Ca2+-induced biochemical changes in human erythrocytes and their relation to microvesiculation

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins I: Localization at Plasma Membranes and Extracellular Compartments

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of plasma membranes (PMs) of all eukaryotic organisms studied so far by covalent linkage to a highly conserved glycolipid rather than a transmembrane domain. Since their first description, experimental data have been accumulating for the capability of GPI-APs to be released from PMs into the surrounding milieu. It became evident that this release results in distinct arrangements of GPI-APs which are compatible with the aqueous milieu upon loss of their GPI anchor by (proteolytic or lipolytic) cleavage or in the course of shielding of the full-length GPI anchor by incorporation into extracellular vesicles, lipoprotein-like particles and (lyso)phospholipid- and cholesterol-harboring micelle-like complexes or by association with GPI-binding proteins or/and other full-length GPI-APs. In mammalian organisms, the (patho)physiological roles of the released GPI-APs in the extracellular environment, such as blood and tissue cells, depend on the molecular mechanisms of their release as well as the cell types and tissues involved, and are controlled by their removal from circulation. This is accomplished by endocytic uptake by liver cells and/or degradation by GPI-specific phospholipase D in order to bypass potential unwanted effects of the released GPI-APs or their transfer from the releasing donor to acceptor cells (which will be reviewed in a forthcoming manuscript).

The Redox Balance and Membrane Shedding in RBC Production, Maturation, and Senescence

Membrane shedding in the form of extracellular vesicles plays a key role in normal physiology and pathology. Partial disturbance of the membrane-cytoskeleton linkage and increased in the intracellular Ca content are considered to be mechanisms underlying the process, but it is questionable whether they constitute the primary initiating steps. Homeostasis of the redox system, which depends on the equilibrium between oxidants and antioxidants, is crucial for many cellular processes. Excess oxidative power results in oxidative stress, which affects many cellular components, including the membrane. Accumulating evidence suggests that oxidative stress indirectly affects membrane shedding most probably by affecting the membrane-cytoskeleton and the Ca content. In red blood cells (RBCs), changes in both the redox system and membrane shedding occur throughout their life-from birth-their production in the bone marrow, to death-aging in the peripheral blood and removal by macrophages in sites of the reticuloendothelial system. Both oxidative stress and membrane shedding are disturbed in diseases affecting the RBC, such as the hereditary and acquired hemolytic anemias (i.e., thalassemia, sickle cell anemia, and autoimmune hemolytic anemia). Herein, I review some data-based and hypothetical possibilities that await experimental confirmation regarding some aspects of the interaction between the redox system and membrane shedding and its role in the normal physiology and pathology of RBCs.

Lysophosphatidic acid-induced pro-thrombotic phosphatidylserine exposure and ionophore-induced microvesiculation is mediated by the scramblase TMEM16F in erythrocytes

Recent studies indicate that erythrocytes actively modulate blood clotting and thrombus formation. The lipid mediator lysophosphatidic acid (LPA) is produced by activated platelets, and triggers a signaling process in erythrocytes. This results in cellular calcium uptake and exposure of phosphatidylserine (PS) at the cell surface, thereby generating activated membrane binding sites for factors of the clotting cascade. Moreover, erythrocytes of patients with a bleeding disorder and mutations in the scramblase TMEM16F show impaired PS exposure and microvesiculation upon treatment with calcium ionophore. We report that TMEM16F inhibitors tannic acid (TA) and epigallocatechin-3-gallate (EGCG) inhibit LPA-induced PS exposure and calcium uptake at low micromolar concentrations; fluoxetine, an antidepressant and a known activator of TMEM16F, enhances these processes. These effectors likewise modulate erythrocyte PS exposure and microvesicle shedding induced by calcium ionophore treatment. Further, LPA-treated erythrocytes triggered thrombin generation in platelet-free plasma which was partially impaired in the presence of TA and EGCG. Thus, this study suggests that LPA activates the scramblase TMEM16F in erythrocytes, thereby possibly mediating a pro-thrombotic function in these cells. EGCG as well as fluoxetine, substances with potentially high plasma concentrations due to alimentation or medical treatment, should be considered as potential effectors of systemic hemostatic regulation.

Nitrite-derived nitric oxide reduces hypoxia-inducible factor 1α-mediated extracellular vesicle production by endothelial cells

INTRODUCTION

Extracellular vesicles (EVs) are small, spherical particles enclosed by a phospholipid bilayer (∼30-1000 nm) released from multiple cell types, and have been shown to have pathophysiological roles in a plethora of disease states. The transcription factor hypoxia-inducible factor-1 (HIF-1) allows for adaptation of cellular physiology in hypoxia and may permit the enhanced release of EVs under such conditions. Nitric oxide (NO) plays a pivotal role in vascular homeostasis, and can modulate the cellular response to hypoxia by preventing HIF-1 accumulation. We aimed to selectively target HIF-1 via sodium nitrite (NaNO₂) addition, and examine the effect on endothelial EV, size, concentration and function, and delineate the role of HIF-1 in EV biogenesis.

METHODS

Endothelial (HECV) cells were exposed to hypoxic conditions (1% O₂, 24 h) and compared to endothelial cells exposed to normoxia (21% O₂) with and without the presence of sodium nitrite (NaNO₂) (30 μM). Allopurinol (100 μM), an inhibitor of xanthine oxidoreductase, was added both alone and in combination with NaNO₂ to cells exposed to hypoxia. EV and cell preparations were quantified by nanoparticle tracking analysis and confirmed by electron microscopy. Western blotting and siRNA were used to confirm the role of HIF-1α and HIF-2α in EV biogenesis. Flow cytometry and time-resolved fluorescence were used to assess the surface and intravesicular protein content.

RESULTS

Endothelial (HECV) cells exposed to hypoxia (1% O₂) produced higher levels of EVs compared to cells exposed to normoxia. This increase was confirmed using the hypoxia-mimetic agent desferrioxamine. Treatment of cells with sodium nitrite (NaNO₂) reduced the hypoxic enhancement of EV production. Treatment of cells with the xanthine oxidoreductase inhibitor allopurinol, in addition to NaNO₂ attenuated the NaNO₂-attributed suppression of hypoxia-mediated EV release. Transfection of cells with HIF-1α siRNA, but not HIF-2α siRNA, prior to hypoxic exposure prevented the enhancement of EV release.

CONCLUSION

These data provide evidence that hypoxia enhances the release of EVs in endothelial cells, and that this is mediated by HIF-1α, but not HIF-2α. Furthermore, the reduction of NO₂^- to NO via xanthine oxidoreductase during hypoxia appears to inhibit HIF-1α-mediated EV production.

