J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, Dolejškova 3, 18223 Prague 8, Czech Republic.
Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland.
Langmuir. 2020 Sep 8;36(35):10438-10447. doi: 10.1021/acs.langmuir.0c01590. Epub 2020 Aug 28.
Cholesterol renders mammalian cell membranes more compact by reducing the amount of voids in the membrane structure. Because of this, cholesterol is known to regulate the ability of cell membranes to prevent the permeation of water and water-soluble molecules through the membranes. Meanwhile, it is also known that even seemingly tiny modifications in the chemical structure of cholesterol can lead to notable changes in membrane properties. The question is, how significantly do these small changes in cholesterol structure affect the permeability barrier function of cell membranes? In this work, we applied fluorescence methods as well as atomistic molecular dynamics simulations to characterize changes in lipid membrane permeability induced by cholesterol oxidation. The studied 7β-hydroxycholesterol (7β-OH-chol) and 27-hydroxycholesterol (27-OH-chol) represent two distinct groups of oxysterols, namely, ring- and tail-oxidized cholesterols, respectively. Our previous research showed that the oxidation of the cholesterol tail has only a marginal effect on the structure of a lipid bilayer; however, oxidation was found to disturb membrane dynamics by introducing a mechanism that allows sterol molecules to move rapidly back and forth across the membrane-bobbing. Herein, we show that bobbing of 27-OH-chol accelerates fluorescence quenching of NBD-lipid probes in the inner leaflet of liposomes by dithionite added to the liposomal suspension. Systematic experiments using fluorescence quenching spectroscopy and microscopy led to the conclusion that the presence of 27-OH-chol increases membrane permeability to the dithionite anion. Atomistic molecular dynamics simulations demonstrated that 27-OH-chol also facilitates water transport across the membrane. The results support the view that oxysterol bobbing gives rise to successive perturbations to the hydrophobic core of the membrane, and these perturbations promote the permeation of water and small water-soluble molecules through a lipid bilayer. The observed impairment of permeability can have important consequences for eukaryotic organisms. The effects described for 27-OH-chol were not observed for 7β-OH-chol which represents ring-oxidized sterols.
胆固醇通过减少膜结构中的空隙量使哺乳动物细胞膜更加紧凑。由于这个原因，胆固醇被认为可以调节细胞膜防止水和水溶性分子通过膜渗透的能力。同时，人们也知道，胆固醇化学结构的微小变化也会导致膜性质的显著变化。问题是，胆固醇结构的这些微小变化对细胞膜的渗透性屏障功能有多大影响？在这项工作中，我们应用荧光方法和原子分子动力学模拟来表征胆固醇氧化引起的脂质膜通透性的变化。所研究的 7β-羟胆固醇（7β-OH-chol）和 27-羟胆固醇（27-OH-chol）分别代表两种不同的氧化固醇组，即环氧化固醇和尾氧化固醇。我们之前的研究表明，胆固醇尾的氧化对脂质双层结构只有很小的影响；然而，氧化被发现通过引入一种允许固醇分子在膜内快速来回移动的机制来干扰膜动力学。在这里，我们表明，二硫代硫酸盐加入到脂质体悬浮液中，会加速 27-OH-chol 在脂质体内部叶层中的 NBD-脂质探针的荧光猝灭。使用荧光猝灭光谱学和显微镜的系统实验得出的结论是，27-OH-chol 的存在增加了二硫代硫酸盐阴离子对膜的通透性。原子分子动力学模拟表明，27-OH-chol 也促进了水在膜中的传输。这些结果支持这样一种观点，即氧化固醇的摆动会引起膜疏水区的连续扰动，这些扰动会促进水和小的水溶性分子通过脂质双层的渗透。观察到的渗透性损伤可能对真核生物有重要影响。对于代表环氧化固醇的 7β-OH-chol，没有观察到 27-OH-chol 所表现出的渗透性增强效应。
