Several conserved classes of guidance molecules direct axon guidance in C. elegans and other organisms. The UNC-6/netrin diffusible guidance cue is required for axon guidance along the dorsal-ventral axis in C. elegans, Drosophila and vertebrates. The UNC-5 guidance receptor in combination with UNC-40, a homolog of the cell surface proteins DCC and neogenin, mediates repulsive response to the UNC-6 which is expressed in ventral nerve cord and ventral epidermis, to orient growing axons in a dorsal direction. Attractive responses to UNC-6 require UNC-40 but not UNC-5. Previous studies suggested that SAX-3, a homolog of the transmembrane proteins ROBO (Roundabout), mediates repulsive responses to dorsally expressed SLT-1/Slit, one of the ligands for SAX-3, to orient axons in a ventral direction (1).
ev752 has AVM ventralward axon guidance defects. We have rescued
ev752 mutations with a cosmid fragment encoding a novel molecule with two putative galactose binding domains and one transmembrane domain. We also found a point mutation in the gene, which causes a missense mutation of the conserved cysteine in the second galactose binding domain. To elucidate the distribution of the galactose binding protein (GBP), we used GFP transcriptional reporter lines. GFP is expressed in some of the neurons in head, tail, BDU neurons, PVM touch cell, motor neurons, body wall muscles and uterus and vulva junctions. From the GFP tanslational reporter lines, GFP tagged GBP localized exclusively in cell membranes. Although the GFP expression was not visible in AVM, GBP expressed in touch neurons using the
mec-7 promoter rescued the
ev752 mutant defects, suggesting it works cell-autonomously. We have found that the
ev752 mutation enhances the AVM and PVM guidance defects of
unc-6 null or
unc-40 null alleles, suggesting that GBP functions in an axon guidance pathway that acts in parallel with UNC-6 and UNC-40. We have also found that the
ev752 mutation does not enhance the AVM or PVM guidance defects of a
slt-1 null allele or a
sax-3 strong allele (
ky123), suggesting that the GBP functions in the same pathway as SLT-1 and SAX-3. Mis/over expression of SLT-1 by
myo-3::
slt-1 causes neomorphic axon guidance defects, such as AVM axons that grow posteriorly. These defects are suppressed by
sax-3 mutations (1), but not by the
ev752 mutation. We are now further examining the effects of mis/over expression of GBP and SLT-1 in the background of
ev752,
slt-1, and
sax-3 mutations. 1. Hao JC, Wu TW, Fujisawa K, Culotti JG, Gengyo-Ando K, Mitani S, Moulder G, Barstead R,Tessier-Lavigne M, Bargmann CI, Neuron 32: 25-38 (2001)