Anoctamins/TMEM16 Proteins: Chloride Channels Flirting with Lipids and Extracellular Vesicles

Anoctamin (ANO)/TMEM16 proteins exhibit diverse functions in cells throughout the body and are implicated in several human diseases. Although the founding members ANO1 (TMEM16A) and ANO2 (TMEM16B) are Ca²⁺-activated Cl^- channels, most ANO paralogs are Ca²⁺-dependent phospholipid scramblases that serve as channels facilitating the movement (scrambling) of phospholipids between leaflets of the membrane bilayer. Phospholipid scrambling significantly alters the physical properties of the membrane and its landscape and has vast downstream signaling consequences. In particular, phosphatidylserine exposed on the external leaflet of the plasma membrane functions as a ligand for receptors vital for cell-cell communication. A major consequence of Ca²⁺-dependent scrambling is the release of extracellular vesicles that function as intercellular messengers by delivering signaling proteins and noncoding RNAs to alter target cell function. We discuss the physiological implications of Ca²⁺-dependent phospholipid scrambling, the extracellular vesicles associated with this activity, and the roles of ANOs in these processes.

Ionomycin causes susceptibility to phospholipase A2 while temperature-induced increases in membrane fluidity fail: possible involvement of actin fragmentation

A diminution in the order of membrane lipids, which occurs during apoptosis, has been shown to correlate with increased membrane susceptibility to hydrolysis by secretory phospholipase A2. Studies with artificial membranes, however, have demonstrated that the relationship between membrane order and hydrolysis is more complex than suggested thus far by cell studies. To better resolve this relationship, this study focused on comparisons between increasing temperature and calcium ionophore as means of decreasing membrane order in S49 cells. Although these two treatments caused comparable changes in apparent membrane order as detected by steady-state fluorescence measurements, only ionophore treatment enhanced phospholipase activity. Experiments with exogenously-added phosphatidylserine indicated that the difference was not due to the presence of that anionic phospholipid in the outer membrane leaflet. Instead, analysis of the equilibration kinetics of various cationic membrane probes revealed that the difference could relate to the spacing of membrane lipids. Specifically, ionophore treatment increased that spacing while temperature only affected overall membrane order and fluidity. To consider the possibility that the distinction with ionophore might relate to the actin cytoskeleton, cells were stained with phalloidin and imaged via confocal microscopy. Ionophore caused disruption of actin fibers while increased temperature did not. This apparent connection between membrane hydrolysis and the cytoskeleton was further corroborated by examining the relationship among these events during apoptosis stimulated by thapsigargin.

Membrane properties involved in calcium-stimulated microparticle release from the plasma membranes of S49 lymphoma cells

This study answered the question of whether biophysical mechanisms for microparticle shedding discovered in platelets and erythrocytes also apply to nucleated cells: cytoskeletal disruption, potassium efflux, transbilayer phospholipid migration, and membrane disordering. The calcium ionophore, ionomycin, disrupted the actin cytoskeleton of S49 lymphoma cells and produced rapid release of microparticles. This release was significantly inhibited by interventions that impaired calcium-activated potassium current. Microparticle release was also greatly reduced in a lymphocyte cell line deficient in the expression of scramblase, the enzyme responsible for calcium-stimulated dismantling of the normal phospholipid transbilayer asymmetry. Rescue of the scrambling function at high ionophore concentration also resulted in enhanced particle shedding. The effect of membrane physical properties was addressed by varying the experimental temperature (32–42°C). A significant positive trend in the rate of microparticle release as a function of temperature was observed. Fluorescence experiments with trimethylammonium diphenylhexatriene and Patman revealed significant decrease in the level of apparent membrane order along that temperature range. These results demonstrated that biophysical mechanisms involved in microparticle release from platelets and erythrocytes apply also to lymphocytes.

Interplay of host-pathogen microvesicles and their role in infectious disease

The release of extracellular vesicles, whether MVs (microvesicles) or exosomes, from host cells or intracellular pathogens is likely to play a significant role in the infection process. Host MVs may fuse with pathogen surfaces to deliver host complement regulatory proteins. They may also deliver cytokines that enhance invasion. Decoy functions are also possible. Whereas host MVs may direct pathogens away from their target cells, pathogen MVs may in turn redirect complement membrane-attack complexes away from their target pathogen. An understanding of the mechanisms of this interplay, bringing about both immune evasion and enhanced invasion, will help to direct future research with a view to rendering pathogens more susceptible to immune attack or in improving drug efficacy. It should also be possible to use MVs or exosomes isolated directly from the pathogens, or from the cells infected with pathogens, to provide alternative vaccination strategies.

A role for activated endothelial cells in red blood cell clearance: implications for vasopathology

BACKGROUND

Phosphatidylserine exposure by red blood cells is acknowledged as a signal that initiates phagocytic removal of the cells from the circulation. Several disorders and conditions are known to induce phosphatidylserine exposure. Removal of phosphatidylserine-exposing red blood cells generally occurs by macrophages in the spleen and liver. Previously, however, we have shown that endothelial cells are also capable of erythrophagocytosis. Key players in the erythrophagocytosis by endothelial cells appeared to be lactadherin and α(v)-integrin. Phagocytosis via the phosphatidylserine-lactadherin-α(v)-integrin pathway is the acknowledged route for removal of apoptotic innate cells by phagocytes.

DESIGN AND METHODS

Endothelial cell phagocytosis of red blood cells was further explored using a more (patho)physiological approach. Red blood cells were exposed to oxidative stress, induced by tert-butyl hydroperoxide. After opsonization with lactadherin, red blood cells were incubated with endothelial cells to study erythrophagocytosis and examine cytotoxicity.

RESULTS

Red blood cells exposed to oxidative stress show alterations such as phosphatidylserine exposure and loss of deformability. When incubated with endothelial cells, marked erythrophagocytosis occurred in the presence of lactadherin under both static and flow conditions. As a consequence, intracellular organization was disturbed and endothelial cells were seen to change shape ('rounding up'). Increased expression of apoptotic markers indicated that marked erythrophagocytosis has cytotoxic effects.

CONCLUSIONS

Activated endothelial cells show significant phagocytosis of phosphatidylserine-exposing and rigid red blood cells under both static and flow conditions. This results in a certain degree of cytotoxicity. We postulate that activated endothelial cells play a role in red blood cell clearance in vivo. Significant erythrophagocytosis can induce endothelial cell loss, which may contribute to vasopathological effects as seen, for instance, in sickle cell disease.

Red blood cells release factors with growth and survival bioactivities for normal and leukemic T cells

Human red blood cells are emerging as a cell type capable to regulate biological processes of neighboring cells. Hereby, we show that human red blood cell conditioned media contains bioactive factors that favor proliferation of normal activated T cells and leukemic Jurkat T cells, and therefore called erythrocyte-derived growth and survival factors. Flow cytometry and electron microscopy in parallel with bioactivity assays revealed that the erythrocyte factors are present in the vesicle-free supernatant, which contains up to 20 different proteins. The erythrocyte factors are thermosensitive and do not contain lipids. Native polyacrylamide gel electrophoresis followed by passive elution and mass spectrometry identification reduced the potential erythrocyte factors to hemoglobin and peroxiredoxin II. Two-dimensional differential gel electrophoresis of the erythrocyte factors revealed the presence of multiple hemoglobin oxy-deoxy states and peroxiredoxin II isoforms differing in their isoelectric point akin to the presence of β-globin chains. Our results show that red blood cells release protein factors with the capacity to sustain T-cell growth and survival. These factors may have an unforeseen role in sustaining malignant cell growth and survival in vivo.

