Acid and neutral sphingomyelinases of rat brain. Activity in developing brain and regional distribution in adult brain

Cocaine attenuates acid sphingomyelinase activity during establishment of addiction-related behavior-A translational study in rats and monkeys

Cocaine addiction is a severe psychiatric condition for which currently no effective pharmacotherapy is available. Brain mechanisms for the establishment of addiction-related behaviors are still not fully understood, and specific biomarkers for cocaine use are not available. Sphingolipids are major membrane lipids, which shape neuronal membrane composition and dynamics in the brain. Here, we investigated how chronic cocaine exposure during establishment of addiction-related behaviors affects the activity of the sphingolipid rheostat controlling enzymes in the brain of rats. As we detected specific effects on several enzymes in the brain, we tested whether the activity of selected enzymes in the blood may serve as potential biomarker for cocaine exposure in non-human primates (Callithrix penicillata). We found that intravenous cocaine self-administration led to a reduced mRNA expression of Cers1, Degs1 and Degs2, and Smpd1 in the prefrontal cortex of rats, as well as a reduction of Cers4 expression in the striatum. These effects reversed after 10 days of abstinence. Monkeys showed a robust cocaine-induced place preference (CPP). This coincided with a reduction in blood acid sphingomyelinase (ASM) activity after CPP establishment. This effect normalized after 15 days of abstinence. Altogether, these findings suggest that the establishment of cocaine addiction-related behaviors coincides with changes in the activity of sphingolipid controlling enzymes. In particular, blood ASM levels may serve as a translational biomarker for recent cocaine exposure.

Neutral sphingomyelinase mediates the co-morbidity trias of alcohol abuse, major depression and bone defects

Mental disorders are highly comorbid and occur together with physical diseases, which are often considered to arise from separate pathogenic pathways. We observed in alcohol-dependent patients increased serum activity of neutral sphingomyelinase. A genetic association analysis in 456,693 volunteers found associations of haplotypes of SMPD3 coding for NSM-2 (NSM) with alcohol consumption, but also with affective state, and bone mineralisation. Functional analysis in mice showed that NSM controls alcohol consumption, affective behaviour, and their interaction by regulating hippocampal volume, cortical connectivity, and monoaminergic responses. Furthermore, NSM controlled bone-brain communication by enhancing osteocalcin signalling, which can independently supress alcohol consumption and reduce depressive behaviour. Altogether, we identified a single gene source for multiple pathways originating in the brain and bone, which interlink disorders of a mental-physical co-morbidity trias of alcohol abuse-depression/anxiety-bone disorder. Targeting NSM and osteocalcin signalling may, thus, provide a new systems approach in the treatment of a mental-physical co-morbidity trias.

Sphingolipids in Alzheimer's disease, how can we target them?

Altered levels of sphingolipids and their metabolites in the brain, and the related downstream effects on neuronal homeostasis and the immune system, provide a framework for understanding mechanisms in neurodegenerative disorders and for developing new intervention strategies. In this review we will discuss: the metabolites of sphingolipids that function as second messengers; and functional aberrations of the pathway resulting in Alzheimer's disease (AD) pathophysiology. Focusing on the central product of the sphingolipid pathway ceramide, we describ approaches to pharmacologically decrease ceramide levels in the brain and we argue on how the sphingolipid pathway may represent a new framework for developing novel intervention strategies in AD. We also highlight the possible use of clinical and non-clinical drugs to modulate the sphingolipid pathway and sphingolipid-related biological cascades.

SMPD3 deficiency perturbs neuronal proteostasis and causes progressive cognitive impairment

Neutral sphingomyelinase smpd3 is most abundantly expressed in neurons of brain. The function of SMPD3 has remained elusive. Here, we report a pathogenetic nexus between absence of SMPD3 in the Golgi compartment (GC) of neurons of the smpd3-/- mouse brain, inhibition of Golgi vesicular protein transport and progressive cognitive impairment. Absence of SMPD3 activity in the Golgi sphingomyelin cycle impedes remodeling of the lipid bilayer, essential for budding and multivesicular body formation. Importantly, we show that inhibition of the Golgi vesicular protein transport causes accumulation of neurotoxic proteins APP, Aβ and phosphorylated Tau, dysproteostasis, unfolded protein response, and apoptosis, which ultimately manifests in progressive cognitive decline, similar to the pathognomonic signatures of familial and sporadic forms of Alzheimer´s disease. This discovery might contribute to the search for other primary pathogenic mechanisms, which link perturbed lipid bilayer structures and protein processing and transport in the neuronal Golgi compartment and neurodegeneration and cognitive deficits.

