Pore Line Tension Research Articles

Free energy of hydrophilic and hydrophobic pores in lipid bilayers by free energy perturbation of a restraint.

The free energy of pore formation in lipid bilayers has been previously calculated using a variety of reaction coordinates. Here, we use free energy perturbation of a cylindrical lipid exclusion restraint to compute the free energy profile as a function of pore radius in dimyristoylphosphatidylcholine (DMPC) and dioleoylphosphatidylcholine (DOPC) bilayers. Additionally restraining the headgroups to lie on the membrane surface allows us to also calculate the free energy profile of hydrophobic pores, i.e., cylindrical pores lined by acyl chains. For certain pore radii, the free energy of wetting of hydrophobic pores is calculated using the density bias method. It is found that wetting of hydrophobic pores becomes thermodynamically favorable at 5.0 Å for DMPC and 6.5 Å for DOPC, although significant barriers prevent spontaneous wetting of the latter on a nanosecond time scale. The free energy of transformation of hydrophilic pores to hydrophobic ones is also calculated using free energy perturbation of headgroup restraints along the bilayer normal. This quantity, along with wetting and pore growth free energies, provides complete free energy profiles as a function of radius. Pore line tension values for the hydrophilic pores obtained from the slope of the free energy profiles are 37.6 pN for DMPC and 53.7 pN for DOPC. The free energy profiles for the hydrophobic pores are analyzed in terms of elementary interfacial tensions. It is found that a positive three-phase line tension is required to explain the results. The estimated value for this three-phase line tension (51.2 pN) lies within the expected range.

Effect of Lipid Structure and Material Properties on the Membrane Stability to Pore Formation

Biological membranes are natural barriers that preserve the integrity of the cell and its organelles. Even temporary damage of the cell membrane can break its normal homeostasis that will start the recovery mechanisms, apoptosis and inflammatory reactions. Formation of pores, through defects, in the lipid bilayer of biological membranes can lead to serious disruption of the normal lifecycle of the cells and the development of pathologies.A two-pronged approach was used to study the energetics of pore formation in lipid membranes assembled by isoprenoid lipids (e.g., DPhPC and Di-O-PhPC). The first prong studied the line tension of DPhPC, Di-O-PhPC, DOPC, and POPC using MD simulations and experiments with an electrical breakdown technique. Resistance to pore formation can be characterized by pore edge line tension, which describes the energy barrier height that should be overcome to irreversibly expand the pore, i.e. to rupture the membrane. A method of measuring pore line tension in MD, determining the lateral tension of the lipid membrane with the static pore, and experiments revealed that branched lipid significantly increased the pore energy and membrane resistance to pore formation. Additionally, work on GUVs and MD showed that low-weight solvent significantly lowers pore edge energy. The second prong used MD to quantify other key material properties that can affect pore formation (i.e., the compressibility modulus, bending modulus, per area free energy change with respect to curvature, and the intrinsic curvature). DPhPC's bending modulus is about half that of DPPC, but DPhPC's compressibility modulus is about two times larger than DPPC. Finally, DPhPC's effect on monolayer spontaneous curvature is similar to that of a PE lipid. Thus both structural peculiarities of lipid tails and solvents strongly influence membrane stability.

Frontiers of Complex Disease Mechanisms: Membrane Surface Tension May Link Genotype to Phenotype in Glaucoma.

