Complete exchange of phosphatidylcholine from intact erythrocytes after protein crosslinking

Flow cytometric quantitation of red blood cell vesicles in thalassemia

BACKGROUND

Thalassemia is a hereditary hemolytic anemia caused by mutations in the globin gene complex. Circulatory disturbances including arterial and venous thrombosis have also been noted in these patients. Aggregability of abnormal RBC and the high level of membrane-derived microparticles stemming from activated platelets and other blood cells are thought to be responsible for the associated thrombotic risk. Destruction of RBC is also thought to be an important pathophysiological consequence, particularly through the formation of circulating vesicles. To our knowledge, there has been no attempt to quantitatively evaluate the number of RBC vesicles in thalassemia. This prompted us to study the level of RBC vesicles in the peripheral blood of thalassemia patients using quantitative flow cytometry.

METHODS

Whole blood from each subject was doubly stained for RBC and platelet or annexin V markers, together with the known density TruCount beads. RBC vesicles were gated according to their forward/side scatter and RBC marker. Percentage of RBC vesicles and their absolute number were analyzed by flow cytometry.

RESULTS

Our data indicated that RBC vesicles were annexin V-positive. The number of annexin V-positive events was higher than their intact RBCs. RBC vesicles were present in both normal and thalassemic blood samples, but the numbers of RBC vesicles were significantly higher in thalassemia. Both the percentage and the absolute number of RBC vesicles were especially marked in splenectomized subjects with beta-thalassemia/Hemoglobin E. When clinical and hematological indices were compared with RBC vesicles, there was an inverse relationship between the degree of severity in thalassemia patients and the number of RBC vesicles.

CONCLUSION

Flow cytometric quantitation of RBC vesicles is simple, reliable and may offer new insights in to study of the relationship between defective hemoglobin synthesis, RBC perturbation and pathophysiological complications in thalassemia.

Chemical control of phospholipid distribution across bilayer membranes

Most biological membranes possess an asymmetric transbilayer distribution of phospholipids. Endogenous enzymes expend energy to maintain the arrangement by promoting the rate of phospholipid translocation, or flip-flop. Researchers have discovered ways to modify this distribution through the use of chemicals. This review presents a critical analysis of the phospholipid asymmetry data in the literature followed by a brief overview of the maintenance and physiological consequences of phospholipid asymmetry, and finishes with a list of chemical ways to alter phospholipid distribution by enhancement of flip-flop.

Involvement of ATP-dependent aminophospholipid translocation in maintaining phospholipid asymmetry in diamide-treated human erythrocytes

Crosslinking of membrane skeletal proteins such as spectrin by oxidation of their SH-groups can be provoked by treatment of intact erythrocytes with diamide. Shortly after exposure of human erythrocytes to diamide and despite the transverse destabilization of the lipid bilayer that was observed in these cells (Franck, P.F.H., Op den Kamp, J.A.F., Roelofsen, B. and Van Deenen, L.L.M. (1986) Biochim. Biophys. Acta 857, 127-130), no abnormalities could be detected regarding the asymmetric distribution of the phospholipids when probed by either the prothrombinase assay or brief exposure of the cells to a modified phospholipase A2 with enhanced membrane penetrating capacity. This asymmetry appeared to undergo dramatic changes however, when the ATP content of the cytosol had decreased to less than 10% of its original level during prolonged incubation of the treated cells. These observations indicate that the initial maintenance of phospholipid asymmetry in diamide-treated erythrocytes can be solely ascribed to the action of the ATP-dependent aminophospholipid translocase. This view is supported by experiments involving radiolabeled phospholipids of which trace amounts had been inserted into the outer membrane leaflet of diamide-treated red cells and which still showed a preferential translocation of both aminophospholipids in favour of the inner monolayer, be it that the efficiency of the translocase was found to be impaired when compared to control cells.

Glutathione-dependent inactivation of sodium-dependent phosphate transport across rat renal brush-border membrane

Thiol/disulfide is fundamental in protein function; we previously observed an inhibitory effect of thiol oxidants on the Na-dependent phosphate (Pi) uptake into renal brush border membrane vesicles (BBMV). We examined whether oxidation of glutathione (GSH) is involved in the mechanism. Vesicular thiols were measured by liquid chromatography. BBMV were incubated with reagents before an influx of Pi. Diamide (5 mM) reduced the capacity of the Pi uptake. Subsequent treatment with dithiothreitol (5 mM) blocked the inhibitory effect of diamide. Vesicular GSH was not modified only by the incubation, whereas it was oxidized by the treatment with diamide, and reduced by dithiothreitol. Furthermore, in vivo treatment with cAMP provided GSH-depleted BBMV without any influence on Pi uptake. Diamide did not inhibit the transport of Pi into GSH-depleted vesicles, but it did inhibit the uptake when GSH was introduced into the vesicles. In conclusion, a GSH-dependent mechanism is involved in the inhibitory effect of diamide on sodium-dependent Pi transport across the renal brush-border membrane.

