The assembly of the major desmosome glycoproteins of Madin-Darby canine kidney cells

ER-to-Golgi blockade of nascent desmosomal cadherins in SERCA2-inhibited keratinocytes: Implications for Darier's disease

Darier's disease (DD) is an autosomal dominantly inherited skin disorder caused by mutations in sarco/endoplasmic reticulum Ca²⁺ -ATPase 2 (SERCA2), a Ca²⁺ pump that transports Ca²⁺ from the cytosol to the endoplasmic reticulum (ER). Loss of desmosomes and keratinocyte cohesion is a characteristic feature of DD. Desmosomal cadherins (DC) are Ca²⁺ -dependent transmembrane adhesion proteins of desmosomes, which are mislocalized in the lesional but not perilesional skin of DD. We show here that inhibition of SERCA2 by 2 distinct inhibitors results in accumulation of DC precursors in keratinocytes, indicating ER-to-Golgi transport of nascent DC is blocked. Partial loss of SERCA2 by siRNA has no such effect, implicating that haploinsufficiency is not sufficient to affect nascent DC maturation. However, a synergistic effect is revealed between SERCA2 siRNA and an ineffective dose of SERCA2 inhibitor, and between an agonist of the ER Ca²⁺ release channel and SERCA2 inhibitor. These results suggest that reduction of ER Ca²⁺ below a critical level causes ER retention of nascent DC. Moreover, colocalization of DC with ER calnexin is detected in SERCA2-inhibited keratinocytes and DD epidermis. Collectively, our data demonstrate that loss of SERCA2 impairs ER-to-Golgi transport of nascent DC, which may contribute to DD pathogenesis.

Loss of flotillin expression results in weakened desmosomal adhesion and Pemphigus vulgaris-like localisation of desmoglein-3 in human keratinocytes

Desmosomes are adhesion plaques that mediate cell-cell adhesion in many tissues, including the epidermis, and generate mechanical resistance to tissues. The extracellular domains of desmosomal cadherin proteins, desmogleins and desmocollins, are required for the interaction with cadherins of the neighbouring cells, whereas their cytoplasmic tails associate with cytoplasmic proteins which mediate connection to intermediate filaments. Disruption of desmosomal adhesion by mutations, autoantibodies or bacterial toxins results in severe human disorders of e.g. the skin and the heart. Despite the vital role of desmosomes in various tissues, the details of their molecular assembly are not clear. We here show that the two members of the flotillin protein family directly interact with the cytoplasmic tails of desmogleins. Depletion of flotillins in human keratinocytes results in weakened desmosomal adhesion and reduced expression of desmoglein-3, most likely due to a reduction in the desmosomal pool due to increased turnover. In the absence of flotillins, desmoglein-3 shows an altered localisation pattern in the cell-cell junctions of keratinocytes, which is highly similar to the localisation observed upon treatment with pemphigus vulgaris autoantibodies. Thus, our data show that flotillins, which have previously been connected to the classical cadherins, are also of importance for the desmosomal cell adhesion.

Structural and functional diversity of desmosomes

Desmosomes anchor intermediate filaments at sites of cell contact established by the interaction of cadherins extending from opposing cells. The incorporation of cadherins, catenin adaptors, and cytoskeletal elements resembles the closely related adherens junction. However, the recruitment of intermediate filaments distinguishes desmosomes and imparts a unique function. By linking the load-bearing intermediate filaments of neighboring cells, desmosomes create mechanically contiguous cell sheets and, in so doing, confer structural integrity to the tissues they populate. This trait and a well-established role in human disease have long captured the attention of cell biologists, as evidenced by a publication record dating back to the mid-1860s. Likewise, emerging data implicating the desmosome in signaling events pertinent to organismal development, carcinogenesis, and genetic disorders will secure a prominent role for desmosomes in future biological and biomedical investigations.

