Notch Family

Notch Ligands

Canonical Notch ligands of the mammalian Delta/Serrate/Lag-2 (DSL) family belong to either the Delta-like (DLL1, DLL3, DLL4) or the Jagged (Jagged 1, Jagged 2) class of proteins. Delta-like ligands are structural homologs of the Drosophila Delta protein, while Jagged ligands are homologs of the Drosophila Serrate protein. Both the Delta-like and Jagged canonical Notch ligands are type I transmembrane proteins with an extracellular domain containing a DSL motif and multiple tandem EGF-like repeats. The DSL motif, along with sequences in the N-terminus and the first two EGF-like repeats, are required for Notch binding and signaling. Despite similarities in the two classes of DSL family ligands, the Jagged proteins are unique in that they contain almost twice as many EGF-like repeats as the Delta-like proteins and some of these repeats have conserved insertions with unknown functions. Additionally, the Jagged proteins contain a membrane proximal, cysteine-rich region that is not present in the Delta-like proteins. Both the Delta-like and Jagged proteins activate Notch signaling in trans with the ligand expressed on one cell activating the Notch receptor expressed on a neighboring cell. In contrast, cis interactions between the DSL ligands and Notch on the same cell inhibit Notch signaling.

While Notch signaling is primarily activated by the Jagged and Delta-like canonical Notch ligands, a number of unrelated, non-canonical Notch ligands have also been identified. These include the membrane-associated proteins, Delta-like 1 (Dlk-1)/Pref-1 and Delta/Notch-like EGF-related receptor (DNER), the GPI-linked neural cell adhesion molecules, F3/Contactin-1 and NB3/Contactin-6, and the soluble proteins, CCN3/NOV, MAGP-1, and MAGP-2. Dlk-1/Pref-1 and DNER are integral membrane proteins that lack a DSL domain, but are otherwise structurally similar to the Delta-like proteins. While Dlk-1/Pref-1 has been reported to interact with Notch in cis and compete with trans-activating Notch ligands to inhibit Notch signaling, DNER functions as a trans ligand to activate Notch signaling. Similarly, the GPI-linked F3/Contactin-1 and NB3/Contactin-6 proteins, which are structurally distinct from Dlk-1/Pref-1 and DNER, are also thought to interact with Notch in trans and activate CSL-independent Notch signaling. Additionally, three secreted proteins in vertebrates have been identified as Notch ligands. All three of these proteins, CCN3/NOV, and the microfibril-associated glycoproteins, MAGP-1 and MAGP-2, activate Notch signaling when both the ligand and the Notch receptor are expressed in the same cell, suggesting that they may activate autocrine signaling.

Notch Receptors

In mammals, there are four highly conserved Notch receptors (Notch-1-4). These receptors are bipartite, type I transmembrane proteins consisting of an extracellular domain with 29-36 EGF-like repeats that is noncovalently linked to a smaller transmembrane and intracellular domain. Ligand binding is mediated through the 11^th and 12^th EGF-like repeats in the Notch extracellular domain. Adjacent to the EGF-like repeats are three cysteine-rich, Lin-12/Notch repeats, known as the LNR region, which has been suggested to be involved in down regulating Notch receptor activity. This is followed by a transmembrane segment and a cytoplasmic domain that contains a RAM region, six adjacent ankyrin repeats that mediate binding to the CSL transcription factor, a glutamine-rich domain, and a PEST sequence that may be involved in mediating the stability of the receptor. Additionally, Notch-1 and Notch-2 contain a cytokine response region just downstream of the ankyrin repeats and a transcriptional activation domain that mediate their interactions with transcriptional co-activators. Functionally, the four Notch receptors are also distinct due to differences in their temporal and cellular expression, coupled with the differences in their ligand-binding affinities.

Notch Signaling

Canonical Notch signaling involves a transmembrane DSL ligand on one cell binding to the extracellular domain of the Notch receptor on an adjacent cell. Notch ligand-receptor binding results in: 1) the proteolytic cleavage of Notch by TNF-converting enzyme (TACE/ADAM17) and 2) the ubiquitination of the intracellular domain of the Notch ligand and endocytosis of the Notch ligand/Notch extracellular domain complex into the ligand-expressing cell. Cleavage of Notch by TACE/ADAM17 is followed by a second cleavage of the receptor by Presenilin-dependent gamma-secretase, resulting in the release of the Notch intracellular domain (NICD). The NICD then translocates to the nucleus, where it forms a complex with CSL/RBPj, MAML, and transcriptional co-activators to regulate the transcription of Notch target genes. In addition to this canonical Notch signaling pathway, there appears to be crosstalk between Notch signaling and other signaling pathways, including Akt/mTOR, NF kappa B, Wnt/beta-Catenin, and others.

Notch signaling is highly conserved in multicellular organisms and is important for specifying cell fates, regulating pattern formation, defining boundaries between different cell types, and modulating cell proliferation and differentiation. It is required for multiple developmental processes, including vasculogenesis, angiogenesis, somatogenesis, myogenesis, neurogenesis, and hematopoiesis. Similar to other growth factors, mutations in the Notch signaling pathway are associated with developmental defects. Additionally, Notch signaling has been shown to have context-dependent tumor suppressive or oncogenic effects in many different types of cancer.