Charybdotoxin blocks with high affinity the Ca-activated K+ channel of Hb A and Hb S red cells: individual differences in the number of channels

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.

Emerging drug targets for sickle cell disease: shedding light on new knowledge and advances at the molecular level

INTRODUCTION

In sickle cell disease (SCD), a single amino acid substitution at β6 of the hemoglobin (Hb) chain replaces glutamate with valine, forming HbS instead of the normal adult HbA. Loss of a negative charge, and the conformational change in deoxygenated HbS molecules, enables formation of HbS polymers. These not only distort red cell morphology but also have other profound effects so that this simple etiology belies a complex pathogenesis with multiple complications. Although SCD represents a common severe inherited disorder with life-long consequences, approved treatments remain inadequate. Hydroxyurea is currently the most effective, with a handful of newer treatments, but there remains a real need for novel, efficacious therapies.

AREAS COVERED

This review summarizes important early events in pathogenesis to highlight key targets for novel treatments.

EXPERT OPINION

A thorough understanding of early events in pathogenesis closely associated with the presence of HbS is the logical starting point for identification of new targets rather than concentrating on more downstream effects. We discuss ways of reducing HbS levels, reducing the impact of HbS polymers, and of membrane events perturbing cell function, and suggest using the unique permeability of sickle cells to target drugs specifically into those more severely compromised.

Mechanistic ion channel interactions in red cells of patients with Gárdos channelopathy

In patients with Gárdos channelopathy (p.R352H), an increased concentration of intracellular Ca2+ was previously reported. This is a surprising finding because the Gárdos channel (KCa3.1) is a K+ channel. Here, we confirm the increased intracellular Ca2+ for patients with the KCa3.1 mutation p.S314P. Furthermore, we provide the concept of KCa3.1 activity resulting in a flickering of red blood cell (RBC) membranepotential, which activates the CaV2.1 channel allowing Ca2+ to enter the RBC. Activity of the nonselective cation channel Piezo1 modulates the aforementioned interplay in away that a closed Piezo1 is in favor of the KCa3.1-CaV2.1 interaction. In contrast, Piezo1 openings compromise the membrane potential flickering, thus limiting the activity of CaV2.1. With the compound NS309, we mimic a gain-of-function mutation of KCa3.1. Assessing the RBC Ca2+ response by Fluo-4-based flow cytometry and by measuring the membrane potential using the Macey-Bennekou-Egée method, we provide data that support the concept of the KCa3.1/CaV2.1/Piezo1 interplay as a partial explanation for an increased number of high Ca2+ RBCs. With the pharmacological inhibition of KCa3.1 (TRAM34 and Senicapoc), CaV2.1 (ω-agatoxin TK), and Piezo1 (GsMTx-4), we could project the NS309 behavior of healthy RBCs to the RBCs of Gárdos channelopathy patients.

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 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.

Erythrocytes as a biological model for screening of xenobiotics toxicity

Erythrocytes are the main cells in circulation. They are devoid of internal membrane structures and easy to be isolated and handled providing a good model for different assays. Red blood cells (RBCs) plasma membrane is a multi-component structure that keeps the cell morphology, elasticity, flexibility and deformability. Alteration of membrane structure upon exposure to xenobiotics could induce various cellular abnormalities and releasing of intracellular components. Therefore the morphological changes and extracellular release of haemoglobin [hemolysis] and increased content of extracellular adenosine triphosphate (ATP) [as signs of membrane stability] could be used to evaluate the cytotoxic effects of various molecules. The nucleated RBCs from birds, fish and amphibians can be used to evaluate genotoxicity of different xenobiotics using comet, DNA fragmentation and micronucleus assays. The RBCs could undergo programmed cell death (eryptosis) in response to injury providing a useful model to analyze some mechanisms of toxicity that could be implicated in apoptosis of nucleated cells. Erythrocytes are vulnerable to peroxidation making it a good biological membrane model for analyzing the oxidative stress and lipid peroxidation of various xenobiotics. The RBCs contain a large number of enzymatic and non-enzymatic antioxidants. The changes of the RBCs antioxidant capacity could reflect the capability of xenobiotics to generate reactive oxygen species (ROS) resulting in oxidative damage of tissue. These criteria make RBCs a valuable in vitro model to evaluate the cytotoxicity of different natural or synthetic and organic or inorganic molecules by cellular damage measures.

'Gardos Channelopathy': a variant of hereditary Stomatocytosis with complex molecular regulation

The Gardos channel is a Ca²⁺ sensitive, K⁺ selective channel present in several tissues including RBCs, where it is involved in cell volume regulation. Recently, mutations at two different aminoacid residues in KCNN4 have been reported in patients with hereditary xerocytosis. We identified by whole exome sequencing a new family with two members affected by chronic hemolytic anemia carrying mutation R352H in the KCNN4 gene. No additional mutations in genes encoding for RBCs cytoskeletal, membrane or channel proteins were detected. We performed functional studies on patients’ RBCs to evaluate the effects of R352H mutation on the cellular properties and eventually on the clinical phenotype. Gardos channel hyperactivation was demonstrated in circulating erythrocytes and erythroblasts differentiated ex-vivo from peripheral CD34+ cells. Pathological alterations in the function of multiple ion transport systems were observed, suggesting the presence of compensatory effects ultimately preventing cellular dehydration in patient’s RBCs; moreover, flow cytometry and confocal fluorescence live-cell imaging showed Ca²⁺ overload in the RBCs of both patients and hypersensitivity of Ca²⁺ uptake by RBCs to swelling. Altogether these findings suggest that the ‘Gardos channelopathy’ is a complex pathology, to some extent different from the common hereditary xerocytosis.

Functional characterization and modified rescue of novel AE1 mutation R730C associated with overhydrated cation leak stomatocytosis

We report the novel, heterozygous AE1 mutation R730C associated with dominant, overhydrated, cation leak stomatocytosis and well-compensated anemia. Parallel elevations of red blood cell cation leak and ouabain-sensitive Na(+) efflux (pump activity) were apparently unaccompanied by increased erythroid cation channel-like activity, and defined ouabain-insensitive Na(+) efflux pathways of nystatin-treated cells were reduced. Epitope-tagged AE1 R730C at the Xenopus laevis oocyte surface exhibited severely reduced Cl(-) transport insensitive to rescue by glycophorin A (GPA) coexpression or by methanethiosulfonate (MTS) treatment. AE1 mutant R730K preserved Cl(-) transport activity, but R730 substitution with I, E, or H inactivated Cl(-) transport. AE1 R730C expression substantially increased endogenous oocyte Na(+)-K(+)-ATPase-mediated (86)Rb(+) influx, but ouabain-insensitive flux was minimally increased and GPA-insensitive. The reduced AE1 R730C-mediated sulfate influx did not exhibit the wild-type pattern of stimulation by acidic extracellular pH (pH(o)) and, unexpectedly, was partially rescued by exposure to sodium 2-sulfonatoethyl methanethiosulfonate (MTSES) but not to 2-aminoethyl methanethiosulfonate hydrobromide (MTSEA) or 2-(trimethylammonium)ethyl methanethiosulfonate bromide (MTSET). AE1 R730E correspondingly exhibited acid pH(o)-stimulated sulfate uptake at rates exceeding those of wild-type AE1 and AE1 R730K, whereas mutants R730I and R730H were inactive and pH(o) insensitive. MTSES-treated oocytes expressing AE1 R730C and untreated oocytes expressing AE1 R730E also exhibited unprecedented stimulation of Cl(-) influx by acid pH(o). Thus recombinant cation-leak stomatocytosis mutant AE1 R730C exhibits severely reduced anion transport unaccompanied by increased Rb(+) and Li(+) influxes. Selective rescue of acid pH(o)-stimulated sulfate uptake and conferral of acid pH(o)-stimulated Cl(-) influx, by AE1 R730E and MTSES-treated R730C, define residue R730 as critical to selectivity and regulation of anion transport by AE1.