Aminophospholipid translocase in the plasma membrane of Friend erythroleukemic cells can induce an asymmetric topology for phosphatidylserine but not for phosphatidylethanolamine

The ATP-dependent translocation of phospholipids in the plasma membrane of intact Friend erythroleukemic cells (FELCs) was studied in comparison with that in the membrane of mature murine erythrocytes. This was done by following the fate of radiolabeled phospholipid molecules, previously inserted into the outer monolayer of the plasma membranes by using a non-specific lipid transfer protein. The transbilayer equilibration of these probe molecules was monitored by treating the cells--under essentially non-lytic conditions--with phospholipases A2 of different origin. Rapid reorientations of the newly introduced aminophospholipids in favour of the inner membrane leaflet were observed in fresh mouse erythrocytes; the inward translocation of phosphatidylcholine (PC) in this membrane proceeded relatively slow. In FELCs, on the other hand, all three glycerophospholipids equilibrated over both halves of the plasma membrane very rapidly, i.e. within 1 h; nevertheless, an asymmetric distribution in favour of the inner monolayer was only observed for phosphatidylserine (PS). Lowering the ATP-level in the FELCs caused a reduction in the rate of inward translocation of both aminophospholipids, but not of that of PC, indicating that this translocation of PS and phosphatidylethanolamine (PE) is clearly ATP-dependent. Hence, the situation in the plasma membrane of the FELC is rather unique in a sense that, though an ATP-dependent translocase is present and active both for PS and PE, its activity results in an asymmetric distribution of PS, but not of PE. This remarkable situation might be the consequence of the fact that, in contrast to the mature red cell, this precursor cell still lacks a complete membrane skeletal network.

Hemolytic activity of a cyclic peptide Ro09-0198 isolated from Streptoverticillium

Ro09-0198, a cyclic peptide isolated from culture filtrates of Streptoverticillium griseoverticillatum, induced lysis of erythrocytes. Preincubation of the peptide with phosphatidylethanolamine reduced the hemolytic activity, whereas other phospholipids present in erythrocytes in nature had no effect. A study of the structural requirements on phosphatidylethanolamine necessary for interaction with the peptide indicates that Ro09-0198 recognizes strictly a particular chemical structure of phosphatidylethanolamine: dialkylphosphoethanolamine as well as 1-acylglycerophosphoethanolamine showed the same inhibitory effect on hemolysis induced by Ro09-0198 as diacylphosphatidylethanolamine, whereas phosphoethanolamine gave no inhibitory effect. Neither phosphatidyl-N-monomethylethanolamine nor alkylphosphopropanolamine had an inhibitory effect. Consequently, the hydrophobic chain is necessary for the interaction and the phosphoethanolamine moiety is exactly recognized by the peptide. Ro-09-0198-induced hemolysis was temperature-dependent and the sensitivity of hemolysis differed greatly among animal species.

Enhanced transbilayer mobility of phospholipids in malaria-infected monkey erythrocytes: a spin-label study

Using a recently described technique of electron spin resonance spectroscopy, we determined the rate of transbilayer mobility (flip) of the four major simian-erythrocyte phospholipids in the erythrocyte membrane after infection by Plasmodium knowlesi. The development of the malarial parasite induces a very large increase in the flip rate of these phospholipids (mainly during the ring stage and at the beginning of trophozoite maturation). The half flip time fell from 2 and 3 hr in the case of choline phospholipids in healthy erythrocytes to less than 15 min in erythrocytes infected with the last stage of the parasite.

Studies on sickled erythrocytes provide evidence that the asymmetric distribution of phosphatidylserine in the red cell membrane is maintained by both ATP-dependent translocation and interaction with membrane skeletal proteins

In order to study factors which are involved in maintenance of phosphatidylserine (PS) asymmetry within the human red cell membrane, we measured the effect of ATP-depletion and of membrane skeleton/lipid bilayer uncoupling induced by sickling on the distribution of PS within the membrane bilayer of sickle cells. Trace amounts of radiolabeled PS were introduced into the outer membrane leaflet of both fresh and ATP-depleted reversibly sickled cells (RSCs), using a non-specific lipid transfer protein purified from bovine liver. The equilibration of the newly introduced PS over the two halves of the bilayer was monitored by treatment of the cells with phospholipase A2 which selectively hydrolyzes only those molecules present in the outer membrane leaflet. Within 1 h after insertion into fresh RSCs, only 10% of the labeled PS was accessible to the action of phospholipase A2. This fraction was markedly increased when the cells were subsequently deoxygenated. Prolonged deoxygenation of RSCs, deprived of their ATP after incorporation of radiolabeled PS, caused enhanced phospholipase A2-induced hydrolysis of radiolabeled PS. Similarly, phospholipase A2-induced hydrolysis of endogenous PS in intact RSCs was markedly enhanced when ATP-depleted, but not when fresh cells, were incubated under nitrogen for 3.5 h. Deoxygenated ATP-depleted RSCs markedly enhanced the rate of thrombin formation in the presence of purified coagulation factors Xa, Va, prothrombin and Ca2+. This enhancement appeared to be dependent on the duration of incubation under nitrogen. This phenomenon, indicating the presence of increasing amounts of endogenous PS in the outer membrane leaflet, was not observed when either fresh RSCs or ATP-depleted normal erythrocytes were incubated under nitrogen. Our present observations provide evidence that, in addition to the interaction of PS with the skeletal proteins, an ATP-dependent translocation of PS is required to maintain its absolute asymmetric distribution in the human erythrocyte membrane.

