Myelin contains neutral sphingomyelinase activity that is stimulated by tumor necrosis factor-alpha

Inhibition of neutral sphingomyelinase 2 promotes remyelination

Myelination requires a highly organized synthesis of multiple lipid species that regulate myelin curvature and compaction. For reasons that are not understood, central nervous system remyelinated axons often have thin myelin sheaths with a disorganized structure susceptible to secondary demyelination. We found that expression of the sphingomyelin hydrolase neutral sphingomyelinase 2 (nSMase2) during the differentiation of oligodendrocyte progenitor cells (OPCs) to myelinating oligodendrocytes changes their response to inflammatory cytokines. OPCs do not express nSMase2 and exhibit a protective/regenerative response to tumor necrosis factor-α and interleukin-1β. Oligodendrocytes express nSMase2 and exhibit a stress response to cytokine challenge that includes an overproduction of ceramide, a sphingolipid that forms negative curvatures in membranes. Pharmacological inhibition or genetic deletion of nSMase2 in myelinating oligodendrocytes normalized the ceramide content of remyelinated fibers and increased thickness and compaction. These results suggest that inhibition of nSMase2 could improve the quality of myelin and stabilize structure.

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.

IFNγ-Stimulated Dendritic Cell Exosomes for Treatment of Migraine Modeled Using Spreading Depression

Migraine is a common headache disorder characterized by unilateral, intense headaches. In migraine with aura, the painful headache is preceded by focal neurological symptoms that can be visual, sensory, or motor in nature. Spreading depression (the most likely cause of migraine with aura and perhaps related headache pain) results in increased neuronal excitability and related increases in inflammation and production of reactive oxygen species. This in turn can promote the transformation of low-frequency, episodic migraine into higher-frequency and eventually chronic migraine. Though migraine affects 11% of adults worldwide, with 3% experiencing chronic headache, existing therapies offer only modest benefits. Here, we focus on the mechanisms by which environmental enrichment (i.e., volitionally increased intellectual, social, and physical activity) mitigates spreading depression. In prior work, we have shown that exposure to environmental enrichment reduces susceptibility to spreading depression in rats. This protective effect is at least in part due to environmental enrichment-mediated changes in the character of serum exosomes produced by circulating immune cells. We went on to show that environmental enrichment-mimetic exosomes can be produced by stimulating dendritic cells with low levels of interferon gamma (a cytokine that is phasically increased during environmental enrichment). Interferon gamma-stimulated dendritic cell exosomes (IFNγ-DC-Exos) significantly improve myelination and reduce oxidative stress when applied to hippocampal slice cultures. Here, we propose that they may also be effective against spreading depression. We found that administration of IFNγ-DC-Exos reduced susceptibility to spreading depression in vivo and in vitro, suggesting that IFNγ-DC-Exos may be a potential therapeutic for migraine.

Phasic Treatment with Interferon Gamma Stimulates Release of Exosomes that Protect Against Spreading Depression

The detrimental effects of T-cell-secreted interferon gamma (IFNγ) on oxidative stress (OS) and demyelination in multiple sclerosis (MS) are well recognized. Recently, we demonstrated that IFNγ-mediated damage to myelin also increases susceptibility to spreading depression (SD; the likely basis of migraine with aura). However, before onset of MS, induction of physiological levels of IFNγ, like that produced by environmental enrichment (EE), protects against demyelination and OS. Accordingly, we focused on the potential for physiological levels of IFNγ to protect against SD. EE, which occurs with a moderate and phasic increase in proinflammatory cytokines, reduces migraine frequency. Thus, we applied phasic or pulsed IFNγ to brain slice cultures to emulate EE. This treatment reduced OS, increased myelin basic protein, a marker for myelin, and reduced susceptibility to SD. Building on our research on exosomes in EE-based neuroprotection, we found that IFNγ stimulation of slice cultures induced release of exosomes, likely from the microglia that produce the same protective effects as IFNγ treatment when applied to naive cultures. Finally, nasal administration of IFNγ to rats recapitulated in vitro effects, reducing OS, increasing myelin, and reducing SD. These results support phasic IFNγ signaling as a therapeutic target for prevention of SD and, by extension, migraine.

