Merocyanine 540 recognizes membrane abnormalities of erythrocytes in chronic myelogenous leukemia

Merocyanine 540 is a fluorescent dye which stains erythrocytes that have lost their normal membrane phospholipid asymmetry. Because erythrocytes from patients with chronic myelogenous leukemia have been reported to display this abnormal phenotype, peripheral blood erythrocytes from such patients were examined for their ability to stain with the dye. Erythrocytes from all patients with active disease states stained, whereas neither erythrocytes from normal, healthy individuals nor from a patient whose disease symptoms were eliminated by chemotherapy stained. These results suggest that merocyanine 540 may have utility in the clinical evaluation of chronic myelogenous leukemia.     

Heat-induced alterations in monkey erythrocyte membrane phospholipid organization and skeletal protein structure and interactions

Rhesus monkey erythrocytes were subjected to heating at 50 degrees C for 5-15 min, and the heat-induced effects on the membrane structure were ascertained by analysing the membrane phospholipid organization and membrane skeleton dynamics and interactions in the heated cells. Membrane skeleton dynamics and interactions were determined by measuring the Tris-induced dissociation of the Triton-insoluble membrane skeleton (Triton shells), the spectrin-actin extractability at low ionic strength, spectrin self-association and spectrin binding to normal monkey erythrocyte membrane inside-out vesicles (IOVs). The Tris-induced Triton shell dissociation and spectrin-actin extractability were markedly decreased by the erythrocyte heating. Also, the binding of the heated erythrocyte membrane spectrin-actin with the IOVs was much smaller than that observed with the normal erythrocyte spectrin-actin. Further, the spectrin structure was extensively modified in the heated cells, as compared to the normal erythrocytes. Transbilayer phospholipid organization was ascertained by employing bee venom and pancreatic phospholipases A2, fluorescamine, and Merocyanine 540 as the external membrane probes. The amounts of aminophospholipids hydrolysed by phospholipases A2 or labeled by fluorescamine in intact erythrocytes considerably increased after subjecting them to heating at 50 degrees C for 15 min. Also, the fluorescent dye Merocyanine 540 readily stained the 15-min-heated cells but not the fresh erythrocytes. Unlike these findings, the extent of aminophospholipid hydrolysis in 5-min-heated cells by phospholipases A2 depended on the incubation time. While no change in the membrane phospholipid organization could be detected in 10 min, prolonged incubations led to the increased aminophospholipid hydrolysis. Similarly, fluorescamine failed to detect any change in the transbilayer phospholipid distribution soon after the 5 min heating, but it labeled greater amounts of aminophospholipids in the 5-min-heated cells, as compared to normal cells, after incubating them for 4 h at 37 degrees C. These results have been discussed to analyse the role of membrane skeleton in maintaining the erythrocyte membrane phospholipid asymmetry. It has been concluded that both the ATP-dependent aminophospholipid pump and membrane bilayer-skeleton interactions are required to maintain the transbilayer phospholipid asymmetry in native erythrocyte membrane.     

Membrane phospholipid organization in calcium-loaded human erythrocytes

Intracellular Ca2+ levels in human erythrocytes were increased by incubating them with variable concentrations of Ca2+ in the presence of ionophore A23187. Experiments were done to confirm that the Ca2+ loading did induce changes in the cell shape and membrane protein composition. The effect of the increased cytoplasmic Ca2+ levels on the membrane phospholipid organization was analysed using bee venom and pancreatic phospholipases A2, Merocyanine 540 and fluorescamine as the external membrane probes. About 20% phosphatidylethanolamine (PE) and 0% phosphatidylserine (PS) were hydrolysed by the phospholipases in intact control cells, whereas in identical conditions these enzymes readily degraded, 20-30% PE and 7-30% PS, in Ca2+-loaded erythrocytes, depending on the cytoplasmic Ca2+ concentration. Also, Merocyanine 540 failed to stain the fresh or control erythrocytes, but it labeled the cells loaded with Ca2+. Furthermore, fluorescamine labeled approx. 20% PE in fresh or control erythrocytes while in identical conditions, significantly higher amounts of PE were modified in intact Ca2+-loaded cells. These results demonstrate that Ca2+ loading in human erythrocytes leads to loss of the transbilayer phospholipid asymmetry, and suggest that, together with spectrin, polypeptides 2.1 and 4.1 may also play an important role in maintaining the asymmetric distribution of various phospholipids across the erythrocyte membrane bilayer.     

