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

Plakophilin-3 Binds the Membrane and Filamentous Actin without Bundling F-Actin

Plakophilin-3 is a ubiquitously expressed protein found widely in epithelial cells and is a critical component of desmosomes. The plakophilin-3 carboxy-terminal domain harbors nine armadillo repeat motifs with largely unknown functions. Here, we report the 5 Å cryogenic electron microscopy (cryoEM) structure of the armadillo repeat motif domain of plakophilin-3, one of the smaller cryoEM structures reported to date. We find that this domain is a monomer or homodimer in solution. In addition, using an in vitro actin co-sedimentation assay, we show that the armadillo repeat domain of plakophilin-3 directly interacts with F-actin. This feature, through direct interactions with actin filaments, could be responsible for the observed association of extra-desmosomal plakophilin-3 with the actin cytoskeleton directly attached to the adherens junctions in A431 epithelial cells. Further, we demonstrate, through lipid binding analyses, that plakophilin-3 can effectively be recruited to the plasma membrane through phosphatidylinositol-4,5-bisphosphate-mediated interactions. Collectively, we report on novel properties of plakophilin-3, which may be conserved throughout the plakophilin protein family and may be behind the roles of these proteins in cell-cell adhesion.

Desmosomes undergo dynamic architectural changes during assembly and maturation

Desmosomes are macromolecular cell-cell junctions critical for maintaining adhesion and resisting mechanical stress in epithelial tissue. Desmosome assembly and the relationship between maturity and molecular architecture are not well understood. To address this, we employed a calcium switch assay to synchronize assembly followed by quantification of desmosome nanoscale organization using direct Stochastic Optical Reconstruction Microscopy (dSTORM). We found that the organization of the desmoplakin rod/C-terminal junction changed over the course of maturation, as indicated by a decrease in the plaque-to-plaque distance, while the plaque length increased. In contrast, the desmoplakin N-terminal domain and plakoglobin organization (plaque-to-plaque distance) were constant throughout maturation. This structural rearrangement of desmoplakin was concurrent with desmosome maturation measured by E-cadherin exclusion and increased adhesive strength. Using two-color dSTORM, we showed that while the number of individual E-cadherin containing junctions went down with the increasing time in high Ca²⁺, they maintained a wider desmoplakin rod/C-terminal plaque-to-plaque distance. This indicates that the maturation state of individual desmosomes can be identified by their architectural organization. We confirmed these architectural changes in another model of desmosome assembly, cell migration. Desmosomes in migrating cells, closest to the scratch where they are assembling, were shorter, E-cadherin enriched, and had wider desmoplakin rod/C-terminal plaque-to-plaque distances compared to desmosomes away from the wound edge. Key results were demonstrated in three cell lines representing simple, transitional, and stratified epithelia. Together, these data suggest that there is a set of architectural programs for desmosome maturation, and we hypothesize that desmoplakin architecture may be a contributing mechanism to regulating adhesive strength.

Plakophilin 3 and Par3 facilitate desmosomes' association with the apical junctional complex

Desmosomes (DSMs), together with adherens junctions (AJs) and tight junctions (TJs), constitute the apical cell junctional complex (AJC). While the importance of the apical and basolateral polarity machinery in the organization of AJs and TJs is well established, how DSMs are positioned within the AJC is not understood. Here we use highly polarized DLD1 cells as a model to address how DSMs integrate into the AJC. We found that knockout (KO) of the desmosomal ARM protein Pkp3, but not other major DSM proteins, uncouples DSMs from the AJC without blocking DSM assembly. DLD1 cells also exhibit a prominent extraDSM pool of Pkp3, concentrated in tricellular (tC) contacts. Probing distinct apicobasal polarity pathways revealed that neither the DSM’s association with AJC nor the extraDSM pool of Pkp3 are abolished in cells with defects in Scrib module proteins responsible for basolateral membrane development. However, a loss of the apical polarity protein, Par3, completely eliminates the extraDSM pool of Pkp3 and disrupts AJC localization of desmosomes, dispersing these junctions along the entire length of cell–cell contacts. Our data are consistent with a model whereby Par3 facilitates DSM assembly within the AJC, controlling the availability of an assembly competent pool of Pkp3 stored in tC contacts.

Regulation of intestinal epithelial intercellular adhesion and barrier function by desmosomal cadherin desmocollin-2

The role of desmosomal cadherin desmocollin-2 (Dsc2) in regulating barrier function in intestinal epithelial cells (IECs) is not well understood. Here, we report the consequences of silencing Dsc2 on IEC barrier function in vivo using mice with inducible intestinal-epithelial-specific Dsc2 knockdown (KD) (Dsc2ERΔIEC). While the small intestinal gross architecture was maintained, loss of epithelial Dsc2 influenced desmosomal plaque structure, which was smaller in size and had increased intermembrane space between adjacent epithelial cells. Functional analysis revealed that loss of Dsc2 increased intestinal permeability in vivo, supporting a role for Dsc2 in the regulation of intestinal epithelial barrier function. These results were corroborated in model human IECs in which Dsc2 KD resulted in decreased cell-cell adhesion and impaired barrier function. It is noteworthy that Dsc2 KD cells exhibited delayed recruitment of desmoglein-2 (Dsg2) to the plasma membrane after calcium switch-induced intercellular junction reassembly, while E-cadherin accumulation was unaffected. Mechanistically, loss of Dsc2 increased desmoplakin (DP I/II) protein expression and promoted intermediate filament interaction with DP I/II and was associated with enhanced tension on desmosomes as measured by a Dsg2-tension sensor. In conclusion, we provide new insights on Dsc2 regulation of mechanical tension, adhesion, and barrier function in IECs.

Dual Functional States of R406W-Desmin Assembly Complexes Cause Cardiomyopathy With Severe Intercalated Disc Derangement in Humans and in Knock-In Mice

BACKGROUND

Mutations in the human desmin gene cause myopathies and cardiomyopathies. This study aimed to elucidate molecular mechanisms initiated by the heterozygous R406W-desmin mutation in the development of a severe and early-onset cardiac phenotype.

METHODS

We report an adolescent patient who underwent cardiac transplantation as a result of restrictive cardiomyopathy caused by a heterozygous R406W-desmin mutation. Sections of the explanted heart were analyzed with antibodies specific to 406W-desmin and to intercalated disc proteins. Effects of the R406W mutation on the molecular properties of desmin were addressed by cell transfection and in vitro assembly experiments. To prove the genuine deleterious effect of the mutation on heart tissue, we further generated and analyzed R405W-desmin knock-in mice harboring the orthologous form of the human R406W-desmin.

