Palmitoylation of acetylated tubulin and association with ceramide-rich platforms is critical for ciliogenesis.

Microtubules are polymers composed of αß-tubulin subunits that provide structure to cells and play a crucial role in in the development and function of neuronal processes and cilia, microtubule-driven extensions of the plasma membrane that have sensory (primary cilia) or motor (motile cilia) functions. To stabilize microtubules in neuronal processes and cilia, α tubulin is modified by the posttranslational addition of an acetyl group, or acetylation. We discovered that acetylated tubulin in microtubules interacts with the membrane sphingolipid, ceramide. However, the molecular mechanism and function of this interaction are not understood. Here, we show that in human induced pluripotent stem cell-derived neurons, ceramide stabilizes microtubules, which indicates a similar function in cilia. Using proximity ligation assays, we detected complex formation of ceramide with acetylated tubulin in Chlamydomonas reinhardtii flagella and cilia of human embryonic kidney (HEK293T) cells, primary cultured mouse astrocytes, and ependymal cells. Using incorporation of palmitic azide and click chemistry-mediated addition of fluorophores, we show that a portion of acetylated tubulin is S-palmitoylated. S-palmitoylated acetylated tubulin is colocalized with ceramide-rich platforms in the ciliary membrane, and it is coimmunoprecipitated with Arl13b, a GTPase that mediates transport of proteins into cilia. Inhibition of S-palmitoylation with 2-bromo palmitic acid or inhibition of ceramide biosynthesis with fumonisin B1 reduces formation of the Arl13b-acetylated tubulin complex and its transport into cilia, concurrent with impairment of ciliogenesis. Together, these data show, for the first time, that ceramide-rich platforms mediate membrane anchoring and interaction of S-palmitoylated proteins that are critical for cilium formation, stabilization, and function.

Role of 1-Deoxysphingolipids in docetaxel neurotoxicity.

A major dose-limiting side effect of docetaxel chemotherapy is peripheral neuropathy. Patients' symptoms include pain, numbness, tingling and burning sensations, and motor weakness in the extremities. The molecular mechanism is currently not understood, and there are no treatments available. Previously, we have shown an association between neuropathy symptoms of patients treated with paclitaxel and the plasma levels of neurotoxic sphingolipids, the 1-deoxysphingolipids (1-deoxySL) (Kramer et al, FASEB J, 2015). 1-DeoxySL are produced when the first enzyme of the sphingolipid biosynthetic pathway, serine palmitoyltransferase (SPT), uses L-alanine as a substrate instead of its canonical amino acid substrate, L-serine. In the current investigation, we tested whether 1-deoxySL accumulate in the nervous system following systemic docetaxel treatment in mice. In dorsal root ganglia (DRG), we observed that docetaxel (45 mg/kg cumulative dose) significantly elevated the levels of 1-deoxySL and L-serine-derived ceramides, but not sphingosine-1-phosphate (S1P). S1P is a bioactive sphingolipid and a ligand for specific G-protein-coupled receptors. In the sciatic nerve, docetaxel decreased 1-deoxySL and ceramides. Moreover, we show that in primary DRG cultures, 1-deoxysphingosine produced neurite swellings that could be reversed with S1P. Our results demonstrate that docetaxel chemotherapy up-regulates sphingolipid metabolism in sensory neurons, leading to the accumulation of neurotoxic 1-deoxySL. We suggest that the neurotoxic effects of 1-deoxySL on axons can be reversed with S1P.

Ectopic expression of ceramide synthase 2 in neurons suppresses neurodegeneration induced by ceramide synthase 1 deficiency.