Ceramides in Alzheimer's Disease: Key Mediators of Neuronal Apoptosis Induced by Oxidative Stress and Aβ Accumulation

Alzheimer's disease (AD), the most common chronic and progressive neurodegenerative disorder, is characterized by extracellular deposits of amyloid β-peptides (Aβ) and intracellular deposits of hyperphosphorylated tau (phospho-tau) protein. Ceramides, the major molecules of sphingolipid metabolism and lipid second messengers, have been associated with AD progression and pathology via Aβ generation. Enhanced levels of ceramides directly increase Aβ through stabilization of β-secretase, the key enzyme in the amyloidogenic processing of Aβ precursor protein (APP). As a positive feedback loop, the generated oligomeric and fibrillar Aβ induces a further increase in ceramide levels by activating sphingomyelinases that catalyze the catabolic breakdown of sphingomyelin to ceramide. Evidence also supports important role of ceramides in neuronal apoptosis. Ceramides may initiate a cascade of biochemical alterations, which ultimately leads to neuronal death by diverse mechanisms, including depolarization and permeabilization of mitochondria, increased production of reactive oxygen species (ROS), cytochrome c release, Bcl-2 depletion, and caspase-3 activation, mainly by modulating intracellular signalling, particularly along the pathways related to Akt/PKB kinase and mitogen-activated protein kinases (MAPKs). This review summarizes recent findings related to the role of ceramides in oxidative stress-driven neuronal apoptosis and interplay with Aβ in the cascade of events ending in neuronal degeneration.

Spreading depression transiently disrupts myelin via interferon-gamma signaling

Multiple sclerosis and migraine with aura are clinically correlated and both show imaging changes suggestive of myelin disruption. Furthermore, cortical myelin loss in the cuprizone animal model of multiple sclerosis enhances susceptibility to spreading depression, the likely underlying cause of migraine with aura. Since multiple sclerosis pathology involves inflammatory T cell lymphocyte production of interferon-gamma and a resulting increase in oxidative stress, we tested the hypothesis that spreading depression disrupts myelin through similar signaling pathways. Rat hippocampal slice cultures were initially used to explore myelin loss in spreading depression, since they contain T cells, and allow for controlled tissue microenvironment. These experiments were then translated to the in vivo condition in neocortex. Spreading depression in slice cultures induced significant loss of myelin integrity and myelin basic protein one day later, with gradual recovery by seven days. Myelin basic protein loss was abrogated by T cell depletion, neutralization of interferon-gamma, and pharmacological inhibition of neutral sphingomyelinase-2. Conversely, one day after exposure to interferon-gamma, significant reductions in spreading depression threshold, increases in oxidative stress, and reduced levels of glutathione, an endogenous neutral sphingomyelinase-2 inhibitor, emerged. Similarly, spreading depression triggered significant T cell accumulation, sphingomyelinase activation, increased oxidative stress, and reduction of gray and white matter myelin in vivo. Myelin disruption is involved in spreading depression, thereby providing pathophysiological links between multiple sclerosis and migraine with aura. Myelin disruption may promote spreading depression by enhancing aberrant excitability. Thus, preservation of myelin integrity may provide novel therapeutic targets for migraine with aura.

Role of cytokines as mediators and regulators of microglial activity in inflammatory demyelination of the CNS

As the resident innate immune cells of the central nervous system (CNS), microglia fulfil a critical role in maintaining tissue homeostasis and in directing and eliciting molecular responses to CNS damage. The human disease Multiple Sclerosis and animal models of inflammatory demyelination are characterized by a complex interplay between degenerative and regenerative processes, many of which are regulated and mediated by microglia. Cellular communication between microglia and other neural and immune cells is controlled to a large extent by the activity of cytokines. Here we review the role of cytokines as mediators and regulators of microglial activity in inflammatory demyelination, highlighting their importance in potentiating cell damage, promoting neuroprotection and enhancing cellular repair in a context-dependent manner.

