Suicide for survival--death of infected erythrocytes as a host mechanism to survive malaria

Stimulation of suicidal erythrocyte death by oridonin

Oridonin triggers apoptosis of cancer cells and was suggested as anticancer agent. Oridonin is partially effective through mitochondrial depolarization and partially by modifying gene expression. Erythrocytes lack mitochondria and nuclei but may undergo eryptosis, a suicidal cell death characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include increase of cytosolic Ca(2+)-activity, ATP depletion and ceramide formation. The present study explored, whether oridonin triggers eryptosis. Cytosolic Ca(2+)-concentration was estimated from Fluo3-fluorescence, cell volume from forward scatter in FACS analysis, phosphatidylserine exposure from binding of fluorescent annexin V, hemolysis from hemoglobin release, ATP concentration utilizing a luciferin-luciferase assay and ceramide abundance utilizing fluorescent anti-ceramide antibodies. A 48 h exposure to oridonin (≥25μM) significantly increased cytosolic Ca(2+)-concentration, increased ceramide formation, decreased forward scatter and triggered annexin V-binding (the latter in >20% of the erythrocytes). Oridonin didn't decrease ATP concentration and hemolysed <5% of erythrocytes. The effects of oridonin on annexin V binding were partially reversed in the nominal absence of Ca(2+) and by the addition of amiloride (1mM). The present observations reveal a completely novel effect of oridonin, i.e. triggering of Ca(2+) entry and ceramide formation as well as suicidal death of erythrocytes.

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.

Apoptosis induced by parasitic diseases

Fatalities caused by parasitic infections often occur as a result of tissue injury that results from a form of host-cell death known as apoptosis. However, instead of being pathogenic, parasite-induced apoptosis may facilitate host survival. Consequently, it is of utmost importance to decipher and understand the process and the role of apoptosis induced or controlled by parasites in humans. Despite this, few studies provide definitive knowledge of parasite-induced host-cell apoptosis. Here, the focus is on a consideration of host-cell apoptosis as either a pathogenic feature or as a factor enabling parasite survival and development.

Cell death by apoptotic-like mechanisms could be described as a ride to death with a return ticket, as initiation of the pathway may be reversed, with the potential that it could be manipulated for therapeutic purposes. The management of host-cell apoptosis could thus be an adjunctive factor for parasitic disease treatment. Evidence that the apoptotic process could be reversed by anti-apoptotic drugs has recently been obtained, leading to the possibility of host-cell rescue after injury. An important issue will be to predict the beneficial or deleterious effects of controlling human cell death by apoptotic-like mechanisms during parasitic diseases.

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.

Schistosoma mansoni soluble egg antigens trigger erythrocyte cell death

Estimated to affect nearly 300 million people worldwide, schistosomiasis is caused by parasitic flatworms of the genus Schistosoma. The major pathological consequences of chronic schistosomiasis are associated with soluble egg antigens (SEA) secreted from schistosome egg deposits in liver and other organs. The vigorous immune responses induced by egg antigens result in granuloma formation and other pathophysiological symptoms such as hepatosplenomegaly and anemia. Risk of anemia correlates with infection intensity and the level of host hemoglobin is inversely proportional to egg count. Schistosomiasis-associated anemia could be multifactorial, but the potential link and molecular underpinnings are unclear. Here, we evaluate whether S. mansoni SEA affects survival of mouse erythrocytes. Erythrocytes incubated with different concentrations of SEA were tested for various markers of erythrocyte cell death. Erythrocytes exposed to SEA exhibit elevated intracellular Ca(2+) levels as measured by Fluo-3 AM fluorescence in flow cytometry, and they also display concentration-dependent, Ca(2+)-dependent, and heat-sensitive increases in phosphatidyl serine exposure. Further, SEA-exposed erythrocytes show increased fluorescence using the in situ apoptosis marker CaspACE FITC, indicating the involvement of caspase-mediated cell deformation. Taken together, these results offer several lines of experimental evidence for SEA-induced erythrocyte cell death and may provide new insights into factors contributing to schistosomiasis-associated anemia.

Protective effect of amiodarone in malaria

According to previous observations, amiodarone triggers suicidal erythrocyte death or eryptosis, which is characterized by cell shrinkage and exposure of phosphatidylserine at the erythrocyte surface. Eryptosis may in turn accelerate the clearance of Plasmodium-infected erythrocytes. The present study tested whether amiodarone augments phosphatidylserine exposure of Plasmodium-infected erythrocytes, interferes with the development of parasitemia and thus influences the course of malaria. The in vitro infection of human erythrocytes with Plasmodium falciparum (strain BinH) increased annexin V-binding, an effect significantly augmented by amiodarone (10 microM). Amiodarone further significantly decreased intraerythrocytic DNA/RNA content (> or =5 microM) and in vitro parasitemia (> or =1 microM). Following infection of mice with Plasmodiumberghei ANKA by intraperitoneal injection of parasitized murine erythrocytes (1x10(6)) amiodarone (intraperitoneal 50mg/kg b.w.) significantly decreased the parasitemia and increased the survival of P. berghei-infected mice (from 0% to 70% 26 days after infection). Moreover, treatment with amiodarone significantly increased the percentage of PS-exposing infected erythrocytes. In conclusion, amiodarone inhibits intraerythrocytic growth of P. falciparum, enhances suicidal death of infected erythrocytes, decreases parasitemia following P. berghei infection and supports host survival during malaria.

