Red cell membrane disorders

Significant advances have been made in our understanding of the structural basis for altered cell function in various inherited red cell membrane disorders with reduced red cell survival and resulting hemolytic anemia. The current review summarizes these advances as they relate to defining the molecular and structural basis for disorders involving altered membrane structural organization (hereditary spherocytosis [HS] and hereditary elliptocytosis [HE]) and altered membrane transport function (hereditary overhydrated stomatocytosis and hereditary xerocytosis). Mutations in genes encoding membrane proteins that account for these distinct red cell phenotypes have been identified. These molecular insights have led to improved understanding of the structural basis for altered membrane function in these disorders. Weakening of vertical linkage between the lipid bilayer and spectrin-based membrane skeleton leads to membrane loss in HS. In contrast, weakening of lateral linkages among different skeletal proteins leads to membrane fragmentation and decreased surface area in HE. The degrees of membrane loss and resultant increases in cell sphericity determine the severity of anemia in these two disorders. Splenectomy leads to amelioration of anemia by increasing the circulatory red cell life span of spherocytic red cells that are normally sequestered by the spleen. Disordered membrane cation permeability and resultant increase or decrease in red cell volume account for altered cellular deformability of hereditary overhydrated stomatocytosis and hereditary xerocytosis, respectively. Importantly, splenectomy is not beneficial in these two membrane transport disorders and in fact contraindicated due to severe postsplenectomy thrombotic complications.

Management of infected pancreatic necrosis-the "step up" approach and minimal access retroperitoneal pancreatic necrosectomy

Infected pancreatic necrosis (IPN) is associated with high morbidity and mortality. It is increasingly being recognized that noninvasive management, radiological guided drainage, and minimally invasive procedures rather than the traditionally advocated open necrosectomy are associated with a better outcome in IPN. We present a patient with IPN who was managed with the now popular "step up" approach and describe the procedure of Minimal access retroperitoneal pancreatic necrosectomy.

Primary hematopoietic cells from DBA patients with mutations in RPL11 and RPS19 genes exhibit distinct erythroid phenotype in vitro

Diamond-Blackfan anemia (DBA) is caused by aberrant ribosomal biogenesis due to ribosomal protein (RP) gene mutations. To develop mechanistic understanding of DBA pathogenesis, we studied CD34⁺ cells from peripheral blood of DBA patients carrying RPL11 and RPS19 ribosomal gene mutations and determined their ability to undergo erythroid differentiation in vitro. RPS19 mutations induced a decrease in proliferation of progenitor cells, but the terminal erythroid differentiation was normal with little or no apoptosis. This phenotype was related to a G₀/G₁ cell cycle arrest associated with activation of the p53 pathway. In marked contrast, RPL11 mutations led to a dramatic decrease in progenitor cell proliferation and a delayed erythroid differentiation with a marked increase in apoptosis and G₀/G₁ cell cycle arrest with activation of p53. Infection of cord blood CD34⁺ cells with specific short hairpin (sh) RNAs against RPS19 or RPL11 recapitulated the two distinct phenotypes in concordance with findings from primary cells. In both cases, the phenotype has been reverted by shRNA p53 knockdown. These results show that p53 pathway activation has an important role in pathogenesis of DBA and can be independent of the RPL11 pathway. These findings shed new insights into the pathogenesis of DBA.

Diamond-Blackfan anemia, ribosome and erythropoiesis

Diamond-Blackfan anemia is a rare inherited bone marrow failure syndrome (five to seven cases per million live births) characterized by an aregenerative, usually macrocytic anemia with an absence or less than 5% of erythroid precursors (erythroblastopenia) in an otherwise normal bone marrow. The platelet and the white cell counts are usually normal but neutropenia, thrombopenia or thrombocytosis have been noted at diagnosis. In 40 to 50% of DBA patients, congenital abnormalities mostly in the cephalic area and in thumbs and upper limbs have been described. Recent analysis did show a phenotype/genotype correlation. Congenital erythroblastopenia of DBA is the first human disease identified to result from defects in ribosomal biogenesis. The first ribosomal gene involved in DBA, ribosomal protein (RP) gene S19 (RPS19 gene), was identified in 1999. Subsequently, mutations in 12 other RP genes out of a total of 78 RP genes have been identified in DBA. All RP gene mutations described to date are heterozygous and dominant inheritance has been documented in 40 to 45% of affected individuals. As RP mutations are yet to be identified in approximately 50% of DBA cases, it is likely that other yet to be identified genes involved in ribosomal biogenesis or other pathways may be responsible for DBA phenotype.

Functional interaction between Rh proteins and the spectrin-based skeleton in erythroid and epithelial cells

We summarize the different experimental approaches which provide evidence that direct interaction of Rh and RhAG to ankyrin-R constitutes, together with the AE-1 (Band 3)-ankyrin-protein 4.2 and GPC-protein 4.1-p55 complexes, another major anchoring site between the red cell membrane bilayer and the underlying spectrin-based skeleton. The observations that some residues of the ankyrin binding site are mutated in Rh and RhAG proteins from some weak D and Rh(null) variants, respectively, suggest that the Rh-RhAG/ankyrin-R interaction plays a crucial role in the biosynthesis and/or the stability of the Rh complex in the red cell membrane. Similarly, binding to ankyrin G is required for cell surface expression of the non-erythroid member of the Rh protein family, RhBG, at the basolateral membrane domain of polarized epithelial cells. The next challenge will be to determine whether binding to the membrane skeleton may be critical for the emerging ammonium transport function of Rh proteins in erythroid and non-erythroid cells.

Murine recessive hereditary spherocytosis, sph/sph, is caused by a mutation in the erythroid alpha-spectrin gene

INTRODUCTION

Spectrin, a heterodimer of alpha- and beta-subunits, is the major protein component of the red blood cell membrane skeleton. The mouse mutation, sph, causes an alpha-spectrin-deficient hereditary spherocytosis with the severe phenotype typical of recessive hereditary spherocytosis in humans. The sph mutation maps to the erythroid alpha-spectrin locus, Spna1, on Chromosome 1.

MATERIALS AND METHODS

Scanning electron microscopy, osmotic gradient ektacytometry, cDNA cloning, RT-PCR, nucleic acid sequencing, and Northern blot analyses were used to characterize the wild type and sph alleles of the Spna1 locus.

RESULTS

Our results confirm the spherocytic nature of sph/sph red blood cells and document a mild spherocytic transition in the +/sph heterozygotes. Sequencing of the full length coding region of the Spna1 wild type allele from the C57BL/6J strain of mice reveals a 2414 residue deduced amino acid sequence that shows the typical 106-amino-acid repeat structure previously described for other members of the spectrin protein family. Sequence analysis of RT-PCR clones from sph/sph alpha-spectrin mRNA identified a single base deletion in repeat 5 that would cause a frame shift and premature termination of the protein. This deletion was confirmed in sph/sph genomic DNA. Northern blot analyses of the distribution of Spna1 mRNA in non-erythroid tissues detects the expression of 8, 2.5 and 2.0 kb transcripts in adult heart.

