Membrane cation and anion transport activities in erythrocytes of hereditary spherocytosis: effects of different membrane protein defects

Nrf2 Plays a Key Role in Erythropoiesis during Aging

Aging is characterized by increased oxidation and reduced efficiency of cytoprotective mechanisms. Nuclear factor erythroid-2-related factor (Nrf2) is a key transcription factor, controlling the expression of multiple antioxidant proteins. Here, we show that Nrf2-/- mice displayed an age-dependent anemia, due to the combined contributions of reduced red cell lifespan and ineffective erythropoiesis, suggesting a role of Nrf2 in erythroid biology during aging. Mechanistically, we found that the expression of antioxidants during aging is mediated by activation of Nrf2 function by peroxiredoxin-2. The absence of Nrf2 resulted in persistent oxidation and overactivation of adaptive systems such as the unfolded protein response (UPR) system and autophagy in Nrf2-/- mouse erythroblasts. As Nrf2 is involved in the expression of autophagy-related proteins such as autophagy-related protein (Atg) 4-5 and p62, we found impairment of late phase of autophagy in Nrf2-/- mouse erythroblasts. The overactivation of the UPR system and impaired autophagy drove apoptosis of Nrf2-/- mouse erythroblasts via caspase-3 activation. As a proof of concept for the role of oxidation, we treated Nrf2-/- mice with astaxanthin, an antioxidant, in the form of poly (lactic-co-glycolic acid) (PLGA)-loaded nanoparticles (ATS-NPs) to improve its bioavailability. ATS-NPs ameliorated the age-dependent anemia and decreased ineffective erythropoiesis in Nrf2-/- mice. In summary, we propose that Nrf2 plays a key role in limiting age-related oxidation, ensuring erythroid maturation and growth during aging.

Mitapivat reprograms the RBC metabolome and improves anemia in a mouse model of hereditary spherocytosis

Hereditary spherocytosis (HS) is the most common, nonimmune, hereditary, chronic hemolytic anemia after hemoglobinopathies. The genetic defects in membrane function causing HS lead to perturbation of the RBC metabolome, with altered glycolysis. In mice genetically lacking protein 4.2 (4.2-/-; Epb42), a murine model of HS, we showed increased expression of pyruvate kinase (PK) isoforms in whole and fractioned RBCs in conjunction with abnormalities in the glycolytic pathway and in the glutathione (GSH) system. Mitapivat, a PK activator, metabolically reprogrammed 4.2-/- mouse RBCs with amelioration of glycolysis and the GSH cycle. This resulted in improved osmotic fragility, reduced phosphatidylserine positivity, amelioration of RBC cation content, reduction of Na/K/Cl cotransport and Na/H-exchange overactivation, and decrease in erythroid vesicles release in vitro. Mitapivat treatment significantly decreased erythrophagocytosis and beneficially affected iron homeostasis. In mild-to-moderate HS, the beneficial effect of splenectomy is still controversial. Here, we showed that splenectomy improves anemia in 4.2-/- mice and that mitapivat is noninferior to splenectomy. An additional benefit of mitapivat treatment was lower expression of markers of inflammatory vasculopathy in 4.2-/- mice with or without splenectomy, indicating a multisystemic action of mitapivat. These findings support the notion that mitapivat treatment should be considered for symptomatic HS.

Erythrocyte pyruvate kinase activation in red cell disorders

PURPOSE OF REVIEW

In red cells, pyruvate kinase is a key enzyme in the final step of glycolytic degradative process, which generates a constant energy supply via ATP production. This commentary discusses recent findings on pyruvate kinase activators as new therapeutic option in hereditary red cell disorders such as thalassemic syndromes or sickle cell disease (SCD).

RECENT FINDINGS

Mitapivat and etavopivat are two oral pyruvate kinase activators. Studies in a mouse model for β thalassemia have shown beneficial effects of mitapivat on both red cell survival and ineffective erythropoiesis, with an amelioration of iron homeostasis. This was confirmed in a proof-of-concept study in patients with nontransfusion-dependent thalassemias. Both mitapivat and etavopivat have been evaluated in mouse models for SCD, showing an increased 2-3DPG/ATP ratio and a reduction in haemolysis as well as in sickling. These data were confirmed in proof-of-concept clinical studies with both molecules carried in patients with SCD.

SUMMARY

Preclinical and clinical evidence indicate that pyruvate kinase activators represent new therapeutic option in hemoglobinopathies or SCD. Other red cell disorders such as hereditary spherocytosis or hereditary anaemias characterized by defective erythropoiesis might represent additional areas to investigate the therapeutic impact of pyruvate kinase activators.