Sphingolipids exhibit extreme functional and chemical diversity that is in part determined by their hydrophobic moiety, ceramide. In mammals, the fatty acyl chain length variation of ceramides is determined by six (dihydro)ceramide synthase (CerS) isoforms. Previously, we and others showed that mutations in the major neuron-specific CerS1, which synthesizes 18-carbon fatty acyl (C18) ceramide, cause elevation of long-chain base (LCB) substrates and decrease in C18 ceramide and derivatives in the brain, leading to neurodegeneration in mice and myoclonus epilepsy with dementia in humans. Whether LCB elevation or C18 ceramide reduction leads to neurodegeneration is unclear. Here, we ectopically expressed CerS2, a nonneuronal CerS producing C22-C24 ceramides, in neurons of Cers1-deficient mice. Surprisingly, the Cers1 mutant pathology was almost completely suppressed. Because CerS2 cannot replenish C18 ceramide, the rescue is likely a result of LCB reduction. Consistent with this hypothesis, we found that only LCBs, the substrates common for all of the CerS isoforms, but not ceramides and complex sphingolipids, were restored to the wild-type levels in the Cers2-rescued Cers1 mutant mouse brains. Furthermore, LCBs induced neurite fragmentation in cultured neurons at concentrations corresponding to the elevated levels in the CerS1-deficient brain. The strong association of LCB levels with neuronal survival both in vivo and in vitro suggests high-level accumulation of LCBs is a possible underlying cause of the CerS1 deficiency-induced neuronal death.

Increased liver tumor formation in neutral sphingomyelinase-2-deficient mice.

Sphingolipids are key signaling lipids in cancer. Genome-wide studies have identified neutral SMase-2 (nSMase2), an enzyme generating ceramide from SM, as a potential repressor for hepatocellular carcinoma. However, little is known about the sphingolipids regulated by nSMase2 and their roles in liver tumor development. We discovered growth of spontaneous liver tumors in 27.3% (9 of 33) of aged male nSMase2-deficient (fro/fro) mice. Lipidomics analysis showed a marked increase of SM in the tumor. Unexpectedly, tumor tissues presented with more than a 7-fold increase of C16-ceramide, concurrent with upregulation of ceramide synthase 5. The fro/fro liver tumor, but not adjacent tissue, exhibited substantial accumulation of lipid droplets, suggesting that nSMase2 deficiency is associated with tumor growth and increased neutral lipid generation in the tumor. Tumor tissue expressed significantly increased levels of CD133 and EpCAM mRNA, two markers of liver cancer stem-like cells (CSCs) and higher levels of phosphorylated signal transducer and activator of transcription 3, an essential regulator of stemness. CD133(+) cells showed strong labeling for SM and ceramide. In conclusion, these results suggest that SMase-2 deficiency plays a role in the survival or proliferation of CSCs, leading to spontaneous tumors, which is associated with tumor-specific effects on lipid homeostasis.

Novel function of ceramide for regulation of mitochondrial ATP release in astrocytes.

We reported that amyloid ß peptide (Aß42) activated neutral SMase 2 (nSMase2), thereby increasing the concentration of the sphingolipid ceramide in astrocytes. Here, we show that Aß42 induced mitochondrial fragmentation in wild-type astrocytes, but not in nSMase2-deficient cells or astrocytes treated with fumonisin B1 (FB1), an inhibitor of ceramide synthases. Unexpectedly, ceramide depletion was concurrent with rapid movements of mitochondria, indicating an unknown function of ceramide for mitochondria. Using immunocytochemistry and super-resolution microscopy, we detected ceramide-enriched and mitochondria-associated membranes (CEMAMs) that were codistributed with microtubules. Interaction of ceramide with tubulin was confirmed by cross-linking to N-[9-(3-pent-4-ynyl-3-H-diazirine-3-yl)-nonanoyl]-D-erythro-sphingosine (pacFACer), a bifunctional ceramide analog, and binding of tubulin to ceramide-linked agarose beads. Ceramide-associated tubulin (CAT) translocated from the perinuclear region to peripheral CEMAMs and mitochondria, which was prevented in nSMase2-deficient or FB1-treated astrocytes. Proximity ligation and coimmunoprecipitation assays showed that ceramide depletion reduced association of tubulin with voltage-dependent anion channel 1 (VDAC1), an interaction known to block mitochondrial ADP/ATP transport. Ceramide-depleted astrocytes contained higher levels of ATP, suggesting that ceramide-induced CAT formation leads to VDAC1 closure, thereby reducing mitochondrial ATP release, and potentially motility and resistance to Aß42 Our data also indicate that inhibiting ceramide generation may protect mitochondria in Alzheimer's disease.

