The roles of nectins in cell adhesions: cooperation with other cell adhesion molecules and growth factor receptors

Nectins are Ca(2+)-independent Ig-like cell adhesion molecules (CAMs) which homophilically and heterophilically interact in trans with nectins and form cell-cell adhesion. This cell-cell adhesion is involved in the formation of many types of cell-cell junctions such as adherens junctions, tight junctions, and synaptic junctions, cooperatively with other CAMs such as cadherins and claudins. Nectins transduce signals cooperatively with integrin alpha(v)beta(3), and regulate formation of cell-cell junctions. In addition, nectin interacts in cis with PDGF receptor and regulates its signaling for anti-apoptosis. Furthermore, nectin interacts in trans with nectin-like molecule-5 (Necl-5) and regulate cell movement and proliferation. We describe cooperative roles of nectins with other CAMs and growth factor receptors.     

Role of multiple bonds between the single cell adhesion molecules, nectin and cadherin, revealed by high sensitive force measurements

Nectins and cadherins, members of cell adhesion molecules (CAMs), are the primary mediators for various types of cell-cell junctions. Here, intermolecular force microscopy (IFM) with force sensitivity at sub-picoNewtons is used to characterize the extracellular trans-interactions between paired nectins and paired cadherins at the single molecule level. Three and four different bound states between paired nectins and paired cadherins are, respectively, identified and characterized based on bond strength distributions where each bound state has a unique lifetime and bond length. The results indicate that multiple domains of nectins act uncooperatively, as a zipper-like multiply bonded system whereas those of cadherins act cooperatively, as a parallel-like multiply bonded system, consistent with a "fork initiation and zipper" hypothesis for the formation of cell-cell adhesion. The observed dynamic properties among multiple bonds are expected to be advantageous such that nectins search adaptively in the cell-cell exploratory recognition process while cadherins slowly stabilize in the cell-cell zippering process.     

Afadin- and alpha-actinin-binding protein ADIP directly binds beta'-COP, a subunit of the coatomer complex

Afadin DIL domain-interacting protein (ADIP) is a novel protein that binds both afadin and alpha-actinin and localizes at adherens junctions, which are formed by nectins and cadherins, cell-cell adhesion molecules. Afadin is an actin filament (F-actin)-binding protein which connects nectins to the actin cytoskeleton. alpha-Actinin is another F-actin-binding protein that is indirectly associated with cadherins through the catenin complex. ADIP is at least partly involved in the physical association of nectins and cadherins. We show here that ADIP furthermore binds beta'-COP, a subunit of the coatomer complex. ADIP co-localizes with beta'-COP at the Golgi complex in Madin Darby canine kidney and normal rat kidney cells. These results suggest that ADIP is involved in vesicle trafficking from the Golgi to the endoplasmic reticulum and through the Golgi complex by interacting with the coatomer complex.     

Cooperative role of nectin-nectin and nectin-afadin interactions in formation of nectin-based cell-cell adhesion

The nectin cell adhesion molecules interact in trans with each other through their extracellular regions and with afadin through their cytoplasmic tails, forming adherens junctions in cooperation with cadherins. In a single cell, Necl-5 (nectin-like molecule-5) localizes at the leading edge and regulates directional cell movement in response to a chemoattractant. In such a single cell, afadin also localizes at the leading edge without interacting with nectins or Necl-5. It remains unknown how the nectin-nectin and nectin-afadin interactions are initiated when moving cells contact each other to initiate the formation of adherens junctions. We show here that the Necl-5-nectin interaction induced by cell-cell contact enhances the nectin-afadin interaction. This interaction then enhances the nectin-nectin interaction, which further enhances the nectin-afadin interaction in a positive feedback manner. Thus, the Necl-5-nectin, nectin-nectin, and nectin-afadin interactions cooperatively increase the clustering of the nectin-afadin complex at the cell-cell contact sites, promoting the formation of the nectin-based cell-cell adhesion.     

