Desmosome regulation and signaling in disease

Disassembling a cancer puzzle: Cell junctions and plasma membrane as targets for anticancer therapy

Despite an enhanced permeability and retention effect typical of many solid tumors, drug penetration is not always sufficient. Possible strategies for the drug delivery improvement are a modification of the tumor cell-to-cell junctions and usage of cell membrane permeabilization proteins. In this review we discuss epithelial cell junctions as targets for a combined anticancer therapy and propose new possible sources of such agents. We suggest considering viral and bacterial pathogens disrupting epithelial layers as plentiful sources of new therapeutic agents for increasing tumor permeability for other effector agents. We also observe the application of pore forming proteins and peptides of different origin for cytoplasmic delivery of anti-cancer agents and consider the main obstacles of their use in vivo.

Genetics of and pathogenic mechanisms in arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited heart disease, associated with a high risk of sudden cardiac death. ARVC has been termed a 'disease of the desmosome' based on the fact that in many cases, it is caused by mutations in genes encoding desmosomal proteins at the specialised intercellular junctions between cardiomyocytes, the intercalated discs. Desmosomes maintain the structural integrity of the ventricular myocardium and are also implicated in signal transduction pathways. Mutated desmosomal proteins are thought to cause detachment of cardiac myocytes by the loss of cellular adhesions and also affect signalling pathways, leading to cell death and substitution by fibrofatty adipocytic tissue. However, mutations in desmosomal proteins are not the sole cause for ARVC as mutations in non-desmosomal genes were also implicated in its pathogenesis. This review will consider the pathology, genetic basis and mechanisms of pathogenesis for ARVC.

Ultrastructural changes in endometrial desmosomes of desmoglein 2 mutant mice

The intercellular binding of desmosomal junctions is mediated by cadherins of the desmoglein (Dsg) and desmocollin (Dsc) type. Dsg2 mutant mice with deletion of a substantial segment of the extracellular EC1-EC2 domain, which is believed to participate in homo- and heterophilic desmosomal cadherin interactions, develop cardiac fibrosis and ventricular dilation. Widening of the intercellular cleft and complete intercalated disc ruptures can be observed in the hearts of these mice. Since a reduced litter size of homozygous Dsg2 mutant mice was noted and a functional correlation between desmosomes and embryo implantation has been deduced from animal studies, we looked for an alteration of desmosomes in uterine endometrial epithelium. Shape and number of desmosomes as well as the expression of Dsg2 and the desmosomal plaque protein desmoplakin (Dsp) were investigated by electron microscopy and immunohistochemistry in 12 oestrous-dated mice (7 wild type and 5 homozygous Dsg2 mutant mice) at the age of 9-17 weeks. The immunohistochemical detection of Dsg2 was diminished in the mutants and the number of desmosomes was significantly reduced as revealed by electron microscopy. In addition, the intercellular desmosomal space measured in electron micrographs was considerably widened in the Dsg2 mutants. The increased intercellular spacing can be explained by the partial deletion of the extracellular EC1-EC2 domain of Dsg2. Whether these changes explain the reduced number of offspring of homozygous Dsg2 mutant mice remains to be further investigated.

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

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

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.

Hereditary palmoplantar keratodermas. Part I. Non-syndromic palmoplantar keratodermas: classification, clinical and genetic features

The term palmoplantar keratoderma (PPK) indicates any form of persistent thickening of the epidermis of palms and soles and includes genetic as well as acquired conditions. We review the nosology of hereditary PPKs that comprise an increasing number of entities with different prognoses, and a multitude of associated cutaneous and extracutaneous features. On the basis of the phenotypic consequences of the underlying genetic defect, hereditary PPKs may be divided into the following: (i) non-syndromic, isolated PPKs, which are characterized by a unique or predominant palmoplantar involvement; (ii) non-syndromic PPKs with additional distinctive cutaneous and adnexal manifestations, here named complex PPKs; (iii) syndromic PPKs, in which PPK is associated with specific extracutaneous manifestations. To date, the diagnosis of the different hereditary PPKs is based mainly on clinical history and features combined with histopathological findings. In recent years, the exponentially increasing use of next-generation sequencing technologies has led to the identification of several novel disease genes, and thus substantially contributed to elucidate the molecular basis of such a heterogeneous group of disorders. Here, we focus on hereditary non-syndromic isolated and complex PPKs. Syndromic PPKs are reviewed in the second part of this 2-part article, where other well-defined genetic diseases, which may present PPK among their phenotypic manifestations, are also listed and diagnostic and therapeutic approaches for PPKs are summarized.

