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• W1982799568 abstract "In their recent papers Gedde et al. (Gedde et al., 1997Gedde M.M. Davis D.K. Huestis W.H. Cytoplasmic pH and human erythrocyte shape.Biophys. J. 1997; 72: 1234-1246Abstract Full Text PDF PubMed Scopus (34) Google Scholar, Gedde and Huestis, 1997Gedde M.M. Huestis W.H. Membrane potential and human erythrocyte shape.Biophys. J. 1997; 72: 1220-1233Abstract Full Text PDF PubMed Scopus (38) Google Scholar) reexamined various hypotheses on the mechanism of shape changes of human erythrocytes. Their careful examination suggests that the shape is determined by the cytoplasmic pH and not by the transmembrane potential (TMP) (Glaser et al., 1987Glaser R. Fujii T. Müller P. Tamura E. Herrmann A. Erythrocyte shape dynamics: influence of electrolyte conditions and membrane potential.Biomed. Biochim. Acta. 1987; 46: 327-333Google Scholar). Nevertheless, the authors were unable to identify a molecular mechanism explaining their findings. They discussed five different mechanisms, one of which (cell water content) is only able to modulate shape changes. Three other mechanisms, an ionic gel model for spectrin (Stokke et al., 1986Stokke B.T. Mikkelsen A. Elgsaeter A. Spectrin, human erythrocyte shapes, and mechanochemical properties.Biophys. J. 1986; 49: 319-327Abstract Full Text PDF PubMed Scopus (36) Google Scholar), pH-dependent intramembrane band 3 aggregation (Elgsaeter et al., 1976Elgsaeter A. Shotton D.M. Branton D. Intramembrane particle aggregation in erythrocyte ghosts. II. The influence of spectrin aggregation.Biochim. Biophys. Acta. 1976; 426: 101-122Crossref PubMed Scopus (197) Google Scholar), and electrostatic effects of pH-titratable lipids (Gedde et al., 1995Gedde M.M. Yang E. Huestis W.H. Shape response of human erythrocytes at altered cell pH.Blood. 1995; 86: 1595-1599PubMed Google Scholar), were shown to be incapable of explaining the shape behavior of intact cells. Gedde et al. favored a mechanism in which a pH-dependent insertion of an unspecified “cell protein into the red cell inner leaflet” induces shape changes based on the bilayer couple model. The bilayer couple model is a generally accepted mechanism for erythrocyte shape changes and was proved by changing the ratio of lipids or lipophilic compounds of the two monolayers. However, under physiological conditions, transbilayer phospholipid movement is too slow to explain shape changes that occur in a matter of seconds (see Brumen et al., 1993Brumen M. Heinrich R. Herrmann A. Müller P. Mathematical modelling of lipid transbilayer movement in the human erythrocyte plasma membrane.Eur. Biophys. J. 1993; 22: 213-223Crossref PubMed Scopus (26) Google Scholar). Gedde et al. probably overlooked a new approach which proposes that the anion-exchange protein band 3 plays a major role in erythrocyte shape. Band 3 is a good candidate protein because many of its known characteristics match the required properties. Wong, 1994Wong P. Mechanism of control of erythrocyte shape: a possible relationship to band 3.J. Theor. Biol. 1994; 171: 197-205Crossref PubMed Scopus (17) Google Scholar favored a mechanism in which anions transported by band 3 govern the folding and unfolding of spectrin. In contrast, Gimsa and Ried, 1995Gimsa J. Ried C. Do band 3 protein conformational changes mediate shape changes of human erythrocytes?.Mol. Membr. Biol. 1995; 12: 247-254Crossref PubMed Scopus (62) Google Scholar proposed a band 3 conformation-controlled bilayer couple (CCBC) model. It assumes that a conformational change of band 3 changes the ratio of inwardly and outwardly facing anion-binding sites of the intramembraneous domain of band 3. In particular, the synchronized recruitment of a certain conformation should significantly alter the membrane monolayer area ratio. This model further assumes that conformers favoring the external orientation occupy an increased volume in the external monolayer (Gimsa and Ried, 1995Gimsa J. Ried C. Do band 3 protein conformational changes mediate shape changes of human erythrocytes?.Mol. Membr. Biol. 1995; 12: 247-254Crossref PubMed Scopus (62) Google Scholar). This notion is backed by well-known properties of band 3 (Bamberg and Passow, 1992Bamberg E. Passow H. The band 3 proteins: anion transporters, binding proteins and senescence antigens. Progress in Cell Research. 2. Elsevier, Amsterdam, New York, Oxford1992Google Scholar). Physiologically, the protein ensures the fast exchange of internal and external anions such as chloride and bicarbonate, at cycles higher than 104 s−1. It operates by a ping-pong mechanism that exposes the transport site to the opposite sides of the membrane. This mechanism excludes a transition of the unloaded transporter and suggests that the average orientation of the binding site depends on ligand affinities, as well as on the amount of ligands available at the two membrane sides. Consequently, a lack of external anions will prevent the binding site from returning to the inside, thereby inducing a conformational change of the protein. Then a shape change according to the bilayer couple model is only retarded by the viscoelastic cell properties. There are more than 1 million copies of band 3 per human erythrocyte, and they occupy ∼10% of the total membrane area. It can be estimated that the sum of all cross-sectional areas of the protein’s access channels corresponds to ∼1% of the total membrane area. Thus, this area is greater than that needed for a profound change in the erythrocyte shape. Because anions, the ligands of band 3, are usually distributed according to the TMP, the CCBC model suggests an influence on the equilibrium distribution of conformers and a correlation of cell shape and TMP. An additional mechanism may be the direct recruitment of internal conformers by a positive TMP (Wyatt and Cherry, 1992Wyatt K. Cherry R.J. Effect of membrane potential on band 3 conformation in the human erythrocyte membrane detected by triplet state quenching experiments.Biochemistry. 