Cultured granule cells and astrocytes from cerebellum differ in metabolizing sphingosine

Extracellular vesicles released by microglia and macrophages carry endocannabinoids which foster oligodendrocyte differentiation

Introduction

Microglia and macrophages can influence the evolution of myelin lesions through the production of extracellular vesicles (EVs). While microglial EVs promote in vitro differentiation of oligodendrocyte precursor cells (OPCs), whether EVs derived from macrophages aid or limit OPC maturation is unknown.

Methods

Immunofluorescence analysis for the myelin protein MBP was employed to evaluate the impact of EVs from primary rat macrophages on cultured OPC differentiation. Raman spectroscopy and liquid chromatography-mass spectrometry was used to define the promyelinating lipid components of myelin EVs obtained in vitro and isolated from human plasma.

Results and discussion

Here we show that macrophage-derived EVs do not promote OPC differentiation, and those released from macrophages polarized towards an inflammatory state inhibit OPC maturation. However, their lipid cargo promotes OPC maturation in a similar manner to microglial EVs. We identify the promyelinating endocannabinoids anandamide and 2-arachidonoylglycerol in EVs released by both macrophages and microglia in vitro and circulating in human plasma. Analysis of OPC differentiation in the presence of the endocannabinoid receptor antagonists SR141716A and AM630 reveals a key role of vesicular endocannabinoids in OPC maturation. From this study, EV-associated endocannabinoids emerge as important mediators in microglia/macrophage-oligodendrocyte crosstalk, which may be exploited to enhance myelin repair.

Combined Inhibition of Hedgehog and HDAC6: In Vitro and In Vivo Studies Reveal a New Role for Lysosomal Stress in Reducing Glioblastoma Cell Viability

Glioblastoma multiforme (GBM) is the most common and malignant brain tumor in adults. The invasiveness and the rapid progression that characterize GBM negatively impact patients’ survival. Temozolomide (TMZ) is currently considered the first-choice chemotherapeutic agent. Unfortunately, over 50% of patients with GBM do not respond to TMZ treatment, and the mutation-prone nature of GBM enables the development of resistance mechanisms. Therefore, efforts have been devoted to the dissection of aberrant pathways involved in GBM insurgence and resistance in order to identify new therapeutic targets. Among them, sphingolipid signaling, Hedgehog (Hh) pathway, and the histone deacetylase 6 (HDAC6) activity are frequently dysregulated and may represent key targets to counteract GBM progression. Given the positive correlation between Hh/HDAC6/sphingolipid metabolism in GBM, we decided to perform a dual pharmacological inhibition of Hh and HDAC6 through cyclopamine and tubastatin A, respectively, in a human GMB cell line and zebrafish embryos. The combined administration of these compounds elicited a more significant reduction of GMB cell viability than did single treatments in vitro and in cells orthotopically transplanted in the zebrafish hindbrain ventricle. We demonstrated, for the first time, that the inhibition of these pathways induces lysosomal stress which results in an impaired fusion of lysosomes with autophagosomes and a block of sphingolipid degradation in GBM cell lines. This condition, which we also recapitulated in zebrafish embryos, suggests an impairment of lysosome-dependent processes involving autophagy and sphingolipid homeostasis and might be instrumental in the reduction of GBM progression.

Sphingolipids and their role in health and disease in the central nervous system

Sphingolipids (SLs) are lipids derived from sphingosine, and their metabolism involves a broad and complex network of reactions. Although SLs are widely distributed in the body, it is well known that they are present in high concentrations within the central nervous system (CNS). Under physiological conditions, their abundance and distribution in the CNS depend on brain development and cell type. Consequently, SLs metabolism impairment may have a significant impact on the normal CNS function, and has been associated with several disorders, including sphingolipidoses, Parkinson's, and Alzheimer's. This review summarizes the main SLs characteristics and current knowledge about synthesis, catabolism, regulatory pathways, and their role in physiological and pathological scenarios in the CNS.

