IgG Binds to Desmoglein 3 in Desmosomes and Causes a Desmosomal Split Without Keratin Retraction in a Pemphigus Mouse Model

A Novel In Vivo Active Pemphigus Model Targeting Desmoglein1 and Desmoglein3: A Tool Representing All Pemphigus Variants

Background: Pemphigus is a life-threatening blistering autoimmune disease. Several forms, characterized by the presence of autoantibodies against different autoantigens, have been described. In Pemphigus Vulgaris (PV), autoantibodies target the cadherin Desmoglein 3 (DSG3), while in Pemphigus foliaceous (PF) autoantibodies target the cadherin Desmoglein 1 (DSG1). Another variant, mucocutaneous Pemphigus, is characterized by the presence of IgG against both DSG1 and DSG3. Moreover, other forms of Pemphigus characterized by the presence of autoantibodies against other autoantigens have been described. With regard to animal models, one can distinguish between passive models, where pathological IgG are transferred into neonatal mice, and active models, where B cells deriving from animals immunized against a specific autoantigen are transferred into immunodeficient mice that develop the disease. Active models recreate PV and a form of Pemphigus characterized by the presence of IgG against the cadherin Desmocollin 3 (DSC3). Further approaches allow to collect sera or B/T cells from mice immunized against a specific antigen to evaluate the mechanisms underlying the onset of the disease. Objective: To develop and characterize a new active model of Pemphigus where mice express auto antibodies against either DSG1 alone, or DSG1 and DSG3, thereby recapitulating PF and mucocutaneous Pemphigus, respectively. In addition to the existing models, with the active models reported in this work, it will be possible to recapitulate and mimic the main forms of pemphigus in adult mice, thus allowing a better understanding of the disease in the long term, including the benefit/risk ratio of new therapies. Results: The new DSG1 and the DSG1/DSG3 mixed models were developed as proposed. Immunized animals, and subsequently, animals that received splenocytes from the immunized donors produce a high concentration of circulating antibodies against the specific antigens. The severity of the disease was assessed by evaluating the PV score, evidencing that the DSG1/DSG3 mixed model exhibits the most severe symptoms among those analyzed. Alopecia, erosions, and blistering were observed in the skin of DSG1, DSG3 and DSG1/DSG3 models, while lesions in the mucosa were observed only in DSG3 and DSG1/DSG3 animals. The effectiveness of the corticosteroid Methyl-Prednisolone was evaluated in the DSG1 and DSG1/DSG3 models, that showed only partial responsiveness.

Apremilast prevents blistering in human epidermis and stabilizes keratinocyte adhesion in pemphigus

Pemphigus vulgaris is a life-threatening blistering skin disease caused by autoantibodies destabilizing desmosomal adhesion. Current therapies focus on suppression of autoantibody formation and thus treatments directly stabilizing keratinocyte adhesion would fulfill an unmet medical need. We here demonstrate that apremilast, a phosphodiesterase 4 inhibitor used in psoriasis, prevents skin blistering in pemphigus vulgaris. Apremilast abrogates pemphigus autoantibody-induced loss of keratinocyte cohesion in ex-vivo human epidermis, cultured keratinocytes in vitro and in vivo in mice. In parallel, apremilast inhibits keratin retraction as well as desmosome splitting, induces phosphorylation of plakoglobin at serine 665 and desmoplakin assembly into desmosomal plaques. We established a plakoglobin phospho-deficient mouse model that reveals fragile epidermis with altered organization of keratin filaments and desmosomal cadherins. In keratinocytes derived from these mice, intercellular adhesion is impaired and not rescued by apremilast. These data identify an unreported mechanism of desmosome regulation and propose that apremilast stabilizes keratinocyte adhesion and is protective in pemphigus.

In Vitro, Ex Vivo, and In Vivo Models for the Study of Pemphigus

Pemphigus is a life-threatening autoimmune disease. Several phenotypic variants are part of this family of bullous disorders. The disease is mainly mediated by pathogenic autoantibodies, but is also directed against two desmosomal adhesion proteins, desmoglein 1 (DSG1) and 3 (DSG3), which are expressed in the skin and mucosae. By binding to their antigens, autoantibodies induce the separation of keratinocytes, in a process known as acantholysis. The two main Pemphigus variants are Pemphigus vulgaris and foliaceus. Several models of Pemphigus have been described: in vitro, ex vivo and in vivo, passive or active mouse models. Although no model is ideal, different models display specific characteristics that are useful for testing different hypotheses regarding the initiation of Pemphigus, or to evaluate the efficacy of experimental therapies. Different disease models also allow us to evaluate the pathogenicity of specific Pemphigus autoantibodies, or to investigate the role of previously not described autoantigens. The aim of this review is to provide an overview of Pemphigus disease models, with the main focus being on active models and their potential to reproduce different disease subgroups, based on the involvement of different autoantigens.

