Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Cadherins

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_39

Synonyms

Historical Background: The Larger Cadherin Superfamily

Cadherins comprise a large protein superfamily of calcium-dependent proteins associated with the plasma membrane. They are hallmarked by at least two consecutive extracellular cadherin repeats (ECs) in their extracellular domain. Epithelial cadherin (E-cadherin or cadherin-1) is the founding family member and represents the classic or type-I cadherins. These cadherins mediate specific and strong cell-cell adhesion and play key roles in local organization of cytoskeletal structures and in signaling cascades, as documented below. The mature E-cadherin contains an ectodomain of about 550 amino acids (AA) comprising five ECs, a single transmembrane (TM) domain, and a cytoplasmic domain (CD) of about 150 AA. The CD has two highly conserved domains for binding either p120ctn and related protein family members or β-catenin and the homologous plakoglobin (γ-catenin), all of which belong to the armadillo protein family (reviewed by van Roy and Berx 2008) (Fig. 1a). Association of p120ctn with the juxtamembrane domain (JMD) of cadherins protects them from premature endocytosis and hence stabilizes the junctions. On the other hand, in a classical cadherin–catenin complex, also referred to as CCC, β-catenin forms a molecular bridge between the cadherins and α-catenins, the latter of which are vinculin-related proteins that bind F-actin. However, a model consisting of a stable CCC has been criticized (reviewed in Nelson and Fuchs 2010). Indeed, such complex was found mainly in biochemical experiments. Although monomeric α-catenin could integrate into such CCC, it does not associate with F-actin, whereas homodimeric α-catenin indeed binds F-actin but dissociates from the CCC. This discrepancy was clarified in two ways. Other actin-binding linker molecules, such as Eplin and Myosin VI, might be involved in anchoring cadherin-dependent junctions to the cytoskeleton (Fig. 1b, c). Equally important, mechanical tension regulates the affinity of α-catenin for its binding partners, including actin-binding vinculin (le Duc et al. 2010). Such stretched junctional complex then becomes tightly anchored to the cortical actomyosin cytoskeleton, which in turn is stabilized in the junction by, for instance, vinculin recruited Ena/VASP proteins (Fig. 1c). Cell-cell adhesion is much strengthened under these conditions so that weak focal junctions evolve to tight zonula adherens junctions, which, however, remain under highly dynamic regulation and can contribute significantly to morphogenic tissue dynamics (Takeichi 2014; Lecuit and Yap 2015).
Cadherins, Fig. 1

Alternative states of the classic cadherin-catenin complex (CCC) at the junction between two neighboring epithelial cells (see text for details and references). (a) Nascent junction: Transmembrane classic cadherins form zipper-like intercellular junctions based on homophilic molecular interactions between their EC1 and EC2 domains. Their cytoplasmic domains contain conserved binding sites for the armadillo proteins p120ctn and β-catenin (although not drawn that way, p120ctn and β-catenin can interact simultaneously with a single cadherin molecule). p120ctn and β-catenin also have roles in the cytoplasm and nucleus (not detailed here). Further, the CCC contains monomeric α-catenin, which, via partly unknown intermediate proteins (?), links the CCC to the actin cytoskeleton. In a nascent junction, F-actin grows and branches by the action of the Arp2/3 protein complex. Certain δ-protocadherins (including protocadherin-10/OL-protocadherin) interfere with formation of tight CCC-dependent junctions and induce concerted cell migration by recruiting a Nap1-Abi-WASP/WAVE complex, leading to focal activation of Ena/VASP molecules, what promotes growth of F-actin filaments. (b) Molecular interactions in an established CCC-dependent junction. Monomeric α-catenin in the CCC can associate with the actin-bundling protein Eplin. On the other hand, it can dissociate from the CCC, dimerize, and locally bundle F-actin. Dimeric α-catenin is thought to inhibit the Arp2/3 complex, so that F-actin grows merely linearly by the action of formins. (c) In recent years, evidence has been mounting that tighter cadherin-dependent junctions arise in cells under mechanical tension, for examples, during morphogenetic movements and wound repair. While in cells not under tension (depicted on the right) both vinculin and α-catenin are in a less active, folded configuration, they become unfolded when tension is applied. Local tension is generated by the action of myosin-II on F-actin (depicted on the left). This stretches α-catenin to a configuration that recruits and activates vinculin. Vinculin in turn, with the help of Ena/VASP proteins, binds to and reinforces the actomyosin complex. In this way, the reinforced mechanical coupling between the CCC and the cytoskeleton protects the junction against the tension while transmitting the force over the tissue. In addition to α-catenin- and vinculin-regulated mechanosensitive junctional complexes, also direct interactions between E-cadherin and F-actin, like the Myosin VI linkage depicted, can contribute (Modified after van Roy and Berx (2008), Yonemura et al. (2010), and Lecuit and Yap (2015))

Cadherins were originally discovered as mediators of specific cell-cell adhesive structures, often called adherens junctions (AJ) (Gumbiner 2005; van Roy and Berx 2008; van Roy 2013). The basic and well-documented model for this function involves two modes of interaction between the most amino-terminal ECs (called EC1): homophilic (between identical cadherin species) and homotypic (between identical cell types). Both cis (between neighboring cadherin molecules in the same plasma membrane) and trans interactions (between apposing cadherin molecules) have been described in considerable detail (van Roy and Berx 2008). Cell-cell adhesion mediated by classic cadherins is considered the basis of cell sorting out during both embryonic development and the morphogenesis of most organs (Gumbiner 2005; Halbleib and Nelson 2006; Niessen et al. 2011; van Roy 2013; Suzuki and Hirano 2016). Although the cadherin activity during these important processes might be explained by cadherins serving as cell-type specific molecular glues, it is increasingly clear that signaling by and to cadherins plays major roles. Indeed, cadherins are required not only for rigid, static cell-cell contacts, but also for regulation of dynamic morphogenetic cell movements by active remodeling of intercellular junctions and the associated cytoskeletal structures (Harris and Tepass 2010; Takeichi 2014).

