Maintenance of lipid asymmetry in red blood cells and ghosts: effect of divalent cations and serum albumin on the transbilayer distribution of phosphatidylserine

Alterations in the expression pattern of RBC membrane associated proteins (RMAPs) in whole body γ-irradiated Sprague Dawley rats

PURPOSE

Peripheral blood serum/plasma proteins are frequently studied for their potential use as radiation exposure biomarkers. Here we report RBC membrane associated proteins (RMAPs), which show alterations in expression level following whole-body γ-irradiation of rats at sub-lethal/lethal doses.

MATERIALS AND METHODS

RBCs from peripheral blood of Sprague Dawley rats were segregated using the Ficoll-Hypaque method, and membrane fractions were hypotonically isolated at various time points (6 h, 24 h, 48 h) after γ-irradiation at 2 Gy, 5 Gy, and 7.5 Gy doses. Following purification of proteins from these fractions, two-dimensional electrophoresis (2-DE) was carried out. Treatment induced differentially expressed protein spots (≥2 fold increase/decrease) were picked up, trypsinized, and identified using LC-MS/MS analysis. Western immunoblots using protein specific antibodies were used to confirm the results. Gene ontology and interactions of these proteins were also studied.

RESULTS

From a number of differentially expressed radiation-responsive 2-DE protein spots detected, eight were identified unequivocally using LC-MS/MS. Out of these, actin, cytoplasmic 1 (ACTB) showed detectable yet insignificant variation (<50%) in expression. In contrast, peroxiredoxin-2 (PRDX2) and 26S proteasome regulatory subunit RPN11 (PSMD14) were the two most prominently over-expressed proteins. Five more proteins, namely tropomyosin alpha-3 chain (TPM3), exosome component 6 (EXOSC6), isoform 4 of tropomyosin alpha-1 chain (TPM1), serum albumin (ALB), and the 55 kDa erythrocyte membrane protein (P55) showed distinct alteration in their expression at different time-points and doses. ALB, EXOSC6, and PSMD14 were the most responsive at 2 Gy, albeit at different time-points. While EXOSC6 and PSMD14 showed maximum over-expression (5-12 fold) at 6 h post-irradiation, ALB expression increased progressively (4 up to 7 fold) from 6 h to 48 h. TPM1 showed over-expression (2-3 fold) at all doses and time-points tested. TPM3 showed a dose-dependent response at all time-points studied; with no variation at 2 Gy, ∼2 fold increase at 5 Gy, and 3-6 fold at the highest dose used (7.5 Gy). The p55 protein was over-expressed (∼2.5 fold) only transiently at 24 h following the lethal (7.5 Gy) dose.

CONCLUSION

This is the first study to report γ-radiation induced alterations in the RBC membrane associated proteins. We are further evaluating the potential of these proteins as radiation biomarkers. Due to the abundance and easy use of RBCs, this approach can prove very useful for detecting ionizing radiation exposure.

Pros and cons of phospholipid asymmetry in erythrocytes

Phospholipids of erythrocyte are found as bilayer with choline containing phospholipid like phosphatidyl choline and sphingomylein in the outer layer and amine containing phospholipid like phosphatidyl ethanolamine and phosphatidyl serine in the inner layer. This arrangement is known as phospholipid asymmetry. Lipid asymmetry is maintained throughout the entire life span of red blood cell and is disturbed when cells enter into the stage of apoptosis. We here discuss some of the conditions in which phospholipid asymmetry of erythrocyte is maintained and disturbed and the various detection methods to check the distortion phospholipid asymmetry of it.

The photolytic activity of poly-arginine cell penetrating peptides conjugated to carboxy-tetramethylrhodamine is modulated by arginine residue content and fluorophore conjugation site

Upon light irradiation, Fluorophore-cell-penetrating peptide (Fl-CPP) conjugates can disrupt the integrity of biological membranes. This activity can in turn be used to photoinduce the disruption of endocytic organelles and promote the delivery of entrapped macromolecules such as proteins or RNAs into live cells. Recent mechanistic studies have shown that ROS production by the fluorophore and a latent lytic ability of CPPs act in synergy to elicit photolysis. However, how the structure of fluorophore-CPP conjugates impacts this synergistic activity remains unclear. Herein, using red blood cells (RBCs) as a model of biological membranes, we show that the number of arginine residues in a CPP as well as the position of fluorophore with respect to the CPP dramatically affect the photolytic activity of a fluorophore-CPP conjugate. These factors should therefore be considered for the development of effective photoinducible delivery agents.

Etiology of autistic features: the persisting neurotoxic effects of propionic acid

BACKGROUND

Recent clinical observations suggest that certain gut and dietary factors may transiently worsen symptoms in autism. Propionic acid (PA) is a short chain fatty acid and an important intermediate of cellular metabolism. Although PA has several beneficial biological effects, its accumulation is neurotoxic.

METHODS

Two groups of young Western albino male rats weighing about 45 to 60 grams (approximately 21 days old) were used in the present study. The first group consisted of oral buffered PA-treated rats that were given a neurotoxic dose of 250 mg/kg body weight/day for three days, n = eight; the second group of rats were given only phosphate buffered saline and used as a control. Biochemical parameters representing oxidative stress, energy metabolism, neuroinflammation, neurotransmission, and apoptosis were investigated in brain homogenates of both groups.

RESULTS

Biochemical analyses of brain homogenates from PA-treated rats showed an increase in oxidative stress markers (for example, lipid peroxidation), coupled with a decrease in glutathione (GSH) and glutathione peroxidase (GPX) and catalase activities. Impaired energy metabolism was ascertained through the decrease of lactate dehydrogenase and activation of creatine kinase (CK). Elevated IL-6, TNFα, IFNγ and heat shock protein 70 (HSP70) confirmed the neuroinflammatory effect of PA. Moreover, elevation of caspase3 and DNA fragmentation proved the pro-apoptotic and neurotoxic effect of PA to rat pups

CONCLUSION

By comparing the results obtained with those from animal models of autism or with clinical data on the biochemical profile of autistic patients, this study showed that the neurotoxicity of PA as an environmental factor could play a central role in the etiology of autistic biochemical features.

Impaired plasma phospholipids and relative amounts of essential polyunsaturated fatty acids in autistic patients from Saudi Arabia

Backgrounds

Autism is a developmental disorder characterized by social and emotional deficits, language impairments and stereotyped behaviors that manifest in early postnatal life. This study aims to compare the relative concentrations of essential fatty acids (Linoleic and α- linolenic), their long chain polyunsaturated fatty acids and phospholipids in plasma of autistic patients from Saudi Arabia with age-matching controls.

Methods

25 autistic children aged 3-15 years and 16 healthy children as control group were included in this study. Relative concentration of essential fatty acids/long chain polyunsaturated fatty acids and omega-3/omega-6 fatty acid series together with phosphatidylethanolamine, phosphatidylserine and phosphatidylcholine were measured in plasma of both groups.

Results

Remarkable alteration of essential fatty acids/long chain polyunsaturated fatty acids, omeg-3/omega-6 and significant lower levels of phospholipids were reported. Reciever Operating characteristics (ROC) analysis of the measured parameters revealed a satisfactory level of sensitivity and specificity.

