Coordinated expression of desmoglein 1 and desmocollin 1 regulates intercellular adhesion

Melanoma cells repress Desmoglein 1 in keratinocytes to promote tumor cell migration

Melanoma is an aggressive cancer typically arising from transformation of melanocytes residing in the basal layer of the epidermis, where they are in direct contact with surrounding keratinocytes. The role of keratinocytes in shaping the melanoma tumor microenvironment remains understudied. We previously showed that temporary loss of the keratinocyte-specific cadherin, Desmoglein 1 (Dsg1), controls paracrine signaling between normal melanocytes and keratinocytes to stimulate the protective tanning response. Here, we provide evidence that melanoma cells hijack this intercellular communication by secreting factors that keep Dsg1 expression low in the surrounding keratinocytes, which in turn generate their own paracrine signals that enhance melanoma spread through CXCL1/CXCR2 signaling. Evidence suggests a model whereby paracrine signaling from melanoma cells increases levels of the transcriptional repressor Slug, and consequently decreases expression of the Dsg1 transcriptional activator Grhl1. Together, these data support the idea that paracrine crosstalk between melanoma cells and keratinocytes resulting in chronic keratinocyte Dsg1 reduction contributes to melanoma cell movement associated with tumor progression.

Desmocollin-1 is associated with pro-metastatic phenotype of luminal A breast cancer cells and is modulated by parthenolide

BACKGROUND

Desmocollin-1 (DSC1) is a desmosomal transmembrane glycoprotein that maintains cell-to-cell adhesion. DSC1 was previously associated with lymph node metastasis of luminal A breast tumors and was found to increase migration and invasion of MCF7 cells in vitro. Therefore, we focused on DSC1 role in cellular and molecular mechanisms in luminal A breast cancer and its possible therapeutic modulation.

METHODS

Western blotting was used to select potential inhibitor decreasing DSC1 protein level in MCF7 cell line. Using atomic force microscopy we evaluated effect of DSC1 overexpression and modulation on cell morphology. The LC-MS/MS analysis of total proteome on Orbitrap Lumos and RNA-Seq analysis of total transcriptome on Illumina NextSeq 500 were performed to study the molecular mechanisms associated with DSC1. Pull-down analysis with LC-MS/MS detection was carried out to uncover DSC1 protein interactome in MCF7 cells.

RESULTS

Analysis of DSC1 protein levels in response to selected inhibitors displays significant DSC1 downregulation (p-value ≤ 0.01) in MCF7 cells treated with NF-κB inhibitor parthenolide. Analysis of mechanic cell properties in response to DSC1 overexpression and parthenolide treatment using atomic force microscopy reveals that DSC1 overexpression reduces height of MCF7 cells and conversely, parthenolide decreases cell stiffness of MCF7 cells overexpressing DSC1. The LC-MS/MS total proteome analysis in data-independent acquisition mode shows a strong connection between DSC1 overexpression and increased levels of proteins LACRT and IGFBP5, increased expression of IGFBP5 is confirmed by RNA-Seq. Pathway analysis of proteomics data uncovers enrichment of proliferative MCM_BIOCARTA pathway including CDK2 and MCM2-7 after DSC1 overexpression. Parthenolide decreases expression of LACRT, IGFBP5 and MCM_BIOCARTA pathway specifically in DSC1 overexpressing cells. Pull-down assay identifies DSC1 interactions with cadherin family proteins including DSG2, CDH1, CDH3 and tyrosine kinase receptors HER2 and HER3; parthenolide modulates DSC1-HER3 interaction.

CONCLUSIONS

Our systems biology data indicate that DSC1 is connected to mechanisms of cell cycle regulation in luminal A breast cancer cells, and can be effectively modulated by parthenolide.

Significance of anti-desmocollin autoantibodies in pemphigus

The major autoantigens for pemphigus are desmogleins (Dsgs), cell-cell adhesive structure proteins, one of the desmosomal cadherins. Recent progress in molecular biology has revealed that IgG autoantibodies of classical pemphigus react with Dsg1 or Dsg3. Desmocollins (Dscs) also belong to the cadherin supergene family that provides structure to the desmosomes and play an important role in cell-to-cell adhesion. In addition to the presence of four desmosomal Dsg isoforms, i.e. Dsg1-4, Dsc1, 2 and 3, all of which are derived from different genes, Dsc1 has been previously identified as the target antigen of IgA autoantibodies in the subcorneal pustular dermatosis (SPD)-type of intercellular IgA dermatosis. In addition to the IgA anti-Dsc1 autoantiboides, the presence of IgG anti-Dsc autoantibodies is described in patients of some autoimmune bullous diseases. In particular, the current pemphigus detecting autoantibodies to Dscs has shown a tendency in atypical variants of pemphigus. Therefore, autoantibodies against Dscs alone may cause detachment of cell-cell adhesion in the epidermis in some pemphigus. However, except for the findings of a few in vitro and in vivo studies, there is currently no clear evidence for the pathogenicity of anti-Dsc autoantibodies in pemphigus, whereas significance of anti-Dsg autoantibodies is well established. This article describes the structure and function of the Dscs, and explores the evidence regarding the pathogenic role of anti-Dsc autoantibodies in pemphigus.

Elastic versus brittle mechanical responses predicted for dimeric cadherin complexes

Cadherins are a superfamily of adhesion proteins involved in a variety of biological processes that include the formation of intercellular contacts, the maintenance of tissue integrity, and the development of neuronal circuits. These transmembrane proteins are characterized by ectodomains composed of a variable number of extracellular cadherin (EC) repeats that are similar but not identical in sequence and fold. E-cadherin, along with desmoglein and desmocollin proteins, are three classical-type cadherins that have slightly curved ectodomains and engage in homophilic and heterophilic interactions through an exchange of conserved tryptophan residues in their N-terminal EC1 repeat. In contrast, clustered protocadherins are straighter than classical cadherins and interact through an antiparallel homophilic binding interface that involves overlapped EC1 to EC4 repeats. Here we present molecular dynamics simulations that model the adhesive domains of these cadherins using available crystal structures, with systems encompassing up to 2.8 million atoms. Simulations of complete classical cadherin ectodomain dimers predict a two-phased elastic response to force in which these complexes first softly unbend and then stiffen to unbind without unfolding. Simulated α, β, and γ clustered protocadherin homodimers lack a two-phased elastic response, are brittle and stiffer than classical cadherins and exhibit complex unbinding pathways that in some cases involve transient intermediates. We propose that these distinct mechanical responses are important for function, with classical cadherin ectodomains acting as molecular shock absorbers and with stiffer clustered protocadherin ectodomains facilitating overlap that favors binding specificity over mechanical resilience. Overall, our simulations provide insights into the molecular mechanics of single cadherin dimers relevant in the formation of cellular junctions essential for tissue function.

