The expression of desmoglein isoforms in cultured human keratinocytes is regulated by calcium, serum, and protein kinase C

Desmoglein-2 and COVID-19 complications: insights into its role as a biomarker, pathogenesis and clinical implications

Desmoglein-2 (DSG2) has emerged as a potential biomarker for coronavirus disease 2019 (COVID-19) complications, particularly cardiac and cardiovascular involvement. The expression of DSG2 in lung tissues has been detected at elevated levels, and circulating DSG2 levels correlate with COVID-19 severity. DSG2 may contribute to myocardial injury, cardiac dysfunction and vascular endothelial dysfunction in COVID-19. Monitoring DSG2 levels could aid in risk stratification, early detection and prognostication of COVID-19 complications. However, further research is required to validate DSG2 as a biomarker. Such research will aim to elucidate its precise role in pathogenesis, establishing standardized assays for its measurement and possibly identifying therapeutic targets.

What protein kinases are crucial for acantholysis and blister formation in pemphigus vulgaris? A systematic review

Pemphigus vulgaris (PV) is a potentially fatal autoimmune blistering disease characterized by cell-cell detachment (or acantholysis) and blister formation. While the signaling mechanisms that associate with skin/mucosal blistering are being elucidated, specific treatment strategies targeting PV-specific pathomechanisms, particularly kinase signaling, have yet to be established. Hence, the aim of this review was to systematically evaluate molecules in the class of kinases that are essential for acantholysis and blister formation and are therefore candidates for targeted therapy. English articles from PubMed and Scopus databases were searched, and included in vitro, in vivo, and human studies that investigated the role of kinases in PV. We selected studies, extracted data and assessed risk of bias in duplicates and the results were reported according to the methodology outlined by the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA). The risk of bias assessment was performed on in vivo studies utilizing SYRCLE's risk of bias tool. Thirty-five studies were included that satisfied the pathogenicity criterion of kinases in PV, the vast majority being experimental models that used PV sera (n = 13) and PV-IgG (n = 22). Inhibition of kinase activity (p38MAPK, PKC, TK, c-Src, EGFR, ERK, mTOR, BTK, and CDK2) was achieved mostly by pharmacological means. Overall, we found substantial evidence that kinase inhibition reduced PV-associated phosphorylation events and keratinocyte disassociation, prevented acantholysis, and blocked blister formation. However, the scarce adherence to standardized reporting systems and the experimental protocols/models used did limit the internal and external validity of these studies. In summary, this systematic review highlighted the pathogenic intracellular events mediated by kinases in PV acantholysis and presented kinase signaling as a promising avenue for translational research. In particular, the molecules identified and discussed in this study represent potential candidates for the development of mechanism-based interventions in PV.

Characterisation of the novel spontaneously immortalized and invasively growing human skin keratinocyte line HaSKpw

We here present the spontaneously immortalised cell line, HaSKpw, as a novel model for the multistep process of skin carcinogenesis. HaSKpw cells were established from the epidermis of normal human adult skin that, without crisis, are now growing unrestricted and feeder-independent. At passage 22, clonal populations were established and clone7 (HaSKpwC7) was further compared to the also spontaneously immortalized HaCaT cells. As important differences, the HaSKpw cells express wild-type p53, remain pseudodiploid, and show a unique chromosomal profile with numerous complex aberrations involving chromosome 20. In addition, HaSKpw cells overexpress a pattern of genes and miRNAs such as KRT34, LOX, S100A9, miR21, and miR155; all pointing to a tumorigenic status. In concordance, HaSKpw cells exhibit reduced desmosomal contacts that provide them with increased motility and a highly migratory/invasive phenotype as demonstrated in scratch- and Boyden chamber assays. In 3D organotypic cultures, both HaCaT and HaSKpw cells form disorganized epithelia but only the HaSKpw cells show tumorcell-like invasive growth. Together, HaSKpwC7 and HaCaT cells represent two spontaneous (non-genetically engineered) “premalignant” keratinocyte lines from adult human skin that display different stages of the multistep process of skin carcinogenesis and thus represent unique models for analysing skin cancer development and progression.

Proteomic analysis of desmosomes reveals novel components required for epidermal integrity

Desmosomes are cell–cell adhesions necessary for the maintenance of tissue integrity in the skin and heart. While the core components of desmosomes have been identified, peripheral components that modulate canonical or noncanonical desmosome functions still remain largely unexplored. Here we used targeted proximity labeling approaches to further elaborate the desmosome proteome in epidermal keratinocytes. Quantitative mass spectrometry analysis identified all core desmosomal proteins while uncovering a diverse array of new constituents with broad molecular functions. By individually targeting the inner and outer dense plaques, we defined proteins enriched within these subcompartments. We validated a number of these novel desmosome-associated proteins and find that many are membrane proximal proteins that show a dependence on functional desmosomes for their cortical localization. We further explored the mechanism of localization and function of two novel desmosome-associated adaptor proteins enriched in the desmosome proteome, Crk and Crk-like (CrkL). These proteins interacted with Dsg1 and rely on Dsg1 and desmoplakin for robust cortical localization. Epidermal deletion of both Crk and CrkL resulted in perinatal lethality with defects in desmosome morphology and keratin organization, thus demonstrating the utility of this dataset in identifying novel proteins required for desmosome-dependent epidermal integrity.

Single-Cell Analysis Suggests that Ongoing Affinity Maturation Drives the Emergence of Pemphigus Vulgaris Autoimmune Disease

Pemphigus vulgaris (PV) is an autoimmune disease characterized by blistering sores on skin and mucosal membranes, caused by autoantibodies primarily targeting the cellular adhesion protein, desmoglein-3 (Dsg3). To better understand how Dsg3-specific autoantibodies develop and cause disease in humans, we performed a cross-sectional study of PV patients before and after treatment to track relevant cellular responses underlying disease pathogenesis, and we provide an in-depth analysis of two patients by generating a panel of mAbs from single Dsg3-specific memory B cells (MBCs). Additionally, we analyzed a paired sample from one patient collected 15-months prior to disease diagnosis. We find that Dsg3-specific MBCs have an activated phenotype and show signs of ongoing affinity maturation and clonal selection. Monoclonal antibodies (mAbs) with pathogenic activity primarily target epitopes in the extracellular domains EC1 and EC2 of Dsg3, though they can also bind to the EC4 domain. Combining antibodies targeting different epitopes synergistically enhances in vitro pathogenicity.

