Inherited disorders of desmosomes

Atopic Dermatitis-like Genodermatosis: Disease Diagnosis and Management

Eczema is a classical characteristic not only in atopic dermatitis but also in various genodermatosis. Patients suffering from primary immunodeficiency diseases such as hyper-immunoglobulin E syndromes, Wiskott-Aldrich syndrome, immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome, STAT5B deficiency, Omenn syndrome, atypical complete DiGeorge syndrome; metabolic disorders such as acrodermatitis enteropathy, multiple carboxylase deficiency, prolidase deficiency; and other rare syndromes like severe dermatitis, multiple allergies and metabolic wasting syndrome, Netherton syndrome, and peeling skin syndrome frequently perform with eczema-like lesions. These genodermatosis may be misguided in the context of eczematous phenotype. Misdiagnosis of severe disorders unavoidably affects appropriate treatment and leads to irreversible outcomes for patients, which underlines the importance of molecular diagnosis and genetic analysis. Here we conclude clinical manifestations, molecular mechanism, diagnosis and management of several eczema-related genodermatosis and provide accessible advice to physicians.

Translational implications of Th17-skewed inflammation due to genetic deficiency of a cadherin stress sensor

Desmoglein 1 (Dsg1) is a cadherin restricted to stratified tissues of terrestrial vertebrates, which serve as essential physical and immune barriers. Dsg1 loss-of-function mutations in humans result in skin lesions, multiple allergies, and isolated patient keratinocytes exhibit increased pro-allergic cytokine expression. However, the mechanism by which genetic deficiency of Dsg1 causes chronic inflammation is unknown. To determine the systemic response to Dsg1 loss, we deleted the three tandem Dsg1 genes in mice. Whole transcriptome analysis of embryonic Dsg1-/- skin showed a delay in expression of adhesion/differentiation/keratinization genes at E17.5, a subset of which recovered or increased by E18.5. Comparing epidermal transcriptomes from Dsg1-deficient mice and humans revealed a shared IL-17-skewed inflammatory signature. Although the impaired intercellular adhesion observed in Dsg1-/- mice resembles that resulting from anti-Dsg1 pemphigus foliaceus antibodies, pemphigus skin lesions exhibit a weaker IL-17 signature. Consistent with the clinical importance of these findings, treatment of two Dsg1-deficient patients with an IL-12/IL-23 antagonist originally developed for psoriasis resulted in improvement of skin lesions. Thus, beyond impairing the physical barrier, loss of Dsg1 function through gene mutation results in a psoriatic-like inflammatory signature before birth and treatment with a targeted therapy markedly improved skin lesions in patients.

Messenger RNA (mRNA)-based age determination using skin-specific markers of saliva epithelial cells

Background

Age determination is a vital factor in biological identification in forensics. This study was carried out to determine the expression levels of three target genes (Keratin 9 (KRT9), Loricrin (LOR) and Corneodesmosin (CDSN)) in salivary epithelial cells and how they can be used in age determination using reference gene, β-actin. Thirty young adults participated in the study and were divided into three groups according to their ages (16–20, 21–25, and 26–30). Ribonucleic acid (RNA) extraction, complementary deoxyribonucleic acid (cDNA) synthesis and quantitative polymerase chain reaction (qPCR) were performed. Data analysis was done using IBM SPSS Version 26 and the comparative Ct method (2−∆∆Ct method).

Results

CDSN was detected in all the sampled age groups. Though the age group 16–20 had the highest (0.4237) expression of CDSN among the three age groups, there was no significant difference (p > 0.05) in the expression of the gene among the three age groups. The LOR gene was lowly expressed across all age groups used in the study. The expression of the gene did not significantly differ (p > 0.05) between the control and 26–30 years age group, but they were however significantly higher (F = 36.47, p ≤ 0.05) than the expression of the gene in both 16–20 and 21–25 years age groups. The KRT9 gene was expressed only in age groups 16–20 and 26–30 and the expression of the gene did not significantly (p > 0.05) differ between these age groups. Though the expression of all the target genes was low, it was observed that the LOR gene expression varied among 21–25 and 26–30 age groups; therefore, more data and further analyses are still required since this experimental approach for age determination using gene expression is still at an emerging stage.

Conclusion

Although RNA concentration was low and the expression values of the genes were low and could not be used in comparing the expression levels among the three age groups, it can be concluded that the three messenger ribonucleic acid (mRNA) markers CDSN, LOR and KRT9, as well as the ACTB reference mRNA marker analysed via the described qPCR assays, are suitable for identifying epithelial cells in saliva.

