The non-selective voltage-activated cation channel in the human red blood cell membrane: reconciliation between two conflicting reports and further characterisation

Is post-hypertonic lysis of human red blood cells caused by excessive cell volume regulation?

Human red blood cells (RBC) exposed to hypertonic media are subject to post-hypertonic lysis - an injury that only develops during resuspension to an isotonic medium. The nature of post-hypertonic lysis was previously hypothesized to be osmotic when cation leaks were observed, and salt loading was suggested as a cause of the cell swelling upon resuspension in an isotonic medium. However, it was problematic to account for the salt loading since the plasma membrane of human RBCs was considered impermeable to cations. In this study, the hypertonicity-related behavior of human RBCs is revisited within the framework of modern cell physiology, considering current knowledge on membrane ion transport mechanisms - an account still missing. It is recognized here that the hypertonic behavior of human RBCs is consistent with the acute regulatory volume increase (RVI) response - a healthy physiological reaction initiated by cells to regulate their volume by salt accumulation. It is shown by reviewing the published studies that human RBCs can increase cation conductance considerably by activating cell volume-regulated ion transport pathways inactive under normal isotonic conditions and thus facilitate salt loading. A simplified physiological model accounting for transmembrane ion fluxes and membrane voltage predicts the isotonic cell swelling associated with increased cation conductance, eventually reaching hemolytic volume. The proposed involvement of cell volume regulation mechanisms shows the potential to explain the complex nature of the osmotic response of human RBCs and other cells. Cryobiological implications, including mechanisms of cryoprotection, are discussed.

The Gárdos Channel and Piezo1 Revisited: Comparison between Reticulocytes and Mature Red Blood Cells

The Gárdos channel (KCNN4) and Piezo1 are the best-known ion channels in the red blood cell (RBC) membrane. Nevertheless, the quantitative electrophysiological behavior of RBCs and its heterogeneity are still not completely understood. Here, we use state-of-the-art biochemical methods to probe for the abundance of the channels in RBCs. Furthermore, we utilize automated patch clamp, based on planar chips, to compare the activity of the two channels in reticulocytes and mature RBCs. In addition to this characterization, we performed membrane potential measurements to demonstrate the effect of channel activity and interplay on the RBC properties. Both the Gárdos channel and Piezo1, albeit their average copy number of activatable channels per cell is in the single-digit range, can be detected through transcriptome analysis of reticulocytes. Proteomics analysis of reticulocytes and mature RBCs could only detect Piezo1 but not the Gárdos channel. Furthermore, they can be reliably measured in the whole-cell configuration of the patch clamp method. While for the Gárdos channel, the activity in terms of ion currents is higher in reticulocytes compared to mature RBCs, for Piezo1, the tendency is the opposite. While the interplay between Piezo1 and Gárdos channel cannot be followed using the patch clamp measurements, it could be proved based on membrane potential measurements in populations of intact RBCs. We discuss the Gárdos channel and Piezo1 abundance, interdependencies and interactions in the context of their proposed physiological and pathophysiological functions, which are the passing of small constrictions, e.g., in the spleen, and their active participation in blood clot formation and thrombosis.

Modulation of Voltage-Gating and Hysteresis of Lysenin Channels by Cu2+ Ions

The intricate voltage regulation presented by lysenin channels reconstituted in artificial lipid membranes leads to a strong hysteresis in conductance, bistability, and memory. Prior investigations on lysenin channels indicate that the hysteresis is modulated by multivalent cations which are also capable of eliciting single-step conformational changes and transitions to stable closed or sub-conducting states. However, the influence on voltage regulation of Cu2+ ions, capable of completely closing the lysenin channels in a two-step process, was not sufficiently addressed. In this respect, we employed electrophysiology approaches to investigate the response of lysenin channels to variable voltage stimuli in the presence of small concentrations of Cu2+ ions. Our experimental results showed that the hysteretic behavior, recorded in response to variable voltage ramps, is accentuated in the presence of Cu2+ ions. Using simultaneous AC/DC stimulation, we were able to determine that Cu2+ prevents the reopening of channels previously closed by depolarizing potentials and the channels remain in the closed state even in the absence of a transmembrane voltage. In addition, we showed that Cu2+ addition reinstates the voltage gating and hysteretic behavior of lysenin channels reconstituted in neutral lipid membranes in which lysenin channels lose their voltage-regulating properties. In the presence of Cu2+ ions, lysenin not only regained the voltage gating but also behaved like a long-term molecular memory controlled by electrical potentials.

The Chloride Conductance Inhibitor NS3623 Enhances the Activity of a Non-selective Cation Channel in Hyperpolarizing Conditions

Handbooks of physiology state that the strategy adopted by red blood cells (RBCs) to preserve cell volume is to maintain membrane permeability for cations at its minimum. However, enhanced cation permeability can be measured and observed in specific physiological and pathophysiological situations such as in vivo senescence, storage at low temperature, sickle cell anemia and many other genetic defects affecting transporters, membrane or cytoskeletal proteins. Among cation pathways, cation channels are able to dissipate rapidly the gradients that are built and maintained by the sodium and calcium pumps. These situations are very well-documented but a mechanistic understanding of complex electrophysiological events underlying ion transports is still lacking. In addition, non-selective cation (NSC) channels present in the RBC membrane have proven difficult to molecular identification and functional characterization. For instance, NSC channel activity can be elicited by Low Ionic Strength conditions (LIS): the associated change in membrane potential triggers its opening in a voltage dependent manner. But, whereas this depolarizing media produces a spectacular activation of NSC channel, Gárdos channel-evoked hyperpolarization's have been shown to induce sodium entry through a pathway thought to be conductive and termed P_cat. Using the CCCP method, which allows to follow fast changes in membrane potential, we show here (i) that hyperpolarization elicited by Gárdos channel activation triggers sodium entry through a conductive pathway, (ii) that chloride conductance inhibition unveils such conductive cationic conductance, (iii) that the use of the specific chloride conductance inhibitor NS3623 (a derivative of Neurosearch compound NS1652), at concentrations above what is needed for full anion channel block, potentiates the non-selective cation conductance. These results indicate that a non-selective cation channel is likely activated by the changes in the driving force for cations rather than a voltage dependence mechanism per se.

Effects of mobile phone emissions on human red blood cells

Raman spectroscopy was performed on GSM 900 and 1800 MHz mobile phone signal exposed red blood cells (RBCs). The observed changes in the Raman spectra of mobile signal exposed RBCs compared to unexposed control suggest reduced hemoglobin-oxygen affinity for the exposed cells. The possible mechanism may involve activation of the voltage gated membrane Ca²⁺ channels by the mobile phone emissions resulting in an increase in the levels of adenosine triphosphate (ATP) and 2,3-diphosphoglycerate (2,3-DPG) in cells via altered metabolic activities. Further studies carried out with fluorescent Ca²⁺ indicator confirmed increased intracellular Ca²⁺ level in the exposed cells. Since intracellular ATP level influences the shape and mechanics of RBCs, exposed cells were studied using diffraction phase microscopy and optical tweezers. Detectable changes in shape and mechanical properties were observed due to mobile signal exposure.

An Insight into the Stages of Ion Leakage during Red Blood Cell Storage

Packed red blood cells (pRBCs), the most commonly transfused blood product, are exposed to environmental disruptions during storage in blood banks. In this study, temporal sequence of changes in the ion exchange in pRBCs was analyzed. Standard techniques commonly used in electrolyte measurements were implemented. The relationship between ion exchange and red blood cells (RBCs) morphology was assessed with use of atomic force microscopy with reference to morphological parameters. Variations observed in the Na⁺, K⁺, Cl⁻, H⁺, HCO₃⁻, and lactate ions concentration show a complete picture of singly-charged ion changes in pRBCs during storage. Correlation between the rate of ion changes and blood group type, regarding the limitations of our research, suggested, that group 0 is the most sensitive to the time-dependent ionic changes. Additionally, the impact of irreversible changes in ion exchange on the RBCs membrane was observed in nanoscale. Results demonstrate that the level of ion leakage that leads to destructive alterations in biochemical and morphological properties of pRBCs depend on the storage timepoint.