Risk of Human Papillomavirus Infection in Cancer-Prone Individuals: What We Know

Human papillomavirus (HPV) infections cause a significant proportion of cancers worldwide, predominantly squamous cell carcinomas (SCC) of the mucosas and skin. High-risk HPV types are associated with SCCs of the anogenital and oropharyngeal tract. HPV oncogene activities and the biology of SCCs have been intensely studied in laboratory models and humans. What remains largely unknown are host tissue and immune-related factors that determine an individual's susceptibility to infection and/or carcinogenesis. Such susceptibility factors could serve to identify those at greatest risk and spark individually tailored HPV and SCC prevention efforts. Fanconi anemia (FA) is an inherited DNA repair disorder that is in part characterized by extreme susceptibility to SCCs. An increased prevalence of HPV has been reported in affected individuals, and molecular and functional connections between FA, SCC, and HPV were established in laboratory models. However, the presence of HPV in some human FA tumors is controversial, and the extent of the etiological connections remains to be established. Herein, we discuss cellular, immunological, and phenotypic features of FA, placed into the context of HPV pathogenesis. The goal is to highlight this orphan disease as a unique model system to uncover host genetic and molecular HPV features, as well as SCC susceptibility factors.

Desmoglein 3 Order and Dynamics in Desmosomes Determined by Fluorescence Polarization Microscopy

Desmosomes are macromolecular cell-cell junctions that provide adhesive strength in epithelial tissue. Desmosome function is inseparably linked to structure, and it is hypothesized that the arrangement, or order, of desmosomal cadherins in the intercellular space is critical for adhesive strength. However, due to desmosome size, molecular complexity, and dynamics, the role that order plays in adhesion is challenging to study. Herein, we present an excitation resolved fluorescence polarization microscopy approach to measure the spatiotemporal dynamics of order and disorder of the desmosomal cadherin desmoglein 3 (Dsg3) in living cells. Simulations were used to establish order factor as a robust metric for quantifying the spatiotemporal dynamics of order and disorder. Order factor measurements in keratinocytes showed the Dsg3 extracellular domain is ordered at the individual desmosome, single cell, and cell population levels compared to a series of disordered controls. Desmosomal adhesion is Ca²⁺ dependent, and reduction of extracellular Ca²⁺ leads to a loss of adhesion measured by dispase fragmentation assay (λ = 15.1 min). Live cell imaging revealed Dsg3 order decreased more rapidly (λ = 5.5 min), indicating that cadherin order is not required for adhesion. Our results suggest that rapid disordering of cadherins can communicate a change in extracellular Ca²⁺ concentration to the cell, leading to a downstream loss of adhesion. Fluorescence polarization is an effective bridge between protein structure and complex dynamics and the approach presented here is broadly applicable to studying order in macromolecular structures.

Distinct Cellular Basis for Early Cardiac Arrhythmias, the Cardinal Manifestation of Arrhythmogenic Cardiomyopathy, and the Skin Phenotype of Cardiocutaneous Syndromes

RATIONALE

Arrhythmogenic cardiomyopathy is caused primarily by mutations in genes encoding desmosome proteins. Ventricular arrhythmias are the cardinal and typically early manifestations, whereas myocardial fibroadiposis is the pathological hallmark. Homozygous DSP (desmoplakin) and JUP (junction protein plakoglobin) mutations are responsible for a subset of patients with arrhythmogenic cardiomyopathy who exhibit cardiac arrhythmias and dysfunction, palmoplanter keratosis, and hair abnormalities (cardiocutaneous syndromes).