Cambinol, a novel inhibitor of neutral sphingomyelinase 2 shows neuroprotective properties

Ceramide is a bioactive lipid that plays an important role in stress responses leading to apoptosis, cell growth arrest and differentiation. Ceramide production is due in part to sphingomyelin hydrolysis by sphingomyelinases. In brain, neutral sphingomyelinase 2 (nSMase2) is expressed in neurons and increases in its activity and expression have been associated with pro-inflammatory conditions observed in Alzheimer's disease, multiple sclerosis and human immunodeficiency virus (HIV-1) patients. Increased nSMase2 activity translates into higher ceramide levels and neuronal cell death, which can be prevented by chemical or genetic inhibition of nSMase2 activity or expression. However, to date, there are no soluble, specific and potent small molecule inhibitor tool compounds for in vivo studies or as a starting point for medicinal chemistry optimization. Moreover, the majority of the known inhibitors were identified using bacterial, bovine or rat nSMase2. In an attempt to identify new inhibitor scaffolds, two activity assays were optimized as screening platform using the recombinant human enzyme. First, active hits were identified using a fluorescence-based high throughput compatible assay. Then, hits were confirmed using a 14C sphingomyelin-based direct activity assay. Pharmacologically active compounds and approved drugs were screened using this strategy which led to the identification of cambinol as a novel uncompetitive nSMase2 inhibitor (Ki = 7 μM). The inhibitory activity of cambinol for nSMase2 was approximately 10-fold more potent than for its previously known target, silence information regulator 1 and 2 (SIRT1/2). Cambinol decreased tumor necrosis factor-α or interleukin-1 β-induced increases of ceramide and cell death in primary neurons. A preliminary study of cambinol structure and activity allowed the identification of the main structural features required for nSMase2 inhibition. Cambinol and its analogs may be useful as nSMase2 inhibitor tool compounds to prevent ceramide-dependent neurodegeneration.

Sphingolipids in psychiatric disorders and pain syndromes

Despite the high prevalence and devastating impact of psychiatric disorders, little is known about their etiopathology. In this review, we provide an overview on the participation of sphingolipids and enzymes responsible for their metabolism in mechanisms underlying psychiatric disorders. We focus on the pathway from sphingomyelin to proapoptotic ceramide and the subsequent metabolism of ceramide to sphingosine, which is in turn phosphorylated to yield anti-apoptotic sphingosine-1-phosphate (S1P).The sphingomyelinase/ceramide system has been linked to effects of reactive oxygen species and proinflammatory cytokines in the central nervous system as well as to synaptic transmission. Compared to ubiquitously expressed acid sphingomyelinase, acid and neutral ceramidase and neutral sphingomyelinase are highly active in brain regions. Depressed patients show elevated plasma ceramide levels and increased activities of acid sphingomyelinase which is functionally inhibited by many anti-depressive drugs. Exposure to alcohol is associated with an activation of acid and neutral sphingomyelinase observed in cell culture, mouse models and in alcohol-dependent patients and with increased concentrations of ceramide in various organs.Levels of sphingomyelin and ceramide are altered in erythrocytes and post-mortem brain tissues of schizophrenic patients in addition to changes in expression patterns for serine palmitoyltransferase and acid ceramidase leading to impaired myelination. After induction of anxiety-like behavior in animal models, higher serum levels of S1P were reported to lead to neurodegeneration. Correspondingly, S1P infusion appeared to increase anxiety-like behavior. Significantly upregulated levels of the endogenous ceramide catabolite N,N-dimethylsphingosine were observed in rat models of allodynia. Conversely, rats injected intrathecally with N,N-dimethylsphingosine developed mechanical allodynia. Moreover, S1P has been implicated in spinal nociceptive processing.The increasing interest in lipidomics and improved analytical methods led to growing insight into the connection between psychiatric and neurological disorders and sphingolipid metabolism and may once provide new targets and strategies for therapeutic intervention.

Blueberry treatment antagonizes C-2 ceramide-induced stress signaling in muscarinic receptor-transfected COS-7 cells

