Mass spectrometric analysis reveals an increase in plasma membrane polyunsaturated phospholipid species upon cellular cholesterol loading

Ratiometric fluorescence nanoscopy and lifetime imaging of novel Nile Red analogs for analysis of membrane packing in living cells

Subcellular membranes have complex lipid and protein compositions, which give rise to organelle-specific membrane packing, fluidity, and permeability. Due to its exquisite solvent sensitivity, the lipophilic fluorescence dye Nile Red has been used extensively to study membrane packing and polarity. Further improvement of Nile Red can be achieved by introducing electron-donating or withdrawing functional groups. Here, we compare the potential of derivatives of Nile Red with such functional substitutions for super-resolution fluorescence microscopy of lipid packing in model membranes and living cells. All studied Nile Red derivatives exhibit cholesterol-dependent fluorescence changes in model membranes, as shown by spectrally resolved stimulated emission depletion (STED) microscopy. STED imaging of Nile Red probes in cells reveals lower membrane packing in fibroblasts from healthy subjects compared to those from patients suffering from Niemann Pick type C1 (NPC1) disease, a lysosomal storage disorder with accumulation of cholesterol and sphingolipids in late endosomes and lysosomes. We also find small but consistent changes in the fluorescence lifetime of the Nile Red derivatives in NPC1 cells, suggesting altered hydrogen-bonding capacity in their membranes. All Nile Red derivatives are essentially non-fluorescent in water but increase their brightness in membranes, allowing for their use in MINFLUX single molecule tracking experiments. Our study uncovers the potential of Nile Red probes with functional substitutions for nanoscopic membrane imaging.

Cell density-dependent membrane distribution of ganglioside GM3 in melanoma cells

Monosialoganglioside GM3 is the simplest ganglioside involved in various cellular signaling. Cell surface distribution of GM3 is thought to be crucial for the function of GM3, but little is known about the cell surface GM3 distribution. It was shown that anti-GM3 monoclonal antibody binds to GM3 in sparse but not in confluent melanoma cells. Our model membrane study evidenced that monoclonal anti-GM3 antibodies showed stronger binding when GM3 was in less fluid membrane environment. Studies using fluorescent GM3 analogs suggested that GM3 was clustered in less fluid membrane. Moreover, fluorescent lifetime measurement showed that cell surface of high density melanoma cells is more fluid than that of low density cells. Lipidomics and fatty acid supplementation experiment suggested that monounsaturated fatty acid-containing phosphatidylcholine contributed to the cell density-dependent membrane fluidity. Our results indicate that anti-GM3 antibody senses GM3 clustering and the number and/or size of GM3 cluster differ between sparse and confluent melanoma cells.

Long chain ceramides raise the main phase transition of monounsaturated phospholipids to physiological temperature

Little is known about the molecular mechanisms of ceramide-mediated cellular signaling. We examined the effects of palmitoyl ceramide (C16-ceramide) and stearoyl ceramide (C18-ceramide) on the phase behavior of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) using differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SAXS, WAXS). As previously published, the presence of ceramides increased the lamellar gel-to-lamellar liquid crystalline (L_β-L_α) phase transition temperature of POPC and POPE and decreased the L_α-to-inverted hexagonal (L_α-H_II) phase transition temperature of POPE. Interestingly, despite an ~ 30° difference in the main phase transition temperatures of POPC and POPE, the L_β-L_α phase transition temperatures were very close between POPC/C18-ceramide and POPE/C18-ceramide and were near physiological temperature. A comparison of the results of C16-ceramide in published and our own results with those of C18-ceramide indicates that increase of the carbon chain length of ceramide from 16 to 18 and/or the small difference of ceramide content in the membrane dramatically change the phase transition temperature of POPC and POPE to near physiological temperature. Our results support the idea that ceramide signaling is mediated by the alteration of lipid phase-dependent partitioning of signaling proteins.

A Study of the Interaction of a New Benzimidazole Schiff Base with Synthetic and Simulated Membrane Models of Bacterial and Mammalian Membranes

Biological membranes are complex dynamic systems composed of a great variety of carbohydrates, lipids, and proteins, which together play a pivotal role in the protection of organisms and through which the interchange of different substances is regulated in the cell. Given the complexity of membranes, models mimicking them provide a convenient way to study and better understand their mechanisms of action and their interactions with biologically active compounds. Thus, in the present study, a new Schiff base (Bz-Im) derivative from 2-(m-aminophenyl)benzimidazole and 2,4-dihydroxybenzaldehyde was synthesized and characterized by spectroscopic and spectrometric techniques. Interaction studies of (Bz-Im) with two synthetic membrane models prepared with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and DMPC/1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) 3:1 mixture, imitating eukaryotic and prokaryotic membranes, respectively, were performed by applying differential scanning calorimetry (DSC). Molecular dynamics simulations were also developed to better understand their interactions. In vitro and in silico assays provided approaches to understand the effect of Bz-Im on these lipid systems. The DSC results showed that, at low compound concentrations, the effects were similar in both membrane models. By increasing the concentration of Bz-Im, the DMPC/DMPG membrane exhibited greater fluidity as a result of the interaction with Bz-Im. On the other hand, molecular dynamics studies carried out on the erythrocyte membrane model using the phospholipids POPE (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine), SM (N-(15Z-tetracosenoyl)-sphing-4-enine-1-phosphocholine), and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) revealed that after 30 ns of interaction, both hydrophobic interactions and hydrogen bonds were responsible for the affinity of Bz-Im for PE and SM. The interactions of the imine with POPG (1-Palmitoyl-2-Oleoyl-sn-Glycero-3-Phosphoglycerol) in the E. coli membrane model were mainly based on hydrophobic interactions.

The homeoviscous adaptation to dietary lipids (HADL) model explains controversies over saturated fat, cholesterol, and cardiovascular disease risk

SFAs play the leading role in 1 of the greatest controversies in nutrition science. Relative to PUFAs, SFAs generally increase circulating concentrations of LDL cholesterol, a risk factor for atherosclerotic cardiovascular disease (ASCVD). However, the purpose of regulatory mechanisms that control the diet-induced lipoprotein cholesterol dynamics is rarely discussed in the context of human adaptive biology. We argue that better mechanistic explanations can help resolve lingering controversies, with the potential to redefine aspects of research, clinical practice, dietary advice, public health management, and food policy. In this paper we propose a novel model, the homeoviscous adaptation to dietary lipids (HADL) model, which explains changes in lipoprotein cholesterol as adaptive homeostatic adjustments that serve to maintain cell membrane fluidity and hence optimal cell function. Due to the highly variable intake of fatty acids in humans and other omnivore species, we propose that circulating lipoproteins serve as a buffer to enable the rapid redistribution of cholesterol molecules between specific cells and tissues that is necessary with changes in dietary fatty acid supply. Hence, circulating levels of LDL cholesterol may change for nonpathological reasons. Accordingly, an SFA-induced raise in LDL cholesterol in healthy individuals could represent a normal rather than a pathologic response. These regulatory mechanisms may become disrupted secondarily to pathogenic processes in association with insulin resistance and the presence of other ASCVD risk factors, as supported by evidence showing diverging lipoprotein responses in healthy individuals as opposed to those with metabolic disorders such as insulin resistance and obesity. Corresponding with the model, we suggest alternative contributing factors to the association between elevated LDL cholesterol concentrations and ASCVD, involving dietary factors beyond SFAs, such as an increased endotoxin load from diet-gut microbiome interactions and subsequent chronic low-grade inflammation that interferes with fine-tuned signaling pathways.