Collective mechanical responses of cadherin-based adhesive junctions as predicted by simulations

Cadherin-based adherens junctions and desmosomes help stabilize cell-cell contacts with additional function in mechano-signaling, while clustered protocadherin junctions are responsible for directing neuronal circuits assembly. Structural models for adherens junctions formed by epithelial cadherin (CDH1) proteins indicate that their long, curved ectodomains arrange to form a periodic, two-dimensional lattice stabilized by tip-to-tip trans interactions (across junction) and lateral cis contacts. Less is known about the exact architecture of desmosomes, but desmoglein (DSG) and desmocollin (DSC) cadherin proteins are also thought to form ordered junctions. In contrast, clustered protocadherin (PCDH)-based cell-cell contacts in neuronal tissues are thought to be responsible for self-recognition and avoidance, and structural models for clustered PCDH junctions show a linear arrangement in which their long and straight ectodomains form antiparallel overlapped trans complexes. Here, we report all-atom molecular dynamics simulations testing the mechanics of minimalistic adhesive junctions formed by CDH1, DSG2 coupled to DSC1, and PCDHγB4, with systems encompassing up to 3.7 million atoms. Simulations generally predict a favored shearing pathway for the adherens junction model and a two-phased elastic response to tensile forces for the adhesive adherens junction and the desmosome models. Complexes within these junctions first unbend at low tensile force and then become stiff to unbind without unfolding. However, cis interactions in both the CDH1 and DSG2-DSC1 systems dictate varied mechanical responses of individual dimers within the junctions. Conversely, the clustered protocadherin PCDHγB4 junction lacks a distinct two-phased elastic response. Instead, applied tensile force strains trans interactions directly, as there is little unbending of monomers within the junction. Transient intermediates, influenced by new cis interactions, are observed after the main rupture event. We suggest that these collective, complex mechanical responses mediated by cis contacts facilitate distinct functions in robust cell-cell adhesion for classical cadherins and in self-avoidance signaling for clustered PCDHs.

Overlap phenotypes of the left ventricular noncompaction and hypertrophic cardiomyopathy with complex arrhythmias and heart failure induced by the novel truncated DSC2 mutation

Background

The left ventricular noncompaction cardiomyopathy (LVNC) is a rare subtype of cardiomyopathy associated with a high risk of heart failure (HF), thromboembolism, arrhythmia, and sudden cardiac death.

Methods

The proband with overlap phenotypes of LVNC and hypertrophic cardiomyopathy (HCM) complicates atrial fibrillation (AF), ventricular tachycardia (VT), and HF due to the diffuse myocardial lesion, which were diagnosed by electrocardiogram, echocardiogram and cardiac magnetic resonance imaging. Peripheral blood was collected from the proband and his relatives. DNA was extracted from the peripheral blood of proband for high-throughput target capture sequencing. The Sanger sequence verified the variants. The protein was extracted from the skin of the proband and healthy volunteer. The expression difference of desmocollin2 was detected by Western blot.

Results

The novel heterozygous truncated mutation (p.K47Rfs*2) of the DSC2 gene encoding an important component of desmosomes was detected by targeted capture sequencing. The western blots showed that the expressing level of functional desmocollin2 protein (~ 94kd) was lower in the proband than that in the healthy volunteer, indicating that DSC2 p.K47Rfs*2 obviously reduced the functional desmocollin2 protein expression in the proband.

Conclusion

The heterozygous DSC2 p.K47Rfs*2 remarkably and abnormally reduced the functional desmocollin2 expression, which may potentially induce the overlap phenotypes of LVNC and HCM, complicating AF, VT, and HF.

EGFR inhibitors switch keratinocytes from a proliferative to a differentiative phenotype affecting epidermal development and barrier function

BACKGROUND

Cutaneous adverse drug reactions (CADR) associated with oncology therapy involve 45-100% of patients receiving kinase inhibitors. Such adverse reactions may include skin inflammation, infection, pruritus and dryness, symptoms that can significantly affect the patient's quality of life. To prevent severe skin damages dose adjustment or drug discontinuation is often required, interfering with the prescribed oncology treatment protocol. This is particularly the case of Epidermal Growth Factor Receptor inhibitors (EGFRi) targeting carcinomas. Since the EGFR pathway is pivotal for epidermal keratinocytes, it is reasonable to hypothesize that EGFRi also affect these cells and therefore interfere with the epidermal structure formation and skin barrier function.

METHODS

To test this hypothesis, the effects of EGFRi and Vascular Endothelial Growth Factor Receptor inhibitors (VEGFRi) at therapeutically relevant concentrations (3, 10, 30, 100 nM) were assessed on proliferation and differentiation markers of human keratinocytes in a novel 3D micro-epidermis tissue culture model.

RESULTS

EGFRi directly affect basal keratinocyte growth, leading to tissue size reduction and switching keratinocytes from a proliferative to a differentiative phenotype, as evidenced by decreased Ki67 staining and increased filaggrin, desmoglein-1 and involucrin expression compared to control. These effects lead to skin barrier impairment, which can be observed in a reconstructed human epidermis model showing a decrease in trans-epidermal water loss rates. On the other hand, pan-kinase inhibitors mainly targeting VEGFR barely affect keratinocyte differentiation and rather promote a proliferative phenotype.

CONCLUSIONS

This study contributes to the mechanistic understanding of the clinically observed CADR during therapy with EGFRi. These in vitro results suggest a specific mode of action of EGFRi by directly affecting keratinocyte growth and barrier function.

The desmosome as a model for lipid raft driven membrane domain organization

Desmosomes are cadherin-based adhesion structures that mechanically couple the intermediate filament cytoskeleton of adjacent cells to confer mechanical stress resistance to tissues. We have recently described desmosomes as mesoscale lipid raft membrane domains that depend on raft dynamics for assembly, function, and disassembly. Lipid raft microdomains are regions of the plasma membrane enriched in sphingolipids and cholesterol. These domains participate in membrane domain heterogeneity, signaling and membrane trafficking. Cellular structures known to be dependent on raft dynamics include the post-synaptic density in neurons, the immunological synapse, and intercellular junctions, including desmosomes. In this review, we discuss the current state of the desmosome field and put forward new hypotheses for the role of lipid rafts in desmosome adhesion, signaling and epidermal homeostasis. Furthermore, we propose that differential lipid raft affinity of intercellular junction proteins is a central driving force in the organization of the epithelial apical junctional complex.

Pemphigus Vulgaris and Foliaceus IgG Autoantibodies Directly Block Heterophilic Transinteraction between Desmoglein and Desmocollin

Anti-desmoglein (Dsg) 1 and Dsg3 IgG autoantibodies in pemphigus foliaceus and pemphigus vulgaris cause blisters through loss of desmosomal adhesion. It is controversial whether blister formation is due to direct inhibition of Dsg, intracellular signaling events causing desmosome destabilization, or both. Recent studies show that heterophilic binding between Dsg and desmocollin (Dsc) is the fundamental adhesive unit of desmosomes. To eliminate cellular contributions to potential pathogenicity of pemphigus antibodies, bead assays coated with recombinant Dsg1, Dsc1, Dsg3, or Dsc3 ectodomains were developed. A mixture of Dsg beads and Dsc beads formed large aggregates, confirming that the heterophilic binding is dominant. The pathogenic anti-Dsg1 and anti-Dsg3 mAbs, which bind the transadhesive interface, blocked the aggregation of Dsg1/Dsc1 and Dsg3/Dsc3 beads, respectively, whereas nonpathogenic mAbs did not. All sera tested from eight patients with pemphigus foliaceus and eight patients with mucosal pemphigus vulgaris with active disease inhibited the adhesion of Dsg1/Dsc1 and Dsg3/Dsc3 beads, respectively. When paired sera obtained from seven patients with pemphigus foliaceus and six patients with pemphigus vulgaris in active disease and remission were compared, the former inhibited aggregation better than the latter. These findings strongly suggest that steric hindrance of heterophilic transinteraction between Dsg and Dsc is important for disease pathology in both pemphigus foliaceus and pemphigus vulgaris.

