The binding of plakoglobin to desmosomal cadherins: patterns of binding sites and topogenic potential

In Vivo Approaches to Understand Arrhythmogenic Cardiomyopathy: Perspectives on Animal Models

Arrhythmogenic cardiomyopathy (AC) is a hereditary cardiac disorder characterized by the gradual replacement of cardiomyocytes with fibrous and adipose tissue, leading to ventricular wall thinning, chamber dilation, arrhythmias, and sudden cardiac death. Despite advances in treatment, disease management remains challenging. Animal models, particularly mice and zebrafish, have become invaluable tools for understanding AC's pathophysiology and testing potential therapies. Mice models, although useful for scientific research, cannot fully replicate the complexity of the human AC. However, they have provided valuable insights into gene involvement, signalling pathways, and disease progression. Zebrafish offer a promising alternative to mammalian models, despite the phylogenetic distance, due to their economic and genetic advantages. By combining animal models with in vitro studies, researchers can comprehensively understand AC, paving the way for more effective treatments and interventions for patients and improving their quality of life and prognosis.

Challenges and Opportunities in the Oral Delivery of Recombinant Biologics

Recombinant biological molecules are at the cutting-edge of biomedical research thanks to the significant progress made in biotechnology and a better understanding of subcellular processes implicated in several diseases. Given their ability to induce a potent response, these molecules are becoming the drugs of choice for multiple pathologies. However, unlike conventional drugs which are mostly ingested, the majority of biologics are currently administered parenterally. Therefore, to improve their limited bioavailability when delivered orally, the scientific community has devoted tremendous efforts to develop accurate cell- and tissue-based models that allow for the determination of their capacity to cross the intestinal mucosa. Furthermore, several promising approaches have been imagined to enhance the intestinal permeability and stability of recombinant biological molecules. This review summarizes the main physiological barriers to the oral delivery of biologics. Several preclinical in vitro and ex vivo models currently used to assess permeability are also presented. Finally, the multiple strategies explored to address the challenges of administering biotherapeutics orally are described.

Dynamic mechanobiology of cardiac cells and tissues: Current status and future perspective

Mechanical forces impact cardiac cells and tissues over their entire lifespan, from development to growth and eventually to pathophysiology. However, the mechanobiological pathways that drive cell and tissue responses to mechanical forces are only now beginning to be understood, due in part to the challenges in replicating the evolving dynamic microenvironments of cardiac cells and tissues in a laboratory setting. Although many in vitro cardiac models have been established to provide specific stiffness, topography, or viscoelasticity to cardiac cells and tissues via biomaterial scaffolds or external stimuli, technologies for presenting time-evolving mechanical microenvironments have only recently been developed. In this review, we summarize the range of in vitro platforms that have been used for cardiac mechanobiological studies. We provide a comprehensive review on phenotypic and molecular changes of cardiomyocytes in response to these environments, with a focus on how dynamic mechanical cues are transduced and deciphered. We conclude with our vision of how these findings will help to define the baseline of heart pathology and of how these in vitro systems will potentially serve to improve the development of therapies for heart diseases.

TROP2 Down-Regulated DSG2 to Promote Gastric Cancer Cell Invasion and Migration by EGFR/AKT and DSG2/PG/β-Catenin Pathways

AIMS

Explore the specific mechanism of TROP2 in promoting cancer in gastric cancer, and provide a basis for the prevention and treatment of gastric cancer. Background Gastric cancer (GC) is the fourth most commonly found cancer and the second highest cause of cancer related death worldwide, TROP2 overexpression is closely related with many cancers including gastrointestinal tumors, DSG2 is an important protein in cell adhesion and its loss is related to cell migration.

OBJECTIVE

Explore the specific mechanism of TROP2 in promoting cancer in gastric cancer, and provide a basis for the prevention and treatment of gastric cancer.

METHOD

DSG2 was identified as an interacting protein of TROP2 in GC cells by co-immunoprecipitation and mass spectrometry. The regulated behavior of TROP2 on DSG2 expression was investigated with TROP2 over-expressure or knockdown. Cell-cell adhesion capacity medicated by DSG2 was evaluated by adhesion related assays. Electron microscope observation was utilized for accessing GC tumor desmosome assembly. Proteins in EGFR/AKT and DSG2/PG/β-catenin pathways were evaluated by western blotting.

RESULT

This study suggests that abundant expression of TROP2 in GC cells lessened DSG2 levels as well as desmosome adhesion, increased cell invasion, migration and promoted malignant progression through EGFR/AKT and DSG2/PG/β-catenin pathways.

CONCLUSION

TROP2 promotes gastric cancer cell invasion and migration by decreasing DSG2 expression through EGFR/AKT and DSG2/PG/β-catenin pathways.

Paradoxical Roles of Desmosomal Components in Head and Neck Cancer

Desmosomes are intercellular adhesion complexes involved in various aspects of epithelial pathophysiology, including tissue homeostasis, morphogenesis, and disease development. Recent studies have reported that the abnormal expression of various desmosomal components correlates with tumor progression and poor survival. In addition, desmosomes have been shown to act as a signaling platform to regulate the proliferation, invasion, migration, morphogenesis, and apoptosis of cancer cells. The occurrence and progression of head and neck cancer (HNC) is accompanied by abnormal expression of desmosomal components and loss of desmosome structure. However, the role of desmosomal components in the progression of HNC remains controversial. This review aims to provide an overview of recent developments showing the paradoxical roles of desmosomal components in tumor suppression and promotion. It offers valuable insights for HNC diagnosis and therapeutics development.

Plakoglobin localization to the cell border restores desmosome function in cells lacking 14-3-3γ

Desmosomes are cell-cell adhesion junctions that anchor intermediate filaments. Loss of 14-3-3γ in HCT116 cells led to defects in desmosome assembly due to a decrease in the transport of Plakoglobin (PG) to the cell border thus disrupting desmosome formation. Desmosome formation in cells lacking 14-3-3γ was restored by artificially localizing PG to the cell border by fusing it to EGFP-f (PG-EGFP-f). These results suggest that a major role of 14-3-3γ in desmosome assembly is to transport PG to the cell border leading to the initiation of desmosome formation.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Modification of cysteine 457 in plakoglobin modulates the proliferation and migration of colorectal cancer cells by altering binding to E-cadherin/catenins

OBJECTIVES

In tissue samples from patients with colorectal cancer (CRC), oxidation of C420 and C457 of plakoglobin (Pg) within tumor tissue was identified by proteomic analysis. The aim of this study was to identify the roles of Pg C420 and C457.

