CENP-E is a putative kinetochore motor that accumulates just before mitosis

Profiling essential genes in human mammary cells by multiplex RNAi screening

By virtue of their accumulated genetic alterations, tumor cells may acquire vulnerabilities that create opportunities for therapeutic intervention. We have devised a massively parallel strategy for screening short hairpin RNA (shRNA) collections for stable loss-of-function phenotypes. We assayed from 6000 to 20,000 shRNAs simultaneously to identify genes important for the proliferation and survival of five cell lines derived from human mammary tissue. Lethal shRNAs common to these cell lines targeted many known cell-cycle regulatory networks. Cell line-specific sensitivities to suppression of protein complexes and biological pathways also emerged, and these could be validated by RNA interference (RNAi) and pharmacologically. These studies establish a practical platform for genome-scale screening of complex phenotypes in mammalian cells and demonstrate that RNAi can be used to expose genotype-specific sensitivities.

Screening for inhibitors of microtubule-associated motor proteins

The mitotic spindle is an important target for cancer chemotherapy. The main protein target for drugs in clinical use is tubulin, the building block of microtubules. In recent years, other proteins of the mitotic spindle have been identified as potential targets for the development of more specific drugs with the hope that these will have fewer side effects than known antimitotics (taxanes, vinca alkaloids). The human genome contains more than 40 members of the kinesin superfamily, with at least 12 of these involved in mitosis and cytokinesis. HsEg5 (also called KSP, kinesin spindle protein), a member of the kinesin-5 family, involved in the formation of the bipolar spindle, is a very promising target for cancer chemotherapy with specific inhibitors in Phase I and II clinical trails. Several successful approaches exist today to screen Eg5 for inhibitors, including phenotype-based assays and simple in vitro assays that explore the intrinsic enzymatic ATPase activity of Eg5. Here, we describe a robust and straightforward in vitro method to rapidly screen Eg5 for inhibitors. The assay can easily be adapted to other mitotic kinesins that may be identified in the future as potential drug targets, or simply to obtain specific kinesin inhibitors for use in "chemical genetics" to study the function of this important class of proteins.

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.

Astrin is required for the maintenance of sister chromatid cohesion and centrosome integrity

Faithful chromosome segregation in mitosis requires the formation of a bipolar mitotic spindle with stably attached chromosomes. Once all of the chromosomes are aligned, the connection between the sister chromatids is severed by the cysteine protease separase. Separase also promotes centriole disengagement at the end of mitosis. Temporal coordination of these two activities with the rest of the cell cycle is required for the successful completion of mitosis. In this study, we report that depletion of the microtubule and kinetochore protein astrin results in checkpoint-arrested cells with multipolar spindles and separated sister chromatids, which is consistent with untimely separase activation. Supporting this idea, astrin-depleted cells contain active separase, and separase depletion suppresses the premature sister chromatid separation and centriole disengagement in these cells. We suggest that astrin contributes to the regulatory network that controls separase activity.

A loss of function screen identifies nine new radiation susceptibility genes

Genomic instability is considered a hallmark of carcinogenesis, and dysfunction of DNA repair and cell cycle regulation in response to DNA damage caused by ionizing radiation are thought to be important factors in the early stages of genomic instability. We performed cell-based functional screening using an RNA interference library targeting 200 genes in human cells. We identified three known and nine new radiation susceptibility genes, eight of which are linked directly or potentially with cell cycle progression. Cell cycle analysis on four of the genes not previously linked to cell cycle progression demonstrated that one, ZDHHC8, was associated with the G2/M checkpoint in response to DNA damage. Further study of the 12 radiation susceptibility genes identified in this screen may help to elucidate the molecular mechanisms of cell cycle progression, DNA repair, cell death, cell growth and genomic instability, and to develop new radiation sensitizing agents for radiotherapy.

The human kinesin Kif18A is a motile microtubule depolymerase essential for chromosome congression

BACKGROUND

The accurate alignment of chromosomes at the spindle equator is fundamental for the equal distribution of the genome in mitosis and thus for the genetic integrity of eukaryotes. Although it is well established that chromosome movements are coupled to microtubule dynamics, the underlying mechanism is not well understood.

RESULTS

By combining RNAi-depletion experiments with in vitro biochemical assays, we demonstrate that the human kinesin Kif18A is a motile microtubule depolymerase essential for chromosome congression in mammalian tissue culture cells. We show that in vitro Kif18A is a slow plus-end-directed kinesin that possesses microtubule depolymerizing activity. Notably, Kif18A like its yeast ortholog Kip3p depolymerizes longer microtubules more quickly than shorter ones. In vivo, Kif18A accumulates in mitosis where it localizes close to the plus ends of kinetochore microtubules. The depletion of Kif18A induces aberrantly long mitotic spindles and loss of tension across sister kinetochores, resulting in the activation of the Mad2-dependent spindle-assembly checkpoint. Live-cell microscopy studies revealed that in Kif18A-depleted cells, chromosomes move at reduced speed and completely fail to align at the spindle equator.

