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

Sphingomyelinases: their regulation and roles in cardiovascular pathophysiology

Sphingomyelinases (SMases) hydrolyse sphingomyelin, releasing ceramide and creating a cascade of bioactive lipids. These lipids include sphingosine and sphingosine-1-phosphate, all of which have a specific signalling capacity. Sphingomyelinase activation occurs in different cardiovascular system cell types, namely cardiac myocytes, endothelial and vascular smooth muscle cells, mediating cell proliferation, cell death, and contraction of cardiac and vascular myocytes. Three main types of SMases contribute to cardiovascular physiology: the lysosomal and secreted acidic SMases (L- and S-ASMases, respectively) and the membrane neutral SMase (NSMase). These three enzymes have common activators, including ischaemia/reperfusion stress and proinflammatory cytokines, but they differ in their enzymatic properties and subcellular locations that determine the final effect of enzyme activation. This review focuses on the recent advances in the understanding of ASMase and NSMase pathways and their specific contribution to cardiovascular pathophysiology. Current knowledge indicates that the inhibitors of the different SMase types are potential tools for the treatment of cardiovascular diseases. Acid SMase inhibitors could be tools against post-ischaemia reperfusion injury and in the treatment of atherosclerosis. Neutral SMase inhibitors could be tools for the treatment of atherosclerosis, heart failure, and age-related decline in vasomotion. However, the design of bioavailable and more specific SMase-type inhibitors remains a challenge.

Functional and structural characterization of chimeras of a bacterial genotoxin and human type I DNAse

Chimeras composed of the cdtB gene of a novel bacterial genotoxin and the human type I DNAse I gene were constructed and their products characterized relative to the biochemical and enzymatic properties of the native proteins. The product of a cdtB/DNAse I chimera formed a heterotrimer with the CdtA and CdtC subunits of the genotoxin, and targeted mutations increased the specific activity of the hybrid protein. Expression of active chimeric gene products established that the CdtB protein is an atypical divalent cation-dependent endonuclease and demonstrated the potential for genetically engineering a new class of therapeutic agent for inhibiting the proliferation of cancer cells.

Role of neutral sphingomyelinases in aging and inflammation

Aging is characterized by changes in the organism's immune functions and stress response, which in the elderly leads to increased incidence of complications and mortality following inflammatory stress. Alterations in the neuro-endocrine axes and overall decline in the immune system play an essential role in this process. Overwhelming evidence however suggests that many cellular cytokine signaling pathways are also affected, thus underscoring the idea that both, "cellular" and "systemic" changes contribute to aging. IL-1beta for example, induces more potent cellular responses in hepatocytes isolated from aged animals then in hepatocytes from young rats. This phenomenon is referred to as IL-1b hyperresponsiveness and is linked to abnormal regulation of various acute phase proteins during aging.Evidence has consistently indicated that activation of neutral sphingomyelinase and the resulting accumulation of ceramide mediate cellular responses to LPS, IL-1beta, and TNFalpha in young animals. More recent studies identified the cytokine-inducible neutral sphingomyelinase with nSMase2 (smpd3) that is localized in the plasma membrane and mediates cellular responses to IL-1beta and TNFalpha. Intriguingly, constitutive up-regulation of nSMase2 occurs in aging and it underlies the hepatic IL-1b hyperresponsiveness. The increased activity of nSMases2 in aging is caused by a substantial decline in hepatic GSH content linking thereby oxidative stress to the onset of pro-inflammatory state in liver. nSMase2 apparently follows a pattern of regulation consisting with "developmental-aging" continuum, since in animal models of delayed aging, like calorie-restricted animals, the aging-associated changes in NSMase activity and function are reversed.

Identification of Mg2+ -dependent neutral sphingomyelinase 1 as a mediator of heat stress-induced ceramide generation and apoptosis

Neutral sphingomyelinases (SMases) are involved in the induction of ceramide-mediated proapoptotic signaling under heat stress conditions. Although ceramide is an important mediator of apoptosis, the neutral SMase that is activated under heat stress has not been identified. In this study, we cloned an Mg(2+)-dependent neutral SMase from a zebrafish embryonic cell cDNA library using an Escherichia coli expression-cloning vector. Screening of the clones using an SMase activity assay with C(6)-7-nitro-2-1,3-benzoxadiazol-4-yl-sphingomyelin as the substrate resulted in the isolation of one neutral SMase cDNA clone. This cDNA encoded a polypeptide of 420 amino acids (putative molecular weight: 46,900) containing two predicted transmembrane domains in its C-terminal region. The cloned neutral SMase 1 acted as a mediator of stress-induced apoptosis. Bacterially expressed recombinant neutral SMase 1 hydrolyzed [choline-methyl-(14)C]sphingomyelin optimally at pH 7.5 in the presence of an Mg(2+) ion. In zebrafish embryonic cells, the endogenous SMase enzyme was localized in the microsomal fraction. In FLAG-tagged SMase-overexpressing cells, neutral SMase 1 colocalized with a Golgi marker in a cytochemical analysis. Inactivation of the enzyme by an antisense phosphorothioate oligonucleotide repressed the induction of ceramide generation, caspase-3 activation, and apoptotic cell death by heat stress. Thus, neutral SMase 1 participates in an inducible ceramide-mediating, proapoptotic signaling pathway that operates in heat-induced apoptosis in zebrafish embryonic cells.

Regulation of neutral sphingomyelinase-2 (nSMase2) by tumor necrosis factor-alpha involves protein kinase C-delta in lung epithelial cells

Neutral sphingomyelinases (N-SMases) are major candidates for stress-induced ceramide production, but there is still limited knowledge of the regulatory mechanisms of the cloned N-SMase enzyme-nSMase2. We have reported that p38 mitogen-activated protein kinase (MAPK) was upstream of nSMase2 in tumor necrosis-alpha (TNF-alpha)-stimulated A549 cells ( J Biol Chem 282: 1384-1396, 2007 ). Here, we report a role for protein kinase C (PKC) in mediating TNF-induced translocation of nSMase2 from the Golgi to the plasma membrane (PM). Pharmacological inhibition of PKCs prevented TNF-stimulated nSMase2 translocation to the PM in A549 cells. Using phorbol 12-myristate 13-acetate (PMA) as a tool to dissect PKC responses, we found that PMA induced nSMase2 translocation to the PM in a time- and dose-dependent manner. Pharmacological inhibitors and specific siRNA implicated the novel PKCs, specifically PKC-delta, in both TNF and PMA-stimulated nSMase2 translocation. However, PMA did not increase in vitro N-SMase activity and PKC-delta did not regulate TNF-induced N-SMase activity. Furthermore, PKC-delta and nSMase2 did not coimmunoprecipitate, suggesting that other signaling proteins may be involved. PMA-stimulated nSMase2 translocation was independent of p38 MAPK, and neither PKC inhibitors nor small interfering RNA had significant effects on TNF-stimulated p38 MAPK activation, indicating that PKC-delta does not act through p38 MAPK in regulating nSMase2. Finally, down-regulation of PKC-delta inhibited induction of vascular cell and intercellular adhesion molecules, previously identified as downstream of nSMase2 in A549 cells. Taken together, these data implicate PKC-delta as a regulator of nSMase2 and, for the first time, identify nSMase2 as a point of cross-talk between the PKC and sphingolipid pathways.