Presently, mammalian genomes are known to contain over 100 genes encoding cadherins or cadherin-related proteins (Hulpiau and van Roy 2011; Suzuki and Hirano 2016). Some of them are more recent evolvements, such as desmogleins and desmocollins, which form the backbone of desmosomes (strong cell junctions in tissues under high physical stress, like epidermis and heart). The so-called clustered protocadherins form another recently evolved cadherin subtype. Protocadherins seem to be less involved in tight cell-cell adhesion and may play greater roles in cell recognition and signaling. On the other hand, recent phylogenetic analyses have shown the existence of six ancient types of cadherins and cadherin-related proteins (Hulpiau and van Roy 2011) (Fig. 2). These are the classical cadherins (CDH), the FAT cadherins, the FAT-like cadherins, the Dachsous cadherins (DCHS), the flamingo (Fmi) or CELSR cadherins, and several cadherin-related proteins, such as cadherin-related 23 (CDHR23). Several of these cadherin family members were originally discovered in the fruit fly, and their genetic analyses have revealed important functions in signaling. The protocadherins emerged slightly later in evolution. They were originally thought to be specific for deuterostomes, but recent studies revealed their presence in, for instance, cnidarians although not in the fruit fly or the nematode, which probably lost protocadherins during evolution.
Cadherins, Fig. 2

Domain structure of representative members of the cadherin superfamily in human (Homo sapiens, Hs). All proteins are drawn to scale and aligned at their transmembrane domains (TM). They are depicted as precursor proteins with their extracellular N-terminal ends on the left. Sizes (number of AA) are indicated on the right and range between about 700 and 5000 AA. The cadherin families are exemplified as follows (from top to bottom). The CDH family is represented by E-cadherin (CDH1), cadherin-11, and cadherin-13. The latter has an atypical glycosyl phosphatidyl inositol anchor (GPI). The 7TM cadherin family (or CELSR/Flamingo family) is represented by CELSR1. These proteins have a seven-TM domain. The protocadherin (PCDH) superfamily is represented by protocadherin-α6 for the subfamily of clustered PCDHs, by protocadherin-1 for the subfamily of δ1-PCDHs, and by protocadherin-8 for the subfamily of δ2-PCDHs. The cadherin-related proteins are represented by FAT4, by DCHS1, and by cadherin-related protein 23. The very long ectodomains of FAT4 and DCHS1 have been shown to be interrupted by hairpin-like bends (see also Fig. 7) due to the presence of EC-EC linkers lacking calcium-binding properties (Tsukasaki et al. 2014). The following protein domains are annotated: 7TM, seven-transmembrane domain; CBD, (conserved cadherin-specific) β-catenin binding domain; CE, Cysteine-rich EGF repeat-like domain; CM1 to CM3, conserved motifs in the CDs of particular protocadherins; EC, extracellular cadherin repeat; FB, Flamingo box; GAIN, GPCR autoproteolysis-inducing domain; HRD, Hormone receptor domain (in CELSR/Flamingo proteins); JMD, (conserved cadherin-specific) juxtamembrane domain, binding p120ctn; LAG, laminin A globular domain; Pro-d, prodomain. UCD, unique cytoplasmic domain (Modified after van Roy 2014)

From these studies, several hallmarks became clear: Ancient family members always have long ectodomains with numerous ECs, which during evolution became progressively shorter (Hulpiau and van Roy 2011). Current classic and desmosomal cadherins have 5 ECs, whereas protocadherins have 6 to 7 ECs (Fig. 2). This, together with the finding of many long cadherin-like proteins in unicellular choanoflagellates, indicates that the cadherin family members originally served in extracellular sensing and cell-cell recognition rather than in specific cell-cell adhesion.

Cadherins characteristically have an ectodomain that is largely or exclusively composed of ECs, but their TM domains and in particular their CDs are much more diverse (Fig. 2). Quite often, cadherins have their own subfamily-specific CD, often with one to several motifs. These CDs are conserved within the same subfamily, but apparently they are not related to each other and often not at all to the “standard” CD of classic cadherins. Cadherin-13, also called T-cadherin (truncated), stands out because it is the only known cadherin lacking a TM domain. Instead, it is linked to the cell surface by a glycosylphosphatidylinositol (GPI) anchor (reviewed by Berx and van Roy in Nelson and Fuchs 2010). Moreover, the recently resolved structure of its EC1 domain is unusual. The flamingo/CELSR cadherins have, besides an ectodomain of nine ECs, a 7-TM domain, which is exceptional for cadherins. At the outer side of the plasma membrane, the cadherin ectodomains are mainly involved in homophilic bonds (i.e.,between like molecules), linking neighboring cells in specialized junctions. Recent findings have revealed quite some variation in cadherin binding modes, deviating from the “classic” zipper-like structure of ligated E-cadherin arrays (Fig. 1b) (Shapiro in Suzuki and Hirano 2016). This seems to serve very well various molecule-specific functions.

