Endocytosis in sickle erythrocytes: a mechanism for elevated intracellular Ca2+ levels

A hypothesis of target cell formation in sickle cell disease

A fraction of erythrocytes appear as target cells in stained blood smears in sickle cell disease, due to a inheritance of the hemoglobin variant Hb S, polymerizing upon deoxygenation. These cells appear in a three dimension as thin cups. A process of their formation in this disease is proposed based on a band 3-based mechanism of the erythrocyte shape control, able to explain the erythrocyte echinocytosis by glucose depletion. It indicates that their formation is due to a stomatocytogenic slow outward transport of the dibasic form of endogenous Pi with an H(+) by band 3, promoted by the decrease of the Donnan ratio, which decreases cell pH and volume, attributed by a decrease of cell KCl concentration by the higher efflux of K(+)Cl(-) cotransport and Ca(2+) activation of the Gardos channel. Its implications are briefly discussed with respect to target cells per se, target cell formation in other hemoglobinopathies, acquired and inherited disorders of the lipid metabolism and dehydrated hereditary stomatocytosis as well as a stomatocyte presence in a double heterozygote of Hb S and Hb C and of an involvement of the process of target cell formation in acanthocytosis in acquired and inherited disorders.

Ion transport pathology in the mechanism of sickle cell dehydration

Polymers of deoxyhemoglobin S deform sickle cell anemia red blood cells into sickle shapes, leading to the formation of dense, dehydrated red blood cells with a markedly shortened life-span. Nearly four decades of intense research in many laboratories has led to a mechanistic understanding of the complex events leading from sickling-induced permeabilization of the red cell membrane to small cations, to the generation of the heterogeneity of age and hydration condition of circulating sickle cells. This review follows chronologically the major experimental findings and the evolution of guiding ideas for research in this field. Predictions derived from mathematical models of red cell and reticulocyte homeostasis led to the formulation of an alternative to prevailing gradualist views: a multitrack dehydration model based on interactive influences between the red cell anion exchanger and two K(+) transporters, the Gardos channel (hSK4, hIK1) and the K-Cl cotransporter (KCC), with differential effects dependent on red cell age and variability of KCC expression among reticulocytes. The experimental tests of the model predictions and the amply supportive results are discussed. The review concludes with a brief survey of the therapeutic strategies aimed at preventing sickle cell dehydration and with an analysis of the main open questions in the field.

Sickle cell anaemia: progress in pathogenesis and treatment

The phenotypic expression of sickle cell anaemia varies greatly among patients and longitudinally in the same patient. It influences all aspects of the life of affected individuals including social interactions, intimate relationships, family relations, peer interactions, education, employment, spirituality and religiosity. The clinical manifestations of sickle cell anaemia are protean and fall into three major categories: anaemia and its sequelae;pain and related issues; andorgan failure including infection. Recent studies on the pathogenesis of sickle cell anaemia have centred on the sequence of events that occur between polymerisation of deoxy haemoglobin (Hb) S and vaso-occlusion. Cellular dehydration, inflammatory response and reperfusion injury seem to be important pathophysiological mechanisms. Management of sickle cell anaemia continues to be primarily palliative in nature, including supportive, symptomatic and preventative approaches to therapy. Empowerment and education are the major aspects of supportive care. Symptomatic management includes pain management, blood transfusion and treatment of organ failure. Pain managment should follow certain priniciples that include assessment, individualisation of therapy and proper utilisation of opioid and nonopioid analgesics in order to acheive adequate pain relief. Blood selected for transfusion should be leuko-reduced and phenotypically matched for the C, E and Kell antigens. Exchange transfusion is indicated in patients who are transfused chronically in order to prevent or delay the onset of iron-overload. Acute chest syndrome is the most common form of organ failure and its management should be agressive, including adequate ventilation, multiple antibacterials and simple or exchange blood transfusion depending on its severity. Preventitive therapy includes prophylactic penicillin in infants and children, blood transfusion (preferably exchange transfusion) in patients with stroke, and hydroxyurea in patients with frequent acute painful episodes. Bone marrow and cord blood transplantation have been successful modalities of curative therapy in selected children with sickle cell anaemia. Newer approaches to preventative therapy include cellular rehydration with agents that inhibit the Gardos channel or the KCl co-transport channel. Curative gene therapy continues to be investigational at the level of the test tube and transgenic mouse models.

