Synchronizing chromosome segregation by flux-dependent force equalization at kinetochores

Springs, clutches and motors: driving forward kinetochore mechanism by modelling

As a mechanical system, the kinetochore can be viewed as a set of interacting springs, clutches and motors; the problem of kinetochore mechanism is now one of understanding how these functional modules assemble, disassemble and interact with one another to give rise to the emergent properties of the system. The sheer complexity of the kinetochore system points to a future requirement for data-driven mathematical modelling and statistical analysis based on quantitative empirical measurement of sister kinetochore trajectories. Here, we review existing models of chromosome motion in the context of recent advances in our understanding of kinetochore molecular biology.

Spindle pole mechanics studied in mitotic asters: dynamic distribution of spindle forces through compliant linkages

During cell division, chromosomes must faithfully segregate to maintain genome integrity, and this dynamic mechanical process is driven by the macromolecular machinery of the mitotic spindle. However, little is known about spindle mechanics. For example, spindle microtubules are organized by numerous cross-linking proteins yet the mechanical properties of those cross-links remain unexplored. To examine the mechanical properties of microtubule cross-links we applied optical trapping to mitotic asters that form in mammalian mitotic extracts. These asters are foci of microtubules, motors, and microtubule-associated proteins that reflect many of the functional properties of spindle poles and represent centrosome-independent spindle-pole analogs. We observed bidirectional motor-driven microtubule movements, showing that microtubule linkages within asters are remarkably compliant (mean stiffness 0.025 pN/nm) and mediated by only a handful of cross-links. Depleting the motor Eg5 reduced this stiffness, indicating that Eg5 contributes to the mechanical properties of microtubule asters in a manner consistent with its localization to spindle poles in cells. We propose that compliant linkages among microtubules provide a mechanical architecture capable of accommodating microtubule movements and distributing force among microtubules without loss of pole integrity-a mechanical paradigm that may be important throughout the spindle.

Fluorescent speckle microscopy

INTRODUCTION

Fluorescent speckle microscopy (FSM) is a live imaging and quantitative measurement technique used for analyzing motion and turnover of macromolecular assemblies in vivo and in vitro. It differs from related imaging techniques such as photobleaching and photoactivation in its use of substantially lower concentrations of fluorescently labeled assembly subunits. When small numbers of labeled subunits and large numbers of unlabeled subunits become randomly incorporated together into a macromolecular structure, the random distribution of fluorophores generates nonuniform fluorescence intensity patterns that appear as distinct puncta against low background fluorescence. These puncta, called speckles, serve as fiduciary markers so that motion and turnover of the structure are visualized. Computational analysis of speckle image data transforms FSM into a powerful tool for high-resolution quantitative analysis of macromolecular assembly dynamics. Successful application of FSM depends on the ability to reliably generate and image speckles, which are characterized by their weak emission signals, and to effectively extract quantitative information through computational analysis of speckle image data, which are characterized by their stochastic fluctuations, low signal-to-noise ratios, and high spatiotemporal complexity. This article aims to provide a practical introduction to basic principles, experimental implementation, and computational data analysis of FSM. Examples are used to show the application of FSM in analyzing the dynamic organization and assembly/disassembly of cytoskeletal filament networks, an area in which FSM analysis has found great success.

Anaphase B spindle dynamics in Drosophila S2 cells: Comparison with embryo spindles

Background

In the Drosophila melanogaster syncytial blastoderm stage embryo anaphase B is initiated by a cell cycle switch in which the suppression of microtubule minus end depolymerization and spatial reorganization of the plus ends of outwardly sliding interpolar microtubules triggers spindle elongation. RNA interference in Drosophila cultured S2 cells may present a useful tool for identifying novel components of this switch, but given the diversity of spindle design, it is important to first determine the extent of conservation of the mechanism of anaphase B in the two systems.

Results

The basic mechanism, involving an inverse correlation between poleward flux and spindle elongation is qualitatively similar in these systems, but quantitative differences exist. In S2 cells, poleward flux is only partially suppressed and the rate of anaphase B spindle elongation increases with the extent of suppression. Also, EB1-labelled microtubule plus ends redistribute away from the poles and towards the interpolar microtubule overlap zone, but this is less pronounced in S2 cells than in embryos. Finally, as in embryos, tubulin FRAP experiments revealed a reduction in the percentage recovery after photobleaching at regions proximal to the pole.

Conclusions

The basic features of the anaphase B switch, involving the suppression of poleward flux and reorganization of growing microtubule plus ends, is conserved in these systems. Thus S2 cells may be useful for rapidly identifying novel components of this switch. The quantitative differences likely reflect the adaptation of embryonic spindles for rapid, streamlined mitoses.