Principles of bioactive lipid signalling: lessons from sphingolipids

It has become increasingly difficult to find an area of cell biology in which lipids do not have important, if not key, roles as signalling and regulatory molecules. The rapidly expanding field of bioactive lipids is exemplified by many sphingolipids, such as ceramide, sphingosine, sphingosine-1-phosphate (S1P), ceramide-1-phosphate and lyso-sphingomyelin, which have roles in the regulation of cell growth, death, senescence, adhesion, migration, inflammation, angiogenesis and intracellular trafficking. Deciphering the mechanisms of these varied cell functions necessitates an understanding of the complex pathways of sphingolipid metabolism and the mechanisms that regulate lipid generation and lipid action.

Mutations in the neutral sphingomyelinase gene SMPD3 implicate the ceramide pathway in human leukemias

Ceramide is a lipid second messenger derived from the hydrolysis of sphingomyelin by sphingomyelinases (SMases) and implicated in diverse cellular responses, including growth arrest, differentiation, and apoptosis. Defects in the neutral SMase (nSMase) gene Smpd3, the primary regulator of ceramide biosynthesis, are responsible for developmental defects of bone; regulation of ceramide levels have been implicated in macrophage differentiation, but this pathway has not been directly implicated in human cancer. In a genomic screen for gene copy losses contributing to tumorigenesis in a mouse osteosarcoma model, we identified a somatic homozygous deletion specifically targeting Smpd3. Reconstitution of SMPD3 expression in mouse tumor cells lacking the endogenous gene enhanced tumor necrosis factor (TNF)-induced reduction of cell viability. Nucleotide sequencing of the highly conserved SMPD3 gene in a large panel of human cancers revealed mutations in 5 (5%) of 92 acute myeloid leukemias (AMLs) and 8 (6%) of 131 acute lymphoid leukemias (ALLs), but not in other tumor types. In a subset of these mutations, functional analysis indicated defects in protein stability and localization. Taken together, these observations suggest that disruption of the ceramide pathway may contribute to a subset of human leukemias.

Neutral sphingomyelinase (SMPD3) deficiency causes a novel form of chondrodysplasia and dwarfism that is rescued by Col2A1-driven smpd3 transgene expression

Neutral sphingomyelinase SMPD3 (nSMase2), a sphingomyelin phosphodiesterase, resides in the Golgi apparatus and is ubiquitously expressed. Gene ablation of smpd3 causes a generalized prolongation of the cell cycle that leads to late embryonic and juvenile hypoplasia because of the SMPD3 deficiency in hypothalamic neurosecretory neurons. We show here that this novel form of combined pituitary hormone deficiency is characterized by the perturbation of the hypothalamus-pituitary growth axis, associated with retarded chondrocyte development and enchondral ossification in the epiphyseal growth plate. To study the contribution by combined pituitary hormone deficiency and by the local SMPD3 deficiency in the epiphyseal growth plate to the skeletal phenotype, we introduced the full-length smpd3 cDNA transgene under the control of the chondrocyte-specific promoter Col2a1. A complete rescue of the smpd3(-/-) mouse from severe short-limbed skeletal dysplasia was achieved. The smpd3(-/-) mouse shares its dwarf and chondrodysplasia phenotype with the most common form of human achondrodysplasia, linked to the fibroblast-growth-factor receptor 3 locus, not linked to deficits in the hypothalamic-pituitary epiphyseal growth plate axis. The rescue of smpd3 in vivo has implications for future research into dwarfism and, particularly, growth and development of the skeletal system and for current screening and future treatment of combined dwarfism and chondrodysplasia.

Targeting the conversion of ceramide to sphingosine 1-phosphate as a novel strategy for cancer therapy

Sphingolipids not only function as structural components of cell membranes but also act as signaling molecules to regulate fundamental cellular responses, such as cell death and differentiation, proliferation and certain types of inflammation. Particularly the cellular balance between ceramide and sphingosine 1-phosphate seems to be crucial for a cell's decision to either undergo apoptosis or proliferate, two events which are implicated in tumor development and growth. Whereas ceramide possesses proapoptotic capacity in many cell types, sphingosine 1-phosphate acts as a counterplayer able to induce cell proliferation and protect cells from undergoing apoptosis. Therefore, tipping the balance in favour of ceramide production, i.e. by inhibiting ceramidase or sphingosine kinase activities has potential to support its proapoptotic action and hence represents a promising rational approach to effective cancer therapy. This review highlights most recent data on the regulation of cellular sphingolipid formation and their potential implication in tumor development, and provides perspectives for their use as targets in molecular intervention therapy.

Effect of tricyclodecan-9-yl potassium xanthate (D609) on phospholipid metabolism and cell death during oxygen-glucose deprivation in PC12 cells

Alterations in lipid metabolism play an integral role in neuronal death in cerebral ischemia. Here we used an in vitro model, oxygen-glucose deprivation (OGD) of rat pheochromocytoma (PC12) cells, and analyzed changes in phosphatidylcholine (PC) and sphingomyelin (SM) metabolism. OGD (4-8 h) of PC12 cells triggered a dramatic reduction in PC and SM levels, and a significant increase in ceramide. OGD also caused increases in phosphatidylcholine-phospholipase C (PC-PLC) and phospholipase D (PLD) activities and PLD2 protein expression, and reduction in cytidine triphosphate:phosphocholine cytidylyltransferase-alpha (CCTalpha, the rate-limiting enzyme in PC synthesis) protein expression and activity. Phospholipase A2 activity and expression were unaltered during OGD. Increased neutral sphingomyelinase activity during OGD could account for SM loss and increased ceramide. Surprisingly, treatment with PC-PLC inhibitor tricyclodecan-9-yl potassium xanthate (D609) aggravated cell death in PC12 cells during OGD. D609 was cytotoxic only during OGD; cell death could be prevented by inclusion of sera, glucose or oxygen. During OGD, D609 caused further loss of PC and SM, depletion of 1,2-diacylglycerol (DAG), increase in ceramide and free fatty acids (FFA), cytochrome c release from mitochondria, increases in intracellular Ca2+ ([Ca2+]i), poly-ADP ribose polymerase (PARP) cleavage and phosphatidylserine externalization, indicative of apoptotic cell death. Exogenous PC during OGD in PC12 cells with D609 attenuated PC, SM loss, restored DAG, attenuated ceramide levels, decreased cytochrome c release, PARP cleavage, annexin V binding, attenuated the increase in [Ca2+]i, FFA release, and significantly increased cell viability. Exogenous PC may have elicited these effects by restoring membrane PC levels. A tentative scheme depicting the mechanism of action of D609 (inhibiting PC-PLC, SM synthase, PC synthesis at the CDP-choline-1,2-diacylglycerol phosphocholine transferase (CPT) step and causing mitochondrial dysfunction) has been proposed based on our observations and literature.

