Alterations in human erythrocyte shape and the state of spectrin and phospholipid phosphorylation induced by cholesterol depletion

Spicule movement on RBCs during echinocyte formation and possible segregation in the RBC membrane

We use phase contrast microscopy of red blood cells to observe the transition between the initial discocyte shape and a spiculated echinocyte form. During the early stages of this change, spicules can move across the surface of the cell; individual spicules can also split apart into pairs. One possible explanation of this behaviour is that the membrane forms large scale domains in association with the spicules. The spicules are formed initially at the rim of the cell and then move at speeds of up to 3 μm/min towards the centre of the disc. Spicule formation that was reversed and then allowed to proceed a second time resulted in spicules at reproducible places, a shape memory effect that implies that the cytoskeleton contributes towards stopping the spicule movement. The splitting of the spicules produces a well-defined shape change with an increase in membrane curvature associated with formation of the daughter pair of spicules; the total boundary length around the spicules also increases. Following the model in which the spicules are associated with lipid domains, these observations suggest an experimental procedure that could potentially be applied to the calculation of the line tension of lipid domains in living cells.

Anti-atherogenic effects of centipede acidic protein in rats fed an atherogenic diet

AIM OF THE STUDY

To investigate the effects of centipede acidic protein (CAP) on atherosclerotic rats and the mechanisms involved.

MATERIALS AND METHODS

Male Sprague-Dawley rats were randomly divided into five groups: control, atherosclerotic, low-dose CAP (L-CAP), high-dose CAP (H-CAP) and simvastatin group (n=12 in each group). Atherosclerotic model was established by a single dose of vitamin D(3) and an atherogenic diet. Rats of H-CAP and simvastatin groups simultaneously received CAP or simvastatin daily for 6 weeks. At the completion of the experiment, the changes in lipid profile, hemorrheology, nitric oxide (NO), endothelin-1 (ET-1), malondialdehyde (MDA) and superoxide desmutase (SOD) were measured, and the histological changes in aorta and liver were observed.

RESULTS

Treatment of atherosclerotic rats with either low or high doses of CAP led not only to significant decreases in plasma total cholesterol, triglyceride, low density lipoprotein and increase in plasma high density lipoprotein, but also to improvement of the hemorrheologic abnormalities. On the other hand, CAP suppressed the lipid peroxidation, regulated the levels of ET-1 and NO. From the histopathological examination, treatment with CAP ameliorated the pathological changes in thoracic aorta and liver in atherosclerotic rats.

CONCLUSIONS

These results suggest that CAP significantly suppress the development of atherosclerosis, improves the hemorrheological disturbances and histopathological changes in the atherogenic diet fed rat model. These effects may partly attribute to reverse of dyslipidemia, inhibition of lipid peroxidation, regulation of NO and ET-1 system.

Membrane changes in rat erythrocyte ghosts on ghee feeding

Alterations in membrane lipid composition is known to result in functional and structural changes in the membrane, and dietary lipids play an important role in this change. It was of interest to study the influence of ghee feeding to the rat on membrane structure and function. The activities of membrane bound enzymes Na+ K+ ATPase and Acetylcholinesterase were studied as an index of membrane changes. Male weanling rats were fed 2.5% fresh or thermally oxidized ghee in the diet for a period of 8 weeks. The control rats were fed groundnut oil. A decrease of 28% in the membrane fluidity of erythrocyte ghost membranes was observed in the oxidized ghee fed group at 37 degrees C, by fluorescence polarization measurements using 1,6-Diphenyl-1,3,5-hexatriene as a probe. The activities of Na+ K+ ATPase and Acetylcholinesterase showed an increase of 65 and 200% respectively after feeding oxidized ghee (2.5%). Also changes in Na+, K+ and ATP kinetics were observed in these rats. Increased membrane lipid peroxidation (80%) and C/PL ratio (11%) in the oxidized ghee fed group was observed. Marginal changes in the fatty acid composition were also seen. Further, an increase in the osmotic fragility of erythrocytes was observed in the oxidized ghee fed rats. It is inferred from these experiments that consumption of oxidized ghee with the diet affects the erythrocyte ghost membrane structure and function at 2.5% level, whereas consumption of fresh ghee has no effect on the erythrocyte membrane.

