Macromolecular recognition through electrostatic repulsion.

In the process of genetic translation, each aminoacyl-tRNA synthetase specifically aminoacylates its cognate tRNAs and rejects the 19 other species of tRNAs. A decrease in the specificity of this reaction can result in misincorporations of amino acids into proteins and be deleterious to the cell. In the case of tyrosyl-tRNA synthetase from Bacillus stearothermophilus, the change of residue Glu152 into Ala results in erroneous interactions with non-cognate tRNAs. To analyse how Glu152 contributes to the discrimination between tRNAs by tyrosyl-tRNA synthetase, 11 changes to this residue were created by mutagenesis. The misaminoacylations of tRNA(Phe) and tRNA(Val) with tyrosine in vitro (on a scale going from 1 to 30) and the toxicity of tyrosyl-tRNA synthetase in vivo (on a scale from 1 to 10(7)) increased in a correlated way when the nature of the side chain in position 152 varied from negatively charged to uncharged then to positively charged. The aminoacylation of tRNA(Tyr) was unaffected by the mutations. The results show that the role of Glu152 in the discrimination between tRNAs is purely negative, that it acts by electrostatic repulsion of non-cognate tRNAs and that this mechanism has been conserved throughout evolution.     

Evidence for long-range electrostatic repulsion between HeLa cells

Agglutination curves obtained on addition of low molecular weight poly-l-lysines (mol. wt 4 000–23 000) to HeLa cells, show a deviation from linearity at low polymer concentration. This probably indicates the existence of a ζ-potential which has to be lowered before agglutination can take place. Experiments with dilysine support the assumption that cell surface charge is lowered on addition of low concentrations of short chain poly-l-lysines.Long poly-l-lysine molecules (mol. wts 70 000; 100 000) yield linear agglutination curves already at the lowest polymer concentrations. This might indicate that these polymers are able to bridge the original repulsion gap between HeLa cells.After removal of peripheral sialic acid by neuraminidase, linear agglutination curves are obtained with all poly-l-lysines irrespective of their chain lengths. This is interpreted as evidence for involvement of sialic acid residues in the charge organization responsible for electrostatic repulsion.The magnitude of the presumed repulsion effect is shown to vary with the cell density at the time the HeLa cells were harvested from the culture. The largest repulsion effect is obtained with cells from density inhibited cultures which also have the lowest tendency for mutual adhesion. With fast growing cells from low density cultures linear agglutination curves are obtained with short chain poly-l-lysines. This is interpreted as evidence for a strong diminishment or absence of long-range electrostatic repulsion between such cells.     

A correlation between the effects of calcium on intercellular adhesion and electrostatic repulsion between cells.

Calcium ions enchance the mutual adhesiveness of HeLa cells harvested from suspension cultures in which growth is density inhibited. No significant effect of calcium is observed on the mutual adhesiveness of HeLa cells from fast growing suspension cultures. Agglutinative titration of the cells using poly-L-lysine, mol. wt 15000, shows that calcium ions reduce the strength of the repulsive forces on density inhibited HeLa cells. The agglutination curve of the nonrepulsive fast growing HeLa cells is not significantly modified by the addition of calcium. The results support the conclusion that the effect of calcium on the mutual adhesiveness of density inhibited cells is due to a weakening of the repulsive forces on these cells.     

Red blood cells

Red blood cells are derived from haemopoietic stem cells in bone marrow. Following a series of maturation steps, directed largely by the hormone erythropoietin (Epo), red cells enucleate and enter the circulatory system. In circulation these small, flexible biconcave cells containing haemoglobin transport O₂ from the lungs to the periphery, and CO₂ back from the periphery to the lungs. The most common disorders associated with red blood cells are anaemias. While there are numerous causes of anaemia, the reduced capacity for gaseous exchange is the underlying theme. Over the past 15 years, recombinant Epo has been used extremely successfully in the treatment of several forms of anaemia. The single gene disorders collectively known as haemoglobinopathies represent one of the best opportunities for gene therapy.     

Red blood cells as an antigen-delivery system.

The use of adjuvants is usually required to induce strong immunological responses to protein antigens. However, in many cases these adjuvants cannot be extensively applied in human and veterinary vaccinations because of associated inflammatory reactions or granuloma formation. We show here that protein antigens (bovine serum albumin, hog liver uricase, and yeast hexokinase), coupled to autologous red blood cells by way of a biotin-avidin-biotin bridge, elicit an immunological response in mice similar to or higher than that obtained by the use of Freund's adjuvant. Quantities as low as 0.5 micrograms/mouse are high enough to generate these immunological responses. Furthermore, splenocytes of mice immunized by red blood cell-coupled antigens can be used to generate hybridomas secreting monoclonal antibodies. Thus, the delivery of antigens by autologous red blood cells is an effective way to avoid the use of adjuvants for producing anti-peptide antibodies and possibly to generate peptide vaccines.     

Role of electrostatic repulsion in the acidic molten globule of cytochrome c.

