The REF-1 Family of bHLH Transcription Factors Pattern C. elegans Embryos through Notch-Dependent and Notch-Independent Pathways

Abstract

Much of the patterning of early C. elegans embryos involves a series of Notch interactions that occur in rapid succession and have distinct outcomes; however, none of the targets for these interactions have been identified. We show that the REF-1 family of bHLH transcription factors is a major target of Notch signaling in all these interactions and that most examples of Notch-mediated transcriptional repression can be attributed to REF-1 activities. The REF-1 family is expressed and has similar functions in both Notch-dependent and Notch-independent pathways, and this dual mode of deployment is used repeatedly to pattern the embryo. REF-1 proteins are unusual in that they contain two different bHLH domains and lack the distinguishing characteristics of Hairy/Enhancer of Split (HES) bHLH proteins that are Notch targets in other systems. Our results show that the highly divergent REF-1 proteins are nonetheless HES-like bHLH effectors of Notch signaling.

emb-4Is a Conserved Gene Required for Efficient Germline-Specific Chromatin Remodeling DuringCaenorhabditis elegansEmbryogenesis

“…ref-1 is a transcription factor that is downstream of lag-1, and in the absence of Notch signaling its expression is either abrogated or mislocalized (Neves and Priess 2005). While emb-4(qm31)V;ref-1Tgfp animals are extremely sick and produce very few embryos, GFP expression appears normal, suggesting that emb-4 does not directly affect expression of ref-1 (data not shown).…”

Notch Signaling Is Antagonized by SAO-1, a Novel GYF-Domain Protein That Interacts with the E3 Ubiquitin Ligase SEL-10 in Caenorhabditis elegans

“…Second, the fact that both lf and hyperactive glp-1 alleles cause more embryonic lethality in sao-1-deficient embryos prevents a simple interpretation of embryonic lethality, and instead points to the complexity of properly regulating GLP-1 activity in the embryo. Third, interpretation of glp-1(lf); sao-1 double mutants is complicated by the existence of LIN-12 receptor that can mediate some embryonic Notch signaling in the absence of GLP-1, and whose expression is normally repressed in Notch-induced embryonic cells (Neves and Priess 2005). Decreased GLP-1 activity in early embryonic interactions has been shown to derepress lin-12 and other Notch signaling component genes; the effect of such inappropriately expressed LIN-12 could be exacerbated by the removal of a downregulator of Notch signaling, together causing ectopic Notch activation in subsequent cells (see Discussion for another possibility).…”

Notch-GATA synergy promotes endoderm-specific expression ofref-1inC. elegans

“…For example, the Drosophila cut and sim genes are Notch targets in the wing and the ventrolateral ectoderm, respectively (Morel and Schweisguth, 2000;Guss et al, 2001). Similarly, the C. elegans ref-1 gene is a direct target of multiple Notch interactions during embryogenesis, but does not appear to be expressed in several postembryonic Notch interactions (Neves and Priess, 2005). This specificity suggests that CSL proteins must cooperate directly or indirectly with tissuerestricted factors to control target gene expression.…”

“…The ref-1 gene is a direct target of many, though not all, Notch-mediated cell interactions in embryogenesis, and in addition is expressed in several cells independently of Notch signaling (Neves and Priess, 2005;Ross et al, 2005;Lanjuin et al, 2006). Expression of ref-1 in some cells involves DNA sequences located over 8 kb upstream of the start codon (Ross et al, 2005), in contrast to most C. elegans genes that have very small promoter regions (Okkema and Krause, 2005).…”