Encephalopathy caused by ablation of very long acyl chain ceramide synthesis may be largely due to reduced galactosylceramide levels

Direct tandem mass spectrometric profiling of sulfatides in dry urinary samples for screening of metachromatic leukodystrophy

BACKGROUND

Prediagnostic steps in suspected metachromatic leukodystrophy (MLD) rely on clinical chemical methods other than enzyme assays. We report a new diagnostic method which evaluates changes in the spectrum of molecular types of sulfatides (3-O-sulfogalactosyl ceramides) in MLD urine.

METHODS

The procedure allows isolation of urinary sulfatides by solid-phase extraction on DEAE-cellulose membranes, transportation of a dry membrane followed by elution and tandem mass spectrometry (MS/MS) analysis in the clinical laboratory. Major sulfatide isoforms are normalized to the least variable component of the spectrum, which is the indigenous C18:0 isoform. This procedure does not require the use of specific internal standards and minimizes errors caused by sample preparation and measurement.

RESULTS

Urinary sulfatides were analyzed in a set of 21 samples from patients affected by sulfatidosis. The combined abundance of the five most elevated isoforms, C22:0, C22:0-OH, C24:0, C24:1-OH, and C24:0-OH sulfatides, was found to give the greatest distinction between MLD-affected patients and a control group.

CONCLUSIONS

The method avoids transportation of liquid urine samples and generates stable membrane-bound sulfatide samples that can be stored at ambient temperature. MS/MS sulfatide profiling targeted on the most MLD-representative isoforms is simple with robust results and is suitable for screening.

Diverse functions of ceramide in cancer cell death and proliferation

Ceramide, a bioactive sphingolipid, is now at the forefront of cancer research. Classically, ceramide is thought to induce death, growth inhibition, and senescence in cancer cells. However, it is now clear that this simple picture of ceramide no longer holds true. Recent studies suggest that there are diverse functions of endogenously generated ceramides, which seem to be context dependent, regulated by subcellular/membrane localization and presence/absence of direct targets of these lipid molecules. For example, different fatty-acid chain lengths of ceramide, such as C(16)-ceramide that can be generated by ceramide synthase 6 (CerS6), have been implicated in cancer cell proliferation, whereas CerS1-generated C(18)-ceramide mediates cell death. The dichotomy of ceramides' function in cancer cells makes some of the metabolic enzymes of ceramide synthesis potential drug targets (such as Cers6) to prevent cancer growth in breast and head and neck cancers. Conversely, activation of CerS1 could be a new therapeutic option for the development of novel strategies against lung and head and neck cancers. This chapter focuses on recent discoveries about the mechanistic details of mainly de novo-generated ceramides and their signaling functions in cancer pathogenesis, and about how these mechanistic information can be translated into clinically relevant therapeutic options for the treatment of cancer.

Ceramide synthases expression and role of ceramide synthase-2 in the lung: insight from human lung cells and mouse models

Increases in ceramide levels have been implicated in the pathogenesis of both acute or chronic lung injury models. However, the role of individual ceramide species, or of the enzymes that are responsible for their synthesis, in lung health and disease has not been clarified. We now show that C24- and C16-ceramides are the most abundant lung ceramide species, paralleled by high expression of their synthetic enzymes, ceramide synthase 2 (CerS2) and CerS5, respectively. Furthermore, the ceramide species synthesis in the lung is homeostatically regulated, since mice lacking very long acyl chain C24-ceramides due to genetic deficiency of CerS2 displayed a ten-fold increase in C16-ceramides and C16-dihydroceramides along with elevation of acid sphingomyelinase and CerS5 activities. Despite relatively preserved total lung ceramide levels, inhibition of de novo sphingolipid synthesis at the level of CerS2 was associated with significant airflow obstruction, airway inflammation, and increased lung volumes. Our results suggest that ceramide species homeostasis is crucial for lung health and that CerS2 dysfunction may predispose to inflammatory airway and airspace diseases.

