Are desmosomes more than tethers for intermediate filaments?

Desmosomes are intercellular adhesive junctions that anchor intermediate filaments at membrane-associated plaques in adjoining cells, thereby forming a three-dimensional supracellular scaffolding that provides tissues with mechanical strength. But desmosomes have also recently been recognized as sensors that respond to environmental and cellular cues by modulating their assembly state and, possibly, their signalling functions.     

The desmosome and pemphigus

Desmosomes are patch-like intercellular adhering junctions ("maculae adherentes"), which, in concert with the related adherens junctions, provide the mechanical strength to intercellular adhesion. Therefore, it is not surprising that desmosomes are abundant in tissues subjected to significant mechanical stress such as stratified epithelia and myocardium. Desmosomal adhesion is based on the Ca(2+)-dependent, homo- and heterophilic transinteraction of cadherin-type adhesion molecules. Desmosomal cadherins are anchored to the intermediate filament cytoskeleton by adaptor proteins of the armadillo and plakin families. Desmosomes are dynamic structures subjected to regulation and are therefore targets of signalling pathways, which control their molecular composition and adhesive properties. Moreover, evidence is emerging that desmosomal components themselves take part in outside-in signalling under physiologic and pathologic conditions. Disturbed desmosomal adhesion contributes to the pathogenesis of a number of diseases such as pemphigus, which is caused by autoantibodies against desmosomal cadherins. Beside pemphigus, desmosome-associated diseases are caused by other mechanisms such as genetic defects or bacterial toxins. Because most of these diseases affect the skin, desmosomes are interesting not only for cell biologists who are inspired by their complex structure and molecular composition, but also for clinical physicians who are confronted with patients suffering from severe blistering skin diseases such as pemphigus. To develop disease-specific therapeutic approaches, more insights into the molecular composition and regulation of desmosomes are required.     

Desmosomes in the Heart: A Review of Clinical and Mechanistic Analyses

Abstract

Desmosomes have long been appreciated as intercellular junctions that are vital for maintaining the structural integrity of stratified epithelia. More recent clinical investigations of patients with diseases such as arrhythmogenic cardiomyopathy have further highlighted the importance of desmosomes in cardiac tissue, where they help to maintain coordination of cardiac myocytes. Here, we review clinical and mechanistic studies that provide insight into the functions of desmosomal proteins in skin and heart during homeostasis and in disease. While intercellular junctions are organized differently in cardiac and epithelial tissues, studies conducted in epithelial systems may inform our understanding of cardiac desmosomes. We explore traditional and non-traditional roles of desmosomal proteins, ranging from adhesive capacities to nuclear functions. Finally, we discuss how these studies can guide future investigations focused on determining the molecular mechanisms by which desmosomal mutations promote the development of cardiac diseases.     

Rescuing desmoplakin function in extra-embryonic ectoderm reveals the importance of this protein in embryonic heart, neuroepithelium, skin and vasculature

Desmosomes mediate intercellular adhesion through desmosomal cadherins, which interface with plakoglobin (PG) and desmoplakin (DP) to associate with the intermediate filament (IF) cytoskeleton. Desmosomes first assemble in the E3.5 mouse trophectoderm, concomitant with establishment of epithelial polarity and appearance of a blastocoel cavity. Increasing in size and number, desmosomes continue their prominence in extra-embryonic tissues, but as development proceeds, they also become abundant in a number of embryonic tissues, including heart muscle, epidermis and neuroepithelium. Previously, we explored the functional importance of desmosomes by ablating the Dsp gene. Homozygous Dsp mutant embryos progressed through implantation, but did not survive beyond E6.5, owing to a loss or instability of desmosomes and tissue integrity. We have now rescued the extra-embryonic tissues by aggregation of tetraploid (wild-type) and diploid (Dsp mutant) morulae. These animals survive several days longer, but die shortly after gastrulation, with major defects in the heart muscle, neuroepithelium and skin epithelium, all of which possess desmosomes, as well as the microvasculature, which does not. Interestingly, although wild-type endothelial cells of capillaries do not form desmosomes, they possess unusual intercellular junctions composed of DP, PG and VE-cadherin. The severity in phenotype and the breadth of defects in the Dsp mutant embryo is greater than PG mutant embryos, substantiating redundancy between PG and other armadillo proteins (e.g. beta-catenin). The timing of lethality is similar to that of the VE-cadherin null embryo, suggesting that a participating cause of death may be a defect in vasculature, not reported for PG null embryos.     