Ceramide and neurodegeneration: susceptibility of neurons and oligodendrocytes to cell damage and death

Neurodegenerative disorders are marked by extensive neuronal apoptosis and gliosis. Although several apoptosis-inducing agents have been described, understanding of the regulatory mechanisms underlying modes of cell death is incomplete. A major breakthrough in delineation of the mechanism of cell death came from elucidation of the sphingomyelin (SM)-ceramide pathway that has received worldwide attention in recent years. The SM pathway induces apoptosis, differentiation, proliferation, and growth arrest depending upon cell and receptor types, and on downstream targets. Sphingomyelin, a plasma membrane constituent, is abundant in mammalian nervous system, and ceramide, its primary catabolic product released by activation of either neutral or acidic sphingomyelinase, serves as a potential lipid second messenger or mediator molecule modulating diverse cellular signaling pathways. Neutral sphingomyelinase (NSMase) is a key enzyme in the regulated activation of the SM cycle and is particularly sensitive to oxidative stress. In a context of increasing clarification of the mechanisms of neurodegeneration, we thought that it would be useful to review details of recent findings that we and others have made concerning different pro-apoptotic neurotoxins including proinflammatory cytokines, hypoxia-induced SM hydrolysis and ceramide production that induce cell death in human primary neurons and primary oligodendrocytes: redox sensitive events. What has and is emerging is a vista of therapeutically important ceramide regulation affecting a variety of different neurodegenerative and neuroinflammatory disorders.

The type 1 TNF receptor and its associated adapter protein, FAN, are required for TNFalpha-induced sickness behavior

RATIONALE

During the course of an infection, the pro-inflammatory cytokine tumor necrosis factor alpha (TNFalpha) acts in the brain to trigger development of behavioral responses, collectively termed sickness behavior. Biological activities of TNFalpha can be mediated by TNF receptor type 1 (TNF-R1) and type 2 (TNF-R2). TNFalpha activates neutral sphingomyelinase through the TNF-R1 adapter protein FAN (factor associated with neutral sphingomyelinase activation), but a behavioral role of FAN in the brain has never been reported.

OBJECTIVES

We hypothesized that TNFalpha-induced sickness behavior requires TNF-R1 and that FAN is a necessary component for this response.

MATERIALS AND METHODS

We determined the role of brain TNF-R1 in sickness behavior by administering an optimal amount of TNFalpha intracerebroventricularly (i.c.v., 50 ng/mouse) to wild-type (WT), TNF-R1-, TNF-R2-, and FAN-deficient mice. Sickness was assessed by decreased social exploration of a novel juvenile, induction of immobility, and loss of body weight.

RESULTS

TNF-R1-deficient mice were resistant to the sickness-inducing properties of i.c.v. TNFalpha, whereas both TNF-R2-deficient and WT mice were fully responsive. Furthermore, the complete absence of TNFalpha-induced sickness behavior in FAN-deficient mice provided in vivo evidence that FAN-dependent TNF-R1 signaling is critical for this central action of TNFalpha.

CONCLUSIONS

This is the first report to demonstrate that TNFalpha-induced sickness behavior is fully mediated by TNF-R1 and that the adaptor protein FAN is a necessary intracellular intermediate for sickness behavior.

In vitro and in vivo pharmacological models to assess demyelination and remyelination

In making a selection of cellular tools and animal models for generating screening assays in the search for new drugs, one needs to take into consideration the practicality of their use in the drug discovery process. Conducting high-throughput primary screens using libraries of small molecules, close to 1 million members in size, requires the generation of large numbers of cells which are easily acquired, reliably enriched, and reproducibly responsive to standard positive controls. These cells need to be similar in form and function to their counterparts in human disease. In vitro assays that can be mechanized by using robots can therefore save time and costs. In selecting in vivo models, consideration must be given to the species and strain of animal chosen, the appropriateness of the model to human disease, the extent of animal husbandry required during the in-life pharmacological assessment, the technical aspects of generating the model and harvesting the tissues for analyses, the cost of research tools in terms of time and money (demyelinating and remyelinating agents, amount of compound to be generated), and the length of time required for drug testing in the model. A consideration of the translational aspects of the in vivo model compared to those used in the clinic is also important. These themes will be developed with examples for drug discovery in the field of CNS demyelination and repair, specifically as it pertains to multiple sclerosis.

