Desmosomes and hemidesmosomes

Compositionally different desmosomes in the various compartments of the human hair follicle

Hair follicles are complex organs of the skin, in morphological and ontogenic continuity with the epidermis. We have examined the location of desmosomal cadherins and desmosomal plaque proteins in the hair follicle of adult and fetal human scalp skin by immunohistochemistry and have established a localization "map" of the hair follicle. Using antibodies against the plaque proteins desmoplakin I and II, plakoglobin, and plakophilin 1, we have found that these occur in most, if not all hair follicle desmosomes, whereas plakophilin 2 was absent, except in the basal cells of the outer root sheath, where a weak reactivity was found. By contrast, the desmosomal cadherins were mostly differentially synthesized, displaying a complicated map. While desmocollin Dsc3 was detected in all cell types examined, Dsc1 was detected only in the outer root sheath companion cell layer and the inner root sheath, and Dsc2 showed practically a mutually exclusive presence. Desmoglein Dsg2 was observed in basal cells of the outer root sheath as well as in the central cell layers of the subinfundibular outer rood sheath, matrix cells and trichocytes, in partial overlap with the otherwise different immunopositive reactions of Dsg1 and Dsg3. We have also determined when these proteins are synthesized during fetal hair follicle development. The differential molecular composition of desmosomes is discussed in relation to possible functional differences between the individual cell types.

Erythema multiforme associated human autoantibodies against desmoplakin I and II: biochemical characterization and passive transfer studies into newborn mice

The demonstration of circulating autoantibodies directed against the constitutive desmosomal plaque proteins desmoplakin (dp) I and II in mucocutaneous lesions in a subset of patients with erythema multiforme major, suggests that humoral immune mechanisms may play a role in the pathogenesis of this severe skin disease. In this study we identified a specific peptide sequence--YSYSYS--representing an antigenic binding site for the human autoantibodies. This epitope is localized at the extreme carboxy terminal domain of dp thought to be responsible for the assembly of keratin filaments with desmosomes. To test the possibility whether these antibodies may exert any pathologic effects in vivo, human autoantibodies were affinity purified on a corresponding synthetic peptide matrix and peptide-specific antibodies were raised in rabbits. After repeated subcutaneous injections into newborn mice, affinity-purified human autoantibodies and anti-peptide rabbit IgG were detected on desmosomal plaques of keratinocytes overlying the injection site. Histologic and electron microscopic examinations showed hydropic degeneration of basal and suprabasal keratinocytes, dyskeratosis, signs of suprabasal acantholysis, and keratin filaments detached from the desmosomal plaques clumping around the nucleus. We demonstrate that autoantibodies are directed to an epitope within a dp domain crucial for the interaction of keratin filaments with desmosomes, and, when injected subcutaneously into newborn mice, produce pathologic changes. These findings imply that autoantibodies to dp could impair the function of desmosome-keratin filament complexes suggesting a pathogenic role in vivo.

Aspects of the structure and assembly of desmosomes

Desmosomes are found principally in epithelial cells and consist of disc-like plaques, the extracellular face of which is paired with that of a neighbouring cell. There is increasing evidence that desmosomes are adhesive structures, and that two types of desmosomal glycoproteins, the desmogleins (Dsg) and desmocollins (Dsc) both Ca(2+)-binding cadherin-like molecules, perform this role in adhesion through interaction of their extracellular domains. A number of isoforms of Dsg and Dsc are present in specific tissues. The cytoplasmic side of the plaque is attached to intermediate filaments through desmoplakin, a major plaque protein. Also associated with desmosomes are plakoglobin and beta-catenin, suggesting that the adhesive function of desmosomes might be mediated by signal transduction. Formation of desmosomes can be studied by growing epithelial cells in low-Ca2+ medium (LCM, < 0.1 mM), where desmosomal proteins are either synthesized but not assembled, or form partially assembled but unstable half-desmosomes. Addition of Ca2+ (to about 2mM) initiates cell contact and, in the case of half-desmosomes, leads to stabilization by incorporation into membranes and formation of typical paired structures. In cases where such pre-assembled structures are not formed, recruitment of desmosomal proteins appears to occur by vesicular transport of desmocollins and desmogleins to the cell surface, where association is made with plakoglobin and later, with desmoplakin. Although much remains to be learned of the assembly process, specific interacting domains of the molecular components are being recognized. Desmosome assembly is part of a coordinated pattern of junction formation which accompanies the establishment of cell polarity, resulting in differentiation of apical and basolateral cell surfaces. Desmosomes are now being regarded, not as static and inert structures, but as membrane specializations linked to systems involved in cell-cell communication as well as adhesion.

Demystified ... adhesion molecules

The cell adhesion molecules are ubiquitous recognition molecules that allow cells to communicate with one another and their environment. Through these molecules, complex alterations in the cytoplasmic messenger pathways and the microfilamentous cytoskeleton can lead to profound alterations in cell division, differentiation, behaviour, and function (fig 9). It is difficult to conceive of a group of molecules that could be more important to pathologists and to their understanding of disease processes.

Beta- and gamma-catenin expression in thyroid carcinomas

Cadherins are calcium-dependent cell-cell adhesion molecules whose intracellular domain forms a complex with proteins required for their function, called catenins. Down-regulation of cadherins has frequently been detected in many types of human carcinomas, being associated with tumour progression. The present study investigates the immunohistochemical expression of E-cadherin and beta- and gamma-catenin in 27 human thyroid carcinomas. E-cadherin immunoreactivity was found to be decreased at cell-cell contacts in 8/15 (53 per cent) papillary, 5/7 (71 per cent) follicular, and 5/5 (100 per cent) anaplastic carcinomas. Beta-catenin membrane localization was found to be decreased in 6/15 (40 per cent) papillary, 2/7 (28 per cent) follicular, and 5/5 (100 per cent) anaplastic carcinomas. Gamma-catenin expression was partially or totally lost in 13/15 (86 per cent) papillary, 6/7 (85 per cent) follicular, and 5/5 (100 per cent) anaplastic carcinomas. A normal pattern of expression for these three molecules was observed in areas of normal tissue in each sample. These data indicate that in addition to E-cadherin, catenins are also down-regulated at cell-cell junctions in thyroid tumours and could represent potentially useful differentiation and/or transformation markers. The high frequency of alterations of gamma-catenin expression found in thyroid carcinomas suggests an important role for this gene product in thyroid carcinogenesis.

Contributions of extracellular and intracellular domains of full length and chimeric cadherin molecules to junction assembly in epithelial cells

The integrity of cell-cell junctions in epithelial cells depends on functional interactions of both extracellular and intracellular domains of cadherins with other junction proteins. To examine the roles of the different domains of E-cadherin and desmoglein in epithelial junctions, we stably expressed full length desmoglein 1 and chimeras of E-cadherin and desmoglein 1 in A431 epithelial cells. Full length desmoglein 1 was able to incorporate into or disrupt endogenous desmosomes depending on expression level. Each of the chimeric cadherin molecules exhibited distinct localization patterns at the cell surface. A chimera of the desmoglein 1 extracellular domain and the E-cadherin intracellular domain was distributed diffusely at the cell surface while the reverse chimera, comprising the E-cadherin extracellular domain and the desmoglein 1 intracellular domain, localized in large, sometimes contiguous patches at cell-cell interfaces. Nevertheless, both constructs disrupted desmosome assembly. Expression of constructs containing the desmoglein 1 cytoplasmic domain resulted in approximately a 3-fold decrease in E-cadherin bound to plakoglobin and a 5- to 10-fold reduction in the steady-state levels of the endogenous desmosomal cadherins, desmoglein 2 and desmocollin 2, possibly contributing to the dominant negative effect of the desmoglein 1 tail. In addition, biochemical analysis of protein complexes in the stable lines revealed novel in vivo protein interactions. Complexes containing beta-catenin and desmoglein 1 were identified in cells expressing constructs containing the desmoglein 1 tail. Furthermore, interactions were identified between endogenous E-cadherin and the chimera containing the E-cadherin extracellular domain and the desmoglein 1 intracellular domain providing in vivo evidence for previously predicted lateral interactions of E-cadherin extracellular domains.

