Curved single-bilayers in the region of the anomalous swelling: Effect of curvature and chain length

Energetic and Structural Insights into Phospholipid Transfer from Membranes with Different Curvatures by Time-Resolved Neutron Scattering

Understanding how lipid dynamics change with membrane curvature is important given that biological membranes constantly change their curvature and morphology through membrane fusion and endo-/exocytosis. Here, we used time-resolved small-angle neutron scattering and time-resolved fluorescence to characterize the properties and dynamics of phospholipids in vesicles with different curvatures. Dissociation of phospholipids from vesicles required traversing an energy barrier comprising positive enthalpy and negative entropy. However, lipids in membranes with high positive curvature have dense acyl chain packing and loose headgroup packing, leading to hydrophobic hydration due to water penetration into the membrane. These properties were found to lower the hydrophobic hydration enhancement associated with phospholipid dissociation and mitigate the acyl chain packing of lipids adjacent to the space created by the lipid dissociation, resulting in an increase in activation entropy. The results of this study provide important insights into the functions of biomembranes in relation to their dynamic structural changes.

Calorimetry of extracellular vesicles fusion to single phospholipid membrane

Extracellular vesicles (EVs)-mediated communication relies not only on the delivery of complex molecular cargoes as lipids, proteins, genetic material, and metabolites to their target cells but also on the modification of the cell surface local properties induced by the eventual fusion of EVs' membranes with the cells' plasma membrane. Here we applied scanning calorimetry to study the phase transition of single phospholipid (DMPC) monolamellar vesicles, investigating the thermodynamical effects caused by the fusion of doping amounts of mesenchymal stem cells-derived EVs. Specifically, we studied EVs-induced consequences on the lipids distributed in the differently curved membrane leaflets, having different density and order. The effect of EV components was found to be not homogeneous in the two leaflets, the inner (more disordered one) being mainly affected. Fusion resulted in phospholipid membrane flattening associated with lipid ordering, while the transition cooperativity, linked to membrane domains' coexistence during the transition process, was decreased. Our results open new horizons for the investigation of the peculiar effects of EVs of different origins on target cell membrane properties and functionality.

Aβ Beyond the AD Pathology: Exploring the Structural Response of Membranes Exposed to Nascent Aβ Peptide

The physiological and pathological roles of nascent amyloid beta (Aβ) monomers are still debated in the literature. Their involvement in the pathological route of Alzheimer’s Disease (AD) is currently considered to be the most relevant, triggered by their aggregation into structured oligomers, a toxic species. Recently, it has been suggested that nascent Aβ, out of the amyloidogenic pathway, plays a physiological and protective role, especially in the brain. In this emerging perspective, the study presented in this paper investigated whether the organization of model membranes is affected by contact with Aβ in the nascent state, as monomers. The outcome is that, notably, the rules of engagement and the resulting structural outcome are dictated by the composition and properties of the membrane, rather than by the Aβ variant. Interestingly, Aβ monomers are observed to favor the tightening of adjacent complex membranes, thereby affecting a basic structural event for cell-cell adhesion and cell motility.

Formation of intermediate gel-liquid crystalline phase on medium-chain phosphatidylcholine bilayers: Phase transitions depending on the bilayer packing

The bilayer phase transitions of medium-chain phosphatidylcholines with linear saturated acyl chains (C_n = 12, 13 and 14) were measured by high-pressure light-transmittance measurements and differential scanning calorimetry to investigate the formation of intermediate gel-liquid crystalline phase called L_x phase. The constructed phase diagrams showed that there existed a distinct region of the L_x phase between ripple gel (P_β') and liquid crystalline (L_α) phase for multilamellar vesicle bilayers of C12PC and C13PC. The L_x phase of the C12PC bilayer was metastable at all pressures and disappeared at a higher pressure. In the C13PC bilayer, the L_x phase was stable and also disappeared at a higher pressure but its region markedly shrunk. By contrast, the L_x phase was not detected for the C14PC bilayer. Effects of other factors such as vesicle size and solvent substitution on the L_x phase of the C13PC bilayer were also examined. A decrease in vesicle size and solvent substitution from water to 50 wt% ethylene glycol solution promoted the L_x-phase formation as opposed to the effects of acyl-chain elongation and pressurization. The fluorescence data of the C13PC bilayer with different vesicle sizes showed that the L_x phase is caused by the difference of local packing in the bilayer. Considering these facts, we concluded that the L_x phase is an intermediate gel-L_α phase that has gel-phase monolayers with negative curvature and L_α-phase monolayers with positive curvature. The formation mechanism of the L_x-phase in stacked bilayers and dispersed vesicles is also explainable by this difference in packing state.

