Desmoglein 2 compensates for desmoglein 3 but does not control cell adhesion via regulation of p38 mitogen-activated protein kinase in keratinocytes

Desmosomal Hyper-adhesion Affects Direct Inhibition of Desmoglein Interactions in Pemphigus

During differentiation, keratinocytes acquire a strong, hyper-adhesive state, where desmosomal cadherins interact calcium ion independently. Previous data indicate that hyper-adhesion protects keratinocytes from pemphigus vulgaris autoantibody-induced loss of intercellular adhesion, although the underlying mechanism remains to be elucidated. Thus, in this study, we investigated the effect of hyper-adhesion on pemphigus vulgaris autoantibody-induced direct inhibition of desmoglein (DSG) 3 interactions by atomic force microscopy. Hyper-adhesion abolished loss of intercellular adhesion and corresponding morphological changes of all pathogenic antibodies used. Pemphigus autoantibodies putatively targeting several parts of the DSG3 extracellular domain and 2G4, targeting a membrane-proximal domain of DSG3, induced direct inhibition of DSG3 interactions only in non-hyper-adhesive keratinocytes. In contrast, AK23, targeting the N-terminal extracellular domain 1 of DSG3, caused direct inhibition under both adhesive states. However, antibody binding to desmosomal cadherins was not different between the distinct pathogenic antibodies used and was not changed during acquisition of hyper-adhesion. In addition, heterophilic DSC3-DSG3 and DSG2-DSG3 interactions did not cause reduced susceptibility to direct inhibition under hyper-adhesive condition in wild-type keratinocytes. Taken together, the data suggest that hyper-adhesion reduces susceptibility to autoantibody-induced direct inhibition in dependency on autoantibody-targeted extracellular domain but also demonstrate that further mechanisms are required for the protective effect of desmosomal hyper-adhesion in pemphigus vulgaris.

JNK inhibition enhances cell-cell adhesion impaired by desmoglein 3 gene disruption in keratinocytes

c-Jun NH2-terminal protein kinase (JNK) and p38 are stress-activated mitogen-activated protein kinases (MAPK) that are phosphorylated by various stimuli. It has been reported that the loss of desmoglein (DSG) 3, a desmosomal transmembrane core molecule, in keratinocytes impairs cell-cell adhesion accompanied by p38 MAPK activation. To understand the biological role of DSG3 in desmosomes and its relationship with stress-activated MAPKs, we established DSG3 knockout keratinocytes (KO cells). Wild-type cells showed a linear localization of DSG1 to cell-cell contacts, whereas KO cells showed a remarkable reduction despite the increased protein levels of DSG1. Cell-cell adhesion in KO cells was impaired over time, as demonstrated by dispase-based dissociation assays. The linear localization of DSG1 to cell-cell contacts and the strength of cell-cell adhesion were promoted by the pharmacological inhibition of JNK. Conversely, pharmacological activation of JNK, but not p38 MAPK, in wild-type cells reduced the linear localization of DSG1 in cell-cell contacts. Our data indicate that DSG1 and DSG2 in KO cells cannot compensate for the attenuation of cell-cell adhesion strength caused by DSG3 deficiency and that JNK inhibition restores the strength of cell-cell adhesion by increasing the linear localization of DSG1 in cell-cell contacts in KO cells. Inhibition of JNK signaling may improve cell-cell adhesion in diseases in which DSG3 expression is impaired.

Proteomic insights into mental health status: plasma markers in young adults

Global emphasis on enhancing prevention and treatment strategies necessitates an increased understanding of the biological mechanisms of psychopathology. Plasma proteomics is a powerful tool that has been applied in the context of specific mental disorders for biomarker identification. The p-factor, also known as the "general psychopathology factor", is a concept in psychopathology suggesting that there is a common underlying factor that contributes to the development of various forms of mental disorders. It has been proposed that the p-factor can be used to understand the overall mental health status of an individual. Here, we aimed to discover plasma proteins associated with the p-factor in 775 young adults in the FinnTwin12 cohort. Using liquid chromatography-tandem mass spectrometry, 13 proteins with a significant connection with the p-factor were identified, 8 of which were linked to epidermal growth factor receptor (EGFR) signaling. This exploratory study provides new insight into biological alterations associated with mental health status in young adults.

Tacrolimus reverses pemphigus vulgaris serum-induced depletion of desmoglein in HaCaT cells via inhibition of heat shock protein 27 phosphorylation

BACKGROUND

Glucocorticoids are the first-line treatment for Pemphigus vulgaris (PV), but its serious side effects can be life-threatening for PV patients. Tacrolimus (FK506) has been reported to have an adjuvant treatment effect against PV. However, the mechanism underlying the inhibitory effect of FK506 on PV-IgG-induced acantholysis is unclear.

OBJECTIVE

The objective of this study was to explore the effect of FK506 on desmoglein (Dsg) expression and cell adhesion in an immortalized human keratinocyte cell line (HaCaT cells) stimulated with PV sera.

