Is the filaggrin-histidine-urocanic acid pathway essential for stratum corneum acidification?

Clinical efficacy of a multilamellar cream on skin physiology and microbiome in an epidermal stress model: A controlled double-blinded study

INTRODUCTION

Stratum corneum (SC) is essential for skin barrier function, mitigating water loss and shielding against potentially harmful substances and allergens. The SC's lipid matrix, arranged in a lamellar structure, is integral to its protective role. Our study explores the restoration effects of a multilamellar cream with an acidic pH compared to a basic placebo cream on skin physiology and its interaction with the skin microbiome after stress induction via tape stripping (TS).

MATERIALS AND METHODS

In this double-blind study, 14 healthy participants aged 21-58 years were assessed pre- and post-tape stripping, followed by a 14 days application of a multilamellar test cream and a placebo cream with evaluations on days 7, 14 and 17 for sustained effects. Skin physiology was analysed in terms of epidermal barrier function, SC hydration and surface pH. The microbiome was analysed by 16S rRNA amplicon sequencing the 16S rRNA gene using Illumina MiSeq, with subsequent species identification.

RESULTS

Our study showed significant improvements in skin barrier repair and SC hydration with verum, particularly after 14 days of application, while both creams initially enhanced stratum corneum hydration. No significant changes in surface-pH were detected. The skin microbiome analysis revealed that TS slightly decreased alpha diversity, a trend that verum significantly reversed, enhancing diversity beyond baseline levels after 14 days. Overall, while both creams contributed to a broader microbial phyla diversity over time, no significant changes in the abundance of specific genera or species were noted between treatments.

DISCUSSION AND CONCLUSION

Our study delineates the efficacy of a pH-optimized multilamellar cream in enhancing epidermal barrier recovery and SC hydration post-sequential TS, in contrast to an unstructured basic placebo. Verum cream significantly improved skin barrier function and SC hydration at day 14, with sustained effects evident beyond the treatment period. Furthermore, the multilamellar formulation facilitated the restitution of cutaneous microbiome diversity, a key indicator of healthy skin ecology, underscoring the symbiotic relationship between barrier integrity and microbial composition. These findings underscore the importance of multilamellar emollient structures in dermatological therapeutics, with potential implications for the design of advanced skincare interventions that holistically support cutaneous resilience and homeostasis.

Prolyl Endopeptidase Is Involved in Filaggrinolysis and Cornification

FLG is a well-known biomarker of atopic dermatitis and skin dryness. Its full proteolysis (or filaggrinolysis) produces the major constituents of the natural moisturizing factor. Some proteases/peptidases remain to be identified in this multistep process. Mining 16 omics analyses, we identified prolyl endopeptidase (PREP) as a candidate peptidase. Indirect immunofluorescence and confocal analysis demonstrated its localization in the granular and deep cornified layers, where it colocalized with FLG. Tandem mass spectroscopy and fluorescent quenching activity assays showed that PREP cleaved several synthetic peptides derived from the FLG sequence, at the carboxyl side of an internal proline. Deimination of these peptides increased PREP enzymatic efficiency. Specific inhibition of PREP in reconstructed human epidermis using benzyloxycarbonyl-pro-prolinal induced the accumulation of FLG monomers. Downregulation of PREP expression in reconstructed human epidermis using RNA interference confirmed the impact of PREP on FLG metabolism and highlighted a more general role of PREP in keratinocyte differentiation. Indeed, quantitative global proteomic, western blotting, and RT-qPCR analyses showed a strong reduction in the expression of bleomycin hydrolase, known to be involved in filaggrinolysis, and of several other actors of cornification such as loricrin. Consequently, at the functional level, the transepidermal electric resistance was drastically reduced.

Correlation of non-invasive psycho-physiological and skin-physiological measures

INTRODUCTION

Psychological stress alters epidermal barrier function. While intensive studies on the underlying mechanism have been performed in mice, human studies are limited. Non-invasive skin-physiology measures have not yet been directly linked to non-invasive psycho-physiological assessments.

METHODS

Standard measures of (I) transepidermal water loss prior to and after experimental barrier perturbation via tape stripping, (II) skin surface pH, (III) electrodermal activity, and (IV) heart rate function were taken over a 24 h time period. To document perceived stress, a standardized stress self-assessment questionnaire, namely the Trierer Inventar zum chronischen Stress (TICS), was utilized.

RESULTS

Twenty healthy, Caucasian (Fitzpatrick skin phototype I-II), female volunteers (21-32 years, mean age 27, SD = 3.67 years) were included in this study (random sample). Significant correlations were shown for 24 h delta transepidermal water loss changes, that is, barrier repair kinetics (sympathetic activity) and heart rate variability (parasympathetic activity). Further correlations were noted for electrodermal activity and skin surface pH. Perceived stress, as documented by the TICS questionnaire, did not correlate with psycho- and skin physiological parameters, respectively.

CONCLUSION

The presented approaches may provide a basis for non-invasive objective research on the correlation between psychological stressors and epidermal barrier function.

L-4-thiazolylalanine (Protinol), a novel non-proteinogenic amino acid, demonstrates epidermal and dermal efficacy with clinically observable benefits

OBJECTIVE

Facial skin undergoes major structural and functional changes as a result of intrinsic and extrinsic factors. The goal of the current work is to demonstrate L-4-thiazolylalaine (L4, Protinol), a non-proteinogenic amino acid shown to stimulate the production of dermal proteins by fibroblasts, is an alternative efficacious topical ingredient for visible signs of ageing.

