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

Intermembrane lipid transfer during Trypanosoma cruzi-induced erythrocyte membrane destabilization

The ability of Trypanosoma cruzi to induce erythrocyte membrane destabilization in vitro was studied. Epimastigote forms adhered to human erythrocytes and caused fusion or lysis of the red cells, depending on the conditions of the interaction. Red cells were fused in the presence of calcium, while haemolysis was induced in the absence of the cation. Dextran 60 C facilitated fusion but delayed lysis. Optimum pH and temperature for fusion were 7.4 and 37 degrees C, respectively. Lipid alterations were produced in the plasma membrane of the red cell during the interaction with the parasite. A Ca(2+)-independent increase of lysophospholipids and free fatty acids was common to both the lysis and fusion processes. A relative increase of 1,2-diacylglycerides was unique to the fusion process and these changes were dependent on Ca2+. The transfer of free fatty acids and lysophospholipids from T. cruzi to erythrocyte membranes was demonstrated using parasites pre-labelled with radioactive phospholipids. Pre-treatment of parasites with exogenous phospholipase A2 abolished the fusogenicity, while lysis was increased. Neither fusion nor haemolysis occurred when the parasites were pre-treated with fatty acid free albumin, phospholipase A2 inhibitors or when these compounds were present in the medium during the parasite-erythrocyte interaction. Our results suggest that T. cruzi induces erythrocyte membrane destabilization in vitro by transfer of lipid material in a calcium independent manner and that this ion is necessary for other membrane alterations that lead to erythrocyte fusion.

Human erythrocyte shape regulation: interaction of metabolic and redox status

The echinocyte-to-discocyte shape recovery of metabolically depleted erythrocytes is compromised by sulfhydryl reducing agents (Truong, H.-T.N., Ferrell, J.E., Jr. and Huestis, W.H. (1986) Blood 67, 214-221). In the presence of dithiothreitol (DTT) and sugars, crenated cells recover normal discoid shape transiently, but then develop the invaginations and intracellular inclusions of stomatocytes. The stomatogenic effects of DTT were investigated in erythrocytes recovering from crenation induced by several independent mechanisms. Cells crenated by direct manipulation of the membrane bilayer (lysophosphatidylcholine incorporation) recovered discoid shape similarly in the presence and absence of the reducing agent. In contrast, resealed ghosts and cells crenated by Mg2+ depletion or Ca2+ loading did not maintain stable discoid morphology in the presence of DTT, proceeding further to form stomatocytes. Thus cell crenation by expedients that involve cellular metabolic processes develop a redox-related morphological instability that is not found in amphipath-crenated cells.

Generation of phosphatidic acid during calcium-loading of human erythrocytes. Evidence for a phosphatidylcholine-hydrolyzing phospholipase D

We have studied the mechanism by which calcium-loading of human erythrocytes stimulates phospholipid turnover and generates diacylglycerol and phosphatidic acid. Using quantitative measurement of individual phospholipid classes, we have demonstrated that the amount of phosphatidic acid generated during calcium-loading of intact red cells exceeds the amount of diacylglycerol formed by phospholipase-C-mediated hydrolysis of the polyphosphoinositol lipids and that addition of the diacylglycerol kinase inhibitor, R59022, only partly inhibited this increase. Thus, in contrast to current explanations, the phosphatidic acid generated following calcium-loading of erythrocytes cannot be solely explained by the action of a polyphosphoinositol-lipid-specific phospholipase C with subsequent phosphorylation of diacylglycerol to phosphatidic acid. Our data demonstrate that calcium-loading of intact erythrocytes, but not of red cell ghost membranes, causes a small but significant decrease in the relative amount of phosphatidylcholine (PtdCho). In order to identify the mechanisms responsible for calcium-mediated hydrolysis of PtdCho, we encapsulated Ptd[Me-14C]Cho-containing rat liver microsomes into erythrocytes and studied the generation of [Me-14C]choline and phospho[Me-14C]choline. We found that choline was the only detectable 14C-labeled product. Furthermore, incubation of erythrocytes with calcium under hypotonic conditions and in the presence of [14C]PtdCho vesicles and ethanol resulted in the formation of [14C]phosphatidylethanol. Together, these results suggest that the loss of PtdCho during calcium-loading of human erythrocytes is caused by a previously unrecognized PtdCho-hydrolyzing phospholipase D, resulting in direct generation of phosphatidic acid. Analysis of the molecular species composition of PtdCho, phosphatidic acid, and diradylglycerol, confirm the simultaneous actions of PtdCho-hydrolyzing and polyphosphoinositol-lipid-hydrolyzing phospholipases in calcium-loaded human erythrocytes.

The role of inositol phospholipids in the association of band 4.1 with the human erythrocyte membrane

Band 4.1 is a major protein of the erythrocyte membrane skeleton. It promotes the binding of spectrin to F-actin and may anchor the skeletal network to the plasma membrane via its association with integral membrane proteins. Here, we have investigated the involvement of inositol phospholipids in the binding of band 4.1 to erythrocyte membranes using membrane vesicles stripped of all peripheral proteins at alkaline pH. Trypsinization of these vesicles allows the discrimination of two classes of band 4.1 binding sites: trypsin-sensitive sites (60-65% of the total), largely or exclusively on band 3, and trypsin-resistant sites (35-40% of the total), composed, at least in part, of the glycophorins. ATP depletion or activation of erythrocyte phosphoinositol phospholipase C led to a reduction in membrane phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] content by 20-70% in different experiments. The resulting decrease of band 4.1 binding to vesicles by was variable, but averaged about 15-20%. The same treatments led to an average decrease in the band 4.1 binding capacity of trypsinized vesicles of 55%. Since this is equivalent to a 20% decrease in the binding capacity of non-trypsinized vesicles (consistent with the above result), it indicates that PtdIns(4,5)P2 regulates the binding of band 4.1 only to trypsin-resistant binding sites (and to only a subset of these) accounting for about 15-20% of total band 4.1 binding sites on membranes. We found that hydrolysis of > 95% of PtdIns(4,5)P2 with exogenous phospholipase C-delta (PLC delta) resulted in no further decrease in band 4.1 binding to vesicles than did hydrolysis of 65-70% of PtdIns(4,5)P2 which is accessible to erythrocyte phosphoinositol phospholipase C. This suggests that only 65-70% of total membrane PtdIns(4,5)P2 is involved in regulating band 4.1 binding. Significantly, the pool of PtdIns(4,5)P2 involved is the same pool which can be hydrolysed by erythrocyte phosphoinositol phospholipase C, and which has been shown to be metabolically labile in erythrocytes. The membrane binding capacity for band 4.1 found in this study (averaging 1000 micrograms/mg vesicle protein) is considerably higher than that found in previous studies. The results are consistent with the existence of a binding site for band 4.1 on each copy of the major transmembrane proteins (band 3 and the glycophorins). These results provide new insights into the involvement of membrane inositol phospholipids in cytoskeletal-membrane interactions.

