Serum from pemphigus vulgaris reduces desmoglein 3 half-life and perturbs its de novo assembly to desmosomal sites in cultured keratinocytes

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.

Plakophilin 1 but not plakophilin 3 regulates desmoglein clustering

Plakophilins (Pkp) are desmosomal plaque proteins crucial for desmosomal adhesion and participate in the regulation of desmosomal turnover and signaling. However, direct evidence that Pkps regulate clustering and molecular binding properties of desmosomal cadherins is missing. Here, keratinocytes lacking either Pkp1 or 3 in comparison to wild type (wt) keratinocytes were characterized with regard to their desmoglein (Dsg) 1- and 3-binding properties and their capability to induce Dsg3 clustering. As revealed by atomic force microscopy (AFM), both Pkp-deficient keratinocyte cell lines showed reduced membrane availability and binding frequency of Dsg1 and 3 at cell borders. Extracellular crosslinking and AFM cluster mapping demonstrated that Pkp1 but not Pkp3 is required for Dsg3 clustering. Accordingly, Dsg3 overexpression reconstituted cluster formation in Pkp3- but not Pkp1-deficient keratinocytes as shown by AFM and STED experiments. Taken together, these data demonstrate that both Pkp1 and 3 regulate Dsg membrane availability, whereas Pkp1 but not Pkp3 is required for Dsg3 clustering.

Internalization of Non-Clustered Desmoglein 1 without Depletion of Desmoglein 1 from Adhesion Complexes in An Experimental Model of the Autoimmune Disease Pemphigus Foliaceus

Serum antibodies against desmoglein 1 (Dsg1) are known to induce the clinical and histological manifestations of pemphigus foliaceus (PF), autoimmune bullous disease targeting skin. The basic pathophysiological phenomenon of PF blistering is the disruption of epithelial integrity in the granular layer of the epidermis due to separation of keratinocytes from one another, or acantholysis. In this report we investigate the changes in subcellular distribution of Dsg1 in response to serum of patients with PF by using an in vitro model of PF. Immunofluorescence analysis on HaCaT cells indicates that non-clustered Dsg1 is markedly internalized after exposure to serum. However, binding of PF IgG to Dsg1-rich adhesion complexes (desmosomes) does not cause disruption of such structures nor depletion of clustered Dsg1, as revealed by colocalization of PF IgG and Dsg1 in a punctate staining on cell membrane 24 hours after treatment. Furthermore, morphological studies demonstrate that the dramatic alterations induced by PF sera are not the result of apoptotic programs. Taken together, our data strongly suggest that anti-Dsg1 antibodies from PF serum could cause the internalization of non-clustered Dsg1 and perturb the formation of new desmosomes but not directly disrupt Dsg1-containing junctions when stable contacts are already formed.

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.

MAPKAP kinase 2 (MK2)-dependent and -independent models of blister formation in pemphigus vulgaris

Pemphigus vulgaris (PV) is an autoimmune blistering disease characterized by autoantibodies to the keratinocyte adhesion protein desmoglein (Dsg) 3. Previous studies suggest that PV pathogenesis involves p38 mitogen activated protein kinase-dependent and -independent pathways. However, p38 is a difficult protein to study and therapeutically target because it has four isoforms and multiple downstream effectors. In the current study, we identify MAPKAP kinase 2 (MK2) as a downstream effector of p38 signaling in PV and describe MK2-dependent and -independent mechanisms of blister formation using passive transfer of human anti-Dsg IgG4 mAbs to neonatal mice. In human keratinocytes, PV mAbs activate MK2 in a dose-dependent manner. MK2 is also activated in human pemphigus skin blisters, causing translocation of MK2 from the nucleus to the cytosol. Small molecule inhibition of MK2 and silencing of MK2 expression block PV mAb-induced Dsg3 endocytosis in human keratinocytes. Additionally, small molecule inhibition and genetic deletion of p38α and MK2 inhibit spontaneous, but not induced, suprabasal blisters by PV mAbs in mouse passive transfer models. Collectively, these data suggest that MK2 is a key downstream effector of p38 that can modulate PV autoantibody pathogenicity. MK2 inhibition may be a valuable adjunctive therapy for control of pemphigus blistering.

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.