Age decline in the activity of the Ca2+-sensitive K+ channel of human red blood cells

The Ca(2+)-sensitive K(+) channel of human red blood cells (RBCs) (Gardos channel, hIK1, hSK4) was implicated in the progressive densification of RBCs during normal senescence and in the mechanism of sickle cell dehydration. Saturating RBC Ca(2+) loads were shown before to induce rapid and homogeneous dehydration, suggesting that Gardos channel capacity was uniform among the RBCs, regardless of age. Using glycated hemoglobin as a reliable RBC age marker, we investigated the age-activity relation of Gardos channels by measuring the mean age of RBC subpopulations exceeding a set high density boundary during dehydration. When K(+) permeabilization was induced with valinomycin, the oldest and densest cells, which started nearest to the set density boundary, crossed it first, reflecting conservation of the normal age-density distribution pattern during dehydration. However, when Ca(2+) loads were used to induce maximal K(+) fluxes via Gardos channels in all RBCs (F(max)), the youngest RBCs passed the boundary first, ahead of the older RBCs, indicating that Gardos channel F(max) was highest in those young RBCs, and that the previously observed appearance of uniform dehydration concealed a substantial degree of age scrambling during the dehydration process. Further analysis of the Gardos channel age-activity relation revealed a monotonic decline in F(max) with cell age, with a broad quasi-Gaussian F(max) distribution among the RBCs.

Evidence for a protective role of the Gardos channel against hemolysis in murine spherocytosis

It has been shown that mice with complete deficiency of all 4.1R protein isoforms (4.1-/-) exhibit moderate hemolytic anemia, with abnormal erythrocyte morphology (spherocytosis) and decreased membrane stability. Here, we characterized the Gardos channel function in vitro and in vivo in erythrocytes of 4.1-/- mice. Compared with wild-type, the Gardos channel of 4.1-/- erythrocytes showed an increase in Vmax (9.75 +/- 1.06 vs 6.08 +/- 0.09 mM cell x minute; P < .04) and a decrease in Km (1.01 +/- 0.06 vs 1.47 +/- 1.02 microM; P < .03), indicating an increased sensitivity to activation by intracellular calcium. In vivo function of the Gardos channel was assessed by the oral administration of clotrimazole, a well-characterized Gardos channel blocker. Clotrimazole treatment resulted in worsening of anemia and hemolysis, with decreased red cell survival and increased numbers of circulating hyperchromic spherocytes and microspherocytes. Clotrimazole induced similar changes in 4.2-/- and band 3+/- mice, indicating that these effects of the Gardos channel are shared in different models of murine spherocytosis. Thus, potassium and water loss through the Gardos channel may play an important protective role in compensating for the reduced surface-membrane area of hereditary spherocytosis (HS) erythrocytes and reducing hemolysis in erythrocytes with cytoskeletal impairments.

Ca2+-dependent K+ channels from rat olfactory cilia characterized in planar lipid bilayers

Olfactory cilia contain cyclic nucleotide-gated and Ca2+-dependent Cl- conductances that underlie excitatory chemotransduction, and a Ca2+-dependent K+ (KCa) conductance, apparently involved in inhibitory transduction. Previous single-channel patch-clamp studies on olfactory cilia revealed four different KCas, with different conductances and kinetics. Here, we further characterized these channels in planar bilayers, where blockers could be properly tested. All four ciliary KCas were observed: The 16 pS channel, K0.5,Ca=40 microM and apamin-sensitive; the 30 and 50 pS channel, K0.5,Ca=59 microM, clotrimazole-sensitive and charybdotoxin-insensitive; the 60 pS channel, clotrimazole-sensitive and charybdotoxin-insensitive; and the 210 pS channel, K0.5,Ca=63 microM, blocked by charybdotoxin and iberiotoxin. The presence of the 16 and 210 pS channels was confirmed by immunoblotting.

Butoxyacetic acid-induced hemolysis of rat red blood cells: effect of external osmolarity and cations

Hemolysis is the principal toxicity of acute exposure to ethylene glycol monobutyl ether (EGBE) in rats. EGBE itself is not an active hemolytic agent, but its metabolite, butoxyacetic acid (BAA) formed as a result of dehydrogenase activity is a potent hemolysin. Here we address the role of osmolarity and cation composition of the suspending buffers in the mechanism of BAA-induced hemolysis of rat red blood cells in vitro. Rat erythrocytes were protected from BAA-induced cell swelling and hemolysis by the addition of sucrose to the suspending media. Hemolysis and cell swelling were also reduced by replacing external sodium with potassium. When calcium was not present in the suspending medium or when chelated by EGTA, hemolysis was increased after 2 h incubation with 1 mM or 2 mM BAA. Addition of as little as 0.05 mM CaCl(2) reduced hemolysis significantly while the addition of MgCl(2) had no effect. The dose-response relationship between BAA concentration and hemolysis determined in the presence or absence of calcium showed an increased effect of BAA in the absence of calcium. BAA-induced spherocytosis and cell fragmentation were more pronounced in the absence of calcium. The time course of BAA-induced hemolysis in the presence and absence of calcium demonstrated that the effect of calcium is to delay the onset of hemolysis. Increased intracellular calcium as a result of exposure to BAA was verified by atomic absorption spectroscopy. Charybdotoxin, an inhibitor of the calcium activated potassium channel, blocked the protective effect of calcium suggesting that the delay of onset of hemolysis in the presence of calcium is due to potassium loss caused by this channel. We conclude that the mode of action of BAA is to cause a colloid osmotic lysis of the rat red blood cell. Hemolysis requires external sodium and is associated with calcium uptake. Calcium appears to delay the onset of hemolysis. We speculate that: (1) BAA causes sodium and calcium to enter the cell; (2) calcium initially has a protective effect via the calcium activated potassium channel which facilitates the loss of potassium thereby, compensating for the osmotic effect of increased cell sodium; (3) calcium subsequently may have other deleterious effects through activation of proteases and externalization of phosphatidylserine in the exterior leaflet of the membrane.

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.

Distribution of dehydration rates generated by maximal Gardos-channel activation in normal and sickle red blood cells

The Ca(2+)-activated K+ channels of human red blood cells (RBCs) (Gardos channels, hIK1, hSK4) can mediate rapid cell dehydration, of particular relevance to the pathophysiology of sickle cell disease. Previous investigations gave widely discrepant estimates of the number of Gardos channels per RBC, from as few as 1 to 3 to as many as 300, with large cell-to-cell differences, suggesting that RBCs could differ extensively in their susceptibility to dehydration by elevated Ca2+. Here we investigated the distribution of dehydration rates induced by maximal and uniform Ca2+ loads in normal (AA) and sickle (SS) RBCs by measuring the time-dependent changes in osmotic fragility and RBC volume distributions. We found a remarkable conservation of osmotic lysis and volume distribution profiles during Ca(2+)-induced dehydration, indicating overall uniformity of dehydration rates among AA and SS RBCs. In light of these results, alternative interpretations were suggested for the previously proposed low estimates and heterogeneity of channel numbers per cell. The results support the view that stochastic Ca2+ permeabilization rather than Gardos-channel variation is the main determinant selecting which SS cells dehydrate through Gardos channels in each sickling episode.

