Thinking outside the cell: how cadherins drive adhesion

Getting a grip on adhesion: Cadherin switching and collagen signaling

Epithelial-mesenchymal transition (EMT) is a developmental biological process that is hijacked during tumor progression. Cadherin switching, which disrupts adherens junctions and alters cadherin-associated signaling pathways, is common during EMT. In many tumors, substantial extracellular matrix (ECM) is deposited. Collagen is the most abundant ECM constituent and it mediates specific signaling pathways by binding to integrins and discoidin domain receptors (DDRs). The interaction of the collagen receptors results in activation of signaling pathways that promote tumor progression including an induction of the cadherin switching. DDR inhibitors have demonstrated anticancer therapeutic efficacy preclinically by inhibiting the collagen signaling. Understanding how collagen signaling impacts cellular processes including EMT and cadherin switching is of great interest especially given the strong interest in stromal targeted therapies for desmoplastic cancers.

Cadherin Related Family Member 2 Acts As A Tumor Suppressor By Inactivating AKT In Human Hepatocellular Carcinoma

Cadherin related family member 2 (CDHR2) belongs to the protocadherin family and is abundant in normal liver, kidney, and colon tissues, but weakly expressed in cancers arising from these tissues. In this study, we demonstrated that CDHR2 was highly expressed in para-cancer tissues of human hepatocellular carcinoma (HCC), but significantly downregulated or silenced in 85.7% (6/7) of HCC cell lines by both semi-quantitative PCR and western blot, and 79.1% (19/24) and 80.2% (89/111) of tumor tissues from patients with HCC by semi-quantitative PCR, and immunohistochemistry, respectively. Interestingly, CpG islands in the promoter of CDHR2 gene were hypermethylated in HCC cell lines and tissues compared with the para-cancer tissues by methylation-specific PCR analysis, leading to transcriptional repression and silencing of CDHR2 in HCC. In addition, CDHR2 overexpression by lentiviral vectors had suppressive effects on HCC cell growth and proliferation, as evidenced by prolonged cell doubling time and reduced colony-forming ability in vitro, as well as by decreased tumorigenicity in vivo. Mechanistically, CDHR2 overexpression resulted in AKT dephosphorylation along with downregulation of cyclooxygenase-2 (COX2), a downstream target of AKT. This effect was reversed by myristoylated AKT, a constitutively active form of AKT, suggesting an involvement of CDHR2-AKT-COX2 axis in the suppression of HCC growth. Taken together, our study identified CDHR2 as a novel tumor suppressor in HCC and provided a new therapeutic target for HCC.

Isothermal Titration Calorimetry of Be2+ with Phosphatidylserine Models Guides All-Atom Force-Field Development for Lipid-Ion Interactions

Beryllium has multiple industrial applications but exposure to its dust during manufacturing is associated with developing chronic inflammation in lungs known as berylliosis. Besides binding to specific alleles of MHC-II, Be²⁺ was recently found to compete with Ca²⁺ for binding sites on phosphatidylserine-containing membranes and inhibit recognition of this lipid by phagocytes. Computational studies of possible molecular targets for this small toxic dication are impeded by the absence of a reliable force field. This study introduces parameters for Be²⁺ for the CHARMM36 additive force field that represent interactions with water, including free energy of hydration and ion-monohydrate interaction energy and separation distance; and interaction parameters describing Be²⁺ affinity for divalent ion binding sites on lipids, namely phosphoryl and carboxylate oxygens. Results from isothermal titration calorimetry experiments for the binding affinities of Be²⁺ to dimethyl phosphate and acetate ions reveal that Be²⁺ strongly binds to phosphoryl groups. Revised interaction parameters for Be²⁺ with these types of oxygens reproduce experimental affinities in solution simulations. Surface tensions calculated from simulations of DOPS monolayers with varied concentrations of Be²⁺ are compared with prior results from Langmuir monolayer experiments, verifying the compacting effect that produces greater surface tensions (lower pressures) for Be²⁺-bound monolayers at the same surface area in comparison with K⁺. The new parameters will enable simulations that should reveal the mechanism of Be²⁺ interference with molecular recognition and signaling processes.

Relative Binding Affinity Prediction of Charge-Changing Sequence Mutations with FEP in Protein-Protein Interfaces

Building on the substantial progress that has been made in using free energy perturbation (FEP) methods to predict the relative binding affinities of small molecule ligands to proteins, we have previously shown that results of similar quality can be obtained in predicting the effect of mutations on the binding affinity of protein–protein complexes. However, these results were restricted to mutations which did not change the net charge of the side chains due to known difficulties with modeling perturbations involving a change in charge in FEP. Various methods have been proposed to address this problem. Here we apply the co-alchemical water approach to study the efficacy of FEP calculations of charge changing mutations at the protein–protein interface for the antibody–gp120 system investigated previously and three additional complexes. We achieve an overall root mean square error of 1.2 kcal/mol on a set of 106 cases involving a change in net charge selected by a simple suitability filter using side-chain predictions and solvent accessible surface area to be relevant to a biologic optimization project. Reasonable, although less precise, results are also obtained for the 44 more challenging mutations that involve buried residues, which may in some cases require substantial reorganization of the local protein structure, which can extend beyond the scope of a typical FEP simulation. We believe that the proposed prediction protocol will be of sufficient efficiency and accuracy to guide protein engineering projects for which optimization and/or maintenance of a high degree of binding affinity is a key objective.

Response of epithelial cell and tissue shape to external forces in vivo

How actomyosin generates forces at epithelial adherens junctions has been extensively studied. However, less is known about how a balance between internal and external forces establishes epithelial cell, tissue and organ shape. We use the Drosophila egg chamber to investigate how contractility at adherens junction in the follicle epithelium is modulated to accommodate and resist forces arising from the growing germline. We find that between stages 6 and 9 adherens junction tension in the post-mitotic epithelium decreases, suggesting that the junctional network relaxes to accommodate germline growth. At that time, a prominent medial Myosin II network coupled to corrugating adherens junctions develops. Local enrichment of medial Myosin II in main body follicle cells resists germline-derived forces, thus constraining apical areas and consequently cuboidal cell shapes at stage 9. At the tissue and organ level, local reinforcement of medial-junctional architecture ensures the timely contact of main body cells with the expanding oocyte and imposes circumferential constraints on the germline guiding egg elongation. Our study provides insight into how adherens junction tension promotes cell and tissue shape transitions while integrating growth and shape of an internally enclosed structure in vivo.

A Mechanically Weak Extracellular Membrane-Adjacent Domain Induces Dimerization of Protocadherin-15

The cadherin superfamily of proteins is defined by the presence of extracellular cadherin (EC) "repeats" that engage in protein-protein interactions to mediate cell-cell adhesion, cell signaling, and mechanotransduction. The extracellular domains of nonclassical cadherins often have a large number of EC repeats along with other subdomains of various folds. Protocadherin-15 (PCDH15), a protein component of the inner-ear tip link filament essential for mechanotransduction, has 11 EC repeats and a membrane adjacent domain (MAD12) of atypical fold. Here we report the crystal structure of a pig PCDH15 fragment including EC10, EC11, and MAD12 in a parallel dimeric arrangement. MAD12 has a unique molecular architecture and folds as a ferredoxin-like domain similar to that found in the nucleoporin protein Nup54. Analytical ultracentrifugation experiments along with size-exclusion chromatography coupled to multiangle laser light scattering and small-angle x-ray scattering corroborate the crystallographic dimer and show that MAD12 induces parallel dimerization of PCDH15 near its membrane insertion point. In addition, steered molecular dynamics simulations suggest that MAD12 is mechanically weak and may unfold before tip-link rupture. Sequence analyses and structural modeling predict the existence of similar domains in cadherin-23, protocadherin-24, and the "giant" FAT and CELSR cadherins, indicating that some of them may also exhibit MAD-induced parallel dimerization.