METHODS

A cell culture model of PV was established by stimulating HaCaT cells with 5% PV sera with or without FK506 and clobetasol propionate (CP) treatment. The effects of PV sera on intercellular junctions and protein levels of p38 mitogen-activated protein kinase (p38MAPK), heat shock protein 27 (HSP27), and Dsg were assayed using western blot analysis, immunofluorescence staining, and a keratinocyte dissociation assay.

RESULTS

PV sera-induced downregulation of Dsg3 was observed in HaCaT cells and was blocked by FK506 and/or CP. Immunofluorescence staining revealed that linear deposits of Dsg3 on the surface of HaCaT cells in the PV sera group disappeared and were replaced by granular and agglomerated fluorescent particles on the cell surface; however, this effect was reversed by FK506 and/or CP treatment. Furthermore, cell dissociation assays showed that FK506 alone or in combination with CP increased cell adhesion in HaCaT cells and ameliorated loss of cell adhesion induced by PV sera. Additionally, FK506 noticeably decreased the PV serum-induced phosphorylation of HSP 27, but had no effect on p38MAPK phosphorylation.

CONCLUSION

FK506 reverses PV-IgG induced-Dsg depletion and desmosomal dissociation in HaCaT cells, and this effect may be obtained by inhibiting HSP27 phosphorylation.

Dsg3 epitope-specific signalling in pemphigus

Introduction

Pemphigus is an autoantibody driven disease that impairs the barrier function of the skin and mucosa by disrupting desmosomes and thereby impeding cellular cohesion. It is known that the different clinical phenotypes of pemphigus vulgaris (PV) and pemphigus foliaceus (PF) are dependent on the autoantibody profile and target antigens that, amongst others, are primarily desmoglein (Dsg)1 and/or Dsg3 for PV and Dsg1 for PF. However, it was reported that autoantibodiesagainst different epitopes of Dsg1 and Dsg3 can be pathogenic or not. The underlying mechanisms are very complex and involve both direct inhibition of Dsg interactions and downstream signalling. The aim of this study was to find out whether there is target-epitope-specific Dsg3 signalling by comparing the effects of the two pathogenic murine IgGs, 2G4 and AK23.

Methods

Dispase-based dissociation assay, Western Blot analysis, Stimulated emission depletion microscopy, Fura-based Ca2+ flux measurements, Rho/Rac G-Protein-linked immunosorbent assay, Enzyme-linked immunosorbent assay.

Results

The IgGs are directed against the EC5 and EC1 domain of Dsg3, respectively. The data show that 2G4 was less effective in causing loss of cell adhesion, compared to AK23. STED imaging revealed that both autoantibodies had similar effects on keratin retraction and reduction of desmosome number whereas only AK23 induced Dsg3 depletion. Moreover, both antibodies induced phosphorylation of p38MAPK and Akt whereas Src was phosphorylated upon treatment with AK23 only. Interestingly, Src and Akt activation were p38MAPK-dependent. All pathogenic effects were rescued by p38MAPK inhibition and AK23-mediated effects were also ameliorated by Src inhibition.

Discussion

The results give first insights into pemphigus autoantibody-induced Dsg3 epitope-specific signalling which is involved in pathogenic events such as Dsg3 depletion.

Role of ADAM10 and ADAM17 in the Regulation of Keratinocyte Adhesion in Pemphigus Vulgaris

The severe autoimmune blistering disease Pemphigus vulgaris (PV) is mainly caused by autoantibodies (IgG) against desmoglein (Dsg) 3 and Dsg1. The mechanisms leading to the development of blisters are not fully understood, but intracellular signaling seems to play an important role. Sheddases ADAM10 and ADAM17 are involved in the turnover of the desmosomal cadherin Dsg2 and ADAM10 has been shown to contribute to acantholysis in a murine pemphigus model. In the present study, we further examined the role of ADAM10 and ADAM17 both in keratinocyte adhesion and in the pathogenesis of PV. First, we found that inhibition of ADAM10 enhanced adhesion of primary human keratinocytes but not of immortalized keratinocytes. In dissociation assays, inhibition of ADAM10 shifted keratinocyte adhesion towards a hyperadhesive state. However, ADAM inhibition did neither modulate protein levels of Dsg1 and Dsg3 nor activation of EGFR at Y1068 and Y845. In primary human keratinocytes, inhibition of ADAM10, but not ADAM17, reduced loss of cell adhesion and fragmentation of Dsg1 and Dsg3 immunostaining in response to a PV1-IgG from a mucocutaneous PV patient. Similarly, inhibition of ADAM10 in dissociation assay decreased fragmentation of primary keratinocytes induced by a monoclonal antibody against Dsg3 and by PV-IgG from two other patients both suffering from mucosal PV. However, such protective effect was not observed in both cultured cells and ex vivo disease models, when another mucocutaneous PV4-IgG containing more Dsg1 autoantibodies was used. Taken together, ADAM10 modulates both hyperadhesion and PV-IgG-induced loss of cell adhesion dependent on the autoantibody profile.

