Effect of osmotic stress on membrane fusion on solid substrate

Vesicle fusion and release in neurons under dynamic mechanical equilibrium

Vesicular fusion plays a pivotal role in cellular processes, involving stages like vesicle trafficking, fusion pore formation, content release, and membrane integration or separation. This dynamic process is regulated by a complex interplay of protein assemblies, osmotic forces, and membrane tension, which together maintain a mechanical equilibrium within the cell. Changes in cellular mechanics or external pressures prompt adjustments in this equilibrium, highlighting the system's adaptability. This review delves into the synergy between intracellular proteins, structural components, and external forces in facilitating vesicular fusion and release. It also explores how cells respond to mechanical stress, maintaining equilibrium and offering insights into vesicle fusion mechanisms and the development of neurological disorders.

Suspended phospholipid bilayers: A new biological membrane mimetic

HYPOTHESIS

The attractive interaction between a cationic surfactant monolayer at the air-water interface and vesicles, incorporating anionic lipids, is sufficient to drive the adsorption and deformation of the vesicles. Osmotic rupture of the vesicles produces a continuous lipid bilayer beneath the monolayer.

EXPERIMENTAL

Specular neutron reflectivity has been measured from the surface of a purpose-built laminar flow trough, which allows for rapid adsorption of vesicles, the changes in salt concentration required for osmotic rupture of the adsorbed vesicles into a bilayer, and for neutron contrast variation of the sub-phase without disturbing the monolayer.

FINDINGS

The neutron reflectivity profiles measured after vesicle addition are consistent with the adsorption and flattening of the vesicles beneath the monolayer. An increase in the buffer salt concentration results in further flattening and fusion of the adsorbed vesicles, which are ruptured by a subsequent decrease in the salt concentration. This process results in a continuous, high coverage, bilayer suspended 11 Åbeneath the monolayer. As the bilayer is not constrained by a solid substrate, this new mimetic is well-suited to studying the structure of lipid bilayers that include transmembrane proteins.

Membrane fusion of phospholipid bilayers under high pressure: Spherical and irreversible growth of giant vesicles

Membrane fusion of giant vesicles (GVs) for binary bilayers of unsaturated phospholipids, dioleoylphosphatidyl-ethanolamine (DOPE) having an ability to promote membrane fusion, and its homolog dioleoylphosphatidylcholine (DOPC) having an ability to form GV, was investigated under atmospheric and high pressure. While DOPC formed GVs in the presence of inorganic salts with a multivalent metal ion under atmospheric pressure, an equimolar mixture of DOPE and DOPC formed GVs both in the absence and the presence of LaCl₃. We examined the change in size and shape of the GVs of this binary mixture in the absence and presence of LaCl₃ as a function of time under atmospheric and high pressure. The size and shape of the GVs in the absence of LaCl₃ under atmospheric and high pressure and those in the presence of LaCl₃ under atmospheric pressure hardly changed with time. By contrast, the GV in the presence of LaCl₃ under high pressure gradually changed in the size and shape with time on a time scale of several hours. Namely, the GV became larger than the original GV due to accelerated membrane fusion and its shape became more spherical. This pressure-induced membrane fusion was completely irreversible, and the growth rate was correlated with the applied pressure. The reason for the GV growth by applying pressure was considered on the basis of thermodynamic phase diagrams. We concluded that the growth is attributable to a closer packing of lipid molecules in the bilayer resulting from their preference of smaller volumes under high pressure. Furthermore, the molecular mechanism of the pressure-induced membrane fusion was explored by observing the fusion of two GVs with almost the same size. From their morphological changes, we revealed that the fusion is caused by the actions of Laplace and osmotic pressure.

