Mechanical Equilibrium of Biological Membranes

The nature of mechanical and electrical forces on biological membranes in relation to mechanical equilibrium is examined. The presence of a double layer of electric charge is shown to give rise to an effective pressure drop across a curved membrane of finite thickness. For certain geometric shapes of a membrane, the magnitude of the pressure drop due to electrostatic forces may set a limit on the hydrostatic pressure drop that the membrane can support without buckling. The results are applied to the equilibrium shape of the red blood cell.     

右美托咪定对红细胞变形性影响的体外研究

目的:研究右美托咪定(DEX)对离体红细胞变形性的影响,探讨其作用机制。方法:采健康志愿者人血5 m L制成2%红细胞悬液,分成空白对照组(C组),低浓度DEX组(DL组)、中浓度DEX组(DM组)、高浓度DEX组(DH组),单纯育亨宾(Y组)以及育亨宾+DEX组(YD组),每组9例。将各组样本放入37℃恒温震荡培养箱中60 min后取出,测定红细胞变形性指数(EI)、红细胞内一氧化氮(NO)含量以及内皮型一氧化氮合酶(e NOS)含量。结果:与C组比较,DL组、DM组、DH组及YD组中EI、红细胞内NO、e NOS的含量增高(P0.05);Y组含量差异无统计学意义(P0.05)。与YD组比较,DM组EI、红细胞内NO及e NOS含量增高(P0.05)。结论:DEX可以提高离体红细胞的变形能力,其可能机制是通过激活红细胞膜上肾上腺素受体,激活红细胞内e NOS使细胞内NO增多有关。     

Disorders in Magnesium Balance Decrease Erythrocyte Deformability in Athletes

Blood rheology was compared with the magnesium status in athletes (N = 45), who were divided into tertiles with a decreased ≤ 0.86 mmol/l), moderate (0.86–0.97 mmol/l), or high ≥0.97 mmol/l) Mg content in the blood serum. One-way ANOVA revealed differences in the erythrocyte suspension viscosity, total globulins, triglycerides, high density lipoprotein cholesterol (HDL Ch), plasma viscosity, and acid resistance of erythrocytes. According to discriminant analysis, key differences were associated with the erythrocyte deformability (ED), which was decreased in the lower and upper tertiles. The decrease in ED directly correlated with Mg content in the lower tertile and with HDL Ch in the upper tertile. The athletes with the increased Mg had increased plasma viscosity, which was due to elevated globulins and triglycerides. Thus, Mg imbalance in the athletes was associated with a decrease in ED.__________Translated from Fiziologiya Cheloveka, Vol. 31, No. 3, 2005, pp. 133–136.Original Russian Text Copyright © 2005 by Melnikov, Vikulov.     

cAMP-Signalling Regulates Gametocyte-Infected Erythrocyte Deformability Required for Malaria Parasite Transmission

Blocking Plasmodium falciparum transmission to mosquitoes has been designated a strategic objective in the global agenda of malaria elimination. Transmission is ensured by gametocyte-infected erythrocytes (GIE) that sequester in the bone marrow and at maturation are released into peripheral blood from where they are taken up during a mosquito blood meal. Release into the blood circulation is accompanied by an increase in GIE deformability that allows them to pass through the spleen. Here, we used a microsphere matrix to mimic splenic filtration and investigated the role of cAMP-signalling in regulating GIE deformability. We demonstrated that mature GIE deformability is dependent on reduced cAMP-signalling and on increased phosphodiesterase expression in stage V gametocytes, and that parasite cAMP-dependent kinase activity contributes to the stiffness of immature gametocytes. Importantly, pharmacological agents that raise cAMP levels in transmissible stage V gametocytes render them less deformable and hence less likely to circulate through the spleen. Therefore, phosphodiesterase inhibitors that raise cAMP levels in P. falciparum infected erythrocytes, such as sildenafil, represent new candidate drugs to block transmission of malaria parasites.     