Tyrosine Phosphorylation Modulates Peroxiredoxin-2 Activity in Normal and Diseased Red Cells

Peroxiredoxin-2 (Prx2) is the third most abundant cytoplasmic protein in red blood cells. Prx2 belongs to a well-known family of antioxidants, the peroxiredoxins (Prxs), that are widely expressed in mammalian cells. Prx2 is a typical, homodimeric, 2-Cys Prx that uses two cysteine residues to accomplish the task of detoxifying a vast range of organic peroxides, H₂O₂, and peroxynitrite. Although progress has been made on functional characterization of Prx2, much still remains to be investigated on Prx2 post-translational changes. Here, we first show that Prx2 is Tyrosine (Tyr) phosphorylated by Syk in red cells exposed to oxidation induced by diamide. We identified Tyr-193 in both recombinant Prx2 and native Prx2 from red cells as a specific target of Syk. Bioinformatic analysis suggests that phosphorylation of Tyr-193 allows Prx2 conformational change that is more favorable for its peroxidase activity. Indeed, Syk-induced Tyr phosphorylation of Prx2 enhances in vitro Prx2 activity, but also contributes to Prx2 translocation to the membrane of red cells exposed to diamide. The biologic importance of Tyr-193 phospho-Prx2 is further supported by data on red cells from a mouse model of humanized sickle cell disease (SCD). SCD is globally distributed, hereditary red cell disorder, characterized by severe red cell oxidation due to the pathologic sickle hemoglobin. SCD red cells show Tyr-phosphorylated Prx2 bound to the membrane and increased Prx2 activity when compared to healthy erythrocytes. Collectively, our data highlight the novel link between redox related signaling and Prx2 function in normal and diseased red cells.

Density, heterogeneity and deformability of red cells as markers of clinical severity in hereditary spherocytosis

Hereditary spherocytosis (HS) originates from defective anchoring of the cytoskeletal network to the transmembrane protein complexes of the red blood cell (RBC). Red cells in HS are characterized by membrane instability and reduced deformability and there is marked heterogeneity in disease severity among patients. To unravel this variability in disease severity, we analyzed blood samples from 21 HS patients with defects in ankyrin, band 3, α-spectrin or β-spectrin using red cell indices, eosin-5-maleimide binding, microscopy, the osmotic fragility test, Percoll density gradients, vesiculation and ektacytometry to assess cell membrane stability, cellular density and deformability. Reticulocyte counts, CD71 abundance, band 4.1 a:b ratio, and glycated hemoglobin were used as markers of RBC turnover. We observed that patients with moderate/severe spherocytosis have short-living erythrocytes of low density and abnormally high intercellular heterogeneity. These cells show a prominent decrease in membrane stability and deformability and, as a consequence, are quickly removed from the circulation by the spleen. In contrast, in mild spherocytosis less pronounced reduction in deformability results in prolonged RBC lifespan and, hence, cells are subject to progressive loss of membrane. RBC from patients with mild spherocytosis thus become denser before they are taken up by the spleen. Based on our findings, we conclude that RBC membrane loss, cellular heterogeneity and density are strong markers of clinical severity in spherocytosis.

Thermal sensitivity and haemolysis of erythrocytes with membranopathy

Measuring the impedance of heated suspensions of erythrocytes and erythrocyte ghost membranes, two thermally-induced alterations are registered in the plasma membrane at T_A (denaturation of spectrin with inducing temperature at 49,5 °C) and T_G (hyperthermic activation of basal ion permeability with inducing temperature at 60.7 °C). In this study erythrocytes from 9 healthy patients and 15 patients with hemolytic anemia were studied and divided into four groups depending on their T_A and T_G top temperatures. The T_A and T_G of erythrocytes with hemoglobinopathy were the same as those of control erythrocytes while those of erythrocytes with membranopathy were significantly reduced. In erythrocytes with severe membranopathy, the T_G was decreased by about 5 °C. In latter cells the normal value of T_G was restored and the resistance to thermal haemolysis was increased by 90% after the specific stabilization of band 3 protein by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS). Obtained results indicate the involvement of band 3 in the membrane alteration at T_G and in the heat target responsible for thermal haemolysis.

Squeezing for Life - Properties of Red Blood Cell Deformability

Deformability is an essential feature of blood cells (RBCs) that enables them to travel through even the smallest capillaries of the human body. Deformability is a function of (i) structural elements of cytoskeletal proteins, (ii) processes controlling intracellular ion and water handling and (iii) membrane surface-to-volume ratio. All these factors may be altered in various forms of hereditary hemolytic anemia, such as sickle cell disease, thalassemia, hereditary spherocytosis and hereditary xerocytosis. Although mutations are known as the primary causes of these congenital anemias, little is known about the resulting secondary processes that affect RBC deformability (such as secondary changes in RBC hydration, membrane protein phosphorylation, and RBC vesiculation). These secondary processes could, however, play an important role in the premature removal of the aberrant RBCs by the spleen. Altered RBC deformability could contribute to disease pathophysiology in various disorders of the RBC. Here we review the current knowledge on RBC deformability in different forms of hereditary hemolytic anemia and describe secondary mechanisms involved in RBC deformability.