Effects of prolactin on intracellular stored calcium in the course of bovine oocyte maturation in vitro

At present there are divergent opinions as to the role of prolactin (PRL) in the mechanisms of meiotic regulation in mammals. We investigated the effects of bovine PRL (bPRL) on bovine oocyte maturation in different culture systems and varying levels of intracellular stored calcium ([Ca2+]is) in the oocytes. Cumulus-oocyte complexes (COC) were incubated in TCM 199 containing either 10% fetal calf serum (FCS) in the absence (System 1) or presence (System 2) of FSH and estradiol, or 6 mg/mL bovine serum albumin (BSA) in the presence of FSH and estradiol (System 3). Levels of [Ca2+]is in oocytes were determined by using the fluorophore chlortetracycline. The addition of 50 ng/mL bPRL to different culture media increased the percentage of oocytes at telophase I and metaphase II stages (Systems 1 and 2) and/or decreased the percentage of oocytes with degenerated chromosomes (Systems 1 and 3). Compared with the control, lower levels of [Ca2+]is were observed in oocytes cultured for 2.5 h in those systems in which bPRL decreased the rate of oocytes with degenerated chromosomes (1.27+/-0.11 vs. 1.67+/-0.09 arbitrary units (AU) in System 1, P<0.001 and 1.27+/-0.12 vs. 1.52+/-0.04 AU in System 3, P<0.001). These findings show that the effects of bPRL on bovine oocyte maturation depend on the composition of the culture system and that the decline in the rate of oocytes with degenerated chromosomes in response to bPRL may be the result of the decrease in [Ca2+ ]is levels at early stages of oocyte maturation.

Modulation of rat incisor odontoblast plasma membrane-associated Ca2+ with nifedipine

In addition to the Ca2+ portion freely dissociated in the cytosol, another Ca2+ pool is associated with plasma membranes and intracellular organelle membranes. This Ca2+ portion is of importance for regulation of, among other things, the cell cycle, actin-mediated processes, and cell morphology. In the literature, dihydropyridines have been reported to influence this membrane-associated pool of Ca2+ under certain conditions. The aim of this investigation was to study possible modulations of plasma membrane-associated Ca2+ upon treatment with nifedipine in vitro in a Ca2+-transporting cell, the dentin-forming odontoblast. The membrane-associated portion of Ca2+ in dissected dentinogenically active rat incisor odontoblasts was monitored by fluorescence spectrophotometry using chlortetracycline as a probe. In addition, images of chlortetracycline-Ca2+ binding were obtained by fluorescence microscopy. It was found that membrane-associated Ca2+ decreased by the dihydropyridine nifedipine, whereas this Ca2+ pool was unaffected by the cellular polarization state, which was in contrast to cytosolic free Ca2+ as measured by fura-2. The results show that the odontoblast plasma membrane-associated Ca2+-pool can be modulated by nifedipine, thus being dependent on the conformational state of the L-type Ca2+ channels.

Prolactin in follicular fluid and intracellular store calcium in follicular cells are related to morphological signs of ovarian follicle atresia in cows: work in progress

