Model for anaphase B: role of three mitotic motors in a switch from poleward flux to spindle elongation

Unveiling inter-embryo variability in spindle length over time: Towards quantitative phenotype analysis

How can inter-individual variability be quantified? Measuring many features per experiment raises the question of choosing them to recapitulate high-dimensional data. Tackling this challenge on spindle elongation phenotypes, we showed that only three typical elongation patterns describe spindle elongation in C. elegans one-cell embryo. These archetypes, automatically extracted from the experimental data using principal component analysis (PCA), accounted for more than 95% of inter-individual variability of more than 1600 experiments across more than 100 different conditions. The two first archetypes were related to spindle average length and anaphasic elongation rate. The third archetype, accounting for 6% of the variability, was novel and corresponded to a transient spindle shortening in late metaphase, reminiscent of kinetochore function-defect phenotypes. Importantly, these three archetypes were robust to the choice of the dataset and were found even considering only non-treated conditions. Thus, the inter-individual differences between genetically perturbed embryos have the same underlying nature as natural inter-individual differences between wild-type embryos, independently of the temperatures. We thus propose that beyond the apparent complexity of the spindle, only three independent mechanisms account for spindle elongation, weighted differently in the various conditions. Interestingly, the spindle-length archetypes covered both metaphase and anaphase, suggesting that spindle elongation in late metaphase is sufficient to predict the late anaphase length. We validated this idea using a machine-learning approach. Finally, given amounts of these three archetypes could represent a quantitative phenotype. To take advantage of this, we set out to predict interacting genes from a seed based on the PCA coefficients. We exemplified this firstly on the role of tpxl-1 whose homolog tpx2 is involved in spindle microtubule branching, secondly the mechanism regulating metaphase length, and thirdly the central spindle players which set the length at anaphase. We found novel interactors not in public databases but supported by recent experimental publications.

Computational Studies Reveal How Passive Cross-Linkers Regulate Anaphase Spindle Elongation

In eukaryotic cell division, a series of events are organized to produce two daughter cells. The spindle elongation in anaphase B is essential for providing enough space to maintain cell size and distribute sister chromatids properly, which is associated with microtubules and microtubule-associated proteins such as kinesin-5 Eg5 and the Ase1-related protein, PRC1. The available experimental data indicated that after the start of anaphase B more PRC1 proteins can bind to the antiparallel microtubule pairs in the spindle but the excess amount of PRC1 proteins can lead to the failure of cell division, indicating that PRC1 proteins can regulate the spindle elongation in a concentration-dependent manner. However, the underlying mechanism of the PRC1 proteins regulating the spindle elongation has not been explained up to now. Here, we use a simplified model, where only the two important participants (kinesin-5 Eg5 motors and PRC1 proteins) are considered, to study the spindle elongation during anaphase B. We first show that only in the appropriate range of the PRC1 concentration can the spindle elongation complete properly. Furthermore, we explore the underlying mechanism of PRC1 as a regulator for spindle elongation.

Coordination, cooperation, competition, crowding and congestion of molecular motors: Theoretical models and computer simulations

Cytoskeletal motor proteins are biological nanomachines that convert chemical energy into mechanical work to carry out various functions such as cell division, cell motility, cargo transport, muscle contraction, beating of cilia and flagella, and ciliogenesis. Most of these processes are driven by the collective operation of several motors in the crowded viscous intracellular environment. Imaging and manipulation of the motors with powerful experimental probes have been complemented by mathematical analysis and computer simulations of the corresponding theoretical models. In this article, we illustrate some of the key theoretical approaches used to understand how coordination, cooperation and competition of multiple motors in the crowded intra-cellular environment drive the processes that are essential for biological function of a cell. In spite of the focus on theory, experimentalists will also find this article as an useful summary of the progress made so far in understanding multiple motor systems.

Enucleation of the C. elegans embryo revealed dynein-dependent spacing between microtubule asters

The intracellular positioning of the centrosome, a major microtubule-organizing center, is important for cellular functions. One of the features of centrosome positioning is the spacing between centrosomes; however, the underlying mechanisms are not fully understood. To characterize the spacing activity in Caenorhabditis elegans embryos, a genetic setup was developed to produce enucleated embryos. The centrosome was duplicated multiple times in the enucleated embryo, which enabled us to characterize the chromosome-independent spacing activity between sister and non-sister centrosome pairs. We found that the timely spacing depended on cytoplasmic dynein, and we propose a stoichiometric model of cortical and cytoplasmic pulling forces for the spacing between centrosomes. We also observed dynein-independent but non-muscle myosin II-dependent movement of centrosomes in the later cell cycle phase. The spacing mechanisms revealed in this study are expected to function between centrosomes in general, regardless of the presence of a chromosome/nucleus between them, including centrosome separation and spindle elongation.

Endoplasmic reticulum membranes are continuously required to maintain mitotic spindle size and forces

Membrane organelle function, localization, and proper partitioning upon cell division depend on interactions with the cytoskeleton. Whether membrane organelles also impact the function of cytoskeletal elements remains less clear. Here, we show that acute disruption of the ER around spindle poles affects mitotic spindle size and function in Drosophila syncytial embryos. Acute ER disruption was achieved through the inhibition of ER membrane fusion by the dominant-negative cytoplasmic domain of atlastin. We reveal that when centrosome-proximal ER membranes are disrupted, specifically at metaphase, mitotic spindles become smaller, despite no significant changes in microtubule dynamics. These smaller spindles are still able to mediate sister chromatid separation, yet with decreased velocity. Furthermore, by inducing mitotic exit, we found that nuclear separation and distribution are affected by ER disruption. Our results suggest that ER integrity around spindle poles is crucial for the maintenance of mitotic spindle shape and pulling forces. In addition, ER integrity also ensures nuclear spacing during syncytial divisions.

Kinetochore- and chromosome-driven transition of microtubules into bundles promotes spindle assembly

Mitotic spindle assembly is crucial for chromosome segregation and relies on bundles of microtubules that extend from the poles and overlap in the middle. However, how these structures form remains poorly understood. Here we show that overlap bundles arise through a network-to-bundles transition driven by kinetochores and chromosomes. STED super-resolution microscopy reveals that PRC1-crosslinked microtubules initially form loose arrays, which become rearranged into bundles. Kinetochores promote microtubule bundling by lateral binding via CENP-E/kinesin-7 in an Aurora B-regulated manner. Steric interactions between the bundle-associated chromosomes at the spindle midplane drive bundle separation and spindle widening. In agreement with experiments, theoretical modeling suggests that bundles arise through competing attractive and repulsive mechanisms. Finally, perturbation of overlap bundles leads to inefficient correction of erroneous kinetochore-microtubule attachments. Thus, kinetochores and chromosomes drive coarsening of a uniform microtubule array into overlap bundles, which promote not only spindle formation but also chromosome segregation fidelity.

