Sphingomyelinase D, a novel probe for cellular sphingomyelin: effects on cholesterol homeostasis in human skin fibroblasts

Active cholesterol 20 years on

This review considers the following hypotheses, some well-supported and some speculative. Almost all of the sterol molecules in plasma membranes are associated with bilayer phospholipids in complexes of varied strength and stoichiometry. These complexes underlie many of the material properties of the bilayer. The small fraction of cholesterol molecules exceeding the binding capacity of the phospholipids is thermodynamically active and serves diverse functions. It circulates briskly among the cell membranes, particularly through contact sites linking the organelles. Active cholesterol provides the upstream feedback signal to multiple mechanisms governing plasma membrane homeostasis, pegging the sterol level to a threshold set by its phospholipids. Active cholesterol could also be the cargo for various inter-organelle transporters and the form excreted from cells by reverse transport. Furthermore, it is integral to the function of caveolae; a mediator of Hedgehog regulation; and a ligand for the binding of cytolytic toxins to membranes. Active cholesterol modulates a variety of plasma membrane proteins-receptors, channels and transporters-at least in vitro.

Variability of cholesterol accessibility in human red blood cells measured using a bacterial cholesterol-binding toxin

Cholesterol partitions into accessible and sequestered pools in cell membranes. Here, we describe a new assay using fluorescently-tagged anthrolysin O, a cholesterol-binding bacterial toxin, to measure accessible cholesterol in human red blood cells (RBCs). Accessible cholesterol levels were stable within individuals, but varied >10 fold among individuals. Significant variation was observed among ethnic groups (Blacks>Hispanics>Whites). Variation in accessibility of RBC cholesterol was unrelated to the cholesterol content of RBCs or plasma, but was associated with the phospholipid composition of the RBC membranes and with plasma triglyceride levels. Pronase treatment of RBCs only modestly altered cholesterol accessibility. Individuals on hemodialysis, who have an unexplained increase in atherosclerotic risk, had significantly higher RBC cholesterol accessibility. Our data indicate that RBC accessible cholesterol is a stable phenotype with significant inter-individual variability. Factors both intrinsic and extrinsic to the RBC contribute to variation in its accessibility. This assay provides a new tool to assess cholesterol homeostasis among tissues in humans.

DOI:http://dx.doi.org/10.7554/eLife.23355.001

Sphingomyelinase D inhibits store-operated Ca2+ entry in T lymphocytes by suppressing ORAI current

Sphingomyelinase D suppresses Orai current in human T cells and decreases cytokine production, providing a mechanism whereby certain bacteria could inhibit the immune system.

Infections caused by certain bacteria including Mycobacterium tuberculosis and Corynebacterium pseudotuberculosis provoke inflammatory responses characterized by the formation of granulomas with necrotic foci—so-called caseous necrosis. The granulomas of infected animals show prominent infiltration by T lymphocytes, and T cell depletion increases host mortality. Notorious zoonotic C. pseudotuberculosis secretes sphingomyelinase (SMase) D, a phospholipase that cleaves off the choline moiety of sphingomyelin, a phospholipid found primarily in the outer leaflet of host cell plasma membranes. Experimental C. pseudotuberculosis strains that lack SMase D are markedly less infectious and unable to spread in hosts, indicating that this enzyme is a crucial virulence factor for sustaining the caseous lymphadenitis infections caused by this microbe. However, the molecular mechanism by which SMase D helps bacteria evade the host’s immune response remains unknown. Here, we find that SMase D inhibits store-operated Ca²⁺ entry (SOCE) in human T cells and lowers the production of the SOCE-dependent cytokines interleukin-2, which is critical for T cell growth, proliferation, and differentiation, and tumor necrosis factor α, which is crucial for the formation and maintenance of granulomas in microbial infections. SMase D inhibits SOCE through a previously unknown mechanism, namely, suppression of Orai1 current, rather than through altering gating of voltage-gated K⁺ channels. This finding suggests that, whereas certain genetic mutations abolish Orai1 activity causing severe combined immunodeficiency (SCID), bacteria have the ability to suppress Orai1 activity with SMase D to create an acquired, chronic SCID-like condition that allows persistent infection. Thus, in an example of how virulence factors can disrupt key membrane protein function by targeting phospholipids in host cell membranes, our study has uncovered a novel molecular mechanism that bacteria can use to thwart host immunity.

