Biophysics of sphingolipids I. Membrane properties of sphingosine, ceramides and other simple sphingolipids

Biophysical properties of sphingosine, ceramides and other simple sphingolipids

Some of the simplest sphingolipids, namely sphingosine, ceramide and their phosphorylated compounds [sphingosine 1-phosphate (Sph-1-P) and ceramide 1-phosphate (Cer-1-P)], are potent metabolic regulators. Each of these lipids modifies in marked and specific ways the physical properties of the cell membranes, in what can be the basis for some of their physiological actions. The present paper is an overview of the mechanisms by which these sphingolipid signals, sphingosine and ceramide, in particular, are able to modify the properties of cell membranes.

Live cell imaging of membrane/cytoskeleton interactions and membrane topology

We elucidate the interaction between actin and specific membrane components, using real time live cell imaging, by delivering probes that enable access to components, that cannot be accessed genetically. We initially investigated the close interplay between Phosphatidylinositol 4,5-bisphosphate (PIP2) and the F-actin network. We show that, during the early stage of cell adhesion, PIP2 forms domains within the filopodia membrane. We studied these domains alongside cell spreading and observed that these very closely follow the actin tread-milling. We show that this mechanism is associated with an active transport of PIP2 rich organelles from the cell perinuclear area to the edge, along actin fibers. Finally, mapping other phospholipids and membrane components we observed that the PIP2 domains formation is correlated with sphingosine and cholesterol rafts.

Early and late HIV-1 membrane fusion events are impaired by sphinganine lipidated peptides that target the fusion site

Lipid-conjugated peptides have advanced the understanding of membrane protein functions and the roles of lipids in the membrane milieu. These lipopeptides modulate various biological systems such as viral fusion. A single function has been suggested for the lipid, binding to the membrane and thus elevating the local concentration of the peptide at the target site. In the present paper, we challenged this argument by exploring in-depth the antiviral mechanism of lipopeptides, which comprise sphinganine, the lipid backbone of DHSM (dihydrosphingomyelin), and an HIV-1 envelope-derived peptide. Surprisingly, we discovered a partnership between the lipid and the peptide that impaired early membrane fusion events by reducing CD4 receptor lateral diffusion and HIV-1 fusion peptide-mediated lipid mixing. Moreover, only the joint function of sphinganine and its conjugate peptide disrupted HIV-1 fusion protein assembly and folding at the later fusion steps. Via imaging techniques we revealed for the first time the direct localization of these lipopeptides to the virus–cell and cell–cell contact sites. Overall, the findings of the present study may suggest lipid–protein interactions in various biological systems and may help uncover a role for elevated DHSM in HIV-1 and its target cell membranes.

We show that sphinganine lipidated peptides affect membrane fusion, modulate the membrane and disrupt protein assembly. In addition the findings may aid in deciphering the function of DHSM in biological membranes.

N-Acyl Chain in Ceramide and Sphingomyelin Determines Their Mixing Behavior, Phase State, and Surface Topography in Langmuir Films

Sphingolipids are membrane lipids composed by a long chain aminediol base, usually sphingosine, with a N-linked fatty acyl chain whose quality depends on the membrane type. The effect of length and unsaturation of the N-acyl chain on the mixing behavior of different sphingolipids has scarcely been studied, and in this work this issue is addressed employing Langmuir monolayers at the air-water interface, in order to assess the surface mixing in binary mixtures of different species of sphingomyelins and ceramides. The dependence on the monolayer composition of the mean molecular area, perpendicular dipole moment, domain segregation, and surface topography, as well as the film elasticity and optical thickness were studied. The results indicate that composition-dependent favorable interactions among sphingomyelin and ceramide occur as a consequence of complementary lateral packing and increased acyl chain ordering; the phase state of the components appears as a major factor determining miscibility among sphingomyelins and ceramides even in cases where the lipids have a considerable hydrocarbon chain length mismatch.

Ceramide-lipid interactions studied by MD simulations and solid-state NMR

Ceramides play a key modulatory role in many cellular processes, which results from their effect on the structure and dynamics of biological membranes. In this study, we investigate the influence of C16-ceramide (C16) on the biophysical properties of DMPC lipid bilayers using solid-state NMR and atomistic molecular dynamics (MD) simulations. MD simulations and NMR measurements were carried out for a pure DMPC bilayer and for a 20% DMPC-C16 mixture. Calculated key structural properties, namely area per lipid, chain order parameters, and mass density profiles, indicate that C16 has an ordering effect on the DMPC bilayer. Furthermore, the simulations predict that specific hydrogen-bonds form between DMPC and C16 molecules. Multi-nuclear solid-state NMR was used to verify these theoretical predictions. Chain order parameters extracted from (13)C(1)H dipole couplings were measured for both lipid and ceramide and follow the trend suggested by the MD simulations. Furthermore, (1)H-MAS NMR experiments showed a direct contact between ceramide and lipids.

