Flaky tail mouse denotes human atopic dermatitis in the steady state and by topical application with Dermatophagoides pteronyssinus extract

Aquaporin-9-expressing neutrophils are required for the establishment of contact hypersensitivity

Aquaporin-9 (AQP9), a water/glycerol channel protein, is expressed in several immune cells including neutrophils; however, its role in immune response remains unknown. Here we show the involvement of AQP9 in hapten-induced contact hypersensitivity (CHS), as a murine model of skin allergic contact dermatitis, using AQP9 knockout (AQP9^−/−) mice. First, the CHS response to hapten dinitrofluorobenzene (DNFB) was impaired in AQP9^−/− mice compared with wild-type (WT) mice. Adoptive transfer of sensitized AQP9^−/− draining lymph node (dLN) cells into WT recipients resulted in a reduced CHS response, indicating impaired sensitization in AQP9^−/− mice. Second, administration of WT neutrophils into AQP9^−/− mice during sensitization rescued the impaired CHS response. Neutrophil recruitment to dLNs upon hapten application was attenuated by AQP9 deficiency. Coincidentally, AQP9^−/− neutrophils showed a reduced CC-chemokine receptor 7 (CCR7) ligand-induced migration efficacy, which was attributed to the attenuated recruitment of neutrophils to dLNs. Furthermore, we found that neutrophil deficiency, observed in AQP9^−/− or neutrophil-depleted mice, decreased IL-17A production by dLN cells, which might be responsible for T cell activation during a subsequent CHS response. Taken together, these findings suggest that AQP9 is required for the development of sensitization during cutaneous acquired immune responses via regulating neutrophil function.

Atopic dermatitis-like skin lesions with IgE hyperproduction and pruritus in KFRS4/Kyo rats

BACKGROUND

Rats showing spontaneous atopic dermatitis (AD)-like skin lesions were observed in the Kyoto Fancy Rat Stock 4 (KFRS4) strain breeding colony.

OBJECTIVE

To establish the KFRS4 rat as a model of AD.

METHODS

The clinical symptoms of AD-like skin lesions were assessed by scoring the degree of dermatitis and examining scratching behavior. The transepidermal water loss was measured to evaluate skin barrier function. Cells infiltrating the skin lesions were identified using histological and immunohistological analyses. IgE and cytokine levels were measured to examine immune status. An ointment treatment experiment was carried out to characterize dermatitis in the KFRS4 rats.

RESULTS

Dermatitis initially appeared around 4 months of age and rapidly worsened from 6 to 8 months of age. The skin lesions accompanied scratching behavior and were predominantly observed in females. The increased transepidermal water loss indicated skin barrier dysfunction. Extensive infiltration of eosinophils, mast cells and lymphocytes was observed in the skin lesions. The plasma IgE level increased in accord with increasing severity of dermatitis. The Th2 and Th17 cytokine mRNA levels were significantly higher in the skin-draining lymph nodes than those in the non-skin-draining lymph nodes. It was demonstrated that betamethasone improved the symptoms of dermatitis. These findings demonstrated that dermatitis in the KFRS4 rats closely resembled that seen in human AD.

CONCLUSION

Female KFRS4 rats have the potential to serve as an animal model of human AD.

Filaggrin deficiency promotes the dissemination of cutaneously inoculated vaccinia virus

BACKGROUND

Eczema vaccinatum is a life-threatening complication of smallpox vaccination in patients with atopic dermatitis (AD) characterized by dissemination of vaccinia virus (VV) in the skin and internal organs. Mutations in the filaggrin (FLG) gene, the most common genetic risk factor for AD, confer a greater risk for eczema herpeticum in patients with AD, suggesting that it impairs the response to cutaneous viral infections.

OBJECTIVE

We sought to determine the effects of FLG deficiency on the response of mice to cutaneous VV inoculation.

METHODS

VV was inoculated by means of scarification of unsensitized skin or skin topically sensitized with ovalbumin in FLG-deficient flaky tail (ft/ft) mice or wild-type (WT) control mice. The sizes of primary and satellite skin lesions were measured, and hematoxylin and eosin staining was performed. VV genome copy numbers and cytokine mRNA levels were measured by using quantitative PCR.

RESULTS

VV inoculation in unsensitized skin of ft/ft mice, independent of the matted hair mutation, resulted in larger primary lesions, more abundant satellite lesions, heavier viral loads in internal organs, greater epidermal thickness, dermal cellular infiltration, and higher local Il17a, Il4, Il13, and Ifng mRNA levels than in WT control mice. VV inoculation at sites of topical ovalbumin application amplified all of these features in ft/ft mice but had no detectable effect in WT control mice. The number of satellite lesions and the viral loads in internal organs after cutaneous VV inoculation were significantly reduced in both unsensitized and topically sensitized ft/ftxIl17a(-/-) mice.