RESULTS

Microscopic analysis of the explanted heart revealed desmin aggregates and the absence of desmin filaments at intercalated discs. Structural changes within intercalated discs were revealed by the abnormal organization of desmoplakin, plectin, N-cadherin, and connexin-43. Next-generation sequencing confirmed the DES variant c.1216C>T (p.R406W) as the sole disease-causing mutation. Cell transfection studies disclosed a dual behavior of R406W-desmin with both its integration into the endogenous intermediate filament system and segregation into protein aggregates. In vitro, R406W-desmin formed unusually thick filaments that organized into complex filament aggregates and fibrillar sheets. In contrast, assembly of equimolar mixtures of mutant and wild-type desmin generated chimeric filaments of seemingly normal morphology but with occasional prominent irregularities. Heterozygous and homozygous R405W-desmin knock-in mice develop both a myopathy and a cardiomyopathy. In particular, the main histopathologic results from the patient are recapitulated in the hearts from R405W-desmin knock-in mice of both genotypes. Moreover, whereas heterozygous knock-in mice have a normal life span, homozygous animals die at 3 months of age because of a smooth muscle-related gastrointestinal phenotype.

CONCLUSIONS

We demonstrate that R406W-desmin provokes its severe cardiotoxic potential by a novel pathomechanism, where the concurrent dual functional states of mutant desmin assembly complexes underlie the uncoupling of desmin filaments from intercalated discs and their structural disorganization.

The cell-cell junctions of mammalian testes. III. Absence of an endothelial cell layer covering the peritubular wall of the seminiferous tubules-an immunocytochemical correction of a 50-year-old error in the literature

In the molecular biological and ultrastructural studies of the peritubular wall cells encasing the seminiferous tubules of mammalian testes, we found it necessary to characterize the outermost cell layer bordering on the interstitial space in detail. For half a century, the extremely thin cells of this monolayer have in the literature been regarded as part of a lymphatic endothelium, in particular in rodents. However, our double-label immunofluorescence microscopical results have shown that in all six mammalian species examined, including three rodent ones (rat, mouse, guinea pig), this classification is not correct: the very attenuated cells of this monolayer are not of lymphatic endothelial nature as they do not contain established endothelial marker molecules. In particular, they do not contain claudin-5-positive tight junctions, VE-cadherin-positive adherens junctions, "lymph vessel endothelium hyaluronan receptor 1" (LYVE-1), podoplanin, protein myozap and "von Willebrand Factor" (vWF). By contrast and as controls, all these established marker molecules for the lymphatic endothelial cell type are found in the endothelia of the lymph and-partly also-blood vessels located nearby in the interstitial space. Thus, our results provide evidence that the monolayer cells covering the peritubular wall do not contain endothelial marker molecules and hence are not endothelial cells. We discuss possible methodological reasons for the maintenance of this incorrect cell type classification in the literature and emphasize the value of molecular analyses using multiple cell type-specific markers, also with respect to physiology and medical sciences.

A rim-and-spoke hypothesis to explain the biomechanical roles for cytoplasmic intermediate filament networks

Textbook images of keratin intermediate filament (IF) networks in epithelial cells and the functional compromization of the epidermis by keratin mutations promulgate a mechanical role for this important cytoskeletal component. In stratified epithelia, keratin filaments form prominent radial spokes that are focused onto cell-cell contact sites, i.e. the desmosomes. In this Hypothesis, we draw attention to a subset of keratin filaments that are apposed to the plasma membrane. They form a rim of filaments interconnecting the desmosomes in a circumferential network. We hypothesize that they are part of a rim-and-spoke arrangement of IFs in epithelia. From our review of the literature, we extend this functional role for the subplasmalemmal rim of IFs to any cell, in which plasma membrane support is required, provided these filaments connect directly or indirectly to the plasma membrane. Furthermore, cytoplasmic IF networks physically link the outer nuclear and plasma membranes, but their participation in mechanotransduction processes remain largely unconsidered. Therefore, we also discuss the potential biomechanical and mechanosensory role(s) of the cytoplasmic IF network in terms of such a rim (i.e. subplasmalemmal)-and-spoke arrangement for cytoplasmic IF networks.

Desmosomal adhesion in vivo

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

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.

Mechanistic basis of desmosome-targeted diseases

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

In vitro functional analyses of arrhythmogenic right ventricular cardiomyopathy-associated desmoglein-2-missense variations

Background

Although numerous sequence variants in desmoglein-2 (DSG2) have been associated with arrhythmogenic right ventricular cardiomyopathy (ARVC), the functional impact of new sequence variations is difficult to estimate.

Methodology/Principal Findings

To test the functional consequences of DSG2-variants, we established an expression system for the extracellular domain and the full-length DSG2 using the human cell line HT1080. We established new tools to investigate ARVC-associated DSG2 variations and compared wild-type proteins and proteins with one of the five selected variations (DSG2-p.R46Q, -p.D154E, -p.D187G, -p.K294E, -p.V392I) with respect to prodomain cleavage, adhesion properties and cellular localisation.

Conclusions/Significance

The ARVC-associated DSG2-p.R46Q variation was predicted to be probably damaging by bioinformatics tools and to concern a conserved proprotein convertase cleavage site. In this study an impaired prodomain cleavage and an influence on the DSG2-properties could be demonstrated for the R46Q-variant leading to the classification of the variant as a potential gain-of-function mutant. In contrast, the variants DSG2-p.K294E and -p.V392I, which have an arguable impact on ARVC pathogenesis and are predicted to be benign, did not show functional differences to the wild-type protein in our study. Notably, the variants DSG2-p.D154E and -p.D187G, which were predicted to be damaging by bioinformatics tools, had no detectable effects on the DSG2 protein properties in our study.