The Role of Desmoglein 1 in Gap Junction Turnover Revealed through the Study of SAM Syndrome

An effective epidermal barrier requires structural and functional integration of adherens junctions, tight junctions, gap junctions (GJ), and desmosomes. Desmosomes govern epidermal integrity while GJs facilitate small molecule transfer across cell membranes. Some patients with severe dermatitis, multiple allergies, and metabolic wasting (SAM) syndrome, caused by biallelic desmoglein 1 (DSG1) mutations, exhibit skin lesions reminiscent of erythrokeratodermia variabilis, caused by mutations in connexin (Cx) genes. We, therefore, examined whether SAM syndrome-causing DSG1 mutations interfere with Cx expression and GJ function. Lesional skin biopsies from SAM syndrome patients (n = 7) revealed decreased Dsg1 and Cx43 plasma membrane localization compared with control and nonlesional skin. Cultured keratinocytes and organotypic skin equivalents depleted of Dsg1 exhibited reduced Cx43 expression, rescued upon re-introduction of wild-type Dsg1, but not Dsg1 constructs modeling SAM syndrome-causing mutations. Ectopic Dsg1 expression increased cell-cell dye transfer, which Cx43 silencing inhibited, suggesting that Dsg1 promotes GJ function through Cx43. As GJA1 gene expression was not decreased upon Dsg1 loss, we hypothesized that Cx43 reduction was due to enhanced protein degradation. Supporting this, PKC-dependent Cx43 S368 phosphorylation, which signals Cx43 turnover, increased after Dsg1 depletion, while lysosomal inhibition restored Cx43 levels. These data reveal a role for Dsg1 in regulating epidermal Cx43 turnover.

Host Invasion by Pathogenic Amoebae: Epithelial Disruption by Parasite Proteins

The epithelium represents the first and most extensive line of defence against pathogens, toxins and pollutant agents in humans. In general, pathogens have developed strategies to overcome this barrier and use it as an entrance to the organism. Entamoeba histolytica, Naegleriafowleri and Acanthamoeba spp. are amoebae mainly responsible for intestinal dysentery, meningoencephalitis and keratitis, respectively. These amoebae cause significant morbidity and mortality rates. Thus, the identification, characterization and validation of molecules participating in host-parasite interactions can provide attractive targets to timely intervene disease progress. In this work, we present a compendium of the parasite adhesins, lectins, proteases, hydrolases, kinases, and others, that participate in key pathogenic events. Special focus is made for the analysis of assorted molecules and mechanisms involved in the interaction of the parasites with epithelial surface receptors, changes in epithelial junctional markers, implications on the barrier function, among others. This review allows the assessment of initial host-pathogen interaction, to correlate it to the potential of parasite invasion.

Inhibition of N-glycosylation by tunicamycin attenuates cell-cell adhesion via impaired desmosome formation in normal human epidermal keratinocytes

N-Glycosylation affects protein functions such as location, stability, and susceptibility to proteases. Desmosomes in keratinocytes are essential to maintain epidermal tissue integrity to protect against environmental insults. However, it is not yet known whether N-glycosylation affects desmosomal functions in primary keratinocytes. Tunicamycin is an inhibitor of N-glycosylation that has been a useful tool in glycobiology. Therefore, we investigated the effect of inhibiting N-glycosylation by tunicamycin treatment on desmosomes in primary keratinocytes. In our experiments, cell–cell adhesive strength was reduced in tunicamycin-treated primary keratinocytes. TEM showed that desmosome formation was impaired by tunicamycin. Desmogleins (Dsgs) 1 and 3, which constitute the core structure of desmosomes, were well transported to the cell–cell borders, but the amount decreased and showed an aberrant distribution at the cell borders in tunicamycin-treated keratinocytes. The stability of both desmoglein proteins was also reduced, and they were degraded through both proteasomal and lysosomal pathways, although inhibiting degradation did not restore the cell–cell adhesion. Finally, tunicamycin induced desmosomal instability, enhancing their disassembly. In conclusion, these results indicate that N-glycosylation is critical to the desmosome complex to maintain cell–cell adhesive strength in primary keratinocytes.

Interaction of host and Staphylococcus aureus protease-system regulates virulence and pathogenicity

Staphylococcus aureus causes various health care- and community-associated infections as well as certain chronic TH2 driven inflammatory diseases. It is a potent pathogen with serious virulence and associated high morbidity. Severe pathogenicity is accredited to the S. aureus secreted virulence factors such as proteases and host protease modulators. These virulence factors promote adhesion and invasion of bacteria through damage of tight junction barrier and keratinocytes. They inhibit activation and transmigration of various immune cells such as neutrophils (and neutrophil proteases) to evade opsono-phagocytosis and intracellular bacterial killing. Additionally, they protect the bacteria from extracellular killing by disrupting integrity of extracellular matrix. Platelet activation and agglutination is also impaired by these factors. They also block the classical as well as alternative pathways of complement activation and assist in spread of infection through blood and tissue. As these factors are exquisite factors of S. aureus mediated disease development, we have focused on review of diversification of various protease-system associated virulence factors, their structural building, diverse role in disease development and available therapeutic counter measures. This review summarises the role of protease-associated virulence factors during invasion and progression of disease.

Cell-Cell Junctions Organize Structural and Signaling Networks

Cell-cell junctions link cells to each other in tissues, and regulate tissue homeostasis in critical cell processes that include tissue barrier function, cell proliferation, and migration. Defects in cell-cell junctions give rise to a wide range of tissue abnormalities that disrupt homeostasis and are common in genetic abnormalities and cancers. Here, we discuss the organization and function of cell-cell junctions primarily involved in adhesion (tight junction, adherens junction, and desmosomes) in two different epithelial tissues: a simple epithelium (intestine) and a stratified epithelium (epidermis). Studies in these tissues reveal similarities and differences in the organization and functions of different cell-cell junctions that meet the requirements for the specialized functions of each tissue. We discuss cell-cell junction responses to genetic and environmental perturbations that provide further insights into their roles in maintaining tissue homeostasis.

Desmosomal cadherin association with Tctex-1 and cortactin-Arp2/3 drives perijunctional actin polymerization to promote keratinocyte delamination

The epidermis is a multi-layered epithelium that serves as a barrier against water loss and environmental insults. Its morphogenesis occurs through a tightly regulated program of biochemical and architectural changes during which basal cells commit to differentiate and move towards the skin's surface. Here, we reveal an unexpected role for the vertebrate cadherin desmoglein 1 (Dsg1) in remodeling the actin cytoskeleton to promote the transit of basal cells into the suprabasal layer through a process of delamination, one mechanism of epidermal stratification. Actin remodeling requires the interaction of Dsg1 with the dynein light chain, Tctex-1 and the actin scaffolding protein, cortactin. We demonstrate that Tctex-1 ensures the correct membrane compartmentalization of Dsg1-containing desmosomes, allowing cortactin/Arp2/3-dependent perijunctional actin polymerization and decreasing tension at E-cadherin junctions to promote keratinocyte delamination. Moreover, Dsg1 is sufficient to enable simple epithelial cells to exit a monolayer to form a second layer, highlighting its morphogenetic potential.

