Sequence and structural determinants of strand swapping in cadherin domains: do all cadherins bind through the same adhesive interface?

Arrhythmogenic cardiomyopathy-related cadherin variants affect desmosomal binding kinetics

Cadherins are calcium dependent adhesion proteins that establish and maintain the intercellular mechanical contact by bridging the gap between adjacent cells. Desmoglein-2 (Dsg2) and desmocollin-2 (Dsc2) are tissue specific cadherin isoforms of the cell-cell contact in cardiac desmosomes. Mutations in the DSG2-gene and in the DSC2-gene are related to arrhythmogenic right ventricular cardiomyopathy (ARVC) a rare but severe heart muscle disease. Here, several possible homophilic and heterophilic binding interactions of wild-type Dsg2, wild-type Dsc2, as well as one Dsg2- and two Dsc2-variants, each associated with ARVC, are investigated. Using single molecule force spectroscopy (SMFS) with atomic force microscopy (AFM) and applying Jarzynski's equality the kinetics and thermodynamics of Dsg2/Dsc2 interaction can be determined. The free energy landscape of Dsg2/Dsc2 dimerization exposes a high activation energy barrier, which is in line with the proposed strand-swapping binding motif. Although the binding motif is not affected by any of the mutations, the binding kinetics of the interactions differ significantly from the wild-type. While wild-type cadherins exhibit an average complex lifetime of approx. 0.3 s interactions involving a variant consistently show - lifetimes that are substantially larger. The lifetimes of the wild-type interactions give rise to the picture of a dynamic adhesion interface consisting of continuously dissociating and (re)associating molecular bonds, while the delayed binding kinetics of interactions involving an ARVC-associated variant might be part of the pathogenesis. Our data provide a comprehensive and consistent thermodynamic and kinetic description of cardiac cadherin binding, allowing detailed insight into the molecular mechanisms of cell adhesion.

Antiparallel dimer structure of CELSR cadherin in solution revealed by high-speed atomic force microscopy

Cadherin EGF LAG seven-pass G-type receptors (CELSR) cadherins, members of the cadherin superfamily, and adhesion G-protein-coupled receptors, play a vital role in cell-cell adhesion. The mutual binding of the extracellular domains (ectodomains) of CELSR cadherins between cells is crucial for tissue formation, including the establishment of planar cell polarity, which directs the proper patterning of cells. CELSR cadherins possess nine cadherin ectodomains (EC1-EC9) and noncadherin ectodomains. However, the structural and functional mechanisms of the binding mode of CELSR cadherins have not been determined. In this study, we investigated the binding mode of CELSR cadherins using single-molecule fluorescence microscopy, high-speed atomic force microscopy (HS-AFM), and bead aggregation assay. The fluorescence microscopy analysis results indicated that the trans-dimer of the CELSR cadherin constitutes the essential adhesive unit between cells. HS-AFM analysis and bead aggregation assay results demonstrated that EC1-EC8 entirely overlap and twist to form antiparallel dimer conformations and that the binding of EC1-EC4 is sufficient to sustain bead aggregation. The interaction mechanism of CELSR cadherin may elucidate the variation of the binding mechanism within the cadherin superfamily and physiological role of CELSR cadherins in relation to planar cell polarity.

The tip link protein Cadherin-23: From Hearing Loss to Cancer

Cadherin-23 is an atypical member of the cadherin superfamily, with a distinctly long extracellular domain. It has been known to be a part of the tip links of the inner ear mechanosensory hair cells. Several studies have been carried out to understand the role of Cadherin-23 in the hearing mechanism and defects in the CDH23 have been associated with hearing impairment resulting from defective or absence of tip links. Recent studies have highlighted the role of Cadherin-23 in several pathological conditions, including cancer, suggesting the presence of several unknown functions. Initially, it was proposed that Cadherin-23 represents a yet unspecified subtype of Cadherins; however, no other proteins with similar characteristics have been identified, till date. It has a unique cytoplasmic domain that does not bear a β-catenin binding region, but has been demonstrated to mediate cell-cell adhesions. Several protein interacting partners have been identified for Cadherin-23 and the roles of their interactions in various cellular mechanisms are yet to be explored. This review summarizes the characteristics of Cadherin-23 and its roles in several pathologies including cancer.