Cytoskeletal proteolysis during calcium-induced morphological transitions of human erythrocytes

An analysis was made of the rate, extent, and reversibility of the morphological transitions which were induced in human erythrocytes after loading with 150 microM or 1 mM Ca2+. The rate and extent of proteolytic cleavage of cytoskeletal proteins were monitored simultaneously, particularly those of the ankyrins and band 4.1, and were found not to reflect the rate of shape change. These observations were made when intact cells were incubated either in a buffer which supported glycolysis or in a simple isotonic Tris buffer without glucose. The composition of the buffer affected the initial morphology of the cells, the rate of morphological transition, the rate of proteolysis of cytoskeletal proteins, and the extent and kinetics of the reversal of morphology from the echinocyte to discocyte after removal of the ionophore A23187 and Ca2+. The morphology of cells transformed to spheroechinocytes by loading metabolically depleted cells for 15 min with 1 mM Ca2+, and which retained 50% band 2.1, was reversed in the presence of substrates for ATP synthesis to that of a mixture of 60% stage 1 echinocytes plus 25% discocytes, suggesting that ankyrin may not be essential for the maintenance of a disc-like morphology. Echinocytes which were depleted of greater than 50% band 4.1 were unable to undergo the transition back to discs.

Temperature-reversible eruptions of vesicles in model membranes studied by NMR

Deuterium (2H) and phosphorus (31P) nuclear magnetic resonance (NMR) and freeze-fracture electron microscopy were used to study spontaneous vesiculation in model membranes composed of POPC:POPS with or without cholesterol. The NMR spectra indicated the presence of a central isotropic line, the intensity of which is reversibly and linearly dependent upon temperature in the L alpha phase, with no hysteresis when cycling between higher and lower temperatures. Freeze-fracture microscopy showed small, apparently connected vesicles that were only present when the samples were frozen (for freeze-fracture) from an initial temperature of 40-60 degrees C, and absent when the samples are frozen from an initial temperature of 20 degrees C. Analysis of motional narrowing was consistent with the isotropic lines being due to lateral diffusion in (and tumbling of) small vesicles (diameters approximately 50 nm). These results were interpreted in terms of current theories of shape fluctuations in large unilamellar vesicles which predict that small daughter vesicles may spontaneously "erupt" from larger parent vesicles in order to expel the excess area created by thermal expansion of the bilayer surface at constant volume. Assuming that all the increased area due to increasing temperature is associated with the isotropic lines, the NMR results allowed a novel estimate of the coefficient of area expansion alpha A in multilamellar vesicles (MLVs) which is in good agreement with micromechanical measurements upon giant unilamellar vesicles of similar composition. Experiments performed on unilamellar vesicles, which had been placed upon glass beads, confirmed that alpha A determined in this way is unchanged compared with the MLV case. Addition of the highly positively charged (extrinsic) myelin basic protein (MBP) to a POPC:POPS system showed that membrane eruptions of the type described here occur in response to the presence of this protein.

Formation of large, membrane skeleton-free erythrocyte vesicles as a function of the intracellular pH and temperature

Vesiculation of intact erythrocytes can be induced by decreasing their intracellular pH and then heating the red cell suspension to a critical temperature value. While at intracellular pH 6 vesiculation begins at 45 degrees C, further decrease in the intracellular pH lowers the critical temperature. In addition, the critical temperature value can be modified by varying the length of the interval between titration and heating as well as by changing the temperature during this interval. The vesicles are large (1-3.5 micron in diameter), haemoglobin-containing and completely free of skeletal proteins. Pretreatment of the cells with diamide and 2,4-dinitrophenol had no substantial effect on vesiculation, while N-ethylmaleimide, chlorpromazine and wheat germ agglutinin proved to be inhibitory. Increasing the osmolarity of the incubation medium markedly decreased the critical temperature: red cells suspended in a solution of 600 mosM NaCl vesiculated at 42 degrees C instead of 45 degrees C when the intracellular pH was decreased to 6. We propose that the vesiculation is due to a purely physicochemical molecular mechanism which affects the state and dimension of the membrane skeleton. We also discuss the possible role of an altered haemoglobin-membrane interaction in preventing low pH-induced intramembrane particle aggregation in the membrane skeleton-free vesicles.

Uncoupling of the spectrin-based skeleton from the lipid bilayer in sickled red cells

The distribution of spectrin and band 3 in deoxygenated reversibly sickled cells was visualized by immunofluorescence and immunoelectron microscopy. Antibodies against band 3, the major lipid-associated transmembrane protein, labeled the entire cell body, including the entire length of the long protruding spicule, whereas antibodies against spectrin labeled only the cell body and the base region of the spicules. The results suggest that the formation of long spicules during sickling is associated with a continuous polymerization of hemoglobin S polymers, presumably through gaps in the spectrin-actin meshwork, and a subsequent uncoupling of the lipid bilayer from the submembrane skeleton.

