Separation of phosphoinositides and other phospholipids by two-dimensional thin-layer chromatography

Molecular basis of product recognition during PIP5K-mediated production of PI(4,5)P2 with positive feedback

The ability for cells to localize and activate peripheral membrane-binding proteins is critical for signal transduction. Ubiquitously important in these signaling processes are phosphatidylinositol phosphate (PIP) lipids, which are dynamically phosphorylated by PIP lipid kinases on intracellular membranes. Functioning primarily at the plasma membrane, phosphatidylinositol-4-phosphate 5-kinases (PIP5K) catalyzes the phosphorylation of PI(4)P to generate most of the PI(4,5)P2 lipids found in eukaryotic plasma membranes. Recently, we determined that PIP5K displays a positive feedback loop based on membrane-mediated dimerization and cooperative binding to its product, PI(4,5)P2. Here, we examine how two motifs contribute to PI(4,5)P2 recognition to control membrane association and catalysis of PIP5K. Using a combination of single molecule TIRF microscopy and kinetic analysis of PI(4)P lipid phosphorylation, we map the sequence of steps that allow PIP5K to cooperatively engage PI(4,5)P2. We find that the specificity loop regulates the rate of PIP5K membrane association and helps orient the kinase to more effectively bind PI(4,5)P2 lipids. After correctly orienting on the membrane, PIP5K transitions to binding PI(4,5)P2 lipids near the active site through a motif previously referred to as the substrate or PIP-binding motif (PIPBM). The PIPBM has broad specificity for anionic lipids and serves a role in regulating membrane association in vitro and in vivo. Overall, our data supports a two-step membrane-binding model where the specificity loop and PIPBM act in concert to help PIP5K orient and productively engage anionic lipids to drive the positive feedback during PI(4,5)P2 production.

Molecular mechanisms of spontaneous curvature and softening in complex lipid bilayer mixtures

Membrane reshaping is an essential biological process. The chemical composition of lipid membranes determines their mechanical properties and thus the energetics of their shape. Hundreds of distinct lipid species make up native bilayers, and this diversity complicates efforts to uncover what compositional factors drive membrane stability in cells. Simplifying assumptions, therefore, are used to generate quantitative predictions of bilayer dynamics based on lipid composition. One assumption commonly used is that "per lipid" mechanical properties are both additive and constant-that they are an intrinsic property of lipids independent of the surrounding composition. Related to this is the assumption that lipid bulkiness, or "shape," determines its curvature preference, independently of context. In this study, all-atom molecular dynamics simulations on three separate multilipid systems were used to explicitly test these assumptions, applying methodology recently developed to isolate properties of single lipids or nanometer-scale patches of lipids. The curvature preference experienced by populations of lipid conformations were inferred from their redistribution on a dynamically fluctuating bilayer. Representative populations were extracted by both structural similarity and semi-automated hidden Markov model analysis. The curvature preferences of lipid dimers were then determined and compared with an additive model that combines the monomer curvature preference of both the individual lipids. In all three systems, we identified conformational subpopulations of lipid dimers that showed non-additive curvature preference, in each case mediated by a special chemical interaction (e.g., hydrogen bonding). Our study highlights the importance of specific chemical interactions between lipids in multicomponent bilayers and the impact of interactions on bilayer stiffness. We identify two mechanisms of bilayer softening: diffusional softening, driven by the dynamic coupling between lipid distributions and membrane undulations, and conformational softening, driven by the inter-conversion between distinct dimeric conformations.

Membrane-mediated dimerization potentiates PIP5K lipid kinase activity

The phosphatidylinositol 4-phosphate 5-kinase (PIP5K) family of lipid-modifying enzymes generate the majority of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] lipids found at the plasma membrane in eukaryotic cells. PI(4,5)P2 lipids serve a critical role in regulating receptor activation, ion channel gating, endocytosis, and actin nucleation. Here, we describe how PIP5K activity is regulated by cooperative binding to PI(4,5)P2 lipids and membrane-mediated dimerization of the kinase domain. In contrast to constitutively dimeric phosphatidylinositol 5-phosphate 4-kinase (PIP4K, type II PIPK), solution PIP5K exists in a weak monomer-dimer equilibrium. PIP5K monomers can associate with PI(4,5)P2-containing membranes and dimerize in a protein density-dependent manner. Although dispensable for cooperative PI(4,5)P2 binding, dimerization enhances the catalytic efficiency of PIP5K through a mechanism consistent with allosteric regulation. Additionally, dimerization amplifies stochastic variation in the kinase reaction velocity and strengthens effects such as the recently described stochastic geometry sensing. Overall, the mechanism of PIP5K membrane binding creates a broad dynamic range of lipid kinase activities that are coupled to the density of PI(4,5)P2 and membrane-bound kinase.

Nir1 constitutively localizes at ER-PM junctions and promotes Nir2 recruitment for PIP2 homeostasis

Homeostatic regulation of plasma membrane (PM) phosphatidylinositol 4,5-bisphosphate (PIP₂) in receptor-stimulated cells is mediated by the lipid transfer protein Nir2. Nir2 is dynamically recruited to endoplasmic reticulum–plasma membrane (ER–PM) junctions to facilitate replenishment of PM PIP₂ hydrolyzed during receptor-mediated signaling. However, our knowledge regarding the activation and sustainment of Nir2-mediated replenishment of PM PIP₂ is limited. Here, we describe the functions of Nir1 as a positive regulator of Nir2 and PIP₂ homeostasis. In contrast to the family proteins Nir2 and Nir3, Nir1 constitutively localizes at ER–PM junctions. Nir1 potentiates Nir2 targeting to ER–PM junctions during receptor-mediated signaling and is required for efficient PM PIP₂ replenishment. Live-cell imaging and biochemical analysis reveal that Nir1 interacts with Nir2 via a region between the FFAT motif and the DDHD domain. Combined, results from this study identify Nir1 as an ER–PM junction localized protein that promotes Nir2 recruitment for PIP₂ homeostasis.

Quantification of phosphoinositides reveals strong enrichment of PIP2 in HIV-1 compared to producer cell membranes

Human immunodeficiency virus type 1 (HIV-1) acquires its lipid envelope during budding from the plasma membrane of the host cell. Various studies indicated that HIV-1 membranes differ from producer cell plasma membranes, suggesting budding from specialized membrane microdomains. The phosphoinositide PI(4,5)P₂ has been of particular interest since PI(4,5)P₂ is needed to recruit the viral structural polyprotein Gag to the plasma membrane and thus facilitates viral morphogenesis. While there is evidence for an enrichment of PIP₂ in HIV-1, fully quantitative analysis of all phosphoinositides remains technically challenging and therefore has not been reported, yet. Here, we present a comprehensive analysis of the lipid content of HIV-1 and of plasma membranes from infected and non-infected producer cells, resulting in a total of 478 quantified lipid compounds, including molecular species distribution of 25 different lipid classes. Quantitative analyses of phosphoinositides revealed strong enrichment of PIP₂, but also of PIP₃, in the viral compared to the producer cell plasma membrane. We calculated an average of ca. 8,000 PIP₂ molecules per HIV-1 particle, three times more than Gag. We speculate that the high density of PIP₂ at the HIV-1 assembly site is mediated by transient interactions with viral Gag polyproteins, facilitating PIP₂ concentration in this microdomain. These results are consistent with our previous observation that PIP₂ is not only required for recruiting, but also for stably maintaining Gag at the plasma membrane. We believe that this quantitative analysis of the molecular anatomy of the HIV-1 lipid envelope may serve as standard reference for future investigations.