Peroxiredoxin-2: A Novel Regulator of Iron Homeostasis in Ineffective Erythropoiesis

AIMS

Iron overload (IO) is a life-threatening complication of chronic hemolytic disorders such as β-thalassemia. IO results in severe cellular oxidative damage, leading to organ failure. Peroxiredoxin-2 (Prx2), a typical 2-cysteine-(Cys)-peroxiredoxin, is an important component of the cytoprotective system, but its response to IO is still to be fully defined.

RESULTS

We studied the effects of IO on Prx2-knockout mice (Prx2^-/-). The absence of Prx2 enhanced toxicity due to IO on erythropoiesis. We found that IO failed to induce the typical hepcidin (Hamp) upregulation in Prx2^-/- mice due to its failure to activate the signal transducer and activator of transcription-3 (STAT3) with intact Jak2 signaling. In Prx2^-/- mice, the loss of Hamp response was also observed after administration of a single dose of oral iron. When lipopolysaccharide (LPS) was used to explore IL6-STAT3 activation in Prx2^-/- mice, STAT3 activation and Hamp upregulation were once again defective. Treatment with PEP-fusion-recombinant-Prx2 (PEP Prx2) significantly increased STAT3 activation with upregulation of Hamp expression in both IO- and LPS-exposed Prx2^-/- mice. We also confirmed the beneficial effects of PEP Prx2 on Hamp expression through STAT3 activation in β-thalassemic mice.

INNOVATION

We propose that Prx2 plays a key role in responding to cytotoxicity of IO, directly targeting STAT3-transcriptional factor in a Jak2-independent fashion and regulating Hamp in response to canonical stimuli.

CONCLUSION

Collectively, our data highlight a novel role of Prx2 in iron homeostasis. Prx2 is a key cytoprotector against IO that is induced either by iron supplementation or due to chronic hemolysis as in β-thalassemia. Prx2 is required to support STAT3 transcriptional activity and regulation of Hamp expression. Antioxid. Redox Signal. 28, 1-14.

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.

A new molecular link between defective autophagy and erythroid abnormalities in chorea-acanthocytosis

Chorea-acanthocytosis is one of the hereditary neurodegenerative disorders known as the neuroacanthocytoses. Chorea-acanthocytosis is characterized by circulating acanthocytes deficient in chorein, a protein of unknown function. We report here for the first time that chorea-acanthocytosis red cells are characterized by impaired autophagy, with cytoplasmic accumulation of active Lyn and of autophagy-related proteins Ulk1 and Atg7. In chorea-acanthocytosis erythrocytes, active Lyn is sequestered by HSP90-70 to form high-molecular-weight complexes that stabilize and protect Lyn from its proteasomal degradation, contributing to toxic Lyn accumulation. An interplay between accumulation of active Lyn and autophagy was found in chorea-acanthocytosis based on Lyn coimmunoprecipitation with Ulk1 and Atg7 and on the presence of Ulk1 in Lyn-containing high-molecular-weight complexes. In addition, chorein associated with Atg7 in healthy but not in chorea-acanthocytosis erythrocytes. Electron microscopy detected multivesicular bodies and membrane remnants only in circulating chorea-acanthocytosis red cells. In addition, reticulocyte-enriched chorea-acanthocytosis red cell fractions exhibited delayed clearance of mitochondria and lysosomes, further supporting the impairment of authophagic flux. Because autophagy is also important in erythropoiesis, we studied in vitro CD34+-derived erythroid precursors. In chorea-acanthocytosis, we found (1) dyserythropoiesis; (2) increased active Lyn; (3) accumulation of a marker of autophagic flux and autolysososme degradation; (4) accumlation of Lamp1, a lysosmal membrane protein, and LAMP1-positive aggregates; and (5) reduced clearance of lysosomes and mitochondria. Our results uncover in chorea-acanthocytosis erythroid cells an association between accumulation of active Lyn and impaired autophagy, suggesting a link between chorein and autophagic vesicle trafficking in erythroid maturation.