Effects of divalent cations on lipid flip-flop in the human erythrocyte membrane

Treatment of human erythrocytes with ionophore A23187 (10 mumol.l-1) and Ca2+ (0.05-0.5 mmol.l-1) or Sr2+ (0.2-1 mmol.l-1) in results in a concentration-dependent acceleration of the transmembrane reorientation (flip) of the lipid probes lysophosphatidylcholine and palmitoylcarnitine to the inner membrane leaflet after their primary insertion into the outer leaflet. Mg2+, Mn2+, Zn2+ and La3+ do not accelerate flip. Ca2(+)-induced flip acceleration depends also on the ionophore concentration. It is reversed by removal of Ca2+ with EDTA. A causal role of Ca2(+)-induced membrane protein degradation and decrease of the polyphosphoinositide level in flip acceleration could be excluded. Likewise, calmodulin-dependent processes are probably not involved since the calmodulin antagonist calmidazolium (2-10 mumol.l-1) does not suppress but even enhances the Ca2(+)-induced flip acceleration. The same is true for the Ca2+ antagonist flunarizine. These drugs do not alter flip rate in the absence of Ca2+. At high Ca2+ (1-5 mmol.l-1) an initial flip acceleration is followed by flip normalization. High concentrations of Mn2+ and Mg2+ slow down flip rates. The selective acceleration of flip by Ca2+ and Sr2+ is discussed to be due to a local detachment of the membrane skeleton from the bilayer, whereas the unselective slow down of flip by divalent cations might be due to a stabilization of the membrane bilayer by the cations. After loading of cells with Ca2+ (but not with Mn2+) the inner membrane leaflet phospholipid phosphatidylserine becomes rapidly exposed to the outer membrane surface, as detectable by its accessibility to phospholipase A2 (5 min).(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Membrane Vesiculation Induced by Amphiphiles

Amphiphilic compounds, having the ability to alter the shape of erythrocytes, and to protect erythrocytes against hypotonic haemolysis, were studied for their ability to induce membrane vesiculation in human erythrocytes. About 40 amphiphilic compounds (including surfactants, bile salts, fatty acids, lysolecithin, phenothiazines, propanolol and benzyl alcohol) were studied. At a concentration where they exhibit maximum protection against hypotonic haemolysis, all the amphiphiles induced release of microvesicles from the membrane. Amphiphiles altering the shape of erythrocytes to sphero-echinocytes, induced a release of exovesicles, whereas amphiphiles altering the shape of erythrocytes to stomatocytes caused release of endovesicles. Some amphiphiles induced complex shape alterations by first causing sphero-echinocytes and then stomatocytes. These amphiphiles induced release of both exovesicles and endovesicles.

Echinocytosis and microvesiculation of human erythrocytes induced by insertion of merocyanine 540 into the outer membrane leaflet

Echinocytosis and release of microvesicles from human erythrocytes treated with the impermeant fluorescent dye merocyanine 540 (MC540) has been correlated with the extent of dye binding to intact cells and ghosts. At 20 degrees C binding appeared to saturate at about 9.3.10(6) molecules per cell (3.6 mol/100 mol phospholipid), equivalent to an expansion of the outer leaflet lipid area of about 2.7%. Stage 3 echinocytes were formed upon binding of (3-4).10(6) molecules of MC540/cell (about 1.3 mol/100 mol phospholipid), equivalent to an expansion of the outer leaflet lipid area of about 1.0%. Negligible release of microvesicles was observed with MC540 at 20 degrees C. Binding of MC540 to permeable ghosts was approximately twice that to cells suggesting that there was no selective binding to the unsaturated (more fluid) phospholipids which are concentrated in the inner lipid leaflet of the membrane. At 37 degrees C apparent maximal binding of MC540 was about 3.2 mol/100 mol phospholipid and correlated with the maximal release of microvesicles from the cells as measured by release of phospholipid and acetylcholinesterase. These results are discussed in relation to the bilayer couple hypothesis of Sheetz and Singer (Proc. Natl. Acad. Sci. USA 71 (1974) 4457-4461).

Vesiculation induced by amphiphiles in erythrocytes

The ability of shape-transforming cationic, anionic, zwitterionic, and nonionic amphiphiles to induce vesiculation in human erythrocytes was studied. At concentrations where they exhibit maximum protection against hypotonic haemolysis (CAHmax) echinocytogenic amphiphiles induced a rapid release of exovesicles. Following 5 min of incubation, the vesicle release (acetylcholinesterase release) amounted from 4% (sodium alkyl sulphates) to 13% (zwittergents) of the total acetylcholinesterase activity of the erythrocytes. At concentrations corresponding to CAH50 the vesicle release was less than 15% of that released at CAHmax. The size and the appearance of the vesicles varied with the type of amphiphile. Stomatocytogenic amphiphiles which do not pass the erythrocytes through echinocytic stages, did not induce release of exovesicles. Electron and fluorescence microscopic observations of erythrocytes treated with stomatocytogenic amphiphiles strongly indicated that an endovesiculation had occurred. Amphiphiles which pass the erythrocytes through echinocytic stages before stomatocytic shapes are attained, induced a release of both exo- and endovesicles.

Red cell membrane phosphatidylinositol kinase activity in hemolytic anemias and myeloproliferative diseases

Phosphatidylinositol kinase activity was determined in red cell membranes from 85 healthy individuals, 20 patients with hereditary hemolytic anemia and 24 patients with myeloproliferative disorder. Increased activity was found in all ten cases of sickle red disease and seven among ten cases of other hereditary hemolytic anemias. These increases had no correlation with the reticulocyte count nor with the red cell shape. An unexpected decreased activity was found in several cases of myeloproliferative disorders, especially in polycythemia vera, with a negative correlation with the reticulocyte count. The mechanism(s) and significance of the phosphatidylinositol kinase abnormalities in these different groups of diseases remain to determine.

The separation of [32P]inositol phosphates by ion-pair chromatography: optimization of the method and biological applications

We have developed an ion-pair reverse-phase HPLC method to measure inositol phosphates in 32P-labeled cells. The different chromatographic parameters were analyzed to optimize the resolution of the 32P-labeled metabolites. Analysis of inositol phosphates in biological samples was improved by a single charcoal pretreatment which eliminated interfering nucleotides without removing inositol phosphates. The kinetics of production of inositol phosphates in calcium-activated erythrocytes, vasopressin-stimulated hepatocytes, and thrombin-activated platelets were analyzed. Original data on the activation of phosphoinositide phospholipase C were obtained in intact erythrocytes by direct measurement of inositol (1,4,5)P3. Data from agonist-stimulated hepatocytes and platelets were consistent with those from previous studies. In conclusion, this technique offers many advantages over the methodologies currently employed involving anion-exchange chromatography and [3H]inositol labeling: (i) 32P labeling is less expensive and more efficient than 3H labeling and can be used with all types of cells without permeabilization treatments and (ii) ion-pair HPLC gives good resolution of inositol phosphates from nucleotides with shorter retention times, and long reequilibration periods are not required.

