Structure and assembly of desmosome junctions: biosynthesis, processing, and transport of the major protein and glycoprotein components in cultured epithelial cells

Inhibition of N-glycosylation by tunicamycin attenuates cell-cell adhesion via impaired desmosome formation in normal human epidermal keratinocytes

N-Glycosylation affects protein functions such as location, stability, and susceptibility to proteases. Desmosomes in keratinocytes are essential to maintain epidermal tissue integrity to protect against environmental insults. However, it is not yet known whether N-glycosylation affects desmosomal functions in primary keratinocytes. Tunicamycin is an inhibitor of N-glycosylation that has been a useful tool in glycobiology. Therefore, we investigated the effect of inhibiting N-glycosylation by tunicamycin treatment on desmosomes in primary keratinocytes. In our experiments, cell–cell adhesive strength was reduced in tunicamycin-treated primary keratinocytes. TEM showed that desmosome formation was impaired by tunicamycin. Desmogleins (Dsgs) 1 and 3, which constitute the core structure of desmosomes, were well transported to the cell–cell borders, but the amount decreased and showed an aberrant distribution at the cell borders in tunicamycin-treated keratinocytes. The stability of both desmoglein proteins was also reduced, and they were degraded through both proteasomal and lysosomal pathways, although inhibiting degradation did not restore the cell–cell adhesion. Finally, tunicamycin induced desmosomal instability, enhancing their disassembly. In conclusion, these results indicate that N-glycosylation is critical to the desmosome complex to maintain cell–cell adhesive strength in primary keratinocytes.

ER-to-Golgi blockade of nascent desmosomal cadherins in SERCA2-inhibited keratinocytes: Implications for Darier's disease

Darier's disease (DD) is an autosomal dominantly inherited skin disorder caused by mutations in sarco/endoplasmic reticulum Ca²⁺ -ATPase 2 (SERCA2), a Ca²⁺ pump that transports Ca²⁺ from the cytosol to the endoplasmic reticulum (ER). Loss of desmosomes and keratinocyte cohesion is a characteristic feature of DD. Desmosomal cadherins (DC) are Ca²⁺ -dependent transmembrane adhesion proteins of desmosomes, which are mislocalized in the lesional but not perilesional skin of DD. We show here that inhibition of SERCA2 by 2 distinct inhibitors results in accumulation of DC precursors in keratinocytes, indicating ER-to-Golgi transport of nascent DC is blocked. Partial loss of SERCA2 by siRNA has no such effect, implicating that haploinsufficiency is not sufficient to affect nascent DC maturation. However, a synergistic effect is revealed between SERCA2 siRNA and an ineffective dose of SERCA2 inhibitor, and between an agonist of the ER Ca²⁺ release channel and SERCA2 inhibitor. These results suggest that reduction of ER Ca²⁺ below a critical level causes ER retention of nascent DC. Moreover, colocalization of DC with ER calnexin is detected in SERCA2-inhibited keratinocytes and DD epidermis. Collectively, our data demonstrate that loss of SERCA2 impairs ER-to-Golgi transport of nascent DC, which may contribute to DD pathogenesis.

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.

Desmosomes in vivo

The structure, function, and regulation of desmosomal adhesion in vivo are discussed. Most desmosomes in tissues exhibit calcium-independent adhesion, which is strongly adhesive or “hyperadhesive”. This is fundamental to tissue strength. Almost all studies in culture are done on weakly adhesive, calcium-dependent desmosomes, although hyperadhesion can be readily obtained in confluent cell culture. Calcium dependence is a default condition in vivo, found in wounds and embryonic development. Hyperadhesion appears to be associated with an ordered arrangement of the extracellular domains of the desmosomal cadherins, which gives rise to the intercellular midline identified in ultrastructural studies. This in turn probably depends on molecular order in the desmosomal plaque. Protein kinase C downregulates hyperadhesion and there is preliminary evidence that it may also be regulated by tyrosine kinases. Downregulation of desmosomes in vivo may occur by internalisation of whole desmosomes rather than disassembly. Hyperadhesion has implications for diseases such as pemphigus.

Serum from pemphigus vulgaris reduces desmoglein 3 half-life and perturbs its de novo assembly to desmosomal sites in cultured keratinocytes

Defects of cell-cell adhesion underlie disruption of epithelial integrity observed in patients with pemphigus vulgaris (PV), an autoimmune disease characterized by severe mucosal erosions and skin blisters. Pathogenic PV autoantibodies found in patients' sera target desmoglein 3 (Dsg3), a major component of the desmosome, but how does this phenomenon affect Dsg-dependent adhesion and lead to acantholysis still remains controversial. Here, we show that PV serum determines a reduction of Dsg3 half-life in HaCaT keratinocytes, although the total amount of Dsg3 remains unchanged. Immunofluorescence studies suggest that PV IgG exert their effect prevalently by binding non-desmosomal Dsg3 without causing its massive internalization. Furthermore, PV IgG targeting desmosome-assembled Dsg3 do not induce depletion of Dsg3 from the adhesion sites. Conversely, incorporation of PV IgG-Dsg3 complexes into new forming desmosomes appears perturbed. With our study, the basic biochemical changes of Dsg3 in an in vitro model of PV have been defined.

