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Keywords:

  • desmosome;
  • cadherins;
  • desmoplakin;
  • hyperadhesion;
  • proteases;
  • pemphigus;
  • cystatin A;
  • ADAM17;
  • ARVC

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Infectious and auto-immune diseases of desmosomes
  5. Inherited desmosomal disorders
  6. Other desmosome-related disorders
  7. Non-adhering functions of desmosomal components in disease
  8. Acknowledgements
  9. Author contributions
  10. References

Cell–cell connectivity is an absolute requirement for the correct functioning of cells, tissues and entire organisms. At the level of the individual cell, direct cell–cell adherence and communication is mediated by the intercellular junction complexes: desmosomes, adherens, tight and gap junctions. A broad spectrum of inherited, infectious and auto-immune diseases can affect the proper function of intercellular junctions and result in either diseases affecting specific individual tissues or widespread syndromic conditions. A particularly diverse group of diseases result from direct or indirect disruption of desmosomes—a consequence of their importance in tissue integrity, their extensive distribution, complex structure, and the wide variety of functions their components accomplish. As a consequence, disruption of desmosomal assembly, structure or integrity disrupts not only their intercellular adhesive function but also their functions in cell communication and regulation, leading to such diverse pathologies as cardiomyopathy, epidermal and mucosal blistering, palmoplantar keratoderma, woolly hair, keratosis, epidermolysis bullosa, ectodermal dysplasia and alopecia. Here, as well as describing the importance of the other intercellular junctions, we focus primarily on the desmosome, its structure and its role in disease. We will examine the various pathologies that result from impairment of desmosome function and thereby demonstrate the importance of desmosomes to tissues and to the organism as a whole. Copyright © 2011 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Infectious and auto-immune diseases of desmosomes
  5. Inherited desmosomal disorders
  6. Other desmosome-related disorders
  7. Non-adhering functions of desmosomal components in disease
  8. Acknowledgements
  9. Author contributions
  10. References

Connections between individual cells are a hallmark of, and an absolute requirement for, multicellular life. Connectivity in terms of intercellular communication can be accomplished between distantly separated cells within an organism by various means, such as nervous transmission, hormonal signalling in endocrine or paracrine fashions, or through specialized fluids that function specifically to link distant cells, such as the blood or lymph. Meanwhile, direct cell–cell connections are mediated by intercellular junction complexes; a diverse group of organelles—including desmosomes, adherens, tight and gap junctions—which facilitate adherence and communication between individual cells and maintain the integrity of larger tissues. The importance of intercellular junctions, as with many other features of the human body, is most amply demonstrated by their involvement in a wide-ranging variety of diseases, of genetic, auto-immune, cancerous and infectious aetiology. This review will examine the importance of cell–cell connectivity by discussing the basis of diseases involving the intercellular junctions. Particular attention will be paid to desmosomes, whose wide distribution and key roles in maintaining tissue integrity and in cellular signalling mean that a broad variety of diseases can result from their dysfunction or dysregulation.

Non-desmosomal intercellular junctions in disease

Adherens junctions can be grouped with desmosomes as ‘anchoring’ or ‘adhering’ junctions, whose role primarily features organizing and tethering the cytoskeleton (condensed actin filaments in the case of adherens junctions, intermediate filaments for desmosomes) to the plasma membrane, and maintaining close physical association between cells 1. Sharing a kind of structural homology with desmosomes, adherens junctions are assembled from: classical cadherins (such as E-, N- and P-cadherin)—calcium-dependent transmembrane proteins extending out from the cell surface, which bind cadherins on adjacent cells, originally named for the tissue in which they are were thought to be mainly expressed (eg E-cadherin in epithelium, N-cadherin in neuronal tissue) 2; armadillo proteins (such as β-catenin and plakoglobin (PG)—the latter also in desmosomes), which bind to intracellular domains of the cadherins and have important roles in signalling pathways and adherence 3; and cytoskeletal adaptor proteins (such as α-catenin), which were thought to provide a link between the armadillo proteins and the actin cytoskeleton until it was shown that α-catenin in complex with a cadherin and β-catenin cannot bind F-actin 4. Diseases that result from mutations in adherens junction components reflect their wide distribution in many tissues. For example, deficiencies of P-cadherin have been reported in the related conditions hypotrichosis with juvenile macular dystrophy (HJMD) 5 and ectodermal dysplasia, ectrodactyly and macular dystrophy (EEM syndrome) 6. Both feature early hair loss and progressive degeneration of the central retina, with the latter accompanied by hypodontia and limb defects such as hand or foot malformation, the affected bodily sites correlating with areas of strong P-cadherin expression during development 7. As well as its function in adherens junctions, β-catenin is also a crucial component of the Wnt signalling pathway, which regulates development and homeostasis via gene expression, cell growth, survival and polarity 8. Wnt pathway signalling is regulated by phosphorylation and subsequent degradation of β-catenin under normal circumstances, meaning that somatic mutations in β-catenin itself, or in those proteins involved in its phosphorylation or degradation [including Dishevelled, Axin, Adenomatous Polyposis Coli (APC) and glycogen synthase kinase-3β] can lead to constitutive β-catenin activation and thus cancer development 8. Mutations in these proteins have been observed in a very wide variety of tumour types and are reviewed well by Polakis 9 and Xueling and Xin 10, among others.

