The extracellular architecture of adherens junctions revealed by crystal structures of type I cadherins

Structural and functional diversity of desmosomes

Desmosomes anchor intermediate filaments at sites of cell contact established by the interaction of cadherins extending from opposing cells. The incorporation of cadherins, catenin adaptors, and cytoskeletal elements resembles the closely related adherens junction. However, the recruitment of intermediate filaments distinguishes desmosomes and imparts a unique function. By linking the load-bearing intermediate filaments of neighboring cells, desmosomes create mechanically contiguous cell sheets and, in so doing, confer structural integrity to the tissues they populate. This trait and a well-established role in human disease have long captured the attention of cell biologists, as evidenced by a publication record dating back to the mid-1860s. Likewise, emerging data implicating the desmosome in signaling events pertinent to organismal development, carcinogenesis, and genetic disorders will secure a prominent role for desmosomes in future biological and biomedical investigations.

Principles of E-cadherin supramolecular organization in vivo

BACKGROUND

E-cadherin plays a pivotal role in tissue morphogenesis by forming clusters that support intercellular adhesion and transmit tension. What controls E-cadherin mesoscopic organization in clusters is unclear.

RESULTS

We use 3D superresolution quantitative microscopy in Drosophila embryos to characterize the size distribution of E-cadherin nanometric clusters. The cluster size follows power-law distributions over three orders of magnitude with exponential decay at large cluster sizes. By exploring the predictions of a general theoretical framework including cluster fusion and fission events and recycling of E-cadherin, we identify two distinct active mechanisms setting the cluster-size distribution. Dynamin-dependent endocytosis targets large clusters only, thereby imposing a cutoff size. Moreover, interactions between E-cadherin clusters and actin filaments control the fission in a size-dependent manner.

CONCLUSIONS

E-cadherin clustering depends on key cortical regulators, which provide tunable and local control over E-cadherin organization. Our data provide the foundation for a quantitative understanding of how E-cadherin distribution affects adhesion and might regulate force transmission in vivo.

Identification and characterization of CDH1 germline variants in sporadic gastric cancer patients and in individuals at risk of gastric cancer

Objective

To screen and characterize germline variants for E-cadherin (CDH1) in non-hereditary gastric cancer (GC) patients and in subjects at risk of GC.

Methods

59 GCs, 59 first degree relatives (FDRs) of GC, 20 autoimmune metaplastic atrophic gastritis (AMAGs) and 52 blood donors (BDs) were analyzed for CDH1 by direct sequencing, structural modelling and bioinformatics. Functional impact on splicing was assessed for intronic mutations. E-cadherin/β-catenin immunohistochemical staining and E-cadherin mRNA quantification using RT-PCR were performed.

Results

In GCs, 4 missense variants (p.G274S; p.A298T; p.T470I; p.A592T), 1 mutation in the 5′UTR (−71C>G) and 1 mutation in the intronic IVS12 (c.1937-13T>C) region were found. First pathogenic effect of p.A298T mutation was predicted by protein 3D modelling. The novel p.G274S mutation showed a no clear functional significance. Moreover, first, intronic IVS12 (c.1937-13T>C) mutation was demonstrated to lead to an aberrant CDH1 transcript with exon 11 deletion. This mutation was found in 2 GCs and in 1 BD. In FDRs, we identified 4 variants: the polymorphic (p.A592T) and 3 mutations in untranslated regions with unidentified functional role except for the 5′UTR (−54G>C) that had been found to decrease CDH1 transcription. In AMAGs, we detected 2 alterations: 1 missense (p.A592T) and 1 novel variant (IVS1 (c.48+7C>T)) without effect on CDH1 splicing. Several silent and polymorphic substitutions were found in all the groups studied.

Conclusions

Overall our study improves upon the current characterization of CDH1 mutations and their functional role in GC and in individuals at risk of GC. Mutations found in untranslated regions and data on splicing effects deserve a particular attention like associated with a reduced E-cadherin amount. The utility of CDH1 screening, in addition to the identification of other risk factors, could be useful for the early detection of GC in subjects at risk (i.e. FDRs and AMAGs), and warrants further study.

Mining the O-mannose glycoproteome reveals cadherins as major O-mannosylated glycoproteins

The metazoan O-mannose (O-Man) glycoproteome is largely unknown. It has been shown that up to 30% of brain O-glycans are of the O-Man type, but essentially only alpha-dystroglycan (α-DG) of the dystrophin-glycoprotein complex is well characterized as an O-Man glycoprotein. Defects in O-Man glycosylation underlie congenital muscular dystrophies and considerable efforts have been devoted to explore this O-glycoproteome without much success. Here, we used our SimpleCell strategy using nuclease-mediated gene editing of a human cell line (MDA-MB-231) to reduce the structural heterogeneity of O-Man glycans and to probe the O-Man glycoproteome. In this breast cancer cell line we found that O-Man glycosylation is primarily found on cadherins and plexins on β-strands in extracellular cadherin and Ig-like, plexin and transcription factor domains. The positions and evolutionary conservation of O-Man glycans in cadherins suggest that they play important functional roles for this large group of cell adhesion glycoproteins, which can now be addressed. The developed O-Man SimpleCell strategy is applicable to most types of cell lines and enables proteome-wide discovery of O-Man protein glycosylation.