It is known that prolactin (PRL) is the third pituitary hormone serving gonadotropic function in mammals. However, its role in the regulation of ovarian folliculogenesis and, in particular, its relationship to follicular atresia as well as the mechanism of its influence on follicular cells are poorly understood. We investigated PRL levels in follicular fluids (FFs) and intracellular store calcium ([Ca2+]is) in cell walls of bovine ovarian follicles with diameters of 10 to 20 mm and their relationship to follicular atresia. Ovarian follicles were categorized on the basis of macroscopic criteria and of microscopic examination of granulosa cell (GC) smears. Prolactin concentrations in FFs were measured by RIA and levels of [Ca2+]is in follicular cells were determined by using the fluorophore chlortetracycline. Compared to atretic follicles, morphologically normal follicles were characterized by higher concentrations of PRL in FFs (P < 0.001) and lower contents of [Ca2+]is in follicular cells (P < 0.01). Furthermore, follicles containing no more than 20% of pycnotic GCs had higher levels of PRL in their fluids than those containing over 40% of pycnotic GCs (P < 0.05). Finally, the direct effect of PRL on [Ca2+]is content in follicular cells was studied in vitro. Compared to control, PRL decreased (P < 0.001) the levels of [Ca2+]is in the cells after 24 h culture of follicular walls from morphologically normal follicles in TCM 199 supplemented by 10% fetal calf serum. Our findings suggest that the decline of PRL concentrations in FFs and the rise of [Ca2+]is contents in follicular cells are related to atresia of large bovine follicles and that there appears to be a relationship between the two biochemical parameters.

Assessment of painful episode frequency in sickle-cell disease

Frequency of painful episodes in sickle-cell disease is considered to be related to clinical severity and possibly to other aspects of the disease. Measurements of frequency often include only hospital-related or more severe, longer-lasting episodes. Since painful episodes, however, may regularly occur in nonhospital settings or be shorter-lasting with possible different pathologic effects, we measured all painful episodes in 10 adults with sickle-cell disease for 1.0-3.8 years, using a daily questionnaire. The results were related to other indices of disease severity and to possible precipitating factors, such as cold weather and menses. Sixty-one percent (on average) of the total number of episodes (243) were nonhospital-related, and 33% (on average) were shorter-lasting. Episode frequencies, whether determined as total, hospital-related, nonhospital-related, or shorter-lasting, were not related to each other or to other indicators of disease severity. The highest incidence of episode frequency occurred in the winter. The association of episodes with menses was moderately close in individual patients. The findings suggest that nonhospital-related painful episodes and shorter-lasting episodes may contribute significantly to episode frequency. Measurement of frequency of all painful episodes would require consideration when evaluating episode frequency and its relationship to disease severity, to possible precipitating factors of episodes, and to treatment of the disease, and for study of the natural course of the disease.

Pathophysiology of vaso-occlusion

Sickle cell anemia is a very complex disease. Vascular occlusion, the major event that accompanies SS, is itself a complex and multifactional process. Microvascular occlusion results in acute painful crises, whereas macrovascular occlusion seems to be the cause of organ failure. Understanding the basic pathophysiologic events of vascular occlusion may elucidate the clinical manifestations of SS, its natural history, and complications, and it may give new insights into preventive and curative therapy.

Ca2+ accumulation and loss by aberrant endocytic vesicles in sickle erythrocytes

Sickle cells contain internal vesicles which accumulate Ca2+. As shown here, the membrane enclosing the vesicles contains the plasma membrane Ca(2+)-ATPase, or Ca2+ pump, as judged by staining with an antibody directed against the protein. Moreover, the number of cells containing such vesicles increases upon deoxygenation. These findings argue strongly that the vesicles arise by endocytosis from the plasma membrane, and explain how they accumulate Ca2+. When sickle cells are depleted of ATP, Ca2+ is lost from the vesicles, as judged by the disappearance of staining with the Ca2+/membrane probe chlortetracycline (CTC), without a corresponding loss of antibody staining. This loss of Ca2+ can be inhibited by nitrendipine, a Ca2+ channel blocker. These results suggest that the vesicle membrane allows outward passage of Ca2+ by a nitrendipine-sensitive pathway, which can be overcome by the inward-directed activity of the Ca2+ pump of the vesicle membrane. If so, the Ca2+ which vesicles contain is in dynamic equilibrium with the cytoplasm of the sickle erythrocyte.