Sustained activation of sphingomyelin synthase by 2-hydroxyoleic acid induces sphingolipidosis in tumor cells

The mechanism of action of 2-hydroxyoleic acid (2OHOA), a potent antitumor drug, involves the rapid and specific activation of sphingomyelin synthase (SMS), leading to a 4-fold increase in SM mass in tumor cells. In the present study, we investigated the source of the ceramides required to sustain this dramatic increase in SM. Through radioactive and fluorescent labeling, we demonstrated that sphingolipid metabolism was altered by a 24 h exposure to 2OHOA, and we observed a consistent increase in the number of lysosomes and the presence of unidentified storage materials in treated cells. Mass spectroscopy revealed that different sphingolipid classes accumulated in human glioma U118 cells after exposure to 2OHOA, demonstrating a specific effect on C16-, C20-, and C22-containing sphingolipids. Based on these findings, we propose that the demand for ceramides required to sustain the SMS activation (ca. 200-fold higher than the basal level) profoundly modifies both sphingolipid and phospholipid metabolism. As the treatment is prolonged, tumor cells fail to adequately metabolize sphingolipids, leading to a situation resembling sphingolipidosis, whereby cell viability is compromised.

The complexity of sphingolipid biosynthesis in the endoplasmic reticulum

Unlike the synthesis of other membrane lipids, sphingolipid synthesis is compartmentalized between the endoplasmic reticulum and the Golgi apparatus. The initial steps of sphingolipid synthesis, from the activity of serine palmitoyltransferase through to dihydroceramide desaturase, take place in the endoplasmic reticulum, but the further metabolism of ceramide to sphingomyelin and complex glycosphingolipids takes place mostly in the Golgi apparatus. Studies over the last decade or so have revealed unexpected levels of complexity in the sphingolipid biosynthetic pathway, mainly due to either the promiscuity of some enzymes towards their substrates, or the tight selectivity of others towards specific substrates. We now discuss two enzymes in this pathway, namely serine palmitoyltransferase (SPT) and ceramide synthase (CerS), and one lipid transport protein, CERT. For SPT and CERT, significant structural information is available, and for CerS, significant information has recently been obtained that sheds light of the roles of the specific ceramide species that are produced by each of the CerS. We consider the mechanisms by which specificity is generated and speculate on the reasons that sphingolipid biosynthesis is so complex. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

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.

Ceramide synthases: reexamining longevity

The ceramide synthase (CerS) enzymes catalyze the formation of (dihydro) ceramide, and thereby provide critical complexity to all sphingolipids (SLs) with respect to their acyl chain length. This review summarizes the progress in the field of CerS from the time of their discovery more than a decade ago as Longevity assurance (Lass) genes in yeast, until the recent development of CerS-deficient mouse models. Human hereditary CerS disorders are yet to be discovered. However, the recent findings in CerS mutant animals highlight the important physiological role of these enzymes. The fundamental findings with respect to CerS structure, function, localization, and regulation are discussed, as well as CerS roles in maintaining longevity in vivo.

Genetic disorders of simple sphingolipid metabolism

A better understanding of the functions sphingolipids play in living organisms can be achieved by analyzing the biochemical and physiological changes that result from genetic alterations of sphingolipid metabolism. This review summarizes the current knowledge gained from studies both on human patients and mutant animals (mice, cats, dogs, and cattle) with genetic disorders of sphingolipid metabolism. Genetic alterations affecting the biosynthesis, transport, or degradation of simple sphingolipids are discussed.