Dynamics and regulation of epithelial adherens junctions: recent discoveries and controversies

Adherens junctions (AJs) are evolutionarily conserved plasma-membrane structures that mediate cell-cell adhesions in multicellular organisms. They are organized by several types of adhesive integral membrane proteins, most notably cadherins and nectins that are clustered and stabilized by a number of cytoplasmic scaffolds. AJs are key regulators of tissue architecture and dynamics via control of cell proliferation, polarity, shape, motility, and survival. They are absolutely critical for normal tissue morphogenesis and their disruption results in pathological abnormalities in different tissues. Although the field of adherens-junction research dramatically progressed in recent years, a number of important questions remain controversial and poorly understood. This review outlines basic principles that regulate organization of AJs in mammalian epithelia and discusses recent advances and standing controversies in the field. A special attention is paid to the regulation of AJs by vesicle trafficking and the intracellular cytoskeleton as well as roles and mechanisms of adherens-junction disruption during tumor progression and tissue inflammation.

Mechanism of E-cadherin dimerization probed by NMR relaxation dispersion

Epithelial cadherin (E-cadherin), a member of the classical cadherin family, mediates calcium-dependent homophilic cell-cell adhesion. Crystal structures of classical cadherins reveal an adhesive dimer interface featuring reciprocal exchange of N-terminal β-strands between two protomers. Previous work has identified a putative intermediate (called the "X-dimer") in the dimerization pathway of wild-type E-cadherin EC1-EC2 domains, based on crystal structures of mutants not capable of strand swapping and on deceleration of binding kinetics by mutations at the X-dimer interface. In the present work, NMR relaxation dispersion spectroscopy is used to directly observe and characterize intermediate states without the need to disrupt the strand-swapped binding interface by mutagenesis. The results indicate that E-cadherin forms strand-swapped dimers predominantly by a mechanism in which formation of a weak and short-lived X-dimer-like state precedes the conformational changes required for formation of the mature strand-swapped dimeric structure. Disruption of this intermediate state through mutation reduces both association and dissociation rates by factors of ~10(4), while minimally perturbing affinity. The X-dimer interface lowers the energy barrier associated with strand swapping and enables E-cadherins to form strand-swapped dimers at a rate consistent with residence times in adherens junctions.

A mechanobiological perspective on cadherins and the actin-myosin cytoskeleton

Classical cadherin receptors mediate morphogenetic cell-cell interactions within many tissues of the body. Their biological impact often entails cooperation between cadherin adhesion and the actin cytoskeleton, but how this may occur and – even more urgently – how this leads to morphogenetic outcomes are questions that remain poorly understood. Here, we suggest that the emerging field of cadherin mechanobiology provides a useful new perspective from which to revisit these issues. We propose that the actin cytoskeleton can be considered as an active agent that mediates how cadherin junctions resist, sense and transduce forces between cells.

Mechanistic basis of desmosome-targeted diseases

Desmosomes are dynamic junctions between cells that maintain the structural integrity of skin and heart tissues by withstanding shear forces. Mutations in component genes cause life-threatening conditions including arrhythmogenic right ventricular cardiomyopathy, and desmosomal proteins are targeted by pathogenic autoantibodies in skin blistering diseases such as pemphigus. Here, we review a set of newly discovered pathogenic alterations and discuss the structural repercussions of debilitating mutations on desmosomal proteins. The architectures of native desmosomal assemblies have been visualized by cryo-electron microscopy and cryo-electron tomography, and the network of protein domain interactions is becoming apparent. Plakophilin and desmoplakin mutations have been discovered to alter binding interfaces, structures, and stabilities of folded domains that have been resolved by X-ray crystallography and NMR spectroscopy. The flexibility within desmoplakin has been revealed by small-angle X-ray scattering and fluorescence assays, explaining how mechanical stresses are accommodated. These studies have shown that the structural and functional consequences of desmosomal mutations can now begin to be understood at multiple levels of spatial and temporal resolution. This review discusses the recent structural insights and raises the possibility of using modeling for mechanism-based diagnosis of how deleterious mutations alter the integrity of solid tissues.

Tuning the kinetics of cadherin adhesion

Cadherins are Ca(2+)-dependent cell-cell adhesion proteins that maintain the structural integrity of the epidermis; their principle function is to resist mechanical force. This review summarizes the biophysical mechanisms by which classical cadherins tune adhesion and withstand mechanical stress. We first relate the structure of classical cadherins to their equilibrium binding properties. We then review the role of mechanical perturbations in tuning the kinetics of cadherin adhesion. In particular, we highlight recent studies that show that cadherins form three types of adhesive bonds: catch bonds, which become longer lived and lock in the presence of tensile force; slip bonds, which become shorter lived when pulled; and ideal bonds, which are insensitive to tugging.

Binding to F-actin guides cadherin cluster assembly, stability, and movement

Binding of cadherin to F-actin cooperates with the cadherin cis-interface to stabilize cadherin adhesion clusters and is required for their directional movement.

The cadherin extracellular region produces intercellular adhesion clusters through trans- and cis-intercadherin bonds, and the intracellular region connects these clusters to the cytoskeleton. To elucidate the interdependence of these binding events, cadherin adhesion was reconstructed from the minimal number of structural elements. F-actin–uncoupled adhesive clusters displayed high instability and random motion. Their assembly required a cadherin cis-binding interface. Coupling these clusters with F-actin through an α-catenin actin-binding domain (αABD) dramatically extended cluster lifetime and conferred direction to cluster motility. In addition, αABD partially lifted the requirement for the cis-interface for cluster assembly. Even more dramatic enhancement of cadherin clustering was observed if αABD was joined with cadherin through a flexible linker or if it was replaced with an actin-binding domain of utrophin. These data present direct evidence that binding to F-actin stabilizes cadherin clusters and cooperates with the cis-interface in cadherin clustering. Such cooperation apparently synchronizes extracellular and intracellular binding events in the process of adherens junction assembly.