Mechanisms Causing Acantholysis in Pemphigus-Lessons from Human Skin

Pemphigus vulgaris (PV) is an autoimmune bullous skin disease caused primarily by autoantibodies (PV-IgG) against the desmosomal adhesion proteins desmoglein (Dsg)1 and Dsg3. PV patient lesions are characterized by flaccid blisters and ultrastructurally by defined hallmarks including a reduction in desmosome number and size, formation of split desmosomes, as well as uncoupling of keratin filaments from desmosomes. The pathophysiology underlying the disease is known to involve several intracellular signaling pathways downstream of PV-IgG binding. Here, we summarize our studies in which we used transmission electron microscopy to characterize the roles of signaling pathways in the pathogenic effects of PV-IgG on desmosome ultrastructure in a human ex vivo skin model. Blister scores revealed inhibition of p38MAPK, ERK and PLC/Ca2+ to be protective in human epidermis. In contrast, inhibition of Src and PKC, which were shown to be protective in cell cultures and murine models, was not effective for human skin explants. The ultrastructural analysis revealed that for preventing skin blistering at least desmosome number (as modulated by ERK) or keratin filament insertion (as modulated by PLC/Ca2+) need to be ameliorated. Other pathways such as p38MAPK regulate desmosome number, size, and keratin insertion indicating that they control desmosome assembly and disassembly on different levels. Taken together, studies in human skin delineate target mechanisms for the treatment of pemphigus patients. In addition, ultrastructural analysis supports defining the specific role of a given signaling molecule in desmosome turnover at ultrastructural level.

Marked to Die-Cell Death Mechanisms for Keratinocyte Acantholysis in Pemphigus Diseases

Pemphigus is a group of blistering autoimmune diseases causing painful skin lesions, characterized by acantholysis and by the production of autoantibodies against, mainly, adhesion proteins. We reviewed the literature for molecules and/ or features involved in the 12 cell death pathways described by Nomenclature Committee on Cell Death, taking place in pemphigus patients, cell lines, or human skin organ cultures treated with sera or IgG from pemphigus patients or in pemphigus mouse models, and found 61 studies mentioning 97 molecules involved in cell death pathways. Among the molecules, most investigated were pleiotropic molecules such as TNF and CASP3, followed by FASL and CASP8, and then by FAS, BAX, BCL2, and TP53, all involved in more than one pathway but interpreted to function only within apoptosis. Most of these previous investigations focused only on apoptosis, but four recent studies, using TUNEL assays and/or electron microscopy, disqualified this pathway as a previous event of acantholysis. For PV, apoptolysis was suggested as a cell death mechanism based on pathogenic autoantibodies diversity, mitochondrial dysfunction, and p38 MAPK signaling. To answer those many questions that remain on cell death and pemphigus, we propose well-controlled, statistically relevant investigations on pemphigus and cell death pathways besides apoptosis, to overcome the challenges of understanding the etiopathology of pemphigus diseases.

Autoantibody-Specific Signalling in Pemphigus

Pemphigus is a severe autoimmune disease impairing barrier functions of epidermis and mucosa. Autoantibodies primarily target the desmosomal adhesion molecules desmoglein (Dsg) 1 and Dsg 3 and induce loss of desmosomal adhesion. Strikingly, autoantibody profiles in pemphigus correlate with clinical phenotypes. Mucosal-dominant pemphigus vulgaris (PV) is characterised by autoantibodies (PV-IgG) against Dsg3 whereas epidermal blistering in PV and pemphigus foliaceus (PF) is associated with autoantibodies against Dsg1. Therapy in pemphigus is evolving towards specific suppression of autoantibody formation and autoantibody depletion. Nevertheless, during the acute phase and relapses of the disease additional treatment options to stabilise desmosomes and thereby rescue keratinocyte adhesion would be beneficial. Therefore, the mechanisms by which autoantibodies interfere with adhesion of desmosomes need to be characterised in detail. Besides direct inhibition of Dsg adhesion, autoantibodies engage signalling pathways interfering with different steps of desmosome turn-over. With this respect, recent data indicate that autoantibodies induce separate signalling responses in keratinocytes via specific signalling complexes organised by Dsg1 and Dsg3 which transfer the signal of autoantibody binding into the cell. This hypothesis may also explain the different clinical pemphigus phenotypes.

Evolving Mechanisms in the Pathophysiology of Pemphigus Vulgaris: A Review Emphasizing the Role of Desmoglein 3 in Regulating p53 and the Yes-Associated Protein

The immunobullous condition Pemphigus Vulgaris (PV) is caused by autoantibodies targeting the adhesion proteins of desmosomes, leading to blistering in the skin and mucosal membrane. There is still no cure to the disease apart from the use of corticosteroids and immunosuppressive agents. Despite numerous investigations, the pathological mechanisms of PV are still incompletely understood, though the etiology is thought to be multifactorial. Thus, further understanding of the molecular basis underlying this disease process is vital to develop targeted therapies. Ample studies have highlighted the role of Desmoglein-3 (DSG3) in the initiation of disease as DSG3 serves as a primary target of PV autoantibodies. DSG3 is a pivotal player in mediating outside-in signaling involved in cell junction remodeling, cell proliferation, differentiation, migration or apoptosis, thus validating its biological function in tissue integrity and homeostasis beyond desmosome adhesion. Recent studies have uncovered new activities of DSG3 in regulating p53 and the yes-associated protein (YAP), with the evidence of dysregulation of these pathways demonstrated in PV. The purpose of this review is to summarize the earlier and recent advances highlighting our recent findings related to PV pathogenesis that may pave the way for future research to develop novel specific therapies in curing this disease.