Electrostatic Interactions in Protein Structure, Folding, Binding, and Condensation

Charged and polar groups, through forming ion pairs, hydrogen bonds, and other less specific electrostatic interactions, impart important properties to proteins. Modulation of the charges on the amino acids, e.g., by pH and by phosphorylation and dephosphorylation, have significant effects such as protein denaturation and switch-like response of signal transduction networks. This review aims to present a unifying theme among the various effects of protein charges and polar groups. Simple models will be used to illustrate basic ideas about electrostatic interactions in proteins, and these ideas in turn will be used to elucidate the roles of electrostatic interactions in protein structure, folding, binding, condensation, and related biological functions. In particular, we will examine how charged side chains are spatially distributed in various types of proteins and how electrostatic interactions affect thermodynamic and kinetic properties of proteins. Our hope is to capture both important historical developments and recent experimental and theoretical advances in quantifying electrostatic contributions of proteins.

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.

Concentration dependent survival and neural differentiation of murine embryonic stem cells cultured on polyethylene glycol dimethacrylate hydrogels possessing a continuous concentration gradient of n-cadherin derived peptide His-Ala-Val-Asp-Lle

N-cadherin cell-cell signaling plays a key role in the structure and function of the nervous system. However, few studies have incorporated bioactive signaling from n-cadherin into tissue engineering matrices. The present study uses a continuous gradient approach in polyethylene glycol dimethacrylate hydrogels to identify concentration dependent effects of n-cadherin peptide, His-Ala-Val-Asp-Lle (HAVDI), on murine embryonic stem cell survival and neural differentiation. The n-cadherin peptide was found to affect the expression of pluripotency marker, alkaline phosphatase, in murine embryonic stem cells cultured on n-cadherin peptide containing hydrogels in a concentration dependent manner. Increasing n-cadherin peptide concentrations in the hydrogels elicited a biphasic response in neurite extension length and mRNA expression of neural differentiation marker, neuron-specific class III β-tubulin, in murine embryonic stem cells cultured on the hydrogels. High concentrations of n-cadherin peptide in the hydrogels were found to increase the expression of apoptotic marker, caspase 3/7, in murine embryonic stem cells compared to that of murine embryonic stem cell cultures on hydrogels containing lower concentrations of n-cadherin peptide. Increasing the n-cadherin peptide concentration in the hydrogels facilitated greater survival of murine embryonic stem cells exposed to increasing oxidative stress caused by hydrogen peroxide exposure. The combinatorial approach presented in this work demonstrates concentration dependent effects of n-cadherin signaling on mouse embryonic stem cell behavior, underscoring the need for the greater use of systematic approaches in tissue engineering matrix design in order to understand and optimize bioactive signaling in the matrix for tissue formation.

STATEMENT OF SIGNIFICANCE

Single cell encapsulation is common in tissue engineering matrices. This eliminates cellular access to cell-cell signaling. N-cadherin, a cell-cell signaling molecule, plays a vital role in the development of neural tissues, but has not been well studied as a bioactive signaling element in neural tissue engineering matrices. The present study uses a systematic continuous gradient approach to identify concentration dependent effects of n-cadherin derived peptide, HAVDI, on the survival and neural differentiation of murine embryonic stem cells. This work underscores the need for greater use to combinatorial strategies to understand the effect complex bioactive signaling, such as n-cadherin, and the need to optimize the concentration of such bioactive signaling within tissue engineering matrices for maximal cellular response.

A Computational Model for Kinetic Studies of Cadherin Binding and Clustering

Cadherin is a cell-surface transmembrane receptor that mediates calcium-dependent cell-cell adhesion and is a major component of adhesive junctions. The formation of intercellular adhesive junctions is initiated by trans binding between cadherins on adjacent cells, which is followed by the clustering of cadherins via the formation of cis interactions between cadherins on the same cell membranes. Moreover, classical cadherins have multiple glycosylation sites along their extracellular regions. It was found that aberrant glycosylation affects the adhesive function of cadherins and correlates with metastatic phenotypes of several cancers. However, a mechanistic understanding of cadherin clustering during cell adhesion and the role of glycosylation in this process is still lacking. Here, we designed a kinetic model that includes multistep reaction pathways for cadherin clustering. We further applied a diffusion-reaction algorithm to numerically simulate the clustering process using a recently developed coarse-grained model. Using experimentally measured rates of trans binding between soluble E-cadherin extracellular domains, we conducted simulations of cadherin-mediated cell-cell binding kinetics, and the results are quantitatively comparable to experimental data from micropipette experiments. In addition, we show that incorporating cadherin clustering via cis interactions further increases intercellular binding. Interestingly, a two-phase kinetic profile was derived under the assumption that glycosylation regulates the kinetic rates of cis interactions. This two-phase profile is qualitatively consistent with experimental results from micropipette measurements. Therefore, our computational studies provide new, to our knowledge, insights into the molecular mechanism of cadherin-based cell adhesion.

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.

