Temperature sensitivity of suicidal erythrocyte death

Hemocompatibility studies in nanotoxicology: Hemolysis or eryptosis? (A review)

Hemocompatibility evaluation is an important step in nanotoxicological studies. It is generally accepted that nanomaterials promote lysis of erythrocytes, blood clotting, alter phagocytosis, and upregulate pro-inflammatory cytokines. However, there are no standardized guidelines for testing nanomaterials hemocompatibility despite the fact that nanomaterials enter the bloodstream and interact with blood cells. In this review, the current knowledge on the ability of nanomaterials to induce distinct cell death modalities of erythrocytes is highlighted primarily focusing on hemolysis and eryptosis. This review aims to summarize the molecular mechanisms underlying erythrotoxicity of nanomaterials and critically compare the sensitivity and efficiency of hemolysis or eryptosis assays for nanomaterials blood compatibility testing. The list of eryptosis-inducing nanomaterials is growing, but it is still difficult to generalize how physico-chemical properties of nanoparticles affect eryptosis degree and molecular mechanisms involved. Thus, another aim of this review is to raise the awareness of eryptosis as a nanotoxicological tool to encourage the corresponding studies. It is worthwhile to consider adding eryptosis to in vitro nanomaterials hemocompatibility testing protocols and guidelines.

SARS-CoV-2 Impact on Red Blood Cell Morphology

Severe COVID-19 alters the biochemical and morphological characteristics of blood cells in a wide variety of ways. To date, however, the vast majority of research has been devoted to the study of leukocytes, while erythrocyte morphological changes have received significantly less attention. The aim of this research was to identify erythrocyte morphology abnormalities that occur in COVID-19, compare the number of different poikilocyte types, and measure erythrocyte sizes to provide data on size dispersion. Red blood cells obtained from 6 control donors (800-2200 cells per donor) and 5 COVID-19 patients (800-1900 cells per patient) were examined using low-voltage scanning electron microscopy. We did not discover any forms of erythrocyte morphology abnormalities that would be specific to COVID-19. Among COVID-19 patients, we observed an increase in the number of acanthocytes (p = 0.01) and a decrease in the number of spherocytes (p = 0.03). In addition, our research demonstrates that COVID-19 causes an increase in the median (p = 0.004) and interquartile range (p = 0.009) when assessing erythrocyte size. The limitation of our study is a small number of participants.

Molecular Mechanisms and Pathophysiological Significance of Eryptosis

Despite lacking the central apoptotic machinery, senescent or damaged RBCs can undergo an unusual apoptosis-like cell death, termed eryptosis. This premature death can be caused by, or a symptom of, a wide range of diseases. However, various adverse conditions, xenobiotics, and endogenous mediators have also been recognized as triggers and inhibitors of eryptosis. Eukaryotic RBCs are unique among their cell membrane distribution of phospholipids. The change in the RBC membrane composition of the outer leaflet occurs in a variety of diseases, including sickle cell disease, renal diseases, leukemia, Parkinson’s disease, and diabetes. Eryptotic erythrocytes exhibit various morphological alterations such as shrinkage, swelling, and increased granulation. Biochemical changes include cytosolic Ca2+ increase, oxidative stress, stimulation of caspases, metabolic exhaustion, and ceramide accumulation. Eryptosis is an effective mechanism for the elimination of dysfunctional erythrocytes due to senescence, infection, or injury to prevent hemolysis. Nevertheless, excessive eryptosis is associated with multiple pathologies, most notably anemia, abnormal microcirculation, and prothrombotic risk; all of which contribute to the pathogenesis of several diseases. In this review, we provide an overview of the molecular mechanisms, physiological and pathophysiological relevance of eryptosis, as well as the potential role of natural and synthetic compounds in modulating RBC survival and death.

Clot-targeted magnetic hyperthermia permeabilizes blood clots to make them more susceptible to thrombolysis

BACKGROUND

Thrombolysis is a frontline treatment for stroke, which involves the application of tissue plasminogen activator (tPA) to trigger endogenous clot-degradation pathways. However, it is only effective within 4.5 h of symptom onset because of clot contraction preventing tPA permeation into the clot. Magnetic hyperthermia (MH) mediated by tumor-targeted magnetic nanoparticles is used to treat cancer by using local heat generation to trigger apoptosis of cancer cells.