Desmoplakin acts as a tumor suppressor by inhibition of the Wnt/β-catenin signaling pathway in human lung cancer

Desmosomes are intercellular junctions that confer strong cell-cell adhesion, thus conferring resistance against mechanical stress on epithelial tissues. A body of evidence indicates that decreased expression of desmosomal proteins is associated with poor prognosis in various cancers. As a key component of desmosomal plaque proteins, the functional role of desmoplakin (DSP) in cancer is not yet elucidated. Here, we reported the anti-tumorigenic activity of DSP in non-small cell lung cancer (NSCLC). We found by DSP DNA methylation that DSP expression was downregulated in 8 out of 11 lung cancer cell lines and in 34 out of 56 primary lung tumors . Ectopic expression of DSP in the NSCLC cell line H157 significantly inhibited cell proliferation, anchorage-independent growth, migration and invasion and also increased the sensitivity of NSCLC cells to apoptosis induced by an anticancer drug, gemcitabine. Furthermore, overexpression of DSP enhanced expression of plakoglobin (γ-catenin), resulting in decreased T-cell factor/lymphoid enhancer factor (TCF/LEF)-dependent transcriptional activity and reduced expression of the Wnt/β-catenin target genes Axin2 and matrix metalloproteinase MMP14. In accordance, DSP suppression by small interfering RNA resulted in downregulation of plakoglobin and upregulation of β-catenin and MMP14. Taken together, these data suggest that DSP is inactivated in lung cancer by an epigenetic mechanism, increases the sensitivity to anticancer drug-induced apoptosis and has tumor-suppressive function, possibly through inhibition of the Wnt/β-catenin signaling pathway in NSCLC cells. The epigenetic regulation of DSP and its ability to increase the sensitivity to anticancer drug-induced apoptosis has potential implications for clinical application.

A dual function of Bcl11b/Ctip2 in hippocampal neurogenesis

The transcription factor Bcl11b/Ctip2 promotes hippocampal progenitor proliferation and neural differentiation in a non-cell autonomous manner by regulating the expression of the cell adhesion molecule Desmoplakin. Forebrain-specific ablation causes defective spatial learning and memory.

The development of the dentate gyrus is characterized by distinct phases establishing a durable stem-cell pool required for postnatal and adult neurogenesis. Here, we report that Bcl11b/Ctip2, a zinc finger transcription factor expressed in postmitotic neurons, plays a critical role during postnatal development of the dentate gyrus. Forebrain-specific ablation of Bcl11b uncovers dual phase-specific functions of Bcl11b demonstrated by feedback control of the progenitor cell compartment as well as regulation of granule cell differentiation, leading to impaired spatial learning and memory in mutants. Surprisingly, we identified Desmoplakin as a direct transcriptional target of Bcl11b. Similarly to Bcl11b, postnatal neurogenesis and granule cell differentiation are impaired in Desmoplakin mutants. Re-expression of Desmoplakin in Bcl11b mutants rescues impaired neurogenesis, suggesting Desmoplakin to be an essential downstream effector of Bcl11b in hippocampal development. Together, our data define an important novel regulatory pathway in hippocampal development, by linking transcriptional functions of Bcl11b to Desmoplakin, a molecule known to act on cell adhesion.

Plakophilin-2: a cell-cell adhesion plaque molecule of selective and fundamental importance in cardiac functions and tumor cell growth

Within the characteristic ensemble of desmosomal plaque proteins, the armadillo protein plakophilin-2 (Pkp2) is known as a particularly important regulatory component in the cytoplasmic plaques of various other cell-cell junctions, such as the composite junctions (areae compositae) of the myocardiac intercalated disks and in the variously-sized and -shaped complex junctions of permanent cell culture lines derived therefrom. In addition, Pkp2 has been detected in certain protein complexes in the nucleoplasm of diverse kinds of cells. Using a novel set of highly sensitive and specific antibodies, both kinds of Pkp2, the junctional plaque-bound and the nuclear ones, can also be localized to the cytoplasmic plaques of diverse non-desmosomal cell-cell junction structures. These are not only the puncta adhaerentia and the fasciae adhaerentes connecting various types of highly proliferative non-epithelial cells growing in culture but also some very proliferative states of cardiac interstitial cells and cardiac myxomata, including tumors growing in situ as well as fetal stages of heart development and cultures of valvular interstitial cells. Possible functions and assembly mechanisms of such Pkp2-positive cell-cell junctions as well as medical consequences are discussed.

Epithelial junction opener JO-1 improves monoclonal antibody therapy of cancer

The efficacy of monoclonal antibodies (mAb) used to treat solid tumors is limited by intercellular junctions which tightly link epithelial tumor cells to each another. In this study, we define a small, recombinant adenovirus serotype 3-derived protein, termed junction opener 1 (JO-1), which binds to the epithelial junction protein desmoglein 2 (DSG2). In mouse xenograft models employing Her2/neu- and EGFR-positive human cancer cell lines, JO-1 mediated cleavage of DSG2 dimers and activated intracellular signaling pathways which reduced E-cadherin expression in tight junctions. Notably, JO-1-triggered changes allowed for increased intratumoral penetration of the anti-Her2/neu mAb trastuzumab (Herceptin) and improved access to its target receptor, Her2/neu, which is partly trapped in tight junctions. This effect translated directly into increased therapeutic efficacy of trastuzumab in mouse xenograft models using breast, gastric, and ovarian cancer cells that were Her2/neu-positive. Furthermore, combining JO-1 with the EGFR-targeting mAb cetuximab (Erbitux) greatly improved therapeutic outcomes in a metastatic model of EGFR-positive lung cancer. A combination of JO-1 with an approach that triggered transient degradation of tumor stroma proteins elicited eradication of tumors. Taken together, our findings offer preclinical proof of concept to employ JO-1 in combination with mAb therapy.

Desmosomes: new perpetrators in tumour suppression

Adherens junctions, which are intercellular adhesive complexes that are crucial for maintaining epithelial homeostasis, are downregulated in many cancers to promote tumour progression. However, the role of desmosomes - adhesion complexes that are related to adherens junctions - in carcinogenesis has remained elusive. Recent studies using mouse genetic approaches have uncovered a role for desmosomes in tumour suppression, demonstrating that desmosome downregulation occurs before that of adherens junctions to drive tumour development and early invasion, suggesting a two-step model of adhesion dysfunction in cancer progression.

E-cadherin and plakoglobin recruit plakophilin3 to the cell border to initiate desmosome assembly

A decrease in the levels of the desmosomal plaque protein, plakophilin3 (PKP3), leads to a decrease in desmosome size and cell-cell adhesion. To test the hypothesis that PKP3 is required for desmosome formation, the recruitment of desmosomal components to the cell surface was studied in the PKP3 knockdown clones. The PKP3 knockdown clones showed decreased cell border staining for multiple desmosomal proteins, when compared to vector controls, and did not form desmosomes in a calcium switch assay. Further analysis demonstrated that PKP3, plakoglobin (PG) and E-cadherin are present at the cell border at low concentrations of calcium. Loss of either PG or E-cadherin led to a decrease in the levels of PKP3 and other desmosomal proteins at the cell border. The results reported here are consistent with the model that PG and E-cadherin recruit PKP3 to the cell border to initiate desmosome formation.