Effect of dietary (n-9), (n-6) and (n-3) fatty acids on membrane lipid composition and morphology of rat erythrocytes

Studies focused on the intake of different dietary fats have shown changes in membrane lipid composition and, as a result, alterations in membrane physical properties. These changes affect erythrocyte morphology, receptor activity and oxygen transport, among others. Here, we compare the effects of diets exclusively differing in the type of fat (olive oil rich in monounsaturates, sunflower oil rich in n-6 polyunsaturates and fish oil rich in n-3 polyunsaturates) on fatty acid composition of plasma and erythrocyte membranes and erythrocyte morphology under scanning electron microscopy in rats. Monounsaturates are highest in animals fed olive oil diets; as are linoleic and arachidonic acids in sunflower oil-fed animals and n-3 PUFAs in fish oil-fed animals. The lowest levels of arachidonic acid are found in fish oil-fed animals and so are n-3 PUFAs in sunflower oil-fed animals. Our results show that sunflower oil-fed animals present lower discocyte, the major cell shape related to tissue oxygen supply, and higher codocyte percentages than olive oil- and fish oil-fed groups. Echinocyte percentage is higher in fish oil-fed animals with respect to the other two groups. The collective data indicate that olive oil elevates monounsaturates and the number of discocytes, pointing out a possible beneficial aspect of this dietary fat.

Does cholesterol depletion have adverse effects on blood rheology?

Blood viscosity (eta B) at shear rates 10 and 100s-1, plasma viscosity (eta P), hematocrit (Hct), and whole blood cholesterol (Chol) were measured in 50 patients with a history of myocardial infarction or unstable angina pectoris. Erythrocyte morphology was also studied by scanning electron microscopy to determine the proportion of nondiscocytic erythrocytes (NDE). There was a significant positive correlation between Chol and eta P (r = 0.41, P < 0.004) and a highly significant negative correlation (r = -0.69, P < 0.001) between Chol and Tk, a viscometric index of erythrocyte rigidity based on relative blood viscosity at high shear (eta B/eta P) corrected for Hct. This latter result indicates Chol reduction in this population may increase erythrocyte rigidity. Twenty-five patients with Chol values in the range 4.0-8.0 mmol/L were commenced on a standard lipid-lowering diet and after eight weeks half were also given pravastatin (40 mg daily). After thirty-two weeks Chol had fallen significantly more in the pravastatin group (28%) than in the diet only group (11%, P = 0.005). There was no change in eta P for either group but a significant increase in Tk for the pravastatin group only (P = 0.011). The change in total cholesterol (delta Chol) for each patient over thirty-two weeks was negatively correlated with both the change in the index of erythrocyte rigidity (delta Tk) (r = -0.40, P = 0.044) and the change in the proportion of nondiscocytic erythrocytes (delta NDE) (r = -0.47, P = 0.026). These data suggest that cholesterol reduction within the normolipemic range may be associated with unfavorable changes in blood rheology.

Phosphoinositide phosphorylation and shape changes produced by phosmet-oxon in human erythrocytes

1. "In vitro" incubation of red blood cells with phosmetoxon induced crenated and invaginated forms. 2. [32P] phosphate incorporation was greater in membranes from erythrocytes exposed to 300 nM phosmetoxon for 10 min than in control cells. 3. The highest incorporation was for phosphatidylinositol (PI), followed by phosphatidylinositol phosphate (PIP) and phosphatidylinositolbiphosphate (PIP2). 4. An activation of phosphatidylinositol (PI) kinase was detected with 150 and 300 nM of the pesticide, while there was no change in poliphosphoinositides (PPI) phosphodiesterase activity. 5. Results suggest an association between changes in PI kinase activity, the phosphorylation cycle of phosphatidylinositols and alterations in erythrocyte morphology induced by phosmetoxon.

Red blood cell morphology after a 100-km run

We studied the red blood cells of male athletes competing in a 100-km run. The RBC, haematocrit, and red cell indices were unaffected. The red cell morphology was studied in a wet preparation with light microscopy. A slight stomatocytosis was observed after the run compared to a control day (12.4 +/- 1.8% vs 4.3 +/- 1.8%, P less than 0.05). This difference persisted when runner red cells were incubated in buffer. Red cells from a normal donor did not undergo stomatocytic transformation when incubated in runner plasma. These data indicate that stomatocytosis after long-distance running is not caused by a plasmatic factor but is a change in the red cell membrane itself.