Surface behavior of sphingomyelins with very long chain polyunsaturated fatty acids and effects of their conversion to ceramides

Molecular species of sphingomyelin (SM) with nonhydroxy (n) and 2-hydroxy (h) very long chain polyunsaturated fatty acids (n- and h-28:4, 30:5, and 32:5) abound in rat spermatogenic cells and spermatozoa. These SMs are located on the sperm head, where they are converted to the corresponding ceramides (Cer) after the completion of the acrosomal reaction, as induced in vitro. The aim of this study was to look into the surface properties of these unique SM species and how these properties change by the SM → Cer conversion. After isolation by HPLC, these SMs were organized in Langmuir films and studied alone, in combination with different proportions of Cer, and during their conversion to Cer by sphingomyelinase. Compression isotherms for all six SMs under study were compatible with a liquid-expanded (LE) state and showed large molecular areas. Only the longest SMs (n-32:5 and h-32:5 SM) underwent a phase transition upon cooling. Interestingly, the abundant h-28:4 Cer exhibited a highly compressible liquid-condensed (LC) phase compatible with a high conformational freedom of Cer molecules but with the characteristic low diffusional properties of the LC phase. In mixed films of h-28:4 SM/h-28:4 Cer, the components showed favorable mixing in the LE phase. The monolayer exhibited h-28:4 Cer-rich domains both in premixed films and when formed by the action of sphingomyelinase on pure h-28:4 SM films. Whereas the SMs from sperm behaved in a way similar to that of shorter acylated SMs, the corresponding Cers showed atypical rheological properties that may be relevant to the membrane structural rearrangements that take place on the sperm head after the completion of the acrosomal reaction.

Recent progress and new perspectives in lymphoma glycobiology

Glycosylation has recently become one of the most significant subjects in tumor biology, and cell surface glycosylation is closely associated with various biological phenomena in tumor cells. However, the biological significance of cell surface glycosylation and sialic acid linked to glycans in human malignant lymphoma is not well elucidated. We have determined that 1) sialylation or loss of N-glycosylation is closely associated with a worse prognosis in human diffuse large B-cell lymphoma (DLBCL), and 2) glycosylation or sialic acid on the surface of lymphoma cells plays significant roles in cell adhesion or invasion to the extracellular matrix, cell growth, apoptosis and cell death. In the present review, the biological functions of glycosylation or sialic acid in human malignant lymphoma are discussed.

A molecular level understanding of interaction between FTY720 (Fingolimod hydrochloride) and DMPC multilamellar vesicles

This work focuses on the molecular level understanding of interaction between FTY720 (Fingolimod hydrochloride) and dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles (MLVs) as a drug molecule carrier by investigating the structural changes, solubilisation effect and thermotropic phase behaviour.

A cost-benefit analysis of the physical mechanisms of membrane curvature

Many cellular membrane-bound structures exhibit distinct curvature that is driven by the physical properties of their lipid and protein constituents. Here we review how cells manipulate and control this curvature in the context of dynamic events such as vesicle-mediated membrane traffic. Lipids and cargo proteins each contribute energy barriers that must be overcome during vesicle formation. In contrast, protein coats and their associated accessory proteins drive membrane bending using a variety of interdependent physical mechanisms. We survey the energy costs and drivers involved in membrane curvature, and draw a contrast between the stochastic contributions of molecular crowding and the deterministic assembly of protein coats. These basic principles also apply to other cellular examples of membrane bending events, including important disease-related problems such as viral egress.

Expression analysis and enzymatic characterization of phospholipase Cδ4 from olive flounder (Paralichthys olivaceus)

Phospholipase Cδ4 (PLCδ4) plays a significant role in cell proliferation, tumorigenesis, and in an early stage of fertilization. Despite the characterization of the mammalian PLCδ4, extensive study in aquatic organisms has not been carried out so far. Here, we performed the molecular and biochemical characterization of flatfish Paralichthys olivaceus PLCδ4 (PoPLCδ4) to understand its enzymatic properties and physiological functions. The olive flounder PLCδ4 cDNA has an open reading frame (ORF) of 2,268 bp, and encodes a 755 amino acid polypeptide with a predicted molecular weight of 86 kDa. All the characteristic domains found in mammalian PLCδ isoforms (PH domain, EF hands, an X-Y catalytic region, and a C2 domain) were found to be present in PoPLCδ4. The mRNA expression analysis of PoPLCδ4 showed that PoPLCδ4 is predominantly expressed in the brain, eye and heart tissues. Like other mammalian PLCδ proteins, the enzyme activity of recombinant PoPLCδ4 to phosphatidylinositol-4,5-bis-phosphate (PIP2) was noted to be concentration- and Ca(2+)-dependent. The structural features and biochemical characteristics of PoPLCδ4 were found to be similar to those of mammalian PLCδ4. This is the first demonstration of the expression analysis and enzymatic characterization of piscine PLCδ4.