The molten globule has been assumed to be a major intermediate state of protein folding. To extend our understanding of protein folding it is important to elucidate the thermodynamic mechanism of conformational stability of the molten globule. To clarify the role of electrostatic charge repulsion in the stability of the acidic molten globule state, we prepared a series of acetylated horse ferricytochrome c species with various degrees of charge repulsion. On the basis of circular dichroism measurement, we show that the stability of the acidic molten globule is determined by a balance of electrostatic repulsions between positive residues, which favor the extended conformation, and the opposing forces, which stabilize the molten globule. These results provide a clear example of charge repulsions producing unfolding of the compact protein structure, and suggest that the reversibly denatured conformation of ferricytochrome c under physiological conditions (i.e. neutral pH, ambient temperature and no denaturant) is the molten globule.     

Erythrocyte aggregation: bridging by macromolecules and electrostatic repulsion by sialic acid.

Relation between aggregating force (of fibrinogen and IgG) and disaggregating force (due to electrostatic repulsion among erythrocytes) in erythrocyte aggregation was investigated with a rheoscope combining a video camera, an image analyzer and a computer. (i) Erythrocyte aggregation was augmented with the increase of molecular weight of bridging macromolecules as far as examined for fibrinogen and the degradation products and IgG and the related macromolecules, and the augmentation seemed to be dependent on the molecular length of macromolecules. In accelerating the erythrocyte aggregation, fibrinogen was more effective than IgG, and some interaction between fibrinogen and IgG in their coexistence was suggested. (ii) The decrease of sialic acid content on the erythrocyte surface accelerated IgG-induced erythrocyte aggregation much greater than fibrinogen-induced one. (iii) Counteraction between aggregating force and disaggregating force in leading to erythrocyte aggregation was discussed relating to molecular length of bridging macromolecule and electrostatic repulsive force by sialic acid.     

Red blood cells protect endothelial cells against H2O2-mediated but not hyperoxia-induced damage

We studied the effect of intact red blood cells on the exogenous H2O2-mediated damage as well as on the hyperoxia-induced injury of cultured endothelial cells. Red blood cells protected endothelial cells against H2O2-mediated injury efficiently, but had no effect on the hyperoxia-induced damage. Failure of red blood cells to protect endothelial cells against hyperoxia-induced injury was not due to hemolysis. Furthermore, hyperoxia-exposed red blood cells were still capable of protecting endothelial cells against H2O2-mediated damage.     

Red blood cells under mechanical stress

The effect of mechanical stress on erythrocytes suspended in various media was studied. The ability of the cells to increase their glucose consumption was found to be the major criterion allowing to divide the media into two groups. In plasma, serum or in Ringer's solution supplemented with albumin and glucose the energy consumption by mechanically stressed erythrocytes increased 20 to 50%; no morphological changes of the cells were observed either in suspension or on Giemsa smears. The cells behaved in the same way in Mg2(+)-free medium. The other group included protein-free medium (Ringer's solution supplemented with glucose) and Ca2(+)-free Ringer's solution supplemented with albumin and glucose; under these conditions erythrocytes were unable to raise their energy consumption in response to mechanical stress, and after some period structural impairment of the membrane could be observed on Giemsa smears. No differences in metabolism-associated nucleotide concentrations (ATP, ADP, NAD, NADP) were observed between the samples. Resealed red cell ghosts with high concentrations of intracellular components were prepared as a model of cells with damaged membrane. In these ghosts (with low ATP concentration) mechanical stress produced increased proportions of echinocytes, even in the "native" suspension. These results have confirmed the vital role of the energy-consuming contractile apparatus in the erythrocyte membrane, and supplied a clue to the role of Ca2+ in its activation and to the influence of extracellular proteins on the maintenance of in red cell shape.     

Formation of giant liposomes promoted by divalent cations: critical role of electrostatic repulsion.

Spontaneous formation of giant unilamellar liposomes in a gentle hydration process, as well as the adhesion energy between liposomal membranes, has been found to be dependent on the concentration of divalent alkali cations, Ca2+ or Mg2+, in the medium. With electrically neutral phosphatidylcholine (PC), Ca2+ or Mg2+ at 1-30 mM greatly promoted liposome formation compared to low yields in nonelectrolyte or potassium chloride solutions. When negatively charged phosphatidylglycerol (PG) was mixed at 10%, the yield was high in nonelectrolytes but liposomes did not form at 3-10 mM CaCl2. In the adhesion test with micropipette manipulation, liposomal membranes adhered to each other only in a certain range of CaCl2 concentrations, which agreed with the range where liposome did not form. The adhesion range shifted to higher Ca2+ concentrations as the amount of PG was increased. These results indicate that the divalent cations bind to and add positive charges to the lipids, and that membranes are separated and stabilized in the form of unilamellar liposomes when net charges on the membranes produce large enough electrostatic repulsion. Under the assumption that the maximum of adhesion energy within an adhesive range corresponds to exact charge neutralization by added Ca2+, association constants of PC and PG for Ca2+ were estimated at 7.3 M(-1) and 86 M(-1), respectively, in good agreement with literature values.     