Signaling by Classic Cadherin-Catenin Complexes

Many major mechanisms are known to influence the roles of classic cadherins in cell structure and function. (1) Specific interactions between cells are influenced by the generation and stabilization versus destabilization and internalization of junctions composed of cadherins. This also has consequences for other types of junctions, such as tight junctions and gap junctions, not composed of cadherins. (2) By forming either static or dynamic cell-cell junctions, cadherins have a flexible influence on cytoskeletal structures, such as cortical F-actin/myosin-II (actomyosin) networks as well as microtubules, which in turn influence the form, polarity, and behavior of cells. (3) Either by physically sequestering β-catenin and p120ctn in stable junctions at the cell surface or by functional control of these armadillo proteins, cadherins can modulate the signaling roles of these key proteins in the cytoplasm and/or nucleus. Reciprocally, stable exposition of classic cadherins at the cell surface is due to p120-mediated inhibition of endocytosis, and stable association of β-catenin is needed for a functional CCC. (4) Cadherin ligation by itself is an excellent inhibitor of cell proliferation and a stimulator of cell polarity and differentiation, because cadherins engaged in adhesion serve as a hub in various signaling pathways and particularly in those involving small GTPases. A selection of published studies illustrates and consolidates these various signaling possibilities.
  1. 1.

    During both normal embryogenesis and pathological processes, such as invasive cancer, cadherin type switching is a frequent occurrence that forms the basis for the migration of particular cells and for generation of new structures with new functions (reviewed by van Roy 2014). A notable example is the dynamic but highly controlled behavioral change of cells derived from the neural crest.

     
  2. 2.

    On the one hand, junctional p120ctn stabilizes the cadherins, but on the other hand, cadherins counteract cytoplasmic p120ctn, which can modulate members of the Rho GTPase family (reviewed by Pieters et al. 2012a). Also, it has recently become clear that junctional αE-catenin can serve as a tension transducer for the CCC (Yonemura et al. 2010) (Fig. 1c). When an external force is applied to epithelial cells, for instance, during wound healing, actomyosin-driven tension induces an unfolded conformation in αE-catenin so that it can recruit vinculin to the CCC. This serves as a positive feedback because mechanical coupling between actomyosin networks and the CCC is reinforced in this way. At the same time, the force-dependent αE-catenin/vinculin interaction strengthens the intercellular junctions and contributes to tissue integrity despite the external force (Takeichi 2014; Lecuit and Yap 2015). On the other hand, αE-catenin dimers were shown to preferentially dissociate from the CCC and to locally block the binding of the actin nucleating and branching factor ARP2/3 complex (reviewed by Tan et al. in Suzuki and Hirano 2016) (Fig. 1b). At the same time, binding of αE-catenin to formin is promoted in a way that facilitates the formation of elongated F-actin fibers that can associate effectively with myosin-II (Fig. 1b and c). More recently, it was demonstrated that AJs interact also with microtubules, either the plus ends of microtubules via β-catenin or the minus ends via p120ctn, Plekha7 and Nezha (Camsap3) (reviewed by Harris and Tepass 2010; Takeichi 2014). Association of the AJ with microtubules facilitates junctional assembly and also allows the AJ to influence intracellular structuring. For instance, the orientation of both symmetric and asymmetric cell divisions are influenced by AJs (reviewed by Harris and Tepass 2010).

     
  3. 3.

    By sequestering β-catenin in the junctions, this important signaling molecule is prevented from entering the nucleus and activating Lef/Tcf-dependent gene activation ( β-catenin). Similarly, p120ctn in complex with classic cadherins cannot shuttle into the nucleus, where it would contribute to Wnt signaling by retrieving the transcriptional inhibitor Kaiso to the cytoplasm. In the case of E-cadherin, p120ctn binding to the JMD of classic cadherins shields both a di-leucine motif required in mammalian cells for clathrin-mediated endocytosis and a Tyr residue that, when phosphorylated, is recognized by Hakai, an E3 ubiquitin ligase (reviewed by Niessen et al. 2011) (see next section).

     
  4. 4.
    Native cadherin ligation, that is, the interaction in trans between cadherins across cell-cell boundaries, can result in two types of signaling: increased juxtacrine signaling by neighboring noncadherin molecules and a direct effect on cadherin-mediated signaling. To dissociate the first type of adhesion-dependent processes from the second type, researchers triggered cell-associated cadherins with, for instance, beads coated with recombinant cadherin ectodomains. Studies of this type have revealed a major role for small GTPases of the Rho family (Kooistra et al. 2007; Niessen et al. 2011; Pieters et al. 2012b; Ratheesh et al. in van Roy 2013). Although the effects seen are complex and much dependent on the cadherin type involved, as well as on cell type and cell activation status, several useful models have emerged. For instance, E-cadherin ligation induces local and transient activation of Rac1 and Cdc42 GTPases (Fig. 3). This might occur by activation of appropriate GEFs (guanine nucleotide exchange factors) such as Tiam1 and Vav2, and it appears to be dependent on association of p120ctn with cadherin. Two potential mediators of cadherin-induced GTPase activation are the protein tyrosine kinase c-Src and the lipid kinase PI3K, which are both stimulated upon E-cadherin ligation and lead to Tiam1 and Vav2 activation (Ratheesh et al. in van Roy 2013), while also Rap1 is involved. This is a Ras-like small GTPase, which includes Vav2 and Tiam1 among its numerous effectors (Kooistra et al. 2007).
    Cadherins, Fig. 3