The voltage-dependent nonselective cation current in human red blood cells studied by means of whole-cell and nystatin-perforated patch-clamp techniques

Human red blood cells (RBC) can be studied by means of whole-cell and nystatin-perforated patch-clamp techniques. In 85% of the whole-cell experiments (n=86) and 69% of the perforated-patch recordings (n=13), steps to positive potentials, from a holding potential of 0 mV, induced a slow-activating non-inactivating persistent outward current which reverted at about 0 mV. The current activation phase fitted well with a two-component exponential curve. Half-maximal conductance was reached at about 42 mV. Na+ and K+ carried this current, which was not affected by 20 nM charybdotoxin or 20 mM TEA, but was reduced following a partial substitution of extracellular Cl- by tartrate. This current has characteristics similar to the single-channel currents already described in RBC and may be involved in the rapid adaptations of these cells in the circulation.

The paradox of hemoglobin SC disease

Homozygous HbC gene results only in mild hemolytic anemia. In HbSC disease red cells contain equal levels of HbS and HbC. It is a paradox that HbSC exhibit a moderately severe phenotype in spite of being a mixture of HbS trait and HbC trait, neither of which has significant pathology. Why does the combination of these two Hbs result in a serious disease? The short answer is that HbC enhances, by dehydrating the SC red cell, the pathogenic properties of HbS, resulting in a clinically significant disorder, but somewhat milder that sickle cell anemia (SCA). Nevertheless, retinnitis proliferans, osteonecrosis, and acute chest syndrome have equal or higher incidence in HbSC disease compared to SCA. This pathogenic trick is accomplished by HbC inducing, by mechanisms not fully understood, an increase in the activity of K:Cl cotransport that induces the lost of K(+) and consequently of intracellular water. This event creates a sufficient increase of MCHC, so that the lower levels of HbS found in SC red cells can polymerize rapidly and effectively. This situation offers a unique opportunity: if we could inhibit the effect of HbC on K(+) transport we can cure the disease.

The Gárdos channel: a review of the Ca2+-activated K+ channel in human erythrocytes

Ca(2+)-dependent K(+) efflux from human erythrocytes was first described in the 1950s. Subsequent studies revealed that a K(+)-specific membrane protein (the Gárdos channel) was responsible for this phenomenon (the Gárdos effect). In recent years several types of Ca-activated K(+) channel have been identified and studied in a wide range of cells, with the erythrocyte Gárdos channel serving as both a model for a broader physiological perspective, and an intriguing component of erythrocyte function. The existence of this channel has raised a number of questions. For example, what is its role in the establishment and maintenance of ionic distribution across the red cell membrane? What role might it play in erythrocyte development? To what extent is it active in circulating erythrocytes? What are the cell-physiological implications of its dysfunction?This review summarises current knowledge of this membrane protein with respect to its function and structure, its physiological roles (some putative) and its contribution to various disease states, and it provides an introduction to adaptable NMR methods, which is our own area of technical expertise, for such ion transport analysis.

Small-conductance calcium-activated K+ channels are expressed in pancreatic islets and regulate glucose responses

Glucose-stimulated insulin secretion is associated with transients of intracellular Ca(2+) concentration [Ca(2+)](i) in the pancreatic beta-cell. We identified the expression and function of specific small-conductance Ca(2+)-activated K(+) (SK) channel genes in insulin-secreting cells. The presence of mRNA for SK1, -2, -3, and -4 (intermediate-conductance Ca(2+)-activated K(+) 1 [IK1]) channels was demonstrated by RT-PCR in rodent islets and insulinoma cells. SK2 and -3 proteins in mouse islets were detected by immunoblot and immunocytochemistry. In the tTA-SK3 tet-off mouse, a normal amount of SK3 protein was present in islets, but it became undetectable after exposure to doxycycline (DOX), which inhibits the transcription of the tTA-SK3 gene. The SK/IK channel-blockers apamin, dequalinium, and charybdotoxin caused increases in average [Ca(2+)](i) levels and in frequency of [Ca(2+)](i) oscillations in wild-type mouse islets. In SK3-tTA tet-off mice, the addition of apamin with glucose and tetraethylammonium (TEA) caused a similar elevation in [Ca(2+)](i), which was greatly diminished after DOX suppression of SK3 expression. We conclude that SK1, -2, -3, and IK1 (SK4) are expressed in islet cells and insulin-secreting cells and are able to influence glucose-induced calcium responses, thereby regulating insulin secretion.

Membrane transport in sickle cell disease

We have reviewed here a number of membrane transport events in red cells from normal individuals and sickle cell patients which respond to changes in O(2) tension. Some deoxygenation-induced changes in membrane permeability are unique to HbS cells and contribute to their dehydration and subsequent sickling. Polymerization of HbS, or specific oxidant damage (or altered redox potential), is a likely factor underlying the abnormal behavior. The key regulatory sites within the membrane or associated proteins remain uncertain and their identity will form the focus of future research. A model for sickle cell dehydration is presented. Inhibition of these permeability changes represents possible avenues for future chemotherapy to ameliorate the condition.

Modulation of Gardos channel activity by cytokines in sickle erythrocytes

It has recently been shown that the Gardos channel activity of mouse erythrocytes can be modified by endothelins, suggesting a functional linkage between endothelin receptors and the Gardos channel. Using (86)Rubidium ((86)Rb) influx, effects were estimated of proinflammatory molecules such as platelet activator factor (PAF), endothelin-1 (ET-1), interleukin-10 (IL-10), and regulated on activation normal T cells expressed and secreted (RANTES) on the Gardos channel activity in human normal and sickle red cells. It was found that PAF (EC(50): 15 +/- 7 nM), RANTES (EC(50), 9 +/- 6 ng/mL [1.2 +/- 0.8 nM]), IL-10 (EC(50), 11 +/- 8 ng/mL [204 +/- 148 nM]), and ET-1 (EC(50), 123 +/- 34 nM) induce a significant increase in Gardos channel activity-between 28% and 84%-over the control. In addition, these agents modify the Gardos channel affinity for internal Ca(++) (K(0.5)) by 2- to 6-fold. Biochemical evidence is provided for the presence of ET receptor subtype B in sickle and normal red cells. Furthermore, it was found that ET-1, PAF, RANTES, and IL-10 induce a significant increase in red cell density (P <.05). These data suggest that activation of the Gardos channel is functionally coupled to receptor motifs such as C-X-C (PAF), C-C (RANTES), and ET receptor subtype B. Thus, cell volume regulation or erythrocyte hydration states might be altered by activation of the Gardos channel by cytokines in vivo. The role of these mediators in promoting sickle cell dehydration in vivo is under investigation.

Oxygen-sensitive cation transport in sickle cells

There are many examples of O2-sensitive solute transport in vertebrate red cells. The response is selective, specific, and conserved across the entire vertebrate spectrum. A number of possible physiological roles have been proposed, but abnormal responses to O2 may also be important pathologically. Significant alterations in O2 dependence of red cell cation transport are observed in sickle cell disease (and also following exposure to oxidants) and probably contribute to its pathophysiology. In this paper, we review some of the features of O2-sensitive solute transporters in red cells and possible reasons for the abnormal response in sickle cells. Our aim is to identify specific, novel pharmacological inhibitors of these abnormal pathways and thereby ameliorate the disease.