Desmosomes and Intermediate Filaments: Their Consequences for Tissue Mechanics

Adherens junctions (AJs) and desmosomes connect the actin and keratin filament networks of adjacent cells into a mechanical unit. Whereas AJs function in mechanosensing and in transducing mechanical forces between the plasma membrane and the actomyosin cytoskeleton, desmosomes and intermediate filaments (IFs) provide mechanical stability required to maintain tissue architecture and integrity when the tissues are exposed to mechanical stress. Desmosomes are essential for stable intercellular cohesion, whereas keratins determine cell mechanics but are not involved in generating tension. Here, we summarize the current knowledge of the role of IFs and desmosomes in tissue mechanics and discuss whether the desmosome-keratin scaffold might be actively involved in mechanosensing and in the conversion of chemical signals into mechanical strength.

Mutations of desmoglein-2 in sudden death from arrhythmogenic right ventricular cardiomyopathy and sudden unexplained death

Desmoglein-2 (DSG2), a member of the desmosomal cadherin superfamily, has been linked to arrhythmogenic right ventricular cardiomyopathy (ARVC)which may cause life-threatening ventricular arrhythmias and sudden death. Fatal arrhythmias resulting in sudden death also occur in the absence of morphologic cardiac abnormalities at autopsy. We sequenced all 15 exons of DSG2 in DNA extracted from post-mortem heart tissues of 25 patients dying with ARVC and 25 from sudden unexplained death (SUD). The primers were designed using the Primer Express 3.0 software. Direct sequencing for both sense and antisense strands was performed with a BigDye Terminator DNA sequencing kit on a 3130 xl Genetic Analyzer. Mutation damage prediction was made using Mutation Taster, Polyphen and SIFT software. 2 DSG2 mutations (p. S1026Q fsX12, p. G678R)in two ARVC samples and 2 DSG2 mutations(p. E 896K, p. A858 V) in two SUD samples were identified, all the mutations were novel. We concluded that DSG2 mutations may not specific for ARVC and may be related to the fatal arrhythmic events even in patients with a morphological normal heart.

Salivary proteomics in bisphosphonate-related osteonecrosis of the jaw

OBJECTIVE

The objective of this study was to identify differentially expressed salivary proteins in bisphosphonate-related osteonecrosis of the jaw (BRONJ) patients that could serve as biomarkers for BRONJ diagnosis.

SUBJECTS AND METHODS

Whole saliva obtained from 20 BRONJ patients and 20 controls were pooled within groups. The samples were analyzed using iTRAQ-labeled two-dimensional liquid chromatography-tandem mass spectrometry.

RESULTS

Overall, 1340 proteins were identified. Of these, biomarker candidates were selected based on P-value (<0.001), changes in protein expression (≥1.5-fold increase or decrease), and unique peptides identified (≥2). Three comparisons made between BRONJ and control patients identified 200 proteins to be differentially expressed in BRONJ patients. A majority of these proteins were predicted to have a role in drug metabolism and immunological and dermatological diseases. Of all the differentially expressed proteins, we selected metalloproteinase-9 and desmoplakin for further validation. Immunoassays confirmed increased expression of metalloproteinase-9 in individual saliva (P = 0.048) and serum samples (P = 0.05) of BRONJ patients. Desmoplakin was undetectable in saliva. However, desmoplakin levels tended to be lower in BRONJ serum than controls (P = 0.157).

CONCLUSIONS

Multiple pathological reactions are involved in BRONJ development. One or more proteins identified by this study may prove to be useful biomarkers for BRONJ diagnosis. The role of metalloproteinase-9 and desmoplakin in BRONJ requires further investigation.

The GEF Bcr activates RhoA/MAL signaling to promote keratinocyte differentiation via desmoglein-1

The GEF Bcr promotes RhoA-dependent actin remodeling and MAL/SRF signaling in keratinocytes, which in turn promotes differentiation via regulation of desmoglein-1 expression.

Although much is known about signaling factors downstream of Rho GTPases that contribute to epidermal differentiation, little is known about which upstream regulatory proteins (guanine nucleotide exchange factors [GEFs] or GTPase-activating proteins [GAPs]) are involved in coordinating Rho signaling in keratinocytes. Here we identify the GEF breakpoint cluster region (Bcr) as a major upstream regulator of RhoA activity, stress fibers, and focal adhesion formation in keratinocytes. Loss of Bcr reduced expression of multiple markers of differentiation (such as desmoglein-1 [Dsg1], keratin-1, and loricrin) and abrogated MAL/SRF signaling in differentiating keratinocytes. We further demonstrated that loss of Bcr or MAL reduced levels of Dsg1 mRNA in keratinocytes, and ectopic expression of Dsg1 rescued defects in differentiation seen upon loss of Bcr or MAL signaling. Taken together, these data identify the GEF Bcr as a regulator of RhoA/MAL signaling in keratinocytes, which in turn promotes differentiation through the desmosomal cadherin Dsg1.