Urban legends: pemphigus vulgaris

Pemphigus vulgaris (PV) is the most common type of pemphigus. PV pathogenesis is still debated, and treatment remains challenging. We investigated five controversial topics: (1) What are the target antigens in PV? (2) Do desmogleins adequately address PV pathophysiology? (3) How does acantholysis occur in PV? (4) Is PV still a lethal disease? (5) What is the role of rituximab (RTX) in PV treatment? Results from extensive literature searches suggested the following: (1) Target antigens of PV include a variety of molecules and receptors that are not physically compartmentalized within the epidermis. (2) PV is caused by a variety of autoantibodies to keratinocyte self-antigens, which concur to cause blistering by acting synergistically. (3) The concept of apoptolysis distinguishes the unique mechanism of autoantibody-induced keratinocyte damage in PV from other known forms of cell death. (4) PV remains potentially life-threatening largely because of treatment side effects, but it is uncertain which therapies carry the highest likelihood of lethal risk. (5) RTX is a very promising treatment option in patients with widespread recalcitrant or life-threatening PV. RTX's cost is an issue, its long-term side effects are still unknown, and randomized controlled trials are needed to establish the optimal dosing regimen.

Characterization of a Novel Oral Glucocorticoid System and Its Possible Role in Disease

Synthetic corticosteroids are used widely for the treatment of a variety of diseases of the mouth. However, little is known as to whether the oral mucosa is able to modulate the local concentration of active corticosteroids or to produce steroids de novo. This has important clinical implications, because tissue-specific regulation of glucocorticoids is a key determinant of the clinical efficacy of these drugs. In the present study, we show that oral fibroblasts and keratinocytes expressed ACTH receptor (MC2R), glucocorticoid receptor (GR), and 11β-hydroxysteroid dehydrogenases (11β-HSDs). Unlike keratinocytes, fibroblasts lacked 11β-HSD2 and could not effectively deactivate exogenously administered cortisol. However, both cell types were able not only to activate cortisone into the active form cortisol, but also to synthesize cortisol de novo following stimulation with ACTH. 11β-HSD2, the enzyme controlling cortisol deactivation, exhibited different patterns of expression in normal (squamous epithelium and salivary glands) and diseased oral mucosa (squamous cell carcinoma and mucoepidermoid carcinoma). Blocking of endogenous cortisol catabolism in keratinocytes with the 11β-HSD2 inhibitor 18β-glycyrrhetinic acid mimicked the effect of exogenous administration of hydrocortisone and partially prevented the detrimental effects induced by pemphigus vulgaris sera. Analysis of the data demonstrates that a novel, non-adrenal glucocorticoid system is present in the oral mucosa that may play an important role in disease.

Keratinocytes synthesize and activate cortisol

The bioavailability of circulating and/or endogenous hydrocortisone (cortisol) in epidermal cells is a key determinant in inflammatory disease and chronic wounds. It is not known, however, whether epidermal cells can regulate tissue cortisol and whether they are capable of producing endogenous glucocorticoids. In the present study, we show by microarray analysis that epidermal cells express mRNAs to all the major enzymes involved in the metabolic chain from cholesterol to cortisol, including cytocrome P450 chain, 11β-hydroxysteroid dehydrogenases (HSD11Bs), adrenocorticotropic hormone (ACTH) receptor (MC2R), and glucocorticoid receptor. The two enzymes mediating activation/deactivation of cortisone to cortisol, namely HSD11B1 and HSD11B2, were expressed at the protein level in cultured keratinocytes as well as human skin samples, as shown by Western blotting and immunohistochemistry, respectively. In functional assays, we show that keratinocytes are not only able to activate cortisone to cortisol in a HSD11B-dependent manner but also silencing of either HSD11B1 or HSD11B2 specifically modulates the bioavailability of the inactive glucocorticoid and the active steroid, respectively. A further key observation was that keratinocytes responded to stimulation with ACTH by a significant increase in the de novo synthesis of cortisol. Taken together, we provide evidence for a novel non-adrenal steroideal system in human keratinocytes.

Deregulation of PERK in the autoimmune disease pemphigus vulgaris occurs via IgG-independent mechanisms

BACKGROUND

Serum and IgG isolated from patients with the autoimmune blistering disease pemphigus vulgaris (PV) trigger complex intracellular pathways in keratinocytes, including alterations of the cell cycle and metabolism, which ultimately lead to cell-cell detachment (acantholysis). We have shown previously that one of the earliest pathogenic events in PV is the activation of protein kinases, including the PKR-like endoplasmic reticulum (ER) kinase PERK.

OBJECTIVES

In the present study we investigated in more detail the role of PERK in the pathogenesis of PV.

METHODS

PERK levels were assessed by Western blotting and in-cell enzyme-linked immunosorbent assay, and PERK expression was silenced by siRNA technology. The effects of PV sera/IgG on keratinocyte cultures were investigated by flow cytometry, MTT and adhesion assays.