Atomic Force Microscopy Provides New Mechanistic Insights into the Pathogenesis of Pemphigus

Autoantibodies binding to the extracellular domains of desmoglein (Dsg) 3 and 1 are critical in the pathogenesis of pemphigus by mechanisms leading to impaired function of desmosomes and blister formation in the epidermis and mucous membranes. Desmosomes are highly organized protein complexes which provide strong intercellular adhesion. Desmosomal cadherins such as Dsgs, proteins of the cadherin superfamily which interact via their extracellular domains in Ca2+-dependent manner, are the transmembrane adhesion molecules clustered within desmosomes. Investigations on pemphigus cover a wide range of experimental approaches including biophysical methods. Especially atomic force microscopy (AFM) has recently been applied increasingly because it allows the analysis of native materials such as cultured cells and tissues under near-physiological conditions. AFM provides information about the mechanical properties of the sample together with detailed interaction analyses of adhesion molecules. With AFM, it was recently demonstrated that autoantibodies directly inhibit Dsg interactions on the surface of living keratinocytes, a phenomenon which has long been considered the main mechanism causing loss of cell cohesion in pemphigus. In addition, AFM allows to study how signaling pathways altered in pemphigus control binding properties of Dsgs. More general, AFM and other biophysical studies recently revealed the importance of keratin filaments for regulation of Dsg binding and keratinocyte mechanical properties. In this mini-review, we reevaluate AFM studies in pemphigus and keratinocyte research, recapitulate what is known about the interaction mechanisms of desmosomal cadherins and discuss the advantages and limitations of AFM in these regards.

c-Src/Cav1-dependent activation of the EGFR by Dsg2

The desmosomal cadherin, desmoglein 2 (Dsg2), is deregulated in a variety of human cancers including those of the skin. When ectopically expressed in the epidermis of transgenic mice, Dsg2 activates multiple mitogenic signaling pathways and increases susceptibility to tumorigenesis. However, the molecular mechanism responsible for Dsg2-mediated cellular signaling is poorly understood. Here we show overexpression as well as co-localization of Dsg2 and EGFR in cutaneous SCCs in vivo. Using HaCaT keratinocytes, knockdown of Dsg2 decreases EGFR expression and abrogates the activation of EGFR, c-Src and Stat3, but not Erk1/2 or Akt, in response to EGF ligand stimulation. To determine whether Dsg2 mediates signaling through lipid microdomains, sucrose density fractionation illustrated that Dsg2 is recruited to and displaces Cav1, EGFR and c-Src from light density lipid raft fractions. STED imaging confirmed that the presence of Dsg2 disperses Cav1 from the cell-cell borders. Perturbation of lipid rafts with the cholesterol-chelating agent MβCD also shifts Cav1, c-Src and EGFR out of the rafts and activates signaling pathways. Functionally, overexpression of Dsg2 in human SCC A431 cells enhances EGFR activation and increases cell proliferation and migration through a c-Src and EGFR dependent manner. In summary, our data suggest that Dsg2 stimulates cell growth and migration by positively regulating EGFR level and signaling through a c-Src and Cav1-dependent mechanism using lipid rafts as signal modulatory platforms.

The interaction between Staphylococcus aureus SdrD and desmoglein 1 is important for adhesion to host cells

Staphylococcus aureus is known as a frequent colonizer of the skin and mucosa. Among bacterial factors involved in colonization are adhesins such as the microbial surface components recognizing adhesive matrix molecules (MSCRAMMs). Serine aspartate repeat containing protein D (SdrD) is involved in adhesion to human squamous cells isolated from the nose. Here, we identify Desmoglein 1 (Dsg1) as a novel interaction partner for SdrD. Genetic deletion of sdrD in S. aureus NCTC8325-4 through allelic replacement resulted in decreased bacterial adherence to Dsg1- expressing HaCaT cells in vitro. Complementary gain-of-function was demonstrated by heterologous expression of SdrD in Lactococcus lactis, which increased adherence to HaCaT cells. Also ectopic expression of Dsg1 in HEK293 cells resulted in increased adherence of S. aureus NCTC8325-4 in vitro. Increased adherence of NCTC8325-4, compared to NCTC8325-4ΔsdrD, to the recombinant immobilized Dsg1 demonstrated direct interaction between SdrD and Dsg1. Specificity of SdrD interaction with Dsg1 was further verified using flow cytometry and confirmed binding of recombinant SdrD to HaCaT cells expressing Dsg1 on their surface. These data demonstrate that Dsg1 is a host ligand for SdrD.

In Vitro Model of the Epidermis: Connecting Protein Function to 3D Structure

Much of our understanding of the biological processes that underlie cellular functions in humans, such as cell-cell communication, intracellular signaling, and transcriptional and posttranscriptional control of gene expression, has been acquired from studying cells in a two-dimensional (2D) tissue culture environment. However, it has become increasingly evident that the 2D environment does not support certain cell functions. The need for more physiologically relevant models prompted the development of three-dimensional (3D) cultures of epithelial, endothelial, and neuronal tissues (Shamir & Ewald, 2014). These models afford investigators with powerful tools to study the contribution of spatial organization, often in the context of relevant extracellular matrix and stromal components, to cellular and tissue homeostasis in normal and disease states.

Desmosomes: regulators of cellular signaling and adhesion in epidermal health and disease

Desmosomes are intercellular junctions that mediate cell-cell adhesion and anchor the intermediate filament network to the plasma membrane, providing mechanical resilience to tissues such as the epidermis and heart. In addition to their critical roles in adhesion, desmosomal proteins are emerging as mediators of cell signaling important for proper cell and tissue functions. In this review we highlight what is known about desmosomal proteins regulating adhesion and signaling in healthy skin-in morphogenesis, differentiation and homeostasis, wound healing, and protection against environmental damage. We also discuss how human diseases that target desmosome molecules directly or interfere indirectly with these mechanical and signaling functions to contribute to pathogenesis.

Senescent cancer-associated fibroblasts secrete active MMP-2 that promotes keratinocyte dis-cohesion and invasion

Background:

Previous studies have demonstrated that senescent cancer-associated fibroblasts (CAFs) derived from genetically unstable oral squamous cell carcinomas (GU-OSCC), unlike non-senescent CAFs from genetically stable carcinomas (GS-OSCC), promoted keratinocyte invasion in vitro in a paracrine manner. The mechanism by which this occurs is unclear.

Methods:

Previous work to characterise the senescent-associated secretory phenotype (SASP) has used antibody arrays, technology that is limited by the availability of suitable antibodies. To extend this work in an unbiased manner, we used 2D gel electrophoresis and mass spectroscopy for protein identification. Matrix metalloproteinases (MMPs) were investigated by gelatin zymography and western blotting. Neutralising antibodies were used to block key molecules in the functional assays of keratinocyte adhesion and invasion.

Results:

Among a variety of proteins that were differentially expressed between CAFs from GU-OSCC and GS-OSCC, MMP-2 was a major constituent of senescent CAF-CM derived from GU-OSCC. The presence of active MMP-2 was confirmed by gelatine zymography. MMP-2 derived from senescent CAF-CM induced keratinocyte dis-cohesion and epithelial invasion into collagen gels in a TGF-β-dependent manner.

Conclusions:

Senescent CAFs from GU-OSCC promote a more aggressive oral cancer phenotype by production of active MMP-2, disruption of epithelial adhesion and induction of keratinocyte invasion.

Activation of the δ-opioid receptor promotes cutaneous wound healing by affecting keratinocyte intercellular adhesion and migration

BACKGROUND AND PURPOSE

In addition to its analgesic functions, the peripheral opioid receptor system affects skin homeostasis by influencing cell differentiation, migration and adhesion; also, wound healing is altered in δ-opioid receptor knockout mice (DOPr^–/–). Hence, we investigated δ-opioid receptor effects on the expression of several proteins of the desmosomal junction complex and on the migratory behaviour of keratinocytes.

EXPERIMENTAL APPROACH

Expression levels of desmosomal cadherins in wild-type and DOPr^–/– mice, and the morphology of intercellular adhesion in human keratinocytes were analysed by immunofluorescence. To investigate the δ-opioid receptor activation pathway, protein expression was studied using Western blot and its effect on cellular migration determined by in vitro live cell migration recordings from human keratinocytes.

