ω-O-Acylceramides but not ω-hydroxy ceramides are required for healthy lamellar phase architecture of skin barrier lipids

The skin barrier in mammals is essential for prevention of water and electrolyte loss, as well as protection from the penetration of harmful chemicals and pathogenic microbes. Impairment of skin barrier function can cause or aggravate skin disorders, including dry skin, psoriasis, and atopic dermatitis. Although the epidermis is a highly organized stratified epithelium consisting of four distinctive layers: the innermost stratum basale, the stratum spinosum, the stratum granulosum and the uppermost stratum corneum, its barrier function is provided mainly by specialized structures in the stratum corneum and tight junctions in the stratum granulosum. The unique stratum corneum components include cross-linked, insoluble proteins of corneocytes forming the cornified envelope and its associated, external membrane monolayer, called the cornified lipid envelope, as well as the intercellular lipid lamellae, which are mainly composed of ceramides, cholesterol and free fatty acids. Among the epidermal ceramides with marked molecular heterogeneity (at least 12 classes in humans), acylceramides are essential for physical and functional organization of lipids in the stratum corneum interstices, and thereby the barrier function of the skin. Impaired biosynthesis or processing of acylceramides causes ichthyosis, characterized by dry, scaly and thickened skin. It is known that acylceramide is an unusual ceramide species whose N-acyl chain is composed of ω-hydroxylated ultra-long chain FAs esterified at the ω-position with linoleic acid; their production necessitates the transacylase activity of patatin-like phospholipase domains comprising 1. (PNPLA1). ω-O-acylCer levels are dramatically reduced and ω-hydroxylated Cers (ω-OHCers) accumulate in ichthyosis caused by a malfunctioning PNPLA1.

In a new study published in the Journal of Lipid Research researchers from Charles University: Lukáš Opálka, Veronika Ondrejčeková, Linda Svatošová, and led by Professor Kateřina Vávrová in collaboration with Dr. Jason Meyer at Vanderbilt University Medical Center and Dr. Franz Radner at University of Graz examined the function of the linoleate moiety in ω-O-acylCers in the construction of the skin lipid barrier. The ultrastructures of skin barrier lipids and their precursors in mice with functional and deficient PNPLA1, respectively, were examined by transmission electron microscopy.. They constructed a few lipid models mimicking the skin barrier lipids in healthy and PNPLA1-deficient skin barrier. By applying different concentrations of ω-O-acylCer/ω-OHCer, the lamellar organization of these models was evaluated by a variety of techniques such as X-ray Diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) to determine the lipid lamellar arrangement, chain packing, order, and transitions. Water loss measurements and two permeants were used to examine the lipid models’ permeabilities while they were sandwiched between Franz cells.

The research team showed that, in addition to corneocyte lipid envelope formation, the linoleate chain in ω-O-acylCers is crucial for organizing together the free stratum corneum lipids into the long periodicity lamellar phase (LPP) and the barrier lipid precursors in the lamellar bodies. The authors showed the lamellar bodies of Pnpla1-/- animals have single bands instead of paired bands, indicating that the linoleate tails of ω-O-acylCer are present in the electrondense layer in the centre of the paired lamella in WT mice, like a lipid droplet lens. Although under ideal circumstances, ω-OHCers form a medium lamellar phase with an 11 nm-repeat distance and permeability barrier properties similar to the long periodicity lamellar phase, X-ray diffraction, infrared spectroscopy, and permeability studies indicated that ω-OHCers could not replace ω-O-acylCers. Moreover, the authors used in vitro lipid models to attempt to replicate some of the distinctive characteristics of the lipid lamellae in the absence of the ω-O-acylCer linoleate tails to better understand the behavior of stratum corneum lipids in Pnpla1-/- mice. Following the TEM pictures of Pnpla1-/- mice, the LPP architecture was lost in the models in which ω-O-acylCers were switched out for ω-OHCers.

In conclusion, the findings of Professor Kateřina Vávrová and her colleagues revealed that, in addition to their function in CLE formation, ω-O-acylCers are crucial for stratum corneum lipid assembly into the LPP and lamellae pairing in lamellar bodies. ω-OHCers, which lack the linoleate moiety, under the right circumstances can form the MLP, a nearly 11 nm lamella with LPP-like permeability properties. The MLP, in contrast to the LPP, is more susceptible to environmental factors during assembly and may split into two phases with weaker barrier characteristics. According to the lipid models that combine ω-OHCers and ω-O-acylCers, ω-OHCers do not necessarily disrupt the lipid organization, and substitution of ω-O-acylCers may be an effective treatment for people who have PNPLA1 deficiency. In conclusion, the evidence points to the possibility that patients with PNPLA1 deficiency may benefit from supplementing their skin with ω-O-acylCer. The study by Professor Kateřina Vávrová and her colleagues contribute to a better understanding of the skin barrier formation, and should be useful in providing novel therapeutic strategies for treatment of patients with skin barrier disorders.