Detrimental and protective action of microglial extracellular vesicles on myelin lesions: astrocyte involvement in remyelination failure

Microglia are highly plastic immune cells which exist in a continuum of activation states. By shaping the function of oligodendrocyte precursor cells (OPCs), the brain cells which differentiate to myelin-forming cells, microglia participate in both myelin injury and remyelination during multiple sclerosis. However, the mode(s) of action of microglia in supporting or inhibiting myelin repair is still largely unclear. Here, we analysed the effects of extracellular vesicles (EVs) produced in vitro by either pro-inflammatory or pro-regenerative microglia on OPCs at demyelinated lesions caused by lysolecithin injection in the mouse corpus callosum. Immunolabelling for myelin proteins and electron microscopy showed that EVs released by pro-inflammatory microglia blocked remyelination, whereas EVs produced by microglia co-cultured with immunosuppressive mesenchymal stem cells promoted OPC recruitment and myelin repair. The molecular mechanisms responsible for the harmful and beneficial EV actions were dissected in primary OPC cultures. By exposing OPCs, cultured either alone or with astrocytes, to inflammatory EVs, we observed a blockade of OPC maturation only in the presence of astrocytes, implicating these cells in remyelination failure. Biochemical fractionation revealed that astrocytes may be converted into harmful cells by the inflammatory EV cargo, as indicated by immunohistochemical and qPCR analyses, whereas surface lipid components of EVs promote OPC migration and/or differentiation, linking EV lipids to myelin repair. Although the mechanisms through which the lipid species enhance OPC maturation still remain to be fully defined, we provide the first demonstration that vesicular sphingosine 1 phosphate stimulates OPC migration, the first fundamental step in myelin repair. From this study, microglial EVs emerge as multimodal and multitarget signalling mediators able to influence both OPCs and astrocytes around myelin lesions, which may be exploited to develop novel approaches for myelin repair not only in multiple sclerosis, but also in neurological and neuropsychiatric diseases characterized by demyelination.

Sphingosine 1-phosphate receptors regulate TLR4-induced CXCL5 release from astrocytes and microglia

Sphingosine 1-phosphate receptors (S1PR) are G protein-coupled and compose a family with five subtypes, S1P1R-S1P5R. The drug Gilenya^® (Novartis, Basel, Switzerland) (Fingolimod; FTY720) targets S1PRs and was the first oral therapy for patients with relapsing-remitting multiple sclerosis (MS). The phosphorylated form of FTY720 (pFTY720) binds S1PRs causing initial agonism, then subsequent receptor internalization and functional antagonism. Internalization of S1P1R attenuates sphingosine 1-phosphate (S1P)-mediated egress of lymphocytes from lymph nodes, limiting aberrant immune function in MS. pFTY720 also exerts direct actions on neurons and glial cells which express S1PRs. In this study, we investigated the regulation of pro-inflammatory chemokine release by S1PRs in enriched astrocytes and microglial cultures. Astrocytes and microglia were stimulated with lipopolysaccharide (LPS) and increases in C-X-C motif chemokine 5 (CXCL5), also known as LIX (lipopolysaccharide-induced CXC chemokine) expression were quantified. Results showed that pFTY720 attenuated LPS-induced CXCL5 (LIX) protein release from astrocytes, as did the S1P1R selective agonist, SEW2871. In addition, pFTY720 blocked messenger ribonucleic acid (mRNA) transcription of the chemokines, (i) CXCL5/LIX, (ii) C-X-C motif chemokine 10 (CXCL10) also known as interferon gamma-induced protein 10 (IP10) and (iii) chemokine (C-C motif) ligand 2 (CCL2) also known as monocyte chemoattractant protein 1 (MCP1). Interestingly, inhibition of sphingosine kinase attenuated LPS-induced increases in mRNA levels of all three chemokines, suggesting that LPS-TLR4 (Toll-like receptor 4) signalling may enhance chemokine expression via S1P-S1PR transactivation. Lastly, these observations were not limited to astrocytes since we also found that pFTY720 attenuated LPS-induced release of CXCL5 from microglia. These data highlight a role for S1PR signalling in regulating the levels of chemokines in glial cells and support the notion that pFTY720 efficacy in multiple sclerosis may involve the direct modulation of astrocytes and microglia.

