Beyond structure: mechanism and dynamics of intercellular adhesion

Sticking together: Harnessing cadherin biology for tissue engineering

Directing cell behavior and building a tissue for therapeutic impact is the main goal of regenerative medicine, for which scientists need to modulate the interaction of cells with biomaterials. The focus of the field thus far has been on the incorporation of cues from the extracellular matrix but we propose that scientists take lessons from cell-cell adhesion proteins, more specifically cadherin biology, as these proteins make multicellularity possible. In this perspective, we re-examine cadherins through the lens of a tissue engineer for the purpose of advancing regenerative medicine. Furthermore, we summarize exciting developments in biomaterials inspired by cadherins and discuss some challenges and opportunities for the future. STATEMENT OF SIGNIFICANCE: Tissue engineers need tools to direct cell behavior. To date, tissue engineers have designed many sophisticated materials to positively influence cell behavior but are faced with the challenge where these materials sometimes work and sometimes fail. This uncertainty is a big unanswered question that challenges the community. We propose that tissue engineering could be more successful if they would take lessons from cell-cell adhesion proteins, more specifically cadherin biology. In the article, we discuss key structural and functional characteristics that make cadherins ideal for tissue engineering approaches. Furthermore, by providing a state-of-the-art overview of exemplary studies that have used cadherins to influence cell behavior, we show tissue engineers that they already have the tools necessary to incorporate this knowledge.

Probing the recognition specificity of αVβ1 integrin and syndecan-4 using force spectroscopy

The knowledge on how cells interact with microenvironment is particularly important in understanding the interaction of cancer cells with surrounding stroma, which affects cell migration, adhesion, and metastasis. The main cell surface receptors responsible for the interaction with extracellular matrix (ECM) are integrins, however, they are not the only ones. Integrins are accompanied to other molecules such as syndecans. The role of the latter has not yet been fully established. In our study, we would like to answer the question of whether integrins and syndecans, possessing similar functions, share also similar unbinding properties. By using single molecule force spectroscopy (SMFS), we conducted measurements of the unbinding properties of α_Vβ₁ and syndecan-4 in the interaction with vitronectin (VN), which, as each ECM protein, possesses two binding sites specific to integrins and syndecans. The unbinding force and the kinetic off rate constant derived from SMFS describe the stability of single molecular complex. Obtained data show one barrier transition for each complex. The proposed model shows that the unbinding of α_Vβ₁ from VN proceeds before the unbinding of SDC-4. However, despite different unbinding kinetics, the access to both receptors is needed for cell growth and proliferation.

CXCL12/CXCR4 Axis Upregulates Twist to Induce EMT in Human Glioblastoma

In recent decades, the chemokine receptor CXCR4 and its ligand CXCL12 have been extensively reported to be associated with tumorigenesis. In addition, Twist signaling induces the epithelial-mesenchymal transition (EMT) process in glioblastoma development. In the present study, in vitro assays were used to investigate the role of CXCR4 and Twist in human glioblastoma. We explored the impact of CXCR4 and Twist on human glioblastoma using in vitro protein and gene assays. We found the administration of CXCL12 upregulated the expression of p-ERK, p-AKT, Twist, N-cadherin, and MMP9 in U87 cells, whereas the increase of E-cadherin protein was affected. Subsequently, Twist activity and EMT signaling were directly influenced by PD98059 and LY294002. Most importantly, the genetic silencing of Twist inhibited CXCL12-induced EMT occurrence, including proliferation, migration, and tumor formation of U87 cells. In conclusion, CXCL12/CXCR4 pathway activates ERK and PI3K/AKT signaling to upregulate Twist pathway, leading to the progression of EMT in human glioblastoma. Our study creates a new stage for molecule-targeted therapy of human glioblastoma.