Influence of hypercholesterolemia on morphological and rheological characteristics of erythrocytes

The changes in shape and rheological parameters of erythrocytes in cholesterol-fed rabbits (fed with normal diet plus 0.5% cholesterol for a duration of 1 year) and in vitro cholesterol enrichment of erythrocytes in cholesterol enriched plasma were studied. This process affects the shape of erythrocytes. In rabbits the actual shape change begins after 6 weeks of cholesterol feeding. The shape changes in rabbits and in vitro cholesterol-enrichment are similar. The extent of shape changes depends on the amount of cholesterol incorporated into the membrane. The rheological parameters such as blood viscosity, osmotic fragility and deformability are measured. The whole blood viscosity is increased and deformability decreased. The cells become osmotically more fragile in cholesterol-fed rabbits but are resistant to osmotic lysis in in vitro cholesterol enrichment. The altered lipid composition may be responsible for the observed changes in these parameters.

Erythrocyte and plasma cholesterol exchange in sickle cell anemia

The effects of sickling on cholesterol exchange between red cell membranes and serum lipoproteins were studied by following the movement of tritiated cholesterol incorporated into erythrocytes. The initial rate of this exchange was greater in sickled cells than in normal cells. One quarter of the cholesterol in the sickled cells is quickly exchanged with plasma lipoproteins. After 15 minutes, the rate becomes identical for these two types of cells, reaching similar equilibrium at end. The sickling of red cells would explain the observed differences, although conditions of hypoxia and the saturation of the incubation medium with oxygen tend respectively to accentuate and to cancel this phenomenon.

On the mechanism of action of lonidamine: a study on human erythrocyte membrane

The incubation of human erythrocytes with increasing levels of the antineoplastic drug Lonidamine clearly indicated a dose-dependent effect on the lipid composition of the plasma membranes. A selective transfer of phosphatidylcholine and cholesterol from membrane to incubation medium and the consequent enrichment in phosphatidylethanolamine of the membrane itself was observed. Moreover, the membranes were found to contain increasing levels of the drug which was incorporated at a constant membrane/medium partition ratio. The changes in composition appeared to be consistent with morphological alterations observed by scanning and freeze-fracture electron microscopy demonstrating changes in cell shape, the presence of numerous intracellular vesicles, and a membrane protein rearrangement. The analysis of intact red cells by nuclear magnetic resonance ruled out the possibility that the alterations described above could be due to an ATP depletion. This further confirmed that cell membranes were the primary target of the Lonidamine action, the previously described energy metabolism impairment being a consequence of a selective damage of cellular membranes, probably originating from the incorporation of the drug into the lipid bilayer.

Cholesterol depletion affects the Ca2+ influx but not the Ca2+ pump in human erythrocytes

Control and cholesterol-depleted human erythrocytes were loaded with permeant Ca2+ chelators (Benz2-AM or Quin2-AM) in order to increase their exchangeable Ca2+ pool and to measure both Ca2+ fluxes and [Ca]i (free cytoplasmic calcium concentration). The fluxes were independent of the concentration and of the nature of the intracellular chelator. The ATP content was not decreased by more than 50% under our experimental conditions. Cholesterol depletion (up to 28%) induced a decrease in both Ca2+ fluxes and [Ca]i which was proportional to the extent of the depletion. It is shown that cholesterol depletion primarily altered the properties of the system responsible for Ca2+ entry causing a diminution of the [Ca]i. This, in turn, induced a diminution of the activity of the Ca2+ pump without affecting the properties of this pump.

Flicker spectroscopy of erythrocytes. A sensitive method to study subtle changes of membrane bending stiffness

