New Aspects of Bilayer Lipid Membranes for the Analysis of Ion Channel Functions

The bilayer lipid membrane (BLM) is the main structural component of cell membranes, in which various membrane proteins are embedded. Artificially formed BLMs have been used as a platform in studies of the functions of membrane proteins, including various ion channels. In this review, we summarize recent advances that have been made on artificial BLM systems for the analysis of ion channel functions. We focus on two BLM-based systems, cell-membrane mimicry and four-terminal BLM systems. As a cell-membrane-mimicking system, an efficient screening platform for the evaluation of drug side effects that act on a cell-free synthesized channel has been developed, and its prospects for use in personalized medicine will be discussed. In the four-terminal BLMs, we introduce "lateral voltage" to BLM systems as a novel input to regulate channel activities, in addition to the traditional transmembrane voltages. Such state-of-the-art technologies and new system setups are predicted to pave the way for a variety of applications, in both fundamental physiology and in drug discovery.

Simultaneous measurement of surface and bilayer tension in a microfluidic chip

Freestanding lipid bilayers are one of the most used model systems to mimic biological cell membranes. To form an unsupported bilayer, we employ two aqueous fingers in a microfluidic chip surrounded by an oily phase that contains lipids. Upon pushing two aqueous fingers forward, their interface becomes decorated with a lipid monolayer and eventually zip to form a bilayer when the monolayers have nanoscopic contact with each other. Using this straightforward approach, the quick and easy bilayer formation is facilitated by oil draining into the microfluidic device material consisting of polydimethylsiloxane. However, the oil drainage limits the lifetime of a bilayer to about 1 h. We demonstrate that this drainage can be managed, resulting in superior bilayer stability and an increased lifetime of several hours when using a pressure-controlled system. Applying different pressures to the aqueous fingers in the microfluidic chip, the formed bilayer can even be bent to a desired curvature. Extracting the contact angle and the resulting curvature of the bilayer region, for a given applied pressure difference, both the bilayer tension and the surface tension of each lipid monolayer can be derived from a single experiment using the Young Laplace pressure equation.

Mechanically stable solvent-free lipid bilayers in nano- and micro-tapered apertures for reconstitution of cell-free synthesized hERG channels

The self-assembled bilayer lipid membrane (BLM) is the basic component of the cell membrane. The reconstitution of ion channel proteins in artificially formed BLMs represents a well-defined system for the functional analysis of ion channels and screening the effects of drugs that act on them. However, because BLMs are unstable, this limits the experimental throughput of BLM reconstitution systems. Here we report on the formation of mechanically stable solvent-free BLMs in microfabricated apertures with defined nano- and micro-tapered edge structures. The role of such nano- and micro-tapered structures on the stability of the BLMs was also investigated. Finally, this BLM system was combined with a cell-free synthesized human ether-a-go-go-related gene channel, a cardiac potassium channel whose relation to arrhythmic side effects following drug treatment is well recognized. Such stable BLMs as these, when combined with a cell-free system, represent a potential platform for screening the effects of drugs that act on various ion-channel genotypes.

Transmembrane Signaling with Lipid-Bilayer Assemblies as a Platform for Channel-Based Biosensing

Artificial and natural lipid membranes that elicit transmembrane signaling is are useful as a platform for channel-based biosensing. In this account we summarize our research on the design of transmembrane signaling associated with lipid bilayer membranes containing nanopore-forming compounds. Channel-forming compounds, such as receptor ion-channels, channel-forming peptides and synthetic channels, are embedded in planar and spherical bilayer lipid membranes to develop highly sensitive and selective biosensing methods for a variety of analytes. The membrane-bound receptor approach is useful for introducing receptor sites on both planar and spherical bilayer lipid membranes. Natural receptors in biomembranes are also used for designing of biosensing methods.

Measurement of membrane tension of free standing lipid bilayers via laser-induced surface deformation spectroscopy

Non-invasive measurement of the membrane tension of free-standing black lipid membranes (BLMs), with sensitivity on the order of μN m(-1), was achieved using laser-induced surface deformation (LISD) spectroscopy. A BLM was vertically formed via the folding method and aqueous phases with different refractive indices were added on each side in order to induce radiation pressure by a laser beam. The dynamic response of the deformed BLMs was measured under periodic intensity modulation and their tensions could be estimated. The dependence of membrane tension on the cholesterol concentration of BLMs composed of phosphatidylcholine and phosphatidylethanolamine was investigated, with the membrane tension increasing from 1.3 μN m(-1) to 68.1 μN m(-1) when the cholesterol concentration increased from zero to 33%. These tension values are much smaller than some of those previously reported, because this method does not suppress membrane fluctuation unlike other conventional methods. Our LISD system can be a promising tool for the measurement of membrane tension in BLMs.

Single channel and ensemble hERG conductance measured in droplet bilayers

The human ether-a-go-go related gene (hERG) encodes the potassium channel Kv11.1, which plays a key role in the cardiac action potential and has been implicated in cardiac disorders as well as a number of off-target pharmaceutical interactions. The electrophysiology of this channel has been predominantly studied using patch clamp, but lipid bilayers have the potential to offer some advantages, including apparatus simplicity, ease of use, and the ability to control the membrane and solution compositions. We made membrane preparations from hERG-expressing cells and measured them using droplet bilayers, allowing measurement of channel ensemble currents and 13.5 pS single channel currents. These currents were ion selective and were blockable by E-4031 and dofetilide in a dose-dependent manner, allowing determination of IC50 values of 17 nM and 9.65 μM for E-4031 and dofetilide, respectively. We also observed time- and voltage- dependent currents following step changes in applied potential that were similar to previously reported patch clamp measurements.

Pore-forming compounds as signal transduction elements for highly sensitive biosensing

Pore-forming compounds are attracting much attention due to the signal transduction ability for the development of highly sensitive biosensing. In this review, we describe an overview of the recent advances made by our group in the design of molecular sensing interfaces of spherical and planar lipid bilayers and natural bilayers. The potential uses of pore-forming compounds, such as gramicidin and MCM-41, in lipid bilayers and natural glutamate receptor channels in biomembrane are presented.