Loss of phospholipid asymmetry in dilauroylphosphatidylcholine induced plasma membrane vesicles from human platelets

Incubation of human platelets with unilamellar vesicles composed of dilauroylphosphatidylcholine (DLPC) induces shedding of small vesicular structures from the platelet plasma membrane. No significant cell lysis is observed during the process of shedding. Isolated spicules contain the major membrane glycoproteins, Ib, IIb, and IIIa, which are used to define the sidedness of the spicule membrane. These glycoproteins are completely susceptible to chymotrypsin treatment, whereas cytoskeletal proteins are inaccessible towards this enzyme. This demonstrates that the spicule membranes have a right-side-out orientation in as far as membrane proteins are concerned. Isolated spicules were 30-fold more active than platelets in stimulating prothrombin conversion to thrombin by the prothrombinase complex (factors Xa, Va and Ca2+). The increased prothrombinase activity reflects an increased amount of phosphatidylserine in the outer leaflet of the spicule membrane. Protein analysis of platelet spicules and native platelets reveals a number of differences, the most conspicuous of which is the virtual absence of myosin in the spicule preparations. It is proposed that a lack of myosin produces a different cytoskeletal organization in the spicules. This enables phosphatidylserine to become exposed at the outer surface of the spicule membrane.

An intracellular simian malarial parasite (Plasmodium knowlesi) induces stage-dependent alterations in membrane phospholipid organization of its host erythrocyte

The membrane phospholipid organization in monkey erythrocytes harbouring different developmental stages of the simian malarial parasite Plasmodium knowlesi was studied using phospholipase A2 from two different sources and Merocyanine 540 as the external-membrane probes. Experiments were done to confirm that the phospholipases did not penetrate into the infected cells or hydrolyse phospholipids during membrane isolation. The parasite-free erythrocyte membrane was isolated by differential centrifugation or by using the cationic beads Affi-Gel 731. The purity of the membranes was established by optical and electron microscopy, and by assaying the parasite-specific enzyme glutamate dehydrogenase. About 10% of the phosphatidylethanolamine and none of phosphatidylserine were hydrolysed by the phospholipases in intact normal monkey erythrocytes. However, accessibility of these aminophospholipids to the enzymes was significantly enhanced in the infected cells under identical conditions. The degree of this enhancement depended on the developmental stage of the intracellular parasite, but not on the parasitaemia levels in the infected monkeys, and increased with the parasite growth inside the cells. Analogously, Merocyanine 540 was found to label the trophozoite- or schizont-infected erythrocytes, but not the ring-infected or normal cells. These results demonstrate that the intracellular malarial parasite produces stage-dependent alterations in the membrane phospholipid organization of its host erythrocyte.

Phospholipid organization in monkey erythrocytes upon Plasmodium knowlesi infection

The phospholipid organization in monkey erythrocytes upon Plasmodium knowlesi infection has been studied. Parasitized and nonparasitized erythrocytes from malaria-infected blood were separated and pure erythrocyte membranes from parasitized cells were isolated using Affi-Gel beads. In this way, the phospholipid content and composition of the membrane of nonparasitized cells, the erythrocyte membrane of parasitized cells and the parasite could be determined. The phospholipid content and composition of the erythrocyte membranes of nonparasitized and parasitized cells and erythrocytes from chloroquine-treated monkeys cured from malaria, were the same as in normal erythrocytes. The phospholipid content of the parasite increased during its development, but its composition remained unchanged. Three independent techniques, i.e., treatment of intact cells with phospholipase A2 and sphingomyelinase C, fluorescamine labeling of aminophospholipids and a phosphatidylcholine-transfer protein-mediated exchange procedure have been applied to assess the disposition of phospholipids in: erythrocytes from healthy monkeys, nonparasitized and parasitized erythrocytes from monkeys infected with Plasmodium knowlesi, and erythrocytes from monkeys that had been cured from malaria by chloroquine treatment. The results obtained by these experiments do not show any abnormality in phospholipid asymmetry in the erythrocyte from malaria-infected (splenectomized) monkeys, neither in the nonparasitized cells, nor in the parasitized cells at any stage of parasite development. Nevertheless, a considerable degree of lipid bilayer destabilization in the membrane of the parasitized cells is apparent from the enhanced exchangeability of the PC from those cells, as well as from their increased permeability towards fluorescamine.

Effect of neuraminidase treatment on the topological distribution of phospholipids in chick brain microsomes

The desialylation of chick brain microsomal membranes affects the transbilayer distribution of phospholipids. When intact microsomes were treated with neuraminidase, less phosphatidylcholine and sphingomyelin could be hydrolysed with phospholipase C under experimental conditions which allowed the hydrolysis of the phospholipids of the external leaflet only. In contrast, the accessibility of phosphatidylethanolamine and phosphatidylserine to the external probes (trinitrobenzene sulfonic acid or phospholipase C) was not affected. After neuraminidase treatment of a microsomal fraction, less phosphatidylcholine, newly synthesized through the cytidine pathway, could be hydrolysed by phospholipase C, whereas the reaction of newly synthesized phosphatidylethanolamine molecules with trinitrobenzene sulfonic acid was not affected. The results suggest that in biological membranes some choline phospholipid molecules may interact with the sialyl residue of sialocompounds. This interaction may contribute to the maintenance of phospholipid asymmetry in brain membranes.

