Analysis of phospho- and phosphonosphingolipids by high-performance liquid chromatography

A simple and efficient method for the separation of phosphosphingolipids including phosphonosphingolipids by high-performance liquid chromatography is described. A mixture of authentic lipids consisting of sphingomyelin, ceramide phosphorylethanolamine, ceramide 2-aminoethylphosphonate, and ceramide N-methylaminoethylphosphonate was completely separated using a silica gel (Zorbax SIL) column with acetonitrile-methanol-water 72:40:10 (v/v) as eluting solvent. The elution of these sphingolipids was monitored directly with an ultraviolet spectromonitor at 207 nm. The practical limit of detection of each sphingolipid was about 0.2 microgram or 0.3 nmol. Using this method, we found that from one to four different phosphono- and/or phosphosphingolipids in fresh-water shellfish can be routinely identified and reproducibly quantified.     

Ablation of Ceramide Synthase 2 Causes Chronic Oxidative Stress Due to Disruption of the Mitochondrial Respiratory Chain

Ceramide is a key intermediate in the pathway of sphingolipid biosynthesis and is an important intracellular messenger. We recently generated a ceramide synthase 2 (CerS2) null mouse that cannot synthesize very long acyl chain (C22-C24) ceramides. This mouse displays severe and progressive hepatopathy. Significant changes were observed in the sphingolipid profile of CerS2 null mouse liver, including elevated C16-ceramide and sphinganine levels in liver and in isolated mitochondrial fractions. Because ceramide may be involved in reactive oxygen species (ROS) formation, we examined whether ROS generation was affected in CerS2 null mice. Levels of a number of anti-oxidant enzymes were elevated, as were lipid peroxidation, protein nitrosylation, and ROS. ROS were generated from mitochondria due to impaired complex IV activity. C16-ceramide, sphingosine, and sphinganine directly inhibited complex IV activity in isolated mitochondria and in mitoplasts, whereas other ceramide species, sphingomyelin, and diacylglycerol were without effect. A fluorescent analog of sphinganine accumulated in mitochondria. Heart mitochondria did not display a substantial alteration in the sphingolipid profile or in complex IV activity. We suggest that C16-ceramide and/or sphinganine induce ROS formation through the modulation of mitochondrial complex IV activity, resulting in chronic oxidative stress. These results are of relevance for understanding modulation of ROS signaling by sphingolipids.     

Palmitate increases sphingosine-1-phosphate in C2C12 myotubes via upregulation of sphingosine kinase message and activity

Studies in skeletal muscle demonstrate that elevation of plasma FFAs increases the sphingolipid ceramide. We aimed to determine the impact of FFA oversupply on total sphingolipid profiles in a skeletal muscle model. C2C12 myotubes were treated with palmitate (PAL). Lipidomics analysis revealed pleiotropic effects of PAL on cell sphingolipids not limited to ceramides. ¹³C labeling demonstrated that PAL activated several branches of sphingolipid synthesis by distinct mechanisms. Intriguingly, PAL increased sphingosine-1-phosphate independently of de novo synthesis. Quantitative real-time PCR demonstrated that PAL increased sphingosine kinase 1 (SK1) mRNA by approximately 4-fold. This was accompanied by a 2.3-fold increase in sphingosine kinase enzyme activity. This upregulation did not occur upon treatment with oleate, suggesting some level of specificity for PAL. These findings were recapitulated in the diet-induced obesity mouse model, in which high-fat feeding increased SK1 message in skeletal muscle over 2.3-fold. These data suggest that the impact of elevated FFA on sphingolipids reaches beyond ceramides and de novo sphingolipid synthesis. Moreover, these findings identify PAL as a novel regulatory stimulus for SK1.     

Effects of Bacteroides Phosphosphingolipids on Murine Neutrophils

Bacteroides and related species formerly classified in the genus Bacteroides have phosphosphingolipids as their membrane lipid. To evaluate the possible involvement of bacterial sphingolipids in the infection process, the effects of Bacteroides sphingolipids on murine neutrophils were examined in this study. Observation by differential interference contrast microscopy showed morphological changes in murine neutrophils in the presence of the sphingolipids, indicating biological activity by the Bacteroides sphingolipids. The lipids dose-dependently inhibited superoxide release from the neutrophils stimulated with phorbol myristate acetate. This observation suggests that bactericidal activity of the neutrophils may be affected by Bacteroides sphingolipids. Taken together, these findings provide evidence that Bacteroides sphingolipids may contribute to the pathogenesis of mixed infections by direct inhibition of neutrophil function.     

