Lipid and protein structures in the permeability barrier of mammalian epidermis

A novel function for transglutaminase 1: attachment of long-chain omega-hydroxyceramides to involucrin by ester bond formation

Transglutaminases (TGases) are defined as enzymes capable of forming isopeptide bonds by transfer of an amine onto glutaminyl residues of a protein. Here we show that the membrane-bound form of the TGase 1 enzyme can also form ester bonds between specific glutaminyl residues of human involucrin and a synthetic analog of epidermal specific omega-hydroxyceramides. The formation of a approximately 5-nm-thick lipid envelope on the surface of epidermal keratinocytes is an important component of normal barrier function. The lipid envelope consists of omega-hydroxyceramides covalently linked by ester bonds to cornified envelope proteins, most abundantly to involucrin. We synthesized an analog of natural omega-hydroxyceramides N-[16-(16-hydroxyhexadecyl)oxypalmitoyl]sphingosine (lipid Z). When recombinant human TGase 1 and involucrin were reacted on the surface of synthetic lipid vesicles containing lipid Z, lipid Z was attached to involucrin and formed saponifiable protein-lipid adducts. By mass spectroscopy and sequencing of tryptic lipopeptides, the ester linkage formation used involucrin glutamine residues 107, 118, 122, 133, and 496 by converting the gamma-carboxamido groups to lipid esters. Several of these residues have been found previously to be attached to ceramides in vivo. Mass spectrometric analysis after acetonide derivatization also revealed that ester formation involved primarily the omega-hydroxyl group of lipid Z. Our data reveal a dual role for TGase 1 in epidermal barrier formation and provide insights into the pathophysiology of lamellar ichthyosis resulting from defects of TGase 1 enzyme.

Sphingolipid activator proteins are required for epidermal permeability barrier formation

The epidermal permeability barrier is maintained by extracellular lipid membranes within the interstices of the stratum corneum. Ceramides, the major components of these multilayered membranes, derive in large part from hydrolysis of glucosylceramides mediated by stratum corneum beta-glucocerebrosidase (beta-GlcCerase). Prosaposin (pSAP) is a large precursor protein that is proteolytically cleaved to form four distinct sphingolipid activator proteins, which stimulate enzymatic hydrolysis of sphingolipids, including glucosylceramide. Recently, pSAP has been eliminated in a mouse model using targeted deletion and homologous recombination. In addition to the extracutaneous findings noted previously, our present data indicate that pSAP deficiency in the epidermis has significant consequences including: 1) an accumulation of epidermal glucosylceramides together with below normal levels of ceramides; 2) alterations in lipids that are bound by ester linkages to proteins of the cornified cell envelope; 3) a thickened stratum lucidum with evidence of scaling; and 4) a striking abnormality in lamellar membrane maturation within the interstices of the stratum corneum. Together, these results demonstrate that the production of pSAP, and presumably mature sphingolipid activator protein generation, is required for normal epidermal barrier formation and function. Moreover, detection of significant amounts of covalently bound omega-OH-GlcCer in pSAP-deficient epidermis suggests that deglucosylation to omega-OH-Cer is not a requisite step prior to covalent attachment of lipid to cornified envelope proteins.

Induction of selected lipid metabolic enzymes and differentiation-linked structural proteins by air exposure in fetal rat skin explants

The epidermal permeability barrier of premature infants matures rapidly following birth. Previous studies suggest that air exposure could contribute to this acceleration, because: (i) development of a structurally and functionally mature barrier accelerates when fetal rat skin explants are incubated at an air-medium interface, and (ii) occlusion with a water-impermeable membrane prevents this acceleration. To investigate further the effects of air exposure on epidermal barrier ontogenesis, we compared the activities of several key enzymes of lipid metabolism and gene expression of protein markers of epidermal differentiation in fetal rat skin explants grown immersed versus air exposed. The rate-limiting enzymes of cholesterol (HMG CoA reductase) and ceramide (serine palmitoyl transferase) synthesis were not affected. In contrast, the normal developmental increases in activities of glucosylceramide synthase and cholesterol sulfotransferase, responsible for the synthesis of glucosylceramides and cholesterol sulfate, respectively, were accelerated further by air exposure. Additionally, two enzymes required for the final stages of barrier maturation and essential for normal stratum corneum function, beta-glucocerebrosidase, which converts glucosylceramide to ceramide, and steroid sulfatase, which desulfates cholesterol sulfate, also increased with air exposure. Furthermore, filaggrin and loricrin mRNA levels, and filaggrin, loricrin, and involucrin protein levels all increased with air exposure. Finally, occlusion with a water-impermeable membrane prevented both the air-exposure-induced increase in lipid enzyme activity, and the expression of loricrin, filaggrin, and involucrin. Thus, air exposure stimulates selected lipid metabolic enzymes and the gene expression of key structural proteins in fetal epidermis, providing a biochemical basis for air-induced acceleration of permeability barrier maturation in premature infants.