Previous research has shown that muscarinic receptors (MAChRs) show loss of sensitivity in aging and AD and are selectively sensitive to oxidative stress (OS). Thus, COS-7 cells transfected (tn) with MAChR subtype M1 show > OS sensitivity [as reflected in the ability of the cell to extrude or sequester Ca(2+) following depolarization (recovery) by oxotremorine (oxo) and exposure to dopamine (DA) or amyloid beta (Abeta)] than M3-transfected COS-7 cells. Blueberry (BB) extract pretreatment prevented these deficits. Research has also indicated that C2 ceramide (Cer) has several age-related negative cellular effects (e.g., OS). When these cells were treated with Cer, the significant decrements in the ability of both types of tn cells to initially respond to oxo were antagonized by BB treatment. Present experiments assessed signaling mechanisms involved in BB protection in the presence or absence of DA, Abeta, and/or Cer in this model. Thus, control or BB-treated M1 and M3 tn COS-7 cells were exposed to DA or Abeta(42) in the presence or absence of Cer. Primarily, results showed that the effects of DA or Abeta(42) were to increase stress (e.g., PKCgamma, p38MAPK) and protective signals (e.g., pMAPK). Cer also appeared to raise several of the stress and protective signals in the absence of the other stressors, including PKCgamma, pJNK, pNfkappaB, p53, and p38MAPK, while not significantly altering MAPK, or Akt. pArc was, however, increased by Cer in both types of transfected cells. The protective effects of BB when combined with Cer generally showed greater protection when BB extract was applied prior to Cer, except for one protective signal (pArc) where a greater effect was seen in the M3 cells exposed to Abeta(42.) In the absence of the Abeta(42) or DA, for several of the stress signals (e.g., pNfkappaB, p53), BB lowered their Cer-induced increases in M1- and M3-transfected cells. We are exploring these interactions further, but it is clear that increases in ceramide, to the same levels as are seen in aging, can have profound effects on calcium clearance and signaling during oxidative stress.

The ceramide transporter and the Goodpasture antigen binding protein: one protein--one function?

The Goodpasture antigen-binding protein (GPBP) and its splice variant the ceramide transporter (CERT) are multifunctional proteins that have been found to play important roles in brain development and biology. However, the function of GPBP and CERT is controversial because of their involvement in two apparently unrelated research fields: GPBP was initially isolated as a protein associated with collagen IV in patients with the autoimmune disease Goodpasture syndrome. Subsequently, a splice variant lacking a serine-rich domain of 26 amino acids (GPBPDelta26) was found to mediate the cytosolic transport of ceramide and was therefore (re)named CERT. The two splice forms likely carry out different functions in specific sub-cellular localizations. Selective GPBP knockdown induces extensive apoptosis and tissue loss in the brain of zebrafish. GPBP/GPBPDelta26 knock-out mice die as a result of structural and functional defects in endoplasmic reticulum and mitochondria. Because both mitochondria and ceramide play an important role in many biological events that regulate neuronal differentiation, cellular senescence, proliferation and cell death, we propose that GPBP and CERT are pivotal in neurodegenerative processes. In this review, we discuss the current state of knowledge on GPBP and CERT, including the molecular and biochemical characterization of GPBP in the field of autoimmunity as well as the fundamental research on CERT in ceramide transport, biosynthesis, localization, metabolism and cell homeostasis.

Is coenzyme Q a key factor in aging?

Coenzyme Q (Q) is a key component for bioenergetics and antioxidant protection in the cell. During the last years, research on diseases linked to Q-deficiency has highlighted the essential role of this lipid in cell physiology. Q levels are also affected during aging and neurodegenerative diseases. Therefore, therapies based on dietary supplementation with Q must be considered in cases of Q deficiency such as in aging. However, the low bioavailability of dietary Q for muscle and brain obligates to design new mechanisms to increase the uptake of this compound in these tissues. In the present review we show a complete picture of the different functions of Q in cell physiology and their relationship to age and age-related diseases. Furthermore, we describe the problems associated with dietary Q uptake and the mechanisms currently used to increase its uptake or even its biosynthesis in cells. Strategies to increase Q levels in tissues are indicated.

Sphingolipids in apoptosis, survival and regeneration in the nervous system

Simple sphingolipids such as ceramide, sphingosine and sphingosine 1-phosphate are key regulators of diverse cellular functions. Their roles in the nervous system are supported by extensive evidence derived primarily from studies in cultured cells. More recently animal studies and studies with human samples have revealed the importance of ceramide and its metabolites in the development and progression of neurodegenerative disorders. The roles of sphingolipids in neurons and glial cells are complex, cell dependent, and many times contradictory. In this review I will summarize the effects elicited by ceramide and ceramide metabolites in cells of the nervous system, in particular those effects related to cell survival and death, emphasizing the molecular mechanisms involved. I also discuss recent evidence for the implication of sphingolipids in the development and progression of certain dementias.

IGF-I protects cortical neurons against ceramide-induced apoptosis via activation of the PI-3K/Akt and ERK pathways; is this protection independent of CREB and Bcl-2?

Current understanding of IGF-I-mediated neuroprotection implies the activation of phosphatidylinositol-3-kinase (PI-3K), which leads to the activation of Akt/Protein Kinase B. In non-neuronal cells, Akt phosphorylates and activates the transcription factor CREB, implicated in the transcription of the anti-apoptotic bcl-2 gene. This paper further analyses the anti-apoptotic IGF-I action in neurons. We show that IGF-I protects cortical neurons against ceramide-induced apoptosis. Ceramide decreases Akt phosphorylation during apoptotic process whereas a simultaneous treatment with IGF-I increases Akt phosphorylation. Analysis of the signal transduction pathways revealed that IGF-I induces CREB phosphorylation via PI-3K and ERK, whereas simultaneous ceramide and IGF-I treatment decreases CREB phosphorylation. Although an overexpression of Bcl-2 protects cortical neurons against ceramide-induced apoptosis, our data indicate that the Bcl-2 protein level is not modulated during IGF-I, ceramide and/or LY294002 treatment. In consequence, we demonstrated that IGF protects neurons against ceramide-induced apoptosis and that IGF-I protection involves the PI-3K/Akt and ERK pathways; this protection may be independent of CREB and Bcl-2.