Role of PKC and ERK Signaling in Epidermal Blistering and Desmosome Regulation in Pemphigus

Desmosomes reinforce cohesion of epithelial cells at the interface between adjacent cells. They include the cadherin-type adhesion molecules desmoglein 1 (Dsg1) and Dsg3. Pemphigus vulgaris (PV) is an autoimmune disease in which circulating autoantibodies (PV-IgG) targeting Dsg1 and 3 cause characteristic epidermal blister formation. It has been shown that PV-IgG binding induced activation of kinases such as ERK and PKC, and inhibition of these signaling pathways prevented loss of cell cohesion in cell cultures. However, the role of Erk and PKC in blister formation and regulation of desmosome ultrastructure in human skin are unknown. Accordingly, we assessed the role of PKC and ERK signaling pathways in blister formation and regulation of desmosome ultrastructure in human epidermis. Here we performed electron microscopy analyses using human skin explants injected with PV-IgG together with inhibitors for PKC or ERK signaling. Inhibition of PKC was not effective to prevent suprabasal blister formation or ultrastructural alterations of desmosomes. In contrast, inhibition of ERK signaling significantly ameliorated blister formation and decrease in the number of desmosomes whereas shortening and splitting of desmosomes and keratin filament insertion were not different from samples treated with PV-IgG alone. However, apical desmosomes between basal and suprabasal cells remained unaltered when ERK signaling was inhibited. Therefore, our results show that inhibition of ERK but not PKC signaling appears to be effective to ameliorate blistering and alterations of desmosome ultrastructure triggered by PV-IgG in human skin.

Autoantibody Signaling in Pemphigus Vulgaris: Development of an Integrated Model

Pemphigus vulgaris (PV) is an autoimmune skin blistering disease effecting both cutaneous and mucosal epithelia. Blister formation in PV is known to result from the binding of autoantibodies (autoAbs) to keratinocyte antigens. The primary antigenic targets of pathogenic autoAbs are known to be desmoglein 3, and to a lesser extent, desmoglein 1, cadherin family proteins that partially comprise the desmosome, a protein structure responsible for maintaining cell adhesion, although additional autoAbs, whose role in blister formation is still unclear, are also known to be present in PV patients. Nevertheless, there remain large gaps in knowledge concerning the precise mechanisms through which autoAb binding induces blister formation. Consequently, the primary therapeutic interventions for PV focus on systemic immunosuppression, whose side effects represent a significant health risk to patients. In an effort to identify novel, disease-specific therapeutic targets, a multitude of studies attempting to elucidate the pathogenic mechanisms downstream of autoAb binding, have led to significant advancements in the understanding of autoAb-mediated blister formation. Despite this enhanced characterization of disease processes, a satisfactory explanation of autoAb-induced acantholysis still does not exist. Here, we carefully review the literature investigating the pathogenic disease mechanisms in PV and, taking into account the full scope of results from these studies, provide a novel, comprehensive theory of blister formation in PV.

Cell-Cell Junctions Organize Structural and Signaling Networks

Cell-cell junctions link cells to each other in tissues, and regulate tissue homeostasis in critical cell processes that include tissue barrier function, cell proliferation, and migration. Defects in cell-cell junctions give rise to a wide range of tissue abnormalities that disrupt homeostasis and are common in genetic abnormalities and cancers. Here, we discuss the organization and function of cell-cell junctions primarily involved in adhesion (tight junction, adherens junction, and desmosomes) in two different epithelial tissues: a simple epithelium (intestine) and a stratified epithelium (epidermis). Studies in these tissues reveal similarities and differences in the organization and functions of different cell-cell junctions that meet the requirements for the specialized functions of each tissue. We discuss cell-cell junction responses to genetic and environmental perturbations that provide further insights into their roles in maintaining tissue homeostasis.

Pemphigus-A Disease of Desmosome Dysfunction Caused by Multiple Mechanisms

Pemphigus is a severe autoimmune-blistering disease of the skin and mucous membranes caused by autoantibodies reducing desmosomal adhesion between epithelial cells. Autoantibodies against the desmosomal cadherins desmogleins (Dsgs) 1 and 3 as well as desmocollin 3 were shown to be pathogenic, whereas the role of other antibodies is unclear. Dsg3 interactions can be directly reduced by specific autoantibodies. Autoantibodies also alter the activity of signaling pathways, some of which regulate cell cohesion under baseline conditions and alter the turnover of desmosomal components. These pathways include Ca²⁺, p38MAPK, PKC, Src, EGFR/Erk, and several others. In this review, we delineate the mechanisms relevant for pemphigus pathogenesis based on the histology and the ultrastructure of patients’ lesions. We then dissect the mechanisms which can explain the ultrastructural hallmarks detectable in pemphigus patient skin. Finally, we reevaluate the concept that the spectrum of mechanisms, which induce desmosome dysfunction upon binding of pemphigus autoantibodies, finally defines the clinical phenotype.