A novel role of nectins in inhibition of the E-cadherin-induced activation of Rac and formation of cell-cell adherens junctions

Nectins are Ca(2+)-independent immunoglobulin (Ig)-like cell-cell adhesion molecules. The trans-interactions of nectins recruit cadherins to the nectin-based cell-cell adhesion, resulting in formation of cell-cell adherens junctions (AJs) in epithelial cells and fibroblasts. The trans-interaction of E-cadherin induces activation of Rac small G protein, whereas the trans-interactions of nectins induce activation of not only Rac but also Cdc42 small G protein. We showed by the fluorescent resonance energy transfer (FRET) imaging that the trans-interaction of E-cadherin induced dynamic activation and inactivation of Rac, which led to dynamic formation and retraction of lamellipodia. Moreover, we found here that the nectins, which did not trans-interact with other nectins (non-trans-interacting nectins), inhibited the E-cadherin-induced activation of Rac and reduced the velocity of the formation of the E-cadherin-based cell-cell AJs. The inhibitory effect of non-trans-interacting nectins was suppressed by the activation of Cdc42 induced by the trans-interactions of nectins. These results indicate a novel role of nectins in regulation of the E-cadherin-induced activation of Rac and formation of cell-cell AJs.     

Trans-interactions of nectins induce formation of filopodia and Lamellipodia through the respective activation of Cdc42 and Rac small G proteins

Nectins and afadin constitute a novel cell-cell adhesion system that plays a cooperative role with cadherins in the organization of adherens junctions (AJs). Nectins are Ca(2+)-independent immunoglobulin-like cell-cell adhesion molecules, and afadin is a nectin- and actin filament-binding protein that connects nectins to the actin cytoskeleton. Rac and Cdc42 small G proteins have been implicated in the organization of AJs, but their modes of action remain unknown. The trans-interaction of E-cadherin has recently been shown to induce the activation of Rac, but not that of Cdc42. We show here that the trans-interactions of nectins induce the formation of filopodia and lamellipodia through the respective activation of Cdc42 and Rac. The Cdc42 activation is necessary, but not sufficient, for the Rac-induced formation of lamellipodia, whereas the Rac activation is not necessary for the Cdc42-induced formation of filopodia. These effects of nectins require their cytoplasmic tail but not their association with afadin. We propose here the functional relationship between nectins and the small G proteins in the organization of AJs.     

Involvement of the Interaction of Afadin with ZO-1 in the Formation of Tight Junctions in Madin-Darby Canine Kidney Cells

Tight junctions (TJs) and adherens junctions (AJs) are major junctional apparatuses in epithelial cells. Claudins and junctional adhesion molecules (JAMs) are major cell adhesion molecules (CAMs) at TJs, whereas cadherins and nectins are major CAMs at AJs. Claudins and JAMs are associated with ZO proteins, whereas cadherins are associated with β- and α-catenins, and nectins are associated with afadin. We previously showed that nectins first form cell-cell adhesions where the cadherin-catenin complex is recruited to form AJs, followed by the recruitment of the JAM-ZO and claudin-ZO complexes to the apical side of AJs to form TJs. It is not fully understood how TJ components are recruited to the apical side of AJs. We studied the roles of afadin and ZO-1 in the formation of TJs in Madin-Darby canine kidney (MDCK) cells. Before the formation of TJs, ZO-1 interacted with afadin through the two proline-rich regions of afadin and the SH3 domain of ZO-1. During and after the formation of TJs, ZO-1 dissociated from afadin and associated with JAM-A. Knockdown of afadin impaired the formation of both AJs and TJs in MDCK cells, whereas knockdown of ZO-1 impaired the formation of TJs, but not AJs. Re-expression of full-length afadin restored the formation of both AJs and TJs in afadin-knockdown MDCK cells, whereas re-expression of afadin-ΔPR1–2, which is incapable of binding to ZO-1, restored the formation of AJs, but not TJs. These results indicate that the transient interaction of afadin with ZO-1 is necessary for the formation of TJs in MDCK cells.     

Crosstalk between different adhesion molecules

Cell adhesion molecules mediate cell-cell and cell-extracellular matrix adhesions, and coordination between these molecules is essential for tissue formation and morphogenesis. Crosstalk between integrins and cadherins may result from a physical response to integrin-mediated adhesion, complex cell differentiation processes, or direct signaling pathways linking the two adhesion systems. Nectins have recently been shown to regulate the organization of cadherins into adherens junctions and the formation of tight junctions by several processes. Furthermore, protocadherins can interact with extracellular matrix proteins or function by regulating classical cadherins.     

Biology and pathology of nectins and nectin-like molecules

Immunoglobulin-like nectins contribute to the formation of a variety of cell-cell junctions, acting cooperatively with, or independently of, cadherins. In addition, nectins heterophilically trans-interact with nectin-like molecules (Necls), which are involved in cell adhesion, migration, and proliferation, and assist or modify their functions. On the other hand, nectins and Necls serve as viral receptors and are associated with human diseases (including cancer) when mutated or upregulated.     