Microbial sphingomyelinase induces RhoA-mediated reorganization of the apical brush border membrane and is protective against invasion

Both commensal and pathogenic microbes that colonize the GI tract can synthesize and secrete spingomyelinase enzymes that cleave membrane sphingomyelin, leaving the ceramide component intact in the cell membrane. This study examines how this reaction affects the structure and function of host enterocytes and mucosal defense.

The apical brush border membrane (BBM) of intestinal epithelial cells forms a highly structured and dynamic environmental interface that serves to regulate cellular physiology and block invasion by intestinal microbes and their products. How the BBM dynamically responds to pathogenic and commensal bacterial signals can define intestinal homeostasis and immune function. We previously found that in model intestinal epithelium, the conversion of apical membrane sphingomyelin to ceramide by exogenous bacterial sphingomyelinase (SMase) protected against the endocytosis and toxicity of cholera toxin. Here we elucidate a mechanism of action by showing that SMase induces a dramatic, reversible, RhoA-dependent alteration of the apical cortical F-actin network. Accumulation of apical membrane ceramide is necessary and sufficient to induce the actin phenotype, and this coincides with altered membrane structure and augmented innate immune function as evidenced by resistance to invasion by Salmonella.

The ATP-binding cassette transporter-2 (ABCA2) regulates esterification of plasma membrane cholesterol by modulation of sphingolipid metabolism

The ATP-binding cassette transporters are a large family (~48 genes divided into seven families A-G) of proteins that utilize the energy of ATP-hydrolysis to pump substrates across lipid bilayers against a concentration gradient. The ABC "A" subfamily is comprised of 13 members and transport sterols, phospholipids and bile acids. ABCA2 is the most abundant ABC transporter in human and rodent brain with highest expression in oligodendrocytes, although it is also expressed in neurons. Several groups have studied a possible connection between ABCA2 and Alzheimer's disease as well as early atherosclerosis. ABCA2 expression levels have been associated with changes in cholesterol and sphingolipid metabolism. In this paper, we hypothesized that ABCA2 expression level may regulate esterification of plasma membrane-derived cholesterol by modulation of sphingolipid metabolism. ABCA2 overexpression in N2a neuroblastoma cells was associated with an altered bilayer distribution of the sphingolipid ceramide that inhibited acylCoA:cholesterol acyltransferase (ACAT) activity and cholesterol esterification. In contrast, depletion of endogenous ABCA2 in the rat schwannoma cell line D6P2T increased esterification of plasma membrane cholesterol following treatment with exogenous bacterial sphingomyelinase. These findings suggest that control of ABCA2 expression level may be a key locus of regulation for esterification of plasma membrane-derived cholesterol through modulation of sphingolipid metabolism.

Sphingomyelinase D activity in model membranes: structural effects of in situ generation of ceramide-1-phosphate

The toxicity of Loxosceles spider venom has been attributed to a rare enzyme, sphingomyelinase D, which transforms sphingomyelin to ceramide-1-phosphate. The bases of its inflammatory and dermonecrotic activity, however, remain unclear. In this work the effects of ceramide-1-phosphate on model membranes were studied both by in situ generation of this lipid using a recombinant sphingomyelinase D from the spider Loxosceles laeta and by pre-mixing it with sphingomyelin and cholesterol. The systems of choice were large unilamellar vesicles for bulk studies (enzyme kinetics, fluorescence spectroscopy and dynamic light scattering) and giant unilamellar vesicles for fluorescence microscopy examination using a variety of fluorescent probes. The influence of membrane lateral structure on the kinetics of enzyme activity and the consequences of enzyme activity on the structure of target membranes containing sphingomyelin were examined. The findings indicate that: 1) ceramide-1-phosphate (particularly lauroyl ceramide-1-phosphate) can be incorporated into sphingomyelin bilayers in a concentration-dependent manner and generates coexistence of liquid disordered/solid ordered domains, 2) the activity of sphingomyelinase D is clearly influenced by the supramolecular organization of its substrate in membranes and, 3) in situ ceramide-1-phosphate generation by enzymatic activity profoundly alters the lateral structure and morphology of the target membranes.