RESULTS

We show that PERK is activated in keratinocytes exposed to PV serum, as demonstrated by an increase in phosphorylated PERK levels and phosphorylation of eIF2α. Decreased expression of PERK by siRNA reduced the effects of PV serum on the cell cycle and keratinocyte viability, two key events in PV pathophysiology. As impairment of metabolic activity in PV is partially due to non-IgG serum factors, we then investigated the activation of PERK in keratinocytes incubated with whole PV serum, purified PV IgG and IgG-depleted PV serum. The data demonstrated that PV sera depleted of IgG, but not PV IgG, triggered PERK phosphorylation and this correlated with a marked reduction of metabolic activity in keratinocytes exposed to IgG-free serum. Knockdown of PERK by siRNA abrogated the changes in the cell cycle and apoptosis induced by IgG-depleted PV serum. Finally, the reduction of metabolic activity observed in keratinocytes exposed to IgG-depleted PV serum was almost absent in PERK-deficient cells.

CONCLUSIONS

Taken together, the results demonstrate that activation of PERK participates in the reduction of metabolic activity and cell viability seen in PV and that this phenomenon depends on non-IgG factors. PERK activation may represent a novel signalling mechanism linking ER stress and acantholysis in PV.

Controversial role of antibodies against linear epitopes of desmoglein 3 in pemphigus vulgaris, as revealed by semiquantitative living cell immunofluorescence microscopy and in-cell ELISA

A novel explanation of pemphigus vulgaris (PV) pathogenesis suggests that serum autoantibodies may affect desmoglein 3 (Dsg3)-mediated adhesion by triggering depletion of Dsg3 from desmosomes. Furthermore, abrogation of Dsg3 from the cell seems to depend on anti-Dsg3 pemphigus IgG. In this study we sought to gain more insights into the role of PV IgG recognizing non-conformational epitopes of Dsg3 (anti-Dsg3-L IgG) by semi-quantitative living cell immunofluorescence (LCIF) microscopy, in-cell ELISA and morphometric analysis of acantholysis. Our data demonstrate that PV serum and PV IgG can induce acantholysis and reduce the total amount of Dsg3 in cultured keratinocytes, whereas anti-Dsg3-L IgG fail to do so when administered at concentrations comparable to those present in pathogenic PV sera. However, the Dsg3-depleting activity of such polyclonal anti-Dsg3 IgG was acquired when used at 1 microg/ml. Interestingly, both PV sera and IgG, including anti-Dsg3-L IgG, caused early depletion of surface Dsg3 while slightly affecting the total cell content of Dsg3 until late acantholysis. This raises a possibility that depletion of Dsg3 from cell membrane and reduction of the total cellular levels of Dsg3 represent distinct phenomena in PV acantholysis. Taken together, our data demonstrate that anti-Dsg3 PV IgG against linear epitopes of Dsg3 can induce acantholytic changes of keratinocytes in a dose- and time-dependent manner. Specifically, both morphological and biochemical changes suggestive of acantholysis are seen only at high IgG concentrations. We conclude that anti-Dsg3L IgG play a minor role in experimental PV under physiologic conditions.

Captopril modulates acetylcholinesterase in human keratinocytes

Human keratinocytes synthesize and secrete non-neuronal acetylcholine, which acts as a local cell signaling molecule, regulating functions like proliferation, cell adhesion, motility, desmosomal cell contact, and glandular activity. The keratinocyte acetylcholine axis is composed of the enzymes mediating acetylcholine synthesis (acetyltransferase) and degradation (acetylcholinesterase), and two classes of acetylcholine receptors. In this study we investigated the effect of captopril, an ACE-inhibitor, on acetylcholinesterase and acetylcholine secretion in human keratinocytes. We analyzed the level of acetylcholinesterase in HaCat and NHEK cells by RT-PCR and Western blotting analysis. In addition, the effect of captopril on AChE activity was evaluated. We found that captopril induces a strong AChE up-regulation leading to ACh degradation and reduced secretion. Our results suggest that acantholysis induced by ACE-inhibitors might be linked to altered level of Ach.

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.

Serum of patients with oral pemphigus vulgaris impairs keratinocyte wound repair in vitro: a time-lapse study on the efficacy of methylprednisolone and pyridostigmine bromide

OBJECTIVES

Pemphigus vulgaris (PV) is an autoimmune blistering disease affecting primarily oral mucosa and skin. Among the drugs used for the therapy of pemphigus, both methylprednisolone (MP) and pyridostigmine bromide (PBr) can prevent acantholysis in vitro. However, their putative therapeutic properties in regenerating PV-like lesions and promoting the healing process still remain to be demonstrated. To address this issue, here we have developed a model for studying the process of epithelial cleft regeneration in PV by artificially wounding keratinocyte monolayers.