Cleavage of BP180, a 180-kDa bullous pemphigoid antigen, yields a 120-kDa collagenous extracellular polypeptide

The hemidesmosome (HD) is a cell-to-substrate adhesion apparatus found in stratified and complex epithelia. One of the putative cell-matrix adhesion molecules present in the HD is the 180-kDa bullous pemphigoid antigen (BP180), also termed type XVII collagen. In our previous study, using a monoclonal antibody (mAb) 1337, we have detected a 120-kDa collagenase-sensitive polypeptide in the HD fraction (Uematsu, J. and Owaribe, K. (1993) Cell Struct. Funct. 18, 588 (abstr.)). The present study was undertaken to assess the relation of the 120-kDa polypeptide to this BP180. Immunofluorescence microscopy of bovine skin revealed the basement membrane zone of skin to be stained clearly with mAb 1337, whereas the lateral surfaces of basal cells, which were decorated by typical antibodies against BP180, were not. The antibody did not detect HDs in cultured cells but rather in the culture medium. These results indicate a localization of mAb 1337 antigen distinct from BP180. However, the same polypeptide was also recognized by monoclonal antibodies to the extracellular but not the cytoplasmic part of BP180, and found to react with a polyclonal antibody against the non-collagenous 16A domain of BP180. Therefore, the polypeptide was identified as an extracellular fragment of BP180. mAb 1337 immunoprecipitated the 120-kDa fragment from the medium, but not the 180-kDa molecule of BP180 extracted from cultured cells, indicating that the antibody specifically recognizes the fragment. The mAb 1337 apparently recognizes a unique epitope that is exposed or formed by the cleavage. Hence, the staining pattern observed for bovine skin demonstrated the presence of the 120-kDa extracellular fragment. Rotary shadow electron microscopy of affinity-purified 120-kDa fragments demonstrated that they have the unique molecular shape consisting of a central rod and a flexible tail, without the globular head that is present in the BP180 molecule. From these results, we conclude that mAb 1337 shows unique epitope specificity, recognizing only the 120-kDa extracellular fragment of BP180, which is constitutively cleaved on the cell surface as a 120-kDa fragment both in in vivo and in vitro.

Absence of PS integrins or laminin A affects extracellular adhesion, but not intracellular assembly, of hemiadherens and neuromuscular junctions in Drosophila embryos

We have examined the role of integrins in the formation of the cell junctions that connect muscles to epidermis (muscle attachments) and muscles to neurons (neuromuscular junctions). To this end we have analyzed muscle attachments and neuromuscular junctions ultrastructurally in single or double mutant Drosophila embryos lacking PS1 integrin (alphaPS1betaPS), PS2 integrin (alphaPS2betaPS), and/or their potential extracellular ligand laminin A. At the muscle attachments PS integrins are essential for the adhesion of hemiadherens junctions (HAJs) to extracellular matrix, but not for their intracellular link to the cytoskeleton. The PS2 integrin is only expressed in the muscles, but it is essential for the adhesion of muscle and epidermal HAJs to electron dense extracellular matrix. It is also required for adhesion of muscle HAJs to a less electron dense form of extracellular matrix, the basement membrane. The PS1 integrin is expressed in epidermal cells and can mediate adhesion of the epidermal HAJs to the basement membrane. The ligands involved in adhesion mediated by both PS integrins seem distinct because adhesion mediated by PS1 appears to require the extracellular matrix component laminin A, while adhesion mediated by PS2 integrin does not. At neuromuscular junctions the formation of functional synapses occurs normally in embryos lacking PS integrins and/or laminin A, but the extent of contact between neuronal and muscle surfaces is altered significantly. We suggest that neuromuscular contact in part requires basement membrane adhesion to the general muscle surface, and this form of adhesion is completely abolished in the absence of laminin A.

Immunohistochemical study of desmosomes in oral squamous cell carcinoma: correlation with cytokeratin and E-cadherin staining, and with tumour behaviour

Reduction or loss of the intercellular junctions known as desmosomes may contribute to the invasive and metastatic behaviour of various carcinomas. Previous studies have shown that metastasis of oral squamous cell carcinomas of the head and neck correlates with a reduction in immunohistochemical staining for desmoplakin and desmoglein at the invasion front. The primary aim of the present study was to extend these observations to include a third component of desmosomes, the glycoprotein desmocollin. An additional aim was to determine whether the differentiation status of tumours is reflected in their staining for cytokeratins 1, 13, and 19, and, if so, whether these parameters correlate with desmosomal staining and/or metastasis. The study included 54 primary tumours of which 28 showed lymph node metastases. The results of this investigation show that tumours can be divided into three groups according to whether they have lost staining for no, one or more than one desmosomal component. A statistically significant correlation was found between the number of desmosomal components lost and metastasis. Tumours could also be divided into five groups according to their staining for different combinations of cytokeratins. Furthermore, differentiation status as indicated both histologically and by cytokeratin staining correlated with reduced desmosomal staining and metastasis. Tumours were also examined for intensity of staining for the adhesion molecule E-cadherin. Reduction in E-cadherin staining was correlated with mode of invasion and with reduction in desmosomal staining, but not with poor differentiation as indicated by cytokeratin staining. The results of this extensive study reinforce the view that adhesive junctions and adhesion molecules contribute to the suppression of tumour invasion and metastasis.

Immunohistochemical staining for desmogleins 1 and 2 in keratinocytic neoplasms with squamous phenotype: actinic keratosis, keratoacanthoma and squamous cell carcinoma of the skin

Desmosomes are intercellular junctions that have been shown to be down-regulated in certain types of carcinoma and that may play a role in suppression of invasion and metastasis. This paper describes an immunohistochemical study of three types of epidermal neoplasms with monoclonal antibody to desmoglein in order to determine how desmosomal staining correlates with the clinical, biological and histopathological features of these neoplasms. Actinic keratosis (AK) is the most common keratinocytic premalignant neoplasm that was reported to have a 10-20% rate of malignant transformation into squamous cell carcinoma (SCC). Keratoacanthoma (KA) is a benign neoplasm that involutes spontaneously after a few months of rapid growth. SCC is a malignant tumour capable of metastasis. Electron microscope studies of KA and SCC showed significantly reduced staining for desmosomes in SCC but not in KA. We have examined staining for desmoglein using the monoclonal antibody 33-3D, a mouse IgM monoclonal antibody, that recognizes the cytoplasmic domains of desmoglein (Dsg)1 and Dsg2 on frozen sections. Immunohistochemical staining of normal skin with this antibody revealed strong pericellular localization of the antigen, outlining the cell membranes of the keratinocytes. A series of 30 AKs, 12 KAs and 24 SCCs was stained immunohistochemically with 33-3D monoclonal antibody. All examined KAs showed extensive pericellular staining for Dsg. By contrast, juxtanuclear staining for Dsg was noted in 12 SCCs, and completely negative staining in seven SCCs. The five remaining SCCs showed focal pericellular staining for the Dsg marker. The most common finding in AK was focal pericellular staining for Dsg, with complete absence of staining in dysplastic areas (25 cases). In five cases negative pericellular staining in dysplastic areas was associated with juxtanuclear accumulation of the Dsg marker. A strong negative correlation between Dsg staining and degree of dysplasia was obtained. The Dsg pattern in KA is similar to normal epidermis and shows a clear difference between KA and SCC. AK has a limited loss of Dsg expression in a SCC-like pattern that is congruent with its premalignant nature. As the stain works on frozen tissue, it may be helpful for rapid differentiation in selected cases in cutaneous oncology and Mohs micrographic surgery. This antibody may also have great potential for the detection of the effects of chemopreventive agents in skin cancer.