Homophilic interaction and deformation of E-cadherin and cadherin 7 probed by single molecule force spectroscopy

Cadherin-mediated adhesion plays a crucial role in multicellular organisms. Dysfunction within this adhesion system has major consequences in many pathologies, including cancer invasion and metastasis. However, mechanisms controlling cadherin recognition and adhesive strengthening are only partially understood. Here, we investigated the homophilic interactions and mechanical stability of the extracellular (EC) domains of E-cadherin and cadherin 7 using atomic force microscopy and magnetic tweezers. Besides exhibiting stronger interactions, E-cadherin also showed more efficient force-induced self-strengthening of interactions than cadherin 7. In addition, the distributions of the unbinding forces for both cadherins partially overlap with those of the unfolding forces, indicating that partial unfolding/deformation of the cadherin EC domains may take place during their homophilic interactions. These conformational changes may be involved in cadherins physiology function and contribute to the significant differences in adhesive strength mediated by type I and type II cadherins.

Molecular logic of neuronal self-recognition through protocadherin domain interactions

Self-avoidance, a process preventing interactions of axons and dendrites from the same neuron during development, is mediated in vertebrates through the stochastic single-neuron expression of clustered protocadherin protein isoforms. Extracellular cadherin (EC) domains mediate isoform-specific homophilic binding between cells, conferring cell recognition through a poorly understood mechanism. Here, we report crystal structures for the EC1-EC3 domain regions from four protocadherin isoforms representing the α, β, and γ subfamilies. All are rod shaped and monomeric in solution. Biophysical measurements, cell aggregation assays, and computational docking reveal that trans binding between cells depends on the EC1-EC4 domains, which interact in an antiparallel orientation. We also show that the EC6 domains are required for the formation of cis-dimers. Overall, our results are consistent with a model in which protocadherin cis-dimers engage in a head-to-tail interaction between EC1-EC4 domains from apposed cell surfaces, possibly forming a zipper-like protein assembly, and thus providing a size-dependent self-recognition mechanism.

Cadherin flexibility provides a key difference between desmosomes and adherens junctions

Desmosomes and adherens junctions are intercellular adhesive structures essential for the development and integrity of vertebrate tissue, including the epidermis and heart. Their cell adhesion molecules are cadherins: type 1 cadherins in adherens junctions and desmosomal cadherins in desmosomes. A fundamental difference is that desmosomes have a highly ordered structure in their extracellular region and exhibit calcium-independent hyperadhesion, whereas adherens junctions appear to lack such ordered arrays, and their adhesion is always calcium-dependent. We present here the structure of the entire ectodomain of desmosomal cadherin desmoglein 2 (Dsg2), using a combination of small-angle X-ray scattering, electron microscopy, and solution-based biophysical techniques. This structure reveals that the ectodomain of Dsg2 is flexible even in the calcium-bound state and, on average, is shorter than the type 1 cadherin crystal structures. The Dsg2 structure has an excellent fit with the electron tomography reconstructions of human desmosomes. This fit suggests an arrangement in which desmosomal cadherins form trans interactions but are too far apart to interact in cis, in agreement with previously reported observations. Cadherin flexibility may be key to explaining the plasticity of desmosomes that maintain tissue integrity in their hyperadhesive form, but can adopt a weaker, calcium-dependent adhesion during wound healing and early development.

Single-cell identity generated by combinatorial homophilic interactions between α, β, and γ protocadherins

Individual mammalian neurons stochastically express distinct repertoires of α, β, and γ protocadherin (Pcdh) proteins, which function in neural circuit assembly. We report that all three subfamilies of clustered Pcdhs can engage in specific homophilic interactions, that cell surface delivery of Pcdhα isoforms requires cis interactions with other Pcdhs, and that the extracellular cadherin domain EC6 plays a critical role in this process. Examination of homophilic interactions between specific combinations of multiple Pcdh isoforms revealed that Pcdh combinatorial recognition specificities depend on the identity of all of the expressed isoforms. A single mismatched Pcdh isoform can interfere with these combinatorial homophilic interactions. A theoretical analysis reveals that assembly of Pcdh isoforms into multimeric recognition units and the observed tolerance for mismatched isoforms can generate cell surface diversity sufficient for single-cell identity. However, the competing demands of nonself discrimination and self-recognition place limitations on the mechanisms by which homophilic recognition units can function.