MATERIALS AND METHODS

The experimental model was established by scratching confluent monolayers to simulate the epithelial cleft; then, wound regeneration in the presence of submaximal concentrations of PV sera was studied by time-lapse microscopy, with or without the addition of MP and PBr in the culture medium.

RESULTS

Pemphigus vulgaris serum inhibited epithelial cleft repair of wounded monolayers. Indeed, in the presence of 10% (v/v) PV serum, keratinocytes reached only 2% confluence within 72 h vs an almost complete healing of controls. When administered together with PV sera, MP significantly (P < 0.01) enhanced wound fill by 30% after 72 h. PV-associated wound repair was significantly (P < 0.05) ameliorated by PBr by 24 h and keratinocytes reached 20% confluence after 72 h. Interestingly, neither MP nor PBr could accelerate wound healing when compared with untreated control monolayers.

CONCLUSIONS

In PV, MP and PBr exert their curative effects in part by enhancing the regeneration properties of keratinocytes. Indeed, our data suggest that both drugs can specifically counterbalance the detrimental effects of PV serum on keratinocyte wound healing. These findings provide an explanation for the efficacy of MP and PBr in the treatment of PV lesions in human skin and oral mucosa.

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.

Involvement of the apoptotic mechanism in pemphigus foliaceus autoimmune injury of the skin

Pemphigus foliaceus (PF) is an organ-specific autoimmune skin disease characterized by subcorneal epidermal cell detachment (acantholysis) and pathogenic autoantibodies against desmoglein 1. The mechanism responsible for pemphigus autoantibody-induced epidermal injury is not fully understood. In this study, we used the IgG passive transfer mouse model of PF to investigate the relevance of the apoptotic mechanism in pemphigus pathogenesis. TUNEL-positive epidermal cells and increased oligonucleosomes in the epidermal cytosolic fractions were detected in the diseased mice. Time course study reveals that TUNEL-positive epidermal cells appear before intraepidermal blisters. Moreover, the proapoptotic factor Bax was up-regulated at the earlier time points (2 and 4 h), whereas the antiapoptotic factor Bcl-x(L) was down-regulated at the later time points (6, 8, and 20 h) post-PF IgG injection by Western blot analysis. The active forms of caspase-3 and -6 were detected at the later time period (6, 8, and 20 h). Administration of Ac-DEVD-cmk, a peptide-based caspase-3/7 inhibitor, protected mice from developing intraepidermal blisters and clinical disease induced by PF IgG. The same protective effect was also observed using a broad-spectrum caspase inhibitor, Bok-D-fmk. Collectively, these findings show that biochemical events of apoptosis are provoked in the epidermis of mice injected with PF autoantibodies. Caspase activation may contribute to acantholytic blister formation in PF.

Advances in pemphigus and its endemic pemphigus foliaceus (Fogo Selvagem) phenotype: a paradigm of human autoimmunity

Pemphigus encompasses a group of organ specific, antibody mediated autoimmune diseases of the skin characterized by keratinocyte detachment that leads to the development of blisters and erosions, which can become life-threatening. The pathogenic autoantibodies recognize desmogleins, which are members of the desmosomal cadherin family of cell adhesion molecules. Desmoglein 3 is targeted in pemphigus vulgaris while desmoglein 1 is targeted in pemphigus foliaceus and its endemic form, Fogo Selvagem. This review will briefly define the salient features of pemphigus and the proposed steps in pathogenesis. We will then summarize the most recent advances in three important areas of investigation: (i) epidemiologic, genetic, and immunologic features of Fogo Selvagem, (ii) molecular mechanisms of injury to the epidermis, and (iii) novel therapeutic strategies targeting specific steps in disease pathogenesis. The advances in each of these three seemingly separate areas contribute to the overall understanding of the pemphigus disease model. These recent advancements also underscore the dynamic interplay between the treatment of patients in a clinical setting and basic science research and have led to an integrative understanding of disease pathogenesis and treatment, allowing pemphigus to serve as a paradigm of human autoimmunity.

