Changes in nuclear and actin mechanics from G1 to G2 affect nuclear integrity

The structural integrity of the nucleus is dependent on nuclear mechanical elements of chromatin and lamins resisting antagonistic actin cytoskeleton forces. Force imbalance results in nuclear blebbing, rupture and cellular dysfunction found in many human diseases. Here, we used the fluorescent ubiquitin cell cycle indicator (FUCCI) cells to determine how cell cycle changes affect the nucleus and actin force balance. Whereas nuclear blebs were present equally throughout interphase, nuclear blebs formed predominantly in G1 and then persisted into G2. Actin-based nuclear confinement and focal adhesion density was greater in G1 versus G2 cells. Removal of focal adhesions through treatment with an inhibitor resulted in decreased nuclear confinement and blebbing, supporting this as the underlying mechanism. Upon artificial confinement, G2 nuclei ruptured more than G1 nuclei. Single nucleus micromanipulation force measurements confirmed that G1 nuclei were stiffer than G2 nuclei in both the chromatin-based and lamin-based nuclear stiffness regimes. Decreased nuclear stiffness can be explained by loss of peripheral H3K9me3 from G1 to G2, recapitulated by H3K9me3 inhibition through treatment with chaetocin. Cell cycle-based changes in nuclear and actin mechanics impact nuclear integrity and shape.