Inter- and intra-individual differences in human stratum corneum lipid content related to physical parameters of skin barrier function in vivo

For a full understanding of the properties of the human skin barrier, physical macroscopic parameters of barrier function must be correlated to the structural organization of the barrier on a molecular level. This study was undertaken to relate differences in the relative composition of the three main lipid classes of human stratum corneum, i.e., free fatty acids, cholesterol, and ceramides, to differences in transepidermal water loss, stratum corneum electrical impedance, and corneometer value. A new high performance liquid chromatography/light scattering detection-based analysis method recently developed was used for collection of quantitative lipid data in conjunction with gas chromatography/mass spectrometry/flame ionization detection measurements on the free fatty acid fraction. After subtraction of contaminating lipid fractions we have estimated the molar ratio of the human skin barrier lipid composition to be, respectively, 15% cholesterol esters, 16% saturated long chain free fatty acids, 32% cholesterol, and 37% ceramides. The inter-individual difference in the relative amount of free fatty acids, cholesterol, and ceramides, respectively, can be >100% in the individual case. It was found that the relative amount of ceramides to cholesterol is larger in the wrist area, paralleled by a higher transepidermal water loss and corneometer value as well as different skin electrical impedance values as compared with the upper forearm area. We conclude that the site-dependent differences in the stratum corneum lipid composition are small compared with the large inter-individual variation. Interestingly, in the individual case, no correlation was registered between relative ceramide content and barrier properties.

cDNA cloning and characterization of sciellin, a LIM domain protein of the keratinocyte cornified envelope

Sciellin is a precursor of the cornified envelopes of mammalian keratinizing tissues. We have cloned the cDNA encoding sciellin by screening a human keratinocyte expression library with a sciellin-specific monoclonal antibody. The composite cDNA of 2.35 kilobase pairs encodes a protein of 75.3 kDa with a pI of 10.09. The translated sequence has a central domain containing 16 repeats of 20 amino acids each that is rich in Gln and Lys residues, which are potential transglutaminase substrates, and a carboxyl domain, which contains a single LIM motif. Sciellin cDNA probes hybridize to bands of 3.4 and 4.4 kilobase pairs on Northern blots of cultured human keratinocyte RNA. The gene was mapped to human chromosome band 13q22 by fluorescence in situ hybridization. Radiation hybrid mapping demonstrated that sciellin is linked to the sequence tagged site marker WI-457 with a logarithm of the odds score of 7.77. In situ hybridization of human foreskin tissue sections demonstrated that sciellin is expressed in the stratum granulosum. Immunofluorescent staining with a polyclonal rabbit antibody made to a recombinant sciellin protein showed peripheral cytoplasmic localization in the upper cell layers of epidermis and in stratified squamous epithelia such as the oral cavity, esophagus, and vagina. Simple and columnar epithelia, with the exception of the amnion, showed no reaction.

Expression and regulation of mRNA for putative fatty acid transport related proteins and fatty acyl CoA synthase in murine epidermis and cultured human keratinocytes

The epidermis has a requirement for fatty acids in order to synthesize cellular membranes and the extracellular lipid lamellar membranes in the stratum corneum. Despite high endogenous production of fatty acids the transport of exogenous essential fatty acids into the epidermis is an absolute requirement. Fatty acid uptake by keratinocytes has been shown to be mediated by a transport system. In this study we determined in murine epidermis and human cultured keratinocytes the expression of three putative fatty acid transport related proteins and fatty acyl CoA synthase, an enzyme that facilitates the uptake of fatty acids by promoting their metabolism. In cultured human keratinocytes, mRNA for fatty acid transport protein (FATP), plasma membrane fatty acid binding protein (FABP-pm), and fatty acyl CoA synthase (FACS) were detectable. Differentiation, induced by high calcium, did not affect FATP mRNA levels, but resulted in an approximately 50% increase in FACS mRNA, while decreasing FABP-pm mRNA by 50%. Fatty acid translocase (FAT) mRNA was not detected in cultured human keratinocytes. In murine epidermis, FATP, FABP-pm, FACS, and FAT mRNA were all present. Barrier disruption by either tape stripping or acetone treatment increased FAT mRNA levels by approximately 2-fold without affecting FATP, FABP-pm, or FACS. Occlusion with an impermeable membrane immediately after barrier disruption completely blocked the increase in FAT mRNA levels, indicating that this increase is related to barrier disruption rather than a nonspecific injury effect. In summary, this study demonstrates that several putative fatty acid transport related proteins as well as fatty acyl CoA synthase are expressed in keratinocytes and epidermis, and that the expression of these proteins may be regulated by differentiation and/ or barrier disruption.

Sphingolipid functions in Saccharomyces cerevisiae: comparison to mammals

Many roles for sphingolipids have been identified in mammals. Available data suggest that sphingolipids and their intermediates also have diverse roles in Saccharomyces cerevisiae. These roles include signal transduction during the heat stress response, regulation of calcium homeostasis or components in calcium-mediated signaling pathways, regulation of the cell cycle, and functions as components in trafficking of secretory vesicles from the endoplasmic reticulum to the Golgi apparatus and as the lipid moiety in many glycosylphosphatidylinositol-anchored proteins. S. cerevisiae is likely to be the first organism in which all genes involved in sphingolipid metabolism are identified. This information will provide an unprecedented opportunity to determine, for the first time in any organism, how sphingolipid synthesis is regulated. Through the use of both genetic and biochemical techniques, the identification of the complete array of processes regulated by sphingolipid signals is likely to be possible, as is the quantification of the physiological contribution of each.