The DSPII splice variant is crucial for desmosome-mediated adhesion in HaCaT keratinocytes

Desmosomes are intercellular junctions specialised for strong adhesion that are prominent in the epidermis and heart muscle. Defective desmosomal function due to inherited mutations in the constitutive desmosomal gene desmoplakin (DSP) causes skin or heart disorders and in some instances both. Different mutations have different disease-causing molecular mechanisms as evidenced by the varying phenotypes resulting from mutations affecting different domains of the same protein, but the majority of these mechanisms remain to be determined. Here, we studied two mutations in DSP that lead to different dosages of the two major DSP splice variants, DSPI and DSPII, and compared their molecular mechanisms. One of the mutations results in total DSP haploinsufficiency and is associated with autosomal dominant striate palmoplantar keratoderma (PPK). The other leads to complete absence of DSPI and the minor isoform DSPIa but normal levels of DSPII, and is associated with autosomal recessive epidermolytic PPK, woolly hair and severe arrhythmogenic dilated cardiomyopathy. Using siRNA treatments to mimic these two mutations and additionally a DSPII-specific siRNA, we found striking differences between DSP isoforms with respect to keratinocyte adhesion upon cellular stress with DSPII being the key component in intermediate filament (IF) stability and desmosome-mediated adhesion. In addition, reduction in DSP expression reduced the amount of plakophilin 1, desmocollin (DSC) 2 and DSC3 with DSPI having a greater influence than DSPII on the expression levels of DSC3. These results suggest that the two major DSP splice variants are not completely redundant in function and that DSPII dosage is particularly important for desmosomal adhesion in the skin.

The desmosome: cell science lessons from human diseases

Human skin diseases have revealed fundamental mechanisms by which cytoskeletal proteins contribute to tissue architecture and function. In particular, the analysis of epidermal blistering disorders and the role of keratin gene mutations in these diseases has led to significant increases in our understanding of intermediate filament biology. The major cell-surface attachment site for intermediate filament networks is the desmosome, an adhesive intercellular junction prominent in the epidermis and the heart. During the past decade, substantial progress has been made in understanding the molecular basis of a variety of epidermal autoimmune diseases, skin fragility syndromes, and disorders that involve a combination of heart and skin defects caused by perturbations in desmosome structure and function. These human diseases reveal key roles for desmosomes in maintaining tissue integrity, but also suggest functions for desmosomal components in signal transduction pathways and epidermal organization.

Serum from pemphigus vulgaris reduces desmoglein 3 half-life and perturbs its de novo assembly to desmosomal sites in cultured keratinocytes

Defects of cell-cell adhesion underlie disruption of epithelial integrity observed in patients with pemphigus vulgaris (PV), an autoimmune disease characterized by severe mucosal erosions and skin blisters. Pathogenic PV autoantibodies found in patients' sera target desmoglein 3 (Dsg3), a major component of the desmosome, but how does this phenomenon affect Dsg-dependent adhesion and lead to acantholysis still remains controversial. Here, we show that PV serum determines a reduction of Dsg3 half-life in HaCaT keratinocytes, although the total amount of Dsg3 remains unchanged. Immunofluorescence studies suggest that PV IgG exert their effect prevalently by binding non-desmosomal Dsg3 without causing its massive internalization. Furthermore, PV IgG targeting desmosome-assembled Dsg3 do not induce depletion of Dsg3 from the adhesion sites. Conversely, incorporation of PV IgG-Dsg3 complexes into new forming desmosomes appears perturbed. With our study, the basic biochemical changes of Dsg3 in an in vitro model of PV have been defined.

Impaired trafficking of the desmoplakins in cultured Darier's disease keratinocytes

Darier's disease is an autosomal dominantly inherited skin disorder characterized by loss of adhesion between epidermal cells, breakdown of desmosome-keratin filaments, and abnormal keratinization. ATP2A2 has been identified as the causative gene of Darier's disease. This gene encodes the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) isoform 2 pump, which transports Ca2+ from the cytosol into the endoplasmic reticulum lumen to maintain a low cytosolic Ca2+ concentration. Using indirect immunofluorescence and biochemical analysis, we investigated the distribution of key desmosomal proteins in normal human and Darier's disease keratinocytes under various calcium conditions. We show that inhibition of SERCA by thapsigargin in normal human keratinocytes impairs the trafficking of the desmoplakins, desmoglein, and desmocollin to the cell surface; these proteins show a diffuse cytoplasmic distribution and, together with plakoglobin, form detergent-insoluble aggregates. In Darier's disease keratinocytes, only the trafficking of desmoplakin is significantly inhibited; in these cells, desmoplakin forms insoluble aggregates when extracted with mild detergent. In contrast, the transmembrane proteins desmoglein and desmocollin are efficiently transported to the cell surface. These proteins, along with plakoglobin, remain equally distributed between detergent-soluble and -insoluble fractions. We also demonstrate an interaction between SERCA2 and desmoplakin during differentiation. Our results provide further insights into the critical role of calcium ATPases in maintaining epidermal integrity.