CONCLUSION

FLG deficiency predisposes to eczema vaccinatum. This is mediated primarily through production of IL-17A.

Mast cells are dispensable in a genetic mouse model of chronic dermatitis

Chronic inflammatory skin diseases, such as atopic dermatitis, affect a large percentage of the population, but the role of different immune cells in the pathogenesis of these disorders is largely unknown. Recently, we found that mice lacking fibroblast growth factor receptor 1 (Fgfr1) and Fgfr2 (K5-R1/R2 mice) in the epidermis have a severe impairment in the epidermal barrier, which leads to the development of a chronic inflammatory skin disease that shares many features with human atopic dermatitis. Using Fgfr1-/Fgfr2-deficient mice, we analyzed the consequences of the loss of mast cells. Mast cells accumulated and degranulated in the skin of young Fgfr1-/Fgfr2-deficient mice, most likely as a consequence of increased expression of the mast cell chemokine Ccl2. The increase in mast cells occurred before the development of histological abnormalities, indicating a functional role of these cells in the inflammatory skin phenotype. To test this hypothesis, we mated the Fgfr1-/Fgfr2-deficient mice with mast cell-deficient CreMaster mice. Surprisingly, loss of mast cells did not or only mildly affect keratinocyte proliferation, epidermal thickness, epidermal barrier function, accumulation and activation of different immune cells, or expression of different proinflammatory cytokines in the skin. These results reveal that mast cells are dispensable for the development of chronic inflammation in response to a defect in the epidermal barrier.

Mast cells and basophils in cutaneous immune responses

Mast cells and basophils share some functions in common and are generally associated with T helper 2 (Th2) immune responses, but taking basophils as surrogate cells for mast cell research or vice versa for several decades is problematic. Thus far, their in vitro functions have been well studied, but their in vivo functions remained poorly understood. New research tools for their functional analysis in vivo have revealed previously unrecognized roles for mast cells and basophils in several skin disorders. Newly developed mast cell-deficient mice provided evidence that mast cells initiate contact hypersensitivity via activating dendritic cells. In addition, studies using basophil-deficient mice have revealed that basophils were responsible for cutaneous Th2 skewing to haptens and peptide antigens but not to protein antigens. Moreover, human basophils infiltrate different skin lesions and have been implicated in the pathogenesis of skin diseases ranging from atopic dermatitis to autoimmune diseases. In this review, we will discuss the recent advances related to mast cells and basophils in human and murine cutaneous immune responses.

Lessons learned from mice and man: mimicking human allergy through mouse models

The relevance of using mouse models to represent human allergic pathologies is still unclear. Recent studies suggest the limitations of using models as a standard for assessing immune response and tolerance mechanisms, as mouse models often do not sufficiently depict human atopic conditions. Allergy is a combination of aberrant responses to innocuous environmental agents and the subsequent TH2-mediated inflammatory responses. In this review, we will discuss current paradigms of allergy - specifically, TH2-mediated and IgE-associated immune responses - and current mouse models used to recreate these TH2-mediated pathologies. Our overall goal is to highlight discrepancies that exist between mice and men by examining the advantages and disadvantages of allergic mouse models with respect to the human allergic condition.

Epidermal barriers

The epidermis functions as a physical barrier to the external environment and works to prevent loss of water from the skin. Numerous factors have been implicated in the formation of epidermal barriers, such as cornified envelopes, corneocytes, lipids, junctional proteins, proteases, protease inhibitors, antimicrobial peptides, and transcription factors. This review illustrates human diseases (ichthyoses) and animal models in which the epidermal barrier is disrupted or dysfunctional at steady state owing to ablation of one or more of the above factors. These diseases and animal models help us to understand the complicated mechanisms of epidermal barrier formation and give further insights on epidermal development.

Epidermal permeability barrier defects and barrier repair therapy in atopic dermatitis

Atopic dermatitis (AD) is a multifactorial inflammatory skin disease perpetuated by gene-environmental interactions and which is characterized by genetic barrier defects and allergic inflammation. Recent studies demonstrate an important role for the epidermal permeability barrier in AD that is closely related to chronic immune activation in the skin during systemic allergic reactions. Moreover, acquired stressors (e.g., Staphylococcus aureus infection) to the skin barrier may also initiate inflammation in AD. Many studies involving patients with AD revealed that defective skin barriers combined with abnormal immune responses might contribute to the pathophysiology of AD, supporting the outside-inside hypothesis. In this review, we discuss the recent advances in human and animal models, focusing on the defects of the epidermal permeability barrier, its immunologic role and barrier repair therapy in AD.