Dsg1 and Dsg3 Composition of Desmosomes Across Human Epidermis and Alterations in Pemphigus Vulgaris Patient Skin

Desmosomes are important epidermal adhesion units and signalling hubs, which play an important role in pemphigus pathogenesis. Different expression patterns of the pemphigus autoantigens desmoglein (Dsg)1 and Dsg3 across different epidermal layers have been demonstrated. However, little is known about changes in desmosome composition in different epidermal layers or in patient skin. The aim of this study was thus to characterize desmosome composition in healthy and pemphigus skin using super-resolution microscopy. An increasing Dsg1/Dsg3 ratio from lower basal (BL) to uppermost granular layer (GL) was observed. Within BL desmosomes, Dsg1 and Dsg3 were more homogeneously distributed whereas superficial desmosomes mostly comprised one of the two molecules or domains containing either one but not both. Extradesmosomal, desmoplakin (Dp)-independent, co-localization of Dsg3 with plakoglobin (Pg) was found mostly in BL and extradesmosomal Dsg1 co-localization with Pg in all layers. In contrast, in the spinous layer (SL) most Dsg1 and Dsg3 staining was confined to desmosomes, as revealed by the co-localization with Dp. In pemphigus patient skin, Dsg1 and Dsg3 immunostaining was altered especially along blister edges. The number of desmosomes in patient skin was reduced significantly in basal and spinous layer keratinocytes with only few split desmosomes found. In addition, Dsg1-Pg co-localization at the apical BL and Dsg3-Pg co-localization in SL were significantly reduced in patients, suggesting that that extradesmosomal Dsg molecules were affected. These results support the hypothesis that pemphigus is a desmosome assembly disease and may help to explain histopathologic differences between pemphigus phenotypes.

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.

Structure and regulation of desmosomes in intercalated discs: Lessons from epithelia

For electromechanical coupling of cardiomyocytes, intercalated discs (ICDs) are pivotal as highly specialized intercellular contact areas. ICD consists of adhesive contacts, such as desmosomes and adherens junctions (AJs) that are partially intermingled and thereby form an area composita to provide mechanical strength, as well as gap junctions (GJ) and sodium channels for excitation propagation. In contrast, in epithelia, mixed junctions with features of desmosomes and AJs are regarded as transitory primarily during the formation of desmosomes. The anatomy of desmosomes is defined by a typical ultrastructure with dense intracellular plaques anchoring the cadherin-type adhesion molecules to the intermediate filament cytoskeleton. Desmosomal diseases characterized by impaired adhesive and signalling functions of desmosomal contacts lead to arrhythmogenic cardiomyopathy when affecting cardiomyocytes and cause pemphigus when manifesting in keratinocytes or present as cardiocutaneous syndromes when both cell types are targeted by the disease, which underscores the high biomedical relevance of these cell contacts. Therefore, comparative analyses regarding the structure and regulation of desmosomal contacts in cardiomyocytes and epithelial cells are helpful to better understand disease pathogenesis. In this brief review, we describe the structural properties of ICD compared to epithelial desmosomes and suggest that mechanisms regulating adhesion may at least in part be comparable. Also, we discuss whether phenomena such as hyperadhesion or the bidirectional regulation of desmosomes to serve as signalling hubs in epithelial cells may also be relevant for ICD.

Desmoglein2 Regulates Claudin2 Expression by Sequestering PI-3-Kinase in Intestinal Epithelial Cells

Inflammation-induced reduction of intestinal desmosomal cadherin Desmoglein 2 (Dsg2) is linked to changes of tight junctions (TJ) leading to impaired intestinal epithelial barrier (IEB) function by undefined mechanisms. We characterized the interplay between loss of Dsg2 and upregulation of pore-forming TJ protein Claudin2. Intraperitoneal application of Dsg2-stablising Tandem peptide (TP) attenuated impaired IEB function, reduction of Dsg2 and increased Claudin2 in DSS-induced colitis in C57Bl/6 mice. TP blocked loss of Dsg2-mediated adhesion and upregulation of Claudin2 in Caco2 cells challenged with TNFα. In Dsg2-deficient Caco2 cells basal expression of Claudin2 was increased which was paralleled by reduced transepithelial electrical resistance and by augmented phosphorylation of AKT^Ser473 under basal conditions. Inhibition of phosphoinositid-3-kinase proved that PI-3-kinase/AKT-signaling is critical to upregulate Claudin2. In immunostaining PI-3-kinase dissociated from Dsg2 under inflammatory conditions. Immunoprecipitations and proximity ligation assays confirmed a direct interaction of Dsg2 and PI-3-kinase which was abrogated following TNFα application. In summary, Dsg2 regulates Claudin2 expression by sequestering PI-3-kinase to the cell borders in intestinal epithelium.

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.

Ca2+ signalling is critical for autoantibody-induced blistering of human epidermis in pemphigus

BACKGROUND

Pemphigus is a severe bullous autoimmune skin disease. Pemphigus foliaceus (PF) is characterized by antidesmoglein (Dsg) 1 IgG causing epidermal blistering; mucosal pemphigus vulgaris (mPV) by anti-Dsg3 IgG inducing erosions in the mucosa; and mucocutaneous pemphigus vulgaris (PV) by affecting both, with autoantibodies targeting Dsg1 and Dsg3.

