Neuroprotective effect of neuron-specific deletion of the C16 ceramide synthetic enzymes in an animal model of multiple sclerosis

Ceramide C16 is a sphingolipid detected at high levels in several neurodegenerative disorders, including multiple sclerosis (MS). It can be generated de novo or from the hydrolysis of other sphingolipids, such as sphingomyelin or through the recycling of sphingosine, in what is known as the salvage pathway. While the myelin damage occurring in MS suggests the importance of the hydrolytic and salvage pathways, the growing interest on the importance of diet in demyelinating disorders, prompted us to investigate the involvement of de novo ceramide C16 synthesis on disease severity. A diet rich in saturated fats such as palmitic acid, as found in many highly processed foods, provides substrates for the ceramide C16 synthetic enzymes ceramide synthase 6 (CERS6) and 5 (CERS5), which are expressed in the central nervous system. Using the experimental autoimmune encephalomyelitis (EAE) model of inflammatory demyelination, we show here that mice with CamK2a+ neuronal specific deletion of both CerS6 and CerS5 show a milder course of EAE than wild type mice, even when fed a diet enriched in palmitic acid. At a cellular level, neurons lacking both CerS6 and CerS5 are protected from the mitochondrial dysfunction arising from exposure to oxidative stress and palmitic acid in the medium. These data underscore the importance of a healthy diet avoiding processed foods for demyelinating disorders and identifies endogenous neuronal synthesis of ceramide C16 as an important determinant of disease severity.

The Multifaceted Impact of Bioactive Lipids on Gut Health and Disease

Bioactive lipids have a multifaceted role in health and disease and are recognized to play an important part in gut immunity and disease conditions such as inflammatory bowel disease and colon cancer. Advancements in lipidomics, enabled by mass spectrometry and chromatographic techniques, have enhanced our understanding of lipid diversity and functionality. Bioactive lipids, including short-chain fatty acids, saturated fatty acids, omega-3 fatty acids, and sphingolipids, exhibit diverse effects on inflammation and immune regulation. Short-chain fatty acids like butyrate demonstrate anti-inflammatory properties, enhancing regulatory T cell function, gut barrier integrity, and epigenetic regulation, making them promising therapeutic targets for inflammatory bowel disease and colon cancer. Conversely, saturated fatty acids promote inflammation by disrupting gut homeostasis, triggering oxidative stress, and impairing immune regulation. Omega-3 lipids counteract these effects, reducing inflammation and supporting immune balance. Sphingolipids exhibit complex roles, modulating immune cell trafficking and inflammation. They can exert protective effects or exacerbate colitis depending on their source and context. Additionally, eicosanoids can also prevent pathology through prostaglandin defense against damage to epithelial barriers. This review underscores the importance of dietary lipids in shaping gut health and immunity and also highlights the potential use of lipids as therapeutic strategies for managing inflammatory conditions and cancer.

Antioxidant Therapies in the Treatment of Multiple Sclerosis

Several studies have proposed a potential role for oxidative stress in the development of multiple sclerosis (MS). For this reason, it seems tentative to think that treatment with antioxidant substances could be useful in the treatment of this disease. In this narrative review, we provide a summary of the current findings on antioxidant treatments, both in experimental models of MS, especially in experimental autoimmune encephalomyelitis (EAE) and in the cuprizone-induced demyelination model, and clinical trials in patients diagnosed with MS. Practically all the antioxidants tested in experimental models of MS have shown improvement in clinical parameters, in delaying the evolution of the disease, and in improving histological and biochemical parameters, including decreased levels of markers of inflammation and oxidative stress in the central nervous system and other tissues. Only a few clinical trials have been carried out to investigate the potential efficacy of antioxidant substances in patients with MS, most of them in the short term and involving a short series of patients, so the results of these should be considered inconclusive. In this regard, it would be desirable to design long-term, randomized, multicenter clinical trials with a long series of patients, assessing several antioxidants that have demonstrated efficacy in experimental models of MS.

Metabolomics of Multiple Sclerosis Lesions Demonstrates Lipid Changes Linked to Alterations in Transcriptomics-Based Cellular Profiles

BACKGROUND AND OBJECTIVES

People with multiple sclerosis (MS) have a dysregulated circulating metabolome, but the metabolome of MS brain lesions has not been studied. The aims of this study were to identify differences in the brain tissue metabolome in MS compared with controls and to assess its association with the cellular profile of corresponding tissue.

METHODS

MS tissues included samples from the edge and core of chronic active or inactive lesions and periplaque white matter (WM). Control specimens were obtained from normal WM. Metabolomic analysis was performed using mass-spectrometry coupled with liquid/gas chromatography and subsequently integrated with single-nucleus RNA-sequencing data by correlating metabolite abundances with relative cell counts, as well as individual genes using Multiomics Factor Analysis (MOFA).

RESULTS

Seventeen samples from 5 people with secondary progressive MS and 8 samples from 6 controls underwent metabolomic profiling identifying 783 metabolites. MS lesions had higher levels of sphingosines (false discovery rate-adjusted p-value[q] = 2.88E-05) and sphingomyelins and ceramides (q = 2.15E-07), but lower nucleotide (q = 0.05), energy (q = 0.001), lysophospholipid (q = 1.86E-07), and monoacylglycerol (q = 0.04) metabolite levels compared with control WM. Periplaque WM had elevated sphingomyelins and ceramides (q = 0.05) and decreased energy metabolites (q = 0.01) and lysophospholipids (q = 0.05) compared with control WM. Sphingolipids and membrane lipid metabolites were positively correlated with astrocyte and immune cell abundances and negatively correlated with oligodendrocytes. On the other hand, long-chain fatty acid, endocannabinoid, and monoacylglycerol pathways were negatively correlated with astrocyte and immune cell populations and positively correlated with oligodendrocytes. MOFA demonstrated associations between differentially expressed metabolites and genes involved in myelination and lipid biosynthesis.

DISCUSSION

MS lesions and perilesional WM demonstrated a significantly altered metabolome compared with control WM. Many of the altered metabolites were associated with altered cellular composition and gene expression, indicating an important role of lipid metabolism in chronic neuroinflammation in MS.

Emerging roles and therapeutic potentials of sphingolipids in pathophysiology: emphasis on fatty acyl heterogeneity

Sphingolipids not only exert structural roles in cellular membranes, but also act as signaling molecules in various physiological and pathological processes. A myriad of studies have shown that abnormal levels of sphingolipids and their metabolic enzymes are associated with a variety of human diseases. Moreover, blood sphingolipids can also be used as biomarkers for disease diagnosis. This review summarizes the biosynthesis, metabolism, and pathological roles of sphingolipids, with emphasis on the biosynthesis of ceramide, the precursor for the biosynthesis of complex sphingolipids with different fatty acyl chains. The possibility of using sphingolipids for disease prediction, diagnosis, and treatment is also discussed. Targeting endogenous ceramides and complex sphingolipids along with their specific fatty acyl chain to promote future drug development will also be discussed.

