Evidence for heterogeneity in the 160/165 × 10(3) Mr glycoprotein components of desmosomes

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: 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.

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

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

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

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

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.

Epithelial cells retain junctions during mitosis

It has long been known that cells show reduced cell-substratum adhesion during mitosis in tissue culture, but it is not generally known whether cell-cell adhesion is also reduced. Epithelial cells, both in culture and in tissues, are linked together by several different types of intercellular junctions. Are these junctions disassembled when epithelial cells divide? Cultured epithelial cells were fluorescently stained for desmosomes, tight junctions and zonulae adherentes, and large numbers of dividing cells examined by light microscopy. The results suggested that all three types of intercellular junctions were retained throughout cell division and no evidence for internalization of junctions was obtained. The persistence of intercellular junctions by cultured cells during division was confirmed by electron microscopy. In order to determine whether intercellular junctions were similarly retained by dividing cells in tissues, human colonic mucosal crypt cells and basal keratinocytes were studied by electron microscopy. Both cell types retained intercellular junctions during division. Dividing basal keratinocytes also possessed hemidesmosomal contact with the basement membrane. It is suggested that retention of cellular junctions during division is important for maintenance of tissue integrity and organization.

Dynamic expression of pemphigus and desmosomal antigens by cultured keratinocytes

Dynamic expression of pemphigus antigens by cultured human and mouse keratinocytes was compared with that of desmosome-associated molecules and cellular markers relating to epidermal differentiation. Plakoglobin was detected in localized areas of keratinocyte sheets in low Ca2+ (0.15 mM) KGM medium. In minimum essential medium (MEM) containing 1.8 mM Ca2+, plakoglobin was expressed in the intercellular spaces (ICS) throughout the keratinocyte sheet. Desmoplakin I and II, which were present in the cytoplasm of keratinocytes in the low Ca2+ medium, moved to the cell surface after the medium was changed to MEM. Desmoglein 1 and pemphigus vulgaris (PV) antigens were observed in the ICS of both the monolayers and stratified areas in the MEM. Pemphigus foliaceus (PF) antigens, frequently together with desmoglein 1, involucrin and keratins specific for the upper layer of the epidermis, were expressed by stratified keratinocytes but not the cells in the monolayers. The Western blotting study of the cultured keratinocyte extract showed 160- and 130-kDa bands positive for desmoglein 1 antigens and a 130-kDa band stained with PV sera. These findings suggest that although desmoglein 1 molecules bear PF antigenic sites, their expression pattern by cultured keratinocytes is closely related to that of PV rather than PF antigens. The PF antigenic sites may be formed on desmoglein 1 during epidermal differentiation.

The molecular biology of desmosomes and hemidesmosomes: "what's in a name"?

Desmosomes are junctions involved in intercellular adhesion of epithelial cells and hemidesmosomes are junctions involved in adhesion of epithelia to basement membranes. Both are characterised at the ultrastructural level by dense cytoplasmic plaques which are linked to the intermediate filament cytoskeleton of the cells. The plaques strongly resemble each other suggesting a relationship between the two kinds of junctions, as implied by their names. Recent characterisation of the molecular components of the junctions shows they are, in fact, quite unrelated implying that structural similarity is fortuitous. The molecular biology raises many fascinating problems relating to their structure and function.

Evidence for a role of the monoclonal antibody E48 defined antigen in cell-cell adhesion in squamous epithelia and head and neck squamous cell carcinoma

Monoclonal antibody (MAb) E48 recognizes a 20- to 22-kDa antigen expressed by human squamous and transitional epithelia and their neoplastic counterparts. Histochemical examination of these tissues revealed distinct surface labeling with MAb E48. To investigate the subcellular localization of the E48 antigen we have performed electron microscopical analysis. In cells of normal oral mucosa, the E48 antigen was expressed on the plasmalemma, particularly associated with desmosomes, suggesting involvement of the E48 antigen in intercellular adhesion. Furthermore, the level of expression of the E48 antigen appeared to be influenced by the cellular organization. In squamous cell carcinoma (SCC) cell lines grown in vitro as subconfluent monolayer cultures, the E48 antigen expression was low. However, E48 antigen expression increased when SCC cells were grown to confluency. E48 antigen expression was similarly high when SCC cell lines were cultured under conditions promoting three-dimensional growth either as colonies within floating collagen gels or as xenograft in tumor-bearing nude mice. Further evidence for the involvement of the E48 antigen in cell-cell adhesion was found when SCC cells were grown within collagen gels in the presence of MAb E48: no spherical colonies were formed, but cells grew out to colonies composed of single cells. Moreover, in this culture system the percentage of SCC cells growing out to colonies was decreased by the presence of MAb E48. These findings indicate that the E48 antigen is involved in the structural organization of squamous tissue and might have a role in intercellular adhesion.