Cleavage of desmoglein 3 can explain its depletion from keratinocytes in pemphigus vulgaris

We have previously demonstrated that serum of patients with pemphigus vulgaris induces reduction of desmoglein 3 (Dsg3) half-life in keratinocytes (FEBS Lett 2006: 580: 3276). This phenomenon seems to occur as a consequence of the progressive depletion of Dsg3 from desmosomes. Here we reported that reduction of full-length Dsg3 may be due to its progressive cleavage, leading to the formation of two fragmentation products with apparent molecular masses of about 60 kDa (fragment 1) and 70 kDa (fragment 2), as revealed by Western blotting. Unexpectedly, analysis of fragmentation pattern suggested cleavage to occur intracellularly. Consistently, fragment 1 was shed and localized within the cytosol, as shown by living cell immunofluorescence microscopy. Total amounts of full-length plakoglobin and Dsg1 were apparently unchanged. Taken together, our findings provide evidence that proteolytic processing of Dsg3 can lead to depletion of Dsg3 from the cell.

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.

At least three phosphorylation events induced by pemphigus vulgaris sera are pathogenically involved in keratinocyte acantholysis

Intercellular adhesion among keratinocytes is guaranteed by desmosomes. Disruption of desmosomal integrity leads to cell-cell detachment or acantholysis, as it classically occurs in pemphigus vulgaris (PV), an autoimmune blistering disease of skin and mucous membranes. While purified PV IgG seems to trigger intracellular signaling that crucially involves p38 MAPK, keratinocyte acantholysis induced by whole PV serum may recruit a number of additional signals. In this study, the Pro-Q Diamond Phosphoprotein Assay was used to investigate the overall changes in protein phosphorylation levels in an in vitro model of PV. We showed that keratinocytes exposed to whole PV sera underwent at least three early and transient phosphorylation events. Two bands with apparent molecular masses of 35 and 45 kDa were found to be phosphorylated within 1 min after incubation with PV sera. A third band of about 80 kDa reached the peak of phosphorylation level after 3 hours. Morphologic evidence of cell shrinkage and acantholysis were late events and did not correlate temporally with kinase activation, suggesting that cytoskeleton reorganization is a downstream phenomenon. Interestingly, pharmacological abrogation of PV-specific protein phosphorylation was able to inhibit the cell-cell detachment, rounding up, and redistribution of Dsg3 in keratinocytes. Thus, at least three phosphorylation events are pathogenically involved in pemphigus acantholysis.

The most widespread desmosomal cadherin, desmoglein 2, is a novel target of caspase 3-mediated apoptotic machinery

Apoptotic cells are known to regulate the ordered dismantling of intercellular contacts through caspase activity. Despite the important role of desmoglein (Dsg) 2 in epithelial cell-cell adhesion, the fate of this widespread desmosomal cadherin during apoptosis is yet poorly understood. Here, by means of pharmacological approaches, we investigated whether Dsg2 was targeted by caspases in HaCaT and HT-29 cell lines undergoing staurosporine (STS)-induced apoptosis. Results showed that STS induced a caspase-dependent form of cell-death in both keratinocytes (HaCaT) and enterocytes (HT-29), that associated with progressive depletion of Dsg2 from cell lysates. The proteolytic processing of full-length Dsg2 resulted in the appearance of a 70-kDa fragment which was released into the cytosol. Consistently, immunofluorescence studies revealed that Dsg2 staining was abolished from cell surface whereas the cytoplasmic region of Dsg2 did localize intracellularly. Plakoglobin (Pg) also underwent cleavage and detached from Dsg2. Apoptotic changes paralleled with progressive loss of intercellular adhesion strength. All these biochemical, morphological, and functional changes were regulated by caspase 3. Indeed, in the presence of the caspase 3-inhibitor z-DEVD-fmk, full-length Dsg2 protein levels were preserved, whereas the amount of the 70-kDa fragment was maintained on control levels. Furthermore, cells pretreated with z-DEVD-fmk retained the membrane labeling of Dsg2. Taken together, our data demonstrate that the apoptotic processing of Dsg2 is mediated by caspase 3 in epithelial cells.

