Isolation of cDNAs encoding desmosomal plaque proteins: evidence that bovine desmoplakins I and II are derived from two mRNAs and a single gene

Clustering of desmosomal cadherins by desmoplakin is essential for cell-cell adhesion

AIM

Desmoplakin (Dp) is a crucial component of the desmosome, a supramolecular cell junction complex anchoring intermediate filaments. The mechanisms how Dp modulates cell-cell adhesion are only partially understood. Here, we studied the impact of Dp on the function of desmosomal adhesion molecules, desmosome turnover and intercellular adhesion.

METHODS

CRISPR/Cas9 was used for gene editing of human keratinocytes which were characterized by Western blot and immunostaining. Desmosomal ultrastructure and function were assessed by electron microscopy and cell adhesion assays. Single molecule binding properties and localization of desmosomal cadherins were studied by atomic force microscopy and super-resolution imaging.

RESULTS

Knockout (ko) of Dp impaired cell cohesion to drastically higher extents as ko of another desmosomal protein, plakoglobin (Pg). In contrast to Pg ko, desmosomes were completely absent in Dp ko. Binding properties of the desmosomal adhesion molecules desmocollin (Dsc) 3 and desmoglein (Dsg) 3 remained unaltered under loss of Dp. Dp was required for assembling desmosomal cadherins into large clusters, as Dsg2 and Dsc3, adhesion molecules primarily localized within desmosomes, were redistributed into small puncta in the cell membrane of Dp ko cells. Additional silencing of desmosomal cadherins in Dp ko did not further increase loss of intercellular adhesion.

CONCLUSION

Our data demonstrate that Dp is essential for desmosome formation but does not influence intercellular adhesion on the level of individual cadherin binding properties. Rather, macro-clustering of desmosomal adhesion molecules through Dp is crucial. These results may help to better understand severe diseases which are caused by Dp dysfunction.

Desmoplakin interacts with the coil 1 of different types of intermediate filament proteins and displays high affinity for assembled intermediate filaments

The interaction of intermediate filaments (IFs) with the cell-cell adhesion complexes desmosomes is crucial for cytoskeletal organization and cell resilience in the epidermis and heart. The intracellular desmosomal protein desmoplakin anchors IFs to the cell adhesion complexes predominantly via its four last carboxy-terminal domains (C-terminus). However, it remains unclear why the C-terminus of desmoplakin interacts with different IF types or if there are different binding affinities for each type of IFs that may influence the stability of cell-specific adhesion complexes. By yeast three-hybrid and fluorescence binding assays, we found that the coiled-coil 1 of the conserved central rod domain of the heterodimeric cytokeratins (Ks) 5 and 14 (K5/K14) was required for their interaction with the C-terminus of desmoplakin, while their unique amino head- and C-tail domains were dispensable. Similar findings were obtained in vitro with K1/K10, and the type III IF proteins desmin and vimentin. Binding assays testing the C-terminus of desmoplakin with assembled K5/K14 and desmin IFs yielded an apparent affinity in the nM range. Our findings reveal that the same conserved domain of IF proteins binds to the C-terminus of desmoplakin, which may help explain the previously reported broad binding IF-specificity to desmoplakin. Our data suggest that desmoplakin high-affinity binding to diverse IF proteins ensures robust linkages of IF cytoskeleton and desmosomes that maintain the structural integrity of cellular adhesion complexes. In summary, our results give new insights into the molecular basis of the IF-desmosome association.

Structural and functional diversity of desmosomes

Desmosomes anchor intermediate filaments at sites of cell contact established by the interaction of cadherins extending from opposing cells. The incorporation of cadherins, catenin adaptors, and cytoskeletal elements resembles the closely related adherens junction. However, the recruitment of intermediate filaments distinguishes desmosomes and imparts a unique function. By linking the load-bearing intermediate filaments of neighboring cells, desmosomes create mechanically contiguous cell sheets and, in so doing, confer structural integrity to the tissues they populate. This trait and a well-established role in human disease have long captured the attention of cell biologists, as evidenced by a publication record dating back to the mid-1860s. Likewise, emerging data implicating the desmosome in signaling events pertinent to organismal development, carcinogenesis, and genetic disorders will secure a prominent role for desmosomes in future biological and biomedical investigations.