Cathepsin D is the main lysosomal enzyme involved in the degradation of alpha-synuclein and generation of its carboxy-terminally truncated species

Alpha-synuclein is likely to play a key role in the development of Parkinson's disease as well as other synucleinopathies. In animal models, overexpression of full-length or carboxy-terminally truncated alpha-synuclein has been shown to produce pathology. Although the proteosome and lysosome have been proposed to play a role in the degradation of alpha-synuclein, the enzyme(s) involved in alpha-synuclein clearance and generation of its carboxy-terminally truncated species have not been identified. In this study, the role of cathepsin D and calpain I in these processes was analyzed. In vitro experiments, using either recombinant or endogenous alpha-synuclein as substrates and purified cathepsin D or lysosomes, demonstrated that cathepsin D degraded alpha-synuclein very efficiently, and that limited proteolysis resulted in the generation of carboxy-terminally truncated species. Purified calpain I also cleaved alpha-synuclein, but carboxy-terminally truncated species were not the main cleavage products, and calpain I activity present in cellular lysates was not able to degrade the protein. Knockdown of cathepsin D in cells overexpressing wild-type alpha-synuclein increased total alpha-synuclein levels by 28% and lysosomal alpha-synuclein by 2-fold. In in vitro experiments, pepstatin A completely blocked the degradation of alpha-synuclein in purified lysosomes. Furthermore, lysosomes isolated from cathepsin D knockdown cells showed a marked reduction in alpha-synuclein degrading activity, indicating that cathepsin D is the main lysosomal enzyme involved in alpha-synuclein degradation. Our findings suggest that upregulation of cathepsin D could be an additional therapeutic strategy to lessen alpha-synuclein burden in synucleinopathies.     

Entamoeba invadens: Sphingolipids metabolic regulation is the main component of a PKC signaling pathway in controlling cell growth and proliferation

The sphingolipids biosynthesis pathway generates bioactive molecules crucial to the regulation of physiological processes. We have recently reported that DAG (diacylglycerol) generated during sphingomyelin synthesis, plays an important role in PKC (protein kinase C) activation, necessary for the transit through the cell cycle (G1 to S transition) and cell proliferation (Cerbon and Lopez-Sanchez, 2003. Diacylglycerol generated during sphingomyelin synthesis is involved in protein kinase C activation and cell proliferation in Madin-Darby canine kidney cells. Biochem. J. 373, 917-924). Since pathogenic Entamoeba invadens synthesize the sphingolipids inositol-phosphate ceramide (IPC) and ethanolamine-phosphate ceramide (EPC) as well as sphingomyelin (SM), we decided to investigate when during growth initiation, the synthesis of sphingolipids takes place, DAG is generated and PKC is activated. We found that during the first 6 h of incubation there was a significant increase in the synthesis of all three sphingolipids, accompanied by a progressive increment (up to 4-fold) in the level of DAG, and particulate PKC activity was increased 4-8 times. The enhanced DAG levels coincided with decrements in the levels of sphingoid bases, conditions adequate for the activation of PKC. Moreover, we found that inhibition of sphingolipid synthesis with myriocin, specific inhibitor of the synthesis of sphinganine, reduce DAG generation, PKC activation and cell proliferation. All these inhibitory processes were restored by metabolic complementation with exogenous d-erythrosphingosine, indicating that the DAG generated during sphingolipid synthesis was necessary for PKC activation and cell proliferation. Also, we show that PI (phosphatidylinositol), PE (phosphatidylethanolamine) and PC (phosphatidylcholine) are the precursors of their respective sphingolipids (IPC, EPC and SM), and therefore sources of DAG to activate PKC.     

Sphingolipids in Neuroblastoma: Their Role in Drug Resistance Mechanisms

Disseminated neuroblastoma usually calls for chemotherapy as the primary approach for treatment. Treatment failure is often attributable to drug resistance. This involves a variety of cellular mechanisms, including increased drug efflux through expression of ATP-binding cassette transporters (e.g., P-glycoprotein) and the inability of tumor cells to activate or propagate the apoptotic response. In recent years it has become apparent that sphingolipid metabolism and the generation of sphingolipid species, such as ceramide, also play a role in drug resistance. This may involve an autonomous mechanism, related to direct effects of sphingolipids on the apoptotic response, but also a subtle interplay between sphingolipids and ATP-binding cassette transporters. Here, we present an overview of the current understanding of the multiple levels at which sphingolipids function in drug resistance, with an emphasis on sphingolipid function in neuroblastoma and how modulation of sphingolipid metabolism may be used as a novel treatment paradigm.     

