Resistance of Human Erythrocyte Membranes to Triton X-100 and C12E8

Membrane Rearrangements in the Maturation of Circulating Human Reticulocytes

Red blood cells (RBCs) begin their circulatory life as reticulocytes (Retics) after their egress from the bone marrow where, as R1 Retics, they undergo significant rearrangements in their membrane and intracellular components, via autophagic, proteolytic, and vesicle-based mechanisms. Circulating, R2 Retics must complete this maturational process, which involves additional loss of significant amounts of membrane and selected membrane proteins. Little is known about the mechanism(s) at the basis of this terminal differentiation in the circulation, which culminates with the production of a stable biconcave discocyte. The membrane of R1 Retics undergoes a selective remodeling through the release of exosomes that are enriched in transferrin receptor and membrane raft proteins and lipids, but are devoid of Band 3, glycophorin A, and membrane skeletal proteins. We wondered whether a similar selective remodeling occurred also in the maturation of R2 Retics. Peripheral blood R2 Retics, isolated by an immunomagnetic method, were compared with mature circulating RBCs from the same donor and their membrane protein and lipid content was analyzed. Results show that both Band 3 and spectrin decrease from R2 Retics to RBCs on a "per cell" basis. Looking at membrane proteins that are considered as markers of membrane rafts, flotillin-2 appears to decrease in a disproportionate manner with respect to Band 3. Stomatin also decreases but in a more proportionate manner with respect to Band 3, hinting at a heterogeneous nature of membrane rafts. High resolution lipidomics analysis, on the contrary, revealed that those lipids that are typically representative of the membrane raft phase, sphingomyelin and cholesterol, are enriched in mature RBCs with respct to Retics, relative to total cell lipids, strongly arguing in favor of the selective retention of at least certain subclasses of membrane rafts in RBCs as they mature from Retics. Our hypothesis that rafts serve as additional anchoring sites for the lipid bilayer to the underlying membrane-skeleton is corroborated by the present results. It is becoming ever more clear that a proper lipid composition of the reticulocyte is necessary for the production of a normal mature RBC.

Continuous Change in Membrane and Membrane-Skeleton Organization During Development From Proerythroblast to Senescent Red Blood Cell

Within the context of erythropoiesis and the possibility of producing artificial red blood cells (RBCs) in vitro, a most critical step is the final differentiation of enucleated erythroblasts, or reticulocytes, to a fully mature biconcave discocyte, the RBC. Reviewed here is the current knowledge about this fundamental maturational process. By combining literature data with our own experimental evidence we propose that the early phase in the maturation of reticulocytes to RBCs is driven by a membrane raft-based mechanism for the sorting of disposable membrane proteins, mostly the no longer needed transferrin receptor (TfR), to the multivesicular endosome (MVE) as cargo of intraluminal vesicles that are subsequently exocytosed as exosomes, consistently with the seminal and original observation of Johnstone and collaborators of more than 30 years ago (Pan BT, Johnstone RM. Cell. 1983;33:967-978). According to a strikingly selective sorting process, the TfR becomes cargo destined to exocytosis while other molecules, including the most abundant RBC transmembrane protein, band 3, are completely retained in the cell membrane. It is also proposed that while this process could be operating in the early maturational steps in the bone marrow, additional mechanism(s) must be at play for the final removal of the excess reticulocyte membrane that is observed to occur in the circulation. This processing will most likely require the intervention of the spleen, whose function is also necessary for the continuous remodeling of the RBC membrane all along this cell's circulatory life.

Molecular features of nonionic detergents involved in the binding kinetics and solubilization efficiency, as studied in model (Langmuir films) and biological (Erythrocytes) membranes

