Centripetal nuclear shape fluctuations associate with chromatin condensation towards mitosis

Abstract

The cell nucleus plays a central role in several key cellular processes, including chromosome organisation, replication and transcription. Recent work intriguingly suggests an association between nuclear mechanics and cell-cycle progression, but many aspects of this connection remain unexplored. Here, by monitoring nuclear shape fluctuations at different cell cycle stages, we uncover increasing inward fluctuations in late G2 and early mitosis, which are initially transient, but develop into instabilities that culminate into nuclear-envelope breakdown in mitosis. Perturbation experiments and correlation analysis reveal an association of these processes with chromatin condensation. We propose that the contrasting forces between an extensile stress and centripetal pulling from chromatin condensation could link mechanically chromosome condensation and nuclear- envelope breakdown, the two main nuclear processes during mitosis.Significance StatementThe nucleus was recently shown to exhibit shape fluctuations that vary with cell-cycle stage, but we know very little about the possible links between nuclear mechanics and cell cycle- progression. Through flickering analysis, this study monitors radius and nuclear envelope fluctuations across the cell cycle. The authors discover that as the cell cycle progresses towards mitosis, localised inward invaginations of the nuclear shape form initially transiently and gradually increasing their amplitude, in association with chromatin condensation. This phenomenon develops into nuclear envelope breakdown, suggesting a novel link between cell cycle, chromatin mechanics and nuclear shape fluctuations.

Force-biased nuclear import sets nuclear-cytoplasmic volumetric coupling by osmosis

“…The apparent tension of the nucleus, estimating σ N or σ 0 is around 10 −6 N/m in nuclear shape fluctuations experiments (Chu et al, 2017; Introini et al, 2021), but this value is an underestimation of the actual tension, since it is affected by excess flickering due to transient deformations of active origin (decreasing the effective tension with respect to the actual mechanical tension). We can assume that this value is a lower bound for the real mechanical tension σ 0 .…”

“…Additionally, in yeast, the fraction of q of nuclearly-localized to cytoplasmically localized proteins is about 1/2 (Kumar et al, 2002). However, since this factor is a major determinant of the NC volume ratio, and we find ratios that are about 0.2, we conclude that this factor must be smaller in mammalian cells. The apparent tension of the nucleus, estimating σ N or σ 0 is around 10 −6 N/m in nuclear shape fluctuations experiments (Chu et al, 2017; Introini et al, 2021), but this value is an underestimation of the actual tension, since it is affected by excess flickering due to transient deformations of active origin (decreasing the effective tension with respect to the actual mechanical tension). We can assume that this value is a lower bound for the real mechanical tension σ 0 . We assume that cytoskeletal forces are compressive, in line with the recent literature (but we have also considered the opposite case, see below).…”

Current Methods and Pipelines for Image-Based Quantitation of Nuclear Shape and Nuclear Envelope Abnormalities

“…Looking at the flickering of the nucleus, i.e., the changes in the nucleus' contour on the time scale of seconds, Chu et al [36], were able to quantify the mean square amplitude of the fluctuations and showed that this measure correlates with cell cycle stage. Quantitative analysis of nuclear flickering can also yield NE properties such as the NE bending modulus [37].…”