Ceramide induces neuronal apoptosis through mitogen-activated protein kinases and causes release of multiple mitochondrial proteins

Ceramide accumulates in neurons during various disorders associated with acute or chronic neurodegeneration. In these studies, we investigated the mechanisms of ceramide-induced apoptosis in primary cortical neurons using exogenous C(2) ceramide as well as inducing endogenous ceramide accumulation using inhibitors of glucosylceramide synthetase. Ceramide induced the translocation of certain, but not all, pro-apoptotic mitochondrial proteins: cytochrome c, Omi, SMAC, and AIF were released from the mitochondria, whereas Endonuclease G was not. Ceramide also selectively altered the phosphorylation state of members of the MAPK superfamily, causing dephosphorylation of ERK1/2 and hyperphosphorylation of p38 MAP kinases, but not affecting the phosphorylation of JNK or ERK5. Inhibitors of the p38 MAP kinase pathway (SB-202190 or SB-203580) and an inhibitor of the ERK1/2 pathway (U0126) reduced ceramide-induced neuronal death. These p38 and ERK1/2 inhibitors appear to block ceramide-activated apoptotic signaling upstream of the mitochondria, as they attenuated mitochondrial release of cytochrome c, Omi, AIF, and SMAC, as well as reducing ceramide-induced caspase-3 activation.

Identification of three competitive inhibitors for membrane-associated, Mg2+-dependent and neutral 60 kDa sphingomyelinase activity

Methanol extracts of domestic plants of Korea were evaluated as a potential inhibitor of neutral pH optimum and membrane-associated 60 kDa sphingomyelinase (N-SMase) activity. In this study, we partially purified N-SMase from bovine brain membranes using ammonium sulfate. It was purified approximately 163-fold by the sequential use of DE52, Butyl-Toyopearl, DEAE-Cellulose, and Phenyl-5PW column chromatographies. The purified N-SMase activity was assayed in the presence of the plant extracts of three hundreds species. Based on the in vitro assay, three plant extracts significantly inhibited the N-SMase activity in a time- and concentration-dependent manner. To further examine the inhibitory pattern, a Dixon plot was constructed for each of the plant extracts. The extracts of Abies nephrolepis, Acer tegmentosum, and Ginkgo biloba revealed a competitive inhibition with the inhibition constant (Ki) of 11.9 microg/ mL, 9.4 microg/mL, and 12.9 microg/mL, respectively. These extracts also inhibited in a dose-dependent manner the production of ceramide induced by serum deprivation in human neuroblastoma cell line SH-SY5Y.

Coenzyme Q-dependent functions of plasma membrane in the aging process

Coenzyme Q (Q) is reduced in plasma membrane and mitochondria by NAD(P)H-dependent reductases providing reducing equivalents to maintain both respiratory chain and antioxidant protection. Reactive oxygen species (ROS) are accumulated in the aging process originating mainly in mitochondria but also in other membranes, such as plasma membrane partially by the loss of electrons from the semiquinone. The reduction of Q by NAD(P)H-dependent reductases in plasma membrane is responsible for providing its antioxidant capacity, preventing both the lipid peroxidation chain and the activation of the ceramide-dependent apoptosis pathway. Both Q content and its reductases are decreased in plasma membrane of aging mammals. Calorie restriction, which extends mammal life span, increases the content of Q in the plasma membrane and also activates Q reductases in this membrane. Both lipid peroxidation and ceramide production are decreased in the plasma membrane in calorie-restricted animals. Plasma membrane is, then, an important cellular component to control the aging process through its concentration and redox state of Q.

Neutral sphingomyelinase 2 (smpd3) in the control of postnatal growth and development

Neutral sphingomyelinases sphingomyelin phosphodiesterase (SMPD)2 and -3 hydrolyze sphingomyelin to phosphocholine and ceramide. smpd2 is expressed ubiquitously, and smpd3 is expressed predominantly in neurons of the CNS. Their activation and the functions of the released ceramides have been associated with signaling pathways in cell growth, differentiation, and apoptosis. However, these cellular responses remain poorly understood. Here we describe the generation and characterization of the smpd3(-/-) and smpd2(-/-)smpd3(-/-) double mutant mouse, which proved to be devoid of neutral sphingomyelinase activity. SMPD3 plays a pivotal role in the control of late embryonic and postnatal development: the smpd3-null mouse develops a novel form of dwarfism and delayed puberty as part of a hypothalamus-induced combined pituitary hormone deficiency. Our studies suggest that SMPD3 is segregated into detergent-resistant subdomains of Golgi membranes of hypothalamic neurosecretory neurons, where its transient activation modifies the lipid bilayer, an essential step in the Golgi secretory pathway. The smpd3(-/-) mouse might mimic a form of human combined pituitary hormone deficiency.