Trypanosoma cruzi: involvement of proteolytic activity during cell fusion induced by epimastigote form

Human erythrocytes were fused by Trypanosoma cruzi from 7 and 14 day old culture (stationary and declination phases, respectively) while only lysis was induced by 4 day old culture parasite (exponential phase). Lysis and erythrocyte fusion were studied by phase contrast microscopy, measuring of hemolysis and gel electrophoresis. The fusogenicity is Ca2+-dependent while lysis is delayed in the absence of exogenous Ca2+. The proteolysis of erythrocyte protein bands 1, 2, 2.1, 2.3 and 3 are common features of both fusion and lysis processes. Nevertheless the breakdown rate of ankyrin (band 2.1) and band 3 are different in fused or in lysed cells. The lysis process is associated with a faster degradation of band 2.1 and increase of band 2.3 than in the case of the fusion process. By contrast, degradation of band 3 occurs faster in the fusion than in the lytic event. Treatment of fusogenic parasites but not erythrocytes with TPCK, soybean trypsin inhibitor or FCS inhibited to some extent the fusion process and the decrease of bands 1, 2, 2.1, 2.3 and 3. The results suggest that proteases from fusogenic parasites may be directly or indirectly involved in the proteolysis of band 2.1 in a way related to induction of fusion.

Endogenous calcium in sickle cells does not activate polyphosphoinositide phospholipase C

Sickle-cell-anaemia erythrocytes (SS cells) are known to have a high Ca2+ content (particularly the dense cell fraction) and to take up Ca2+ on deoxygenation. It has been reported that this high Ca2+ was responsible for the activation of the Ca2+-dependent K+ loss, and of the Ca2+-sensitive polyphosphoinositide phospholipase C (PIC) in dense SS cells. We found that, either in the total population of SS cells or in the light or dense fractions, the content of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] was not changed, whereas that of phosphatidylinositol 4-phosphate was increased and that of phosphatidic acid (PtdOH) was decreased compared with normal (AA) erythrocytes. Deoxygenation-induced Ca2+ entry into SS cells did not change the concentration or, in 32P-prelabelled cells, the radioactivity of polyphosphoinositides and PtdOH. It also failed to induce the formation of inositol 1,4,5-trisphosphate, the product of PtdIns(4,5)P2 hydrolysis by PIC, which was measured by an original method using ion-pair reverse-phase h.p.l.c. Thus there was no evidence of an endogenous Ca2+ effect on the PIC activity in SS cells, in agreement with the demonstration that the excess Ca2+ in SS cells is compartmentalized into internal vesicles and unavailable as free Ca2+. The 32P incorporation in polyphosphoinositides and PtdOH was markedly higher in SS than in AA cells, but this increase was the same in both dense and light SS cells. The increase in the turnover of these phospholipids in SS cells is consistent either with an activation of the lipid kinases and phosphatases or with perturbation in the metabolic compartmentation of these lipids.

Membrane bilayer balance and platelet shape: morphological and biochemical responses to amphipathic compounds

Activated platelets adopt a characteristic spiculate morphology. A wide variety of anionic and zwitterionic amphipathic compounds were found to effect a similar shape change and to cause the open canalicular system to become less prominent. Several cationic amphipaths reversed thrombin-, PAF-, and amphipath-induced spiculation and restored the discoid shape. Higher concentrations of cationic amphipaths caused the cells to assume spheroid and indented forms, and caused the canalicular system to appear more prominent. Three amphipaths were studied further to address possible mechanisms underlying their morphological effects. Dilauroylphosphatidylcholine was found to induce spiculation without causing the changes in protein phosphorylation and inositide metabolism generally associated with platelet activation. Two other amphipaths, chlorpromazine (which induced sphering) and dilauroylphosphatidylserine (which caused spiculation followed by sphering) caused specific changes in protein and/or lipid phosphorylation, which may be responsible for some, but not all, of the morphological effects of these compounds. To account for these findings, we propose that platelet shape can be influenced by changes in the plasma membrane bilayer balance. Agents that bind to the membrane outer monolayer are accommodated by spiculation; those that bind to the inner monolayer are accommodated by sphering.

Storage of human red blood cells and platelets. Some aspects concerning the factors leading to storage lesion characterized as morphological changes and vesiculation. Minireview based on a doctoral thesis

1. Storage renders erythrocytes more responsive to thermally induced morphological changes, especially the shedding of microvesicles. 4-8 week old cells can be morphologically "rejuvenated" by heating. 2. If pH increases during storage of platelets an extensive loss of small particles occurs. The platelet disintegration is associated with a loss in the metabolic activity, discharge of LDH, increased susceptibility to phospholipid hydrolysis by phospholipase C and is found to be initiated during the actual preparation of platelet concentrates. 3. Activation of platelets during preparation can be decreased by shortening the first centrifugation time or by using adenine in the anticoagulant. 4. A 4 hour prestorage of the whole blood unit prior to centrifugation strongly decreases the activation of platelets upon stimuli and results in platelet concentrates much more stable to storage.

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.

Spin label study of erythrocyte deformability. Ca2+-induced loss of deformability and the effects of stomatocytogenic reagents on the deformability loss in human erythrocytes in shear flow

The Ca2+-induced loss of deformability in human erythrocytes and the recovery of the lost deformability by stomatocytogenic reagents were investigated by means of a new flow electron paramagnetic resonance (EPR) spin label method, which provides information on deformation and orientation characteristics of spin labeled erythrocytes in shear flow. The Ca2+-induced loss of deformability is attributed mainly to the increase in intracellular viscosity resulting from efflux of intracellular potassium ions and water (Gardos effect). Partial recovery of the lost deformability is demonstrated in the presence of stomatocytogenic reagents, such as chlorpromazine, trifluoperazine, W-7, and calmidazolium (R24571). The recovery can not be explained solely by suppression of the Gardos effect due to the reagents. Incorporation of an optimal amount of the reagents into the membrane appears to compensate for the membrane modification due to Ca2+ ions to restore a part of the lost deformability.

