Unique Keratinization Process in Psoriasis: Late Differentiation Markers Are Abolished Because of the Premature Cell Death

ΔNp63 Regulates Homeostasis, Stemness, and Suppression of Inflammation in the Adult Epidermis

The p63 transcription factor is critical for epidermis formation in embryonic development, but its role in the adult epidermis is poorly understood. In this study, we show that acute genetic ablation of ΔNp63, the main p63 isoform, in adult epidermis disrupts keratinocyte proliferation and self-maintenance and, unexpectedly, triggers an inflammatory psoriasis-like condition. Mechanistically, single-cell RNA sequencing revealed the downregulation of cell cycle genes, upregulation of differentiation markers, and induction of several proinflammatory pathways in ΔNp63-ablated keratinocytes. Intriguingly, ΔNp63-ablated cells disappear by 3 weeks after ablation, at the expense of the remaining nonablated cells. This is not associated with active cell death and is likely due to reduced self-maintenance and enhanced differentiation. Indeed, in vivo wound healing, a physiological readout of the epidermal stem cell function, is severely impaired upon ΔNp63 ablation. We found that the Wnt signaling pathway (Wnt10A, Fzd6, Fzd10) and the activator protein 1 (JunB, Fos, FosB) factors are the likely ΔNp63 effectors responsible for keratinocyte proliferation/stemness and suppression of differentiation, respectively, whereas IL-1a, IL-18, IL-24, and IL-36γ are the likely negative effectors responsible for suppression of inflammation. These data establish ΔNp63 as a critical node that coordinates epidermal homeostasis, stemness, and suppression of inflammation, upstream of known regulatory pathways.

Mucosal expression of PI3, ANXA1, and VDR discriminates Crohn's disease from ulcerative colitis

Differential diagnosis of inflammatory bowel disease (IBD) to Crohn's disease (CD) or ulcerative colitis (UC) is crucial for treatment decision making. With the aim of generating a clinically applicable molecular-based tool to classify IBD patients, we assessed whole transcriptome analysis on endoscopy samples. A total of 408 patient samples were included covering both internal and external samples cohorts. Whole transcriptome analysis was performed on an internal cohort of FFPE IBD samples (CD, n = 16 and UC, n = 17). The 100 most significantly differentially expressed genes (DEG) were tested in two external cohorts. Ten of the DEG were further processed by functional enrichment analysis from which seven were found to show consistent significant performance in discriminating CD from UC: PI3, ANXA1, VDR, MTCL1, SH3PXD2A-AS1, CLCF1, and CD180. Differential expression of PI3, ANXA1, and VDR was reproduced by RT-qPCR, which was performed on an independent sample cohort of 97 patient samples (CD, n = 44 and UC, n = 53). Gene expression levels of the three-gene profile, resulted in an area under the curve of 0.84 (P = 0.02) in discriminating CD from UC, and therefore appear as an attractive molecular-based diagnostic tool for clinicians to distinguish CD from UC.

Exploring the potential of Halomonas levan and its derivatives as active ingredients in cosmeceutical and skin regenerating formulations

Demand on natural products that contain biological ingredients mimicking growth factors and cytokines made natural polysaccharides popular in pharmaceutical and cosmetic industries. Levan is the β-(2-6) linked, nontoxic, biocompatible, water-soluble, film former fructan polymer that has diverse applications in pharmacy and cosmeceutical industries with its moisturizing, whitening, anti-irritant, anti-aging and slimming activities. Driven by the limited reports on few structurally similar levan polymers, this study presents the first systematic investigation on the effects of structurally different extremophilic Halomonas levan polysaccharides on human skin epidermis cells. In-vitro experiments with microbially produced linear Halomonas levan (HL), its hydrolyzed, (hHL) and sulfonated (ShHL) derivatives as well as enzymatically produced branched levan (EL) revealed increased keratinocyte and fibroblast proliferation (113-118 %), improved skin barrier function through induced expressions of involucrin (2.0 and 6.43 fold changes for HL and EL) and filaggrin (1.74 and 3.89 fold changes for hHL and ShHL) genes and increased type I collagen (2.63 for ShHL) and hyaluronan synthase 3 (1.41 for HL) gene expressions together with fast wound healing ability within 24 h (100 %, HL) on 2D wound models clearly showed that HL and its derivatives have high potential to be used as natural active ingredients in cosmeceutical and skin regenerating formulations.

Classification and biomarker gene selection of pyroptosis-related gene expression in psoriasis using a random forest algorithm

Background: Psoriasis is a chronic and immune-mediated skin disorder that currently has no cure. Pyroptosis has been proved to be involved in the pathogenesis and progression of psoriasis. However, the role pyroptosis plays in psoriasis remains elusive.

Methods: RNA-sequencing data of psoriasis patients were obtained from the Gene Expression Omnibus (GEO) database, and differentially expressed pyroptosis-related genes (PRGs) between psoriasis patients and normal individuals were obtained. A principal component analysis (PCA) was conducted to determine whether PRGs could be used to distinguish the samples. PRG and immune cell correlation was also investigated. Subsequently, a novel diagnostic model comprising PRGs for psoriasis was constructed using a random forest algorithm (ntree = 400). A receiver operating characteristic (ROC) analysis was used to evaluate the classification performance through both internal and external validation. Consensus clustering analysis was used to investigate whether there was a difference in biological functions within PRG-based subtypes. Finally, the expression of the kernel PRGs were validated in vivo by qRT-PCR.

Results: We identified a total of 39 PRGs, which could distinguish psoriasis samples from normal samples. The process of T cell CD4 memory activated and mast cells resting were correlated with PRGs. Ten PRGs, IL-1β, AIM2, CASP5, DHX9, CASP4, CYCS, CASP1, GZMB, CHMP2B, and CASP8, were subsequently screened using a random forest diagnostic model. ROC analysis revealed that our model has good diagnostic performance in both internal validation (area under the curve [AUC] = 0.930 [95% CI 0.877–0.984]) and external validation (mean AUC = 0.852). PRG subtypes indicated differences in metabolic processes and the MAPK signaling pathway. Finally, the qRT-PCR results demonstrated the apparent dysregulation of PRGs in psoriasis, especially AIM2 and GZMB.

Conclusion: Pyroptosis may play a crucial role in psoriasis and could provide new insights into the diagnosis and underlying mechanisms of psoriasis.