Defective maintenance of pH of stratum corneum is correlated with preferential emergence and exacerbation of atopic-dermatitis-like dermatitis in flaky-tail mice

BACKGROUND

Neutralization of stratum corneum (SC) pH, which is induced by a variety of stimuli, such as scratching, use of soap and inflammation, can stimulate activity of serine protease (SPase). Activation of SPase induces production of thymic stromal lymphopoietin (TSLP) through protease-activated receptor-2. Both reduced expression of natural moisturizing factors, which are required for maintenance of SC pH, and the preferential development of atopic dermatitis (AD)-like dermatitis are found in flaky-tail mice (FTM) with a loss-of-function mutation in filaggrin.

OBJECTIVE

We examined possible correlations between disturbance of responses to an exogenous stimulus of SC neutralization and the preferential emergence of AD-like dermatitis in FTM.

METHODS

FTM and wild-type mice (C57BL/6) were subjected to an SC-neutralization stimulus via application of 1,1,3,3-tetramethylguanidine (TMG). TMG was applied to young mice at a time when FTM had not yet developed significant dermatitis, and we examined their ability to maintain SC acidity and several parameters associated with AD-like dermatitis.

RESULTS

The recovery of SC pH after the application of TMG was delayed in FTM, presumably because of unchanged expression of Na(+)/H(+) antiporter 1, which is involved in maintenance of SC acidity. Cutaneous inflammation with elevated SPase activity and serum levels of TSLP, thymus and activation-regulated chemokine and IgE were induced only in TMG-treated FTM.

CONCLUSION

Our results suggest that defective maintenance of pH of SC is correlated with emergence and exacerbation of AD-like dermatitis in FTM.

Possible new therapeutic strategy to regulate atopic dermatitis through upregulating filaggrin expression

BACKGROUND

Nonsense mutations in filaggrin (FLG) represent a significant genetic factor in the cause of atopic dermatitis (AD).

OBJECTIVE

It is of great importance to find drug candidates that upregulate FLG expression and to determine whether increased FLG expression controls the development of AD.

METHODS

We screened a library of bioactives by using an FLG reporter assay to find candidates that promoted FLG mRNA expression using a human immortalized keratinocyte cell line (HaCaT). We studied the effect of the compound on keratinocytes using the human skin equivalent model. We examined the effect of the compound on AD-like skin inflammation in NC/Nga mice.

RESULTS

JTC801 promoted FLG mRNA and protein expression in both HaCaT and normal human epidermal keratinocytes. Intriguingly, JTC801 promoted the mRNA and protein expression levels of FLG but not the mRNA levels of other makers for keratinocyte differentiation, including loricrin, keratin 10, and transglutaminase 1, in a human skin equivalent model. In addition, oral administration of JTC801 promoted the protein level of Flg and suppressed the development of AD-like skin inflammation in NC/Nga mice.

CONCLUSION

This is the first observation that the compound, which increased FLG expression in human and murine keratinocytes, attenuated the development of AD-like skin inflammation in mice. Our findings provide evidence that modulation of FLG expression can be a novel therapeutic target for AD.

Tmem79/Matt is the matted mouse gene and is a predisposing gene for atopic dermatitis in human subjects

BACKGROUND

Atopic dermatitis (AD) is a major inflammatory condition of the skin caused by inherited skin barrier deficiency, with mutations in the filaggrin gene predisposing to development of AD. Support for barrier deficiency initiating AD came from flaky tail mice, which have a frameshift mutation in Flg and also carry an unknown gene, matted, causing a matted hair phenotype.

OBJECTIVE

We sought to identify the matted mutant gene in mice and further define whether mutations in the human gene were associated with AD.

METHODS

A mouse genetics approach was used to separate the matted and Flg mutations to produce congenic single-mutant strains for genetic and immunologic analysis. Next-generation sequencing was used to identify the matted gene. Five independently recruited AD case collections were analyzed to define associations between single nucleotide polymorphisms (SNPs) in the human gene and AD.

RESULTS

The matted phenotype in flaky tail mice is due to a mutation in the Tmem79/Matt gene, with no expression of the encoded protein mattrin in the skin of mutant mice. Matt(ft) mice spontaneously have dermatitis and atopy caused by a defective skin barrier, with mutant mice having systemic sensitization after cutaneous challenge with house dust mite allergens. Meta-analysis of 4,245 AD cases and 10,558 population-matched control subjects showed that a missense SNP, rs6684514, [corrected] in the human MATT gene has a small but significant association with AD.

CONCLUSION

In mice mutations in Matt cause a defective skin barrier and spontaneous dermatitis and atopy. A common SNP in MATT has an association with AD in human subjects.

A homozygous nonsense mutation in the gene for Tmem79, a component for the lamellar granule secretory system, produces spontaneous eczema in an experimental model of atopic dermatitis

BACKGROUND

Flaky tail (ma/ma Flg(ft/ft)) mice have a frameshift mutation in the filaggrin (Flg(ft)) gene and are widely used as a model of human atopic dermatitis associated with FLG mutations. These mice possess another recessive hair mutation, matted (ma), and develop spontaneous dermatitis under specific pathogen-free conditions, whereas genetically engineered Flg(-/-) mice do not.