OBJECTIVES

To develop clot-targeting magnetic nanoparticles to deliver MH to the surface of human blood clots, and to assess whether this can improve the efficacy of thrombolysis of contracted blood clots.

METHODS

Clot-targeting magnetic nanoparticles were developed by functionalizing iron oxide nanoparticles with an antibody recognizing activated integrin αIIbβ3 (PAC-1). The magnetic properties of the PAC-1-tagged magnetic nanoparticles were characterized and optimized to deliver clot-targeted MH.

RESULTS

Clot-targeted MH increases the efficacy of tPA-mediated thrombolysis in contracted human blood clots, leading to a reduction in clot weight. MH increases the permeability of the clots to tPA, facilitating their breakdown. Scanning electron microscopy reveals that this effect is elicited through enhanced fibrin breakdown and triggering the disruption of red blood cells on the surface of the clot. Importantly, endothelial cells viability in a three-dimensional blood vessel model is unaffected by exposure to MH.

CONCLUSIONS

This study demonstrates that clot-targeted MH can enhance the thrombolysis of contracted human blood clots and can be safely applied to enhance the timeframe in which thrombolysis is effective.

18F-fluorodeoxyglucose positron emission tomography-computed tomography for assessing organ distribution of stressed red blood cells in mice

Red blood cells (RBCs) stressed by high temperature are similar to senescent or damaged RBCs in pathological conditions. RBCs can be efficiently labelled with ¹⁸F-fluorodeoxyglucose (FDG). The aim of this study was to assess stressed RBCs erythrophagocytosis and organ distribution in vivo with the application of ¹⁸F-FDG PET/CT. RBCs were induced under high temperature (48 °C) to prepare stressed RBCs. Fluorescence-activated cell sorting (FACS) was used to analyse reactive oxygen species (ROS) generation, intracellular Ca²⁺ concentration and membrane phosphatidylserine (PS) externalization of RBCs. ¹⁸F-FDG was used to label RBCs and assess the erythrophagocytosis. Finally, ¹⁸F-FDG PET/CT was applied to reveal and measure the organ distribution of stressed RBCs in mice. Compared with untreated RBCs, stressed RBCs decreased in cell volume and increased in ROS level, intracellular Ca²⁺ concentration, and PS exposure. RBCs could be labelled by ¹⁸F-FDG. Stressed RBCs tended to be phagocytosed by macrophages via assessment of FACS and radioactivity. ¹⁸F-FDG PET/CT imaging showed that stressed RBCs were mainly trapped in spleen, while untreated RBCs remained in circulation system. Thus, stressed RBCs can be effectively labelled by ¹⁸F-FDG and tend to be trapped in spleen of mice as assessed by PET/CT.

Febrile Temperature Elevates the Expression of Phosphatidylserine on Plasmodium falciparum (FCR3CSA) Infected Red Blood Cell Surface Leading to Increased Cytoadhesion

During the asexual intra-erythrocytic cycle, Plasmodium (P.) falciparum exports parasitic proteins to the surface of infected red blood cells (iRBCs) facilitating its cytoadhesion to various endothelial host receptors. This adhesive behavior is a critical contributor towards disease manifestation. However, little is known about the influence of recurring elevated temperature – a common symptom of the malaria infection – on the adhesive properties of iRBCs to endothelial receptors. To address this, we performed dual-micropipette step-pressure technique between P. falciparum (strain FCR3CSA) iRBCs and Chinese Hamster Ovary cells expressing Chondroitin sulfate A (CHO-CSA) after transient iRBCs incubation at febrile temperatures which revealed increase in adhesion parameters. Furthermore, flow cytometry analysis revealed an increase in phosphatidylserine (PS) expression on the iRBC surface following exposure to febrile temperature. The adhesion between iRBCs and CHO-CSA cells was remarkably reduced in presence of soluble Annexin V, indicating the mediation of PS on the adhesion events. Our results suggest that elevated PS recruitment on iRBC under thermally stressed conditions contributes to the increased adhesive behavior of iRBCs CSA-binding phenotype to CHO-CSA.

