The outermost layer of the skin, the stratum corneum (SC), is comprised of corneocyte cells surrounded by a dense extracellular lipid matrix. The SC functions as the body’s primary barrier to external penetrants and prevents water loss. More specifically, the barrier function of the SC is localized to the extracellular lipid matrix, which is the only continuous path through which permeants can pass through the SC. In contrast to the phospholipid-based bilayer membranes that are abundant throughout the body, the lipid matrix of the SC forms largely dehydrated multilamellar structures that are primarily composed of ceramides along with cholesterol and free fatty acids and no phospholipids. The multilayer arrangement allows the two-tailed ceramides to adopt either a hairpin conformation, where both tails are in the same layer, but also an extended conformation, where the tails occupy separate layers. Evidence from recent studies using Fourier transform infrared spectroscopy (FTIR)
1 as well as nuclear magnetic resonance (NMR) experiments,
2 support the hypothesis that a majority of ceramides in the SC are in the extended conformation. However, the exact fraction of extended ceramides and the effect of extended ceramides on the membrane structural properties are unknown and are difficult to study experimentally. To gain a better understanding of the effect of extended ceramides on the structure and arrangement of SC lipid membranes, we use molecular dynamics simulations. In previous work, we developed a coarse-grained model and procedure to simulate the self-assembly of multilayer SC lipid membranes.
3 Here we extend this prior approach, applying constraints to the ceramides that allow us to control the fraction that adopt extended conformations. These self-assembled coarse-grained membranes are reversed-mapped to the atomistic level to allow detailed examination of the properties. As the fraction of extended ceramides increased, we observed measurable variations in structure, lipid arrangement and hydrogen bonding. In addition, agreement with FTIR and X-ray scattering experiments improved with increases in the extended fraction, providing additional support for the hypothesis that realistic SC lipid membranes contain high fractions of extended ceramides.
(1) Beddoes, C. M.; Gooris, G. S.; Foglia, F.; Ahmadi, D.; Barlow, D. J.; Lawrence, M. J.; Demé, B.; Bouwstra, J. A. Arrangement of Ceramides in the Skin: Sphingosine Chains Localize at a Single Position in Stratum Corneum Lipid Matrix Models. Langmuir 2020, 36 (34), 10270–10278. https://doi.org/10.1021/acs.langmuir.0c01992.
(2) Engberg, O.; KováÄik, A.; Pullmannová, P.; JuhaÅ¡Äik, M.; Opálka, L.; Huster, D.; Vávrová, K. The Sphingosine and Acyl Chains of Ceramide [NS] Show Very Different Structure and Dynamics That Challenge Our Understanding of the Skin Barrier. Angew Chem Int Ed 2020, 59 (40), 17383–17387. https://doi.org/10.1002/anie.202003375.
(3) Shamaprasad, P.; Moore, T. C.; Xia, D.; Iacovella, C. R.; Bunge, A. L.; McCabe, C. Multiscale Simulation of Ternary Stratum Corneum Lipid Mixtures: Effects of Cholesterol Composition. Langmuir 2022, 38 (24), 7496–7511.https://doi.org/10.1021/acs.langmuir.2c00471.
(4) Shamaprasad, P.; Frame, C. O.; Moore, T. C.; Yang, A.; Iacovella, C. R.; Bouwstra, J. A.; Bunge, A. L.; McCabe, C. Using Molecular Simulation to Understand the Skin Barrier. Prog Lipid Res 2022, 88, 101184. https://doi.org/10.1016/j.plipres.2022.101184.