Phospholipid asymmetry in human erythrocyte ghosts

Using phospholipase digestion and the fluorescent probe merocyanine 540 the maintenance of phospholipid asymmetry in the plasma membrane of human erythrocyte ghosts was investigated. Digestion with phospholipase A2 indicated that ghosts prepared in the presence of Mg++ as the only divalent cation retained the normal phospholipid asymmetry characteristic of intact erythrocytes. These ghosts, like normal erythrocytes, also failed to stain with merocyanine 540. However, the presence of as little as 5-10 microM Ca++ during ghost preparation resulted in ghosts in which lipid asymmetry had been abolished, as indicated by phospholipase digestion. Moreover, these ghosts stained with merocyanine 540. In contrast to ghosts, intact erythrocytes treated with ionophore required millimolar levels of Ca++ ions to disrupt membrane lipid asymmetry. To discover the reason for this difference in behavior between ghosts and intact cells, ghosts were prepared from preswollen cells using only small volumes of buffer for lysis. These experiments demonstrated that as the cellular contents of erythrocytes are diluted, the asymmetric arrangement of phospholipids becomes more sensitive to disruption by Ca++.     

Role of membrane-associated cytoskeleton in maintenance of membrane structure

Various structural components of biological membranes are asymmetrically localized in the two surfaces of the membrane bilayer. This asymmetry is absolute for membrane (glyco) proteins, but only a partial asymmetry has been observed for membrane phospholipids. In the red cell membrane, choline-phospholipids are localized mainly in the outer monolayer whereas aminophospholipids are distributed almost exclusively in the inner monolayer. Several evidences are now available to suggest that this distribution of membrane phospholipids in red cells is directly or indirectly maintained by the membrane-associated cytoskeleton (membrane skeleton). This belief is well supported by the previous as well as recent studies carried out in the authors laboratory. Previously, it has been shown that lipid-lipid interactions play no major role in maintaining the transmembrane phospholipid asymmetry in erythrocytes, and that the asymmetry is lost upon covalent crosslinking of the major membrane skeletal protein, spectrin. The recent data presented here further shows that degradation or denaturation of spectrin indices rapid transbilayer movement of membrane phospholipids in the cells which, in turn, leads to more random phospholipid distributions across the membrane. These studies taken together strongly suggest that the skeleton-membrane associations are the major determinants of the transmembrane phospholipid asymmetry in erythrocytes, and that the dissociation of the skeleton from the membrane bilayer probably results in generation of new reorientation sites for phospholipids in the membrane. Communication No 3648 from C.D.R.I., Lucknow.     

Phospholipid asymmetry in the membranes of intact human erythrocytes and in spectrin-free microvesicles derived from them

Phospholipase A₂ from bee venom and Naja naja has been used to study the orientation of phospholipids present in the membrane of intact human erythrocytes and in spectrin-free microvesicles derived from the cells by treatment with Ca²⁺ and A23187. Little difference between the cells and microvesicles was observed in the apparent accessibility of phospholipids to the enzyme, suggesting that the original lipid asymmetry was maintained in the absence of spectrin. However, incubation of the microvesicles for 16 h at 37°C did lead to partial loss of asymmetry in the transmembrane distribution of phosphatidylcholine and phosphatidylethanolamine but not of phosphatidylserine. Despite the similarity of lipid asymmetry in cells and fresh microvesicles, the latter were about 40-fold more sensitive to phospholipase treatment than were cells. Although they retained the lipid asymmetry of intact cells, the microvesicles resembled ghosts in their great sensitivity to phospholipase A₂ attack, suggesting that the lipid packing in microvesicles and ghosts was similar. This conclusion was supported by the results of experiments with a fluorescent probe Merocyanine 540.     

