Curvature effects on lipid packing and dynamics in liposomes revealed by coarse grained molecular dynamics simulations

The molecular packing details of lipids in planar bilayers are well characterized. For curved bilayers, however, little data is available. In this paper we study the effect of temperature and membrane composition on the structural and dynamical properties of a liposomal membrane in the limit of high curvature (liposomal diameter of 15-20 nm), using coarse grained molecular dynamics simulations. Both pure dipalmitoyl phosphatidylcholine (DPPC) liposomes and binary mixtures of DPPC and either dipalmitoyl phosphatidylethanolamine (DPPE) or polyunsaturated dilinoleylphosphatidylcholine (DLiPC) lipids are modeled. We take special care in the equilibration of the liposomes requiring lipid flip-flopping, which can be facilitated by the temporary insertion of artificial pores. The equilibrated liposomes show some remarkable properties. Curvature induces membrane thinning and reduces the thermal expansivity of the membrane. In the inner monolayer the lipid head groups are very closely packed and dehydrated, and the lipids tails relatively disordered. The opposite packing effects are seen in the outer monolayer. In addition, we noticed an increased tendency of the lipid tails to backfold toward the interface in the outer monolayer. The distribution of lipids over the monolayers was found to be strongly temperature dependent. Higher temperatures favor more equally populated monolayers. Relaxation times of the lipid tails were found to increase with increasing curvature, with the lipid tails in the outer monolayer showing a significant slower dynamics compared to the lipid tails in the inner monolayer. In the binary systems there is a clear tendency toward partial transversal demixing of the two components, with especially DPPE enriched in the inner monolayer. This observation is in line with a static shape concept which dictates that inverted-cone shaped lipids such as DPPE and DLiPC would prefer the concave volume of the inner monolayer. However, our results for DLiPC show that another effect comes into play that is almost equally strong and provides a counter-acting driving force toward the outer, rather than the inner monolayer. This effect is the ability of the polyunsaturated tails of DLiPC to backfold, which is advantageous in the outer monolayer. We speculate that polyunsaturated lipids in biological membranes may play an important role in stabilizing both positive and negative regions of curvature.</p

The 2018 biomembrane curvature and remodeling roadmap

The importance of curvature as a structural feature of biological membranes has been recognized for many years and has fascinated scientists from a wide range of different backgrounds. On the one hand, changes in membrane morphology are involved in a plethora of phenomena involving the plasma membrane of eukaryotic cells, including endo-and exocytosis, phagocytosis and filopodia formation. On the other hand, a multitude of intracellular processes at the level of organelles rely on generation, modulation, and maintenance of membrane curvature to maintain the organelle shape and functionality. The contribution of biophysicists and biologists is essential for shedding light on the mechanistic understanding and quantification of these processes. Given the vast complexity of phenomena and mechanisms involved in the coupling between membrane shape and function, it is not always clear in what direction to advance to eventually arrive at an exhaustive understanding of this important research area. The 2018 Biomembrane Curvature and Remodeling Roadmap of Journal of Physics D: Applied Physics addresses this need for clarity and is intended to provide guidance both for students who have just entered the field as well as established scientists who would like to improve their orientation within this fascinating area

A tethering complex drives the terminal stage of SNARE-dependent membrane fusion.

Membrane fusion in eukaryotic cells mediates the biogenesis of organelles, vesicular traffic between them, and exo- and endocytosis of important signalling molecules, such as hormones and neurotransmitters. Distinct tasks in intracellular membrane fusion have been assigned to conserved protein systems. Tethering proteins mediate the initial recognition and attachment of membranes, whereas SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein complexes are considered as the core fusion engine. SNARE complexes provide mechanical energy to distort membranes and drive them through a hemifusion intermediate towards the formation of a fusion pore. This last step is highly energy-demanding. Here we combine the in vivo and in vitro fusion of yeast vacuoles with molecular simulations to show that tethering proteins are critical for overcoming the final energy barrier to fusion pore formation. SNAREs alone drive vacuoles only into the hemifused state. Tethering proteins greatly increase the volume of SNARE complexes and deform the site of hemifusion, which lowers the energy barrier for pore opening and provides the driving force. Thereby, tethering proteins assume a crucial mechanical role in the terminal stage of membrane fusion that is likely to be conserved at multiple steps of vesicular traffic. We therefore propose that SNAREs and tethering proteins should be considered as a single, non-dissociable device that drives fusion. The core fusion machinery may then be larger and more complex than previously thought

