The nature of the epidermal barrier: biochemical aspects

Models of the Three-Component Bilayer of Stratum Corneum: A Molecular Simulation Study

The construction of the stratum corneum (SC) is crucial to the problems of transdermal drug delivery. SC consists of the keratinocyte layers and the lipid matrix surrounding it. Among them, the lipid matrix is the barrier for many exogenous molecules, mainly composed of ceramides (CERs), free fatty acids (FFA), and cholesterol (CHOL). In this work, we developed single-component (CERs, CER-NS, and CER-EOS) and six three-component models, and each model was simulated by using the GROMOS-54A7 force field. Short-period phase (SPP) and long-period phase (LPP) systems were established separately, and area per lipid (APL), thickness, order of carbon chain (SCD), and density distribution were analyzed. The transition of CER-NS and CER-EOS in LPP was observed. The results of hydrogen bonds in the lipid systems indicated that a strong hydrogen-bond network was formed between the skin-lipid bilayers. Umbrella sampling method simulations were performed to calculate the free energy change of ethanol moving into the skin-lipid bilayer. The results revealed that ethanol molecules pulled some water molecules into the membrane when they passed through SPP-1. Our findings provided some insights and models of the stratum corneum that could be used for the subsequent mechanism of macromolecule permeation through membranes in drugs, cosmetics, and so on.

Investigating the nanostructure of a CER[NP]/CER[AP]-based stratum corneum lipid matrix model: A combined neutron diffraction & molecular dynamics simulations approach

The human skin provides a physiochemical and biological protective barrier due to the unique structure of its outermost layer known as the Stratum corneum. This layer consists of corneocytes and a multi-lamellar lipid matrix forming a composite, which is a major determining factor for the barrier function of the Stratum corneum. A substantiated understanding of this barrier is necessary, as controlled breaching or modulation of the same is also essential for various health and personal care applications such as topical drug delivery and cosmetics to a name few. In this study, we discuss the state-of-the-art of neutron diffraction techniques, using specifically deuterated lipids, combined with the information obtained from molecular models using molecular dynamics simulations, to understand the structure and barrier function of the Stratum corneum lipid matrix. As an example, the effect of ceramide concentration on a lipid lamella system consisting of CER[NP]/CER[AP]/Cholesterol/free fatty acid (deprotonated) is studied. This study demonstrates the usefulness of the combined approach of neutron diffraction and molecular dynamics simulations for effective analysis of the model systems created for the Stratum corneum lipid matrix. The optimization of force fields by comparison with experimental data is furthermore an important step in the direction of providing a predictive quality.

Using molecular simulation to understand the skin barrier

Skin's effectiveness as a barrier to permeation of water and other chemicals rests almost entirely in the outermost layer of the epidermis, the stratum corneum (SC), which consists of layers of corneocytes surrounded by highly organized lipid lamellae. As the only continuous path through the SC, transdermal permeation necessarily involves diffusion through these lipid layers. The role of the SC as a protective barrier is supported by its exceptional lipid composition consisting of ceramides (CERs), cholesterol (CHOL), and free fatty acids (FFAs) and the complete absence of phospholipids, which are present in most biological membranes. Molecular simulation, which provides molecular level detail of lipid configurations that can be connected with barrier function, has become a popular tool for studying SC lipid systems. We review this ever-increasing body of literature with the goals of (1) enabling the experimental skin community to understand, interpret and use the information generated from the simulations, (2) providing simulation experts with a solid background in the chemistry of SC lipids including the composition, structure and organization, and barrier function, and (3) presenting a state of the art picture of the field of SC lipid simulations, highlighting the difficulties and best practices for studying these systems, to encourage the generation of robust reproducible studies in the future. This review describes molecular simulation methodology and then critically examines results derived from simulations using atomistic and then coarse-grained models.

Mammalian Epidermis: A Compendium of Lipid Functionality

Mammalian epidermis is a striking example of the role of lipids in tissue biology. In this stratified epithelium, highly specialized structures are formed that leverage the hydrophobic properties of lipids to form an impermeable barrier and protect the humid internal environment of the body from the dry outside. This is achieved through tightly regulated lipid synthesis that generates the molecular species unique to the tissue. Beyond their fundamental structural role, lipids are involved in the active protection of the body from external insults. Lipid species present on the surface of the body possess antimicrobial activity and directly contribute to shaping the commensal microbiota. Lipids belonging to a variety of classes are also involved in the signaling events that modulate the immune responses to environmental stress as well as differentiation of the epidermal keratinocytes themselves. Recently, high-resolution methods are beginning to provide evidence for the involvement of newly identified specific lipid molecules in the regulation of epidermal homeostasis. In this review we give an overview of the wide range of biological functions of mammalian epidermal lipids.