Probing Cadherin Interactions in Zebrafish with E- and N-Cadherin Missense Mutants

Cadherins are cell adhesion molecules that regulate numerous adhesive interactions during embryonic development and adult life. Consistent with these functions, when their expression goes astray cells lose their normal adhesive properties resulting in defective morphogenesis, disease, and even metastatic cancer. In general, classical cadherins exert their effect by homophilic interactions via their five characteristic extracellular (EC) repeats. The EC1 repeat provides the mechanism for cadherins to dimerize with each other whereas the EC2 repeat may facilitate dimerization. Less is known about the other EC repeats. Here, we show that a zebrafish missense mutation in the EC5 repeat of N-cadherin is a dominant gain-of-function mutation and demonstrate that this mutation alters cell adhesion almost to the same degree as a zebrafish missense mutation in the EC1 repeat of N-cadherin. We also show that zebrafish E- and N-cadherin dominant gain-of-function missense mutations genetically interact. Perturbation of cell adhesion in embryos that are heterozygous mutant at both loci is similar to that observed in single homozygous mutants. Introducing an E-cadherin EC5 missense allele into the homozygous N-cadherin EC1 missense mutant more radically affects morphogenesis, causing synergistic phenotypes consistent with interdependent functions being disrupted. Our studies indicate that a functional EC5 repeat is critical for cadherin-mediated cell affinity, suggesting that its role may be more important than previously thought. These results also suggest the possibility that E- and N-cadherin have heterophilic interactions during early morphogenesis of the embryo; interactions that might help balance the variety of cell affinities needed during embryonic development.

Morphological Plasticity of Human Melanoma Cells Is Determined by Nanoscopic Patterns of E- and N-Cadherin Interactions

Loss of E-cadherin and concomitant upregulation of N-cadherin is known as the cadherin switch, and has been implicated in melanoma progression. Mechanistically, homophilic ligation of N-cadherin-expressing melanoma cells with N-cadherin presented within the microenvironment is thought to facilitate invasion. However, the biophysical aspects governing molecular specificity and function of such interactions remain unclear. By using precisely defined nano-patterns of N- or E-cadherin (with densities tunable by more than one order of magnitude from 78 to 1,128 ligands/μm²), we analyzed adhesion and spreading of six different human melanoma cell lines with distinct constitutive cadherin expression patterns. Cadherin-mediated homophilic cell interactions (N/N and E/E) with cadherin-functionalized nano-matrices revealed an unexpected functional dichotomy inasmuch as melanoma cell adhesion was cadherin density-dependent, while spreading and lamellipodia formation were independent of cadherin density. Surprisingly, E-cadherin-expressing melanoma cells also interacted with N-cadherin-presenting nano-matrices, suggesting heterophilic (N/E) interactions. However, cellular spreading in these cases occurred only at high densities of N-cadherin (i.e., >285 ligands/μm²). Overall, our approach using nano-patterned biomimetic surfaces provides a platform to further refine the roles of cadherins in tumor cell behavior and it revealed an intriguing flexibility of mutually compensating N- and E-cadherin interactions relevant for melanoma progression.

Vascular Endothelial (VE)-Cadherin, Endothelial Adherens Junctions, and Vascular Disease

Endothelial cell-cell adherens junctions (AJs) supervise fundamental vascular functions, such as the control of permeability and transmigration of circulating leukocytes, and the maintenance of existing vessels and formation of new ones. These processes are often dysregulated in pathologies. However, the evidence that links dysfunction of endothelial AJs to human pathologies is mostly correlative. In this review, we present an update of the molecular organization of AJ complexes in endothelial cells (ECs) that is mainly based on observations from experimental models. Furthermore, we report in detail on a human pathology, cerebral cavernous malformation (CCM), which is initiated by loss-of-function mutations in the genes that encode the three cytoplasmic components of AJs (CCM1, CCM2, and CCM3). At present, these represent a unique example of mutations in components of endothelial AJs that cause human disease. We describe also how studies into the defects of AJs in CCM are shedding light on the crucial regulatory mechanisms and signaling activities of these endothelial structures. Although these observations are specific for CCM, they support the concept that dysfunction of endothelial AJs can directly contribute to human pathologies.

An extracellular Leptospira interrogans leucine-rich repeat protein binds human E- and VE-cadherins

Pathogenic Leptospira bacteria are the causative agents of leptospirosis, a zoonotic disease affecting animals and humans worldwide. These pathogenic species have the ability to rapidly cross host tissue barriers by a yet unknown mechanism. A comparative analysis of pathogens and saprophytes revealed a higher abundance of genes encoding proteins with leucine-rich repeat (LRR) domains in the genomes of pathogens. In other bacterial pathogens, proteins with LRR domains have been shown to be involved in mediating host cell attachment and invasion. One protein from the pathogenic species Leptospira interrogans, LIC10831, has been previously analysed via X-ray crystallography, with findings suggesting it may be an important bacterial adhesin. Herein we show that LIC10831 elicits an antibody response in infected animals, is actively secreted by the bacterium, and binds human E- and VE-cadherins. These results provide biochemical and cellular evidences of LRR protein-mediated host-pathogen interactions and identify a new multireceptor binding protein from this infectious Leptospira species.

Importance of integrity of cell-cell junctions for the mechanics of confluent MDCK II cells

Intercellular junctions are important mechanical couplers between cells in epithelial layers providing adhesion and intercellular communication. Regulation of the junctions occurs in cellular processes such as layer formation, epithelial-to-mesenchymal transition, embryogenesis, and cancer progression. Many studies addressed the role of force generation in cells for establishing lateral cell-cell junctions and the role of cellular force transmission in tissue formation and maintenance. Our atomic force microscopy- (AFM) based study shed light on the role of both, tight junctions and adherens junctions for the mechanical properties of individual epithelial cells that are part of a confluent monolayer. We found that tight junctions are important for the establishment of a functional barrier-forming layer but impairing them does not reduce the mechanical integrity of cells. Depletion of ZO-1 results in a weak increase in cortical tension. An opposite effect was observed for disruption of E-cadherin-mediated adherens junctions using DTT. Opening of adherens junctions leads to substantial alterations of cellular mechanics such as reduced overall stiffness, but these changes turned out to be reversible after re-establishing disulfide bridges in E-cadherin by removal of DTT. We found that regulatory mechanisms exist that preserve mechanical integrity during recovery of disrupted adherens junctions.

Beyond Cell-Cell Adhesion: Sensational Cadherins for Hearing and Balance

Cadherins form a large family of proteins often involved in calcium-dependent cellular adhesion. Although classical members of the family can provide a physical bond between cells, a subset of special cadherins use their extracellular domains to interlink apical specializations of single epithelial sensory cells. Two of these cadherins, cadherin-23 (CDH23) and protocadherin-15 (PCDH15), form extracellular "tip link" filaments that connect apical bundles of stereocilia on hair cells essential for inner-ear mechanotransduction. As these bundles deflect in response to mechanical stimuli from sound or head movements, tip links gate hair-cell mechanosensitive channels to initiate sensory perception. Here, we review the unusual and diverse structural properties of these tip-link cadherins and the functional significance of their deafness-related missense mutations. Based on the structural features of CDH23 and PCDH15, we discuss the elasticity of tip links and models that bridge the gap between the nanomechanics of cadherins and the micromechanics of hair-cell bundles during inner-ear mechanotransduction.

