Functional tight junction barrier localizes in the second layer of the stratum granulosum of human epidermis.

BACKGROUND: Mammalian epidermis has two diffusion barriers, the stratum corneum (SC) and tight junctions (TJs). We reported previously that a single living cell layer exists between the SC and TJ-forming keratinocytes in mice; however, the exact location of the TJ barrier in human epidermis has not been defined. OBJECTIVE: To investigate the precise distribution of epidermal TJs in relation to various cell-cell junction proteins and the SC and to clarify the barrier function of TJs against macromolecules in human skin. METHODS: The localization of various junctional proteins was investigated in human skin sections and in the roofs of bullae formed by ex vivo exfoliative toxin (ET) treatment in three dimensions. ET and single-chain variable fragments (scFv) against desmoglein 1 were used as large diffusion probes. RESULTS: Human stratum granulosum (SG) cells have a distinct distribution of TJ, adherens junction, and desmosome proteins in the uppermost three layers (SG1-SG3 from the surface inward). Ex vivo injection of ET or scFv demonstrated that only SG2-SG2 junctions function as a TJ barrier, limiting the inside-out diffusion of these proteins. The roofs of bullae formed by ex vivo ET treatment consisted of SC, SG1 cells, and TJ-forming SG2 cells, probably mimicking bulla formation in bullous impetigo. CONCLUSION: Human epidermis has three SG cell layers with distinct properties just beneath the SC, of which only SG2 cells have functional TJs. Our results suggest that human epidermal TJs between SG2 cells form a paracellular diffusion barrier against soluble proteins, including immunoglobulins and bacterial toxins.

Autoimmune reactivity against precursor form of desmoglein 1 in healthy Tunisians in the area of endemic pemphigus foliaceus.

BACKGROUND: Desmoglein 1 (Dsg1), the pemphigus foliaceus (PF) antigen, is produced as a precursor (preDsg1) and is transported to the cell surface as the mature form (matDsg1). Recent studies show that B cells from North American individuals without pemphigus can potentially produce anti-preDsg1 IgG antibodies, but ELISA screening of large numbers of normal people in North America and Japan hardly ever shows circulating antibodies against preDsg1 or matDsg1. In contrast, in Tunisia, where PF is endemic, anti-Dsg1 IgGs are frequently detected in healthy individuals. OBJECTIVE: To characterize these anti-Dsg1 antibodies from normal individuals in Tunisia. METHODS: Sera from 16 healthy individuals and 9 PF patients in the endemic PF area in Tunisia, and sera from Japanese non-endemic PF patients were analyzed by immunoprecipitation-immunoblotting using recombinant proteins of preDsg1, matDsg1, and domain-swapped Dsg1/Dsg2 molecules. RESULTS: Sera from normal Tunisian individuals reacted to preDsg1 alone (8/16) or more strongly to preDsg1 than to matDsg1 (7/16), while those from all Tunisian PF patients and Japanese non-endemic PF patients reacted similarly to preDsg1 and matDsg1, or preferentially to matDsg1. The epitopes recognized by anti-Dsg1 IgGs from normal Tunisian individuals were more frequently found in the C-terminal extracellular domains (EC3 to EC5), while those in Tunisian endemic PF patients were more widely distributed throughout the extracellular domains, suggesting IgGs against EC1 and EC2 developed during disease progression. CONCLUSIONS: These findings indicate that IgG autoantibodies against Dsg1 are mostly raised against preDsg1 and/or C-terminal domains of Dsg1 in healthy Tunisians in the endemic area of PF.

The mapping of linear B-cell epitope regions in the extracellular parts of the desmoglein 1 and 3 proteins: recognition of immobilized peptides by pemphigus patients' serum autoantibodies.