OBJECTIVE

To determine phenotypic consequences of deletion of Dsp in a subset of cells common to the heart and skin.

METHODS AND RESULTS

Expression of CSPG4 (chondroitin sulfate proteoglycan 4) was detected in epidermal keratinocytes and the cardiac conduction system. CSPG4pos cells constituted ≈5.6±3.3% of the nonmyocyte cells in the mouse heart. Inducible postnatal deletion of Dsp under the transcriptional control of the Cspg4 locus led to ventricular arrhythmias, atrial fibrillation, atrioventricular conduction defects, and death by 4 months of age. Cardiac arrhythmias occurred early and in the absence of cardiac dysfunction and excess cardiac fibroadipocytes, as in human arrhythmogenic cardiomyopathy. The mice exhibited palmoplantar keratosis and progressive alopecia, leading to alopecia totalis, associated with accelerated proliferation and impaired terminal differentiation of keratinocytes. The phenotype is similar to human cardiocutaneous syndromes caused by homozygous mutations in DSP.

CONCLUSIONS

Deletion of Dsp under the transcriptional regulation of the CSPG4 locus led to lethal cardiac arrhythmias in the absence of cardiac dysfunction or fibroadiposis, palmoplantar keratosis, and alopecia, resembling the human cardiocutaneous syndromes. The findings offer a cellular basis for early cardiac arrhythmias in patients with arrhythmogenic cardiomyopathy and cardiocutaneous syndromes.

Epidermal Growth Factor Receptor neddylation is regulated by a desmosomal-COP9 (Constitutive Photomorphogenesis 9) signalosome complex

Cell junctions are scaffolds that integrate mechanical and chemical signaling. We previously showed that a desmosomal cadherin promotes keratinocyte differentiation in an adhesion-independent manner by dampening Epidermal Growth Factor Receptor (EGFR) activity. Here we identify a potential mechanism by which desmosomes assist the de-neddylating COP9 signalosome (CSN) in attenuating EGFR through an association between the Cops3 subunit of the CSN and desmosomal components, Desmoglein1 (Dsg1) and Desmoplakin (Dp), to promote epidermal differentiation. Silencing CSN or desmosome components shifts the balance of EGFR modifications from ubiquitination to neddylation, inhibiting EGFR dynamics in response to an acute ligand stimulus. A reciprocal relationship between loss of Dsg1 and neddylated EGFR was observed in a carcinoma model, consistent with a role in sustaining EGFR activity during tumor progression. Identification of this previously unrecognized function of the CSN in regulating EGFR neddylation has broad-reaching implications for understanding how homeostasis is achieved in regenerating epithelia.

The outer layer of skin – the epidermis – forms a critical barrier against a range of stresses from the environment. The epidermis itself consists of multiple layers of cells that are constantly being renewed. New cells are made in the deepest layer and move upwards until they eventually reach the skin’s surface. During this journey, the cells change the molecules they make in a process called epidermal differentiation.

To maintain an effective barrier, the epidermis must balance the division of cells in the deepest layer with the differentiation of cells in the layers above. When activated, a protein called the Epidermal Growth Factor Receptor (or EGFR for short) encourages cells in the deepest layer to divide. However, it remains poorly understood how the balance between cells dividing and cells differentiating is achieved.

The desmosome is a structure that can link together cells within the epidermis. Najor et al. now report a new interaction between the desmosome and a very large protein complex called the COP9- signalosome known to remove protein-based tags from other proteins. The experiments show that the COP9-signalosome results in the removal of these tags from EGFR. The status of the tags on EGFR regulates whether or not it is found at the cell surface. Najor et al. propose that that the desmosome acts as a scaffold and holds the COP9 signalosome close to EGFR. The enzyme in the COP9 signalosome then removes protein-based tags from EGFR, which triggers a series of events that remove EGFR from the cell surface. This dampens down the signals EGFR would normally send to make cells divide, and allows differentiation to proceed.