Inhibition of p38MAPK signalling prevents epidermal blistering and alterations of desmosome structure induced by pemphigus autoantibodies in human epidermis

BACKGROUND

Pemphigus vulgaris (PV) is a skin blistering disease caused by autoantibodies targeting the desmosomal adhesion proteins desmoglein (Dsg) 3 and 1. The mechanisms underlying pemphigus skin blistering are not fully elucidated but p38 mitogen-activated protein kinase (p38MAPK) activation is one of the signalling events necessary for full loss of cell cohesion. However, it is unclear whether ultrastructural hallmarks of desmosome morphology as observed in patients' lesions are mediated by p38MAPK signalling.

OBJECTIVES

In this study, we tested the relevance of p38MAPK for blister formation and the ultrastructural changes induced by PV autoantibodies in human skin.

METHODS

Human skin samples were injected with IgG fractions of one patient suffering from mucocutaneous PV (mcPV-IgG), one from mucosal-dominant PV (mdPV-IgG) or AK23, a pathogenic monoclonal Dsg3 antibody derived from a pemphigus mouse model. Samples were processed for histological and electron microscopy analyses.

RESULTS

mcPV-IgG and AK23 but not mdPV-IgG reduced desmosome size, caused interdesmosomal widening and formation of split desmosomes, and altered keratin filament insertion. In contrast, full epidermal blister formation and lower desmosome number were evident in tissue samples exposed to mcPV-IgG only. Pharmacological inhibition of p38MAPK blunted the reduction of desmosome number and size, ameliorated interdesmosomal widening and loss of keratin insertion and prevented mcPV-IgG-induced blister formation.

CONCLUSIONS

Our data demonstrate that blistering can be prevented by inhibition of p38MAPK in the human epidermis. Moreover, typical morphological alterations induced by mcPV-IgG such as interdesmosomal widening and the reduction of desmosome size at least in part require p38MAPK signalling.

Mechanisms of Acantholysis in Pemphigus: Mechanical or Inflammatory?

Pemphigus is a rare autoimmune disease, involving the skin and mucous epithelia, characterized by flaccid blisters and erosions. Histologically, the basic abnormality in all forms of pemphigus is the separation of keratinocytes from one another, a process known as acantholysis. There is direct evidence that autoantibodies against desmoglein, a transmembrane desmosomal component, are critical in its pathogenesis, but the exact mechanism that induces acantholysis is yet unknown. Actually, different studies suggest three possible mechanisms: sterical impedance, intracellular signalling and apoptosis. Understanding these processes should show new therapeutic perspective.

Claws, Disorder, and Conformational Dynamics of the C-Terminal Region of Human Desmoplakin

Multicellular organisms consist of cells that interact via elaborate adhesion complexes. Desmosomes are membrane-associated adhesion complexes that mechanically tether the cytoskeletal intermediate filaments (IFs) between two adjacent cells, creating a network of tough connections in tissues such as skin and heart. Desmoplakin (DP) is the key desmosomal protein that binds IFs, and the DP·IF association poses a quandary: desmoplakin must stably and tightly bind IFs to maintain the structural integrity of the desmosome. Yet, newly synthesized DP must traffic along the cytoskeleton to the site of nascent desmosome assembly without "sticking" to the IF network, implying weak or transient DP···IF contacts. Recent work reveals that these contacts are modulated by post-translational modifications (PTMs) in DP's C-terminal tail (DPCTT). Using molecular dynamics simulations, we have elucidated the structural basis of these PTM-induced effects. Our simulations, nearing 2 μs in aggregate, indicate that phosphorylation of S2849 induces an "arginine claw" in desmoplakin's C-terminal tail. If a key arginine, R2834, is methylated, the DPCTT preferentially samples conformations that are geometrically well-suited as substrates for processive phosphorylation by the cognate kinase GSK3. We suggest that DPCTT is a molecular switch that modulates, via its conformational dynamics, DP's overall efficacy as a substrate for GSK3. Finally, we show that the fluctuating DPCTT can contact other parts of DP, suggesting a competitive binding mechanism for the modulation of DP···IF interactions.

Super-Resolution Microscopy Reveals Altered Desmosomal Protein Organization in Tissue from Patients with Pemphigus Vulgaris

Pemphigus vulgaris (PV) is an autoimmune epidermal blistering disease in which autoantibodies (IgG) are directed against the desmosomal cadherin desmoglein 3 (Dsg3). In order to better understand how PV IgG alters desmosome morphology and function in vivo, PV patient biopsies were analyzed by structured illumination microscopy (SIM), a form of super-resolution fluorescence microscopy. In patient tissue, desmosomal proteins were aberrantly clustered and localized to PV IgG-containing endocytic linear arrays. Patient IgG also colocalized with markers for lipid rafts and endosomes. Additionally, steady-state levels of Dsg3 were decreased and desmosomes were reduced in size in patient tissue. Desmosomes at blister sites were occasionally split, with PV IgG decorating the extracellular faces of split desmosomes. Desmosome splitting was recapitulated in vitro by exposing cultured keratinocytes both to PV IgG and to mechanical stress, demonstrating that splitting at the blister interface in patient tissue is due to compromised desmosomal adhesive function. These findings indicate that Dsg3 clustering and endocytosis are associated with reduced desmosome size and adhesion defects in PV patient tissue. Further, this study reveals that super-resolution optical imaging is powerful approach for studying epidermal adhesion structures in normal and diseased skin.