Basic residue at position 14 is not required for fast assembly and disassembly kinetics in neural cadherin

In spite of their structural similarities, epithelial (E-) and neural (N-) cadherin are expressed at different types of synapses and differ significantly in their dimerization kinetics. Recent studies proposed a transient intermediate in E-cadherin as the key requirement for rapid disassembly kinetics of the adhesive dimer. This E-cadherin intermediate comprises four intermolecular ionic and H-bonding interactions between adhesive partners. These interactions are not preserved in N-cadherin except for a basic residue at the 14th position, which could stabilize the intermediate through either H-bonding or ionic interactions with the partner protomer. To investigate the origin of the rapid dimerization kinetics of N-cadherin in the presence of calcium, studies reported here systematically test the role of ionic and H-bonding interactions in dimerization kinetics using R14S, R14A, and R14E mutants of N-cadherin. Analytical size-exclusion chromatographic and bead aggregation studies showed two primary results. First, N-cadherin/R14S and N-cadherin/R14A mutants showed fast assembly and disassembly kinetics in the calcium-saturated state similar to that of wild-type N-cadherin. These results indicate that the fast disassembly of the calcium-saturated dimer of N-cadherin does not require a basic residue at the 14th position. Second, the dimerization kinetics of N-cadherin/R14E were slow in the calcium-saturated state, indicating that negative charge destabilizes the intermediate state. Taken together, these results indicate that the basic residue at the 14th position does not promote rapid dimerization kinetics but that an acidic amino acid in that position significantly impairs dimerization kinetics.

Conservation weighting functions enable covariance analyses to detect functionally important amino acids

The explosive growth in the number of protein sequences gives rise to the possibility of using the natural variation in sequences of homologous proteins to find residues that control different protein phenotypes. Because in many cases different phenotypes are each controlled by a group of residues, the mutations that separate one version of a phenotype from another will be correlated. Here we incorporate biological knowledge about protein phenotypes and their variability in the sequence alignment of interest into algorithms that detect correlated mutations, improving their ability to detect the residues that control those phenotypes. We demonstrate the power of this approach using simulations and recent experimental data. Applying these principles to the protein families encoded by Dscam and Protocadherin allows us to make testable predictions about the residues that dictate the specificity of molecular interactions.

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.

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.

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.

Origin of metazoan cadherin diversity and the antiquity of the classical cadherin/β-catenin complex

The evolution of cadherins, which are essential for metazoan multicellularity and restricted to metazoans and their closest relatives, has special relevance for understanding metazoan origins. To reconstruct the ancestry and evolution of cadherin gene families, we analyzed the genomes of the choanoflagellate Salpingoeca rosetta, the unicellular outgroup of choanoflagellates and metazoans Capsaspora owczarzaki, and a draft genome assembly from the homoscleromorph sponge Oscarella carmela. Our finding of a cadherin gene in C. owczarzaki reveals that cadherins predate the divergence of the C. owczarzaki, choanoflagellate, and metazoan lineages. Data from these analyses also suggest that the last common ancestor of metazoans and choanoflagellates contained representatives of at least three cadherin families, lefftyrin, coherin, and hedgling. Additionally, we find that an O. carmela classical cadherin has predicted structural features that, in bilaterian classical cadherins, facilitate binding to the cytoplasmic protein β-catenin and, thereby, promote cadherin-mediated cell adhesion. In contrast with premetazoan cadherin families (i.e., those conserved between choanoflagellates and metazoans), the later appearance of classical cadherins coincides with metazoan origins.

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.

Spontaneous dimerization of titin protein Z1Z2 domains induces strong nanomechanical anchoring

Muscle elasticity strongly relies on the mechanical anchoring of the giant protein titin to both the sarcomere M-band and the Z-disk. Such strong attachment ensures the reversible dynamics of the stretching-relaxing cycles determining the muscle passive elasticity. Similarly, the design of biomaterials with enhanced elastic function requires experimental strategies able to secure the constituent molecules to avoid mechanical failure. Here we show that an engineered titin-mimicking protein is able to spontaneously dimerize in solution. Our observations reveal that the titin Z1Z2 domains are key to induce dimerization over a long-range distance in proteins that would otherwise remain in their monomeric form. Using single molecule force spectroscopy, we measure the threshold force that triggers the noncovalent transition from protein dimer to monomer, occurring at ∼700 piconewtons. Such extremely high mechanical stability is likely to be a natural protective mechanism that guarantees muscle integrity. We propose a simple molecular model to understand the force-induced dimer-to-monomer transition based on the geometric distribution of forces occurring within a dimeric protein under mechanical tension.

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.

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.

