Department of Physics, Kyushu University, Fukuoka, Japan.
Department of Physics, University of Tokyo, Tokyo, Japan.
Biophys J. 2023 Nov 21;122(22):4395-4413. doi: 10.1016/j.bpj.2023.10.018. Epub 2023 Oct 20.
The glassy cytoplasm, crowded with bio-macromolecules, is fluidized in living cells by mechanical energy derived from metabolism. Characterizing the living cytoplasm as a nonequilibrium system is crucial in elucidating the intricate mechanism that relates cell mechanics to metabolic activities. In this study, we conducted active and passive microrheology in eukaryotic cells, and quantified nonthermal fluctuations by examining the violation of the fluctuation-dissipation theorem. The power spectral density of active force generation was estimated following the Langevin theory extended to nonequilibrium systems. However, experiments performed while regulating cellular metabolic activity showed that the nonthermal displacement fluctuation, rather than the active nonthermal force, is linked to metabolism. We discuss that mechano-enzymes in living cells do not act as microscopic objects. Instead, they generate meso-scale collective fluctuations with displacements controlled by enzymatic activity. The activity induces structural relaxations in glassy cytoplasm. Even though the autocorrelation of nonthermal fluctuations is lost at long timescales due to the structural relaxations, the nonthermal displacement fluctuation remains regulated by metabolic reactions. Our results therefore demonstrate that nonthermal fluctuations serve as a valuable indicator of a cell's metabolic activities, regardless of the presence or absence of structural relaxations.
充满生物大分子的玻璃状细胞质在活细胞中通过代谢产生的机械能而流化。将活细胞质描述为非平衡系统对于阐明将细胞力学与代谢活动相关联的复杂机制至关重要。在这项研究中，我们在真核细胞中进行了主动和被动微流变学研究，并通过检查对涨落耗散定理的违背来量化非热涨落。根据扩展到非平衡系统的 Langevin 理论来估计主动力产生的功率谱密度。然而，在调节细胞代谢活性的同时进行的实验表明，与代谢相关的是非热位移涨落，而不是主动非热力。我们讨论了活细胞中的机械酶不是作为微观物体起作用。相反，它们产生由酶活性控制的具有位移的介观集体涨落。该活性诱导玻璃状细胞质的结构松弛。即使由于结构松弛，非热涨落的自相关在长时间尺度上丢失，但非热位移涨落仍受代谢反应的调节。因此，我们的结果表明，无论是否存在结构松弛，非热涨落都可以作为细胞代谢活动的有价值的指标。