The adherens junction: a mosaic of cadherin and nectin clusters bundled by actin filaments

Cadherin and nectin are distinct transmembrane proteins of adherens junctions. Their ectodomains mediate adhesion while their cytosolic regions couple the adhesive contact to the cytoskeleton. Both these proteins are essential for adherens junction formation and maintenance. However, some basic aspects of these proteins, such as their organization in adherence junctions, have remained open. Therefore, using super-resolution microscopy and live-imaging, we focused on the subjunctional distribution of these proteins. We showed that cadherin and nectin in the junctions of A431 cells and human keratinocytes are located in separate clusters. The size of each cluster is independent of that of the adjacent clusters and can significantly fluctuate over time. Several nectin and cadherin clusters that constitute an individual adherens junction are united by the same actin filament bundle. Surprisingly, interactions between each cluster and F-actin are not uniform since neither vinculin nor LIM domain actin-binding proteins match the boundaries of cadherin or nectin clusters. Thus, the adherens junction is not a uniform structure but a mosaic of different adhesive units with very diverse modes of interaction with the cytoskeleton. We propose that such a mosaic architecture of adherence junctions is important for the fast regulation of their dynamics.

Expression of recombinant glycoproteins in mammalian cells: towards an integrative approach to structural biology

Mammalian cells are rapidly becoming the system of choice for the production of recombinant glycoproteins for structural biology applications. Their use has enabled the structural investigation of a whole new set of targets including large, multi-domain and highly glycosylated eukaryotic cell surface receptors and their supra-molecular assemblies. We summarize the technical advances that have been made in mammalian expression technology and highlight some of the structural insights that have been obtained using these methods. Looking forward, it is clear that mammalian cell expression will provide exciting and unique opportunities for an integrative approach to the structural study of proteins, especially of human origin and medically relevant, by bridging the gap between the purified state and the cellular context.

Delineation of the key aspects in the regulation of epithelial monolayer formation

The formation, maintenance, and repair of epithelial barriers are of critical importance for whole-body homeostasis. However, the molecular events involved in epithelial tissue maturation are not fully established. To this end, we investigated the molecular processes involved in renal epithelial proximal-tubule monolayer maturation utilizing transcriptomic, metabolomic, and functional parameters. We uncovered profound dynamic alterations in transcriptional regulation, energy metabolism, and nutrient utilization over the maturation process. Proliferating cells exhibited high glycolytic rates and high transcript levels for fatty acid synthesis genes (FASN), whereas matured cells had low glycolytic rates, increased oxidative capacity, and preferentially expressed genes for beta oxidation. There were dynamic alterations in the expression and localization of several adherens (CDH1, -4, and -16) and tight junction (TJP3 and CLDN2 and -10) proteins. Genes involved in differentiated proximal-tubule function, cilium biogenesis (BBS1), and transport (ATP1A1 and ATP1B1) exhibited increased expression during epithelial maturation. Using TransAM transcription factor activity assays, we could demonstrate that p53 and FOXO1 were highly active in matured cells, whereas HIF1A and c-MYC were highly active in proliferating cells. The data presented here will be invaluable in the further delineation of the complex dynamic cellular processes involved in epithelial cell regulation.

E-cadherin polarity is determined by a multifunction motif mediating lateral membrane retention through ankyrin-G and apical-lateral transcytosis through clathrin

We report a highly conserved motif in the E-cadherin juxtamembrane domain that determines apical-lateral polarity by conferring both restricted mobility at the lateral membrane and transcytosis of apically mis-sorted protein to the lateral membrane. Mutations causing either increased lateral membrane mobility or loss of apical-lateral transcytosis result in partial mis-sorting of E-cadherin in Madin-Darby canine kidney cells. However, loss of both activities results in complete loss of polarity. We present evidence that residues required for restricted mobility mediate retention at the lateral membrane through interaction with ankyrin-G, whereas dileucine residues conferring apical-lateral transcytosis act through a clathrin-dependent process and function in an editing pathway. Ankyrin-G interaction with E-cadherin is abolished by the same mutations resulting in increased E-cadherin mobility. Clathrin heavy chain knockdown and dileucine mutation of E-cadherin both cause the same partial loss of polarity of E-cadherin. Moreover, clathrin knockdown causes no further change in polarity of E-cadherin with dileucine mutation but does completely randomize E-cadherin mutants lacking ankyrin-binding. Dileucine mutation, but not loss of ankyrin binding, prevented transcytosis of apically mis-sorted E-cadherin to the lateral membrane. Finally, neurofascin, which binds ankyrin but lacks dileucine residues, exhibited partial apical-lateral polarity that was abolished by mutation of its ankyrin-binding site but was not affected by clathrin knockdown. The polarity motif thus integrates complementary activities of lateral membrane retention through ankyrin-G and apical-lateral transcytosis of mis-localized protein through clathrin. Together, the combination of retention and editing function to ensure a high fidelity steady state localization of E-cadherin at the lateral membrane.

