Dipyridamole inhibits sickling-induced cation fluxes in sickle red blood cells

Erythroid anion transport, nitric oxide, and blood pressure

Glycophorin A and glycophorin B are structural membrane glycoproteins bound in the band 3 multiprotein complexes on human red blood cells (RBCs). Band 3 is an erythroid-specific anion exchanger (AE1). AE1-mediated HCO3 - transport provides the substrate for the enzyme-catalyzed conversion HCO3 - (aq) ⇌ CO2(g), which takes place inside the RBCs. Bicarbonate transport via AE1 supports intravascular acid-base homeostasis and respiratory excretion of CO2. In the past decade, we conducted several comparative physiology studies on Taiwanese people having the glycophorin variant GPMur RBC type (which accompanies greater AE1 expression). We found that increased anion transport across the erythrocyte membrane not only enhances gas exchange and lung functions but also elevates blood pressure (BP) and reduces nitric oxide (NO)-dependent vasodilation and exhaled NO fraction (FeNO) in healthy individuals with GP.Mur. Notably, in people carrying the GPMur blood type, the BP and NO-dependent, flow-mediated vasodilation (FMD) are both more strongly correlated with individual hemoglobin (Hb) levels. As blood NO and nitrite (NO2 -) are predominantly scavenged by intraerythrocytic Hb, and NO2 - primarily enters RBCs via AE1, could a more monoanion-permeable RBC membrane (i.e., GPMur/increased AE1) enhance NO2 -/NO3 - permeability and Hb scavenging of NO2 - and NO to affect blood pressure? In this perspective, a working model is proposed for the potential role of AE1 in intravascular NO availability, blood pressure, and clinical relevance.

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.

Pathophysiology and recent therapeutic insights of sickle cell disease

Sickle cell disease (SCD) is an autosomal recessive blood disorder which occurs due to point mutation in the β-globin chain of hemoglobin. Since the past decades, various therapies have been put forth, which are based on obstructing pathophysiological mechanisms of SCD including inhibition of Gardos channel and cation fluxes which in turn prevents sickle erythrocyte destruction and dehydration. The pharmacological approaches are based on the mechanism of reactivating γ-globin expression by utilizing fetal hemoglobin (HbF)-inducing drugs such as hydroxyurea. In SCD, gene therapy could be considered as a promising tool which involves modifying mutation at the gene-specific target by either promoting insertion or deletion of globins. Although there are various therapies emerged so far in the treatment of SCD, many of them have faced a major setback in most of developing countries in terms of cost, unavailability of expertise, and suitable donor. Therefore, in addition to pathophysiological aspects, this review will discuss new advancements and approaches made in the therapeutic domain of SCD including a viewpoint of modulating hemoglobin in SCD by the intervention of probiotics.

The Evolving Pharmacotherapeutic Landscape for the Treatment of Sickle Cell Disease

Sickle cell disease (SCD) is an extremely heterogeneous disease that has been associated with global morbidity and early mortality. More effective and inexpensive therapies are needed. During the last five years, the landscape of the pharmacotherapy of SCD has changed dramatically. Currently, 54 drugs have been used or under consideration to use for the treatment of SCD. These fall into 3 categories: the first category includes the four drugs (Hydroxyurea, L-Glutamine, Crizanlizumab tmca and Voxelotor) that have been approved by the United States Food and Drug Administration (FDA) based on successful clinical trials. The second category includes 22 drugs that failed, discontinued or terminated for now and the third category includes 28 drugs that are actively being considered for the treatment of SCD. Crizanlizumab and Voxelotor are included in the first and third categories because they have been used in more than one trial. New therapies targeting multiple pathways in the complex pathophysiology of SCD have been achieved or are under continued investigation. The emerging trend seems to be the use of multimodal drugs (i.e. drugs that have different mechanisms of action) to treat SCD similar to the use of multiple chemotherapeutic agents to treat cancer.

