Field theoretic study of bilayer membrane fusion III: membranes with leaves of different composition

We extend previous work on homogeneous bilayers to calculate the barriers to fusion of planar bilayers that contain two different amphiphiles, a lamellar former and a hexagonal former, with different compositions of the two in each leaf. Self-consistent field theory is employed, and both standard and alternative pathways are explored. We first calculate these barriers as the amount of hexagonal former is increased equally in both leaves to levels appropriate to the plasma membrane of human red blood cells. We follow these barriers as the composition of hexagonal formers is then increased in the cis layer and decreased in the trans layer, again to an extent comparable to the biological system. We find that, while the fusion pathway exhibits two barriers in both the standard and alternative pathways, in both cases the magnitudes of these barriers are comparable to one another, and small, on the order of 13 k(B)T. As a consequence, one expects that once the bilayers are brought sufficiently close to one another to initiate the process, fusion should occur rapidly.

A microscopic model calculation of the phase diagram of ternary mixtures of cholesterol and saturated and unsaturated phospholipids

The authors solved a microscopic model that describes mixtures of a saturated phospholipid, an unsaturated phospholipid, and cholesterol. The method employed was the self-consistent field approximation. The model was capable of producing several classes of phase diagram, but only one of them showed a liquid-liquid coexistence region. The phospholipids in the cholesterol-rich liquid are more ordered than those in the cholesterol-poor liquid. Within this model, coexistence of two liquids in the ternary system is intimately tied to such coexistence in the binary cholesterol-saturated phospholipid system.

Phase separation in bilayer lipid membranes: effects on the inner leaf due to coupling to the outer leaf

The combined effects of the tendency of cholesterol to order lipids in the liquid phase and the coupling between lipids in the two leaves of a bilayer are investigated theoretically utilizing a Landau free energy. We show that as a consequence of these combined effects, lateral phase separation in the outer leaf between cholesterol-rich and -poor liquids causes a similar, but weaker, phase separation in the inner leaf. Just as the areal density of lipids in the outer leaf increases in the cholesterol-rich regions, so the areal density of lipids also increases in the inner leaf. Thus, the areal density in the inner leaf varies spatially, reflecting spatial variations of the areal density in the outer leaf. This provides a mechanism for proteins attached to the inner leaf via a hydrocarbon tether to respond to variations in the composition of the outer leaf. We also note that the effect of coupling between the leaves should be observable in artificial bilayers.

Self-consistent field study of the alignment by an electric field of a cylindrical phase of block copolymer

Self-consistent field theory is applied to a film of cylindrical-forming block copolymer subject to a surface field which tends to align the cylinders parallel to electrical plates, and to an external electric field tending to align them perpendicular to the plates. The Maxwell equations and self-consistent field equations are solved exactly, numerically, in real space. By comparing the free energies of different configurations, we show that for weak surface fields, the phase of cylinders parallel to the plates makes a direct transition to a phase in which the cylinders are aligned with the field throughout the sample. For stronger surface fields, there is an intermediate phase in which cylinders in the interior of the film, aligned with the field, terminate near the plates. For surface fields which favor the minority block, there is a boundary layer of hexagonal symmetry at the plates in which the monomers favored by the surface field occupy a larger area than they would if the cylinders extended to the surface.

Phase separation of saturated and mono-unsaturated lipids as determined from a microscopic model

A molecular model is proposed of a bilayer consisting of fully saturated dipalmitoylphosphatidylcholine (DPPC) and mono-unsaturated dioleoylphosphatidylcholine (DOPC). The model not only encompasses the constant density within the hydrophobic core of the bilayer, but also the tendency of chain segments to align. It is solved within self-consistent field theory. A model bilayer of DPPC undergoes a main-chain transition to a gel phase, while a bilayer of DOPC does not do so above zero degrees centigrade because of the double bond which disrupts order. We examine structural and thermodynamic properties of these membranes and find our results in reasonable accord with experiment. In particular, order-parameter profiles are in good agreement with NMR experiments. A phase diagram is obtained for mixtures of these lipids in a membrane at zero tension. The system undergoes phase separation below the main-chain transition temperature of the saturated lipid. Extensions to the ternary DPPC, DOPC, and cholesterol system are outlined.

Phase diagram of a ternary mixture of cholesterol and saturated and unsaturated lipids calculated from a microscopic model

We employ a molecular model to study a ternary mixture of saturated lipid, with tails of 16 carbons, a monounsaturated lipid with tails of 18 carbons, and cholesterol. The model, solved within mean-field theory, produces several forms of phase diagrams depending upon the relative strengths of interactions, but only one that shows the coexistence of two liquid phases observed in experiment. The lipids in the phase rich in cholesterol are more ordered than those in the other. The binary cholesterol, saturated lipid system also exhibits liquid, liquid coexistence.

