Substrate-Induced Structure and Molecular Dynamics in a Lipid Bilayer Membrane

Kinetics of Ganglioside-Rich Supported Lipid Bilayer Formation with Tracer Vesicle Fluorescence Imaging

Gangliosides, forming a class of lipids complemented by sugar chains, influence the lateral distribution of membrane proteins or membrane-binding proteins, act as receptors for viruses and bacterial toxins, and mediate several types of cellular signaling. Gangliosides incorporated into supported lipid bilayers (SLBs) have been widely applied as a model system to examine these biological processes. In this work, we explored how ganglioside composition affects the kinetics of SLB formation using the vesicle rupturing method on a solid surface. We imaged the attachment of vesicles and the subsequent SLB formation using the time-lapse total internal reflection fluorescence microscopy technique. In the early phase, the ganglioside type and concentration influence the adsorption kinetics of vesicles and their residence/lifetime on the surface before rupturing. Our data confirm that a simultaneous rupturing of neighboring surface-adsorbed vesicles forms microscopic lipid patches on the surface and it is triggered by a critical coverage of the vesicles independent of their composition. In the SLB growth phase, lipid patches merge, forming a continuous SLB. The propagation of patch edges catalyzes the process and depends on the ganglioside type. Our pH-dependent experiments confirm that the polar/charged head groups of the gangliosides have a critical role in these steps and phases of SLB formation kinetics.

PDMS as a Substrate for Lipid Bilayers

PDMS (polydimethylsiloxane) is a cheap, optically clear polymer that is elastic and can be easily and quickly fabricated into a wide array of microscale and nanoscale architectures, making it a versatile substrate for biophysical experiments on cell membranes. It is easy to imagine many new experiments will be devised that require a bilayer to be placed upon a substrate that is flexible or easily cast into a desired geometry, such as in lab-on-a-chip, organ-on-chip, and microfluidic applications, or for building accurate membrane models that replicate the surface structure and elasticity of the cytoskeleton. However, PDMS has its limitations, and the extent to which the behavior of membranes is affected on PDMS has not been fully explored. We use AFM and fluorescence optical microscopy to investigate the use of PDMS as a substrate for the formation and study of supported lipid bilayers (SLBs). Lipid bilayers form on plasma-treated PDMS and show free diffusion and normal phase transitions, confirming its suitability as a model bilayer substrate. However, lipid-phase separation on PDMS is severely restricted due to the pinning of domains to surface roughness, resulting in the cessation of lateral hydrodynamic flow. We show the high-resolution porous structure of PDMS and the extreme smoothing effect of oxygen plasma treatment used to hydrophilize the surface, but this is not flat enough to allow domain formation. We also observe bilayer degradation over hour timescales, which correlates with the known hydrophobic recovery of PDMS, and establish a critical water contact angle of 30°, above which bilayers degrade or not form at all. Care must be taken as incomplete surface oxidation and hydrophobic recovery result in optically invisible membrane disruption, which will also be transparent to fluorescence microscopy and lipid diffusion measurements in the early stages.

Domain Localization by Graphene Oxide in Supported Lipid Bilayers

The gel-phase domains in a binary supported lipid bilayer (SLB) comprising dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC) were localized on graphene oxide (GO) deposited on a SiO₂/Si substrate. We investigated the distribution of the gel-phase domains and the liquid crystalline (L_α) phase regions in DOPC+DPPC-SLB on thermally oxidized SiO₂/Si substrates with GO flakes to understand the mechanism of the domain localization on GO. Fluorescence microscopy and atomic force microscopy revealed that the gel-phase domains preferably distributed on GO flakes, whereas the fraction of the L_α-phase increased on the bare SiO₂ surface which was not covered with the GO flakes. The gel-phase domain was condensed on GO more effectively at the lower cooling rate. We propose that nucleation of the gel-phase domain preferentially occurred on GO, whose surface has amphiphilic property, during the gel-phase domain formation. The domains of the liquid ordered (L_o) phase were also condensed on GO in a ternary bilayer containing cholesterol that was phase-separated to the L_o phase and the liquid disordered phase. Rigid domains segregates on GO during their formation process, leaving fluid components to the surrounding region of GO.

Characterization of Phase Separated Planar Lipid Bilayer Membrane by Fluorescence Ratio Imaging and Scanning Probe Microscope

The lipid membrane forms nanodomains (rafts) and shows heterogeneous properties. These nanodomains relate to significant roles in various cell functions, and thus the analysis of the nanodomains in phase-separated lipid membranes is important to clarify the function and role of the nanodomains. However, the lipid membrane possesses small-sized nanodomains and shows a small height difference between the nanodomains and their surroundings at certain lipid compositions. In addition, nanodomain analysis sometimes requires highly sensitive and expensive apparatus, such as a two-photon microscope. These have prevented the analysis by the conventional fluorescence microscope and by the topography of the scanning probe microscope (SPM), even though these are promising methods in macroscale and microscale analysis, respectively. Therefore, this study aimed to overcome these problems in nanodomain analysis. We successfully demonstrated that solvatochromic dye, LipiORDER, could analyze the phase state of the lipid membrane at the macroscale with low magnification lenses. Furthermore, we could prove that the phase mode of SPM was effective in the visualization of specific nanodomains by properties difference as well as topographic images of SPM. Hence, this combination method successfully gave much information on the phase state at the micro/macro scale, and thus this would be applied to the analysis of heterogeneous lipid membranes.

