Mechanism of spontaneous inside-out vesiculation of red cell membranes

Opsonization-independent antigen-specific recognition by myeloid phagocytes expressing monoclonal antibodies

This report demonstrates a novel class of innate immune cells designated "variable immunoreceptor-expressing myeloids" (VIREMs). Using single-cell transcriptomics and genome-wide epigenetic profiling, we establish that VIREMs are myeloid cells unrelated to lymphocytes. We visualize the phenotype of B-VIREMs that are capable of genetically recombining and expressing antibody genes, the exclusive hallmark function of B lymphocytes. These cells, designated B-VIREMs, display monoclonal antibody cell surface signatures and regularly circulate in the blood of healthy individuals. Single-cell data reveal clonal expansion of circulating B-VIREMs as a dynamic response to disease stimuli. Live-cell imaging models suggest that B-VIREMs load their own Fc receptors with endogenous antibodies during vesicle transport to the cell surface. A first cloned B-VIREM-derived antibody (Vab1) specifically binds stomatin, a ubiquitous scaffold protein that is strictly expressed intracellularly, allowing Vab1-bearing macrophages to phagocytose cell debris without requiring prior opsonization. Our results suggest important antigen-specific tissue maintenance functionalities in these innate immune cells.

Biophysical Tools and Concepts Enable Understanding of Asexual Blood Stage Malaria

Forces and mechanical properties of cells and tissues set constraints on biological functions, and are key determinants of human physiology. Changes in cell mechanics may arise from disease, or directly contribute to pathogenesis. Malaria gives many striking examples. Plasmodium parasites, the causative agents of malaria, are single-celled organisms that cannot survive outside their hosts; thus, thost-pathogen interactions are fundamental for parasite’s biological success and to the host response to infection. These interactions are often combinations of biochemical and mechanical factors, but most research focuses on the molecular side. However, Plasmodium infection of human red blood cells leads to changes in their mechanical properties, which has a crucial impact on disease pathogenesis because of the interaction of infected red blood cells with other human tissues through various adhesion mechanisms, which can be probed and modelled with biophysical techniques. Recently, natural polymorphisms affecting red blood cell biomechanics have also been shown to protect human populations, highlighting the potential of understanding biomechanical factors to inform future vaccines and drug development. Here we review biophysical techniques that have revealed new aspects of Plasmodium falciparum invasion of red blood cells and cytoadhesion of infected cells to the host vasculature. These mechanisms occur differently across Plasmodium species and are linked to malaria pathogenesis. We highlight promising techniques from the fields of bioengineering, immunomechanics, and soft matter physics that could be beneficial for studying malaria. Some approaches might also be applied to other phases of the malaria lifecycle and to apicomplexan infections with complex host-pathogen interactions.

Osmotic Vesicle Collapse of Sealed Inside-Out Membrane Vesicles From Red Blood Cells

The preparation of plasma membrane vesicles from a large variety of cells has contributed a wealth of information on the identity and vectorial properties of membrane transporters and enzymes. Vesicles from red blood cell (RBC) membranes are generated in media of extremely low tonicity. For functional studies, it is required to suspend the vesicles in higher tonicity media in order to bring the concentrations of the substrates of transporters and enzymes under investigation within the physiological ranges. We investigated the effects of hypertonic transitions on the vesicle morphology using transmission electron microscopy. The results show that hypertonic transitions cause an irreversible osmotic collapse of sealed membrane vesicles. Awareness of the collapsed condition of vesicles during functional studies is critical for the proper interpretation of experimental results.

To Close or to Collapse: The Role of Charges on Membrane Stability upon Pore Formation

Resealing of membrane pores is crucial for cell survival. Membrane surface charge and medium composition are studied as defining regulators of membrane stability. Pores are generated by electric field or detergents. Giant vesicles composed of zwitterionic and negatively charged lipids mixed at varying ratios are subjected to a strong electric pulse. Interestingly, charged vesicles appear prone to catastrophic collapse transforming them into tubular structures. The spectrum of destabilization responses includes the generation of long-living submicroscopic pores and partial vesicle bursting. The origin of these phenomena is related to the membrane edge tension, which governs pore closure. This edge tension significantly decreases as a function of the fraction of charged lipids. Destabilization of charged vesicles upon pore formation is universal-it is also observed with other poration stimuli. Disruption propensity is enhanced for membranes made of lipids with higher degree of unsaturation. It can be reversed by screening membrane charge in the presence of calcium ions. The observed findings in light of theories of stability and curvature generation are interpreted and mechanisms acting in cells to prevent total membrane collapse upon poration are discussed. Enhanced membrane stability is crucial for the success of electroporation-based technologies for cancer treatment and gene transfer.

Bursting and Reassembly of Giant Double Emulsion Drops Form Polymer Vesicles

Polymeric vesicles are excellent building blocks of synthetic compartmentalized systems such as protocells and artificial organelles. In such applications, the efficient encapsulation of materials into the vesicles is an essential requirement. However, common encapsulation techniques can be time-consuming, demand special equipment or have limited efficiency for large components, such as proteins and nanoparticles. Here, we describe a simple method to create cargo-filled polymer vesicles based on bursting and reassembly of giant double emulsion droplets (DED). Due to their large average diameter of 2 mm, DEDs eventually burst in the aqueous medium, producing polymeric film fragments. These fragments rapidly reassemble into smaller vesicles in a process involving folding, fusion and vesiculation. The daughter vesicles have an average diameter of 10 μm, representing a two-order of magnitude size reduction compared to the original DED, and can efficiently encapsulate components present in solution by entrapment of the aqueous medium during vesicle reassembly.

SARS-CoV-2 genomic and subgenomic RNAs in diagnostic samples are not an indicator of active replication

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) was first detected in late December 2019 and has spread worldwide. Coronaviruses are enveloped, positive sense, single-stranded RNA viruses and employ a complicated pattern of virus genome length RNA replication as well as transcription of genome length and leader containing subgenomic RNAs. Although not fully understood, both replication and transcription are thought to take place in so-called double-membrane vesicles in the cytoplasm of infected cells. Here we show detection of SARS-CoV-2 subgenomic RNAs in diagnostic samples up to 17 days after initial detection of infection and provide evidence for their nuclease resistance and protection by cellular membranes suggesting that detection of subgenomic RNAs in such samples may not be a suitable indicator of active coronavirus replication/infection.

