Junctional complexes in various epithelia

The epithelia of a number of glands and cavitary organs of the rat and guinea pig have been surveyed, and in all cases investigated, a characteristic tripartite junctional complex has been found between adjacent cells. Although the complex differs in precise arrangement from one organ to another, it has been regularly encountered in the mucosal epithelia of the stomach, intestine, gall bladder, uterus, and oviduct; in the glandular epithelia of the liver, pancreas, parotid, stomach, and thyroid; in the epithelia of pancreatic, hepatic, and salivary ducts; and finally, between the epithelial cells of the nephron (proximal and distal convolution, collecting ducts). The elements of the complex, identified as zonula occludens (tight junction), zonula adhaerens (intermediary junction), and macula adhaerens (desmosome), occupy a juxtaluminal position and succeed each other in the order given in an apical-basal direction. The zonula occludens (tight junction) is characterized by fusion of the adjacent cell membranes resulting in obliteration of the intercellular space over variable distances. Within the obliterated zone, the dense outer leaflets of the adjoining cell membranes converge to form a single intermediate line. A diffuse band of dense cytoplasmic material is often associated with this junction, but its development varies from one epithelium to another. The zonula adhaerens (intermediate junction) is characterized by the presence of an intercellular space ( approximately 200 A) occupied by homogeneous, apparently amorphous material of low density; by strict parallelism of the adjoining cell membranes over distances of 0.2 to 0.5 micro; and by conspicuous bands of dense material located in the subjacent cytoplasmic matrix. The desmosome or macula adhaerens is also characterized by the presence of an intercellular space ( approximately 240 A) which, in this case, contains a central disc of dense material; by discrete cytoplasmic plaques disposed parallel to the inner leaflet of each cell membrane; and by the presence of bundles of cytoplasmic fibrils converging on the plaques. The zonula occludens appears to form a continuous belt-like attachment, whereas the desmosome is a discontinuous, button-like structure. The zomula adhaerens is continuous in most epithelia but discontinuous in some. Observations made during experimental hemoglobinuria in rats showed that the hemoglobin, which undergoes enough concentration in the nephron lumina to act as an electron-opaque mass tracer, does not penetrate the intercellular spaces beyond the zonula occludens. Similar observations were made in pancreatic acini and ducts where discharged zymogen served as a mass tracer. Hence the tight junction is impervious to concentrated protein solutions and appears to function as a diffusion barrier or "seal." The desmosome and probably also the zonula adhaerens may represent intercellular attachment devices.

Granulation tissue as a contractile organ. A study of structure and function

CONTRACTING GRANULATION TISSUES CONTAIN FIBROBLASTS THAT DEVELOP CHARACTERISTICS TYPICAL OF SMOOTH MUSCLE: (a) They contain an extensive cytoplasmic fibrillar system. (b) They show immunofluorescent labeling of their cytoplasm with human anti-smooth muscle serum. (c) The nuclei show complicated folds and indentations, indicative of cellular contraction. (d) There are cell-to-cell and cell-to-stroma attachments. (e) It is possible to extract similar quantities of actomyosin (having the same adenosine triphosphatase activity) from granulation tissue and from pregnant rat uterus. (f) Strips of granulation tissue, when tested pharmacologically in vitro, behave similarly to smooth muscle. All these data support the view that, under certain conditions, fibroblasts can differentiate into a cell type structurally and functionally similar to smooth muscle and that this cell, the "myo-fibroblast," plays an important role in connective tissue contraction.

Mutations in the desmosomal protein plakophilin-2 are common in arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is associated with fibrofatty replacement of cardiac myocytes, ventricular tachyarrhythmias and sudden cardiac death. In 32 of 120 unrelated individuals with ARVC, we identified heterozygous mutations in PKP2, which encodes plakophilin-2, an essential armadillo-repeat protein of the cardiac desmosome. In two kindreds with ARVC, disease was incompletely penetrant in most carriers of PKP2 mutations.

Leaving the neighborhood: molecular mechanisms involved during epithelial-mesenchymal transition

Several molecular mechanisms contribute directly and mechanically to the loss of epithelial phenotype. During epithelial-mesenchymal transition (EMT), adherens junctions and desmosomes are at least partially dissociated. At the same time, a massive cytoskeleton reorganization takes place, involving the rho family and the remodeling of the actin microfilament mesh. Numerous pathways have been described in vitro that control phenotype transition in specific cell models. In vivo developmental studies suggest that transcriptional control, activated by a specific pathway involving Ras, Src and potentially the Wnt pathway, is an essential step. Recent functional and localization experiments indicate that the slug/snail family of transcription factors functions overall as an epithelial phenotype repressor and could represent a key EMT contributor.