Disruption of phospholipid asymmetry in erythrocyte vesicles deficient in spectrin

The transbilayer distribution of phospholipids in right-side-out and inside-out vesicles derived from human erythrocytes was studied by phospholipase A2 digestion assays and by staining with the fluorescent dye merocyanine 540. In both types of vesicles, the normal asymmetric distribution of phospholipids characteristic of intact cells was disrupted. Because both types of vesicles are deficient in spectrin, the major protein of the cytoskeletal network which normally underlies the membrane, these results support the contention that spectrin is involved in the maintenance of phospholipid asymmetry.     

Abnormal membrane protein methylation and merocyanine 540 fluorescence in sickle erythrocyte membranes

Sickle cell erythrocytes exhibit reduced carboxyl methylation of membrane proteins compared to normal erythrocytes. This altered methylation in sickle membrane proteins is also observable when extracted membranes, both intact and alkali treated, were used as substrates for the homologous protein methylase II (S-adenosylmethionine:protein-carboxyl O-methyltransferase, EC. 2.1.1.24). However, when glycophorin A, one of the major methyl acceptors in both membranes, was extracted by lithium diiodosalicylate and used as the methyl acceptor, the proteins from both membranes were methylated equally, suggesting an involvement of membrane structure in membrane-bound protein methylation. Merocyanine 540 (MC-540), a fluorescent probe, was used to determine if the membranes differed in organization. Incubation of both normal and sickle erythrocytes membranes with MC-540 produced a marked increase in extrinsic fluorescence, reflecting a relatively nonpolar environment for the dye bound to the membranes. The fluorescence from sickle cell ghosts was only 87% as intense as that from normal ghosts, while the actual amount of MC-540 associated with sickle cell membranes was only 62% of normal. These data suggest that differences exist in the distribution of surface charges on these plasma membranes. These results are consistent with the hypothesis that abnormal levels of membrane protein methylation observed in sickle erythrocytes may be a result of abnormal membrane organization characteristic to sickle cell anemia.     

A model for the extracellular release of PAF: the influence of plasma membrane phospholipid asymmetry.

Recent studies suggesting that cellular activation leads to enhanced transbilayer movement of phospholipids and loss of plasma membrane phospholipid asymmetry lead us to hypothesize that such events may govern the release of PAF, a potent, but variably release, lipid mediator synthesized by numerous inflammatory cells. To model these membrane events, we studied the transbilayer movement of PAF across the human erythrocyte and erythrocyte ghost plasma membrane, membranes with documented phospholipid asymmetry which can be deliberately manipulated. Utilizing albumin to extract outer leaflet PAF, transbilayer movement of PAF was shown to be significantly enhanced in erythrocytes and ghosts altered to lose membrane asymmetry when compared to movement in those with native membrane asymmetry. Verification of membrane changes was demonstrated using merocyanine 540 (MC540), a dye which preferentially stains loosely packed or hydrophobic membranes, and acceleration of the modified Russell's viper venom clotting assay by externalized anionic phospholipids. Utilizing the erythrocyte ghost loaded with PAF in either the outer or the inner leaflet, enhanced transbilayer movement to the opposite leaflet was seen to accompany loss of membrane asymmetry. Studies utilizing ghosts loaded with albumin intracellularly demonstrated that 'acceptor' molecules binding PAF further influence the disposition of PAF across the plasma membrane. Taken together, these findings suggest that the net release of PAF from activated inflammatory cells will depend on localization of PAF to the plasma membrane, transbilayer movement, which is facilitated by alteration of membrane phospholipid asymmetry, and removal from the membrane by extracellular and intracellular 'acceptor' molecules.     

Increased adherence of oxidant-treated human and bovine erythrocytes to cultured endothelial cells