Red blood cells imaging and antigen-antibody interaction measurement

In the present study the atomic force microscope (AFM) was used to image the surface morphology of red blood cells (RBC) for the first time. The AFM yielded very reproducible images without appreciable modifications of the sample surfaces. In addition to this topographical imaging, we have developed an experimental approach to measure the binding strength between antibody (anti-A), and the RBC antigen A, when reversible bonds between specific molecules such as antigen and antibody mediate the adhesion. The experimental results suggest that the procedure established here may be used for specific antibody detection. This study has also enhanced our understanding under physiological conditions of molecular interaction in particular antigen-antibody.     

Red blood cell targeting to smooth muscle cells

Monoclonal antibody discriminating between endothelial and smooth muscle cells is suggested to be used as a vector for directed transport of drugs to injured (denuded) areas of blood vessel wall. An in vitro model system was used in the studies: vascular smooth muscle or endothelial cells grown on plastic surface were treated with specific mouse monoclonal antibody recognizing an antigen localized on the surface of smooth muscle rather than endothelial cells; then erythrocytes coated with secondary (rabbit antimouse) antibodies were added. The results were analyzed spectrophotometrically or with scanning electron microscopy. Under the experimental conditions, erythrocytes, possible 'containers' for carrying the drugs, were found to bind only to smooth muscle cells. The data show that antibody provides absolute discrimination between endothelial and smooth muscle cells and, thus, may be used as a vector for drug targeting.     

Red blood cells augment leukocyte rolling in a virtual blood vessel

Leukocyte rolling and arrest on the vascular endothelium is a central event in normal and pathological immune responses. However, rigorous estimation of the fluid and surface forces involved in leukocyte-endothelial interactions has been difficult due to the particulate, non-Newtonian nature of blood. Here we present a Lattice-Boltzmann approach to quantify forces exerted on rolling leukocytes by red blood cells in a "virtual blood vessel." We report that the normal force imparted by erythrocytes is sufficient to increase leukocyte binding and that increases in tangential force and torque can promote rolling of previously adherent leukocytes. By simulating changes in hematocrit we show that a close "envelopment" of the leukocyte by the red blood cells is necessary to produce significant changes in the forces. This novel approach can be applied to a large number of biological and industrial problems involving the complex flow of particulate suspensions.     

Interaction between quaternary ammonium ions in the pore of potassium channels. Evidence against an electrostatic repulsion mechanism

We have examined the interaction between internal and external ions in the pore of potassium channels. We found that external tetraethylammonium was able to antagonize block of Shaker channels by internal TEA when the external and internal solutions contained K(+) ions. This antagonism was absent in solutions with Rb(+) as the only permeant ion. An externally applied trivalent TEA analogue, gallamine, was less effective than the monovalent TEA in inhibiting block by internal TEA. In addition, block by external TEA was little affected by changes in the concentration of internal K(+) ions, but was increased by the presence of internal Na(+) ions in the pore. These results demonstrate that external and internal TEA ions, likely located at opposite ends of the pore selectivity filter, do not experience a mutual electrostatic repulsion. We found that these results can be simulated by a simple 4-barrier-3-site permeation model in which ions compete for available binding sites without long-range electrostatic interactions.     

Kinetics of specific and nonspecific adhesion of red blood cells on glass.

Fixed spherical human red blood cells suspended in 17% sucrose were allowed to adhere on either clean glass surfaces or glass surfaces preincubated with antibodies specific to a certain blood group antigen. The adhesion experiments were performed in an impinging jet apparatus, in which the cells are subjected to stagnation point flow. The objective of this study was to compare the efficiencies of nonspecific and specific (antigen-antibody mediated) adhesion of red blood cells on glass surfaces. The efficiency was defined as the ratio of the experimental adhesion rate to that calculated based on numerical solutions of the mass transfer equation, taking into account hydrodynamic interactions as well as colloidal forces. The efficiency for nonspecific adhesion was nearly unity at flow rates lower than 85 microliter/s (corresponding to a wall shear rate, Gw, of 30 s-1 at a radial distance of 110 microns from the stagnation point). The values of efficiency dropped at higher flow rates, due to an increase in the tangential force. The critical deposition concentration is found to occur at 120-150 mM NaCl, which is consistent with the theoretically predicted values. At low salt concentrations, the experimental values are higher than the theoretical ones. Similar discrepancies have been found in many colloidal systems. Introducing steric repulsion by adsorbing a layer of albumin molecules on the glass completely prevents nonspecific adhesion at flow rates below 60 microliter/s (Gw congruent to 15 s-1). The efficiency of specific adhesion depends both on the concentration of antibody molecules on the surface and the flow rate. Normal red cells adhere more readily through antigen-antibody bonds than fixed cells. Fixed spherical cells have a higher adhesion efficiency than fixed biconcave ones.