Desmoglein-1, differentiation, and disease

Desmoglein-1 (DSG1), a desmosomal protein, maintains the structure of epidermis through its adhesive function. However, heterozygous mutations in DSG1 in humans result in abnormal differentiation, as does downregulation of DSG1 in human skin organ culture, suggesting that it may have important signaling functions. In this issue of the JCI, Harmon et al. elucidate how the binding of the DSG1 cytoplasmic tail to the scaffolding protein Erbin decreases signaling through the Ras-Raf pathway to promote stratification and differentiation of keratinocytes in the epidermis.

Cadherin defects in inherited human diseases

The tight control of cell-cell connectivity mediated by cadherins is a key issue in human health and disease. The human genome contains over 115 genes encoding cadherins and cadherin-like proteins. Defects in about 21 of these proteins (8 classical, 5 desmosomal, 8 atypical cadherins) have been linked to inherited disorders in humans, including skin and hair disorders, cardiomyopathies, sensory defects associated with deafness and blindness, and psychiatric disorders. With the advent of exome and genome sequencing techniques, we can anticipate the discovery of yet more evidence for the involvement of additional cadherins. Elucidation of the related physiopathological mechanisms underlying these conditions should help to clarify the roles played by these cadherins in tissues and the ways in which defects in different cadherins cause such a wide spectrum of associated phenotypes. These disorders also constitute disparate model systems for investigations of the relative contributions of mechanical adhesive strength and intracellular signaling pathways to the pathogenic process for a given cadherin.

Gastro-oesophageal reflux disease is associated with up-regulation of desmosomal components in oesophageal mucosa

AIMS

Gastro-oesophageal reflux disease (GERD) is associated with impaired epithelial barrier function. This study was aimed at investigating the role of desmosomal proteins in relation to GERD.

METHODS AND RESULTS

Ninety-five patients with GERD-related symptoms (erosive, n = 51; non-erosive, n = 44) and 27 patients lacking those symptoms were included. Endoscopic and histological characterization of oesophagitis was performed according to the Los Angeles and Ismeil-Beigi criteria, respectively. Multiple biopsies were taken from the oesophageal mucosa of each patient. Gene expression analysis of plakoglobin, desmoglein-1, desmoglein-2 and desmoglein-3 was performed by quantitative real time (RT)-polymerase chain reaction and immunohistochemistry in the oesophageal mucosa. Routine histology revealed specific GERD-related alterations, such as dilatation of intercellular spaces (DIS), basal cell hyperplasia (BCH), and elongation of the papillae, in the oesophageal mucosa of patients with GERD, as compared with controls (all parameters: P < 0.05). All four genes and corresponding proteins were found to be up-regulated by between 1.7 and 8.1-fold (transcript level, P < 0.05; protein level, P < 0.05). Induced gene expression levels of plakoglobin, desmoglein-1 and desmoglein-2 correlated significantly with DIS and BCH.

CONCLUSIONS

Taken together, the uniform up-regulation of desmosomal genes/proteins in the oesophageal mucosa of patients with GERD supports the concept of architectural and molecular changes in the desmosomal compartment in the pathogenesis of GERD.

Human epidermal desmosome-enriched tissue fractions for analytical and prospective studies

Here, we report a method, adapted to the human epidermis, allowing isolation of desmosomes in small tissue fractions. The methods previously developed for animal skin did not work efficiently with human tissue. Enrichment of desmosomes was performed by the association of two incubation steps in acidic solutions containing detergent NP-40 at two different concentrations followed by a sonication step. The suspension was centrifuged twice: first to remove the heavy cell fragments and then at 16000 g on a discontinuous sucrose gradient. A desmosome-enriched fraction (DsF) was collected at the 30-50% sucrose interface. We demonstrate by immunoelectron microscopy and by western blotting that the central part of the desmosome structure is preserved as well as the antigenicity of its components. Our approach, allowing a significant enrichment of the cell fractions containing desmosomes, can be used to immunize animals and create new antibodies directed against desmosomal components. Using this strategy, new and so far poorly studied molecules incorporated into the desmosome cores could be targeted more easily.