Membrane-impermeable cross-linking provides evidence for homophilic, isoform-specific binding of desmosomal cadherins in epithelial cells

Desmosomes and adherens junctions are cadherin-based protein complexes responsible for cell-cell adhesion of epithelial cells. Type 1 cadherins of adherens junctions show specific homophilic adhesion that plays a major role in developmental tissue segregation. The desmosomal cadherins, desmocollin and desmoglein, occur as several different isoforms with overlapping expression in some tissues where different isoforms are located in the same desmosomes. Although adhesive binding of desmosomal cadherins has been investigated in a variety of ways, their interaction in desmosome-forming epithelial cells has not been studied. Here, using extracellular homobifunctional cross-linking, we provide evidence for homophilic and isoform-specific binding between the Dsc2, Dsc3, Dsg2, and Dsg3 isoforms in HaCaT keratinocytes and show that it represents trans interaction. Furthermore, the cross-linked adducts are present in the detergent-insoluble fraction, and electron microscopy shows that extracellular cross-linking probably occurs in desmosomes. We found no evidence for either heterophilic or cis interaction, but neither can be completely excluded by our data. Mutation of amino acid residues Trp-2 and Ala-80 that are important for trans interaction in classical cadherin adhesive binding abolished Dsc2 binding, indicating that these residues are also involved in desmosomal adhesion. These interactions of desmosomal cadherins may be of key importance for their ordered arrangement within desmosomes that we believe is essential for desmosomal adhesive strength and the maintenance of tissue integrity.

Intercellular junction assembly, dynamics, and homeostasis

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

Desmosomes: adhesive strength and signalling in health and disease

Desmosomes are intercellular junctions whose primary function is strong intercellular adhesion, known as hyperadhesion. In the present review, we discuss how their structure appears to support this function as well as how they are assembled and down-regulated. Desmosomal components also have signalling functions that are important in tissue development and remodelling. Their adhesive and signalling functions are both compromised in genetic and autoimmune diseases that affect the heart, skin and mucous membranes. We conclude that much work is required on structure–function relationships within desmosomes in vivo and on how they participate in signalling processes to enhance our knowledge of tissue homoeostasis and human disease.

Desmosomal molecules in and out of adhering junctions: normal and diseased States of epidermal, cardiac and mesenchymally derived cells

Current cell biology textbooks mention only two kinds of cell-to-cell adhering junctions coated with the cytoplasmic plaques: the desmosomes (maculae adhaerentes), anchoring intermediate-sized filaments (IFs), and the actin microfilament-anchoring adherens junctions (AJs), including both punctate (puncta adhaerentia) and elongate (fasciae adhaerentes) structures. In addition, however, a series of other junction types has been identified and characterized which contain desmosomal molecules but do not fit the definition of desmosomes. Of these special cell-cell junctions containing desmosomal glycoproteins or proteins we review the composite junctions (areae compositae) connecting the cardiomyocytes of mature mammalian hearts and their importance in relation to human arrhythmogenic cardiomyopathies. We also emphasize the various plakophilin-2-positive plaques in AJs (coniunctiones adhaerentes) connecting proliferatively active mesenchymally-derived cells, including interstitial cells of the heart and several soft tissue tumor cell types. Moreover, desmoplakin has also been recognized as a constituent of the plaques of the complexus adhaerentes connecting certain lymphatic endothelial cells. Finally, we emphasize the occurrence of the desmosomal transmembrane glycoprotein, desmoglein Dsg2, out of the context of any junction as dispersed cell surface molecules in certain types of melanoma cells and melanocytes. This broadening of our knowledge on the diversity of AJ structures indicates that it may still be too premature to close the textbook chapters on cell-cell junctions.

Desmosomal plakophilins in the prostate and prostatic adenocarcinomas: implications for diagnosis and tumor progression

The plakophilins, members of the armadillo-repeat family, consist of three different proteins (PKP1-3) that are specifically recruited to desmosomal plaques in a highly cell type-specific manner. Using immunofluorescence, immunoelectron microscopy, and immunoblot, we found that all three plakophilins occurred in luminal and basal cells of the pseudostratified prostate epithelium. The analysis of 135 cases of prostatic adenocarcinomas grouped into tumors with low (Gleason score < or = 6), intermediate (Gleason score 7), and high Gleason score (8 < or = Gleason score < or = 10) showed that the expression of PKP1 was reduced or lost in adenocarcinomas with high Gleason scores. The expression of PKP2 was unchanged in all prostatic adenocarcinomas analyzed. In contrast, PKP3 expression was increased in carcinomas with high Gleason scores in comparison with carcinomas with low Gleason scores. In DU 145 cell lines with either overexpression or knockdown of PKP3, both imbalances resulted in fewer desmosomal cell contacts. In addition, overexpression of PKP3 in DU 145 cells led to an augmentation in proliferation rate. Our data imply that both loss of PKP1 and up-regulation of PKP3 expression are biologically important events in prostate cancer and are associated with a more aggressive phenotype.

Plakophilins: multifunctional scaffolds for adhesion and signaling

Armadillo family proteins known as plakophilins have been characterized as structural components of desmosomes that stabilize and strengthen adhesion by enhancing attachments with the intermediate filament cytoskeleton. However, plakophilins and their close relatives are emerging as versatile scaffolds for multiple signaling and metabolic processes that not only facilitate junction dynamics but also more globally regulate diverse cellular activities. While perturbation of plakophilin functions contribute to inherited diseases and cancer pathogenesis, the functional significance of the multiple PKP isoforms and the mechanisms by which their behaviors are regulated remain to be elucidated.

