[Desmosomal structures in the cytoplasm of normal and abnormal keratinocytes (author's transl)]

Desmosome dualism - most of the junction is stable, but a plakophilin moiety is persistently dynamic

Desmosomes, strong cell-cell junctions of epithelia and cardiac muscle, link intermediate filaments to cell membranes and mechanically integrate cells across tissues, dissipating mechanical stress. They comprise five major protein classes - desmocollins and desmogleins (the desmosomal cadherins), plakoglobin, plakophilins and desmoplakin - whose individual contribution to the structure and turnover of desmosomes is poorly understood. Using live-cell imaging together with fluorescence recovery after photobleaching (FRAP) and fluorescence loss and localisation after photobleaching (FLAP), we show that desmosomes consist of two contrasting protein moieties or modules: a very stable moiety of desmosomal cadherins, desmoplakin and plakoglobin, and a highly mobile plakophilin (Pkp2a). As desmosomes mature from Ca2+ dependence to Ca2+-independent hyper-adhesion, their stability increases, but Pkp2a remains highly mobile. We show that desmosome downregulation during growth-factor-induced cell scattering proceeds by internalisation of whole desmosomes, which still retain a stable moiety and highly mobile Pkp2a. This molecular mobility of Pkp2a suggests a transient and probably regulatory role for Pkp2a in desmosomes. This article has an associated First Person interview with the first author of the paper.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

Ultrastructural changes of the palatal mucosa following application of 4-nitroquinoline-l-oxide (4NQO) in rats subjected to major salivary gland excision

OBJECTIVE

It has been suggested that saliva exerts a protective role against the carcinogenic effect of various substances in the oral cavity. The objective of this study was to examine the ultrastructural changes of the palatal mucosa caused by the application of 4-nitroquinoline-l-oxide (4NQO) in the presence or absence of saliva.

STUDY DESIGN

Wistar-Furth rats subjected and not subjected to total bilateral excision of the major salivary glands were either painted with an aqueous solution of 4NQO or with propylene glycol only (controls). Two animals of each group were humanely killed periodically. The areas of the palatal lesions were immediately sliced and processed for TEM examination.

RESULTS

Ultrastructurally, the progressive changes to squamous cell carcinoma were observed in the animals painted with 4NQO. In the desalivated animals group, the ultrastructural alterations appeared earlier than in the group with salivary glands.

CONCLUSIONS

Saliva appeared to delay but not hinder tumor induction by 4NQO.

Hyper-adhesion in desmosomes: its regulation in wound healing and possible relationship to cadherin crystal structure

The resistance of tissues to physical stress is dependent upon strong cell-cell adhesion in which desmosomes play a crucial role. We propose that desmosomes fulfil this function by adopting a more strongly adhesive state, hyper-adhesion, than other junctions. We show that the hyper-adhesive desmosomes in epidermis resist disruption by ethylene glycol bis(2-aminoethyl ether)-N,N,N′N′-tetraacetic acid (EGTA) and are thus independent of Ca2+. We propose that Ca2+ independence is the normal condition for tissue desmosomes. Ca2+ independence is associated with an organised arrangement of the intercellular adhesive material exemplified by a dense midline. When epidermis is wounded, desmosomes in the wound-edge epithelium lose hyper-adhesiveness and become Ca2+ dependent, i.e. readily dissociated by EGTA. Ca2+-dependent desmosomes lack a midline and show narrowing of the intercellular space. We suggest that this indicates a less-organised, weakly adhesive arrangement of the desmosomal cadherins, resembling classical cadherins in adherens junctions. Transition to Ca2+ dependence on wounding is accompanied by relocalisation of protein kinase C α to desmosomal plaques suggesting that an `inside-out' transmembrane signal is responsible for changing desmosomal adhesiveness. We model hyper-adhesive desmosomes using the crystal packing observed for the ectodomain of C-cadherin and show how the regularity of this 3D array provides a possible explanation for Ca2+ independence.

Epidermolysis bullosa herpetiformis (Dowling-Meara type) exhibits ultrastructural derangement of tonofilaments and desmosomes

Ultrastructural and immunohistochemical studies of clinically intact skin obtained from three severe neonatal cases of epidermolysis bullosa herpetiformis (Dowling-Meara type) demonstrated disorders in the assembly of keratin intermediate filaments and desmosomes of the keratinocytes. During mitosis, K5- and K14-positive and K1- and K10-negative tonofilaments were disrupted and formed spherical bodies associated with intracytoplasmic desmosomes by invagination of the desmosomes and the adjacent plasma membrane. During the invagination process, destructive changes in the internalized membrane were noted. These were accompanied by gradual loss of reactivity with a monoclonal antibody ZK31, which detected plasma membrane adjacent to the attachment plaques of desmosomes. However, the reactivity of the attachment plaques of the internalized desmosomes for desmoplakins and desmoglein did not decline during the process of internalization. In the suprabasal layers of the epidermis, filamentous substructures and K1 and K10 appeared at the periphery of the spherical bodies. Simultaneously, the desmosomes that were sparsely located in the lower epidermis, increased in number as cell differentiation progressed. Thus, the keratinocytes attained an almost normal appearance with respect to tonofilaments and desmosomes by the time they reached the upper layer of the epidermis. These findings may be relevant to the mechanism responsible for the clinical appearance of the herpetiform blisters in epidermolysis bullosa herpetiformis, which are also characterized by spontaneous involution during childhood or when exposed to high ambient temperatures.