mRNA-based skin identification for forensic applications

Although the identification of human skin cells is of important relevance in many forensic cases, there is currently no reliable method available. Here, we present a highly specific and sensitive messenger RNA (mRNA) approach for skin identification, meeting the key requirements in forensic analyses. We examined 11 candidate genes with skin-specific expression, as ascertained from expression databases and the literature, as well as five candidate reference genes ascertained from previous studies, in skin samples and in other forensically relevant tissues. We identified mRNA transcripts from three genes CDSN, LOR and KRT9, showing strong over-expression in skin samples relative to samples from forensic body fluids, making them suitable markers for skin identification. Out of the candidate reference genes tested, only ACTB showed similarly high expression in skin and body-fluid samples, providing a suitable reference marker for quantitative real-time PCR (qPCR) analysis of skin. Analyses of palmar and thumbprint skin samples indicate that our qPCR approach for the three skin-targeted mRNA markers, as well as the reference mRNA marker ACTB, is highly sensitive, allowing successful detection of minute amounts of skin material including full, half and quarter thumbprints, albeit with decreased success in decreasing print material. Furthermore, thumbprints stored for 6.5 months provided similar results relative to freshly analysed samples implying reasonable time-wise stability of the three skin-targeted mRNAs as well as the ACTB reference mRNA. Our study represents the first attempt towards reliable mRNA-based skin identification in forensic applications with particular relevance for future trace/touched object analyses in forensic case work. Although the approach for skin identification introduced here can be informative when applied on its own, we recommend for increased reliability the integration of (one or more of) the skin-targeted mRNA markers presented here into multiplex assays additionally including mRNA markers targeting alternative cell types expected in forensic samples.

The cardiac desmosome and arrhythmogenic cardiomyopathies: from gene to disease

Intercellular communication is essential for proper cardiac function. Mechanical and electrical activity need to be synchronized so that the work of individual myocytes transforms into the pumping function of the organ. Mechanical continuity is provided by desmosomes and adherens junctions, while gap junctions provide a pathway for passage of ions and small molecules between cells. These complexes preferentially reside at the site of end-end contact between myocytes, within the intercalated disc. Recognition that some forms of arrhythmogenic cardiomyopathy are caused by mutations in desmosomal protein genes has galvanized interest in the biology of the desmosome and its interactions with other junctional molecules. This review presents the cellular and molecular biology of the desmosome, current knowledge on the relation of desmosomal mutations and disease phenotypes, and an overview of the molecular pathophysiology of arrhythmogenic right ventricular cardiomyopathy. Clinical experience and results from cellular and animal models provide insights into the intercalated disc as a functional unit and into the basic substrates that underlie pathogenesis and arrhythmogenesis of arrhythmogenic right ventricular cardiomyopathy.

Cadherins as targets for genetic diseases

The 6-billion human population provides a vast reservoir of mutations, which, in addition to the opportunity of detecting very subtle defects, including specific cognitive dysfunctions as well as late appearing disorders, offers a unique background in which to investigate the roles of cell-cell adhesion proteins. Here we focus on inherited human disorders involving members of the cadherin superfamily. Most of the advances concern monogenic disorders. Yet, with the development of single nucleotide polymorphism (SNP) association studies, cadherin genes are emerging as susceptibility genes in multifactorial disorders. Various skin and heart disorders revealed the critical role played by desmosomal cadherins in epidermis, hairs, and myocardium, which experience high mechanical stress. Of particular interest in that respect is the study of Usher syndrome type 1 (USH1), a hereditary syndromic form of deafness. Studies of USH1 brought to light the crucial role of transient fibrous links formed by cadherin 23 and protocadherin 15 in the cohesion of the developing hair bundle, the mechanoreceptive structure of the auditory sensory cells, as well as the involvement of these cadherins in the formation of the tip-link, a key component of the mechano-electrical transduction machinery. Finally, in line with the well-established role of cadherins in synaptic formation, maintenance, strength, and plasticity, a growing number of cadherin family members, especially protocadherins, have been found to be involved in neuropsychiatric disorders.

Cancer biomarker discovery: the entropic hallmark

Background

It is a commonly accepted belief that cancer cells modify their transcriptional state during the progression of the disease. We propose that the progression of cancer cells towards malignant phenotypes can be efficiently tracked using high-throughput technologies that follow the gradual changes observed in the gene expression profiles by employing Shannon's mathematical theory of communication. Methods based on Information Theory can then quantify the divergence of cancer cells' transcriptional profiles from those of normally appearing cells of the originating tissues. The relevance of the proposed methods can be evaluated using microarray datasets available in the public domain but the method is in principle applicable to other high-throughput methods.