METHODS

Human CRC tissues, CRC and breast cancer cells, and normal mouse colon were prepared to validate Pg oxidation. MC38 cells were co-transfected with E-cadherin plus wild type (WT) or mutant (C420S or C457S) Pg to evaluate protein interactions and cellular localization, proliferation, and migration.

RESULTS

Pg was more oxidized in stage III CRC tumor tissue than in non-tumor tissue. Similar oxidation of Pg was elicited by H₂O₂ treatment in normal colon and cancer cells. C457S Pg exhibited diminished binding to E-cadherin and α-catenin, and reduced the assembly of E-cadherin-α-/β-catenin complexes. Correspondingly, immunofluorescent analysis of Pg cellular localization suggested impaired binding of C457S Pg to membranes. Cell migration and proliferation were also suppressed in C457S-expressing cells.

DISCUSSION

Pg appears to be redox-sensitive in cancer, and the C457 modification may impair cell migration and proliferation by affecting its interaction with the E-cadherin/catenin axis. Our findings suggest that redox-sensitive cysteines of Pg may be the targets for CRC therapy.

Normalization of Naxos plakoglobin levels restores cardiac function in mice

Arrhythmogenic cardiomyopathy (AC) is associated with mutations in genes encoding intercalated disc proteins and ultimately results in sudden cardiac death. A subset of patients with AC have the autosomal recessive cardiocutaneous disorder Naxos disease, which is caused by a 2-base pair deletion in the plakoglobin-encoding gene JUP that results in a truncated protein with reduced expression. In mice, cardiomyocyte-specific plakoglobin deficiency recapitulates many aspects of human AC, and overexpression of the truncated Naxos-associated plakoglobin also results in an AC-like phenotype; therefore, it is unclear whether Naxos disease results from loss or gain of function consequent to the plakoglobin mutation. Here, we generated 2 knockin mouse models in which endogenous Jup was engineered to express the Naxos-associated form of plakoglobin. In one model, Naxos plakoglobin bypassed the nonsense-mediated mRNA decay pathway, resulting in normal levels of the truncated plakoglobin. Moreover, restoration of Naxos plakoglobin to WT levels resulted in normal heart function. Together, these data indicate that a gain of function in the truncated form of the protein does not underlie the clinical phenotype of patients with Naxos disease and instead suggest that insufficiency of the truncated Naxos plakoglobin accounts for disease manifestation. Moreover, these results suggest that increasing levels of truncated or WT plakoglobin has potential as a therapeutic approach to Naxos disease.

Integrating animal models and in vitro tissue models to elucidate the role of desmosomal proteins in diseases

Desmosomes are intercellular junctions that provide tissues with structural stability. These junctions might also act as signaling centers that transmit environmental clues to the cell, thereby affecting cell differentiation, migration, and proliferation. The importance of desmosomes is underscored by devastating skin and heart diseases caused by mutations in desmosomal genes. Recent observations suggest that abnormal desmosomal protein expression might indirectly contribute to skin disorders previously not linked to these proteins. For example, it has been postulated that reduced desmosomal protein expression occurs in patients affected by Ankyloblepharon-ectodermal defects-cleft lip/palate syndrome (AEC), a skin fragility disorder caused by mutations in the transcription factor TP63. Currently, it is not clear how these changes in desmosomal gene expression contribute to AEC. We will discuss new approaches that combine in vitro and in vivo models to elucidate the role of desmosomal gene deregulation in human skin diseases such as AEC.

In VivoFunction of Desmosomes

Desmosomes are morphologically and biochemically defined cell-cell junctions that are required for maintaining the mechanical integrity of skin and the heart in adult mammals. Furthermore, since mice with null mutations in desmosomal plaque proteins (plakoglobin and desmoplakin) die in utero, it is also evident that desmosomes are indispensable for normal embryonic development. This review focuses on the role of desmosomes in vivo. We will summarize the effects of mutations in desmosomal genes on pre- and post-embryonic development of mouse and man and discuss recent findings relating to the specific role of desmosomal cadherins in skin differentiation and homeostasis.

Flotillins directly interact with γ-catenin and regulate epithelial cell-cell adhesion

Flotillin-1 and flotillin-2 are two homologous, membrane raft associated proteins. Although it has been reported that flotillins are involved in cell adhesion processes and play a role during breast cancer progression, thus making them interesting future therapeutic targets, their precise function has not been well elucidated. The present study investigates the function of these proteins in cell-cell adhesion in non-malignant cells. We have used the non-malignant epithelial MCF10A cells to study the interaction network of flotillins within cell-cell adhesion complexes. RNA interference was used to examine the effect of flotillins on the structure of adherens junctions and on the association of core proteins, such as E-cadherin, with membrane rafts. We here show that the cadherin proteins of the adherens junction associate with flotillin-2 in MCF10A cells and in various human cell lines. In vitro, flotillin-1 and flotillin-2 directly interact with γ-catenin which is so far the only protein known to be present both in the adherens junction and the desmosome. Mapping of the interaction domain within the γ-catenin sequence identified the Armadillo domains 6-8, especially ARM domain 7, to be important for the association with flotillins. Furthermore, depletion of flotillins significantly influenced the morphology of the adherens junction in human epithelial MCF10A cells and altered the association of E-cadherin and γ-catenin with membrane rafts. Taken together, these observations suggest a functional role for flotillins, especially flotillin-2, in cell-cell adhesion in non-malignant epithelial cells.

The molecular composition and function of desmosomes

Desmosomes are intercellular adhesive junctions that are particularly prominent in tissues experiencing mechanical stress, such as the heart and epidermis. Whereas the related adherens junction links actin to calcium-dependent adhesion molecules known as classical cadherins, desmosomes link intermediate filaments (IF) to the related subfamily of desmosomal cadherins. By tethering these stress-bearing cytoskeletal filaments to the plasma membrane, desmosomes serve as integrators of the IF cytoskeleton throughout a tissue. Recent evidence suggests that IF attachment in turn strengthens desmosomal adhesion. This collaborative arrangement results in formation of a supracellular network, which is critical for imparting mechanical integrity to tissues. Diseases and animal models targeting desmosomal components highlight the importance of desmosomes in development and tissue integrity, while the downregulation of individual protein components in cancer metastasis and wound healing suggests their importance in cell homeostasis. This chapter will provide an update on desmosome composition, function, and regulation, and will also discuss recent work which raises the possibility that desmosome proteins do more than play a structural role in tissues where they reside.