Metabolism and biological functions of two phosphorylated sphingolipids, sphingosine 1-phosphate and ceramide 1-phosphate

Sphingolipids are major lipid constituents of the eukaryotic plasma membrane. Without certain sphingolipids, cells and/or embryos cannot survive, indicating that sphingolipids possess important physiological functions that are not substituted for by other lipids. One such role may be signaling. Recent studies have revealed that some sphingolipid metabolites, such as long-chain bases (LCBs; sphingosine (Sph) in mammals), long-chain base 1-phosphates (LCBPs; sphingosine 1-phosphate (S1P) in mammals), ceramide (Cer), and ceramide 1-phosphate (C1P), act as signaling molecules. The addition of phosphate groups to LCB/Sph and Cer generates LCBP/S1P and C1P, respectively. These phospholipids exhibit completely different functions than those of their precursors. In this review, we describe recent advances in understanding the functions of LCBP/S1P and C1P in mammals and in the yeast Saccharomyces cerevisiae. Since LCB/Sph, LCBP/S1P, Cer, and C1P are mutually convertible, regulation of not only the total amount of the each lipid but also of the overall balance in cellular levels is important. Therefore, we describe in detail their metabolic pathways, as well as the genes involved in each reaction.

Redox regulation of neutral sphingomyelinase-1 activity in HEK293 cells through a GSH-dependent mechanism

Phospholipases are essential enzymes in cellular signalling processes such as cellular differentiation, proliferation and apoptosis. Based on its high degree of homology with sequences of prokaryote SMases, a type of Mg(2+)-dependent PLC (nSMase-1) was recently discovered which displayed strong redox dependence for activity in vitro [F. Rodrigues-Lima, A.C. Fensome, M. Josephs, J. Evans, R.J. Veldman, M. Katan (2000), J. Biol. Chem. 275 (36) 28316-28325]. The aim of this work was to test the hypothesis that glutathione could be a natural regulator of nSMase-1 activity ex vivo. We studied how altering glutathione levels and redox ratio modulate nSMase-1 activity in a HEK293 cell line that ectopically overexpressed the nSMase-1 gene. Diminishing total glutathione with BSO without altering significantly the GSH/GSSG ratio did not affect nSMase-1 activity. Treatment of cells with diamide produced a transient decrease of total glutathione and a sharp, but also transient, decrease of the GSH/GSSG ratio. Under these conditions, nSMase-1 activity was temporarily activated and then returned to normal levels. Simultaneous treatment with BSO and diamide that resulted in permanent decreases of total glutathione and GSH/GSSG redox ratio produced a sustained activation of nSMase-1 activity. Taken together, these data indicate that altering the GSH/GSSG ratio by increasing GSSG or decreasing GSH levels, but not the total concentration of glutathione, modulates nSMase-1 activity. Our findings are the first evidence supporting the ex vivo regulation of nSMase-1 through a redox glutathione-dependent mechanism.

Analysis of membrane topology of neutral sphingomyelinase 2

Neutral sphingomyelinase 2 (nSMase2), which has two hydrophobic segments at its NH(2)-terminus, plays an important role in ceramide-mediated cell regulation. Here, we investigated the membrane topology of nSMase2. When a double-tagged nSMase2 at both the NH(2) and COOH termini, was overexpressed in MCF-7 cells, the signals from both tags were detected in the inner leaflet of the plasma membrane. Furthermore, insertion of a tag into the internal sequence and green fluorescent protein-fused deletion mutants revealed that the entire catalytic region of the protein was located on the cytosolic face of the membranes and each hydrophobic segment is integrated into the membranes, but unlikely to span the entire membrane. These results indicate the presence of the enzyme in the inner leaflet of plasma membrane.

Aging of the brain, neurotrophin signaling, and Alzheimer's disease: is IGF1-R the common culprit?

The last decade has revealed that the lifespan of an organism can be modulated by the signaling pathway that acts downstream of the insulin/insulin-like growth factor 1 receptors (IR/IGF1-R), indicating that there is a "program" that drives the process of aging. New results have now linked the same pathway to the neurogenic capacities of the aging brain, to neurotrophin signaling, and to the molecular pathogenesis of Alzheimer's disease. Therefore, a common signaling cascade now seems to link aging to age-associated pathologies of the brain, suggesting that pharmacologic approaches aimed at the modulation of this pathway can serve to delay the onset of age-associated disorders and improve the quality of life. Work from a wide range of fields performed with different approaches has already identified some of the signaling molecules that act downstream of IGF1-R, and has revealed that a delicate checkpoint exists to balance excessive growth/"immortality" and reduced growth/"senescence" of a cell. Future research will determine how far the connection goes and how much of it we can influence.

Neutral sphingomyelinase 2 is palmitoylated on multiple cysteine residues. Role of palmitoylation in subcellular localization

The neutral sphingomyelinases (nSMases) are considered major candidates for mediating the stress-induced production of ceramide. nSMase2, which has two hydrophobic segments near the NH(2)-terminal region, has been reported to be located at the plasma membrane and play important roles in ceramide-mediated signaling. In this study, we found that nSMase2 is palmitoylated on multiple cysteine residues via thioester bonds. Site-directed mutagenesis of cysteine residues to alanine indicated that two cysteine clusters of the enzyme are multiply palmitoylated; one cluster is located between the two hydrophobic segments, and the second one is located in the middle of the catalytic region of the protein. When overexpressed in the confluent phase of MCF-7 cells, wild-type nSMase2 was strictly localized in the plasma membranes, and the cysteine mutants of each palmitoylated cysteine cluster were seen not only at the plasma membrane but also in some punctate structures. Furthermore, mutation of all potential palmitoylation sites resulted in a dramatic reduction in the plasma membrane distribution and an increase in the punctate structures. The palmitoylation-deficient mutant was directed to lysosomes and rapidly degraded. Palmitoylation had no effect on enzyme activity but affected membrane-association properties of the protein. Finally, the catalytic region of nSMase2 where palmitoylation occurs was found to be localized at the inner leaflet of the plasma membrane. In summary, the results from this study reveal for the first time the palmitoylation of nSMase2 via thioester bonds and its importance in the subcellular localization and stability of this protein.

Sphingolipid Metabolism in Systemic Inflammation

The inflammatory response - induced and regulated by a variety of mediators such as cytokines, prostaglandins, and reactive oxygen species (ROS) - is the localized host’s response of the tissue to injury, irritation, or infection. In a very similar and stereotyped sequence, the mediators are thought to induce an acute phase response orchestrated by an array of substances produced locally or near the source or origin of the inflammatory response. Despite its basically protective function, the response can become inappropriate in intensity or duration damaging host tissues or interfering with normal metabolism. Thus, inflammation is the cause and/or consequence of a diversity of diseases and plays a major role in the development of remote organ failure. Better knowledge of the underlying mechanisms of these processes is, therefore, a fundamental pre-requisite fostering the molecular understanding of novel therapeutic targets or diagnostic variables.

Sphingolipid Metabolism in Systemic Inflammation

The inflammatory response — induced and regulated by a variety of mediators such as cytokines, prostaglandins, and reactive oxygen species (ROS) — is the localized host’s response of the tissue to injury, irritation, or infection. In a very similar and stereotyped sequence, the mediators are thought to induce an acute phase response orchestrated by an array of substances produced locally or near the source or origin of the inflammatory response. Despite its basically protective function, the response can become inappropriate in intensity or duration damaging host tissues or interfering with normal metabolism. Thus, inflammation is the cause and/or consequence of a diversity of diseases and plays a major role in the development of remote organ failure. Better knowledge of the underlying mechanisms of these processes is, therefore, a fundamental pre-requisite fostering the molecular understanding of novel therapeutic targets or diagnostic variables.

