Expression of full-length desmosomol glycoproteins (desmocollins) is not sufficient to confer strong adhesion on transfected L929 cells

Activation of cell-surface proteases promotes necroptosis, inflammation and cell migration

Necroptosis is a programmed, caspase-independent cell death that is morphologically similar to necrosis. TNF-induced necroptosis is mediated by receptor-interacting protein kinases, RIP1 and RIP3, and the mixed lineage kinase domain-like (MLKL). After being phosphorylated by RIP3, MLKL is translocated to the plasma membrane and mediates necroptosis. However, the execution of necroptosis and its role in inflammation and other cellular responses remain largely elusive. In this study, we report that MLKL-mediated activation of cell-surface proteases of the a disintegrin and metalloprotease (ADAM) family promotes necroptosis, inflammation and cell migration. ADAMs are specifically activated at the early stage of necroptosis when MLKL is phosphorylated and translocated to the cell plasma membrane. Activation of ADAMs induces ectodomain shedding of diverse cell-surface proteins including adhesion molecules, receptors, growth factors and cytokines. Importantly, the shedding of cell-surface proteins disrupts cell adhesion and accelerates necroptosis, while the soluble fragments of the cleaved proteins trigger the inflammatory responses. We also demonstrate that the shedding of E-cadherin ectodomain from necroptotic cells promotes cell migration. Thus, our study provides a novel mechanism of necroptosis-induced inflammation and new insights into the physiological and pathological functions of this unique form of cell death.

Desmosome assembly and dynamics

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

The molecular composition and function of desmosomes

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

Desmosomal cadherins utilize distinct kinesins for assembly into desmosomes

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

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

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.

Autoimmunity to desmocollin 3 in pemphigus vulgaris

Pemphigus vulgaris is a blistering disease associated with autoantibodies to the desmosomal adhesion protein, desmoglein 3. Genetic deficiency of desmoglein 3 in mice mimics autoimmunity to desmoglein 3 in pemphigus vulgaris, with mucosal-dominant blistering in the suprabasal layer of the epidermis. Mice with an epidermal-specific deletion of desmocollin 3, the other major desmosomal cadherin isoform expressed in the basal epidermis, develop suprabasal blisters in skin that are histologically identical to those observed in pemphigus vulgaris, suggesting that desmocollin 3 might be a target of autoantibodies in some pemphigus vulgaris patients. We now demonstrate that desmocollin 3 is an autoantigen in pemphigus vulgaris, illustrated in a patient with mucosal-dominant blistering. Six of 38 pemphigus vulgaris and one of 85 normal serum samples immunoprecipitate desmocollin 3 (P = 0.003). Incubation of patient IgG with human keratinocytes causes loss of intercellular adhesion, and adsorption with recombinant desmocollin 3 specifically prevents this pathogenic effect. Additionally, anti-desmocollin 3 sera cause loss of keratinocyte cell surface desmocollin 3, but not desmoglein 3 by immunofluorescence, indicating distinct cellular pathogenic effects in anti-desmocollin and anti-desmoglein pemphigus, despite their identical clinical presentations. These data demonstrate that desmocollin 3 is a pathogenic autoantigen in pemphigus vulgaris and suggest that pemphigus vulgaris is a histological reaction pattern that may result from autoimmunity to desmoglein 3, desmocollin 3, or both desmosomal cadherins.

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.

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.

Desmosome structure, composition and function

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

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

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

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.

Tight junction biology and kidney dysfunction

The epithelial tight junction (TJ) has three major functions. As a "gate," it serves as a regulatory barrier separating and maintaining biological fluid compartments of different composition. As a "fence," it generates and maintains the apicobasal polarity of cells that form the confluent epithelium. Finally, the TJ proteins form a trafficking and signaling platform that regulates cell growth, proliferation, differentiation, and dedifferentiation. Six examples are selected that illustrate the emerging link between TJ dysfunction and kidney disease. First, the glomerular slit diaphragm (GSD) is evolved, in part, from the TJ and, on maturation, exhibits all three functions of the TJ. GSD dysfunction leads to proteinuria and, in some instances, podocyte dedifferentiation and proliferation. Second, accumulating evidence supports epithelial-mesenchymal transformation (EMT) as a major player in renal fibrosis, the final common pathway that leads to end-stage renal failure. EMT is characterized by a loss of cell-cell contact and apicobasal polarity, which are hallmarks of TJ dysfunction. Third, in autosomal dominant polycystic kidney disease, mutations of the polycystins may disrupt their known interactions with the apical junction complex, of which the TJ is a major component. This can lead to disturbances in epithelial polarity regulation with consequent abnormal tubulogenesis and cyst formation. Fourth, evidence for epithelial barrier and polarity dysregulation in the pathogenesis of ischemic acute renal failure will be summarized. Fifth, the association between mutations of paracellin-1, the first TJ channel identified, and clinical disorders of magnesium and calcium wasting and bovine renal fibrosis will be used to highlight an integral TJ protein that can serve multiple TJ functions. Finally, the role of WNK4 protein kinase in shunting chloride across the TJ of the distal nephron will be addressed.

