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Single-Cell Biophysics: Measurement, Modulation, and Modeling
Saturday Speaker Abstracts
19
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.