Desmocollins form a distinct subset of the cadherin family of cell adhesion molecules

Rupture strength of living cell monolayers

To fulfil their function, epithelial tissues need to sustain mechanical stresses and avoid rupture. Although rupture is usually undesired, it is central to some developmental processes, for example, blastocoel formation. Nonetheless, little is known about tissue rupture because it is a multiscale phenomenon that necessitates comprehension of the interplay between mechanical forces and biological processes at the molecular and cellular scales. Here we characterize rupture in epithelial monolayers using mechanical measurements, live imaging and computational modelling. We show that despite consisting of only a single layer of cells, monolayers can withstand surprisingly large deformations, often accommodating several-fold increases in their length before rupture. At large deformation, epithelia increase their stiffness multiple fold in a process controlled by a supracellular network of keratin filaments. Perturbing the keratin network organization fragilized the monolayers and prevented strain-stiffening. Although the kinetics of adhesive bond rupture ultimately control tissue strength, tissue rheology and the history of deformation set the strain and stress at the onset of fracture.

Writing, erasing and reading histone lysine methylations

Histone modifications are key epigenetic regulatory features that have important roles in many cellular events. Lysine methylations mark various sites on the tail and globular domains of histones and their levels are precisely balanced by the action of methyltransferases ('writers') and demethylases ('erasers'). In addition, distinct effector proteins ('readers') recognize specific methyl-lysines in a manner that depends on the neighboring amino-acid sequence and methylation state. Misregulation of histone lysine methylation has been implicated in several cancers and developmental defects. Therefore, histone lysine methylation has been considered a potential therapeutic target, and clinical trials of several inhibitors of this process have shown promising results. A more detailed understanding of histone lysine methylation is necessary for elucidating complex biological processes and, ultimately, for developing and improving disease treatments. This review summarizes enzymes responsible for histone lysine methylation and demethylation and how histone lysine methylation contributes to various biological processes.

Functional Role of G9a Histone Methyltransferase in Cancer

Post-translational modifications of DNA and histones are epigenetic mechanisms, which affect the chromatin structure, ultimately leading to gene expression changes. A number of different epigenetic enzymes are actively involved in the addition or the removal of various covalent modifications, which include acetylation, methylation, phosphorylation, ubiquitination, and sumoylation. Deregulation of these processes is a hallmark of cancer. For instance, G9a, a histone methyltransferase responsible for histone H3 lysine 9 (H3K9) mono- and dimethylation, has been observed to be upregulated in different types of cancer and its overexpression has been associated with poor prognosis. Key roles played by these enzymes in various diseases have led to the hypothesis that these molecules represent valuable targets for future therapies. Several small molecule inhibitors have been developed to specifically block the epigenetic activity of these enzymes, representing promising therapeutic tools in the treatment of human malignancies, such as cancer. In this review, the role of one of these epigenetic enzymes, G9a, is discussed, focusing on its functional role in regulating gene expression as well as its implications in cancer initiation and progression. We also discuss important findings from recent studies using epigenetic inhibitors in cell systems in vitro as well as experimental tumor growth and metastasis assays in vivo.

Expression of mRNA, before and after freezing, in bovine blastocysts cultured under different conditions

Production methods and culture systems have been shown to affect blastocyst mRNA expression and cryopreservability, which may serve as sensitive indicators of embryo quality and developmental competence. In the present study, the impact of four established culture conditions for producing bovine blastocysts (in vitro production, IVP; gamete intra-fallopian transfer, GIFT; transfer of cleaved stages into the oviduct, CLVT; multiple ovulation embryo transfer, MOET) was assessed, in terms of both cryosurvival and levels of mRNA expression of several selected genes (occludin, desmocollin 2, solute carrier family 2 member 3, BAX, BCL-XL, heat shock protein 1A, aquaporin 3, DNA methyltransferase 1a) detected with RT-qPCR. At 24 hours post-thawing, blastocysts derived from in vitro production showed a significantly higher re-expansion rate compared to the other groups. At later times, this difference was no longer significant. Before freezing, embryos of the MOET group showed significantly more desmocollin 2 mRNA compared to embryos produced using other culture methods. After freezing, significant upregulation was found in transcripts of heat shock protein 1A in embryos of all groups; of solute carrier family 2 member 3, only in IVP derived embryos; of BAX, BCL-XL, occludin, desmocollin 2, only in the MOET and IVP groups. Aquaporin 3 and DNA methyltransferase 1a were neither up- nor downregulated in blastocysts of any group. In conclusion, these findings suggest that, after freezing, embryos seem to have switched on mRNA synthesis, an active metabolism, operational cell connections, and are prepared for hatching and beyond.

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.

Identification of novel Cry1Ac binding proteins in midgut membranes from Heliothis virescens using proteomic analyses

Proteins such as aminopeptidases and alkaline phosphatases, both glycosyl-phosphatidyl-inositol (GPI) anchored proteins, were previously identified as Cry1Ac binding proteins in the Heliothis virescens midgut. To identify additional toxin binding proteins, brush border membrane vesicles from H. virescens larvae were treated with phosphatidyl inositol phospholipase C, and released proteins were resolved by two-dimensional electrophoresis. Protein spots selected by their ability to bind Cry1Ac were identified by MALDI-TOF mass spectrometry coupled to peptide mass fingerprinting (PMF) and database searching. As in previous studies, H. virescens alkaline phosphatase was identified as a Cry1Ac binding protein. V-ATP synthase subunit A and actin were identified as novel Cry1Ac binding proteins in H. virescens. Additional toxin-binding proteins were predicted based on MS/MS fragmentation and de novo sequencing, providing amino acid sequences that were used in database searches to identify a phosphatase and a putative protein of the cadherin superfamily as additional Cry1Ac binding proteins.

Discriminating roles of desmosomal cadherins: Beyond desmosomal adhesion

The desmosomal cadherins, which include desmogleins and desmocollins, are Ca(2+)-dependent adhesion molecules that cooperate to make up the adhesive core of intercellular junctions known as desmosomes. The roles of desmosomal cadherins in epidermal integrity and as targets in human cutaneous disease have been well established. However, the molecular basis of these disorders is still poorly understood, due in part to a lack of fundamental knowledge about the organization of the adhesive interface and molecular machinery that dictates the proper presentation of desmogleins and desmocollins on the cell surface. Further, the diversity of the desmosomal cadherin family, and their individualized expression patterns within complex tissues, suggests that these adhesion molecules may have differentiation-specific functions that transcend their roles in intercellular adhesion. Here we will review the most recent data from our own group and others that are beginning to unveil the diverse properties and functions of this complex family of adhesion molecules.