Expression and localization of laminin-5 subunits during mouse tooth development

Tooth morphogenesis is regulated by epithelial-mesenchymal interactions mediated by the basement membrane (BM). Laminins are major glycoprotein components of the BMs, which are involved in several cellular activities. The expression and localization of the alpha3, beta3, and gamma2 laminin-5 subunits have been analyzed by in situ hybridization and immunohistochemistry during mouse molar development. Initially (E12), mRNAs of all subunits were detected in the entire dental epithelium and the corresponding proteins were located in the BM. During cap formation (E13-14), transcripts for the alpha3 and gamma2 subunits were localized in the outer dental epithelium (ODE), whereas the beta3 subunit mRNA was present in the inner dental epithelium (IDE). During the early bell stage (E16), immunoreactivity for all subunits disappeared from the BM along the IDE, although intense signals for beta3 mRNA were detectable in cells of the IDE. Subsequently, when the dentinal matrix was secreted by odontoblasts (E18-19.5), mRNAs of all three subunits were re-expressed by ameloblasts, and the corresponding proteins were detected in ameloblasts and in the enamel matrix. Tissue recombination experiments demonstrated that when E16 IDE or ODE was associated with E18 dental papilla mesenchyme, immunostaining for all laminin-5 subunits disappeared from the BM, whereas when cultured with non-dental limb bud mesenchyme, they remained positive after 48 hr of culture. These results suggest that the temporospatial expression of laminin-5 subunits in tooth development, which appears to be differentially controlled by the dental mesenchyme, might be related to the enamel organ histo-morphogenesis and the ameloblast differentiation.

Molecular and functional analysis of cadherin-based adherens junctions

Adherens junctions are specialized forms of cadherin-based adhesive contacts important for tissue organization in developing and adult organisms. Cadherins form protein complexes with cytoplasmic proteins (catenins) that convert the specific, homophilic-binding capacity of the extracellular domain into stable cell adhesion. The extracellular domains of cadherins form parallel dimers that possess intrinsic homophilic-binding activity. Cytoplasmic interactions can influence the function of the ectodomain by a number of potential mechanisms, including redistribution of binding sites into clusters, providing cytoskeletal anchorage, and mediating physiological regulation of cadherin function. Adherens junctions are likely to serve specific, specialized functions beyond the basic adhesive process. These functions include coupling cytoskeletal force generation to strongly adherent sites on the cell surface and the regulation of intracellular signaling events.

A structural scaffolding of intermediate filaments in health and disease

The cytoplasm of animal cells is structured by a scaffolding composed of actin microfilaments, microtubules, and intermediate filaments. Intermediate filaments, so named because their 10-nanometer diameter is intermediate between that of microfilaments (6 nanometers) and microtubules (23 nanometers), assemble into an anastomosed network within the cytoplasm. In combination with a recently identified class of cross-linking proteins that mediate interactions between intermediate filaments and the other cytoskeletal networks, evidence is reviewed here that intermediate filaments provide a flexible intracellular scaffolding whose function is to structure cytoplasm and to resist stresses externally applied to the cell. Mutations that weaken this structural framework increase the risk of cell rupture and cause a variety of human disorders.

Signaling and adhesion activities of mammalian beta-catenin and plakoglobin in Drosophila

The armadillo protein of Drosophila and its vertebrate homologues, beta-catenin and plakoglobin, are implicated in cell adhesion and wnt signaling. Here, we examine the conservation of these two functions by assaying the activities of mammalian beta-catenin and plakoglobin in Drosophila. We show that, in the female germ line, both mammalian beta-catenin and plakoglobin complement an armadillo mutation. We also show that shotgun mutant germ cells (which lack Drosophila E-cadherin) have a phenotype identical to that of armadillo mutant germ cells. It therefore appears that armadillo's role in the germ line is solely in a complex with Drosophila E-cadherin (possibly an adhesion complex), and both beta-catenin and plakoglobin can function in Drosophila cadherin complexes. In embryonic signaling assays, we find that plakoglobin has no detectable activity whereas beta-catenin's activity is weak. Surprisingly, when overexpressed, either in embryos or in wing imaginal disks, both beta-catenin and plakoglobin have dominant negative activity on signaling, an effect also obtained with COOH-terminally truncated armadillo. We suggest that the signaling complex, which has been shown by others to comprise armadillo and a member of the lymphocyte enhancer binding factor-1/T cell factor-family, may contain an additional factor that normally binds to the COOH-terminal region of armadillo.

Desmosomes are regulated by protein kinase C in primary rat epithelial cells

In the present study, we addressed the possible relevance of protein kinase C (PKC) in the regulation of intracytoplasmic desmosome assembly. Treatment of cultured rat lingual and epidermal keratinocytes with a potent and highly selective PKC inhibitor (GF109203X) induced an increase in granular labelling for major desmosomal proteins, desmoplakins, desmoglein and plakoglobin, both intracellularly and at the cell surface. This was associated with the formation of ultrastructurally recognizable desmosomes deep in the cytoplasm and increase in intercellular desmosome number. In contrast, PKC activation upon short exposure to 12-O-tetradecanoylphorbol 13-acetate (TPA) resulted in altered cell morphology, loss of intercellular contact and accumulation of desmosomal proteins in the juxtanuclear zone. On the other hand, PKC depletion by long term TPA treatment re-established cell-cell contact, where desmosomal markers were exclusively redistributed. Taken together, these results suggest that inhibition of PKC is required for intracytoplasmic as well as intercellular desmosome assembly, whereas its activation may regulate disassembly process.

TGFbeta3 promotes transformation of chicken palate medial edge epithelium to mesenchyme in vitro

Epithelial-mesenchymal transformation plays an important role in the disappearance of the midline line epithelial seam in rodent palate, leading to confluence of the palate. The aim of this study was to test the potential of the naturally cleft chicken palate to become confluent under the influence of growth factors, such as TGFbeta3, which are known to promote epithelial-mesenchymal transformation. After labeling medial edge epithelia with carboxyfluorescein, palatal shelves (E8-9) with or without beak were dissected and cultured on agar gels. TGFbeta1, TGFbeta2 or TGFbeta3 was added to the chemically defined medium. By 24 hours in culture, medial edge epithelia form adherent midline seams in all paired groups without intact beaks. After 72 hours, seams in the TGFbeta3 groups disappear and palates become confluent due to epithelial-mesenchymal transformation, while seams remain mainly epithelial in control, TGFbeta1 and TGFbeta2 groups. Epithelium-derived mesenchymal cells are identified by carboxyfluorescein fluorescence with confocal microscopy and by membrane-bound carboxyfluorescein isolation bodies with electron microscopy. Labeled fibroblasts completely replace the labeled epithelia of origin in TGFbeta3-treated palates without beaks. Single palates are unable to undergo transformation, and paired palatal shelves with intact beaks do not adhere or undergo transformation, even when treated with TGFbeta3. Thus, physical contact of medial edge epithelia and formation of the midline seam are necessary for epithelial-mesenchymal transformation to be triggered. We conclude that there may be no fundamental difference in developmental potential of the medial edge epithelium for transformation to mesenchyme among reptiles, birds and mammals. The bird differs from other amniotes in having developed a beak and associated craniofacial structures that seemingly keep palatal processes separated in vivo. Even control medial edge epithelia partly transform to mesenchyme if placed in close contact. However, exogenous TGFbeta3 is required to achieve complete confluence of the chicken palate.