Desmosomes in acquired disease

Desmosomes are cell-cell junctions that mediate adhesion and couple the intermediate filament cytoskeleton to sites of cell-cell contact. This architectural arrangement integrates adhesion and cytoskeletal elements of adjacent cells. The importance of this robust adhesion system is evident in numerous human diseases, both inherited and acquired, which occur when desmosome function is compromised. This review focuses on autoimmune and infectious diseases that impair desmosome function. In addition, we discuss emerging evidence that desmosomal genes are often misregulated in cancer. The emphasis of our discussion is placed on the way in which human diseases can inform our understanding of basic desmosome biology and in turn, the means by which fundamental advances in the cell biology of desmosomes might lead to new treatments for acquired diseases of the desmosome.

Desmosomal adhesion in vivo

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

Nanorobotic investigation identifies novel visual, structural and functional correlates of autoimmune pathology in a blistering skin disease model

There remain major gaps in our knowledge regarding the detailed mechanisms by which autoantibodies mediate damage at the tissue level. We have undertaken novel strategies at the interface of engineering and clinical medicine to integrate nanoscale visual and structural data using nanorobotic atomic force microscopy with cell functional analyses to reveal previously unattainable details of autoimmune processes in real-time. Pemphigus vulgaris is a life-threatening autoimmune blistering skin condition in which there is disruption of desmosomal cell-cell adhesion structures that are associated with the presence of antibodies directed against specific epithelial proteins including Desmoglein (Dsg) 3. We demonstrate that pathogenic (blister-forming) anti-Dsg3 antibodies, distinct from non-pathogenic (non-blister forming) anti-Dsg3 antibodies, alter the structural and functional properties of keratinocytes in two sequential steps - an initial loss of cell adhesion and a later induction of apoptosis-related signaling pathways, but not full apoptotic cell death. We propose a “2-Hit” model for autoimmune disruption associated with skin-specific pathogenic autoantibodies. These data provide unprecedented details of autoimmune processes at the tissue level and offer a novel conceptual framework for understanding the action of self-reactive antibodies.

Hyper-adhesion: a unique property of desmosomes

Hyper-adhesion is a unique, strongly adhesive form of desmosomal adhesion that functions to maintain tissue integrity. In this short review, we define hyper-adhesion, summarise the evidence for it in culture and in vivo, discuss its role in development, wound healing, and skin disease, and speculate about its molecular and cellular basis.

Desmosomal adhesion and pemphigus vulgaris: the first half of the story

Pemphigus vulgaris (PV) is a paradigm of autoimmune disease affecting intercellular adhesion. The mechanisms that lead to cell-cell detachment (acantholysis) have crucial therapeutic implications and are currently undergoing major scrutiny. The first part of this review focuses on the classical view of the pathogenesis of PV, which is dominated by the cell adhesion molecules of the desmosome, namely desmogleins (Dsgs). Cloning of the DSG3 gene, generation DSG3 knock-out mice and isolation of monoclonal anti-Dsg3 IgG have aided to clarify the pathogenic mechanisms of PV, which are in part dependent on the fate of desmosomal molecules. These include perturbation of the desmosomal network at the transcriptional, translational, and interaction level, kinase activation, proteinase-mediated degradation, and hyper-adhesion. By the use of PV models, translational research has in turn helped shed light into the basic structure, function, and dynamics of assembly of desmosomal cadherins. The combined efforts of basic and applied research has resulted in tremendous advance into the understanding of epidermal adhesion and helped debunk old myths on the supposedly unique role of desmogleins in the mechanisms of cell-cell detachment in PV.

Signaling dependent and independent mechanisms in pemphigus vulgaris blister formation

Pemphigus vulgaris (PV) is an autoimmune epidermal blistering disease caused by autoantibodies directed against the desmosomal cadherin desmoglein-3 (Dsg3). Significant advances in our understanding of pemphigus pathomechanisms have been derived from the generation of pathogenic monoclonal Dsg3 antibodies. However, conflicting models for pemphigus pathogenicity have arisen from studies using either polyclonal PV patient IgG or monoclonal Dsg3 antibodies. In the present study, the pathogenic mechanisms of polyclonal PV IgG and monoclonal Dsg3 antibodies were directly compared. Polyclonal PV IgG cause extensive clustering and endocytosis of keratinocyte cell surface Dsg3, whereas pathogenic mouse monoclonal antibodies compromise cell-cell adhesion strength without causing these alterations in Dsg3 trafficking. Furthermore, tyrosine kinase or p38 MAPK inhibition prevents loss of keratinocyte adhesion in response to polyclonal PV IgG. In contrast, disruption of adhesion by pathogenic monoclonal antibodies is not prevented by these inhibitors either in vitro or in human skin explants. Our results reveal that the pathogenic activity of polyclonal PV IgG can be attributed to p38 MAPK-dependent clustering and endocytosis of Dsg3, whereas pathogenic monoclonal Dsg3 antibodies can function independently of this pathway. These findings have important implications for understanding pemphigus pathophysiology, and for the design of pemphigus model systems and therapeutic interventions.