SARS-CoV-2 genomic and subgenomic RNAs in diagnostic samples are not an indicator of active replication

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) was first detected in late December 2019 and has spread worldwide. Coronaviruses are enveloped, positive sense, single-stranded RNA viruses and employ a complicated pattern of virus genome length RNA replication as well as transcription of genome length and leader containing subgenomic RNAs. Although not fully understood, both replication and transcription are thought to take place in so-called double-membrane vesicles in the cytoplasm of infected cells. Here we show detection of SARS-CoV-2 subgenomic RNAs in diagnostic samples up to 17 days after initial detection of infection and provide evidence for their nuclease resistance and protection by cellular membranes suggesting that detection of subgenomic RNAs in such samples may not be a suitable indicator of active coronavirus replication/infection.

Hybrid Erythrocyte Liposomes: Functionalized Red Blood Cell Membranes for Molecule Encapsulation

The modification of erythrocyte membrane properties provides a new tool towards improved drug delivery and biomedical applications. The fabrication of hybrid erythrocyte liposomes is presented by doping red blood cell membranes with synthetic lipid molecules of different classes (PC, PS, PG) and different degrees of saturation (14:0, 16:0-18:1). The respective solubility limits are determined, and material properties of the hybrid liposomes are studied by a combination of X-ray diffraction, epi-fluorescent microscopy, dynamic light scattering (DLS), Zeta potential, UV-vis spectroscopy, and Molecular Dynamics (MD) simulations. Membrane thickness and lipid orientation can be tuned through the addition of phosphatidylcholine lipids. The hybrid membranes can be fluorescently labelled by incorporating Texas-red DHPE, and their charge modified by incorporating phosphatidylserine and phosphatidylglycerol. By using fluorescein labeled dextran as an example, it is demonstrated that small molecules can be encapsulated into these hybrid liposomes.

Inverted erythrocyte membranes demonstrate β2GPI-antiphospholipid antibody interactions and membrane crosslinking

INTRODUCTION

The antiphospholipid syndrome (APS) is an acquired autoimmune disorder predisposing patients to thrombosis or pregnancy complications. Since inverted erythrocyte membranes (iEMs) might provide a physiologically relevant source of anionic phospholipids, we studied the interactions of phospholipid-binding proteins and APS antibodies using iEMs.

MATERIALS & METHODS

iEMs were prepared from packed erythrocytes by hypotonic lysis. Phosphatidylserine (PS) exposure was confirmed by annexin A5 (A5) binding using fluorescence microscopy and flow cytometry. Binding of β₂-glycoprotein I (β₂GPI)-IgG immune complexes to iEMs was investigated with gel electrophoresis, western blot and flow cytometry. Functional involvement in coagulation was documented in the thrombin generation assay.

RESULTS

iEMs readily precipitated purified β₂GPI as well as β₂GPI from normal plasma and APS plasma. The plasma of APS patients provided higher levels of IgG binding to iEMs relative to healthy controls. Thrombin generation increased with increasing concentrations of iEMs, documenting that coagulation proteins bound to the exposed phospholipids. The LA effect was also distinguished in thrombin generation when comparing APS patients, as indicated by an increased lag time. Agglutination was observed after incubation with APS patient plasma and this was augmented by anti-human globulin.

CONCLUSIONS

In conclusion, iEMs can provide a more physiological approach than phospholipid vesicle-based tests for investigating APS and are more amenable to standardization than platelet membranes.

Synthesis of Nanogels via Cell Membrane-Templated Polymerization

The synthesis of biomimetic hydrogel nanoparticles coated with a natural cell membrane is described. Compared to the existing strategy of wrapping cell membranes onto pre-formed nanoparticle substrates, this new approach forms the cell membrane-derived vesicles first, followed by growing nanoparticle cores in situ. It adds significant controllability over the nanoparticle properties and opens unique opportunities for a broad range of biomedical applications.

Dynamic morphology and cytoskeletal protein changes during spontaneous inside-out vesiculation of red blood cell membranes

Vesicle preparations from cell plasma membranes, red blood cells in particular, are extensively used in transport and enzymic studies and in the fields of drug delivery and drug-transport interactions. Here we investigated the role of spectrin–actin, the main components of the red cell cortical cytoskeleton, in a particular mechanism of vesicle generation found to be relevant to the egress process of Plasmodium falciparum merozoites from infected red blood cells. Plasma membranes from red blood cells lysed in ice-cold media of low ionic strength and free of divalent cations spontaneously and rapidly vesiculate upon incubation at 37 °C rendering high yields of inside-out vesicles. We tested the working hypothesis that the dynamic shape transformations resulted from changes in spectrin–actin configuration within a disintegrating cytoskeletal mesh. We showed that cytoskeletal-free membranes behave like a two-dimensional fluid lacking shape control, that spectrin–actin remain attached to vesiculating membranes for as long as spontaneous movement persists, that most of the spectrin–actin detachment occurs terminally at the time of vesicle sealing and that naked membrane patches increasingly appear during vesiculation. These results support the proposed role of spectrin–actin in spontaneous vesiculation. The implications of these results to membrane dynamics and to the mechanism of merozoite egress are discussed.

Exploration of interactions between membrane proteins embedded in supported lipid bilayers and their antibodies by reflectometric interference spectroscopy-based sensing

A microfluidic reflectometric interference spectroscopy (RIfS)-based sensor was fabricated to investigate the activity of multidrug resistance-associated protein 1 (MRP1), applied as a model membrane protein.

