Spectrin tetramer-dimer equilibrium and the stability of erythrocyte membrane skeletons

Role of Spectrin in Endocytosis

Cytoskeletal spectrin is found in (non)erythroid cells. Eukaryotic endocytosis takes place for internalizing cargos from extracellular milieu. The role of spectrin in endocytosis still remains poorly understood. Here, I summarize current knowledge of spectrin function, spectrin-based cytoskeleton and endocytosis of erythrocytes, and highlight how spectrin contributes to endocytosis and working models in different types of cells. From an evolutionary viewpoint, I discuss spectrin and endocytosis in a range of organisms, particularly in plants and yeast where spectrin is absent. Together, the role of spectrin in endocytosis is related to its post-translational modification, movement/rearrangement, elimination (by proteases) and meshwork fencing.

Supramolecular reactions of metallo-architectures: Ag2-double-helicate/Zn4-grid, Pb4-grid/Zn4-grid interconversions, and Ag2-double-helicate fusion

Supramolecular reactions are of importance in many fields. We report herein three examples where complexes of hydrazone-based ligands are involved. A Ag2-double-helicate was converted, by treatment with Zn(OTf)2, into a Zn4-grid (exchange of metal ions and change of the nature of the initial complex). A Pb4-grid was converted, upon reaction with ZnCl2 or ZnBr2, into a Zn4-grid (exchange of metal ions, but conservation of the nature of the initial complex). The reverse conversions were also achieved. The fusion of a Ag2-double-helicate with another Ag2-double-helicate was performed (exchange of ligands, but conservation of the nature of the complexes) and resulted in a mixture of three helicates (two homostranded ones and one heterostranded one).

Mechanical control of the sense of touch by β-spectrin

The ability to sense and respond to mechanical stimuli emanates from sensory neurons and is shared by most, if not all, animals. Exactly how such neurons receive and distribute mechanical signals during touch sensation remains mysterious. Here, we show that sensation of mechanical forces depends on a continuous, pre-stressed spectrin cytoskeleton inside neurons. Mutations in the tetramerization domain of Caenorhabditis elegans β-spectrin (UNC-70), an actin-membrane crosslinker, cause defects in sensory neuron morphology under compressive stress in moving animals. Through atomic force spectroscopy experiments on isolated neurons, in vivo laser axotomy and fluorescence resonance energy transfer imaging to measure force across single cells and molecules, we show that spectrin is held under constitutive tension in living animals, which contributes to elevated pre-stress in touch receptor neurons. Genetic manipulations that decrease such spectrin-dependent tension also selectively impair touch sensation, suggesting that such pre-tension is essential for efficient responses to external mechanical stimuli.

Understanding pathogenic single-nucleotide polymorphisms in multidomain proteins--studies of isolated domains are not enough

Studying the effects of pathogenic mutations is more complex in multidomain proteins when compared with single domains: mutations occurring at domain boundaries may have a large effect on a neighbouring domain that will not be detected in a single-domain system. To demonstrate this, we present a study that utilizes well-characterized model protein domains from human spectrin to investigate the effect of disease-and non-disease-causing single point mutations occurring at the boundaries of human spectrin repeats. Our results show that mutations in the single domains have no clear correlation with stability and disease; however, when studied in a tandem model system, the disease-causing mutations are shown to disrupt stabilizing interactions that exist between domains. This results in a much larger decrease in stability than would otherwise have been predicted, and demonstrates the importance of studying such mutations in the correct protein context.

Red cell membrane: past, present, and future

As a result of natural selection driven by severe forms of malaria, 1 in 6 humans in the world, more than 1 billion people, are affected by red cell abnormalities, making them the most common of the inherited disorders. The non-nucleated red cell is unique among human cell type in that the plasma membrane, its only structural component, accounts for all of its diverse antigenic, transport, and mechanical characteristics. Our current concept of the red cell membrane envisions it as a composite structure in which a membrane envelope composed of cholesterol and phospholipids is secured to an elastic network of skeletal proteins via transmembrane proteins. Structural and functional characterization of the many constituents of the red cell membrane, in conjunction with biophysical and physiologic studies, has led to detailed description of the way in which the remarkable mechanical properties and other important characteristics of the red cells arise, and of the manner in which they fail in disease states. Current studies in this very active and exciting field are continuing to produce new and unexpected revelations on the function of the red cell membrane and thus of the cell in health and disease, and shed new light on membrane function in other diverse cell types.

Disorders of red cell membrane

Studies during the last three decades have enabled the development of detailed molecular insights into the structural basis of altered function in various inherited red cell membrane disorders. This review highlights our current understanding of molecular and mechanistic insights into various inherited red cell membrane disorders involving either altered membrane structural organization (hereditary spherocytosis, hereditary elliptocytosis and hereditary ovalocytosis) or altered membrane transport function (hereditary stomatocytosis). The molecular basis for the vast majority of cases of hereditary spherocytosis, elliptocytosis and ovalocytosis have been fully defined while little progress has been made in defining the molecular basis for hereditary stomatocytosis. Mutations in a number of distinct genes account for hereditary spherocytosis and elliptocytosis, while a single genetic defect accounts for all cases of hereditary ovalocytosis. Based on these molecular insights, a comprehensive understanding of the structural basis for altered membrane function has been developed. Loss of vertical linkage between membrane skeleton and lipid bilayer leads to membrane loss in hereditary spherocytosis, while weakening of lateral linkages between skeletal proteins leads to membrane fragmentation and surface area loss in hereditary elliptocytosis. Importantly, the severity of anaemia in both these disorders is directly related to extent of membrane surface area loss. Splenectomy results in amelioration of anaemia.

Membrane perturbations of erythrocyte ghosts by spectrin release

The cytoskeleton plays an important role in the stability and function of the membrane. Spectrin release from erythrocyte ghosts makes the membrane more fragile. However, the detail of membrane fragility has remained unclear. In the present study, the effects of incubation temperatures and polyamines on the membrane structure of ghosts under hypotonic conditions have been examined. Upon exposure of ghosts to a hypotonic buffer at 0-37 degrees C, reduction of ghost volume, spectrin release and decrease of band 3-cytoskeleton interactions were clearly observed above 30 degrees C. However, such changes were completely inhibited by spermine and spermidine. Interestingly, conformational changes of spectrin induced at 37 degrees C or 49 degrees C were not suppressed by both polyamines. Flow cytometry of fluorescein isothiocyanate-labelled ghosts exposed to 37 degrees C demonstrated the two peaks corresponding to ghosts with normal spectrin content and decreased one. Taken together, these results indicate that the degree of spectrin release from the membrane under hypotonic conditions is not same in all ghosts, and that polyamines inhibit the spectrin release followed by changes in the membrane structure, but not conformational changes of spectrin.

