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.

Defective ceramide synthases in mice cause reduced amplitudes in electroretinograms and altered sphingolipid composition in retina and cornea.

Complex sphingolipids are strongly expressed in neuronal tissue and contain ceramides in their backbone. Ceramides are synthesized by six ceramide synthases (CerS1-6). Although it is known that each tissue has a unique profile of ceramide synthase expression and ceramide synthases are implicated in several neurodegenerative disorders, the expression of ceramide synthase isoforms has not been investigated in the retina. Here we demonstrate CerS1, CerS2 and CerS4 expression in mouse retina and cornea, with CerS4 ubiquitously expressed in all retinal neurons and Müller cells. To test whether ceramide synthase deficiency affects retinal function, we compared electroretinograms and retina morphology between wild-type and CerS1-, CerS2- and CerS4-deficient mice. Electroretinograms were strongly reduced in amplitude in ceramide synthase-deficient mice, suggesting that signalling in the outer retina is affected. However, the number of photoreceptors and cone outer segment length were unaltered and no changes in retinal layer thickness or synaptic structures were found. Mass spectrometric analyses of ceramides, hexosyl-ceramides and sphingomyelins showed that C20 to C24 acyl-containing species were decreased whereas C16-containing species were increased in the retina of ceramide synthase-deficient mice. Similar but smaller changes were also found in the cornea. Thus, we hypothesize that the replacement of very long-chain fatty acyl residues by shorter C16 residues may affect the electrical properties of retina and cornea, and alter receptor-mediated signal transduction, vesicle-mediated synaptic transmission or corneal light transmission. Future studies need to identify the molecular targets of ceramides or derived sphingolipids in light signal transduction and transmission in the eye.

Altered epidermal lipid processing and calcium distribution in the KID syndrome mouse model Cx26S17F.

The keratitis-ichthyosis-deafness (KID) syndrome is caused by mutations in the gap junctional channel protein connexin 26 (Cx26), among them the mutation Cx26S17F. Heterozygous Cx26S17F mice resemble the human KID syndrome, i.e. exhibiting epidermal hyperplasia and hearing impairments. Newborn Cx26S17F mice show a defective epidermal water barrier as well as altered epidermal lipid secretion and location. Linoleoyl ω-esterified ceramides are strongly decreased on the skin surface of Cx26S17F mice. Moreover, the epidermal calcium gradient is altered in the mutant mice. These alterations may be caused by an abnormal Cx26S17F channel function that leads to a defective epidermal water barrier, which in turn may trigger the hyperproliferation seen in the KID syndrome.

Exacerbation of experimental autoimmune encephalomyelitis in ceramide synthase 6 knockout mice is associated with enhanced activation/migration of neutrophils.

Ceramides are mediators of inflammatory processes. In experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), we observed that CerS6 mRNA expression was upregulated 15-fold in peripheral blood leukocytes before the onset of EAE symptoms. In peripheral blood leukocytes from MS patients, a 3.9-fold upregulation was found. Total genetic deletion of CerS6 and the selective deletion of CerS6 in peripheral blood leucocytes exacerbated the progression of clinical symptoms in EAE mice. This was associated with enhanced leukocyte, predominantly neutrophil infiltration and enhanced demyelination in the lumbar spinal cord of EAE mice. Interferon-gamma/tumor necrosis factor alpha (IFN-γ/TNF-α) and granulocyte colony-stimulating factor (G-CSF) both drive EAE development and induce expression of the integrin CD11b and the chemokine receptor C-X-C motif chemokine receptor 2 (CXCR2), and we found they also induce CerS6 expression. In vivo, the genetic deletion of CerS6 enhanced the activation/migration of neutrophils, as reflected by an enhanced upregulation of CD11b and CXCR2. In vitro, the genetic deletion of CerS6 enhanced the activation status of IFN-γ/TNF-α-stimulated neutrophils, as shown by increased expression of nitric oxide and CD11b and an increased adhesion capacity. In G-CSF-stimulated neutrophils, the migration status was enhanced, as reflected by an elevated level of CXCR2 and an increased migration capacity. These data suggest that CerS6/C16-Cer mediates feedback regulation by inhibiting the formation of CD11b and CXCR2, which are induced either by IFN-γ/TNF-α or by G-CSF, respectively. We conclude that CerS6/C16-Cer mediates anti-inflammatory effects during the development of EAE and MS possibly by suppressing the migration and deactivation of neutrophils.