Modulation of erythrocyte vesiculation by amphiphilic drugs

Release of acetylcholinesterase-containing vesicles from human erythrocyte membranes induced by dimyristoylphosphatidylcholine (DMPC) was inhibited by exposure of red cells to cationic amphiphilic drugs like tetracaine, chlorpromazine and primaquine which all are known to induce stomatocyte formation. On the other hand, the process was facilitated when red cells were exposed to crenators like the anionic drugs indomethacin and phenylbutazone or when DMPC was added to calcium-loaded red cells. The results suggest that agents which are known to modulate red cell shape do also influence the vesiculation behavior of the cells.

Multiple metabolic pools of phosphoinositides and phosphatidate in human erythrocytes incubated in a medium that permits rapid transmembrane exchange of phosphate

1. A Hepes-based medium has been devised which allows rapid Pi exchange across the plasma membrane of the human erythrocyte. This allows the metabolically labile phosphate pools of human erythrocytes to come to equilibrium with [32P]Pi in the medium after only 5 h in vitro. 2. After 5-7 h incubation with [32P]Pi in this medium, only three phospholipids, phosphatidic acid (PtdOH), phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2) are radioactively labelled. The concentrations of PtdIns4P and PtdIns4,5P2 remain constant throughout the incubation, so this labelling process is a reflection of the steady-state turnover of their monoester phosphate groups. 3. During such incubations, the specific radioactivities of the monoesterified phosphates of PtdIns4, PtdIns4,5P2 and PtdOH come to a steady value after 5 h that is only 25-30% of the specific radioactivity of the gamma-phosphate of ATP at that time. We suggest that this is a consequence of metabolic heterogeneity. This heterogeneity is not a result of the heterogeneous age distribution of the erythrocytes in human blood. Thus it appears that there is metabolic compartmentation of these lipids within cells, such that within a time-scale of a few hours only 25-30% of these three lipids are actively metabolized. 4. The phosphoinositidase C of intact human erythrocytes, when activated by Ca2+-ionophore treatment, only hydrolyses 50% of the total PtdIns4,5P2 and 50% of 32P-labelled PtdIns4,5P2 present in the cells: this enzyme does not discriminate between the metabolically active and inactive compartments of lipids in the erythrocyte membrane. Hence at least four metabolic pools of PtdIns4P and PtdIns4,5P2 are distinguishable in the human erythrocyte plasma membrane. 5. The mechanisms by which multiple non-mixing metabolic pools of PtdOH, PtdIns4P and PtdIns4,5P2 are sustained over many hours in the plasma membranes of intact erythrocytes are unknown, although some possible explanations are considered.

The action of sphingomyelinase from Bacillus cereus on ATP-depleted bovine erythrocyte membranes and different lipid composition of liposomes

The presence of cholesterol or phosphatidylethanolamine in sphingomyelin liposomes enhanced 2- to 10-fold the breakdown of sphingomyelin by sphingomyelinase from Bacillus cereus. On the other hand, the presence of phosphatidylcholine was either without effect or slightly stimulative at a higher molar ratio of phosphatidylcholine to sphingomyelin (3/1). In the bovine erythrocytes and their ghosts, the increase by 40-50% or the decrease by 10-23% in membranous cholesterol brought about acceleration or deceleration of enzymatic degradation of sphingomyelin by 50 or 40-50%, respectively. The depletion of ATP (less than 0.9 mg ATP/100 ml packed erythrocytes) enhanced K+ leakage from, and hot hemolysis (lysis without cold shock) of, bovine erythrocytes but decelerated the breakdown of sphingomyelin and hot-cold hemolysis (lysis induced by ice-cold shock to sphingomyelinase-treated erythrocytes), either in the presence of 1 mM MgCl2 alone or in the presence of 1 mM MgCl2 and 1 mM CaCl2. Also, ATP depletion enhanced the adsorption of sphingomyelinase onto bovine erythrocyte membranes in the presence of 1 mM CaCl2 up to 81% of total activity, without appreciable K+ leakage and hot or hot-cold hemolysis. These results suggest that the presence of cholesterol or phosphatidylethanolamine in biomembranes makes the membranes more susceptible to the attack of sphingomyelinase from B. cereus and that the segregation of lipids and proteins in the erythrocyte membranes by ATP depletion causes the deceleration of sphingomyelin hydrolysis despite the enhanced enzyme adsorption onto the erythrocyte membranes.

Calcium-induced alterations in the levels and subcellular distribution of proteolytic enzymes in human red blood cells

Human red cells were treated with 100 microM Ca2+ and ionophore A 23187. This treatment induces remarkable changes in the activities of the two major proteolytic systems of red cells, i.e. Ca2+-dependent neutral proteinase and acid endopeptidases. Ca2+-dependent neutral proteinase undergoes intracellularly preliminary activation of the inactive proenzyme species, followed by eventual inactivation through self-proteolysis. Transient activation is shown by selective degradation of cytoskeletal proteins known to be targets of this enzyme system. Concomitantly, acid endopeptidase activity is substantially released from the membrane into the cytosol. Preliminary inactivation of the Ca2+-dependent neutral proteinase by exposure of Glucose 6-phosphate dehydrogenase-deficient red cells to auto-oxidizing divicine prevents alterations induced by Ca2+ loading on cytoskeletal membrane proteins, while leaving solubilization of acid endopeptidase activity unaffected. The two events, although dependent on Ca2+ loading, are therefore unrelated to each other.

The 240-kDa subunit of human erythrocyte spectrin binds calmodulin at micromolar calcium concentrations

The binding of the isolated alpha-subunit of human erythrocyte spectrin to calmodulin is demonstrated by partitioning in aqueous two-phase systems. The affinity of the alpha-subunit for calmodulin is slightly higher than that of the spectrin dimer, whereas the beta-subunit interacts only very weakly. The binding is in all cases calcium-dependent and is abolished on addition of chlorpromazine. At an ionic strength close to physiological conditions, about 1 microM free calcium is required to induce maximum binding of calmodulin to spectrin dimer.

Changes in membrane polypeptides, polyphosphoinositides and phosphatidate in dense fractions of sickle cells

When sickle erythrocytes were fractionated on discontinuous isotonic stractan gradients the denser fractions, which were rich in irreversibly sickled cells contained less polyphosphoinositides and more phosphatidate than either lighter sickle cell fractions or normal cells. These changes could be due to activation of a polyphosphoinositide phosphodiesterase in the denser cells. Membrane polypeptide analysis of the denser fractions also showed a marked depletion of band 4.1 and a protein of molecular mass about 110 kDa but an increased amount of a 180 kDa polypeptide which might be a breakdown product of ankyrin. These biochemical alterations could be consequences of Ca2+ accumulation in the denser sickle cells and may contribute to the structural alterations which give rise to irreversibly sickled cells.