Upregulation of plakophilin-2 and its acquisition to adherens junctions identifies a novel molecular ensemble of cell-cell-attachment characteristic for transformed mesenchymal cells

In contrast to the desmosome-containing epithelial and carcinoma cells, normal and malignantly transformed cells derived from mesenchymal tissues and tumors are connected only by adherens junctions (AJs) containing N-cadherins and/or cadherin-11, anchored in a cytoplasmic plaque assembled by alpha- and beta-catenin, plakoglobin, proteins p120 and p0071. Here, we report that the AJs of many malignantly transformed cell lines are characterized by the additional presence of plakophilin-2 (Pkp2), a protein hitherto known only as a major component of desmosomal plaques, i.e., AJs of epithelia and carcinomatous cells. This massive acquisition of Pkp2 and its integration into AJ plaques of a large number of transformed cell lines is demonstrated with biochemical and immunolocalization techniques. Upregulation of Pkp2 and its integration into AJs has also been noted in some soft tissue tumors insitu and some highly proliferative colonies of cultured mesenchymal stem cells. As Pkp2 has recently been identified as a functionally important major regulatory organizer in AJs and related junctions in epithelial cells and cardiomyocytes, we hypothesize that the integration of Pkp2 into AJs of "soft tissue tumor" cells also can serve functions in the upregulation of proliferation, the promotion of malignant growth in general as well as the close-packing of diverse kinds of cells and the metastatic behavior of such tumors. We propose to examine its presence in transformed mesenchymal cells and related tumors and to use it as an additional diagnostic criterion.

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.

Cordial connections: molecular ensembles and structures of adhering junctions connecting interstitial cells of cardiac valves in situ and in cell culture

Remarkable efforts have recently been made in the tissue engineering of heart valves to improve the results of valve transplantations and replacements, including the design of artificial valves. However, knowledge of the cell and molecular biology of valves and, specifically, of valvular interstitial cells (VICs) remains limited. Therefore, our aim has been to determine and localize the molecules forming the adhering junctions (AJs) that connect VICs in situ and in cell culture. Using biochemical and immunolocalization methods at the light- and electron-microscopic levels, we have identified, in man, cow, sheep and rat, the components of VIC-connecting AJs in situ and in cell culture. These AJs contain, in addition to the transmembrane glycoproteins N-cadherin and cadherin-11, the typical plaque proteins alpha- and beta-catenin as well as plakoglobin and p120, together with minor amounts of protein p0071, i.e. a total of five plaque proteins of the armadillo family. While we can exclude the occurrence of desmogleins, desmocollins and desmoplakin, we have noted with surprise that AJs of VICs in cell cultures, but not those growing in the valve tissue, contain substantial amounts of the desmosomal plaque protein, plakophilin-2. Clusters of AJs occur not only on the main VIC cell bodies but are also found widely dispersed on their long filopodia thus forming, in the tissue, a meshwork that, together with filopodial attachments to paracrystalline collagen fiber bundles, establishes a three-dimensional suprastructure, the role of which is discussed with respect to valve formation, regeneration and function.

Endothelial and virgultar cell formations in the mammalian lymph node sinus: endothelial differentiation morphotypes characterized by a special kind of junction (complexus adhaerens)

The lymph node sinus are channel structures of unquestionable importance in immunology and pathology, specifically in the filtering of the lymph, the transport and processing of antigens, the adhesion and migration of immune cells, and the spread of metastatic cancer cells. Our knowledge of the cell and molecular biology of the sinus-forming cells is still limited, and the origin and biological nature of these cells have long been a matter of debate. Here, we review the relevant literature and present our own experimental results, in particular concerning molecular markers of intercellular junctions and cell differentiation. We show that both the monolayer cells lining the sinus walls and the intraluminal virgultar cell meshwork are indeed different morphotypes of the same basic endothelial cell character, as demonstrated by the presence of a distinct spectrum of general and lymphatic endothelial markers, and we therefore refer to these cells as sinus endothelial/virgultar cells (SEVCs). These cells are connected by unique adhering junctions, termed complexus adhaerentes, characterized by the transmembrane glycoprotein VE-cadherin, combined with the desmosomal plaque protein desmoplakin, several adherens junction plaque proteins including alpha- and beta-catenin and p120 catenin, and components of the tight junction ensemble, specifically claudin-5 and JAM-A, and the plaque protein ZO-1. We show that complexus adhaerentes are involved in the tight three-dimensional integration of the virgultar network of SEVC processes along extracellular guidance structures composed of paracrystalline collagen bundle "stays". Overall, the SEVC system might be considered as a local and specific modification of the general lymphatic vasculature system. Finally, physiological and pathological alterations of the SEVC system will be presented, and the possible value of the molecular markers described in histological diagnoses of autochthonous lymph node tumors will be discussed.

Subtypes of melanocytes and melanoma cells distinguished by their intercellular contacts: heterotypic adherens junctions, adhesive associations, and dispersed desmoglein 2 glycoproteins

In the tissue integration of melanocytes and melanoma cells, an important role is attributed to cell adhesion molecules, notably the cadherins. In cultured melanoma cells, we have previously described a more heterogeneous repertoire of cadherins than normal, including some melanoma subtypes synthesizing the desmosomal cadherin, desmoglein 2, out of the desmosomal context. Using biochemical and immunological characterization of junctional molecules, confocal laser scanning, and electron and immunoelectron microscopy, we now demonstrate homo- and heterotypic cell-cell adhesions of normal epidermal melanocytes. In human epidermis, both in situ and in cell culture, melanocytes and keratinocytes are connected by closely aligned membranes that are interspersed by small puncta adhaerentia containing heterotypic complexes of E- and P-cadherin. Moreover, melanocytes growing in culture often begin to synthesize desmoglein 2, which is dispersed over extended areas of intimate adhesive cell-cell associations. As desmoglein 2 is not found in melanocytes in situ, we hypothesize that its synthesis is correlated with cell proliferation. Indeed, in tissue microarrays, desmoglein 2 has been demonstrated in a sizable subset of nevi and primary melanomas. The biological meanings of these cell-cell adhesion molecule arrangements, the possible diagnostic and prognostic significance of these findings, and the implications of the heterogeneity types of melanomas are discussed.

Differential expression pattern of protein ARVCF in nephron segments of human and mouse kidney

The protein ARVCF is a member of the p120 subfamily of armadillo proteins whose members have been described to occur in junction-bound and non-junction-bound forms. Studies on ARVCF were constrained because the endogenous protein was difficult to detect with the available reagents. We have generated novel monoclonal and polyclonal antibodies usable for biochemical and localization studies. By systematic immunohistochemical analysis of various tissues protein ARVCF is prominently detected in mouse, bovine and human kidney. Using antibodies against specific markers of nephron segments protein ARVCF is localized in proximal tubules according to double label immunofluorescence. Besides its occurrence in proximal tubules of adult kidney and in renal cell carcinoma derived from proximal tubules ARVCF is also detected in maturing nephrons in early mouse developmental stages such as, for example, 15 days of gestation (E15). Immunoblotting of total extracts of cultured cells of renal origin showed that ARVCF is detected in all human and murine cultured cells analyzed. Upon immunolocalization ARVCF is mostly detected in the cytoplasm occurring in a fine granular form. This prominent cytoplasmic localization of ARVCF in cultured cells and its occurrence in proximal tubules implies an involvement of ARVCF in specific functional processes of proximal tubules of kidney.

