Mutations in the
fax-1 gene cause defects in axon pathfinding of neurons that extend axons in the left ventral nerve cord bundle (see abstract by Wightman et al.) The
fax-1 gene encodes a predicted protein that is a member of the steroid hormone receptor superfamily of transcription factors, suggesting that FAX-1 regulates the transcription of genes that function in axon pathfinding.
fax-1 mutations also cause additional defects in nervous system function:
fax-1 mutants are uncoordinated (Unc), foraging abnormal (Fab) and defective in chemotaxis to volatile odorants (Che). The neurons that extend axons along the left ventral cord bundle are not known to be involved in any of these processes. Therefore,
fax-1 is likely to function in cells other than those that extend axons along the left ventral nerve cord. We have constructed several
fax-1::GFP reporter fusions to determine in which cells
fax-1 is expressed. Approximately 15 neurons or neuron-pairs express
fax-1::GFP in embryos, larvae and adults. Post-embryonic expression includes the sensory neurons CEPDR/L and URXR/L, 3 pharyngeal neurons (likely M1, M5 and MI), 3-4 pairs of ring interneurons (likely including RICR/L, and possibly AIYR/L), the interneurons AVKR/L, 4 neurons in the retro-vesicular ganglion (likely including SABD and SABVR/L), the interneuron PVPL, a dorso-rectal neuron (possibly DVA) and the migrating distal tip cells. We have not observed expression of
fax-1::GFP in the motorneurons of the ventral nerve cord or the command interneurons AVA, AVB, AVD and PVC. These results suggest that
fax-1 might be required in interneurons for normal response to volatile odorants and might be required in SAB's and/or DVA for normal locomotion. The basis of the foraging defect remains mysterious, although it could be related to
fax-1 function in the pharynx. Comparative analysis of
fax-1::GFP expression patterns from fusion genes that contain different portions of the
fax-1 5'-flanking sequence have allowed us to begin sorting out where
fax-1 may be required for movement. A
fax-1(
del2.3)::GFP transgene that contains a 2.3 kb deletion of distal 5' flanking sequence is expressed in a subset of the full
fax-1::GFP pattern: the CEPD's, the 3 pharyngeal neurons, RICR/L and 4 retrovesicular neurons including the SAB's. A genomic fragment that contains the equivalent 2.3 kb deletion of 5' flanking sequence rescues
fax-1 locomotary defects, but does not rescue
fax-1 axon pathfinding defects. Taken together, these data suggest that
fax-1 might be required in the SAB's for normal movement, since these are the only
fax-1(
del2.3)::GFP expressing neurons known to form neuro-muscular junctions. In order to address issues of
fax-1 function in the nervous system more directly, and initiate biochemical analysis, we are expressing FAX-1 protein in E. coli in preparation for production of antibodies to FAX-1.