Emerging disease-modifying therapies for sickle cell disease

Sickle cell disease afflicts millions of people worldwide and approximately 100,000 Americans. Complications are myriad and arise as a result of complex pathological pathways ‘downstream’ to a point mutation in DNA, and include red blood cell membrane damage, inflammation, chronic hemolytic anemia with episodic vaso-occlusion, ischemia and pain, and ultimately risk of cumulative organ damage with reduced lifespan of affected individuals. The National Heart, Lung, and Blood Institute’s 2014 evidence-based guideline for sickle cell disease management states that additional research is needed before investigational curative therapies will be widely available to most patients with sickle cell disease. To date, sickle cell disease has been cured by hematopoietic stem cell transplantation in approximately 1,000 people, most of whom were children, and significantly ameliorated by gene therapy in a handful of subjects who have only limited follow-up thus far. During a timespan in which over 20 agents were approved for the treatment of cystic fibrosis by the Food and Drug Administration, similar approval was granted for only two drugs for sickle cell disease (hydroxyurea and L-glutamine) despite the higher prevalence of sickle cell disease. This trajectory appears to be changing, as the lack of multimodal agent therapy in sickle cell disease has spurred engagement among many in academia and industry who, in the last decade, have developed new drugs poised to prevent complications and alleviate suffering. Identified therapeutic strategies include fetal hemoglobin induction, inhibition of intracellular HbS polymerization, inhibition of oxidant stress and inflammation, and perturbation of the activation of the endothelium and other blood components (e.g. platelets, white blood cells, coagulation proteins) involved in the pathophysiology of sickle cell disease. In this article, we present a crash-course review of disease-modifying approaches (minus hematopoietic stem cell transplant and gene therapy) for patients with sickle cell disease currently, or recently, tested in clinical trials in the era following approval of hydroxyurea.

Sickle cell dehydration: Pathophysiology and therapeutic applications

Cell dehydration is a distinguishing characteristic of sickle cell disease and an important contributor to disease pathophysiology. Due to the unique dependence of Hb S polymerization on cellular Hb S concentration, cell dehydration promotes polymerization and sickling. In double heterozygosis for Hb S and C (SC disease) dehydration is the determining factor in disease pathophysiology. Three major ion transport pathways are involved in sickle cell dehydration: the K-Cl cotransport (KCC), the Gardos channel (KCNN4) and Psickle, the polymerization induced membrane permeability, most likely mediated by the mechano-sensitive ion channel PIEZO1. Each of these pathways exhibit unique characteristics in regulation by oxygen tension, intracellular and extracellular environment, and functional expression in reticulocytes and mature red cells. The unique dependence of K-Cl cotransport on intracellular Mg and the abnormal reduction of erythrocyte Mg content in SS and SC cells had led to clinical studies assessing the effect of oral Mg supplementation. Inhibition of Gardos channel by clotrimazole and senicapoc has led to Phase 1,2,3 trials in patients with sickle cell disease. While none of these studies has resulted in the approval of a novel therapy for SS disease, they have highlighted the key role played by these pathways in disease pathophysiology.

Cation Homeostasis in Red Cells From Patients With Sickle Cell Disease Heterologous for HbS and HbC (HbSC Genotype)

Sickle cell disease (SCD) in patients of HbSC genotype is considered similar, albeit milder, to that in homozygous HbSS individuals--but with little justification. In SCD, elevated red cell cation permeability is critical as increased solute loss causes dehydration and encourages sickling. Recently, we showed that the KCl cotransporter (KCC) activity in red cells from HbSC patients correlated significantly with disease severity, but that in HbSS patients did not. Two transporters involved in red cell dehydration, the conductive channels Psickle and the Gardos channel, behaved similarly in red cells from the two genotypes, but were significantly less active in HbSC patients. By contrast, KCC activity was quantitatively greater in HbSC red cells. Results suggest that KCC is likely to have greater involvement in red cell dehydration in HbSC patients, which could explain its association with disease severity in this genotype. This work supports the hypothesis that SCD in HbSC patients is a distinct disease entity to that in HbSS patients. Results suggest the possibility of designing specific treatments of particular benefit to HbSC patients and a rationale for the development of prognostic markers, to inform early treatment of children likely to develop more severe complications of the disease.

