Lipid and protein structures in the permeability barrier of mammalian epidermis

Primary Prevention of Canine Atopic Dermatitis: Breaking the Cycle-A Narrative Review

Canine atopic dermatitis (cAD) is a common and distressing skin condition in dogs, affecting up to 30% of the canine population. It not only impacts their quality of life but also that of their owners. Like human atopic dermatitis (hAD), cAD has a complex pathogenesis, including genetic and environmental factors. Current treatments focus on managing clinical signs, but they can be costly and have limitations. This article emphasizes the importance of preventing cAD from developing in the first place. Understanding the role of the skin's protective barrier is crucial, as its dysfunction plays a vital role in both hAD and cAD. hAD prevention studies have shown promising results in enhancing the skin barrier, but more research is needed to support more robust conclusions. While hAD primary prevention is currently a focal point of intensive investigation in human medicine, research on cAD primary prevention remains under-researched and almost non-existent. Pioneering effective prevention strategies for cAD holds immense potential to enhance the quality of life for both dogs and their owners. Additionally, it bears the promise of a translational impact on human research. Hence, further exploration of this crucial topic is not only relevant but also timely and imperative, warranting support and encouragement.

Two C18 hydroxy-cyclohexenone fatty acids from mammalian epidermis: Potential relation to 12R-lipoxygenase and covalent binding of ceramides

A key requirement in forming the water permeability barrier in the mammalian epidermis is the oxidation of linoleate esterified in a skin-specific acylceramide by the sequential actions of 12R-lipoxygenase, epidermal lipoxygenase-3, and the epoxyalcohol dehydrogenase SDR9C7 (short-chain dehydrogenase-reductase family 7 member 9). By mechanisms that remain unclear, this oxidation pathway promotes the covalent binding of ceramides to protein, forming a critical structure of the epidermal barrier, the corneocyte lipid envelope. Here, we detected, in porcine, mouse, and human epidermis, two novel fatty acid derivatives formed by KOH treatment from precursors covalently bound to protein: a "polar" lipid chromatographing on normal-phase HPLC just before omega-hydroxy ceramide and a "less polar" lipid nearer the solvent front. Approximately 100 μg of the novel lipids were isolated from porcine epidermis, and the structures were established by UV-spectroscopy, LC-MS, GC-MS, and NMR. Each is a C18 fatty acid and hydroxy-cyclohexenone with the ring on carbons C9-C14 in the polar lipid and C8-C13 in the less polar lipid. Overnight culture of [14C]linoleic acid with whole mouse skin ex vivo led to recovery of the 14C-labeled hydroxy-cyclohexenones. We deduce they are formed from covalently bound precursors during the KOH treatment used to release esterified lipids. KOH-induced intramolecular aldol reactions from a common precursor can account for their formation. Discovery of these hydroxy-cyclohexenones presents an opportunity for a reverse pathway analysis, namely to work back from these structures to identify their covalently bound precursors and relationship to the linoleate oxidation pathway.

Modulation of function and structure of stratum corneum in sphingomyelin synthase 2-deficient mice

Sphingomyelin synthase (SMS) synthesizes sphingomyelin (SM) from ceramide (Cer), a precursor of Cer. The effects of SMS deficiency on stratum corneum (SC) barrier function and SC lamellar structure are unknown. In this report, permeation of hydrophilic and lipophilic compounds through full-thickness skin or stripped skin of SMS2-knockout (KO) and wild-type (WT) mice was examined. Furthermore, small-angle and wide-angle X-ray scattering (SAXS and WAXS) measurements of the SC were performed as a function of temperature to analyze the lamellar structure and hydrocarbon chain packing, where a SC sample was changed from 10 °C to 120 °C at 2 °C/min and the X-ray diffraction profile in the small-angle region and the wide-angle region was observed. Skin permeability of the hydrophilic compound increased significantly for SMS2-KO mice when compared with that of WT mice. In contrast, no difference was observed in the penetration of lipophilic compounds in the skin of both SMS2-KO and WT mice. In SC of SMS2-KO mice, two sharp SAXS peaks were observed due to the lamellar structure with a repetition period of 4.8 nm. The WAXS revealed that the intensity ratio R_0.42/0.37 of the 0.42 nm peak at 2.4 nm^-1 to the 0.37 nm peak at 2.7 nm^-1 was smaller in the SMS2-KO mouse than in the WT mouse. Due to the temperature dependence of the WAXS, the peaks of 2.4 and 2.7 nm^-1 remained until the higher temperatures in SMS2-KO mouse SC than those in WT mouse SC. The results of X-ray diffraction suggest that deficiency of SMS2 may cause the appearance of highly ordered structures of SC, which in turn may reduce the barrier function of SC.

Low-flux electron diffraction study on body site dependence of stratum corneum structures in human skin

For analysis of the structure of human skin stratum corneum (SC), we introduced low-flux electron diffraction (ED) and developed a new statistical analysis method for obtained ED intensity profiles. By use of this method we compared the differences in the intercellular lipid organization on the SC corneocytes collected at human forehead, cheek, and forearm by the grid-stripping method. As a result, we found a significant regional difference in the distribution of lipid hydrocarbon chain packing domains in the SC; the ring-type ED pattern with orthorhombic symmetry was more often observed in the forearm SC than in the forehead and cheek SCs. We also found that the dependence of the background electron diffraction intensity on the modulus of the scattering vector differed significantly among them. The present method for the analysis of a large number of ED patterns of noninvasively obtained SC samples could be a powerful tool to scrutinize the structural difference between the SCs under various experimental conditions.

Triglycerides: How to Manage Patients with Elevated Triglycerides and When to Refer?