Current knowledge of the implication of lipid mediators in psoriasis

The skin is an organ involved in several biological processes essential to the proper functioning of the organism. One of these essential biological functions of the skin is its barrier function, mediated notably by the lipids of the stratum corneum, and which prevents both penetration from external aggression, and transepidermal water loss. Bioactive lipid mediators derived from polyunsaturated fatty acids (PUFAs) constitute a complex bioactive lipid network greatly involved in skin homeostasis. Bioactive lipid mediators derived from n-3 and n-6 PUFAs have well-documented anti- and pro-inflammatory properties and are recognized as playing numerous and complex roles in the behavior of diverse skin diseases, including psoriasis. Psoriasis is an inflammatory autoimmune disease with many comorbidities and is associated with enhanced levels of pro-inflammatory lipid mediators. Studies have shown that a high intake of n-3 PUFAs can influence the development and progression of psoriasis, mainly by reducing the severity and frequency of psoriatic plaques. Herein, we provide an overview of the differential effects of n-3 and n-6 PUFA lipid mediators, including prostanoids, hydroxy-fatty acids, leukotrienes, specialized pro-resolving mediators, N-acylethanolamines, monoacylglycerols and endocannabinoids. This review summarizes current findings on lipid mediators playing a role in the skin and their potential as therapeutic targets for psoriatic patients.

Causal Biological Network Model for Inflammasome Signaling Applied for Interpreting Transcriptomic Changes in Various Inflammatory States

Virtually any stressor that alters the cellular homeostatic state may result in an inflammatory response. As a critical component of innate immunity, inflammasomes play a prominent role in the inflammatory response. The information on inflammasome biology is rapidly growing, thus creating the need for structuring it into a model that can help visualize and enhance the understanding of underlying biological processes. Causal biological network (CBN) models provide predictive power for novel disease mechanisms and treatment outcomes. We assembled the available literature information on inflammasome activation into the CBN model and scored it with publicly available transcriptomic datasets that address viral infection of the lungs, osteo- and rheumatoid arthritis, psoriasis, and aging. The scoring inferred pathway activation leading to NLRP3 inflammasome activation in these diverse conditions, demonstrating that the CBN model provides a platform for interpreting transcriptomic data in the context of inflammasome activation.

Development of a 3D psoriatic skin model optimized for infiltration of IL-17A producing T cells: Focus on the crosstalk between T cells and psoriatic keratinocytes

Psoriasis is a chronic inflammatory skin disease involving several cell types, including T cells, via the IL-23/IL-17 axis. IL-17A acts on the surrounding epithelial cells thus resulting in an inflammatory feedback loop. The development of immunocompetent models that correctly recapitulate the complex phenotype of psoriasis remains challenging, which also includes both the T cell isolation and activation methods. The purpose of this work was to develop an advanced in vitro 3D psoriatic skin model that enables the study of the impact of T cells on psoriatic epithelial cells. To reach that aim, healthy and psoriatic fibroblasts and keratinocytes were used to reproduce this tissue-engineered skin model in which activated T cells, isolated beforehand from human whole blood, have been incorporated. Our study showed that isolation of T cells with the EasySep procedure, followed by activation with PMA/ionomycin, mimicked the psoriatic characteristics in an optimal manner with the production of inflammatory cytokines important in the pathogenesis of psoriasis, as well as increased expression of Ki67, S100A7, elafin and involucrin. This psoriatic model enriched in activated T cells displayed enhanced production of IL-17A, IFN-ƴ, CCL2, CXCL10, IL-1ra, IL-6 and CXCL8 compared with the healthy model and whose increased secretion was maintained over time. In addition, anti-IL17A treatment restored some psoriatic features, including epidermal thickness and basal keratinocytes proliferation, as well as a downregulation of S100A7, elafin and involucrin expression. Altogether, our study demonstrated that this model reflects a proper psoriatic inflammatory environment and is effective for the investigation of epidermal and T cell interaction over time. STATEMENT OF SIGNIFICANCE: The aim of this study was to provide an innovative 3D immunocompetent human psoriatic skin model. To our knowledge, this is the first immunocompetent model that uses skin cells from psoriatic patients to study the impact of IL-17A on pathological cells. Through the use of this model, we demonstrated that the T-cell enriched psoriatic model differs from T-cell enriched healthy model, highlighting efficient crosstalk between pathologic epithelial cells and T cells. This advanced preclinical model further mimics the original psoriatic skin and will prove relevant in predicting clinical outcomes, thereby decreasing inaccurate predictions of compound effects.

Molecular Pathogenesis of Psoriasis and Biomarkers Reflecting Disease Activity

Psoriasis is a chronic inflammatory skin disease induced by multifactorial causes and is characterized by bothersome, scaly reddish plaques, especially on frequently chafed body parts, such as extensor sites of the extremities. The latest advances in molecular-targeted therapies using biologics or small-molecule inhibitors help to sufficiently treat even the most severe psoriatic symptoms and the extra cutaneous comorbidities of psoriatic arthritis. The excellent clinical effects of these therapies provide a deeper understanding of the impaired quality of life caused by this disease and the detailed molecular mechanism in which the interleukin (IL)-23/IL-17 axis plays an essential role. To establish standardized therapeutic strategies, biomarkers that define deep remission are indispensable. Several molecules, such as cytokines, chemokines, antimicrobial peptides, and proteinase inhibitors, have been recognized as potent biomarker candidates. In particular, blood protein markers that are repeatedly measurable can be extremely useful in daily clinical practice. Herein, we summarize the molecular mechanism of psoriasis, and we describe the functions and induction mechanisms of these biomarker candidates.

IL-1 Family Antagonists in Mouse and Human Skin Inflammation

Interleukin (IL)-1 family cytokines initiate inflammatory responses, and shape innate and adaptive immunity. They play important roles in host defense, but excessive immune activation can also lead to the development of chronic inflammatory diseases. Dysregulated IL-1 family signaling is observed in a variety of skin disorders. In particular, IL-1 family cytokines have been linked to the pathogenesis of psoriasis and atopic dermatitis. The biological activity of pro-inflammatory IL-1 family agonists is controlled by the natural receptor antagonists IL-1Ra and IL-36Ra, as well as by the regulatory cytokines IL-37 and IL-38. These four anti-inflammatory IL-1 family members are constitutively and highly expressed at steady state in the epidermis, where keratinocytes are a major producing cell type. In this review, we provide an overview of the current knowledge concerning their regulatory roles in skin biology and inflammation and their therapeutic potential in human inflammatory skin diseases. We further highlight some common misunderstandings and less well-known observations, which persist in the field despite recent extensive interest for these cytokines.