Synthesis and structure-activity relationships of cetiedil analogues as blockers of the Ca(2+)-activated K+ permeability of erythrocytes

Cetiedil, [2-cyclohexyl-2-(3-thienyl)ethanoic acid 2-(hexahydro-1H-azepin-1-yl)ethyl ester], which blocks the intermediate calcium-activated potassium ion permeability (IK(Ca)) in red blood cells, was used as a lead for investigating structure-activity relationships with the aim of determining the pharmacophore and of synthesizing agents of greater potency. A series of compounds having structures related to cetiedil was made and tested on rabbit erythrocytes. Channel blocking activity within the series was found to correlate well with octanol-water partition coefficients but not with the specific chemical structure of the acid moiety. However, whereas log P for the compounds spans a range of values over 4 orders of magnitude, potency only increases by 2 orders. This suggests that hydrophobic interactions with an active site on the channel are probably not the main determinants of activity. It seems more likely that increased lipophilicity enhances access to the channel, probably from within the cell membrane. In keeping with this interpretation, cetiedil methoiodide was found to be inactive. Triphenylethanoic was found to be a more effective acid grouping than 2-cyclohexyl-2-(3-thienyl)ethanoic, and its 2-(hexahydro-1H-azepin-l-yl)ethyl ester (11) was approximately 3 times more potent than cetiedil. The 9-benzylfluoren-9-yl carboxylic acid ester (21) was found to be approximately 9 times more active than cetiedil, and replacing -CO(2)- in 21 by an ethynyl (-C identical to C-) linkage (compound 26, UCL 1608) increased potency by some 15-fold over that of cetiedil.

Calmodulin binding to the C-terminus of the small-conductance Ca2+-activated K+ channel hSK1 is affected by alternative splicing

We identified three splice variants of hSK1 whose C-terminal structures are determined by the independent deletion of two contiguous nucleotide sequences. The upstream sequence extends 25 bases in length, is initiated by a donor splice site within exon 8, and terminates at the end of the exon. The downstream sequence consists of nine bases that compose exon 9. When the upstream sequence (hSK1(-)(25b)) or both sequences (hSK1(-)(34b)) are deleted, truncated proteins are encoded in which the terminal 118 amino acids are absent. The binding of calmodulin to these variants is diminished, particularly in the absence of Ca2+ ions. The first 20 amino acids of the segment deleted from hSK1(-)(25b) and hSK1(-)(34b) contain a 1-8-14 Ca2+ calmodulin binding motif, and synthetic oligopeptides based on this region bind calmodulin better in the presence than absence of Ca2+ ions. When the downstream sequence (hSK1(-)(9b)) alone is deleted, only the three amino acids A452, Q453, and K454 are removed, and calmodulin binding is not reduced. On the basis of the relative abundance of mRNA encoding each of the four isoforms, the full-length variant appears to account for most hSK1 in the human hippocampus, while hSK1(-)(34b) predominates in reticulocytes, and hSK1(-)(9b) is especially abundant in human erythroleukemia cells in culture. We conclude that the binding of calmodulin by hSK1 can be modulated through alternative splicing.

O2 dependence of K+ transport in sickle cells: the effect of different cell populations and the substituted benzaldehyde 12C79

The molecular basis of sickle cell disease (SCD) is well known but the pathophysiology is poorly understood. It remains intractable to therapy. Hyperactivity of several membrane transport systems, including the K+-Cl- cotransporter (termed KCC), cause HbS-containing red cells (termed HbS cells) to dehydrate and sickle, leading to the development of sickle cell crises (SCCs). Contrary to normal red cells (HbA cells), KCC in HbS cells is active at low O2 tensions (PO2s), remaining responsive to low pH or urea. Since these stimuli are usually encountered in hypoxic regions, the abnormal O2 dependence increases the contribution of KCC to dehydration, and hence development of SCCs. These differences with HbA cells may be due to the younger population of cells or to polymerization of HbS. We used 86Rb+ as a K+ congener to investigate the activity of KCC at different PO2s, and density gradient separation to investigate different red cell fractions. We found no correlation of O2 dependence with cell fractions. We also used the substituted benzaldehyde 12C79 to increase the O2 affinity of HbS and found that its effect on HbS O2 saturation and cell sickling correlated with that on both Cl--independent and Cl--dependent K+ transport, implying that, at low PO2s, KCC activity correlated with HbS polymerization. The importance of these results to understanding the pathophysiology of SCD, and for the design of chemotherapeutic agents to ameliorate or prevent SCC, is discussed.

Effect of dimethyl adipimidate on K+ transport and shape change in red blood cells from sickle cell patients

Dimethyl adipimidate (DMA) reduces K+ loss from, and dehydration of, red cells containing haemoglobin S (HbS cells). Three membrane transporters may contribute to these processes: the deoxygenation-induced cation-selective channel (Psickle), the Ca2+-activated K+ channel (or Gardos channel) and the K+-CI- cotransporter (KCC). We show that DMA inhibited all three pathways in deoxygenated HbS cells. The Gardos channel could be activated following Ca2+ loading. Considerable KCC activity was present in oxygenated HbS cells, showing a selective action of DMA on the transporter in deoxygenated cells. Inhibition of sickling correlated strongly with that of Psickle and moderately with that of KCC activity. We conclude that DMA does not inhibit the K+ pathways directly, but acts mainly by preventing HbS polymerisation and sickling. These findings are relevant to the development of novel chemotherapeutic agents for amelioration of sickle cell disease.

Endothelins activate Ca(2+)-gated K(+) channels via endothelin B receptors in CD-1 mouse erythrocytes

Cell dehydration mediated by Ca(2+)-activated K(+) channels plays an important role in the pathogenesis of sickle cell disease. CD-1 mouse erythrocytes possess a Ca(2+)-activated K(+) channel (Gardos channel) with maximal velocity (V(max)) of 0.154 +/- 0.02 mmol. l cells(-1). min(-1) and an affinity constant (K(0.5)) for Ca(2+) of 286 +/- 83 nM in the presence of A-23187. Cells pretreated with 500 nM endothelin-1 (ET-1) increased their V(max) by 88 +/- 9% (n = 8) and decreased their K(0.5) for Ca(2+) to 139 +/- 63 nM (P < 0.05; n = 4). Activation of the Gardos channel resulted in an EC(50) of 75 +/- 20 nM for ET-1 and 374 +/- 97 nM for ET-3. Analysis of the affinity of unlabeled ET-1 for its receptor showed two classes of binding sites with apparent dissociation constants of 167 +/- 51 and 785 +/- 143 nM and with capacity of binding sites of 298 +/- 38 and 1,568 +/- 211 sites/cell, respectively. The Gardos channel was activated by the endothelin B (ET(B)) receptor agonist IRL 1620 and inhibited by BQ-788, demonstrating the involvement of ET(B) receptors. Calphostin C inhibited 73% of ET-1-induced Gardos activation and 84% of the ET-1-induced membrane protein kinase C activity. Thus endothelins regulate erythrocyte Gardos channels via ET(B) receptors and a calphostin-sensitive mechanism.

Mild spherocytosis and altered red cell ion transport in protein 4. 2-null mice

Protein 4.2 is a major component of the red blood cell (RBC) membrane skeleton. We used targeted mutagenesis in embryonic stem (ES) cells to elucidate protein 4.2 functions in vivo. Protein 4. 2-null (4.2(-/-)) mice have mild hereditary spherocytosis (HS). Scanning electron microscopy and ektacytometry confirm loss of membrane surface in 4.2(-/-) RBCs. The membrane skeleton architecture is intact, and the spectrin and ankyrin content of 4. 2(-/-) RBCs are normal. Band 3 and band 3-mediated anion transport are decreased. Protein 4.2(-/-) RBCs show altered cation content (increased K+/decreased Na+)resulting in dehydration. The passive Na+ permeability and the activities of the Na-K-2Cl and K-Cl cotransporters, the Na/H exchanger, and the Gardos channel in 4. 2(-/-) RBCs are significantly increased. Protein 4.2(-/-) RBCs demonstrate an abnormal regulation of cation transport by cell volume. Cell shrinkage induces a greater activation of Na/H exchange and Na-K-2Cl cotransport in 4.2(-/-) RBCs compared with controls. The increased passive Na+ permeability of 4.2(-/-) RBCs is also dependent on cell shrinkage. We conclude that protein 4.2 is important in the maintenance of normal surface area in RBCs and for normal RBC cation transport.

