Universal glass-forming behavior of in vitro and living cytoplasm

Kenji Nishizawa,Daisuke Mizuno,Kei Fujiwara,Miho Yanagisawa,Masahiro Ikenaga,Nobushige Nakajo

doi:10.1038/s41598-017-14883-y

Abstract

Physiological processes in cells are performed efficiently without getting jammed although cytoplasm is highly crowded with various macromolecules. Elucidating the physical machinery is challenging because the interior of a cell is so complex and driven far from equilibrium by metabolic activities. Here, we studied the mechanics of in vitro and living cytoplasm using the particle-tracking and manipulation technique. The molecular crowding effect on cytoplasmic mechanics was selectively studied by preparing simple in vitro models of cytoplasm from which both the metabolism and cytoskeletons were removed. We obtained direct evidence of the cytoplasmic glass transition; a dramatic increase in viscosity upon crowding quantitatively conformed to the super-Arrhenius formula, which is typical for fragile colloidal suspensions close to jamming. Furthermore, the glass-forming behaviors were found to be universally conserved in all the cytoplasm samples that originated from different species and developmental stages; they showed the same tendency for diverging at the macromolecule concentrations relevant for living cells. Notably, such fragile behavior disappeared in metabolically active living cells whose viscosity showed a genuine Arrhenius increase as in typical strong glass formers. Being actively driven by metabolism, the living cytoplasm forms glass that is fundamentally different from that of its non-living counterpart.

Highlights

Elucidating the mechanics of the cytoplasm is essential for understanding cell behaviors because cytoplasm governs the dynamics of core functional molecules essential for living systems[1,2,3]
The mechanics of living cytoplasm might be affected by cytoskeletal networks, metabolic activity, and molecular crowding in a complex manner[20,21]
We investigated the mechanics in living cells by comparing them with several simple in vitro models of the cytoplasm that lack cytoskeletons and metabolic activity: a globular biomacromolecule solution and three types of cell extracts (Escherichia coli, Xenopus eggs, and HeLa cells)

Summary

Introduction

Elucidating the mechanics of the cytoplasm is essential for understanding cell behaviors because cytoplasm governs the dynamics of core functional molecules essential for living systems[1,2,3]. Physical activation by molecular motors directly induces out-of-equilibrium fluctuations and further alter the mechanics of a living cell’s interior[16] Glass transition in such activity-driven systems[17,18,19] was implied with the anomalous fluctuations observed in living cells[20]. Viscosity of a living cell’s interior retains the Arrhenius-like exponential increase over a wide range of concentrations, that is typical for strong glass formers[8] These results indicate that living cells are not fluidized by their metabolic activity; they still maintain glass-forming property but are driven toward the qualitatively altered state that can be attained in activity-driven systems. We discuss the possible mechanism for the loss of fragility, in relation to the homogenization/relaxation with active stirring in living cells

Methods

Results

Conclusion