Imaging Sphingomyelin- and Cholesterol-Enriched Domains in the Plasma Membrane Using a Novel Probe and Super-Resolution Microscopy

In this chapter, we show the visualization of lipid domains using a specific lipid-binding protein and super-resolution microscopy. Lipid rafts are plasma membrane domains enriched in both sphingolipids and sterols that play key roles in various physiological events. We identified a novel protein that specifically binds to a complex of sphingomyelin (SM) and cholesterol (Chol). The isolated protein, nakanori, labels the SM/Chol complex at the outer leaflet of the plasma membrane in mammalian cells. Structured illumination microscopic images suggested that the influenza virus buds from the edges of the SM/Chol domains in MDCK cells. Furthermore, a photoactivated localization microscopy analysis indicated that the SM/Chol complex forms domains in the outer leaflet, just above the phosphatidylinositol 4,5-bisphosphate domains in the inner leaflet. These observations provide significant insight into the structure and function of lipid rafts.

Complementary probes reveal that phosphatidylserine is required for the proper transbilayer distribution of cholesterol

Cholesterol is an essential component of metazoan cellular membranes and it helps to maintain the structural integrity and fluidity of the plasma membrane. Here, we developed a cholesterol biosensor, termed D4H, based on the fourth domain of Clostridium perfringens theta-toxin, which recognizes cholesterol in the cytosolic leaflet of the plasma membrane and organelles. The D4H probe disassociates from the plasma membrane upon cholesterol extraction and after perturbations in cellular cholesterol trafficking. When used in combination with a recombinant version of the biosensor, we show that plasmalemmal phosphatidylserine is essential for retaining cholesterol in the cytosolic leaflet of the plasma membrane. In vitro experiments reveal that 1-stearoy-2-oleoyl phosphatidylserine can induce phase separation in cholesterol-containing lipid bilayers and shield cholesterol from cholesterol oxidase. Finally, the altered transbilayer distribution of cholesterol causes flotillin-1 to relocalize to endocytic organelles. This probe should be useful in the future to study pools of cholesterol in the cytosolic leaflet of the plasma membrane and organelles.

Aging bone marrow mesenchymal stromal cells have altered membrane glycerophospholipid composition and functionality

Human mesenchymal stem/stromal cells (hMSC) are increasingly used in advanced cellular therapies. The clinical use of hMSCs demands sequential cell expansions. As it is well established that membrane glycerophospholipids (GPL) provide precursors for signaling lipids that modulate cellular functions, we studied the effect of the donor's age and cell doublings on the GPL profile of human bone marrow MSC (hBMSC). The hBMSCs, which were harvested from five young and five old adults, showed clear compositional changes during expansion seen at the level of lipid classes, lipid species, and acyl chains. The ratio of phosphatidylinositol to phosphatidylserine increased toward the late-passage samples. Furthermore, 20:4n-6-containing species of phosphatidylcholine and phosphatidylethanolamine accumulated while the species containing monounsaturated fatty acids (FA) decreased during passaging. Additionally, in the total FA pool of the cells, 20:4n-6 increased, which happened at the expense of n-3 polyunsaturated FAs, especially 22:6n-3. The GPL and FA correlated with the decreased immunosuppressive capacity of hBMSCs during expansion. Our observations were further supported by alterations in the gene expression levels of several enzymes involved in lipid metabolism and immunomodulation. The results show that extensive expansion of hBMSCs harmfully modulates membrane GPLs, affecting lipid signaling and eventually impairing functionality.

Quantitative analysis of sphingomyelin by high-performance liquid chromatography after enzymatic hydrolysis

Sphingomyelin is the most abundant sphingolipid in mammalian cells and is mostly present in the plasma membrane. A new analytical method using high-performance liquid chromatography (HPLC) was developed to quantify sphingomyelin in mouse plasma and tissues, 3T3-L1 cells, rat aortic smooth muscle cells, and HT-29 cells. Sphingomyelin and dihydrosphingomyelin, an internal standard, were separated by high-performance thin-layer chromatography and simultaneously hydrolyzed with sphingolipid ceramide N-deacylase and sphingomyelinase to release sphingosine and dihydrosphingosine, respectively. Sphingomyelin content was measured by HPLC following o-phthalaldehyde derivatization. Sphingomyelin concentrations in 3T3-L1 cells, rat aortic smooth muscle cells, and HT-29 cells were 60.10 ± 0.24, 62.69 ± 0.08, and 58.38 ± 0.37 pmol/μg protein, respectively, whereas those in brain, kidney, and liver of ICR mice were 55.60 ± 0.43, 43.75 ± 0.21, and 22.26 ± 0.14 pmol/μg protein. The sphingomyelin concentration in mouse plasma was 407.40 ± 0.31 μM. The limits of detection and quantification for sphingomyelin were 5 and 20 pmol, respectively, in the HPLC analysis with fluorescence detection. This sensitivity was sufficient for analyzing sphingomyelin in biological samples. In conclusion, this analytical method is a sensitive and specific technique for quantifying sphingomyelin and was successfully applied to diverse biological samples with excellent reproducibility.

Acyl chains of phospholipase D transphosphatidylation products in Arabidopsis cells: a study using multiple reaction monitoring mass spectrometry

Background

Phospholipases D (PLD) are major components of signalling pathways in plant responses to some stresses and hormones. The product of PLD activity is phosphatidic acid (PA). PAs with different acyl chains do not have the same protein targets, so to understand the signalling role of PLD it is essential to analyze the composition of its PA products in the presence and absence of an elicitor.

Methodology/Principal findings

Potential PLD substrates and products were studied in Arabidopsis thaliana suspension cells treated with or without the hormone salicylic acid (SA). As PA can be produced by enzymes other than PLD, we analyzed phosphatidylbutanol (PBut), which is specifically produced by PLD in the presence of n-butanol. The acyl chain compositions of PBut and the major glycerophospholipids were determined by multiple reaction monitoring (MRM) mass spectrometry. PBut profiles of untreated cells or cells treated with SA show an over-representation of 160/18∶2- and 16∶0/18∶3-species compared to those of phosphatidylcholine and phosphatidylethanolamine either from bulk lipid extracts or from purified membrane fractions. When microsomal PLDs were used in in vitro assays, the resulting PBut profile matched exactly that of the substrate provided. Therefore there is a mismatch between the acyl chain compositions of putative substrates and the in vivo products of PLDs that is unlikely to reflect any selectivity of PLDs for the acyl chains of substrates.

Conclusions

MRM mass spectrometry is a reliable technique to analyze PLD products. Our results suggest that PLD action in response to SA is not due to the production of a stress-specific molecular species, but that the level of PLD products per se is important. The over-representation of 160/18∶2- and 16∶0/18∶3-species in PLD products when compared to putative substrates might be related to a regulatory role of the heterogeneous distribution of glycerophospholipids in membrane sub-domains.

Phosphatidylserine dynamics in cellular membranes

The distribution and dynamics of phosphatidylserine are studied in the plasma membrane and in organellar membranes of live cells using two novel fluorescent probes in combination with various biophysical techniques, including fluorescence correlation spectroscopy and single-particle tracking.

