Omega-O-acylceramide, a lipid essential for mammalian survival.

The prevention of water loss through the skin is critical for terrestrial mammalian species. This function is served by the epidermal permeability barrier, which resides primarily in the extracellular domains of the stratum corneum, the outermost layer of skin, and its highly ordered lamellar membranes composed primarily of free fatty acids, cholesterol, and ceramides (Cer). The dominant lipids in these lamellae are Cer, which comprise a heterogeneous group of chemically distinct species. One particular subfamily of Cer, which is unique to the outer layers of the epidermis of terrestrial mammals, is omega (omega)-O-acylCer (or acylCer). Myriad evidence suggests that these acylCer play critical roles in barrier function. The formation of these epidermal acylCer requires several metabolic steps, including synthesis of very long chain fatty acids, omega-hydroxylation of the fatty acids, and esterification at the omega-hydroxy group with primarily linoleic acid. The authors previously demonstrated that a cytochrome P-450-type enzyme is involved in omega-hydroxylation during acylCer generation and that inhibition of omega-hydroxylation leads to a barrier abnormality in murine epidermis. More recently, we discovered that lack of normal elongation of very long chain fatty acid (or ELOVL) 4 function in mutant ELOVL4 knock-in mice causes acylCer deficiency associated with abnormal barrier formation and neonatal lethality. These results indicate not only that acylCer are critical lipid components for mammalian survival, but also that keratinocytes deploy a complex metabolic pathway leading to the formation of these unique Cer.