Protease inhibitors-based therapy induces acquired spherocytic-like anaemia and ineffective erythropoiesis in chronic hepatitis C virus patients

BACKGROUND & AIMS

The addition of protease inhibitors, boceprevir (BOC) or telaprevir (TRV), to peg-interferon and ribavirin (PR) increases the incidence of anaemia in patients with chronic hepatitis C virus (HCV) infection. Although genetic variants in inosine triphosphatase (ITPA) gene have been linked to the haemolytic anaemia induced by PR, the mechanism sustaining severe anaemia during triple therapy is still unknown. This study aims to elucidate the molecular mechanisms underlying anaemia in chronic HCV patients with combined therapy.

METHODS

We studied 59 patients with chronic HCV genotype-1: 29 treated with TRV/PR and 30 with BOC/PR. We evaluated biochemical and haematological parameters, red cell index at baseline, 4, 12, 16 and 24 weeks of treatment; in a subgroup, we performed functional studies: osmotic fragility, red cell membrane protein separation, mass spectrometry analysis, quantification of erythroid microparticles release. IL28B and ITPA polymorphisms were also evaluated.

RESULTS

We found early acute normochromic normocytic haemolytic anaemia (4-8 weeks) followed by a late macrocytic hypo-regenerative anaemia with inappropriate low reticulocyte count (12-24 weeks). Studies on red cells revealed: (i) presence of spherocytes; (ii) increased osmotic fragility; (iii) abnormalities in red cell membrane protein composition; (iv) reduced membrane-cytoskeleton stability; (v) increased release of erythroid microparticles. ITPA polymorphisms impacted only the early phase of anaemia.

CONCLUSIONS

The bimodal pattern of anaemia in chronic HCV patients on triple therapy might be because of acquired spherocytic-like anaemia in the early phase, followed by hyporegenerative anaemia, most likely related to the combined effects of PR and TRV or BOC on erythropoiesis.

Abnormal red cell features associated with hereditary neurodegenerative disorders: the neuroacanthocytosis syndromes

PURPOSE OF REVIEW

This review discusses the mechanisms involved in the generation of thorny red blood cells (RBCs), known as acanthocytes, in patients with neuroacanthocytosis, a heterogenous group of neurodegenerative hereditary disorders that include chorea-acanthocytosis (ChAc) and McLeod syndrome (MLS).

RECENT FINDINGS

Although molecular defects associated with neuroacanthocytosis have been identified recently, their pathophysiology and the related RBC abnormalities are largely unknown. Studies in ChAc RBCs have shown an altered association between the cytoskeleton and the integral membrane protein compartment in the absence of major changes in RBC membrane composition. In ChAc RBCs, abnormal Lyn kinase activation in a Syk-independent fashion has been reported recently, resulting in increased band 3 tyrosine phosphorylation and perturbation of the stability of the multiprotein band 3-based complexes bridging the membrane to the spectrin-based membrane skeleton. Similarly, in MLS, the absence of XK-protein, which is associated with the spectrin-actin-4.1 junctional complex, is associated with an abnormal membrane protein phosphorylation state, with destabilization of the membrane skeletal network resulting in generation of acanthocytes.

SUMMARY

A novel mechanism in generation of acanthocytes involving abnormal Lyn activation, identified in ChAc, expands the acanthocytosis phenomenon toward protein-protein interactions, controlled by phosphorylation-related abnormal signaling.

Oxidative stress and β-thalassemic erythroid cells behind the molecular defect

β-thalassemia is a worldwide distributed monogenic red cell disorder, characterized by the absence or reduced β-globin chain synthesis. Despite the extensive knowledge of the molecular defects causing β-thalassemia, less is known about the mechanisms responsible for the associated ineffective erythropoiesis and reduced red cell survival, which sustain anemia of β-thalassemia. The unbalance of alpha-gamma chain and the presence of pathological free iron promote a severe red cell membrane oxidative stress, which results in abnormal β-thalassemic red cell features. These cells are precociously removed by the macrophage system through two mechanisms: the removal of phosphatidylserine positive cells and through the natural occurring antibody produced against the abnormally clustered membrane protein band 3. In the present review we will discuss the changes in β-thalassemic red cell homeostasis related to the oxidative stress and its connection with production of microparticles and with malaria infection. The reactive oxygen species (ROS) are also involved in ineffective erythropoiesis of β-thalassemia through still partially known pathways. Novel cytoprotective systems such as ASHP, eIF2α, and peroxiredoxin-2 have been suggested to be important against ROS in β-thalassemic erythropoiesis. Finally, we will discuss the results of the major in vitro and in vivo studies with antioxidants in β-thalassemia.