High Throughput Quantitative Glycomics and Glycoform-focused Proteomics of Murine Dermis and Epidermis

Despite recent advances in our understanding of the significance of the protein glycosylation, the throughput of protein glycosylation analysis is still too low to be applied to the exhaustive glycoproteomic analysis. Aiming to elucidate the N-glycosylation of murine epidermis and dermis glycoproteins, here we used a novel approach for focused proteomics. A gross N-glycan profiling (glycomics) of epidermis and dermis was first elucidated both qualitatively and quantitatively upon N-glycan derivatization with novel, stable isotope-coded derivatization reagents followed by MALDI-TOF(/TOF) analysis. This analysis revealed distinct features of the N-glycosylation profile of epidermis and dermis for the first time. A high abundance of high mannose type oligosaccharides was found to be characteristic of murine epidermis glycoproteins. Based on this observation, we performed high mannose type glycoform-focused proteomics by direct tryptic digestion of protein mixtures and affinity enrichment. We identified 15 glycoproteins with 19 N-glycosylation sites that carry high mannose type glycans by off-line LC-MALDI-TOF/TOF mass spectrometry. Moreover the relative quantity of microheterogeneity of different glycoforms present at each N-glycan binding site was determined. Glycoproteins identified were often contained in lysosomes (e.g. cathepsin L and gamma-glutamyl hydrolase), lamellar granules (e.g. glucosylceramidase and cathepsin D), and desmosomes (e.g. desmocollin 1, desmocollin 3, and desmoglein). Lamellar granules are organelles found in the terminally differentiating cells of keratinizing epithelia, and desmosomes are intercellular junctions in vertebrate epithelial cells, thus indicating that N-glycosylation of tissue-specific glycoproteins may contribute to increase the relative proportion of high mannose glycans. The striking roles of lysosomal enzymes in epidermis during lipid remodeling and desquamation may also reflect the observed high abundance of high mannose glycans.

Impaired trafficking of the desmoplakins in cultured Darier's disease keratinocytes

Darier's disease is an autosomal dominantly inherited skin disorder characterized by loss of adhesion between epidermal cells, breakdown of desmosome-keratin filaments, and abnormal keratinization. ATP2A2 has been identified as the causative gene of Darier's disease. This gene encodes the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) isoform 2 pump, which transports Ca2+ from the cytosol into the endoplasmic reticulum lumen to maintain a low cytosolic Ca2+ concentration. Using indirect immunofluorescence and biochemical analysis, we investigated the distribution of key desmosomal proteins in normal human and Darier's disease keratinocytes under various calcium conditions. We show that inhibition of SERCA by thapsigargin in normal human keratinocytes impairs the trafficking of the desmoplakins, desmoglein, and desmocollin to the cell surface; these proteins show a diffuse cytoplasmic distribution and, together with plakoglobin, form detergent-insoluble aggregates. In Darier's disease keratinocytes, only the trafficking of desmoplakin is significantly inhibited; in these cells, desmoplakin forms insoluble aggregates when extracted with mild detergent. In contrast, the transmembrane proteins desmoglein and desmocollin are efficiently transported to the cell surface. These proteins, along with plakoglobin, remain equally distributed between detergent-soluble and -insoluble fractions. We also demonstrate an interaction between SERCA2 and desmoplakin during differentiation. Our results provide further insights into the critical role of calcium ATPases in maintaining epidermal integrity.

The Autoantigen of Anti-p200 Pemphigoid Is an Acidic Noncollagenous N-Linked Glycoprotein of the Cutaneous Basement Membrane

Anti-p200 pemphigoid is an autoimmune subepidermal blistering disease characterized by autoantibodies to a 200-kDa protein (p200) of the dermal-epidermal junction (DEJ). p200 has been demonstrated to be distinct from all major DEJ autoantigens and is thought to be important for cell-matrix adhesion. This study provides the first biochemical characterization of p200. Differential extraction experiments demonstrated that efficient recovery of p200 from the dermis was strongly dependent on the presence of reducing agents, suggesting that it forms highly insoluble oligomers and/or is extensively cross-linked to other extracellular matrix components by disulfide bonding. p200 was resistant to digestion with bacterial collagenase, whereas this treatment did degrade major collagenous proteins of the dermis, including type I, VI, and VII collagen. This finding firmly established the noncollagenous nature of p200. N-Glycosidase F reduced the molecular size of the p200 autoantigen from 200 to 190 kDa without decreasing its immunoreactivity. In contrast, digestion of p200 with neuraminidase, O-glycosidase, chondroitinase ABC, and heparitinase I had no effect on its electrophoretic mobility. These data suggest that the p200 molecule contains N-glycans but lacks O-linked oligosaccharides and chondroitin/heparan sulfate side chains. Two-dimensional gel electrophoresis demonstrated that p200 is an acidic protein with an isoelectric point of 5.4 to 5.6. Six different p200-specific sera recognized an identical protein spot of two-dimensionally separated dermal extracts, confirming that patients with this novel autoimmune disease indeed form a single pathobiochemical entity.

Desmosome assembly in MDCK cells: transport of precursors to the cell surface occurs by two phases of vesicular traffic and involves major changes in centrosome and Golgi location during a Ca(2+) shift

Desmosome formation in MDCK cells was investigated using a Ca(2+) shift. Following preliminary treatment with cycloheximide at 37 degrees C, continued surface transport and subsequent endocytosis were minimized by incubating cells at 19 degrees C to trap nascent glycoproteins within the Golgi body. Release into high Ca(2+) medium (HCM) at 37 degrees C resulted in junction formation as well as relocation of the Golgi body and centrosomes to a subapical location. Desmosome formation occurred in two stages over 2 h, the first occurring within 30 min of the shift to HCM, in 60-nm vesicles containing chiefly Dsc2 and lower concentrations of Dsg and E-cadherin distributed to the entire cell surface. Much of this material was subsequently endocytosed. The second stage involved transport of Dsg, E-cadherin, plakoglobin, and beta-catenin, in more complex vesicles some 200 nm in size, directed to possible nucleation sites on the developing basolateral surface. Plaque proteins such as desmoplakin I/II were added subsequently. Stage-two vesicles, but possibly not those of stage one, were accessible to endocytic markers via retrograde transport from multivesicular bodies prelabeled at 19 degrees C.

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.

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.