The desmosome and pemphigus

Desmosomes are patch-like intercellular adhering junctions ("maculae adherentes"), which, in concert with the related adherens junctions, provide the mechanical strength to intercellular adhesion. Therefore, it is not surprising that desmosomes are abundant in tissues subjected to significant mechanical stress such as stratified epithelia and myocardium. Desmosomal adhesion is based on the Ca(2+)-dependent, homo- and heterophilic transinteraction of cadherin-type adhesion molecules. Desmosomal cadherins are anchored to the intermediate filament cytoskeleton by adaptor proteins of the armadillo and plakin families. Desmosomes are dynamic structures subjected to regulation and are therefore targets of signalling pathways, which control their molecular composition and adhesive properties. Moreover, evidence is emerging that desmosomal components themselves take part in outside-in signalling under physiologic and pathologic conditions. Disturbed desmosomal adhesion contributes to the pathogenesis of a number of diseases such as pemphigus, which is caused by autoantibodies against desmosomal cadherins. Beside pemphigus, desmosome-associated diseases are caused by other mechanisms such as genetic defects or bacterial toxins. Because most of these diseases affect the skin, desmosomes are interesting not only for cell biologists who are inspired by their complex structure and molecular composition, but also for clinical physicians who are confronted with patients suffering from severe blistering skin diseases such as pemphigus. To develop disease-specific therapeutic approaches, more insights into the molecular composition and regulation of desmosomes are required.

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.

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.

The area composita of adhering junctions connecting heart muscle cells of vertebrates. V. The importance of plakophilin-2 demonstrated by small interference RNA-mediated knockdown in cultured rat cardiomyocytes

In the adult mammalian heart, the cardiomyocytes are connected by large polar arrays of closely spaced or even fused composite, plaque-bearing adhering junctions (areae compositae, ACs), in a region usually termed "intercalated disk" (ID). We have recently reported that during late embryogenesis and postnatally these polar assemblies of AC-junction structures are gradually formed as replacements of distinct embryonal junctions representing desmosomes and fasciae adhaerentes which then may amalgamate to the fused AC structures, in some regions occupying more than 90% of the total ID area. Previous gene knockout results as well as mutation analyses of specific human cardiomyopathies have suggested that among the various AC constituents, the desmosomal plaque protein, plakophilin-2, plays a particularly important role in the formation, architectural organization and stability of these junctions interconnecting mature cardiomyocytes. To examine this hypothesis, we have decided to study losses of--or molecular alterations in--such AC proteins with respect to their effects on myocardiac organization and functions. Here we report that plakophilin-2 is indeed of obvious importance for myocardial architecture and cell-cell coupling of rat cardiomyocytes growing in culture. We show that siRNA-mediated reduction of the cardiomyocyte content of plakophilin-2 but not of some other major plaque components such as desmoplakin results in progressive disintegration--and losses--of AC junction structures and that numerous variously sized vesicles appear, which are plaque protein-associated as demonstrable by immunofluorescence and immunoelectron microscopy. The importance of plakophilin-2 as a kind of "organizer" protein in the formation, stabilization and functions of the AC structure and the ID architecture is discussed in relation to other junction proteins and to causes of certain cardiomyopathies.

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.

Desmosomes: new perspectives on a classic

Desmosomes are highly specialized anchoring junctions that link intermediate filaments to sites of intercellular adhesion, thus facilitating the formation of a supracellular scaffolding that distributes mechanical forces throughout a tissue. These junctions are thus particularly important for maintaining the integrity of tissues that endure physical stress, such as the epidermis and myocardium. The importance of the classic mechanical functions of desmosomal constituents is underscored by pathologies reported in animal models and an ever-expanding list of human mutations that target both desmosomal cadherins and their associated cytoskeletal anchoring proteins. However, the notion that desmosomes are static structures that exist simply to glue cells together belies their susceptibility to remodeling in response to environmental cues and their important tissue-specific roles in cell behavior and signaling. Here, we review the molecular blueprint of the desmosome and models for assembling its protein components to form an adhesive interface and the desmosomal plaque. We also discuss emerging evidence of supra-adhesive roles for desmosomal proteins in regulating tissue morphogenesis and homeostasis. Finally, we highlight the dynamic nature of these adhesive organelles, examining mechanisms in health and disease for modulating adhesive strength and stability of desmosomes.

The area composita of adhering junctions connecting heart muscle cells of vertebrates - IV: coalescence and amalgamation of desmosomal and adhaerens junction components - late processes in mammalian heart development

In the adult mammalian heart, the cardiomyocytes and thus their terminally anchored myofibrillar bundles are connected by large arrays of closely spaced or even fused adhering junctions (AJs), termed "intercalated disks" (IDs). In recent years, the ID complex has attracted special attention as it has become clear that several human hereditary cardiomyopathies are caused by mutations of genes encoding ID marker proteins, in particular some that are also known as constituents of epithelial desmosomes. Previously, we have shown that in the mature myocardial ID the compositional differences between desmosome-like and adhaerens junctions are, by and large, lost and a composite hybrid structure, the area composita, is formed. We now report results from immunofluorescence and (immuno-)electron microscopic studies of heart formation during mouse embryogenesis and postnatal growth and show that the formation of the IDs with extended area composita structures is a late, primarily postnatal process. While up to birth small distinct desmosomes and AJs are resolved as predominant ID structures, areae compositae of increasing sizes and merged marker protein patterns occupy most of the IDs in the mature heart. Differences in the patterns of ID formation and amalgamation of the two ensembles of junction proteins in time and space are also demonstrated. Together with corresponding observations during rat and human heart development our results indicate that ID topogenesis and area composita formation are also late developmental processes in other mammals. We discuss the importance of the ID and the areae compositae in cardiac functions and, consequently, in cardiomyopathies and possible myocardial regeneration processes.

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.