The F-BAR domains from srGAP1, srGAP2 and srGAP3 regulate membrane deformation differently

Coordination of membrane deformation and cytoskeletal dynamics lies at the heart of many biological processes critical for cell polarity, motility and morphogenesis. We have recently shown that Slit-Robo GTPase-activating protein 2 (srGAP2) regulates neuronal morphogenesis through the ability of its F-BAR domain to regulate membrane deformation and induce filopodia formation. Here, we demonstrate that the F-BAR domains of two closely related family members, srGAP1 and srGAP3 [designated F-BAR(1) and F-BAR(3), respectively] display significantly different membrane deformation properties in non-neuronal COS7 cells and in cortical neurons. F-BAR(3) induces filopodia in both cell types, though less potently than F-BAR(2), whereas F-BAR(1) prevents filopodia formation in cortical neurons and reduces plasma membrane dynamics. These three F-BAR domains can heterodimerize, and they act synergistically towards filopodia induction in COS7 cells. As measured by fluorescence recovery after photobleaching, F-BAR(2) displays faster molecular dynamics than F-BAR(3) and F-BAR(1) at the plasma membrane, which correlates well with its increased potency to induce filopodia. We also show that the molecular dynamic properties of F-BAR(2) at the membrane are partially dependent on F-Actin. Interestingly, acute phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] depletion in cells does not interfere with plasma membrane localization of F-BAR(2), which is compatible with our result showing that F-BAR(2) binds to a broad range of negatively-charged phospholipids present at the plasma membrane, including phosphatidylserine (PtdSer). Overall, our results provide novel insights into the functional diversity of the membrane deformation properties of this subclass of F-BAR-domains required for cell morphogenesis.

Lipid analysis by thin-layer chromatography--a review of the current state

High-performance thin-layer chromatography (HPTLC) is a widely used, fast and relatively inexpensive method of separating complex mixtures. It is particularly useful for smaller, apolar compounds and offers some advantages over HPLC. This review gives an overview about the special features as well as the problems that have to be considered upon the HPTLC analysis of lipids. The term "lipids" is used here in a broad sense and comprises fatty acids and their derivatives as well as substances related biosynthetically or functionally to these compounds. After a short introduction regarding the stationary phases and the methods how lipids can be visualized on an HPTLC plate, the individual lipid classes will be discussed and the most suitable solvent systems for their separation indicated. The focus will be on lipids that are most abundant in biological systems, i.e. cholesterol and its derivates, glycerides, sphingo- and glycolipids as well as phospholipids. Finally, a nowadays very important topic, the combination between HPTLC and mass spectrometric (MS) detection methods will be discussed. It will be shown that this is a very powerful method to investigate the identities of the HPTLC spots in more detail than by the use of common staining methods. Future aspects of HPTLC in the lipid field will be also discussed.

Regulation of the actin cytoskeleton-plasma membrane interplay by phosphoinositides

The plasma membrane and the underlying cortical actin cytoskeleton undergo continuous dynamic interplay that is responsible for many essential aspects of cell physiology. Polymerization of actin filaments against cellular membranes provides the force for a number of cellular processes such as migration, morphogenesis, and endocytosis. Plasma membrane phosphoinositides (especially phosphatidylinositol bis- and trisphosphates) play a central role in regulating the organization and dynamics of the actin cytoskeleton by acting as platforms for protein recruitment, by triggering signaling cascades, and by directly regulating the activities of actin-binding proteins. Furthermore, a number of actin-associated proteins, such as BAR domain proteins, are capable of directly deforming phosphoinositide-rich membranes to induce plasma membrane protrusions or invaginations. Recent studies have also provided evidence that the actin cytoskeleton-plasma membrane interactions are misregulated in a number of pathological conditions such as cancer and during pathogen invasion. Here, we summarize the wealth of knowledge on how the cortical actin cytoskeleton is regulated by phosphoinositides during various cell biological processes. We also discuss the mechanisms by which interplay between actin dynamics and certain membrane deforming proteins regulate the morphology of the plasma membrane.

Electrostatic sequestration of PIP2 on phospholipid membranes by basic/aromatic regions of proteins

The basic effector domain of myristoylated alanine-rich C kinase substrate (MARCKS), a major protein kinase C substrate, binds electrostatically to acidic lipids on the inner leaflet of the plasma membrane; interaction with Ca2+/calmodulin or protein kinase C phosphorylation reverses this binding. Our working hypothesis is that the effector domain of MARCKS reversibly sequesters a significant fraction of the L-alpha-phosphatidyl-D-myo-inositol 4,5-bisphosphate (PIP2) on the plasma membrane. To test this, we utilize three techniques that measure the ability of a peptide corresponding to its effector domain, MARCKS(151-175), to sequester PIP2 in model membranes containing physiologically relevant fractions (15-30%) of the monovalent acidic lipid phosphatidylserine. First, we measure fluorescence resonance energy transfer from Bodipy-TMR-PIP2 to Texas Red MARCKS(151-175) adsorbed to large unilamellar vesicles. Second, we detect quenching of Bodipy-TMR-PIP2 in large unilamellar vesicles when unlabeled MARCKS(151-175) binds to vesicles. Third, we identify line broadening in the electron paramagnetic resonance spectra of spin-labeled PIP2 as unlabeled MARCKS(151-175) adsorbs to vesicles. Theoretical calculations (applying the Poisson-Boltzmann relation to atomic models of the peptide and bilayer) and experimental results (fluorescence resonance energy transfer and quenching at different salt concentrations) suggest that nonspecific electrostatic interactions produce this sequestration. Finally, we show that the PLC-delta1-catalyzed hydrolysis of PIP2, but not binding of its PH domain to PIP2, decreases markedly as MARCKS(151-175) sequesters most of the PIP2.

Phospholipid composition of cell-derived microparticles determined by one-dimensional high-performance thin-layer chromatography

Microparticles in the circulation activate the coagulation system and may activate the complement system via C-reactive protein upon conversion of membrane phospholipids by phospholipases. We developed a sensitive and reproducible method to determine the phospholipid composition of microparticles. Samples were applied to horizontal, one-dimensional high-performance thin-layer chromatography (HPTLC). Phospholipids were separated on HPTLC by chloroform:ethyl acetate:acetone:isopropanol:ethanol:methanol:water:acetic acid (30:6:6:6:16:28:6:2); visualized by charring with 7.5% Cu-acetate (w/v), 2.5% CuSO(4) (w/v), and 8% H(3)PO(4) (v/v) in water; and quantified by photodensitometric scanning. Erythrocyte membranes were used to validate the HPTLC system. Microparticles were isolated from plasma of healthy individuals (n = 10). On HPTLC, mixtures of (purified) phospholipids, i.e., lysophosphatidylcholine, phosphatidylcholine (PC), sphingomyelin (SM), lysophosphatidylserine, phosphatidylserine, lysophosphatidylethanolamine, phosphatidylethanolamine (PE), and phosphatidylinositol, could be separated and quantified. All phospholipids were detectable in erythrocyte ghosts, and their quantities fell within ranges reported earlier. Quantitation of phospholipids, including extraction, was highly reproducible (CV < 10%). Microparticles contained PC (59%), SM (20.6%), and PE (9.4%), with relatively minor (<5%) quantities of other phospholipids. HPTLC can be used to study the phospholipid composition of cell-derived microparticles and may also be a useful technique for the analysis of other samples that are available only in minor quantities.

