Pushing the envelope: force balance between the spindle and nuclear envelope during fission yeast closed mitosis

When eukaryotic cells divide, the chromosomes must be duplicated and segregated into two new daughter nuclei, each of which are isolated from the cytoplasm by the nuclear envelope. The mitotic spindle has the critical task of segregating chromosomes. To do so, it must self-assemble and then generate force to pull chromosomes to the right place at the right time. In many eukaryotes, the nuclear envelope breaks down during this process, while in others, chromosome segregation happens inside an intact nuclear envelope. In the fission yeast S. pombe, which has closed mitosis, changes to nuclear envelope shape and topology accompany spindle elongation. While it is assumed that this elongation drives nuclear shape changes, the mechanics underlying these transitions are very poorly understood. To probe this question, we molecularly and mechanically perturb mitotic S. pombe cells. We sever specific mitotic structures, including the spindle and the nuclear envelope, by laser ablation, and we quantify the subsequent response to these acute perturbations using live cell confocal microscopy. The dynamics of these responses report on the material properties of the spindle, nuclear envelope, and the nucleoplasm. We perform a similar set of experiments in S. japonicus, the phylogenetic cousin of S. pombe, which goes through semi-open mitosis, in which nuclear envelope closure is not required. We find differences in spindle mechanics between the two species, suggesting how features of spindle organization may evolve to match their mechanical constraints.