A deficiency of ceramide biosynthesis causes cerebellar purkinje cell neurodegeneration and lipofuscin accumulation.

Sphingolipids, lipids with a common sphingoid base (also termed long chain base) backbone, play essential cellular structural and signaling functions. Alterations of sphingolipid levels have been implicated in many diseases, including neurodegenerative disorders. However, it remains largely unclear whether sphingolipid changes in these diseases are pathological events or homeostatic responses. Furthermore, how changes in sphingolipid homeostasis shape the progression of aging and neurodegeneration remains to be clarified. We identified two mouse strains, flincher (fln) and toppler (to), with spontaneous recessive mutations that cause cerebellar ataxia and Purkinje cell degeneration. Positional cloning demonstrated that these mutations reside in the Lass1 gene. Lass1 encodes (dihydro)ceramide synthase 1 (CerS1), which is highly expressed in neurons. Both fln and to mutations caused complete loss of CerS1 catalytic activity, which resulted in a reduction in sphingolipid biosynthesis in the brain and dramatic changes in steady-state levels of sphingolipids and sphingoid bases. In addition to Purkinje cell death, deficiency of CerS1 function also induced accumulation of lipofuscin with ubiquitylated proteins in many brain regions. Our results demonstrate clearly that ceramide biosynthesis deficiency can cause neurodegeneration and suggest a novel mechanism of lipofuscin formation, a common phenomenon that occurs during normal aging and in some neurodegenerative diseases.

Ceramide-mediated orchestration of oxidative stress response through filopodia-derived small extracellular vesicles.

Extracellular vesicles (EVs) are shed from the plasma membrane, but the regulation and function of these EVs remain unclear. We found that oxidative stress induced by H2O2 in Hela cells stimulated filopodia formation and the secretion of EVs. EVs were small (150 nm) and labeled for CD44, indicating that they were derived from filopodia. Filopodia-derived small EVs (sEVs) were enriched with the sphingolipid ceramide, consistent with increased ceramide in the plasma membrane of filopodia. Ceramide was colocalized with neutral sphingomyelinase 2 (nSMase2) and acid sphingomyelinase (ASM), two sphingomyelinases generating ceramide at the plasma membrane. Inhibition of nSMase2 and ASM prevented oxidative stress-induced sEV shedding but only nSMase2 inhibition prevented filopodia formation. nSMase2 was S-palmitoylated and interacted with ASM in filopodia to generate ceramide for sEV shedding. sEVs contained nSMase2 and ASM and decreased the level of these two enzymes in oxidatively stressed Hela cells. A novel metabolic labeling technique for EVs showed that oxidative stress induced secretion of fluorescent sEVs labeled with NBD-ceramide. NBD-ceramide-labeled sEVs transported ceramide to mitochondria, ultimately inducing cell death in a proportion of neuronal (N2a) cells. In conclusion, using Hela cells we provide evidence that oxidative stress induces interaction of nSMase2 and ASM at filopodia, which leads to shedding of ceramide-rich sEVs that target mitochondria and propagate cell death.

The S1P receptor 1 antagonist Ponesimod reduces TLR4-induced neuroinflammation and increases Aß clearance in 5XFAD mice.