Monozygotic twins with sickle cell anemia and discordant clinical courses: clinical and laboratory studies

We describe a rare set of monozygotic twins with coexistent sickle cell anemia and alpha-/alpha alpha thalassemia who have asynchronous painful crises of different frequency and severity. Studies include measurements of cell deformability and other hemorheologic tests, cell density distribution, the percentage of irreversibly sickled cells, adherence of red cells to endothelial cells, membrane heme and membrane free iron, calcium containing internal vesicles and serum antioxidants. Results of these studies, including estimates of organ damage (bone, spleen, retina), were similar except for an increase in red cell membrane free iron in the patient with more frequent and severe painful crises. The study supports the concept that non-inherited factors are important contributors to the frequency and severity of painful crises in sickle cell anemia.

Use of chlortetracycline fluorescence for the detection of Ca storing intracellular vesicles in normal human erythrocytes

The uptake of chlortetracycline (CTC) and the nature of the fluorescence of CTC was studied in intact human erythrocytes from apparently healthy donors. The uptake of CTC at 22 degrees C proceeded with a t1/2 of about 3 min, and after 15 min a stable equilibrium was achieved with an intracellular accumulation by a factor of 5-6 relative to the medium concentration. The accumulation did not change in the range of CTC concentrations tested (20-500 microM). The Ca specificity of the CTC fluorescence spectrum was confirmed by Ca depletion of red cells using A23187 in the presence of EGTA and 0.2 mM Mg. This procedure decreased the total intracellular calcium content by about 70% and reduced the fluorescence intensity to one-fourth. Fluorescence microscopy of red cells incubated with 100 microM CTC at 22 degrees C showed that the fluorescence originated mainly from the red cell membrane. In addition, in about 15% of erythrocytes one or more fluorescent dots (diameter greater than 0.2 less than 1 microns) were detected. The fluorescence of the dots and membranes was related to calcium, as evidenced by the reduction of their intensity in Ca depleted cells. The number of erythrocytes with fluorescent dots and the frequency of the dots per cell was largely unaffected by lowering the incubation temperature to 0 degrees C, indicating that the dots most probably do not represent endocytotic artifacts induced by CTC. The number of dots was increased in erythrocytes preincubated with primaquine, demonstrating that CTC fluorescence can be applied to monitor the appearance of intracellular Ca storing vesicles. It is concluded that in (at least) 15% of erythrocytes obtained from apparently healthy donors intracellular vesicles containing Ca can be detected by CTC fluorescence microscopy.

Alteration in protein kinase C activity and subcellular distribution in sickle erythrocytes

In agreement with previous data, membrane protein phosphorylation was found to be altered in intact sickle cells (SS) relative to intact normal erythrocytes (AA). Similar changes were observed in their isolated membranes. The involvement of protein kinase C (PKC) in this process was investigated. The membrane PKC content in SS cells, measured by [3H]phorbol ester binding, was about 6-times higher than in AA cells. In addition, the activity of the enzyme, measured by histone phosphorylation was also found to be increased in SS cell membranes but decreased in their cytosol compared to the activity in AA cell membranes and cytosol. The increase in membrane PKC activity was observed mostly in the light fraction of SS cells, fractionated by density gradient, whereas the decrease in cytosolic activity was only observed in the dense fraction. PKC activity, measured in cells from the blood of reticulocyte-rich patients, exhibited an increase in both membranes and cytosol, thus explaining some of the effects observed in the SS cell light fraction, which is enriched in reticulocytes. The increase in PKC activity in the membranes of SS cells is partly explained by their young age but the loss of PKC activity in their cytosol, particularly in that of the dense fraction, seems to be specific to SS erythrocytes. The relative decrease in membrane PKC activity between the dense and the light fractions of SS cells might be related to oxidative inactivation of the enzyme.