Frequency analysis of thermally excited surface undulations of erythrocytes leading to the flicker phenomenon is applied to determine biochemically and physically induced modulations of the membrane curvature elasticity. Flicker spectra of individual cells fixed to the window of a flow chamber by polylysine are taken by phase contrast microscopy, enabling investigations of the reversibility of the structural modifications. The spectra may be approximated by Lorentzian lines in most cases. By measuring the amplitude (at zero frequency) and the line width, effects of the structural changes on the curvature elastic constant, Kc, and the wavelength distribution of the undulations may be studied separately. Effect of physically induced modifications: The temperature dependence of the flicker spectra are taken from 10 degrees C to 37 degrees C. Above 20 degrees C, Kc decreases with increasing temperature whereas the reverse holds below this limit. The latter anomalous behaviour is explained in terms of a conformational change associated with protein and lipid lateral phase separation. The bending stiffness increases when the cells swell osmotically, owing to surface tension effects. The dependence of the flicker spectra on the viscosity of the suspension medium agrees with the theoretical prediction. Biochemically and drug induced modifications: 5 vol% of ethanol leads to a pronounced and reversible suppression of the long wavelength undulations without altering the discoid cell shape and without affecting the bending stiffness appreciably. Adsorption of dextran to the glycocalix increases Kc by a factor of 1.6 at saturation. The bending stiffness is increased by a factor of 1.3 after cross-linking the proteins with the SH-oxidizing agent diamid. Injection of Ca++ into the cell via ionophores evokes (within 10 min) the formation of fine--probably spectrin free--spicules. This leads to an increase in Kc by a factor of 1.3 which is explained in terms of a lateral condensation of the spectrin/actin network. The spicule formation and Kc change is completely reversible (within 2 min) after perfusion with Ca++-free buffer. Cholesterol depletion leads first to a continuous increase in Kc without change of the cell shape whereas a sudden discocyte- to echinocyte transformation sets in below a critical steroid content. The latter transition is also observed in cell suspensions and is reminiscent of a phase transition. The anti-tumor drug actinomycin D evokes an increase in the bending stiffness Kc by a factor of two, suggesting that its effect is at least partially due to a modulation of the membrane structure.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct regulatory effect of cholesterol on the calmodulin stimulated calcium pump of cardiac sarcolemma

A great controversy has been set for several years related to an indirect versus a direct effect of cholesterol upon the muscle calcium pump. Employing an enriched cardiac sarcolemma preparation and a (Ca2+, Mg2+)-ATPase fraction isolated from this preparation, this study demonstrates that cholesterol directly interacts with the sarcolemmal calcium pump importantly inhibiting its enzyme activity. It was discovered that this inhibition can be in part explained by a total sensitivity loss of the pump to calmodulin. These results can be considered of importance in the correlation of plasma membrane cholesterol levels with deficiencies in calcium transport and cardiac muscle cell damage.

Effect of membrane cholesterol depletion on the immunoreactivity of the D antigen and IgG anti-D

The immunoreactivity of the main Rh antigen (D) and its corresponding antibody, as determined by a ti-IgG to IgG combining ratio, antibody dissociation and antigen accessibility to antibody, was examined in cholesterol depleted human red cells and ghost membranes. The anti-IgG reactivity of IgG anti-D bound to cholesterol depleted red cells and ghosts was demonstrably enhanced in vitro and in electron microscopy studies, particularly in ghosts. Dissociation of cell bound anti-D during buffer incubation was greater after cholesterol depletion, especially in ghosts. There was also reduced binding of anti-D to cholesterol-depleted cells as previously reported. All these effects appeared to be independent of endogenous or exogenous proteolysis in either cholesterol-depleted membranes or controls as judged from membrane electrophoretic analyses. A2C, an agent which increases membrane fluidity, had no effect on anti-D binding or the antiglobulin reactivity of cell bound IgG. A reduction in anti-D binding also was observed in red cells depleted of cholesterol following immobilization of membrane proteins by glutaraldehyde crosslinking. The findings show that cholesterol depletion not only affects the antigen but also Rh antibody reactivity. They also suggest that factors other than vertical antigen movement in a fluid bilayer may influence the behavior of the D antigen in cholesterol-modified erythrocytes.

Phosphoinositide reorganization in human erythrocyte membrane upon cholesterol depletion

