Studies of red-cell membrane function in heterozygous beta thalassaemia and other hypochromic anaemias

The erythrocyte membrane properties of beta thalassaemia heterozygotes and their consequences for Plasmodium falciparum invasion

Malaria parasites such as Plasmodium falciparum have exerted formidable selective pressures on the human genome. Of the human genetic variants associated with malaria protection, beta thalassaemia (a haemoglobinopathy) was the earliest to be associated with malaria prevalence. However, the malaria protective properties of beta thalassaemic erythrocytes remain unclear. Here we studied the mechanics and surface protein expression of beta thalassaemia heterozygous erythrocytes, measured their susceptibility to P. falciparum invasion, and calculated the energy required for merozoites to invade them. We found invasion-relevant differences in beta thalassaemic cells versus matched controls, specifically: elevated membrane tension, reduced bending modulus, and higher levels of expression of the major invasion receptor basigin. However, these differences acted in opposition to each other with respect to their likely impact on invasion, and overall we did not observe beta thalassaemic cells to have lower P. falciparum invasion efficiency for any of the strains tested.

Epistatic interactions between genetic disorders of hemoglobin can explain why the sickle-cell gene is uncommon in the Mediterranean

Several human genetic disorders of hemoglobin have risen in frequency because of the protection they offer against death from malaria, sickle-cell anemia being a canonical example. Here we address the issue of why this highly protective mutant, present at high frequencies in subSaharan Africa, is uncommon in Mediterranean populations that instead harbor a diverse range of thalassemic hemoglobin disorders. We demonstrate that these contrasting profiles of malaria-protective alleles can arise and be stably maintained by two well-documented phenomena: an alleviation of the clinical severity of alpha- and beta-thalassemia in compound thalassemic genotypes and a cancellation of malaria protection when alpha-thalassemia and the sickle-cell trait are coinherited. The complex distribution of globin mutants across Africa and the Mediterranean can therefore be explained by their specific intracellular interactions.

The membrane characteristics of Plasmodium falciparum-infected and -uninfected heterozygous alpha(0)thalassaemic erythrocytes

The alpha thalassaemias are the commonest known human genetic disorders. Although they have almost certainly risen to their current frequencies through natural selection by malaria, the precise mechanism of malaria protection remains unknown. We have investigated the characteristics of red blood cells (RBCs) from individuals heterozygous for alpha(0)thalassaemia (-/alphaalpha) from a range of perspectives. On the basis of the hypothesis that defects in membrane transport could be relevant to the mechanism of malaria protection, we investigated sodium and potassium transport and the activity of the Plamodium falciparum-induced choline channel but found no significant differences in -/alphaalpha RBCs. Using flow cytometry, we found that thalassaemic P. falciparum-infected RBCs (IRBCs) bound 44% more antibody from immune plasma than control IRBCs. This excess binding was abrogated by predigestion of IRBCs with trypsin but was not directed at the variant surface molecule PfEMP1. Furthermore, we found no evidence for altered cytoadhesion of alpha-thalassaemic IRBCs to the endothelial receptors intercellular adhesion molecule-1 (ICAM-1), CD36 or thrombospondin. We hypothesize that altered red-cell membrane band 3 protein may be a target for enhanced antibody binding to alpha-thalassaemic IRBCs and could be involved in the mechanism of malaria protection.

Pathophysiology of thalassaemia

Most of the major clinical manifestations of the beta-thalassaemias can be related to the deleterious effects of imbalanced globin chain synthesis on erythroid maturation and red cell survival. The destruction of red cell progenitors and their progeny results from an extremely complex series of mechanisms all related to the presence of excess alpha-globin chain production. These include mechanical damage, interference with cell division and oxidative destruction of both organelles and components of the red cell membrane. The unequal distribution of gamma-globin chains between different precursors, and the intense selection of those with relatively higher levels of gamma chain production, lead to an extremely heterogeneous cell population in the peripheral blood. Iron overload, due to increased gastrointestinal absorption and blood transfusion is the major cause of tissue damage, morbidity and death.