Lysophosphatidic Acid-Activated Calcium Signaling Is Elevated in Red Cells from Sickle Cell Disease Patients

(1) Background: It is known that sickle cells contain a higher amount of Ca2+ compared to healthy red blood cells (RBCs). The increased Ca2+ is associated with the most severe symptom of sickle cell disease (SCD), the vaso-occlusive crisis (VOC). The Ca2+ entry pathway received the name of Psickle but its molecular identity remains only partly resolved. We aimed to map the involved Ca2+ signaling to provide putative pharmacological targets for treatment. (2) Methods: The main technique applied was Ca2+ imaging of RBCs from healthy donors, SCD patients and a number of transgenic mouse models in comparison to wild-type mice. Life-cell Ca2+ imaging was applied to monitor responses to pharmacological targeting of the elements of signaling cascades. Infection as a trigger of VOC was imitated by stimulation of RBCs with lysophosphatidic acid (LPA). These measurements were complemented with biochemical assays. (3) Results: Ca2+ entry into SCD RBCs in response to LPA stimulation exceeded that of healthy donors. LPA receptor 4 levels were increased in SCD RBCs. Their activation was followed by the activation of Gi protein, which in turn triggered opening of TRPC6 and CaV2.1 channels via a protein kinase Cα and a MAP kinase pathway, respectively. (4) Conclusions: We found a new Ca2+ signaling cascade that is increased in SCD patients and identified new pharmacological targets that might be promising in addressing the most severe symptom of SCD, the VOC.

Calcium/protein kinase C signaling mechanisms in shear-induced mechanical responses of red blood cells

Red blood cell (RBC) deformability has vital importance for microcirculation in the body, as RBCs travel in narrow capillaries under shear stress. Deformability can be defined as a remarkable cell ability to change shape in response to an external force which allows the cell to pass through the narrowest blood capillaries. Previous studies showed that RBC deformability could be regulated by Ca²⁺/protein kinase C (PKC) signaling mechanisms due to the phosphorylative changes in RBC membrane proteins by kinases and phosphatases. We investigated the roles of Ca²⁺/PKC signaling pathway on RBC mechanical responses and impaired RBC deformability under continuous shear stress (SS). A protein kinase C inhibitor Chelerythrine, a tyrosine phosphatase inhibitor Calpeptin, and a calcium channel blocker Verapamil were applied into human blood samples in 1 micromolar concentration. Samples with drugs were treated with or without 3 mM Ca²⁺. A shear stress at 5 Pa level was applied to each sample continuously for 300 s. RBC deformability was measured by a laser-assisted optical rotational cell analyzer (LORRCA) and was calculated as the change in elongation index (EI) of RBC upon a range of shear stress (SS, 0.3-50 Pa). RBC mechanical stress responses were evaluated before and after continuous SS through the parameterization of EI-SS curves. The drug administrations did not produce any significant alterations in RBC mechanical responses when they were applied alone. However, the application of the drugs together with Ca²⁺ substantially increased RBC deformability compared to calcium alone. Verapamil significantly improved Ca²⁺-induced impairments of deformability both before and after 5 Pa SS exposure (p < 0.0001). Calpeptin and Chelerythrine significantly ameliorated impaired deformability only after continuous SS (p < 0.05). Shear-induced improvements of deformability were conserved by the drug administrations although shear-induced deformability was impaired when the drugs were applied with calcium. The blocking of Ca²⁺ channel by Verapamil improved impaired RBC mechanical responses independent of the SS effect. The inhibition of tyrosine phosphatase and protein kinase C by Calpeptin and Chelerythrine, respectively, exhibited ameliorating effects on calcium-impaired deformability with the contribution of shear stress. The modulation of Ca²⁺/PKC signaling pathway could regulate the mechanical stress responses of RBCs when cells are under continuous SS exposure. Shear-induced improvements in the mechanical properties of RBCs by this signaling mechanism could facilitate RBC flow in the microcirculation of pathophysiological disorders, wherein Ca²⁺ homeostasis is disturbed and RBC deformability is reduced.

Intracellular Ca2+ Concentration and Phosphatidylserine Exposure in Healthy Human Erythrocytes in Dependence on in vivo Cell Age

After about 120 days of circulation in the blood stream, erythrocytes are cleared by macrophages in the spleen and the liver. The “eat me” signal of this event is thought to be the translocation of phosphatidylserine from the inner to the outer membrane leaflet due to activation of the scramblase, while the flippase is inactivated. Both processes are triggered by an increased intracellular Ca²⁺ concentration. Although this is not the only mechanism involved in erythrocyte clearance, in this minireview, we focus on the following questions: Is the intracellular-free Ca²⁺ concentration and hence phosphatidylserine exposure dependent on the erythrocyte age, i.e. is the Ca²⁺ concentration, progressively raising during the erythrocyte aging in vivo? Can putative differences in intracellular Ca²⁺ and exposure of phosphatidylserine to the outer membrane leaflet be measured in age separated cell populations? Literature research revealed less than dozen of such publications with vastly contradicting results for the Ca²⁺ concentrations but consistency for a lack of change for the phosphatidylserine exposure. Additionally, we performed reanalysis of published data resulting in an ostensive illustration of the situation described above. Relating these results to erythrocyte physiology and biochemistry, we can conclude that the variation of the intracellular free Ca²⁺ concentration is limited with 10 μM as the upper level of the concentration. Furthermore, we propose the hypothesis that variations in measured Ca²⁺ concentrations may to a large extent depend on the experimental conditions applied but reflect a putatively changed Ca²⁺ susceptibility of erythrocytes in dependence of in vivo cell age.

Calcium Channels and Calcium-Regulated Channels in Human Red Blood Cells

Free Calcium (Ca²⁺) is an important and universal signalling entity in all cells, red blood cells included. Although mature mammalian red blood cells are believed to not contain organelles as Ca²⁺ stores such as the endoplasmic reticulum or mitochondria, a 20,000-fold gradient based on a intracellular Ca²⁺ concentration of approximately 60 nM vs. an extracellular concentration of 1.2 mM makes Ca²⁺-permeable channels a major signalling tool of red blood cells. However, the internal Ca²⁺ concentration is tightly controlled, regulated and maintained primarily by the Ca²⁺ pumps PMCA1 and PMCA4. Within the last two decades it became evident that an increased intracellular Ca²⁺ is associated with red blood cell clearance in the spleen and promotes red blood cell aggregability and clot formation. In contrast to this rather uncontrolled deadly Ca²⁺ signals only recently it became evident, that a temporal increase in intracellular Ca²⁺ can also have positive effects such as the modulation of the red blood cells O₂ binding properties or even be vital for brief transient cellular volume adaptation when passing constrictions like small capillaries or slits in the spleen. Here we give an overview of Ca²⁺ channels and Ca²⁺-regulated channels in red blood cells, namely the Gárdos channel, the non-selective voltage dependent cation channel, Piezo1, the NMDA receptor, VDAC, TRPC channels, Ca_V2.1, a Ca²⁺-inhibited channel novel to red blood cells and i.a. relate these channels to the molecular unknown sickle cell disease conductance P_sickle. Particular attention is given to correlation of functional measurements with molecular entities as well as the physiological and pathophysiological function of these channels. This view is in constant progress and in particular the understanding of the interaction of several ion channels in a physiological context just started. This includes on the one hand channelopathies, where a mutation of the ion channel is the direct cause of the disease, like Hereditary Xerocytosis and the Gárdos Channelopathy. On the other hand it applies to red blood cell related diseases where an altered channel activity is a secondary effect like in sickle cell disease or thalassemia. Also these secondary effects should receive medical and pharmacologic attention because they can be crucial when it comes to the life-threatening symptoms of the disease.

Red Blood Cell Passage of Small Capillaries Is Associated with Transient Ca2+-mediated Adaptations

When red blood cells (RBCs) pass constrictions or small capillaries they need to pass apertures falling well below their own cross section size. We used different means of mechanical stimulations (hypoosmotic swelling, local mechanical stimulation, passing through microfluidic constrictions) to observe cellular responses of human RBCs in terms of intracellular Ca²⁺-signaling by confocal microscopy of Fluo-4 loaded RBCs. We were able to confirm our in vitro results in a mouse dorsal skinfold chamber model showing a transiently increased intracellular Ca²⁺ when RBCs were passing through small capillaries in vivo. Furthermore, we performed the above-mentioned in vitro experiments as well as measurements of RBCs filterability under various pharmacological manipulations (GsMTx-4, TRAM-34) to explore the molecular mechanism of the Ca²⁺-signaling. Based on these experiments we conclude that mechanical stimulation of RBCs activates mechano-sensitive channels most likely Piezo1. This channel activity allows Ca²⁺ to enter the cell, leading to a transient activation of the Gardos-channel associated with K⁺, Cl^-, and water loss, i.e., with a transient volume adaptation facilitating the passage of the RBCs through the constriction.