A fluorescent assay for ceramide synthase activity

The sphingolipids are a diverse family of lipids with important roles in membrane compartmentalization, intracellular signaling, and cell-cell recognition. The central sphingolipid metabolite is ceramide, formed by the transfer of a variable length fatty acid from coenzyme A to a sphingoid base, generally sphingosine or dihydrosphingosine (sphinganine) in mammals. This reaction is catalyzed by a family of six ceramide synthases (CerS1-6). CerS activity is usually assayed using either radioactive substrates or LC-MS/MS. We describe a CerS assay with fluorescent, NBD-labeled sphinganine as substrate. The assay is readily able to detect endogenous CerS activity when using amounts of cell or tissue homogenate protein that are lower than those reported for the radioactive assay, and the Michaelis-Menten constant was essentially the same for NBD-sphinganine and unlabeled sphinganine, indicating that NBD-sphinganine is a good substrate for these enzymes. Using our assay, we confirm that the new clinical immunosuppressant FTY720 is a competitive inhibitor of CerS activity, and show that inhibition requires the compound's lipid tail and amine headgroup. In summary, we describe a fluorescent assay for CerS activity that circumvents the need to use radioactive substrates, while being more accessible and cheaper than LC-MS based assays.

Modulation of ceramide synthase activity via dimerization

Ceramide, the backbone of all sphingolipids, is synthesized by a family of ceramide synthases (CerS) that each use acyl-CoAs of defined chain length for N-acylation of the sphingoid long chain base. CerS mRNA expression and enzymatic activity do not always correlate with the sphingolipid acyl chain composition of a particular tissue, suggesting post-translational mechanism(s) of regulation of CerS activity. We now demonstrate that CerS activity can be modulated by dimer formation. Under suitable conditions, high M(r) CerS complexes can be detected by Western blotting, and various CerS co-immunoprecipitate. CerS5 activity is inhibited in a dominant-negative fashion by co-expression with catalytically inactive CerS5, and CerS2 activity is enhanced by co-expression with a catalytically active form of CerS5 or CerS6. In a constitutive heterodimer comprising CerS5 and CerS2, the activity of CerS2 depends on the catalytic activity of CerS5. Finally, CerS dimers are formed upon rapid stimulation of ceramide synthesis by curcumin. Together, these data demonstrate that ceramide synthesis can be regulated by the formation of CerS dimers and suggest a novel way to generate the acyl chain composition of ceramide (and downstream sphingolipids), which may depend on the interaction of CerS with each other.

Ablation of ceramide synthase 2 strongly affects biophysical properties of membranes

Little is known about the effects of altering sphingolipid (SL) acyl chain structure and composition on the biophysical properties of biological membranes. We explored the biophysical consequences of depleting very long acyl chain (VLC) SLs in membranes prepared from lipid fractions isolated from a ceramide synthase 2 (CerS2)-null mouse, which is unable to synthesize C22-C24 ceramides. We demonstrate that ablation of CerS2 has different effects on liver and brain, causing a significant alteration in the fluidity of the membrane and affecting the type and/or extent of the phases present in the membrane. These changes are a consequence of the depletion of VLC and unsaturated SLs, which occurs to a different extent in liver and brain. In addition, ablation of CerS2 causes changes in intrinsic membrane curvature, leading to strong morphological alterations that promote vesicle adhesion, membrane fusion, and tubule formation. Together, these results show that depletion of VLC-SLs strongly affects membrane biophysical properties, which may compromise cellular processes that critically depend on membrane structure, such as trafficking and sorting.

Chain length-specific properties of ceramides

Ceramides are a class of sphingolipids that are abundant in cell membranes. They are important structural components of the membrane but can also act as second messengers in various signaling pathways. Until recently, ceramides and dihydroceramides were considered as a single functional class of lipids and no distinction was made between molecules with different chain lengths. However, based on the development of high-throughput, structure-specific and quantitative analytical methods to measure ceramides, it has now become clear that in cellular systems the amounts of ceramides differ with respect to their chain length. Further studies have indicated that some functions of ceramides are chain-length dependent. In this review, we discuss the chain length-specific differences of ceramides including their pathological impact on Alzheimer's disease, inflammation, autophagy, apoptosis and cancer.