Vitamin E and the Peroxidizability of Erythrocyte Membranes in Neonates

We showed the increased susceptibility of neonatal biomembranes to oxidation by a kinetic analysis using an azo compound as a free-radical initiator and red blood cell (RBC) ghosts as a model membrane. When the RBC ghosts were oxidized, oxygen consumption was suppressed during the induction period in which membrane tocopherol was consumed at a constant rate, while increased oxygen uptake was observed after the tocopherol was exhausted. The total tocopherol content was similar in cord, maternal, and adult RBC ghosts, and there were no differences in the induction period (t/_inh) among the three types of ghosts. While the oxygen uptake rate during the induction period (R_inh) was similar in cord and adult ghosts, the rate in the subsequent phase (R_p) was considerably faster in the cord ghosts. Fatty acid analysis in the membrane lipids showed that the active bisallylic hydrogen (active H) content was greater in cord ghosts than in adult ghosts. The active H content closely correlated with the R_p, but did not with the R_inh. The kinetic chain length (KCL), i.e., the ratio of the rate of propagation to that of initiation, was calculated from R_p and tocopherol consumption rate and KCL values were higher in cord ghosts than in adult ghosts. The faster R_p and the higher KCL of the cord ghosts were attributable to a greater active H content rather than to the tocopherol content.     

生物立方膜

脂质立方液晶在药物载体方面有着较为广泛的应用.然而在生物体内,有一种与之类似的结构,称为立方膜.具体而言,立方膜就是指含有脂蛋白的三维周期性脂质双分子单层、双层或多层的纳米曲面结构.亚细胞器中的这种生物立方膜结构可能也能作为药物载体,同时有抗氧化、紫外滤光等潜在作用.本文将主要介绍立方膜的研究进展、形成机理,以及在自然界中的存在情况,及其功能和潜在的应用价值.     

pH-Dependent Effects of Chlorpromazine on Liposomes and Erythrocyte Membranes

Abstract

Chlorpromazine (CPZ) is an amphipathic antipsychotic drug that binds to erythrocytes reaching in this way the central nervous system. CPZ is a basic molecule with pK = 8.6. This paper reports on CPZ-induced lysis of red blood cells and liposomes. Haemolysis was tested under hypotonic conditions, in the pH range 5.0–10.0. Cell sensitivity towards CPZ increased with increasing pH. Increasing pH caused also a decrease in the critical micellar concentrations of CPZ. These results are interpreted in terms of a competition between repulsive electrostatic forces and attractive hydrophobic forces, that would act both in pure CPZ and in mixed CPZ-phospholipid micelles. In order to eliminate possible pH effects mediated by red blood cell proteins, experiments were carried out in which CPZ induced release of a fluorescent dye from liposomes (large unilamellar vesicles). The latter observations confirmed that membrane sensitivity towards CPZ was increased at higher pH.     

The Interaction of the Polyene Antibiotic Lucensomycin with Cholesterol in Erythrocyte Membranes and in Model Systems: II. Cooperative Effects in Erythrocyte Membranes

Fluorometric titration curves of erythrocyte membranes with increasing lucensomycin have a sigmoid shape. This behavior, which was not present when colloidal cholesterol suspensions were used, is, however, not peculiar to the membrane structure, being also present in cholesterol-containing phospholipid micelles. Addition of acetic acid induced or increased sigmoidicity. This behavior can either be due to a true cooperativity in binding or to different fluorescence yields of the various lucensomycin-membrane complexes. The latter hypothesis appears to be slightly favored.     

Cation-impermeable Inside-out and Right-side-out Vesicles from Human Erythrocyte Membranes

The two surfaces of a membrane can be compared by isolating sealed vesicles which bare one side or the other to impermeable probe molecules. For this purpose, inside-out (IO) and right-side-out (RO) vesicles have been generated from human red blood cell membrane ghosts and partially resolved on dextran density gradients¹. We now report further characterization of this system, showing (a) that vesicles can be made which are impermeable to small solutes, including Na⁺ and K⁺, even though the parent ghosts are quite unsealed; (b) that the dextran gradient fractionation serves to separate sealed vesicles (which happened to be IO and RO, respectively, in the original study¹); (c) that impermeable RO vesicles form instead of IO vesicles if the published protocol is modified by the untimely addition of trace divalent cations; and (d) that the vesicle fractions can and must be accurately monitored for sealing and orientation to avoid misinterpretations such as those encountered in a previous sidedness study².     