C-Jun N-terminal kinases/c-Jun and p38 pathways cooperate in ceramide-induced neuronal apoptosis

Understanding the regulation of the apoptotic program in neurons by intracellular pathways is currently a subject of great interest. Recent results suggest that c-Jun N-terminal kinases (JNK), mitogen-activated protein kinases and the transcription factor c-Jun are important regulators of this cell death program in post-mitotic neurons following survival-factor withdrawal. Our study demonstrates that ceramide levels increase upon survival-factor withdrawal in primary cultured cortical neurons. Furthermore, survival-factor withdrawal or addition of exogenous c(2)-ceramide induces JNK pathway activation in these cells. Western blot analyses of JNK and c-Jun using phospho-specific antibodies reveal that JNK and subsequent c-Jun phosphorylation occur hours before the initiation of apoptosis, reflected morphologically by neurite retraction and fragmentation, cell-body shrinkage and chromatin fragmentation. Immunocytochemistry using the same antibodies shows that phospho-JNK are localized in the neurites of control neurons and translocate to the nucleus where phospho-c-Jun concurrently appears upon ceramide-induced apoptosis. To determine if ceramide-induced c-Jun activation is responsible for the induction of the apoptotic program, we performed transient transfections of a dominant negative form of c-Jun, truncated in its transactivation region. Our results show that DNc-Jun partially protects cortical neurons from ceramide-induced apoptosis. Treatment of dominant negative c-Jun-expressing neurons with the pharmacological inhibitor of p38 kinase, SB203580, completely blocked neuronal death. Thus our data show that p38 and JNK/c-Jun pathways cooperate to induce neuronal apoptosis.

Ceramide-induced apoptosis in cortical neurons is mediated by an increase in p38 phosphorylation and not by the decrease in ERK phosphorylation

Ceramide, the central molecule of the sphingomyelin pathway, serves as a second messenger for cellular functions ranging from proliferation and differentiation to growth arrest and apoptosis. In this study we show that c2-ceramide induces apoptosis in primary cortical neuron cultures and that this effect correlates with differential modulation of mitogen-activated protein kinase (MAPK) cascades. Phosphorylation of extracellular signal-regulated kinases (ERKs) and their upstream activators MAPK kinases (MEKs), as measured by immunoblotting is rapidly decreased by c2-ceramide. However, the MEK inhibitor PD98059 alone does not induce apoptosis and in combination with c2-ceramide it does not modify c2-ceramide-induced apoptosis. Treatment with c2-ceramide increases p38 and c-Jun N-terminal kinase (JNK) phosphorylation before and during caspase-3 activation. The p38 inhibitor SB203580 partially protects cortical neurons against c2-ceramide-induced apoptosis, implicating the p38 pathway in this process. The c2-ceramide treatment also increases levels of c-jun, c-fos and p53 mRNA in primary cortical neuron cultures, but this is independent of p38 activation. Our study further elucidates the time-courses of MAPK cascade modulation, and of c-jun, c-fos and p53 activation during c2-ceramide-induced neuronal apoptosis. It reveals that one of the activated kinases, p38, is necessary for this apoptosis.

Cloning and characterization of the mammalian brain-specific, Mg2+-dependent neutral sphingomyelinase

The enzymatic breakdown of sphingomyelin by sphingomyelinases is considered the major source of the second messenger ceramide. Studies on the contribution of the various described acidic and neutral sphingomyelinases to the signaling pool of ceramide have been hampered by the lack of molecular data on the neutral sphingomyelinases (nSMases). We recently identified a mammalian nSMase, an integral membrane protein with remote similarity to bacterial sphingomyelinases. However, its ubiquitous expression pattern is in contrast to previous findings that sphingomyelinase activity is found mainly in brain tissues. By using an improved database search method, combined with phylogenetic analysis, we identified a second mammalian nSMase (nSMase2) with predominant expression in the brain. The sphingomyelinase activity of nSMase2 has a neutral pH optimum, depends on Mg(2+) ions, and is activated by unsaturated fatty acids and phosphatidylserine. Immunofluorescence reveals a neuron-specific punctate perinuclear staining, which colocalizes with a Golgi marker in a number of cell lines. The likely identity of nSMase2 with cca1, a rat protein involved in contact inhibition of 3Y1 fibroblasts, suggests a role for this enzyme in cell cycle arrest. Both mammalian nSMases are members of a superfamily of Mg(2+)-dependent phosphohydrolases, which also contains nucleases, inositol phosphatases, and bacterial toxins.