Evidence that "pinin", reportedly a differentiation-specific desmosomal protein, is actually a widespread nuclear protein

A protein recently described as a desmosome-specific molecule involved in anchoring intermediate-sized filaments (IFs) to the desmosomal plaque, and hence named "pinin" [43], has been known in our laboratory for several years as a strictly nuclear protein occurring in a wide range of cell types, including many that are totally devoid of desmosomes. Using a series of specific antibodies we have localized the protein in the nucleoplasm of cultured cells, blood cells and solid tissues of diverse vertebrate species, from oocytes to erythrocytes of amphibia and from liver to connective tissue and fibroblasts in mammals. Desmosomes have consistently been negative, and the nuclear specificity of the immunolocalization reactions has also been directly demonstrated by double-label immunofluorescence microscopy. From our results we conclude that this nuclear protein, characterized by a domain exceptionally rich in serine residues and hence termed DRS-protein, occurs in at least two genetically different forms in a diffusible state as well as in special ribonucleoprotein-particles, "speckles" [6], and is a widespread if not ubiquitous nuclear protein. Consequently it must serve nuclear functions rather than "pinning" IFs to plasma membranes and does not provide a new reliable marker for desmosomes and epithelial or myocardial differentiation.

The amino-terminal domain of desmoplakin binds to plakoglobin and clusters desmosomal cadherin-plakoglobin complexes

The desmosome is a highly organized plasma membrane domain that couples intermediate filaments to the plasma membrane at regions of cell-cell adhesion. Desmosomes contain two classes of cadherins, desmogleins, and desmocollins, that bind to the cytoplasmic protein plakoglobin. Desmoplakin is a desmosomal component that plays a critical role in linking intermediate filament networks to the desmosomal plaque, and the amino-terminal domain of desmoplakin targets desmoplakin to the desmosome. However, the desmosomal protein(s) that bind the amino-terminal domain of desmoplakin have not been identified. To determine if the desmosomal cadherins and plakoglobin interact with the amino-terminal domain of desmoplakin, these proteins were co-expressed in L-cell fibroblasts, cells that do not normally express desmosomal components. When expressed in L-cells, the desmosomal cadherins and plakoglobin exhibited a diffuse distribution. However, in the presence of an amino-terminal desmoplakin polypeptide (DP-NTP), the desmosomal cadherins and plakoglobin were observed in punctate clusters that also contained DP-NTP. In addition, plakoglobin and DP-NTP were recruited to cell-cell interfaces in L-cells co-expressing a chimeric cadherin with the E-cadherin extracellular domain and the desmoglein-1 cytoplasmic domain, and these cells formed structures that were ultrastructurally similar to the outer plaque of the desmosome. In transient expression experiments in COS cells, the recruitment of DP-NTP to cell borders by the chimera required co-expression of plakoglobin. Plakoglobin and DP-NTP co-immunoprecipitated when extracted from L-cells, and yeast two hybrid analysis indicated that DP-NTP binds directly to plakoglobin but not Dsg1. These results identify a role for desmoplakin in organizing the desmosomal cadherin-plakoglobin complex and provide new insights into the hierarchy of protein interactions that occur in the desmosomal plaque.

Differential requirement of Grb2 and PI3-kinase in HGF/SF-induced cell motility and tubulogenesis

Hepatocyte growth factor/scatter factor (HGF/SF) is a multifunctional cytokine that induces mitogenesis, motility, invasion, and morphogenesis of several epithelial and endothelial cell lines in culture. The receptor for HGF/SF has been identified as the Met tyrosine kinase. To investigate the signaling pathways that are involved in these events, we have generated chimeric receptors containing the extracellular domain of the colony stimulating factor-1 (CSF-1) receptor fused to the transmembrane and intracellular domains of the Met receptor (MET). Madin-Darby canine kidney (MDCK) epithelial cells, expressing the CSF-MET chimera dissociate, scatter and form branching tubules in response to CSF-1. However, cells expressing a mutant CSF-MET receptor containing a phenylalanine substitution for tyrosine 1356 (Y1356F) are unable to scatter or form branching tubules following stimulation with CSF-1. Tyrosine 1356 is essential for the recruitment of multiple substrates including Grb2, the p85 subunit of PI3-kinase, and PLC gamma. To investigate the role of these signaling pathways, we have generated a mutant receptor that selectively fails to associate with Grb2, and have treated MDCK cells with potent inhibitors of PLC gamma, PI3-kinase, and p70S6K, a downstream target of PI3-kinase. Our results implicate pathways downstream from PI3-kinase in cell dissociation and scatter, whereas pathways downstream from Grb2 are required for branching tubulogenesis in MDCK cells.

Regulation of desmosomal cell adhesion in human tumour cells by polyunsaturated fatty acids

Desmosomes are key structures in cell-cell adhesion. In this study we examined the effect of n-6 essential fatty acids on the expression of desmoglein (Dsg), desmosomal cadherin and the formation of desmosomes in E-cadherin negative human breast, colon and lung cancer cells and melanoma cells. Electron microscopy revealed that cells cultured with gamma linolenic acid (GLA) showed increased cell-cell adhesion together with an increase in the formation of desmoglein-containing desmosomes. Western blotting studies of cellular proteins demonstrated that, following culture with fatty acids, Dsg expression was modified, with the greatest increase seen after GLA treatment. Other fatty acids increased Dsg expression, but to a lesser extent. It is concluded that GLA regulates desmosome-mediated cell-cell adhesion in human cancer cells, particularly in cells without E-cadherin.

Cloning and analysis of cDNA encoding murine pinin

Pinin is a cell junction-associated protein involved in the stabilization of the desmosome-intermediate filament complex in various epithelial tissues. Utilizing a cDNA probe derived from canine pinin, we isolated overlapping cDNA clones encoding murine full-length pinin. The total cDNA contained an open reading frame of 2175 nucleotides coding for 725 amino acids as well as a 3'- and a 5'-untranslated regions of 620 and 18 nucleotides, respectively. The overall predicted amino acid sequence of mouse pinin displayed strong identities to those of canine and human pinin, with the exception of a stretch of 38 amino acids which were found to be deleted in mouse pinin. There were several discernible domains found within mouse pinin. These included three coiled-coil domains, a small stretch of glycine loops, a short glutamine-proline-rich domain and a polyserine domain.