The antinuclear antibody dense fine speckled pattern and possible clinical associations: An indication of a proinflammatory microenvironment

BACKGROUND

Indirect immunofluorescence (IIF) is the most prevalent screening antinuclear antibody test for systemic autoimmune rheumatic disease (SARD). Certain IIF patterns have known antibody and disease associations, but the dense fine speckled (ANA-DFS) pattern has no confirmed clinical associations. Our objective was to determine the prevalence of SARD among a group of ANA-DFS positive individuals and to identify final diagnoses among non-SARD individuals in order to determine possible clinical associations with the ANA-DFS pattern.

METHODS

A retrospective study of 425 patients from a university health care system with a positive ANA-DFS pattern consecutively between August 2017 and September 2018. Sera samples underwent ANA testing by IIF on HEp-2 cell substrates (Euroimmun, Germany). Clinical information was retrieved from electronic health records and stored in a de-identified database.

RESULTS

The prevalence of SARD was 24%. Undetermined diagnosis (17%), skin disorders (12.1%), and fibromyalgia/chronic pain syndrome/chronic fatigue syndrome (11.8%) were the most common non-SARD diagnoses. Taking into account past medical history, the most common non-SARD were atopic disorders (21.2%), fibromyalgia/chronic pain syndrome/chronic fatigue syndrome (17.6%), and skin disorders (16.7%).

CONCLUSIONS

The ANA-DFS pattern may be indicative of an underlying antigen-antibody interaction that plays a role in either the initiation or propagation of immunologic reactions. DFS70/LEDGF is a transcription factor involved in cell survival and stress protection, and autoantibodies may inhibit its function. It is likely that there are other antibodies producing the ANA-DFS pattern besides anti-DFS70/LEDGF, and more research is necessary to identify additional antibody specificities. The ANA-DFS pattern may be an indicator of a proinflammatory microenvironment given the high frequency of symptomatic patients and disease processes with an immunologic basis (including SARD).

Indirubin inhibits Wnt/β-catenin signal pathway via promoter demethylation of WIF-1

Background

Psoriasis is a common inflammatory skin disease. Abnormal proliferation of keratinocytes is one of the psoriatic histopathological features. Indirubin has an essential effect on the proliferation and activation of keratinocytes; however, in psoriasis, the specific mechanism of action of indirubin on keratinocytes is unclear. In the present study, we revealed the effects of indirubin on DNA methyltransferase 1 (DNMT1), wnt inhibitory factor 1 (wif-1), and wnt/β-catenin signal pathway, in the meantime, we explored the effects of indirubin on proliferation, cell cycle and the apoptosis of HaCaT cells.

Methods

The expression of DNMT1, wif-1, Frizzled2, Frizzled5, and β-catenin in HaCaT cells treated with different concentrations of indirubin were detected by Western blotting and quantitative real-time polymerase chain reaction (qRT-PCR). The expression levels of DNMT1 and wif-1 were observed after treated with different concentrations of indirubin by enzyme-linked immunosorbent assay (ELISA). The wif-1 promoter methylation status was detected by DNA methylation-specific PCR (MSP). The transcriptional activities of wif-1 and β-catenin were discovered by a luciferase reporter gene system. Cell viability was determined by Cell Counting Kit-8 (CCK8) method. The cell cycle was detected by flow cytometry. The apoptotic cells were surveyed by the apoptosis kit. The expression of Inolucrin, Loricrin, Filaggrin, Keratin 17, and transcriptional activation of transglutaminase 1(TGase1) were detected by Western blotting.

Results

Indirubin inhibited the expression of DNMT1 and the methylation of the wif-1 promoter. In the wnt signal pathway, indirubin restored the protein expression of wif-1 and inhibited expression of Frizzled2, Frizzled5, and β-catenin. Besides, indirubin inhibited the proliferation of HaCaT cells, induced apoptosis, and arrest cell cycle. We also reported that indirubin could down-regulate the expression of Involucrin, TGase 1, and keratin 17, but the expression of Filaggrin and Loricrin had no significant effect.

Conclusion

Our research showed that indirubin promoted the demethylation of wif-1 and suppressed the wnt/β-catenin signal pathway, thereby exerted an anti-proliferative effect. This study reveals the anti-proliferation mechanism of indirubin, which may provide an effective option for the treatment of proliferative diseases.

Homeostatic Function of Dermokine in the Skin Barrier and Inflammation

Dermokine is a chiefly skin-specific secreted glycoprotein localized in the upper epidermis, and its family consists of three splice variants in mice and five in humans. To investigate the pathophysiological impact of dermokine, we generated mice deficient for two (βγ) or all dermokine isoforms (αβγ). Both variants, especially dermokine αβγ-deficient mice exhibited scale and wrinkle formation resembling ichthyosis accompanied by transepidermal water imbalance at the neonatal stage. Several dermokine αβγ-deficient mice died by postnatal day 21 when reared under low humidity. Moreover, the cornified envelope was vulnerable, and skin barrier lipid ceramides were reduced in the epidermis of dermokine αβγ-deficient mice. cDNA microarray and quantitative reverse transcriptase-PCR assays of the epidermis revealed the upregulation of small proline-rich protein and late cornified envelope family members, as well as antimicrobial peptides in the dermokine αβγ-deficient mice. These barrier gene signatures were similar to that seen in psoriasis, whereas recent studies demonstrated that congenital ichthyosis has gene profiles resembling psoriasis. In line with these findings, adult dermokine αβγ-deficient mice exhibited aggravated phenotypes in psoriasis-like dermatitis models but not in allergic dermatitis models. Dermokine may play a regulatory role in inflammatory dyskeratotic diseases, such as congenital ichthyosis and psoriasis, in the crosstalk between barrier dysfunction and inflammation.

Mimicking cigarette smoke exposure to assess cutaneous toxicity

Cigarette smoke stands among the most toxic environmental pollutants and is composed of thousands of chemicals including polycyclic aromatic hydrocarbons (PAHs). Despite restrict cigarette smoking ban in indoor or some outdoor locations, the risk of non-smokers to be exposed to environmental cigarette smoke is not yet eliminated. Beside the well-known effects of cigarette smoke to the respiratory and cardiovascular systems, a growing literature has shown during the last 3 decades its noxious effects also on cutaneous tissues. Being the largest organ as well as the interface between the outer environment and the body, human skin acts as a natural shield which is continuously exposed to harmful exogenous agents. Thus, a prolonged and/or repetitive exposure to significant levels of toxic smoke pollutants may have detrimental effects on the cutaneous tissue by disrupting the epidermal barrier function and by exacerbating inflammatory skin disorders (i.e. psoriasis, atopic dermatitis). With the development of very complex skin tissue models and sophisticated cigarette smoke exposure systems it has become important to better understand the toxicity pathways induced by smoke pollutants in more realistic laboratory conditions to find solutions for counteracting their effects. This review provides an update on the skin models currently available to study cigarette smoke exposure and the known pathways involved in cutaneous toxicity. In addition, the article will briefly cover the inflammatory skin pathologies potentially induced and/or exacerbated by cigarette smoke exposure.