On the Origin of the Anomalous Behavior of Lipid Membrane Properties in the Vicinity of the Chain-Melting Phase Transition

Biomembranes are key objects of numerous studies in biology and biophysics of great importance to medicine. A few nanometers thin quasi two-dimensional liquid crystalline membranes with bending rigidity of a few kT exhibit unusual properties and they are the focus of theoretical and experimental physics. The first order chain-melting phase transition of lipid membranes is observed to be accompanied by a pseudocritical behavior of membrane physical-chemical properties. However, the investigation of the nature of the anomalous swelling of a stack of lipid membranes in the vicinity of the transition by different groups led to conflicting conclusions about the level of critical density fluctuations and their impact on the membrane softening. Correspondingly, conclusions about the contribution of Helfrich's undulations to the effect of swelling were different. In our work we present a comprehensive complementary neutron small-angle and spin-echo study directly showing the presence of significant critical fluctuations in the vicinity of the transition which induce membrane softening. However, contrary to the existing paradigm, we demonstrate that the increased undulation forces cannot explain the anomalous swelling. We suggest that the observed effect is instead determined by the dominating increase of short-range entropic repulsion.

Direct comparison of elastic incoherent neutron scattering experiments with molecular dynamics simulations of DMPC phase transitions

Neutron scattering techniques have been employed to investigate 1,2-dimyristoyl-sn -glycero-3-phosphocholine (DMPC) membranes in the form of multilamellar vesicles (MLVs) and deposited, stacked multilamellar-bilayers (MLBs), covering transitions from the gel to the liquid phase. Neutron diffraction was used to characterise the samples in terms of transition temperatures, whereas elastic incoherent neutron scattering (EINS) demonstrates that the dynamics on the sub-macromolecular length-scale and pico- to nano-second time-scale are correlated with the structural transitions through a discontinuity in the observed elastic intensities and the derived mean square displacements. Molecular dynamics simulations have been performed in parallel focussing on the length-, time- and temperature-scales of the neutron experiments. They correctly reproduce the structural features of the main gel-liquid phase transition. Particular emphasis is placed on the dynamical amplitudes derived from experiment and simulations. Two methods are used to analyse the experimental data and mean square displacements. They agree within a factor of 2 irrespective of the probed time-scale, i.e. the instrument utilized. Mean square displacements computed from simulations show a comparable level of agreement with the experimental values, albeit, the best match with the two methods varies for the two instruments. Consequently, experiments and simulations together give a consistent picture of the structural and dynamical aspects of the main lipid transition and provide a basis for future, theoretical modelling of dynamics and phase behaviour in membranes. The need for more detailed analytical models is pointed out by the remaining variation of the dynamical amplitudes derived in two different ways from experiments on the one hand and simulations on the other.

Experimental aspects of colloidal interactions in mixed systems of liposome and inorganic nanoparticle and their applications

In the past few years, growing attention has been devoted to the study of the interactions taking place in mixed systems of phospholipid membranes (for instance in the form of vesicles) and hard nanoparticles (NPs). In this context liposomes (vesicles) may serve as versatile carriers or as a model system for biological membranes. Research on these systems has led to the observation of novel hybrid structures whose morphology strongly depends on the charge, composition and size of the interacting colloidal species as well as on the nature (pH, ionic strength) of their dispersing medium. A central role is played by the phase behaviour of phospholipid bilayers which have a tremendous influence on the liposome properties. Another central aspect is the incorporation of nanoparticles into vesicles, which is intimately linked to the conditions required for transporting a nanoparticle through a membrane. Herein, we review recent progress made on the investigations of the interactions in liposome/nanoparticle systems focusing on the particularly interesting structures that are formed in these hybrid systems as well as their potential applications.

Thermally gated liposomes: a closer look

A homologous series of pore-forming amphiphiles (PFAs), derived from cholic acid, lysine, and spermine, have been used as "thermal gates" for releasing sucrose from liposomes made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (sodium salt) [DPPG]. Binding measurements have established that these PFAs are fully bound to these liposomes in their gel state and that their transfer to fluid phase membranes is negligible. Release experiments have shown that thermal gating is sensitive to both the size and the concentration of the PFA that are used. Increases in the extent of release of sucrose with increasing temperature that have been found in the gel/fluid coexistence region indicate the existence of heterogeneity among the liposomes.

Reduced sterol-phospholipid recognition in curved fluid bilayers

Nearest-neighbor recognition experiments have been carried out in fluid liposomal membranes made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol using exchangeable dimers derived from 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine and cholesterol. In cholesterol-rich bilayers, the association between these two exchangeable lipids was reduced as the curvature of the membrane increases; that is, when the diameter of the liposomes was below ca. 200 nm. In sharp contrast, the mixing of these exchangeable lipids was close to random in the cholesterol-poor membranes, regardless of their curvature. The biological implications of these findings are briefly discussed.