Slug signaling is up-regulated by CCL21/CCR7 [corrected] to induce EMT in human chondrosarcoma

In recent decades, the CXC chemokine receptor 7 (CCR7) [corrected] and its ligand CCL21 have been extensively reported to be associated with tumorigenesis. Meanwhile, Slug signaling induces the epithelial-mesenchymal transition (EMT) process in chondrosarcoma development. In the present study, we explored the functions of CCL21/CCR7 [corrected] in Slug-mediated EMT in the chondrosarcoma. We analyzed protein expression of CCR7 [corrected] and Slug in human chondrosarcoma samples. Effects of CCR7 [corrected] on chondrosarcoma cells were assessed by in vitro assays. Additionally, CCR7 [corrected] pathways were further investigated by pharmacological and genetic approaches. We found that the altered CCR7 [corrected] (81.7 %) and Slug (85.0 %) expression in human chondrosarcoma tissues were significantly associated with grade, recurrence, and 5-year overall survival. According to in vitro assays, CCL21 stimulation induced the expression of phosph-ERK, phosph-AKT, Slug and N-cadherin in SW1353 cells, while the expression of E-cadherin was down-regulated. Furthermore, Slug signaling modulated E- to N-cadherin switch, which was influenced by the kinase inhibitor PD98059 and LY294002. In addition, the genetic silencing of Slug inhibited the capacity of migration and invasion of SW1353 cells. In conclusion, CCL21/CCR7 [corrected] pathway activates ERK and PI3K/AKT signallings to up-regulate Slug pathway, leading to the occurrence of EMT process in human chondrosarcoma. This study lays a new foundation for molecule-targeted therapy of human chondrosarcoma.

Cadherin recognition and adhesion

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

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Cadherin exits the junction by switching its adhesive bond

Intercellular traction forces or lateral alignment of cadherin molecules can influence adherens junction dynamics by altering the cadherin dimerization interface.

The plasticity of cell–cell adhesive structures is crucial to all normal and pathological morphogenetic processes. The molecular principles of this plasticity remain unknown. Here we study the roles of two dimerization interfaces, the so-called strand-swap and X dimer interfaces of E-cadherin, in the dynamic remodeling of adherens junctions using photoactivation, calcium switch, and coimmunoprecipitation assays. We show that the targeted inactivation of the X dimer interface blocks the turnover of catenin-uncoupled cadherin mutants in the junctions of A-431 cells. In contrast, the junctions formed by strand-swap dimer interface mutants exhibit high instability. Collectively, our data demonstrate that the strand-swap interaction is a principal cadherin adhesive bond that keeps cells in firm contact. However, to leave the adherens junction, cadherin reconfigures its adhesive bond from the strand swap to the X dimer type. Such a structural transition, controlled by intercellular traction forces or by lateral cadherin alignment, may be the key event regulating adherens junction dynamics.

Strong reversible Fe3+-mediated bridging between dopa-containing protein films in water

Metal-containing polymer networks are widespread in biology, particularly for load-bearing exoskeletal biomaterials. Mytilus byssal cuticle is an especially interesting case containing moderate levels of Fe(3+) and cuticle protein-mussel foot protein-1 (mfp-1), which has a peculiar combination of high hardness and high extensibility. Mfp-1, containing 13 mol % of dopa (3, 4-dihydroxyphenylalanine) side-chains, is highly positively charged polyelectrolyte (pI approximately 10) and didn't show any cohesive tendencies in previous surface forces apparatus (SFA) studies. Here, we show that Fe(3+) ions can mediate unusually strong interactions between the positively charged proteins. Using an SFA, Fe(3+) was observed to impart robust bridging (W(ad) approximately 4.3 mJ/m(2)) between two noninteracting mfp-1 films in aqueous buffer approaching the ionic strength of seawater. The Fe(3+) bridging between the mfp-1-coated surfaces is fully reversible in water, increasing with contact time and iron concentration up to 10 microM; at 100 microM, Fe(3+) bridging adhesion is abolished. Bridging is apparently due to the formation of multivalent dopa-iron complexes. Similar Fe-mediated bridging (W(ad) approximately 5.7 mJ/m(2)) by a smaller recombinant dopa-containing analogue indicates that bridging is largely independent of molecular weight and posttranslational modifications other than dopa. The results suggest that dopa-metal interactions may provide an energetic new paradigm for engineering strong, self-healing interactions between polymers under water.