Untangling desmosomal knots with electron tomography

Cell adhesion by adherens junctions and desmosomes relies on interactions between cadherin molecules. However, the molecular interfaces that define molecular specificity and that mediate adhesion remain controversial. We used electron tomography of plastic sections from neonatal mouse skin to visualize the organization of desmosomes in situ. The resulting three-dimensional maps reveal individual cadherin molecules forming discrete groups and interacting through their tips. Fitting of an x-ray crystal structure for C-cadherin to these maps is consistent with a flexible intermolecular interface mediated by an exchange of amino-terminal tryptophans. This flexibility suggests a novel mechanism for generating both cis and trans interactions and for propagating these adhesive interactions along the junction.

The differential fate of cadherins during T-cell-induced keratinocyte apoptosis leads to spongiosis in eczematous dermatitis

Recently we have shown that T-cell-mediated keratinocyte apoptosis plays a key pathogenetic role in the formation of eczematous dermatitis. Spongiosis, the histologic hallmark of eczematous dermatitis, is characterized by impairment of cohesion between epidermal keratinocytes. It is conceivable that the intercellular junction of keratinocytes is an early target of apoptosis-inducing T cells. In this study, we demonstrate that the induction of keratinocyte apoptosis is accompanied by a rapid cleavage of E-cadherin and loss of coimmunoprecipitated beta-catenin. In situ examination of E-cadherin expression and cellular distribution in acute eczematous dermatitis revealed a reduction in keratinocyte membrane E-cadherin in areas of spongiosis. In contrast, the in vitro and in vivo expression of desmosomal cadherins during early apoptosis remained unchanged. Therefore, induction of keratinocyte apoptosis by skin-infiltrating T cells, subseqent cleavage of E-cadherin, and resisting desmosomal cadherins suggests a mechanism for spongiosis formation in eczematous dermatitis.

Perinuclear and cytoplasmic distribution of desmoglein in esophageal squamous cell carcinomas

The desmosomal glycoproteins desmoglein (Dsg) and desmocollin (Dsc) are members of the cadherin family of cell adhesion molecules. They play an important role in epithelial adhesion. To observe the distribution pattern of Dsg in esophageal squamous cell carcinomas (SCC), immunohistochemical and immunoelectron microscopic analyses were performed. Immunohistochemically, normal esophageal squamous cells strongly expressed Dsg at the cell-cell boundaries, while moderately differentiated esophageal SCC cells showed a perinuclear distribution in addition to the cell boundary staining. At the ultrastructural level, the reaction product was concentrated at the desmosomes in the cell membrane region of normal epithelial cells, but was reduced at the membrane and found throughout the cytoplasm as well as in the surrounding outer nuclear envelope in SCC cells. These results demonstrate an aberrant distribution of Dsg in SCC cells. This may have important consequences for invasion and metastasis, as it may indicate loosened intercellular adhesion.

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.

Involvement of desmoplakin phosphorylation in the regulation of desmosomes by protein kinase C, in HeLa cells

In the present study, we have examined how modulation of protein kinase C (PKC) activity affected desmosome organization in HeLa cells. Immunofluorescence and electron microscopy showed that PKC activation upon short exposure to 12-O-tetradecanoylphorbol 13-acetate (TPA) resulted in a reduction of intercellular contacts, splitting of desmosomes and dislocation of desmosomal components from the cell periphery towards the cytoplasm. As determined by immunoblot analysis of Triton X-100-soluble and -insoluble pools of proteins, these morphological changes were not correlated with modifications in the extractability of both desmoglein and plakoglobin, but involved almost complete solubilization of the desmosomal plaque protein, desmoplakin. Immunoprecipitation experiments and immunoblotting with anti-phosphoserine, antiphosphothreonine and anti-phosphotyrosine antibodies revealed that desmoplakin was mainly phosphorylated on serine and tyrosine residues in both treated and untreated cells. While phosphotyrosine content was not affected by PKC activation, phosphorylation on serine residues was increased by about two-fold. This enhanced serine phosphorylation coincided with the increase in the protein solubility, suggesting that phosphorylation of desmoplakin may be a mechanism by which PKC mediates desmosome disassembly. Consistent with the loss of PKC activity, we also showed that down-modulation of the kinase (in response to prolonged TPA treatment) or its specific inhibition (by GF 109203X) had opposite effects and increased desmosome formation. Taken together, these results clearly demonstrate an important role for PKC in the regulation ofdesmosomal junctions in HeLa cells, and identify serine phosphorylation of desmoplakin as a crucial event in this pathway.

Desmosomes: intercellular adhesive junctions specialized for attachment of intermediate filaments

Cell-cell adhesion is thought to play important roles in development, in tissue morphogenesis, and in the regulation of cell migration and proliferation. Desmosomes are adhesive intercellular junctions that anchor the intermediate filament network to the plasma membrane. By functioning both as an adhesive complex and as a cell-surface attachment site for intermediate filaments, desmosomes integrate the intermediate filament cytoskeleton between cells and play an important role in maintaining tissue integrity. Recent observations indicate that tissue integrity is severely compromised in autoimmune and genetic diseases in which the function of desmosomal molecules is impaired. In addition, the structure and function of many of the desmosomal molecules have been determined, and a number of the molecular interactions between desmosomal proteins have now been elucidated. Finally, the molecular constituents of desmosomes and other adhesive complexes are now known to function not only in cell adhesion, but also in the transduction of intracellular signals that regulate cell behavior.

Compositionally different desmosomes in the various compartments of the human hair follicle

Hair follicles are complex organs of the skin, in morphological and ontogenic continuity with the epidermis. We have examined the location of desmosomal cadherins and desmosomal plaque proteins in the hair follicle of adult and fetal human scalp skin by immunohistochemistry and have established a localization "map" of the hair follicle. Using antibodies against the plaque proteins desmoplakin I and II, plakoglobin, and plakophilin 1, we have found that these occur in most, if not all hair follicle desmosomes, whereas plakophilin 2 was absent, except in the basal cells of the outer root sheath, where a weak reactivity was found. By contrast, the desmosomal cadherins were mostly differentially synthesized, displaying a complicated map. While desmocollin Dsc3 was detected in all cell types examined, Dsc1 was detected only in the outer root sheath companion cell layer and the inner root sheath, and Dsc2 showed practically a mutually exclusive presence. Desmoglein Dsg2 was observed in basal cells of the outer root sheath as well as in the central cell layers of the subinfundibular outer rood sheath, matrix cells and trichocytes, in partial overlap with the otherwise different immunopositive reactions of Dsg1 and Dsg3. We have also determined when these proteins are synthesized during fetal hair follicle development. The differential molecular composition of desmosomes is discussed in relation to possible functional differences between the individual cell types.