Fluorescently labeled neomycin as a probe of phosphatidylinositol-4, 5-bisphosphate in membranes

Phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)), a minor component of the plasma membrane, is important in signal transduction, exocytosis, and ion channel activation. Thus fluorescent probes suitable for monitoring the PI(4,5)P(2) distribution in living cells are valuable tools for cell biologists. We report here three experiments that show neomycin labeled with either fluorescein or coumarin can be used to detect PI(4,5)P(2) in model phospholipid membranes. First, addition of physiological concentrations of PI(4,5)P(2) (2%) to lipid vesicles formed from mixtures of phosphatidylcholine (PC) and phosphatidylserine (PS) enhances the binding of labeled neomycin significantly (40-fold for 5:1 PC/PS vesicles). Second, physiological concentrations of inositol-1,4,5-trisphosphate (10 microM I(1,4,5)P(3)) cause little translocation of neomycin from PC/PS/PI(4,5)P(2) membranes to the aqueous phase, whereas the same concentrations of I(1,4,5)P(3) cause significant translocation of the green fluorescent protein/phospholipase C-delta pleckstrin homology (GFP-PH) constructs from membranes (Hirose et al., Science, 284 (1999) 1527). Third, fluorescence microscopy observations confirm that one can distinguish between PC/PS vesicles containing either 0 or 2% PI(4, 5)P(2) by exposing a mixture of the vesicles to labeled neomycin. Thus fluorescently labeled neomycin could complement GFP-PH constructs to investigate the location of PI(4,5)P(2) in cell membranes.

Quantitative Analysis of Apoptotic Cell Death Using Proton Nuclear Magnetic Resonance Spectroscopy

Quantification of apoptotic cell death in vivo has become an important area of investigation in patients with acute lymphoblastic leukemia (ALL). We have devised a noninvasive analytical method to estimate the percentage of apoptotic lymphoblasts in doxorubicin-treated Jurkat T-cell ALL cultures, using proton nuclear magnetic resonance spectroscopy (1H NMR). We have found that the ratio of the methylene (CH2 ) resonance (at 1.3 ppm) to the methyl (CH3 ) resonance (at 0.9 ppm) signal intensity, as observed by 1H NMR, is directly proportional to the percentage of apoptotic lymphoblasts in vitro. The correlation between the CH2/CH3 signal intensity ratio and the percentage of apoptotic lymphoblasts was optimal 24 to 28 hours after doxorubicin treatment (r2 = .947, N = 27 samples). There was also a direct temporal relationship between an increase in the CH2/CH3 signal intensity ratio and the onset of apoptosis as detected by nuclear morphologic analysis, fluorescein-annexin V flow cytometry, and DNA gel electrophoresis. Thin-layer chromatography confirmed that a dynamic and/or compositional change of the plasma membrane, rather than increases in lipase activity or fatty acid production, appears to account for the increase in the CH2/CH3 signal intensity ratio during apoptosis. 1H NMR may have clinical utility for the early noninvasive assessment of chemotherapeutic efficacy in patients with ALL.

Quantitative Analysis of Apoptotic Cell Death Using Proton Nuclear Magnetic Resonance Spectroscopy

Quantification of apoptotic cell death in vivo has become an important area of investigation in patients with acute lymphoblastic leukemia (ALL). We have devised a noninvasive analytical method to estimate the percentage of apoptotic lymphoblasts in doxorubicin-treated Jurkat T-cell ALL cultures, using proton nuclear magnetic resonance spectroscopy (1H NMR). We have found that the ratio of the methylene (CH2 ) resonance (at 1.3 ppm) to the methyl (CH3 ) resonance (at 0.9 ppm) signal intensity, as observed by 1H NMR, is directly proportional to the percentage of apoptotic lymphoblasts in vitro. The correlation between the CH2/CH3 signal intensity ratio and the percentage of apoptotic lymphoblasts was optimal 24 to 28 hours after doxorubicin treatment (r2 = .947, N = 27 samples). There was also a direct temporal relationship between an increase in the CH2/CH3 signal intensity ratio and the onset of apoptosis as detected by nuclear morphologic analysis, fluorescein-annexin V flow cytometry, and DNA gel electrophoresis. Thin-layer chromatography confirmed that a dynamic and/or compositional change of the plasma membrane, rather than increases in lipase activity or fatty acid production, appears to account for the increase in the CH2/CH3 signal intensity ratio during apoptosis. 1H NMR may have clinical utility for the early noninvasive assessment of chemotherapeutic efficacy in patients with ALL.

Plasma fatty acid levels in South African interethnic male high school pupils at different ultimate risks of coronary heart disease

While coronary heart disease (CHD) is responsible for about a quarter of deaths in South African white, coloured and Indian populations, the rate is very low in the black population. Dietary risk factors for CHD include high intakes of saturated fatty acids and cholesterol, with low intakes of mono and polyunsaturated fatty acids. In a characterization study, plasma fatty acid profiles have been determined in seven groups of high school pupils aged 16-18 years, namely, white, rural black, urban black and middle-class and lower socio-economic class coloureds and Indians. Plasma fatty acids were measured by gas liquid chromatography in 20 subjects from each group. Contrary to expectation, urban and rural black pupils had the highest mean molar composition of saturated fatty acids, largely myristic (14:0) and palmitic (16:0) acids. Rural blacks and middle-class coloureds had the highest molar percentage composition of monounsaturated fatty acids, primarily oleic acid (18:1). Again unexpectedly, white, coloured and Indian groups had the highest level of polyunsaturated fatty acids, due entirely to linoleic acid (18:2). As the findings are unrelated to proneness to CHD mortality in the particular communities studied, plasma fatty acid levels are not predictive of the disease. Epidemiologically, there are many examples of lack of correlation in serum lipid levels and proneness to CHD.

Bicarbonate/CO2 induces rapid activation of phospholipase A2 and renders boar spermatozoa capable of undergoing acrosomal exocytosis in response to progesterone

We tested whether capacitation, a phenomenon that renders spermatozoa capable of undergoing acrosomal exocytosis, may be completed rapidly after a short exposure to bicarbonate/CO2. We found that, in the presence of Ca2+, a 10-min exposure of boar spermatozoa to bicarbonate led to a partial activation of phospholipase A2, primed spermatozoa for a major subsequent activation of this enzyme upon stimulation with progesterone and furthermore rendered spermatozoa capable of undergoing exocytosis in response to this steroid. These results suggest that capacitation may be completed in a relatively short period of time and open up new possibilities for unravelling molecular mechanisms underlying this process.

