Antibodies to the kinesin motor domain and CENP-E inhibit microtubule depolymerization-dependent motion of chromosomes in vitro

Kinesin-7 CENP-E is essential for chromosome alignment and spindle assembly of mouse spermatocytes

Genome stability depends on chromosome congression and alignment during cell division. Kinesin-7 CENP-E is critical for kinetochore-microtubule attachment and chromosome alignment, which contribute to genome stability in mitosis. However, the functions and mechanisms of CENP-E in the meiotic division of male spermatocytes remain largely unknown. In this study, by combining the use of chemical inhibitors, siRNA-mediated gene knockdown, immunohistochemistry, and high-resolution microscopy, we have found that CENP-E inhibition results in chromosome misalignment and metaphase arrest in dividing spermatocyte during meiosis. Strikingly, we have revealed that CENP-E regulates spindle organization in metaphase I spermatocytes and cultured GC-2 spd cells. CENP-E depletion leads to spindle elongation, chromosome misalignment, and chromosome instability in spermatocytes. Together, these findings indicate that CENP-E mediates the kinetochore recruitment of BubR1, spindle assembly checkpoint and chromosome alignment in dividing spermatocytes, which finally contribute to faithful chromosome segregation and chromosome stability in the male meiotic division.

Anaphase A

Anaphase A is the motion of recently separated chromosomes to the spindle pole they face. It is accompanied by the shortening of kinetochore-attached microtubules. The requisite tubulin depolymerization may occur at kinetochores, at poles, or both, depending on the species and/or the time in mitosis. These depolymerization events are local and suggest that cells regulate microtubule dynamics in specific places, presumably by the localization of relevant enzymes and microtubule-associated proteins to specific loci, such as pericentriolar material and outer kinetochores. Motor enzymes can contribute to anaphase A, both by altering microtubule stability and by pushing or pulling microtubules through the cell. The generation of force on chromosomes requires couplings that can both withstand the considerable force that spindles can generate and simultaneously permit tubulin addition and loss. This chapter reviews literature on the molecules that regulate anaphase microtubule dynamics, couple dynamic microtubules to kinetochores and poles, and generate forces for microtubule and chromosome motion.

Leaving no-one behind: how CENP-E facilitates chromosome alignment

Chromosome alignment and biorientation is essential for mitotic progression and genomic stability. Most chromosomes align at the spindle equator in a motor-independent manner. However, a subset of polar kinetochores fail to bi-orient and require a microtubule motor-based transport mechanism to move to the cell equator. Centromere Protein E (CENP-E/KIF10) is a kinesin motor from the Kinesin-7 family, which localizes to unattached kinetochores during mitosis and utilizes plus-end directed microtubule motility to slide mono-oriented chromosomes to the spindle equator. Recent work has revealed how CENP-E cooperates with chromokinesins and dynein to mediate chromosome congression and highlighted its role at aligned chromosomes. Additionally, we have gained new mechanistic insights into the targeting and regulation of CENP-E motor activity at the kinetochore. Here, we will review the function of CENP-E in chromosome congression, the pathways that contribute to CENP-E loading at the kinetochore, and how CENP-E activity is regulated during mitosis.

Kinesin-7 CENP-E regulates cell division, gastrulation and organogenesis in development

Kinesin-7 CENP-E motor protein is essential for chromosome alignment and kinetochore-microtubule attachment in cell division. Human CENP-E has recently identified to be linked with the microcephalic primordial dwarfism syndromes associated with a smaller head, brain malformations and a prominent nose. However, the roles of CENP-E in embryonic development remain largely unknown. In this study, we find that zebrafish CENP-E inhibition results in defects in early zygote cleavage, including asymmetric cell division, cell cycle arrest and the developmental abnormalities. We also demonstrate that CENP-E ablation in cultured cells leads to chromosome misalignment, spindle abnormalities and interruptions of the cell cycle. These observations suggest that CENP-E plays a key role in early cell division and cell cycle progression. Furthermore, we also find that CENP-E inhibition results in the defects in the epiboly, the developmental arrest, the smaller head and the abnormal embryo during zebrafish embryogenesis. Our data demonstrate new functions of CENP-E in development and provide insights into its essential roles in organogenesis.

Kinesin-7 CENP-E regulates chromosome alignment and genome stability of spermatogenic cells

Kinesin-7 CENP-E is an essential kinetochore motor required for chromosome alignment and congression. However, the specific functions of CENP-E in the spermatogenic cells during spermatogenesis remain unknown. In this study, we find that CENP-E proteins are expressed in the spermatogonia, spermatocytes, and the elongating spermatids. CENP-E inhibition by specific inhibitor GSK923295 results in the disruption of spermatogenesis and cell cycle arrest of spermatogenic cells. Both spermatogonia and spermatocytes are arrested in metaphase and several chromosomes are not aligned at the equatorial plate. We find that CENP-E inhibition leads to chromosome misalignment, the spindle disorganization, and the formation of the aneuploidy cells. Furthermore, the inhibition of CENP-E results in the defects in the formation of spermatids, including the sperm head condensation and the sperm tail formation. We have revealed that kinesin-7 CENP-E is essential for chromosome alignment and genome stability of the spermatogenic cells.

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.

α-Fodrin is required for the organization of functional microtubules during mitosis

The cytoskeleton protein α-fodrin plays a major role in maintaining structural stability of membranes. It was also identified as part of the brain γ-tubulin ring complex, the major microtubule nucleator. Here, we investigated the requirement of α-fodrin for microtubule spindle assembly during mitotic progression. We found that α-fodrin depletion results in abnormal mitosis with uncongressed chromosomes, leading to prolonged activation of the spindle assembly checkpoint and a severe mitotic delay. Further, α-fodrin repression led to the formation of shortened spindles with unstable kinetochore-microtubule attachments. We also found that the mitotic kinesin CENP-E had reduced levels at kinetochores to likely account for the chromosome misalignment defects in α-fodrin-depleted cells. Importantly, we showed these cells to exhibit reduced levels of detyrosinated α-tubulin, which primarily drives CENP-E localization. Since proper microtubule dynamics and chromosome alignment are required for completion of normal mitosis, this study reveals an unforeseen role of α-fodrin in regulating mitotic progression. Future studies on these lines of observations should reveal important mechanistic insight for fodrin's involvement in cancer.