Ceramide synthases as potential targets for therapeutic intervention in human diseases

Ceramide is located at a key hub in the sphingolipid metabolic pathway and also acts as an important cellular signaling molecule. Ceramide contains one acyl chain which is attached to a sphingoid long chain base via an amide bond, with the acyl chain varying in length and degree of saturation. The identification of a family of six mammalian ceramide synthases (CerS) that synthesize ceramide with distinct acyl chains, has led to significant advances in our understanding of ceramide biology, including further delineation of the role of ceramide in various pathophysiologies in both mice and humans. Since ceramides, and the complex sphingolipids generated from ceramide, are implicated in disease, the CerS might potentially be novel targets for therapeutic intervention in the diseases in which the ceramide acyl chain length is altered. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Ceramides in the pathophysiology of the anterior segment of the eye

PURPOSE

Sphingolipid (SL) research reached a peak in the past years. Yet this positive trend was not evident for eye research as the relative number of studies centered on SLs is decreasing. Our aim is to encourage the inclusion of SL metabolites in studies of ocular pathophysiology by summarizing recent findings and current awareness concerning ceramides in the anterior segment of the eye.

METHODS

Review of literature relating to ceramides as bioactive lipids and the extent to which their particular nature was investigated in ocular pathophysiology.

RESULTS

Ceramides are rare but indispensable lipids that influence cellular responses through their effects on membrane biophysical properties or direct interaction with target proteins. Their biological significance is increased by variability and adaptability as there are tens of enzymes designed to modulate their function. The eye offers a set of unique environments where ceramides or other SLs have not been extensively studied. Not surprisingly, ceramides were associated with apoptosis in the metabolically active tissues, while little is known about its effects on the biophysical properties of the tears or lens lipids. More so, there are still aspects of the ocular homeostasis control where SLs contribution has not been investigated to date (e.g. pathogen aggression).

CONCLUSIONS

Ceramides and SL metabolism still receive increasing attention and have proven to be a significant metabolite in many research fields (e.g. cancer, stress response and inflammation) and there are yet many questions that they will aid answer. With the present work, we seek to increase awareness of these lipids also in eye research and to highlight their importance as common regulators of various diseases.

Shear and compression rheology of Langmuir monolayers of natural ceramides: solid character and plasticity

The present work addresses the fundamental question of membrane elasticity of ceramide layers with a special focus on the plastic regime. The compression and shear viscoelasticity of egg-ceramide Langmuir monolayers were investigated using oscillatory surface rheology in the linear regime and beyond. High compression and shear moduli were measured at room temperature-a clear signature for a solid behavior. At deformations larger than one per mill, egg-ceramide monolayers display plastic features characterized by a decrease of the storage modulus followed by a viscous regime typical of fluid lipids. This behavior is accompanied by a marked decrease of the loss modulus with increasing stress above a yield point. The results permit to univocally classify ceramide monolayers as 2D solids able to undergo plastic deformations, at the difference of typical fluid lipid monolayers. These unusual features are likely to have consequences in the mechanical behavior of ceramide-rich emplacements in biological membranes.

Lipid Phase Separation and Protein-Ganglioside Clustering in Supported Bilayers Are Induced by Photorelease of Ceramide

Photolysis of 6-bromo-7-hydroxycoumarinyl-caged ceramide was used to generate ceramide with spatial and temporal control in supported lipid bilayers prepared from mixtures of caged ceramide and phospholipids. The caged ceramide molecules are randomly distributed in fluid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, and upon photolysis with long wavelength UV light small ordered ceramide domains are formed that phase separate from the bulk fluid membrane. Irradiation of a spatially restricted area leads to the transient formation of ceramide-enriched gel phase domains that equilibrate via lipid diffusion with the surrounding unirradiated membrane. Photorelease of C16-ceramide in supported bilayers prepared from POPC, caged ceramide and the ganglioside GM1 (90:10:1 molar ratio) results in partitioning of a ganglioside-protein complex into the ceramide-enriched domains, modeling some aspects of ceramide's behavior in cells. The photo-uncaging strategy used here for delivery of ceramide in bilayers provides a novel and useful alternative to the enzymatic generation of ceramide in sphingomyelin-containing membranes. The ability to control membrane phase separation behavior and the clustering of membrane-anchored proteins illustrates the potential of photo-uncaging for studying the compartmentalization of ceramide in cellular membranes.

Myristate-derived d16:0 sphingolipids constitute a cardiac sphingolipid pool with distinct synthetic routes and functional properties

BACKGROUND

Myristate is a novel potential substrate for sphingoid base synthesis.

RESULTS

Myocardial sphingoid base synthesis utilizes myristate; these sphingolipids are functionally non-redundant with canonical sphingoid bases.

CONCLUSION

d16:0 and d16:1 sphingolipids constitute an appreciable proportion of cardiac dihydrosphingosine and dihydroceramide, with distinct biological roles.