Gap junctions serve to permit intercellular communication, a function vital for controlling homeostasis in organisms and for responses to external stimuli, which they accomplish by allowing the transfer of ions (including Ca2+ and Mg2+) and small molecules of < 1 kDa (such as cAMP, cGMP and ATP) between cells 11. Gap junctions consist of plaques of many small channels, each the product of two hexameric hemi-channels on closely apposed cell membranes. Each hemi-channel is a homo- or heteromeric hexamer made up of connexins (Cx), a protein family of 21 members in humans, whose members are named according to their molecular mass (eg Cx26, weighing approximately 26 kDa) 12. Connexins are differentially expressed in the human body, with multiple types expressed in any single tissue type. The range of disorders—including skin disease, non-syndromic deafness, neuropathy, and syndromic diseases (affecting multiple tissue types) 13—associated with Cx mutations illustrates their wide distribution and physiological importance. Connexins' widespread role is further demonstrated by the fact that different mutations in the same connexin gene can lead to completely different phenotypes, with either dominant or recessive inheritance. For example, mutations in GJB3 (the gene encoding Cx31) can result in non-syndromic deafness, erythrokeratoderma variabilis alone, or peripheral neuropathy with deafness (reviewed by Scott and Kelsell 11). Deafness as a result of connexin mutations (particularly affecting Cx26) accounts for a major proportion of genetic non-syndromic hearing loss in many ethnic groups 14, 15.

The primary role of the tight junctions is thought to be in the maintenance of the epithelial barrier, where they function to regulate the permeability of the paracellular pathway between epithelial cells; tightly sealing the barrier to the point of high resistance in tissues such as the bladder, whilst providing a more permeable barrier—to facilitate water and nutrient absorption—in the intestinal epithelium, for example 16. The best characterized of the proteins that make up tight junctions are the claudins, a transmembrane protein family of at least 24 members, expressed in tissue-specific combinations 17. Although almost no information is known about how claudins oligomerize into the higher-order structures seen in tight junctions, or now they adhere across the intercellular space, their first extracellular loop has been shown to define to which ions (and non-ionic solutes) a tight junction is permeable—defining its ionic charge selectivity 18. Proteins associated with claudins at the tight junction include the cytoplasmic ZO-1, -2 and -3 family and other transmembrane proteins, such as occludin and tricellulin. The tight junction cytoplasmic plaque and non-claudin transmembrane proteins are reviewed by Schneeberger and Lynch 19. Mutations in a variety of the genes encoding the claudins constitute the majority of tight junction-associated disease. CLDN16 (claudin 16) loss-of-function mutations, for example, cause hypomagnesaemia, hypercalciuria and nephrocalcinosis, as a result of impaired Ca2+ and Mg2+ reabsorbtion via cation-selective tight junctions in the renal tubule 20, 21, whereas mutations in CLDN19 (claudin 19) can cause a similar syndrome accompanied by visual impairment 22. Mutations in the associated proteins ZO-2 23 and tricellulin 24 can lead to familial hypercholanaemia and non-syndromic deafness, respectively.

Desmosome structure and component function

Desmosomes not only provide mechanical stability but also facilitate cell–cell communication through signal transmission. Desmosomal structures have been observed in various tissue types that experience mechanical stress, such as the intestinal mucosa, gallbladder, uterus and oviduct, liver, pancreas, stomach, salivary and thyroid glands and the epithelial cells of the nephron, but are most abundant in the skin and myocardium 25–28. A primary function of desmosomes is the anchoring of cytoskeletal keratin intermediate filaments in the epidermis, desmin intermediate filaments in the heart and vimentin intermediate filaments in meningeal cells and the follicular dendritic cells of lymph nodes to the cell membrane 29.

The desmosome's structure was first observed by the Italian pathologist Bizzozero (1864); its structure has since been analysed by techniques such as electron microscopy (EM) to reveal a complex structure and organisation. The desmosome divides into three parallel identifiable zones, arranged symmetrically on the cytoplasmic faces of the plasma membranes of bordering cells and separated by the extracellular domain, which in mature desmosomes is bisected by a dense midline 30. Each desmosomal plaque consists of a thick outer dense plaque and a translucent inner dense plaque 31. The five major desmosomal components are the desmosomal cadherins, represented by desmogleins (DSG1-4) and desmocollins (DSC1-3), the armadillo family members, plakoglobin (PG) and the plakophilins (PKP1-3), and the plakin linker protein desmoplakin (DSP), which anchors the intermediate keratin filaments. The structure of a desmosome is illustrated in Figure 1.

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Figure 1. Desmosome structure. (A) EM of a young desmosome, which lacks a dense midline, connecting two adjacent cells in a keratinocyte monolayer. (B) Schematic representation of desmosomal structure. Desmogleins (DSGs) and desmocollins (DSCs) constitute the extracellular link between cells, and are responsible for the appearance of the dense midline; they form homo- and heterodimers, whose intracellular tails bind the Armadillo family proteins plakoglobin (PG) and the plakophilins (PKPs), which serve to link cadherins to the plakin protein desmoplakin (DSP), which in turn tethers desmosomes to the intermediate filament network

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Desmosomal cadherins and Ca2+-mediated hyperadhesion

Desmogleins (DSGs) and desmocollins (DSCs) are transmembrane components that bridge adjacent cells and are embedded in the cytoplasmic plaques, and have been identified to form the dense midline seen in mature desmosomes. They share 30% amino acid identity between each other and with classical cadherins 32, with DSC genes being more closely related to the classical cadherins 33.