ABC transporters in multidrug resistance and pharmacokinetics, and strategies for drug development

Multidrug resistance (MDR) is a serious problem that hampers the success of cancer pharmacotherapy. A common mechanism is the overexpression of ATP-binding cassette (ABC) efflux transporters in cancer cells such as P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1) and breast cancer resistance protein (BCRP/ABCG2) that limit the exposure to anticancer drugs. One way to overcome MDR is to develop ABC efflux transporter inhibitors to sensitize cancer cells to chemotherapeutic drugs. The complete clinical trials thus far have showen that those tested chemosensitizers only add limited or no benefits to cancer patients. Some MDR modulators are merely toxic, and others induce unwanted drug-drug interactions. Actually, many ABC transporters are also expressed abundantly in the gastrointestinal tract, liver, kidney, brain and other normal tissues, and they largely determine drug absorption, distribution and excretion, and affect the overall pharmacokinetic properties of drugs in humans. In addition, ABC transporters such as P-gp, MRP1 and BCRP co-expressed in tumors show a broad and overlapped specificity for substrates and MDR modulators. Thus reliable preclinical assays and models are required for the assessment of transporter-mediated flux and potential effects on pharmacokinetics in drug development. In this review, we provide an overview of the role of ABC efflux transporters in MDR and pharmacokinetics. Preclinical assays for the assessment of drug transport and development of MDR modulators are also discussed.

Red blood cell membrane dynamics during malaria parasite egress

Precisely how malaria parasites exit from infected red blood cells to further spread the disease remains poorly understood. It has been shown recently, however, that these parasites exploit the elasticity of the cell membrane to enable their egress. Based on this work, showing that parasites modify the membrane's spontaneous curvature, initiating pore opening and outward membrane curling, we develop a model of the dynamics of the red blood cell membrane leading to complete parasite egress. As a result of the three-dimensional, axisymmetric nature of the problem, we find that the membrane dynamics involve two modes of elastic-energy release: 1), at short times after pore opening, the free edge of the membrane curls into a toroidal rim attached to a membrane cap of roughly fixed radius; and 2), at longer times, the rim radius is fixed, and lipids in the cap flow into the rim. We compare our model with the experimental data of Abkarian and co-workers and obtain an estimate of the induced spontaneous curvature and the membrane viscosity, which control the timescale of parasite release. Finally, eversion of the membrane cap, which liberates the remaining parasites, is driven by the spontaneous curvature and is found to be associated with a breaking of the axisymmetry of the membrane.

Self-similar curling of a naturally curved elastica

We consider the curling of an initially flat but naturally curved elastica on a hard, nonadhesive surface. Combining theory, simulations, and experiments, we find novel behavior, including a constant front velocity and a self-similar shape of the curl that scales in size as t(1/3) at long times after the release of one end of the elastica. The front velocity is selected by matching the self-similar solution with a roll of nearly constant curvature located near the free end.

Cell phones and male infertility: a review of recent innovations in technology and consequences

Cell phones have become a vital part of everyday life. However, the health risks associated with their usage are often overlooked. Recently, evidence from several studies supports a growing claim that cell phone usage may have a detrimental effect on sperm parameters leading to decreased male fertility. Nonetheless, other studies showed no conclusive link between male infertility and cell phone usage. The ambiguity of such results is attributed to the lack of a centralized assay for measuring inflicted damage caused by cell phones. Study design, ethics, and reproducibility are all aspects which must be standardized before any conclusions can be made.

Malaria: surprising mechanism of merozoite egress revealed

A recent study reveals that the intraerythrocytic asexual reproduction cycle of Plasmodium falciparum ends with the ruptured erythrocyte membrane curling outwards, buckling, everting and vesiculating. Analogy with the sequence seen during spontaneous inside-out vesiculation of erythrocyte membranes suggests that the parasite co-opts pre-existing cytoskeletal conformations to facilitate terminal merozoite dispersal.

A novel mechanism for egress of malarial parasites from red blood cells

The culminating step of the intraerythrocytic development of Plasmodium falciparum, the causative agent of malaria, is the spectacular release of multiple invasive merozoites on rupture of the infected erythrocyte membrane. This work reports for the first time that the whole process, taking place in time scales as short as 400 milliseconds, is the result of an elastic instability of the infected erythrocyte membrane. Using high-speed differential interference contrast (DIC) video microscopy and epifluorescence, we demonstrate that the release occurs in 3 main steps after osmotic swelling of the infected erythrocyte: a pore opens in ~ 100 milliseconds, ejecting 1-2 merozoites, an outward curling of the erythrocyte membrane is then observed, ending with a fast eversion of the infected erythrocyte membrane, pushing the parasites forward. It is noteworthy that this last step shows slight differences when infected erythrocytes are adhering. We rationalize our observations by considering that during the parasite development, the infected erythrocyte membrane acquires a spontaneous curvature and we present a subsequent model describing the dynamics of the curling rim. Our results show that sequential erythrocyte membrane curling and eversion is necessary for the parasite efficient angular dispersion and might be biologically essential for fast and numerous invasions of new erythrocytes.

Quantitative imaging of human red blood cells infected with Plasmodium falciparum

During its 48 h asexual reproduction cycle, the malaria parasite Plasmodium falciparum ingests and digests hemoglobin in excess of its metabolic requirements and causes major changes in the homeostasis of the host red blood cell (RBC). A numerical model suggested that this puzzling excess consumption of hemoglobin is necessary for the parasite to reduce the colloidosmotic pressure within the host RBC, thus preventing lysis before completion of its reproduction cycle. However, the validity of the colloidosmotic hypothesis appeared to be compromised by initial conflicts between model volume predictions and experimental observations. Here, we investigated volume and membrane area changes in infected RBCs (IRBCs) using fluorescence confocal microscopy on calcein-loaded RBCs. Substantial effort was devoted to developing and testing a new threshold-independent algorithm for the precise estimation of cell volumes and surface areas to overcome the shortfalls of traditional methods. We confirm that the volume of IRBCs remains almost constant during parasite maturation, suggesting that the reported increase in IRBCs' osmotic fragility results from a reduction in surface area and increased lytic propensity on volume expansion. These results support the general validity of the colloidosmotic hypothesis, settle the IRBC volume debate, and help to constrain the range of parameter values in the numerical model.