Stratum Corneum Function: A Structural Study with Dynamic Synchrotron X-ray Diffraction Experiments

Studies on the effectiveness of substances such as drugs and cosmetics that act on the skin require structural evidence at the molecular level in the stratum corneum to clarify their interaction with intercellular lipid and soft keratin. For this purpose, when applying the substances to the stratum corneum X-ray diffraction experiment is one of the powerful tools. To detect minute structural changes in a stratum corneum sample, using a "solution cell", dynamic synchrotron X-ray diffraction measurements were performed when applying aqueous solution of the substances to the stratum corneum: (1) It was found that a surfactant, sodium dodecyl sulfate, significantly disrupted the long-period lamellar structure. (2) To study the effects of water, structural modifications of the short-period lamellar structure and the soft keratin in corneocytes were measured as a function of time. At the initial water content of 15 wt%, the spacings of the short-period lamellar structure and the soft keratin increased toward those at the water content of 25 wt%, that is a key water content in the stratum corneum. (3) Nanoparticles composed of assembly of amphiphilic molecules are one of the leading pharmaceutical formulations. When the nanoparticles were applied, a new assembly of amphiphilic molecules originated from the nanoparticle appeared. This phenomenon suggests that the formation of the new assembly at the surface of skin is concerned with the release of the drug from the nanoparticles. (4) When ethanol was applied to the stratum corneum, only the liquid state in the intercellular lipid matrix was dissolved. After the removal of ethanol from this stratum corneum, the ordered hydrocarbon-chain packing structures appeared. From this fact we would propose that the liquid state region is the main pathway for hydrophobic drugs with a small molecular weight in connection with the so-called 500 Da rule. Here, not only the technique but also the background to these studies and the characteristic results obtained from these studies are explained.

How to control interactions of cellulose-based biomaterials with skin: the role of acidity in the contact area

Being able to control the interactions of biomaterials with living tissues and skin is highly desirable for many biomedical applications. This is particularly the case for cellulose-based materials which provide one of the most versatile platforms for tissue engineering due to their strength, biocompatibility and abundance. Achieving such control, however, requires detailed molecular-level knowledge of the dominant interaction mechanisms. Here, we employed both biased and unbiased atomic-scale molecular dynamics simulations to explore how cellulose crystals interact with model stratum corneum bilayers, ternary mixtures of ceramides, cholesterol, and free fatty acids. Our findings show that acidity in the contact area directly affects binding between cellulose and the stratum corneum bilayer: Protonation of free fatty acids in the bilayer promotes attractive cellulose-bilayer interactions. We identified two major factors that control the cellulose-skin interactions: (i) the electrostatic repulsion between a cellulose crystal and the charged (anionic due to deprotonated fatty acids) surface of a stratum corneum bilayer and (ii) the cellulose-stratum corneum hydrogen bonding. When less than half of the fatty acids in the bilayer are protonated, the first factor dominates and there is no binding to skin. At a larger degree of fatty acid protonation the cellulose-stratum corneum hydrogen bonding prevails yielding a tight binding. Remarkably, we found that ceramide molecules are the key component in hydrogen bonding with cellulose. Overall, our findings highlight the critical role of fatty acid protonation in biomaterial-stratum corneum interactions and can be used for optimizing the surface properties of cellulose-based materials aimed at biomedical applications such as wound dressings.

Fourier transform infrared absorption (FTIR) on dry stratum corneum, corneocyte-lipid interfaces: experimental and vibrational spectroscopy calculations

Many questions concerning the biophysical and physiological properties of skin are still open. Skin aging, permeability, dermal absorption, hydration, and drug transdermal delivery, are few examples of processes with unveiled underlying mechanisms. In this work, it is presented a comparison between Fourier transform infrared absorption (FTIR) of dry stratum corneum and stratum corneum under lipase action supported by first-principles density functional vibrational calculations. The molecular structure of stratum corneum was modeled by an archetype of its hygroscopic proteic portion inside the corneocytes, the natural moisturizing factor, coupled to glycerol molecules which represent the lipid fraction of stratum corneum. Vibrational spectra were calculated and compared to experimental data obtained on the animal model of stratum corneum. The experimental results indicated prominent spectral differences between dry and lipase-treated stratum corneum. Principal components analysis and hyerarchical clustering indicated that 1200, 1650, and 1695 cm^-1 bands are the most influential on the discrimination. It is noticed that bands in the fingerprint region (800-1800 cm^-1) were correctly assigned. Moreover, the calculations revealed the existence of two coupled vibration between the hydroxyl group of lipid and methylene (1120 and 1160 cm^-1), which are of special interest since they probe the lipid-amino acid coupling. The model was also able to predict the shear modulus of dry stratum corneum in excellent agreement with the reported values from the literature. Other physical/chemical properties could be calculated exploring the chemical accuracy and molecular resolution of this model. Research in dermatology, cosmetology, and biomedical engineering in the specific topics of drug delivery and/or mechanical properties of skin are examples of fields that would potentially take advantage of this approach.

Nature or Nurture: Can Prey-Based Diets Influence Species-Specific Physiological Performance Traits of Epidermal Lipid Content and Cutaneous Water Loss?

Epidermal lipids serve as the primary barrier to cutaneous water loss (CWL) and play a significant role in water conservation and homeostasis. Previous studies have shown the correlation between increased aridity of habitats and the amount of epidermal lipids among species. Generally, increased amounts of epidermal lipids lower skin permeability. Species-specific differences in CWL and prey preferences between two sympatric snake species, the Northern Cottonmouth (Agkistrodon piscivorus) and the Eastern Copperhead (Agkistrodon contortrix), motivated us to question if prey-base can result in these observed species-specific differences in CWL. We experimentally controlled the diets for a captive colony of Northern Cottonmouths (A. piscivorus) by feeding either fish (Notemigonus crysoleucas) or mice (Mus musculus) to investigate if diet can affect the quantity and quality of epidermal lipids and the rates of CWL. Snakes fed mice gained consistently more mass, but diet treatments did not affect growth rate. We found no significant differences in quantitative lipid content or rates of CWL between diet treatments. An analysis for qualitative lipid content using infrared spectrophotometry also showed no diet effect, thus suggesting that lipid content and CWL are strong species-specific physiological performance traits not influenced by recent dietary history. While there is some evidence that epidermal permeability may be variable under certain environmental conditions (e.g., humidity), our findings show that diet has no effect and that a shift in prey preference may not influence or enhance physiological performance for decreasing CWL.