Desmosome assembly in MDCK cells: transport of precursors to the cell surface occurs by two phases of vesicular traffic and involves major changes in centrosome and Golgi location during a Ca(2+) shift

Desmosome formation in MDCK cells was investigated using a Ca(2+) shift. Following preliminary treatment with cycloheximide at 37 degrees C, continued surface transport and subsequent endocytosis were minimized by incubating cells at 19 degrees C to trap nascent glycoproteins within the Golgi body. Release into high Ca(2+) medium (HCM) at 37 degrees C resulted in junction formation as well as relocation of the Golgi body and centrosomes to a subapical location. Desmosome formation occurred in two stages over 2 h, the first occurring within 30 min of the shift to HCM, in 60-nm vesicles containing chiefly Dsc2 and lower concentrations of Dsg and E-cadherin distributed to the entire cell surface. Much of this material was subsequently endocytosed. The second stage involved transport of Dsg, E-cadherin, plakoglobin, and beta-catenin, in more complex vesicles some 200 nm in size, directed to possible nucleation sites on the developing basolateral surface. Plaque proteins such as desmoplakin I/II were added subsequently. Stage-two vesicles, but possibly not those of stage one, were accessible to endocytic markers via retrograde transport from multivesicular bodies prelabeled at 19 degrees C.

Extracellularly truncated desmoglein 1 compromises desmosomes in MDCK cells

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

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.

Antisense expression of a desmocollin gene in MDCK cells alters desmosome plaque assembly but does not affect desmoglein expression

The desmocollins are one of two types of putative adhesive proteins present in the desmosome type of cell junctions, the other type being the desmogleins; both are members of the cadherin superfamily. Each type of desmosomal cadherin occurs as a number of isoforms which have differing tissue distribution; within stratifying epithelia some isoforms occur only suprabasally. We have sought to analyse desmocollin function by reducing the amount of protein using antisense gene expression in the widely studied Madin-Darby canine kidney (MDCK) cell line. Although this is a simple epithelial cell line, we show by Northern blot analysis that it expresses multiple isoforms of the desmosomal cadherins. Desmocollins DSC2 and DSC3 and desmogleins DSG2 and DSG3 (the pemphigus vulgaris antigen PVA) were detected, but DSC1 and DSG1, which are present exclusively in the suprabasal layers of the epidermis, were absent. The major desmocollin isoform was the type 2 (DSC2). A DSC2 clone isolated from a MDCK cDNA library had the same cell adhesion recognition sequence (Phe-Ala-Thr) as human, bovine and mouse type 2 isoforms. This sequence appears diagnostic for the three desmocollin isoforms. This cDNA clone was used to isolate a genomic DSC2 clone; antisense expression of this clone in MDCK cells resulted in a drastic reduction of desmocollin protein as judged by Western blots; Dsc3 was not upregulated to compensate for the loss of Dsc2. This antisense expression significantly altered desmosome assembly. There was a loss of punctate staining evident when using a desmosome plaque protein (desmoplakin) antibody. Electron microscopy revealed that there was a reduction in the number of desmosomes and a notable increase in the asymmetry of plaques between adjacent cells. Immunolabelling showed that similar levels of desmogleins and E-cadherin were present. Immunoelectron microscopy also showed that many vesicular structures were labelled, at intervals along the lateral membranes between cells. The distinctive loose organization of the remaining desmosomes may originate in modifications to the targeting and incorporation of proteins into fully assembled plaques. Other junctions were unaffected and the cells maintained their integrity as a confluent monolayer.