Activation of both MAP kinase and phosphatidylinositide 3-kinase by Ras is required for hepatocyte growth factor/scatter factor-induced adherens junction disassembly

Hepatocyte growth factor/scatter factor (HGF/SF) stimulates the motility of epithelial cells, initially inducing centrifugal spreading of colonies followed by disruption of cell-cell junctions and subsequent cell scattering. In Madin-Darby canine kidney cells, HGF/SF-induced motility involves actin reorganization mediated by Ras, but whether Ras and downstream signals regulate the breakdown of intercellular adhesions has not been established. Both HGF/SF and V12Ras induced the loss of the adherens junction proteins E-cadherin and beta-catenin from intercellular junctions during cell spreading, and the HGF/SF response was blocked by dominant-negative N17Ras. Desmosomes and tight junctions were regulated separately from adherens junctions, because they were not disrupted by V12Ras. MAP kinase, phosphatidylinositide 3-kinase (PI 3-kinase), and Rac were required downstream of Ras, because loss of adherens junctions was blocked by the inhibitors PD098059 and LY294002 or by dominant-inhibitory mutants of MAP kinase kinase 1 or Rac1. All of these inhibitors also prevented HGF/SF-induced cell scattering. Interestingly, activated Raf or the activated p110alpha subunit of PI 3-kinase alone did not induce disruption of adherens junctions. These results indicate that activation of both MAP kinase and PI 3-kinase by Ras is required for adherens junction disassembly and that this is essential for the motile response to HGF/SF.

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.

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

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

Vascular smooth muscle cells of H-2Kb-tsA58 transgenic mice. Characterization of cell lines with distinct properties

BACKGROUND

The vascular wall is composed of at least two different populations of smooth muscle cells that are distinct in their structure and protein composition. According to the developmental stage of tissue taken for culture, the ratio between cells of epithelioid phenotype and spindle-shaped cells is variable. In particular, the epithelioid cells display characteristic features associated with immaturity. Because their increased appearance can be observed in endothelial denudation, the represent a dedifferentiated, proliferative smooth muscle cell type with a repair function in vascular injury.

METHODS AND RESULTS

To investigate this cellular heterogeneity, we established vascular smooth muscle cell lines from H-2Kb-tsA58 transgenic mice. Due to temperature-sensitive expression of the SV 40 large T-antigen in cells derived from this mouse strain, our smooth muscle lines were conditionally immortalized from the onset of their life in culture. Thus, we were able to clone cell lines representing the two different phenotypes described so far. Epithelioid cells derived from newborn animals are characterized by their expression of cytokeratins and the development of tight junctional complexes. Spindle-shaped cells, which could be isolated from newborn or adult animals, corresponded in phenotype and protein expression to smooth muscle cell lines established previously.

CONCLUSIONS

The special properties of vascular smooth muscle cells of the epithelioid phenotype suggest an endothelial replacement function in the course of injury to the vascular wall.

Conformational epitopes of pemphigus antigens (Dsg1 and Dsg3) are calcium dependent and glycosylation independent

The target molecule of pemphigus autoantibodies is a transmembrane desmosomal component, desmoglein 3 (Dsg3) in pemphigus vulgaris (PV) and Dsg1 in pemphigus foliaceus (PF). In this study, we examined the effects of calcium and glycosylation on the anti-genicity of the pemphigus antigens and on the generation of conformational epitopes. We used recombinant baculovirus proteins, PVIg and PFIg, which are considered to reflect accurately the native conformation of the extracellular domain of their respective proteins Dsg3 and Dsg1. These baculoproteins could immunoadsorb heterogeneous autoantibodies from the corresponding sera of PV and PF patients, completely blocking indirect immunofluorescence staining of normal human skin. Chelating calcium from the solution containing the baculoproteins using ethylenediaminetetraacetic acid (EDTA) or ethyleneglycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) abolished immunoadsorption by both PVIg and PFIg; however, immunoadsorption by the baculoproteins was restored after dialysis against 1 mM calcium. Nonglycosylated forms of both baculoproteins produced in the presence of tunicamycin retained their immunoadsorptive ability. Furthermore, immunoadsorption by the baculo-proteins was prevented irreversibly by treatment with low pH, high pH, and boiling, but not with the non-ionic detergent Nonidet P-40. These findings indicate that formation of the conformational epitopes on the pemphigus antigens is dependent on calcium but independent of glycosylation, and provide direct evidence that calcium plays an important role in determining the antigenic properties of the pemphigus antigens.

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.

Internalisation of desmosomes and their entry into the endocytic pathway via late endosomes in MDCK cells. Possible mechanisms for the modulation of cell adhesion by desmosomes during development

MDCK cells grown in media with normal levels of Ca2+ (approximately 2 mM) contain internalised desmosomes, referred to as desmosome-associated vacuoles (DAVs). The DAVs consist of one to three plaques retained in the plane of a surrounding vacuolar membrane, and their entry into the endocytic pathway has been investigated using HRP, cationized ferritin and BSA/gold in combination with electron microscopy and immunogold labelling of frozen sections. Endocytic tracers supplied from the apical and basolateral surfaces to filter-grown MDCK cells met in a common perinuclear compartment but DAVs were not labelled during short (5-30 minutes) pulses of marker, whether applied apically or basolaterally. Only when the tracers were taken up from the basolateral surface and then chased for periods of 2–18 hours, were DAVs labelled. It is proposed that entry of an endocytic tracer to DAVs occurs by the association of the desmosomal vacuole with late endosomes. Immunolabelling studies with antibodies to desmosomal components (to Dsg, DPI/II), to HRP and to the cation-independent mannose 6-phosphate receptor (MPR), confirmed that Dsg and DPI/II are located within DAVs and late endosomes, but not in early endosomes. Passage of Dsg, but to a lesser extent DPI/II, was detected in MPR- structures (lysosomes). DAV-like structures have also been observed in developing tissues such as mouse kidney. Such engulfment may provide a general mechanism for handling insoluble junctional proteins, particularly where rapid morphogenetic changes are occurring in the pattern of cell-cell adhesion.

The specificity of integrin-ligand interactions in cultured human renal epithelium

Members of the beta 1 integrin family are present at the basolateral membrane of human renal tubular epithelium in vivo and at the ventral surfaces of cultured renal epithelial cells, at the sites appropriate for cell substratum adhesion. In this study we have proven that these molecules are indeed functional in mediating cell substratum attachment in normal human renal epithelium by using monoclonal antibodies to integrin alpha subunits to block initial cell attachment. The importance of arginine-glycine-aspartic acid (RGD) recognition by cell surface receptors in various extracellular ligands was also examined using synthetic peptides. RGDS peptide strongly inhibited attachment to plain plastic or fibronectin-coated substrata but had no effect on cell adhesion to laminin-coated coverslips.