The neuropil of the glomeruli of the olfactory bulb

The neuropil of the glomeruli of the rat olfactory bulb has been studied with the electron microscope with a view to elucidating the type of processes involved - dendrites, appendages and axons - their cellular identity, and the synaptic relationships they establish. The problems encountered in defining these are considered and criteria based on the previous study of neuron types and on examination of serial sections are put forward. The glomeruli are large structures containing many thousands of processes and are the sole site of termination of the olfactory receptor axons. The terminals of the latter are characteristically electron-dense, allowing identification in normal material; they run through the glomeruli making many synapses by means of spherical vesicles and asymmetrical thickenings on to all types of dendritic profile. The glomerular dendritic arborizations of mitral and tufted cells, which are indistinguishable from each other, start as large, fairly regular, pale profiles but become increasingly varicose as they branch and diminish in size. They regularly show groups of spherical vesicles, often in association with asymmetrical synaptic thickenings directed from the dendrite; these are typically associated with return, reciprocal, synapses of the symmetrical type from profiles containing large flattened vesicles. These latter profiles are those of the dendrites and gemmules of periglomerular cells; the dendrites are of irregular outline and give rise to many appendages, mostly gemmules making synaptic contact with mitral or tufted cell dendrites. A small number of pale axon terminals containing either small or large flattened vesicles, derived from short-axon and periglomerular cells respectively, synapse with symmetrical thickenings on to the periglomerular cell dendritic processes. Close associations of particular types of axo-dendritic and dendro-dendritic synapses on interconnecting processes, termed synaptic patterns, are described and their significance considered. The nature of the glomerular interactions is discussed and then placed in the context of other, smaller glomeruli in the central nervous system; certain common principles of glomeruli are suggested.

Compound and digenic heterozygosity contributes to arrhythmogenic right ventricular cardiomyopathy

OBJECTIVES

The aim of this study was to define the genetic basis of arrhythmogenic right ventricular cardiomyopathy (ARVC).

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy, characterized by right ventricular fibrofatty replacement and arrhythmias, causes sudden death. Autosomal dominant inheritance, reduced penetrance, and 7 desmosome-encoding causative genes are known. The basis of low penetrance is poorly understood.

METHODS

Arrhythmogenic right ventricular cardiomyopathy probands and family members were enrolled, blood was obtained, lymphoblastoid cell lines were immortalized, deoxyribonucleic acid was extracted, polymerase chain reaction (PCR) amplification of desmosome-encoding genes was performed, PCR products were sequenced, and diseased tissue samples were studied for intercellular junction protein distribution with confocal immunofluorescence microscopy and antibodies against key proteins.

RESULTS

We identified 21 variants in plakophilin-2 (PKP2) in 38 of 198 probands (19%), including missense, nonsense, splice site, and deletion/insertion mutations. Pedigrees showed wide intra-familial variability (severe early-onset disease to asymptomatic individuals). In 9 of 38 probands, PKP2 variants were identified that were encoded in trans (compound heterozygosity). The 38 probands hosting PKP2 variants were screened for other desmosomal genes mutations; second variants (digenic heterozygosity) were identified in 16 of 38 subjects with PKP2 variants (42%), including desmoplakin (DSP) (n = 6), desmoglein-2 (DSG2) (n = 5), plakophilin-4 (PKP4) (n = 1), and desmocollin-2 (DSC2) (n = 1). Heterozygous mutations in non-PKP 2 desmosomal genes occurred in 14 of 198 subjects (7%), including DSP (n = 4), DSG2 (n = 5), DSC2 (n = 3), and junctional plakoglobin (JUP) (n = 2). All variants occurred in conserved regions; none was identified in 700 ethnic-matched control subjects. Immunohistochemical analysis demonstrated abnormalities of protein architecture.

CONCLUSIONS

These data suggest that the genetic basis of ARVC includes reduced penetrance with compound and digenic heterozygosity. Disturbed junctional cytoarchitecture in subjects with desmosomal mutations confirms that ARVC is a disease of the desmosome and cell junction.