The influence of membrane physical properties on microvesicle release in human erythrocytes

Exposure of human erythrocytes to elevated intracellular calcium causes fragments of the cell membrane to be shed as microvesicles. This study tested the hypothesis that microvesicle release depends on microscopic membrane physical properties such as lipid order, fluidity, and composition. Membrane properties were manipulated by varying the experimental temperature, membrane cholesterol content, and the activity of the trans-membrane phospholipid transporter, scramblase. Microvesicle release was enhanced by increasing the experimental temperature. Reduction in membrane cholesterol content by treatment with methyl-β-cyclodextrin also facilitated vesicle shedding. Inhibition of scramblase with R5421 impaired vesicle release. These data were interpreted in the context of membrane characteristics assessed previously by fluorescence spectroscopy with environment-sensitive probes such as laurdan, diphenylhexatriene, and merocyanine 540. The observations supported the following conclusions: 1) calcium-induced microvesicle shedding in erythrocytes relates more to membrane properties detected by diphenylhexatriene than by the other probes; 2) loss of trans-membrane phospholipid asymmetry is required for microvesicle release.

PACS Codes: 87.16.dj, 87.16.dt

Curvature-dependent lateral distribution of raft markers in the human erythrocyte membrane

The distribution of raft markers in curved membrane exvaginations and invaginations, induced in human erythrocytes by amphiphile-treatment or increased cytosolic calcium level, was studied by fluorescence microscopy. Cholera toxin subunit B and antibodies were used to detect raft components. Ganglioside GM1 was enriched in membrane exvaginations (spiculae) induced by cytosolic calcium and amphiphiles. Stomatin and the cytosolic proteins synexin and sorcin were enriched in spiculae when induced by cytosolic calcium, but not in spiculae induced by amphiphiles. No enrichment of flotillin-1 was detected in spiculae. Analyses of the relative protein content of released exovesicles were in line with the microscopic observations. In invaginations induced by amphiphiles, the enrichment of ganglioside GM1, but not of the integral membrane proteins flotillin-1 and stomatin, was observed. Based on the experimental results and theoretical considerations we suggest that membrane skeleton-detached, laterally mobile rafts may sort into curved or flat membrane regions dependent on their intrinsic molecular shape and/or direct interactions between the raft elements.

Tin triggers suicidal death of erythrocytes

Suicidal erythrocyte death or eryptosis is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine (PS) exposure at the erythrocyte surface. Triggers of eryptosis include increase in cytosolic Ca(2+) activity, formation of ceramide and energy depletion. Excessive eryptosis contributes to several anemic conditions. Intoxication with inorganic tin(II) may lead to anemia. The present study therefore explored whether tin influences eryptosis. To this end, erythrocytic phosphatidylserine exposure was estimated from annexin V-binding, cell volume from forward scatter, cytosolic Ca(2+) activity from Fluo3 fluorescence, ceramide formation from binding of fluorescent antibodies and cytosolic ATP utilizing a luciferin-luciferase assay kit. Under control conditions, eryptosis was observed in less than 5% of the erythrocytes. Exposure to tin (1-100 microm) significantly increased the percentage of PS-exposing erythrocytes and decreased cell volume. The effect was paralleled by an increase in the cytosolic Ca(2+) concentration, ceramide formation and a decrease of intracellular ATP concentration. In conclusion, tin triggers eryptosis, an effect at least partially due to Ca(2+ )entry, ceramide formation and ATP depletion. The effect could contribute to tin-induced anemia.

RBC NOS: regulatory mechanisms and therapeutic aspects

Nitric oxide (NO), one of the most important vascular signaling molecules, is primarily produced by endothelial NO synthase (eNOS). eNOS is tightly regulated by its substrate l-arginine, cofactors and diverse interacting proteins. Interestingly, an NO synthase (NOS) was described within red blood cells (RBC NOS), and it was recently shown to significantly contribute to the intravascular NO pool and to regulate physiologically relevant mechanisms. However, the regulatory mechanisms and clinical implications of RBC NOS are unknown. The aim of this review is to highlight intracellular RBC NOS interactions and the role of RBC NOS in RBC homeostasis. Furthermore, macro- and microvascular diseases affected by RBC-derived NO are discussed.

Myoblast fusion and inositol phospholipid breakdown: causal relationship or coincidence?

The fusion of embryonic chick myoblasts has been examined in culture. Cells were prepared from 12-day-old chick embryonic breast muscle and cultured for 50 h at a Ca2+ concentration in the medium of 10(-7) M. During this period they attain fusion competence. Addition of 1.4 mM-Ca2+ to these cells elicits rapid fusion. Changes in the metabolism of myoblast phospholipids in response to the raised Ca2+ concentration have been examined. Only the inositol phospholipids are affected. Phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate are rapidly broken down and 1,2-diacylglycerol and phosphatidic acid are synthesized. Myoblast fusion has also been found to be stimulated by a factor present in chick embryo extract, probably of neuronal origin. A receptor-mediated mechanism for myoblast fusion is proposed. This envisages the polyphosphoinositides acting as a fusion block, either themselves or by their binding to membrane proteins. The inositol phospholipid breakdown could result in a more fluid membrane and the breakdown products 1,2-diacylglycerol and phosphatidic acid, two known fusogens, could stimulate fusion.

A role of phosphatidylserine externalization in clearance of erythrocytes exposed to stress but not in eliminating aging populations of erythrocyte in mice

Age dependent changes in phosphatidylserine (PS) externalization were studied in mouse erythrocytes of different age groups (range 1-55 days) by using a newly developed double in vivo biotinylation (DIB) technique. Around 3-4% of the erythrocytes freshly released in the circulation were PS(+) but this proportion fell rapidly to 1% or less and did not increase at later time points. Blocking erythrocyte clearance from the circulation by in vivo depletion of macrophages (by treatment with clodronate loaded liposomes) for up to 7 days did not result in accumulation of PS(+) erythrocytes in the circulation indicating that the low percentage of PS(+) cells within old erythrocytes (age >40 days) was not related to the clearance of PS(+) erythrocytes by macrophages. In vitro treatment with stress inducing agents like deoxyglucose or Ca(++)/calcium ionophore resulted in a marked induction of PS externalization in mouse erythrocytes and this effect was most prominent in the youngest erythrocyte population (age <10 days). Kinetics of clearance of different age groups of stress exposed erythrocytes after intravenous infusion into recipient mice indicated that the young erythrocytes were cleared at fastest rate from the circulation as compared to erythrocytes of older age groups. Within young erythrocytes exposed to stress, PS(+) erythrocytes were preferentially cleared. Taken together our results suggest that PS externalization is unlikely to have a role in the removal of old erythrocytes from blood circulation but may have a role in the clearance of stressed and damaged young erythrocytes in blood circulation.