Intracellular Desmoglein-2 cleavage sensitizes epithelial cells to apoptosis in response to pro-inflammatory cytokines

Desmosomal cadherins mediate intercellular adhesion and have also been shown to regulate homeostatic signaling in epithelial cells. We have previously reported that select pro-inflammatory cytokines induce Dsg2 ectodomain cleavage and shedding from intestinal epithelial cells (IECs). Dsg2 extracellular cleaved fragments (Dsg2 ECF) function to induce paracrine pro-proliferative signaling in epithelial cells. In this study, we show that exposure of IECs to pro-inflammatory cytokines interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α) resulted in Dsg2 intracellular cleavage and generation of a ~55 kDa fragment (Dsg2 ICF). Dsg2 intracellular cleavage is mediated by caspase-8 and occurs prior to Dsg2 extracellular cleavage and the execution of apoptosis. Expression of exogenous Dsg2 ICF in model IECs resulted in increased sensitivity to apoptotic stimuli and apoptosis execution. Additionally, expression of the Dsg2 ICF repressed the anti-apoptotic Bcl-2 family member proteins Bcl-X_L and Mcl1. Taken together, our findings identify a novel mechanism by which pro-inflammatory mediators induce modification of Dsg2 to activate apoptosis and eliminate damaged cells, while also promoting release of Dsg2 ECF that promotes proliferation of neighboring cells and epithelial barrier recovery.

Crosslinking Between Trichocyte Keratins and Keratin Associated Proteins

Trichocyte keratins differ considerably from their epithelial cousins in having a higher number of cysteine residues, of which the greater proportion are located in the head and tail regions of these proteins. Coupled with this is the presence of a large number of keratin associated proteins in these fibres that are high in their cysteine content, the high sulfur proteins and ultra-high sulfur proteins. Thus it is the crosslinking that occurs between the cysteines in the keratins and KAPs that is an important determinant in the functionality of wool and hair fibres. Studies have shown the majority of the cysteine residues are involved in internal crosslinking in the KAPs leaving only a few specific cysteines to interact with the keratins, with most evidence pointing to interactions between these KAP cysteines and the keratin head groups.

Adherens junctions and desmosomes are damaged by Entamoeba histolytica: Participation of EhCPADH complex and EhCP112 protease

Entamoeba histolytica trophozoites adhere to epithelium at the cell-cell contact and perturb tight junctions disturbing the transepithelial electrical resistance. Behind tight junctions are the adherens junctions (AJs) that reinforce them and the desmosomes (DSMs) that maintain the epithelium integrity. The damage produced to AJs and DMSs by this parasite is unknown. Here, we studied the effect of the trophozoites, the EhCPADH complex, and the EhCP112 recombinant enzyme (rEhCP112) on AJ and DSM proteins. We found that trophozoites degraded β-cat, E-cad, Dsp l/ll, and Dsg-2 with the participation of EhCPADH and EhCP112. After contact of epithelial cells with trophozoites, immunofluorescence and transmission electron microscopy assays revealed EhCPADH and rEhCP112 at the intercellular space where they colocalised with β-cat, E-cad, Dsp l/ll, and Dsg-2. Moreover, our results suggested that rEhCP112 could be internalised by caveolae and clathrin-coated vesicles. Immunoprecipitation assays showed the interaction of EhCPADH with β-cat and Dsp l/ll. Besides, in vivo assays demonstrated that rEhCP112 concentrates at the cellular borders of the mouse intestine degrading E-cad and Dsp I/II. Our research gives the first clues on the trophozoite attack to AJs and DSMs and point out the role of the EhCPADH and EhCP112 in the multifactorial event of trophozoites virulence.

Loss of desmoglein 2 promotes tumorigenic behavior in pancreatic cancer cells

The ability to maintain cell-cell adhesion is crucial for tissue integrity and organization. Accordingly, loss of cohesiveness plays a critical role in cancer invasion and metastasis. Desmosomes are cell junctions providing strong intercellular adhesive strength and dysregulation of desmosomal constituents contributes to cancer progression through altered cell signaling pathways. Here, we focused on the desmosomal adhesion molecules Desmoglein 2 (Dsg2) and Desmocollin 2 (Dsc2), and their contribution to migration and invasion in pancreatic cancer cells. Silencing of Dsg2 but not Dsc2 resulted in loss of cell cohesion and enhanced migration, and invasion of pancreatic adenocarcinoma cells. To identify potential pathways regulated by Dsg2, we performed kinase arrays and detected the activity of ERK and growth factor receptors to be significantly enhanced in Dsg2-deficient cells. Consequently, inhibition of ERK phosphorylation in Dsg2 knockdown cells normalized migration. Loss of Dsg2 resulted in reduced levels of the desmosomal adapter protein and transcriptional regulator Plakoglobin (PG) in an ERK-dependent manner, whereas other desmosomal molecules were not altered. Overexpression of PG rescued enhanced migration induced by silencing of Dsg2. These results identify a novel pro-migratory pathway of pancreatic cancer cells in which loss of Dsg2 reduces the levels of PG via deregulated MAPK signaling.

Intermediate Filaments and the Plasma Membrane

A variety of intermediate filament (IF) types show intricate association with plasma membrane proteins, including receptors and adhesion molecules. The molecular basis of linkage of IFs to desmosomes at sites of cell-cell interaction and hemidesmosomes at sites of cell-matrix adhesion has been elucidated and involves IF-associated proteins. However, IFs also interact with focal adhesions and cell-surface molecules, including dystroglycan. Through such membrane interactions, it is well accepted that IFs play important roles in the establishment and maintenance of tissue integrity. However, by organizing cell-surface complexes, IFs likely regulate, albeit indirectly, signaling pathways that are key to tissue homeostasis and repair.

EphA2 proteomics in human keratinocytes reveals a novel association with afadin and epidermal tight junctions

EphA2 is a receptor tyrosine kinase that helps to maintain epidermal tissue homeostasis. A proximity-dependent biotin identification (BioID) approach was used to identify proteins in close proximity to EphA2 within primary human keratinocytes and three-dimensional (3D) reconstituted human epidermis (RHE) cultures to map a putative protein interaction network for this membrane receptor that exhibits a polarized distribution in stratified epithelia. Although a subset of known EphA2 interactors were identified in the BioID screen, >97% were uniquely detected in keratinocytes with over 50% of these vicinal proteins only present in 3D human epidermal culture. Afadin (AFDN), a cytoskeletal and junction-associated protein, was present in 2D and 3D keratinocyte cultures, and validated as a so-far-unknown EphA2-interacting protein. Loss of EphA2 protein disrupted the subcellular distribution of afadin and occludin in differentiated keratinocytes, leading to impairment of tight junctions. Collectively, these studies illustrate the use of the BioID approach in order to map receptor interaction networks in 3D human epithelial cultures, and reveal a positive regulatory role for EphA2 in the organization of afadin and epidermal tight junctions.