The area composita of adhering junctions connecting heart muscle cells of vertebrates – III: Assembly and disintegration of intercalated disks in rat cardiomyocytes growing in culture

For cell and molecular biological studies of heart formation and function cell cultures of embryonal, neonatal or adult hearts of various vertebrates, notably rat and chicken, have been widely used. As the myocardium-specific cell-cell junctions, the intercalated disks (ID), have recently been found to be particularly sensitive to losses of - or mutations in - certain cytoskeletal proteins, resulting in cardiac damages, we have examined the ID organization in primary cultures of cardiomyocytes obtained from neonatal rats. Using immunofluorescence and immunoelectron microscopy, we have studied the major ID components for up to 2 weeks in culture, paying special attention to spontaneously beating, individual cardiomyocytes and myocardial cell colonies. While our results demonstrate the formation of some ID-like cardiomyocyte-connecting junction arrays, they also reveal a variety of structural disorders such as rather extended, junction-free ID regions, sac-like invaginations and endocytotic blebs as well as accumulations of intracytoplasmic structures suggestive of endocytosed forms of junction-derived vesicles or of junction fragments resembling fascia adhaerens elements. Moreover, we have noticed a novel type of small, obviously plaque-free cytoplasmic vesicles containing one or both of the desmosomal cadherins, desmocollin Dsc2 and desmoglein Dsg2. We conclude that cardiomyocyte cultures are useful model systems for studies of certain aspects of myocardiac differentiation and functions but, on the other hand, show progressive disintegration and deterioration. The potential value of molecular markers and reagents in studies of myocardial pathology as well as in the monitoring of myocardial differentiation of so-called stem cells is discussed.

Bortezomib inhibits cell-cell adhesion and cell migration and enhances epidermal growth factor receptor inhibitor-induced cell death in squamous cell cancer

The lack of cell-cell adhesion and increased migration are key characteristics of cancer cells. The loss of expression of cell adhesion components and overexpression of components critical for cell migration, such as focal adhesion kinase (FAK), correlate with poor prognosis. Because alteration of protein turnover affects the expression levels and, in turn, may influence protein function, we investigated the effects of the proteasome inhibitor bortezomib on cell adhesion and migration in oral squamous cell cancer cell lines SCC68 and SCC15. Following treatment with bortezomib, protein levels of adherens junction components such as E-cadherin were unchanged. The desmosomal linker protein desmoplakin level was increased, whereas the protein level of the desmosomal cadherin, desmoglein 2, was diminished. Reduced desmoglein 2 levels correlated with the diminished strength of mechanical cell-cell adhesion. The protein level of the epidermal growth factor receptor (EGFR) increased after proteasome inhibition and EGFR inhibition with the EGFR-specific tyrosine kinase inhibitor PKI166 was able to restore cell-cell adhesion. Furthermore, we found that the combination of PKI166 with bortezomib enhanced the rate of cell death. Although the FAK protein level was unchanged following bortezomib treatment, recruitment of FAK phosphorylated at tyrosine residue 397 to the periphery of the cell was induced. Migration was reduced following treatment with bortezomib, which could potentially be explained by a prominent but disorganized actin fiber network revealed through immunofluorescence. Collectively, our results suggest that proteasome inhibition using bortezomib affects cell adhesion and cell migration profoundly and provides a rationale for its clinical use in conjunction with an EGFR inhibitor.

Desmoglein autoimmunity in the pathogenesis of pemphigus

The most characteristic feature of pemphigus is a loss of cohesion between keratinocytes, resulting in formation of blisters and erosions on the mucosal membranes and the skin. Identification of circulating antibodies which bind to desmogleins (Dsg), transmembrane proteins involved in assembly of the desmosomes, led to the immediate realization that these antibodies may be pathogenic by interfering with desmosomal function. Despite extensive experimental evidence documenting the presence of the anti-Dsg response, its pathogenic relevance is still debated. At the current stage of the knowledge it seems likely that anti-Dsg imunoglobulins may play a role in pemphigus via interference with cellular Dsg trafficking and by activation of specific signalling pathways rather than by simple interference with desmosomal adhesion.

Plakophilins: Multifunctional proteins or just regulators of desmosomal adhesion?

Plakophilins 1-3 are members of the p120(ctn) family of armadillo-related proteins. The plakophilins have been characterized as desmosomal proteins, whereas p120(ctn) and the closely related delta-catenin, ARVCF and p0071 associate with adherens junctions and play essential roles in stabilizing cadherin mediated adhesion. Recent evidence suggests that plakophilins are essential components of the desmosomal plaque where they interact with desmosomal cadherins as well as the cytoskeletal linker protein desmoplakin. Plakophilins stabilize desmosomal proteins at the plasma membrane and therefore may function in a manner similar to p120(ctn) in the adherens junctions. The three plakophilins reveal distinct expression patterns, and although partially redundant in their function, mediate distinct effects on desmosomal adhesion. Besides a structural role, a function in signaling has been postulated in analogy to other armadillo proteins such as beta-catenin. At least plakophilins 1 and 2 are also localized in the nucleus, and all three proteins occur in a cytoplasmic pool. This review aims to summarize the current knowledge of plakophilin function in the context of cell adhesion, signaling and their putative role in diseases.

Carboxyl Terminus of Plakophilin-1 Recruits It to Plasma Membrane, whereas Amino Terminus Recruits Desmoplakin and Promotes Desmosome Assembly

Plakophilins are armadillo repeat-containing proteins, initially identified as desmosomal plaque proteins that have subsequently been shown to also localize to the nucleus. Loss of plakophilin-1 is the underlying cause of ectodermal dysplasia/skin fragility syndrome, and skin from these patients exhibits desmosomes that are reduced in size and number. Thus, it has been suggested that plakophilin-1 plays an important role in desmosome stability and/or assembly. In this study, we used a cell culture system (A431DE cells) that expresses all of the proteins necessary to assemble a desmosome, except plakophilin-1. Using this cell line, we sought to determine the role of plakophilin-1 in de novo desmosome assembly. When exogenous plakophilin-1 was expressed in these cells, desmosomes were assembled, as assessed by electron microscopy and immunofluorescence localization of desmoplakin, into punctate structures. Deletion mutagenesis experiments revealed that amino acids 686-726 in the carboxyl terminus of plakophilin-1 are required for its localization to the plasma membrane. In addition, we showed that amino acids 1-34 in the amino terminus were necessary for subsequent recruitment of desmoplakin to the membrane and desmosome assembly.