Cross talk between focal adhesion kinase and cadherins: role in regulating endothelial barrier function

A layer of endothelial cells attached to their underlying matrices by complex transmembrane structures termed focal adhesion (FA) proteins maintains the barrier property of microvascular endothelium. FAs sense the physical properties of the extracellular matrix (ECM) and organize the cytoskeleton accordingly. The close association of adherens junction (AJ) protein, cadherin, with the cytoskeleton is known to be essential in coordinating the appropriate mechanical properties to cell-cell contacts. Recently, it has become clear that a crosstalk exists between focal adhesion kinase (FAK) and cadherin that regulates signaling at intercellular endothelial junctions. This review discusses recent advances in our understanding of the dynamic regulation of the molecular connections between FAK and the cadherin complex and cadherin-catenin-actin interaction-dependent changes as well as the role of small GTPases in endothelial barrier regulation. This review also discusses how a signaling network regulates a range of cellular processes important for barrier function and diseases.

Prolines in βA-sheet of neural cadherin act as a switch to control the dynamics of the equilibrium between monomer and dimer

Neural cadherins dimerize through the formation of calcium-dependent strand-crossover structures. Dimerization of cadherins leads to cell-cell adhesion in multicellular organisms. Strand-crossover dimer forms exclusively between the first N-terminal extracellular modules (EC1) of the adhesive partners via swapping of their βA-sheets and docking of tryptophan-2 in the hydrophobic pocket. In the apo-state wild-type cadherin is predominantly monomer, which indicates that the dimerization is energetically unfavorable in the absence of calcium. Addition of calcium favors dimer formation by creating strain in the monomer and lowering the energetic barrier between monomer and dimer. Dynamics of the monomer-dimer equilibrium is vital for plasticity of synapses. Prolines recurrently occur in proteins that form strand-crossover dimer and are believed to be the source of the strain in the monomer. N-cadherins have two proline residues in the βA-sheet. We focused our studies on the role of these two prolines in calcium-dependent dimerization. Spectroscopic, electrophoretic, and chromatopgraphic studies showed that mutations of both prolines to alanines increased the dimerization affinity by ~20-fold and relieved the requirement of calcium in dimerization. The P5A and P6A mutant formed very stable dimers that required denaturation of protein to disassemble in the apo conditions. In summary, the proline residues act as a switch to control the dynamics of the equilibrium between monomer and dimer which is crucial for the plasticity of synapses.

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

Adherens junctions, which play a central role in intercellular adhesion, comprise clusters of type I classical cadherins that bind via extracellular domains extended from opposing cell surfaces. We show that a molecular layer seen in crystal structures of E- and N-cadherin ectodomains reported here and in a previous C-cadherin structure corresponds to the extracellular architecture of adherens junctions. In all three ectodomain crystals, cadherins dimerize through a trans adhesive interface and are connected by a second, cis, interface. Assemblies formed by E-cadherin ectodomains coated on liposomes also appear to adopt this structure. Fluorescent imaging of junctions formed from wild-type and mutant E-cadherins in cultured cells confirm conclusions derived from structural evidence. Mutations that interfere with the trans interface ablate adhesion, whereas cis interface mutations disrupt stable junction formation. Our observations are consistent with a model for junction assembly involving strong trans and weak cis interactions localized in the ectodomain.

Molecular design principles underlying β-strand swapping in the adhesive dimerization of cadherins

Cell adhesion by classical cadherins is mediated by dimerization of their EC1 domains through the “swapping” of N-terminal β-strands. We use molecular simulations, measurements of binding affinities, and x-ray crystallography to provide a detailed picture of the structural and energetic factors that control the adhesive dimerization of cadherins. We show that strand swapping in EC1 is driven by conformational strain in cadherin monomers which arises from the anchoring of their short N-terminal strand at one end by the conserved Trp2 and at the other by ligation to Ca²⁺ ions. We also demonstrate that a conserved pro-pro motif functions to avoid the formation of an overly tight interface where affinity differences between different cadherins, crucial at the cellular level, are lost. We use these findings to design site-directed mutations which transform a monomeric EC2-EC3 domain cadherin construct, into a strand-swapped dimer.

Structure and binding mechanism of vascular endothelial cadherin: a divergent classical cadherin

Vascular endothelial cadherin (VE-cadherin), a divergent member of the type II classical cadherin family of cell adhesion proteins, mediates homophilic adhesion in the vascular endothelium. Previous investigations with a bacterially produced protein suggested that VE-cadherin forms cell surface trimers that bind between apposed cells to form hexamers. Here we report studies of mammalian-produced VE-cadherin ectodomains suggesting that, like other classical cadherins, VE-cadherin forms adhesive trans dimers between monomers located on opposing cell surfaces. Trimerization of the bacterially produced protein appears to be an artifact that arises from a lack of glycosylation. We also present the 2.1-Å-resolution crystal structure of the VE-cadherin EC1-2 adhesive region, which reveals homodimerization via the strand-swap mechanism common to classical cadherins. In common with type II cadherins, strand-swap binding involves two tryptophan anchor residues, but the adhesive interface resembles type I cadherins in that VE-cadherin does not form a large nonswapped hydrophobic surface. Thus, VE-cadherin is an outlier among classical cadherins, with characteristics of both type I and type II subfamilies.

