Lipid and protein structures in the permeability barrier of mammalian epidermis

The proteins elafin, filaggrin, keratin intermediate filaments, loricrin, and small proline-rich proteins 1 and 2 are isodipeptide cross-linked components of the human epidermal cornified cell envelope

The cornified cell envelope (CE) is a 15-nm thick layer of insoluble protein deposited on the intracellular side of the cell membrane of terminally differentiated stratified squamous epithelia. The CE is thought to consist of a complex amalgam of proteins cross-linked by isodipeptide bonds formed by the action of transglutaminases, but little is known about how or in which order the several putative proteins are cross-linked together. In this paper, CEs purified from human foreskin epidermis were digested in two steps by proteinase K, which released as soluble peptides about 30% and then another 35% of CE protein mass, corresponding to approximately the outer third (cytoplasmic surface) and middle third, respectively. Following fractionation, 145 unique peptides containing two or more sequences cross-linked by isodipeptide bond(s) were sequenced. Based on these data, most (94% molar mass) of the outer third of CE structure consists of intra- and interchain cross-linked loricrin, admixed with SPR1 and SPR2 proteins as bridging cross-links between loricrin. Likewise, the middle third of CE structure consists largely of cross-linked loricrin and SPR proteins, but is mixed with the novel protein elafin which also forms cross-bridges between loricrin. In addition, cross-links involving loricrin and keratins 1, 2e, and 10 or filaggrin were recovered in both levels. The data establish for the first time that these several proteins are indeed cross-linked protein components of the CE structure. In addition, the data support a model for the intermediate to final stages of CE assembly: the proteins elafin, SPR1 and SPR2, and loricrin begin to be deposited on a preformed scaffold; later, elafin deposition decreases as loricrin and SPR accumulation continues to effect final assembly. The recovery of cross-links involving keratins further suggests that the subjacent cytoplasmic keratin intermediate filament-filaggrin network is anchored to the developing CE during these events.

Stratum corneum dynamic function measurements after moisturizer or irritant application

Two simple tests were conducted which allowed the quantification of parameters that characterize the stratum corneum (SC) dynamic functions in vivo under physiological conditions after moisturizer applications for 1 h and after irritation with different concentrations of sodium lauryl sulphate (SLS; 0.5-4%) applied under occlusion for 15 min or 24 h. Both tests, the sorption-desorption test (SDT) and the moisture accumulation test (MAT), were performed with a Nova Dermal Phase Meter 9003. The following parameters were quantified: prehydration state (SDT, MAT), hygroscopicity, water-holding capacity (SDT), water accumulation velocity and water accumulation (MAT). These procedures allowed the demonstration of the water-holding effect of urea contained in moisturizers. Differences between the long and the short application time of SLS were characterized by differences in SC dynamic functions while the hydration state was not changed. An effect on transepidermal water loss (TEWL) was noted only after the long application time, although the MAT clearly showed dynamic parameters to be changed after 15 min of treatment. These tests were simple in practice and allowed the demonstration of functional modifications of the SC while other parameters remained unchanged. They gave insight into possible action mechanisms of urea and SLS in the SC.

Percutaneous penetration of dimethylnitrosamine through human skin in vitro: application from cosmetic vehicles

Human skin penetration of N-dimethylnitrosamine (DMN) from three vehicles has been determined in vitro. When applied as an infinite dose in isopropyl myristate (IPM, 1 microgram/microliter) the average total absorption over 48 hr was 2.6 +/- 1.2% of the applied dose (all data presented are expressed as means +/- standard errors). When applied as a finite dose in a representative oil-in-water emulsion vehicle the average total absorption over 48 hr was 4.0 +/- 0.3% of the applied dose. When applied as a finite dose in a representative shampoo vehicle for 10 min followed by rinsing (i.e. to represent in-use exposure conditions) the average total absorption over 48 hr was 1.1 +/- 0.1% of the applied dose. Approximately 72% of the DMN in the applied shampoo vehicle was removed by rinsing. There was considerable evaporative loss of DMN from the IPM and oil-in-water emulsion vehicles, such that absorption was complete within 3 hr of application. The overall data indicate that DMN can penetrate the skin rapidly but that in practice the amount actually available for penetration is significantly reduced by high permeant volatility. In contrast, application of N-nitrosodiethanolamine (NDELA) at a concentration of 1 microgram/microliter as an infinite dose generated an average total absorption over 48 hr of 23.6 +/- 6.4%, representing a total flux of 103.9 +/- 28.4 micrograms/cm2. In the case of NDELA, no evaporative loss was evident.

