Simulation and modeling of artificial bilayer lipid membranes under the voltage-clamp conditions

抄録

Introduction

All cells in our body are covered by cell membranes. Membrane proteins in cell membranes are essential for biological activities because they are responsible for signal transduction through the membranes. The lipid bilayer, which is the basic structure of the cell membrane, can be artificially formed and is widely used as a model cell membrane system. When isolated or synthesized membrane proteins are embedded in the lipid bilayer, this membrane will be a useful functional analysis system for various membrane proteins. We have reported stable artificial lipid bilayer and proposed them as a membrane platform for evaluating the functions of the membrane proteins, especially ion channels. Recently, we have constructed a membrane system in which a lateral voltage can be applied in addition to a conventional transmembrane voltage. The lateral voltage effectively enhanced the transmembrane current in ion-channel-incorporated lipid bilayer systems [1]. However, the working mechanism of the lateral voltage to increase the channel activities has not yet been clarified. In this study, we constructed a simulation model of an artificial membrane platform using continuum modeling and simulated the effect of the lateral voltage on the membrane properties of lipid bilayer.

Methods

An artificial bilayer lipid membrane is formed across a microaperture in a 12 µm-thick Teflon film. In order to apply the lateral voltage inside the lipid bilayer, two titanium (Ti) electrodes are deposited around the aperture, and a protective SiO₂ layer is formed on the surface of the electrodes. The diameter of the microaperture is 80-150 µm. The thickness of Ti electrodes and SiO₂ insulating layers is 200 nm and 300 nm, respectively. We modeled the artificial lipid bilayer platform using the finite element analysis software COMSOL Multiphysics 5.4. To describe the processes in this system, we used two physics modules of electrostatic and transport of diluted species which are described by the Poisson equation and Nernst-Planck equation, respectively.

Results and Discussion

First, we constructed a model in which a transmembrane voltage is applied to a bilayer lipid membrane of 5 nm thick and 200 nm wide. Both sides of the membrane were filled with KCl buffer (102.5 nm long and 200 nm wide). The relative permittivity of the bilayer lipid membrane and KCl buffer was 2.2 and 75, respectively. A potential of +100 mV was applied to the KCl buffer at one side of the membrane, and the KCl buffer at the other side was grounded at 0 mV. The ionic concentrations of potassium and chloride ions in the buffer were set to 150 mM. The Poisson-Boltzmann equation was incorporated to reproduce the electric double layer at the electrode-KCl buffer interface. Changing the ionic concentrations of the buffer from 150 mM to 10 mM resulted in an expansion of the thickness of the electric double layer from about 3 nm to about 20 nm, indicating that the electric double layer was introduced into the lipid bilayer model. In the presentation, we will also discuss the results when a lateral voltage is applied to the inside of the lipid bilayer.

References

[1] T. Ma, et al., Faraday Discuss., 233, 244-256 (2022).