Pathogenic mutations in the chromokinesin KIF22 disrupt anaphase chromosome segregation

The chromokinesin KIF22 generates forces that contribute to mitotic chromosome congression and alignment. Mutations in the α2 helix of the motor domain of KIF22 have been identified in patients with abnormal skeletal development, and we report the identification of a patient with a novel mutation in the KIF22 tail. We demonstrate that pathogenic mutations do not result in a loss of KIF22's functions in early mitosis. Instead, mutations disrupt chromosome segregation in anaphase, resulting in reduced proliferation, abnormal daughter cell nuclear morphology, and, in a subset of cells, cytokinesis failure. This phenotype could be explained by a failure of KIF22 to inactivate in anaphase. Consistent with this model, constitutive activation of the motor via a known site of phosphoregulation in the tail phenocopied the effects of pathogenic mutations. These results suggest that the motor domain α2 helix may be an important site for regulation of KIF22 activity at the metaphase to anaphase transition. In support of this conclusion, mimicking phosphorylation of α2 helix residue T158 also prevents inactivation of KIF22 in anaphase. These findings demonstrate the importance of both the head and tail of the motor in regulating the activity of KIF22 and offer insight into the cellular consequences of preventing KIF22 inactivation and disrupting force balance in anaphase.

Microtubule rescue at midzone edges promotes overlap stability and prevents spindle collapse during anaphase B

During anaphase B, molecular motors slide interpolar microtubules to elongate the mitotic spindle, contributing to the separation of chromosomes. However, sliding of antiparallel microtubules reduces their overlap, which may lead to spindle breakage, unless microtubules grow to compensate sliding. How sliding and growth are coordinated is still poorly understood. In this study, we have used the fission yeast S. pombe to measure microtubule dynamics during anaphase B. We report that the coordination of microtubule growth and sliding relies on promoting rescues at the midzone edges. This makes microtubules stable from pole to midzone, while their distal parts including the plus ends alternate between assembly and disassembly. Consequently, the midzone keeps a constant length throughout anaphase, enabling sustained sliding without the need for a precise regulation of microtubule growth speed. Additionally, we found that in S. pombe, which undergoes closed mitosis, microtubule growth speed decreases when the nuclear membrane wraps around the spindle midzone.

Kinesin-14 motors participate in a force balance at microtubule plus-ends to regulate dynamic instability

Kinesin-14 motors represent an essential class of molecular motors that bind to microtubules and then walk toward the microtubule minus-end. However, whether these motors can interact with growing plus-ends of microtubules to impact the lengthening of microtubules remains unknown. We found that Kinesin-14 motors could bind to a protein that resides at growing microtubule plus-ends and then pull this protein away from the growing end. This interaction acted to disrupt microtubule growth and decrease microtubule lengths in cells, likely by exerting minus-end–directed forces at the microtubule tip to alter the configuration of the growing microtubule plus-end. This work demonstrates general principles for the diverse roles that force-generating molecular motors can play in regulating cellular processes.

Kinesin-14 molecular motors represent an essential class of proteins that bind microtubules and walk toward their minus-ends. Previous studies have described important roles for Kinesin-14 motors at microtubule minus-ends, but their role in regulating plus-end dynamics remains controversial. Kinesin-14 motors have been shown to bind the EB family of microtubule plus-end binding proteins, suggesting that these minus-end–directed motors could interact with growing microtubule plus-ends. In this work, we explored the role of minus-end–directed Kinesin-14 motor forces in controlling plus-end microtubule dynamics. In cells, a Kinesin-14 mutant with reduced affinity to EB proteins led to increased microtubule lengths. Cell-free biophysical microscopy assays were performed using Kinesin-14 motors and an EB family marker of growing microtubule plus-ends, Mal3, which revealed that when Kinesin-14 motors bound to Mal3 at growing microtubule plus-ends, the motors subsequently walked toward the minus-end, and Mal3 was pulled away from the growing microtubule tip. Strikingly, these interactions resulted in an approximately twofold decrease in the expected postinteraction microtubule lifetime. Furthermore, generic minus-end–directed tension forces, generated by tethering growing plus-ends to the coverslip using λ-DNA, led to an approximately sevenfold decrease in the expected postinteraction microtubule growth length. In contrast, the inhibition of Kinesin-14 minus-end–directed motility led to extended tip interactions and to an increase in the expected postinteraction microtubule lifetime, indicating that plus-ends were stabilized by nonmotile Kinesin-14 motors. Together, we find that Kinesin-14 motors participate in a force balance at microtubule plus-ends to regulate microtubule lengths in cells.

Anaphase B: Long-standing models meet new concepts

Mitotic cell divisions ensure stable transmission of genetic information from a mother to daughter cells in a series of generations. To ensure this crucial task is accomplished, the cell forms a bipolar structure called the mitotic spindle that divides sister chromatids to the opposite sides of the dividing mother cell. After successful establishment of stable attachments of microtubules to chromosomes and inspection of connections between them, at the heart of mitosis, the cell starts the process of segregation. This spectacular moment in the life of a cell is termed anaphase, and it involves two distinct processes: depolymerization of microtubules bound to chromosomes, which is also known as anaphase A, and elongation of the spindle or anaphase B. Both processes ensure physical separation of disjointed sister chromatids. In this chapter, we review the mechanisms of anaphase B spindle elongation primarily in mammalian systems, combining different pioneering ideas and concepts with more recent findings that shed new light on the force generation and regulation of biochemical modules operating during spindle elongation. Finally, we present a comprehensive model of spindle elongation that includes structural, biophysical, and molecular aspects of anaphase B.

Modelling Nuclear Morphology and Shape Transformation: A Review

As one of the most important cellular compartments, the nucleus contains genetic materials and separates them from the cytoplasm with the nuclear envelope (NE), a thin membrane that is susceptible to deformations caused by intracellular forces. Interestingly, accumulating evidence has also indicated that the morphology change of NE is tightly related to nuclear mechanotransduction and the pathogenesis of diseases such as cancer and Hutchinson–Gilford Progeria Syndrome. Theoretically, with the help of well-designed experiments, significant progress has been made in understanding the physical mechanisms behind nuclear shape transformation in different cellular processes as well as its biological implications. Here, we review different continuum-level (i.e., energy minimization, boundary integral and finite element-based) approaches that have been developed to predict the morphology and shape change of the cell nucleus. Essential gradients, relative advantages and limitations of each model will be discussed in detail, with the hope of sparking a greater research interest in this important topic in the future.

Modeling reveals cortical dynein-dependent fluctuations in bipolar spindle length

Proper formation and maintenance of the mitotic spindle is required for faithful cell division. Although much work has been done to understand the roles of the key molecular components of the mitotic spindle, identifying the consequences of force perturbations in the spindle remains a challenge. We develop a computational framework accounting for the minimal force requirements of mitotic progression. To reflect early spindle formation, we model microtubule dynamics and interactions with major force-generating motors, excluding chromosome interactions that dominate later in mitosis. We directly integrate our experimental data to define and validate the model. We then use simulations to analyze individual force components over time and their relationship to spindle dynamics, making it distinct from previously published models. We show through both model predictions and biological manipulation that rather than achieving and maintaining a constant bipolar spindle length, fluctuations in pole-to-pole distance occur that coincide with microtubule binding and force generation by cortical dynein. Our model further predicts that high dynein activity is required for spindle bipolarity when kinesin-14 (HSET) activity is also high. To the best of our knowledge, our results provide novel insight into the role of cortical dynein in the regulation of spindle bipolarity.