Evidence of key role of Cdk2 overexpression in pemphigus vulgaris

The pathogenesis of pemphigus vulgaris (PV) is still poorly understood. Autoantibodies present in PV patients can promote detrimental effects by triggering altered transduction of signals, which results in a final acantholysis. To investigate mechanisms involved in PV, cultured keratinocytes were treated with PV serum. PV sera were able to promote the cell cycle progression, inducing the accumulation of cyclin-dependent kinase 2 (Cdk2). Microarray analysis on keratinocytes detected that PV serum induced important changes in genes coding for one and the same proteins with known biological functions involved in PV disease (560 differentially expressed genes were identified). Then, we used two different approaches to investigate the role of Cdk2. First, small interfering RNA depletion of Cdk2 prevented cell-cell detachment induced by PV sera. Second, pharmacological inhibition of Cdk2 activity through roscovitine prevented blister formation and acantholysis in the mouse model of the disease. In vivo PV serum was found to alter multiple different pathways by microarray analysis (1463 differentially expressed genes were identified). Major changes in gene expression induced by roscovitine were studied through comparison of effects of PV serum alone and in association with roscovitine. The most significantly enriched pathways were cell communication, gap junction, focal adhesion, adherens junction, and tight junction. Our data indicate that major Cdk2-dependent multiple gene regulatory events are present in PV. This alteration may influence the evolution of PV and its therapy.

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.

Mucocutaneous pemphigus vulgaris carrying high-titre antidesmoglein 1 antibodies with skin lesions resembling pemphigus erythematosus

Patients with pemphigus vulgaris (PV) who have both antidesmoglein (Dsg)1 and anti-Dsg3 antibodies usually develop flaccid blisters on skin and mucous membranes. We report a case of PV with crusting skin lesions resembling pemphigus erythematosus, the localized variant of pemphigus foliaceus (PF). Notably, the patient had high titres of anti-Dsg1 IgG, as assessed by ELISA. We then established an in vitro model of pemphigus, and found that patient's serum was able to induce suprabasilar acantholysis in mouse skin culture. However, epidermal splitting also occurred within the granular layer, suggesting that the pathogenic potential of such a high-titre anti-Dsg1 serum was intermediate between PV and PF. Thus, the levels of anti-Dsg1 antibodies could play a role in determining the clinical phenotype of pemphigus.

Vesicular and Bullous Disorders: Pemphigus

Pemphigus is a chronic, autoimmune disease involving the skin and Malpighian mucous membranes. Pemphigus leads to progressive blistering and subsequent erosions. This article describes the etiology and treatment of pemphigus.

If pemphigus vulgaris IgG are the cause of acantholysis, new IgG-independent mechanisms are the concause

Pemphigus vulgaris (PV) is a disease of epidermal adhesion. Its pathogenesis is currently traced back to the action of autoantibodies against antigens located within the intercellular substance of keratinocytes, such as desmogleins and acetylcholine receptors. In the present paper, we sought to elucidate the non-IgG-mediated effects of PV sera on keratinocytes. Results showed that PV sera depleted of IgG were able to induce well-defined changes on keratinocyte morphology and metabolic activity. Indeed, PV IgG-free sera determined marked alterations on cell shape, accompanied by partial loss of keratinocyte-keratinocyte interactions within 48 h after treatment. Furthermore, PV IgG-depleted sera caused a sharp reduction of cell viability along with a less sustained weakening of intercellular adhesion strength. In light of the above findings, loss of cell-cell adhesion in PV occurs as a result of the cooperating action of both IgG and non-IgG-mediated mechanisms. These data have remarkable consequences on experimental models of PV and might open new "biological" approaches to its therapy. Thus, researchers are well advised that PV pathophysiology cannot be faithfully reproduced by leaving non-IgG serum factors out of consideration.

The connexin turnover, an important modulating factor of the level of cell-to-cell junctional communication: comparison with other integral membrane proteins

The constituent proteins of gap junctions, called "connexins" (Cxs) in chordates, are generally renewed several times a day, in approximately the same rate range as many other integral plasma membrane proteins and the proteins of other channels, other intercellular junctions or different membrane receptors. This permanent renewal turns on a fine-tuned balance among various processes, such as gene transcription, mRNA stability and processing, protein synthesis and oligomerization, posttranslational modifications, transport to the plasma membrane, anchoring to the cytoskeleton, connexon aggregation and docking, regulation of endocytosis and controlled degradations of the proteins. Subtle changes at one or some of these steps would represent an exquisite level of regulation that extends beyond the rapid channel opening and closure events associated with channel gating; membrane channels and receptors are constantly able to answer to physiological requirements to either up- or downregulate their activity. The Cx turnover rate thereby appears to be a key component in the regulation of any protein, particularly of gap junctional proteins. However, the physiological stimuli that control the assembly of Cxs into gap junctions and their degradation remain poorly understood.

