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Review
. 2017 Jun 5;216(6):1525-1531.
doi: 10.1083/jcb.201612064. Epub 2017 May 10.

The mechanics of microtubule networks in cell division

Scott Forth  1 , Tarun M Kapoor  2
Affiliations
Review

The mechanics of microtubule networks in cell division

Scott Forth et al. J Cell Biol. .

Abstract

The primary goal of a dividing somatic cell is to accurately and equally segregate its genome into two new daughter cells. In eukaryotes, this process is performed by a self-organized structure called the mitotic spindle. It has long been appreciated that mechanical forces must be applied to chromosomes. At the same time, the network of microtubules in the spindle must be able to apply and sustain large forces to maintain spindle integrity. Here we consider recent efforts to measure forces generated within microtubule networks by ensembles of key proteins. New findings, such as length-dependent force generation, protein clustering by asymmetric friction, and entropic expansion forces will help advance models of force generation needed for spindle function and maintaining integrity.

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Figures

Figure 1.
Figure 1.
Mechanics of the spindle microtubule network. (A) Schematic depicting the three major classes of microtubules of the spindle. K-fibers (red), interpolar microtubules (green), and astral microtubules (purple) are shown. (B) Calibrated microneedles inserted into a spindle are used to apply mechanical perturbations perpendicular to the long axis of the spindle. At fast (seconds) and slow (minutes) timescales, the spindle is more elastic, whereas at intermediate (tens of seconds) timescales, the spindle is more viscous. (C) A Zener-type model of a viscoelastic solid, which includes elastic spring-like elements linked to k-fiber and interpolar microtubule stiffness and viscous damping terms linked to cross-linking dynamics, describes the measured mechanical properties of the spindle. (D) Microneedles inserted near the spindle poles allow for the application of force along the spindle’s long axis.
Figure 2.
Figure 2.
Motor proteins within overlapping filaments. (A) Microtubules cross-linked by kinesin-5 with different overlap lengths. Longer overlaps can recruit more motor protein molecules, resulting in an increase in relative microtubule sliding forces. (B) Force generation by kinesin-5 ensembles scales with the length of microtubule overlap. (C) Microtubules can be cross-linked by motor proteins with different directional preferences. (D) When both kinesin-5 and kinesin-14 cross-link microtubule pairs at different ratios, directional microtubule sliding or fluctuations without a preferred directed motion are observed. A stable balance point with no relative microtubule motion cannot be achieved with motor proteins alone.
Figure 3.
Figure 3.
Nonmotor proteins within overlapping filaments. (A) Nonmotor MAPs can generate frictional resistance when moving along the lattice surface. (B) Some proteins, such as NuMA, EB1, and Kip3, have been shown to exhibit asymmetric friction, where moving toward one end of the microtubule results in increased resistance compared with motion in the opposite direction. (C) Cross-linking proteins whose microtubule binding domains possess frictional asymmetry can move directionally within fluctuating microtubule bundles. (D) Cross-linking proteins undergo diffusion, which can result in an entropic force that slides microtubules and opposes reduction in overlap lengths.
Figure 4.
Figure 4.
Proposed model of spindle force map. Sources of force production within the dense spindle microtubule network include overlap length–dependent pushing and braking forces (green), viscous frictional drag (red), entropic expansion by diffusible cross-linkers (purple), protein clustering by frictional asymmetry (orange), and fluctuations arising from mixtures of plus end– and minus end–directed motor proteins (blue/green).
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