ER Membrane Lipid Composition and Metabolism: Lipidomic Analysis

Plant ER membranes are the major site of biosynthesis of several lipid families (phospholipids, sphingolipids, neutral lipids such as sterols and triacylglycerols). The structural diversity of lipids presents considerable challenges to comprehensive lipid analysis. This chapter will briefly review the various biosynthetic pathways and will detail several aspects of the lipid analysis: lipid extraction, handling, separation, detection, identification, and data presentation. The different tools/approaches used for lipid analysis will also be discussed in relation to the studies to be carried out on lipid metabolism and function.

The receptor kinase FERONIA regulates phosphatidylserine localization at the cell surface to modulate ROP signaling

Cells maintain a constant dialog between the extracellular matrix and their plasma membrane to fine tune signal transduction processes. We found that the receptor kinase FERONIA (FER), which is a proposed cell wall sensor, modulates phosphatidylserine plasma membrane accumulation and nano-organization, a key regulator of Rho GTPase signaling in Arabidopsis. We demonstrate that FER is required for both Rho-of-Plant 6 (ROP6) nano-partitioning at the membrane and downstream production of reactive oxygen species upon hyperosmotic stimulus. Genetic and pharmacological rescue experiments indicate that phosphatidylserine is required for a subset of, but not all, FER functions. Furthermore, application of FER ligand shows that its signaling controls both phosphatidylserine membrane localization and nanodomains formation, which, in turn, tunes ROP6 signaling. Together, we propose that a cell wall-sensing pathway controls via the regulation of membrane phospholipid content, the nano-organization of the plasma membrane, which is an essential cell acclimation to environmental perturbations.

Sterols and Sphingolipids as New Players in Cell Wall Building and Apical Growth of Nicotiana tabacum L. Pollen Tubes

Pollen tubes are tip-growing cells that create safe routes to convey sperm cells to the embryo sac for double fertilization. Recent studies have purified and biochemically characterized detergent-insoluble membranes from tobacco pollen tubes. These microdomains, called lipid rafts, are rich in sterols and sphingolipids and are involved in cell polarization in organisms evolutionarily distant, such as fungi and mammals. The presence of actin in tobacco pollen tube detergent-insoluble membranes and the preferential distribution of these domains on the apical plasma membrane encouraged us to formulate the intriguing hypothesis that sterols and sphingolipids could be a "trait d'union" between actin dynamics and polarized secretion at the tip. To unravel the role of sterols and sphingolipids in tobacco pollen tube growth, we used squalestatin and myriocin, inhibitors of sterol and sphingolipid biosynthesis, respectively, to determine whether lipid modifications affect actin fringe morphology and dynamics, leading to changes in clear zone organization and cell wall deposition, thus suggesting a role played by these lipids in successful fertilization.

Lysophosphatidic acid acyltransferases: a link with intracellular protein trafficking in Arabidopsis root cells?

Phosphatidic acid (PA) and lysophosphatidic acid acyltransferases (LPAATs) might be critical for the secretory pathway. Four extra-plastidial LPAATs (LPAAT2, 3, 4, and 5) were identified in Arabidopsis thaliana. These AtLPAATs display a specific enzymatic activity converting lysophosphatidic acid to PA and are located in the endomembrane system. We investigate a putative role for AtLPAATs 3, 4, and 5 in the secretory pathway of root cells through genetical (knockout mutants), biochemical (activity inhibitor, lipid analyses), and imaging (live and immuno-confocal microscopy) approaches. Treating a lpaat4;lpaat5 double mutant with the LPAAT inhibitor CI976 produced a significant decrease in primary root growth. The trafficking of the auxin transporter PIN2 was disturbed in this lpaat4;lpaat5 double mutant treated with CI976, whereas trafficking of H+-ATPases was unaffected. The lpaat4;lpaat5 double mutant is sensitive to salt stress, and the trafficking of the aquaporin PIP2;7 to the plasma membrane in the lpaat4;lpaat5 double mutant treated with CI976 was reduced. We measured the amounts of neo-synthesized PA in roots, and found a decrease in PA only in the lpaat4;lpaat5 double mutant treated with CI976, suggesting that the protein trafficking impairment was due to a critical PA concentration threshold.

The enpp4 ectonucleotidase regulates kidney patterning signalling networks in Xenopus embryos

The enpp ectonucleotidases regulate lipidic and purinergic signalling pathways by controlling the extracellular concentrations of purines and bioactive lipids. Although both pathways are key regulators of kidney physiology and linked to human renal pathologies, their roles during nephrogenesis remain poorly understood. We previously showed that the pronephros was a major site of enpp expression and now demonstrate an unsuspected role for the conserved vertebrate enpp4 protein during kidney formation in Xenopus. Enpp4 over-expression results in ectopic renal tissues and, on rare occasion, complete mini-duplication of the entire kidney. Enpp4 is required and sufficient for pronephric markers expression and regulates the expression of RA, Notch and Wnt pathway members. Enpp4 is a membrane protein that binds, without hydrolyzing, phosphatidylserine and its effects are mediated by the receptor s1pr5, although not via the generation of S1P. Finally, we propose a novel and non-catalytic mechanism by which lipidic signalling regulates nephrogenesis.

Massé and colleagues identify enpp4 as a key regulator in the development of the kidney in Xenopus. The gene signalling pathways regulated by this ectonucleotidase are described and lipidic signalling regulatory mechanisms are explored.