Although many monogenic diseases are understood based upon structural changes of gene products, less progress has been made concerning polygenic disease mechanisms. This article presents a new interdisciplinary approach to understand complex diseases, especially their genetic polymorphisms. I focus upon primary open angle glaucoma (POAG). Although elevated intraocular pressure (IOP) and oxidative stress are glaucoma hallmarks, the linkages between these factors and cell death are obscure. Reactive oxygen species (ROS) promote the formation of oxidatively truncated phosphoglycerides (OTP), free fatty acids, lysophosphoglycerides, oxysterols, and other chemical species that promote membrane disruption and decrease membrane surface tension. Several POAG-linked gene polymorphisms identify proteins that manage damaged lipids and/or influence membrane surface tension. POAG-related genes expected to participate in these processes include: ELOVL5, ABCA1, APOE4, GST, CYP46A1, MYOC, and CAV. POAG-related gene products are expected to influence membrane surface tension, strength, and repair. I propose that heightened IOP overcomes retinal ganglion cell (RGC) membrane compressive strength, weakened by damaged lipid accumulation, to form pores. The ensuing structural failure promotes apoptosis and blindness. The linkage between glaucoma genotype and phenotype is mediated by physical events. Force balancing between the IOP and compressive strength regulates pore nucleation; force balancing between pore line tension and membrane surface tension regulates pore growth. Similar events may contribute to traumatic brain injury, Alzheimer's disease, and macular degeneration.

Edge Structure of through Pore in Lipid Membrane

Theoretical model of edge structure of through pore in lipid membrane was developed. We assumed that membrane monolayers are not necessary in close contact with each other everywhere; instead, hydrophobic void can appear between monolayers of the membrane near the pore edge. If formed, the void is supposed to be filled by organic solvent or low-polarity impurities, which are usually present in artificial and cell membranes. The monolayer of pore edge was divided to three regions: almost vertical monolayer, almost horizontal monolayer, and horizontal monolayer of bilayer part of the membrane. All three regions were assumed continuously conjugated. Pore line tension was calculated in the framework of theory of elasticity of liquid crystals adapted to lipid membranes, by taking into account splay and tilt deformations. Line tension depends on the value of surface tension at the boundary of lipid tails with the void. For sufficiently high surface tension the void collapses. Calculated values of pore edge line tension are in good agreement with available experimental data both for membranes containing and lacking organic solvent or low-polarity impurities.

Effect of Monovalent Ion Concentration in Molecular Simulation of Electroporation

Monovalent ion concentration gradients, regulated by ion pumps and leak channels, are critical components of many cellular functions. This dynamic balance is disturbed by the electroporative permeabilization of the cell membrane, which bypasses the normal membrane barriers to transmembrane ion flux. A better understanding of ion transport during and after electroporation will enable more efficient and more effective utilization of this method in biomedicine and biotechnology.Molecular dynamics (MD) simulations provide a view of the behavior of ions and biomolecular structures at the molecular level. Previous MD studies have demonstrated some of the effects of monovalent ions on phospholipid bilayers, including decreased area per lipid, higher ordering in head group vertical orientations, and decreased lateral lipid mobility [1-4]. MD simulations of porated membranes have also shown that the binding of monovalent cations to phospholipids can increase pore line tension, which leads to a decrease in pore lifetime [5]. In this work we employ MD simulations to systematically study the effects of varying the concentration of Na+, K+, and Cl− in POPC lipid bilayer systems during different stages of electropore formation.1.Gurtovenko, A.A. and I. Vattulainen, J Phys Chem B, 2008. 112(7): p. 1953-62.2.Vacha, R., et al., Journal of Physical Chemistry A, 2009. 113(26): p. 7235-7243.3.Vacha, R., et al., J Phys Chem B, 2010. 114(29): p. 9504-9.4.Jurkiewicz, P., et al., Biochim Biophys Acta, 2012. 1818(3): p. 609-16.5.Leontiadou, H., A.E. Mark, and S.J. Marrink, Biophys J, 2007. 92(12): p. 4209-15.