OBJECTIVES

To characterize the Ca²⁺ flux pathway and delineate its importance in pemphigus pathogenesis and clinical phenotypes caused by different antibody profiles.

METHODS

Immunoprecipitation, Ca²⁺ flux analysis, Western blotting, immunofluorescence staining, dissociation assays and a human skin ex vivo model were used.

RESULTS

PV IgG and PF IgG, but neither Dsg3-specific monoclonal antibody (AK23) nor mPV IgG, caused Ca²⁺ influx in primary human keratinocytes. Phosphatidylinositol 4-kinase α interacts with Dsg1 but not with Dsg3. Its downstream target - phospholipase-C-γ1 (PLC) - was activated by PV IgG and PF IgG but not AK23 or mPV IgG. PLC releases inositol 1,4,5-trisphosphate (IP3) causing IP3 receptor (IP3R) activation and Ca²⁺ flux from the endoplasmic reticulum into the cytosol, which stimulates Ca²⁺ release-activated channels (CRAC)-mediated Ca²⁺ influx. Inhibitors against PLC, IP3R and CRAC effectively blocked PV IgG and PF IgG-induced Ca²⁺ influx; ameliorated alterations of Dsg1 and Dsg3 localization, and reorganization of keratin and actin filaments; and inhibited loss of cell adhesion in vitro. Finally, inhibiting PLC or IP3R was protective against PV IgG-induced blister formation and redistribution of Dsg1 and Dsg3 in human skin ex vivo.

CONCLUSIONS

Ca²⁺ -mediated signalling is important for epidermal blistering and dependent on the autoantibody profile, which indicates different roles for signalling complexes organized by Dsg1 and Dsg3. Interfering with PLC and Ca²⁺ signalling may be a promising approach to treat epidermal manifestations of pemphigus.

Clustering of desmosomal cadherins by desmoplakin is essential for cell-cell adhesion

AIM

Desmoplakin (Dp) is a crucial component of the desmosome, a supramolecular cell junction complex anchoring intermediate filaments. The mechanisms how Dp modulates cell-cell adhesion are only partially understood. Here, we studied the impact of Dp on the function of desmosomal adhesion molecules, desmosome turnover and intercellular adhesion.

METHODS

CRISPR/Cas9 was used for gene editing of human keratinocytes which were characterized by Western blot and immunostaining. Desmosomal ultrastructure and function were assessed by electron microscopy and cell adhesion assays. Single molecule binding properties and localization of desmosomal cadherins were studied by atomic force microscopy and super-resolution imaging.

RESULTS

Knockout (ko) of Dp impaired cell cohesion to drastically higher extents as ko of another desmosomal protein, plakoglobin (Pg). In contrast to Pg ko, desmosomes were completely absent in Dp ko. Binding properties of the desmosomal adhesion molecules desmocollin (Dsc) 3 and desmoglein (Dsg) 3 remained unaltered under loss of Dp. Dp was required for assembling desmosomal cadherins into large clusters, as Dsg2 and Dsc3, adhesion molecules primarily localized within desmosomes, were redistributed into small puncta in the cell membrane of Dp ko cells. Additional silencing of desmosomal cadherins in Dp ko did not further increase loss of intercellular adhesion.

CONCLUSION

Our data demonstrate that Dp is essential for desmosome formation but does not influence intercellular adhesion on the level of individual cadherin binding properties. Rather, macro-clustering of desmosomal adhesion molecules through Dp is crucial. These results may help to better understand severe diseases which are caused by Dp dysfunction.

Targeting desmosomal adhesion and signalling for intestinal barrier stabilization in inflammatory bowel diseases-Lessons from experimental models and patients

Inflammatory bowel diseases (IBD) such as Crohn's disease (CD) and Ulcerative colitis (UC) have a complex and multifactorial pathogenesis which is incompletely understood. A typical feature closely associated with clinical symptoms is impaired intestinal epithelial barrier function. Mounting evidence suggests that desmosomes, which together with tight junctions (TJ) and adherens junctions (AJ) form the intestinal epithelial barrier, play a distinct role in IBD pathogenesis. This is based on the finding that desmoglein (Dsg) 2, a cadherin-type adhesion molecule of desmosomes, is required for maintenance of intestinal barrier properties both in vitro and in vivo, presumably via Dsg2-mediated regulation of TJ. Mice deficient for intestinal Dsg2 show increased basal permeability and are highly susceptible to experimental colitis. In several cohorts of IBD patients, intestinal protein levels of Dsg2 are reduced and desmosome ultrastructure is altered suggesting that Dsg2 is involved in IBD pathogenesis. In addition to its adhesive function, Dsg2 contributes to enterocyte cohesion and intestinal barrier function. Dsg2 is also involved in enterocyte proliferation, barrier differentiation and induction of apoptosis, in part by regulation of p38MAPK and EGFR signalling. In IBD, the function of Dsg2 appears to be compromised via p38MAPK activation, which is a critical pathway for regulation of desmosomes and is associated with keratin phosphorylation in IBD patients. In this review, the current findings on the role of Dsg2 as a novel promising target to prevent loss of intestinal barrier function in IBD patients are discussed.