Loricrin expression is coordinated with other epidermal proteins and the appearance of lipid lamellar granules in development

In mouse, epidermal development proceeds from a single basal cell layer covered by a specialized single cell layer called the periderm at E14 to a fully differentiated stratified squamous epithelium at E18. To determine when loricrin, a major cell envelope component, is expressed during development, we examined fetal skin from mice of gestational ages E13 through E19 and compared the temporal pattern of loricrin expression with that of other differentiation markers. We found that loricrin mRNA and protein were expressed by E16, following the expression of keratins K1 and K10 and preceding the expression of profilaggrin. Interestingly, both loricrin and profilaggrin were initially expressed focally in areas corresponding to more advanced morphologic stages of maturation. Because the cornified envelope is a composite structure consisting of both protein and lipid components, we also monitored the appearance of lipid lamellar granules during epidermal development. These granules were first evident at E16 and the extrusion of lipids from the granules into the intercellular space occurred at E17, prior to the cross linking of loricrin into the cell envelope. Our results document that loricrin is expressed and accumulates at the cell periphery subsequent to the extrusion of lipids, but prior to processing of profilaggrin. We suggest that the sequential regulation of these events is critical for formation of epidermal barrier function during development.

Cutaneous lipid synthesis during late fetal development in the rat

Lipid synthesis in fetal skin may be important both for the development of a mature epidermal permeability barrier and for growth. In these studies, we measured cutaneous cholesterol, sphingolipid and fatty acid synthesis during the critical period of epidermal barrier development in fetal rats to determine whether barrier function influences synthetic rates. In addition, the activities of HMG CoA reductase, serine palmitoyl transferase and acetyl coenzyme A carboxylase were evaluated. In whole skin, synthesis of cholesterol, ceramide, sphingomyelin and fatty acid decreased from day 17 to day 21 of gestation, as did the activity of HMG CoA reductase, serine palmitoyl transferase and acetyl coenzyme A carboxylase. In both the epidermis and dermis, a decrease in cholesterol, ceramide, sphingomyelin and fatty acid synthesis was measured over days 19-21 of gestation. Epidermal HMG CoA reductase activity also decreased over this same time period. In summary, epidermal and dermal synthetic rates and enzyme activity were highest early in gestation when the barrier was least competent and decreased as competence was achieved. Since other studies with mature animals have revealed that epidermal synthetic rates and enzyme activity are highest when barrier disruption is maximal, enhanced epidermal lipid synthesis precedes the establishment of a competent barrier in both fetal and mature rodents.

In vivo distribution of free long-chain sphingoid bases in the human stratum corneum by high-performance liquid chromatographic analysis of strippings

Conditions were established for the in vivo determination of free long-chain bases (phytosphingosine, sphingosine and sphinganine) in human stratum corneum layers by reversed-phase HPLC analysis of adhesive-tape strippings. Long-chain bases were first extracted from individual strippings with pure methanol in an ultrasound bath, then o-phthalaldehyde derivatives were separated on a C18 column with a gradient from methanol-water (80:20, v/v) to pure methanol. By performing a second series of strippings for protein determination it was possible to express the amounts of free long-chain bases per milligram of protein for each individual stratum corneum layer. Co-elution of endogenous phytosphingosine, sphingosine and spinganine was demonstrated in separate experiments by addition of standards to typical strips. From a study of three human volunteers, average concentrations of free long-chain bases in the stratum corneum were found to be 2.8, 1.2 and 0.5 nmol/mg of protein for phytosphingosine, sphingosine and sphinganine, respectively.

Polymorphism in stratum corneum lipids

Fourier transform infrared spectroscopy (FTIR) was employed to investigate the thermotropic phase behavior of stratum corneum lipid multilamellae. Stratum corneum (SC), the uppermost layer of mammalian skin, is unusual in many respects. It has been demonstrated that the lipids of the stratum corneum provide the primary electrical and transport resistance in the skin. These lipids are unusual in their composition, structure and localization; they contain only cholesterol, fatty acids and ceramides and they form broad, multi-lamellar sheets which are located extracellularly. The FTIR results from both the symmetric CH2 stretching and the CH2 scissoring vibrations suggest that the SC lipids exhibit polymorphic phase behavior below the main phase transition temperature. The multiple phases are most likely crystalline mixtures of different alkyl chain packings, along with solid-liquid phases. Similarities between the FTIR results reported here for SC lipids and those obtained for cholesterol-containing gel phase phospholipids suggest that the non-uniform distribution of cholesterol occurs in each system.