KEY RESULTS

Expression of the desmosomal cadherins, desmogleins 1 and 4, was up-regulated in skin from DOPr^–/– mice, and down-regulated in δ-opioid receptor-overexpressing human keratinocytes. The localization of desmoplakin expression was rearranged from linear arrays emanating from cell borders to puncta in cell periphery, resulting in less stable intercellular adhesion. Migration and wound recovery were enhanced in human keratinocyte monolayers overexpressing δ-opioid receptors in vitro. These δ-opioid receptor effects were antagonized by specific PKCα/β inhibition indicating they were mediated through the PKC signalling pathway. Finally, cells overexpressing δ-opioid receptors developed characteristically long but undirected protrusions containing filamentous actin and δ-opioid receptors, indicating an enhanced migratory phenotype.

CONCLUSION AND IMPLICATIONS

Opioid receptors affect intercellular adhesion and wound healing mechanisms, underlining the importance of a cutaneous neuroendocrine system in wound healing and skin homeostasis.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2

Structural and functional diversity of desmosomes

Desmosomes anchor intermediate filaments at sites of cell contact established by the interaction of cadherins extending from opposing cells. The incorporation of cadherins, catenin adaptors, and cytoskeletal elements resembles the closely related adherens junction. However, the recruitment of intermediate filaments distinguishes desmosomes and imparts a unique function. By linking the load-bearing intermediate filaments of neighboring cells, desmosomes create mechanically contiguous cell sheets and, in so doing, confer structural integrity to the tissues they populate. This trait and a well-established role in human disease have long captured the attention of cell biologists, as evidenced by a publication record dating back to the mid-1860s. Likewise, emerging data implicating the desmosome in signaling events pertinent to organismal development, carcinogenesis, and genetic disorders will secure a prominent role for desmosomes in future biological and biomedical investigations.

The Th2 cytokine, interleukin-4, abrogates the cohesion of normal stratum corneum in mice: implications for pathogenesis of atopic dermatitis

There is mounting evidence that Th2 cytokines adversely affect skin barrier functions and contribute to the pathogenesis of atopic dermatitis (AD). AD is also characterized by abnormal cohesion in the stratum corneum (SC). However, the contribution of Th2 cytokines to this abnormality remains unknown. This study examined the effects of IL-4, a prototypic Th2 cytokine, on the cohesion of the SC. Structural and physiological assessments revealed that repeated intradermal injections of IL-4 compromised the cohesion of the SC of normal hairless mice. Two potential mechanisms were explored to account for the altered cohesion. First, IL-4 decreased the amount of corneodesmosomes and down-regulated the expression of desmoglein 1, but not of corneodesmosin (CDSN) or loricrin expression, in murine skin and in cultured human keratinocytes (KC). IL-4 did not affect the skin surface pH, and in situ zymography revealed no net change in total serine protease activity in the IL-4-treated SC. Yet, IL-4 enhanced expression of kallikrein (KLK)7, while simultaneously down-regulating KLK5 and KLK14. Finally, IL-4 did not alter the expression of the lympho-epithelial Kazal-type inhibitor (LEKTI) in KC. This study suggests that IL-4 abrogates the cohesion of SC primarily by reducing epidermal differentiation.

The molecular composition and function of desmosomes

Desmosomes are intercellular adhesive junctions that are particularly prominent in tissues experiencing mechanical stress, such as the heart and epidermis. Whereas the related adherens junction links actin to calcium-dependent adhesion molecules known as classical cadherins, desmosomes link intermediate filaments (IF) to the related subfamily of desmosomal cadherins. By tethering these stress-bearing cytoskeletal filaments to the plasma membrane, desmosomes serve as integrators of the IF cytoskeleton throughout a tissue. Recent evidence suggests that IF attachment in turn strengthens desmosomal adhesion. This collaborative arrangement results in formation of a supracellular network, which is critical for imparting mechanical integrity to tissues. Diseases and animal models targeting desmosomal components highlight the importance of desmosomes in development and tissue integrity, while the downregulation of individual protein components in cancer metastasis and wound healing suggests their importance in cell homeostasis. This chapter will provide an update on desmosome composition, function, and regulation, and will also discuss recent work which raises the possibility that desmosome proteins do more than play a structural role in tissues where they reside.

Retinoid Induces the Degradation of Corneodesmosomes and Downregulation of Corneodesmosomal Cadherins: Implications on the Mechanism of Retinoid-induced Desquamation

BACKGROUND

Topical retinoids induce skin fragility. As corneodesmosomes are important adhesion structures in the epidermal cohesion, an effect of retinoids on corneodesmosomes has been suspected.

OBJECTIVE

The aim of this study was to investigate the effect of retinoid on the expression of corneodesmosomal components including desmoglein (DSG) 1, desmocollin (DSC) 1, corneodesmosin (CDSN) and kallikrein (KLK)s.

METHODS

2% all-trans-retinol or ethanol was applied to the back of hairless mice for five days, and the structure of the stratum corneum was examined by transmission electron microscopy. The cultured human keratinocytes were treated with all-trans-retinoic acid (RA) in low or high calcium media for 24 hours.

RESULTS

Topical retinol increased corneocyte detachment and degradation of corneodesmosomes. RA significantly decreased DSG1 and DSC1 expression at the mRNA and protein levels in keratinocytes that were cultured in both low- and high-calcium media. On the other hand, CDSN mRNA levels did not decrease in low-calcium media or increase in high-calcium media after RA treatment. KLK5 and KLK7 expression did not increase after RA treatment.

CONCLUSION

Our results indicate that DSG1 and DSC1 downregulation by RA could be related to the increased degradation of corneodesmosomes and consequent desquamation induced by retinoids.

Inflammatory skin and bowel disease linked to ADAM17 deletion

We performed genetic and immunohistochemical studies in a sister and brother with autosomal recessive neonatal inflammatory skin and bowel lesions. The girl died suddenly at 12 years of age from parvovirus B19-associated myocarditis; her brother had mild cardiomyopathy. We identified a loss-of-function mutation in ADAM17, which encodes a disintegrin and metalloproteinase 17 (also called tumor necrosis factor α [TNF-α]-converting enzyme, or TACE), as the probable cause of this syndrome. Peripheral-blood mononuclear cells (PBMCs) obtained from the brother at 17 years of age showed high levels of lipopolysaccharide-induced production of interleukin-1β and interleukin-6 but impaired release of TNF-α. Despite repeated skin infections, this young man has led a relatively normal life. (Funded by Barts and the London Charity and the European Commission Seventh Framework Programme.).

RNAi-mediated inhibition of the desmosomal cadherin (desmoglein 3) impairs epithelial cell proliferation

OBJECTIVES

  Desmoglein 3 (Dsg3) is a desmosomal adhesion protein expressed in basal and immediate suprabasal layers of skin. Importance of Dsg3 in cell-cell adhesion and maintenance of tissue integrity is illustrated by findings of keratinocyte dissociation in the autoimmune disease, pemphigus vulgaris, where autoantibodies target Dsg3 on keratinocyte surfaces and cause Dsg3 depletion from desmosomes. However, recognition of possible participation of involvement of Dsg3 in cell proliferation remains controversial. Currently, available evidence suggests that Dsg3 may have both anti- and pro-proliferative roles in keratinocytes. The aim of this study was to use RNA interference (RNAi) strategy to investigate effects of silencing Dsg3 in cell-cell adhesion and cell proliferation in two cell lines, HaCaT and MDCK.