Defining the involvement of proteinases in pemphigus vulgaris: evidence of matrix metalloproteinase-9 overexpression in experimental models of disease

Pemphigus vulgaris (PV) acantholysis represents a complex phenomenon wherein a number of factors cooperates. PV serum is known to modulate important cellular events, including kinase activity, transcriptional regulation, and proteinase expression. Indeed, transduction of signals to the cell triggered by PV serum may induce proteinase up-regulation potentially responsible for disruption of epidermal adhesion and, ultimately, blister formation. Here, we sought to investigate this hypothesis by using both in vivo and in vitro models of PV. Microarray analysis on mouse skin tissues suggested that the equilibrium between extracellular proteinases and their inhibitors moved towards enhanced proteolytic activity in PV neonatal mouse model, at least on the transcriptional level. Conversely, genes codifying cell adhesion proteins were dramatically down-regulated. The effects of PV serum on the protein level were then studied in vitro both in keratinocyte monolayers and skin organ cultures focusing on matrix metalloproteinase (MMP) 9 expression and activity. By means of Western blotting, zymography, and living cell immunofluorescence studies, we showed that MMP-9 was early overexpressed in keratinocytes exposed to PV serum, and subsequently secreted in the culture medium. However, we failed to demonstrate extracellular activation of MMP-9, since it was found in its 92 kDa inactive form in serum-free culture supernatants. Taken together, our data demonstrated that proteinase expression, particularly of MMP-9, is modulated by PV serum and associated with PV acantholysis.

Anti-desmoglein 3 (Dsg3) Monoclonal Antibodies Deplete Desmosomes of Dsg3 and Differ in Their Dsg3-depleting Activities Related to Pathogenicity

Pemphigus vulgaris (PV) is an autoimmune blistering disease, characterized by the loss of cell-cell adhesion between epidermal keratinocytes and the presence of autoantibody against desmoglein 3 (Dsg3), which provides adhesive integrity to desmosomes between adjacent keratinocytes. We have previously shown that PV-IgG purified from patients depletes desmosomes of Dsg3. However, PV-IgG contains not only antibodies against a variety of different epitopes of Dsg3 but also against other unknown antigens. Therefore, we examined whether the Dsg3-depleting activity of PV-IgG is generated specifically by anti-Dsg3 activity in a human squamous cell carcinoma cell line (DJM-1) and normal human keratinocytes by using four different pathogenic and nonpathogenic monoclonal antibodies against Dsg3. We demonstrate that these monoclonal antibodies deplete cells and desmosomes of Dsg3, as PV-IgG does. Individual monoclonal anti-Dsg3 antibodies display characteristic limits to their Dsg3-depleting activity, which correlates with their pathogenic activities. In combination, these antibodies exert a cumulative or synergistic effect, which may explain the potent Dsg3-depleting capability of PV-IgG, which is polyclonal. Finally, although Dsg3-depletion activity correlated with AK-monoclonal antibody pathogenicity in mouse models, the residual level of Dsg3, when below approximately 50%, does not correlate with the adhesive strength index in the present study. This may suggest that although the Dsg3 depletion is not indicative for adhesive strength, the level of Dsg3 can be used as a read-out of pathogenic changes within the cell and that the Dsg3 depletion from desmosomes plays an important role in skin fragility or susceptibility to blister formation in PV patients.

Plakoglobin-dependent disruption of the desmosomal plaque in pemphigus vulgaris

We recently reported that the pathogenesis of pemphigus vulgaris (PV), an autoimmune blistering skin disorder, is driven by the accumulation of c-Myc secondary to abrogation of plakoglobin (PG)-mediated transcriptional c-Myc suppression. PG knock-out mouse keratinocytes express high levels of c-Myc and resemble PVIgG-treated wild-type keratinocytes in most respects. However, they fail to accumulate nuclear c-Myc and loose intercellular adhesion in response to PVIgG-treatment like wild-type keratinocytes. This suggested that PG is also required for propagation of the PVIgG-induced events between augmented c-Myc expression and acantholysis. Here, we addressed this possibility by comparing PVIgG-induced changes in the desmosomal organization between wild-type and PG knock-out keratinocytes. We found that either bivalent PVIgG or monovalent PV-Fab (known to trigger blister formation in vivo) disrupt the linear organization of all major desmosomal components along cell borders in wild-type keratinocytes, simultaneously with a reduction in intercellular adhesive strength. In contrast, PV-Fab failed to affect PG knock-out keratinocytes while PVIgG cross-linked their desmosomal cadherins without significantly affecting desmoplakin. These results identify PG as a principle effector of the PVIgG-induced signals downstream of c-Myc that disrupt the desmosomal plaque at the plasma membrane.