Structural aspects of adherens junctions and desmosomes

The cadherin-containing intercellular junctions, adherens junctions and desmosomes share an overall logical organization in which the extracellular regions of the cadherins on opposing cells interact, while their cytoplasmic domains are linked to the cytoskeleton through protein assemblies. In adherens junctions, beta-catenin binds to the cytoplasmic domain of cadherins and to alpha-catenin, which links the cadherin/beta-catenin complex to the actin cytoskeleton. In desmosomes, the beta-catenin homolog plakoglobin binds to desmosomal cadherins. The desmosomal cadherin/plakoglobin complex is linked to the intermediate filament system by the protein desmoplakin. In the past decade, components of these systems have been purified to homogeneity and studied biochemically and structurally, providing the beginnings of a mechanistic description of junction architecture and dynamics.

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.

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.

Interactions of plakoglobin and beta-catenin with desmosomal cadherins: basis of selective exclusion of alpha- and beta-catenin from desmosomes

Plakoglobin and beta-catenin are homologous armadillo repeat proteins found in adherens junctions, where they interact with the cytoplasmic domain of classical cadherins and with alpha-catenin. Plakoglobin, but normally not beta-catenin, is also a structural constituent of desmosomes, where it binds to the cytoplasmic domains of the desmosomal cadherins, desmogleins and desmocollins. Here, we report structural, biophysical, and biochemical studies aimed at understanding the molecular basis of selective exclusion of beta-catenin and alpha-catenin from desmosomes. The crystal structure of the plakoglobin armadillo domain bound to phosphorylated E-cadherin shows virtually identical interactions to those observed between beta-catenin and E-cadherin. Trypsin sensitivity experiments indicate that the plakoglobin arm domain by itself is more flexible than that of beta-catenin. Binding of plakoglobin and beta-catenin to the intracellular regions of E-cadherin, desmoglein1, and desmocollin1 was measured by isothermal titration calorimetry. Plakoglobin and beta-catenin bind strongly and with similar thermodynamic parameters to E-cadherin. In contrast, beta-catenin binds to desmoglein-1 more weakly than does plakoglobin. beta-Catenin and plakoglobin bind with similar weak affinities to desmocollin-1. Full affinity binding of desmoglein-1 requires sequences C-terminal to the region homologous to the catenin-binding domain of classical cadherins. Although pulldown assays suggest that the presence of N- and C-terminal beta-catenin "tails" that flank the armadillo repeat region reduces the affinity for desmosomal cadherins, calorimetric measurements show no significant effects of the tails on binding to the cadherins. Using purified proteins, we show that desmosomal cadherins and alpha-catenin compete directly for binding to plakoglobin, consistent with the absence of alpha-catenin in desmosomes.

Discovering the molecular components of intercellular junctions--a historical view

The organization of metazoa is based on the formation of tissues and on tissue-typical functions and these in turn are based on cell-cell connecting structures. In vertebrates, four major forms of cell junctions have been classified and the molecular composition of which has been elucidated in the past three decades: Desmosomes, which connect epithelial and some other cell types, and the almost ubiquitous adherens junctions are based on closely cis-packed glycoproteins, cadherins, which are associated head-to-head with those of the hemi-junction domain of an adjacent cell, whereas their cytoplasmic regions assemble sizable plaques of special proteins anchoring cytoskeletal filaments. In contrast, the tight junctions (TJs) and gap junctions (GJs) are formed by tetraspan proteins (claudins and occludins, or connexins) arranged head-to-head as TJ seal bands or as paracrystalline connexin channels, allowing intercellular exchange of small molecules. The by and large parallel discoveries of the junction protein families are reported.

Plakoglobin has both structural and signalling roles in zebrafish development

Plakoglobin, or gamma-catenin, is found in both desmosomes and adherens junctions and participates in Wnt signalling. Mutations in the human gene are implicated in the congenital heart disorder, arrhythmogenic right ventricular cardiomyopathy (ARVC), but the signalling effects of plakoglobin loss in ARVC have not been established. Here we report that knockdown of plakoglobin in zebrafish results in decreased heart size, reduced heartbeat, cardiac oedema, reflux of blood between heart chambers and a twisted tail. Wholemount in situ hybridisation shows reduced expression of the heart markers nkx2.5 at 24 hours post fertilisation (hpf), and cmlc2 and vmhc at 48 hpf, while there is lack of restriction of the valve markers notch1b and bmp4 at 48 hpf. Wnt target gene expression was examined by semi-quantitative RT-PCR and found to be increased in morphant embryos indicating that plakoglobin is antagonistic to Wnt signalling. Co-expression of the Wnt inhibitor, Dkk1, rescues the cardiac phenotype of the plakoglobin morphant. beta-catenin protein expression is increased in morphant embryos as is its colocalisation with E-cadherin in adherens junctions. Endothelial cells at the atrioventricular boundary of morphant hearts have an aberrant morphology, indicating problems with valvulogenesis. Morphants also have decreased numbers of desmosomes and adherens junctions in the intercalated discs. These results establish the zebrafish as a model for ARVC caused by loss of plakoglobin function and indicate that there are signalling as well as structural consequences of this loss.

Anchoring junctions as drug targets: role in contraceptive development

In multicellular organisms, cell-cell interactions are mediated in part by cell junctions, which underlie tissue architecture. Throughout spermatogenesis, for instance, preleptotene leptotene spermatocytes residing in the basal compartment of the seminiferous epithelium must traverse the blood-testis barrier to enter the adluminal compartment for continued development. At the same time, germ cells must also remain attached to Sertoli cells, and numerous studies have reported extensive restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface during germ cell movement across the seminiferous epithelium. Furthermore, the proteins and signaling cascades that regulate adhesion between testicular cells have been largely delineated. These findings have unveiled a number of potential "druggable" targets that can be used to induce premature release of germ cells from the seminiferous epithelium, resulting in transient infertility. Herein, we discuss a novel approach with the aim of developing a nonhormonal male contraceptive for future human use, one that involves perturbing adhesion between Sertoli and germ cells in the testis.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

A novel dominant mutation in plakoglobin causes arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited disorder associated with arrhythmias and sudden death. A recessive mutation in the gene encoding plakoglobin has been shown to cause Naxos disease, a cardiocutaneous syndrome characterized by ARVC and abnormalities of hair and skin. Here, we report, for the first time, a dominant mutation in the gene encoding plakoglobin in a German family with ARVC but no cutaneous abnormalities. The mutation (S39_K40insS) is predicted to insert an extra serine residue at position 39 in the N-terminus of plakoglobin. Analysis of a biopsy sample of the right ventricle from the proband showed markedly decreased localization of plakoglobin, desmoplakin, and connexin43 at intercalated discs in cardiac myocytes. A yeast-two-hybrid screen revealed that the mutant protein established novel interactions with histidine-rich calcium-binding protein and TGF beta induced apoptosis protein 2. Immunoblotting and confocal microscopy in human embryonic kidney 293 (HEK293) cell lines transfected to stably express either wild-type or mutant plakoglobin protein showed that the mutant protein was apparently ubiquitylated and was preferentially located in the cytoplasm, suggesting that the S39_K40insS mutation may increase plakoglobin turnover via proteasomal degradation. HEK293 cells expressing mutant plakoglobin also showed higher rates of proliferation and lower rates of apoptosis than did cells expressing the wild-type protein. Electron microscopy showed smaller and fewer desmosomes in cells expressing mutant plakoglobin. Taken together, these observations suggest that the S39_K40insS mutation affects the structure and distribution of mechanical and electrical cell junctions and could interfere with regulatory mechanisms mediated by Wnt-signaling pathways. These results implicate novel molecular mechanisms in the pathogenesis of ARVC.