Neutral sphingomyelinases and nSMase2: Bridging the gaps

There is strong evidence indicating a role for ceramide as a second messenger in processes such as apoptosis, cell growth and differentiation, and cellular responses to stress. Ceramide formation from the hydrolysis of sphingomyelin is considered to be a major pathway of stress-induced ceramide production with magnesium-dependent neutral sphingomyelinase (N-SMase) identified as a prime candidate in this pathway. The recent cloning of a mammalian N-SMase-nSMase2- and generation of nSMase2 knockout/mutant mice have now provided vital tools with which to further study the regulation and roles of this enzyme in both a physiological and pathological context. In the present review, we summarize current knowledge on N-SMase relating this to what is known about nSMase2. We also discuss the future areas of nSMase2 research important for molecular understanding of this enzyme and its physiological roles.

Sphingolipids and cell signaling: involvement in apoptosis and atherogenesis

This review considers various functional aspects of cell sphingolipids (sphingomyelin, ceramides) and lysosphingolipids (sphingosine-1-phosphate (S1P) and sphingosine phosphorylcholine). Good evidence now exists that they are actively involved in numerous cell-signaling processes. The enzymes responsible for formation and interconversion of cell sphingolipids (sphingomyelinases, ceramidase, sphingosine kinase, S1P-lyase) exhibit high sensitivity to various stimulating factors. This determines the content of individual cell sphingolipids and therefore the mode of cell response. Special attention is paid to preferential localization of sphingolipids in the rigid plasma membrane domains (rafts) coupled to many signal proteins. The suggestion is discussed that ceramide signaling may be based on the modification of fine molecular interactions in lipid rafts, resulting in its clusterization inducing the signal transduction. The review also highlights involvement of sphingolipids in cell proliferation, apoptosis, and in processes implicated to atherosclerosis.

Role for neutral sphingomyelinase-2 in tumor necrosis factor alpha-stimulated expression of vascular cell adhesion molecule-1 (VCAM) and intercellular adhesion molecule-1 (ICAM) in lung epithelial cells: p38 MAPK is an upstream regulator of nSMase2

Neutral sphingomyelinases (N-SMases) are major candidates for stress-induced ceramide production. However, there is little information on the physiological regulation and roles of the cloned N-SMase enzyme, nSMase2. In this study, nSMase2 was found to translocate acutely to the plasma membrane of A549 epithelial cells in response to tumor necrosis factor alpha (TNF-alpha) in a time- and dose-dependent manner. Additionally, TNF-alpha increased N-SMase activity rapidly and transiently both endogenously and in cells overexpressing nSMase2. Furthermore, the translocation of nSMase2 was regulated by p38-alpha MAPK, but not ERK or JNK, and the increase in endogenous N-SMase activity was abrogated by p38 MAPK inhibition. In addition, both p38-alpha MAPK and nSMase2 were implicated in the TNF-alpha-stimulated up-regulation of the adhesion proteins vascular cell adhesion molecule-1 (VCAM) and intercellular adhesion molecule-1 (ICAM), but this was largely independent of NF-kappaB activation. These data reveal p38 MAPK as an upstream regulator of nSMase2 and indicate a role for nSMase2 in pro-inflammatory responses induced by TNF-alpha as a regulator of adhesion proteins.

Acid sphingomyelinase deficiency increases susceptibility to fatal alphavirus encephalomyelitis

Sindbis virus (SV), an enveloped virus with a single-stranded, plus-sense RNA genome, is the prototype alphavirus in the Togaviridae family. In mice, SV infects neurons and can cause apoptosis of immature neurons. Sphingomyelin (SM) is the most prevalent cellular sphingolipid, is particularly abundant in the nervous systems of mammals, and is required for alphavirus fusion and entry. The level of SM is tightly regulated by sphingomyelinases. A defect in acid sphingomyelinase (ASMase) results in SM storage and subsequent intracellular accumulation of SM. To better understand the role of the SM pathway in SV pathogenesis, we have characterized SV infection of transgenic mice deficient in the ASMase gene. ASMase knockout (ASM-KO) mice were more susceptible to SV infection than wild-type (WT) or heterozygous (Het) animals. Titers of SV were higher in the brains of ASM-KO mice than in the brains of WT mice. More SV RNA was detected by in situ hybridization, more SV protein was detected by immunohistochemistry, and more terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling-positive cells were present in the cortex and hippocampus of ASM-KO mice than in those of WT or Het mice. Interleukin-6 (IL-6), but not IL-1beta or tumor necrosis factor alpha, was elevated in infected ASM-KO mice compared to levels in WT or Het mice, but studies with IL-6-KO mice and recombinant SV expressing IL-6 showed no role for IL-6 in fatal disease. Together these data indicate that the increase in susceptibility of ASM-KO mice to SV infection was the result of more-rapid replication and spread of SV in the nervous system and increased neuronal death.

Golgi alkaline ceramidase regulates cell proliferation and survival by controlling levels of sphingosine and S1P

Sphingosine-1-phosphate (S1P), a sphingolipid metabolite, promotes cell proliferation and survival whereas its precursor, sphingosine, has the opposite effects. However, much remains unknown about their regulation. Here we identify a novel human ceramidase (haCER2) that regulates the levels of both sphingosine and S1P by controlling the hydrolysis of ceramides. haCER2 is localized to the Golgi complex and is highly expressed in the placenta. High ectopic expression of haCER2 caused fragmentation of the Golgi complex and growth arrest in HeLa cells due to sphingosine accumulation. Low ectopic expression of haCER2 increased S1P without sphingosine accumulation, promoting cell proliferation in serum-free medium. This proliferative effect was suppressed by dimethylsphingosine, an inhibitor of the S1P formation, or by the RNA interference (RNAi) -mediated inhibition of S1P(1,) a G-protein-coupled receptor for S1P. The RNAi-mediated down-regulation of haCER2 enhanced the serum deprivation-induced growth arrest and apoptosis of HeLa cells, which was inhibited by addition of exogenous S1P. Serum deprivation up-regulated both haCER2 mRNA and activity in HeLa cells. haCER2 mRNA is also up-regulated in some tumors. Taken together, these results suggest that haCER2 is important for the generation of S1P and S1P-mediated cell proliferation and survival, but that its overexpression may cause cell growth arrest due to an accumulation of sphingosine.

Ceramide/sphingosine/sphingosine 1-phosphate metabolism on the cell surface and in the extracellular space

Sphingolipid metabolites, ceramide, sphingosine, and sphingosine 1-phosphate, have emerged as a new class of lipid biomodulators of various cell functions. These metabolites are known to function not only as intracellular second messengers, but also in the extracellular space. Sphingosine 1-phosphate especially has numerous functions as an important extracellular mediator that binds to cell surface S1P receptors. Recent studies have also shown that sphingolipid-metabolizing enzymes function not only in intracellular organelles but also in the extracellular spaces, including the outer leaflet of the plasma membrane. This review focuses on the metabolic enzymes (acid and alkaline sphingomyelinases, neutral ceramidase, and sphingosine kinase) that are involved in the production of the sphingolipid metabolites in these extracellular spaces, and on the metabolic pathway itself.