IAP insertion in the murine LamB3 gene results in junctional epidermolysis bullosa

The laminin-5 molecule functions in the attachment of various epithelia to basement membranes. Mutations in the laminin-5-coding genes have been associated with Herlitz junctional epidermolysis bullosa (HJEB), a severe and often lethal blistering disease of humans. Here we report the characterization of a spontaneous mouse mutant with an autosomal recessive blistering disease. These mice exhibit sub-epithelial blisters of the skin and mucosal surfaces and abnormal hemidesmosomes lacking sub-basal dense plates. By linkage analysis the genetic defect was localized to a 2-cM region on distal Chromosome (Chr) 1 where a laminin-5 subunit gene, LamB3, was previously localized. LamB3 mRNA and laminin-5 protein were undetectable by Northern blot analysis and immunohistochemical methods, respectively. DNA sequence analysis indicated that the LamB3 genetic defect resulted from disruption of the coding sequence by insertion of an intracisternal-A particle (IAP) at an exon/intron junction. These findings suggest a role for laminin-5 in hemidesmosome formation and indicate that the LamB3(IAP) mutant is a useful mouse model for HJEB.

The epithelial cell default-phenotype hypothesis and its implications for cancer

The expression of epithelial cell adhesion and cytoskeletal genes is orchestrated by an apparently unique set of rules. No tissue-specific transactivator proteins have been found to drive them; only ubiquitous factors are utilized. In non-epithelial cells, they are actively repressed. Moreover, it was recently found that a single protein (adenovirus E1a) coordinately represses non-epithelial genes while inducing epithelial genes. A simple model is offered to explain how epithelial gene expression is coordinated. Under this model, the epithelial cell gene expression program is a transcriptional 'default'; that is, it occurs in the absence of tissue-specific transactivation. Conversion to this default requires only that mesenchymal transactivators are not expressed, or that central 'integrator' proteins are inactive. In their absence, mesenchymal gene expression cannot occur. Moreover, because the repressors cease to be expressed, the epithelial genes are induced. Oncogenes generally cause the breakdown of the epithelial phenotype--generating carcinomas--so genes such as E1a that cause epithelial conversion may prove useful for both understanding and controlling cancer.

Direct Ca2+-dependent heterophilic interaction between desmosomal cadherins, desmoglein and desmocollin, contributes to cell-cell adhesion

Human fibrosarcoma cells, HT-1080, feature extensive adherens junctions, lack mature desmosomes, and express a single known desmosomal protein, Desmoglein 2 (Dsg2). Transfection of these cells with bovine Desmocollin 1a (Dsc1a) caused dramatic changes in the subcellular distribution of endogenous Dsg2. Both cadherins clustered in the areas of the adherens junctions, whereas only a minor portion of Dsg2 was seen in these areas in the parental cells. Deletion mapping showed that intact extracellular cadherin-like repeats of Dsc1a (Arg1-Thr170) are required for the translocation of Dsg2. Deletion of the intracellular C-domain that mediates the interaction of Dsc1a with plakoglobin, or the CSI region that is involved in the binding to desmoplakin, had no effect. Coimmunoprecipitation experiments of cell lysates stably expressing Dsc1a with anti-Dsc or -Dsg antibodies demonstrate that the desmosomal cadherins, Dsg2 and Dsc1a, are involved in a direct Ca2+-dependent interaction. This conclusion was further supported by the results of solid phase binding experiments. These showed that the Dsc1a fragment containing cadherin-like repeats 1 and 2 binds directly to the extracellular portion of Dsg in a Ca2+-dependent manner. The contribution of the Dsg/ Dsc interaction to cell-cell adhesion was tested by coculturing HT-1080 cells expressing Dsc1a with HT-1080 cells lacking Dsc but expressing myc-tagged plakoglobin (MPg). In the latter cells, MPg and the endogenous Dsg form stable complexes. The observed specific coimmunoprecipitation of MPg by anti-Dsc antibodies in coculture indicates that an intercellular interaction between Dsc1 and Dsg is involved in cell-cell adhesion.

The zinc-finger protein slug causes desmosome dissociation, an initial and necessary step for growth factor-induced epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is an essential morphogenetic process during embryonic development. It can be induced in vitro by hepatocyte growth factor/scatter factor (HGF/SF), or by FGF-1 in our NBT-II cell model for EMT. We tested for a central role in EMT of a zinc-finger protein called Slug. Slug mRNA and protein levels were increased transiently in FGF-1-treated NBT-II cells. Transient or stable transfection of Slug cDNA in NBT-II cells resulted in a striking disappearance of the desmosomal markers desmoplakin and desmoglein from cell-cell contact areas, mimicking the initial steps of FGF-1 or HGF/SF- induced EMT. Stable transfectant cells expressed Slug protein and were less epithelial, with increased cell spreading and cell-cell separation in subconfluent cultures. Interestingly, NBT-II cells transfected with antisense Slug cDNA were able to resist EMT induction by FGF-1 or even HGF/SF. This antisense effect was suppressed by retransfection with Slug sense cDNA. Our results indicate that Slug induces the first phase of growth factor-induced EMT, including desmosome dissociation, cell spreading, and initiation of cell separation. Moreover, the antisense inhibition experiments suggest that Slug is also necessary for EMT.

Frontiers in keratodermas: pushing the envelope

A clinically and genetically heterogeneous group of disorders, known collectively as the palmoplantar keratodermas, are unified by the phenotypic characteristic of a thickening of the skin over the palms and soles. Although spectacular progress has been made in understanding the basis of many genodermatoses, the genetic defects causing many of the keratodermas are still largely unknown. These unusual phenotypes are beginning to capture the attention of investigators in epidermal biology, and several compelling lines of evidence point to the cornified cell envelope and structural components of the desmosome as potential underlying targets of disease. It is anticipated that understanding the molecular basis of the keratodermas will underscore the importance of the integrity of the cell envelope and the desmosome, and provide new insights into the mechanisms of epidermal differentiation and related disorders.

Polarisation-dependent association of plectin with desmoplakin and the lateral submembrane skeleton in MDCK cells

The intermediate filament-binding protein plectin and cytokeratin were localised at the cellular periphery of fully polarised Madin-Darby canine kidney (MDCK) cells, whereas vimentin was primarily found in a perinuclear network. Confocal and immunoelectron microscopy revealed that plectin was restricted to areas underlying the lateral plasma membrane. It colocalised with fodrin, a component of the submembrane skeleton, and was closely associated with desmosomal plaque structures. Biochemically, plectin was shown to interact directly with immunoprecipitated desmoplakin in vitro. Upon loss of cell polarity in low calcium medium, plectin redistributed to a cytoplasmic vimentin- and cytokeratin-related network, clearly distinct from diffusely distributed fodrin and internalised desmoplakin structures. The structural reorganisation of plectin was also reflected by an increased solubility of the protein in Triton X-100/high salt, and a decrease in its half-life from approximately 20 to approximately 5 hours. Furthermore, unlike cytokeratins and vimentin, desmoplakin and fodrin did not associate with plectin attached to magnetic beads in cell lysates of unpolarised cells, while all proteins formed a stable complex in polarised cells. Altogether, these data indicate that plectin is involved in the anchorage of intermediate filaments to desmosomes and to the submembrane skeleton in polarised MDCK cells.