MATERIALS AND METHODS

  Cells were transfected with siRNA, and knockdown of Dsg3 was assessed by western blotting, fluorescence-activated cell sorting and confocal microscopy. Cell-cell adhesion was analysed using the hanging drop/fragmentation assay, and cell proliferation by colony forming efficiency, BrdU incorporation, cell counts and organotypic culture.

RESULTS

  Silencing Dsg3 caused defects in cell-cell adhesion and concomitant reduction in cell proliferation in both HaCaT and MDCK cells.

CONCLUSION

  These findings suggest that Dsg3 depletion by RNAi reduces cell proliferation, which is likely to be secondary to a defect in cell-cell adhesion, an essential function required for cell differentiation and morphogenesis.

The extent of desmoglein 3 depletion in pemphigus vulgaris is dependent on Ca(2+)-induced differentiation: a role in suprabasal epidermal skin splitting?

Pemphigus vulgaris (PV) is an autoimmune disease of the skin and mucous membranes and is characterized by development of autoantibodies against the desmosomal cadherins desmoglein (Dsg) 3 and Dsg1 and formation of intraepidermal suprabasal blisters. Depletion of Dsg3 is a critical mechanism in PV pathogenesis. Because we did not detect reduced Dsg3 levels in keratinocytes cultured for longer periods under high-Ca(2+) conditions, we hypothesized that Dsg depletion depends on Ca(2+)-mediated keratinocyte differentiation. Our data indicate that depletion of Dsg3 occurs specifically in deep epidermal layers both in skin of patients with PV and in an organotypic raft model of human epidermis incubated using IgG fractions from patients with PV. In addition, Dsg3 depletion and loss of Dsg3 staining were prominent in cultured primary keratinocytes and in HaCaT cells incubated in high-Ca(2+) medium for 3 days, but were less pronounced in HaCaT cultures after 8 days. These effects were dependent on protein kinase C signaling because inhibition of protein kinase C blunted both Dsg3 depletion and loss of intercellular adhesion. Moreover, protein kinase C inhibition blocked suprabasal Dsg3 depletion in cultured human epidermis and blister formation in a neonatal mouse model. Considered together, our data indicate a contribution of Dsg depletion to PV pathogenesis dependent on Ca(2+)-induced differentiation. Furthermore, prominent depletion in basal epidermal layers may contribute to the suprabasal cleavage plane observed in PV.

A role for caveolin-1 in desmoglein binding and desmosome dynamics

Desmoglein-2 (Dsg2) is a desmosomal cadherin that is aberrantly expressed in human skin carcinomas. In addition to its well-known role in mediating intercellular desmosomal adhesion, Dsg2 regulates mitogenic signaling that may promote cancer development and progression. However, the mechanisms by which Dsg2 activates these signaling pathways and the relative contribution of its signaling and adhesion functions in tumor progression are poorly understood. In this study we show that Dsg2 associates with caveolin-1 (Cav-1), the major protein of specialized membrane microdomains called caveolae, which functions in both membrane protein turnover and intracellular signaling. Sequence analysis revealed that Dsg2 contains a putative Cav-1-binding motif. A permeable competing peptide resembling the Cav-1 scaffolding domain bound to Dsg2, disrupted normal Dsg2 staining and interfered with the integrity of epithelial sheets in vitro. Additionally, we observed that Dsg2 is proteolytically processed; resulting in a 95-kDa ectodomain shed product and a 65-kDa membrane-spanning fragment, the latter of which localizes to lipid rafts along with full-length Dsg2. Disruption of lipid rafts shifted Dsg2 to the non-raft fractions, leading to the accumulation of these proteins. Interestingly, Dsg2 proteolytic products are elevated in vivo in skin tumors from transgenic mice overexpressing Dsg2. Collectively, these data are consistent with the possibility that accumulation of truncated Dsg2 protein interferes with desmosome assembly and/or maintenance to disrupt cell-cell adhesion. Furthermore, the association of Dsg2 with Cav-1 may provide a mechanism for regulating mitogenic signaling and modulating the cell-surface presentation of an important adhesion molecule, both of which could contribute to malignant transformation and tumor progression.

Kallikrein-5 promotes cleavage of desmoglein-1 and loss of cell-cell cohesion in oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) ranks among the top 8 causes of cancer death worldwide, with only a 60% 5-year survival rate, highlighting the need for discovery of novel biomarkers and therapeutic targets. We have previously reported that expression of a panel of serine proteinase kallikreins (KLK 5, 7, 8, and 10) is correlated with formation of more aggressive OSCC tumors in a murine orthotopic OSCC model and is elevated in human OSCC. Current studies focus on understanding the potential role of KLK5 in OSCC progression. In initial studies, KLK levels in malignant OSCC cells (SCC25) were compared with cells from normal oral mucosa (OKF/6) and pre-malignant oral keratinocytes (pp126) using qPCR. A marked elevation of all KLKs was observed in aggressive SCC25 cells relative to OKF/6 cells. In normal skin, KLKs are involved in desquamation during epidermal differentiation via proteolytic cleavage of the desmosomal cadherin component desmoglein 1 (Dsg1). As loss of cell-cell cohesion is prevalent in tumor metastasis, Dsg1 integrity was evaluated. Results show that SCC25 cells exhibit cleavage of Dsg1, which is blocked by proteinase inhibitor treatment as well as by siRNA silencing of KLK5 expression. Furthermore, cell-cell aggregation assays demonstrate that silencing of KLK5 enforces cell-cell adhesion; conversely, overexpression of KLK5 in normal oral mucosal cells (OKF/6) enhances cell dispersal. These data suggest that KLK5 may promote metastatic dissemination of OSCC by promoting loss of junctional integrity through cleavage of desmoglein 1.

Desmosomes in the testis: Moving into an unchartered territory

Desmosomes are cell-cell junctions that link to cytoplasmic intermediate filaments, and they are known to mediate robust and stable adhesion in organs such as the skin and heart. Desmosomes are also present between apposing Sertoli cells at the blood-testis barrier, and between Sertoli cells and all germ cells up to, but not including, step 8 spermatids in the seminiferous epithelium. Unfortunately, they remain to be one of the least studied cell junction types in the seminiferous epithelium of the mammalian testis. In this article, we briefly discuss how kinases and the actin cytoskeleton relate to the study of desmosomes in the testis. It is hoped that this information is used to initiate more studies on the biology of the desmosome in the future.