OBJECTIVE

We identified and characterized the gene responsible for the matted hair and dermatitis phenotype in flaky tail mice.

METHODS

We narrowed down the responsible region by backcrossing ma/ma mice with wild-type mice and identified the mutation using next-generation DNA sequencing. We attempted to rescue the matted phenotype by introducing the wild-type matted transgene. We characterized the responsible gene product by using whole-mount immunostaining of epidermal sheets.

RESULTS

We demonstrated that ma, but not Flg(ft), was responsible for the dermatitis phenotype and corresponded to a Tmem79 gene nonsense mutation (c.840C>G, p.Y280*), which encoded a 5-transmembrane protein. Exogenous Tmem79 expression rescued the matted hair and dermatitis phenotype of Tmem79(ma/ma) mice. Tmem79 was mainly expressed in the trans-Golgi network in stratum granulosum cells in the epidermis in both mice and humans. The Tmem79(ma/ma) mutation impaired the lamellar granule secretory system, which resulted in altered stratum corneum formation and a subsequent spontaneous dermatitis phenotype.

CONCLUSIONS

The Tmem79(ma/ma) mutation is responsible for the spontaneous dermatitis phenotype in matted mice, probably as a result of impaired lamellar granule secretory system and altered stratum corneum barrier function.

Oral administration of poly-γ-glutamate ameliorates atopic dermatitis in Nc/Nga mice by suppressing Th2-biased immune response and production of IL-17A

Atopic dermatitis (AD) is a chronic inflammatory skin disease that is closely related to dysregulation of the T helper type 1 and 2 (Th1)/Th2 balance. A previous study showed that high molecular mass poly-γ-glutamate (γ-PGA) isolated from Bacillus subtilis sp. Chungkookjang induces the production of IL-12 from dendritic cells (DCs). Here, we investigated the effect of γ-PGA on AD-like skin disease using an Nc/Nga mouse model. In vitro, γ-PGA activated DCs and induced IL-12 production in mice. In vivo, oral administration of γ-PGA markedly reduced the AD symptoms, similar to the response seen in the dexamethasone (Dex)-treated group. Treatment with γ-PGA also decreased the serum levels of IgG1, the skin levels of Th2 cytokines, the extent of skin inflammation, and the accumulation of mast cells. Furthermore, γ-PGA was effective against established AD, significantly decreasing serum IgE and Th2 cytokines in the inflamed tissue. Interestingly, the production of IL-17A in splenocytes was also suppressed by γ-PGA, indicating that it inhibits both Th2 and Th17 immune responses. Collectively, these results suggest that oral administration of γ-PGA could be a therapeutic strategy for treating AD via the modulation of Th2-biased immune responses in an Nc/Nga mouse model.

Spontaneous atopic dermatitis-like symptoms in a/a ma ft/ma ft/J flaky tail mice appear early after birth

Loss-of-function mutations in human profilaggrin gene have been identified as the cause of ichthyosis vulgaris (IV), and as a major predisposition factor for atopic dermatitis (AD). Similarly, flaky tail (a/a ma ft/ma ft/J) mice were described as a model for IV, and shown to be predisposed to eczema. The aim of this study was to correlate the flaky tail mouse phenotype with human IV and AD, in order to dissect early molecular events leading to atopic dermatitis in mice and men, suffering from filaggrin deficiency. Thus, 5-days old flaky tail pups were analyzed histologically, expression of cytokines was measured in skin and signaling pathways were investigated by protein analysis. Human biopsies of IV and AD patients were analyzed histologically and by real time PCR assays. Our data show acanthosis and hyperproliferation in flaky tail epidermis, associated with increased IL1β and thymic stromal lymphopoietin (TSLP) expression, and Th2-polarization. Consequently, NFκB and Stat pathways were activated, and IL6 mRNA levels were increased. Further, quantitative analysis of late epidermal differentiation markers revealed increased Small proline-rich protein 2A (Sprr2a) synthesis. Th2-polarization and Sprr2a increase may result from high TSLP expression, as shown after analysis of 5-days old K14-TSLP tg mouse skin biopsies. Our findings in the flaky tail mouse correlate with data obtained from patient biopsies of AD, but not IV. We propose that proinflammatory cytokines are responsible for acanthosis in flaky tail epidermis, and together with the Th2-derived cytokines lead to morphological changes. Accordingly, the a/a ma ft/ma ft/J mouse model can be used as an appropriate model to study early AD onset associated with profilaggrin deficiency.

The stratum corneum: the rampart of the mammalian body

BACKGROUND

The stratum corneum (SC) is the outermost region of the epidermis and plays key roles in cutaneous barrier function in mammals. The SC is composed of 'bricks', represented by flattened, protein-enriched corneocytes, and 'mortar', represented by intercellular lipid-enriched layers. As a result of this 'bricks and mortar' structure, the SC can be considered as a 'rampart' that encloses water and solutes essential for physiological homeostasis and that protects mammals from physical, chemical and biological assaults.