Quenching Efficiency of Quantum Dots Conjugated to Lipid Bilayers on Graphene Oxide Evaluated by Fluorescence Single Particle Tracking

A single particle observation of quantum dots (QDs) was performed on lipid bilayers formed on graphene oxide (GO). The long-range fluorescence quenching of GO has been applied to biosensing for various biomolecules. We demonstrated the single particle observation of a QD on supported lipid bilayers in this study, aiming to detect the quenching efficiency of lipid and protein molecules in a lipid bilayer by fluorescence single particle tacking (SPT). A single lipid bilayer or double lipid bilayers were formed on GO flakes deposited on a thermally oxidized silicon substrate by the vesicle fusion method. The QDs were conjugated on the lipid bilayers, and single particle images of the QDs were obtained under the quenching effect of GO. The quenching efficiency of a single QD was evaluated from the fluorescence intensities on the regions with and without GO. The quenching efficiency reflecting the layer numbers of the lipid bilayers was obtained.

Physical Properties and Reactivity of Microdomains in Phosphatidylinositol-Containing Supported Lipid Bilayer

We characterized the size, distribution, and fluidity of microdomains in a lipid bilayer containing phosphatidylinositol (PI) and revealed their roles during the two-dimensional assembly of a membrane deformation protein (FBP17). The morphology of the supported lipid bilayer (SLB) consisting of PI and phosphatidylcholine (PC) on a mica substrate was observed with atomic force microscope (AFM). Single particle tracking (SPT) was performed for the PI+PC-SLB on the mica substrate by using the diagonal illumination setup. The AFM topography showed that PI-derived submicron domains existed in the PI+PC-SLB. The spatiotemporal dependence of the lateral lipid diffusion obtained by SPT showed that the microdomain had lower fluidity than the surrounding region and worked as the obstacles for the lipid diffusion. We observed the two-dimensional assembly of FBP17, which is one of F-BAR family proteins included in endocytosis processes and has the function generating lipid bilayer tubules in vitro. At the initial stage of the FBP17 assembly, the PI-derived microdomain worked as a scaffold for the FBP17 adsorption, and the fluid surrounding region supplied FBP17 to grow the FBP17 domain via the lateral molecular diffusion. This study demonstrated an example clearly revealing the roles of two lipid microregions during the protein reaction on a lipid bilayer.

Cholesterol-induced microdomain formation in lipid bilayer membranes consisting of completely miscible lipids

Recently, we reported that a ternary lipid bilayer comprising phosphatidylethanolamine (PE), phosphatidylcholine (PC), which were both derived from chicken egg, and cholesterol (Chol) generates microdomains that function as specific fusion sites for proteoliposomes. Chol-induced microdomain formation in a completely miscible lipid bilayer is an exceptional phenomenon. Numerous studies have elucidated the formation of domains in liquid ordered (L_o) and liquid disordered (L_d) phases of ternary bilayers, which comprise two partially miscible lipids and Chol. Herein, we investigated the composition and mechanism of formation of these unique microdomains in supported lipid bilayers (SLBs) using a fluorescence microscope and an atomic force microscope (AFM). We prepared ternary SLBs using egg-derived PC (eggPC), Chol and three different types of PE: egg-derived PE, 1-palmitoyl-2-oleoyl-PE, and 1,2-didocosahexaenoyl-PE (diDHPE). Fluorescence microscopy observations revealed that fluid and continuous SLBs were formed at PE concentrations (C_PE) of ≥6 mol%. Fluorescence recovery after photobleaching measurement revealed that the microdomain was more fluid than the surrounding region that showed typical diffusion coefficient of the L_o phase. The microdomains were observed as depressions in the AFM topographies. Their area fraction (θ) increased with C_PE, and diDHPE produced a significantly large θ among the three PEs. The microdomains in the PE+eggPC+Chol-SLBs were rich in polyunsaturated PE and were in the L_d-like phase. Associating eggPC and Chol caused polyunsaturated PE to segregate, resulting in a microdomain formation by conferring the umbrella effect on Chol, entropic effect of disordered acyl chains, and π-π interactions in the hydrophobic core.