Gold stimulates Ca2+ entry into and subsequent suicidal death of erythrocytes

The suicidal death of erythrocytes, eryptosis, is characterized by cell shrinkage and cell membrane scrambling leading to phosphatidylserine exposure at the erythrocyte surface. Erythrocyte cell membrane scrambling is stimulated by increase of cytosolic Ca2+ concentration ([Ca2+](i)) and formation of ceramide. Phosphatidylserine (PS) exposing cells are rapidly cleared from circulating blood. Ca2+ entry and/or ceramide formation and thus eryptosis are triggered by lead, mercury, aluminium, and copper ions. The present study explored whether eryptosis could be similarly triggered by exposure to gold. To this end, erythrocytes from healthy volunteers were exposed to AuCl and phosphatidylserine exposure (annexin V binding), cell volume (forward scatter), [Ca2+](i) (Fluo3-dependent fluorescence), and ceramide formation (anti-ceramide-FITC fluorescence) were determined by flow cytometry. Exposure of erythrocytes to low concentrations of AuCl (> or =0.75microg/ml) increased [Ca2+](i) but did not affect ceramide formation. AuCl at concentrations > or =0.5microg/ml significantly increased the number of PS exposing erythrocytes and decreased forward scatter at low concentrations of AuCl pointing to cell shrinkage. Aurothiomalate (> or =1microg/ml), a gold containing drug effective against rheumatoid arthritis, similarly triggered PS exposure of erythrocytes. The present observations disclose a novel action of gold, which may well contribute to side effects during treatment with gold preparations.

Stimulation of suicidal erythrocyte death by amantadine

Amantadine is an effective drug for treatment of both, Parkinson's disease and viral infections. Side effects of amantadine include anemia, which may limit its therapeutic use. The cause of amantatine induced anemia is ill defined. At least in theory, the anemia could partially result from suicidal erythrocyte death or eryptosis, which accelerates the clearance of circulating erythrocytes. Eryptosis is characterized by cell shrinkage and cell membrane scrambling leading to phosphatidylserine exposure at the cell surface. Triggers of erythrocyte membrane scrambling include an increase of cytosolic Ca2+ concentration ([Ca2+]i) resulting from activation of Ca2+-permeable cation channels. The present study has been performed to test for an effect of amantadine on eryptosis. Erythrocytes from healthy volunteers were exposed to amantadine and annexin V binding (disclosing phosphatidylserine exposure), forward scatter (reflecting cell volume), and Fluo3-dependent fluorescence (reflecting [Ca2+]i) were determined by flow cytometry. Exposure of erythrocytes to amantadine (> or =0.2 microg/ml) increased [Ca2+]i and triggered annexin V binding, and increased forward scatter. The effect on annexin V binding was virtually abolished in the absence of extracellular Ca2+. The present observations disclose mechanisms presumably contributing to amantadine induced anemia.

Investigation on the early events of apoptosis in senescent erythrocytes with special emphasis on intracellular free calcium and loss of phospholipid asymmetry in chronic renal failure

BACKGROUND

The pathophysiological link between increased blood concentrations of factors responsible for the derangement and erythrocyte membrane functions in chronic renal failure (CRF) patients are not thoroughly elucidated. We studied the erythrocyte characteristics and phospholipid asymmetry loss in CRF patients with different grades of uremia and also examined the involvement of intracellular free Ca(2+) in early events of apoptosis in uremic erythrocytes.

METHODS

The studied population consisted of 90, age and sex matched control subjects (Group I) and 238 CRF cases divided into 3 groups (Group II, III and IV) according to urea concentrations and complexity of secondary complications. Erythrocyte membrane fluidity determined by binding of MC540. Intracellular free Ca(2+) concentration was determined by the 2-wavelength method by using fluorescent calcium-sensitive probe FURA-2AM. Measurement of erythrocyte phosphatidylserine exposure by flow cytometry using Annexin V-FITC.

RESULTS

Cholesterol shedding increased with increasing severity of uremic complications. Erythrocytes from Group II show mild echinocyte or formation of spicules on the erythrocyte membrane surface whereas in Group III and IV they were echinocytic. Binding of MC540 was significantly higher with progression of uremic complications. Surface charge of uremic erythrocyte membrane was significantly reduced when compared with control subjects. Intracellular free Ca(2+) was positively correlated with binding of MC540 and surface hydrophobicity. The phosphatidylserine exposure of erythrocytes was significantly higher (p<0.001) in uremic patients when compared with controls.

CONCLUSIONS

Phosphatidylserine (PS) exposure erythrocytes were significantly increased in uremic patients when compared with controls. Uremic complications predisposes to membrane damages in erythrocytes.

Intracellular free calcium concentration and calcium transport in human erythrocytes of lead-exposed workers

Erythrocytes are the route of lead distribution to organs and tissues. The effect of lead on calcium homeostasis in human erythrocytes and other excitable cells is not known. In the present work we studied the effect of lead intoxication on the uptake and efflux (measured as (Ca(2+)-Mg(2+))-ATPase activity) of calcium were studied in erythrocytes obtained from lead-exposed workers. Blood samples were taken from 15 workers exposed to lead (blood lead concentration 74.4+/-21.9 microg/dl) and 15 non-exposed workers (9.9+/-2 microg/dl). In erythrocytes of lead-exposed workers, the intracellular free calcium was 79+/-13 nM, a significantly higher concentration (ANOVA, P<0.01) than the one detected in control (30+/-9 nM). The enhanced intracellular free calcium was associated with a higher osmotic fragility and with important modifications in erythrocytes shape. The high intracellular free calcium in lead-exposed workers was also related to a 100% increase in calcium incorporation and to 50% reduction of (Ca(2+)-Mg(2+))-ATPase activity. Lipid peroxidation was 1.7-fold higher in erythrocytes of lead-exposed workers as compared with control. The alteration on calcium equilibrium in erythrocytes is discussed in light of the toxicological effects in lead-exposed workers.