Activation of Cdc42 by trans interactions of the cell adhesion molecules nectins through c-Src and Cdc42-GEF FRG

Nectins, Ca2+ -independent immunoglobulin-like cell-cell adhesion molecules, initiate cell-cell adhesion by their trans interactions and recruit cadherins to cooperatively form adherens junctions (AJs). In addition, the trans interactions of nectins induce the activation of Cdc42 and Rac small G proteins, which increases the velocity of the formation of AJs. We examined here how nectins induce the activation of Cdc42 in MDCK epithelial cells and L fibroblasts. Nectins recruited and activated c-Src at the nectin-based cell-cell adhesion sites. FRG, a GDP/GTP exchange factor specific for Cdc42, was then recruited there, tyrosine phosphorylated by c-Src, and activated, causing an increase in the GTP-bound active form of Cdc42. Inhibition of the nectin-induced activation of c-Src suppressed the nectin-induced activation of FRG and Cdc42. Inhibition of the nectin-induced activation of FRG or depletion of FRG by RNA interference suppressed the nectin-induced activation of Cdc42. These results indicate that nectins induce the activation of Cdc42 through c-Src and FRG locally at the nectin-based cell-cell adhesion sites.     

Roles and modes of action of nectins in cell-cell adhesion

Nectins are Ca(2+)-independent immunoglobulin (Ig)-like cell-cell adhesion molecules (CAMs), which comprise a family consisting of four members. Each nectin homophilically and heterophilically trans-interacts and causes cell-cell adhesion. Biochemical, cell biological, and knockout mice studies have revealed that nectins play important roles in formation of many types of cell-cell junctions and cell-cell contacts, including cadherin-based adherens junctions (AJs) and synapses. Mode of action of nectins in the formation of AJs has extensively been investigated. Nectins form initial cell-cell adhesion and recruit E-cadherin to the nectin-based cell-cell adhesion sites. In addition, nectins induce activation of Cdc42 and Rac small G proteins, which eventually enhances the formation of cadherin-based AJs through the reorganization of the actin cytoskeleton. Nectins furthermore heterophilically trans-interact with nectin-like molecules (Necls), other Ig-like CAMs, and assist or modify their various functions, such as cell adhesion, migration, and proliferation. We describe here the roles and modes of action of nectins as CAMs.     

Tage4/Nectin-like molecule-5 heterophilically trans-interacts with cell adhesion molecule Nectin-3 and enhances cell migration

Malignant transformation of cells causes disruption of cell-cell adhesion, enhancement of cell motility, and invasion into surrounding tissues. Nectins have both homophilic and heterophilic cell-cell adhesion activities and organize adherens junctions in cooperation with cadherins. We examined here whether Tage4, which was originally identified to be a gene overexpressed in colon carcinoma and has a domain structure similar to those of nectins, is involved in cell adhesion and/or migration. Tage4 heterophilically trans-interacted with nectin-3, but not homophilically with Tage4. Expression of Tage4 was markedly elevated in NIH3T3 cells transformed by an oncogenic Ki-Ras (V12Ras-NIH3T3 cells) as compared with that of wild-type NIH3T3 cells. trans-Interaction of Tage4 with nectin-3 enhanced motility of V12Ras-NIH3T3 cells. Tage4 did not bind afadin, a nectin- and actin filament-binding protein that connects nectins to the actin cytoskeleton and cadherins through catenins. Thus, Tage4 heterophilically trans-interacts with nectin-3 and regulates cell migration. Tage4 is tentatively re-named here nectin-like molecule-5 (necl-5) on the basis of its function and domain structure similar to those of nectins.     

Implications of nectin-like molecule-2/IGSF4/RA175/SgIGSF/TSLC1/SynCAM1 in cell-cell adhesion and transmembrane protein localization in epithelial cells

Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules that play roles in organization of a variety of cell-cell junctions in cooperation with or independently of cadherins. Four nectins have been identified. Five nectin-like molecules, which have domain structures similar to those of nectins, have been identified, and we characterized here nectin-like molecule-2 (Necl-2)/IGSF4/RA175/SgIGSF/TSLC1/SynCAM1. Necl-2 showed Ca2+-independent homophilic cell-cell adhesion activity. It furthermore showed Ca2+-independent heterophilic cell-cell adhesion activity with Necl-1/TSLL1/SynCAM3 and nectin-3. Necl-2 was widely expressed in rat tissues examined. Necl-2 localized at the basolateral plasma membrane in epithelial cells of the mouse gall bladder, but not at specialized cell-cell junctions, such as tight junctions, adherens junctions, and desmosomes. Nectins bind afadin, whereas Necl-2 did not bind afadin but bound Pals2, a membrane-associated guanylate kinase family member known to bind Lin-7, implicated in the proper localization of the Let-23 protein in Caenorhabditis elegans, the homologue of mammalian epidermal growth factor receptor. These results indicate the unique localization of Necl-2 and its possible involvement in localization of a transmembrane protein(s) through Pals2.     