Coordinated expression of desmoglein 1 and desmocollin 1 regulates intercellular adhesion

Desmoglein 1 (Dsg1) is a component of desmosomes present in the upper epidermis and can be targeted by autoimmune antibodies or bacterial toxins, resulting in skin blistering diseases. These defects in tissue integrity are believed to result from compromised desmosomal adhesion; yet, previous attempts to directly test the adhesive roles of desmosomal cadherins using normally non-adherent L cells have yielded mixed results. Here, two complementary approaches were used to better resolve the molecular determinants for Dsg1-mediated adhesion: (1) a tetracycline-inducible system was used to modulate the levels of Dsg1 expressed in L cell lines containing desmocollin 1 (Dsc1) and plakoglobin (PG) and (2) a retroviral gene delivery system was used to introduce Dsg1 into normal human epidermal keratinocytes (NHEK). By increasing Dsg1 expression relative to Dsc1 and PG, we were able to demonstrate that the ratio of Dsg1:Dsc1 is a critical determinant of desmosomal adhesion in fibroblasts. The distribution of Dsg1 was organized at areas of cell-cell contact in the multicellular aggregates that formed in these suspension cultures. Similarly, the introduction of Dsg1 into NHEKs was capable of increasing the aggregation of single cell suspensions and further enhanced the adhesive strength of intact epithelial sheets. Endogenous Dsc1 levels were also increased in NHEKs containing Dsg1, providing further support for the coordination of these two desmosomal cadherins in regulating adhesive structures. These Dsg1-mediated effects on intercellular adhesion were directly related to the presence of an intact extracellular domain as ETA, a toxin that specifically cleaves this desmosomal cadherin, inhibited adhesion in both fibroblasts and keratinocytes. Collectively, these observations demonstrate that Dsg1 promotes the formation of intercellular adhesion complexes and suggest that the relative level of Dsg and Dsc expressed at the cell surface regulates this adhesive process.

Isolation of Desmosomes from the Epidermis of Xenopus laevis and Immunochemical Characterization of the Xenopus Desmosomal Cadherins

Here we report a new method of isolating epidermal desmosomes from Xenopus laevis, and a major constituent of desmosomes designated as Xenopus desmogleins (XDsg). Isolation of desmosomes from Xenopus laevis epidermis was carried out by a two step-incubation with different concentrations of NP-40. After discontinuous sucrose gradient centrifugation at 30,000 g for 60 min, a pure desmosomal fraction was obtained at 30%/40% interface. In the SDS-PAGE of isolated desmosomes, at least 12 bands (XDB1 to XDB12) were observed over a 75 kD region. Among them, three bands (XDB3, XDB7, XDB8; estimated MW 175, 124, and 112 kD respectively) were recognized as glycoproteins based on ConA binding. Monospecific polyclonal antibody against XDB3 cross-reacted with bovine Dsgs and vis-a-vis anti-bovine Dsgs with XDB3. By contrast, monospecific antibody against bovine Dsc a/b did not cross-react with either XDB7 or XDB8. Heterogeneous molecular constituents of desmosomal adhesion molecule, which have been observed among different bovine tissues, were confirmed in a phylogenetically different animal, Xenopus laevis. Combined results with other evidence could suggest an alternative system for desmosome-mediated cell adhesion.

Molecular interactions between desmosomal cadherins

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

Desmosomal adhesion regulates epithelial morphogenesis and cell positioning

Desmosomes are intercellular junctions of epithelia and are of widespread importance in the maintenance of tissue architecture. We provide evidence that desmosomal adhesion has a function in epithelial morphogenesis and cell-type-specific positioning. Blocking peptides corresponding to the cell adhesion recognition (CAR) sites of desmosomal cadherins block alveolar morphogenesis by epithelial cells from mammary lumen. Desmosomal CAR-site peptides also disrupt positional sorting of luminal and myoepithelial cells in aggregates formed by the reassociation of isolated cells. We demonstrate that desmosomal cadherins and E-cadherin are comparably involved in epithelial morphoregulation. The results indicate a wider role for desmosomal adhesion in morphogenesis than has previously been considered.