Impact of oxLDL on Cholesterol-Rich Membrane Rafts

Numerous studies have demonstrated that cholesterol-rich membrane rafts play critical roles in multiple cellular functions. However, the impact of the lipoproteins on the structure, integrity and cholesterol composition of these domains is not well understood. This paper focuses on oxidized low-density lipoproteins (oxLDLs) that are strongly implicated in the development of the cardiovascular disease and whose impact on membrane cholesterol and on membrane rafts has been highly controversial. More specifically, we discuss three major criteria for the impact of oxLDL on membrane rafts: distribution of different membrane raft markers, changes in membrane cholesterol composition, and changes in lipid packing of different membrane domains. We also propose a model to reconcile the controversy regarding the relationship between oxLDL, membrane cholesterol, and the integrity of cholesterol-rich membrane domains.

Cell cholesterol homeostasis: mediation by active cholesterol

Recent evidence suggests that the major pathways mediating cell cholesterol homeostasis respond to a common signal: active membrane cholesterol. Active cholesterol is the fraction that exceeds the complexing capacity of the polar bilayer lipids. Increments in plasma membrane cholesterol exceeding this threshold have an elevated chemical activity (escape tendency) and redistribute via diverse transport proteins to both circulating plasma lipoproteins and intracellular organelles. Active cholesterol thereby prompts several feedback responses. It is the substrate for its own esterification and for the synthesis of regulatory side-chain oxysterols. It also stimulates manifold pathways that down-regulate the biosynthesis, curtail the ingestion and increase the export of cholesterol. Thus, the abundance of cell cholesterol is tightly coupled to that of its polar lipid partners through active cholesterol.

oxLDL-induced decrease in lipid order of membrane domains is inversely correlated with endothelial stiffness and network formation

Oxidized low-density lipoprotein (oxLDL) is a major factor in development of atherosclerosis. Our earlier studies have shown that exposure of endothelial cells (EC) to oxLDL increases EC stiffness, facilitates the ability of the cells to generate force, and facilitates EC network formation in three-dimensional collagen gels. In this study, we show that oxLDL induces a decrease in lipid order of membrane domains and that this effect is inversely correlated with endothelial stiffness, contractility, and network formation. Local lipid packing of cell membrane domains was assessed by Laurdan two-photon imaging, endothelial stiffness was assessed by measuring cellular elastic modulus using atomic force microscopy, cell contractility was estimated by measuring the ability of the cells to contract collagen gels, and EC angiogenic potential was estimated by visualizing endothelial networks within the same gels. The impact of oxLDL on endothelial biomechanics and network formation is fully reversed by supplying the cells with a surplus of cholesterol. Furthermore, exposing the cells to 7-keto-cholesterol, a major oxysterol component of oxLDL, or to another cholesterol analog, androstenol, also results in disruption of lipid order of membrane domains and an increase in cell stiffness. On the basis of these observations, we suggest that disruption of lipid packing of cholesterol-rich membrane domains plays a key role in oxLDL-induced changes in endothelial biomechanics.

Ceramide activates JNK to inhibit a cAMP-gated K+ conductance and Cl- secretion in intestinal epithelia

Sphingomyelinases (SMases) hydrolyze membrane sphingomyelin to ceramide and are expressed by diverse host and microbial cell types populating mucosal surfaces. Exogenous bacterial SMase acts on the basolateral membrane of polarized human intestinal epithelial cells to repress the cAMP-induced Cl(-) secretory response, but how this occurs is unknown. We show here that SMase acts by down-regulating a cAMP-gated basolateral membrane K(+) conductance. Neither phosphocholine, ceramide-1-phosphate, nor sphingosine-1-phosphate recapitulates this effect, indicating that ceramide production is the decisive factor. Basolaterally applied SMase induced the phosphorylation of c-Jun NH(2)-terminal kinase (JNK), and inhibition of JNK rescued the effect of SMase on cAMP-dependant secretion. SMase secreted by normal human fibroblasts specifically recapitulated the effect on cAMP-induced Cl(-) secretion, indicating that cell types inhabiting the subepithelial space can provide such an activity to the basolateral membrane of intestinal enterocytes in trans. Thus, conversion of sphingomyelin to ceramide in basolateral membranes of intestinal cells rapidly activates JNK to inhibit a cAMP-gated K(+) conductance and thereby attenuates Cl(-) secretion. These results define a novel lipid-mediated pathway for regulation of salt and water homeostasis at mucosal surfaces.