Mechanisms by Which Kinesin-5 Motors Perform Their Multiple Intracellular Functions

Bipolar kinesin-5 motor proteins perform multiple intracellular functions, mainly during mitotic cell division. Their specialized structural characteristics enable these motors to perform their essential functions by crosslinking and sliding apart antiparallel microtubules (MTs). In this review, we discuss the specialized structural features of kinesin-5 motors, and the mechanisms by which these features relate to kinesin-5 functions and motile properties. In addition, we discuss the multiple roles of the kinesin-5 motors in dividing as well as in non-dividing cells, and examine their roles in pathogenetic conditions. We describe the recently discovered bidirectional motility in fungi kinesin-5 motors, and discuss its possible physiological relevance. Finally, we also focus on the multiple mechanisms of regulation of these unique motor proteins.

Mitotic poleward flux: Finding balance between microtubule dynamics and sliding

Continuous poleward motion of microtubules in metazoan mitotic spindles has been fascinating generations of cell biologists over the last several decades. In human cells, this so-called poleward flux was recently shown to be driven by the coordinated action of four mitotic kinesins. The sliding activities of kinesin-5/EG5 and kinesin-12/KIF15 are sequentially supported by kinesin-7/CENP-E at kinetochores and kinesin-4/KIF4A on chromosome arms, with the individual contributions peaking during prometaphase and metaphase, respectively. Although recent data elucidate the molecular mechanism underlying this cellular phenomenon, the functional roles of microtubule poleward flux during cell division remain largely elusive. Here, we discuss potential contribution of microtubule flux engine to various essential processes at different stages of mitosis.

The metaphase spindle at steady state - Mechanism and functions of microtubule poleward flux

The mitotic spindle is a bipolar cellular structure, built from tubulin polymers, called microtubules, and interacting proteins. This macromolecular machine orchestrates chromosome segregation, thereby ensuring accurate distribution of genetic material into the two daughter cells during cell division. Powered by GTP hydrolysis upon tubulin polymerization, the microtubule ends exhibit a metastable behavior known as the dynamic instability, during which they stochastically switch between the growth and shrinkage phases. In the context of the mitotic spindle, dynamic instability is furthermore regulated by microtubule-associated proteins and motor proteins, which enables the spindle to undergo profound changes during mitosis. This highly dynamic behavior is essential for chromosome capture and congression in prometaphase, as well as for chromosome alignment to the spindle equator in metaphase and their segregation in anaphase. In this review we focus on the mechanisms underlying microtubule dynamics and sliding and their importance for the maintenance of shape, structure and dynamics of the metaphase spindle. We discuss how these spindle properties are related to the phenomenon of microtubule poleward flux, highlighting its highly cooperative molecular basis and role in keeping the metaphase spindle at a steady state.

The multifunctional spindle midzone in vertebrate cells at a glance

During anaphase, a microtubule-containing structure called the midzone forms between the segregating chromosomes. The midzone is composed of an antiparallel array of microtubules and numerous microtubule-associated proteins that contribute to midzone formation and function. In many cells, the midzone is an important source of signals that specify the location of contractile ring assembly and constriction. The midzone also contributes to the events of anaphase by generating forces that impact chromosome segregation and spindle elongation; some midzone components contribute to both processes. The results of recent experiments have increased our understanding of the importance of the midzone, a microtubule array that has often been overlooked. This Journal of Cell Science at a Glance article will review, and illustrate on the accompanying poster, the organization, formation and dynamics of the midzone, and discuss open questions for future research.

Mechanical Mechanisms of Chromosome Segregation

Chromosome segregation—the partitioning of genetic material into two daughter cells—is one of the most crucial processes in cell division. In all Eukaryotes, chromosome segregation is driven by the spindle, a microtubule-based, self-organizing subcellular structure. Extensive research performed over the past 150 years has identified numerous commonalities and contrasts between spindles in different systems. In this review, we use simple coarse-grained models to organize and integrate previous studies of chromosome segregation. We discuss sites of force generation in spindles and fundamental mechanical principles that any understanding of chromosome segregation must be based upon. We argue that conserved sites of force generation may interact differently in different spindles, leading to distinct mechanical mechanisms of chromosome segregation. We suggest experiments to determine which mechanical mechanism is operative in a particular spindle under study. Finally, we propose that combining biophysical experiments, coarse-grained theories, and evolutionary genetics will be a productive approach to enhance our understanding of chromosome segregation in the future.

Spindle scaling mechanisms

The mitotic spindle robustly scales with cell size in a plethora of different organisms. During development and throughout evolution, the spindle adjusts to cell size in metazoans and yeast in order to ensure faithful chromosome separation. Spindle adjustment to cell size occurs by the scaling of spindle length, spindle shape and the velocity of spindle assembly and elongation. Different mechanisms, depending on spindle structure and organism, account for these scaling relationships. The limited availability of critical spindle components, protein gradients, sequestration of spindle components, or post-translational modification and differential expression levels have been implicated in the regulation of spindle length and the spindle assembly/elongation velocity in a cell size-dependent manner. In this review, we will discuss the phenomenon and mechanisms of spindle length, spindle shape and spindle elongation velocity scaling with cell size.

Ase1 domains dynamically slow anaphase spindle elongation and recruit Bim1 to the midzone

How cells regulate microtubule cross-linking activity to control the rate and duration of spindle elongation during anaphase is poorly understood. In this study, we test the hypothesis that PRC1/Ase1 proteins use distinct microtubule-binding domains to control the spindle elongation rate. Using the budding yeast Ase1, we identify unique contributions for the spectrin and carboxy-terminal domains during different phases of spindle elongation. We show that the spectrin domain uses conserved basic residues to promote the recruitment of Ase1 to the midzone before anaphase onset and slow spindle elongation during early anaphase. In contrast, a partial Ase1 carboxy-terminal truncation fails to form a stable midzone in late anaphase, produces higher elongation rates after early anaphase, and exhibits frequent spindle collapses. We find that the carboxy-terminal domain interacts with the plus-end tracking protein EB1/Bim1 and recruits Bim1 to the midzone to maintain midzone length. Overall, our results suggest that the Ase1 domains provide cells with a modular system to tune midzone activity and control elongation rates.

Stoichiometric interactions explain spindle dynamics and scaling across 100 million years of nematode evolution

The spindle shows remarkable diversity, and changes in an integrated fashion, as cells vary over evolution. Here, we provide a mechanistic explanation for variations in the first mitotic spindle in nematodes. We used a combination of quantitative genetics and biophysics to rule out broad classes of models of the regulation of spindle length and dynamics, and to establish the importance of a balance of cortical pulling forces acting in different directions. These experiments led us to construct a model of cortical pulling forces in which the stoichiometric interactions of microtubules and force generators (each force generator can bind only one microtubule), is key to explaining the dynamics of spindle positioning and elongation, and spindle final length and scaling with cell size. This model accounts for variations in all the spindle traits we studied here, both within species and across nematode species spanning over 100 million years of evolution.