Accelerated apoptotic death and in vivo turnover of erythrocytes in mice lacking functional mitogen- and stress-activated kinase MSK1/2

The mitogen- and stress-activated kinase MSK1/2 plays a decisive role in apoptosis. In analogy to apoptosis of nucleated cells, suicidal erythrocyte death called eryptosis is characterized by cell shrinkage and cell membrane scrambling leading to phosphatidylserine (PS) externalization. Here, we explored whether MSK1/2 participates in the regulation of eryptosis. To this end, erythrocytes were isolated from mice lacking functional MSK1/2 (msk^−/−) and corresponding wild-type mice (msk^+/+). Blood count, hematocrit, hemoglobin concentration and mean erythrocyte volume were similar in both msk^−/− and msk^+/+ mice, but reticulocyte count was significantly increased in msk^−/− mice. Cell membrane PS exposure was similar in untreated msk^−/− and msk^+/+ erythrocytes, but was enhanced by pathophysiological cell stressors ex vivo such as hyperosmotic shock or energy depletion to significantly higher levels in msk^−/− erythrocytes than in msk^+/+ erythrocytes. Cell shrinkage following hyperosmotic shock and energy depletion, as well as hemolysis following decrease of extracellular osmolarity was more pronounced in msk^−/− erythrocytes. The in vivo clearance of autologously-infused CFSE-labeled erythrocytes from circulating blood was faster in msk^−/− mice. The spleens from msk^−/− mice contained a significantly greater number of PS-exposing erythrocytes than spleens from msk^+/+ mice. The present observations point to accelerated eryptosis and subsequent clearance of erythrocytes leading to enhanced erythrocyte turnover in MSK1/2-deficient mice.

Dynamic memory of a single voltage-gated potassium ion channel: A stochastic nonequilibrium thermodynamic analysis

In this work, we have studied the stochastic response of a single voltage-gated potassium ion channel to a periodic external voltage that keeps the system out-of-equilibrium. The system exhibits memory, resulting from time-dependent driving, that is reflected in terms of dynamic hysteresis in the current-voltage characteristics. The hysteresis loop area has a maximum at some intermediate voltage frequency and disappears in the limits of low and high frequencies. However, the (average) dissipation at long-time limit increases and finally goes to saturation with rising frequency. This raises the question: how diminishing hysteresis can be associated with growing dissipation? To answer this, we have studied the nonequilibrium thermodynamics of the system and analyzed different thermodynamic functions which also exhibit hysteresis. Interestingly, by applying a temporal symmetry analysis in the high-frequency limit, we have analytically shown that hysteresis in some of the periodic responses of the system does not vanish. On the contrary, the rates of free energy and internal energy change of the system as well as the rate of dissipative work done on the system show growing hysteresis with frequency. Hence, although the current-voltage hysteresis disappears in the high-frequency limit, the memory of the ion channel is manifested through its specific nonequilibrium thermodynamic responses.

Conductance hysteresis in the voltage-dependent anion channel

Hysteresis in the conductance of voltage-sensitive ion channels is observed when the transmembrane voltage is periodically varied with time. Although this phenomenon has been used in studies of gating of the voltage-dependent anion channel, VDAC, from the outer mitochondrial membrane for nearly four decades, full hysteresis curves have never been reported, because the focus was solely on the channel opening branches of the hysteresis loops. We studied the hysteretic response of a multichannel VDAC system to a triangular voltage ramp the frequency of which was varied over three orders of magnitude, from 0.5 mHz to 0.2 Hz. We found that in this wide frequency range the area encircled by the hysteresis curves changes by less than a factor of three, suggesting broad distribution of the characteristic times and strongly non-equilibrium behavior. At the same time, quasi-equilibrium two-state behavior is observed for hysteresis branches corresponding to VDAC opening. This enables calculation of the usual equilibrium gating parameters, gating charge and voltage of equipartitioning, which were found to be almost insensitive to the ramp frequency. To rationalize this peculiarity, we hypothesize that during voltage-induced closure and opening the system explores different regions of the complex free energy landscape, and, in the opening branch, follows quasi-equilibrium paths.

Identification of signalling cascades involved in red blood cell shrinkage and vesiculation

Even though red blood cell (RBC) vesiculation is a well-documented phenomenon, notably in the context of RBC aging and blood transfusion, the exact signalling pathways and kinases involved in this process remain largely unknown. We have established a screening method for RBC vesicle shedding using the Ca²⁺ ionophore ionomycin which is a rapid and efficient method to promote vesiculation. In order to identify novel pathways stimulating vesiculation in RBC, we screened two libraries: the Library of Pharmacologically Active Compounds (LOPAC) and the Selleckchem Kinase Inhibitor Library for their effects on RBC from healthy donors. We investigated compounds triggering vesiculation and compounds inhibiting vesiculation induced by ionomycin. We identified 12 LOPAC compounds, nine kinase inhibitors and one kinase activator which induced RBC shrinkage and vesiculation. Thus, we discovered several novel pathways involved in vesiculation including G protein-coupled receptor (GPCR) signalling, the phosphoinositide 3-kinase (PI3K)–Akt (protein kinase B) pathway, the Jak–STAT (Janus kinase–signal transducer and activator of transcription) pathway and the Raf–MEK (mitogen-activated protein kinase kinase)–ERK (extracellular signal-regulated kinase) pathway. Moreover, we demonstrated a link between casein kinase 2 (CK2) and RBC shrinkage via regulation of the Gardos channel activity. In addition, our data showed that inhibition of several kinases with unknown functions in mature RBC, including Alk (anaplastic lymphoma kinase) kinase and vascular endothelial growth factor receptor 2 (VEGFR-2), induced RBC shrinkage and vesiculation.

After screening two libraries of small bioactive molecules and kinase inhibitors, we identified several signalling pathways to be involved in red blood cell (RBC) shrinkage and vesiculation. These include the Jak (Janus kinase)–STAT (signal transducer and activator of transcription) pathway, phosphoinositide 3-kinase (PI3K)–Akt pathway, the Raf–MEK (mitogen-activated protein kinase kinase)–ERK (extracellular signal-regulated kinase) pathway and GPCR (G protein-coupled receptor) signalling.

N-methyl-D-aspartate receptors in human erythroid precursor cells and in circulating red blood cells contribute to the intracellular calcium regulation

The presence of N-methyl-d-aspartate receptor (NMDAR) was previously shown in rat red blood cells (RBCs) and in a UT-7/Epo human myeloid cell line differentiating into erythroid lineage. Here we have characterized the subunit composition of the NMDAR and monitored its function during human erythropoiesis and in circulating RBCs. Expression of the NMDARs subunits was assessed in erythroid progenitors during ex vivo erythropoiesis and in circulating human RBCs using quantitative PCR and flow cytometry. Receptor activity was monitored using a radiolabeled antagonist binding assay, live imaging of Ca(2+) uptake, patch clamp, and monitoring of cell volume changes. The receptor tetramers in erythroid precursor cells are composed of the NR1, NR2A, 2C, 2D, NR3A, and 3B subunits of which the glycine-binding NR3A and 3B and glutamate-binding NR2C and 2D subunits prevailed. Functional receptor is required for survival of erythroid precursors. Circulating RBCs retain a low number of the receptor copies that is higher in young cells compared with mature and senescent RBC populations. In circulating RBCs the receptor activity is controlled by plasma glutamate and glycine. Modulation of the NMDAR activity in RBCs by agonists or antagonists is associated with the alterations in whole cell ion currents. Activation of the receptor results in the transient Ca(2+) accumulation, cell shrinkage, and alteration in the intracellular pH, which is associated with the change in hemoglobin oxygen affinity. Thus functional NMDARs are present in erythroid precursor cells and in circulating RBCs. These receptors contribute to intracellular Ca(2+) homeostasis and modulate oxygen delivery to peripheral tissues.

A model for the hysteresis observed in gating of lysenin channels

The pore-forming toxin lysenin self-inserts to form conductance channels in natural and artificial lipid membranes containing sphingomyelin. The inserted channels exhibit voltage regulation and hysteresis of the macroscopic current during the application of positive periodic voltage stimuli. We explored the bi-stable behavior of lysenin channels and present a theoretical approach for the mechanism of the hysteresis to explain its static and dynamic components. This investigation develops a model to incorporate the role of charge accumulation on the bilayer lipid membrane in influencing the channel conduction state. Our model is supported by experimental results and also provides insight into the temperature dependence of lysenin channel hysteresis. Through this work we gain perspective into the mechanism of how the response of a channel protein is determined by previous stimuli.