Developmental control of plant Rho GTPase nano-organization by the lipid phosphatidylserine

Rho guanosine triphosphatases (GTPases) are master regulators of cell signaling, but how they are regulated depending on the cellular context is unclear. We found that the phospholipid phosphatidylserine acts as a developmentally controlled lipid rheostat that tunes Rho GTPase signaling in Arabidopsis Live superresolution single-molecule imaging revealed that the protein Rho of Plants 6 (ROP6) is stabilized by phosphatidylserine into plasma membrane nanodomains, which are required for auxin signaling. Our experiments also revealed that the plasma membrane phosphatidylserine content varies during plant root development and that the level of phosphatidylserine modulates the quantity of ROP6 nanoclusters induced by auxin and hence downstream signaling, including regulation of endocytosis and gravitropism. Our work shows that variations in phosphatidylserine levels are a physiological process that may be leveraged to regulate small GTPase signaling during development.

The multistress-induced Translocator protein (TSPO) differentially modulates storage lipids metabolism in seeds and seedlings

Translocator proteins (TSPO) are conserved membrane proteins extensively studied in mammals, but their function is still unclear. Angiosperm TSPO are transiently induced by abiotic stresses in vegetative tissues. We showed previously that constitutive expression of the Arabidopsis TSPO (AtTSPO) could be detrimental to the cell. Degradation of AtTSPO requires an active autophagy pathway. We show here that genetic modifications of TSPO expression in plant and yeast cells reduce the levels of cytoplasmic lipid droplets (LD). Transgenic Arabidopsis seedlings overexpressing AtTSPO contain less LD as compared with wild type (WT). LD levels were increased in Arabidopsis AtTSPO knockout (KO) seedlings. Deletion of the Schizosaccharomyces pombe TSPO resulted in an increase in LD level in the cell. As compared with the WT, the mutant strain was more sensitive to cerulenin, an inhibitor of fatty acids and sterol biosynthesis. We found that in contrast with seedlings, overexpression of AtTSPO (OE) resulted in an up to 50% increase in seeds fatty acids as compared with WT. A time course experiment revealed that after 4 days of seed imbibition, the levels of triacylglycerol (TAG) was still higher in the OE seeds as compared with WT or KO seeds. However, the de novo synthesis of phospholipids and TAG after 24 h of imbibition was substantially reduced in OE seeds as compared with WT or KO seeds. Our findings support a plant TSPO role in energy homeostasis in a tissue-specific manner, enhancing fatty acids and LD accumulation in mature seeds and limiting LD levels in seedlings.

A Combinatorial Lipid Code Shapes the Electrostatic Landscape of Plant Endomembranes

Membrane surface charge is critical for the transient, yet specific recruitment of proteins with polybasic regions to certain organelles. In eukaryotes, the plasma membrane (PM) is the most electronegative compartment of the cell, which specifies its identity. As such, membrane electrostatics is a central parameter in signaling, intracellular trafficking, and polarity. Here, we explore which are the lipids that control membrane electrostatics using plants as a model. We show that phosphatidylinositol-4-phosphate (PI4P), phosphatidic acidic (PA), and phosphatidylserine (PS) are separately required to generate the electrostatic signature of the plant PM. In addition, we reveal the existence of an electrostatic territory that is organized as a gradient along the endocytic pathway and is controlled by PS/PI4P combination. Altogether, we propose that combinatorial lipid composition of the cytosolic leaflet of organelles not only defines the electrostatic territory but also distinguishes different functional compartments within this territory by specifying their varying surface charges.

The odd one out: Arabidopsis reticulon 20 does not bend ER membranes but has a role in lipid regulation

Reticulons are integral ER membrane proteins characterised by a reticulon homology domain comprising four transmembrane domains which results in the proteins sitting in the membrane in a W-topology. Here we report on a novel subgroup of reticulons with an extended N-terminal domain and in particular on arabidopsis reticulon 20. Using high resolution confocal microscopy we show that reticulon 20 is located in a unique punctate pattern on the ER membrane. Its closest homologue reticulon 19 labels the whole ER. Other than demonstrated for the other members of the reticulon protein family RTN20 and 19 do not display ER constriction phenotypes on over expression. We show that mutants in RTN20 or RTN19, respectively, display a significant change in sterol composition in roots indicating a role in lipid regulation. A third homologue in this family -3BETAHSD/D1- is unexpectedly localised to ER exit sites resulting in an intriguing location difference for the three proteins.

Constraint-Based Modeling Highlights Cell Energy, Redox Status and α-Ketoglutarate Availability as Metabolic Drivers for Anthocyanin Accumulation in Grape Cells Under Nitrogen Limitation