Much has been learned about the role of exofacial phosphatidylserine (PS) in apoptosis and blood clotting using annexin V. However, because annexins are impermeant and unable to bind PS at low calcium concentration, they are unsuitable for intracellular use. Thus little is known about the topology and dynamics of PS in the endomembranes of normal cells. We used two new probes—green fluorescent protein (GFP)–LactC2, a genetically encoded fluorescent PS biosensor, and 1-palmitoyl-2-(dipyrrometheneboron difluoride)undecanoyl-sn-glycero-3-phospho-l-serine (TopFluor-PS), a synthetic fluorescent PS analogue—to examine PS distribution and dynamics inside live cells. The mobility of PS was assessed by a combination of advanced optical methods, including single-particle tracking and fluorescence correlation spectroscopy. Our results reveal the existence of a sizable fraction of PS with limited mobility, with cortical actin contributing to the confinement of PS in the plasma membrane. We were also able to measure the dynamics of PS in endomembrane organelles. By targeting GFP-LactC2 to the secretory pathway, we detected the presence of PS in the luminal leaflet of the endoplasmic reticulum. Our data provide new insights into properties of PS inside cells and suggest mechanisms to account for the subcellular distribution and function of this phospholipid.

Quantitative analysis of the lipidomes of the influenza virus envelope and MDCK cell apical membrane

Analysis of the lipid composition of influenza virus–infected cells provides support for the membrane raft-based biogenesis model.

The influenza virus (IFV) acquires its envelope by budding from host cell plasma membranes. Using quantitative shotgun mass spectrometry, we determined the lipidomes of the host Madin–Darby canine kidney cell, its apical membrane, and the IFV budding from it. We found the apical membrane to be enriched in sphingolipids (SPs) and cholesterol, whereas glycerophospholipids were reduced, and storage lipids were depleted compared with the whole-cell membranes. The virus membrane exhibited a further enrichment of SPs and cholesterol compared with the donor membrane at the expense of phosphatidylcholines. Our data are consistent with and extend existing models of membrane raft-based biogenesis of the apical membrane and IFV envelope.

Role of lipids in virus replication

Viruses intricately interact with and modulate cellular membranes at several stages of their replication, but much less is known about the role of viral lipids compared to proteins and nucleic acids. All animal viruses have to cross membranes for cell entry and exit, which occurs by membrane fusion (in enveloped viruses), by transient local disruption of membrane integrity, or by cell lysis. Furthermore, many viruses interact with cellular membrane compartments during their replication and often induce cytoplasmic membrane structures, in which genome replication and assembly occurs. Recent studies revealed details of membrane interaction, membrane bending, fission, and fusion for a number of viruses and unraveled the lipid composition of raft-dependent and -independent viruses. Alterations of membrane lipid composition can block viral release and entry, and certain lipids act as fusion inhibitors, suggesting a potential as antiviral drugs. Here, we review viral interactions with cellular membranes important for virus entry, cytoplasmic genome replication, and virus egress.

Sterols have higher affinity for sphingomyelin than for phosphatidylcholine bilayers even at equal acyl-chain order

The interaction between cholesterol and phospholipids in bilayer membranes is important for the formation and maintenance of membrane structure and function. However, cholesterol does not interact favorably with all types of phospholipids and, for example, prefers more ordered sphingomyelins (SMs) over phosphatidylcholines (PCs). The reason for this preference is not clear. Here we have studied whether acyl-chain order could be responsible for the preferred sterol interaction with SMs. Acyl-chain order was deduced from diphenylhexatriene anisotropy and from the deuterium order parameter obtained by (2)H-NMR on bilayers made from either 14:0/14:0((d27))-PC, or 14:0((d27))-SM. Sterol/phospholipid interaction was determined from sterol bilayer partitioning. Cholestatrienol (CTL) was used as a fluorescence probe for cholesterol, because its relative membrane partitioning is similar to cholesterol. When CTL was allowed to reach equilibrium partitioning between cyclodextrins and unilamellar vesicles made from either 14:0/14:0-PC or 14:0-SM, the molar-fraction partitioning coefficient (K(x)) was approximately twofold higher for SM bilayers than for PC bilayers. This was even the case when the temperature in the SM samples was raised to achieve equal acyl-chain order, as determined from 1,6-diphenyl-1,3,5-hexatriene (DPH) anisotropy and the deuterium order parameter. Although the K(x) did increase with acyl-chain order, the higher K(x) for SM bilayers was always evident. At equal acyl-chain order parameter (DPH anisotropy), the K(x) was also higher for 14:0-SM bilayers than for bilayers made from either 14:0/15:0-PC or 15:0-/14:0-PC, suggesting that minor differences in chain length or molecular asymmetry are not responsible for the difference in K(x). We conclude that acyl-chain order affects the bilayer affinity of CTL (and thus cholesterol), but that it is not the cause for the preferred affinity of sterols for SMs over matched PCs. Instead, it is likely that the interfacial properties of SMs influence and stabilize interactions with sterols in bilayer membranes.

Role of lipid rafts in liver health and disease

Liver diseases are an increasingly common cause of morbidity and mortality; new approaches for investigation of mechanisms of liver diseases and identification of therapeutic targets are emergent. Lipid rafts (LRs) are specialized domains of cellular membranes that are enriched in saturated lipids; they are small, mobile, and are key components of cellular architecture, protein partition to cellular membranes, and signaling events. LRs have been identified in the membranes of all liver cells, parenchymal and non-parenchymal; more importantly, LRs are active participants in multiple physiological and pathological conditions in individual types of liver cells. This article aims to review experimental-based evidence with regard to LRs in the liver, from the perspective of the liver as a whole organ composed of a multitude of cell types. We have gathered up-to-date information related to the role of LRs in individual types of liver cells, in liver health and diseases, and identified the possibilities of LR-dependent therapeutic targets in liver diseases.

Sensing phosphatidylserine in cellular membranes

Phosphatidylserine, a phospholipid with a negatively charged head-group, is an important constituent of eukaryotic cellular membranes. On the plasma membrane, rather than being evenly distributed, phosphatidylserine is found preferentially in the inner leaflet. Disruption of this asymmetry, leading to the appearance of phosphatidylserine on the surface of the cell, is known to play a central role in both apoptosis and blood clotting. Despite its importance, comparatively little is known about phosphatidylserine in cells: its precise subcellular localization, transmembrane topology and intracellular dynamics are poorly characterized. The recent development of new, genetically-encoded probes able to detect phosphatidylserine within live cells, however, is leading to a more in-depth understanding of the biology of this phospholipid. This review aims to give an overview of the current methods for phosphatidylserine detection within cells, and some of the recent realizations derived from their use.

Glycosphingolipid storage leads to the enhanced degradation of the B cell receptor in Sandhoff disease mice

Glycosphingolipid storage diseases are a group of inherited metabolic diseases in which glycosphingolipids accumulate due to their impaired lysosomal breakdown. Splenic B cells isolated from NPC1, Sandhoff, GM1-gangliosidosis and Fabry disease mouse models showed large (20- to 30-fold) increases in disease specific glycosphingolipids and up to a 4-fold increase in cholesterol. The magnitude of glycosphingolipid storage was in the order NPC1 > Sandhoff approximately GM1 gangliosidosis > Fabry. Except for Fabry disease, glycosphingolipid storage led to an increase in the lysosomal compartment and altered glycosphingolipid trafficking. In order to investigate the consequences of storage on B cell function, the levels of surface expression of B cell IgM receptor and its associated components were quantitated in Sandhoff B cells, since they are all raft-associated on activation. Both the B cell receptor, CD21 and CD19 had decreased cell surface expression. In contrast, CD40 and MHC II, surface receptors that do not associate with lipid rafts, were unchanged. Using a pulse chase biotinylation procedure, surface B cell receptors on a Sandhoff lymphoblast cell line were found to have a significantly decreased half-life. Increased co-localization of fluorescently conjugated cholera toxin and lysosomes was also observed in Sandhoff B cells. Glycosphingolipid storage leads to the enhanced formation of lysosomal lipid rafts, altered endocytic trafficking and increased degradation of the B cell receptor.