Bovine erythrocytes, which normally lack phosphatidyl choline in their membranes, when treated with either H2O2 or diamide (1-3 mM), showed a partial appearance of phosphatidyl ethanolamine (PE 40%) and phosphatidyl serine (PS, 30-33%) in the external leaflet of the bilayer and a concomitant increased (four- to five-fold) propensity to adhere to cultured bovine aortic endothelial cells. Similar treatment of normal human erythrocytes caused an alteration in the organization of the phospholipid bilayer and also resulted in their increased adherence to endothelial cells derived either from human umbilical vein or bovine aorta. Treatment of RBCs with H2O2 at low concentration (0.5 mM) resulted in cross-linking of spectrin without significant changes in the orientation of aminophospholipids but the RBCs exhibited 15-20% increase in adherence to endothelial cells. Pretreatment of either human or bovine erythrocytes with antioxidants such as vitamin E (2 mM) prevented both oxidant-induced reorganization of phospholipids in the bilayer and enhancement of adherence to endothelial cells. Introduction of either phosphatidyl serine or phosphatidyl ethanolamine but not phosphatidyl choline into erythrocyte membranes increased their adherence to endothelial cells threefold. Oxidant-treated RBCs exhibited enhanced binding and fluorescence of Merocyanine 540 dye (MC-540), which is sensitive to the packing of lipids in the lipid bilayer. On flow cytometric analysis, 78% of H2O2 (0.5 mM)-treated erythrocytes compared to 30% of untreated RBCs exhibited MC-540 binding and fluorescence, indicating differences in the lipid packing in the outer leaflet of the bilayer. Oxidant-treated erythrocytes adhere preferentially to endothelial cells rather than to bovine aortic smooth muscle cells and skin fibroblasts. It is suggested that the alterations in the erythrocyte membrane surface due to spectrin cross-linking and the organization of the phospholipids concomitant with less ordered packing in the external leaflet of the bilayer, either induced by oxidative manipulation in normal RBC or in pathological erythrocytes, play a role in erythrocyte-endothelial cell interaction.     

Loss of membrane phospholipid asymmetry during activation of blood platelets and sickled red cells; mechanisms and physiological significance

Membrane phospholipid asymmetry is considered to be a general property of biological membranes. Detailed information is presently available on the non-random orientation of phospholipids in red cell- and platelet membranes. The outer leaflet of the lipid bilayer membrane is rich in choline-phospholipids, whereas amino-phospholipids are abundant in the inner leaflet. Studies with blood platelets have shown that these asymmetries are not maintained when the cells are activated in various ways. Undoing the normal asymmetry of membrane phospholipids in activated blood cells is presumably mediated by increased transbilayer movement of phospholipids. This process, which leads to increased exposure of negatively charged phosphatidylserine at the outer surface, plays an important physiological role in local blood clotting reactions. A similar phenomenon occurs in sickled red cells. Phospholipid vesicles breaking off from reversibly sickled cells contribute similarly to intravascular clotting in the crisis phase of sickle cell disease.The loss of membrane phospholipid asymmetry in activated platelets seems to be strictly correlated with degradation of cytoskeletal proteins by endogenous calpain. It is remarkable that membrane phospholipid asymmetry can be (partly) restored when activated platelets are treated with reducing agents. This leads to disappearance of phosphatidylserine from the outer leaflet where it was previously exposed during cell activation. These observations will be discussed in relation to two mechanisms which have been recognized to play a role in the regulation of membrane phospholipid asymmetry; i.e. the interaction of aminophospholipids to cytoskeletal proteins, and the involvement of a phospholipid-translocase catalyzing outward-inward transbilayer movement of amino-phospholipids.     

Rapid loss and restoration of lipid asymmetry by different pathways in resealed erythrocyte ghosts.

The normal asymmetric distribution of phospholipids across the plasma membrane of erythrocytes can be abolished by lysing and resealing cells in the presence of Ca2+. In the present study, using flow cytometric analysis of the binding of merocyanine 540 to monitor transbilayer phospholipid distribution, Ca(2+)-induced loss of asymmetry is shown to be independent from the aminophospholipid translocase which catalyzes movement of normally internal phospholipids from the outer to the inner leaflet of the membrane. Loss of asymmetry is rapid, temperature-sensitive, and occurs in an uninterrupted, intact bilayer, rather than by diffusion of lipids through the hemolytic pore. Addition of ATP during lysis reverses loss of asymmetry, and this restoration can be blocked by inhibitors of the aminophospholipid translocase. These results suggest that the ATP-dependent translocase is essential for recovery of asymmetry, in turn suggesting that separate mechanisms mediate the loss and the recovery of lipid asymmetry in erythrocytes.     