CONCLUSION

These results predict the heart as an additional site where alpha-spectrin mutations may produce a phenotype and raise the possibility that a novel functional class of small alpha-spectrin isoforms may exist.

Recombinant erythropoietin therapy as an alternative to blood transfusions in infants with hereditary spherocytosis

INTRODUCTION

In hereditary spherocytosis, erythropoiesis has been described as 'sluggish' during the first months of life. The lack of appropriate erythropoietic response to compensate for increased red cell destruction necessitates blood transfusions in 70-80% of hereditary spherocytosis-affected infants during their first year of life. After this period, less than 30% require regular transfusion support. This transient requirement for transfusion led us to wonder whether anemic hereditary spherocytosis infants, like anemic premature infants, could benefit from recombinant erythropoietin therapy (rHu-Epo).

MATERIAL AND METHODS

In 16 hereditary spherocytosis infants (age range 16-119 days) with severe anemia, a compassionate open preliminary study was performed. rHu-Epo treatment (1000 IU/kg/week) was instituted together with iron supplementation. Hemoglobin values and reticulocyte counts were repeatedly assessed.

RESULTS

In 13 out of 16 infants, prompt increases in reticulocyte counts were noted after the first week of treatment with 1000 IU/kg/week of rHu-Epo. During treatment with Epo these infants maintained clinically acceptable levels of hemoglobin and did not require blood transfusions. As the infants grew and began to mount an adequate erythropoietic response, the rHu-Epo dose could be tapered and the treatment could be discontinued before the age of nine months.

CONCLUSION

Epo treatment in most hereditary spherocytosis infants appears to be effective in the management of anemia and could serve as a valuable alternative to packed RBC transfusions.

Analysis of integral membrane protein contributions to the deformability and stability of the human erythrocyte membrane

Three major hypotheses have been proposed to explain the role of membrane-spanning proteins in establishing/maintaining membrane stability. These hypotheses ascribe the essential contribution of integral membrane proteins to (i) their ability to anchor the membrane skeleton to the lipid bilayer, (ii) their capacity to bind and stabilize membrane lipids, and (iii) their ability to influence and regulate local membrane curvature. In an effort to test these hypotheses in greater detail, we have modified both the membrane skeletal and lipid binding interactions of band 3 (the major membrane-spanning and skeletal binding protein of the human erythrocyte membrane) and have examined the impact of these modifications on erythrocyte membrane morphology, deformability, and stability. The desired changes in membrane skeletal and protein-lipid interactions were induced by 1) reaction of the cells with 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS), an inhibitor of band 3-mediated anion transport that dissociates band 3 into dimers (increasing its surface area in contact with lipid) and severs band 3 linkages to the membrane skeleton; 2) a fragment of ankyrin that ruptures the same ankyrin-band 3 bridge to the membrane skeleton, but drives the band 3 subunit equilibrium toward the tetramer (i.e. decreasing the band 3 surface area in contact with lipid); and 3) an antibody to the ankyrin-binding site on band 3 that promotes the same changes in band 3 skeletal and lipid interactions as the ankyrin fragment. We observed that although DIDS induced echinocytic morphological changes in the treated erythrocytes, it had little impact on either membrane deformability or stability. In contrast, resealing of either the ankyrin fragment or anti-band 3 IgG into erythrocytes caused spontaneous membrane fragmentation and loss of deformability/stability. Because these and other new observations cannot all be reconciled with any single hypothesis on membrane stability, we suggest that more than one hypothesis may be operative and provide an explanation of how each might individually contribute to net membrane stability.

Automated quantitation of hemoglobin-based blood substitutes in whole blood samples

It is necessary to develop methods for accurate monitoring of cell-free hemoglobin in circulation. Routine monitoring of circulating cell-free hemoglobin will be useful for evaluating the efficacy of blood substitute administration andfor determining the clearance rates of the blood substitute from circulation. In addition, discriminating between cell-free hemoglobin and cell-associated hemoglobin will enable accurate determination of RBC indices, mean cell hemoglobin and mean corpuscular hemoglobin concentration, in individuals receiving hemoglobin-based blood substitutes. As colorimetric methods used by hematology analyzers to quantitate the hemoglobin value of a blood sample cannot distinguish between cell-associated and cell-free hemoglobin, it is currently not feasible to quantitate the levels of hemoglobin substitutes in circulation. The advent of a technology that measures volume and hemoglobin concentration of individual RBCs provides an alternative strategy for quantitating the cell-associated hemoglobin in a blood sample. We document that the combined use of cell-based and colorimetric hemoglobin measurements provides accurate discrimination between cell-associated and cell-free hemoglobin over a wide range of hemoglobin levels. This strategy should enable rapid and accurate monitoring of the levels of cell-free hemoglobin substitutes in the circulation of recipients of these blood substitutes.

Temporal differences in membrane loss lead to distinct reticulocyte features in hereditary spherocytosis and in immune hemolytic anemia

Spherocytic red cells with reduced membrane surface area are a feature of hereditary spherocytosis (HS) and some forms of autoimmune hemolytic anemia (AIHA). It is generally assumed that membrane loss in spherocytic red cells occurs during their sojourn in circulation. The structural basis for membrane loss in HS is improper assembly of membrane proteins, whereas in AIHA it is due to partial phagocytosis of circulating red cells by macrophages. A hypothesis was formed that these different mechanisms should lead to temporal differences in surface area loss during red cell genesis and during sojourn in circulation in these 2 spherocytic syndromes. It was proposed that cell surface loss could begin at the reticulocyte stage in HS, whereas surface area loss in AIHA involves only circulating mature red cells. The validity of this hypothesis was established by documenting differences in cellular features of reticulocytes in HS and AIHA. Using a novel technique to quantitate cell surface area, the decreased membrane surface area of both reticulocytes and mature red cells in HS compared with normal cells was documented. In contrast, in AIHA only mature red cells but not reticulocytes exhibited decreased membrane surface area. These data imply that surface area loss in HS, but not in AIHA, is already present at the circulating reticulocyte stage. These findings imply that loss of cell surface area is an early event during genesis of HS red cells and challenge the existing concepts that surface area loss in HS occurs predominantly during the sojourn of mature red cells in circulation.