Regulation of tumor phenotypes by caveolin-1 and sphingolipid-controlled membrane signaling complexes

Aberrant (glyco)sphingolipid expression deeply affects several properties of tumor cells that are involved in tumor progression and metastasis formation: cell adhesion (to the extracellular matrix or to the endothelium of blood vessels), motility, recognition and invasion of host tissues. In particular, (glyco)sphingolipids might contribute to the modulation of integrin-dependent interactions of tumor cells (determining their adhesion, motility and invasiveness) with the extracellular matrix as well as with host cells present in the stromal compartment of the tumor. A model based on solid experimental evidence has been proposed: (glyco)sphingolipids at the cell surface interact with plasma membrane receptors (e.g., integrin receptors and growth factor receptors) and adapter molecules (including tetraspanins) forming signaling complexes that are able to influence the activity of signal transduction molecules oriented at the cytosolic surface of the plasma membrane (mainly the Src kinases pathway members). The function of these signaling complexes appears to be strictly dependent on their (glyco)sphingolipid composition, and likely on specific sphingolipid-protein interactions. From this point of view, particularly intriguing is the connection between (glyco)sphingolipids and caveolin-1, a membrane protein that plays multiple roles as a suppressor of tumor growth and metastasis in ovarian, breast and colon human carcinomas.     

Identification of ISC1 (YER019w) as inositol phosphosphingolipid phospholipase C in Saccharomyces cerevisiae

Sphingolipids have emerged as novel bioactive mediators in eukaryotic cells including yeast. It has been proposed that sphingomyelin (SM) hydrolysis and the concomitant generation of ceramide are involved in various stress responses in mammalian cells. The yeast Saccharomyces cerevisiae has inositol phosphosphingolipids (IPS) instead of SM and glycolipids, and synthesis of IPS is indispensable to its growth. Although the genes responsible for the synthesis of IPS have been identified, the gene(s) for the degradation of IPS has not been reported. Here we show that ISC1 (YER019w), which has homology to bacterial neutral sphingomyelinase (SMase), encodes IPS phospholipase C (IPS-PLC). First, we observed that overexpression of ISC1 greatly increased neutral SMase activity, and this activity was dependent on the presence of phosphatidylserine. Cells deleted in ISC1 demonstrated negligible neutral SMase activity. Because yeast cells have IPS instead of SM, we investigated whether IPS are the physiologic substrates of this enzyme. Lysates of ISC1-overexpressing cells demonstrated very high PLC activities on IPS. Deletion of ISC1 eliminated endogenous IPS-PLC activities. Labeling yeast cells with [(3)H]dihydrosphingosine showed that IPS were increased in the deletion mutant cells. This study identifies the first enzyme involved in catabolism of complex sphingolipids in S. cerevisiae.     

Separation and identification of major plant sphingolipid classes from leaves

Sphingolipids are major components of the plasma membrane, tonoplast, and other endomembranes of plant cells. Previous compositional analyses have focused only on individual sphingolipid classes because of the widely differing polarities of plant sphingolipids. Consequently, the total content of sphingolipid classes in plants has yet to be quantified. In addition, the major polar sphingolipid class in the model plant Arabidopsis thaliana has not been previously determined. In this report, we describe the separation and quantification of sphingolipid classes from A. thaliana leaves using hydrolysis of sphingolipids and high performance liquid chromatography (HPLC) analysis of o-phthaldialdehyde derivatives of the released long-chain bases to monitor the separation steps. An extraction solvent that contained substantial proportions of water was used to solubilized >95% of the sphingolipids from leaves. Neutral and charged sphingolipids were then partitioned by anion exchange solid phase extraction. HPLC analysis of the charged lipid fraction from A. thaliana revealed only one major anionic sphingolipid class, which was identified by mass spectrometry as hexose-hexuronic-inositolphosphoceramide. The neutral sphingolipids were predominantly composed of monohexosylceramide with lesser amounts of ceramides. Extraction and separation of sphingolipids from soybean and tomato showed that, like A. thaliana, the neutral sphingolipids consisted of ceramide and monohexosylceramides; however, the major polar sphingolipid was found to be N-acetyl-hexosamine-hexuronic-inositolphosphoceramide. In extracts from A. thaliana leaves, hexosehexuronic-inositolphosphoceramides, monohexosylceramides, and ceramides accounted for approximately 64, 34, and 2% of the total sphingolipids, respectively, suggesting an important role for the anionic sphingolipids in plant membranes.     