    Interactions between ligation of nectins, ligation of classic cadherins and activation of small GTPases. Nectins are members of the immunoglobulin(Ig)-like family of adhesion proteins. They homopolymerize in cis and heteropolymerize in trans. Their C-terminal ends bind to the PDZ domain of afadin. Afadin binds to a variety of proteins, including α-catenin, p120ctn, and Rap1, a Ras-like small GTPase. Rap1 is converted into its GTP-bound active form by the action of either Rap-GEF C3G or PDZ-GEF1. This occurs initially in nascent E-cadherin-containing junctions as C3G binds transiently to the cytoplasmic domain of E-cadherin (where it competes with β-catenin). Nectins promote C3G activation and junctional maturation via a c-Src/CRK signaling pathway. In this macromolecular complex, strengthening of the binding of p120ctn to E-cadherin stimulates activation of Rac1 and recruits p190RhoGAP to the junction. The latter is activated by both reactive oxygen species (ROS) and tyrosine phosphorylation (probably mediated by c-Src), and this leads to inactivation of RhoA and activation of IQGAP1. The latter is an F-actin cross-linking protein counteracting E-cadherin endocytosis and in that way stimulating junctional maturation. Cadherin ligation by itself stimulates both c-Src and PI3K, leading to activation of the GEF proteins Vav2 and Tiam1, which in turn activate Rac1 and Cdc42. Upon junctional maturation, C3G on the cadherin tail is displaced by β-catenin, which interacts with scaffold proteins of the MAGI/S-SCAM family. Interaction of these scaffold proteins with PDZ-GEF1 further increases the local concentration of active Rap1-GTP, which in turn further activates Vav2 and Tiam1. (Modified after van Roy and Berx (2008), Pieters et al. (2012b), and van Roy (2013))

     

Rap1 by itself can be activated upon cadherin ligation in at least four possible ways (Fig. 3). (a) The E-cadherin cytoplasmic domain transiently recruits the Rap-GEF C3G in nascent junctions. (b) Junctional β-catenin recruits MAGI scaffold proteins and PDZ-GEF1, and this macromolecular complex activates Rap1. (c) Nectins, which are immunoglobulin-like adhesion molecules, promote cadherin-dependent junction formation (reviewed by van Roy and Berx 2008) and also induce Rap1 activation by a process involving c-Src, Crk, and C3G (Hoshino et al. 2005). (d) Various RTKs can associate with classic cadherins (see below) and trigger signaling pathways leading to Rap1 activation. Interestingly, afadin (AF6), an actin-binding adaptor protein, couples the cytoplasmic domains of nectins to junctional p120ctn and α-catenin (Mandai et al. in van Roy 2013) (Fig. 3). Afadin has a Rap1 binding domain and is indeed recruited to nascent junctions through the action of Rap1. There, it strengthens the binding of p120ctn to E-cadherin what counteracts the endocytosis of unligated E-cadherin and hence contributes to maturation of cadherin-based and nectin-based junctions (Hoshino et al. 2005; Fujiwara et al. in Suzuki and Hirano 2016).

When and where are such cadherin-modulated signals important? Nascent epithelial junctions show a restricted membrane zone with active GTP-Rac1, recruitment of Arp2/3, and increased lamellipodial activity at the site of the expanding intercellular contacts (reviewed by Niessen et al. 2011). Subsequent expansion and completion of cell-cell adhesion is dependent on RhoA activation at the distal edges of the intercellular contacts, what locally induces actomyosin contractility. The cadherin-induced activation of Rac1 (and also Cdc42) can have different consequences, including actin nucleation through activation of WASP-WAVE family proteins and the Arp2/3 complex, local inhibition of RhoA by activation of p190RhoGAP, and inhibition of E-cadherin endocytosis through IQGAP1, which is an F-actin cross-linking protein and a downstream target of Rac and Cdc42 (Ratheesh et al. in van Roy 2013) (Fig. 3).

Interestingly, similar signaling molecules are at play also in neural synapses (reviewed by Brigidi and Bamji 2011). Postsynaptic spine morphology is indeed dependent on modulation of small GTPases by p120ctn, the related neural δ-catenin, and α-catenin. So, clustering of postsynaptic N-cadherin with associated p120ctn and α-catenin leads to recruitment of afadin, which in turn recruits the Rac-GEF kalirin-7, promoting Rac1-dependent widening of the spine head. Moreover, p120ctn and δ-catenin can inhibit RhoA, which regulates spine length and density. These findings collectively show that functional interaction of catenins, be it β-catenin, p120ctn or α-catenin, with small GTPases and actin remodeling proteins is a recurrent theme that has evident implications for correct tissue morphogenesis (reviewed by Harris and Tepass 2010).

