Dehydration response of sickle cells to sickling-induced Ca(++) permeabilization

The Function of Ion Channels and Membrane Potential in Red Blood Cells: Toward a Systematic Analysis of the Erythroid Channelome

Erythrocytes represent at least 60% of all cells in the human body. During circulation, they experience a huge variety of physical and chemical stimulations, such as pressure, shear stress, hormones or osmolarity changes. These signals are translated into cellular responses through ion channels that modulate erythrocyte function. Ion channels in erythrocytes are only recently recognized as utmost important players in physiology and pathophysiology. Despite this awareness, their signaling, interactions and concerted regulation, such as the generation and effects of “pseudo action potentials”, remain elusive. We propose a systematic, conjoined approach using molecular biology, in vitro erythropoiesis, state-of-the-art electrophysiological techniques, and channelopathy patient samples to decipher the role of ion channel functions in health and disease. We need to overcome challenges such as the heterogeneity of the cell population (120 days lifespan without protein renewal) or the access to large cohorts of patients. Thereto we will use genetic manipulation of progenitors, cell differentiation into erythrocytes, and statistically efficient electrophysiological recordings of ion channel activity.

Chronic Kidney Disease amongst Sickle Cell Anaemia Patients at the University of Maiduguri Teaching Hospital, Northeastern Nigeria: A Study of Prevalence and Risk Factors

Background

Involvement of the kidneys in patients with sickle cell anaemia is a well recognised chronic complication. This study seeks to determine the prevalence of chronic kidney disease in patients with homozygous sickle cell disease (HbSS) and to identify risk factors associated with its development.

Methodology

The subjects consisted of adolescents and adults with HbSS recruited sequentially from the adult haematology outpatient clinic and Daycare ward of the unit. Clinical variables including age at diagnosis of SCA, the frequency of vaso-occlusive crisis and transfusion therapy, as well as laboratory data including haematological profile and renal function tests were obtained. The glomerular filtration rate was estimated (eGFR) using the ‘modification of diet in renal disease’ (MDRD) formula.

Results

Two hundred and eighty-four HbSS patients were recruited. The prevalence of CKD amongst them was 38.9%. Further stratification of the patients based on eGFR showed that sixty-nine (26.8%) had hyperfiltration; 35 (13.6%) stage 1 CKD; 53 (20.6%) stage 2 CKD; 33 (12.8%) stage 3a CKD; 28 (10.9%) stage 3b CKD; 30 (11.7%) stage 4 CKD and 9 (3.5%) had end stage renal disease. There was significant association between eGFR and clinical parameters such as age (r −0.353, p=0.000), SBP (r −0.148, p= 0.021), DBP (r −0.213, p=0.001) and total number of blood received (r −0.276, p=0.000); and laboratory parameters such as PCV (r 0.371, p=0.000); urea ( r 0.527, p=000); creatinine (r 0.625, p=0.000) and uric acid ( r −0.419, p=0.000).

Conclusions

The present study has revealed a high prevalence of CKD amongst patients with SCA in our region. Clinical and laboratory predictors of CKD using eGFR were identified to include age, SBP, number of units of blood transfusion, PCV, urea, creatinine and uric acid levels.

