Many Ig domain containing transmembrane proteins including e.g.
sax-3/Robo,
unc-40/Frazzled, and the receptor phosphotyrosine phosphatase LAR have been implicated in correct embryonic axon pathfinding. In addition, we have recently described a novel mechanism of postembryonic axon maintenance involving a member of the secreted Ig domain family of ZIG proteins.We report her that the Ig domain containing receptor tyrosine kinase EGL-15/FGFR which has been widely studied in the context of sex myoblast migration is also involved in the proper development and axon maintenance of PVQ interneurons in the ventral nerve cord. We observe three different classes of defects: embryonic axon extension defects (type 1), embryonic cross-over defects at the ventral midline (type 2) and post-embryonic axon maintenance defects (type 3). Intriguingly, neither of the two canonical FGF ligand mutants
let-756 and
egl-17 nor a
let-756;
egl-17 double mutant show similar defects suggesting that a novel ligand might interact with EGL-15/FGFR in these functions. We demonstrate that a larval lethal
egl-15 null allele (
n1456) shows type 1 and 2 defects. Lethality of
egl-15 null alleles is rescued by a
let-60 gain-of-function mutation (
n1046). In analogy, in
let-60(gf);
egl-15 (
n1456) double mutant animals the axon extension defect (type 1) is rescued while cross-over defects (type 2) are not. This shows that FGFR mediated ras signaling is essential for axon extension of PVQ interneurons. Interestingly, the canonical
egl-15 allele (
n484) shows no type 1 or type 2 defects. However, we observe post-embryonic maintenance defects (type 3). This demonstrates that the embryonic and post-embryonic defects are genetically separable. To elucidate the molecular basis for this allele specificity we have sequenced the
egl-15(
n484) allele and identified a nonsense mutation in an alternatively spliced exon 5. This exon was confirmed by RT-PCR analyses and is consistent with previous reports (Lo et al., ECWM 2002 abstr. 165; Goodman et al., 2000 ECWM abstr. 98). In addition, we have identified at least 4 different C terminal splice variants suggesting that up to eight different
egl-15 transcripts might exist. Using isoform specific rescuing constructs we will show how different splice variants are involved in specific aspects of neuronal development and maintenance. In conclusion, we have identified EGL-15 as the first protein that is involved in both axon development and maintenance. But what is(are) the ligand(s) in this context? One intriguing possibility is that the aforementioned ZIG proteins may be involved as maintenance factors with EGL-15/FGFR, a hypothesis we are currently testing experimentally.