Reversible inhibition of the platelet procoagulant response through manipulation of the Gardos channel

The platelet procoagulant response requires a sustained elevation of the intracellular Ca2+ concentration, [Ca2+]i, causing exposure of phosphatidylserine (PS) at the outer surface of the plasma membrane. An increased [Ca2+]i also activates Ca2+-dependent K+ channels. Here, we investigated the contribution of the efflux of K+ ions on the platelet procoagulant response in collagen-thrombin–activated platelets using selective K+ channel blockers. The Gardos channel blockers clotrimazol, charybdotoxin, and quinine caused a similar decrease in prothrombinase activity as well as in the number of PS-exposing platelets detected by fluorescence-conjugated annexin A5. Apamin and iberiotoxin, inhibitors of other K+ channels, were without effect. Only clotrimazol showed a significant inhibition of the collagen-plus-thrombin–induced intracellular calcium response. Clotrimazol and charybdotoxin did not inhibit aggregation and release under the conditions used. Inhibition by Gardos channel blockers was reversed by valinomycin, a selective K+ ionophore. The impaired procoagulant response of platelets from a patient with Scott syndrome was partially restored by pretreatment with valinomycin, suggesting a possible defect of the Gardos channel in this syndrome. Collectively, these results provide evidence for the involvement of efflux of K+ ions through Ca2+-activated K+ channels in the procoagulant response of platelets, opening potential strategies for therapeutic interventions.

Bordetella adenylate cyclase toxin induces a cascade of morphological changes of sheep erythrocytes and localizes into clusters in erythrocyte membranes

Adenylate cyclase toxin (CyaA) of Bordetella pertussis penetrates the membrane of eukaryotic cells, producing high levels of intracellular cAMP, as well as hemolysis that results from the formation of cation-selective toxin channels in the membrane. Using several microscopical approaches we studied the effects of CyaA action on the morphology of sheep erythrocytes during early phases preceding lysis and examined localization of CyaA molecules within the erythrocyte membrane. CyaA induced a cascade of morphological changes of erythrocytes, such as shrinkage, formation of membrane projections, and blebs and swelling. The use of an enzymatically inactive CyaA-AC- toxoid that is unable to produce cAMP and of a CyaA-E581K mutant exhibiting higher hemolytic activity than with CyaA showed that the hemolytic activity is responsible for the induction of morphological changes of erythrocytes. Further, immunolabeling of inserted CyaA-232/FLAG molecules with specific anti-FLAG antibodies and IgG-gold particles indicated a clustered distribution of CyaA molecules in erythrocyte membrane. This was confirmed by immunofluorescence and confocal microscopy, which revealed uniform stoichiometry of CyaA clusters, suggesting CyaA binding into specific domains in erythrocyte membrane. Indeed, a decrease of CyaA binding after cholesterol depletion of erythrocytes suggests toxin targeting and binding to membrane microdomains (rafts).

The how and why of exocytic vesicles

The purpose of this review is to draw the attention of general readers to the importance of cellular exocytic vesiculation as a normal mechanism of development and subsequent adjustment to changing conditions, focusing on red cell (RBC) vesiculation. Recent studies have emphasized the possible role of these microparticles as diagnostic and investigative tools. RBCs lose membrane, both in vivo and during ex vivo storage, by the blebbing of microvesicles from the tips of echinocytic spicules. Microvesicles shed by RBCs in vivo are rapidly removed by the reticuloendothelial system. During storage, this loss of membrane contributes to the storage lesion and the accumulation of the microvesicles are believed to be thrombogenic and thus to be clinically important.

Divalent cations increase lipid order in erythrocytes and susceptibility to secretory phospholipase A2

Elevated concentrations of intracellular calcium in erythrocytes increase membrane order and susceptibility to secretory phospholipase A2. We hypothesize that calcium aids the formation of domains of ordered lipids within erythrocyte membranes by interacting directly with the inner leaflet of the cell membrane. The interface of these domains with regions of more fluid lipids may create an environment with weakened neighbor-neighbor interactions that would facilitate phospholipid migration into the active site of bound secretory phospholipase A2. This hypothesis was investigated by determining the effects of seven other divalent ions on erythrocyte membrane properties. Changes in membrane order were assessed with steady-state fluorescence spectroscopy and two-photon microscopy with an environment-sensitive probe, laurdan. Each ion increased apparent membrane order in model membranes and in erythrocytes when introduced with an ionophore, suggesting that direct binding to the inner face of the membrane accounts for the effects of calcium on membrane fluidity. Furthermore, the degree to which ions affected membrane properties correlated with the ionic radius and electronegativity of the ions. Lastly, erythrocytes became more susceptible to enzyme hydrolysis in the presence of elevated intracellular levels of nickel and manganese, but not magnesium. These differences appeared related to the ability of the ions to induce a transition in erythrocyte shape.

Differential sorting of tyrosine kinases and phosphotyrosine phosphatases acting on band 3 during vesiculation of human erythrocytes

One of the most intensively studied post-translational modifications of erythrocyte proteins is the phosphorylation of tyrosine residues of band 3, which is strictly regulated in vivo by PTKs (protein-tyrosine kinases) and PTPs (protein-phosphotyrosine phosphatases). Two PTKs (p72(syk) and p56/53(lyn)) and two PTP activities (PTP1B and SHPTP-2) have been immunologically identified so far in mature human erythrocytes. We have shown previously that band 3 undergoes tyrosine phosphorylation upon a decrease in cell volume, as occurs when erythrocytes treated with Ca(2+)/Ca(2+) ionophore (A23187) lose KCl and release microvesicles. Similar levels of band 3 tyrosine phosphorylation in vesicles and in the parent cells are induced by this treatment. However, we have found that tyrosine phosphorylation of band 3 in vesicles is more stable than in whole erythrocytes. Examination of how the identified PTPs and PTKs are partitioned between the vesicles and the remnant cells during vesiculation reveals that PTP1B, unlike the PTKs, is retained entirely in the parent cell compartment. Since a tight association between PTP1B and band 3 has been documented previously, we have investigated the partitioning of PTP1B and band 3 between the membrane and the membrane-skeletal fractions prepared from resting or Ca(2+)/A23187-treated cells. Our results rule out the possibility that the preferential retention of PTP1B within the cell was due to an increase in the amount of membrane-skeleton-associated band 3 (and of PTP1B) during the release of spectrin-free vesicles, suggesting a more complex modality of interaction of PTP1B with band 3 in the erythrocyte membrane. Analysis of erythrocytes of different cell ages revealed that PTP1B, unlike the other enzymes examined, was quantitatively conserved during erythrocyte aging. This suggests important roles for the down-regulation of tyrosine phosphorylation of band 3 in erythrocyte physiology, and for vesiculation as a mechanism of human erythrocyte senescence.

Alterations in sensitivity to calcium and enzymatic hydrolysis of membranes from sickle cell disease and trait erythrocytes

Normally, human erythrocytes display several responses to elevated intracellular calcium levels. These include a shape transition from discocyte to spherocyte, shedding of microvesicles into the extracellular fluid, and enhanced susceptibility to the hydrolytic action of secretory phospholipase A(2). These responses to elevated intracellular calcium were all blunted in erythrocytes containing hemoglobin S. The reduction of both the shape transition and the shedding of microvesicles were greater than the impairment of phospholipase susceptibility, and both correlated strongly with the intracellular content of hemoglobin S. In contrast to the response to elevated intracellular calcium, erythrocytes containing hemoglobin S displayed a 2.5-fold increase in basal susceptibility to phospholipase A(2) compared to control erythrocytes in the absence of ionophore. The effect was more prominent among samples from patients heterozygous for hemoglobin S than in samples from homozygous individuals. These results reveal additional abnormalities in the membranes of sickle cell erythrocytes beyond those described previously and demonstrate that red blood cells from both heterozygous and homozygous are affected. Furthermore, they suggest a possible means by which sickle cell disease and trait patients may display enhanced vulnerability to inflammatory stimuli.