Force-generating mechanisms of anaphase in human cells

What forces drive chromosome segregation remains one of the most challenging questions in cell division. Even though the duration of anaphase is short, it is of utmost importance for genome fidelity that no mistakes are made. Seminal studies in model organisms have revealed different mechanisms operating during chromosome segregation in anaphase, but the translation of these mechanisms to human cells is not straightforward. Recent work has shown that kinetochore fiber depolymerization during anaphase A is largely motor independent, whereas spindle elongation during anaphase B is coupled to sliding of interpolar microtubules in human cells. In this Review, we discuss the current knowledge on the mechanisms of force generation by kinetochore, interpolar and astral microtubules. By combining results from numerous studies, we propose a comprehensive picture of the role of individual force-producing and -regulating proteins. Finally, by linking key concepts of anaphase to most recent data, we summarize the contribution of all proposed mechanisms to chromosome segregation and argue that sliding of interpolar microtubules and depolymerization at the kinetochore are the main drivers of chromosome segregation during early anaphase in human cells.

Central-spindle microtubules are strongly coupled to chromosomes during both anaphase A and anaphase B

Spindle microtubules, whose dynamics vary over time and at different locations, cooperatively drive chromosome segregation. Measurements of microtubule dynamics and spindle ultrastructure can provide insight into the behaviors of microtubules, helping elucidate the mechanism of chromosome segregation. Much work has focused on the dynamics and organization of kinetochore microtubules, that is, on the region between chromosomes and poles. In comparison, microtubules in the central-spindle region, between segregating chromosomes, have been less thoroughly characterized. Here, we report measurements of the movement of central-spindle microtubules during chromosome segregation in human mitotic spindles and Caenorhabditis elegans mitotic and female meiotic spindles. We found that these central-spindle microtubules slide apart at the same speed as chromosomes, even as chromosomes move toward spindle poles. In these systems, damaging central-spindle microtubules by laser ablation caused an immediate and complete cessation of chromosome motion, suggesting a strong coupling between central-spindle microtubules and chromosomes. Electron tomographic reconstruction revealed that the analyzed anaphase spindles all contain microtubules with both ends between segregating chromosomes. Our results provide new dynamical, functional, and ultrastructural characterizations of central-spindle microtubules during chromosome segregation in diverse spindles and suggest that central-spindle microtubules and chromosomes are strongly coupled in anaphase.

The evolution of spindles and their mechanical implications for cancer metastasis

The mitotic spindle has long been known to play a crucial role in mitosis, orchestrating the segregation of chromosomes into two daughter cells during mitosis with high fidelity. Intracellular forces generated by the mitotic spindle are increasingly well understood, and recent work has revealed that the efficiency and the accuracy of mitosis is ensured by the scaling of mitotic spindle size with cell size. However, the role of the spindle in cancer progression has largely been ignored. Two recent studies point toward the role of mitotic spindle evolution in cancer progression through extracellular force generation. Cancer cells with lengthened spindles exhibit highly increased metastatic potential. Further, interpolar spindle elongation drives protrusive extracellular force generation along the mitotic axis to allow mitotic elongation, a morphological change that is required for cell division. Together, these findings open a new research area studying the role of the mitotic spindle evolution in cancer metastasis.

Kinesin-6 regulates cell-size-dependent spindle elongation velocity to keep mitosis duration constant in fission yeast

The length of the mitotic spindle scales with cell size in a wide range of organisms during embryonic development. Interestingly, in C. elegans embryos, this goes along with temporal regulation: larger cells speed up spindle assembly and elongation. We demonstrate that, similarly in fission yeast, spindle length and spindle dynamics adjust to cell size, which allows to keep mitosis duration constant. Since prolongation of mitosis was shown to affect cell viability, this may resemble a mechanism to regulate mitosis duration. We further reveal how the velocity of spindle elongation is regulated: coupled to cell size, the amount of kinesin-6 Klp9 molecules increases, resulting in an acceleration of spindle elongation in anaphase B. In addition, the number of Klp9 binding sites to microtubules increases overproportionally to Klp9 molecules, suggesting that molecular crowding inversely correlates to cell size and might have an impact on spindle elongation velocity control.

Bidirectional motility of kinesin-5 motor proteins: structural determinants, cumulative functions and physiological roles

Mitotic kinesin-5 bipolar motor proteins perform essential functions in mitotic spindle dynamics by crosslinking and sliding antiparallel microtubules (MTs) apart within the mitotic spindle. Two recent studies have indicated that single molecules of Cin8, the Saccharomyces cerevisiae kinesin-5 homolog, are minus end-directed when moving on single MTs, yet switch directionality under certain experimental conditions (Gerson-Gurwitz et al., EMBO J 30:4942-4954, 2011; Roostalu et al., Science 332:94-99, 2011). This finding was unexpected since the Cin8 catalytic motor domain is located at the N-terminus of the protein, and such kinesins have been previously thought to be exclusively plus end-directed. In addition, the essential intracellular functions of kinesin-5 motors in separating spindle poles during mitosis can only be accomplished by plus end-directed motility during antiparallel sliding of the spindle MTs. Thus, the mechanism and possible physiological role of the minus end-directed motility of kinesin-5 motors remain unclear. Experimental and theoretical studies from several laboratories in recent years have identified additional kinesin-5 motors that are bidirectional, revealed structural determinants that regulate directionality, examined the possible mechanisms involved and have proposed physiological roles for the minus end-directed motility of kinesin-5 motors. Here, we summarize our current understanding of the remarkable ability of certain kinesin-5 motors to switch directionality when moving along MTs.

Mitotic spindle: kinetochore fibers hold on tight to interpolar bundles

When a cell starts to divide, it forms a spindle, a micro-machine made of microtubules, which separates the duplicated chromosomes. The attachment of microtubules to chromosomes is mediated by kinetochores, protein complexes on the chromosome. Spindle microtubules can be divided into three major classes: kinetochore microtubules, which form k-fibers ending at the kinetochore; interpolar microtubules, which extend from the opposite sides of the spindle and interact in the middle; and astral microtubules, which extend towards the cell cortex. Recent work in human cells has shown a close relationship between interpolar and kinetochore microtubules, where interpolar bundles are attached laterally to kinetochore fibers almost all along their length, acting as a bridge between sister k-fibers. Most of the interpolar bundles are attached to a pair of sister kinetochore fibers and vice versa. Thus, the spindle is made of modules consisting of a pair of sister kinetochore fibers and a bundle of interpolar microtubules that connects them. These interpolar bundles, termed bridging fibers, balance the forces acting at kinetochores and support the rounded shape of the spindle during metaphase. This review discusses the structure, function, and formation of kinetochore fibers and interpolar bundles, with an emphasis on how they interact. Their connections have an impact on the force balance in the spindle and on chromosome movement during mitosis because the forces in interpolar bundles are transmitted to kinetochore fibers and hence to kinetochores through these connections.