Continual assembly of half-desmosomal structures in the absence of cell contacts and their frustrated endocytosis: a coordinated Sisyphus cycle

It is widely assumed that the coordinate assembly of desmosomal cadherins and plaque proteins into desmosome-typical plaque-coated membrane domains, capable of anchoring intermediate-sized filaments (IF), requires cell-to-cell contacts and a critical extracellular Ca2+ concentration. To test this hypothesis we studied several cell lines grown for years in media with less than 0.1 mM Ca2+ to steady-state low Ca2+ medium (LCM) conditions, particularly the human keratinocyte line HaCaT devoid of any junctional cell contact (HaCaT-L cells). Using immunolocalization and vesicle fractionation techniques, we found that the transmembrane glycoprotein, desmoglein (Dsg), colocalized with the plaque proteins, desmoplakin and plakoglobin. The sites of coassembly of desmosomal molecules in HaCaT-L cells as well as in HaCaT cells directly brought into LCM were identified as asymmetric plaque-coated plasma membrane domains (half-desmosomes) or as special plaque-associated cytoplasmic vesicles, most of which had formed endocytotically. The surface exposure of Dsg in these half-desmosomes was demonstrated by the binding, in vivo, of antibodies specific for an extracellular Dsg segment which also could cross-bridge them into symmetric quasi-desmosomes. Otherwise, these half-desmosomes were shown in LCM to be taken up endocytotically. Half-desmosomal assemblies were also seen in uncoupled cells in normal Ca2+ medium. We conclude that, in the absence of intercellular contacts, assembly of desmosomal proteins at the cell surface takes place, resulting in transient half-desmosomes which then, in LCM and without a stable partner connection to the adjacent cell, can be endocytotically resumed. This frustrated cycle of synthesis and assembly maintains an ensemble of molecules characteristic of epithelial differentiation and the potential to form desmosomes, even when the final junctional structure cannot be formed. We propose that these half-desmosomal structures are general cell structures of epithelial and other desmosome-forming cells.

Desmosome assembly and disassembly are regulated by reversible protein phosphorylation in cultured epithelial cells

Desmosomes are one component of the intercellular junctional complex in epithelia. In cultures of epithelial cells, desmosome assembly can be regulated by modulating the calcium concentrations of the growth media. At present, very little is known about the intracellular signal transduction mechanisms that regulate desmosome assembly and disassembly in response to changing extracellular calcium concentrations. We have used inhibitors of protein kinases and phosphatases in a combined biochemical and morphological approach to analyze the role of protein phosphorylation in the assembly and disassembly of desmosomes in Madin-Darby canine kidney epithelial cells. Our results suggest that desmosomal proteins (desmoplakins I/II and desmoglein 1) are primarily phosphorylated on serine residues. Electron microscopic analyses of desmosome assembly upon induction of cell-cell contact, in the presence of protein kinase inhibitor, H-7, revealed an apparently normal assembly of desmosomes. However, complete disassembly of desmosomes was inhibited by H-7 upon removal of extracellular calcium. Under these conditions, although desmosomes split, desmosomal plaques and their associated cytokeratin filaments can not be internalized. In contrast, treatment of the cultures with okadaic acid (OA), an inhibitor of protein phosphatases, inhibited desmosome assembly but had no effect on disassembly. In addition, the inhibitory effect of okadaic acid on desmosome assembly was specific to this junction since we observed apparently normal tight junction and adherens junction in okadaic acid-treated cultures. These results suggest that assembly and disassembly of desmosomes may be regulated by extracellular Ca2+ via reversible protein phosphorylation involving both protein kinase and protein phosphatases.

Internalisation of desmosomes and their entry into the endocytic pathway via late endosomes in MDCK cells. Possible mechanisms for the modulation of cell adhesion by desmosomes during development

MDCK cells grown in media with normal levels of Ca2+ (approximately 2 mM) contain internalised desmosomes, referred to as desmosome-associated vacuoles (DAVs). The DAVs consist of one to three plaques retained in the plane of a surrounding vacuolar membrane, and their entry into the endocytic pathway has been investigated using HRP, cationized ferritin and BSA/gold in combination with electron microscopy and immunogold labelling of frozen sections. Endocytic tracers supplied from the apical and basolateral surfaces to filter-grown MDCK cells met in a common perinuclear compartment but DAVs were not labelled during short (5-30 minutes) pulses of marker, whether applied apically or basolaterally. Only when the tracers were taken up from the basolateral surface and then chased for periods of 2–18 hours, were DAVs labelled. It is proposed that entry of an endocytic tracer to DAVs occurs by the association of the desmosomal vacuole with late endosomes. Immunolabelling studies with antibodies to desmosomal components (to Dsg, DPI/II), to HRP and to the cation-independent mannose 6-phosphate receptor (MPR), confirmed that Dsg and DPI/II are located within DAVs and late endosomes, but not in early endosomes. Passage of Dsg, but to a lesser extent DPI/II, was detected in MPR- structures (lysosomes). DAV-like structures have also been observed in developing tissues such as mouse kidney. Such engulfment may provide a general mechanism for handling insoluble junctional proteins, particularly where rapid morphogenetic changes are occurring in the pattern of cell-cell adhesion.