Hypertriglyceridemia (HTG) is among the most common dyslipidemias seen in clinical practice. Studies in recent years have demonstrated a causal relationship between triglyceride-rich lipoproteins (TRL) and cardiovascular disease (CVD). This is primarily due to enhanced atherogenicity of cholesterol-enriched remnants, the metabolic byproducts of TRLs. Other factors influencing atherogenicity of TRLs include apolipoprotein CIII-directed proinflammatory signaling pathways and triglyceride enrichment of low-density lipoprotein that results in overabundance of small dense atherogenic particles within a prooxidative milieu that serves as the gateway for unregulated incorporation by vascular wall macrophages. HTG is caused by familial and metabolic disorders as well as selected medications that impair TRL hydrolysis.

Three-Dimensional Skin Tissue Printing with Human Skin Cell Lines and Mouse Skin-Derived Epidermal and Dermal Cells

Since the skin covers most surfaces of the body, it is susceptible to damage, which can be fatal depending on the degree of injury to the skin because it defends against external attack and protects internal structures. Various types of artificial skin are being studied for transplantation to repair damaged skin, and recently, the production of replaceable skin using three-dimensional (3D) bioprinting technology has also been investigated. In this study, skin tissue was produced using a 3D bioprinter with human skin cell lines and cells extracted from mouse skin, and the printing conditions were optimized. Gelatin was used as a bioink, and fibrinogen and alginate were used for tissue hardening after printing. Printed skin tissue maintained a survival rate of 90% or more when cultured for 14 days. Culture conditions were established using 8 mM calcium chloride treatment and the skin tissue was exposed to air to optimize epidermal cell differentiation. The skin tissue was cultured for 14 days after differentiation induction by this optimized culture method, and immunofluorescent staining was performed using epidermal cell differentiation markers to investigate whether the epidermal cells had differentiated. After differentiation, loricrin, which is normally found in terminally differentiated epidermal cells, was observed in the cells at the tip of the epidermal layer, and cytokeratin 14 was expressed in the lower cells of the epidermis layer. Collectively, this study may provide optimized conditions for bioprinting and keratinization for three-dimensional skin production.

Biomolecular modifications during keratinocyte differentiation: Raman spectroscopy and chromatographic techniques

From the basal layer until the stratum corneum, lipid and protein biomarkers associated with morphological changes denote keratinocyte differentiation and characterize each epidermis layer. Herein, we followed keratinocyte differentiation in the early stages using HaCaT cells over a period of two weeks by two complementary analytical techniques: Raman microspectroscopy and high-performance liquid chromatography coupled with high resolution mass spectrometry. A high concentration of calcium in the medium induced HaCaT cell differentiation in vitro. The results from both techniques underlined the keratinocyte passage from the granular layer (day 9) to the stratum corneum layer (day 13). After 13 days of differentiation, we observed a strong increase in the lipid content, decrease in proteins, decrease in DNA, and a decrease in glucosylceramides/ceramides and sphingomyelins/ceramides ratios.

The chemistry and histology of sexually dimorphic mental glands in the freshwater turtle, Mauremys leprosa

Despite evidence from anatomy, behavior and genomics indicating that the sense of smell in turtles is important, our understanding of chemical communication in this group is still rudimentary. Our aim was to describe the microanatomy of mental glands (MGs) in a freshwater turtle, Mauremys leprosa (Geoemydidae), and to assess the chemical composition of their secretions with respect to variation among individuals and between sexes. MGs are paired sac-like organs on the gular region of the neck and are dimorphic in this species with males having fully functional holocrine glands while those of females appear non-secretory and vestigial. In adult males, the glandular epithelium of the inner portion of the gland provides exocytotic products as well as cellular debris into the lumen of the gland. The contents of the lumen can be secreted through the narrow duct portion of the gland ending in an orifice on the surface of the skin. Females have invaginated structures similar in general outline to male glands, but lack a glandular epithelium. Using gas chromatography coupled to mass spectrometry, we identified a total of 61 compounds in mental gland secretions, the most numerous being carboxylic acids, carbohydrates, alkanes, steroids and alcohols. The number of compounds per individual varied widely (mean (median) ± SD = 14.54 (13) ± 8.44; min = 3; max = 40), but only cholesterol was found in all samples. We found that the relative abundances of only six chemicals were different between the sexes, although males tended to have larger amounts of particular compounds. Although the lipid fraction of mental gland secretions is rich in chemical compounds, most occur in both sexes suggesting that they are metabolic byproducts with no role in chemical signaling. However, the relative amounts of some compounds tended to be higher in males, with significantly larger amounts of two carboxylic acids and one steroid, suggesting their putative involvement in chemical communication.

Predicting Skin Permeability by Means of Computational Approaches: Reliability and Caveats in Pharmaceutical Studies

The skin is the main barrier between the internal body environment and the external one. The characteristics of this barrier and its properties are able to modify and affect drug delivery and chemical toxicity parameters. Therefore, it is not surprising that permeability of many different compounds has been measured through several in vitro and in vivo techniques. Moreover, many different in silico approaches have been used to identify the correlation between the structure of the permeants and their permeability, to reproduce the skin behavior, and to predict the ability of specific chemicals to permeate this barrier. A significant number of issues, like interlaboratory variability, experimental conditions, data set building rationales, and skin site of origin and hydration, still prevent us from obtaining a definitive predictive skin permeability model. This review wants to show the main advances and the principal approaches in computational methods used to predict this property, to enlighten the main issues that have arisen, and to address the challenges to develop in future research.