Dsg2 Upregulation as a Rescue Mechanism in Pemphigus

In pemphigus vulgaris (PV), autoantibodies directed against the desmosomal cadherin desmoglein (Dsg) 3 cause loss of intercellular adhesion. It is known that Dsg3 interactions are directly inhibited by autoantibody binding and that Dsg2 is upregulated in epidermis of PV patients. Here, we investigated whether heterophilic Dsg2-Dsg3 interactions occur and would modulate PV pathogenesis. Dsg2 was upregulated in PV patients’ biopsies and in a human ex vivo pemphigus skin model. Immunoprecipitation and cell-free atomic force microscopy (AFM) experiments demonstrated heterophilic Dsg2-Dsg3 interactions. Similarly, in Dsg3-deficient keratinocytes with severely disturbed intercellular adhesion Dsg2 was upregulated in the desmosome containing fraction. AFM revealed that Dsg2-Dsg3 heterophilic interactions showed binding frequency, strength, Ca²⁺-dependency and catch-bond behavior comparable to homophilic Dsg3-Dsg3 or homophilic Dsg2-Dsg2 interactions. However, heterophilic Dsg2-Dsg3 interactions had a longer lifetime compared to homophilic Dsg2-Dsg2 interactions and PV autoantibody-induced direct inhibition was significantly less pronounced for heterophilic Dsg2-Dsg3 interactions compared to homophilic Dsg3 interactions. In contrast, a monoclonal anti-Dsg2 inhibitory antibody reduced heterophilic Dsg2-Dsg3 and homophilic Dsg2-Dsg2 binding to the same degree and further impaired intercellular adhesion in Dsg3-deficient keratinocytes. Taken together, the data demonstrate that Dsg2 undergoes heterophilic interactions with Dsg3, which may attenuate autoantibody-induced loss of keratinocyte adhesion in pemphigus.

Transgenic Kallikrein 14 Mice Display Major Hair Shaft Defects Associated with Desmoglein 3 and 4 Degradation, Abnormal Epidermal Differentiation, and IL-36 Signature

Netherton syndrome is a rare autosomal recessive skin disease caused by loss-of-function mutations in SPINK5 encoding LEKTI protein that results in unopposed activity of epidermal kallikrein-related peptidases (KLKs), mainly KLK5, KLK7, and KLK14. Although the function of KLK5 and KLK7 has been previously studied, the role of KLK14 in skin homeostasis and its contribution to Netherton syndrome pathogenesis remains unknown. We generated a transgenic murine model overexpressing human KLK14 (TghKLK14) in stratum granulosum. TghKLK14 mice revealed increased proteolytic activity in the granular layers and in hair follicles. Their hair did not grow and displayed major defects with hyperplastic hair follicles when hKLK14 was overexpressed. TghKLK14 mice displayed abnormal epidermal hyperproliferation and differentiation. Ultrastructural analysis revealed cell separation in the hair cortex and increased thickness of Huxley's layer. Desmoglein (Dsg) 2 staining was increased, whereas Dsg3 and Dsg4 were markedly reduced. In vitro studies showed that hKLK14 directly cleaves recombinant human DSG3 and recombinant human DSG4, suggesting that their degradation contributes to hair abnormalities. Their skin showed an inflammatory signature, with enhanced expression of IL-36 family members and their downstream targets involved in innate immunity. This in vivo study identifies KLK14 as an important contributor to hair abnormalities and skin inflammation seen in Netherton syndrome.

Microtubule-associated protein 4 phosphorylation regulates epidermal keratinocyte migration and proliferation

Both cell migration and proliferation are indispensable parts of reepithelialization during skin wound healing, which is a complex process for which the underlying molecular mechanisms are largely unknown. Here, we identify a novel role for microtubule-associated protein 4 (MAP4), a cytosolic microtubule-binding protein that regulates microtubule dynamics through phosphorylation modification, as a critical regulator of epidermal wound repair. We showed that MAP4 phosphorylation was induced in skin wounds. In an aberrant phosphorylated MAP4 mouse model, hyperphosphorylation of MAP4 (S737 and S760) accelerated keratinocyte migration and proliferation and skin wound healing. Data from both primary cultured keratinocytes and HaCaT cells in vitro revealed the same results. The promigration and proproliferation effects of MAP4 phosphorylation depended on microtubule rearrangement and could be abolished by MAP4 dephosphorylation. We also identified p38/MAPK as an upstream regulator of MAP4 phosphorylation in keratinocytes. Our findings provide new insights into the molecular mechanisms underlying wound-associated keratinocyte migration and proliferation and identify potential targets for the remediation of defective wound healing.

