Characterization of one of the the REF-1 Family Members, HLH-25, in C. elegans

The REF-1 family proteins are distinguished by the presence of two basic helix-loop helix domains. The REF-1 family members are considered functional homologs of the Hairy/Enhancer of Split in humans. HLH-25 is one of the six members of the REF-1 family. HLH-25 has not been studied extensively. In preliminary studies from our laboratory, genes identified by microarray analysis of hlh-25 mutants were essential for embryogenesis, larval development, and growth. Thus, the present study was designed to further characterize HLH-25 and to more precisely define its role during embryonic and larval development. The gene encoding HLH-25 is actively expressed in embryos, larvae and adults. In the absence of hlh-25, animals show a 54% embryonic lethality, a reduced brood size, an increased number of unfertilized eggs, a slower movement rate, a longer life span, and a longer dauer recovery. The human tumor suppressor PTEN homolog, daf-18 is one of the HLH-25 target genes. The regulation of daf-18 through HLH-25 is the responsible for changes to the life span and dauer recovery in hlh-25 mutant animals

HES-Mediated Repression of Pten in Caenorhabditis elegans

The Hairy/Enhancer-of-Split (HES) group of transcription factors controls embryonic development, often by acting downstream of the Notch signaling pathway; however, little is known about postembryonic roles of these proteins. In Caenorhabditis elegans, the six proteins that make up the REF-1 family are considered to be HES orthologs that act in both Notch dependent and Notch-independent pathways to regulate embryonic events. To further our understanding of how the REF-1 family works to coordinate post-embryonic cellular events, we performed a functional characterization of the REF-1 family member, HLH-25. We show that, after embryogenesis, hlh-25 expression persists throughout every developmental stage, including dauer, into adulthood. Like animals that carry loss-of-function alleles in genes required for normal cell cycle progression, the phenotypes of hlh-25 animals include reduced brood size, unfertilized oocytes, and abnormal gonad morphology. Using gene expression microarray, we show that the HLH-25 transcriptional network correlates with the phenotypes of hlh-25 animals, and that the C. elegans Pten ortholog, daf-18, is one major hub in the network. Finally, we show that HLH-25 regulates C. elegans lifespan and dauer recovery, which correlates with a role in the transcriptional repression of daf-18 activity. Collectively, these data provide the first genetic evidence that HLH-25 may be a functional ortholog of mammalian HES1, which represses PTEN activity in mice and human cells