Motor axon navigation relies on Fidgetin-like 1-driven microtubule plus end dynamics

Fassier et al. identify Fidgetin-like 1 (Fignl1) as a key growth cone (GC)-enriched microtubule (MT)-associated protein in motor circuit wiring. They show that Fignl1 modulates motor GC morphology and steering behavior by down-regulating EB binding at MT plus ends and promoting MT depolymerization beneath the cell cortex.

During neural circuit assembly, extrinsic signals are integrated into changes in growth cone (GC) cytoskeleton underlying axon guidance decisions. Microtubules (MTs) were shown to play an instructive role in GC steering. However, the numerous actors required for MT remodeling during axon navigation and their precise mode of action are far from being deciphered. Using loss- and gain-of-function analyses during zebrafish development, we identify in this study the meiotic clade adenosine triphosphatase Fidgetin-like 1 (Fignl1) as a key GC-enriched MT-interacting protein in motor circuit wiring and larval locomotion. We show that Fignl1 controls GC morphology and behavior at intermediate targets by regulating MT plus end dynamics and growth directionality. We further reveal that alternative translation of Fignl1 transcript is a sophisticated mechanism modulating MT dynamics: a full-length isoform regulates MT plus end–tracking protein binding at plus ends, whereas shorter isoforms promote their depolymerization beneath the cell cortex. Our study thus pinpoints Fignl1 as a multifaceted key player in MT remodeling underlying motor circuit connectivity.

Kinesin-5 mediated chromosome congression in insect spindles

INTRODUCTION

The microtubule motor protein kinesin-5 is well known to establish the bipolar spindle by outward sliding of antiparallel interpolar microtubules. In yeast, kinesin-5 also facilitates chromosome alignment "congression" at the spindle equator by preferentially depolymerizing long kinetochore microtubules (kMTs). The motor protein kinesin-8 has also been linked to chromosome congression. Therefore, we sought to determine whether kinesin-5 or kinesin-8 facilitates chromosome congression in insect spindles.

METHODS

RNAi of the kinesin-5 Klp61F and kinesin-8 Klp67A were performed separately in Drosophila melanogaster S2 cells to test for inhibited chromosome congression. Klp61F RNAi, Klp67A RNAi, and control metaphase mitotic spindles expressing fluorescent tubulin and fluorescent Cid were imaged, and their fluorescence distributions were compared.

RESULTS

RNAi of Klp61F with a weak Klp61F knockdown resulted in longer kMTs and less congressed kinetochores compared to control over a range of conditions, consistent with kinesin-5 length-dependent depolymerase activity. RNAi of the kinesin-8 Klp67A revealed that kMTs relative to the spindle lengths were not longer compared to control, but rather that the spindles were longer, indicating that Klp67A acts preferentially as a length-dependent depolymerase on interpolar microtubules without significantly affecting kMT length and chromosome congression.

CONCLUSIONS

This study demonstrates that in addition to establishing the bipolar spindle, kinesin-5 regulates kMT length to facilitate chromosome congression in insect spindles. It expands on previous yeast studies, and it expands the role of kinesin-5 to include kMT assembly regulation in eukaryotic mitosis.

Eml1 loss impairs apical progenitor spindle length and soma shape in the developing cerebral cortex

The ventricular zone (VZ) of the developing cerebral cortex is a pseudostratified epithelium that contains progenitors undergoing precisely regulated divisions at its most apical side, the ventricular lining (VL). Mitotic perturbations can contribute to pathological mechanisms leading to cortical malformations. The HeCo mutant mouse exhibits subcortical band heterotopia (SBH), likely to be initiated by progenitor delamination from the VZ early during corticogenesis. The causes for this are however, currently unknown. Eml1, a microtubule (MT)-associated protein of the EMAP family, is impaired in these mice. We first show that MT dynamics are perturbed in mutant progenitor cells in vitro. These may influence interphase and mitotic MT mechanisms and indeed, centrosome and primary cilia were altered and spindles were found to be abnormally long in HeCo progenitors. Consistently, MT and spindle length regulators were identified in EML1 pulldowns from embryonic brain extracts. Finally, we found that mitotic cell shape is also abnormal in the mutant VZ. These previously unidentified VZ characteristics suggest altered cell constraints which may contribute to cell delamination.

A novel immunofluorescence method to visualize microtubules in the antiparallel overlaps of microtubule-plus ends in the anaphase and telophase midzone

Cell division, in which duplicated chromosomes are separated into two daughter cells, is the most dynamic event during cell proliferation. Chromosome movement is powered mainly by microtubules, which vary in morphology and are organized into characteristic structures according to mitotic progression. During the later stages of mitosis, antiparallel microtubules form the spindle midzone, and the irregular formation of the midzone often leads to failure of cytokinesis, giving rise to the unequal segregation of chromosomes. However, it is difficult to analyze the morphology of these microtubules because microtubules in the antiparallel overlaps of microtubule-plus ends in the midzone are embedded in highly electron-dense matrices, impeding the access of anti-tubulin antibodies to their epitopes during immunofluorescence staining. Here, we developed a novel method to visualize selectively antiparallel microtubule overlaps in the midzone. When cells are air-dried before fixation, aligned α-tubulin staining is observed and colocalized with PRC1 in the center of the midzone of anaphase and telophase cells, suggesting that antiparallel microtubule overlaps can be visualized by this method. In air-dried cells, mCherry-α-tubulin fluorescence and β-tubulin staining show almost the same pattern as α-tubulin staining in the midzone, suggesting that the selective visualization of antiparallel microtubule overlaps in air-dried cells is not attributed to an alteration of the antigenicity of α-tubulin. Taxol treatment extends the microtubule filaments of the midzone in air-dried cells, and nocodazole treatment conversely decreases the number of microtubules, suggesting that unstable microtubules are depolymerized during the air-drying method. It is of note that the air-drying method enables the detection of the disruption of the midzone and premature midzone formation upon Aurora B and Plk1 inhibition, respectively. These results suggest that the air-drying method is suitable for visualizing microtubules in the antiparallel overlaps of microtubule-plus ends of the midzone and for detecting their effects on midzone formation.

Microtubule Sliding within the Bridging Fiber Pushes Kinetochore Fibers Apart to Segregate Chromosomes

During cell division, mitotic spindle microtubules segregate chromosomes by exerting forces on kinetochores. What forces drive chromosome segregation in anaphase remains a central question. The current model for anaphase in human cells includes shortening of kinetochore fibers and separation of spindle poles. Both processes require kinetochores to be linked with the poles. Here we show, by combining laser ablation, photoactivation, and theoretical modeling, that kinetochores can separate without any attachment to one spindle pole. This separation requires the bridging fiber, a microtubule bundle that connects sister kinetochore fibers. Bridging fiber microtubules in intact spindles slide apart with kinetochore fibers, indicating strong crosslinks between them. We conclude that sliding of microtubules within the bridging fibers drives pole separation and pushes kinetochore fibers poleward by the friction of passive crosslinks between these fibers. Thus, sliding within the bridging fiber works together with the shortening of kinetochore fibers to segregate chromosomes.