A tension-independent mechanism reduces Aurora B-mediated phosphorylation upon microtubule capture by CENP-E at the kinetochore

During mitosis, Aurora B kinase is required for forming proper bi-oriented kinetochore-microtubule attachments. Current models suggest that tension exerted between a pair of sister-kinetochores (inter-kinetochore stretch) produces a spatial separation of Aurora B kinase from kinetochore-associated microtubule binding substrates, such as the Knl1-Mis12-Ndc80 (KMN) network, resulting in a decrease of phosphorylation and, thus, an increase of affinity for microtubules. Using Single-Molecule High-Resolution Colocalization (SHREC) microscopy analysis of the kinetochore-associated motor CENP-E, we now show that CENP-E undergoes structural rearrangements prior to and after tension generation at the kinetochore, and displays a bi-modal Gaussian distribution on a pair of bi-oriented sister kinetochores. The conformational change of CENP-E depends on its microtubule-stimulated motor motility and the highly flexible coiled-coil between its motor and kinetochore-binding tail domains. Chemical inhibition of the motor motility or perturbations of the coiled-coil domain of CENP-E increases Aurora B-mediated Ndc80 phosphorylation in a tension-independent manner. Metaphase chromosome misalignment caused by CENP-E inhibition can be rescued by chemical inhibition of Aurora B kinase. Furthermore, a pair of monotelic sister-kinetochores shows asymmetric levels of Aurora B-mediated phosphorylation in mono-polar spindles depending on CENP-E motor activity. These results collectively suggest a tension-independent mechanism to reduce Aurora B-mediated phosphorylation of outer kinetochore components in response to microtubule capture by CENP-E.

Phosphatases in Mitosis: Roles and Regulation

Mitosis requires extensive rearrangement of cellular architecture and of subcellular structures so that replicated chromosomes can bind correctly to spindle microtubules and segregate towards opposite poles. This process originates two new daughter nuclei with equal genetic content and relies on highly-dynamic and tightly regulated phosphorylation of numerous cell cycle proteins. A burst in protein phosphorylation orchestrated by several conserved kinases occurs as cells go into and progress through mitosis. The opposing dephosphorylation events are catalyzed by a small set of protein phosphatases, whose importance for the accuracy of mitosis is becoming increasingly appreciated. This review will focus on the established and emerging roles of mitotic phosphatases, describe their structural and biochemical properties, and discuss recent advances in understanding the regulation of phosphatase activity and function.

Assessing the Contributions of Motor Enzymes and Microtubule Dynamics to Mitotic Chromosome Motions

During my graduate work with Keith Porter, I became fascinated by the mitotic spindle, an interest that has motivated much of my scientific work ever since. I began spindle studies by using electron microscopes, instruments that have made significant contributions to our understanding of spindle organization. Such instruments have helped to elucidate the distributions of spindle microtubules, the interactions among them, their molecular polarity, and their associations with both kinetochores and spindle poles. Our lab has also investigated some processes of spindle physiology: microtubule dynamics, the actions of microtubule-associated proteins (including motor enzymes), the character of forces generated by specific spindle components, and factors that control mitotic progression. Here, I give a personal perspective on some of this intellectual history and on what recent discoveries imply about the mechanisms of chromosome motion.

Mechanisms of Chromosome Congression during Mitosis

Chromosome congression during prometaphase culminates with the establishment of a metaphase plate, a hallmark of mitosis in metazoans. Classical views resulting from more than 100 years of research on this topic have attempted to explain chromosome congression based on the balance between opposing pulling and/or pushing forces that reach an equilibrium near the spindle equator. However, in mammalian cells, chromosome bi-orientation and force balance at kinetochores are not required for chromosome congression, whereas the mechanisms of chromosome congression are not necessarily involved in the maintenance of chromosome alignment after congression. Thus, chromosome congression and maintenance of alignment are determined by different principles. Moreover, it is now clear that not all chromosomes use the same mechanism for congressing to the spindle equator. Those chromosomes that are favorably positioned between both poles when the nuclear envelope breaks down use the so-called "direct congression" pathway in which chromosomes align after bi-orientation and the establishment of end-on kinetochore-microtubule attachments. This favors the balanced action of kinetochore pulling forces and polar ejection forces along chromosome arms that drive chromosome oscillatory movements during and after congression. The other pathway, which we call "peripheral congression", is independent of end-on kinetochore microtubule-attachments and relies on the dominant and coordinated action of the kinetochore motors Dynein and Centromere Protein E (CENP-E) that mediate the lateral transport of peripheral chromosomes along microtubules, first towards the poles and subsequently towards the equator. How the opposite polarities of kinetochore motors are regulated in space and time to drive congression of peripheral chromosomes only now starts to be understood. This appears to be regulated by position-dependent phosphorylation of both Dynein and CENP-E and by spindle microtubule diversity by means of tubulin post-translational modifications. This so-called "tubulin code" might work as a navigation system that selectively guides kinetochore motors with opposite polarities along specific spindle microtubule populations, ultimately leading to the congression of peripheral chromosomes. We propose an integrated model of chromosome congression in mammalian cells that depends essentially on the following parameters: (1) chromosome position relative to the spindle poles after nuclear envelope breakdown; (2) establishment of stable end-on kinetochore-microtubule attachments and bi-orientation; (3) coordination between kinetochore- and arm-associated motors; and (4) spatial signatures associated with post-translational modifications of specific spindle microtubule populations. The physiological consequences of abnormal chromosome congression, as well as the therapeutic potential of inhibiting chromosome congression are also discussed.