Antisense expression of a desmocollin gene in MDCK cells alters desmosome plaque assembly but does not affect desmoglein expression

The desmocollins are one of two types of putative adhesive proteins present in the desmosome type of cell junctions, the other type being the desmogleins; both are members of the cadherin superfamily. Each type of desmosomal cadherin occurs as a number of isoforms which have differing tissue distribution; within stratifying epithelia some isoforms occur only suprabasally. We have sought to analyse desmocollin function by reducing the amount of protein using antisense gene expression in the widely studied Madin-Darby canine kidney (MDCK) cell line. Although this is a simple epithelial cell line, we show by Northern blot analysis that it expresses multiple isoforms of the desmosomal cadherins. Desmocollins DSC2 and DSC3 and desmogleins DSG2 and DSG3 (the pemphigus vulgaris antigen PVA) were detected, but DSC1 and DSG1, which are present exclusively in the suprabasal layers of the epidermis, were absent. The major desmocollin isoform was the type 2 (DSC2). A DSC2 clone isolated from a MDCK cDNA library had the same cell adhesion recognition sequence (Phe-Ala-Thr) as human, bovine and mouse type 2 isoforms. This sequence appears diagnostic for the three desmocollin isoforms. This cDNA clone was used to isolate a genomic DSC2 clone; antisense expression of this clone in MDCK cells resulted in a drastic reduction of desmocollin protein as judged by Western blots; Dsc3 was not upregulated to compensate for the loss of Dsc2. This antisense expression significantly altered desmosome assembly. There was a loss of punctate staining evident when using a desmosome plaque protein (desmoplakin) antibody. Electron microscopy revealed that there was a reduction in the number of desmosomes and a notable increase in the asymmetry of plaques between adjacent cells. Immunolabelling showed that similar levels of desmogleins and E-cadherin were present. Immunoelectron microscopy also showed that many vesicular structures were labelled, at intervals along the lateral membranes between cells. The distinctive loose organization of the remaining desmosomes may originate in modifications to the targeting and incorporation of proteins into fully assembled plaques. Other junctions were unaffected and the cells maintained their integrity as a confluent monolayer.

Modulation of hepatocyte growth factor-induced scattering of HT29 colon carcinoma cells. Involvement of the MAPK pathway

Hepatocyte growth factor (HGF)/scatter factor modulates the motility of HT29 colon carcinoma cells in vitro by inducing morphological changes that depend on the type of extra-cellular matrix (ECM) ligand; HGF-induced scattering of HT29 cells is observed if cells are grown on plastic coated with serum proteins but not laminin. The absence of scattering correlates with a lack of cell spreading on laminin and it is not due to impaired HGF induced tyrosine phosphorylation of the E-cadherin/desmosome component, (gamma)-catenin, or lack of activation of mitogen activated protein kinase (MAPK). Treatment of HT29 cells with phorbol 12-myristate, 13-acetate (PMA), but not arachidonic acid, restored the ability of the cells to spread on laminin in an integrin-dependent manner. Moreover, the addition of both PMA and HGF restored the ability of these cells to scatter on laminin in a synergistic manner. This event correlated with increased tyrosine phosphorylation of paxillin and activation of MAPK. Moreover, when the MEK (MAPK kinase)/MAPK pathway was blocked by the MEK inhibitor PD098059, HGF-induced scattering of HT29 cells was blocked. Thus, HGF modulation of HT29 cell motility is regulated by both integrin and growth factor-dependent signaling and implicates MAPK in the modulation of intercellular adhesion and epithelial cell motility.

Signaling and adhesion activities of mammalian beta-catenin and plakoglobin in Drosophila

The armadillo protein of Drosophila and its vertebrate homologues, beta-catenin and plakoglobin, are implicated in cell adhesion and wnt signaling. Here, we examine the conservation of these two functions by assaying the activities of mammalian beta-catenin and plakoglobin in Drosophila. We show that, in the female germ line, both mammalian beta-catenin and plakoglobin complement an armadillo mutation. We also show that shotgun mutant germ cells (which lack Drosophila E-cadherin) have a phenotype identical to that of armadillo mutant germ cells. It therefore appears that armadillo's role in the germ line is solely in a complex with Drosophila E-cadherin (possibly an adhesion complex), and both beta-catenin and plakoglobin can function in Drosophila cadherin complexes. In embryonic signaling assays, we find that plakoglobin has no detectable activity whereas beta-catenin's activity is weak. Surprisingly, when overexpressed, either in embryos or in wing imaginal disks, both beta-catenin and plakoglobin have dominant negative activity on signaling, an effect also obtained with COOH-terminally truncated armadillo. We suggest that the signaling complex, which has been shown by others to comprise armadillo and a member of the lymphocyte enhancer binding factor-1/T cell factor-family, may contain an additional factor that normally binds to the COOH-terminal region of armadillo.

Desmosomes are regulated by protein kinase C in primary rat epithelial cells

In the present study, we addressed the possible relevance of protein kinase C (PKC) in the regulation of intracytoplasmic desmosome assembly. Treatment of cultured rat lingual and epidermal keratinocytes with a potent and highly selective PKC inhibitor (GF109203X) induced an increase in granular labelling for major desmosomal proteins, desmoplakins, desmoglein and plakoglobin, both intracellularly and at the cell surface. This was associated with the formation of ultrastructurally recognizable desmosomes deep in the cytoplasm and increase in intercellular desmosome number. In contrast, PKC activation upon short exposure to 12-O-tetradecanoylphorbol 13-acetate (TPA) resulted in altered cell morphology, loss of intercellular contact and accumulation of desmosomal proteins in the juxtanuclear zone. On the other hand, PKC depletion by long term TPA treatment re-established cell-cell contact, where desmosomal markers were exclusively redistributed. Taken together, these results suggest that inhibition of PKC is required for intracytoplasmic as well as intercellular desmosome assembly, whereas its activation may regulate disassembly process.

Hierarchical expression of desmosomal cadherins during stratified epithelial morphogenesis in the mouse

Desmosomes contain two heterogeneous families of specialized cadherins (desmogleins or Dsgs and desmocollins or Dscs), subtypes of which are known to be expressed in tissue-specific and differentiation-dependent patterns in adult epithelial tissues. To examine the temporal and spatial order in which the individual desmosomal cadherins are expressed during stratified epithelial development we have obtained partial cDNA clones of all six murine desmosomal cadherins and have carried out in situ hybridization analysis on E12.5 to E16.5 mouse embryos. The results indicate that the type 2, type 3 and type 1 desmosomal cadherin messages are not obligatorily expressed as pairs during stratified epithelial morphogenesis. Instead the individual genes appear to be transcribed in hierarchical, overlapping temporal and spatial patterns extending from DSG2 to DSC1. DSG2 was the most uniformly expressed message in all E12.5 epithelia, gradually becoming confined to the basal cell layers during epithelial stratification indicating that its transcription was restricted to undifferentiated cells. In contrast, DSC2 message was expressed variably in early epithelia and was strongly upregulated in the suprabasal cell layers during the stratification of wet-surfaced epithelia. DSC3 message was expressed before that of DSG3 in the dental and lingual epithelium where its spatial distribution matched that of DSG2, but after DSG3 in the non-glandular gastric epithelium. DSC3 transcripts became confined to the lower layers of stratifying epithelia but were usually less basally restricted than those of DSG2. Like DSC2, DSG3 mRNA was strongly upregulated in the suprabasal layers of wet-surfaced epithelia as they stratified. Upregulation of DSG1 message was temporally linked to that of DSG3 in all tissues apart from the non-glandular gastric epithelium.