Lower DSC1 expression is related to the poor differentiation and prognosis of head and neck squamous cell carcinoma (HNSCC)

PURPOSE

Although desmocollins have an important position in cancer-related research, there are little reports about the relations between cancers and desmocollin 1 (DSC1). The present study was designed to investigate the correlations between DSC1 and head and neck squamous cell carcinoma (HNSCC).

METHODS

First we analyzed the GEO database; then, HNSCC and pericarcinous tissues were collected to verify the results. DSC1 expression was detected by western blot and real-time PCR. The co-expression genes of DSC1 were extracted from Cancer Cell Line Encyclopedia database (CCLE database), and their correlation was analyzed in The Cancer Genome Atlas HNSCC database (TCGA HNSCC database). Next the gene ontology analysis (GO) was carried out. Moreover, we suppressed DSC1 in FaDu cell to investigate the internal mechanism.

RESULTS

GEO database showed that DSC1 was higher in HNSCC and patients with higher DSC1 had unfavorable prognosis. The results of the samples showed that DSC1 was significantly higher in HNSCC than in normal tissue, which was consistent with the results of GEO database. The co-expression genes of DSC1 were extracted from CCLE database and verified in TCGA HNSCC database. It revealed that DSC1 was related to cell signal transduction. In FaDu/siDSC1 cells, the proliferation and migration were decreased compared to FaDu cells, and the expression levels of β-catenin, c-myc and cyclin D1 down-regulated significantly.

CONCLUSIONS

The increased expression of DSC1 can promote the occurrence of HNSCC and is associated with tumor. The increased expression of DSC1 also indicates a poor prognosis of the patients with HNSCC.

Structural basis of adhesive binding by desmocollins and desmogleins

Desmosomes are intercellular adhesive junctions that impart strength to vertebrate tissues. Their dense, ordered intercellular attachments are formed by desmogleins (Dsgs) and desmocollins (Dscs), but the nature of trans-cellular interactions between these specialized cadherins is unclear. Here, using solution biophysics and coated-bead aggregation experiments, we demonstrate family-wise heterophilic specificity: All Dsgs form adhesive dimers with all Dscs, with affinities characteristic of each Dsg:Dsc pair. Crystal structures of ectodomains from Dsg2 and Dsg3 and from Dsc1 and Dsc2 show binding through a strand-swap mechanism similar to that of homophilic classical cadherins. However, conserved charged amino acids inhibit Dsg:Dsg and Dsc:Dsc interactions by same-charge repulsion and promote heterophilic Dsg:Dsc interactions through opposite-charge attraction. These findings show that Dsg:Dsc heterodimers represent the fundamental adhesive unit of desmosomes and provide a structural framework for understanding desmosome assembly.

Inflammation-induced desmoglein-2 ectodomain shedding compromises the mucosal barrier

Proinflammatory cytokines promote desmoglein-2 (Dsg2) ectodomain shedding in intestinal epithelial cells. Epithelial exposure to Dsg2 ectodomains compromises intercellular adhesion while also promoting proliferation. These findings identify mechanisms by which mucosal inflammation–induced cleavage of Dsg2 influences intestinal epithelial homeostasis.

Desmosomal cadherins mediate intercellular adhesion and control epithelial homeostasis. Recent studies show that proteinases play an important role in the pathobiology of cancer by targeting epithelial intercellular junction proteins such as cadherins. Here we describe the proinflammatory cytokine-induced activation of matrix metalloproteinase 9 and a disintegrin and metalloproteinase domain–containing protein 10, which promote the shedding of desmosomal cadherin desmoglein-2 (Dsg2) ectodomains in intestinal epithelial cells. Epithelial exposure to Dsg2 ectodomains compromises intercellular adhesion by promoting the relocalization of endogenous Dsg2 and E-cadherin from the plasma membrane while also promoting proliferation by activation of human epidermal growth factor receptor 2/3 signaling. Cadherin ectodomains were detected in the inflamed intestinal mucosa of mice with colitis and patients with ulcerative colitis. Taken together, our findings reveal a novel response pathway in which inflammation-induced modification of columnar epithelial cell cadherins decreases intercellular adhesion while enhancing cellular proliferation, which may serve as a compensatory mechanism to promote repair.

150th Anniversary Series: Desmosomes and the Hallmarks of Cancer

Desmosomes represent adhesive, spot-like intercellular junctions that in association with intermediate filaments mechanically link neighboring cells and stabilize tissue architecture. In addition to this structural function, desmosomes also act as signaling platforms involved in the regulation of cell proliferation, differentiation, migration, morphogenesis, and apoptosis. Thus, deregulation of desmosomal proteins has to be considered to contribute to tumorigenesis. Proteolytic fragmentation and downregulation of desmosomal cadherins and plaque proteins by transcriptional or epigenetic mechanisms were observed in different cancer entities suggesting a tumor-suppressive role. However, discrepant data in the literature indicate that context-dependent differences based on alternative intracellular, signal transduction lead to altered outcome. Here, modulation of Wnt/β-catenin signaling by plakoglobin or desmoplakin and of epidermal growth factor receptor signaling appears to be of special relevance. This review summarizes current evidence on how desmosomal proteins participate in carcinogenesis, and depicts the molecular mechanisms involved.

Desmosomal adhesion in vivo

Desmosomes are intercellular junctions that provide strong adhesion or hyper-adhesion in tissues. Here, we discuss the molecular and structural basis of this with particular reference to the desmosomal cadherins (DCs), their isoforms and evolution. We also assess the role of DCs as regulators of epithelial differentiation. New data on the role of desmosomes in development and human disease, especially wound healing and pemphigus, are briefly discussed, and the importance of regulation of the adhesiveness of desmosomes in tissue dynamics is considered.

Different roles of cadherins in the assembly and structural integrity of the desmosome complex

Adhesion between cells is established by the formation of specialized intercellular junctional complexes, such as desmosomes. Desmosomes contain isoforms of two members of the cadherin superfamily of cell adhesion proteins, desmocollins (Dsc) and desmogleins (Dsg), but their combinatorial roles in desmosome assembly are not understood. To uncouple desmosome assembly from other cell-cell adhesion complexes, we used micro-patterned substrates of Dsc2aFc and/or Dsg2Fc and collagen IV; we show that Dsc2aFc, but not Dsg2Fc, was necessary and sufficient to recruit desmosome-specific desmoplakin into desmosome puncta and produce strong adhesive binding. Single-molecule force spectroscopy showed that monomeric Dsc2a, but not Dsg2, formed Ca(2+)-dependent homophilic bonds, and that Dsg2 formed Ca(2+)-independent heterophilic bonds with Dsc2a. A W2A mutation in Dsc2a inhibited Ca(2+)-dependent homophilic binding, similar to classical cadherins, and Dsc2aW2A, but not Dsg2W2A, was excluded from desmosomes in MDCK cells. These results indicate that Dsc2a, but not Dsg2, is required for desmosome assembly through homophilic Ca(2+)- and W2-dependent binding, and that Dsg2 might be involved later in regulating a switch to Ca(2+)-independent adhesion in mature desmosomes.