Regulation of ceramide synthase 6 in a spontaneous experimental autoimmune encephalomyelitis model is sex dependent.

Ceramides (Cer) are mediators of inflammatory processes. In a chronic experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis (MS), we observed a significant elevation of C16-Cer and its synthesizing enzyme, ceramide synthase(CerS)6, in the lumbar spinal cord. In the present study, we have confirmed that C16-Cer and CerS6 are also upregulated in the lumbar spinal cord in a spontaneous relapse-remitting EAE model, using SJL mice overexpressing a transgenic T cell receptor (TCR1640). CerS6 was found to be expressed in macrophages, T cells and B cells in EAE lesions. In macrophages, we demonstrated that interferon gamma (IFN-γ)-induced CerS6 upregulation was amplified by 17ß-estradiol, an action that was further accompanied by increased upregulation of tumor necrosis factor alpha (TNF-α). Accordingly, CerS6 and TNF-α expression was upregulated predominantly in the spinal cord in female TCR1640 mice, which usually develop the relapse-remitting form of EAE, while male TCR1640 mice showed an attenuated regulation of CerS6 and TNF-α and exhibit mostly chronic disease progression. Furthermore, expression of TNFR2, one of two receptors of TNF-α, which is linked to neuroprotection and remyelination, was also upregulated to a greater extent during EAE in female TCR1640 mice in comparison to male TCR1640 mice. Taken together, our results confirm the upregulation of CerS6 and C16-Cer in an adjuvant-independent, physiological EAE model and further suggest an anti-inflammatory role of CerS6 in the regulation of the disease course in female TCR1640 mice via TNF-α/TNFR2.

Ceramide synthase 4 deficiency in mice causes lipid alterations in sebum and results in alopecia.

Five ceramide synthases (CerS2-CerS6) are expressed in mouse skin. Although CerS3 has been shown to fulfill an essential function during skin development, neither CerS6- nor CerS2-deficient mice show an obvious skin phenotype. In order to study the role of CerS4, we generated CerS4-deficient mice (Cers4-/-) and CerS4-specific antibodies. With these biological tools we analysed the tissue distribution and determined the cell-type specific expression of CerS4 in suprabasal epidermal layers of footpads as well as in sebaceous glands of the dorsal skin. Loss of CerS4 protein leads to an altered lipid composition of the sebum, which is more solidified and therefore might cause progressive hair loss due to physical blocking of the hair canal. We also noticed a strong decrease in C20 1,2-alkane diols consistent with the decrease of wax diesters in the sebum of Cers4-/- mice. Cers4-/- mice at 12 months old display additional epidermal tissue destruction due to dilated and obstructed pilary canals. Mass spectrometric analyses additionally show a strong decrease in C20-containing sphingolipids.

Inactivation of ceramide synthase 6 in mice results in an altered sphingolipid metabolism and behavioral abnormalities.

The N-acyl chain length of ceramides is determined by the specificity of different ceramide synthases (CerS). The CerS family in mammals consists of six members with different substrate specificities and expression patterns. We have generated and characterized a mouse line harboring an enzymatically inactive ceramide synthase 6 (CerS6KO) gene and lacz reporter cDNA coding for ß-galactosidase directed by the CerS6 promoter. These mice display a decrease in C16:0 containing sphingolipids. Relative to wild type tissues the amount of C16:0 containing sphingomyelin in kidney is ∼35%, whereas we find a reduction of C16:0 ceramide content in the small intestine to about 25%. The CerS6KO mice show behavioral abnormalities including a clasping abnormality of their hind limbs and a habituation deficit. LacZ reporter expression in the brain reveals CerS6 expression in hippocampus, cortex, and the Purkinje cell layer of the cerebellum. Using newly developed antibodies that specifically recognize the CerS6 protein we show that the endogenous CerS6 protein is N-glycosylated and expressed in several tissues of mice, mainly kidney, small and large intestine, and brain.