The effect of the nonionic detergents Brij-98 and Brij-58 over human erythrocytes was studied through quantitative hemolysis and in Langmuir films. Hemolytic tests revealed that Brijs are stronger membrane solubilizers than Triton X-100 (TX-100), with effective detergent/lipid ratios of 0.18 and 0.37 for Brij-98 and Brij-58, respectively. Experiments with Langmuir films provided significant information on the kinetics and thermodynamics of detergent-membrane interaction. The adsorption (k_a) and desorption (k_d) rate constants of Brijs were lower than those of TX-100. In the case of k_a, that is probably due to their larger hydrophilic head (with twice (20) the oxyethylene units of TX-100). As for the thermodynamic binding constant, the linear and longer hydrophobic acyl chains of Brijs favor their stabilization in-between the lipids, through London van der Waals forces. Consequently, K_b,m values of Brij-98 (12,500 M^-1) and Brij-58 (19,300 M^-1) resulted higher than TX-100 (7500 M^-1), in agreement with results from the hemolytic tests. Furthermore, Brij-58 binds with higher affinity than Brij-98 to bilayers and monolayers, despite its shorter (palmitic) hydrocarbon chain, showing that unsaturation restrains the detergent insertion into these environments. Our results provide significant information about the mechanism of interaction between Brijs and membranes, supporting their distinct solubilization effect.

Encapsulation of ropivacaine in a combined (donor-acceptor, ionic-gradient) liposomal system promotes extended anesthesia time

Ropivacaine is a local anesthetic with similar potency but lower systemic toxicity than bupivacaine, the most commonly used spinal anesthetic. The present study concerns the development of a combined drug delivery system for ropivacaine, comprised of two types of liposomes: donor multivesicular vesicles containing 250 mM (NH₄)₂SO₄ plus the anesthetic, and acceptor large unilamellar vesicles with internal pH of 5.5. Both kinds of liposomes were composed of hydrogenated soy-phosphatidylcholine:cholesterol (2:1 mol%) and were prepared at pH 7.4. Dynamic light scattering, transmission electron microscopy and electron paramagnetic resonance techniques were used to characterize the average particle size, polydispersity, zeta potential, morphology and fluidity of the liposomes. In vitro dialysis experiments showed that the combined liposomal system provided significantly longer (72 h) release of ropivacaine, compared to conventional liposomes (~45 h), or plain ropivacaine (~4 h) (p <0.05). The pre-formulations tested were significantly less toxic to 3T3 cells, with toxicity increasing in the order: combined system < ropivacaine in donor or acceptor liposomes < ropivacaine in conventional liposomes < plain ropivacaine. The combined formulation, containing 2% ropivacaine, increased the anesthesia duration up to 9 h after subcutaneous infiltration in mice. In conclusion, a promising drug delivery system for ropivacaine was described, which can be loaded with large amounts of the anesthetic (2%), with reduced in vitro cytotoxicity and extended anesthesia time.

Biophysical approaches in the study of biomembrane solubilization: quantitative assessment and the role of lateral inhomogeneity

Detergents are amphiphilic molecules widely used to solubilize biological membranes and/or extract their components. Nevertheless, because of the complex composition of biomembranes, their solubilization by detergents has not been systematically studied. In this review, we address the solubilization of erythrocytes, which provide a relatively simple, robust and easy to handle biomembrane, and of biomimetic models, to stress the role of the lipid composition on the solubilization process. First, results of a systematic study on the solubilization of human erythrocyte membranes by different series of non-ionic (Triton, CxEy, Brij, Renex, Tween), anionic (bile salts) and zwitterionic (ASB, CHAPS) detergents are shown. Such quantitative approach allowed us to propose Resat-the effective detergent/lipid molar ratio in the membrane for the onset of hemolysis as a new parameter to classify the solubilization efficiency of detergents. Second, detergent-resistant membranes (DRMs) obtained as a result of the partial solubilization of erythrocytes by TX-100, C12E8 and Brij detergents are examined. DRMs were characterized by their cholesterol, sphingolipid and specific proteins content, as well as lipid packing. Finally, lipid bilayers of tuned lipid composition forming liposomes were used to investigate the solubilization process of membranes of different compositions/phases induced by Triton X-100. Optical microscopy of giant unilamellar vesicles revealed that pure phospholipid membranes are fully solubilized, whereas the presence of cholesterol renders the mixture partially or even fully insoluble, depending on the composition. Additionally, Triton X-100 induced phase separation in raft-like mixtures, and selective solubilization of the fluid phase only.