Conclusion

Essential fatty acids/long chain polyunsaturated fatty acids and omeg-3/omega-6 ratios, phosphatidylethanolamine, phosphatidylserine and phosphatidylcholine could be used as potential biomarkers that point to specific mechanisms in the development of autism and may help tailor treatment or prevention strategies.

ATP8A1 activity and phosphatidylserine transbilayer movement

The asymmetric distribution of the amino-containing phospholipids, phosphatidyl-serine (PS) and phosphatidyl-ethanolamine (PE), across the two leaflets of red blood cell (RBC) membrane is essential to the function and survival of the cell. PS and PE are sequestered in the inner leaflet by an ATP-dependent transport activity of a membrane protein known as the RBC flippase that specifically moves amino-phospholipids from the outer to the inner leaflet. The enucleated RBC lacks the means to replace damaged enzymes and inactivation of the flippase can lead to the unwarranted exposure of PS on the cell surface. Loss in the ability to maintain phospholipid asymmetry is exacerbated in RBC disorders and PS-exposing RBCs present in the circulation play a significant role in the pathology of hemoglobinopathies. We identified the Atp8a1 protein, a member of the family of the P(4)-type ATPases, as a RBC flippase candidate. Atp8a1 is expressed in RBC precursors and is present in the membrane of mature red cells. The flippase activity of the protein was established in purified secretory vesicles of Saccharomyces cerevisiae. ATPase activity was stimulated by PS and PE. In addition, Atp8a1 can move PS molecules across the leaflets of the vesicle membrane in presence of ATP.

Deep coverage mouse red blood cell proteome: a first comparison with the human red blood cell

Mice have close genetic/physiological relationships to humans, breed rapidly, and can be genetically modified, making them the most used mammal in biomedical research. Because the red blood cell (RBC) is the sole gas transporter in vertebrates, diseases of the RBC are frequently severe; much research has therefore focused on RBC and cardiovascular disorders of mouse and humans. RBCs also host malaria parasites. Recently we presented an in-depth proteome for the human RBC. Here we present directly comparable data for the mouse RBC as membrane-only, soluble-only, and combined membrane-bound/soluble proteomes (comprising, respectively, 247, 232, and 165 proteins). All proteins were identified, validated, and categorized in terms of subcellular localization, protein family, and function, and in comparison with the human RBC, were classified as orthologs, family-related, or unique. Splice isoforms were identified, and polypeptides migrating with anomalous apparent molecular weights were grouped into putatively ubiquitinated or partially degraded complexes. Overall there was close concordance between mouse and human proteomes, confirming the unexpected RBC complexity. Several novel findings in the human proteome have been confirmed here. This comparison sheds light on several open issues in RBC biology and provides a departure point for more comprehensive understanding of RBC function.

In-depth analysis of the membrane and cytosolic proteome of red blood cells

In addition to transporting oxygen and carbon dioxide to and from the tissues, a range of other functions are attributed to red blood cells (RBCs) of vertebrates. Diseases compromising RBC performance in any of these functions warrant in-depth study. Furthermore, the human RBC is a vital host cell for the malaria parasite. Much has been learned from classical biochemical approaches about RBC composition and membrane organization. Here, we use mass spectrometry (MS)-based proteomics to characterize the normal RBC protein profile. The aim of this study was to obtain the most complete and informative human RBC proteome possible by combining high-accuracy, high-sensitivity protein identification technology (quadrupole time of flight and Fourier transform MS) with selected biochemical procedures for sample preparation. A total of 340 membrane proteins and 252 soluble proteins were identified, validated, and categorized in terms of subcellular localization, protein family, and function. Splice isoforms of proteins were identified, and polypeptides that migrated with anomalously high or low apparent molecular weights could be grouped into either ubiquitinylated, partially degraded, or ester-linked complexes. Our data reveal unexpected complexity of the RBC proteome, provide a wealth of data on its composition, shed light on several open issues in RBC biology, and form a departure point for comprehensive understanding of RBC functions.

The effect of PS content on the ability of natural membranes to fuse with positively charged liposomes and lipoplexes

Supramolecular aggregates containing cationic lipids have been widely used as transfection mediators due to their ability to interact with negatively charged DNA molecules and biological membranes. First steps of the process leading to transfection are partly electrostatic, partly hydrophobic interactions of liposomes/lipoplexes with cell and/or endosomal membrane. Negatively charged compounds of biological membranes, namely glycolipids, glycoproteins and phosphatidylserine (PS), are responsible for such events as adsorption, hemifusion, fusion, poration and destabilization of natural membranes upon contact with cationic liposomes/lipoplexes. The present communication describes the dependence of interaction of cationic liposomes with natural and artificial membranes on the negative charge of the target membrane, charges which in most cases were generated by charging the PS content or its exposure. The model for the target membranes were liposomes of variable content of PS or PG (phosphatidylglycerol) and erythrocyte membranes in which the PS and other anionic compound content/exposure was modified in several ways. Membranes of increased anionic phospholipid content displayed increased fusion with DOTAP (1,2-dioleoyl-3-trimethylammoniumpropane) liposomes, while erythrocyte membranes partly depleted of glycocalix, its sialic acid, in particular, showed a decreased fusion ability. The role of the anionic component is also supported by the fact that erythrocyte membrane inside-out vesicles fused easily with cationic liposomes. The data obtained on erythrocyte ghosts of normal and disrupted asymmetry, in particular, those obtained in the presence of Ca(2+), indicate the role of lipid flip-flop movement catalyzed by scramblase. The ATP-depletion of erythrocytes also induced an increased sensitivity to hemoglobin leakage upon interactions with DOTAP liposomes. Calcein leakage from anionic liposomes incubated with DOTAP liposomes was also dependent on surface charge of the target membranes. In all experiments with the asymmetric membranes the fusion level markedly increased with an increase of temperature, which supports the role of membrane lipid mobility. The decrease in positive charge by binding of plasmid DNA and the increase in ionic strength decreased the ability of DOTAP liposomes/lipoplexes to fuse with erythrocyte ghosts. Lower pH promotes fusion between erythrocyte ghosts and DOTAP liposomes and lipoplexes. The obtained results indicate that electrostatic interactions together with increased mobility of membrane lipids and susceptibility to form structures of negative curvature play a major role in the fusion of DOTAP liposomes with natural and artificial membranes.

Natural phosphatidylcholine is actively translocated across the plasma membrane to the surface of mammalian cells

The cell surface of eukaryotic cells is enriched in choline phospholipids, whereas the aminophospholipids are concentrated at the cytosolic side of the plasma membrane by the activity of one or more P-type ATPases. Lipid translocation has been investigated mostly by using short chain lipid analogs because assays for endogenous lipids are inherently complicated. In the present paper, we optimized two independent assays for the translocation of natural phosphatidylcholine (PC) to the cell surface based on the hydrolysis of outer leaflet phosphoglycerolipids by exogenous phospholipase A2 and the exchange of outer leaflet PC by a transfer protein. We report that PC reached the cell surface in the absence of vesicular traffic by a pathway that involved translocation across the plasma membrane. In erythrocytes, PC that was labeled at the inside of the plasma membrane was translocated to the cell surface with a half-time of 30 min. This translocation was probably mediated by an ATPase, because it required ATP and was vanadate-sensitive. The inhibition of PC translocation by glibenclamide, an inhibitor of various ATP binding cassette transporters, and its reduction in erythrocytes from both Abcb1a/1b and Abcb4 knockout mice, suggest the involvement of ATP binding cassette transporters in natural PC cell surface translocation. The relative importance of the outward translocation of PC as compared with the well characterized fast inward translocation of phosphatidylserine for the overall asymmetric phospholipid organization in plasma membranes remains to be established.

