Chapter Three - Canonical and Non-Canonical Notch Ligands
Introduction
The Notch pathway functions as a core signaling system during embryonic development and is also required for the regulation of tissue homeostasis and stem cell maintenance in the adult (Artavanis-Tsakonas et al., 1999, Gridley, 1997, Gridley, 2003). Ligand-induced Notch signaling directs the specification of a variety of cell types and contributes to tissue patterning and morphogenesis through effects on cellular differentiation, proliferation, survival, and apoptosis (Bray, 2006, Fiuza and Arias, 2007). Given the widespread usage of the Notch pathway in different cell types and cellular processes, it is not surprising that defects in Notch ligands are associated with hereditary diseases such as Alagille syndrome and spondylocostal dysostosis and that aberrant ligand expression is detected in several cancers (Koch and Radtke, 2007, Leong and Karsan, 2006, Piccoli and Spinner, 2001, Turnpenny et al., 2007).
The canonical ligands that bind and activate Notch receptors are integral cell surface proteins, and thus activation of Notch signaling is dependent on direct cell-to-cell interactions. The transmembrane nature of Notch ligands serves to limit signaling to local cell interactions and additionally provides a signaling system for cells to communicate directly with their neighbors. Interestingly, during certain developmental processes, ligands have been found to activate Notch expressed on the surface of distantly located cells. Such long range signaling may utilize actin-based cellular projections to deliver activating signals to Notch at distant sites (de Joussineau et al., 2003). In support of such a model, the ligand Delta appears to concentrate in filopodia-like projections, possibly inducing and stabilizing these structures to facilitate long-range signaling (de Joussineau et al., 2003, Renaud and Simpson, 2001). Similarly, the Caenorhabditis elegans, distal tip cell has long cellular processes that contain the ligand Lag2 and appear to extend all the way to the mitotic/meiotic border where they regulate proliferation of the germ line through activation of the Notch homolog Glp1 (Fitzgerald and Greenwald, 1995).
Signaling induced by Notch cells following engagement with ligand cells involves a series of proteolytic cleavages in Notch to release the intracellular domain (ICD) that functions directly as the biologically active signal transducer (Kopan and Ilagan, 2009). During maturation and trafficking to the cell surface, the Notch receptor is processed by a furin-like protease to produce an intramolecular heterodimer that predisposes Notch to proteolytic activation by ligand. Interactions with ligand cells result in an extracellular juxtamembrane cleavage in Notch catalyzed by an A-Disintegrin-And-Metalloprotease (ADAM), which is followed by an intramembrane cleavage by γ-secretase to release the Notch intracellular domain (NICD) from the membrane (Fig. 3.1). NICD translocates to the nucleus where it functions directly in signal transduction through complexing with the CSL (CBF1, Su(H), LAG1) DNA binding protein and transcriptional coactivators to switch on expression of Notch target genes such as hairy and enhancer of split (HES) family. The mechanism and details of Notch transcriptional activation are covered extensively in Chapter 8. In addition to the well-characterized role for the activation of Notch signaling through cell–cell interactions (trans-interactions), ligands can also interact with Notch cell autonomously (cis-interactions) leading to inhibition of Notch signaling. The nature and mechanisms underlying the inhibitory role of Notch ligands will be discussed in Section 3 of this review. Additional characteristics of canonical and noncanonical Notch ligands required to activate signaling are discussed below.