Hereditary elliptocytosis: spectrin and protein 4.1R

Hereditary elliptocytosis (HE) is a common disorder of erythrocyte shape, occurring especially in individuals of African and Mediterranean ancestry, presumably because elliptocytes confer some resistance to malaria. The principle lesion in HE is mechanical weakness or fragility of the erythrocyte membrane skeleton due to defects in alpha-spectrin, beta-spectrin, or protein 4.1. Numerous mutations have been described in the genes encoding these proteins, including point mutations, gene deletions and insertions, and mRNA processing defects. Several mutations have been identified in a number of individuals on the same genetic background, suggesting a "founder effect." The majority of HE patients are asymptomatic, but some may experience hemolytic anemia, splenomegaly, and intermittent jaundice.

Conservation of heterochromatin protein 1 function

Heterochromatin represents a cytologically visible state of heritable gene repression. In the yeast, Schizosaccharomyces pombe, the swi6 gene encodes a heterochromatin protein 1 (HP1)-like chromodomain protein that localizes to heterochromatin domains, including the centromeres, telomeres, and the donor mating-type loci, and is involved in silencing at these loci. We identify here the functional domains of swi6p and demonstrate that the chromodomain from a mammalian HP1-like protein, M31, can functionally replace that of swi6p, showing that chromodomain function is conserved from yeasts to humans. Site-directed mutagenesis, based on a modeled three-dimensional structure of the swi6p chromodomain, shows that the hydrophobic amino acids which lie in the core of the structure are critical for biological function. Gel filtration, gel overlay experiments, and mass spectroscopy show that HP1 proteins can self-associate, and we suggest that it is as oligomers that HP1 proteins are incorporated into heterochromatin complexes that silence gene activity.

The spectrin skeleton of newly-invaginated plasma membrane

As a cell's shape and volume change. its surface area must re-adjust. How is the plasma membrane's spectrin skeleton implicated? For erythrocytes, cells of fixed surface area, spectrin responses to mechanical disturbances have been studied, but for more typical cells with changeable surface areas, they have not. In rapidly shrinking cells, surface membrane at an adherent substratum invaginates, forming transient vacuole-like dilations (VLDs). We exploited this readily inducible surface area perturbation to pose a simple question: is newly invaginated plasma membrane naked or is it supported by a spectrin skeleton? The spectrin skeleton was examined immunocytochemically in L6 cells (rat skeletal muscle) before and after VLD formation, using fixation in cold methanol and 4112, an antibody against beta-fodrin and beta-spectrin. 4112 was visualized by confocal fluorescence microscopy, while paired phase contrast images independently located the VLDs. To generate VLDs, cells were hypotonically swelled then reshrunk in isotonic medium. Swollen L6 cells maintained their plasma membrane (sarcolemma) spectrin skeleton. Within minutes of subsequent shrinkage, VLDs of 1-2 microm diameter invaginated at the substratum surface of myotubes. Both sarcolemma and VLDs were lined by a relatively uniform spectrin skeleton. Z-series suggested that some of the spectrin skeleton-lined sarcolemma became internalized as vacuoles.

Elasticity of the red cell membrane and its relation to hemolytic disorders: an optical tweezers study

We have used optical tweezers to study the elasticity of red cell membranes; force was applied to a bead attached to a permeabilized spherical ghost and the force-extension relation was obtained from the response of a second bead bound at a diametrically opposite position. Interruption of the skeletal network by dissociation of spectrin tetramers or extraction of the actin junctions engendered a fourfold reduction in stiffness at low applied force, but only a twofold change at larger extensions. Proteolytic scission of the ankyrin, which links the membrane skeleton to the integral membrane protein, band 3, induced a similar effect. The modified, unlike the native membranes, showed plastic relaxation under a prolonged stretch. Flaccid giant liposomes showed no measurable elasticity. Our observations indicate that the elastic character is at least as much a consequence of the attachment of spectrin as of a continuous membrane-bound network, and they offer a rationale for formation of elliptocytes in genetic conditions associated with membrane-skeletal perturbations. The theory of Parker and Winlove for elastic deformation of axisymmetric shells (accompanying paper) allows us to determine the function BH(2) for the spherical saponin-permeabilized ghost membranes (where B is the bending modulus and H the shear modulus); taking the literature value of 2 x 10(-19) Nm for B, H then emerges as 2 x 10(-6) Nm(-1). This is an order of magnitude higher than the value reported for intact cells from micropipette aspiration. Reasons for the difference are discussed.

Competition between Na(+) and Li(+) for unsealed and cytoskeleton-depleted human red blood cell membrane: a (23)Na multiple quantum filtered and (7)Li NMR relaxation study

Evidence for competition between Li(+) and Na(+) for binding sites of human unsealed and cytoskeleton-depleted human red blood cell (csdRBC) membranes was obtained from the effect of added Li(+) upon the (23)Na double quantum filtered (DQF) and triple quantum filtered (TQF) NMR signals of Na(+)-containing red blood cell (RBC) membrane suspensions. We found that, at low ionic strength, the observed quenching effect of Li(+) on the (23)Na TQF and DQF signal intensity probed Li(+)/Na(+) competition for isotropic binding sites only. Membrane cytoskeleton depletion significantly decreased the isotropic signal intensity, strongly affecting the binding of Na(+) to isotropic membrane sites, but had no effect on Li(+)/Na(+) competition for those sites. Through the observed (23)Na DQF NMR spectra, which allow probing of both isotropic and anisotropic Na(+) motion, we found anisotropic membrane binding sites for Na(+) when the total ionic strength was higher than 40 mM. This is a consequence of ionic strength effects on the conformation of the cytoskeleton, in particular on the dimer-tetramer equilibrium of spectrin. The determinant involvement of the cytoskeleton in the anisotropy of Na(+) motion at the membrane surface was demonstrated by the isotropy of the DQF spectra of csdRBC membranes even at high ionic strength. Li(+) addition initially quenched the isotropic signal the most, indicating preferential Li(+)/Na(+) competition for the isotropic membrane sites. High ionic strength also increased the intensity of the anisotropic signal, due to its effect on the restructuring of the membrane cytoskeleton. Further Li(+) addition competed with Na(+) for those sites, quenching the anisotropic signal. (7)Li T(1) relaxation data for Li(+)-containing suspensions of unsealed and csdRBC membranes, in the absence and presence of Na(+) at low ionic strength, showed that cytoskeleton depletion does not affect the affinity of Na(+) for the RBC membrane, but increases the affinity of Li(+) by 50%. This clearly indicates that cytoskeleton depletion favors Li(+) relative to Na(+) binding, and thus Li(+)/Na(+) competition for its isotropic sites. Thus, this relaxation technique proves to be very sensitive to alkali metal binding to the membrane, detecting a more pronounced steric hindrance effect of the cytoskeleton network to binding of the larger hydrated Li(+) ion to the membrane phosphate groups.