Dynamic quantitative photothermal monitoring of cell death of individual human red blood cells upon glucose depletion

Red blood cells (RBCs) have been found to undergo "programmed cell death," or eryptosis, and understanding this process can provide more information about apoptosis of nucleated cells. Photothermal (PT) response, a label-free photothermal noninvasive technique, is proposed as a tool to monitor the cell death process of living human RBCs upon glucose depletion. Since the physiological status of the dying cells is highly sensitive to photothermal parameters (e.g., thermal diffusivity, absorption, etc.), we applied linear PT response to continuously monitor the death mechanism of RBC when depleted of glucose. The kinetics of the assay where the cell's PT response transforms from linear to nonlinear regime is reported. In addition, quantitative monitoring was performed by extracting the relevant photothermal parameters from the PT response. Twofold increases in thermal diffusivity and size reduction were found in the linear PT response during cell death. Our results reveal that photothermal parameters change earlier than phosphatidylserine externalization (used for fluorescent studies), allowing us to detect the initial stage of eryptosis in a quantitative manner. Hence, the proposed tool, in addition to detection of eryptosis earlier than fluorescence, could also reveal physiological status of the cells through quantitative photothermal parameter extraction.

Targeting glutathione by dimethylfumarate protects against experimental malaria by enhancing erythrocyte cell membrane scrambling

The balance between GSH-levels and oxidative stress is critical for cell survival. The GSH-levels of erythrocytes are dramatically decreased during infection with Plasmodium spp. We therefore investigated the consequences of targeting GSH for erythrocyte and Plasmodium survival in vitro and in vivo using dimethylfumarate (DMF) at therapeutically established dosage. We first show that noninfected red blood cells (RBC) exposed to DMF undergo changes typical of apoptosis or eryptosis, such as cell shrinkage and cell membrane scrambling with subsequent phosphatidylserine (PS) exposure. DMF did not induce appreciable hemolysis. DMF-triggered PS exposure was mediated by intracellular GSH depletion and reversed by the antioxidative N-acetyl-l-cysteine. DMF treatment controlled intraerythrocyte DNA amplification and in vitro parasitemia of Plasmodium falciparum-infected RBC. In vivo, DMF treatment had no effect on RBC count or GSH levels in noninfected mice. Consistent with its effects on infected RBC, DMF treatment abrogated parasitemia and enhanced the survival of mice infected with Plasmodium berghei from 0% to 60%. In conclusion, DMF sensitizes the erythrocytes to the effect of Plasmodium infection on PS exposure, thus accelerating the clearance of infected erythrocytes. Accordingly, DMF treatment favorably influences the clinical course of malaria. As DMF targets mechanisms within the host cell, it is not likely to generate resistance of the pathogen.

Temperature sensitivity of suicidal erythrocyte death

BACKGROUND

Fever and hyperthermia are frequently associated with anaemia. Under most clinical conditions, they are considered to be two mutually independent clinical consequences of a common cause. The present study explored the possibility that anaemia results from temperature-sensitive suicidal erythrocyte death or eryptosis. Eryptosis is characterised by cell shrinkage and cell membrane scrambling with phosphatidylserine (PS) exposure at the erythrocyte surface. It is triggered by increase in the cytosolic Ca(2+) activity on the one hand and by ceramide formation on the other.

MATERIAL AND METHODS

Annexin V-binding was utilised to disclose PS exposure, forward scatter to analyse cell volume, Fluo 3 fluorescence to estimate cytosolic Ca(2+) activity, binding of fluorescent antibodies to determine ceramide abundance and a luciferin/luciferase-based assay to measure the cytosolic ATP concentration.

RESULTS

Graded increases in temperature from 37 to 41 degrees C decreased the forward scatter and stimulated annexin V-binding of human erythrocytes. The effect was accompanied by increased cytosolic Ca(2+) activity, decrease of the cellular ATP content and a moderate rise in ceramide formation. The effect of hyperthermia on annexin V-binding was significantly blunted by the leukotriene receptor CysLT1 antagonist cinalukast (1 microM).

CONCLUSIONS

Hyperthermia stimulates Ca(2+) entry into erythrocytes leading to cell shrinkage and PS exposure. As PS-exposing erythrocytes are rapidly cleared from circulating blood, the eryptosis during hyperthermia may cause anaemia.