Compound heterozygous desmoplakin mutations result in a phenotype with a combination of myocardial, skin, hair, and enamel abnormalities

Desmoplakin (DP) anchors the intermediate filament cytoskeleton to the desmosomal cadherins and thereby confers structural stability to tissues. In this study, we present a patient with extensive mucocutaneous blisters, epidermolytic palmoplantar keratoderma, nail dystrophy, enamel dysplasia, and sparse woolly hair. The patient died at the age of 14 years from undiagnosed cardiomyopathy. The skin showed hyperplasia and acantholysis in the mid- and lower epidermal layers, whereas the heart showed extensive fibrosis and fibrofatty replacement in both ventricles. Immunofluorescence microscopy showed a reduction in the C-terminal domain of DP in the skin and oral mucosa. Sequencing of the DP gene showed undescribed mutations in the maternal and paternal alleles. Both mutations affected exon 24 encoding the C-terminal domain. The paternal mutation, c.6310delA, leads to a premature stop codon. The maternal mutation, c.7964 C to A, results in a substitution of an aspartic acid for a conserved alanine residue at amino acid 2655 (A2655D). Structural modeling indicated that this mutation changes the electrostatic potential of the mutated region of DP, possibly altering functions that depend on intermolecular interactions. To conclude, we describe a combination of DP mutation phenotypes affecting the skin, heart, hair, and teeth. This patient case emphasizes the importance of heart examination of patients with desmosomal genodermatoses.

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.

Perturbed intraepithelial differentiation of corneal epithelium in c-Fos-null mice

PURPOSE

AP-1 is a transcription factor that plays a pivotal role in regulating cellular homeostasis and which may modulate the differentiation of corneal epithelial cells. We examined the role of c-Fos in the differentiation of corneal epithelial cells by using c-Fos-deficient (c-fos (-/-)) mice.

METHODS

Ten adult c-fos (-/-) mice and ten control (c-fos (+/-) or c-fos (+/+)) mice were used. The expression patterns of the mRNA and protein of keratin 12 (K12) were determined to examine the differentiation of cornea-type epithelium. To evaluate the intraepithelial differentiation from basal cells to superficial cells, the ultrastructure of the corneal epithelium was studied. We focused on the formation of desmosomes in the superficial, suprabasal, and basal cell layers, and also on the hemidesmosomes. The number of desmosomes in each epithelial layer was statistically analyzed by using an unpaired t test. The expressions of keratin 14 (K14), desmoglein, E-cadherin, occludin, connexin 43, filaggrin, loricrin, and involucrin were examined to analyze epithelial differentiation.

RESULTS

The mRNA and protein of K12 were expressed in the corneal epithelium of c-fos (-/-) and control mice. Ultrastructural observations showed that the number of desmosomes between the basal cells of the corneal epithelia was similar in c-fos (-/-) and control mice. However, there were fewer desmosomes between suprabasal cells and between superficial cells in c-fos (-/-) mice than in control mice. The number of hemidesmosomes in the corneal epithelial cells in c-Fos-null mice was similar to that in control mice. The expressions of the other epithelial cell differentiation markers were not affected by the absence of c-Fos. Ultrastructural observations showed a disarrangement of the corneal epithelium in the c-Fos-null mice.

CONCLUSIONS

The absence of c-Fos disturbs the formation of desmosomes in the superficial layers of the corneal epithelium, suggesting a perturbation of intraepithelial differentiation from the basal epithelial cells to the suprabasal and superficial epithelial cells.

