The life and death of an organism rely on correct cell division, which occurs through the process of mitosis. Although the biochemical signalling and morphogenetic processes during mitosis are well understood, the importance of mechanical forces and material properties is only just starting to be discovered. Recent studies have revealed that the layer of proteins beneath the cell membrane—the so-called cell cortex—stiffens during mitosis, but it is as yet unclear whether mechanical changes occur in the rest of the material in the cell, contained in the cytoplasm. Here we show that, in contrast to the cortical stiffening, the interior of the cell undergoes a softening and an increase in dissipative timescale, similar to viscoelastic relaxation. These mechanical changes are accompanied by a decrease in the active forces that drive particle mobility. Using optical tweezers to perform microrheology measurements, we capture the complex active and passive material states of the cytoplasm using six relevant parameters, of which only two vary considerably during mitosis. We demonstrate a role switch between microtubules and actin that could contribute to the observed softening.
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Python code used for analysing active microrheological and fluctuation analysis data is available at https://github.com/shmuen/Active_passive_microrheology.
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We thank T. Münker for technical support and helpful discussions. We are thankful for critical comments by E. Raz and M. Reichman-Fried. We thank R. Wedlich-Söldner for the MDCK II cell line expressing H2B-mCherry and GFP-LifeAct and the HeLa line expressing H2B-mCherry, and we thank A. Sivan for his help generating patterns. We thank C. Brennecka for critical revision of the manuscript. S.H., M.B. and T.B. were supported by the Interdisciplinary Center for Clinical Research (IZKF) Münster (Bet1/013/17). T.B. was supported by the European Research Council ERC-Consolidator grant PolarizeMe (771201) and by the DFG under Germany’s Excellence Strategy (EXC 2067/1- 390729940).
The authors declare no competing interests.
Peer review information Nature Physics thanks Dimitrije Stamenovic and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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The parameters show a similar behaviour of mechanical properties as in MDCK cells. a) Power-law exponents α and β of viscoelastic fit for different phases of mitosis. b) Prefactor cα of viscoelastic fit. c) Prefactor cβ of viscoelastic fit. d) Prefactor e0 of power-law fit to active energy. e) Power-law exponent ν of fit to active energy. ncells: 20, 11, 23, 11, 10.
Interphase cells forced into a round shape comparable to dividing cells show increased stiffness. a) Fluorescent images of interphase cell forced into a round shape (left), cell in metaphase (middle) and cell arrested in mitosis by STC (right). Red: H2B; cyan: actin. b) Prefactors cα, cβ and e0. cα shows drastic increase of stiffness in interphase cells forced into a round shape. c) Exponents α, β and ν. Only ν shows a significant increase, which is not seen in mitotic cells. d) Fold changes of all parameters of cells forced into a round shape compared to flat interphase cells. Error bars indicate normalized error. The found differences cannot explain the changes found in mitotic cells. In round interphase cells stiffness increases, while stiffness drastically decreases in mitotic cells. ncells: 57, 63.
Extended Data Fig. 4 Comparison of passive and active mechanical parameters of MDCK cells in prophase and arrested in mitosis by STC.
The parameters do not show a significant difference between cells in prophase and cells treated with STC, which arrests the cells in mitosis. ncells: 15, 36. Data shows mean ± SE.
Extended Data Fig. 5 Comparison of material property parameters in mitotic cells after treatment with different actin drugs.
Treatment with cytochalasin B (20.9 μM), cytochalasin D (10 μM) and latrunculin A (474 nM) all lead to similar parameters. ncells: 55, 58, 47.
Top: Immunostained cells in interphase and treated with STC; blue: nucleus, Hoechst; red: microtubules, anti-α-tubulin; cyan: actin, phalloidin; magenta: 1 μm particle, covalently bound dye. The yellow circle indicates the area around the particle that was analysed. Bottom: mean radial intensity of particle, actin and microtubule signal of 21 particles in interphase cells (left) and 10 particles in STC treated cells (right). Error indicates SD.
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Hurst, S., Vos, B.E., Brandt, M. et al. Intracellular softening and increased viscoelastic fluidity during division. Nat. Phys. 17, 1270–1276 (2021). https://doi.org/10.1038/s41567-021-01368-z