Targeted inactivation of plectin reveals essential function in maintaining the integrity of skin, muscle, and heart cytoarchitecture

Previous studies suggest that plectin, a versatile cytoskeletal linker protein, has an important role in maintaining the structural integrity of diverse cells and tissues. To establish plectin's function in a living organism, we have disrupted its gene in mice. Plectin (-/-) mice died 2-3 days after birth exhibiting skin blistering caused by degeneration of keratinocytes. Ultrastructurally, hemidesmosomes and desmosomes appeared unaffected. In plectin-deficient mice, however, hemidesmosomes were found to be significantly reduced in number and apparently their mechanical stability was altered. The skin phenotype of these mice was similar to that of patients suffering from epidermolysis bullosa simplex (EBS)-MD, a hereditary skin blistering disease with muscular dystrophy, caused by defects in the plectin gene. In addition, plectin (-/-) mice revealed abnormalities reminiscent of minicore myopathies in skeletal muscle and disintegration of intercalated discs in heart. Our results clearly demonstrate a general role of plectin in the reinforcement of mechanically stressed cells. Plectin (-/-) mice will provide a useful tool for the study of EBS-MD, and possibly other types of plectin-related myopathies involving skeletal and cardiac muscle, in an organism amenable to genetic manipulation.

Intercellular attachment in the epithelium of Hydra as revealed by electron microscopy

In Hydra adjacent epithelial cells are bound firmly to each other by desmosomes of a type not described in detail hitherto. The most prominent feature of these desmosomes is the presence of a series of parallel lamellae which bridge the intercellular space and connect the two apposed cell surfaces directly. These structures, here termed intercellular attachment lamellae, display two peaks of density about 50 A apart. These dense lines appear in some instances to be continuous with the outer dense components of the plasma unit membranes of the attached cells. The presence of prominent lamellae in intercellular attachments is sufficiently distinctive to deserve special terminology; accordingly, the term septate desmosome is proposed. It is noted that septate desmosomes may have been seen in other animals in instances where published electron micrographs show cross-striations or prominent connections in regions of intercellular attachment. It is suggested that septate desmosomes in Hydra, in addition to binding cells firmly to each other, form barriers to the movement of water into intercellular spaces and thus help to protect the organism's internal environment. Observations on the use of phosphotungstic acid for improving contrast in materials embedded in epoxy resins are also recorded.

Mutation analysis and molecular genetics of epidermolysis bullosa

Cutaneous basement membrane zone (BMZ) consists of a number of attachment structures that are critical for stable association of the epidermis to the underlying dermis. These include hemidesmosomes, anchoring filaments and anchoring fibrils which form an interconnecting network extending from the intracellular milieu of basal keratinocytes across the dermal-epidermal basement membrane to the underlying dermis. Aberrations in this network structure, e.g. due to genetic lesions in the corresponding genes, can result in fragility of the skin at the level of the cutaneous BMZ. The prototype of such diseases is epidermolysis bullosa (EB), a heterogeneous group of genodermatoses characterized by fragility and blistering of the skin, often associated with extracutaneous manifestations, and inherited either in an autosomal dominant or autosomal recessive manner. Based on constellations of the phenotypic manifestations, severity of the disease, and the level of tissue separation within the cutaneous BMZ, EB has been divided into clinically distinct subcategories, including the simplex, hemidesmosomal, junctional and dystrophic variants. Elucidation of BMZ gene/protein systems and development of mutation detection strategies have allowed identification of mutations in 10 different BMZ genes which can explain the clinical heterogeneity of EB. These include mutations in the type VII collagen gene (COL7A1) in the dystrophic (severely scarring) forms of EB; mutations in the laminin 5 genes (LAMA3, LAMB3 and LAMC2) in a lethal (Herlitz) variant of junctional EB; aberrations in the type XVII collagen gene (COL17A1) in non-lethal forms of junctional EB; mutations in the alpha6 and beta4 integrin genes in a distinct hemidesmosomal variant of EB with congenital pyloric atresia; and mutations in the plectin gene (PLEC1) in a form of EB associated with late-onset muscular dystrophy. Identification of mutations in these gene/protein systems attests to their critical importance in the overall stability of the cutaneous BMZ. Furthermore, elucidation of mutations in different variants of EB has direct clinical applications in terms of refined classification, improved genetic counseling, and development of DNA-based prenatal testing in families with EB.