The heat shock protein Hsp27 controls mitochondrial function by modulating ceramide generation

Sphingolipids have key functions in membrane structure and cellular signaling. Ceramide is the central molecule of the sphingolipid metabolism and is generated by ceramide synthases (CerS) in the de novo pathway. Despite their critical function, mechanisms regulating CerS remain largely unknown. Using an unbiased proteomics approach, we find that the small heat shock protein 27 (Hsp27) interacts specifically with CerS1 but not other CerS. Functionally, our data show that Hsp27 acts as an endogenous inhibitor of CerS1. Wild-type Hsp27, but not a mutant deficient in CerS1 binding, inhibits CerS1 activity. Additionally, silencing of Hsp27 enhances CerS1-generated ceramide accumulation in cells. Moreover, phosphorylation of Hsp27 modulates Hsp27-CerS1 interaction and CerS1 activity in acute stress-response conditions. Biologically, we show that Hsp27 knockdown impedes mitochondrial function and induces lethal mitophagy in a CerS1-dependent manner. Overall, we identify an important mode of CerS1 regulation and CerS1-mediated mitophagy through protein-protein interaction with Hsp27.

Natural Products and Small Molecules Targeting Cellular Ceramide Metabolism to Enhance Apoptosis in Cancer Cells

Molecular targeting strategies have been used for years in order to control cancer progression and are often based on targeting various enzymes involved in metabolic pathways. Keeping this in mind, it is essential to determine the role of each enzyme in a particular metabolic pathway. In this review, we provide in-depth information on various enzymes such as ceramidase, sphingosine kinase, sphingomyelin synthase, dihydroceramide desaturase, and ceramide synthase which are associated with various types of cancers. We also discuss the physicochemical properties of well-studied inhibitors with natural product origins and their related structures in terms of these enzymes. Targeting ceramide metabolism exhibited promising mono- and combination therapies at preclinical stages in preventing cancer progression and cemented the significance of sphingolipid metabolism in cancer treatments. Targeting ceramide-metabolizing enzymes will help medicinal chemists design potent and selective small molecules for treating cancer progression at various levels.

Sphingolipids in thyroid eye disease

Graves' disease (GD) is caused by an autoimmune formation of autoantibodies and autoreactive T-cells against the thyroid stimulating hormone receptor (TSHR). The autoimmune reaction does not only lead to overstimulation of the thyroid gland, but very often also to an immune reaction against antigens within the orbital tissue leading to thyroid eye disease, which is characterized by activation of orbital fibroblasts, orbital generation of adipocytes and myofibroblasts and increased hyaluronan production in the orbit. Thyroid eye disease is the most common extra-thyroidal manifestation of the autoimmune Graves' disease. Several studies indicate an important role of sphingolipids, in particular the acid sphingomyelinase/ceramide system and sphingosine 1-phosphate in thyroid eye disease. Here, we discuss how the biophysical properties of sphingolipids contribute to cell signaling, in particular in the context of thyroid eye disease. We further review the role of the acid sphingomyelinase/ceramide system in autoimmune diseases and its function in T lymphocytes to provide some novel hypotheses for the pathogenesis of thyroid eye disease and potentially allowing the development of novel treatments.

Ceramide synthase 6 impacts T-cell allogeneic response and graft-versus-host disease through regulating N-RAS/ERK pathway

Allogeneic hematopoietic cell transplantation (allo-HCT) is an effective immunotherapy for various hematologic malignances, predominantly through potent graft-versus-leukemia (GVL) effect. However, the mortality after allo-HCT is because of relapse of primary malignancy and followed by graft-vs-host-disease (GVHD) as a major cause of transplant-related mortality. Hence, strategies to limit GVHD while preserving the GVL effect are highly desirable. Ceramide, which serves a central role in sphingolipid metabolism, is generated by ceramide synthases (CerS1–6). In this study, we found that genetic or pharmacologic targeting of CerS6 prevented and reversed chronic GVHD (cGVHD). Furthermore, specific inhibition of CerS6 with ST1072 significantly ameliorated acute GVHD (aGVHD) while preserving the GVL effect, which differed from FTY720 that attenuated aGVHD but impaired GVL activity. At the cellular level, blockade of CerS6 restrained donor T cells from migrating into GVHD target organs and preferentially reduced activation of donor CD4 T cells. At the molecular level, CerS6 was required for optimal TCR signaling, CD3/PKCθ co-localization, and subsequent N-RAS activation and ERK signaling, especially on CD4⁺ T cells. The current study provides rationale and means for targeting CerS6 to control GVHD and leukemia relapse, which would enhance the efficacy of allo-HCT as an immunotherapy for hematologic malignancies in the clinic.

Sapropterin (BH4) Aggravates Autoimmune Encephalomyelitis in Mice

Depletion of the enzyme cofactor, tetrahydrobiopterin (BH4), in T-cells was shown to prevent their proliferation upon receptor stimulation in models of allergic inflammation in mice, suggesting that BH4 drives autoimmunity. Hence, the clinically available BH4 drug (sapropterin) might increase the risk of autoimmune diseases. The present study assessed the implications for multiple sclerosis (MS) as an exemplary CNS autoimmune disease. Plasma levels of biopterin were persistently low in MS patients and tended to be lower with high Expanded Disability Status Scale (EDSS). Instead, the bypass product, neopterin, was increased. The deregulation suggested that BH4 replenishment might further drive the immune response or beneficially restore the BH4 balances. To answer this question, mice were treated with sapropterin in immunization-evoked autoimmune encephalomyelitis (EAE), a model of multiple sclerosis. Sapropterin-treated mice had higher EAE disease scores associated with higher numbers of T-cells infiltrating the spinal cord, but normal T-cell subpopulations in spleen and blood. Mechanistically, sapropterin treatment was associated with increased plasma levels of long-chain ceramides and low levels of the poly-unsaturated fatty acid, linolenic acid (FA18:3). These lipid changes are known to contribute to disruptions of the blood-brain barrier in EAE mice. Indeed, RNA data analyses revealed upregulations of genes involved in ceramide synthesis in brain endothelial cells of EAE mice (LASS6/CERS6, LASS3/CERS3, UGCG, ELOVL6, and ELOVL4). The results support the view that BH4 fortifies autoimmune CNS disease, mechanistically involving lipid deregulations that are known to contribute to the EAE pathology.

Ceramide Metabolism and Parkinson's Disease-Therapeutic Targets

Ceramide is a bioactive sphingolipid involved in numerous cellular processes. In addition to being the precursor of complex sphingolipids, ceramides can act as second messengers, especially when they are generated at the plasma membrane of cells. Its metabolic dysfunction may lead to or be a consequence of an underlying disease. Recent reports on transcriptomics and electrospray ionization mass spectrometry analysis have demonstrated the variation of specific levels of sphingolipids and enzymes involved in their metabolism in different neurodegenerative diseases. In the present review, we highlight the most relevant discoveries related to ceramide and neurodegeneration, with a special focus on Parkinson's disease.