Activation of early components of complement targets myelin and oligodendrocytes in the aged rhesus monkey brain

The disruption and loss of myelin in the white matter are some of the major changes that occur in the brain with age. In vitro studies suggest a role of the complement system in the catabolic breakdown of myelin membranes. This study presents findings on activation of the early components of complement cascade in the brains of both young and aged rhesus monkeys with evidence of increased complement activation in aged animals. Complement containing oligodendrocytes (CAOs) containing C3d and C4d complement activation products bound to oligodendrocytes and myelinated fibers were found in the brain of normal young and old animals. The CAOs, which also contained activated microglia, were distributed throughout the whole brain and in significantly greater numbers in the aged monkeys. These findings, together with the demonstration of covalent binding of the C3 fragments to myelin, suggest the initiation of the complement cascade by myelin and oligodendrocytes, which are known classical complement activators. Activation of terminal complement components was not demonstrable in the CAOs. Taken together the findings support the concept that activation of early components of complement in the brain may be a normal biological process that involves the metabolism of myelin and oligodendrocytes and up-regulates with age.

Sphingolipids in human lens membranes: an update on their composition and possible biological implications

The unique nature of the most abundant phospholipids in human lens membranes remained overlooked until the 1990s when it was possible to discern dihydrosphingomyelins (DHSMs) from the more common sphingomyelins (SMs). Unlike in other mammalian membranes, DHSMs comprise nearly half of the phospholipids in adult human lenses. Compared to SMs with a trans double bond between carbons 4 and 5 of the sphingoid backbone, the absence of this unsaturation site in DHSMs allows the participation of the OH group on C3 in intermolecular H-bonds and leads to stronger interlipid interactions with both neighboring DHSMs and cholesterol. Phospholipid compositional changes with age and lens region observed in mammals with various life spans and lens growth rates, suggest that the highest levels of DHSMs along with the lowest amounts of phosphatidylcholines and SMs are found in lenses with the lowest growth rate, namely human lenses. The participation of phospholipid metabolites in the control of mitosis and elongation of lens cells is plausible and deserves investigation.

Fatty acid synthesizing enzymes intrinsic to myelin

A recent study showing incorporation of acetyl groups from neuronal N-acetylaspartate into myelin lipids suggested the presence of fatty acid synthesizing enzymes in myelin that utilize the acetyl groups liberated by myelin-associated aspartoacylase [J. Neurochem. 78 (2001) 736]. We report here detection of the fatty acid synthase (FAS) complex and acetyl-CoA carboxylase (ACC) in purified myelin. The activity of myelin FAS was approximately half that of cytosolic FAS and, unlike the latter, required detergent for activation. Intrinsic association of FAS with myelin was indicated by failure to remove the activity with NaCl or Na-taurocholate. Myelin-associated ACC was approximately 10% of cytosolic ACC in myelin isolated by gradient centrifugation, and this was reduced by half following osmotic shock; this suggested bimodal distribution of myelin ACC, some being loosely associated within inter-lamellar cytoplasmic spaces and the remainder more firmly associated in a manner that resists NaCl/Na-taurocholate treatments. These results, in combination with earlier findings, provide a possible mechanism for the observed incorporation of neuronal NAA acetyl groups into myelin lipids.

Transcription factor expression and cellular redox in immature oligodendrocyte cell death: effect of Bcl-2

Multiple sclerosis (MS) is characterized by the progressive damage or loss of oligodendrocytes. In an effort to better understand the causes of oligodendrocyte destruction in MS plaques, we treated immature oligodendrocytes with glucose oxidase, ceramide, or brefeldin A. These treatments model the different mechanisms by which oligodendrocytes are thought to die. We report that the AP-1 and Egr-1 transcription factors are induced within an hour of treatment. Of the AP-1 proteins studied, c-Jun was expressed at the highest level, followed by JunD, c-Fos, and Fra-2, although different treatments induced slightly different levels of expression. Bcl-2 overexpression protects against all treatments, to differing degrees. Although Bcl-2 did not have a dramatic effect on AP-1 or Egr-1 induction within the first 3 h, it caused a lowering of steady-state redox levels with a concomitant increase in cellular glutathione. We propose that the lowering of cellular redox and the upregulation of glutathione are responsible in part for the protective properties of Bcl-2.