Entry of muscle satellite cells into the cell cycle requires sphingolipid signaling

Adult skeletal muscle is able to repeatedly regenerate because of the presence of satellite cells, a population of stem cells resident beneath the basal lamina that surrounds each myofiber. Little is known, however, of the signaling pathways involved in the activation of satellite cells from quiescence to proliferation, a crucial step in muscle regeneration. We show that sphingosine-1-phosphate induces satellite cells to enter the cell cycle. Indeed, inhibiting the sphingolipid-signaling cascade that generates sphingosine-1-phosphate significantly reduces the number of satellite cells able to proliferate in response to mitogen stimulation in vitro and perturbs muscle regeneration in vivo. In addition, metabolism of sphingomyelin located in the inner leaflet of the plasma membrane is probably the main source of sphingosine-1-phosphate used to mediate the mitogenic signal. Together, our observations show that sphingolipid signaling is involved in the induction of proliferation in an adult stem cell and a key component of muscle regeneration.

Equilibrium in lung schistosomula sphingomyelin breakdown and biosynthesis allows very small molecules, but not antibody, to access proteins at the host-parasite interface

The mechanism by which lung-stage schistosomula expose proteins at the host-parasite interface to nutrient, but not antibody, uptake has been obscure. We have found that Schistosoma mansoni and Schistosoma haematobium larvae emerging from host lung at a pH of around 7.5, and fixed with diluted formaldehyde (HCHO), readily bind specific antibodies in indirect membrane immunofluorescence. Data on inhibitors and activators of parasite tegument-bound, magnesium-dependent, neutral sphingomyelinase (nSMase), and sphingomyelin biosynthesis inhibitors revealed that equilibrium in schistosomular sphingomyelin breakdown and biosynthesis prevents antibody binding, yet permits access of small HO-CH2-OH polymers to interact with and cross-link proteins at the host-parasite interface, allowing for their serological visualization.

IL-1beta induces a MyD88-dependent and ceramide-mediated activation of Src in anterior hypothalamic neurons

The proinflammatory cytokine interleukin 1beta (IL-1beta), acting at IL-1R1 receptors, affects neuronal signaling under both physiological and pathophysiological conditions. The molecular mechanism of the rapid synaptic actions of IL-1beta in neurons is not known. We show here that within minutes of IL-1beta exposure, the firing rate of anterior hypothalamic (AH) neurons in culture was inhibited. This effect was prevented by pre-exposure of the cells to the Src family inhibitor, PP2, suggesting the involvement of Src in the hyperpolarizing effects of IL-1beta. The IL-1beta stimulation of neurons induced a rapid increase in the phosphorylation of the tyrosine kinase Src and kinase suppressor of Ras (ceramide activated protein kinase (CAPK)/KSR) in neurons grown on glia from IL-1RI(-/-) mice. These effects of IL-1beta were dependent on the association of the cytosolic adaptor protein, MyD88, to the IL-1 receptor, and on the activation of the neutral sphingomyelinase, leading to production of ceramide. A cell-permeable analog of ceramide mimicked the effects of IL-1beta on the cultured AH neurons. These results suggest that ceramide may be the second messenger of the fast IL-1beta actions in AH neurons, and that this IL-1beta/ceramide pathway may underlie the fast non-transcription-dependent, electrophysiological effects of IL-1beta observed in AH neurons in vivo.

Sphingolipids as modulators of cancer cell death: potential therapeutic targets

Through modifications in the fine membrane structure, cell-cell or cell-matrix interactions, and/or modulation of intracellular signaling pathways, sphingolipids can affect the tumorigenic potential of numerous cell types. Whereas ceramide and its metabolites have been described as regulators of cell growth and apoptosis, these lipids as well as other sphingolipid molecules can modulate the ability of malignant cells to grow and resist anticancer treatments, and their susceptibility to non-apoptotic cell deaths. This review summarizes our current knowledge on the properties of sphingolipids in the regulation of cancer cell death and tumor development. It also provides an update on the potential perspectives of manipulating sphingolipid metabolism and using sphingolipid analogues in anticancer therapy.

Sphingolipid signaling and redox regulation

Sphingolipids including ceramide and its derivatives such as ceramide-1-phosphate, glycosyl-ceramide, and sphinogosine (-1-phosphate) are now recognized as novel intracellular signal mediators for regulation of inflammation, apoptosis, proliferation, and differentiation. One of the important and regulated steps in these events is the generation of these sphingolipids via hydrolysis of sphingomyelin through the action of sphingomyelinases (SMase). Several lines of evidence suggest that reactive oxygen species (ROS; O2-, H2O2, and OH-,) and reactive nitrogen species (RNS; NO, and ONOO-) and cellular redox potential, which is mainly regulated by cellular glutathione (GSH), are tightly linked to the regulation of SMase activation. On the other hand, sphingolipids are also known to play an important role in maintaining cellular redox homeostasis through regulation of NADPH oxidase, mitochondrial integrity, and antioxidant enzymes. Therefore, this paper reviews the relationship between cellular redox and sphingolipid metabolism and its biological significance.

Shift in sphingolipid metabolism leads to an accumulation of ceramide in senescence

Ceramide mediates the effects of several agonists leading to differentiation, apoptosis or senescence. We previously showed that ceramide becomes elevated in senescent fibroblasts. In the present study, senescent cultures of Wi-38 fibroblasts and human umbilical-vein endothelial cells were compared to low-passage cultures in order to identify which of the several pathways is predominantly responsible for the increased ceramide. We found that senescent cells take up the ceramide precursor [(3)H]palmitic acid and convert it to ceramide at essentially equivalent rates to their low-passage counterparts, suggesting that, as a whole, the inherent steps are unaltered. Analysis of subsequent steps, however, revealed changes in ceramide metabolism. The rate of ceramide conversion to sphingomyelin was reduced while glucosylceramide synthesis differed between the cell lines, while the rate of the reverse reactions tended to be increased in senescent cells. We also found a decrease in acidic but not alkaline ceramidase. The data show an overall change in favor increased ceramide levels. Of all of the pathways, neutral sphingomyelinase appears to be the most likely source of the senescence-associated ceramide. The relevance to mitosis and apoptosis are discussed.

nSMase2 activation and trafficking are modulated by oxidative stress to induce apoptosis

We have previously shown that accumulation of ceramide, triggered by hydrogen peroxide (H(2)O(2)), induces apoptosis of human airway epithelial (HAE) cells. Under oxidant exposure, a lung sphingomyelinase (SMase) is activated and displays continued ceramide generation and pro-apoptotic signaling, thus leading to the pathological apoptosis that causes lung injury. In a search for a specific SMase that is modulated by oxidative stress, we recently cloned nSMase2 from monkey lung tissue and HAE cells. Here, we show that this nSMase2 is up-regulated by an oxidant (H(2)O(2)) and is inhibited by an antioxidant (glutathione (GSH)). Moreover, nSMase2 subcellular localization is governed by oxidant exposure, which leads to its preferential trafficking to the plasma membrane, where it generates ceramide and induces apoptosis. On the other hand, exposure to GSH results in nSMase2 trafficking to the nucleus, where it neither generates ceramide nor induces apoptosis.