Anthocyanin biosynthesis is regulated by environmental factors (such as light, temperature, and water availability) and nutrient status (such as carbon, nitrogen, and phosphate nutrition). Previous reports show that low nitrogen availability strongly enhances anthocyanin accumulation in non carbon-limited plant organs or cell suspensions. It has been hypothesized that high carbon-to-nitrogen ratio would lead to an energy excess in plant cells, and that an increase in flavonoid pathway metabolic fluxes would act as an "energy escape valve," helping plant cells to cope with energy and carbon excess. However, this hypothesis has never been tested directly. To this end, we used the grapevine Vitis vinifera L. cultivar Gamay Teinturier (syn. Gamay Freaux or Freaux Tintorier, VIVC #4382) cell suspension line as a model system to study the regulation of anthocyanin accumulation in response to nitrogen supply. The cells were sub-cultured in the presence of either control (25 mM) or low (5 mM) nitrate concentration. Targeted metabolomics and enzyme activity determinations were used to parametrize a constraint-based model describing both the central carbon and nitrogen metabolisms and the flavonoid (phenylpropanoid) pathway connected by the energy (ATP) and reducing power equivalents (NADPH and NADH) cofactors. The flux analysis (2 flux maps generated, for control and low nitrogen in culture medium) clearly showed that in low nitrogen-fed cells all the metabolic fluxes of central metabolism were decreased, whereas fluxes that consume energy and reducing power, were either increased (upper part of glycolysis, shikimate, and flavonoid pathway) or maintained (pentose phosphate pathway). Also, fluxes of flavanone 3β-hydroxylase, flavonol synthase, and anthocyanidin synthase were strongly increased, advocating for a regulation of the flavonoid pathway by alpha-ketoglutarate levels. These results strongly support the hypothesis of anthocyanin biosynthesis acting as an energy escape valve in plant cells, and they open new possibilities to manipulate flavonoid production in plant cells. They do not, however, support a role of anthocyanins as an effective mechanism for coping with carbon excess in high carbon to nitrogen ratio situations in grape cells. Instead, constraint-based modeling output and biomass analysis indicate that carbon excess is dealt with by vacuolar storage of soluble sugars.

ER Membrane Lipid Composition and Metabolism: Lipidomic Analysis

Plant ER membranes are the major site of biosynthesis of several lipid families (phospholipids, sphingolipids, neutral lipids such as sterols and triacylglycerols). The structural diversity of lipids presents considerable challenges to comprehensive lipid analysis. This chapter will briefly review the various biosynthetic pathways and will detail several aspects of the lipid analysis: lipid extraction, handling, separation, detection, identification, and data presentation. The different tools/approaches used for lipid analysis will also be discussed in relation to the studies to be carried out on lipid metabolism and function.

The Qb-SNARE Memb11 interacts specifically with Arf1 in the Golgi apparatus of Arabidopsis thaliana

The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are critical for the function of the secretory pathway. The SNARE Memb11 is involved in membrane trafficking at the ER-Golgi interface. The aim of the work was to decipher molecular mechanisms acting in Memb11-mediated ER-Golgi traffic. In mammalian cells, the orthologue of Memb11 (membrin) is potentially involved in the recruitment of the GTPase Arf1 at the Golgi membrane. However molecular mechanisms associated to Memb11 remain unknown in plants. Memb11 was detected mainly at the cis-Golgi and co-immunoprecipitated with Arf1, suggesting that Arf1 may interact with Memb11. This interaction of Memb11 with Arf1 at the Golgi was confirmed by in vivo BiFC (Bimolecular Fluorescence Complementation) experiments. This interaction was found to be specific to Memb11 as compared to either Memb12 or Sec22. Using a structural bioinformatic approach, several sequences in the N-ter part of Memb11 were hypothesized to be critical for this interaction and were tested by BiFC on corresponding mutants. Finally, by using both in vitro and in vivo approaches, we determined that only the GDP-bound form of Arf1 interacts with Memb11. Together, our results indicate that Memb11 interacts with the GDP-bound form of Arf1 in the Golgi apparatus.

Characterisation of detergent-insoluble membranes in pollen tubes of Nicotiana tabacum (L.)

Pollen tubes are the vehicle for sperm cell delivery to the embryo sac during fertilisation of Angiosperms. They provide an intriguing model for unravelling mechanisms of growing to extremes. The asymmetric distribution of lipids and proteins in the pollen tube plasma membrane modulates ion fluxes and actin dynamics and is maintained by a delicate equilibrium between exocytosis and endocytosis. The structural constraints regulating polarised secretion and asymmetric protein distribution on the plasma membrane are mostly unknown. To address this problem, we investigated whether ordered membrane microdomains, namely membrane rafts, might contribute to sperm cell delivery. Detergent insoluble membranes, rich in sterols and sphingolipids, were isolated from tobacco pollen tubes. MALDI TOF/MS analysis revealed that actin, prohibitins and proteins involved in methylation reactions and in phosphoinositide pattern regulation are specifically present in pollen tube detergent insoluble membranes. Tubulins, voltage-dependent anion channels and proteins involved in membrane trafficking and signalling were also present. This paper reports the first evidence of membrane rafts in Angiosperm pollen tubes, opening new perspectives on the coordination of signal transduction, cytoskeleton dynamics and polarised secretion.

Phospholipid biosynthesis increases in RHD3-defective mutants

RHD3, a member of the ER-shaping dynamin-like GTPases, is required in the transition from a cisternal to a tubular ER architecture during cell growth. The aberrant ER morphology in rhd3 mutants may be correlated with alterations of the ER lipid bilayer. We analyzed the lipid fraction of rhd3 mutants at qualitative and quantitative levels. We observed an increase of the amount of phospholipids but also of proteins in the mutants, indicating an overall increase of ER membranes. This increase may indicate that phospholipid biosynthesis is deregulated in rhd3 mutants. It was shown that overexpression of PIS1 and PIS2 (involved in phosphatidylinositol biosynthesis) induces the synthesis of phosphatidylinositol (PI) but also of phosphatidic acid and that overexpression of PIS1 also induces the synthesis of phosphatidylethanolamine and diacylglycerol. (1) We wondered whether PIS1 or PIS2 could be linked to the increase of the amount of phospholipids in rhd3 mutants. To answer, we measured the phospholipid composition in the double mutants rhd3-7/pis1 and rhd3-7/pis2. The phospholipid increase in the rhd3 mutant was compensated in rhd3-7/pis1 but not rhd3-7/pis2. Our results suggest a possible deregulation of PIS1 in the rhd3 mutant.