Mannose 6-phosphate-independent membrane association of cathepsin D, glucocerebrosidase, and sphingolipid-activating protein in HepG2 cells.

The membrane association of the lysosomal enzymes cathepsin D and glucocerebrosidase and its naturally occurring sphingolipid activating protein was studied in HepG2 cells. We differentially permeabilized cells with low concentrations of saponin, at which secretory proteins rinsed out completely, whereas integral membrane proteins were not released. All relevant intracellular compartments were shown to be permeabilized by saponin. Metabolic labeling showed that early precursors of cathepsin D, sphingolipid activating protein, and glucocerebrosidase were completely released from the cells, whereas more than 80% of the high molecular mass intermediates were retained by the cells. Treatment of permeabilized cells with 10 mM mannose 6-phosphate released only 50% of the cell-associated cathepsin D. Glucocerebrosidase remained membrane-associated, but cathepsin D and sphingolipid activating protein were released from the cells after proteolytic processing. Sphingolipid activating proteins and cathepsin D behaved similarly during biosynthesis and showed similar sensitivity to mannose 6-phosphate. The membrane association of the intermediate form of cathepsin D was independent of the presence of N-linked oligosaccharides. Subcellular fractionation on sucrose gradients showed that the lysosomal proteins became membrane-associated probably in the Golgi complex, and that both mannose 6-phosphate-dependent and mannose 6-phosphate-independent membrane association occur in the same compartments. We conclude that, in HepG2 cells, cathepsin D, sphingolipid activating protein, and glucocerebrosidase exhibit MPR-independent membrane association which is acquired in the same compartments beyond the rough endoplasmic reticulum.     

Molecular facets of sphingolipids: Mediators of diseases

Sphingolipids constitute a biologically active lipid class that is significantly important from both structural and regulatory aspects. The manipulation of sphingolipid metabolism is currently being studied as a novel strategy for cancer therapy. The basics of this therapeutic approach lie in the regulation property of sphingolipids on cellular processes, which are important in a cell's fate, such as cell proliferation, apoptosis, cell cycle arrest, senescence, and inflammation. Furthermore, the mutations in the enzymes catalyzing some specific reactions in the sphingolipid metabolism cause mortal lysosomal storage diseases like Fabry, Gaucher, Niemann-Pick, Farber, Krabbe, and Metachromatic Leukodystrophy. Therefore, the alteration of the sphingolipid metabolic pathway determines the choice between life and death. Understanding the sphingolipid metabolism and regulation is significant for the development of new therapeutic approaches for all sphingolipid-related diseases, as well as for cancer. An important feature of the sphingolipid metabolic pathway is the compartmentalization into endoplasmic reticulum, the Golgi apparatus, lysosome and plasma membrane, and this compartmentalization makes the transport of sphingolipids critical for proper functioning. This paper focuses on the structures, metabolic pathways, localization, transport mechanisms, and diseases of sphingolipids in Saccharomyces cerevisiae and humans, and provides the latest comprehensive information on sphingolipid research.     

Sli2 (Ypk1), a homologue of mammalian protein kinase SGK, is a downstream kinase in the sphingolipid-mediated signaling pathway of yeast