Modulation of the oncogenic potential of beta-catenin by the subcellular distribution of plakoglobin

Plakoglobin (Pg) and beta-catenin are homologous proteins that function in cell-cell adhesion and signaling. The cadherin-associated form of these proteins mediates adhesion, whereas the cytosolic/nuclear form has a signaling role. Despite their interactions with common cellular partners, beta-catenin has a well-documented oncogenic potential while Pg has a less characterized tumor suppressor activity. We showed previously that Pg overexpression in Pg-deficient SCC9 cells (SCC9-Pg-WT) induced Bcl-2 expression and inhibited apoptosis. To assess the exact role of Pg in Bcl-2 expression, we generated and characterized SCC9 transfectants expressing Pg with a restricted cytoplasmic (Pg-NES) or nuclear (Pg-NLS) distribution. We show that Bcl-2 was expressed regardless of Pg localization, although its level was substantially lower in SCC9-Pg-NLS cells. Bcl-2 expression coincided with increased nuclear beta-catenin levels (Pg-NES) or a decrease in the level of total and nuclear beta-catenin associated with N-cadherin and alpha-catenin (Pg-WT and -NLS) cells. Bcl-2 expression also was induced in SCC9 cells overexpressing beta-catenin. In contrast, SCC9 cells expressing mutant Pg proteins, unable to interact with N-cadherin and alpha-catenin, had noticeably lower Bcl-2 levels. Our data suggest that Bcl-2 expression is induced by beta-catenin and modulated by Pg. We show that the inhibition of beta-catenin-dependent TCF transactivation had no effect on Bcl-2 levels, suggesting that induction of Bcl-2 expression by beta-catenin and its modulation by Pg may involve factors other than, or in addition, to, TCF. These results provide a possible mechanism for the tumor suppressor activity of Pg via its role as a regulator of the oncogenic potential beta-catenin.

Desmosome structure, composition and function

Desmosomes are intercellular junctions of epithelia and cardiac muscle. They resist mechanical stress because they adopt a strongly adhesive state in which they are said to be hyper-adhesive and which distinguishes them from other intercellular junctions; desmosomes are specialised for strong adhesion and their failure can result in diseases of the skin and heart. They are also dynamic structures whose adhesiveness can switch between high and low affinity adhesive states during processes such as embryonic development and wound healing, the switching being signalled by protein kinase C. Desmosomes may also act as signalling centres, regulating the availability of signalling molecules and thereby participating in fundamental processes such as cell proliferation, differentiation and morphogenesis. Here we consider the structure, composition and function of desmosomes, and their role in embryonic development and disease.

Homo- and heterotypic cell contacts in malignant melanoma cells and desmoglein 2 as a novel solitary surface glycoprotein

During progression of melanomas, a crucial role has been attributed to alterations of cell-cell adhesions, specifically, to a "cadherin switch" from E- to N-cadherin (cad). We have examined the adhesion of melanoma cells to each other and to keratinocytes. When different human melanoma cell lines were studied by protein analysis and immunofluorescence microscopy, six of eight lines contained N-cad, three E-cad, and five P-cad, and some lines had more than one cad. Surprisingly, two N-cad-positive lines, MeWo and C32, also contained desmoglein 2 (Dsg2), a desmosomal cad previously not reported for melanomas, whereas other desmosome-specific proteins were absent. This finding was confirmed by reverse transcriptase-PCR, immunoprecipitation, and matrix-assisted laser desorption ionization-time of flight analyses. Double-label confocal and immunoelectron microscopy showed N-cad, alpha- and beta-catenin in plaque-bearing puncta adhaerentia, whereas Dsg2 was distributed rather diffusely over the cell surface. In cocultures with HaCaT keratinocytes Dsg2 was found in heterotypic cell contact regions. Correspondingly, immunohistochemistry revealed Dsg2 in five of 10 melanoma metastases. Together, we show that melanoma cell adhesions are more heterogeneous than expected and that certain cells devoid of desmosomes contain Dsg2 in a non-junction-restricted form. Future studies will have to clarify the diagnostic and prognostic significance of these different adhesion protein subtypes.

Requirement of plakophilin 2 for heart morphogenesis and cardiac junction formation

Plakophilins are proteins of the armadillo family that function in embryonic development and in the adult, and when mutated can cause disease. We have ablated the plakophilin 2 gene in mice. The resulting mutant mice exhibit lethal alterations in heart morphogenesis and stability at mid-gestation (E10.5–E11), characterized by reduced trabeculation, disarrayed cytoskeleton, ruptures of cardiac walls, and blood leakage into the pericardiac cavity. In the absence of plakophilin 2, the cytoskeletal linker protein desmoplakin dissociates from the plaques of the adhering junctions that connect the cardiomyocytes and forms granular aggregates in the cytoplasm. By contrast, embryonic epithelia show normal junctions. Thus, we conclude that plakophilin 2 is important for the assembly of junctional proteins and represents an essential morphogenic factor and architectural component of the heart.

beta-catenin-dependent and -independent effects of DeltaN-plakoglobin on epidermal growth and differentiation

Both beta-catenin and plakoglobin can stimulate the expression of Lef/Tcf target genes in vitro. beta-Catenin is known to associate with Lef/Tcf factors and to participate directly in transactivation in vivo, whereas the role of plakoglobin in transcriptional regulation has been less studied. To analyze the functions of plakoglobin in vivo, we generated transgenic mice expressing in the epidermis N-terminally truncated plakoglobin (DeltaN122-PG) lacking the glycogen synthase kinase 3beta phosphorylation sites and therefore protected against degradation (transgenic line K5-DeltaN122-PG). The expression of DeltaN122-PG led to the formation of additional hair germs, hyperplastic hair follicles, and noninvasive hair follicle tumors, a phenotype reminiscent of that induced by expression of N-terminally truncated beta-catenin. However, if expressed in beta-catenin-null epidermis, DeltaN122-PG did not induce new hair follicle germs and follicular tumors. Thus, DeltaN122-PG cannot substitute for beta-catenin in its signaling functions in vivo and the phenotype observed in K5-DeltaN122-PG mouse skin must be due to the aberrant activation of beta-catenin signaling. On the other hand, the expression of DeltaN122-PG in beta-catenin-null skin significantly increased the survival rate of mutant mice, rescued differentiation, and limited excessive proliferation in the interfollicular epidermis, suggesting that plakoglobin may be involved in the intracellular signaling events essential for epidermal differentiation.