Probing the interactions among sphingosine and phytosphingosine ceramides with non- and alpha-hydroxylated acyl chains in skin lipid model membranes

Ceramides (Cers) are significant constituents of the stratum corneum (SC), the uppermost skin layer responsible for skin barrier properties. Cers are a heterogeneous group of lipids whose mutual interactions are still unclear. To better understand these interactions, we characterized model membranes containing stearic acid, cholesterol, cholesterol sulfate and one or more of the following ceramides: N-stearoyl-sphingosine (CerNS), N-stearoyl-phytosphingosine (CerNP) and N-(2-hydroxy)stearoyl-phytosphingosine (CerAP). Small angle X-ray scattering and FTIR spectroscopy were used to study lipid arrangement, phase separation and thermotropic behaviour. In the one-Cer systems, the membranes with CerNP showed strong hydrogen bonding and significant phase separation, even after phase transition, while the systems containing CerAP and CerNS had increased lipid miscibility. The multi-Cer systems exhibited different behaviour. In particular, the membrane containing all three Cers was a highly miscible system with narrow one-step phase transition, which, of all the studied samples, occurred at the lowest temperatures. Our results show that even a small variation in Cer structure results in substantially different phase behaviour, which is further affected by the presence of other Cer subclasses. Interestingly, the phase behaviour of the most complex three-Cer system was simpler than that of the others, highlighting the importance of lipid diversity in real SC.

Simultaneous Measurements of Structure and Water Permeability in an Isolated Human Skin Stratum Corneum Sheet

Stratum corneum (SC), the outermost layer of human skin, acts as an intelligent physicochemical interface between the inside and the outside of our body. To make clear the relationship between structure and physical barrier properties of SC, we developed a method that enables us to simultaneously acquire X-ray diffraction (XD) patterns and transepidermal water loss (TEWL) values using a spread SC sheet isolated from human skin. The synchrotron X-ray was incident on the SC sheet surface at an angle of 45° to avoid interference between the two kinds of measurements. Detailed comparison between XD and TEWL data suggested that the thermal behavior of water permeability is closely related to the thermal expansion of the lattice spacings of the hexagonal phases above 40 °C and to the existence ratio of the orthorhombic phase below 40 °C. Thus, the new method we developed can give useful information on the mechanism of water permeation in SC without ambiguity caused by separate measurements of structure and water permeability with different samples.

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.

The long periodicity phase (LPP) controversy part I: The influence of a natural-like ratio of the CER[EOS] analogue [EOS]-br in a CER[NP]/[AP] based stratum corneum modelling system: A neutron diffraction study

This study used neutron diffraction to investigate a ceramide-[NP] C24/[AP] C24 /[EOS]-br C30/cholesterol/lignoceric acid (0.6: 0.3: 0.1: 0.7: 1) based stratum corneum modelling system. By adding specifically deuterated ceramides-[NP]-D₃, [AP]-D₃, and [EOS]-br-D₃, detailed information on the lamellar and the nanostructure of the system was obtained. For the short periodicity phase a natural-like lamellar repeat distance of 5.47 ± 0.02 nm was observed, similar to the [NP]/[AP] base system without the [EOS]-br. Unlike in this system the ceramides here were slightly tilted, hinting towards a slightly less natural arrangement. Due to the deuteration it was possible to observe that the long ceramide chains were overlapping in the lamellar mid-plane. This is considered to be an important feature for the natural stratum corneum. Despite the presence of a ceramide [EOS] analogue - able to form a long phase arrangement - no distinct long periodicity phase was formed, despite a slightly higher than natural ω-acyl ceramide ratio of 10 mol%. The deuterated variant of this ceramide determined that the very long ceramide was integrated into the short periodicity phase, spanning multiple layers instead. The - compared to the base system - unchanged repeat distance highlights the stability of this structure. Furthermore, the localisation of the very long ceramide in the short periodicity phase indicates the possibility of a crosslinking effect and thus a multilayer stabilizing role for the ceramide [EOS]. It can be concluded, that additionally to the mere presence of ceramide-[EOS] more complex conditions have to be met in order to form this long phase. This has to be further investigated in the future.

Impact of the ceramide subspecies on the nanostructure of stratum corneum lipids using neutron scattering and molecular dynamics simulations. Part I: impact of CER[NS]

For this study mixtures based on the ceramides [NS] (NS = non-hydroxy-sphingosine) and [AP] (AP = α-hydroxy-phytosphingosine) in a 2:1 and 1:2 ratio, together with cholesterol and lignoceric acid, were investigated. These mixtures are modelling the uppermost skin layer, the stratum corneum. Neutron diffraction, utilizing specifically deuterated ceramide molecules, was used to obtain a maximum amount of experimental detail. Highly detailed molecular dynamics simulations were used to generate even more information from the experimental data. It was possible to observe a single lamellar phase for both systems. They had a lamellar repeat distance of 5.43 ± 0.05 nm for the [NS]/[AP] 2:1 and a slightly shorter one of 5.34 ± 0.05 nm for the 1:2 system. The structure and water content was uninfluenced by excess humidity. Both the experimental and simulation data indicated slightly tilted ceramides, with their C24 chains overlapping in the lamellar mid-plane. This arrangement is well comparable to systems investigated before. The structure of both systems, except for the differing repeat distance, looks similar at first. However, on a smaller scale there were various distinct differences, demonstrating only low redundancy between the different ceramide species, despite only minor chemical differences. The mainly ceramide [AP] determined 1:2 system has a slightly smaller repeat distance. This is a result of a tighter arrangement of the lipids chain along the bilayer normal and increased overlapping of the long chains in the lamellar middle. For the CER[NS] some novel features could be shown, despite it being the overall most investigated ceramide. These include the low adaptability to changed lateral interactions, leading to an increased chain opening. This effect could explain its low miscibility with other lipids. The investigated model systems allows it to directly compare results from the literature which have used ceramide [NS] to the most recent studies using the phytosphingosine ceramides such as ceramide [AP].