Ca2+ promotes erythrocyte band 3 tyrosine phosphorylation via dissociation of phosphotyrosine phosphatase from band 3

The anion-exchange band 3 protein is the main erythrocyte protein that is phosphorylated by protein tyrosine kinase (PTK). We have previously identified a band 3-associated phosphotyrosine phosphatase (PTP) that is normally highly active and prevents the accumulation of band 3 phosphotyrosine. Band 3 tyrosine phosphorylation can be induced by inhibition of PTP (vanadate, thiol oxidation), activation of PTK (hypertonic NaCl) or intracellular increased Ca(2+) (mechanism unknown). We now show that there is inhibition of dephosphorylation of band 3 in Ca(2+)/ionophore-treated erythrocytes and in membranes isolated from the treated cells. These membranes exhibit phosphatase activity upon the addition of exogenous substrate. Dephosphorylation of the endogenous substrate (band 3) can be activated in these membranes by the addition of Mg(2+). Thus the inability of PTP to dephosphorylate the band 3 phosphotyrosine is not due to inhibition of the enzyme itself. Ca(2+) rise in the erythrocyte causes dissociation of PTP from band 3, thus leaving the kinase unopposed. This is shown by a significant diminution in band 3/PTP co-precipitation. Addition of Mg(2+) to these membranes leads to reassociation of band 3 with PTP. The Ca(2+)-induced inhibition of band 3 dephosphorylation may be due to Ca(2+)-dependent alterations in membrane components and structure, affecting the interaction of band 3 with PTP. The Ca(2+)-induced tyrosine phosphorylation, involving an apparent PTP inhibition via dissociation from the substrate, may play a role in signal transduction pathways and in certain pathological disorders associated with increased cell Ca(2+).

Ca(++)-dependent vesicle release from erythrocytes involves stomatin-specific lipid rafts, synexin (annexin VII), and sorcin

Cytosolic Ca(++) induces the shedding of microvesicles and nanovesicles from erythrocytes. Atomic force microscopy was used to determine the sizes of these vesicles and to resolve the patchy, fine structure of the microvesicle membrane. The vesicles are highly enriched in glycosyl phosphatidylinositol-linked proteins, free of cytoskeletal components, and depleted of the major transmembrane proteins. Both types of vesicles contain 2 as-yet-unrecognized red cell proteins, synexin and sorcin, which translocate from the cytosol to the membrane upon Ca(++) binding. In nanovesicles, synexin and sorcin are the most abundant proteins after hemoglobin. In contrast, the microvesicles are highly enriched in stomatin. The membranes of both microvesicles and nanovesicles contain lipid rafts. Stomatin is the major protein of the microvesicular lipid rafts, whereas synexin and sorcin represent the major proteins of the nanovesicular rafts in the presence of Ca(++). Interestingly, the raft proteins flotillin-1 and flotillin-2 are not found in the vesicles but remain in the red cell membrane. These data indicate the presence of different types of lipid rafts in the erythrocyte membrane with distinct fates after Ca(++) entry. Synexin, which is known to be vital to the process of membrane fusion, is suggested to be a key component in the process of vesicle release from erythrocytes.

Human mature red blood cells express caspase-3 and caspase-8, but are devoid of mitochondrial regulators of apoptosis

Although proteases of the caspase family are essential mediators of apoptosis in nucleated cells, in anucleate cells their presence and potential functions are almost completely unknown. Human erythrocytes are a major cell population that does not contain a cell nucleus or other organelles. However, during senescence they undergo certain morphological alterations resembling apoptosis. In the present study, we found that mature erythrocytes contain considerable amounts of caspase-3 and -8, whereas essential components of the mitochondrial apoptotic cascade such as caspase-9, Apaf-1 and cytochrome c were missing. Strikingly, although caspases of erythrocytes were functionally active in vitro, they failed to become activated in intact erythrocytes either during prolonged storage or in response to various proapoptotic stimuli. Following an increase of cytosolic calcium, instead the cysteine protease calpain but not caspases became activated and mediated fodrin cleavage and other morphological alterations such as cell shrinkage. Our results therefore suggest that erythrocytes do not have a functional death system. In addition, because of the presence of procaspases and the absence of a cell nucleus and mitochondria erythrocytes may be an attractive system to dissect the role of certain apoptosis-regulatory pathways.

Temperature transition of human hemoglobin at body temperature: effects of calcium

We studied the effects of calcium ion concentration on the temperature dependence of rheological behavior of human red blood cells (RBCs) and concentrated hemoglobin solutions. Our previous study (G. M. Artmann, C. Kelemen, D. Porst, G. Büldt, and S. Chien, 1998, Biophys. J., 75:3179-3183) showed a critical temperature (Tc) of 36.4 +/- 0.3 degrees C at which the RBCs underwent a transition from non-passage to passage through 1.3 microm micropipettes in response to an aspiration pressure of -2.3 kPa. An increase in intracellular Ca2+ concentration by using the ionophore A23187 reduced the passability of intact RBCs through small micropipettes above T(c); the micropipette diameter needed for >90% passage increased to 1.7 microm. Viscometry of concentrated hemoglobin solutions (45 and 50 g/dl) showed a sudden viscosity transition at 36 +/- 1 degrees C (Tc(eta)) at all calcium concentrations investigated. Below Tc(eta), the viscosity value of the concentrated hemoglobin solution at 1.8 mM Ca(2+) was higher than that at other concentrations (0.2 microM, 9 mM, and 18 mM). Above Tc(eta), the viscosity was almost Ca2+ independent. At 1.8 mM Ca2+ and 36 +/- 1 degrees C, the activation energy calculated from the viscometry data showed a strong dependence on the hemoglobin concentration. We propose that the transition of rheological behavior is attributable to a high-to-low viscosity transition mediated by a partial release of the hemoglobin-bound water.