Sphingolipids and cellular cholesterol homeostasis. Effect of ceramide on cholesterol trafficking and HMG CoA reductase activity

We previously showed that degradation of cellular sphingomyelin (SM) by SMase C results in a greater stimulation of cholesterol translocation to endoplasmic reticulum, compared to its degradation by SMase D. Here we investigated the hypothesis that the effect of SMase C is partly due to the generation of ceramide, rather than due to depletion of SM alone. Inhibition of hydroxymethylglutaryl CoA reductase (HMGCR) activity was used as a measure of cholesterol translocation. Treatment of fibroblasts with SMase C resulted in a 90% inhibition of HMGCR, whereas SMase D treatment inhibited it by 29%. Treatment with exogenous ceramides, or increasing the endogenous ceramide levels also inhibited HMGCR by 60-80%. Phosphorylation of HMGCR was stimulated by SMase C or exogenous ceramide. The effects of ceramide and SMase D were additive, indicating the independent effects of SM depletion and ceramide generation. These results show that ceramide regulates sterol trafficking independent of cellular SM levels.

Sphingomyelin functions as a novel receptor for Helicobacter pylori VacA

The vacuolating cytotoxin (VacA) of the gastric pathogen Helicobacter pylori binds and enters epithelial cells, ultimately resulting in cellular vacuolation. Several host factors have been reported to be important for VacA function, but none of these have been demonstrated to be essential for toxin binding to the plasma membrane. Thus, the identity of cell surface receptors critical for both toxin binding and function has remained elusive. Here, we identify VacA as the first bacterial virulence factor that exploits the important plasma membrane sphingolipid, sphingomyelin (SM), as a cellular receptor. Depletion of plasma membrane SM with sphingomyelinase inhibited VacA-mediated vacuolation and significantly reduced the sensitivity of HeLa cells, as well as several other cell lines, to VacA. Further analysis revealed that SM is critical for VacA interactions with the plasma membrane. Restoring plasma membrane SM in cells previously depleted of SM was sufficient to rescue both toxin vacuolation activity and plasma membrane binding. VacA association with detergent-resistant membranes was inhibited in cells pretreated with SMase C, indicating the importance of SM for VacA association with lipid raft microdomains. Finally, VacA bound to SM in an in vitro ELISA assay in a manner competitively inhibited by lysenin, a known SM-binding protein. Our results suggest a model where VacA may exploit the capacity of SM to preferentially partition into lipid rafts in order to access the raft-associated cellular machinery previously shown to be required for toxin entry into host cells.

Sensitivity to toxins produced by pathogenic bacteria is largely dictated by the presence or absence of toxin receptors on the plasma membrane of host cells. VacA is an important toxin produced by the pathogenic bacterium Helicobacter pylori, which infects the human stomach and causes gastric ulcer disease and stomach cancer. VacA binds and enters human cells, and induces several changes resulting ultimately in the death of the intoxicated cells. However, the identity of the VacA receptor responsible for toxin binding and function has remained a topic of debate. In this paper, we demonstrate that sphingomyelin, a lipid on the surface of cells with important membrane structural and signaling properties, functions as a VacA receptor. We demonstrate that VacA binds to sphingomyelin, and that presence or absence of sphingomyelin on the plasma membrane dictates how much VacA binds to the cell surface, and therefore, how sensitive cells are to the toxin. The identification of sphingomyelin also provides a conceptual framework for how VacA may enter cells through specialized functional domains on the surface of cells. This is the first example of a bacterial toxin that exploits sphingomyelin as a receptor, and future work will focus on developing strategies to block VacA interactions with sphingomyelin, thereby protecting cells from the downstream consequences of toxin action.