Identification of the Ca²⁺ entry pathway involved in deoxygenation-induced phosphatidylserine exposure in red blood cells from patients with sickle cell disease

Phosphatidylserine (PS) exposure in red blood cells (RBCs) from sickle cell disease (SCD) patients is increased compared to levels in normal individuals and may participate in the anaemic and ischaemic complications of SCD. Exposure is increased by deoxygenation and occurs with elevation of intracellular Ca²⁺ to low micromolar levels. The Ca²⁺ entry step has not been defined but a role for the deoxygenation-induced pathway, P_sickle, is postulated. Partial P_sickle inhibitors 4-acetamido-4′-isothiocyanostilbene-2,2′-disulphonic acid (SITS), 4,4′-dithiocyano-2,2′-stilbene-disulphonic acid (DIDS) and dipyridamole inhibited deoxygenation-induced PS exposure (DIDS IC₅₀, 118 nM). Inhibitors and activators of other pathways (including these stimulated by depolarisation, benzodiazepines, glutamate and stretch) were without effect. Zn²⁺ and Gd³⁺ stimulated PS exposure to high levels. In the case of Zn²⁺, this effect was independent of oxygen (and hence HbS polymerisation and RBC sickling) but required extracellular Ca²⁺. The effect was completely abolished when Zn²⁺ (100 μM) was added to RBCs suspended in autologous plasma, implying a requirement of high levels of free Zn²⁺.

The conductance of red blood cells from sickle cell patients: ion selectivity and inhibitors

The abnormally high cation permeability in red blood cells (RBCs) from patients with sickle cell disease (SCD) occupies a central role in pathogenesis. Sickle RBC properties are notably heterogeneous, however, thus limiting conventional flux techniques that necessarily average out the behaviour of millions of cells. Here we use the whole-cell patch configuration to characterise the permeability of single RBCs from patients with SCD in more detail. A non-specific cation conductance was reversibly induced upon deoxygenation and was permeable to both univalent (Na+, K+, Rb+) and also divalent (Ca2+, Mg2+) cations. It was sensitive to the tarantula spider toxin GsMTx-4. Mn2+ caused partial, reversible inhibition. The aromatic aldehyde o-vanillin also irreversibly inhibited the deoxygenation-induced conductance, partially at 1mM and almost completely at 5mM. Nifedipine, amiloride and ethylisopropylamiloride were ineffective. In oxygenated RBCs, the current was pH sensitive showing a marked increase as pH fell from 7.4 to 6, with no change apparent when pH was raised from 7.4 to 8. The effects of acidification and deoxygenation together were not additive. Many features of this deoxygenation-induced conductance (non-specificity for cations, permeability toCa2+ andMg2+, pH sensitivity, reversibility, partial inhibition by DIDS and Mn2+) are shared with the flux pathway sometimes referred to as Psickle. Sensitivity to GsMTx-4 indicates its possible identity as a stretch-activated channel. Sensitivity to o-vanillin implies that activation requires HbS polymerisation but since the conductance was observed in whole-cell patches, results suggest that bulk intracellular Hb is not involved; rather a membrane-bound subfraction is responsible for channel activation. The ability to record P(sickle)-like activity in single RBCs will facilitate further studies and eventual molecular identification of the pathway involved.

Effects of disodium cromoglycate on cationic exchange of deoxygenated sickle cells

In the present work, we explored the way in which cromoglycate, a drug used to treat allergies acts on ion movements in sickle cells. Cells were either slowly deoxygenated by overnight exposure to nitrogen or acutely deoxygenated by exposure to metabisulfite, a strong reducing agent which induces sickling of red blood sickle cells. Flushing the cells with nitrogen increased the intracellular concentration of Na(+) and decreased the intracellular concentration of K(+) and the sum of the concentrations of the two cations. One hundred nM cromoglycate inhibited the decrease of intracellular K(+) and the increase of intracellular Na(+) induced by deoxygenation (n=17). Metabisulfite (100mM) increased the intracellular concentration of Ca(2+) (measured by Fura Red) (n=15) and the shape of the cells (measured by light scattering) (n=9). One μM cromoglycate partially inhibited these two responses. In conclusion, cromoglycate partially inhibits abnormal K(+) loss, Ca(2+) entry pathways or Ca(2+) channels opened by cell deoxygenation and ensuing membrane modifications and prevents cell sickling.