Mechanisms by which intracellular calcium induces susceptibility to secretory phospholipase A2 in human erythrocytes

Exposure of human erythrocytes to the calcium ionophore ionomycin rendered them susceptible to the action of secretory phospholipase A(2) (sPLA(2)). Analysis of erythrocyte phospholipid metabolism by thin-layer chromatography revealed significant hydrolysis of both phosphatidylcholine and phosphatidylethanolamine during incubation with ionomycin and sPLA(2). Several possible mechanisms for the effect of ionomycin were considered. Involvement of intracellular phospholipases A(2) was excluded since inhibitors of these enzymes had no effect. Assessment of membrane oxidation by cis-parinaric acid fluorescence and comparison to the oxidants diamide and phenylhydrazine revealed that oxidation does not participate in the effect of ionomycin. Incubation with ionomycin caused classical physical changes to the erythrocyte membrane such as morphological alterations (spherocytosis), translocation of aminophospholipids to the outer leaflet of the membrane, and release of microvesicles. Experiments with phenylhydrazine, KCl, quinine, merocyanine 540, the calpain inhibitor E-64d, and the scramblase inhibitor R5421 revealed that neither phospholipid translocation nor vesicle release was required to induce susceptibility. Results from fluorescence spectroscopy and two-photon excitation scanning microscopy using the membrane probe laurdan argued that susceptibility to sPLA(2) is a consequence of increased order of membrane lipids.

Mechanism of selective release of membrane proteins from human erythrocytes in the presence of liposomes

Incubation of erythrocytes with liposomes results in the release of shed vesicles rich in glycosyl-phosphatidylinositol (GPI)-anchored proteins but poor in transmembranous proteins. We investigated the mechanisms of membrane protein polarization by examining the effect of the interaction between spectrin and membrane proteins on the release of a transmembranous protein, band 3, and a GPI-anchored protein, acetylcholinesterase (AChE), from erythrocyte ghosts. Polymerization of spectrin resulted in a 30-fold decrease in the released amount of band 3 per constant amount of shed vesicles but did not affect the amount of released AChE per constant amount of shed vesicles. On the other hand, the amount of released band 3 per constant amount of shed vesicles increased by cleaving the cytoplasmic part of band 3. Our results first demonstrated that the diffusibility of membrane proteins determined by steric hindrance between membrane proteins and protein mesh primarily determines the ease of localization of membrane proteins into shed vesicles. Taken together with the recent biophysical studies, we built a "fence selection model" that retrograding spectrin mesh sweeps diffusing band 3 molecules from the tip of the membrane crenated area toward the entry of the crenated area, but not AChE molecules. Our study describes a novel method for isolation of a large number of vesicles containing special and intact membrane proteins from cells not by using detergents or organic solvents, but by utilizing the fence effect between the cytoskeleton and membrane proteins.

Extensive electroporation abolishes experimentally induced shape transformations of erythrocytes: a consequence of phospholipid symmetrization?

As shown in earlier work (M.M. Henszen et al., Mol. Membr. Biol. 14 (1997) 195-204), exposure of erythrocytes to single brief electric field pulses (5-7 kV cm(-1)) enhances the transbilayer mobility of phospholipids and produces echinocytes which can subsequently be transformed into stomatocytes in an ATP-dependent process. These shape transformations arise from partly reversible changes of the transbilayer disposition of phospholipids, in agreement with the bilayer couple concept. Extensive membrane modification by repetitive (</=20) field pulses followed by 20 h incubation at 37 degrees C is now shown to produce discocytic cells which are resistant to many established shape-transforming treatments, including (A) single electric field pulses, Ca(2+) incorporation and exposure to membrane active amphiphiles, but also (B) metabolic depletion, binding of band 3 ligands, alkaline pH and contact with glass surfaces. The suppression of type A effects can readily be interpreted by a complete symmetrization of the phospholipids in extensively field pulse-modified cells which prevents shape transformations related to the asymmetric disposition of the phospholipids. This symmetrization could be further substantiated by more direct determinations of the transbilayer distribution of phospholipids. Suppression of shape transformations of type B may indicate an involvement of phospholipid asymmetry in these processes on a yet unknown mechanistic basis. Alternatively we discuss field pulse-induced alterations of the disposition of peripheral proteins or of the conformation of integral membrane proteins as mechanisms interfering with shape transformations of type B.

Catabolism of the human erythrocyte C3b/C4b receptor (CR1, CD35): vesiculation and/or proteolysis?

Human erythrocytes (E) react by exocytosis of membrane vesicles to various stresses including the fixation of the membrane attack complex of Complement. E from normal individuals loose a notable proportion of their initial number of surface CR1 molecules during the ageing process. An acquired decrease of CR1 on E also occurs in pathological conditions such as Systemic Lupus Erythematosus or AIDS. The present study investigated whether calcium ionophore A23187 (Ca-ion) induced vesicle formation of human E in vitro is responsible for a preferential loss of CR1 as well as whether CR1 molecules at the surface of Ca-ion treated E or vesicles are: (i) functional, (ii) native or protease degraded, or (iii) more clustered than CR1 on native E. A study of E from 137 normal individuals showed that a one-hour Ca-ion induced vesicle formation preferentially removed one third of E surface CR1. Kinetic experiments suggested that all surface CR1 could be removed from E upon longer incubation times. CR1 molecules on vesicles were still able to inhibit Complement activation, and were found in larger clusters than on native E. These data suggest that a significant part of surface CR1 molecules may be removed from E by vesicle formation during the life of E in normal individuals. This phenomenon could be exacerbated in pathological conditions.

Activation of the alternative pathway of complement by calcium-loaded erythrocytes resulting from loss of membrane phospholipid asymmetry

The aminophospholipids phosphatidylserine (PS) and phosphatidylethanolamine (PE) are exposed on the outer membrane leaflet of deoxygenated and irreversibly sickled erythrocytes and senescent normal cells. PS exposure on erythrocytes results in the expression of procoagulant activity for the conversion of prothrombin to thrombin. Because liposomes or vesicles composed of aminophospholipids can activate the alternative pathway of complement, the possibility that increased exposure of PS and PE on intact erythrocytes would also make them capable of activating the alternative pathway was examined. Loss of normal membrane phospholipid asymmetry was induced by incubation of erythrocytes with calcium (Ca2+) and the calcium ionophore A23187. PS exposure on 60% of erythrocytes was confirmed by binding of fluorescein isothiocyanate-conjugated annexin V. Expression of procoagulant activity, measured with the Russell's viper venom clotting assay, was significantly increased on the Ca2+/A23187-treated erythrocytes. In addition, the erythrocytes became capable of activating the alternative pathway of complement, as judged by an increase in cell-bound C3b after incubation with serum and a decrease in alternative pathway hemolytic activity of the serum. The effect could be reversed by incubation of the Ca2+/A23187-treated erythrocytes under conditions that induced recovery of normal membrane phospholipid asymmetry. In contrast, tetrathionate-treated erythrocytes showed no increase in binding of annexin V and no procoagulant activity and failed to activate the alternative pathway of complement. These findings demonstrate that loss of phospholipid asymmetry in erythrocytes not only results in expression of procoagulant activity but also renders the cells capable of activating the alternative pathway of complement.