Desmosomal cadherins are formed of five extracellular cadherin repeats containing Ca2+-binding sites and a cell-adhesion recognition (CAR) site 34, 35. All DSC gene products undergo alternative splicing to generate a complete DSC ‘a’ form and a shortened DSC ‘b’ form of the protein by the insertion of a mini-exon containing a stop codon, the length of their carboxy-terminal domain being the only difference between the two isoforms 36.

The desmosomal cadherins show complex developmental and differentiation-specific patterns of expression 28, which implies that desmosomes within different tissues are biochemically and functionally distinct. The precise role for the tissue-specific expression patterns of desmosomal cadherins is not fully understood, but manipulation of desmosomal cadherin expression suggests that tight regulation of their expression pattern is critical to tissue homeostasis 37. Within the epidermis, these genes are differentially expressed as keratinocytes undergo terminal differentiation 38, 28: DSG1 and DSC1 are strongly expressed in the granular and spinous layers, their levels decreasing in the lower levels of the epidermis 39–41; DSG2 and DSC2 are expressed in all desmosome-bearing tissues and represent the predominant isoforms in simple epithelia 42, 43, and are mainly expressed in the basal layer of stratified epidermis 32, 41. DSG4 is primarily expressed in the hair follicle and is restricted to the granular layer in stratified epithelia 44. DSG1, DSG3 and DSG4, and DSC1 and DSG3, are predominantly expressed in the epidermis, whereas DSG2 and DSC2 are highly expressed in the myocardium 45. Figure 2 summarizes the differential expression patterns of desmosomal cadherins in the epidermis.

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Figure 2. Desmosomal cadherin expression in the epidermis. This figure illustrates the differential distribution and relative expression levels of the desmosomal cadherins—the DSGs and DSCs—in the epidermis. The location and depth of blistering observed in diseases such as pemphigus vulgaris and foliaceus reflects this distribution

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Within the cornified layer of the epidermis (stratum corneum), desmosomes are modified into corneodesmosomes; structures that contain DSG1, DSC1 and corneodesmosin as their major extracellular constituents. The relative thickness of the stratum corneum is stabilized by the controlled degradation of corneodesmosomes, any modifications at this level leading to severe barrier defects 46. Indeed, mutations in the corneodesmosin (CDSN) gene are responsible for inherited skin and hair defects, such as peeling skin syndrome 47, 48 and hypotrichosis 49, 50. Meanwhile, impaired transport of cellular lipids and corneodesmosomal-associated proteases into the intercellular space of the epidermis, as a result of mutations in the transporter protein ABCA12, underlies the pathogenesis of the severe and often lethal congenital disorder harlequin ichthyosis 51, 52.

DSGs and DSCs are required for strong cell–cell adhesion 53 via their interaction with each other across the intercellular space, in a homophilic and/or heterophilic manner; the difference between the two types of interaction remaining unclear. The first types of junction to form between adjacent cells are adherens junctions, which create a first point of contact between cells and facilitate the subsequent formation of desmosomes in response to cell–cell contact, but also by raising extracellular levels of calcium (Ca2+) 54, 55. Via several binding motifs within their structure, DSGs and DSCs can bind Ca2+ and assume a rigidified functional conformation 56, and thereby increase the level of adhesion between neighbouring cells and create what has been described as ‘the dense midline of desmosomes’. In low-Ca2+ conditions the desmosomal plaque components and membrane proteins are transported to the plasma membrane in separate compartments, but when desmosomal assembly is triggered, cadherin and DSP complexes do not associate as in normal Ca2+ conditions and remain separate 57. It has been observed that during the early stages of desmosome formation the assembly can reverse between the mature and young phases, but ultimately desmosomes mature and can no longer be dissociated by calcium depletion 54; the adhesion strength in cultured keratinocytes increases after 6 days in culture due to this phenomenon 57 in a similar way to intact epithelia in vivo58, 59. This is referred to as hyperadhesion and represents the result of high-affinity and stable adhesive binding of desmosomal components into mature structures; it has not been observed in adherens or tight junctions, making it specific to desmosomes and explaining the hyperadhesive state of keratinocytes 60.

There are various situations in which desmosomes switch from a Ca2+-independent to a Ca2+-dependent state. It has been observed that upon wounding of keratinocyte cell cultures, the desmosomes of cells situated at the edge of the wound lose their hyperadhesiveness and become Ca2+-dependent 58, permitting the cell motility required for wound re-epithelialization. Other situations include mitotically active basal cells and during tumour invasion. Kimura et al60 have shown that the transition from a Ca2+-dependent to a Ca2+-independent state with the induction of hyperadhesion proceeds via modulation of protein kinase C-α (PKCα) signalling.

Although the passage of desmosomes from a less adhesive to a hyperadhesive state is Ca2+-dependent, O-glycosylation of the desmosomal plaque component PG has also been shown to augment desmosomal adhesion in keratinocytes 61.

Whilst Ca2+ ions regulate the transition of desmosomes from a hyperadhesive state into a less adhesive state and vice versa, there are other components, such as p120-catenin an armadillo-repeat protein, which regulate the formation of desmosomes. Kanno et al have observed that the interaction of p120-catenin with desmosomal cadherins such as DSG3 leads to desmosome remodelling by maintaining free DSG3 at the cell surface before being internalized into desmosomes 62.