Molecular mechanisms of kinesin-14 motors in spindle assembly and chromosome segregation

During eukaryote cell division, molecular motors are crucial regulators of microtubule organization, spindle assembly, chromosome segregation and intracellular transport. The kinesin-14 motors are evolutionarily conserved minus-end-directed kinesin motors that occur in diverse organisms from simple yeasts to higher eukaryotes. Members of the kinesin-14 motor family can bind to, crosslink or slide microtubules and, thus, regulate microtubule organization and spindle assembly. In this Commentary, we present the common subthemes that have emerged from studies of the molecular kinetics and mechanics of kinesin-14 motors, particularly with regard to their non-processive movement, their ability to crosslink microtubules and interact with the minus- and plus-ends of microtubules, and with microtubule-organizing center proteins. In particular, counteracting forces between minus-end-directed kinesin-14 and plus-end-directed kinesin-5 motors have recently been implicated in the regulation of microtubule nucleation. We also discuss recent progress in our current understanding of the multiple and fundamental functions that kinesin-14 motors family members have in important aspects of cell division, including the spindle pole, spindle organization and chromosome segregation.

Merotelic kinetochore attachment in oocyte meiosis II causes sister chromatids segregation errors in aged mice

Mammalian oocyte chromosomes undergo 2 meiotic divisions to generate haploid gametes. The frequency of chromosome segregation errors during meiosis I increase with age. However, little attention has been paid to the question of how aging affects sister chromatid segregation during oocyte meiosis II. More importantly, how aneuploid metaphase II (MII) oocytes from aged mice evade the spindle assembly checkpoint (SAC) mechanism to complete later meiosis II to form aneuploid embryos remains unknown. Here, we report that MII oocytes from naturally aged mice exhibited substantial errors in chromosome arrangement and configuration compared with young MII oocytes. Interestingly, these errors in aged oocytes had no impact on anaphase II onset and completion as well as 2-cell formation after parthenogenetic activation. Further study found that merotelic kinetochore attachment occurred more frequently and could stabilize the kinetochore-microtubule interaction to ensure SAC inactivation and anaphase II onset in aged MII oocytes. This orientation could persist largely during anaphase II in aged oocytes, leading to severe chromosome lagging and trailing as well as delay of anaphase II completion. Therefore, merotelic kinetochore attachment in oocyte meiosis II exacerbates age-related genetic instability and is a key source of age-dependent embryo aneuploidy and dysplasia.

Theoretical model of mitotic spindle microtubule growth for FRAP curve interpretation

Background

Spindle FRAP curves depend on the kinetic parameters of microtubule polymerization and depolymerization. The empirical FRAP curve proposed earlier permits determination of only one such dynamic parameter, commonly called the "tubulin turnover". The aim of our study was to build a FRAP curve based on an already known kinetic model of microtubule growth.

Results

A numerical expression that describes the distribution of polymerizing and depolymerizing microtubule ends as a function of four kinetic parameters is presented. In addition, a theoretical FRAP curve for the metaphase spindle is constructed using previously published dynamic parameters.

Conclusion

The numerical expression we elaborated can replace the empirical FRAP curve described earlier for a spindle comprising fluorescently marked microtubules. The curve we generated fits well the experimental data.

Physical determinants of bipolar mitotic spindle assembly and stability in fission yeast

Mitotic spindles use an elegant bipolar architecture to segregate duplicated chromosomes with high fidelity. Bipolar spindles form from a monopolar initial condition; this is the most fundamental construction problem that the spindle must solve. Microtubules, motors, and cross-linkers are important for bipolarity, but the mechanisms necessary and sufficient for spindle assembly remain unknown. We describe a physical model that exhibits de novo bipolar spindle formation. We began with physical properties of fission-yeast spindle pole body size and microtubule number, kinesin-5 motors, kinesin-14 motors, and passive cross-linkers. Our model results agree quantitatively with our experiments in fission yeast, thereby establishing a minimal system with which to interrogate collective self-assembly. By varying the features of our model, we identify a set of functions essential for the generation and stability of spindle bipolarity. When kinesin-5 motors are present, their bidirectionality is essential, but spindles can form in the presence of passive cross-linkers alone. We also identify characteristic failed states of spindle assembly-the persistent monopole, X spindle, separated asters, and short spindle, which are avoided by the creation and maintenance of antiparallel microtubule overlaps. Our model can guide the identification of new, multifaceted strategies to induce mitotic catastrophes; these would constitute novel strategies for cancer chemotherapy.

Anaphase B

Anaphase B spindle elongation is characterized by the sliding apart of overlapping antiparallel interpolar (ip) microtubules (MTs) as the two opposite spindle poles separate, pulling along disjoined sister chromatids, thereby contributing to chromosome segregation and the propagation of all cellular life. The major biochemical “modules” that cooperate to mediate pole–pole separation include: (i) midzone pushing or (ii) braking by MT crosslinkers, such as kinesin-5 motors, which facilitate or restrict the outward sliding of antiparallel interpolar MTs (ipMTs); (iii) cortical pulling by disassembling astral MTs (aMTs) and/or dynein motors that pull aMTs outwards; (iv) ipMT plus end dynamics, notably net polymerization; and (v) ipMT minus end depolymerization manifest as poleward flux. The differential combination of these modules in different cell types produces diversity in the anaphase B mechanism. Combinations of antagonist modules can create a force balance that maintains the dynamic pre-anaphase B spindle at constant length. Tipping such a force balance at anaphase B onset can initiate and control the rate of spindle elongation. The activities of the basic motor filament components of the anaphase B machinery are controlled by a network of non-motor MT-associated proteins (MAPs), for example the key MT cross-linker, Ase1p/PRC1, and various cell-cycle kinases, phosphatases, and proteases. This review focuses on the molecular mechanisms of anaphase B spindle elongation in eukaryotic cells and briefly mentions bacterial DNA segregation systems that operate by spindle elongation.

Kinesin-5 Contributes to Spindle-length Scaling in the Evolution of Cancer toward Metastasis

During natural evolution, the spindles often scale with cell sizes to orchestrate accurate chromosome segregation. Whether in cancer evolution, when the constraints on genome integrity are relaxed, cancer cells may evolve the spindle to confer other advantages has not been investigated. Using invasion as a selective pressure in vitro, we found that a highly metastatic cancer clone displays a lengthened metaphase spindle, with faster spindle elongation that correlates with transiently elevated speed of cell migration. We found that kinesin-5 is upregulated in this malignant clone, and weak inhibition of kinesin-5 activity could revert the spindle to a smaller aspect ratio, decrease the speed of spindle pole separation, and suppress post-mitotic cell migration. A correlation was found between high aspect ratio and strong metastatic potential in cancers that evolved and were selected in vivo, implicating that the spindle aspect ratio could serve as a promising cellular biomarker for metastatic cancer clones.

Inducible fluorescent speckle microscopy

Generation of high-contrast and high-signal fluorescent 3D speckles allows fluorescent speckle microscopy to be performed in readily available libraries of cell lines and primary tissues for the measurement of microtubule turnover and sliding.