E-cadherin binds to desmoglein to facilitate desmosome assembly

Desmosomes are adhesive junctions composed of two desmosomal cadherins: desmocollin (Dsc) and desmoglein (Dsg). Previous studies demonstrate that E-cadherin (Ecad), an adhesive protein that interacts in both trans (between opposing cells) and cis (on the same cell surface) conformations, facilitates desmosome assembly via an unknown mechanism. Here we use structure-function analysis to resolve the mechanistic roles of Ecad in desmosome formation. Using AFM force measurements, we demonstrate that Ecad interacts with isoform 2 of Dsg via a conserved Leu-175 on the Ecad cis binding interface. Super-resolution imaging reveals that Ecad is enriched in nascent desmosomes, supporting a role for Ecad in early desmosome assembly. Finally, confocal imaging demonstrates that desmosome assembly is initiated at sites of Ecad mediated adhesion, and that Ecad-L175 is required for efficient Dsg2 and desmoplakin recruitment to intercellular contacts. We propose that Ecad trans interactions at nascent cell-cell contacts initiate the recruitment of Dsg through direct cis interactions with Ecad which facilitates desmosome assembly.

α-Catenin Structure and Nanoscale Dynamics in Solution and in Complex with F-Actin

As a core component of the adherens junction, α-catenin stabilizes the cadherin/catenin complexes to the actin cytoskeleton for the mechanical coupling of cell-cell adhesion. α-catenin also modulates actin dynamics, cell polarity, and cell-migration functions that are independent of the adherens junction. We have determined the solution structures of the α-catenin monomer and dimer using in-line size-exclusion chromatography small-angle X-ray scattering, as well as the structure of α-catenin dimer in complex to F-actin filament using selective deuteration and contrast-matching small angle neutron scattering. We further present the first observation, to our knowledge, of the nanoscale dynamics of α-catenin by neutron spin-echo spectroscopy, which explicitly reveals the mobile regions of α-catenin that are crucial for binding to F-actin. In solution, the α-catenin monomer is more expanded than either protomer shown in the crystal structure dimer, with the vinculin-binding M fragment and the actin-binding domain being able to adopt different configurations. The α-catenin dimer in solution is also significantly more expanded than the dimer crystal structure, with fewer interdomain and intersubunit contacts than the crystal structure. When in complex to F-actin, the α-catenin dimer has an even more open and extended conformation than in solution, with the actin-binding domain further separated from the main body of the dimer. The α-catenin-assembled F-actin bundle develops into an ordered filament packing arrangement at increasing α-catenin/F-actin molar ratios. Together, the structural and dynamic studies reveal that α-catenin possesses dynamic molecular conformations that prime this protein to function as a mechanosensor protein.

Integrin Clustering Matters: A Review of Biomaterials Functionalized with Multivalent Integrin-Binding Ligands to Improve Cell Adhesion, Migration, Differentiation, Angiogenesis, and Biomedical Device Integration

Material systems that exhibit tailored interactions with cells are a cornerstone of biomaterial and tissue engineering technologies. One method of achieving these tailored interactions is to biofunctionalize materials with peptide ligands that bind integrin receptors present on the cell surface. However, cell biology research has illustrated that both integrin binding and integrin clustering are required to achieve a full adhesion response. This biophysical knowledge has motivated researchers to develop material systems biofunctionalized with nanoscale clusters of ligands that promote both integrin occupancy and clustering of the receptors. These materials have improved a wide variety of biological interactions in vitro including cell adhesion, proliferation, migration speed, gene expression, and stem cell differentiation; and improved in vivo outcomes including increased angiogenesis, tissue healing, and biomedical device integration. This review first introduces the techniques that enable the fabrication of these nanopatterned materials, describes the improved biological effects that have been achieved, and lastly discusses the current limitations of the technology and where future advances may occur. Although this technology is still in its nascency, it will undoubtedly play an important role in the future development of biomaterials and tissue engineering scaffolds for both in vitro and in vivo applications.

Pathogenic IgG4 autoantibodies from endemic pemphigus foliaceus recognize a desmoglein-1 conformational epitope

Fogo Selvagem (FS), the endemic form of pemphigus foliaceus, is mediated by pathogenic IgG4 autoantibodies against the amino-terminal extracellular cadherin domain of the desmosomal cadherin desmoglein 1 (Dsg1). Here we define the detailed epitopes of these pathogenic antibodies. Proteolytic footprinting showed that IgG4 from 95% of FS donor sera (19/20) recognized a 16-residue peptide (A129LNSMGQDLERPLELR144) from the EC1 domain of Dsg1 that overlaps the binding site for an adhesive-partner desmosomal cadherin molecule. Mutation of Dsg1 residues M133 and Q135 reduced the binding of FS IgG4 autoantibodies to Dsg1 by ∼50%. Molecular modeling identified two nearby EC1 domain residues (Q82 and V83) likely to contribute to the epitope. Mutation of these residues completely abolished the binding of FS IgG4 to Dsg1. Bead aggregation assays showed that native binding interactions between Dsg1 and desmocollin 1 (Dsc1), which underlie desmosome structure, were abolished by Fab fragments of FS IgG4. These results further define the molecular mechanism by which FS IgG4 autoantibodies interfere with desmosome structure and lead to cell-cell detachment, the hallmark of this disease.

Spatial and temporal organization of cadherin in punctate adherens junctions

Adherens junctions (AJs) play a fundamental role in tissue integrity; however, the organization and dynamics of the key AJ transmembrane protein, E-cadherin, both inside and outside of AJs, remain controversial. Here we have studied the distribution and motility of E-cadherin in punctate AJs (pAJs) of A431 cells. Using single-molecule localization microscopy, we show that pAJs in these cells reach more than 1 μm in length and consist of several cadherin clusters with crystal-like density interspersed within sparser cadherin regions. Notably, extrajunctional cadherin appears to be monomeric, and its density is almost four orders of magnitude less than observed in the pAJ regions. Two alternative strategies of tracking cadherin motion within individual junctions show that pAJs undergo actin-dependent rapid-on the order of seconds-internal reorganizations, during which dense clusters disassemble and their cadherins are immediately reused for new clusters. Our results thus modify the classical view of AJs by depicting them as mosaics of cadherin clusters, the short lifetimes of which enable stable overall morphology combined with rapid internal rearrangements.

Junctional Adhesion Molecules (JAMs): The JAM-Integrin Connection

Junctional adhesion molecules (JAMs) are cell surface adhesion receptors of the immunoglobulin superfamily. JAMs are involved in a variety of biological processes both in the adult organism but also during development. These include processes such as inflammation, angiogenesis, hemostasis, or epithelial barrier formation, but also developmental processes such as hematopoiesis, germ cell development, and development of the nervous system. Several of these functions of JAMs depend on a physical and functional interaction with integrins. The JAM – integrin interactions in trans regulate cell-cell adhesion, their interactions in cis regulate signaling processes originating at the cell surface. The JAM – integrin interaction can regulate the function of the JAM as well as the function of the integrin. Beyond the physical interaction with integrins, JAMs can regulate integrin function through intracellular signaling indicating an additional level of JAM – integrin cross-talk. In this review, we describe the various levels of the functional interplay between JAMs and integrins and the role of this interplay during different physiological processes.

Integrin-based diffusion barrier separates membrane domains enabling the formation of microbiostatic frustrated phagosomes

Candida albicans hyphae can reach enormous lengths, precluding their internalization by phagocytes. Nevertheless, macrophages engulf a portion of the hypha, generating incompletely sealed tubular phagosomes. These frustrated phagosomes are stabilized by a thick cuff of F-actin that polymerizes in response to non-canonical activation of integrins by fungal glycan. Despite their continuity, the surface and invaginating phagosomal membranes retain a strikingly distinct lipid composition. PtdIns(4,5)P₂ is present at the plasmalemma but is not detectable in the phagosomal membrane, while PtdIns(3)P and PtdIns(3,4,5)P₃ co-exist in the phagosomes yet are absent from the surface membrane. Moreover, endo-lysosomal proteins are present only in the phagosomal membrane. Fluorescence recovery after photobleaching revealed the presence of a diffusion barrier that maintains the identity of the open tubular phagosome separate from the plasmalemma. Formation of this barrier depends on Syk, Pyk2/Fak and formin-dependent actin assembly. Antimicrobial mechanisms can thereby be deployed, limiting the growth of the hyphae.

