A novel hemidesmosomal plaque component: tissue distribution and incorporation into assembling hemidesmosomes in an in vitro model

The hemidesmosome and its associated structures, such as anchoring fibrils, form a complex structure, the polypeptide composition of which has only recently begun to be elucidated. We describe the characterization of a monoclonal antibody, mAb6A5, directed against a 200-kDa polypeptide found in the cytoplasmic-most area of the hemidesmosomal plaque. This 200-kDa polypeptide is immunologically distinct from the 180- and 230-kDa hemidesmosomal plaque components recognized by bullous pemphigoid (BP) autoantibodies. mAb6A5 recognizes hemidesmosomes of stratified squamous epithelia in a number of species, including human tissue. mAb6A5 also recognizes pseudo-stratified epithelium, but not simple or transitional epithelia. During de novo hemidesmosome assembly in an in vitro model of epithelial wound healing, the 200-kDa polypeptide is in most instances deposited at the epithelial-stromal interface after plaque components recognized by BP autoantibodies, but before the collagen type VII component of anchoring fibrils. We discuss possible mechanisms of hemidesmosomal plaque assembly.     

Formation of hemidesmosomes in vitro by a transformed rat bladder cell line

Two hemidesmosomal plaque components of 230 and 180 kD have recently been characterized using autoantibodies in the serum samples of bullous pemphigoid (BP) patients (Klatte, D. H., M. A. Kurpakus, K. A. Grelling, and J. C. R. Jones. 1989, J. Cell Biol. 109:3377-3390). These BP autoantibodies generate the type of staining patterns that one would predict for formed hemidesmosomes, i.e., a punctate staining pattern towards the substratum; in less than 50% of various primary epithelial and transformed epidermal cell lines even when such cells are maintained in culture for prolonged periods. In contrast, affinity- purified human autoantibodies against the 230-kD hemidesmosomal plaque component generate intense immunofluorescence staining along the region of cell-substratum interaction in the rat bladder tumor cell line 804G maintained on uncoated glass cover-slips. This pattern is distinct from that observed in the 804G cells using an antibody preparation directed against vinculin, a component of adhesion plaques. Ultrastructural analyses of the 804G cells reveals that hemidesmosome-like structures occur along the basal surface of cells where they abut the substratum. These structures are present in 804G cells maintained in culture in reduced levels of Ca2+ and are recognized by autoantibodies directed against the 230-kD hemidesmosomal plaque component as determined by immunogold ultrastructural localization. To study hemidesmosome appearance in this cell line, 804G cells were trypsinized and then allowed to readhere to glass coverslips. In rounded, unattached 804G cells, hemidesmosome-like plaque structures occur along the cell surface. These structures are recognized by the 230-kD autoantibodies. At 1 h after plating, hemidesmosomes are observed along the substratum attached surface of cells. Protein synthesis is not required for the appearance of these hemidesmosomes. Within 4 h of plating, autoantibody staining and hemidesmosomes appear towards the cell periphery. Subsequently, the polypeptide recognized by the BP autoantibodies becomes concentrated in the perinuclear region, where there are numerous hemidesmosomes. We propose that the hemidesmosomes in 804G cells are involved in cell-substratum adhesion. We discuss possible mechanisms of assembly of hemidesmosomes in the 804G cells. Indeed, the 804G cells should prove an invaluable cell line for the biochemical and molecular dissection of hemidesmosome structure, function, and assembly.     

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.     