State of the art in Stratum Corneum research: The biophysical properties of ceramides

This review is summarizing an important part of the state of the art in stratum corneum research. A complete overview on discoveries about the general biophysical and physicochemical properties of the known ceramide species' is provided. The ceramides are one of the three major components of the lipid matrix and mainly govern its properties and structure. They are shown to exhibit very little redundancy, despite the minor differences in their chemical structure. The results are discussed, compared to each other as well as the current base of knowledge. New interesting aspects and concepts are concluded or suggested. A novel interpretation of the 3-dimensional structure of the lipid matrix and its influence on the barrier function will be discussed. The most important conclusion is the presentation of a new and up to date theoretical model of the nanostructure of the short periodicity phase. The model suggests three perpendicular layers: The rigid head group region, the rigid chain region and, a liquid-like overlapping middle layer. The general principle of the skin barrier function is highlighted in regard to this structure and the ceramides biophysical and physicochemical properties. As a result of these considerations, the entropy vs. enthalpy principle is introduced, shedding light on the function as well as the effectiveness of the skin barrier. Additionally, general ideas to effectively overcome this barrier principle for dermal and transdermal delivery of actives or how to use it for specific targeting of the stratum corneum are proposed.

Vertebrates as a Bactericidal Agent

In Brazil, although a large number of animals are used in traditional medicine (at least 354 species), information about their biological activities is scarce. In this context, the objective of this study was to evaluate the bactericidal potential of zootherapeutic by-products from animals used in Brazilian traditional medicine and discuss the ecological and cultural consequences of such practices. The species analyzed were: Tupinambis merianae (skin), Iguana iguana (skin and body fat), Crotalus durissus (skin and body fat), Boa constrictor (skin), Euphractus sexcinctus (body fat) and Coendou prehensilis (quills). Experiments were performed with standard clinical strains of Escherichia coli (EC-ATCC10536) and Staphylococcus aureus (SA-ATCC 25923). For the microbiological assay, the zootherapeutics were evaluated using serial microdilutions. The results indicate that none of the samples possess inhibitory activity against standard bacterial strains. The in vitro ineffectiveness of the analyzed products demonstrate a necessity for new pharmacological research that encompass a large number of species of medicinal animals as well as highlight the importance of zootherapy in the context of plans for animal conservation.

Synthesis and Self-Assembly of Amphiphilic Star/Linear-Dendritic Polymers: Effect of Core versus Peripheral Branching on Reverse Micelle Aggregation

A series of branched polymers, consisting of a poly(ethylene glycol) (PEG) core and lipophilic peripheral dendrons, were synthesized and their self-assembly into reverse micelles studied toward the ultimate goal of carrier-mediated transdermal drug delivery. More specifically, this investigation systematically explores the structure-property contributions arising from location and extent of branching by varying the number of branch points at the core and the generation of dendrons at the polar/nonpolar interface. For branching at the core, PEGs were selected with one, two or four arms, with one terminal functionality per arm. For peripheral branching, end groups were modified with polyester dendrons (of dendritic generations 0, 1, and 2) for each of the three cores. Finally, lauric acid (LA) was used to esterify the periphery, yielding a library of branched, amphiphilic polymers. Characterization of these materials via MALDI-TOF MS, GPC and NMR confirmed their exceptionally well-defined structure. Furthermore, atomic force microscopy (AFM) and dynamic light scattering (DLS) confirmed these polymers' abilities to make discrete aggregates. As expected, increased multiplicity of branching resulted in more compact aggregates; however, the location of branching (core vs periphery) did not seem as important in defining aggregate size as the extent of branching. Finally, computational modeling of the branched amphiphile series was explored to elucidate the macromolecular interactions governing self-assembly in these systems.

Evaluation of anionic surfactants effects on the skin barrier function based on skin permeability

Anionic surfactants are often used for cleaning and pharmaceutical purposes because of their strong surfactancy and foaming property. However, they are rarely ingested orally, the skin is a part of the human body most affected by surfactants. Barrier function of the skin is very strong, but the anionic surfactants can cause serious damages to it. Recently, amino acid-based surfactants have attracted attention as a safer option owing to their biocompatibility. Cytotoxicity examinations revealed that the amino acid-based surfactants are superior to sulfate-based surfactants. However, a systematical and comprehensive study related to the effect of these surfactants on skin barrier function has not yet been reported. In this work, skin permeation test using the skin of hairless mice and HPLC method is carried out. The material transmission speed through skin in a steady state was different between each surfactant treatment. We performed a comprehensive analysis of the effect of surfactants on skin barrier function and defined Transmission Index as an index for the degree of effect of surfactants. Glutamate series amino acid-based surfactant were effective to Transmission Index and we guessed the cause was due to adsorption. Based on the finding this study, we suggest using adsorptive property as a measure to the effect on the skin barrier function.

Abnormal Sphingolipid World in Inflammation Specific for Lysosomal Storage Diseases and Skin Disorders

Research in recent years has shown that sphingolipids are essential signalling molecules for the proper biological and structural functioning of cells. Long-term studies on the metabolism of sphingolipids have provided evidence for their role in the pathogenesis of a number of diseases. As many inflammatory diseases, such as lysosomal storage disorders and some dermatologic diseases, including psoriasis, atopic dermatitis and ichthyoses, are associated with the altered composition and metabolism of sphingolipids, more studies precisely determining the responsibilities of these compounds for disease states are required to develop novel pharmacological treatment opportunities. It is worth emphasizing that knowledge from the study of inflammatory metabolic diseases and especially the possibility of their treatment may lead to insight into related metabolic pathways, including those involved in the formation of the epidermal barrier and providing new approaches towards workable therapies.

