Epidermal 1-O-acylceramides appear with the establishment of the water permeability barrier in mice and are produced by maturating keratinocytes

1-O-Acylceramides (1-OACs) have a fatty acid esterified to the 1-hydroxyl of the sphingosine head group of the ceramide, and recently we identified these lipids as natural components of human and mouse epidermis. Here we show epidermal 1-OACs arise shortly before birth during the establishment of the water permeability barrier in mice. Fractionation of human epidermis indicates 1-OACs concentrate in the stratum corneum. During in vitro maturation into reconstructed human epidermis, human keratinocytes dramatically increase 1-OAC levels indicating they are one source of epidermal 1-OACs. In search of potential enzymes responsible for 1-OAC synthesis in vivo, we analyzed mutant mice with deficiencies of ceramide synthases (Cers2, Cers3, or Cers4), diacylglycerol acyltransferases (Dgat1 or Dgat2), elongase of very long fatty acids 3 (Elovl3), lecithin cholesterol acyltransferase (Lcat), stearoyl-CoA desaturase 1 (Scd1), or acidic ceramidase (Asah1). Overall levels of 1-OACs did not decrease in any mouse model. In Cers3 and Dgat2-deficient epidermis they even increased in correlation with deficient skin barrier function. Dagt2 deficiency reshapes 1-OAC synthesis with an increase in 1-OACs with N-linked non-hydroxylated fatty acids and a 60% decrease compared to control in levels of 1-OACs with N-linked hydroxylated palmitate. As none of the single enzyme deficiencies we examined resulted in a lack of 1-OACs, we conclude that either there is functional redundancy in forming 1-OAC and more than one enzyme is involved, and/or an unknown acyltransferase of the epidermis performs the final step of 1-OAC synthesis, the implications of which are discussed.

Cross-calibration of a combined electrostatic and time-of-flight analyzer for energy- and charge-state-resolved spectrometry of tin laser-produced plasma

We present the results of the calibration of a channeltron-based electrostatic analyzer operating in time-of-flight mode (ESA-ToF) using tin ions resulting from laser-produced plasma, over a wide range of charge states and energies. Specifically, the channeltron electron multiplier detection efficiency and the spectrometer resolution are calibrated, and count rate effects are characterized. With the obtained overall response function, the ESA-ToF is shown to accurately reproduce charge-integrated measurements separately and simultaneously obtained from a Faraday cup (FC), up to a constant factor the finding of which enables absolute cross-calibration of the ESA-ToF using the FC as an absolute benchmark. Absolute charge-state-resolved ion energy distributions are obtained from ns-pulse Nd:YAG-laser-produced microdroplet tin plasmas in a setting relevant for state-of-the-art extreme ultraviolet nanolithography.

Endogenous levels of 1-O-acylceramides increase upon acidic ceramidase deficiency and decrease due to loss of Dgat1 in a tissue-dependent manner

Except for epidermis and liver, little is known about endogenous expression of 1-O-acylceramides (1-OACs) in mammalian tissue. Therefore, we screened several organs (brain, lung, liver, spleen, lymph nodes, heart, kidney, thymus, small intestine, and colon) from mice for the presence of 1-OACs by LC-MS². In most organs, low levels of about 0.25-1.3 pmol 1-OACs/mg wet weight were recorded. Higher levels were detected in liver, small and large intestines, with about 4-13 pmol 1-OACs/mg wet weight. 1-OACs were esterified mainly with palmitic, stearic, or oleic acids. Esterification with saturated very long-chain fatty acids, as in epidermis, was not observed. Western-type diet induced 3-fold increased 1-OAC levels in mice livers while ceramides were unaltered. In a mouse model of Farber disease with a decrease of acid ceramidase activity, we observed a strong, up to 50-fold increase of 1-OACs in lung, thymus, and spleen. In contrast, 1-OAC levels were reduced 0.54-fold in liver. Only in lung 1-OAC levels correlated to changes in ceramide levels - indicating tissue-specific mechanisms of regulation. Glucosylceramide synthase deficiency in liver did not cause changes in 1-OAC or ceramide levels, whereas increased ceramide levels in glucosylceramide synthase-deficient small intestine caused an increase in 1-OAC levels. Deficiency of Dgat1 in mice resulted in a reduction of 1-OACs to 30% in colon, but not in small intestine and liver, going along with constant free ceramides levels. From these data, we conclude that Dgat1 as well as lysosomal lipid metabolism contribute in vivo to homeostatic 1-OAC levels in an organ-specific manner.

1-O-acylceramides are natural components of human and mouse epidermis

The lipid-rich stratum corneum functions as a barrier against pathogens and desiccation inter alia by an unbroken meshwork of extracellular lipid lamellae. These lamellae are composed of cholesterol, fatty acids, and ceramides (Cers) in an equimolar ratio. The huge class of skin Cers consists of three groups: group I, "classical" long and very long chain Cers; group II, ultra-long chain Cers; and group III, ω-esterified ultra-long chain Cers, which are esterified either with linoleic acid or with cornified envelope proteins and are required for the water permeability barrier. Here, we describe 1-O-acylceramides as a new class of epidermal Cers in humans and mice. These Cers contain, in both the N- and 1-O-position, long to very long acyl chains. They derive from the group I of classical Cers and make up 5% of all esterified Cers. Considering their chemical structure and hydrophobicity, we presume 1-O-acylceramides to contribute to the water barrier homeostasis. Biosynthesis of 1-O-acylceramides is not dependent on lysosomal phospholipase A2. However, glucosylceramide synthase deficiency was followed by a 7-fold increase of 1-O-acylceramides, which then contributed 30% to all esterified Cers. Furthermore, loss of neutral glucosylceramidase resulted in decreased levels of a 1-O-acylceramide subgroup. Therefore, we propose 1-O-acylceramides to be synthesized at endoplasmic reticulum-related sites.

Loss of ceramide synthase 3 causes lethal skin barrier disruption

The stratum corneum as the outermost epidermal layer protects against exsiccation and infection. Both the underlying cornified envelope (CE) and the intercellular lipid matrix contribute essentially to these two main protective barriers. Epidermis-unique ceramides with ultra-long-chain acyl moities (ULC-Cers) are key components of extracellular lipid lamellae (ELL) and are bound to CE proteins, thereby contributing to the cornified lipid envelope (CLE). Here, we identified human and mouse ceramide synthase 3 (CerS3), among CerS1-6, to be exclusively required for the ULC-Cer synthesis in vitro and of mouse CerS3 in vivo. Deficiency of CerS3 in mice results in complete loss of ULC-Cers (≥C26), lack of continuous ELL and a non-functional CLE. Consequently, newborn mutant mice die shortly after birth from transepidermal water loss. Mutant skin is prone to Candida albicans infection highlighting ULC-Cers to be pivotal for both barrier functions. Persistent periderm, hyperkeratosis and deficient cornification are hallmarks of mutant skin demonstrating loss of Cers to trigger a keratinocyte maturation arrest at an embryonic pre-barrier stage.