High specificity of human secretory class II phospholipase A2 for phosphatidic acid

Lysophosphatidic acid (LPA) is a potent lipid second messenger which stimulates platelet aggregation, cell proliferation and smooth-muscle contraction. The phospholipase A2 (PLA2)-catalysed hydrolysis of phosphatidic acid (PA) is thought to be a primary synthetic route for LPA. Of the multiple forms of PLA2 present in human tissues, human secretory class-II PLA2 (hs-PLA2) has been implicated in the production of LPA from platelets and whole blood cells challenged with inflammatory stimuli. To explore further the possibility that hs-PLA2 is involved in the production of LPA, we rigorously measured the phospholipid head group specificity of hs-PLA2 by a novel PLA2 kinetic system using polymerized mixed liposomes. Kinetic analysis of recombinant hs-PLA2 demonstrates that hs-PLA2 strongly prefers PA as substrate over other phospholipids found in the mammalian plasma membrane including phosphatidylserine (PS), phosphatidylcholine (PC) and phosphatidylethanolamine (PE). The order of preference is PA >> PE approximately PS > PC. To identify amino acid residues of hs-PLA2 that are involved in its unique substrate specificity, we mutated two residues, Glu-56 and Lys-69, which were shown to interact with the phospholipid head group in the X-ray-crystallographic structure of the hs-PLA2-transition-state-analogue complex. The K69Y mutant showed selective inactivation toward PA whereas the E56K mutant displayed a most pronounced inactivation to PE. Thus it appears that Lys-69 is at least partially involved in the PA specificity of hs-PLA2 and Glu-56 in the distinction between PE and PC. In conjunction with a recent cell study [Fourcade, Simon, Viode, Rugani, Leballe, Ragab, Fournie, Sarda and Chap (1995) Cell 80, 919-927], these studies suggest that hs-PLA2 can rapidly hydrolyse PA molecules exposed to the outer layer of cell-derived microvesicles and thereby produce LPA.

Tyrosine phosphorylation of band 3 protein in Ca2+/A23187-treated human erythrocytes

Human erythrocytes were induced to release membrane vesicles by treatment with Ca2+ and ionophore A23187. In addition to the biochemical changes already known to accompany loading of human erythrocytes with Ca2+, the present study reveals that tyrosine phosphorylation of the anion exchanger band 3 protein also occurs. The relationship between tyrosine phosphorylation of band 3 and membrane vesiculation was analysed using quinine (a non-specific inhibitor of the Ca(2+)-activated K+ channel, and the only known inhibitor of Ca(2+)-induced vesiculation) and charybdotoxin, a specific inhibitor of the apamin-insensitive K(+)-channel. Both inhibitors suppressed tyrosine phosphorylation of band 3. In the presence of quinine, membrane vesiculation was also suppressed. In contrast, at the concentration of charybdotoxin required to suppress tyrosine phosphorylation of band 3, membrane vesiculation was only mildly inhibited (16-23% inhibition), suggesting that tyrosine phosphorylation of band 3 is not necessary for membrane vesiculation. Phosphorylation of band 3 was in fact observed when erythrocytes were induced to shrink in a Ca(2+)-independent manner, e.g. by treatment with the K+ ionophore valinomycin or with hypertonic solutions. These observations suggest that band 3 tyrosine phosphorylation occurs when cell volume regulation is required.

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.

Secretory phospholipase A2 generates the novel lipid mediator lysophosphatidic acid in membrane microvesicles shed from activated cells

Nonpancreatic secretory phospholipase A2 (sPLA2) displays proinflammatory properties; however, its physiological substrate is not identified. Although inactive toward intact cells, sPLA2 hydrolyzed phospholipids in membrane microvesicles shed from Ca(2+)-loaded erythrocytes as well as from platelets and from whole blood cells challenged with inflammatory stimuli. sPLA2 was stimulated upon degradation of sphingomyelin (SPH) and produced lysophosphatidic acid (LPA), which induced platelet aggregation. Finally, lysophospholipid-containing vesicles and sPLA2 were detected in inflammatory fluids in relative proportions identical to those used in vitro. We conclude that upon loss of phospholipid asymmetry, cell-derived microvesicles provide a preferential substrate for sPLA2. SPH hydrolysis, which is provoked by various cytokines, regulates sPLA2 activity, and the novel lipid mediator LPA can be generated by this pathway.

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

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

Streptolysin-O induces release of glycosylphosphatidylinositol-anchored alkaline phosphatase from ROS cells by vesiculation independently of phospholipase action

Streptolysin-O (SLO), a cholesterol-binding agent, was used for studies on the release of glycosylphosphatidylinositol (GPI)-anchored alkaline phosphatase (AP) from ROS cells. Treatment of cells with SLO resulted in a time- and concentration-dependent release of AP into the extracellular medium. This release was potentiated by Ca2+ and bovine serum, but not by GPI-specific phospholipase D (GPI-PLD) purified from bovine serum. The released AP distributed to the detergent phase after Triton X-114 phase separation. This result suggested that the released AP contained an intact GPI anchor, and thus both proteolysis and anchor degradation by anchor-specific hydrolases, including GPI-PLD, as the potential mechanisms for SLO-mediated AP release were ruled out. The released AP sedimented at 100,000 g. A substantial amount of lipids was detected in the 100,000 g pellet. Cholesterol and sphingomyelin were enriched in SLO-released material, compared with intact cells. These results were consistent with vesiculation as the mechanism for SLO induction of AP release. Two other cholesterol-binding agents, saponin and digitonin, were also able to release AP, possibly by a similar vesiculation mechanism, whereas others, including nystatin, filipin and beta-escin, failed to elicit any AP release. Eight GPI-anchored proteins were identified in ROS cells, and all were substantially enriched in the vesicles released by SLO. Taken together, these results do not provide any support for the hypothesis that the clustering of GPI-anchored proteins in the plasma membrane is responsible for their resistance to GPI-PLD cleavage.

HDL and apolipoprotein A-I protect erythrocytes against the generation of procoagulant activity

The appearance of anionic lipids on the extracellular surface of cells is required for the formation of the procoagulant complex that leads to the activation of prothrombin. Procoagulant activity would be expected to be inhibited by substances that stabilize the membrane structure and hence inhibit the transbilayer diffusion of phosphatidylserine from the cytoplasmic to the extracellular surface of the plasma membrane. The generation of procoagulant activity in human erythrocytes by A23187 and Ca2+ is inhibited by apolipoprotein A-I, its amphipathic peptide analogues, and high-density lipoprotein (HDL). These agents do not inhibit the Ca2+ loading of erythrocytes by A23187, nor do they inhibit the activation of prothrombin once the cells have been incubated at 37 degrees C with A23187 and Ca2+. Transbilayer diffusion of fluorescently labeled phosphatidylserine is inhibited by apolipoprotein A-I. These findings indicate that class A amphipathic helixes as well as lipoprotein particles and liposomes inhibit the transbilayer diffusion of phospholipids and procoagulant activity. This activity may contribute to the protective role of HDL against arteriosclerosis and thrombosis.