Implications for lipids during replication of enveloped viruses

Enveloped viruses, which include many medically important viruses such as human immunodeficiency virus, influenza virus and hepatitis C virus, are intracellular parasites that acquire lipid envelopes from their host cells. Success of replication is intimately linked to their ability to hijack host cell mechanisms, particularly those related to membrane dynamics and lipid metabolism. Despite recent progress, our knowledge of lipid mediated virus-host interactions remains highly incomplete. In addition, diverse experimental systems are used to study different stages of virus replication thus complicating comparisons. This review aims to present a unifying view of the widely diverse strategies used by enveloped viruses at distinct stages of their replication cycles.

The lipidomes of vesicular stomatitis virus, semliki forest virus, and the host plasma membrane analyzed by quantitative shotgun mass spectrometry

Although enveloped virus assembly in the host cell is a crucial step in the virus life cycle, it remains poorly understood. One issue is how viruses include lipids in their membranes during budding from infected host cells. To analyze this issue, we took advantage of the fact that baby hamster kidney cells can be infected by two different viruses, namely, vesicular stomatitis virus and Semliki Forest virus, from the Rhabdoviridae and Togaviridae families, respectively. We purified the host plasma membrane and the two different viruses after exit from the host cells and analyzed the lipid compositions of the membranes by quantitative shotgun mass spectrometry. We observed that the lipid compositions of these otherwise structurally different viruses are virtually indistinguishable, and only slight differences were detected between the viral lipid composition and that of the plasma membrane. Taken together, the facts that the lipid compositions of the two viruses are so similar and that they strongly resemble the composition of the plasma membrane suggest that these viruses exert little selection in including lipids in their envelopes.

Cholesterol Supplementation During Production Increases the Infectivity of Retroviral and Lentiviral Vectors Pseudotyped with the Vesicular Stomatitis Virus Glycoprotein (VSV-G)

Cholesterol, a major component of plasma membrane lipid rafts, is important for assembly and budding of enveloped viruses, including influenza and HIV-1. Cholesterol depletion impairs virus assembly and infectivity. This study examined the effects of exogenous cholesterol addition (delivered as a complex with methyl beta cyclodextrin) on the production of Molony murine leukemia virus retroviral vector and HIV-1-based lentiviral vector pseudotyped with the vesicular stomatitis virus glycoprotein (VSV-G). Cholesterol supplementation before and during vector production enhanced the infectivity of retroviral and lentiviral vectors up to 4-fold and 6-fold, respectively. In contrast, the amount of retroviral vector produced was unchanged, and that of lentiviral vector was increased less than two-fold. Both free cholesterol and cholesterol ester content in 293-gag-pol producer cells increased with cholesterol addition. In contrast, the phospholipids headgroup composition was essentially unchanged by cholesterol supplementation in 293-gag-pol packaging cells. Based on these results, it is proposed that cholesterol supplementation increases the infectivity of VSV-G-pseudotyped retroviral and lentiviral vectors, possibly by altering the composition of the producer cell membrane where the viral vectors are assembled and bud, and/or by changing the lipid composition of the viral vectors.

Progressive ordering with decreasing temperature of the phospholipids of influenza virus

Using linewidth and spinning sideband intensities of lipid hydrocarbon chain resonances in proton magic angle spinning NMR spectra, we detected the temperature-dependent phase state of naturally occurring lipids of intact influenza virus without exogenous probes. Increasingly, below 41 degrees C ordered and disordered lipid domains coexisted for the viral envelope and extracts thereof. At 22 degrees C much lipid was in a gel phase, the fraction of which reversibly increased with cholesterol depletion. Diffusion measurements and fluorescence microscopy independently confirmed the existence of gel-phase domains. Thus the existence of ordered regions of lipids in biological membranes is now demonstrated. Above the physiological temperatures of influenza infection, the physical properties of viral envelope lipids, regardless of protein content, were indistinguishable from those of the disordered fraction. Viral fusion appears to be uncorrelated to ordered lipid content. Lipid ordering may contribute to viral stability at lower temperatures, which has recently been found to be critical for airborne transmission.

Endocytic trafficking of sphingomyelin depends on its acyl chain length

To study the principles of endocytic lipid trafficking, we introduced pyrene sphingomyelins (PyrSMs) with varying acyl chain lengths and domain partitioning properties into human fibroblasts or HeLa cells. We found that a long-chain, ordered-domain preferring PyrSM was targeted Hrs and Tsg101 dependently to late endosomal compartments and recycled to the plasma membrane in an NPC1- and cholesterol-dependent manner. A short-chain, disordered domain preferring PyrSM recycled more effectively, by using Hrs-, Tsg101- and NPC1-independent routing that was insensitive to cholesterol loading. Similar chain length-dependent recycling was observed for unlabeled sphingomyelins (SMs). The findings 1) establish acyl chain length as an important determinant in the endocytic trafficking of SMs, 2) implicate ESCRT complex proteins and NPC1 in the endocytic recycling of ordered domain lipids to the plasma membrane, and 3) introduce long-chain PyrSM as the first fluorescent lipid tracing this pathway.

Incorporation of functional HN-F glycoprotein-containing complexes into newcastle disease virus is dependent on cholesterol and membrane lipid raft integrity

Newcastle disease virus assembles in plasma membrane domains with properties of membrane lipid rafts, and disruption of these domains by cholesterol extraction with methyl-beta-cyclodextrin resulted in the release of virions with irregular protein composition, abnormal particle density, and reduced infectivity (J. P. Laliberte, L. W. McGinnes, M. E. Peeples, and T. G. Morrison, J. Virol. 80:10652-10662, 2006). In the present study, these results were confirmed using Niemann-Pick syndrome type C cells, which are deficient in normal membrane rafts due to mutations affecting cholesterol transport. Furthermore, cholesterol extraction of infected cells resulted in the release of virions that attached to target cells at normal levels but were defective in virus-cell membrane fusion. The reduced fusion capacity of particles released from cholesterol-extracted cells correlated with significant loss of HN-F glycoprotein-containing complexes detected in the virion envelopes of these particles and with detection of cell-associated HN-F protein-containing complexes in extracts of cholesterol-extracted cells. Extraction of cholesterol from purified virions had no effect on virus-cell attachment, virus-cell fusion, particle infectivity, or the levels of glycoprotein-containing complexes. Taken together, these results suggest that cholesterol and membrane rafts are required for the formation or maintenance of HN-F glycoprotein-containing complexes in cells but not the stability of preformed glycoprotein complexes once assembled into virions.

Cyclodextrins but not compactin inhibit the lateral diffusion of membrane proteins independent of cholesterol

Cholesterol and glycosphingolipid-enriched membrane domains, termed lipid rafts, were proposed to play important roles in trafficking and signaling events. These functions are inhibited following putative disruption of rafts by cholesterol depletion, commonly induced by treatment with methyl-beta-cyclodextrin (MbetaCD). However, several studies showed that the lateral diffusion of membrane proteins is inhibited by MbetaCD, suggesting that it may have additional effects on membrane organization unrelated to cholesterol removal. Here, we investigated this possibility by comparison of the effects of cholesterol depletion by MbetaCD and by metabolic inhibition (compactin), and of treatment with alpha-CD, which does not bind cholesterol. The studies employed two series of proteins (Ras and influenza hemagglutinin), each containing as internal controls related mutants that differ in raft association. Mild MbetaCD treatment retarded the lateral diffusion of both raft and non-raft mutants, whereas similar cholesterol reduction (30-33%) by metabolic inhibition enhanced selectively the diffusion of the raft-associated mutants. Moreover, alpha-CD also inhibited the diffusion of raft and non-raft mutants, despite its lack of effect on cholesterol content. These findings suggest that the widely used treatment with CD to reduce cholesterol has additional, cholesterol-independent effects on membrane protein mobility, which do not necessarily distinguish between raft and non-raft proteins.