METHODS

In vitro studies using 3D human skin tissue models were performed to show changes in protein and gene expression of key dermal markers in samples treated with 0.3% L4 compared to vehicle control. In vivo evaluation of skin turnover was measured in volunteers after treatment with L4 compared to retinol. Skin biopsies (n = 30) were taken to investigate epidermal and dermal changes in cases treated with L4 and compared to retinol. Finally, a clinical evaluation (n = 28) was conducted to assess the efficacy of L4 over a base formulation using various ageing parameters within a population of women 46-66 years old with mild-to-moderate wrinkles.

RESULTS

In vitro studies on 3D tissues displayed significant changes in the dermal matrix via an increase in HA and pro-collagen I production and a decrease in the expression of inflammatory genes. In vivo biopsy studies demonstrated that L4 and retinol independently increased epidermal thickness and collagen remodelling significantly more compared with the base formula. Clinical evaluation showed firmer and smoother skin at day 28 post-treatment with L4 over the vehicle control without causing side effects such as redness or irritation.

CONCLUSION

L4 is a novel, multi-functional ingredient which offers a superior alternative to currently available technologies for improving epidermal and dermal parameters that change during ageing and photodamage.

Skin Barrier Function: The Interplay of Physical, Chemical, and Immunologic Properties

An intact barrier function of the skin is important in maintaining skin health. The regulation of the skin barrier depends on a multitude of molecular and immunological signaling pathways. By examining the regulation of a healthy skin barrier, including maintenance of the acid mantle and appropriate levels of ceramides, dermatologists can better formulate solutions to address issues that are related to a disrupted skin barrier. Conversely, by understanding specific skin barrier disruptions that are associated with specific conditions, such as atopic dermatitis or psoriasis, the development of new compounds could target signaling pathways to provide more effective relief for patients. We aim to review key factors mediating skin barrier regulation and inflammation, including skin acidity, interleukins, nuclear factor kappa B, and sirtuin 3. Furthermore, we will discuss current and emerging treatment options for skin barrier conditions.

Ovol1/2 loss-induced epidermal defects elicit skin immune activation and alter global metabolism

Skin epidermis constitutes the outer permeability barrier that protects the body from dehydration, heat loss, and myriad external assaults. Mechanisms that maintain barrier integrity in constantly challenged adult skin and how epidermal dysregulation shapes the local immune microenvironment and whole-body metabolism remain poorly understood. Here, we demonstrate that inducible and simultaneous ablation of transcription factor-encoding Ovol1 and Ovol2 in adult epidermis results in barrier dysregulation through impacting epithelial-mesenchymal plasticity and inflammatory gene expression. We find that aberrant skin immune activation then ensues, featuring Langerhans cell mobilization and T cell responses, and leading to elevated levels of secreted inflammatory factors in circulation. Finally, we identify failure to gain body weight and accumulate body fat as long-term consequences of epidermal-specific Ovol1/2 loss and show that these global metabolic changes along with the skin barrier/immune defects are partially rescued by immunosuppressant dexamethasone. Collectively, our study reveals key regulators of adult barrier maintenance and suggests a causal connection between epidermal dysregulation and whole-body metabolism that is in part mediated through aberrant immune activation.

Molecular Mechanisms of Scombroid Food Poisoning

Scombroid food poisoning (SFP) is a foodborne disease that develops after consumption of fresh fish and, rarely, seafood that has fine organoleptic characteristics but contains a large amount of exogenous histamine. SFP, like other food pseudo-allergic reactions (FPA), is a disorder that is clinically identical to allergic reactions type I, but there are many differences in their pathogenesis. To date, SFP has been widespread throughout the world and is an urgent problem, although exact epidemiological data on incidence varies greatly. The need to distinguish SFP from true IgE-associated allergy to fish and seafood is one of the most difficult examples of the differential diagnosis of allergic conditions. The most important difference is the absence of an IgE response in SFP. The pathogenesis of SFP includes a complex system of interactions between the body and chemical triggers such as exogenous histamine, other biogenic amines, cis-urocanic acid, salicylates, and other histamine liberators. Because of the wide range of molecular pathways involved in this process, it is critical to understand their differences. This may help predict and prevent poor outcomes in patients and contribute to the development of adequate hygienic rules and regulations for seafood product safety. Despite the vast and lengthy history of research on SFP mechanisms, there are still many blank spots in our understanding of this condition. The goals of this review are to differentiate various molecular mechanisms of SFP and describe methods of hygienic regulation of some biogenic amines that influence the concentration of histamine in the human body and play an important role in the mechanism of SFP.

