The many facets of Notch ligands

Abstract

The Notch signaling pathway regulates a diverse array of cell types and cellular processes and is tightly regulated by ligand binding. Both canonical and noncanonical Notch ligands have been identified that may account for some of the pleiotropic nature associated with Notch signaling. This review focuses on the molecular mechanisms by which Notch ligands function as signaling agonists and antagonists, and discusses different modes of activating ligands as well as findings that support intrinsic ligand signaling activity independent of Notch. Post-translational modification, proteolytic processing, endocytosis and membrane trafficking, as well as interactions with the actin cytoskeleton may contribute to the recently appreciated multifunctionality of Notch ligands. The regulation of Notch ligand expression by other signaling pathways provides a mechanism to coordinate Notch signaling with multiple cellular and developmental cues. The association of Notch ligands with inherited human disorders and cancer highlights the importance of understanding the molecular nature and activities intrinsic to Notch ligands. Oncogene (2008) 27, 5148-5167; doi:10.1038/onc.2008.229.

Figure 1

Protein structure of the DSL family of ligands. Red boxes, DSL domain; white boxes, EGF repeat; grey boxes, calcium-binding EGF repeat; red box with diagonal lines, DSL with non-conserved cysteine spacing. See text for details. All DSL ligands contain a N-terminal signal sequence (not shown). Structures are based on the following protein sequences from GenBank: Drosophila Serrate, P18168; Drosophila Delta, P10041; human Jagged1, XP056118; human Jagged2, Q9Y219; human Delta-like1, O00548; Xenopus X-Delta-2, AAB37131; human Delta-like3, Q9NYJ7; human Delta-like4, Q9NR61; C. elegans LAG-2, P45442; C. elegans APX-1, P41990; C. elegans ARG-1, T16213; C. elegans DSL-1, AAC04450. The drawing is approximately to scale. NT, N-terminal domain; DSL, Delta/Serrate/LAG-2 domain; EGF, epidermal growth factor-like; CR, cysteine-rich region; TM, transmembrane domain; PDZL; PDZ ligand motif.

Figure 2

Effects of proteolytic cleavage of DSL ligands on Notch signaling. Mammalian and Drosophila DSL ligands undergo juxtamembrane and intramembrane cleavages. Juxtamembrane cleavage of mammalian and Drosophila DSL ligands by A-Disintegrin-And-Metalloproteases (ADAMs) results in shedding of the extracellular domain (ECD). The shed/soluble ECD may be inactive or can act as either an agonist or antagonist of Notch signaling depending on its state of clustering. In mammalian cells, the membrane-tethered fragment containing the intracellular domain (TMICD) undergoes sequential intramembrane γ-secretase, releasing the intracellular domain (ICD) from its membrane tether. The released ICD can translocate to the nucleus and activate gene transcription suggesting that ligand-Notch interactions can trigger bi-directional signaling. Unlike TMICD generated from mammalian DSL ligands, the Drosophila Delta TMICD fragment is not further processed and could antagonize Notch signaling in trans. A thiol-sensitive activity (TSA) catalyzes ADAM-independent intramembrane cleavage of Drosophila Delta resulting in cleavage products that may or may not remain membrane-tethered. If the ECD containing fragment (ECDTM) remains membrane-tethered, it could act as a Notch signaling antagonist either in cis or in trans. If ECDTM is released from the membrane it could act as proposed for soluble ECD. If the ICD containing intramembrane cleavage product TMICD^TSA remains membrane-tethered, it could act as a Notch signaling antagonist in trans. Alternatively, the ICD may be released from the membrane, translocate to the nucleus and activate gene transcription. EC = extracellular; PM = plasma membrane; IC = intracellular.

Figure 3

Non-canonical ligands reported to affect Notch signaling. Accession numbers for human proteins: Jagged1, XP056118; Delta-like1, O00548; DNER, Q8NFT8; DLK-1, P80370; Jedi, Q5VY43; F3/Contactin, Q12860; NB-3, Q9UQ52; MAGP-1, P55001; MAGP-2, Q13361; CCN3/NOV, P48745. See text for details. All non-canonical ligands contain a N-terminal signal sequence (not shown). The drawings are approximately to scale. NT, conserved N-terminal domain found in DSL ligands; DSL, Delta/Serrate/LAG-2 domain; EGF, 6-cysteine epidermal growth factor repeat; cys, cysteine; CR, cysteine-rich domain; TM, transmembrane domain; PDZL, PDZ ligand; aa, amino acids; EMI, emilin-like domain; EGF-like, EGF-like motif with 8 cysteines that is not laminin-like; Ig-CAM, immunoglobulin-containing cell adhesion molecule domain; FNIII, fibronectin type III domain; GPI, glycosylphosphatidylinositol; Q, glutamine-rich region; FReD, fibrinogen-related domain; MBD, matrix binding domain; RGD, integrin binding motif; IGFBP, insulin-like growth factor-binding protein-like domain; VWF-C, von Willebrand factor type C-like domain; TSP-1, thrombospondin type 1-like domain; CTCK, C-terminal cysteine knot domain. *Only full-length constructs were tested for binding.