The balance between cell division and differentiation is a fundamental process that is affected in many skin conditions, including psoriasis and atopic dermatitis. EGFR is also commonly overactive in cancers. As such, understanding how epidermal differentiation and cell division are controlled will shed light on a variety of disorders, allowing for the potential development of new treatments for these diseases.

The role of desmosomes in carcinogenesis

Desmosomes, which are intercellular adhesive complexes, are essential for the maintenance of epithelial homeostasis. They are located at the cell membrane, where they act as anchors for intermediate filaments. Downregulation of desmosome proteins in various cancers promotes tumor progression. However, the role of desmosomes in carcinogenesis is still being elucidated. Recent studies revealed that desmosome family members play a crucial role in tumor suppression or tumor promotion. This review focuses on studies that provide insights into the role of desmosomes in carcinogenesis and address their molecular functions.

Roles of Grainyhead-like transcription factors in cancer

The mammalian homologs of the D. melanogaster Grainyhead gene, Grainyhead-like 1-3 (GRHL1, GRHL2 and GRHL3), are transcription factors implicated in wound healing, tubulogenesis and cancer. Their induced target genes encode diverse epithelial cell adhesion molecules, while mesenchymal genes involved in cell migration and invasion are repressed. Moreover, GRHL2 suppresses the oncogenic epithelial-mesencyhmal transition, thereby acting as a tumor suppressor. Mechanisms, some involving established cancer-related signaling/transcription factor pathways (for example, Wnt, TGF-β, mir200, ZEB1, OVOL2, p63 and p300) and translational implications of the Grainyhead proteins in cancer are discussed in this review article.

Molecular and cytoskeletal regulations in epidermal development

At the surface of the body, the epidermis covers great depth in its developmental regulation. While many genes have been shown to be important for skin development through their associations with disease phenotypes in mice and human, it is in the past decade that the intricate interplay between various molecules become gradually revealed through sophisticated genetic models and imaging analyses. In particular, there is increasing evidence suggesting that cytoskeleton-associated proteins, including adhesion proteins and the crosslinker proteins may play critical roles in regulating epidermis development. We here provide a broad overview of the various molecules involved in epidermal development with special emphasis on the cytoskeletal components.

Phosphorylation of Pkp1 by RIPK4 regulates epidermal differentiation and skin tumorigenesis

Tissue homeostasis of skin is sustained by epidermal progenitor cells localized within the basal layer of the skin epithelium. Post-translational modification of the proteome, such as protein phosphorylation, plays a fundamental role in the regulation of stemness and differentiation of somatic stem cells. However, it remains unclear how phosphoproteomic changes occur and contribute to epidermal differentiation. In this study, we survey the epidermal cell differentiation in a systematic manner by combining quantitative phosphoproteomics with mammalian kinome cDNA library screen. This approach identified a key signaling event, phosphorylation of a desmosome component, PKP1 (plakophilin-1) by RIPK4 (receptor-interacting serine-threonine kinase 4) during epidermal differentiation. With genome-editing and mouse genetics approach, we show that loss of function of either Pkp1 or Ripk4 impairs skin differentiation and enhances epidermal carcinogenesis in vivo Phosphorylation of PKP1's N-terminal domain by RIPK4 is essential for their role in epidermal differentiation. Taken together, our study presents a global view of phosphoproteomic changes that occur during epidermal differentiation, and identifies RIPK-PKP1 signaling as novel axis involved in skin stratification and tumorigenesis.

The desmoplakin-intermediate filament linkage regulates cell mechanics

Desmoplakin connects desmosomal core components to intermediate filaments at sites of cell–cell adhesion. Modulating the strength of this linkage using desmoplakin mutants led to alterations in cell–substrate and cell–cell forces and cell stiffness as assessed by micropillar arrays and atomic force microscopy. Perturbation of the actin cytoskeleton leads to abrogation of these effects.