Sphingolipid Metabolism and Signaling in Skeletal Muscle: From Physiology to Physiopathology

Sphingolipids represent one of the major classes of eukaryotic lipids. They play an essential structural role, especially in cell membranes where they also possess signaling properties and are capable of modulating multiple cell functions, such as apoptosis, cell proliferation, differentiation, and inflammation. Many sphingolipid derivatives, such as ceramide, sphingosine-1-phosphate, and ganglioside, have been shown to play many crucial roles in muscle under physiological and pathological conditions. This review will summarize our knowledge of sphingolipids and their effects on muscle fate, highlighting the role of this class of lipids in modulating muscle cell differentiation, regeneration, aging, response to insulin, and contraction. We show that modulating sphingolipid metabolism may be a novel and interesting way for preventing and/or treating several muscle-related diseases.

Progranulin deficiency confers resistance to autoimmune encephalomyelitis in mice

Progranulin is a secreted neurotrophin that assists in the autophagolysosomal pathways that contribute to MHC-mediated antigen processing, pathogen removal, and autoimmunity. We showed that patients with multiple sclerosis (MS) have high levels of circulating progranulin and that its depletion in a mouse model by a monoclonal antibody aggravates MS-like experimental autoimmune encephalomyelitis (EAE). However, unexpectedly, progranulin-deficient mice (Grn^−/−) were resistant to EAE, and this resistance was fully restored by wild-type bone marrow transplantation. FACS analyses revealed a loss of MHC-II-positive antigen-presenting cells in Grn^−/− mice and a reduction in the number of CD8+ and CD4+ T-cells along with a strong increase in the number of scavenger receptor class B (CD36+) phagocytes, suggesting defects in antigen presentation along with a compensatory increase in phagocytosis. Indeed, bone marrow-derived dendritic cells from Grn^−/− mice showed stronger uptake of antigens but failed to elicit antigen-specific T-cell proliferation. An increase in the number of CD36+ phagocytes was associated with increased local inflammation at the site of immunization, stronger stimulation-evoked morphological transformation of bone marrow-derived macrophages to phagocytes, an increase in the phagocytosis of E. coli particles and latex beads and defects in the clearance of the material. Hence, the outcomes in the EAE model reflect the dichotomy of progranulin-mediated immune silencing and autoimmune mechanisms of antigen recognition and presentation, and our results reveal a novel progranulin-dependent pathway in autoimmune encephalomyelitis.

Roles of Ceramides and Other Sphingolipids in Immune Cell Function and Inflammation

Ceramides are bioactive sphingolipids that support the structure of the plasma membrane and mediate numerous cell-signaling events in eukaryotic cells. The finding that ceramides act as second messengers transducing cellular signals has attracted substantial attention in several fields of Biology. Since all cells contain lipid plasma membranes, the impact of various ceramides, ceramide synthases, ceramide metabolites, and other sphingolipids has been implicated in a vast range of cellular functions including, migration, proliferation, response to external stimuli, and death. The roles of lipids in these functions widely differ among the diverse cell types. Herein, we discuss the roles of ceramides and other sphingolipids in mediating the function of various immune cells; particularly dendritic cells, neutrophils, and macrophages. In addition, we highlight the main studies describing effects of ceramides in inflammation, specifically in various inflammatory settings including insulin resistance, graft-versus-host disease, immune suppression in cancer, multiple sclerosis, and inflammatory bowel disease.

Sphingolipids in neurodegeneration (with focus on ceramide and S1P)

For many decades, research on sphingolipids associated with neurodegenerative disease focused on alterations in glycosphingolipids, particularly glycosylceramides (cerebrosides), sulfatides, and gangliosides. This seemed quite natural since many of these glycolipids are constituents of myelin and accumulated in lipid storage diseases (sphingolipidoses) resulting from enzyme deficiencies in glycolipid metabolism. With the advent of recognizing ceramide and its derivative, sphingosine-1-phosphate (S1P), as key players in lipid cell signaling and regulation of cell death and survival, research focus shifted toward these two sphingolipids. Ceramide and S1P are invoked in a plethora of cell biological processes participating in neurodegeneration such as ER stress, autophagy, dysregulation of protein and lipid transport, exosome secretion and neurotoxic protein spreading, neuroinflammation, and mitochondrial dysfunction. Hence, it is timely to discuss various functions of ceramide and S1P in neurodegenerative disease and to define sphingolipid metabolism and cell signaling pathways as potential targets for therapy.

Ceramide Synthase 6: Comparative Analysis, Phylogeny and Evolution

Ceramide synthase 6 (CerS6, also known as LASS6) is one of the six members of ceramide synthase gene family in humans. Comparisons of CerS6 amino acid sequences and structures as well as of CerS6 gene structures/locations were conducted using data from several vertebrate genome projects. A specific role for the CerS6 gene and protein has been identified as the endoplasmic reticulum C₁₄- and C₁₆-ceramide synthase. Mammalian CerS6 proteins share 90⁻100% similarity among different species, but are only 22⁻63% similar to other CerS family members, suggesting that CerS6 is a distinct gene family. Sequence alignments, predicted transmembrane, lumenal and cytoplasmic segments and N-glycosylation sites were also investigated, resulting in identification of the key conserved residues, including the active site as well as C-terminus acidic and serine residues. Mammalian CerS6 genes contain ten exons, are primarily located on the positive strands and transcribed as two major isoforms. The human CERS6 gene promoter harbors a large CpG island (94 CpGs) and multiple transcription factor binding sites (TFBS), which support precise transcriptional regulation and signaling functions. Additional regulation is conferred by 15 microRNA (miRNA) target sites identified in the CERS6 3'-UTR region. Phylogenetic analysis of the vertebrate CerS1⁻6 gene families relationships supports a major role for the CerS6 enzyme that is strongly conserved throughout vertebrate evolution.

Dietary phytol reduces clinical symptoms in experimental autoimmune encephalomyelitis (EAE) at least partially by modulating NOX2 expression

Multiple sclerosis (MS) is an inflammatory, demyelinating disease of the central nervous system. We investigated the effect of phytol in an animal model of MS, experimental autoimmune encephalomyelitis (EAE), as phytol, a plant-derived diterpene alcohol, exerts anti-inflammatory and redox-protective actions. We observed a significant amelioration of clinical symptoms in EAE C57BL/6N mice fed prophylactically with a phytol-enriched diet. Demyelination, DNA damage, and infiltration of immune cells, specifically T_H1 cells, into the central nervous system were reduced in phytol-fed EAE mice. Furthermore, phytol reduced T-cell proliferation ex vivo. Phytanic acid - a metabolite of phytol - also reduced T-cell proliferation, specifically that of T_H1 cells. Additionally, phytol-enriched diet increased the mRNA expression of nicotinamide adenine dinucleotide phosphate oxidase (NOX) 2 in white blood cells in the lymph nodes. Accordingly, phytol lost its anti-inflammatory effects in chimeric EAE C57BL/6N mice whose peripheral cells lack NOX2, indicating that phytol mediates its effects in peripheral cells via NOX2. Moreover, the effects of phytol on T-cell proliferation were also NOX2-dependent. In contrast, the T-cell subtype alterations and changes in proliferation induced by phytanic acid, the primary metabolite of phytol, were NOX2-independent. In conclusion, phytol supplementation of the diet leads to amelioration of EAE pathology in both a NOX2-dependent and a NOX2-independent manner via yet unknown mechanisms.