The importance of cardiolipin synthase for mitochondrial ultrastructure, respiratory function, plant development, and stress responses in Arabidopsis

Cardiolipin (CL) is the signature phospholipid of the mitochondrial inner membrane. In animals and yeast (Saccharomyces cerevisiae), CL depletion affects the stability of respiratory supercomplexes and is thus crucial to the energy metabolism of obligate aerobes. In eukaryotes, the last step of CL synthesis is catalyzed by CARDIOLIPIN SYNTHASE (CLS), encoded by a single-copy gene. Here, we characterize a cls mutant in Arabidopsis thaliana, which is devoid of CL. In contrast to yeast cls, where development is little affected, Arabidopsis cls seedlings are slow developing under short-day conditions in vitro and die if they are transferred to long-day (LD) conditions. However, when transferred to soil under LD conditions under low light, cls plants can reach the flowering stage, but they are not fertile. The cls mitochondria display abnormal ultrastructure and reduced content of respiratory complex I/complex III supercomplexes. The marked accumulation of tricarboxylic acid cycle derivatives and amino acids demonstrates mitochondrial dysfunction. Mitochondrial and chloroplastic antioxidant transcripts are overexpressed in cls leaves, and cls protoplasts are more sensitive to programmed cell death effectors, UV light, and heat shock. Our results show that CLS is crucial for correct mitochondrial function and development in Arabidopsis under both optimal and stress conditions.

Cell wall constrains lateral diffusion of plant plasma-membrane proteins

A cell membrane can be considered a liquid-phase plane in which lipids and proteins theoretically are free to diffuse. Numerous reports, however, describe retarded diffusion of membrane proteins in animal cells. This anomalous diffusion results from a combination of structuring factors including protein-protein interactions, cytoskeleton corralling, and lipid organization into microdomains. In plant cells, plasma-membrane (PM) proteins have been described as relatively immobile, but the control mechanisms that structure the PM have not been studied. Here, we use fluorescence recovery after photobleaching to estimate mobility of a set of minimal PM proteins. These proteins consist only of a PM-anchoring domain fused to a fluorescent protein, but their mobilities remained limited, as is the case for many full-length proteins. Neither the cytoskeleton nor membrane microdomain structure was involved in constraining the diffusion of these proteins. The cell wall, however, was shown to have a crucial role in immobilizing PM proteins. In addition, by single-molecule fluorescence imaging we confirmed that the pattern of cellulose deposition in the cell wall affects the trajectory and speed of PM protein diffusion. Regulation of PM protein dynamics by the plant cell wall can be interpreted as a mechanism for regulating protein interactions in processes such as trafficking and signal transduction.

Cell wall constrains lateral diffusion of plant plasma-membrane proteins

A cell membrane can be considered a liquid-phase plane in which lipids and proteins theoretically are free to diffuse. Numerous reports, however, describe retarded diffusion of membrane proteins in animal cells. This anomalous diffusion results from a combination of structuring factors including protein-protein interactions, cytoskeleton corralling, and lipid organization into microdomains. In plant cells, plasma-membrane (PM) proteins have been described as relatively immobile, but the control mechanisms that structure the PM have not been studied. Here, we use fluorescence recovery after photobleaching to estimate mobility of a set of minimal PM proteins. These proteins consist only of a PM-anchoring domain fused to a fluorescent protein, but their mobilities remained limited, as is the case for many full-length proteins. Neither the cytoskeleton nor membrane microdomain structure was involved in constraining the diffusion of these proteins. The cell wall, however, was shown to have a crucial role in immobilizing PM proteins. In addition, by single-molecule fluorescence imaging we confirmed that the pattern of cellulose deposition in the cell wall affects the trajectory and speed of PM protein diffusion. Regulation of PM protein dynamics by the plant cell wall can be interpreted as a mechanism for regulating protein interactions in processes such as trafficking and signal transduction.

Glucosylceramide biosynthesis is involved in Golgi morphology and protein secretion in plant cells

Lipids have an established role as structural components of membranes or as signalling molecules, but their role as molecular actors in protein secretion is less clear. The complex sphingolipid glucosylceramide (GlcCer) is enriched in the plasma membrane and lipid microdomains of plant cells, but compared to animal and yeast cells, little is known about the role of GlcCer in plant physiology. We have investigated the influence of GlcCer biosynthesis by glucosylceramide synthase (GCS) on the efficiency of protein transport through the plant secretory pathway and on the maintenance of normal Golgi structure. We determined that GlcCer is synthesized at the beginning of the plant secretory pathway [mainly endoplasmic reticulum (ER)] and that D,L-threo-1-phenyl-2-decanoyl amino-3-morpholino-propanol (PDMP) is a potent inhibitor of plant GCS activity in vitro and in vivo. By an in vivo confocal microscopy approach in tobacco leaves infiltrated with PDMP, we showed that the decrease in GlcCer biosynthesis disturbed the transport of soluble and membrane secretory proteins to the cell surface, as these proteins were partly retained intracellularly in the ER and/or Golgi. Electron microscopic observations of Arabidopsis thaliana root cells after high-pressure freezing and freeze substitution evidenced strong morphological changes in the Golgi bodies, pointing to a link between decreased protein secretion and perturbations of Golgi structure following inhibition of GlcCer biosynthesis in plant cells.