Investigation of a CER[NP]- and [AP]-Based Stratum Corneum Modeling Membrane System: Using Specifically Deuterated CER Together with a Neutron Diffraction Approach

Neutron diffraction was used as a tool to investigate the lamellar as well as molecular nanostructure of ceramide-[NP]/ceramide-[AP]/cholesterol/lignoceric acid model systems with a nativelike 2:1 ratio and a 1:2 ratio to study the influence of the ceramide-[AP]. By using mixtures together with cholesterol and free fatty acids as well as a humidity and temperature chamber while measuring, natural conditions were simulated as closely as possible. Despite its simplicity, the system simulated the native stratum corneum lipid matrix fairly closely, showing a similar lamellar thickness with a repeat distance of 5.45 ± 0.1 nm and a similar arrangement with overlapping long C24 chains. Furthermore, despite the very minor chemical difference between ceramide-[NP] and ceramide-[AP], which is only a single OH group, it was possible to demonstrate substantial differences between the structural influence of the two ceramides. Ceramide-[AP] could be concluded to be arranged in such a way that its C24 chain in both ratios is somehow shorter than that of ceramide-[NP], not overlapping as much with the opposite lamellar leaflet. Furthermore, in the unnatural 1:2 ratio, the higher ceramide-[AP] content causes an increased tilt of the ceramide acyl chains. This leads to even less overlapping within the lamellar midplane, whereas the repeat distance stays the same as for the ceramide-[NP]-rich system. In this nativelike 2:1 ratio, the chains are arranged mostly straight, and the long C24 chains show a broad overlapping region in the lamellar midplane.

Effects of cosmetics on the skin microbiome of facial cheeks with different hydration levels

Basic cosmetics was used by volunteers belonging to high (HHG) and low (LHG) hydration groups for 4 weeks, and bacterial communities and biophysical parameters in facial skin were analyzed. Hydration level increases and transepidermal water loss and roughness decreases were observed in both groups after cosmetic use. Bacterial diversity was greater in LHG than HHG, and increased after cosmetic use in both groups. Bray–Curtis dissimilarities that were higher in LHG than HHG increased in HHG after cosmetic use, whereas they decreased in LHG. The phyla Actinobacteria, Proteobacteria, Firmicutes, and Bacteroidetes and the genera Propionibacterium, Ralstonia, Burkholderia, Staphylococcus, Corynebacterium, Cupriavidus, and Pelomonas were identified as common groups and they were not significantly different between LHG and HHG except for Propionibacterium that was more abundant in HHG. After cosmetic use, Propionibacterium, Staphylococcus, and Corynebacterium decreased, whereas Ralstonia, not a core genus, increased, as did KEGG categories of lipid metabolism and xenobiotics biodegradation and metabolism, suggesting that Ralstonia in skin may have the ability to metabolize cosmetics components. Bacterial communities after cosmetic use were different from those in both LHG and HHG before the cosmetic use, indicating that bacterial communities in LHG were not shifted to resemble those in HHG by cosmetics use.

Determination of the influence of C24 D/(2R)- and L/(2S)-isomers of the CER[AP] on the lamellar structure of stratum corneum model systems using neutron diffraction

This study was able to investigate the different influence of the d- and l-ceramide [AP] on the lamellar as well as molecular nanostructure of stratum corneum simulating lipid model mixtures. In this case, neutron diffraction together with specifically deuterated ceramide was used as an effective tool to investigate the lamellar and the molecular nanostructure of the mixtures. It could clearly be demonstrated, that both isomers show distinctly different characteristics, even though the variation between both is only a single differently arranged OH-group. The l-ceramide [AP] promotes a crystalline like phase behaviour even if mixed with ceramide [NP], cholesterol and free fatty acids. The d-ceramide [AP] only shows crystalline-like features if mixed only with cholesterol and free fatty acids but adopts a native-like behaviour if additionally mixed with ceramide [NP]. It furthermore demonstrates that the l-ceramide [AP] should not be used for any applications concerning ceramide substitution. It could however possibly serve its own purpose, if this crystalline like behaviour has some kind of positive influence on the SC or can be utilized for any practical applications. The results obtained in this study demonstrate that the diastereomers of ceramide [AP] are an attractive target for further research because their influence on the lamellar as well as the nanostructure is exceptionally strong. Additionally, the results furthermore show a very strong influence on hydration of the model membrane. With these properties, the d-ceramide [AP] could be effectively used to simulate native like behaviour even in very simple mixtures and could also have a strong impact on the native stratum corneum as well as high relevance for dermal ceramide substitution. The unnatural l-ceramide [AP] on the other hand should be investigated further, to assess its applicability.