Expression of epidermal keratins and the cornified envelope protein involucrin is influenced by permeability barrier disruption

In previous studies we have shown that experimental permeability barrier disruption leads to an increase in epidermal lipid and DNA synthesis. Here we investigate whether barrier disruption also influences keratins and cornified envelope proteins as major structural keratinocyte proteins. Cutaneous barrier disruption was achieved in hairless mouse skin by treatments with acetone +/- occlusion, sodium dodecyl sulfate, or tape-stripping. As a chronic model for barrier disruption, we used essential fatty acid deficient mice. Epidermal keratins were determined by one- and two-dimensional gel electrophoresis, immunoblots, and anti-keratin antibodies in biopsy samples. In addition, the expression of the cornified envelope proteins loricrin and involucrin after barrier disruption was determined by specific antibodies in human skin. Acute as well as chronic barrier disruption resulted in the induction of the expression of keratins K6, K16, and K17. Occlusion after acute disruption led to a slight reduction of keratin K6 and K16 expression. Expression of basal keratins K5 and K14 was reduced after both methods of barrier disruption. Suprabasal keratin K10 expression was increased after acute barrier disruption and K1 as well as K10 expression was increased after chronic barrier disruption. Loricrin expression in mouse and in human skin was unchanged after barrier disruption. In contrast, involucrin expression, which was restricted to the granular and upper spinous layers in normal human skin, showed an extension to the lower spinous layers 24 h after acetone treatment. In summary, our results document that acute or chronic barrier disruption leads to expression of keratins K6, K16, and K17 and to a premature expression of involucrin. We suggest that the coordinated regulation of lipid, DNA, keratin, and involucrin synthesis is critical for epidermal permeability barrier function.

Up-regulation of glucosylceramide synthase expression and activity during human keratinocyte differentiation

During keratinocyte differentiation, the glycolipid, glucosylceramide (GlcCer), is thought to be synthesized, stored in intracellular lamellar granules and eventually extruded into the intercellular space where GlcCer is hydrolyzed to ceramide, a major component of the epidermal permeability barrier. Previous studies showed that GlcCer synthase (GCS) activity increases during keratinocyte differentiation; however, the mechanism by which GCS activity is regulated was not established. In the present study, we prepared anti-peptide antibodies and amplified cDNA probes based on the cDNA sequence for human GCS (Ichikawa, S., Sakiyama, H., Suzuki, G., Hidari, K. I.-P. J., and Hirabayashi, Y. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 4638-4643) in order to study GCS expression during keratinocyte differentiation. Confluent human keratinocytes in culture were induced to terminally differentiate by elevation of Ca+2 in the medium without exogenous hormones or growth factors. GlcCer synthesis assayed in situ using a fluorescent ceramide analog increased approximately 5-fold during keratinocyte differentiation, peaking at day 6. Fluorescence microscopy studies of living keratinocytes showed that fluorescent ceramide and/or its metabolites accumulated in the Golgi in undifferentiated cells but targeted to unique vesicular structures that may be derived from the trans-Golgi region. Expression of both GCS mRNA, a approximately 3. 8-kilobase transcript on Northern blots, and GCS protein, a approximately 38-kDa polypeptide detected by Western blotting, increased dramatically (approximately 5-fold) during differentiation, reaching a maximum at about day 8. These results suggest that GCS is up-regulated at the transcriptional level during keratinocyte differentiation and provide the first direct evidence for GCS up-regulation in any cell type.

Cornified cell envelope assembly: a model based on electron microscopic determinations of thickness and projected density

In stratifying squamous epithelia, the cornified cell envelope (CE), a peripheral layer of crosslinked protein, is assembled sequentially from precursor proteins initially dispersed in the cytoplasm. Its major component is loricrin (37 kDa in mouse), which contributes from approx. 60% to >80% of the protein mass in different tissues. Despite its importance to the mechanical resilience and impenetrability of these tissues, detailed information has not been obtained on CE structure, even on such basic properties as its thickness or uniformity across a given CE or from tissue to tissue. To address this issue, we have studied CEs isolated from three murine epithelia, namely epidermis, forestomach and footpad, by electron microscopy of metal-shadowed specimens and scanning transmission electron microscopy (STEM) of unstained specimens. The former data reveal that the cytoplasmic surface is smoothly textured whereas the extracellular surface is corrugated, and that the average thickness is 15.3+/−1.2 nm, and strikingly uniform. Measurements of mass-per-unit-area from the STEM images yielded values of approx. 7.0+/−0.8 kDa/nm2, which were remarkably consistent over all three tissues. These data imply that the mature CE has a uniquely defined thickness. To explain its uniformity, we postulate that loricrin forms a molecular monolayer, not a variable number of multiple layers. In this scenario, the packing density is one loricrin monomer per 7 nm2, and loricrin should have an elongated shape, 2.5-3.0 nm wide by approx. 11 nm long. Moreover, we anticipate that any inter-tissue variations in the mechanical properties of CEs should depend more on protein composition and cross-linking pattern than on the thickness of the protein layer deposited.

Patterned acquisition of skin barrier function during development

Skin barrier function is conferred by the outer layer of epidermis, the stratum corneum, and is essential for terrestrial life. Quantitative trans-epidermal water loss assays show that barrier forms late in embryogenesis, permitting the foetus to survive a terrestrial environment at birth. Using qualitative in situ assays for skin permeability, we show that barrier forms in a patterned manner late in mouse gestation. Barrier forms at specific epidermal sites, then spreads around the embryo as a moving front. The moving front of permeability change is accompanied by multiple changes in the outer, stratum corneum-precursor cells. We use the permeability assays to show that final stages of cornified envelope assembly are coordinated with initial stages of barrier formation. Hence the whole-mount permeability assays record developmental acquisition of a known, essential component of the adult barrier. We demonstrate the authenticity of the whole-mount assays after maternal glucocorticoid therapy (known to accelerate barrier formation) and in additional species including the rat where barrier formation is well characterized by TEWL assay (Aszterbaum, M., Menon, G. K., Feingold, K. R. and Williams, M. L. Pediatr. Res. 31, 308–317). The demonstration of patterned barrier formation in other species suggests patterned change as the universal mode of embryonic barrier acquisition. These results highlight the importance of patterning as a mode of epidermal maturation during development.