Enhanced eryptosis of erythrocytes from gene-targeted mice lacking annexin A7

Annexin A7 is a ubiquitously expressed Ca(2+)- and phospholipid-binding protein. Erythrocytes from mice lacking annexin A7 (anxA7(-/-)) are deformed and relatively resistant to osmotic swelling. In normal erythrocytes, hyperosmotic shock, Cl(-) removal, and energy depletion (glucose removal) trigger PGE(2) formation, which stimulates Ca(2+)-permeable cation channels, increases cytosolic Ca(2+) activity ([Ca(2+)](i)), and thus triggers suicidal death of erythrocytes or eryptosis, characterized by scrambling of the cell membrane with phosphatidylserine exposure at the cell surface. The present experiments explored the influence of annexin A7 deficiency on eryptosis. In erythrocytes from annexin A7-deficient mice (anxA7(-/-)) and wild-type mice (anxA7(+/+)), PGE(2) formation was determined utilizing an immunoassay, ion channel activity by whole-cell patch clamp recording, [Ca(2+)](i) by fluo3 fluorescence, and phosphatidylserine exposure by binding of annexin A5 in fluorescence activated cell sorter (FACS) analysis. Erythrocyte number and hematocrit were significantly smaller in blood from anx7(-/-) than in anx7(+/+) mice. Cl(-)-removal (replacement with gluconate) stimulated PGE(2)-formation, activated cation currents, increased [Ca(2+)](i), and triggered phosphatidylserine exposure, effects significantly more pronounced in anx7(-/-) than in anx7(+/+) erythrocytes. Hyperosmotic shock (addition of 400 mM sucrose) and glucose depletion (removal of glucose) similarly increased cytosolic Ca(2+) activity and triggered phosphatidylserine exposure, effects again significantly more pronounced in anx7(-/-) than in anx7(+/+) erythrocytes. The effects of Cl(-) removal on PGE(2) formation and the cation current, as well as the effect of hypertonic cell shrinkage on [Ca(2+)](i) and cell membrane scrambling, were blunted following inhibition of cyclooxygenase by aspirin or diclofenac. In conclusion, lack of annexin A7 sensitizes the erythrocytes for "proapoptotic" Ca(2+) overload, an effect shortening the life span of the affected erythrocytes and, thus, leading to anemia.

Beneficial effect of aurothiomalate on murine malaria

Background

Premature death of Plasmodium-infected erythrocytes is considered to favourably influence the clinical course of malaria. Aurothiomalate has previously been shown to trigger erythrocyte death or eryptosis, which is characterized by cell membrane scrambling leading to phosphatidylserine exposure at the cell surface. Phosphatidylserine-exposing cells are rapidly cleared from circulating blood. The present study thus tested whether sodium aurothiomalate influences the intraerythrocytic parasite development in vitro and the clinical course of murine malaria in vivo.

Methods

Human erythrocytes were infected with Plasmodium falciparum BinH in vitro and mice were infected (intraperitoneal injection of 1 × 10⁶ parasitized murine erythrocytes) with Plasmodium berghei ANKA in vivo.

Results

Exposure to aurothiomalate significantly decreased the in vitro parasitemia of P. falciparum-infected human erythrocytes without influencing the intraerythrocytic DNA/RNA content. Administration of sodium aurothiomalate in vivo (daily 10 mg/kg b.w. s.c. from the 8^th day of infection) enhanced the percentage of phosphatidylserine-exposing infected and noninfected erythrocytes in blood. All nontreated mice died within 30 days of infection. Aurothiomalate-treatment delayed the lethal course of malaria leading to survival of more than 50% of the mice 30 days after infection.

Conclusions

Sodium aurothiomalate influences the survival of Plasmodium berghei-infected mice, an effect only partially explained by stimulation of eryptosis.

Effect of phytic acid on suicidal erythrocyte death

Phytic acid, an anticarcinogenic food component, stimulates apoptosis of tumor cells. Similar to apoptosis, human erythrocytes may undergo suicidal death or eryptosis, characterized by cell membrane scrambling and cell shrinkage. Triggers of eryptosis include energy depletion. Phytate intake could cause anemia, an effect attributed to iron complexation. The present experiments explored whether phytic acid influences eryptosis. Supernatant hemoglobin concentration was determined to reveal hemolysis, annexin V-binding in FACS analysis was utilized to identify erythrocytes with scrambled cell membrane, forward scatter in FACS analysis was taken as a measure of cell volume, and a luciferin-luciferase assay was employed to determine erythrocyte ATP content. As a result, phytic acid (>or=1 mM) did not lead to significant hemolysis, but significantly increased the percentage of annexin V-binding erythrocytes, significantly decreased forward scatter, and significantly decreased cellular ATP content. In conclusion, phytic acid stimulates suicidal human erythrocyte death, an effect paralleling its proapoptotic effect on nucleated cells.