Lidocaine action and conformational changes in cytoskeletal protein network in human red blood cells

The mechanism of action of lidocaine, which is commonly used clinically as a local anesthetic, was studied in human red blood cells. The influx of [14C]lidocaine through the cell membrane induced reversible transformation of human red blood cells from discocytes to stomatocytes. This change in shape depended on the lidocaine concentration and required both ATP and carbonic anhydrase. The lidocaine-induced shape change occurred as a result of spectrin aggregation, which altered the intracellular environment of the human red blood cells, mediated by carbonic anhydrase and activation of vacuolar type H(+)-ATPase (V-ATPase). Lidocaine controlled the influx of 22Na into the human red blood cells in a concentration-dependent manner. When incubated in media containing 6-chloro-9-[(4-diethylamino)-1-methyl-butyl]amino-2-methoxyacridine (mepacrine), an inhibitor of Na+ channels, human red blood cells changed shape from discocytes to stomatocytes and the intracellular pH decreased. This phenomenon was very similar to the shape change induced by lidocaine. These results suggest that the mode of action of lidocaine is related to a conformational change in the cytoskeletal protein network.

Lipid peroxidation of erythrocytes during anemia of the hamsters infected with Leishmania donovani

Visceral leishmaniasis has been found to be associated with severe anemia and premature lysis of erythrocytes. Peroxidative damage of red cells has been noted in several hemolytic anemias. Present study shows enhanced formation of methemoglobin in hamsters infected with Leishmania donovani. Increased formation of malonyldialdehyde and diene conjugate has been noted in the erythrocytes of the infected animals with the progress of anemia. Results showed decreased activities of protective enzymes like superoxide dismutase, catalase and glutathione reductase against peroxidative attack. An increase in the membrane cholesterol/phospholipid ratio and a decrease in membrane fluidity of erythrocytes were observed under the diseased condition. Densitometric scan after SDS-PAGE of red cell membrane of the infected animals revealed significant degradation of band 3 and band 4.1 proteins. The results suggest that alteration in the membrane may lead to reduced life span of the red cells in experimental visceral leishmaniasis.

Cooperative action between band 3 and glycophorin A in human erythrocytes: immobilization of band 3 induced by antibodies to glycophorin A

The ability of transmembrane receptor proteins to change their association with the cytoskeleton in response to ligand binding seems to be a key mechanism of signal transduction across membranes. To investigate the molecular features of this mechanism we have used the red cell membrane as a model system to study signal transduction through the integral protein, glycophorin A. In these studies the lateral mobility of integral proteins was measured in situ by fluorescence recovery after photobleaching, and membrane rigidity was characterized by micropipette aspiration technique. We found that binding either a monoclonal antibody or its monovalent Fab to the exoplasmic domain of glycophorin A in normal red cells immobilized the receptor and rigidified the membrane. Further, immobilization and rigidification did not occur when antibodies were bound to Miltenberger V cells containing a mutant form of glycophorin A lacking the cytoplasmic domain. These results imply that the site of the immobilization/rigidification lies within the membrane skeletal structure, not in exofacial receptor crosslinking, and requires the extended cytoplasmic domain of normal glycophorin A. In addition, we found that glycophorin A immobilization and membrane skeletal rigidification were accompanied by immobilization of band 3 receptors. This unexpected result indicates a cooperative coupling between liganded glycophorin A, band 3, and the membrane skeleton. We speculate that cooperation of this type may represent a general mechanism for cytoskeletal linkage and transformation initiated by receptors with short cytoplasmic sequences, such as integrins.

The self-association of ovine erythrocyte spectrin

1. Spectrin extracted from ovine erythrocyte membranes at low temperature shows association behaviour similar to that reported for human and bovine erythrocytes. 2. The spectrin tetramer is the predominant oligomer, the dimer is well represented, and smaller amounts of hexamer and higher oligomers are present. 3. The estimates of parameters describing the self-association of purified ovine spectrin studied by sedimentation equilibrium analysis were found to be indistinguishable from those obtained for human spectrin under the same conditions, within the precision of the measurements. 4. The data suggest that the cooperative isodesmic model may be general for spectrin, and not a peculiarity of the human.

Sodium interaction with ordered structures in mammalian red blood cells detected by Na-23 double quantum NMR

Na-23 double and triple quantum filtered NMR spectra of intact dog and human red blood cells were measured with the pulse sequence 90 degrees-tau/2-180 degrees-tau/2-theta degrees-t1-theta degrees-t2(Acq). For theta = 90 degrees the triple quantum filtered spectra exhibited the typical multiple quantum filtered lineshape, characteristic of isotropic media, while the double quantum filtered ones presented a superposition of two signals, whose proportion depended on the creation time tau. This effect is due to the formation of both second and third rank tensors. The formation of the second rank tensor, T21 results from non-zero residual quadrupolar interaction and is related to the anisotropic motion of sodium ions. Measurements of the double quantum filtered spectra with theta = 54.7 degrees enabled the detection of the contribution of T21 exclusively. No residual quadrupolar interaction was detected for sodium in the cytoplasm, while unsealed ghosts displayed the double quantum filtered spectral pattern, similar to that of intact cells. The anisotropy of motion of the sodium at the plasma membrane of mammalian erythrocytes depended on the integrity of the cytoskeleton network. Theoretical analysis of the double quantum filtered spectra gave a value of residual quadrupolar splitting of approximately 20 Hz for intact unsealed ghosts. The data presented prove that double quantum filtering is a sensitive technique for detection of motional anisotropies in biological systems.

Requirement of cytoplasmic components for lidocaine-induced shape change in human erythrocytes

To clarify the mechanism underlying local anesthetic-induced changes in the shape of human erythrocytes from discocytes to stomatocytes, we treated erythrocytes with lidocaine, a cationic drug. Analysis of the erythrocyte membrane and cytoplasm by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed that the intensities of the stained bands of 62 kDa, 28 kDa and 22 kDa depended on the extent of the shape change induced by lidocaine. The change in the intensity of the 28 kDa band was particularly marked. We identified the cytoplasmic substances, i.e., the 28 kDa and 22 kDa peptides, as carbonic anhydrase (CA) and glutathione peroxidase (GSH Px)1, respectively, by immunoblotting. The 62 kDa peptide was identified as Hb by column chromatography and SDS-PAGE analysis. To identify the protein responsible for the lidocaine-induced shape change, we incorporated CA and GSH Px into ATP-MgCl2-resealed ghosts. The shape of the resealed ghosts changed upon addition of lidocaine, but only in the presence of CA. These results suggest that ATP and CA are required for the shape changes induced by lidocaine.