Keratinocyte differentiation is stimulated by activators of the nuclear hormone receptor PPARalpha

Peroxisome proliferator activated receptors (PPAR) belong to the superfamily of nuclear hormone receptors that heterodimerize with the retinoid X receptor and regulate transcription of several genes involved in lipid metabolism and adipocyte differentiation. Because of the role of 1,25-dihydroxyvitamin D3 and retinoic acid working through similar receptors (the vitamin D receptor and retinoic acid receptor, respectively) on keratinocyte differentiation, we have examined the effects of activators of PPARalpha on keratinocyte differentiation. The rate of cornified envelope formation was increased 3-fold in keratinocytes maintained in low calcium (0.03 mM) and incubated in the presence of clofibric acid, a potent PPARalpha activator. Involucrin, a cornified envelope precursor, and the cross-linking enzyme transglutaminase, were increased at both the message level (2-7-fold) and the protein level (4-12-fold) by clofibric acid. Furthermore, physiologic doses of the fatty acids oleic acid, linoleic acid, and eicosatetraynoic acid, which are also activators of PPARalpha, also induced involucrin and transglutaminase protein and mRNA. In contrast, the PPARgammaligand prostaglandin J2 had no effect on protein or mRNA levels of involucrin or transglutaminase. Levels of involucrin and transglutaminase mRNA and protein were induced by clofibric acid in keratinocytes incubated in 1.2 mM calcium, a concentration which by itself induces keratinocyte differentiation. Finally, PPARalpha activators inhibit DNA synthesis. This study demonstrates that PPARalpha activators, including putative endogenous ligands such as fatty acids, induce differentiation and inhibit proliferation in keratinocytes, and suggests a regulatory role for the PPARalpha in epidermal homeostasis.

Phase coexistence in cholesterol-fatty acid mixtures and the effect of the penetration enhancer Azone

Small and wide-angle X-ray diffraction was used to study the phase behaviour of cholesterol-fatty acid mixtures in an attempt to understand lipid interaction occurring in the stratum corneum, the outermost layer of skin. The effect of the penetration enhancer Azone was investigated as well. It was found that equimolar mixtures of cholesterol, palmitic acid and oleic acid (with the acids neutralised to 41 mol%) in 25% (wt/wt) water typically showed three phases at room temperature, two crystalline and one gel phase. The crystalline phases consisted mainly of palmitic acid:soap and cholesterol, respectively. The water present was unevenly distributed and was associated with the gel phase. Both cholesterol and palmitic acid seemed to be depleted from their crystalline phases by Azone. The electrostatic effects on titration of fatty acids in lamellar aggregates were calculated in view of the present results, and the effects of phase separation were discussed.

The role of 18-methyleicosanoic acid in the structure and formation of mammalian hair fibres

Although branched chain fatty acids perform many functions in biological systems, the importance of the anteiso 18 methyleicosanoic acid (MEA) has only recently been recognized. In this first review on MEA its role and distribution is explored. MEA has been found in minor amounts in the fatty acid components of a wide range of biological materials, but the current interest results from it being the major covalently bound fatty acid in mammalian hair fibres, a finding which is unusual because protein-bound fatty acids are typically straight-chain, even-numbered acids (C14-C18). MEA is released by surface restricted reagents indicating that it is located exclusively in or on the surface of the cuticle cells, a conclusion that has been verified by analysis of isolated cuticle cells, X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectroscopy (SIMS) studies support these results in that they show the surface of the cuticle to be predominantly hydrocarbon. When either neutral hydroxylamine or acidic chlorine solutions are applied to hair and wool fibres fatty acids are liberated, indicating the presence of thioester bonds. Calculations, based on fatty acid and amino acid analysis, indicate that approximately one residue in 10 of the cuticular membrane protein is a fatty acid thioester of cysteine. Removal of this covalently linked fatty acid renders the fibre hydrophilic, thus offering a chemical explanation for many technological and cosmetic treatments of mammalian fibres. Examination of the fibre surface and that of isolated cuticle cells by transmission electron microscopy (TEM) confirms the presence of a thin non-staining continuous layer surrounding the cuticle cells. Alkaline treatments which remove the bound fatty acids were found to disrupt this layer. TEM examination of developing hair fibres has indicated that the fatty acid layer on the upper surface and scale edges of the cuticle cell differs from that of the underside of the cell. Similar structural studies of hair from patients with maple syrup urine disease (MSUD) support the findings that thioester-bound MEA is limited to the upper surface of fibre cuticle cells. The current model proposed for the boundary layer consists of crosslinked protein with surface thioester-linked fatty acids, forming a continuous hydrophobic layer on the upper surface and scale edges of the cells.

