(E-172) Drop-like nuclear deformation in endothelial cells

Introduction:: Mechanical stresses from the extracellular environment, such as shear stresses, are propagated to the endothelial nucleus. How the nucleus mechanically responds to these stresses is not well understood. We have recently proposed the “nuclear drop model” for explaining nuclear deformation in cells. Key features of the nuclear drop model are 1) the nucleus deforms at constant surface area and volume, 2) folds/wrinkles in the nuclear lamina allow facile deformation, and 3) the nuclear lamina resists extension by cellular stresses once the folds in the lamina are fully unfolded, resulting in a surface tension in the nuclear drop and a nuclear pressure. Here we tested the nuclear drop model in human umbilical vein endothelial cells (HUVEC).


Materials and Methods:: Microfluidic vessel chips were used to seed HUVECs under the shear stress of 1 dyne/cm² for 15 hours. HUVECs were treated by IL-6 (25 ng/mL), an inflammatory cytokine. Lamin A or lamin B1 was immunostained in the vessel chips and confocal fluorescence microscopy was performed. Nuclear shape irregularities were quantified with an Elliptical Fourier analysis. Nuclear volume and surface area was quantified with IMARIS software. YAP nuclear translocation was also quantified.


Results, Conclusions, and Discussions:: We found that disruption of F-actin filaments with cytochalasin D treatment caused the appearance of wrinkles/folds in the lamin B1-labelled nuclear lamina, which were absent in the control cells (Figure 1A). Nuclear surface area and volume were unchanged between these conditions, consistent with the nuclear drop model (Figure 1B and C). We performed an Elliptical Fourier analysis on confocal fluorescent images of the lamin A stained nucleus to calculate the Elliptical Fourier Coefficient (EFC) ratio, which is a quantitative and mathematically explainable measure of nuclear surface irregularities. The EFC ratio was significantly lower in cytochalasin treated cells compared to control, consistent with the presence of excess area in rounded endothelial nuclei. Treatment with IL-6, an inflammatory cytokine, in HUVECs exposed to shear stress also caused the appearance of folds/wrinkles in some cells. The EFC ratio directly correlated with YAP translocation to the nucleus (Figure 2), consistent with the hypothesis that YAP can translocate to the nucleus upon a development of surface tension in the nuclear lamina. These results collectively suggest that the nucleus undergoes drop-like deformation in endothelial cells.


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