Eryptosis in health and disease: A paradigm shift towards understanding the (patho)physiological implications of programmed cell death of erythrocytes

During the course of their natural ageing and upon injury, anucleate erythrocytes can undergo an unconventional apoptosis-like cell death, termed eryptosis. Eryptotic erythrocytes display a plethora of morphological alterations including volume reduction, membrane blebbing and breakdown of the membrane phospholipid asymmetry resulting in phosphatidylserine externalization which, in turn, mediates their phagocytic recognition and clearance from the circulation. Overall, the eryptosis machinery is tightly orchestrated by a wide array of endogenous mediators, ion channels, membrane receptors, and a host of intracellular signaling proteins. Enhanced eryptosis shortens the lifespan of circulating erythrocytes and confers a procoagulant phenotype; this phenomenon has been tangibly implicated in the pathogenesis of anemia, deranged microcirculation, and increased prothrombotic risk associated with a multitude of clinical conditions. Herein, we reviewed the molecular mechanisms dictating eryptosis and erythrophagocytosis and critically analyzed the current evidence leading to the pathophysiological ramifications of eryptotic cell death in the context of human disease.

Influences of breed, sex and age on seasonal changes in haematological variables of tropical goat kids

Abstract. The influences of breed, sex and age on seasonal changes in haematological variables of kids (1–4 months old) belonging to Red Sokoto (n  =  60) and Sahel (n  =  60) goats were studied at the peaks of the cold-dry, hot-dry and rainy seasons in a West Africa Guinea savanna climate. The results showed that, during the hot-dry season, Sahel goat kids had significantly higher (P  <  0.05) packed cell volume (PCV) and red blood cell (RBC) count but lower (P  <  0.05) mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC) and total leucocyte count than Red Sokoto kids. Similarly, younger kids of both breed had significantly higher (P  <  0.05) PCV and RBC but lower MCV, MCH and MCHC compared with the older kids during the hot-dry season. Younger kids (1–2 months) of both breeds exhibited significantly (P  <  0.05) high PCV and RBC but low MCH and MCHC during the hot-dry season as compared with the cold-dry and rainy seasons, with the magnitude of the change being greater in Sahel kids. More remarkable seasonal fluctuations in haematological parameters were observed in buck kids than doe kids of both breeds. Multivariate analysis revealed a clear distinction between the change in haematological parameters during the cold-dry as compared with the hot-dry and rainy seasons, with MCV having the strongest discriminating power (0.91*). In conclusion, breed, age and sex variations in haematological variables of goats were more pronounced in the hot-dry season, during which the seasonal changes were more dramatic in kids of Sahel goats, as well as in younger and male kids of both breeds. These findings may be useful in the management of tropical goat kids in different seasons.

Stimulation of Erythrocyte Cell Membrane Scrambling by C-Reactive Protein

Background: Eryptosis, the suicidal erythrocyte death characterized by cell shrinkage and phosphatidylserine-translocation, is triggered by fever and inflammation. Signaling includes increased cytosolic Ca2+-activity ([Ca2+]i), caspase activation, and ceramide. Inflammation is associated with increased plasma concentration of C-reactive protein (CRP). The present study explored whether CRP triggers eryptosis. Methods: Phosphatidylserine abundance at the cell surface was estimated from annexin-V-binding, cell volume from forward scatter, [Ca2+]i from Fluo3-fluorescence, ceramide abundance and caspase-3-activity utilizing FITC-conjugated antibodies. Moreover, blood was drawn from patients with acute appendicitis (9♀,11♂) and healthy volunteers (10♀,10♂) for determination of CRP, blood count and phosphatidylserine. Results: A 48h CRP treatment significantly increased the percentage of annexin-V-binding cells (≥5µg/ml), [Ca2+]i (≥5µg/ml), ceramide (20µg/ml) and caspase-activity (20µg/ml). Annexin-V-binding was significantly blunted by caspase inhibitor zVAD (10µM). The percentage of phosphatidylserine-exposing erythrocytes in freshly drawn blood was significantly higher in appendicitis patients (1.83±0.21%) than healthy volunteers (0.81±0.09%), and significantly higher following a 24h incubation of erythrocytes from healthy volunteers to patient plasma than to plasma from healthy volunteers. The percentage of phosphatidylserine-exposing erythrocytes correlated with CRP plasma concentration. Conclusion: C-reactive protein triggers eryptosis, an effect at least partially due to increase of [Ca2+]i, increase of ceramide abundance and caspase activation.