Methodology/Principal Findings

Using melanoma and prostate cancer datasets we illustrate how it is possible to employ Shannon Entropy and the Jensen-Shannon divergence to trace the transcriptional changes progression of the disease. We establish how the variations of these two measures correlate with established biomarkers of cancer progression. The Information Theory measures allow us to identify novel biomarkers for both progressive and relatively more sudden transcriptional changes leading to malignant phenotypes. At the same time, the methodology was able to validate a large number of genes and processes that seem to be implicated in the progression of melanoma and prostate cancer.

Conclusions/Significance

We thus present a quantitative guiding rule, a new unifying hallmark of cancer: the cancer cell's transcriptome changes lead to measurable observed transitions of Normalized Shannon Entropy values (as measured by high-througput technologies). At the same time, tumor cells increment their divergence from the normal tissue profile increasing their disorder via creation of states that we might not directly measure. This unifying hallmark allows, via the the Jensen-Shannon divergence, to identify the arrow of time of the processes from the gene expression profiles, and helps to map the phenotypical and molecular hallmarks of specific cancer subtypes. The deep mathematical basis of the approach allows us to suggest that this principle is, hopefully, of general applicability for other diseases.

Desmosomes: adhesive strength and signalling in health and disease

Desmosomes are intercellular junctions whose primary function is strong intercellular adhesion, known as hyperadhesion. In the present review, we discuss how their structure appears to support this function as well as how they are assembled and down-regulated. Desmosomal components also have signalling functions that are important in tissue development and remodelling. Their adhesive and signalling functions are both compromised in genetic and autoimmune diseases that affect the heart, skin and mucous membranes. We conclude that much work is required on structure–function relationships within desmosomes in vivo and on how they participate in signalling processes to enhance our knowledge of tissue homoeostasis and human disease.

Exploring the Nature of Desmosomal Cadherin Associations in 3D

Desmosomes are a complex assembly of protein molecules that mediate adhesion between adjacent cells. Desmosome composition is well established and spatial relationships between components have been identified. Intercellular cell-cell adhesion is created by the interaction of extracellular domains of desmosomal cadherins, namely, desmocollins and desmogleins. High-resolution methods have provided insight into the structural interactions between cadherins. However, there is a lack of understanding about the architecture of the intact desmosomes and the physical principles behind their adhesive strength are unclear. Electron Tomography (ET) studies have offered three-dimensional visual data of desmosomal cadherin associations at molecular resolution. This review discusses the merits of two cadherin association models represented using ET. We discuss the possible role of sample preparation on the structural differences seen between models and the possibility of adaptive changes in the structure as a direct consequence of mechanical stress and stratification.

Mutations of plakophilin-2 in Chinese with arrhythmogenic right ventricular dysplasia/cardiomyopathy

Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is an inherited heart muscle disease associated with increased risks of sudden death, particularly in young, otherwise healthy, patients. The pathologic features are progressive myocardial atrophy and fibrofatty replacement. Plakophilin-2 (PKP2) is reported as the most common ARVD/C-causing gene in Western countries. In this study we aimed to determine the prevalence of PKP2 mutations in Chinese patients with ARVD/C and their phenotype characteristics. Genotype and phenotype were investigated in a cohort of 18 unrelated Chinese patients with a clinical diagnosis of ARVD/C. Direct sequencing of PKP2 led to the identification of 5 novel heterozygous mutations (R158K, Q211X, L419S, A793D, and N852fsX930) in 39% of patients (7 of 18) with ARVD/C. Among them, N852fsX930 was found in 3 unrelated young patients who presented with symptomatic ventricular tachyarrhythmia. Nevertheless, no significant difference could be detected between patients with ARVD/C with (n = 7) and without (n = 11) PKP2 mutations with regard to the phenotype characteristics and clinical outcomes. Decreased penetrance was prominent in family members. In conclusion, 5 novel PKP2 mutations were identified in a cohort of symptomatic Chinese patients with ARVD/C. N852fsX930 appeared to be a hot-spot mutation in which patients presented with a severe ARVD/C phenotype, and 2/3 had early onset of arrhythmic events. No significant difference was found in phenotype characteristics between patients with ARVD/C with and without PKP2 mutations. The decreased penetrance indicated that an ARVD/C diagnosis cannot solely rely on genotyping results.