Interactions between ankyrin-G, Plakophilin-2, and Connexin43 at the cardiac intercalated disc

RATIONALE

The early description of the intercalated disc defined 3 structures, all of them involved in cell-cell communication: desmosomes, gap junctions, and adherens junctions. Current evidence demonstrates that molecules not involved in providing a physical continuum between cells also populate the intercalated disc. Key among them is the voltage-gated sodium channel complex. An important component of this complex is the cytoskeletal adaptor protein Ankyrin-G (AnkG).

OBJECTIVE

To test the hypothesis that AnkG partners with desmosome and gap junction molecules and exerts a functional effect on intercellular communication in the heart.

METHODS AND RESULTS

We used a combination of microscopy, immunochemistry, patch-clamp, and optical mapping to assess the interactions between AnkG, Plakophilin-2, and Connexin43. Coimmunoprecipitation studies from rat heart lysate demonstrated associations between the 3 molecules. With the use of siRNA technology, we demonstrated that loss of AnkG expression caused significant changes in subcellular distribution and/or abundance of PKP2 and Connexin43 as well as a decrease in intercellular adhesion strength and electric coupling. Regulation of AnkG and of Na(v)1.5 by Plakophilin-2 was also demonstrated. Finally, optical mapping experiments in AnkG-silenced cells demonstrated a shift in the minimal frequency at which rate-dependence activation block was observed.

CONCLUSIONS

These experiments support the hypothesis that AnkG is a key functional component of the intercalated disc at the intersection of 3 complexes often considered independent: the voltage-gated sodium channel, gap junctions, and the cardiac desmosome. Possible implications to the pathophysiology of inherited arrhythmias (such as arrhythmogenic right ventricular cardiomyopathy) are discussed.

Cell contacts in the mouse mammary gland. I. Normal gland in postnatal development and the secretory cycle

The nature and distribution of cell contacts have been examined in thin sections and freeze-fracture replicas of mammary gland samples from female C3H/Crgl mice at stages from birth through pregnancy, lactation, and postweaning involution. Epithelial cells of major mammary ducts at all stages examined are linked at their luminal borders by junctional complexes consisting of tight junctions, variable intermediate junctions, occasional small gap junctions, and one or more series of desmosomes. Scattered desmosomes and gap junctions link ductal epithelial and myoepithelial cells in all combinations; hemidesmosomes attach myoepithelial cells to the basal lamina. Freeze-fracture replicas confirm the erratic distribution of gap junctions and reveal a loose, irregular network of ridges comprising the continuous tight-junctional belts. Alveoli develop early in gestation and initially resemble ducts. Later, as alveoli and small ducts become actively secretory, they lose all desmosomes and most intermediate junctions, whereas tight and gap junctions persist, The tight-junctional network becomes compact and orderly, its undulating ridges oriented predominantly parallel to the luminal surface. It is suggested that these changes in junctional morphology, occurring in secretory cells around parturition, may be related to the greatly enhanced rate of movement of milk precursors and products through the lactating epithelium, or to the profound and recurrent changes in shape of secretory cells that occur in relation to myoepithelial cell contraction, or to both.

Cloning and sequencing of rat plectin indicates a 466-kD polypeptide chain with a three-domain structure based on a central alpha-helical coiled coil

We have determined the complete cDNA sequence of rat plectin from a number of well-characterized overlapping lambda gt11 clones. The 4,140-residue predicted amino acid sequence (466,481 D) is consistent with a three-domain structural model in which a long central rod domain, having mainly an alpha-helical coiled coil conformation, is flanked by globular NH2- and COOH-terminal domains. The plectin sequence has a number of repeating motifs. The rod domain has five subregions approximately 200-residues long in which there is a strong repeat in the charged amino acids at 10.4 residues that may be involved in association between plectin molecules. The globular COOH-terminal domain has a prominent six-fold tandem repeat, with each repeat having a strongly conserved central region based on nine tandem repeats of a 19-residue motif. The plectin sequence has several marked similarities to that of desmoplakin (Green, K. J., D. A. D. Parry, P. M. Steinert, M. L. A. Virata, R. M. Wagner, B. D. Angst, and L.A. Nilles. 1990. J. Biol. Chem. 265:2,603-2,612), which has a shorter coiled-coil rod domain with a similar 10.4 residue charge periodicity and a COOH-terminal globular domain with three tandem repeats homologous to the six found in plectin. The plectin sequence also has homologies to that of the bullous pemphigoid antigen. Northern blot analysis indicated that there is a significant degree of conservation of plectin genes between rat, human, and chicken and that, as shown previously at the protein level, plectin has a wide tissue distribution. There appeared to be a single rat plectin gene that gave rise to a 15-kb message. Expression of polypeptides encoded by defined fragments of plectin cDNA in E. coli has also been used to localize the epitopes of a range of monoclonal and serum antibodies. This enabled us to tentatively map a sequence involved in plectin-vimentin and plectin-lamin B interactions to a restricted region of the rod domain.