Desmoplakin expression and distribution in cultured rat bladder epithelial cells of varying tumorigenic potential

The expression and distribution of the desmosomal plaque proteins, desmoplakins (DPs) I and II, were studied in nontumorigenic (RBE-8) and a series of tumorigenic (AY34, R-4909, SS-24B, RBTCC-8, and 804G) rat bladder epithelial cell lines. These cell lines ranged from slow-growing papillary transitional cells (AY34) to rapidly metastatic carcinoma cells (RBTCC-8). DPs I and II were shown by immunoblotting and Northern analysis to be present in nontumorigenic RBE-8 cells as well as in all of the tumorigenic cell lines, albeit in differing amounts. Immunofluorescence microscopy revealed striking differences in DP distribution, corresponding in general with increases in tumorigenic potential. Whereas DPs of normal RBE-8 cells and less tumorigenic AY34 cells were localized predominantly at cell interfaces, the more tumorigenic lines exhibited a high proportion of DP in the form of cytoplasmic dots, a distribution reminiscent of that seen in epithelial cells maintained in low levels of extracellular calcium. In 804G cells, which represented the most extreme example of this phenomenon, the majority of DPs were organized as cytoplasmic dots. Electron microscopy revealed intermediate filament (IF)-associated spots in the cytoplasm as well as an elaborate array of IF-associated plaques at the cell-substratum interface. The IF-associated spots in the cytoplasm reacted with anti-DP antibody in immunogold labeling experiments while those at the cell-substratum did not react. In more dense cultures of 804G cells, certain cells stratified and expressed increased amounts of DP followed by the induction of new keratins including those of the skin type. Decreasing extracellular calcium resulted in a rearrangement of DP in each cell line; staining at cell-cell interfaces disappeared and was replaced with a pattern of cytoplasmic dots. These results demonstrate a possible relationship between desmosome assembly and/or maintenance and tumorigenic potential.

Distribution of desmoplakin in normal cultured human keratinocytes and in basal cell carcinoma cells

In cultured human keratinocytes (NHEK) maintained in medium containing low levels of Ca2+ (0.04 mM) desmoplakin is a component of certain electron-dense bodies in the cytoplasm. These bodies are associated with bundles of intermediate filaments. Upon elevation of the level of Ca2+ in the culture medium to 1.2 mM, desmoplakin first appears at sites of cell-cell contact in association with bundles of intermediate filaments. Subsequently, desmoplakin becomes incorporated into desmosomes in a manner comparable to that seen in mouse keratinocytes (Jones and Goldman: Journal of Cell Biology 101:506-517, 1985). NHEK cells maintained for 24 hr at Ca2+ concentrations between 0.04 mM and 0.18 mM were processed for immunofluorescence, immunoelectron, and conventional electron microscopical analysis. In NHEK cells grown at Ca2+ concentrations of 0.11 mM, desmoplakin appears to be localized in electron-dense bodies associated with intermediate filaments at sites of cell-cell contact in the absence of formed desmosomes. At a Ca2+ concentration of 0.13 mM desmoplakin is arrayed like beads on a "string" of intermediate filaments at areas of cell-cell association. At 0.15 mM, desmosome formation occurs, and desmoplakin is associated with the desmosomal plaque. In basal cell carcinoma cells desmoplakin is not restricted to desmosomes but also occurs in certain electron-dense bodies morphologically similar to those seen in NHEK maintained in low levels of Ca2+ and during early stages of desmosome assembly. We discuss the possibility of "cycling" of desmoplakin through these bodies in proliferative cells.