ISP-1 is a new type of immunosuppressant, the structure of which is homologous to that of sphingosine. In a previous study, ISP-1 was found to inhibit mammalian serine palmitoyltransferase, the primary enzyme involved in sphingolipid biosynthesis, and to reduce the intracellular pool of sphingolipids. ISP-1 induces the apoptosis of cytotoxic T cells, which is triggered by decreases in the intracellular levels of sphingolipids. In this study, the inhibition of yeast (Saccharomyces cerevisiae) proliferation by ISP-1 was observed. This ISP-1-induced growth inhibition was also triggered by decreases in the intracellular levels of sphingolipids. In addition, DNA duplication without cytokinesis was detected in ISP-1-treated yeast cells on flow cytometry analysis. We have cloned multicopy suppressor genes of yeast which overcome the lethal sphingolipid depletion induced by ISP-1. One of these genes, SLI2, is synonymous with YPK1, which encodes a serine/threonine kinase. Kinase-dead mutants of YPK1 did not show any resistance to ISP-1, leading us to predict that the kinase activity of the Ypk1 protein should be essential for this resistance to ISP-1. Ypk1 protein overexpression had no effect on sphingolipid biosynthesis by the yeast. Furthermore, both the phosphorylation and intracellular localization of the Ypk1 protein were regulated by the intracellular sphingolipid levels. These data suggest that the Ypk1 protein is a downstream kinase in the sphingolipid-mediated signaling pathway of yeast. The Ypk1 protein was reported to be a functional homologue of the mammalian protein kinase SGK, which is a downstream kinase of 3-phosphoinositide-dependent kinase 1 (PDK1). PDK1 phosphotidylinositol (PI) is regulated by PI-3,4,5-triphosphate and PI-3,4-bisphosphate through the pleckstrin homology (PH) domain. Overexpression of mammalian SGK also overcomes the sphingolipid depletion in yeast. Taking both the inability to produce PI-3,4, 5-triphosphate and PI-3,4-bisphosphate and the lack of a PH domain in the yeast homologue of PDK1, the Pkh1 protein, into account, these findings further suggest that yeast may use sphingolipids instead of inositol phospholipids as lipid mediators.     

Cancer and sphingolipid storage disease therapy using novel synthetic analogs of sphingolipids

Sphingolipid metabolites have become recognized for their participation in cell functions and signaling events that control a wide array of cellular activities. Two main sphingolipids, ceramide and sphingosine-1-phosphate, are involved in signaling pathways that regulate cell proliferation, apoptosis, motility, differentiation, angiogenesis, stress responses, protein synthesis, carbohydrate metabolism, and intracellular trafficking. Ceramide and S1P often exert opposing effects on cell survival, ceramide being pro-apoptotic and S1P generally promoting cell survival. Therefore, the conversion of one of these metabolites to the other by sphingolipid enzymes provides a vast network of regulation and provides a useful therapeutic target. Here we provide a survey of the current knowledge of the roles of sphingolipid metabolites in cancer and in lipid storage disease. We review our attempts to interfere with this network of regulation and so provide new treatments for a range of diseases. We synthesized novel analogs of sphingolipids which inhibit the hydrolysis of ceramide or its conversion to more complex sphingolipids. These analogs caused elevation of ceramide levels, leading to apoptosis of a variety of cancer cells. Administration of a synthetic analog to tumor-bearing mice resulted in reduction and even disappearance of the tumors. Therapies for sphingolipid storage diseases, such as Niemann-Pick and Gaucher diseases were achieved by two different strategies: inhibition of the biosynthesis of the substrate (substrate reduction therapy) and protection of the mutated enzyme (chaperone therapy). Sphingolipid metabolism was monitored by the use of novel fluorescent sphingolipid analogs. The results described in this review indicate that our synthetic analogs could be developed both as anticancer drugs and for the treatment of sphingolipid storage diseases.     

Non-specific phospholipase C4 hydrolyzes phosphosphingolipids and phosphoglycerolipids and promotes rapeseed growth and yield

Phosphorus is a major nutrient vital for plant growth and development, with a substantial amount of cellular phosphorus being used for the biosynthesis of membrane phospholipids. Here, we report that NON-SPECIFIC PHOSPHOLIPASE C4 (NPC4) in rapeseed (Brassica napus) releases phosphate from phospholipids to promote growth and seed yield, as plants with altered NPC4 levels showed significant changes in seed production under different phosphate conditions. Clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9)-mediated knockout of BnaNPC4 led to elevated accumulation of phospholipids and decreased growth, whereas overexpression (OE) of BnaNPC4 resulted in lower phospholipid contents and increased plant growth and seed production. We demonstrate that BnaNPC4 hydrolyzes phosphosphingolipids and phosphoglycerolipids in vitro, and plants with altered BnaNPC4 function displayed changes in their sphingolipid and glycerolipid contents in roots, with a greater change in glycerolipids than sphingolipids in leaves, particularly under phosphate deficiency conditions. In addition, BnaNPC4-OE plants led to the upregulation of genes involved in lipid metabolism, phosphate release, and phosphate transport and an increase in free inorganic phosphate in leaves. These results indicate that BnaNPC4 hydrolyzes phosphosphingolipids and phosphoglycerolipids in rapeseed to enhance phosphate release from membrane phospholipids and promote growth and seed production.     

