[论文解读] Nonlinear conductance, rectification and mechanosensitive channel formation of lipid membranes

Karis Amata Zecchi, Thomas Heimburg|arXiv (Cornell University)|Aug 7, 2020

Lipid Membrane Structure and Behavior参考文献 64被引用 14

一句话总结

本文表明，纯脂质膜可表现出电压门控、机械敏感的离子通道形成，具有整流的电流-电压（I-V）关系，类似于蛋白质通道。通过在不同深度（改变压力梯度）的脂质膜上进行膜片钳记录，作者识别出两种电导机制：一种是电压无关的漏电流，另一种是产生离散通道样事件的电压门控孔道开放过程。关键发现是，仅脂质膜即可表现出非线性、向外整流的I-V行为以及依赖于电压和机械应力的孔道形成，表明脂质通道可能在生物膜中作为或并行于蛋白质通道发挥作用。

ABSTRACT

There is mounting evidence that lipid bilayers display conductive properties. However, when interpreting the electrical response of biological membranes to voltage changes, they are commonly considered as inert insulators. However, lipid bilayers under voltage-clamp conditions do not only display current traces with discrete conduction-steps indistinguishable from those attributed to the presence of protein channels. In current-voltage (I-V) plots they may also display outward rectification, i.e., voltage-gating. Surprisingly, this has even been observed in chemically symmetric lipid bilayers. Here, we investigate this phenomenon using a theoretical framework that models the electrostrictive effect of voltage on lipid membranes in the presence of a spontaneous polarization, which can be recognized by a voltage offset in electrical measurements. It can arise from an asymmetry of the membrane, for example from a nonzero spontaneous curvature of the membrane. This curvature can be caused by voltage via the flexoelectric effect, or by hydrostatic pressure differences across the membrane. Here, we describe I-V relations for lipid membranes formed at the tip of patch pipettes situated close to an aqueous surface. We measured at different depths relative to air/water surface, resulting in different pressure gradients across the membrane. Both linear and nonlinear I-V profiles were observed. Nonlinear conduction consistently takes the form of outward rectified currents. We explain the conductance properties by two mechanisms: One leak current with constant conductance without pores, and a second process that is due to voltage-gated pore opening correlating with the appearance of channel-like conduction steps. In some instances, these nonlinear I-V relations display a voltage regime in which dI/dV is negative. This has also been previously observed in the presence of sodium channels. ...

研究动机与目标

• 研究不含蛋白质的纯脂质膜中非线性电导和整流行为的起源。
• 确定脂质膜是否可独立于蛋白质通道形成电压门控和机械敏感的离子通道。
• 检验假设：脂质双分子层中的热涨落和电致收缩效应可导致类似通道的电导事件。
• 比较脂质膜与电压门控钾通道在开放概率和电导动力学方面的I-V行为。

提出的方法

• 在与水相表面接触的膜片电极尖端形成的脂质膜上进行膜片钳记录。
• 系统性改变电极深度以调节跨膜压力梯度，并研究其对I-V曲线的影响。
• 使用双指数拟合分析电流记录，以识别快速（电容性）和慢速（结构性）弛豫过程。
• 对电致收缩效应和脂质膜中的自发极化进行理论建模，结合弯曲电致伸缩效应和电压偏移效应。
• 将膜片实验结果与黑脂质膜（BLM）实验进行比较，以评估对称性与整流性的差异。
• 对脂质膜的开放概率曲线与霍奇金-赫胥黎和KvAP钾通道的曲线进行定量比较。

实验结果

研究问题

• RQ1 纯脂质膜是否可在无任何蛋白质或大分子组分的情况下表现出电压门控、整流的I-V行为？
• RQ2 跨膜压力梯度在调节脂质膜中通道形成与电导中的作用是什么？
• RQ3 脂质膜中电导的动力学与电压依赖性与电压门控钾通道相比如何？
• RQ4 导致脂质膜中观测到的I-V曲线的两种不同电导机制是什么？
• RQ5 非线性I-V行为，包括负微分电导（dI/dV < 0），能否通过膜涨落与电压门控的热力学一致模型来解释？

主要发现

• 在膜片电极尖端形成的脂质膜中，I-V曲线始终表现出向外整流，表明存在电压门控孔道开放。
• 观察到非线性I-V行为，包括负微分电导区域（dI/dV < 0），这一特征也见于钠通道。
• 电导机制由两部分组成：电压无关的漏电流和产生离散通道样事件的电压门控孔道形成过程。
• 孔道形成被证实既具有电压门控性又具有机械敏感性，这由I-V曲线和孔道出现频率随深度变化的证据支持。
• 脂质膜中孔道的开放概率与霍奇金-赫胥黎钾通道及KvAP通道非常相似，特别是在激活的电压依赖性方面。
• 黑脂质膜（BLMs）表现出对称的I-V曲线，无整流或电压偏移，与膜片钳测得的脂质膜所呈现的非对称、整流行为形成鲜明对比。