SIGNIFICANCE

This pool of sphingolipids may play a heretofore unsuspected role in myocardial pathology or protection. The enzyme serine palmitoyltransferase (SPT) catalyzes the formation of the sphingoid base "backbone" from which all sphingolipids are derived. Previous studies have shown that inhibition of SPT ameliorates pathological cardiac outcomes in models of lipid overload, but the metabolites responsible for these phenotypes remain unidentified. Recent in vitro studies have shown that incorporation of the novel subunit SPTLC3 broadens the substrate specificity of SPT, allowing utilization of myristoyl-coenzyme A (CoA) in addition to its canonical substrate palmitoyl-CoA. However, the relevance of these findings in vivo has yet to be determined. The present study sought to determine whether myristate-derived d16 sphingolipids are represented among myocardial sphingolipids and, if so, whether their function and metabolic routes were distinct from those of palmitate-derived d18 sphingolipids. Data showed that d16:0 sphingoid bases occurred in more than one-third of total dihydrosphingosine and dihydroceramides in myocardium, and a diet high in saturated fat promoted their de novo production. Intriguingly, d16-ceramides demonstrated highly limited N-acyl chain diversity, and in vitro enzyme activity assays showed that these bases were utilized preferentially to canonical bases by CerS1. Functional differences between myristate- and palmitate-derived sphingolipids were observed in that, unlike d18 sphingolipids and SPTLC2, d16 sphingolipids and SPTLC3 did not appear to contribute to myristate-induced autophagy, whereas only d16 sphingolipids promoted cell death and cleavage of poly(ADP-ribose) polymerase in cardiomyocytes. Thus, these results reveal a previously unappreciated component of cardiac sphingolipids with functional differences from canonical sphingolipids.

The onset of Triton X-100 solubilization of sphingomyelin/ceramide bilayers: effects of temperature and composition

The early stages of Triton X-100 solubilization of bilayers consisting of sphingomyelin/ceramide (SM/Cer) mixtures have been studied using a combination of calorimetric and spectroscopic techniques. Compositions based on sphingomyelin, containing up to 30 mol% Cer, at 4, 20 and 50°C have been examined. The presence of Cer does not modify the affinity (in terms of ΔG of binding per mol total lipid) of the SM-based bilayers for Triton X-100, although it does increase the amount of detergent required for the onset of solubilization. At 50°C more detergent was required to solubilize the SM/Cer bilayers than at 20°C. The data can be rationalized in terms of lipid and detergent geometries and interactions (Lichtenberg et al., 2013).

Cholesteryl phosphocholine--a study on its interactions with ceramides and other membrane lipids

We prepared cholesteryl phosphocholine (CholPC) by chemical synthesis and studied its interactions with small (ceramide and cholesterol) and large headgroup (sphingomyelin (SM) and phosphatidylcholine) colipids in bilayer membranes. We established that CholPC could form bilayers (giant uni- and multilamellar vesicles, as well as extruded large unilamellar vesicles) with both ceramides and cholesterol (initial molar ratio 1:1). The extruded bilayers appeared to be fluid, although highly ordered, even when the ceramide had an N-linked palmitoyl acyl chain. In binary systems containing CholPC and either palmitoyl SM or 1,2-dipalmitoyl-sn-glycero-3-phospholine, CholPC markedly destabilized the gel phase of the respective large headgroup lipid. In 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, CholPC was much less efficient than cholesterol in ordering the acyl chains. In complex bilayers containing POPC and cholesterol or palmitoyl ceramide, CholPC appeared to prefer interacting with the small headgroup lipids over POPC. When the degree of order in CholPC/PCer bilayers was compared to Chol/PSM bilayers, CholPC/PCer bilayers were more disordered (based on DPH anisotropy). This finding may result from different headgroup orientation and dynamics in CholPC and PSM. Our results overall can be understood if one takes into account the molecular shape of CholPC (large polar headgroup and modest size hydrophobic part) when interpreting molecular interactions between small and large headgroup colipids. The results are also consistent with the proposed umbrella model" for explaining cholesterol/colipid interactions.

Molecular properties of various structurally defined sphingomyelins -- correlation of structure with function

Sphingomyelins are important phospholipids in plasma membranes of most cells. Because of their dominantly saturated nature, they affect the lateral structure of membranes, and contribute to the regulation of cholesterol distribution within membranes, and in cells. However, the abundance of molecular species present in cells also implies that sphingomyelins have other, more specific functions. Many of these functions are currently unknown, but are under extensive study. Mostly model membrane studies have shown that sphingomyelins (and other sphingolipids), in contrast to glycerophospholipids, have important hydrogen bonding properties which in several important ways confer specific functional properties to this abundant class of membrane phospholipids. The often very asymmetric nature of sphingomyelins, arising from mismatch in length between the long chain base and N-acyl chains, also impose specific properties (e.g., interdigitation) to sphingomyelins not seen with glycerophospholipids. In this review, the latest sphingomyelin literature will be scrutinized, and an effort will be made to correlate the molecular structure of sphingomyelin with functional properties. In particular, the effects of head group properties, interfacial hydrogen bonding, long chain base hydroxylation, N-acyl chain hydroxylation, and N-acyl chain methyl-branching will be discussed.