Yield Strength of Human Erythrocyte Membranes to Impulsive Stretching

Deformability while remaining viable is an important mechanical property of cells. Red blood cells (RBCs) deform considerably while flowing through small capillaries. The RBC membrane can withstand a finite strain, beyond which it ruptures. The classical yield areal strain of 2–4% for RBCs is generally accepted for a quasi-static strain. It has been noted previously that this threshold strain may be much larger with shorter exposure duration. Here we employ an impulse-like forcing to quantify this yield strain of RBC membranes. In the experiments, RBCs are stretched within tens of microseconds by a strong shear flow generated from a laser-induced cavitation bubble. The deformation of the cells in the strongly confined geometry is captured with a high-speed camera and viability is successively monitored with fluorescence microscopy. We find that the probability of cell survival is strongly dependent on the maximum strain. Above a critical areal strain of ∼40%, permanent membrane damage is observed for 50% of the cells. Interestingly, many of the cells do not rupture immediately and exhibit ghosting, but slowly obtain a round shape before they burst. This observation is explained with structural membrane damage leading to subnanometer-sized pores. The cells finally lyse from the colloidal osmotic pressure imbalance.     

Yield Strength of Human Erythrocyte Membranes to Impulsive Stretching

Deformability while remaining viable is an important mechanical property of cells. Red blood cells (RBCs) deform considerably while flowing through small capillaries. The RBC membrane can withstand a finite strain, beyond which it ruptures. The classical yield areal strain of 2–4% for RBCs is generally accepted for a quasi-static strain. It has been noted previously that this threshold strain may be much larger with shorter exposure duration. Here we employ an impulse-like forcing to quantify this yield strain of RBC membranes. In the experiments, RBCs are stretched within tens of microseconds by a strong shear flow generated from a laser-induced cavitation bubble. The deformation of the cells in the strongly confined geometry is captured with a high-speed camera and viability is successively monitored with fluorescence microscopy. We find that the probability of cell survival is strongly dependent on the maximum strain. Above a critical areal strain of ∼40%, permanent membrane damage is observed for 50% of the cells. Interestingly, many of the cells do not rupture immediately and exhibit ghosting, but slowly obtain a round shape before they burst. This observation is explained with structural membrane damage leading to subnanometer-sized pores. The cells finally lyse from the colloidal osmotic pressure imbalance.     

Theory of the Sphering of Red Blood Cells

A rigorous mathematical solution of the sphering of a red blood cell is obtained under the assumptions that the red cells is a fluid-filled shell and that it can swell into a perfect sphere in an appropriate hypotonic medium. The solution is valid for finite strain of the cell membrane provided that the membrane is isotropic, elastic and incompressible. The most general nonlinear elastic stress-strain law for the membrane in a state of generalized plane stress is used. A necessary condition for a red cell to be able to sphere is that its extensional stiffness follow a specific distribution over the membrane. This distribution is strongly influenced by the surface tension in the cell membrane. A unique relation exists between the extensional stiffness, pressure differential, surface tension, and the ratio of the radius of the sphere to that of the undeformed red cell. The functional dependence of this stiffness distribution on various physical parameters is presented. A critique of some current literature on red cell mechanics is presented.     