The Action of Red Cell Calcium Ions on Human Erythrophagocytosis in Vitro

In the present work we have studied in vitro the effect of increasing red cell Ca²⁺ ions on human erythrophagocytosis by peripheral monocyte-derived autologous macrophages. In addition, the relative contribution to phagocytosis of phosphatidylserine exposure, autologous IgG binding, complement deposition and Gárdos channel activity was also investigated. Monocytes were obtained after ficoll-hypaque fractionation and induced to transform by adherence to glass coverslips, for 24 h at 37°C in a RPMI medium, containing 10% fetal calf serum. Red blood cells (RBC) were loaded with Ca²⁺ using 10 μM A23187 and 1 mM Ca-EGTA buffers, in the absence of Mg²⁺. Ca²⁺-loaded cells were transferred to above coverslips and incubated for 2 h at 37°C under various experimental conditions, after which phagocytosis was assessed by light microscopy. Confirming earlier findings, phagocytosis depended on internal Ca²⁺. Accordingly; it was linearly raised from about 2–15% by increasing the free Ca²⁺ content of the loading solution from 0.5 to 20 μM, respectively. Such a linear increase was virtually doubled by the presence of 40% autologous serum. At 7 μM Ca²⁺, the phagocytosis degree attained with serum was practically equal to that obtained with either 2 mg/ml affinity-purified IgG or 40% IgG-depleted serum. However, phagocytosis was reduced to levels found with Ca²⁺ alone when IgG-depleted serum was inactivated by heat, implying an involvement of complement. On the other hand, phagocytosis in the absence of serum was markedly reduced by preincubating macrophages with phosphatidylserine-containing liposomes. In contrast, a similar incubation in the presence of serum affected it partially whereas employing liposomes made only of phosphatidylcholine essentially had no effect. Significantly, the Gárdos channel inhibitors clotrimazole (2 μM) and TRAM-34 (100 nM) fully blocked serum-dependent phagocytosis. These findings show that a raised internal Ca²⁺ promotes erythrophagocytosis by independently triggering phosphatidylserine externalization, complement deposition and IgG binding. Serum appeared to stimulate phagocytosis in a way dependent on Gárdos activity. It seems likely that Ca²⁺ promoted IgG-binding to erythrocytes via Gárdos channel activation. This can be an important signal for clearance of senescent human erythrocytes under physiological conditions.

Disorders of erythrocyte hydration

The erythrocyte contains a network of pathways that regulate salt and water content in the face of extracellular and intracellular osmotic perturbations. This allows the erythrocyte to maintain a narrow range of cell hemoglobin concentration, a process critical for normal red blood cell function and survival. Primary disorders that perturb volume homeostasis jeopardize the erythrocyte and may lead to its premature destruction. These disorders are marked by clinical, laboratory, and physiologic heterogeneity. Recent studies have revealed that these disorders are also marked by genetic heterogeneity. They have implicated roles for several proteins, PIEZO1, a mammalian mechanosensory protein; GLUT1, the glucose transporter; SLC4A1, the anion transporter; RhAG, the Rh-associated glycoprotein; KCNN4, the Gardos channel; and ABCB6, an adenosine triphosphate-binding cassette family member, in the maintenance of erythrocyte volume homeostasis. Secondary disorders of erythrocyte hydration include sickle cell disease, thalassemia, hemoglobin CC, and hereditary spherocytosis, where cellular dehydration may be a significant contributor to disease pathology and clinical complications. Understanding the pathways regulating erythrocyte water and solute content may reveal innovative strategies to maintain normal volume in disorders associated with primary or secondary cellular dehydration. These mechanisms will serve as a paradigm for other cells and may reveal new therapeutic targets for disease prevention and treatment beyond the erythrocyte.

Is Increased Intracellular Calcium in Red Blood Cells a Common Component in the Molecular Mechanism Causing Anemia?

For many hereditary disorders, although the underlying genetic mutation may be known, the molecular mechanism leading to hemolytic anemia is still unclear and needs further investigation. Previous studies revealed an increased intracellular Ca²⁺ in red blood cells (RBCs) from patients with sickle cell disease, thalassemia, or Gardos channelopathy. Therefore we analyzed RBCs' Ca²⁺ content from 35 patients with different types of anemia (16 patients with hereditary spherocytosis, 11 patients with hereditary xerocytosis, 5 patients with enzymopathies, and 3 patients with hemolytic anemia of unknown cause). Intracellular Ca²⁺ in RBCs was measured by fluorescence microscopy using the fluorescent Ca²⁺ indicator Fluo-4 and subsequent single cell analysis. We found that in RBCs from patients with hereditary spherocytosis and hereditary xerocytosis the intracellular Ca²⁺ levels were significantly increased compared to healthy control samples. For enzymopathies and hemolytic anemia of unknown cause the intracellular Ca²⁺ levels in RBCs were not significantly different. These results lead us to the hypothesis that increased Ca²⁺ levels in RBCs are a shared component in the mechanism causing an accelerated clearance of RBCs from the blood stream in channelopathies such as hereditary xerocytosis and in diseases involving defects of cytoskeletal components like hereditary spherocytosis. Future drug developments should benefit from targeting Ca²⁺ entry mediating molecular players leading to better therapies for patients.

