Stomatocytosis is absent in "stomatin"-deficient murine red blood cells.
To examine the relationship between erythrocyte membrane protein 7. 2b deficiency and the hemolytic anemia of human hereditary stomatocytosis, we created 7.2b knock-out mice by standard gene targeting approaches. Immunoblots showed that homozygous knock-out mice completely lacked erythrocyte protein 7.2b. Despite the absence of protein 7.2b, there was no hemolytic anemia and mouse red blood cells (RBCs) were normal in morphology, cell indices, hydration status, monovalent cation content, and ability to translocate lipids. The absence of the phenotype of hereditary stomatocytosis implies that protein 7.2b deficiency plays no direct role in the etiology of this disorder and casts doubt on the previously proposed role of this protein as a mediator of cation transport in RBC. (+info)
Vesicle deformation by an axial load: from elongated shapes to tethered vesicles.
A sufficiently large force acting on a single point of the fluid membrane of a flaccid phospholipid vesicle is known to cause the formation of a narrow bilayer tube (tether). We analyze this phenomenon by means of general mathematical methods allowing us to determine the shapes of strongly deformed vesicles including their stability. Starting from a free vesicle with an axisymmetric, prolate equilibrium shape, we consider an axial load that pulls (or pushes) the poles of the vesicle apart. Arranging the resulting shapes of strained vesicles in dependence of the axial deformation and of the area difference of monolayers, phase diagrams of stable shapes are presented comprising prolate shapes with or without equatorial mirror symmetry. For realistic values of membrane parameters, we study the force-extension relation of strained vesicles, and we demonstrate in detail how the initially elongated shape of an axially stretched vesicle transforms into a shape involving a membrane tether. This tethering transition may be continuous or discontinuous. If the free vesicle is mirror symmetric, the mirror symmetry is broken as the tether forms. The stability analysis of tethered shapes reveals that, for the considered vesicles, the stable shape is always asymmetric (polar), i.e., it involves only a single tether on one side of the main vesicle body. Although a bilayer tube formed from a closed vesicle is not an ideal cylinder, we show that, for most practical purposes, it is safe to assume a cylindrical geometry of tethers. This analysis is supplemented by the documentation of a prototype experiment supporting our theoretical predictions. It shows that the currently accepted model for the description of lipid-bilayer elasticity (generalized bilayer couple model) properly accounts for the tethering phenomenon. (+info)
Monte Carlo simulation of two-component bilayers: DMPC/DSPC mixtures.
In this paper, we describe a relatively simple lattice model of a two-component, two-state phospholipid bilayer. Application of Monte Carlo methods to this model permits simulation of the observed excess heat capacity versus temperature curves of dimyristoylphosphatidylcholine (DMPC)/distearoylphosphatidylcholine (DSPC) mixtures as well as the lateral distributions of the components and properties related to these distributions. The analysis of the bilayer energy distribution functions reveals that the gel-fluid transition is a continuous transition for DMPC, DSPC, and all DMPC/DSPC mixtures. A comparison of the thermodynamic properties of DMPC/DSPC mixtures with the configurational properties shows that the temperatures characteristics of the configurational properties correlate well with the maxima in the excess heat capacity curves rather than with the onset and completion temperatures of the gel-fluid transition. In the gel-fluid coexistence region, we also found excellent agreement between the threshold temperatures at different system compositions detected in fluorescence recovery after photobleaching experiments and the temperatures at which the percolation probability of the gel clusters is 0.36. At every composition, the calculated mole fraction of gel state molecules at the fluorescence recovery after photobleaching threshold is 0.34 and, at the percolation threshold of gel clusters, it is 0.24. The percolation threshold mole fraction of gel or fluid lipid depends on the packing geometry of the molecules and the interchain interactions. However, it is independent of temperature, system composition, and state of the percolating cluster. (+info)
Cell membrane dynamics and the induction of apoptosis by lipid compounds.
To investigate the induction of apoptosis by some lipid compounds which are a potent inducer of apoptosis, the plasma membrane fluidity of U937 cells was measured using the fluorescent probe, pyrene. The increase of the membrane fluidity was observed immediately after the treatment of cells with lipid inducers. We also found that the trigger of apoptosis was pulled within 30 min after treatment. Data from the dynamic light scattering experiment indicated that lipid inducers were dissolved to form the emulsion. At the very early stage of apoptosis, possibly, the well-controlled transfer of lipid inducers from the emulsion to the lipid layer of cells can bring about the increase of membrane dynamics which might lead to the induction of apoptosis. (+info)
The interaction of ubiquinone-3 with phospholipid membranes.
