The chirality of the mitotic spindle provides a passive mechanical response to forces and depends on microtubule motors and crosslinkers

Abstract

ABSTRACTMechanical forces produced by motor proteins and microtubule dynamics within the mitotic spindle are crucial for the movement of chromosomes and their segregation into the daughter cells. In addition to linear forces, rotational forces or torques are present in the spindle, reflected in the left-handed twisted shapes of microtubule bundles that make the spindle chiral. However, the biological role and molecular origins of spindle chirality are unknown. By developing methods to measure spindle twist, we show that spindles are most chiral near the metaphase-to-anaphase transition. To test whether spindles react to external forces by changing the twist, we compressed the spindles along their axis, which resulted in stronger left-handed twist. Inhibition or depletion of motor proteins that perform chiral stepping, Eg5/kinesin-5 or Kif18A/kinesin-8, decreased the left-handed twist or led to right-handed twist, suggesting that these motors regulate the twist by rotating microtubules around one another within the antiparallel overlaps. Right-handed twist was also observed after depletion of the microtubule crosslinker PRC1 or the nucleator augmin, indicating that PRC1 contributes to the twist by constraining microtubule rotation, and augmin by nucleating antiparallel bridging microtubules. The observed switch from left-handed to right-handed twist reveals the existence of competing mechanisms that promote twisting in opposite directions. As round spindles were more twisted than elongated ones, we suggest that bending and twisting moments are generated by similar molecular mechanisms. We propose a physiological role for spindle chirality in providing a passive mechanical response to forces, decreasing the risk of spindle breakage under high load.