Quantitative Profiling of Nanoscale Liposome Deformation by a Localized Surface Plasmon Resonance Sensor

Characterizing the shape of sub-100 nm, biological soft-matter particulates (e.g., liposomes and exosomes) adsorbed at a solid-liquid interface remains a challenging task. Here, we introduce a localized surface plasmon resonance (LSPR) sensing approach to quantitatively profile the deformation of nanoscale, fluid-phase 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes contacting a titanium dioxide substrate. Experimental and theoretical results validate that, due to its high sensitivity to the spatial proximity of phospholipid molecules near the sensor surface, the LSPR sensor can discriminate fine differences in the extent of ionic strength-modulated liposome deformation at both low and high surface coverages. By contrast, quartz crystal microbalance-dissipation (QCM-D) measurements performed with equivalent samples were qualitatively sensitive to liposome deformation only at saturation coverage. Control experiments with stiffer, gel-phase 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes verified that the LSPR measurement discrimination arises from the extent of liposome deformation, while the QCM-D measurements yield a more complex response that is also sensitive to the motion of adsorbed liposomes and coupled solvent along with lateral interactions between liposomes. Collectively, our findings demonstrate the unique measurement capabilities of LSPR sensors in the area of biological surface science, including competitive advantages for probing the shape properties of adsorbed, nanoscale biological particulates.

Influence of Divalent Cations on Deformation and Rupture of Adsorbed Lipid Vesicles

The fate of adsorbed lipid vesicles on solid supports depends on numerous experimental parameters and typically results in the formation of a supported lipid bilayer (SLB) or an adsorbed vesicle layer. One of the poorly understood questions relates to how divalent cations appear to promote SLB formation in some cases. The complexity arises from the multiple ways in which divalent cations affect vesicle-substrate and vesicle-vesicle interactions as well as vesicle properties. These interactions are reflected, e.g., in the degree of deformation of adsorbed vesicles (if they do not rupture). It is, however, experimentally challenging to measure the extent of vesicle deformation in real-time. Herein, we investigated the effect of divalent cations (Mg(2+), Ca(2+), Sr(2+)) on the adsorption of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid vesicles onto silicon oxide- and titanium oxide-coated substrates. The vesicle adsorption process was tracked using the quartz crystal microbalance-dissipation (QCM-D) and localized surface plasmon resonance (LSPR) measurement techniques. On silicon oxide, vesicle adsorption led to SLB formation in all cases, while vesicles adsorbed but did not rupture on titanium oxide. It was identified that divalent cations promote increased deformation of adsorbed vesicles on both substrates and enhanced rupture on silicon oxide in the order Ca(2+) > Mg(2+) > Sr(2+). The influence of divalent cations on different factors in these systems is discussed, clarifying experimental observations on both substrates. Taken together, the findings in this work offer insight into how divalent cations modulate the interfacial science of supported membrane systems.

Preservation of Supported Lipid Membrane Integrity from Thermal Disruption: Osmotic Effect

Preservation of structural integrity under various environmental conditions is one major concern in the development of the supported lipid membrane (SLM)-based devices. It is common for SLMs to experience temperature shifts from manufacture, processing, storage, and transport to operation. In this work, we studied the thermal adaption of the supported membranes on silica substrates. Homogenous SLMs with little defects were formed through the vesicle fusion method. The mass and fluidity of the bilayers were found to deteriorate from a heating process but not a cooling process. Fluorescence characterizations showed that the membranes initially budded as a result of heating-induced lipid lateral area expansion, followed by the possible fates including maintenance, retraction, and fission, among which the last contributes to the irreversible compromise of the SLM integrity and spontaneous release of the interlipid stress accumulated. Based on the mechanism, we developed a strategy to protect SLMs from thermal disruption by increasing the solute concentration in medium. An improved preservation of the membrane mass and fluidity against the heating process was observed, accompanied by a decrease in the retraction and fission of the buds. Theoretical analysis revealed a high osmotic energy penalty for the fission, which accounts for the depressed disruption. This osmotic-based protection strategy is facile, solute nonspecific, and long-term efficient and has little impact on the original SLM properties. The results may help broaden SLM applications and sustain the robustness of SLM-based devices under multiple thermal conditions.