Marked enrichment of the alkenylacyl subclass of plasma ethanolamine glycerophospholipid with eicosapentaenoic acid in human subjects consuming a fish oil concentrate

Alteration in human platelet fatty acid levels with the consumption of fish oils containing eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) have been well documented, but changes in the fatty acid composition of plasma plasmalogenic phospholipid under similar circumstances have not been delineated. In the present study, subjects consumed the fish oil concentrate (MaxEPA) for 6 wk followed immediately by a 6-wk recovery period with no fish oil ingestion. Plasma total choline glycerophospholipid (GPC) and ethanolamine glycerophospholipid (GPE) subclasses isolated from blood samples obtained at 0, 3, 6, 9 and 12 wk of the experimental period were analyzed for fatty acid composition via thin-layer and gas-liquid chromatographic techniques. Consumption of fish oil for 3 or 6 wk significantly elevated the content of n-3 fatty acids while concomitantly decreasing n-6 fatty acid levels in plasma total GPC and in diacyl and alkenylacyl (plasmalogen) GPE. Alkenylacyl GPE exhibited the greatest alteration of both n-3 and n-6 fatty acid levels. Following 6 wk of supplementation with fish oil, EPA rose by 24.6 mol% in alkenylacyl GPE compared to increases of 6.7 and 7.1 mol% in diacyl GPE and total GPC, respectively. The increase in EPA (from 5.0 to 29.6 mol%) in plasma alkenylacyl GPE represents amongst the highest enrichment of EPA in any lipid yet reported in human subjects. DHA also rose by 8.0, 4.8, and 3.1 mol% in alkenylacyl GPE, diacyl GPE, and total GPC, respectively. Alkenylacyl GPE exhibited the greatest mol% decline (by 18.7 mol%) in arachidonic acid (AA, 20:4n-6) following 6 wk of fish oil supplementation. The corresponding decreases of AA in diacyl GPE and total GPC were 8.7 and 1.8 mol%, respectively. Following the 6 wk recovery period, n-3 and n-6 fatty acid levels had returned to pre-supplementation values. The marked enrichment of alkenylacyl GPE in n-3 fatty acids, especially EPA, may be of significance with respect to a unique role for this plasma phospholipid subclass in attenuating certain lipoprotein-mediated cardiovascular effects as observed with fish/fish oil consumption.

Epidermal growth factor stimulates hydrolysis of phosphatidylinositol 4,5-bisphosphate, generation of diacylglycerol and exocytosis in mouse spermatozoa

Mouse spermatozoa stimulated with epidermal growth factor (EGF) or zona pellucida (ZP) experienced phosphatidylinositol 4,5-bisphosphate hydrolysis, diacylglycerol (DAG) generation and acrosomal exocytosis. The agonists showed additive effects but the action of EGF is likely to be mediated by a distinct receptor because maximal stimulation achieved with EGF was enhanced further by ZP. Generation of DAG and exocytosis stimulated by EGF were inhibited by tyrphostin A48, indicating that tyrosine kinase activity mediates EGF action. On the other hand, pertussis toxin did not affect the EGF-induced formation of DAG or exocytosis, ruling out the involvement of sperm Gi-like proteins. These results indicate that EGF could be an important co-factor in the initiation of exocytosis in spermatozoa.

Wheat germ agglutinin stabilization of erythrocyte shape: role of bilayer balance and the membrane skeleton

The effects of wheat germ agglutinin (WGA), Limulus lectin, and concanavalin A on cell shape changes were examined in human erythrocytes. These agents inhibited echinocytosis in cells having elevated cytosolic Ca2+ or incorporated foreign phosphatidylcholine, but had no effect on cell stomatocytosis in response to incorporated phosphatidylserine. The role of the membrane skeleton in this selective membrane fixation was examined. WGA inhibited echinocytosis in cells previously depleted of polyphosphoinositides to reduce membrane skeleton binding to transmembrane proteins, treated with phorbol ester to enhance protein 4.1 phosphorylation, heat-treated to denature spectrin, alkylated with p-chloromercuribenzoate to dissociate glycophorin from the membrane skeleton, or subjected to elevated cell 2,3-diphosphoglycerate to alter organization of the spectrin-actin-protein 4.1 complex. Limulus lectin and increased concentrations of WGA also stabilized discoid shape in pronase-digested cells containing no detectable intact glycophorin. In contrast, cell digestion with sialidase abolished the shape-stabilizing effect of WGA. The results suggest that the membrane skeleton is not involved in WGA shape stabilization. Rather, they suggest that glycoproteins and glycolipids interact with the lectin to stabilize cell surface molecular associations, forming a superficial calyx that inhibits outward, but not inward, membrane bending.

Quantitation of the fatty acid composition of phosphatidic acid by capillary gas chromatography electron-capture detection with picomole sensitivity

We describe a relatively simple and sensitive method to measure femtomole amounts of phosphatidic acid in cells. Phosphatidic acid was extracted from cells in the presence of 1-heptadecanoyl-2-heptadecanoyl-sn-glycero-3-phosphate as an internal standard, purified by two-dimensional thin-layer chromatography, and hydrolyzed to its constituent free fatty acids which were then derivatized to the corresponding pentafluorobenzyl esters. Pentafluorobenzyl esters of fatty acids were analyzed by gas chromatography with electron-capture detection. Long-chain fatty acids were resolved with excellent signal-to-noise ratios. Using heptadecanoic acid as an internal standard for quantitation, as little as 1 fmol of pentafluorobenzyl ester of stearic acid was detected with a linear response up to 10 pmol. Linear detector responses were obtained for all major classes of fatty acids. For phosphatidic acid measurement, the detection limit was at least 50 fmol thus achieving a 1000-fold increase in sensitivity compared to the most sensitive of the previously described methods. An example is provided of quantitating phosphatidic acid from minute amounts of biological samples such as islets of Langerhans.

Protein kinase C in hydrozoans: involvement in metamorphosis of Hydractinia and in pattern formation of Hydra

A wealth of information has suggested the involvement of protein kinase C (PKC) in metamorphosis of Hydractinia echinata and in pattern formation of Hydra magnipapillata. We have identified a Ca²⁺- and phospholipid-dependent kinase activity in extracts of both species. The enzyme was characterized as being similar to mammalian PKC by ion exchange chromatography. Gel filtration experiments revealed a molecular weight of about 70 kD. In phosphorylation assays of endogenous Hydractinia proteins, a protein with a molecular weight of 22.5 kD was found to be phoshorylated upon addition of phosphatidylserine. Bacterial induction of metamorphosis of Hydractinia echinata caused an increase in endogenous diacylglycerol, the physiological activator of PKC, suggesting that the bacterial inducer acts by activating receptor-regulated phospholipid metabolism. Exogenous diacylglycerol leads to membrane translocation of PKC, indicative of an activation. On the basis of our results and those of Freeman and Ridgway (1990) a model for the biochemical events during metamorphosis is presented.

Erythrocyte band 3 protein strongly interacts with phosphoinositides

85% of the phosphorus coisolated with band 3 protein during separation of the intrinsic proteins of the human erythrocyte membrane by zonal electrophoresis in high concentrations of acetic acid was found to be derived from phosphoinositides, mainly phosphatidylinositol 4,5-bisphosphate. When native band 3 protein and pyrene-labelled phospholipids were present in micelles of the nonionic detergent nonaethyleneglycol lauryl ether, strong resonance energy transfer was observed between the tryptophan residues and phosphatidylinositol 4,5-bisphosphate and, to a smaller degree, phosphatidylinositol 4-phosphate. We conclude that band 3 protein strongly interacts with phosphoinositides, in particular with phosphatidylinositol 4,5-bisphosphate.