STUDIES ON AN EPITHELIAL (GLAND) CELL JUNCTION. II. SURFACE STRUCTURE

The surface structure of a gland epithelium (Drosophila salivary gland), particularly that at the junction between cells, was examined under the electron microscope. The junctional surface, which in the preceding paper was shown to be highly permeable to ions, has the following structural characteristics. About two-thirds of it are profusely infolded; the surface membranes of adjoining cells interdigitate and present desmosomes. The width of the intercellular space varies considerably. The remainder of the junctional surface, the third that abuts on the lumen, is rather straight. Here, the cell membranes are aligned parallel at a distance of 150 A, and interconnected at regular intervals of 100 A. The connecting material has a high electron opacity, and is about as thick as the cell membranes, but, unlike the latter, has no resolvable unit membrane structure. The surface at the cell base, which in the preceding paper was shown to be rather impermeable, is infolded and resembles the infolded junctional region. The luminal surface exhibits microvilli. Critical surface dimensions are given, and the implications of surface structure in intercellular permeability are discussed.

Comparative structural analysis of desmoplakin, bullous pemphigoid antigen and plectin: members of a new gene family involved in organization of intermediate filaments

Desmoplakins (DP) and bullous pemphigoid antigen (BPA) are major plaque components of the desmosome and hemidesmosome, respectively. These cell adhesion structures are both associated intimately with the intermediate filament (IF) network. Structural analyses of DP and BPA sequences have indicated that these molecules are likely to form extended dumbbell-shaped dimers with a central rod and globular end domains. Recent sequence data have indicated that the N-terminal domains of both DP and BPA (like their C-terminal domains) are highly related: the former contain regions of heptad repeats that are predicted to form several alpha-helical bundles. Comparisons of DP and BPA protein sequences with that of plectin (PL), a 466 kDa IF-associated protein, have also revealed large scale homology. Identities between their N-terminal domains are: DP:BPA = 35%, DP:PL = 32%, BPA:PL = 40%, suggesting that BPA is more closely related to PL than DP in this region. In the C-terminal domains, which contain a 38-residue repeating motif, however, DP and PL are closer relatives (identities: DP:BPA = 38%, BPA:PL = 40%, DP:PL = 49%). The central domains of all three proteins have extensive heptad repeat substructure, express the same periodic distribution of charged residues, and are predicted to form two-stranded alpha-helical coiled-coil ropes. These observations suggest that DP, BPA and PL belong to a new gene family encoding proteins involved in IF organization.

Human epidermis reconstructed by culture: is it "normal"?

Human keratinocytes were grown on a dermal equivalent (or lattice) at the liquid-air interface in an attempt to reconstitute a functional epidermis in vitro. Although the multilayered epithelium thus obtained is well differentiated, as shown by the presence of keratohyaline granules and horny layer, several differences from its in vivo counterpart were also observed: In the reconstructed epidermis, basal keratinocytes do not have the cuboidal shape found in vivo; they synthesize bullous pemphigoid antigen and laminin, but the distribution of these antigens is not linear as in vivo; they contain the plasma-membrane antigens restricted to the basal layer in vivo (VM1, BC1), but these antigens are not polarized; lack of polarization is also evidenced by the distribution of actin. Differentiation markers appear but with a topography slightly different from that of epidermis in vivo; the 67-kD keratin does not appear in the first suprabasal layer as in vivo but above; involucrin, which appears in the granular layers in vivo appears as soon as the cells leave the basal layer. psi 3 antigen and fibronectin found in vivo only in hyperproliferative epidermis (wound healing, psoriasis) are detected. Hyperproliferation would also explain the unexpected straining of basal cells by KL1 monoclonal antibody. Because of the potential clinical or pharmacologic use of artificial epidermis, the question of whether the epidermis obtained in vitro can be considered as "normal" is discussed.