Biomimetic Stratum Corneum Liposome Models: Lamellar Organization and Permeability Studies

Abstract

The stratum corneum (SC), the outer layer of the skin, plays a crucial role as a barrier protecting the underlying cells from external stress. The SC comprises three key components: ceramide (CER), free fatty acid (FFA), and cholesterol, along with small fractions of cholesterol sulfate and cholesterol ester. In order to gain a deeper understanding about the interdependence of the two major components, CER and FFA, on the organizational, structural, and functional properties of the SC layer, a library of SC lipid liposome (SCLL) models was developed by mixing CER (phytosphingosine or sphingosine), FFA (oleic acid, palmitic acid, or stearic acid), cholesterol, and cholesterol sulfate. Self-assembly of the SC lipids into lamellar phases was first confirmed by small-angle X-ray scattering. Short periodicity and long periodicity phases were identified for SCLLs containing phytosphingosines and sphingosine CERs, respectively. Furthermore, unsaturation in the CER acyl and FFA chains reduced the lipid conformational ordering and packing density of the liposomal bilayer, which were measured by differential scanning calorimetry and Fourier transform infrared spectroscopy. The introduction of unsaturation in the CER and/or FFA chains also impacted the lamellar integrity and permeability. This extensive library of SCLL models exhibiting physiologically relevant lamellar phases with defined structural and functional properties may potentially be used as a model system for screening pharmaceuticals or cosmetic agents.

Keywords: biophysical characterization; ceramide; cholesterol; free fatty acid; lamellar organization; permeability; stratum corneum.

Scheme 1

The stratum corneum (SC) and SC lipid liposome (SCLL) lipid organization. SC lipids are organized into short periodicity phase (SPP) and long periodicity phase (LPP). The model (right) represents a proposed SC lipid liposome (SCLL) model.

Figure 1

The chemical structures of the ceramides (CERs) and free fatty acids (FFAs) used. NP = CERs consisting of a nonhydroxylated (N) fatty acid moiety and a trihydroxylated phytophingosine (P) moiety. Two types of CER (NP), decorated with C18:0 or C18:1 fatty acids, are studied. AP = CERs consisting of an α-hydroxylated (A) fatty acid moiety and a trihydroxylated phytophingosine (P) moiety. NS = CERs consisting of a nonhydroxylated (N) fatty acid moiety and a sphingosine (S) moiety. The bovine CER contains a mixture consisting of C18:0 or C24:1 as the fatty acid moiety.

Figure 2

SAXS diffraction profiles of SCLLs. (A) CER3-FFAs. (B) CER3B-FFAs. (C) CER6-FFAs. (D) Bovine CER-FFAs. Roman numeral mark “(I)” denotes the diffraction peaks attributed to SPP (48.3 Å–57.1 Å). * Represents crystalline phase of CER 3 (39.3 Å). “(a)” denotes additional SPP (41.9 Å–44.9 Å). “(i)” and “(ii)” represent 1st- and 2nd-order diffraction order of phase with repeat distance 62.8 Å–69.8 Å. “(1b)” and “(2b)” represent 1st- and 2nd-order diffraction of 78.6 Å–83.8 Å phase. The peaks “(1)” and “(2)” indicate the 1st- and 2nd-order diffraction of the LPP (125.7 Å). The LPP phase for Bovine-CER-OA with periodicity 139.6 Å is indicated by the hash sign “(#)”.

Figure 3

The DSC thermograms of the SCLLs recorded in the heating cycle. (A) CER3-FFAs. (B) CER3B-FFAs. (C) CER6-FFAs. (D) Bovine CER-FFAs.

Figure 4

The dye-release study with calcein-loaded SCLLs composed of different CERs and FFAs. Values represent % of calcein release induced by 0.004% Triton X-100 at 40 µM vesicle concentration. The values were taken about 20 min after the addition of Triton X-100. Error bars refer to SEM of three independent experiments.