Cisplatin combined with hyperthermia kills HepG2 cells in intraoperative blood salvage but preserves the function of erythrocytes

The safe use of intraoperative blood salvage (IBS) in cancer surgery remains controversial. Here, we investigated the killing effect of cisplatin combined with hyperthermia on human hepatocarcinoma (HepG2) cells and erythrocytes from IBS in vitro. HepG2 cells were mixed with concentrated erythrocytes and pretreated with cisplatin (50, 100, and 200 μg/ml) alone at 37 °C for 60 min and cisplatin (25, 50, 100, and 200 μg/ml) combined with hyperthermia at 42 °C for 60 min. After pretreatment, the cell viability, colony formation and DNA metabolism in HepG2 and the Na(+)-K(+)-ATPase activity, 2,3-diphosphoglycerate (2,3-DPG) concentration, free hemoglobin (Hb) level, osmotic fragility, membrane phosphatidylserine externalization, and blood gas variables in erythrocytes were determined. Pretreatment with cisplatin (50, 100, and 200 μg/ml) combined with hyperthermia (42 °C) for 60 min significantly decreased HepG2 cell viability, and completely inhibited colony formation and DNA metabolism when the HepG2 cell concentration was 5×10(4) ml(-1) in the erythrocyte (P<0.01). Erythrocytic Na(+)-K(+)-ATPase activity, 2,3-DPG level, phosphatidylserine externalization, and extra-erythrocytic free Hb were significantly altered by hyperthermia plus high concentrations of cisplatin (100 and 200 μg/ml) (P<0.05), but not by hyperthermia plus 50 μg/ml cisplatin (P>0.05). In conclusion, pretreatment with cisplatin (50 μg/ml) combined with hyperthermia (42 °C) for 60 min effectively eliminated HepG2 cells from IBS but did not significantly affect erythrocytes in vitro.

Ceramide in the regulation of eryptosis, the suicidal erythrocyte death

Similar to apoptosis of nucleated cells, erythrocytes may undergo eryptosis, a suicidal death characterized by cell shrinkage and phospholipid scrambling of the cell membrane leading to phosphatidylserine exposure at the cell surface. As eryptotic erythrocytes are rapidly cleared from circulating blood, excessive eryptosis may lead to anemia. Moreover, eryptotic erythrocytes may adhere to the vascular wall and thus impede microcirculation. Stimulators of eryptosis include osmotic shock, oxidative stress and energy depletion. Mechanisms involved in the stimulation eryptosis include ceramide formation which may result from phospholipase A2 dependent formation of platelet activating factor (PAF) with PAF dependent stimulation of sphingomyelinases. Enhanced erythrocytic ceramide formation is observed in fever, sepsis, HUS, uremia, hepatic failure, and Wilson's disease. Enhanced eryptosis is further observed in iron deficiency, phosphate depletion, dehydration, malignancy, malaria, sickle-cell anemia, beta-thalassemia and glucose-6-phosphate dehydrogenase-deficiency. Moreover, eryptosis is triggered by osmotic shock and a wide variety of xenobiotics, which are again partially effective by enhancing ceramide abundance. Ceramide formation is inhibited by high concentrations of urea. As shown in Wilson's disease, pharmacological interference with ceramide formation may be a therapeutic option in the treatment of eryptosis inducing clinical disorders.

Enhanced suicidal erythrocyte death in acute cardiac failure

BACKGROUND

A common complication of acute cardiac failure (AHF) is anaemia, which negatively influences the clinical outcome. Causes of anaemia include enhanced eryptosis, a suicidal erythrocyte death characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation. Signalling triggering eryptosis include oxidative stress, increase of cytosolic Ca(2+) -activity ([Ca(2+) ]i ) and ceramide. The present study explored whether AHF is associated with accelerated eryptosis.

MATERIALS AND METHODS

Erythrocytes were drawn from healthy volunteers (n = 10) and patients hospitalized for AHF (n = 22). Phosphatidylserine exposure was estimated from annexin-V-binding, cell volume from forward scatter, [Ca(2+) ]i from Fluo3-fluorescence, ceramide abundance utilizing specific antibodies and reactive oxygen species (ROS) abundance from 2',7'-dichlorodihydrofluorescein diacetate (DCFDA) fluorescence, as determined by flow cytometry.