Does diamide treatment of intact human erythrocytes cause a loss of phospholipid asymmetry?

Diamide-treated human erythrocytes have been compared with native red cells as to the accessibility of their amino phospholipids to both phospholipase A2 hydrolysis and fluorescamine labeling. In agreement with observations by others (Haest, C.W.M., Plasa, G., Kamp, D. and Deuticke, B. (1978) Biochim. Biophys. Acta 509, 21-32), treatment of intact human erythrocytes with diamide resulted in considerably enhanced degradation of amino phospholipids upon subsequent incubation of the cells with bee venom phospholipase A2. The hydrolysis of phosphatidylethanolamine (PE) in control cells reached a plateau value at 5% after 10 min. In diamide-treated cells, on the other hand, PE hydrolysis did not level off. Contrastingly, dose-response curves recorded for the labeling of PE with the very fast reacting NH2-group-specific reagent, fluorescamine, showed identical results for both native and diamide-treated erythrocytes. In each of these two cases, a plateau was reached after approx. 15% of the PE had been labeled. These results strongly suggest that the enhanced phospholipase-A2-induced hydrolysis of amino phospholipids in diamide-treated erythrocytes may reflect a destabilization of the lipid bilayer, rather than an in situ loss of phospholipid asymmetry.

Outside-inside translocation of aminophospholipids in the human erythrocyte membrane is mediated by a specific enzyme

When human erythrocytes are incubated with spin-labeled analogues of sphingomyelin, phosphatidylcholine, phosphatidylserine, or phosphatidylethanolamine, with a short beta chain (C5) bearing a doxyl group at the fourth carbon position, the labeled lipids incorporate readily in the outer monolayer. The incorporation is followed in fresh erythrocytes by a selective inward diffusion of the amino derivatives. This observation led us to postulate the existence of a selective ATP-dependent system that would flip aminophospholipids from the outer to the inner monolayer [Seigneuret, M., & Devaux, P. F. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 3751-3755]. This study further examines the nature of this selective transport and demonstrates that it is mediated by a specific membrane protein. By measurement of the initial rate of transverse diffusion of spin-labeled lipids incorporated at various concentrations in the membrane outer leaflet of packed erythrocytes, apparent Km values were determined for the phosphatidylserine and phosphatidylethanolamine analogues. A ratio of approximately equal to 1/9.4 [corrected] was obtained (KmPS/KmPE). Using spin-labels bearing either a 14N or a 15N isotope, we have carried out competition experiments allowing us to measure simultaneously the transport of two different phospholipids. By this procedure, we show that phosphatidylserine and phosphatidylethanolamine compete for the same transport site but that phosphatidylserine has a higher affinity, in agreement with a lower apparent Km. On the other hand, the slow diffusion of the phosphatidylcholine or sphingomyelin analogues has no influence on the transport of phosphatidylserine or phosphatidylethanolamine. Experiments carried out in ghosts loaded with ATP enabled us to determine the activation energies for phosphatidylserine and phosphatidylcholine transverse diffusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid molecular shape affects erythrocyte morphology: a study involving replacement of native phosphatidylcholine with different species followed by treatment of cells with sphingomyelinase C or phospholipase A2

In a previous report it was shown that the replacement of native erythrocyte phosphatidylcholine (PC) with different PC species which have defined acyl chain compositions can lead to morphological changes (Kuypers, F.A., W. Berendsen, B. Roelofsen, J. A. F. Op den Kamp, and L.L.M. van Deenen, 1984, J. Cell Biol., 99:2260-2267). It was proposed that differences in molecular shape between the introduced PC species and normal erythrocyte PC caused the membrane to bend outwards or inwards, depending on the shape of the PC exchanged. To support this proposal, two requirements would have to be fulfilled: the exchange reaction would take place only with the outer lipid monolayer of the erythrocyte, and the extent of lipid transbilayer movement would be restricted. If this theory is correct, any treatment causing unilateral changes in lipid molecular shape should lead to predictable morphological changes. Since this hypothesis is a refinement of the coupled bilayer hypothesis, but so far lacks experimental support, we have sought other means to change lipid molecular shape unilaterally. Shape changes of human erythrocytes were induced by the replacement of native PC by various PC species using a phosphatidylcholine-specific transfer protein: by hydrolysis of phospholipids in intact cells using sphingomyelinase C or phospholipase A2, and by the combination of both procedures. The morphological changes were predictable; additive when both treatments were applied, and explicable on the basis of the geometry of the lipid molecules involved. The results strongly support the notion that lipid molecular shape affects erythrocyte morphology.

