The
fax-1 nuclear hormone receptor and
unc-42 homeobox gene control interneuron identities in C. elegans.
fax-1 is the ortholog of unfulfilled in Drosophila and PNR/NR3E3 in vertebrates, where it functions in the development and function of mushroom bodies and photoreceptors, respectively. The
fax-1 and
unc-42 transcription factors function in specifying the identities of an overlapping subset of nematode interneurons, including the command interneurons AVA and AVE, which function in coordinated movements. Both genes are required for the expression of neuron-specific genes, including glutamate receptors subunits, and axon pathfinding. Mutations in both
fax-1 and
unc-42 cause an incompletely-penetrant slow-growth phenotype that arises from temporary arrest after hatching at the L1 stage. L1 arrest has been shown to be controlled by the insulin-like signaling pathway that also controls dauer formation and longevity. The
daf-2 insulin receptor is a primary mediator of insulin signaling in C. elegans. Strong
daf-2 mutations cause L1 arrest, while weak
daf-2 mutations cause dauer-arrest. Both
fax-1 and
unc-42 mutations cause a fully-penetrant late embryonic arrest in combination with a weak
daf-2 mutation. The arrest can be reversed by a mutation in the
daf-16 forkhead transcription factor, which functions downstream of
daf-2, but not by mutations in the parallel TGFb pathway. Arrested
fax-1;
daf-2 and
unc-42;
daf-2 embryos typically displayed normal L1 morphology, but remained coiled in a broken eggshell in a state of extreme quiescence. Arrested embryos exhibited weak or no pharyngeal pumping. Nonetheless, arrested embryos could be prompted to vigorous movement by stimulation with green light. The arrest could also be partially reversed by a mutation in
egl-4, which is required for sleep-like quiescence. We also examined the strong
daf-2(
e979) mutation and found that it displays a complex phenotype, including morphological collapse at elongation in mid-embryogenesis, late embryo quiescence (like the double mutants), and L1 arrested animals that display fairly normal pumping and activity. These observations indicate that the
fax-1 and
unc-42 transcription factors function in an insulin pathway that controls arousal and developmental progression out of embryogenesis. Given that both
fax-1 and
unc-42 are required for the development of a limited set of interneurons, these experiments suggest a previously unappreciated role for interneuron function and insulin signaling in regulating developmental arrest and arousal. Supported by NIGMS.