Calcium content of the erythrocytes: a sensitive and easy handling method for measuring free calcium ions, and modulation of the Ca2+ ion concentration by the calcium antagonists nifedipine and pentoxifylline

1. A method for determining free Ca2+-ions in the erythrocyte is described, using a commercially available ORION-Ca-electrode and calomel reference electrode assembly, where changes in free Ca2+-ion concentration upon addition of 0.01% digitonin could be measured. 2. The average value found for fresh cells from 20 healthy donors at 37 degrees C (pH = 7.4) was 0.20 +/- 0.04 mumol/L referred to a haematocrit of 10%. 3. Decrease of the simultaneously determined adenosinetriphosphate (ATP) concentration indicates that ATP is presumably needed to activate the Ca-ATPase. 4. In vitro addition of the calcium antagonists pentoxifylline and nifedipine, respectively, induced a normalization of the intraerythrocytic Ca2+-ion concentration after previous increase with the ion carrier ionophore A23187. 5. The advantages and possible clinical applications of this method are discussed.

Endogenous calcium in sickle cells does not activate polyphosphoinositide phospholipase C

Sickle-cell-anaemia erythrocytes (SS cells) are known to have a high Ca2+ content (particularly the dense cell fraction) and to take up Ca2+ on deoxygenation. It has been reported that this high Ca2+ was responsible for the activation of the Ca2+-dependent K+ loss, and of the Ca2+-sensitive polyphosphoinositide phospholipase C (PIC) in dense SS cells. We found that, either in the total population of SS cells or in the light or dense fractions, the content of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] was not changed, whereas that of phosphatidylinositol 4-phosphate was increased and that of phosphatidic acid (PtdOH) was decreased compared with normal (AA) erythrocytes. Deoxygenation-induced Ca2+ entry into SS cells did not change the concentration or, in 32P-prelabelled cells, the radioactivity of polyphosphoinositides and PtdOH. It also failed to induce the formation of inositol 1,4,5-trisphosphate, the product of PtdIns(4,5)P2 hydrolysis by PIC, which was measured by an original method using ion-pair reverse-phase h.p.l.c. Thus there was no evidence of an endogenous Ca2+ effect on the PIC activity in SS cells, in agreement with the demonstration that the excess Ca2+ in SS cells is compartmentalized into internal vesicles and unavailable as free Ca2+. The 32P incorporation in polyphosphoinositides and PtdOH was markedly higher in SS than in AA cells, but this increase was the same in both dense and light SS cells. The increase in the turnover of these phospholipids in SS cells is consistent either with an activation of the lipid kinases and phosphatases or with perturbation in the metabolic compartmentation of these lipids.

Rheology of the sickle cell disorders

The sickling process causes secondary changes in cell shape, size, cation and water content, and membrane structure that contribute to the impairment of intrinsic cell deformability (Figure 2). This rheological defect is partially compensated by a low haematocrit, which moderates the rise in whole-blood viscosity, and by a rise in cardiac output which increases capillary flow velocity (Berger and King, 1982). A delicate balance exists between these mechanisms and any local disturbance of this balance by pathological changes in factors extrinsic to the sickle cell (Figure 2) can precipitate vaso-occlusion. There is still considerable controversy over the site (arteriolar, capillary, or venular) of vaso-occlusion, the type of sickle cell (reversibly sickled or irreversibly sickled) that is primarily involved, and the relative importance of extra-erythrocytic precipitating factors such as stasis, hypoxia, hyperosmolality, acidosis, alteration in temperature, acute-phase rise in plasma proteins and leukocytes, prothrombotic changes in coagulation factors and platelets, and adhesion of blood cells to vascular endothelium (Figure 2). A low-grade hypercoagulable state has been described in patients with SS (Leichtman and Brewer, 1978; Richardson et al, 1979) which may be related to the procoagulant effect of the shift of phosphatidyl serine to the outer lipid bilayer of the sickle cell (Chiu et al, 1981; Franck et al, 1985). Platelets appear to accumulate at sites of vaso-occlusion (Siegel et al, 1985) and their migration to the vessel wall may be enhanced by the presence of poorly deformable erythrocytes (Aarts et al, 1984). Endothelial cell damage in the arterial or venous circulation may also contribute (Klug et al, 1982). Thus vaso-occlusion appears to result from a complex interaction between blood cells, plasma proteins and endothelium and any one of several precipitating factors may disturb the fragile steady state and cause a painful crisis. The study of sickle cells by rheological methods has considerable potential for investigating the pathophysiology of vaso-occlusive episodes in the SCD and for monitoring, both in vitro and ex vivo, the efficacy of antisickling compounds. Because of the multiple intrinsic and extrinsic factors that contribute to the rheological defect, it is not yet known which of these should be the primary target for an antisickling agent. In-vitro rheological studies in which different metabolic stresses can be applied to intact sickle cells in the presence of a putative antisickling drug should help to answer this question.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular calcium content of human erythrocytes: relation to sodium transport systems