Advances in Current Drugs and Formulations for the Management of Atopic Dermatitis

Atopic dermatitis (AD) is a chronic, relapsing inflammatory skin disease with a complex pathophysiology. Treatment of AD remains challenging owing to the presence of a wide spectrum of clinical phenotypes and limited response to existing therapies. However, recent genetic, immunological, and pathophysiological insights into the disease mechanism resulted in the invention of novel therapeutic drug candidates. This review provides a comprehensive overview of current therapies and assesses various novel drug delivery strategies currently under clinical investigation. Further, this review majorly emphasizes on various topical treatments including emollient therapies, barrier repair agents, topical corticosteroids (TCS), phosphodiesterase 4 (PDE4) inhibitors, calcineurin inhibitors, and Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway inhibitors. It also discusses biological and systemic therapies, upcoming treatments based on ongoing clinical trials. Additionally, this review scrutinized the use of pharmaceutical inactive ingredients in the approved topical dosage forms for AD treatment.

Revisiting the Roles of Filaggrin in Atopic Dermatitis

The discovery in 2006 that loss-of-function mutations in the filaggrin gene (FLG) cause ichthyosis vulgaris and can predispose to atopic dermatitis (AD) galvanized the dermatology research community and shed new light on a skin protein that was first identified in 1981. However, although outstanding work has uncovered several key functions of filaggrin in epidermal homeostasis, a comprehensive understanding of how filaggrin deficiency contributes to AD is still incomplete, including details of the upstream factors that lead to the reduced amounts of filaggrin, regardless of genotype. In this review, we re-evaluate data focusing on the roles of filaggrin in the epidermis, as well as in AD. Filaggrin is important for alignment of keratin intermediate filaments, control of keratinocyte shape, and maintenance of epidermal texture via production of water-retaining molecules. Moreover, filaggrin deficiency leads to cellular abnormalities in keratinocytes and induces subtle epidermal barrier impairment that is sufficient enough to facilitate the ingress of certain exogenous molecules into the epidermis. However, although FLG null mutations regulate skin moisture in non-lesional AD skin, filaggrin deficiency per se does not lead to the neutralization of skin surface pH or to excessive transepidermal water loss in atopic skin. Separating facts from chaff regarding the functions of filaggrin in the epidermis is necessary for the design efficacious therapies to treat dry and atopic skin.

Ni2+-Assisted Hydrolysis May Affect the Human Proteome; Filaggrin Degradation Ex Vivo as an Example of Possible Consequences

Deficiency in a principal epidermal barrier protein, filaggrin (FLG), is associated with multiple allergic manifestations, including atopic dermatitis and contact allergy to nickel. Toxicity caused by dermal and respiratory exposures of the general population to nickel-containing objects and particles is a deleterious side effect of modern technologies. Its molecular mechanism may include the peptide bond hydrolysis in X1-S/T-c/p-H-c-X2 motifs by released Ni2+ ions. The goal of the study was to analyse the distribution of such cleavable motifs in the human proteome and examine FLG vulnerability of nickel hydrolysis. We performed a general bioinformatic study followed by biochemical and biological analysis of a single case, the FLG protein. FLG model peptides, the recombinant monomer domain human keratinocytes in vitro and human epidermis ex vivo were used. We also investigated if the products of filaggrin Ni2+-hydrolysis affect the activation profile of Langerhans cells. We found X1-S/T-c/p-H-c-X2 motifs in 40% of human proteins, with the highest abundance in those involved in the epidermal barrier function, including FLG. We confirmed the hydrolytic vulnerability and pH-dependent Ni2+-assisted cleavage of FLG-derived peptides and FLG monomer, using in vitro cell culture and ex-vivo epidermal sheets; the hydrolysis contributed to the pronounced reduction in FLG in all of the models studied. We also postulated that Ni-hydrolysis might dysregulate important immune responses. Ni2+-assisted cleavage of barrier proteins, including FLG, may contribute to clinical disease associated with nickel exposure.

Epidermal Hyaluronan in Barrier Alteration-Related Disease

In skin, although the extracellular matrix (ECM) is highly developed in dermis and hypodermis, discrete intercellular spaces between cells of the living epidermal layers are also filled with ECM components. Herein, we review knowledge about structure, localization and role of epidermal hyaluronan (HA), a key ECM molecule. HA is a non-sulfated glycosaminoglycan non-covalently bound to proteins or lipids. Components of the basal lamina maintain some segregation between the epidermis and the underlying dermis, and all epidermal HA is locally synthesized and degraded. Functions of HA in keratinocyte proliferation and differentiation are still controversial. However, through interactions with partners, such as the TSG-6 protein, HA is involved in the formation, organization and stabilization of the epidermal ECM. In addition, epidermal HA is involved in the formation of an efficient epidermal barrier made of cornified keratinocytes. In atopic dermatitis (AD) with profuse alterations of the epidermal barrier, HA is produced in larger amounts by keratinocytes than in normal skin. Epidermal HA inside AD lesional skin is located in enlarged intercellular spaces, likely as the result of disease-related modifications of HA metabolism.

Towards Optimal pH of the Skin and Topical Formulations: From the Current State of the Art to Tailored Products

Acidic pH of the skin surface has been recognized as a regulating factor for the maintenance of the stratum corneum homeostasis and barrier permeability. The most important functions of acidic pH seem to be related to the keratinocyte differentiation process, the formation and function of epidermal lipids and the corneocyte lipid envelope, the maintenance of the skin microbiome and, consequently, skin disturbances and diseases. As acknowledged extrinsic factors that affect skin pH, topically applied products could contribute to skin health maintenance via skin pH value control. The obtained knowledge on skins’ pH could be used in the formulation of more effective topical products, which would add to the development of the so-called products ‘for skin health maintenance’. There is a high level of agreement that topical products should be acidified and possess pH in the range of 4 to 6. However, formulators, dermatologists and consumers would benefit from some more precise guidance concerning favorable products pH values and the selection of cosmetic ingredients which could be responsible for acidification, together with a more extensive understanding of the mechanisms underlaying the process of skin acidification by topical products.