RESULTS

In AHF-patients, haemoglobin concentration (11·5 ± 0·5 g/dL), and haematocrit (35·6 ± 1·2%) were significantly lower than haemoglobin concentration (14·1 ± 0·4 g/dL), and haematocrit (40·1 ± 1·0%) in healthy volunteers, even though reticulocyte number was significantly higher in AHF patients (2·3 ± 0·3%) than in healthy volunteers (1·1 ± 0·2%). The percentage of erythrocytes exposing phosphatidylserine was significantly higher in AHF patients (1·8 ± 0·1%) than in healthy volunteers (1·2 ± 0·2%). The forward scatter was significantly lower and the ROS abundance significantly larger in AHF patients than in healthy volunteers. In erythrocytes drawn from healthy volunteers, phosphatidylserine and ROS abundance was increased to significantly higher values following a 24 h treatment with plasma from AHF patients than with plasma from healthy volunteers.

CONCLUSION

AHF leads to anaemia despite increased reticulocyte number and at least partially due to enhanced eryptosis. Underlying mechanisms include oxidative stress imposed by a plasma borne component.

Thermostability of IFN-γ and IP-10 release assays for latent infection with Mycobacterium tuberculosis: A TBnet study

INTRODUCTION

Interferon-γ (IFN-γ) inducible protein 10kD (IP-10) and IFN-γ release assays (IGRAs) are immunodiagnostic tests aiming to identify the presence of specific cellular immune responses, interpreted as markers for latent infection with Mycobacterium tuberculosis. Incubation at higher temperatures could affect IFN-γ and IP-10 responsiveness in order to improve the performance of IP-10 release assays and IGRAs.

AIM

The aim of this study was to assess the robustness of whole blood based IP-10 release assay and IGRAs and the effect of hyper-thermic incubation (39 °C) on the diagnostic accuracy of IP-10 release assay and IGRAs.

RESULTS

We included 65 patients with confirmed pulmonary tuberculosis and 160 healthy controls from 6 European centres collaborating in the TBnet. In patients, IP-10 responses increased 1.07 (IQR 0.90-1.36) fold and IFN-γ responses decreased 0.88 (IQR 0.57-1.02) fold, with 39 °C compared to 37 °C incubation temperature. At 37 °C IGRA sensitivity was 85% and IP-10 sensitivity was 82%, whereas specificity was 97% for both tests (p > 0.8). These minor changes observed as a result of hyper-thermic incubation were not sufficient to impact IGRA and IP-10 release assay test performance.

CONCLUSION

The performance of IGRA and IP-10 release assays is robust despite variations in the incubation temperature between 37 °C and 39 °C.

Enhanced eryptosis following juglone exposure

Juglone, a quinone isolated from Juglans mandshurica Maxim, has previously been shown to be effective against malignancy. The effect is at least partially due to stimulation of suicidal death or apoptosis of tumour cells. On the other hand, juglone has been shown to counteract apoptosis, for example, of neurons. In analogy to apoptosis of nucleated cells, erythrocytes may enter eryptosis, a suicidal death characterized by cell shrinkage and breakdown of phosphatidylserine asymmetry of the cell membrane with phosphatidylserine exposure at the erythrocyte surface. Stimulators of eryptosis include increase in cytosolic Ca(2+) activity [(Ca(2+) )i]. This study explored whether juglone stimulates eryptosis. To this end, erythrocyte volume was estimated from forward scatter, phosphatidylserine exposure at the erythrocyte surface from FITC annexin V binding, ceramide abundance from binding of fluorescent antibodies in flow cytometry and cytosolic ATP with a luciferin-luciferase-based assay. As a result, a 24-hr exposure of human erythrocytes to juglone (5 μM) significantly decreased erythrocyte forward scatter. Juglone (1-5 μM) significantly increased the percentage of annexin V binding cells. Juglone (5 μM) significantly increased ceramide abundance at the erythrocyte surface and decreased erythrocyte ATP concentration. The effect of juglone (10 μM) on annexin V binding was slightly but significantly blunted by removal of extracellular Ca(2+) and by addition of protein kinase C (PKC) inhibitor staurosporine (1 μM). In conclusion, juglone stimulates suicidal erythrocyte death or eryptosis at least in part by upregulation of ceramide abundance, energy depletion and activation of PKC.

