Force-generation and dynamic instability of microtubule bundles

Abstract

Individual dynamic microtubules can generate pushing or pulling forces when their growing or shrinking ends are in contact with cellular objects such as the cortex or chromosomes. These microtubules can operate in parallel bundles, for example when interacting with mitotic chromosomes. Here, we investigate the forcegenerating capabilities of a bundle of growing microtubules and study the effect that force has on the cooperative dynamics of such a bundle. We used an optical tweezers setup to study microtubule bundles growing against a microfabricated rigid barrier in vitro. We show that multiple microtubules can generate a pushing force that increases linearly with the number of microtubules present. In addition, the bundle can cooperatively switch to a shrinking state, due to a force-induced coupling of the dynamic instability of single microtubules. In the presence of GMPCPP, bundle catastrophes no longer occur, and high bundle forces are reached more effectively. We reproduce the observed behavior with a simple simulation of microtubule bundle dynamics that takes into account previously measured force effects on single microtubules. Using this simulation, we also show that a constant compressive force on a growing bundle leads to oscillations in bundle length that are of potential relevance for chromosome oscillations observed in living cells.chromosome oscillations ͉ optical tweezers ͉ simulations

Minimal model for collective kinetochore–microtubule dynamics

“…Individual MTs are "dynamically unstable," spontaneously switching between a polymerizing state and a depolymerizing state (1) with growth, shortening, and switching rates that are regulated by the forces exerted at the MT tips (2)(3)(4)(5)(6). For many eukaryotes, however, multiple MTs are connected to each kinetochore, giving rise to collective MT behavior that is not well understood and can be entirely different from the behavior of individual MTs.…”

“…Another model proposes that oscillations occur via a general mechanobiochemical feedback (25). Models of force-dependent MTs interacting with the same object also exhibit cooperative behavior (5,(26)(27)(28)(29). However, these models do not explain error correction dynamics.…”

Sufficient conditions for the additivity of stall forces generated by multiple filaments or motors

“…On the other hand, in similar experiments performed on microtubules, researchers have found that the growth velocity of the filament growing against an immobile barrier decreases with increased resistance -from 1.2µm per minute at zero load to 0.2µm per minute at 4pN to 5pN force, which is the putative stall force in this case [39]. In another experiment based on microtubules, using optical tweezers, Laan et al [42] report that stall forces of 2.7pN, 5.5pN and 8.1pN are produced by groups of filaments. They in-terpreted this as containing one, two and three filaments, respectively.…”

“…To understand collective force generation by polymerizing biofilaments, researchers typically resort to in vitro experiments in which biofilaments are polymerised either in the form of bundles or branched sheets against a barrier, which resists their motion by producing reaction * These two authors contributed equally forces [36,[39][40][41][42][43]. One important focus of these studies is a quantity called the stall force, which is defined as the resisting force at which the mean growth velocity of the collection of filaments is zero, and is the maximum pushing force generated by these filaments.…”

“…A number of experiments on force generation by an assembly of biofilaments have been reported in the literature [36,39,[41][42][43][44][45][46][47][48][49]. In view of the overall content and objective of our paper we will focus only on the experiments related to single and parallel bundles of growing biofilaments.…”