1992; 31: 4650-4656Crossref PubMed Scopus (16) Google Scholar); this is probably experienced in the access channel (Jennings et al., 1990Jennings M.L. Schulz R.K. Allen M. Effects of membrane potential on electrically silent transport. Potential-independent translocation and asymmetric potential-dependent substrate binding to the red blood cell anion exchange protein.J. Gen. Physiol. 1990; 96: 991-1012Crossref PubMed Scopus (25) Google Scholar). However, under physiological conditions, around neutral external pH, band 3 conformers are asymmetrically distributed, with ∼90% of the transport sites facing the inside. Thus recruitment of the binding site under physiological conditions (e.g., by the lack of anions in the vicinity of a negatively charged glass surface) will involve almost all band 3 proteins and may induce a conformational change leading to a more drastic shape effect than recruitment of the internal conformation. It is known that alkaline pH changes the distribution asymmetry, so that ∼80% of the binding sites face the outside (Bjerrum, 1992Bjerrum P.J. The human erythrocyte anion transport protein, band 3.J. Gen. Physiol. 1992; 100: 301-339Crossref PubMed Scopus (15) Google Scholar). In light of the CCBC model, particularly at alkaline pH, this pH dependence corresponds to the pH shape dependence of figure 1 in Gedde and Huestis, 1997Gedde M.M. Huestis W.H. Membrane potential and human erythrocyte shape.Biophys. J. 1997; 72: 1220-1233Abstract Full Text PDF PubMed Scopus (38) Google Scholar. Another hint of a significant role of band 3 not discussed by the authors is that many specific band 3 inhibitors can induce shape effects at extremely low doses. Most of them by far are echinocytogenic. When their effect on the band 3 conformation was investigated, all of the echinocytogenic inhibitors were found to recruit the external conformation (4,4′-diisothiocyanatostilben-2,2′-disulfonic acid (DIDS), 4-acetamido-4′-isothiocyanatostilben-2,2′-disulfonate, furosemide, N-ethylmaleimide, salicylic acid, etc.). DIDS instantaneously induces echinocytes, overriding any possible correspondence with the TMP or pH. We found that even 4 nM DIDS was sufficient to form echinocytes (see also Blank et al., 1994Blank M.E. Hoefner D.M. Diedrich D.F. Morphology and volume alterations of human erythrocytes caused by the anion transporter inhibitors, DIDS and p-azidobenzylphlorizin.Biochim. Biophys. Acta. 1994; 1192: 223-233Crossref PubMed Scopus (19) Google Scholar). At our cell concentration, this DIDS concentration corresponded to an estimated concentration ratio of DIDS (in solution) to band 3 (overall content of cell suspension) of 1:1. It can be deduced that the echinocytogenic effect is specific for the inhibitory reaction of DIDS, which in parallel recruits external conformers (Bamberg and Passow, 1992Bamberg E. Passow H. The band 3 proteins: anion transporters, binding proteins and senescence antigens. Progress in Cell Research. 2. Elsevier, Amsterdam, New York, Oxford1992Google Scholar). Nevertheless, it must be expected that DIDS or other inhibitors induce specific deformations of the protein. Consequently, the induced changes of the protein’s volume in the two monolayers may vary for different inhibitors and from those induced by pH, leading to different stages of echinocytosis. Of course, the CCBC model does not exclude other mechanisms, like changing protein interactions in the cytoskeleton or the redistribution of membrane compounds, from influencing the erythrocyte shape. However, the redistribution of the major species of lipids after a pH jump, e.g., from 7.4 to 5.8, is rather slow. Moreover, the transbilayer steady-state distribution of these lipids most probably does not correlate with the equilibrium erythrocyte shape according to the bilayer couple model (Libera et al., 1997Libera J. Pomorski T. Müller P. Herrmann A. Influence of pH on phospholipid redistribution in human erythrocyte membrane.Blood. 1997; 90: 1684-1693PubMed Google Scholar). Furthermore, one must ask whether equilibrium investigations may disclose a general mechanism that can explain quasiequilibrium cell shapes, as well as very quick changes. Surprisingly, the CCBC model may play a role in the mechanisms behind both observations: very rapid shape changes due to synchronized recruitment of the binding sites and a subsequent conformational change, and the erythrocyte’s equilibrium shape due to the average conformer distribution, such as that at alkaline pH." @default.
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• W1982799568 title "A Possible Molecular Mechanism Governing Human Erythrocyte Shape" @default.
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