Interleukin-2 receptors and interleukin-2-mediated signaling in myelin: activation of diacylglycerol kinase and phosphatidylinositol 3-kinase

Myelin was previously shown to possess neurotransmitter and cytokine receptors that trigger well-defined signaling mechanisms within the multilamellar structure. The present study reveals the presence of an interleukin-2 (IL-2) receptor in isolated mouse CNS myelin that responds to recombinant mouse IL-2 by activating diacylglycerol kinase (DAGK) and phosphoinositide 3-kinase (PI3K); additional evidence suggests participation by protein tyrosine kinase. Activation of myelin DAGK by IL-2 occurred in brain stem tissue mince and was blocked by chelerythrin chloride, indicating an essential role for myelin-localized protein kinase C. Two inhibitors of PI3K, wortmannin and LY294002, blocked endogenous PI3K as well as that enhanced by IL-2. Activation of PI3K by IL-2 was also blocked by tyrphostin A25, a selective inhibitor of PTK, suggesting activation of the latter by IL-2 is upstream to PI3K activation. This reaction resulted in tyrosine phosphorylation of a protein tentatively identified as the p85 subunit of PI3K. Developmental changes were noted in that receptor density and signaling activity were robust during the period of rapid myelination and declined rapidly thereafter.

Ubiquinol inhibition of neutral sphingomyelinase in liver plasma membrane: specific inhibition of the Mg2+-dependent enzyme and role of isoprenoid chain

In this work, the specificity of ubiquinol as inhibitor of the neutral sphingomyelinases present at the plasma membrane (Mg(2+)-dependent and -independent) and structural requirements for such inhibition have been studied. Our results have shown that ubiquinol specifically inhibits Mg(2+)-dependent neutral sphingomyelinase activity in isolated liver plasma membranes, but no significant participation of the Mg(2+)-independent enzyme was observed. Both the reduction state of the (hydro)quinone ring and the length of the hydrophobic side chain were important determinants in neutral sphingomyelinase inhibition. Ubiquinols inhibited the nSMase more efficiently than ubiquinones, and hydrophobic homologs with six or nine isoprene units were the most effective inhibitors. Inhibition of nSMase by ubiquinols displayed similarities with inhibition by manumycin and the hydroquinones F11334's, suggesting that these compounds could act as structural analogs of ubiquinol. Beyond its participation in mitochondrial energy metabolism, and as antioxidant, this novel role for ubiquinol as a neutral sphingomyelinase inhibitor should be considered an important factor to regulate lipid signaling at the plasma membrane that could be related to its beneficial effects on cells, tissues, and organisms.

Glial cells as targets for cytotoxic immune mediators

Oligodendrocytes and Schwann cells are the glia principally responsible for the synthesis and maintenance of myelin. Damage may occur to these cells in a number of conditions, but perhaps the most studied are the idiopathic inflammatory demyelinating diseases, multiple sclerosis in the CNS, and Guillain-Barré syndrome and its variants in the peripheral nervous system (PNS). This article explores the effects on these cells of cytotoxic immunological and inflammatory mediators: similarities are revealed, of which perhaps the most important is the sensitivity of both Schwann cells and oligodendrocytes to many such agents. This area of research is, however, characterised and complicated by numerous and often very substantial inter-observer discrepancies. Marked variability in cell culture techniques, and in assays of cell damage and death, provide artifactual explanations for some of this variability; true inter-species differences also contribute. Not the least important conclusion centres on the limited capacity of in vitro studies to reveal disease mechanisms: cell culture findings merely illustrate possibilities which must then be tested ex vivo using human tissue samples affected by the relevant disease.