Electroperturbation of the human skin barrier in vitro: II. Effects on stratum corneum lipid ordering and ultrastructure

In transdermal iontophoresis, drugs can be driven across the skin by electrorepulsion, but their transport can also be enhanced by electrical perturbation of the skin barrier. Our objective was to study perturbing effects of electrical current on human stratum corneum lipid fine structure combining techniques including freeze-fracture electron microscopy. Human stratum corneum was subjected to pulsed constant currents, varying from 0.013-13 mA.cm-2. The voltage across the stratum corneum was high-frequency-sampled and s.c. impedence values derived from it. Upon termination of the current, skin samples were rapidly frozen and processed for freeze-fracture electron microscopy or subjected to X-ray diffraction analysis. Initially a rapid decrease of the resistance and, overall, a rapid increase of the capacitances was observed; generally, these effects became more pronounced with increasing current density. Wide- and small-angle X-ray diffractograms of human stratum corneum exposed for 1 h to the highest current indicated a disordering of both the lateral packaging arrangement and long-range lamellar stacking of the intercellular lipids of stratum corneum. Furthermore, an increase in the stratum corneum hydration level as a result of electrical current application was observed. On electron micrographs of freeze-fracture replicas of human stratum corneum, exposed for 1 h to current densities between 0.013 and 13 mA.cm-2, perturbations of the intercellular lipid structure were observed in accordance with the results of X-ray diffraction; these perturbations aggravated with increasing current density. Together, the data suggest that both the lateral and the longitudinal disordering of the intercellular lipids observed with X-ray diffraction may be responsible for the appearance of perturbed structures observed with freeze-fracture electron microscopy. The lipid disordering may be due to polarization of the lipid head groups induced by the electrical field, followed by mutual repulsion.

A novel approach to the understanding of human skin barrier function

The basis for externally caused skin disorders is penetration of the skin barrier. A recent model for the skin barrier, the domain mosaic model, based on current knowledge of the physics of lipid bilayer organization gave tentative explanations for several aspects of function. It is demonstrated here that a development of the model explains how the requirements are met for a water-tight structure that will still allow a controlled, minute loss of water, the perspiratio insensibilis, necessary for maintaining plasticity of the keratin. A major advantage of the extended model is that it allows an interpretation of the changes imposed on the structure when in contact with detergents and/or penetration enhancers.

Direct evidence that involucrin is a major early isopeptide cross-linked component of the keratinocyte cornified cell envelope

Involucrin was the first protein to be identified as a likely constituent of the insoluble cornified cell envelope (CE) of stratified squamous epithelia. However, to date, direct isolation from CEs of involucrin cross-linked by way of the transglutaminase-induced isopeptide bond has not been reported. We have treated human foreskin CEs with methanol/KOH (saponification) to hydrolyze off much of the lipids. By immunogold electron microscopy, this exposed large amounts of involucrin epitopes as well as of desmoplakin, a desmosomal structural protein. About 20% of the total CE protein could be solubilized by proteolytic digestion after saponification, of which involucrin was the most abundant. Subsequent amino acid sequencing revealed many peptides involving involucrin cross-linked either to itself or to a variety of other known CE protein components, including cystatin alpha, desmoplakin, elafin, keratins, members of the small proline-rich superfamily, loricrin, and unknown proteins related to the desmoplakin family. Specific glutamines or lysines of involucrin were used to cross-link the different proteins, such as glutamines 495 and 496 to desmoplakin, glutamine 288 to keratins, and lysines 468, 485, and 508 and glutamines 465 and 489 for interchain involucrin cross-links. Many identical peptides were obtained from immature CEs isolated from the inner living cell layers of foreskin epidermis. The multiple cross-linked partners of involucrin provide experimental confirmation that involucrin is an important early scaffold protein in the CE. Further, these data suggest that there is significant redundancy in the structural organization of the CE.

Transdermal delivery of peptides by iontophoresis

Transdermal administration by iontophoresis (enhanced transport via the skin using the driving force of an applied electric field) has been successfully demonstrated but no formal relationship between peptide sequence/structure and efficiency of delivery has been established. There are notable examples, such as the lipophilic leutinizing hormone releasing hormone (LHRH) analogs, Nafarelin and Leuprolide, that exhibit down-regulation of their own transport across the skin under the influence of an iontophoretic current. The hypothesis that this phenomenon is due to neutralization of the skin's net negative charge by these cationic peptides was examined with LHRH oligopeptides. The impact of these compounds on the electroosmotic flow of solvent into the skin, which is induced by iontophoresis and which contributes significantly to the electrotransport of large, positively charged ions, was examined and quantified. Close juxtaposition of cationic and lipophilic residues profoundly inhibited electroosmosis and, presumably, peptide flux. The results indicate that the lipophilicity of the oligopeptides facilitates van der Waals interactions with hydrophobic patches along the transport route, thereby permitting the positively charged oligopeptide to interact with carboxylate side chains that give the skin its net negative charge at neutral pH. The lipophilic, cationic oligopeptide, therefore, becomes anchored in the transport path, neutralizing the original charge of the membrane, and completely altering the permselective properties of the skin.

Hair keratinization in health and disease

The cells of the epidermis and its derivative, the hair follicle, undergo processes of terminal differentiation that involves the synthesis and assembly of classes of protein and enzymes to form the stratum corneum of the epidermis, and the hair fiber and its cuticle. Using genetic linkage and DNA sequencing methods, we now know that mutations in several genes encoding epidermal keratins or a transglutaminase enzyme cause ichthyosis-related diseases. Similar methods have now suggested that mutations in hair keratin genes underlie some cases of monilethrix, and a deficiency in a cuticle lipid metabolizing enzyme causes maple syrup urine disease. It is to be expected that further application of these methods will elucidate the molecular bases of other genetic hair diseases.