Metabolic Profiling of Glycerophospholipid Synthesis in Fibroblasts Loaded with Free Cholesterol and Modified Low Density Lipoproteins

Currently, the detailed regulation of major pathways of glycerophospholipid synthesis upon cholesterol loading is largely unknown. Therefore, a detailed lipid metabolic profiling using stable isotope-labeled choline, ethanolamine, and serine was performed by quantitative electrospray ionization tandem mass spectrometry (ESI-MS/MS) in free cholesterol (FC), oxidized (Ox-LDL) and enzymatically modified LDL (E-LDL)-loaded primary human skin fibroblasts. As previously described, an adaptive induction of phosphatidylcholine (PC) synthesis via CDP-choline was found upon FC loading. In contrast to PC, CDP-ethanolamine-mediated phosphatidylethanolamine (PE) synthesis was inhibited by FC incubation. Furthermore, FC induced a shift toward polyunsaturated PE and PC species, which was mediated primarily by PE biosynthesis but not PE remodeling, whereas PC species were shifted mainly by fatty acid (FA) remodeling of existing PC. Modified lipoprotein incubation revealed rather different effects on glycerophospholipid synthesis. E-LDL greatly enhanced PC synthesis, whereas Ox-LDL did not change PC synthesis. Addition of different free FAs (FFA) with and without FC coincubation, as major components of E-LDL, clearly indicated an incorporation of FFA into newly synthesized PC and PE species as well as FFA as important driving force for PC synthesis. Because FC and FFA are known to affect lipid membrane properties including membrane curvature, these data support that CTP:phosphocholine cytidylyl-transferase activity and consequently PC synthesis are regulated by modulation of membrane characteristics at the cellular level. In conclusion, the application of high throughput metabolic profiling of major glycerophospholipid pathways by ESI-MS/MS is a powerful tool to unravel mechanisms underlying the regulation of cellular lipid metabolism.

Overexpression of OSBP-related protein 2 (ORP2) in CHO cells induces alterations of phospholipid species composition

We have previously shown that overexpression of human OSBP-related protein 2 (ORP2) in Chinese hamster ovary (CHO) cells results in increased efflux and reduced esterification of cholesterol. The ORP2-expressing cells also have a reduced level of triacylglycerols. We investigated the effects of ORP2 expression on the phospholipid (PL) molecular species and the neutral lipid (NL) fatty acid composition of CHO cells cultured in the presence or absence of serum lipoproteins. In the presence of lipoproteins, ORP2/CHO cells display an increase in polyunsaturated PL species, and polyunsaturated fatty acids (PUFA) in the diminished NL pool are reduced. The increase of polyunsaturated PL may represent a compensatory response to alterations in cholesterol metabolism. Upon lipoprotein deprivation, the ORP2/CHO cells display a drop in polyunsaturated and an increase in mono and diunsaturated PL species. Our results suggest that this is due to defective recycling of PUFA from the diminished NL pool to PL. Furthermore, the PL PUFA, which are elevated in ORP2/CHO cells, are most likely subject to more rapid turnover than the NL-associated pool. The results provide evidence for a delicate integration of cholesterol, PL, and NL metabolism and a role of ORP2 as a regulator of the cellular lipidome.

Shotgun lipidomics: multidimensional MS analysis of cellular lipidomes

Shotgun lipidomics, comprised of intrasource separation, multidimensional mass spectrometry and computer-assisted array analysis, is an emerging powerful technique in lipidomics. Through effective intrasource separation of predetermined groups of lipid classes based on their intrinsic electrical propensities, analyses of lipids from crude extracts of biologic samples can be directly and routinely performed. Appropriate multidimensional array analysis of lipid pseudomolecular ions and fragments can be performed leading to the identification and quantitation of targeted lipid molecular species. Since most biologic lipids are linear combinations of aliphatic chains, backbones and head groups, a rich repertoire of multiple lipid building blocks present in discrete combinations represent experimental observables that can be computer reconstructed in conjunction with their pseudomolecular ions to directly determine the lipid molecular structures from a lipid extract. Through this approach, dramatic increases in the accessible dynamic range for ratiometric quantitation and discrimination of isobaric molecular species can be achieved without any prior column chromatography or operator-dependent supervision. At its current state of development, shotgun lipidomics can analyze over 20 lipid classes, hundreds of lipid molecular species and more than 95% of the mass content of a cellular lipidome. Thus, understanding the biochemical mechanisms underlying lipid-mediated disease states will be greatly facilitated by the power of shotgun lipidomics.

Defective insulin receptor activation and altered lipid rafts in Niemann-Pick type C disease hepatocytes

Niemann-Pick type C (NPC) disease is a neuro-visceral cholesterol storage disorder caused by mutations in the NPC-1 or NPC-2 gene. In the present paper, we studied IR (insulin receptor) activation and the plasma-membrane lipid assembly in primary hepatocytes from control and NPC1-/- mice. We have previously reported that, in hepatocytes, IR activation is dependent on cholesterol-sphingolipid rafts [Vainio, Heino, Mansson, Fredman, Kuismanen, Vaarala and Ikonen (2002) EMBO Rep. 3, 95-100]. We found that, in NPC hepatocytes, IR levels were up-regulated and the receptor activation was compromised. Defective IR activation was reproduced in isolated NPC plasma-membrane preparations, which displayed an increased cholesterol content and saturation of major phospholipids. The NPC plasma membranes were less fluid than control membranes as indicated by increased DPH (1,6-diphenyl-1,3,5-hexatriene) fluorescence anisotropy values. Both in NPC hepatocytes and plasma-membrane fractions, the association of IR with low-density DRMs (detergent-resistant membranes) was increased. Moreover, the detergent resistance of both cholesterol and phosphatidylcholine were increased in NPC membranes. Finally, cholesterol removal inhibited IR activation in control membranes but restored IR activation in NPC membranes. Taken together, the results reveal a lipid imbalance in the NPC hepatocyte, which increases lipid ordering in the plasma membrane, alters the properties of lipid rafts and interferes with the function of a raft-associated plasma-membrane receptor. Such a mechanism may participate in the pathogenesis of NPC disease and contribute to insulin resistance in other disorders of lipid metabolism.