A novel method to investigate pemphigus-induced keratinocyte dysmorphisms through living cell immunofluorescence microscopy

Pemphigus vulgaris (PV) blistering occurs as a result of the disruption of intercellular contacts among keratinocytes, or acantholysis. The hallmark of PV acantholysis in vitro is considered to be the retraction of keratin intermediate filaments (KIF) onto the nucleus, which parallels with loss of cell-cell adhesion and rounding up of keratinocytes. However, the fine morphological changes of keratinocytes as well as the fate of cell adhesion structures cannot be appreciated on immunofluorescence by the simple cytokeratin staining. In this paper, we show that acantholytic dysmorphisms are sharply investigated by using PV IgG as a primary antibody on metabolically quiescent living cells. Indeed, PV IgG recognise a wide spectrum of molecules and enabled us to monitor the main changes occurring in acantholytic keratinocytes, including cell shrinkage with the appearance of prickle-like processes, detachment of keratinocytes from one another and collapse of cytoskeleton-bound proteins along nuclear periphery. This method has wider applications as it could be useful for staining cell periphery of keratinocytes and changes in cell shape. Furthermore, images displayed clear and sharp contours because living cell microscopy allows to avoid antigen distortion due to cell manipulation, which usually precedes the immunolabelling.

Pemphigus vulgaris immunoglobulin G can recognize a 130 000 MW antigen other than desmoglein 3 on peripheral blood mononuclear cell surface

Pemphigus vulgaris (PV) is considered to be an autoimmune disease affecting skin and mucous membranes. Traditionally, PV autoantibodies are thought to recognize antigens located in the intercellular substance (ICS) of keratinocytes; antigens represented mainly by the desmosomal cadherin desmoglein 3 (Dsg3). Accordingly, titres of anti-ICS and anti-Dsg3 immunoglobulin G (IgG) are considered to be major laboratory criteria when making a diagnosis of PV. In this paper, we demonstrated for the first time that PV IgG bind antigen(s) expressed on the surface of peripheral blood mononuclear cells (PBMC), as revealed by immunofluorescence studies. This novel autoantigen is immunoprecipitated by PV IgG as a 130 000 molecular weight protein. However, Western blot analysis of the immunocomplexes failed to show reactivity with anti-Dsg3 monoclonal and polyclonal antibodies. Taken together, our data provide strong evidence that PV autoimmunity targets a 130 000 antigen other than Dsg3 on PBMC. This shifting from epidermis to blood cells may open new perspectives for a better understanding of pemphigus autoimmunity and more rational approaches to its treatment.

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.

Searching for experimental models of Pemphigus vulgaris

The current knowledge on Pemphigus vulgaris (PV) pathophysiology suggests that blister formation relies on both PV IgG and non-IgG serum factors activity. PV autoimmunity seems to develop against both desmoglein 1/3 and acetylcholine receptors leading to transduction of signals to the cell mediated by phosphorilation events. Serum factors other than IgG also participate to PV acantholysis through apoptotic or cytokine-mediated mechanisms. Apart from the role played by each actor within the acantholysis, however, the current scenario arises important methodological issues. For example, the use of PV IgG or monoclonal anti-Dsg3 antibodies to experimentally reproduce the disease appears inadequate, as it does not take into account the role of non-IgG factors. On the basis of the above observations and those from our laboratories, here we propose that using whole sera from PV patients with active disease represents the most faithful manner to mimic the disease.

Changes in desmoglein 1 expression and subcellular localization in cultured keratinocytes subjected to anti-desmoglein 1 pemphigus autoimmunity

The complexity of pemphigus acantholysis together with the weak expression of desmoglein 1 (Dsg1) in cultured keratinocytes have made the study on the pathogenic action of anti-Dsg1 antibodies quite difficult. The pathophysiology of the acantholytic phenomenon could depend on the reduction of Dsg1 adhesion function occurring after its massive internalization or decrease of its synthesis. Here, we have investigated this hypothesis by using sera of patients having antibodies against Dsg1 or monoclonal anti-Dsg1 antibodies to simulate pemphigus autoimmunity in Dsg1-rich keratinocytes. Similar to pemphigus foliaceus (PF) and vulgaris (PV) sera, monoclonal anti-Dsg1 antibodies induced transient internalization of Dsg1 and reduced the adhesion strength among keratinocytes. However, binding of IgG to Dsg1 did not determine its early depletion from the adhesion complexes but reduced the amount of Dsg1 found in the Triton X-100 soluble pool of proteins. Taken together, our results represent the first demonstration that anti-Dsg1 antibodies induce similar alterations on the subcellular distribution of Dsg1 irrespective of the disease where they come from. Furthermore, the present study provides insight into the mechanisms underlying epithelial blistering observed in the skin type of pemphigus.