Assembly of contractile actin bundles is required for interphase NE rupture in the primary nucleus. (A) Still images from time-lapse imaging of U2OS cells expressing 3xGFP-NLS (GFP-NLS), histone 2B (H2B)-mCherry, and LmnB1 shRNA. Chromatin herniation is indicated by localization of H2B-mCherry and GFP-NLS to a nucleus bleb. NE rupture is indicated by GFP-NLS localization to the cytoplasm. NE repair is indicated by GFP-NLS reaccumulation in the nucleus. Time in hours:minutes. (B) Experiment schematic for time-lapse imaging of NE rupture in U2OS GFP-NLS shLmnB1 cells. (C and D) Quantification of NE rupture after treatment with indicated drugs. Graph represents pooled data (columns) from 3 experiments (points, n > 200 cells/expt.). ***, P < 0.001; ns, P > 0.05, Fisher’s exact test. Bleb, blebbistatin; CytoD, cytochalasin D; Noc, nocodazole. (E) Representative images of micronuclei (arrows) in cells treated for 18 h with indicated drugs and labeled with rhodamine-phalloidin (rho-phal) to visualize F-actin. The micronucleus in the top image is intact and the bottom one is disrupted. (F) Experiment schematic (top) for analysis of micronucleus disruption. Quantification (bottom) of disrupted micronuclei frequency 3 h after cell replating and 18 h after addition of indicated drug. Graph represents pooled data (columns) from three independent experiments (points, n > 100 cells). ns, P > 0.05, Fisher’s exact test. Bars, 10 µm.