Hydrogen peroxide oxidation induces the transfer of phospholipids from the membrane into the cytosol of human erythrocytes

The effects of oxidative damage on membrane phospholipid organization were examined in human erythrocytes. Exposure to H2O2 induced shape changes in these cells; normal discocytes became echinocytic, and stomatocytes generated by foreign phosphatidylserine incorporation reverted to discoid morphology. H2O2 treatment also inhibited phosphatidylserine transport from the outer to inner membrane monolayer, consistent with earlier reports on oxidative sensitivity of the aminophospholipid translocator. The morphological changes are consistent with movement of inner monolayer lipids to the outer monolayer, as might be expected if aminophospholipid sequestration is compromised. However, lipid extraction and prothrombinase activation assays showed no increased exposure of phosphatidylserine on the cell surface. Instead, phosphatidylserine was found associated with the cytosolic fraction of H2O2-treated cells. These observations suggest that oxidative damage alters the lipid organization of erythrocyte membranes, not by randomizing the lipid classes within the bilayer, but by inducing extraction of inner monolayer components into the cytosol.

Phospholipase D and exocytosis of the ram sperm acrosome

We have investigated whether phospholipase D (PLD) is involved in events leading to acrosomal exocytosis. Ram spermatozoa pre-labelled with [3H]alkyl-lysophosphatidylcholine and stimulated with the ionophore A23187 (1 microM) and Ca2+ (3 mM) in the presence of ethanol, showed a slow time-dependent increase in [3H]phosphatidic acid and [3H]phosphatidylethanol (PEt), the latter being clear evidence of PLD activity. Unlabelled cells similarly treated underwent acrosomal exocytosis. However, [3H]PEt formation was inhibited by high Ca2+ concentrations, although such conditions result in maximal acrosomal exocytosis. Treatment with A23187/Ca2+ led to a fast generation of [3H]alkyl-diglyceride and an increase in 1,2-diacylglycerol mass, which preceded [3H]PEt formation. The rises in [3H]alkyl-diglyceride and 1,2-diacylglycerol mass took place regardless of the presence or absence of ethanol. Inclusion of propranolol, a phosphatidic acid phosphohydrolase inhibitor, did not affect the early rise of labelled or unlabelled 1,2-diglycerides either. Stimulation of spermatozoa with A23187/Ca2+ in the presence of either ethanol or propranolol did not affect the occurrence of acrosomal exocytosis. Taken together, these results indicate that although Ca2+ entry triggers a late activation of PLD, this enzyme is not involved in the early generation of diglycerides. Moreover, they suggest that PLD does not make a substantial contribution in events leading to exocytosis of the sperm acrosome. Therefore, generation of diglycerides may take place primarily via phospholipase C.

Role of cell cholesterol in modulating vincristine uptake and resistance

The relationship between cell-membrane permeability to vincristine and cholesterol/phospholipid levels was studied in L5178Y murine leukemic lymphoblasts and in 2 multidrug-resistant cell sublines, VCR/P60 and VCR/P200, which expressed increasing levels of vincristine resistance. The uptake of 3H-vincristine was measured in all cell lines and in cholesterol-depleted and -reloaded L5178Y and VCR/P200 cells. The initial rate of drug entry in resistant cells was lower than that measured in the parental cell line and it decreased as the relative resistance increased. An increment of cholesterol content, characterized in resistant cells, was directly proportional to the relative resistance to vincristine. Cholesterol depletion in both sensitive and resistant cells resulted in an increase in the rate of vincristine uptake, which reverted to the respective basal levels when each cell line was cholesterol-reloaded. The rate of drug uptake was inversely correlated with the molar ratio of cholesterol to phospholipids. Although both VCR/P cell sublines, but not the sensitive parental cells, expressed the P-glycoprotein in their plasma membrane, there were no differences in drug efflux and retention between resistant and parental cells. These results indicate that cholesterol modulates the permeation of vincristine through the plasma membrane and strongly suggest that increased levels of cholesterol/phospholipid account for the lower drug accumulation and greater resistance in these multidrug-resistant cells.

Analysis of complex mixtures of phospholipid classes from cell membranes using two-dimensional thin-layer chromatography and scanning laser densitometry

Increasing recognition of the important roles served by membrane phospholipids in cellular metabolic and signal transduction processes has stimulated interest in examining potential phospholipid abnormalities in patients with psychiatric disorders. This report describes a method, based on several novel modifications of existing techniques, for concurrently analyzing nanomolar amounts of nine phospholipid classes in a single aliquot of membrane extract. With this method, diverse phospholipid classes are first separated by two-dimensional thin-layer chromatography, and then determined using two-dimensional scanning laser densitometry. The method is able to quantitate even small amounts of specific phospholipid classes, corresponding to < 10 ng of lipid phosphorus. The sensitivity of this method allows it to be readily applied to clinical studies involving membranes from cell types that are obtainable only in small quantities.

Alteration of membrane fatty acid composition and inositol phosphate metabolism in HT-29 human colon cancer cells

The present study was designed to investigate the role of membrane fatty acid (FA) composition on inositol phosphate (InsP) release by a human colon tumor cell line. Cells were supplemented for five days in culture with 0, 10, 30, or 100 microM sodium stearate (18:0), linoleate [18:2(omega-6)], or linolineate [18:3(omega-3)]. These FAs were supplied as a complex with FA-free bovine serum albumin. InsP release was examined in these cells with or without stimulation with deoxycholic acid (DCA) after they were labeled with [3H]myoinositol. FA enrichment was found to influence inositol incorporation into membrane lipids. Although 18:0 had no effect, 18:2(omega-6) decreased the incorporation. On the other hand, 18:3(omega-3) increased the incorporation of inositol compared with the cells supplemented with the other FAs, but they were not different from control. Basal release of total InsP was elevated only with supplementation of 10 and 30 microM 18:3(omega-3). FA supplementation with 18:0 at 30 microM and 18:2 at 30 and 100 microM resulted in downregulation of bsal release of InsP. Enrichment of HT-29 cell membranes with polyunsaturated FAs resulted in a significant increase in stimulated release of InsP, but this was not seen with saturated FA supplementation. At 10 microM supplementation, 18:2 had the greatest effect on stimulated InsP release. This effect of 18:2 disappeared at 30 microM. However, the increase in the stimulated InsP release caused by 18:3 occurred at 10 and 30 microM. DCA-stimulated release of InsP was not downregulated by any FA supplementation. This study showed that enrichment of the membranes with polyunsaturated FAs increases the response of the phosphatidylinositol cycle to DCA stimulation. In addition, enrichment with 18:3(omega-3) increases the basal turnover of InsP. It is concluded that alteration of membrane FAs has a profound effect on the phosphatidylinositol cycle.

Platelet hyperreactivity in hyperlipidaemia with specific reference to platelet lipids and fatty acid composition

Platelet function after thrombin stimulation, the fatty acid composition of individual phospholipids, and levels of lipid components (cholesterol, cholesterol ester, phospholipids and triglycerides) were determined in total membranes of platelets from hyperlipidaemic (HL) and control subjects. Platelet aggregation, thromboxane (TX) B2 production and serotonin release was significantly greater in HL patients than in controls. Levels of platelet cholesterol, total phospholipids, cholesterol ester and triglycerides, were significantly higher in the HL patients. Small differences between the two groups were observed for the phospholipid fatty acid patterns. However, levels of arachidonic acid (AA) were significantly higher in phosphatidylinositol (PI) of HL patients (40.01 +/- 6.59 mol%) as compared to the controls (31.52 +/- 9.91 mol%) (P = 0.002). The higher levels of AA in PI, which is considered a donor pool for eicosanoid synthesis, may be an additional mechanism for the well documented platelet hyperfunction and greater TXB2 production in hyperlipidaemic subjects.