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.

Effect of chloride channel inhibitors on cytosolic Ca2+ levels and Ca2+-activated K+ (Gardos) channel activity in human red blood cells

DIDS, NPPB, tannic acid (TA) and AO1 are widely used inhibitors of Cl(-) channels. Some Cl(-) channel inhibitors (NPPB, DIDS, niflumic acid) were shown to affect phosphatidylserine (PS) scrambling and, thus, the life span of human red blood cells (hRBCs). Since a number of publications suggest Ca(2+) dependence of PS scrambling, we explored whether inhibitors of Cl(-) channels (DIDS, NPPB) or of Ca(2+)-activated Cl(-) channels (DIDS, NPPB, TA, AO1) modified intracellular free Ca(2+) concentration ([Ca(2+)]i) and activity of Ca(2+)-activated K(+) (Gardos) channel in hRBCs. According to Fluo-3 fluorescence in flow cytometry, a short treatment (15 min, +37 °C) with Cl(-) channels inhibitors decreased [Ca(2+)]i in the following order: TA > AO1 > DIDS > NPPB. According to forward scatter, the decrease of [Ca(2+)]i was accompanied by a slight but significant increase in cell volume following DIDS, NPPB and AO1 treatments. TA treatment resulted in cell shrinkage. According to whole-cell patch-clamp experiments, TA activated and NPPB and AO1 inhibited Gardos channels. The Cl(-) channel blockers further modified the alterations of [Ca(2+)]i following ATP depletion (glucose deprivation, iodoacetic acid, 6-inosine), oxidative stress (1 mM t-BHP) and treatment with Ca(2+) ionophore ionomycin (1 μM). The ability of the Cl(-) channel inhibitors to modulate PS scrambling did not correlate with their influence on [Ca(2+)]i as TA and AO1 had a particularly strong decreasing effect on [Ca(2+)]i but at the same time enhanced PS exposure. In conclusion, Cl(-) channel inhibitors affect Gardos channels, influence Ca(2+) homeostasis and induce PS exposure of hRBCs by Ca(2+)-independent mechanisms.

Physiology and pathophysiology of eryptosis

SUMMARY

Suicidal erythrocyte death (eryptosis) is characterized by cell shrinkage, cell membrane blebbing, and cell membrane phospholipid scrambling with phosphatidylserine exposure at the cell surface. Eryptotic cells adhere to the vascular wall and are rapidly cleared from circulating blood. Eryptosis is stimulated by an increase in cytosolic Ca(2)+ activity, ceramide, hyperosmotic shock, oxidative stress, energy depletion, hyperthermia, and a wide variety of xenobiotics and endogenous substances. Inhibitors of eryptosis include erythropoietin and nitric oxide. Enhanced eryptosis is observed in diabetes, renal insufficiency, hemolytic uremic syndrome, sepsis, mycoplasma infection, malaria, iron deficiency, sickle cell anemia, beta-thalassemia, glucose-6-phosphate dehydrogenase-(G6PD) deficiency, hereditary spherocytosis, paroxysmal nocturnal hemoglobinuria, Wilson's disease, myelodysplastic syndrome, and phosphate depletion. Eryptosis is further enhanced in gene-targeted mice with deficient annexin 7, cGMP-dependent protein kinase type I (cGKI), AMP-activated protein kinase (AMPK), anion exchanger 1 (AE1), adenomatous polyposis coli (APC), and Klotho, as well as in mouse models of sickle cell anemia and thalassemia. Decreased eryptosis is observed in mice with deficient phosphoinositide-dependent kinase 1 (PDK1), platelet activating factor (PAF) receptor, transient receptor potential channel 6 (TRPC6), janus kinase 3 (JAK3), and taurine transporter (TAUT). Eryptosis may be a useful mechanism to remove defective erythrocytes prior to hemolysis. Excessive eryptosis may, however, compromise microcirculation and lead to anemia.