Membrane skeleton-bilayer interaction is not the major determinant of membrane phospholipid asymmetry in human erythrocytes

Transbilayer phospholipid distribution, membrane skeleton dissociation/association, and spectrin structure have been analysed in human erythrocytes after subjecting them to heating at 50 degrees C for 15 min. The membrane skeleton dissociation/association was determined by measuring the Tris-induced dissociation of Triton-insoluble membrane skeletons (Triton shells), the spectrin-actin extractability under low ionic conditions, and the binding of spectrin-actin with normal erythrocyte membrane inside-out vesicles (IOVs). The spectrin structure was ascertained by measuring the spectrin dimer-to-tetramer ratio as well as the spectrin tryptophan fluorescence. Both the Tris-induced Triton shell dissociation and the spectrin-actin extractability under low ionic conditions were considerably reduced by the heat treatment. Also, the binding of heated erythrocyte spectrin-actin to IOVs was significantly smaller than that observed with the normal cell spectrin-actin. Further, the quantity of spectrin dimers was appreciably increased in heat-treated erythrocytes as compared to the normal cells. This change in the spectrin dimer-to-tetramer ratio was accompanied by marked changes in the spectrin tryptophan fluorescence. In spite of these heat-induced alterations in structure and bilayer interactions of the membrane skeleton, the inside-outside glycerophospholipid distribution remained virtually unaffected in the heat-treated cells, as judged by employing bee venom and pancreatic phospholipase A2, fluorescamine and Merocyanine 540 as the external membrane probes. These results strongly indicate that membrane bilayer-skeleton interaction is not the major factor in determining the transbilayer phospholipid asymmetry in human erythrocyte membrane.     

Membrane-associated cytoskeleton and transbilayer phospholipid asymmetry

Earlier studies have suggested that the membrane-associated cytoskeleton (membrane skeleton) in erythrocytes plays a major role in maintaining the transmembrane phospholipid asymmetry. But recently, it has been proposed that an ATP-dependent aminophospholipid pump is the sole determinant of this asymmetry in these cells. A critical analysis of the published data along with some unpublished results from the author's laboratory, however, indicate that both membrane skeleton and ATP-dependent aminophospholipid pump are required for maintaining the membrane phospholipid asymmetry in native erythrocytes.     

Effect of propionyl-L-carnitine treatment on membrane phospholipid fatty acid turnover in diabetic rat erythrocytes

In this work we have examined the effect of the oral administration of propionyl-L-carnitine (PLC) on the membrane phospholipid fatty acid turnover of erythrocytes from streptozotocin-induced diabetic rats. A statistically significant reduction in radioactive palmitate, oleate, and linoleate, but not arachidonate, incorporation into membrane phosphatidylcholine (PC) of diabetic rat erythrocytes with respect to control animals was found. Changes in radioactive fatty acid incorporation were also found in diabetic red cell phosphatidylethanolamine (PE), though they were not statistically significant. Oral propionyl-L-carnitine (PLC) treatment of diabetic rats partially restored the ability of intact red cells to reacylate membrane PC with palmitate and oleate, and reacylation with linoleate was fully restored. The analysis of the membrane phospholipid fatty acid composition revealed a consistent increase of linoleate levels in diabetic rat red cells, and a modest decrease of palmitate, oleate and arachidonate. The phospholipid fatty acid composition of diabetic red blood cells was not affected by the PLC treatment. Lysophosphatidylcholine acyl-CoA transferase (LAT) specific activity measured with either palmitoyl-CoA or oleyl-CoA was significantly reduced in diabetic erythrocyte membranes in comparison to controls. In addition LAT kinetic parameters of diabetic erythrocytes were altered. The reduced LAT activity could be partially corrected by PLC treatment of diabetic rats. Our data suggest that the impaired erythrocyte membrane physiological expression induced by the diabetic disease may be attenuated by the beneficial activity of PLC on the red cell membrane phospholipid fatty acid turnover.Abbreviations LAT lysophosphatidylcholine acyl-CoA transferase - PC phosphatidylcholine - PE phosphatidylethanolamine - PLC propionyl-L-carnitine - STZ streptozotocin     

Effect of vanadium compounds on the lipid organization of liposomes and cell membranes