Theory and simulations of adhesion receptor dimerization on membrane surfaces

The equilibrium constants of trans and cis dimerization of membrane bound (2D) and freely moving (3D) adhesion receptors are expressed and compared using elementary statistical-thermodynamics. Both processes are mediated by the binding of extracellular subdomains whose range of motion in the 2D environment is reduced upon dimerization, defining a thin reaction shell where dimer formation and dissociation take place. We show that the ratio between the 2D and 3D equilibrium constants can be expressed as a product of individual factors describing, respectively, the spatial ranges of motions of the adhesive domains, and their rotational freedom within the reaction shell. The results predicted by the theory are compared to those obtained from a novel, to our knowledge, dynamical simulations methodology, whereby pairs of receptors perform realistic translational, internal, and rotational motions in 2D and 3D. We use cadherins as our model system. The theory and simulations explain how the strength of cis and trans interactions of adhesive receptors are affected both by their presence in the constrained intermembrane space and by the 2D environment of membrane surfaces. Our work provides fundamental insights as to the mechanism of lateral clustering of adhesion receptors after cell-cell contact and, more generally, to the formation of lateral microclusters of proteins on cell surfaces.

Regulation and repair of the alveolar-capillary barrier in acute lung injury

Considerable progress has been made in understanding the basic mechanisms that regulate fluid and protein exchange across the endothelial and epithelial barriers of the lung under both normal and pathological conditions. Clinically relevant lung injury occurs most commonly from severe viral and bacterial infections, aspiration syndromes, and severe shock. The mechanisms of lung injury have been identified in both experimental and clinical studies. Recovery from lung injury requires the reestablishment of an intact endothelial barrier and a functional alveolar epithelial barrier capable of secreting surfactant and removing alveolar edema fluid. Repair mechanisms include the participation of endogenous progenitor cells in strategically located niches in the lung. Novel treatment strategies include the possibility of cell-based therapy that may reduce the severity of lung injury and enhance lung repair.

Cadherin selectivity filter regulates endothelial sieving properties

The molecular basis of endothelial protein sieving, the critical vascular barrier function that restricts flow of large plasma proteins into tissues while allowing small molecules and water to pass, is not understood. Here, we address this issue using a novel assay to detect macromolecular penetrance at microdomains of endothelial adherens junctions. Adherens junctions, as detected by cadherin-GFP expression, were distributed in the cell perimeter as high- or low-density segments. Low but not high-density segments permitted penetrance of a 70-kDa fluorescent dextran, a molecule of equivalent size to albumin. Expression of a cadherin mutant that abrogates strand-swap adhesive binding in the cadherin EC1 ectodomain, or alternatively of an α-actinin-1 mutant that inhibits F-actin bundling, increased both cadherin mobility and 70 kDa dextran penetrance at high-density segments. These findings suggest that adhesive interactions in the cadherin EC1 domain, which underlie adherens junction structure, are critical determinants of endothelial macromolecular sieving.

Stability and dynamics of cell-cell junctions

Adherens junctions display dual properties of robustness and plasticity. In multicellular organisms, they support both strong cell-cell adhesion and rapid cell-cell contact remodeling during development and wound healing. The core components of adherens junctions are clusters of cadherin molecules, which mediate cell-cell adhesion through homophilic interactions in trans. Interactions of cadherins with the actin cytoskeleton are essential for providing both stability and plasticity to adherens junctions. Cadherins regulate the turnover of actin by regulating its polymerization and anchor tensile actomyosin networks at the cell cortex. In turn, actin regulates cadherin turnover by regulating its endocytosis and actomyosin networks exert forces driving remodeling of cell-cell contacts. The interplay between adherens junctions and contractile actomyosin networks has striking outcomes during epithelial morphogenesis. Their integrated dynamics result in different morphogenetic patterns shaping tissues and organs.

The evolutionary origin of epithelial cell-cell adhesion mechanisms

A simple epithelium forms a barrier between the outside and the inside of an organism, and is the first organized multicellular tissue found in evolution. We examine the relationship between the evolution of epithelia and specialized cell-cell adhesion proteins comprising the classical cadherin/β-catenin/α-catenin complex (CCC). A review of the divergent functional properties of the CCC in metazoans and non-metazoans, and an updated phylogenetic coverage of the CCC using recent genomic data reveal: (1) The core CCC likely originated before the last common ancestor of unikonts and their closest bikont sister taxa. (2) Formation of the CCC may have constrained sequence evolution of the classical cadherin cytoplasmic domain and β-catenin in metazoa. (3) The α-catenin-binding domain in β-catenin appears to be the favored mutation site for disrupting β-catenin function in the CCC. (4) The ancestral function of the α/β-catenin heterodimer appears to be an actin-binding module. In some metazoan groups, more complex functions of α-catenin were gained by sequence divergence in the non-actin-binding (N-, M-) domains. (5) Allosteric regulation of α-catenin may have evolved for more complex regulation of the actin cytoskeleton.

Coordinating Rho and Rac: the regulation of Rho GTPase signaling and cadherin junctions

Cadherin-based cell-cell adhesions are dynamic structures that mediate tissue organization and morphogenesis. They link cells together, mediate cell-cell recognition, and influence cell shape, motility, proliferation, and differentiation. At the cellular level, operation of classical cadherin adhesion systems is coordinated with cytoskeletal dynamics, contractility, and membrane trafficking to support productive interactions. Cadherin-based cell signaling is critical for the coordination of these many cellular processes. Here, we discuss the role of Rho family GTPases in cadherin signaling. We focus on understanding the pathways that utilize Rac and Rho in junctional biology, aiming to identify the mechanisms of upstream regulation and define how the effects of these activated GTPases might regulate the actin cytoskeleton to modulate the cellular processes involved in cadherin-based cell-cell interactions.