IL-36β Promotes Inflammatory Activity and Inhibits Differentiation of Keratinocytes In Vitro

Objective Psoriasis is an immune-mediated inflammatory disease. Despite advances in the study of its pathogenesis, the exact development mechanism of psoriasis remains to be fully elucidated. Hyperproliferative epidermis plays a crucial role in psoriasis. This study aimed to investigate the effects of interleukin-36β (IL-36β) on keratinocyte dysfunction in vitro. Methods Human keratinocyte cell lines, HaCaT cells, were treated with 0 (control), 50 or 100 ng/ml IL-36β respectively for 24 h. Cell viability was determined with a cell counting kit-8 assay. Flow cytometry was used to assess the effects of IL-36β on apoptosis and cell cycle distribution. Expressions of the differentiation markers, such as keratin 10 and involucrin, were evaluated by quantitative real-time polymerase chain reaction (RT-qPCR). Expressions of the inflammatory cytokines, IL-1β and IL-6 were tested by ELISA. Results CCK8 assay showed the survival rate had no significant difference between the control and treated group (P > 0.05). Flow cytometry analysis showed cell cycle arrest at S phase in the IL-36β-treated groups compared with the control group (P < 0.05). RT-qPCR verified the decreased mRNA expressions of keratin 10 and involucrin in the IL-36β-treated groups compared with the negative control (P < 0.01). ELISA showed 100 ng/ml IL-36β enhanced levels of IL-1β and IL-6 in culture supernatants of HaCaT cells compared with the negative control (P < 0.05). Conclusion Taken together, these findings suggest that IL-36β could induce cell cycle arrest at S phase, inhibit keratin 10 and involucrin expressions and promote inflammatory activity in HaCaT cell lines.

Infiltration of T Cells into a Three-Dimensional Psoriatic Skin Model Mimics Pathological Key Features

Psoriasis is an autoimmune chronic dermatosis that is T cell-mediated, characterized by epidermal thickening, aberrant epidermal differentiation and inflammatory infiltrates, with a dominant Th1 and Th17 profile. Additional in vitro models are required to study the complex interactions between activated T cells and skin cells, and to develop new, more effective treatments. We have therefore sought to model this psoriatic inflammation by the generation of tissue-engineered immunocompetent tissues, and we have investigated the response of activated T-cell infiltration in models produced with lesional psoriatic skin cells on major hallmarks of psoriasis. The immunocompetent lesional skin model displayed a delayed onset of epidermal differentiation, an hyperproliferation of the basal keratinocytes, a drastic increase in the secretion of proinflammatory cytokines, and a disturbed expression of key transcription factors, as observed in lesional plaques, suggesting a crucial importance of combining the pathological phenotype of cutaneous cells to T cells in order to generate a relevant model for psoriasis. Finally, we found this skin model to be responsive to methotrexate treatment, making it a valuable tool for drug development.

Orf virus (ORFV) infection in a three-dimensional human skin model: Characteristic cellular alterations and interference with keratinocyte differentiation

ORF virus (ORFV) is the causative agent of contagious ecthyma, a pustular dermatitis of small ruminants and humans. Even though the development of lesions caused by ORFV was extensively studied in animals, only limited knowledge exists about the lesion development in human skin. The aim of the present study was to evaluate a three-dimensional (3D) organotypic culture (OTC) as a human skin model for ORFV infection considering lesion development, replication of the virus, viral gene transcription and modulation of differentiation of human keratinocytes by ORFV. ORFV infection of OTC was performed using the ORFV isolate B029 derived from a human patient. The OTC sections showed a similar structure of stratified epidermal keratinocytes as human foreskin and a similar expression profile of the differentiation markers keratin 1 (K1), K10, and loricrin. Upon ORFV infection, OTCs exhibited histological cytopathic changes including hyperkeratosis and ballooning degeneration of the keratinocytes. ORFV persisted for 10 days and was located in keratinocytes of the outer epidermal layers. ORFV-specific early, intermediate and late genes were transcribed, but limited viral spread and restricted cell infection were noticed. ORFV infection resulted in downregulation of K1, K10, and loricrin at the transcriptional level without affecting proliferation as shown by PCNA or Ki-67 expression. In conclusion, OTC provides a suitable model to study the interaction of virus with human keratinocytes in a similar structural setting as human skin and reveals that ORFV infection downregulates several differentiation markers in the epidermis of the human skin, a hitherto unknown feature of dermal ORFV infection in man.

Three Constituents of Moringa oleifera Seeds Regulate Expression of Th17-Relevant Cytokines and Ameliorate TPA-Induced Psoriasis-Like Skin Lesions in Mice

As a folk medicine, Moringa oleifera L. is used effectively to treat inflammatory conditions and skin diseases. However, its mechanism of action is not well understood, limiting its medical use. We isolated and identified three compounds, namely niazirin, marumoside A and sitosterol-3-O-β-d-glucoside, from the seeds of Moringa oleifera, and studied their effects on the expression of Th17-relevant cytokines (IL-12/IL-23 p40, IL-17A, IL-22 and IL-23 p19) using lipopolysaccharide-stimulated THP-1 cells. Additionally, as Th17 plays a critical role in the pathogenesis of psoriasis, we used a 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced psoriasis-like skin lesion mouse model to study their potential therapeutic application in vivo. The compounds suppressed the expression of IL-12/IL-23 p40, IL-17A, IL-22 and IL-23 p19 in vitro, and in vivo they ameliorated psoriasis-like skin lesions, decreased IL-17A mRNA expression, and increased the expression of keratinocyte differentiation markers. To our knowledge, this is the first report regarding the mechanism and therapeutic application of Moringa oleifera seeds to treat psoriasis-like lesions in vivo.