Curling and local shape changes of red blood cell membranes driven by cytoskeletal reorganization

Human red blood cells (RBCs) lack the actin-myosin-microtubule cytoskeleton that is responsible for shape changes in other cells. Nevertheless, they can display highly dynamic local deformations in response to external perturbations, such as those that occur during the process of apical alignment preceding merozoite invasion in malaria. Moreover, after lysis in divalent cation-free media, the isolated membranes of ruptured ghosts show spontaneous inside-out curling motions at the free edges of the lytic hole, leading to inside-out vesiculation. The molecular mechanisms that drive these rapid shape changes are unknown. Here, we propose a molecular model in which the spectrin filaments of the RBC cortical cytoskeleton control the sign and dynamics of membrane curvature depending on whether the ends of the filaments are free or anchored to the bilayer. Computer simulations of the model reveal that curling, as experimentally observed, can be obtained either by an overall excess of weakly-bound filaments throughout the cell, or by the flux of such filaments toward the curling edges. Divalent cations have been shown to arrest the curling process, and Ca2+ ions have also been implicated in local membrane deformations during merozoite invasion. These effects can be replicated in our model by attributing the divalent cation effects to increased filament-membrane binding. This process converts the curl-inducing loose filaments into fully bound filaments that arrest curling. The same basic mechanism can be shown to account for Ca2+-induced local and dynamic membrane deformations in intact RBCs. The implications of these results in terms of RBC membrane dynamics under physiological, pathological, and experimental conditions is discussed.

Ultrastructural analysis of lysosome reactions to inside-out cell membrane vesicles in a cell-free system

Lysosome reactions were ultrastructurally analyzed using a cell-free system with inside-out cell membrane vesicles (IOVs) prepared from rat erythrocyte ghosts in an alkaline buffer and with wheat germ agglutinin-coated colloidal gold particles (WGA-CGs). The submembranous surface coat in the ghosts was depleted from the IOVs' outer surfaces. When lysosomes from rat liver were incubated with these IOVs, some of the trilaminar membranes of the lysosomes and IOVs came into close contact and formed a five-laminar structure without an intermembranous gap. In other reactions, the membranes of both structures formed one continuous trilaminar membrane along the margin of contact and ruffling five-laminar structures in other regions. Several lysosomes exhibited invaginating hollows or projections that entrapped or encircled the IOVs. Similar five-laminar structures were seen at a few points of contact between the IOVs and the hollowing or projecting membranes. In contrast, such reactions were much rarer when IOVs with reconstituted spectrins and actins on their outer surface were used. The formation of tubuliform pits with membrane-bound WGA-CGs was also observed after the incubation of lysosomes with WGA-CGs. These observations suggest that lysosomes fuse with cytoskeleton-depleted IOVs, wrap arm-like projections around them, enclose them by invagination or incorporate their membrane-bound macromolecules through the process of tubuliform invagination. Furthermore, the fusion and wrapping processes are not necessarily independent.

Bursting of sensitive polymersomes induced by curling

Polymersomes, which are stable and robust vesicles made of block copolymer amphiphiles, are good candidates for drug carriers or micro/nanoreactors. Polymer chemistry enables almost unlimited molecular design of responsive polymersomes whose degradation upon environmental changes has been used for the slow release of active species. Here, we propose a strategy to remotely trigger instantaneous polymersome bursting. We have designed asymmetric polymer vesicles, in which only one leaflet is composed of responsive polymers. In particular, this approach has been successfully achieved by using a UV-sensitive liquid-crystalline copolymer. We study experimentally and theoretically this bursting mechanism and show that it results from a spontaneous curvature of the membrane induced by the remote stimulus. The versatility of this mechanism should broaden the range of applications of polymersomes in fields such as drug delivery, cosmetics and material chemistry.

Unusual "erythroid loops" in canine blood smears after viper-bite envenomation

Northern Italy is a habitat for many species of viper; Vipera aspis is responsible for most reported bites of humans and animals. Five dogs of different breeds and ages were presented by their owners to a veterinary clinic in northern Italy between September 2004 and August 2007 with a history of being bitten by a viper within the past 2 hours. On physical examination, all of the dogs were depressed, had pale mucous membranes, and had a painful area consistent with a recent viper bite wound on the nose (2), distal front leg (2), or lip (1). Hemoglobinemia and hemoglobinuria were observed in plasma and urine from all dogs. CBCs were done at the time of presentation, daily for 4 days, and 10 days after presentation in all dogs with the exception of one dog that died after 2 days. All dogs had an acute decrease in HCT within 24 hours of presentation; all dogs had neutrophilia, 3/5 had a mildly toxic left shift, and 4/5 had thrombocytopenia. On Diff-Quik-stained blood smears, moderate numbers of echinocytes, spherocytes, and erythrocyte ghosts were observed. In addition, moderate numbers of unusual erythrocyte membrane-like structures ("erythroid loops") were observed. The loops were annular in shape and sometimes disrupted, appearing as thin pale blue bands. Erythrocyte morphologic abnormalities decreased by day 3 and were no longer observed on day 10. The unique appearance of the erythroid loops together with evidence for intravascular hemolysis and other erythrocyte morphologic changes suggest they may be a consequence of erythrocyte lysis. Echinocytes, spherocytes, and erythrocyte ghosts are known to result from the action of phospholipase in viper venom; however, erythroid loops have not been reported previously and their exact mechanism of formation is unknown.

Malaria: endless fascination with merozoite release

The erythrocytic asexual reproduction cycle of Plasmodium falciparum, the most lethal malaria parasite in humans, starts with the invasion of a red blood cell by a merozoite and ends with the release of up to 32 new copies of itself in about 48 hours. A new study reveals that merozoite release is an explosive event ensuring the dispersal of these non-motile parasites for optimal re-invasion of new red cells.

Interactions of mefloquine with ABC proteins, MRP1 (ABCC1) and MRP4 (ABCC4) that are present in human red cell membranes

Human erythrocyte membranes express the multidrug resistance-associated proteins, MRP1, MRP4 and 5, that collectively can efflux oxidised glutathione, glutathione conjugates and cyclic nucleotides. It is already known that the quinoline derivative, MK-571, is a potent inhibitor of MRP-mediated transport. We here examine whether the quinoline-based antimalarial drugs, amodiaquine, chloroquine, mefloquine, primaquine, quinidine and quinine, also interact with erythrocyte MRPs with consequences for their access to the intracellular parasites or for efflux of oxidised glutathione from infected cells. Using inside-out vesicles prepared from human erythrocytes we have shown that mefloquine and MK-571 inhibit transport of 3 microM [(3)H]DNP-SG known to be mediated by MRP1 (IC(50) 127 and 1.1 microM, respectively) and of 3.3 microM [(3)H]cGMP thought but not proven to be mediated primarily by MRP4 (IC(50) 21 and 0.41 microM). They also inhibited transport in membrane vesicles prepared from tumour cells expressing MRP1 or MRP4 and blocked calcein efflux from MRP1-overexpressing cells and BCECF efflux from MRP4-overexpressing cells. Both stimulated ATPase activity in membranes prepared from MRP1 and MRP4-overexpressing cells and inhibited activity stimulated by quercetin or PGE(1), respectively. Neither inhibited [alpha-(32)P]8-azidoATP binding confirming that the interactions are not at the ATP binding site. These results demonstrate that mefloquine and MK-571 both inhibit transport of other substrates and stimulate ATPase activity and thus may themselves be substrates for transport. But at concentrations achieved clinically mefloquine is unlikely to affect the MRP1-mediated transport of GSSG across the erythrocyte membrane.