Pemphigus autoimmunity: hypotheses and realities

The goal of contemporary research in pemphigus vulgaris and pemphigus foliaceus is to achieve and maintain clinical remission without corticosteroids. Recent advances of knowledge on pemphigus autoimmunity scrutinize old dogmas, resolve controversies, and open novel perspectives for treatment. Elucidation of intimate mechanisms of keratinocyte detachment and death in pemphigus has challenged the monopathogenic explanation of disease immunopathology. Over 50 organ-specific and non-organ-specific antigens can be targeted by pemphigus autoimmunity, including desmosomal cadherins and other adhesion molecules, PERP cholinergic and other cell membrane (CM) receptors, and mitochondrial proteins. The initial insult is sustained by the autoantibodies to the cell membrane receptor antigens triggering the intracellular signaling by Src, epidermal growth factor receptor kinase, protein kinases A and C, phospholipase C, mTOR, p38 MAPK, JNK, other tyrosine kinases, and calmodulin that cause basal cell shrinkage and ripping desmosomes off the CM. Autoantibodies synergize with effectors of apoptotic and oncotic pathways, serine proteases, and inflammatory cytokines to overcome the natural resistance and activate the cell death program in keratinocytes. The process of keratinocyte shrinkage/detachment and death via apoptosis/oncosis has been termed apoptolysis to emphasize that it is triggered by the same signal effectors and mediated by the same cell death enzymes. The natural course of pemphigus has improved due to a substantial progress in developing of the steroid-sparing therapies combining the immunosuppressive and direct anti-acantholytic effects. Further elucidation of the molecular mechanisms mediating immune dysregulation and apoptolysis in pemphigus should improve our understanding of disease pathogenesis and facilitate development of steroid-free treatment of patients.

A glass of red wine to keep vascular disease at bay, but what about pemphigus vulgaris?

Pemphigus vulgaris is a rare autoimmune blistering disease, involving the skin and mucous epithelia, which is characterized by flaccid blisters and erosions. It is caused by the presence of autoantibodies directed against desmoglein, a glycoprotein that plays a critical role in cell-cell attachment. Upon a predisposing genetic background, different agents have been shown to act as triggers for the pathogenesis of pemphigus. The most evident association is with drug intake, while the role of diet is often underestimated. The aim of this article is to review the possible role of tannins, a group of phenolic metabolites that are widely distributed in almost all plant foods and beverages, particularly red wine, as a trigger for pemphigus vulgaris.

Desmosome disassembly in response to pemphigus vulgaris IgG occurs in distinct phases and can be reversed by expression of exogenous Dsg3

Pemphigus vulgaris (PV) is an epidermal blistering disorder caused by antibodies directed against the desmosomal cadherin desmoglein-3 (Dsg3). The mechanism by which PV IgG disrupts adhesion is not fully understood. To address this issue, primary human keratinocytes (KCs) and patient IgG were used to define the morphological, biochemical, and functional changes triggered by PV IgG. Three phases of desmosome disassembly were distinguished. Analysis of fixed and living KCs demonstrated that PV IgG cause rapid Dsg3 internalization, which likely originates from a non-junctional pool of Dsg3. Subsequently, Dsg3 and other desmosomal components rearrange into linear arrays that run perpendicular to cell contacts. Dsg3 complexes localized at the cell surface are transported in a retrograde manner along with these arrays before being released into cytoplasmic vesicular compartments. These changes in Dsg3 distribution are followed by depletion of detergent-insoluble Dsg3 pools and by the loss of cell adhesion strength. Importantly, this process of disassembly can be prevented by expressing exogenous Dsg3, thereby driving Dsg3 biosynthesis and desmosome assembly. These data support a model in which PV IgG cause the loss of cell adhesion by altering the dynamics of Dsg3 assembly into desmosomes and the turnover of cell surface pools of Dsg3 through endocytic pathways.

Efficacy of intravenous immunoglobulin (IVIG) affinity-purified anti-desmoglein anti-idiotypic antibodies in the treatment of an experimental model of pemphigus vulgaris

Pemphigus vulgaris is a rare life-threatening autoimmune bullous disease caused by immunoglobulin G (IgG) autoantibodies directed against desmogleins 1 and 3. Previously, we showed that intravenous immunoglobulin (IVIG) ameliorates anti-desmoglein-induced experimental pemphigus vulgaris in newborn naive mice. The aim of this study was to examine the efficacy of anti-anti-desmoglein-specific IVIG in a similar model. Pemphigus-vulgaris-specific IVIG (PV-sIVIG) was affinity-purified from IVIG on a column of single-chain variable fragment (scFv) anti-desmogleins 1 and 3. The anti-idiotypic activity of PV-sIVIG was confirmed by enzyme-linked immunosorbent assay, inhibition assay. After induction of pemphigus by injection of anti-desmogleins 1 and 3 scFv to newborn mice, the animals were treated with PV-sIVIG, IVIG (low or high dose) or IgG from a healthy donor (n = 10 each). The skin was examined 24-48 h later, and samples of affected areas were analysed by histology and immunofluorescence. In vitro study showed that PV-sIVIG significantly inhibited anti-desmogleins 1 and 3 scFv binding to recombinant desmoglein-3 in a dose-dependent manner. Specificity was confirmed by inhibition assay. In vivo analysis revealed cutaneous lesions of pemphigus vulgaris in mice injected with normal IgG (nine of 10 mice) or low-dose IVIG (nine of 10 mice), but not in mice treated with PV-sIVIG (none of 10) or high-dose IVIG (none of 10). On immunopathological study, PV-sIVIG and regular IVIG prevented the formation of acantholysis and deposition of IgG in intercellular spaces. In conclusion, the PV-sIVIG preparation is more effective than native IVIG in inhibiting anti-desmoglein-induced pemphigus vulgaris in mice and might serve as a future therapy in patients with the clinical disease.