Mutations in the desmoglein 1 gene in five Pakistani families with striate palmoplantar keratoderma

BACKGROUND

Striate palmoplantar keratoderma (SPPK; OMIM #148700) is a rare autosomal dominant genodermatosis characterized by linear hyperkeratosis on the digits and hyperkeratosis on the palms and soles. SPPK is known to be caused by heterozygous mutations in either the desmoglein 1 (DSG1), desmoplakin (DSP), or keratin 1 (KRT1) genes.

OBJECTIVE

To define the molecular basis of SPPK in five Pakistani families showing a clear autosomal dominant inheritance pattern of SPPK.

METHODS

Based on previous reports of DSG1 mutations in SPPK, we performed direct sequencing of the DSG1 gene of all five families.

RESULTS

Mutation analysis resulted in the identification of one recurrent mutation (p.R26X) and four novel mutations (c.Ivs4-2A>G, c.515C>T, c.Ivs9-3C>G, and c.1399delA) in the DSG1 gene. Each mutation is predicted to cause haploinsufficiency of DSG1 protein.

CONCLUSION

The results of our study further underscore the significance of the desmoglein gene family in diseases of epidermal integrity.

Cryoelectron tomography of isolated desmosomes

Desmosomes are a complex assembly of protein molecules that form at the cell surface and mediate cell-cell adhesion. Much is known about the composition of desmosomes and there is an established consensus for the location of and interactions between constituent proteins within the assembly. Furthermore, X-ray crystallography has determined atomic structures of isolated domains from several constituent proteins. Nevertheless, there is a lack of understanding about the architecture of the intact assembly and the physical principles behind the adhesive strength of desmosomes therefore remain vague. We have used electron tomography to address this problem. In previous work, we investigated the in situ structure of desmosomes from newborn mouse skin preserved by freeze-substitution and imaged in resin-embedded thin sections. In our present work, we have isolated desmosomes from cow snout and imaged them in the frozen unstained state. Although not definitive, the resulting images provide support for the irregular groupings of cadherin molecules seen previously in mouse skin.

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.

A perspective of pemphigus from bedside and laboratory-bench

Pemphigus represents a distinct organ-specific acquired autoimmune disease characterized by intra-epidermal blistering, which is induced by autoantibodies against desmosomal cadherins, desmoglein 1 (Dsg1), and Dsg3. Pemphigus is currently divided into three distinct varieties, i.e., pemphigus vulgaris (PV), pemphigus foliaceus (PF) and other variants of pemphigus (mostly associated with inflammation), depending on clinical features, the level of separation in the epidermis, and immunologic characteristics of auto-antigens. Blistering pathomechanisms differ for each of the types of pemphigus. Pemphigus, which results from autoantibodies against desmogleins and possibly to other proteins, binds to the cell surface antigens. This binding may cause steric hindrance to homophilic adhesion of desmogleins, and may, in turn, lead to internalization of desmogleins and inhibition of desmogleins' integration into desmosomes, resulting in the formation of Dsg3-depleted desmosomes in PV or Dsg1-depleted desmosomes in PF. Furthermore, PV-IgG activates an "outside-in" signaling pathway to induce disassembly of desmosomal components from the inside of the cells by phosphorylation of proteins, including Dsg3. On the other hand, Pemphigus-IgG-augmented signaling pathways may be linked to the secretion of cytokines such as in case of pemphigus herpetiformis and chemokines that initiate or activate inflammation. In this article, the classification of pemphigus and the characteristic pathomechanisms for acantholysis will be reviewed, with particular emphasis on the molecular and biochemical cell biology of these diseases.

Perturbed desmosomal cadherin expression in grainy head-like 1-null mice

In Drosophila, the grainy head (grh) gene plays a range of key developmental roles through the regulation of members of the cadherin gene family. We now report that mice lacking the grh homologue grainy head-like 1 (Grhl1) exhibit hair and skin phenotypes consistent with a reduction in expression of the genes encoding the desmosomal cadherin, desmoglein 1 (Dsg1). Grhl1-null mice show an initial delay in coat growth, and older mice exhibit hair loss as a result of poor anchoring of the hair shaft in the follicle. The mice also develop palmoplantar keratoderma, analogous to humans with DSG1 mutations. Sequence analysis, DNA binding, and chromatin immunoprecipitation experiments demonstrate that the human and mouse Dsg1 promoters are direct targets of GRHL1. Ultrastructural analysis reveals reduced numbers of abnormal desmosomes in the interfollicular epidermis. These findings establish GRHL1 as an important regulator of the Dsg1 genes in the context of hair anchorage and epidermal differentiation, and suggest that cadherin family genes are key targets of the grainy head-like genes across 700 million years of evolution.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

