The Notch signaling pathway consists of multiple types of receptors and ligands, whose interactions can be tuned by Fringe glycosyltransferases. A major challenge is to determine how these components control the specificity and directionality of Notch signaling in developmental contexts. Here, we analyzed same-cell (cis) Notch-ligand interactions for Notch1, Dll1, and Jag1, and their dependence on Fringe protein expression in mammalian cells. We found that Dll1 and Jag1 can cis-inhibit Notch1, and Fringe proteins modulate these interactions in a way that parallels their effects on trans interactions. Fringe similarly modulated Notch-ligand cis interactions during Drosophila development. Based on these and previously identified interactions, we show how the design of the Notch signaling pathway leads to a restricted repertoire of signaling states that promote heterotypic signaling between distinct cell types, providing insight into the design principles of the Notch signaling system, and the specific developmental process of Drosophila dorsal-ventral boundary formation.
In animals, cells use a process called Notch signaling to communicate with neighboring cells. During this process, a protein known as a DSL ligand from one cell binds to a protein called a Notch receptor on a neighboring cell. This triggers a series of events in the neighboring cell that change how the genes in this cell are expressed. Notch signaling is involved in many processes including the early growth of embryos, the formation of organs and limbs, and the maintenance of stem cells throughout adult life.
Enzymes called Fringe enzymes can control Notch signaling by blocking or promoting the formation of the DSL ligand-Notch receptor pairs. It is also possible for a DSL ligand and a Notch receptor from the same cell to interact. This is thought to be important because it prevents an individual cell from sending and receiving Notch signals at the same time.
There are several different DSL ligands, Notch receptors and Fringe enzymes, so it is difficult to determine which configurations of receptors, ligands and Fringe enzymes can enable Notch signals to be sent or received. To address this problem, LeBon et al. investigated how Fringe enzymes acted on several different DSL-Notch receptor pairs in mammalian cells, and also in fruit flies. They focused in particular on the interactions that occurred within the same cell, as the role of Fringe enzymes in this type of interaction has not been examined previously.
The experiments revealed that Fringe proteins modify specific same-cell interactions in a way that enables a cell to receive one type of Notch signal from a neighboring cell and send a different type of Notch signal to another cell at the same time. More generally, these results show how an unconventional, ‘bottom-up’ approach can reveal the design principles of cell signaling systems, and suggest that it should now be possible to use these principles to try to understand which cell types send signals to which other cell types in many different contexts.