The area composita of adhering junctions connecting heart muscle cells of vertebrates. I. Molecular definition in intercalated disks of cardiomyocytes by immunoelectron microscopy of desmosomal proteins

Among sarcomeric muscles the cardiac muscle cells are unique by, inter alia, a systemic and extended cell-cell contact structure, the intercalated disk (ID), comprising frequent and closely spaced arrays of plaque-coated cell-cell adhering junctions (AJs). As some of these junctions may look somewhat like desmosomes and others like fasciae adhaerentes, the dogma has emerged in the literature that IDs contain - like epithelial cells - both kinds of AJs formed by - for the most - mutually exclusive molecular ensembles. This, however, is not the case. In comprehensive immunoelectron microscopic studies of mammalian (human, bovine, rat, mouse) and non-mammalian (chicken, amphibia, fishes) heart muscle tissues, we have localized major constituents of the desmosomal plaques of polar epithelia, desmoplakin, plakophilin-2 and plakoglobin, as well as the desmosomal cadherins, desmoglein Dsg2 and desmocollin Dsc2, in both kinds of ID AJs, independent of the specific morphological appearance. The desmosomal molecules are not restricted to the desmosome-like-looking junctions but can also be detected in junctions appearing similar to the zonula or fascia adhaerens structures. These AJs of cardiac ID are therefore subsumed under the collective term area composita. We discuss our results with respect to the importance of ID junction molecules for the formation, maintenance and function of the heart, particularly in relation to recent findings that deletions of - or mutations in - genes encoding such proteins can cause severe, sometimes lethal damages.

Histogenesis of human renal cell carcinoma by using electron microscopy and immunohistochemical techniques

Electron microscopy and immunohistochemical techniques are powerful tools for the determination of tissue origin. Both techniques have been used in the current experiment for histogenesis of renal cell carcinoma. Fifty kidney tumors were subjected to immunohistochemical detection for intermediate filaments cytokeratin and vimentin, which are normally expressed in epithelial tissue and mesenchymal tissues, respectively. Twenty cases of the above were examined by electron microscopy for detection of ultrastructure features. From each kidney, two specimens were taken, one from the diseased area and another far from it to serve as a control. Immunohistochemical study revealed in cases of renal cell carcinoma, cytokeratin and vimentin were expressed alone in 44% of cases, and 40% of cases, respectively. Twelve percent of cases were coexpressed with both cytokeratin and vimentin. Electron microscopic study of diseased specimens revealed the expression of desmosomes which was observed in almost all tumor specimens. The expression of the vimentin in some cases either alone or with cytokeratin was interpreted as a change in the characters of some tumor cells which indicates the need for additional techniques in such cases to get the proper interpretation. The prevalence of the expression of cytokeratin and the persistence existence of desmosomes indicate the epithelial origin of the tumor. This data is very beneficial for determination of line of therapy and follow up of the patients. The results confirm the power of combined use of both immunohistochemistry and electron microscopy in the field of histogenesis.

Mechanisms of plakoglobin-dependent adhesion: desmosome-specific functions in assembly and regulation by epidermal growth factor receptor

Plakoglobin (PG) is a member of the Armadillo family of adhesion/signaling proteins that can be incorporated into both adherens junctions and desmosomes. Loss of PG results in defects in the mechanical integrity of heart and skin and decreased adhesive strength in keratinocyte cultures established from the skin of PG knock-out (PG-/-) mice, the latter of which cannot be compensated for by overexpressing the closely related beta-catenin. In this study, we examined the mechanisms of PG-regulated adhesion in murine keratinocytes. Biochemical and morphological analyses indicated that junctional incorporation of desmosomal, but not adherens junction, components was impaired in PG-/- cells compared with PG+/- controls. Re-expression of PG, but not beta-catenin, in PG-/- cells largely reversed these effects, indicating a key role for PG in desmosome assembly. Epidermal growth factor (EGF) receptor activation resulted in Tyr phosphorylation of PG, which was accompanied by a loss of desmoplakin from desmosomes and decreased adhesive strength following 18-h EGF treatment. Importantly, introduction of a phosphorylation-deficient PG mutant into PG null cells prevented the EGF receptor-dependent loss of desmoplakin from junctions, attenuating the effects of long term EGF treatment on cell adhesion. Therefore, PG is essential for maintaining and regulating adhesive strength in keratinocytes largely through its contributions to desmosome assembly and structure. As a target for modulation by EGF, regulation of PG-dependent adhesion may play an important role during wound healing and tumor metastasis.

Desmosomal cell adhesion in mammalian development

Defects in desmosome-mediated cell-cell adhesion can lead to tissue fragility syndromes. Both inherited and acquired diseases caused by desmosomal defects have been described. The two organs that appear most vulnerable to these defects are the skin with its appendages, and the heart. Furthermore, the analysis of genetically engineered mice has led to the discovery that desmosomal proteins are also required for normal embryonic development. Knockout mice for several desmosomal proteins die in utero. Depending on the protein studied, death occurs either around the time of implantation, at mid-gestation or shortly before birth. So far, it appears that structural defects leading to abnormal histo-architecture and tissue fragility are the main cause of death, i.e. there is no evidence that loss of a desmosomal protein would abort specific cell lineages or differentiation programs. Nevertheless, we are only beginning to understand the functions of individual desmosomal proteins during development. This review focuses on the role of desmosomes during mouse embryonic development.

A role for plakophilin-1 in the initiation of desmosome assembly

Plakophilins (pkp-1, -2, and -3) comprise a family of armadillo-repeat containing proteins that are found in the desmosomal plaque and in the nucleus. Plakophilin-1 is most highly expressed in the suprabasal layers of the epidermis and loss of plakophilin-1 expression results in skin fragility-ectodermal dysplasia syndrome, which is characterized by a reduction in the number and size of desmosomes in the epithelia of affected individuals. To investigate the role of plakophilin-1 during desmosome formation, we fused plakophilin-1 to the hormone-binding domain of the estrogen receptor to create a fusion protein (plakophilin-1/ER) that can be activated in cell culture by the addition of 4-hydroxytamoxifen. When plakophilin-1/ER was expressed in A431 cells it was incorporated into endogenous desmosomes and did not disrupt desmosome formation. A derivative of A431 cells (A431D) do not form desmosomes, even though they express all the components believed to be necessary for desmosome assembly. Expression and activation of plakophilin-1/ER in A431D cells resulted in punctate desmoplakin staining on the cell surface. Co-expression of a classical cadherin (N-cadherin) and plakophilin-1/ER in A431D cells resulted in punctate desmoplakin staining at cell-cell borders. These data suggest that plakophilin-1 can induce assembly of desmosomal components in A431D cells in the absence of a classical cadherin; however a classical cadherin (N-cadherin) is required to direct assembly of desmosomes between adjacent cells. The activatable plakophilin-1/ER system provides a unique culture system to study the assembly of the desmosomal plaque in culture.