3D-Organotypic Cultures to Unravel Molecular and Cellular Abnormalities in Atopic Dermatitis and Ichthyosis Vulgaris

Atopic dermatitis (AD) is characterized by dry and itchy skin evolving into disseminated skin lesions. AD is believed to result from a primary acquired or a genetically-induced epidermal barrier defect leading to immune hyper-responsiveness. Filaggrin (FLG) is a protein found in the cornified envelope of fully differentiated keratinocytes, referred to as corneocytes. Although FLG null mutations are strongly associated with AD, they are not sufficient to induce the disease. Moreover, most patients with ichthyosis vulgaris (IV), a monogenetic skin disease characterized by FLG homozygous, heterozygous, or compound heterozygous null mutations, display non-inflamed dry and scaly skin. Thus, all causes of epidermal barrier impairment in AD have not yet been identified, including those leading to the Th2-predominant inflammation observed in AD. Three dimensional organotypic cultures have emerged as valuable tools in skin research, replacing animal experimentation in many cases and precluding the need for repeated patient biopsies. Here, we review the results on IV and AD obtained with epidermal or skin equivalents and consider these findings in the context of human in vivo data. Further research utilizing complex models including immune cells and cutaneous innervation will enable finer dissection of the pathogenesis of AD and deepen our knowledge of epidermal biology.

Skin barrier in atopic dermatitis: beyond filaggrin

Atopic dermatitis is a chronic inflammatory skin disease with a complex pathogenesis, where changes in skin barrier and imbalance of the immune system are relevant factors. The skin forms a mechanic and immune barrier, regulating water loss from the internal to the external environment, and protecting the individual from external aggressions, such as microorganisms, ultraviolet radiation and physical trauma. Main components of the skin barrier are located in the outer layers of the epidermis (such as filaggrin), the proteins that form the tight junction (TJ) and components of the innate immune system. Recent data involving skin barrier reveal new information regarding its structure and its role in the mechanic-immunological defense; atopic dermatitis (AD) is an example of a disease related to dysfunctions associated with this complex.

Skin pH Is the Master Switch of Kallikrein 5-Mediated Skin Barrier Destruction in a Murine Atopic Dermatitis Model

Elevated skin surface pH has been reported in patients with atopic dermatitis. In this study, we explored the role of skin pH in the pathogenesis of atopic dermatitis using the NC/Tnd murine atopic dermatitis model. Alkalinization of the skin of asymptomatic NC/Tnd mice housed in specific pathogen-free conditions induced kallikrein 5 and activated protease-activated receptor 2, resulting in thymic stromal lymphopoietin secretion and a cutaneous T-helper 2 allergic response. This was associated with increased transepidermal water loss and development of eczematous lesions in these specific pathogen-free NC/Tnd mice, which normally do not suffer from atopic dermatitis. Injection of recombinant thymic stromal lymphopoietin also induced scratching behavior in the specific pathogen-free NC/Tnd mice. Thymic stromal lymphopoietin production and dermatitis induced by alkalinization of the skin could be blocked by the protease-activated receptor 2 antagonist ENMD-1068. In contrast, weak acidification of eczematous skin in conventionally housed NC/Tnd mice reduced kallikrein 5 activity and ameliorated the dermatitis. Onset of the dermatitis was associated with increased epidermal filaggrin expression and impaired activity of the sodium/hydrogen exchanger 1, a known regulator of skin pH. We conclude that alterations in skin pH directly modulate kallikrein 5 activity leading to skin barrier dysfunction, itch, and dermatitis via the protease-activated receptor 2-thymic stromal lymphopoietin pathway.

Metabolomics of Human Amniotic Fluid and Maternal Plasma during Normal Pregnancy

Metabolic profiles of amniotic fluid and maternal blood are sources of valuable information about fetus development and can be potentially useful in diagnosis of pregnancy disorders. In this study, we applied 1H NMR-based metabolic profiling to track metabolic changes occurring in amniotic fluid (AF) and plasma (PL) of healthy mothers over the course of pregnancy. AF and PL samples were collected in the 2nd (T2) and 3rd (T3) trimester, prolonged pregnancy (PP) until time of delivery (TD). A multivariate data analysis of both biofluids reviled a metabolic switch-like transition between 2nd and 3rd trimester, which was followed by metabolic stabilization throughout the rest of pregnancy probably reflecting the stabilization of fetal maturation and development. The differences were further tested using univariate statistics at α = 0.001. In plasma the progression from T2 to T3 was related to increasing levels of glycerol, choline and ketone bodies (3-hydroxybutyrate and acetoacetate) while pyruvate concentration was significantly decreased. In amniotic fluid, T2 to T3 transition was associated with decreasing levels of glucose, carnitine, amino acids (valine, leucine, isoleucine, alanine, methionine, tyrosine, and phenylalanine) and increasing levels of creatinine, succinate, pyruvate, choline, N,N-dimethylglycine and urocanate. Lactate to pyruvate ratio was decreased in AF and conversely increased in PL. The results of our study, show that metabolomics profiling can be used to better understand physiological changes of the complex interdependencies of the mother, the placenta and the fetus during pregnancy. In the future, these results might be a useful reference point for analysis of complicated pregnancies.