cGMP (guanosine 3′,5′-cyclic monophosphate) transport across human erythrocyte membranes

Human erythrocytes produce cGMP that can be eliminated by phosphodiesterases or active efflux transporters. The efflux can be studied under controlled conditions as ATP-dependent uptake into inside-out membrane vesicles. However, widely differing values for the transport rates have been reported. We have here examined factors that influence the uptake rates measured and thus may explain these discrepancies. Both the ionic composition of the buffer used during uptake and the mode of vesicle preparation were found to affect the observed transport rates. Furthermore it was apparent that different blood donors expressed on their erythrocytes different amounts of both MRP4 and MRP5, transporters that have been putatively linked to cGMP efflux across erythrocyte membranes. These differences in expression were reflected in differences in rates of cGMP uptake into inside-out erythrocyte membrane vesicles. Calculations based on the transport rates observed using vesicles suggest that efflux may be the principal means for eliminating cGMP from human erythrocytes.

The effects of erythrocyte membranes on the nucleation of sickle hemoglobin

Pathology in sickle cell disease begins with nucleation-dependent polymerization of deoxyhemoglobin S into stiff, rodlike fibers that deform and rigidify red cells. We have measured the effect of erythrocyte membranes on the rate of homogeneous nucleation in sickle hemoglobin, using preparations of open ghosts (OGs) with intact cytoskeletons from sickle (SS) and normal adult (AA) red cells. Nucleation rates were measured by inducing polymerization by laser photolysis of carboxy sickle hemoglobin and observing stochastic variation of replicate experiments of the time for the scattering signals to reach 10% of their respective maxima. By optical imaging of membrane fragments added to a hemoglobin solution we contrast the rate of nucleation immediately adjacent to membrane fragments with nucleation in a region of the same solution but devoid of membranes. From analysis of 29,272 kinetic curves obtained, we conclude that the effect of AA OGs is negligible (10% enhancement of nucleation rates +/-20%), whereas SS OGs caused 80% enhancement (+/-20%). In red cells, where more membrane surface is available to Hb, this implies enhancement of nucleation by a factor of 6. These experiments represent a 10-fold improvement in precision over previous approaches and are the first direct, quantitative measure of the impact of erythrocyte membranes on the homogeneous nucleation process that is responsible for polymer initiation in sickle cell disease.

cGMP and glutathione-conjugate transport in human erythrocytes

The nature of cGMP transport in human erythrocytes, its relationship to glutathione conjugate transport, and possible mediation by multidrug resistance-associated proteins (MRPs) have been investigated. MRP1, MRP4 and MRP5 are detected in immunoblotting studies with erythrocytes. MRP1 and MRP5 are also detected in multidrug resistant COR-L23/R and MOR/R cells but at greatly reduced levels in the parent, drug sensitive COR-L23/P cells. MRP4 is detected in MOR/R but not COR-L23/R cells. Uptake of cGMP into inside-out membrane vesicles prepared by a spontaneous, one-step vesiculation process is shown to be by a low affinity system that accounts for more than 80% of the transport at all concentrations above 3 micro m. This transport is reduced by MRP inhibitors and substrates including MK-571, methotrexate, estradiol 17-beta-d-glucuronide, and S(2,4-dinitrophenyl)glutathione (DNP-SG) and also by glibenclamide and frusemide but not by the monoclonal Ig QCRL-3 that inhibits high-affinity transport of DNP-SG by MRP1. It is concluded that the cGMP exporter is distinct from MRP1 and has properties similar to those reported for MRP4. Furthermore the evidence suggests that the protein responsible for cGMP transport is the same as that mediating low-affinity DNP-SG transport in human erythrocytes.

Effect of divalent counterions on asymmetrically charged lipid bilayers

We study an asymmetrically charged lipid bilayer in which the inner layer is negatively charged, while the outer one is neutral. In particular, we focus on the interplay between the asymmetrical charge distribution and counterion valency in determining the preferred curvature state of the bilayer. We show that, at low ionic strength, an entropic effect associated with counterion release tends to expand the inner layer-surprisingly, the excess charge of the bilayer opposes this. In the absence of multivalent ions, the entropic effect is dominant and gives rise to a tendency towards negative mean curvature. The presence of divalent counterions, however, counterbalances the entropic effect-they tend to shrink the inner layer, leading to positive mean curvature.

Effects of clotrimazole on transport mediated by multidrug resistance associated protein 1 (MRP1) in human erythrocytes and tumour cells

Clotrimazole has been shown to have potent anti-malarial activity in vitro, one possible mechanism being inhibition of oxidized glutathione (GSSG) export from the infected human red blood cells or from the parasite itself. Efflux of GSSG from normal erythrocytes is mediated by a high affinity glutathione S-conjugate transporter. This paper shows that transport of the model substrate, 3 microm dinitrophenyl S-glutathione, across erythrocyte membranes is inhibited by multidrug resistance-associated protein 1 (MRP1)-specific antibody, QCRL-3, strongly suggesting that the high affinity transport is mediated by MRP1. The rates of transport observed with membrane vesicles prepared from erythrocytes or from multidrug resistant tumour cells show a similar pattern of responses to applied reduced glutathione, GSSG and MRP1 inhibitors (indomethacin, MK571) further supporting the conclusion that the high affinity transporter is MRP1. In both erythrocytes and MRP1-expressing tumour cells, MRP1-associated transport is inhibited by clotrimazole over the range 2-20 microm, and the inhibitory effect leads to increases in accumulation of MRP1 substrates, vincristine and calcein, and decreases in calcein efflux from intact MRP1-expressing human tumour cells. It also results in increased sensitivity to daunorubicin of the multidrug resistant cells, L23/R but not the sensitive parent L23/P cells. These results demonstrate that clotrimazole can inhibit the MRP1 which is present in human erythrocytes, an effect that may contribute to, though not fully account for, its anti-malarial action.

