OSM-11 Facilitates LIN-12 Notch Signaling during Caenorhabditis elegans Vulval Development

Abstract

Notch signaling is critical for cell fate decisions during development. Caenorhabditis elegans and vertebrate Notch ligands are more diverse than classical Drosophila Notch ligands, suggesting possible functional complexities. Here, we describe a developmental role in Notch signaling for OSM-11, which has been previously implicated in defecation and osmotic resistance in C. elegans. We find that complete loss of OSM-11 causes defects in vulval precursor cell (VPC) fate specification during vulval development consistent with decreased Notch signaling. OSM-11 is a secreted, diffusible protein that, like previously described C. elegans Delta, Serrate, and LAG-2 (DSL) ligands, can interact with the lineage defective-12 (LIN-12) Notch receptor extracellular domain. Additionally, OSM-11 and similar C. elegans proteins share a common motif with Notch ligands from other species in a sequence defined here as the Delta and OSM-11 (DOS) motif. osm-11 loss-of-function defects in vulval development are exacerbated by loss of other DOS-motif genes or by loss of the Notch ligand DSL-1, suggesting that DOS-motif and DSL proteins act together to activate Notch signaling in vivo. The mammalian DOS-motif protein Deltalike1 (DLK1) can substitute for OSM-11 in C. elegans development, suggesting that DOS-motif function is conserved across species. We hypothesize that C. elegans OSM-11 and homologous proteins act as coactivators for Notch receptors, allowing precise regulation of Notch receptor signaling in developmental programs in both vertebrates and invertebrates.

Author Summary

The classic view of Notch receptor activation involves receptor binding to transmembrane Notch ligands that contain a conserved DSL (Delta, Serrate, and LAG-2) domain. Here, we find that the Caenorhabditis elegans OSM-11 protein is a novel ligand of the well-characterized Notch signal transduction pathway and plays a role in cell fate specification during development. OSM-11 is a secreted, diffusible protein whose loss decreases Notch signaling in vivo. OSM-11 and related C. elegans proteins do not contain a DSL domain, but contain a conserved motif we have named DOS (Delta and OSM-11) that is also found in the extracellular domain of known Notch ligands in organisms other than C. elegans. The functional mammalian homolog of OSM-11 is the secreted protein Deltalike1 (Dlk1), also known as Preadipocyte Factor 1 (PREF1), which plays a poorly defined role in Notch signaling regulating obesity and other developmental decisions. This suggests that Notch ligands are split into two complementary coligand families that act together to regulate Notch signaling in developmental contexts. In addition to regulating development, DOS ligands play roles in osmotic stress and C. elegans behavior, suggesting previously unsuspected roles for Notch signaling across species.

Abstract

The C. elegans OSM-11 protein acts with DSL ligands to activate Notch signaling in cell fate specification and defines a conserved family of potential Notch co-ligands.