The frequency of polymorphic variants of filaggrin gene and clinical atopic dermatitis

Introduction

As far as pathogenesis of the atopic dermatitis (AD) is concerned, the roles of an impaired epidermal barrier and cornified cell envelope are widely emphasized.

Aim

The assessment of mutations of the filaggrin gene and their connection with the clinical picture of AD as well as selected allergological and environmental indicators.

Material and methods

105 patients with diagnosed AD on the basis of diagnostic criteria were included. For every patient of the examined group, quantitative determination of the total concentration of IgE and the concentration of IgE antibodies to selected allergens were examined. For all patients, studies were performed by means of analysis of two genomic gene variants of profilaggrin (FLG) – R501X and 2282del4.

Results

Loss-of-function mutations in the filaggrin gene were shown in 12 (11.4%) patients in the examined group. All patients in the study group who developed one of the tested loss-of-function mutations in the filaggrin gene demonstrated an extrinsic, allergic form of atopic dermatitis. A significant association (p = 0.0002) between the presence of one of the tested loss-of-function mutations in the filaggrin gene and elevated levels of total concentration of immunoglobulin E was shown.

Conclusions

Patients with AD of null mutations in the filaggrin gene demonstrate a relationship with the total and specific concentration of immunoglobulin E, specifically higher concentrations of IgE against aeroallergens and alimentary allergens as well as elevated levels of total immunoglobulin E.

Atopic March from Atopic Dermatitis to Asthma-Like Lesions in NC/Nga Mice Is Accelerated or Aggravated by Neutralization of Stratum Corneum but Partially Inhibited by Acidification

Prolonged and/or repeated damage to the skin barrier followed by atopic dermatitis (AD) is an initial step in atopic march that ultimately progresses to respiratory allergy. Maintaining normal stratum corneum (SC) acidity has been suggested as a therapeutic or preventive strategy for barrier impairment caused by skin inflammation. We determined whether a representative AD murine model, NC/Nga mice, develops airway inflammation after repeated epicutaneous application followed by inhalation of house dust mite (HDM), implying atopic march, and whether prolongation of non-proper SC acidity accelerates respiratory allergy. HDM was applied to the skin of NC/Nga mice, accompanied by the application of neutral cream (pH 7.4) or acidic cream (pH 2.8) for 6 weeks. Intranasal inhalation of HDM was administered daily during the last 3 days. Repeated epicutaneous applications followed by inhalation of HDM in NC/Nga mice induced an atopic march-like progression from AD lesions to respiratory allergy. Concurrent neutral cream treatment accelerated or aggravated the allergic inflammation in the skin and respiratory system, whereas an acidic cream partially alleviated these symptoms. Collectively, we developed an atopic march in NC/Nga mice by HDM application, and found that prevention of a neutral environment in the SC may be an interventional method to inhibit the march.

The Importance of Acidification in Atopic Eczema: An Underexplored Avenue for Treatment

Atopic dermatitis is a form of dermatitis commonly seen in children and adults. Its pathophysiology is complex and is centered on the barrier function of the epidermis. An important aspect of the skin’s barrier is pH, which in turn affects a number of parameters such as the skin flora, protease function, and mediators of inflammation and pruritus. Normal pH for non-neonatal skin is acidic and ranges from 4 to 6. Skin pH in atopic dermatitis patients is often increased into the neutral to basic range, and the resulting cascade of changes contributes to the phenotype of atopic dermatitis. Therefore, the maintenance of normal skin pH remains an important topic in understanding and treating atopic dermatitis. This article will review skin pH and its impact on normal barrier function, pathological pH changes in atopic dermatitis, and the therapeutic considerations related to restoring and maintaining pH balance.

From the outside-in: Epidermal targeting as a paradigm for atopic disease therapy

Atopic dermatitis (AD) is a chronic inflammatory skin disorder which can precede asthma and allergic rhinitis in a disease trajectory known as the atopic march. The pathophysiology of AD includes cutaneous inflammation, disrupted epidermal barrier function, xerosis and propensity to secondary infections. AD had previously been thought to arise from the systemic atopic immune response and therapies are therefore directed towards ameliorating Th2-mediated inflammation. However in recent years the focus has shifted towards primary defects in the skin barrier as an initiating event in AD. Links between loss-of-function variants in the gene encoding filaggrin and disrupted activity of epidermal serine proteases and AD have been reported. Based on these observations, a mechanism has been described by which epidermal barrier dysfunction may lead to inflammation and allergic sensitization. Exogenous and endogenous stressors can further exacerbate inherited barrier abnormalities to promote disease activity. Pathways underlying progression of the atopic march remain unclear, but recent findings implicate thymic stromal lymphopoietin as a factor linking AD to subsequent airway inflammation in asthma. This new appreciation of the epidermis in the development of AD should lead to deployment of more specific strategies to restore barrier function in atopic patients and potentially halt the atopic march.

