EXPERIMENTAL MANIPULATION OF DESMOSOME FORMATION

In the corneal epithelium of the embryonic chick there is a 3- to 4-fold increase in desmosomes between the 15th and 16th days of incubation which has not been noted in earlier studies of this tissue. This finding has made it feasible to study the effects of the local cell environment on desmosome formation. Cells of 15-day corneas which were forming desmosomes rapidly, were dispersed and combined in culture with cells from 10-day corneas which were forming few desmosomes. Surfaces of the same 15-day cell which were confronted with either another 15-day cell or a 10-day cell were compared. Desmosomes formed preferentially on the surface adjacent to a like cell. When 15-day cells were confronted with pigment cells, desmosomes formed almost exclusively on the surface adjacent to a like cell. Evidence for such localized differences on the same cell surface emphasize the importance of the immediate cell environment in desmosome formation. The observation that single desmosome plaques form occasionally on lateral cell surfaces has been noted previously. This finding was confirmed.     

Formation of junctions and cell sorting in aggregates of chick and mouse cells

The frequency of desmosome formation was examined in aggregates of old cells, which form many junctions, combined with young cells, which form few. Cells of chick corneal epithelium and mouse epidermis, which can be distinguished morphologically, were combined. Desmosomes between these cell types are stable. Further, young cells make more desmosomes than they otherwise would on those surfaces adjoining old cells. Desmosomes increase in number in aggregates while cell sorting is occurring. Cells consistently sort, with those which form most desmosomes lying internally. Gap junctions and intermediate junctions are also present, but are uncommon. A carbohydrate cell-surface coat has regenerated by the time desmosome formation starts. The possible relation of desmosome formation to cell sorting is discussed.     

A role for plakophilin-1 in the initiation of desmosome assembly

Plakophilins (pkp-1, -2, and -3) comprise a family of armadillo-repeat containing proteins that are found in the desmosomal plaque and in the nucleus. Plakophilin-1 is most highly expressed in the suprabasal layers of the epidermis and loss of plakophilin-1 expression results in skin fragility-ectodermal dysplasia syndrome, which is characterized by a reduction in the number and size of desmosomes in the epithelia of affected individuals. To investigate the role of plakophilin-1 during desmosome formation, we fused plakophilin-1 to the hormone-binding domain of the estrogen receptor to create a fusion protein (plakophilin-1/ER) that can be activated in cell culture by the addition of 4-hydroxytamoxifen. When plakophilin-1/ER was expressed in A431 cells it was incorporated into endogenous desmosomes and did not disrupt desmosome formation. A derivative of A431 cells (A431D) do not form desmosomes, even though they express all the components believed to be necessary for desmosome assembly. Expression and activation of plakophilin-1/ER in A431D cells resulted in punctate desmoplakin staining on the cell surface. Co-expression of a classical cadherin (N-cadherin) and plakophilin-1/ER in A431D cells resulted in punctate desmoplakin staining at cell-cell borders. These data suggest that plakophilin-1 can induce assembly of desmosomal components in A431D cells in the absence of a classical cadherin; however a classical cadherin (N-cadherin) is required to direct assembly of desmosomes between adjacent cells. The activatable plakophilin-1/ER system provides a unique culture system to study the assembly of the desmosomal plaque in culture.     

Intermediate filaments and the initiation of desmosome assembly

The desmosome junction is an important component in the cohesion of epithelial cells, especially epidermal keratinocytes. To gain insight into the structure and function of desmosomes, their morphogenesis has been studied in a primary mouse epidermal (PME) cell culture system. When these cells are grown in approximately 0.1 mM Ca2+, they contain no desmosomes. They are induced to form desmosomes when the Ca2+ level in the culture medium is raised to approximately 1.2 mM Ca2+. PME cells in medium containing low levels of Ca2+, and then processed for indirect immunofluorescence using antibodies directed against desmoplakins (desmosomal plaque proteins), display a pattern of discrete fluorescent spots concentrated mainly in the perinuclear region. Double label immunofluorescence using keratin and desmoplakin antibodies reveals that the desmoplakin-containing spots and the cytoplasmic network of tonofibrils (bundles of intermediate filaments [IFB]) are in the same juxtanuclear region. Within 1 h after the switch to higher levels of Ca2+, the spots move toward the cell surface, primarily to areas of cell-cell contact and not to free cell surfaces. This reorganization occurs at the same time that tonofibrils also move toward cell surfaces in contact with neighboring cells. Once the desmoplakin spots have reached the cell surface, they appear to aggregate to form desmosomes. These immunofluorescence observations have been confirmed by immunogold ultrastructural localization. Preliminary biochemical and immunological studies indicate that desmoplakin appears in whole cell protein extracts and in Triton high salt insoluble residues (i.e., cytoskeletal preparations consisting primarily of IFB) prepared from PME cells maintained in medium containing both low and normal Ca2+ levels. These findings show that certain desmosome components are preformed in the cytoplasm of PME cells. These components undergo a dramatic reorganization, which parallels the changes in IFB redistribution, upon induction of desmosome formation. The reorganization depends upon both the extracellular Ca2+ level and the establishment of cell-to-cell contacts. Furthermore, the data suggests that desmosomes do not act as organizing centers for the elaboration of IFB. Indeed, we postulate that the movement of IFB and preformed desmosomal components to the cell surface is an important initiating event in desmosome morphogenesis.     