Targeted disruption of the pemphigus vulgaris antigen (desmoglein 3) gene in mice causes loss of keratinocyte cell adhesion with a phenotype similar to pemphigus vulgaris

In patients with pemphigus vulgaris (PV), autoantibodies against desmoglein 3 (Dsg3) cause loss of cell-cell adhesion of keratinocytes in the basal and immediate suprabasal layers of stratified squamous epithelia. The pathology, at least partially, may depend on protease release from keratinocytes, but might also result from antibodies interfering with an adhesion function of Dsg3. However, a direct role of desmogleins in cell adhesion has not been shown. To test whether Dsg3 mediates adhesion, we genetically engineered mice with a targeted disruption of the DSG3 gene. DSG3 -/- mice had no DSG3 mRNA by RNase protection assay and no Dsg3 protein by immunofluorescence (IF) and immunoblots. These mice were normal at birth, but by 8-10 d weighed less than DSG3 +/- or +/+ littermates, and at around day 18 were grossly runted. We speculated that oral lesions (typical in PV patients) might be inhibiting food intake, causing this runting. Indeed, oropharyngeal biopsies showed erosions with histology typical of PV, including suprabasilar acantholysis and "tombstoning" of basal cells. EM showed separation of desmosomes. Traumatized skin also had crusting and suprabasilar acantholysis. Runted mice showed hair loss at weaning. The runting and hair loss phenotype of DSG3 -/- mice is identical to that of a previously reported mouse mutant, balding (bal). Breeding indicated that bal is coallelic with the targeted mutation. We also showed that bal mice lack Dsg3 by IF, have typical PV oral lesions, and have a DSG3 gene mutation. These results demonstrate the critical importance of Dsg3 for adhesion in deep stratified squamous epithelia and suggest that pemphigus autoantibodies might interfere directly with such a function.

The distribution of the desmosomal protein, plakophilin 1, in human skin and skin tumors

Desmosomes are predominant among the types of plaque-bearing adhering junctions found in human skin. These structures contain a set of desmosomal cadherins and cytoplasmic plaque proteins, the synthesis of which is differentiation dependent. As plakophilin 1, a member of the armadillo gene family, is an important accessory desmosomal plaque protein, we raised several monoclonal antibodies specific for this protein and applied immunohistochemical and immunoblotting procedures to study the distribution of plakophilin 1 in desmosomes in adult and fetal skin, psoriatic epidermis, various epithelial skin tumors, and keratinocyte sheets grown in culture. In epidermis, the spinous layers were prominently immunostained by plakophilin 1 antibodies, whereas the basal cell layer was only weakly stained and the stratum corneum was entirely unstained. The staining observed in psoriatic epidermis was somewhat heterogeneous. In hair follicles, the outer root sheath (ORS) was delineated in its suprabasal cell layers, with variable staining in its upper and lower parts. All basal cells of the ORS remained unstained, as did upper inner root sheath (IRS) and matrix cells of lower bulb. In eccrine sweat glands, the reaction was confined to inner dermal ductal cells, with the acini remaining unstained. The desmosomal immunostaining observed in basal cell carcinomas (BCCs) and squamous cell carcinomas (SCCs) was very heterogeneous: In general, junctions in well-differentiated stratified tumor regions were more intensely stained than sections of poorly differentiated and invasively growing BCCs and SCCs. Plakophilin 1 was also prominent in the desmosomes of keratinocyte sheets grown in culture. The cell type-specific, i.e., differentiation-dependent, distribution of desmosomal plakophilin 1 is discussed in relation both to the stratification of the cutaneous epithelia and to tumor differentiation and growth.

Hailey-Hailey disease keratinocytes: normal assembly of cell-cell junctions in vitro

The blisters in the inherited disorder, Hailey-Hailey disease, may be caused by defective epidermal junctional complexes. We evaluated these structural complexes in vivo and in vitro. We induced a vesicular lesion in the apparently normal skin of a patient with Hailey-Hailey disease and studied a biopsy of this lesion by transmission electron microscopy. To determine whether acantholysis was related to a defect in the number or assembly of intercellular junctions, we cultured Hailey-Hailey disease keratinocytes in medium containing 0.1 mM Ca2+ and increased the [Ca2+] to 1.1 mM in order to induce assembly of cell-cell junctions. Keratinocytes were examined by double immunofluorescence with antibodies to the desmosome protein, desmoplakin, and the adherens junction protein, vinculin, at intervals after the increase in [Ca2+]. Characteristic Hailey-Hailey disease histopathology was observed by electron microscopy of the patient's skin after trauma, but we found no splitting of desmosomes. Based on the location, intensity, and rate of change of immunofluorescent staining, Hailey-Hailey and normal keratinocytes did not differ in their ability to assemble desmosomes and adherens junctions. Furthermore, we observed no significant morphologic differences between normal and Hailey-Hailey keratinocytes cultured in low and high [Ca2+]-containing media; Hailey-Hailey cells contained abundant normal-appearing desmosomes in 1.1 mM [Ca2+]. Since Hailey-Hailey disease keratinocytes can assemble normal-appearing adherens junctions and desmosomes in vitro, the functional defect may not lie in assembly of cell-cell adhering junctions, or additional perturbation may be required to expose the defect.

Desmosomes: differentiation, development, dynamics and disease

Recent evidence on the distribution of desmosomal glycoprotein isoforms that shows their combined expression in individual desmosomes has strengthened the belief that the latter are involved in epithelial differentiation and morphogenesis. It has been shown that cellular interactions and protein kinase C can modulate the adhesive properties of desmosomes in epithelial cell sheets. Genetic studies indicate the involvement of desmosomal components in cancer and epidermal diseases.