Pathophysiology of a sickle cell trait mouse model: human alpha(beta)(S) transgenes with one mouse beta-globin allele

As a potential model for sickle cell trait (AS), we examined mice containing one normal mouse beta-globin allele in combination with a human hemoglobin S (h(alpha)beta(S)) transgene (m(beta)/hS). The mice segregated into two subpopulations containing low and high proportions of hemoglobin S (m(beta)/hS1 and m(beta)/hS2, respectively) that was associated with one or two human h(alpha)beta(S) transgenes. We noted striking kidney pathology (cortical cysts, hyperplastic tubules, and glomerulonephritis), increasing with age and with greater severity in m(beta)/hS1. mBeta/hS2 animals were largely tolerant to 5% O(2) for 1 h, whereas 80% of m(beta)/hS1 mice died, exhibiting acute sequestration of erythrocytes in spleen, liver, and heart. These pathologies appear to result from a decreased oxygen affinity of the hybrid (human alpha/mouse beta) hemoglobins with a mild beta-thalassemia phenotype. Thus, these mouse models of sickle trait seem to manifest their renal pathology and sensitivity to hypoxia by mechanisms related to low tissue oxygen delivery and are different from the human syndrome. Analyses of parameters such as P(50), red cell indices, and genetic background are necessary in establishing potential relevance of any mouse model of the sickle cell syndromes.

Transport mechanisms in Plasmodium-infected erythrocytes: lipid rafts and a tubovesicular network

The mature human erythrocyte is a simple cell that is devoid of intracellular organelles and does not show endocytic or phagocytic activity at the plasma membrane. However, following infection by Plasmodium, the erythrocyte undergoes several morphological and functional changes. Parasite-derived proteins are exported into the erythrocyte cytoplasm and to the membrane, while several proteins are localised to the parasitophorous vacuolar membrane and to the tubovesicular membranous network structures surrounding the parasite. Recent evidence indicates that multiple host proteins, independent of the type of their membrane anchor, that exist in detergent-resistant membrane (DRM) rafts or microdomains enter this apicomplexan vacuole. The internalised host components along with the parasite-encoded transmembrane protein PfEXP1 can be detected as DRM rafts in the vacuole. It appears that in Plasmodium-infected erythrocytes lipid rafts may play a role in endovacuolation and macromolecular transport.

Structural and functional characterization of protein 4.1R-phosphatidylserine interaction: potential role in 4.1R sorting within cells

Erythrocyte protein 4.1R is a multifunctional protein that binds to various membrane proteins and to phosphatidylserine. In the present study, we report two important observations concerning 4.1R-phosphatidylserine interaction. Biochemically, a major finding of the present study is that 4.1R binding to phosphatidylserine appears to be a two-step process in which 4.1R first interacts with serine head group of phosphatidylserine through the positively charged amino acids YKRS and subsequently forms a tight hydrophobic interaction with fatty acid moieties. 4.1R failed to dissociate from phosphatidylserine liposomes under high ionic strength but could be released specifically by phospholipase A(2) but not by phospholipase C or D. Biochemical analyses showed that acyl chains were associated with 4.1R released by phospholipase A(2). Importantly, the association of acyl chains with 4.1R impaired its ability to interact with calmodulin, band 3, and glycophorin C. Removal of acyl chains restored 4.1R binding. These data indicate that acyl chains of phosphatidylserine play an important role in its interaction with 4.1R and on 4.1R function. In terms of biological significance, we have obtained evidence that 4.1R-phosphatidylserine interaction may play an important role in cellular sorting of 4.1R.

Short survival of phosphatidylserine-exposing red blood cells in murine sickle cell anemia

Several transgenic murine models for sickle cell anemia have been developed that closely reproduce the biochemical and physiological disorders in the human disease. A comprehensive characterization is described of hematologic parameters of mature red blood cells, reticulocytes, and red cell precursors in the bone marrow and spleen of a murine sickle cell model in which erythroid cells expressed exclusively human alpha, gamma, and betaS globin. Red cell survival was dramatically decreased in these anemic animals, partially compensated by considerable enhancement in erythropoietic activity. As in humans, these murine sickle cells contain a subpopulation of phosphatidylserine-exposing cells that may play a role in their premature removal. Continuous in vivo generation of this phosphatidylserine-exposing subset may have a significant impact on the pathophysiology of sickle cell disease.

Elastic thickness compressibilty of the red cell membrane

We have used an ultrasensitive force probe and optical interferometry to examine the thickness compressibility of the red cell membrane in situ. Pushed into the centers of washed-white red cell ghosts lying on a coverglass, the height of the microsphere-probe tip relative to its closest approach on the adjacent glass surface revealed the apparent material thickness, which began at approximately 90 nm per membrane upon detection of contact (force approximately 1-2 pN). With further impingement, the apparent thickness per membrane diminished over a soft compliant regime that spanned approximately 40 nm and stiffened on approach to approximately 50 nm under forces of approximately 100 pN. The same force-thickness response was obtained on recompression after retraction of the probe, which demonstrated elastic recoverability. Scaled by circumferences of the microspheres, the forces yielded energies of compression per area which exhibited an inverse distance dependence resembling that expected for flexible polymers. Attributed to the spectrin component of the membrane cytoskeleton, the energy density only reached one thermal energy unit (k(B)T) per spectrin tetramer near maximum compression. Hence, we hypothesized that the soft compliant regime probed in the experiments represented the compressibility of the outer region of spectrin loops and that the stiff regime < 50 nm was the response of a compact mesh of spectrin backed by a hardcore structure. To evaluate this hypothesis, we used a random flight theory for the entropic elasticity of polymer loops to model the spectrin network. We also examined the possibility that additional steric repulsion and apparent thickening could arise from membrane thermal-bending excitations. Fixing the energy scale to k(B)T/spectrin tetramer, the combined elastic response of a network of ideal polymer loops plus the membrane steric interaction correlated well with the measured dependence of energy density on distance for a statistical segment length of approximately 5 nm for spectrin (i.e., free chain end-to-end length of approximately 29 nm) and a hardcore limit of approximately 30 nm for underlying structure.

The Role of Cholesterol and Glycosylphosphatidylinositol-anchored Proteins of Erythrocyte Rafts in Regulating Raft Protein Content and Malarial Infection

Human erythrocytes are terminally differentiated, nonendocytic cells that lack all intracellular organelles. Here we show that their plasma membranes contain detergent-resistant membrane rafts that constitute a small fraction (4%) of the total membrane protein, with a complex mixture of proteins that differentially associate with rafts. Depletion of raft-cholesterol abrogates association of all proteins with no significant effect on cholesterol:protein ratios in the rest of the membrane, lipid asymmetry, deformability, or transport properties of the bilayer, indicating that cholesterol is critical for protein assembly into rafts and suggesting that rafts have little influence on several erythrocyte functions. Erythrocytes from patients with paroxysmal nocturnal hemoglobinuria, which lack glycosylphosphatidylinositol-anchored proteins, show significant elevation in raft-cholesterol but no increase in raft protein association, suggesting that raft assembly does not require glycosylphosphatidylinositol-anchored proteins, raft proteins do not bind directly to cholesterol, and only threshold levels of raft-cholesterol are critical for protein recruitment. Loss of glycosylphosphatidylinositol-anchored proteins had no effect on erythrocytic infection by malarial parasite or movement of raft markers into the parasite's vacuole. However, infection is blocked following raft-cholesterol disruption, suggesting that erythrocyte rafts can be functionally exploited and providing the first evidence for the involvement of host rafts in an apicomplexan infection.