Inhibition of desmosome formation with tunicamycin and with lectin in corneal cell aggregates

Desmosome formation in chick corneal epithelium can be used as a tool to study cell interaction. In the present work, the possible role of carbohydrates in initiating junction formation is investigated. When corneal cells of 15-day chicks are dispersed, then aggregated in rotating medium, desmosomes form on a regular schedule and can be quantitated as desmosomes per micrometer of cell membrane cross section. Tunicamycin, at 0.05 μg/ml has little effect on incorporation of leucine into TCA-precipitable cell fractions, but lowers mannose incorporation, and inhibits desmosome formation. The inhibition is counteracted by the presence of leupeptin, a protease inhibitor. This finding can be interpreted as favoring a role of stabilization, rather than recognition for the carbohydrate moiety. Cytochalasin B inhibits cell surface turnover in isolated cells and enhances desmosome formation. Enhancement occurs even in the presence of tunicamycin. Ferritin-conjugated succinyl-Con A will label surfaces of freshly dispersed cells and when cells are aggregated in its presence, the label is internalized in cytoplasmic vacuoles and desmosomes do not form. To test further the possibility that a lectin binding component of the surface may be a specific recognition factor, cells were aggregated in the presence of a number of sugars. None inhibited junction formation. Thus, evidence favors a stabilizing role for carbohydrates, acting at some point in the process of junction formation and leaves open the possibility that a “recognition” function may also be involved.     

Organization of cytokeratin bundles by desmosomes in rat mammary cells

In a rat mammary epithelial cell line, LA-7, cytokeratin bundles recognized in immunofluorescence by a monoclonal antibody (24B42) disappear after trypsinization of cultures and are gradually reformed after replating. We have followed the time course of cytokeratin filament reappearance by growing cells in low calcium medium (0.1 mM) which prevents desmosome formation, and then shifting to high calcium (1.8 mM) to start the process. By fixing the cells at various intervals and staining them in immunofluorescence for 24B42 cytokeratin and for desmosomal proteins, we found that cell to cell contact and desmosome formation are prerequisites for keratin filament formation in these cells. EGTA treatment, by disassembling desmosomes, causes the cytokeratin filaments to disappear and the 24B42 protein to pass into a soluble form in this cell line, as ascertained by 100,000 g fractionation and immunoenzymatic assay. Cycloheximide treatment also causes cytokeratin filaments to disappear, indicating that protein synthesis is needed for normal filament maintenance. In another related cell line (106A-10a) and in HeLa cells, trypsinization and EGTA exposure do not cause a complete loss of 24B42 immunofluorescence, although distinct filaments disappear, indicating the presence in these cells of different organizing centers, besides desmosomes, for cytokeratin bundle formation. LA7 cells therefore seem to have a cytokeratin system strictly dependent on the presence of desmosomes, which act as an organizing center for filament assembly. 106A-10a cells (also rich in desmosomes) and HeLa cells (showing instead a reduced number of desmosomes) have a cytokeratin system partially or totally independent from that of desmosomes, with different organizing centers.     