Genetic alterations at the Bpag1 locus in dt mice and their impact on transcript expression

The dystonin/Bpag1 gene encodes several tissue-specific alternatively spliced transcripts that encode cytoskeletal binding proteins. These various isoforms are necessary for maintaining the structural integrity of epithelial, neural, and muscle tissues. Mutations in the dystonin/Bpag1 gene cause dystonia musculorum (dt), a hereditary neuropathy of the mouse characterized by the progressive degeneration of sensory neurons. Several dt mutant alleles exist, most of which have arisen through spontaneous mutations. In this article we demonstrate that the dt locus encodes 107 exons spanning 400 kb. The high frequency of occurrence of spontaneous dt mutants may therefore be a result of the large size of the gene. Analysis of genomic DNA from several dt spontaneous mutant alleles, dt(24J), dt(27J), dt(Alb), and dt(Frk), shows a deletion of the central portion of the gene in dt(Alb) but no large rearrangements or deletions in the other alleles. These other alleles likely have small deletions or rearrangements, or point mutations. To determine the impact of the known and unknown mutations on transcript levels, RT-PCR was performed to detect various coding regions of the dystonin/Bpag1 transcripts in brain and muscle from multiple dt alleles: dt(Tg4), dt(Alb), dt(24J), dt(27J), and dt(Frk). With the exception of dt(Frk), reduced transcript levels were observed for all alleles tested. Such alterations likely result in reduced or absent dystonin/Bpag1 protein levels. Thus, distinct genetic defects lead to a common outcome of reduced transcript expression causing the same phenotype in multiple dt alleles.

Loss of desmoplakin tail causes lethal acantholytic epidermolysis bullosa

The cytoplasmic plaque protein desmoplakin (DP), which is located in desmosomes, plays a major role in epithelial and muscle cell adhesion by linking the transmembrane cadherins to the cytoplasmic intermediate filament network. Mutations of DP may cause striate palmoplantar keratoderma, arrhythmogenic right ventricular dysplasia, skin fragility/woolly hair syndrome, Naxos-like disease, and Carvajal syndrome. DP must be indispensable, because DP-/- mice are early abortive. Here, we report a patient with severe fragility of skin and mucous membranes caused by genetic truncation of the DP tail. The new phenotype is lethal in the neonatal period because of immense transcutaneous fluid loss. The phenotype also comprised universal alopecia, neonatal teeth, and nail loss. Histology showed suprabasal clefting and acantholysis throughout the spinous layer, mimicking pemphigus. Electron microscopy revealed disconnection of keratin intermediate filaments from desmosomes. Immunofluorescence staining of DP showed a distinct punctate intercellular pattern in the patient's skin. Protein analysis revealed expression of truncated DP polypeptides. Mutational analysis of the patient demonstrated compound heterozygosity for two DP mutations, 6079C-->T (R1934X) and 6370delTT, respectively. Aberrant mRNA transcripts that predict premature termination of translation with loss of the three intermediate filament-binding subdomains in the DP tail were detected by RT-PCR. The new dramatic phenotype, which we named "lethal acantholytic epidermolysis bullosa," underscores the paramount role of DP in epidermal integrity.

Plakophilins--hard work in the desmosome, recreation in the nucleus?

The linkage of the different types of cytoskeletal proteins to cell adhesion structures at the cytoplasmic membrane and the connection of these contact sites to corresponding sites of adjacent cells is a prerequisite for integrity and stability of cells and tissues. The structurally most prominent types of such cell-cell adhesion complexes are the desmosomes (maculae adhaerentes), which are found in all epithelia and certain non-epithelial tissues. As an element of the cytoskeleton, intermediate filaments are connected to the adhesive desmosomal transmembrane proteins by the cytoplasmic desmosomal plaque proteins. At least three different types of proteins are found in the desmosomal plaque, one of which is represented by the plakophilins, a recently described sub-family of sequence-related armadillo-repeat proteins. Consisting of three isoforms, plakophilins (plakophilin 1 to 3, PKP 1 to 3) are located in all desmosomes in a differentiation-dependent manner. While PKP 2 and PKP 3 are part of almost all desmosome-bearing cell types (PKP 2 except for differentiated cells of stratified epithelia and PKP 3 for hepatocytes and cardiomyocytes), PKP 1 is restricted to desmosomes of cells of stratified and complex epithelia. Besides the architectural function that plakophilins seem to fulfill in the desmosomes, at least PKP 1 and 2 are also localized in the nucleus independently of any differentiation-related processes and with an up to now enigmatic function in this compartment. In the following article we want to summarize the current knowledge concerning structure, function and regulation of the plakophilins that has been achieved during the last decade.