Is Ki-67, keratin 16, involucrin, and filaggrin immunostaining sufficient to diagnose inflammatory linear verrucous epidermal nevus? A report of eight cases and a comparison with psoriasis vulgaris

Inflammatory linear verrucous epidermal nevus and linear psoriasis are sometimes hard to differentiate clinically and pathologically. Although immunohistochemical expression of keratin 10 (K10), K16, Ki-67, and involucrin may be useful for differentiating both entities, these results have been reported in only a few cases. We collected data from 8 patients with inflammatory linear verrucous epidermal nevus, 11 with psoriasis vulgaris, and 8 healthy controls and evaluated immunohistochemical expression of Ki-67, K16, involucrin, and filaggrin among them. Ki-67 and K16 overexpression was similar in inflammatory linear verrucous epidermal nevus and psoriasis vulgaris compared with normal skin. Although staining for involucrin showed discontinuous expression in parakeratotic regions in 4 inflammatory linear verrucous epidermal nevus cases, it was continuous in the other 4 cases and in all psoriasis vulgaris cases. Filaggrin expression was present in hyperkeratotic regions but scarce in parakeratotic areas in both inflammatory linear verrucous epidermal nevus and psoriasis vulgaris. The immunostaining pattern of Ki-67, K16, involucrin, and filaggrin may be insufficient to discriminate inflammatory linear verrucous epidermal nevus from psoriasis vulgaris.

Deciphering psoriasis. A bioinformatic approach

BACKGROUND

Psoriasis is an immune-mediated, inflammatory and hyperproliferative disease of the skin and joints. The cause of psoriasis is still unknown. The fundamental feature of the disease is the hyperproliferation of keratinocytes and the recruitment of cells from the immune system in the region of the affected skin, which leads to deregulation of many well-known gene expressions.

OBJECTIVE

Based on data mining and bioinformatic scripting, here we show a new dimension of the effect of psoriasis at the genomic level.

METHODS

Using our own pipeline of scripts in Perl and MySql and based on the freely available NCBI Gene Expression Omnibus (GEO) database: DataSet Record GDS4602 (Series GSE13355), we explore the extent of the effect of psoriasis on gene expression in the affected tissue.

RESULTS

We give greater insight into the effects of psoriasis on the up-regulation of some genes in the cell cycle (CCNB1, CCNA2, CCNE2, CDK1) or the dynamin system (GBPs, MXs, MFN1), as well as the down-regulation of typical antioxidant genes (catalase, CAT; superoxide dismutases, SOD1-3; and glutathione reductase, GSR). We also provide a complete list of the human genes and how they respond in a state of psoriasis.

CONCLUSION

Our results show that psoriasis affects all chromosomes and many biological functions. If we further consider the stable and mitotically inheritable character of the psoriasis phenotype, and the influence of environmental factors, then it seems that psoriasis has an epigenetic origin. This fit well with the strong hereditary character of the disease as well as its complex genetic background.

In vitro psoriasis models with focus on reconstructed skin models as promising tools in psoriasis research

Psoriasis is a complex chronic immune-mediated inflammatory cutaneous disease associated with the development of inflammatory plaques on the skin. Studies proved that the disease results from a deregulated interplay between skin keratinocytes, immune cells and the environment leading to a persisting inflammatory process modulated by pro-inflammatory cytokines and activation of T cells. However, a major hindrance to study the pathogenesis of psoriasis more in depth and subsequent development of novel therapies is the lack of suitable pre-clinical models mimicking the complex phenotype of this skin disorder. Recent advances in and optimization of three-dimensional skin equivalent models have made them attractive and promising alternatives to the simplistic monolayer cultures, immunological different in vivo models and scarce ex vivo skin explants. Moreover, human skin equivalents are increasing in complexity level to match human biology as closely as possible. Here, we critically review the different types of three-dimensional skin models of psoriasis with relevance to their application potential and advantages over other models. This will guide researchers in choosing the most suitable psoriasis skin model for therapeutic drug testing (including gene therapy via siRNA molecules), or to examine biological features contributing to the pathology of psoriasis. However, the addition of T cells (as recently applied to a de-epidermized dermis-based psoriatic skin model) or other immune cells would make them even more attractive models and broaden their application potential. Eventually, the ultimate goal would be to substitute animal models by three-dimensional psoriatic skin models in the pre-clinical phases of anti-psoriasis candidate drugs. Impact statement The continuous development of novel in vitro models mimicking the psoriasis phenotype is important in the field of psoriasis research, as currently no model exists that completely matches the in vivo psoriasis skin or the disease pathology. This work provides a complete overview of the different available in vitro psoriasis models and suggests improvements for future models. Moreover, a focus was given to psoriatic skin equivalent models, as they offer several advantages over the other models, including commercial availability and validity. The potential and reported applicability of these models in psoriasis pre-clinical research is extensively discussed. As such, this work offers a guide to researchers in their choice of pre-clinical psoriasis model depending on their type of research question.

IL-36γ inhibits differentiation and induces inflammation of keratinocyte via Wnt signaling pathway in psoriasis

Psoriasis is a common inflammatory skin disease characterized by abnormal keratinocyte inflammation and differentiation that has a major impact on patients' quality of life. IL-36γ, a member of IL-36 cytokine family, is highly expressed in psoriasis and plays an important role in inflammation response and differentiation. However, the function of IL-36γ in differentiation and inflammation of keratinocyte in psoriasis has not been clearly identified. Thus, this study aimed to investigate the role of IL-36γ on differentiation and inflammation in HaCaT cells. HaCaT cells were divided into three groups: (1) Control group; (2) IL-36γ (100 ng/mL) group; (3) IL-36γ (100 ng/mL) + IWP-2 (1μM) group. Real time PCR was used to detect gene expression; the inflammation cytokines were examined by ELISA. We showed that treatment of HaCaT cells with IL-36γ significantly upregulated the expression levels of β-catenin, cyclin D1, and ki-67. IL-36γ also promoted the production of the inflammatory cytokines IFN-γ, IL-1β and IL-6, suppressed the expression of filaggrin, involucrin, keratin 1 and keratin 5. Meanwhile, we demonstrated the role of IWP-2, an inhibitor of the Wnt signaling pathway, in IL-36γ-treated HaCaT cells. Collectively, our findings suggest that IL-36γ inhibits differentiation and induces inflammation of keratinocyte via Wnt signaling pathway in psoriasis, this indicated that downregulation of IL-36γ may be a potential therapeutic option in psoriasis.

Interaction of Wnt5a with Notch1 is Critical for the Pathogenesis of Psoriasis

Background

Psoriasis is characterized by uncontrolled hyperproliferation, aberrant differentiation, and dermal infiltration of immune cells. Recent studies have reported that Wnt5a and Notch1 signaling are altered in psoriatic skin lesions.