Conformation and elasticity of the isolated red blood cell membrane skeleton

We studied the structure and elasticity of membrane skeletons from human red blood cells (RBCs) during and after extraction of RBC ghosts with nonionic detergent. Optical tweezers were used to suspend individual cells inside a flow chamber, away from all surfaces; this procedure allowed complete exchange of medium while the low-contrast protein network of the skeleton was observed by high resolution, video-enhanced differential interference-contrast (DIC) microscopy. Immediately following extraction in a 5 mM salt buffer, skeletons assumed expanded, nearly spherical shapes that were uncorrelated with the shapes of their parent RBCs. Judging by the extent of thermal undulations and by their deformability in small flow fields, the bending rigidity of skeletons was markedly lower than that of either RBCs or ghosts. No further changes were apparent in skeletons maintained in this buffer for up to 40 min at low temperatures (T less than 10 degrees C), but skeletons shrank when the ionic strength of the buffer was increased. When the salt concentration was raised to 1.5 M, shrinkage remained reversible for approximately 1 min but thereafter became irreversible. When maintained in 1.5 M salt buffer for longer periods, skeletons continued to shrink, lost flexibility, and assumed irregular shapes: this rigidification was irreversible. At this stage, skeletons closely resembled those isolated in standard bulk preparations. We propose that the transformation to the rigid, irreversibly shrunken state is a consequence of spectrin dimer-dimer reconnections and that these structural rearrangements are thermally activated. We also measured the salt-dependent size of fresh and bulk extracted skeletons. Our measurements suggest that, in situ, the spectrin tethers are flexible, with a persistence length of approximately 10 nm at 150 mM salt.

An 88-kDa protein of Plasmodium falciparum is related to the band-3-binding domain of human erythrocyte ankyrin

Three tryptic-peptide sequences of an 88-kDa pair of phosphoproteins of the malaria parasite Plasmodium falciparum were determined. They exhibit a striking similarity to corresponding sequences of the 89-kDa domain of human erythrocyte ankyrin. [35S]Methionine labeling of the two proteins demonstrated their parasitic origin. Using an appropriate oligonucleotide probe, Southern-blot analysis of genomic malaria DNA and Northern-blot analysis of malaria RNA suggest the existence of ankyrin-related sequences in the parasite genome and the presence of an ankyrin-related transcript of about 3.2 kb. Our studies provide further evidence of malaria-specific analogues of host-cell proteins, implying an unusual kind of parasite/host interaction.

Membrane-altering effects of velnacrine and N-methylacridinium: relevance to tacrine and Alzheimer's disease

The interaction of pharmacological agents potentially useful in Alzheimer's disease, 9-amino-1,2,3,4-tetrahydroacridine (THA or tacrine) and its major metabolite velnacrine (or HP-029), along with related compounds with cytoskeletal proteins in human erythrocyte membrane was investigated using electron paramagnetic resonance spin labeling techniques. The results suggest that: (1) the position of the positive charge of tacrine may be important in the mechanism of its interaction with the membrane cytoskeleton; (2) like tacrine, velnacrine also strengthens cytoskeletal protein-protein interactions in erythrocyte membranes, but appears to be only about half as potent as tacrine. These results are discussed with relevance to therapeutic use of these agents in Alzheimer's disease.

Altered erythrocyte protein kinase C activity and membrane protein phosphorylation in chronic myelogenous leukemia

The membrane protein kinase C (PKC) content was found to be higher in erythrocytes form patients suffering from chronic myelogenous leukemia (CML) compared to normal erythrocytes. PKC activity was also higher in the cytosol and after translocation to the membrane, as assessed by histone phosphorylation. The increased PKC activity in CML erythrocytes was associated with abnormal phosphorylation of protein 4.1. Since phosphorylation-dephosphorylation mechanisms are likely candidates for controlling membrane protein associations, the altered PKC activity may be one of the factors responsible for altered thermal sensitivity and mechanical stability of CML erythrocytes.

Heat-induced alterations in monkey erythrocyte membrane phospholipid organization and skeletal protein structure and interactions

Rhesus monkey erythrocytes were subjected to heating at 50 degrees C for 5-15 min, and the heat-induced effects on the membrane structure were ascertained by analysing the membrane phospholipid organization and membrane skeleton dynamics and interactions in the heated cells. Membrane skeleton dynamics and interactions were determined by measuring the Tris-induced dissociation of the Triton-insoluble membrane skeleton (Triton shells), the spectrin-actin extractability at low ionic strength, spectrin self-association and spectrin binding to normal monkey erythrocyte membrane inside-out vesicles (IOVs). The Tris-induced Triton shell dissociation and spectrin-actin extractability were markedly decreased by the erythrocyte heating. Also, the binding of the heated erythrocyte membrane spectrin-actin with the IOVs was much smaller than that observed with the normal erythrocyte spectrin-actin. Further, the spectrin structure was extensively modified in the heated cells, as compared to the normal erythrocytes. Transbilayer phospholipid organization was ascertained by employing bee venom and pancreatic phospholipases A2, fluorescamine, and Merocyanine 540 as the external membrane probes. The amounts of aminophospholipids hydrolysed by phospholipases A2 or labeled by fluorescamine in intact erythrocytes considerably increased after subjecting them to heating at 50 degrees C for 15 min. Also, the fluorescent dye Merocyanine 540 readily stained the 15-min-heated cells but not the fresh erythrocytes. Unlike these findings, the extent of aminophospholipid hydrolysis in 5-min-heated cells by phospholipases A2 depended on the incubation time. While no change in the membrane phospholipid organization could be detected in 10 min, prolonged incubations led to the increased aminophospholipid hydrolysis. Similarly, fluorescamine failed to detect any change in the transbilayer phospholipid distribution soon after the 5 min heating, but it labeled greater amounts of aminophospholipids in the 5-min-heated cells, as compared to normal cells, after incubating them for 4 h at 37 degrees C. These results have been discussed to analyse the role of membrane skeleton in maintaining the erythrocyte membrane phospholipid asymmetry. It has been concluded that both the ATP-dependent aminophospholipid pump and membrane bilayer-skeleton interactions are required to maintain the transbilayer phospholipid asymmetry in native erythrocyte membrane.