Magnetic Levitation Coupled with Portable Imaging and Analysis for Disease Diagnostics

Currently, many clinical diagnostic procedures are complex, costly, inefficient, and inaccessible to a large population in the world. The requirements for specialized equipment and trained personnel require that many diagnostic tests be performed at remote, centralized clinical laboratories. Magnetic levitation is a simple yet powerful technique and can be applied to levitate cells, which are suspended in a paramagnetic solution and placed in a magnetic field, at a position determined by equilibrium between a magnetic force and a buoyancy force. Here, we present a versatile platform technology designed for point-of-care diagnostics which uses magnetic levitation coupled to microscopic imaging and automated analysis to determine the density distribution of a patient's cells as a useful diagnostic indicator. We present two platforms operating on this principle: (i) a smartphone-compatible version of the technology, where the built-in smartphone camera is used to image cells in the magnetic field and a smartphone application processes the images and to measures the density distribution of the cells and (ii) a self-contained version where a camera board is used to capture images and an embedded processing unit with attached thin-film-transistor (TFT) screen measures and displays the results. Demonstrated applications include: (i) measuring the altered distribution of a cell population with a disease phenotype compared to a healthy phenotype, which is applied to sickle cell disease diagnosis, and (ii) separation of different cell types based on their characteristic densities, which is applied to separate white blood cells from red blood cells for white blood cell cytometry. These applications, as well as future extensions of the essential density-based measurements enabled by this portable, user-friendly platform technology, will significantly enhance disease diagnostic capabilities at the point of care.

Sickle cell detection using a smartphone

Sickle cell disease affects 25% of people living in Central and West Africa and, if left undiagnosed, can cause life threatening "silent" strokes and lifelong damage. However, ubiquitous testing procedures have yet to be implemented in these areas, necessitating a simple, rapid, and accurate testing platform to diagnose sickle cell disease. Here, we present a label-free, sensitive, and specific testing platform using only a small blood sample (<1 μl) based on the higher density of sickle red blood cells under deoxygenated conditions. Testing is performed with a lightweight and compact 3D-printed attachment installed on a commercial smartphone. This attachment includes an LED to illuminate the sample, an optical lens to magnify the image, and two permanent magnets for magnetic levitation of red blood cells. The sample is suspended in a paramagnetic medium with sodium metabisulfite and loaded in a microcapillary tube that is inserted between the magnets. Red blood cells are levitated in the magnetic field based on equilibrium between the magnetic and buoyancy forces acting on the cells. Using this approach, we were able to distinguish between the levitation patterns of sickle versus control red blood cells based on their degree of confinement.

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.

Deoxygenation-induced and Ca(2+) dependent phosphatidylserine externalisation in red blood cells from normal individuals and sickle cell patients

Phosphatidylserine (PS) is usually confined to the inner leaflet of the red blood cell (RBC) membrane. It may become externalised in various conditions, however, notably in RBCs from patients with sickle cell disease (SCD) where exposed PS may contribute to anaemic and ischaemic complications. PS externalisation requires both inhibition of the aminophospholipid translocase (or flippase) and activation of the scramblase. Both may follow from elevation of intracellular Ca(2+). Flippase inhibition occurs at low [Ca(2+)](i), about 1μM, but [Ca(2+)](i) required for scrambling is reported to be much higher (around 100μM). In this work, FITC-labelled lactadherin and FACS were used to measure externalised PS, with [Ca(2+)](i) altered using bromo-A23187 and EGTA/Ca(2+) mixtures. Two components of Ca(2+)-induced scrambling were apparent, of high (EC(50) 1.8±0.3μM) and low (306±123μM) affinity, in RBCs from normal individuals and the commonest SCD genotypes, HbSS and HbSC. The high affinity component was lost in the presence of unphysiologically high [Mg(2+)] but was unaffected by high K(+) (90mM) or vanadate (1mM). The high affinity component accounted for PS scrambling in ≥2/3rd RBCs. It is likely to be most significant in vivo and may be involved in the pathophysiology of SCD or other conditions involving eryptosis.