Intraneuronal N-acetylaspartate supplies acetyl groups for myelin lipid synthesis: evidence for myelin-associated aspartoacylase

Despite its growing use as a radiological indicator of neuronal viability, the biological function of N-acetylaspartate (NAA) has remained elusive. This is due in part to its unusual metabolic compartmentalization wherein the synthetic enzyme occurs in neuronal mitochondria whereas the principal metabolizing enzyme, N-acetyl-L-aspartate amidohydrolase (aspartoacylase), is located primarily in white matter elements. This study demonstrates that within white matter, aspartoacylase is an integral component of the myelin sheath where it is ideally situated to produce acetyl groups for synthesis of myelin lipids. That it functions in this manner is suggested by the fact that myelin lipids of the rat optic system are well labeled following intraocular injection of [14C-acetyl]NAA. This is attributed to uptake of radiolabeled NAA by retinal ganglion cells followed by axonal transport and transaxonal transfer of NAA into myelin, a membrane previously shown to contain many lipid synthesizing enzymes. This study identifies a group of myelin lipids that are so labeled by neuronal [14C]NAA, and demonstrates a different labeling pattern from that produced by neuronal [14C]acetate. High performance liquid chromatographic analysis of the deproteinated soluble materials from the optic system following intraocular injection of [14C]NAA revealed only the latter substance and no radiolabeled acetate, suggesting little or no hydrolysis of NAA within mature neurons of the optic system. These results suggest a rationale for the unusual compartmentalization of NAA metabolism and point to NAA as a neuronal constituent that is essential for the formation and/or maintenance of myelin. The relevance of these findings to Canavan disease is discussed.

The role of neutral sphingomyelinase in interleukin-1beta signal transduction in mouse cerebral cortex cells

The cytokine interleukin-1 (IL-1) is an important mediator of neuroimmune interactions, though it has not been established precisely how the IL-lbeta signal is transmitted in nerve cells. This study demonstrates the involvement of the sphingomyelin cascade in IL-1beta signal transduction in the P2 membrane fraction of the mouse cerebral cortex. The key role of the membrane enzyme neutral sphingomyelinase in initiating the sphingomyelin signal transduction pathway for this cytokine is supported. The stimulating activity of IL-1beta on sphingomyelinase activity in the P2 fraction of the cerebral cortex was found to be dose-dependent. Studies using this membrane fraction from mice lacking the IL-1 type I receptor due to genomic mutations, along with studies using an IL-1 receptor antagonist. yielded data showing that IL-1beta binding with the type I receptor is a necessary event for activation of neutral sphingomyelinase. The results obtained here lead to the conclusion that the action of IL-1beta in the CNS is mediated by the IL-1 type I receptor and activation of neutral sphingomyelinase as the initiating enzyme of the sphingomyelin cascade.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Identification of multiple forms of membrane-associated neutral sphingomyelinase in bovine brain

Many different stimuli such as bioactive agents and environmental stresses are known to cause the activation of sphingomyelinase (SMase), which hydrolyzes sphingomyelin to generate ceramide as a second messenger playing a key role in differentiation and apoptosis in various cell types. Here we identified multiple forms of the membrane-associated neutral SMase (N-mSMase) activity in bovine brain. They could be classified into two groups according to extracting agents: group T-mSMase, extracted with 0.2% Triton X-100, and group S-mSMase, extracted with 0.5 M (NH(4))(2)SO(4). Group T-mSMase: alpha, beta, gamma, and delta, which were extensively purified from 40,000-g pellets of bovine brain homogenates by 3,150-, 5,275-, 1,665-, and 2,556-fold over the membrane extracts, respectively, by sequential use of several column chromatographies. On the other hand, S-mSMase was eluted as two active peaks of S-mSMase epsilon and zeta in a phenyl-5PW hydrophobic HPLC column and further purified by 1,119- and 976-fold over 40,000-g pellets of the homogenates, respectively. These highly purified N-mSMase enzyme preparations migrated as several bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and showed many different features in biochemical properties such as pH dependence, Mg(2+) requirements, and effects of detergents. Taken together, our data strongly suggest that mammalian brain N-mSMase may exist as multiple forms different in both its chromatographic profiles and biochemical properties.