A novel di-acidic motif facilitates ER export of the syntaxin SYP31

It is generally accepted that ER protein export is largely influenced by the transmembrane domain (TMD). The situation is unclear for membrane-anchored proteins such as SNAREs, which are anchored to the membrane by their TMD at the C-terminus. For example, in plants, Sec22 and SYP31 (a yeast Sed5 homologue) have a 17 aa TMD but different locations (ER/Golgi and Golgi), indicating that TMD length alone is not sufficient to explain their targeting. To establish the identity of factors that influence SNARE targeting, mutagenesis and live cell imaging experiments were performed on SYP31. It was found that deletion of the entire N-terminus domain of SYP31 blocked the protein in the ER. Several deletion mutants of different parts of this N-terminus domain indicated that a region between the SNARE helices Hb and Hc is required for Golgi targeting. In this region, replacement of the aa sequence MELAD by GAGAG or MALAG retained the protein in the ER, suggesting that MELAD may function as a di-acidic ER export motif EXXD. This suggestion was further verified by replacing the established di-acidic ER export motif DLE of a type II Golgi protein AtCASP and a membrane-anchored type I chimaera, TMcCCASP, by MELAD or GAGAG. The MELAD motif allowed the proteins to reach the Golgi, whereas the motif GAGAG was found to be insufficient to facilitate ER protein export. Our analyses indicate that we have identified a novel and transplantable di-acidic motif that facilitates ER export of SYP31 and may function for type I and type II proteins in plants.

Immunolocalization and high affinity interactions of acyl-CoAs with proteins: an original study with anti-acyl-CoA antibodies

Anti-acyl-Coenzyme A (acyl-CoA) antibodies were used to detect fatty acyl-CoAs in cultured rat hippocampal neurons, in which important lipid metabolism and transport occur. Hippocampus was chosen because of his involvement in many cerebral functions and diseases. Immunofluorescence experiments showed an intense labelling within neurites and cell bodies. Labelling seems to be associated with vesicles and membrane domains. We have shown by immunoblot experiments that the labelling corresponded to acyl-CoAs which were in strong interaction with proteins, without being covalently bound to them. Immunoprecipitation experiments, followed by proteomic analysis, showed that anti-acyl-CoA antibodies were also able to immunoprecipitate multiprotein complexes, principally related to vesicle trafficking and/or to membrane rafts.

Anti-phospholipid antibodies in serum from patients with Guillain-Barré syndrome

OBJECTIVE

The Guillain-Barré syndrome (GBS) is an acute inflammatory polyneuropathy related to autoimmunity. However, no conclusive etiological concept has yet been found. We examined the variation in autoantibodies to lipids in serum of GBS patients in response to the course of the disease but investigated titer modifications during treatment with gamma-globulin.

DESIGN AND SETTING

Prospective clinical study in a 14-bed general ICU.

PATIENTS

Nine patients with GBS and nine controls were included in the study.

MEASUREMENTS AND RESULTS

Four blood samples were obtained before and after treatment. Serum samples, diluted 1:60, were tested by enzyme-linked immunosorbent assay for IgM, IgA, and IgG antibodies to phosphatidylcholine, phosphatidylinositol, cardiolipin, phosphatidic acid, phosphatidylserine, phosphatidylglycerol, phosphatidylethanolamine, sphingomyelin, and gangliosides. Anti-phospholipid antibodies of the IgM, IgA, and IgG families were detected in all GBS patients but in none of the controls. Phosphatidylinositol, cardiolipin, phosphatidylcholine, and phosphatidic acid were the main antigens. All patients developed anti-phosphatidylinositol antibodies of the IgM family and anti-cardiolipin antibodies of the IgA and IgG families. A decrease in the level of anti-phospholipid autoantibodies was observed after 1 day of treatment with gamma-globulin. Two days after ending gamma-globulin administration the IgG antibodies increased again.

CONCLUSIONS

Our findings suggest that in GBS there is an extensive immune reaction, which is altered after gamma-globulin treatment. Anti-cardiolipin and anti-phosphatidylinositol antibodies could be useful markers for the response to treatment.

Chromatographic purification of an insoluble histidine tag recombinant Ykt6p SNARE from Arabidopsis thaliana over-expressed in E. coli

In order to undertake in plant cell the study of the endoplasmic reticulum (ER)-Golgi apparatus (GA) protein and/or lipid vesicular transport pathway, expressed sequence tag (EST) coding for a homologue to the yeast soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) Ykt6p has been cloned in Arabidopsis thaliana by reverse transcription polymerase chain reaction (RT-PCR). The corresponding protein was over-expressed as a recombinant histidine-tag (his-tag) protein in E. coli. Starting from one litter of culture, an ultrasonic homogenization was performed for cell disruption and after centrifugation the Arabidopsis Ykt6p SNARE present in inclusion bodies in the pellet was solubilized. After centrifugation, the clarified feedstock obtained was injected onto an immobilized metal affinity chromatography (IMAC) in presence of 6 M guanidine and on-column refolding was performed. Folded and subsequently purified (94% purity) recombinant protein was obtained with 82% of recovery.

Lipids are co-eluted with immunoglobulins G during purification by recombinant streptococcal protein G affinity chromatography

The efficiency of recombinant streptococcal protein G (rec-spG) affinity column chromatography in purifying immunoglobulins G (IgG) from lipids has been studied, with particular reference to IgG fractions from bronchoalveolar lavage (BAL) fluid and serum samples from different sources. It was found that the IgG fractions purified by rec-spG affinity column chromatography also contained cholesterol and phospholipids.