Glucolipotoxicity impairs ceramide flow from the endoplasmic reticulum to the Golgi apparatus in INS-1 β-cells

Accumulating evidence suggests that glucolipotoxicity, arising from the combined actions of elevated glucose and free fatty acid levels, acts as a key pathogenic component in type II diabetes, contributing to β-cell dysfunction and death. Endoplasmic reticulum (ER) stress is among the molecular pathways and regulators involved in these negative effects, and ceramide accumulation due to glucolipotoxicity can be associated with the induction of ER stress. Increased levels of ceramide in ER may be due to enhanced ceramide biosynthesis and/or decreased ceramide utilization. Here, we studied the effect of glucolipotoxic conditions on ceramide traffic in INS-1 cells in order to gain insights into the molecular mechanism(s) of glucolipotoxicity. We showed that glucolipotoxicity inhibited ceramide utilization for complex sphingolipid biosynthesis, thereby reducing the flow of ceramide from the ER to Golgi. Glucolipotoxicity impaired both vesicular- and CERT-mediated ceramide transport through (1) the decreasing of phospho-Akt levels which in turn possibly inhibits vesicular traffic, and (2) the reducing of the amount of active CERT mainly due to a lower protein levels and increased protein phosphorylation to prevent its localization to the Golgi. In conclusion, our findings provide evidence that glucolipotoxicity-induced ceramide overload in the ER, arising from a defect in ceramide trafficking may be a mechanism that contributes to dysfunction and/or death of β-cells exposed to glucolipotoxicity.

Fingolimod protects cultured cortical neurons against excitotoxic death

Fingolimod (FTY720), a novel drug approved for the treatment of relapsing-remitting multiple sclerosis, activates different sphingosine-1-phosphate receptor (S1PR) subtypes. Its primary mechanism of action is to reduce the egress of T lymphocytes from secondary lymphoid organs, thus restraining neuroinflammation and autoimmunity. However, recent evidence suggests that the action of FTY720 involves S1PRs expressed by cells resident in the CNS, including neurons. Here, we examined the effect of FTY720, its active metabolite, FTY720-P, and sphingosine-1-phosphate (S1P) on neuronal viability using a classical in vitro model of excitotoxic neuronal death. Mixed cultures of mouse cortical cells were challenged with toxic concentrations of N-methyl-d-aspartate (NMDA) for 10 min, and neuronal death was assessed 20 h later. FTY720, FTY720-P, and S1P were all neuroprotective when applied 18-20 h prior to the NMDA pulse. Neuroprotection was attenuated by pertussis toxin, and inhibited by the selective type-1 S1PR (S1P1R) antagonist, W146, and by inhibitors of the mitogen associated protein kinase (MAPK) and the phosphatidylinositol-3-kinase (PtdIns-3-K) pathways. Both FTY720 and FTY720-P retained their protective activity in pure cultures of mouse or rat cortical neurons. These data offer the first direct demonstration that FTY720 and its active metabolite protect neurons against excitotoxic death.