Sphingolipids

A significant corpus of work over the last decade has firmly established an important role for sphingolipids in a variety of important biological processes. Such processes include signaling events related to cell growth, differentiation, programmed cell death, and stress responses. These processes not only involve those sphingolipids that accumulate as a result of a variety of inherited lysosomal storage disorders, but, in addition, sphingolipids associated with long-chain base metabolism. This article reviews the chemical properties, pathways, regulated metabolism, and signaling function of sphingolipids. In addition, the potential roles of sphingolipids in renal-specific processes are considered. While a variety of cellular functions have been ascribed to sphingolipids, in many cases proof of the concept has yet to be well established. Thus, a number of critical questions can be posed in interpreting these studies. Several of these questions are considered.

Activation of neutral sphingomyelinase by IL-1beta requires the type 1 interleukin 1 receptor

The cytokine interleukin 1beta (IL-1beta) plays an important role in host defence reactions and neuro-immune interactions but it is still not clear which of the two interleukin 1 receptor subtypes is coupled to activation of neutral sphingomyelinase (nSMase) by IL-1beta. To investigate involvement of neutral sphingomyelinase (nSMase) in central IL-1beta effects we used P(2)fractions of brain cerebral cortex from wild-type mice and mice deficient in the type 1 IL-1 receptor. IL-1beta (human, recombinant) was shown to activate, in a dose-dependent manner, nSMase in the P(2)brain fraction of the wild-type mice while in the knock-out mice the stimulatory effect of IL-1beta on nSMase was absent. In the presence of an IL-1 receptor antagonist (IL-1ra), IL-1beta did not activate nSMase either in the cortex of wild-type or knock-out mice. These data suggest that nSMase, a key enzyme of the sphingomyelin signal transduction pathway, might be involved in IL-1beta signalling in the brain and that activation of the enzyme requires the IL-1 receptor type 1.

Mechanisms of damage to myelin and oligodendrocytes and their relevance to disease

Oligodendrocytes synthesize and maintain myelin in the central nervous system (CNS). Damage may occur to these cells in a number of conditions, including infections, exposure to toxins, injury, degeneration, or autoimmune disease, arising both in the course of human disease and in experimental animal models of demyelination and dysmyelination; multiple sclerosis is the commonest human demyelinating disorder. Conventional classical accounts of the pathology of this and other myelin diseases have given great insights into their core features, but there remain considerable uncertainties concerning the timing, means and cause(s) of oligodendrocyte and myelin damage. At present, therapeutic efforts largely concentrate on immune manipulation and damage limitation, an approach that has produced only modest effects in multiple sclerosis. One reason for this must be the limited understanding of the mechanisms underlying cell damage - clearly, successful therapeutic strategies for preserving the oligodendrocyte-myelin unit must depend on knowledge of how oligodendrocyte damage and death occurs. In this review, mechanisms of oligodendrocyte and myelin damage are considered, and attempts made to relate them to disease processes, clinical and experimental. The hallmarks of different cell death processes are described, and oligodendrocyte-myelin injury by cellular and soluble mediators is discussed, both in vitro and invivo. Recent developments concerning the pathological involvement of oligodendrocytes in neurodegenerative disease are summarized. Finally, these neuropathological and applied neurobiological observations are drawn together in the context of multiple sclerosis.