激光生物学技术研究高脂血症大鼠红细胞变形能力改变及其机制

目的:通过研究红细胞膜流动性以及红细胞骨架结构的改变,进一步探讨高脂血症大鼠红细胞变形能力改变的机制。方法:16只Wistar大鼠随机分为两组:高血症组和对照组。高脂组给予高脂饮食。16周后,腹主动脉采血,采用酶比色法检测血浆甘油三脂、胆固醇含量;并利用激光衍射法测定红细胞变形指数、取向指数,荧光偏振法测定红细胞膜流动性,激光共聚焦显微镜观测红细胞骨架改变和红细胞F-actin的含量。结果:发现高脂血症大鼠红细胞的变形指数、取向指数以及红细胞膜的流动性显著降低(P<0.05),红细胞形态和骨架发生改变,F-actin含量显著降低(P<0.05)。结论:高脂血症大鼠红细胞变形能力降低与红细胞膜结构改变有一定的关系。     

Iron-Induced Oxidative Stress in Erythrocyte Membranes of Non-Insulin-Dependent Diabetic Nigerians

The presence of higher level of endogenous free radical reaction products in the erythrocyte ghost membrane (EGM) of Non-insulin-dependent diabetes mellitus (NIDDM) subjects compared with that of normal healthy controls has been demonstrated. The EGMs of NIDDM subjects were also shown to be more susceptible to exogenously generated oxidative stress than those of normal healthy individuals. The decreased level of reactive thiol groups in the EGM of NIDDM individuals supported this observation. We propose that the presence of significant levels of non-heme iron in the EGM of NIDDM subjects is an indication of the potential for iron-catalysed production of hydroxy and other toxic radicals which could cause continuous oxidative stress and tissue damage. Oxygen free radicals could therefore be responsible for most of the erythrocyte abnormalities associated with non-insulin-dependent diabetes and could indeed be intimately involved in the mechanism of tissue damage in diabetic complications.     

Atomic Force Microscopy of Biological Membranes

Atomic force microscopy (AFM) is an ideal method to study the surface topography of biological membranes. It allows membranes that are adsorbed to flat solid supports to be raster-scanned in physiological solutions with an atomically sharp tip. Therefore, AFM is capable of observing biological molecular machines at work. In addition, the tip can be tethered to the end of a single membrane protein, and forces acting on the tip upon its retraction indicate barriers that occur during the process of protein unfolding. Here we discuss the fundamental limitations of AFM determined by the properties of cantilevers, present aspects of sample preparation, and review results achieved on reconstituted and native biological membranes.     

Constitutive Model of Erythrocyte Membranes with Distributions of Spectrin Orientations and Lengths

We present an analytical hyperelastic constitutive model of the red blood cell (erythrocyte) membrane based on recently improved characterizations of density and microscopic structure of its spectrin network from proteomics and cryo-electron tomography. The model includes distributions of both orientations and natural lengths of spectrin and updated copy numbers of proteins. By applying finite deformation to the spectrin network, we obtain the total free energy and stresses in terms of invariants of shear and area deformation. We generalize an expression of the initial shear modulus, which is independent of the number of molecular orientations within the network and also derive a simplified version of the model. We apply the model and its simplified version to analyze micropipette aspiration computationally and analytically and explore the effect of local cytoskeletal density change. We also explore the discrepancies among shear modulus values measured using different experimental techniques reported in the literature. We find that the model exhibits hardening behavior and can explain many of these discrepancies. Moreover, we find that the distribution of natural lengths plays a crucial role in the hardening behavior when the correct copy numbers of proteins are used. The initial shear modulus values we obtain using our current model (5.9–15.6 pN/μm) are close to the early estimates (6–9 pN/μm). This new, to our knowledge, constitutive model establishes a direct connection between the molecular structure of spectrin networks and constitutive laws and also defines a new picture of a much denser spectrin network than assumed in prior studies.     

Fluctuations and Fractal Noise in Biological Membranes

Our understanding of cell structure and function derives from applications of a variety of physical and life science disciplines, methods and models to an important physiological process, namely, the exchange and transport of ions and molecules across biological membranes. We know that ion transport through membranes arises from a diversity of interrelated and interactive physical and chemical phenomena over a wide range of spatial and temporal scales. Among these phenomena common to all cellular structure and function include metabolism, kinetics of molecules, chemically mediated alteration of cell membrane electrical potential, membrane ion conductance, electrical signal propagation, and modulation by chemo- and mechanoreceptive mechanisms. This review focuses on the unique information contained in fluctuations in electrical properties associated with cell membrane ion transport. Received: 19 May 2000/Revised: 10 July 2000