Strain-specific variations in cation content and transport in mouse erythrocytes

Studies of ion transport pathophysiology in hematological disorders and tests of possible new therapeutic agents for these disorders have been carried out in various mouse models because of close functional similarities between mouse and human red cells. We have explored strain-specific differences in erythrocyte membrane physiology in 10 inbred mouse strains by determining erythrocyte contents of Na(+), K(+), and Mg(2+), and erythrocyte transport of ions via the ouabain-sensitive Na-K pump, the amiloride-sensitive Na-H exchanger (NHE1), the volume and chloride-dependent K-Cl cotransporter (KCC), and the charybdotoxin-sensitive Gardos channel (KCNN4). Our data reveal substantial strain-specific and sex-specific differences in both ion content and trans-membrane ion transport in mouse erythrocytes. These differences demonstrate the feasibility of identifying specific quantitative trait loci for erythroid ion transport and content in genetically standardized inbred mouse strains.

Hereditary red cell membrane disorders and laboratory diagnostic testing

This overview describes two groups of nonimmune hereditary hemolytic anemias caused by defects in membrane proteins located in distinct layers of the red cell membrane. Hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and hereditary pyropoikilocytosis (HPP) represent disorders of the red cell cytoskeleton. Hereditary stomatocytoses represents disorders of cation permeability in the red cell membrane. The current laboratory screening tests for HS are the osmotic fragility test, acid glycerol lysis time test (AGLT), cryohemolysis test, and eosin-5'-maleimide (EMA)-binding test. For atypical HS, SDS-polyacrylamide gel electrophoresis of erythrocyte membrane proteins is carried out to confirm the diagnosis. The diagnosis of HE/HPP is based on abnormal red cell morphology and the detection of protein 4.1R deficiency or spectrin variants using gel electrophoresis. None of screening tests can detect all HS cases. Some testing centers (a survey of 25 laboratories) use a combination of tests (e.g., AGLT and EMA). No specific screening test for hereditary stomatocytoses is available. The preliminary diagnosis is based on presenting a compensated hemolytic anemia, macrocytosis, and a temperature or time dependent pseudohyperkalemia in some patients. Both the EMA-binding test and the osmotic fragility test may help in differential diagnosis of HS and hereditary stomatocytosis.

Modularity of CHIP/LDB transcription complexes regulates cell differentiation

Transcription is the first step through which the cell operates, via its repertoire of transcription complexes, to direct cellular functions and cellular identity by generating the cell-specific transcriptome. The modularity of the composition of constituents of these complexes allows the cell to delicately regulate its transcriptome. In a recent study we have examined the effects of reducing the levels of specific transcription co-factors on the function of two competing transcription complexes, namely CHIP-AP and CHIP-PNR which regulate development of cells in the thorax of Drosophila. We found that changing the availability of these co-factors can shift the balance between these complexes leading to transition from utilization of CHIP-AP to CHIP-PNR. This is reflected in change in the expression profile of target genes, altering developmental cell fates. We propose that such a mechanism may operate in normal fly development. Transcription complexes analogous to CHIP-AP and CHIP-PNR exist in mammals and we discuss how such a shift in the balance between them may operate in normal mammalian development.

A novel erythroid anion exchange variant (Gly796Arg) of hereditary stomatocytosis associated with dyserythropoiesis

BACKGROUND

Stomatocytoses are a group of inherited autosomal dominant hemolytic anemias and include overhydrated hereditary stomatocytosis, dehydrated hereditary stomatocytosis, hereditary cryohydrocytosis and familial pseudohyperkalemia.

DESIGN AND METHODS

We report a novel variant of hereditary stomatocytosis due to a de novo band 3 mutation (p. G796R-band3 CEINGE) associated with a dyserythropoietic phenotype. Band 3 genomic analysis, measurement at of hematologic parameters and red cell indices and morphological analysis of bone marrow were carried out. We then evaluated the red cell membrane permeability and ion transport systems by functional studies of the patient's erythrocytes and Xenopus oocytes transfected with mutated band 3. We analyzed the red cell membrane tyrosine phosphorylation profile and the membrane association of the tyrosine kinases Syk and Lyn from the Src-family-kinase group, since the activity of the membrane cation transport pathways is related to cyclic phosphorylation-dephosphorylation events.

RESULTS

The patient showed mild hemolytic anemia with circulating stomatocytes together with signs of dyserythropoiesis. Her red cells displayed increased Na(+) content with decreased K(+)content and abnormal membrane cation transport activities. Functional characterization of band 3 CEINGE in Xenopus oocytes showed that the mutated band 3 is converted from being an anion exchanger (Cl(-), HCO(3)(-)) to being a cation pathway for Na(+) and K(+). Increased tyrosine phosphorylation of some red cell membrane proteins was observed in diseased erythrocytes. Syk and Lyn membrane association was increased in the patient's red cells compared to in normal controls, indicating perturbation of phospho-signaling pathways involved in cell volume regulation events.