High sensitivity optical measurement of skin gloss

We demonstrate a low-cost optical method for measuring the gloss properties with improved sensitivity in the low gloss regime, relevant for skin gloss properties. The gloss estimation method is based on, on the one hand, the slope of the intensity gradient in the transition regime between specular and diffuse reflection and on the other on the sum over the intensities of pixels above threshold, derived from a camera image obtained using unpolarized white light illumination. We demonstrate the improved sensitivity of the two proposed methods using Monte Carlo simulations and experiments performed on ISO gloss calibration standards with an optical prototype. The performance and linearity of the method was compared with different professional gloss measurement devices based on the ratio of specular to diffuse intensity. We demonstrate the feasibility for in-vivo skin gloss measurements by quantifying the temporal evolution of skin gloss after application of standard paraffin cream bases on skin. The presented method opens new possibilities in the fields of cosmetology and dermatopharmacology for measuring the skin gloss and resorption kinetics and the pharmacodynamics of various external agents.

Stratum corneum modulation by chemical enhancers and lipid nanostructures: implications for transdermal drug delivery

Skin is the outermost and largest protective covering of the body. The uppermost layer of the skin, stratum corneum also called the horny layer is composed of keratin-filled cells covered by a lipid matrix which shields the skin from physical and chemical entrants. The lipid lamellar structure comprises of ceramides, cholesterol, fatty acids and proteins. Chemical enhancers that mimic the lamellar chemistry, reversibly fluidize the latter can be utilized for enhancing transport of cargo across the epidermis into the dermis. This review deals with the stratum corneum chemistry, mechanisms to modulate its packing with the aid of chemical enhancers, biophysical techniques for characterization and applications in the design of nature-inspired biocompatible lipid nanostructures for transdermal delivery of drugs and bioactive agents.

Raman active components of skin cancer

Raman spectroscopy (RS) has shown great potential in noninvasive cancer screening. Statistically based algorithms, such as principal component analysis, are commonly employed to provide tissue classification; however, they are difficult to relate to the chemical and morphological basis of the spectroscopic features and underlying disease. As a result, we propose the first Raman biophysical model applied to in vivo skin cancer screening data. We expand upon previous models by utilizing in situ skin constituents as the building blocks, and validate the model using previous clinical screening data collected from a Raman optical fiber probe. We built an 830nm confocal Raman microscope integrated with a confocal laser-scanning microscope. Raman imaging was performed on skin sections spanning various disease states, and multivariate curve resolution (MCR) analysis was used to resolve the Raman spectra of individual in situ skin constituents. The basis spectra of the most relevant skin constituents were combined linearly to fit in vivo human skin spectra. Our results suggest collagen, elastin, keratin, cell nucleus, triolein, ceramide, melanin and water are the most important model components. We make available for download (see supplemental information) a database of Raman spectra for these eight components for others to use as a reference. Our model reveals the biochemical and structural makeup of normal, nonmelanoma and melanoma skin cancers, and precancers and paves the way for future development of this approach to noninvasive skin cancer diagnosis.

Histological study of white rhinoceros integument

In this study, we report findings from a microscopic analysis of the white rhinoceros (Ceratotherium simum) integumentary ultrastructure. Skin samples from the cheek, shoulder, flank and rump were taken from a 46-year-old female southern white rhinoceros and examined using H&E and elastic histological stains. The epidermis was thickest in the flank (1.003 mm) followed by the rump, cheek and shoulder. The stratum corneum comprised more than half the epidermal thickness. Numerous melanin granules were found in the basal and spinosum layers. The epidermal-dermal junction was characterized by abundant papillary folds increasing surface contact between integument layers. Most of the dermal thickness consisted of organized collagen bundles with scattered elastic fibers. Collagen fiber bundles were thickest in the flank (210.9 μm) followed by shoulder, rump and cheek. Simple coiled sweat glands were present in the dermis, but hair and sebaceous glands were absent. Together, these data suggest the white rhinoceros has a unique integumentary system among large terrestrial herbivores.

Phospholipase Cδ1 regulates p38 MAPK activity and skin barrier integrity

Keratinocytes undergo a unique type of programmed cell death known as cornification, which leads to the formation of the stratum corneum (SC), the main physical barrier of the epidermis. A defective epidermal barrier is a hallmark of the two most common inflammatory skin disorders, psoriasis, and atopic dermatitis. However, the detailed molecular mechanisms of skin barrier formation are not yet fully understood. Here, we showed that downregulation of phospholipase C (PLC) δ1, a Ca²⁺-mobilizing and phosphoinositide-metabolizing enzyme abundantly expressed in the epidermis, impairs the barrier functions of the SC. PLCδ1 downregulation also impairs localization of tight junction proteins. Loss of PLCδ1 leads to a decrease in intracellular Ca²⁺ concentrations and nuclear factor of activated T cells activity, along with hyperactivation of p38 mitogen-activated protein kinase (MAPK) and inactivation of RhoA. Treatment with a p38 MAPK inhibitor reverses the barrier defects caused by PLCδ1 downregulation. Interestingly, this treatment also attenuates psoriasis-like skin inflammation in imiquimod-treated mice. These findings demonstrate that PLCδ1 is essential for epidermal barrier integrity. This study also suggests a possible link between PLCδ1 downregulation, p38 MAPK hyperactivation, and barrier defects in psoriasis-like skin inflammation.