Anaphase A: Disassembling Microtubules Move Chromosomes toward Spindle Poles

The separation of sister chromatids during anaphase is the culmination of mitosis and one of the most strikingly beautiful examples of cellular movement. It consists of two distinct processes: Anaphase A, the movement of chromosomes toward spindle poles via shortening of the connecting fibers, and anaphase B, separation of the two poles from one another via spindle elongation. I focus here on anaphase A chromosome-to-pole movement. The chapter begins by summarizing classical observations of chromosome movements, which support the current understanding of anaphase mechanisms. Live cell fluorescence microscopy studies showed that poleward chromosome movement is associated with disassembly of the kinetochore-attached microtubule fibers that link chromosomes to poles. Microtubule-marking techniques established that kinetochore-fiber disassembly often occurs through loss of tubulin subunits from the kinetochore-attached plus ends. In addition, kinetochore-fiber disassembly in many cells occurs partly through 'flux', where the microtubules flow continuously toward the poles and tubulin subunits are lost from minus ends. Molecular mechanistic models for how load-bearing attachments are maintained to disassembling microtubule ends, and how the forces are generated to drive these disassembly-coupled movements, are discussed.

Biophysics of Microtubule End Coupling at the Kinetochore

The main physiological function of mitotic kinetochores is to provide durable attachment to spindle microtubules, which segregate chromosomes in order to partition them equally between the two daughter cells. Numerous kinetochore components that can bind directly to microtubules have been identified, including ATP-dependent motors and various microtubule-associated proteins with no motor activity. A major challenge facing the field is to explain chromosome motions based on the biochemical and structural properties of these individual kinetochore components and their assemblies. This chapter reviews the molecular mechanisms responsible for the motions associated with dynamic microtubule tips at the single-molecule level, as well as the activities of multimolecular ensembles called couplers. These couplers enable persistent kinetochore motion even under load, but their exact composition and structure remain unknown. Because no natural or artificial macro-machines function in an analogous manner to these molecular nano-devices, understanding their underlying biophysical mechanisms will require conceptual advances.

Mitosis

SUMMARYAll eukaryotic cells prepare for cell division by forming a "mitotic spindle"-a bipolar machine made from microtubules (MTs) and many associated proteins. This device organizes the already duplicated DNA so one copy of each chromosome attaches to each end of the spindle. Both formation and function of the spindle require controlled MT dynamics, as well as the actions of multiple motor enzymes. Spindle-driven motions separate the duplicated chromosomes into two distinct sets that are then moved toward opposite ends of the cell. The two cells that subsequently form by cytokinesis, therefore, contain all the genes needed to grow and divide again.

Cenp-meta is required for sustained spindle checkpoint

Cenp-E is a kinesin-like motor protein required for efficient end-on attachment of kinetochores to the spindle microtubules. Cenp-E immunodepletion in Xenopus mitotic extracts results in the loss of mitotic arrest and massive chromosome missegregation, whereas its depletion in mammalian cells leads to chromosome segregation defects despite the presence of a functional spindle assembly checkpoint (SAC). Cenp-meta has previously been reported to be the Drosophila homolog of vertebrate Cenp-E. In this study, we show that cenp-metaΔ mutant neuroblasts arrest in mitosis when treated with colchicine. cenp-metaΔ mutant cells display a mitotic delay. Yet, despite the persistence of the two checkpoint proteins Mad2 and BubR1 on unattached kinetochores, these cells eventually enter anaphase and give rise to highly aneuploid daughter cells. Indeed, we find that cenp-metaΔ mutant cells display a slow but continuous degradation of cyclin B, which eventually triggers the mitotic exit observed. Thus, our data provide evidence for a role of Cenp-meta in sustaining the SAC response.

Kinetochore kinesin CENP-E is a processive bi-directional tracker of dynamic microtubule tips

During vertebrate mitosis, the centromere-associated kinesin CENP-E (centromere protein E) transports misaligned chromosomes to the plus ends of spindle microtubules. Subsequently, the kinetochores that form at the centromeres establish stable associations with microtubule ends, which assemble and disassemble dynamically. Here we provide evidence that after chromosomes have congressed and bi-oriented, the CENP-E motor continues to play an active role at kinetochores, enhancing their links with dynamic microtubule ends. Using a combination of single-molecule approaches and laser trapping in vitro, we demonstrate that once reaching microtubule ends, CENP-E converts from a lateral transporter into a microtubule tip-tracker that maintains association with both assembling and disassembling microtubule tips. Computational modelling of this behaviour supports our proposal that CENP-E tip-tracks bi-directionally through a tethered motor mechanism, which relies on both the motor and tail domains of CENP-E. Our results provide a molecular framework for the contribution of CENP-E to the stability of attachments between kinetochores and dynamic microtubule ends.

New insights into the mechanism for chromosome alignment in metaphase

During mitosis, duplicated sister chromatids are properly aligned at the metaphase plate of the mitotic spindle before being segregated into two daughter cells. This requires a complex process to ensure proper interactions between chromosomes and spindle microtubules. The kinetochore, the proteinaceous complex assembled at the centromere region on each chromosome, serves as the microtubule attachment site and powers chromosome movement in mitosis. Numerous proteins/protein complexes have been implicated in the connection between kinetochores and dynamic microtubules. Recent studies have advanced our understanding on the nature of the interface between kinetochores and microtubule plus ends in promoting and maintaining their stable attachment. These efforts have demonstrated the importance of this process to ensure accurate chromosome segregation, an issue which has great significance for understanding and controlling abnormal chromosome segregation (aneuploidy) in human genetic diseases and in cancer progression.