Objective

We aimed to investigate the interaction of Wnt5a with Notch 1 with respect to inflammation-mediated epidermal hyperproliferation in psoriasis.

Methods

Expression of Wnt5a and Notch1 signaling-related proteins were examined in psoriatic skin biopsies. Wnt5a was upregulated in human keratinocytes by treating the cells with its recombinant form (rWnt5a).

Results

In psoriatic lesions, expression of Wnt5a increased while that of Notch1 decreased when compared to that in non-lesional and normal skin. Treatment with rWnt5a increased the proliferation of keratinocytes and increased their secretion of interleukin (IL)-23, IL-12, and tumor necrosis factor (TNF)-α. Further, exposure of keratinocytes to IL-1α, TNF-α, transforming growth factor-α, and interferon-γ downregulated Notch1 as well as HES 1, which is downstream to Notch1, but increased the Wnt5a levels. The upregulated Wnt5a in keratinocytes downregulated both Notch1 and HES1.

Conclusion

Our data suggest that Wnt5a and Notch1 signaling exert counteracting influences on each other and are involved, in part, in the pathomechanism of psoriasis.

Intraindividual genome expression analysis reveals a specific molecular signature of psoriasis and eczema

Previous attempts to gain insight into the pathogenesis of psoriasis and eczema by comparing their molecular signatures were hampered by the high interindividual variability of those complex diseases. In patients affected by both psoriasis and nonatopic or atopic eczema simultaneously (n = 24), an intraindividual comparison of the molecular signatures of psoriasis and eczema identified genes and signaling pathways regulated in common and exclusive for each disease across all patients. Psoriasis-specific genes were important regulators of glucose and lipid metabolism, epidermal differentiation, as well as immune mediators of T helper 17 (TH17) responses, interleukin-10 (IL-10) family cytokines, and IL-36. Genes in eczema related to epidermal barrier, reduced innate immunity, increased IL-6, and a TH2 signature. Within eczema subtypes, a mutually exclusive regulation of epidermal differentiation genes was observed. Furthermore, only contact eczema was driven by inflammasome activation, apoptosis, and cellular adhesion. On the basis of this comprehensive picture of the pathogenesis of psoriasis and eczema, a disease classifier consisting of NOS2 and CCL27 was created. In an independent cohort of eczema (n = 28) and psoriasis patients (n = 25), respectively, this classifier diagnosed all patients correctly and also identified initially misdiagnosed or clinically undifferentiated patients.

Beta-catenin expression in psoriasis

BACKGROUND

Psoriasis is a common inflammatory skin disease characterized by abnormal keratinocyte proliferation and differentiation. Beta-catenin participates in intercellular adhesion. Catenins are proteins found in complexes with cadherin cell adhesion molecules of cells. The role of catenin in regulating keratinocyte stem cell differentiation and hair follicle morphogenesis has been extensively reported.

AIMS AND OBJECTIVES

is to study β-catenin expression in lesional and non-lesional psoriatic skin to throw light upon its possible role in the pathogenesis of psoriasis.

MATERIALS AND METHODS

Biopsies were taken from 20 patients with psoriasis vulgaris and from 10 normal controls. The distribution of Beta catenin was investigated using polycolonal rabbits B-catenin antibody-1 by immunohistochemical method.

RESULTS

In this study membranous β-catenin expression was significantly demonstrated in the control group then the non-lesional areas in comparison to the lesional areas (P < 0.001). Nuclear β-catenin staining expression was significantly more demonstrated in lesional and non-lesional areas in comparison to the control cases (P < 0.001).

CONCLUSIONS

The down regulation of membranous β-catenin expression in lesional psoriatic skin might reflect a useful phenotypic marker of hyperprolifration of keratinocytes in psoriasis. Moreover, the mild down regulation of membranous β-catenin expression in non lesional psoriatic skin may provide clues about incipient structural abnormalities in the pathogenesis of psoriasis, providing an early diagnostic indicator for evolution to a generalized form of the disease. Nuclear β-catenin expression was not found in the control group but was demonstrated in lesional and moderately in non-lesional reflecting its role in kerationcyte proliferation.

The antipsoriatic agent monomethylfumarate has antiproliferative, prodifferentiative, and anti-inflammatory effects on keratinocytes

Monomethylfumarate (MMF) is thought to be the bioactive ingredient of the drug Fumaderm (Biogen Idec, Cambridge, MA), licensed in Germany since 1994 for the treatment of moderate-to-severe psoriasis. Psoriasis is a common inflammatory hyperproliferative skin disorder that involves cross-talk between different cell types, including immune cells and keratinocytes. Psoriatic lesions are characterized by hyperproliferation, aberrant differentiation, and inflammation, with the psoriatic cytokine network maintained by communication between immune cells and keratinocytes. Recently, there is increasing evidence regarding the pivotal role of keratinocytes in mediating the disease process, and these cells can be regarded as safe therapeutic targets. From the data available on human subjects treated with Fumaderm, MMF is an effective antipsoriatic agent with known effects on immune cells. However, little is known about its direct effects on keratinocytes. We hypothesized that MMF has direct antiproliferative, prodifferentiative, and anti-inflammatory effects on keratinocytes. Indeed, MMF dose-dependently inhibited [(3)H]thymidine incorporation into DNA, indicating a direct antiproliferative action on keratinocytes. MMF significantly increased the protein level of keratin 10, the early keratinocyte differentiation marker, and the activity of transglutaminase, a late differentiation marker. These results are consistent with an ability of MMF to promote keratinocyte differentiation and inhibit proliferation, thereby improving psoriatic lesions. In 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced keratinocytes, MMF significantly inhibited the expression of the proinflammatory cytokines, tumor necrosis factor-α (TNFα), interleukin-6, and interleukin-1α as well as the production of TNFα. Our results support the notion that MMF has direct antiproliferative, prodifferentiative, and anti-inflammatory effects on keratinocytes, highlighting its potential use as a multifactorial antipsoriatic agent.

Pharmacogenetics of topical and systemic treatment of psoriasis

Psoriasis is a chronic inflammatory skin disease. The cause of psoriasis is unknown, although genetics may play a key role in its development. Treatment of the disease varies with severity. Topical drugs, such as corticosteroids, coal tar, retinoids and vitamin D analogs, are commonly used to treat mild psoriasis. Phototherapy and systemic drugs, such as calcineurin inhibitors, methotrexate, acitretin and biological drugs, are usually used to treat moderate-to-severe psoriasis. Not all patients respond well to treatment, and some can develop severe adverse effects. Interindividual differences in several genes may explain this variation in response to treatment. Pharmacogenetics and pharmacogenomics can facilitate more personalized medicine and prevent the adverse effects associated with treatment.