Arrestant Effect of Human Scalp Components on Head Louse (Phthiraptera: Pediculidae) Behavior

Relevant evidence has shown that parasites process host-related information using chemical, visual, tactile, or auditory cues. However, the cues that are involved in the host-parasite interaction between Pediculus humanus capitis (De Geer 1767) and humans have not been identified yet. In this work, we studied the effect of human scalp components on the behavior of adult head lice. Filter paper segments were rubbed on volunteers' scalps and then placed in the experimental arena, where adult head lice were individually tested. The movement of the insects was recorded for each arena using the software EthoVision. Average movement parameters were calculated for the treatments in the bioassays such as total distance, velocity, number of times a head louse crossed between zones of the arena, and time in each zone of the arena. We found that scalp components induced head lice to decrease average locomotor activity and to remain arrested on the treated paper. The effect of the ageing of human scalp samples in the response of head lice was not statistically significant (i.e., human scalp samples of 4, 18, 40, and 60 h of ageing did not elicit a significant change in head louse behavior). When we analyzed the effect of the sex in the response of head lice to human scalp samples, males demonstrated significant differences. Our results showed for the first time the effect of host components conditioning head lice behavior. We discuss the role of these components in the dynamic of head lice infestation.

Depth-selective photothermal IR spectroscopy of skin: potential application for non-invasive glucose measurement

Photothermal depth profiling is applied to total internal reflection enhanced photothermal deflection spectroscopy (TIR-PTDS) in order to study skin characteristicsin vivoand to improve the sensing technique for non-invasive glucose monitoring.

Chemical penetration enhancers in stratum corneum - Relation between molecular effects and barrier function

Skin is attractive for drug therapy because it offers an easily accessible route without first-pass metabolism. Transdermal drug delivery is also associated with high patient compliance and through the site of application, the drug delivery can be locally directed. However, to succeed with transdermal drug delivery it is often required to overcome the low permeability of the upper layer of the skin, the stratum corneum (SC). One common strategy is to employ so-called penetration enhancers that supposedly act to increase the drug passage across SC. Still, there is a lack of understanding of the molecular effects of so-called penetration enhancers on the skin barrier membrane, the SC. In this study, we provide a molecular characterization of how different classes of compounds, suggested as penetration enhancers, influence lipid and protein components in SC. The compounds investigated include monoterpenes, fatty acids, osmolytes, surfactant, and Azone. We employ natural abundance (13)C polarization transfer solid-state nuclear magnetic resonance (NMR) on intact porcine SC. With this method it is possible to detect small changes in the mobility of the minor fluid lipid and protein SC components, and simultaneously obtain information on the major fraction of solid SC components. The balance between fluid and solid components in the SC is essential to determine macroscopic material properties of the SC, including barrier and mechanical properties. We study SC at different hydration levels corresponding to SC in ambient air and under occlusion. The NMR studies are complemented with diffusion cell experiments that provide quantitative data on skin permeability when treated with different compounds. By correlating the effects on SC molecular components and SC barrier function, we aim at deepened understanding of diffusional transport in SC, and how this can be controlled, which can be utilized for optimal design of transdermal drug delivery formulations.

mTOR inhibition by rapamycin increases ceramide synthesis by promoting transforming growth factor-β1/Smad signaling in the skin

Although mammalian target of rapamycin (mTOR) mediates a wide variety of biological functions, little information is available on the effect of mTOR on the functions of skin cells. In this study, we investigated effects of mTOR inhibition by rapamycin on ceramide synthesis in the skin of rats and human keratinocytes and its regulatory mechanisms. The phosphorylation of p70 S6 kinase, which indicates mTOR activation, was induced in the skin of rats fed a high-fat diet, but this abnormality was reversed by supplementation with rapamycin. Ceramide levels and the mRNA levels of serine palmitoyltransferase (SPT) and transforming growth factor (TGF)-β1 were suppressed in the skin of rats fed high-fat diets, but this abnormality was reversed by supplementation with rapamycin. TGF-β1-induced SPT mRNA expression was blocked by SB525334, an inhibitor of TGF-β1-induced Smad2/3 nuclear localization, in human keratinocytes. Rapamycin-induced SPT mRNA expression was blocked by an anti-TGF-β1 antibody or SB525334 in human keratinocytes. These results show that mTOR inhibition by rapamycin increases ceramide synthesis by promoting TGF-β1/Smad signaling in the skin.

Skin lipids: localization of ceramide and fatty acid in the unit cell of the long periodicity phase

The lipid matrix of the skin's stratum corneum plays a key role in the barrier function, which protects the body from desiccation. The lipids that make up this matrix consist of ceramides, cholesterol, and free fatty acids, and can form two coexisting crystalline lamellar phases: the long periodicity phase (LPP) and the short periodicity phase (SPP). To fully understand the skin barrier function, information on the molecular arrangement of the lipids in the unit cell of these lamellar phases is very desirable. To determine this arrangement in previous studies, we examined the molecular arrangement of the SPP. In this study, neutron diffraction studies were performed to obtain information on the molecular arrangement of the LPP. The diffraction pattern reveals nine diffraction orders attributed to the LPP with a repeating unit of 129.4 ± 0.5 Å. Using D2O/H2O contrast variation, the scattering length density profiles were calculated for protiated samples and samples that included either the perdeuterated acyl chain of the most abundant ceramide or the most abundant perdeuterated fatty acid. Both perdeuterated chains are predominantly located in the central part of the unit cell with substantial interdigitation of the acyl chains in the unit cell center. However, a fraction of the perdeuterated chains is also located near the border of the unit cell with their acyl chains directing toward the center. This arrangement of lipids in the LPP unit cell corresponds with the location of their lipid headgroups at the border and also inside of the unit cell at a well-defined position (±21 Å from the unit cell center), indicative of a three-layer lipid arrangement within the 129.4 ± 0.5 Å repeating unit.