Immunochemical characterization of three components of the hemidesmosome and their expression in cultured epithelial cells

Treatment of bovine tongue mucosa with 1 M KCl induced a split in the lamina densa of the basement membrane zone (BMZ). The epithelium was then separated from the underlying connective tissue. Electron microscopic analysis of the stripped epithelium revealed that hemidesmosomes and their associated intermediate filaments (IF) remain along the basal surface of the epithelium. This surface was solubilized in an SDS/urea-containing buffer. Characterization of components of this protein mixture was undertaken using human autoantibodies from bullous pemphigoid (BP) patients that have been shown to recognize hemidesmosomal plaque elements (Mutasim, D. F., Y. Takahashi, R. S. Labib, G. J. Anhalt, H. P. Patel, and L. A. Diaz. 1985. J. Invest. Dermatol. 84:47-53) and by production of mAbs. Affinity-purified autoantibodies directed against 180- and 240-kD polypeptides present in the protein preparation generated strong immunofluorescence staining patterns along the BMZ of bovine tongue mucosa. Furthermore, immunogold localization revealed that these two polypeptides are associated with the hemidesmosomal plaque. A mAb preparation directed against a 125-kD polypeptide present in this same protein mixture lamina lucida side of the hemidesmosome. Autoantibodies in BP serum samples, affinity- purified 180-kD autoantibodies and the mAb preparation generated a punctate stain along the substratum attached surface of epithelial cells maintained on glass substrata for approximately 1 wk. The spots appeared to be associated with bundles of IF in cultured mouse keratinocytes. These monospecific antibody probes should prove invaluable for the study of hemidesmosome structure, assembly, and function.     

Both type-I hemidesmosomes and adherens-type junctions contribute to the cell-substratum adhesion system in myoepithelial cells

Myoepithelial cells present in exocrine glands cause secretion from the glands by contraction. They have mixed characteristics with regard to cytoskeletal elements, containing both epithelial-type intermediate filaments and smooth muscle-type myofilaments. For further characterization, myoepithelial cells from bovine apocrine sweat glands and tracheal glands were here examined with special attention to the cell-substratum adhesion system. Immunofluorescence microscopy using a panel of antibodies against adherens-type junctional and hemidesmosomal proteins demonstrated two types of cell-substratum junctions in myoepithelial cells from both glands. Type-I hemidesmosomes (HDs) consisting of plectin, BP230, integrin alpha6beta4, and BP180 were thus observed as punctate arrays longitudinally arranged along myoepithelial cell surfaces, while adherens-type junctions were similarly evident as linear rib-like structures. Double-label immunofluoresence revealed the two junctions to be distributed in a mutually exclusive or independent manner. Electron microscopy further demonstrated that apocrine myoepithelial cells surround secretory epithelial cells completely, without any gaps, HDs being abundant along the basement membrane, but with no distinct structures in the inter-hemidesmosomal regions. Immunoelectron microscopy, however, revealed an interhemidesmosomal localization of vinculin, pointing to the existence of adherens-type junctions. Secretory epithelial cells in tracheal glands were found not to be completely covered with myoepithelial cells, so that more than half of them are directly attached to the basement membrane, where they form type II-HDs lacking BP230 and BP180, but no detectable adherens junctions, like epidermal basal cells and sebaceous gland cells. These observations demonstrate that, in addition to their cytoskeleton, myoepithelial cells have both epithelial- and smooth muscle-type cell-substratum adhesion structures, i.e. HDs and dense plaque-like adherens junctions.     

The tetraspan molecule CD151, a novel constituent of hemidesmosomes, associates with the integrin alpha6beta4 and may regulate the spatial organization of hemidesmosomes