Functional expression and cellular localization of a mouse epidermal lipoxygenase

Three distinct murine lipoxygenase genes have been functionally characterized: 5-lipoxygenase (Chen, X.-S., Naumann, T. A., Kurre, U. , Jenkins, N. A., Copeland, N. G., and Funk, C. D. (1995) J. Biol. Chem. 270, 17993-17999), platelet-type 12-lipoxygenase and leukocyte-type 12-lipoxygenase (Chen, X.-S., Kurre, U., Jenkins, N. A., Copeland, N. G., and Funk, C. D. (1994) J. Biol. Chem. 269, 13979-13987). Here, we describe the cloning and functional characterization of a fourth lipoxygenase gene in mice. Using a polymerase chain reaction-based approach together with partial sequence information from a genomic clone, we isolated a novel lipoxygenase cDNA from the RNA of 3-6-day-old mouse epidermis. The open reading frame predicts a 662-amino acid lipoxygenase that displays 60% identity with both murine 12-lipoxygenase isozymes and 40% identity to 5-lipoxygenase; the sequence is identical to a genomic sequence reported recently (van Dijk, K. W., Steketee, K., Havekes, L., Frants, R., and Hofker, M. (1995) Biochim. Biophys. Acta 1259, 4-8). A full-length clone was expressed in human embryonic kidney 293 cells and homogenates from disrupted cells produced 12-hydroxyeicosatetraenoic acid (12-HETE) and minor amounts of 15-HETE from arachidonic acid. Chiral phase analysis indicated that the 12-HETE is exclusively the 12S enantiomer. In situ hybridization revealed highly specific expression of epidermal lipoxygenase in differentiated keratinocytes of the epidermis and in restricted regions of the root sheath and bulb of hair follicles. High expression was also detected in conjunctiva of the eyelid and in cells of Meibomian and preputial (sebaceous) glands. A 2. 4-kilobase mRNA was detected in mouse epidermis by Northern blot analysis and its abundance was not affected by phorbol ester treatment. The epidermal lipoxygenase gene (Aloxe) resides on mouse chromosome 11 closely linked with the two 12-lipoxygenase genes (Alox12p and Alox12l).

Induction of ceramide glucosyltransferase activity in cultured human keratinocytes. Correlation with culture differentiation

Ceramides are the major component of the extracellular lipids that comprise the epidermal permeability barrier. They are derived from glucosylceramides (GlcCer) upon their extrusion from lamellar granules into the extracellular space in the upper layers of the epidermis. To better understand the regulation of the unique pathway for ceramide production in epidermis, we have studied the activity of the enzyme responsible for GlcCer synthesis, ceramide glucosyltransferase (CerGlc transferase), during keratinocyte culture differentiation. Human keratinocyte cultures were expanded in low calcium keratinocyte growth medium (KGM) and then switched to either normal calcium KGM (nKGM) or "complete" Dulbecco's modified Eagle's medium/Ham's F-12 (3:1) supplemented with 10% fetal bovine serum (cDMEM). At 7 and 10 days after the medium switch, electron microscopy revealed that cDMEM cultures were more fully differentiated morphologically and contained numerous lamellar granules. The GlcCer/DNA content of cDMEM cultures increased to 6 times that of day 0 cultures and was nearly 4 times greater than that of nKGM cultures, whereas the total lipid/DNA content of cDMEM cultures increased to only 1.8 times that of day 0 cultures and was approximately 1.2 times that of nKGM cultures. CerGlc transferase activity/DNA increased 6 times in cDMEM cultures but <1.5 times in nKGM cultures. By contrast, beta-glucocerebrosidase activity, which is responsible for the conversion of GlcCer to ceramide, increased to a similar extent in both differentiating culture systems. Treatment of cultures with the reversible CerGlc transferase inhibitor, DL-threo-1-phenyl-2-(palmitoylamino)-3-morpholino-1-propanol, prevented the increase of GlcCer in cDMEM cultures, and blocked conversion of exogenously added ceramide to GlcCer. A low level of CerGlc transferase activity, relative to that in differentiated keratinocytes, was detected in cultures of other human cell types. These results indicate that CerGlc transferase activity is induced during epidermal differentiation and that regulation of this enzyme may be an important determinant of the specialized production and compartmentalization of epidermal sphingolipids.

Sodium lauryl sulfate (SLS) induced irritant contact dermatitis: a correlation study between ceramides and in vivo parameters of irritation

Sodium lauryl sulfate (SLS), a surfactant frequently used in the induction of experimental irritant contact dermatitis in animals and in humans, characteristically induces a dose-related increase in TEWL (transepidermal water loss). Ceramides are considered to be important in the regulation of the skin barrier. We therefore examined the relationship between initial ceramide content of stratum corneum and induced changes in skin color (erythema) and barrier function, after SLS application under occlusion (1% and 3% in water) to the forearm of 14 volunteers. Stratum corneum sheets were removed, stratum corneum lipids extracted, and ceramide composition determined from chromatograms (TLC) using densitometry. After determining baseline skin color and TEWL at each area, 2 samples of stratum corneum were obtained from each volunteer. Clinical and instrumental controls of the SLS-induced irritation were performed at 24, 48, 72 and 96 h. Erythema was evaluated by colorimetry: barrier impairment by changes in TEWL. We found inverse correlations between baseline ceramide 61 (weight) and the 24 h erythema score for SLS 3%, between ceramide 1 and 24 h TEWL, and between ceramide 611 and 72 h TEWL for SLS 3%. Our findings suggest that low levels of these ceramides may determine a proclivity to SLS-induced irritant contact dermatitis.