Biophysics of mitosis

Mitosis is the process by which eukaryotic cells organize and segregate their chromosomes in preparation for cell division. It is accomplished by a cellular machine composed largely of microtubules (MTs) and their associated proteins. This article reviews literature on mitosis from a biophysical point of view, drawing attention to the assembly and motility processes required to do this complex job with precision. Work from both the recent and the older literature is integrated into a description of relevant biological events and the experiments that probe their mechanisms. Theoretical work on specific subprocesses is also reviewed. Our goal is to provide a document that will expose biophysicists to the fascination of this quite amazing process and provide them with a good background from which they can pursue their own research interests in the subject.

Mitosis futures: the past is prologue

The mechanisms by which cells organize and segregate their chromosomes have been under close scrutiny for years, and significant progress has been made in understanding how mitosis works. Modern cell biology has identified most of the molecules that underlie mitotic spindle function, but the ways in which they are organized and controlled to make an effective and accurate cellular machine are exciting subjects for future study.

Tubulin depolymerization may be an ancient biological motor

The motions of mitotic chromosomes are complex and show considerable variety across species. A wealth of evidence supports the idea that microtubule-dependent motor enzymes contribute to this variation and are important both for spindle formation and for the accurate completion of chromosome segregation. Motors that walk towards the spindle pole are, however, dispensable for at least some poleward movements of chromosomes in yeasts, suggesting that depolymerizing spindle microtubules can generate mitotic forces in vivo. Tubulin protofilaments that flare outward in association with microtubule shortening may be the origin of such forces, because they can move objects that are appropriately attached to a microtubule wall. For example, some kinetochore-associated proteins can couple experimental objects, such as microspheres, to shortening microtubules in vitro, moving them over many micrometers. Here, we review recent evidence about such phenomena, highlighting the force-generation mechanisms and different coupling strategies. We also consider bending filaments of the tubulin-like protein FtsZ, which form rings girding bacteria at their sites of cytokinesis. Mechanical similarities between these force-generation systems suggest a deep phylogenetic relationship between tubulin depolymerization in eukaryotic mitosis and FtsZ-mediated ring contraction in bacteria.

Kinetochore microtubules, analyzed by correlated light and immunoelectron microscopy

The movement of chromosomes in mitosis requires spindle microtubules, as well as a set of specific motor proteins located at the kinetochores of the chromosomes. The exact mechanisms of chromosome movement have remained ambiguous for many years. Cumulating evidence indicates that chromosome movement in early mitosis occurs by lateral sliding of kinetochores along the surface of microtubules. We provide here the protocol for an immunological staining method of microtubules that allows electron microscopic analysis of spindle microtubules over a long distance and that has helped clarifying this biological question. The technique involves the use of ultra-small immunogold, enhanced by silver.

Contrasting models for kinetochore microtubule attachment in mammalian cells

Kinetochore function is mediated through its interaction with microtubule plus ends embedded in the kinetochore outer plate. Here, we compare and evaluate current models for kinetochore microtubule attachment, beginning with a brief review of the molecular, biochemical, cellular, and structural studies upon which these models are based. The majority of these studies strongly support a model in which the kinetochore outer plate is a network of fibers that form multiple weak attachments to each microtubule, chiefly through the Ndc80 complex. Multiple weak attachments enable kinetochores to remain attached to microtubule plus ends that are continually growing and shrinking. It is unlikely that rings or "kinetochore fibrils" have a significant role in kinetochore microtubule attachment, but such entities could have a role in stabilizing attachment, modifying microtubule dynamics, and harnessing the energy released from microtubule disassembly. It is currently unclear whether kinetochores control and coordinate the dynamics of individual kinetochore microtubules.

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.

Mechanisms of chromosome behaviour during mitosis

For over a century, scientists have strived to understand the mechanisms that govern the accurate segregation of chromosomes during mitosis. The most intriguing feature of this process, which is particularly prominent in higher eukaryotes, is the complex behaviour exhibited by the chromosomes. This behaviour is based on specific and highly regulated interactions between the chromosomes and spindle microtubules. Recent discoveries, enabled by high-resolution imaging combined with the various genetic, molecular, cell biological and chemical tools, support the idea that establishing and controlling the dynamic interaction between chromosomes and microtubules is a major factor in genomic fidelity.

Reduced expression of cenp-e in human hepatocellular carcinoma

Background

CENP-E, one of spindle checkpoint proteins, plays a crucial role in the function of spindle checkpoint. Once CENP-E expression was interrupted, the chromosomes can not separate procedurally, and may result in aneuploidy which is a hallmark of most solid cancers, such as hepatocellular carcinoma (HCC). We investigate the expression of CENP-E in human hepatocellular carcinoma,. and analyze the effect of low CENP-E expression on chromosome separation in normal liver cell line (LO2).

Methods

We determined its levels in HCC and para-cancerous tissues, human hepatocellular carcinoma-derived cell line (HepG2) and LO2 cell line using real time quantitative PCR (QPCR) and Western blot. Further to know whether reduction in CENP-E expression impairs chromosomes separation in LO2 cells. we knocked down CENP-E using shRNA expressing vector and then count the aneuploid in LO2 cells using chromosomal counts assay.

Results

We found that both CENP-E mRNA and protein levels were significantly reduced in HCC tissues and HepG2 cells compared with para-cancerous tissues and LO2 cells, respectively. A significantly-increased proportion of aneuploid in these down-knocked LO2 cells compared with those treated with control shRNA vector.

Conclusions

Together with other results, these results reveal that CENP-E expression was reduced in human HCC tissue, and low CENP-E expression result in aneuploidy in LO2 cells.