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.

Clustered cadherin genes: a sequence-ready contig for the desmosomal cadherin locus on human chromosome 18

We describe the assembly of a cosmid and PAC contig of approximately 700 kb on human chromosome 18q12 spanning the DSC and DSG genes coding for the desmocollins and desmogleins. These are members of the cadherin superfamily of calcium-dependent cell adhesion proteins present in the desmosome type of cell junction found especially in epithelial cells. They provide the strong cell-cell adhesion generated by this type of cell junction for which expression of both a desmocollin and a desmoglein is required. In the autoimmune skin diseases pemphigus foliaceous and pemphigus vulgaris (PV), where the autoantigens are, respectively, encoded by the DSG1 and DSG3 genes, severe areas of acantholysis (cell separation), potentially life-threatening in the case of PV, are evident. Dominant mutations in the DSG1 gene causing striate palmoplantar keratoderma result in hyperkeratosis of the skin on the parts of the body where pressure and abrasion are greatest, viz., on the palms and soles. These genes are also candidate tumor suppressor genes in squamous cell carcinomas and other epithelial cancers. We have screened two chromosome 18-specific cosmid libraries by hybridization with previously isolated YAC clones and DSC and DSG cDNAs, and a whole genome PAC library, both by hybridization with the YACs and by screening by PCR using cDNA sequences and YAC end sequence. The contigs were extended by further PCR screens using STSs generated by vectorette walking from the ends of the cosmids and PACs, together with sequence from PAC ends. Despite screening of two libraries, the cosmid contig still had four gaps. The PAC contig filled these gaps and in fact covered the whole locus. The positions of 45 STSs covering the whole of this region are presented. The desmocollin and desmoglein genes, which are about 30-35 kb in size, are quite well separated at approximately 20-30 kb apart and are arranged in two clusters, one DSC cluster and one DSG cluster, which are transcribed outward from the interlocus region. The order of the genes is correlated with the spatial order of gene expression in the developing mouse embryo, and this, and previous transgenic experiments, suggests that long-range genetic elements that coordinate expression of these genes may be present. The complete bacterial clone contig described in this paper is thus a resource not only for future sequencing but also for investigations into the control of expression of these clustered genes.

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.

Aspects of the structure and assembly of desmosomes

Desmosomes are found principally in epithelial cells and consist of disc-like plaques, the extracellular face of which is paired with that of a neighbouring cell. There is increasing evidence that desmosomes are adhesive structures, and that two types of desmosomal glycoproteins, the desmogleins (Dsg) and desmocollins (Dsc) both Ca(2+)-binding cadherin-like molecules, perform this role in adhesion through interaction of their extracellular domains. A number of isoforms of Dsg and Dsc are present in specific tissues. The cytoplasmic side of the plaque is attached to intermediate filaments through desmoplakin, a major plaque protein. Also associated with desmosomes are plakoglobin and beta-catenin, suggesting that the adhesive function of desmosomes might be mediated by signal transduction. Formation of desmosomes can be studied by growing epithelial cells in low-Ca2+ medium (LCM, < 0.1 mM), where desmosomal proteins are either synthesized but not assembled, or form partially assembled but unstable half-desmosomes. Addition of Ca2+ (to about 2mM) initiates cell contact and, in the case of half-desmosomes, leads to stabilization by incorporation into membranes and formation of typical paired structures. In cases where such pre-assembled structures are not formed, recruitment of desmosomal proteins appears to occur by vesicular transport of desmocollins and desmogleins to the cell surface, where association is made with plakoglobin and later, with desmoplakin. Although much remains to be learned of the assembly process, specific interacting domains of the molecular components are being recognized. Desmosome assembly is part of a coordinated pattern of junction formation which accompanies the establishment of cell polarity, resulting in differentiation of apical and basolateral cell surfaces. Desmosomes are now being regarded, not as static and inert structures, but as membrane specializations linked to systems involved in cell-cell communication as well as adhesion.