Structure and regulation of the envoplakin gene

Envoplakin, a member of the plakin family of proteins, is a component of desmosomes and the epidermal cornified envelope. To understand how envoplakin expression is regulated, we have analyzed the structure of the mouse envoplakin gene and characterized the promoters of both the human and mouse genes. The mouse gene consists of 22 exons and maps to chromosome 11E1, syntenic to the location of the human gene on 17q25. The exon-intron structure of the mouse envoplakin gene is common to all members of the plakin family: the N-terminal protein domain is encoded by 21 small exons, and the central rod domain and the C-terminal globular domain are coded by a single large exon. The C terminus shows the highest sequence conservation between mouse and human envoplakins and between envoplakin and the other family members. The N terminus is also conserved, with sequence homology extending to Drosophila Kakapo. A region between nucleotides -101 and 288 was necessary for promoter activity in transiently transfected primary keratinocytes. This region is highly conserved between the human and mouse genes and contains at least two different positively acting elements identified by site-directed mutagenesis and electrophoretic mobility shift assays. Mutation of a GC box binding Sp1 and Sp3 proteins or a combined E box and Krüppel-like element interacting with unidentified nuclear proteins virtually abolished promoter activity. 600 base pairs of the mouse upstream sequence was sufficient to drive expression of a beta-galactosidase reporter gene in the suprabasal layers of epidermis, esophagus, and forestomach of transgenic mice. Thus, we have identified a regulatory region in the envoplakin gene that can account for the expression pattern of the endogenous protein in stratified squamous epithelia.

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.

Defining the interactions between intermediate filaments and desmosomes

Desmoplakin (DP), plakoglobin (PG), and plakophilin 1 (PP1) are desmosomal components lacking a transmembrane domain, thus making them candidate linker proteins for connecting intermediate filaments and desmosomes. Using deletion and site-directed mutagenesis, we show that remarkably, removal of approximately 1% of DP's sequence obliterates its ability to associate with desmosomes. Conversely, when linked to a foreign protein, as few as 86 NH2-terminal DP residues are sufficient to target to desmosomes efficiently. In in vitro overlay assays, the DP head specifically associates with itself and with desmocollin 1a (Dsc1a). In similar overlay assays, PP1 binds to DP and Dsc1a, and to a lesser extent, desmoglein 1 (Dsg1), while PG binds to Dsg1 and more weakly to Dsc1a and DP. Interestingly, like DP, PG and PP1 associate with epidermal keratins, although PG is considerably weaker in its ability to do so. As judged by overlay assays, the amino terminal head domain of type II keratins appears to have a special importance in establishing these connections. Taken together, our findings provide new insights into the complexities of the links between desmosomes and intermediate filaments (IFs). Our results suggest a model whereby at desmosome sites within dividing epidermal cells, DP and PG anchor to desmosomal cadherins and to each other, forming an ordered array of nontransmembrane proteins that then bind to keratin IFs. As epidermal cells differentiate, PP1 is added as a molecular reinforcement to the plaque, enhancing anchorage to IFs and accounting at least partially for the increase in numbers and stability of desmosomes in suprabasal cells.

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.

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.