Keratins Regulate p38MAPK-Dependent Desmoglein Binding Properties in Pemphigus

Keratins are crucial for the anchorage of desmosomes. Severe alterations of keratin organization and detachment of filaments from the desmosomal plaque occur in the autoimmune dermatoses pemphigus vulgaris and pemphigus foliaceus (PF), which are mainly caused by autoantibodies against desmoglein (Dsg) 1 and 3. Keratin alterations are a structural hallmark in pemphigus pathogenesis and correlate with loss of intercellular adhesion. However, the significance for autoantibody-induced loss of intercellular adhesion is largely unknown. In wild-type (wt) murine keratinocytes, pemphigus autoantibodies induced keratin filament retraction. Under the same conditions, we used murine keratinocytes lacking all keratin filaments (KtyII k.o.) as a model system to dissect the role of keratins in pemphigus. KtyII k.o. cells show compromised intercellular adhesion without antibody (Ab) treatment, which was not impaired further by pathogenic pemphigus autoantibodies. Nevertheless, direct activation of p38MAPK via anisomycin further decreased intercellular adhesion indicating that cell cohesion was not completely abrogated in the absence of keratins. Direct inhibition of Dsg3, but not of Dsg1, interaction via pathogenic autoantibodies as revealed by atomic force microscopy was detectable in both cell lines demonstrating that keratins are not required for this phenomenon. However, PF-IgG shifted Dsg1-binding events from cell borders toward the free cell surface in wt cells. This led to a distribution pattern of Dsg1-binding events similar to KtyII k.o. cells under resting conditions. In keratin-deficient keratinocytes, PF-IgG impaired Dsg1-binding strength, which was not different from wt cells under resting conditions. In addition, pathogenic autoantibodies were capable of activating p38MAPK in both KtyII wt and k.o. cells, the latter of which already displayed robust p38MAPK activation under resting conditions. Since inhibition of p38MAPK blocked autoantibody-induced loss of intercellular adhesion in wt cells and restored baseline cell cohesion in keratin-deficient cells, we conclude that p38MAPK signaling is (i) critical for regulation of cell adhesion, (ii) regulated by keratins, and (iii) targets both keratin-dependent and -independent mechanisms.

Tuning Inner-Ear Tip-Link Affinity Through Alternatively Spliced Variants of Protocadherin-15

Human hearing relies upon the tip-to-tip interaction of two nonclassical cadherins, protocadherin-15 (PCDH15) and cadherin-23 (CDH23). Together, these proteins form a filament called the tip link that connects neighboring stereocilia of mechanosensitive hair cells. As sound waves enter the cochlea, the stereocilia deflect and tension is applied to the tip link, opening nearby transduction channels. Disruption of the tip link by loud sound or calcium chelators eliminates transduction currents and illustrates that tip-link integrity is critical for mechanosensing. Tip-link remodeling after disruption is a dynamic process, which can lead to the formation of atypical complexes that incorporate alternatively spliced variants of PCDH15. These variants are categorized into six groups (N1-N6) based upon differences in the first two extracellular cadherin (EC) repeats. Here, we characterized the two N-terminal EC repeats of all PCDH15 variants (pcdh15(N1) to pcdh15(N6)) and combined these variants to test complex formation. We solved the crystal structure of a new complex composed of CDH23 EC1-2 (cdh23) and pcdh15(N2) at 2.3 Å resolution and compared it to the canonical cdh23-pcdh15(N1) complex. While there were subtle structural differences, the binding affinity between cdh23 and pcdh15(N2) is ∼6 times weaker than cdh23 and pcdh15(N1) as determined by surface plasmon resonance analysis. Steered molecular dynamics simulations predict that the unbinding force of the cdh23-pcdh15(N2) complex can be lower than the canonical tip link. Our results demonstrate that alternative heterophilic tip-link structures form stable protein-protein interactions in vitro and suggest that homophilic PCDH15-PCDH15 tip links form through the interaction of additional EC repeats.

O-mannosylation and N-glycosylation: two coordinated mechanisms regulating the tumour suppressor functions of E-cadherin in cancer

Dysregulation of tumor suppressor protein E-cadherin is an early molecular event in cancer. O-mannosylation profile of E-cadherin is a newly-described post-translational modification crucial for its adhesive functions in homeostasis. However, the role of O-mannosyl glycans in E-cadherin-mediated cell adhesion in cancer and their interplay with N-glycans remains largely unknown. We herein demonstrated that human gastric carcinomas exhibiting a non-functional E-cadherin display a reduced expression of O-mannosyl glycans concomitantly with increased modification with branched complex N-glycans. Accordingly, overexpression of MGAT5-mediated branched N-glycans both in gastric cancer cells and transgenic mice models led to a significant decrease of O-mannosyl glycans attached to E-cadherin that was associated with impairment of its tumour suppressive functions. Importantly, overexpression of protein O-mannosyltransferase 2 (POMT2) induced a reduced expression of branched N-glycans which led to a protective effect of E-cadherin biological functions. Overall, our results reveal a newly identified mechanism of (dys)regulation of E-cadherin that occur through the interplay between O-mannosylation and N-glycosylation pathway.

Deep analysis of N-cadherin/ADH-1 interaction: a computational survey

Due to the considerable role of N-cadherin in cancer metastasis, tumor growth, and progression, inhibition of this protein has been highly regarded in recent years. Although ADH-1 has been known as an appropriate inhibitor of N-cadherin in clinical trials, its chemical nature and binding mode with N-cadherin have not been precisely specified yet. Accordingly, in this study, quantum mechanics calculations were used to investigate the chemical nature of ADH-1. These calculations clarify the molecular properties of ADH-1 and determine its reactive sites. Based on the results, the oxygen atoms are suitable for electrophilic reactivity, while the hydrogen atoms that are connected to nitrogen atoms are the favorite sites for nucleophilic reactivity. The higher electronegativity of the oxygen atoms makes them the most reactive portions in this molecule. Molecular docking and molecular dynamics (MD) simulation have also been applied to specify the binding mode of ADH-1 with N-cadherin and determine the important residues of N-cadherin involving in the interaction with ADH-1. Moreover, the verified model by MD simulation has been studied to extract the free energy value and find driving forces. These calculations and molecular electrostatic potential map of ADH-1 indicated that hydrophobic and electrostatic interactions are almost equally involved in the implantation of ADH-1 in the N-cadherin binding site. The presented results not only enable a closer examination of N-cadherin in complex with ADH-1 molecule, but also are very beneficial in designing new inhibitors for N-cadherin and can help to save time and cost in this field.

Using thermal scanning assays to test protein-protein interactions of inner-ear cadherins

Protein-protein interactions play a crucial role in biological processes such as cell-cell adhesion, immune system-pathogen interactions, and sensory perception. Understanding the structural determinants of protein-protein complex formation and obtaining quantitative estimates of their dissociation constant (KD) are essential for the study of these interactions and for the discovery of new therapeutics. At the same time, it is equally important to characterize protein-protein interactions in a high-throughput fashion. Here, we use a modified thermal scanning assay to test interactions of wild type (WT) and mutant variants of N-terminal fragments (EC1+2) of cadherin-23 and protocadherin-15, two proteins essential for inner-ear mechanotransduction. An environmentally sensitive fluorescent dye (SYPRO orange) is used to monitor melting temperature (Tm) shifts of protocadherin-15 EC1+2 (pcdh15) in the presence of increasing concentrations of cadherin-23 EC1+2 (cdh23). These Tm shifts are absent when we use proteins containing deafness-related missense mutations known to disrupt cdh23 binding to pcdh15, and are increased for some rationally designed mutants expected to enhance binding. In addition, surface plasmon resonance binding experiments were used to test if the Tm shifts correlated with changes in binding affinity. We used this approach to find a double mutation (cdh23(T15E)- pcdh15(G16D)) that enhances binding affinity of the cadherin complex by 1.98 kJ/mol, roughly two-fold that of the WT complex. We suggest that the thermal scanning methodology can be used in high-throughput format to quickly compare binding affinities (KD from nM up to 100 μM) for some heterodimeric protein complexes and to screen small molecule libraries to find protein-protein interaction inhibitors and enhancers.