Desmosomal adhesion inhibits invasive behavior

Recent studies of human disease and transgenic animal experiments have clearly demonstrated the importance of desmosomes in normal tissue architecture. Furthermore, desmosomal components are down-regulated in certain types of carcinomas, suggesting a possible role for desmosomes in suppression of invasion and metastasis. However, there is no functional evidence to support such a hypothesis. To obtain such evidence, we needed to generate desmosomal adhesion in an invasive cell line. We show that expression of multiple desmosomal components (the desmosomal cadherins, desmocollin and desmoglein, and the armadillo protein, plakoglobin) in nonadhesive L929 fibroblasts generates adhesion in aggregation assays. This adhesion is specifically blocked by short peptides corresponding to the putative cell adhesion recognition sites of desmocollin and desmoglein. This result provides an experimental demonstration of the functional importance of the cell adhesion recognition sites of desmocollin and desmoglein and indicates that both desmosomal cadherins are specifically involved in this adhesion. Moreover, whereas parental L929 cells are strongly invasive into collagen gels, we show that invasion is substantially inhibited in cells transfected with desmosomal components. Invasion is restored by treating the transfected cells with anti-adhesion peptides, indicating that desmosomal adhesion specifically blocks invasion in culture. Our results support the suggestion that desmosomes have a role in suppression of tumor spreading.

Contributions of extracellular and intracellular domains of full length and chimeric cadherin molecules to junction assembly in epithelial cells

The integrity of cell-cell junctions in epithelial cells depends on functional interactions of both extracellular and intracellular domains of cadherins with other junction proteins. To examine the roles of the different domains of E-cadherin and desmoglein in epithelial junctions, we stably expressed full length desmoglein 1 and chimeras of E-cadherin and desmoglein 1 in A431 epithelial cells. Full length desmoglein 1 was able to incorporate into or disrupt endogenous desmosomes depending on expression level. Each of the chimeric cadherin molecules exhibited distinct localization patterns at the cell surface. A chimera of the desmoglein 1 extracellular domain and the E-cadherin intracellular domain was distributed diffusely at the cell surface while the reverse chimera, comprising the E-cadherin extracellular domain and the desmoglein 1 intracellular domain, localized in large, sometimes contiguous patches at cell-cell interfaces. Nevertheless, both constructs disrupted desmosome assembly. Expression of constructs containing the desmoglein 1 cytoplasmic domain resulted in approximately a 3-fold decrease in E-cadherin bound to plakoglobin and a 5- to 10-fold reduction in the steady-state levels of the endogenous desmosomal cadherins, desmoglein 2 and desmocollin 2, possibly contributing to the dominant negative effect of the desmoglein 1 tail. In addition, biochemical analysis of protein complexes in the stable lines revealed novel in vivo protein interactions. Complexes containing beta-catenin and desmoglein 1 were identified in cells expressing constructs containing the desmoglein 1 tail. Furthermore, interactions were identified between endogenous E-cadherin and the chimera containing the E-cadherin extracellular domain and the desmoglein 1 intracellular domain providing in vivo evidence for previously predicted lateral interactions of E-cadherin extracellular domains.

Immunohistochemical study of desmosomes in oral squamous cell carcinoma: correlation with cytokeratin and E-cadherin staining, and with tumour behaviour

Reduction or loss of the intercellular junctions known as desmosomes may contribute to the invasive and metastatic behaviour of various carcinomas. Previous studies have shown that metastasis of oral squamous cell carcinomas of the head and neck correlates with a reduction in immunohistochemical staining for desmoplakin and desmoglein at the invasion front. The primary aim of the present study was to extend these observations to include a third component of desmosomes, the glycoprotein desmocollin. An additional aim was to determine whether the differentiation status of tumours is reflected in their staining for cytokeratins 1, 13, and 19, and, if so, whether these parameters correlate with desmosomal staining and/or metastasis. The study included 54 primary tumours of which 28 showed lymph node metastases. The results of this investigation show that tumours can be divided into three groups according to whether they have lost staining for no, one or more than one desmosomal component. A statistically significant correlation was found between the number of desmosomal components lost and metastasis. Tumours could also be divided into five groups according to their staining for different combinations of cytokeratins. Furthermore, differentiation status as indicated both histologically and by cytokeratin staining correlated with reduced desmosomal staining and metastasis. Tumours were also examined for intensity of staining for the adhesion molecule E-cadherin. Reduction in E-cadherin staining was correlated with mode of invasion and with reduction in desmosomal staining, but not with poor differentiation as indicated by cytokeratin staining. The results of this extensive study reinforce the view that adhesive junctions and adhesion molecules contribute to the suppression of tumour invasion and metastasis.