Incorporation and translocation of aminophospholipids in human erythrocytes

Cell morphology changes are used to examine the interaction of exogenous phosphatidylserine and phosphatidylethanolamine with human erythrocytes. Short-chain saturated lipids transfer from liposomes to cells, inducing shape changes that are indicative of their incorporation into, and in some cases translocation across, the cell membrane bilayer. Dioleoylphosphatidylserine and low concentrations of dilauroyl- and dimyristoylphosphatidylserine induce stomatocytosis. At higher concentrations, dilauroylphosphatidylserine and dimyristoylphosphatidylserine induce a biphasic shape change: the cells crenate initially but rapidly revert to a discocytic and eventually stomatocytic shape. The extent of these shape changes is dose dependent and increases with increasing hydrophilicity of the phospholipid. Cells treated with dilauroylphosphatidylethanolamine and bovine brain lysophosphatidylserine exhibit a similar biphasic shape change but revert to discocytes rather than stomatocytes. These shape changes are not a result of vesicle--cell fusion nor can they be accounted for by cholesterol depletion. The reversion from crenated to stomatocytic forms is dependent on intracellular ATP and Mg2+ concentrations and the state of protein sulfhydryl groups. The present results are consistent with the existence of a Mg2+- and ATP-dependent protein in erythrocytes that selectively translocates aminophospholipids to the membrane inner monolayer engendering aminophospholipid asymmetry.

Solute-induced acceleration of transbilayer movement and its implications on models of blood-brain barrier

Hexylglycerol accelerates the transbilayer (flip-flop) movement of phospholipids, lysophospholipids and peptides. For example, lysophosphatidylcholine added to dimyristoylphosphatidylcholine vesicles activates the action of pig pancreatic phospholipase A2 (Jain and DeHaas (1983) Biochim. Biophys. Acta 736, 157-162) This activating effect is dissipated slowly after mixing, and no activation is observed when the lysophospholipid molecules are equally distributed on both sides of the bilayer. The half time for transbilayer movement of lysophosphatidylcholine is about 7 h, and it is accelerated over 100-fold in the presence of n-hexylglycerol, as well as by a variety of other amphipathic solutes including n-alkanols, ketamine, and flufenamic acid. Hexylglycerol also accelerates the rate of transbilayer movement of an amphipathic hexapeptide bocLALALW, as well as of the phosphatidylcholine molecules in erythrocyte membrane. These effects are observed without any change in the gross bilayer organization as judged by 31P-NMR. Biophysical significance of such solute induced acceleration of transbilayer movement of amphipathic solutes is discussed to account for the effect of alkylglycerols on blood brain barrier.

Abnormal transbilayer mobility of phosphatidylcholine in hereditary pyropoikilocytosis reflects the increased heat sensitivity of the membrane skeleton

We determined whether the membrane defect in hereditary pyropoikilocytosis (HPP) is associated with thermally induced changes in the lipid bilayer, the stability of which was probed by the rate of translocation of phosphatidylcholine (PC) over the two leaflets. [14C]PC was incorporated into the outer leaflet of the lipid bilayer of the intact erythrocytes using a PC-specific phospholipid exchange protein. The transbilayer equilibration of this PC was determined by measuring the time-dependent changes in its accessibility to exogenous phospholipase A2. The rate of transbilayer equilibration of PC was increased in HPP cells at 37 degrees C when compared to normal erythrocytes (rate constants, 0.07 +/- 0.02 and 0.03 +/- 0.01 h-1, respectively). A further dramatic increase in PC transbilayer equilibration was noted in HPP cells incubated at 44 degrees C (rate constant, 0.15 +/- 0.02 h-1). A similar marked acceleration in transbilayer movement of PC was also seen in normal erythrocytes when incubated at 46 degrees C (rate constant, 0.13 +/- 0.03 h-1). Despite the enhanced transbilayer mobility of PC in HPP cells when compared to normal erythrocytes, no major alteration in the asymmetric distribution could be observed when probed with phospholipase A2. Since changes in transbilayer mobility of PC and cell morphology occur in HPP cells at lower temperature than in normal red cells, it may be concluded that the enhanced thermal sensitivity of spectrin is the major factor responsible for these changes. Our results therefore support the view that the structural integrity of the skeletal network is essential for stabilization of the lipid bilayer of the red cell membrane.

Uncoupling of the membrane skeleton from the lipid bilayer. The cause of accelerated phospholipid flip-flop leading to an enhanced procoagulant activity of sickled cells

We have previously reported that the normal membrane phospholipid organization is altered in sickled erythrocytes. More recently, we presented evidence of enhanced transbilayer movement of phosphatidylcholine (PC) in deoxygenated reversibly sickled cells (RSC) and put forward the hypothesis that these abnormalities in phospholipid organization are confined to the characteristic protrusions of these cells. To test this hypothesis, we studied the free spicules released from RSC by repeated sickling and unsickling as well as the remnant despiculated cells. The rate of transbilayer movement of PC in the membrane of deoxygenated remnant despiculated cells was determined by following the fate of 14C-labelled PC, previously introduced into the outer monolayer under fully oxygenated conditions using a PC-specific phospholipid exchange protein from beef liver. The rate of transbilayer movement of PC in the remnant despiculated cells was significantly slower than in deoxygenated native RSC and was not very much different from that in oxygenated native RSC or irreversibly sickled cells. The free spicules had the same lipid composition as the native cells, but were deficient in spectrin. These spicules markedly enhanced the rate of thrombin formation in the presence of purified prothrombinase (Factor Xa, Factor Va, and Ca2+) and prothrombin, indicating the exposure of a significant fraction of phosphatidylserine (PS) in the outer monolayer. This effect was not observed when the spicules in this assay were replaced by normal erythrocytes, deoxygenated native RSC, or a deoxygenated sample of RSC after repetitive sickling/unsickling. The results are interpreted to indicate that the destabilization of the lipid bilayer in sickled cells, expressed by the enhanced flip-flop of PC and the exposure of PS in the outer monolayer, occurs predominantly in those parts of the membrane that are in spicular form.