Desmosomal transmembrane glycoproteins desmogleins (Dsg) 1 and 3 are targets of life-threatening autoimmune blistering disorders such as Pemphigus vulgaris (PV) and Pemphigus foliaceus (PF). In these diseases, pemphigus autoantibodies are produced against Dsg1 and Dsg3 proteins. The autoantibodies bind to these transmembrane elements leading to a loss of desmosomal cell-cell adhesion and clinically, to the presence of blisters and erosions. Identification, characterization, and detailed analysis of the binding sites of autoantibodies have an outstanding importance in understanding the immunopathology of the disease and also in the design of novel diagnostics. Here, we describe the localization of the B-cell epitope regions of Dsg1 and Dsg3 proteins' extracellular parts recognized by IgG-type serum autoantibodies of patients with PV and PF. In our study, overlapping pentadecapeptides were synthesized on hydroxypropyl methacrylate pins based on the results of in silico predictions. To detect the interaction between the serum autoantibodies and the immobilized synthetic peptides, modified Enzyme Linked Immunosorbent Assay (ELISA) was performed with pin-attached peptides testing the serum samples of ten patients and four healthy donors. We identified five possible epitope regions (aa86-110, aa196-220, aa226-250, aa326-340, and aa486-520) within the extracellular part of the Dsg1 and four possible epitope regions (aa64-78, aa330-344, aa375-399, and aa446-460) within that of the Dsg3 protein sequence using these methods. Our data showed that serum autoantibodies of patients, previously identified as Dsg1 and Dsg3 positive, are able to recognize continuous linear epitope regions of both Dsg1 and Dsg3 proteins using pin-bound overlapping peptides in modified ELISAs.

Plakoglobin rescues adhesive defects induced by ectodomain truncation of the desmosomal cadherin desmoglein 1: implications for exfoliative toxin-mediated skin blistering.

Desmoglein 1 (Dsg1) is a desmosomal cadherin that is essential to epidermal integrity. In the blistering diseases bullous impetigo and staphylococcal scalded-skin syndrome, pathogenesis depends on cleavage of Dsg1 by a bacterial protease, exfoliative toxin A, which removes residues 1 to 381 of the Dsg1 ectodomain. However, the cellular responses to Dsg1 cleavage that precipitate keratinocyte separation to induce blister formation are unknown. Here, we show that ectodomain-deleted Dsg1 (Δ381-Dsg1) mimics the toxin-cleaved cadherin, disrupts desmosomes, and reduces the mechanical integrity of keratinocyte sheets. In addition, we demonstrate that truncated Dsg1 remains associated with its catenin partner, plakoglobin, and causes a reduction in the levels of endogenous desmosomal cadherins in a dose-dependent manner, leading us to hypothesize that plakoglobin sequestration by truncated Dsg1 destabilizes other cadherins. Accordingly, a triple-point mutant of the ectodomain-deleted cadherin, which is uncoupled from plakoglobin, does not impair adhesion, indicating that this interaction is essential to the pathogenic potential of truncated Dsg1. Moreover, we demonstrate that increasing plakoglobin levels rescues cadherin expression, desmosome organization, and functional adhesion in cells expressing Δ381-Dsg1 or treated with exfoliative toxin A. Finally, we report that histone deacetylase inhibition up-regulates desmosomal cadherins and prevents the loss of adhesion induced by Dsg1 truncation. These findings further our understanding of the mechanism of exfoliative toxin-induced pathology and suggest novel strategies to suppress blistering in bulbous impetigo and staphylococcal scalded-skin syndrome.

Removal of amino-terminal extracellular domains of desmoglein 1 by staphylococcal exfoliative toxin is sufficient to initiate epidermal blister formation.

BACKGROUND: In both bullous impetigo and staphylococcal scalded-skin syndrome (SSSS), exfoliative toxins (ETs) produced by Staphylococcus aureus cause superficial intraepidermal blisters. ETs are known to cleave specifically a single peptide bond in the extracellular domains 3 and 4 of desmoglein (Dsg) 1. However, the precise mechanisms underlying ET-induced epidermal blister formation remain poorly understood. OBJECTIVE: To determine whether cleavage of Dsg1 by an ET is sufficient to induce blister formation in vivo or if the subsequent internalization of cleaved Dsg1 or other desmosomal components is required. METHODS: Skin samples obtained from neonatal mice injected with ETA were analyzed by time-lapse immunofluorescence and transmission electron microscopy for desmosomal components. RESULTS: Epidermal blister formation was observed as early as 60 min after ETA treatment. At this time, the amino-terminal extracellular domains of Dsg1 disappeared from the surface of keratinocytes, while the cleaved carboxy-terminal domain of Dsg1 (Dsg1-C) as well as the extracellular domains of desmocollin 1 (Dsc1-N) remained on the cell surface. Half-split desmosomes with intracytoplasmic dense plaques and attached tonofilaments were recognized ultrastructurally on the split surface of keratinocytes at 60 min. Subsequent to this, Dsg1-C and Dsc1-N gradually disappeared from the surface layer of keratinocytes. CONCLUSION: Our findings suggest that the removal of amino-terminal extracellular domains of Dsg1 by ETs is sufficient to initiate epidermal blister formation in bullous impetigo and SSSS.