Expression of the "skin-type" desmosomal cadherin DSC1 is closely linked to the keratinization of epithelial tissues during mouse development

Desmosomal junctions contain two classes of desmosomal cadherin, the desmocollins and the desmogleins, each of which occurs as three distinct isoforms. To investigate the role of the "skin-type" desmosomal cadherins (desmocollin 1 and desmoglein 1) in the formation of keratinized epithelial structures, we have now cloned full-length mouse desmocollin 1 complementary deoxyribonucleic acid and examined the expression of desmocollin 1 and desmoglein 1 and messages during murine embryonic development by in situ hybridization. In the general body epidermis, desmocollin 1 and desmoglein 1 transcripts both showed considerable upregulation at 15.5 d, which is after the onset of stratification and before the start of keratinization. Before this the epidermis expressed low levels of desmocollin 1 message, although the desmoglein 1 signal was always stronger and more extensive. In the tongue, expression of desmocollin 1 message occurred several days after desmoglein 1 and coincided with the formation of the keratinizing filiform papillae. Desmoglein 1 message was also detected in epithelial tissues in which desmocollin 1 was absent, suggesting that expression of the two "skin-type" desmosomal cadherins was not tightly coupled during embryonic development. Human desmocollin 1 monoclonal antibodies that cross-reacted with mouse skin and tongue indicated that desmocollin 1 protein was first expressed in those outermost epithelial cells destined to form the keratinized layers of the stratum corneum or the papillae. The results suggest that expression of desmocollin 1 is closely associated with the keratinization of epithelial tissues during mouse development.

Expression of component desmosomal proteins in uterine endometrial carcinoma and their relation to cellular differentiation

BACKGROUND

While the assessment of the malignancy of neoplasms is based on morphologic studies of cells and tissues, use of objective molecular markers is leading to a better understanding and more biologically meaningful classification of neoplasms. In recent years, changes in the expression of cell adhesion molecules, especially E-cadherin, catenin, and adenomatous polyposis coli (APC), in carcinomas have attracted the attention of researchers. However, little is known about desmosomes in the uterine endometrium or in endometrial carcinomas. In this study, we semiquantified the desmosomal components desmoplakin I and II and desmoglein, in tissue sections using confocal laser scanning microscopy (LSM), and examined their relationship to the pathological type, the occurrence of lymph node metastases, and the extent of myometrial invasion.

METHOD

Frozen sections of 31 specimens of normal endometrium, 5 specimens of atypical hyperplasia, and 41 specimens of endometrial carcinoma were stained by the immunofluorescence method using antidesmoplakin I and II and antidesmoglein, and these markers were then semiquantified in tissue sections by LSM.

RESULTS

The expression and location of desmoplakin I and II and desmoglein were similar, and their expression decreased with loss of differentiation. The expression was lower in cases of lymph node metastasis than in negative cases and was lower in the cases with > one-half myometrial invasion than in cases with < one-half myometrial invasion.

CONCLUSIONS

Reduction of desmoplakin I and II and desmoglein expression may play an important role in the invasiveness and metastatic activity of human endometrial carcinoma. They can therefore be used as differentiation markers for endometrial carcinoma.

Desmosomal cadherin binding domains of plakoglobin

Plakoglobin is a major component of both desmosomes and adherens junctions. At these sites it binds to the cytoplasmic domains of cadherin cell-cell adhesion proteins and regulates their adhesive and cytoskeletal binding functions. Plakoglobin also forms distinct cytosolic protein complexes that function in pathways of tumor suppression and cell fate determination. Recent studies in Xenopus suggest that cadherins inhibit the signaling functions of plakoglobin presumably by sequestering this protein at the membrane and depleting its cytosolic pool. To understand the reciprocal regulation between desmosomal cadherins (desmoglein and desmocollin) and plakoglobin, we have sought to identify the binding domains involved in the formation of these protein complexes. Plakoglobin comprises 13 central repeats flanked by amino-terminal and carboxyl-terminal domains. Our results show that repeats 1-4 are involved in binding desmoglein-1. In contrast, the interaction of plakoglobin with desmocollin-1a is sensitive to deletion of either end of the central repeat domain. The binding sites for two adherens junction components, alpha-catenin and classical cadherins, overlap these sites. Competition among these proteins for binding sites on plakoglobin may therefore account for the distinct composition of adherens junctions and desmosomes.

Immunological identification and characterization of the desmosomal cadherin Dsg2 in coupled and uncoupled epithelial cells and in human tissues

Cells of epithelia, but also of certain other tissues such as myocardium and the dendritic reticulum of lymph node follicles, are interconnected by numerous intercellular junctions termed desmosomes. These are characterized by a set of transmembrane glycoproteins, i.e. the desmosomal cadherins, desmoglein(s) and desmocollin(s). Using cDNA-derived hybridization probes, we have previously shown that different desmogleins exist (Dsg1-3) and that only one Dsg isoform, Dsg2, is found in diverse kinds of tissues, tumors and cultured cell lines whereas the synthesis of Dsg1 and Dsg3 is much more restricted, primarily to stratified epithelia [51]. We now report immunocytochemical results obtained with a series of monoclonal and polyclonal antibodies specific for either the aminoterminal extracellular portion or the carboxyterminal cytoplasmic segment of Dsg2. These antibodies detect Dsg2 in all tissues possessing desmosomes, including human stratified and single-layered polar epithelia, as well as non-epithelial tissues such as myocardium and lymph node follices. They also react with the desmosomes of carcinomas and of diverse cultured epithelium-derived cell lines. Moreover, antibodies specific for extracellular domain regions of Dsg2 react with the "half-desmosomes" present on the surfaces of uncoupled intact epithelial cells. Remarkably, in stratified squamous epithelia the Dsg2-reaction is not homogeneous, as this glycoprotein is detected only in the basal cell layer and appears to be absent from suprabasal strata. The potential value of Dsg2-specific antibodies in histology and in tumor diagnosis as well as in studies of the mechanisms desmosomal cell coupling is discussed.

Expression of E-cadherin in skin carcinomas

In this study, we investigated the expression of E-cadherin in 31 cases of human skin carcinoma including basal cell carcinoma (BCC), squamous cell carcinoma (SCC), Paget's disease, Bowen's disease (invasive type), and trichilemmal carcinoma, by immunohistochemical staining using a monoclonal antibody specific for E-cadherin. Similar to the E-cadherin expression in normal epidermis, E-cadherin was strongly expressed in all samples of BCC on the cell borders, whereas marked decrease or loss of E-cadherin expression was found in the tumor cells of SCC, Paget's disease, and Bowen's disease (invasive type). On the other hand, E-cadherin expression of trichilemmal carcinoma was slightly reduced. Considering the clinical and histological features of these skin carcinoma, the reduction of E-cadherin expression is considered to be associated with the invasion and metastasis of human skin carcinoma.

Integration of intermediate filaments into cellular organelles

The intermediate filaments represent core components of the cytoskeleton and are known to interact with several membranous organelles. Classic examples of this are the attachment of keratin filaments to the desmosomes and the association of the lamin filament meshwork with the inner nuclear membrane. At this point, the molecular mechanisms by which the filaments link to membranes are not clearly understood. However, since a substantial body of information has been amassed, the time is now ripe for comparing notes and formulating working hypotheses. With this objective in mind, we review here pioneering studies on this subject, together with work that has appeared more recently in the literature.