Motor-independent targeting of CLASPs to kinetochores by CENP-E promotes microtubule turnover and poleward flux

Efficient chromosome segregation during mitosis relies on the coordinated activity of molecular motors with proteins that regulate kinetochore attachments to dynamic spindle microtubules [1]. CLASPs are conserved kinetochore- and microtubule-associated proteins encoded by two paralog genes, clasp1 and clasp2, and have been previously implicated in the regulation of kinetochore microtubule dynamics [2-4]. However, it remains unknown how CLASPs work in concert with other proteins to form a functional kinetochore microtubule interface. Here we have identified mitotic interactors of human CLASP1 via a proteomic approach. Among these, the microtubule plus-end-directed motor CENP-E [5] was found to form a complex with CLASP1 that colocalizes to multiple structures of the mitotic apparatus in human cells. We found that CENP-E recruits both CLASP1 and CLASP2 to kinetochores independently of its motor activity or the presence of microtubules. Depletion of CLASPs or CENP-E by RNA interference in human cells causes a significant and comparable reduction of kinetochore microtubule poleward flux and turnover rates and rescues spindle bipolarity in Kif2a-depleted cells. We conclude that CENP-E integrates two critical functions that are important for accurate chromosome movement and spindle architecture: one relying directly on its motor activity, and the other involving the targeting of key microtubule regulators to kinetochores.

Characterization of novel diaryl oxazole-based compounds as potential agents to treat pancreatic cancer

A series of diaryl- and fluorenone-based analogs of the lead compound UA-62784 [4-(5-(4-methoxyphenyl)oxazol-2-yl)-9H-fluoren-9-one] was synthesized with the intention of improving upon the selective cytotoxicity of UA-62784 against human pancreatic cancer cell lines with a deletion of the tumor suppressor gene deleted in pancreas cancer locus 4 (DPC-4, SMAD-4). Over 80 analogs were synthesized and tested for antitumor activity against pancreatic cancer (PC) cell lines (the PC series). Despite a structural relationship to UA-62784, which inhibits the mitotic kinesin centromere protein E (CENP-E), none of the analogs was selective for DPC-4-deleted pancreatic cancer cell lines. Furthermore, none of the analogs was a potent or selective inhibitor of four different mitotic kinesins (mitotic kinesin-5, CENP-E, mitotic kinesin-like protein-1, and mitotic centromere-associated kinesin). Therefore, other potential mechanisms of action were evaluated. A diaryl oxazole lead analog from this series, PC-046 [5-(4-methoxyphenyl)-2-(3-(3-methoxyphenyl)pyridin-4-yl) oxazole], was shown to potently inhibit several protein kinases that are overexpressed in human pancreatic cancers, including tyrosine receptor kinase B, interleukin-1 receptor-associated kinase-4, and proto-oncogene Pim-1. Cells exposed to PC-046 exhibit a cell cycle block in the S-phase followed by apoptotic death and necrosis. PC-046 effectively reduced MiaPaca-2 tumor growth in severe combined immunodeficiency mice by 80% compared with untreated controls. The plasma half-life was 7.5 h, and cytotoxic drug concentrations of >3 muM were achieved in vivo in mice. The diaryl oxazole series of compounds represent a new chemical class of anticancer agents that inhibit several types of cancer-relevant protein kinases.

Sequential assembly of centromeric proteins in male mouse meiosis

The assembly of the mitotic centromere has been extensively studied in recent years, revealing the sequence and regulation of protein loading to this chromosome domain. However, few studies have analyzed centromere assembly during mammalian meiosis. This study specifically targets this approach on mouse spermatocytes. We have found that during prophase I, the proteins of the chromosomal passenger complex Borealin, INCENP, and Aurora-B load sequentially to the inner centromere before Shugoshin 2 and MCAK. The last proteins to be assembled are the outer kinetochore proteins BubR1 and CENP-E. All these proteins are not detected at the centromere during anaphase/telophase I and are then reloaded during interkinesis. The loading sequence of the analyzed proteins is similar during prophase I and interkinesis. These findings demonstrate that the interkinesis stage, regularly overlooked, is essential for centromere and kinetochore maturation and reorganization previous to the second meiotic division. We also demonstrate that Shugoshin 2 is necessary for the loading of MCAK at the inner centromere, but is dispensable for the loading of the outer kinetochore proteins BubR1 and CENP-E.

The centromere is a chromosome domain essential for the correct partitioning of chromosomes during mitotic and meiotic cell divisions. The characterization of the centromeric proteins and their sequential assembly have been extensively studied in mammalian mitosis, since defective chromosome segregation is associated with birth defects and cancer. However, few studies have analyzed the centromere assembly during meiosis, a special cell division leading to the production of haploid gametes. Here, we analyze the sequence of loading of several centromeric and kinetochoric proteins during male mouse meiosis. We show that during both meiotic divisions, the proteins of the chromosomal passenger complex Borealin, INCENP, and Aurora-B load sequentially to the inner centromere before Shugoshin 2 and MCAK. The outer kinetochore proteins BubR1 and CENP-E are the last ones to be assembled. We also demonstrate, using a knockout mouse for Sgol2, that the inner centromeric protein Shugoshin 2 is dispensable for the loading of the outer kinetochore proteins BubR1 and CENP-E, but necessary for the assembly of MCAK. This study shows that the analysis of the behavior of different centromere proteins during meiosis can offer new insights concerning centromere organization.

Kinesin-8 from fission yeast: a heterodimeric, plus-end-directed motor that can couple microtubule depolymerization to cargo movement

Fission yeast expresses two kinesin-8s, previously identified and characterized as products of the klp5(+) and klp6(+) genes. These polypeptides colocalize throughout the vegetative cell cycle as they bind cytoplasmic microtubules during interphase, spindle microtubules, and/or kinetochores during early mitosis, and the interpolar spindle as it elongates in anaphase B. Here, we describe in vitro properties of these motor proteins and some truncated versions expressed in either bacteria or Sf9 cells. The motor-plus-neck domain of Klp6p formed soluble dimers that cross-linked microtubules and showed both microtubule-activated ATPase and plus-end-directed motor activities. Full-length Klp5p and Klp6p, coexpressed in Sf9 cells, formed soluble heterodimers with the same activities. The latter recombinant protein could also couple microbeads to the ends of shortening microtubules and use energy from tubulin depolymerization to pull a load in the minus end direction. These results, together with the spindle localizations of these proteins in vivo and their requirement for cell viability in the absence of the Dam1/DASH kinetochore complex, support the hypothesis that fission yeast kinesin-8 contributes both to chromosome congression to the metaphase plate and to the coupling of spindle microtubules to kinetochores during anaphase A.