Sphingomyelinase-induced adhesion of eryptotic erythrocytes to endothelial cells

Eryptosis, the suicidal erythrocyte death, leads to cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Eryptotic erythrocytes adhere to the vascular wall by binding of phosphatidylserine to the CXC chemokine ligand 16 (CXCL16). Stimulators of eryptosis include increased cytosolic Ca(2+) activity, energy depletion, and activation of ceramide-producing sphingomyelinase. The present study explored whether sphingomyelinase triggers erythrocyte adhesion to endothelial cells. To this end, human erythrocytes were exposed for 6 h to bacterial sphingomyelinase (1-10 mU/ml) and phosphatidylserine exposure was estimated from fluorescent annexin-V-binding, cell volume from forward scatter in FACS-analysis, erythrocyte adhesion to human umbilical vein endothelial cells (HUVEC) from trapping of labeled erythrocytes in a flow chamber under flow conditions at arterial shear rates, and CXCL16 protein abundance utilizing Western blotting and FACS analysis of fluorescent antibody binding. As a result, sphingomyelinase (≥1 mU/ml) triggered cell shrinkage, phosphatidylserine exposure and erythrocyte adhesion to HUVEC, effects blunted by Ca(2+) removal. Adhesion was significantly blunted by phosphatidylserine-coating annexin-V (5 μl/ml), following addition of neutralizing antibodies against endothelial CXCL16 (4 μg/ml) and following silencing of the CXCL16 gene with small interfering RNA. Pretreatment of HUVEC with sphingomyelinase upregulated CXCL16 protein abundance. Six hours pretreatment of HUVEC with sphingomyelinase (10 mU/ml) or C6-ceramide (50 μM) augmented erythrocyte adhesion following a 30-min treatment with Ca(2+) ionophore ionomycin (1 μM) or following energy depletion by 48-h glucose removal. Thus exposure to sphingomyelinase or C6-ceramide triggers eryptosis followed by phosphatidylserine- and CXCL16-sensitive adhesion of eryptotic erythrocytes to HUVEC.

Killing me softly - suicidal erythrocyte death

Similar to nucleated cells, erythrocytes may undergo suicidal death or eryptosis, which is characterized by cell shrinkage, cell membrane blebbing and cell membrane phospholipid scrambling. Eryptotic cells are removed and thus prevented from undergoing hemolysis. Eryptosis is stimulated by Ca(2+) following Ca(2+) entry through unspecific cation channels. Ca(2+) sensitivity is enhanced by ceramide, a product of acid sphingomyelinase. Eryptosis is triggered by hyperosmolarity, oxidative stress, energy depletion, hyperthermia and a wide variety of xenobiotics and endogenous substances. Eryptosis is inhibited by nitric oxide, catecholamines and a variety of further small molecules. Erythropoietin counteracts eryptosis in part by inhibiting the Ca(2+)-permeable cation channels but by the same token may foster formation of erythrocytes, which are particularly sensitive to eryptotic stimuli. Eryptosis is triggered in several clinical conditions such as iron deficiency, diabetes, renal insufficiency, myelodysplastic syndrome, phosphate depletion, sepsis, haemolytic uremic syndrome, mycoplasma infection, malaria, sickle-cell anemia, beta-thalassemia, glucose-6-phosphate dehydrogenase-(G6PD)-deficiency, hereditary spherocytosis, paroxysmal nocturnal hemoglobinuria, and Wilson's disease. Enhanced eryptosis is observed in mice with deficient annexin 7, cGMP-dependent protein kinase type I (cGKI), AMP-activated protein kinase AMPK, anion exchanger AE1, adenomatous polyposis coli APC and Klotho as well as in mouse models of sickle cell anemia and thalassemia. Eryptosis is decreased in mice with deficient phosphoinositide dependent kinase PDK1, platelet activating factor receptor, transient receptor potential channel TRPC6, janus kinase JAK3 or taurine transporter TAUT. If accelerated eryptosis is not compensated by enhanced erythropoiesis, clinically relevant anemia develops. Eryptotic erythrocytes may further bind to endothelial cells and thus impede microcirculation.