Lipid sorting by ceramide and the consequences for membrane proteins

We mimicked the effect of sphingomyelinase activity on lipid mixtures of palmitoyl-oleoyl-phosphatidylcholine, sphingomyelin, ceramide, and 10 mol % cholesterol. Using x-ray diffraction experiments in combination with osmotic stress we found, in agreement with previous studies, that ceramide induces a coexistence of L(α) and L(β) domains. A detailed structural analysis of the coexisting domains demonstrated an increase of lipid packing density and membrane thickness in the L(α) domains upon increasing overall ceramide levels. This provides evidence for a ceramide-driven accumulation of cholesterol in the L(α) domains, in support of previous reports. We further determined the bending rigidities of the coexisting domains and found that the accumulation of cholesterol in the L(α) domains stabilizes their bending rigidity, which experiences a dramatic drop in the absence of cholesterol. Deriving experimental estimates for the spontaneous curvature and Gaussian modulus of curvature, we show, using a simple geometric model for ion channels, that in this way changes in the conformational equilibrium of membrane proteins can be kept small.

Human tRNA(Sec) associates with HeLa membranes, cell lipid liposomes, and synthetic lipid bilayers

We have shown previously that simple RNA structures bind pure phospholipid liposomes. However, binding of bona fide cellular RNAs under physiological ionic conditions is shown here for the first time. Human tRNA(Sec) contains a hydrophobic anticodon-loop modification: N⁶-isopentenyladenosine (i⁶A) adjacent to its anticodon. Using a highly specific double-probe hybridization assay, we show mature human tRNA(Sec) specifically retained in HeLa intermediate-density membranes. Further, isolated human tRNA(Sec) rebinds to liposomes from isolated HeLa membrane lipids, to a much greater extent than an unmodified tRNA(Sec) transcript. To better define this affinity, experiments with pure lipids show that liposomes forming rafts or including positively charged sphingosine, or particularly both together, exhibit increased tRNA(Sec) binding. Thus tRNA(Sec) residence on membranes is determined by several factors, such as hydrophobic modification (likely isopentenylation of tRNA(Sec)), lipid structure (particularly lipid rafts), or sphingosine at a physiological concentration in rafted membranes. From prior work, RNA structure and ionic conditions also appear important. tRNA(Sec) dissociation from HeLa liposomes implies a mean membrane residence of 7.6 min at 24°C (t(1/2) = 5.3 min). Clearly RNA with a 5-carbon hydrophobic modification binds HeLa membranes, probably favoring raft domains containing specific lipids, for times sufficient to alter biological fates.

Sphingolipids: regulators of crosstalk between apoptosis and autophagy

Apoptosis and autophagy are two evolutionarily conserved processes that maintain homeostasis during stress. Although the two pathways utilize fundamentally distinct machinery, apoptosis and autophagy are highly interconnected and share many key regulators. The crosstalk between apoptosis and autophagy is complex, as autophagy can function to promote cell survival or cell death under various cellular conditions. The molecular mechanisms of crosstalk are beginning to be elucidated and have critical implications for the treatment of various diseases, such as cancer. Sphingolipids are a class of bioactive lipids that mediate many key cellular processes, including apoptosis and autophagy. By targeting several of the shared regulators, sphingolipid metabolites differentially regulate the induction of apoptosis and autophagy. Importantly, individual sphingolipid species appear to "switch" autophagy toward cell survival (e.g., sphingosine-1-phosphate) or cell death (e.g., ceramide, gangliosides). This review assesses the current understanding of sphingolipid-induced apoptosis and autophagy to address how sphingolipids mediate the "switch" between the cell survival and cell death. As sphingolipid metabolism is frequently dysregulated in cancer, sphingolipid-modulating agents, or sphingomimetics, have emerged as a novel chemotherapeutic strategy. Ultimately, a greater understanding of sphingolipid-mediated crosstalk between apoptosis and autophagy may be critical for enhancing the chemotherapeutic efficacy of these agents.

Ceramide sphingolipid signaling mediates Tumor Necrosis Factor (TNF)-dependent toxicity via caspase signaling in dopaminergic neurons

BACKGROUND

Dopaminergic (DA) neurons in the ventral midbrain selectively degenerate in Parkinson's disease (PD) in part because their oxidative environment in the substantia nigra (SN) may render them vulnerable to neuroinflammatory stimuli. Chronic inhibition of soluble Tumor Necrosis Factor (TNF) with dominant-negative TNF inhibitors protects DA neurons in rat models of parkinsonism, yet the molecular mechanisms and pathway(s) that mediate TNF toxicity remain(s) to be clearly identified. Here we investigated the contribution of ceramide sphingolipid signaling in TNF-dependent toxicity.

RESULTS

Ceramide dose-dependently reduced the viability of DA neuroblastoma cells and primary DA neurons and pharmacological inhibition of sphingomyelinases (SMases) with three different inhibitors during TNF treatment afforded significant neuroprotection by attenuating increased endoplasmic reticulum (ER) stress, loss of mitochondrial membrane potential, caspase-3 activation and decreases in Akt phosphorylation. Using lipidomics mass spectrometry we confirmed that TNF treatment not only promotes generation of ceramide, but also leads to accumulation of several atypical deoxy-sphingoid bases (DSBs). Exposure of DA neuroblastoma cells to atypical DSBs in the micromolar range reduced cell viability and inhibited neurite outgrowth and branching in primary DA neurons, suggesting that TNF-induced de novo synthesis of atypical DSBs may be a secondary mechanism involved in mediating its neurotoxicity in DA neurons.