Determinants of erythrocyte hydration

PURPOSE OF REVIEW

Maintenance of cellular water and solute homeostasis is critical for survival of the erythrocyte. Inherited or acquired disorders that perturb this homeostasis jeopardize the erythrocyte, leading to its premature destruction. This study reviews recent progress in our understanding the determinants of erythrocyte hydration and its related disorders.

RECENT FINDINGS

The molecular and genetic bases of primary disorders of erythrocyte hydration are poorly understood. Recent studies have implicated roles for the anion transporter, SLC4A1, and the Rh-associated glycoprotein, RhAG. The most common secondary disorder associated with perturbed hydration of the erythrocyte is sickle cell disease, in which dehydration contributes to disease pathology and clinical complications. Advances in understanding the mechanisms regulating erythrocyte solute and water content, particularly associated with KCl cotransport and Gardos channel activation, have revealed novel signaling mechanisms controlling erythrocyte hydration. These signaling pathways may provide innovative strategies to prevent erythrocyte dehydration in sickle cell disease.

SUMMARY

Clinical, translational and biologic studies all contribute to our knowledge of erythrocyte hydration. Understanding the mechanisms controlling erythrocyte water and solute homeostasis will serve as a paradigm for other cells and may reveal new therapeutic targets for disease prevention and treatment.

Effect of interleukin-8 and RANTES on the Gardos channel activity in sickle human red blood cells: role of the Duffy antigen receptor for chemokines

We investigated the effects of the chemokines IL-8 and RANTES on the activity of the Gardos channel (GC) of sickle red blood cells (SSRBCs). SSRBCs expressing the Duffy antigen receptor for chemokines (DARC) incubated under oxygenated conditions exhibit GC activation. The deoxygenation-stimulated K(+) loss via the GC is activated by the chemokines in the Duffy-positive SSRBCs. The percentage of cells with high density is 17 times higher in the Duffy-positive group. These findings are consistent with a greater susceptibility of Duffy-positive SSRBCs to inflammatory chemokines leading to GC activation and cellular dehydration and suggest a coupling, promoted by the sickling process, between DARC and the GC.

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.

Senicapoc (ICA-17043): a potential therapy for the prevention and treatment of hemolysis-associated complications in sickle cell anemia

Sickle cell disease (SCD) is characterized by hemolytic as well as vaso-occlusive complications. The development of treatments for this inherited disease is based on an understanding of its pathophysiology. Polymerization of sickle hemoglobin is dependent on several independent factors, including the intracellular hemoglobin concentration. The hydration state (and intracellular hemoglobin concentration) of the sickle erythrocyte depends on the loss of solute and osmotically obliged water through specific pathways. Senicapoc (also known as ICA-17043) is a potent blocker of the Gardos channel, a calcium-activated potassium channel of intermediate conductance, in the red blood cell. Preclinical studies and studies in transgenic models of SCD show that inhibition of potassium efflux through the Gardos channel is associated with an increased hemoglobin level, decreased dense cells and decreased hemolysis. Senicapoc is well tolerated when administered to SCD patients and produces dose-dependent increases in hemoglobin and decreases in markers of hemolysis. Despite the lack of a reduction in the frequency of pain episodes, the increasing recognition that hemolysis contributes to the development of several SCD-related complications suggests that by decreasing hemolysis, senicapoc may yet prove to be beneficial in this disease.

Gardos pathway to sickle cell therapies?

In this issue of Blood, Ataga and colleagues report that treatment of sickle cell disease patients with senicapoc, a Gardos channel inhibitor, reduces the number of dehydrated cells, increases hemoglobin levels, and diminishes hemolysis.