Immunolocalization of long-chain acyl-CoAs in plant cells

Long chain acyl-Coenzyme A esters (acyl-CoAs) are key substrates in many enzymic reactions of lipid metabolism. Due to their amphiphilic nature, the membrane localization of these molecules cannot be established by subcellular membrane fractionation and usual biochemical studies. We have developed another approach based on ultrastructural immunogold cytochemistry. To preserve the acyl-CoA membrane content, the plant material was freeze substituted and cryoembedded after short aldehyde fixation followed by quick freezing. Using Arabidopsis thaliana root cells and specific antibodies raised against acyl-CoAs, we show that acyl-CoAs are mainly localized in endoplasmic reticulum membranes. Our results demonstrate the value of cryo-methods for the accurate localization of labile metabolites in plant cells.

Anti-phospholipid antibodies in diabetes mellitus

The presence of autoantibodies to phospholipids may be associated with various pathological disorders; diabetes could be one of them because of the changes occurring in lipid metabolism but there are only few reports examining this question, and they are not always leading to the same conclusions because of the differences in the procedures or in the phospholipids tested. We carried out a systematic comparative study of diabetic serum antibody binding to all phospholipids, anionic and zwitterionic, by a quantitative ELISA. The implication of the hydrophobic moiety of the lipids was also studied: the presence of autoantibodies to the fatty acyl chains was investigated. Our results show the presence of anti-phospholipid antibodies in diabetic sera, particularly anti-phosphatidylinositol and anti-phosphatidylcholine which have never been tested before, and appear to be associated with macroangiopathic complications. The antigenic epitopes are mainly the polar heads as no antibody binding to the hydrophobic moiety was observed. We discuss the relation of those antibodies to the angiopathic complications and to the direct effects of hyperglycemia on lipid antigenicity.

Autoantibodies to lipids in bronchoalveolar lavage fluid of patients with acute respiratory distress syndrome

OBJECTIVE

To investigate the presence of autoantibodies to lipids in the bronchoalveolar lavage (BAL) fluid from adult patients with acute respiratory distress syndrome (ARDS).

DESIGN

Analysis of immunoglobulin G (IgG) in BAL fluid by electrophoresis followed by immunoblotting and characterization of immunoglobulins as antilipid autoantibodies.

SETTING

Intensive care unit of a university hospital and two research university laboratories.

SUBJECTS

Twenty-seven mechanically ventilated patients in total, including nine patients with ARDS and two control groups.

INTERVENTIONS

Patients were ventilated with a mechanical ventilation mode. Six aliquots of 20-mL sterile normal saline at 37 degrees C were infused through the working channel of the bronchoscope.

MEASUREMENTS

Total protein, detection of IgG by electrophoresis followed by immunoblotting, and characterization of IgG by enzyme-linked immunosorbent assay using different lipids as target antigens.

MAIN RESULTS

Antiphospholipid autoantibodies are present in BAL fluid of ARDS patients. Among the phospholipids tested, phosphatidic acid and phosphatidylserine gave the most significant activity. The IgG fraction, purified from BAL fluids by affinity chromatography, gave the same pattern of binding as that of the BAL fluid.

CONCLUSION

The presence of antiphospholipid autoantibodies in BAL fluid suggests involvement of autoimmune mechanisms in the pathogenesis of ARDS.

The characteristics of bronchoalveolar lavage from a patient with antiphospholipid syndrome who developed acute respiratory distress syndrome

The purpose of this study was to investigate the biochemical characteristics as well as the occurrence and specificity of antiphospholipid antibodies in the bronchoalveolar lavage (BAL) fluid from a patient with both antiphospholipid antibodies syndrome (APS) and acute respiratory distress syndrome (ARDS). Proteins, lipids, cells and autoantibodies were determined. Immunoglobulins were purified with affinity chromatography. Autoantibody identification was assessed with enzyme-linked immunosorbent assay (ELISA) and with electrophoresis, followed by immunoblotting and revelation with antihuman IgG-peroxidase conjugate. Antiphospholipid antibodies were found to be present in the BAL fluid as well as in the serum from a patient with APS. Specifically, antiphosphatidylserine and antiphosphatidic acid IgG antibodies in the BAL fluid and antiphosphatidylcholine and anticardiolipin IgG antibodies in the serum were detected at high levels. BAL fluid protein and the percentage of neutrophils were found to be increased. A quantitative as well as qualitative deficiency of surfactant phospholipids was also observed. Antibodies directed against surfactant phospholipids could cause surfactant abnormalities and an inflammatory reaction. These disorders may be one of the causes of the ARDS or a factor in the perpetuation of the inflammation.

Phosphatidylserine delivery to endoplasmic reticulum-derived vesicles of plant cells depends on two biosynthetic pathways

Vesicles formed from endoplasmic reticulum (ER) by a cell-free system of leek cells (Allium porrum) are enriched in phosphatidylserine (PS), especially species containing very long chain fatty acids (VLCFA, at least 20 carbon atoms). In plant cells, PS is formed either by PS synthase or the serine exchange enzyme, although it is not known which pathway(s) contribute(s) to PS delivery in the ER-derived vesicles (EV), nor to what extent this occurs. Taking advantage of a cell-free system, we have shown that PS enrichment originates mainly from the serine exchange enzyme which is the only pathway that synthesizes the VLCFA-PS species. On the other hand, both enzymes synthesize PS with long chain fatty acids (up to 18 carbon atoms), but these species are given to the EV by PS synthase.