The effect of cholesterol depletion on the activity of phosphatidylinositol/phosphatidylinositol 4-phosphate and diacylglycerol kinases and polyphosphoinositide phosphodiesterase has been studied in isolated membranes of human normal and cholesterol-depleted erythrocytes. Polyphosphoinositide synthesis (phosphatidylinositol/phosphatidylinositol 4-phosphate kinase activities) were found to depend on the permeability and sidedness characteristics of the membrane vesicles, which could limit the accessibility of ATP for the enzymes. When measured under proper conditions, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate synthesis were decreased in cholesterol-depleted membranes as compared with control membranes. The same level of synthesis could be obtained in both membranes by the addition of phosphatidylinositol (and Triton X-100) or of phosphatidylinositol 4-phosphate. Phosphatidic acid synthesis (diacylglycerol kinase activity) was also decreased in cholesterol-depleted membranes as compared with control membranes when measured in the presence of Ca2+. Addition of diolein (and Triton X-100) caused a large increase in phosphatidic acid synthesis which reached approximately the same level in both membranes. This showed that the apparent inhibition of polyphosphoinositide and phosphatidic acid synthesis was not due to a loss or to an inactivation of the kinases. Ca2+-activated polyphosphoinositide phosphodiesterase promoted the hydrolysis of 65-70% of the polyphosphoinositides in control and of only 45-55% in cholesterol-depleted membranes without changing the Ca2+ concentration for half-maximum hydrolysis (1 microM). Upon addition of sodium oleate, the extent of polyphosphoinositide hydrolysis became identical in both membranes, indicating again that there was no loss nor inactivation of the polyphosphoinositide phosphodiesterase in the cholesterol-depleted membranes. Since the concentration of the polyphosphoinositides was not changed by cholesterol depletion [Giraud, M'Zali, Chailley & Mazet (1984) Biochim. Biophys. Acta 778, 191-200], the reduction in both their synthesis and degradation observed here could be attributed to a reorganization of the phosphoinositides in membrane domains where they were not accessible to the kinases and phosphodiesterase. The reduction in phosphatidic acid synthesis was likely caused by a reduction in the total amount of the substrate diacylglycerol in cholesterol-depleted membranes as already shown [Giraud, M'Zali, Chailley & Mazet (1984) Biochim. Biophys. Acta 778, 191-200].

Relation of red cell membrane properties to invasion by Plasmodium falciparum

The effects of changes in red cell membrane properties on invasion by Plasmodium falciparum have been studied by varying the cholesterol content and the intracellular concentration of polyamines. Increased cholesterol content is known to cause large reductions in the internal fluidity of the phospholipid bilayer and a change in its preferred direction of bending, but does not cause changes in gross mechanical rigidity. Polyamines, on the other hand, are thought to increase the cohesion of the membrane cytoskeleton and impede translational diffusion of transmembrane particles, as well as increase the mechanical rigidity of the membrane. Cells with membranes augmented by 50% in cholesterol show no reduction in their susceptibility to parasitic invasion, whereas an increase in cytosolic polyamine (especially spermine) concentration leads to strong inhibition of invasion. In neither case is the development of the intracellular parasite affected. We conclude that it is the macroscopic, rather than the microscopic rheoelastic properties of the membrane that influence the invasion process. Depletion of membrane cholesterol leads to a substantial reduction in parasitaemia; it is suggested that this is linked to the reduced phosphorus incorporation into spectrin in these cells. Polyamines may exert a significant effect at physiological concentrations and the possibility must be considered that the elevated polyamine levels found in red cells in sickle cell disease may account for the protection against P. falciparum.

Incorporation and translocation of aminophospholipids in human erythrocytes

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

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

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

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.

Influence of cholesterol content on red cell membrane viscoelasticity and fluidity

The purpose of this investigation was to correlate the viscoelastic properties and lipid fluidity of the red blood cell membrane to its lipid composition. The viscoelastic properties of human red cells that had been enriched or depleted in cholesterol were determined by the micropipette technique. The lipid fluidity of the outer and inner leaflets of the erythrocyte membrane was concurrently assessed by steady state fluorescence depolarization. The elastic modulus and the viscosity moduli of the erythrocyte membrane showed no significant differences between the cholesterol-modified and the control cells. Cholesterol enrichment decreased the lipid fluidity of the outer membrane leaflet alone, and cholesterol depletion increased the fluidity mainly of the inner leaflet.

The coordination of sterol and phospholipid synthesis in cultured myogenic cells. Effect of cholesterol synthesis inhibition on the synthesis of phosphatidylcholine

