Bunner, S. | bioRxiv Science

bioRxiv2025

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 to resist antagonistic actin cytoskeleton forces. Imbalance results in nuclear blebbing, rupture, and cellular dysfunction found in many human diseases. We used Fluorescent Ubiquitin Cell Cycle Indicator (FUCCI) cells to determine how cell cycle changes affect the nucleus and actin force balance. While nuclear blebs are present equally throughout interphase, nuclear blebs form predominantly in G1 and then persist into G2 due to increased actin-based nuclear confinement and focal adhesion density in G1 vs. G2 cells. Upon artificial confinement, G2 nuclei ruptured more than G1 nuclei. Single nucleus micromanipulation force measurements confirmed that G1 nuclei are stronger 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 via Chaetocin. Cell cycle-based changes in nuclear and actin mechanics impact nuclear integrity and shape.

cell biology↗

bioRxiv2024

DNA density is a better indicator of a nuclear bleb than lamin B loss

Nuclear blebs are herniations of the nucleus that occur in diseased nuclei that cause nuclear rupture leading to cellular dysfunction. Chromatin and lamins are two of the major structural components of the nucleus that maintain its shape and function, but their relative roles in nuclear blebbing remain elusive. Lamin B is reported to be lost in blebs by qualitative data while quantitative studies reveal a spectrum of lamin B levels in nuclear blebs dependent on perturbation and cell type. Chromatin has been reported to be decreased or de-compacted in nuclear blebs, but again the data are not conclusive. To determine the composition of nuclear blebs, we compared the immunofluorescence intensity of lamin B and DNA in the main nucleus body and nuclear bleb across cell types and perturbations. Lamin B nuclear bleb levels varied drastically across MEF wild type and chromatin or lamins perturbations, HCT116 lamin B1-GFP imaging, and human disease model cells of progeria and prostate cancer. However, DNA concentration was consistently decreased to about half that of the main nucleus body across all measured conditions. Using Partial Wave Spectroscopic (PWS) microscopy to measure chromatin density in the nuclear bleb vs body we find similar results that DNA is consistently less dense in nuclear blebs. Thus, our data spanning many different cell types and perturbations supports that decreased DNA is a better marker of a nuclear bleb than lamin B levels that vary widely.

cell biology↗