Regulation of erythrocyte ghost membrane mechanical stability by chlorpromazine

Chlorpromazine (CPZ), a widely used tranquilizer, is known to induce stomatocytic shape changes in human erythrocytes. However, the effect of CPZ on membrane mechanical properties of erythrocyte membranes has not been documented. In the present study we show that CPZ induces a dose-dependent increase in mechanical stability of erythrocyte ghost membrane. Furthermore, we document that spectrin specifically binds to CPZ intercalated into inside-out vesicles depleted of all peripheral proteins. These findings imply that CPZ-induced mechanical stabilization of the erythrocyte ghost membranes may be mediated by direct binding of spectrin to the bilayer. Membrane active drugs that partition into lipid bilayer can thus induce cytoskeletal protein interactions with the membrane and modulate membrane material properties.

Glycophorin A dimerization and band 3 interaction during erythroid membrane biogenesis: in vivo studies in human glycophorin A transgenic mice

Band 3 and glycophorin A (GPA) are the 2 most abundant integral proteins in the human erythrocyte membrane. Earlier studies suggested that the 2 proteins may associate not only in the mature erythrocyte membrane, but also during their posttranslational processing and intracellular trafficking. The purpose of this study was to directly examine the GPA-band 3 interaction in vivo and determine the nature of this association during erythroid membrane biogenesis. Transgenic mice were generated expressing the human glycophorin A gene and were used to examine how the induction of human GPA expression affected the levels of murine GPA and band 3 expression in the red cell membrane. Murine GPA expression was reduced in erythrocytes expressing human GPA, whereas the level of band 3 expression remained constant, implying a tight coupling of band 3 and GPA expression in the membrane of mature red cells. In vivo GPA dimerization was not modulated solely by the GPA transmembrane motif, but the distance between this motif and the basic residues on the cytoplasmic side of the transmembrane domain may also be important. In addition, GPA monomers with varying degrees of glycosylation dimerized, providing clear evidence that carbohydrate structures on the extracellular domain do not affect dimerization. The association between the multiple transmembrane-spanning protein, band 3, and the single transmembrane-spanning sialoglycoprotein, GPA, may serve as a model for interactions of other multi-pass and single-pass polypeptides during membrane biogenesis.

Evidence for linkage of familial Diamond-Blackfan anemia to chromosome 8p23.3-p22 and for non-19q non-8p disease

Diamond-Blackfan anemia (DBA) is a rare congenital hypoplastic anemia that usually presents early in infancy and is inherited in 10% to 20% of cases. Linkage analysis has shown that DBA in many of both dominant and recessive DBA families mapped to chromosome 19q13.2 leading to the cloning of a gene on chromosome 19q13.2 that encodes a ribosomal protein, RPS19. However, subsequently, mutations of the RPS19 gene have only been identified in 25% of all patients with DBA. This study analyzed 14 multiplex DBA families, 9 of which had 19q13.2 haplotypes inconsistent with 19q linkage. A genome-wide search for linked loci suggested the presence of a second DBA locus in a 26.4-centimorgan (cM) interval on human chromosome 8p. Subsequently, 24 additional DBA families were ascertained and all 38 families were analyzed with additional polymorphic markers on chromosome 8p. In total, 18 of 38 families were consistent with linkage to chromosome 8p with a maximal LOD score with heterogeneity of 3.55 at D8S277 assuming 90% penetrance. The results indicate the existence of a second DBA gene in the 26.4-cM telomeric region of human chromosome 8p23.3-p22, most likely within an 8.1-cM interval flanked by D8S518 and D8S1825. Seven families were inconsistent with linkage to 8p or 19q and did not reveal mutations in the RPS19 gene, suggesting further genetic heterogeneity. (Blood. 2001;97:2145-2150)

Erythrocytic vacuolar rafts induced by malaria parasites

Studies in the past year displaced long-standing dogmas and provided many new molecular insights into how proteins and solutes move between the erythrocyte plasma membrane and the malarial vacuole. Highlights include a demonstration that (1) detergent-resistant membrane (DRM) rafts exist in the red cell membrane and their resident proteins are detected as rafts in the plasmodial vacuole, (2) a voltage-gated channel in the infected red cell membrane mediates uptake of extracellular nutrient solutes, and (3) intraerythrocytic membranes transport a parasite-encoded adherence antigen to the red cell surface.

Macrothrombocytopenia with abnormal demarcation membranes in megakaryocytes and neutropenia with a complete lack of sialyl-Lewis-X antigen in leukocytes--a new syndrome?

A new megathrombocytopenic syndrome with giant platelets in peripheral blood and severe thrombocytopenia was diagnosed in a 4-month-old boy. His clinical course included repeated hemorrhagic incidents leading to death at age 37 months. Bone marrow ultrastructural analysis revealed numerous dystrophic megakaryocytes with giant membrane complexes. Although these features were similar to those described for megakaryocytes in mice lacking the gene for transcription factor p45-NF-E2, no abnormalities in the p45-NF-E2 gene could be documented. Platelet membrane analysis showed a reduction in glycoprotein (GP) Ib, but normal content of GPIIb and GPIIIa. Analysis of genes encoding for GPIb alpha and beta, GPV, and GPIX ruled out the possibility that the observed platelet abnormality is a variant of Bernard-Soulier syndrome. A moderate neutropenia was associated with a complete lack of expression of sialyl-Lewis-X on the surface of polymorphonuclear neutrophils. A common defect in posttranslational modification of glycoproteins could account for the diverse cellular abnormalities.

Diamond-Blackfan anemia

Diamond-Blackfan Anemia (DBA) is a rare, congenital hypoplastic anemia often diagnosed early in infancy. A moderate to severe aregenerative anemia is found in association with erythroblastopenia in an otherwise normocellular bone marrow. In 40% of these infants with DBA, diverse developmental abnormalities are also noted. A majority of patients with DBA respond to steroid therapy. Recent molecular studies have identified mutations in the gene encoding the ribosomal protein RPS19 on chromosome 19 in 25% of patients with DBA. In another subset of patients, linkage analysis has identified another locus on chromosome 8p in association with DBA. There are, however, other cases of DBA that are linked neither to the RPS19 gene nor to the locus on 8p, implying the involvement of yet-to-be-defined genetic defects in the cause of DBA. The pathogenesis of DBA is still to be fully defined and it is anticipated that further molecular studies will lead to a better understanding of this complex disease.