Cell type-specific desmosomal plaque proteins of the plakoglobin family: plakophilin 1 (band 6 protein)

Desmosomes represent a special type of the plaque-bearing adhering junctions, characteristic of certain pathways of cell differentiation, which compositionally are not identical in the various kinds of desmosome-forming cells. While all desmosomes contain the cytoplasmic plaque proteins desmoplakin I and plakoglobin, they can vary in their specific complement of desmosomal cadherins and by the presence of additional plaque proteins. We have raised monoclonal antibodies recognizing one such 'accessory' plaque protein, the cytokeratin-binding, basic protein plakophilin 1, originally introduced as 'band 6 protein' or 'polypeptide D6', which is an abundant desmosomal component in certain epithelia. Using such antibodies, we have isolated cDNA clones encoding the bovine and the human protein and determined their complete amino acid sequences. The mRNAs, which on Northern blot tests appear as two bands corresponding to approximately 4 and 2.4 kb (bovine) or 5 and 2.6 kb (human), code for 727 amino acids (calculated mol. wt. 80,180; IEP 9.25) in bovine and 726 amino acids (mol. wt. 80,496; IEP 9.34) in human plakophilin. Sequence analyses have revealed the presence of 9.2 repeated units of the arm-motif sequence, confirming our previous conclusion that this protein is a member of a larger family of proteins including, inter alia, several membrane-associated plaque proteins such as vertebrate plakoglobin and beta-catenin as well as the product of the armadillo gene of Drosophila. The plakophilin antibodies and cDNA probes have also allowed us to examine its synthesis in various tissues and cell cultures. While we confirm the occurrence of the protein in cytoskeletal fractions from various stratified squamous, complex, glandular duct and bladder epithelia, where it can be localized to desmosomes, we have, surprisingly, also identified the protein, although at lower amounts, in cytoskeletal fractions from several cultured cell lines in which the protein has not been consistently localized to desmosomes by immunofluorescence microscopy. Examples include cultured cells derived from certain simple epithelia such as the kidney-derived line MDBK and cultured calf lens cells. We have also found that, in all plakophilin 1-positive cells examined, a pool of diffusible ('soluble') cytoplasmic plakophilin exists, including cell lines such as human mammary carcinoma MCF-7 cells in which this soluble plakophilin seems to be the only detectable form. In addition, we have identified some soluble proteins conspicuously cross-reacting with plakophilin 1. Possible functions of plakophilin and its potential value as a marker for specific states of cell differentiation are discussed, particularly with respect to tumor diagnosis.

Making a connection: direct binding between keratin intermediate filaments and desmosomal proteins

In epidermal cells, keratin intermediate filaments connect with desmosomes to form extensive cadherin-mediated cytoskeletal architectures. Desmoplakin (DPI), a desmosomal component lacking a transmembrane domain, has been implicated in this interaction, although most studies have been conducted with cells that contain few or no desmosomes, and efforts to demonstrate direct interactions between desmoplakin and intermediate filaments have not been successful. In this report, we explore the biochemical nature of the connections between keratin filaments and desmosomes in epidermal keratinocytes. We show that the carboxy terminal "tail" of DPI associates directly with the amino terminal "head" of type II epidermal keratins, including K1, K2, K5, and K6. We have engineered and purified recombinant K5 head and DPI tail, and we demonstrate direct interaction in vitro by solution-binding assays and by ligand blot assays. This marked association is not seen with simple epithelial type II keratins, vimentin, or with type I keratins, providing a possible explanation for the greater stability of the epidermal keratin filament architecture over that of other cell types. We have identified an 18-amino acid residue stretch in the K5 head that is conserved only among type II epidermal keratins and that appears to play some role in DPI tail binding. This finding might have important implications for understanding a recent point mutation found within this binding site in a family with a blistering skin disorder.

A case of bullous pemphigoid with antidesmoplakin autoantibodies

Paraneoplastic pemphigus, a recently identified disease entity, is associated with autoantibodies against a variety of epidermal proteins including desmoplakins I and II, and the 230-kDa bullous pemphigoid (BP) antigen. We report an 84-year-old Japanese man who had typical clinical and histopathological features of BP, but in whom indirect immunofluorescence, using normal human skin as the substrate, revealed concomitant serum antibasement membrane zone and antikeratinocyte cell surface autoantibodies. His serum showed reactivity similar to that which is seen with antidesmoplakin monoclonal antibody on immunofluorescence of cardiac muscle and urinary bladder. With immunoblotting, using various antigen sources, the patient's serum reacted with desmoplakins I and II, and with both the 230- and 180-kDa BP antigens. Immunogold electron microscopy also indicated the presence of antidesmoplakin antibodies. Although the significance of the antidesmoplakin antibodies in this patient is unknown, the findings in this case may provide an insight into understanding the occurrence of antidesmoplakin antibodies in paraneoplastic pemphigus.

