Notch Signaling

Serrate-Mediated Cis-Inhibition

Sci. Signal.  23 Apr 2013:
Vol. 6, Issue 272, pp. ec90
DOI: 10.1126/scisignal.2004263

Engagement of the transmembrane receptor Notch by one of its transmembrane ligands present on an adjacent (sending) cell results in proteolytic cleavage that releases the Notch intracellular domain from the membrane so that it can enter the nucleus to promote target gene expression in the receiving cell. However, when Notch binds to a ligand present on the same cell, signaling is inhibited. Although transactivation of Notch signaling is well described, the mechanisms underlying cis-inhibition are less clear but rely on the relative abundance of the ligand and receptor and require only the extracellular domain of the ligand. Fleming et al. examined the extracellular domain of the Notch ligand Serrate (Ser) to identify the portions required for cis-inhibition in Drosophila melanogaster. By deletion analysis, the authors demonstrated that 3 of the 14 epidermal growth factor–like repeats (EGF repeats 4 to 6) in the extracellular domain were required for Ser-mediated cis-inhibition but dispensable for Ser-mediated transactivation in an in vivo wing patterning assay. EGF repeats 4 to 6 were also required for the dominant-negative activity exhibited by forms of Ser lacking the intracellular domain. Deletion of EGF repeats 4 to 6 did not affect the abundance or subcellular distribution of Ser, or its internalization by the sending cell in the context of transactivation. A soluble form of EGF repeats 4 to 6 had no effect on Notch signaling in an in vivo assay, so this region was necessary but not sufficient to mediate cis-inhibition. In a competition assay using cultured fly S2 cells, Ser present on the same cell as Notch competed with Ser present on another cell for binding to Notch, but a Ser mutant lacking EGF repeat 6 was unable to compete in cis for binding to Notch. In many developmental contexts where Notch signaling plays a role, cells are initially developmentally equivalent and produce both receptor and ligand. Subsequent interactions between the cells lead to the emergence of sending and receiving cell types. These findings represent an important step in understanding cis-inhibition, which is likely to play a key role in this process.

R. J. Fleming, K. Hori, A. Sen, G. V. Filloramo, J. M. Langer, R. A. Obar, S. Artavanis-Tsakonas, A. C. Maharaj-Best, An extracellular region of Serrate is essential for ligand-induced cis-inhibition of Notch signaling. Development 140, 2039–2049 (2013). [Abstract] [Full Text]