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

Epidermal omega-O-acylceramides (omega-O-acylCers) are essential components of a competent skin barrier. These unusual sphingolipids with ultralong N-acyl chains contain linoleic acid esterified to the terminal hydroxyl of the N-acyl, the formation of which requires the transacylase activity of patatin-like phospholipase domain containing 1 (PNPLA1). In ichthyosis with dysfunctional PNPLA1, omega-O-acylCer levels are significantly decreased, and omega-hydroxylated Cers (omega-OHCers) accumulate. Here, we explore the role of the linoleate moiety in omega-O-acylCers in the assembly of the skin lipid barrier. Ultrastructural studies of skin samples from neonatal Pnpla1(+/+) and Pnpla1(-/-) mice showed that the linoleate moiety in omega-O-acylCers is essential for lamellar pairing in lamellar bodies, as well as for stratum corneum lipid assembly into the long periodicity lamellar phase. To further study the molecular details of omega-O-acylCer deficiency on skin barrier lipid assembly, we built in vitro lipid models composed of major stratum corneum lipid subclasses containing either omega-O-acylCer (healthy skin model), omega-OHCer (Pnpla1(-/-) model), or combination of the two. X-ray diffraction, infrared spectroscopy, and permeability studies indicated that omega-OHCers could not substitute for omega-O-acylCers, although in favorable conditions, they form a medium lamellar phase with a 10.8 nm-repeat distance and permeability barrier properties similar to long periodicity lamellar phase. In the absence of omega-O-acylCers, skin lipids were prone to separation into two phases with diminished barrier properties. The models combining omega-OHCers with omega-O-acylCers indicated that accumulation of omega-OHCers does not prevent omega-O-acylCer-driven lamellar stacking. These data suggest that omega-O-acylCer supplementation may be a viable therapeutic option in patients with PNPLA1 deficiency.

Automated summary

This study explored the importance of the linoleate moiety in epidermal omega-O-acylceramides (omega-O-acylCers) in the assembly of the skin lipid barrier. The results showed that the linoleate moiety is essential for lamellar pairing in skin lipid structures, and omega-hydroxylated Cers (omega-OHCers) cannot substitute for omega-O-acylCers. These findings suggest that omega-O-acylCer supplementation may be a potential therapeutic option for patients with PNPLA1 deficiency.