New insights into the evolution of metazoan cadherins and catenins

E-Cadherin and β-catenin are the best studied representatives of the superfamilies of transmembrane cadherins and intracellular armadillo catenins, respectively. However, in over 600 million years of multicellular animal evolution, these two superfamilies have diversified remarkably both structurally and functionally. Although their basic building blocks, respectively, the cadherin repeat domain and the armadillo repeat domain, predate metazoans, the specific and complex domain compositions of the different family members and their functional roles in cell adhesion and signaling appear to be key features for the emergence of multicellular animal life. Basal animals such as placozoans and sponges have a limited number of distinct cadherins and catenins. The origin of vertebrates, in particular, coincided with a large increase in the number of cadherins and armadillo proteins, including modern "classical" cadherins, protocadherins, and plakophilins. Also, α-catenins increased. This chapter introduces the many different family members and describes the putative evolutionary relationships between them.

[Regulation of intercellular adhesion during epithelial morphogenesis]

The epithelium is one of the most abundant tissues in metazoans. It is required to generate stable chemical and mechanical barriers between physiological compartments (fluid matrix/external environment). This function is based on multiple intercellular junctions, which insulate and stabilize cell-cell contacts in the tissue. Despite this apparent robustness, epithelia can be extensively remodeled during wound healing, embryogenesis and tumor progression. The capacity to be remodeled while keeping tissue cohesion requires a perfect balance between stability and plasticity of intercellular junctions. The balance is partially regulated by intercellular adhesion, which is mostly based on adherens junctions and the transmembrane protein E-cadherin. The aim of this review is to report the molecular basis of the balance between plasticity and robustness in the epithelium. We will first present the minimal physical framework used to describe epithelial cell shape. We will then describe the main processes involved in intercellular adhesion regulation and their functions during epithelial morphogenesis. Eventually, we will analyze the relationship and the coupling between adhesive forces and cortical tension.

Wnt-11 and Fz7 reduce cell adhesion in convergent extension by sequestration of PAPC and C-cadherin

Wnt-11/frizzled-7 reduces the lateral clustering of C-cadherin by capturing the protocadherin PAPC and C-cadherin into distinct adhesion-modulating complexes.

Wnt-11/planar cell polarity signaling polarizes mesodermal cells undergoing convergent extension during Xenopus laevis gastrulation. These shape changes associated with lateral intercalation behavior require a dynamic modulation of cell adhesion. In this paper, we report that Wnt-11/frizzled-7 (Fz7) controls cell adhesion by forming separate adhesion-modulating complexes (AMCs) with the paraxial protocadherin (PAPC; denoted as AMCP) and C-cadherin (denoted as AMCC) via distinct Fz7 interaction domains. When PAPC was part of a Wnt-11–Fz7 complex, its Dynamin1- and clathrin-dependent internalization was blocked. This membrane stabilization of AMCP (Fz7/PAPC) by Wnt-11 prevented C-cadherin clustering, resulting in reduced cell adhesion and modified cell sorting activity. Importantly, Wnt-11 did not influence C-cadherin internalization; instead, it promoted the formation of AMCC (Fz7/Cadherin), which competed with cis-dimerization of C-cadherin. Because PAPC and C-cadherin did not directly interact and did not form a joint complex with Fz7, we suggest that Wnt-11 triggers the formation of two distinct complexes, AMCC and AMCP, that act in parallel to reduce cell adhesion by hampering lateral clustering of C-cadherin.

Remarkable disparity in mechanical response among the extracellular domains of type I and II cadherins

Cadherins, a large family of calcium-dependent adhesion molecules, are critical for intercellular adhesion. While crystallographic structures for several cadherins show clear structural similarities, their relevant adhesive strengths vary and their mechanisms of adhesion between types I and II cadherin subfamilies are still unclear. Here, stretching of cadherins was explored experimentally by atomic force microscopy and computationally by steered molecular dynamics (SMD) simulations, where partial unfolding of the E-cadherin ectodomains was observed. The SMD simulations on strand-swapping cadherin dimers displayed similarity in binding strength, suggesting contributions of other mechanisms to explain the strength differences of cell adhesion in vivo. Systematic simulations on the unfolding of the extracellular domains of type I and II cadherins revealed diverse pathways. However, at the earliest stage, a remarkable similarity in unfolding was observed for the various type I cadherins that was distinct from that for type II cadherins. This likely correlates positively with their distinct adhesive properties, suggesting that the initial forced deformation in type I cadherins may be involved in cadherin-mediated adhesion. An animated Interactive 3D Complement (I3DC) is available in Proteopedia at http://proteopedia.org/w/Journal:JBSD:25.

A bigger picture: classical cadherins and the dynamic actin cytoskeleton

Classical cadherin adhesion receptors influence tissue integrity in health and disease. Their biological function is intimately linked to the actin cytoskeleton. To date, research has largely focused on identifying the molecular mechanisms that physically couple cadherin to cortical actin filaments. However, the junctional cytoskeleton is dynamic. Recent developments in understanding how filament dynamics and organization in the junctional cytoskeleton are controlled provide new insights into how the actin cytoskeleton regulates cadherin junctions in health and disease.