Persea americana Mill. Seed: Fractionation, Characterization, and Effects on Human Keratinocytes and Fibroblasts

Methanolic avocado (Persea americana Mill., Lauraceae) seed extracts were separated by preparative HSCCC. Partition and HSCCC fractions were principally characterized by LC-ESI-MS/MS analysis. Their in vitro influence was investigated on proliferation, differentiation, cell viability, and gene expression on HaCaT and normal human epidermal keratinocytes (NHEK) and normal human dermal fibroblasts (NHDF). The methanol-water partition (M) from avocado seeds and HSCCC fraction 3 (M.3) were mostly composed of chlorogenic acid and its isomers. Both reduced NHDF but enhanced HaCaT keratinocytes proliferation. HSCCC fraction M.2 composed of quinic acid among chlorogenic acid and its isomers inhibited proliferation and directly induced differentiation of keratinocytes as observed on gene and protein level. Furthermore, M.2 increased NHDF proliferation via upregulation of growth factor receptors. Salidrosides and ABA derivatives present in HSCCC fraction M.6 increased NHDF and keratinocyte proliferation that resulted in differentiation. The residual solvent fraction M.7 contained among low concentrations of ABA derivatives high amounts of proanthocyanidins B1 and B2 as well as an A-type trimer and stimulated proliferation of normal cells and inhibited the proliferation of immortalized HaCaT keratinocytes.

Gardiquimod inhibits the expression of calcium-induced differentiation markers in HaCaT cells

Toll-like receptor 7 (TLR7) is an important member in pattern recognition receptors families. TLR7 signal pathway is involved in the physiological process in many type cells, but the impact of TRL7 on differentiation in the human keratinocytes is still unknown. In this study, we investigated the expression of TLR7 in keratinocytes, and the effect of TLR7 agonist gardiquimod on the expression of calcium (Ca(2+))-induced keratinocytes differentiation markers in HaCaT cells. Immunohistochemistry and western-blotting analysis showed that TLR7 is expressed in basal keratinocytes of normal skin and in the human keratinocyte cell line HaCaT, but not expressed in the keratinocytes of psoriasis lesions. Pretreatment with gardiquimod could down-regulate Ca(2+)-induced differentiation marker expression and activate Raf-MEK-ERK and PI3K-AKT signal pathways in HaCaT cells. However, specific inhibitors studies showed that the down-regulation of the differentiation markers expression by gardiquimod was not dependent on the activation of these two pathways. TLR7 may play an important role in the pathogenesis of psoriasis through regulating the differentiation of the keratinocytes, and will give a new insight into the psoriasis.

Genetics of psoriasis and pharmacogenetics of biological drugs

Psoriasis is a chronic inflammatory disease of the skin. The causes of psoriasis are unknown, although family and twin studies have shown genetic factors to play a key role in its development. The many genes associated with psoriasis and the immune response include TNFα, IL23, and IL12. Advances in knowledge of the pathogenesis of psoriasis have enabled the development of new drugs that target cytokines (e.g., etanercept, adalimumab, and infliximab, which target TNFα, and ustekinumab, which targets the p40 subunit of IL23 and IL12). These drugs have improved the safety and efficacy of treatment in comparison with previous therapies. However, not all patients respond equally to treatment, possibly owing to interindividual genetic variability. In this review, we describe the genes associated with psoriasis and the immune response, the biological drugs used to treat chronic severe plaque psoriasis, new drugs in phase II and III trials, and current knowledge on the implications of pharmacogenomics in predicting response to these treatments.

The influence of 13-cis retinoic acid on human meibomian gland epithelial cells

PURPOSE

Meibomian gland dysfunction (MGD) is a primary cause of dry eye disease. One of the risk factors for MGD is exposure to 13-cis retinoic acid (13-cis RA), a metabolite of vitamin A. However, the mechanism is not well understood. We hypothesize that 13-cis RA inhibits cell proliferation, promotes cell death, alters gene and protein expressions, and attenuates cell survival pathways in human meibomian gland epithelial cells.

METHODS

To test our hypotheses, immortalized human meibomian gland epithelial cells were cultured with or without 13-cis RA for varying doses and time. Cell proliferation, cell death, gene expression, and proteins involved in proliferation/survival and inflammation were evaluated.

RESULTS

We found that 13-cis RA inhibited cell proliferation, induced cell death, and significantly altered the expression of 6726 genes, including those involved in cell proliferation, cell death, differentiation, keratinization, and inflammation, in human meibomian gland epithelial cells. Further, 13-cis RA also reduced the phosphorylation of Akt and increased the generation of interleukin-1β and matrix metallopeptidase 9.

CONCLUSIONS

Exposure to 13-cis RA inhibits cell proliferation, increases cell death, alters gene expression, changes signaling pathways, and promotes inflammatory mediator and protease expression in meibomian gland epithelial cells. These effects may be responsible, at least in part, for the 13-cis RA-related induction of MGD.

Identifying targets for topical RNAi therapeutics in psoriasis: assessment of a new in vitro psoriasis model

Diseases of the skin are amenable to RNAi-based therapies and targeting key components in the pathophysiology of psoriasis using RNAi may represent a successful new therapeutic strategy. We aimed to develop a straightforward and highly reproducible in vitro psoriasis model useful to study the effects of gene knockdown by RNAi and to identify new targets for topical RNAi therapeutics. We evaluated the use of keratinocytes derived from psoriatic plaques and normal human keratinocytes (NHKs). To induce a psoriatic phenotype in NHKs, combinations of pro-inflammatory cytokines (IL-1α, IL-17A, IL-6 and TNF-α) were tested. The model based on NHK met our needs of a reliable and predictive preclinical model, and this model was further selected for gene expression analyses, comprising a panel of 55 psoriasis-associated genes and five micro-RNAs (miRNAs). Gene silencing studies were conducted by using small interfering RNAs (siRNAs) and miRNA inhibitors directed against potential target genes such as CAMP and DEFB4 and miRNAs such as miR-203. We describe a robust and highly reproducible in vitro psoriasis model that recapitulates expression of a large panel of genes and miRNAs relevant to the pathogenesis of psoriasis. Furthermore, we show that our model is a powerful first step model system for testing and screening RNAi-based therapeutics.