Immunomolecular mapping of adherens junction and desmosomal components in normal human epidermis

Adherens junctions (AJs) are cell-cell and cell-matrix junctions that are known to comprise the transmembrane and cytoplasmic components linked to the f-actin cytoskeleton. Although the presence of AJs han been confirmed in normal human epidermis, previous studies immunolocalizing AJ-related antigens have been controversial. The purpose of this study was to produce a more precise molecular mapping of AJs and their constituents in relation to desmosomes in normal human epidermal keratinocytes. Using an electron microscope (EM) method to optimally fix plasma membranes. AJ structures were typically seen as a narrowing of the intercellular space between two keratinocytes that was distinct from desmosomes and gap junctions. Such structures were consistently found more frequently in the upper epidermis than in the basal layer. Immunogold electron microscopy showed an absence of the AJ components (E-cadherin and beta-catenin) from desmosomal areas but they were present at interdesmosomal areas at sites of close membrane association. Conversely, the desmosomal components plakoglobin and plakophilin 1 were restricted only to the outer attachment plaque of the desmosome. These results further confirm that AJs have a distinct molecular composition and distribution from desmosomes and that they regularly occur between desmosomes along the keratinocyte plasma membrane to provide alternative cell-cell adhesion mechanisms.

Expression of desmosomal proteins in squamous cell carcinomas of the skin

BACKGROUND

Desmosomal proteins are well established markers of epithelial differentiation. Down-regulation of desmosomal proteins has been suggested to be a sign of reduced adhesiveness in metastasizing cells.

METHODS

We examined actinic keratoses, Bowen's disease, and squamous cell carcinoma (SCC) of the skin for the expression of desmosomal proteins using isoform-specific antibodies on paraffin-embedded sections. Evaluation was performed qualitatively in comparison to the epidermis and semiquantitatively using an area-intensity-score.

RESULTS

We found no qualitative correlation of desmoplakin or plakoglobin expression with risk of metastasis. Plakophilin 1, desmoglein 1, and the desmocollins 1-3 were found to be heterogeneously expressed in all neoplasms without significant correlation to aggressive tumor behavior. Plakophilin 2 was not expressed in any of the neoplasms examined. As most striking finding, desmoglein 2 was up-regulated qualitatively in half of all neoplasms examined and showed a significant higher proportion of positive cells in high-risk SCC than in low-risk SCC.

CONCLUSIONS

Desmosomal proteins are highly regulated in cutaneous SCC. Only desmoglein 2 expression correlates with risk of metastasis. Desmosomes may still be functional in metastasizing tumor cells.

Down-Regulation and Altered Localization of γ-Catenin in Cisplatin-Resistant Adenocarcinoma Cells

Resistance to cisplatin, one of the most widely used anticancer chemotherapeutic agents, is a major clinical problem. There is no effective way to predict development of cisplatin resistance in cancers. As determined by reverse transcription-polymerase chain reaction and Western blotting, the expression of gamma-catenin, an adherens junction protein, was decreased in KB-CP20 and 7404-CP20 cells compared with parental-sensitive cells. Short-term treatment with cisplatin of the parental cells resulted in proteolysis of gamma-catenin as evaluated in membrane pellet preparations, and the extent of cleavage increased as cisplatin concentration was raised from 1 to 5 microg/ml during 1 h of treatment. Uncleaved cytoplasmic gamma-catenin increased under the same conditions. These biochemical results were supported by confocal microscopy, which showed a loss of gamma-catenin from adherens plaques after cisplatin treatment. Cleavage of gamma-catenin was specific to cisplatin treatment in that cleavage did not occur after treatment with doxorubicin and cytosine arabinoside. Pretreatment of KB and 7404 cells with cisplatin for 1 h resulted in reduced uptake of [(14)C]carboplatin, suggesting that the biochemical changes induced by cisplatin treatment, including cleavage of gamma-catenin, could affect the ability of cells to internalize platinum compounds. Cells transfected with the gamma-catenin gene are sensitive to cisplatin compared with cells transfected with a control vector. Our data suggest that proteolysis and altered localization of gamma-catenin are early markers for the response of cells to cisplatin, and reduced levels of gamma-catenin in resistant cells may indicate an important role for gamma-catenin in mediating or modulating the toxicity of cisplatin in cancer cells.

Cadherins as modulators of cellular phenotype

Cadherins are transmembrane glycoproteins that mediate calcium-dependent cell-cell adhesion. The cadherin family is large and diverse, and proteins are considered to be members of this family if they have one or more cadherin repeats in their extracellular domain. Cadherin family members are the transmembrane components of a number of cellular junctions, including adherens junctions, desmosomes, cardiac junctions, endothelial junctions, and synaptic junctions. Cadherin function is critical in normal development, and alterations in cadherin function have been implicated in tumorigenesis. The strength of cadherin interactions can be regulated by a number of proteins, including the catenins, which serve to link the cadherin to the cytoskeleton. Cadherins have been implicated in a number of signaling pathways that regulate cellular behavior, and it is becoming increasingly clear that integration of information received from cell-cell signaling, cell-matrix signaling, and growth factor signaling determines ultimate cellular phenotype and behavior.

Assessment of splice variant-specific functions of desmocollin 1 in the skin

Desmocollin 1 (Dsc1) is part of a desmosomal cell adhesion receptor formed in terminally differentiating keratinocytes of stratified epithelia. The dsc1 gene encodes two proteins (Dsc1a and Dsc1b) that differ only with respect to their COOH-terminal cytoplasmic amino acid sequences. On the basis of in vitro experiments, it is thought that the Dsc1a variant is essential for assembly of the desmosomal plaque, a structure that connects desmosomes to the intermediate filament cytoskeleton of epithelial cells. We have generated mice that synthesize a truncated Dsc1 receptor that lacks both the Dsc1a- and Dsc1b-specific COOH-terminal domains. This mutant transmembrane receptor, which does not bind the common desmosomal plaque proteins plakoglobin and plakophilin 1, is integrated into functional desmosomes. Interestingly, our mutant mice did not show the epidermal fragility previously observed in dsc1-null mice. This suggests that neither the Dsc1a- nor the Dsc1b-specific COOH-terminal cytoplasmic domain is required for establishing and maintaining desmosomal adhesion. However, a comparison of our mutants with dsc1-null mice suggests that the Dsc1 extracellular domain is necessary to maintain structural integrity of the skin.