CONCLUSIONS

We conclude that TNF/TNFR1-dependent activation of SMases generates ceramide and sphingolipid species that promote degeneration and caspase-dependent cell death of DA neurons. Ceramide and atypical DSBs may represent novel drug targets for development of neuroprotective strategies that can delay or attenuate the progressive loss of nigral DA neurons in patients with PD.

Sphingopeptides: dihydrosphingosine-based fusion inhibitors against wild-type and enfuvirtide-resistant HIV-1

Understanding the structural organization of lipids in the cell and viral membranes is essential for elucidating mechanisms of viral fusion that lead to entry of enveloped viruses into their host cells. The HIV lipidome shows a remarkable enrichment in dihydrosphingomyelin, an unusual sphingolipid formed by a dihydrosphingosine backbone. Here we investigated the ability of dihydrosphingosine to incorporate into the site of membrane fusion mediated by the HIV envelope (Env) protein. Dihydrosphingosine as well as cholesterol, fatty acid, and tocopherol was conjugated to highly conserved, short HIV‐1 Env‐derived peptides with no antiviral activity otherwise. We showed that dihydrosphingosine exclusively endowed nanomolar antiviral activity to the peptides (IC₅₀ as low as 120 nM) in HIV‐1 infection on TZM‐bl cells and on Jurkat T cells, as well as in the cell‐cell fusion assay. These sphingopeptides were active against enfuvirtide‐resistant and wild‐type CXCR4 and CCR5 tropic HIV strains. The anti‐HIV activity was determined by both the peptides and their dihydrosphingosine conjugate. Moreover, their mode of action involved accumulation in the cells and viruses and binding to membranes enriched in sphingomyelin and cholesterol. The data suggest that sphingopeptides are recruited to the HIV membrane fusion site and provide a general concept in developing inhibitors of sphingolipid‐mediated biological systems.—Ashkenazi, A., Viard, M., Unger, L., Blumenthal, R., Shai, Y. Sphingopeptides: dihydrosphingosine‐based fusion inhibitors against wild‐type and enfuvirtide‐resistant HIV‐1. FASEB J. 26, 4628–4636 (2012). http://www.fasebj.org

Ionizing radiations increase the activity of the cell surface glycohydrolases and the plasma membrane ceramide content

We detected significant levels of β-glucosidase, β-galactosidase, sialidase Neu3 and sphingomyelinase activities associated with the plasma membrane of fibroblasts from normal and Niemann-Pick subjects and of cells from breast, ovary, colon and neuroblastoma tumors in culture. All of the cells subjected to ionizing radiations showed an increase of the activity of plasma membrane β-glucosidase, β-galactosidase and sialidase Neu3, in addition of the well known increase of activity of plasma membrane sphingomyelinase, under similar conditions. Human breast cancer cell line T47D was studied in detail. In these cells the increase of activity of β-glucosidase and β-galactosidase was parallel to the increase of irradiation dose up to 60 Gy and continued with time, at least up to 72 h from irradiation. β-glucosidase increased up to 17 times and β-galactosidase up to 40 times with respect to control. Sialidase Neu3 and sphingomyelinase increased about 2 times at a dose of 20 Gy but no further significant differences were observed with increase of radiation dose and time. After irradiation, we observed a reduction of cell proliferation, an increase of apoptotic cell death and an increase of plasma membrane ceramide up to 3 times, with respect to control cells. Tritiated GM3 ganglioside has been administered to T47D cells under conditions that prevented the lysosomal catabolism. GM3 became component of the plasma membranes and was transformed into LacCer, GlcCer and ceramide. The quantity of ceramide produced in irradiated cells was about two times that of control cells.

Solid character of membrane ceramides: a surface rheology study of their mixtures with sphingomyelin

The compression and shear viscoelasticities of egg-ceramide and its mixtures with sphingomyelin were investigated using oscillatory surface rheology performed on Langmuir monolayers. We found high values for the compression and shear moduli for ceramide, compatible with a solid-state membrane, and extremely high surface viscosities when compared to typical fluid lipids. A fluidlike rheological behavior was found for sphingomyelin. Lateral mobilities, measured from particle tracking experiments, were correlated with the monolayer viscosities through the usual hydrodynamic relationships. In conclusion, ceramide increases the solid character of sphingomyelin-based membranes and decreases their fluidity, thus drastically decreasing the lateral mobilities of embedded objects. This mechanical behavior may involve important physiological consequences in biological membranes containing ceramides.