Expression of distinct desmocollin isoforms in human epidermis

Previous evidence suggested the presence of two distinct desmocollin isoforms in human epidermis. These isoforms have now been distinguished at the protein level using monoclonal and polyclonal antibodies against N-terminal fragments of desmosomal glycoprotein (DG) IV/V isolated from plantar callus and antibodies against a fusion protein containing the extracellular domain of DGII/III. Immune blotting of glycoprotein fractions from whole epidermis, plantar callus, psoriatic scales and cultured keratinocytes showed that intact DGIV/V and its proteolytic fragments consistently migrated faster than DGII/III during SDS-PAGE. The apparent Mr difference between the two isoforms was in the range 2-5 kD. DGIV/V was the predominant species in epidermal tissue but was much less prominent in cultured cells by immune-blotting and immune precipitation. This is consistent with the differentiation-related expression of desmocollins revealed by immunofluorescence. DGIV/V was strongly expressed in the upper spinous/granular layer of the epidermis whereas DGII/III was more prominent in the basal layers of the tissue. The DGIV/V monoclonal (LH50) recognized an N-terminal, Ca(++)-sensitive epitope, because its staining of unfixed epidermal tissue was markedly influenced by Ca++ levels. Ca++ inhibition was observed at concentrations as low as 50 microM, suggesting its possible physiologic significance. Ca++ inhibition of LH50 binding was also observed in an enzyme-linked immunosorbent assay system using denatured glycoproteins although higher concentrations were required. It remains to be seen whether direct effects of Ca++ on desmocollin conformation are involved in the regulation of keratinization by extracellular Ca++.

Stratification-related expression of isoforms of the desmosomal cadherins in human epidermis

Desmosomal junctions are abundant in epidermis and contain two classes of transmembrane glycoprotein, the desmocollins and the desmogleins, which are members of the cadherin superfamily of Ca(2+)-dependent cell adhesion molecules. The desmocollin subfamily includes DGIV/V and DGII/III while the desmoglein subfamily includes DGI, HDGC and the autoantigen of the blistering skin disease pemphigus vulgaris (PVA). There are also several non-glycosylated proteins, including the desmoplakins and plakoglobin, present in the desmosomal plaque, which forms a link between the glycoproteins and the cytokeratin intermediate filaments. To provide a picture of the expression of the desmosomal genes and their products in epidermis, we have used in situ hybridisation and immunofluorescence staining on sections of human foreskin. We find that, as expected, desmoplakin DPI/II and plakoglobin are expressed throughout the epidermis, gradually accumulating during differentiation, which probably reflects the increased numbers of desmosomes. In contrast, while keratin 14 and the hemidesmosomal component bullous pemphigoid antigen I (BPAGI) are basal-specific, desmocollin DGIV/V is expressed only in the upper spinous/granular layers of the epidermis, whereas DGII/III expression is enriched in the basal layers. Amongst the desmogleins, expression of DGI appears similar to desmoplakin and plakoglobin; PVA is more prevalent in the lower spinous layers, whereas HDGC expression is detected basally but not suprabasally. The major desmosomal cadherin transcripts are desmocollin DGIV/V and desmoglein DGI. The resultant changes in desmosomal composition and structure may reflect the maturation of desmosomes, presumably being related to the need for changes in cell adhesion during stratification, terminal differentiation, and desquamation, and point to the desmosome being a key player in epidermal differentiation.

Contributions of cytoplasmic domains of desmosomal cadherins to desmosome assembly and intermediate filament anchorage

To examine the potential of cytoplasmic portions ("tails") of desmosomal cadherins for assembly of desmosome plaque structures and anchorage of intermediate filaments (IFs), we transfected cultured human A-431 carcinoma cells, abundant in desmosomes and cytokeratin IFs, with constructs encoding chimeric proteins in which the transmembranous region of connexin 32 had been fused with tails of desmocollin (Dsc) or desmoglein (Dsg). The results show that the tail of the long splice form a of Dsc, but not its shorter splice form b, contains sufficient information to recruit desmoplakin and plakoglobin to connexon membrane paracrystals (gap junctions) and to form a novel kind of plaque at which cytokeratin IFs attach. By contrast, chimeras containing a Dsg tail, which accumulated in the plasma membrane, showed a dominant-negative effect: they not only were unable to form gap junction structures and plaques but also led to the disappearance of all endogenous desmosomes and the detachment of IFs from the plasma membrane.

Disorganization of microfilaments and intermediate filaments interferes with the assembly and stability of desmosomes in MDCK epithelial cells

To investigate the possible role(s) of cytoskeletal elements in desmosome assembly we have studied the effects of cytostatic drugs on the assembly of desmosomes in MDCK epithelial cells. We showed previously [Pasdar et al.: Cell Motil. Cytoskeleton 23:201-213, 1992] that selective disruption of microtubules has no effect on desmosome assembly. Here, we have treated MDCK cells with cytochalasin B and a combination of cytochalasin B and nocodazole and analysed the effects of desmosome assembly. Immunofluorescence analysis of MDCK cultures following drug treatment indicated complete disruption of actin microfilaments and disorganization of cytokeratin intermediate filaments. Biochemical analysis of newly synthesized desmosomal membrane core glycoproteins as well as the cell adhesion protein E-cadherin revealed no effect of these drugs on the kinetics of synthesis, intracellular processing, or transport to the plasma membrane either in the presence or absence of cell-cell contact. However, morphological analyses revealed a significant disruption in the spatial organization of desmosomal proteins and E-cadherin. Drug treatment in the absence of cell-cell contact resulted in the disruption of the normally observed homogeneous punctate staining pattern and appearance of aggregate staining. Induction of cell-cell contact in these cultures resulted in redistribution of some of the aggregate staining to the plasma membrane. In contrast to control cultures, significant amount of intracellular staining was retained for all desmosomal proteins. Biochemical analyses of turnover rates of newly synthesized desmosomal proteins indicated a significant decrease in metabolic stability of these proteins while the turnover rate of E-cadherin was not significantly different among control and drug-treated cultures. Taken together, these results suggest that intact actin and cytokeratin filaments are necessary for the stability, efficient assembly, and spatial organization of the junctional components at the membrane. The regulatory role of cytokeratins and actin filaments in assembly and stability of desmosomes on the plasma membrane is discussed.