Immune response towards the amino-terminus of desmoglein 1 prevails across different activity stages in nonendemic pemphigus foliaceus.

BACKGROUND: Pemphigus foliaceus (PF) is a blistering skin disease mediated by antibodies to desmoglein (Dsg) 1. The two major subtypes are nonendemic and endemic PF. A previous study in endemic PF demonstrated that changes in antibody epitope could modulate disease relapse and remission. OBJECTIVES: To characterize the frequency of immunoreactivity to various Dsg1 extracellular (EC) domains in nonendemic PF and to study if there is any change in epitope profile across various activity stages. METHODS: Sera from 34 patients with nonendemic PF were selected. To map the conformational epitopes by immunoprecipitation-immunoblotting, we constructed five Dsg1/Dsg2 domain-swapped molecules, with each molecule representing one EC domain of Dsg1 on a backbone of Dsg2. RESULTS: Dsg1 EC1, EC2, EC3, EC4 and EC5 domains were recognized by 88%, 50%, 13%, 22% and 0% of active PF sera, respectively. Immunoreactivity to EC3 or EC4 often cosegregated with that to either EC1 or EC2. Longitudinal follow-up of 21 patients with PF for a median of 16 months revealed that, in most cases, immunoreactivity to the amino-terminus of Dsg1 persisted across various activity stages; only two patients lost their EC1 reactivity upon remission and changed their major epitope(s) to EC2 ± EC3. CONCLUSIONS: Most of the anti-Dsg1 antibodies in nonendemic PF bind to the amino-terminus of Dsg1, a region critical for intercellular adhesion of cadherins, and this skewed amino-terminal immunoreactivity prevails across various activity stages in most patients, even upon remission. These findings are valuable for understanding the biology of Dsg-mediated cellular adhesion as well as for the development of epitope-based monitoring and therapeutic strategies.

Interactions of plakoglobin and beta-catenin with desmosomal cadherins: basis of selective exclusion of alpha- and beta-catenin from desmosomes.

Plakoglobin and beta-catenin are homologous armadillo repeat proteins found in adherens junctions, where they interact with the cytoplasmic domain of classical cadherins and with alpha-catenin. Plakoglobin, but normally not beta-catenin, is also a structural constituent of desmosomes, where it binds to the cytoplasmic domains of the desmosomal cadherins, desmogleins and desmocollins. Here, we report structural, biophysical, and biochemical studies aimed at understanding the molecular basis of selective exclusion of beta-catenin and alpha-catenin from desmosomes. The crystal structure of the plakoglobin armadillo domain bound to phosphorylated E-cadherin shows virtually identical interactions to those observed between beta-catenin and E-cadherin. Trypsin sensitivity experiments indicate that the plakoglobin arm domain by itself is more flexible than that of beta-catenin. Binding of plakoglobin and beta-catenin to the intracellular regions of E-cadherin, desmoglein1, and desmocollin1 was measured by isothermal titration calorimetry. Plakoglobin and beta-catenin bind strongly and with similar thermodynamic parameters to E-cadherin. In contrast, beta-catenin binds to desmoglein-1 more weakly than does plakoglobin. beta-Catenin and plakoglobin bind with similar weak affinities to desmocollin-1. Full affinity binding of desmoglein-1 requires sequences C-terminal to the region homologous to the catenin-binding domain of classical cadherins. Although pulldown assays suggest that the presence of N- and C-terminal beta-catenin "tails" that flank the armadillo repeat region reduces the affinity for desmosomal cadherins, calorimetric measurements show no significant effects of the tails on binding to the cadherins. Using purified proteins, we show that desmosomal cadherins and alpha-catenin compete directly for binding to plakoglobin, consistent with the absence of alpha-catenin in desmosomes.

The desmoglein-specific cytoplasmic region is intrinsically disordered in solution and interacts with multiple desmosomal protein partners.