Sphingosine-1-phosphate rapidly increases cortisol biosynthesis and the expression of genes involved in cholesterol uptake and transport in H295R adrenocortical cells

In the acute phase of adrenocortical steroidogenesis, adrenocorticotrophin (ACTH) activates a cAMP/PKA-signaling pathway that promotes the transport of free cholesterol to the inner mitochondrial membrane. We have previously shown that ACTH rapidly stimulates the metabolism of sphingolipids and the secretion of sphingosine-1-phosphate (S1P) in H295R cells. In this study, we examined the effect of S1P on genes involved in the acute phase of steroidogenesis. We show that S1P increases the expression of steroidogenic acute regulatory protein (StAR), 18-kDa translocator protein (TSPO), low-density lipoprotein receptor (LDLR), and scavenger receptor class B type I (SR-BI). S1P-induced StAR mRNA expression requires Gα(i) signaling, phospholipase C (PLC), Ca(2+)/calmodulin-dependent kinase II (CamKII), and ERK1/2 activation. S1P also increases intracellular Ca(2+), the phosphorylation of hormone sensitive lipase (HSL) at Ser(563), and cortisol secretion. Collectively, these findings identify multiple roles for S1P in the regulation of glucocorticoid biosynthesis.

Chain length-specific properties of ceramides

Ceramides are a class of sphingolipids that are abundant in cell membranes. They are important structural components of the membrane but can also act as second messengers in various signaling pathways. Until recently, ceramides and dihydroceramides were considered as a single functional class of lipids and no distinction was made between molecules with different chain lengths. However, based on the development of high-throughput, structure-specific and quantitative analytical methods to measure ceramides, it has now become clear that in cellular systems the amounts of ceramides differ with respect to their chain length. Further studies have indicated that some functions of ceramides are chain-length dependent. In this review, we discuss the chain length-specific differences of ceramides including their pathological impact on Alzheimer's disease, inflammation, autophagy, apoptosis and cancer.

Sphingolipid metabolism, oxidant signaling, and contractile function of skeletal muscle

SIGNIFICANCE

Sphingolipids are a class of bioactive lipids that regulate diverse cell functions. Ceramide, sphingosine, and sphingosine-1-phosphate accumulate in tissues such as liver, brain, and lung under conditions of cellular stress, including oxidative stress. The activity of some sphingolipid metabolizing enzymes, chiefly the sphingomyelinases, is stimulated during inflammation and in response to oxidative stress. Ceramide, the sphingomyelinase product, as well as the ceramide metabolite, sphingosine-1-phosphate, can induce the generation of more reactive oxygen species, propagating further inflammation.

RECENT ADVANCES

This review article summarizes information on sphingolipid biochemistry and signaling pertinent to skeletal muscle and describes the potential influence of sphingolipids on contractile function.

CRITICAL ISSUES

It encompasses topics related to (1) the pathways for complex sphingolipid biosynthesis and degradation, emphasizing sphingolipid regulation in various muscle fiber types and subcellular compartments; (2) the emerging evidence that implicates ceramide, sphingosine, and sphingosine-1-phosphate as regulators of muscle oxidant activity, and (3) sphingolipid effects on contractile function and fatigue.

FUTURE DIRECTIONS

We propose that prolonged inflammatory conditions alter ceramide, sphingosine, and sphingosine-1-phosphate levels in skeletal muscle and that these changes promote the weakness, premature fatigue, and cachexia that plague individuals with heart failure, cancer, diabetes, and other chronic inflammatory diseases.

Mitochondrial ceramide-rich macrodomains functionalize Bax upon irradiation

Background

Evidence indicates that Bax functions as a “lipidic” pore to regulate mitochondrial outer membrane permeabilization (MOMP), the apoptosis commitment step, through unknown membrane elements. Here we show mitochondrial ceramide elevation facilitates MOMP-mediated cytochrome c release in HeLa cells by generating a previously-unrecognized mitochondrial ceramide-rich macrodomain (MCRM), which we visualize and isolate, into which Bax integrates.

Methodology/Principal Findings

MCRMs, virtually non-existent in resting cells, form upon irradiation coupled to ceramide synthase-mediated ceramide elevation, optimizing Bax insertion/oligomerization and MOMP. MCRMs are detected by confocal microscopy in intact HeLa cells and isolated biophysically as a light membrane fraction from HeLa cell lysates. Inhibiting ceramide generation using a well-defined natural ceramide synthase inhibitor, Fumonisin B1, prevented radiation-induced Bax insertion, oligomerization and MOMP. MCRM deconstruction using purified mouse hepatic mitochondria revealed ceramide alone is non-apoptogenic. Rather Bax integrates into MCRMs, oligomerizing therein, conferring 1–2 log enhanced cytochrome c release. Consistent with this mechanism, MCRM Bax isolates as high molecular weight “pore-forming” oligomers, while non-MCRM membrane contains exclusively MOMP-incompatible monomeric Bax.

Conclusions/Significance

Our recent studies in the C. elegans germline indicate that mitochondrial ceramide generation is obligate for radiation-induced apoptosis, although a mechanism for ceramide action was not delineated. Here we demonstrate that ceramide, generated in the mitochondrial outer membrane of mammalian cells upon irradiation, forms a platform into which Bax inserts, oligomerizes and functionalizes as a pore. We posit conceptualization of ceramide as a membrane-based stress calibrator, driving membrane macrodomain organization, which in mitochondria regulates intensity of Bax-induced MOMP, and is pharmacologically tractable in vitro and in vivo.

