During differentiation, neurons extend axons to their targets, form synaptic connections and begin to exhibit attributes of mature neurons. Neurons must retain their appropriate differentiated state and express the correct complement of 'terminal differentiation' genes throughout the life of the organism to maintain a functional nervous system.
sra-6::gfp is expressed exclusively and continuously in PVQ, ASH and ASI soon after the cells are born during embryogenesis and serves as a terminal differentiation marker for PVQ. Mutations in
unc-119 and five other genes that disrupt axon growth were found to also eliminate expression of
sra-6::gfp in PVQ soon after hatching. Expression of
sra-6::gfp was high in embryos and newly hatched L1 larvae, yet the GFP signal was greatly reduced within 2-3 hours and was typically not detectable after 5-6 hours. Mutation of all six genes caused a similar time course in the extinction of the GFP signal, supporting the idea that these genes act in a common pathway. Expression in ASH and ASI was not altered and unrelated axon outgrowth mutations did not affect
sra-6::gfp expression in PVQ, indicating that axon outgrowth defects per se do not perturb
sra-6::gfp expression. We examined three additional genes expressed in PVQ and found that
egl-47::gfp and
gpa-14::gfp expression was reduced after hatching but that
gpa-9::gfp expression was not affected by mutation of these six genes. These observations indicate that these six axon outgrowth genes act in a process that maintains the correct expression profile or differentiated state of PVQ after hatching.
To understand further how these genes affect PVQ differentiation, we undertook a genetic screen to isolate mutations that restore expression of
sra-6::gfp in PVQ in
unc-119 mutants. Eight suppressors that define two genes were isolated.
unc-119 animals harboring a suppresser mutation still exhibit a strong Dpy and Unc
unc-119 phenotype and PVQ still has axon outgrowth defects. As such, these suppressors are specific to the restoration of
sra-6::gfp expression in PVQ and do not appear to rescue other
unc-119 functions. We are testing whether these suppressors can restore
egl-47::gfp and
gpa-14::gfp expression in PVQ as well as whether they can suppress the
sra-6::gfp expression defects caused by mutation of the other five axon growth genes. We are addressing whether
unc-119 acts autonomously in PVQ and whether it is needed continuously or transiently during development. We are also investigating whether the
unc-119 process affects
zag-1 and
pag-3, which are needed for PVQ differentiation and block expression of
sra-6::gfp in PVQ when mutated.