Membrane skeleton-bilayer interaction is not the major determinant of membrane phospholipid asymmetry in human erythrocytes

Transbilayer phospholipid distribution, membrane skeleton dissociation/association, and spectrin structure have been analysed in human erythrocytes after subjecting them to heating at 50 degrees C for 15 min. The membrane skeleton dissociation/association was determined by measuring the Tris-induced dissociation of Triton-insoluble membrane skeletons (Triton shells), the spectrin-actin extractability under low ionic conditions, and the binding of spectrin-actin with normal erythrocyte membrane inside-out vesicles (IOVs). The spectrin structure was ascertained by measuring the spectrin dimer-to-tetramer ratio as well as the spectrin tryptophan fluorescence. Both the Tris-induced Triton shell dissociation and the spectrin-actin extractability under low ionic conditions were considerably reduced by the heat treatment. Also, the binding of heated erythrocyte spectrin-actin to IOVs was significantly smaller than that observed with the normal cell spectrin-actin. Further, the quantity of spectrin dimers was appreciably increased in heat-treated erythrocytes as compared to the normal cells. This change in the spectrin dimer-to-tetramer ratio was accompanied by marked changes in the spectrin tryptophan fluorescence. In spite of these heat-induced alterations in structure and bilayer interactions of the membrane skeleton, the inside-outside glycerophospholipid distribution remained virtually unaffected in the heat-treated cells, as judged by employing bee venom and pancreatic phospholipase A2, fluorescamine and Merocyanine 540 as the external membrane probes. These results strongly indicate that membrane bilayer-skeleton interaction is not the major factor in determining the transbilayer phospholipid asymmetry in human erythrocyte membrane.

Spectrin modifications in a heterozygous case of both hereditary elliptocytosis and beta-thalassemia

The clinical and hematological parameters of a patient described here, who inherited the genes of both hereditary elliptocytosis (HE) and beta-thalassemia, seem to reflect a mutual enhancement of the two diseases. The coexistence of the two pathologies is probably also responsible for the observed changes in spectrin: the appearance of an extra spectrin band between tetramers and dimers on denaturing gel electrophoresis and the metabolic-dependent reduction in spectrin amount. It is assumed that the instability of the skeletal network that results from the HE pathology caused increased exposure of the spectrin molecule to oxidative damage that usually occurs in thalassemic red cells. The products of such oxidation may have led to abnormal spectrin associations which finally resulted in the above changes.

Effect of L-carnitine and acetyl-L-carnitine on the human erythrocyte membrane stability and deformability

In this study we examined the effect of carnitine and acetylcarnitine on the human erythrocyte membrane stability and membrane deformability. Since erythrocyte membranes are impermeable to these compounds, we resealed erythrocyte ghosts in the presence of different concentrations of carnitine or acetylcarnitine. Resealed ghosts can be adequately studied in their cellular deformability and membrane stability properties by means of ektacytometry. Both carnitine and acetylcarnitine alter the membrane stability but not membrane deformability of the red cell membrane. Resealed ghosts containing 20, 50, 150, and 300 microM carnitine had 1.1, 1.6, 0.9, and 0.7 times the normal stability. While resealed ghosts containing 20, 50, 150, and 300 microM acetylcarnitine had 1.1, 1.5, 1.3, and 1.2 times the normal stability. Such changes were found to be reversible. We also conducted SDS PAGE of cytoskeletal membrane proteins from membrane fragments and residual membranes produced during membrane stability analysis, and unsheared resealed membranes in those samples where we observed an increase or a decrease of membrane stability. No changes in the cytoskeletal membrane proteins were noticed, even when the samples, prior SDS PAGE analysis, were treated with or without dithiothreitol. In addition, fluorescence steady state anisotropy of DPH in the erythrocyte membrane treated with carnitine or acetylcarnitine shows no modification of the lipid order parameter. Our results would suggest that both carnitine and its acetyl-ester, at physiological concentrations, may increase membrane stability in mature erythrocytes, most likely via a specific interaction with one or more cytoskeletal proteins, and that this effect would manifest when the erythrocytes are subjected to high shear stress.

Spin-labelling studies of the interaction of 9-amino-1,2,3,4-tetrahydroacridine (THA), a proposed drug for the treatment of Alzheimer's disease, with erythrocyte membranes

ESR spin labels specific for skeletal proteins or cell-surface sialic acid have been used to monitor the interaction of 9-amino-1,2,3,4-tetrahydroacridine (THA) and its structural analogs with human erythrocyte membranes. The results suggest that THA significantly increases skeletal protein-protein interactions and may secondarily alter the physical state of the opposite side of the membrane. The fully aromatic analog of THA, 9-aminoacridine, showed even more pronounced effects on skeletal proteins than did THA. These results are discussed in relation to possible interaction sites of THA in erythrocyte ghosts and to potential mechanisms by which THA reportedly increases mental function of victims of Alzheimer's disease.

Ultracentrifugal analysis of the junction complexes of the red cell membrane cytoskeletal network: application to hereditary spherocytosis and metabolically depleted cells

A method has been developed for the assessment of the number of spectrin dimer units associated with each actin protofilament junction, in the membrane cytoskeletal network (i.e. the degree of branching) of the red cell. Ghosts are first exposed to elevated temperature at low ionic strength to dissociate some 65% of the spectrin tetramers (that link the network junctions) into dimers, without causing their release from the actin filaments. Non-ionic detergent is then added to solubilize the membrane itself with its intrinsic proteins, so as to liberate the cytoskeletal material, and the mixture is immediately examined in the analytical ultracentrifuge. The predominant components observed are isolated junctions (20 S), free spectrin dimers and the residual undissociated cytoskeletal material, with very minor components, probably corresponding to multiple junctions, linked by spectrin tetramers. The junction boundary is homogeneous within the accuracy of measurement and is taken to correspond to a complex containing six spectrin dimers, known to predominate in situ. About 17% of the total network is liberated in this form and 12% as free spectrin dimers. In hereditary spherocytosis both the size of the junction complex (as reflected by its sedimentation coefficient) and the proportion of the complex and of free spectrin liberated are indistinguishable from normal values. We conclude that the reported deficit of spectrin in hereditary spherocytosis is not reflected by a lower degree of branching of the network, and, if the membrane area is not correspondingly reduced, this must mean that the junctions are more widely spaced and the spectrin tetramers therefore more extended. In metabolically depleted cells, in which the cytoskeletal proteins are known to be extensively dephosphorylated, there is no change in the sedimentation pattern and thus no detectable loss of spectrin from the junctions or weakening in the cohesion of the cytoskeletal network.