Fibrils connect microtubule tips with kinetochores: a mechanism to couple tubulin dynamics to chromosome motion

Kinetochores of mitotic chromosomes are coupled to spindle microtubules in ways that allow the energy from tubulin dynamics to drive chromosome motion. Most kinetochore-associated microtubule ends display curving "protofilaments," strands of tubulin dimers that bend away from the microtubule axis. Both a kinetochore "plate" and an encircling, ring-shaped protein complex have been proposed to link protofilament bending to poleward chromosome motion. Here we show by electron tomography that slender fibrils connect curved protofilaments directly to the inner kinetochore. Fibril-protofilament associations correlate with a local straightening of the flared protofilaments. Theoretical analysis reveals that protofilament-fibril connections would be efficient couplers for chromosome motion, and experimental work on two very different kinetochore components suggests that filamentous proteins can couple shortening microtubules to cargo movements. These analyses define a ring-independent mechanism for harnessing microtubule dynamics directly to chromosome movement.

The Dam1 ring binds microtubules strongly enough to be a processive as well as energy-efficient coupler for chromosome motion

Accurate chromosome segregation during mitotic division of budding yeast depends on the multiprotein kinetochore complex, Dam1 (also known as DASH). Purified Dam1 heterodecamers encircle microtubules (MTs) to form rings that can function as "couplers," molecular devices that transduce energy from MT disassembly into the motion of a cargo. Here we show that MT depolymerization develops a force against a Dam1 ring that is sixfold larger than the force exerted on a coupler that binds only one side of an MT. Wild-type rings slow depolymerization fourfold, but rings that include a mutant Dam1p with truncated C terminus slow depolymerization less, consistent with the idea that this tail is part of a strong bond between rings and MTs. A molecular-mechanical model for Dam1-MT interaction predicts that binding between this flexible tail and the MT wall should cause a Dam1 ring to wobble, and Fourier analysis of moving, ring-attached beads corroborates this prediction. Comparison of the forces generated against wild-type and mutant complexes confirms the importance of tight Dam1-MT association for processive cargo movement under load.

CENP-E combines a slow, processive motor and a flexible coiled coil to produce an essential motile kinetochore tether

The mitotic kinesin centromere protein E (CENP-E) is an essential kinetochore component that directly contributes to the capture and stabilization of spindle microtubules by kinetochores. Although reduction in CENP-E leads to high rates of whole chromosome missegregation, neither its properties as a microtubule-dependent motor nor how it contributes to the dynamic linkage between kinetochores and microtubules is known. Using single-molecule assays, we demonstrate that CENP-E is a very slow, highly processive motor that maintains microtubule attachment for long periods. Direct visualization of full-length Xenopus laevis CENP-E reveals a highly flexible 230-nm coiled coil separating its kinetochore-binding and motor domains. We also show that full-length CENP-E is a slow plus end–directed motor whose activity is essential for metaphase chromosome alignment. We propose that the highly processive microtubule-dependent motor activity of CENP-E serves to power chromosome congression and provides a flexible, motile tether linking kinetochores to dynamic spindle microtubules.

The mitotic kinesin CENP-E is a processive transport motor

In vivo studies suggest that centromeric protein E (CENP-E), a kinesin-7 family member, plays a key role in the movement of chromosomes toward the metaphase plate during mitosis. How CENP-E accomplishes this crucial task, however, is not clear. Here we present single-molecule measurements of CENP-E that demonstrate that this motor moves processively toward the plus end of microtubules, with an average run length of 2.6 +/- 0.2 mum, in a hand-over-hand fashion, taking 8-nm steps with a stall force of 6 +/- 0.1 pN. The ATP dependence of motor velocity obeys Michaelis-Menten kinetics with K(M,ATP) = 35 +/- 5 muM. All of these features are remarkably similar to those for kinesin-1-a highly processive transport motor. We, therefore, propose that CENP-E transports chromosomes in a manner analogous to how kinesin-1 transports cytoplasmic vesicles.

Microtubule/organelle motility assays

This unit describes an in vitro assay that uses video-enhanced differential interference contrast (VE-DIC) microscopy to examine the motile interactions between isolated organelle fractions and microtubules (MTs). The method can be used to dissect the molecular requirements for organelle movement and membrane trafficking. A field of axoneme-nucleated MTs, growing and shortening as they would in a living cell (dynamic MTs), is generated in a simple microscope perfusion chamber. Various combinations of isolated endoplasmic reticulum (ER) and Golgi apparatus organelles, cytosol containing motor proteins and other soluble factors, nucleotides, and specific pharmacological reagents are then added to the dynamic MT, and the motile interactions between the organelles and MTs are observed by VE-DIC microscopy.

Molecular architecture of the kinetochore-microtubule interface

Segregation of the replicated genome during cell division in eukaryotes requires the kinetochore to link centromeric DNA to spindle microtubules. The kinetochore is composed of a number of conserved protein complexes that direct its specification and assembly, bind to spindle microtubules and regulate chromosome segregation. Recent studies have identified more than 80 kinetochore components, and are revealing how these proteins are organized into the higher order kinetochore structure, as well as how they function to achieve proper chromosome segregation.