The role of cadherin-11 in microcystin-LR-induced migration and invasion in colorectal carcinoma cells

The present study aimed to explore whether microcystin-LR (MC-LR; a well-known cyanobacterial toxin produced in eutrophic lakes or reservoirs) induced tumor progression by activating cadherin-11(CDH11). A previous tumor metastasis PCR array demonstrated that MC-LR exposure resulted in a significant increase in the expression of CDH11. In the present study, to confirm the effect of the MC-LR treatment on CDH11 expression, HT-29 cell migration and invasion following MC-LR treatment were tested by Transwell assays, and protein levels of CDH11 were tested by immunofluorescence and western blot analysis. The results demonstrated that MC-LR activated CDH11 expression in addition to cell migration and invasion in HT-29 cells. To further investigate the association between MC-LR-induced CDH11 upregulation, and higher motility and invasiveness in HT-29 cells, knockdown of CDH11 using small interfering RNA (siRNA) in HT-29 cells was performed. Subsequent Transwell assays confirmed that MC-LR-induced enhancement of migration and invasion was significantly decreased following CDH11 knockdown by CDH11-siRNA in HT-29 cells. The results from the present study indicate that MC-LR may act as a CDH11 activator to promote HT-29 cell migration and invasion.

The role of ubiquitination and deubiquitination in the regulation of cell junctions

Maintenance of cell junctions plays a crucial role in the regulation of cellular functions including cell proliferation, permeability, and cell death. Disruption of cell junctions is implicated in a variety of human disorders, such as inflammatory diseases and cancers. Understanding molecular regulation of cell junctions is important for development of therapeutic strategies for intervention of human diseases. Ubiquitination is an important type of post-translational modification that primarily regulates endogenous protein stability, receptor internalization, enzyme activity, and protein-protein interactions. Ubiquitination is tightly regulated by ubiquitin E3 ligases and can be reversed by deubiquitinating enzymes. Recent studies have been focusing on investigating the effect of protein stability in the regulation of cell-cell junctions. Ubiquitination and degradation of cadherins, claudins, and their interacting proteins are implicated in epithelial and endothelial barrier disruption. Recent studies have revealed that ubiquitination is involved in regulation of Rho GTPases’ biological activities. Taken together these studies, ubiquitination plays a critical role in modulating cell junctions and motility. In this review, we will discuss the effects of ubiquitination and deubiquitination on protein stability and expression of key proteins in the cell-cell junctions, including junction proteins, their interacting proteins, and small Rho GTPases. We provide an overview of protein stability in modulation of epithelial and endothelial barrier integrity and introduce potential future search directions to better understand the effects of ubiquitination on human disorders caused by dysfunction of cell junctions.

Arrhythmogenic cardiomyopathy related DSG2 mutations affect desmosomal cadherin binding kinetics

Cadherins are calcium dependent adhesion proteins that establish the intercellular mechanical contact by bridging the gap to adjacent cells. Desmoglein-2 (Dsg2) is a specific cadherin of the cell-cell contact in cardiac desmosomes. Mutations in the DSG2-gene are regarded to cause arrhythmogenic (right ventricular) cardiomyopathy (ARVC) which is a rare but severe heart muscle disease. The molecular pathomechanisms of the vast majority of DSG2 mutations, however, are unknown. Here, we investigated the homophilic binding of wildtype Dsg2 and two mutations which are associated with ARVC. Using single molecule force spectroscopy and applying Jarzynski’s equality we determined the kinetics and thermodynamics of Dsg2 homophilic binding. Notably, the free energy landscape of Dsg2 dimerization exposes a high activation barrier which is in line with the proposed strand-swapping binding motif. Although the binding motif is not directly affected by the mutations the binding kinetics differ significantly from the wildtype. Furthermore, we applied a dispase based cell dissociation assay using HT1080 cell lines over expressing Dsg2 wildtype and mutants, respectively. Our molecular and cellular results consistently demonstrate that Dsg2 mutations can heavily affect homophilic Dsg2 interactions. Furthermore, the full thermodynamic and kinetic description of Dsg2 dimerization provides a consistent model of the so far discussed homophilic cadherin binding.

miR-9a modulates maintenance and ageing of Drosophila germline stem cells by limiting N-cadherin expression

Ageing is characterized by a decline in stem cell functionality leading to dampened tissue regeneration. While the expression of microRNAs across multiple species is markedly altered with age, the mechanism by which they govern stem cell-sustained tissue regeneration is unknown. We report that in the Drosophila testis, the conserved miR-9a is expressed in germline stem cells and its levels are significantly elevated during ageing. Transcriptome and functional analyses show that miR-9a directly regulates the expression of the adhesion molecule N-cadherin (N-cad). miR-9a null mutants maintain a higher number of stem cells even in the aged tissue. Remarkably, this rise fails to improve tissue regeneration and results in reduced male fertility. Similarly, overexpression of N-cad also results in elevated stem cell number and decreased regeneration. We propose that miR-9a downregulates N-cad to enable adequate detachment of stem cells toward differentiation, thus providing the necessary directionality toward terminal differentiation and spermatogenesis.In the Drosophila testis, ageing leads to loss of germline stem cells. Here, the authors show that, during ageing in Drosophila, miR-9a is upregulated in male germline stem cells and regulates their proliferation by targeting N-cadherin.

Keratins Regulate the Adhesive Properties of Desmosomal Cadherins through Signaling

Tightly controlled intercellular adhesion is crucial for the integrity and function of the epidermis. The keratin filament cytoskeleton anchors desmosomes, supramolecular complexes required for strong intercellular adhesion. We tested whether keratin filaments control cell adhesion by regulating the adhesive properties of desmosomal cadherins such as desmoglein (Dsg) 3. Atomic force microscopy and fluorescence recovery after photobleaching experiments showed reduced Dsg3 adhesive forces and membrane stability in murine keratinocytes lacking all keratin filaments. Impairment of the actin cytoskeleton also resulted in decreased Dsg3 immobilization but did not affect Dsg3 binding properties, indicating that the latter are exclusively controlled by keratins. Reduced binding forces were dependent on p38 mitogen-activated protein kinase activity, which was deregulated in keratin-deficient cells. In contrast, inhibition of protein kinase C signaling, which is known to be controlled by keratins, promoted and spatially stabilized Dsg3-mediated interactions in the membrane. These results show a previously unreported mechanism for how keratins stabilize intercellular adhesion on the level of single desmosomal adhesion molecules.

The mechanical properties of individual cell spheroids

The overall physical properties of tissues emerge in a complex manner from the properties of the component cells and other constituent materials from which the tissue is formed, across multiple length scales ranging from nanometres to millimetres. Recent studies have suggested that interfacial tension between cells contributes significantly to the mechanical properties of tissues and that the overall surface tension is determined by the ratio of adhesion tension to cortical tension. Using cavitation rheology (CR), we have measured the interfacial properties and the elastic modulus of spheroids formed from HEK cells. By comparing the work of bubble formation with deformation of the cell spheroid at different length scales, we have estimated the cortical tension for HEK cells. This innovative approach to understanding the fundamental physical properties associated with tissue mechanics may guide new approaches for the generation of materials to replace or regenerate damaged or diseased tissues.

Sporadic Early-Onset Diffuse Gastric Cancers Have High Frequency of Somatic CDH1 Alterations, but Low Frequency of Somatic RHOA Mutations Compared With Late-Onset Cancers

BACKGROUND & AIMS

Early-onset gastric cancer, which develops in patients younger than most gastric cancers, is usually detected at advanced stages, has diffuse histologic features, and occurs more frequently in women. We investigated somatic genomic alterations associated with the unique characteristics of sporadic diffuse gastric cancers (DGCs) from younger patients.