Roles and modes of action of nectins in cell–cell adhesion

Nectins are Ca²⁺-independent immunoglobulin (Ig)-like cell–cell adhesion molecules (CAMs), which comprise a family consisting of four members. Each nectin homophilically and heterophilically trans-interacts and causes cell–cell adhesion. Biochemical, cell biological, and knockout mice studies have revealed that nectins play important roles in formation of many types of cell–cell junctions and cell–cell contacts, including cadherin-based adherens junctions (AJs) and synapses. Mode of action of nectins in the formation of AJs has extensively been investigated. Nectins form initial cell–cell adhesion and recruit E-cadherin to the nectin-based cell–cell adhesion sites. In addition, nectins induce activation of Cdc42 and Rac small G proteins, which eventually enhances the formation of cadherin-based AJs through the reorganization of the actin cytoskeleton. Nectins furthermore heterophilically trans-interact with nectin-like molecules (Necls), other Ig-like CAMs, and assist or modify their various functions, such as cell adhesion, migration, and proliferation. We describe here the roles and modes of action of nectins as CAMs.     

Recruitment of nectin-3 to cell-cell junctions through trans-heterophilic interaction with CD155, a vitronectin and poliovirus receptor that localizes to alpha(v)beta3 integrin-containing membrane microdomains

Nectins present a novel class of Ig superfamily adhesion molecules that, cooperatively with cadherins, establish and maintain cell-cell adherens junctions. CD155, the cognate receptor for poliovirus, undergoes cell-matrix contacts by binding to the extracellular matrix protein vitronectin. The significant homology of nectins with CD155 prompted us to investigate the possibility of their interaction. We determined that nectin-3 binds CD155 and its putative mouse homologue Tage4 in cell-based ligand binding assays. Coculture of nectin-3- and CD155-expressing HeLa cells led to CD155-dependent recruitment of nectin-3 to cell-cell contacts. In a heterologous coculture system with CD155 expressing mouse neuroblastoma cells, HeLa cell-expressed nectin-3 was recruited to contact sites with CD155 bearing neurites. CD155 and nectin-3 colocalized to epithelial cell-cell junctions in renal proximal tubules and in the amniotic membrane. Efficient interaction depended on CD155 dimerization, which appears to be aided by cell type-specific cofactors. We furthermore found CD155 to codistribute with alpha(v) integrin microdomains on the surface of transfected mouse fibroblasts and at amniotic epithelial cell junctions. Our findings demonstrate the possible trans-interaction between the bona fide cell-cell adherens type adhesion system (cadherin/nectin) and the cell-matrix adhesion system (integrin/CD155) by virtue of their nectin-3 and CD155 components, respectively.     

Vav2 as a Rac-GDP/GTP exchange factor responsible for the nectin-induced, c-Src- and Cdc42-mediated activation of Rac

Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules that form homo- and hetero-trans-dimers (trans-interactions). Nectins first form cell-cell contact and then recruit cadherins to the nectin-based cell-cell contact sites to form adherens junctions cooperatively with cadherins. In addition, the trans-interactions of nectins induce the activation of Cdc42 and Rac small G proteins, which enhances the formation of adherens junctions by forming filopodia and lamellipodia, respectively. The trans-interactions of nectins first recruit and activate c-Src at the nectin-based cell-cell contact sites. c-Src then phosphorylates and activates FRG, a Cdc42-GDP/GTP exchange factor (GEF) for Cdc42. The activation of both c-Src and Cdc42 by FRG is necessary for the activation of Rac, but the Rac-GEF responsible for this activation of Rac remains unknown. We showed here that the nectin-induced activation of Rac was inhibited by a dominant negative mutant of Vav2, a Rac-GEF. Nectins recruited and tyrosine-phosphorylated Vav2 through c-Src at the nectin-based cell-cell contact sites, whereas Cdc42 was not necessary for the nectin-induced recruitment of Vav2 or the nectin-induced, c-Src-mediated tyrosine phosphorylation of Vav2. Cdc42 activated through c-Src then enhanced the GEF activity of tyrosine-phosphorylated Vav2 on Rac1. These results indicate that Vav2 is a GEF responsible for the nectin-induced, c-Src-, and Cdc42-mediated activation of Rac.     