Stimulation of prothrombinase activity by the nonapeptide Thr-Trp-Ala-Arg-Asn-Ser-Tyr-Asn-Val, a segment of a plant thionin

Pyrularia thionin (PT) is a basic 47 amino acid peptide isolated from the nuts of Pyrularia pubera. Its structure and properties have been studied in some detail. Its receptor site is a domain of membrane phosphatidyl serine (PS), where it binds with a relatively high specificity. A segment of its covalent structure, the nonapeptide Thr-Trp-Ala-Arg-Asn-Ser-Tyr-Asn-Val, designated serine nonapeptide (SNP), corresponds to amino acids 7-15 of the thionin, except for the position 12 (Ser), which substitutes for Cys, to give stability. This peptide represents what we consider to be the active site of the thionin, and it also binds to PS domains, but less tightly than thionin does. The peptide has an effect on the prothrombinase assay using the chromophore S2238 to measure the thrombin produced by the prothrombinase complex. It is shown that SNP stimulates the prothrombinase complex activity, instead of inhibiting it, as would be expected if it simply covered the PS sites on the membrane of erythrocyte ghosts, used in the prothrombinase assay. SNP appears to substitute for Va in the prothrombinase complex reaction, in a Ca(2+) independent manner, being even more effective in the absence than in the presence of ghosts. In the clotting system, SNP can also substitute for Factor Va.

Identification of a functional role for lipid asymmetry in biological membranes: Phosphatidylserine-skeletal protein interactions modulate membrane stability

Asymmetric distribution of phospholipids is ubiquitous in the plasma membranes of many eukaryotic cells. The majority of the aminophospholipids are located in the inner leaflet whereas the cholinephospholipids are localized predominantly in the outer leaflet. Several functional roles for asymmetric phospholipid distribution in plasma membranes have been suggested. Disruption of lipid asymmetry creates a procoagulant surface on platelets and serves as a trigger for macrophage recognition of apoptotic cells. Furthermore, the dynamic process of phospholipid translocation regulates important cellular events such as membrane budding and endocytosis. In the present study, we used the red cell membrane as the model system to explore the contribution of phospholipid asymmetry to the maintenance of membrane mechanical properties. We prepared two different types of membranes in terms of their phospholipid distribution, one in which phospholipids were scrambled and the other in which the asymmetric distribution of phospholipids was maintained and quantitated their mechanical properties. We documented that maintenance of asymmetric distribution of phospholipids resulted in improved membrane mechanical stability. The greater difficulty in extracting the spectrin-actin complex at low-ionic strength from the membranes with asymmetric phospholipid distribution further suggested the involvement of interactions between aminophospholipids in the inner leaflet and skeletal proteins in modulating mechanical stability of the red cell membrane. These findings have enabled us to document a functional role of lipid asymmetry in regulating membrane material properties.

A unique phospholipid organization in bovine erythrocyte membranes

Ruminant erythrocytes are remarkable for their choline-phospholipid anomalies; namely, low or absent phosphatidylcholine (PC) along with high sphingomyelin levels. Here, we report another anomaly in bovine erythrocytes that affects aminophospholipids: phosphatidylethanolamine (PE) shows an extreme asymmetry, with only 2% of the total present in the outer leaflet. Furthermore, we found that phospholipase A(2), an enzyme located on the external surface of the erythrocytes, shows higher activity against PC than against PE. In addition, we observed that acylation of PE is by far the most important biosynthetic event in this system. We propose that deacylation of PE and PC by phospholipase A(2) to generate lysocompounds, followed by selective reacylation of lyso-PE in the inner leaflet, can account for the compositional and architectural peculiarities of bovine erythrocyte membranes.

Dynamics of membrane penetration of the fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group attached to an acyl chain of phosphatidylcholine

Location and dynamic reorientation of the fluorophore 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) covalently attached to a short (C6) or a long (C12) sn2 acyl chain of a phosphatidylcholine molecule was investigated by fluorescence and solid-state NMR spectroscopy. 2H NMR lipid chain order parameters indicate a perturbation of the phospholipid packing density in the presence of NBD. Specifically, a decrease of molecular order was found for acyl chain segments of the lower, more hydrophobic region. Molecular collision probabilities determined by 1H magic angle spinning nuclear Overhauser enhancement spectroscopy indicate a highly dynamic reorientation of the probe in the membrane due to thermal fluctuations. A broad distribution of the fluorophore in the lipid bilayer is observed with a preferential location in the upper acyl chain/glycerol region. The distribution of the NBD group in the membrane is quite similar for both the long- and the short-chain analog. However, a slight preference of the NBD group for the lipid-water interface is found for C12-NBD-PC in comparison with C6-NBD-PC. Indeed, as shown by dithionite fluorescence assay, the long-chain analog reacts more favorably with dithionite, indicating a better accessibility of the probe by dithionite present in the aqueous phase. Forces determining the location of the fluorophore in the lipid water interface are discussed.

Rapid flip-flop of phospholipids in endoplasmic reticulum membranes studied by a stopped-flow approach

The transbilayer movement of short-chain spin-labeled and fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) phospholipid analogs in rat liver microsomes is measured by stopped-flow mixing of labeled microsomes with bovine serum albumin (BSA) solution. Extraction of analogs from the outer leaflet of microsomes to BSA can be directly monitored in conjunction with electron paramagnetic resonance or fluorescence spectroscopy by taking advantage of the fact that the signal of spin-labeled or fluorescent analogs bound to BSA is different from that of the analogs inserted into membranes. From the signal kinetics, the transbilayer movement and the distribution of analogs in microsomal membranes can be derived provided the extraction of analogs by BSA is much faster in comparison to the transbilayer movement of analogs. Half-times of the back-exchange for spin-labeled and fluorescent analogs were <3.5 and <9.5 s, respectively. The unprecedented time resolution of the assay revealed that the transbilayer movement of spin-labeled analogs is much faster than previously reported. The half-time of the movement was about 16 s or even less at room temperature. Transmembrane movement of NBD-labeled analogs was six- to eightfold slower than that of spin-labeled analogs.

Lateral organisation of membrane lipids. The superlattice view

Most biological membranes are extremely complex structures consisting of hundreds or even thousands of different lipid and protein molecules. The prevailing view regarding the organisation of these membranes is based on the fluid-mosaic model proposed by Singer and Nicholson in 1972. According to this model, phospholipids together with some other lipids form a fluid bilayer in which these lipids are diffusing very rapidly laterally. The idea of rapid lateral diffusion implies that, in general, the different lipid species would be randomly distributed in the plain of the membrane. However, there are recent data indicating that the components tend to adopt regular (superlattice-like) distributions in fluid, mixed bilayers. Based on this, a superlattice model of membranes has been proposed. This superlattice model is intriguing because it allows only a limited certain number of 'critical' compositions. These critical compositions could play a key role in the regulation of the lipid compositions of biological membranes. Furthermore, such putative critical compositions could explain how compositionally distinct organelles can exist despite of rapid inter-organelle membrane traffic. In this review, these intriguing predictions are discussed along with the basic principles of the model and the evidence supporting it.