Continual assembly of desmosomes within stable intercellular contacts of epithelial A-431 cells

Subclones of human carcinoma-derived A-431 cell line stably producing fusion proteins consisting of the enhanced green fluorescent protein and either human desmoglein 2 (Dsg-GFP) or human plakoglobin (GFP-Pg) were used to examine the behavior of desmosomes in living cells. Immunofluorescence microscopy of the fixed cells showed that both fusion proteins, which were expressed in significantly lower levels relative to their endogenous counterparts, were efficiently recruited into desmosomes. Time-lapse confocal imaging of these cells reveals that such GFP-labeled desmosomes (GFP desmosomes) are stable structures which exhibit various dynamic and motile activities. The most notable are independent lateral mobility and fusion. Furthermore, the continual assembly of new nascent desmosomes is observed within stable contacts located at the middle of the epithelial sheet. A new GFP desmosome appears as a closely apposed group of fine patches which after a few minutes aggregate into a single structure. These three dynamic processes resulted in constant changes of desmosome distribution, numbers, and sizes. In addition, fluorescence recovery after photobleaching experiments showed that fine patches of desmosomal proteins may participate in desmosome maintenance. Such a diverse range of dynamic activities of desmosomes apparently produces flexible but tight cell-cell adhesion required for different morphogenetic events in epithelial structures.This work was supported by grant AR44016-04 from the National Institutes of Health     

Identification of actin-, alpha-actinin-, and vinculin-containing plaques at the lateral membrane of epithelial cells

In this paper, a new type of spot desmosome-like junction (type II plaque) is described that is scattered along the entire lateral plasma membrane of rat and human intestinal epithelium. Ultrastructurally type II plaques differed from the classical type of epithelial spot desmosome ("macula adherens", further denoted as type I desmosome) by weak electron density of the membrane-associated plaque material, association of the plaques with microfilaments rather than intermediate filaments, and poorly visible material across the intercellular space. Thus, type II plaques resemble cross-sections of the zonula adherens. Immunofluorescence-microscopic studies were done using antibodies to a main protein associated with the plaques of type I desmosomes (desmoplakin I) and to the three major proteins located at the plaques of the zonula adherens (actin, alpha-actinin, and vinculin). Two types of plaques were visualized along the lateral surface of intestinal and prostatic epithelium: (a) the type I desmosomes, which were labeled with anti-desmoplakin but did not bind antibodies to actin, alpha-actinin, and vinculin, and (b) a further set of similarly sized plaques, which bound antibodies to actin, alpha-actinin, and vinculin but were not stained with anti-desmoplakin. Three-dimensional computer reconstruction of serial sections double-labeled with anti-desmoplakin and anti-alpha-actinin further confirmed that both types of plaques are spatially completely separated from each other along the lateral plasma membrane. The computer graphs further revealed that the actin-, alpha-actinin-, and vinculin-containing plaques have the tendency to form clusters, a feature also typical of type II plaques. It is suggested that the type II plaques represent spot desmosome-like intercellular junctions, which, like the zonula adherens, appear to be linked to the actin filament system. As the type II plaques cover a considerable part of the lateral cell surface, they might play a particular role in controlling cellular shape and intercellular adhesion.     

Down-Regulation of Desmosomes in Cultured Cells: The Roles of PKC,Microtubules and Lysosomal/Proteasomal Degradation

Desmosomes are intercellular adhesive junctions of major importance for tissue integrity. To allow cell motility and migration they are down-regulated in epidermal wound healing. Electron microscopy indicates that whole desmosomes are internalised by cells in tissues, but the mechanism of down-regulation is unclear. In this paper we provide an overview of the internalisation of half-desmosomes by cultured cells induced by calcium chelation. Our results show that: (i) half desmosome internalisation is dependent on conventional PKC isoforms; (ii) microtubules transport internalised half desmosomes to the region of the centrosome by a kinesin-dependent mechanism; (iii) desmosomal proteins remain colocalised after internalisation and are not recycled to the cell surface; (iv) internalised desmosomes are degraded by the combined action of lysosomes and proteasomes. We also confirm that half desmosome internalisation is dependent upon the actin cytoskeleton. These results suggest that half desmosomes are not disassembled and recycled during or after internalisation but instead are transported to the centrosomal region where they are degraded. These findings may have significance for the down-regulation of desmosomes in wounds.     