STRUCTURES AND FUNCTIONS

The corneocyte cytoskeleton contains tight bundles of keratin intermediate filaments aggregated with filaggrin monomers, which are subsequently degraded into natural moisturizing compounds by various proteases, including caspase 14. A cornified cell envelope is formed on the inner surface of the corneocyte plasma membrane by transglutaminase-catalysed cross-linking of involucrin and loricrin. Ceramides form a lipid envelope by covalently binding to the cornified cell envelope, and extracellular lamellar lipids play an important role in permeability barrier function. Corneodesmosomes are the main adhesive structures in the SC and are degraded by certain serine proteases, such as kallikreins, during desquamation.

CLINICAL RELEVANCE

The roles of the different SC components, including the structural proteins in corneocytes, extracellular lipids and some proteins associated with lipid metabolism, have been investigated in genetically engineered mice and in naturally occurring hereditary skin diseases, such as ichthyosis, ichthyosis syndrome and atopic dermatitis in humans, cattle and dogs.

Protease activity enhances production of thymic stromal lymphopoietin and basophil accumulation in flaky tail mice

Epidermal barrier abnormality due to filaggrin deficiency is an important predisposing factor in the development of atopic dermatitis (AD). In addition, the expression of thymic stromal lymphopoietin (TSLP) in keratinocytes (KCs), induced by barrier disruption, can promote type 2 helper T-cell polarization. Protease activity, including protease-activated receptor-2 (PAR-2), is also known to be involved in epidermal barrier function in AD. However, to our knowledge, the relationship between protease activity and filaggrin deficiency from the perspective of AD has not been elucidated. Flaky tail (Flg(ft)) mice, known to have a mutation in the filaggrin gene, were used to assess the role of protease in KCs in the steady state and the mite-induced AD-like skin inflammation model. In the steady state, the expression and activity levels of endogenous proteases, kallikreins 5, 7, and 14, in the skin and TSLP were higher in Flg(ft) than in control mice. In addition, activation of PAR-2 by its agonist induced the production of TSLP in KCs of Flg(ft) mice, which was abrogated by a newly developed PAR-2 antagonist. Application of the PAR-2 antagonist improved symptoms and basophil accumulation in Flg(ft) mice treated with mite extracts. These results suggest that possibly through the PAR-2 activation in KCs, filaggrin deficiency induces TSLP production and basophil accumulation, which play important roles in the establishment of AD.

Topical treatment with basic fibroblast growth factor promotes wound healing and barrier recovery induced by skin abrasion

It has been reported that basic fibroblast growth factor (bFGF) promotes the healing of skin ulceration by inducing fibroblast proliferation, yet the role of bFGF on epidermal barrier function, especially from the perspective of scratch-induced skin abrasion, remains unknown. To this end, we initially developed an epidermal abrasion mouse model induced by scratching with a stainless-steel wire brush, and examined the effects of bFGF on the wound healing induced by skin abrasion. This procedure induced a significant elevation of transepidermal water loss (TEWL) in a scratch-count-dependent manner. This elevated TEWL was significantly decreased following topical application of bFGF to the skin. In addition, bFGF increased the expression of Ki67 in keratinocytes following mechanical scratching. These results suggest that bFGF enhances keratinocyte proliferation, which, in turn, repairs the skin barrier disruption and wounds caused by scratching in mice. Consistently, bFGF stimulated proliferation of normal human epidermal keratinocytes (NHEK). Intriguingly, the effect of bFGF and other growth factors on NHEK proliferation was additive. However, high cell density diminished the effect of bFGF on NHEK proliferation. This particular result can be explained by our observation that FGF receptor mRNA expression in NHEK was low under conditions of high cell density. Our findings suggest that bFGF stimulates keratinocyte proliferation, especially in a lower cell density environment, to repair skin wound in accord with skin barrier recovery.

Update of immune events in the murine contact hypersensitivity model: toward the understanding of allergic contact dermatitis

Allergic contact dermatitis (ACD) is one of the most common skin diseases, consisting of sensitization and elicitation phases. With the advancement of technology and the discovery of new types of immune cells, our knowledge of the immunological mechanisms of contact hypersensitivity (CHS) as a murine model of ACD has expanded significantly in the past decade. For example, by introducing regulatory T cells, CD4(+) T-helper 17 cells, and Langerin-positive dermal dendritic cells, the initiation and termination mechanism of CHS has been revealed. In addition, the role of mast cells in CHS, long a matter of debate, has become apparent by developing conditional mast cell-deficient mice. Moreover, the role of the innate immunity system, such as that of Toll-like receptor signaling, has made a breakthrough in this field. In this review, we will integrate the recent advancement of immunological mechanisms of both the sensitization and elicitation phases of CHS into the classic view, and we will discuss updated mechanisms on its development and future directions.