Long-term evaluation of the beneficial effect of subtotal splenectomy for management of hereditary spherocytosis

Clinical manifestations of hereditary spherocytosis (HS) can be abrogated by splenectomy. However, concerns exist regarding exposure of patients to a lifelong risk for overwhelming infections and, to a lesser extent, to vascular complications after total splenectomy. In the search for alternative treatment modalities, we assessed, in a previous pilot study, the potential usefulness of subtotal splenectomy in a small population of patients. During a mean follow-up period of 3.5 years, subtotal splenectomy was shown to be effective in decreasing the hemolytic rate, while maintaining the phagocytic function of the spleen. In the current study, we evaluated the clinical and biologic features of 40 patients with HS who underwent subtotal splenectomy and were monitored for periods ranging from 1 to 14 years. The beneficial effect of subtotal splenectomy included a sustained decrease in hemolytic rate and a continued maintenance of phagocytic function of the splenic remnant. However, mild-to-moderate hemolysis was persistent and accounted for secondary gallstone formation and aplastic crisis in a small subset of patients. Surprisingly, regrowth of the remnant spleen did not seem to have a major impact on the beneficial outcomes of these individuals. Our results suggest that subtotal splenectomy appears to be a reasonable treatment option for management of patients with HS, especially young children.

Defective spectrin integrity and neonatal thrombosis in the first mouse model for severe hereditary elliptocytosis

Mutations affecting the conversion of spectrin dimers to tetramers result in hereditary elliptocytosis (HE), whereas a deficiency of human erythroid alpha- or beta-spectrin results in hereditary spherocytosis (HS). All spontaneous mutant mice with cytoskeletal deficiencies of spectrin reported to date have HS. Here, the first spontaneous mouse mutant, sph(Dem)/ sph(Dem), with severe HE is described. The sph(Dem) mutation is the insertion of an intracisternal A particle element in intron 10 of the erythroid alpha-spectrin gene. This causes exon skipping, the in-frame deletion of 46 amino acids from repeat 5 of alpha-spectrin and alters spectrin dimer/tetramer stability and osmotic fragility. The disease is more severe in sph(Dem)/sph(Dem) neonates than in alpha-spectrin-deficient mice with HS. Thrombosis and infarction are not, as in the HS mice, limited to adults but occur soon after birth. Genetic background differences that exist between HE and HS mice are suspect, along with red blood cell morphology differences, as modifiers of thrombosis timing. sph(Dem)/sph(Dem) mice provide a unique model for analyzing spectrin dimer- to-tetramer conversion and identifying factors that influence thrombosis.

Lutheran blood group glycoprotein and its newly characterized mouse homologue specifically bind alpha5 chain-containing human laminin with high affinity

Lutheran blood group glycoproteins (Lu gps) are receptors for the extracellular matrix protein, laminin. Studies suggest that Lu gps may contribute to vaso-occlusion in sickle cell disease and it has recently been shown that sickle cells adhere to laminin isoforms containing the alpha5 chain (laminin 10/11). Laminin alpha5 is present in the subendothelium and is also a constituent of bone marrow sinusoids, suggesting a role for the Lu/laminin interaction in erythropoiesis. The objectives of the current study were to define more precisely the molecular interactions of the extracellular and intracellular regions of human Lu and to clone and characterize a mouse homologue. To this end, complementary DNA and genomic clones for the mouse homologue were sequenced and the mouse Lu gene mapped to a region on chromosome 7 with conserved synteny with human 19q13.2. Mouse and human Lu gps are highly conserved (72% identity) at the amino acid sequence level and both mouse and human Lu gps specifically bind laminin 10/11 with high affinity. Furthermore, the first 3, N-terminal, immunoglobulin superfamily domains of human Lu are critical for this interaction. The results indicated that the cytoplasmic domain of BRIC 221-labeled human Lu gp is linked with the spectrin-based skeleton, affording the speculation that this interaction may be critical for signal transduction. These results further support a role for Lu gps in sickle cell disease and indicate the utility of mouse models to explore the function of Lu gp-laminin 10/11 interaction in normal erythropoiesis and in sickle cell disease.

The malaria-infected red blood cell: structural and functional changes

The asexual stage of malaria parasites of the genus Plasmodium invade red blood cells of various species including humans. After parasite invasion, red blood cells progressively acquire a new set of properties and are converted into more typical, although still simpler, eukaryotic cells by the appearance of new structures in the red blood cell cytoplasm, and new proteins at the red blood cell membrane skeleton. The red blood cell undergoes striking morphological alterations and its rheological properties are considerably altered, manifesting as red blood cells with increased membrane rigidity, reduced deformability and increased adhesiveness for a number of other cells including the vascular endothelium. Elucidation of the structural changes in the red blood cell induced by parasite invasion and maturation and an understanding of the accompanying functional alterations have the ability to considerably extend our knowledge of structure-function relationships in the normal red blood cell. Furthermore, interference with these interactions may lead to previously unsuspected means of reducing parasite virulence and may lead to the development of novel antimalarial therapeutics.

Plasmodium falciparum histidine-rich protein 1 associates with the band 3 binding domain of ankyrin in the infected red cell membrane

Infection of erythrocytes by the malaria parasite Plasmodium falciparum results in the export of several parasite proteins into the erythrocyte cytoplasm. Changes occur in the infected erythrocyte due to altered phosphorylation of proteins and to novel interactions between host and parasite proteins, particularly at the membrane skeleton. In erythrocytes, the spectrin based red cell membrane skeleton is linked to the erythrocyte plasma membrane through interactions of ankyrin with spectrin and band 3. Here we report an association between the P. falciparum histidine-rich protein (PfHRP1) and phosphorylated proteolytic fragments of red cell ankyrin. Immunochemical, biochemical and biophysical studies indicate that the 89 kDa band 3 binding domain and the 62 kDa spectrin-binding domain of ankyrin are co-precipitated by mAb 89 against PfHRP1, and that native and recombinant ankyrin fragments bind to the 5' repeat region of PfHRP1. PfHRP1 is responsible for anchoring the parasite cytoadherence ligand to the erythrocyte membrane skeleton, and this additional interaction with ankyrin would strengthen the ability of PfEMP1 to resist shear stress.

Protein 4.1R core domain structure and insights into regulation of cytoskeletal organization

The crystal structure of the core domain (N-terminal 30 kDa domain) of cytoskeletal protein 4.1R has been determined and shows a cloverleaf-like architecture. Each lobe of the cloverleaf contains a specific binding site for either band 3, glycophorin C/D or p55. At a central region of the molecule near where the three lobes are joined are two separate calmodulin (CaM) binding regions. One of these is composed primarily of an alpha-helix and is Ca 2+ insensitive; the other takes the form of an extended structure and its binding with CaM is dramatically enhanced by the presence of Ca 2+, resulting in the weakening of protein 4.1R binding to its target proteins. This novel architecture, in which the three lobes bind with three membrane associated proteins, and the location of calmodulin binding sites provide insight into how the protein 4.1R core domain interacts with membrane proteins and dynamically regulates cell shape in response to changes in intracellular Ca2+ levels.