Mechanisms of fingolimod's efficacy and adverse effects in multiple sclerosis

Until recently, all approved multiple sclerosis (MS) disease treatments were administered parenterally. Oral fingolimod was approved in September 2010 by the US Food and Drug Administration to reduce relapses and disability progression in relapsing forms of MS. In the clinical trials that led to approval, fingolimod reduced not only acute relapses and magnetic resonance imaging lesion activity but also disability progression and brain volume loss, suggesting preservation of tissue. Fingolimod's mechanism of action in MS is not known with certainty. Its active form, fingolimod-phosphate (fingolimod-P), is a sphingosine 1-phosphate receptor (S1PR) modulator that inhibits egress of lymphocytes from lymph nodes and their recirculation, potentially reducing trafficking of pathogenic cells into the central nervous system (CNS). Fingolimod also readily penetrates the CNS, and fingolimod-P formed in situ may have direct effects on neural cells. Fingolimod potently inhibits the MS animal model, experimental autoimmune encephalomyelitis, but is ineffective in mice with selective deficiency of the S1P₁ S1PR subtype on astrocytes despite normal expression in the immune compartment. These findings suggest that S1PR modulation by fingolimod in both the immune system and CNS, producing a combination of beneficial anti-inflammatory and possibly neuroprotective/reparative effects, may contribute to its efficacy in MS. In clinical trials, fingolimod was generally safe and well tolerated. Its interaction with S1PRs in a variety of tissues largely accounts for the reported adverse effects, which were seen more frequently with doses 2.5 to 10x the approved 0.5 mg dose. Fingolimod's unique mechanism of action distinguishes it from all other currently approved MS therapies.

Sphingosine kinase 1 is up-regulated during hypoxia in U87MG glioma cells. Role of hypoxia-inducible factors 1 and 2

Sphingosine 1-phosphate (S1P), a sphingolipid metabolite that plays an important role in the regulation of cell survival, growth, migration, and angiogenesis, acts both inside the cells and as an extracellular mediator through binding to five G protein-coupled receptors (S1P(1-5)). Sphingosine kinase 1 (SK1), the enzyme responsible for S1P production, is overexpressed in many solid tumors, including gliomas. One common feature of these tumors is the presence of "hypoxic regions," characterized by cells expressing high levels of hypoxia-inducible factors HIF-1alpha and HIF-2alpha, two transcription regulators that modulate the levels of proteins with crucial roles in tumor progression. So far, nothing is known about the role and the regulation of SK1 during tumor-induced hypoxia or about SK1 regulation and HIFs. Here we investigated the role of HIF-1alpha and HIF-2alpha in the regulation of SK1 during hypoxic stress in glioma-derived U87MG cells. We report that hypoxia increases SK1 mRNA levels, protein expression, and enzyme activity, followed by intracellular S1P production and S1P release. Interestingly, knockdown of HIF-2alpha by small interfering RNA abolished the induction of SK1 and the production of extracellular S1P after CoCl(2) treatment, whereas HIF-1alpha small interfering RNA resulted in an increase of HIF-2alpha and of SK1 protein levels. Moreover, using chromatin immunoprecipitation analysis, we demonstrate that HIF-2alpha binds the SK1 promoter. Functionally, we demonstrate that conditioned medium from hypoxia-treated tumor cells results in neoangiogenesis in human umbilical vein endothelial cells in a S1P receptor-dependent manner. These studies provide evidence of a link between S1P production as a potent angiogenic agent and the hypoxic phenotype observed in many tumors.

Phosphorylated FTY720 promotes astrocyte migration through sphingosine-1-phosphate receptors

Sphingosine-1-phosphate (S1P) receptors are widely expressed in the central nervous system where they are thought to regulate glia cell function. The phosphorylated version of fingolimod/FTY720 (FTY720P) is active on a broad spectrum of S1P receptors and the parent compound is currently in phase III clinical trials for the treatment of multiple sclerosis. Here, we aimed to identify which cell type(s) and S1P receptor(s) of the central nervous system are targeted by FTY720P. Using calcium imaging in mixed cultures from embryonic rat cortex we show that astrocytes are the major cell type responsive to FTY720P in this assay. In enriched astrocyte cultures, we detect expression of S1P1 and S1P3 receptors and demonstrate that FTY720P activates Gi protein-mediated signaling cascades. We also show that FTY720P as well as the S1P1-selective agonist SEW2871 stimulate astrocyte migration. The data indicate that FTY720P exerts its effects on astrocytes predominantly via the activation of S1P1 receptors, whereas S1P signals through both S1P1 and S1P3 receptors. We suggest that this distinct pharmacological profile of FTY720P, compared with S1P, could play a role in the therapeutic effects of FTY720 in multiple sclerosis.