Structure and interactions of desmosomal and other cadherins

The cadherin superfamily of cell-cell adhesion molecules is now known to include proteins of the desmosome as well as of the adherens type of junction. The desmosomal cadherins consist of two families of proteins, the desmocollins and the desmogleins, both of which are represented by different isoforms which are differentially expressed in epidermis. The desmocollins are quite similar to the classic cadherins in overall structure, but with alternatively spliced variants; the desmogleins have extra cytoplasmic sequences added onto the basic cadherin structure. The cytoplasmic domains are specialized for binding to 'mediator' proteins, such as plakoglobin, which interconnect to the intermediate filament system rather than the actin filaments as do the classic cadherins.

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)

Desmosome assembly in MDCK epithelial cells does not require the presence of functional microtubules

Desmosomes, complex multisubunit structures that assemble at sites of cell-cell contact, are important components of the epithelial junctional complex. Desmosome assembly requires the coordinated interaction at the plasma membrane of at least 8 cytoplasmic and integral membrane proteins organized into two structurally and functionally distinct domains, the cytoplasmic plaque and membrane core. Previous studies (Pasdar et al., J. Cell Biol., 113:645-655) provided evidence that cytokeratin filaments and microtubules may regulate transfer and assembly of cytoplasmic plaque and membrane core proteins, respectively. To determine directly the role of microtubules in these processes, Madin-Darby canine kidney (MDCK) cells were treated with nocodazole or colchicine to disrupt the microtubular network. Biochemical analysis of the different components of the cytoplasmic plaque and membrane core domains revealed little or no effect of nocodazole or colchicine on the kinetics of synthesis, post-translational modifications, transfer of proteins to the plasma membrane or their metabolic stability in the presence or absence of cell-cell contact. Likewise, immunofluorescence analysis of desmosome formation demonstrated an apparently normal desmosome assembly in the presence of nocodazole or colchicine upon induction of cell-cell contact. These results indicate that an intact microtubular network is not necessary for the processing or transport of the desmosomal membrane core glycoproteins to the plasma membrane in the absence or presence of cell-cell contact. Furthermore, the integration of the cytoplasmic plaque and membrane core domains induced by cell-cell contact at the plasma membranes of adjacent cells does not require the presence of functional microtubules.

Transmembrane molecular assemblies regulated by the greater cadherin family

The cadherin family of cell-cell adhesion molecules is turning out to be much more diverse than previously thought, with members involved in several kinds of intercellular junctions. The adhesive specificity and cytoskeletal interaction of these members varies. Their cytoplasmic terminals are specialized for binding several families of 'mediator' proteins which interconnect to the actin or intermediate filament systems. These multi-molecular complexes have roles not only in cell-cell adhesion, but also in intracellular signalling of developmental information.

Desmosomal glycoprotein DGI, a component of intercellular desmosome junctions, is related to the cadherin family of cell adhesion molecules

Among the variety of specialized intercellular junctions, those of the adherens type have the most obvious association with cytoskeletal elements. This may be with the actin microfilament system as in the zonula adherens or with intermediate filaments as in the macula adherens, or desmosome. In the former case, it is clear that transmembrane glycoproteins of the cadherin family are important adhesive components of the molecular assembly. We now show for desmosomes that a major glycoprotein component (desmosomal glycoprotein DGI) has extensive homology with the cadherins, defining an extended family, but also has unique features in its cytoplasmic domain that are likely to be relevant to the association with intermediate rather than actin filaments. A novel 282-residue extension contains repeats of approximately 29 amino acid residues predicted to have an antiparallel beta-sheet structure, followed by a glycine-rich sequence. As in the cadherins, the extracellular domain contains possible Ca2(+)-binding sequences and a potential protease processing site. The cell adhesion recognition region (His-Ala-Val) of the cadherins is modified to Arg-Ala-Leu.

Sugars protect desmosome and corneosome glycoproteins from proteolysis

Adhesional glycoproteins of desmosomes possess asparagine-linked, complex oligosaccharide side chains. We investigated the potential of these sugars to protect the core proteins of desmosomes and corneosomes (modified stratum corneum desmosomes) against proteolysis. Isolated pig ear epidermis was exposed sequentially to individual hydrolases, and their effect monitored ultrastructurally. Two major steps were employed: (1) glycosidases, to remove stepwise the sugars in a typical complex oligosaccharide chain; and (2) proteolysis using both endopeptidases and an exopeptidase. Controls were exposed to the same sequence of buffers, but without enzymes. Proteases alone induced no major changes in desmosomes or corneosomes compared with controls. Glycosidases alone, or proteases followed by glycosidases, caused mild fragmentation of the desmosomal interspace, but no widening. However, dramatic changes occurred when glycosidase treatment was followed by proteolysis. The interspace of both desmosomes and corneosomes was extensively digested, and consequently widened, causing loose packing of the epidermis. These findings indicate that sugars are potentially anti-proteolytic in both desmosomes and corneosomes. Sugars may, therefore, be a factor in preventing premature desquamation, by protecting desmosomes and corneosomes against extracellular proteases derived from membrane-coating granules.