Further analysis of pemphigus autoantibodies and their use in studies on the heterogeneity, structure, and function of desmosomes

Pemphigus is an autoimmune disease that causes blistering of human epidermis. We have recently shown that autoantibodies in the serum of three pemphigus patients bind to desmosomes (Jones, J. C. R., J. Arnn, L. A. Staehelin, and R. D. Goldman, 1984, Proc. Natl. Acad. Sci. USA., 81:2781-2785), and we suggested that pemphigus blisters form, at least in part, from a specific antibody-induced disruption of desmosomes in the epidermis. In this paper, experiments are described that extend our initial observations. 13 pemphigus serum samples, which include four known pemphigus vulgaris (Pv) and four known pemphigus foliaceus (Pf) serum samples, have been analyzed by both immunofluorescence and by immunoblotting using cell-free desmosome preparations. Tissue sections of mouse skin processed for double indirect immunofluorescence using each of the pemphigus serum samples and a rabbit antiserum directed against a component of the desmosomal plaque (desmoplakin) show similar punctate cell surface staining patterns. This suggests that all 13 pemphigus serum samples contain autoantibodies that recognize desmosomes. These autoantibodies appear specific for stratified squamous epithelial cell desmosomes and do not recognize desmosomes of other tissues (e.g., mouse heart and mouse intestine). Cultured mouse keratinocytes, which possess well-defined desmosomes, were processed for indirect immunofluorescence using the pemphigus serum samples. Eight of the 13 sera (including the four known Pv samples but not the known Pf sera) stain desmosomes in these preparations. By double indirect immunofluorescence the desmoplakin antiserum stains a double fluorescent line along the contacting edges of cultured keratinocytes, whereas the positive pemphigus serum samples stain a single fluorescent line along this same border. We believe that these pemphigus autoantibodies recognize extracellular antigens located somewhere within the region between the two apposing membranes that comprise the desmosome. The pemphigus sera exhibit positive immunoblotting reactions with desmosome-enriched fractions obtained from bovine tongue epithelium. Three serum samples (including two of the four known Pf serum samples) react with 160- and 165-kD desmosome-associated polypeptides (Koulu, L., A. Kusimi, M. S. Steinberg, V. Klaus-Kovtun, and J. R. Stanley, 1984, J. Exp. Med., 160:1509-1518). Another eight serum samples (including the four known Pv sera) recognize a 140-kD desmosome-associated polypeptide. We propose that the antigens recognized by these human autoantibodies may play important roles in the adhesion of cells within the epidermis.(ABSTRACT TRUNCATED AT 400 WORDS)     

Extracellularly truncated desmoglein 1 compromises desmosomes in MDCK cells

The formation and stability of epithelial tissue involves cell adhesion and the connection of the intermediate filaments of contiguous cells, mediated by desmosomes. The cadherin family members Desmocollins (Dsc) and Desmogleins (Dsg) mediate desmosome extracellular adhesion. The main intracellular molecules identified linking Dscs and Dsgs with the intermediate filament network are Plakoglobin (PG), Plakophilins (PPs) and Desmoplakin (DP). Previous studies on desmosome-mediated adhesion have focused on the intracellular domains of Dsc and Dsg because of their capacity to interact with PG, PPs and DP. This study examines the role of the extracellular domain of Dsg1 upon desmosome stability in MDCK cells. Dsg1 was constructed containing an extracellular deletion (Dsg delta 1EC) and was expressed in MDCK cells. A high expressor Dsg delta 1EC/MDCK clone was obtained and analysed for its capacity to form desmosomes in cell monolayers and when growing under mechanical stress in three-dimensional collagen cultures. Phenotypic changes associated with the ectopic expression of Dsg1 delta EC in MDCK cells were: disturbance of the cytokeratin network, a change in the quality and number of desmosomes and impairment of the formation of cysts in suspension cultures. Interestingly, Dsg1 delta EC was not localized in desmosomes, but was still able to maintain its intracytoplasmic interaction with PG, suggesting that the disruptive effects were largely due to PG and/or PP sequestration.     

ISOLATION OF EPIDERMAL DESMOSOMES

A method is reported for the isolation of desmosomes in a high yield and of a purity suitable for biochemical analysis. The procedure utilizes the selective solubilizing action of citric acid-sodium citrate (CASC) buffer, pH 2.6, on the non-cornified layers of cow nose epidermis, followed by discontinuous sucrose density gradient centrifugation. Electron microscopy with both thin sections of pellets and unfixed spread preparations reveals that after centrifugation, desmosomes are located mainly at the 55–60% sucrose interface. In the desmosome preparation thus obtained, the characteristic desmosome structure is well preserved, showing the midline, unit membranes, and dense plaques. Furthermore, removal of the epidermal filament bundles by the solubilizing action of CASC buffer has revealed a finely filamentous layer on the cytoplasmic surface of the plaques. The dimensions, location, and appearance of this layer correspond with those of the "connecting component" which has been previously suggested as being responsible for the attachment of epidermal filament bundles to the desmosome.     