Desmosomes in vivo

The structure, function, and regulation of desmosomal adhesion in vivo are discussed. Most desmosomes in tissues exhibit calcium-independent adhesion, which is strongly adhesive or “hyperadhesive”. This is fundamental to tissue strength. Almost all studies in culture are done on weakly adhesive, calcium-dependent desmosomes, although hyperadhesion can be readily obtained in confluent cell culture. Calcium dependence is a default condition in vivo, found in wounds and embryonic development. Hyperadhesion appears to be associated with an ordered arrangement of the extracellular domains of the desmosomal cadherins, which gives rise to the intercellular midline identified in ultrastructural studies. This in turn probably depends on molecular order in the desmosomal plaque. Protein kinase C downregulates hyperadhesion and there is preliminary evidence that it may also be regulated by tyrosine kinases. Downregulation of desmosomes in vivo may occur by internalisation of whole desmosomes rather than disassembly. Hyperadhesion has implications for diseases such as pemphigus.

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.

Significance of autoimmunity to non-desmoglein targets in pemphigus

The pathogenesis of pemphigus vulgaris (PV) is a highly controversial, "hot" topic that has received considerable enrichment in recent years by both clinical and basic researchers. On the one hand, the classical view of desmogleins (Dsg) as main targets of this autoimmune disease is supported by the characterization of pathogenic anti-Dsg3 antibodies from both patients and animal models. On the other hand, fundamental doubt has been raised towards this monopathogenic view by several independent factors: (1) pemphigus lesions can be induced in Dsg3-knockout (KO) mice; (2) pemphigus sera contain multiple autoantibodies against different adhesion molecules and also cholinergic receptors; (3) experimental inhibition of PV IgG induced acantholysis can be obtained by interference with different signaling cascades regulating both calcium homeostasis and apoptosis; and (4) cholinergic agonists exhibit anti-acantholytic activity both in vitro and in vivo. The field is open for controlled clinical trials and further basic research to unfold the true story of the pemphigus enigma and provide the basis for a better treatment of pemphigus patients.

Pemphigus antibody induced phosphorylation of keratinocyte proteins

The pemphigus family of autoimmune blistering diseases is characterized by an autoantibody response to desmosomal cadherins in epithelia. Autoantibodies against desmogleins, desmosome cell adhesion molecules, induce loss of cell-cell adhesion that is characterized clinically by blister formation. The mechanism by which these autoantibodies induce loss of cell-cell adhesion is under active investigation, but appears to involve a coordinated intracellular response including activation of intracellular signaling and phosphorylation of a number of proteins in the target keratinocyte. Activation of p38 mitogen activated protein kinase may have a critical role in the acantholytic mechanism as inhibitors of p38MAPK block the ability of pemphigus IgG to induce blistering in pemphigus animal models.

Loss of the desmosomal protein perp enhances the phenotypic effects of pemphigus vulgaris autoantibodies

Pemphigus vulgaris (PV) is an autoimmune bullous disease in which autoantibodies against proteins of the desmosomal adhesion complex perturb desmosomal function, leading to intercellular adhesion defects in the oral mucosa and skin. Previous studies have demonstrated a central role for downregulation of the desmosomal cadherin desmoglein 3 (DSG3) in the pathogenesis of PV. However, the effects of non-cadherin desmosomal proteins in modulating the cellular manifestations of PV remain poorly understood. Here, we characterize the expression and functional importance of Perp, a newly discovered tetraspan desmosomal protein, in PV. Our data demonstrate that PV autoantibodies disrupt Perp expression at the membrane and trigger its internalization along with DSG3 into the endosomal pathway, where it is ultimately targeted to the lysosome for degradation. We further show that Perp deficiency exacerbates the pathogenic effects of PV autoantibodies on keratinocytes by enhancing both the depletion of desmosomal DSG3 and intercellular adhesion defects. Together, our findings highlight the importance of non-cadherin desmosomal proteins in modulating PV phenotypes and provide new insight into Perp's role in the desmosome.

Apoptosis is not required for acantholysis in pemphigus vulgaris

The autoimmune blistering skin disease pemphigus vulgaris (PV) is caused primarily by autoantibodies against desmosomal cadherins. It was reported that apoptosis can be detected in pemphigus skin lesions and that apoptosis can be induced by PV-IgG in cultured keratinocytes. However, the role of apoptosis in PV pathogenesis is unclear at present. In this study, we provide evidence that apoptosis is not required for acantholysis in PV. In skin lesions from two PV patients, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) positivity, but not cleaved caspase-3, was detected in single keratinocytes in some lesions but was completely absent in other lesions from the same patients. In cultures of human keratinocytes (HaCaT and normal human epidermal keratinocytes), PV-IgG from three different PV patients caused acantholysis, fragmented staining of Dsg 3 staining, and cytokeratin retraction in the absence of nuclear fragmentation, TUNEL positivity, and caspase-3 cleavage and hence in the absence of detectable apoptosis. To further rule out the contribution of apoptotic mechanisms, we used two different approaches that are effective to block apoptosis induced by various stimuli. Inhibition of caspases by z-VAD-fmk as well as overexpression of Fas-associated death domain-like interleukin-1β-converting enzyme (FLICE)-like inhibitory proteins FLIPL and FLIPS to inhibit receptor-mediated apoptosis did not block PV-IgG-induced effects, indicating that apoptosis was not required. Taken together, we conclude that apoptosis is not a prerequisite for skin blistering in PV but may occur secondary to acantholysis.

