L1- and NCAM-type cell adhesion molecules represent distinct protein families that function as specific receptors for different axon guidance cues. However, both L1 and NCAM proteins promote axonal growth by inducing neuronal tyrosine kinase activity and are coexpressed in subsets of axon tracts in arthropods and vertebrates. We have studied the functional requirements for the Drosophila L1- and NCAM-type proteins, Neuroglian (Nrg) and Fasciclin II (FasII), during postembryonic sensory axon guidance. The rescue of the Neuroglian loss-of-function (LOF) phenotype by transgenically expressed L1- and NCAM-type proteins demonstrates a functional interchangeability between these proteins in Drosophila photoreceptor pioneer axons, where both proteins are normally coexpressed. In contrast, the ectopic expression of Fasciclin II in mechanosensory neurons causes a strong enhancement of the axonal misguidance phenotype. Moreover, our findings demonstrate that this functionally redundant specificity to mediate axon guidance has been conserved in their vertebrate homologs, L1-CAM and NCAM.
This work was supported by an EMBO short-term fellowship to L.V.K.; an FPI predoctoral fellowship to E.V.; grants from Vera and Carl Johan Michelsen's Foundation and the Danish Cancer Society to E.B.; a grant from the Lundbeck Foundation to V.B.; NICHD HD29388, NSF IBN-0132819, and SCRF No.1797 grants to M.H.; and by the MCYT SAF2001-1628 (part from FEDER) grant to L.G-A.