Phosphoinositide-specific phospholipase C-delta 1 binds with high affinity to phospholipid vesicles containing phosphatidylinositol 4,5-bisphosphate

We studied the binding of phosphoinositide-specific phospholipase C-delta 1 (PLC-delta) to vesicles containing the negatively charged phospholipids phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylserine (PS). PLC-delta did not bind significantly to large unilamellar vesicles formed from the zwitterionic lipid phosphatidylcholine (PC) but bound strongly to vesicles formed from mixtures of PC and PIP2. The apparent association constant for the putative 1:1 complex formed between PLC-delta and PIP2 was Ka congruent to 10(5) M-1. The binding strength increased further (Ka congruent to 10(6) M-1) when the vesicles also contained 30% PS. High-affinity binding of PLC-delta to PIP2 did not require Ca2+. PLC-delta bound only weakly to vesicles formed from mixtures of PC and either PS or phosphatidylinositol (PI); binding increased as the mole fraction of acidic lipid in the vesicles increased. We also studied the membrane binding of a small basic peptide that corresponds to a conserved region of PLC. Like PLC-delta, the peptide bound weakly to vesicles containing monovalent negatively charged lipids; unlike PLC-delta, it did not bind strongly to vesicles containing PIP2. Our data suggest that a significant fraction of the PLC-delta in a cell could be bound to PIP2 on the cytoplasmic surface of the plasma membrane.

Carbachol stimulation of phospholipase A2 and insulin secretion in pancreatic islets

Arachidonic acid has been implicated as a second messenger in insulin secretion by islets of Langerhans. D-Glucose, the major physiological stimulus, increases unesterified arachidonate accumulation in islets. We now show, for the first time, that the muscarinic agonist carbachol, at concentrations which stimulate insulin secretion, causes a rapid and nearly 3-fold increase in arachidonic acid accumulation in islets. The combination of glucose and carbachol has an additive effect on unesterified arachidonate release. There is a large component of secretagogue-induced arachidonate accumulation that is independent of extracellular Ca2+. Carbachol stimulation of arachidonic acid release is mediated by activation of phospholipase A2, as demonstrated by early increases in endogenous lysophosphatidylcholine. In addition to phospholipase A2 activation, carbachol-induced arachidonic acid accumulation also appears to involve diacylglycerol hydrolysis, since the diacylglycerol lipase inhibitor RG80267 partly inhibited arachidonic acid accumulation. In contrast, glucose-induced arachidonic acid accumulation appears to reflect diacylglycerol hydrolysis entirely. Our observations indicate that phospholipase A2 has an important role in muscarinic-induced insulin secretion.

Calcium-independent effects of TMB-8. Modification of phospholipid metabolism in neuroblastoma cells by inhibition of choline uptake

TMB-8 [8-(NN-diethylamino)-octyl-3,4,5-trimethoxybenzoate] blocks agonist-stimulated release of Ca2+ from intracellular sites in many cell lines and is often used to distinguish between dependence on extracellular and intracellular Ca2+. In N1E-115 neuroblastoma cells, TMB-8 did not alter the resting cytosolic Ca2+ concentration in unstimulated cells, yet phospholipid metabolism was greatly affected. At concentrations of TMB-8 (25-150 microM) that inhibit Ca2+ release, phosphatidylcholine formation was inhibited, whereas synthesis of phosphatidylinositol, phosphatidylglycerol and phosphatidylserine was stimulated. Unlike other cationic amphipathic compounds, TMB-8 did not inhibit phosphatidate phosphatase or enzymes in the pathway from choline to phosphatidylcholine. Choline transport was the major site of action. TMB-8 was a competitive inhibitor (Ki = 10 microM) of low-affinity (Kt = 20 microM) choline transport. When added at the same time as labelled precursor, TMB-8 also decreased cellular uptake of phosphate and inositol, but not that of ethanolamine or serine. In prelabelled cells, continued uptake and incorporation of phosphate and inositol were not affected. Under these conditions phosphatidylinositol synthesis was increased 2-fold and, like the effect on phosphatidylcholine, reached a plateau at 100 microM-TMB-8. Phosphatidylglycerol synthesis increased linearly with TMB-8 concentration to 40-fold stimulation at 150 microM, suggesting a selective effect on synthesis of phosphatidylglycerol from CDP-diacylglycerol. Phosphatidylserine synthesis was also increased up to 3-fold. These Ca(2+)-independent effects limit the use of TMB-8 in studies of cell signalling that involve stimulated phosphatidylinositol and phosphatidylcholine metabolism.

Physiological state and the sensitivity of liver mitochondrial outer membrane carnitine palmitoyltransferase to malonyl-CoA. Correlations with assay temperature, salt concentration and membrane lipid composition

1. Liver mitochondrial outer membranes were pre-exposed to media of low (20 mM phosphate) or high salt concentration (20 mM phosphate + 0.3 M KCl) before assay of carnitine palmitoyltransferase (CPT) at 25 degrees C. 2. With membranes from fed rats, exposure to high salt decreased sensitivity of CPT to malonyl-CoA whereas high salt increased sensitivity of CPT to malonyl-CoA in membranes from 48 hr-fasted rats. These changes were paralleled by alterations in the KD for high affinity binding of [14C]malonyl-CoA to outer membranes. 3. Decreasing the CPT assay temperatures from 25 to 10 degrees C caused qualitatively similar changes to those seen on exposure to high salt. 4. The relative content of sphingomyelin was increased 2-fold and 4-fold in liver mitochondrial outer membranes from fasted and diabetic rats respectively. Fasting had no effect on the content of cholesterol whereas diabetes decreased this by a third.

The role of diacylglycerol in the exocytosis of the sperm acrosome. Studies using diacylglycerol lipase and diacylglycerol kinase inhibitors and exogenous diacylglycerols

When ram spermatozoa were treated with Ca2+ and the ionophore A23187 to induce acrosomal exocytosis, a rise in diacylglycerol (DAG) mass was observed, concomitant with a rapid breakdown of [32P]P1-labelled phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate and a rise in [32P]Pi-labelled phosphatidate. Inclusion of the DAG lipase inhibitor RHC 80267 resulted in further but biphasic increases in DAG; there was an increasing accumulation of DAG with concentrations of RHC 80267 up to 10 microM, whereas higher concentrations produced lessening accumulation. Inclusion of RHC 80267 in the ionophore induction system also resulted in significant accelerations of the onset of exocytosis. In spermatozoa stimulated with Ca2+/A23187 and the DAG kinase inhibitor R59022, a similar increase in DAG levels together with stimulation of acrosomal exocytosis were observed. Preincubation of spermatozoa with sn-1-oleoyl-2-acetylglycerol, rac-1-oleoyl-2-acetylglycerol, sn-1,2-dioctanoylglycerol and sn-1,3-dioctanoylglycerol before treatment with Ca2+/A23187 resulted in a dose-dependent stimulation of exocytosis by all these isomers. Neomycin inhibited Ca2+/A23187-induced generation of DAG together with polyphosphoinositide breakdown, as well as acrosomal exocytosis. Inclusion of exogenous DAG, however, overcame the inhibitory effect of neomycin on exocytosis. Our results suggest that DAG has a key role in acrosomal exocytosis and that it acts as a messenger rather than as a substrate from which other active metabolites are generated. The lack of stereospecificity shown by the exogenous DAGs implies that DAG does not act by stimulating protein kinase C, but the metabolite's actual target in the sperm cell is as yet unclear.