Evolving concepts of neonatal skin

Skin barrier function is crucial to health. Importantly, the skin operates as an air-liquid, a liquid-liquid, and an immunological barrier. The skin's physical and chemical structures, as well as its microbiome, function to create, maintain, and repair this complex barrier.

Basis for enhanced barrier function of pigmented skin

Humans with darkly pigmented skin display superior permeability barrier function in comparison with humans with lightly pigmented skin. The reduced pH of the stratum corneum (SC) of darkly pigmented skin could account for enhanced function, because acidifying lightly pigmented human SC resets barrier function to darkly pigmented levels. In SKH1 (nonpigmented) versus SKH2/J (pigmented) hairless mice, we evaluated how a pigment-dependent reduction in pH could influence epidermal barrier function. Permeability barrier homeostasis is enhanced in SKH2/J versus SKH1 mice, correlating with a reduced pH in the lower SC that colocalizes with the extrusion of melanin granules. Darkly pigmented human epidermis also shows substantial melanin extrusion in the outer epidermis. Both acute barrier disruption and topical basic pH challenges accelerate reacidification of SKH2/J (but not SKH1) SC, while inducing melanin extrusion. SKH2/J mice also display enhanced expression of the SC acidifying enzyme, secretory phospholipase A2f (sPLA2f). Enhanced barrier function of SKH2/J mice could be attributed to enhanced activity of two acidic pH-dependent, ceramide-generating enzymes, β-glucocerebrosidase and acidic sphingomyelinase, leading to accelerated maturation of SC lamellar bilayers. Finally, organotypic cultures of darkly pigmented human keratinocytes display enhanced barrier function in comparison with lightly pigmented cultures. Together, these results suggest that the superior barrier function of pigmented epidermis can be largely attributed to the pH-lowering impact of melanin persistence/extrusion and enhanced sPLA2f expression.

Red face revisited: Endogenous dermatitis in the form of atopic dermatitis and seborrheic dermatitis

Atopic dermatitis and seborrheic dermatitis are multifactorial dermatitides that are known collectively as endogenous dermatitis. Both conditions can affect the face, but they have clinical, epidemiological, and physiopathological peculiarities that distinguish them from each other. These two diseases are very common all around the world. Atopic dermatitis is associated with xerosis and increased susceptibility to irritants and proteins; patients with this condition have a tendency to develop asthma, allergic rhinitis, and systemic manifestations that are mediated by immunoglobulin E. Seborrheic dermatitis is a moderate chronic dermatitis that is restricted to regions with a high production of sebum and areas that have cutaneous folds. There are many studies about pathophysiology related to the immunology and genetics of atopic dermatitis, but little is known about the genetic and immunological markers of seborrheic dermatitis.

Cohabitation--relationships of corynebacteria and staphylococci on human skin

Skin microbiome main cultivable aerobes in human are coagulase-negative staphylococci and lipophilic corynebacteria. Staphylococcus strains (155) belonging to 10 species and 105 strains of Corynebacterium belonging to nine species from the skin swabs of healthy male volunteers were investigated to determine their enzymatic activity to main metabolic substrates: carbohydrates, proteins, lipids, and response to factors present on the skin such as osmotic pressure, pH, and organic acids. The results showed that lipophilic corynebacteria have different capacity for adaptation on the skin than staphylococci. Most of Corynebacterium spp. expressed lack of proteinase, phospholipase, and saccharolytic enzymes activity. Corynebacteria were also more sensitive than Staphylococcus spp. to antimicrobial agents existing on human skin, especially to low pH. These characters can explain domination of Staphylococcus genera on healthy human skin. It can be suggested that within these two bacterial genus, there exists conceivable cooperation and reciprocal protection which results in their quantitative ratio. Such behavior must be considered as crucial for the stability of the population on healthy skin.

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.

sPLA2 and the epidermal barrier

The mammalian epidermis provides both an interface and a protective barrier between the organism and its environment. Lipid, processed into water-impermeable bilayers between the outermost layers of the epidermal cells, forms the major barrier that prevents water from exiting the organism, and also prevents toxins and infectious agents from entering. The secretory phospholipase 2 (sPLA2) enzymes control important processes in skin and other organs, including inflammation and differentiation. sPLA2 activity contributes to epidermal barrier formation and homeostasis by generating free fatty acids, which are required both for formation of lamellar membranes and also for acidification of the stratum corneum (SC). sPLA2 is especially important in controlling SC acidification and establishment of an optimum epidermal barrier during the first postnatal week. Several sPLA2 isoforms are present in the epidermis. We find that two of these isoforms, sPLA2 IIA and sPLA2 IIF, localize to the upper stratum granulosum and increase in response to experimental barrier perturbation. sPLA2F(-/-) mice also demonstrate a more neutral SC pH than do their normal littermates, and their initial recovery from barrier perturbation is delayed. These findings confirm that sPLA2 enzymes perform important roles in epidermal development, and suggest that the sPLA2IIF isoform may be central to SC acidification and barrier function. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Filaggrin deficiency leads to impaired lipid profile and altered acidification pathways in a 3D skin construct