Field theoretic study of bilayer membrane fusion: II. Mechanism of a stalk-hole complex

We use self-consistent field theory to determine structural and energetic properties of intermediates and transition states involved in bilayer membrane fusion. In particular, we extend our original calculations from those of the standard hemifusion mechanism, which was studied in detail in the first article of this series, to consider a possible alternative to it. This mechanism involves non-axial stalk expansion, in contrast to the axially symmetric evolution postulated in the classical mechanism. Elongation of the initial stalk facilitates the nucleation of holes and leads to destabilization of the fusing membranes via the formation of a stalk-hole complex. We study properties of this complex in detail, and show how transient leakage during fusion, previously predicted and recently observed in experiment, should vary with lipid architecture and tension. We also show that the barrier to fusion in the alternative mechanism is lower than that of the standard mechanism by a few k(B)T over most of the relevant region of system parameters, so that this alternative mechanism is a viable alternative to the standard pathway. We emphasize that any mechanism, such as this alternative one, which affects, even modestly, the line tension of a hole in a membrane, affects greatly the ability of that membrane to undergo fusion.

Field theoretic study of bilayer membrane fusion. I. Hemifusion mechanism

Self-consistent field theory is used to determine structural and energetic properties of metastable intermediates and unstable transition states involved in the standard stalk mechanism of bilayer membrane fusion. A microscopic model of flexible amphiphilic chains dissolved in hydrophilic solvent is employed to describe these self-assembled structures. We find that the barrier to formation of the initial stalk is much smaller than previously estimated by phenomenological theories. Therefore its creation it is not the rate-limiting process. The relevant barrier is associated with the rather limited radial expansion of the stalk into a hemifusion diaphragm. It is strongly affected by the architecture of the amphiphile, decreasing as the effective spontaneous curvature of the amphiphile is made more negative. It is also reduced when the tension is increased. At high tension the fusion pore, created when a hole forms in the hemifusion diaphragm, expands without bound. At very low membrane tension, small fusion pores can be trapped in a flickering metastable state. Successful fusion is severely limited by the architecture of the lipids. If the effective spontaneous curvature is not sufficiently negative, fusion does not occur because metastable stalks, whose existence is a seemingly necessary prerequisite, do not form at all. However if the spontaneous curvature is too negative, stalks are so stable that fusion does not occur because the system is unstable either to a phase of stable radial stalks, or to an inverted-hexagonal phase induced by stable linear stalks. Our results on the architecture and tension needed for successful fusion are summarized in a phase diagram.

A new mechanism of model membrane fusion determined from Monte Carlo simulation

We have carried out extensive Monte Carlo simulations of the fusion of tense apposed bilayers formed by amphiphilic molecules within the framework of a coarse-grained lattice model. The fusion pathway differs from the usual stalk mechanism. Stalks do form between the apposed bilayers, but rather than expand radially to form an axial-symmetric hemifusion diaphragm of the trans leaves of both bilayers, they promote in their vicinity the nucleation of small holes in the bilayers. Two subsequent paths are observed. 1) The stalk encircles a hole in one bilayer creating a diaphragm comprised of both leaves of the other intact bilayer, which ruptures to complete the fusion pore. 2) Before the stalk can encircle a hole in one bilayer, a second hole forms in the other bilayer, and the stalk aligns and encircles them both to complete the fusion pore. Both pathways give rise to mixing between the cis and trans leaves of the bilayer and allow for transient leakage.

[A method for worst-case study of coupling between medical equipment]

The increasing use of electronic devices in operating theatres and intensive-care units leads to possible interactions of such devices and consequently to serious immunity problems. This paper presents a numerical near field worst-case-study of the coupling between coagulators and ECG devices under consideration of the human body. For this purpose a special body model has been developed which meets the geometrical dimensions given in DIN 33402 part 2. The model's electrical properties correspond to a weighted average of those of different tissues of the human body. Different parameters (coagulation frequency, positions of electrodes, etc.) are considered.

Theory of the effect of unsaturation on the main-chain transition

We calculate the temperature of the main transition of lipids as a function of chain length for saturated lipids, and as a function of location of a cis double bond for lipids with one or two monounsaturated chains. We employ an extension of Marcelja's self-consistent field method which contains only two interaction parameters, and include many more chain orientations. Agreement with experiment on both the variation with chain length and with location of the double bond is satisfactory.

Alcohol intake, tumour necrosis factor-alpha, leptin and craving: factors of a possibly vicious circle?

AIMS

Since the appetite-regulating peptide leptin was recently found to be highly correlated with both craving for alcohol and lifetime ethanol intake, the aim of our study was to test the hypothesis whether tumour necrosis factor-alpha (TNF-alpha) might be the factor that links alcohol intake with elevated leptin levels.

METHODS

TNF-alpha, leptin, and alcohol craving were assessed in male alcohol addicts at the onset of alcohol withdrawal and in matched controls.

RESULTS

Increased leptin plasma levels in alcohol addicts correlated significantly with an enhanced secretion of TNF-alpha, which was itself related to the duration of alcohol misuse.

CONCLUSIONS

Since leptin was shown to be associated with alcohol craving, a possible vicious circle is suggested, including the components: alcohol intake, increase of TNF-alpha, enhanced leptin secretion, enhanced alcohol craving, and consecutively increased alcohol intake.