The effects of ubiquinone-3 (UQ) on dipalmitoylphosphatidylcholine (DPPC) membrane were studied by surface monolayer, differential scanning calorimetry (DSC) and fluorescence techniques. DPPC and UQ are proved to be freely miscible in the mixed monolayer at an air/water interface, and to be partially miscible in bulk phase, i.e. bilayer and solid phase. There is a condensing interaction between UQ and DPPC in the UQ/DPPC mixed monolayers. The solubility of UQ in the DPPC is about 20 mole% and the solubility of DPPC in UQ is about 10 mole%. The membrane fluidity of DPPC was increased by the addition of UQ and the phase transition temperature was decreased. (+info)
Interaction of tumor and normal blood cells with ethylene oxide and propylene oxide block copolymers.
Ethylene oxide and propylene oxide block copolymers (pluronics) are widely known as agents that promote drug penetration across biological barriers. We have studied the interaction of normal and malignant blood cells with pluronics L61 and P85 that have different hydrophobicity. SP2/0 myeloma cells accumulated pluronics while normal cells adsorb most of the polymer on the surface. Interaction of pluronics with cells resulted in drastic changes of membrane microviscosity. Tumor cell membrane microviscosity decreased after pluronics adsorption, in contrast to normal cells, whose membrane microviscosity was enhanced. We suppose that sensitivity of tumor cell membrane microviscosity to the pluronics action correlates with its permeability for molecular substances. (+info)
The yeast multidrug resistance pump, Pdr5p, confers reduced drug resistance in erg mutants of Saccharomyces cerevisiae.
Mutants of Saccharomyces cerevisiae bearing lesions in the ergosterol biosynthetic pathway exhibit a pleiotropic drug-sensitive phenotype. This has been reported to result from an increased permeability of the membranes of the mutant strains to different drugs. As disruption of the yeast multidrug resistance protein, Pdr5p, results in a similar pleiotropic drug-sensitive phenotype, the possibility that Pdr5p may be functioning with a reduced efficiency in these altered sterol backgrounds was examined. To do this, the function of Pdr5p in isogenic strains of S. cerevisiae that have disruptions in the late stages of the ergosterol biosynthesis pathway (ERG6, ERG2, ERG3, ERG4) was studied. A reduced ability of Pdr5p to confer resistance to different drugs in these strains was observed, which did not appear to be dependent solely on the permeability of the membrane towards the drug. A simultaneous examination was made of how the lipid composition might be altering the efficiency of Pdr5p by similar studies in strains lacking phosphatidylserine synthase (encoded by CHO1). The results indicated that the drug sensitivity of the erg strains is, to a significant extent, a result of the reduced efficiency of the Pdr5p efflux pump, and that the membrane environment plays an important role in determining the drug resistance conferred by Pdr5p. (+info)
Induction of acetylcholinesterase release from erythrocytes in the presence of liposomes.
When human erythrocytes are incubated with liposomes, the release of acetylcholinesterase (AChE) occurs following an induction period [Cook et al. (1980) Biochemistry 19, 4601-4607]. However, the mechanism of the induction has not been elucidated. We examined the relationships among the lipid transfer from liposomes to erythrocytes, the morphological change of erythrocytes, the fluidity of the erythrocyte membrane and the start of AChE release. The AChE release into the liposomes and into shed-vesicle fractions started simultaneously after an induction period. The morphological index (MI) of erythrocytes was approximately 2.8 at the beginning of the release, regardless of the induction period. AChE was not released from the erythrocytes of index 2.8 even in the presence of liposomes if the MI remained at 2.8. Therefore, for the release, erythrocytes needed a further increase of the MI from 2.8. As the rate of lipid transfer increased, the induction period became shorter. No significant lipid release from erythrocytes was detected during the induction period. The initiation of the AChE release was not simply affected by the change in the membrane fluidity of erythrocytes upon interaction with liposomes. These results first demonstrate that AChE release into the shed-vesicle and liposome fractions is triggered by a further increase of the MI from 2.8, which is induced by lipid transfer from liposomes to erythrocytes. (+info)