Section snippets
Canonical Notch Ligand Structure
The majority of Notch signaling is induced by a family of DSL ligands that are characterized by the presence of a DSL (Delta, Serrate, and Lag2) domain (Henderson et al., 1994, Tax et al., 1994). The mammalian DSL ligands are classified as either Delta-like (Dll1, Dll3, and Dll4) or Serrate (Jagged)-like (Jagged1 and Jagged2) based on homology to their Drosophila prototypes Delta and Serrate (Kopan and Ilagan, 2009). DSL ligands are type 1 transmembrane proteins that share a common modular
Canonical Ligands as Inhibitors of Notch Signaling
The Notch receptors and DSL ligands are widely expressed during development, and in many cases, interacting cells express both ligands and receptors. Cells take on distinct fates because Notch signaling is consistently activated in only one of the two interacting cells, indicating that the signaling polarity must be highly regulated. The relative levels of Notch and its ligands present on interacting cells are thought to establish the signaling polarity necessary to ensure that the correct cell
Glycosylation
The Notch ligands and receptors undergo O- and N-linked glycan modifications at conserved sequences within specific EGF repeats; however, only O-fucose and O-glucose additions to Notch have so far been reported to affect signaling. N-glycan modifications of Notch, on the other hand, do not appear to alter Notch-dependent development in mice (Haltiwanger and Lowe, 2004). Glycosylation of Notch both positively and negatively regulates signaling induced by ligands, presumably through modulating
Ligand Endocytosis in Activation of Notch Signaling
A requirement for direct cell-to-cell interactions is a hallmark of Notch signaling; however, the transmembrane property of the ligands may underlie the basic mechanism of Notch activation that is dependent on ligand endocytosis. Specifically, in the absence of endocytosis, ligands accumulate at the cell surface but fail to activate signaling (Itoh et al., 2003, Nichols et al., 2007a, Parks et al., 2000). That ligands need to be internalized by the signal-sending cell to activate Notch on the
Regulation of DSL Ligand Activity by Proteolysis
DSL ligands undergo proteolytic cleavage by ADAMs and γ-secretase as described for Notch; however, in contrast to signaling induced by Notch proteolysis, proteolytic removal of cell surface ligand can either inhibit or enhance Notch signaling. Although Notch proteolysis generates an intracellular fragment that acts as the signal transducer, it is less clear if the cleavage products generated by ligand proteolysis have intrinsic activity (Fig. 3.4). A detailed review describing the proteases
DSL Ligand Interactions with PDZ-Domain Containing Proteins
The vertebrate DSL ligands Dll1, Dll4, and Jagged1 have PDZ-binding motifs at their carboxy-termini (Pintar et al., 2007), which mediate interactions with PDZ-containing scaffold/adaptor proteins (Ascano et al., 2003, Estrach et al., 2007, Mizuhara et al., 2005, Pfister et al., 2003, Six et al., 2004, Wright et al., 2004). While being dispensable for both ligand activation (Ascano et al., 2003, Mizuhara et al., 2005, Six et al., 2004, Wright et al., 2004) and inhibition of Notch signaling (
Regulation of DSL Ligand Expression Patterns
Notch signaling can both positively and negatively regulate DSL ligand expression, such that defects in Notch signaling are associated with increased expression of Dll1 (Barrantes et al., 1999, de la Pompa et al., 1997) or Dll4 (Suchting et al., 2007). On the other hand, Notch inductive signals upregulate DSL ligand expression, which is necessary for proper wing margin formation in flies (Doherty et al., 1996) as well as somite formation and patterning in vertebrates (Barrantes et al., 1999,
Noncanonical Ligands
In contrast to other signaling systems that employ a large number of activating ligands, there are only four mammalian ligands known to activate Notch receptors. It is difficult to account for the pleiotropic affects of Notch given this limited number of DSL ligands; however, the identification of noncanonical ligands expands the repertoire of ligands reported to activate signaling. Unlike the activating canonical ligands that contain a DSL domain required to interact with Notch (Fig. 3.2),
Conclusions and Future Directions
Although unique ligand–receptor combinations have been identified that induce specific cellular responses, the molecular mechanisms underlying ligand-specific signaling remains an outstanding question in the field. Moreover, given the direct and somewhat simple signaling mechanism ascribed to Notch, it is unclear how different Notch ligands could induce distinct signaling responses. It will be important to determine if different ligand–Notch complexes recruit unique signaling effectors and
Acknowledgments
We thank Abdiwahab Musse and Jason Tchieu for help with the illustrations and Alison Miyamoto for contributions to the material previously published in Oncogene (2008) 27, 5148–5167. The authors acknowledge the National Institute of Health NIH (GW), Jonsson Comprehensive Cancer Center (JCCF) (LMK), and Association of International Cancer Research (AICR) (BD) for financial support.
References (276)
- et al.
Microfibril-associate glycoprotein-2 (MAGP-2) promotes angiogenic cell sprouting by blocking notch signaling in endothelial cells
Microvasc. Res.
(2008) - et al.
Instruction of distinct CD4 T helper cell fates by different notch ligands on antigen-presenting cells
Cell
(2004) - et al.
Bidirectional ephrin/Eph signaling in synaptic functions
Brain Res.
(2007) - et al.
The C-terminal PDZ-ligand of JAGGED1 is essential for cellular transformation
J. Biol. Chem.
(2003) - et al.
Dlk acts as a negative regulator of Notch1 activation through interactions with specific EGF-like repeats
Exp. Cell Res.
(2005) - et al.
Bearded family members inhibit Neuralized-mediated endocytosis and signaling activity of Delta in Drosophila
Dev. Cell
(2006) - et al.
Interaction between Notch signalling and Lunatic fringe during somite boundary formation in the mouse
Curr. Biol.
(1999) - et al.
Mind bomb1 is a ubiquitin ligase essential for mouse embryonic development and Notch signaling
Mech. Dev.
(2005) - et al.
A role of receptor Notch in ligand cis-inhibition in Drosophila
Curr. Biol.
(2010) - et al.