Morphologically homogeneous red blood cells present a heterogeneous response to hormonal stimulation

Red blood cells (RBCs) are among the most intensively studied cells in natural history, elucidating numerous principles and ground-breaking knowledge in cell biology. Morphologically, RBCs are largely homogeneous, and most of the functional studies have been performed on large populations of cells, masking putative cellular variations. We studied human and mouse RBCs by live-cell video imaging, which allowed single cells to be followed over time. In particular we analysed functional responses to hormonal stimulation with lysophosphatidic acid (LPA), a signalling molecule occurring in blood plasma, with the Ca²⁺ sensor Fluo-4. Additionally, we developed an approach for analysing the Ca²⁺ responses of RBCs that allowed the quantitative characterization of single-cell signals. In RBCs, the LPA-induced Ca²⁺ influx showed substantial diversity in both kinetics and amplitude. Also the age-classification was determined for each particular RBC and consecutively analysed. While reticulocytes lack a Ca²⁺ response to LPA stimulation, old RBCs approaching clearance generated robust LPA-induced signals, which still displayed broad heterogeneity. Observing phospatidylserine exposure as an effector mechanism of intracellular Ca²⁺ revealed an even increased heterogeneity of RBC responses. The functional diversity of RBCs needs to be taken into account in future studies, which will increasingly require single-cell analysis approaches. The identified heterogeneity in RBC responses is important for the basic understanding of RBC signalling and their contribution to numerous diseases, especially with respect to Ca²⁺ influx and the associated pro-thrombotic activity.

Calcium in red blood cells-a perilous balance

Ca2+ is a universal signalling molecule involved in regulating cell cycle and fate, metabolism and structural integrity, motility and volume. Like other cells, red blood cells (RBCs) rely on Ca2+ dependent signalling during differentiation from precursor cells. Intracellular Ca2+ levels in the circulating human RBCs take part not only in controlling biophysical properties such as membrane composition, volume and rheological properties, but also physiological parameters such as metabolic activity, redox state and cell clearance. Extremely low basal permeability of the human RBC membrane to Ca2+ and a powerful Ca2+ pump maintains intracellular free Ca2+ levels between 30 and 60 nM, whereas blood plasma Ca2+ is approximately 1.8 mM. Thus, activation of Ca2+ uptake has an impressive impact on multiple processes in the cells rendering Ca2+ a master regulator in RBCs. Malfunction of Ca2+ transporters in human RBCs leads to excessive accumulation of Ca2+ within the cells. This is associated with a number of pathological states including sickle cell disease, thalassemia, phosphofructokinase deficiency and other forms of hereditary anaemia. Continuous progress in unravelling the molecular nature of Ca2+ transport pathways allows harnessing Ca2+ uptake, avoiding premature RBC clearance and thrombotic complications. This review summarizes our current knowledge of Ca2+ signalling in RBCs emphasizing the importance of this inorganic cation in RBC function and survival.

Effect of bacterial peptidoglycan on erythrocyte death and adhesion to endothelial cells

Peptidoglycans, bacterial wall components, have previously been shown to trigger eryptosis, the suicidal erythrocyte death, characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Phosphatidylserine exposing erythrocytes adhere to the vascular wall at least partially by interaction of erythrocytic phosphatidylserine with endothelial CXC chemokine ligand 16 (CXCL16). The present study explored whether peptidoglycan exposure fosters the adhesion of erythrocytes to human umbilical vein endothelial cells (HUVEC). To this end, HUVEC were treated for 48 h with peptidoglycan (10 μg/ml) and CXCL16 abundance determined by confocal microscopy and FACS analysis. Moreover, human erythrocytes were exposed for 48 h to peptidoglycan (10 μg/ml) and phosphatidylserine exposure estimated from binding of fluorescent annexin-V, cell volume from forward scatter in FACS analysis and erythrocyte adhesion to human umbilical vein endothelial cells (HUVEC) from trapping of labeled erythrocytes in a flow chamber. As a result, bacterial peptidoglycan exposure was followed by increased CXCL16 expression in HUVEC as well as erythrocyte shrinkage, phosphatidylserine exposure and adhesion to HUVEC under flow conditions at arterial shear rates. The adhesion was significantly attenuated but not abrogated in the presence of either, erythrocyte phosphatidylserine-coating annexin-V (5 μl/ml) or CXCL16 neutralizing antibody directed against endothelial CXCL16 (4 μg/ml). In conclusion, exposure to peptidoglycan increases endothelial CXCL16 expression and leads to eryptosis followed by phosphatidylserine- and CXCL16-mediated adhesion of eryptotic erythrocytes to vascular endothelial cells.

Adhesion of klotho-deficient eryptotic erythrocytes to endothelial cells

AIM

Suicidal erythrocyte death or eryptosis is characterized by cell shrinkage and phosphatidylserine exposure at the cell surface. Eryptotic erythrocytes may adhere to the vascular wall by binding of phosphatidylserine to endothelial CXC chemokine ligand 16 (CXCL16). Triggers of eryptosis include osmotic shock or energy depletion. Susceptibility to eryptosis is modified by Klotho, a protein with profound effect on ageing and lifespan. Klotho deficiency leads to accelerated ageing and early death. The percentage of eryptotic erythrocytes is significantly larger in klotho-deficient mice (klotho(-/-) ) than in their wild-type littermates (klotho(+/+) ). The present study explored whether the accelerated eryptosis of klotho-deficient mice is paralleled by enhanced adhesion.

METHODS

Phosphatidylserine-exposing erythrocytes were identified by measurement of annexin V binding and adhesion to human umbilical vein endothelial cells (HUVEC) from trapping of labelled erythrocytes in a flow chamber.

RESULTS

Annexin V binding was higher in klotho(-/-) erythrocytes than in klotho(+/+) erythrocytes. Osmotic shock for 1 h (addition of 550 mm sucrose) and energy depletion (12-h glucose depletion) increased annexin V binding to values again significantly larger in klotho(-/-) erythrocytes than in klotho(+/+) erythrocytes. klotho(-/-) erythrocytes were particularly sensitive to osmotic shock. Both osmotic shock and energy depletion enhanced erythrocyte adhesion, an effect again more pronounced in klotho(-/-) erythrocytes than in klotho(+/+) erythrocytes. The adhesion was significantly decreased by coating of phospatidylserine with annexin V (5 μL mL(-1) ) or by coating of CXCL16 with neutralizing antibodies (4 μg mL(-1) ).

CONCLUSIONS

klotho(-/-) erythrocytes are particularly sensitive to osmotic shock, and enhanced eryptosis of klotho(-/-) erythrocytes is paralleled by enhanced adhesion to endothelial CXCL16.

Adhesion of annexin 7 deficient erythrocytes to endothelial cells

Annexin 7 deficiency has previously been shown to foster suicidal death of erythrocytes or eryptosis, which is triggered by increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) and characterized by cell shrinkage and cell membrane scrambling with subsequent phosphatidylserine exposure at the cell surface. Eryptosis following increase of [Ca(2+)](i) by Ca(2+) ionophore ionomycin, osmotic shock or energy depletion was more pronounced in erythrocytes from annexinA7-deficient mice (anxA7(-/-)) than in erythrocytes from wild type mice (anxA7(+/+)). As phosphatidylserine exposure is considered to mediate adhesion of erythrocytes to the vascular wall, the present study explored adhesion of erythrocytes from anx7(-/-) and anx7(+/+)-mice following increase of [Ca(2+)](i) by Ca(2+) ionophore ionomycin (1 µM for 30 min), hyperosmotic shock (addition of 550 mM sucrose for 2 hours) or energy depletion (removal of glucose for 12 hours). Phosphatidylserine exposing erythrocytes were identified by annexin V binding, cell volume estimated from forward scatter in FACS analysis and adhesion to human umbilical vein endothelial cells (HUVEC) utilizing a flow chamber. As a result, ionomycin, sucrose addition and glucose removal all triggered phosphatidylserine-exposure, decreased forward scatter and enhanced adhesion of erythrocytes to human umbilical vein endothelial cells (HUVEC), effects significantly more pronounced in anx7(-/-) than in anx7(+/+)-erythrocytes. Following ischemia, morphological renal injury was significantly higher in anx7(-/-) than in anx7(+/+)-mice. The present observations demonstrate that enhanced eryptosis of annexin7 deficient cells is paralleled by increased adhesion of erythrocytes to the vascular wall, an effect, which may impact on microcirculation during ischemia.