Coexpression of both types of desmosomal cadherin and plakoglobin confers strong intercellular adhesion

Desmosomes are unique intercellular junctions in that they invariably contain two types of transmembrane cadherin molecule, desmocollins and desmogleins. In addition they possess a distinct cytoplasmic plaque structure containing a few major proteins including desmoplakins and the armadillo family member plakoglobin. Desmosomal cadherins are putative cell-cell adhesion molecules and we have tested their adhesive capacity using a transfection approach in mouse L cells. We find that L cells expressing either one or both of the desmosomal cadherins desmocollin 2a or desmoglein 1 display weak cell-cell adhesion activity that is Ca2+-dependent. Both homophilic and heterophilic adhesion could be detected. However, co-expression of plakoglobin with both desmosomal cadherins, but not with desmoglein 1 alone, resulted in a dramatic potentiation of cell-cell aggregation and the accumulation of detergent-insoluble desmosomal proteins at points of cell-cell contact. The effect of plakoglobin seems to be due directly to its interaction with the desmosomal cadherins rather than to its signalling function. The data suggest that the desmosome may obligatorily contain two cadherins and is consistent with a model in which desmocollins and desmogleins may form side by side heterodimers in contrast to the classical cadherins that are homodimeric. Plakoglobin may function by potentiating dimer formation, accretion of dimers to cell-cell contact sites or desmosomal cadherin stability.

The amino-terminal domain of desmoplakin binds to plakoglobin and clusters desmosomal cadherin-plakoglobin complexes

The desmosome is a highly organized plasma membrane domain that couples intermediate filaments to the plasma membrane at regions of cell-cell adhesion. Desmosomes contain two classes of cadherins, desmogleins, and desmocollins, that bind to the cytoplasmic protein plakoglobin. Desmoplakin is a desmosomal component that plays a critical role in linking intermediate filament networks to the desmosomal plaque, and the amino-terminal domain of desmoplakin targets desmoplakin to the desmosome. However, the desmosomal protein(s) that bind the amino-terminal domain of desmoplakin have not been identified. To determine if the desmosomal cadherins and plakoglobin interact with the amino-terminal domain of desmoplakin, these proteins were co-expressed in L-cell fibroblasts, cells that do not normally express desmosomal components. When expressed in L-cells, the desmosomal cadherins and plakoglobin exhibited a diffuse distribution. However, in the presence of an amino-terminal desmoplakin polypeptide (DP-NTP), the desmosomal cadherins and plakoglobin were observed in punctate clusters that also contained DP-NTP. In addition, plakoglobin and DP-NTP were recruited to cell-cell interfaces in L-cells co-expressing a chimeric cadherin with the E-cadherin extracellular domain and the desmoglein-1 cytoplasmic domain, and these cells formed structures that were ultrastructurally similar to the outer plaque of the desmosome. In transient expression experiments in COS cells, the recruitment of DP-NTP to cell borders by the chimera required co-expression of plakoglobin. Plakoglobin and DP-NTP co-immunoprecipitated when extracted from L-cells, and yeast two hybrid analysis indicated that DP-NTP binds directly to plakoglobin but not Dsg1. These results identify a role for desmoplakin in organizing the desmosomal cadherin-plakoglobin complex and provide new insights into the hierarchy of protein interactions that occur in the desmosomal plaque.

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.

Direct Ca2+-dependent heterophilic interaction between desmosomal cadherins, desmoglein and desmocollin, contributes to cell-cell adhesion

Human fibrosarcoma cells, HT-1080, feature extensive adherens junctions, lack mature desmosomes, and express a single known desmosomal protein, Desmoglein 2 (Dsg2). Transfection of these cells with bovine Desmocollin 1a (Dsc1a) caused dramatic changes in the subcellular distribution of endogenous Dsg2. Both cadherins clustered in the areas of the adherens junctions, whereas only a minor portion of Dsg2 was seen in these areas in the parental cells. Deletion mapping showed that intact extracellular cadherin-like repeats of Dsc1a (Arg1-Thr170) are required for the translocation of Dsg2. Deletion of the intracellular C-domain that mediates the interaction of Dsc1a with plakoglobin, or the CSI region that is involved in the binding to desmoplakin, had no effect. Coimmunoprecipitation experiments of cell lysates stably expressing Dsc1a with anti-Dsc or -Dsg antibodies demonstrate that the desmosomal cadherins, Dsg2 and Dsc1a, are involved in a direct Ca2+-dependent interaction. This conclusion was further supported by the results of solid phase binding experiments. These showed that the Dsc1a fragment containing cadherin-like repeats 1 and 2 binds directly to the extracellular portion of Dsg in a Ca2+-dependent manner. The contribution of the Dsg/ Dsc interaction to cell-cell adhesion was tested by coculturing HT-1080 cells expressing Dsc1a with HT-1080 cells lacking Dsc but expressing myc-tagged plakoglobin (MPg). In the latter cells, MPg and the endogenous Dsg form stable complexes. The observed specific coimmunoprecipitation of MPg by anti-Dsc antibodies in coculture indicates that an intercellular interaction between Dsc1 and Dsg is involved in cell-cell adhesion.