Involvement of afadin in the formation and remodeling of synapses in the hippocampus

In the hippocampus, synapses are formed between mossy fiber terminals and CA3 pyramidal cell dendrites and comprise highly developed synaptic junctions (SJs) and puncta adherentia junctions (PAJs). Dynamic remodeling of synapses in the hippocampus is implicated in learning and memory. Components of both the nectin-afadin and cadherin-catenin cell adhesion systems exclusively accumulate at PAJs. We investigated the role of afadin at synapses in mice in which the afadin gene was conditionally inactivated in hippocampal neurons. In these mutant mice, the signals for not only nectins, but also N-cadherin and β-catenin, were hardly detected in the CA3 area, in addition to loss of the signal for afadin, resulting in disruption of PAJs. Ultrastructural analysis revealed an increase in the number of perforated synapses, suggesting the instability of SJs. These results indicate that afadin is involved not only in the assembly of nectins and cadherins at synapses, but also in synaptic remodeling.     

The roles of cadherins and nectins in interneuronal synapse formation

Cadherins are Ca(2+)-dependent intercellular adhesion molecules (CAMs) and they play key roles in the intercellular junctions of a wide variety of cells, including interneuronal synapses. Nectins are Ca(2+)-independent immunoglobulin-like CAMs and they are also involved in the organization of various types of intercellular junctions, including interneuronal synapses, either in cooperation with or independently of cadherins. Intercellular adhesion through nectins induces activation of Cdc42 and Rac small G proteins, leading to a reorganization of the actin cytoskeleton, gene expression, and cell polarization.     

Roles of cell-adhesion molecules nectin 1 and nectin 3 in ciliary body development

Nectins are Ca2+-independent immunoglobulin-like cell-cell-adhesion molecules consisting of four members. Nectins homophilically and heterophilically trans-interact to form a variety of cell-cell junctions, including cadherin-based adherens junctions in epithelial cells and fibroblasts in culture, synaptic junctions in neurons, and Sertoli cell-spermatid junctions in the testis, in cooperation with, or independently of, cadherins. To further explore the function of nectins, we generated nectin 1-/- and nectin 3-/-)mice. Both nectin 1-/- and nectin 3-/- mice showed a virtually identical ocular phenotype, microphthalmia, accompanied by a separation of the apex-apex contact between the pigment and non-pigment cell layers of the ciliary epithelia. Immunofluorescence and immunoelectron microscopy revealed that nectin 1 and nectin 3, but not nectin 2, localized at the apex-apex junctions between the pigment and non-pigment cell layers of the ciliary epithelia. However, nectin 1-/- and nectin 3-/- mice showed no impairment of the apicolateral junctions between the pigment epithelia where nectin 1, nectin 2 and nectin 3 localized, or of the apicolateral junctions between the non-pigment epithelia where nectin 2 and nectin 3, but not nectin 1, localized. These results indicate that the heterophilic trans-interaction between nectin 1 and nectin 3 plays a sentinel role in establishing the apex-apex adhesion between the pigment and non-pigment cell layers of the ciliary epithelia that is essential for the morphogenesis of the ciliary body.     

Regulation by nectin of the velocity of the formation of adherens junctions and tight junctions

Cadherins are key Ca(2+)-dependent cell-cell adhesion molecules at adherens junctions (AJs) in fibroblasts and epithelial cells, whereas claudins are key Ca(2+)-independent cell-cell adhesion molecules at tight junctions (TJs) in epithelial cells. The formation and maintenance of TJs are dependent on the formation and maintenance of AJs. Nectins are Ca(2+)-independent immunoglobulin-like cell-cell adhesion molecules which comprise a family of four members, nectin-1, -2, -3, and -4, and are involved in the formation of AJs in cooperation with cadherins, and the subsequent formation of TJs. We show here that the velocity of the formation of the E-cadherin-based AJs is increased by overexpression of nectin-1 and is reduced by addition of the nectin-1 inhibitors to the medium in L cells stably expressing E-cadherin and Madin-Darby canine kidney cells. Moreover, the velocity of the formation of the claudin-based TJs is increased by overexpression of nectin-1 and is reduced by addition of the nectin-1 inhibitors to the medium in Madin-Darby canine kidney cells. These results indicate that nectins regulate the velocity of the formation of the E-cadherin-based AJs and the subsequent formation of the claudin-based TJs.