The
fax-1 nuclear hormone receptor is required for specifying aspects of AVK neuron identity. Mutations in
fax-1 cause defects in axon pathfinding and expression of the FMRFamide-related neuropeptide precursor gene
flp-1 .
fax-1 mutants also exhibit movement, chemotaxis and foraging behavior defects, suggesting that the
fax-1 gene also functions in the development of other neurons.
fax-1 encodes the C. elegans NR2E3 receptor, an ortholog of human PNR, which is expressed exclusively in the retina. This group of NHR's is closely related to Drosophila Tailless and its vertebrate and C. elegans orthologs. Mutations in human PNR are the likely cause of an inherited progressive human blindness called enhanced S-cone syndrome. Individuals carrying mutations in PNR have night blindness, enhanced sensitivity to blue light, and often suffer progressive retinal degeneration that eventually leads to blindness. Although direct evidence of the developmental events leading to these phenotypes is lacking, there has been speculation that the disease results from an identity defect in the specification of photoreceptors; rods and red and green cones may be converted into blue (S-) cones. Given the apparent role for
fax-1 in specifying neuron identity in C. elegans, the study of the mechanism of
fax-1 function serves as a relevant model for understanding the human disease. We are testing whether human PNR can function for
fax-1 in nematodes. A full-length PNR clone was not able to rescue a
fax-1 mutant, however the ligand-binding domains (LBD) of the two proteins show little relationship, suggesting that a chimeric protein consisting of the PNR DNA-binding domain (DBD) fused to the FAX-1 LBD may be functional. We determined the expression pattern of
fax-1 by generating antibodies to FAX-1 in mice. FAX-1 protein is observed in nuclei of mid-stage embryos, just after neurogenesis but before most axon outgrowth occurs, and in all larval stages and adults. We observe FAX-1 protein in 18 neurons consistently at all stages, including the AVK's, AIY's, RIC's, MI, and DVA. The additional 5 pairs of neurons (all in anterior ganglia) have not been definitively identified, but likely include AVA, AVE, and another interneuron pair that is either AVB, AVH or AVJ. The expression of FAX-1 in a neuron pair involved in chemotaxis (AIY) and in neuron pairs involved in coordination of movement (AVA, AVB, AVE) provide potential sites for
fax-1 function in these behaviors. FAX-1 is also expressed transiently in the dtc's in L2-L4 larvae and the transverse (T) vulval cells of the 2 o lineages (P5.ppal/r and P7.papl/r) in L4 larvae. The timed expression of FAX-1 in non-neuronal cells that are undergoing movement (migration and morphogenesis, respectively) is tantalizing, however we observe no gonad or vulval phenotypes in
fax-1 mutants. Given that
fax-1 is a likely transcriptional regulator and its apparent role in specifying neuron identity, we are studying the regulatory circuits in which
fax-1 participates. Previous analysis showed that
flp-1 expression requires
fax-1 . Because
unc-42 , a homeobox gene, is also expressed in AVK's we examined the regulatory relationships among these genes. Expression of
flp-1 is more severely compromised in
unc-42 mutants than
fax-1 mutants. Furthermore, four pairs of anterior FAX-1-positive neurons fail to accumulate FAX-1 in
unc-42 mutants, suggesting a regulatory hierarchy in which
unc-42 regulates
fax-1 and other genes, which in turn regulate
flp-1 . FAX-1 also does not accumulate in AIY's of
ttx-3 mutants, suggesting that
fax-1 also functions downstream of the
ttx-3 LIM homeobox gene. We are also attempting to determine the position of
fax-1 in regulatory hierarchies in other neurons. Our efforts to identify downstream targets of
fax-1 regulation have focused on one-hybrid strategies and candidate sequence gel-shift studies. Finally, we have also begun studying other NHR's that are closely-related to
fax-1 . Among these is
nhr-111 (F44G3.9), which shows relationship to
fax-1 in both the DBD and LBD.
nhr-111::gfp fusions are expressed in a single pair of anterior ganglion neurons in embryos and a second pair of cells in the mid-body.