The desmoglein-specific cytoplasmic region (DSCR) is a conserved region of unknown structure and function that uniquely defines the desmoglein family of cell adhesion molecules. It is the site of caspase cleavage during apoptosis, and its mutation is linked to cardiomyopathy. Here, we reveal that a 276-residue DSCR construct of human desmoglein 1 is intrinsically disordered and forms an interaction hub for desmosomal proteins. In solution, it contains 6.5% helical and 10.3% beta-strand structure based on circular dichroism spectroscopy. A single monomeric state with a predominantly unfolded structure is found by size-exclusion chromatography and analytical ultracentrifugation. Thermal stability assays and nuclear magnetic resonance spectroscopy reveal a nonglobular structure under a range of solution conditions. However, the introduction of detergent micelles increases structure to 18% helical and 16% beta-strand character, suggesting an inducible structure. The DSCR exhibits weak but specific interactions with plakoglobin, the plakin domain of desmoplakin, plakophilin 1, and the cytoplasmic domain of desmocollin 1. The desmoglein 1 membrane proximal region also interacts with all four DSCR ligands, strongly with plakoglobin and plakophilin and more weakly with desmoplakin and desmocollin 1. Thus, the DSCR is an intrinsically disordered functional domain with an inducible structure that, along with the membrane proximal region, forms a flexible scaffold for cytoplasmic assembly at the desmosome.

Peptides Targeting the Desmoglein 3 Adhesive Interface Prevent Autoantibody-induced Acantholysis in Pemphigus.

Pemphigus vulgaris (PV) autoantibodies directly inhibit desmoglein (Dsg) 3-mediated transinteraction. Because cellular signaling also seems to be required for PV pathogenesis, it is important to characterize the role of direct inhibition in pemphigus acantholysis to allow establishment of new therapeutic approaches. Therefore, we modeled the Dsg1 and Dsg3 sequences into resolved cadherin structures and predicted peptides targeting the adhesive interface of both Dsg3 and Dsg1. In atomic force microscopy single molecule experiments, the self-designed cyclic single peptide specifically blocked homophilic Dsg3 and Dsg1 transinteraction, whereas a tandem peptide (TP) consisting of two combined single peptides did not. TP did not directly block binding of pemphigus IgG to their target Dsg antigens but prevented PV-IgG-induced inhibition of Dsg3 transinteraction in cell-free (atomic force microscopy) and cell-based (laser tweezer) experiments, indicating stabilization of Dsg3 bonds. Similarly, PV-IgG-mediated acantholysis and disruption of Dsg3 localization in HaCaT keratinocytes was partially blocked by TP. This is the first evidence that direct inhibition of Dsg3 binding is important for PV pathogenesis and that peptidomimetics stabilizing Dsg transinteraction may provide a novel approach for PV treatment.

Immunological hotspots analyzed by docking simulations: evidence for a general mechanism in pemphigus vulgaris pathology and transformation.

BACKGROUND: Pemphigus vulgaris (PV) is an acquired autoimmune blistering disorder in which greater than 80% of active patients produce autoantibodies to the desmosomal protein desmogelin 3 (Dsg3). As the disease progresses, 40-50% of patients may also develop reactivity to a second component of the desmosomal complex, desmogelin 1 (Dsg1). T cells are clearly required for the production of autoantibodies in PV. However, few T-cell specificities within Dsg3 or Dsg1 have been reported to date, and the precise role of T-cells in disease pathogenesis and evolution remains poorly understood. In particular, no studies have addressed the immunological mechanisms that underlie the observed clinical heterogeneity in pemphigus. We report here a structure-based technique for the screening of DRB1*0402-specific immunological (T-cell epitope) hotspots in both Dsg3 and Dsg1 glycoproteins. RESULTS: High predictivity was obtained for DRB1*0402 (r2 = 0.90, s = 1.20 kJ/mol, q2 = 0.82, spress = 1.61 kJ/mol) predictive model, compared to experimental data. In silico mapping of the T-cell epitope repertoires in Dsg3 and Dsg1 glycoproteins revealed that the potential immunological hotspots of both target autoantigens are highly conserved, despite limited sequence identity (54% identical, 72% similar). A similar number of well-conserved (18%) high-affinity binders were predicted to exist within both Dsg3 and Dsg1, with analogous distribution of binding registers. CONCLUSION: This study provides interesting new insights into the possible mechanism for PV disease progression. Our data suggests that the potential T-cell epitope repertoires encoded in Dsg1 and Dsg3 is substantially overlapping, and it may be possible to apply a common, antigen-specific therapeutic strategy with efficacy across distinct clinical phases of disease.