Desmosomal adhesiveness is developmentally regulated in the mouse embryo and modulated during trophectoderm migration

During embryonic development tissues remain malleable to participate in morphogenetic movements but on completion of morphogenesis they must acquire the toughness essential for independent adult life. Desmosomes are cell-cell junctions that maintain tissue integrity especially where resistance to mechanical stress is required. Desmosomes in adult tissues are termed hyper-adhesive because they adhere strongly and are experimentally resistant to extracellular calcium chelation. Wounding results in weakening of desmosomal adhesion to a calcium-dependent state, presumably to facilitate cell migration and wound closure. Since desmosomes appear early in mouse tissue development we hypothesised that initial weak adhesion would be followed by acquisition of hyper-adhesion, the opposite of what happens on wounding. We show that epidermal desmosomes are calcium-dependent until embryonic day 12 (E12) and become hyper-adhesive by E14. Similarly, trophectodermal desmosomes change from calcium-dependence to hyper-adhesiveness as blastocyst development proceeds from E3 to E4.5. In both, development of hyper-adhesion is accompanied by the appearance of a midline between the plasma membranes supporting previous evidence that hyper-adhesiveness depends on the organised arrangement of desmosomal cadherins. By contrast, adherens junctions remain calcium-dependent throughout but tight junctions become calcium-independent as desmosomes mature. Using protein kinase C (PKC) activation and PKCα-/- mice, we provide evidence suggesting that conventional PKC isoforms are involved in developmental progression to hyper-adhesiveness. We demonstrate that modulation of desmosomal adhesion by PKC can regulate migration of trophectoderm. It appears that tissue stabilisation is one of several roles played by desmosomes in animal development.     

On the spatial relationship between fibroblasts and myogenic cells during early development of skeletal muscles

The spatial relation between fibroblasts and myoblasts as well as myotubes in the extensor muscles of the forearm of mouse embryos was investigated with routine electron microscopic techniques. From day 13 of embryonic development onwards, fibroblasts and myogenic cells may be distinguished by their ultrastructure from each other. Above all the fibroblasts are characterized by their long cell processes. Specialized cell contacts between them are visible. Fibroblasts often encircle some myogenic cells indicating the division of the muscles into smaller bundles. Over long distances, the intercellular gap between both cell types is narrow and measures about 15 nm. Sometimes specialized intercellular junctions can be seen. They resemble simple desmosomes and seem to be mechanical cell junctions. After the establishment of the basal lamina around the myogenic cells such specialized cell contacts are no longer visible. It seems possible that the fibroblasts act as mechanical counterpart for the organization of the myogenic cells.     

Desmosomal Adhesion In Vivo

Abstract

Desmosomes are intercellular junctions that provide strong adhesion or hyper-adhesion in tissues. Here, we discuss the molecular and structural basis of this with particular reference to the desmosomal cadherins (DCs), their isoforms and evolution. We also assess the role of DCs as regulators of epithelial differentiation. New data on the role of desmosomes in development and human disease, especially wound healing and pemphigus, are briefly discussed, and the importance of regulation of the adhesiveness of desmosomes in tissue dynamics is considered.     

Down-Regulation of Desmosomes in Cultured Cells: The Roles of PKC,Microtubules and Lysosomal/Proteasomal Degradation

Desmosomes are intercellular adhesive junctions of major importance for tissue integrity. To allow cell motility and migration they are down-regulated in epidermal wound healing. Electron microscopy indicates that whole desmosomes are internalised by cells in tissues, but the mechanism of down-regulation is unclear. In this paper we provide an overview of the internalisation of half-desmosomes by cultured cells induced by calcium chelation. Our results show that: (i) half desmosome internalisation is dependent on conventional PKC isoforms; (ii) microtubules transport internalised half desmosomes to the region of the centrosome by a kinesin-dependent mechanism; (iii) desmosomal proteins remain colocalised after internalisation and are not recycled to the cell surface; (iv) internalised desmosomes are degraded by the combined action of lysosomes and proteasomes. We also confirm that half desmosome internalisation is dependent upon the actin cytoskeleton. These results suggest that half desmosomes are not disassembled and recycled during or after internalisation but instead are transported to the centrosomal region where they are degraded. These findings may have significance for the down-regulation of desmosomes in wounds.     

Formation of junctions and cell sorting in aggregates of chick and mouse cells

The frequency of desmosome formation was examined in aggregates of old cells, which form many junctions, combined with young cells, which form few. Cells of chick corneal epithelium and mouse epidermis, which can be distinguished morphologically, were combined. Desmosomes between these cell types are stable. Further, young cells make more desmosomes than they otherwise would on those surfaces adjoining old cells. Desmosomes increase in number in aggregates while cell sorting is occurring. Cells consistently sort, with those which form most desmosomes lying internally. Gap junctions and intermediate junctions are also present, but are uncommon. A carbohydrate cell-surface coat has regenerated by the time desmosome formation starts. The possible relation of desmosome formation to cell sorting is discussed.     