Protein zero, a nervous system adhesion molecule, triggers epithelial reversion in host carcinoma cells

Protein zero (P(o)) is the immunoglobulin gene superfamily glycoprotein that mediates the self-adhesion of the Schwann cell plasma membrane that yields compact myelin. HeLa is a poorly differentiated carcinoma cell line that has lost characteristic morphological features of the cervical epithelium from which it originated. Normally, HeLa cells are not self-adherent. However, when P(o) is artificially expressed in this line, cells rapidly aggregate, and P(o) concentrates specifically at cell-cell contact sites. Rows of desmosomes are generated at these interfaces, the plasma membrane localization of cingulin and ZO-1, proteins that have been shown to be associated with tight junctions, is substantially increased, and cytokeratins coalesce into a cohesive intracellular network. Immunofluorescence patterns for the adherens junction proteins N-cadherin, alpha-catenin, and vinculin, and the desmosomal polypeptides desmoplakin, desmocollin, and desmoglein, are also markedly enhanced at the cell surface. Our data demonstrate that obligatory cell-cell adhesion, which in this case is initially brought about by the homophilic association of P(o) molecules across the intercellular cleft, triggers pronounced augmentation of the normally sluggish or sub-basal cell adhesion program in HeLa cells, culminating in suppression of the transformed state and reversion of the monolayer to an epithelioid phenotype. Furthermore, this response is apparently accompanied by an increase in mRNA and protein levels for desmoplakin and N-cadherin which are normally associated with epithelial junctions. Our conclusions are supported by analyses of ten proteins we examined immunochemically (P(o), cingulin, ZO-1, desmoplakin, desmoglein, desmocollin, N-cadherin, alpha-catenin, vinculin, and cytokeratin-18), and by quantitative polymerase chain reactions to measure relative amounts of desmoplakin and N-cadherin mRNAs. P(o) has no known signaling properties; the dramatic phenotypic changes we observed are highly likely to have developed in direct response to P(o)-induced cell adhesion. More generally, the ability of this "foreign" membrane adhesion protein to stimulate desmosome and adherens junction formation by augmenting well-studied cadherin-based adhesion mechanisms raises the possibility that perhaps any bona fide cell adhesion molecule, when functionally expressed, can engage common intracellular pathways and trigger reversion of a carcinoma to an epithelial-like phenotype.

The desmocollins of human foreskin epidermis: identification and chromosomal assignment of a third gene and expression patterns of the three isoforms

A third human desmocollin, designated DSC3, was identified in foreskin epidermis by reverse transcriptase-polymerase chain reaction (PCR) using degenerate desmocollin primers. cDNA clones covering the entire coding sequence of the longer DSC3 splice variant were isolated and sequenced. Sequence comparisons indicated that this new desmocollin showed greater homology (67% amino acid identity) with the original human desmocollin (now designated DSC2) than with DSC1 (52% amino acid identity) although it had a unique potential cell adhesion recognition site (YAS). DSC3 was assigned to chromosome 18 by PCR analysis of rodent-human somatic cell hybrids, where it appears to be closely linked to all the other desmosomal cadherin genes. The expression of the three human desmocollins was examined in foreskin epidermis by in situ hybridization with 3'-untranslated riboprobes and by immunofluorescence with isoform-specific anti-peptide antibodies. DSC1 was present in the upper spinous/granular layers but not in the basal/lower spinous layers of the tissue. DSC2 and DSC3 were present in most of the living layers of the epidermis. DSC1 was not detected in any of the nonkeratinizing human epithelia examined (buccal mucosa, cervix, esophagus), indicating that it is specific for the keratinizing epithelium of the epidermis. However, all these internal epithelia expressed DSC2 and DSC3, and both were present in most of the living layers of the tissues including the basal layers.

Characterisation of a desmocollin isoform (bovine DSC3) exclusively expressed in lower layers of stratified epithelia

Desmocollins are cadherin-like glycoproteins involved in cell adhesion and plaque formation in desmosome junctions. Three distinct isoforms, the products of different genes, have been found in bovine tissues. We have reported previously that one of these, DSC3, is expressed only in basal and lower suprabasal layers of stratified epithelia. Using RT-PCR we have now obtained the complete cDNA coding sequence of mature bovine DSC3. It has alternatively spliced ‘a’ and ‘b’ forms found in other desmocollins but is unique in having a 43 instead of a 46 base pair exon. We have characterised a monoclonal antibody, 07–4G, which is specific for the Dsc3 protein, recognising an epitope in the extracellular domain. Immunofluorescent staining with 07–4G confirms that this isoform is found only in stratified epithelia, being strongly expressed in the basal cell layers of these tissues. The intensity of expression fades gradually in the suprabasal layers and disappears completely below the upper limit of desmosome expression. These results suggest that Dsc3 plays an important role in cell epithelial differentiation.

Desmosome assembly and disassembly are regulated by reversible protein phosphorylation in cultured epithelial cells

Desmosomes are one component of the intercellular junctional complex in epithelia. In cultures of epithelial cells, desmosome assembly can be regulated by modulating the calcium concentrations of the growth media. At present, very little is known about the intracellular signal transduction mechanisms that regulate desmosome assembly and disassembly in response to changing extracellular calcium concentrations. We have used inhibitors of protein kinases and phosphatases in a combined biochemical and morphological approach to analyze the role of protein phosphorylation in the assembly and disassembly of desmosomes in Madin-Darby canine kidney epithelial cells. Our results suggest that desmosomal proteins (desmoplakins I/II and desmoglein 1) are primarily phosphorylated on serine residues. Electron microscopic analyses of desmosome assembly upon induction of cell-cell contact, in the presence of protein kinase inhibitor, H-7, revealed an apparently normal assembly of desmosomes. However, complete disassembly of desmosomes was inhibited by H-7 upon removal of extracellular calcium. Under these conditions, although desmosomes split, desmosomal plaques and their associated cytokeratin filaments can not be internalized. In contrast, treatment of the cultures with okadaic acid (OA), an inhibitor of protein phosphatases, inhibited desmosome assembly but had no effect on disassembly. In addition, the inhibitory effect of okadaic acid on desmosome assembly was specific to this junction since we observed apparently normal tight junction and adherens junction in okadaic acid-treated cultures. These results suggest that assembly and disassembly of desmosomes may be regulated by extracellular Ca2+ via reversible protein phosphorylation involving both protein kinase and protein phosphatases.