Enhanced suicidal erythrocyte death in mice carrying a loss-of-function mutation of the adenomatous polyposis coli gene

Loss-of-function mutations in human adenomatous polyposis coli (APC) lead to multiple colonic adenomatous polyps eventually resulting in colonic carcinoma. Similarly, heterozygous mice carrying defective APC (apc^Min/+) suffer from intestinal tumours. The animals further suffer from anaemia, which in theory could result from accelerated eryptosis, a suicidal erythrocyte death triggered by enhanced cytosolic Ca²⁺ activity and characterized by cell membrane scrambling and cell shrinkage. To explore, whether APC-deficiency enhances eryptosis, we estimated cell membrane scrambling from annexin V binding, cell size from forward scatter and cytosolic ATP utilizing luciferin–luciferase in isolated erythrocytes from apc^Min/+ mice and wild-type mice (apc^+/+). Clearance of circulating erythrocytes was estimated by carboxyfluorescein-diacetate-succinimidyl-ester labelling. As a result, apc^Min/+ mice were anaemic despite reticulocytosis. Cytosolic ATP was significantly lower and annexin V binding significantly higher in apc^Min/+ erythrocytes than in apc^+/+ erythrocytes. Glucose depletion enhanced annexin V binding, an effect significantly more pronounced in apc^Min/+ erythrocytes than in apc^+/+ erythrocytes. Extracellular Ca²⁺ removal or inhibition of Ca²⁺ entry with amiloride (1 mM) blunted the increase but did not abrogate the genotype differences of annexin V binding following glucose depletion. Stimulation of Ca²⁺-entry by treatment with Ca²⁺-ionophore ionomycin (10 μM) increased annexin V binding, an effect again significantly more pronounced in apc^Min/+ erythrocytes than in apc^+/+ erythrocytes. Following retrieval and injection into the circulation of the same mice, apc^Min/+ erythrocytes were more rapidly cleared from circulating blood than apc^+/+ erythrocytes. Most labelled erythrocytes were trapped in the spleen, which was significantly enlarged in apc^Min/+ mice. The observations point to accelerated eryptosis and subsequent clearance of apc^Min/+ erythrocytes, which contributes to or even accounts for the enhanced erythrocyte turnover, anaemia and splenomegaly in those mice.

Carbon monoxide-sensitive apoptotic death of erythrocytes

Carbon monoxide (CO) intoxication severely interferes with the oxygen-transporting function of haemoglobin. Beyond that, CO participates in the regulation of apoptosis. CO could be generated from CO-releasing molecules (CORM), such as the tricarbonyl-dichlororuthenium (II) dimer (CORM-2), which is presently considered for the treatment of vascular dysfunction, inflammation, tissue ischaemia and organ rejection. CORM-2 is at least partially effective by modifying gene expression and mitochondrial potential. Erythrocytes lack nuclei and mitochondria but may undergo suicidal cell death or eryptosis, characterized by cell shrinkage and phospholipid scrambling of the cell membrane. Eryptosis is triggered by the increase in cytosolic Ca²⁺ activity ([Ca²⁺](i)). The present study explored whether CORM-2 influences eryptosis. To this end, [Ca²⁺](i) was estimated from Fluo-3-fluorescence, cell volume from forward scatter, phospholipid scrambling from annexin-V-binding and haemolysis from haemoglobin release. CO-binding haemoglobin (COHb) was estimated utilizing a blood gas analyser. As a result, exposure of erythrocytes for 24 hr to CORM-2 (≥5 μM) significantly increased COHb, [Ca²⁺](i) , forward scatter, annexin-V-binding and haemolysis. Annexin-V-binding was significantly blunted by 100% oxygen and was virtually abolished in the nominal absence of Ca²⁺. In conclusion, CORM-2 stimulates cell membrane scrambling of erythrocytes, an effect largely due to Ca²⁺ entry and partially reversed by O₂.

Dynamic adhesion of eryptotic erythrocytes to endothelial cells via CXCL16/SR-PSOX

Suicidal death of erythrocytes, or eryptosis, is characterized by cell shrinkage and cell membrane scrambling leading to phosphatidylserine exposure at the cell surface. Eryptosis is triggered by increase of cytosolic Ca2+activity, which may result from treatment with the Ca2+ionophore ionomycin or from energy depletion by removal of glucose. The present study tested the hypothesis that phosphatidylserine exposure at the erythrocyte surface fosters adherence to endothelial cells of the vascular wall under flow conditions at arterial shear rates and that binding of eryptotic cells to endothelial cells is mediated by the transmembrane CXC chemokine ligand 16 (CXCL16). To this end, human erythrocytes were exposed to energy depletion (for 48 h) or treated with the Ca2+ionophore ionomycin (1 μM for 30 min). Phosphatidylserine exposure was quantified utilizing annexin-V binding, cell volume was estimated from forward scatter in FACS analysis, and erythrocyte adhesion to human vascular endothelial cells (HUVEC) was determined in a flow chamber model. As a result, both, ionomycin and glucose depletion, triggered eryptosis and enhanced the percentage of erythrocytes adhering to HUVEC under flow conditions at arterial shear rates. The adhesion was significantly blunted in the presence of erythrocyte phosphatidylserine-coating annexin-V (5 μl/ml), of a neutralizing antibody against endothelial CXCL16 (4 μg/ml), and following silencing of endothelial CXCL16 with small interfering RNA. The present observations demonstrate that eryptotic erythrocytes adhere to endothelial cells of the vascular wall in part by interaction of phosphatidylserine exposed at the erythrocyte surface with endothelial CXCL16.

Dicoumarol activates Ca2+-permeable cation channels triggering erythrocyte cell membrane scrambling

Dicoumarol, a widely used anticoagulant, may cause anemia, which may result from enhanced erythrocyte loss due to bleeding or due to accelerated erythrocyte death. Erythrocytes may undergo suicidal death or eryptosis, characterized by cell shrinkage and phospholipid scrambling of the cell membrane. Eryptosis may be triggered by increase of cytosolic Ca(2+)-activity ([Ca(2+)](i)). The present study explored, whether dicoumarol induces eryptosis. [Ca(2+)](i) was estimated from Fluo3-fluorescence, cation channel activity utilizing whole cell patch clamp, cell volume from forward scatter, phospholipid scrambling from annexin-V-binding, and hemolysis from haemoglobin release. Exposure of erythrocytes for 48 hours to dicoumarol (=10 μM) significantly increased [Ca(2+)](i), enhanced cation channel activity, decreased forward scatter, triggered annexin-V-binding and elicited hemolysis. Following exposure to 30 μM dicoumarol, annexin-V-binding affected approximately 15%, and hemolysis 2% of treated erythrocytes. The stimulation of annexin-V-binding by dicoumarol was abrogated in the nominal absence of Ca(2+). In conclusion, dicoumarol stimulates suicidal death of erythrocytes by stimulating Ca(2+) entry and subsequent triggering of Ca(2+) dependent cell membrane scrambling.

Regulation of phosphatidylserine exposure in red blood cells

The exposure of phosphatidylserine (PS) on the outer membrane leaflet of red blood cells (RBCs) serves as a signal for eryptosis, a mechanism for the RBC clearance from blood circulation. The process of PS exposure was investigated as function of the intracellular Ca(2+) content and the activation of PKCα in human and sheep RBCs. Cells were treated with lysophosphatidic acid (LPA), 4-bromo-A23187, or phorbol-12 myristate-13 acetate (PMA) and analysed by flow cytometry, single cell fluorescence video imaging, or confocal microscopy. For human RBCs, no clear correlation existed between the number of cells with an elevated Ca(2+) content and PS exposure. Results are explained by three different mechanisms responsible for the PS exposure in human RBCs: (i) Ca(2+)-stimulated scramblase activation (and flippase inhibition) by LPA, 4-bromo-A23187, and PMA; (ii) PKC activation by LPA and PMA; and (iii) enhanced lipid flop caused by LPA. In sheep RBCs, only the latter mechanism occurs suggesting absence of scramblase activity.