Brain sphingosine-1-phosphate receptors: implication for FTY720 in the treatment of multiple sclerosis

Multiple sclerosis (MS) is an autoimmune, neurological disability with unknown etiology. The current therapies available for MS work by an immunomodulatory action, preventing T-cell- and macrophage-mediated destruction of brain-resident oligodendrocytes and axonal loss. Recently, FTY720 (fingolimod) was shown to significantly reduce relapse rates in MS patients and is currently in Phase III clinical trials. This drug attenuates trafficking of harmful T cells entering the brain by regulating sphingosine-1-phosphate (S1P) receptors. Here, we outline the direct roles that S1P receptors play in the central nervous system (CNS) and discuss additional modalities by which FTY720 may provide direct neuroprotection in MS.

Phosphorylated FTY720 stimulates ERK phosphorylation in astrocytes via S1P receptors

Sphingosine-1-phosphate receptors (S1P1-5) are activated by the endogenous agonist S1P and are expressed in the central nervous system. In astrocytes, activation of S1P receptors leads to phosphorylation of extracellular-signal regulated kinase (ERK), a signaling cascade which plays intimate roles in cell proliferation. Fingolimod (FTY720) is in phase III clinical trials for the treatment of multiple sclerosis and its phosphorylated version (FTY720P) activates S1P receptors. We examined the effects of FTY720P on ERK phosphorylation and determined which S1P receptor subtype(s) mediated this signaling event. FTY720P augmented ERK phosphorylation in cortical cultures prepared from embryonic day 18 rat brains and was blocked by an MEK inhibitor or by pertussis toxin. Co-localisation of phosphorylated ERK occurred in glial fibrillary acidic protein (GFAP) positive astrocytes but not neurons or oligodendrocytes. Furthermore, FTY720P stimulated ERK phosphorylation in highly enriched astrocyte cultures made from postnatal day 2 rat cortices. The effects of FTY720P were mimicked by selective S1P1 receptor agonists and blocked by S1P1 receptor antagonists. Collectively, these results demonstrate that FTY720P mediates ERK phosphorylation in astrocytes via the activation of S1P1 receptors.

Sphingosine-1-phosphate and calcium signaling in cerebellar astrocytes and differentiated granule cells

S1P is involved in the regulation of multiple biological processes (cell survival, growth, migration and differentiation) both in neurons and glial cells. The study was aimed at investigating the possible effects of S1P on calcium signaling in cerebellar astrocytes and differentiated granule cells. In cerebellar astrocytes S1P is able to mediate calcium signaling mainly through Gi protein coupled receptors, whereas in differentiated neurons it failed to evoke any calcium signaling, despite acting both extracellularly and intracellularly. The data indicate strict cell specificity in S1P-evoked calcium response, which could be relevant to communication between neurons and glial cells in the cerebellum.

Sphingolipid metabolism in neural cells

Sphingolipids were discovered more than a century ago in the brain. Cerebrosides and sphingomyelins were named so because they were first isolated from neural tissue. Although glycosphingolipids and especially those containing sialic acid in their oligosaccharide moiety are particularly abundant in the brain, sphingolipids are ubiquitous cellular membrane components. They form cell- and species-specific profiles at the cell surfaces that characteristically change in development, differentiation, and oncogenic transformation, indicating the significance of these lipid molecules for cell-cell and cell-matrix interactions as well as for cell adhesion, modulation of membrane receptors and signal transduction. This review summarizes sphingolipid metabolism with emphasis on aspects particularly relevant in neural cell types, including neurons, oligodendrocytes and neuroblastoma cells. In addition, the reader is briefly introduced into the methodology of lipid evaluation techniques and also into the putative physiological functions of glycosphingolipids and their metabolites in neural tissue.