Roles of sphingosine 1-phosphate on tumorigenesis

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid with a variety of biological activities. It is generated from the conversion of ceramide to sphingosine by ceramidase and the subsequent conversion of sphingosine to S1P, which is catalyzed by sphingosine kinases. Through increasing its intracellular levels by sphingolipid metabolism and binding to its cell surface receptors, S1P regulates several physiological and pathological processes, including cell proliferation, migration, angiogenesis and autophagy. These processes are responsible for tumor growth, metastasis and invasion and promote tumor survival. Since ceramide and S1P have distinct functions in regulating in cell fate decision, the balance between the ceramide/sphingosine/S1P rheostat becomes a potent therapeutic target for cancer cells. Herein, we summarize our current understanding of S1P signaling on tumorigenesis and its potential as a target for cancer therapy.

Ceramide in stress response

Evidence has consistently indicated that activation of sphingomyelinases and/or ceramide synthases and the resulting accumulation of ceramide mediate cellular responses to stressors such as lipopolysaccharide, interleukin 1beta, tumor necrosis factor alpha, serum deprivation, irradiation and various antitumor treatments. Recent studies had identified the genes encoding most of the enzymes responsible for the generation of ceramide and ongoing research is aimed at characterizing their individual functions in cellular response to stress. This chapter discusses the seminal and more recent discoveries in regards to the pathways responsible for the accumulation of ceramide during stress and the mechanisms by which ceramide affects cell functions. The former group includes the roles of neutral sphingomyelinase 2, serine palmitoyltransferase, ceramide synthases, as well as the secretory and endosomal/lysosomal forms of acid sphingomyelinase. The latter summarizes the mechanisms by which ceramide activate its direct targets, PKCzeta, PP2A and cathepsin D. The ability of ceramide to affect membrane organization is discussed in the light of its relevance to cell signaling. Emerging evidence to support the previously assumed notion that ceramide acts in a strictly structure-specific manner are also included. These findings are described in the context of several physiological and pathophysiological conditions, namely septic shock, obesity-induced insulin resistance, aging and apoptosis of tumor cells in response to radiation and chemotherapy.

Effect of ceramide acyl chain length on skin permeability and thermotropic phase behavior of model stratum corneum lipid membranes

Stratum corneum ceramides play an essential role in the barrier properties of skin. However, their structure-activity relationships are poorly understood. We investigated the effects of acyl chain length in the non-hydroxy acyl sphingosine type (NS) ceramides on the skin permeability and their thermotropic phase behavior. Neither the long- to medium-chain ceramides (8-24 C) nor free sphingosine produced any changes of the skin barrier function. In contrast, the short-chain ceramides decreased skin electrical impedance and increased skin permeability for two marker drugs, theophylline and indomethacin, with maxima in the 4-6C acyl ceramides. The thermotropic phase behavior of pure ceramides and model stratum corneum lipid membranes composed of ceramide/lignoceric acid/cholesterol/cholesterol sulfate was studied by differential scanning calorimetry and infrared spectroscopy. Differences in thermotropic phase behavior of these lipids were found: those ceramides that had the greatest impact on the skin barrier properties displayed the lowest phase transitions and formed the least dense model stratum corneum lipid membranes at 32°C. In conclusion, the long hydrophobic chains in the NS-type ceramides are essential for maintaining the skin barrier function. However, this ability is not shared by their short-chain counterparts despite their having the same polar head structure and hydrogen bonding ability.

Sphingolipids and expression regulation of genes in cancer

Sphingolipids including glycosphingolipids have myriad effects on cell functions and affect cancer in aspects of tumorigenesis, metastasis and tumor response to treatments. Bioactive ones like ceramide, sphingosine 1-phosphate and globotriaosylceramide initiate and process cellular signaling to alter cell behaviors immediately responding to oncogenic stress or treatment challenges. Recent studies pinpoint that sphingolipid-mediated gene expression has long and profound impacts on cancer cells, and these play crucial roles in tumor progression and in treatment outcome. More than 10 sphingolipids and glycosphingolipids selectively mediate expressions of approximately 50 genes including c-myc, p21, c-fos, telomerase reverse transcriptase, caspase-9, Bcl-x, cyclooxygenase-2, matrix metalloproteinases, integrins, Oct-4, glucosylceramide synthase and multidrug-resistant gene 1. By diverse functions of these genes, sphingolipids enduringly affect cellular processes of mitosis, apoptosis, migration, stemness of cancer stem cells and cellular resistance to therapies. Mechanistic studies indicate that sphingolipids regulate particular gene expression by modulating phosphorylation and acetylation of proteins that serve as transcription factors (β-catenin, Sp1), repressor of transcription (histone H3), and regulators (SRp30a) in RNA splicing. Disclosing molecular mechanisms by which sphingolipids selectively regulate particular gene expression, instead of other relevant ones, requires understanding of the exact roles of individual lipid instead of a group, the signaling pathways that are implicated in and interaction with proteins or other lipids in details. These studies not only expand our knowledge of sphingolipids, but can also suggest novel targets for cancer treatments.