KEY MESSAGES

Phytol diet ameliorates EAE pathology. Phytol diet reduces demyelination, immune cell infiltration, and T-cell proliferation. Phytol diet increases NOX2 mRNA expression in white blood cells in the lymph nodes. Phytol mediates its effects in peripheral cells via NOX2. Effects of phytol on T-cell proliferation were NOX2-dependent.

Adoptive Transfer of Ceramide Synthase 6 Deficient Splenocytes Reduces the Development of Colitis

Sphingolipids regulate critical cellular processes including inflammation. Ceramide, which serves a central role in sphingolipid metabolism, is generated by six ceramide synthases (CerS) that differ in substrate specificity. CerS6 preferentially generates C₁₆-ceramide and its mRNA is highly expressed in immune tissues. In this study we analyzed how deficiency of CerS6 impacts on the development of colitis using an adoptive transfer model. Adoptive transfer of CerS6-deficient splenocytes, which have significantly decreased levels of C₁₆-ceramide, showed that CerS6-deficiency protected against the development of colitis. However, adoptively transferred cells isolated from the lamina propria of the large intestine from wild type or CerS6-deficient groups showed no differences in the percentages of immune-suppressive regulatory T cells, pro-inflammatory Th17 cells, or their ability to express IL-17. In vitro polarization of wild type or CerS6-deficient splenocytes also revealed no defects in the development of T cell subsets. Our data suggest that protection from colitis following adoptive transfer of CerS6-deficient splenocytes maybe related to their ability to migrate and proliferate in vivo rather than subset development or cytokine expression.

Sphingosine Toxicity in EAE and MS: Evidence for Ceramide Generation via Serine-Palmitoyltransferase Activation

Multiple sclerosis (MS) is a demyelinating disorder characterized by massive neurodegeneration and profound axonal loss. Since myelin is enriched with sphingolipids and some of them display toxicity, biological function of sphingolipids in demyelination has been investigated in MS brain tissues. An elevation of sphingosine with a decrease in monoglycosylceramide and psychosine (myelin markers) was observed in MS white matter and plaque compared to normal brain tissue. This indicated that sphingosine toxicity might mediate oligodendrocyte degeneration. To explain the source of sphingosine accumulation, total sphingolipid profile was investigated in Lewis rats after inducing experimental autoimmune encephalomyelitis (EAE) and also in human oligodendrocytes in culture. An intermittent increase in ceramide followed by sphingosine accumulation in EAE spinal cord along with a stimulation of serine-palmitoyltransferase (SPT) activity was observed. Apoptosis was identified in the lumbar spinal cord, the most prominent demyelinating area, in the EAE rats. TNFα and IFNγ stimulation of oligodendrocytes in culture also led to an accumulation of ceramide with an elevation of sphingosine. Ceramide elevation was drastically blocked by myriocin, an inhibitor of SPT, and also by FTY720. Myriocin treatment also protected oligodendrocytes from cytokine mediated apoptosis or programmed cell death. Hence, we propose that sphingosine toxicity may contribute to demyelination in both EAE and MS, and the intermittent ceramide accumulation in EAE may, at least partly, be mediated via SPT activation, which is a novel observation that has not been previously reported.

Dysregulation of lysophosphatidic acids in multiple sclerosis and autoimmune encephalomyelitis

Abstract

Bioactive lipids contribute to the pathophysiology of multiple sclerosis. Here, we show that lysophosphatidic acids (LPAs) are dysregulated in multiple sclerosis (MS) and are functionally relevant in this disease. LPAs and autotaxin, the major enzyme producing extracellular LPAs, were analyzed in serum and cerebrospinal fluid in a cross-sectional population of MS patients and were compared with respective data from mice in the experimental autoimmune encephalomyelitis (EAE) model, spontaneous EAE in TCR¹⁶⁴⁰ mice, and EAE in Lpar2^-/- mice. Serum LPAs were reduced in MS and EAE whereas spinal cord LPAs in TCR¹⁶⁴⁰ mice increased during the ‘symptom-free’ intervals, i.e. on resolution of inflammation during recovery hence possibly pointing to positive effects of brain LPAs during remyelination as suggested in previous studies. Peripheral LPAs mildly re-raised during relapses but further dropped in refractory relapses. The peripheral loss led to a redistribution of immune cells from the spleen to the spinal cord, suggesting defects of lymphocyte homing. In support, LPAR2 positive T-cells were reduced in EAE and the disease was intensified in Lpar2 deficient mice. Further, treatment with an LPAR2 agonist reduced clinical signs of relapsing-remitting EAE suggesting that the LPAR2 agonist partially compensated the endogenous loss of LPAs and implicating LPA signaling as a novel treatment approach.

Graphical abstract

Graphical summary of lysophosphatidic signaling in multiple sclerosis

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-017-0446-4) contains supplementary material, which is available to authorized users.

Ceramide synthesis regulates T cell activity and GVHD development

Allogeneic hematopoietic cell transplantation (allo-HCT) is an effective immunotherapy for a variety of hematologic malignances, yet its efficacy is impeded by the development of graft-versus-host disease (GVHD). GVHD is characterized by activation, expansion, cytokine production, and migration of alloreactive donor T cells. Hence, strategies to limit GVHD are highly desirable. Ceramides are known to contribute to inflammation and autoimmunity. However, their involvement in T-cell responses to alloantigens is undefined. In the current study, we specifically characterized the role of ceramide synthase 6 (CerS6) after allo-HCT using genetic and pharmacologic approaches. We found that CerS6 was required for optimal T cell activation, proliferation, and cytokine production in response to alloantigen and for subsequent induction of GVHD. However, CerS6 was partially dispensable for the T cell-mediated antileukemia effect. At the molecular level, CerS6 was required for efficient TCR signal transduction, including tyrosine phosphorylation, ZAP-70 activation, and PKCθ/TCR colocalization. Impaired generation of C16-ceramide was responsible for diminished allogeneic T cell responses. Furthermore, targeting CerS6 using a specific inhibitor significantly reduced T cell activation in mouse and human T cells in vitro. Our study provides a rationale for targeting CerS6 to control GVHD, which would enhance the efficacy of allo-HCT as an immunotherapy for hematologic malignancies in the clinic.

