A gelation transition enables the self-organization of bipolar metaphase spindles

Abstract

The mitotic spindle is a highly dynamic bipolar structure that emerges from the self-organization of microtubules, molecular motors, and other proteins. Sustained motor-driven poleward flows of short dynamic microtubules play a key role in the bipolar organization of spindles. However, it is not understood how the local activity of motor proteins generates these large-scale coherent poleward flows. Here, we combine experiments and simulations to show that a gelation transition enables long-ranged microtubule transport causing spindles to self-organize into two oppositely polarized microtubule gels. Laser ablation experiments reveal that local active stresses generated at the spindle midplane propagate through the structure thereby driving global coherent microtubule flows. Simulations show that microtubule gels undergoing rapid turnover can exhibit long stress relaxation times, in agreement with the long-ranged flows observed in experiments. Finally, we show that either disrupting such flows or decreasing the network connectivity can lead to a microtubule polarity reversal in spindles both in the simulations and in the experiments. Thus, we uncover an unexpected connection between spindle rheology and architecture in spindle self-organization.

Kinetochore microtubules flux poleward along fixed centrosome-anchored microtubules during the metaphase ofC. elegansone-cell embryo

“…It can be assisted by severing enzymes like katanin, spastin and fidgetin (Zhang et al, 2007). Furthermore, treadmilling is plainly consistent with the gel-like behaviour of the spindle (Dalton et al, 2022) and the classic flux measurements using photobleaching/photoconversion experiments (Barisic et al, 2021; Mitchison and Salmon, 1992). They all suggest a mostly solid motion of the spindle microtubule network at steady state.…”

“…In other organisms, the poleward flux involves all the microtubules, although their dynamics may differ (Conway et al, 2022; Risteski et al, 2022; Zhai et al, 1995). Dalton and co-authors proposed that the spindle undergoes gelation to account for the solid poleward motion of the microtubules within spindles prepared from Xenopus laevis extracts (Dalton et al, 2022). In contrast, we propose that only the kMTs undergo a poleward flux.…”

Cross-linker design determines microtubule network organization by opposing motors

“…The mitotic spindle, which segregates the chromosomes during eukaryotic cell division, is a paradigm for self-organizing active filament networks. It is composed of dynamic microtubules, interconnected by motile cross-linkers, forming an active gel continuously driven by internal stresses while maintaining steady-state shape ( 1 – 5 ). The metaphase spindle can be conceptualized as a central extensile nematic network of antiparallel microtubules that gradually turns into polar microtubule networks at the spindle poles ( 6 , 7 ).…”

Acentrosomal spindles assemble from branching microtubule nucleation near chromosomes

“…Based on this framework and mechanism we provide here for how microtubule nucleation around chromosomes occurs, the time is now ripe to further investigate how molecular motors organize these initial chromosomal branched networks into a functioning bipolar spindle. This issue is only now starting to be quantitatively addressed 49 .…”