Amino acid sequence of bovine muzzle epithelial desmocollin derived from cloned cDNA: a novel subtype of desmosomal cadherins

Desmosomes are cell-type-specific intercellular junctions found in epithelium, myocardium and certain other tissues. They consist of assemblies of molecules involved in the adhesion of specific cell types and in the anchorage of cell-type-specific cytoskeletal elements, the intermediate-size filaments, to the plasma membrane. To explore the individual desmosomal components and their functions we have isolated DNA clones encoding the desmosomal glycoprotein, desmocollin, using antibodies and a cDNA expression library from bovine muzzle epithelium. The cDNA-deduced amino-acid sequence of desmocollin (presently we cannot decide to which of the two desmocollins, DC I or DC II, this clone relates) defines a polypeptide with a calculated molecular weight of 85,000, with a single candidate sequence of 24 amino acids sufficiently long for a transmembrane arrangement, and an extracellular aminoterminal portion of 561 amino acid residues, compared to a cytoplasmic part of only 176 amino acids. Amino acid sequence comparisons have revealed that desmocollin is highly homologous to members of the cadherin family of cell adhesion molecules, including the previously sequenced desmoglein, another desmosome-specific cadherin. Using riboprobes derived from cDNAs for Northern-blot analyses, we have identified an mRNA of approximately 6 kb in stratified epithelia such as muzzle epithelium and tongue mucosa but not in two epithelial cell culture lines containing desmosomes and desmoplakins. The difference may indicate drastic differences in mRNA concentration or the existence of cell-type-specific desmocollin subforms. The molecular topology of desmocollin(s) is discussed in relation to possible functions of the individual molecular domains.

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.

Desmoglein shows extensive homology to the cadherin family of cell adhesion molecules

Desmoglein is a major adhesive component of the desmosome. It is also at least one of the antigenic targets of pathogenic antibodies circulating in the sera of patients with the blistering disease Pemphigus foliaceus. To examine the molecular basis of desmosomal adhesion and to further our understanding of its disruption in various bullous disorders we have cloned cDNAs encoding four of the extracellular domains of desmoglein. The predicted amino acid sequence of these clones shows extensive homology with the cadherin class of calcium-dependent cell adhesion molecules. Desmoglein represents a novel subtype of this family.

The hemidesmosomal plaque. I. Characterization of a major constituent protein as a differentiation marker for certain forms of epithelia

To examine whether constituent proteins of hemidesmosomal structures can be used as markers for certain pathways of epithelial differentiation we have examined the occurrence of the major M- approximately 230,000 plaque protein, the "bullous pemphigoid" (BP) antigen. Several bovine, rat and human tissues and bovine cell culture lines were examined, using different human autoantibody preparations in immunocytochemistry and immunoblotting. We report that this protein, also unequivocally identified by cDNA cloning from expression libraries and DNA sequencing, occurs not only in different stratified epithelia but also, apparently always in hemidesmosomal structures, in urothelium of bladder and the complex epithelia of trachea, bronchus and several glands, notably myoepithelium-containing skin glands, the mammary gland and salivary glands. The protein is absent, however, in all single-layered epithelia and in several tissues reported to have subplasmalemmal densities structurally similar to hemidesmosomes, such as Purkinje fibers of heart, meninges and perineuria. A mammary-gland-derived epithelial cell line (BMGE + H) is particularly rich in hemidesmosomes. This has been used to study the endocytotic uptake of hemidesmosome-containing plasma membrane domains into cytoplasmic vesicles upon detachment of cell sheets during treatment with dispase, a proteolytic enzyme. We propose to use the Mr- approximately 230,000 plaque protein as a marker selective for certain subsets of epithelial cell types and epithelium-derived tumors in studies of fetal and tumor development, including differentiation diagnosis of carcinomas.

Pemphigus foliaceus antigen: characterization of a keratinocyte envelope associated pool and preparation of a soluble immunoreactive fragment

In both the endemic and sporadic forms of pemphigus foliaceus (PF), antiepidermal autoantibodies against desmoglein I are present. Desmoglein I is a highly insoluble 160-kD transmembrane glycoprotein of the desmosomal core. The detailed immunochemical characterization of the epitope(s) recognized by the PF autoantibodies is hampered by its large molecular weight and the insolubility of desmoglein I in nondenaturing buffers. This study was designed to identify alternative methods that could yield soluble immunoreactive PF antigen (Ag) from normal human epidermis. The presence of PF Ag in human epidermis and in its soluble or insoluble fractions was monitored by indirect immunofluorescence, immunoadsorption of PF sera, and immunoprecipitation of radiolabeled fractions. The PF Ag from trypsin-resistant, radiolabeled cell envelope preparations was cleaved by papain and immunoprecipitated by PF sera. A 50-kD peptide, isoelectric at pH 5.5-5.8, was immunoprecipitated by sera from all patients with endemic PF (n = 15) or idiopathic PF (n = 4), and by two of four pemphigus vulgaris sera, but by no control sera (n = 7). This study shows that a significant fraction of the PF Ag is insoluble, trypsin-resistant, and is associated with the cornified cell envelope fraction, but an Ag fragment can be obtained in a small molecular weight, soluble, and immunoreactive form by papain digestion. This 50-kD papain fragment is more amenable to detailed chemical and immunologic characterization than the native molecule.

Regulation of desmosome assembly in epithelial cells: kinetics of synthesis, transport, and stabilization of desmoglein I, a major protein of the membrane core domain

Desmosomes are composed of two morphologically and biochemically distinct domains, a cytoplasmic plaque and membrane core. We have initiated a study of the synthesis and assembly of these domains in Madin-Darby canine kidney (MDCK) epithelial cells to understand the mechanisms involved in the formation of desmosomes. Previously, we reported the kinetics of assembly of two components of the cytoplasmic plaque domain, Desmoplakin I/II (Pasdar, M., and W. J. Nelson. 1988. J. Cell Biol. 106:677-685 and 106:687-699. We have now extended this analysis to include a major glycoprotein component of the membrane core domain, Desmoglein I (DGI; Mr = 150,000). Using metabolic labeling and inhibitors of glycoprotein processing and intracellular transport, we show that DGI biosynthesis is a sequential process with defined stages. In the absence of cell-cell contact, DGI enters a Triton X-100 soluble pool and is core glycosylated. The soluble DGI is then transported to the Golgi complex where it is first complex glycosylated and then titrated into an insoluble pool. The insoluble pool of DGI is subsequently transported to the plasma membrane and is degraded rapidly (t1/2 less than 4 h). Although this biosynthetic pathway occurs independently of cell-cell contact, induction of cell-cell contact results in dramatic increases in the efficiency and rate of titration of DGI from the soluble to the insoluble pool, and its transport to the plasma membrane where DGI becomes metabolically stable (t1/2 greater than 24 h). Taken together with our previous study of DPI/II, we conclude that newly synthesized components of the cytoplasmic plaque and membrane core domains are processed and assembled with different kinetics indicating that, at least initially, each domain is assembled separately in the cell. However, upon induction of cell-cell contact there is a rapid titration of both components into an insoluble and metabolically stable pool at the plasma membrane that is concurrent with desmosome assembly.

