Stratum corneum lipid matrix: Location of acyl ceramide and cholesterol in the unit cell of the long periodicity phase

Abstract

The extracellular lipid matrix in the skin's outermost layer, the stratum corneum, is crucial for the skin barrier. The matrix is composed of ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs) and involves two lamellar phases: the short periodicity phase (SPP) and the long periodicity phase (LPP). To understand the skin barrier thoroughly, information about the molecular arrangement in the unit cell of these lamellar phases is paramount. Previously we examined the molecular arrangement in the unit cell of the SPP. Furthermore X-ray and neutron diffraction revealed a trilayer arrangement of lipids within the unit cell of the LPP [D. Groen et al., Biophysical Journal, 97, 2242-2249, 2009]. In the present study, we used neutron diffraction to obtain more details about the location of lipid (sub)classes in the unit cell of the LPP. The diffraction pattern revealed at least 8 diffraction orders of the LPP with a repeating unit of 129.6±0.5Å. To determine the location of lipid sub(classes) in the unit cell, samples were examined with either only protiated lipids or selectively deuterated lipids. The diffraction data obtained by means of D2O/H2O contrast variation together with a gradual replacement of one particular CER, the acyl CER, by its partly deuterated counterpart, were used to construct the scattering length density profiles. The acyl chain of the acyl CER subclass is located at a position of ~21.4±0.2Å from the unit cell centre of the LPP. The position and orientation of CHOL in the LPP unit cell were determined using tail and head-group deuterated forms of the sterol. CHOL is located with its head-group positioned ~26±0.2Å from the unit cell centre. This allows the formation of a hydrogen bond with the ester group of the acyl CER located in close proximity. Based on the positions of the deuterated moieties of the acyl CER, CHOL and the previously determined location of two other lipid subclasses [E.H. Mojumdar et al., Biophysical Journal, 108, 2670-2679, 2015], a molecular model is proposed for the unit cell of the LPP. In this model CHOL is located in the two outer layers of the LPP, while CER EOS is linking the two outer layers with the central lipid layers. Finally the two other lipid subclasses are predominantly located in the central layer of the LPP.

Synthesis of specific deuterated derivatives of the long chained stratum corneum lipids [EOS] and [EOP] and characterization using neutron scattering

“…The long‐chain acid with the linoleoyl residue is bound to sphingosine (CER[EOS]), to phytosphingosine (CER[EOP]), and 6‐hydroxysphingosine (CER[EOH]). CER[EOS] appears to play a key role in one model of the nanostructure of the SC lipids because it is stated to form the long periodicity phase of the SC lipids . This model is still an issue of scientific debate .…”

“…According to reaction pathway B, starting material was octanoic acid (9). First, 9 was transformed into the dianion by double deprotonation in a modified way according to Creger.…”

“…CER[EOS] appears to play a key role in one model of the nanostructure of the SC lipids because it is stated to form the long periodicity phase of the SC lipids. 8,9 This model is still an issue of scientific debate. [10][11][12] Therefore, additional studies are necessary concerning the impact of CER [EOS].…”

Preferential arrangement of lipids in the long-periodicity phase of a stratum corneum matrix model

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With the use of model systems, previous work has identified the relative positions of several of the lipid subclasses (17)(18)(19). CERs with a nonhydroxyl C24 acyl chain linked to a C18 sphingosine (NS) and FFAs were located in the central region of the LPP trilayer structure, while CHOL remained exclusively in the outer region (18).

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Microplasma Drug Delivery

“…There is a minimal amount of required cholesterol for the formation of the long periodicity phase of ceramides (without cholesterol, long periodicity phase is not formed). Cholesterol does not influence only the presence of long periodicity phase but also the packing density in the long periodicity phase [39]. Cholesterol is very important for correct skin barrier function [40].…”