Abnormalities in the erythrocyte membrane in acute lymphoid leukaemia

Erythrocytes from patients suffering from acute lymphoid leukaemia (ALL) show decreased proportions of spectrin tetrameters and altered spatial distribution of band 4.1 and ankyrins. These abnormalities of the cytoskeleton are probably responsible for altered membrane fluidity and transbilayer distribution of phosphatidylethanolamine in ALL. ALL is associated with severe anaemia and usually, but not always, with overproduction of lymphocytes. To our knowledge, this is the first report of abnormalities in the erythrocyte membrane in ALL which may, in part, be responsible for the observed anaemia.

Elasticity of the human red cell membrane skeleton. Effects of temperature and denaturants

The molecular basis for the elasticity of the human erythrocyte membrane was explored. Skeletons were released from ghosts in Triton X-100 and their dimensions followed by dark-field microscopy and packed volume. The rest size of skeletons was assumed to reflect the balance point between expansion (deformation) driven by electrostatic repulsions among the excess of fixed negative charges on the proteins and contraction (recovery) driven by their elasticity. The size of skeletons decreased with increasing temperature. This finding suggests that entropy drives elasticity. The requisite entropy change could be associated with either the configurational freedom of flexible protein chains or with the solvation of side chains exposed during protein dissociation (hydrophobic effects). To distinguish between these two alternatives, we tested the impact of two weak denaturants, 10% ethanol and 20 nM lithium 3,5-diiodosalicylate. Both agents reversibly promoted the expansion of skeletons, presumably by reducing their elasticity. Since the conformation of random coils and globular proteins should not be significantly altered by these mild treatments, this finding strongly suggests a role for weak interdomain and/or interprotein associations. We conclude that the elasticity of the red cell membrane skeleton may not derive from the configurational entropy of flexible coils. Rather, the elastic energy may arise from reversible dissociations of weak but specific intramolecular and/or intermolecular contacts, presumably within deformed spectrin filaments.

The spectrin network as a barrier to lateral diffusion in erythrocytes. A percolation analysis

The spectrin network on the cytoplasmic surface of an erythrocyte can be modeled as a triangular lattice of spectrin tetramers (Tsuji, A., and S. Ohnishi, 1986. Biochemistry. 25:6133-6139). The tetramers act as barriers to protein diffusion, while dissociated dimer pairs, single dimers, and missing tetramers do not. Diffusion in the presence of these barriers is shown to be equivalent to bond percolation on the honeycomb lattice. Monte Carlo calculations for this system then yield the relative diffusion constant of a mobile integral protein as a function of the fraction of spectrin tetramers. At high concentrations of spectrin tetramer, long-range diffusion is blocked, but short-range diffusion is still possible. Monte Carlo calculations yield the average distance over which short-range diffusion can occur, as a function of the fraction of spectrin tetramers. Applications to erythrocyte development and hereditary hemolytic anemia are discussed.

Multiple protein 4.1 isoforms produced by alternative splicing in human erythroid cells

Protein 4.1 is a multifunctional structural protein located in the erythrocyte membrane skeleton and in many nonerythroid cells. Molecular characterization of cloned protein 4.1 sequences from human reticulocytes has revealed the existence of multiple transcripts of the protein 4.1 gene that may encode a family of closely related protein isoforms. Several independently isolated cDNAs were sequenced and demonstrated to encode four different protein 4.1 species having identical primary sequences, except for the presence or absence of discrete peptides in the 8-kDa spectrin/actin binding domain (21 amino acids) and near the carboxyl terminus (43 and 34 amino acids). The same four protein 4.1 isoforms were detected when reticulocyte protein 4.1 mRNA sequences were reverse transcribed into cDNA and enzymatically amplified in vitro by using protein 4.1-specific oligonucleotide primers and the polymerase chain reaction. The finding of multiple protein 4.1 isoforms raises the possibility that the many binding functions ascribed to protein 4.1 may reside in distinct structural isoforms. Since only a single protein 4.1 gene appears to be expressed in erythrocytes, it is likely that these isoforms are produced by alternative mRNA splicing from a common protein 4.1 pre-mRNA. Multiple RNA splicing pathways are thus operative in the protein 4.1 gene even within a single cell lineage, human erythroid cells.

A flow EPR study of deformation and orientation characteristics of erythrocyte ghosts: a possible effect of an altered state of cytoskeletal network

Using the flow EPR technique, we investigated the resealed ghost deformability in shear flow and the effects of the altered state of cytoskeletal network induced by hypotonic incubation of ghosts. Isotonically resealed ghosts in the presence of Mg-ATP, in which alteration of cytoskeletal network is not effected, have smooth biconcave discoid shapes, and show a flow orientation and deformation behavior similar to that of erythrocytes, except that higher viscosities are required to induce the same degrees of deformation and orientation as in erythrocytes. The flow behavior of resealed ghosts is Mg-ATP dependent, and the shape of the ghosts resealed without Mg-ATP is echinocytic. In contrast, the ghosts resealed by hypotonic incubation show a markedly reduced deformability even with Mg-ATP present. Nonreducing, nondenaturing polyacrylamide gel electrophoresis (PAGE) of the low ionic strength extracts from hypotonically resealed ghosts reveals a shift of the spectrin tetramer-dimer equilibrium toward the dimers. In the maleimide spin-labeled ghosts, the ratios of the weakly immobilized to the strongly immobilized EPR intensities are larger in hypotonically resealed ghosts than in the isotonically resealed ghosts, indicating an enhanced mobility in the spectrin structure in the former. Photomicrographs of hypotonically resealed ghosts show slightly stomatocytic transformations. These data suggest that the shape and the deformability loss in hypotonically resealed ghosts are related to an alteration of the spectrin tetramer-dimer equilibrium in the membrane. Thus, the shift of the equilibrium is likely to affect the regulation of the membrane deformability both in normal and pathological cells such as hereditary elliptocytes.

Abnormal erythrocyte membrane cytoskeleton structure in chronic myelogenous leukaemia

Chronic myelogenous leukaemia (CML) is a haematologic malignancy characterised by excessive growth of myeloid cells and their progenitors. Our studies show that there are several abnormalities in CML red blood cells. The proportion of spectrin dimers compared to tetramers extracted from membranes at 4 degrees C, under low ionic strength conditions, increased in CML erythrocytes. These also displayed abnormal thermal sensitivity (between 45 and 46 instead of 49 degrees C). Decreased spectrin tetramer formation observed in several hereditary anaemias has been associated with decreased red cell deformability leading to splenic sequestration. This could also be one of the causes of the severe anaemia observed in CML. Crosslinking with the bifunctional reagent, dimethyl adipimidate (8.6 A) showed significant organizational modification of not only spectrin, but other cytoskeletal components such as ankyrin, bands 4.2 and 5. Enhanced concanavalin A agglutinability of CML erythrocytes also suggests altered topographic distribution of a functionally important membrane protein, band 3.