Armadillo proteins

PG (also called γ-catenin), together with PKPs1–3 63, 64 (all members of the armadillo family), are adaptor proteins with important roles in facilitating the adhesion of DSP to keratin intermediate filaments, in regulating clustering of the desmosomal components and in mediating important signal transduction pathways. PG is formed of 12 arm repeats that share 65% amino acid identity with β-catenin, the equivalent protein associated with adherens junctions. The central armadillo domain of PG interacts with DSP, which in turn tethers intermediate filaments to the desmosomal plaque. PG can also translocate to adherens junctions and bind E-cadherin in the same manner as β-catenin, but its higher affinity for DSP may explain why PG and not β-catenin locates to desmosomes 65.

Although PKP1 and PKP2 can also localize to the nucleus, the PKPs are found predominantly in desmosomes 44. PKPs1–3 share 50–55% sequence similarity with the armadillo repeats of other armadillo family proteins 64. Based on structural analysis studies, PKPs contain nine arm repeat domains 66, with 21 additional amino acids added to PKP1 and 44 amino acids added to PKP2. Both PKP 1 and PKP2 exist in two isoforms, a shorter ‘a’ form and a longer ‘b’ form 67, 68, with the short ‘a’ form more predominant.

PKPs show tissue- and differentiation-specific patterns of expression similar to the desmosomal cadherins. It has been observed that, while PKP3 shows expression throughout simple and all layers of stratified epithelia, apart for hepatocytes, PKP1 is mostly expressed in the suprabasal layers of stratified epithelia and PKP2 expression extends to simple epithelia, lower layers of stratified epithelia and non-epithelial tissues such as cardiac muscle—where it is the only isoform—and lymph nodes 67–72.

PKPs appear to play a role in the clustering of desmosomal proteins during the formation of desmosomes. The N-terminal head domain of PKP1 can associate with DSG1, PG, keratin and actin filaments and ultimately with DSP through what appears to be a robust association which drives DSP recruitment to cell–cell junctions 73–76. PKP3 interacts with the largest number of desmosomal proteins, including DSP, PG, DSG1-3, DSC3a and 3b and DSC1a and 2a 64. PKP2 does play an important role in transport of DSP to the plasma membrane during desmosome assembly, but does so less efficiently than PKP1 1, 77. The mechanism behind PKP1 and PKP3 mediated-desmosomal assembly it is not yet fully determined, although it appears that PKP2 functions as a scaffold for PKC-α and regulates DSP association with intermediate filaments 1, 78, 79.

The plakin family

DSP, the most abundant desmosomal protein, plays a key role as the linker between the plasma membrane and keratin intermediate filaments 44. The protein is predicted to form homodimers through a α-helical coiled-coil rod domain which also interconnects a globular amino-terminus, responsible for binding the arm proteins PG and PKPs, and a carboxy-terminus domain, responsible for the attachment of intermediate filaments 28, 80–83. Until recently only two isoforms of DSP (DSPI and DSPII) have been known. As with the ‘a’ and ‘b’ forms of desmocollins, DSPI and DSPII isoforms are produced as a result of alternative mRNA splicing, with DSPII the shorter isoform of the two. Both are widely expressed in numerous tissues, although DSPII expression is reduced/absent from the heart and from simple epithelia 84. A minor DSP isoform derived from DSPI, named DSPIα, produced by the alternative splicing of DSPI mRNA, has recently been described and is detectable in lower levels than the dominant isoforms, although it presents a similar tissue distribution 85.

Franke et al have observed, by immunogold labelling of DSP, that in normal heart muscle DSP is located in all plaques of the desmosome-like and fascia adherens-type junctions, with a very intense signal within the desmosome-like junctions 86. Several studies in vivo and in vitro support the importance of DSP in desmosome assembly and function, and show that it appears to play a pivotal role in the development of epidermis, neuroepithelium, heart and blood vessels 87, 88. In keratinocytes, DSPII appears to play a more significant role than DSPI in maintaining robust adhesion, suggesting cell type-specific functions for the DSP isoforms 89.

Infectious and auto-immune diseases of desmosomes

  1. Top of page
  2. Abstract
  3. Introduction
  4. Infectious and auto-immune diseases of desmosomes
  5. Inherited desmosomal disorders
  6. Other desmosome-related disorders
  7. Non-adhering functions of desmosomal components in disease
  8. Acknowledgements
  9. Author contributions
  10. References

The crucial role of desmosomes in mediating intracellular adhesion is demonstrated by the consequences of both auto-immune and infectious diseases, which target the desmosomal cadherins.

Pemphigus vulgaris and pemphigus foliaceus

The auto-immune diseases pemphigus vulgaris (PV) and pemphigus foliaceus (PF) are a pair of potentially fatal conditions characterized by loss of keratinocyte cell–cell adhesion (acantholysis) and blister formation, in the epidermis (in PF) or mucosal membranes (PV) 90. Pemphigus auto-antibodies target the DSGs, with DSG3 identified as the auto-antigen in PV 91 and DSG1 the auto-antigen in PF 92. Among those patients with early-stage PV, involving only mucous membrane lesions, only anti-DSG3 auto-antibodies are found, whilst those with later-stage disease, featuring skin lesions in addition to mucosal, have sera containing both anti-DSG1 and anti-DSG3 auto-antibodies 93.

