A secreted Delta1-Fc fusion protein functions both as an activator and inhibitor of Notch1 signaling.

Signaling induced through interactions between DSL (Delta, serrate, LAG-2) ligand-signaling cells and Notch-responding cells influences the developmental fate of a wide variety of invertebrate and vertebrate cell types. Consistently with a requirement for direct cell-cell interactions, secreted DSL ligands expressed in flies do not appear to activate Notch signaling but rather produce phenotypes reminiscent of losses in Notch signaling. In contrast, secreted DSL ligands expressed in Caenorhabditis elegans or supplied to mammalian cells in culture produce effects indicative of Notch activation. In fact, engineered secreted DSL ligands have been used to study Notch signaling in neurogenesis, gliogenesis, hematopoeisis, neurite morphogenesis and ligand-induced nuclear translocation of the Notch intracellular domain. Using a recombinant, secreted form of the DSL ligand Delta1, we found that antibody-induced oligomerization (termed "clustering") was required for this soluble ligand to bind specifically to Notch1-expressing cells, undergo internalization, and activate downstream signaling. Interestingly, clustering with either limiting or excess antibody led to ligand binding in the absence of Notch signaling, indicating that ligand binding is necessary but not sufficient for activation of Notch signaling. Moreover, such antibody clustering conditions blocked Notch1 signaling induced by membrane-bound DSL ligands. We propose that multimerization influences whether ligand binding to Notch results in activation or inhibition of downstream signaling and suggest that differences in ligand presentation might account for why secreted forms of DSL ligands have been reported to function as agonists and antagonists of Notch signal transduction.