Activity-dependent glassy cell mechanics II: Non-thermal fluctuations under metabolic activity

The glassy cytoplasm, crowded with bio-macromolecules, is fluidized in living cells by mechanical energy derived from metabolism. Characterizing the living cytoplasm as a non-equilibrium 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 non-thermal fluctuations from the violation of the fluctuation-dissipation theorem (FDT). The power spectral density corresponding to active force generation was then estimated following the Langevin theory extended to non-equilibrium systems. Experiments performed while regulating cellular metabolic activity showed that the non-thermal displacement fluctuation, rather than the active non-thermal force, directly correlates with metabolism. We discuss how mechano-enzymes in living cells do not act as a collection of microscopic objects; rather, they generate meso-scale collective fluctuations that directly correlate with enzymatic activity. The correlation is lost at long time scales because of the mesoscopic structural relaxations induced by the metabolic activities. Since the efficiency with which energy is converted to non-thermal fluctuations decreases as the cytoplasm becomes fluidic, the fluidization of the cytoplasm stops at critical jamming. Regardless of the presence or absence of structural relaxations in the cytoplasm, we demonstrate that non-thermal fluctuations in a probe particle can serve as a valuable indicator of those metabolic activities that typically perturb the mechanical environment within cells.