Passive transport pathways for Ca(2+) and Co(2+) in human red blood cells. (57)Co(2+) as a tracer for Ca(2+) influx

The passive transport of calcium and cobalt and their interference were studied in human red cells using (45)Ca and (57)Co as tracers. In ATP-depleted cells, with the ATP concentration reduced to about 1μM, the progress curve for (45)Ca uptake at 1mM rapidly levels off with time, consistent with a residual Ca-pump activity building up at increasing [Ca(T)](c) to reach at [Ca(T)](c) about 5μmol(lcells)(-1) a maximal pump rate that nearly countermands the passive Ca influx, resulting in a linear net uptake at a low level. In ATP-depleted cells treated with vanadate, supposed to cause Ca-pump arrest, a residual pump activity is still present at high [Ca(T)](c). Moreover, vanadate markedly increases the passive Ca(2+) influx. The residual Ca-pump activity in ATP-depleted cells is fuelled by breakdown of the large 2,3-DPG pool, rate-limited by the sustainable ATP-turnover at about 40-50μmol(lcells)(-1)h(-1). The apparent Ca(2+) affinity of the Ca-pump appears to be markedly reduced compared to fed cells. The 2,3-DPG breakdown can be prevented by inhibition of the 2,3-DPG phosphatase by tetrathionate, and under these conditions the (45)Ca uptake is markedly increased and linear with time, with the unidirectional Ca influx at 1mM Ca(2+) estimated at 50-60μmol(lcells)(-1)h(-1). The Ca influx increases with the extracellular Ca(2+) concentration with a saturating component, with K(½(Ca)) about 0.3mM, plus a non-saturating component. From (45)Ca-loaded, ATP-depleted cells the residual Ca-pump can also be detected as a vanadate- and tetrathionate-sensitive efflux. The (45)Ca efflux is markedly accelerated by external Ca(2+), both in control cells and in the presence of vanadate or tetrathionate, suggesting efflux by carrier-mediated Ca/Ca exchange. The (57)Co uptake is similar in fed cells and in ATP-depleted cells (exposed to iodoacetamide), consistent with the notion that Co(2+) is not transported by the Ca-pump. The transporter is thus neither SH-group nor ATP or phosphorylation dependent. The (57)Co uptake shows several similarities with the (45)Ca uptake in ATP-depleted cells supplemented with tetrathionate. The uptake is linear with time, and increases with the cobalt concentration with a saturating component, with J(max) about 16μmol(lcells)(-1)h(-1) and K(½(Co)) about 0.1mM, plus a non-saturating component. The (57)Co and (45)Ca uptake shows mutual inhibition, and at least the stochastic Ca(2+) influx is inhibited by Co(2+). The (57)Co and (45)Ca uptake are both insensitive to the 1,4-dihydropyridine Ca-channel blocker nifedipine, even at 100μM. The (57)Co uptake is increased at high negative membrane potentials, indicating that the uptake is at least partially electrogenic. The (57)Co influx amounts to about half the (45)Ca influx in ATP-depleted cells. It is speculated that the basal Ca(2+) and Co(2+) uptake could be mediated by a common transporter, probably with a channel-like and a carrier-mediated component, and that (57)Co could be useful as a tracer for at least the channel-like Ca(2+) entry pathway in red cells, since it is not itself transported by the Ca-pump and, moreover, is effectively buffered in the cytosol by binding to hemoglobin, without interfering with Ca(2+) buffering. The molecular identity of the putative common transporter(s) remains to be defined.

Bi-stability, hysteresis, and memory of voltage-gated lysenin channels

Lysenin, a 297 amino acid pore-forming protein extracted from the coelomic fluid of the earthworm E. foetida, inserts constitutively open large conductance channels in natural and artificial lipid membranes containing sphingomyelin. The inserted channels show voltage regulation and slowly close at positive applied voltages. We report on the consequences of slow voltage-induced gating of lysenin channels inserted into a planar Bilayer Lipid Membrane (BLM), and demonstrate that these pore-forming proteins constitute memory elements that manifest gating bi-stability in response to variable external voltages. The hysteresis in macroscopic currents dynamically changes when the time scale of the voltage variation is smaller or comparable to the characteristic conformational equilibration time, and unexpectedly persists for extremely slow-changing external voltage stimuli. The assay performed on a single lysenin channel reveals that hysteresis is a fundamental feature of the individual channel unit and an intrinsic component of the gating mechanism. The investigation conducted at different temperatures reveals a thermally stable reopening process, suggesting that major changes in the energy landscape and kinetics diagram accompany the conformational transitions of the channels. Our work offers new insights on the dynamics of pore-forming proteins and provides an understanding of how channel proteins may form an immediate record of the molecular history which then determines their future response to various stimuli. Such new functionalities may uncover a link between molecular events and macroscopic processing and transmission of information in cells, and may lead to applications such as high density biologically-compatible memories and learning networks.

Band 3 missense mutations and stomatocytosis: insight into the molecular mechanism responsible for monovalent cation leak

Missense mutations in the erythroid band 3 protein (Anion Exchanger 1) have been associated with hereditary stomatocytosis. Features of cation leaky red cells combined with functional expression of the mutated protein led to the conclusion that the AE1 point mutations were responsible for Na(+) and K(+) leak through a conductive mechanism. A molecular mechanism explaining mutated AE1-linked stomatocytosis involves changes in AE1 transport properties that become leaky to Na(+) and K(+). However, another explanation suggests that point-mutated AE1 could regulate a cation leak through other transporters. This short paper intends to discuss these two alternatives.

Lysophosphatidic acid induced red blood cell aggregation in vitro

Under physiological conditions healthy RBCs do not adhere to each other. There are indications that RBCs display an intercellular adhesion under certain (pathophysiological) conditions. Therefore we investigated signaling steps starting with transmembrane calcium transport by means of calcium imaging. We found a lysophosphatidic acid (LPA) concentration dependent calcium influx with an EC(50) of 5 μM LPA. Downstream signaling was investigated by flow cytometry as well as by video-imaging comparing LPA induced with "pure" calcium mediated phosphatidylserine exposure and concluded the coexistence of two branches of the signaling pathway. Finally we performed force measurements with holographic optical tweezers (HOT): The intercellular adhesion of RBCs (aggregation) exceeds a force of 25 pN. These results support (i) earlier data of a RBC associated component in thrombotic events under certain pathophysiological conditions and (ii) the concept to use RBCs in studies of cellular adhesion behavior, especially in combination with HOT. The latter paves the way to use RBCs as model cells to investigate molecular regulation of cellular adhesion processes.

Stimulation of suicidal erythrocyte death by benzethonium

Benzethonium, an antimicrobial surfactant widely used as preservative of pharmaceuticals, topical wound care product and oral disinfectant, triggers apoptosis of several cell types. The apoptosis is preceded and possibly triggered by mitochondrial depolarization. Even though lacking mitochondria, erythrocytes may similarly undergo suicidal cell death or eryptosis. Hallmarks of eryptosis include cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Eryptosis may be triggered by energy depletion, which leads to increase of cytosolic Ca(2+)-activity with subsequent Ca(2+)-sensitive cell shrinkage and cell membrane scrambling. Ca(2+)-sensitivity is enhanced by ceramide. The present study explored the effect of benzethonium on eryptosis. Cell membrane scrambling was estimated from binding of fluorescent annexin V to phosphatidylserine, cell volume from forward scatter in FACS analysis, cytosolic Ca(2+)-concentration from Fluo3-fluorescence, hemolysis from hemoglobin release, lactate formation by colorimetry and ceramide utilizing fluorescent antibodies. A 48 hours exposure to benzethonium (=5μM) significantly increased cytosolic Ca(2+)-concentration, decreased forward scatter and triggered annexin V-binding affecting some 30% of the erythrocytes at 5 μM benzethonium. Only 5% of treated erythrocytes were hemolytic. The effects of benzethonium on annexin V binding were blunted in the nominal absence of Ca(2+) and in the presence of amiloride (1 mM) but not in the presence of the pancaspase inhibitor zVAD (10 μM). Benzethonium further significantly enhanced the effect of glucose depletion on cytosolic Ca(2+)-concentration and annexin V-binding, but significantly blunted the effect of glucose depletion on forward scatter. Benzethonium (5 μM) significantly enhanced lactic acid formation but not ceramide abundance. The present observations disclose a novel effect of benzethonium, i.e. triggering of suicidal death of erythrocytes.