Membrane-impermeable cross-linking provides evidence for homophilic, isoform-specific binding of desmosomal cadherins in epithelial cells

Desmosomes and adherens junctions are cadherin-based protein complexes responsible for cell-cell adhesion of epithelial cells. Type 1 cadherins of adherens junctions show specific homophilic adhesion that plays a major role in developmental tissue segregation. The desmosomal cadherins, desmocollin and desmoglein, occur as several different isoforms with overlapping expression in some tissues where different isoforms are located in the same desmosomes. Although adhesive binding of desmosomal cadherins has been investigated in a variety of ways, their interaction in desmosome-forming epithelial cells has not been studied. Here, using extracellular homobifunctional cross-linking, we provide evidence for homophilic and isoform-specific binding between the Dsc2, Dsc3, Dsg2, and Dsg3 isoforms in HaCaT keratinocytes and show that it represents trans interaction. Furthermore, the cross-linked adducts are present in the detergent-insoluble fraction, and electron microscopy shows that extracellular cross-linking probably occurs in desmosomes. We found no evidence for either heterophilic or cis interaction, but neither can be completely excluded by our data. Mutation of amino acid residues Trp-2 and Ala-80 that are important for trans interaction in classical cadherin adhesive binding abolished Dsc2 binding, indicating that these residues are also involved in desmosomal adhesion. These interactions of desmosomal cadherins may be of key importance for their ordered arrangement within desmosomes that we believe is essential for desmosomal adhesive strength and the maintenance of tissue integrity.

Experimental human cell and tissue models of pemphigus

Pemphigus is a chronic mucocutaneous autoimmune bullous disease that is characterized by loss of cell-cell contact in skin and/or mucous membranes. Past research has successfully identified desmosomes as immunological targets and has demonstrated that acantholysis is initiated through direct binding of IgG. The exact mechanisms of acantholysis, however, are still missing. Experimental model systems have contributed considerably to today's knowledge and are still a favourite tool of research. In this paper we will describe to what extent human cell and tissue models represent the in vivo situation, for example, organ cultures of human skin, keratinocyte cultures, and human skin grafted on mice and, furthermore, how suitable they are to study the pathogenesis of pemphigus. Organ cultures closely mimic the architecture of the epidermis but are less suitable to answer posed biochemical questions. Cultured keratinocyte monolayers are convenient in this respect, but their desmosomal make-up in terms of adhesion molecules does not exactly reflect the in vivo situation. Reconstituted skin is a relatively new model that approaches organ culture. In models of human skin grafted on mice, acantholysis can be studied in actual human skin but now with all the advantages of an animal model.

Human embryonic stem cell-derived keratinocytes exhibit an epidermal transcription program and undergo epithelial morphogenesis in engineered tissue constructs

Human embryonic stem (hES) cells are an attractive source of cellular material for scientific, diagnostic, and potential therapeutic applications. Protocols are now available to direct hES cell differentiation to specific lineages at high purity under relatively defined conditions; however, researchers must establish the functional similarity of hES cell derivatives and associated primary cell types to validate their utility. Using retinoic acid to initiate differentiation, we generated high-purity populations of keratin 14+ (K14) hES cell-derived keratinocyte (hEK) progenitors and performed microarray analysis to compare the global transcriptional program of hEKs and primary foreskin keratinocytes. Transcriptional patterns were largely similar, though gene ontology analysis identified that genes associated with signal transduction and extracellular matrix were upregulated in hEKs. In addition, we evaluated the ability of hEKs to detect and respond to environmental stimuli such as Ca(2+), serum, and culture at the air-liquid interface. When cultivated on dermal constructs formed with collagen gels and human dermal fibroblasts, hEKs survived and proliferated for 3 weeks in engineered tissue constructs. Maintenance at the air-liquid interface induced stratification of surface epithelium, and immunohistochemistry results indicated that markers of differentiation (e.g., keratin 10, involucrin, and filaggrin) were localized to suprabasal layers. Although the overall tissue morphology was significantly different compared with human skin samples, organotypic cultures generated with hEKs and primary foreskin keratinocytes were quite similar, suggesting these cell types respond to this microenvironment in a similar manner. These results represent an important step in characterizing the functional similarity of hEKs to primary epithelia.

RNA interference in keratinocytes and an organotypic model of human epidermis

Gene silencing approaches afford investigators the ability to gain important insight into the normal functional requirements of specific epidermal proteins and promise to yield a powerful therapeutic means to dampen the level of proteins that are mutated or frequently overexpressed in skin disease. The efficient and tractable delivery of siRNAs into epidermal keratinocytes is seminal to this process. Here, we describe techniques for transient and long-term silencing of a representative gene product, namely desmoglein 1, in primary human epidermal keratinocytes maintained as submerged cultures or three-dimensional organotypic raft cultures. As a complement to epidermal-specific gene targeting strategies in mice, these technical approaches permit relatively rapid loss-of-function studies purely in keratinocytes without some of the potential influences present in situ, such as an immune system or vasculature.

Desmoglein 4 is regulated by transcription factors implicated in hair shaft differentiation

The hair fiber is made of specialized keratinocytes, known as trichocytes, that primarily express hair keratins, which are cemented by a multitude of keratin-associated proteins (KAPs). The hair keratins form the intermediate filament cytoskeleton of the trichocytes, which are linked to abundant cell-cell adhesion junctions, called desmosomes. Desmoglein 4 (DSG4) is the major desmosomal cadherin expressed in the hair shaft cortex where the hair keratins are highly expressed. In humans, mutations affecting either the hair keratins or DSG4 lead to beaded hair phenotypes with features of monilethrix. In this work, we postulated that the regulatory pathways governing the expression of hair shaft components, such as hair keratins and DSG4, are shared. Therefore, we studied the transcriptional regulation of DSG4 by transcription factors/pathways that are known regulators of hair keratin or KAP expression. We show that HOXC13, LEF1 and FOXN1 repress DSG4 transcription and provide in vitro and in vivo evidence correlating the Notch pathway with the activation and/or maintenance of DSG4 expression in the hair follicle.

Increased expression of Dsg2 in malignant skin carcinomas: A tissue-microarray based study

Desmoglein 2 (Dsg2), a transmembrane cadherin of the desmosomal cell-cell adhesion structure, is downregulated with epithelial differentiation. We recently demonstrated that overexpression of Dsg2 in epidermal keratinocytes deregulates multiple signaling pathways associated with increased growth rate, anchorage-independent cell survival, and the development of skin tumors. While changes in Dsg2 expression have been observed in neoplastic lesions, the correlation of expression levels and localization of Dsg2 and the state of tumor development has not been fully established. Here we generated a highly sensitive Dsg2 antibody (Ab10) and characterized that antibody along with a previously developed Dsg2 specific antibody 10D2. Using these antibodies in immunostaining of tissue microarrays, we show a dramatic upregulation of Dsg2 expression in certain human epithelial malignancies including basal cell carcinomas (BCC; n = 12), squamous cell carcinomas (SCC; n = 57), carcinomas of sebaceous and sweat glands (n = 12), and adenocarcinomas (n = 3). Dsg2 expression was completely absent in malignant fibrosarcomas (n = 16) and melanomas (n = 15). While Dsg2 expression was consistently strong in BCC, it varied in SCC with a minor correlation between a decrease of Dsg2 expression and tumor differentiation. In summary, we have identified Dsg2 as a potential novel marker for epithelial-derived malignancies.