Characterization of human RhCG and mouse Rhcg as novel nonerythroid Rh glycoprotein homologues predominantly expressed in kidney and testis

In mammals, the Rh family includes the variable Rh polypeptides and invariant RhAG glycoprotein. These polytopic proteins are confined to the erythroid lineage and are assembled into a multisubunit complex essential for Rh antigen expression and plasma membrane integrity. Here, we report the characterization of RhCG and Rhcg, a pair of novel Rh homologues present in human and mouse nonerythroid tissues. Despite sharing a notable similarity to the erythroid forms, including the 12-transmembrane topological fold, the RHCG/Rhcg pair is distinct in chromosome location, genomic organization, promoter structure, and tissue-specific expression. RHCG and Rhcg map at 15q25 of human chromosome 15 and the long arm of mouse chromosome 7, respectively, each having 11 exons and a CpG-rich promoter. Northern blots detected kidney and testis as the major organs of RHCG or Rhcg expression. In situ hybridization revealed strong expression of Rhcg in the kidney collecting tubules and testis seminiferous tubules. Confocal imaging of transiently expressed green fluorescence protein fusion proteins localized RhCG exclusively to the plasma membrane, a distribution confirmed by cellular fractionation and Western blot analysis. In vitro translation and ex vivo expression showed that RhCG carries a complex N-glycan, probably at the (48)NLS(50) sequon of exoloop 1. These results pinpoint RhCG and Rhcg as novel polytopic membrane glycoproteins that may function as epithelial transporters maintaining normal homeostatic conditions in kidney and testis.

Regulation of protein 4.1R, p55, and glycophorin C ternary complex in human erythrocyte membrane

Three binary protein-protein interactions, glycophorin C (GPC)-4.1R, GPC-p55, and p55-4.1R, constitute the GPC-4.1R-p55 ternary complex in the erythrocyte membrane. Little is known regarding the molecular basis for the interaction of 4.1R with either GPC or p55 and regarding the role of 4.1R in regulating the various protein-protein interactions that constitute the GPC-4.1R-p55 ternary complex. In the present study, we present evidence that sequences in the 30-kDa domain encoded by exon 8 and exon 10 of 4.1R constitute the binding interfaces for GPC and p55, respectively. We further show that 4.1R increases the affinity of p55 binding to GPC by an order of magnitude, implying that 4.1R modulates the interaction between p55 and GPC. Finally, we document that binding of calmodulin to 4.1R decreases the affinity of 4.1R interactions with both p55 and GPC in a Ca(2+)-dependent manner, implying that the GPC-4.1R-p55 ternary protein complex can undergo dynamic regulation in the erythrocyte membrane. Taken together, these findings have enabled us to identify an important role for 4.1R in regulating the GPC-4.1R-p55 ternary complex in the erythrocyte membrane.

Vacuolar uptake of host components, and a role for cholesterol and sphingomyelin in malarial infection

Erythrocytes, which are incapable of endocytosis or phagocytosis, can be infected by the malaria parasite Plasmodium falciparum. We find that a transmembrane protein (Duffy), glycosylphosphatidylinositol (GPI)-anchored and cytoplasmic proteins, associated with detergent-resistant membranes (DRMs) that are characteristic of microdomains in host cell membranes, are internalized by vacuolar parasites, while the major integral membrane and cytoskeletal proteins are not. The internalized host proteins and a plasmodial transmembrane resident parasitophorous vacuolar membrane (PVM) protein are detected in DRMs associated with vacuolar parasites. This is the first report of a host transmembrane protein being recruited into an apicomplexan vacuole and of the presence of vacuolar DRMs; it establishes that integral association does not preclude protein internalization into the P.FALCIPARUM: vacuole. Rather, as shown for Duffy, intracellular accumulation occurs at the same rate as that seen for a DRM-associated GPI-anchored protein. Furthermore, novel mechanisms regulated by the DRM lipids, sphingomyelin and cholesterol, mediate (i) the uptake of host DRM proteins and (ii) maintenance of the intracellular vacuole in the non-endocytic red cell, which may have implications for intracellular parasitism and pathogenesis.

Identification and characterization of a newly recognized population of high-Na+, low-K+, low-density sickle and normal red cells

We describe a population of sickle cell anemia red cells (SS RBCs) ( approximately 4%) and a smaller fraction of normal RBCs (<0.03%) that fail to dehydrate when permeabilized to K(+) with either valinomycin or elevated internal Ca(2+). The nonshrinking, valinomycin-resistant (val-res) fractions, first detected by flow cytometry of density-fractionated SS RBCs, constituted up to 60% of the lightest, reticulocyte-rich (R1) cell fraction, and progressively smaller portions of the slightly denser R2 cells and discocytes. R1 val-res RBCs had a mean cell hemoglobin concentration of approximately 21 g of Hb per dl, and many had an elongated shape like "irreversibly sickled cells," suggesting a dense SS cell origin. Of three possible explanations for val-res cells, failure of valinomycin to K(+)-permeabilize the cells, low co-ion permeability, or reduced driving K(+) gradient, the latter proved responsible: Both SS and normal val-res RBCs were consistently high-Na(+) and low-K(+), even when processed entirely in Na-free media. Ca(2+) + A23187-induced K(+)-permeabilization of SS R1 fractions revealed a similar fraction of cal-res cells, whose (86)Rb uptake showed both high Na/K pump and leak fluxes. val-res/cal-res RBCs might represent either a distinct erythroid genealogy, or an "end-stage" of normal and SS RBCs. This paper focuses on the discovery, basic characterization, and exclusion of artifactual origin of this RBC fraction. Many future studies will be needed to clarify their mechanism of generation and full pathophysiological significance.

Ca(2+)-dependent and Ca(2+)-independent calmodulin binding sites in erythrocyte protein 4.1. Implications for regulation of protein 4.1 interactions with transmembrane proteins

In vitro protein binding assays identified two distinct calmodulin (CaM) binding sites within the NH(2)-terminal 30-kDa domain of erythrocyte protein 4.1 (4.1R): a Ca(2+)-independent binding site (A(264)KKLWKVCVEHHTFFRL) and a Ca(2+)-dependent binding site (A(181)KKLSMYGVDLHKAKDL). Synthetic peptides corresponding to these sequences bound CaM in vitro; conversely, deletion of these peptides from a 30-kDa construct reduced binding to CaM. Thus, 4.1R is a unique CaM-binding protein in that it has distinct Ca(2+)-dependent and Ca(2+)-independent high affinity CaM binding sites. CaM bound to 4.1R at a stoichiometry of 1:1 both in the presence and absence of Ca(2+), implying that one CaM molecule binds to two distinct sites in the same molecule of 4.1R. Interactions of 4.1R with membrane proteins such as band 3 is regulated by Ca(2+) and CaM. While the intrinsic affinity of the 30-kDa domain for the cytoplasmic tail of erythrocyte membrane band 3 was not altered by elimination of one or both CaM binding sites, the ability of Ca(2+)/CaM to down-regulate 4. 1R-band 3 interaction was abrogated by such deletions. Thus, regulation of protein 4.1 binding to membrane proteins by Ca(2+) and CaM requires binding of CaM to both Ca(2+)-independent and Ca(2+)-dependent sites in protein 4.1.