The desmosomal protein desmoglein 1 aids recovery of epidermal differentiation after acute UV light exposure

Epidermal structure is damaged by exposure to UV light, but the molecular mechanisms governing structural repair are largely unknown. UVB (290-320 nm wavelengths) exposure before induction of differentiation reduced expression of differentiation-associated proteins, including desmoglein 1 (Dsg1), desmocollin 1 (Dsc1), and keratins 1 and 10 (K1/K10), in a dose-dependent manner in normal human epidermal keratinocytes (NHEKs). The UVB-induced reduction in both Dsg1 transcript and protein was associated with reduced binding of the p63 transcription factor to previously unreported enhancer regulatory regions of the Dsg1 gene. As Dsg1 promotes epidermal differentiation in addition to participating in cell-cell adhesion, the role of Dsg1 in aiding differentiation after UVB damage was tested. Compared with controls, depleting Dsg1 via short hairpin RNA resulted in further reduction of Dsc1 and K1/K10 expression in monolayer NHEK cultures and in abnormal epidermal architecture in organotypic skin models recovering from UVB exposure. Ectopic expression of Dsg1 in keratinocyte monolayers rescued the UVB-induced differentiation defect. Treatment of UVB-exposed monolayer or organotypic cultures with trichostatin A, a histone deacetylase inhibitor, partially restored differentiation marker expression, suggesting a potential therapeutic strategy for reversing UV-induced impairment of epidermal differentiation after acute sun exposure.

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.

Loss of the desmosomal cadherin desmoglein-2 suppresses colon cancer cell proliferation through EGFR signaling

Desmosomal cadherins mediate cell-cell adhesion in epithelial tissues and have been known to be altered in cancer. We have previously shown that one of the two intestinal epithelial desmosomal cadherins, desmocollin-2 (Dsc2) loss promotes colonic epithelial carcinoma cell proliferation and tumor formation. In this study we show that loss of the other intestinal desmosomal cadherin, desmoglein-2 (Dsg2) that pairs with Dsc2, results in decreased epithelial cell proliferation and suppressed xenograft tumor growth in mice. Dsg2-deficient cells demonstrated a compensatory increase in Dsc2 expression, and small interfering RNA-mediated loss of Dsc2 restored proliferation in Dsg2-deficient cells. Dsg2 downregulation inhibited epidermal growth factor receptor (EGFR) signaling and cell proliferation through altered phosphorylation of EGFR and downstream extracellular signal-regulated kinase activation in parallel with inhibited EGFR receptor internalization. Additionally, we demonstrated a central role of Dsc2 in controlling EGFR signaling and cell proliferation in intestinal epithelial cells. Consistent with these findings, analyses of human colon cancers demonstrated increased Dsg2 protein expression. Taken together, these data demonstrate that partner desmosomal cadherins Dsg2 and Dsc2 play opposing roles in controlling colonic carcinoma cell proliferation through differential effects on EGFR signaling.

Mechanistic basis of desmosome-targeted diseases

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

Desmosome assembly and dynamics

Desmosomes are intercellular junctions that anchor intermediate filaments (IFs) to the plasma membrane, forming a supracellular scaffold that provides mechanical resilience to tissues. This anchoring function is accomplished by specialized members of the cadherin family and associated cytoskeletal linking proteins, which together form a highly organized membrane core flanked by mirror-image cytoplasmic plaques. Due to the biochemical insolubility of desmosomes, the mechanisms that govern assembly of these components into a functional organelle remained elusive. Recently developed molecular reporters and live cell imaging approaches have provided powerful new tools to monitor this finely tuned process in real time. Here we discuss studies that are beginning to decipher the machinery and regulation governing desmosome assembly and homeostasis in situ and how these mechanisms are affected during disease pathogenesis.

Bidirectional binding property of high glycine-tyrosine keratin-associated protein contributes to the mechanical strength and shape of hair

Since their first finding in wool 50years ago, keratin-associated proteins (KAPs), which are classified into three groups; high sulfur (HS) KAPs, ultra high sulfur (UHS) KAPs, and high glycine-tyrosine (HGT) KAPs, have been the target of curiosity for scientists due to their characteristic amino acid sequences. While HS and UHS KAPs are known to function in disulfide bond crosslinking, the function of HGT KAPs remains unknown. To clarify the function as well as the binding partners of HGT KAPs, we prepared KAP8.1 and other KAP family proteins, the trichocyte intermediate filament proteins (IFP) K85 and K35, the head domain of K85, and the C subdomain of desmoplakin C-terminus (DPCT-C) and investigated the interactions between them in vitro. Western blot analysis and isothermal titration calorimetry (ITC) indicate that KAP8.1 binds to the head domain of K85, which is helically aligned around the axis of the intermediate filament (IF). From these results and transmission electron microscopy (TEM) observations of bundled filament complex in vitro, we propose that the helical arrangement of IFs found in the orthocortex, which is uniquely distributed on the convex fiber side of the hair, is regulated by KAP8.1. Structure-dependent binding of DPCT-C to trichocyte IFP was confirmed by Western blotting, ITC, and circular dichroism. Moreover, DPCT-C also binds to some HGT KAPs. It is probable that such bidirectional binding property of HGT KAPs contribute to the mechanical robustness of hair.

Desmoglein-1/Erbin interaction suppresses ERK activation to support epidermal differentiation

Genetic disorders of the Ras/MAPK pathway, termed RASopathies, produce numerous abnormalities, including cutaneous keratodermas. The desmosomal cadherin, desmoglein-1 (DSG1), promotes keratinocyte differentiation by attenuating MAPK/ERK signaling and is linked to striate palmoplantar keratoderma (SPPK). This raises the possibility that cutaneous defects associated with SPPK and RASopathies share certain molecular faults. To identify intermediates responsible for executing the inhibition of ERK by DSG1, we conducted a yeast 2-hybrid screen. The screen revealed that Erbin (also known as ERBB2IP), a known ERK regulator, binds DSG1. Erbin silencing disrupted keratinocyte differentiation in culture, mimicking aspects of DSG1 deficiency. Furthermore, ERK inhibition and the induction of differentiation markers by DSG1 required both Erbin and DSG1 domains that participate in binding Erbin. Erbin blocks ERK signaling by interacting with and disrupting Ras-Raf scaffolds mediated by SHOC2, a protein genetically linked to the RASopathy, Noonan-like syndrome with loose anagen hair (NS/LAH). DSG1 overexpression enhanced this inhibitory function, increasing Erbin-SHOC2 interactions and decreasing Ras-SHOC2 interactions. Conversely, analysis of epidermis from DSG1-deficient patients with SPPK demonstrated increased Ras-SHOC2 colocalization and decreased Erbin-SHOC2 colocalization, offering a possible explanation for the observed epidermal defects. These findings suggest a mechanism by which DSG1 and Erbin cooperate to repress MAPK signaling and promote keratinocyte differentiation.

The C-terminal unique region of desmoglein 2 inhibits its internalization via tail-tail interactions

Tail–tail interactions of desmoglein 2, promoted by its C-terminal unique region, inhibit its internalization, stabilizing it at the cell surface and promoting intercellular adhesion.