Protein kinase C isoenzymes differentially regulate the differentiation-dependent expression of adhesion molecules in human epidermal keratinocytes

Epidermal expression of adhesion molecules such as desmogleins (Dsg) and cadherins is strongly affected by the differentiation status of keratinocytes. We have previously shown that certain protein kinase C (PKC) isoforms differentially alter the growth and differentiation of human epidermal HaCaT keratinocytes. In this paper, using recombinant overexpression and RNA interference, we define the specific roles of the different PKC isoenzymes in modulation of expression of adhesion molecules in HaCaT keratinocytes. The level of Dsg1, a marker of differentiating keratinocytes, was antagonistically regulated by two Ca-independent 'novel' nPKC isoforms; i.e. it increased by the differentiation-promoting nPKCdelta and decreased by the growth-promoting nPKCepsilon. The expression of Dsg3 (highly expressed in proliferating epidermal layers) was conversely regulated by these isoenzymes, and was also inhibited by the differentiation inducer Ca-dependent 'conventional' cPKCalpha. Finally, the expression of P-cadherin (a marker of proliferating keratinocytes) was regulated by all of the examined PKCs, also in an antagonistic manner (inhibited by cPKCalpha/nPKCdelta and stimulated by cPKCbeta/nPKCepsilon). Collectively, the presented results strongly argue for the marked, differential, and in some instances antagonistic roles of individual Ca-dependent and Ca-independent PKC isoforms in the regulation of expression of adhesion molecules of desmosomes and adherent junctions in human epidermal keratinocytes.

Possible role of natural killer cells in pemphigus vulgaris - preliminary observations

Pemphigus vulgaris (PV) is an autoimmune blistering disease that affects the skin and multiple mucous membranes, and is caused by antibodies to desmoglein (Dsg) 1 and 3. Natural killer (NK) cells have a role in autoimmunity, but their role in PV is not known. NK cells in the peripheral blood leucocytes (PBL) of 15 untreated Caucasian patients with active PV were studied and compared with healthy controls for the expression of major histocompatibility complex (MHC) class II and co-stimulatory molecules. CD56+ CD16- CD3- NK or CD56+ CD16+ CD3- NK cells from the PBL of PV patients co-express MHC class II and co-stimulatory molecule B7-H3 without exogenous stimulation. CD4+ T cells from the PBL and perilesional skin of PV patients were co-cultured with CD56+ CD3- NK cells from the PBL of the same patients; in the presence of Dsg3 peptides underwent statistically significant proliferation, indicating that NK cells functioned as antigen-presenting cells. Supernatants from these co-cultures and serum of the same patients with active PV had statistically significantly elevated levels of interleukin (IL)-6, IL-8 and interferon-gamma, compared with controls indicating that the NK cells stimulated CD4+ T cells to produce proinflammatory cytokines. In these experiments, we present preliminary evidence that NK cells may play a role in the pathobiology of PV.

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

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

An integrated systems biology approach to understanding the rules of keratinocyte colony formation

Closely coupled in vitro and in virtuo models have been used to explore the self-organization of normal human keratinocytes (NHK). Although it can be observed experimentally, we lack the tools to explore many biological rules that govern NHK self-organization. An agent-based computational model was developed, based on rules derived from literature, which predicts the dynamic multicellular morphogenesis of NHK and of a keratinocyte cell line (HaCat cells) under varying extracellular Ca++ concentrations. The model enables in virtuo exploration of the relative importance of biological rules and was used to test hypotheses in virtuo which were subsequently examined in vitro. Results indicated that cell-cell and cell-substrate adhesions were critically important to NHK self-organization. In contrast, cell cycle length and the number of divisions that transit-amplifying cells could undergo proved non-critical to the final organization. Two further hypotheses, to explain the growth behaviour of HaCat cells, were explored in virtuo-an inability to differentiate and a differing sensitivity to extracellular calcium. In vitro experimentation provided some support for both hypotheses. For NHKs, the prediction was made that the position of stem cells would influence the pattern of cell migration post-wounding. This was then confirmed experimentally using a scratch wound model.

Changes in localization of human discs large (hDlg) during keratinocyte differentiation is associated with expression of alternatively spliced hDlg variants

Alternative spliced variants of the human discs large (hDlg) tumour suppressor are characterized by combinations of insertions. Here, using insertions I2- and I3-specific antibodies, we show that I2 and I3 variants have distinct distributions in epidermal and cervical epithelia. In skin and cervix, I3 variants are found in the cytoplasm. Cytoplasmic localization of I3 variants decreases as cervical keratinocytes differentiate, concomitant with relocalization to the cell periphery. I2 variants are found at the cell periphery of differentiated epidermal and cervical keratinocytes. Nuclear localization of I2 variants was evident in both tissues, with concentration of nuclear I2 variants in basal and parabasal cervical keratinocytes. A prominent nuclear localization of hDlg in cells of hyperproliferative layers of psoriatic lesions, but not in mature differentiated keratinocytes, together with I2 redistribution in differentiating keratinocytes, suggests that nuclear hDlg functions may be pertinent to growth of undifferentiated cells. Supporting our findings in squamous tissues, a decrease of nuclear hDlg and an increase of membrane-bound and cytoplasmic hDlg upon calcium-induced keratinocyte differentiation were not concomitant processes. Furthermore, we confirm that the exit of I2 variants from the nucleus is linked to stimulation of epithelial differentiation. The dynamic redistribution of hDlg also correlated with a marked increase in the expression of I3 variants while the level of I2 variants showed only a moderate decrease. Because changes in the intracellular distribution of hDlg splice variants, and in their expression levels, correlate with changes in differentiation state we hypothesize that the different hDlg isoforms play distinct roles at various stages of epithelial differentiation.

Metalloproteinase 9 is the outer executioner of desmoglein 3 in apoptotic keratinocytes

OBJECTIVE

To investigate the specific matrix metalloproteinases (MMPs) targeting desmoglein 3 (Dsg3) in apoptotic keratinocytes.

METHOD

Inhibitor studies on cultured keratinocytes and Western blot analysis.

RESULTS

Blocking of MMP-9 activity strongly reduces shedding of Dsg3 from cell surface. MMP-2 has a less relevant role in the cleavage of Dsg3 while other MMPs, such as MMP-1, -3, and -8, do not target Dsg3.

CONCLUSION

Apoptic keratinocytes impair the extracellular domain of cell surface Dsg3 by MMP-9 activity. The discovery of a specific targeting of Dsg3 could be useful to understand the pathophysiology of diseases in which Dsg3 is affected.

Suprabasal Dsg2 expression in transgenic mouse skin confers a hyperproliferative and apoptosis-resistant phenotype to keratinocytes

Desmoglein 2 (Dsg2), a component of the desmosomal cell-cell adhesion structure, has been linked to invasion and metastasis in squamous cell carcinomas. However, it is unknown whether – and if so how – Dsg2 contributes to the malignant phenotype of keratinocytes. In this study, we addressed the consequences of Dsg2 overexpression under control of the involucrin promoter (Inv-Dsg2) in the epidermis of transgenic mice. These mice exhibited epidermal hyperkeratosis with slightly disrupted early and late differentiation markers, but intact epidermal barrier function. However, Inv-Dsg2 transgene expression was associated with extensive epidermal hyperplasia and increased keratinocyte proliferation in basal and suprabasal epidermal strata. Cultured Inv-Dsg2 keratinocytes showed enhanced cell survival in the anchorage-independent state that was critically dependent on EGF receptor activation and NF-κB activity. Consistent with the hyperproliferative and apoptosis-resistant phenotype of Inv-Dsg2 transgenic keratinocytes, we observed enhanced activation of multiple growth and survival pathways, including PI 3-kinase/AKT, MEK-MAPK, STAT3 and NF-κB, in the transgenic skin in situ. Finally, Inv-Dsg2 transgenic mice developed intraepidermal skin lesions resembling precancerous papillomas and were more susceptible to chemically induced carcinogenesis. In summary, overexpression of Dsg2 in epidermal keratinocytes deregulates multiple signaling pathways associated with increased growth rate, anchorage-independent cell survival, and the development of skin tumors in vivo.