Hyperphosphorylation of beta-catenin on serine-threonine residues and loss of cell-cell contacts induced by calyculin A and okadaic acid in human epidermal cells

Phosphorylation and dephosphorylation events may critically control junction assembly and stability, as well as regulate the formation of the cadherin-cytoskeleton complex, thus influencing the adhesive function of cells. In the present study, we have used specific activators and inhibitors of protein kinases and phosphatases to analyze the role of protein phosphorylation in the maintenance of epithelial architecture. Okadaic acid and calyculin A cell treatments induced two major effects: a dramatic alteration of the keratin network of epidermal cells and a complete disruption of cell-cell contacts. This loss in cell-cell contacts was not tissue and species restricted and the interactions of keratinocytes with the matrix were not involved. The observed changes were highly specific for these drugs and were obtained in the range of concentrations corresponding to the inhibition of protein phosphatase 1 (PP1). They were time- and dose-dependent, and reversible, excluding a cytotoxic effect of the drugs. A decrease in electrophoretic mobility of beta-catenin, a major protein involved in the regulation of intercellular adherens junctions, was observed in keratinocytes and fibroblasts treated with okadaic acid and calyculin A, suggesting a change in the protein phosphorylation level and/or protein conformation. Data from beta-catenin immunocomplex autoradiography performed after 32P in vivo incorporation in untreated and okadaic acid or calyculin A-treated HaCaT cells, demonstrated a higher level of phosphorylation of beta-catenin in treated cells compared to untreated ones. Analysis of 32P-labeled phosphoaminoacids demonstrated that beta-catenin was exclusively phosphorylated on serine-threonine residues but not on tyrosine residues. Immunoprecipitations and Western blotting using anti-phosphoserine and anti-phosphotyrosine antibodies confirmed these data. The change in beta-catenin phosphorylation on serine-threonine residues may play a role in the control of the cohesion between epithelial cells and may be involved in the regulation of the transduction signal.

Keith R. Porter Lecture, 1996. Of mice and men: genetic disorders of the cytoskeleton

Since the time when I was a postdoctoral fellow under the supervision of Dr. Howard Green, then at the Massachusetts Institute of Technology, I have been interested in understanding the molecular mechanisms underlying growth, differentiation, and development in the mammalian ectoderm. The ectoderm gives rise to epidermal keratinocytes and to neurons, which are the only two cell types of the body that devote most of their protein-synthesizing machinery to developing an elaborate cytoskeletal architecture composed of 10-nm intermediate filaments (IFs). Our interest is in understanding the architecture of the cytoskeleton in keratinocytes and in neurons, and in elucidating how perturbations in this architecture can lead to degenerative diseases of the skin and the nervous system. I will concentrate on the intermediate filament network of the skin and its associated genetic disorders, since this has been a long-standing interest of my laboratory at the University of Chicago.

The subcellular distribution of the high molecular mass protein, HD1, is determined by the cytoplasmic domain of the integrin beta 4 subunit

The high molecular mass protein, HD1, is a structural protein present in hemidesmosomes as well as in distinct adhesion structures termed type II hemidesmosomes. We have studied the distribution and expression of HD1 in the GD25 cells, derived from murine embryonal stem cells deficient for the beta 1 integrin subunit. We report here that these cells possess HD1 but not BP230 or BP180; two other hemidesmosomal constituents, and express only traces of the alpha 6 beta 4 integrin. By immunofluorescence and interference reflection microscopy HD1 was found together with vinculin at the end of actin filaments in focal contacts. In OVCAR-4 cells, derived from a human ovarian carcinoma which, like GD25 cells, only weakly express alpha 6 beta 4, HD1 was also localized in focal contacts. Upon transfection of both GD25 and OVCAR-4 cells with cDNA for the human beta 4 subunit the subcellular distribution of HD1 changed significantly. HD1 is then no longer present in focal contacts but in other structures at cell-substrate contacts, colocalized with alpha 6 beta 4. These junctional complexes are probably the equivalent of the type II hemidesmosomes. Transfection of GD25 cells with beta 1 cDNA did not affect the distribution of HD1, which indicates that the localization of HD1 in focal contacts was not due to the absence of beta 1. Moreover, in GD25 cells transfected with cDNA encoding a beta 4/beta 1 chimera, in which the cytoplasmic domain of beta 4 was replaced by that of beta 1, the distribution of HD1 was unaffected. Our findings indicate that the cytoplasmic domain of beta 4 determines the subcellular distribution of HD1 and emphasize the important role of alpha 6 beta 4 in the assembly of hemidesmosomes and other junctional adhesive complexes containing HD1.

Protein phosphatases maintain the organization and structural interactions of hepatic keratin intermediate filaments

The importance of protein phosphatases in the maintenance of cytoskeletal structure is supported by the serious liver injury caused by microcystin-LR, a hepatotoxic inhibitor of type-1 and type-2A serine/threonine protein phosphatases. We used the microcystin-LR-induced cell injury as a model to study the roles of protein dephosphorylation in maintaining cytoskeletal structure and cellular interactions in primary rat hepatocyte cultures. Confocal microscopy revealed that the first visible effect of microcystin-LR is disruption of desmoplakin organization at the cell surface, indicating dissociation of desmosomes. This effect is followed by a dramatic reorganization of both the intermediate filament (keratins 8 and 18) and microfilament networks, resulting in a merged structure in which the intermediate filaments are organized around a condensed actin core. Keratin 8, keratin 18 and desmoplakin I/II are the major cytoskeleton-associated targets for microcystin-LR-induced phosphorylation. Hyperphosphorylation of keratin 8 and 18 is accompanied by an increased keratin solubility, which correlates with the observed morphological effects. Phosphopeptide mapping shows that four specific tryptic phosphopeptides are highly phosphorylated predominantly in the soluble pool of keratin 18, whereas keratin 8 shows no indications of such assembly state-specific sites. Phosphopeptide maps of keratins phosphorylated in vivo and in vitro indicate that Ca2+/calmodulin-dependent kinase may be involved in regulating the serine-specific phosphorylation of both keratin 8 and keratin 18, while cAMP-dependent protein kinase does not seem to play a major role in this context. Taken together, our results show that the interactions between keratin intermediate filaments and desmosomes as well as the assembly states of their main constituent proteins, are directly regulated by serine/threonine kinase/phosphatase equilibria.

JSID Tanioku Memorial Lecture 1996. Genetic disorders of keratins and their associated proteins

It has recently been demonstrated that genetic defects in keratin genes cause a number of different skin disorders, including epidermolysis bullosa simplex (EBS), epidermolytic hyperkeratosis (EH), the EH form of epidermal nevi, epidermolytic and non-epidermolytic forms of palmoplantar keratoderma (EPPK and PPK) and pachyonychia congenita (PC). In this review, I describe the research that led to this discovery.

Mouse embryonic submandibular gland epithelium loses its tissue integrity during early branching morphogenesis

During the development of the mouse submandibular gland, the epithelium undergoes not only shape changes to produce extensively branched lobules and stalk, but also changes in cell arrangement from a cell mass to a cavitated cell sheet. The present study examined the organization in the developing epithelium of intercellular adhesion systems and of actin-containing microfilaments. E-cadherin and beta-catenin, which are components of cell-to-cell adherens junctions in epithelial cells, were distributed along the cell periphery of almost the entire epithelium of the submandibular gland at all stages examined and were mainly localized at the apical region of the oral epithelium. Actin-containing microfilaments, which are associated with cell-to-cell adherens junctions, showed a distribution similar to that of those molecules. In contrast, although the distributions of desmoplakins I/II, major desmosomal proteins, and ZO-1 (a tight junction protein) were seen in the oral epithelium and proximal stalk of the submandibular gland epithelium, signals representing these molecules were absent from or much reduced in the submandibular gland epithelium of the cell mass at the 12- and 13-day stages. In the 14-day gland, they strongly appeared in the cells facing the appearing lumens, whereas they were weakly scattered within the terminal lobules that were still a part of the cell mass. These findings suggest that cell-to-cell adhesion systems are differentially regulated during the epithelial morphogenesis of the submandibular gland and that the integrity of the submandibular gland epithelium is lost during the early stages of development, indicating the tissue to be a rather plastic structure.