Signaling by Posttranslational Modification of Classic Cadherins

Of the wide variety of possible posttranslational modifications that might affect and modulate classic cadherins and associated proteins, (de)phosphorylation and proteolytic processing have been studied in most detail. For instance, the cytoplasmic domains of classic cadherins as well as the associated armadillo proteins p120ctn and β-catenin are efficient substrates for various tyrosine kinases, both receptor tyrosine kinases (RTKs) and nonreceptor tyrosine kinases such as Src, Fyn, and Fer (reviewed by Niessen et al. 2011; Pieters et al. 2012b; Tan et al. in Suzuki and Hirano 2016). Tyrosine phosphorylation of cadherin and catenin is generally thought to perturb their functions, but there is ample evidence that weak or transient phosphorylation under physiological conditions stimulates cadherin-mediated adhesion. Ligation of E-cadherin stimulates the c-Src signaling pathway in a delicate biphasic way: weak signals are supportive of E-cadherin-based cell-cell contacts in a positive feedback (see also Fig. 3), whereas strong signals inhibit E-cadherin functionality in a negative feedback (reviewed by Niessen et al. 2011).

Interestingly, cadherins can interact, in a cadherin-type specific way, with transmembrane signaling proteins like RTKs and phosphotyrosine phosphatases, and this influences both transcellular and intracelluar signaling. For instance, the E-cadherin ectodomain interacts physically with the ectodomains of EGFR and the HGF-receptor c-Met (Fig. 4a). These interactions either stimulate or inhibit ligand-induced receptor activities, through co-recruitment of RTKs by E-cadherin to the cell surface or by co-endocytosis of E-cadherin with the receptor (reviewed by van Roy and Berx 2008). On the other hand, the associated RTKs can negatively affect E-cadherin functionality by tyrosine phosphorylation of E-cadherin and the associated catenins.
Cadherins, Fig. 4

Tyrosine phosphorylation of the cadherin/catenin complex (CCC) influences its stability. (a) The ectodomain of E-cadherin forms a complex (bidirectional arrows) with the epidermal growth factor receptor (EGFR). Triggering of dimeric EGFR by its ligand EGF activates its tyrosine kinase (TK) activity, leading to cross-phosphorylation of EGFR and phosphorylation of several components of the CCC on Tyr (Y) residues (arrows and -YP annotations). This leads to release of catenins from the CCC, which exposes a di-Leu motif on the cytoplasmic domain of E-cadherin. Recognition of this motif by the clathrin adaptor AP2 induces endocytosis of E-cadherin. Moreover, the P-Tyr on the cytoplasmic domain of E-cadherin is recognized by the E3 ubiquitin ligase Hakai. Ubiquitinated E-cadherin is quickly endocytosed. (b) On the other hand, junctional p120ctn can associate with the nonreceptor tyrosine kinase Fer. This leads to activation of protein Tyr phosphatases (PTP1B, SHP2) in complex with junctional β-catenin, which counteracts the action of other tyrosine kinases, including EGFR, on β-catenin. Not shown here is that increased Fer activity at the junctions leads to Tyr phosphorylation also of β-catenin, stimulating its release from the CCC. Hence, p120ctn is a critical regulator of cell-cell adhesion by maintaining a balance between kinase and phosphatase activities at the junctions (Modified after Pieters et al. 2012b)

In the case of N-cadherin, the ectodomain has been demonstrated to interact molecularly and functionally with the FGF-receptor ectodomain to prevent FGFR internalization and allow sustained receptor activation and downstream signaling (reviewed by Berx and van Roy in Nelson and Fuchs 2010; van Roy 2014). In endothelial cells, VEGFR2 associates with the CD instead of the ectodomain of VE-cadherin (cadherin-5). This interaction prevents receptor internalization, but in this case also VEGFR2 receptor-induced signaling, as VE-cadherin (but not N-cadherin) recruits the tyrosine phosphatase Dep1. A similar attenuation occurs with respect to the FGFR1 receptor. This combined effect inhibits endothelial cell proliferation and migration and promotes in this way vascular stability, a regulatory process disturbed by N-cadherin upregulation in tumor-associated vasculature (Giampietro et al. 2012; reviewed by van Roy 2014). VE-cadherin is tyrosine phosphorylated in response to a wide spectrum of signals, including triggering by VEGF and various inflammatory mediators. This protein modification leads to weakening of endothelial junctions and is counteracted by the endothelial-specific transmembrane tyrosine phosphatase VE-PTP (reviewed by Dejana and Vestweber in van Roy 2013).

Generally, the cellular response to cadherin/catenin phosphorylation appears to be much influenced by the cadherin type, the cell type, and the microenvironment. Therefore, the following interactions serve as representative examples rather than as rules of thumb. In adherent cells expressing N-cadherin, the cadherin-associated p120ctn recruits the nonreceptor tyrosine kinase Fer, which activates the protein Tyr phosphatase PTP1B (PTPN1) when this is complexed to junctional β-catenin (Fig. 4b). This promotes junctional integrity (reviewed by Pieters et al. 2012b). Interestingly, a similar mechanism of junctional stabilization occurs in excitatory synapses, where junctional p120ctn at the presynaptic membrane recruits Fer, leading to activation of the Tyr phosphatase SHP2 (PTPN11), which in turn dephosphorylates β-catenin and promotes junctional integrity at synapses (Lee et al. 2008) (Fig. 4b). Further, synaptic functionality is promoted by various molecular interactions with junctional N-cadherin, β-catenin, and p120ctn homologs (reviewed by Brigidi and Bamji 2011).