One of the origins of plasma membrane phosphatidylserine in plant cells is a local synthesis by a serine exchange activity

In plant cells, as in animal cells, the endoplasmic reticulum (ER) is considered to be the major site of phospholipid synthesis, and it has been shown that phosphatidylserine (PS) reaches the plasma membrane via the vesicular ER-Golgi-plasma membrane pathway in leek cells. However, it has never been determined whether the plasma membrane of leek cells is able to synthesize PS. We have analyzed the distribution of PS synthesizing enzymes along the vesicular pathway. In ER, Golgi and plasma membrane fractions isolated from leek cells, we have measured the activity of the two biosynthetic pathways leading to the synthesis of PS, i.e. serine exchange and CTP cytidylyltransferase plus PS synthase. We have found a high serine exchange activity in the plasma membrane fraction, and then determined that this membrane is able to synthesize both long chain fatty acid- and very long chain fatty acid-containing PS. Therefore, the PS in the plasma membrane of leek cells has two different origins: the intracellular vesicular pathway from the ER and a local synthesis in the plasma membrane.

Immunocytochemistry of lipids: chemical fixatives have dramatic effects on the preservation of tissue lipids

We report here the effects of chemical fixatives on lipids studied under conditions simulating the immunogold labelling of phosphatidylserine. Using anti-phosphatidylserine antibodies, it is shown that the labelling intensity of a phosphatidylserine/phosphatidylcholine coating depends largely on the conditions of fixation. In fact, the usual aldehydic fixatives washed out most of the phostphatidylserine, thus preventing the binding of anti-phosphatidylserine antibodies. This was confirmed on biological samples such as rat liver and brain by measuring the loss of radiolabelled lipids during the fixation procedure. Furthermore, the complete procedure of tissue preparation for electron microscopical observation was investigated. The loss of (radiolabelled) lipids was studied in tissue samples during fixation and resin embedding. The results showed that the classical procedure (glutaraldehyde fixation followed by epoxy resin embedding) results in the loss of 73-91% of the tissue lipids whereas in unfixed, freeze-substituted samples, more than 76% of the tissue lipids are preserved.

ATP-Dependent formation of phosphatidylserine-rich vesicles from the endoplasmic reticulum of leek cells

Leek (Allium porrum) plasma membrane is enriched in phosphatidylserine (PS) by the vesicular pathway, in a way similar to that already observed in animal cells (B. Sturbois-Balcerzak, D.J. Morre, O. Loreau, J.P. Noel, P. Moreau, C. Cassagne [1995] Plant Physiol Biochem 33: 625-637). In this paper we document the formation of PS-rich small vesicles from leek endoplasmic reticulum (ER) membranes upon addition of ATP and other factors. The omission of ATP or its replacement by ATPgamma-S prevents vesicle formation. These vesicles correspond to small structures (70-80 nm) and their phospholipid composition, characterized by a PS enrichment, is compatible with a role in PS transport. Moreover, the PS enrichment over phosphatidylinositol in the ER-derived vesicles is the first example, to our knowledge, of phospholipid sorting from the ER to ER-derived vesicles in plant cells.

Antibodies to long-chain acyl-CoAs. A new tool for lipid biochemistry

Antibodies directed against long chain acyl-CoAs (having 16 and 18 carbon atoms) have been prepared and are reported for the first time. A modified ELISA procedure adapted to these amphiphilic molecules has been developed: it is a rapid, simple and sensitive test permitting to detect as little as 3 pmol of acyl-CoA. These antibodies represent a new tool for studying long-chain acyl-CoAs. Their use in an immunochemical approach for the study of protein-acyl-CoA interactions is presented.

Immunogold labelling of fatty acyl chains

For the first time, antibodies against a hydrophobic hapten have been used for immunogold labelling of a lipid antigen (BSA-C18:1 conjugate) coated on polystyrene. The labelling was visualised either directly in transmission electron microscopy or in light microscopy after silver enhancement. Good recognition of the fatty acyl chain was obtained even after treatment of the antigen coat with various cross-linking fixatives used for electron microscopy, i.e. formaldehyde, glutaraldehyde and osmium tetroxide.

Cell-free transfer of phospholipids between the endoplasmic reticulum and the Golgi apparatus of leek seedlings

The transfer of lipids between the endoplasmic reticulum and the Golgi apparatus was investigated in vitro using a cell-free system from leek seedlings. Lipids of the donor membranes (endoplasmic reticulum) were radiolabeled either by incubating leek seedlings with [1-14C]acetate or [3H]acetate. Acceptor membranes (Golgi apparatus) were unlabeled and immobilized on nitrocellulose strips. The assay measured the lipid transfer resulting from both an ATP-independent process and an ATP- and cytosol-dependent process. A significant ATP- and cytosol-dependent lipid transfer was observed only in the case of the endoplasmic reticulum as donor and the Golgi apparatus as acceptor. Lipids transferred in an ATP-dependent manner were chiefly phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine. The stimulation of lipid transfer by ATP as compared to the ATP-independent process was +79% (PC), +123% (PS) and +69% (PE). On the other hand, PI was not transferred in an ATP-dependent manner (the stimulation by ATP was only 20%). This supports the theory that a sorting of phospholipids takes place in the donor membrane. Moreover, a formation of lysoPC was observed only in the presence of ATP (+330%). The ATP-dependent lipid transfer was inhibited by N-ethylmaleimide, indicating the involvement of cytosolic (but no phospholipid transfer proteins) or membrane proteins in the transfer process. The ATP-dependent transfer of lipids was also diminished at 12 degrees C showing the sensitivity to low temperatures of the transfer of lipids between the endoplasmic reticulum and the Golgi apparatus.