Lipid translocation across the plasma membrane of mammalian cells

The plasma membrane, which forms the physical barrier between the intra- and extracellular milieu, plays a pivotal role in the communication of cells with their environment. Exchanging metabolites, transferring signals and providing a platform for the assembly of multi-protein complexes are a few of the major functions of the plasma membrane, each of which requires participation of specific membrane proteins and/or lipids. It is therefore not surprising that the two leaflets of the membrane bilayer each have their specific lipid composition. Although membrane lipid asymmetry has been known for many years, the mechanisms for maintaining or regulating the transbilayer lipid distribution are still not completely understood. Three major players have been presented over the past years: (1) an inward-directed pump specific for phosphatidylserine and phosphatidylethanolamine, known as aminophospholipid translocase; (2) an outward-directed pump referred to as 'floppase' with little selectivity for the polar headgroup of the phospholipid, but whose actual participation in transport of endogenous lipids has not been well established; and (3) a lipid scramblase, which facilitates bi-directional migration across the bilayer of all phospholipid classes, independent of the polar headgroup. Whereas a concerted action of aminophospholipid translocase and floppase could, in principle, account for the maintenance of lipid asymmetry in quiescent cells, activation of the scramblase and concomitant inhibition of the aminophospholipid translocase causes a collapse of lipid asymmetry, manifested by exposure of phosphatidylserine on the cell surface. In this article, each of these transporters will be discussed, and their physiological importance will be illustrated by the Scott syndrome, a bleeding disorder caused by impaired lipid scrambling. Finally, phosphatidylserine exposure during apoptosis will be briefly discussed in relation to inhibition of translocase and simultaneous activation of scramblase.

Flow cytometric detection of transbilayer movement of fluorescent phospholipid analogues across the boar sperm plasma membrane: elimination of labeling artifacts

Reliable protocols were established for investigating asymmetric distributions of 6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino-caproyl (C6NBD) phospholipids in the plasma membrane of boar sperm cells under physiological conditions. A method based on fluorescence resonance energy transfer was used to ensure that incorporation of the fluorescent phospholipids into the sperm proceeded via monomeric transfer. The total amount of incorporated phospholipid fluorescence and the proportion of translocated phospholipid fluorescence were determined by flow cytometric analysis before, and after, dithionite destruction of outer leaflet fluorescence. Catabolism of incorporated fluorescent phospholipids was blocked with phenylmethylsulfonyl fluoride. Membrane-damaged cells were detected with impermeant DNA stains, thereby enabling their exclusion from subsequent analyses of the flow cytometric data, whence it could be demonstrated that the labeled phospholipids were incorporated only via the outer plasma membrane leaflet in living sperm cells. Phospholipid uptake and internalization was followed at 38 degrees C. After 1 hr of labeling, about 96% of the incorporated C6NBD-phosphatidylserine, 80% of C6NBD-phosphatidylethanolamine, 18% of C6NBD-phosphatidylcholine, and 4% of C6NBD-sphingomyelin were found to have moved across the plasma membrane bilayer to the interior of the spermatozoa. These inward movements of fluorescent phospholipids were ATP-dependent and could be blocked with sulfhydryl reagents. Movements from the inner to the outer leaflet of the sperm plasma membrane were minimal for intact fluorescent phospholipids, but were rapid and ATP-independent for fluorescent lipid metabolites. The described method enables, for the first time, assessment of changes in lipid asymmetry under fertilizing conditions.

Internalization of phospholipids from the plasma membrane of human osteoblasts depends on the lipid head group

The redistribution of spin- or fluorescence-labeled phospholipid analogs across the plasma membrane of human osteoblast cells, either in suspension or grown as monolayers, was investigated. After incorporation into the outer membrane leaflet, analogs of the aminophospholipids phosphatidylserine and phosphatidylethanolamine moved rapidly to the inner monolayer, whereas the choline-containing analogs of phosphatidylcholine and sphingomyelin disappeared more slowly from the outer leaflet. The fast inward movement of the aminophospholipids became reduced after lowering the intracellular ATP, suggesting the presence of an aminophospholipid translocase activity in the plasma membrane of these cells. From these data, a transverse phospholipid asymmetry in osteoblasts can be inferred with the aminophospholipids mainly concentrated in the inner monolayer and the choline-containing phospholipids in the outer leaflet. A similar pattern of phospholipid internalization was inferred for osteoblasts from human osteoporotic bones and for a human osteosarcoma cell line. The relevance of the enrichment of phosphatidylserine in the cytoplasmic membrane leaflet for calcification in skeletal tissues is emphasized.

Regulatory mechanisms of transmembrane phospholipid distributions and pathophysiological implications of transbilayer lipid scrambling

The various phospholipid classes that comprise mammalian cell membranes are distributed over both leaflets of the bilayer in a non-random fashion. While a specific and ATP-dependent transporter is responsible for rapid inward movement of aminophospholipids, its inhibition does not lead to spontaneous redistribution of lipids. Conditions of cellular activation which are accompanied with increased levels of intracellular Ca2+ may cause a collapse of lipid asymmetry by switching on an ATP-independently operating scramblase, which accelerates bidirectional movement of all phospholipid classes. The most prominent change in transmembrane lipid distribution is surface exposure of phosphatidylserine (PS), the more so since conditions which activate scramblase in most if not all cases lead to inhibition of aminophospholipid translocase activity, which will prevent PS from being pumped back to the inner leaflet of the membrane. Surface-exposed PS serves at least two important physiological functions: it promotes blood coagulation and offers a recognition signal for clearance by macrophages and other cells of the reticuloendothelial system. As such, PS exposure may form an important early event in the process of apoptosis to ensure rapid removal of these cells in order to avoid release of their inflammatory contents. Defective regulation of transbilayer lipid distribution may result in clinical manifestations such as in the Scott syndrome, a bleeding disorder caused by an impaired scramblase activity. Conversely, excessive PS exposure may lead to thrombosis or may explain formation of so-called antiphospholipid antibodies as occurring in patients with antiphospholipid syndrome.

Compensating lipid fluxes generated by the aminophospholipid translocase

By a combined kinetic and thermodynamic model on the transbilayer dynamics and asymmetric distribution of lipids in the red blood cell, compensating lipid fluxes to the exoplasmic leaflet have been analysed, counterbalancing the active transport of aminophospholipids to the cytoplasmic monolayer by the aminophospholipid translocase. The compensating fluxes are assumed to be of passive nature generated by forces of lateral mechanical stress and of lipid concentration differences between the two monolayers. These forces are shown to be caused and maintained by the operation of the aminophospholipid translocase. Simulations reveal that a reduction of the compensating fluxes upon ATP-depletion can be attributed to the inhibition of the aminophospholipid translocase. Thus, a Mg(2+)- and ATP-dependence of the outward movement of phospholipid analogues in the plasma membrane of red blood cells can be expected independent of the existence and operation of an ATP-dependent 'floppase' activity.