Differences in the actions of some blockers of the calcium-activated potassium permeability in mammalian red cells

1. The actions of some inhibitors of the Ca2+-activated K+ permeability in mammalian red cells have been compared. 2. Block of the permeability was assessed from the reduction in the net loss of K+ that followed the application of the Ca2+ ionophore A23187 (2 microM) to rabbit red cells suspended at a haematocrit of 1% in a low potassium solution ([K]0 0.12-0.17 mM) at 37 degrees C. Net movement of K+ was measured using a K+-sensitive electrode placed in the suspension. 3. The concentrations (microM +/- s.d.) of the compounds tested causing 50% inhibition of K+ loss were: quinine, 37 +/- 3; cetiedil, 26 +/- 1; the cetiedil congeners UCL 1269, UCL 1274 and UCL 1495, approximately 150, 8.2 +/- 0.1, 0.92 +/- 0.03 respectively; clotrimazole, 1.2 +/- 0.1; nitrendipine, 3.6 +/- 0.5 and charybdotoxin, 0.015 +/- 0.002. 4. The characteristics of the block suggested that compounds could be placed in two groups. For one set (quinine, cetiedil, and the UCL congeners), the concentration-inhibition curves were steeper (Hill coefficient, nH, > or = 2.7) than for the other (clotrimazole, nitrendipine, charybdotoxin) for which nH approximately 1. 5. Compounds in the first set alone became less active on raising the concentration of K+ in the external solution to 5.4 mM. 6. The rate of K+ loss induced by A23187 slowed in the presence of high concentrations of cetiedil and its analogues, suggesting a use-dependent component to the inhibitory action. This was not seen with clotrimazole. 7. The blocking action of the cetiedil analogue UCL 1274 could not be overcome by an increase in external Ca2+ and its potency was unaltered when K+ loss was induced by the application of Pb2+ (10 microM) rather than by A23187. 8. These results, taken with the findings of others, suggest that agents that block the red cell Ca2+-activated K+ permeability can be placed in two groups with different mechanisms of action. The differences can be explained by supposing that clotrimazole and charybdotoxin act at the outer face of the channel whereas cetiedil and its congeners may block within it, either at or near the K+ binding site that determines the flow of K+.

Two Distinct Pathways Mediate the Formation of Intermediate Density Cells and Hyperdense Cells From Normal Density Sickle Red Blood Cells

In sickle cell anemia (SS), some red blood cells dehydrate, forming a hyperdense (HD) cell fraction (&gt;1.114 g/mL; mean corpuscular hemoglobin concentration [MCHC], &gt;46 g/dL) that contains many irreversibly sickled cells (ISCs), whereas other SS red blood cells dehydrate to an intermediate density (ID; 1.090 to 1.114 g/mL; MCHC, 36 to 46 g/dL). This study asks if the potassium-chloride cotransporter (K:Cl) and the calcium-dependent potassium channel [K(Ca2+)] are participants in the formation of one or both types of dense SS red blood cells. We induced sickling by exposing normal density (ND; 1.080 to 1.090 g/mL; MCHC, 32 to 36 g/dL) SS discocytes to repetitive oxygenation-deoxygenation (O-D) cycles in vitro. At physiologic Na+, K+, and Cl−, and 0.5 to 2 mmol/L Ca2+, the appearance of dense cells was time- and pH-dependent. O-D cycling at pH 7.4 in 5% CO2-equilibrated buffer generated only ID cells, whereas O-D cycling at pH 6.8 in 5% CO2-equilibrated buffer generated both ID and HD cells, the latter taking more than 8 hours to form. At 22 hours, 35% ± 17% of the parent ND cells were recovered in the ID fraction and 18% ± 11% in the HD fraction. Continuous deoxygenation (N2/5% CO2) at pH 6.8 generated both ID and HD cells, but many of these cells had multiple projections, clearly different from the morphology of endogenous dense cells and ISCs. Continuous oxygenation (air/5% CO2) at pH 6.8 resulted in less than 10% dense cell (ID + HD) formation. ATP depletion substantially increased HD cell formation and moderately decreased ID cell formation. HD cells formed after 22 hours of O-D cycling at pH 6.8 contained fewer F cells than did ID cells, suggesting that HD cell formation is particularly dependent on HbS polymerization. EGTA chelation of buffer Ca2+ inhibited HD but not ID cell formation, and increasing buffer Ca2+ from 0.5 to 2 mmol/L promoted HD but not ID cell formation in some SS patients. Substitution of nitrate for Cl− inhibited ID cell formation, as did inhibitors of the K:Cl cotransporter, okadaic acid, and [(dihydroindenyl) oxy]alkanoic acid (DIOA). Conversely, inhibitors of K(Ca2+), charybdotoxin and clotrimazole, inhibited HD cell formation. The combined use of K(Ca2+) and K:Cl inhibitors nearly eliminated dense cell (ID + HD cell) formation. In summary, dense cells formed by O-D cycling for 22 hours at pH 7.4 cycling are predominately the ID type, whereas dense cells formed by O-D cycling for 22 hours at pH 6.8 are both the ID and HD type, with the latter low in HbF, suggesting that HD cell formation has a greater dependency on HbS polymerization. A combination of K:Cl cotransport and the K(Ca2+) activities account for the majority of dense cells formed, and these pathways can be driven independently. We propose a model in which reversible sickling-induced K+ loss by K:Cl primarily generates ID cells and K+ loss by the K(Ca2+) channel primarily generates HD cells. These results imply that both pathways must be inhibited to completely prevent dense SS cell formation and have potential therapeutic implications.

Two Distinct Pathways Mediate the Formation of Intermediate Density Cells and Hyperdense Cells From Normal Density Sickle Red Blood Cells