Desmosomal cadherins, desmogleins (Dsgs) and desmocollins, make up the adhesive core of intercellular junctions called desmosomes. A critical determinant of epithelial adhesive strength is the level and organization of desmosomal cadherins on the cell surface. The Dsg subclass of desmosomal cadherins contains a C-terminal unique region (Dsg unique region [DUR]) with unknown function. In this paper, we show that the DUR of Dsg2 stabilized Dsg2 at the cell surface by inhibiting its internalization and promoted strong intercellular adhesion. DUR also facilitated Dsg tail–tail interactions. Forced dimerization of a Dsg2 tail lacking the DUR led to decreased internalization, supporting the conclusion that these two functions of the DUR are mechanistically linked. We also show that a Dsg2 mutant, V977fsX1006, identified in arrhythmogenic right ventricular cardiomyopathy patients, led to a loss of Dsg2 tail self-association and underwent rapid endocytosis in cardiac muscle cells. Our observations illustrate a new mechanism desmosomal cadherins use to control their surface levels, a key factor in determining their adhesion and signaling roles.

Desmosomal cadherins utilize distinct kinesins for assembly into desmosomes

Desmogleins and desmocollins are transported to the plasma membrane by different kinesin motors, providing a potential mechanism to tailor desmosome structure and function during development and epithelial remodeling.

The desmosomal cadherins, desmogleins (Dsgs) and desmocollins (Dscs), comprise the adhesive core of intercellular junctions known as desmosomes. Although these adhesion molecules are known to be critical for tissue integrity, mechanisms that coordinate their trafficking into intercellular junctions to regulate their proper ratio and distribution are unknown. We demonstrate that Dsg2 and Dsc2 both exhibit microtubule-dependent transport in epithelial cells but use distinct motors to traffic to the plasma membrane. Functional interference with kinesin-1 blocked Dsg2 transport, resulting in the assembly of Dsg2-deficient junctions with minimal impact on distribution of Dsc2 or desmosomal plaque components. In contrast, inhibiting kinesin-2 prevented Dsc2 movement and decreased its plasma membrane accumulation without affecting Dsg2 trafficking. Either kinesin-1 or -2 deficiency weakened intercellular adhesion, despite the maintenance of adherens junctions and other desmosome components at the plasma membrane. Differential regulation of desmosomal cadherin transport could provide a mechanism to tailor adhesion strength during tissue morphogenesis and remodeling.

Cell-cell connectivity: desmosomes and disease

Cell-cell connectivity is an absolute requirement for the correct functioning of cells, tissues and entire organisms. At the level of the individual cell, direct cell-cell adherence and communication is mediated by the intercellular junction complexes: desmosomes, adherens, tight and gap junctions. A broad spectrum of inherited, infectious and auto-immune diseases can affect the proper function of intercellular junctions and result in either diseases affecting specific individual tissues or widespread syndromic conditions. A particularly diverse group of diseases result from direct or indirect disruption of desmosomes--a consequence of their importance in tissue integrity, their extensive distribution, complex structure, and the wide variety of functions their components accomplish. As a consequence, disruption of desmosomal assembly, structure or integrity disrupts not only their intercellular adhesive function but also their functions in cell communication and regulation, leading to such diverse pathologies as cardiomyopathy, epidermal and mucosal blistering, palmoplantar keratoderma, woolly hair, keratosis, epidermolysis bullosa, ectodermal dysplasia and alopecia. Here, as well as describing the importance of the other intercellular junctions, we focus primarily on the desmosome, its structure and its role in disease. We will examine the various pathologies that result from impairment of desmosome function and thereby demonstrate the importance of desmosomes to tissues and to the organism as a whole.

RNAi-mediated inhibition of the desmosomal cadherin (desmoglein 3) impairs epithelial cell proliferation

OBJECTIVES

  Desmoglein 3 (Dsg3) is a desmosomal adhesion protein expressed in basal and immediate suprabasal layers of skin. Importance of Dsg3 in cell-cell adhesion and maintenance of tissue integrity is illustrated by findings of keratinocyte dissociation in the autoimmune disease, pemphigus vulgaris, where autoantibodies target Dsg3 on keratinocyte surfaces and cause Dsg3 depletion from desmosomes. However, recognition of possible participation of involvement of Dsg3 in cell proliferation remains controversial. Currently, available evidence suggests that Dsg3 may have both anti- and pro-proliferative roles in keratinocytes. The aim of this study was to use RNA interference (RNAi) strategy to investigate effects of silencing Dsg3 in cell-cell adhesion and cell proliferation in two cell lines, HaCaT and MDCK.

MATERIALS AND METHODS

  Cells were transfected with siRNA, and knockdown of Dsg3 was assessed by western blotting, fluorescence-activated cell sorting and confocal microscopy. Cell-cell adhesion was analysed using the hanging drop/fragmentation assay, and cell proliferation by colony forming efficiency, BrdU incorporation, cell counts and organotypic culture.

RESULTS

  Silencing Dsg3 caused defects in cell-cell adhesion and concomitant reduction in cell proliferation in both HaCaT and MDCK cells.

CONCLUSION

  These findings suggest that Dsg3 depletion by RNAi reduces cell proliferation, which is likely to be secondary to a defect in cell-cell adhesion, an essential function required for cell differentiation and morphogenesis.

Kallikrein-5 promotes cleavage of desmoglein-1 and loss of cell-cell cohesion in oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) ranks among the top 8 causes of cancer death worldwide, with only a 60% 5-year survival rate, highlighting the need for discovery of novel biomarkers and therapeutic targets. We have previously reported that expression of a panel of serine proteinase kallikreins (KLK 5, 7, 8, and 10) is correlated with formation of more aggressive OSCC tumors in a murine orthotopic OSCC model and is elevated in human OSCC. Current studies focus on understanding the potential role of KLK5 in OSCC progression. In initial studies, KLK levels in malignant OSCC cells (SCC25) were compared with cells from normal oral mucosa (OKF/6) and pre-malignant oral keratinocytes (pp126) using qPCR. A marked elevation of all KLKs was observed in aggressive SCC25 cells relative to OKF/6 cells. In normal skin, KLKs are involved in desquamation during epidermal differentiation via proteolytic cleavage of the desmosomal cadherin component desmoglein 1 (Dsg1). As loss of cell-cell cohesion is prevalent in tumor metastasis, Dsg1 integrity was evaluated. Results show that SCC25 cells exhibit cleavage of Dsg1, which is blocked by proteinase inhibitor treatment as well as by siRNA silencing of KLK5 expression. Furthermore, cell-cell aggregation assays demonstrate that silencing of KLK5 enforces cell-cell adhesion; conversely, overexpression of KLK5 in normal oral mucosal cells (OKF/6) enhances cell dispersal. These data suggest that KLK5 may promote metastatic dissemination of OSCC by promoting loss of junctional integrity through cleavage of desmoglein 1.