Desmosomes: new perspectives on a classic

Desmosomes are highly specialized anchoring junctions that link intermediate filaments to sites of intercellular adhesion, thus facilitating the formation of a supracellular scaffolding that distributes mechanical forces throughout a tissue. These junctions are thus particularly important for maintaining the integrity of tissues that endure physical stress, such as the epidermis and myocardium. The importance of the classic mechanical functions of desmosomal constituents is underscored by pathologies reported in animal models and an ever-expanding list of human mutations that target both desmosomal cadherins and their associated cytoskeletal anchoring proteins. However, the notion that desmosomes are static structures that exist simply to glue cells together belies their susceptibility to remodeling in response to environmental cues and their important tissue-specific roles in cell behavior and signaling. Here, we review the molecular blueprint of the desmosome and models for assembling its protein components to form an adhesive interface and the desmosomal plaque. We also discuss emerging evidence of supra-adhesive roles for desmosomal proteins in regulating tissue morphogenesis and homeostasis. Finally, we highlight the dynamic nature of these adhesive organelles, examining mechanisms in health and disease for modulating adhesive strength and stability of desmosomes.

Broken hearts, woolly hair, and tattered skin: when desmosomal adhesion goes awry

Desmosomal cadherins constitute the adhesive core of desmosomes. Different desmosomal cadherins are differentially expressed in a tissue-specific as well as differentiation-dependent manner. The skin and the heart are two examples of tissues whose vital functions require the ability to endure mechanical stress, and therefore, rely on the integrity of desmosomal adhesion. When this adhesion is compromised via mutations in genes encoding desmosomal cadherins or associated plaque proteins, both tissues can suffer the consequences. Open questions revolve around whether the resulting phenotypes are solely because of physical disruption of cell adhesion or whether these events are coupled with signaling mechanisms that influence many additional cellular processes. In this review, we focus on new developments in desmosomal adhesion with an emphasis on the skin, hair, and heart.

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.

Desmosomes are unaltered during infections by attaching and effacing pathogens

The human attaching and effacing (A/E) intestinal pathogens enterohemorrhagic Escherichia coli (EHEC), enteropathogenic E. coli (EPEC), and the murine A/E pathogen Citrobacter rodentium cause serious diarrhea in their hosts. These bacteria alter numerous host cell components, including organelles, the host cell cytoskeleton, and tight junctions during the infectious process. One of the proteins that contribute to the intermediate filament network in host cells, cytokeratin-18, is extensively altered during EPEC infections. Based on this, we tested the hypothesis that desmosomes, the only intercellular junctions that interact with intermediate filaments, are also influenced by A/E pathogen infections. We found that the desmosomal transmembrane proteins desmoglein and desmocollin, as well as the desmosome plaque protein desmoplakin, all remain unchanged during EPEC infection in vitro. This evidence is corroborated by the unaltered localization of desmoglein and desmoplakin in vivo in mice infected with C. rodentium for 7 days. Electron microscopic analysis of 7-day C. rodentium-infected murine colonocytes also show no observable differences in the desmosomes when compared to uninfected controls. Our data suggest that, unlike tight junctions, the desmosome protein levels and localization, as well as desmosome morphology, are unaltered during A/E pathogenesis.

Desmoglein autoimmunity in the pathogenesis of pemphigus

The most characteristic feature of pemphigus is a loss of cohesion between keratinocytes, resulting in formation of blisters and erosions on the mucosal membranes and the skin. Identification of circulating antibodies which bind to desmogleins (Dsg), transmembrane proteins involved in assembly of the desmosomes, led to the immediate realization that these antibodies may be pathogenic by interfering with desmosomal function. Despite extensive experimental evidence documenting the presence of the anti-Dsg response, its pathogenic relevance is still debated. At the current stage of the knowledge it seems likely that anti-Dsg imunoglobulins may play a role in pemphigus via interference with cellular Dsg trafficking and by activation of specific signalling pathways rather than by simple interference with desmosomal adhesion.

Desmosomes: just cell adhesion or is there more?