Soluble oligomers of amyloid-beta peptide induce neuronal apoptosis by activating a cPLA2-dependent sphingomyelinase-ceramide pathway

Recent data have revealed that soluble oligomeric amyloid-beta peptide (Abeta) may be the proximate effectors of neuronal injuries and death in Alzheimer's disease (AD) by unknown mechanisms. Consistently, we recently demonstrated the critical role of a redox-sensitive cytosolic calcium-dependent phospholipase A2 (cPLA2)-arachidonic acid (AA) pathway in Abeta oligomer-induced cell death. According to the involvement of oxidative stress and polyunsaturated fatty acids like AA in the regulation of sphingomyelinase (SMase) activity, the present study underlines the role of SMases in soluble Abeta-induced apoptosis. Soluble Abeta oligomers induced the activation of both neutral and acidic SMases, as demonstrated by the direct measurement of their enzymatic activities, by the inhibitory effects of both specific neutral and acidic SMase inhibitors, and by gene knockdown using antisense oligonucleotides. Furthermore, soluble Abeta-mediated activation of SMases and subsequent cell death were found to be inhibited by antioxidant molecules and a cPLA2-specific inhibitor or antisense oligonucleotide. We also demonstrate that sphingosine-1-phosphate is a potent neuroprotective factor against soluble Abeta oligomer-induced cell death and apoptosis by inhibiting soluble Abeta-induced activation of acidic sphingomyelinase. These results suggest that Abeta oligomers induce neuronal death by activating neutral and acidic SMases in a redox-sensitive cPLA2-AA pathway.

Structural basis of the sphingomyelin phosphodiesterase activity in neutral sphingomyelinase from Bacillus cereus

Sphingomyelinase (SMase) from Bacillus cereus (Bc-SMase) hydrolyzes sphingomyelin to phosphocholine and ceramide in a divalent metal ion-dependent manner. Bc-SMase is a homologue of mammalian neutral SMase (nSMase) and mimics the actions of the endogenous mammalian nSMase in causing differentiation, development, aging, and apoptosis. Thus Bc-SMase may be a good model for the poorly characterized mammalian nSMase. The metal ion activation of sphingomyelinase activity of Bc-SMase was in the order Co2+ > or = Mn2+ > or = Mg2+ >> Ca2+ > or = Sr2+. The first crystal structures of Bc-SMase bound to Co2+, Mg2+, or Ca2+ were determined. The water-bridged double divalent metal ions at the center of the cleft in both the Co2+- and Mg2+-bound forms were concluded to be the catalytic architecture required for sphingomyelinase activity. In contrast, the architecture of Ca2+ binding at the site showed only one binding site. A further single metal-binding site exists at one side edge of the cleft. Based on the highly conserved nature of the residues of the binding sites, the crystal structure of Bc-SMase with bound Mg2+ or Co2+ may provide a common structural framework applicable to phosphohydrolases belonging to the DNase I-like folding superfamily. In addition, the structural features and site-directed mutagenesis suggest that the specific beta-hairpin with the aromatic amino acid residues participates in binding to the membrane-bound sphingomyelin substrate.

Novel tumor necrosis factor-responsive mammalian neutral sphingomyelinase-3 is a C-tail-anchored protein

Two genes encoding neutral sphingomyelinases-1 and -2 (sphingomyelin phosphodiesterases-2 and -3) have been recently identified that hydrolyze sphingomyelin to phosphorylcholine and ceramide. Data bank searches using a peptide sequence derived from a previously purified bovine neutral sphingomyelinase (nSMase) allowed us to identify a cDNA encoding a novel human sphingomyelinase, nSMase3, that shows only a little homology to nSMase1 and -2. nSMase3 was biochemically characterized by overexpression in a yeast strain, JK9-3ddeltaIsc1p, lacking endogenous SMase activity. Similar to nSMase2, nSMase3 is Mg2+-dependent and shows optimal activity at pH 7, which is enhanced in the presence of phosphatidylserine and inhibited by scyphostatin. nSMase3 is ubiquitously expressed as a 4.6-kb mRNA species. nSMase3 lacks an N-terminal signal peptide, yet contains a 23-amino-acid transmembrane domain close to the C terminus, which is indicative for the family of C-tail-anchored integral membrane proteins. Cellular localization studies with hemagglutinin-tagged nSMase3 demonstrated colocalization with markers of the endoplasmic reticulum as well as with Golgi markers. Tumor necrosis factor stimulates rapid activation of nSMase3 in MCF7 cells with peak activity at 1.5 min, which was impaired by expression of dominant negative FAN.

Bone-related gene profiles in developing calvaria

Generating a comprehensive understanding of osteogenesis-related gene profiles is very important in the development of new treatments for osteopenic conditions. Developing calvaria undergoes a typical intramembranous bone-forming process. To identify genes associated with osteoblast differentiation, we isolated total RNAs from parietal bones, that represent active osteoblasts, and sutural mesenchyme, that represents osteoprogenitor cells, and comprehensively analyzed their gene expression profiles using an oligo-based Affymetrix microarray chip containing 22,690 probes. About 2100 genes with "Present" calls had more than 2-fold higher expression in bone compared to sutures while 73 of these genes had more than 8-fold expression. Some of these genes are already known to be bone-related biomarkers: VitD receptor, bone sialoprotein, osteocalcin, osteopontin, MMP13, etc. Eight genes were selected and subjected to confirmation by quantitative real-time RT-PCR analyses. All the genes tested showed higher expression in bones, ranging from 5- to 140-fold. Several of these genes are ESTs while others are already known but their functions in osteogenesis were not previously known. Most genes of the BMP and FGF families probed in the Genechip analysis were more highly expressed in bone tissues compared to suture. All differentially-expressed Runx and Dlx family genes also showed higher expression in bone. These results imply that our data is valid and can be used as a good standard for the mining of osteogenesis-related genes.

Characterization of point mutations in the cdtA gene of the cytolethal distending toxin of Actinobacillus actinomycetemcomitans

The Cdt is a family of gram-negative bacterial toxins that typically arrest eukaryotic cells in the G0/G1 or G2/M phase of the cell cycle. The toxin is a heterotrimer composed of the cdtA, cdtB and cdtC gene products. Although it has been shown that the CdtA protein subunit binds to cells in culture and in an enzyme-linked immunosorbent assay (CELISA) the precise mechanisms by which CdtA interacts with CdtB and CdtC has not yet been clarified. In this study we employed a random mutagenesis strategy to construct a library of point mutations in cdtA to assess the contribution of individual amino acids to binding activity and to the ability of the subunit to form biologically active holotoxin. Single unique amino acid substitutions in seven CdtA mutants resulted in reduced binding of the purified recombinant protein to Chinese hamster ovary cells and loss of binding to the fucose-containing glycoprotein, thyroglobulin. These mutations clustered at the 5'- and 3'-ends of the cdtA gene resulting in amino acid substitutions that resided outside of the aromatic patch region and a conserved region in CdtA homologues. Three of the amino acid substitutions, at positions S165N (mutA81), T41A (mutA121) and C178W (mutA221) resulted in gene products that formed holotoxin complexes that exhibited a 60% reduction (mutA81) or loss (mutA121, mutA221) of proliferation inhibition. A similar pattern was observed when these mutant holotoxins were tested for their ability to induce cell cycle arrest and to convert supercoiled DNA to relaxed and linear forms in vitro. The mutations in mutA81 and mutA221 disrupted holotoxin formation. The positions of the amino acid substitutions were mapped in the Haemophilus ducreyi Cdt crystal structure providing some insight into structure and function.