Membrane-impermeable cross-linking provides evidence for homophilic, isoform-specific binding of desmosomal cadherins in epithelial cells

Desmosomes and adherens junctions are cadherin-based protein complexes responsible for cell-cell adhesion of epithelial cells. Type 1 cadherins of adherens junctions show specific homophilic adhesion that plays a major role in developmental tissue segregation. The desmosomal cadherins, desmocollin and desmoglein, occur as several different isoforms with overlapping expression in some tissues where different isoforms are located in the same desmosomes. Although adhesive binding of desmosomal cadherins has been investigated in a variety of ways, their interaction in desmosome-forming epithelial cells has not been studied. Here, using extracellular homobifunctional cross-linking, we provide evidence for homophilic and isoform-specific binding between the Dsc2, Dsc3, Dsg2, and Dsg3 isoforms in HaCaT keratinocytes and show that it represents trans interaction. Furthermore, the cross-linked adducts are present in the detergent-insoluble fraction, and electron microscopy shows that extracellular cross-linking probably occurs in desmosomes. We found no evidence for either heterophilic or cis interaction, but neither can be completely excluded by our data. Mutation of amino acid residues Trp-2 and Ala-80 that are important for trans interaction in classical cadherin adhesive binding abolished Dsc2 binding, indicating that these residues are also involved in desmosomal adhesion. These interactions of desmosomal cadherins may be of key importance for their ordered arrangement within desmosomes that we believe is essential for desmosomal adhesive strength and the maintenance of tissue integrity.     

金鱼精巢支持细胞间连接和血睾屏障

Freeze-fracture and etching technique combined with thin sectioning and lanthanum impregnation has been used for the study of Sertoli cell junctions and the blood-testis barrier formation in goldfish testis with lobular organization. Some observations and results are first given in this paper. The results of experiments can be summarized as the following: 1). Sertoli cell junctions are compound junctions of tight junctions, desmosomes and gap junctions. Tight junctions usually appear as parallel or network like ridges on the P face and fine grooves on the E face at the freeze-etching replicas. Desmosomes and gap junctions often are located between or nearby the ridges of tight junctions. In addition, endoplasmic reticulum cristae near the junction area can also be observed. 2). The number, area and density of each individual junction vary with the development and differentiation stages of germinal cells in the cyst. 3). Tight junctions can be observed at any stage during germinal cell differentiation through the period of spermatogenesis and spermiogenesis. However, they appear morphologically different as type I and type II. 4). Lanthanum can partially penetrate into the intercellular spaces of spermatogonium and early primary spermatocyte but can't penetrate after the stage of late primary spermatocyte. 5). The blood-testis barrier formation starts at the stage of pachytene spermatocytes. The formation of the blood-testis barrier is the result of the development of the tight junction from type I to type II.     

Sec3-containing Exocyst Complex Is Required for Desmosome Assembly in Mammalian Epithelial Cells

The Exocyst is a conserved multisubunit complex involved in the docking of post-Golgi transport vesicles to sites of membrane remodeling during cellular processes such as polarization, migration, and division. In mammalian epithelial cells, Exocyst complexes are recruited to nascent sites of cell–cell contact in response to E-cadherin–mediated adhesive interactions, and this event is an important early step in the assembly of intercellular junctions. Sec3 has been hypothesized to function as a spatial landmark for the development of polarity in budding yeast, but its role in epithelial cells has not been investigated. Here, we provide evidence in support of a function for a Sec3-containing Exocyst complex in the assembly or maintenance of desmosomes, adhesive junctions that link intermediate filament networks to sites of strong intercellular adhesion. We show that Sec3 associates with a subset of Exocyst complexes that are enriched at desmosomes. Moreover, we found that membrane recruitment of Sec3 is dependent on cadherin-mediated adhesion but occurs later than that of the known Exocyst components Sec6 and Sec8 that are recruited to adherens junctions. RNA interference-mediated suppression of Sec3 expression led to specific impairment of both the morphology and function of desmosomes, without noticeable effect on adherens junctions. These results suggest that two different exocyst complexes may function in basal–lateral membrane trafficking and will enable us to better understand how exocytosis is spatially organized during development of epithelial plasma membrane domains.     

Desmosomal cadherins in zebrafish epiboly and gastrulation

Background  

The desmosomal cadherins (DCs), desmocollin (Dsc) and desmoglein (Dsg), are the adhesion molecules of desmosomes, intercellular adhesive junctions of epithelia and cardiac muscle. Both the DCs and desmosomes have demonstrably essential roles in mammalian development. In order to initiate their study in a more tractable developmental system we have characterised zebrafish DCs and examined their roles in early zebrafish development.     