In sickle cell anemia (SS), some red blood cells dehydrate, forming a hyperdense (HD) cell fraction (>1.114 g/mL; mean corpuscular hemoglobin concentration [MCHC], >46 g/dL) that contains many irreversibly sickled cells (ISCs), whereas other SS red blood cells dehydrate to an intermediate density (ID; 1.090 to 1.114 g/mL; MCHC, 36 to 46 g/dL). This study asks if the potassium-chloride cotransporter (K:Cl) and the calcium-dependent potassium channel [K(Ca2+)] are participants in the formation of one or both types of dense SS red blood cells. We induced sickling by exposing normal density (ND; 1.080 to 1.090 g/mL; MCHC, 32 to 36 g/dL) SS discocytes to repetitive oxygenation-deoxygenation (O-D) cycles in vitro. At physiologic Na+, K+, and Cl−, and 0.5 to 2 mmol/L Ca2+, the appearance of dense cells was time- and pH-dependent. O-D cycling at pH 7.4 in 5% CO2-equilibrated buffer generated only ID cells, whereas O-D cycling at pH 6.8 in 5% CO2-equilibrated buffer generated both ID and HD cells, the latter taking more than 8 hours to form. At 22 hours, 35% ± 17% of the parent ND cells were recovered in the ID fraction and 18% ± 11% in the HD fraction. Continuous deoxygenation (N2/5% CO2) at pH 6.8 generated both ID and HD cells, but many of these cells had multiple projections, clearly different from the morphology of endogenous dense cells and ISCs. Continuous oxygenation (air/5% CO2) at pH 6.8 resulted in less than 10% dense cell (ID + HD) formation. ATP depletion substantially increased HD cell formation and moderately decreased ID cell formation. HD cells formed after 22 hours of O-D cycling at pH 6.8 contained fewer F cells than did ID cells, suggesting that HD cell formation is particularly dependent on HbS polymerization. EGTA chelation of buffer Ca2+ inhibited HD but not ID cell formation, and increasing buffer Ca2+ from 0.5 to 2 mmol/L promoted HD but not ID cell formation in some SS patients. Substitution of nitrate for Cl− inhibited ID cell formation, as did inhibitors of the K:Cl cotransporter, okadaic acid, and [(dihydroindenyl) oxy]alkanoic acid (DIOA). Conversely, inhibitors of K(Ca2+), charybdotoxin and clotrimazole, inhibited HD cell formation. The combined use of K(Ca2+) and K:Cl inhibitors nearly eliminated dense cell (ID + HD cell) formation. In summary, dense cells formed by O-D cycling for 22 hours at pH 7.4 cycling are predominately the ID type, whereas dense cells formed by O-D cycling for 22 hours at pH 6.8 are both the ID and HD type, with the latter low in HbF, suggesting that HD cell formation has a greater dependency on HbS polymerization. A combination of K:Cl cotransport and the K(Ca2+) activities account for the majority of dense cells formed, and these pathways can be driven independently. We propose a model in which reversible sickling-induced K+ loss by K:Cl primarily generates ID cells and K+ loss by the K(Ca2+) channel primarily generates HD cells. These results imply that both pathways must be inhibited to completely prevent dense SS cell formation and have potential therapeutic implications.

HbS-Oman Heterozygote: A New Dominant Sickle Syndrome

Hemoglobin (Hb) S-Oman has two mutations in the β-chains. In addition to the classic βS mutation (β6 Glu → Val), it contains a second mutation in the same chain (β121 Glu → Lys) identical to that of HbOARAB. We have studied a pedigree of heterozygous carriers of HbS-Oman that segregates into two types of patients: those expressing about 20% HbS-Oman and concomitant −/ thalassemia and those with about 14% of HbS-Oman and concomitant −/− thalassemia. The higher expressors of S-Oman have a sickle cell anemia (SS) clinical syndrome of moderate intensity, while the lower expressors have no clinical syndrome, and are comparable to the solitary case first described in Oman. In addition, the higher expressors exhibit a unique form of irreversibly sickled cell reminiscent of a “yarn and knitting needle” shape, in addition to folded and target cells. The CSAT of S-Oman is identical to that of S-Antilles, another supersickling hemoglobin, whose carriers express the abnormal hemoglobin at 40% to 50%, with a very similar clinical picture to HbS-Oman. Because the level of expression is so different and the clinical picture so similar, and based on the hemolysates CSAT’s, we conclude that HbS-Oman produces pathology beyond its sickling tendencies. A clue for this additional pathogenesis is found in the fact that homozygous HbOARAB, which has the same second substitution as S-Oman, has a moderately severe hemolytic anemia; when HbOARAB is combined with HbS, it makes the phenotype of this double heterozygote as severe as SS. Properties of HbS-Oman red blood cells (RBCs) include reticulocytes that are much denser than normal (similar to those of SC and CC disease), a decrease in the Km for Ca2+ needed to activate the Gardos’ channel (making this transporter more sensitive to Ca2+), increased association of HbS-Oman with the RBC membrane, the presence of dense cells by isopycnic gradient, the presence of folded cells, and abundant nidus of polymerization under the membrane. Other properties include a clear increase in volume and N-ethylmaleimide–stimulated K:Cl cotransport in RBCs expressing more than 20% HbS-Oman. We conclude that the pathology of heterozygous S-Oman is the product of the sickling properties of the β6 Val mutation which are enhanced by the second mutation at β121. In addition, the syndrome is further enhanced by a hemolytic anemia induced by the mutation at β121. We speculate that this pathology results from the abnormal association of the highly positively charged HbS-Oman (3 charges different from normal hemoglobin) with the RBC membrane.

HbS-Oman Heterozygote: A New Dominant Sickle Syndrome

Hemoglobin (Hb) S-Oman has two mutations in the β-chains. In addition to the classic βS mutation (β6 Glu → Val), it contains a second mutation in the same chain (β121 Glu → Lys) identical to that of HbOARAB. We have studied a pedigree of heterozygous carriers of HbS-Oman that segregates into two types of patients: those expressing about 20% HbS-Oman and concomitant −/ thalassemia and those with about 14% of HbS-Oman and concomitant −/− thalassemia. The higher expressors of S-Oman have a sickle cell anemia (SS) clinical syndrome of moderate intensity, while the lower expressors have no clinical syndrome, and are comparable to the solitary case first described in Oman. In addition, the higher expressors exhibit a unique form of irreversibly sickled cell reminiscent of a “yarn and knitting needle” shape, in addition to folded and target cells. The CSAT of S-Oman is identical to that of S-Antilles, another supersickling hemoglobin, whose carriers express the abnormal hemoglobin at 40% to 50%, with a very similar clinical picture to HbS-Oman. Because the level of expression is so different and the clinical picture so similar, and based on the hemolysates CSAT’s, we conclude that HbS-Oman produces pathology beyond its sickling tendencies. A clue for this additional pathogenesis is found in the fact that homozygous HbOARAB, which has the same second substitution as S-Oman, has a moderately severe hemolytic anemia; when HbOARAB is combined with HbS, it makes the phenotype of this double heterozygote as severe as SS. Properties of HbS-Oman red blood cells (RBCs) include reticulocytes that are much denser than normal (similar to those of SC and CC disease), a decrease in the Km for Ca2+ needed to activate the Gardos’ channel (making this transporter more sensitive to Ca2+), increased association of HbS-Oman with the RBC membrane, the presence of dense cells by isopycnic gradient, the presence of folded cells, and abundant nidus of polymerization under the membrane. Other properties include a clear increase in volume and N-ethylmaleimide–stimulated K:Cl cotransport in RBCs expressing more than 20% HbS-Oman. We conclude that the pathology of heterozygous S-Oman is the product of the sickling properties of the β6 Val mutation which are enhanced by the second mutation at β121. In addition, the syndrome is further enhanced by a hemolytic anemia induced by the mutation at β121. We speculate that this pathology results from the abnormal association of the highly positively charged HbS-Oman (3 charges different from normal hemoglobin) with the RBC membrane.

Effects of PKC alpha activation on Ca2+ pump and K(Ca) channel in deoxygenated sickle cells

We have previously shown that a pretreatment with phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), reduced deoxygenation-induced K+ loss and Ca2+ uptake and prevented cell dehydration in sickle anemia red blood cells (SS cells) (H. Fathallah, E. Coezy, R.-S. De Neef, M.-D. Hardy-Dessources, and F. Giraud. Blood 86: 1999-2007, 1995). The present study explores the detailed mechanism of this PMA-induced inhibition. The main findings are, first, the detection of PKC alpha and PKC zeta in normal red blood cells and the demonstration that both isoforms are expressed at higher levels in SS cells. The alpha-isoform only is translocated to the membrane and activated by PMA and by elevation of cytosolic Ca2+. Second, PMA is demonstrated to activate Ca2+ efflux in deoxygenated SS cells by a direct stimulation of the Ca2+ pump. PMA, moreover, inhibits deoxygenation-induced, charybdotoxin-sensitive K+ efflux in SS cells. This inhibition is partly indirect and explained by the reduced deoxygenation-induced rise in cytosolic Ca2+ resulting from Ca2+ pump stimulation. However, a significant inhibition of the Ca2+-activated K+ channels (K(Ca) channels) by PMA can also be demonstrated when the channels are activated by Ca2+ plus ionophore, under conditions in which the Ca2+ pump is operating near its maximal extrusion rate, but swamped by Ca2+ plus ionophore. The data thus suggest a PKC alpha-mediated phosphorylation both of the Ca2+ pump and of the K(Ca) channel or an auxiliary protein.