Role of calcium in phosphatidylserine externalisation in red blood cells from sickle cell patients

Phosphatidylserine exposure occurs in red blood cells (RBCs) from sickle cell disease (SCD) patients and is increased by deoxygenation. The mechanisms responsible remain unclear. RBCs from SCD patients also have elevated cation permeability, and, in particular, a deoxygenation-induced cation conductance which mediates Ca²⁺ entry, providing an obvious link with phosphatidylserine exposure. The role of Ca²⁺ was investigated using FITC-labelled annexin. Results confirmed high phosphatidylserine exposure in RBCs from SCD patients increasing upon deoxygenation. When deoxygenated, phosphatidylserine exposure was further elevated as extracellular [Ca²⁺] was increased. This effect was inhibited by dipyridamole, intracellular Ca²⁺ chelation, and Gardos channel inhibition. Phosphatidylserine exposure was reduced in high K⁺ saline. Ca²⁺ levels required to elicit phosphatidylserine exposure were in the low micromolar range. Findings are consistent with Ca²⁺ entry through the deoxygenation-induced pathway (P_sickle), activating the Gardos channel. [Ca²⁺] required for phosphatidylserine scrambling are in the range achievable in vivo.

P-glycoprotein, multidrug resistance-associated proteins and human organic anion transporting polypeptide influence the intracellular accumulation of atazanavir

BACKGROUND

Drug efflux (for example, P-glycoprotein [P-gp], multidrug resistance-associated proteins [MRPs] and breast cancer resistance protein [BCRP]) and influx (for example, human organic anion transporting polypeptide [hOCTP] or human organic anion transporting polypeptide [hOATP]) transporters alter the cellular concentrations of some HIV protease inhibitors (HPIs). Here, we studied the lipophilicity and uptake of [(3)H]-atazanavir (ATV) in CEM (parental), CEM(VBL) (P-gp-overexpressing), CEM(E1000) (MRP1-overexpressing) and peripheral blood mononuclear cells (PBMCs), and evaluate the effects of modulators of drug transporters on uptake.

METHODS

Lipophilicity was measured by octanol/saline partition method. The influence of influx/efflux transporters on uptake was evaluated in the absence and presence of inhibitors of P-gp (GPV031), P-gp/BCRP (tariquidar and GF120918), P-gp/MRP1 (dipyridamole and daidzein), MRP1/2 (frusemide and genistein), hOATP/hOCTP (estrone-3-sulfate [E-3-S]) and hOATP/hOCTP/MRP (probenecid). The effects of a number of HPIs on uptake were also evaluated. Data from digitonin permeabilized cells allowed the evaluation of the contribution of cellular binding to total drug uptake, whereas the inhibitory effect of ATV on P-gp was assessed by daunomycin efflux/uptake assays.

RESULTS

[(3)H]-ATV is lipophilic and accumulates in the cultured cells as follows: CEM>CEM(E1000)>CEM(VBL). Tariquidar, GF120918 and daidzein significantly increased the uptake of [(3)H]-ATV in the cultured cells. By contrast, only daidzein and tipranavir significantly increased uptake in PBMCs, with tariquidar and frusemide devoid of effects, whereas dipyridamole, E-3-S, GPV031 and genistein significantly decreased accumulation. ATV inhibits P-gp activity; manipulation of uptake with digitonin suggests binding of [(3)H]-ATV to P-gp.

CONCLUSIONS

[(3)H]-ATV is lipophilic, a P-gp, MRP and hOATP substrate and an inhibitor of P-gp. Concomitant administration of ATV with drugs and dietary components (for example, daidzein and genistein) that interact with these transporters could alter its pharmacokinetics.

Hypoxia activates a Ca2+-permeable cation conductance sensitive to carbon monoxide and to GsMTx-4 in human and mouse sickle erythrocytes

Background

Deoxygenation of sickle erythrocytes activates a cation permeability of unknown molecular identity (Psickle), leading to elevated intracellular [Ca²⁺] ([Ca²⁺]_i) and subsequent activation of K_Ca 3.1. The resulting erythrocyte volume decrease elevates intracellular hemoglobin S (HbSS) concentration, accelerates deoxygenation-induced HbSS polymerization, and increases the likelihood of cell sickling. Deoxygenation-induced currents sharing some properties of Psickle have been recorded from sickle erythrocytes in whole cell configuration.