A cytoskeletal spring in cochlear outer hair cells

Normal hearing in mammals depends on an active mechanical filter, within the cochlea, which separates different sound frequencies before neural encoding. Experiments on the intact cochlea indicate that the critical cellular components underlying the process are probably the outer hair cells which are strategically placed to influence movement of the basilar membrane. This idea is attractive because isolated cells can generate axial forces at acoustic frequencies when electrically stimulated. The mechanical properties of cells are largely determined by structures closely associated with the plasma membrane. We show here, using light and electron microscopy, that beneath this membrane lies a lattice of crosslinked circumferential filaments which are pitched at a mean angle of 15 degrees to the transverse axis of the cell. The lattice is sufficient to retain the shape of the cell following demembranation and mechanical deformation. The structure of the lattice allows it to be described as a coiled helical spring but with longitudinal stiffness primarily determined by the crosslinks. Direct measurements of longitudinal stiffness reported here indicate that the lattice contributes 5-10% of the stiffness. We propose that the 'circumferential lattice' ensures that outer hair cells can act as directed force generators within the organ of Corti, a prerequisite in current descriptions of cochlear micromechanics.

Regulation of band 3 mobilities in erythrocyte ghost membranes by protein association and cytoskeletal meshwork

Rotational diffusion of erythrocyte anion channel protein band 3 was measured in ghost membranes by observing time-resolved phosphorescence anisotropy decays of eosinyl-5-maleimide covalently attached to the protein. Experiments were carried out under conditions similar to those employed by Tsuji and Ohnishi (1986) for translational diffusion measurement of band 3 [(1986) Biochemistry 25, 6133-6139] to allow direct comparison of rotational and translational diffusion of band 3. Detailed analysis of diffusive properties of band 3 in ghost membranes was made on the basis of these rotational and translational diffusion data. Rotational diffusion measurements indicated that there are at least three populations of band 3 molecules with high, low, and no rotational mobilities in the time scale of 10(-4)-10(-2) s. These populations are in equilibrium, and the fractional ratios are strongly temperature dependent. At 26 degrees C, 44% of band 3 molecules are mobile (16% have an average rotational correlation time of 0.19 ms, and 28% have an average correlation time of 2.4 ms), and 56% are immobile. These results correlate well with translational diffusion data which indicated 40% mobile and 60% immobile fractions of band 3. The rotational diffusion data together with the translational diffusion data by Tsuji and Ohnishi (1986) and Golan and Veatch [(1980) Proc. Natl. Acad. Sci. U.S.A. 77, 2537-2541] suggest that immobilization of band 3 is largely caused by binding of band 3 oligomers to ankyrin, which abolishes both rotational and translational diffusion of band 3.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of erythrocyte membrane material properties by Ca2+ and calmodulin. Implications for their role in regulation of skeletal protein interactions

Skeletal proteins of the red blood cell apparently play an important role in regulating membrane material properties of deformability and stability. However, the role of various intracellular constituents in regulating membrane properties has not been clearly defined. To determine whether Ca2+ and calmodulin might play a role in this regulation, we measured the membrane stability and deformability of resealed ghosts prepared in the presence of varying concentrations of Ca2+ and calmodulin (CaM). For membranes resealed in the presence of Ca2+ and physiologic concentrations of CaM (2-8 microM), membrane stability decreased with increasing Ca2+ concentrations (greater than 1.0 microM). Moreover, Ca2+ and CaM-induced alterations in membrane stability were completely reversible. In the absence of CaM, an equivalent decrease in membrane stability was seen only when Ca2+ concentration was two orders of magnitude higher (greater than 100 microM). Calmodulin did not alter membrane stability in the absence of Ca2+. Compared with these changes in membrane stability, membrane deformability decreased only at Ca2+ concentrations greater than 100 microM, and calmodulin had no effect on Ca2+-induced decrease in membrane deformability. Examination of the effects of Ca2+ and CaM on various membrane interactions have enabled us to suggest that spectrin-protein 4.1-actin interaction may be one of the targets for the effect of Ca2+ and CaM. These results imply that Ca2+ and calmodulin can regulate membrane stability through modulation of skeletal protein interactions, and that these protein interactions are of a dynamic nature on intact membranes.

Cell surface energy and membrane associated actin in lymphocytes

We have shown previously that membrane associated actin correlates with the migratory abilities of lymphocytes during recirculation, and that cell surface energy correlates with the adhesiveness of lymphocytes to other cells. In this study, measurements of actin content and cell surface energy have been made for various lymphocyte subpopulations to examine the possibility that recirculation ability may be related to nonspecific adhesiveness. We have found that: both cell surface energy and actin content combine to provide a consistent explanation for the relative rates of recirculation of various lymphocyte subpopulations, and cell surface energies and actin contents vary independently in these lymphocyte subpopulations. Comparison of the actin contents and cell surface energies of metastatic and nonmetastatic lymphoma cell lines indicated that the differences in metastatic potential were more likely attributable to specific receptor-ligand interactions than to nonspecific adhesiveness. Cell surface energy and actin content are consistent with the greater adhesiveness of B cells than T cells to nylon wool, providing a physical basis for this common cell separation technique.

Direct involvement of spectrin thiols in maintaining erythrocyte membrane thermal stability and spectrin dimer self-association

Human erythrocytes vesiculate upon exposure to temperatures of 49 degrees C and above. Pretreatment of the cells with the thiol-alkylating agent N-ethylmaleimide (NEM) lowers the temperature needed to produce the same effect. Concomitant with the cells' heat susceptibility, skeletal mechanical instability and an increase in spectrin dissociation have been reported (Smith and Palek (1983) Blood 62, 1190). In the present study, similar results were achieved by preincubation of the cells with diamide, which could be reversed by reduction with dithiothreitol. Another oxidative agent, sodium tetrathionate, could only induce the temperature susceptibility, with little effect on spectrin dissociation. Incubation of spectrin solutions with NEM or diamide caused decreased association of spectrin dimers and increased dissociation of spectrin tetramers. Estimation of membrane and spectrin thiols in the treated cells showed that NEM was effective while blocking less than 20% of the thiols. Diamide and tetrathionate blocked more than 50% of the thiols, but were less effective than NEM. It is suggested that some very defined population of thiols is essential for spectrin self-association and for membrane thermal stability. They are more available to NEM than to diamide and less so to tetrathionate. Other thiols participate in maintaining the membrane thermal stability only.

