Lipid membrane nanotubes are abundant in living cells, even though tubules are energetically less stable than sheet-like structures. According to membrane elastic theory, the tubular endoplasmic reticulum (ER), with its high area-to-volume ratio, appears to be particularly unstable. We explore how tubular membrane structures can nevertheless be induced and why they persist. In Monte Carlo simulations of a fluid–elastic membrane model subject to thermal fluctuations and without constraints on symmetry, we find that a steady increase in the area-to-volume ratio readily induces tubular structures. In simulations mimicking the ER wrapped around the cell nucleus, tubules emerge naturally as the membrane area increases. Once formed, a high energy barrier separates tubules from the thermodynamically favored sheet-like membrane structures. Remarkably, this barrier persists even at large area-to-volume ratios, protecting tubules against shape transformations despite enormous driving forces toward sheet-like structures. Molecular dynamics simulations of a molecular membrane model confirm the metastability of tubular structures. Volume reduction by osmotic regulation and membrane area growth by lipid production and by fusion of small vesicles emerge as powerful factors in the induction and stabilization of tubular membrane structures.

中文翻译：

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脂质膜纳米管的形成和稳定性

即使小管在能量上比片状结构不稳定，脂质膜纳米管在活细胞中也很丰富。根据膜弹性理论，管状的内质网（ER）具有高的体积体积比，似乎特别不稳定。我们探索了如何仍然可以诱导管状膜结构以及它们为什么持续存在。在受热波动影响且不受对称性约束的流体-弹性膜模型的蒙特卡洛模拟中，我们发现面积/体积比的稳定增加很容易诱发管状结构。在模拟包裹在细胞核周围的ER的模拟中，随着膜面积的增加，肾小管自然出现。一旦形成，高能垒将小管与热力学上有利的片状膜结构分开。值得注意的是 即使在大面积体积比的情况下，这种屏障仍然存在，即使对片状结构有巨大的驱动力，也可以保护小管不受形状变化的影响。分子膜模型的分子动力学模拟证实了管状结构的亚稳定性。通过渗透调节和通过脂质生产以及通过小囊泡融合而引起的膜面积增长来减少体积，已成为诱导和稳定管状膜结构的有力因素。