Isolation of the intercellular glycoproteins of desmosomes

To characterize the desmosome components that mediate intercellular adhesion and cytoskeletal-plasma membrane attachment, we prepared whole desmosomes and isolated desmosomal intercellular regions (desmosomal "cores") from the living cell layers of bovine muzzle epidermis. The tissue was disrupted in a nonionic detergent at low pH, sonicated, and the insoluble residue fractionated by differential centrifugation and metrizamide gradient centrifugation. Transmission electron microscopic analyses reveal that a fraction obtained after differential centrifugation is greatly enriched in whole desmosomes that possess intracellular plaques. Metrizamide gradient centrifugation removes most of the plaque material, leaving the intercellular components and the adjoining plasma membranes. Sodium dodecyl sulfate polyacrylamide gel electrophoresis coupled with methods that reveal carbohydrate-containing moieties on gels demonstrate that certain proteins present in whole desmosomes are glycosylated. These glycoproteins are specifically and greatly enriched in the desmosome cores of which they are the principal protein constituents, and thus may function as the intercellular adhesive of the desmosome.     

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.     

THE ULTRASTRUCTURE OF CARDIAC DESMOSOMES IN THE TOAD AND THEIR RELATIONSHIP TO THE INTERCALATED DISC

The fine structure of desmosomes and intercalated discs in the toad heart is discussed. A definite relationship between the dense components of these structures and the dense region of the Z band is demonstrated. The dense region of the Z band characteristically widens at its approach to the plasma membrane, and often terminates beneath it in a distinct discoidal plaque. Cardiac desmosomes appear to be structures which result from the intimate apposition of plaques of Z band material. These desmosomes retain the Z band function as sites of attachment for myofilaments. The suggestion is made that rotation of a desmosome through 90° and splitting of filaments from the adjacent sarcomere could result in the formation of a simple step-like intercalated disc. Intermediate stages in this process are illustrated. Complex discs present in the toad probably represent the alignment of groups of simple discs produced by contractile forces. Possible physiologic functions of the disc and desmosome are discussed. Other morphologic features of toad cardiac cells include a distinct amorphous outer coat to the sarcolemma, a prominent N band, and a granular sarcoplasm with poorly developed reticulum.     

The relationship between intermediate filaments and microfilaments before and during the formation of desmosomes and adherens-type junctions in mouse epidermal keratinocytes

Actin, keratin, vinculin and desmoplakin organization were studied in primary mouse keratinocytes before and during Ca2+-induced cell contact formation. Double-label fluorescence shows that in cells cultured in low Ca2+ medium, keratin-containing intermediate filament bundles (IFB) and desmoplakin-containing spots are both concentrated towards the cell center in a region bounded by a series of concentric microfilament bundles (MFB). Within 5-30 min after raising Ca2+ levels, a discontinuous actin/vinculin-rich, submembranous zone of fluorescence appears at cell-cell interfaces. This zone is usually associated with short, perpendicular MFB, which become wider and longer with time. Later, IFB and the desmoplakin spots are seen aligned along the perpendicular MFB as they become redistributed to cell-cell interfaces where desmosomes form. Ultrastructural analysis confirms that before the Ca2+ switch, IFB and desmosomal components are found predominantly within the perimeter defined by the outermost of the concentric MFB. Individual IF often splay out, becoming interwoven into these MFB in the region of cell-substrate contact. In the first 30 min after the Ca2+ switch, areas of submembranous dense material (identified as adherens junctions), which are associated with the perpendicular MFB, can be seen at newly formed cell-cell contact sites. By 1-2 h, IFB-desmosomal component complexes are aligned with the perpendicular MFB as the complexes become redistributed to cell-cell interfaces. Cytochalasin D treatment causes the redistribution of actin into numerous patches; keratin-containing IFB undergo a concomitant redistribution, forming foci that coincide with the actin-containing aggregates. These results are consistent with an IF-MF association before and during desmosome formation in the primary mouse epidermal keratinocyte culture system, and with the temporal and spatial coordination of desmosome and adherens junction formation.     