CD151 is a cell surface protein that belongs to the tetraspan superfamily. It associates with other tetraspan molecules and certain integrins to form large complexes at the cell surface. CD151 is expressed by a variety of epithelia and mesenchymal cells. We demonstrate here that in human skin CD151 is codistributed with alpha3beta1 and alpha6beta4 at the basolateral surface of basal keratinocytes. Immunoelectron microscopy showed that CD151 is concentrated in hemidesmosomes. By immunoprecipitation from transfected K562 cells, we established that CD151 associates with alpha3beta1 and alpha6beta4. In beta4-deficient pyloric atresia associated with junctional epidermolysis bullosa (PA-JEB) keratinocytes, CD151 and alpha3beta1 are clustered together at the basal cell surface in association with patches of laminin-5. Focal adhesions are present at the periphery of these clusters, connected with actin filaments, and they contain both CD151 and alpha3beta1. Transient transfection studies of PA-JEB cells with beta4 revealed that the integrin alpha6beta4 becomes incorporated into the alpha3beta1-CD151 clusters where it induces the formation of hemidesmosomes. As a result, the amount of alpha3beta1 in the clusters diminishes and the protein becomes restricted to the peripheral focal adhesions. Furthermore, CD151 becomes predominantly associated with alpha6beta4 in hemidesmosomes, whereas its codistribution with alpha3beta1 in focal adhesions becomes partial. The localization of alpha6beta4 in the pre-hemidesmosomal clusters is accompanied by a strong upregulation of CD151, which is at least partly due to increased cell surface expression. Using beta4 chimeras containing the extracellular and transmembrane domain of the IL-2 receptor and the cytoplasmic domain of beta4, we found that for recruitment of CD151 into hemidesmosomes, the beta4 subunit must be associated with alpha6, confirming that integrins associate with tetraspans via their alpha subunits. CD151 is the only tetraspan identified in hemidesmosomal structures. Others, such as CD9 and CD81, remain diffusely distributed at the cell surface.In conclusion, we show that CD151 is a major component of (pre)-hemidesmosomal structures and that its recruitment into hemidesmosomes is regulated by the integrin alpha6beta4. We suggest that CD151 plays a role in the formation and stability of hemidesmosomes by providing a framework for the spatial organization of the different hemidesmosomal components.     

The complement of desmosomal plaque proteins in different cell types

Desmosomal plaque proteins have been identified in immunoblotting and immunolocalization experiments on a wide range of cell types from several species, using a panel of monoclonal murine antibodies to desmoplakins I and II and a guinea pig antiserum to desmosomal band 5 protein. Specifically, we have taken advantage of the fact that certain antibodies react with both desmoplakins I and II, whereas others react only with desmoplakin I, indicating that desmoplakin I contains unique regions not present on the closely related desmoplakin II. While some of these antibodies recognize epitopes conserved between chick and man, others display a narrow species specificity. The results show that proteins whose size, charge, and biochemical behavior are very similar to those of desmoplakin I and band 5 protein of cow snout epidermis are present in all desmosomes examined. These include examples of simple and pseudostratified epithelia and myocardial tissue, in addition to those of stratified epithelia. In contrast, in immunoblotting experiments, we have detected desmoplakin II only among cells of stratified and pseudostratified epithelial tissues. This suggests that the desmosomal plaque structure varies in its complement of polypeptides in a cell-type specific manner. We conclude that the obligatory desmosomal plaque proteins, desmoplakin I and band 5 protein, are expressed in a coordinate fashion but independently from other differentiation programs of expression such as those specific for either epithelial or cardiac cells.     

Expression of annexin VI (p68, 67 kDa-calelectrin) in normal human tissues: evidence for developmental regulation in B- and T-lymphocytes.

This paper describes the tissue distribution of annexin VI, a Ca(2+)-dependent phospholipid binding protein, and a member of the annexin super-gene family. In order to determine whether annexin VI expression correlated with a particular functional phenotype, an extensive series of non-pathological human tissues were examined, in which annexin VI was detected either immunohistochemically or by immunofluorescence, using a rabbit polyclonal anti(human annexin VI)-IgG of known specificity. Although most tissues investigated were found to express annexin VI, the protein was usually confined to highly specific cell types within each tissue, the staining generally appearing cytoplasmic and diffuse. There was particularly good correlation between annexin VI expression and hormone secreting cells, with positive staining in the islet cells of the pancreas, the Leydig cells of the testis and the cells of the adrenal cortex. A notable exception was the parathyroid gland, which lacked detectable annexin VI. Although the protein was absent in most epithelia, it was expressed strongly in certain secretory epithelia; e.g. the ductal epithelial cells of the salivary glands and non-lactating breast, and the sweat glands and their ducts. The observation that the epithelial cells of lactating breast failed to stain for annexin VI suggests functional regulation of protein expression in this tissue. However, the most interesting finding was that annexin VI expression appeared to be developmentally regulated in B- and T-lymphocyte differentiation, with negative staining in the proliferating B cells of the germinal centre of the lymph nodes, but strong staining in the mature small lymphocytes of the cortex, mantle zone and paracortex.     