Increasing acetyl-CoA metabolism attenuates injury and alters spinal cord lipid content in mice subjected to experimental autoimmune encephalomyelitis

Acetate supplementation increases brain acetyl-CoA metabolism, alters histone and non-histone protein acetylation, increases brain energy reserves, and is anti-inflammatory and neuroprotective in rat models of neuroinflammation and neuroborreliosis. To determine the impact acetate supplementation has on a mouse model of multiple sclerosis, we quantified the effect treatment had on injury progression, spinal cord lipid content, phospholipase levels, and myelin structure in mice subjected to experimental autoimmune encephalomyelitis (EAE). EAE was induced by inoculating mice with a myelin oligodendrocyte glycoprotein peptide fragment (MOG_35-55 ), and acetate supplementation was maintained with 4 g/kg glyceryl triacetate by a daily oral gavage. Acetate supplementation prevented the onset of clinical signs in mice subject to EAE compared to control-treated mice. Furthermore, acetate supplementation prevented the loss of spinal cord ethanolamine and choline glycerophospholipid and phosphatidylserine in mice subjected to EAE compared to EAE animals treated with water. Treatment increased saturated and monounsaturated fatty acid levels in phosphatidylserine compared to controls suggesting that acetate was utilized to increase spinal cord fatty acid content. Also, acetate supplementation prevented the loss of spinal cord cholesterol in EAE animals but did not change cholesteryl esters. Treatment significantly increased GD3 and GD1a ganglioside levels in EAE mice when compared to EAE mice treated with water. Treatment returned levels of phosphorylated and non-phosphorylated cytosolic phospholipase A₂ (cPLA₂ ) levels back to baseline and based on FluoroMyelin™ histochemistry maintained myelin structural characteristics. Overall, these data suggest that acetate supplementation may modulate lipid metabolism in mice subjected to EAE.

Hepatic inflammatory cytokine production can be regulated by modulating sphingomyelinase and ceramide synthase 6

Chronic inflammation is associated with the pathogenesis of type 2 diabetes and diabetic complications, and palmitate has been nominated as a candidate for the molecular link between these disorders. Recently, a crucial role of ceramide in inflammation and metabolic diseases has been reported. Therefore, in this study, we investigated whether ceramide formation is involved in palmitate‑induced hepatic inflammation in vitro and in vivo. Ceramide can be generated either by the de novo pathway or by sphingomyelin degradation, and six different ceramide synthases (CerS) determine the specific acyl chain length of ceramide in mammals. We examined the roles of CerS and sphingomyelinases (SMases) in the secretion of inflammatory cytokines, such as tumour necrosis factor (TNF)‑α, interleukin (IL)‑1β, and IL‑6 in Hep3B cells. Among the six CerS, CerS6 overexpression uniquely elevated TNF‑α secretion via p38 mitogen‑activated protein kinase (MAPK) activation. In addition, the treatment of CerS6 overexpressing cells with palmitate synergistically increased cytokine secretion. However, neither palmitate treatment nor CerS6 overexpression altered lipopolysaccharide (LPS)-induced cytokine secretion. Instead, the activation of acidic (A)‑SMase was involved in LPS‑induced cytokine secretion via the MAPK/NF‑κB pathway. Finally, the suppression of ceramide generation via A‑SMase inhibition or de novo ceramide synthesis decreased high‑fat diet‑induced hepatic cytokine production in vivo. On the whole, our results revealed that CerS6 played a role in TNF‑α secretion, and palmitate augmented inflammatory responses in pathophysiological conditions in which CerS6 is overexpressed. In addition, A‑SMase activation was shown to be involved in LPS‑induced inflammatory processes, suggesting that the modulation of CerS6 and A‑SMase may be a therapeutic target for controlling hepatic inflammation.

Tetrahydrobiopterin Attenuates DSS-evoked Colitis in Mice by Rebalancing Redox and Lipid Signalling

BACKGROUND AND AIMS

Guanosine triphosphate cyclohydrolase [GCH1] governs the production of the enzyme cofactor tetrahydrobiopterin [BH4] which is essential for biogenic amine synthesis, lipid metabolism via alkylglycerol monooxygenase [AGMO], and redox coupling of nitric oxide synthases [NOSs]. Inflammation-evoked unequal regulation of GCH1 and NOS or AGMO may cause redox stress and lipid imbalances.

METHODS

The present study assessed potential therapeutic effects of rebalancing these systems with BH4 in experimental colitis in mice.

RESULTS

Oral treatment with BH4 as a suspension of crushed tablets attenuated colitis, whereas inhibition of its production had opposite effects: aggravated weight loss, epithelial haemorrhages and ulcers, neutrophil infiltrates, production of reactive oxygen species, and unfavourable profile changes of endocannabinoids, ceramides, and lysophosphatidic acids. Conversely, oral BH4 normalised biopterin, reduced in vivo activity of oxidases and peroxidases in the inflamed gut, favoured nitric oxide over hydrogen peroxide, and maintained normal levels of lipid signalling molecules. BH4 favoured thereby resident CD3+CD8+ and regulatory CD3+CD25+ intraepithelial T cells that are important for epithelial integrity.

CONCLUSIONS

BH4 protected against colitis in mice via two major pathways: [i] by reduction of oxidative stress; and [ii] by re-orchestration of alkyl- and acylglycerolipid signalling via AGMO. Oral treatment with BH4 is a safe approved supplementary therapy for genetic BH4 deficiency and did not excessively increase systemic BH4 levels. Therefore, one may consider repurposing of oral BH4 as an adjunctive treatment for colitis.

The enigma of ceramide synthase regulation in mammalian cells

Ceramide synthases (CerS) are key enzymes in the lipid metabolism of eukaryotic cells. Their products, ceramides (Cer), are components of cellular membranes but also mediate signaling functions in physiological processes such as proliferation, skin barrier function and cerebellar development. In pathophysiological processes such as multiple sclerosis and tumor progression, ceramide levels are altered, which can be ascribed, partly, to dysregulation of CerS gene transcription. Most publications deal with the effects of altered ceramide levels on physiological and pathophysiological processes, but the regulation of the appropriate CerS is frequently not investigated. This is insufficient for the clarification of the role of ceramides, because most ceramide species are generated by at least two CerS. The mechanisms of CerS regulation are manifold and it seems that each CerS isoform is regulated individually. For this reason, we discuss the different CerS separately in this review. From transcriptional regulation to alteration of protein activity, the possibilities to influence CerS are diverse. Furthermore, CerS are influenced by a variety of molecules including hormones and lipids. Without claiming completeness, we provide a résumé of the regulatory mechanisms for each CerS in mammalian cells and how dysregulation of these mechanisms during physiological processes may lead to pathophysiological processes.