Effects of 2-hydroxyoleic acid on the structural properties of biological and model plasma membranes

Genetic hypertension is associated with alterations in lipid metabolism, membrane lipid composition and membrane-protein function. 2-Hydroxyoleic acid (2OHOA) is a new antihypertensive molecule that regulates the structure of model membranes and their interaction with certain peripheral signalling proteins in vitro. While the effect of 2OHOA on elevated blood pressure is thought to arise through its influence on signalling proteins, its effects on membrane lipid composition remain to be assessed. 2OHOA administration altered the lipid membrane composition of hypertensive and normotensive rat plasma membranes, and increased the fluidity of reconstituted liver membranes from hypertensive rats. In spontaneously hypertensive rats (SHR), treatment with 2OHOA increased the cholesterol and sphingomyelin content while decreasing that of phosphatidylserine-phosphatidylinositol lipids. In addition, monounsaturated fatty acid levels increased as well as the propensity of reconstituted membranes to form HII-phases. These data suggest that 2OHOA regulates lipid metabolism that is altered in hypertensive animals, and that it affects the structural properties of liver plasma membranes in SHR. These changes in the structural properties of the plasma membrane may modulate the activity of signalling proteins that associate with the cell membrane such as the Galphaq/11 protein and hence, signal transduction.

Lipid rafts in membrane-cytoskeleton interactions and control of cellular biomechanics: actions of oxLDL

Membrane-cytoskeleton coupling is known to play major roles in a plethora of cellular responses, such as cell growth, differentiation, polarization, motility, and others. In this review, the authors discuss the growing amount of evidence indicating that membrane-cytoskeleton interactions are regulated by the lipid composition of the plasma membrane, suggesting that cholesterol-rich membrane domains (lipid rafts), including caveolae, are essential for membrane-cytoskeleton coupling. Several models for raft-cytoskeleton interactions are discussed. Also described is the evidence suggesting that raft-cytoskeleton interactions play key roles in several cytoskeleton-dependent processes, particularly in the regulation of cellular biomechanical properties. To address further the physiological significance of raft-cytoskeleton coupling, the authors focus on the impact of oxidized low density lipoproteins, one of the major cholesterol carriers and proatherogenic factors, on the integrity of lipid rafts/caveolae, and on the organization of the cytoskeleton. Finally, the authors review the recent studies showing that oxLDL and cholesterol depletion have similar impacts on the biomechanical properties of vascular endothelial cells, which in turn affect endothelial angiogenic potential.

Role of sphingomyelin and ceramide in the regulation of the activity and fatty acid specificity of group V secretory phospholipase A2

We previously showed that group V secretory phospholipase A(2) (sPLA(2)V) is inhibited by sphingomyelin (SM), but activated by ceramide. Here, we investigated the effect of sphingolipid structure on the activity and acyl specificity of sPLA(2)V. Degradation of HDL SM to ceramide, but not to ceramide phosphate, stimulated the activity by 6-fold, with the release of all unsaturated fatty acids being affected equally. Ceramide-enrichment of HDL similarly stimulated the release of unsaturated fatty acids. Incorporation of SM into phosphatidylcholine (PC) liposomes preferentially inhibited the hydrolysis of 16:0-20:4 PC. Conversely, SMase C treatment or ceramide incorporation resulted in preferential stimulation of hydrolysis of 16:0-20:4 PC. The presence of a long chain acyl group in ceramide was essential for the activation, and long chain diacylglycerols were also effective. However, ceramide phosphate was inhibitory. These studies show that SM and ceramide in the membranes and lipoproteins not only regulate the activity of phospholipases, but also the release of arachidonate, the precursor of eicosanoids.

Protection of membrane cholesterol by sphingomyelin against free radical-mediated oxidation

Although the free radical-mediated oxidation of free cholesterol (FC) is critical in the generation of regulatory sterols and in atherogenesis, the physiological regulation of this process is poorly understood. We tested the hypothesis that sphingomyelin (SM), a major phospholipid of cell membranes, which is closely associated with FC, protects FC against oxidation, because of its unique structure, and affinity to the sterol. We employed phosphatidylcholine (PC) liposomes containing varying amounts of SM, and either radioactive FC or a fluorescent analog, dehydroergosterol (DHE), and determined the oxidative decay of the sterol in presence of 2,2'-azo-bis(2-amidinopropane hydrochloride) (AAPH). Incorporation of 25 mol% of SM in the liposomes inhibited the oxidation of FC or DHE by up to 50%. This inhibition was specific for SM among phospholipids, and was abolished by sphingomyelinase treatment. SM was not degraded during the oxidation reaction, and its effect was not dependent on the nature of the oxidizing agent, because it also inhibited sterol oxidation by FeSO(4)/ascorbate, and by cholesterol oxidase. These studies show that SM plays a physiological role in the regulation of cholesterol oxidation by free radicals.