METHODS

We conducted whole exome and RNA sequence analyses of 80 resected DGC samples from patients 45 years old or younger in Korea. Patients with pathogenic germline mutations in CDH1, TP53, and ATM were excluded from the onset of this analysis, given our focus on somatic alterations. We used MutSig2CV to evaluate the significance of mutated genes. We recruited 29 additional early-onset Korean DGC samples and performed SNP6.0 array and targeted sequencing analyses of these 109 early-onset DGC samples (54.1% female, median age, 38 years). We compared the SNP6.0 array and targeted sequencing data of the 109 early-onset DGC samples with those from diffuse-type stomach tumor samples collected from 115 patients in Korea who were 46 years or older (late onset) at the time of diagnosis (controls; 29.6% female, median age, 67 years). We compared patient survival times among tumors from different subgroups and with different somatic mutations. We performed gene silencing of RHOA or CDH1 in DGC cells with small interfering RNAs for cell-based assays.

RESULTS

We identified somatic mutations in the following genes in a significant number of early-onset DGCs: the cadherin 1 gene (CDH1), TP53, ARID1A, KRAS, PIK3CA, ERBB3, TGFBR1, FBXW7, RHOA, and MAP2K1. None of 109 early-onset DGC cases had pathogenic germline CDH1 mutations. A higher proportion of early-onset DGCs had mutations in CDH1 (42.2%) or TGFBR1 (7.3%) compared with control DGCs (17.4% and 0.9%, respectively) (P < .001 and P = .014 for CDH1 and TGFBR1, respectively). In contrast, a smaller proportion of early-onset DGCs contained mutations in RHOA (9.2%) than control DGCs (19.1%) (P = .033). Late-onset DGCs in The Cancer Genome Atlas also contained less frequent mutations in CDH1 and TGFBR1 and more frequent RHOA mutations, compared with early-onset DGCs. Early-onset DGCs from women contained significantly more mutations in CDH1 or TGFBR1 than early-onset DGCs from men. CDH1 alterations, but not RHOA mutations, were associated with shorter survival times in patients with early-onset DGCs (hazard ratio, 3.4; 95% confidence interval, 1.5-7.7). RHOA activity was reduced by an R5W substitution-the RHOA mutation most frequently detected in early-onset DGCs. Silencing of CDH1, but not RHOA, increased migratory activity of DGC cells.

CONCLUSIONS

In an integrative genomic analysis, we found higher proportions of early-onset DGCs to contain somatic mutations in CDH1 or TGFBR1 compared with late-onset DGCs. However, a smaller proportion of early-onset DGCs contained somatic mutations in RHOA than late-onset DGCs. CDH1 alterations, but not RHOA mutations, were associated with shorter survival times of patients, which might account for the aggressive clinical course of early-onset gastric cancer. Female predominance in early-onset gastric cancer may be related to relatively high rates of somatic CDH1 and TGFBR1 mutations in this population.

Structural origins of clustered protocadherin-mediated neuronal barcoding

Clustered protocadherins mediate neuronal self-recognition and non-self discrimination-neuronal "barcoding"-which underpin neuronal self-avoidance in vertebrate neurons. Recent structural, biophysical, computational, and cell-based studies on protocadherin structure and function have led to a compelling molecular model for the barcoding mechanism. Protocadherin isoforms assemble into promiscuous cis-dimeric recognition units and mediate cell-cell recognition through homophilic trans-interactions. Each recognition unit is composed of two arms extending from the membrane proximal EC6 domains. A cis-dimeric recognition unit with each arm coding adhesive trans homophilic specificity can generate a zipper-like assembly that in turn suggests a chain termination mechanism for self-vs-non-self-discrimination among vertebrate neurons.

Sustained α-catenin Activation at E-cadherin Junctions in the Absence of Mechanical Force

Mechanotransduction at E-cadherin junctions has been postulated to be mediated in part by a force-dependent conformational activation of α-catenin. Activation of α-catenin allows it to interact with vinculin in addition to F-actin, resulting in a strengthening of junctions. Here, using E-cadherin adhesions reconstituted on synthetic, nanopatterned membranes, we show that activation of α-catenin is dependent on E-cadherin clustering, and is sustained in the absence of mechanical force or association with F-actin or vinculin. Adhesions were formed by filopodia-mediated nucleation and micron-scale assembly of E-cadherin clusters, which could be distinguished as either peripheral or central assemblies depending on their relative location at the cell-bilayer adhesion. Whereas F-actin, vinculin, and phosphorylated myosin light chain associated only with the peripheral assemblies, activated α-catenin was present in both peripheral and central assemblies, and persisted in the central assemblies in the absence of actomyosin tension. Impeding filopodia-mediated nucleation and micron-scale assembly of E-cadherin adhesion complexes by confining the movement of bilayer-bound E-cadherin on nanopatterned substrates reduced the levels of activated α-catenin. Taken together, these results indicate that although the initial activation of α-catenin requires micron-scale clustering that may allow the development of mechanical forces, sustained force is not required for maintaining α-catenin in the active state.

Effect of monocytes/macrophages on the osteogenic differentiation of adipose-derived mesenchymal stromal cells in 3D co-culture spheroids

This study aimed to investigate the distinctive roles of the monocytes and macrophages on osteogenic differentiation of adipose-derived mesenchymal stromal cells (ADMSCs) in 3D spheroid co-cultures. We hypothesized that monocytes or macrophages (subtypes pro-inflammatory M1 and pro-wound healing M2) would affect the osteogenic differentiation of ADMSCs in 3D spheroids and that cell-cell interactions between monocytes/macrophages and ADMSCs play an important role in the osteogenic differentiation process of ADMSCs. The obtained results indicated that the osteogenic differentiation of ADMSCs was inhibited by monocytes and both macrophage subtypes in 3D spheroids. Monocytes and M2 macrophages had a stronger inhibiting effect than M1 macrophages. Cell-cell interactions mediated by N-cadherin likely played a role in the inhibiting effect of monocytes/macrophages on the osteogenic differentiation of ADMSCs.

Screening of potentially crucial genes and regulatory factors involved in epithelial ovarian cancer using microarray analysis

The present study aimed to screen potential genes implicated in epithelial ovarian cancer (EOC) and to further understand the molecular pathogenesis of EOC. In order to do this, datasets GSE14407 (containing 12 human ovarian cancer epithelia samples and 12 normal epithelia samples) and GSE29220 (containing 11 salivary transcriptomes from ovarian cancer patients with serous papillary adenocarcinoma and 11 matched controls) were obtained from the Gene Expression Omnibus. Differentially expressed genes (DEGs) within these datasets were screened using the Linear Models for Microarray Data package, and potential gene functions were predicted by functional and pathway enrichment analyses. Additionally, module analysis of protein-protein interaction networks was performed using MCODE software in Cytoscape. The potential microRNAs (miRNAs/miRs) and transcription factors (TFs) regulating DEGs were also analyzed, and the integrated TF-DEG and miRNA-DEG regulatory networks were visualized with Cytoscape. In total, 31 upregulated DEGs and 64 downregulated DEGs were screened. The upregulated DEGs, such as centromere protein F (CENPF) and ubiquitin like with PHD and ring finger domains 1 (UHRF1), were significantly associated with the cell cycle and were regulated by the TF nuclear transcription factor Y (NF-Y). CENPF was modulated by miR-373, and UHRF1 was regulated by miR-146a. The downregulated DEGs, such as aldehyde dehydrogenase 1 family member A2 (ALDH1A2), were distinctly involved in the response to estrogen stimulus and modulated by tumor protein 53 (TP53); protocadherin 9 (PCDH9) was regulated by TP53, miR-92b-3p and miR-137. The DEGs, including CENPF, UHRF1, ALDH1A2 and PCDH9, and a set of gene regulators, including all NFY genes, TP53, miR-373, miR-146a, miR-92b-3p and miR-137, may be involved in the pathogenesis of EOC.