When RTKs or oncogenic tyrosine kinases are active, phosphotyrosine sites on p120ctn and cadherin cytoplasmic domains recruit SH2-containing kinases and phosphatases. These enzymes act in a complex and concerted manner on junctional, cytoskeletal, and signaling protein substrates. A major consequence can be phosphorylation of critical Tyr residues on β-catenin, leading to disassembly of β-catenin from cadherin tails on the one hand and from α-catenin on the other hand (Fig. 4a). Moreover, junctional p120ctn can also recruit the Ser/Thr kinase CK1ε, which upon activation (for instance by Wnt signaling) phosphorylates and activates several Wnt receptor components, which form a macromolecular complex with the E-cadherin/catenin complex (Casagolda et al. 2010) (Fig. 5). However, CK1ε also phosphorylates the cadherin tail and p120ctn. This leads to dissociation of both β-catenin and p120ctn/CK1ε from E-cadherin. While cytoplasmic stabilized β-catenin mediates Wnt signaling, this signaling is expected to be terminated for two reasons: the removal of CK1ε from the signalosome and the destabilization of E-cadherin. Indeed, release of p120ctn from the E-cadherin tail removes Fer and tyrosine phosphatases from the junctional complex and exposes Tyr residues in the cadherin tail. When these residues become phosphorylated, they serve as docking sites for the E3 ubiquitin ligase Hakai; this results in ubiquitination and internalization of E-cadherin (Fig. 4a) (Pieters et al. 2012b).
Cadherins, Fig. 5

Action of casein kinase-1ε (CK1ε) on a macromolecular complex of E-cadherin with the Wnt receptor complex modulates the Wnt signaling pathway. (a) In the absence of the Wnt ligand, the Ser/Thr kinase CK1ε associates with junctional p120ctn, whereas E-cadherin associates with the Wnt co-receptor LRP5/6 (bidirectional arrows). Upon binding of Wnt, the complex grows by association between LRP5/6 and the Wnt receptor Frizzled (Fz), followed by recruitment of Dishevelled (Dvl). By then, CK1ε becomes activated and phosphorylates LRP5/6, Dvl, p120ctn, and the cytoplasmic tail of E-cadherin (arrows). (b) On the one hand, these phosphorylation events trigger Wnt signaling by recruiting (and thus inhibiting) Axin and GSK-3β, while β-catenin is released from the CCC and can shuttle in a stabilized form to the nucleus, where it stimulates transcription. On the other hand, phosphorylated E-cadherin dissociates from both LRP and p120ctn, and this increases the turnover of the junction, what arrests the stimulatory influence on the Wnt receptor complex (Modified after Casagolda et al. (2010)

Posttranslational processing is another means for modulating the adhesive and signaling functions of cadherin-catenin complexes. Briefly, furin-mediated removal of a prodomain is essential for initiating adhesion by classic cadherins (reviewed by van Roy and Berx 2008). Moreover, the ectodomain of classic cadherins is shed upon cleavage by various metalloproteases, including transmembrane ADAMs. This phenomenon is probably relevant to cancer progression (reviewed by Berx and van Roy in Nelson and Fuchs 2010). In turn, the remaining membrane-associated cadherin fragments can be further cleaved by presenilin-1/γ-secretase or by caspase-3 (Fig. 6). The final C-terminal fragments (CTF) can retain biological activity by binding various proteins, including armadillo-family catenins, and by translocation into the nucleus (reviewed by Berx and van Roy in Nelson and Fuchs 2010; Niessen et al. 2011).
Cadherins, Fig. 6

Junctional E-cadherin is sensitive to proteolytic processing. Action of metalloproteases (MMPs) and ADAMs (A Disintegrin And Metalloprotease) can lead to shedding of the ectodomain of cadherins. This reduces intercellular adhesion directly as well as indirectly, by interfering elsewhere in the tissue or organism with the clustering/signaling by full-length cadherins. The remaining cadherin fragment is released from the plasma membrane by the actions of presenilin (γ-secretase) and then caspase-3. The final C-terminal fragment, which is still bound to the armadillo catenins, might shuttle as a complex to the nucleus, or it might release these catenins, which are then expected to translocate separately into the nucleus. In either case, transcription of specific target genes would be activated

Signaling by Nonclassic Cadherins

Compared to classic cadherins, knowledge on nonclassic and cadherin-related atypical cadherins is still sparse. Nevertheless, it points to multiple signaling functions.

Cadherins with atypical membrane association are the GPI-anchored cadherin-13 and flamingo/CELSR with 7-TM domains. Upon homophilic ligation to endothelial cell surfaces, cadherin-13 becomes linked to Grp78/BiP. The latter is normally ER-retained but can be secreted under particular conditions. Formation of this complex with Grp78 triggers the antiapoptotic Akt kinase pathway. This has important consequences for tumor-associated angiogenesis (reviewed by Berx and van Roy in Nelson and Fuchs 2010; reviewed by Imai-Okano and Hirano in Suzuki and Hirano 2016). The flamingo (Fmi)/CELSR proteins are particularly well known for their roles in planar cell polarity (PCP), both in fruit fly (where it is also called starry night or stan) and in vertebrates (Gray et al. 2011; reviewed by Shi et al. in Suzuki and Hirano 2016). PCP has turned out to be central for important biological processes as diverse as convergence and extension movements during embryonic development, inner ear development, hair follicle and cilia polarization, tangential neuronal migration, and development of heart, kidney, and lung (Fig. 7, middle). At the distal side of polarized cells, Fmi/CELSR forms preferentially a molecular complex with Frizzled receptors, while at the proximal side preferentially a complex of Fmi/CELSR with Van gogh (Vang, also called strabismus or stan) is formed (Fig. 7, bottom). In addition to these heterophilic interactions, Fmi/CELSR molecules interact in trans across distal-proximal membrane boundaries in a homophilic modus, but not so at lateral cell boundaries. Unfortunately, the structures of these various molecular interactions have not been reported. Anyhow, cellular asymmetry is assumed to be generated on the basis of the heterophilic interaction partners of Fmi/CELSR. Outside these asymmetric junctional complexes, Fmi/CELSR is unstable due to a high rate of endocytosis.
Cadherins, Fig. 7