Shape changes in human erythrocytes induced by replacement of the native phosphatidylcholine with species containing various fatty acids

Phosphatidylcholine-specific transfer protein from beef liver has been used to replace native phosphatidylcholine (PC) molecules from intact human erythrocytes by a variety of PC species differing in fatty acid composition. These replacements changed neither the total phospholipid content of the membrane, nor the composition of this fraction in terms of the various phospholipid classes. The morphology of the erythrocyte was not modified when native PC was replaced by 1-palmitoyl,2-oleoyl PC, 1-palmitoyl,2-linoleoyl PC, egg PC, or PC isolated from rat liver microsomes. Replacement with the disaturated species 1,2-dimyristoyl PC, 1,2-dipalmitoyl PC, and 1,2-distearoyl PC resulted in the formation of echinocytes and, at higher levels of replacement, in spheroechinocytes. Echinocyte-like erythrocytes were also observed after replacement with 1-palmitoyl,2-arachidonoyl PC, whereas stomatocytes were formed upon replacement with PC species containing two unsaturated fatty acids, e.g., 1,2-dioleoyl PC and 1,2-dilinoleoyl PC. The observations show that the erythrocyte membrane structure and the overall discoid cell shape of the human erythrocyte are optimally stabilized by PC species that contain one saturated and one mono- or diunsaturated fatty acid, and that the cell tolerates only limited variations in the species composition of its PC.

Lipid organization in erythrocyte membrane microvesicles

The aminophospholipids of microvesicles released from human erythrocytes on storage or prepared from erythrocyte ghosts by shearing under pressure are susceptible to the action of 2,4,6-trinitrobenzenesulphonic acid. The aminophospholipids of the former vesicles are also susceptible to attack by phospholipase A2. Under the same conditions, the aminophospholipids of erythrocytes undergo little reaction. This suggests that the phospholipids in microvesicle membranes are more randomly distributed than those in erythrocyte membranes. Measurements have also been made of the ability of filipin to react with the cholesterol of sealed and unsealed erythrocyte ghosts and of microvesicles prepared from them. From the initial rates of reaction, it was concluded that there is no preferential transfer of cholesterol molecules from one side of the bilayer to the other during the formation of the microvesicles.

Stabilizing factors of phospholipid asymmetry in the erythrocyte membrane

Transbilayer reorientation (flip) of exogenous lysophospholipids and changes of the transbilayer distribution of endogenous phospholipids were studied in human erythrocytes and membrane vesicles. (1) Exogenous lysophosphatidylserine irreversibly accumulates in the inner membrane layer of resealed ghosts of human erythrocytes. (2) This accumulation even occurs after complete loss of asymmetric distribution of endogenous phosphatidylethanolamine and partial loss of phosphatidylserine asymmetry in diamide-treated cells. (3) Formation of inside-out and right-side-out vesicles from erythrocyte membranes results in a loss of endogenous phospholipid asymmetry as well as of the ability to establish asymmetry of exogenous lysophosphatidylserine. Rates of transbilayer reorientation of lysophospholipids for the vesicles, however, are comparable to those for intact cells. (4) Loss of endogenous asymmetry of phosphatidylserine is also observed in vesicles isolated from erythrocytes after heat denaturation of spectrin. The asymmetry in the residual cells is maintained. (5) In contrast to the loss of asymmetry of phosphatidylethanolamine and of phosphatidylserine, the asymmetry of sphingomyelin is completely maintained in the vesicles. (6) The stability of phospholipid asymmetry in the native cell is discussed in terms of a limitation of access of phospholipids to hypothetical reorientation sites. Such a limitation may either be the result of interaction of phospholipids with the membrane skeleton as in case of phosphatidylserine and phosphatidylethanolamine, or the result of lipid-lipid interactions as in case of sphingomyelin.

ATP-dependent asymmetric distribution of spin-labeled phospholipids in the erythrocyte membrane: relation to shape changes

Spin-labeled analogs of phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine have been used to study phospholipid transverse diffusion and asymmetry in the human erythrocyte membrane. Ascorbate reduction was used to assess the transbilayer distribution of the labels. All three spin-labeled phospholipids initially incorporated into the outer leaflet of the membrane. On fresh erythrocytes at 5 degrees C, the phosphatidylcholine label remained mainly in the outer leaflet. In contrast, the phosphatidylserine and phosphatidylethanolamine labels underwent rapid transverse diffusion that led to their asymmetric distribution in favor of the inner leaflet. The latter effect was reversibly inhibited after ATP depletion of the erythrocytes and could be reproduced on resealed erythrocyte ghosts only if hydrolyzable Mg-ATP was included in the internal medium. It is suggested that an ATP-driven transport of amino phospholipids toward the inner leaflet could be the major cause of the phospholipid asymmetry in the erythrocyte membrane. It is also proposed that the same mechanism could explain the ATP requirement of the maintenance of the erythrocyte membrane discoid shape.