Core-multishell nanocarriers: Transport and release of dexamethasone probed by soft X-ray spectromicroscopy

Label-free detection of core-multishell (CMS) nanocarriers and the anti-inflammatory drug dexamethasone is reported. Selective excitation by tunable soft X-rays in the O 1s-regime is used for probing either the CMS nanocarrier or the drug. Furthermore, the drug loading efficiency into CMS nanocarriers is determined by X-ray spectroscopy. The drug-loaded nanocarriers were topically applied to human skin explants providing insights into the penetration and drug release processes. It is shown that the core-multishell nanocarriers remain in the stratum corneum when applied for 100min to 1000min. Dexamethasone, if applied topically to human ex vivo skin explants using different formulations, shows a vehicle-dependent penetration behavior. Highest local drug concentrations are found in the stratum corneum as well as in the viable epidermis. If the drug is loaded to core-multishell nanocarriers, the concentration of the free drug is low in the stratum corneum and is enhanced in the viable epidermis as compared to other drug formulations. The present results provide insights into the penetration of drug nanocarriers as well as the mechanisms of controlled drug release from CMS nanocarriers in human skin. They are also compared to related work using dye-labeled nanocarriers and dyes that were used as model drugs.

Evaporation of Sunscreen Films: How the UV Protection Properties Change

We have investigated the evaporation of thin sunscreen films and how the light absorption and the derived sun protection factor (SPF) change. For films consisting of solutions of common UV filters in propylene glycol (PG) as solvent, we show how evaporation generally causes three effects. First, the film area can decrease by dewetting leading to a transient increase in the average film thickness. Second, the film thins by evaporative loss of the solvent. Third, precipitation of the UV filter occurs when solvent loss causes the solubility limit to be reached. These evaporation-induced changes cause the UV absorbance of the film to decrease with resultant loss of SPF over the time scale of the evaporation. We derive an approximate model which accounts semiquantitatively for the variation of SPF with evaporation. Experimental results for solutions of different UV filters on quartz, different skin mimicking substrates, films with added nanoparticles, films with an added polymer and films with fast-evaporating decane as solvent (instead of slow evaporating PG) are discussed and compared with model calculations. Addition of either nanoparticles or polymer suppress film dewetting. Overall, it is hoped that the understanding gained about the mechanisms whereby film evaporation affects the SPF will provide useful guidance for the formulation of more effective sunscreens.

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.

A rhenium tris-carbonyl derivative as a model molecule for incorporation into phospholipid assemblies for skin applications

A rhenium tris-carbonyl derivative (fac-[Re(CO)3Cl(2-(1-dodecyl-1H-1,2,3,triazol-4-yl)-pyridine)]) was incorporated into phospholipid assemblies, called bicosomes, and the penetration of this molecule into skin was monitored using Fourier-transform infrared microspectroscopy (FTIR). To evaluate the capacity of bicosomes to promote the penetration of this derivative, the skin penetration of the Re(CO)3 derivative dissolved in dimethyl sulfoxide (DMSO), a typical enhancer, was also studied. Dynamic light scattering results (DLS) showed an increase in the size of the bicosomes with the incorporation of the Re(CO)3 derivative, and the FTIR microspectroscopy showed that the Re(CO)3 derivative incorporated in bicosomes penetrated deeper into the skin than when dissolved in DMSO. When this molecule was applied on the skin using the bicosomes, 60% of the Re(CO)3 derivative was retained in the stratum corneum (SC) and 40% reached the epidermis (Epi). Otherwise, the application of this molecule via DMSO resulted in 95% of the Re(CO)3 derivative being in the SC and only 5% reaching the Epi. Using a Re(CO)3 derivative with a dodecyl-chain as a model molecule, it was possible to determine the distribution of molecules with similar physicochemical characteristics in the skin using bicosomes. This fact makes these nanostructures promising vehicles for the application of lipophilic molecules inside the skin.

Protection effect of cosmetics on human skin under simulated rigorous environment

BACKGROUND/PURPOSE

The efficacy of cosmetics on human skin measured under normal mild laboratory environment might be discounted by exterior environment factors such as wind, UV exposure, etc. Few studies have focused on the 'genuine' efficacy of cosmetics on human skin during exposure to external rigorous environment. The aim of this study was to develop a model for the evaluation of the efficacy of cosmetics on human skin under simulated rigorous environment.

METHODS

We measured skin water content and transepidermal water loss (TEWL) before and after products application under simulated windy and sub-erythema UV exposure treatment in a constant temperature and humidity laboratory.

RESULTS

The results showed that the products had higher water content and lower TEWL at 2, 4 and 8 h of products application, and ameliorated the skin moisturization situation after a 3-week continuous use of the products. In addition, the products might protect the skin to maintain the normal water content and TEWL under the simulated windy or sub-erythema UV exposure treatment in our laboratory when compared with untreated area.

CONCLUSION

Our data indicate that this model may provide a more accurate evaluation for the genuine moisturizing effect of cosmetics under external natural climate.

Thematic review series: skin lipids. Antimicrobial lipids at the skin surface

The skin surface represents our interface with the external environment, and as such, is our first line of defense against microbial colonization and infection. Lipids at the skin surface are thought to underlie at least part of an antimicrobial barrier. Some of these lipids are synthesized in the epidermis and are carried to the surface as cells differentiate, whereas others are secreted onto the surface from the sebaceous glands. One such group, free sphingoid bases, are known to have broad antimicrobial activity, and our previous studies demonstrate their presence at the skin surface. Free sphingoid bases may be generated by enzymatic hydrolysis of epidermal ceramides. In addition, our preliminary results demonstrate potent antibacterial activity associated with two specific fatty acids derived from sebaceous triglycerides. Most remarkably, one of these fatty acids (sapienic acid, C16:1Delta6), in combination with a low concentration of ethanol, is very effective against methicillin-resistant Staphylococcus aureus (MRSA). In fact, this combination was far more effective than mupirocin with or without ethanol. Mupirocin is a "gold standard" for activity against MRSA.