Sphingosine-1-phosphate is released by cerebellar astrocytes in response to bFGF and induces astrocyte proliferation through Gi-protein-coupled receptors

The mitogenic role of sphingosine-1-phosphate (S1P) and its involvement in basic fibroblast growth factor (bFGF)-induced proliferation were examined in primary cultures of cerebellar astrocytes. Exposure to bFGF resulted in a rapid increase of extracellular S1P formation, bFGF inducing astrocytes to release S1P, but not sphingosine kinase, in the extracellular milieu. The SK inhibitor N,N-dimethylsphingosine inhibited S1P release as well as bFGF-induced growth stimulation. S1P application in quiescent astrocytes caused a dose-dependent increase in DNA synthesis. This gliotrophic effect was induced by a brief exposure to low nanomolar S1P, mimicked by the S1P receptor agonist dihydro-S1P, and inhibited by pertussis toxin (PTX), an inactivator of G(i)/G(o)-proteins. S1P also induced activation of extracellular signal-regulated kinase that was inhibited again by PTX. Moreover, the S1P lyase inhibitor 4-deoxypyridoxine induced the cellular accumulation of S1P but did not affect DNA synthesis. These results support the view that S1P exerted a mitogenic effect on cerebellar astrocytes extracellularly, most likely through cell surface S1P receptors. In agreement, mRNAs for S1P1, S1P2, and S1P3 receptors are expressed in cerebellar astrocytes (Anelli et al., 2005. J Neurochem 92:1204-1215). Ceramide, a negative regulator of astrocyte proliferation and down-regulated by bFGF (Riboni et al., 2002. Cerebellum 1:129-135), efficiently inhibited S1P-induced proliferation. The S1P action appears to be part of an autocrine/paracrine cascade stimulated by bFGF and, together with ceramide down-regulation, essential for astrocytes to respond to bFGF. The results suggest that S1P and bFGF/S1P may play an important role in physiopathological glial proliferation, such as brain development, reactive gliosis and brain tumor formation.

Extracellular release of newly synthesized sphingosine-1-phosphate by cerebellar granule cells and astrocytes

Sphingosine-1-phosphate (S1P) is a potent biomediator that can act as either an intracellular or an intercellular messenger. In the nervous system it exerts a wide range of actions, and specific membrane receptors for it have been identified in various regions. However, the physiological origin of extracellular S1P in the nervous system is largely unknown. We investigated cerebellar granule cells at different stages of differentiation and astrocytes in primary cultures as possible origins of extracellular S1P. Although these cells show marked differences in S1P metabolism, we found that they can all release S1P and express mRNAs for S1P specific receptors. Extracellular S1P derives from the export of newly synthesized intracellular S1P, and not from the action of a released sphingosine kinase. S1P release is rapid, efficient, and can be regulated by exogenous stimuli. Phorbol ester treatment resulted in an increase in sphingosine kinase 1 activity in the membranes, accompanied by a significant increase in extracellular S1P. S1P release in cells from the cerebellum emerges as a regulated mechanism, possibly related to a specific pool of newly synthesized S1P. To our knowledge, this is the first evidence of the extracellular release of S1P by primary cells from the CNS, which supports a role of S1P as autocrine/paracrine physiological messenger in the cerebellum.

Acid ceramidase overexpression prevents the inhibitory effects of saturated fatty acids on insulin signaling

Recent studies indicate that insulin resistance and type 2 diabetes result from the accumulation of lipids in tissues not suited for fat storage, such as skeletal muscle and the liver. To elucidate the mechanisms linking exogenous fats to the inhibition of insulin action, we evaluated the effects of free fatty acids (FFAs) on insulin signal transduction in cultured C2C12 myotubes. As we described previously (Chavez, J. A., and Summers, S. A. (2003) Arch. Biochem. Biophys. 419, 101-109), long-chain saturated FFAs inhibited insulin stimulation of Akt/protein kinase B, a central regulator of glucose uptake and anabolic metabolism. Moreover, these FFAs stimulated the de novo synthesis of ceramide and sphingosine, two sphingolipids shown previously to inhibit insulin action. To determine the contribution of either sphingolipid in FFA-dependent inhibition of insulin action, we generated C2C12 myotubes that constitutively overexpress acid ceramidase (AC), an enzyme that catalyzes the lysosomal conversion of ceramide to sphingosine. AC overexpression negated the inhibitory effects of saturated FFAs on insulin signaling while blocking their stimulation of ceramide accumulation. By contrast, AC overexpression stimulated the accrual of sphingosine. These results support a role for aberrant accumulation of ceramide, but not sphingosine, in the inhibition of muscle insulin sensitivity by exogenous FFAs.