Complexus adhaerentes, a new group of desmoplakin-containing junctions in endothelial cells: II. Different types of lymphatic vessels

In diverse mammalian species, including (man, cow and rat) the very flat endothelial cells of lymphatic vessels of various organs, including the retothelial meshwork of sinus of lymph nodes, are connected by zonula-like plaque-bearing junctions which differ from the similarly structured junctions of blood vessel endothelia by the presence of desmoplakin or an as yet unknown but closely related plaque protein. These extended junctions, which also contain plakoglobin but none of the presently known desmogleins and desmocollins, are therefore different from the spot-like desmosomes (maculae adhaerentes) present in epithelia, myocardium and dendritic reticulum cells of lymphatic follicles, and are collectively subsumed under the new category of complexus adhaerentes, including the 'syndesmos' connecting the processes of the retothelial cells. The lymphatic endothelial cells possessing these special desmoplakin-containing junctions also contain the calcium-dependent transmembrane glycoproteins, V-cadherin and cadherin 5, of which the latter has also been partly localized to regions with desmoplakin-positive junctions. Possible functional reasons for the formation and maintenance of complexus adhaerentes are discussed as well as the potential value of reagents which allow their identification in relation to physiology and pathology.

Human autoantibodies against desmoplakins in paraneoplastic pemphigus

Recently, a previously unrecognized autoantibody mediated blistering disease, paraneoplastic pemphigus has been described. Paraneoplastic pemphigus is associated with lymphoid malignancies, thymomas, and poorly differentiated sarcomas. Serum of affected patients contain pathogenic autoantibodies that immunoprecipitate from normal keratinocytes a characteristic complex of four polypeptides with M(r) of 250, 230, 210, and 190 kD. As our preliminary studies indicated that the 250-kD and the 210-kD antigens comigrated with desmoplakins I and II, we investigated the possibility that autoantibodies against the desmoplakins were a component of this autoimmune syndrome. 11 sera from affected patients were tested by indirect immunofluorescence against desmosome containing tissues, immunoprecipitation of metabolically labeled keratinocytes, and Western immunoblotting of desmoplakins I and II that had been purified to homogeneity from pig tongue epithelium. By indirect immunofluorescence, 9 of 11 sera showed strong binding to epithelial and nonepithelial desmosomes, and 2 were weakly reactive. All 11 immunoprecipitated 250- and 210-kD bands of variable intensity that comigrated with bands identified by a murine monoclonal antidesmoplakin antibody, and immunoblotting confirmed binding of the serum autoantibodies to purified desmoplakins. This demonstrates that paraneoplastic pemphigus is the first human autoimmune syndrome in which autoantibodies against the desmoplakins are a prominent component of the humoral autoimmune response.

The human gene (DSG3) coding for the pemphigus vulgaris antigen is, like the genes coding for the other two known desmogleins, assigned to chromosome 18

Pemphigus vulgaris (PV) is a potentially lethal skin disease in which epidermal blisters occur as the result of the loss of cell-cell adhesion caused by the action of autoantibodies against a keratinocyte cell surface glycoprotein, the PV antigen (PVA). This latter protein is a member of the desmoglein subfamily of the cadherin superfamily of cell-cell adhesion molecules, present in the desmosome type of intercellular junction. The other two known desmogleins are DGI, which is a target antigen in another autoantibody-mediated blistering disease of the epidermis, pemphigus foliaceous, and HDGC, which is expressed in the basal layer of the epidermis and in the simple epithelium of, for example, the colon. Genes coding for DGI (DSG1) and HDGC (DSG2) have previously been assigned to human chromosome 18. We now present evidence, using a polymerase chain reaction assay, that DSG3, the gene coding for PVA, is assigned to the same chromosome.