Targeted Inactivation of Murine Band 3 (AE1) Gene Produces a Hypercoagulable State Causing Widespread Thrombosis In Vivo

Only 5% to 10% of band 3 null mice survive the neonatal period. To determine the cause of death, 3 adult and 11 newborn band 3 null mice were submitted for histopathologic examination. All but 1 pup showed evidence of thrombosis including: (1) large thrombotic lesions in the heart, which were partially organized, calcified in some fields, and endothelialized, indicating a process that developed premortem (3 of 3 adults and 6 of 11 pups). (2) Subcapsular necrotic areas in the liver suggestive of premortem ischemic events caused by arteriolar occlusions (8 of 11 pups). (3) Large vein thrombi (4 of 11 pups). To investigate the etiology of this hypercoagulable state, we have used the Russell’s viper venom test (RVV) to show that red blood cells (RBCs) from band 3 null mice significantly shorten the RVV clotting time of normal plasma in a dose-dependent fashion, whereas RBCs from normal mice have no effect, suggesting that the membrane of band 3 null RBCs provides a suitable surface for activation of the prothrombinase complex. Using flow cytometry, we have examined the phosphatidylserine (PS)-specific binding of fluorescein isothiocyanate (FITC)-annexin V to normal and band 3 null RBCs. A subpopulation of cells (3% to 5% of RBCs) with increased FITC-annexin V binding was detected in band 3 null RBCs as compared with normal RBCs. Furthermore, the entire cell population of band 3 null RBCs shows a measurable increase in the mean fluorescence intensity, suggesting that band 3 null RBCs may have increased PS exposure on the outer membrane leaflet. These findings are further supported by direct fluorescence microscopy of normal and band 3 null RBCs labeled with FITC-annexin V. Based on these observations, we postulate that the high mortality of band 3 null mice may be related to a hypercoagulable state, which appears to originate from changes in the phospholipid composition of the membrane leading to PS exposure on the outer leaflet.

© 1998 by The American Society of Hematology.

Targeted Inactivation of Murine Band 3 (AE1) Gene Produces a Hypercoagulable State Causing Widespread Thrombosis In Vivo

Only 5% to 10% of band 3 null mice survive the neonatal period. To determine the cause of death, 3 adult and 11 newborn band 3 null mice were submitted for histopathologic examination. All but 1 pup showed evidence of thrombosis including: (1) large thrombotic lesions in the heart, which were partially organized, calcified in some fields, and endothelialized, indicating a process that developed premortem (3 of 3 adults and 6 of 11 pups). (2) Subcapsular necrotic areas in the liver suggestive of premortem ischemic events caused by arteriolar occlusions (8 of 11 pups). (3) Large vein thrombi (4 of 11 pups). To investigate the etiology of this hypercoagulable state, we have used the Russell’s viper venom test (RVV) to show that red blood cells (RBCs) from band 3 null mice significantly shorten the RVV clotting time of normal plasma in a dose-dependent fashion, whereas RBCs from normal mice have no effect, suggesting that the membrane of band 3 null RBCs provides a suitable surface for activation of the prothrombinase complex. Using flow cytometry, we have examined the phosphatidylserine (PS)-specific binding of fluorescein isothiocyanate (FITC)-annexin V to normal and band 3 null RBCs. A subpopulation of cells (3% to 5% of RBCs) with increased FITC-annexin V binding was detected in band 3 null RBCs as compared with normal RBCs. Furthermore, the entire cell population of band 3 null RBCs shows a measurable increase in the mean fluorescence intensity, suggesting that band 3 null RBCs may have increased PS exposure on the outer membrane leaflet. These findings are further supported by direct fluorescence microscopy of normal and band 3 null RBCs labeled with FITC-annexin V. Based on these observations, we postulate that the high mortality of band 3 null mice may be related to a hypercoagulable state, which appears to originate from changes in the phospholipid composition of the membrane leading to PS exposure on the outer leaflet.

© 1998 by The American Society of Hematology.

Out-to-in translocation of butanetriol-containing phospholipid analogs in human erythrocyte membrane

Fluorescent butanetriol-containing phospholipid analogs were synthesized by replacing the glycerol moiety in 1-hexadecanoyl-2-[6-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) aminohexanoyl]-sn-glycero-3-phosphocholine, -phosphoethanolamine, -phosphoserine and 1-hexadecanoyl-2-[12-N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)aminododecanoyl]-sn-glycero-3-phosphocholine, -phosphoethanolamine, -phosphoserine by the 1,3,4-butanetriol residue, and their out-to-in translocation in the human erythrocyte membrane studied by 'back exchanging' the outer surface-incorporated phospholipids using bovine serum albumin. The results of these studies indicate that the replacement of the glycerol moiety by the 1,3,4-butanetriol residue in aminophospholipids does not effect their out-to-in translocation in the human erythrocyte membrane. Furthermore, since earlier study by Arora and Gupta (Biochim. Biophys. Acta 1324 (1997) 47-60) has shown that the conformation of the 1,3,4-butanetriol phospholipids possess the backbone conformation similar to that of glycerophospholipids, it is suggested that besides the normal phospholipid polar head-group, a normal phospholipid interface conformation may also be required for the aminophospholipid-translocase interactions.

Redistribution of phosphatidylethanolamine and phosphatidylserine precedes reperfusion-induced apoptosis

Although cardiomyocyte death and infarction associated with ischemia-reperfusion are traditionally believed to be induced via necrosis, recent studies implicated apoptotic cell death in ischemic reperfused tissue. To examine whether myocardial ischemic reperfusion injury is mediated by apoptotic cell death, isolated perfused rat hearts were subjected to 15 and 30 min of ischemia as well as 15 min of ischemia followed by 30, 90, or 120 min of reperfusion. At the end of each experiment, hearts were processed for the evaluation of apoptosis and DNA laddering. Apoptosis was studied by visualizing the apoptotic cardiomyocytes by direct fluorescence detection of digoxigenin-labeled genomic DNA using APOPTAG in situ apoptosis detection kit. DNA laddering was evaluated by subjecting the DNA obtained from cardiomyocytes to 1.8% agarose gel electrophoresis and photographed under ultraviolet illumination. In addition, high-performance thin-layer chromatography (HPTLC) of aminophospholipids labeled with 2,4,6-trinitrobenzenesulfonate was performed to evaluate phospholipid topography in cardiomyocytes. The results of our study revealed apoptotic cells only in the 90- and 120-min reperfused hearts as demonstrated by the intense fluorescence of the immunostained digoxigenin-labeled genomic DNA when observed under fluorescence microscope. None of the ischemic hearts showed any evidence of apoptosis. These results corroborated with the findings of DNA fragmentation that showed increased ladders of DNA bands in the 120-min reperfused hearts, representing integer multiples of the internucleosomal DNA length (approximately 180 bp). Two-dimensional HPTLC of the phospholipids obtained from the cardiomyocytes and transbilayer organization of the phosphatidylethanolamine (PE) and phosphatidylserine (PS) in the myocytes indicated translocation of both PE and PS from the inner leaflet to the outer leaflet of the membrane as early as after 20 min of ischemia. These results demonstrate that the redistribution of PS and PE precedes the apototic cell death and DNA fragmentation associated with the reperfusion of ischemic myocardium, suggesting that ischemia may trigger the signal for apoptosis although it becomes evident during reperfusion.