Characterization and subcellular localization of murine and human magnesium-dependent neutral sphingomyelinase

Sphingomyelinases (SMases) catalyze the hydrolysis of sphingomyelin, an essential lipid constituent of the plasma membrane, lysosomal membranes, endoplasmic reticulum, and the Golgi membrane stacks of mammalian cells. In this study, we report the biochemical and functional characterization and subcellular localization of magnesium-dependent nSMase1 from overexpressing human embryonic kidney (HEK293) cells. Site-directed mutagenesis of conserved residues probably involved in the enzymatic sphingomyelin cleavage as well as the removal of one or both putative transmembrane domains lead to the complete loss of enzymatic activity of human nSMase1 expressed in HEK293 cells. Polyclonal antibodies raised against recombinant mammalian nSMase1 immunoprecipitated and inactivated the enzyme in membrane extracts of overexpressing HEK293 cells and different murine tissues. Cell fractionation combined with immunoprecipitation studies localized the nSMase1 protein predominantly in the microsomal fraction. The enzyme colocalized with marker proteins of the endoplasmic reticulum and the Golgi apparatus in immunocytochemistry. Anti-nSMase1 antibodies did not affect the nSMase activity in the plasma membrane fraction and membrane extracts from murine brain. Our study leads to the conclusion that nSMase1 is one of at least two mammalian neutral sphingomyelinases with different subcellular localization, tissue specificity, and enzymatic properties.

Sphingomyelin-degrading pathways in human cells role in cell signalling

The ubiquitous sphingophospholipid sphingomyelin (SM) can be hydrolysed in human cells to ceramide by different sphingomyelinases (SMases). These enzymes exert a dual role, enabling not only the turnover of membrane SM and the degradation of exogenous (lipoprotein) SM, but also the signal-induced generation of the lipid second messenger ceramide. This review focuses on the function(s) of the different SMases in living cells. While both lysosomal and non-lysosomal pathways that ensure SM hydrolysis in intact cells can be distinguished, the precise contribution of each of these SM-cleaving enzymes to the production of ceramide as a signalling molecule remains to be clarified.

Ceramide prevents motoneuronal cell death through inhibition of oxidative signal

We previously reported that cell death of rat spinal motoneurons, induced by trophic factor-deprivation, was attenuated by the application of exogenous cell-permeable ceramide (C6-Cer), or bacterial sphingomyelinase (SMase). Recently, motoneuronal cell death was demonstrated to be mediated by the generation of reactive oxygen species (ROS), including superoxide and peroxinitrite. In this study, to investigate the protective mechanism of ceramide (Cer), we examined the effects of Cer and sphingolipid metabolites against ROS generation and oxidative injury in enriched motoneuron cultures. Staining with C-DCDHF-DA, a fluorescent probe for detection of ROS, demonstrated that application of C6-Cer (2.5 mM) or bacterial SMase inhibited the increase of ROS generation. C6-dihydro-Cer, a biologically inactive analogue of C6-Cer, sphingosine, and sphingosine-1-phosphate did not affect ROS generation. This specificity corresponded to the results of cell survival assays. In addition, C6-Cer was shown to specifically inhibit ROS-induced reactions, such as tyrosine nitration and lipid peroxidation, in studies using antibodies against peroxinitrite and 4-hydroxinonenal, respectively. A potent neurotrophin for motoneurons, GDNF, had inhibitory effects against ROS generation and ROS-induced reactions. C6-Cer was also effective in the prevention of cytotoxicity induced by 1-buthionine-sulfoximine, an inhibitor of glutathione synthesis. These observations suggest that Cer plays a protective role in spinal motoneurons through inhibition of oxidative signals.

Application of exogenous ceramide to cultured rat spinal motoneurons promotes survival or death by regulation of apoptosis depending on its concentrations

The membrane lipid ceramide (Cer) has been shown to be involved in the survival and dendritic growth of cerebellar Purkinje cells and hippocampal neurons. We examined the effects of Cer on isolated rat spinal motoneurons. Basal neuronal cell death due to apoptosis occurs under these culture conditions. This cell death was prevented by treatment with 2.5 microM of D-erythro-N-hexsanoylsphingosine (C6-Cer), a cell-permeable analogue, and the surviving cell number was increased approximately 1.6-fold compared with the control cell number on 5 days in vitro (DIV). Application of the same amount of C6-Cer improved axonal elongation. Conversely, addition of 10 microM of C6-Cer led all motoneurons to apoptotic cell death by 2DIV. A stereo isomer, threo-C6-Cer, which is not metabolized to C6-glucosylceramide, also promoted survival, death, and axonal growth in the same manners as C6-Cer. However, C6-dihydro-Cer, a biologically inactive analogue, had no effects on survival or death, indicating that the presence of a double bond in the sphingosine base is essential for its activity. In addition, treatment with bacterial sphingomyelinase, which generates endogenous Cer, increases motoneuron survival and axonal growth. These observations suggest that Cer, but not its metabolites, regulates survival and development of spinal motoneurons, depending on its intracellular concentration.