The influence of vanadate on the adsorption properties of Merocyanine 540 (MC540) to UMR cells was studied by means of specrofluorometry. An increment in the fluorescence was observed in the osteoblasts incubated with 0.1 mM vanadate. This effect could be interpreted in terms of vanadate inhibitory effects on aminotraslocase activity. However, vanadate promotes a similar behavior to that found in UMR 106 cells when it was added to lipid vesicles composed of phosphatidylcholine. The effect of vanadium in different oxidation states, such as vanadate(V) and vanadyl(IV) on lipid membrane properties was examined in large unilamellar vesicles by means of spectrofluorometry employing different probes. Merocyanine 540 and 1,6-diphenylhexatriene were used in order to sense the changes at interfacial and hydrophobic core of membranes, respectively. In contrast to vanadate, vanadyl decreased the fluorescence of MC540. Both vanadium compounds slightly perturbed the hydrocarbon core. The results can be interpreted by the specific adsorption of both compounds on the polar head groups of phospholipid and suggest a possible influence of vanadium compounds on the lipid organization of cell membranes.     

A fluorescent dye which recognizes mature peripheral erythrocytes of myeloproliferative disorders

Based on the previous finding that erythrocytes from patients with chronic myelogenous leukemia stain with the fluorescent dye merocyanine 540, erythrocytes from patients with other myeloproliferative disorders were examined for their ability to bind the membrane probe. As assessed by both fluorescence staining and a quantitative dye removal assay, all samples of erythrocytes from patients with chronic myelogenous leukemia, polycythemia vera, myelofibrosis with myeloid metaplasia and essential thrombocythemia bound more dye than did erythrocytes from normal, healthy individuals. Erythrocytes from three of six patients with acute myelogenous leukemia also showed increased affinity for the dye. In contrast, erythrocytes from three patients with acute lymphocytic leukemia and one with unclassifiable leukemia bound only normal amounts of dye. The procedures described may be useful as a supplemental aid to diagnosis of myeloproliferative disorders or for investigation of hematological diseases where multilineage involvement is suspected.     

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.     

Surface expression of phosphatidylserine on macrophages is required for phagocytosis of apoptotic thymocytes

Cells generally maintain an asymmetric distribution of phospholipids across the plasma membrane bilayer, restricting the phospholipid, phosphatidylserine (PS), to the inner leaflet of the plasma membrane. When cells undergo apoptosis, this asymmetric transbilayer distribution is lost, bringing PS to the surface where it acts as a signal for engulfment by phagocytes. The fluorescent dye merocyanine 540 specifically stains the plasma membrane of apoptotic cells which have lost their asymmetric distribution of phospholipids. However, it also stains non-apoptotic macrophages, suggesting that phospholipid asymmetry may not be maintained in these cells, and thus that they may express PS on their surface. Here, the PS-binding protein, annexin V, was used to show that in fact normal macrophages do express PS on their surface. Furthermore, pre-treating macrophages with annexin V was found to inhibit phagocytosis of apoptotic thymocytes and thymocytes on which PS expression was artificially induced, but did not inhibit phagocytosis of latex beads or Fc receptor-mediated phagocytosis of opsonized erythrocytes. These results indicate that PS is constitutively expressed on the surface of macrophages and is functionally significant for the phagocytosis of PS-expressing target cells.     

The parasitophorous vacuole membrane of Plasmodium falciparum: demonstration of vesicle formation using an immunoprobe

We have applied several immunolabeling techniques using a monoclonal antibody to a Plasmodium falciparum antigen to differentiate morphologically dissimilar membranous structures present in infected erythrocytes. Evidence is presented that cytoplasmic clefts, multimembranous structures and vesicles within the infected cell originate from the parasitophorous vacuole membrane by a process described as budding off. The parasitophorous vacuole membrane and related structures in infected, parasitized erythrocytes reacted with the cyanine dye Merocyanine 540, demonstrating that they are accessible to molecules from the extracellular environment. Immunogold labeling of freeze-fractured preparations and of thin sections of parasitized cells using pre- and post-embedding techniques revealed that each of the membranous structures carried a common parasite antigen, QF 116, which was identified by monoclonal antibody 8E7/55.