Plakoglobin: kinetics of synthesis, phosphorylation, stability, and interactions with desmoglein and E-cadherin

We have analyzed the kinetics of synthesis, phosphorylation, and stability of the soluble and insoluble plakoglobin (PG) and their interactions with Dsg1 and E-cadherin in Madin-Darby canine kidney (MDCK) epithelial cells in the absence of cell adhesion and after the induction of cell-cell contact. Using a combination of biochemical and morphological approaches, we show that newly synthesized PG enters a soluble:insoluble pool of proteins in a 60:40 ratio regardless of cell-cell contact. Following synthesis, PG is increasingly found in the insoluble pool. Although cell-cell contact does not effect either the size of each pool or the rate or efficiency of the transfer from the soluble into the insoluble pool, it results in a significant increase in the metabolic stability of the newly synthesized insoluble PG. The soluble PG initially forms separate complexes with E-cadherin and Dsg1. PG-Dsg1 complexes become insoluble and localize to the desmosome. PG-E-cadherin complexes remain soluble and are distributed intracellularly. The insoluble PG and E-cadherin detected at the cell periphery remain distinctly separate, as demonstrated previously [Hinck et al., 1994: J. Cell Biol. 125:1327-1340; Nathke et al., 1994: J. Cell Biol. 125:1341-1352]. In addition, we detected a separate pool of PG which is not associated with either Dsg1 or E-cadherin and after the induction of cell-cell contact becomes primarily insoluble and is distributed along the lateral membrane. Phosphorylation analysis showed that there is a significantly greater amount of phosphorylated PG in the soluble pool than in the insoluble pool. In addition the soluble pool is both serine and threonine phosphorylated, whereas the insoluble PG is primarily phosphorylated on serine residues.

Adhesion molecules. I: Keratinocyte-keratinocyte interactions; cadherins and pemphigus

During the last few years, considerable progress has been made in our understanding of the structure and function of cadherins and of the pathophysiology of pemphigus. Cadherins are a multiple gene family of Ca(++)-dependent cell adhesion molecules with a typical single-spanning transmembrane structure. Cadherins have two major subfamilies, classic cadherin and desmosomal cadherin. Classic cadherins, including E-, P-, and N-cadherins, are characterized by a homophilic binding specificity. They localize at adherens junctions and mediate physiologic interaction with the involvement of cytoplasmic anchoring molecules, catenins, and the actin-based cytoskeleton network. Desmosomal cadherins, the desmocollins and desmogleins, localize at desmosomes and are linked to the intermediate keratin filaments network via plakoglobin and desmoplakin. Molecular cloning has demonstrated that the autoantigens of both pemphigus vulgaris and pemphigus foliaceus are members of the desmoglein subfamily of the cadherin supergene family. Thus, pemphigus is characterized as an anti-cadherin autoimmune disease. Furthermore, a baculovirus recombinant protein of pemphigus vulgaris antigen was capable of absorbing out the pathogenic autoantibodies from patients' sera, providing a possibility of antigen-specific therapeutic strategies for pemphigus.

Identification of amino acid sequence motifs in desmocollin, a desmosomal glycoprotein, that are required for plakoglobin binding and plaque formation

By transfecting epithelial cells with gene constructs encoding chimeric proteins of the transmembrane part of the gap junction protein connexin 32 in combination with various segments of the cytoplasmic part of the desmosomal cadherin desmocollin 1a, we have determined that a relatively short sequence element is necessary for the formation of desmosome-like plaques and for the specific anchorage of bundles of intermediate-sized filaments (IFs). Deletion of as little as the carboxyl-terminal 37 aa resulted in a lack of IF anchorage and binding of the plaque protein plakoglobin, as shown by immunolocalization and immunoprecipitation experiments. In addition, we show that the sequence requirements for the recruitment of desmoplakin, another desmosomal plaque protein, differ and that a short (10 aa) segment of the desmocollin 1a tail, located close to the plasma membrane, is also required for the binding of plakoglobin, as well as of desmoplakin, and also for IF anchorage. The importance of the carboxyl-terminal domain, homologous in diverse types of cadherins, is emphasized, as it must harbor, in a mutually exclusive pattern, the information for assembly of the IF-anchoring desmosomal plaque in desmocollins and for formation of the alpha-/beta-catenin- and vinculin-containing, actin filament-anchoring plaque in E- and N-cadherin.

Cloning, sequence analysis and expression pattern of mouse desmocollin 2 (DSC2), a cadherin-like adhesion molecule

Desmocollins are cadherin-like adhesion molecules of desmosomes. We have determined the full cDNA sequence of a murine desmocollin, the homologue of human and bovine type 2 desmocollins (DSC2), and studied its tissue distribution and expression in stratified epithelia. An 8.5 day mouse embryo cDNA library was screened yielding overlapping clones which encoded the mouse DSC2. This gene has an open reading frame of 2710 base pairs (bp) encoding a polypeptide of 902 amino acids (aa). The polypeptide comprises a signal peptide, a precursor peptide, and a mature protein of 766 aa having an extracellular domain of 549 aa, a single transmembrane domain and a cytoplasmic domain of 184 aa. Like other desmocollins, murine DSC2 has two products, Dsc2a and Dsc2b, produced by alternative splicing of a 46 bp exon which encodes 11 COOH-terminal aa followed by an in-frame stop codon. Inclusion of this exon forms Dsc2b which is 54 aa shorter than Dsc2a. Mouse Dsc2a shows 75.7% amino acid identity to human and 63.3% identity to bovine Dsc2a. The mouse desmocollin is also homologous to the cadherins; 32.2% to the most closely related typical cadherin, human N-cadherin. DSC2 is ubiquitously expressed in epithelial tissues and the heart of adult mice and from the blastocyst stage of development. In situ hybridization shows that the gene is most strongly expressed suprabasally in stratified epithelia, similar to the expression of bovine DSC2.

Band 6 protein, a major constituent of desmosomes from stratified epithelia, is a novel member of the armadillo multigene family

Desmosomes are intercellular adhering junctions characteristic of epithelial cells. Several constitutive proteins--desmoplakin, plakoglobin and the transmembrane glycoproteins desmoglein and desmocollin--have been identified as fundamental constituents of desmosomes in all tissues. A number of additional and cell type-specific constituents also contribute to desmosomal plaque formation. Among these proteins is the band 6 polypeptide (B6P). This positively charged, non-glycosylated protein is a major constituent of the plaque in stratified and complex glandular epithelia. Using an overlay assay we show that purified keratins bind in vitro to B6P. Thus B6P may play a role in ordering intermediate filament networks of adjacent epithelial cells. To characterize the structure of B6P in the desmosome we have isolated cDNA clones representing the entire coding sequence. The predicted amino acid sequence of human B6P shows strong sequence homology with a murine p120 protein, which is a substrate of protein tyrosine kinase receptors and of p60v-src. P120 and B6P show amino-terminal domains differing distinctly in length and sequence. These are followed in both proteins by 460 residues that display a series of imperfect repeats corresponding to the repeats in the cadherin binding proteins armadillo, plakoglobin and beta-catenin. Over this repeat region B6P and p120 share 33% sequence identity (54% similarity). These sequence characteristics define B6P as a novel member of the armadillo multigene family and raise the question of whether the structural proteins B6P, plakoglobin, beta-catenin and armadillo share some function. Since armadillo, plakoglobin, beta-catenin and p120 seem involved in signal transduction this may also hold for B6P. The amino-terminal region of B6P (residues 1 to 263) shows no significant homology to any known protein sequence. It may therefore be involved in unique functions of B6P.