Methodology/Principal Findings

We now show by cell-attached and nystatin-permeabilized patch clamp recording from sickle erythrocytes of mouse and human that deoxygenation reversibly activates a Ca²⁺- and cation-permeable conductance sensitive to inhibition by Grammastola spatulata mechanotoxin-4 (GsMTx-4; 1 µM), dipyridamole (100 µM), DIDS (100 µM), and carbon monoxide (25 ppm pretreatment). Deoxygenation also elevates sickle erythrocyte [Ca²⁺]_i, in a manner similarly inhibited by GsMTx-4 and by carbon monoxide. Normal human and mouse erythrocytes do not exhibit these responses to deoxygenation. Deoxygenation-induced elevation of [Ca²⁺]_i in mouse sickle erythrocytes did not require KCa3.1 activity.

Conclusions/Significance

The electrophysiological and fluorimetric data provide compelling evidence in sickle erythrocytes of mouse and human for a deoxygenation-induced, reversible, Ca²⁺-permeable cation conductance blocked by inhibition of HbSS polymerization and by an inhibitor of strctch-activated cation channels. This cation permeability pathway is likely an important source of intracellular Ca²⁺ for pathologic activation of KCa3.1 in sickle erythrocytes. Blockade of this pathway represents a novel therapeutic approach for treatment of sickle disease.

Non-electrolyte permeability of deoxygenated sickle cells compared

The passive permeability pathways of red cells are poorly defined, with the exception of the Gardos channel. Several cation and anion pathways can be induced by a variety of manoeuvres, however, including treatment with oxidants, low ionic strength (LIS), shrinkage, swelling and also infection with the intra-erythrocytic malaria parasite. Several of these stimuli (malaria, swelling, LIS), in addition, also activate a non-electrolyte this permeability. Sickle cells uniquely show a deoxygenation-induced pathway, which is termed P(sickle) and is usually considered to be a conductive cationic pathway. In this report, we explore further the extent to which this permeability pathway of deoxygenated sickle cells is available for non-electrolyte transport. We show that a number of solutes are permeable, with greater permeability to sugars (notably lactose and maltose) and smaller molecules, and less to charged or zwitterionic species. Red cells from heterozygous HbSC patients also showed deoxygenation-induced haemolysis in isosmotic sucrose solution, though to a slightly lesser extent than for red cells from homozygous sickle cell patients. In contrast to sickle cells, red cells from beta-thalassaemic patients did not show haemolysis in isosmotic sucrose solutions, regardless of the O(2) tension. Of the secondary cellular changes resulting from incubation in non-electrolyte solutions (which include imposition of a highly positive membrane potential, marked intracellular alkalinisation and cell shrinkage), none appear to correlate with activation of the non-electrolyte permeability. Rather, findings indicate that it is low ionic strength per se that is responsible. Normal red cells also show changes in ionic and non-electrolyte permeability in low ionic strength media, and these permeabilities are compared to those found in deoxygenated sickle cells. The extent to which these different permeabilities in normal and sickle red cells can be ascribed to one or more common pathways remains to be determined.

Abnormal permeability pathways in human red blood cells

A number of situations that result in abnormal permeability pathways in human red blood cells (RBCs) have been investigated. In sickle cell disease (SCD), RBCs contain HbS, rather than the normal HbA. When deoxygenated, an abnormal conductance pathway, termed P(sickle), is activated, which contributes to cell dehydration, largely through allowing Ca(2+) entry and subsequent activation of the Gardos channel. Whole-cell patch-clamp recordings from sickle RBCs show a deoxygenated-induced conductance, absent from normal RBCs, which shares some of the properties of P(sickle): equivalent Na(+) and K(+) permeability, significant Ca(2+) conductance, partial inhibition by DIDS and also Zn(2+). Gd(3+) markedly attenuates conductance in both normal and sickle RBCs. In addition, deoxygenated sickle cells, but not oxygenated ones or normal RBCs regardless of the oxygen tension, undergo haemolysis in isosmotic non-electrolyte solutions. Non-electrolyte entry was confirmed radioisotopically whilst haemolysis was inhibited by DIDS. These findings suggest that under certain circumstances P(sickle) may also be permeable to non-electrolytes. Finally, RBCs from certain patients with hereditary stomatocytosis have a mutated band 3, which appears able to act as a conductance pathway for univalent cations. These results extend our understanding of the abnormal permeability pathways of RBCs.