Immune clearance of phosphatidylserine-expressing cells by phagocytes. The role of beta2-glycoprotein I in macrophage recognition

The function of beta2-glycoprotein I (beta2GPI), a 50-kDa serum glycoprotein, is not completely understood but has been suggested to be involved in the regulation of thrombosis (Brighton, T. A., Hogg, P. J., Dai, Y.-P., Murray, B. H., Choing, B. H., and Chesterman, C. N. (1996) Br. J. Haematol. 93, 185-194) and the clearance of phosphatidylserine (PS)-expressing cells (Chonn, A., Semple S. C., and Cullis P. R. (1995) J. Biol. Chem. 270, 25845-25849). To further understand the role of this protein, we characterized the ability of beta2GPI to interact with PS vesicles and influence their uptake by macrophages in vitro. beta2GPI bound to and precipitated vesicles containing anionic but not zwitterionic phospholipids in a gel diffusion assay. beta2GPI also inhibited the procoagulant activity of PS liposomes. In vitro phagocytosis studies showed 20-fold greater uptake of PS liposomes over phosphatidylcholine liposomes. This enhanced uptake was maintained even after PS was "shielded" with beta2GPI and further increased upon the addition of beta2GPI antibodies. Similar to liposomes, PS-expressing apoptotic thymocytes and lipid symmetric red blood cell ghosts bound beta2GPI. Macrophage uptake of these cells was also maintained or enhanced in the presence of beta2GPI and further increased upon the addition of beta2GPI antibodies. It is concluded that beta2GPI can play a critical role in hemostasis by influencing both thrombosis and the clearance of PS-expressing cells.

Meta-stability of the hemifusion intermediate induced by glycosylphosphatidylinositol-anchored influenza hemagglutinin

Fusion between influenza virus and target membranes is mediated by the viral glycoprotein hemagglutinin (HA). Replacement of the transmembrane domain of HA with a glycosylphosphatidylinositol (GPI) membrane anchor allows lipid mixing but not the establishment of cytoplasmic continuity. This observation led to the proposal that the fusion mechanism passes through an intermediate stage corresponding to hemifusion between outer monolayers. We have used confocal fluorescence microscopy to study the movement of probes for specific bilayer leaflets of erythrocytes fusing with HA-expressing cells. N-Rh-PE and NBD-PC were used for specific labeling of the outer and inner membrane leaflet, respectively. In the case of GPI-HA-induced fusion, different behaviors of lipid transfer were observed, which include 1) exclusive movement of N-Rh-PE (hemifusion), 2) preferential movement of N-Rh-PE relative to NBD-PC, and 3) equal movement of both lipid analogs. The relative population of these intermediate states was dependent on the time after application of a low pH trigger for fusion. At early time points, hemifusion was more common and full redistribution of both bilayers was rare, whereas later full redistribution of both probes was frequently observed. In contrast to wild-type HA, the latter was not accompanied by mixing of the cytoplasmic marker Lucifer Yellow. We conclude that 1) the GPI-HA-mediated hemifusion intermediate is meta-stable and 2) expansion of an aqueous fusion pore requires the transmembrane and/or cytoplasmic domain of HA.

Rapid kinetics of insertion and accessibility of spin-labeled phospholipid analogs in lipid membranes: a stopped-flow electron paramagnetic resonance approach

Spin-labeled phospholipid analogs have been employed to probe the transbilayer distribution of endogenous phospholipids in various membrane systems. To determine the transmembrane distribution of the spin-labeled analogs, the analogs are usually inserted into the membrane of interest and subsequently the amount of analog in the outer membrane leaflet is determined either by chemical reduction with ascorbate or by back-exchange to bovine serum albumin (BSA). For accurate determination of the transbilayer distribution of analogs, both the kinetics of incorporation and those of accessibility of analogs to ascorbate or BSA have to be fast in comparison to their transbilayer movement. By means of stopped-flow electron paramagnetic resonance (EPR) spectroscopy, we have studied the kinetics of incorporation of the spin-labeled phosphatidylcholine (PC) analog 1-palmitoyl-2-(4-doxylpentanoyl)-sn-glycero-3-phosphocholine (SL-PC) and of its accessibility to chemical reduction and to back-exchange at room temperature. Incorporation of SL-PC into the outer leaflet of egg phosphatidylcholine (EPC) and red cell ghost membranes was essentially completed within 5 s. Ninety percent of the SL-PC molecules located in the outer membrane leaflet of those membranes were extracted by BSA within 15 s. All exterior-facing SL-PC molecules were reduced by ascorbate in a pseudo-first-order reaction within 60 s in EPC membranes and within 90 s in red cell ghost membranes. The rate of the reduction process could be enhanced by approximately 30-fold when 6-O-phenyl-ascorbic acid was used instead of ascorbate as the reducing agent. The results are discussed in light of assaying rapid transbilayer movement of spin-labeled analogs in biological membranes.

Regulatory mechanisms in maintenance and modulation of transmembrane lipid asymmetry: pathophysiological implications

The two leaflets of the plasma membrane of eukaryotic cells differ in lipid composition: the outer leaflet comprises mainly neutral choline containing phospholipids, whereas the aminophospholipids reside almost exclusively in the cytoplasmic leaflet. The importance of transmembrane lipid asymmetry may be judged from the fact that the cell invests energy to maintain this situation for which at least two regulatory mechanisms are held responsible. A translocase, selective for aminophospholipids, acts as an ATP-dependent pump for rapid inward movement of phosphatidylserine (PS) and phosphatidylethanolamine; in addition, a non-selective, but also ATP-dependent pump causes outward movement of phospholipids, be it at a much lower rate compared to the inward transport by the aminophospholipid translocase. These two systems, acting in concert, are thought to be the main players in the maintenance of a dynamic equilibrium of the phospholipids over both membrane leaflets. Dissipation of membrane lipid asymmetry can be elicited in different cell types under a variety of conditions; in particular, platelets upon activation rapidly lose their normal plasma membrane lipid distribution, but also in other blood cells, lipid asymmetry can be lost, be it at a much lower rate and extent than in platelets. A putative protein, referred to as "scramblase' has been described, which requires the continuous presence of elevated intracellular Ca(2+)-levels, to allow a rapid, non-selective and bidirectional transbilayer movement of phospholipids. Although scrambling of lipids does not require ATP as such, preliminary studies suggest the possible involvement of one or more phosphorylated proteins. The most prominent consequence of the loss of phospholipid asymmetry is exposure of PS in the outer leaflet of the plasma membrane. Surface-exposed PS serves several important physiological functions: it promotes assembly of enzyme complexes of the coagulation cascade, it forms a signal for cell-cell recognition, which is important for cell scavenging processes. Surface-exposure of PS is an early phenomenon of apoptosis and appears to be involved in efficient removal of these cells. In addition, PS in the outer leaflet of cells is thought to play a role in cell fusion processes. It may be clear from the foregoing, that the amount of PS present at the cell surface needs to be tightly controlled, and that an impairment of this process leads to either excessive- or diminished exposition of PS which may have several pathophysiological consequences.