Cytoplasmic desmosomes and intermediate filament disturbance following acrylamide treatment in cultured rat keratinocytes

The present paper describes disturbances in the organization of tonofilaments and desmosomes of rat lingual and epidermal keratinocytes after treatment of the cells with acrylamide in culture. This treatment induced changes in cell shape, reduction of intercellular adhesion and a perinuclear accumulation of cytoplasmic organelles. Using specific antibodies for cytokeratins, the filaments were disorganized particularly in the perinuclear region. In untreated cells, keratin filament labelling was very weak or absent above and below the nucleus thus leaving a black nuclear space in fluorescine microscopy. Following acrylamide treatment, the keratin filament labelling covered the nuclear space which indicated the accumulation of these filaments all around the nucleus. Furthermore, the desmosomal junctions were often associated with thick keratin bundles. Antibodies for desmoplakins revealed a reduction in intercellular labelling and stronger cytoplasmic labelling. Ultrastructurally, well-developed long tonofilaments were found to associate with large desmosomal junctions. Furthermore, small-sized desmosomal structures were identified within the cytoplasm. Morphologically, these were identical to cell surface desmosomes and were almost always associated with well-developed tonofilaments. The effect of acrylamide on the protein kinase A activity might be implicated in the disturbances of the desmosome-intermediate filament complex and in the initiation of contractile forces necessary for perinuclear accumulation of intermediate filaments and for the formation of intact cytoplasmic desmosomes. The acrylamide-induced intermediate filament and desmosomal changes may provide valuable information on the mechanism of intact cytoplasmic desmosome formation in several skin diseases and in squamous cell carcinoma.

Different modes of internalization of proteins associated with adhaerens junctions and desmosomes: experimental separation of lateral contacts induces endocytosis of desmosomal plaque material

The distribution and fate of two junctional complexes, zonula adhaerens and desmosomes, after dissociation of cell-cell contacts is described in MDBK cells. Junctions were split between adjacent cells by treatment with EGTA and proteins associated with the plaques of zonulae adhaerentes and desmosomes were localized by immunological methods. Splitting of these junctions is accompanied by the dislocation of desmosomal plaque protein from the cell periphery and its distribution in punctate arrays over the whole cytoplasm. By contrast, vinculin associated with zonulae adhaerentes is still seen at early times (0.5-1 h) in a conspicuous belt-like structure which, however, is displaced from the plasma membrane. Strong vinculin staining is maintained on leading edges of free cell surfaces. Electron microscopy of EGTA-treated cells exposed to colloidal gold particles reveals the disappearance of junctional structures from the cell periphery and the concomitant appearance of a distinct class of gold particle-containing vesicles which are coated by dense plaques. These vesicle plaques react with antibodies to desmosomal plaque proteins and are associated with filaments of the cytokeratin type. In the same cells, extended dense aggregates are seen which are most probably the membrane-detached vinculin-rich material from the zonula adhaerens . The experiments show that, upon release from their junction-mediated connections with adjacent cells, major proteins associated with the cytoplasmic side of the junctions remain, for several hours, clustered within plaques displaced from the cell surface. While plaque material of adhaerens junctions containing vinculin is recovered in large belt-like aggregates, desmosomal plaque protein remains attached to membrane structures and appears on distinct vesicles endocytotically formed from half-desmosomal equivalents.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural studies of epidermal lesions in pityriasis lichenoides chronica: occurrence of tubular aggregates and intracytoplasmic desmosomes

Very early skin lesions from four patients with pityriasis lichenoides chronica were examined by electron microscopy. Aggregates of tubular structures, which resembled those reported in endothelium in lupus erythematosus and are currently being observed in an increasing variety of conditions, were noticed in the epidermal cells of all patients. These cells also showed intracytoplasmic desmosomes, and isolated mitochondria and myelin-like figures in intercellular spaces, in addition to the occurrence of vacuoles and lysosomes in the cytoplasm, dilatation of endoplasmic reticulum and alteration of mitochondria. Occasionally, epidermal cells were seen to be individually filled with fibrils. These ultrastructural findings seem to indicate that in pityriasis lichenoides chronica some injury to epidermal cells occurs at the early stage of pathological processes, as suggested by light microscope studies.

Microfilaments in human epithelial cancer cells

The occurence, distribution, and ultrastructural morphology of microfilaments in malignant epithelial cells of invasive squamous cell carcinoma of human oral cavity were studied by electron microscopy. The findings are compared with those in malignant oral epithelial cells of carcinoma-in-situ. In the malignant cells of invasive carcinoma, microfilaments 50-70 A in diameter are prominent in the cortical cytoplasm of the lateral and basal cell surfaces, adjacent and parallel to the plasma membrane, and extending into cell processes and microvillous extensions. Additional microfilaments are found to run from the peripheral cytoplasm to the perinuclear region. The microfilaments are aggregated into bundles aligned parallel to the long axis of the cell and display foci of increased electron density. They also tend to be aggregated into complex polygonal arrays. These microfilaments are similar in organization, concentration and ultrastructural architecture to those of various other nonmuscle cells, where they are thought to be capable of contraction and associated with cell motility. The presence of a microfilament system believed to be associated with contractile and motile cell processes may be an important characteristic of malignant cells of invasive tumors. The lack of abundant organized microfilaments in malignant cells in the absence of tumor invasion, and the presence of a prominent microfilament system in cells of invasive tumors, suggest that the microfilaments are related to the invasive properties of malignant tumor cells.