Single-Cell Biophysics: Measurement, Modulation, and Modeling

Single-Cell Biophysics: Measurement, Modulation, and Modeling

Saturday Speaker Abstracts

Shape Transformation of the Nuclear Envelope during Closed Mitosis Qian Zhu, Chuanhai Fu, Yuan Lin . The University of Hong Kong, Hong Kong, Hong Kong.

Lower eukaryotes, such as fission yeast , undergo closed mitosis during which the nuclear envelope (NE) stays intact but changes shape dramatically, usually from a sphere to an ellipsoid and then to a dumbbell for wild-type cells. In comparison, the NE in gene-deletion mutants of the yeast can undergo asymmetric division which often involves tethering or budding of the nuclear membrane. Here we report a combined experimental and theoretical study to examine this intriguing phenomenon. Specifically, shape evolution of the cell nuclei in the wild-type and different mutants, with known gene defects, of fission yeast was closely monitored with live-cell imaging at high temporal resolution. Interestingly, it was found that structural deficiencies in one or both SPBs will cause the improper assembly and anchoring of mitotic spindle microtubules and ultimately lead to the formation of a single or multiple tethers. On the theoretical side, a physical model was also developed to predict the nuclear shape during mitosis based on energetic considerations. Our model suggests that, in addition to the bending rigidity and surface energy of the nuclear membrane, the spatial distribution of internally generated forces on the NE plays a key role in its shape transformation, with forces localized on both poles of the cell resulting in membrane tethering while a load distribution over a broad area typically leading to the formation of two equal-sized spherical daughter nuclei. These results provide physical explanations on how complex shapes of the nuclear envelope are developed during cell division as well as elucidate their correlations with structural alterations in the nuclear-cytoskeleton, as indicated in our experiments.

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