Disorganization of microfilaments and intermediate filaments interferes with the assembly and stability of desmosomes in MDCK epithelial cells

To investigate the possible role(s) of cytoskeletal elements in desmosome assembly we have studied the effects of cytostatic drugs on the assembly of desmosomes in MDCK epithelial cells. We showed previously [Pasdar et al.: Cell Motil. Cytoskeleton 23:201-213, 1992] that selective disruption of microtubules has no effect on desmosome assembly. Here, we have treated MDCK cells with cytochalasin B and a combination of cytochalasin B and nocodazole and analysed the effects of desmosome assembly. Immunofluorescence analysis of MDCK cultures following drug treatment indicated complete disruption of actin microfilaments and disorganization of cytokeratin intermediate filaments. Biochemical analysis of newly synthesized desmosomal membrane core glycoproteins as well as the cell adhesion protein E-cadherin revealed no effect of these drugs on the kinetics of synthesis, intracellular processing, or transport to the plasma membrane either in the presence or absence of cell-cell contact. However, morphological analyses revealed a significant disruption in the spatial organization of desmosomal proteins and E-cadherin. Drug treatment in the absence of cell-cell contact resulted in the disruption of the normally observed homogeneous punctate staining pattern and appearance of aggregate staining. Induction of cell-cell contact in these cultures resulted in redistribution of some of the aggregate staining to the plasma membrane. In contrast to control cultures, significant amount of intracellular staining was retained for all desmosomal proteins. Biochemical analyses of turnover rates of newly synthesized desmosomal proteins indicated a significant decrease in metabolic stability of these proteins while the turnover rate of E-cadherin was not significantly different among control and drug-treated cultures. Taken together, these results suggest that intact actin and cytokeratin filaments are necessary for the stability, efficient assembly, and spatial organization of the junctional components at the membrane. The regulatory role of cytokeratins and actin filaments in assembly and stability of desmosomes on the plasma membrane is discussed.

Structure and interactions of desmosomal and other cadherins

The cadherin superfamily of cell-cell adhesion molecules is now known to include proteins of the desmosome as well as of the adherens type of junction. The desmosomal cadherins consist of two families of proteins, the desmocollins and the desmogleins, both of which are represented by different isoforms which are differentially expressed in epidermis. The desmocollins are quite similar to the classic cadherins in overall structure, but with alternatively spliced variants; the desmogleins have extra cytoplasmic sequences added onto the basic cadherin structure. The cytoplasmic domains are specialized for binding to 'mediator' proteins, such as plakoglobin, which interconnect to the intermediate filament system rather than the actin filaments as do the classic cadherins.

Temporal changes in the expression and distribution of adhesion molecules during liver development and regeneration