Failure to express GAP-43 leads to disruption of a multipotent precursor and inhibits astrocyte differentiation

The nervous system-specific protein GAP-43 is significantly upregulated in neurons and glia that are differentiating. In P19 EC cells that do not express GAP-43, neurogenesis is inhibited; many immature neurons apoptose and the survivors do not mature morphologically. Here we show that the initial defect is in an early precursor with characteristics of a neural stem cell, which failed to respond normally to retinoic acid (RA). As a consequence, its progeny had altered cell fates: In addition to the neuronal defects previously reported, RC1-labeled radial glia failed to exit the cell cycle, accumulated, and failed to acquire GFAP immunoreactivity. However, leukemia inhibitory factor (LIF) could stimulate GFAP expression suggesting that astrocytes not derived from radial glia are less affected by absence of GAP-43. Differentiation of radial glia-derived astrocytes was also inhibited in glial cultures from GAP-43 (-/-) cerebellum, and in GAP-43 (-/-) telencephalon in vivo, differentiation of astrocytes derived from both radial and nonradial glia lineages were both affected: In the glial wedge, GFAP-labeled radial glia-derived astrocytes were reduced consistent with the interpretation that they may be unable to deflect GAP-43 (-/-) commissural axons toward the midline. At the midline, both radial and nonradial glia-derived astrocytes were also decreased although it fused normally. The results demonstrate that GAP-43 expressed in multipotent precursors is required for appropriate cell fate commitment, and that its absence affects astrocyte as well as neuronal differentiation.

Ceramide in nitric oxide inhibition of glioma cell growth. Evidence for the involvement of ceramide traffic

The treatment of C6 glioma cells with the nitric oxide donor, PAPANONOate ((Z)-[N-(3-ammoniopropyl)-N-(n-propyl)amino]diazen-1-ium-1,2-diolate), resulted in a dose-dependent inhibition of cell proliferation. This was associated to a rapid and significant increase of ceramide levels and was mimicked by treatments that augment cellular ceramide. Metabolic experiments with radioactive sphingosine, serine, and choline showed that nitric oxide strongly reduced the utilization of ceramide for the biosynthesis of both sphingomyelin and glucosylceramide. Nevertheless, nitric oxide did not modify the activity of different enzymes of ceramide metabolism. The possibility that nitric oxide impairs the availability of ceramide for sphingolipid biosynthesis was then investigated. The metabolism of N-hexanoyl-[(3)H]sphingosine demonstrated that nitric oxide did not affect the biosynthesis of N-hexanoyl-[(3)H]sphingolipids but inhibited the metabolic utilization of long chain [(3)H]ceramide, synthesized in the endoplasmic reticulum (ER) from the recycled [(3)H]sphingosine. Moreover, results obtained with fluorescent ceramides, brefeldin A, ATP depletion, as well as in a ceramide transport assay indicate that nitric oxide impairs the traffic of ceramide from ER to Golgi apparatus. All this supports that, in glioma cells, the modulation of ceramide traffic can contribute to the regulation of its intracellular levels and participate in the nitric oxide-activated signaling pathway involved in the control of cell proliferation.

Metabolic formation of ceramide-1-phosphate in cerebellar granule cells: evidence for the phosphorylation of ceramide by different metabolic pathways

Aiming to investigate the possible production of ceramide-1-phosphate from complex sphingolipid metabolism in neurons, we administered radiolabeled sphingolipids to cerebellar granule cells and inspected the formation of labeled ceramide-1-phosphate in different experimental conditions. We report that differentiated granule cells are capable to form Cer-1-P via ceramide derived from SM degradation at the plasma membrane level. Moreover we observed that ceramide-1-phosphate can be also produced from a metabolic pathway not involving SM degradation. In particular, we obtained evidence that ceramide, synthesized via the recycling of sphingosine produced from ganglioside catabolism, can also be the precursor of ceramide-1-phosphate. We also found that undifferentiated and differentiated granule cells display different capacities to phosphorylate Cer produced by the two different metabolic pathways. The results here obtained demonstrate that cerebellar neurons are able to metabolically produce ceramide-1-phosphate and support that this molecule may serve a potential role in sphingoid-mediated signaling in the nervous system.