Deoxygenation-induced non-electrolyte pathway in red cells from sickle cell patients

Red cells from patients with sickle cell disease contain HbS rather than the normal HbA (here termed HbS cells). On deoxygenation, HbS cells exhibit a distinctive solute permeability pathway, P(sickle), activated stochastically, and partially inhibited by DIDS and dipyridamole. It is often referred to as a cation channel although its permeability characteristics remain vague and its molecular identity is unknown. We show that, in contrast to normal red cells, a proportion of HbS cells underwent haemolysis when deoxygenated in isosmotic non-electrolyte solutions. Haemolysis was stochastic: cells unlysed after an initial deoxygenation pulse showed lysis when harvested, reoxygenated and subsequently exposed to a second period of deoxygenation. O(2) dependence of haemolysis was similar to that of P(sickle) activation. Haemolysis was accompanied by high rates of sucrose influx, and both haemolysis and sucrose influx were inhibited by DIDS and dipyridamole. Sucrose influx was only detected as ionic strength was reduced below 80 mM. These findings are consistent with the postulate that deoxygenation of HbS cells, under certain conditions, activates a novel non-electrolyte pathway. Their significance lies in understanding the nature of the deoxygenation-induced permeability in HbS cells, together with its relationship with novel pathways induced by a variety of manipulations in normal red cells.

The effect of deoxygenation on whole-cell conductance of red blood cells from healthy individuals and patients with sickle cell disease

Red blood cells from patients with sickle cell disease (SCD) exhibit increased electrogenic cation permeability, particularly following deoxygenation and hemoglobin (Hb) polymerisation. This cation permeability, termed P(sickle), contributes to cellular dehydration and sickling, and its inhibition remains a major goal for SCD treatment. Nevertheless, its characteristics remain poorly defined, its molecular identity is unknown, and effective inhibitors have not been established. Here, patch-clamp methodology was used to record whole-cell currents in single red blood cells from healthy individuals and patients with SCD. Oxygenated normal red blood cells had a low membrane conductance, unaffected by deoxygenation. Oxygenated HbS cells had significantly increased conductance and, on deoxygenation, showed a further rise in membrane conductance. The deoxygenation-induced pathway was variable in magnitude. It had equal permeability to Na(+) and K(+), but was less permeable to NMDG(+) and Cl(-). Conductance to Ca(2+) was also of a similar magnitude to that of monovalent cations. It was inhibited by DIDS (100 microM), Zn(2+) (100 microM), and by Gd(3+) (IC(50) of approximately 2 microM). It therefore shares some properties with P(sickle). These findings represent the first electrical recordings of single HbS cells and will facilitate progress in understanding altered red blood cell cation transport characteristics of SCD.

Role of adhesion molecules and vascular endothelium in the pathogenesis of sickle cell disease

A number of lines of evidence now support the hypothesis that vaso-occlusion and several of the sequelae of sickle cell disease (SCD) arise, at least in part, from adhesive interactions of sickle red blood cells, leukocytes, and the endothelium. Both experimental and genetic evidence provide support for the importance of these interactions. It is likely that future therapies for SCD might target one or more of these interactions.