Reorientation rates and asymmetry of distribution of lysophospholipids between the inner and outer leaflet of the erythrocyte membrane

Labelled lysophospholipids were inserted into the outer layer of the erythrocyte membrane and their reorientation (flip) to the inner layer quantified by following the increase of the fraction of lysophospholipids not extractable by albumin. Flip rate constants were calculated from the kinetics of equilibration of the lysophospholipids between two compartments, the outer and the inner leaf of the bilayer, in the early phase of the flip kinetics where correction for non-enzymatic hydrolysis and acylation could be omitted. The distribution of a lysophospholipid finally attained reflects its affinity for the two layers. Whereas lysophosphatidylcholine has a slight preference for the outer layer of the membrane, lysophosphatidylserine spontaneously concentrates in the inner layer up to a ratio of 4:1. This asymmetry mimics the distribution of phosphatidylserine in the native membrane. Flip rates depend on membrane lipid compositions. They are enhanced by cholesterol depletion. Comparison of various mammalian species demonstrates that erythrocytes with a higher phosphatidylcholine/sphingomyelin ratio and high content of polyunsaturated fatty acids (mouse and rat) have a high transbilayer mobility, in contrast to erythrocytes with a low phosphatidylcholine/sphingomyelin ratio and a low content of polyunsaturated fatty acids (ox). Molecular properties of lysophospholipids influence their transbilayer mobility. Flip rates of lysophospholipids are enhanced not only by unsaturation of their fatty acid, but also by a negative net charge on the headgroup. This indicates that the strongly asymmetric distribution of phosphatidylserine in the native erythrocyte membrane, which is maintained for the lifespan of the cell, does not result from a lack of transbilayer mobility.

Cross-linking of SH-groups in the erythrocyte membrane enhances transbilayer reorientation of phospholipids. Evidence for a limited access of phospholipids to the reorientation sites

Oxidation of erythrocyte membrane SH-groups and concomitant cross-linking of spectrin, which induce a partial loss of phospholipid asymmetry (Haest, C.W.M., Plasa, G., Kamp, D. and Deuticke, B. (1978) Biochim. Biophys. Acta 509, 21-32) are now shown to result in a remarkable increase of the rates of transbilayer reorientation of exogenously incorporated lysophospholipids. Reorientation of both, neutral lysophosphatidylcholine and of negatively charged lysophosphatidylserine is enhanced. A decrease of the activation energy of the reorientation process as well as quantitative changes of the dependence of reorientation on the lysophosphatidylcholine and cholesterol content of the membrane indicate formation of new reorientation sites or modification of existing sites. A common mechanism may underly the formation of reorientation sites and the occurrence of leaks for small solutes (Deuticke, B., Poser, B., Lütkemeier, P. and Haest, C.W.M. (1983) Biochim. Biophys. Acta 731, 196-210) subsequent to oxidation of membrane SH-groups. Whereas exogenous lysophospholipids completely equilibrate between the two lipid layers regardless of the extent of oxidation of SH-groups, endogenous inner layer phospholipids become available for reorientation in a graded way. Native phospholipid asymmetry is therefore not the result of a low transbilayer mobility of phospholipids, but probably due to a lack of access of inner layer phospholipids to the reorientation sites.

Inhibition of malarial invasion of red cells by chemical and immunochemical linking of spectrin molecules

The invasion of resealed human red cell ghosts by Plasmodium falciparum, and those from monkey cells by P. knowlesi, was strongly inhibited by anti-spectrin antibodies introduced into their cytoplasm. Univalent F(ab)1 fragments gave no such effect, but a combination of these fragments and goat anti-rabbit immunoglobulin, to restore bifunctionality, caused perceptible inhibition of invasion. Disulphide cross-links introduced between spectrin molecules in intact red cells by the membrane-permeant oxidizing agent, diamide, again led to inhibition of invasion. This effect was largely reversed by reduction of the cross-links. Gel electrophoresis was used to confirm that cross-linking was essentially confined to spectrin, and that extended covalent networks were not formed. It follows that local formation of bridges, whether by antibodies or oxidation of thiol groups, functions by inhibiting a local rearrangement of the cytoskeleton that forms a step in the invasion process.

The sphingomyelin pools in the outer and inner layer of the human erythrocyte membrane are composed of different molecular species

Analyses of the fatty acid composition of the outer and inner pools of sphingomyelin in the human erythrocyte membrane revealed significant differences in molecular species composition of these two pools. The sphingomyelin in the inner monolayer, representing 15-20% of the total sphingomyelin content of this membrane, is characterized by a relatively high content (73%) of fatty acids, which have less than 20 carbon atoms, whereas these account for only 31% of the total fatty acids in the sphingomyelin in the outer leaflet. On the other hand, the ratio saturated/unsaturated fatty acids in the two pools is similar. Significant differences are also observed for the fatty acid composition of the sphingomyelin in human serum when compared to that in the outer monolayer of the corresponding red cell. These results are interpreted to indicate an (almost) complete absence of transbilayer movements of sphingomyelin molecules in the human erythrocyte membrane, whereas an exchange of this phospholipid between the red cell membrane and serum is either virtually absent, or affects only a minor fraction of the sphingomyelin in the outer membrane layer.