Increased keratinocyte proliferation initiated through downregulation of desmoplakin by RNA interference

The intercellular adhesive junction desmosomes are essential for the maintenance of tissue structure and integrity in skin. Desmoplakin (Dp) is a major obligate plaque protein which plays a fundamental role in anchoring intermediate filaments to desmosomal cadherins. Evidence from hereditary human disease caused by mutations in the gene encoding Dp, e.g. Dp haploinsufficiency, suggests that alterations in Dp expression result not only in the disruption of tissue structure and integrity but also could evoke changes in keratinocyte proliferation. We have used transient RNA interference (RNAi) to downregulate Dp specifically in HaCaT keratinocytes. We showed that this Dp downregulation also caused reduced expression of several other desmosomal proteins. Increased cell proliferation and enhanced G(1)-to-S-phase entry in the cell cycle, as monitored by colonial cellular density and BrdU incorporation, were seen in Dp RNAi-treated cells. These proliferative changes were associated with elevated phospho-ERK1/2 and phospho-Akt levels. Furthermore, this increase in phospho-ERK/1/2 and phospho-Akt levels was sustained in Dp RNAi-treated cells at confluence whereas in control cells there was a significant reduction in phosphorylation of ERK1/2. This study indicates that Dp may participate in the regulation of keratinocyte cell proliferation by, in part at least, regulating cell cycle progression.

Stem/progenitor cell-like properties of desmoglein 3dim cells in primary and immortalized keratinocyte lines

We showed previously that primary keratinocytes selected for low desmoglein 3 (Dsg3) expression levels exhibited increased colony-forming efficiency and heightened proliferative potential relative to cells with higher Dsg3 expression levels, characteristics consistent with a more "stem/progenitor cell-like" phenotype. Here, we have confirmed that Dsg3(dim) cells derived from cultured primary human adult keratinocytes have comparability with alpha(6)(bri)/CD71(dim) stem cells in terms of colony-forming efficiency. Moreover, these Dsg3(dim) cells exhibit increased reconstituting ability in in vitro organotypic culture on de-epidermalized dermis (DED); they are small, actively cycling cells, and they express elevated levels of various p63 isoforms. In parallel, using the two immortalized keratinocyte cell lines HaCaT and NTERT, we obtained essentially similar though occasionally different findings. Thus, reduced colony-forming efficiency by Dsg3(bri) cells consistently was observed in both cell lines even though the cell cycle profile and levels of p63 isoforms in the bri and dim populations differed between these two cell lines. Dsg3(dim) cells from both immortalized lines produced thicker and better ordered hierarchical structural organization of reconstituted epidermis relative to Dsg3(bri) and sorted control cells. Dsg3(dim) HaCaT cells also show sebocyte-like differentiation in the basal compartment of skin reconstituted after a 4-week organotypic culture. No differences in percentages of side population cells (also a putative marker of stem cells) were detected between Dsg3(dim) and Dsg3(bri) populations. Taken together our data indicate that Dsg3(dim) populations from primary human adult keratinocytes and long-term established keratinocyte lines possess certain stem/progenitor cell-like properties, although the side population characteristic is not one of these features. Disclosure of potential conflicts of interest is found at the end of this article.

Discriminating roles of desmosomal cadherins: Beyond desmosomal adhesion

The desmosomal cadherins, which include desmogleins and desmocollins, are Ca(2+)-dependent adhesion molecules that cooperate to make up the adhesive core of intercellular junctions known as desmosomes. The roles of desmosomal cadherins in epidermal integrity and as targets in human cutaneous disease have been well established. However, the molecular basis of these disorders is still poorly understood, due in part to a lack of fundamental knowledge about the organization of the adhesive interface and molecular machinery that dictates the proper presentation of desmogleins and desmocollins on the cell surface. Further, the diversity of the desmosomal cadherin family, and their individualized expression patterns within complex tissues, suggests that these adhesion molecules may have differentiation-specific functions that transcend their roles in intercellular adhesion. Here we will review the most recent data from our own group and others that are beginning to unveil the diverse properties and functions of this complex family of adhesion molecules.

Localisation of bullous pemphigoid antigen 180 (BP180) in cultured human keratinocytes: functionally relevant modification by calcium

The expression of BP180 has previously been demonstrated to be influenced by both calcium (Ca(2+)) concentration and binding of anti-BP180-antibodies in cultured keratinocytes of the skin squamous cell carcinoma line DJM-1. Here, BP180 expression was studied in cultured normal human epidermal keratinocytes by confocal laser scanning microscopy. We exploited an experimental system, in which BP180 was previously shown to mediate, upon incubation with anti-BP180 antibodies, a specific signal-transducing event that leads to the release of inflammatory mediators, such as IL-8 from cultured normal human epidermal keratinocytes (NHEK). We found that without addition of BP180-specific IgG, BP180 is predominantly expressed on the cell surface irrespective of the Ca(2+) concentration. In contrast, cell surface BP180 was greatly reduced in NHEK kept in high Ca(2+) medium after incubation with BP180-specific IgG for 12 h compared to low Ca(2+) medium. This effect was seen with antibodies to both N- and C-terminal fragments of the BP180 ectodomain, respectively. In addition, a slightly higher BP180 expression was found in NHEK cultured in low compared to high Ca(2+) medium by Western blotting. Interestingly, in contrast to NHEK kept under low Ca(2+ )conditions, in NHEK grown in high Ca(2+) medium, no elevated levels of IL-8 were released after treatment of cells with anti-BP180 IgG compared to normal IgG. Our data indicate that the Ca(2+)-modulated expression of BP180 is functionally relevant. This finding sheds further light on the complex pathomechanism in blister formation of BP180-related autoimmune blistering skin diseases.

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

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

The Differentiation-dependent Desmosomal Cadherin Desmoglein 1 Is a Novel Caspase-3 Target That Regulates Apoptosis in Keratinocytes

Although a number of cell adhesion proteins have been identified as caspase substrates, the potential role of differentiation-specific desmosomal cadherins during apoptosis has not been examined. Here, we demonstrate that UV-induced caspase cleavage of the human desmoglein 1 cytoplasmic tail results in distinct 17- and 140- kDa products, whereas metalloproteinase-dependent shedding of the extracellular adhesion domain generates a 75-kDa product. In vitro studies identify caspase-3 as the preferred enzyme that cleaves desmoglein 1 within its unique repeating unit domain at aspartic acid 888, part of a consensus sequence not conserved among the other desmosomal cadherins. Apoptotic processing leads to decreased cell surface expression of desmoglein 1 and re-localization of its C terminus diffusely throughout the cytoplasm over a time course comparable with the processing of other desmosomal proteins and cytoplasmic keratins. Importantly, whereas classic cadherins have been reported to promote cell survival, short hairpin RNA-mediated suppression of desmoglein 1 in differentiated keratinocytes protected cells from UV-induced apoptosis. Collectively, our results identify desmoglein 1 as a novel caspase and metalloproteinase substrate whose cleavage likely contributes to the dismantling of desmosomes during keratinocyte apoptosis and also reveal desmoglein 1 as a previously unrecognized regulator of apoptosis in keratinocytes.