Brij detergents reveal new aspects of membrane microdomain in erythrocytes

Membrane microdomains enriched in cholesterol, sphingolipids (rafts), and specific proteins are involved in important physiological functions. However their structure, size and stability are still controversial. Given that detergent-resistant membranes (DRMs) are in the liquid-ordered state and are rich in raft-like components, they might correspond to rafts at least to some extent. Here we monitor the lateral order of biological membranes by characterizing DRMs from erythrocytes obtained with Brij-98, Brij-58, and TX-100 at 4 °C and 37 °C. All DRMs were enriched in cholesterol and contained the raft markers flotillin-2 and stomatin. However, sphingomyelin (SM) was only found to be enriched in TX-100-DRMs - a detergent that preferentially solubilizes the membrane inner leaflet - while Band 3 was present solely in Brij-DRMs. Electron paramagnetic resonance spectra showed that the acyl chain packing of Brij-DRMs was lower than TX-100-DRMs, providing evidence of their diverse lipid composition. Fatty acid analysis revealed that the SM fraction of the DRMs was enriched in lignoceric acid, which should specifically contribute to the resistance of SM to detergents. These results indicate that lipids from the outer leaflet, particularly SM, are essential for the formation of the liquid-ordered phase of DRMs. At last, the differential solubilization process induced by Brij-98 and TX-100 was monitored using giant unilamellar vesicles. This study suggests that Brij and TX-100-DRMs reflect different degrees of lateral order of the membrane microdomains. Additionally, Brij DRMs are composed by both inner and outer leaflet components, making them more physiologically relevant than TX-100-DRMs to the studies of membrane rafts.

Direct visualization of the action of Triton X-100 on giant vesicles of erythrocyte membrane lipids

The raft hypothesis proposes that microdomains enriched in sphingolipids, cholesterol, and specific proteins are transiently formed to accomplish important cellular tasks. Equivocally, detergent-resistant membranes were initially assumed to be identical to membrane rafts, because of similarities between their compositions. In fact, the impact of detergents in membrane organization is still controversial. Here, we use phase contrast and fluorescence microscopy to observe giant unilamellar vesicles (GUVs) made of erythrocyte membrane lipids (erythro-GUVs) when exposed to the detergent Triton X-100 (TX-100). We clearly show that TX-100 has a restructuring action on biomembranes. Contact with TX-100 readily induces domain formation on the previously homogeneous membrane of erythro-GUVs at physiological and room temperatures. The shape and dynamics of the formed domains point to liquid-ordered/liquid-disordered (Lo/Ld) phase separation, typically found in raft-like ternary lipid mixtures. The Ld domains are then separated from the original vesicle and completely solubilized by TX-100. The insoluble vesicle left, in the Lo phase, represents around 2/3 of the original vesicle surface at room temperature and decreases to almost 1/2 at physiological temperature. This chain of events could be entirely reproduced with biomimetic GUVs of a simple ternary lipid mixture, 2:1:2 POPC/SM/chol (phosphatidylcholine/sphyngomyelin/cholesterol), showing that this behavior will arise because of fundamental physicochemical properties of simple lipid mixtures. This work provides direct visualization of TX-100-induced domain formation followed by selective (Ld phase) solubilization in a model system with a complex biological lipid composition.

Membrane rafts of the human red blood cell

The cell type of election for the study of cell membranes, the mammalian non-nucleated erythrocyte, has been scarcely considered in the research of membrane rafts of the plasma membrane. However, detergent-resistant-membranes (DRM) were actually first described in human erythrocytes, as a fraction resisting solubilization by the nonionic detergent Triton X-100. These DRMs were insoluble entities of high density, easily pelleted by centrifugation, as opposed to the now accepted concept of lipid raft-like membrane fractions as material floating in low-density regions of sucrose gradients. The present article reviews the available literature on membrane rafts/DRMs in human erythrocytes from an historical point of view, describing the experiments that provided the solution to the above described discrepancy and suggesting possible avenue of research in the field of membrane rafts that, moving from the most studied model of living cell membrane, the erythrocyte's, could be relevant also for other cell types.