The understanding of cytoskeleton dynamics has benefited from the capacity to generate fluorescent fiducial marks on cytoskeleton components. Here we show that light-induced imprinting of three-dimensional (3D) fluorescent speckles significantly improves speckle signal and contrast relative to classic (random) fluorescent speckle microscopy. We predict theoretically that speckle imprinting using photobleaching is optimal when the laser energy and fluorophore responsivity are related by the golden ratio. This relation, which we confirm experimentally, translates into a 40% remaining signal after speckle imprinting and provides a rule of thumb in selecting the laser power required to optimally prepare the sample for imaging. This inducible speckle imaging (ISI) technique allows 3D speckle microscopy to be performed in readily available libraries of cell lines or primary tissues expressing fluorescent proteins and does not preclude conventional imaging before speckle imaging. As a proof of concept, we use ISI to measure metaphase spindle microtubule poleward flux in primary cells and explore a scaling relation connecting microtubule flux to metaphase duration.

Mechanism for Anaphase B: Evaluation of "Slide-and-Cluster" versus "Slide-and-Flux-or-Elongate" Models

Elongation of the mitotic spindle during anaphase B contributes to chromosome segregation in many cells. Here, we quantitatively test the ability of two models for spindle length control to describe the dynamics of anaphase B spindle elongation using experimental data from Drosophila embryos. In the slide-and-flux-or-elongate (SAFE) model, kinesin-5 motors persistently slide apart antiparallel interpolar microtubules (ipMTs). During pre-anaphase B, this outward sliding of ipMTs is balanced by depolymerization of their minus ends at the poles, producing poleward flux, while the spindle maintains a constant length. Following cyclin B degradation, ipMT depolymerization ceases so the sliding ipMTs can push the poles apart. The competing slide-and-cluster (SAC) model proposes that MTs nucleated at the equator are slid outward by the cooperative actions of the bipolar kinesin-5 and a minus-end-directed motor, which then pulls the sliding MTs inward and clusters them at the poles. In assessing both models, we assume that kinesin-5 preferentially cross-links and slides apart antiparallel MTs while the MT plus ends exhibit dynamic instability. However, in the SAC model, minus-end-directed motors bind the minus ends of MTs as cargo and transport them poleward along adjacent, parallel MT tracks, whereas in the SAFE model, all MT minus ends that reach the pole are depolymerized by kinesin-13. Remarkably, the results show that within a narrow range of MT dynamic instability parameters, both models can reproduce the steady-state length and dynamics of pre-anaphase B spindles and the rate of anaphase B spindle elongation. However, only the SAFE model reproduces the change in MT dynamics observed experimentally at anaphase B onset. Thus, although both models explain many features of anaphase B in this system, our quantitative evaluation of experimental data regarding several different aspects of spindle dynamics suggests that the SAFE model provides a better fit.

Intercentrosomal angular separation during mitosis plays a crucial role for maintaining spindle stability

Cell division through proper spindle formation is one of the key puzzles in cell biology. In most mammalian cells, chromosomes spontaneously arrange to achieve a stable bipolar spindle during metaphase which eventually ensures proper segregation of the DNA into the daughter cells. In this paper, we present a robust three-dimensional mechanistic model to investigate the formation and maintenance of a bipolar mitotic spindle in mammalian cells under different physiological constraints. Using realistic parameters, we test spindle viability by measuring the spindle length and studying the chromosomal configuration. The model strikingly predicts a feature of the spindle instability arising from the insufficient intercentrosomal angular separation and impaired sliding of the interpolar microtubules. In addition, our model successfully reproduces chromosomal patterns observed in mammalian cells, when activity of different motor proteins is perturbed.

Fission yeast kinesin-8 controls chromosome congression independently of oscillations

In higher eukaryotes, efficient chromosome congression relies, among other players, on the activity of chromokinesins. Here, we provide a quantitative analysis of kinetochore oscillations and positioning in Schizosaccharomyces pombe, a model organism lacking chromokinesins. In wild-type cells, chromosomes align during prophase and, while oscillating, maintain this alignment throughout metaphase. Chromosome oscillations are dispensable both for kinetochore congression and stable kinetochore alignment during metaphase. In higher eukaryotes, kinesin-8 family members control chromosome congression by regulating their oscillations. By contrast, here, we demonstrate that fission yeast kinesin-8 controls chromosome congression by an alternative mechanism. We propose that kinesin-8 aligns chromosomes by controlling pulling forces in a length-dependent manner. A coarse-grained model of chromosome segregation implemented with a length-dependent process that controls the force at kinetochores is necessary and sufficient to mimic kinetochore alignment, and prevents the appearance of lagging chromosomes. Taken together, these data illustrate how the local action of a motor protein at kinetochores provides spatial cues within the spindle to align chromosomes and to prevent aneuploidy.

Highlighted Article: Quantitative analysis in S. pombe reveals that chromosome oscillations are dispensable for kinetochore congression in mitosis. Kinesin-8 controls chromosome congression through length-dependent pulling forces.

Direct interaction between centralspindlin and PRC1 reinforces mechanical resilience of the central spindle

During animal cell division, the central spindle, an anti-parallel microtubule bundle structure formed between segregating chromosomes during anaphase, cooperates with astral microtubules to position the cleavage furrow. Because the central spindle is the only structure linking the two halves of the mitotic spindle, it is under mechanical tension from dynein-generated cortical pulling forces, which determine spindle positioning and drive chromosome segregation through spindle elongation. The central spindle should be flexible enough for efficient chromosome segregation while maintaining its structural integrity for reliable cytokinesis. How the cell balances these potentially conflicting requirements is poorly understood. Here, we demonstrate that the central spindle in C. elegans embryos has a resilient mechanism for recovery from perturbations by excess tension derived from cortical pulling forces. This mechanism involves the direct interaction of two different types of conserved microtubule bundlers that are crucial for central spindle formation, PRC1 and centralspindlin.

Motor function in interpolar microtubules during metaphase

We analyze experimental motility assays of microtubules undergoing small fluctuations about a "balance point" when mixed in solution of two different kinesin motor proteins, KLP61F and Ncd. It has been proposed that the microtubule movement is due to stochastic variations in the densities of the two species of motor proteins. We test this hypothesis here by showing how it maps onto a one-dimensional random walk in a random environment. Our estimate of the amplitude of the fluctuations agrees with experimental observations. We point out that there is an initial transient in the position of the microtubule where it will typically move of order its own length. We compare the physics of this gliding assay to a recent theory of the role of antagonistic motors on restricting interpolar microtubule sliding of a cell's mitotic spindle during prometaphase. It is concluded that randomly positioned antagonistic motors can restrict relative movement of microtubules, however they do so imperfectly. A variation in motor concentrations is also analyzed and shown to lead to greater control of spindle length.

Chromatids segregate without centrosomes during Caenorhabditis elegans mitosis in a Ran- and CLASP-dependent manner

Laser ablation of centrosomes in one-cell Caenorhabditis elegans embryos shows that chromatids can segregate independently of centrosomes and also independently of the activity of kinetochore microtubules during mitosis. CLASP and RanGTP are required to generate this centrosome-independent force, whereas SPD-1 and BMK-1 act as brakes to oppose it.