Phlorhizin protects against erythrocyte cell membrane scrambling

Phlorhizin interferes with glucose transport. Glucose depletion triggers suicidal erythrocyte death or eryptosis, characterized by cell shrinkage and cell membrane scrambling. Eryptosis is further triggered by oxidative stress. The present study explored whether phlorhizin influences eryptosis following glucose depletion or oxidative stress. Cell membrane scrambling was estimated from annexin binding, cell volume from forward scatter (FSC), and cytosolic Ca(2+) concentration from Fluo-3 fluorescence. Phlorhizin (10-100 μM) added alone did not modify scrambling, FSC, or Fluo-3 fluorescence. Glucose depletion (48 h) significantly increased Fluo-3 fluorescence, decreased FSC, and increased annexin binding, effects in part significantly blunted by phlorhizin (annexin binding ≥ 10 μM, FSC ≥ 50 μM). Oxidative stress (30 min 0.3 mM tert-butylhydroperoxide) again significantly increased Fluo-3 fluorescence and triggered annexin binding, effects again in part significantly blunted by phlorhizin (Fluo-3 fluorescence ≥ 50 μM, annexin-binding ≥ 10 μM). Phlorhizin did not blunt the cell shrinkage induced by oxidative stress. The present observations disclose a novel effect of phlorhizin, that is, an influence on suicidal erythrocyte death following energy depletion and oxidative stress.

Stimulation of erythrocyte phospholipid scrambling by enniatin A

SCOPE

Enniatin A, a peptide antibiotic and common food contaminant, triggers mitochondrial dysfunction and apoptosis. Even though lacking mitochondria, erythrocytes may similarly undergo suicidal cell death or eryptosis. Eryptosis is characterized by cell shrinkage and cell membrane phospholipid scrambling. Triggers of phospholipid scrambling include energy depletion and increase in cytosolic Ca(2+) activity ([Ca(2+) ](i) ). The present study explored whether enniatin A triggers phospholipid scrambling.

METHODS AND RESULTS

Phospholipid scrambling was estimated from annexin-V-binding, cell volume from forward scatter (FSC), [Ca(2+) ](i) from Fluo3-fluorescence, cytosolic ATP-concentration ([ATP](i) ) using a luciferase assay and hemolysis from hemoglobin release. Exposure of erythrocytes for 48 h to enniatin A (≥ 2.5 μM) significantly increased [Ca(2+) ](i) , decreased [ATP](i) , decreased FSC, triggered annexin-V-binding and elicited hemolysis. Annexin-V-binding affected 25%, and hemolysis 2% of treated erythrocytes. Decreased [ATP](i) by glucose depletion for 48 h was similarly followed by increased [Ca(2+) ](i) , decreased FSC and annexin-V-binding. Enniatin A augmented the effect on [Ca(2+) ](i) and annexin-V-binding, but not on FSC. Annexin-V-binding was blunted by Ca(2+) removal, by the cation channel inhibitor amiloride (1 mM), by the protein kinase C inhibitor staurosporine (500 nM) but not by the pancaspase inhibitor zVAD (10 μM).

CONCLUSION

The food contaminant enniatin A triggers ATP depletion and increases cytosolic Ca(2+) activity, effects resulting in suicidal erythrocyte death.

Stimulation of human red blood cells leads to Ca2+-mediated intercellular adhesion

Red blood cells (RBCs) are a major component of blood clots, which form physiologically as a response to injury or pathologically in thrombosis. The active participation of RBCs in thrombus solidification has been previously proposed but not yet experimentally proven. Holographic optical tweezers and single-cell force spectroscopy were used to study potential cell-cell adhesion between RBCs. Irreversible intercellular adhesion of RBCs could be induced by stimulation with lysophosphatidic acid (LPA), a compound known to be released by activated platelets. We identified Ca(2+) as an essential player in the signaling cascade by directly inducing Ca(2+) influx using A23187. Elevation of the internal Ca(2+) concentration leads to an intercellular adhesion of RBCs similar to that induced by LPA stimulation. Using single-cell force spectroscopy, the adhesion of the RBCs was identified to be approximately 100 pN, a value large enough to be of significance inside a blood clot or in pathological situations like the vasco-occlusive crisis in sickle cell disease patients.

Inhibition of suicidal erythrocyte death by blebbistatin

Blebbistatin, a myosin II inhibitor, interferes with myosin-actin interaction and microtubule assembly. By influencing cytoskeletal dynamics blebbistatin counteracts apoptosis of several types of nucleated cells. Even though lacking nuclei and mitochondria, erythrocytes may undergo suicidal cell death or eryptosis, which is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include energy depletion and osmotic shock, which enhance cytosolic Ca(2+) activity with subsequent Ca(2+)-sensitive cell shrinkage and cell membrane scrambling. The present study explored the effect of blebbistatin on eryptosis. Cell membrane scrambling was estimated from binding of annexin V to phosphatidylserine at the erythrocyte surface, cell volume from forward scatter in fluorescence-activated cell sorting analysis and cytosolic Ca(2+) concentration from Fluo3 fluorescence. Exposure to blebbistatin on its own (1-50 μM) did not significantly modify cytosolic Ca(2+) concentration, forward scatter, or annexin V binding. Glucose depletion (48 h) was followed by a significant increase of Fluo3 fluorescence and annexin V binding, effects significantly blunted by blebbistatin (Fluo3 fluorescence ≥ 25 μM, annexin V binding ≥ 10 μM). Osmotic shock (addition of 550 mM sucrose) again significantly increased Fluo3 fluorescence and annexin binding, effects again significantly blunted by blebbistatin (Fluo3 fluorescence ≥ 25 μM, annexin V binding ≥ 25 μM). The present observations disclose a novel effect of blebbistatin, i.e., an influence on Ca(2+) entry and suicidal erythrocyte death following energy depletion and osmotic shock.

Beauvericin induced erythrocyte cell membrane scrambling

Beauvericin is a mycotoxin with antiviral, antibacterial, nematicidal, insecticidal, cytotoxic, and apoptotic activity. Similar to nucleated cells erythrocytes may undergo suicidal death or eryptosis, which is characterized by cell shrinkage and phosphatidylserine exposure at the erythrocyte surface. Eryptosis may be triggered by energy depletion leading to increase of cytosolic Ca²+ activity. The present study thus explored whether beauvericin is able to trigger eryptosis and influence eryptosis following energy depletion. Cell membrane scrambling was estimated from binding of annexin V to phosphatidylserine at the erythrocyte surface, cell volume from forward scatter in FACS analysis, cytosolic Ca²+ concentration from Fluo3 fluorescence, cytosolic ATP concentration from a luciferase-assay and ion channel activity with whole cell patch clamp. Exposure to beauvericin (≥ 5 μM) significantly decreased erythrocyte ATP concentration and increased cytosolic Ca²+ concentration as well as annexin V-binding. The effect of beauvericin on annexin V binding was significantly blunted by removal of extracellular Ca²+. Glucose depletion (48 h) was followed by, increase of Fluo3 fluorescence, decrease of forward scatter and increase of annexin V-binding. Beauvericin (≥ 1 μM) augmented the effect of glucose withdrawal on Fluo3 fluorescence and annexin V-binding, but significantly blunted the effect of glucose withdrawal on forward scatter, an effect paralleled by inhibition of Ca²+ activated K+ channels. The present observations disclose novel effects of beauvericin, i.e. stimulation of Ca²+ entry with subsequent cell membrane scrambling and inhibition of Ca²+ activated K+ channels with blunting of cell shrinkage.

Stimulation of suicidal erythrocyte death by α-lipoic acid

α-lipoic acid, a nutrient with both, antioxidant and oxidant activity induces apoptosis in a variety of cells. Owing to its proapoptotic potency α-lipoic acid has been suggested for the therapy of cancer. α-Lipoic acid stimulates apoptosis by induction of oxidative stress and subsequent activation of caspases. Oxidative stress could similarly trigger caspase activation and suicidal erythrocyte death or eryptosis, which is characterized by cell membrane scrambling and cell shrinkage. Eryptosis is triggered by increase of cytosolic Ca(2+) concentration and/or ceramide formation. The present study explored whether α -lipoic acid influences eryptosis. Cell membrane scrambling was estimated from binding of annexin V to phosphatidylserine at the erythrocyte surface, cell volume from forward scatter in FACS analysis, cytosolic Ca(2+) concentration from Fluo3 fluorescence, caspase activation and ceramide formation utilizing respective antibodies, cytosolic ATP concentration from a luciferase-assay. Within 48 hours, exposure to α-lipoic acid (10 - 75 mM) significantly decreased forward scatter, increased cytosolic Ca(2+) concentration, decreased ATP concentration, activated caspase 3, stimulated formation of ceramide and triggered annexin V-binding. Glucose depletion (48 h) was followed by decrease of forward scatter and increase of annexin V-binding, effects significantly augmented in the presence of α-lipoic acid (20 mM). Oxidative stress (30 min 0.3 mM tert-butylhydroperoxide) similarly triggered annexin binding, an effect slightly but significantly blunted by α-lipoic acid. In conclusion, α-lipoic acid triggers eryptosis but by the same token counteracts eryptosis during oxidative stress. α-lipoic acid sensitive eryptosis may lead to anemia and derangements of microcirculation.