Oxidation-induced calcium-dependent dehydration of normal human red blood cells

Phenazine-methosulphate (PMS) is a strong oxidant that induces reactive oxygen species (ROS) formation in cells. Though it has been shown that PMS increases the red blood cell (RBC) membrane permeability to K(+), the hypotheses on the mechanism of PMS-induced effects are contradictory and there are no data on volume changes induced by this oxidant. Therefore, the influence of the PMS + ascorbate oxidative system on the volume of normal human RBCs was studied. In a Ca(2 + )-containing medium, PMS + ascorbate caused dehydration (shrinking) of RBCs judged by: (1) changes in the density and osmotic resistance distributions of RBCs, and (2) a decrease in their low-angle scattering assessed by FACS analysis. The dehydration resulted from activation of the Gardos channels, was PMS and ascorbate concentration-dependent, was associated with broadening of the density and osmotic resistance distributions of the RBCs, and decreased in the presence of the taxifolin and rutin antioxidants. These findings contribute to a better understanding of the physiology and pathology of oxidatively-modified RBCs and may be of practical significance in estimating the antioxidant activity of various substances.

Regulation of K-Cl cotransport: from function to genes

This review intends to summarize the vast literature on K-Cl cotransport (COT) regulation from a functional and genetic viewpoint. Special attention has been given to the signaling pathways involved in the transporter's regulation found in several tissues and cell types, and more specifically, in vascular smooth muscle cells (VSMCs). The number of publications on K-Cl COT has been steadily increasing since its discovery at the beginning of the 1980s, with red blood cells (RBCs) from different species (human, sheep, dog, rabbit, guinea pig, turkey, duck, frog, rat, mouse, fish, and lamprey) being the most studied model. Other tissues/cell types under study are brain, kidney, epithelia, muscle/smooth muscle, tumor cells, heart, liver, insect cells, endothelial cells, bone, platelets, thymocytes and Leishmania donovani. One of the salient properties of K-Cl-COT is its activation by cell swelling and its participation in the recovery of cell volume, a process known as regulatory volume decrease (RVD). Activation by thiol modification with N-ethylmaleimide (NEM) has spawned investigations on the redox dependence of K-Cl COT, and is used as a positive control for the operation of the system in many tissues and cells. The most accepted model of K-Cl COT regulation proposes protein kinases and phosphatases linked in a chain of phosphorylation/dephosphorylation events. More recent studies include regulatory pathways involving the phosphatidyl inositol/protein kinase C (PKC)-mediated pathway for regulation by lithium (Li) in low-K sheep red blood cells (LK SRBCs), and the nitric oxide (NO)/cGMP/protein kinase G (PKG) pathway as well as the platelet-derived growth factor (PDGF)-mediated mechanism in VSMCs. Studies on VSM transfected cells containing the PKG catalytic domain demonstrated the participation of this enzyme in K-Cl COT regulation. Commonly used vasodilators activate K-Cl COT in a dose-dependent manner through the NO/cGMP/PKG pathway. Interaction between the cotransporter and the cytoskeleton appears to depend on the cellular origin and experimental conditions. Pathophysiologically, K-Cl COT is altered in sickle cell anemia and neuropathies, and it has also been proposed to play a role in blood pressure control. Four closely related human genes code for KCCs (KCC1-4). Although considerable information is accumulating on tissue distribution, function and pathologies associated with the different isoforms, little is known about the genetic regulation of the KCC genes in terms of transcriptional and post-transcriptional regulation. A few reports indicate that the NO/cGMP/PKG signaling pathway regulates KCC1 and KCC3 mRNA expression in VSMCs at the post-transcriptional level. However, the detailed mechanisms of post-transcriptional regulation of KCC genes and of regulation of KCC2 and KCC4 mRNA expression are unknown. The K-Cl COT field is expected to expand further over the next decades, as new isoforms and/or regulatory pathways are discovered and its implication in health and disease is revealed.

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.

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.