Mechanisms of mitotic spindle assembly and function

The mitotic spindle is the macromolecular machine that segregates chromosomes to two daughter cells during mitosis. The major structural elements of the spindle are microtubule polymers, whose intrinsic polarity and dynamic properties are critical for bipolar spindle organization and function. In most cell types, spindle microtubule nucleation occurs primarily at two centrosomes, which define the spindle poles, but microtubules can also be generated by the chromosomes and within the spindle itself. Many associated factors help organize the spindle, including molecular motors and regulators of microtubule dynamics. The past decade has provided a wealth of information on the molecular players that are critical for spindle assembly as well as a high-resolution view of the intricate movements and dynamics of the spindle microtubules and the chromosomes. In this chapter we provide a historical account of the key observations leading to current models of spindle assembly, as well as an up-to-date status report on this exciting field.

Modeling of chromosome motility during mitosis

Chromosome motility is a highly regulated and complex process that ultimately achieves proper segregation of the replicated genome. Recent modeling studies provide a computational framework for investigating how microtubule assembly dynamics, motor protein activity and mitotic spindle mechanical properties are integrated to drive chromosome motility. Among other things, these studies show that metaphase chromosome oscillations can be explained by a range of assumptions, and that non-oscillatory states can be achieved with modest changes to the model parameters. In addition, recent microscopy studies provide new insight into the nature of the coupling between force on the kinetochore and kinetochore-microtubule assembly/disassembly. Together, these studies facilitate advancement toward a unified model that quantitatively predicts chromosome motility.

Mitotic kinesin-like protein 2 binds and colocalizes with papillomavirus E2 during mitosis

MKlp2 is a kinesin-like motor protein of the central mitotic spindle required for completion of cytokinesis. Papillomavirus E2 is a sequence specific DNA binding protein that regulates viral transcription and replication and is responsible for partitioning viral episomes into daughter cells during cell division. We demonstrate that MKlp2 specifically associates with the E2 protein during mitosis. Using chromatin immunoprecipitation, we show viral genomes are in complex with MKlp2 only within this stage of cell cycle. By immunofluorescence, a subpopulation of papillomavirus E2 colocalizes with MKlp2 in the midbody/midplate during late mitosis. We conclude that during specific stages of mitosis, the papillomavirus E2 protein binds to MKlp2, and infer that association with this motor protein ensures viral genome partitioning during cytokinesis.

A driving and coupling "Pac-Man" mechanism for chromosome poleward translocation in anaphase A

During mitosis, chromatid harnesses its kinetochore translocation at the depolymerizing microtubule ends for its poleward movement in anaphase A. The force generation mechanism for such movement remains unknown. Analysis of the current experimental results shows that the bending energy release from the bound tubulin subunits alone cannot provide sufficient driving force. Additional contribution from effective electrostatic attractions between the kinetochore and the microtubule is needed for kinetochore translocation. Interestingly, as the kinetochore moves to inside the microtubule, the microtubule tip is free to bend outward so that the instantaneous distance between the kinetochore and the microtubule tip is much closer than the rest of the microtubule. This close contact yields much larger electrostatic attraction than that from the rest of the microtubule under physiological ionic conditions. As a result, the effective electrostatic interaction hinders the further kinetochore poleward translocation until the microtubule tip dissociates. Thus, the kinetochore translocation is strongly coupled at the depolymerizing microtubule end. This driving-coupling mechanism indicates that the kinetochore velocity is largely controlled by the microtubule dissociation rate, which explains the insensitivity of kinetochore velocity to its viscous drag and the large redundancy in its stalling force.

Xorbit/CLASP links dynamic microtubules to chromosomes in the Xenopus meiotic spindle

A family of microtubule (MT)-binding proteins, Orbit/multiple asters/cytoplasmic linker protein–associated protein, has emerged as an important player during mitosis, but their functional mechanisms are poorly understood. In this study, we used meiotic egg extracts to gain insight into the role of the Xenopus laevis homologue Xorbit in spindle assembly and function. Xorbit immunodepletion or its inhibition by a dominant-negative fragment resulted in chromosome alignment defects and aberrant MT structures, including monopolar and small spindles. Xorbit-depleted extracts failed to nucleate MTs around chromatin-coated beads, indicating its essential requirement for spindle assembly in the absence of centrosomes and kinetochores. Xorbit's MT stabilizing effect was most apparent during anaphase, when spindle MTs depolymerized rapidly upon Xorbit inhibition. Biochemical interaction between a COOH-terminal Xorbit fragment and the kinetochore-associated kinesin centromeric protein E may contribute to Xorbit's role in chromosome congression. We propose that Xorbit tethers dynamic MT plus ends to kinetochores and chromatin, providing a stabilizing activity that is crucial for spindle assembly and chromosome segregation.

A bidirectional kinesin motor in live Drosophila embryos

Spindle assembly and elongation involve poleward and away-from-the-pole forces produced by microtubule dynamics and spindle-associated motors. Here, we show that a bidirectional Drosophila Kinesin-14 motor that moves either to the microtubule plus or minus end in vitro unexpectedly causes only minor spindle defects in vivo. However, spindles of mutant embryos are longer than wild type, consistent with increased plus-end motor activity. Strikingly, suppressing spindle dynamics by depriving embryos of oxygen causes the bidirectional motor to show increased accumulation at distal or plus ends of astral microtubules relative to wild type, an effect not observed for a mutant motor defective in motility. Increased motor accumulation at microtubule plus ends may be due to increased slow plus-end movement of the bidirectional motor under hypoxia, caused by perturbation of microtubule dynamics or inactivation of the only other known Drosophila minus-end spindle motor, cytoplasmic dynein. Negative-stain electron microscopy images are consistent with highly cooperative motor binding to microtubules, and gliding assays show dependence on motor density for motility. Mutant effects of the bidirectional motor on spindle function may be suppressed under normal conditions by motor: motor interactions and minus-end movement induced by spindle dynamics. These forces may also bias wild-type motor movement toward microtubule minus ends in live cells. Our findings link motor : motor interactions to function in vivo by showing that motor density, together with cellular dynamics, may influence motor function in live cells.