The role of pump number and intracellular sodium and potassium in determining Na,K pump activity in human erythrocytes

The factors that determine the activity of the Na,K pump in vivo were investigated by measuring Na,K pump activity under in vivo conditions in human red cells and relating it to the intracellular content of sodium ([Na]i) and potassium ([K]i) and the number of pump units per cell (pump number). Na,K pump activity was measured as ouabain-sensitive K+ influx, pump number was determined from the maximal binding of 3H-ouabain to intact cells, and [Na]i and [K]i were measured by atomic absorption spectrophotometry in washed, packed cells. In the 81 samples studied, pump activity per cell was significantly correlated with pump number (r = .64, P less than 0.001), but was negatively correlated with [Na]i (r = -.28, P less than 0.02) and was not correlated with [K]i. An inverse relationship was found between pump number and [Na]i. When pump activity was expressed as activity per pump unit, rather than per cell, a significant relationship was seen between pump activity and [Na]i (r = .50, P less than 0.001), and a negative correlation existed between the activity per pump unit and [K]i (r = -.29, P less than 0.01). The effect of intracellular Na+ at physiologic levels on pump activity was not strong, with the activity per pump unit increasing only 25% with a doubling of [Na]i. These results indicate that pump number is the major determinant of pump activity in human red cells in vivo, while [Na]i and [K]i are of secondary importance.(ABSTRACT TRUNCATED AT 250 WORDS)

Furosemide-sensitive Na+ and K+ transport and human erythrocyte volume

The relationship between cation transport and cell volume in human erythrocytes was investigated by measuring ouabain-sensitive K+ influx, ouabain-resistant, furosemide-sensitive K+ influx, and ouabain + furosemide-resistant K+ influx, and maximal ouabain binding in microcytic, normocytic and macrocytic red cells. A significant correlation was found between the mean corpuscular volume and furosemide-sensitive K+ influx normalized either to cell number (r = 0.636, P less than 0.001) or to cell volume (r = 0.488, P less than 0.001). No relationship was seen between mean corpuscular volume and ouabain-sensitive K+ influx, and the number of ouabain-binding sites per cell was only weakly correlated with mean corpuscular volume (r = 0.337, P less than 0.05). A slight, negative relationship existed between mean corpuscular volume and ouabain + furosemide-resistant K+ influx expressed per volume of cells (r = -0.359, P less than 0.01), and an apparent relationship between furosemide-sensitive K+ influx and mean corpuscular hemoglobin concentration (r = 0.446, P less than 0.01) disappeared when microcytic samples were excluded from analysis. Furosemide-sensitive transport, including Na+ influx and K+ and Na+ efflux, was completely absent in microcytic cells from one patient with alpha-thalassemia minor. In addition, these cells exhibited a furosemide-resistant, Cl(-)-dependent K+ influx. Exposure of normal erythrocytes to hypotonic conditions (196 mosM) increased furosemide-sensitive K+ influx by a mean of 45% (P less than 0.05), while exposure to hypertonic conditions (386 mosM) had no significant effect. The results indicate that furosemide-sensitive transport and cell volume are interrelated in human erythrocytes. However, the inability to fully recreate this relationship with in vitro manipulation of cell volume suggest that this relationship is established prior to red cell maturation.

The erythrocyte membranes in beta-thalassemia. Lower sialic acid levels in glycophorin

The sialic acids content of glycophorin of thalassemic erythrocyte membranes is about 25% lower than in glycophorin of normal erythrocyte membranes. Glycophorin extracted from old thalassemic erythrocytes separated by density centrifugation, has about half the sialic acids content found in glycophorin extracted from young thalassemic erythrocytes. Possible sialidase activty was sought in the plasma and erythrocyte membranes of thalassemic erythrocytes. No increased sialidase activity was detected in the plasma of the patients as compared to that of normal donors. Thus, other sites for sialidase activity, or other possibilities have to be explored to account for the increased sialic acid hydrolysis of glycophorin of the thalassemic erythrocytes.

Increased potassium permeability induced in vitro by menadione in normal human red cells

Normal red blood cells, preincubated for 75 min with 1.15 mM menadione sodium bisulfite lose potassium and water on subsequent incubation at 37 degrees C for 24 h without menadione. The potassium loss is increased by addition of calcium and prevented by addition of glucose. Since normal red cells treated with menadione behave like untreated hypochromic cells, both from beta-thalassaemia or iron deficiency anaemia in respect to membrane permeability to potassium, it may be supposed that menadione induces in normal red cells an abnormality similar to that naturally occurring in hypochromic cells.

The diagnosis of beta-thalassemia trait: a review

Diagnostic and screening methods for beta-thalassemia trait are reviewed in the order of their development, including identification by homozygous offspring, erythrocyte morphology, osmotic fragility, hemoglobin composition, globin synthetic rates, and red blood cell indices.