Multiscale Simulation of Ternary Stratum Corneum Lipid Mixtures: Effects of Cholesterol Composition

Abstract

Molecular dynamics simulations of mixtures of the ceramide nonhydroxy-sphingosine (NS), cholesterol, and a free fatty acid are performed to gain molecular-level understanding of the structure of the lipids found in the stratum corneum layer of skin. A new coarse-grained force field for cholesterol was developed using the multistate iterative Boltzmann inversion (MS-IBI) method. The coarse-grained cholesterol force field is compatible with previously developed coarse-grained force fields for ceramide NS, free fatty acids, and water and validated against atomistic simulations of these lipids using the CHARMM force field. Self-assembly simulations of multilayer structures using these coarse-grained force fields are performed, revealing that a large fraction of the ceramides adopt extended conformations, which cannot occur in the single bilayer in water structures typically studied using molecular simulation. Cholesterol fluidizes the membrane by promoting packing defects, and an increase in cholesterol content is found to reduce the bilayer thickness due to an increase in interdigitation of the C 24 lipid tails, consistent with experimental observations. Using a reverse-mapping procedure, a self-assembled coarse-grained multilayer system is used to construct an equivalent structure with atomistic resolution. Simulations of this atomistic structure are found to closely agree with experimentally derived neutron scattering length density profiles. Significant interlayer hydrogen bonding is observed in the inner layers of the atomistic multilayer structure that are not found in the outer layers in contact with water or in equivalent bilayer structures. This work highlights the importance of simulating multilayer structures, as compared to the more commonly studied bilayer systems, to enable more appropriate comparisons with multilayer experimental membranes. These results also provide validation of the efficacy of the MS-IBI derived coarse-grained force fields and the framework for multiscale simulation.

Using molecular simulation to understand the skin barrier

“…Interdigitation of all lipids might exceed that of individual lipid components if interdigitation between different lipid components (e.g., CER tails with FFA tails) is larger than interdigitation between the same lipid component. Thus, for the equimolar mixture of CER NS C24, CHOL and FFA C24 (fully protonated) at 305 K, an overall interdigitation of 10.5 Å from CG simulation [ 110 ] agrees well with FFA C24 (10.6 Å), which was the component with the largest interdigitation in atomistic simulation from Wang and Klauda [ 148 ] (interdigitation values for CER and CHOL were 6.4 Å and 0.88 Å, respectively). Reanalysis of trajectories of Moore et al [ 115 ] for the same system reveal similar values to Wang and Klauda [ 148 ] with 7.3 Å, 0.1 Å, and 8.7 Å, respectively for CER, CHOL, and FFA, and an overall interdigitation of 8.1 Å between all lipid components.…”

“…5e with a very thin water slab above and below each bilayer). As examples of this latter situation, approximately 35% of the CERs were in the extended conformation in the interior bilayers of both a four- and a six-leaflet stack containing CER NS C24:CHOL:FFA C24 at a 1:0.5:1 molar ratio that self-assembled (using a coarse-grained model described in Section 4.2.2 ) with <3 intermembrane water molecules per lipid [ 110 ].…”

“…Because CHOL has a larger cross-sectional area per tail than either CERs or FFAs, APT is only suitable for evaluating lipid packing in SC lipid mixtures without CHOL. To address this issue, Shamaprasad et al [ 110 ] proposed a new metric, the normalized lipid area (NLA), in which the APL is divided by the effective number of hydrocarbon tails per lipid, where FFAs have one, CERs have two, and CHOL has 1.9 (estimated from the ratio of the experimental cross sectional areas for CHOL and hydrocarbon chain; 38 Å 2 and 20 Å 2 [ 141 ]). Although NLA could, like the calculation of APT, be multiplied by , here we choose to define it within the plane normal to the membrane interface (i.e., without ) because is likely to be close to one.…”

“…The d HH method has the disadvantage of being unsuitable for some multicomponent systems where there may be no clear headgroup peak or where there are multiple peaks from the headgroup locations of the different components. Additionally, care must be taken in comparing d HH results for bilayers with headgroup-headgroup interfaces as compared to bilayers with headgroup-water interfaces; systems with headgroup-headgroup interfaces may exhibit only a single broad peak in this regime, resulting in a larger d HH value than that calculated for otherwise identical systems with headgroup-water interfaces [ 110 ].…”

Current insights into skin lipids and their roles in cutaneous health and disease

“…The use of molecular simulation is an efficient system to study the complexity of stratum corneum lipids and understand the physicochemical properties, organization and configurations necessary for correct barrier function [60 ▪▪ ]. Recent insights based on in-silico model studies include a detailed understanding of the orientation of the ceramide headgroups and localization of phytosphingosine ceramides in model membranes [61 ▪▪ ,62], requirement of ω-O-acylceramides for the correct architecture of lamellar phase in barrier lipids [63 ▪▪ ], and the effect of cholesterol composition on stratum corneum lipid organization [64]. Finally, recent advances in high-resolution cryo-electron microscopy allowed for elucidation of the structure of the skin's barrier and its formation, providing further understanding of the role of stratum corneum lipids [65 ▪ ].…”