With its well-defined neuronal architecture and powerful genetics, C. elegans is a useful model system for studies of neuronal differentiation. In an effort to identify genes that specify motor neuron fate, we screened for mutations that selectively alter gene expression in B-class motor neurons. Surprisingly, this screen revealed new alleles of
rpm-1 (Regulator of Presynaptic Morphology). The RPM-1 protein, and its homologues Highwire (Hiw) (Drosophila) and PHR1 (vertebrates) localize to presynaptic regions adjacent to the active zone where they are believed to function as E3 ubiquitin ligases. Mutations in
rpm-1/hiw/Phr1 disrupt synaptic structure and signaling. This effect may result from the loss of ubiquitin-dependent protein degradation. Genetic epistasis experiments indicate that RPM-1 normally downregulates
pmk-3 (
p38 MAPK) activity. We have now shown that
pmk-3 also suppresses disregulation of B-class motor neuron markers in
rpm-1 mutants. This finding suggests that RPM-1 regulates transcription of specific target genes via a MAPK signaling pathway.
We adopted two approaches to identify the targets of RPM-1. We assumed that a global, microarray-based approach would reveal the transcriptional targets of RPM-1 throughout the nervous system. Using the mRNA-tagging method, we identified ~150 genes regulated at a 2 fold or greater level in
rpm-1 mutants. We have confirmed mis-regulation of one of these targets,
tbb-6, in an
rpm-1 mutant background. In a second approach to identify
rpm-1 pathway genes, we developed a genetic screen to find suppressors of an Rpm-1 gene expression defect, i.e., ectopic
unc-129::GFP in VB motor neurons. The validity of this assay was established by showing that
pmk-3, a dominant suppressor of the Rpm-1 synaptic defect, is also a dominant suppressor of the Rpm-1 gene expression phenotype. A screen of ~5000 mutagenized F1 animals yielded 8 suppressors of Rpm-1 (
sorp-1 to
sorp-8).
sorp-5 is likely to encode a novel
rpm-1 suppressor locus because it maps to Chromosome II, and thus is not linked to other known MAPK signaling genes (
dlk-1,
mkk-4,
pmk-3). We hypothesize that
sorp-5 functions downstream of the MAPK signaling cascade to regulate gene expression in the nervous system.