Towards a quantitative understanding of mitotic spindle assembly and mechanics

The 'simple' view of the mitotic spindle is that it self-assembles as a result of microtubules (MTs) randomly searching for chromosomes, after which the spindle length is maintained by a balance of outward tension exerted by molecular motors on the MTs connecting centrosomes and chromosomes, and compression generated by other motors on the MTs connecting the spindle poles. This picture is being challenged now by mounting evidence indicating that spindle assembly and maintenance rely on much more complex interconnected networks of microtubules, molecular motors, chromosomes and regulatory proteins. From an engineering point of view, three design principles of this molecular machine are especially important: the spindle assembles quickly, it assembles accurately, and it is mechanically robust--yet malleable. How is this design achieved with randomly interacting and impermanent molecular parts? Here, we review recent interdisciplinary studies that have started to shed light on this question. We discuss cooperative mechanisms of spindle self-assembly, error correction and maintenance of its mechanical properties, speculate on analogy between spindle and lamellipodial dynamics, and highlight the role of quantitative approaches in understanding the mitotic spindle design.

Photoactivatable-GFP-α-tubulin as a tool to study microtubule plus-end turnover in living human cells

The development of photactivatable (PA) variants of Green fluorescent protein (GFP) has allowed the dynamics of spatially restricted protein pools within living cells to be determined. Over the last 5 years, experiments utilizing PA-GFP fused to α-tubulin have provided important insights into the mechanisms that control microtubule dynamics in living cells. In this chapter, we describe the methodology required to generate stable cell lines expressing photoactivatable-GFP-α-tubulin and to derive quantitative measurements of tubulin turnover at microtubules plus-ends in living cells.

The perpetual movements of anaphase

One of the most extraordinary events in the lifetime of a cell is the coordinated separation of sister chromatids during cell division. This is truly the essence of the entire mitotic process and the reason for the most profound morphological changes in cytoskeleton and nuclear organization that a cell may ever experience. It all occurs within a very short time window known as "anaphase", as if the cell had spent the rest of its existence getting ready for this moment in an ultimate act of survival. And there is a good reason for this: no space for mistakes. Problems in the distribution of chromosomes during cell division have been correlated with aneuploidy, a common feature observed in cancers and several birth defects, and the main cause of spontaneous abortion in humans. In this paper, we critically review the mechanisms of anaphase chromosome motion that resisted the scrutiny of more than 100 years of research, as part of a tribute to the pioneering work of Miguel Mota.

Mitotic force generators and chromosome segregation

The mitotic spindle uses dynamic microtubules and mitotic motors to generate the pico-Newton scale forces that are needed to drive the mitotic movements that underlie chromosome capture, alignment and segregation. Here, we consider the biophysical and molecular basis of force-generation for chromosome movements in the spindle, and, with reference to the Drosophila embryo mitotic spindle, we briefly discuss how mathematical modeling can complement experimental analysis to illuminate the mechanisms of chromosome-to-pole motility during anaphase A and spindle elongation during anaphase B.

Directly probing the mechanical properties of the spindle and its matrix

Several recent models for spindle length regulation propose an elastic pole to pole spindle matrix that is sufficiently strong to bear or antagonize forces generated by microtubules and microtubule motors. We tested this hypothesis using microneedles to skewer metaphase spindles in Xenopus laevis egg extracts. Microneedle tips inserted into a spindle just outside the metaphase plate resulted in spindle movement along the interpolar axis at a velocity slightly slower than microtubule poleward flux, bringing the nearest pole toward the needle. Spindle velocity decreased near the pole, which often split apart slowly, eventually letting the spindle move completely off the needle. When two needles were inserted on either side of the metaphase plate and rapidly moved apart, there was minimal spindle deformation until they reached the poles. In contrast, needle separation in the equatorial direction rapidly increased spindle width as constant length spindle fibers pulled the poles together. These observations indicate that an isotropic spindle matrix does not make a significant mechanical contribution to metaphase spindle length determination.

Finding the middle ground: how kinetochores power chromosome congression

Genomic stability requires error-free chromosome segregation during mitosis. Chromosome congression to the spindle equator precedes chromosome segregation in anaphase and is a hallmark of metazoan mitosis. Here we review the current knowledge and concepts on the processes that underlie chromosome congression, including initial attachment to spindle microtubules, biorientation, and movements, from the perspective of the kinetochore.