Atopic dermatitis results in intrinsic barrier and immune abnormalities: implications for contact dermatitis

Atopic dermatitis (AD), as well as irritant contact dermatitis (ICD) and allergic contact dermatitis (ACD), are common skin diseases. These diseases are characterized by skin inflammation mediated by activated innate immunity or acquired immune mechanisms. Although AD, ICD, and ACD can be encountered in pure forms by allergists and dermatologists, patients with AD often present with increased frequency of ICD and ACD. Although a disturbed barrier alone could potentiate immune reactivity in patients with AD through increased antigen penetration, additional immune mechanisms might explain the increased susceptibility of atopic patients to ICD and ACD. This review discusses cellular pathways associated with increased skin inflammation in all 3 conditions and presents mechanisms that might contribute to the increased rate of ICD and ACD in patients with AD.

Reduced expression of epidermal growth factor receptor, E-cadherin, and occludin in the skin of flaky tail mice is due to filaggrin and loricrin deficiencies

Disruption of skin barrier function leads to increases in the percutaneous transfer of allergens and the incidence of atopic dermatitis. Flaky tail (Flg(ft)) mice have been used as a model of atopic dermatitis with skin barrier dysfunction. Although Flg(ft) mice are known to have filaggrin mutation, the mechanism responsible for the skin barrier dysfunction that they display needs to be determined, especially for the roles of epidermal adhesion and junction proteins. Herein, we report the decreased expression of epidermal growth factor receptor (EGFR), E-cadherin, occludin, and SIRT1 in the skin of Flg(ft) mice, compared with those in C57BL/6J mice. Administration of N-acetyl-L-cysteine, an antioxidant, in the drinking water improved these protein expressions in the skin of Flg(ft) mice. Notably, we discovered that loricrin expression was suppressed in Flg(ft) mice. In vitro experiments showed that filaggrin small interfering RNA, loricrin small interfering RNA, or SIRT1 inhibitor sirtinol suppressed the expression levels of EGFR, E-cadherin, and occludin in a human immortalized keratinocyte cell line (HaCaT cells). Our findings suggest that the observed reductions in EGFR, E-cadherin, and occludin expression were due to filaggrin deficiency accompanied with subsequent loricrin deficiency and disruption of the SIRT1 pathway in the skin of Flg(ft) mice.

Altered stratum corneum barrier and enhanced percutaneous immune responses in filaggrin-null mice

BACKGROUND

Loss-of-function mutations in filaggrin are major predisposing factors for atopic dermatitis. Although various reports suggest a critical role for filaggrin in stratum corneum (SC) barrier formation, the lack of filaggrin-null (Flg(-/-)) mice has hampered detailed in vivo analysis of filaggrin's functions.

OBJECTIVE

We sought to generate Flg(-/-) mice and to assess the effect of filaggrin loss on SC barrier function and percutaneous immune responses.

METHODS

We generated Flg(-/-) mice using gene targeting and assessed the morphology, hydration, mechanical strength, and antigen permeability of their SC. Percutaneous immune responses were evaluated through irritant- and hapten-induced contact hypersensitivity studies and by measuring humoral responses to epicutaneous sensitization with protein antigen.

RESULTS

Newborn Flg(-/-) mice exhibited dry scaly skin. Despite marked decreases in natural moisturizing factor levels, which are filaggrin degradation products, SC hydration and transepidermal water loss were normal. Microscopic analyses suggested premature shedding of SC layers, and indeed, increased desquamation under mechanical stress was demonstrated. Loss of keratin patterns, which are critical for corneocyte stabilization, is likely attributable to fragility in the Flg(-/-) SC. Antigens penetrated the Flg(-/-) SC more efficiently, leading to enhanced responses in hapten-induced contact hypersensitivity and higher serum levels of anti-ovalbumin IgG(1) and IgE.

CONCLUSION

Complete filaggrin deficiency led to altered barrier integrity and enhanced sensitization, which are important factors in early-phase atopic dermatitis. Flg(-/-) mice should provide a valuable tool to further explore additional factors the dysfunction of which leads to uncontrolled inflammation in patients with atopic diseases.