We have compared by immunocytochemistry and immunoblotting the expression and distribution of adhesion molecules participating in cell-matrix and cell-cell interactions during embryonic development and regeneration of rat liver. Fibronectin and the fibronectin receptor, integrin alpha 5 beta 1, were distributed pericellularly and expressed at a steady level during development from the 16th day of gestation and in neonate and adult liver. AGp110, a nonintegrin fibronectin receptor was first detected on the 17th day of gestation in a similar, nonpolarized distribution on parenchymal cell surfaces. At that stage of development haemopoiesis is at a peak in rat liver and fibronectin and receptors alpha 5 beta 1 and AGp110 were prominent on the surface of blood cell precursors. During the last 2 d of gestation (20th and 21st day) hepatocytes assembled around lumina. AGp110 was initially depolarized on the surface of these acinar cells but then confined to the lumen and to newly-formed bile canaliculi. At birth, a marked increase occurred in the canalicular expression of AGp110 and in the branching of the canalicular network. Simultaneously, there was enhanced expression of ZO-1, a protein component of tight junctions. On the second day postpartum, presence of AGp110 and of protein constituents of desmosomes and intermediate junctions, DGI and E-cadherin, respectively, was notably enhanced in cellular fractions insoluble in nonionic detergents, presumably signifying linkage of AGp110 with the cytoskeleton and assembly of desmosomal and intermediate junctions. During liver regeneration after partial hepatectomy, AGp110 remained confined to apical surfaces, indicating a preservation of basic polarity in parenchymal cells. A decrease in the extent and continuity of the canalicular network occurred in proliferating parenchyma, starting 24 h after resection in areas close to the terminal afferent blood supply of portal veins and spreading to the rest of the liver within the next 24 h. Distinct acinar structures, similar to the ones in prenatal liver, appeared at 72 h after hepatectomy. Restoration of the normal branching of the biliary tree commenced at 72 h. At 7 d postoperatively acinar formation declined and one-cell-thick hepatic plates, as in normal liver, were observed.

Desmocollins form a distinct subset of the cadherin family of cell adhesion molecules

The desmosomal adhesive core is formed by four major components: desmoglein (Mr, 165,000), desmocollins I and II (Mr, 120,000 and 110,000, respectively), and a Mr 22,000 protein. Here, we report the cloning and sequencing of cDNAs encoding a bovine desmocollin. The open reading frame found in the longest cDNA, 5 kilobases, contains a region encoding a protein of 839 amino acids. The features of the deduced amino acid sequence imply that the mature 707-amino acid desmocollin is a type I transmembrane protein that is produced by proteolytic cleavage of an 810-amino acid precursor. The ectodomain of desmocollin contains repeats that show extensive sequence similarity to members of the cadherin family of calcium-dependent cell adhesion molecules. A comparison of the amino acid sequences of desmocollin, desmoglein, and the cadherins shows that although these intercellular junctional adhesion molecules share a consensus sequence in their adhesive domains that defines them as a family, several features, including the divergence in the sequence of their cytoplasmic tails, divide them into three distinct subtypes.

Desmoglein shows extensive homology to the cadherin family of cell adhesion molecules

Desmoglein is a major adhesive component of the desmosome. It is also at least one of the antigenic targets of pathogenic antibodies circulating in the sera of patients with the blistering disease Pemphigus foliaceus. To examine the molecular basis of desmosomal adhesion and to further our understanding of its disruption in various bullous disorders we have cloned cDNAs encoding four of the extracellular domains of desmoglein. The predicted amino acid sequence of these clones shows extensive homology with the cadherin class of calcium-dependent cell adhesion molecules. Desmoglein represents a novel subtype of this family.

The hemidesmosomal plaque. I. Characterization of a major constituent protein as a differentiation marker for certain forms of epithelia

To examine whether constituent proteins of hemidesmosomal structures can be used as markers for certain pathways of epithelial differentiation we have examined the occurrence of the major M- approximately 230,000 plaque protein, the "bullous pemphigoid" (BP) antigen. Several bovine, rat and human tissues and bovine cell culture lines were examined, using different human autoantibody preparations in immunocytochemistry and immunoblotting. We report that this protein, also unequivocally identified by cDNA cloning from expression libraries and DNA sequencing, occurs not only in different stratified epithelia but also, apparently always in hemidesmosomal structures, in urothelium of bladder and the complex epithelia of trachea, bronchus and several glands, notably myoepithelium-containing skin glands, the mammary gland and salivary glands. The protein is absent, however, in all single-layered epithelia and in several tissues reported to have subplasmalemmal densities structurally similar to hemidesmosomes, such as Purkinje fibers of heart, meninges and perineuria. A mammary-gland-derived epithelial cell line (BMGE + H) is particularly rich in hemidesmosomes. This has been used to study the endocytotic uptake of hemidesmosome-containing plasma membrane domains into cytoplasmic vesicles upon detachment of cell sheets during treatment with dispase, a proteolytic enzyme. We propose to use the Mr- approximately 230,000 plaque protein as a marker selective for certain subsets of epithelial cell types and epithelium-derived tumors in studies of fetal and tumor development, including differentiation diagnosis of carcinomas.