BACKGROUND: Previously, we showed that the sphingosine-1-phosphate (S1P) transporter spinster 2 (Spns2) mediates activation of microglia in response to amyloid ß peptide (Aß). Here, we investigated if Ponesimod, a functional S1P receptor 1 (S1PR1) antagonist, prevents Aß-induced activation of glial cells and Alzheimer's disease (AD) pathology. METHODS: We used primary cultures of glial cells and the 5XFAD mouse model to determine the effect of Aß and Ponesimod on glial activation, Aß phagocytosis, cytokine levels and pro-inflammatory signaling pathways, AD pathology, and cognitive performance. FINDINGS: Aß42 increased the levels of TLR4 and S1PR1, leading to their complex formation. Ponesimod prevented the increase in TLR4 and S1PR1 levels, as well as the formation of their complex. It also reduced the activation of the pro-inflammatory Stat1 and p38 MAPK signaling pathways, while activating the anti-inflammatory Stat6 pathway. This was consistent with increased phagocytosis of Aß42 in primary cultured microglia. In 5XFAD mice, Ponesimod decreased the levels of TNF-α and CXCL10, which activate TLR4 and Stat1. It also increased the level of IL-33, an anti-inflammatory cytokine that promotes Aß42 phagocytosis by microglia. As a result of these changes, Ponesimod decreased the number of Iba-1+ microglia and GFAP+ astrocytes, and the size and number of amyloid plaques, while improving spatial memory as measured in a Y-maze test. INTERPRETATION: Ponesimod targeting S1PR1 is a promising therapeutic approach to reprogram microglia, reduce neuroinflammation, and increase Aß clearance in AD. FUNDING: NIHR01AG064234, RF1AG078338, R21AG078601, VAI01BX003643.

Cell density-dependent reduction of dihydroceramide desaturase activity in neuroblastoma cells.

We applied a metabolic approach to investigate the role of sphingolipids in cell density-induced growth arrest in neuroblastoma cells. Our data revealed that sphingolipid metabolism in neuroblastoma cells significantly differs depending on the cells' population context. At high cell density, cells exhibited G0/G1 cell-cycle arrest and reduced ceramide, monohexosylceramide, and sphingomyelin, whereas dihydroceramide was significantly increased. In addition, our metabolic-labeling experiments showed that neuroblastoma cells at high cell density preferentially synthesized very long chain (VLC) sphingolipids and dramatically decreased synthesis of sphingosine-1-phosphate (S1P). Moreover, densely populated neuroblastoma cells showed increased message levels of both anabolic and catabolic enzymes of the sphingolipid pathway. Notably, our metabolic-labeling experiments indicated reduced dihydroceramide desaturase activity at confluence, which was confirmed by direct measurement of dihydroceramide desaturase activity in situ and in vitro. Importantly, we could reduce dihydroceramide desaturase activity in low-density cells by applying conditional media from high-density cells, as well as by adding reducing agents, such as DTT and L-cysteine to the media. In conclusion, our data suggest a role of the sphingolipid pathway, dihydroceramides desaturase in particular, in confluence-induced growth arrest in neuroblastoma cells.

Function of ceramide transfer protein for biogenesis and sphingolipid composition of extracellular vesicles.

The formation of extracellular vesicles (EVs) is induced by the sphingolipid ceramide. How this pathway is regulated is not entirely understood. Here, we report that the ceramide transport protein (CERT) mediates a non-vesicular transport of ceramide between the endoplasmic reticulum (ER) and the multivesicular endosome at contact sites. The process depends on the interaction of CERT's PH domain with PI4P generated by PI4KIIα at endosomes. Furthermore, a complex is formed between the START domain of CERT, which carries ceramide, and the Tsg101 protein, which is part of the endosomal sorting complex required for transport (ESCRT-I). Inhibition of ceramide biosynthesis reduces CERT-Tsg101 complex formation. Overexpression of CERT increases EV secretion while its inhibition reduces EV formation and the concentration of ceramides and sphingomyelins in EVs. In conclusion, we discovered a function of CERT in regulating the sphingolipid composition and biogenesis of EVs, which links ceramide to the ESCRT-dependent pathway.

Neutral Sphingomyelinase 2 Mediates Oxidative Stress Effects on Astrocyte Senescence and Synaptic Plasticity Transcripts.