Role of Dsg1- and Dsg3-Mediated Signaling in Pemphigus Autoantibody-Induced Loss of Keratinocyte Cohesion

Pemphigus is an autoimmune dermatosis in which mucocutaneous blisters are induced primarily by autoantibodies against Desmoglein (Dsg) 1 and 3. Pemphigus vulgaris (PV) usually is associated with autoantibodies against Dsg3 whereas pemphigus foliaceus (PF) patients present autoantibodies against Dsg1. Several signaling pathways were proposed to cause loss of keratinocyte adhesion. However, relevance of different signaling pathways and role of Dsg1 and 3 to trigger signaling are not fully understood. Here, we show that Ca²⁺ chelation reduced PV-IgG- and PF-IgG-mediated loss of HaCaT keratinocyte cohesion whereas EGFR inhibition did not inhibit effects of PF-IgG. PV-IgG activated EGFR in a Src-dependent manner whereas both PV-IgG and PF-IgG caused Ca²⁺ influx independent of EGFR. ERK activation was Src-dependent in response to PV-IgG but not PF-IgG. To delineate the roles of Dsg isoforms to trigger signaling pathways, Dsg3- and Dsg2-deficient HaCaT keratinocyte cell lines were generated using CRISPR/Cas9. Dsg3- but not Dsg2-deficient cells were protected against PV-IgG-induced loss of cell adhesion. Ca²⁺ influx and ERK activation in response to PF-IgG were preserved in both cell lines.

Desmogleins as signaling hubs regulating cell cohesion and tissue/organ function in skin and heart - EFEM lecture 2018

Cell-cell contacts are crucial for intercellular cohesion and formation of endothelial and epithelial barriers. Desmosomes are the adhesive contacts providing mechanical strength to epithelial intercellular adhesion and therefore are most abundant in tissues subjected to high mechanical stress such as the epidermis and heart muscle. Desmogleins (Dsg) besides intercellular adhesion serve as signalling hubs regulating cell behaviour. In desmosomal diseases such as the autoimmune blistering skin disease pemphigus or arrhythmic cardiomyopathy (AC), which is caused by mutations of desmosomal components of cardiomyocyte intercalated discs, the adhesive and signalling functions of desmosomes are impaired. Therefore, our goal is to elucidate the mechanisms regulating adhesion of desmosomes in order to develop new strategies to treat desmosomal diseases. For pemphigus, we have provided evidence that intracellular signalling is required for loss of keratinocyte cohesion and have characterized a first disease-relevant adhesion receptor consisting of Dsg3 and p38MAPK. We propose that signalling patterns correlate with autoantibody profiles and thereby define the clinical phenotypes of pemphigus. Besides direct modulation of signalling pathways we have demonstrated that peptide-mediated crosslinking of Dsg molecules can abolish skin blistering in vivo. A similar approach may be effective to stabilize adhesion in cardiomyocytes of AC hearts. Since we observed that the adrenergic β1-receptor is localized at intercalated discs we evaluate signalling pathways regulating cardiomyocyte cohesion. With adrenergic signalling we have reported a first mechanism to stabilize desmosomal adhesion in intercalated discs and proposed a new function of the sympathicus in the heart we refer to as positive adhesiotropy.

Desmoglein 3 - Influence on oral carcinoma cell migration and invasion

Desmoglein 3 (Dsg3) is an adhesion receptor in desmosomes, but its role in carcinoma cell migration and invasion is mostly unknown. Our aim was to quantitatively analyse the motion of Dsg3-modified carcinoma cells in 2D settings and in 3D within tumour microenvironment mimicking (TMEM) matrices. We tested mutant constructs of C-terminally truncated Dsg3 (∆238 and ∆560), overexpressed full-length (FL) Dsg3, and empty vector control (Ct) of buccal mucosa squamous cell carcinoma (SqCC/Y1) cells. We captured live cell images and analysed migration velocities and accumulated and Euclidean distances. We compared rodent collagen and Matrigel^® with human Myogel TMEM matrices for these parameters in 3D sandwich, in which we also tested the effects of monoclonal antibody AK23, which targets the EC1 domain of Dsg3. In monolayer culture, FL and both truncated constructs migrated faster and had higher accumulated distances than Ct cells. However, in the 3D assays, only the mutants invaded faster relative to Ct cells. Of the mutants, the shorter form (Δ238) exhibited faster migration and invasion than Δ560 cells. In the Transwell, all of the cells invaded faster through Myogel than Matrigel^® coated wells. In 3D sandwich, AK23 antibody inhibited only the invasion of FL cells. We conclude that different experimental 2D and 3D settings can markedly influence the movement of oral carcinoma cells with various Dsg3 modifications.

Keratins Regulate the Adhesive Properties of Desmosomal Cadherins through Signaling

Tightly controlled intercellular adhesion is crucial for the integrity and function of the epidermis. The keratin filament cytoskeleton anchors desmosomes, supramolecular complexes required for strong intercellular adhesion. We tested whether keratin filaments control cell adhesion by regulating the adhesive properties of desmosomal cadherins such as desmoglein (Dsg) 3. Atomic force microscopy and fluorescence recovery after photobleaching experiments showed reduced Dsg3 adhesive forces and membrane stability in murine keratinocytes lacking all keratin filaments. Impairment of the actin cytoskeleton also resulted in decreased Dsg3 immobilization but did not affect Dsg3 binding properties, indicating that the latter are exclusively controlled by keratins. Reduced binding forces were dependent on p38 mitogen-activated protein kinase activity, which was deregulated in keratin-deficient cells. In contrast, inhibition of protein kinase C signaling, which is known to be controlled by keratins, promoted and spatially stabilized Dsg3-mediated interactions in the membrane. These results show a previously unreported mechanism for how keratins stabilize intercellular adhesion on the level of single desmosomal adhesion molecules.