Plakoglobin is O-glycosylated close to the N-terminal destruction box

Plakoglobin provides a key linkage in protein chains that connect desmosomal and classical cadherins to the cytoskeleton. It is also present in a significant cytosolic pool that has the capacity to impact on canonical Wnt signaling by competing for interaction with partner proteins of beta-catenin. The closely related protein, beta-catenin, is rapidly targeted for proteasomal degradation by phosphorylation of a "destruction box" within the N-terminal domain. Inhibition of this process forms the basis of Wnt signaling. This destruction box is also found in the N-terminal domain of plakoglobin. We report that plakoglobin is modified by the addition of O-GlcNAc at a single site in close proximity to the destruction box. O-GlcNAc modification has been proposed to counteract phosphorylation, provide protection from proteasomal degradation, mediate signal transduction, silence transcription, and regulate multimolecular protein assembly. This finding has potential implications for understanding the roles of plakoglobin.

Cytoskeletal proteins in uterine epithelial cells only partially return to the pre-receptive state after the period of receptivity

Immunohistochemical staining of 5 cytoskeletal proteins (actin, alpha-actinin, gelsolin, plectin and plakoglobin) was used to investigate changes in distribution patterns of these proteins after the period of uterine receptivity for blastocyst implantation in the rat. Actin was found throughout the cytoplasm but it was concentrated along the apical plasma membrane on day 1 of pregnancy, decreased by day 6 and then increased again at day 9. Alpha-actinin and gelsolin were localized in distinctive bands along the apical plasma membrane at day 6 of pregnancy but became diffusely distributed at day 9. Plectin was localized along the apical and basal plasma membranes at day 6 but in higher amounts apically and at day 9, it was concentrated in apical and basal zones in the cells. Plakoglobin was found along the lateral and basal membranes with increased intensity along the apical third of the lateral plasma membrane from day 6 to day 9 of pregnancy. These results show that all 5 cytoskeletal proteins redistributed after the period of uterine receptivity: some exhibited a similar pattern of labelling to that found during the prereceptive state, whereas others only partially returned to the pre-receptive state. This change in distribution patterns may reflect differences in the epithelial barrier function before and after the period of receptivity.

De novo formation of desmosomes in cultured cells upon transfection of genes encoding specific desmosomal components

Desmosomes are cell junctions and cytoskeleton-anchoring structures of epithelia, the myocardium, and dendritic reticulum cells of lymphatic follicles whose major components are known. Using cultured HT-1080 SL-1 fibrosarcoma-derived cells and transfection of cDNAs encoding specific desmosomal components, we have determined a minimum ensemble of proteins sufficient to introduce de novo structures, which, by morphology and functional competence, are indistinguishable from authentic desmosomes. In a more refined analysis, the influence of the desmosomal proteins desmoplakin (Dp), plakoglobin (Pg), and plakophilin 2 (Pp2) on the lateral clustering of the desmosomal transmembrane-glycoprotein desmoglein 2 (Dsg) was examined. We found that for efficient clustering of desmoglein 2 and desmosome structure formation, all three major plaque proteins-desmoplakin, plakoglobin, and plakophilin 2- were necessary. Furthermore, in this cell model, plakophilin 2 was capable of directing desmoplakin to adhaerens junctions (AJ), whereas plakoglobin was crucial for the segregation of desmosomal and AJ components. These results are discussed with respect to the variability in cell junction composition observed in various nonepithelial tissues.

Desmosomal cadherins

New evidence from blocking desmosomal adhesion with anti-adhesion peptides reveals a role for desmosomes in cell positioning in morphogenesis. Desmosomal adhesion is necessary for the stability of adherens junctions in epithelial cell sheets. Knockout and mis-expression of desmosomal cadherins in mice suggests that they may function directly or indirectly in regulating epidermal differentiation. Protein kinase C signalling and tyrosine phosphorylation appear to regulate desmosomal adhesion. There are new insights into the role of desmosomal cadherins in autoimmune, infectious and genetic disease.

Strict correlation between uPAR and plakoglobin expression in pemphigus vulgaris

BACKGROUND

Recent studies have reported nuclear delocalization of plakoglobin in acantholytic pemphigus vulgaris cells. The objective of this study was to evaluate the role of plakoglobin in the pathogenesis of acantholysis in pemphigus vulgaris (PV) and its relation with the urokinase-type plasminogen activator receptor (uPAR) expression.

MATERIALS AND METHODS

Plakoglobin and uPAR expressions were evaluated by immunohistochemistry in 22 cases of PV at various stages of the disease, and as controls in 18 specimens of skin/oral mucosa from healthy patients.

RESULTS

Healthy skin/normal oral mucosa showed strong plakoglobin expression in the basal and spinous layers with prevalent cellular membrane distribution; the intensity of staining progressively decreased toward the superficial layers of the epithelium. In PV patients, a progressive displacement of the plakoglobin signal toward the nucleus was found in 18/22 of the cases. Healthy skin/normal oral mucosa showed low uPAR expression with prevalent cellular membrane distribution. In the PV patients, strong uPAR expression was present in the acantholytic cells in 16/22 of the cases. There was direct correlation (p < 0.05) between the uPAR expression and nuclear plakoglobin.

CONCLUSIONS

The uPAR overexpression in acantholytic PV may be considered a direct consequence of plakoglobin abnormal distribution. Nuclear delocalization of plakoglobin, a direct consequence of plakoglobin-Dsg-3 dissociation induced by PV IgG, probably induces uPAR overexpression. This evidence suggests a central role for plakoglobin in PV pathogenesis because of its delocalization toward the nucleus, which is the probable cause of the uPAR gene expression.

Both the dimerization and immunochemical properties of E-cadherin EC1 domain depend on Trp(156) residue

Using site-directed mutagenesis, we show in this paper that the adhesive interface detected in cadherin crystals is unlikely to mediate adhesive interaction between myc- and flag-tagged E-cadherin molecules in human A-431 cells. We also found that a critical residue within this interface, His(233), is part of the epitope for mAb SHE78-7. This epitope was accessible to the antibody in the adhesive E-cadherin dimers, which is consistent with uninvolvement of the site containing His(233) in cell-cell adhesion. However, both the adhesive dimerization and the integrity of the SHE78-7 epitope depended on the same intramolecular interaction between Trp(156) and its hydrophobic pocket. Our data suggest that this interaction may have an important regulatory function in controlling the surface topology of the NH(2)-terminal domain of E-cadherin.