Protein kinase C activation upregulates intercellular adhesion of alpha-catenin-negative human colon cancer cell variants via induction of desmosomes

The alpha-catenin molecule links E-cadherin/ beta-catenin or E-cadherin/plakoglobin complexes to the actin cytoskeleton. We studied several invasive human colon carcinoma cell lines lacking alpha-catenin. They showed a solitary and rounded morphotype that correlated with increased invasiveness. These round cell variants acquired a more normal epithelial phenotype upon transfection with an alpha-catenin expression plasmid, but also upon treatment with the protein kinase C (PKC) activator 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Video registrations showed that the cells started to establish elaborated intercellular junctions within 30 min after addition of TPA. Interestingly, this normalizing TPA effect was not associated with alpha-catenin induction. Classical and confocal immunofluorescence showed only minor TPA-induced changes in E-cadherin staining. In contrast, desmosomal and tight junctional proteins were dramatically rearranged, with a conversion from cytoplasmic clusters to obvious concentration at cell-cell contacts and exposition at the exterior cell surface. Electron microscopical observations revealed the TPA-induced appearance of typical desmosomal plaques. TPA-restored cell-cell adhesion was E-cadherin dependent as demonstrated by a blocking antibody in a cell aggregation assay. Addition of an antibody against the extracellular part of desmoglein-2 blocked the TPA effect, too. Remarkably, the combination of anti-E-cadherin and anti-desmoglein antibodies synergistically inhibited the TPA effect. Our studies show that it is possible to bypass the need for normal alpha-catenin expression to establish tight intercellular adhesion by epithelial cells. Apparently, the underlying mechanism comprises upregulation of desmosomes and tight junctions by activation of the PKC signaling pathway, whereas E-cadherin remains essential for basic cell-cell adhesion, even in the absence of alpha-catenin.

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.

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.

Analysis of desmosomal cadherin-adhesive function and stoichiometry of desmosomal cadherin-plakoglobin complexes

Desmosomes are intercellular adhesive junctions that associate with the intermediate filament cytoskeleton. The two major classes of transmembrane desmosomal glycoproteins, desmogleins and desmocollins, are widely considered to function as adhesion molecules. This assumption is based in part on their homology to the cadherin family of calcium-dependent homophilic adhesion molecules. In addition, autoantibodies from pemphigus patients bind directly to desmoglein family members and are thought to cause epidermal blistering by inhibiting the function of these cadherins. To directly test the ability of the desmosomal cadherins to mediate adhesion, desmoglein-1 (Dsg1), desmocollin-2 (Dsc2a) and plakoglobin were expressed in mouse L cell fibroblasts. Similar to catenin:classical cadherin complexes, plakoglobin:Dsc2a complexes exhibited an approximately 1:1 stoichiometry; however, plakoglobin:Dsg1 complexes exhibited a 6:1 stoichiometry. When L cells expressing the desmosomal cadherins were tested for the ability to aggregate in suspension, L cells expressing E-cadherin exhibited extensive aggregation, but L cells expressing Dsg1 or Dsc2a did not aggregate. In addition, L cells co-expressing Dsg1, Dsc2a, and plakoglobin failed to aggregate. The cytoplasmic domain of E-cadherin is thought to play a central role in the adhesive function of E-cadherin by providing a link to the actin cytoskeleton. Therefore, two chimeric cadherins comprising the cytoplasmic domain of E-cadherin and the extracellular domain of either Dsg1 or Dsc2a were expressed in L cells. Both chimeras formed a complex with alpha- and beta-catenin. Nevertheless, neither of these chimeras supported aggregation of L cells when expressed individually or when co-expressed. These data suggest that the extracellular domains of the desmosomal cadherins exhibit functional properties distinct from those of the classical cadherins, such as E-cadherin.