Desmoglein 2 regulates the intestinal epithelial barrier via p38 mitogen-activated protein kinase

Intestinal epithelial barrier properties are maintained by a junctional complex consisting of tight junctions (TJ), adherens junctions (AJ) and desmosomes. Desmoglein 2 (Dsg2), an adhesion molecule of desmosomes and the only Dsg isoform expressed in enterocytes, is required for epithelial barrier properties and may contribute to barrier defects in Crohn’s disease. Here, we identified extradesmosomal Dsg2 on the surface of polarized enterocytes by Triton extraction, confocal microscopy, SIM and STED. Atomic force microscopy (AFM) revealed Dsg2-specific binding events along the cell border on the surface of enterocytes with a mean unbinding force of around 30pN. Binding events were blocked by an inhibitory antibody targeting Dsg2 which under same conditions activated p38MAPK but did not reduce cell cohesion. In enterocytes deficient for Dsg2, p38MAPK activity was reduced and both barrier integrity and reformation were impaired. Dsc2 rescue did not restore p38MAPK activity indicating that Dsg2 is required. Accordingly, direct activation of p38MAPK in Dsg2-deficient cells enhanced barrier reformation demonstrating that Dsg2-mediated activation of p38MAPK is crucial for barrier function. Collectively, our data show that Dsg2, beside its adhesion function, regulates intestinal barrier function via p38MAPK signalling. This is in contrast to keratinocytes and points towards tissue-specific signalling functions of desmosomal cadherins.

Different signaling patterns contribute to loss of keratinocyte cohesion dependent on autoantibody profile in pemphigus

Pemphigus is an autoimmune blistering skin disease caused primarily by autoantibodies against desmoglein (Dsg)1 and 3. Here, we characterized the mechanisms engaged by pemphigus IgG from patients with different clinical phenotypes and autoantibody profiles. All pemphigus vulgaris (PV) and pemphigus foliaceus (PF) IgG and AK23, a monoclonal mouse antibody against Dsg3, caused loss of cell cohesion, cytokeratin retraction and p38MAPK activation. Strong alterations in Dsg3 distribution were caused by mucosal (aDsg3 antibodies), mucocutaneous (aDsg1 + aDsg3) as well as atypical (aDsg3) PV-IgG. All PV-IgG fractions and AK23 compromised Dsg3 but not Dsg1 binding and enhanced Src activity. In contrast, rapid Ca²⁺ influx and Erk activation were induced by mucocutaneous PV-IgG and pemphigus foliaceus (PF) IgG (aDsg1) whereas cAMP was increased by mucosal and mucocutaneous PV-IgG only. Selective inhibition of p38MAPK, Src or PKC blocked loss of keratinocyte cohesion in response to all autoantibody fractions whereas Erk inhibition was protective against mucocutaneous PV-IgG and PF-IgG only. These results demonstrate that signaling patterns parallel the clinical phenotype as some mechanisms involved in loss of cell cohesion are caused by antibodies targeting Dsg3 whereas others correlate with autoantibodies against Dsg1. The concept of key desmosome regulators may explain observations from several experimental models of pemphigus.

Plakophilin-2 loss promotes TGF-β1/p38 MAPK-dependent fibrotic gene expression in cardiomyocytes

Members of the desmosome protein family are integral components of the cardiac area composita, a mixed junctional complex responsible for electromechanical coupling between cardiomyocytes. In this study, we provide evidence that loss of the desmosomal armadillo protein Plakophilin-2 (PKP2) in cardiomyocytes elevates transforming growth factor β1 (TGF-β1) and p38 mitogen-activated protein kinase (MAPK) signaling, which together coordinate a transcriptional program that results in increased expression of profibrotic genes. Importantly, we demonstrate that expression of Desmoplakin (DP) is lost upon PKP2 knockdown and that restoration of DP expression rescues the activation of this TGF-β1/p38 MAPK transcriptional cascade. Tissues from PKP2 heterozygous and DP conditional knockout mouse models also exhibit elevated TGF-β1/p38 MAPK signaling and induction of fibrotic gene expression in vivo. These data therefore identify PKP2 and DP as central players in coordination of desmosome-dependent TGF-β1/p38 MAPK signaling in cardiomyocytes, pathways known to play a role in different types of cardiac disease, such as arrhythmogenic or hypertrophic cardiomyopathy.