Effect of sickling on dimyristoylphosphatidylcholine-induced vesiculation in sickle red blood cells

To study the effect of sickling on dimyristoylphosphatidylcholine (DMPC)-induced vesiculation, sickle (SS) red blood cells were incubated with sonicated suspensions of DMPC under either room air or nitrogen. Like normal red cells, when sickle cells were incubated with DMPC under oxygenated conditions, incorporation of DMPC into the erythrocyte membrane occurred, followed by echinocytic shape transformation and subsequent release of membrane vesicles. On the other hand, when SS cells were induced to sickle by deoxygenation, DMPC-induced vesiculation of these cells was dramatically reduced. However, upon reoxygenation, release of vesicles from these sickle erythrocytes occurred immediately. When SS cells were incubated under hypertonic (500 mosM) and deoxygenated conditions (where hemoglobin polymerization occurs but red cells do not show the typical sickle morphology), a similar decrease in the extent of vesiculation was observed. Experiments with radiolabelled lipid vesicles indicated that incorporation of DMPC into erythrocyte membranes occurred in all cases and therefore was not the limiting factor in the reduction of vesiculation in deoxygenated SS cells. Taken together, these results indicate that cellular viscosity and membrane rigidity, both of which are influenced by hemoglobin polymerization, are two important factors in process of vesicle release from sickle erythrocytes.

Endocytosis in sickle erythrocytes: a mechanism for elevated intracellular Ca2+ levels

Staining of sickle cells with the fluorescent probes chlortetracycline (a Ca2+ probe) and diindocarbocyanine (a general membrane probe) revealed the presence of Ca2+-containing vesicles which are not found in normal erythrocytes. These vesicles increase in number upon deoxygenation, and are apparently formed by endocytosis, as judged by the use of the extracellular fluorescent probe lucifer yellow. The presence of vesicles is not restricted to any particular morphological or density class of cells in the general population.

Regulation of the association of membrane skeletal protein 4.1 with glycophorin by a polyphosphoinositide

Many of the physical properties of the erythrocyte membrane appear to depend on the membrane skeleton, which is attached to the membrane through associations with transmembrane proteins. A membrane skeletal protein, protein 4.1, is pivotal in the assembly of the membrane skeleton because of its ability to promote associations between spectrin and actin. Protein 4.1 also binds to the membrane through at least two sites: a high-affinity site on the glycophorins and a site of lower affinity associated with band 3 (ref. 11). The glycophorin-protein 4.1 association has been proposed to be involved in maintenance of cell shape. Here we show that the association between glycophorin and protein 4.1 is regulated by a polyphosphoinositide cofactor. This observation suggests a mechanism which may explain the recently reported dependence of red cell shape on the level of polyphosphoinositides in the membrane.

The influence of cellular ATP levels on dimyristoylphosphatidylcholine-induced release of vesicles from human erythrocytes

Release of membrane vesicles from human erythrocytes was induced by modulation of red cell ATP levels, by incubation of erythrocytes with sonicated dimyristoylphosphatidylcholine (DMPC) suspensions, or by a sequential combination of both procedures. When red blood cell ATP levels were decreased prior to incubation with DMPC, the lag-time between addition of the lipid and beginning of vesiculation was reduced. Furthermore, the rate of vesicle release itself was accelerated. Experiments carried out with a rapid ATP depletion technique showed that the onset of vesiculation and the release were most evidently accelerated in those cases where echinocytes had been formed prior to the addition of DMPC. The results suggest that red blood cells with reduced cellular ATP levels or an altered cell shape are more susceptible to a further perturbation of the membrane by addition of exogenous DMPC.

The effects of phorbol ester, diacylglycerol, phospholipase C and Ca2+ ionophore on protein phosphorylation in human and sheep erythrocytes

Treatment of human or sheep erythrocytes with PMA (phorbol myristate acetate) enhanced [32P]phosphate labelling of membrane polypeptides of approx. 100, 80 and 46 kDa. The 80 kDa and 46 kDa polypeptides coincided with bands 4.1 and 4.9 respectively on Coomassie-Blue-stained gels. Similar but smaller effects were obtained by treating human cells with 1-oleoyl-2-acetyl-rac-glycerol (OAG), exogenous bacterial phospholipase C or ionophore A23187 + Ca2+, each of which treatments would be expected to raise the concentration of membrane diacylglycerol. In contrast, sheep cells, which do not increase their content of diacylglycerol when treated with phospholipase C or A23187 + Ca2+, only showed enhanced phosphorylation with OAG. Neither human nor sheep cells showed any enhanced [32P]phosphate labelling of phosphoproteins when treated with 1-mono-oleoyl-rac-glycerol. It is concluded that diacylglycerol from a variety of sources can activate erythrocyte protein kinase C, but that the most effective diacylglycerol is that derived from endogenous polyphosphoinositides. In contrast with bacterial phospholipase C and A23187, which stimulate synthesis of phosphatidate by increasing the cell-membrane content of diacylglycerol in human erythrocytes, PMA, OAG or 1-mono-oleoyl-rac-glycerol caused no change in phospholipid metabolism.

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++.

Changes in morphology and in polyphosphoinositide turnover of human erythrocytes after cholesterol depletion

Human erythrocytes were cholesterol-depleted (5-25%) by incubation with phosphatidylcholine vesicles in media containing Ca2+ at different concentrations (0, 28 nM, 5 microM or 1 mM). After removal of the vesicles, the cells were reincubated with [32P]phosphate in the same media. Control (incubated in buffer alone) and cholesterol-maintained erythrocytes (incubated with cholesterol/phosphatidylcholine vesicles) were treated similarly. Cholesterol depletion induced the conversion of the cells into stomatocytes III and spherostomatocytes and decreased the turnover rate of phosphatidylinositol phosphate and of phosphatidylinositol bisphosphate. None of these effects were observed in cholesterol-maintained cells. In cholesterol-depleted cells, they occurred without changes in the ATP specific activity or in the polyphosphoinositide concentrations. Moreover, these modifications of shape and of lipid metabolism were proportional to the extent of the cholesterol depletion and were independent of the external Ca2+ concentration. In contrast, other effects of cholesterol depletion, a decrease in the turnover rate of phosphatidic acid, a decrease in diacylglycerol and in phosphatidic acid concentrations were dependent on the external Ca2+ concentration. Thus it appears that the shape change was not correlated with a change in the concentrations of these phospholipids or of diacylglycerol and therefore cannot be explained by a bilayer couple mechanism involving these phospholipids. However, the spherostomatocytic transformation was correlated with the decrease in the turnover rate of the polyphosphoinositides, but not with the turnover rate of phosphatidic acid, suggesting a role for the turnover of the polyphosphoinositides in the maintenance of the erythrocyte shape.