LI-cadherin cis-dimerizes in the plasma membrane Ca(2+) independently and forms highly dynamic trans-contacts

LI-cadherin belongs to the family of 7D-cadherins that is characterized by a low sequence similarity to classical cadherins, seven extracellular cadherin repeats (ECs), and a short cytoplasmic domain. Nevertheless, LI-cadherins mediates Ca²⁺-dependent cell–cell adhesion and induces an epitheloid cellular phenotype in non-polarized CHO cells. Whereas several studies suggest that classical cadherins cis-dimerize in a Ca²⁺-dependent manner and interact in trans by strand-swapping tryptophan 2 of EC1, little is known about the molecular interactions of LI-cadherin, which lacks tryptophan 2. We thus expressed fluorescent LI-cadherin fusion proteins in HEK293 and CHO cells, analyzed their cell–cell adhesive properties and studied their cellular distribution, cis-interaction, and lateral diffusion in the presence and absence of Ca²⁺. LI-cadherin highly concentrates in cell contact areas but rapidly leaves those sites upon Ca²⁺ depletion and redistributes evenly on the cell surface, indicating that it is only kept in the contact areas by trans-interactions. Fluorescence resonance energy transfer analysis of LI-cadherin-CFP and -YFP revealed that LI-cadherin forms cis-dimers that resist Ca²⁺ depletion. As determined by fluorescence redistribution after photobleaching, LI-cadherin freely diffuses in the plasma membrane as a cis-dimer (D = 0.42 ± 0.03 μm²/s). When trapped by trans-binding in cell contact areas, its diffusion coefficient decreases only threefold to D = 0.12 ± 0.01 μm²/s, revealing that, in contrast to classical and desmosomal cadherins, trans-contacts formed by LI-cadherin are highly dynamic.

Cadherin recognition and adhesion

Classical cadherins are the principle adhesive proteins at cohesive intercellular junctions, and are essential proteins for morphogenesis and tissue homeostasis. Because subtype-dependent differences in cadherin adhesion are at the heart of cadherin functions, several structural and biophysical approaches have been used to elucidate relationships between cadherin structures, biophysical properties of cadherin bonds, and cadherin-dependent cell functions. Some experimental approaches appeared to provide conflicting views of the cadherin binding mechanism. However, recent structural and biophysical data, as well as computer simulations generated new insights into classical cadherin binding that increasingly reconcile diverse experimental findings. This review summarizes these recent findings, and highlights both the consistencies and remaining challenges needed to generate a comprehensive model of cadherin interactions that is consistent with all available experimental data.

X-interface is not the explanation for the slow disassembly of N-cadherin dimers in the apo state

In spite of structural similarities Epithelial- (E-) and Neural- (N-) cadherins are expressed at two types of synapses and differ significantly in dimer disassembly kinetics. Recent studies suggested that the formation of an X-dimer intermediate in E-cadherin is the key requirement for rapid disassembly of the adhesive dimer (Harrison et al., Nat Struct Mol Biol 2010;17:348-357 and Hong et al., J Cell Biol 2011;192:1073-1083). The X-interface in E-cadherin involves three noncovalent interactions, none of which is conserved in N-cadherin. Dimer disassembly is slow at low calcium concentration in N-cadherin, which may be due to the differences in the X-interface residues. To investigate the origin of the slow disassembly kinetics we introduced three point mutations into N-cadherin to provide the opportunity for the formation of X-interface interactions. Spectroscopic studies showed that the triple mutation did not affect the stability or the calcium-binding affinity of the X-enabled N-cadherin mutant. Analytical size exclusion chromatography was used to assay for the effect of the mutation on the rate of dimer disassembly. Contrary to our expectation, the disassembly of dimers of the X-enabled N-cadherin mutant was as slow as seen for wild-type N-cadherin in the apo-state. Thus, the differences in the X-interface residues are not the origin of slow disassembly kinetics of N-cadherin in the apo-state.

The Na-K-ATPase α₁β₁ heterodimer as a cell adhesion molecule in epithelia

The ion gradients generated by the Na-K-ATPase play a critical role in epithelia by driving transepithelial transport of various solutes. The efficiency of this Na-K-ATPase-driven vectorial transport depends on the integrity of epithelial junctions that maintain polar distribution of membrane transporters, including the basolateral sodium pump, and restrict paracellular diffusion of solutes. The review summarizes the data showing that, in addition to pumping ions, the Na-K-ATPase located at the sites of cell-cell junction acts as a cell adhesion molecule by interacting with the Na-K-ATPase of the adjacent cell in the intercellular space accompanied by anchoring to the cytoskeleton in the cytoplasm. The review also discusses the experimental evidence on the importance of a specific amino acid region in the extracellular domain of the Na-K-ATPase β(1) subunit for the Na-K-ATPase trans-dimerization and intercellular adhesion. Furthermore, a possible role of N-glycans linked to the Na-K-ATPase β(1) subunit in regulation of epithelial junctions by modulating β(1)-β(1) interactions is discussed.