Spectrin and related molecules

This review begins with a complete discussion of the erythrocyte spectrin membrane skeleton. Particular attention is given to our current knowledge of the structure of the RBC spectrin molecule, its synthesis, assembly, and turnover, and its interactions with spectrin-binding proteins (ankyrin, protein 4.1, and actin). We then give a historical account of the discovery of nonerythroid spectrin. Since the chicken intestinal form of spectrin (TW260/240) and the brain form of spectrin (fodrin) are the best characterized of the nonerythroid spectrins, we compare these molecules to RBC spectrin. Studies establishing the existence of two brain spectrin isoforms are discussed, including a description of the location of these spectrin isoforms at the light- and electron-microscope level of resolution; a comparison of their structure and interactions with spectrin-binding proteins (ankyrin, actin, synapsin I, amelin, and calmodulin); a description of their expression during brain development; and hypotheses concerning their potential roles in axonal transport and synaptic transmission.

Crosslinking of isolated cytoskeletal proteins with hemoglobin: a possible damage inflicted to the red cell membrane

Crosslinking of isolated red cell membrane cytoskeletal proteins and hemoglobin mediated by H2O2 was studied. The products of spectrin and hemoglobin interaction were demonstrated electrophoretically to be high-molecular-weight polypeptides crosslinked by nondisulfide covalent bonds. The molecular weight of the protein bands correlated with various combinations of spectrin and hemoglobin chains and the relative amount of the different products was dependent on the molar ratio of the interacting proteins. Free hemin caused spectrin crosslinking as well, but globin in the absence of hemin was inactive. Since the H2O2-mediated reaction resulted in reduction of the spectrin tryptophan fluorescence, the latter was used to monitor the reaction progress under various conditions. Both oxyhemoglobin and methemoglobin were found to be most efficient, whereas cyanmethemoglobin and hemichrome were relatively inactive. Analysis of the data implied that tryptophan oxidation as well as spectrin conformational changes follow an iron-induced crosslinking of the interacting proteins. Actin, the second major protein in the red cell cytoskeleton, behaved similarly to spectrin. The intrinsic fluorescence intensity of both G- and F-actin was decreased upon addition of H2O2 to the mixture of hemoglobin and each of the actin forms. SDS-polyacrylamide gel electrophoresis revealed that G-actin crosslinked one or two hemoglobin chains. F-actin-hemoglobin interaction induced by H2O2 produced very high aggregates that could not penetrate the gel. It is suggested that crosslinking of cytoskeletal proteins in red cells containing membrane-associated hemoglobin provides a rationale for the loss of membrane flexibility.

Visualization of the hexagonal lattice in the erythrocyte membrane skeleton

The isolated membrane skeleton of human erythrocytes was studied by high resolution negative staining electron microscopy. When the skeletal meshwork is spread onto a thin carbon film, clear images of a primarily hexagonal lattice of junctional F-actin complexes crosslinked by spectrin filaments are obtained. The regularly ordered network extends over the entire membrane skeleton. Some of the junctional complexes are arranged in the form of pentagons and septagons, approximately 3 and 8%, respectively. At least five forms of spectrin crosslinks are detected in the spread skeleton including a single spectrin tetramer linking two junctional complexes, three-armed Y-shaped spectrin molecules linking three junctional complexes, three-armed spectrin molecules connecting two junctional complexes with two arms bound to one complex and the third arm bound to the adjacent complex, double spectrin filaments linking two junctional complexes, and four-armed spectrin molecules linking two junctional complexes. Of these, the crosslinks of single spectrin tetramers and three-armed molecules are the most abundant and represent 84 and 11% of the total crosslinks, respectively. These observations are compatible with the presence of spectrin tetramers and oligomers in the erythrocyte membrane skeleton. Globular structures (9-12 nm in diameter) are attached to the majority of the spectrin tetramers or higher order oligomer-like molecules, approximately 80 nm from the distal ends of the spectrin tetramers. These globular structures are ankyrinor ankyrin/band 3-containing complexes, since they are absent when ankyrin and residual band 3 are extracted from the skeleton under hypertonic conditions.

Modulation of erythrocyte membrane mechanical stability by 2,3-diphosphoglycerate in the neonatal poikilocytosis/elliptocytosis syndrome

To explain the transient anemia and poikilocytosis seen during infancy in hereditary elliptocytosis (HE), we resealed erythrocyte (RBC) ghosts from affected children or their elliptocytic parents with 2,3-diphosphoglycerate (DPG) (0-8 mM), a compound that dissociates membrane skeletons, then measured ghost mechanical stability in the ektacytometer. Without added 2,3-DPG, ghost mechanical stability was subnormal in infantile poikilocytosis (IP) and HE but was even more abnormal in hereditary pyropoikilocytosis (HPP). Addition of 2,3-DPG (2.55 mM) to IP or HE ghosts, decreased their stability to that of HPP ghosts (without 2,3-DPG). Nonphysiological 2,3-DPG levels (6-8 mM) were required to elicit a similar effect in normal ghosts. The data suggest that free 2,3-DPG, present in neonatal RBC as a consequence of diminished binding to HbF, may render HE susceptible to in vivo fragmentation. The developmental switch from fetal to adult hemoglobin, by diminishing available free 2,3-DPG, may explain the abatement of poikilocytosis and hemolytic anemia that accompanies maturation.

The molecular basis of erythrocyte shape

Recent discoveries about the molecular organization and physical properties of the mammalian erythrocyte membrane and its associated structural proteins can now be used to explain, and may eventually be used to predict, the shape of the erythrocyte. Such explanations are possible because the relatively few structural proteins of the erythrocyte are regularly distributed over the entire cytoplasmic surface of the cell membrane and because the well-understood topological associations of these proteins seem to be stable in comparison with the time required for the cell to change shape. These simplifications make the erythrocyte the first nonmuscle cell for which it will be possible to extend our knowledge of molecular interactions to the next hierarchical level of organization that deals with shape and shape transformations.

Phenylhydrazine-induced changes in erythrocyte membrane surface lipid packing

Phenylhydrazine-induced oxidative damage in red cells results in increased binding of merocyanine 540, a fluorescence probe sensitive to changes in lipid packing. Fluorescence polarization studies with diphenylhexatriene did not reveal major changes in order parameters both in intact red cells and lysates treated with phenylhydrazine. These fluorescence studies indicate that major changes are observed in membrane lipids. Analytical studies of membrane phospholipids revealed a significant decrease in phosphatidylethanolamine. The results of the fluorescence and lipid studies, taken in association with our previously reported findings on spectrin and other cytoskeletal protein degradation in red cells exposed to phenylhydrazine, suggests that degradation of cytoskeleton membrane proteins is also responsible for changes in the lipid bilayer surface of the red cell membrane.