During mitosis, chromosomes are connected to a microtubule-based spindle. Current models propose that displacement of the spindle poles and/or the activity of kinetochore microtubules generate mechanical forces that segregate sister chromatids. Using laser destruction of the centrosomes during Caenorhabditis elegans mitosis, we show that neither of these mechanisms is necessary to achieve proper chromatid segregation. Our results strongly suggest that an outward force generated by the spindle midzone, independently of centrosomes, is sufficient to segregate chromosomes in mitotic cells. Using mutant and RNAi analysis, we show that the microtubule-bundling protein SPD-1/MAP-65 and BMK-1/kinesin-5 act as a brake opposing the force generated by the spindle midzone. Conversely, we identify a novel role for two microtubule-growth and nucleation agents, Ran and CLASP, in the establishment of the centrosome-independent force during anaphase. Their involvement raises the interesting possibility that microtubule polymerization of midzone microtubules is continuously required to sustain chromosome segregation during mitosis.

The microtubule cross-linker Feo controls the midzone stability, motor composition, and elongation of the anaphase B spindle in Drosophila embryos

The antiparallel MT-MT cross-linking nonmotor MAP, Feo, controls the organization, stability, and motor composition of the Drosophila embryo anaphase B spindle midzone, thereby facilitating the kinesin-5-driven sliding filament mechanism underlying proper anaphase B spindle elongation and chromosome segregation.

Chromosome segregation during anaphase depends on chromosome-to-pole motility and pole-to-pole separation. We propose that in Drosophila embryos, the latter process (anaphase B) depends on a persistent kinesin-5–generated interpolar (ip) microtubule (MT) sliding filament mechanism that “engages” to push apart the spindle poles when poleward flux is turned off. Here we investigated the contribution of the midzonal, antiparallel MT-cross-linking nonmotor MAP, Feo, to this “slide-and-flux-or-elongate” mechanism. Whereas Feo homologues in other systems enhance the midzone localization of the MT-MT cross-linking motors kinesin-4, -5 and -6, the midzone localization of these motors is respectively enhanced, reduced, and unaffected by Feo. Strikingly, kinesin-5 localizes all along ipMTs of the anaphase B spindle in the presence of Feo, including at the midzone, but the antibody-induced dissociation of Feo increases kinesin-5 association with the midzone, which becomes abnormally narrow, leading to impaired anaphase B and incomplete chromosome segregation. Thus, although Feo and kinesin-5 both preferentially cross-link MTs into antiparallel polarity patterns, kinesin-5 cannot substitute for loss of Feo function. We propose that Feo controls the organization, stability, and motor composition of antiparallel ipMTs at the midzone, thereby facilitating the kinesin-5–driven sliding filament mechanism underlying proper anaphase B spindle elongation and chromosome segregation.

Sliding filaments and mitotic spindle organization

Mitosis depends upon the action of the mitotic spindle, a subcellular machine that uses microtubules (MTs) and motors to assemble itself and to coordinate chromosome segregation. Recent work illuminates how the motor-driven poleward sliding of MTs - nucleated at centrosomes, chromosomes and on pre-existing MTs - contributes to spindle assembly and length control.

KIF4A and PP2A-B56 form a spatially restricted feedback loop opposing Aurora B at the anaphase central spindle

The mitotic kinase Aurora B is concentrated at the anaphase central spindle by the kinesin MKlp2 during mitotic exit and cytokinesis. This pool of Aurora B phosphorylates substrates including the kinesin KIF4A to regulate central spindle length. In this paper, we identify a counteracting system in which PP2A-B56γ and -ε, but not PP2A-B56α, -β, and -δ, are maintained at the central spindle by KIF4A. Biochemical assays show that PP2A-B56γ can dephosphorylate the T799 Aurora B site on KIF4A and thereby counteract the Aurora B- and microtubule-stimulated ATPase activity of KIF4A. In agreement with these observations, combined silencing of PP2A-B56γ and -ε resulted in increased phosphorylation of KIF4A T799 and decreased central spindle growth in anaphase B. Furthermore, reduced turnover of regulatory phosphorylation on another Aurora B substrate MKlp1 was observed, suggesting that PP2A-B56γ and -ε play a general role opposing Aurora B at the central spindle. KIF4A and PP2A-B56γ and -ε therefore create a spatially restricted negative feedback loop counteracting Aurora B in anaphase.

Measuring microtubule polarity in spindles with second-harmonic generation

The spatial organization of microtubule polarity, and the interplay between microtubule polarity and protein localization, is thought to be crucial for spindle assembly, anaphase, and cytokinesis, but these phenomena remain poorly understood, in part due to the difficulty of measuring microtubule polarity in spindles. We develop and implement a method to nonperturbatively and quantitatively measure microtubule polarity throughout spindles using a combination of second-harmonic generation and two-photon fluorescence. We validate this method using computer simulations and by comparison to structural data on spindles obtained from electron tomography and laser ablation. This method should provide a powerful tool for studying spindle organization and function, and may be applicable for investigating microtubule polarity in other systems.

Modelling chromosome dynamics in mitosis: a historical perspective on models of metaphase and anaphase in eukaryotic cells

Mitosis is the process by which the genome is segregated to form two identical daughter cells during cell division. The process of cell division is essential to the maintenance of every form of life. However, a detailed quantitative understanding of mitosis has been difficult owing to the complexity of the process. Indeed, it has been long recognized that, because of the complexity of the molecules involved, their dynamics and their properties, the mitotic events that mediate the segregation of the genome into daughter nuclei cannot be fully understood without the contribution of mathematical/quantitative modelling. Here, we provide an overview of mitosis and describe the dynamic and mechanical properties of the mitotic apparatus. We then discuss several quantitative models that emerged in the past decades and made an impact on our understanding of specific aspects of mitosis, including the motility of the chromosomes within the mitotic spindle during metaphase and anaphase, the maintenance of spindle length during metaphase and the switch to spindle elongation that occurs during anaphase.

Analysis of mitotic protein dynamics and function in Drosophila embryos by live cell imaging and quantitative modeling

Mitosis depends upon the mitotic spindle, a dynamic protein machine that uses ensembles of dynamic microtubules (MTs) and MT-based motor proteins to assemble itself, control its own length (pole-pole spacing), and segregate chromosomes during anaphase A (chromosome-to-pole motility) and anaphase B (spindle elongation). In this review, we describe how the molecular and biophysical mechanisms of these processes can be analyzed in the syncytial Drosophila embryo by combining (1) time-lapse imaging and other fluorescence light microscopy techniques to study the dynamics of mitotic proteins such as tubulins, mitotic motors, and chromosome or centrosome proteins; (2) the perturbation of specific mitotic protein function using microinjected inhibitors (e.g., antibodies) or mutants to infer protein function; and (3) mathematical modeling of the qualitative models derived from these experiments, which can then be used to make predictions which are in turn tested experimentally. We provide details of the methods we use for embryo preparation, fluorescence imaging, and mathematical modeling.