Notch-Mediated Cell Adhesion

Abstract

Notch family members are generally recognized as signaling molecules that control various cellular responses in metazoan organisms. Early fly studies and our mammalian studies demonstrated that Notch family members are also cell adhesion molecules; however, information on the physiological roles of this function and its origin is limited. In this review, we discuss the potential present and ancestral roles of Notch-mediated cell adhesion in order to explore its origin and the initial roles of Notch family members dating back to metazoan evolution. We hypothesize that Notch family members may have initially emerged as cell adhesion molecules in order to mediate multicellularity in the last common ancestor of metazoan organisms.

Keywords: Delta/Delta-like; Notch; Serrate/Jagged; cell adhesion; metazoan evolution.

Figure 1

Notch family members in flies and mammals. (A) Flies have one Notch receptor, while mammals have four (Notch1-Notch4) with different numbers (36, 36, 34, and 29, respectively) of EGF repeats [16,17,18,19,20,21,22,23]. Most of mammalian Notch receptors are expressed as furin-processed heterodimers, while most surface dNotch is the unprocessed full-length form [29,30,31,32]. The core ligand-binding site of dNotch and mammalian Notch1 is located within the 12th EGF repeat [41], while other EGF repeats also contribute to ligand binding [13,42,43,44,45,46]. Many EGF repeats, including the 12th, bind to Ca²⁺ ions, which are critical for protein structures and ligand binding [47,48,49]. Notch receptors form a dimer, or even a multimer, on the cell surface [50,51,52]. The ICD contains several domains that mediate nuclear translocation, interactions with CSL (i.e., Ankyrin repeats), and transcriptional activity (not depicted). LNR, Lin12-Notch repeats, HD, heterodimerization domain, and TM, transmembrane domain; (B) DSL ligands have a conserved N-terminal MNNL (module at the N-terminus of Notch ligands) domain, followed by a DSL domain and multiple tandem EGF-like repeats. MNNL, DSL, and the first three EGF repeats are all required for receptor binding [45,53,54,55,56,57,58]; however, other domains may also contribute to receptor binding [55,59]. Serrate is distinguishable from Delta by its larger number of EGF repeats and the presence of an additional cysteine-rich region (Cys). Some ligands have an intracellular PDZL (PSD-95/Dlg/Zo-1-ligand) motif that mediates interaction with PDZ-containing scaffold/adaptor proteins [60].

Figure 2

Canonical Notch signaling. (A and B) Canonical Notch signaling requires the successive cleavage of Notch by ADAMs and the γ-secretase complex. (1) Upon ligand binding, endocytosis in ligand-expressing cells is proposed to generate a pulling force that induces (A) conformational changes in dNotch ECD or (B) the dissociation of heterodimerized mammalian Notch ECD [24,65,66,67]; (2) This allows ADAMs to access the cleavage site of Notch ECD remaining on the membrane [24]. After γ-secretase processing, the released Notch ICD induces transcription of target genes.

Figure 3

Mammalian Notch family members function as adhesion molecules. Representative photomicrographs of adherent mast cells after the removal of non-adherent cells are shown. Many adherent mast cells spread on stromal cells with a deformed morphology at 37 °C, while those on OP9-Dll1 cultured on ice tethered, maintaining their original morphology. Photomicrographs are from reference [14] (Copyright 2010. The American Association of Immunologists, Inc., Bethesda, MD, USA).

Figure 4

Possible explanations for long-lasting Notch-mediated cell adhesion accompanied by Notch signaling activation in mammals. (A) Although adhesion and signaling functions are traditionally considered to be incompatible, our findings suggest that they are compatible; (B) the turnover hypothesis. The function of adhesion is transient, consistent with the traditional view. However, newly-formed Notch and DSL ligands successively form Notch receptor-ligand interactions, and, thus, cells appear to stably adhere to each other; and (C) the dissociation-resistant interaction hypothesis. There are two forms of Notch receptor-ligand interactions, i.e., dissociation-resistant (1) and dissociation-sensitive (2). The former assumes long-lasting cell adhesion, while the latter assumes signaling activation.

Figure 5

A summary of Notch-mediated cell adhesion in organisms described in this review. The simplified phylogenic tree is depicted based on reference [152]. The last common ancestors of each group are indicated as filled circles at nodes. Branches are collapsed at the base of Metazoa to reflect current uncertainty about the branch order of the most basal metazoan phyla [152]. In Ctenophora, P. bachei has the orthologue of Delta [127], while M. leidyi has no diagnostic domains for Delta [126]. In the column for Notch-mediated cell adhesion, the existence of the cell adhesion function in Notch family members is shown as open circles. D, Delta-like; J, Jagged.