The different patterns of disease in PV and PF reflect the distribution and expression levels of DSG1 and DSG3, which differ significantly between the epidermis and mucosa 94. In addition, the pattern of blister formation in the two conditions is influenced by the apparent ability of DSG3 to compensate for adhesion defects caused by the presence of anti-DSG1 antibodies, and vice versa—the so-called ‘DSG compensation theory’ 94. For example, blisters in PF form only in the superficial epidermis because, as shown in Figure 2, this is the only area in which DSG1 is expressed in the absence of co-expressed DSG3. Meanwhile, the deeper blisters in later-stage PV reflect DSG3 expression in the absence of DSG1 in more basal epidermal layers.

The mechanism by which anti-DSG antibodies result in a loss of intracellular adhesion, and thus blister formation, has been a matter of some controversy. Early research into the pathology of pemphigus auto-antibodies investigated the potential of a role for proteases, particularly of the plasminogen activator (PA) family, in indirectly mediating acantholysis and blister formation 95, 96 secondary to auto-antibody binding. However, Mahoney et al97 demonstrated the pathogenicity of pemphigus auto-antibodies in a PA knock-out mouse model, demonstrating that these proteases are not required for blister formation and indicating that blister formation is a direct result of the effect of pemphigus auto-antibodies on DSGs.

The DSG3 epitopes recognized by auto-antibodies in PV patients have been found to reside particularly in the DSG3 amino-terminal domain, the region implicated in forming the adhesive interface between neighbouring desmosomes 98, 99, suggesting direct steric interference in DSG ectodomain interactions as the cause of acantholysis. However, keratinocytes have been shown to remain adhesive, with desmosomes intact, when cells are incubated at 4 °C with PV auto-antibodies bound to DSG3, with several hours incubation at 37 °C instead required before significant loss of adhesion is observed 100.

The consequent suggestion that a cellular response by keratinocytes to PV auto-antibodies may be required for loss of adhesion is supported by observations of DSG endocytosis, followed by desmosome disassembly, in response to autoantibody binding in PV 1. PV auto-antibodies have been shown to cause rapid depletion of DSG3 from the membrane compartment of cultured cells 101. This has been found to occur by endocytosis in sequential temporal phases, beginning with its internalization from non-desmosome-associated clusters at the cell surface (and delivered to early endosomes) following initial binding of PV auto-antibodies 102. This represents internalization of newly-synthesized DSG3 not yet incorporated into desmosomes, and is also observed in studies utilizing anti-DSG3 monoclonal antibodies 103. Indeed, the pathogenicity of anti-DSG3 monoclonal antibodies correlates with the extent to which they are capable of depleting DSG3 from desmosomes 104. This is followed by a re-arrangement of desmosomes into linear array structures, which serve as sites for endocytosis of desmosomal DSG3, resulting in junctional DSG3 depletion and loss of cell adhesion 105, 106. Significant internalization of desmosomal DSG3 does not occur until 24 h after treatment of keratinocytes with anti-DSG3 monoclonal antibodies 103, and occurs in a clathrin- and dynamin-independent fashion 107. Thus, by inducing DSG3 internalization and thus desmosome disassembly, PV auto-antibodies disrupt normal desmosomal homeostasis and trigger loss of keratinocyte cell–cell adhesion and acantholysis 38.

Bullous impetigo and staphylococcal scalded-skin syndrome

The infectious disease bullous impetigo, and its generalized form staphylococcal scalded skin syndrome (SSSS), conditions which mostly affect children under 5 years of age and immunocompromised adults 108, are both characterized by epidermal blister formation as a result of keratinocyte acantholysis. Both of these related conditions share a startling clinical similarity to PF 109, with the histopathology of adult PF skin and that of neonatal mice with bullous impetigo being indistinguishable 110. This is a result of the fact that the same desmosomal cadherin, DSG1, is affected in both cases, on this occasion by the exfoliative toxins (ETs), peptide toxins produced by some strains of pathogenic Staphylococcus aureus bacteria 110, 111. The three known ETs affecting humans (ETA, ETB and ETD; murine and canine-specific forms also exist) are glutamate-specific serine proteases which specifically cleave a single peptide bond in DSG1 (but not the closely related DSG3), within its extracellular domain 112 and at a Ca2+-binding site in which the Ca2+ ion is required for cleavage to take place 113. This removes residues 1–381 of the DSG1 ectodomain to produce the truncated protein Δ381-DSG1. Direct DSG1 cleavage in this way serves to disrupt keratinocyte cell–cell adhesion, leading to the formation of blisters. The location and depth of the blisters which result from ET action most likely again reflect the DSG compensation theory, as outlined for PF, in that the blisters form in epidermal strata where little to no DSG3 exists which could compensate for the loss of DSG1 function.

Studies using time-lapse immunofluorescence and electron microscopy have shown that blister formation in mice occurs following removal of the DSG1 N-terminal domain, while the C-terminal remnant of cleaved DSG1 remains at the cell surface 114. This suggests that cleavage of DSG1 alone is sufficient to induce epidermal blister formation, rather than, for example, internalization of cleaved DSG1 being required. Work by Simpson et al115 has further demonstrated that Δ381-DSG1 remains bound to its catenin partner PG, effectively sequestering PG and causing a dose-dependent reduction in other desmosomal cadherins. This factor, along with the fact that increased PG reduces cadherin expression, suggests falling desmosomal cadherin levels as a result of PG sequestration by truncated DSG1 as a potential cellular mechanism of acantholysis and blistering in bullous impetigo and SSSS.

