Ceramides are key components of the skin's permeability barrier. In atopic dermatitis, pathological hydrolysis of ceramide precursors - glucosylceramides and sphingomyelin - into lysosphingolipids, specifically glucosylsphingosine (GS) and sphingosine-phosphorylcholine (SPC), and free fatty acids (FFAs) has been proposed to contribute to impaired skin barrier function. This study investigated whether replacing ceramides with lysosphingolipids and FFAs in skin lipid barrier models would exacerbate barrier dysfunction. When applied topically to human stratum corneum sheets, SPC and GS increased water loss, decreased electrical impedance, and slightly disordered lipid chains. In lipid models containing isolated human stratum corneum ceramides, reducing ceramides by ≥ 30% significantly increased permeability to four markers, likely due to loss of long-periodicity phase (LPP) lamellae and phase separation within the lipid matrix, as revealed by X-ray diffraction and infrared spectroscopy. However, when the missing ceramides were replaced by lysosphingolipids and FFAs, no further increase in permeability was observed. Conversely, these molecules partially mitigated the negative effects of ceramide deficiency, particularly with 5%-10% SPC, which reduced permeability even compared to control with "healthy" lipid composition. These findings suggest that while ceramide deficiency is a key factor in skin barrier dysfunction, the presence of lysosphingolipids and FFAs does not aggravate lipid structural or functional damage, but may provide partial compensation, raising further questions about the behavior of lyso(sphingo)lipids in rigid multilamellar lipid environments, such as the stratum corneum, that warrant further investigation.

• Keywords
• ceramide, fatty acid, glucosylsphingosine, lipid model, lysolipid, permeability, skin barrier, sphingosine-phosphorylcholine,
• MeSH
• Ceramides * metabolism   deficiency   MeSH
• Phosphorylcholine analogs & derivatives   MeSH
• Skin * metabolism   drug effects   MeSH
• Fatty Acids, Nonesterified metabolism   MeSH
• Humans MeSH
• Lysophospholipids * metabolism   MeSH
• Permeability MeSH
• Sphingolipids * metabolism   MeSH
• Sphingosine analogs & derivatives   metabolism   pharmacology   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Ceramides * MeSH
• Phosphorylcholine MeSH
• Fatty Acids, Nonesterified MeSH
• Lysophospholipids * MeSH
• Sphingolipids * MeSH
• Sphingosine MeSH
• sphingosine phosphorylcholine MeSH Browser

Omega-O-acyl ceramides such as 32-linoleoyloxydotriacontanoyl sphingosine (Cer[EOS]) are essential components of the lipid skin barrier, which protects our body from excessive water loss and the penetration of unwanted substances. These ceramides drive the lipid assembly to epidermal-specific long periodicity phase (LPP), structurally much different than conventional lipid bilayers. Here, we synthesized Cer[EOS] with selectively deuterated segments of the ultralong N-acyl chain or deuterated or 13C-labeled linoleic acid and studied their molecular behavior in a skin lipid model. Solid-state 2H NMR data revealed surprising molecular dynamics for the ultralong N-acyl chain of Cer[EOS] with increased isotropic motion toward the isotropic ester-bound linoleate. The sphingosine moiety of Cer[EOS] is also highly mobile at skin temperature, in stark contrast to the other LPP components, N-lignoceroyl sphingosine acyl, lignoceric acid, and cholesterol, which are predominantly rigid. The dynamics of the linoleic chain is quantitatively described by distributions of correlation times and using dynamic detector analysis. These NMR results along with neutron diffraction data suggest an LPP structure with alternating fluid (sphingosine chain-rich), rigid (acyl chain-rich), isotropic (linoleate-rich), rigid (acyl-chain rich), and fluid layers (sphingosine chain-rich). Such an arrangement of the skin barrier lipids with rigid layers separated with two different dynamic "fillings" i) agrees well with ultrastructural data, ii) satisfies the need for simultaneous rigidity (to ensure low permeability) and fluidity (to ensure elasticity, accommodate enzymes, or antimicrobial peptides), and iii) offers a straightforward way to remodel the lamellar body lipids into the final lipid barrier.

• Keywords
• NMR spectroscopy, lipid assembly, lipid chain order, long periodicity phase, molecular dynamics, neutron diffraction, stratum corneum models,
• MeSH
• Ceramides chemistry   MeSH
• Epidermis MeSH
• Skin chemistry   MeSH
• Linoleic Acid * MeSH
• Sphingosine analysis   MeSH
• Molecular Dynamics Simulation * MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Ceramides MeSH
• Linoleic Acid * MeSH
• Sphingosine MeSH