The translation of mechanical forces into biochemical signals plays a central role in guiding normal physiological processes during tissue development and homeostasis. Interfering with this process contributes to cardiovascular disease, cancer progression, and inherited disorders. The actin-based cytoskeleton and its associated adherens junctions are well-established contributors to mechanosensing and transduction machinery; however, the role of the desmosome–intermediate filament (DSM–IF) network is poorly understood in this context. Because a force balance among different cytoskeletal systems is important to maintain normal tissue function, knowing the relative contributions of these structurally integrated systems to cell mechanics is critical. Here we modulated the interaction between DSMs and IFs using mutant forms of desmoplakin, the protein bridging these structures. Using micropillar arrays and atomic force microscopy, we demonstrate that strengthening the DSM–IF interaction increases cell–substrate and cell–cell forces and cell stiffness both in cell pairs and sheets of cells. In contrast, disrupting the interaction leads to a decrease in these forces. These alterations in cell mechanics are abrogated when the actin cytoskeleton is dismantled. These data suggest that the tissue-specific variability in DSM–IF network composition provides an opportunity to differentially regulate tissue mechanics by balancing and tuning forces among cytoskeletal systems.

[Effects of embryonic lead exposure on food intake and bowel movement in offspring rats and possible mechanisms]

OBJECTIVE

To study the effects of embryonic lead exposure on food intake and bowel movement in offspring rats and possible mechanisms.

METHODS

Sprague-Dawley rats were given 0.1% (low-dose lead exposure group) or 0.2% (high-dose lead exposure group) lead acetate freely during pregnancy to establish an animal model of embryonic lead exposure. A blank control group was also established. The male offspring rats were enrolled in the study, and 10 male offspring rats from each group were selected to observe the changes in food intake, bowel movement, gastric emptying, intestine propulsion, and pathological inflammatory response in the gastric mucosa. Eight offspring rats were selected from each group, and electron microscopy and immunohistochemistry were used to observe the changes in the ultrastructure of jejunal microvilli and cell junction and the expression of cholecystokinin-8 (CCK-8) and motilin (MTL) in the feeding center, in order to reveal the possible mechanisms for abnormal gastrointestinal motility in offspring rats induced by embryonic lead exposure.

RESULTS

Compared with the control group, the low- and high-dose lead exposure groups had a significant reduction in daily food intake, a significant increase in water content of feces, a significant reduction in fecal pellet weight, and a significant increase in small intestine propulsion (P<0.05). The high-dose lead exposure group had a significant reduction in gastric emptying ability compared with the control group (P<0.05). Compared with the control group, the lead exposure groups had significantly greater pathological inflammatory changes in the gastric mucosa (P<0.05), significant reductions in the number and length of the jejunal microvilli and the number of epithelial desmosome junctions (P<0.05), a significant increase in the macula densa gap (P<0.05), and significant increases in the expression of MTL and CCK-8 in the feeding center (P<0.05), in a dose-dependent manner.

CONCLUSIONS

The degree of gastrointestinal structural injury and expression levels of MTL and CCK-8 in the feeding center are lead dose-dependent, which may be important mechanisms for changes in food intake, bowel movement, and digestive functions in offspring rats induced by embryonic lead exposure.

Transgenic mice overexpressing desmocollin-2 (DSC2) develop cardiomyopathy associated with myocardial inflammation and fibrotic remodeling

Background

Arrhythmogenic cardiomyopathy is an inherited heart muscle disorder leading to ventricular arrhythmias and heart failure, mainly as a result of mutations in cardiac desmosomal genes. Desmosomes are cell-cell junctions mediating adhesion of cardiomyocytes; however, the molecular and cellular mechanisms underlying the disease remain widely unknown. Desmocollin-2 is a desmosomal cadherin serving as an anchor molecule required to reconstitute homeostatic intercellular adhesion with desmoglein-2. Cardiac specific lack of desmoglein-2 leads to severe cardiomyopathy, whereas overexpression does not. In contrast, the corresponding data for desmocollin-2 are incomplete, in particular from the view of protein overexpression. Therefore, we developed a mouse model overexpressing desmocollin-2 to determine its potential contribution to cardiomyopathy and intercellular adhesion pathology.