Calpromotin, a cytoplasmic protein, is associated with the formation of dense cells in sickle cell anemia

We have tested the hypothesis that dense cell formation in sickle cell disease is associated with increased binding of calpromotin to the membrane, an event that occurs during the activation of calcium-dependent potassium transport. By SDS polyacrylamide gel electrophoresis, we found that sickle cell membranes contained more calpromotin than did normal membranes when stained with Coomassie brilliant blue or when transferred to nitrocellulose paper and immunostained with horseradish peroxidase. Also, the membranes from dense sickle cells contained significantly (P = 0.00055) higher levels of calpromotin, 2.62+/-1.59 microg/mg membrane protein, compared to light sickle cells, 1.40+/-0.70 microg/mg membrane protein, when measured by an enzyme-linked immunosorbent assay. The ratio of calpromotin associated with dense cell membranes to light cell membranes was significantly greater than 1.0 (P < 0.00005). Transmission electron micrographs of immunogold-labelled membranes supported the increase in calpromotin binding in dense sickle cell membranes. In addition, the immunogold probe demonstrated clustering, which was not observed in light sickle cell membranes nor in normal membranes. Finally, we incubated HbSS cells in vitro using a repetitive deoxygenation/ reoxygenation procedure to produce dense cells and then measured the levels of calpromotin associated with their membranes. As expected, the levels of calpromotin bound to the membrane doubled during the procedure relative to the basal levels at the beginning of the incubation. The correlation coefficient, calculated between the increase in dense cell formation and the increase in calpromotin associated with the membrane, was statistically significant (P = 0.038). The results demonstrate that an increase in calpromotin binding to the membrane is associated with dense cell formation presumably through the activation of the calcium-dependent potassium channel.

Modulation of Calcium Channels in Human Erythroblasts by Erythropoietin

Erythropoietin (Epo) induces a dose-dependent increase in intracellular free Ca2+ ([Ca2+]i ) in human erythroblasts, which is dependent on extracellular Ca2+ and blocked by high doses of nifedipine or Ni2+. In addition, pretreatment of human erythroblasts with mouse antihuman erythropoietin receptor antibody but not mouse immunopure IgG blocked the Epo-induced [Ca2+]i increase, indicating the specificity of the Ca2+ response to Epo stimulation. In this study, the erythropoietin-regulated calcium channel was identified by single channel recordings. Use of conventional whole cell patch-clamp failed to detect Epo-induced whole cell Ca2+ current. To minimize washout of cytosolic constituents, we next used nystatin perforated patch, but did not find any Epo-induced whole cell Ca2+ current. Using Ba2+ (30 mmol/L) as charge carrier in cell-attached patches, we detected single channels with unitary conductance of 3.2 pS, reversal potential of +72 mV, and whose unitary current (at +10 mV) increased monotonically with increasing Ba2+ concentrations. Channel open probability did not appreciably change over the voltage range (−50 to +30 mV) tested. Epo (2 U/mL) increased both mean open time (from 4.27 ± 0.75 to 11.15 ± 1.80 ms) and open probability (from 0.26 ± 0.06 to 2.56 ± 0.59%) of this Ba2+-permeable channel. Our data strongly support the conclusion that the Epo-induced [Ca2+]i increase in human erythroblasts is mediated via Ca2+ entry through a voltage-independent Ca2+ channel.

Modulation of Calcium Channels in Human Erythroblasts by Erythropoietin

Erythropoietin (Epo) induces a dose-dependent increase in intracellular free Ca2+ ([Ca2+]i ) in human erythroblasts, which is dependent on extracellular Ca2+ and blocked by high doses of nifedipine or Ni2+. In addition, pretreatment of human erythroblasts with mouse antihuman erythropoietin receptor antibody but not mouse immunopure IgG blocked the Epo-induced [Ca2+]i increase, indicating the specificity of the Ca2+ response to Epo stimulation. In this study, the erythropoietin-regulated calcium channel was identified by single channel recordings. Use of conventional whole cell patch-clamp failed to detect Epo-induced whole cell Ca2+ current. To minimize washout of cytosolic constituents, we next used nystatin perforated patch, but did not find any Epo-induced whole cell Ca2+ current. Using Ba2+ (30 mmol/L) as charge carrier in cell-attached patches, we detected single channels with unitary conductance of 3.2 pS, reversal potential of +72 mV, and whose unitary current (at +10 mV) increased monotonically with increasing Ba2+ concentrations. Channel open probability did not appreciably change over the voltage range (−50 to +30 mV) tested. Epo (2 U/mL) increased both mean open time (from 4.27 ± 0.75 to 11.15 ± 1.80 ms) and open probability (from 0.26 ± 0.06 to 2.56 ± 0.59%) of this Ba2+-permeable channel. Our data strongly support the conclusion that the Epo-induced [Ca2+]i increase in human erythroblasts is mediated via Ca2+ entry through a voltage-independent Ca2+ channel.

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.

Review: sickle cell disease: present and future treatment

Over the past few decades, the life expectancy of patients with sickle cell disease has improved. This has been because of better supportive care and greater awareness of the complications of this disorder. Recent successes of neonatal screening, childhood prophylactic penicillin, and, perhaps, hydroxyurea in adults may further extend the life of sickle cell disease patients. This review will do the following: 1) briefly highlight the major aspects of the conventional treatment of sickle cell disease, 2) address the present use of hydroxyurea in more depth, and 3) succinctly preview what the near-term future of treatment may bring.

High-conductance calcium-activated potassium channels; structure, pharmacology, and function

High-conductance calcium-activated potassium (maxi-K) channels comprise a specialized family of K+ channels. They are unique in their dual requirement for depolarization and Ca2+ binding for transition to the open, or conducting, state. Ion conduction through maxi-K channels is blocked by a family of venom-derived peptides, such as charybdotoxin and iberiotoxin. These peptides have been used to study function and structure of maxi-K channels, to identify novel channel modulators, and to follow the purification of functional maxi-K channels from smooth muscle. The channel consists of two dissimilar subunits, alpha and beta. The alpha subunit is a member of the slo Ca(2+)-activated K+ channel gene family and forms the ion conduction pore. The beta subunit is a structurally unique, membrane-spanning protein that contributes to channel gating and pharmacology. Potent, selective maxi-K channel effectors (both agonists and blockers) of low molecular weight have been identified from natural product sources. These agents, together with peptidyl inhibitors and site-directed antibodies raised against alpha and beta subunit sequences, can be used to anatomically map maxi-K channel expression, and to study the physiologic role of maxi-K channels in various tissues. One goal of such investigations is to determine whether maxi-K channels represent novel therapeutic targets.