Induction of Experimental Autoimmune Encephalomyelitis in Mice and Evaluation of the Disease-dependent Distribution of Immune Cells in Various Tissues

Multiple sclerosis is presumed to be an inflammatory autoimmune disease, which is characterized by lesion formation in the central nervous system (CNS) resulting in cognitive and motor impairment. Experimental autoimmune encephalomyelitis (EAE) is a useful animal model of MS, because it is also characterized by lesion formation in the CNS, motor impairment and is also driven by autoimmune and inflammatory reactions. One of the EAE models is induced with a peptide derived from the myelin oligodendrocyte protein (MOG)35-55 in mice. The EAE mice develop a progressive disease course. This course is divided into three phases: the preclinical phase (day 0 - 9), the disease onset (day 10 - 11) and the acute phase (day 12 - 14). MS and EAE are induced by autoreactive T cells that infiltrate the CNS. These T cells secrete chemokines and cytokines which lead to the recruitment of further immune cells. Therefore, the immune cell distribution in the spinal cord during the three disease phases was investigated. To highlight the time point of the disease at which the activation/proliferation/accumulation of T cells, B cells and monocytes starts, the immune cell distribution in lymph nodes, spleen and blood was also assessed. Furthermore, the levels of several cytokines (IL-1β, IL-6, IL-23, TNFα, IFNγ) in the three disease phases were determined, to gain insight into the inflammatory processes of the disease. In conclusion, the data provide an overview of the functional profile of immune cells during EAE pathology.

Synthesis and degradation pathways, functions, and pathology of ceramides and epidermal acylceramides

Ceramide (Cer) is a structural backbone of sphingolipids and is composed of a long-chain base and a fatty acid. Existence of a variety of Cer species, which differ in chain-length, hydroxylation status, and/or double bond number of either of their hydrophobic chains, has been reported. Ceramide is produced by Cer synthases. Mammals have six Cer synthases (CERS1-6), each of which exhibits characteristic substrate specificity toward acyl-CoAs with different chain-lengths. Knockout mice for each Cer synthase show corresponding, isozyme-specific phenotypes, revealing the functional differences of Cers with different chain-lengths. Cer diversity is especially prominent in epidermis. Changes in Cer levels, composition, and chain-lengths are associated with atopic dermatitis. Acylceramide (acyl-Cer) specifically exists in epidermis and plays an essential role in skin permeability barrier formation. Accordingly, defects in acyl-Cer synthesis cause the cutaneous disorder ichthyosis with accompanying severe skin barrier defects. Although the molecular mechanism by which acyl-Cer is generated was long unclear, most genes involved in its synthesis have been identified recently. In Cer degradation pathways, the long-chain base moiety of Cer is converted to acyl-CoA, which is then incorporated mainly into glycerophospholipids. This pathway generates the lipid mediator sphingosine 1-phosphate. This review will focus on recent advances in our understanding of the synthesis and degradation pathways, physiological functions, and pathology of Cers/acyl-Cers.

Alkaline ceramidase 3 deficiency aggravates colitis and colitis-associated tumorigenesis in mice by hyperactivating the innate immune system

Increasing studies suggest that ceramides differing in acyl chain length and/or degree of unsaturation have distinct roles in mediating biological responses. However, still much remains unclear about regulation and role of distinct ceramide species in the immune response. Here, we demonstrate that alkaline ceramidase 3 (Acer3) mediates the immune response by regulating the levels of C18:1-ceramide in cells of the innate immune system and that Acer3 deficiency aggravates colitis in a murine model by augmenting the expression of pro-inflammatory cytokines in myeloid and colonic epithelial cells (CECs). According to the NCBI Gene Expression Omnibus (GEO) database, ACER3 is downregulated in immune cells in response to lipopolysaccharides (LPS), a potent inducer of the innate immune response. Consistent with these data, we demonstrated that LPS downregulated both Acer3 mRNA levels and its enzymatic activity while elevating C(18:1)-ceramide, a substrate of Acer3, in murine immune cells or CECs. Knocking out Acer3 enhanced the elevation of C(18:1)-ceramide and the expression of pro-inflammatory cytokines in immune cells and CECs in response to LPS challenge. Similar to Acer3 knockout, treatment with C(18:1)-ceramide, but not C18:0-ceramide, potentiated LPS-induced expression of pro-inflammatory cytokines in immune cells. In the mouse model of dextran sulfate sodium-induced colitis, Acer3 deficiency augmented colitis-associated elevation of colonic C(18:1)-ceramide and pro-inflammatory cytokines. Acer3 deficiency aggravated diarrhea, rectal bleeding, weight loss and mortality. Pathological analyses revealed that Acer3 deficiency augmented colonic shortening, immune cell infiltration, colonic epithelial damage and systemic inflammation. Acer3 deficiency also aggravated colonic dysplasia in a mouse model of colitis-associated colorectal cancer. Taken together, these results suggest that Acer3 has an important anti-inflammatory role by suppressing cellular or tissue C(18:1)-ceramide, a potent pro-inflammatory bioactive lipid and that dysregulation of ACER3 and C(18:1)-ceramide may contribute to the pathogenesis of inflammatory diseases including cancer.

Inhibition of ceramide de novo synthesis as a postischemic strategy to reduce myocardial reperfusion injury

The injury caused by myocardial reperfusion after ischemia can be contained by interventions aimed at reducing the inflammation and the oxidative stress that underlie exacerbation of tissue damage. Sphingolipids are a class of structural and signaling lipid molecules; among them, the inflammation mediator ceramide accumulates in the myocardium upon ischemia/reperfusion. Here, we show that, after transient coronary occlusion in mice, an increased de novo ceramide synthesis takes place at reperfusion in the ischemic area surrounding necrosis (area at risk). This correlates with the enhanced expression of the first and rate-limiting enzyme of the de novo pathway, serine palmitoyltransferase (SPT). The intraventricular administration at reperfusion of myriocin, an inhibitor of SPT, significantly protected the area at risk from damage, reducing the infarcted area by 40.9 % relative to controls not treated with the drug. In the area at risk, myriocin downregulated ceramide, reduced the content in other mediators of inflammation and reactive oxygen species, and activated the Nrf2-HO1 cytoprotective response. We conclude that an enhanced ceramide synthesis takes part in ischemia/reperfusion injury and that myriocin treatment can be proposed as a strategy for myocardial pharmacological postconditioning.

Ceramide synthases in biomedical research

Sphingolipid metabolism consists of multiple metabolic pathways that converge upon ceramide, one of the key molecules among sphingolipids (SLs). In mammals, ceramide synthesis occurs via N-acylation of sphingoid backbones, dihydrosphingosine (dhSo) or sphingosine (So). The reaction is catalyzed by ceramide synthases (CerS), a family of enzymes with six different isoforms, with each one showing specificity towards a restricted group of acyl-CoAs, thus producing ceramides (Cer) and dihydroceramides (dhCer) with different fatty acid chain lengths. A large body of evidence documents the role of both So and dhSo as bioactive molecules, as well as the involvement of dhCer and Cer in physiological and pathological processes. In particular, the fatty acid composition of Cer has different effects in cell biology and in the onset and progression of different diseases. Therefore, modulation of CerS activity represents an attractive target in biomedical research and in finding new treatment modalities. In this review, we discuss functional, structural and biochemical features of CerS and examine CerS inhibitors that are currently available.