Manipulation of the phosphatidylethanolamine pool in the human red cell membrane affects its Mg2+-ATPase activity

Decreasing the size of the outer leaflet pool of phosphatidylethanolamine (PE) in the erythrocyte membrane by treatment of intact cells with either phospholipase A2, or trinitrobenzenesulphonic acid (TNBS), causes a corresponding decrease in Mg(2+)-ATPase activity as determined in their respective ghosts. Also, incubation of ghosts with Ro09-0198, a cyclic peptide from Streptoverticillium which is known to interact specifically with PE, causes a decrease in Mg(2+)-ATPase activity which is dependent on the amount of peptide added. These three different approaches, all causing a decrease in endogenous PE, thus result in a concomitant decrease in Mg(2+)-ATPase activity which reaches a plateau level at approximately 25% residual activity. Hence, it is inferred that the complementary fraction (75%) of the total Mg(2+)-ATPase in the red cell membrane is closely related to the functioning of its aminophospholipid specific translocase as it mediates a (continuous) transport of PE molecules from outer to inner membrane leaflet. This view is supported by the observation that an increase in the total amount of PE in the membrane by decarboxylation of an appreciable fraction of its PS, results in a considerable increase in Mg(2+)-ATPase activity.

Effect of X31 influenza virus fusion on phosphatidylserine asymmetry in erythrocytes

Influenza virus fusion is mediated by its fusion protein, hemagglutinin (HA). HA undergoes a low pH dependent conformational change that results in insertion into the cell membrane bilayer, formation of a fusion pore, and merging of membrane lipids and establishment of cytoplasmic continuity. Erythrocytes, which can serve as targets of influenza virus fusion, display an asymmetric transbilayer arrangement of their phospholipids. The effect of influenza virus fusion on erythrocyte phosphatidylserine asymmetry was determined. Influenza virus were bound to erythrocytes containing the fluorescent membrane probe NBD-PS in the inner leaflet. Induction of fusion by exposure to a low pH environment resulted in movement of PS to the outer leaflet of the cell as well as hemolysis. Insertion of the fusion protein into erythrocytes and subsequent fusion can be distinguished from hemolysis by examining the interaction of a soluble form of HA (BHA) with cells and by monitoring viral fusion at low temperatures. No hemolysis was observed under either condition. BHA binding and insertion into cells did not affect the asymmetry of PS. Incubation of influenza virus fusion at pH 5, 0 degrees C resulted in complete fusion but no outward movement of PS was observed. These findings suggest the viral fusion pore does not involve a rearrangement of the transbilayer phospholipid organization of the target membrane.

Binding and phagocytosis of apoptotic vascular smooth muscle cells is mediated in part by exposure of phosphatidylserine

Apoptosis of vascular smooth muscle cells has recently been demonstrated to occur in vitro and in vivo. Uptake of apoptotic cells into adjacent normal cells appears to be rapid and specific. We have investigated binding and phagocytosis of apoptotic vascular smooth muscle cells by normal smooth muscle cell monolayers. Vascular smooth muscle cells were infected with the proto-oncogene c-myc or the adenovirus E1A gene, induced to undergo apoptosis in low-serum conditions, and then incubated with normal smooth muscle cells. Apoptosis was accompanied by a marked increase in exposure of phosphatidylserine on the outer surface of the cell, which was recognized by binding to annexin V. Liposomes containing phosphatidylserine but not phosphatidylinositol inhibited uptake of apoptotic cells in a dose-dependent manner to a maximum of 50% inhibition; annexin V also inhibited the uptake of apoptotic cells in a dose-dependent and calcium-dependent manner. Binding of apoptotic bodies did not appear to be mediated by endogenous annexin V, as evidenced by the inability of an antibody to annexin V to inhibit uptake. Smooth muscle cells were also able to recognize exposed phosphatidylserine on other cell types, as judged by their ability to bind erythrocytes having a high degree of exposed phosphatidylserine. We conclude that smooth muscle cells express phosphatidylserine during apoptosis, and this exposure partly mediates binding and phagocytosis of dead cells. This mechanism may be important in promoting rapid cell removal in the vessel wall.

Calcium stimulation of procoagulant activity in human erythrocytes. ATP dependence and the effects of modifiers of stimulation and recovery

The human erythrocyte membrane is generally considered to have no procoagulant activity. The normal membrane is characterized as having an asymmetric distribution of phospholipid species such that negatively charged and aminophospholipids are predominantly located on the inner leaflet of the membrane bilayer. Elevation of cytoplasmic Ca2+ in erythrocytes produces an assortment of biochemical and structural responses that include diminished phospholipid asymmetry and an elevation in procoagulant activity. Maintenance of the normal asymmetric distribution of phospholipid species is believed to be largely mediated by a phospholipid translocase mechanism. We have utilized a recently developed single-step kinetic assay of procoagulant activity to investigate the mechanisms of Ca2+ stimulation of procoagulant activity and recovery from the procoagulant state upon removal of Ca2+. This study demonstrated that stimulation of procoagulant activity by elevated cytoplasmic Ca2+ is greatly diminished in ATP-depleted erythrocytes. Phospholipid translocase inhibitors failed to fully inhibit recovery from the procoagulant state after removal of Ca2+. The data indicate that recovery of endogenous lipid from a procoagulant cofiguration may not be entirely mediated by the phospholipid translocase. Additionally, the data are inconsistent with the phospholipid translocase mediating the Ca(2+)-induced elevation of procoagulant activity, although the involvement of other protein(s) is indicated.

Production and characterization of transformed B-lymphocytes expressing the membrane defect of Scott syndrome

Scott syndrome is a bleeding disorder associated with an isolated defect in expression of membrane coagulant activity by stimulated platelets. This defect represents a decrease in platelet membrane binding sites for coagulation factors Va and VIIIa, reflecting diminished surface exposure of phosphatidylserine (PS). To gain insight into the cellular and genetic basis for this disorder, B-lymphocytes from a patient with Scott syndrome and from normal donors were immortalized by EBV-transformation, and tested for their capacity to expose plasma membrane PS in response to the Ca2+ ionophore, A23187. Upon incubation with A23187, EBV-lymphoblasts derived from normal donors consistently induced surface expression of PS in > 70% of all cells, as detected by membrane association of the PS-binding proteins, factor Va or annexin V. PS exposure in these cells was maximal after 5 min, and saturated at < 100 microM external free [Ca2+]. By contrast, < 30% of Scott syndrome lymphoblasts exposed PS, and saturation was not observed at > 1 mM external free [Ca2+]. Single-cell clones derived from the Scott lymphoblasts all exhibited a diminished response to A23187 comparable with that of the parental cells, suggesting that all lymphocytes from this patient share this membrane abnormality. Hybridomas prepared by fusion of Scott lymphoblasts with the myeloma cell line UC-LUC showed responses to Ca2+ ionophore comparable to those observed for normal lymphoblasts and for hybridomas prepared by fusion of normal lymphoblasts with UC-LUC. This correction of the Scott abnormality suggests possible complementation of an aberrant gene(s) responsible for this disorder.