E-cadherin and Src associate with extradesmosomal Dsg3 and modulate desmosome assembly and adhesion

Desmosomes provide strong intercellular cohesion essential for the integrity of cells and tissues exposed to continuous mechanical stress. For desmosome assembly, constitutively synthesized desmosomal cadherins translocate to the cell-cell border, cluster and mature in the presence of Ca(2+) to stable cell contacts. As adherens junctions precede the formation of desmosomes, we investigated in this study the relationship between the classical cadherin E-cadherin and the desmosomal cadherin Desmoglein 3 (Dsg3), the latter of which is indispensable for cell-cell adhesion in keratinocytes. By using autoantibodies from patients with the blistering skin disease pemphigus vulgaris (PV), we showed in loss of function studies that E-cadherin compensates for effects of desmosomal disassembly. Overexpression of E-cadherin reduced the loss of cell cohesion induced by PV autoantibodies and attenuated activation of p38 MAPK. Silencing of E-cadherin abolished the localization of Dsg3 at the membrane and resulted in a shift of Dsg3 from the cytoskeletal to the non-cytoskeletal protein pool which conforms to the notion that E-cadherin regulates desmosome assembly. Mechanistically, we identified a complex consisting of extradesmosomal Dsg3, E-cadherin, β-catenin and Src and that the stability of this complex is regulated by Src. Moreover, Dsg3 and E-cadherin are phosphorylated on tyrosine residues in a Src-dependent manner and Src activity is required for recruiting Dsg3 to the cytoskeletal pool as well as for desmosome maturation towards a Ca(2+)-insensitive state. Our data provide new insights into the role of E-cadherin and the contribution of Src signaling for formation and maintenance of desmosomal junctions.

Cell cycle- and cancer-associated gene networks activated by Dsg2: evidence of cystatin A deregulation and a potential role in cell-cell adhesion

Cell-cell adhesion is paramount in providing and maintaining multicellular structure and signal transmission between cells. In the skin, disruption to desmosomal regulated intercellular connectivity may lead to disorders of keratinization and hyperproliferative disease including cancer. Recently we showed transgenic mice overexpressing desmoglein 2 (Dsg2) in the epidermis develop hyperplasia. Following microarray and gene network analysis, we demonstrate that Dsg2 caused a profound change in the transcriptome of keratinocytes in vivo and altered a number of genes important in epithelial dysplasia including: calcium-binding proteins (S100A8 and S100A9), members of the cyclin protein family, and the cysteine protease inhibitor cystatin A (CSTA). CSTA is deregulated in several skin cancers, including squamous cell carcinomas (SCC) and loss of function mutations lead to recessive skin fragility disorders. The microarray results were confirmed by qPCR, immunoblotting, and immunohistochemistry. CSTA was detected at high level throughout the newborn mouse epidermis but dramatically decreased with development and was detected predominantly in the differentiated layers. In human keratinocytes, knockdown of Dsg2 by siRNA or shRNA reduced CSTA expression. Furthermore, siRNA knockdown of CSTA resulted in cytoplasmic localization of Dsg2, perturbed cytokeratin 14 staining and reduced levels of desmoplakin in response to mechanical stretching. Both knockdown of either Dsg2 or CSTA induced loss of cell adhesion in a dispase-based assay and the effect was synergistic. Our findings here offer a novel pathway of CSTA regulation involving Dsg2 and a potential crosstalk between Dsg2 and CSTA that modulates cell adhesion. These results further support the recent human genetic findings that loss of function mutations in the CSTA gene result in skin fragility due to impaired cell-cell adhesion: autosomal-recessive exfoliative ichthyosis or acral peeling skin syndrome.

Atomic force microscopy identifies regions of distinct desmoglein 3 adhesive properties on living keratinocytes

Desmosomes provide strong cell-cell adhesion which is crucial for the integrity of tissues such as the epidermis. However, nothing is known about the distribution and binding properties of desmosomal adhesion molecules on keratinocytes. Here we used atomic force microscopy (AFM) to simultaneously visualize the topography of living human keratinocytes and the distribution and binding properties of the desmosomal adhesion molecule desmoglein 3 (Dsg3). Using recombinant Dsg3 as sensor, binding events were detectable diffusely and in clusters on the cell surface and at areas of cell-cell contact. This was blocked by removing Ca(2+) and by addition of Dsg3-specific antibodies indicating homophilic Dsg3 binding. Binding forces of Dsg3 molecules were lower on the cell surface compared to areas of cell-cell contact. Our data for the first time directly demonstrate the occurrence of Dsg3 molecules outside of desmosomes and show that Dsg3 adhesive properties differ depending on their localization. From the clinical editor: Using atomic force microscopy in the study of keratinocytes, this study directly demonstrates the occurrence of desmoglein 3 molecules outside of desmosomes and reveales that the adhesive properties of these molecules do differ depending on their localization.

Remodeling of keratin-coupled cell adhesion complexes

Epithelial cells constitute the main barrier between the inside and outside of organs, acting as gatekeepers of their structure and integrity. Hemidesmosomes and desmosomes are respectively cell-matrix and cell-cell adhesions coupled to the intermediate filament cytoskeleton. These adhesions ensure mechanical integrity of the epithelial barrier. Although desmosomes and hemidesmosomes are essential in maintaining strong cell-cell and cell-matrix adhesions, there is an emerging view that they should be remodeled in order to maintain epithelial homeostasis. Here we review the adhesion properties of desmosomes and hemidesmosomes, as well as the mechanisms driving their remodeling. We also discuss recent data suggesting that keratin-coupled adhesion complexes can sense the biomechanical cellular environment and participate in the cellular response to such external cues.