Differential expression and cell envelope incorporation of small proline-rich protein 1 in different cornified epithelia

In the final stages of terminal differentiation in the epidermis and other squamous epithelia, a approximately 15 nm thick protein layer called the cornified cell envelope (CE) assembles on the keratinocytes' inner surface. Its constituent proteins are covalently crosslinked by the action of transglutaminases. Recent studies have indicated that the expression of CE precursor proteins may vary in different tissues. To investigate such variations further, we have studied the CEs of two different keratinizing epithelia of mouse: epidermis and forestomach, with particular focus on their contents of loricrin and the small proline-rich proteins (SPRs). To this end, we have applied electron microscopic immunocytochemistry and estimated the CE protein compositions by mathematical modeling of their amino acid compositions. Ultrastructurally, forestomach resembles the epidermis in having well defined cornified and granular layers. Minor but significant differences are: in forestomach, striated material resembling lamellar granules is intercalated between the cornified squames; and in forestomach granular layer cells, loricrin-containing L-granules are more abundant, and filaggrin-containing F-granules less abundant than in epidermis. In forestomach, dense labeling with anti-SPR1 antibody was observed at the margin of cornified layer cells; and in the granular layer, diffuse but positive labeling of both cytoplasm and nucleus. In contrast, epidermis was uniformly negative. Isolated forestomach CEs (but not epidermal CEs), labeled positively on the cytoplasmic side, consistent with the presence of covalently crosslinked SPR1. Our compositional analysis predicts the content of loricrin in forestomach CEs to be very high (approximately 65%), as in the epidermis, and accompanied by approximately 18% content of total SPRs. Of these, a substantial proportion should be SPR1, according to our immunolabeling data. In contrast, epidermal CEs are calculated to have a much lower amount of SPRs or SPR-like proteins (approximately 8%), with a negligible content of SPR1. Thus both kinds of CEs have loricrin as their major constituent but differ in their respective complements of SPRs, which are thought to inter-connect loricrin molecules in the final phase of CE assembly. Applying a basic concept of materials science, it may be that the observed differences in their SPR contents reflect differences in the mechanical and chemical properties required for the function of the respective CEs.

X-ray diffraction analysis of isolated skin lipids: reconstitution of intercellular lipid domains

Low- and wide-angle X-ray diffraction were used to determine the structural organization of lipids isolated from the stratum corneum extracellular matrix that forms the major water permeability barrier in mammalian epidermis. Hydrated pig skin ceramides gave a single low-angle reflection of about 62 angstroms and a wide-angle-reflection at 4.15 angstroms. The addition of either cholesterol or fatty acid, the other major lipid components of the skin stratum corneum extracellular matrix, modified this diffraction pattern, depending on the lipid mole ratios. In the absence of water, lipid mixtures exhibited lipid phase separation, as shown by low- and wide-angle reflections typical of a separate cholesterol phase. However, a hydrated 2:1:1 mole ratio of ceramide:cholesterol:palmitic acid (similar to that found in stratum corneum) produced a diffraction pattern with a single sharp wide-angle reflection at 4.10 angstroms and low-angle reflections which indexed as the first eight orders of a single repeat period of 130 angstroms. The repeat period and intensity distribution of the low-angle data were similar to those found in intact stratum corneum [White et al. (1988) Biochemistry 27, 3725-3732; Bouwstra et al. (1994) Biochim. Biophys. Acta 1212, 183-192]. Higher concentrations of cholesterol or palmitic acid resulted in lipid phase separations. The 130 angstrom repeat period decreased only about 3 angstroms as water was removed by incubation in low-relative humidity atmospheres. The 130 angstrom repeat period depended on the presence of a particular ceramide, N-(omega-acyloxy)-acylsphingosine, which is found only in the epidermis. In contrast, 2:1:1 mixtures of brain ceramide:cholesterol:palmitic acid gave reflections of 56 and 34 angstroms. These results indicate that a structure with dimensions similar to those of the lamellar repeating unit found in skin stratum corneum does not depend on the presence of protein but does depend on the presence of specific skin ceramides and appropriate concentrations of cholesterol and fatty acid.

Epidermal barrier ontogenesis: maturation in serum-free media and acceleration by glucocorticoids and thyroid hormone but not selected growth factors

Because the cutaneous permeability barrier develops late in gestation, prematurity may result in increased morbidity and mortality due to barrier incompetence. The purpose of the present study was to develop an in vitro model of barrier ontogenesis in order to identify those factors critical for fetal barrier formation. Skin explants from gestational day 17 fetal rats (term is 22 days) were incubated in hormone- and serum-free media. After 4 d in culture, a multi-layered stratum corneum (SC) developed that demonstrated a membrane pattern of fluorescence using the hydrophobic probe, nile red, and the deposition of mature lamellar unit structures throughout the SC interstices, ultrastructurally. Transepidermal water loss rates declined during explant culture such that after 4 d a competent barrier was present. Similarly, lanthanum permeation studies showed tracer penetration into all cell layers in 2-d explants, whereas it did not penetrate above the stratum granulosum in 4-d explants. Thus, the chronology of epidermal development in the explants precisely mirrored that observed in utero. Treatment with either 10 nM dexamethasone or 10 nM triiodothyronine accelerated SC development and barrier formation by 2 d. These results indicate that (i) the late events of fetal epidermal development progress in vitro under serum- and growth factor-free conditions, culminating in the formation of a functional barrier, and (ii) both dexamethasone and triiodothyronine accelerate barrier development.