Regulation of the activity and fatty acid specificity of lecithin-cholesterol acyltransferase by sphingomyelin and its metabolites, ceramide and ceramide phosphate

Sphingomyelin (SM), the second most abundant phospholipid in plasma lipoproteins, was previously shown to be a physiological inhibitor of the lecithin-cholesterol acyltransferase (LCAT) reaction. In this study, we investigated the effects of its metabolites, ceramide and ceramide phosphate, on the activity and fatty acid specificity of LCAT in vitro. Treatment of SM-containing substrate with SMase C, which hydrolyzes SM to ceramide, abolished the inhibitory effect of SM, whereas treatment with SMase D, which hydrolyzes it to ceramide phosphate, increased the level of inhibition. Although incorporation of ceramide into the substrate in the absence of SM activated the LCAT reaction only modestly, its co-incorporation with SM neutralized the inhibitory effect of SM. Ceramide phosphate, on the other hand, inhibited the LCAT reaction more strongly than SM. The effects of the sphingolipids on the phospholipase A and cholesterol esterification reactions of the enzyme were similar, indicating that they regulate the binding of phosphatidylcholine (PC) to the active site, rather than the esterification step. Incorporation of ceramide into the substrate stimulated the synthesis of unsaturated cholesteryl esters at the expense of saturated esters. However, these effects on fatty acid specificity disappeared when the PC substrates were incorporated into an inert diether PC matrix, suggesting that ceramide increases the availability of polyunsaturated PCs to the enzyme by altering the macromolecular structure of the substrate particle. Since the plasma ceramide levels are increased during inflammation, these results indicate that the activity and fatty acid specificity of LCAT may be altered during the inflammatory response.

Fumonisin B-glucose reaction products are less toxic when fed to swine

The effects of fumonisin B-glucose reaction products in swine diets was examined. Pigs were fed diets containing 528 micromol of total fumonisin B/kg (FB), 528 micromol of total FB-glucose adducts/kg (FB-G, 122 micromol of unreacted FB/kg), or 0 micromol of total FB/kg for 15 days to test the efficacy of the FB-G reaction products in detoxifying FB. Weight gain in FB pigs was lower than in FB-G or controls, which was correlated with feed intake reduction in FB pigs. Serum aspartate aminotransferase, gamma-glutamyltransferase, and total bilirubin in FB pigs were higher than in FB-G or control pigs. Serum sphinganine/shingosine ratios in FB pigs were higher than in FB-G or control pigs. Microscopic examination of tissues from FB pigs showed generalized liver necrosis and apoptosis with marked cellular pleomorphism and disorganized hepatic cords. The liver and kidneys in the FB-G group appeared to be normal. Tissues of controls were free of lesions. Results suggest that dietary FB-G products are less toxic to swine and may provide an detoxification approach in instances of widespread FB grain contamination (p < 0.05).

Ceramide displaces cholesterol from lipid rafts and decreases the association of the cholesterol binding protein caveolin-1

Addition of exogenous ceramide causes a significant displacement of cholesterol in lipid raft model membranes. However, whether ceramide-induced cholesterol displacement is sufficient to alter the protein composition of caveolin-enriched lipid raft membranes is unknown. Therefore, we examined whether increasing endogenous ceramide levels with bacterial sphingomyelinase (bSMase) depleted cholesterol and changed the protein composition of caveolin-enriched membranes (CEMs) isolated from immortalized Schwann cells. bSMase increased ceramide levels severalfold and decreased the cholesterol content of detergent-insoluble CEMs by 25-50% within 2 h. To examine the effect of ceramide on the protein composition of the CEMs, we performed a quantitative proteomic analysis using stable isotope labeling of cells in culture and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Although ceramide rapidly depleted lipid raft cholesterol, the levels of the cholesterol binding protein caveolin-1 (Cav-1) decreased by 25% only after 8 h. Importantly, replenishing the cells with cholesterol rapidly reversed the loss of Cav-1 from the CEMs. Ceramide-induced cholesterol depletion increased the association of 5'-nucleotidase and ATP synthase beta-subunit with the CEMs but had a minimal effect on changing the abundance of other lipid raft proteins, such as flotillin-1 and G-proteins. These results suggest that the ceramide-induced loss of cholesterol from CEMs may contribute to altering the lipid raft proteome.