Back and forth: the regulation and function of transbilayer phospholipid movement in eukaryotic cells

That some membranes restrict certain lipid species to one side of the bilayer and others to the opposite side has been known for two decades. However, how this asymmetric transbilayer distribution is generated and controlled, how many and what type of membranes are so structured, and even the reason for its existence is just now beginning to be understood. It has been a decade since the discovery of an activity which transports in an ATP-dependent manner only the aminophospholipids from the outer to the inner leaflet of the plasma membrane. This aminophospholipid translocase has yet to be isolated, reconstituted, and identified molecularly. Elevating intracellular Ca2+ allows all the major classes of phospholipids to move freely across the bilayer, scrambling lipids and dissipating asymmetry. The nature of this pathway and its mode of activation by Ca2+ remain to be determined. Though loss of transbilayer asymmetry by blood cells clearly produces a procoagulant surface and increases interactions with the reticuloendothelial system, it remains to be elucidated whether maintenance of blood homeostasis is just one expression of a more general raison d'être for lipid asymmetry. It is these persisting uncertainties and gaps in our knowledge which make the field such an interesting and exciting challenge at the present time.

Phosphatidylserine vesicles inhibit phagocytosis of erythrocytes with a symmetric transbilayer distribution of phospholipids

In the plasma membrane of erythrocytes, the phospholipids are asymmetrically distributed between the two leaflets of the bilayer. Erythrocytes that have lost this asymmetric distribution are more readily phagocytosed in vitro by macrophages. Surface exposure of phosphatidylserine (PS), normally restricted to the inner leaflet of the bilayer, has been suggested as a signal by which macrophages recognize erythrocytes. To test whether lipid-symmetric erythrocytes are recognized by this mechanism, artificial lipid vesicles made of PS were tested for their ability to inhibit phagocytosis. J774 mouse macrophages, which preferentially phagocytose PS vesicles, were incubated with a mixture of lipid-symmetric erythrocytes and vesicles. PS vesicles reduced phagocytosis of lipid-symmetric erythrocytes to the level of lipid-asymmetric erythrocytes, whereas phosphatidylcholine vesicles had only a limited effect. These results support a role for PS in recognition of erythrocytes and lend credence to the more general hypothesis that PS is involved in the recognition of all blood cells by the reticuloendothelial system.

Erythrocyte sodium-lithium countertransport activity is related to membrane fluidity in IDDM patients

Sodium-lithium countertransport (SLC) activity at a standard physiological sodium concentration is raised in uncomplicated IDDM, for which the kinetic mechanism is a raised maximum velocity (Vmax). Diabetic patients with nephropathy do not have raised values for Vmax but a low Michaelis constant (km). Transporter activity could be influenced by its membrane lipid environment. This was assessed in 21 control subjects, 32 uncomplicated diabetic patients, 17 patients with diabetic nephropathy and 11 patients with non-diabetic nephropathy by measuring the fluorescence anisotropy of DPH and TMA-DPH to assess different membrane regions. Standard SLC was higher in all the patient groups compared to the control subjects: 0.307 +/- 0.020 mmol Li/h x 1 cells in uncomplicated IDDM; 0.300 +/- 0.032 in diabetic nephropathy patients and 0.276 +/- 0.019 in non-diabetic nephropathy patients vs 0.216 +/- 0.011 mmol Li/h x 1 cells in control subjects (p < 0.001, p < 0.05, p < 0.05, respectively). This was due to raised Vmax values in the uncomplicated group: 0.528 +/- 0.035 vs 0.385 +/- 0.022 mmol Li/h x 1 cells in control subjects (p = 0.001) and low values for km in the diabetic nephropathy group: 58 (27-170) vs 106 (81-161) mmol/l in control subjects (p < 0.001). Raised SLC in the non-diabetic nephropathy group was largely due to raised Vmax: 0.460 +/- 0.030 mmol Li/h x 1 cells; p = 0.053, with no difference in km: 99.5 (74-137).(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid determination of the transbilayer distribution of NBD-phospholipids in erythrocyte membranes with dithionite

The assessment of the transverse distribution and mobility of NBD-labelled phospholipid analogues in biological membranes by selective chemical destruction of fluorescent label in the outer monolayer with dithionite has been investigated using resealed erythrocyte ghosts as a model system. The distribution of those analogues can be determined in < 30 s directly in the cell suspension provided the permeation of dithionite across the membrane is suppressed. The results were compared with data on translocation of either NBD- or spin-labelled phospholipid analogues obtained with the technique of back exchange to BSA. It is shown that the passage of dithionite can be mediated by anion-transport systems such as band 3 which is inhibited by DIDS. Appropriate conditions for the applicability of the assay were elucidated also using resealed ghosts having fluorescent NBD-taurine in the intracellular lumen. The application of the assay to measure fast translocation processes, e.g. those mediated by the aminophospholipid translocase, is described.

Pyrularia thionin binding to and the role of tryptophan-8 in the enhancement of phosphatidylserine domains in erythrocyte membranes

Pyrularia thionin is a small, strongly basic peptide which interacts readily with cellular and synthetic membranes. With cells it induces hemolysis, depolarizes the cellular membrane with an accompanying influx of Ca2+, and activates an endogenous phospholipase A2. Evidence points toward a binding site involving phosphatidylserine (PS). This study shows that addition of the peptide to erythrocyte membranes as well as to vesicles formed from phospholipids isolated from erythrocyte membranes causes an enhancement of phospholipid domains which are made visible by the use of fluorescence digital imaging microscopy with fluorescent derivatives of PS (NBD-PS) and phosphatidylcholine (NBD-PC). Addition of thionin caused a large increase in NBD-PS domains, with an accompanying enrichment of NBD-PC in another separate domain. Double-labeling experiments performed with a Texas Red derivative of thionin show that the peptide binds to the domain enriched in NBD-PS. P thionin inactivated by modification of Trp-8 with N-bromosuccinimide lost the ability to enhance PS domains, although it bound to the membrane with the same affinity as native P thionin. This shows that binding to the membrane is not in itself sufficient to cause the NBD-PS and NBD-PC redistribution into domains.

The origin of parasitophorous vacuole membrane lipids in malaria-infected erythrocytes

During invasion of an erythrocyte by a malaria merozoite, an indentation develops in the erythrocyte surface at the point of contact between the two cells. This indentation deepens as invasion progresses, until the merozoite is completely surrounded by a membrane known as the parasitophorous vacuole membrane (PVM). We incorporated fluorescent lipophilic probes and phospholipid analogs into the erythrocyte membrane, and followed the fate of these probes during PVM formation with low-light-level video fluorescence microscopy. The concentration of probe in the forming PVM was indistinguishable from the concentration of probe in the erythrocyte membrane, suggesting that the lipids of the PVM are continuous with and derived from the host cell membrane during invasion. In contrast, fluorescently labeled erythrocyte surface proteins were largely excluded from the forming PVM. These data are consistent with a model for PVM formation in which the merozoite induces a localized invagination in the erythrocyte lipid bilayer, concomitant with a localized restructuring of the host cell cytoskeleton.

Neonatal red blood cell lysis induced by hypertonic low ionic strength media

Human neonatal red cells (placental blood) incubated in hypertonic sucrose media showed a significative lytic process in a relatively short time interval. The addition of sodium chloride into the sucrose media reduced the extent of hemolysis. In contrast, the addition of calcium chloride enhanced the hemolysis in these red cells. Calcium-membrane components complex formation that destabilize membrane's bilayer structure would explain the calcium effect above mentioned (on account of the low ionic strength media used and exposed fixed negative charges) This study intends to clarify, in neonatal red cells, the relation between surface charges and cellular stability.

Phospholipids in animal eukaryotic membranes: transverse asymmetry and movement

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