Mammalian O-mannosylation of cadherins and plexins is independent of protein O-mannosyltransferases 1 and 2

Protein O-mannosylation is found in yeast and metazoans, and a family of conserved orthologous protein O-mannosyltransferases is believed to initiate this important post-translational modification. We recently discovered that the cadherin superfamily carries O-linked mannose (O-Man) glycans at highly conserved residues in specific extracellular cadherin domains, and it was suggested that the function of E-cadherin was dependent on the O-Man glycans. Deficiencies in enzymes catalyzing O-Man biosynthesis, including the two human protein O-mannosyltransferases, POMT1 and POMT2, underlie a subgroup of congenital muscular dystrophies designated α-dystroglycanopathies, because deficient O-Man glycosylation of α-dystroglycan disrupts laminin interaction with α-dystroglycan and the extracellular matrix. To explore the functions of O-Man glycans on cadherins and protocadherins, we used a combinatorial gene-editing strategy in multiple cell lines to evaluate the role of the two POMTs initiating O-Man glycosylation and the major enzyme elongating O-Man glycans, the protein O-mannose β-1,2-N-acetylglucosaminyltransferase, POMGnT1. Surprisingly, O-mannosylation of cadherins and protocadherins does not require POMT1 and/or POMT2 in contrast to α-dystroglycan, and moreover, the O-Man glycans on cadherins are not elongated. Thus, the classical and evolutionarily conserved POMT O-mannosylation pathway is essentially dedicated to α-dystroglycan and a few other proteins, whereas a novel O-mannosylation process in mammalian cells is predicted to serve the large cadherin superfamily and other proteins.

Blepharocheilodontic syndrome is a CDH1 pathway-related disorder due to mutations in CDH1 and CTNND1

PURPOSE

Blepharocheilodontic (BCD) syndrome is a rare autosomal dominant condition characterized by eyelid malformations, cleft lip/palate, and ectodermal dysplasia. The molecular basis of BCD syndrome remains unknown.

METHODS

We recruited 11 patients from 8 families and performed exome sequencing for 5 families with de novo BCD syndrome cases and targeted Sanger sequencing in the 3 remaining families.

RESULTS

We identified five CDH1 heterozygous missense mutations and three CTNND1 heterozygous truncating mutations leading to loss-of-function or haploinsufficiency. Establishment of detailed genotype-phenotype correlations was not possible because of the size of the cohort; however, the phenotype seems to appear more severe in case of CDH1 mutations. Functional analysis of CDH1 mutations confirmed their deleterious impact and suggested accelerated E-cadherin degradation.

CONCLUSION

Mutations in CDH1 encoding the E-cadherin were previously reported in hereditary diffuse gastric cancer as well as in nonsyndromic cleft lip/palate. Mutations in CTNND1 have never been reported before. The encoded protein, p120ctn, prevents E-cadherin endocytosis and stabilizes its localization at the cell surface. Conditional deletion of Cdh1 and Ctnnd1 in various animal models induces features reminiscent of BCD syndrome and underlines critical role of the E-cadherin-p120ctn interaction in eyelid, craniofacial, and tooth development. Our data assert BCD syndrome as a CDH1 pathway-related disorder due to mutations in CDH1 and CTNND1 and widen the phenotypic spectrum of E-cadherin anomalies.Genet Med advance online publication 09 March 2017.

Small Molecule-Induced Domain Swapping as a Mechanism for Controlling Protein Function and Assembly

Domain swapping is the process by which identical proteins exchange reciprocal segments to generate dimers. Here we introduce induced domain swapping (INDOS) as a mechanism for regulating protein function. INDOS employs a modular design consisting of the fusion of two proteins: a recognition protein that binds a triggering molecule, and a target protein that undergoes a domain swap in response to binding of the triggering ligand. The recognition protein (FK506 binding protein) is inserted into functionally-inactivated point mutants of two target proteins (staphylococcal nuclease and ribose binding protein). Binding of FK506 to the FKBP domain causes the target domain to first unfold, then refold via domain swap. The inactivating mutations become ‘swapped out’ in the dimer, increasing nuclease and ribose binding activities by 100-fold and 15-fold, respectively, restoring them to near wild-type values. INDOS is intended to convert an arbitrary protein into a functional switch, and is the first example of rational design in which a small molecule is used to trigger protein domain swapping and subsequent activation of biological function.

Biophysical basis of cadherin mediated cell-cell adhesion

Classical cadherin transmembrane cell-cell adhesion proteins play essential roles in tissue morphogenesis and in mediating tissue integrity. Cadherin ectodomains from opposing cells interact to form load-bearing trans dimers that mechanically couple cells. Cell-cell adhesion is believed to be strengthened by cis clustering of cadherins on the same cell surface. This review summarizes biophysical studies of the structure, interaction kinetics and biomechanics of classical cadherin ectodomains. We first discuss the structure and equilibrium binding kinetics of classical cadherin trans and cis dimers. We then discuss how mechanical stimuli alters the kinetics of cadherin interaction and tunes adhesion. Finally, we highlight open questions on the role of mechanical forces in influencing cadherin structure, function and organization on the cell surface.

A Partial Calcium-Free Linker Confers Flexibility to Inner-Ear Protocadherin-15

Tip links of the inner ear are protein filaments essential for hearing and balance. Two atypical cadherins, cadherin-23 and protocadherin-15, interact in a Ca²⁺-dependent manner to form tip links. The largely unknown structure and mechanics of these proteins are integral to understanding how tip links pull on ion channels to initiate sensory perception. Protocadherin-15 has 11 extracellular cadherin (EC) repeats. Its EC3-4 linker lacks several of the canonical Ca²⁺-binding residues, and contains an aspartate-to-alanine polymorphism (D414A) under positive selection in East Asian populations. We present structures of protocadherin-15 EC3-5 featuring two Ca²⁺-binding linker regions: canonical EC4-5 linker binding three Ca²⁺ ions, and non-canonical EC3-4 linker binding only two Ca²⁺ ions. Our structures and biochemical assays reveal little difference between the D414 and D414A variants. Simulations predict that the partial Ca²⁺-free EC3-4 linker exhibits increased flexural flexibility without compromised mechanical strength, providing insight into the dynamics of tip links and other atypical cadherins.

Early events in the assembly of E-cadherin adhesions

E-cadherin is a calcium dependent cell adhesion molecule that is key to the organization of cells in the epithelial tissue. It is a multidomain, trans-membrane protein in which the extracellular domain forms the homotypic, adhesive interaction while the intracellular domain interacts with the actin cytoskeleton through the catenin family of adaptor proteins. A number of recent studies have provided novel insights into the mechanism of adhesion formation by this class of adhesion proteins. Here, we describe an updated view of the process of E-cadherin adhesion formation with an emphasis on the role of molecular mobility, clustering, and active cellular processes.

N-Cadherin, a novel and rapidly remodelling site involved in vasoregulation of small cerebral arteries

KEY POINTS

N-cadherin formed punctate adherens junctions (AJ) along the borders between vascular smooth muscle cells (VSMCs) in the pressurized rat superior cerebellar artery. The formation of N-cadherin AJs in the vessel wall depends on the intraluminal pressure and was responsive to treatment with phenylephrine (PE) (10^-5  m) and ACh (10^-5  m). N-cadherin-coated beads were able to induce clustering of N-cadherin-enhanced green fluorescent protein (EGFP) on the plasma membrane of isolated VSMCs, whereas treatment with PE (10^-5  m) or sodium nitroprusside (10^-5  m) induced a significant increase or decrease in the N-cadherin-EGFP clustering, respectively. Application of pulling force (∼1 nN) to the N-cadherin-coated beads via an atomic force microscope induced a localized mechanical response from the VSMCs that opposed the pulling.