Rac GTPases regulate the morphology and deformability of the erythrocyte cytoskeleton

Actin oligomers are a significant structural component of the erythrocyte cytoskeleton. Rac1 and Rac2 GTPases regulate actin structures and have multiple overlapping as well as distinct roles in hematopoietic cells; therefore, we studied their role in red blood cells (RBCs). Conditional gene targeting with a loxP-flanked Rac1 gene allowed Crerecombinase-induced deletion of Rac1 on a Rac2 null genetic background. The Rac1(-/-);Rac2(-/-) mice developed microcytic anemia with a hemoglobin drop of about 20% and significant anisocytosis and poikilocytosis. Reticulocytes increased more than 2-fold. Rac1(-/-);Rac2(-/-) RBCs stained with rhodamine-phalloidin demonstrated F-actin meshwork gaps and aggregates under confocal microscopy. Transmission electron microscopy of the cytoskeleton demonstrated junctional aggregates and pronounced irregularity of the hexagonal spectrin scaffold. Ektacytometry confirmed that these cytoskeletal changes in Rac1(-/-);Rac2(-/-) erythrocytes were associated with significantly decreased cellular deformability. The composition of the cytoskeletal proteins was altered with an increased actin-to-spectrin ratio and increased phosphorylation (Ser724) of adducin, an F-actin capping protein. Actin and phosphorylated adducin of Rac1(-/-);Rac2(-/-) erythrocytes were more easily extractable by Triton X-100, indicating weaker association to the cytoskeleton. Thus, deficiency of Rac1 and Rac2 GTPases in mice alters actin assembly in RBCs and causes microcytic anemia with reticulocytosis, implicating Rac GTPases as dynamic regulators of the erythrocyte cytoskeleton organization.

Pathophysiologically based drug treatment of sickle cell disease

Sickle cell disease is a systemic disorder that is caused by a mutation (Glu6Val) in the gene that encodes beta globin. The sickle hemoglobin molecule (HbS) is a tetramer of two alpha-globin chains and two sickle beta-globin chains, and has the tendency to polymerize when deoxygenated. HbS facilitates abnormal interactions between the sickle erythrocyte and leukocytes and endothelial cells, which trigger a complex pathobiology. This multifaceted pathophysiology provides the opportunity to interrupt the disease at multiple sites, including polymerization of HbS, erythrocyte density and cell-cell interactions. For example, it is possible to induce higher concentrations of fetal hemoglobin, which disrupts the pathology-initiating step of HbS polymerization. Furthermore, it is possible to improve the hydration of sickle erythrocytes and it might be feasible to counteract the endothelial, inflammatory and oxidative abnormalities of sickle cell disease. A therapeutic approach that targets several sites of pathobiology might be most promising.

Monovalent cation leaks in human red cells caused by single amino-acid substitutions in the transport domain of the band 3 chloride-bicarbonate exchanger, AE1

We identified 11 human pedigrees with dominantly inherited hemolytic anemias in both the hereditary stomatocytosis and spherocytosis classes. Affected individuals in these families had an increase in membrane permeability to Na and K that is particularly marked at 0 degrees C. We found that disease in these pedigrees was associated with a series of single amino-acid substitutions in the intramembrane domain of the erythrocyte band 3 anion exchanger, AE1. Anion movements were reduced in the abnormal red cells. The 'leak' cation fluxes were inhibited by SITS, dipyridamole and NS1652, chemically diverse inhibitors of band 3. Expression of the mutated genes in Xenopus laevis oocytes induced abnormal Na and K fluxes in the oocytes, and the induced Cl transport was low. These data are consistent with the suggestion that the substitutions convert the protein from an anion exchanger into an unregulated cation channel.

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 disease: from membrane pathophysiology to novel therapies for prevention of erythrocyte dehydration

Sickle cell anemia is characterized by the presence of dense dehydrated erythrocytes that have lost most of their K content. Due to the unique dependence of Hb S polymerization on intracellular Hb S concentration, preventing this dehydration should markedly reduce polymerization. The erythrocyte intermediate conductance Ca-activated K channel (hSK4 or KCNN4), first described by Gardos, has been shown to be a major pathway for sickle cell dehydration. Studies with the imidazole antimycotic clotrimazole have shown reduction of sickle cell dehydration in vivo in a small number of patients with sickle cell disease; dose-limiting gastrointestinal and liver toxicities were observed. Based on the chemical structure of clotrimazole metabolites, a novel Gardos channel inhibitor, ICA-17043, has been developed. It has shown substantial activity both in vitro and in vivo in transgenic sickle mice. ICA-17043 is currently in phase 2 human trials. Another potential therapeutic target is the K-Cl cotransport. When sickle erythrocytes are exposed to relatively acidic conditions, they undergo cell shrinkage via activation of this pathway. K-Cl cotransport can be blocked by increasing the abnormally low erythrocyte Mg content of sickle erythrocytes. Oral Mg supplementation has been shown to reduce sickle cell dehydration in vivo in transgenic sickle mice and in patients in two separate clinical trials. Oral Mg pidolate is being tested in clinical trials in homozygous sickle cell disease and in Hb S/HbC (SC) disease, either as a single agent or in combination with hydroxyurea. The ongoing trials will determine the clinical effectiveness of therapies aimed at preventing sickle erythrocyte dehydration.