Enzymatic modification of phospholipids for functional applications and human nutrition

Rapid progress in biochemistry of phospholipids and evolution of modern bioengineering has brought forth a number of novel concepts and technical advancements in the modification of phospholipids for industrial applications and human nutrition. Highlights cover preparation of novel phospholipid analogs based on the latest understanding of pivotal role of phospholipids in manifold biological processes, exploration of remarkable application potentials of phospholipids in meliorating human health, as well as development of new chemical and biotechnological approaches applied to the modification of phospholipids. This work reviews the natural occurrence and structural characteristics of phospholipids, their updated knowledge on manifold biological and nutritional functions, traditional and novel physical and chemical approaches to modify phospholipids as well as their applications to obtain novel phospholipids, and brief introduction of the efforts focusing on de novo syntheses of phospholipids. Special attention is given to the summary of molecular structural characteristics and catalytic properties of multiple phospholipases, which helps to interpret experimental phenomena and to improve reaction design. This will of course provide fundamental bases also for the development of enzymatic technology to produce structured or modified phospholipids.

Spider and Bacterial Sphingomyelinases D Target Cellular Lysophosphatidic Acid Receptors by Hydrolyzing Lysophosphatidylcholine

Bites by Loxosceles spiders can produce severe clinical symptoms, including dermonecrosis, thrombosis, vascular leakage, hemolysis, and persistent inflammation. The causative factor is a sphingomyelinase D (SMaseD) that cleaves sphingomyelin into choline and ceramide 1-phosphate. A similar enzyme, showing comparable bioactivity, is secreted by certain pathogenic corynebacteria and acts as a potent virulence factor. However, the molecular basis for SMaseD toxicity is not well understood, which hampers effective therapy. Here we show that the spider and bacterial SMases D hydrolyze albumin-bound lysophosphatidylcholine (LPC), but not sphingosylphosphorylcholine, with K(m) values ( approximately 20-40 microm) well below the normal LPC levels in blood. Thus, toxic SMases D have intrinsic lysophospholipase D activity toward LPC. LPC hydrolysis yields the lipid mediator lysophosphatidic acid (LPA), a known inducer of platelet aggregation, endothelial hyperpermeability, and pro-inflammatory responses. Introduction of LPA(1) receptor cDNA into LPA receptor-negative cells renders non-susceptible cells susceptible to SmaseD, but only in LPC-containing media. Degradation of circulating LPC to LPA with consequent activation of LPA receptors may have a previously unappreciated role in the pathophysiology of secreted SMases D.

Ceramide selectively displaces cholesterol from ordered lipid domains (rafts): implications for lipid raft structure and function

Ceramide is a membrane lipid involved in a number of crucial biological processes. Recent evidence suggests that ceramide is likely to reside and function within lipid rafts; ordered sphingolipid and cholesterol-rich lipid domains believed to exist within many eukaryotic cell membranes. Using lipid vesicles containing co-existing raft domains and disordered fluid domains, we find that natural and saturated synthetic ceramides displace sterols from rafts. Other raft lipids remain raft-associated in the presence of ceramide, showing displacement is relatively specific for sterols. Like cholesterol-containing rafts, ceramide-rich "rafts" remain in a highly ordered state. Comparison of the sterol-displacing abilities of natural ceramides with those of saturated diglycerides and an unsaturated ceramide demonstrates that tight lipid packing is critical for sterol displacement by ceramide. Based on these results, and the fact that cholesterol and ceramides both have small polar headgroups, we propose that ceramides and cholesterol compete for association with rafts because of a limited capacity of raft lipids with large headgroups to accommodate small headgroup lipids in a manner that prevents unfavorable contact between the hydrocarbon groups of the small headgroup lipids and the surrounding aqueous environment. Minimizing the exposure of cholesterol and ceramide to water may be a strong driving force for the association of other molecules with rafts. Furthermore, displacement of sterol from rafts by ceramide is very likely to have marked effects upon raft structure and function, altering liquid ordered properties as well as molecular composition. In this regard, certain previously observed physiological processes may be a result of displacement. In particular, a direct connection to the previously observed sphingomyelinase-induced displacement of cholesterol from plasma membranes in cells is proposed.