Transforming growth factor-beta stimulates the expression of desmosomal proteins in bronchial epithelial cells

Transforming growth factor-beta 1 (TGF-beta 1) has been shown to induce squamous differentiation of cultured airway epithelial cells. It has also been shown to increase expression of matrix proteins and integrin receptors in cell culture of these and other cells. However, it is unknown if TGF-beta 1 affects expression of genes encoding intercellular junctional proteins. Therefore, we have investigated the effect of TGF-beta 1 on the expression of proteins and mRNAs for desmoplakins (DPs) I and II, desmosomal plaque proteins. Fibronectin, known to be induced by TGF-beta 1 was used as a positive control and tubulin as a negative control. Twenty-four hours after TGF-beta 1 stimulation, DP I and II mRNA levels assessed by Northern blotting analysis had increased significantly (DP I mRNA, 1.8-fold, P less than 0.05; DP II mRNA, 2.4-fold, P less than 0.04), thereby indicating pretranslational regulation of DP expression. By comparison, mRNA for fibronectin increased 8.1-fold whereas mRNA for tubulin was unchanged. Immunofluorescence using the monoclonal anti-DP I and II antibodies revealed dramatic increased expression of punctate DP structures after exposure to TGF-beta 1. Immunoblot analyses with polyclonal anti-DP I antibodies showed increased levels of both DP I (250 kD) and DP II (215 kD), with the DP I increase being more pronounced (DP I, 2.5-fold; DP II, 1.4-fold at 48 h relative to controls), suggesting translational regulation by TGF-beta 1. This study therefore demonstrates the ability of TGF-beta 1 to alter cellular phenotype by altering expression of proteins involved in intercellular junctions.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular structure of the human desmoplakin I and II amino terminus

Desmoplakins (DPs) I and II are closely related proteins found in the innermost region of the desmosomal plaque, which serves as a cell surface attachment site for cytoplasmic intermediate filaments. Overlapping cDNA clones comprising 9.2 kilobases of DP-I, predicted to encode a full-length 310-kDa polypeptide (2677 amino acid residues), have now been identified. Here we report the predicted protein sequence and structural analysis of the N terminus of DP, extending our previous study of the rod and carboxyl domains. The N terminus contains groups of heptad repeats that are predicted to form at least two major alpha-helical-rich bundles. Unlike the rod and carboxyl domains, the N terminus did not display a periodic distribution of charged residues. Northern blot mapping and genomic sequence analysis were also undertaken to examine the organization of the DP mRNAs. A 1-kilobase intron was located at the 3' boundary of a DP-I-specific region; however, instead of an intron at the 5' junction, a possible splice donor site was observed within a potential coding sequence, suggesting alternative RNA splicing from an internal donor site.

Cloning and sequence analysis of desmosomal glycoproteins 2 and 3 (desmocollins): cadherin-like desmosomal adhesion molecules with heterogeneous cytoplasmic domains

Desmosomal glycoproteins 2 and 3 (dg2 and 3) or desmocollins have been implicated in desmosome adhesion. We have obtained a 5.0-kb-long clone for dg3 from a bovine nasal epidermal lambda gt11 cDNA library. Sequence analysis of this clone reveals an open reading frame of 2,517 bases encoding a polypeptide of 839 amino acids. The sequence consists of a signal peptide of 28 amino acids, a precursor sequence of 104 amino acids, and a mature protein of 707 amino acids. The latter has the characteristics of a transmembrane glycoprotein with an extracellular domain of 550 amino acids and a cytoplasmic domain of 122 amino acids. The sequence of a partial clone from the same library shows that dg2 has an alternative COOH terminus that is extended by 54 amino acids. Genomic DNA sequence data show that this arises by splicing out of a 46-bp exon that encodes the COOH-terminal 11 amino acids of dg3 and contains an in-frame stop codon. The extracellular domain of dg3 shows 39.4% protein sequence identity with bovine N-cadherin and 28.4% identity with the other major desmosomal glycoprotein, dg1, or desmoglein. The cytoplasmic domain of dg3 and the partial cytoplasmic domain of dg2 show 23 and 24% identity with bovine N-cadherin, respectively. The results support our previous model for the transmembrane organization of dg2 and 3 (Parrish, E.P., J.E. Marston, D.L. Mattey, H.R. Measures, R. Venning, and D.R. Garrod. 1990. J. Cell Sci. 96:239-248; Holton, J.L., T.P. Kenny, P.K. Legan, J.E. Collins, J.N. Keen, R. Sharma, and D.R. Garrod. 1990. J. Cell Sci. 97:239-246). They suggest that these glycoproteins are specialized for calcium-dependent adhesion in their extracellular domains and, cytoplasmically, for the molecular interactions involved in desmosome plaque formation. Moreover this represents the first example of alternative splicing within the cadherin family of cell adhesion molecules.