Erythrocyte membrane vesiculation and changes in membrane composition during storage in citrate-phosphate-dextrose-adenine-1

Serial studies were made of the membranes of the erythrocytes and the vesicles shed during storage of blood in polyvinyl chloride containers for 35 days in citrate-phosphate-dextrose-adenine anticoagulant. Special precautions were taken to eliminate artifacts created by contaminating leukocytes, platelets and red blood cell ghosts. A total of 15.6% of the cholesterol and 5.2% of the phospholipids of the membranes was lost with no gross change in the gel electrophoretic patterns. The quantity of vesicles found in the supernatant plasma increased during storage and their membranes were characterized by the absence of spectrin, ankyrin, and periodic acid Schiff bands 2 and 3. The ratio of lipids to protein in the vesicles increased as they accumulated perhaps reflecting a rearrangement of the erythrocyte membrane constituents during prolonged maintenance at 4 degrees C.

Phosphoinositide metabolism and the morphology of human erythrocytes

ATP-depleted human erythrocytes lose their smooth discoid shape and adopt a spiny, crenated form. This shape change coincides with the conversion of phosphatidylinositol-4,5-bisphosphate to phosphatidylinositol and phosphatidic acid to diacylglycerol. Both crenation and lipid dephosphorylation are accelerated by iodoacetamide, and both are reversed by nutrient supplementation. The observed changes in lipid populations should shrink the membrane inner monolayer by 0.6%, consistent with estimates of bilayer imbalance in crenated cells. These observations suggest that metabolic crenation arises from a loss of inner monolayer area secondary to the degradation of phosphatidylinositol-4,5-bisphosphate and phosphatidic acid. A related process, crenation after Ca2+ loading, appears to arise from a loss inositides by a different pathway.

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 degrees 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.

Trypanosoma cruzi: fusogenic ability of membranes from cultured epimastigotes in interaction with human syncytiotrophoblast

Trypanosoma cruzi epimastigotes cultured in vitro were disrupted by successive freezing and thawing and subsequent sonication. The total homogenate was fractionated by differential centrifugation to obtain an enriched plasma membrane fraction. The proteins of subcellular parasite fractions were labeled with 131I and their binding to membrane fractions from human placenta syncytiotrophoblast was studied. Syncytiotrophoblast fractions enriched in plasma showed higher specific activity for binding an enriched T. cruzi plasma membrane fraction compared with other fractions of syncytiotrophoblast. The properties of this interaction were studied with digestive enzymes (trypsin and phospholipase A2). The results showed that both proteins and lipids could be involved in this interaction. The Ca2+ requirements for the membrane-membrane interaction are different for the two membranes studied. Also the enriched plasma membrane T. cruzi fraction had a higher capacity to induce fusion processes than the other subcellular fractions. The above results indicate that a preferential syncytiotrophoblast-T. cruzi interaction may occur between the two cell surfaces as compared to intracellular membranes and that the parasite surface is able to induce an instability process leading to membrane fusion. These results may have implications in regard to the mechanism of entry of the parasite into cells.

A cytoplasmic requirement of red cells for invasion by malarial parasites

Human red cells, when lysed by dialysis at high haematocrit against a medium of low ionic strength and then dialysed back to physiological saline at 37 degrees C, give rise to resealed ghosts that are invaded with high efficiency by Plasmodium falciparum parasites. When the haematocrit is reduced, a critical concentration is reached, such that the resealed ghosts no longer support invasion. This indicates that a constituent of the cytoplasm becomes diluted to a concentration below a critical level. This constituent is evidently ATP, for when extraneous ATP is added to the diluent and the dialysate, the susceptibility to invasion is fully restored. This does not occur when the non-hydrolysable analogue, adenylyl-imidodiphosphate (AMP-PNP) is substituted for ATP, whereas the hydrolysable ATP analogue, adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma-S), which can be utilised by kinases, can partly replace ATP. Stimulation of invasion by the addition of 2,3-diphosphoglycerate was also associated with a perceptible rise in ATP concentration. The invasion process does not appear to involve intracellular calcium, for EGTA introduced into the resealed ghost has no detectable effect. Moreover, vanadate in the medium does not appreciably inhibit invasion, and it is thus unlikely that the requirement for ATP is linked to the activity of membrane ion-pump enzymes. An inhibitor of phosphorylation, adenosine, introduced into the cells at high concentration, causes significant inhibition of invasion. The results suggest that ATP is required for maintaining the turnover of phosphoryl groups of membrane-associated proteins, such as spectrin. A basic scheme for the mechanism of the invasion process is suggested. In addition to the effect of ATP, it is also shown that with greater dilution, and in the presence of ATP, there is an abrupt loss of susceptibility to invasion. It is inferred that this is due to the dilution of another essential cytoplasmic constituent to below a critical concentration. This second constituent has not yet been identified.

The fatty acid composition of 1,2-diacylglycerol and polyphosphoinositides from human erythrocyte membranes

The fatty acid compositions of 1,2-diacylglycerol and polyphosphoinositides have been determined in human erythrocyte membranes that have been incubated in the presence or in the absence of Ca2+. The results show that the diacylglycerol that is formed in Ca2+-treated membranes has a fatty acid composition closely similar to that of the inositides, consistent with previous indications that Ca2+ stimulates the activity of a polyphosphoinositide phosphodiesterase in the membranes. In contrast with some previous results, it appears that these plasma-membrane inositides and their derived diacylglycerols are rich in stearic acid and arachidonic acid.