A mediator role of ceramide in the regulation of neuroblastoma Neuro2a cell differentiation

Current studies indicate that ceramide is involved in the regulation of important cell functions, namely cell growth, differentiation, and apoptosis. In the present study, the possible role of ceramide in the differentiation of neuroblastoma Neuro2a cells was investigated. The following results were obtained. (a) Ceramide content of Neuro2a cells, induced to differentiate by retinoic acid (RA) treatment rapidly increased after addition of RA, was maintained at high levels in RA-differentiated cells and returned to the starting levels with removal of RA and reversal of differentiation; under the same conditions, the sphingosine content remained unchanged. (b) After a short pulse with [3H]sphingomyelin or [3H]sphingosine or L-[3H]serine, the metabolic formation of ceramide was markedly higher and more rapid in RA-differentiated than undifferentiated cells. (c) Inhibitors of ceramide biosynthesis (Fumonisin B1, beta-chloroalanine and L-cycloserine) diminished the extent of the differentiating effect of RA and concomitantly Cer content decreased. (d) The activity of neutral sphingomyelinase increased after addition of RA, maintained high levels in RA-differentiated cells, and returned to the initial levels with removal of RA. (e) Experimental conditions that cause an elevation of ceramide content (treatment with sphingosine or ceramide or C2-ceramide or bacterial sphingomyelinase) inhibited cell proliferation and stimulated neurite outgrowth; dihydro-analogues of sphingosine, ceramide, and C2-ceramide had no effect on differentiation. (f) treatment with Fumonisin B1 completely inhibited sphingosine-induced differentiation. These data suggest a specific bioregulatory function of ceramide in the control of Neuro2a cell growth and differentiation and pose the general hypothesis of a mediator role of ceramide in the differentiation of cells of neural origin.

Formation of bioactive sphingoid molecules from exogenous sphingomyelin in primary cultures of neurons and astrocytes

Exogenous sphingomyelin, radiolabelled at the sphingosine moiety, was administered to primary cultures of cerebellar granule cells and astrocytes for different pulse times (20 min-2 h) and the fate of the radioactivity was followed. Ceramide was the main metabolic product in both cells, whereas sphingosine, glucosyl-ceramide and gangliosides GM3 and GD3 were produced only in astrocytes. When endocytosis was prevented and the lysosomal apparatus inactivated, ceramide formation was reduced slightly in granule cells and almost completely blocked in astrocytes, with disappearance of sphingosine, glucosyl-ceramide, GM3 and GD3. These data indicate that (a) ceramide is rapidly produced in cerebellar granule cells and astrocytes, presumably at the level of the plasma membrane in the first cell type, and of the lysosomes in the second one; (b) sphingosine is produced in cerebellar astrocytes by lysosomal sphingomyelin degradation and is partly reused for glucosyl-ceramide and ganglioside biosynthesis.

Ceramide: a new lipid "second messenger"?

1-Alpha, 25-dihydroxyvitamin D3, tumor necrosis factor alpha, and gamma-interferon induce sphingomyelin turnover to ceramide in HL-60 cells. The time frame of the turnover is consistent with ceramide formation being a mediator of growth and differentiation; furthermore, differentiation can be induced by addition of exogenous ceramides alone. Hence, the action of the biologically active form of vitamin D and other nutritionally relevant factors may be mediated by lipid second messenger pathways involving sphingolipids.

An update of the enzymology and regulation of sphingomyelin metabolism

Sphingomyelin is found in plasma membranes and related organelles (such as endocytic vesicles and lysosomes) of all tissues, as well as in lipoproteins. Abnormalities in sphingomyelin metabolism have been associated with atherosclerosis, cancer and genetically transmitted diseases; however, except for Niemann-Pick disease, little is known about the mechanism for these disorders. Sphingomyelin biosynthesis de novo involves ceramide formation from serine and two mol of fatty acyl-CoA followed by addition of the phosphocholine headgroup. The headgroup appears to come from phosphatidylcholine, but other sources have not been ruled out. Factors that influence the rate of sphingomyelin synthesis include the availability of serine and palmitic acid, plus the relative activities of key enzymes of this pathway. Sphingomyelin turnover involves removal of the headgroup and amide-linked fatty acid by sphingomyelinases and ceramidases, respectively, which have been found in both lysosomes (with acidic pH optima) and plasma membranes (with neutral to alkaline pH optima). The enzymes of sphingomyelin turnover release ceramide and free sphingosine from endogenous substrates, which may have implications for the participation of a sphingomyelin/sphingosine cycle as another 'lipid second messenger' system.