Glycosphingolipid Accumulation Inhibits Cholesterol Efflux via the ABCA1/Apolipoprotein A-I Pathway

Cellular glycosphingolipid (GSL) storage is known to promote cholesterol accumulation. Although physical interactions between GSLs and cholesterol are thought to cause intracellular cholesterol "trapping," it is not known whether cholesterol homeostatic mechanisms are also impaired under these conditions. ApoA-I-mediated cholesterol efflux via ABCA1 (ATP-binding cassette transporter A1) is a key regulator of cellular cholesterol balance. Here, we show that apoA-I-mediated cholesterol efflux was inhibited (by up to 53% over 8 h) when fibroblasts were treated with lactosylceramide or the glucocerebrosidase inhibitor conduritol B epoxide. Furthermore, apoA-I-mediated cholesterol efflux from fibroblasts derived from patients with genetic GSL storage diseases (Fabry disease, Sandhoff disease, and GM1 gangliosidosis) was impaired compared with control cells. Conversely, apoA-I-mediated cholesterol efflux from fibroblasts and cholesterol-loaded macrophage foam cells was dose-dependently stimulated (by up to 6-fold over 8 h) by the GSL synthesis inhibitor 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP). Unexpectedly, a structurally unrelated GSL synthesis inhibitor, N-butyldeoxynojirimycin, was unable to stimulate apoA-I-mediated cholesterol efflux despite achieving similar GSL depletion. PDMP was found to up-regulate ABCA1 mRNA and protein expression, thereby identifying a contributing mechanism for the observed acceleration of cholesterol efflux to apoA-I. This study reveals a novel defect in cellular cholesterol homeostasis induced by GSL storage and identifies PDMP as a new agent for enhancing cholesterol efflux via the ABCA1/apoA-I pathway.

Dietary modulation of fatty acid profiles and oxidative status of rat hepatocyte nodules: effect of different n-6/n-3 fatty acid ratios

Male Fischer rats were fed the AIN 76A diet containing varying n-6/n-3 FA ratios using sunflower oil (SFO), soybean oil (SOY), and SFO supplemented with EPA-50 and GLA-80 (GLA) as fat sources. Hepatocyte nodules, induced using diethylnitrosamine followed by 2-acetylaminofluorene/partial hepatectomy promotion, were harvested, with surrounding and respective dietary control tissues, 3 mon after partial hepatectomy. The altered growth pattern of hepatocyte nodules in rats fed SFO is associated with a distinct lipid pattern entailing an increased concentration of PE, resulting in increased levels of 20:4n-6. In addition, there is an accumulation of 18:1 n-9 and 18:2n-6 and a decrease in the end products of the n-3 metabolic pathway in PC, suggesting a dysfunctional delta-6-desaturase enzyme. The hepatocyte nodules of the SFO-fed rats exhibited a significantly reduced lipid peroxidation level that was associated with an increase in the glutathione (GSH) concentration. The low n-6/n-3 FA ratio diets significantly decreased 20:4n-6 in PC and PE phospholipid fractions with a concomitant increase in 20:5n-3, 22:5n-3, and 22:6n-3. The resultant changes in the 20:4/20:5 FA ratio and the 20:3n-6 FA level in the case of the GLA diet suggest a reduction of prostaglandin synthesis of the 2-series. The GLA diet also counteracted the increased level of 20:4n-6 in PE by equalizing the nodule/surrounding ratio. The low n-6/n-3 ratio diets significantly increased lipid peroxidation levels in hepatocyte nodules, mimicking the level in the surrounding and control tissue while GSH was decreased. An increase in n-3 FA levels and oxidative status resulted in a reduction in the number of glutathione-S-transferase positive foci in the liver of the GLA-fed rats. Modulation of cancer development with low n-6/n-3 ratio diets containing specific dietary FA could be a promising tool in cancer intervention in the liver.

Shotgun lipidomics: electrospray ionization mass spectrometric analysis and quantitation of cellular lipidomes directly from crude extracts of biological samples

Lipidomics, after genomics and proteomics, is a newly and rapidly expanding research field that studies cellular lipidomes and the organizational hierarchy of lipid and protein constituents mediating life processes. Lipidomics is greatly facilitated by recent advances in, and novel applications of, electrospray ionization mass spectrometry (ESI/MS). In this review, we will focus on the advances in ESI/MS, which have facilitated the development of shotgun lipidomics and the utility of intrasource separation as an enabling strategy for utilization of 2D mass spectrometry in shotgun lipidomics of biological samples. The principles and experimental details of the intrasource separation approach will be extensively discussed. Other ESI/MS approaches towards the quantitative analyses of global cellular lipidomes directly from crude lipid extracts of biological samples will also be reviewed and compared. Multiple examples of lipidomic analyses from crude lipid extracts employing these approaches will be given to show the power of ESI/MS techniques in lipidomics. Currently, modern society is plagued by the sequelae of lipid-related diseases. It is our hope that the integration of these advances in multiple disciplines will catalyze the development of lipidomics, and such development will lead to improvements in diagnostics and therapeutics, which will ultimately result in the extended longevity and an improved quality of life for humankind.

Toward fingerprinting cellular lipidomes directly from biological samples by two-dimensional electrospray ionization mass spectrometry

Cellular lipids are composed of linear combinations of aliphatic chains covalently attached to backbones and/or head groups that collectively represent the cellular lipidome. Herein we present a novel two-dimensional electrospray ionization mass spectrometric approach to fingerprint most of the major and many of the minor lipid classes in the hepatic cellular lipidome, which collectively represent >80% of the total lipid mass, directly from their chloroform extracts. Through lipid class-selective intrasource ionization and subsequent analysis of two-dimensional cross-peak intensities, the chemical identity and mass composition of individual molecular species of most mouse hepatic lipid classes were determined from its chloroform extract. This new integrated platform provides a robust foundation for the automated analysis of multidimensional mass spectra to advance the level of analytical scripting in lipidomics to a new plateau.

Shotgun lipidomics of phosphoethanolamine-containing lipids in biological samples after one-step in situ derivatization

This article presents a novel methodology for the analysis of ethanolamine glycerophospholipid (PE) and lysoPE molecular species directly from lipid extracts of biological samples. Through brief treatment of lipid extracts with fluorenylmethoxylcarbonyl (Fmoc) chloride, PE and lysoPE species were selectively derivatized to their corresponding carbamates. The reaction solution was infused directly into the ion source of an electrospray ionization mass spectrometer after appropriate dilution. The facile loss of the Fmoc moiety dramatically enhanced the analytic sensitivity and allowed the identification and quantitation of low-abundance molecular species. A detection limitation of attomoles (amoles) per microliter for PE and lysoPE analysis was readily achieved using this technique (at least a 100-fold improvement from our previous method) with a >15,000-fold dynamic range. Through intrasource separation and multidimensional mass spectrometry array analysis of derivatized species, marked improvements in signal-to-noise ratio, molecular species identification, and quantitation can be realized. The procedure is both simple and effective and can be extended to analyze many other lipid classes or other cellular metabolites by adjustments in specific derivatization conditions. Thus, through judicious derivatization, a new dimension exploiting specific functional reactivities in each lipid class can be used in conjunction with shotgun lipidomics to penetrate farther into the low-abundance regime of cellular lipidomes.

New effects in polynucleotide release from cationic lipid carriers revealed by confocal imaging, fluorescence cross-correlation spectroscopy and single particle tracking

We report on new insights into the mechanisms of short single and double stranded oligonucleotide release from cationic lipid complexes (lipoplexes), used in gene therapy. Specifically, we modeled endosomal membranes using giant unilamellar vesicles and investigated the roles of various individual cellular phospholipids in interaction with lipoplexes. Our approach uses a combination of confocal imaging, fluorescence cross-correlation spectroscopy and single particle tracking, revealing several new aspects of the release: (a) phosphatidylserine and phosphatidylethanolamine are equally active in disassembling lipoplexes, while phosphatidylcholine and sphingomyelin are inert; (b) in contrast to earlier findings, phosphatidylethanolamine alone, in the absence of anionic phosphatidylserine triggers extensive release; (c) a double-stranded DNA structure remains well preserved after release; (d) lipoplexes exhibited preferential binding to transient lipid domains, which appear at the onset of lipoplex attachment to originally uniform membranes and vanish after initiation of polynucleotide release. The latter effect is likely related to phosphatidyleserine redistribution in membranes due to lipoplex binding. Real time tracking of single DOTAP/DOPE and DOTAP/DOPC lipoplexes showed that both particles remained compact and associated with membranes up to 1-2 min before fusion, indicating that a more complex mechanism, different from suggested earlier rapid fusion, promotes more efficient transfection by DOTAP/DOPE complexes.