Stabilization and destabilization of cell membranes by multivalent ions

We propose a mechanism for the stabilization and destabilization of cell membranes by multivalent ions with an emphasis on its implications for the division and fusion of cells. We find that multivalent cations preferentially adsorbed onto a membrane dramatically change the membrane stability. They not only reduce the surface charge density of the membrane but also induce a barrier to pore growth. While both of these effects lead to enhanced membrane stability against vesiculation and pore growth, the induced barrier arises from correlated fluctuations of the adsorbed cations and favors closure of a pore. Finally, the addition of a small amount of multivalent anions can reverse the membrane stabilization, providing an effective way to regulate membrane stability.

Packaged merozoite release without immediate host cell lysis

In spite of the extraordinary progress in unravelling the genome of the Plasmodium falciparum parasite, many crucial aspects of its biology remain poorly understood. One largely neglected area is the mechanism of merozoite release from host red blood cells.

Characterization of immunoglobulin binding to isolated human erythrocyte membranes: evidence for selective, temperature-induced binding of naturally occurring autoantibodies to the cytoskeleton

Human plasma contains naturally occurring autoantibodies to the predominant components of the erythrocyte membrane: band 3 and spectrin bands 1 and 2 of the cytoskeleton. The titer of cytoskeletal plasma autoantibodies increases in various hemolytic conditions, suggesting that opsonization of the cytoskeleton may play an important role in the clearance of hemolyzed (not senescent) erythrocytes from the circulation. In this study, we use Alexa Fluor 488 goat anti-human IgG conjugate (Molecular Probes, Eugene, OR, USA), to characterize plasma immunoglobulin binding to erythrocyte membranes from osmotically hemolyzed cells ('ghosts'). The results show that exposure of ghosts to plasma results in 4-fold more immunoglobulin binding to the cytoskeleton than is bound to the proteins contained within the lipid bilayer. Preincubation of the ghosts at 37 degrees C causes 8-fold more immunoglobulin binding to the cytoskeleton compared to bilayer proteins. This temperature-induced change resulted from selective immunoglobulin binding to the cytoskeleton, with no change in immunoglobulin binding to bilayer proteins. However, the rate of increase in cytoskeletal antigenicity at 37 degrees C did correlate with the rate of a conformational change in band 3, a transmembrane protein which serves as a major membrane attachment site for the cytoskeleton. The results of this study suggest that the cytoskeleton is the primary target in the opsonization of hemolyzed erythrocyte membranes by naturally occurring plasma autoantibodies. The conformational changes which occur in ghosts at 37 degrees C are associated with selective exposure of new immunoglobulin binding sites on the cytoskeleton, and with a change in the structure of band 3. We propose a model suggesting that opsonization of the cytoskeleton occurs prior to the decomposition of hemolyzed erythrocytes at 37 degrees C.

Reversible vacuolation of T-tubules in skeletal muscle: mechanisms and implications for cell biology

The majority of investigations of the transverse tubules (T-system) of skeletal muscle have been devoted to their role in excitation-contraction coupling, with particular reference to contact with the sarcoplasmic reticulum and the mechanism of Ca2- release. By contrast, this review is concerned with structural and functional aspects of the vacuolation of T-tubules. It covers experimental procedures used in reversible vacuolation induced by the efflux-influx of glycerol and other small nonelectrolytes, sugars, and ions. The characteristics of the phenomenon, associated alterations in muscle function, and the swelling of analogous structures in nonmuscle cells are considered. Possible functions of reversible vacuolation in water balance, transport, membrane repair, muscle pathology, and fatigue are considered, and the potential application of reversible vacuolation in the transfection of skeletal muscle is discussed. In relation to the possible mechanisms involved in reversible vacuolation, particular attention is given to the dynamic and structural aspects of the opening and closing of T-tubules, the origin of vacuolar membranes, and the localized character of tubular swelling.

Calculation of a Gap restoration in the membrane skeleton of the red blood cell: possible role for myosin II in local repair

Human red blood cells contain all of the elements involved in the formation of nonmuscle actomyosin II complexes (V. M. Fowler. 1986. J. Cell. Biochem. 31:1-9; 1996. Curr. Opin. Cell Biol. 8:86-96). No clear function has yet been attributed to these complexes. Using a mathematical model for the structure of the red blood cell spectrin skeleton (M. J. Saxton. 1992. J. Theor. Biol. 155:517-536), we have explored a possible role for myosin II bipolar minifilaments in the restoration of the membrane skeleton, which may be locally damaged by major mechanical or chemical stress. We propose that the establishment of stable links between distant antiparallel actin protofilaments after a local myosin II activation may initiate the repair of the disrupted area. We show that it is possible to define conditions in which the calculated number of myosin II minifilaments bound to actin protofilaments is consistent with the estimated number of myosin II minifilaments present in the red blood cells. A clear restoration effect can be observed when more than 50% of the spectrin polymers of a defined area are disrupted. It corresponds to a significant increase in the spectrin density in the protein free region of the membrane. This may be involved in a more complex repair process of the red blood cell membrane, which includes the vesiculation of the bilayer and the compaction of the disassembled spectrin network.