Epidermal ADAM17 maintains the skin barrier by regulating EGFR ligand-dependent terminal keratinocyte differentiation

ADAM17 (a disintegrin and metalloproteinase 17) is ubiquitously expressed and cleaves membrane proteins, such as epidermal growth factor receptor (EGFR) ligands, l-selectin, and TNF, from the cell surface, thus regulating responses to tissue injury and inflammation. However, little is currently known about its role in skin homeostasis. We show that mice lacking ADAM17 in keratinocytes (A17(ΔKC)) have a normal epidermal barrier and skin architecture at birth but develop pronounced defects in epidermal barrier integrity soon after birth and develop chronic dermatitis as adults. The dysregulated expression of epidermal differentiation proteins becomes evident 2 d after birth, followed by reduced transglutaminase (TGM) activity, transepidermal water loss, up-regulation of the proinflammatory cytokine IL-36α, and inflammatory immune cell infiltration. Activation of the EGFR was strongly reduced in A17(ΔKC) skin, and topical treatment of A17(ΔKC) mice with recombinant TGF-α significantly improved TGM activity and decreased skin inflammation. Finally, we show that mice lacking the EGFR in keratinocytes (Egfr(ΔKC)) closely resembled A17(ΔKC) mice. Collectively, these results identify a previously unappreciated critical role of the ADAM17-EGFR signaling axis in maintaining the homeostasis of the postnatal epidermal barrier and suggest that this pathway could represent a good target for treatment of epidermal barrier defects.

The human epidermal differentiation complex: cornified envelope precursors, S100 proteins and the 'fused genes' family

  The skin is essential for survival and protects our body against biological attacks, physical stress, chemical injury, water loss, ultraviolet radiation and immunological impairment. The epidermal barrier constitutes the primordial frontline of this defense established during terminal differentiation. During this complex process proliferating basal keratinocytes become suprabasally mitotically inactive and move through four epidermal layers (basal, spinous, granular and layer, stratum corneum) constantly adapting to the needs of the respective cell layer. As a result, squamous keratinocytes contain polymerized keratin intermediate filament bundles and a water-retaining matrix surrounded by the cross-linked cornified cell envelope (CE) with ceramide lipids attached on the outer surface. These cells are concomitantly insulated by intercellular lipid lamellae and hold together by corneodesmosmes. Many proteins essential for epidermal differentiation are encoded by genes clustered on chromosomal human region 1q21. These genes constitute the 'epidermal differentiation complex' (EDC), which is divided on the basis of common gene and protein structures, in three gene families: (i) CE precursors, (ii) S100A and (iii) S100 fused genes. EDC protein expression is regulated in a gene and tissue-specific manner by a pool of transcription factors. Among them, Klf4, Grhl3 and Arnt are essential, and their deletion in mice is lethal. The importance of the EDC is further reflected by human diseases: FLG mutations are the strongest risk factor for atopic dermatitis (AD) and for AD-associated asthma, and faulty CE formation caused by TG1 deficiency causes life-threatening lamellar ichthyosis. Here, we review the EDC genes and the progress in this field.

One remarkable molecule: filaggrin

The discovery, in 2006, that loss-of-function mutations in the filaggrin (FLG) gene are the cause of ichthyosis vulgaris-the most common disorder of keratinization-and also a strong genetic risk factor for atopic eczema, marked a significant breakthrough in the understanding of eczema pathogenesis. Subsequent investigations of the role of FLG-null mutations have identified a series of significant associations with atopic disease phenotypes, including atopic asthma, allergic rhinitis, and peanut allergy. However, many questions remain to be answered in relation to the precise mechanisms by which deficiency of an intracellular protein expressed primarily in the differentiating epidermis may contribute to the development of cutaneous and systemic pathology. This review aims to highlight the key milestones in filaggrin research over the past 25 years, to discuss the mechanistic, clinical, and therapeutic implications, and to consider possible future directions for ongoing investigation.

Increased retinoic acid levels through ablation of Cyp26b1 determine the processes of embryonic skin barrier formation and peridermal development

The process by which the periderm transitions to stratified epidermis with the establishment of the skin barrier is unknown. Understanding the cellular and molecular processes involved is crucial for the treatment of human pathologies, where abnormal skin development and barrier dysfunction are associated with hypothermia and perinatal dehydration. For the first time, we demonstrate that retinoic acid (RA) levels are important for periderm desquamation, embryonic skin differentiation and barrier formation. Although excess exogenous RA has been known to have teratogenic effects, little is known about the consequences of elevated endogenous retinoids in skin during embryogenesis. Absence of cytochrome P450, family 26, subfamily b, polypeptide 1 (Cyp26b1), a retinoic-acid-degrading enzyme, results in aberrant epidermal differentiation and filaggrin expression, defective cornified envelopes and skin barrier formation, in conjunction with peridermal retention. We show that these alterations are RA dependent because administration of exogenous RA in vivo and to organotypic skin cultures phenocopy Cyp26b1(-/-) skin abnormalities. Furthermore, utilizing the Flaky tail (Ft/Ft) mice, a mouse model for human ichthyosis, characterized by mutations in the filaggrin gene, we establish that proper differentiation and barrier formation is a prerequisite for periderm sloughing. These results are important in understanding pathologies associated with abnormal embryonic skin development and barrier dysfunction.