Identification of the Ubiquitous Human Desmoglein, Dsg2, and the Expression Catalogue of the Desmoglein Subfamily of Desmosomal Cadherins

Desmosomes are junctions between epithelial, myocardiac, and certain other kinds of cells. They represent plasma membrane domains enriched in specific transmembrane glycoproteins, notably desmoglein (Dsg) and desmocollin (Dsc), both of which have recently been identified as members of the larger family of Ca²⁺-dependent cell adhesion molecules, the cadherins. Previously described forms of desmoglein have been isolated as proteins and cloned as cDNAs from epidermis and related stratified epithelia but have not been detected in the majority of other desmosome-containing tissues and cell culture lines. Here we present the complete cDNA-derived amino acid (aa) sequence of a different desmoglein polypeptide, termed Dsg2 (1069 aa, mol wt 116,760) and its precursor molecule (1117 aa, mol wt 122,384), which occurs in all human and bovine desmosome-producing tissues, tumors, and cell lines examined, epithelial as well as nonepithelial ones. We conclude that Dsg2, the largest molecule in this protein family, is the fundamental desmoglein common to all desmosome-possessing tissues, including simple epithelia and myocardium, and many cell cultures. Furthermore, in several tissues and cell lines Dsg2 is the only Dsg isoform detected so far. By contrast, the epidermal isoforms Dsg1 and Dsg3 are restricted to certain specialized epithelia, mostly stratified squamous ones. The importance of the junction-specific cadherin Dsg2 in tissue formation and carcinogenesis as well as in the development of autoimmune diseases of the Pemphigus type is discussed. In addition, we propose to use Dsg2 as a general marker common to all epithelial cells and tumors and to use the specific pattern of occurrence of Dsg and Dsc isoforms as an additional criterion for cell typing in tumor diagnosis.     

Molecular characterization and expression of the stratification-related cytokeratins 4 and 15

A number of human cytokeratins are expressed during the development of stratified epithelia from one-layered polar epithelia and continue to be expressed in several adult epithelial tissues. For studies of the regulation of the synthesis of stratification-related cytokeratins in internal tissues, we have prepared cDNA and genomic clones encoding cytokeratin 4, as a representative of the basic (type II) cytokeratin subfamily and cytokeratin 15, as representative of the acidic (type I) subfamily, and determined their nucleotide sequences. The specific expression of mRNAs encoding these two polypeptides in certain stratified tissues and cultured cell lines is demonstrated by Northern blot hybridization. Hybridization in situ with antisense riboprobes and/or synthetic oligonucleotides shows the presence of cytokeratin 15 mRNA in all layers of esophagus, whereas cytokeratin 4 mRNA tends to be suprabasally enriched, although to degrees varying in different regions. We conclude that the expression of the genes encoding these stratification-related cytokeratins starts already in the basal cell layer and does not depend on vertical differentiation and detachment from the basal lamina. Our results also show that simple epithelial and stratification-related cytokeratins can be coexpressed in basal cell layers of certain stratified epithelia such as esophagus. Implications of these findings for epithelial differentiation and the formation of squamous cell carcinomas are discussed.     

A function for the integrin alpha 6 beta 4 in the hemidesmosome.