Mutations in the filaggrin (FLG) gene are strongly associated with common dermatological disorders such as atopic dermatitis. However, the exact underlying pathomechanism is still ambiguous. Here, we investigated the impact of FLG on skin lipid composition, organization, and skin acidification using a FLG knockdown (FLG-) skin construct. Initially, sodium/hydrogen antiporter (NHE-1) activity was sufficient to maintain the acidic pH (5.5) of the reconstructed skin. At day 7, the FLG degradation products urocanic (UCA) and pyrrolidone-5-carboxylic acid (PCA) were significantly decreased in FLG- constructs, but the skin surface pH was still physiological owing to an upregulation of NHE-1. At day 14, secretory phospholipase A2 (sPLA2) IIA, which converts phospholipids to fatty acids, was significantly more activated in FLG- than in FLG+. Although NHE-1 and sPLA2 were able to compensate the FLG deficiency, maintain the skin surface pH, and ensured ceramide processing (no differences detected), an accumulation of free fatty acids (2-fold increase) led to less ordered intercellular lipid lamellae and higher permeability of the FLG- constructs. The interplay of the UCA/PCA and the sPLA2/NHE-1 acidification pathways of the skin and the impact of FLG insufficiency on skin lipid composition and organization in reconstructed skin are described.

Filaggrin - revisited

Profilaggrin (proFLG) and its processing products are critical to the health and appearance of skin. The recent identification of loss-of-function filaggrin (FLG) mutations as a predisposing factor in ichthyosis vulgaris and atopic dermatitis has lead to a resurgent interest in this enigmatic protein. Here, we review the literature on the structure and many functions of proFLG, from its role as a filament-aggregating protein and a source of natural moisturizing factor (NMF), to the more recent discoveries of its role in epidermal barrier formation and its more speculative functions as an antimicrobial and sunscreen. Finally, we discuss the relationship of proFLG with dry skin, the influence of moisturizers on NMF generation and speculate on next generation of FLG research.

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.

Abnormal epidermal barrier in the pathogenesis of atopic dermatitis

Despite the acknowledged contributions of a defective epidermal permeability barrier, dryness of the skin, and the propensity to develop secondary infections to the etiology and pathophysiology of atopic dermatitis (AD), these epidermal changes have, until recently, been assumed to reflect downstream consequences that are secondary phenomena of the primary immunologic abnormality--the historical "inside-outside" view that AD is basically an intrinsic inflammatory disease. In this review, we focused on the role of the epidermal barrier function in the pathophysiology of AD. Specifically, we presented data in support of a barrier-initiated pathogenesis of AD, ie, the "outside-inside" concept. First, we reviewed the evidence on the existence of inherited barrier abnormalities in AD. Reported studies on the possible association of mutations in the filaggrin gene (FLG) and data on human tissue kallikreins (KLKs) and AD have been addressed. We then dealt with the question of the causal link between impaired epidermal barrier and inflammation. Finally, the association between innate immune defense system and the increased avidity of Staphylococcus aureus for atopic skin was examined. Despite very convincing evidence to support the barrier-initiated pathogenesis of AD, the view that AD reflects the downstream consequences of a primary immunologic abnormality cannot be dismissed out of hand. Almost every line of evidence in support of the role of the epidermal barrier as the "driver" of the disease activity can be challenged and at least partially contradicted by opposing evidence. Until more data are available and until all the dust settles around this issue, we should take advantage of what we already know and use our knowledge for practical purposes. Deployment of specific strategies to restore the barrier function in AD means the use of moisturizers as first-line therapy.

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.

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.

Infant epidermal skin physiology: adaptation after birth

BACKGROUND

Functional and structural skin adaptation is a dynamic process which starts immediately after birth in humans and in mammalian skin in general. This adjustment to the extrauterine dry environment is accomplished in the first year of postnatal life of humans.

OBJECTIVES

To assess the dynamic changes in vivo after birth in the molecular composition and skin physiology parameters compared with older children and adults.

METHODS

The molecular composition of the stratum corneum (SC) and the water profile were investigated noninvasively by in vivo Raman confocal microscopy as a function of depth. Functional parameters including transepidermal water loss (characterizing epidermal permeability barrier), capacitance (as an indirect parameter for SC hydration) and skin surface pH were assessed noninvasively. The measurements were performed in 108 subjects divided into six age groups: full-term newborns (1-15 days), babies aged 5-6 weeks, babies aged 6±1 months, children aged 1-2 years, children aged 4-5 years and adults aged 20-35 years.

RESULTS

We showed that skin acidification is still under development during the first weeks of life. While the basal epidermal barrier is competent immediately after birth, the SC is less hydrated in the first 2 weeks of postnatal life. Similar continuous decreasing water content towards the surface for all age groups was observed, whereas this gradient was lower for the newborns. Dynamic changes in the amounts of the natural moisturizing factor constituents were revealed in the period of infancy.

CONCLUSIONS

We demonstrated the relation of formation of an acidic pH as well as underlying mechanisms in the induction of a fully hydrated SC over the first weeks of human life as a dynamic functional adaptation.

Skin barrier disruption: a requirement for allergen sensitization?

For at least half a century, noninvasive techniques have been available to quantify skin barrier function, and these have shown that a number of human skin conditions and disorders are associated with defects in skin permeability. In the past decade, several genes responsible for skin barrier defects observed in both monogenetic and complex polygenic disorders have been elucidated and functionally characterized. This has led to an explosion of work in the past 6 years that has identified pathways connecting epidermal barrier disruption and antigen uptake, as well as the quality and/or magnitude of the antigen-specific adaptive immune response. This review will introduce the notion that diseases arise from the dynamic crosstalk that occurs between skin barrier and the immune system using atopic dermatitis or eczema as the disease prototype. Nevertheless, the concepts put forth are highly relevant to a number of antigen-driven disorders for which skin barrier is at least transiently compromised, such as psoriasis, allergic contact dermatitis, and blistering disorders.