Role of various members of the cadherin superfamily in planar cell polarity (PCP). Central panel: Scheme of a typical tissue with established PCP. Cells are arranged in a sheet with a polarized structure (for instance sensory bristles on a fly wing). Besides having lateral membranes, cells thus have a proximal (anterior) membrane and a distal (posterior) membrane. A spatial gradient of Dachsous (Ds) and an inverse gradient of four-jointed (Fj) contribute to long-range polarity (indicated by the arrow). This is further detailed in the top panel, showing a speculative model of Drosophila cells with PCP. The hairpin-like self-bending ectodomain structures of Fat and DS are based on electron microscopic analyses of mammalian Fat4 and Dchs1 ectodomains (Tsukasaki et al. 2014). The very long Ds protein in the apical region of a distal membrane interacts with Fat in a proximal membrane of a juxtaposed cell (the interaction via the EC1 domains of these two cadherin family members is speculative). This induces cis-dimerization of Fat, followed by phosphorylation of their Fat cytoplasmic domains by the recruited kinase Discs Overgrown (Dco). This leads to inhibition of Fj, a Golgi-localized kinase phosphorylating the ectodomains of both Ds and Fat what modulates their mutual binding affinities. Inverse spatial gradients of Ds and Fj over the tissue (middle panel) are believed to direct PCP. Moreover, the activated Fat dimer triggers, via complex signaling (not shown here), the growth inhibitory Hippo/YAP signaling pathway. One of the mechanisms involved is recruitment of Expanded, a FERM-domain-containing protein, to the apical side of the membrane. Bottom panel: Polarized expression pattern of the core polarity proteins Frizzled (Fz) and Van Gogh (Vang, also named Strabismus/Stbm) in juxtaposed plasma membranes. At the extracellular side, Fz becomes activated by Wnt5 or Wnt 11. At the cytoplasmic side, Fz is associated with Dishevelled (Dvl) and Diego (Dgo), whereas Vang is associated with Prickle (Pk). The asymmetric distribution of these proteins leads to a polarized cytoskeleton structure via modulation of Rho and Rac signaling. Fz and Vang interact heterophilically across the intercellular space in a poorly understood way to which homophilic interaction of Flamingo (CELSR/Fmi) cadherins contribute (the interaction between EC1 and EC9 domains of overlapping Flamingo molecules is speculative) (Modified after Berx and van Roy in Nelson and Fuchs (2010), Sharma and McNeill in van Roy (2013), Tsukasaki et al. (2014), and Shi et al. in Suzuki and Hirano (2016))

While Fmi/CELSR is considered a core protein that mediates PCP at the cellular level, two other atypical cadherin proteins, Fat and Dachsous (Ds), are considered upstream or parallel signaling proteins for PCP, as they mediate PCP at a more global embryonic or organ level (Gray et al. 2011). In mammals, Fat4 is the true ortholog of fly Fat, and Dchs1 is the ortholog of Ds (Hulpiau and van Roy 2011). It is noteworthy that the cytoplasmic domain of FAT4 deviates completely from those of FAT1 to FAT3. Both Fat and Ds have very long ectodomains with numerous EC repeats, and they interact heterophilically with each other at cell junctions. While the ectodomains of classic cadherins adopt a calcium-dependent linear conformation, those of very long cadherin ectodomains seem to be bended due to the presence of several noncalcium binding linkers between ECs (Tsukasaki et al. 2014). Polarity across embryos/organs is mediated both by increased Ds expression at the proximal side and by increased Four-jointed (Fj) at the distal side (Sopko and McNeill 2009; Sharma and McNeill in van Roy 2013) (Fig. 7). Fj is a Golgi kinase that, intriguingly, phosphorylates the ectodomains of Fat and Ds, what modulates their mutual binding affinities. In fly, strong triggering of Fat by Ds leads to cis-dimerization of Fat and to phosphorylation of its C-terminus by the associated Dco (Disc Overgrown, the fly homolog of CK1ε) (Fig. 7, top). Evidence indicates that Dco activity promotes Fat activity in the Hippo pathway, possibly by recruiting Expanded, which is an upstream Hippo regulator, to the subapical membrane (Sharma and McNeill in van Roy 2013). The Hippo or Yap pathway is an emerging signaling network with critical importance for controlling organ size, and its derangement is being reported in an increasing number of human cancer types. Thus, importantly, actively signaling Fat not only mediates PCP but, at least in Drosophila, also activates the Hippo signaling pathway. Evidence for interaction between Fat4 and Hippo signaling exists in zebrafish, but not yet in mammals (Sharma and McNeill in van Roy 2013).