Many epithelial cells appear to use cell-substratum adhesion complexes known as hemidesmosomes as the main means of anchorage to the connective tissue. Initially recognized as distinctive electron-dense images, hemidesmosomes are still poorly understood at the biochemical level. The regulation and mode of their assembly, which is disrupted in certain blistering diseases and is critical to proper wound repair, also remains to be elucidated. The integrin alpha 6 beta 4 is expressed along the basal surface of various epithelial cells. We show here that this integrin localizes to hemidesmosomes as determined by immunoelectron microscopy using antibodies directed against both the extra- and intracytoplasmic domains of alpha 6 beta 4. This result, which agrees with a recent study, suggests a functional role for the alpha 6 beta 4 integrin in the hemidesmosomes. We therefore investigated such a potential role for this integrin using the cultured rat bladder carcinoma cell line 804G, which has the uncommon ability to form hemidesmosomes in vitro when maintained on uncoated glass substrates. By immunoprecipitation and immunofluorescence, we show that 804G cells express alpha 6 beta 4 along their basal surface in a punctate pattern that overlaps with the distribution of hemidesmosomal plaque antigens. However, this pattern is altered when cells are plated in the presence of an antiserum directed against alpha 6 beta 4. Furthermore, no hemidesmosomes are detectable at the ultrastructural level in the alpha 6 beta 4 antibody-treated cells compared with control cells. These results indicate that integrins may play a critical role in assembly and adhesive functions of the hemidesmosome.     

Small proline-rich protein 1 is the major component of the cell envelope of normal human oral keratinocytes

Oral keratinocytes of buccal and gingival tissues undergo a terminal differentiation program to form a protective epithelial barrier as non-keratinized or parakeratinized stratified cells. We have examined the protein composition of cell envelopes (CEs) from normal human buccal and gingival tissues as well as keratinocytes from normal human gingival cells grown in culture. Biochemical and sequencing analyses reveal that the CEs contain 60-70% small proline-rich protein 1a/b (SPR1a/b), together with smaller amounts of involucrin, annexin I and several other known CE proteins. The data imply a specialized role for SPR1 proteins in the unique barrier function requirements of oral epithelia.     

Cytokeratins in normal lung and lung carcinomas. I. Adenocarcinomas, squamous cell carcinomas and cultured cell lines

The various epithelial cells of the lower respiratory tract and the carcinomas derived from them differ markedly in their differentiation characteristics. Using immunofluorescence microscopy and two-dimensional gel electrophoresis of cytoskeletal proteins from microdissected tissues we have considered whether cytokeratin polypeptides can serve as markers of cell differentiation in epithelia from various parts of the human and bovine lower respiratory tract. In addition , we have compared these protein patterns with those found in the two commonest types of human lung carcinoma and in several cultured lung carcinoma cell lines. By immunofluorescence microscopy, broad spectrum antibodies to cytokeratins stain all epithelial cells of the respiratory tract, including basal, ciliated, goblet, and alveolar cells as well as all tumor cells of adenocarcinomas and squamous cell carcinomas. However, in contrast, selective cytokeratin antibodies reveal cell type-related differences. Basal cells of the bronchial epithelium react with antibodies raised against a specific epidermal keratin polypeptide but not with antibodies derived from cytokeratins characteristic of simple epithelia. When examined by two-dimensional gel electrophoresis, the alveolar cells of human lung show cytokeratin polypeptides typical of simple epithelia (nos. 7, 8, 18 and 19) whereas the bronchial epithelium expresses, in addition, basic cytokeratins (no. 5, small amounts of no. 6) as well as the acidic polypeptides nos. 15 and 17. Bovine alveolar cells also differ from cells of the tracheal epithelium by the absence of a basic cytokeratin polypeptide. All adenocarcinomas of the lung reveal a "simple-epithelium-type" cytokeratin pattern (nos. 7, 8, 18 and 19). In contrast, squamous cell carcinomas of the lung contain an unusual complexity of cytokeratins. We have consistently found polypeptides nos. 5, 6, 8, 13, 17, 18 and 19 and, in some cases, variable amounts of cytokeratins nos. 4, 14 and 15. Several established cell lines derived from human lung carcinomas (SK-LU-1, Calu -1, SK-MES-1 and A-549) show a uniform pattern of cytokeratin polypeptides (nos. 7, 8, 18 and 19), similar to that found in adenocarcinomas. In addition, vimentin filaments are produced in all the cell lines examined, except for SK-LU-1.(ABSTRACT TRUNCATED AT 400 WORDS)     