The tubular vacuolation process in amphibian skeletal muscle

The exposure of amphibian muscle to osmotic shock through the introduction and subsequent withdrawal of extracellular glycerol causes 'vacuolation' in the transverse tubules. Such manoeuvres can also electrically isolate the transverse tubules from the surface ('detubulation'), particular if followed by exposures to high extracellular [Ca2+] and/or gradual cooling. This study explored factors influencing vacuolation in Rana temporaria sartorius muscle. Vacuole formation was detected using phase contrast microscopy and through the trapping or otherwise of lissamine rhodamine dye fluorescence within such vacuoles. The preparations were also examined using electron microscopy, for penetration into the transverse tubules and tubular vacuoles of extracellular horseradish peroxidase introduced following the osmotic procedures. These comparisons distinguished for he first time two types of vacuole, 'open' and 'closed', whose lumina were respectively continuous with or detached from the remaining extracellular space. The vacuoles formed closed to and between the Z-lines, but subsequently elongated along the longitudinal axis of the muscle fibres. This suggested an involvement of tubular membrane material; the latter appeared particularly concentrated around such Z-lines in the electron-micrograph stereopairs of thick longitudinal sections. 'Open' vacuoles formed following osmotic shock produced by extracellular glycerol withdrawal from a glycerol-loaded fibre at a stage when one would expect a net water entry to the intracellular space. This suggests that vacuole formation requires active fluid transport into the tubular lumina in response to fibre swelling. 'Closed' vacuoles only formed when the muscle was subsequently exposed to high extracellular [Ca/+] and/or gradual cooling following the initial osmotic shock. Their densities were similar to those shown by 'open' vacuoles in preparations not so treated, suggesting that both vacuole types resulted from a single process initiated by glycerol withdrawal. However, vacuole 'closure' took place well after formation of 'open' vacuoles, over 25 min after glycerol withdrawal. Its time course closely paralleled the development of detubulation reported recently. It was irreversible, in contrast to the reversibility of 'open' vacuole formation. These findings identify electrophysiological 'detubulation' of striated muscle with 'closure' of initially 'open' vacuoles. The reversible formation of open vacuoles is compatible with some normal membrane responses to some physiological stresses such as fatigue, whereas irreversible formation of closed vacuoles might only be expected in pathological situations as in dystrophic muscle.

Osmotic 'detubulation' in frog muscle arises from a reversible vacuolation process

Isolated Rana temporaria sartorius muscle fibres were subject to introduction and subsequent withdrawal of 400 mM extracellular glycerol, exposures to high divalent ion concentrations and then cooling. Tubular detachment was then assessed through changes in the action potential afterdepolarization. (1) The rapid (5-10 min) rather than slow cooling step (30 min) produced a gradual (30 min) development of detubulation arrested by the subsequent replacement of glycerol and reversed by addition of 350 mM sucrose. Such osmotic agents influenced neither resting potentials of intact or detubulated fibres nor action potentials in intact fibres. (2) Full tubular detachment was achieved by 40 min. Laser epifluorescence microscopy demonstrated an accompanying tubular vacuolation through its trapping of a Rhodamine dye. (3) Subsequent re-additions (at 10-80 min) of glycerol restored the afterdepolarization in 30% of detubulated fibres and correspondingly reduced vacuolation. Sustained (> 60 min) exposures to 350 mM sucrose, applied between 30-60 min, both reversed tubular isolation in 70% of detubulated fibres and abolished tubular vacuolation. Finally, results from transient (10-30 min) sucrose exposures resembled the consequences of sustained applications of glycerol, suggesting that detubulation and its reversal result from an osmotic mechanism. (4) Nevertheless, irreversible changes developed after 70-80 min in 70% of detubulated fibres, a process hastened by slow cooling steps in the initial osmotic stress. The present study thus correlates morphological and electrophysiological consequences of applying osmotic shock to skeletal muscle for the first time. It additionally differentiates reversible and irreversible components of detubulation. Finally, it suggests that detubulation results from the similarly reversible vacuolation observed under comparable osmotic conditions, and that such vacuolation can eventually lead to irreversible detubulation.

Surface shape change during fusion of erythrocyte membranes is sensitive to membrane skeleton agents

We previously reported that the induction of membrane fusion between pairs of erythrocyte ghosts is accompanied by the formation of a multipore fusion zone that undergoes an area expansion with condition-dependent characteristics. These characteristics allowed us to hypothesize substantial, if not major, involvement of the spectrin-based membrane skeleton in controlling this expansion. It was also found that the fusion zone, which first appears in phase optics as a flat diaphragm, has a lifetime that is also highly condition-dependent. We report here that 2,3-diphosphoglycerate, wheat germ agglutinin, diamide, and N-ethylmaleimide, all known to have binding sites primarily on skeleton components (including spectrin), have condition-dependent effects on specific components of the fusion zone diameter versus time expansion curve and the flat diaphragm lifetime. We also report a pH/ionic strength condition that causes a dramatic stabilization of flat diaphragms in a manner consistent with the known pH/ionic strength dependence of the spectrin calorimetric transition, thus further supporting the hypothesis of spectrin involvement. Our data suggest that the influence of the membrane skeleton on cell fusion is to restrain the rounding up that takes place after membrane fusion and that it may have variable, rather than fixed, mechanical properties. Data show that WGA, a known ligand for sialic acid, and DPG, a known metabolite, influences the flat diaphragm stability and late period expansion rates, raising the possibility that some of these mechanical properties are biologically regulated.

Band 3 mobility in camelid elliptocytes: implications for erythrocyte shape

Measurements of time-resolved phosphorescence anisotropy were used to monitor the rotational diffusion of eosin-labeled band 3 in membranes of the elliptocytic erythrocytes of alpacas and camels. The rotational freedom of camelid band 3 was more restricted than for human band 3. Removal of the peripheral membrane proteins from human erythrocyte membranes, by high-pH treatment, increased the band 3 rotational freedom. The same high-pH treatment of alpaca and camel erythrocyte membranes failed to alter the rotational freedom of band 3 in these species and also failed to remove ankyrin. Treatment of human and alpaca erythrocyte membranes with trypsin, which removed the cytoplasmic domain of band 3, caused a marked increase in band 3 rotational freedom in both species. We suggest that ankyrin may modulate the rotational freedom of band 3 in camelid erythrocytes, thereby influences the erythrocyte shape and deformability. The rotational freedom of band 3 in sheep, pig, and rat erythrocyte membranes was also examined and found to be slightly greater than for human band 3. This is consistent with the inability of glyceraldehyde-3-phosphate dehydrogenase to bind to band 3 in the erythrocyte membranes of these species.