The freezing process of small lipid vesicles at molecular resolution

At present very little is known about the kinetic barriers which a small vesicle will face during the transformation from the liquid-crystalline to the gel phase, and what the structure of frozen vesicles looks like at the molecular level. The formation of gel domains in the strongly curved bilayer of a small vesicle seems almost paradoxical and is expected to involve large structural reorganizations. In this work we use coarse-grained molecular dynamics simulations to study the kinetic and structural aspects of gel domain formation in small lipid vesicles, specifically dipalmitoylphosphatidylcholine (DPPC) vesicles with a diameter range of 20–40 nm. We observe that cooling of such vesicles below the phase transition temperature does not result in gel phase formation on a microsecond time scale, which we attribute to the presence of an effective area constraint. This area constraint is due to the strongly reduced membrane permeability at lower temperatures, preventing the rapid efflux of water and the required decrease in membrane area to form a gel phase. Control simulations with lamellar bilayers, simulated at fixed area, show that gel phase formation is indeed only possible below a certain threshold area. The effect of lipid asymmetry was also studied with the lamellar setup, and found to be of less importance. To circumvent the kinetic barrier imposed by the effective area constraint of the liposomes, i.e. to mimic the long time behavior, we introduce artificial pores in the membrane facilitating the solvent efflux. In this case, spontaneous gel domains are formed. We identify several stages during the microsecond-long transformation, finally resulting in strongly deformed or ruptured vesicles entirely in the gel state.

The molecular face of lipid rafts in model membranes

Cell membranes contain a large number of different lipid species. Such a multicomponent mixture exhibits a complex phase behavior with regions of structural and compositional heterogeneity. Especially domains formed in ternary mixtures, composed of saturated and unsaturated lipids together with cholesterol, have received a lot of attention as they may resemble raft formation in real cells. Here we apply a simulation model to assess the molecular nature of these domains at the nanoscale, information that has thus far eluded experimental determination. We are able to show the spontaneous separation of a saturated phosphatidylcholine (PC)/unsaturated PC/cholesterol mixture into a liquid-ordered and a liquid-disordered phase with structural and dynamic properties closely matching experimental data. The near-atomic resolution of the simulations reveals remarkable features of both domains and the boundary domain interface. Furthermore, we predict the existence of a small surface tension between the monolayer leaflets that drives registration of the domains. At the level of molecular detail, raft-like lipid mixtures show a surprising face with possible implications for many cell membrane processes

Curvature-Dependent Elastic Properties of Liquid-Ordered Domains Result in Inverted Domain Sorting on Uniaxially Compressed Vesicles

Using a coarse-grained molecular model we study the spatial distribution of lipid domains on a 20-nm-sized vesicle. The lipid mixture laterally phase separates into a raftlike, liquid-ordered (lo) phase and a liquid-disordered phase. As we uniaxially compress the mixed vesicle keeping the enclosed volume constant, we impart tension onto the membrane. The vesicle adopts a barrel shape, which is composed of two flat contact zones and a curved edge. The lo domain, which exhibits a higher bending rigidity, segregates to the highly curved edge. This inverted domain sorting switches to normal domain sorting, where the lo domain prefers the flat contact zone, when we release the contents of the vesicle. We rationalize this domain sorting by a pronounced reduction of the bending rigidity and area compressibility of the lo phase upon bending.

Application of Mean Field Boundary Potentials in Simulations of Lipid Vesicles

A method is presented to enhance the efficiency of simulations of lipid vesicles. The method increases computational speed by eliminating water molecules that either surround the vesicle or reside in the interior of the vesicle, without altering the properties of the water at the membrane interface. Specifically, mean field force approximation (MFFA) boundary potentials are used to replace both the internal and external excess bulk solvent. In addition to reducing the cost of simulating preformed vesicles, the molding effect of the boundary potentials also enhances the formation and equilibration of vesicles from random solutions of lipid in water. Vesicles with diameters in the range from 20 to 60 nm were obtained on a nanosecond time scale, without any noticeable effect of the boundary potentials on their structure.

Stability of Asymmetric Lipid Bilayers Assessed by Molecular Dynamics Simulations

The asymmetric insertion of amphiphiles into biological membranes compromises the balance between the inner and outer monolayers. As a result, area expansion of the receiving leaflet and curvature strain may lead to membrane permeation, shape changes, or membrane fusion events. We have conducted both atomistic and coarse-grained molecular dynamics simulations of dipalmitoyl-phosphatidylcholine (DPPC) bilayers to study the effect of an asymmetric distribution of lipids between the two monolayers on membrane stability. Highly asymmetric lipid bilayers were found to be surprisingly stable within the submicrosecond time span of the simulations. Even the limiting case of a monolayer immersed in water ruptured spontaneously only after at least 20 ns simulation. A thermal shock could destabilize these kinetically trapped states. We also studied mixed systems composed of DPPC and short tail diC8PC lipids, showing that the presence of the cone-shaped short tail lipid facilitates the release of tension in the asymmetric systems via formation of a transmembrane pore. Thus, asymmetric area expansion and curvature stress cooperate to yield bilayer disruption. It appears that, although asymmetric area expansion destabilizes the bilayer structure, the activation energy for transmonolayer lipid re-equilibration is increased. Such a large kinetic barrier can be reduced by lipids with positive spontaneous curvature. These effects are important at the onset of bilayer destabilization phenomena, such as lipid pore formation and membrane fusion, and should be considered for the mechanism of induction of such processes by peptides and proteins.