Computer simulations predict that chromosome movements and rotations accelerate mitotic spindle assembly without compromising accuracy

Abstract

The mitotic spindle self-assembles in prometaphase by a combination of centrosomal pathway, in which dynamically unstable microtubules search in space until chromosomes are captured, and a chromosomal pathway, in which microtubules grow from chromosomes and focus to the spindle poles. Quantitative mechanistic understanding of how spindle assembly can be both fast and accurate is lacking. Specifically, it is unclear how, if at all, chromosome movements and combining the centrosomal and chromosomal pathways affect the assembly speed and accuracy. We used computer simulations and high-resolution microscopy to test plausible pathways of spindle assembly in realistic geometry. Our results suggest that an optimal combination of centrosomal and chromosomal pathways, spatially biased microtubule growth, and chromosome movements and rotations is needed to complete prometaphase in 10 -20 min while keeping erroneous merotelic attachments down to a few percent. The simulations also provide kinetic constraints for alternative error correction mechanisms, shed light on the dual role of chromosome arm volume, and compare well with experimental data for bipolar and multipolar HT-29 colorectal cancer cells.assembly speed and accuracy ͉ merotelic attachments ͉ microtubules ͉ search and capture T he mitotic spindle is a complex molecular machine segregating chromosomes (1, 2). Molecular inventory and general principles of the spindle dynamics are becoming clear (3), but quantitative understanding of spindle mechanics in general and its self-assembly in particular is lacking. The first hypothesis of how the spindle assembles, elegantly called ''search and capture'' (Fig. 1A), was put forward in ref. 4 after the discovery of the dynamic instability phenomenon: Microtubules (MTs) grow and shorten rapidly and repeatedly from the centrosomes in random directions ''searching'' for the kinetochores (KTs), specialized chromosome structures that function as an interface between the chromosomes and the mitotic spindle. Whenever a growing MT plus end runs into a KT, this MT is stabilized; the assembly is complete when all KTs are thus captured transforming two MT asters into a typical bipolar spindle. Capture of a single astral MT by a KT has been visualized directly in newt lung cell cultures (5).How can hundreds of MTs turning over in tens of seconds capture tens of chromosomes within 10-20 min (6) is one of the fundamental questions of mitosis. Mathematical modeling has been instrumental in attempts to answer this question, because it is very hard to experimentally resolve individual MTs, follow their formation, and perturb their dynamics (7). First applications of modeling were the analyses (8, 9) suggesting that the dynamic instability parameters have to be optimized to ensure fast assembly, so that a MT switches from growth to shortening when it is as long as the distance between the centrosome and the chromosome. This analysis was extended (10) to simulate hundreds of MTs searching for tens of KTs in realistic geometry. The simulation...

Towards a quantitative understanding of mitotic spindle assembly and mechanics

“…This 'simple' picture is challenged by recent data suggesting the following: that cooperative dynamics of MTs and chromosomes are needed to assemble the spindle (O'Connell and Khodjakov, 2007), that error correction imposes stringent constraints on these dynamics (Paul et al, 2009), that the force balance and its role in maintaining the spindle length are not that simple (Dumont and Mitchison, 2009a), and that the MT spindle architecture is complex and dynamic (Yang et al, 2007;Needleman et al, 2010). In this Commentary, we focus on the following questions.…”

Contributions of Microtubule Dynamic Instability and Rotational Diffusion to Kinetochore Capture

“…The original picture of search and capture posited that MTs undergo dynamic instability nucleated from centrosomes until they form end-on attachments with KCs. Additional effects such as diffusion of KCs (12,15), a bias in MT growth toward chromosomes (16)(17)(18), MT nucleation from KCs (15,21,22), spatial distribution and rotation of chromosomes (15,19,20), and KC size decreases after capture (20) can make search and capture more rapid. MT rotational diffusion and lateral capture were found to be important mechanisms in fission yeast (24).…”

“…However, the simplest search-and-capture mechanism does not appear rapid enough to capture multiple chromosomes quickly enough to match measured time in mitosis. Extensions to the search-and-capture mechanism that can make KC capture more rapid in large cells or cell extracts include KC diffusion (12,15), MT growth that is spatially biased toward chromosomes (16)(17)(18), chromosome spatial arrangements and rotation (15,19,20), and KC-initiated MTs that can interact with searching MTs (15,21,22). KCs in human cells change size and shape during mitosis, which can both speed up capture and minimize errors (20).…”

Microtubule assembly during mitosis – from distinct origins to distinct functions?

“…This mechanism would naturally lead to the assembly of the bipolar spindle . In the past decade, both experimental and theoretical models have established that additional mechanisms are at work to help the search-andcapture process to connect all 92 kinetochores to the spindle poles in human cells (Magidson et al, 2011;Paul et al, 2009;Wollman et al, 2005).…”