Signaling by Protocadherins

Genuine protocadherins comprise more than half of the cadherin species in mammals and have typical cytoplasmic domains completely different from those of classic cadherins. Reportedly, they have much weaker or even undetectable adhesion potential compared to classic cadherins. Consequently, they are believed to have a signaling function rather than an adhesive function (Kahr et al. in van Roy 2013; Hayashi and Takeichi 2015; Jontes in Suzuki and Hirano 2016). Analyses of protocadherins have been lagging behind those of classic cadherins, and until recently functional studies have been performed only on a few protocadherins and mostly in Xenopus. The cytoplasmic domain of arcadlin, which corresponds to the rat protocadherin-8 (Pcdh8), interacts in synaptic junctions with N-cadherin. Excitation of hippocampal neurons upregulates arcadlin, and this promotes N-cadherin internalization, which is accelerated by the homophilic interaction of arcadlin ectodomains and involves an associated MAPKKK named TAO2β. A similar observation of cross-inhibition involves the induction by activin of PAPC, a Pcdh8 homolog in Xenopus, what is linked to decreased adhesion activity of C-cadherin, a classic cadherin in frog. For several members of the so-called nonclustered δ2-protocadherin subfamily (comprising Pcdh8, Pcdh10, Pcdh17, Pcdh18, and Pcdh19), a specific cytoplasmic interaction with the actin-organizing complex Nap1/WAVE has been demonstrated. Indeed, a functional WIRS motif (WAVE regulatory complex interacting receptor sequence) has been identified in their respective CDs (Chen et al. 2014). At least several of these protocadherins are enriched at sites of cell-cell contact and recruit there the WAVE complex, which in turn recruits lamellipodin (Lpd) and the Ena/VASP regulators of actin polymerization. This leads to weakening of the classic cadherin-catenin complex and facilitation of cell membrane motility at cell-cell contacts (Fig. 1a). This molecular mechanism has been proposed to be instrumental in concerted axonal growth cone extension and guidance (Hayashi and Takeichi 2015; Jontes in Suzuki and Hirano 2016). One could therefore conclude that a general function of protocadherins is to negatively regulate the activity of classic cadherins. However, Pcdh19 in zebrafish acts synergistically with N-cadherin to control cell migration during anterior neurulation, and recent elegant studies in zebrafish have indicated subtle but significant functions of Pcdh19 in regulating neural proliferation, maintenance of cell contacts, neurite fasciculation, and arborization (Cooper et al. 2015; Jontes in Suzuki and Hirano 2016). It will thus be challenging to unravel in detail the underlying mechanisms of protocadherin functions. Meanwhile, mutations in nonclustered protocadherins genes have been identified as causing several neurodevelopmental disorders, and cancer-confined silencing of these genes has been often observed as well (Jontes in Suzuki and Hirano 2016) (van Roy 2014).

The so-called clustered protocadherins (cPcdhs) comprise extended groups of protocadherin-α, protocadherin-β, and protocadherin-γ genes, arranged in tandem on a single chromosomal site. While the ectodomains of each of the encoded cPcdh isoforms are unique, a large part of their cytoplasmic domains is shared among the clustered subfamily members. In neuronal populations, various regulatory mechanisms of cPcdh isoform expression lead to a multitude of different coexpression patterns allowing unique cell recognition properties. Thus, combinatorial expression and heteromultimeric complex formation of cPcdh isoforms has been found to be instrumental in achieving major effects on neuronal development: inhibition of apoptosis, specific synapse formation, and dendritic arborization on the basis of self-avoidance between dendrites of the same origin (Hirayma and Yagi in van Roy 2013; Hayashi and Takeichi 2015; Mah and Weiner in Suzuki and Hirano 2016). It is obvious that the ectodomain variability of cPcdhs plays a key role in these remarkable functions, while knowledge on the signaling by the CDs is still limited. Inhibitory association of these CDs with the tyrosine kinases PYK2 and FAK seems to be involved in, respectively, neuronal survival and dendritic arborization, but many details of signaling by cPcdhs await elucidation.

Summary

This essay deals with selected examples of intra- and cross-cellular signaling by members of the large cadherin superfamily. Much attention is paid to E-cadherin, which, as founder of the superfamily, is an extensively studied “classic” cadherin. As a mediator of specific intercellular junctions, both static and dynamic ones, it has a pivotal role in epithelial cell behavior, tissue formation, and suppression of cancer. Compelling evidence has been gathered that E-cadherin is much more than “molecular glue,” because it serves as a hub for several signaling pathways. On the one hand, association of the armadillo protein p120ctn with the juxtamembrane domain of classic cadherins, as well as activation of various small GTPases, contributes to junctional stability. On the other hand, cadherin ligation generates signals that contribute to cell differentiation and inhibit growth stimulatory signaling, in part by binding via its C-terminal domain the proto-oncogenic β-catenin. Important modulators of cadherin-associated signaling are cytoskeletal components, such as actomyosin, and several enzymes, in particular receptor tyrosine kinases and counteracting phosphatases. E-cadherin is also processed by several proteases to generate various fragments with signaling activity. The many other members of the superfamily include protocadherins, which may act negatively on classic cadherins in various ways. The flamingo/CELSR cadherins and the very extended Dachsous and Fat cadherins play important roles in the signaling towards planar cell polarity. In conclusion, signaling by cadherins is a rapidly growing research field with multiple proven or proposed links to mammalian pathologies. Recent investigations on cadherin-linked signaling raised many intriguing questions that might be addressed by contemporary multidisciplinary approaches.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Molecular Cell Biology Unit, Department of Biomedical Molecular BiologyGhent University and Inflammation Research Center, VIBGhentBelgium