Inherited desmosomal disorders

  1. Top of page
  2. Abstract
  3. Introduction
  4. Infectious and auto-immune diseases of desmosomes
  5. Inherited desmosomal disorders
  6. Other desmosome-related disorders
  7. Non-adhering functions of desmosomal components in disease
  8. Acknowledgements
  9. Author contributions
  10. References

The complexity and relative lack of understanding of how desmosomal components interrelate with each other and with other compartments in a cell-type and differentiation-dependent manner is reflected by the range of genetic disorders arising from mutations affecting the desmosomal genes. The large number of publications reporting various alterations affecting all of the desmosomal genes highlights the phenotypic heterogeneity behind these conditions; different mutations have been shown to result in either cardiac and/or cutaneous disorders, with or without hair implications. One such condition frequently arising from desmosomal mutations is arrhythmogenic right ventricular cardiomyopathy (ARVC), an inherited disorder associated with arrhythmias and sudden cardiac death, and characterized by fibro-fatty replacement of cardiac myocytes.

ARVC-causing mutations have so far been identified in a variety of desmosomal genes and account for 50–70% of ARVC cases. These include non-syndromic ARVC mutations affecting all domains of DSP, which appear to be inherited in a dominant manner. More recently, a study of a German family has reported the first dominantly inherited ARVC-linked mutation in JUP (the gene encoding PG) 116, 117. All these variations cause ARVC without cutaneous abnormalities. Further, studies investigating the role of desmosomal cadherins in human disease, particularly of the heart and skin, have revealed both autosomal dominant and recessive mutations in DSG2 and, more recently, a recessive mutation in DSC2, which also cause ARVC without a cutaneous or hair phenotype 118. However, PKP2 is the gene most frequently associated with ARVC, with both dominant and recessive mutations in this gene identified as being responsible for up to 70% of cases. In the majority of cases PKP2 mutations were found in its carboxy-terminus domain, although other mutations have also been reported 122.

Mutations in other PKPs appear to result in phenotypes ranging from skin fragility to severe autosomal recessive ectodermal dysplasia, both due to a variety of mutations in PKP1, which range from missense and nonsense mutations to splice-site and compound heterozygous changes 119–122. Experiments on PKP2-null mice have shown mid-gestational embryonic lethality caused by cardiac patterning defects and fragility of the myocardium 123, alongside retraction of intermediate filaments from the plasma membrane, demonstrating the importance of PKPs in DSP recruitment and intermediate filament tethering to desmosomes 44. Although no disease-causing mutations have been reported in humans for PKP3, ablation of this isoform in mice results in defective hair follicle morphogenesis, increased keratinocyte proliferation and DSP mislocalization, leading to susceptibility to dermatitis and secondary alopecia 124.

DSP is the most abundant component within the desmosome and it is therefore not surprising that there are a variety of genetic disorders associated with mutations in this gene, resulting in conditions with varying degrees of severity 125. Previous studies have reported that DSP haploinsufficiency may cause the epidermal-thickening disease striate palmoplantar keratoderma (SPPK) 126, compound heterozygosity with amino-terminal missense mutations, and carboxy-terminal nonsense mutations leading to severe keratoderma, skin fragility and woolly hair, or alopecia with or without cardiac involvement 117, 127, 128. For example, a single base-pair deletion mutation in Ecuadorian families, that truncates the intermediate filament-binding site of DSP, leads to the so-called Carvajal syndrome, an ARVC variant which shows predominantly left ventricular involvement, early morbidity and clinical overlaps with dilated cardiomyopathy, and is accompanied by woolly hair and palmoplantar keratoderma (PPK) 128. Another mutation which further truncates the carboxy-terminus of DSP is associated with severe acantholytic epidermolysis bullosa, a lethal disorder which presents complete alopecia, neonatal teeth and nail loss and death due to transcutaneous fluid loss as a result of extensive skin erosion 129.

The critical role of the armadillo-family protein PG in desmosome assembly has also been demonstrated by knock-out studies in mice, which show acantholysis, indicative of compromised desmosome function, and are lethal due to fragility of the myocardium 130, 131. In some cases mouse pups were born but presented epidermal fragility and heart defects and died shortly after birth. In another case, adult mice with a cardiac-restricted deletion of PG presented with myocyte loss, inflammation, fibrosis and cardiac dysfunction, and recapitulated the phenotype of ARVC 132.

Homozygous mutations in the gene encoding PG underlie an autosomal recessive condition known as ‘Naxos disease’ (affected individuals hailing from the Greek island of Naxos), which includes ARVC (presenting without the pronounced left ventricular involvement and early morbidity observed in Carvajal syndrome) alongside woolly hair and PPK 133–135. The epidermal manifestations in Naxos patients are not as severe as those observed in PG-null mice 38. Other recent studies have described a recessive missense mutation affecting JUP in a patient who presented with ARVC, PPK and total alopecia 136, whilst Pigors et al reported a novel lethal phenotype caused by a nonsense mutation in JUP leading to severe congenital skin fragility with generalized epidermolysis, massive transcutaneous fluid loss and no apparent cardiac dysfunction, due to a complete loss of PG in the patient's skin leading to fewer desmosomes and no adhesion structures between keratinocytes 137. Furthermore, Cabral et al have described loss-of-function mutations (eg p.S24X) in the JUP gene, resulting in skin fragility, diffuse PPK and woolly hair without symptoms of cardiomyopathy 138. Again, no PG expression was detectable by immunostaining in skin sections from patients harbouring these mutations.