Trichothecene mycotoxins inhibit phosphoinositide hydrolysis in bovine platelets stimulated with platelet activating factor

The effects of the trichothecene mycotoxins, acetyl T-2 toxin, T-2 toxin, HT-2 toxin, diacetoxyscirpenol (DAS), deoxynivalenol (DON) and T-2 tetraol on phospholipid turnover were determined in bovine platelets prelabelled with [1-14C]arachidonic acid (AA). In resting, non-stimulated platelets exposed to acetyl T-2 toxin, a marked decrease in [1-14C]phosphatidylinositol (PI) along with a marked increase in [1-14C]phosphatidic acid (PA) were observed, whereas T-2 toxin, and HT-2 toxin only induced a significant increase in [1-14C]PA. In contrast, in platelet activating factor (PAF)-stimulated platelets, the mycotoxins were found to suppress both the agonist-induced loss of [1-14C]PI and the appearance of [1-14C]PA with acetyl T-2 toxin being the most effective and T-2 toxin, HT-2 toxin, and DAS essentially equally effective. T-2 tetraol and DON did not affect phospholipid metabolism either in unstimulated or PAF stimulated platelets. The alterations in [1-14C]PI and [1-14C]PA suggest that the inhibitory toxins may activate a specific phospholipase C (PLC) in the unstimulated platelets and then impede further PLC activation in PAF-stimulated platelets.

Regulation of the formation of inositol phosphates by calcium, guanine nucleotides and ATP in digitonin-permeabilized bovine adrenal chromaffin cells

Both micromolar Ca2+ and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) stimulated the formation of inositol phosphates (InsPs) in digitonin-permeabilized chromaffin cells prelabelled with [3H]inositol. The production of InsPs was potentiated by ATP. Guanosine 5'-[beta-thio]diphosphate (GDP[S]) caused a GTP-reversible shift to higher concentrations in the Ca(2+)-concentration-response curve for the release of InsPs without changing the maximal response. GTP[S] caused a shift to lower concentrations of Ca2+ and also increased the maximal response. The effects of GTP[S] and Ca2+ were synergistic. Although as much as 80% of the InsPs were derived from phosphatidylinositol 4-phosphate (PtdInsP) or 4,5-bisphosphate (PtdInsP2), the amount of InsPs produced could be several times the total amount of PtdInsP and PtdInsP2 in the cells and was largely accounted for by a decrease in PtdIns. The levels of labelled PtdInsP and PtdInsP2 increased on stimulation with Ca2+, but decreased on stimulation with GTP[S] or the combination of Ca2+ and GTP[S]. Preincubation with Ca2+ and ATP amplified the subsequent GTP[S]-induced production of InsPs. ATP and its gamma-thio and beta gamma-imido analogues stimulated the formation of InsPs in intact cells. However, only ATP potentiated the responses to Ca2+ and GTP[S] in permeable cells. Our main conclusions are: (1) a GTP-binding protein participates in the Ca(2+)-induced production of InsPs by phospholipase C, and (2) ATP markedly potentiates the stimulated formation of InsPs, an effect with arises from its role in polyphosphoinositide synthesis and does not involve purinergic receptor activation in permeabilized cells. The data also suggest that the different effects of Ca2+ and GTP[S] on polyphosphoinositide synthesis probably contribute to the synergistic action of Ca2+ and GTP[S] on the generation of InsPs.

Inhibition of parathyroid hormone secretion correlates with increased incorporation of 32P into phosphatidylinositol and lysophosphatidylinositol

We have studied the incorporation of radioactive P (32P) into lipids of bovine parathyroid tissue under conditions of stimulated and inhibited hormone secretion. Utilizing low (0.5 mM) and high (3.0 mM) concentrations of calcium to regulate parathyroid hormone secretion, we initially found that the labeling of the cellular phospholipids with 32P was greater in those tissues incubated in high-calcium medium. Thin-layer chromatography of lipid extracts prepared from tissue incubated in either low- or high-calcium media revealed that the increased incorporation of 32P (high or low) was localized primarily to two phospholipids. To determine whether the increases were due directly to the different calcium concentrations, the experiments were performed in media containing normal calcium concentrations (1.25 mM) and low (0.5) or high (3.0) magnesium concentrations to modulate hormone secretion. The results were identical to those obtained using low and high calcium, indicating that the increased 32P incorporation was not an effect of high calcium but rather correlated with the inhibition of hormone secretion. The use of other secretagogues confirmed this correlation. The identity of the two phospholipids was established, by two-dimensional thin-layer chromatography, to be phosphatidylinositol (PI) and lysophosphatidylinositol (LPI). The correlation of increased 32P incorporation with inhibition of secretion led us next to examine isolated secretory granules from tissues exposed to either high-or low-calcium conditions. Thin-layer chromatography of granule lipid extracts yielded chromatograms containing PI and LPI, and the radioactivity of each was greater in the high-calcium sample than in the low-calcium sample.(ABSTRACT TRUNCATED AT 250 WORDS)

Methylmalonic acid metabolism and nervous-system fatty acids in cobalamin-deficient fruit bats receiving supplements of methionine, valine and isoleucine

Cobalamin neuropathy was produced in Cape fruit bats (Rousettus aegyptiacus) by a cobalamin-free diet combined with intermittent exposure to nitrous oxide, which inactivates cobalamin. Cobalamin-deficient bats had low hepatic methylmalonyl-CoA mutase holoenzyme activity, with elevated plasma and urinary methylmalonic acid levels. No significant changes could be demonstrated in the concentration of odd- or branched-chain fatty acids in the nervous system. Supplementation of the cobalamin-free diet with methionine, valine or isoleucine delayed the onset of neuropathy, despite persistence of methylmalonic acid accumulation. Supplementation with any of the three amino acids was associated with elevation of hepatic methionine concentration. The action of valine and isoleucine in delaying the onset of neuropathy can be explained by their methionine-sparing effect. These results emphasize the central role of methionine in the cobalamin neuropathy and do not support the hypothesis that the neuropathy is related to an effect of cobalamin deficiency on the propionic acid pathway.