Nanoscale Substrate Roughness Hinders Domain Formation in Supported Lipid Bilayers

Supported lipid bilayers are model membranes formed at solid substrate surfaces. This architecture renders the membrane experimentally accessible to surface-sensitive techniques used to study their properties, including atomic force microscopy, optical fluorescence microscopy, quartz crystal microbalance, and X-ray/neutron reflectometry, and allows integration with technology for potential biotechnological applications such as drug screening devices. The experimental technique often dictates substrate choice or treatment, and it is anecdotally recognized that certain substrates are suitable for a particular experiment, but the exact influence of the substrate has not been comprehensively investigated. Here, we study the behavior of a simple model bilayer, phase-separating on a variety of commonly used substrates, including glass, mica, silicon, and quartz, with drastically different results. The distinct micron-scale domains observed on mica, identical to those seen in free-floating giant unilamellar vesicles, are reduced to nanometer-scale domains on glass and quartz. The mechanism for the arrest of domain formation is investigated, and the most likely candidate is nanoscale surface roughness, acting as a drag on the hydrodynamic motion of small domains during phase separation. Evidence was found that the physicochemical properties of the surface have a mediating effect, most likely because of the changes in the lubricating interstitial water layer between the surface and bilayer.

Selective Isotopic Labeling Resolves the Gel-to-Fluid Phase Transitions of the Individual Leaflets of a Planar-Supported Phospholipid Bilayer

Knowledge of the thermotropic phase behavior of solid-supported bilayer lipid assemblies is essential to mimick the molecular organization and lateral fluidity of cell membranes. The gel-to-fluid phase transitions in a homologous series of single phospholipid bilayers supported on planar silicon substrates were investigated by temperature-controlled atomic force microscopy and attenuated total reflection infrared spectroscopy to obtain complementary information at the mesoscopic and molecular scales. Symmetric bilayers of dipalmitoylphosphatidylcholine (DPPC) and vertically asymmetric bilayers composed of a leaflet of DPPC and another of acyl-chain-deuterated DPPC (DPPC-d₆₂) were prepared by the Langmuir-Blodgett technique. The selective deuteration of one of the bilayer leaflets enabled the simultaneous monitoring by IR spectroscopy of the acyl chain melting in each leaflet via the spectrally isolated CH₂ and CD₂ stretching vibrations. Two gel-to-fluid transitions were discerned for both the symmetric and asymmetric bilayers in ultrapure water. The deuterium isotope effect observed in free-standing membranes was maintained for the supported bilayers. IR spectroscopy revealed that the melting of one leaflet promotes the disordering of the acyl chains in the adjacent one. The findings suggest that the two leaflet phase transitions do not evolve in isolation. This work sheds insight into the nature of leaflet-leaflet interactions and the thermodynamic properties of surface-confined phospholipid bilayers.

Charge and Coordination Directed Liposome Fusion onto SiO2 and TiO2 Nanoparticles

TiO₂ and SiO₂ are very useful materials for building biointerfaces. A particularly interesting aspect is their interaction with lipid bilayers. Many past research efforts focused on phosphocholine (PC) lipids, which form supported lipid bilayers (SLB) on SiO₂ at physiological conditions but are adsorbed as intact liposomes on TiO₂. Low pH was required to form PC SLBs on TiO₂. This work intends to understand the surface forces and chemistry responsible for such differences. Two charge neutral lipids: 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC) and 2-((2,3-bis(oleoyloxy)propyl)dimethylammonio)ethyl ethyl phosphate (DOCPe) and two negatively charged lipids: 1,2-dioleoyl- sn-glycero-3-phospho-l-serine (DOPS) and 2-((2,3-bis(oleoyloxy)propyl)dimethylammonio)ethyl hydrogen phosphate (DOCP) were used. Using calcein leakage assays, adsorption measurement, cryo-TEM, and washing, we concluded that charge is the dominating factor on SiO₂. The two neutral lipids form SLB on SiO₂ at pH 3 and 7, but the two negatively charged ones cannot form. On TiO₂, both charge and coordination chemistry are important. The two anionic lipids formed SLB from pH 3 to 10. DOCP had stronger affinity than DOPS likely due to the tighter terminal phosphate binding of the former. The two neutral liposomes formed SLB only at pH 3, where phosphate interaction and van der Waals force are deemed important. The pH 3 prepared TiO₂ DOPC SLBs are destabilized at neutral pH, indicating the reversible nature of the interaction. This work has provided new insights into two important materials interacting with common liposomes, which are important for reproducible biosensing, device fabrication, and drug delivery applications.

Morphology and Physical Properties of Hydrophilic-Polymer-Modified Lipids in Supported Lipid Bilayers

Lipid molecules such as glycolipids that are modified with hydrophilic biopolymers participate in the biochemical reactions occurring on cell membranes. Their functions and efficiency are determined by the formation of microdomains and their physical properties. We investigated the morphology and properties of domains induced by the hydrophilic-polymer-modified lipid applying a polyethylene glycol (PEG)-modified lipid as a model modified lipid. We formed supported lipid bilayers (SLBs) using a 0-10 mol % range of PEG-modified lipid concentration ( C_PEG). We studied their morphology and fluidity by fluorescence microscopy, the fluorescence recovery after photobleaching method, and atomic force microscopy (AFM). Fluorescence images showed that domains rich in the PEG-modified lipid appeared and SLB fluidity decreased when C_PEG ≥ 5%. AFM topographies showed that clusters of the PEG-modified lipid appeared prior to domain formation and the PEG-lipid-rich domains were observed as depressions. Frequency-modulation AFM revealed a force-dependent appearance of the PEG-lipid-rich domain.