New insights into functions of erythroid proteins in nonerythroid cells

This article presents new insights into potential roles that three erythrocyte cytoskeletal proteins, protein 4.1, ankyrin, and spectrin, may play in nonerythroid nucleated cells. Each of these proteins is encoded by several closely related genes characterized by complex alternative splicing of their pre-mRNA, thus resulting in the cellular expression of a broad repertoire of isoforms that can adopt tissue- and cell-specific distribution. This could account for the presence of skeletal networks in intracellular organelles such as lysosomes, the Golgi apparatus, or the nucleus. In addition to providing structural support to cell membranes, these cytoskeletal proteins regulate the functions of various transmembrane proteins they interact with, in particular ion channels, as well as the activity of membrane-bound enzymes. Thus, they appear to be key players in major unsuspected cell functions such as protein sorting, dynamics of nuclear architecture during mitosis, or regulation of signal transduction pathways.

Molecular and functional characterization of protein 4.1B, a novel member of the protein 4.1 family with high level, focal expression in brain

Brain-enriched isoforms of skeletal proteins in the spectrin and ankyrin gene families have been described. Here we characterize protein 4.1B, a novel homolog of erythrocyte protein 4.1R that is encoded by a distinct gene. In situ hybridization revealed high level, focal expression of 4.1B mRNA in select neuronal populations within the mouse brain, including Purkinje cells of the cerebellum, pyramidal cells in hippocampal regions CA1-3, thalamic nuclei, and olfactory bulb. Expression was also detected in adrenal gland, kidney, testis, and heart. 4.1B protein exhibits high homology to the membrane binding, spectrin-actin binding, and C-terminal domains of 4.1R, including motifs for interaction with NuMA and FKBP13. cDNA characterization and Western blot analysis revealed multiple spliceoforms of protein 4.1B, with functionally relevant heterogeneity in the spectrin-actin and NuMA binding domains. Regulated alternative splicing events led to expression of unique 4. 1B isoforms in brain and muscle; only the latter possessed a functional spectrin-actin binding domain. By immunofluorescence, 4. 1B was localized specifically at the plasma membrane in regions of cell-cell contact. Together these results indicate that 4.1B transcription is selectively regulated among neuronal populations and that alternative splicing regulates expression of 4.1B isoforms possessing critical functional domains typical of other protein 4.1 family members.

Crystallization and preliminary X-ray crystallographic analysis of the 30 kDa membrane-binding domain of protein 4.1 from human erythrocytes

The 30 kDa membrane-binding domain of protein 4.1 from human erythrocytes has been expressed in Escherichia coli and crystallized in a form suitable for X-ray crystallographic study. Crystals were grown using a salting-in technique. Crystals have a tetragonal plate shape and belong to the C2 space group, with unit-cell parameters a = 163.9, b = 106.5, c = 93.5 A, beta = 95.5 degrees. The crystals diffract to 2.8 A resolution.

Natural history of hereditary spherocytosis during the first year of life

Although hereditary spherocytosis (HS) is a common disorder of the red cell membrane, its clinical and biologic expression at birth and in early infancy has received little attention. In order to obtain insights into the natural history of HS during infancy, we studied 46 neonates, 39 from families in which 1 of the parents had previously been given a diagnosis of HS and 7 presenting with nonimmune hemolytic anemia and no family history of HS. Of these 46 neonates, 23 were subsequently confirmed to have HS and 23 were found to be healthy. The hematologic and biologic analyses carried out in this cohort of 46 newborns enabled us to develop guidelines for early diagnosis of HS. A careful clinical follow-up of 34 HS patients during the first year of life allowed us to define several important clinical features of HS during this period. Hemoglobin values are usually normal at birth but decrease sharply during the subsequent 20 days, which leads, in many cases, to a transient and severe anemia. The anemia is severe enough to warrant blood transfusions in a large number of infants with HS (26 of 34 in our series). The aggravation of anemia appears to be related to the inability of these infants to mount an appropriate erythropoietic response to anemia and to the development of splenic filtering function. These findings indicate that careful monitoring of infants with HS during the first 6 months of life is important for appropriate clinical management. (Blood. 2000;95:393-397)

Mutations in ribosomal protein S19 gene and diamond blackfan anemia: wide variations in phenotypic expression

Mutations of the ribosomal protein S19 (RPS19) gene were recently identified in 10 patients with Diamond Blackfan anemia (DBA). To determine the prevalence of mutations in this gene in DBA and to begin to define the molecular basis for the observed variable clinical phenotype of this disorder, the genomic sequence of the 6 exons and the 5' untranslated region of the RPS19 gene was directly assessed in DBA index cases from 172 new families. Mutations affecting the coding sequence of RPS19 or splice sites were found in 34 cases (19.7%), whereas mutations in noncoding regions were found in 8 patients (4.6%). Mutations included nonsense, missense, splice sites, and frameshift mutations. A hot spot for missense mutations was identified between codons 52 and 62 of the RPS19 gene in a new sequence consensus motif W-[YFW]-[YF]-x-R-[AT]-A-[SA]-x-[AL]-R-[HRK]-[ILV]-Y. No correlation between the nature of mutations and the different patterns of clinical expression, including age at presentation, presence of malformations, and therapeutic outcome, could be documented. Moreover, RPS19 mutations were also found in some first-degree relatives presenting only with isolated high erythrocyte adenosine deaminase activity and/or macrocytosis. The lack of a consistent relationship between the nature of the mutations and the clinical phenotype implies that yet unidentified factors modulate the phenotypic expression of the primary genetic defect in families with RPS19 mutations.

What do mouse gene knockouts tell us about the structure and function of the red cell membrane?

Recent development of knockout mice with targeted deletion of specific genes encoding various red cell membrane proteins has added valuable armamentarium to red cell membrane structure-function studies. In this chapter we will summarize the various recent developments regarding the structure and function of the red cell membrane derived from studies using knockout mice. In addition to being expressed in red cells, all major red cell membrane proteins are also expressed in cells of various tissues. The potential use of knockout mice to decipher the biological functions of red cell membrane proteins in non-erythroid cells is also explored.