Dimeric states of neural- and epithelial-cadherins are distinguished by the rate of disassembly

Epithelial- and neural-cadherins are specifically localized at synapses in neurons which can change the shape and contact surface on a time scale of seconds to months. We have focused our studies on the role of the extracellular domains of cadherins in the dynamics of synapses. The kinetics of dimer disassembly of the first two extracellular domains of E- and N-cadherin, ECAD12 and NCAD12, were studied with analytical size exclusion chromatography and sedimentation velocity. NCAD12 forms three different dimers that are distinguished by assembly conditions and kinetics of dissociation. ECAD12 dimer disassembles rapidly regardless of the calcium concentration, whereas the disassembly of NCAD12 dimers was strongly dependent on calcium concentration. In addition to the apo- and saturated-dimeric forms of NCAD12, there is a third dimeric form that is a slow exchange dimer. This third dimeric form for NCAD12, formed by decalcification of the calcium-saturated dimer, was kinetically trapped in apo-conditions and did not disassemble over a period of months. Sedimentation velocity experiments showed that this dimer, upon addition of calcium, had similar weighted averages as a calcium-saturated dimer. These studies provide evidence that the kinetics of dimer disassembly of the extracellular domains may be a major contributor to the morphological dynamics of synapses in vivo.

The biology of the desmosome-like junction a versatile anchoring junction and signal transducer in the seminiferous epithelium

Mammalian spermatogenesis, a complex process that involves the movement of developing germ cells across the seminiferous epithelium, entails extensive restructuring of Sertoli-Sertoli and Sertoli-germ cell junctions. Presently, it is not entirely clear how zygotene spermatocytes gain entry into the adluminal compartment of the seminiferous epithelium, which is sealed off from the systemic circulation by the Sertoli cell component of the blood-testis barrier, without compromising barrier integrity. To begin to address this question, it is critical that we first have a good understanding of the biology and the regulation of different types of Sertoli-Sertoli and Sertoli-germ cell junctions in the testis. Supported by recent studies in the field, we discuss how crosstalk between different types of junctions contributes to their restructuring during germ cell movement across the blood-testis barrier. We place special emphasis on the emerging role of desmosome-like junctions as signal transducers during germ cell movement across the seminiferous epithelium.

Structure and biochemistry of cadherins and catenins

Classical cadherins mediate specific adhesion at intercellular adherens junctions. Interactions between cadherin ectodomains from apposed cells mediate cell-cell contact, whereas the intracellular region functionally links cadherins to the underlying cytoskeleton. Structural, biophysical, and biochemical studies have provided important insights into the mechanism and specificity of cell-cell adhesion by classical cadherins and their interplay with the cytoskeleton. Adhesive binding arises through exchange of beta strands between the first extracellular cadherin domains (EC1) of partner cadherins from adjacent cells. This "strand-swap" binding mode is common to classical and desmosomal cadherins, but sequence alignments suggest that other cadherins will bind differently. The intracellular region of classical cadherins binds to p120 and beta-catenin, and beta-catenin binds to the F-actin binding protein alpha-catenin. Rather than stably bridging beta-catenin to actin, it appears that alpha-catenin actively regulates the actin cytoskeleton at cadherin-based cell-cell contacts.

Cooperativity between trans and cis interactions in cadherin-mediated junction formation

Intercellullar junctions formed by cadherins, including desmosomes and adherens junctions, comprise two dimensional arrays of "trans" dimers formed between monomers emanating from opposing cell surfaces. Lateral "cis" interfaces between cadherins from the same cell surface have been proposed to play a role in cadherin clustering. Although the molecular details of cis interactions remain uncertain, they must define an anisotropic arrangement where binding is favorable only in certain orientations. Here we report Monte Carlo simulations performed on a 2D lattice constructed to account for the anisotropy in cadherin cis interactions. A crucial finding is that the "phase transition" between freely diffusing cadherin monomers and dimers and a condensed ordered 2D junction formed by dimers alone is a cooperative process involving both trans and cis interactions. Moreover, cis interactions, despite being too weak to be measured in solution, are critical to the formation of an ordered junction structure. We discuss these results in light of available experimental information on cadherin binding free energies that are transformed from their bulk solution values to interaction energies on a 2D lattice.