CONCLUSIONS

Band 3 CEINGE alters function from that of anion exchange to cation transport, affects the membrane tyrosine phosphorylation profile, in particular of band 3 and stomatin, and its presence during red cell development likely contributes to dyserythropiesis.

Current knowledge about the functional roles of phosphorylative changes of membrane proteins in normal and diseased red cells

With the advent of proteomic techniques the number of known post-translational modifications (PTMs) affecting red cell membrane proteins is rapidly growing but the understanding of their role under physiological and pathological conditions is incompletely established. The wide range of hereditary diseases affecting different red cell membrane functions and the membrane modifications induced by malaria parasite intracellular growth represent a unique opportunity to study PTMs in response to variable cellular stresses. In the present review, some of the major areas of interest in red cell membrane research have been considered as modifications of erythrocyte deformability and maintenance of the surface area, membrane transport alterations, and removal of diseased and senescent red cells. In all mentioned research areas the functional roles of PTMs are prevalently restricted to the phosphorylative changes of the more abundant membrane proteins. The insufficient information about the PTMs occurring in a large majority of the red membrane proteins and the general lack of mass spectrometry data evidence the need of new comprehensive, proteomic approaches to improve the understanding of the red cell membrane physiology.

Acute Hemolytic Anemia With Acanthocytosis Associated With High-Dose Misoprostol for Medical Abortion

We report a case of acute hemolytic anemia in a 21-year-old Nigerian woman after high-dose misoprostol (4 mg), used for medical abortion. The major causes of inherited or immune hemolytic anemia were excluded. The patient's peripheral blood smear showed acanthocytes and anisopoikilocytosis, which progressively disappeared in the days postingestion. We evaluated RBC features, and we observed reduced RBC Na+ and K+ content and abnormalities in membrane cation transport pathways and in Ca2+ activated K+ channel (Gardos channel), suggesting possible direct effects of misoprostol on RBCs. Although further studies need to be carried out, the present case suggests that high-dose misoprostol, a prostaglandin E1 analogue, severely affects RBC features and causes an acquired acute hemolytic anemia, which is self-limited when misoprostol is withdrawn.

Erythrocyte membrane defects in hemolytic anemias found through derivative thermal analysis of electric impedance

Hereditary hemolytic anemias originate mainly from defects in hemoglobin and plasma membrane proteins. Here, we propose a new method, thermal analysis of impedance, sensitive to membrane defects. It detects three processes in erythrocyte membrane; fall in membrane capacity at 49.5 degrees C and activation of passive PO(4)(2+) permeability at 37 degrees C and inorganic ions at 61.5 degrees C. The denaturation of spectrin is involved in the first process whilst the anion channel is involved in latter processes. Using this method three persons with xerocytosis were found whereby the fall in membrane capacity and spherization of erythrocytes were both postponed (53 degrees C) compared to control (49.5 degrees C). In contrast to control cells, strong activation of passive permeability for Cl(-) at 37 degrees C and sucrose at 61 degrees C were detected that were both eliminated by pre-inhibition of the anion channel with 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS). In addition, erythrocytes from 15 patients with various forms of anemia were studied in intact state and after refreshment. The results were compared with the data of clinical laboratory and osmotic fragility test. The final conclusion is that this method detects membrane defects with altered spectrin and anion channel syndrome (hereditary xerocytosis, spherocytosis, poikilocytosis and pyropoikilocytosis, elliptocytosis and stomatocytosis) and, after refreshment, helps differentiate them from the anemia with hemoglobinopathy.

Effect of complete protein 4.1R deficiency on ion transport properties of murine erythrocytes

Moderate hemolytic anemia, abnormal erythrocyte morphology (spherocytosis), and decreased membrane stability are observed in mice with complete deficiency of all erythroid protein 4.1 protein isoforms (4.1(-/-); Shi TS et al. J Clin Invest 103: 331, 1999). We have examined the effects of erythroid protein 4.1 (4.1R) deficiency on erythrocyte cation transport and volume regulation. 4.1(-/-) mice exhibited erythrocyte dehydration that was associated with reduced cellular K and increased Na content. Increased Na permeability was observed in these mice, mostly mediated by Na/H exchange with normal Na-K pump and Na-K-2Cl cotransport activities. The Na/H exchange of 4.1(-/-) erythrocytes was markedly activated by exposure to hypertonic conditions (18.2 +/- 3.2 in 4.1(-/-) vs. 9.8 +/- 1.3 mmol/10(13) cell x h in control mice), with an abnormal dependence on osmolality (EC(50) = 417 +/- 42 in 4.1(-/-) vs. 460 +/- 35 mosmol/kgH(2)O in control mice), suggestive of an upregulated functional state. While the affinity for internal protons was not altered (K(0.5) = 489.7 +/- 0.7 vs. 537.0 +/- 0.56 nM in control mice), the V(max) of the H-induced Na/H exchange activity was markedly elevated in 4.1(-/-) erythrocytes (V(max) 91.47 +/- 7.2 compared with 46.52 +/- 5.4 mmol/10(13) cell x h in control mice). Na/H exchange activation by okadaic acid was absent in 4.1(-/-) erythrocytes. Altogether, these results suggest that erythroid protein 4.1 plays a major role in volume regulation and physiologically downregulates Na/H exchange in mouse erythrocytes. Upregulation of the Na/H exchange is an important contributor to the elevated cell Na content of 4.1(-/-) erythrocytes.