The assembly of the major desmosome glycoproteins of Madin-Darby canine kidney cells

Madin-Darby canine kidney (MDCK) cells are unable to form desmosomes when cultured in low-calcium medium [( Ca2+] less than 0.1 meq./l), but can be induced to do so by raising the calcium to physiological concentrations (1-2 meq./l). We have previously demonstrated that this block correlated with increased desmosomal protein turnover. Here we have immunoprecipitated the major desmosome glycoproteins [DGI (150 kDa) and DGII/III (120/100 kDa)] from non-ionic detergent-soluble and -insoluble fractions prepared from metabolically labelled MDCK cells cultured in standard or low-calcium medium. Pulse-chase studies showed that both DGI and DGII/III became unextractable in non-ionic detergent before their arrival at the cell surface, whether cells were grown in standard or low-calcium medium. The non-ionic detergent insolubility of these membrane components is therefore a separate step which precedes the formation of morphologically recognisable desmosomes.

Desmoglein II-derived glycopeptides in human epidermis

Antisera raised against a major 78 kD glycopeptide from pig epidermis were used to identify desmoglein II-derived glycopeptides in the conA-binding material isolated from human epidermis. In whole CaCl2-separated epidermis the antiserum recognized conA-binding components with apparent Mr of 115, 100, 82, 68, 50, 48, and 46 kD. The 82, 68, 48, and 46 kD immunoreactive bands were present in normal stratum corneum and plantar callus. Psoriatic scales contained significantly more of the 82 kD components and less of the 48 and 46 kD bands. Psoriatic scales also contained a major 50 kD conA-binding component unrelated to keratins or desmoglein II. Proteolytic peptide mapping showed that the major immunoreactive bands in normal stratum corneum and plantar callus were also chemically related. The 82 to 46 kD immunoreactive glycopeptides in plantar callus coincided with the major coomassie blue stained bands and were homogeneous on two-dimensional gels suggesting that this tissue may be a valuable source of human desmoglein II-derived glycopeptides. An antiserum directed against the electrophoretically co-purified 48/46 kD glycopeptides from plantar callus recognized the 82 to 46 kD bands in immunoblotting. In indirect immunofluorescence of frozen skin sections this antiserum stained the surface of epidermal cells in the spinous and granular layers of the tissue. In immunogold labeling of paraformaldehyde-fixed skin sections affinity-purified antibodies stained intact desmosomes in spinous and granular cells and desmosomal remnants in the stratum corneum. The results are consistent with our hypothesis that desmoglein II undergoes limited cleavage to stable fragments during terminal differentiation. Proteolytic degradation appears to be incomplete in psoriatic epidermis.

Alignment of desmosomes in stratifying human epidermis

Cultured human epithelial cells stained with antibody to desmosomal proteins by indirect immunofluorescence showed linear arrays of desmosomes en face between stratified cells. To confirm that an extensive linear pattern existed on the cell surface, subconfluent cultures were viewed using scanning electron microscopy. Aligned arrays of blunt protrusions lying parallel to each other and extending in the direction of the long axis of the cell were observed on the surface of groups of superficial cells in intact cultures. That this pattern was indeed related to desmosomal distribution was verified by transmission microscopy of thin sections cut in a plane between the upper and lower surfaces of flattened stratified cells to view desmosomes directly. A similar arrangement of desmosomes was seen in intact tissue, using epidermal sheets separated from newborn foreskin. The same pattern found in flattened cells was sometimes apparent in more rounded basal cells where the cytoplasm was beginning to extend. Since desmosomal plaques are associated with keratin filaments, the alignment of desmosomes must occur in association with cytoskeletal changes as cells become flattened toward the distal epithelial surface. The primary initiation of desmosomal alignment remains to be investigated. However, the present findings demonstrate an increasingly regular membrane-cytoskeletal spatial interaction as stratified epithelial cells of skin mature.

Organization of desmosomal plaque proteins in cells growing at low calcium concentrations

Desmosomes are not formed in epithelial cell cultures growing in media with low (less than or equal to 0.1 mM) concentrations of Ca2+ (LCM) but appear rapidly upon shift to media of normal calcium concentrations (NCM). Previous authors using immunolocalization of desmoplakin, a marker protein for the desmosomal plaque, in LCM-grown cells have interpreted positively stained, dense, cytoplasmic aggregates on intermediate filaments (IF) bundles as preformed plaque units which upon NCM shift would move to the plasma membrane and contribute to desmosome formation. Studying various cell cultures, including primary mouse keratinocytes and human A-431 cells, we show that most, probably all, desmoplakin-positive aggregates in LCM-grown cells are associated with membranous structures, mostly vesicles, and also contain other desmosomal markers, including desmoglein, a transmembrane glycoprotein. We interpret such vesicles as residual desmosome-derived domains endocytosed upon cell dissociation. Only keratinocytes grown for long times (2-4 wk) in LCM are practically free from such vesicles. In addition, we demonstrate that certain cells such as A-431 cells, when passaged in LCM and in the absence of stable junctions, are able to continually assemble "half-desmosomes" on the plasma membrane which in turn can be endocytosed as plaque-bearing vesicles. We also show that in LCM the synthesis of several desmosomal proteins (desmoplakins I and II, plakoglobin, desmoglein, "band 6 protein") continues and that most of the plaque protein, desmoplakin, is diffusely spread over the cytoplasm, apparently in a soluble monodisperse form of approximately 9S. From our results we propose that the plaque proteins occur in small, discrete, diffusible entities in the cytoplasm, in concentrations that are relatively high in LCM and low in NCM, from which they assemble directly, i.e., without intermediate precursor aggregates on IFs in the cytoplasm, on certain plasma membrane domains in a Ca2+ dependent process.

