Miha Fošnarič

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

Abstract 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 μs 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 5-doxyl 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.

Fusion pore stability of peptidergic vesicles

Abstract It is believed that in regulated exocytosis the vesicle membrane fuses with the plasma membrane in response to a physiological stimulus. However, in the absence of stimulation, repetitive transient fusion events are also observed, reflecting a stable state. The mechanisms by which the initial fusion pore attains stability are poorly understood. We modelled energetic stability of the fusion pore by taking into account the anisotropic, intrinsic shape of the membrane constituents and their in-plane ordering in the local curvature of the membrane. We used cell-attached membrane capacitance techniques to monitor the appearance and conductance of single fusion pore events in cultured rat lactotrophs. The results revealed a bell-shaped distribution of the fusion pore conductance with a modal value of 25 pS. The experimentally observed increase of the fusion pore stability with decreasing fusion pore radius agrees well with the theoretical predictions. Moreover, the results revealed a correlation between the amplitude of transient capacitance increases and the fusion pore conductance, indicating that larger vesicles may attain a stable fusion pore with larger fusion pore diameters.

Monte Carlo simulations of complex formation between a mixed fluid vesicle and a charged colloid

Monte Carlo simulations are employed to investigate the ability of a charged fluidlike vesicle to adhere to and encapsulate an oppositely charged spherical colloidal particle. The vesicle contains mobile charges that interact with the colloid and among themselves through a screened electrostatic potential. Both migration of charges on the vesicle surface and elastic deformations of the vesicle contribute to the optimization of the vesicle-colloid interaction. Our Monte Carlo simulations reveal a discontinuous wrapping transition of the colloid as a function of the number of charges on the vesicle. Upon reducing the bending stiffness of the vesicle, the transition terminates in a critical point. At large electrostatic screening length we find a reentrant wrapping-unwrapping behavior upon increasing the total number of charges on the vesicle. We present a simple phenomenological model that qualitatively captures some features of the wrapping transition.

Coupling between vesicle shape and the non-homogeneous lateral distribution of membrane constituents in Golgi bodies

In this work, a hypothesis is presented that could explain the non‐homogeneous lateral distribution of membrane components in Golgi vesicles. It is shown that the non‐homogeneous lateral distribution of membrane components and the specific flattened shape of Golgi vesicles are strongly coupled. In agreement with experimental evidence, it is indicated that some of the membrane components may be concentrated mainly on the curved bulbous rims of the Golgi vesicles, while the other components are distributed predominantly in their flat central part.

Deviatoric elasticity as a possible physical mechanism explaining collapse of inorganic micro and nanotubes

Abstract A mechanism is proposed that explains collapse of the multishell inorganic micro and nanotubes. A single shell is considered as a thin elastic plate with anisotropic properties. The derived elastic energy is expressed by the mean curvature and the curvature deviator. If the tube perimeter exceeds a certain threshold, the collapsed shape corresponds to the absolute minimum of the elastic energy.

Monte Carlo simulations of a polymer confined within a fluid vesicle

Monte Carlo simulations are employed to study a fluid vesicle that contains a single worm-like polymer chain. The contour length of the polymer is about five times the circumference of the nominally spherical vesicle. We vary the degree of polymer confinement in our simulations by increasing the persistence length of the polymer. The vesicle is represented by a randomly triangulated self-avoiding network that can undergo bending deformations. Upon increasing the persistence length of the polymer beyond the size of the vesicle, we observe a transition of the polymer from an isotropic disordered random conformation to an ordered toroidal coil. Concomitantly, the vesicle adopts an oblate shape to allow for some expansion of the polymer coil inside the vesicle. It is convenient to characterize both polymer and vesicle in terms of the asphericity, a quantity derived from the gyration tensor. At the onset of the polymers ordering transition, the asphericity passes through a minimum for both polymer and vesicle. The increase in vesicle asphericity for a semi-flexible polymer can be understood in terms of ground state energy calculations, either for a simplified representation of the vesicle shape (we specifically discuss a disk shape with a semi-toroidal rim) or involving a full vesicle shape optimization. The asphericity of the polymer coil results from conformational fluctuations and can be rationalized using Odijks deflection length of strongly curved semi-flexible polymers.

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 (C12E8) was studied theoretically. In addition, the effects of C12E8 and tetraethylene-glycol dodecylether (C12E4) 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 C12E8-induced membrane inclusions. We found that the torocytic vesicle shape may be stable due to quadrupolar ordering of the C12E8 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 C12E4 drastically changes the proportions of the membrane lipid domains (characterized by different ‘fluidity’), while C12E8 induces much smaller changes in the proportions of the domains. A possible relation between the difference in the effects of C12E8 and C12E4 on the membrane lipid domains, and their distribution between the membrane leaflets, is discussed.

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.

Flexible Membrane Inclusions and Membrane Inclusions Induced by Rigid Globular Proteins

Abstract We present a theoretical approach to the study of flexible membrane inclusions and membrane inclusions induced by rigid membrane-embedded proteins. We derive the contribution to the free energy of the membrane bilayer for both kinds of inclusions. For flexible membrane inclusions, the phenomenological interaction constants that appear in the free energy expression depend on the physical and geometrical properties of the molecules that constitute the inclusion. The cases of constrained and unconstrained local shape perturbations of the membrane around a rigid membrane inclusion are discussed. The total free energy of membrane bilayer with membrane-embedded inclusions (membrane nanodomains) is derived.

On the role of external force of actin filaments in the formation of tubular protrusions of closed membrane shapes with anisotropic membrane components

Biological membranes are composed of different components and there is no a priori reason to assume that all components are isotropic. It was previously shown that the anisotropic properties of membrane components may explain the stability of membrane tubular protrusions even without the application of external force. Our theoretical study focuses on the role of anisotropic membrane components in the stability of membrane tubular structures generated or stabilized by actin filaments. We show that the growth of the actin cytoskeleton inside the vesicle can induce the partial lateral segregation of different membrane components. The entropy of mixing of membrane components hinders the total lateral segregation of the anisotropic and isotropic membrane components. Self-assembled aggregates formed by anisotropic membrane components facilitate the growth of long membrane tubular protrusions. Protrusive force generated by actin filaments favors strong segregation of membrane components by diminishing the opposing effect of mixing entropy.