Thickness dependent effective viscosity of a polymer solution near an interface probed by a quartz crystal microbalance with dissipation method

The solution viscosity near an interface, which affects the solution behavior and the molecular dynamics in the solution, differs from the bulk. This paper measured the effective viscosity of a dilute poly (ethylene glycol) (PEG) solution adjacent to a Au electrode using the quartz crystal microbalance with dissipation (QCM-D) technique. We evidenced that the effect of an adsorbed PEG layer can be ignored, and calculated the zero shear rate effective viscosity to remove attenuation of high shear frequency oscillations. By increasing the overtone n from 3 to 13, the thickness of the sensed polymer solution decreased from ~70 to 30 nm. The zero shear rate effective viscosity of the polymer solution and longest relaxation time of PEG chains within it decrease with increasing solution thickness. The change trends are independent of the relation between the apparent viscosity and shear frequency and the values of the involved parameter, suggesting that the polymer solution and polymer chains closer to a solid substrate have a greater effective viscosity and slower relaxation mode, respectively. This method can study the effect of an interface presence on behavior and phenomena relating to the effective viscosity of polymer solutions, including the dynamics of discrete polymer chains.

Comparison of the different responses of surface plasmon resonance and quartz crystal microbalance techniques at solid–liquid interfaces under various experimental conditions

The different characteristics of surface plasmon resonance and quartz crystal microbalance techniques under different experimental scenarios are discussed.

Shear-induced membrane fusion in viscous solutions

Large unilamellar lipid vesicles do not normally fuse under fluid shear stress. They might deform and open pores to relax the tension to which they are exposed, but membrane fusion occurring solely due to shear stress has not yet been reported. We present evidence that shear forces in a viscous solution can induce lipid bilayer fusion. The fusion of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes is observed in Couette flow with shear rates above 3000 s(-1) provided that the medium is viscous enough. Liposome samples, prepared at different viscosities using a 0-50 wt % range of sucrose concentration, were studied by dynamic light scattering, lipid fusion assays using Förster resonance energy transfer (FRET), and linear dichroism (LD) spectroscopy. Liposomes in solutions with 40 wt % (or more) sucrose showed lipid fusion under shear forces. These results support the hypothesis that under suitable conditions lipid membranes may fuse in response to mechanical-force-induced stress.

Influence of osmotic pressure on adhesion of lipid vesicles to solid supports

The adhesion of lipid vesicles to solid supports represents an important step in the molecular self-assembly of model membrane platforms. A wide range of experimental parameters are involved in controlling this process, including substrate material and topology, lipid composition, vesicle size, solution pH, ionic strength, and osmotic pressure. At present, it is not well understood how the magnitude and direction of the osmotic pressure exerted on a vesicle influence the corresponding adsorption kinetics. In this work, using quartz crystal microbalance with dissipation (QCM-D) monitoring, we have experimentally studied the role of osmotic pressure in the adsorption of zwitterionic vesicles onto silicon oxide. The osmotic pressure was induced by changing the ionic strength of the solvent across an appreciably wider range (from 25 to 1000 mM NaCl outside of the vesicle, and 125 mM NaCl inside of the vesicle, unless otherwise noted) compared to that used in earlier works. Our key finding is demonstration that, by changing osmotic pressure, all three generic types of the kinetics of vesicle adsorption and rupture can be observed in one system, including (i) adsorption of intact vesicles, (ii) adsorption and rupture after reaching a critical vesicle coverage, and (iii) rupture just after adsorption. Furthermore, theoretical analysis of pressure-induced deformation of adsorbed vesicles and a DLVO-type analysis of the vesicle-substrate interaction qualitatively support our observations. Taken together, the findings in this work demonstrate that osmotic pressure can either promote or impede the rupture of adsorbed vesicles on silicon oxide, and offer experimental evidence to support adhesion energy-based models that describe the adsorption and spontaneous rupture of vesicles on solid supports.