Lack of ceramide synthase 2 suppresses the development of experimental autoimmune encephalomyelitis by impairing the migratory capacity of neutrophils

Ceramide synthases (CerS) synthesise ceramides of defined acyl chain lengths, which are thought to mediate cellular processes in a chain length-dependent manner. In experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), we observed a significant elevation of CerS2 and its products, C24-ceramides, in CD11b(+) cells (monocytes and neutrophils) isolated from blood. This result correlates with the clinical finding that CerS2 mRNA expression and C24-ceramide levels were significantly increased by 2.2- and 1.5-fold, respectively, in white blood cells of MS patients. The increased CerS2 mRNA/C24-ceramide expression in neutrophils/monocytes seems to mediate pro-inflammatory effects, since a specific genetic deletion of CerS2 in blood cells or a total genetic deletion of CerS2 significantly delayed the onset of clinical symptoms, due to a reduced infiltration of immune cells, in particular neutrophils, into the central nervous system. CXCR2 chemokine receptors, expressed on neutrophils, promote the migration of neutrophils into the central nervous system, which is a prerequisite for the recruitment of further immune cells and the inflammatory process that leads to the development of MS. Interestingly, neutrophils isolated from CerS2 null EAE mice, as opposed to WT EAE mice, were characterised by significantly lower CXCR2 receptor mRNA expression resulting in their reduced migratory capacity towards CXCL2. Most importantly, G-CSF-induced CXCR2 expression was significantly reduced in CerS2 null neutrophils and their migratory capacity was significantly impaired. In conclusion, our data strongly indicate that G-CSF-induced CXCR2 expression is regulated in a CerS2-dependent manner and that CerS2 thereby promotes the migration of neutrophils, thus, contributing to inflammation and the development of EAE and MS.

Expression of Ceramide Synthase 6 Transcriptionally Activates Acid Ceramidase in a c-Jun N-terminal Kinase (JNK)-dependent Manner

A family of six ceramide synthases with distinct but overlapping substrate specificities is responsible for generation of ceramides with acyl chains ranging from ∼14-26 carbons. Ceramide synthase 6 (CerS6) preferentially generates C14- and C16-ceramides, and we have previously shown that down-regulation of this enzyme decreases apoptotic susceptibility. In this study, we further evaluated how increased CerS6 expression impacts sphingolipid composition and metabolism. Overexpression of CerS6 in HT29 colon cancer cells resulted in increased apoptotic susceptibility and preferential generation of C16-ceramide, which occurred at the expense of very long chain, saturated ceramides. These changes were also reflected in sphingomyelin composition. HT-CerS6 cells had increased intracellular levels of sphingosine, which is generated by ceramidases upon hydrolysis of ceramide. qRT-PCR analysis revealed that only expression of acid ceramidase (ASAH1) was increased. The increase in acid ceramidase was confirmed by expression and activity analyses. Pharmacological inhibition of JNK (SP600125) or curcumin reduced transcriptional up-regulation of acid ceramidase. Using an acid ceramidase promoter driven luciferase reporter plasmid, we demonstrated that CerS1 has no effect on transcriptional activation of acid ceramidase and that CerS2 slightly but significantly decreased the luciferase signal. Similar to CerS6, overexpression of CerS3-5 resulted in an ∼2-fold increase in luciferase reporter gene activity. Exogenous ceramide failed to induce reporter activity, while a CerS inhibitor and a catalytically inactive mutant of CerS6 failed to reduce it. Taken together, these results suggest that increased expression of CerS6 can mediate transcriptional activation of acid ceramidase in a JNK-dependent manner that is independent of CerS6 activity.

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.

Chronic ethanol consumption profoundly alters regional brain ceramide and sphingomyelin content in rodents

Ceramides (CER) are involved in alcohol-induced neuroinflammation. In a mouse model of chronic alcohol exposure, 16 CER and 18 sphingomyelin (SM) concentrations from whole brain lipid extracts were measured using electrospray mass spectrometry. All 18 CER concentrations in alcohol exposed adults increased significantly (range: 25–607%); in juveniles, 6 CER decreased (range: −9 to −37%). In contrast, only three SM decreased in adult and one increased significantly in juvenile. Next, regional identification at 50 μm spatial resolution from coronal sections was obtained with matrix implanted laser desorption/ionization mass spectrometry imaging (MILDI-MSI) by implanting silver nanoparticulate matrices followed by focused laser desorption. Most of the CER and SM quantified in whole brain extracts were detected in MILDI images. Coronal sections from three brain levels show qualitative regional changes in CER-SM ion intensities, as a function of group and brain region, in cortex, striatum, accumbens, habenula, and hippocampus. Highly correlated changes in certain white matter CER-SM pairs occur in regions across all groups, including the hippocampus and the lateral (but not medial) cerebellar cortex of adult mice. Our data provide the first microscale MS evidence of regional lipid intensity variations induced by alcohol.

Hexosylceramides as intrathecal markers of worsening disability in multiple sclerosis

BACKGROUND

Sphingolipids are important components of neurons and the myelin sheath whose levels are altered in multiple sclerosis (MS).

OBJECTIVES

We aimed to determine if cerebrospinal fluid (CSF) sphingolipids can be used as markers of MS disease progression.

METHODS

Using liquid chromatography tandem mass spectrometry, we analysed sphingolipids in CSF from 134 individuals. The MS group included 65 patients divided into 41 relapsing-remitting MS (RRMS) and 24 progressive MS (ProgMS). In addition, a group of 13 early MS/clinically isolated syndrome (EarlyMS) and two control groups consisting of 38 individuals with other neurological diseases (OND) and 18 OND with signs of inflammation (iOND) were analysed. A follow-up study included 17 additional RRMS patients sampled at two time points 4.7±1.7 years apart.

RESULTS

Levels of sphingomyelin (SM)- and hexosylceramide (HexCer)-derived sphingolipids increased in the CSF of patients with MS independently of the fatty acid chain length in RRMS (p<0.05). Levels of palmitic acid (16:0)-containing HexCer (HexCer16:0) increased significantly in ProgMS compared with the OND (p<0.001), iOND (p<0.05) and EarlyMS (p<0.01) groups and correlated with Expanded Disability Status Scale in RRMS in both studies (p=0.048; p=0.027).

CONCLUSION

HexCer16:0 is a promising candidate marker of disease progression in MS, especially in RRMS.