From Hyperactive Connexin26 Hemichannels to Impairments in Epidermal Calcium Gradient and Permeability Barrier in the Keratitis-Ichthyosis-Deafness Syndrome

The keratitis-ichthyosis-deafness (KID) syndrome is characterized by corneal, skin, and hearing abnormalities. KID has been linked to heterozygous dominant missense mutations in the GJB2 and GJB6 genes, encoding connexin26 and 30, respectively. In vitro evidence indicates that KID mutations lead to hyperactive (open) hemichannels, which in some cases is accompanied by abnormal function of gap junction channels. Transgenic mouse models expressing connexin26 KID mutations reproduce human phenotypes and present impaired epidermal calcium homeostasis and abnormal lipid composition of the stratum corneum affecting the water barrier. Here we have compiled relevant data regarding the KID syndrome and propose a mechanism for the epidermal aspects of the disease.

Photothermal deflectometry enhanced by total internal reflection enables non-invasive glucose monitoring in human epidermis

We present TIR-PTD spectroscopy, an IR-pump/VIS-probe method for the measurement of IR absorption spectra by means of photothermal deflectometry (PTD) enhanced by total internal reflection (TIR). It overcomes the limitations of IR spectroscopy for the study of opaque samples and allows molecular fingerprinting of IR-active liquids or solids. Another important advantage of the presented approach over traditional IR spectroscopy methods is the ability to obtain IR information by means of VIS detection, which is generally much cheaper and easier to handle than IR detection. By application of mid-IR TIR-PTD spectroscopy on human skin in vivo, we are demonstrating the correlation between epidermal- and blood-glucose levels on a type 1 diabetic patient.

Altered epidermal lipid processing and calcium distribution in the KID syndrome mouse model Cx26S17F

The keratitis-ichthyosis-deafness (KID) syndrome is caused by mutations in the gap junctional channel protein connexin 26 (Cx26), among them the mutation Cx26S17F. Heterozygous Cx26S17F mice resemble the human KID syndrome, i.e. exhibiting epidermal hyperplasia and hearing impairments. Newborn Cx26S17F mice show a defective epidermal water barrier as well as altered epidermal lipid secretion and location. Linoleoyl ω-esterified ceramides are strongly decreased on the skin surface of Cx26S17F mice. Moreover, the epidermal calcium gradient is altered in the mutant mice. These alterations may be caused by an abnormal Cx26S17F channel function that leads to a defective epidermal water barrier, which in turn may trigger the hyperproliferation seen in the KID syndrome.

The loss of Tm7sf gene accelerates skin papilloma formation in mice

The 3β-hydroxysterol Δ14-reductase, encoded by the Tm7sf2 gene, is an enzyme involved in cholesterol biosynthesis. Cholesterol and its derivatives control epidermal barrier integrity and are protective against environmental insults. To determine the role of the gene in skin cholesterol homeostasis, we applied 12-o-tetradecanoylphorbol-13-acetate (TPA) to the skin of Tm7sf2^+/+ and Tm7sf2^-/- mice. TPA increased skin cholesterol levels by inducing de novo synthesis and up-take only in Tm7sf2^+/+ mouse, confirming that the gene maintains cholesterol homeostasis under stress conditions. Cholesterol sulfate, one of the major players in skin permeability, was doubled by TPA treatment in the skin of wild-type animals but this response was lost in Tm7sf2^-/- mice. The expression of markers of epidermal differentiation concomitant with farnesoid-X-receptor and p38 MAPK activation were also disrupted in Tm7sf2^-/- mice. We then subjected Tm7sf2^+/+ and Tm7sf2^-/- mice to a classical two-stage skin carcinogenesis protocol. We found that the loss of Tm7sf2 increased incidence and multiplicity of skin papillomas. Interestingly, the null genotype showed reduced expression of nur77, a gene associated with resistance to neoplastic transformation. In conclusion, the loss of Tm7sf2 alters the expression of proteins involved in epidermal differentiation by reducing the levels of cholesterol sulfate.

Quantitative proteogenomic profiling of epidermal barrier formation in vitro

BACKGROUND

The barrier function of the epidermis is integral to personal well-being, and defects in the skin barrier are associated with several widespread diseases. Currently there is a limited understanding of system-level proteomic changes during epidermal stratification and barrier establishment.

OBJECTIVE

Here we report the quantitative proteogenomic profile of an in vitro reconstituted epidermis at three time points of development in order to characterize protein changes during stratification.

METHODS

The proteome was measured using data-dependent "shotgun" mass spectrometry and quantified with statistically validated label-free proteomic methods for 20 replicates at each of three time points during the course of epidermal development.

RESULTS

Over 3600 proteins were identified in the reconstituted epidermis, with more than 1200 of these changing in abundance over the time course. We also collected and discuss matched transcriptomic data for the three time points, allowing alignment of this new dataset with previously published characterization of the reconstituted epidermis system.

CONCLUSION

These results represent the most comprehensive epidermal-specific proteome to date, and therefore reveal several aspects of barrier formation and skin composition. The limited correlation between transcript and protein abundance underscores the importance of proteomic analysis in developing a full understanding of epidermal maturation.

Using skin for drug delivery and diagnosis in the critically ill

Skin offers easy access, convenience and non-invasiveness for drug delivery and diagnosis. In principle, these advantages of skin appear to be attractive for critically ill patients given potential difficulties that may be associated with oral and parenteral access in these patients. However, the profound changes in skin physiology that can be seen in these patients provide a challenge to reliably deliver drugs or provide diagnostic information. Drug delivery through skin may be used to manage burn injury, wounds, infection, trauma and the multisystem complications that rise from these conditions. Local anaesthetics and analgesics can be delivered through skin and may have wide application in critically ill patients. To ensure accurate information, diagnostic tools require validation in the critically ill patient population as information from other patient populations may not be applicable.