Ceramide regulation of apoptosis versus differentiation: a walk on a fine line. Lessons from neurobiology

One of the characteristics of ceramide-mediated biology is the variety of biological outcomes observed in response to its intracellular accumulation. The molecular mechanisms that govern the cell "decision-making" in response to ceramide remain largely unclear. In this perspective, the study of neural models has begun to provide important insight into the understanding of these mechanisms that regulate differentiation and cell death. Indeed, differentiation and cell death are among the most common effects elicited by ceramide in most cell types and in neural cells, too. Therefore, the lessons we may learn from the study of ceramide regulation of neurobiology would also shed light on the regulation of ceramide-mediated biology in other cellular models. Since increasing evidence links aberrant metabolism of ceramide to different pathologies, the understanding of the mechanisms underlying these events may represent the key to the design of novel therapeutic approaches.

Nitric oxide production in living neurons is modulated by sphingosine: a fluorescence microscopy study

An investigation was carried out into the possible effect of sphingosine (Sph) on nitric oxide (NO) production in living neurons. Differentiated granule cells were used in a dynamic videoimaging analysis of single cells labeled, simultaneously, with FURA-2 and the NO indicator 4,5-diaminofluorescein. The results demonstrate that Sph exerts a potent inhibitory effect on the Ca2+-dependent production of NO, without modifying the [Ca2+]i. The effect appears to be specific as neither ceramide nor Sph-1-phosphate had any effect on the NO and [Ca2+]i levels. The data demonstrate that Ca2+-dependent NO production is a specific Sph target in living granule cells, suggesting that this bioactive sphingoid plays a relevant role in neuronal NO signaling.

Basic fibroblast growth factor-induced proliferation of primary astrocytes. evidence for the involvement of sphingomyelin biosynthesis

We recently reported that the marked decrease in cellular ceramide in primary astrocytes is an early event associated with the mitogenic activity of basic fibroblast growth factor (bFGF) (Riboni, L., Viani, P., Bassi, R., Stabieini, A., and Tettamanti, G. (2000) GLIA 32, 137-145). Here we show that a rapid activation of sphingomyelin biosynthesis appears to be the major mechanism responsible for the fall in ceramide levels induced by bFGF. When quiescent astrocytes were treated with bFGF, an increased amount of newly synthesized ceramide (from either l-[(3)H]serine or [(3)H]sphingosine) was directed toward the biosynthesis of sphingomyelin. Conversely, bFGF did not appear to affect ceramide levels by other metabolic pathways involved in ceramide turnover such as sphingomyelin degradation and ceramide biosynthesis, degradation, and glucosylation. Enzymatic studies demonstrating a relevant and rapid increase in sphingomyelin synthase activity after bFGF treatment have provided a convincing explanation for the activation of sphingomyelin biosynthesis. The bFGF-induced increase in sphingomyelin synthase appears to depend on a post-translational activation mechanism. Moreover, in the presence of brefeldin A, the activation of sphingomyelin biosynthesis was abolished, suggesting that the enzyme is located in a compartment other than the Golgi apparatus. Also the phosphatidylcholine-specific phospholipase C inhibitor D609 exerted a potent inhibitory effect on sphingomyelin biosynthesis. Finally, we demonstrate that inhibition of sphingomyelin biosynthesis by brefeldin A or D609 led to a significant inhibition of bFGF-stimulated mitogenesis. All this supports that, in primary astrocytes, the early activation of sphingomyelin synthase is involved in the bFGF signaling pathway leading to proliferation.