Protein-4.2 association with band 3 (AE1, SLCA4) in Xenopus oocytes: effects of three natural protein-4.2 mutations associated with hemolytic anemia

We have investigated the effects of coexpression of protein 4.2 and three protein-4.2 variants with band 3 in the Xenopus oocyte expression system. Normal protein 4.2 increased band-3–specific chloride transport in the oocytes. Protein 4.2 also coimmunoprecipitated with band 3 and colocalized with band 3 at the oocyte plasma membrane. The increase in band-3–mediated chloride transport and coimmunoprecipitation of protein 4.2 required the presence of the N-terminal cytoplasmic domain of band 3. Protein 4.2 also localized to the oocyte plasma membrane in the absence of band 3. The protein-4.2 variants 4.2 Tozeur (R310Q) and 4.2 Komatsu (D175Y) had impaired ability to bind to band 3 and these variants did not localize to the oocyte plasma membrane when expressed on their own or when coexpressed with band 3. Unexpectedly, 4.2 Nippon (A142T) behaved similarly to normal protein 4.2. In the absence of a crystal structure of protein 4.2, we propose a homology model of protein 4.2 based on the structure of the sequence-related protein transglutaminase. Using our results in oocytes and this homology model we speculate how these mutations affect protein 4.2 and result in hereditary spherocytosis.

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.

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.

Guidelines for the diagnosis and management of hereditary spherocytosis

Hereditary spherocytosis (HS) is a heterogeneous group of disorders with regard to clinical severity, protein defects and mode of inheritance. It is relatively common in Caucasian populations; most affected individuals have mild or only moderate haemolysis. There is usually a family history, and a typical clinical and laboratory picture so that the diagnosis is often easily made without additional laboratory tests. Atypical cases may require measurement of erythrocyte membrane proteins to clarify the nature of the membrane disorder and in the absence of a family history, occasionally molecular genetic analysis will help to determine whether inheritance is recessive or non-dominant. It is particularly important to rule out stomatocytosis where splenectomy is contraindicated because of the thrombotic risk. Mild HS can be managed without folate supplements and does not require splenectomy. Moderately and severely affected individuals are likely to benefit from splenectomy, which should be performed after the age of 6 years and with appropriate counselling about the infection risk. In all cases careful dialogue between doctor, patient and the family is essential. Laparoscopic surgery, when performed by experienced surgeons, can result in a shorter hospital stay and less pain.

Identification of quantitative trait loci that modify the severity of hereditary spherocytosis in wan, a new mouse model of band-3 deficiency

Defects in red blood cell (RBC) membrane skeleton components cause hereditary spherocytosis (HS). Clinically, HS varies significantly even among individuals with identical gene defects, illustrating the profound effects of genetic background on disease severity. We exploited a new spontaneous mouse model, wan, which arose on the inbred C3H/HeJ strain, to identify quantitative trait loci (QTL) that modify the HS phenotype. Homozygous wan mice have severe HS due to a complete deficiency of erythroid band 3. A QTL analysis of RBC count, hemoglobin, hematocrit, mean corpuscular volume (MCV), and mean corpuscular hemoglobin content (MCHC) was performed in wan/wan mice from an F2 intercross between C3H/HeJ+/wan and CAST/Ei+/+ F1 hybrids. Hematologic and survival data from C3H, CAST/Ei F2 wan homozygotes support the hypothesis that genetic modifiers significantly influence the band-3 null HS phenotype. Significant QTL were identified for the MCV trait only, suggesting that RBC membrane characteristics are a target for modifier gene action. The most significant quantitative trait locus, Hsm1 (hereditary spherocytosis modifier 1), localizes to mouse Chromosome 12 and is dominant. The peak LOD score was obtained with a marker for Spnb1 encoding erythroid β-spectrin, an obvious candidate gene. (Blood. 2004;103: 3233-3240)

Identification of a TAL1 target gene reveals a positive role for the LIM domain-binding protein Ldb1 in erythroid gene expression and differentiation

