The mitotic spindle is chiral due to torques within microtubule bundles

Maja Novak,Barbara Kuzmić,Juraj Simunić,Andreas W Thomae,Zvonimir Boban,Nenad Pavin,Bruno Polak,Iva M Tolić

doi:10.1038/s41467-018-06005-7

Abstract

Mitosis relies on forces generated in the spindle, a micro-machine composed of microtubules and associated proteins. Forces are required for the congression of chromosomes to the metaphase plate and their separation in anaphase. However, besides forces, torques may exist in the spindle, yet they have not been investigated. Here we show that the spindle is chiral. Chirality is evident from the finding that microtubule bundles in human spindles follow a left-handed helical path, which cannot be explained by forces but rather by torques. Kinesin-5 (Kif11/Eg5) inactivation abolishes spindle chirality. Our theoretical model predicts that bending and twisting moments may generate curved shapes of bundles. We found that bundles turn by about −2 deg µm−1 around the spindle axis, which we explain by a twisting moment of roughly −10 pNµm. We conclude that torques, in addition to forces, exist in the spindle and determine its chiral architecture.

Highlights

Mitosis relies on forces generated in the spindle, a micro-machine composed of microtubules and associated proteins
Single optical sections of spindles showed that microtubule bundles are continuous almost from pole to pole and acquire complex curved shapes (Fig. 1a)
We investigated the chirality of spindles in several other conditions: (i) unlabeled HeLa cells with horizontal spindles immunostained for protein regulator of cytokinesis 1 (PRC1) (Fig. 1g; Supplementary Movie 5), (ii) and (iii) live HeLa cells expressing PRC1-green fluorescent protein (GFP), with horizontal (Supplementary Fig. 1c; Supplementary Movie 6) and vertical spindles, (iv) live U2OS cells with vertical spindles, expressing mCherry-α-tubulin (Supplementary Movie 7), and (v) unlabeled U2OS cells with horizontal spindles immunostained for PRC1 (Supplementary Fig. 1c; Supplementary Movie 8)

Summary

Introduction

Mitosis relies on forces generated in the spindle, a micro-machine composed of microtubules and associated proteins. Some of the microtubules that are not associated with kinetochores meet in the central part of the spindle to form antiparallel bundles known as interpolar or overlap bundles[7] These bundles act as a bridge between sister kinetochore fibers and balance the forces at kinetochores in metaphase and anaphase[8,9,10,11,12], and regulate pole separation in anaphase[13]. As well as microtubule dynamics, were included in models that explained chromosome movements during metaphase and anaphase[10,17]. Such one-dimensional models were successful in identification of the most important physical mechanisms of chromosome movements in mitosis. The curved shape of spindles without centrosomes was explained by considering local interactions of short microtubules in a liquid crystal model[20]

Methods

Results

Conclusion