Plakoglobin rescues adhesive defects induced by ectodomain truncation of the desmosomal cadherin desmoglein 1: implications for exfoliative toxin-mediated skin blistering

Desmoglein 1 (Dsg1) is a desmosomal cadherin that is essential to epidermal integrity. In the blistering diseases bullous impetigo and staphylococcal scalded-skin syndrome, pathogenesis depends on cleavage of Dsg1 by a bacterial protease, exfoliative toxin A, which removes residues 1 to 381 of the Dsg1 ectodomain. However, the cellular responses to Dsg1 cleavage that precipitate keratinocyte separation to induce blister formation are unknown. Here, we show that ectodomain-deleted Dsg1 (Δ381-Dsg1) mimics the toxin-cleaved cadherin, disrupts desmosomes, and reduces the mechanical integrity of keratinocyte sheets. In addition, we demonstrate that truncated Dsg1 remains associated with its catenin partner, plakoglobin, and causes a reduction in the levels of endogenous desmosomal cadherins in a dose-dependent manner, leading us to hypothesize that plakoglobin sequestration by truncated Dsg1 destabilizes other cadherins. Accordingly, a triple-point mutant of the ectodomain-deleted cadherin, which is uncoupled from plakoglobin, does not impair adhesion, indicating that this interaction is essential to the pathogenic potential of truncated Dsg1. Moreover, we demonstrate that increasing plakoglobin levels rescues cadherin expression, desmosome organization, and functional adhesion in cells expressing Δ381-Dsg1 or treated with exfoliative toxin A. Finally, we report that histone deacetylase inhibition up-regulates desmosomal cadherins and prevents the loss of adhesion induced by Dsg1 truncation. These findings further our understanding of the mechanism of exfoliative toxin-induced pathology and suggest novel strategies to suppress blistering in bulbous impetigo and staphylococcal scalded-skin syndrome.

Plakoglobin regulates cell motility through Rho- and fibronectin-dependent Src signaling

We previously showed that the cell-cell junction protein plakoglobin (PG) not only suppresses motility of keratinocytes in contact with each other, but also, unexpectedly, of single cells. Here we show that PG deficiency results in extracellular matrix (ECM)-dependent disruption of mature focal adhesions and cortical actin organization. Plating PG⁻/⁻ cells onto ECM deposited by PG+/⁻ cells partially restored normal cell morphology and inhibited PG⁻/⁻ cell motility. In over 70 adhesion molecules whose expression we previously showed to be altered in PG⁻/⁻ cells, a substantial decrease in fibronectin (FN) in PG⁻/⁻ cells stood out. Re-introduction of PG into PG⁻/⁻ cells restored FN expression, and keratinocyte motility was reversed by plating PG⁻/⁻ cells onto FN. Somewhat surprisingly, based on previously reported roles for PG in regulating gene transcription, PG-null cells exhibited an increase, not a decrease, in FN promoter activity. Instead, PG was required for maintenance of FN mRNA stability. PG⁻/⁻ cells exhibited an increase in activated Src, one of the kinases controlled by FN, a phenotype reversed by plating PG⁻/⁻ cells on ECM deposited by PG+/⁻ keratinocytes. PG⁻/⁻ cells also exhibited Src-independent activation of the small GTPases Rac1 and RhoA. Both Src and RhoA inhibition attenuated PG⁻/⁻ keratinocyte motility. We propose a novel role for PG in regulating cell motility through distinct ECM-Src and RhoGTPase-dependent pathways, influenced in part by PG-dependent regulation of FN mRNA stability.

The desmosome

Desmosomes are intercellular junctions that tether intermediate filaments to the plasma membrane. Desmogleins and desmocollins, members of the cadherin superfamily, mediate adhesion at desmosomes. Cytoplasmic components of the desmosome associate with the desmosomal cadherin tails through a series of protein interactions, which serve to recruit intermediate filaments to sites of desmosome assembly. These desmosomal plaque components include plakoglobin and the plakophilins, members of the armadillo gene family. Linkage to the cytoskeleton is mediated by the intermediate filament binding protein, desmoplakin, which associates with both plakoglobin and plakophilins. Although desmosomes are critical for maintaining stable cell-cell adhesion, emerging evidence indicates that they are also dynamic structures that contribute to cellular processes beyond that of cell adhesion. This article outlines the structure and function of the major desmosomal proteins, and explores the contributions of this protein complex to tissue architecture and morphogenesis.

Intercellular junction assembly, dynamics, and homeostasis

Intercellular anchoring junctions are highly specialized regions of the plasma membrane where members of the cadherin family of transmembrane adhesion molecules on opposing cells interact through their extracellular domains, and through their cytoplasmic domains serve as a platform for organizing cytoskeletal anchors and remodelers. Here we focus on assembly of so-called "anchoring" or "adhering" junctions-adherens junctions (AJs) and desmosomes (DSMs), which associate with actin and intermediate filaments, respectively. We will examine how the assembly and function of AJs and DSMs are intimately connected during embryogenesis and in adult cells and tissues, and in some cases even form specialized "mixed" junctions. We will explore signaling and trafficking machineries that drive assembly and remodeling and how these mechanisms are co-opted in human disease.

Ligand targeting of EphA2 enhances keratinocyte adhesion and differentiation via desmoglein 1

Cell–cell adhesion and communication maintains epithelial tissue homeostasis. This study demonstrates that ephrin ligands target the EphA2 receptor to dampen Erk1/2 signaling without affecting cell proliferation and identifies a novel link with desmoglein 1, a desmosomal cadherin that enhances epidermal adhesion and differentiation.

EphA2 is a receptor tyrosine kinase that is engaged and activated by membrane-linked ephrin-A ligands residing on adjacent cell surfaces. Ligand targeting of EphA2 has been implicated in epithelial growth regulation by inhibiting the extracellular signal-regulated kinase 1/2 (Erk1/2)-mitogen activated protein kinase (MAPK) pathway. Although contact-dependent EphA2 activation was required for dampening Erk1/2-MAPK signaling after a calcium switch in primary human epidermal keratinocytes, the loss of this receptor did not prevent exit from the cell cycle. Incubating keratinocytes with a soluble ephrin-A1-Fc peptide mimetic to target EphA2 further increased receptor activation leading to its down-regulation. Moreover, soluble ligand targeting of EphA2 restricted the lateral expansion of epidermal cell colonies without limiting proliferation in these primary cultures. Rather, ephrin-A1-Fc peptide treatment promoted epidermal cell colony compaction and stratification in a manner that was associated with increased keratinocyte differentiation. The ligand-dependent increase in keratinocyte adhesion and differentiation relied largely upon the up-regulation of desmoglein 1, a desmosomal cadherin that maintains the integrity and differentiated state of suprabasal keratinocytes in the epidermis. These data suggest that keratinocytes expressing EphA2 in the basal layer may respond to ephrin-A1–based cues from their neighbors to facilitate entry into a terminal differentiation pathway.

Ordered assembly of the adhesive and electrochemical connections within newly formed intercalated disks in primary cultures of adult rat cardiomyocytes

The intercalated disk (ID) is a complex structure that electromechanically couples adjoining cardiac myocytes into a functional syncitium. The integrity of the disk is essential for normal cardiac function, but how the diverse elements are assembled into a fully integrated structure is not well understood. In this study, we examined the assembly of new IDs in primary cultures of adult rat cardiac myocytes. From 2 to 5 days after dissociation, the cells flatten and spread, establishing new cell-cell contacts in a manner that recapitulates the in vivo processes that occur during heart development and myocardial remodeling. As cells make contact with their neighbors, transmembrane adhesion proteins localize along the line of apposition, concentrating at the sites of membrane attachment of the terminal sarcomeres. Cx43 gap junctions and ankyrin-G, an essential cytoskeletal component of voltage gated sodium channel complexes, were secondarily recruited to membrane domains involved in cell-cell contacts. The consistent order of the assembly process suggests that there are specific scaffolding requirements for integration of the mechanical and electrochemical elements of the disk. Defining the relationships that are the foundation of disk assembly has important implications for understanding the mechanical dysfunction and cardiac arrhythmias that accompany alterations of ID architecture.