The TAL1 (or SCL) gene, originally identified from its involvement by a recurrent chromosomal translocation, encodes a basic helix-loop-helix transcription factor essential for erythropoiesis. Although presumed to regulate transcription, its target genes are largely unknown. We show here that a nuclear complex containing TAL1, its DNA-binding partner E47, zinc finger transcription factor GATA-1, LIM domain protein LMO2, and LIM domain-binding protein Ldb1 transactivates the protein 4.2 (P4.2) gene through two E box GATA elements in its proximal promoter. Binding of this complex to DNA was dependent on the integrity of both E box and GATA sites and was demonstrated to occur on the P4.2 promoter in cells. Maximal transcription in transiently transfected cells required both E box GATA elements and expression of all five components of the complex. This complex was shown, in addition, to be capable of linking in solution double-stranded oligonucleotides corresponding to the two P4.2 E box GATA elements. This DNA-linking activity required Ldb1 and increased with dimethyl sulfoxide-induced differentiation of murine erythroleukemia (MEL) cells. In contrast, enforced expression in MEL cells of dimerization-defective mutant Ldb1, as well as wild-type Ldb1, significantly decreased E box GATA DNA-binding activities, P4.2 promoter activity, and accumulation of P4.2 and beta-globin mRNAs. These studies define a physiologic target for a TAL1- and GATA-1-containing ternary complex and reveal a positive role for Ldb1 in erythroid gene expression and differentiation.

Splenectomy prolongs in vivo survival of erythrocytes differently in spectrin/ankyrin- and band 3–deficient hereditary spherocytosis

Red cell (RBC) deformability and membrane-bound immunoglobulin G (IgG) were studied to better understand premature clearance of erythrocytes in hereditary spherocytosis. Averaged deformability profiles from cells having comparable cell age revealed that splenectomy was more beneficial for spectrin/ankyrin-deficient than for band 3–deficient RBCs. Splenectomy prevented an early loss of young cells in both types of deficiencies. It had an additional beneficial effect on spectrin/ankyrin-deficient but not band 3–deficient RBCs. It prolonged the survival of mature spectrin/ankyrin-deficient RBCs such that they lost their deformability more slowly than RBCs from patients who had not undergone splenectomy. Band 3–deficient RBCs lost their deformability at the same rate before and after splenectomy. In HS patients with band 3 deficiency who underwent splenectomy, RBC deformability inversely correlated with the number of RBC-bound IgG (up to 140 molecules per cell). In spectrin/ankyrin deficiency, RBC-bound IgG remained at control levels (60 IgG or less per cell). It appears that spectrin/ankyrin-deficient RBCs escaped opsonization by releasing band 3–containing vesicles because their band 3 content and deformability dropped in parallel with increasing cell age. Band 3–deficient RBCs did not lose band 3 with increasing cell age. Hence, it is possible that band 3 clusters required for bivalent binding of low-affinity–IgG, naturally occurring antibodies were retained in band 3–deficient RBCs with a relative excess of skeletal proteins but were released from spectrin/ankyrin-deficient RBCs, in which vesicle budding was facilitated by an impaired skeleton.

Splenectomy prolongs in vivo survival of erythrocytes differently in spectrin/ankyrin- and band 3–deficient hereditary spherocytosis

Red cell (RBC) deformability and membrane-bound immunoglobulin G (IgG) were studied to better understand premature clearance of erythrocytes in hereditary spherocytosis. Averaged deformability profiles from cells having comparable cell age revealed that splenectomy was more beneficial for spectrin/ankyrin-deficient than for band 3–deficient RBCs. Splenectomy prevented an early loss of young cells in both types of deficiencies. It had an additional beneficial effect on spectrin/ankyrin-deficient but not band 3–deficient RBCs. It prolonged the survival of mature spectrin/ankyrin-deficient RBCs such that they lost their deformability more slowly than RBCs from patients who had not undergone splenectomy. Band 3–deficient RBCs lost their deformability at the same rate before and after splenectomy. In HS patients with band 3 deficiency who underwent splenectomy, RBC deformability inversely correlated with the number of RBC-bound IgG (up to 140 molecules per cell). In spectrin/ankyrin deficiency, RBC-bound IgG remained at control levels (60 IgG or less per cell). It appears that spectrin/ankyrin-deficient RBCs escaped opsonization by releasing band 3–containing vesicles because their band 3 content and deformability dropped in parallel with increasing cell age. Band 3–deficient RBCs did not lose band 3 with increasing cell age. Hence, it is possible that band 3 clusters required for bivalent binding of low-affinity–IgG, naturally occurring antibodies were retained in band 3–deficient RBCs with a relative excess of skeletal proteins but were released from spectrin/ankyrin-deficient RBCs, in which vesicle budding was facilitated by an impaired skeleton.

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.