Immunohistochemical analysis of the distribution of desmoglein 1 and 2 in the skin of dogs and cats

OBJECTIVE

To compare the distribution of desmoglein (Dsg) 1 and 2 in skin specimens obtained from dogs and cats to provide information about the possible role of the density of Dsg 1 and 2 in the localization of lesions attributable to pemphigus foliaceus in these 2 species.

SAMPLE POPULATION

Skin biopsy specimens obtained from 4 dogs and 4 cats.

PROCEDURE

Biopsy specimens were collected from the muzzle, bridge of the nose, ear, dorsum, abdomen, area adjacent to the teats, and footpads of each animal. Immunohistochemical analysis was performed on formalin-fixed, paraffin-embedded skin samples by use of a biotinylated mouse monoclonal anti-Dsg 1 and 2 antibody raised against bovine muzzle. Color development was performed by use of the streptavidin-biotin-peroxidase method with a chromogenic substrate.

RESULTS

Immunohistochemical staining yielded a positive reaction in skin samples obtained from all anatomic sites. The intensity and distribution of staining were related to the number of layers of the stratum spinosum. No differences were detected between samples obtained from dogs and cats.

CONCLUSIONS AND CLINICAL RELEVANCE

No differences in intensity of Dsg 1 and 2 antigen were observed in the stratum spinosum between skin samples obtained from dogs and cats. Analysis of this result suggests that factors other than the distribution of Dsg may be responsible for the differences in localization of primary clinical lesions in dogs and cats with pemphigus foliaceus.

In vitro keratinocyte dissociation assay for evaluation of the pathogenicity of anti-desmoglein 3 IgG autoantibodies in pemphigus vulgaris

Patients with pemphigus vulgaris (PV) have circulating anti-desmoglein (Dsg) 3 immunoglobulin G (IgG) autoantibodies that induce blister formation. We developed an in vitro quantitative assay to evaluate the pathogenic strength of anti-Dsg3 IgG autoantibodies in blister formation. To obtain intercellular adhesion mediated dominantly by Dsg3, we used primary cultured normal human keratinocytes expressing low level of Dsg2 in the presence of exfoliative toxin A that specifically digests Dsg1. After incubation with various antibodies, monolayers released by dispase were subjected to mechanical stress by pipetting, and the number of cell fragments were counted. When anti-Dsg3 monoclonal antibodies (mAb) obtained from pemphigus model mice were tested, pathogenic AK23 mAb yielded significantly higher number of cell fragments than AK7 or AK20 non-pathogenic mAb. Dissociation scores, defined with AK23 mAb as the positive control, were significantly higher with active stage PV sera (n=10, 77.4+/-21.4) than controls (n=11, 16.0+/-9.6; p=0.003). When pair sera obtained from 6 PV patients in active stage and in remission were compared, the dissociation scores reflected well the disease activity as those in active stage were four to 17 times higher than those in remission. When sera from different patients showing similar ELISA scores but different clinical severity were tested (n=6), the dissociation scores with sera from severe disease activity were significantly higher than those with sera in remission. These findings indicate that this dissociation assay will provide a simple and objective biological method to measure the pathogenic strength of pemphigus autoantibodies.

Coordinated expression of desmoglein 1 and desmocollin 1 regulates intercellular adhesion

Desmoglein 1 (Dsg1) is a component of desmosomes present in the upper epidermis and can be targeted by autoimmune antibodies or bacterial toxins, resulting in skin blistering diseases. These defects in tissue integrity are believed to result from compromised desmosomal adhesion; yet, previous attempts to directly test the adhesive roles of desmosomal cadherins using normally non-adherent L cells have yielded mixed results. Here, two complementary approaches were used to better resolve the molecular determinants for Dsg1-mediated adhesion: (1) a tetracycline-inducible system was used to modulate the levels of Dsg1 expressed in L cell lines containing desmocollin 1 (Dsc1) and plakoglobin (PG) and (2) a retroviral gene delivery system was used to introduce Dsg1 into normal human epidermal keratinocytes (NHEK). By increasing Dsg1 expression relative to Dsc1 and PG, we were able to demonstrate that the ratio of Dsg1:Dsc1 is a critical determinant of desmosomal adhesion in fibroblasts. The distribution of Dsg1 was organized at areas of cell-cell contact in the multicellular aggregates that formed in these suspension cultures. Similarly, the introduction of Dsg1 into NHEKs was capable of increasing the aggregation of single cell suspensions and further enhanced the adhesive strength of intact epithelial sheets. Endogenous Dsc1 levels were also increased in NHEKs containing Dsg1, providing further support for the coordination of these two desmosomal cadherins in regulating adhesive structures. These Dsg1-mediated effects on intercellular adhesion were directly related to the presence of an intact extracellular domain as ETA, a toxin that specifically cleaves this desmosomal cadherin, inhibited adhesion in both fibroblasts and keratinocytes. Collectively, these observations demonstrate that Dsg1 promotes the formation of intercellular adhesion complexes and suggest that the relative level of Dsg and Dsc expressed at the cell surface regulates this adhesive process.

Epidermal keratinocytes: regulation of multiple cell phenotypes by multiple protein kinase C isoforms

Squamous cells form the outermost layers of the epidermis, and though they are readily discarded from the tissue, they serve a vital water barrier function while in the stratum corneum. The generation of cornified or squamous keratinocytes involves a complex, multi-step differentiation process that insures the proper physical and immunological barrier functions of the epidermis are maintained. The regulation of keratinocyte terminal differentiation is influenced by a large number of signaling pathways. This article will review some recent findings regarding the roles of the protein kinase C (PKC) family in normal keratinocyte differentiation, as well as their involvement in skin diseases, especially skin cancer.

Solar-simulated irradiation evokes a persistent and biphasic IL-1alpha response

Exposure of skin to solar-simulated irradiation generates a multitude of adaptive responses including cytokine transcription, synthesis and secretion. Interleukin-1 (IL-1) is one of the cytokines induced in epidermal cells in response to UV irradiation. It displays a broad range of mitogenic and inflammatory activities including fibroblast proliferation and T-cell activation. There are two forms, IL-1alpha and IL-1beta; and IL-1alpha is the predominant form secreted by epidermal keratinocytes. UV-induced modulations of IL-1alpha message levels have been extensively studied within the first 48 h after irradiation, but longer term changes and impact on IL-1alpha cellular protein levels are virtually unexplored. We now report that cells of keratinocyte origin (SCC 12F) respond to a single physiologic dose of solar-simulated irradiation with both early (8 h) and late (72 h) peaks of IL-1alpha mRNA induction. UV-stimulated IL-1alpha secretion is increased above sham-irradiated control secretion for at least 96 h after irradiation. Our study provides evidence that UV-induced adaptive cutaneous responses persist for at least several days, and suggests that different mechanisms may mediate the early vs. late inductions.