With regard to other desmosomal cadherins, mutations leading to loss of DSG4 are responsible for disruptions in hair-follicle differentiation, whilst DSG1 haploinsufficiency leads to SPPK 139–141. No disease-causing mutations in DSG3 have so far been reported. As demonstrated by these examples, a most interesting aspect of inherited desmosomal disease is the wide spectrum of conditions which can arise from mutations in some desmosomal components, such as DSP and PG, in contrast to the restricted phenotypes which result from mutations in the PKPs and desmosomal cadherins. This is illustrated in Figure 3.

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Figure 3. Disease phenotypes resulting from mutations in selected desmosomal components. A wide spectrum of disease phenotypes can result from mutations in either DSP or JUP (A), whilst mutations in the PKPs or desmosomal cadherins result in very limited disease spectra (B). Mutations connected by lines represent reported compound heterozygous mutations. Note that several distinct mutations may be represented by each symbol (eg several dominant mutations in the DSP N-terminus have been reported to cause ARVC alone). Adapted in part from Bolling and Jonkman 116 and Cabral et al131

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Other desmosome-related disorders

  1. Top of page
  2. Abstract
  3. Introduction
  4. Infectious and auto-immune diseases of desmosomes
  5. Inherited desmosomal disorders
  6. Other desmosome-related disorders
  7. Non-adhering functions of desmosomal components in disease
  8. Acknowledgements
  9. Author contributions
  10. References

Darier's disease

The contribution of desmosomal regulation to disease states is well illustrated by the example of Darier's disease (also known as Darier–White disease or dyskeratosis follicularis), an autosomal dominant skin condition characterized by keratotic papules, which form into larger skin lesions 142. Histologically, acantholysis (resulting from impaired desmosome formation and leading to suprabasal cleavage) is prominent, accompanied by abnormal keratinization and rounded keratinocytes 142. Loss-of-function (haploinsufficiency) mutations in ATP2A2, encoding the Ca2+ pump SERCA2, have been identified as the cause of this condition 143.

As SERCA2 is responsible for Ca2+ translocation from the cytosol to the endoplasmic reticulum (ER) lumen 144, reduced SERCA2 function results in depleted ER Ca2+ stores; this can trigger an ER-stress response, leading to apoptosis in some keratinocytes 145, 146 and also to impaired desmosome formation, resulting to acantholysis. Studies using the SERCA2 inhibitor thapsigargin (TG), have demonstrated impaired trafficking of desmosomal components, in particular DSP, and consequent impaired desmosome formation in TG-treated cells compared to controls 147. Meanwhile, cells that have undergone siRNA silencing of SERCA2 display significant reductions in translocation of DSP (reduced by 60%) and PKCα (an important regulator of desmosomal assembly; reduced by 70%) to the plasma membrane 148.

ADAM17 loss-of-function syndrome

ADAM17 is a member of the adisintegrin and metalloprotease (ADAM) family, consisting of membrane-anchored metalloproteases which mediate ectodomain shedding, the proteolytic release of extracellular domains from membrane-bound precursors 149. First described as the tumour necrosis factor-α converting enzyme 150, 151, ADAM17 also cleaves a wide variety of other proteins, including the desmosomal cadherin DSG2 152, and has been shown to cooperate with signalling through the epidermal growth factor receptor (EGFR; a known regulator of desmosomal assembly 153) to regulate DSG2 shedding and endocytic trafficking 154. We have recently described a recessive loss of function mutation in ADAM17 linked to a neonatal-onset inflammatory skin and bowel disease 155, cutaneous features of which include a generalized pustular rash developing into psoriasiform erthyroderma, thickening of both the epidermis and nails, short and disorganized hair with abnormalities of the hair shaft (severe weathering and damaged cuticles) and susceptibility to repeated bacterial infection. Immunofluorescence and western blot studies have revealed significant retention of DSG2 at the keratinocyte cell surface, suggesting that, in part, the skin and hair phenotypes observed may be mediated by dysregulation of DSG cycling.

In addition, one of the patients identified with this mutation died as a consequence of myocarditis caused by Parvovirus B19 155. Considering the very well established role for desmosomal components in ARVC, a link between desmosomal dysregulation and this heart phenotype cannot be discounted. Furthermore, this may suggest a link between infectious myocarditis and desmosomal dysregulation in the absence of ADAM17.

Cystatin A and exfoliative ichthyosis

Cystatin A (CSTA) belongs to the cystatin superfamily of cysteine protease inhibitors and is expressed abundantly in the cytoplasm of epithelial and lymphoid tissues. CSTA is thought to play an important role in many diverse mechanisms, from skin protection against allergens, such as dust mites 156, to regulating the activity of several target proteases in different types of cancers, including tumours of the breast 157, lung 158, prostate 159 and squamous cell carcinomas of the head and neck 160. Some of the target proteases of CSTA, such as cathepsins B, H and L, are observed to be frequently up-regulated in cancer, and appear to facilitate tumour invasion and metastasis through cleavage of cell–cell junctions 160.