Role of anion exchange and thiol groups in the regulation of potassium efflux by lead in human erythrocytes

Pb2+ is thought to enter erythrocytes through anion exchange (AE) and to remain in the cell by binding to thiol groups. To define the role of AE mechanisms and thiol groups in Pb2+ toxicity, we studied the effects of drugs and conditions that modify AE and that modify thiol groups on the ability of Pb2+ to stimulate potassium efflux as measured with 86Rb. The most potent stimulation of 86Rb efflux by Pb2+ occurred when conditions were optimal for the AE mechanism--that is, when bicarbonate was included in the buffer or a buffer made with Nal or NaCl rather than NaClO4 or NaNO3 was used. Furthermore, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfuonic acid, potent inhibitors of the AE mechanism, completely inhibited stimulation of the 86Rb efflux by Pb2+. These conditions or inhibitors did not affect stimulation of the 86Rb efflux by ionomycin plus Ca2+. A role for Ca2+ channels was dismissed because the inorganic Ca2+ channel blockers, Cd2+ or Mn2+, did not prevent stimulation of 86Rb efflux by Pb2+ but did inhibit stimulation by ionomycin plus Ca2+. 86Rb efflux was more sensitive to Pb2+ if erythrocytes were treated for 15 min with thiol-modifying reagents that enter cells, such as iodoacetamide, N-ethylmaleimide, or dithiothreitol, than to reduced glutathione, a thiol-modifying reagent that is not permeable to the cell. Thus, in erythrocytes the AE mechanism and internal thiol groups are critical factors that affect the stimulation of a Ca(2+)-dependent process by Pb2+.

Characterization of Ca(2+)-activated 86Rb+ fluxes in rat C6 glioma cells: a system for identifying novel IKCa-channel toxins

1. The pharmacological characteristics of a putative Ca2+ activated K+ channel (IKCa channel) in rat glioma C6 cells were studied in the presence of the Ca2+ ionophore, ionomycin and various K+ channel blockers, 86Rb+ being used as a radioisotopic tracer for K+. 2. The resting 86Rb+ influx into C6 cells was 318 +/- 20 pmol s-1. The threshold for ionomycin activation of 86Rb+ influx was approx. 100 nM. At ionomycin concentrations above the activation threshold, the initial rate of 86Rb+ influx was proportional to ionophore concentration. Ionomycin-activated 86Rb+ flux was saturable (EC50 = 0.62 +/- 0.03 microM) and was not inhibited by ouabain. 3. Intracellular Ca2+ increased within 30 s from a basal level of 42 +/- 2 nM to 233 +/- 17 nM, after addition of 2 microM ionomycin. During this period, intracellular pH fell from 7.03 +/- 0.04 to 6.87 +/- 0.03 and the cell hyperpolarized from -34 +/- 10 mV to -76 +/- 2 mV. 4. Single channel conductance measurements on inside-out patches in physiological K+ solutions identified a 14 +/- 3 pS CA(2+)-activated K+ current between -25 mV and +50 mV. In symmetrical (100 mM) K+, the single channel conductance was 26 pS. 5. Externally applied quinine (IC50 = 0.12 +/- 0.34 mM) and tetraethylammonium chloride (IC50 = 10 +/- 1.9 mM) inhibited 86Rb+ influx into C6 cells in a concentration-dependent manner. Charybdotoxin (IC50 = 0.5 +/- 0.02 nM) and iberiotoxin (IC50 = 800 +/- 150 nM), as well as the crude venoms from the scorpions Leiurus quinquestriatus and Mesobuthus tamulus, also inhibited 86Rb+ influx. In contrast, apamin and toxin I had no inhibitory effects on 86Rb+ flux. A screen of fractions from cation exchange h.p.l.c. of Mesob. tamulus venom revealed the presence of at least four charybdotoxin-like peptides. One of these was iberiotoxin; the other three are novel toxins. 6. The ionomycin-activated 86Rb+ influx into rat C6 glioma cells has proved to be a valuable pharmacological assay for the screening of toxins and crude venoms which modify intermediate conductance, Ca2+ activated K+ channel activity.

Synthesis and structural characterisation of analogues of the potassium channel blocker charybdotoxin

Charybdotoxin is a 37-residue polypeptide toxin from scorpion venom, which acts by blocking voltage-gated and Ca(2+)-activated K+ channels. We have synthesized charybdotoxin and three mono-substituted analogues using an Fmoc-tBu protocol. The Phe-2 --> Tyr analogues was chosen to introduce a site for Tyr iodination which was distinct from the K+ channel binding surface, while the Glu-12 --> Gln and Arg-19 --> His analogues were studied to probe the roles of charged residues at these positions in the structure and activity of the toxin. The synthetic native molecule was equipped with natural toxin in inhibiting the human erythrocyte Ca(2+)-dependent K+ channel. The affinities of all three analogues for the erythrocyte K+ channel were slightly reduced, with the Arg-19 --> His analogue showing the greatest increase in IC50 (2.30-fold). Two-dimensional 1H-NMR studies of these analogues showed that the Glu-12 to Gln substitution, which appeared to destabilise the N-terminal half of the alpha-helix, possibly due to the weakening of an N-terminal helix capping interaction which is apparent from our NMR data. His-21 has a pKa more than one unit below the value for a non-interacting histidine. Possible reasons for this are that the imidazolium side chain is partly buried and is located near positively charged moieties. Thus, His-21 would be neutral at physiological pH, where charybdotoxin binds to the potassium channel.

Calcium ionophore (A23187) differential effect on red cells from pre and postnatal haemopoiesis

Differences in ionic transport mechanisms through human red cell membranes from pre and postnatal haemopoietic stages of development have been related. The analysis of calcium-sensitive conductive potassium pathways [K+ (Ca2+)] was realized from intracellular calcium level increase after treatment of cells with ionophores. The effect of calcium ionophore (A23187) on potassium transport in human neonatal red cells (nRC) was studied and compared with red cells from adults (aRC). Analysis focused on intra/extracellular potassium redistribution and potassium conductance [K+ (Ca2+)] values in incubated red cells suspensions. A difference was observed between neonatal and adult's red cells only for the first of the parameters referred to. Conductance values of the same order of magnitude for these potassium transport pathways between both cell types suggest that the results presented could rest on different [K+ (Ca2+)] cellular density.

Properties of Ca(2+)-dependent K+ channels of human gingival fibroblasts

Cells in the oral cavity are normally exposed to different temperatures. Ion transport systems are influenced by temperature in other tissues: In particular, changes in intracellular K+ ion can affect cell growth and synthesis of macromolecules. The purpose of this investigation was to identify K+ channels in human gingival fibroblast cells and analyze the effect of temperature on their K+ conduction properties. Ca(2+)-dependent K+ channels with a large conductance (125 pS in symmetrical K(+)-rich solutions) were identified in human gingival fibroblasts and studied by the patch-clamp technique. The open probability of the channels varied with membrane potential between +40 and -100 mV. When the bath temperature was decreased from 40 to 4 degrees C, channel conductance was reduced, but the mean open time of the channels was increased. The activation energies for the conductance and the reciprocal of the mean open time were estimated to be 9.1 and 22.9 kJ/mol, respectively. These values are lower than those reported for these and other types of channels in cells from other tissues. The open probability of the channels was nearly constant in the temperature range studied. These results suggest that the properties of Ca(2+)-dependent K+ channels of gingival fibroblasts remain relatively unchanged when the cells are exposed to a wide range of temperatures.

Intimations of K+ channel structure from a complete functional map of the molecular surface of charybdotoxin

The external vestibules of many K+ channels carry a high-affinity receptor for charybdotoxin, a peptide of known structure. Point mutations of a recombinant toxin identified the residues directly involved in the interaction with a Ca(2+)-activated K+ channel. The interaction surface is formed from 8 of the 37 residues and covers about 25% of the peptide's molecular surface. The shape of the toxin permits a deduced picture of the complementary receptor site in the external vestibule of the K+ channel.