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.

Pathophysiological-based approaches to treatment of sickle cell disease

Sickle hemoglobin (HbS), as a result of its polymer-related and oxidant effects, damages the sickle erythrocyte, provokes inflammation, and causes endothelial injury. All these elements cause the phenotype of sickle cell disease. Novel treatments inhibit HbS polymerization by inducing fetal hemoglobin expression, prevent or repair erythrocyte dehydration by slowing cellular potassium and water loss, and replace HbS-producing erythroid progenitors by stem cell transplantation. Future treatment prospects include gene therapy, interruption of the interaction of sickle cells with the endothelium, inhibition of oxidative damage, and protection of an injured endothelium.

Rehydration of high-density sickle erythrocytes in vitro

Recent studies have identified older, low-density sickle red blood cells (SSRBCs) that were resistant to dehydration by valinomycin, a K(+) ionophore. These cells, thought to derive from dense SSRBCs that have rehydrated, may represent a terminal cellular phase. To study rehydration, we subjected dense SSRBCs (rho > 1.107 g/cc) to either oxygenated incubation or rapid oxygenated/deoxygenated (oxy/deoxy) cycling (70 seconds per cycle). Light cells (rho < 1.087 g/cc) were generated during both oxy incubation (2.9% +/- 2.1%; n = 42) and oxy/deoxy cycling (5.3% +/- 2.4%; n = 42). The rehydrated cells were K(+)-depleted (K(+) = 20 +/- 14 mmol/kg hemoglobin [Hb]) and Na(+)-loaded (Na(+) = 394 +/- 106 mmol/kg Hb), and had high levels of external phosphatidylserine. In the presence of external calcium, the generation of rehydrated SSRBCs was inhibited during oxy/deoxy cycling, but the percentage with external phosphatidylserine increased. The calcium-mediated inhibition of rehydration was reversed by charybdotoxin, implying that rehydration was delayed in some cells by the Ca(++)-activated K(+) channel. Preincubation of dense SSRBCs with DIDS (4,4'-di-isothiocyanato-2,2'-disulfostilbene) inhibited the generation of light cells during fast oxy/deoxy cycling, but not during oxy incubation. These results suggest that the sickling-induced pathway, previously implicated in SSRBC dehydration, may be involved in the deoxy-dependent component of rehydration for dense, K(+)-depleted cells. Light-cell generation was inhibited by 1 mM bumetanide during both oxy incubation and oxy/deoxy cycling, providing evidence that a bumetanide-sensitive, deoxy-independent pathway, previously described in circulating light SSRBCs, also contributes to the rehydration of high-density SSRBCs.

Sickle red cell dehydration: mechanisms and interventions

A critical link between the single molecular defect in sickle cell anemia and the extensive pathology of this disease is the reversible increase in red cell membrane permeability generated by hemoglobin S polymers in the deoxygenated state. This permeability, usually described as P (sickle), triggers a chain of events in which two constitutive transporters of the red cell membrane become activated-the recently cloned intermediate conductance, Ca 2+ -sensitive K channel, and the electroneutral K:Cl cotransporter-leading to sickle cell dehydration. This article reviews knowledge of the dehydration mechanism, stressing the marked heterogeneity of dehydration rates in sickle cell populations, and discusses recent contributions to understanding of the function and regulation of P (sickle), Ca 2+ -sensitive K channel, and K:Cl cotransporter, and of therapies targeted at these transporters.