Interindividual heterogeneity of molecular weight of human brain neutral sphingomyelinase determined by radiation inactivation method

The molecular weight (Mr) of the membrane-bound neutral sphingomyelinase from human brain was determined using the radiation inactivation procedure. Previous studies on three human brains suggested a Mr of 165 +/- 25 kDa (J. Neurochem. 1985, 45:630-632). We now report that in another human brain the neutral sphingomyelinase had a Mr of 740 +/- 100 kDa; this higher Mr was not accompanied by differences in enzymatic properties nor heat-stability.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that neutral sphingomyelinase of cultured murine neuroblastoma cells is oriented externally on the plasma membrane

The activity of the neutral, Mg2+-stimulated sphingomyelinase of cultured neuroblastoma cells (N1E-115) is enriched in the plasma membrane fraction and is reduced following treatment of intact or broken cells with trypsin, alpha-chymotrypsin, papain, and protease. Two protease-sensitive enzymes of the cell interior (lactate dehydrogenase and NADPH-cytochrome c reductase) are not affected by protease treatment of intact cells. These results indicate that the neutral, Mg2+-stimulated sphingomyelinase is oriented externally on the plasma membrane of the cultured neuroblastoma cell.

Stimulation of neutral, magnesium-stimulated sphingomyelinase activity in the neurohypophysis of the rat by hypertonic saline ingestion

Neutral, magnesium-stimulated sphingomyelinase and acid sphingomyelinase activities in the neurohypophysis, adenohypophysis and cerebrum of rats given 2.25% NaCl drinking water for 7 or 14 days were compared with the same enzyme activities in the tissues of control animals allowed free access to distilled drinking water. Neutral sphingomyelinase activity in the neurohypophysis was significantly increased in both experimental groups (7-day, 85.2 +/- 8.4 units/mg protein; 14-day, 110.1 +/- 14.8 units/mg; control, 61.4 +/- 5.5 units/mg). Acid sphingomyelinase activity was significantly but only transiently increased in the tissue (7-day, 73.2 +/- 3.4 units/mg; 14-day, 66.3 +/- 7.5 units/mg; control, 54.3 +/- 2.8 units). Enzyme activities in adenohypophysis and cerebrum of the experimental animals were not significantly different from those of controls. The results suggest a specific role for neutral, Mg2+-stimulated sphinomyelinase in neurosecretion.

Neutral and acid sphingomyelinases of rat brain: somatotopographical distribution and activity following experimental manipulation of the dopaminergic system in vivo

The activities of neutral, magnesium-stimulated, and acid sphingomyelinases were measured in five regions of rat brain. Neutral enzyme activity was 2-3-fold higher in striatum than in parietal cortex and 13-fold higher than in cerebral white matter. Acid sphingomyelinase activity was more evenly distributed throughout these regions. Striatal neutral sphingomyelinase activity was not affected by treatment of rats with reserpine or haloperidol and was reduced (16%) by 6-hydroxydopamine. Striatal acid sphingomyelinase was unaffected by reserpine and 6-hydroxydopamine, and was increased (17%) by haloperidol. We conclude that neutral, magnesium-stimulated sphingomyelinase activity differs in various regions of rat brain and is particularly enriched in the corpus striatum. However, it appears to be a constitutive component of tissue rather than a readily modulated regulatory element of the catecholaminergic system.

Neutral and acid sphingomyelinases: somatotopographical distribution in human brain and distribution in rat organs. A possible relationship with the dopamine system

Acid and neutral sphingomyelinase activities have been measured in 22 regions of human brain, and in several rat organs. In general, acid sphingomyelinase activity was similar in most brain regions examined. By contrast neutral sphingomyelinase activity decreased 30-fold between the globus pallidus and white matter. In grey matter structures activity decreased in the order globus pallidus greater than substantia nigra greater than or equal to putamen greater than head of caudate greater than thalamus greater than cortical structures. Under the conditions of assay and in the presence of several possible donors or acceptors, there was no evidence of transfer of phosphoryl-choline to other lipid acceptors. Acid sphingomyelinase was ubiquitously distributed in all rat tissues examined, highest in liver and lowest in adipose tissue. Neutral sphingomyelinase activity was highest in brain; activity from 25 to 10% of that in brain was observed in testis, adrenal gland and aorta. Activity in the other organs examined was less than 10% of that in brain. We suggest that the neutral enzyme serves a special function in brain, perhaps related to the dopaminergic systems.