Characterization of pinin, a novel protein associated with the desmosome-intermediate filament complex

We have identified a protein named pinin that is associated with the mature desmosomes of the epithelia (Ouyang, P., and S.P. Sugrue. 1992. J. Cell Biol. 118:1477-1488). We suggest that the function of pinin is to pin intermediate filaments to the desmosome. Therefore, pinin may play a significant role in reinforcing the intermediate filament-desmosome complex. cDNA clones coding for pinin were identified, using degenerative oligonucleotide probes that were based on the internal amino acid sequence of pinin for the screening of a cDNA library. Immunoblotting of expressed recombinant proteins with the monoclonal 08L antibody localized the 08L epitope to the carboxyl end of the protein. Polyclonal antibodies directed against fusion proteins immunoidentified the 140-kD protein in tissue extracts. Immunofluorescence analysis, using the antifusion protein antibody, demonstrated pinin at lateral epithelial boundaries, which is consistent with desmosomal localization. The conceptual translation product of the cDNA clones contained three unique domains: (a) a serine-rich domain; (b) a glutamine-proline, glutamine-leucine repeat domain; and (c) an acidic domain rich in glutamic acid. Although the 3' end of the open reading frame of the clone for pinin showed near identity to a partial cDNA isolated for a pig neutrophil phosphoprotein (Bellavite, P., F. Bazzoni, et al. 1990. Biochem. Biophys. Res. Commun. 170:915-922), the remaining sequence demonstrated little homology to known protein sequences. Northern blots of mRNA from chicken corneal epithelium, MDCK cells, and various human tissues indicated that pinin messages exhibit tissue-specific variation in size, ranging from 3.2 to 4.1 kb. Genomic Southern blots revealed the existence of one gene for pinin, suggesting alternative splicing of the mRNA. Expression of the full-length cDNA clones in human 293 cells and monkey COS-7 cells demonstrated that a 140-kD immunoreactive species on Western blots corresponded to pinin. Pinin cDNA transfected into the transformed 293 cells resulted in enhanced cell-cell adhesion. Immunofluorescence staining revealed that the expressed pinin protein was assembled to the lateral boundaries of the cells in contact, which is consistent with the staining pattern of pinin in epithelial cells.

Plakophilins 2a and 2b: constitutive proteins of dual location in the karyoplasm and the desmosomal plaque

Using antibodies and recombinant DNA techniques, we have identified plakophilin 2, a novel desmosomal plaque protein of M(r) 100,000 (estimated from SDS-PAGE), which is a member of the arm-repeat family of proteins and can occur in two splice forms (2a and 2b) because of the insertion of a 44 amino acid (aa)-encoding exon. In its aa sequence (837 and 881 aa, calculated pIs: 9.33 and 9.38, mol wts 92,750 and 97,410 kD), it is conspicuously related to the 80-kD plakophilin 1, with which it shares a central region of 9 repeats of the arm-motif, preceeded by a long head region and followed by a very short (11 aa) carboxy-terminal sequence. Plakophilin 2 and its mRNA have been detected in a wide range of tissues and cell types, including cells devoid of desmosomes. By light and electron microscopical immunolocalization, plakophilin 2 has been localized to plaques of desmosomes of one-layered ("simple") and complex epithelia, carcinomas, diverse epithelium-derived cell culture lines, as well as cardiac tissue and the dendritic reticulum cells of lymphatic germinal centers, i.e., desmosomes in which plakophilin 1 is not detected. However, plakophilin 2 has also been localized in the desmosomes of certain but not all stratified epithelia where it coexists with plakophilin 1. Remarkably, plakophilin 2 is also enriched in the karyoplasm of a wide range of cell types, including many that lack desmosomes and in which, therefore, the nuclear state is the only locally enriched form of plakophilin 2 present. We conclude that plakophilins 2a and 2b are basic nuclear proteins that in certain cell types additionally assemble with other proteins to form the desmosomal plaque and serve general nuclear functions as well as a function specific to many but not all desmosomes.

Cloning and characterization of a new armadillo family member, p0071, associated with the junctional plaque: evidence for a subfamily of closely related proteins

Cell contacts of the adherens type are organized around transmembrane proteins of the cadherin family. Whereas the extracellular domains mediate homophilic interactions between cadherins of neighbouring cells the cytoplasmic domains organize a set of proteins into the junctional plaque. Among these junctional plaque proteins are members of the armadillo gene family, beta-catenin, plakoglobin (gamma-catenin), B6P/plakophilin and p120. These proteins are assumed to play a key role in cell cell signalling through intercellular junctions. Here we report cloning of a cDNA encoding a new armadillo family member, p0071, closely related to p120 and B6P/plakophilin and more distantly related to armadillo, plakoglobin, beta-catenin and other members of the gene family. The deduced amino acid sequence encodes a basic protein of 1,211 amino acids with a central armadillo repeat region which is conserved in sequence and organization of its ten individual motifs between p120, B6P/plakophilin and p0071. In contrast the end domains of the three proteins are variable in size and sequence. The RNA coding for p0071 is expressed in all tissues examined. Using antibodies generated against the armadillo repeat region of the protein we show that p0071 is localized at cell-cell borders and is expressed in the desmosomal plaque of some cultured epithelial cells. The protein seems to be an accessory component of the desmosomal plaque as well as of other adhesion plaques and might be involved in regulating junctional plaque organization and cadherin function. Our data provide evidence for a subfamily of armadillo related proteins that share not only structural features but also have in common their localisation in the junctional plaque. We therefore suggest that family members exert similar functions and might be involved in cell signalling through cell contacts.

Immunolocalization of neurophysin in cytokeratin-positive luteal cells of cows

We have recently detected a subgroup of cytokeratin (CK)-positive luteal cells in the bovine corpus luteum of the early and mid-luteal phase, but not in that of pregnancy. Since, according to the literature, neurophysin (NP)-positive luteal cells behave comparably, simple immunohistochemistry and double labeling were used to identify in serial sections whether the presence of NP co-incided with that of CK. The numbers of CK-positive cells and NP-positive luteal cells were comparable throughout the estrous cycle, decreasing from early to late luteal phase. While few CK-positive cells were found in the former thecal layer during the early luteal phase, many CK-positive cells appeared in the former granulosal layer. NP-positive cells were only detected in the former granulosal layer. During the mid-luteal phase, the CK-positive cells consisted of small and large luteal cells, but only large NP-positive cells were found. Roughly 80% of the large CK-positive cells contained NP, whereas CK was lacking in more than 50% of the NP-positive cells. The corpora lutea of pregnancy contained neither CK-positive nor NP-positive cells. The significance of the simultaneous occurrence of CK and NP remains to be elucidated.

Ultrastructural characterization of internal basement membrane of junctional epithelium at dentogingival border

BACKGROUND

Although a number of electron microscopic observations of the dentogingival border have been reported, observation of the detailed ultrastructure of the internal basement membrane of the junctional epithelium at the border has not yet been done.

METHODS

The internal basement membrane of a rhesus monkey (Macaca mulatta) was ultrastructurally characterized with high-resolution electron microscopy. The head and neck region of a 5-year-old animal was perfused with a fixative containing 1.25% glutaraldehyde, 0.75% formaldehyde, and 0.04% CaCl2 in 0.1 M phosphate buffer, pH 7.4. Isolated mandibular segments were further fixed with 2.5% glutaraldehyde, 2% formaldehyde, and CaCl2 (300 mg/l) in 0.1 M sodium cacodylate buffer, pH 7.4, for 24 hr at 4 degrees C. The segments were then demineralized and cut into small pieces that were postfixed with 1% osmium tetroxide and further processed for electron microscopy.

