Equilibrium and out-of-equilibrium mechanics of living mammalian cytoplasm. (October 2017)

• Record Type: Journal Article
• Authors: Gupta, Satish Kumar; Guo, Ming
• Is Part Of: Journal of the mechanics and physics of solids. Volume 107(2017:Oct.)
• Page Start: 284
• Page End: 293
• Publication Date: 2017-10
• Subjects: Mammalian cytoplasm -- Microrheology -- Optical tweezers -- Non-equilibrium -- Cell mechanics; Mechanics, Applied -- Periodicals; Solids -- Periodicals; Mechanics -- Periodicals; Mécanique appliquée -- Périodiques; Solides -- Périodiques; Mechanics, Applied; Solids; Periodicals; 531.05
• DOI: 10.1016/j.jmps.2017.07.007
• Languages: English
• ISSNs: 0022-5096

Abstract

Highlights: Two distinct regimes of mechanical behavior in living cells are identified. Living mammalian cytoplasm behaves as an equilibrium material at short time scales. It behaves as an out-of-equilibrium material at long time scales. Intracellular fluctuation is used to measure cytoplasmic moduli at high frequencies. Abstract: Living cells are intrinsically non-equilibrium systems. They are driven out of equilibrium by the activity of the molecular motors and other enzymatic processes. This activity along with the ever present thermal agitation results in intracellular fluctuations inside the cytoplasm. In analogy to Brownian motion, the material property of the cytoplasm also influences the characteristics of these fluctuations. In this paper, through a combination of experimentation and theoretical analysis, we show that intracellular fluctuations are indeed due to non-thermal forces at relatively long time-scales, however, are dominated solely by thermal forces at relatively short time-scales. Thus, the cytoplasm of living mammalian cells behaves as an equilibrium material at short time-scales. The mean square displacement of these intracellular fluctuations scales inversely with the cytoplasmic shear modulus in this short time-scale equilibrium regime, and is inversely proportional to the square of the cytoplasmic shear modulus in the long time-scale out-of-equilibrium regime. Furthermore, we deploy passive microrheology based on these fluctuations to extract the mechanical property of the cytoplasm at the high-frequency regime. We show that the cytoplasm of living mammalian cells is a weak elastic gel in this regime; this is in an excellent agreement with an independent micromechanical measurement using optical tweezers.