Desmosomes from a structural perspective

Desmosomes are cell-cell junctions responsible for maintaining the structural integrity of tissues by resisting shear forces. Defects result in diseases of mechanically challenged tissues such as skin and heart. The architectural design represents the key to understanding the strength and durability inherent to desmosomes. A number of different proteins contribute to this architecture, and X-ray crystallography has made considerable progress in defining the atomic structure of various isolated domains. Electron tomography has been used to determine the three-dimensional structure of intact desmosomes in situ. By combining information from X-ray crystallography, cell and molecular biology and electron tomography, it should ultimately be possible to deduce the specific protein interactions that define the mechanical properties of this important adhesive junction.     

桥粒蛋白与离子转运相关通道

桥粒为细胞与细胞之间的一种连接结构,参与细胞间机械应力传导. 在心肌组织中,桥粒与粘着连接及缝隙连接共同构成闰盘,对于维护心肌闰盘结构和功能的完整性具有重要作用. 近年来,越来越多的研究表明,桥粒蛋白基因突变、表达的缺失或功能异常,可引起心肌细胞钠、钾离子通道、缝隙连接蛋白等心肌电活动相关结构的重塑,增加心肌电学异质性,进而促发心律失常. 本文将就桥粒蛋白与离子转运相关通道关系的最新研究进展进行综述.     

Desmosomal adhesion: structural basis,molecular mechanism and regulation (Review)

Desmosomes are adhesive intercellular junctions of epithelia and cardiac muscle. They have an essential function in maintaining the integrity of tissues, which is compromised in human genetic and autoimmune disease that targets desmosomal components. Recent evidence (1) suggests new roles for the function of desmosomal adhesion in tissue morphogenesis, (2) gives new insights into the molecular mechanism of adhesion, (3) indicates that the desmosomal adhesion molecules, desmocollin and desmoglein, may contribute to the regulation of epidermal diffentiation, and (4) shows that the affinity of desmosomal adhesion is regulated by protein kinase C.     

Various types of non-synaptic intercellular contacts in the developing brain of the rat

Peculiarities of ultrastructural organization and localization of early forms of avascular nonsynaptic types of junctions formed in 14-18-day-old rat embryos have been studied; cerebral structures different in their phylogenic relations (the sensomotor cortex and nucleus caudatus) are taken as an example. Five main types of nonsynaptic intercellular junctions have been revealed: desmosome-like, gap, symmetric, asymmetric and mixed junctions. They differ by their ultrastructural organization. These types of junctions make the main types of contacts: soma-somatic, dendro-somatic, dendro-dendritic, axo-somatic, axo-dendritic. Desmosomes form the greatest number of the contacts. The earliest and the most primitive are gap junctions; they, evidently, reflect functional activity of desmosome-like junctions. The mixed junctions, perhaps, reflect the developmental stages of the intercellular contacts of transition from one type of junctions into another. Localization peculiarities of the nonsynaptic intercellular contacts are demonstrated: glomerule-like formations, establishment of numerous contacts looking like a successive chain, and so on. For some other indices a longer period of intercellular contact formation in the nucleus caudatus is noted, comparing the sensomotor cortex, though the latter is a newer structural cerebral formation from the phylogenic point of view.     

Formation of intercellular contacts in myogenesis

In myogenesis in vivo and in the muscle tissue culture certain intercellular junctions have been revealed; they differ in their ultrastructure and functions. For the stage of interaction between a myoblast with another myoblast contacts of adhesive type are distinctive: desmosomes and fasciae adherentes. They are necessary for adhesion of the cells with each other. Besides, gap and punctate contacts occur, serving for exchange of metabolites and electrical conjugation. At more advanced stages of fusion, when the myoblast gets into contact with the early muscle tubule, a bridge contact is observed, resembling the septal one, which is able to transform into a pentalayered (tight) junction. The latter type evidently participates in fusion of the membranes of the interacting cells.     

Desmosomal adhesion: structural basis,molecular mechanism and regulation (Review)

Desmosomes are adhesive intercellular junctions of epithelia and cardiac muscle. They have an essential function in maintaining the integrity of tissues, which is compromised in human genetic and autoimmune disease that targets desmosomal components. Recent evidence (1) suggests new roles for the function of desmosomal adhesion in tissue morphogenesis, (2) gives new insights into the molecular mechanism of adhesion, (3) indicates that the desmosomal adhesion molecules, desmocollin and desmoglein, may contribute to the regulation of epidermal diffentiation, and (4) shows that the affinity of desmosomal adhesion is regulated by protein kinase C.