Involvement of cytoskeletal proteins in the barrier function of the human erythrocyte membrane. III. Permeability of spectrin-depleted inside-out membrane vesicles to hydrophilic nonelectrolytes. Formation of leaks by chemical or enzymatic modification of membrane proteins

Spectrin-depleted inside-out vesicles (IOV's) prepared from human erythrocyte membranes were characterized in terms of size, ground permeability to hydrophilic nonelectrolytes and their sensitivity to modification by SH reagents, DIDS and trypsin. IOV's proved to have the same permeability of their lipid domain to erythritol as native erythrocytes, in contrast to resealed ghosts (Klonk, S. and Deuticke, B. (1992) Biochim. Biophys. Acta 1106, 126-136 (Part I in this series)), which have a residual leak. On the other hand, IOV's have a slightly elevated permeability for mannitol and sucrose, nonelectrolytes which are almost (mannitol) or fully (sucrose) impermeant in the native membrane. These increased fluxes, which have a high activation energy and can be stimulated by phloretin, are, however, also much smaller than the corresponding leak fluxes observed in resealed ghosts. In view of these differences, formation of IOV's can be concluded to go along with partial annealing of barrier defects persisting in the erythrocyte membrane after preparation of resealed ghosts. Oxidation of SH groups of the IOV membrane by diamide produces an enhancement of permeability for hydrophilic nonelectrolytes which is much less pronounced than that induced by a similar treatment of erythrocytes or ghosts (Klonk, S. and Deuticke, B. (1992) Biochim. Biophys. Acta 1106, 126-136 (Part I in this series)). Moreover, proteolytic treatment of the vesicle membrane, although leading to a marked digestion of integral membrane proteins, only induces a minor, saturating increase of permeability, much lower than that in trypsinized resealed ghosts (Klonk, S. and Deuticke, B. (1992) Biochim. Biophys. Acta 1106, 137-142 (Part II of this series)). Since absence of the cytoskeletal proteins, spectrin and actin, is the major difference between IOV's and resealed ghosts, these results may be taken as further evidence for a dependence of the barrier properties of the erythrocyte membrane bilayer domain on its interaction with cytoskeletal elements. In contrast, these barrier properties seem to be rather insensitive to perturbations of integral proteins.

Transbilayer asymmetry of pyrene mobility in human spherocytic red cell membranes

The diffusion-dependent formation of pyrene excimers (excited dimers) was studied in normal and spherocytic red cell membranes. Pyrene emission was alternatively quenched in either bilayer half by non radiative energy transfer to haemoglobin. Pyrene excimer to monomer fluorescence intensity ratio, I'/I, was 0.35 +/- 0.03 (S.E.) in washed red blood cells obtained from normal donors (n = 8) and 0.45 + 0.03 (n = 13) in the corresponding isolated, haemoglobin-free resealed membranes (P less than 0.02). In the spherocytic condition the respective values were 0.28 +/- 0.01 (n = 9) and 0.53 +/- 0.03 (n = 9), P less than 0.001. In contrast to the decrease of I'/I in red cells as compared to isolated membranes, being 22% in normal cells and 47% in spherocytic ones, haemoglobin added to the exofacial side of isolated membranes, respectively, reduced I'/I by 18% and 5%. In normal red cell membranes, pyrene mobility appears to be higher in the inner monolayer than in the outer one. In spherocytic membranes our results indicate an enhanced transmembrane asymmetry in lipid monolayer fluidity, probably due to a defect of the membrane protein skeleton organization.

Quenched flow analysis of exocytosis in Paramecium cells: time course, changes in membrane structure, and calcium requirements revealed after rapid mixing and rapid freezing of intact cells

Synchronous exocytosis in Paramecium cells was analyzed on a subsecond time scale. For this purpose we developed a quenched flow device for rapid mixing and rapid freezing of cells without impairment (time resolution in the millisecond range, dead time approximately 30 ms). Cells frozen at defined times after stimulation with the noncytotoxic secretagogue aminoethyldextran were processed by freeze substitution for electron microscopic analysis. With ultrathin sections the time required for complete extrusion of secretory contents was determined to be less than 80 ms. Using freeze-fracture replicas the time required for resealing of the fused membranes was found to be less than 350 ms. During membrane fusion (visible 30 ms after stimulation) specific intramembranous particles in the cell membrane at the attachment sites of secretory organelles ("fusion rosette") disappear, possibly by dissociation of formerly oligomeric proteins. This hitherto unknown type of rapid change in membrane architecture may reflect molecular changes in protein-protein or protein-lipid interactions, presumably crucial for membrane fusion. By a modification of the quenched flow procedure extracellular [Ca++] during stimulation was adjusted to less than or equal to 3 x 10(-8) M, i.e., below intracellular [Ca++]. Only extrusion of the secretory contents, but not membrane fusion, was inhibited. Thus it was possible to separate both secretory events (membrane fusion from contents extrusion) and to discriminate their Ca++ requirements. We conclude that no Ca++ influx is necessary for induction of membrane fusion.

Two different types of lysis of chromaffin granules characterised by freeze-fracture electron microscopy and photon correlation spectroscopy

When bovine chromaffin granules are incubated in hyperosmolar sucrose solutions and subsequently transferred back towards isoosmolarity they undergo lysis ('hyperosmotic relaxation lysis'). This type of lysis was compared with the common effect of hypotonic lysis by means of photon correlation spectroscopy (PCS) and freeze-fracture electron microscopy. Both methods revealed differences regarding mean sizes and size distributions of granules lysing under either hypotonic or hypertonic conditions. However, the results obtained by these two methods were not consistent. In the case of hypotonic lysis, a nonmonotonic behaviour of the mean diameter as a function of the sucrose concentration was observed by PCS, but not in the micrographs. From EM size determinations we obtained a decrease in the mean diameter and an increase of the width of the distribution due to the appearance of small (50-200 nm) vesicles. Probably these vesicles are intragranular vesicles released during lysis. The maximum in photon correlation spectroscopy (PCS) diameter being 140% of the isotonic diameter is shown to be caused by the changing size distribution and geometry of the lysing granules. In the case of hyperosmotic relaxation, micrographs revealed that originally shrunken, nonspherical granules regained their spherical shape and formed small (60 nm) vesicles upon lysis. In contrast, no difference was observed between the sizes of granules prior to and after hyperosmotic relaxation by means of PCS. The paper discusses the validity of intensity-weighted light scattering data of polydisperse particle suspensions with changing size distributions. The mechanism of hyperosmotic relaxation lysis is considered.