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.

Impact of filaggrin mutations on Raman spectra and biophysical properties of the stratum corneum in mild to moderate atopic dermatitis

BACKGROUND

Atopic dermatitis (AD) is associated with null mutations in the filaggrin (FLG) gene.

OBJECTIVE

To assess the impact of FLG null mutations on biophysical properties and the molecular composition of the stratum corneum (SC) in healthy individuals and AD patients.

METHODS

A total of 196 French adults, including 97 with a history of mild to moderate AD, were genotyped for the three major European FLG mutations. Components of the natural moisturizing factor (NMF), lipids and water content in the SC were determined using Raman spectroscopy. In addition, trans-epidermal water loss, capacitance and pH of the SC were measured.

RESULTS

Stratum corneum concentrations of total NMF, water, ornithine and urocanic acid (UCA) were significantly lower in AD patients than in healthy controls. Null mutations of FLG were detected in 4% of controls and 10% of AD patients. FLG mutations were associated with increased SC levels of lactate, reduced concentrations of most other NMF components and higher disease severity in AD patients. In AD patients without FLG mutations, the content of NMF constituents decreased with increasing disease severity. The concomittant presence of low concentrations of histidine, alanine and either glycine or pyrrolidone-5-carboxylic acid (PCA) in the SC was associated with FLG mutations with 92% specificity.

CONCLUSIONS

Our findings suggest a low prevalence of FLG mutations in mild AD and support an important role for filaggrin in determining the physicochemical parameters of the SC. The combined measurement of several filaggrin breakdown products in the SC may be useful to specifically predict the presence of FLG mutations.

Acidification in the epidermis and the role of secretory phospholipases

The function of the epidermis is to form an effective barrier between the dry, external environment and the interior of the body. The barrier specifically resides in the extracellular lipid membranes of the stratum corneum (SC) and an acidic pH is necessary to maintain its competency against various insults. The purpose of this review is to explore the mechanisms which are postulated to contribute to the acidification of the stratum corneum, including both exogenous and endogenous sources. However, recent research as pointed to several endogenous mechanisms as the major source of acidification, including a sodium/proton pump (NHE1) and free fatty acid conversion from phospholipids by secretory phospholipase A₂ (sPLA₂). sPLA₂ has been shown to play a central role in the formation of the SC “acid mantle” in the early maturation of the epidermis postnatally. Many aspects of this enzyme family are complex and still being elucidated in research and the most recent findings on the localization and functions of sPL A₂-IB, -IIA, -IIC, -IID, -IIE, -IIF, -III, -V, -X and -XII in the epidermis are presented here. Given their role in inflammatory dermatoses, such as psoriasis and atopic dermatitis, understanding this complex enzyme family can lead to novel, life-changing therapies.

Urocanic acid in the skin: a mixed blessing?

Located in the stratum corneum, urocanic acid is a major epidermal chromophore for UVR. This simple molecule has attracted a great deal of research interest over the past half century, initially as a putative "natural sunscreen" and later as a mediator of photoimmunosuppression with a consequent role in photocarcinogenesis. For the first time, Barresi and colleagues provide robust evidence for the photoprotective role of endogenous urocanic acid and reopen the debate on the relative "beneficial" and "detrimental" properties of this molecule.

Protease and protease-activated receptor-2 signaling in the pathogenesis of atopic dermatitis

Proteases in the skin are essential to epidermal permeability barrier homeostasis. In addition to their direct proteolytic effects, certain proteases signal to cells by activating protease-activated receptors (PARs), the G-protein-coupled receptors. The expression of functional PAR-2 on human skin and its role in inflammation, pruritus, and skin barrier homeostasis have been demonstrated. Atopic dermatitis (AD) is a multifactorial inflammatory skin disease characterized by genetic barrier defects and allergic inflammation, which is sustained by gene-environmental interactions. Recent studies have revealed aberrant expression and activation of serine proteases and PAR-2 in the lesional skin of AD patients. The imbalance between proteases and protease inhibitors associated with genetic defects in the protease/protease inhibitor encoding genes, increase in skin surface pH, and exposure to proteolytically active allergens contribute to this aberrant protease/ PAR-2 signaling in AD. The increased protease activity in AD leads to abnormal desquamation, degradation of lipid-processing enzymes and antimicrobial peptides, and activation of primary cytokines, thereby leading to permeability barrier dysfunction, inflammation, and defects in the antimicrobial barrier. Moreover, up-regulated proteases stimulate PAR-2 in lesional skin of AD and lead to the production of cytokines and chemokines involved in inflammation and immune responses, itching sensation, and sustained epidermal barrier perturbation with easier allergen penetration. In addition, PAR-2 is an important sensor for exogenous danger molecules, such as exogenous proteases from various allergens, and plays an important role in AD pathogenesis. Together, these findings suggest that protease activity or PAR-2 may be a future target for therapeutic intervention for the treatment of AD.