The permeability enhancing mechanism of DMSO in ceramide bilayers simulated by molecular dynamics

The lipids of the topmost layer of the skin, the stratum corneum, represent the primary barrier to molecules penetrating the skin. One approach to overcoming this barrier for the purpose of delivery of active molecules into or via the skin is to employ chemical permeability enhancers, such as dimethylsulfoxide (DMSO). How these molecules exert their effect at the molecular level is not understood. We have investigated the interaction of DMSO with gel-phase bilayers of ceramide 2, the predominant lipid in the stratum corneum, by means of molecular dynamics simulations. The simulations satisfactorily reproduce the phase behavior and the known structural parameters of ceramide 2 bilayers in water. The effect of DMSO on the gel-phase bilayers was investigated at various concentrations over the range 0.0-0.6 mol fraction DMSO. The DMSO molecules accumulate in the headgroup region and weaken the lateral forces between the ceramides. At high concentrations of DMSO (> or =0.4 mol fraction), the ceramide bilayers undergo a phase transition from the gel phase to the liquid crystalline phase. The liquid-crystalline phase of ceramides is expected to be markedly more permeable to solutes than the gel phase. The results are consistent with the experimental evidence that high concentrations of DMSO fluidize the stratum corneum lipids and enhance permeability.

Role of ceramide structure and its microenvironment on the conformational order of model stratum corneum lipids mixtures: an approach by FTIR spectroscopy

The aim of this study is to investigate the influence of ceramide head group architecture and free fatty acid (another main class of stratum corneum lipids) or protein (keratin), on the lamellar organization of the ceramide auto-associated in model films mimicking lipid organization within the stratum corneum. FTIR spectroscopy is a powerful technique for investigating the structure of such systems. This technique has already been used to characterize phase transitions of the SC and of related model systems. As temperature is known to modify the conformational order of lipids, we used it as a variable parameter to monitor the differences in the conformational stability of ceramides. Our study included four ceramides: ceramide 2, 3, 5 and 6 which differ by their head group architecture. Two kinds of lipid-lipid interactions were studied: non-polar and polar. We noted some structural factors which participated to the organizational behavior: insaturation of alkyl chain, alpha-hydroxyl on fatty acid moiety and sphingosine or phytosphingosine head group. There is a direct interaction of palmitic acid on alkyl chains organization and a weak interaction with polar head group in presence of keratin, both provoking a destabilization of the ceramidic orthorhombic organization.

Identification of complex mixtures of sphingolipids in the stratum corneum by reversed-phase high-performance liquid chromatography and atmospheric pressure photospray ionization mass spectrometry

Sphingolipids, such as ceramides and cerebrosides, are important molecules in the formation and maintenance of the epidermal barrier to water vapor diffusion. In this paper we explore a new method to identify the sphingolipids found in the stratum corneum (SC), the outer layer of the epidermis, of House sparrows living in Saudi Arabia using reversed-phase high-performance liquid chromatography (HPLC) coupled with atmospheric pressure photo-ionization mass spectrometry (APPI-MS). First, using thin layer chromatography (TLC) we found that the SC contains ceramides, cerebrosides, and free fatty acids along with smaller amounts of cholesterol. Knowing the classes of sphingolipids present in the SC markedly reduced the number of possible molecules present. Then, we identified each sphingolipid molecule in our sample by both negative and positive mode of APPI-MS. We confirmed our identifications by generation of accurate mass data, and by examination of MS/MS spectra for selected molecules. Using APPI-MS, we identified 7 families of cerebrosides, for a total of 97 molecular species, and 4 families of ceramides, for a total of 79 molecules, in the SC of House sparrows, a wider array than would be found in mammals. Carbon chain lengths of fatty acids in the sphingolipids were longer than those that have been reported for mammalian SC; chain lengths of over 40 carbons were common. We also compared our estimates of the quantity of lipids in the SC obtained by HPLC/MS with those from TLC. Estimates of the amount of total ceramides and cerebrosides using TLC differed from those obtained by HPLC/MS by +0.95% and -2.5%, respectively. We conclude that our protocol using reversed-phase HPLC and APPI-MS is an useful method of analyzing complex mixtures of sphingolipids in the SC.

Water relations of tetrapod integument

The vertebrate integument represents an evolutionary compromise between the needs for mechanical protection and those of sensing the environment and regulating the exchange of materials and energy. Fibrous keratins evolved as a means of strengthening the integument while simultaneously providing a structural support for lipids, which comprise the principal barrier to cutaneous water efflux in terrestrial taxa. Whereas lipids are of fundamental importance to water barriers, the efficacy of these barriers depends in many cases on structural features that enhance or maintain the integrity of function. Amphibians are exceptional among tetrapods in having very little keratin and a thin stratum corneum. Thus, effective lipid barriers that are present in some specialized anurans living in xeric habitats are external to the epidermis, whereas lipid barriers of amniotes exist as a lipid-keratin complex within the stratum corneum. Amphibians prevent desiccation of the epidermis and underlying tissues either by evaporating water from a superficial aqueous film, which must be replenished, or by shielding the stratum corneum with superficial lipids. Water barrier function in vertebrates generally appears to be relatively fixed, although various species have`plasticity' to adjust the barrier effectiveness facultatively. While it is clear that both phenotypic plasticity and genetic adaptation can account for covariation between environment and skin resistance to water efflux, studies of the relative importance of these two phenomena are few. Fundamental mechanisms for adjusting the skin water barrier include changes in barrier thickness, composition and physicochemical properties of cutaneous lipids,and/or geometry of the barrier within the epidermis. While cutaneous lipids have been studied extensively in the contexts of disease and cosmetics,relatively little is known about the processes of permeability barrier ontogenesis related to adaptation and environment. Advances in such knowledge have didactic significance for understanding vertebrate evolution as well as practical application to clinical dermatology.