Thinking outside the cell: how cadherins drive adhesion

Cadherins are a superfamily of cell surface glycoproteins whose ectodomains contain multiple repeats of β-sandwich extracellular cadherin (EC) domains that adopt a similar fold to immunoglobulin domains. The best characterized cadherins are the vertebrate 'classical' cadherins, which mediate adhesion via trans homodimerization between their membrane-distal EC1 domains that extend from apposed cells, and assemble intercellular adherens junctions through cis clustering. To form mature trans adhesive dimers, cadherin domains from apposed cells dimerize in a 'strand-swapped' conformation. This occurs in a two-step binding process involving a fast-binding intermediate called the 'X-dimer'. Trans dimers are less flexible than cadherin monomers, a factor that drives junction assembly following cell-cell contact by reducing the entropic cost associated with the formation of lateral cis oligomers. Cadherins outside the classical subfamily appear to have evolved distinct adhesive mechanisms that are only now beginning to be understood.

Structure of TIGIT immunoreceptor bound to poliovirus receptor reveals a cell-cell adhesion and signaling mechanism that requires cis-trans receptor clustering

Nectins (nectin1-4) and Necls [nectin-like (Necl1-5)] are Ig superfamily cell adhesion molecules that regulate cell differentiation and tissue morphogenesis. Adherens junction formation and subsequent cell-cell signaling is initiated by the assembly of higher-order receptor clusters of cognate molecules on juxtaposed cells. However, the structural and mechanistic details of signaling cluster formation remain unclear. Here, we report the crystal structure of poliovirus receptor (PVR)/Nectin-like-5/CD155) in complex with its cognate immunoreceptor ligand T-cell-Ig-and-ITIM-domain (TIGIT). The TIGIT/PVR interface reveals a conserved specific "lock-and-key" interaction. Notably, two TIGIT/PVR dimers assemble into a heterotetramer with a core TIGIT/TIGIT cis-homodimer, each TIGIT molecule binding one PVR molecule. Structure-guided mutations that disrupt the TIGIT/TIGIT interface limit both TIGIT/PVR-mediated cell adhesion and TIGIT-induced PVR phosphorylation in primary dendritic cells. Our data suggest a cis-trans receptor clustering mechanism for cell adhesion and signaling by the TIGIT/PVR complex and provide structural insights into how the PVR family of immunoregulators function.

Adherens junction assembly

Classical cadherins are a family of transmembrane proteins that mediate cell-cell adhesion at adherens junctions. A complex chain of cis- and trans- interactions between cadherin ectodomains establishes a cadherin adhesive cluster. A principal adhesive interaction in such clusters is an exchange of β strands between the first extracellular cadherin domains (EC1). The structure of cadherin adhesive clusters can be modified by other adherens junction proteins including additional transmembrane proteins, nectins and various intracellular proteins that directly or indirectly interact with the intracellular cadherin region. These interactions determine the dynamics and stability of cadherin adhesive structures.

N-Glycosylation Alters Cadherin-Mediated Intercellular Binding Kinetics

These results present direct evidence that the N-glycosylation state of neural cadherin impacts the intrinsic kinetics of cadherin-mediated intercellular binding. Micropipette manipulation measurements quantified the effect of N-glycosylation mutations intercellular binding dynamics. The wild type protein exhibits a two-stage binding process in which a fast, initial binding step is followed by a short lag and second, slower transition to the final binding stage. Mutations that ablate N-glycosylation at three sites on the extracellular domains 2 and 3 (EC2-3) of neural cadherin alter this kinetic fingerprint. Glycosylation does not affect the affinities between the adhesive N-terminal domains, but instead modulates additional cadherin interactions, which govern the dynamics of intercellular binding. These results, together with prior findings that these hypo-glycosylation mutations increase the prevalence of cis dimers on cell membranes, suggest a binding mechanism in which initial adhesion is followed by additional cadherin interactions, which enhance binding but are modulated by N-glycosylation. Given that oncogene expression drives specific changes in N-glycosylation, these results provide insight into possible mechanisms altering cadherin function during tumor progression.

The three-dimensional structure of the cadherin-catenin complex

The cadherin-catenin complex is the major building block of the adherens junction. It is responsible for coupling Ca(2+)-dependent intercellular junctions with various intracellular events, including actin dynamics and signaling pathways. Determination of three-dimensional structures of cadherins, p120 catenin, β-catenin and α-catenin at atomic-level resolution has allowed us to examine how the structure and function of cell adhesion molecules are further modulated by protein-protein interactions. Structural studies of cadherins revealed the strand-swap-dependent and -independent trans-dimerization mechanisms, as well as a potential mechanism for lateral clustering of cadherin trans-dimers. Crystallographic and NMR analyses of p120 catenin revealed that it regulates the stability of cadherin-mediated cell-cell adhesion by associating with the majority of the E-cadherin juxtamembrane domain, including residues implicated in clathrin-mediated endocytosis and Hakai-dependent ubiquitination. Crystal structures of the β-catenin/E-cadherin complex and the β-/α-catenin chimera revealed extensive interactions necessary to form the cadherin/β-catenin/α-catenin ternary complex. Structural characterization of α-catenin has revealed conformational changes within the N-terminal and modulatory domains that are crucial for its role as a mechanosensor of cell-cell adhesion. Further insights into the connection between the cadherin-catenin complex and the actin cytoskeleton are integral to better understand how adjoining cells communicate through cell-cell adhesion.