Inhibition of cation channels in human erythrocytes by spermine

In erythrocytes, spermine concentration decreases gradually with age, which is paralleled by increases of cytosolic Ca²+ concentration, with subsequent cell shrinkage and cell membrane scrambling. Cytosolic Ca²+ was estimated from fluo-3 fluorescence, cell volume from forward scatter, cell membrane scrambling from annexin V binding and cation channel activity with whole-cell patch-clamp in human erythrocytes. Extracellular spermine exerted a dual effect on erythrocyte survival. At 200 μM spermine blunted the increase of intracellular Ca²+, cell shrinkage and annexin V binding following 48 h exposure of cells at +37 °C. In contrast, short exposure (10-30 min) of cells to 2 mM spermine was accompanied by increased cytosolic Ca²+ and annexin binding. Intracellular addition of spermine at subphysiological concentration (0.2 μM) significantly decreased the conductance of monovalent cations (Na+, K+, NMDG+) and of Ca²+. Moreover, spermine (0.2 μM) blunted the stimulation of voltage-independent cation channels by Cl⁻ removal. Spermine (0.2 and 200 μM) added to the extracellular bath solution similarly inhibited the cation conductance in Cl⁻-containing bath solution. The effect of 0.2 μM spermine, but not the effect of 200 μM, was rapidly reversible. Acute addition (250 μM) of a naphthyl acetyl derivative of spermine (200 μM) again significantly decreased basal cation conductance in NaCl bath solution and inhibited voltage-independent cation channels. Spermine is a powerful regulator of erythrocyte cation channel cytosolic Ca²+ activity and, thus, cell survival.

Increased cation conductance in human erythrocytes artificially aged by glycation

Excessive glucose concentrations foster glycation and thus premature aging of erythrocytes. The present study explored whether glycation-induced erythrocyte aging is paralleled by features of suicidal erythrocyte death or eryptosis, which is characterized by cell membrane scrambling with subsequent phosphatidylserine exposure at the cell surface and cell shrinkage. Both are triggered by increases of cytosolic Ca(2+) concentration ([Ca(2+)](i)), which may result from activation of Ca(2+) permeable cation channels. Glycation was accomplished by exposure to high glucose concentrations (40 and 100 mM), phosphatidylserine exposure estimated from annexin binding, cell shrinkage from decrease of forward scatter, and [Ca(2+)](i) from Fluo3-fluorescence in analysis via fluorescence-activated cell sorter. Cation channel activity was determined by means of whole-cell patch clamp. Glycation of total membrane proteins, immunoprecipitated TRPC3/6/7, and immunoprecipitated L-type Ca(2+) channel proteins was estimated by Western blot testing with polyclonal antibodies used against advanced glycation end products. A 30-48-h exposure of the cells to 40 or 100 mM glucose in Ringer solution (at 37 degrees C) significantly increased glycation of membrane proteins, hemoglobin (HbA(1c)), TRPC3/6/7, and L-type Ca(2+) channel proteins, enhanced amiloride-sensitive, voltage-independent cation conductance, [Ca(2+)](i), and phosphatidylserine exposure, and led to significant cell shrinkage. Ca(2+) removal and addition of Ca(2+) chelator EGTA prevented the glycation-induced phosphatidylserine exposure and cell shrinkage after glycation. Glycation-induced erythrocyte aging leads to eryptosis, an effect requiring Ca(2+) entry from extracellular space.

Cell biology and physiology of the uroepithelium

The uroepithelium sits at the interface between the urinary space and underlying tissues, where it forms a high-resistance barrier to ion, solute, and water flux, as well as pathogens. However, the uroepithelium is not simply a passive barrier; it can modulate the composition of the urine, and it functions as an integral part of a sensory web in which it receives, amplifies, and transmits information about its external milieu to the underlying nervous and muscular systems. This review examines our understanding of uroepithelial regeneration and how specializations of the outermost umbrella cell layer, including tight junctions, surface uroplakins, and dynamic apical membrane exocytosis/endocytosis, contribute to barrier function and how they are co-opted by uropathogenic bacteria to infect the uroepithelium. Furthermore, we discuss the presence and possible functions of aquaporins, urea transporters, and multiple ion channels in the uroepithelium. Finally, we describe potential mechanisms by which the uroepithelium can transmit information about the urinary space to the other tissues in the bladder proper.

Modulation of suicidal erythrocyte cation channels by an AMPA antagonist

In neurons alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are heteromeric cation channels composed of different sub-units, including GluA1-GluA4. When expressed without GluA2, AMPA receptors function as Ca²⁺-permeable cation channels. In erythrocytes, activation of Ca²⁺-permeable cation channels triggers suicidal erythrocyte death or eryptosis, which is characterized by cell shrinkage and cell membrane scrambling with subsequent exposure of phosphatidylserine at the cell surface. Activators of the channels and thus eryptosis include removal of extracellular Cl⁻ (replaced by gluconate) and energy depletion (removal of glucose). The present study explored whether GluA1 is expressed in human erythrocytes and whether pharmacological AMPA receptor inhibition modifies Ca²⁺ entry and suicidal death of human erythrocytes. GluA1 protein abundance was determined by confocal microscopy, phosphatidylserine exposure was estimated from annexin V binding, cell volume from forward scatter in FACS analysis, cytosolic Ca²⁺ concentration from Fluo3 fluorescence and channel activity by whole-cell patch-clamp recordings. As a result, GluA1 is indeed expressed in the erythrocyte cell membrane. The AMPA receptor antagonist NBQX (1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide) inhibited the cation channels following Cl⁻ removal and the eryptosis following Cl⁻ removal or energy depletion. The present study reveals a novel action of AMPA receptor antagonists and raises the possibility that GluA1 or a pharmacologically related protein participates in the regulation of Ca²⁺ entry into and suicidal death of human erythrocytes.

Further characterization of cation channels present in the chicken red blood cell membrane

In this paper, we provide an update on cation channels in nucleated chicken erythrocytes. Patch-clamp techniques were used to further characterize the two different types of cation channels present in the membrane of chicken red blood. In the whole-cell mode, with Ringer in the bath and internal K+ saline in the pipette solution, the membrane conductance was generated by cationic currents, since the reversal potential was shifted toward cations equilibrium when the impermeant cation NMDG was substituted to small cations. The membrane conductance could be increased by application of mechanical deformation or by the addition of agonists of the cAMP-dependent pathway. At the unitary level, two different types of cationic channels were revealed and could account for the cationic conductance observed in whole-cell configuration. One of them belongs to the family of stretch-activated cationic channel showing changes in activity under conditions of membrane deformation, whereas the second one belongs to the family of the cAMP activated cationic channels. These two channels could be distinguished according to their unitary conductances and drug sensitivities. The stretch-activated channel was sensitive to Gd(3+) and the cAMP-dependent channel was sensitive to flufenamic acid. Possible role of these channels in cell volume regulation process is discussed.

Vanadate-induced suicidal erythrocyte death

Vanadium, a trace element, as vanadate (VO4(3-)) is known to interfere with a wide variety of enzymes including Ca2+ ATPase and Na+/+ ATPase. VO4(3-) is excreted mainly via the kidney. In renal insufficiency, the impaired VO4(3-) excretion leads to VO4(3-) accumulation in blood.The present study explored the effect of VO4(3-) on eryptosis, the suicidal death of erythrocytes. Eryptosis is characterized by cell shrinkage and phosphatidylserine exposure at the erythrocyte surface. Eryptotic cells are phagocytosed and thus rapidly cleared from circulating blood. Stimulators of eryptosis include an increase of the cytosolic Ca2+ concentration. Erythrocyte Ca2+ activity was estimated from Fluo-3 fluorescence, phosphatidylserine exposure from annexin V-binding, and erythrocyte volume from forward scatter in FACS analysis. Exposure of erythrocytes to VO4(3-) increased cytosolic Ca2+ concentration, enhanced the percentage of annexin V-binding erythrocytes, decreased erythrocyte forward scatter, and lowered the intracellular ATP concentration. In conclusion, VO4(3-) induces eryptosis at least partially through increase of cytosolic Ca2+ concentration, an effect presumably contributing to the development of anemia in chronic renal failure.