Imaging and force spectroscopy on desmoglein 1 using atomic force microscopy reveal multivalent Ca(2+)-dependent, low-affinity trans-interaction.

Desmoglein 1 is a desmosomal member of the cadherin family expressed in stratified epithelia. Desmoglein 1 is the target adhesion molecule of severe blistering skin diseases such as pemphigus or bullous impetigo. However, despite this enormous pathological relevance, the molecular binding properties of desmoglein 1 are largely unknown. Using atomic force microscopic imaging, we found that desmoglein 1 molecules displayed Ca(2+)-dependent conformational changes of the extracellular domains. By single-molecule force-distance cycles, we provide evidence that desmoglein 1 undergoes Ca(2+)-dependent (K (d) = 0.8 mM Ca(2+)) homophilic trans-interaction, which is highly relevant for the contribution of desmoglein 1 homophilic binding to keratinocyte cohesion in distinct epidermal layers. Moreover, while the single-unit unbinding force is comparable to other cadherins (approximately 40 pN at retrace velocity of 300 nm/s), apparent differences with respect to multivalency of interaction and lifetime of single bonds (0.17 s) were observed. Thus, besides the biophysical characterization of desmoglein 1, a main outcome of the study is that desmoglein 1 differs from other members of the cadherin family in terms of some molecular binding properties.

Investigation of SNPs in the porcine desmoglein 1 gene.

BACKGROUND: Desmoglein 1 (DSG1) is the target protein in the skin disease exudative epidermitis in pigs caused by virulent strains of Staphylococcus hyicus. The exfoliative toxins produced by S. hyicus digest the porcine desmoglein 1 (PIG)DSG1 by a very specific reaction. This study investigated the location of single nucleotide polymorphisms (SNPs) in the porcine desmoglein 1 gene (PIG)DSG1 in correlation to the cleavage site as well as if the genotype of the SNPs is correlated to susceptibility or resistance to the disease. RESULTS: DNA from 32 affected and 32 unaffected piglets with exudative epidermitis were diagnosed clinically as affected or unaffected. Two regions of the desmoglein 1 gene were sequenced and genotypes of the SNPs were established. Seven SNPs (823T>C, 828A>G, 829A>G, 830A>T, 831A>T, 838A>C and 1139C>T) were found in the analysed sequences and the allele frequencies were determined for the SNPs resulting in amino acid change. Four of the seven polymorphisms were situated in the motif known to be important for toxin cleavage. The distribution of the genotypes between affected and unaffected animals was analysed. CONCLUSION: The study indicated a possible correlation between the genotypes of two out of seven SNPs found in the porcine desmoglein 1 gene and the susceptibility to exudative epidermitis.

Complete FcRn dependence for intravenous Ig therapy in autoimmune skin blistering diseases.

Numerous mechanisms of action have been proposed for intravenous Ig (IVIG). In this study, we used IgG passive transfer murine models of bullous pemphigoid (BP), pemphigus foliaceus (PF), and pemphigus vulgaris (PV) to test the hypothesis that the effect of IVIG in autoantibody-mediated cutaneous bullous diseases is to accelerate the degradation of pathogenic IgG by saturation of the MHC-like Fc receptor neonatal Fc receptor (FcRn). BP, PF, and PV are organ-specific antibody-mediated diseases in which autoantibodies target the hemidesmosomal antigen BP180 and desmosomal antigens Dsg1 and Dsg3, respectively. Antibodies against BP180, Dsg1, and Dsg3, when injected into neonatal mice, induce the BP, PF, and PV disease phenotypes, respectively. We found that FcRn-deficient mice were resistant to experimental BP, PF, and PV. Circulating levels of pathogenic IgG in FcRn-deficient mice were significantly reduced compared with those in WT mice. Administration of high-dose human IgG (HDIG) to WT mice also drastically reduced circulating pathogenic IgG levels and prevented blistering. In FcRn-deficient mice, no additional protective effect with HDIG was realized. These data demonstrate that the therapeutic efficacy of HDIG treatment in the pemphigus and pemphigoid models is dependent on FcRn. Thus, FcRn is a promising therapeutic target for treating such IgG-mediated autoimmune diseases.