Isolation of a partial clone of desmoplakin-1 by antibody screening of a lambda gt11 library

Desmoplakin-1 is a 280,000 dalton component of the intracellular portion of desmosomes. Complementary DNA (cDNA) coding for a portion of desmoplakin-1 was obtained by screening a lambda gt11 cDNA library constructed specifically for this purpose. A388 cells (a human skin squamous cell carcinoma cell line) were found to produce desmoplakin-1 and mRNA from this cell line was used to prepare a random primed cDNA library. Screening 60,000 recombinants of the library with an affinity purified anti-demoplakin-1 antibody resulted in initial identification of four clones from which one 1500 base pair clone was isolated by plaque purification. The identity of this desmoplakin clone was confirmed by demonstrating that antibody affinity purified on the protein product of the clone bound desmoplakins 1 and 2 in Western blots. Thus, the clone appears to code for the shared region of desmoplakins 1 and 2.

A new high molecular mass protein showing unique localization in desmosomal plaque

A high molecular mass protein of 680 kD was identified and purified from the isolated desmosomes in bovine muzzle epidermal cells. This protein, called "desmoyokin" (from the English, yoke) here, showed no binding ability with keratin filaments in vitro, and its molecule had a characteristic dumbell shape approximately 170 nm in length. We have succeeded in obtaining one monoclonal antibody specific to desmoyokin. By the use of this monoclonal antibody and antidesmoplakin monoclonal antibody, desmoyokin was shown to be colocalized with desmoplakin at the immunofluorescence microscopic level; desmoyokin occurred only in the stratified epithelium, not in the simple epithelium nor in the other tissues. At the electron microscopic level, these two proteins were clearly seen to be sorted out in the plaque of desmosomes with desmoyokin at the periphery and desmoplakin at the center of the disk-shaped desmosomal plaque, suggesting that these two plaque proteins play distinct roles in forming and maintaining the desmosomes in stratified epithelium.

Molecular cloning and amino acid sequence of human plakoglobin, the common junctional plaque protein

Plakoglobin is a major cytoplasmic protein that occurs in a soluble and a membrane-associated form and is the only known constituent common to the submembranous plaques of both kinds of adhering junctions, the desmosomes and the intermediate junctions. Using a partial cDNA clone for bovine plakoglobin, we isolated cDNAs encoding human plakoglobin, determined its nucleotide sequence, and deduced the complete amino acid sequence. The polypeptide encoded by the cDNA was synthesized by in vitro transcription and translation and identified by its comigration with authentic plakoglobin in two-dimensional gel electrophoresis. The identity was further confirmed by comparison of the deduced sequence with the directly determined amino acid sequence of two fragments from bovine plakoglobin. Analysis of the plakoglobin sequence showed the protein (744 amino acids; 81,750 Da) to be unrelated to any other known proteins, highly conserved between human and bovine tissues, and characterized by numerous changes between hydrophilic and hydrophobic sections. Only one kind of plakoglobin mRNA (3.4 kilobases) was found in most tissues, but an additional mRNA (3.7 kilobases) was detected in certain human tumor cell lines. This longer mRNA may be represented by a second type of plakoglobin cDNA, which contains an insertion of 297 nucleotides in the 3' non-coding region.