Characterization and direct quantitation of cerebroside molecular species from lipid extracts by shotgun lipidomics

By using shotgun lipidomics based on the separation of lipid classes in the electrospray ion source (intrasource separation) and two-dimensional (2D) MS techniques (Han, X., and R. W. Gross. 2004. Shotgun lipidomics: electrospray ionization mass spectrometric analysis and quantitation of the cellular lipidomes directly from crude extracts of biological samples. Mass Spectrom. Rev. First published on June 18, 2004; doi: 10.1002/mas.20023, In press), individual molecular species of most major and many minor lipid classes can be quantitated directly from biological lipid extracts. Herein, we extended shotgun lipidomics to the characterization and quantitation of cerebroside molecular species in biological samples. By exploiting the differential fragmentation patterns of chlorine adducts using electrospray ionization (ESI) tandem mass spectrometry, hydroxy and nonhydroxy cerebroside species are readily identified. The hexose (either galactose or glucose) moiety of a cerebroside species can be distinguished by examination of the peak intensity ratio of its product ions at m/z 179 and 89 (i.e., 0.74 +/- 0.10 and 4.8 +/- 0.7 for galactose- and glucose-containing cerebroside species, respectively). Quantitation of cerebroside molecular species (as little as 10 fmol) from chloroform extracts of brain tissue samples was directly conducted by 2D ESI/MS after correction for differences in (13)C-isotopomer intensities. This method was demonstrated to have a greater than 1,000-fold linear dynamic range in the low concentration region; therefore, it should have a wide range of applications in studies of the cellular sphingolipid lipidome.

Pathological cholesterol metabolism fails to modify electrophysiological properties of afflicted neurones in Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is a recessive inherited neurovisceral lipid storage disease characterized by progressive motor impairment and a loss of neurones including those integrated into the motor system. One of the key neuropathological findings is the intracellular accumulation of lysosomes enriched with free cholesterol. This accumulation is due to impaired transport proteins named NPC1 (approx. 95% of the cases) or NPC2 (approx. 5%) responsible for the transport of endocytosed cholesterol from lysomes to plasma membranes. The perturbed lipid-transport in NPC cells leads to an altered lipid composition of the plasma membrane. Available evidence suggests that the lipid matrix influences the electrophysical properties of ion channels in membranes. We therefore evaluated whether electrophysiological properties of NPC neurones differ from healthy neurones. Both, acute brain slices and primary neuronal cell cultures from wildtype and NPC mice, a well-established mouse model for the Niemann-Pick type C disease, were used for a comparison of electrophysiological properties like resting membrane potential, input resistance, action potential amplitudes and synaptic properties of the neurones. In addition we optically recorded the changes of intraneuronal calcium levels elicited by depolarization. Our results show that the characteristics of ion channels in NPC neurones do not differ significantly from wildtype neurones. We therefore conclude that gross alterations of the electrophysiological properties of neurones will probably not initiate or substantially contribute to the development of the motor impairment or other neurological signs of NPC.

Characterization of interaction between cationic lipid-oligonucleotide complexes and cellular membrane lipids using confocal imaging and fluorescence correlation spectroscopy

Complexes formed by cationic liposomes and single-strand oligodeoxynucleotides (CL-ODN) are promising delivery systems for antisense therapy. ODN release from the complexes is an essential step for inhibiting activity of antisense drugs. We applied fluorescence correlation spectroscopy and confocal laser scanning microscopy to monitor CL-ODN complex interaction with membrane lipids leading to ODN release. To model cellular membranes we used giant unilamellar vesicles and investigated the transport of Cy-5-labeled ODNs across DiO-labeled membranes. For the first time, we directly observed that ODN molecules are transferred across the lipid bilayers and are kept inside the giant unilamellar vesicles after release from the carriers. ODN dissociation from the carrier was assessed by comparing diffusion constants of CL-ODN complexes and ODNs before complexation and after release. Freely diffusing Cy-5-labeled ODN (16-nt) has diffusion constant D(ODN) = 1.3 +/- 0.1 x 10(-6) cm2/s. Fluorescence correlation spectroscopy curves for CL-ODN complexes were fitted with two components, which both have significantly slower diffusion in the range of D(CL-ODN) = approximately 1.5 x 10(-8) cm2/s. Released ODN has the mean diffusion constant D = 1.1 +/- 0.2 x 10(-6) cm2/s, which signifies that ODN is dissociated from cationic lipids. In contrast to earlier studies, we report that phosphatidylethanolamine can trigger ODN release from the carrier in the full absence of anionic phosphatidylserine in the target membrane and that phosphatidylethanolamine-mediated release is as extensive as in the case of phosphatidylserine. The presented methodology provides an effective tool for probing a delivery potential of newly created lipid formulations of CL-ODN complexes for optimal design of carriers.

Matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectrometry in lipid and phospholipid research

The interest in the analysis of lipids and phospholipids is continuously increasing due to the importance of these molecules in biochemistry (e.g. in the context of biomembranes and lipid second messengers) as well as in industry. Unfortunately, commonly used methods of lipid analysis are often time-consuming and tedious because they include previous separation and/or derivatization steps. With the development of "soft-ionization techniques" like electrospray ionization (ESI) or matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF), mass spectrometry became also applicable to lipid analysis. The aim of this review is to summarize so far available experiences in MALDI-TOF mass spectrometric analysis of lipids. It will be shown that MALDI-TOF MS can be applied to all known lipid classes and the characteristics of individual lipids will be discussed. Additionally, some selected applications in medicine and biology, e.g. mixture analysis, cell and tissue analysis and the determination of enzyme activities will be described. Advantages and disadvantages of MALDI-TOF MS in comparison to other established lipid analysis methods will be also discussed.

Distribution and Transport of Cholesterol-rich Membrane Domains Monitored by a Membrane-impermeant Fluorescent Polyethylene Glycol-derivatized Cholesterol

Cholesterol-rich membrane domains function in various membrane events as diverse as signal transduction and membrane traffic. We studied the interaction of a fluorescein ester of polyethylene glycol-derivatized cholesterol (fPEG-Chol) with cholesterol-rich membranes both in cells and in model membranes. Unlike filipin and other cholesterol probes, this molecule could be applied as an aqueous dispersion to various samples. When added to live cells, fPEG-Chol distributed exclusively in the outer plasma membrane leaflet and was enriched in microdomains that dynamically clustered by the activation of receptor signaling. The surface-bound fPEG-Chol was slowly internalized via clathrin-independent pathway into endosomes together with lipid raft markers. Noteworthy, fPEG-Chol could be microinjected in the living cells in which we found Golgi apparatus as the sole major organelle to be labeled. PEG-Chol, thus, provides a novel, sensitive probe for unraveling the dynamics of cholesterol-rich microdomains in living cells.