The bovine desmocollin family: a new gene and expression patterns reflecting epithelial cell proliferation and differentiation

We have discovered a third bovine desmocollin gene, DSC3, and studied expression of all three desmocollin genes, DSC1, 2, and 3, by Northern blotting, RT-PCR and in situ hybridization. DSC1 is strongly expressed in epidermis and tongue papillae, showing a "skin"-type pattern resembling that previously described for keratins 1 and 10. Expression is absent from the epidermal basal layer but appears in the immediate suprabasal layers and continues uniformly to the lower granular layer. In tongue epithelium, expression is suprabasal and strictly localized to papillae, being absent from interpapillary regions. In other epithelial low level DSC1 expression is detectable only by RT-PCR. The distribution of Dsc1 glycoproteins, detected by an isoform-specific monoclonal antibody, closely reflects mRNA distribution in epidermis and tongue. DSC2 is ubiquitously expressed in epithelia and cardiac muscle. In stratified epithelia, expression appears immediately suprabasal, continuing weakly to the lower granular layer in epidermis and to just above half epithelial thickness in interpapillary tongue, oesophageal, and rumenal epithelia. DSC3 expression is restricted to the basal and immediately suprabasal layers in stratified epithelia. In deep rete ridges DSC expression strikingly resembles the distribution of stem, transit-amplifying, and terminally differentiating cells described by others. DSC3 expression is strongly basal, DSC2 is strong in 5-10 suprabasal layers, and then weakens to be superseded by strong DSC1. These results suggest that desmocollin isoform expression has important functional consequences in epithelial proliferation, stratification, and differentiation. The data also provide a standard for nomenclature of the desmocollins.

Structure and function of desmosomal transmembrane core and plaque molecules

Desmosomes are intercellular junctions that function in cell-cell adhesion and attachment of intermediate filaments (IF) to the cell surface. Desmogleins and desmocollins are the major components of the transmembrane adhesion complex, whereas desmoplakins (DPs) are the most prominent components of the cytoplasmic plaque. Based on sequence similarity, desmogleins and desmocollins are related to the calcium-dependent homophilic adhesion molecules known as cadherins. Like the classical cadherins, the desmosomal cadherins contain four homologous extracellular domains bearing putative calcium-binding sites, a single transmembrane spanning domain, and a C-terminal cytoplasmic tail. Molecules in the desmoglein subclass contain a unique C-terminal extension within which is found a repeating motif that is predicted to form two beta-strands and two turns. Stable cell lines expressing desmoglein 1 have been generated from normally non-adherent L cell fibroblasts, to study the contribution of this cadherin to desmosomal adhesion. The predicted sequence of desmoplakin (DP) I suggests it will form homodimers comprising a central alpha-helical coiled-coil rod and two globular end domains. The C-terminus contains three regions with significant homology, each of which is made up of a 38-residue motif also found in two other molecules involved in organization of IF, bullous pemphigoid antigen and plectin. Ectopically expressed polypeptides including the C-terminus of DP I specifically align with keratin and vimentin IF in cultured cells, whereas those lacking this domain do not align with IF. The last 68 amino acids of DP are required for alignment along keratin but not vimentin IF, and residues 48-68 from the C-terminal end are critical for this interaction. These results suggest that the C-terminus of DP plays a role in the attachment of IF to the desmosome and that a specific site is necessary for interaction with keratin IF. A sequence at the most N-terminal end of DP appears to be required for efficient incorporation into the desmosomal plaque. Interestingly, this region has not been reported to be present in the homologous bullous pemphigoid antigen or plectin molecules and may represent a desmosomal targeting sequence.

Different effects of protein kinase inhibitors on the localization of junctional proteins at cell-cell contact sites

The protein kinase inhibitor H-7 prevents the assembly of tight junctions in cultured Madin Darby Canine Kidney (MDCK) epithelial cells (Balda et al. (1991) J. Membr. Biol. 122, 193–202; Nigam et al. (1991) Biochem. Biophys. Res. Commun. 181, 548–553); however, its mechanism of action is unknown. To understand the basis of the activity of H-7 and other inhibitors we compared the effect of H-7 on the localization of proteins belonging to tight junctions and adherens-type junctions (zonula adhaerens and desmosome), and on the organization of actin microfilaments. Junction assembly was induced in MDCK cells either by the ‘Ca2+ switch’ procedure or by incubating trypsinized cells at normal extracellular Ca2+, and the cells were then immunofluorescently labeled with antibodies against cingulin, ZO-1, E-cadherin and desmoplakin, and with FITC-phalloidin. Here we show by measuring the transepithelial resistance that, in addition to H-7, H-8 and staurosporine can also significantly block the assembly of tight junctions, whereas HA1004 is poorly active. H-7 inhibited the accumulation of cingulin and ZO-1 in junctional areas most effectively when added during assembly at normal extracellular Ca2+. On the other hand, H-7 did not have major effects on the accumulation of E-cadherin and desmoplakin in the regions of cell-cell contact using either assembly protocol. Electron microscopy confirmed that H-7 does not abolish the formation of adherens-type junctions, suggesting that phosphorylation plays a different role in the assembly of tight junctions versus adherens-type junctions. Finally, in both protocols of junction assembly H-7 caused a major disorganization of actin microfilaments, suggesting that H-7 may prevent TJ assembly through its effect on the cytoskeleton.

Cytoskeletal involvement in the modulation of cell-cell junctions by the protein kinase inhibitor H-7

The protein kinase inhibitor H-7 has been shown to block junction dissociation induced by low extracellular calcium in Madin Darby canine kidney epithelial cells (S. Citi, J. Cell Biol. (1992) 117, 169–178). To understand the basis of this effect, we have examined how H-7 affects the organization of junctions and the actin cytoskeleton in different types of epithelial cells in culture. Immunofluorescence microscopy showed that H-7 confers Ca2+ independence on cultured epithelial lens cells, which lack tight junctions and desmosomes but have microfilament-associated adherens junctions. In these cells, H-7 did not protect N-cadherin from trypsin digestion at low extracellular calcium, suggesting that H-7 does not stabilize the ‘active’ cadherin conformation. In cultured Madin Darby canine kidney cells, H-7 partially prevented the fall in transepithelial resistance induced by cytochalasin D, either alone or in conjunction with calcium chelators. Double-immunofluorescence microscopy showed that H-7 inhibits both the fragmentation of labeling for the tight junction protein cingulin and the condensation of actin into cytoplasmic foci induced by cytochalasin D. Taken together, these observations indicate that H-7 inhibits junction dissociation by affecting the contractility of the adherens junction-associated microfilaments following treatment with calcium chelators or cytochalasin D.