Impaired skin barrier function due to reduced ω- O -acylceramide levels in a mouse model of Sjögren–Larsson syndrome

Sjögren–Larsson syndrome (SLS) is an inherited neurocutaneous disorder whose causative gene encodes the fatty aldehyde dehydrogenase ALDH3A2. To date, the detailed molecular mechanism of the skin pathology of SLS has remained largely unclear. We generated double knockout (DKO) mice for Aldh3a2 and its homolog Aldh3b2 (a pseudogene in humans). These mice showed hyperkeratosis and reduced fatty aldehyde dehydrogenase activity and skin barrier function. The levels of ω- O -acylceramides (acylceramides), which are specialized ceramides essential for skin barrier function, in the epidermis of DKO mice were about 60% of those in wild type mice. In the DKO mice, levels of acylceramide precursors (ω-hydroxy ceramides and triglycerides) were increased, suggesting that the final step of acylceramide production was inhibited. A decrease in acylceramide levels was also observed in human immortalized keratinocytes lacking ALDH3A2 . Differentiated keratinocytes prepared from the DKO mice exhibited impaired long-chain base metabolism. Based on these results, we propose that the long-chain-base–derived fatty aldehydes that accumulate in DKO mice and SLS patients attack and inhibit the enzyme involved in the final step of acylceramide. Our findings provide insight into the pathogenesis of the skin symptoms of SLS, i.e., decreased acylceramide production, and its molecular mechanism.

Direct LC–ESI–MS/MS analysis of plant glucosylceramide and ceramide species with 8E and 8Z isomers of the long chain base

ABSTRACT Glucosylceramides and ceramides with 8E and 8Z isomers of the long chain base are found in plants. These isomers have been difficult to quantify separately using liquid chromatography–tandem mass spectrometry (LC–MS/MS) because the isomers have the same retention time, their precursor and product ions have the same m/z values, and plant ceramide standards are not commercially available. Here we tested trial separations using various ODS columns and prepared plant ceramide standards generated by human glucocerebrosidase (imiglucerase) using commercially available plant glucosylceramide standards as the substrates. Consequently, we were able to quantify the isomers based on differences in retention times in a TSKgel ODS-120A column (Tosoh, Tokyo Japan) using LC–electrospray ionization–MS/MS (LC–ESI–MS/MS).

Biosynthesis of long chain base in sphingolipids in animals, plants and fungi

Long chain base (LCB) is a unique building block found in sphingolipids. The initial step of LCB biosynthesis stems from serine:palmitoyl-CoA transferase enzyme, producing 3-ketodihydrosphingosine with multiple regulatory proteins including small subunit SPT a/b and orosomucoid-like protein1-3. 3-Ketodihydrosphingosine reductase and sphingolipid Δ4-desaturase, both of them poorly characterized mammalian enzymes, play key roles for neurological homeostasis based on their pathogenic mutation in humans. Ceramide synthase in mammals has six isoforms with distinct phenotype in each knockout mouse. In plants and fungi, sphingolipids also contain phytosphingosine due to sphingolipid C4-hydroxylase. In contrast to previous notion that dietary intake might be its major route in animals, emerging evidences suggested that phytosphingosine biosynthesis does occur in some tissues such as the skin by mammalian C4-hydroxylase activity of the DEGS2 gene. This short review summarizes LCB biosynthesis with their associating metabolic pathways in animals, plants and fungi.

Plant-Unique cis/trans Isomerism of Long-Chain Base Unsaturation is Selectively Required for Aluminum Tolerance Resulting from Glucosylceramide-Dependent Plasma Membrane Fluidity

Cis/trans isomerism of the Δ8 unsaturation of long-chain base (LCB) is found only in plant sphingolipids. This unique geometry is generated by sphingolipid LCB Δ8 desaturase SLD which produces both isomers at various ratios, resulting in diverse cis/trans ratios in plants. However, the biological significance of this isomeric diversity remains controversial. Here, we show that the plant-specific cis unsaturation of LCB selectively contributes to glucosylceramide (GlcCer)-dependent tolerance to aluminum toxicity. We established three transgenic rice lines with altered LCB unsaturation profiles. Overexpression of SLD from rice (OsSLD-OX), which preferentially exhibits cis-activity, or Arabidopsis (AtSLD-OX), showing preference for trans-activity, facilitated Δ8 unsaturation in different manners: a slight increase of cis-unsaturated glycosylinositolphosphoceramide (GIPC) in OsSLD-OX, and a drastic increase of trans-unsaturated GlcCer and GIPC in AtSLD-OX. Disruption of LCB Δ4 desaturase (des) significantly decreased the content of GlcCer. Fluorescence imaging analysis revealed that OsSLD-OX and AtSLD-OX showed increased plasma membrane fluidity, whereas des had less fluidity, demonstrating that the isomers universally contributed to increasing membrane fluidity. However, the results of a hydroponic assay showed decreased aluminum tolerance in AtSLD-OX and des compared to OsSLD-OX and the control plants, which did not correlate with membrane fluidity. These results suggest that cis-unsaturated GlcCer, not GIPC, selectively serves to maintain the membrane fluidity specifically associated with aluminum tolerance.