Structural determinants of cadherin-23 function in hearing and deafness

The hair-cell tip link, a fine filament directly conveying force to mechanosensitive transduction channels, is composed of two proteins, protocadherin-15 and cadherin-23, whose mutation causes deafness. However, their molecular structure, elasticity, and deafness-related structural defects are unknown. We present crystal structures of the first and second extracellular cadherin repeats of cadherin-23. Overall, structures show typical cadherin folds, but reveal an elongated N terminus that precludes classical cadherin interactions and contributes to an N-terminal Ca(2+)-binding site. The deafness mutation D101G, in the linker region between the repeats, causes a slight bend between repeats and decreases Ca(2+) affinity. Molecular dynamics simulations suggest that cadherin-23 repeats are stiff and that either removing Ca(2+) or mutating Ca(2+)-binding residues reduces rigidity and unfolding strength. The structures define an uncharacterized cadherin family and, with simulations, suggest mechanisms underlying inherited deafness and how cadherin-23 may bind with itself and with protocadherin-15 to form the tip link.

Two-step adhesive binding by classical cadherins

Crystal structures of classical cadherins have revealed two dimeric configurations. In the first, N-terminal beta-strands of EC1 domains 'swap' between partner molecules. The second configuration (the 'X dimer'), also observed for T-cadherin, is mediated by residues near the EC1-EC2 calcium binding sites, and N-terminal beta-strands of partner EC1 domains, though held adjacent, do not swap. Here we show that strand-swapping mutants of type I and II classical cadherins form X dimers. Mutant cadherins impaired for X-dimer formation show no binding in short-time frame surface plasmon resonance assays, but in long-time frame experiments, they have homophilic binding affinities close to that of wild type. Further experiments show that exchange between monomers and dimers is slowed in these mutants. These results reconcile apparently disparate results from prior structural studies and suggest that X dimers are binding intermediates that facilitate the formation of strand-swapped dimers.

T-cadherin structures reveal a novel adhesive binding mechanism

Vertebrate genomes encode 19 classical cadherins and about 100 nonclassical cadherins. Adhesion by classical cadherins depends on binding interactions in their N-terminal EC1 domains, which swap N-terminal beta-strands between partner molecules from apposing cells. However, strand-swapping sequence signatures are absent from nonclassical cadherins, raising the question of how these proteins function in adhesion. Here, we show that T-cadherin, a glycosylphosphatidylinositol (GPI)-anchored cadherin, forms dimers through an alternative nonswapped interface near the EC1-EC2 calcium-binding sites. Mutations within this interface ablate the adhesive capacity of T-cadherin. These nonadhesive T-cadherin mutants also lose the ability to regulate neurite outgrowth from T-cadherin-expressing neurons. Our findings reveal the likely molecular architecture of the T-cadherin homophilic interface and its requirement for axon outgrowth regulation. The adhesive binding mode used by T-cadherin may also be used by other nonclassical cadherins.

Desmocollin 3-mediated binding is crucial for keratinocyte cohesion and is impaired in pemphigus

Desmocollin (Dsc) 1-3 and desmoglein (Dsg) 1-4, transmembrane proteins of the cadherin family, form the adhesive core of desmosomes. Here we provide evidence that Dsc3 homo- and heterophilic trans-interaction is crucial for epidermal integrity. Single molecule atomic force microscopy (AFM) revealed homophilic trans-interaction of Dsc3. Dsc3 displayed heterophilic interaction with Dsg1 but not with Dsg3. A monoclonal antibody targeted against the extracellular domain reduced homophilic and heterophilic binding as measured by AFM, caused intraepidermal blistering in a model of human skin, and a loss of intercellular adhesion in cultured keratinocytes. Because autoantibodies against Dsg1 are associated with skin blistering in pemphigus, we characterized the role of Dsc3 binding for pemphigus pathogenesis. In contrast to AFM experiments, laser tweezer trapping revealed that pemphigus autoantibodies reduced binding of Dsc3-coated beads to the keratinocyte cell surface. These data indicate that loss of heterophilic Dsc3/Dsg1 binding may contribute to pemphigus skin blistering.