Methods and results

We generated transgenic mice overexpressing DSC2 in cardiac myocytes. Transgenic mice developed a severe cardiac dysfunction over 5 to 13 weeks as indicated by 2D-echocardiography measurements. Corresponding histology and immunohistochemistry demonstrated fibrosis, necrosis and calcification which were mainly localized in patches near the epi- and endocardium of both ventricles. Expressions of endogenous desmosomal proteins were markedly reduced in fibrotic areas but appear to be unchanged in non-fibrotic areas. Furthermore, gene expression data indicate an early up-regulation of inflammatory and fibrotic remodeling pathways between 2 to 3.5 weeks of age.

Conclusion

Cardiac specific overexpression of desmocollin-2 induces necrosis, acute inflammation and patchy cardiac fibrotic remodeling leading to fulminant biventricular cardiomyopathy.

Molecular mechanisms in the pathogenesis of arrhythmogenic cardiomyopathy

The article is based on work presented in the Distinguished Achievement Award lecture at the Society for Cardiovascular Pathology meeting in Seattle, WA, in March 2016. It reviews our current understanding of mechanisms responsible for a highly arrhythmogenic, nonischemic cardiomyopathy. It highlights the armamentarium of powerful methods available to the experimental pathologist in efforts to define how complex cardiovascular diseases work. It concludes with acknowledgment of the need for a far more detailed approach as to how we categorize human disease, a task for which pathologists are especially well positioned.

Pemphigus, a pathomechanism of acantholysis

Autoantibodies to the desmosomal proteins desmoglein 1 and 3 cause pemphigus foliaceus and pemphigus vulgaris, which are characterised by keratinocyte dissociation (acantholysis) and intraepidermal blister formation. The passive transfer of pathogenic anti-desmoglein antibodies induces blisters in mice in vivo and the loss of keratinocyte adhesion in vitro. The pathogenetic mechanisms of acantholysis due to anti-desmoglein autoantibodies are not fully understood. However, recent studies have revealed that signalling-dependent and signalling-independent pathways are operative in the loss of cell adhesion. In this review, we focus on the pathomechanism of acantholysis due to autoantibodies to desmogleins and recent therapeutic approaches.

Palmitoylation of Desmoglein 2 Is a Regulator of Assembly Dynamics and Protein Turnover

Desmosomes are prominent adhesive junctions present between many epithelial cells as well as cardiomyocytes. The mechanisms controlling desmosome assembly and remodeling in epithelial and cardiac tissue are poorly understood. We recently identified protein palmitoylation as a mechanism regulating desmosome dynamics. In this study, we have focused on the palmitoylation of the desmosomal cadherin desmoglein-2 (Dsg2) and characterized the role that palmitoylation of Dsg2 plays in its localization and stability in cultured cells. We identified two cysteine residues in the juxtamembrane (intracellular anchor) domain of Dsg2 that, when mutated, eliminate its palmitoylation. These cysteine residues are conserved in all four desmoglein family members. Although mutant Dsg2 localizes to endogenous desmosomes, there is a significant delay in its incorporation into junctions, and the mutant is also present in a cytoplasmic pool. Triton X-100 solubility assays demonstrate that mutant Dsg2 is more soluble than wild-type protein. Interestingly, trafficking of the mutant Dsg2 to the cell surface was delayed, and a pool of the non-palmitoylated Dsg2 co-localized with lysosomal markers. Taken together, these data suggest that palmitoylation of Dsg2 regulates protein transport to the plasma membrane. Modulation of the palmitoylation status of desmosomal cadherins can affect desmosome dynamics.

Desmoglein-2 is overexpressed in non-small cell lung cancer tissues and its knockdown suppresses NSCLC growth by regulation of p27 and CDK2

PURPOSE

Desmoglein-2 (Dsg2) is a cell adhesion protein of the cadherin superfamily. Altered Dsg2 expression is associated with tumorigenesis. This study determined Dsg2 expression in non-small cell lung cancer (NSCLC) tissue specimens for association with clinicopathological and survival data and then assessed the effect of Dsg2 knockdown on regulation of NSCLC cell malignant behaviors in vitro and in nude mouse xenografts.