A comparative study between human skin substitutes and normal human skin using Raman microspectroscopy

Research in the field of bioengineered skin substitutes is motivated by the need to find new dressings for people affected by skin injuries (burns, diabetic ulcers), and to develop adequate skin models to test new formulations developed in vitro. Thanks to advances in tissue engineering, it is now possible to produce human skin substitutes without any exogenous material, using the self-assembly method developed by the Laboratoire d'Organogénèse Expérimentale. These human skin substitutes consist of a dermis and a stratified epidermis (stratum corneum and living epidermis). Raman microspectroscopy has been used to characterize and compare the molecular organization of skin substitutes with normal human skin. Our results confirm that the stratum corneum is a layer rich in lipids which are well ordered (trans conformers) in both substitutes and normal human skin. The amount of lipids decreases and more gauche conformers appear in the living epidermis in both cases. However, the results also show that there are fewer lipids in the substitutes and that the lipids are more organized in the normal human skin. Concerning the secondary structure of proteins and protein content, the data show that they are similar in the substitutes and in the normal human skin. In fact, the epidermis is rich in α-keratin, whereas the dermis contains mainly type I collagen.

Cross-generational trans fat intake exacerbates UV radiation-induced damage in rat skin

We evaluated the influence of dietary fats on ultraviolet radiation (UVR)-induced oxidative damage in skin of rats. Animals from two consecutive generations born of dams supplemented with fats during pregnancy and breastfeeding were maintained in the same supplementation: soybean-oil (SO, rich in n-6 FA, control group), fish-oil (FO, rich in n-3 FA) or hydrogenated-vegetable-fat (HVF, rich in TFA). At 90 days of age, half the animals from the 2nd generation were exposed to UVR (0.25 J/cm(2)) 3×/week for 12 weeks. The FO group presented higher incorporation of n-3 FA in dorsal skin, while the HVF group incorporated TFA. Biochemical changes per se were observed in skin of the HVF group: greater generation of reactive oxygen species (ROS), lower mitochondrial integrity and increased Na(+)K(+)-ATPase activity. UVR exposure increased skin wrinkles scores and ROS generation and decreased mitochondrial integrity and reduced-glutathione levels in the HVF group. In FO, UVR exposure was associated with smaller skin thickness and reduced levels of protein-carbonyl, together with increased catalase activity and preserved Na(+)K(+)-ATPase function. In conclusion, while FO may be protective, trans fat may be harmful to skin health by making it more vulnerable to UVR injury and thus more prone to develop photoaging and skin cancer.

The important role of stratum corneum lipids for the cutaneous barrier function

The skin protects the body from unwanted influences from the environment as well as excessive water loss. The barrier function of the skin is located in the stratum corneum (SC). The SC consists of corneocytes embedded in a lipid matrix. This lipid matrix is crucial for the lipid skin barrier function. This paper provides an overview of the reported SC lipid composition and organization mainly focusing on healthy and diseased human skin. In addition, an overview is provided on the data describing the relation between lipid modulations and the impaired skin barrier function. Finally, the use of in vitro lipid models for a better understanding of the relation between the lipid composition, lipid organization and skin lipid barrier is discussed. 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. 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.

Caffeic acid induces keratinocyte differentiation by activation of PPAR-α

Objectives

Peroxisome proliferator-activated receptors (PPAR)-α plays an important role in epidermal differentiation and barrier recovery, and topical treatment with PPAR-α agonists restores epidermal homeostasis in essential fatty acid deficiency and permeability barrier in skin disruptions. Therefore, we performed structure-based pharmacophore screening to search for a novel PPAR-α agonist. Caffeic acid was ultimately selected and evaluated for its effects on keratinocyte differentiation and epidermal permeability barrier.

Methods

The transactivation activity of PPAR-responsive element (PPRE) and cornified envelope (CE) formation were assayed. Also, immunoblot analysis and anti-oxidant activity were investigated on caffeic acid.

Key findings

Caffeic acid increases the transactivation activity of PPRE and CE formation in keratinocytes. In addition, caffeic acid promotes the expression of genes and proteins related to CE formation such as involucrin and transglutaminase-1. Additionally, anti-oxidant activity were improved by caffeic acid.

Conclusions

Caffeic acid can promote keratinocyte differentiation and restore skin barrier homeostasis and is suggested to be an appropriate skin therapeutic agent for improving epidermal permeability barrier function.

Formation and functions of the corneocyte lipid envelope (CLE)

Corneocytes in mammalian stratum corneum are surrounded by a monolayer of covalently bound ω-OH-ceramides that form the corneocyte (-bound) lipid envelope (CLE). We review here the structure, composition, and possible functions of this structure, with insights provided by inherited and acquired disorders of lipid metabolism. 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.

Ceramide synthesis in the epidermis

The epidermis and in particular its outermost layer the stratum corneum provides terrestrial vertebrates with a pivotal defensive barrier against water loss, xenobiotics and harmful pathogens. A vital demand for this epidermal permeability barrier is the lipid-enriched lamellar matrix that embeds the enucleated corneocytes. Ceramides are the major components of these highly ordered intercellular lamellar structures, in which linoleic acid- and protein-esterified ceramides are crucial for structuring and maintaining skin barrier integrity. In this review, we describe the fascinating diversity of epidermal ceramides including 1-O-acylceramides. We focus on epidermal ceramide biosynthesis emphasizing its metabolic and topological requirements and discuss enzymes that may be involved in α- and ω-hydroxylation. Finally, we turn to epidermal ceramide regulation, highlighting transcription factors and liposensors recently described to play crucial roles in modulating skin lipid metabolism and epidermal barrier homeostasis. 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.

