series of candidate genes to identify new pathways members, and conducting genetic analysis to order these genes in a pathway.