Spinal cord injury causes chronic liver pathology in rats

Traumatic spinal cord injury (SCI) causes major disruption to peripheral organ innervation and regulation. Relatively little work has investigated these post-SCI systemic changes, however, despite considerable evidence that multiple organ system dysfunction contributes to chronic impairments in health. Because metabolic dysfunction is common after SCI and the liver is a pivotal site for metabolic homeostasis, we sought to determine if liver pathology occurs as a result of SCI in a rat spinal contusion model. Histologic evidence showed excess lipid accumulation in the liver for at least 21 days post-injury after cervical or midthoracic SCI. Lipidomic analysis revealed an acute increase in hepatic ceramides as well as chronically elevated lactosylceramide. Post-SCI hepatic changes also included increased proinflammatory gene expression, including interleukin (IL)-1α, IL-1β, chemokine ligand-2, and tumor necrosis factor-α mRNA. These were coincident with increased CD68+ macrophages in the liver through 21 days post-injury. Serum alanine transaminase, used clinically to detect liver damage, was significantly increased at 21 days post-injury, suggesting that early metabolic and inflammatory damage preceded overt liver pathology. Surprisingly, liver inflammation was even detected after lumbar SCI. Collectively, these results suggest that SCI produces chronic liver injury with symptoms strikingly similar to those of nonalcoholic steatohepatitis (fatty liver disease). These clinically significant hepatic changes after SCI are known to contribute to systemic inflammation, cardiovascular disease, and metabolic syndrome, all of which are more prevalent in persons with SCI. Targeting acute and prolonged hepatic pathology may improve recovery and reduce long-term complications after SCI.

Fingolimod for the treatment of neurological diseases-state of play and future perspectives

Sphingolipids are a fascinating class of signaling molecules derived from the membrane lipid sphingomyelin. They show abundant expression in the brain. Complex sphingolipids such as glycosphingolipids (gangliosides and cerebrosides) regulate vesicular transport and lysosomal degradation and their dysregulation can lead to storage diseases with a neurological phenotype. More recently, simple sphingolipids such ceramide, sphingosine and sphingosine 1-phosphate (S1P) were discovered to signal in response to many extracellular stimuli. Forming an intricate signaling network, the balance of these readily interchangeable mediators is decisive for cell fate under stressful conditions. The immunomodulator fingolimod is the prodrug of an S1P receptor agonist. Following receptor activation, the drug leads to downregulation of the S1P₁ receptor inducing functional antagonism. As the first drug to modulate the sphingolipid signaling pathway, it was marketed in 2010 for the treatment of multiple sclerosis (MS). At that time, immunomodulation was widely accepted as the key mechanism of fingolimod’s efficacy in MS. But given the excellent passage of this lipophilic compound into the brain and its massive brain accumulation as well as the abundant expression of S1P receptors on brain cells, it is conceivable that fingolimod also affects brain cells directly. Indeed, a seminal study showed that the protective effect of fingolimod in experimental autoimmune encephalitis (EAE), a murine MS model, is lost in mice lacking the S1P₁ receptor on astrocytes, arguing for a specific role of astrocytic S1P signaling in MS. In this review, we discuss the role of sphingolipid mediators and their metabolizing enzymes in neurologic diseases and putative therapeutic strategies arising thereof.

PGE2/EP4 signaling in peripheral immune cells promotes development of experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is a T cell-mediated inflammatory autoimmune disease model of multiple sclerosis (MS). The inflammatory process is initiated by activation and proliferation of T cells and monocytes and by their subsequent migration into the central nervous system (CNS), where they induce demyelination and neurodegeneration. Prostaglandin E2 (PGE2) - synthesized by cyclooxygenase 2 (COX-2) - has both pro- and anti-inflammatory potential, which is translated via four different EP receptors. We hypothesized that PGE2 synthesized in the preclinical phase by peripheral immune cells exerts pro-inflammatory properties in the EAE model. To investigate this, we used a bone marrow transplantation model, which enables PGE2 synthesis or EP receptor expression to be blocked specifically in peripheral murine immune cells. Our results reveal that deletion of COX-2 or its EP4 receptor in bone marrow-derived cells leads to a significant delay in the onset of EAE. This effect is due to an impaired preclinical inflammatory process indicated by a reduced level of the T cell activating interleukin-6 (IL-6), reduced numbers of T cells and of the T cell secreted interleukin-17 (IL-17) in the blood of mice lacking COX-2 or EP4 in peripheral immune cells. Moreover, mice lacking COX-2 or EP4 in bone marrow-derived cells show a reduced expression of matrix metalloproteinase 9 (MMP9), which results in decreased infiltration of monocytes and T cells into the CNS. In conclusion, our data demonstrate that PGE2 synthesized by monocytes in the early preclinical phase promotes the development of EAE in an EP4 receptor dependent manner.

Ceramide synthases as potential targets for therapeutic intervention in human diseases

Ceramide is located at a key hub in the sphingolipid metabolic pathway and also acts as an important cellular signaling molecule. Ceramide contains one acyl chain which is attached to a sphingoid long chain base via an amide bond, with the acyl chain varying in length and degree of saturation. The identification of a family of six mammalian ceramide synthases (CerS) that synthesize ceramide with distinct acyl chains, has led to significant advances in our understanding of ceramide biology, including further delineation of the role of ceramide in various pathophysiologies in both mice and humans. Since ceramides, and the complex sphingolipids generated from ceramide, are implicated in disease, the CerS might potentially be novel targets for therapeutic intervention in the diseases in which the ceramide acyl chain length is altered. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Ceramide metabolism in mouse tissue

Ceramides with different N-acyl chains can act as second messengers in various signaling pathways. They are involved in cell processes such as apoptosis, differentiation and inflammation. Ceramide synthases (CerS) are key enzymes in the biosynthesis of ceramides and dihydroceramides. Six isoenzymes (CerS1-6) catalyze the N-acylation of the sphingoid bases, albeit with strictly acyl-Coenzyme A (CoA) chain length specificity. We analyzed the mRNA expression, the protein expression, the specific activity of the CerS, and acyl-CoA, dihydroceramide and ceramide levels in different tissues by LC-MS/MS. Our data indicate that each tissue express a distinct composition of CerS, whereby the CerS mRNA expression levels do not correlate with the respective protein expression levels in the tissues. Furthermore, we found a highly significant negative correlation between the protein expression level of CerS6 and the C16:0-acyl-CoA amounts as well as between the protein expression of CerS2 and C24:0-acyl-CoA amounts. These data indicate that in mouse tissues low substrate availability is compensated by higher CerS protein expression level and vice versa. Apart from the expression level and the specific activity of the CerS, other enzymes of the sphingolipid pathway also influence the composition of ceramides with distinct chain lengths in each cell. Acyl-CoA availability seems to be less important for ceramide composition and might be compensated for by CerS expression/activity.

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 β-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.

Mechanistic perspective on the relationship between pyridoxal 5'-phosphate and inflammation

A variety of inflammatory disease conditions have been found to be associated with low levels of plasma pyridoxal 5'-phosphate (PLP), the active form of vitamin B6 , without any indication of a lower dietary intake of vitamin B6 , excessive catabolism of the vitamin, or congenital defects in its metabolism. The present review was conducted to examine the existing literature in this regard. Current evidence suggests that the inverse association between plasma PLP and inflammation may be the result of mobilization of this coenzyme to the site of inflammation, for use by the PLP-dependent enzymes of the kynurenine pathway of tryptophan degradation, metabolism of the immunomodulatory sphingolipids, ceramide and sphingosine 1-phosphate, and for serine hydroxymethylase for immune cell proliferation.