ABSTRACT

N-cadherin is the major cell-cell adhesion molecule in vascular smooth muscle cells (VSMCs). We tested the hypothesis that N-cadherin is part of a novel mechanosensory mechanism in VSMCs and plays an active role in both the arteriolar myogenic response and during changes in vascular tone induced by vasomotor agonists. Intact and pressurized rat superior cerebellar arteries were labelled for confocal immunofluorescence imaging. N-cadherin formed punctate adherens junctions (AJ) along the borders between VSMCs. When the lumen pressure was raised from 50 to 90 mmHg, both the density and the average size of N-cadherin AJs increased significantly. Similarly, arteriolar constriction with phenylephrine (PE) (10^-5  m) induced a significant increase of N-cadherin AJ density at 50 mmHg, whereas vasodilatation induced by ACh (10^-5  m) was accompanied by a significant decrease in density and size of N-cadherin AJs. An atomic force microscope (AFM) was employed to further examine the mechano-responsive properties of N-cadherin adhesion sites in isolated VSMCs. AFM probes with an attached N-cadherin-coated microbead (5 μm) induced a progressive clustering of N-cadherin-enhanced green fluorescent protein (EGFP) on the VSMC surface. Application of pulling force (∼1 nN) to the N-cadherin-coated-beads with the AFM induced a localized mechanical response from the VSMCs that opposed the pulling. Treatment with PE (10^-5  m) or sodium nitroprusside (10^-5  m) induced a significant increase or decrease of the N-cadherin-EGFP clustering, respectively. These observations provide compelling evidence that N-cadherin AJs are sensitive to pressure and vasomotor agonists in VSMCs and support a functional role of N-cadherin AJs in vasomotor regulation.

Snail2 and Zeb2 repress P-cadherin to define embryonic territories in the chick embryo

Snail and Zeb transcription factors induce epithelial-to-mesenchymal transition (EMT) in embryonic and adult tissues by direct repression of E-cadherin transcription. The repression of E-cadherin transcription by the EMT inducers Snail1 and Zeb2 plays a fundamental role in defining embryonic territories in the mouse, as E-cadherin needs to be downregulated in the primitive streak and in the epiblast, concomitant with the formation of mesendodermal precursors and the neural plate, respectively. Here, we show that in the chick embryo, E-cadherin is weakly expressed in the epiblast at pre-primitive streak stages where it is substituted for by P-cadherin We also show that Snail2 and Zeb2 repress P-cadherin transcription in the primitive streak and the neural plate, respectively. This indicates that E- and P-cadherin expression patterns evolved differently between chick and mouse. As such, the Snail1/E-cadherin axis described in the early mouse embryo corresponds to Snail2/P-cadherin in the chick, but both Snail factors and Zeb2 fulfil a similar role in chick and mouse in directly repressing ectodermal cadherin genes to contribute to the delamination of mesendodermal precursors at gastrulation and the proper specification of the neural ectoderm during neural induction.

Disruption of cell adhesion by an antibody targeting the cell-adhesive intermediate (X-dimer) of human P-cadherin

Human P-cadherin is a cell adhesion protein of the family of classical cadherins, the overexpression of which is correlated with poor prognosis in various types of cancer. Antibodies inhibiting cell-cell adhesion mediated by P-cadherin show clear therapeutic effect, although the mechanistic basis explaining their effectiveness is still unclear. Based on structural, physicochemical, and functional analyses, we have elucidated the molecular mechanism of disruption of cell adhesion by antibodies targeting human P-cadherin. Herein we have studied three different antibodies, TSP5, TSP7, and TSP11, each recognizing a different epitope on the surface of the cell-adhesive domain (EC1). Although all these three antibodies recognized human P-cadherin with high affinity, only TSP7 disrupted cell adhesion. Notably, we demonstrated that TSP7 abolishes cell adhesion by disabling the so-called X-dimer (a kinetic adhesive intermediate), in addition to disrupting the strand-swap dimer (the final thermodynamic state). The inhibition of the X-dimer was crucial for the overall inhibitory effect, raising the therapeutic value of a kinetic intermediary not only for preventing, but also for reversing, cell adhesion mediated by a member of the classical cadherin family. These findings should help to design more innovative and effective therapeutic solutions targeting human P-cadherin.

An elastic element in the protocadherin-15 tip link of the inner ear

Tip link filaments convey force and gate inner-ear hair-cell transduction channels to mediate perception of sound and head movements. Cadherin-23 and protocadherin-15 form tip links through a calcium-dependent interaction of their extracellular domains made of multiple extracellular cadherin (EC) repeats. These repeats are structurally similar, but not identical in sequence, often featuring linkers with conserved calcium-binding sites that confer mechanical strength to them. Here we present the X-ray crystal structures of human protocadherin-15 EC8-EC10 and mouse EC9-EC10, which show an EC8-9 canonical-like calcium-binding linker, and an EC9-10 calcium-free linker that alters the linear arrangement of EC repeats. Molecular dynamics simulations and small-angle X-ray scattering experiments support this non-linear conformation. Simulations also suggest that unbending of EC9-10 confers some elasticity to otherwise rigid tip links. The new structure provides a first view of protocadherin-15's non-canonical EC linkers and suggests how they may function in inner-ear mechanotransduction, with implications for other cadherins.

Structural determinants of adhesion by Protocadherin-19 and implications for its role in epilepsy

Non-clustered δ-protocadherins are homophilic cell adhesion molecules essential for the development of the vertebrate nervous system, as several are closely linked to neurodevelopmental disorders. Mutations in protocadherin-19 (PCDH19) result in a female-limited, infant-onset form of epilepsy (PCDH19-FE). Over 100 mutations in PCDH19 have been identified in patients with PCDH19-FE, about half of which are missense mutations in the adhesive extracellular domain. Neither the mechanism of homophilic adhesion by PCDH19, nor the biochemical effects of missense mutations are understood. Here we present a crystallographic structure of the minimal adhesive fragment of the zebrafish Pcdh19 extracellular domain. This structure reveals the adhesive interface for Pcdh19, which is broadly relevant to both non-clustered δ and clustered protocadherin subfamilies. In addition, we show that several PCDH19-FE missense mutations localize to the adhesive interface and abolish Pcdh19 adhesion in in vitro assays, thus revealing the biochemical basis of their pathogenic effects during brain development.

DOI:http://dx.doi.org/10.7554/eLife.18529.001

As the brain develops, its basic building blocks – cells called neurons – need to form the correct connections with one another in order to give rise to neural circuits. A mistake that leads to the formation of incorrect connections can result in a number of disorders or brain abnormalities.

Proteins called cadherins that are present on the surface of neurons enable them to stick to their correct partners like Velcro. One of these proteins is called Protocadherin-19. However, it was not fully understood how this protein forms an adhesive bond with other Protocadherin-19 molecules, or how some of the proteins within the cadherin family are able to distinguish between one another.

Cooper et al. used X-ray crystallography to visualize the molecular structure of Protocadherin-19 taken from zebrafish in order to better understand the adhesive bond that these proteins form with each other. In addition, the new structure showed the sites of the mutations that cause a form of epilepsy in infant females. From this, Cooper et al. could predict how the mutations would disrupt Protocadherin-19’s shape and function.

The structures revealed that Protocadherin-19 molecules from adjacent cells engage in a “forearm handshake” to form the bond that connects neurons. Some of the mutations that cause epilepsy occur in the region responsible for this Protocadherin-19 forearm handshake. Laboratory experiments confirmed that these mutations impair the formation of the adhesive bond, revealing the molecular basis for some of the mutations that underlie Protocadherin-19-female-limited epilepsy.

Other cadherin molecules may interact via a similar forearm handshake; this could be investigated in future experiments. It also remains to be discovered how brain wiring depends on Protocadherin-19 adhesion in animal development, and how altering these proteins can rewire developing brain circuits.

DOI:http://dx.doi.org/10.7554/eLife.18529.002