AP1 transcription factors in epidermal differentiation and skin cancer

AP1 (jun/fos) transcription factors (c-jun, junB, junD, c-fos, FosB, Fra-1, and Fra-2) are key regulators of epidermal keratinocyte survival and differentiation and important drivers of cancer development. Understanding the role of these factors in epidermis is complicated by the fact that each protein is expressed, at different levels, in multiple cells layers in differentiating epidermis, and because AP1 transcription factors regulate competing processes (i.e., proliferation, apoptosis, and differentiation). Various in vivo genetic approaches have been used to study these proteins including targeted and conditional knockdown, overexpression, and expression of dominant-negative inactivating AP1 transcription factors in epidermis. Taken together, these studies suggest that individual AP1 transcription factors have different functions in the epidermis and in cancer development and that altering AP1 transcription factor function in the basal versus suprabasal layers differentially influences the epidermal differentiation response and disease and cancer development.

Effect of growth factors on the proliferation and gene expression of human meibomian gland epithelial cells

PURPOSE

We hypothesize that growth factors, including epidermal growth factor (EGF) and bovine pituitary extract (BPE), induce proliferation, but not differentiation (e.g., lipid accumulation), of human meibomian gland epithelial cells. We also hypothesize that these actions involve a significant upregulation of genes linked to cell cycle processes, and a significant downregulation of genes associated with differentiation. Our objective was to test these hypotheses.

METHODS

Immortalized human meibomian gland and conjunctival epithelial cells were cultured for varying time periods in the presence or absence of EGF, BPE, EGF + BPE, or serum, followed by cell counting, neutral lipid staining, or RNA isolation for molecular biological procedures.

RESULTS

Our studies show that growth factors stimulate a significant, time-dependent proliferation of human meibomian gland epithelial cells. These effects are associated with a significant upregulation of genes linked to cell cycle, DNA replication, ribosomes, and translation, and a significant decrease in those related to cell differentiation, tissue development, lipid metabolic processes, and peroxisome proliferator-activated receptor signaling. Serum-induced differentiation, but not growth factor-related proliferation, elicits a pronounced lipid accumulation in human meibomian gland epithelial cells. This lipogenic response is unique, and is not duplicated by human conjunctival epithelial cells.

CONCLUSIONS

Our results demonstrate that EGF and BPE stimulate human meibomian gland epithelial cells to proliferate. Further, our findings show that action is associated with an upregulation of cell cycle and translation ontologies, and a downregulation of genetic pathways linked to differentiation and lipid biosynthesis.

A novel control of human keratin expression: cannabinoid receptor 1-mediated signaling down-regulates the expression of keratins K6 and K16 in human keratinocytes in vitro and in situ

Cannabinoid receptors (CB) are expressed throughout human skin epithelium. CB1 activation inhibits human hair growth and decreases proliferation of epidermal keratinocytes. Since psoriasis is a chronic hyperproliferative, inflammatory skin disease, it is conceivable that the therapeutic modulation of CB signaling, which can inhibit both proliferation and inflammation, could win a place in future psoriasis management. Given that psoriasis is characterized by up-regulation of keratins K6 and K16, we have investigated whether CB1 stimulation modulates their expression in human epidermis. Treatment of organ-cultured human skin with the CB1-specific agonist, arachidonoyl-chloro-ethanolamide (ACEA), decreased K6 and K16 staining intensity in situ. At the gene and protein levels, ACEA also decreased K6 expression of cultured HaCaT keratinocytes, which show some similarities to psoriatic keratinocytes. These effects were partly antagonized by the CB1-specific antagonist, AM251. While CB1-mediated signaling also significantly inhibited human epidermal keratinocyte proliferation in situ, as shown by K6/Ki-67-double immunofluorescence, the inhibitory effect of ACEA on K6 expression in situ was independent of its anti-proliferative effect. Given recent appreciation of the role of K6 as a functionally important protein that regulates epithelial wound healing in mice, it is conceivable that the novel CB1-mediated regulation of keratin 6/16 revealed here also is relevant to wound healing. Taken together, our results suggest that cannabinoids and their receptors constitute a novel, clinically relevant control element of human K6 and K16 expression.

The association between GJB2 gene polymorphism and psoriasis: a verification study

Psoriasis is a chronic inflammatory skin disease with multifactorial etiology. Connexin 26 (Cx26), an important gap junction protein, has been found highly expressed in plaques of psoriasis. Recently, genome wide association studies (GWAS) identified one new single nucleotide polymorphism (SNP) in GJB2 gene coding for Cx26 protein associated with psoriasis in Chinese Han population. In this paper, we verified the GWAS data in Chinese Han population. Here we genotyped the polymorphism of GJB2 rs3751385:C>T in 371 psoriasis patients and 330 healthy controls in Chinese Han population using polymerase chain reaction restriction fragment length polymorphism assay (PCR-RFLP). Our case-control assay indicated decreased frequency of the GJB2 rs3751385 C allele in psoriasis patients compared with that in the healthy controls [p = 6.02 × 10(-5), Odds ratio (OR) = 0.793, 95 % confidence interval (CI) 0.706-0.889]. The result suggested that GJB2 gene polymorphism rs3751385:C>T was associated with psoriasis susceptibility of Chinese Han population.

Tight junctions and differentiation--a chicken or the egg question?

Skin barrier function is indispensable to prevent the uncontrolled loss of water and solutes and to protect the body from external assaults. To fulfil this function, keratinocytes undergo a complex pathway of differentiation that terminates in the formation of the stratum corneum. Additionally, tight junctions (TJs), which are cell-cell junctions localized in the stratum granulosum, are involved in the barrier function of the skin. Important biological and clinical roles of TJs are strongly suggested by altered TJ protein levels and distribution in skin diseases like psoriasis, ichthyosis and atopic dermatitis. Because these skin diseases show alterations in differentiation and TJs, it was suggested that changes in TJs might simply be a consequence of altered differentiation. However, in this viewpoint, we like to argue that the situation is not as simple and depends on the specific microenvironment. We discuss three hypotheses regarding the interplay between TJs/TJ proteins and differentiation: (1) TJs/TJ proteins are influenced by differentiation, (2) differentiation is influenced by TJs/TJ proteins, and (3) TJs/TJ proteins and differentiation are independent of each other. In addition, the concept is introduced that both processes are going on at the same time, which means that while one specific TJ protein/barrier component might be influenced by differentiation, the other may influence differentiation.