Validation of the VapoMeter, a closed unventilated chamber system to assess transepidermal water loss vs. the open chamber TewameterR

BACKGROUND/AIM

Transepidermal water loss (TEWL) is one of the most important biophysical parameters for evaluating the efficiency of the human skin water barrier. Different approaches exist to measure TEWL. The most commonly used methodology consists of the open chamber diffusion technique in which the water vapor pressure gradient is measured in g/h m2 according to Fick's law. A typical apparatus is the Tewameter. Recently, a portable device--the VapoMeter--became available with a humidity sensor in a closed chamber.

METHODS

In the present work, the closed chamber VapoMeter is compared with the open chamber Tewameter for its applicability to assess TEWL. A comparative study--including parallel in vivo measurements with both devices--was carried out on human forearm skin.

RESULTS

It could be concluded that both instruments are reliable tools. A good correlation between recordings (r=0.503-0.966) was found with a consistent feature of measuring higher TEWL values for the Tewameter than for the VapoMeter. Probe pressure, probe temperature and relative humidity were revealed to be important parameters inducing significant differences in data outcome.

CONCLUSIONS

From skin barrier damage experiments it became clear that the Tewameter is able to detect significantly smaller differences than the VapoMeter. In addition, the closed chamber device is currently not sensitive enough to discriminate for the effects induced by diurnal rhythm and fluctuations as a function of time. On the other hand, the small and handy VapoMeter allows more flexibility in measuring protocols and in in-use performance.

Silicone Polymers in Scar Remediation: The Role of Migration of Oligomers Through Stratum Corneum

A cross-linked silicone gel for burns scar rehabilitation has been analyzed to determine the oligomeric species that may migrate from the gel into skin. From MALDI–MS (matrix assisted laser desorption ionization–mass spectrometry) these were shown to be hydroxyl-terminated linear oligomers with a mass of up to 2100 Da. The migration of the extracted oligomers through stratum corneum has been determined by breakthrough times using ATR–FTIR (attenuated total reflection Fourier-transform infrared) spectroscopy. The upper value of the estimated diffusion coefficient of 2.5 × 10−9 cm2 s−1, determined from a breakthrough time of 31 h, is similar to that of large hydrophobic solutes, but penetration times from the silicone gel are low (7–10 days) due to the lower concentration of oligomers partitioning into the lipid bilayer of the skin in an aqueous environment.

Ceramide analogue 14S24 ((S)-2-tetracosanoylamino-3-hydroxypropionic acid tetradecyl ester) is effective in skin barrier repair in vitro

Stratum corneum ceramides are fundamental for maintaining the skin barrier properties. Their content is decreased in some skin diseases, e.g. atopic dermatitis, and ceramide supplementation is one of the therapeutic approaches. In the present study we have designed novel ceramide analogue 14S24 ((S)-2-tetracosanoylamino-3-hydroxypropionic acid tetradecyl ester) as a potential barrier-repairing agent. We report a convenient two-step synthesis of this analogue with high yields. The ability of 14S24 to repair the disturbed skin barrier was evaluated in vitro on the porcine skin. After 2h application of 14S24 on the skin disrupted by lipid extraction, the permeability decreased significantly almost to the values of the native skin. The compound is effective in 0.1% aqueous suspension and its effect is comparable with physiological skin lipids under the same condition. The comparison of 14S24 and skin ceramides was made via computer modelling and the in silico physico-chemical parameters are reported. We suggest that allylic hydroxyl, that is essential for the apoptogenic activity of ceramides, is not a necessary component of the skin barrier-forming ceramides. The main result of this study is to demonstrate that simpler and easier-to-synthesise ceramide analogues could be effective in the skin barrier repair.

Barrier function of the skin: "la raison d'être" of the epidermis

The primary function of the epidermis is to produce the protective, semi-permeable stratum corneum that permits terrestrial life. The barrier function of the stratum corneum is provided by patterned lipid lamellae localized to the extracellular spaces between corneocytes. Anucleate corneocytes contain keratin filaments bound to a peripheral cornified envelope composed of cross-linked proteins. The many layers of these specialized cells in the stratum corneum provide a tough and resilient framework for the intercellular lipid lamellae. The lamellae are derived from disk-like lipid membranes extruded from lamellar granules into the intercellular spaces of the upper granular layer. Lysosomal and other enzymes present in the extracellular compartment are responsible for the lipid remodeling required to generate the barrier lamellae as well as for the reactions that result in desquamation. Lamellar granules likely originate from the Golgi apparatus and are currently thought to be elements of the tubulo-vesicular trans-Golgi network. The regulation of barrier lipid synthesis has been studied in a variety of models, with induction of several enzymes demonstrated during fetal development and keratinocyte differentiation, but an understanding of this process at the molecular genetic level awaits further study. Certain genetic defects in lipid metabolism or in the protein components of the stratum corneum produce scaly or ichthyotic skin with abnormal barrier lipid structure and function. The inflammatory skin diseases psoriasis and atopic dermatitis also show decreased barrier function, but the underlying mechanisms remain under investigation. Topically applied "moisturizers" work by acting as humectants or by providing an artificial barrier to trans-epidermal water loss; current work has focused on developing a more physiologic mix of lipids for topical application to skin. Recent studies in genetically engineered mice have suggested an unexpected role for tight junctions in epidermal barrier function and further developments in this area are expected. Ultimately, more sophisticated understanding of epidermal barrier function will lead to more rational therapy of a host of skin conditions in which the barrier is impaired.

