The C. elegans hermaphrodite gonad is bilobed and the U-shaped morphology of the anterior and posterior gonad arms is determined by directional migration of gonadal distal tip cells (DTCs) each located at the distal-most ends of the gonad primordium. To identify the molecules that regulate DTC migration, we have isolated many mutants defective in gonad morphogenesis due to abnormal DTC migration (mig). The DTCs in
mig-24 (
k168) mutants prematurely terminate migration, leading to thick and short gonadal arms and partial sterility.
mig-24 was positionally cloned and found to encode a novel bHLH transcription factor, also designated as
hlh-12. The RNAi knockdown of
mig-24 resulted in the same phenotype as was observed the
mig-24 (
k168) original mutants. The
mig-24 promoter and Venus fusion gene was expressed specifically in DTCs from the L2 to the young adult stages, during which DTCs migrate. Since many bHLH proteins function in heterodimers, we examined the possibility that MIG-24 also forms heterodimers. Among 35 bHLH proteins in C. elegans, HLH-2 was examined, because RNAi of
hlh-2 phenocopies the DTC migration defects observed in
mig-24 (
k168) (Karp and Greenwald, Dev. Biol., 2004). To assess the physical interaction between MIG-24 and HLH-2, co-immunoprecipitation assay was performed. As a result, it was shown that MIG-24 physically interacts with HLH-2. To understand further the biological function of MIG-24, we attempted to identify its transcriptional target genes. Two proteins, LAG-2, a Delta/Serrate-like protein (Henderson et al., Development, 1994) and GON-1, an ADAM-TS family metalloprotease (Blelloch and Kimble, Nature, 1999), are known to be expressed in the DTCs. We examined the expression of fusion genes for
lag-2 or
gon-1 promoters and the reporter GFP gene. We found that the expression of
gon-1p::GFP, but not
lag-2p::GFP, was significantly weakened in the
mig-24 compared to that in the wild-type background, suggesting that
gon-1 is a downstream target of the
mig-24 transcription factor. Furthermore, the chromatin immunoprecipitation (ChIP) assay revealed that MIG-24 can bind to the
gon-1 promoter region, implying the possibility that MIG-24 directly regulates the
gon-1 expression. We propose a model that the MIG-24/HLH-2 heterodimer activates the transcription of
gon-1 in DTCs to remodel the components of the gonadal basement membrane, thereby regulating the migration of DTCs.