Identification of a basic protein of Mr 75,000 as an accessory desmosomal plaque protein in stratified and complex epithelia

Desmosomes are intercellular adhering junctions characterized by a special structure and certain obligatory constituent proteins such as the cytoplasmic protein, desmoglein. Desmosomal fractions from bovine muzzle epidermis contain, in addition, a major polypeptide of Mr approximately 75,000 ("band 6 protein") which differs from all other desmosomal proteins so far identified by its positive charge (isoelectric at pH approximately 8.5 in the denatured state) and its avidity to bind certain type I cytokeratins under stringent conditions. We purified this protein from bovine muzzle epidermis and raised antibodies to it. Using affinity-purified antibodies, we identified a protein of identical SDS-PAGE mobility and isoelectric pH in all epithelia of higher complexity, including representatives of stratified, complex (pseudostratified) and transitional epithelia as well as benign and malignant human tumors derived from such epithelia. Immunolocalization studies revealed the location of this protein along cell boundaries in stratified and complex epithelia, often resolved into punctate arrays. In some epithelia it seemed to be restricted to certain cell types and layers; in rat cornea, for example, it was only detected in upper strata. Electron microscopic immunolocalization showed that this protein is a component of the desmosomal plaque. However, it was not found in the desmosomes of all simple epithelia examined, in the tumors and cultured cells derived thereof, in myocardiac and Purkinje fiber cells, in arachnoideal cells and meningiomas, and in dendritic reticulum cells of lymphoid tissue, i.e., all cells containing typical desmosomes. The protein was also absent in all nondesmosomal adhering junctions. From these results we conclude that this basic protein is not an obligatory desmosomal plaque constituent but an accessory component specific to the desmosomes of certain kinds of epithelial cells with stratified tissue architecture. This suggests that the Mr 75,000 basic protein does not serve general desmosomal functions but rather cell type-specific ones and that the composition of the desmosomal plaque can be different in different cell types. The possible diagnostic value of this protein as a marker in cell typing is discussed.

Kinetics of desmosome assembly in Madin-Darby canine kidney epithelial cells: temporal and spatial regulation of desmoplakin organization and stabilization upon cell-cell contact. I. Biochemical analysis

The functional interaction of cells in the formation of tissues requires the establishment and maintenance of cell-cell contact by the junctional complex. However, little is known biochemically about the mechanism(s) that regulates junctional complex assembly. To address this problem, we have initiated a study of the regulation of assembly of one component of the junctional complex, the desmosome, during induction of cell-cell contact in cultures of Madin-Darby canine kidney epithelial cells. Here we have analyzed two major protein components of the desmosomal plaque, desmoplakins I (Mr of 250,000) and II (Mr of 215,000). Analysis of protein levels of desmoplakins I and II by immunoprecipitation with an antiserum that reacts specifically with an epitope common to both proteins revealed that desmoplakins I and II are synthesized and accumulate at steady state in a ratio of 3-4:1 (in the absence or presence of cell-cell contact). The kinetics of desmoplakins I and II stabilization and assembly were analyzed after partitioning of newly synthesized proteins into a soluble and insoluble protein fraction by extraction of whole cells in a Triton X-100 high salt buffer. In the absence of cell-cell contact, both the soluble and insoluble pools of desmoplakins I and II are unstable and are degraded rapidly (t1/2 approximately 8 h). Upon induction of cell-cell contact, the capacity of the insoluble pool increases approximately three-fold as a proportion of the soluble pool of newly synthesized desmoplakins I and II is titrated into the insoluble pool. The insoluble pool becomes relatively stable (t1/2 greater than 72 h), whereas proteins remaining in the soluble pool (approximately 25-40% of the total) are degraded rapidly (t1/2 approximately 8 h). Furthermore, we show that desmoplakins I and II can be recruited from this unstable soluble pool of protein to the stable insoluble pool upon induction of cell-cell contact 4 h after synthesis; significantly, the stabilization of this population of newly synthesized desmoplakins I and II is blocked by the addition of cycloheximide at the time of cell-cell contact, indicating that the coordinate synthesis of another protein(s) is required for protein stabilization.

Structure and assembly of desmosome junctions: biosynthesis and turnover of the major desmosome components of Madin-Darby canine kidney cells in low calcium medium

Neither stratifying (primary keratinocytes) nor simple (Madin-Darby canine kidney [MDCK] and Madin-Darby bovine kidney [MDBK]) epithelial cell types from desmosomes in low calcium medium (LCM; less than 0.1 mM), but they can be induced to do so by raising the calcium level to physiological concentrations (standard calcium medium [SCM], 2 mM). We have used polyclonal antisera to the major bovine epidermal desmosome components (greater than 100 kD) in a sensitive assay involving immunoprecipitation of the components from metabolically labeled MDCK cell monolayers to investigate the mechanism of calcium-induced desmosome formation. MDCK cells, whether cultured in LCM or SCM, were found to synthesize the desmosome protein, DPI and desmosome glycoproteins DGI and DGII/III with identical electrophoretic mobility, and also, where relevant, with similar carbohydrate addition/processing and proteolytic processing. The timings of these events and of transport of DGI to the cell surface were similar in low and high calcium. Although the rates of synthesis of the various desmosome components were also similar under both conditions, the glycoprotein turnover rates increased dramatically in cells cultured in LCM. The half-lives decreased by a factor of about 7 for DGI and 12 for DGII/III and, consistent with this, MDCK cells labeled for 48 h in SCM had three and six times the amount of DGI and DGII/III, respectively, as cells labeled for 48 h in LCM. The rate of turnover and the levels of DPI were changed in the same direction, but to much lesser extents. Possible mechanisms for the Ca2+-dependent control of desmosome formation are discussed in the light of this new evidence.