Asymmetric Distribution of Phospholipids in Biomembranes

In eukaryotic cells, the biological membrane is characterized by a non-uniform distribution of membrane lipids, vertically as well as laterally. The paradigm for the vertical non-random distribution is the plasma membrane, where phosphatidylcholine (PC), sphingomyelin (SM), and glycosphingolipids are primarily located on the exoplasmic leaflet, while aminophospholipids, including phosphatidylserine (PS) and phosphatidylethanolamine (PE), are generally enriched in the cytoplasmic leaflet. Other minor phospholipids, such as phosphatidic acid and phosphatidylinositol (PI), are also enriched on the cytoplasmic face. Such asymmetrical distribution is related to each lipid regulating various biological events through interaction with other molecules. The clarification of the regulatory mechanism of the distribution and movement of membrane lipids is crucial to understanding the physiological roles of lipids. Here we focus on PS, which has been reported to be involved in apoptosis, blood coagulation and other biological phenomena, and summarize the present understanding of the dynamics of this phospholipid, including biosynthesis, metabolism, transport, and transbilayer movement. We also refer to diseases that have been reported to be related to phospholipid asymmetry.

Age-related changes in neutral sphingomyelin-specific phospholipase C activity in striatum, hippocampus, and frontal cortex: Implication for sensitivity to stress and inflammation

Previous studies show the enrichment of mammalian brain with neutral sphingomyelin-specific phospholipase C (ceramide-phosphocholine phosphodiesterase, EC 3.1.4.12; N-Sase), a key enzyme of sphingolipid metabolism and sphingolipid-induced signaling.

OBJECTIVE

The objective of this study was to evaluate the membrane-associated and cytosolic N-Sase activities in the brain regions associated with behavior (striatum, hippocampus, and frontal cortex).

RESULTS

Results showed higher membrane-associated N-Sase activity as compared to the N-Sase activity in the cytosolic fractions of all the evaluated brain regions. In the hippocampus, the N-Sase activity was significantly higher than in the striatum and cortex. In addition, age-related changes in the hippocampal N-Sase activities were profoundly higher than in the respective fractions isolated from the striatum and cortex. Age-related decreases in the hippocampal and striatal cytosolic N-Sase activities were accompanied by increases in the membrane N-Sase activities in those brain regions. There was a significant increase in the cortical membrane-associated N-Sase activity with age; however, to a much lesser extend than in other brain regions. The increase in the hippocampal membrane-associated N-Sase activity was accompanied by a higher expression of the inflammatory marker, interleukin-1 beta (IL-1 beta), with age. One of the important findings of the present study is the region-specific expression of heat shock protein 70 (hsp70). Frontal cortex showed lower hsp70 expression in both young and old age groups as compared to the striatal and hippocampal hsp70 levels which can contribute to the recently reported higher cortical sensitivity to oxidative stress.

CONCLUSION

In conclusion (a) our results, for the first time to our knowledge, demonstrated the association between the N-Sase activity and the stress/inflammatory markers expression in the brain regions controlling behavior; (b) these findings suggest the role of N-Sase as a contributor to the increased stress and inflammatory sensitivity among the brain regions with age.

Dopamine release in PC12 cells is mediated by Ca2+-dependent production of ceramide via sphingomyelin pathway

A presynaptic membrane disturbance is an essential process for the release of various neurotransmitters. Ceramide, which is a tumor suppressive lipid, has been shown to act as a channel-forming molecule and serve as a precursor of ceramide-1-phosphate, which can disturb the cellular membrane. This study found that while permeable ceramide increases the rate of dopamine release in the presence of a Ca(2+)-ionophore, A23187, permeable ceramide-1-phosphate provoked its release even without the ionophore. The treatment of PC12 cells with the ionophore at concentrations < 2 microM produced ceramide via the sphingomyelin (SM) pathway with a concomitant release of dopamine, and no cell damage was observed. The addition of a Ca(2+) chelator, EGTA, to the medium inhibited the increase in the release of both the ceramide and dopamine. This suggests that ceramide might be produced by Ca(2+) and is implicated in the membrane disturbance associated with the release of dopamine as a result of its conversion to ceramide-1-phosphate. Consistent with these results, this study detected a membrane-associated and neutral pH optimum sphingomyelinase (SMase) whose activity was increased by Ca(2+). Together, these results demonstrate that ceramide can be produced via the activation of a neutral form of SMase through Ca(2+), and is involved in the dopamine release in concert with Ca(2+).

APE-type non-LTR retrotransposons: determinants involved in target site recognition

Non-long terminal repeat (Non-LTR) retrotransposons represent a diverse and widely distributed group of transposable elements and an almost ubiquitous component of eukaryotic genomes that has a major impact on evolution. Their copy number can range from a few to several million and they often make up a significant fraction of the genomes. The members of the dominating subtype of non-LTR retrotransposons code for an endonuclease with homology to apurinic/apyrimidinic endonucleases (APE), and are thus termed APE-type non-LTR retrotransposons. In the last decade both the number of identified non-LTR retrotransposons and our knowledge of biology and evolution of APE-type non-LTR retrotransposons has increased tremendously.

Studies on phosphatidylserine by tandem quadrupole and multiple stage quadrupole ion-trap mass spectrometry with electrospray ionization: structural characterization and the fragmentation processes

Low-energy CAD product-ion spectra of various molecular species of phosphatidylserine (PS) in the forms of [M-H]- and [M-2H+Alk]- in the negative-ion mode, as well as in the forms of [M+H]+, [M+Alk]+, [M-H+2Alk]+, and [M-2H+3Alk]+ (where Alk=Li, Na) in the positive-ion mode contain rich fragment ions that are applicable for structural determination. Following CAD, the [M-H]- ion of PS undergoes dissociation to eliminate the serine moiety (loss of C3H5NO2) to give a [M-H-87]- ion, which equals to the [M-H]- ion of a phoshatidic acid (PA) and give rise to a MS3-spectrum that is identical to the MS2-spectrum of PA. The major fragmentation process for the [M-2H+Alk]- ion of PS arises from primary loss of 87 to give rise to a [M-2H+Alk-87]- ion, followed by loss of fatty acid substituents as acids (RxCO2H, x=1,2) or as alkali salts (e.g., RxCO2Li, x=1,2). These fragmentations result in a greater abundance of [M-2H+Alk-87-R2CO2H]- than [M-2H+Alk-87-R1CO2H]- and a greater abundance of [M-2H+Alk-87-R2CO2Li]- than [M-2H+Alk-87-R1CO2Li]-; while further dissociation of the [M-2H+Alk-87-R2(or 1)CO2Li]- ions gives a preferential formation of the carboxylate anion at sn-1 (R1CO2-) over that at sn-2 (R2CO2-). Other major fragmentation process arises from differential loss of the fatty acid substituents as ketenes (loss of Rx'CH=CO, x=1,2). This results in a more prominent [M-2H+Alk-R2'CH=CO]- ion than [M-2H+Alk-R1'CH=CO]- ion. Ions informative for structural characterization of PS are of low abundance in the MS2-spectra of both the [M+H]+ and the [M+Alk]+ ions, but are abundant in the MS3-spectra. The MS2-spectrum of the [M+Alk]+ ion contains a unique ion corresponding to internal loss of a phosphate group probably via the fragmentation processes involving rearrangement steps. The [M-H+2Alk]+ ion of PS yields a major [M-H+2Alk-87]+ ion, which is equivalent to an alkali adduct ion of a monoalkali salt of PA and gives rise to a greater abundance of [M-H+2Alk-87-R1CO2H]+ than [M-H+2Alk-87-R2CO2H]+. Similarly, the [M-2H+3Alk]+ ion of PS also yields a prominent [M-2H+3Alk-87]+ ion, which undergoes consecutive dissociation processes that involve differential losses of the two fatty acyl substituents. Because all of the above tandem mass spectra contain several sets of ion pairs involving differential losses of the fatty acid substituents as ketenes or as free fatty acids, the identities of the fatty acyl substituents and their positions on the glycerol backbone can be easily assigned by the drastic differences in the abundances of the ions in each pair.