Protein binding and functional characterization of plakophilin 2. Evidence for its diverse roles in desmosomes and beta -catenin signaling

Plakophilins are a subfamily of p120-related arm-repeat proteins that can be found in both desmosomes and the nucleus. Among the three known plakophilin members, plakophilin 1 has been linked to a genetic skin disorder and shown to play important roles in desmosome assembly and organization. However, little is known about the binding partners and functions of the most widely expressed member, plakophilin 2. To better understand the cellular functions of plakophilin 2, we have examined its protein interactions with other junctional molecules using co-immunoprecipitation and yeast two-hybrid assays. Here we show that plakophilin 2 can interact directly with several desmosomal components, including desmoplakin, plakoglobin, desmoglein 1 and 2, and desmocollin 1a and 2a. The head domain of plakophilin 2 is critical for most of these interactions and is sufficient to direct plakophilin 2 to cell borders. In addition, plakophilin 2 is less efficient than plakophilin 1 in localizing to the nucleus and enhancing the recruitment of excess desmoplakin to cell borders in transiently transfected COS cells. Furthermore, plakophilin 2 is able to associate with beta-catenin through its head domain, and the expression of plakophilin 2 in SW480 cells up-regulates the endogenous beta-catenin/T cell factor-signaling activity. This up-regulation by plakophilin 2 is abolished by ectopic expression of E-cadherin, suggesting that these proteins compete for the same pool of signaling active beta-catenin. Our results demonstrate that plakophilin 2 interacts with a broader repertoire of desmosomal components than plakophilin 1 and provide new insight into the possible roles of plakophilin 2 in regulating the signaling activity of beta-catenin.

Molecular interactions between desmosomal cadherins

Desmocollins (Dscs) and desmogleins (Dsgs) are cell-adhesion molecules involved in the formation of desmosome cell-cell junctions and share structural similarities to classical cadherins such as E-cadherin. In order to identify and provide quantitative information on the types of protein-protein interactions displayed by the type 2 isoforms and investigate the role of Ca(2+) in this process, we have developed an Escherichia coli expression system to generate recombinant proteins containing the first two extracellular domains, namely Dsg2(1-2) and Dsc2(1-2). Analytical ultracentrifugation, chemical cross-linking, CD, fluorescence and BIAcore have been used to provide the first direct evidence of Ca(2+) binding to desmosomal cadherins. These studies suggest that Dsc2(1-2) not only exhibits homophilic interactions in solution, but can also form heterophilic interactions with Dsg2(1-2). The latter, on the other hand, shows much weaker homophilic association. Our results further demonstrate that heterophilic interactions are Ca(2+)-dependent, whereas the Ca(2+)-dependence of homophilic association is less clear. Our data indicate that the functional properties of Dsc2(1-2) are more similar to those of classical cadherins, consistent with the observation that Dsc shares a higher level of sequence homology with classical cadherins than does Dsg. In addition to corroborating the conclusions of previously reported transfection studies which suggest the formation of lateral heterodimers and homodimers, our results also provide direct quantitative information on the strength of these interactions which are essential for understanding the adhesion mechanism.

A possible role of catenin dyslocalization in pemphigus vulgaris pathogenesis

BACKGROUND

Pemphigus vulgaris (PV) is an autoimmune blistering disease of the skin and mucosa due to the presence of autoantibodies against the components of desmosomes. To date, less is known about the expression levels of beta- and gamma-catenins in blistering diseases. The objective of this study was to evaluate the role of beta- and gamma-catenins in the pathogenesis of acantholysis in pemphigus vulgaris.

METHODS

beta- and gamma-catenin expression was evaluated by immunohistochemistry in 30 cases of PV at various stages of the disease and, as controls, in 18 specimens of the skin/oral mucosa of healthy patients.

RESULTS

Healthy skin and normal oral mucosa showed a strong beta- and gamma-catenin expression in basal and spinous layers with a prevalent cellular membrane distribution; the intensity of staining progressively decreased toward the superficial layers of epithelium. In PV patients, cytoplasmic expression of gamma-catenin was detected in 28/30 cases, and in 19/30 cases of PV for beta-catenin. Moreover, a progressive displacement of the signal toward the nucleus was found in 14/30 cases for beta-catenin, with dyslocalization toward the nucleus, particularly in areas with intense acantholysis, and in 22/30 cases of PV for gamma-catenin.

CONCLUSIONS

Abnormal distribution of gamma-catenin, consequent to PV IgG, may be considered a direct consequence of Dg3 dissociation from catenin. gamma-catenin likely plays a direct role in PV pathogenesis through its dyslocalization toward the nucleus or indirectly through the beta-catenin dyslocalization toward the nucleus, which is thought to induce transcription of selected target genes, such as uPAR.

A central role for the armadillo protein plakoglobin in the autoimmune disease pemphigus vulgaris

In pemphigus vulgaris (PV), autoantibody binding to desmoglein (Dsg) 3 induces loss of intercellular adhesion in skin and mucous membranes. Two hypotheses are currently favored to explain the underlying molecular mechanisms: (a) disruption of adhesion through steric hindrance, and (b) interference of desmosomal cadherin-bound antibody with intracellular events, which we speculated to involve plakoglobin. To investigate the second hypothesis we established keratinocyte cultures from plakoglobin knockout (PG^−/−) embryos and PG^+/+ control mice. Although both cell types exhibited desmosomal cadherin-mediated adhesion during calcium-induced differentiation and bound PV immunoglobin (IgG) at their cell surface, only PG^+/+ keratinocytes responded with keratin retraction and loss of adhesion. When full-length plakoglobin was reintroduced into PG^−/− cells, responsiveness to PV IgG was restored. Moreover, in these cells like in PG^+/+ keratinocytes, PV IgG binding severely affected the linear distribution of plakoglobin at the plasma membrane. Taken together, the establishment of an in vitro model using PG^+/+ and PG^−/− keratinocytes allowed us (a) to exclude the steric hindrance only hypothesis, and (b) to demonstrate for the first time that plakoglobin plays a central role in PV, a finding that will provide a novel direction for investigations of the molecular mechanisms leading to PV, and on the function of plakoglobin in differentiating keratinocytes.

Are desmosomes more than tethers for intermediate filaments?

Desmosomes are intercellular adhesive junctions that anchor intermediate filaments at membrane-associated plaques in adjoining cells, thereby forming a three-dimensional supracellular scaffolding that provides tissues with mechanical strength. But desmosomes have also recently been recognized as sensors that respond to environmental and cellular cues by modulating their assembly state and, possibly, their signalling functions.