The influence of chromosome flexibility on chromosome transport during anaphase A

The role of protein flexibility in molecular motor function has previously been studied by considering a Brownian ratchet motor that is connected to its cargo by an elastic spring, with the result that the average velocity of the motor/cargo system is increased by reducing the stiffness of the linkage. Here, we extend this investigation to the case of chromosome transport during anaphase A, in which the relevant flexibility is not primarily in the motor/cargo linkage but rather in the cargo itself, i.e., in the chromosome. We model the motor mechanism as an imperfect Brownian ratchet with a built-in opposing load and the chromosome as a collection of discrete segments linked by an elastic energy function that discretizes the potential energy of an elastic rod. Thermal fluctuations are produced in the model by random forces, as in Brownian dynamics. All of the parameters that characterize the chromosome are known or can be estimated from experimental data, as can all but one of the motor parameters, which is adjusted to give the correct transport velocity of normal-length chromosomes. With the parameters so determined, we then reproduce the experimental finding of Nicklas [Nicklas, R. B. (1965) J. Cell Biol. 25, 119-135] that chromosome speed is essentially independent of chromosome length, even though our model contains no "velocity governor." We find instead that this effect is a consequence of chromosome flexibility, as it disappears when stiffer than normal chromosomes are considered.

Force production by disassembling microtubules

Microtubules (MTs) are important components of the eukaryotic cytoskeleton: they contribute to cell shape and movement, as well as to the motions of organelles including mitotic chromosomes. MTs bind motor enzymes that drive many such movements, but MT dynamics can also contribute to organelle motility. Each MT polymer is a store of chemical energy that can be used to do mechanical work, but how this energy is converted to motility remains unknown. Here we show, by conjugating glass microbeads to tubulin polymers through strong inert linkages, such as biotin-avidin, that depolymerizing MTs exert a brief tug on the beads, as measured with laser tweezers. Analysis of these interactions with a molecular-mechanical model of MT structure and force production shows that a single depolymerizing MT can generate about ten times the force that is developed by a motor enzyme; thus, this mechanism might be the primary driving force for chromosome motion. Because even the simple coupler used here slows MT disassembly, physiological couplers may modulate MT dynamics in vivo.

Microtubule capture by CENP-E silences BubR1-dependent mitotic checkpoint signaling

The mitotic checkpoint is the major cell cycle control mechanism for maintaining chromosome content in multicellular organisms. Prevention of premature onset of anaphase requires activation at unattached kinetochores of the BubR1 kinase, which acts with other components to generate a diffusible “stop anaphase” inhibitor. Not only does direct binding of BubR1 to the centromere-associated kinesin family member CENP-E activate its essential kinase, binding of a motorless fragment of CENP-E is shown here to constitutively activate BubR1 bound at kinetochores, producing checkpoint signaling that is not silenced either by spindle microtubule capture or the tension developed at those kinetochores by other components. Using purified BubR1, microtubules, and CENP-E, microtubule capture by the CENP-E motor domain is shown to silence BubR1 kinase activity in a ternary complex of BubR1–CENP-E–microtubule. Together, this reveals that CENP-E is the signal transducing linker responsible for silencing BubR1-dependent mitotic checkpoint signaling through its capture at kinetochores of spindle microtubules.

The dynamic kinetochore-microtubule interface

The kinetochore is a control module that both powers and regulates chromosome segregation in mitosis and meiosis. The kinetochore-microtubule interface is remarkably fluid, with the microtubules growing and shrinking at their point of attachment to the kinetochore. Furthermore, the kinetochore itself is highly dynamic, its makeup changing as cells enter mitosis and as it encounters microtubules. Active kinetochores have yet to be isolated or reconstituted, and so the structure remains enigmatic. Nonetheless, recent advances in genetic, bioinformatic and imaging technology mean we are now beginning to understand how kinetochores assemble, bind to microtubules and release them when the connections made are inappropriate, and also how they influence microtubule behaviour. Recent work has begun to elucidate a pathway of kinetochore assembly in animal cells; the work has revealed that many kinetochore components are highly dynamic and that some cycle between kinetochores and spindle poles along microtubules. Further studies of the kinetochore-microtubule interface are illuminating: (1) the role of the Ndc80 complex and components of the Ran-GTPase system in microtubule attachment, force generation and microtubule-dependent inactivation of kinetochore spindle checkpoint activity; (2) the role of chromosomal passenger proteins in the correction of kinetochore attachment errors; and (3) the function of microtubule plus-end tracking proteins, motor depolymerases and other proteins in kinetochore movement on microtubules and movement coupled to microtubule poleward flux.

Force production by depolymerizing microtubules: a theoretical study

Chromosome movement during mitosis is powered in part by energy released through the depolymerization of kinetochore microtubules (MTs). Strong but indirect evidence suggests the existence of a specialized coupling between kinetochores and MT plus ends that enables this transduction of chemical energy into mechanical work. Analysis of this phenomenon is important for learning how energy is stored within the MT lattice, how it is transduced, and how efficient the process can be, given coupling devices of different designs. Here we use a recently developed molecular-mechanical model of MTs to examine the mechanism of disassembly dependent force generation. Our approach is based on changes in tubulin dimer conformation that occur during MT disassembly. We find that all of the energy of polymerization-associated GTP hydrolysis can be stored as deformations of the longitudinal bonds between tubulin dimers, and its optimal use does not require the weakening of lateral bonds between dimers. Maximum utilization of this stored energy and, hence, the generation of the strongest possible force, is achieved by a protofilament power-stroke mechanism, so long as the coupling device does not restrict full dissociation of the lateral bonds between tubulin dimers.