The notch ligands Dll4 and Jagged1 have opposing effects on angiogenesis
Cell
(2009)
Notch signaling: Fringe really is a glycosyltransferase
Curr. Biol.
Notch induced proteolysis and nuclear localization of the Delta ligand
J. Biol. Chem.
Reciprocal regulatory interactions between the Notch and Ras signaling pathways in the Drosophila embryonic mesoderm
Dev. Biol.
Three modules of zebrafish Mind bomb work cooperatively to promote Delta ubiquitination and endocytosis
Dev. Biol.
The lateral signal for LIN-12/Notch in C. elegans vulval development comprises redundant secreted and transmembrane DSL proteins
Dev. Cell
Distinct biological roles for the notch ligands Jagged-1 and Jagged-2
J. Biol. Chem.
NB-3/Notch1 pathway via Deltex1 promotes neural progenitor cell differentiation into oligodendrocytes
J. Biol. Chem.
Xenopus neuralized is a ubiquitin ligase that interacts with XDelta1 and regulates Notch signaling
Dev. Cell
Notch signaling: distinct ligands induce specific signals during lymphocyte development and maturation
Immunol. Lett.
Dose-dependent effects of the Notch ligand Delta1 on ex vivo differentiation and in vivo marrow repopulating ability of cord blood cells
Blood
Endocytosis-independent mechanisms of Delta ligand proteolysis
Exp. Cell Res.
Bidirectional signaling mediated by ephrin-B2 and EphB2 controls urorectal development
Dev. Biol.
The role of Notch in patterning the human vertebral column
Curr. Opin. Genet. Dev.
Proteolytic processing of delta-like 1 by ADAM proteases
J. Biol. Chem.
Delta/notch-like epidermal growth factor (EGF)-related receptor, a novel EGF-like repeat-containing protein targeted to dendrites of developing and adult central nervous system neurons
J. Biol. Chem.
Asymmetric Rab 11 endosomes regulate delta recycling and specify cell fate in the Drosophila nervous system
Cell
Structural conservation of Notch receptors and ligands
Semin. Cell Dev. Biol.
Autonomous and non-autonomous regulation of mammalian neurite development by Notch1 and Delta1
Curr. Biol.
Further characterization of proteins associated with elastic fiber microfibrils including the molecular cloning of MAGP-2 (MP25)
J. Biol. Chem.
Complementary DNA cloning establishes microfibril-associated glycoprotein (MAGP) to be a discrete component of the elastin-associated microfibrils
J. Biol. Chem.
Making a difference: the role of cell-cell interactions in establishing separate identities for equivalent cells
Cell
Notch signaling in vertebrate development and disease
Mol. Cell. Neurosci.
Maintenance of neuroepithelial progenitor cells by Delta-Notch signalling in the embryonic chick retina
Curr. Biol.
The adhesion force of Notch with Delta and the rate of Notch signaling
J. Cell Biol.
Soluble form of Jagged1: unique product of epithelial keratinocytes and a regulator of keratinocyte differentiation
J. Cell. Biochem.
FGF-dependent Notch signaling maintains the spinal cord stem zone
Genes Dev.
Notch signaling: cell fate control and signal integration in development
Science
Mouse fetal antigen 1 (mFA1), the circulating gene product of mdlk, pref-1 and SCP-1: Isolation, characterization and biology
J. Reprod. Fertil.
Spacing differentiation in the developing Drosophila eye: a fibrinogen-related lateral inhibitor encoded by scabrous
Science
Molecular basis of oligoubiquitin-dependent internalization of membrane proteins in Mammalian cells
Traffic
Composite signalling from Serrate and Delta establishes leg segments in Drosophila through Notch
Development
Notch signalling: A simple pathway becomes complex
Nat. Rev. Mol. Cell Biol.
The atypical mammalian ligand Delta-like homologue 1 (Dlk1) can regulate Notch signalling in Drosophila
BMC Dev. Biol.
PDZ proteins retain and regulate membrane transporters in polarized epithelial cell membranes
Am. J. Physiol. Cell Physiol.
Determinants of Notch-3 receptor expression and signaling in vascular smooth muscle cells: implications in cell-cycle regulation
Circ. Res.
The association of epsin with ubiquitinated cargo along the endocytic pathway is negatively regulated by its interaction with clathrin
Proc. Natl. Acad. Sci. U.S.A.
Notch signaling in differentiation and function of dendritic cells
Immunol. Res.
Notch2, but not Notch1, is required for proximal fate acquisition in the mammalian nephron
Development
Gamma-secretase activity is dispensable for mesenchyme-to-epithelium transition but required for podocyte and proximal tubule formation in developing mouse kidney
Development
Why is delta endocytosis required for effective activation of notch?
Dev. Dyn.
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