Loss of Gap Junction Plaques and Inhibition of Intercellular Communication in Ilimaquinone-treated BICR-M1Rk and NRK Cells

To examine the mechanism(s) and pathways of gap junction formation and removal a novel and reversible inhibitor of protein secretion, ilimaquinone (IQ), was employed. IQ has been reported to cause the vesiculation of Golgi membranes, block protein transport at the cis-Golgi and depolymerize cytoplasmic microtubules. Connexin43 (Cx43) immunolabeling and dye microinjection experiments revealed that gap junction plaques were lost and intercellular communication was inhibited following IQ treatment for 1 hr in BICR-M1R_k rat mammary tumor cells and for 2 hr in normal rat kidney (NRK) cells. Gap junction plaques and intercellular communication recovered within 2 hr when IQ was removed. IQ, however, did not affect the distribution of zonula occludens-1, a protein associated with tight junctions. Western blot analysis revealed that the IQ-induced loss of gap junction plaques was accompanied by a limited reduction in the highly phosphorylated form of Cx43, previously shown to be correlated with gap junction plaques. The presence of IQ inhibited the formation of new gap junction plaques in BICR-M1R_k cells under conditions where preexisting gap junctions were downregulated by brefeldin A treatment. Treatment of BICR-M1R_k and NRK cells with other microtubule depolymerization agents did not inhibit plaque formation or promote rapid gap junction removal. These findings suggest that IQ disrupts intercellular communication by inhibiting the events that are involved in plaque formation and/or retention at the cell surface independent of its effects on microtubules. Our results also suggest that additional factors other than phosphorylation are necessary for Cx43 assembly into gap junction plaques. Received: 16 January 1996/Revised: 20 September 1996     

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.     

Calcium regulation of cell-cell contact and differentiation of epidermal cells in culture. An ultrastructural study

Calcium modulation of keratinocyte growth in culture was studied by both transmission (TEM) and scanning electron microscopy (SEM). Under standard culture conditions (1.2-1.8 mM calcium), cells were connected by desmosomes and stratified to 4-6 cell layers. Many aspects of in vitro epidermal maturation were analogous to the in vivo process, with formation of keratohyalin granules, loss of nuclei, formation of cornified envelopes and shedding of cornified cells containing keratin filaments. When the medium calcium concentration was lowered to 0.02-0.1 mM, the pattern of keratinocyte growth was strikingly changed. Cells grew as a monolayer with no desmosomal connections and proliferated rapidly, shedding largely non-cornified cells into the medium. Large bundles of keratin filaments were concentrated in the perinuclear cytoplasm. The elevation of extracellular calcium to 1.2 mM induced low calcium keratinocytes to stratify, keratinize and cornify in a manner analogous to that seen when plated in standard calcium medium. The earliest calcium-induced ultrastructural change was the asymmetric formation of desmosomes between adjacent cells. Desmosomal plaques with associated tonofilaments were observed 5 min after calcium addition; symmetric desmosomes were formed within 1-2 h. This system is presented as a useful model for the study of the regulation of desmosome assembly and disassembly.     

Contact inhibition of locomotion and the structure of homotypic and heterotypic intercellular contacts in embryonic epithelial cultures

Epithelia from the early chick embryo have been grown in culture and then fixed for electron microscopy so that the ultrastructure of intercellular contacts could be examined. Epithelia were used which showed various forms of contact inhibition of locomotion upon confrontation with one another. Confrontations of hypoblast with hypoblast after 6 days, and endoblast with endoblast after 24 h showed type 1 contact inhibition and formed desmosomes and zonulae adhaerentes with extensive microfilament collinearity between apposed cells. Hypoblast-hypoblast confrontations after 24 h resulted in type 2 contact inhibition with considerable ruffling and position shifting. In this case desmosomes were absent and microfilament collinearity was restricted. Endoblast cells after 24 h in culture show type 2 inhibition with respect to hypoblast monolayers which they infiltrated upon confrontation. Examination of these heterotypic contacts also showed an absence of desmosomes and reduced adhaerens junctions. Intermediate filaments accumulated at all contact sites examined. It is concluded that whereas type 1 contact inhibition of locomotion in these epithelial cells is accompanied by desmosome formation and extensive zonula adhaerens junctions, type 2 inhibition is not. These junctional deficiences may be responsible in part for the cell motility characteristically observed in monolayers of type 2 inhibited cells.