Desmoplakins of Epithelial and Myocardial Desmosomes are Immunologically and Biochemically Related

Abstract. Guinea pig antibodies against desmoplakins from bovine muzzle epidermis showed specific reaction in several epithelial tissues with desmoplakin I (M_r 250,000) and desmoplakin II (M_r 215,000). By immunofluorescence microscopy, prominent punctate staining was observed in various lines of cultured epithelial cells, revealing desmosomal junctions at sites of established cell-to-cell contacts as well as hemidesmosomes and internalized desmosome-derived membrane domains. On frozen tissue sections punctate staining was observed along plasma membranes of epithelial cells, and electron microscopy using the immunoperoxidase technique revealed that the antibodies were specifically localized at the plaques associated with desmosomes and hemidesmosomes. Of a large number of non-epithelial cells examined positive staining was only observed on desmosome-like junctions of myocardial cells and Purkinje fiber cells. In both epithelial and myocardial tissues the antibodies showed a broad range of cross-reactivity between diverse vertebrate species such as man, cow, rodent, and chicken, indicating that desmoplakins contain determinants strongly conserved during evolution. When binding of these antibodies to cytoskeletal polypeptides separated by gel electrophoresis and blotted on nitrocellulose paper sheets was examined, specific reaction was noted with desmoplakin I and, to a variable degree, also desmoplakin II from various epithelial cells. Reaction was also observed with a myocardial polypeptide from bovine and human hearts which had a similar M_r value (250,000) and isoelectric pH range as desmoplakin I. We conclude that desmoplakins are the major proteins present in the desmosomal plaques of both epithelial and myocardial cells and that the desmoplakin polypeptides present in these two different cell types are very similar, if not identical.     

Desmosomal plakophilin 2 as a differentiation marker in normal and malignant tissues

Plakophilin 2 (PKP2) is a widespread protein which shows a remarkable dual location: On the one hand, it appears as a constitutive karyoplasmic protein and on the other it is a desmosomal plaque component of most, probably all, desmosome-possessing tissues and cell culture lines. Here we report on its desmosomal occurrence as revealed by immunocytochemical results obtained with three PKP2-specific murine monoclonal antibodies (mAbs) PP2-62, PP2-86 and PP2-150. These mAbs detect PKP2 in characteristic desmosomes of most normal cells, including simple and stratified epithelia as well as non-epithelial tissues such as myocardium and lymph node follicles. In addition, however, several normal tissues consistently display a differentiation-related PKP2 distribution, for example an absence of immunostaining in the "keratinizing" local specializations of the thymic epithelial reticulum, i.e. Hassall's corpuscles, and the restriction of PKP2 to the stratum basale of most stratified squamous epithelia, in contrast to its absence in upper strata, which contain PKP1- or PKP3-rich desmosomes instead. Taking advantage of the reactivity of mAb PP2-150 with formalin-fixed, paraffin-embedded material, a series of human carcinomas (n = 37) has also been analyzed. The results suggest that mAbs to PKP2 may serve as markers for the identification and characterization of carcinomas derived from--or corresponding to--simple or complex epithelia. Thus consistent PKP2 immunostaining has been observed in all 18 cases of adenocarcinomas tested, but more variable and heterogeneous staining has been noted in squamous cell carcinomas, depending on the specific tumor type. The potential value of such mAbs for cell typing in normal and embryonic tissues and for detecting cell subpopulations with different degrees of differentiation is discussed with respect to their possible application in tumor diagnosis.     