Regulation of epidermal sphingolipid synthesis by permeability barrier function

A mixture of sphingolipids, cholesterol, and free fatty acids forms the intercellular membrane bilayers of the stratum corneum which are presumed to regulate epidermal barrier function. Prior studies have shown that both cholesterol and fatty acid synthesis are rapidly regulated by epidermal barrier requirements. In contrast, the importance of sphingolipids in barrier function has not been directly demonstrated. Here, we have assessed both sphingolipid synthesis by [3H]H2O incorporation and serine palmitoyl transferase (SPT) activity in relation to modulations in barrier function. Incorporation of [3H]H2O into sphingolipids increased after barrier disruption with acetone, with maximal increase (170%) occurring 5-7 h after treatment (P less than 0.005). As barrier function returned to normal over 24 h, incorporation of tritium into sphingolipids normalized. SPT activity also increased after barrier disruption, peaking at 6 h (150%) (P less than 0.05), and returning towards normal by 24 h. Artificial restoration of the barrier with a water vapor-impermeable membrane prevented the increases in both [3H]H2O incorporation into sphingolipids and enzyme activity. Finally, SPT activity was increased in two other models of barrier dysfunction, cellophane tape-stripping and essential fatty acid deficiency. Occlusion normalized SPT activity in both of these models as well. These studies: a) demonstrate a distinctive, delayed increase in epidermal sphingolipid synthesis in response to barrier requirements that contrasts with the immediate responses of cholesterol and fatty acid synthesis; and b) suggest that sphingolipids are important for the maintenance of the epidermal permeability barrier.     

Actin Filaments and Microtubules are Involved in Different Membrane Traffic Pathways That Transport Sphingolipids to the Apical Surface of Polarized HepG2 Cells

In polarized HepG2 hepatoma cells, sphingolipids are transported to the apical, bile canalicular membrane by two different transport routes, as revealed with fluorescently tagged sphingolipid analogs. One route involves direct, transcytosis-independent transport of Golgi-derived glucosylceramide and sphingomyelin, whereas the other involves basolateral to apical transcytosis of both sphingolipids. We show that these distinct routes display a different sensitivity toward nocodazole and cytochalasin D, implying a specific transport dependence on either microtubules or actin filaments, respectively. Thus, nocodazole strongly inhibited the direct route, whereas sphingolipid transport by transcytosis was hardly affected. Moreover, nocodazole blocked “hyperpolarization,” i.e., the enlargement of the apical membrane surface, which is induced by treating cells with dibutyryl-cAMP. By contrast, the transcytotic route but not the direct route was inhibited by cytochalasin D. The actin-dependent step during transcytotic lipid transport probably occurs at an early endocytic event at the basolateral plasma membrane, because total lipid uptake and fluid phase endocytosis of horseradish peroxidase from this membrane were inhibited by cytochalasin D as well. In summary, the results show that the two sphingolipid transport pathways to the apical membrane must have a different requirement for cytoskeletal elements.     

A post-genomic approach to understanding sphingolipid metabolism in Arabidopsis thaliana

AIMS: To highlight the importance of sphingolipids and their metabolites in plant biology. SCOPE: The completion of the arabidopsis genome provides a platform for the identification and functional characterization of genes involved in sphingolipid biosynthesis. Using the yeast Saccharomyces cerevisiae as an experimental model, this review annotates arabidopsis open reading frames likely to be involved in sphingolipid metabolism. A number of these open reading frames have already been subject to functional characterization, though the majority still awaits investigation. Plant-specific aspects of sphingolipid biology (such as enhanced long chain base heterogeneity) are considered in the context of the emerging roles for these lipids in plant form and function. CONCLUSIONS: Arabidopsis provides an excellent genetic and post-genomic model for the characterization of the roles of sphingolipids in higher plants.     

Sphingolipid homeostasis in the web of metabolic routes

Sphingolipids play a key role in cells as structural components of membrane lipid bilayers and signaling molecules implicated in important physiological and pathological processes. Their metabolism is tightly regulated. Mechanisms controlling sphingolipid metabolism are far from being completely understood. However, they already reveal the integration of sphingolipids in the whole metabolic network as signaling devices that coordinate different metabolic pathways. A picture of sphingolipids integrated into metabolic networks might help to understand sphingolipid homeostasis. This review describes recent advances in the regulation of de novo sphingolipid synthesis with a focus on the bridges that exist with other metabolic pathways and the importance of this crosstalk in the control of sphingolipid homeostasis. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.