RESULTS

The internal basement membrane takes the form of both a common basement membrane and, in places, a multilayered basement membrane. This basement membrane was found to have fundamental ultrastructural features common to other basement membranes. That is, the bulk of the basement membrane is composed of a network of irregular anastomosing strands referred to as "cords." Heparan sulfate proteoglycan-containing ribbonlike 4.5-5-nm-wide "double tracks" were also present in this basement membrane. However, the lamina densa of this basement membrane was unusually thick (160 nm) compared with the thickness (30-60 nm) in other basement membranes, and the cords were also much thicker (8.3 +/- 1.9 nm vs. the usual 3-5 nm).

CONCLUSIONS

From these results, it is concluded that this particular basement membrane is specialized for mechanical strength. This specialization is likely to be suitable for the purpose of the tight sealing of the periodontal tissues from the oral environment.

Plectin and human genetic disorders of the skin and muscle. The paradigm of epidermolysis bullosa with muscular dystrophy

Recent progress in understanding the molecular organization of the cutaneous basement membrane zone (BMZ) has revealed an intricate network of structural proteins necessary for stable association of the epidermis to the underlying dermis. Molecular genetics of the cutaneous BMZ has also revealed that defects in as many as nine distinct genes within the dermal-epidermal junction which result in different forms of epidermolysis bullosa (EB), a group of heritable mechano-bullous disorders. We have recently demonstrated that a variant of EB associated with late-onset development of muscular dystrophy (EB-MD, MIM no. 226670) results from mutations in the gene encoding plectin (PLEC1), a cytoskeleton associated attachment protein present in the hemidesmosomal inner plaque and the sarcolemma of the muscle. Consequently, mutations in this multi-functional gene/protein system can result in phenotypic manifestations of EB-MD both in the skin and the muscle. In this overview, we will summarize the domain organization of plectin and the structure of the corresponding gene (PLEC1), as well as the genetic basis of EB-MD in families studied thus far. Elucidation of the molecular basis of this subtype of EB adds to our understanding of the structural and functional complexity of the cutaneous BMZ.

The role of the cytoplasmic domain of alpha 6 integrin in the assembly and function of alpha 6 beta 1 and alpha 6 beta 4

We have studied the role of the cytoplasmic domain of alpha 6 in the assembly and function of the alpha 6 beta 4 integrin, and compared it with the role of alpha 6 in the assembly and function of alpha 6 beta 1, by transfection of cDNAs encoding cytoplasmic mutants of alpha 6 into K562 cells with or without full-length beta 4 cDNA. Des-(1022-1050)-alpha 6, which contains a deletion C-terminal to the GFFKR motif, was expressed in association with beta 1, but associated preferentially with beta 4, whereas the wild-type alpha 6 subunit associated efficiently with beta 1 and beta 4. Des-(1016-1050)-alpha 6, which lacked also the GFFKR sequence, was only expressed at the cell surface when beta 4 was available. Transient expression in COS-7 cells showed that des-(1016-1050)-alpha 6 was retained in the endoplasmic reticulum as a monomer, which suggests that truncation of the cytoplasmic domain reduces the affinity of alpha 6 for beta 1, particularly when the GFFKR sequence is absent. Although the GFFKR motif is not essential for association of alpha 6 with beta 4, it increases the stability of the alpha 6 beta 4 integrin. The cytoplasmic domain of alpha 6 is essential for inside-out and outside-in signaling via the alpha 6 beta 1 receptor, but not for adhesion via alpha 6 beta 4. We show that alpha 6 beta 4 is a constitutively active receptor. Thus, unlike adhesion by most other integrins, adhesion by alpha 6 beta 4 does not seem to depend on any active cellular process. Binding of alpha 6 beta 4 to ligand was only slightly affected by truncation of the alpha 6 cytoplasmic domain N-terminal to the GFFKR sequence and became partially dependent on metabolic energy. These data indicate that truncations of the cytoplasmic domain of the alpha 6 subunit affect the assembly and function of alpha 6 beta 1 more strongly than those of alpha 6 beta 4. This difference may be due to the greater affinity of alpha 6 for beta 4 than for beta 1, which makes alpha 6 beta 4 less susceptible to the effect of truncations.

Desmosomes: differentiation, development, dynamics and disease

Recent evidence on the distribution of desmosomal glycoprotein isoforms that shows their combined expression in individual desmosomes has strengthened the belief that the latter are involved in epithelial differentiation and morphogenesis. It has been shown that cellular interactions and protein kinase C can modulate the adhesive properties of desmosomes in epithelial cell sheets. Genetic studies indicate the involvement of desmosomal components in cancer and epidermal diseases.

Breaking the connection: displacement of the desmosomal plaque protein desmoplakin from cell-cell interfaces disrupts anchorage of intermediate filament bundles and alters intercellular junction assembly

The desmosomal plaque protein desmoplakin (DP), located at the juncture between the intermediate filament (IF) network and the cytoplasmic tails of the transmembrane desmosomal cadherins, has been proposed to link IF to the desmosomal plaque. Consistent with this hypothesis, previous studies of individual DP domains indicated that the DP COOH terminus associates with IF networks whereas NH2-terminal sequences govern the association of DP with the desmosomal plaque. Nevertheless, it had not yet been demonstrated that DP is required for attaching IF to the desmosome. To test this proposal directly, we generated A431 cell lines stably expressing DP NH2-terminal polypeptides, which were expected to compete with endogenous DP during desmosome assembly. As these polypeptides lacked the COOH-terminal IF-binding domain, this competition should result in the loss of IF anchorage if DP is required for linking IF to the desmosomal plaque. In such cells, a 70-kD DP NH2-terminal polypeptide (DP-NTP) colocalized at cell-cell interfaces with desmosomal proteins. As predicted, the distribution of endogenous DP was severely perturbed. At cell-cell borders where endogenous DP was undetectable by immunofluorescence, there was a striking absence of attached tonofibrils (IF bundles). Furthermore, DP-NTP assembled into ultrastructurally identifiable junctional structures lacking associated IF bundles. Surprisingly, immunofluorescence and immunogold electron microscopy indicated that adherens junction components were coassembled into these structures along with desmosomal components and DP-NTP. These results indicate that DP is required for anchoring IF networks to desmosomes and furthermore suggest that the DP-IF complex is important for governing the normal spatial segregation of adhesive junction components during their assembly into distinct structures.

Molecular specializations at nodes and paranodes in peripheral nerve

In the peripheral nervous system, nodes of Ranvier are formed by interactions between myelinating Schwann cells and axons. Nodes have an intricate ultrastructure, and their molecular architecture is similarly complex. A growing list of molecules have been found that are selectively localized to different parts of the nodes. Neural cell adhesion molecule (N-CAM), L1/Ng-CAM, and tenascin/cytotactin are enriched in the nodal basal lamina; hyaluronic acid, versican/hyaluronectin, N-CAM, L1/Ng-CAM, tenascin/cytotactin, and the ganglioside GM1 are enriched in the nodal gap; myelin-associated glycorprotein, oligodendrocyte-myelin glycoprotein, connexin32, E-cadherin, actin, the gangliosides GQ1b and GD1b, the potassium channel KV1.5, and alkaline phosphatase are enriched in the paranodal region of the Schwann cell; voltage-dependent sodium channels and the cytoskeletal proteins spectrin and ankyrin are enriched in the nodal axolemma. Many of these molecules are probably essential for the proper functioning and stability of nodes.