Optimized procedures for investigating changes in human erythrocyte membrane proteins using treatment with calcium and various proteases

In order to establish reproducible sensitive techniques for investigating diseases in which membrane defects have been implicated, human red cell membrane proteins were studied by SDS electrophoresis in ghosts prepared from the blood of 35 separate adult donors using treatment with 1 mmol/l calcium and proteolytic dissection under standardized conditions. The effects of calcium were consistent with increased binding of cytosolic proteins in membranes from fresh blood and endogenous proteolysis in membranes from aged blood. Exogenous proteolysis was most effective with trypsin and was facilitated both by pre-binding this protease to the membrane and by inclusion of 1 mmol/l Ca2+. The nature and extent of these effects was highly dependent on the storage time of the blood. Preliminary indications of cleavage fragment length polymorphism was obtained after treatment with the S. aureus V8 protease but not after trypsin or alpha chymotrypsin treatment.

Limited breakdown of cytoskeletal proteins by an endogenous protease controls Ca2+-induced membrane fusion events in chicken erythrocytes

The profound morphological changes which follow the treatment of chicken erythrocytes with the ionophore A23187 and Ca2+ are associated with a concomitant breakdown of certain membrane-associated proteins including alpha-spectrin, goblin and microtubule-associated proteins (MAPS) which undergo a limited proteolysis to give large, well-defined fragments. The Ca2+-sensitive protease responsible for these changes appears to be present in the soluble fraction of the cells. Treatment with TLCK or iodoacetamide inhibits both the major morphological changes and the proteolytic events but these agents do not prevent the dissociation of microtubules or the activation of endogenous sphingomyelinase which occur in cells with raised levels of intracellular Ca2+. It is suggested that the sphingomyelinase is activated as a consequence of a Ca2+-induced loss of phospholipid asymmetry in the plasma membrane.

Effects of valinomycin, A23187 and repetitive sickling on irreversible sickle cell formation

The formation of irreversibly sickled red cells has been studied by inducing cell shrinkage, ion loss, Ca2+ accumulation and membrane loss either singly or in combination. Valinomycin, A23187+Ca2+ or hypertonic saline caused shrinkage of the cells with retention of the sickled form after reoxygenation. The cells which had retained the sickle shape after treatment with the ionophores and reoxygenation remained sickled after exposure to hypotonic media. These cells were also osmotically insensitive. Retention of the sickled form was not dependent upon membrane loss as induced by repeated sickle-unsickle cycles or by A23187+Ca2+ treatment although repetitive sickling did give rise to shorter, stubbier spicules. Sickled red cells, either the endogenous irreversibly sickled cells or the sickled cells induced by deoxygenation, did not lose membrane by vesicle or spicule loss as normal cells or oxygenated sickle red cells do. Cell water loss without cell membrane loss appears to be an important factor in the irreversible sickling of red cells.

Correlation between distribution of cytoskeletal proteins and release of alkaline phosphatase-rich vesicles by epiphyseal chondrocytes in primary culture

Matrix vesicles, extracellular microstructures known to be involved in endochondral calcification, are rich in alkaline phosphatase and have been shown to contain actin. The mechanism of matrix vesicle formation in chondrocytes is not well understood. Chondrocytes from the epiphyseal growth plate, when grown in primary culture, elaborate alkaline phosphatase-rich vesicles. We examined the distribution of the cytoskeletal proteins actin, myosin, tubulin, and vinculin at various time-points during culture using indirect immunofluorescent labeling. Concomitantly, the production of alkaline phosphatase-containing matrix vesicles was also followed. Cell morphology changed noticeably at two distinct stages during the 22-day culture period: Immediately after release from the growth plate the cells were rounded, but after 4 days of culture they began to spread out and acquire irregular shapes with distinct filopodia. By 13 days, as the cells attained confluency, they reacquired a rounded, polygonal appearance. At all time-points, tubulin was seen as a dense network of microtubules radiating from the perinuclear region throughout the cytoplasm toward the cell periphery. Initially actin was seen in filamentous form, but displayed a punctate distribution focused at contact points during the cell-spreading stage of culture. After confluency, actin was concentrated at cell-cell junctions. Initially, vinculin was diffusely distributed, but became focused in multiple adhesion plaques and at the termini of filopodia during the cell-spreading stage of culture. Following confluency vinculin became concentrated at cell-cell junctions. Myosin was observed at all time-points in small, intensely localized focal points in the cytoplasmic region of the cells and was consistently absent from the nuclear and peripheral regions. The amount of myosin in the cells increased steadily with time in culture. Elaboration of alkaline phosphatase-rich vesicles, which corresponded closely with the rounded morphology of early and late stages of culture, may be correlated with contact inhibition.

Microvesiculation and sphingomyelinase activation in chicken erythrocytes treated with ionophore A23187 and Ca2+

Treatment of chicken erythrocytes with ionophore A23187 and Ca2+ leads to the disappearance of the marginal band of microtubules and to a release of the constraints which normally maintain the nucleus in a central position in the cells. The consequent close apposition of the nucleus to the plasma membrane may allow nuclear-plasma membrane fusion to occur and subsequently results in the release of microvesicles from the hybrid surface membrane. The remnant cells are spherical, and have nuclei which appear to be partly exocytosed. Concomitant with these morphological changes, there is a breakdown of 20-30% of the total cell sphingomyelin by an endogenous sphingomyelinase which does not require Ca2+ and which releases phosphorylcholine only into the cell interior. It is suggested that the pool of sphingomyelin which is broken down as a consequence of Ca2+ entry into the cells is present in the nuclear membrane and that it becomes available to the plasma membrane sphingomyelinase as a result of the close apposition of nucleus and plasma membrane induced by Ca2+.

Deformability of isolated red blood cell membranes

We have used a laser diffraction method (ektacytometry) to directly measure the membrane component of red cell deformability, without contributions from either cell geometry or internal viscosity. This technique was validated by subjecting resealed erythrocyte ghosts to manipulations previously shown to increase the membrane shear modulus. Heating above 45 degrees C, pH greater than 9.0 and less than 5.0, and micromolar concentrations of the cross-linking agents, glutaraldehyde and diamide, all reduced the deformability of resealed erythrocyte ghosts. We have applied this assay to the study of reduced cellular deformability of calcium-loaded red cells, and have shown that, for physiological concentrations of calcium, the effect of calcium on the physical properties of the membrane may be negligible when compared to its effect of promoting cell dehydration and subsequent increased cytoplasmic viscosity.