Plakoglobin and beta-catenin: protein interactions, regulation and biological roles

Beta-catenin can play different roles in the cell, including one as a structural protein at cell-cell adherens junctions and another as a transcriptional activator mediating Wnt signal transduction. Plakoglobin (gamma)-catenin), a close homolog of beta-catenin, shares with beta-catenin common protein partners and can fulfill some of the same functions. The complexing of catenins with various protein partners is regulated by phosphorylation and by intramolecular interactions. The competition between different catenin partners for binding to catenins mediates the cross-talk between cadherin-based adhesion, catenin-dependent transcription and Wnt signaling. Although plakoglobin differs from beta-catenin in its functions and is unable to compensate for defects in Wnt signaling resulting from lack of beta-catenin, recent evidence suggests that plakoglobin plays a unique role in Wnt signaling that is different from that of beta-catenin. The functional difference between catenins is reflected in their differential involvement in embryonic development and cancer progression.

Interaction of plakophilins with desmoplakin and intermediate filament proteins: an in vitro analysis

Plakophilin 1 and 2 (PKP1, PKP2) are members of the arm-repeat protein family. They are both constitutively expressed in most vertebrate cells, in two splice forms named a and b, and display a remarkable dual location: they occur in the nuclei of cells and, in epithelial cells, at the plasma membrane within the desmosomal plaques. We have shown by solid phase-binding assays that both PKP1a and PKP2a bind to intermediate filament (IF) proteins, in particular to cytokeratins (CKs) from epidermal as well as simple epithelial cells and, to some extent, to vimentin. In line with this we show that recombinant PKP1a binds strongly to IFs assembled in vitro from CKs 8/18, 5/14, vimentin or desmin and integrates them into thick (up to 120 nm in diameter) IF bundles extending for several microm. The basic amino-terminal, non-arm-repeat domain of PKP1a is necessary and sufficient for this specific interaction as shown by blot overlay and centrifugation experiments. In particular, the binding of PKP1a to IF proteins is saturable at an approximately equimolar ratio. In extracts from HaCaT cells, distinct soluble complexes containing PKP1a and desmoplakin I (DPI) have been identified by co-immunoprecipitation and sucrose density fractionation. The significance of these interactions of PKP1a with IF proteins on the one hand and desmoplakin on the other is discussed in relation to the fact that PKP1a is not bound - and does not bind - to extended IFs in vivo. We postulate that (1) effective cellular regulatory mechanisms exist that prevent plakophilins from unscheduled IF-binding, and (2) specific desmoplakin interactions with either PKP1, PKP2 or PKP3, or combinations thereof, are involved in the selective recruitment of plakophilins to the desmosomal plaques.

Extracellularly truncated desmoglein 1 compromises desmosomes in MDCK cells

The formation and stability of epithelial tissue involves cell adhesion and the connection of the intermediate filaments of contiguous cells, mediated by desmosomes. The cadherin family members Desmocollins (Dsc) and Desmogleins (Dsg) mediate desmosome extracellular adhesion. The main intracellular molecules identified linking Dscs and Dsgs with the intermediate filament network are Plakoglobin (PG), Plakophilins (PPs) and Desmoplakin (DP). Previous studies on desmosome-mediated adhesion have focused on the intracellular domains of Dsc and Dsg because of their capacity to interact with PG, PPs and DP. This study examines the role of the extracellular domain of Dsg1 upon desmosome stability in MDCK cells. Dsg1 was constructed containing an extracellular deletion (Dsg delta 1EC) and was expressed in MDCK cells. A high expressor Dsg delta 1EC/MDCK clone was obtained and analysed for its capacity to form desmosomes in cell monolayers and when growing under mechanical stress in three-dimensional collagen cultures. Phenotypic changes associated with the ectopic expression of Dsg1 delta EC in MDCK cells were: disturbance of the cytokeratin network, a change in the quality and number of desmosomes and impairment of the formation of cysts in suspension cultures. Interestingly, Dsg1 delta EC was not localized in desmosomes, but was still able to maintain its intracytoplasmic interaction with PG, suggesting that the disruptive effects were largely due to PG and/or PP sequestration.

The function of plakophilin 1 in desmosome assembly and actin filament organization

Plakophilin 1, a member of the armadillo multigene family, is a protein with dual localization in the nucleus and in desmosomes. To elucidate its role in desmosome assembly and regulation, we have analyzed its localization and binding partners in vivo. When overexpressed in HaCaT keratinocytes, plakophilin 1 localized to the nucleus and to desmosomes, and dramatically enhanced the recruitment of desmosomal proteins to the plasma membrane. This effect was mediated by plakophilin 1's head domain, which interacted with desmoglein 1, desmoplakin, and keratins in the yeast two-hybrid system. Overexpression of the armadillo repeat domain induced a striking dominant negative phenotype with the formation of filopodia and long cellular protrusions, where plakophilin 1 colocalized with actin filaments. This phenotype was strictly dependent on a conserved motif in the center of the armadillo repeat domain. Our results demonstrate that plakophilin 1 contains two functionally distinct domains: the head domain, which could play a role in organizing the desmosomal plaque in suprabasal cells, and the armadillo repeat domain, which might be involved in regulating the dynamics of the actin cytoskeleton.

Molecular map of the desmosomal plaque

Recent biochemical and molecular approaches have begun to establish the protein interactions that lead to desmosome assembly. To determine whether these associations occur in native desmosomes we have performed ultrastructural localisation of specific domains of the major desmosomal components and have used the results to construct a molecular map of the desmosomal plaque. Antibodies directed against the amino- and carboxy-terminal domains of desmoplakin, plakoglobin and plakophilin 1, and against the carboxy-terminal domains of desmoglein 3, desmocollin 2a and desmocollin 2b, were used for immunogold labelling of ultrathin cryosections of bovine nasal epidermis. For each antibody, the mean distance of the gold particles, and thus the detected epitope, from the cytoplasmic surface of the plasma membrane was determined quantitatively. Results showed that: (i) plakophilin, although previously shown to bind intermediate filaments in vitro, is localised extremely close to the plasma membrane, rather than in the region where intermediate filaments are seen to insert into the desmosomal plaque; (ii) while the ‘a’ form of desmocollin overlaps with plakoglobin and desmoplakin, the shorter ‘b’ form may be spatially separated from them; (iii) desmoglein 3 extends across the entire outer plaque, beyond both desmocollins; (iv) the amino terminus of desmoplakin lies within the outer dense plaque and the carboxy terminus some 40 nm distant in the zone of intermediate filament attachment. This is consistent with a parallel arrangement of desmoplakin in dimers or higher order aggregates and with the predicted length of desmoplakin II, indicating that desmoplakin I may be folded or coiled. Thus several predictions from previous work were borne out by this study, but in other cases our observations yielded unexpected results. These results have significant implications relating to molecular interactions in desmosomes and emphasise the importance of applying multiple and complementary approaches to biological investigations.