Self-organization of intracellular gradients during mitosis

Gradients are used in a number of biological systems to transmit spatial information over a range of distances. The best studied are morphogen gradients where information is transmitted over many cell lengths. Smaller mitotic gradients reflect the need to organize several distinct events along the length of the mitotic spindle. The intracellular gradients that characterize mitosis are emerging as important regulatory paradigms. Intracellular gradients utilize intrinsic auto-regulatory feedback loops and diffusion to establish stable regions of activity within the mitotic cytosol. We review three recently described intracellular mitotic gradients. The Ran GTP gradient with its elaborate cascade of nuclear transport receptors and cargoes is the best characterized, yet the dynamics underlying the robust gradient of Ran-GTP have received little attention. Gradients of phosphorylation have been observed on Aurora B kinase substrates both before and after anaphase onset. In both instances the phosphorylation gradient appears to result from a soluble gradient of Aurora B kinase activity. Regulatory properties that support gradient formation are highlighted. Intracellular activity gradients that regulate localized mitotic events bare several hallmarks of self-organizing biologic systems that designate spatial information during pattern formation. Intracellular pattern formation represents a new paradigm in mitotic regulation.

Kinesin-13s in mitosis: Key players in the spatial and temporal organization of spindle microtubules

Dynamic microtubules are essential for the process of mitosis. Thus, elucidating when, where, and how microtubule dynamics are regulated is key to understanding this process. One important class of proteins that directly regulates microtubule dynamics is the Kinesin-13 family. Kinesin-13 proteins induce depolymerization uniquely from both ends of the microtubule. This activity coincides with their cellular localization and with their ability to regulate microtubule dynamics to control spindle assembly and kinetochore-microtubule attachments. In this review, we highlight recent findings that dissect the important actions of Kinesin-13 family members and summarize important studies on the regulation of their activity by phosphorylation and by protein-protein interactions.

Mitotic functions of kinesin-5

In all eukaryotic cells, molecular motor proteins play essential roles in spindle assembly and function. The homotetrameric kinesin-5 motors in particular generate outward forces that establish and maintain spindle bipolarity and contribute to microtubule flux. Cell-cycle dependent phosphorylation of kinesin-5 motors regulates their localization to the mitotic spindle. Analysis of live cells further shows that kinesin-5 motors are highly dynamic in the spindle. Understanding the interactions of kinesin-5 motors with microtubules and other spindle proteins is likely to broaden the documented roles of kinesin-5 motors during cell division.

ISWI is a RanGTP-dependent MAP required for chromosome segregation

Chromatin-remodeling factor ISWI is a microtubule-associated protein that contributes to chromosome segregation by stabilizing microtubules during anaphase.

Production of RanGTP around chromosomes induces spindle assembly by activating nuclear localization signal (NLS)–containing factors. Here, we show that the NLS protein ISWI, a known chromatin-remodeling ATPase, is a RanGTP-dependent microtubule (MT)-associated protein. Recombinant ISWI induces MT nucleation, stabilization, and bundling in vitro. In Xenopus culture cells and egg extract, ISWI localizes within the nucleus in interphase and on spindles during mitosis. Depletion of ISWI in egg extracts does not affect spindle assembly, but in anaphase spindle MTs disappear and chromosomes do not segregate. We show directly that ISWI is required for the RanGTP-dependent stabilization of MTs during anaphase independently of its effect on chromosomes. ISWI depletion in Drosophila S2 cells induces defects in spindle MTs and chromosome segregation in anaphase, and the cells eventually stop growing. Our results demonstrate that distinctly from its role in spindle assembly, RanGTP maintains spindle MTs in anaphase through the local activation of ISWI and that this is essential for proper chromosome segregation.

Drosophila S2 cells as a model system to investigate mitotic spindle dynamics, architecture, and function

In order to perpetuate their genetic content, eukaryotic cells have developed a microtubule-based machine known as the mitotic spindle. Independently of the system studied, mitotic spindles share at least one common characteristic--the dynamic nature of microtubules. This property allows the constant plasticity needed to assemble a bipolar structure, make proper kinetochore-microtubule attachments, segregate chromosomes, and finally disassemble the spindle and reform an interphase microtubule array. Here, we describe a variety of experimental approaches currently used in our laboratory to study microtubule dynamics during mitosis using Drosophila melanogaster S2 cells as a model. By using quantitative live cell imaging microscopy in combination with an advantageous labeling background, we illustrate how several cooperative pathways are used to build functional mitotic spindles. We illustrate different ways of perturbing spindle microtubule dynamics, including pharmacological inhibition and RNA interference of proteins that directly or indirectly impair microtubule dynamics. Additionally, we demonstrate the advantage of using fluorescent speckle microscopy to investigate an intrinsic property of spindle microtubules known as poleward flux. Finally, we developed a set of laser microsurgery-based experiments that allow, with unique spatiotemporal resolution, the study of specific spindle structures (e.g., centrosomes, microtubules, and kinetochores) and their respective roles during mitosis.