Ion Transport across Transmembrane Pores

To study the pore-mediated transport of ionic species across a lipid membrane, a series of molecular dynamics simulations have been performed of a dipalmitoyl-phosphatidyl-choline bilayer containing a preformed water pore in the presence of sodium and chloride ions. It is found that the stability of the transient water pores is greatly reduced in the presence of the ions. Specifically, the binding of sodium cations at the lipid/water interface increases the pore line tension, resulting in a destabilization of the pore. However, the application of mechanical stress opposes this effect. The flux of ions through these mechanically stabilized pores has been analyzed. Simulations indicate that the transport of the ions through the pores depends strongly on the size of the water channel. In the presence of small pores (radius <1.5 nm) permeation is slow, with both sodium and chloride permeating at similar rates. In the case in which the pores are larger (radius >1.5 nm), a crossover is observed to a regime where the anion flux is greatly enhanced. Based on these observations, a mechanism for the basal membrane permeability of ions is discussed.

Pore formation in fluctuating membranes

We study the nucleation of a single pore in a fluctuating lipid membrane, specifically taking into account the membrane fluctuations, as well as the shape fluctuations of the pore. For large enough pores, the nucleation free energy is well-described by shifts in the effective membrane surface tension and the pore line tension. Using our framework, we derive the stability criteria for the various pore formation regimes. In addition to the well-known large-tension regime from the classical nucleation theory of pores, we also find a low-tension regime in which the effective line and surface tensions can change sign from their bare values. The latter scenario takes place at sufficiently high temperatures, where the opening of a stable pore of finite size is entropically favorable.

Transient pores in vesicles

AbstractWe present our observations of transient pores in giant unilamellar vesicles, placed under tension, by optical illumination. When the membrane tension reached a certain level, transient pores appeared. Pore opening is driven by the membrane tension, σ, and its closure by the pore's line tension, 𝒯. By use of viscous mixtures of glycerol and water, we slowed down the leak out of the inner liquid in the presence of a pore. This allowed pores to reach large sizes (a few micrometres) and last at least a few seconds so that they could be visualized by fluorescence videomicroscopy. Line tension was inferred from the measurements of the closure velocity of the pores. By addition of cholesterol, which increased 𝒯 (reducing pore lifetimes), or of surfactants, which decreased 𝒯 (increasing pore lifetimes), we demonstrate how 𝒯, and consequently pore lifetimes, can be controlled over nearly two orders of magnitude. Addition of surfactants also has a dramatic effect on vesicle fusion. We discuss how our results can be extended to less viscous aqueous solutions which are more relevant for liposomal drug delivery formulations.© 2003 Society of Chemical Industry

Pro-apoptotic Cleavage Products of Bcl-xL Form Cytochrome c-conducting Pores in Pure Lipid Membranes

During apoptotic cell death, cells usually release apoptogenic proteins such as cytochrome c from the mitochondrial intermembrane space. If Bcl-2 family proteins induce such release by increasing outer mitochondrial membrane permeability, then the pro-apoptotic, but not anti-apoptotic activity of these proteins should correlate with their permeabilization of membranes to cytochrome c. Here, we tested this hypothesis using pro-survival full-length Bcl-x(L) and pro-death Bcl-x(L) cleavage products (DeltaN61Bcl-x(L) and DeltaN76Bcl-x(L)). Unlike Bcl-x(L), DeltaN61Bcl-x(L) and DeltaN76Bcl-x(L) caused the release of cytochrome c from mitochondria in vivo and in vitro. Recombinant DeltaN61Bcl-x(L) and DeltaN76Bcl-x(L), as well as Bcl-x(L), cleaved in situ by caspase 3-possessed intrinsic pore-forming activity as demonstrated by their ability to efficiently permeabilize pure lipid vesicles. Furthermore, only DeltaN61Bcl-x(L) and DeltaN76Bcl-x(L), but not Bcl-x(L), formed pores large enough to release cytochrome c and to destabilize planar lipid bilayer membranes through reduction of pore line tension. Because Bcl-x(L) and its C-terminal cleavage products bound similarly to lipid membranes and formed oligomers of the same size, neither lipid affinity nor protein-protein interactions appear to be solely responsible for the increased membrane-perturbing activity elicited by Bcl-x(L) cleavage. Taken together, these data are consistent with the hypothesis that Bax-like proteins oligomerize to form lipid-containing pores in the outer mitochondrial membrane, thereby releasing intermembrane apoptogenic factors into the cytosol.