We have shown that deficiency of neutral sphingomyelinase 2 (nSMase2), an enzyme generating the sphingolipid ceramide, improves memory in adult mice. Here, we performed sphingolipid and RNA-seq analyses on the cortex from 10-month-old nSMase2-deficient (fro/fro) and heterozygous (+ /fro) mice. fro/fro cortex showed reduced levels of ceramide, particularly in astrocytes. Differentially abundant transcripts included several functionally related groups, with decreases in mitochondrial oxidative phosphorylation and astrocyte activation transcripts, while axon guidance and synaptic transmission and plasticity transcripts were increased, indicating a role of nSMase2 in oxidative stress, astrocyte activation, and cognition. Experimentally induced oxidative stress decreased the level of glutathione (GSH), an endogenous inhibitor of nSMase2, and increased immunolabeling for ceramide in primary + /fro astrocytes, but not in fro/fro astrocytes. ß-galactosidase activity was lower in 5-week-old fro/fro astrocytes, indicating delayed senescence due to nSMase2 deficiency. In fro/fro cortex, levels of the senescence markers C3b and p27 and the proinflammatory cytokines interleukin 1ß, interleukin 6, and tumor necrosis factor α were reduced, concurrent with twofold decreased phosphorylation of their downstream target, protein kinase Stat3. RNA and protein levels of the ionotropic glutamate receptor subunit 2B (Grin2b/NR2B) were increased by twofold, which was previously shown to enhance cognition. This was consistent with threefold reduced levels of exosomes carrying miR-223-3p, a micro-RNA downregulating NR2B. In summary, our data show that nSMase2 deficiency prevents oxidative stress-induced elevation of ceramide and secretion of exosomes by astrocytes that suppress neuronal function, indicating a role of nSMase2 in the regulation of neuroinflammation and cognition.

Extracellular Vesicles Containing Ceramide-Rich Platforms: "Mobile Raft" Isolation and Analysis.

Extracellular vesicles (EVs) are secreted by eukaryotic cells and serve as carriers for a variety of cell signaling factors, including RNAs, proteins, and lipids. We described a unique population of EVs, the membrane of which is highly enriched with the sphingolipid ceramide. We suggested that ceramide in the EV membrane is organized in ceramide-rich platforms (CRPs), a type of lipid raft that mediates interaction of ceramide with ceramide-associated proteins (CAPs). Here, we describe methods using anti-ceramide antibody to isolate ceramide-enriched EVs and detect exosomes after uptake into recipient cells. In addition, we discuss methods for EV analysis using nanoparticle tracking and mass spectrometry. The methods can be extended to the isolation of other types of EVs and "mobile rafts" transported by EVs from donor to recipient cells using antibodies against lipids specific for these EVs.

Cross-Link/Proximity Ligation Assay for Visualization of Lipid and Protein Complexes in Lipid Rafts.

The detection of protein complexes by coimmunoprecipitation or two-hybrid analysis is often limited to cytosolic and soluble proteins, while interaction between membrane proteins or proteins and lipids is hampered by solubilization artefacts or absence of appropriate antibodies to detect a complex. More recently, the proximity ligation assay (PLA) using antibodies for in situ detection of protein complexes in cells and cross-linkable lipid analogs that can be endowed with molecular tags for pull-down assyas were techniques utilized to identify and monitor interaction between proteins and lipids. We have developed a novel technique termed "cross-link/PLA" combining a cross-linkable ceramide analog with PLA and anti-ceramide antibody to visualize lipid-protein complexes in ceramide-rich platforms (CRPs), a particular type of lipid raft. This chapter will discuss experimental protocols and data analysis to use cross-link/PLA for detection and visualization of lipid-protein complexes in CRPs and other types of lipid rafts.

Disruption of ceramide synthesis by CerS2 down-regulation leads to autophagy and the unfolded protein response.