To study the possible role of intracellular Ca (Cai) in controlling the activities of the Na+-K+ pump, the Na+-K+ cotransport and the Na+/Li+ exchange system of human erythrocytes, a method was developed to measure the amount of Ca embodied within the red cell. For complete removal of Ca associated with the outer aspect of the membrane, it proved to be essential to wash the cells in buffers containing less than 20 nM Ca. Ca was extracted by HClO4 in Teflon vessels boiled in acid to avoid Ca contaminations and quantitated by flameless atomic absorption. Cai of fresh human erythrocytes of apparently healthy donors ranged between 0.9 and 2.8 mumol/liter cells. The mean value found in females was significantly higher than in males. The interindividual different Ca contents remained constant over periods of more than one year. Sixty to 90% of Cai could be removed by incubation of the cells with A23187 and EGTA. The activities of the Na+-K+ pump, of Na+-K+ cotransport and Na+/Li+ exchange and the mean cellular hemoglobin content fell with rising Cai; the red cell Na+ and K+ contents rose with Cai. Ca depletion by A23187 plus EGTA as well as chelation of intracellular Ca2+ by quin-2 did not significantly enhance the transport rates. It is concluded that the large scatter of the values of Cai of normal human erythrocytes reported in the literature mainly results from a widely differing removal of Ca associated with the outer aspect of the membrane.

Cation depletion by the sodium pump in red cells with pathologic cation leaks. Sickle cells and xerocytes

The mechanism by which sickle cells and xerocytic red cells become depleted of cations in vivo has not been identified previously. Both types of cells exhibit elevated permeabilities to sodium and potassium, in the case of sickle cells, when deoxygenated. The ouabain-insensitive fluxes of sodium and potassium were equivalent, however, in both cell types under these conditions. When incubated 18 hours in vitro, sickle cells lost cations but only when deoxygenated. This cation depletion was blocked by ouabain, removal of external potassium, or pretreatment with 4,4'-diisothiocyanostilbene-2,2'-disulfonate, which blocks the increase in cation permeability induced by deoxygenation. The loss of cation exhibited by oxygenated xerocytes similarly incubated was also blocked by ouabain. These data support the hypothesis that the elevated "passive" cation fluxes of xerocytes and deoxygenated sickle cells are not directly responsible for cation depletion of these cells; rather, these pathologic leaks interact with the sodium pump to produce a net loss of cellular cation.

Heinz bodies induce clustering of band 3, glycophorin, and ankyrin in sickle cell erythrocytes

In earlier model studies we demonstrated that artificially denatured hemoglobin binds to and clusters the protein, band 3, in the plane of the erythrocyte membrane. To determine whether denatured hemoglobin also clusters band 3 in vivo, we have compared the locations of denatured hemoglobin aggregates (Heinz bodies) with band 3 in sickle cells using phase contrast and immunofluorescence microscopy. We report that where Heinz bodies are found associated with the cytoplasmic surface of the membrane, clusters of band 3 are usually colocalized within the membrane. In contrast, normal erythrocyte membranes and regions of sickle cell membranes devoid of Heinz bodies display an uninterrupted staining of band 3. Similarly, ankyrin and glycophorin are periodically seen to aggregate at Heinz body sites, but the degree of colocalization is lower than for band 3. These data demonstrate that the binding of denatured hemoglobin to the membrane forces a redistribution of several major membrane components.