The coordination of biosynthesis of cholesterol and phosphatidylcholine has been investigated in a myoblast cell line L6, grown in lipid-depleted medium. The addition of 25-hydroxycholesterol or compactin to this medium inhibits cholesterol synthesis by over 95%. The rate of [3H]choline incorporation into phosphatidylcholine begins to decline after 6 h and eventually falls to 45% of control. Measurements of choline flux through the CDPcholine pathway and of the pool sizes of choline-containing intermediates indicate that the formation of CDPcholine is the rate-limiting step in phosphatidylcholine synthesis in L6. The rate of CDPcholine synthesis was measured in vivo by pulse-chase experiments. Culturing cells with 25-hydroxycholesterol or compactin results in an inhibition of this step, which parallels the inhibition of incorporation of [3H]choline into phosphatidylcholine. The specific activities of the enzymes of phosphatidylcholine synthesis were assayed under optimal substrate conditions. Growth in the presence of sterol-synthesis inhibitors for 24 h has a significant, but variable, effect on the activity of microsomal and cytosolic cholinephosphate cytidylytransferase. Inhibition is seen in approximately one-half of the preparations and ranges up to 60%. The degree of inhibition of the enzyme in vitro correlates with an elevation of cytosolic triacylglycerol and phospholipid levels, and is not eliminated by the inclusion of excess stimulatory phospholipids in the assay. The pool sizes of the substrates, cholinephosphate and CTP, are unaffected by cholesterol synthesis inhibition. In contrast to the effects on cholinephosphate cytidylytransferase, the microsomal enzymes glycerol-3-phosphate acyltransferase and choline phosphotransferase are stimulated 2-fold or more. Choline kinase specific activity was inhibited 2-fold after 24 h of treatment with 25-hydroxycholesterol; however, no effect on this step was observed in vivo. These results indicate that the coordination of cholesterol and phosphatidylcholine synthesis involves regulation at the cytidylytransferase-catalyzed step.

A spin-label study of the correlation between stomatocyte formation and membrane fluidization of erythrocytes

Change in the membrane fluidity of human erythrocytes on transformation to stomatocytes was observed by ESR spectroscopy using 12-doxyl stearic acid or its methyl ester as a probe. When the transformation to stomatocytes was induced by four qualitatively different methods, i.e. (a) addition of cationic amphiphilic agents such as chloropromazine, tetracaine, chloroquine or primaquine, (b) addition of Triton X-100, a non-ionic detergent, (c) lowering the pH, and (d) depleting membrane cholesterol, membrane fluidization was always observed. This indicates the existence of a close correlation between stomatocyte formation and increase in the membrane fluidity. Furthermore, since the stomatocytes fixed by diamide treatment exhibited membrane fluidization only in the presence of the reagent, the enhanced membrane fluidity was a direct consequence of the reagent interacting with, and changing the state of, the lipid bilayer itself, and not through the influence of some structural alteration of spectrin. These results provide experimental support for the theoretical prediction made by Brailsford et al. [J. theoret. Biol. 86, 531 (1980)]. Plausible mechanisms for the discocyte-stomatocyte transformation are discussed.

Role of the bilayer in the shape of the isolated erythrocyte membrane

The determinants of cell shape were explored in a study of the crenation (spiculation) of the isolated erythrocyte membrane. Standard ghosts prepared in 5 mM NaPi (pH 8) were plump, dimpled disks even when prepared from echinocytic (spiculated) red cells. These ghosts became crenated in the presence of isotonic saline, millimolar levels of divalent cations, 1 mM 2,4-dinitrophenol or 0.1 mM lysolecithin. Crenation was suppressed in ghosts generated under conditions of minimal osmotic stress, in ghosts from red cells partially depleted of cholesterol, and, paradoxically, in ghosts from red cells crenated by lysolecithin. The susceptibility of ghosts to crenation was lost with time; this process was potentiated by elevated temperature, low ionic strength, and traces of detergents or chlorpromazine. In that ghost shape was influenced by a variety of amphipaths, our results favor the premise that the bilayer and not the subjacent protein reticulum drives ghost crenation. The data also suggest that vigorous osmotic hemolysis induces a redistribution of lipids between the two leaflets of the bilayer which affects membrane contour through a bilayer couple mechanism. Subsequent relaxation of that metastable distribution could account for the observed loss of crenatability.

A rapid and sensitive assay for determination of cholesterol in membrane lipid extracts

A commercially available enzymatic assay (Boehringer Monotest) was modified to allow a rapid and sensitive determination of cholesterol in membrane lipid extracts. This was achieved by adding 0.5% Triton X-100 to the reagent solution. The detergent did not interfere with the assay. The relationship between the amount of cholesterol per assay and the absorbance at 500 nm was linear up to 100 micrograms. The recovery in the assay was better than 95%. The assay was applied to the determination of cholesterol in erythrocyte membrane lipid extracts.