Desmosomes are cell adhesion structures (junctions) that are particularly abundant in cells derived from the ectodermal lineages. These junctions are required to maintain the integrity of organs subjected to mechanical stress, in particular the skin and the heart. This conclusion is partially based on tissue fragility phenotypes observed in mice with null mutations in certain desmosomal genes. Furthermore, patients have been identified that develop severe skin disorders, and even fatal heart diseases, due to impaired desmosome function. Nevertheless, desmosomes are more than cellular glue. New evidence suggests that these junctions can transmit signals from the extracellular environment to the nucleus, for example by controling the cytoplasmic pool of transcriptional co-factors that belong to the armadillo family of desmosomal proteins (i.e. plakoglobin, plakophilins). Understanding the signaling properties of desmosomes will provide new insights into developmental processes such as skin and skin appendage development. Furthermore, there is evidence to suggest that abnormal signaling through these junctions contributes to the symptoms of certain skin and heart diseases.

Developmental functions of the P120-catenin sub-family

For more than a decade, cell, developmental and cancer investigators have brought about a wide interest in the biology of catenin proteins, an attraction being their varied functions within differing cellular compartments. While the diversity of catenin localizations and roles has been intriguing, it has also posed a challenge to the clear interpretation of loss- or gain-of-function developmental phenotypes. The most deeply studied member of the larger catenin family is beta-catenin, whose contributions span areas including cell adhesion and intracellular signaling/ transcriptional control. More recently, attention has been directed towards p120-catenin, which in conjunction with the p120-catenin sub-family members ARVCF- and delta-catenins, are the subjects of this review. Although the requirement for vertebrate versus invertebrate p120-catenin are at variance, vertebrate p120-catenin sub-family members may each inter-link cadherin, cytoskeletal and gene regulatory functions in embryogenesis and disease.

Desmoglein 4 is expressed in highly differentiated keratinocytes and trichocytes in human epidermis and hair follicle

Desmosomes are critical for the tissue integrity of stratified epithelia and their appendages. Desmogleins (DSGs) and desmocollins (DSCs) are transmembrane desmosomal cadherins that interact extracellularly to link neighboring epithelial cells. We recently identified a new member of the DSG family, designated desmoglein 4, whose mutations cause hypotrichosis in human, mouse and rat. In this study, we analyzed in detail the expression domains of human desmoglein 4 protein (DSG4) in human skin relative to differentiation markers and other DSGs. Our results show that DSG4 protein is expressed in the more highly differentiated layers of the epidermis. This expression pattern in vivo is recapitulated in highly differentiated HaCaT human keratinocytes and normal human keratinocytes in vitro. In the human hair follicle, DSG4 is expressed specifically in the hair shaft cortex, the lower hair cuticle, and the upper inner root sheath (IRS) cuticle. Using a green fluorescent protein-tagged version of mouse or rat desmoglein 4 protein (Dsg4) and immuno-electron microscopy, we demonstrate that Dsg4 localizes to desmosomes both in vitro and in vivo. The highly specific expression pattern of DSG4 in the human hair follicle, combined with the phenotype of rodent models and human patients with desmoglein 4 mutations, underscores the importance of this adhesion molecule in the integrity of the hair shaft.

Identification of the junctional plaque protein plakophilin 3 in cytoplasmic particles containing RNA-binding proteins and the recruitment of plakophilins 1 and 3 to stress granules

Recent studies on the subcellular distribution of cytoplasmic plaque proteins of intercellular junctions have revealed that a number of such proteins can also occur in the cyto- and the nucleoplasm. This occurrence in different, and distant locations suggest that some plaque proteins play roles in cytoplasmic and nuclear processes in addition to their involvement in cell-cell adhesive interactions. Plakophilin (PKP) 3, a member of the arm-repeat family of proteins, occurs, in a diversity of cell types, both as an architectural component in plaques of desmosomes and dispersed in cytoplasmic particles. In immuno-selection experiments using PKP3-specific antibodies, we have identified by mass spectrometric analysis the following RNA-binding proteins: Poly (A) binding protein (PABPC1), fragile-X-related protein (FXR1), and ras-GAP-SH3-binding protein (G3BP). Moreover, the RNA-binding proteins codistributed after sucrose gradient centrifugation in PKP3-containing fractions corresponding to 25-35 S and 45-55 S. When cells are exposed to environmental stress (e.g., heat shock or oxidative stress) proteins FXR1, G3BP, and PABPC1 are found, together with PKP3 or PKP1, in "stress granules" known to accumulate stalled translation initiation complexes. Moreover, the protein eIF-4E and the ribosomal protein S6 are also detected in PKP3 particles. Our results show that cytoplasmic PKP3 is constitutively associated with RNA-binding proteins and indicate an involvement in processes of translation and RNA metabolism.