Plasmodium falciparum-infected erythrocytes induce granzyme B by NK cells through expression of host-Hsp70

In the early immune response to Plasmodium falciparum-infected erythrocytes (iRBC), Natural Killer (NK) cells are activated, which suggests an important role in innate anti-parasitic immunity. However, it is not well understood whether NK cells directly recognize iRBC or whether stimulation of NK cells depends mainly on activating signals from accessory cells through cell-to-cell contact or soluble factors. In the present study, we investigated the influence of membrane-bound host Heat shock protein (Hsp) 70 in triggering cytotoxicity of NK cells from malaria-naïve donors or the cell line NK92 against iRBC. Hsp70 and HLA-E membrane expression on iRBC and potential activatory NK cell receptors (NKG2C, CD94) were assessed by flow cytometry and immunoblot. Upon contact with iRBC, Granzyme B (GzmB) production and release was initiated by unstimulated and Hsp70-peptide (TKD) pre-stimulated NK cells, as determined by Western blot, RT-PCR and ELISPOT analysis. Eryptosis of iRBC was determined by Annexin V-staining. Our results suggest that presence of Hsp70 and absence of HLA-E on the membrane of iRBC prompt the infected host cells to become targets for NK cell-mediated cytotoxicity, as evidenced by impaired parasite development. Contact of iRBC with NK cells induced release of GzmB. We propose that following GzmB uptake, iRBC undergo eryptosis via a perforin-independent, GzmB-mediated mechanism. Since NK activity toward iRBC could be specifically enhanced by TKD peptide and abrogated to baseline levels by blocking Hsp70 exposure, we propose TKD as an innovative immunostimulatory agent to be tested as an adjunct to anti-parasitic treatments in vivo.

Mechanisms and significance of eryptosis, the suicidal death of erythrocytes

Eryptosis, the suicidal death of erythrocytes, is characterized by erythrocyte shrinkage, blebbing, and phospholipid scrambling of the cell membrane. Eryptosis is triggered by increased cytosolic Ca(2+) activity, which may result from Ca(2+) entry through PGE(2)-activated Ca(2+)-permeable cation channels. The Ca(2+) sensitivity of the scrambling machinery is enhanced by ceramide, which is formed by an acid sphingomyelinase, an enzyme stimulated by platelet-activating factor. Eryptosis is enhanced in a variety of clinical conditions such as sickle-cell anemia, β-thalassemia, glucose-6-phosphate dehydrogenase deficiency, hereditary spherocytosis, paroxysmal nocturnal hemoglobinuria, myelodysplastic syndrome, phosphate depletion, iron deficiency, sepsis, hemolytic uremic syndrome, renal insufficiency, diabetes, malaria, mycoplasma infection, and Wilson's disease. Eryptosis is enhanced in mouse models of sickle cell anemia and thalassemia, as well as in mice lacking functional annexin 7, cGMP-dependent protein kinase type I, AMP-activated protein kinase, Janus kinase 3, anion exchanger 1, adenomatous polyposis coli, or Klotho. Eryptosis is triggered by osmotic shock, oxidative stress, energy depletion, hyperthermia, and a myriad of small molecules. Eryptosis is inhibited by a variety of substances including nitric oxide and catecholamines. Erythropoietin counteracts eryptosis in part by inhibiting the Ca(2+)-permeable cation channels. Excessive erythropoietin concentrations lead, however, to formation of erythrocytes, which are particularly sensitive to eryptotic stimuli. Accelerated eryptosis may be compensated by enhanced erythropoiesis, which is apparent from reticulocytosis. If the compensation is not sufficient, clinically relevant anemia develops. Beyond that, adhesion of eryptotic erythrocytes to the vascular wall may lead to impairment of microcirculation.

Sphingosine but not sphingosine-1-phosphate stimulates suicidal erythrocyte death

Sphingosine kinase 1 phosphorylates sphingosine, which is converted to ceramide by ceramide synthetase. Ceramide triggers eryptosis, the suicidal erythrocyte death characterized by cell shrinkage and phosphatidylserine (PS) exposure at the erythrocyte surface. Erythrocytes lack sphingosine phosphate-degrading enzymes and thus store large quantities of sphingosine phosphate. The present study explored the influence of sphingosine and sphingosine phosphate on eryptosis. [Ca(2+)](i), was estimated from Fluo3 fluorescence, cell volume from forward scatter and PS exposure from annexin V-binding in FACS analysis. Sphingosine (0.1 - 10 μM) but not sphingosine-1- phosphate (0.1 - 10 μM) increased [Ca(2+)](i), decreased cell volume and increased PS-exposure. The observations disclose sphingosine, but not sphingosine-1-phosphate, as a strong inducer of eryptosis.