Biophysics of cadherin adhesion

Since the identification of cadherins and the publication of the first crystal structures, the mechanism of cadherin adhesion, and the underlying structural basis have been studied with a number of different experimental techniques, different classical cadherin subtypes, and cadherin fragments. Earlier studies based on biophysical measurements and structure determinations resulted in seemingly contradictory findings regarding cadherin adhesion. However, recent experimental data increasingly reveal parallels between structures, solution binding data, and adhesion-based biophysical measurements that are beginning to both reconcile apparent differences and generate a more comprehensive model of cadherin-mediated cell adhesion. This chapter summarizes the functional, structural, and biophysical findings relevant to cadherin junction assembly and adhesion. We emphasize emerging parallels between findings obtained with different experimental approaches. Although none of the current models accounts for all of the available experimental and structural data, this chapter discusses possible origins of apparent discrepancies, highlights remaining gaps in current knowledge, and proposes challenges for further study.

The cytoskeleton and classical cadherin adhesions

This chapter discusses the biochemical and functional links between classical cadherin adhesion systems and the cytoskeleton. Cadherins are best understood to cooperate with the actin cytoskeleton, but there is increasing evidence for the role of junctional microtubules in regulating cadherin biology. Cadherin adhesions and the junctional cytoskeleton are both highly dynamic systems that undergo continual assembly, turnover and remodeling, and yet maintain steady state structures necessary for intercellular adhesion. This requires the functional coordination of cadherins and cadherin-binding proteins, actin regulatory proteins, organizers of microtubule assembly and structure, and signaling pathways. These components act in concert to regulate junctional organization in response to extracellular forces and changing cellular contexts, which is essential for intercellular cohesion and tissue integrity.

A complex of Protocadherin-19 and N-cadherin mediates a novel mechanism of cell adhesion

During embryonic morphogenesis, adhesion molecules are required for selective cell-cell interactions. The classical cadherins mediate homophilic calcium-dependent cell adhesion and are founding members of the large and diverse cadherin superfamily. The protocadherins are the largest subgroup within this superfamily, yet their participation in calcium-dependent cell adhesion is uncertain. In this paper, we demonstrate a novel mechanism of adhesion, mediated by a complex of Protocadherin-19 (Pcdh19) and N-cadherin (Ncad). Although Pcdh19 alone is only weakly adhesive, the Pcdh19-Ncad complex exhibited robust adhesion in bead aggregation assays, and Pcdh19 appeared to play the dominant role. Adhesion by the Pcdh19-Ncad complex was unaffected by mutations that disrupt Ncad homophilic binding but was inhibited by a mutation in Pcdh19. In addition, the complex exhibited homophilic specificity, as beads coated with Pcdh19-Ncad did not intermix with Ncad- or Pcdh17-Ncad-coated beads. We propose a model in which association of a protocadherin with Ncad acts as a switch, converting between distinct binding specificities.

Sensing sound: molecules that orchestrate mechanotransduction by hair cells

Animals use acoustic signals to communicate and to obtain information about their environment. The processing of acoustic signals is initiated at auditory sense organs, where mechanosensory hair cells convert sound-induced vibrations into electrical signals. Although the biophysical principles underlying the mechanotransduction process in hair cells have been characterized in much detail over the past 30 years, the molecular building-blocks of the mechanotransduction machinery have proved to be difficult to determine. We review here recent studies that have both identified some of these molecules and established the mechanisms by which they regulate the activity of the still-elusive mechanotransduction channel.

Crystal structures of Drosophila N-cadherin ectodomain regions reveal a widely used class of Ca²+-free interdomain linkers

Vertebrate classical cadherins mediate selective calcium-dependent cell adhesion by mechanisms now understood at the atomic level. However, structures and adhesion mechanisms of cadherins from invertebrates, which are highly divergent yet function in similar roles, remain unknown. Here we present crystal structures of three- and four-tandem extracellular cadherin (EC) domain segments from Drosophila N-cadherin (DN-cadherin), each including the predicted N-terminal EC1 domain (denoted EC1') of the mature protein. While the linker regions for the EC1'-EC2' and EC3'-EC4' pairs display binding of three Ca(2+) ions similar to that of vertebrate cadherins, domains EC2' and EC3' are joined in a "kinked" orientation by a previously uncharacterized Ca(2+)-free linker. Biophysical analysis demonstrates that a construct containing the predicted N-terminal nine EC domains of DN-cadherin forms homodimers with affinity similar to vertebrate classical cadherins, whereas deleting the ninth EC domain ablates dimerization. These results suggest that, unlike their vertebrate counterparts, invertebrate cadherins may utilize multiple EC domains to form intercellular adhesive bonds. Sequence analysis reveals that similar Ca(2+)-free linkers are widely distributed in the ectodomains of both vertebrate and invertebrate cadherins.

Tandem repeats in proteins: from sequence to structure

The bioinformatics analysis of proteins containing tandem repeats requires special computer programs and databases, since the conventional approaches predominantly developed for globular domains have limited success. Here, I survey bioinformatics tools which have been developed recently for identification and proteome-wide analysis of protein repeats. The last few years have also been marked by an emergence of new 3D structures of these proteins. Appraisal of the known structures and their classification uncovers a straightforward relationship between their architecture and the length of the repetitive units. This relationship and the repetitive character of structural folds suggest rules for better prediction of the 3D structures of such proteins. Furthermore, bioinformatics approaches combined with low resolution structural data, from biophysical techniques, especially, the recently emerged cryo-electron microscopy, lead to reliable prediction of the protein repeat structures and their mode of binding with partners within molecular complexes. This hybrid approach can actively be used for structural and functional annotations of proteomes.