Anisotropic Membrane Inclusions Research Articles

Shape variation of bilayer membrane daughter vesicles induced by anisotropic membrane inclusions

A theoretical model of a two-component bilayer membrane was used in order to describe the influence of anisotropic membrane inclusions on shapes of membrane daughter micro and nano vesicles. It was shown that for weakly anisotropic inclusions the stable vesicle shapes are only slightly out-of-round. In contrast, for strongly anisotropic inclusions the stable vesicle shapes may significantly differ from spheres, i.e. they have a flattened oblate shape at small numbers of inclusions in the membrane, and an elongated cigar-like prolate shape at high numbers of inclusions in the vesicle membrane.

Curvature-induced accumulation of anisotropic membrane components and raft formation in cylindrical membrane protrusions

Coupling between the area density of anisotropic membrane inclusions and local membrane curvature is considered theoretically for a simple case of nearly flat bilayer membrane with thin tubular membrane protrusions. Lateral phase separation, i.e. accumulation of membrane inclusions in tubular membrane protrusions was obtained for strongly anisotropic inclusions if the radius of tubular protrusions is small enough. In accordance with these theoretical predictions we observed persistence of long tubular membrane protrusions devoid of internal rod-like microtubular structure in cells. We suggest that the stability of the tubular membrane protrusions without the inner supporting rod-like cytoskeleton is a consequence of the accumulation of anisotropic membrane components in the bilayer membrane of these protrusions. Based on the presented theoretical and experimental results it is suggested that previously reported concentration of prominin rafts in thin tubular membrane protrusions may be caused by a curvature-induced accumulation of small prominin–lipid complexes (inclusions) in protrusions and their coalescence into larger rafts.

Amphiphile-induced tubular budding of the bilayer membrane

Amphiphile-induced tubular budding of the erythrocyte membrane was studied using transmission electron microscopy. No chiral patterns of the intramembraneous particles were found, either on the cylindrical buds, or on the tubular nanoexovesicles. In agreement with these observations, the tubular budding may be explained by in-plane ordering of anisotropic membrane inclusions in the buds where the difference between the principal membrane curvatures is very large. In contrast to previously reported theories, no direct external mechanical force is needed to explain tubular budding of the bilayer membrane.

Endovesicle formation and membrane perturbation induced by polyoxyethyleneglycolalkylethers in human erythrocytes

Polyoxyethyleneglycolalkylether (C m E n , m=12, n=8) can induce a large torocyte-like endovesicle in human erythrocytes. The present study aimed to examine how variations in the molecular structure of C m E n ( m=10,12,14,16,18; n=1–10,23) affect the occurrence of torocyte endovesicles. Our results show that torocytes occur most frequently when m=12,14 and n=8,9. At this molecular configuration the detergents induce inward membrane bending (stomatocytic S 1–S 2 shapes) resulting in the formation of a large membrane invagination. These detergents have a strong membrane perturbing, i.e., haemolytic, effect. Theoretical calculations indicate that a torocyte-shaped inside-out membrane vesicle can be created from a large membrane invagination due to the impact of laterally mobile anisotropic membrane inclusions. Such inclusions may be detergent–membrane component complexes or unanchored integral membrane proteins. It is shown that a nonhomogeneous lateral distribution of anisotropic membrane inclusions may stabilise the torocyte endovesicle shape, characterised by having opposite membranes in the thin central region of the vesicles separated by a certain distance. Tubular, conical or inverted conical isotropic inclusions cannot do so.

The Many-Body Problem for Anisotropic Membrane Inclusions and the Self-Assembly of “Saddle” Defects into an “Egg Carton”

We calculate the many-body, nonpairwise interaction between N rigid, anisotropic membrane inclusions by modeling them as point-like constraints on the membrane's curvature tensor and by minimizing the membrane's curvature energy. Because multipolar distortions of higher-order decay on very short distances, our calculation gives the correct elastic interaction energy for inclusions separated by distances of the order of several times their size. As an application, we show by thermally equilibrating the many-body elastic energy using a Monte Carlo algorithm, that inclusions shaped as “saddles” attract each other and build an “egg-carton” structure. The latter is reminiscent of some patterns observed in membranes obtained from biological extracts, the origin of which is still mysterious.

Effect of surfactant polyoxyethylene glycol (C12E8) on electroporation of cell line DC3F

Surfactant polyoxyethylene glycol (C12E8) decreases the voltage required for irreversible electroporation in planar lipid bilayers. In our study the effect of non-cytotoxic concentration of C12E8 on cell membrane reversible and irreversible electroporation voltage was investigated in DC3F cell line. Cell suspension was exposed to a train of 8 electric pulses of 100 ms duration, repetition frequency 1 Hz and amplitudes from 0 to 400 V at electrode distance 2 mm. The effect of C12E8 on the reversible and irreversible electroporation was investigated. We found that C12E8 decreases the voltage necessary for irreversible electroporation but has no effect on reversible electropermeabilization. Cell membrane fluidity measured by electron paramagnetic resonance spectrometry, using the spin probe methylester of 5doxyl palmitate was not significantly changed due to the addition of C12E8. Based on this we conclude that the main reason for the observed effect were not the changes in the membrane fluidity. As an alternative explanation we suggest that C12E8 induced anisotropic membrane inclusions may stabilize the hydrophilic pore, by accumulating on a toroidally shaped edge of the pore and attaining favorable orientation. # 2002 Elsevier Science B.V. All rights reserved.

Possible role of anisotropic membrane inclusions in stability of torocyte red blood cell daughter vesicles

The stability of torocyte red blood cell daughter endovesicles induced by octaethylene-glycol dodecylether (C 12E 8) was studied theoretically. In addition, the effects of C 12E 8 and tetraethylene-glycol dodecylether (C 12E 4) on physical properties of the red blood cell membrane were studied experimentally, using the electron spin resonance (ESR) technique. In the theoretical part, it was assumed that the stable vesicle shape corresponds to the minimum of its membrane free energy, which is the sum of the membrane bending energy and the contribution of the C 12E 8-induced membrane inclusions. We found that the torocytic vesicle shape may be stable due to quadrupolar ordering of the C 12E 8 anisotropic inclusions that are embedded in the vesicle membrane. It was also shown how a preference of the membrane inclusions for a specific membrane curvature might lead to their non-homogeneous lateral distribution. In the experimental part, it was shown that C 12E 4 drastically changes the proportions of the membrane lipid domains (characterized by different ‘fluidity’), while C 12E 8 induces much smaller changes in the proportions of the domains. A possible relation between the difference in the effects of C 12E 8 and C 12E 4 on the membrane lipid domains, and their distribution between the membrane leaflets, is discussed.

N-body study of anisotropic membrane inclusions: Membrane mediated interactions and ordered aggregation

We study the collective behavior of inclusions inducing local anisotropic curvatures in a flexible fluid membrane. The N-body interaction energy for general anisotropic inclusions is calculated explicitly, including multi-body interactions. Long-range attractive interactions between inclusions are found to be sufficiently strong to induce aggregation. Monte Carlo simulations show a transition from compact clusters to aggregation on lines or circles. These results might be relevant to proteins in biological membranes or colloidal particles bound to surfactant membranes.

Nontopological saddle-splay and curvature instabilities from anisotropic membrane inclusions.

Anisotropic inclusions are shown to induce spontaneous deviatoric bendings in lipidic membranes, by orienting at right angles across the bilayer. In the limit of strong membrane curvatures, a nonanalytical bending energy term is generated that favors saddlelike and cylindrical shapes, without penalizing spherical ones. An ``egg-carton'' instability results in flat membranes as well as a wormlike shape instability for vesicles.