L-Serine and glycine based ceramide analogues as transdermal permeation enhancers: polar head size and hydrogen bonding

Novel transdermal permeation enhancers related to stratum corneum ceramides were investigated. The synthesis of a series of simple compounds based on two selected amino acids, L-serine and glycine, and their enhancement activities are reported. Glycine derivative 3 enhanced the permeation of theophylline through human skin in vitro 12.5+/-0.5 times. The relationships between properties of the polar head of the compounds and their activity are discussed.

Comparison of effects of different ointment bases on the penetration of ketoprofen through heat-separated human epidermis and artificial lipid barriers

In vitro tests were performed to understand the effects of topical vehicles on the permeability of ketoprofen through artificial lipid barriers and heat-separated human epidermis consisting of stratum corneum and viable epidermis. Ketoprofen was selected as the model penetrant. Human epidermis and artificial membranes made from several mixtures of free fatty acids, cholesterol, cholesteryl ester, ceramides, and triglycerides were used as permeation matrices in untreated conditions and after pretreatment with petrolatum, wool alcohols ointment, or triglycerides. Apparent permeability and diffusion coefficients as well as the solubility of the drug in the artificial lipid matrices were ascertained. The solubilities of drugs and vehicle components that depend on the composition of the lipid matrix predominantly control the permeability of the barrier. Ceramides and cholesterol reduce the permeability and solubility, whereas triglycerides have the opposite effect. The vehicle effects in artificial membranes correspond to those in epidermis observed with samples pretreated with the aforementioned bases. The logarithms of the permeabilities of untreated and pretreated lipid mixtures 3, 4, and 5 are very well correlated with those of the permeabilities of heat-separated epidermis (r > = 0.9868). The artificial mixture containing all five lipids mentioned gives the best approximation to human epidermis. This result indicates comparable vehicle effects although the composition of the artificial mixture was adapted only in a simplified manner to the horny layer lipid phase. This lipid matrix or similar mixtures, therefore, are convenient tools for investigation into the effects of dermatological vehicles.

Ceramides and skin function

Ceramides are the major lipid constituent of lamellar sheets present in the intercellular spaces of the stratum corneum. These lamellar sheets are thought to provide the barrier property of the epidermis. It is generally accepted that the intercellular lipid domain is composed of approximately equimolar concentrations of free fatty acids, cholesterol, and ceramides. Ceramides are a structurally heterogeneous and complex group of sphingolipids containing derivatives of sphingosine bases in amide linkage with a variety of fatty acids. Differences in chain length, type and extent of hydroxylation, saturation etc. are responsible for the heterogeneity of the epidermal sphingolipids. It is well known that ceramides play an essential role in structuring and maintaining the water permeability barrier function of the skin. In conjunction with the other stratum corneum lipids, they form ordered structures. An essential factor is the physical state of the lipid chains in the nonpolar regions of the bilayers. The stratum corneum intercellular lipid lamellae, the aliphatic chains in the ceramides and the fatty acids are mostly straight long-chain saturated compounds with a high melting point and a small polar head group. This means that at physiological temperatures, the lipid chains are mostly in a solid crystalline or gel state, which exhibits low lateral diffusional properties and is less permeable than the state of liquid crystalline membranes, which are present at higher temperatures. The link between skin disorders and changes in barrier lipid composition, especially in ceramides, is difficult to prove because of the many variables involved. However, most skin disorders that have a diminished barrier function present a decrease in total ceramide content with some differences in the ceramide pattern. Formulations containing lipids identical to those in skin and, in particular, some ceramide supplementation could improve disturbed skin conditions. Incomplete lipid mixtures yield abnormal lamellar body contents, and disorder intercellular lamellae, whereas complete lipid mixtures result in normal lamellar bodies and intercellular bilayers. The utilization of physiological lipids according to these parameters have potential as new forms of topical therapy for dermatoses. An alternative strategy to improving barrier function by topical application of the various mature lipid species is to enhance the natural lipid-synthetic capability of the epidermis through the topical delivery of lipid precursors.

Frequency and thermal effects on the enhancement of transdermal transport by sonophoresis

The aims of this work were: (i) to examine the role of ultrasound (US) frequency and intensity on the transport of glucose and mannitol across porcine skin in vitro, (ii) to quantify the energy delivered to the skin during application of low-frequency sonophoresis, and (iii) to 'deconvolute' the thermal effect, induced by US application to the skin, to the enhanced permeability of the cutaneous barrier. Low- (20 kHz) and high-frequency (10 MHz) sonophoresis were first compared. Only low frequency US resulted in significantly increased permeation. Low-frequency, US-induced enhancement of mannitol transport was symmetric; that is, mannitol flux was the same when 'delivered' or 'extracted' from a donor solution (in both cases, the US probe was present on the surface side of the skin). Calorimetry was used to quantify the US energy delivered by the sonicator. Subsequently, the US-enhanced transdermal transport of mannitol, during which a significant (and US intensity-dependent) temperature increase occurred, was compared to that provoked, in the absence of sonophoresis, by a comparable thermal effect. Only 25% of this enhancement was attributable to the increased temperature induced by US. It follows that another mechanism, most probably cavitation, is principally responsible for the lowered skin barrier function observed.