Tight junctions and compositionally related junctional structures in mammalian stratified epithelia and cell cultures derived therefrom

The occurrence of extended tight junction (TJ) structures, including zonulae occludentes (ZO), and the spatial arrangement of TJ proteins in stratified mammalian epithelia has long been controversially discussed. Therefore, we have systematically examined the localization of TJ proteins in diverse stratified epithelial tissues (e.g., epidermis, heel pad, snout, gingiva, tongue, esophagus, exocervix, vagina, urothelium, cornea) of various species (human, bovine, rodents) as well as in human cell culture lines derived from stratified epithelia, by electron microscopy as well as by immunocytochemistry at both the light and the electron microscopic level, using antibodies to TJ proteins such as occludin, claudins 1 and 4, protein ZO-1, cingulin and symplekin. We have found an unexpected diversity of TJ-related structures of which only those showing colocalization with the most restricted transmembrane TJ marker protein, occludin, are presented here. While in epidermis and urothelium occludin is restricted to the uppermost living cell layer, TJ-related junctions are abundant in the upper third or even in the majority of the suprabasal cell layers in other stratified epithelia. Interfollicular epidermis contains, in the stratum granulosum, extended, probably continuous ZO-like structures which can also be traced at least through the Henle cell layer of hair follicles. Similar apical ZO-like structures have been seen in the upper living cell layers of all other stratified epithelia and cell cultures examined, but in most of them we have noticed, in addition, junctional regions showing relatively broad, ribbon-like membrane contacts which in cross-section often appear pentalaminar, with an electron-dense middle lamella ("lamellated TJs", coniunctiones laminosae). In suprabasal layers of several stratified epithelia we have further observed TJ protein-containing junctions of variable sizes which are characterized by a 10-30-nm dense lamina interposed between the two membranes ("sandwich junctions"; iuncturae structae). Moreover, we have often observed variously sized regions in which the intermembrane distance is rather regularly bridged by short rod-like elements ("cross-bridged cell walls"; parietes transtillati), often in close vicinity of TJ-related structures or desmosomes. The significance of these structures and their possible biological importance are discussed.     

Integrin alpha 6/beta 4 complex is located in hemidesmosomes, suggesting a major role in epidermal cell-basement membrane adhesion

The alpha 6/beta 4 complex is a member of the integrin family of adhesion receptors. It is found on a variety of epithelial cell types, but is most strongly expressed on stratified squamous epithelia. Fluorescent antibody staining of human epidermis suggests that the beta 4 subunit is strongly localized to the basal region showing a similar distribution to that of the 230-kD bullous pemphigoid antigen. The alpha 6 subunit is also strongly localized to the basal region but in addition is present over the entire surfaces of basal cells and some cells in the immediate suprabasal region. By contrast staining for beta 1, alpha 2, and alpha 3 subunits was very weak basally, but strong on all other surfaces of basal epidermal cells. These results suggest that different integrin complexes play differing roles in cell-cell and cell-matrix adhesion in the epidermis. Immunoelectron microscopy showed that the alpha 6/beta 4 complex at the basal epidermal surface is strongly localized to hemidesmosomes. This result provides the first well-characterized monoclonal antibody markers for hemidesmosomes and suggests that the alpha 6/beta 4 complex plays a major role in epidermal cell-basement membrane adhesion. We suggest that the cytoplasmic domains of these transmembrane glycoproteins may contribute to the structure of hemidesmosomal plaques. Immunoultrastructural localization of the BP antigen suggests that it may be involved in bridging between hemidesmosomal plaques and keratin intermediate filaments of the cytoskeleton.     