Expression analysis of epithelial cadherin and related proteins in IBH-6 and IBH-4 human breast cancer cell lines

Epithelial cadherin (E-cadherin) is a 120 kDa cell-cell adhesion molecule involved in the establishment of epithelial adherens junctions. It is connected to the actin cytoskeleton by adaptor proteins such as beta-catenin. Loss of E-cadherin expression/function has been related to tumor progression and metastasis. Several molecules associated with down-regulation of E-cadherin have been described, within them neural cadherin, Twist and dysadherin. Human breast cancer cell lines IBH-6 and IBH-4 were developed from ductal primary tumors and show characteristic features of malignant epithelial cells. In this study expression of E-cadherin and related proteins in IBH-6 and IBH-4 cell lines was evaluated. In IBH-6 and IBH-4 cell extracts, only an 89 kDa E-cadherin form (Ecad89) was detected, which is truncated at the C-terminus and is present at low levels. Moreover, no accumulation of the 86 kDa E-cadherin ectodomain and of the 38 kDa CTF1 fragment was observed. IBH-6 and IBH-4 cells showed an intracellular scattered E-cadherin localization. beta-catenin accompanied E-cadherin localization, and actin stress fibers were identified in both cell types. E-cadherin mRNA levels were remarkably low in IBH-6 and IBH-4 cells. The E-cadherin mRNA and genomic sequence encoding exons 14-16 could not be amplified in either cell line. Neither the mRNA nor the protein of neural cadherin and dysadherin were detected. Up-regulation of Twist mRNA was found in both cell lines. In conclusion, IBH-6 and IBH-4 breast cancer cells show down-regulation of E-cadherin expression with aberrant protein localization, and up-regulation of Twist; these features can be related to their invasive/metastatic characteristics.

Design rules for biomolecular adhesion: lessons from force measurements

Cell adhesion to matrix, other cells, or pathogens plays a pivotal role in many processes in biomolecular engineering. Early macroscopic methods of quantifying adhesion led to the development of quantitative models of cell adhesion and migration. The more recent use of sensitive probes to quantify the forces that alter or manipulate adhesion proteins has revealed much greater functional diversity than was apparent from population average measurements of cell adhesion. This review highlights theoretical and experimental methods that identified force-dependent molecular properties that are central to the biological activity of adhesion proteins. Experimental and theoretical methods emphasized in this review include the surface force apparatus, atomic force microscopy, and vesicle-based probes. Specific examples given illustrate how these tools have revealed unique properties of adhesion proteins and their structural origins.

Interaction of fibrin(ogen) with the endothelial cell receptor VE-cadherin: localization of the fibrin-binding site within the third extracellular VE-cadherin domain

Interaction of fibrin with endothelial cells through their receptor VE-cadherin has been implicated in modulation of angiogenesis and inflammation. Previous studies identified the VE-cadherin-binding site in the fibrin betaN-domains formed by the NH(2)-terminal regions of fibrin beta chains and revealed that the recombinant dimeric (beta15-66)(2) fragment mimicking these domains preserves the VE-cadherin-binding properties of fibrin. To test if the other fibrin(ogen) regions/domains are involved in this interaction and localize the complementary fibrin-binding site in VE-cadherin, we prepared several recombinant fragments containing individual extracellular domains of VE-cadherin or combinations thereof, as well as several fragments corresponding to various fibrin(ogen) regions, and tested the interactions between them by ELISA and surface plasmon resonance. The experiments revealed that the betaN-domains are the only fibrin(ogen) regions involved in the interaction with VE-cadherin. They also localized the fibrin-binding site to the third extracellular domain of VE-cadherin and established that the fibrin-binding properties of this domain are not influenced by the presence or absence of the neighboring domains. In addition, the experiments confirmed that calcium ions, which are required to maintain proper conformation and adhesive properties of VE-cadherin, do not influence the fibrin-binding properties of the latter.

Platelet adhesion: a game of catch and release

The interaction of circulating platelets with the vessel wall involves a process of cell catch and release, regulating cell rolling, skipping, or firm adhesion and leading to thrombus formation in flowing blood. In this regard, the interaction of platelet glycoprotein Ibalpha (GPIbalpha) with its adhesive ligand, vWF, is activated by shear force and critical for platelet adhesion to the vessel wall. In this issue of the JCI, Yago and colleagues show how gain-of-function mutations in the GPIbalpha-binding vWF A1 domain disrupt intramolecular interactions within WT vWF A1 that regulate binding to GPIbalpha and flow-enhanced platelet rolling and adhesion (see the related article beginning on page 3195). Together, these studies reveal molecular mechanisms regulating GPIbalpha-vWF bond formation and platelet adhesion under shear stress.