Linking molecular affinity and cellular specificity in cadherin-mediated adhesion

Many cell-cell adhesive events are mediated by the dimerization of cadherin proteins presented on apposing cell surfaces. Cadherin-mediated processes play a central role in the sorting of cells into separate tissues in vivo, but in vitro assays aimed at mimicking this behavior have yielded inconclusive results. In some cases, cells that express different cadherins exhibit homotypic cell sorting, forming separate cell aggregates, whereas in other cases, intermixed aggregates are formed. A third pattern is observed for mixtures of cells expressing either N- or E-cadherin, which form distinct homotypic aggregates that adhere to one another through a heterotypic interface. The molecular basis of cadherin-mediated cell patterning phenomena is poorly understood, in part because the relationship between cellular adhesive specificity and intermolecular binding free energies has not been established. To clarify this issue, we have measured the dimerization affinities of N-cadherin and E-cadherin. These proteins are similar in sequence and structure, yet are able to mediate homotypic cell patterning behavior in a variety of tissues. N-cadherin is found to form homodimers with higher affinity than does E-cadherin and, unexpectedly, the N/E-cadherin heterophilic binding affinity is intermediate in strength between the 2 homophilic affinities. We can account for observed cell aggregation behaviors by using a theoretical framework that establishes a connection between molecular affinities and cell-cell adhesive specificity. Our results illustrate how graded differences between different homophilic and heterophilic cadherin dimerizaton affinities can result in homotypic cell patterning and, more generally, show how proteins that are closely related can, nevertheless, be responsible for highly specific cellular adhesive behavior.

Peptides Targeting the Desmoglein 3 Adhesive Interface Prevent Autoantibody-induced Acantholysis in Pemphigus

Pemphigus vulgaris (PV) autoantibodies directly inhibit desmoglein (Dsg) 3-mediated transinteraction. Because cellular signaling also seems to be required for PV pathogenesis, it is important to characterize the role of direct inhibition in pemphigus acantholysis to allow establishment of new therapeutic approaches. Therefore, we modeled the Dsg1 and Dsg3 sequences into resolved cadherin structures and predicted peptides targeting the adhesive interface of both Dsg3 and Dsg1. In atomic force microscopy single molecule experiments, the self-designed cyclic single peptide specifically blocked homophilic Dsg3 and Dsg1 transinteraction, whereas a tandem peptide (TP) consisting of two combined single peptides did not. TP did not directly block binding of pemphigus IgG to their target Dsg antigens but prevented PV-IgG-induced inhibition of Dsg3 transinteraction in cell-free (atomic force microscopy) and cell-based (laser tweezer) experiments, indicating stabilization of Dsg3 bonds. Similarly, PV-IgG-mediated acantholysis and disruption of Dsg3 localization in HaCaT keratinocytes was partially blocked by TP. This is the first evidence that direct inhibition of Dsg3 binding is important for PV pathogenesis and that peptidomimetics stabilizing Dsg transinteraction may provide a novel approach for PV treatment.

Dynamic properties of a type II cadherin adhesive domain: implications for the mechanism of strand-swapping of classical cadherins

Cadherin-mediated cell adhesion is achieved through dimerization of cadherin N-terminal extracellular (EC1) domains presented from apposed cells. The dimer state is formed by exchange of N-terminal beta strands and insertion of conserved tryptophan indole side chains from one monomer into hydrophobic acceptor pockets of the partner molecule. The present work characterizes individual monomer and dimer states and the monomer-dimer equilibrium of the mouse Type II cadherin-8 EC1 domain using NMR spectroscopy. Limited picosecond-to-nanosecond timescale dynamics of the tryptophan indole moieties for both monomer and dimer states are consistent with well-ordered packing of the N-terminal beta strands intramolecularly and intermolecularly, respectively. However, pronounced microsecond-to-millisecond timescale dynamics of the side chains are observed for the monomer but not the dimer state, suggesting that monomers transiently sample configurations in which the indole moieties are exposed. The results suggest possible kinetic mechanisms for EC1 dimerization.

Built-in loops allow versatility in domain-domain interactions: lessons from self-interacting domains

Compilations of domain-domain interactions based on solved structures suggest there are distinct domain pairs that are used repeatedly in different protein contexts to mediate protein-protein interactions. However, not all protein pairs with the corresponding domains that can potentially mediate interaction do interact, even when they are colocalized and coexpressed. It is conceivable that there are structural and sequence features, below the domain level, that play a role in determining the potential of domains to mediate protein-protein interactions. Here, we discover such features by comparing domains that, on the one hand, mediate homodimerization of proteins and, on the other, occur in different proteins that are documented as monomers. Intriguingly, this comparison uncovered surface loops that can be considered as determinants of the interactions. There are enabling loops, which mediate the domain interactions, and disabling loops that prevent the interactions. The presence of the enabling/disabling loops is consistent with the fulfillment/prevention of the interaction and is highly preserved in evolution. This suggests that, along with the preservation of structural elements that enable interaction, evolution maintains elements intended to prevent unwanted interactions. The enabling and disabling loops discovered in this study have implications in prediction of protein-protein interactions, by pointing to the protein regions that determine the interaction. Our results extend the hierarchy of attributes that collectively establish the modularity of domain-mediated protein-protein interactions.