Plakophilin 3 — a novel cell-type-specific desmosomal plaque protein

Desomosomes are cell-cell adhesion structures of epithelia and some non-epithelial tissues, such as heart muscle and the dendritic reticulum of lymph node follicles, which on their cytoplasmic side anchor intermediate filaments at the plasma membrane. Besides clusters of specific transmembrane glycoproteins of the cadherin family (desmogleins and desmocollins), they contain several desmosomal plaque proteins, such as desmoplakins, plakoglobin, and one or more plakophilins. Using recombinant DNA and immunological techniques, we have identified a novel desmosomal plaque protein that is closely related to plakophilins 1 and 2, both members of the "armadillo-repeat" multigene family, and have named it plakophilin 3 (PKP3). The product of the complete human cDNA defines a protein of 797 amino acids, with a calculated molecular weight of 87.081 kDa and an isoelectric point of pH 10.1. Northern blot analysis has shown that PKP3 mRNA has a size of approximately 2.9 kb and is detectable in the total RNA of cells of stratified and single-layered epithelia. With the help of specific poly- and monoclonal antibodies we have localized PKP3, by immunofluorescence or immunoelectron microscopy, to desmosomes of most simple and almost all stratified epithelia and cell lines derived therefrom, with the remarkable exception of hepatocytes and hepatocellular carcinoma cells. We have also determined the structure of the human PKP3 gene and compared it with that of plakophilin 1 (PKP1). Using fluorescence in situ hybridization, we have localized the human genes for the three known plakophilins to the chromosomes 1q32 (PKP1), 12p11 (PKP2) and 11p15 (PKP3). The similarities and differences of the diverse plakophilins are discussed.     

Tight junction-related structures in the absence of a lumen: occludin, claudins and tight junction plaque proteins in densely packed cell formations of stratified epithelia and squamous cell carcinomas

Tight junctions (TJs), hallmark structures of one-layered epithelia and of endothelia, are of central biological importance as intramembranous "fences" and as hydrophobic "barriers" between lumina represented by liquid- or gas-filled spaces on the one hand and the mesenchymal space on the other. They have long been thought to be absent from stratified epithelia. Recently, however, constitutive TJ proteins and TJ-related structures have also been identified in squamous stratified epithelia, including the epidermis, where they occur in special positions, most prominently in the uppermost living epidermal cell layer, the stratum granulosum. Much to our surprise, however, we have now also discovered several major TJ proteins (claudins 1 and 4, occludin, cingulin, symplekin, protein ZO-1) and TJ-related structures in specific positions of formations of epithelium-derived tissues that lack any lumen and do not border on luminal or body surfaces. Using immunohistochemistry and electron microscopy we have localized TJ proteins and structures in peripheral cells of the Hassall's corpuscles of human and bovine thymi as well as in specific central formations of tumor nests in squamous cell carcinomas, including the so-called "horn pearls". Such structures have even been found in carcinoma metastases. In carcinomas, they often seem to separate certain tumor regions from others or from stroma. The structural significance and the possible functional relevance of the locally restricted synthesis of TJ proteins and of the formations of TJ-related structures are discussed. It is proposed to include the determination of the presence or absence of such proteins and structures in the diagnostic program of tumor pathology.     

Isolation of a hemidesmosome-rich fraction from a human squamous cell carcinoma cell line

Hemidesmosomes are cell-to-matrix adhesion complexes anchoring keratinocytes to basement membranes. For the first time, we present a method to prepare a fraction from human cultured cells that are highly enriched in hemidesmosomal proteins. Using DJM-1 cells derived from human squamous cell carcinoma, accumulation of hemidesmosomes was observed when these cells were cultured for more than 10 days in a commercial serum-free medium without supplemental calcium. Electron microscopy demonstrated that numerous electron-dense adhesion structures were present along the basal cell membranes of DJM-1 cells cultured under the aforementioned conditions. After removing cellular materials using an ammonia solution, hemidesmosomal proteins and deposited extracellular matrix were collected and separated by electrophoresis. There were eight major polypeptides, which were determined to be plectin, BP230, BP180, integrin α6 and β4 subunits, and laminin-332 by immunoblotting and mass spectrometry. Therefore, we designated this preparation as a hemidesmosome-rich fraction. This fraction contained laminin-332 exclusively in its unprocessed form, which may account for the promotion of laminin deposition, and minimal amounts of Lutheran blood group protein, a nonhemidesmosomal transmembrane protein. This hemidesmosome-rich fraction would be useful not only for biological research on hemidesmosomes but also for developing a serum test for patients with blistering skin diseases.