Regulation of mitogen-activated protein kinases by sphingolipid products in oligodendrocytes

Sphingolipid products such as ceramide (cer), sphingosine (sph), and sphingosine-1-phosphate (SPP) are implicated in the regulation of cell growth and apoptosis. We have recently shown that cer, sph, and SPP differentially modulate ionic events in cultured oligodendrocytes (OLGs). Cer but not sph or SPP inhibits the inward rectifier (I(Kir)) in OLGs. To further investigate the role of sphingolipid products in OLGs, we studied the effect of cer, sph, and SPP on OLG survival and on the regulation of mitogen-activated protein kinases (MAPKs). We found that cer, sph, and SPP differentially modulate OLG survival and activation of MAPK members. Cer causes OLG apoptosis, sph causes OLG lysis, and SPP does not affect OLG survival. Cer induces a preferential activation of p38alpha, whereas sph and SPP induce a preferential activation of extracellular signal-regulated kinase 2 (ERK2) in OLGs. In addition, the effect of cer on p38alpha activity is mimicked by the inhibition of I(Kir) with Ba(2+). In contrast, exposure to cer results in increased activity of ERK2 but not of p38alpha in astrocytes. Cer-induced OLG apoptosis is attenuated by a p38 inhibitor, SB203580, and by expression of a p38alpha dominant negative mutant. We conclude that p38alpha is the mediator in cer-induced OLG apoptosis and that cer-induced I(Kir) inhibition may contribute to the sustained activation of p38alpha in OLGs.

Signalling sphingomyelinases: which, where, how and why?

A major lipid signalling pathway in mammalian cells implicates the activation of sphingomyelinase (SMase), which upon cell stimulation hydrolyses the ubiquitous sphingophospholipid sphingomyelin to ceramide. This review summarizes our current knowledge on the nature and regulation of signalling SMase(s). Because of the controversy on the identity of this(these) phospholipase(s), the roles of various SMases in cell signalling are discussed. Special attention is also given to the subcellular site of action of signalling SMases and to the cellular factors that positively or negatively control their activity. These regulating agents include lipids (arachidonic acid, diacylglycerol and ceramide), kinases, proteases, glutathione and other proteins.

Ceramide inhibits inwardly rectifying K+ currents via a Ras- and Raf-1-dependent pathway in cultured oligodendrocytes

Ceramide is a lipid mediator implicated in apoptosis induced by proinflammatory cytokines in many cell types, including oligodendrocytes (OLGs). To determine whether ceramide modulates transmembrane signaling events in OLGs, we studied its effect on intracellular Ca2+ (Cai), resting membrane potential and inwardly rectifying K+ currents (IKir) in cultured neonatal rat OLGs. We report here that (1) exposure to C2-ceramide (cer) rarely increases OLG Cai, whereas sphingosine elicits sustained increase in Cai; (2) cer causes OLG depolarization, an effect mimicked by sphingosine-1-phosphate but not by sphingosine; and (3) cer, but not its inactive analog dihydroceramide, inhibits OLG IKir. The cer effect is attenuated by Ras antibody Y13-259, by protein kinase C inhibitory peptide (19-36), and by suppression of c-Raf-1 expression with antisense raf-1 oligonucleotides. We conclude that cer-induced OLG depolarization is mediated via inhibition of IKir by a Ras- and raf-1-dependent pathway, which results in the phosphorylation of the inward rectifier K+ channel protein.

Cytokines, signal transduction, and inflammatory demyelination: review and hypothesis

The mechanism of focal demyelination in multiple sclerosis has been a long-standing enigma of this disorder. Cytokines, a diverse family of signalling molecules, are viewed as potential mediators of the process based on clinical observations and studies with animal models and tissue/cell culture systems. Myelin and oligodendrocyte (OL) destruction occur in cultured preparations subjected to cytokines such as tumor necrosis factor-alpha (TNF alpha) and lymphotoxin (LT). Many studies have shown these and other cytokines to be elevated at lesion sites and in the CSF of multiple sclerosis (MS) patients, with similar findings in animal models. Some variability in the nature of MS lesion formation has been reported, both OLs and myelin being primary targets. To account for myelin destruction in the presence of apparently functional OLs we hypothesize that cytokines such as TNF alpha and LT alpha contribute to myelin damage through triggering of specific reactions within the myelin sheath. We further propose that neutral sphingomyelinase (SMase) is one such enzyme, two forms of which have been detected in purified myelin. An additional event is accumulation of cholesterol ester, apparently a downstream consequence of cytokine-induced SMase. The resulting lipid changes are viewed as potentially destabilizing to myelin, which may render it more vulnerable to attack by invading and resident phagocytes.