The Flux of Phenolic Compounds through Silicone Membranes

Phenols as a class of molecules have been reported to exhibit higher log maximum fluxes through human stratum corneum, SC, from water, log J_MHAQ, than other classes of molecules. This suggests that their corresponding log maximum fluxes through silicone from water, log J_MPAQ, may be useful to extend the existing n = 63 log J_MPAQ database to include more log J_MPAQ values greater than 0.0. The log J_MPAQ values for n = 7 phenols predicted to give log J_MPAQ values greater than 0.0 based on their log J_MHAQ values have been experimentally determined. These n = 7 new log J_MPAQ values have been added to the existing n = 63 log J_MPAQ database to give a new n = 70 database and the n = 7 literature log J_MHAQ values have been added to the existing n = 48 log J_MHAQ database (matched to the n = 63 log J_MPAQ database) to give a new n = 55 database. The addition of the n = 7 phenols improved the correlations of these flux databases when fitted to the Roberts-Sloan equation, RS, as well as the correlation between the matched experimental (Exp.) log J_MPAQ with the Exp. log J_MHAQ.

The role of sphingolipid metabolism in cutaneous permeability barrier formation

The epidermal permeability barrier of mammalian skin is localized in the stratum corneum. Corneocytes are embedded in an extracellular, highly ordered lipid matrix of hydrophobic lipids consisting of about 50% ceramides, 25% cholesterol and 15% long and very long chain fatty acids. The most important lipids for the epidermal barrier are ceramides. The scaffold of the lipid matrix is built of acylceramides, containing ω-hydroxylated very long chain fatty acids, acylated at the ω-position with linoleic acid. After glucosylation of the acylceramides at Golgi membranes and secretion, the linoleic acid residues are replaced by glutamate residues originating from proteins exposed on the surface of corneocytes. Removal of their glucosyl residues generates a hydrophobic surface on the corneocytes used as a template for the formation of extracellular lipid layers of the water permeability barrier. Misregulation or defects in the formation of extracellular ceramide structures disturb barrier function. Important anabolic steps are the synthesis of ultra long chain fatty acids, their ω-hydroxylation, and formation of ultra long chain ceramides and glucosylceramides. The main probarrier precursor lipids, glucosylceramides and sphingomyelins, are packed in lamellar bodies together with hydrolytic enzymes such as glucosylceramide-β-glucosidase and acid sphingomyelinase and secreted into the intercelullar space between the stratum corneum and stratum granulosum. Inherited defects in the extracellular hydrolytic processing of the probarrier acylglucosylceramides impair epidermal barrier formation and cause fatal diseases: such as prosaposin deficiency resulting in lack of lysosomal lipid binding and transfer proteins, or the symptomatic clinical picture of the "collodion baby" in the absence of glucocerebrosidase. 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.

Clinical use of a ceramide-based moisturizer for treating dogs with atopic dermatitis

In humans, skin barrier dysfunction is thought to be responsible for enhanced penetration of allergens. Similar to conditions seen in humans, canine atopic dermatitis (CAD) is characterized by derangement of corneocytes and disorganization of intercellular lipids in the stratum corenum (SC) with decreased ceramide levels. This study was designed to evaluate the effects of a moisturizer containing ceramide on dogs with CAD. Dogs (n = 20, 3~8 years old) with mild to moderate clinical signs were recruited and applied a moisturizer containing ceramide for 4 weeks. Transepidermal water loss (TEWL), skin hydration, pruritus index for canine atopic dermatitis (PICAD) scores, and canine atopic dermatitis extent and severity index (CADESI) scores of all dogs were evaluated. Skin samples from five dogs were also examined with transmission electron microscopy (TEM) using ruthenium tetroxide. TEWL, PICAD, and CADESI values decreased (p < 0.05) and skin hydration increased dramatically over time (p < 0.05). Electron micrographs showed that the skin barrier of all five dogs was partially restored (p < 0.05). In conclusion, these results demonstrated that moisturizer containing ceramide was effective for treating skin barrier dysfunction and CAD symptoms.

In vivo noninvasive monitoring of glucose concentration in human epidermis by mid-infrared pulsed photoacoustic spectroscopy

The noninvasive determination of glucose in the interstitial layer of the human skin by mid-infrared spectroscopy is reported. The sensitivity for this measurement was obtained by combining the high pulse energy from an external cavity quantum cascade laser (EC-QCL) tunable in the infrared glucose fingerprint region (1000-1220 cm(-1)) focused on the skin, with a detection of the absorbance process by photoacoustic spectroscopy in the ultrasound region performed by a gas cell coupled to the skin. This combination facilitates a quantitative measurement for concentrations of skin glucose in the range from <50 mg/dL to >300 mg/dL, which is the relevant range for the glucose monitoring in diabetes patients. Since the interstitial fluid glucose level is representative of the blood glucose level and follows it without significant delay (<10 min), this method could be applied to establish a noninvasive, painless glucose measurement procedure that is urgently awaited by diabetes patients. We report here the design of the photoacoustic experiments, the spectroscopy of glucose in vivo, and the calibration method for the quantitative determination of glucose in skin. Finally, a preliminary test with healthy volunteers and volunteers suffering from diabetes mellitus demonstrates the viability of a noninvasive glucose monitoring for patients based on the combination of infrared QCL and photoacoustic detection.