Demonstration that anti-phospholipid auto-antibodies react with both anionic and zwitterionic phospholipids

The specificity of anti-phospholipid auto-antibodies present in the serum of systemic lupus erythematosus patients towards the phospholipids has been studied by two different methods. The antibodies have been characterized either after affinity purification or by inhibition experiments. Our results clearly demonstrate that anti-phospholipid auto-antibodies recognize all phospholipids, whatever their polar head.

Fatty acids rendered immunogenic

A method allowing the covalent binding of fatty acids to a carrier was developed. The absence of absorbed fatty acids was controlled by HPTLC. Rabbits were immunized with these immunogens. The antibodies obtained were studied by ELISA and were shown to be directed against the fatty acyl chain, whatever its length.

Synthesis of a monoclonal antibody-indium-111-porphyrin conjugate

Antibodies were labelled with indium-111 with a view to their use in the radio-immunodetection of cancers. The covalent coupling between indium-111 porphyrin and monoclonal antibodies (IgG and F(ab')2 fragment) was achieved using the ester activated method [N-hydroxy-succinimide/1-ethyl-3-(3-dimethylaminopropyl)carbodiimide]. After purification, this provided conjugated with specific activities of 6 muCi/micrograms Mab (9.3 molecules per Mab) or 1 muCi/microgram (F(ab')2 fragment (1.5 molecule per F(ab')2). ELISA procedures suggested the full retention of immunoreactivity by the radiolabelled antibodies.

Autoantibodies against all the phospholipids: a comparative systematic study with systemic lupus erythematosus and healthy sera

The sera of systemic lupus erythematosus patients were tested by ELISA for the presence of autoantibodies against all the phospholipids: cardiolipin, phosphatidic acid, phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol and phosphatidylserine. The quantity of phospholipid coated (in comparison with that initially deposited) on the microtitration plates was precisely evaluated in order to determine if the results (absorbance values) obtained for each phospholipid could be compared directly. The systemic lupus erythematosus sera tested gave positive results for all the phospholipids. The highest level of autoantibodies was observed with phosphatidic acid followed by phosphatidylserine, phosphatidylinositol, cardiolipin, phosphatidylglycerol, phosphatidylcholine and phosphatidylethanolamine. The sera seemed to contain antibodies directed either against all the 8 phospholipids tested or more specifically against one or two phospholipids. The results were compared with those obtained with 17 healthy sera. Much lower values were obtained for the sera of healthy subjects, the majority of which showed a weak binding, similar for all the phospholipids. These results suggest that the anti-phospholipid autoantibodies present in systemic lupus erythematosus sera are significantly higher than those of healthy subjects. It is concluded that in the investigation of anti-phospholipid antibodies, tests should be carried out for all the phospholipids.

Specific immunocytochemical visualization of phosphatidylserine

In order to determine whether anti-phosphatidylserine antibodies are able to detect phosphatidylserine in situ, an immunocytochemical approach has been developed using human platelets. A strong positive reaction was obtained when anti-phosphatidylserine serum was applied to platelets whereas no reaction was observed with preimmune serum, or with immune serum which had been preincubated with 10(-5) M phosphatidylserine, suggesting that phosphatidylserine was indeed specifically detected by the antibodies.

Demonstration of high specificity antibodies against phosphatidylserine

Anti-phosphatidylserine antibodies were raised in rabbits immunized with phosphatidylserine-polyacrylamide gels and with phosphatidylserine-cytochrome c vesicles. A solid-phase immunoenzymatic assay (ELISA) was developed to study the immune serum. The optimal conditions were defined and the technique used to obtain quantitative results. The anti-phosphatidylserine serum diluted to 1/1000 was tested against several phospholipids and was found to be highly specific to phosphatidylserine. This represents the first experimental demonstration of the specificity of antibodies raised against phosphatidylserine.

Natural seric anti-fatty acid antibodies in multiple sclerosis

Naturally occurring anti-oleic acid conjugate antibodies were detected in human sera using an adapted direct immunoenzymatic assay. They were present to a higher level in the sera of patients with multiple sclerosis in acute relapse compared to patients with other neurological diseases or healthy subjects and even patients with multiple sclerosis in progressive form.

Membrane labeling of the cortex of 'aging' rats by anti-fatty acid antibodies

The cortex of 'aging' rat brain exhibits no significant difference in the lipid and fatty acid composition in comparison with that of control rat brain, except for a lowered value of the cholesterol/phospholipids ratio. However, an antiserum raised against oleic acid, specifically labels neurones of the cortex of the 'aging' rat mainly within layers IV and V. An electron microscopical study revealed that immunoreactivity was associated with cytoplasmic vesicular inclusions (lipofuscin) and with membranes. Thus, these anti-fatty acid antibodies may help in the evaluation of local modifications of membranes which are not predictable on the basis of biochemical analysis of lipids.

Antibodies to histamine. Specificity studies and radioimmunological assay

Antibodies against conjugated histamine were raised in rabbits. This amine was coupled to different protein carriers by a bifunctional agent, hexamethylene diisocyanate. The specificity of the antibodies was determined with radioimmunological tests in equilibrium dialysis using an iodinated ligand: 125I-labelled histamine-hexamethylene diisocyanate-glycyl-tyrosine. The latter mimicked the antigenic determinant present in immunogens. Competition experiments were established between the radiolabelled ligand and conjugated histamine, conjugated analogs or unconjugated histamine. Cross-reactivity ratios and affinity constants were calculated from displacement curves, thereby allowing the antibody site to be characterized. The antibodies were found to be highly specific and were used for the assay of histamine in biological samples. For this, polystyrene beads coated with purified antiserum were used to establish a simple and reproducible test.