Ceramide metabolism has come under recent scrutiny because of its role in cellular stress responses. CerS2 (ceramide synthase 2) is one of the six mammalian isoforms of ceramide synthase and is responsible for the synthesis of VLC (very-long-chain) ceramides, e.g. C24, C24:1. To study the role of CerS2 in ceramide metabolism and cellular homoeostasis, we down-regulated CerS2 using siRNA (small interfering RNA) and examined several aspects of sphingolipid metabolism and cell stress responses. CerS2 down-regulation had a broad effect on ceramide homoeostasis, not just on VLC ceramides. Surprisingly, CerS2 down-regulation resulted in significantly increased LC (long-chain) ceramides, e.g. C14, C16, and our results suggested that the increase was due to a ceramide synthase-independent mechanism. CerS2-down-regulation-induced LC ceramide accumulation resulted in growth arrest which was not accompanied by apoptotic cell death. Instead, cells remained viable, showing induction of autophagy and activation of PERK [PKR (double-stranded-RNA-dependent protein kinase)-like endoplasmic reticulum kinase] and IRE1 (inositol-requiring 1) pathways [the latter indicating activation of the UPR (unfolded protein response)].

Ceramide regulates interaction of Hsd17b4 with Pex5 and function of peroxisomes.

The sphingolipid ceramide regulates beta-oxidation of medium and long chain fatty acids in mitochondria. It is not known whether it also regulates oxidation of very long chain fatty acids (VLCFAs) in peroxisomes. Using affinity chromatography, co-immunoprecipitation, and proximity ligation assays we discovered that ceramide interacts with Hsd17b4, an enzyme critical for peroxisomal VLCFA oxidation and docosahexaenoic acid (DHA) generation. Immunocytochemistry showed that Hsd17b4 is distributed to ceramide-enriched mitochondria-associated membranes (CEMAMs). Molecular docking and in vitro mutagenesis experiments showed that ceramide binds to the sterol carrier protein 2-like domain in Hsd17b4 adjacent to peroxisome targeting signal 1 (PTS1), the C-terminal signal for interaction with peroxisomal biogenesis factor 5 (Pex5), a peroxin mediating transport of Hsd17b4 into peroxisomes. Inhibition of ceramide biosynthesis induced translocation of Hsd17b4 from CEMAMs to peroxisomes, interaction of Hsd17b4 with Pex5, and upregulation of DHA. This data indicates a novel role of ceramide as a molecular switch regulating interaction of Hsd17b4 with Pex5 and peroxisomal function.

Side Effects in Cancer Therapy: Are Sphingolipids to Blame?

Cancer patients' quality of life is greatly dependent on the efficacy of treatments and their associated side effects, which can significantly reduce the overall quality of life. Although the effectiveness of cancer treatments has improved over time, adverse effects persist with each treatment. Some side effects, such as paclitaxel-induced peripheral neuropathy, can be dose limiting, thus further reducing the potential of paclitaxel chemotherapy treatment. Premature ovarian failure in young female patients due to radiation and chemotherapy therapy can have devastating infertility consequences. In recent years, a class of lipids known as sphingolipids has been identified as playing a role in the side effects of cancer therapies. Advanced analytical technologies, such as mass spectrometry, have provided great aid in detecting and distinguishing individual sphingolipids at low concentrations. Sphingolipids play an important role in cell proliferation and apoptosis and, importantly, sphingolipid metabolism has been shown to be dysregulated in cancer. The goal of this review is to summarize the latest findings of the role of sphingolipids in the injurious side effects in various cancer treatments. A better understanding of the molecular mechanisms driving these sphingolipid-induced side effects can help develop new drugs and treatments for cancer that have fewer side effects, thus improving treatment efficacy and quality of life.

Ceramide and S1P Signaling in Embryonic Stem Cell Differentiation.

Bioactive sphingolipids are important regulators for stem cell survival and differentiation. Most recently, we have coined the term "morphogenetic lipids" for sphingolipids that regulate stem cells during embryonic and postnatal development. The sphingolipid ceramide and its derivative, sphingosine-1-phosphate (S1P), can act synergistically as well as antagonistically on embryonic stem (ES) cell differentiation. We show here simple as well as state-of-the-art methods to analyze sphingolipids in differentiating ES cells and discuss new protocols to use ceramide and S1P analogs for the guided differentiation of mouse ES cells toward neuronal and glial lineage.

