Proper neural development is directed by numerous kinds of conserved axon guidance molecules, which includes a family of membrane-bound ligands and receptors called ephrins and ephrin receptor tyrosine kinases (EphRTKs). Previous research in the Chin-Sang laboratory has identified the EphRTK VAB-1 and its ephrin ligands EFN-1, EFN-2, EFN-3, and EFN-4 as key signaling molecules in the development of a pair of touch neurons in C. elegans called the posterior lateral microtubule (PLM) (Mohamed and Chin-Sang 2006). Loss-of-function vab-1 mutants show a PLM overextension phenotype while constitutively active vab-1 mutants show a strong premature termination phenotype in the PLM, which suggests that activation of VAB-1 may play a key role in proper PLM termination during development. efn-1 is the only ephrin ligand that shows a PLM overextension phenotype in the single loss-of-function mutant, although the triple efn-2; efn-3; efn-4 loss-of-function mutant also shows a PLM overextension phenotype.
Since efn-1 has the strongest PLM overextension phenotype of the ephrin ligands, we decided to investigate activation of VAB-1 by EFN-1 in vivo. Using ectopic expression of EFN-1 we have assessed its ability to activate the VAB-1 receptor to cause PLM termination when expressed on the same cell (in cis) and on neighboring cells (in trans). The ability of EFN-1 to activate VAB-1 to cause termination of the PLM axon both in cis and in trans in combination with our previous genetic analyses provides evidence that EFN-1 may activate VAB-1 in vivo to provide a stop cue for the PLM during development.
In addition to characterizing the activation of VAB-1 by EFN-1, we have also worked to identify the cellular location where the stop cue is produced to signal the PLM to halt its forward growth during development. We believe the source of the stop cue may be the seam cell V2, one of 10 specialized epithelial cells that run along the sides of the worm. This theory has been tested using genetic and molecular approaches, and our results indicate that there may be positional information exchanged between the PLM and V2, although more research is needed to clarify the nature of this interaction.
Our work adds to the current understanding of the role of EFN-1 in axon guidance in C. elegans, and the ability of epithelial and neuronal cells to communicate to facilitate development, and with further research this knowledge may be applied to human development.