Crystal structure of SmcL, a bacterial neutral sphingomyelinase C from Listeria

Sphingomyelinases C are enzymes that catalyze the hydrolysis of sphingomyelin in biological membranes to ceramide and phosphorylcholine. Various pathogenic bacteria produce secreted neutral sphingomyelinases C that act as membrane-damaging virulence factors. Mammalian neutral sphingomyelinases C, which display sequence homology to the bacterial enzymes, are involved in sphingolipid metabolism and signaling. This article describes the first structure to be determined for a member of the neutral sphingomyelinase C family, SmcL, from the intracellular bacterial pathogen Listeria ivanovii. The structure has been refined to 1.9-A resolution with phases derived by single isomorphous replacement with anomalous scattering techniques from a single iridium derivative. SmcL adopts a DNase I-like fold, and is the first member of this protein superfamily to have its structure determined that acts as a phospholipase. The structure reveals several unique features that adapt the protein to its phospholipid substrate. These include large hydrophobic beta-hairpin and hydrophobic loops surrounding the active site that may bind and penetrate the lipid bilayer to position sphingomyelin in a catalytically competent position. The structure also provides insight into the proposed general base/acid catalytic mechanism, in which His-325 and His-185 play key roles.

Premicellar complexes of sphingomyelinase mediate enzyme exchange for the stationary phase turnover

During the steady state reaction progress in the scooting mode with highly processive turnover, Bacillus cereus sphingomyelinase (SMase) remains tightly bound to sphingomyelin (SM) vesicles (Yu et al., Biochim. Biophys. Acta 1583, 121-131, 2002). In this paper, we analyze the kinetics of SMase-catalyzed hydrolysis of SM dispersed in diheptanoylphosphatidyl-choline (DC7PC) micelles. Results show that the resulting decrease in the turnover processivity induces the stationary phase in the reaction progress. The exchange of the bound enzyme (E*) between the vesicle during such reaction progress is mediated via the premicellar complexes (E(i)#) of SMase with DC7PC. Biophysical studies indicate that in E(i)# monodisperse DC7PC is bound to the interface binding surface (i-face) of SMase that is also involved in its binding to micelles or vesicles. In the presence of magnesium, required for the catalytic turnover, three different complexes of SMase with monodisperse DC7PC (E(i)# with i=1, 2, 3) are sequentially formed with Hill coefficients of 3, 4 and 8, respectively. As a result, during the stationary phase reaction progress, the initial rate is linear for an extended period and all the substrate in the reaction mixture is hydrolyzed at the end of the reaction progress. At low mole fraction (X) of total added SM, exchange is rapid and the processive turnover is limited by the steps of the interfacial turnover cycle without becoming microscopically limited by local substrate depletion or enzyme exchange. At high X, less DC7PC will be monodisperse, E(i)# does not form and the turnover becomes limited by slow enzyme exchange. Transferred NOESY enhancement results show that monomeric DC7PC in solution is in a rapid exchange with that bound to E(i)# at a rate comparable to that in micelles. Significance of the exchange and equilibrium properties of the E(i)# complexes for the interpretation of the stationary phase reaction progress is discussed.

Cytolethal distending toxin generates cell death by inducing a bottleneck in the cell cycle

Cytolethal distending toxin (CDT) is a bacterial protein that is widely distributed among gram-negative bacteria including Escherichia coli, Campylobacter spp., enterohepatic Helicobacter spp., Actinobacillus actinomycetemcomitans and Haemophilus ducreyi. In vitro studies demonstrated that it is able to stop proliferation of various cell lines. The toxin is composed of three subunits designated CDTs A, B and C. The B subunit targets the eukaryotic DNA and triggers a signalling pathway involving different protein kinases which results in a cell block before entering into mitosis. To date, the individual role of the A and C subunits has not been totally elucidated. There are indications that the CDT is also produced in vivo. Its exact role in pathogenesis is not yet clear, but possible actions include inhibition of epithelial cell proliferation, apoptosis of immune cells and inhibition of a fibrotic response.

Activation of sphingomyelinase from Bacillus cereus by Zn2+ hitherto accepted as a strong inhibitor

Sphingomyelinase (SMase) from Bacillus cereus has been known to be activated by Mg2+, Mn2+, and Co2+, but strongly inhibited by Zn2+. In the present study, we investigated the effects of several kinds of metal ions on the catalytic activity of B. cereus SMase, and found that the activity was inhibited by Zn2+ at its higher concentrations or at higher pH values, but unexpectedly activated at lower Zn2+ concentrations or at lower pH values. This result indicates that SMase possesses at least two different binding sites for Zn2+ and that the Zn2+ binding to the high-affinity site can activate the enzyme, whereas the Zn2+ binding to the low-affinity site can inactivate it. We also found that the binding of substrate to the enzyme was independent of the Zn2+ binding to the high-affinity site, but was competitively inhibited by the Zn2+ binding to the low-affinity site. The binding affinity of the metal ions to the site for activating the enzyme was determined to be in the rank-order of Mg2+ = Co2+ < Mn2+ < Zn2+. It was also demonstrated that these four metal ions competed with each other for the same binding site on the enzyme molecule.

Stimulation of CD95-induced apoptosis in T-cells by a subtype specific neutral sphingomyelinase inhibitor

Neutral sphingomyelinase (nSMase) has been supposed to be involved in the activation of anti-apoptotic genes and, thus, could well sustain autoimmune reactions by preventing activation induced death of autoreactive T-cells. When screening cellular extracts for SMase activity in the range between pH 6.5 and 8.5 various murine tissue samples as well as cell lines of murine and human origin displayed peaks of activity, both, at pH 7.0 and 8.0. In contrast, T-cells (human T-cell lymphoma and PHA stimulated murine lymph node cells) and monocytic leukemia cells were lacking SMase activity at pH 8.0. Only one peak of activity was found at pH 7.0. Recently we described an inhibitory compound, C11AG which selectively suppresses nSMase activity. In dose-response assays using cellular extracts the pH 7.0 nSMase turned out to be almost 100-fold more sensitive to the inhibitor than the pH 8.0 nSMase. In Jurkat T-cell lymphoma cells lacking the pH 8.0 nSMase, treatment with C11AG enhanced sensitivity to apoptosis: the concentration of CD95-specific antibody anti-APO1 could be lowered by six-fold in order to induce cell death. Concomitantly the expression of the anti-apoptotic protein A1 was found to be down-regulated. In the joints of arthritic mice, apoptosis of T-cells was stimulated after application of C11AG. Accordingly, C11AG displayed curative effects on experimental arthritis: swelling and inflammation were found to be significantly alleviated.

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.