Eicosanoid-dependent and -independent formation of individual [14C]stearoyl-labelled lysophospholipids in collagen-stimulated human platelets

Low level collagen activation of platelets is mediated via the release of arachidonic acid (AA) from membrane phospholipids and the formation of thromboxane A2 (TxA2). To assess the specific phospholipids undergoing deacylation via phospholipase A2 thereby providing source(s) of releasable AA, we have measured the individual lysophospholipid formations in platelets prelabelled with [14C]stearic acid and incubated with a low level (2 micrograms/ml) or a high level (10 micrograms/ml) of collagen in the absence or presence of BW755C, a dual inhibitor of cyclooxygenase and lipoxygenase activities. Collagen activation resulted in the generation of [14C]stearoyl-labelled lysophosphatidylinositol (lysoPI), lysophosphatidylcholine (lysoPC), lysophosphatidylethanolamine (lysoPE) and lysophosphatidylserine. BW755C significantly inhibited these collagen-induced changes, suggesting that much of the lysophospholipid, and therefore AA release, was eicosanoid-mediated. At the lower level of collagen, considerable generation of [14C]lysoPE was maintained even in the presence of BW755C, suggesting an eicosanoid-independent degradation of phosphatidyl-ethanolamine. The TxA2-dependent release of AA was also investigated in U-46619-stimulated platelets. This TxA2 mimetic induced considerable formation of the 14C-labelled lysophospholipids, including lysoPI and lysoPC, but not lysoPE. These results suggest that an eicosanoid-independent degradation of phosphatidylethanolamine via phospholipase A2 at lower collagen levels may provide a source of the initial AA for conversion to TxA2 and the subsequent deacylation of phosphatidylinositol, phosphatidylcholine, and also phosphatidylserine.

Restricted diffusion of integral membrane proteins and polyphosphoinositides leads to their depletion in microvesicles released from human erythrocytes

The protein and phospholipid composition of microvesicles released from normal human erythrocytes after ATP depletion, on aging or by treatment with merocyanine 540, dimyristoyl phosphatidylcholine or Ca2+/ionophore A23187 has been compared with the composition of the original cell membrane. It has been shown that these microvesicles are depleted of band 3, glycophorin and phosphatidylinositol 4,5-bisphosphate relative to phospholipid by 40% or more. These data are interpreted to mean that less than half of these membrane components are free to diffuse laterally in the lipid bilayer. Acetylcholinesterase was found to be enriched 2-3-fold in microvesicles, possibly because the removal of non-diffusing proteins from the vesiculating region of the lipid bilayer allows more space for freely diffusing proteins like acetylcholinesterase to enter the microvesicle membrane.

Diacylglycerol and phosphatidate production and the exocytosis of the sperm acrosome

We have investigated the production of diacylglycerol (DAG) and phosphatidate (PtdOH) during the exocytosis of the sperm acrosome. Ram spermatozoa treated with Ca2+ and the ionophore A23187 experienced a rapid breakdown of the polyphosphoinositides (PPIs), and a rise in [32P]Pi-labelled PtdOH and DAG mass; PtdOH mass, however, was unaffected. Treatment with Ca2+/A23187 and the DAG kinase inhibitor R59022 resulted in a dose-dependent increase in DAG mass and a concomitant decrease in [32P]PtdOH; such treatment showed a dose-dependent stimulation of acrosomal exocytosis. Pre-incubation with exogenous PtdOHs before stimulation with Ca2+/A23187 did not affect the time-course of exocytosis, whereas treatment with Ca2+/A23187 and exogenous DAGs (dioctanoylglycerol, oleoyl-acetyl-glycerol, or dioleoylglycerol) resulted in a dose-dependent stimulation of acrosomal exocytosis. Our results suggest that DAG, rather than PtdOH, is the important metabolite generated upon PPI hydrolysis; however, since spermatozoa lack protein kinase C, the target of DAG in most cells, a role for DAG in acrosomal exocytosis is as yet unclear.

Evidence that the inositol phospholipids are necessary for exocytosis. Loss of inositol phospholipids and inhibition of secretion in permeabilized cells caused by a bacterial phospholipase C and removal of ATP

We directly manipulated the levels of PtdIns, PtdInsP and PtdInsP2 in digitonin-treated adrenal chromaffin cells with a bacterial phospholipase C (PLC) from Bacillus thuringiensis and by removal of ATP. The PtdIns-PLC acted intracellularly to cause a large decrease in [3H]inositol- or [32P]phosphate-labelled PtdIns, but did not directly hydrolyse PtdInsP or PtdInsP2. [3H]PtdInsP and [3H]PtdInsP2 levels declined markedly, probably because of the action of phosphatases in the absence of synthesis. Removal of ATP also caused marked decreases in [3H]PtdInsP and [3H]PtdInsP2. The decrease in polyphosphoinositide levels by PtdIns-PLC treatment or ATP removal was reflected by the inhibition of the production of inositol phosphates upon subsequent activation of the endogenous PLC by Ca2(+)-dependent catecholamine secretion from permeabilized cells was strongly inhibited by PtdIns-PLC treatment and by ATP removal. Ca2(+)-dependent secretion was similarly correlated with the sum of PtdInsP and PtdInsP2 when the level of these lipids was changed by either manipulation. PtdIns-PLC inhibited only the ATP-dependent component of secretion and did not affect ATP-dependent secretion. Both PtdIns-PLC and ATP removal inhibited the late slow phase of secretion, but had little effect on the initial rapid phase. Although we found a tight correlation between polyphosphoinositide levels and secretion, endogenous phospholipase C activity (stimulated by Ca2+, guanine nucleotides and related agents) was not correlated with secretion. Additional experiments indicated that neither the products of the PtdIns-PLC reaction (diacylglycerol and InsP1) nor the inability to generate products by subsequent activation of the endogenous PLC is likely to account for the inhibition of secretion. Incubation of permeabilized cells with neomycin in the absence of ATP maintained the level of polyphosphoinositides and more than doubled subsequent Ca2(+)-dependent secretion. The data suggest that: (1) Ca2(+)-dependent secretion has a requirement for the presence of inositol phospholipids; (2) the enhancement of secretion by ATP results in part from increased polyphosphoinositide levels; and (3) the role for inositol phospholipids in secretion revealed in these experiments is independent of their being substrates for the generation of diacylglycerol and InsP3.

Albumin-bound docosahexaenoic acid and collagen-induced human platelet reactivity

An in vitro system designed to mimic the effect of various plasma nonesterified (polyunsaturated) fatty acids on platelet function and metabolism was employed. Human platelet aggregation induced by submaximal (1.8 micrograms/ml) collagen stimulation was significantly inhibited by 2 min preincubation with 20 microM albumin-bound docosahexaenoic acid (22:6n-3) (DHA), but not by the other fatty acids tested. [3H]Phosphatidic acid (PA) formation, an indicator of phospholipase C activation following platelet stimulation, was moderately inhibited by eicosapentaenoic acid (20:5n-3), 11,14,17-eicosatrienoic acid (20:3n-3), dihomo-gamma-linolenic acid (20:3n-6), as well as DHA, but not by arachidonic acid (20:4n-6); this inhibition of phospholipase C activation could not explain the differential effect of DHA on platelet aggregation. The decreased production of thromboxane A2 (TxA2), as assessed by [3H]12-hydroxy-5,8,10-heptadecatrienoic acid (HHT) formation, may account for the inhibition of collagen-induced aggregation by 20 microM DHA. Surprisingly, preincubation with 40 microM albumin-bound DHA, even though resulting in greater inhibition of collagen-induced aggregation, had less impact on HHT formation. A small but significant increase in [3H]prostaglandin D2 (PGD2) levels following 3-min collagen stimulation may have contributed to the greater antiaggregatory effect of 40 muM DHA. It is concluded that increased plasma nonesterified DHA may contribute to the dampened platelet activation and altered metabolism following fish oil supplementation of the diet.