Identification of new prognosis factors from the clinical and epidemiologic analysis of a registry of 229 Diamond-Blackfan anemia patients. DBA group of Société d'Hématologie et d'Immunologie Pédiatrique (SHIP), Gesellshaft für Pädiatrische Onkologie und Hämatologie (GPOH), and the European Society for Pediatric Hematology and Immunology (ESPHI)

Diamond-Blackfan anemia (DBA) is a constitutional disease characterized by a specific maturation defect in cells of erythroid lineage. We have assembled a registry of 229 DBA patients, which includes 151 patients from France, 70 from Germany, and eight from other countries. Presence of malformations was significantly and independently associated with familial history of DBA, short stature at presentation (before any steroid therapy), and absence of hypotrophy at birth. Two hundred twenty-two patients were available for long-term follow-up analysis (median, 111.5 mo). Of these individuals, 62.6% initially responded to steroid therapy. Initial steroid responsiveness was found significantly and independently associated with older age at presentation, familial history of DBA, and a normal platelet count at the time of diagnosis. Severe evolution of the disease (transfusion dependence or death) was significantly and independently associated with a younger age at presentation and with a history of premature birth. In contrast, patients with a familial history of the disease experienced a better outcome. Outcome analysis revealed the benefit of reassessing steroid responsiveness during the course of the disease for initially nonresponsive patients. Bone marrow transplantation was successful in 11/13 cases; HLA typing of probands and siblings should be performed early if patients are transfusion dependent, and cord blood should be preserved. Incidence of DBA (assessed for France over a 13-y period) is 7.3 cases per million live births without effect of seasonality on incidence of the disease or on malformative status. Similarly, no parental imprinting effect or anticipation phenomenon could be documented in families with dominant inheritance.

Targeted disruption of the beta adducin gene (Add2) causes red blood cell spherocytosis in mice

Adducins are a family of cytoskeleton proteins encoded by three genes (alpha, beta, gamma). In a comprehensive assay of gene expression, we show the ubiquitous expression of alpha- and gamma-adducins in contrast to the restricted expression of beta-adducin. beta-adducin is expressed at high levels in brain and hematopoietic tissues (bone marrow in humans, spleen in mice). To elucidate adducin's role in vivo, we created beta-adducin null mice by gene targeting, deleting exons 9-13. A 55-kDa chimeric polypeptide is produced from the first eight exons of beta-adducin and part of the neo cassette in spleen but is not detected in peripheral RBCs or brain. beta-adducin null RBCs are osmotically fragile, spherocytic, and dehydrated compared with the wild type, resembling RBCs from patients with hereditary spherocytosis. The lack of beta-adducin in RBCs leads to decreased membrane incorporation of alpha-adducin (30% of normal) and unexpectedly promotes a 5-fold increase in gamma-adducin incorporation into the RBC membrane skeleton. This study demonstrates adducin's importance to RBC membrane stability in vivo.

Mapping the binding domains involved in the interaction between the Plasmodium falciparum knob-associated histidine-rich protein (KAHRP) and the cytoadherence ligand P. falciparum erythrocyte membrane protein 1 (PfEMP1)

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) clusters at electron-dense knob-like structures on the surface of malaria-infected red blood cells and mediates their adhesion to the vascular endothelium. In parasites lacking knobs, vascular adhesion is less efficient, and infected red cells are not able to immobilize successfully under hemodynamic flow conditions even though PfEMP1 is still present on the exterior of the infected red cell. We examined the interaction between the knob-associated histidine-rich protein (KAHRP), the parasite protein upon which knob formation is dependent, and PfEMP1, and we show evidence of a direct interaction between KAHRP and the cytoplasmic region of PfEMP1 (VARC). We have identified three fragments of KAHRP which bind VARC. Two of these KAHRP fragments (K1A and K2A) interact with VARC with binding affinities (K(D(kin))) of 1 x 10(-7) M and 3.3 x 10(-6) M respectively, values comparable to those reported previously for protein-protein interactions in normal and infected red cells. Further experiments localized the high affinity binding regions of KAHRP to the 63-residue histidine-rich and 70-residue 5' repeats. Deletion of these two regions from the KAHRP fragments abolished their ability to bind to VARC. Identification of the critical domains involved in interaction between KAHRP and PfEMP1 may aid development of new therapies to prevent serious complications of P. falciparum malaria.

A novel neuron-enriched homolog of the erythrocyte membrane cytoskeletal protein 4.1

We report the molecular cloning and characterization of 4.1N, a novel neuronal homolog of the erythrocyte membrane cytoskeletal protein 4.1 (4.1R). The 879 amino acid protein shares 70, 36, and 46% identity with 4.1R in the defined membrane-binding, spectrin-actin-binding, and C-terminal domains, respectively. 4.1N is expressed in almost all central and peripheral neurons of the body and is detected in embryonic neurons at the earliest stage of postmitotic differentiation. Like 4.1R, 4.1N has multiple splice forms as evidenced by PCR and Western analysis. Whereas the predominant 4.1N isoform identified in brain is approximately 135 kDa, a smaller 100 kDa isoform is enriched in peripheral tissues. Immunohistochemical studies using a polyclonal 4.1N antibody revealed several patterns of neuronal staining, with localizations in the neuronal cell body, dendrites, and axons. In certain neuronal locations, including the granule cell layers of the cerebellum and dentate gyrus, a distinct punctate-staining pattern was observed consistent with a synaptic localization. In primary hippocampal cultures, mouse 4.1N is enriched at the discrete sites of synaptic contact, colocalizing with the postsynaptic density protein of 95 kDa (a postsynaptic marker) and glutamate receptor type 1 (an excitatory postsynaptic marker). By analogy with the roles of 4.1R in red blood cells, 4.1N may function to confer stability and plasticity to the neuronal membrane via interactions with multiple binding partners, including the spectrin-actin-based cytoskeleton, integral membrane channels and receptors, and membrane-associated guanylate kinases.

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.

Deciphering the nuclear import pathway for the cytoskeletal red cell protein 4.1R

The erythroid membrane cytoskeletal protein 4.1 is the prototypical member of a genetically and topologically complex family that is generated by combinatorial alternative splicing pathways and is localized at diverse intracellular sites including the nucleus. To explore the molecular determinants for nuclear localization, we transfected COS-7 cells with epitope-tagged versions of natural red cell protein 4.1 (4.1R) isoforms as well as mutagenized and truncated derivatives. Two distant topological sorting signals were required for efficient nuclear import of the 4.1R80 isoform: a basic peptide, KKKRER, encoded by alternative exon 16 and acting as a weak core nuclear localization signal (4.1R NLS), and an acidic peptide, EED, encoded by alternative exon 5. 4.1R80 isoforms lacking either of these two exons showed decreased nuclear import. Fusion of various 4.1R80 constructs to the cytoplasmic reporter protein pyruvate kinase confirmed a requirement for both motifs for full NLS function. 4.1R80 was efficiently imported in the nuclei of digitonin-permeabilized COS-7 cells in the presence of recombinant Rch1 (human importin alpha2), importin beta, and GTPase Ran. Quantitative analysis of protein-protein interactions using a resonant mirror detection technique showed that 4.1R80 bound to Rch1 in vitro with high affinity (KD = 30 nM). The affinity decreased at least 7- and 20-fold, respectively, if the EED motif in exon 5 or if 4.1R NLS in exon 16 was lacking or mutated, confirming that both motifs were required for efficient importin-mediated nuclear import of 4.1R80.