Epidermal barrier dysfunction and cutaneous sensitization in atopic diseases

Classic atopic dermatitis is complicated by asthma, allergic rhinitis, and food allergies, cumulatively referred to as atopic diseases. Recent discoveries of mutations in the filaggrin gene as predisposing factors for atopic diseases have refocused investigators' attention on epidermal barrier dysfunction as a causative mechanism. The skin's barrier function has three elements: the stratum corneum (air-liquid barrier), tight junctions (liquid-liquid barrier), and the Langerhans cell network (immunological barrier). Clarification of the molecular events underpinning epidermal barrier function and dysfunction should lead to a better understanding of the pathophysiological mechanisms of atopic diseases.

Filaggrin genotype in ichthyosis vulgaris predicts abnormalities in epidermal structure and function

Although it is widely accepted that filaggrin (FLG) deficiency contributes to an abnormal barrier function in ichthyosis vulgaris and atopic dermatitis, the pathomechanism of how FLG deficiency provokes a barrier abnormality in humans is unknown. We report here that the presence of FLG mutations in Caucasians predicts dose-dependent alterations in epidermal permeability barrier function. Although FLG is an intracellular protein, the barrier abnormality occurred solely via a paracellular route in affected stratum corneum. Abnormal barrier function correlated with alterations in keratin filament organization (perinuclear retraction), impaired loading of lamellar body contents, followed by nonuniform extracellular distribution of secreted organelle contents, and abnormalities in lamellar bilayer architecture. In addition, we observed reductions in corneodesmosome density and tight junction protein expression. Thus, FLG deficiency provokes alterations in keratinocyte architecture that influence epidermal functions localizing to the extracellular matrix. These results clarify how FLG mutations impair epidermal permeability barrier function.

Th2 Cytokines and Atopic Dermatitis

Atopic dermatitis (AD), a chronic relapsing inflammatory skin disease, is increasing in prevalence around the world. Intensive research is ongoing to understand the mechanisms involved in the development of AD and offer new treatment options for patients suffering from AD. In this review, we highlight the importance of allergic Th2 responses in the development of the disease and summarize relevant literature, including genetic studies, studies of human skin and mechanistic studies on keratinocytes and mouse models of AD. We discuss the importance of the skin barrier and review recent findings on the pro-Th2 cytokines TSLP, IL-25, and IL-33, notably their ability to polarize dendritic cells and promote Th2 responses. After a brief update on the contribution of different T-cell subsets to AD, we focus on Th2 cells and the respective contributions of each of the Th2 cytokines (IL-4, IL-13, IL-5, IL-31, and IL-10) to AD. We conclude with a brief discussion of the current gaps in our knowledge and technical limitations.

Barrier dysfunction caused by environmental proteases in the pathogenesis of allergic diseases

Skin barrier dysfunction has emerged as a critical driving force in the initiation and exacerbation of atopic dermatitis and the "atopic march" in allergic diseases. The genetically determined barrier deficiency and barrier disruption by environmental and endogenous proteases in skin and epithelium are considered to increase the risk of sensitization to allergens and contribute to the exacerbation of allergic diseases. Sources of allergens such as mites, cockroaches, fungi, and pollen, produce or contain proteases, which are frequently themselves allergens. Staphylococcus aureus, which heavily colonizes the lesions of atopic dermatitis patients and is known to trigger a worsening of the disease, also produces extracellular proteases. Environmental proteases can cause barrier breakdown in the skin, not only in the epithelium, and stimulate various types of cells through IgE-independent mechanisms. Endogenous protease inhibitors control the functions of environmental and endogenous proteases. In this review, we focus on the barrier dysfunction caused by environmental proteases and roles of endogenous protease inhibitors in the pathogenesis of allergic diseases. Additionally, we examine the subsequent innate response to Th2-skewed adaptive immune reactions.

Upregulation of aquaporin-3 is involved in keratinocyte proliferation and epidermal hyperplasia

Aquaporin-3 (AQP3) is a water/glycerol-transporting protein expressed in keratinocytes of the epidermis. We previously showed that AQP3-mediated transport of water and glycerol is involved in keratinocyte migration and proliferation, respectively. However, the involvement of AQP3 in epidermal hyperplasia in skin diseases, such as atopic dermatitis (AD), is unknown. In this study, we found significantly increased AQP3 transcript and protein expression in the epidermis of human AD lesions. The upregulation of AQP3 expression in human keratinocytes by transfection with human AQP3 DNA plasmid was associated with increased cellular glycerol and ATP, as well as increased cell proliferation. Among several cytokines and chemokines produced in the skin, CCL17, which is highly expressed in AD, was found to be a strong inducer of AQP3 expression and enhanced keratinocyte proliferation. In mouse AD models, AQP3 was strongly overexpressed in the epidermis in wild-type mice. Epidermal hyperplasia was reduced in AQP3-deficient mice, with a decreased number of proliferating keratinocytes. These results suggest the involvement of AQP3 in epidermal hyperplasia by a mechanism involving upregulated AQP3 expression and consequent enhancement of keratinocyte proliferation.