Proteome analysis of Cry4Ba toxin-interacting Aedes aegypti lipid rafts using geLC-MS/MS

Lipid rafts are microdomains in the plasma membrane of eukaryotic cells. Among their many functions, lipid rafts are involved in cell toxicity caused by pore forming bacterial toxins including Bacillus thuringiensis (Bt) Cry toxins. We isolated lipid rafts from brush border membrane vesicles (BBMV) of Aedes aegypti larvae as a detergent resistant membrane (DRM) fraction on density gradients. Cholesterol, aminopeptidase (APN), alkaline phosphatase (ALP) and the raft marker flotillin were preferentially partitioned into the lipid raft fraction. When mosquitocidal Cry4Ba toxin was preincubated with BBMV, Cry4Ba localized to lipid rafts. A proteomic approach based on one-dimensional gel electrophoresis, in-gel trypsin digestion, followed by liquid chromatography-mass spectrometry (geLC-MS/MS) identified a total of 386 proteins. Of which many are typical lipid raft marker proteins including flotillins and glycosylphosphatidylinositol (GPI)-anchored proteins. Identified raft proteins were annotated in silico for functional and physicochemical characteristics. Parameters such as distribution of isoelectric point, molecular mass, and predicted post-translational modifications relevant to lipid raft proteins (GPI anchorage and myristoylation or palmitoylation) were analyzed for identified proteins in the DRM fraction. From a functional point of view, this study identified proteins implicated in Cry toxin interactions as well as membrane-associated proteins expressed in the mosquito midgut that have potential relevance to mosquito biology and vector management.

Subcellular fractionation of stored red blood cells reveals a compartment-based protein carbonylation evolution

During blood banking, erythrocytes undergo storage lesions, altering or degrading their metabolism, rheological properties, and protein content. Carbonylation is a hallmark of protein oxidative lesions, thus of red blood cell oxidative stress. In order to improve global erythrocyte protein carbonylation assessment, subcellular fractionation has been established, allowing us to work on four different protein populations, namely soluble hemoglobin, hemoglobin-depleted soluble fraction, integral membrane and cytoskeleton membrane protein fractions. Carbonylation in erythrocyte-derived microparticles has also been investigated. Carbonylated proteins were derivatized with 2,4-dinitrophenylhydrazine (2,4-DNPH) and quantified by western blot analyses. In particular, carbonylation in the cytoskeletal membrane fraction increased remarkably between day 29 and day 43 (P<0.01). Moreover, protein carbonylation within microparticles released during storage showed a two-fold increase along the storage period (P<0.01). As a result, carbonylation of cytoplasmic and membrane protein fractions differs along storage, and the present study allows explaining two distinct steps in global erythrocyte protein carbonylation evolution during blood banking. This article is part of a Special Issue entitled: Integrated omics.

Direct evidence of lipid rafts by in situ atomic force microscopy

Lipid rafts are membrane microdomains enriched with cholesterol, glycosphingolipids, and proteins. Although they are broadly presumed to play a pivotal role in various cellular functions, there are still fierce debates about the composition, functions, and even existence of lipid rafts. Here high-resolution and time-lapse in situ atomic force microscopy is used to directly confirm the existence of lipid rafts in native erythrocyte membranes. The results indicate some important aspects of lipid rafts: most of the lipid rafts are in the size range of 100-300 nm and have irregular shape; the detergent-resistant membranes consist of cholesterol microdomains and are not likely the same as the lipid rafts; cholesterol contributes significantly to the formation and stability of the protein domains; and Band III is an important protein of lipid rafts in the inner leaflet of erythrocyte membranes, indicating that lipid rafts are exactly the functional domains in plasma membrane. This work provides direct evidence of the presence, size, and main constitutive protein of lipid rafts at a resolution of a few nanometers, which will pave the way for studying their structure and functions in detail.

Curvature factor and membrane solubilization, with particular reference to membrane rafts

The composition of membrane rafts (cholesterol/sphingolipid-rich domains) cannot be fully deduced from the analysis of a detergent-resistant membrane fraction after solubilization in Triton X-100 at 4°C. It is hypothesized that the membrane curvature-dependent lateral distribution of membrane components affects their solubilization. The stomatocytogenic, Triton X-100, cannot effectively solubilize membrane components, especially with regard to the outward membrane curvature.

