Pushing the envelope: force balance in fission yeast closed mitosis

The fission yeast S. pombe divides via closed mitosis, meaning that spindle elongation and chromosome segregation transpire entirely within the complete nuclear envelope. Both the spindle and nuclear envelope must undergo significant conformation changes and exert varying forces on each other during this process. Previous work has demonstrated that nuclear envelope expansion1,2 and spindle pole body (SPB) embedding in the nuclear envelope3 are required for normal S. pombe mitosis, and mechanical modeling has described potential contributions of the spindle to nuclear morphology4,5. However, it is not yet fully clear how and to what extent the nuclear envelope and mitotic spindle each directly shape each other during closed mitosis. Here, we investigate this relationship by observing the behaviors of spindles and nuclei in live mitotic fission yeast following laser ablation. First, we characterize these dynamics in molecularly typical S. pombe spindles, finding them to be stabilized by dense crosslinking, before demonstrating that the compressive force acting on the spindle poles is higher in mitotic cells with greater nuclear envelope tension and that spindle compression can be relieved by lessening nuclear envelope tension via laser ablation. Finally, we use a quantitative model to interpret how these data directly demonstrate that fission yeast spindles and nuclear envelopes are a mechanical pair that can each shape the others morphology.