Accumulation of glycosphingolipids in Niemann-Pick C disease disrupts endosomal transport

Glycosphingolipids are endocytosed and targeted to the Golgi apparatus but are mistargeted to lysosomes in sphingolipid storage disorders. Substrate reduction therapy utilizes imino sugars to inhibit glucosylceramide synthase and potentially abrogate the effects of storage. Niemann-Pick type C (NPC) disease is a disorder of intracellular transport where glycosphingolipids (GSLs) and cholesterol accumulate in endosomal compartments. The mechanisms of altered intracellular trafficking are not known but may involve the mistargeting and disrupted function of proteins associated with GSL membrane microdomains. Membrane microdomains were isolated by Triton X-100 and sucrose density gradient ultracentrifugation. High pressure liquid chromatography and mass spectrometric analysis of NPC1(-/-) mouse brain revealed large increases in GSL. Sphingosine was also found to be a component of membrane microdomains, and in NPC liver and spleen, large increases in cholesterol and sphingosine were found. GSL and cholesterol levels were increased in mutant NPC1-null Chinese hamster ovary cells as well as U18666A and progesterone induced NPC cell culture models. However, inhibition of GSL synthesis in NPC cells with N-butyldeoxygalactonojirimycin led to marked decreases in GSL but only small decreases in cholesterol levels. Both annexin 2 and 6, membrane-associated proteins that are important in endocytic trafficking, show distorted distributions in NPC cells. Altered BODIPY lactosylceramide targeting, decreased endocytic uptake of a fluid phase marker, and mistargeting of annexin 2 (phenotypes associated with NPC) are reversed by inhibition of GSL synthesis. It is suggested that accumulating GSL is part of a mislocalized membrane microdomain and is responsible for the deficit in endocytic trafficking found in NPC disease.

Keratinocytes Rapidly Readjust Ceramide–Sphingomyelin Homeostasis and Contain a Phosphatidylcholine–Sphingomyelin Transacylase

Ceramide as central second messenger of the apoptosis-related sphingomyelin signaling pathway is a potential target for the control of cancer. A complex metabolizing network defines cell type and stage-specific final ceramide concentrations. Successful therapeutic control of ceramide levels requires a knowledge of multiple related turnover rates. The metabolism of ceramide and sphingomyelin was studied in keratinocytes under the condition of an unstimulated sphingomyelin signaling pathway. Preparations enriched in plasma membranes contain a neutral Mg(2+)-dependent sphingomyelinase and a Mg(2+)-independent sphingomyelin synthase that vigorously preserve balanced ceramide and sphingomyelin levels. Ceramide regulates neutral sphingomyelinase. Inhibition of sphingomyelin synthase by D609 treatment results in temporary loss of intercelluar contacts and in cellular shrinking. It is ineffective for sustained elevation of ceramide levels. Ceramide phosphorylating and deacylating activities are insignificant. Recently, fatty-acid remodeling in sphingomyelin was reported as likely to counteract the membrane-rigidifying effects of cholesterol. Keratinocytes transfer fluorescence labeled acyl-chains between phosphatidylcholine and sphingomyelin. A transferase of that kind would allow rapid adjustment of local lipid composition in response to acutely changed conditions. In addition, this transferase might have a function in the formation of the epidermal permeability barrier.

The state of lipid rafts: from model membranes to cells

Lipid raft microdomains were conceived as part of a mechanism for the intracellular trafficking of lipids and lipid-anchored proteins. The raft hypothesis is based on the behavior of defined lipid mixtures in liposomes and other model membranes. Experiments in these well-characterized systems led to operational definitions for lipid rafts in cell membranes. These definitions, detergent solubility to define components of rafts, and sensitivity to cholesterol deprivation to define raft functions implicated sphingolipid- and cholesterol-rich lipid rafts in many cell functions. Despite extensive work, the basis for raft formation in cell membranes and the size of rafts and their stability are all uncertain. Recent work converges on very small rafts <10 nm in diameter that may enlarge and stabilize when their constituents are cross-linked.

Electrospray mass spectrometry of phospholipids

Phospholipids play a central role in the biochemistry of all living cells. These molecules constitute the lipid bilayer defining the outer confines of a cell, but also serve as the structural entities which confine subcellular components. Mass spectrometry has emerged as a powerful tool useful for the qualitative and quantitative analysis of complex phospholipids, including glycerophospholipids and the sphingolipid, sphingomyelin. Collision induced decomposition of both positive and negative molecular ion species yield rich information as to the polar head group of the phospholipid and the fatty-acyl substituents esterified to the glycerophospholipid backbone. This review presents the current level of understanding of the mechanisms involved in the formation of various product ions following collisional activation of molecular ion species generated by electrospray ionization of the common glycerophospholipids, including phosphatidic acid, phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, phosphatidylglycerol, phosphatidylserine, cardiolipin, and sphingomyelin. Recent advances in the application of matrix assisted laser desorption ionization is also considered. Several applications of mass spectrometry applied to phospholipid analysis are presented as they apply to physiology as well as pathophysiology.

Dynamic, yet structured: The cell membrane three decades after the Singer-Nicolson model

The fluid mosaic membrane model proved to be a very useful hypothesis in explaining many, but certainly not all, phenomena taking place in biological membranes. New experimental data show that the compartmentalization of membrane components can be as important for effective signal transduction as is the fluidity of the membrane. In this work, we pay tribute to the Singer-Nicolson model, which is near its 30th anniversary, honoring its basic features, "mosaicism" and "diffusion," which predict the interspersion of proteins and lipids and their ability to undergo dynamic rearrangement via Brownian motion. At the same time, modifications based on quantitative data are proposed, highlighting the often genetically predestined, yet flexible, multilevel structure implementing a vast complexity of cellular functions. This new "dynamically structured mosaic model" bears the following characteristics: emphasis is shifted from fluidity to mosaicism, which, in our interpretation, means nonrandom codistribution patterns of specific kinds of membrane proteins forming small-scale clusters at the molecular level and large-scale clusters (groups of clusters, islands) at the submicrometer level. The cohesive forces, which maintain these assemblies as principal elements of the membranes, originate from within a microdomain structure, where lipid-lipid, protein-protein, and protein-lipid interactions, as well as sub- and supramembrane (cytoskeletal, extracellular matrix, other cell) effectors, many of them genetically predestined, play equally important roles. The concept of fluidity in the original model now is interpreted as permissiveness of the architecture to continuous, dynamic restructuring of the molecular- and higher-level clusters according to the needs of the cell and as evoked by the environment.

Characterisation of a potential biomarker of phospholipidosis from amiodarone-treated rats

A novel and relatively simple analytical method for the separation, characterisation and semi-quantitation of phospholipids (PLs) from extracts of complex biological samples has been developed. This methodology allows PL extracts from cells and tissues to be analysed by liquid chromatography (LC) coupled to electrospray ionisation mass spectrometry (ESI-MS). Complex mixtures of PLs were separated on a high-performance liquid chromatography (HPLC) system using 0.5% ammonium hydroxide in methanol/water/hexane/formate mixture with UV detection at 205 nm. Identification and structural characterisation of molecular species were carried out utilising ESI-MS and MS/MS in the negative ion mode. The abnormal accumulation of PLs (phospholipidosis) was induced in male Sprague-Dawley rats by administration of the cationic amphiphilic drug (CAD), amiodarone. Analysis of the PL profile of liver and lung tissues, lymphocytes and serum from treated rats was carried out using this analytical procedure (LC-ESI/MS/MS). Differences in PL profiles between treated and untreated animals were highlighted by principal component analysis (PCA). This led to the selection of a potential metabolic marker of phospholipidosis (PLD) identified as a lyso-bis-phosphatidic acid (LBPA) derivative, also known as bis(monoglycero)phosphate (BMP). This PL was absent in control animals but was present in quantifiable amounts in all samples from amiodarone-treated rats.