METHODS

qRT-PCR and Western blot were used to detect Dsg2 expression in 28 paired NSCLC and normal tissue samples. Immunohistochemistry was used to detect Dsg2 expression in 70 cases of paraffin-embedded NSCLC tissues. NSCLC A549, H1703, and H1299 cells were cultured with Dsg2 knockdown performed using Dsg2 siRNA. Cell viability, cell cycle, apoptosis, and colony formation were assessed. siRNA-transfected A549 cells were also used to generate tumor xenografts in nude mice.

RESULTS

Both Dsg2 mRNA and protein were highly expressed in NSCLC tissues and associated with NSCLC size, but not with overall survival of patients. Moreover, knockdown of Dsg2 expression reduced NSCLC cell proliferation and arrested them at the G1 phase of the cell cycle, but did not significantly affect NSCLC cell apoptosis. Dsg2 knockdown downregulated cyclin-dependent kinase 2 expression and upregulated p27 expression. Nude mouse xenograft assays showed that Dsg2 knockdown inhibited NSCLC xenograft growth in vivo.

CONCLUSION

This study revealed the importance of Dsg2 in suppression of NSCLC development and progression. Further studies will explore whether restoration of Dsg2 expression is a novel strategy in control of NSCLC.

Molecular organization of the desmosome as revealed by direct stochastic optical reconstruction microscopy

Desmosomes are macromolecular junctions responsible for providing strong cell-cell adhesion. Because of their size and molecular complexity, the precise ultrastructural organization of desmosomes is challenging to study. Here, we used direct stochastic optical reconstruction microscopy (dSTORM) to resolve individual plaque pairs for inner and outer dense plaque proteins. Analysis methods based on desmosomal mirror symmetry were developed to measure plaque-to-plaque distances and create an integrated map. We quantified the organization of desmoglein 3, plakoglobin and desmoplakin (N-terminal, rod and C-terminal domains) in primary human keratinocytes. Longer desmosome lengths correlated with increasing plaque-to-plaque distance, suggesting that desmoplakin is arranged with its long axis at an angle within the plaque. We next examined whether plaque organization changed in different adhesive states. Plaque-to-plaque distance for the desmoplakin rod and C-terminal domains decreased in PKP-1-mediated hyperadhesive desmosomes, suggesting that protein reorganization correlates with function. Finally, in human epidermis we found a difference in plaque-to-plaque distance for the desmoplakin C-terminal domain, but not the desmoplakin rod domain or plakoglobin, between basal and suprabasal cells. Our data reveal the molecular organization of desmosomes in cultured keratinocytes and skin as defined by dSTORM.

Polyclonal breast cancer metastases arise from collective dissemination of keratin 14-expressing tumor cell clusters

Recent genomic studies challenge the conventional model that each metastasis must arise from a single tumor cell and instead reveal that metastases can be composed of multiple genetically distinct clones. These intriguing observations raise the question: How do polyclonal metastases emerge from the primary tumor? In this study, we used multicolor lineage tracing to demonstrate that polyclonal seeding by cell clusters is a frequent mechanism in a common mouse model of breast cancer, accounting for >90% of metastases. We directly observed multicolored tumor cell clusters across major stages of metastasis, including collective invasion, local dissemination, intravascular emboli, circulating tumor cell clusters, and micrometastases. Experimentally aggregating tumor cells into clusters induced a >15-fold increase in colony formation ex vivo and a >100-fold increase in metastasis formation in vivo. Intriguingly, locally disseminated clusters, circulating tumor cell clusters, and lung micrometastases frequently expressed the epithelial cytoskeletal protein, keratin 14 (K14). RNA-seq analysis revealed that K14(+) cells were enriched for desmosome and hemidesmosome adhesion complex genes, and were depleted for MHC class II genes. Depletion of K14 expression abrogated distant metastases and disrupted expression of multiple metastasis effectors, including Tenascin C (Tnc), Jagged1 (Jag1), and Epiregulin (Ereg). Taken together, our findings reveal K14 as a key regulator of metastasis and establish the concept that K14(+) epithelial tumor cell clusters disseminate collectively to colonize distant organs.