Recently, we have identified two unrelated families that presented homozygous loss-of-function mutations in CSTA as being responsible for exfoliative ichthyosis, a condition affecting the intercellular linkage in the basal and suprabasal layers of the epidermis 161. EM of basal and suprabasal layers of affected epidermis has shown increased intercellular gaps and thickening of keratin filaments. Experiments performed using in vitro skin models mirrored that of patient skin, such as breakage of intercellular connections upon stretch-induced stress in CSTA knockdown monolayers and thicker keratin filaments in organotypic cultures lacking CSTA. These observations reveal a previously unknown role for CSTA in basal to suprabasal keratinocyte adhesion.

Non-adhering functions of desmosomal components in disease

  1. Top of page
  2. Abstract
  3. Introduction
  4. Infectious and auto-immune diseases of desmosomes
  5. Inherited desmosomal disorders
  6. Other desmosome-related disorders
  7. Non-adhering functions of desmosomal components in disease
  8. Acknowledgements
  9. Author contributions
  10. References

Desmosomal components in cancer

It has been observed that the desmosome may also play an important role in cancer progression, with various components shown to be up-regulated, down-regulated or modulated in a variety of tumour types. Attempts to clarify the role of desmosomal adhesion, and in investigating the level of desmosomal involvement in cancers, have produced contradictory and confusing results. Studies have shown that an increased expression of desmosome proteins, such as DSG2, DSG3 and PKP3, can be observed in certain cancers of the skin, head and neck, prostate and lung compared with normal tissue, and that this over-expression is associated with enhanced tumour progression 162–166. For example, two reports have shown that PG can transform cells by the specific activation of c-myc in a β-catenin-independent manner 167, whilst concomitantly inhibiting apoptosis by induction of the anti-apoptotic protein Bcl-2 168. Meanwhile, in contrast, the loss or reduction of one or more desmosomal components, including DSG1-3, DSC2, DSC3, PG, PKP1-3 and DSP, has been observed in the development or progression of various human epithelial cancers, such as skin, head and neck, gastric, colorectal, bladder, breast, prostate, cervical and endometrial cancers 169. Further, there are instances in which no obvious changes in the level of desmosomal proteins have been observed during cancer progression 165. Studies on cultured cells have observed that, in some cases, over-expression of desmosomal components promotes proliferation, inhibits apoptosis and increases invasion, characteristics that are advantageous to tumour cells; while conversely, other experiments have shown that over-expression of desmosomal components in cell lines suppresses such tumour-promoting behaviour as invasion and anchorage-independent growth 169.

The desmosomal-related protein PERP (p53 apoptosis effector related to PMP-22) has also been shown to be associated with cancer. PERP is a membrane protein required for desmosome assembly (although its interacting partners remain incompletely understood), and is also transcriptionally activated by the related transcription factors p53 and p63 during DNA damage-induced apoptosis and during the development of stratified epithelia, respectively. Studies have shown that mice with PERP knock-out or loss-of-function mutations develop a dramatic blistering phenotype in the epidermis and stratified epithelia 170, and also have an increased tendency to develop squamous cell carcinomas when exposed to chronic ultraviolet light compared to controls; with the loss of PERP in the skin reducing tumour latency and facilitating tumour progression over differentiation 171, 169. A clear role for PERP in mediating tumour suppression downstream of the p53 and p63 transcription factors has since been identified 169, 170, although its role in the desmosome remains unclear.

Desmosomal components in signalling and morphogenesis

In addition to their role in cell adhesion, desmosomal components have various roles in signalling and regulation in cells. Prominent examples of this feature are the desmosomal cadherins, whose distinct expression patterns in epidermis (illustrated in Figure 2) have been shown to play a role in morphoregulation of that tissue. This can be mediated through either the adhesive function of cadherins 35 or the modulation of intracellular signalling pathways, such as by inhibition of EGFR signalling 172 or via increasing β-catenin stability and signalling 173. Furthermore, studies in which various cadherins are forcibly expressed outside their usual strata in mouse epidermis lead to disruption of normal epidermal structure and function. Phenotypes such as epidermal hyperproliferation and abnormal differentiation 174, 175, gross epidermal scaling 176, acantholysis and blistering (resembling PV) 177 and epidermal fragility alongside ulcerating lesions 178 have been observed in such studies, underlining the morphoregulatory importance of the desmosomal cadherins.

Among the other desmosomal components, the PKPs can function in the nucleus and may play a role in gene regulation or nuclear structure 67, having been found to associate with proteins regulating rRNA and tRNA transcription 179 and elF4A-dependent translation 180, among others. Meanwhile, PG can also function in both the desmosome and the nucleus, regulating Wnt growth factor signalling alongside β-catenin 181, 182 and regulating transcription of Tcf/Lef1 target genes, in both a β-catenin-dependent and -independent fashion 183, 184. Indeed, suppression of Tcf–Lef1 signalling may contribute to the pathology of ARVC, with suppression of DSP expression in mouse myocytes resulting in nuclear localization of PG, consequently causing suppression of Tcf–Lef1 signalling and enhanced cardiac adipogenesis, fibrogenesis and myocyte apoptosis 185.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Infectious and auto-immune diseases of desmosomes
  5. Inherited desmosomal disorders
  6. Other desmosome-related disorders
  7. Non-adhering functions of desmosomal components in disease
  8. Acknowledgements
  9. Author contributions
  10. References