Multiple stages of detergent-erythrocyte membrane interaction--a spin label study

The various stages of the interaction between the detergent Triton X-100 (TTX-100) and membranes of whole red blood cells (RBC) were investigated in a broad range of detergent concentrations. The interaction was monitored by RBC hemolysis-assessed by release of intracellular hemoglobin (Hb) and inorganic phosphate-and by analysis of EPR spectra of a fatty acid spin probe intercalated in whole RBC suspensions, as well as pellets and supernatants obtained upon centrifugation of detergent-treated cells. Hemolysis finished at ca. 0.9mM TTX-100. Spectral analysis and calculation of order parameters (S) indicated that a complex sequence of events takes place, and allowed the characterization of various structures formed in the different stages of detergent-membrane interaction. Upon reaching the end of cell lysis, essentially no pellet was detected, the remaining EPR signal being found almost entirely in the supernatants. Calculated order parameters revealed that whole RBC suspensions, pellets, and supernatants possessed a similar degree of molecular packing, which decreased to a small extent up to 2.5mM detergent. Between 3.2 and 10mM TTX-100, a steep decrease in S was observed for both whole RBC suspensions and supernatants. Above 10mM detergent, S decreased in a less pronounced manner and the EPR spectra approached that of pure TTX-100 micelles. The data were interpreted in terms of the following events: at the lower detergent concentrations, an increase in membrane permeability occurs; the end of hemolysis coincides with the lack of pellet upon centrifugation. Up to 2.5mM TTX-100 the supernatants consist of a (very likely) heterogeneous population of membrane fragments with molecular packing similar to that of whole cells. As the detergent concentration increases, mixed micelles are formed containing lipid and/or protein, approaching the packing found in pure TTX-100 micelles. This analysis is in agreement with the models proposed by Lasch (Biochim. Biophys Acta 1241 (1995) 269-292) and by Le Maire and coworkers (Biochim. Biophys. Acta 1508 (2000) 86-111).

On the association of lipid rafts to the spectrin skeleton in human erythrocytes

Lipid rafts are local inhomogeneities in the composition of the plasma membrane of living cells, that are enriched in sphingolipids and cholesterol in a liquid-ordered state, and proteins involved in receptor-mediated signalling. Interactions between lipid rafts and the cytoskeleton have been observed in various cell types. They are isolated as a fraction of the plasma membrane that resists solubilization by nonionic detergents at 4°C (detergent-resistant membranes, DRMs). We have previously described that DRMs are anchored to the spectrin-based membrane skeleton in human erythrocytes and can be released by increasing the pH and ionic strength of the solubilization medium with sodium carbonate. It was unexplained why this carbonate treatment was necessary and why this requirement was not reported by other workers in this area. We show here that when contaminating leukocytes are present in erythrocyte preparations that are subjected to detergent treatment, the isolation of DRMs can occur without the requirement for carbonate treatment. This is due to the uncontrolled breakdown of erythrocyte membrane components by hydrolases that are released from contaminating neutrophils that lead to proteolytic disruption of the supramolecular assembly of the membrane skeleton. Results presented here corroborate the concept that DRMs are anchored to the membrane skeleton through electrostatic interactions that most likely involve the spectrin molecule.

Formation of two different types of ion channels by amphotericin B in human erythrocyte membranes

The polyene antibiotic amphotericin B (AmB) is known to form aqueous pores in lipid membranes and biological membranes. Here, membrane potential and ion permeability measurements were used to demonstrate that AmB can form two types of selective ion channels in human erythrocytes, differing in their interaction with cholesterol. We show that AmB induced a cation efflux (negative membrane polarization) across cholesterol-containing liposomes and erythrocytes at low concentrations (< or =1.0 x 10(-6) M), but a sharp reversal of such polarization was observed at concentrations greater than 1.0 x 10(-6) M AmB, an indication that aqueous pores are formed. Cation-selective AmB channels are also formed across sterol-free liposomes, but aqueous pores are only formed at AmB concentrations 10 times greater. The effect of temperature on the AmB-mediated K+ efflux across erythrocytes revealed that the energies of activation for channel formation are negative and positive at AmB concentrations that lead predominantly to the formation of cation-selective channels and aqueous pores, respectively. These findings support the conclusion that the two types of AmB channels formed in human erythrocytes differ in their interactions with cholesterol and other membrane components. In effect, a membrane lipid reorganization, as induced by incubation of erythrocytes with tetrathionate, a cross-linking agent of the lipid raft-associated protein spectrin, led to differential changes in the activation parameters for the formation of both types of channels, reflecting the different lipid environments in which such structures are formed.