Primary cilia in stem cells and neural progenitors are regulated by neutral sphingomyelinase 2 and ceramide.

We show here that human embryonic stem (ES) and induced pluripotent stem cell-derived neuroprogenitors (NPs) develop primary cilia. Ciliogenesis depends on the sphingolipid ceramide and its interaction with atypical PKC (aPKC), both of which distribute to the primary cilium and the apicolateral cell membrane in NP rosettes. Neural differentiation of human ES cells to NPs is concurrent with a threefold elevation of ceramide-in particular, saturated, long-chain C16:0 ceramide (N-palmitoyl sphingosine) and nonsaturated, very long chain C24:1 ceramide (N-nervonoyl sphingosine). Decreasing ceramide levels by inhibiting ceramide synthase or neutral sphingomyelinase 2 leads to translocation of membrane-bound aPKC to the cytosol, concurrent with its activation and the phosphorylation of its substrate Aurora kinase A (AurA). Inhibition of aPKC, AurA, or a downstream target of AurA, HDAC6, restores ciliogenesis in ceramide-depleted cells. Of importance, addition of exogenous C24:1 ceramide reestablishes membrane association of aPKC, restores primary cilia, and accelerates neural process formation. Taken together, these results suggest that ceramide prevents activation of HDAC6 by cytosolic aPKC and AurA, which promotes acetylation of tubulin in primary cilia and, potentially, neural processes. This is the first report on the critical role of ceramide generated by nSMase2 in stem cell ciliogenesis and differentiation.

The plant disease resistance gene Asc-1 prevents disruption of sphingolipid metabolism during AAL-toxin-induced programmed cell death.

The nectrotrophic fungus Alternaria alternata f.sp. lycopersici infects tomato plants of the genotype asc/asc by utilizing a host-selective toxin, AAL-toxin, that kills the host cells by inducing programmed cell death. Asc-1 is homologous to genes found in most eukaryotes from yeast to humans, suggesting a conserved function. A yeast strain with deletions in the homologous genes LAG1 and LAC1 was functionally complemented by Asc-1, indicating that Asc-1 functions in an analogous manner to the yeast homologues. Examination of the yeast sphingolipids, which are almost absent in the lag1Deltalac1Delta mutant, showed that Asc-1 was able to restore the synthesis of sphingolipids. We therefore examined the biosynthesis of sphingolipids in tomato by labeling leaf discs with l-[3-3H]serine. In the absence of AAL-toxin, there was no detectable difference in sphingolipid labeling between leaf discs from Asc/Asc or asc/asc leaves. In the presence of pathologically significant concentrations of AAL-toxin however, asc/asc leaf discs showed severely reduced labeling of sphingolipids and increased label in dihydrosphingosine (DHS) and 3-ketodihydrosphingosine (3-KDHS). Leaf discs from Asc/Asc leaves responded to AAL-toxin treatment by incorporating label into different sphingolipid species. The effects of AAL-toxin on asc/asc leaflets could be partially blocked by the simultaneous application of AAL-toxin and myriocin. Leaf discs simultaneously treated with AAL-toxin and myriocin showed no incorporation of label into sphingolipids or long-chain bases as expected. These results indicate that the presence of Asc-1 is able to relieve an AAL-toxin-induced block on sphingolipid synthesis that would otherwise lead to programmed cell death.

Yeast sphingolipid metabolism: clues and connections.

This review of sphingolipid metabolism in the budding yeast Saccharomyces cerevisiae contains information on the enzymes and the genes that encode them, as well as connections to other metabolic pathways. Particular attention is given to yeast homologs, domains, and motifs in the sequence, cellular localization of enzymes, and possible protein-protein interactions. Also included are genetic interactions of special interest that provide clues to the cellular biological roles of particular sphingolipid metabolic pathways and specific sphingolipids.