Alteration of rheological properties of human erythrocytes by crosslinking of membrane proteins

The crosslinking of membrane proteins of human erythrocytes by diamide (diazene dicarboxylic acid bis(N,N-dimethylamide) ) was quantified by 4% polyacrylamide gel electrophoresis in 1% sodium dodecyl sulfate. The relation between the crosslinking of membrane proteins and erythrocyte functions (rheological and oxygen transporting) was quantitatively examined. (i) The crosslinking of membrane protein was induced by diamide, without changing the shape and the contents of intracellular organic phosphates (adenylates and 2,3-diphosphoglycerate). The intensity of spectrin 2 in SDS-polyacrylamide gel electrophoresis decreased proportionally to diamide concentration. The percentage decrease in spectrin 2 (using band 3 as an internal standard) was the most appropriate indicator for crosslinking ("% crosslinking'). (ii) The suspension viscosity of erythrocytes increased in proportion to the percentage of crosslinking, in the range of applied shear rates of 3.76-752 s-1. (iii) Erythrocyte deformability (measured by a high-shear rheoscope) was reduced by the crosslinking. The change was detectable even at 5% crosslinking. (iv) Rouleaux formation (measured by a television image analyzer combined with a low-shear rheoscope) was inhibited by the crosslinking. The inhibition was also sensitively detected at more than 5% crosslinking. (v) Hemoglobin in erythrocytes was chemically modified by higher dose of diamide (probably by the binding of diamide with sulfhydryl groups). Also the oxygen affinity of hemoglobin increased and the heme-heme interaction decreased. (vi) The reduction of the crosslinking of membrane proteins by dithiothreitol apparently reversed the intensity of spectrin bands in SDS-polyacrylamide gel electrophoresis and the erythrocyte functions (the suspension viscosity and the deformability), though not completely.

Red cell membrane abnormalities in chronic myeloid leukaemia

Chronic myeloid leukaemia (CML) is a clonal neoplasm that arises in a stem cell common to granulocytes and erythrocytes. Several abnormalities have been identified in the plasma membranes of granulocytes of CML patients, but to our knowledge no studies have been done on CML erythrocytes. We report here that CML erythrocyte spectrin becomes abnormal due to cross-linking of its two subunits via disulphide bonds. In addition, we show that this cytoskeletal defect in the erythrocytes is associated with loss of transmembrane phospholipid asymmetry. These observations, apart from demonstrating membrane abnormalities in CML erythrocytes, also provide strong support for the view that the asymmetric organization of phospholipids in the red cell membrane is maintained mainly by interactions between spectrin and aminophospholipids.

Formation of aqueous pores in the human erythrocyte membrane after oxidative cross-linking of spectrin by diamide

Oxidation of erythrocyte membrane SH-groups by diamide and tetrathionate induces cross-linking of spectrin (Haest, C.W.M., Kamp, D., Plasa, G. and Deuticke, B. (1977) Biochim. Biophys. Acta 469, 226-230). This cross-linking was now shown to go along with a concentration- and time-dependent enhancement of membrane permeability for hydrophilic nonelectrolytes and ions. The enhancement is specific for oxidative SH-group modifications, is reversible by reduction of the induced disulfides, can be suppressed by a very brief pre-treatment of the cells with low concentrations of N-ethylmaleimide and is strongly temperature-dependent. The pathway of the induced permeability discriminates nonelectrolytes on the basis of molecular size and exhibits a very low activation energy (Ea 3-8 kcal/mol). These findings are reconcilable with the formation of a somewhat inhomogeneous population of aqueous pores with radii probably less than or equal to 0.65 nm. Estimated pore numbers vary with the size of the probe molecule. Assuming a diffusion coefficient as in bulk water within the pore, at least 20 pores per cell have to be postulated; more realistic lower diffusion coefficients increase that number. Alterations of the lipid domain by changes of cholesterol contents and insertion of hexanol or nonionic detergents alter the number or size of the pores. Since aggregation of skeletal and intrinsic membrane proteins also occurs after the SH-oxidation, in parallel to the formation of membrane leaks, one may consider (a) defects in the disturbed bilayer interface, (b) a mismatch between lipid and intrinsic proteins or (c) channels in between aggregated intrinsic proteins as structures forming the pores induced by diamide treatment.

Is erythrocyte membrane phospholipid organisation abnormal in Duchenne muscular dystrophy?

Many of the abnormalities reported in erythrocytes from Duchenne muscular dystrophy (DMD) patients, including alterations in the physical state of the membrane as determined by electron spin resonance, could arise from changes in the lipid components. Although several investigators have shown there to be no compositional differences between normal and DMD erythrocytes, there still exists the hitherto unexplored possibility that the normal asymmetric organisation of the lipids may be deranged in the disease. This report is concerned with the probing of the asymmetric transbilayer distribution of the glycerophospholipids using phospholipase A2 from bee venom. Our results suggest that more phosphatidyl choline (PC) is available for attack by the enzyme in the outer leaflet of the bilayer in DMD erythrocytes than in normals. This may be due to a greater than normal proportion of the PC being present in the outer leaflet or to enhanced transbilayer movement of PC molecules from the inner leaflet. Whichever the explanation, these findings indicate an organisational abnormality in DMD erythrocyte membranes.