Formation of surface membranes in developing mammalian hair fibres

Mammalian hair fibres result from complex mechanisms involving synthesis, assembly and stabilisation of keratin proteins in the follicle. The developing hair shaft consists of outer cuticle cells surrounding cortical and medullary (optional) cell types. Presumptive fibre cuticle (FC) is contained by the inner root sheath (IRS) consisting of IRS cuticle, Huxley and Henle cells which are in turn enclosed in an outer root sheath (ORS) of epidermal-like cells. In the current structural studies we have used energy filtered transmission electron microscopy (Zeiss 902A) on Merino sheep skin biopsies to examine the fine sequence of morphological changes involved in forming the fibre surface membrane and the associated underlying structural bands comprising the a-layer and exocuticle. Prior to the development of the exocuticle, FC cells demonstrate a typical plasma-membrane apposed to IRS cuticle plasma-membranes separated by an intercellular space. The formation of exocuticular lamellae is followed by degradation of the residual FC surface membrane and the appearance of intercellular laminae demonstrating a stained central band. As maturation continues cleavage between IRS cuticle and FC occurs along this central band liberating hair into the pilary canal. The mature surface consists of keratinized cells containing a well developed exocuticle and a-layer coated with paired lamina (presumably two lipid containing bilayers) of material approximately 10-12 nm thick derived from the intercellular laminae. The current observations show FC surface formation is similar to processes occurring in epidermal stratum corneum and that the cuticle surface membrane of mammalian fibres is not derived from a modified plasma-membrane as previously documented.

Development of a stratum corneum and barrier function in an organotypic skin culture

The stratum corneum of human skin is responsible for maintaining the epidermal permeability barrier. We have developed a bilayered skin culture (SC) which forms a corneum 35 +/- 1 cell layers thick 21 days after being raised to the air-liquid (A/L) interface. By the 7th day after raising to the A/L interface the corneocytes were irregularly shaped and had cross-sectional areas (CSA) of > or = 300 microns 2. By the 21st day the corneocytes had assumed polygonal shapes and had a CSA (100-250 microns 2) similar to that of human foreskin. The total lipid (TL) content of the corneum averaged 5-7% of the lyophilized weight. Ceramide content increased from 20% of TL at day 7 of A/L interface culture to 30% at day 21. Triglycerides decreased from 43% to 17% of TL during the same period. Free fatty acids comprised 5.5% of TL at day 21 of A/L interface culture. The intercorneocyte spaces contained stacks of lipid lamellae. However, the stacks lacked the Landmann unit repeat. Abnormal lamellar structures were observed in both the intra- and extracorneocyte spaces. Transepidermal water loss (TEWL) was > 4 mg/cm2 per h throughout the culture period. Lipid supplementation of the culture medium and culturing in a low humidity environment improved barrier function by 50%. However, the effects were not additive. The SC developed a near-normal corneum, but did not achieve barrier competence, due at least partially to abnormalities in lipid composition and organization.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid modification during epidermal cell differentiation

As keratinocytes differentiate into corneocytes of the stratum corneum or epidermal permeability barrier, their lipids are modified so as to fulfill totally different functions. Recent experimentation has clarified the molecular mechanisms by which lipids of membrane origin are targeted to specialized lamellar bodies, where metabolic retailoring makes them suitable for use in the water-impermeable intercellular lamellae. In this latter structure the modified lipids are bound covalently to specialized proteins in a way that encourages the formation of lipid bilayers alternating with lipid monolayers. Only now are potential clues to the molecular regulation of this dramatic lipid transformation becoming apparent.

Covalently bound lipids in keratinizing epithelia

Covalently bound lipids have been identified and compared in keratinizing porcine epithelia including epidermis and oral epithelium from palate and gingiva. Stratum corneum was isolated by tryptic digestion, and after extensive extraction of lipids using a series of chloroform-methanol mixtures, the residual tissue was subjected to alkaline hydrolysis to release covalently bound lipids. The lipids so released were analyzed by quantitative thin-layer chromatography. Stratum corneum from each of the three anatomical sites contained omega-hydroxyceramides, omega-hydroxyacids and fatty acids. In epidermal stratum corneum the total covalently bound lipids represented 2.4% of the dry weight of the tissue, but in the oral epithelia this figure was consistently lower: 0.24% in palatal stratum corneum and 0.20% in gingival stratum corneum. Transmission electron microscopy before and after lipid extraction confirms the presence of a lipid envelope in epidermal stratum corneum and demonstrates the absence of this structure in oral stratum corneum.