Lateral interactions of charges in thin liquid films and the Berezinskii-Kosterlitz-Thouless transition

At certain experimental conditions, foam films drawn from aqueous surfactant solutions can form highly stable Newtonian films. We show that the origin of the film stability can be attributed to specific patterning of surfactant molecules. Due to high dielectric contrast, the charges, together with their electrostatic images, interact as needlelike charged polymers. Below a critical thickness, such quasipolyelectrolytes undergo the Berezinskii-Kosterlitz-Thouless transition from a plasmalike state to a charge neutral one. In the latter, all the charges are bound into dipole pairs. Inherent only in thin films, the effect leads to the surfactant condensation into spots that cannot be observed on a single interface at the same surfactant concentration. We quantify a film resistance to rupture in terms of a pore line tension and show that the corresponding energy barrier needed for pore creation is much greater than the energy of thermal excitations.

Dynamically stabilized pores in bilayer membranes

Zhelev and Needham have recently created large, quasistable pores in artificial lipid bilayer vesicles. Initially created by electroporation, the pores remain open for up to several seconds before quickly snapping shut. This result is surprising, in light of the large line tension for holes in bilayer membranes and the rapid time scale for closure of large pores. We show how pores can be dynamically stabilized via a new feedback mechanism. We also explain quantitatively the observed sudden pore closure as a tangent bifurcation. Finally, we show how Zhelev and Needham's experiment can be used to measure accurately the pore line tension, an important material parameter. For their stearoyloleoylphosphatidylcholine/cholesterol mixture we obtain a line tension of 2.6 x 10(-6) dyn.

Dynamically-Stabilized Pores in Bilayer Membranes

ABSTRACTZhelev and Needham have recently created large, quasi-stable pores in artificial lipid bilayer vesicles. Initially created by electroporation, the pores remain open for up to several seconds before quickly snapping shut. This result is surprising in light of the large line tension for holes in bilayer membranes and the rapid time scale for closure of large pores. We show how pores can be dynamically stabilized via a new feedback mechanism. We also explain quantitatively the observed sudden pore closure as a tangent bifurcation. Finally we show how Zhelev and Need-ham's experiment can be used to measure accurately the pore line tension, an important material parameter. For their SOPC/CHOL mixture we obtain a line tension of 3.0 × 10−6dyn.

Tension-stabilized pores in giant vesicles: determination of pore size and pore line tension

We present the first observations of giant, long-existing, stabilized pores in vesicle membranes. Using a new experimental technique for studying the electropermeabilization of lipid membranes, giant liposomes (from 25 to 56 μm in diameter) were subjected to single, square, electric pulses (duration 150 μs and electric field strength from 63 to 126 kV/m). The liposomes were held by a micropipet and small membrane tensions were created by controlling the pipet suction pressure. The liposomes were loaded with media having different refractive index from the outside solution, and, under these conditions, the formation of pores in the pressurized liposome could be visualized by the jet of inside solution that flowed out from the membrane pore. By adjusting the membrane tension, pores were kept open, and pore lifetimes could be varied from tenths of a second to several seconds. The pore size was determined from the volumetric flow in the pore region and the measured pressure differences across the bilayer. It was clear from the experiments that only one pore remained opened after the pulse. The estimated pore radii were on the order of one micrometer. The pores were in a quasi-stationary state and when they closed they did so spontaneously in a quick process (in milliseconds). The isotropic membrane tension was determined for the same measurements and from determinations of both pore size and dynamic membrane tension the pore line tension was found. The line tension of the pore region was determined for two lipid compositions, stearoyloleoylphosphatidylcholine and stearoyloleoylphosphatidylcholine with 50 mol% cholesterol, and the obtained values for single bilayers were (0.92 ± 0.07)·10 −11 N and (3.05 ± 0.12)·10 −11 N, respectively.