Disruption of desmosome assembly by monovalent human pemphigus vulgaris monoclonal antibodies

The intercellular interactions of the desmosomal cadherins, desmoglein and desmocollin, are required for epidermal cell adhesion. Pemphigus vulgaris (PV) is a potentially fatal autoimmune blistering disease characterized by autoantibodies against desmoglein (Dsg) 3. During calcium-induced desmosome assembly, treatment of primary human keratinocytes with pathogenic monovalent anti-Dsg3 mAbs produced from a PV patient causes a decrease of Dsg3 and desmoplakin but not desmocollin (Dsc) 3 in the Triton-insoluble fraction of cell lysates within 2 hours. Immunofluorescence and antibody ELISA studies suggest that pathogenic mAbs cause internalization of cell-surface Dsg3 but not Dsc3 through early endosomes. Electron microscopy demonstrated a lack of well-formed desmosomes in keratinocytes treated with pathogenic compared to nonpathogenic mAbs. In contrast, pathogenic mAbs caused late depletion of Dsg3 from preformed desmosomes at 24 hours, with effects on multiple desmosomal proteins including Dsc3 and plakoglobin. Together, these studies indicate that pathogenic PV mAbs specifically cause internalization of newly synthesized Dsg3 during desmosome assembly, correlating with their pathogenic activity. Monovalent human PV anti-Dsg mAbs reproduce the effects of polyclonal PV IgG on Dsg3 and will facilitate future studies to further dissect the cellular mechanisms for the loss of cell adhesion in pemphigus.

Autoantibodies in the autoimmune disease pemphigus foliaceus induce blistering via p38 mitogen-activated protein kinase-dependent signaling in the skin

Pemphigus foliaceus (PF) is a human autoimmune blistering disease in which a humoral immune response targeting the skin results in a loss of keratinocyte cell-cell adhesion in the superficial layers of the epidermal epithelium. In PF, desmoglein-1-specific autoantibodies induce blistering. Evidence is beginning to accumulate that activation of signaling may have an important role in the ability of pathogenic pemphigus IgGs to induce blistering and that both p38 mitogen-activated protein kinase (MAPK) and heat shock protein (HSP) 27 are part of this signaling pathway. This study was undertaken to investigate the ability of PF IgGs to activate signaling as well as the contribution of this signaling pathway to blister induction in an in vivo model of PF. Phosphorylation of both p38 MAPK and HSP25, the murine HSP27 homolog, was observed in the skin of PF IgG-treated mice. Furthermore, inhibition of p38 MAPK blocked the ability of PF IgGs to induce blistering in vivo. These results indicate that PF IgG-induced blistering is dependent on activation of p38 MAPK in the target keratinocyte. Rather than influencing the immune system, limiting the autoantibody-induced intracellular signaling response that leads to target end-organ damage may be a more viable therapeutic strategy for the treatment of autoimmune diseases. Inhibition of p38 MAPK may be an effective strategy for the treatment of PF.

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.

Transmission electron microscopy of the preclinical phase of experimental phytophotodermatitis

OBJECTIVE

To examine the epidermis in induced phytophotodermatitis using transmission electron microscopy in order to detect histologic changes even before lesions are visible by light microscopy.

INTRODUCTION

In the first six hours after the experimental induction of phytophotodermatitis, no changes are detectable by light microscopy. Only after 24 hours can keratinocyte necrosis and epidermal vacuolization be detected histologically, and blisters form by 48 hours.

METHODS

The dorsum of four adult rats (Rattus norvegicus) was manually epilated. After painting the right half of the rat with the peel juice of Tahiti lemon, they were exposed to sunlight for eight minutes under general anesthesia. The left side was used as the control and exposed to sunlight only. Biopsies were performed immediately after photoinduction and one and two hours later, and the tissue was analyzed by transmission electron microscopy.

RESULTS

No histological changes were seen on the control side. Immediately after induction, vacuolization in keratinocytes was observed. After one hour, desmosomal changes were also observed in addition to vacuolization. Keratin filaments were not attached to the desmosomal plaque. Free desmosomes and membrane ruptures were also seen. At two hours after induction, similar changes were found, and granular degeneration of keratin was also observed.

DISCUSSION

The interaction of sunlight and psoralens generates a photoproduct that damages keratinocyte proteins, leading to keratinocyte necrosis and blister formation.

CONCLUSIONS

Transmission electron microscopy can detect vacuolization, lesions of the membrane, and desmosomes in the first two hours after experimental induction of phytophotodermatitis.

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.