Ultrastructural organization of avian stratum corneum lipids as the basis for facultative cutaneous waterproofing

The ultrastructure of naked neck epidermis from the ostrich (Struthio camelus) and ventral apterium from watered, and water-deprived, Zebra finches (Taeniopygia [Poephila] guttata castanotis) is presented. The form and distribution of the fully differentiated products of the lipid-enriched multigranular bodies are compared in biopsies post-fixed with osmium tetroxide or ruthenium tetroxide. The fine structure of ostrich epidermis suggests it is a relatively poor barrier to cutaneous water loss (CWL). The fine structure from watered, and 16-hr water-deprived Zebra finches, considered in conjunction with measurements of CWL, confirms previous reports of "facultative waterproofing," and emphasizes the rapidity of tissue response to dehydration. The seemingly counterintuitive facts that one xerophilic avian species, the ostrich, lacks a "good barrier" to CWL, whereas another, the Zebra finch, is capable of forming a good barrier, but does not always express this capability, are discussed. An explanation of these data in comparison to mammals centers on the dual roles of the integument of homeotherms in thermoregulation and conserving body water. It is concluded that birds, whose homeothermic control depends so much on CWL, cannot possess a permanent "good barrier," as such would compromise the heat loss mechanism. Facultative waterproofing (also documented in lizards) protects the organism against sudden reductions in water availability. In birds, and probably in snakes and lizards, facultative waterproofing involves qualitative changes in epidermal cell differentiation. Possible control mechanisms are discussed.

Topical stratum corneum lipids accelerate barrier repair after tape stripping, solvent treatment and some but not all types of detergent treatment

Topical acetone treatment extracts lipids from the stratum corneum, and disrupts the permeability barrier, resulting in a homeostatic response in the viable epidermis that ultimately repairs the barrier. Recently, we have developed an optimal lipid mixture (cholesterol, ceramide, palmitate and linoleate 4.3:2.3:1:1.08) that, when applied topically, accelerates barrier repair following extensive disruption of the barrier by acetone. The present study determined if topical treatment with this optimal lipid mixture would have beneficial effects following disruption of the barrier by petroleum ether, tape stripping, or by detergent treatment. Also, we determined if barrier repair was accelerated after moderate disturbances of barrier function. Following moderate or extensive disruption of the barrier by acetone or petroleum ether (solvents), or tape stripping (mechanical), application of the optimal lipid mixture accelerated barrier repair. Additionally, following barrier disruption with N-laurosarcosine free acid or dodecylbenzensulphuric acid (detergents), the optimal lipid mixture similarly accelerated barrier repair. However, following disruption of the barrier with different detergents, sodium dodecyl sulphate and ammonium lauryl sulphosuccinate, the optimal lipid mixture did not improve barrier recovery. Thus, the optimal lipid mixture is capable of accelerating barrier repair following disruption of the barrier by solvent treatment or tape stripping (mechanical), and by certain detergents such as Sarkosyl and dodecylbenzensulphuric acid. The ability of the optimal lipid mixture to accelerate barrier repair after both moderate and extensive degrees of barrier disruption suggests a potential clinical use for this approach.

Molecular modeling indicates that homodimers form the basis for intermediate filament assembly from human and mouse epidermal keratins

Mammalian epidermal keratin molecules adopt rod-shaped conformations that aggregate to form cytoplasmic intermediate filaments. To investigate these keratin conformations and the basis for their patterns of molecular association, graphical methods were developed to relate known amino acid sequences to probable spacial configurations. The results support the predominantly alpha-helical conformation of keratin chains, interrupted by short non-alpha-helical linkages. However, it was found that many of the linkages have amino acid sequences typical of beta-strand conformations. Space-filling atomic models revealed that the beta-strand sequences would permit the formation of 2-chain and 4-chain cylindrical beta-helices, fully shielding the hydrophobic amino acid chains that alternate with hydrophilic residues in these sequences. Because of the locations of the beta-helical regions in human and mouse stratum corneum keratin chains, only homodimers of the keratins could interact efficiently to form 2-chain and 4-chain beta-helices. Tetramers having the directions and degrees of overlap of constituent dimers that have been identified by previous investigators are also predicted from the interactions of beta-helical motifs. Heterotetramers formed from dissimilar homodimers could combine, through additional beta-helical structures, to form higher oligomers having the dimensions seen in electron microscopic studies. Previous results from chemical crosslinking studies can be interpreted to support the concept of homodimers rather than heterodimers as the basis for keratin filament assembly.

Lipids are covalently attached to rigid corneocyte protein envelopes existing predominantly as beta-sheets: a solid-state nuclear magnetic resonance study

C solid-state nuclear magnetic resonance at natural abundance was used to study isolated corneocyte envelopes from porcine stratum corneum. The presence of lipids covalently attached to the protein envelopes was detected by chemical shifts of methylene and methyl groups of the bound lipids. The corneocyte protein envelopes are rigid, as suggested by efficient 1H to 13C cross polarization and 13C spin-lattice relaxation studies. The chemical shift of the carbonyl carbons of the protein envelopes supports the prediction that the chemically bound lipid envelope is attached to proteins arranged predominantly in the beta-sheet conformation, allowing a dense palisade of ceramide molecules to form a water-impermeable external sheath.