CONCLUSIONS

These studies identify Kif18A as a dual-functional kinesin and a key component of chromosome congression in mammalian cells.

An ent-kaurene that inhibits mitotic chromosome movement and binds the kinetochore protein ran-binding protein 2

Using a chemical genetics screen, we have identified ent-15-oxokaurenoic acid (EKA) as a chemical that causes prolonged mitotic arrest at a stage resembling prometaphase. EKA inhibits the association of the mitotic motor protein centromeric protein E with kinetochores and inhibits chromosome movement. Unlike most antimitotic agents, EKA does not inhibit the polymerization or depolymerization of tubulin. To identify EKA-interacting proteins, we used a cell-permeable biotinylated form that retains biological activity to isolate binding proteins from living cells. Mass spectrometric analysis identified six EKA-binding proteins, including Ran-binding protein 2, a kinetochore protein whose depletion by small interfering RNA causes a similar mitotic arrest phenotype.

Genome-wide transcriptional profiling in human squamous cell carcinoma of the skin identifies unique tumor-associated signatures

The elucidation of specific genetic changes associated with human cancer pathogenesis has focused efforts to relate such changes to the neoplastic phenotype. To further our understanding of the genetic basis of human squamous cell carcinoma (SCC) of the skin, this study used a genome-wide (12 627 sequences) approach to determine transcriptional signatures in lesional and nonlesional sites from five SCC patients. Several novel genes involving the p53 pathway, anti-apoptotic pathways, signal transduction, structural loss and DNA replication, including BCL2A1, MUC4, PTPN11 (SHP2) and FGF9, are upregulated in SCC and could warrant further study regarding their role in disease pathogenesis. SCC pathology is likely combinatorial in nature involving the compounded changes from several cellular processes.

RNA interference-mediated silencing of mitotic kinesin KIF14 disrupts cell cycle progression and induces cytokinesis failure

KIF14 is a microtubule motor protein whose elevated expression is associated with poor-prognosis breast cancer. Here we demonstrate KIF14 accumulation in mitotic cells, where it associated with developing spindle poles and spindle microtubules. Cells at later stages of mitosis were characterized by the concentration of KIF14 at the midbody. Time-lapse microscopy revealed that strong RNA interference (RNAi)-mediated silencing of KIF14 induced cytokinesis failure, causing several rounds of endoreduplication and resulting in multinucleated cells. Additionally, less efficacious KIF14-specific short interfering RNAs (siRNAs) induced multiple phenotypes, all of which resulted in acute apoptosis. Our data demonstrate the ability of siRNA-mediated silencing to generate epiallelic hypomorphs associated with KIF14 depletion. Furthermore, the link we observed between siRNA efficacy and phenotypic outcome indicates that distinct stages during cell cycle progression are disrupted by the differential modulation of KIF14 expression.

Interaction of Skp1 with CENP-E at the midbody is essential for cytokinesis

Centromere-associated protein E (CENP-E) is a kinesin-related microtubule motor protein that is essential for chromosome congression during mitosis. Our previous studies show that microtubule motor CENP-E represents a link between attachment of spindle microtubules and the mitotic checkpoint signaling cascade. However, the molecular function of CENP-E at the midbody had remained elusive. Here we show that CENP-E interacts with Skp1 at the midbody and participates in cytokinesis. CENP-E interacts with Skp1 in vitro and in vivo via its coiled-coil domain. Our yeast two-hybrid assays mapped the binding interfaces to the central stalk region of CENP-E (955-1571 aa) and the C-terminal 33 amino acids of Skp1, respectively. Our immunocytochemical studies revealed that CENP-E targets to the midbody prior to Skp1 and the midbody localization of CENP-E becomes diminished as Skp1 arrives at the midbody. Suppression of Skp1 in mitotic HeLa cells by siRNA resulted in accumulation of telophase cells with elongated inter-cell bridges and with midbodies stretched 2-3 times longer than that of normal cells. These Skp1-eliminated or -suppressed cells accumulate higher level of CENP-E, suggesting that spatiotemporal regulation of CENP-E degradation at the midbody is essential for cytokinesis. Over-expression of Skp1 lacking the CENP-E-binding domain confirmed that Skp1-CENP-E interaction is essential for faithful cytokinesis. We hypothesize that CENP-E degradation is essential for faithful mitotic exit and the proteolysis of CENP-E is mediated by SCF via a direct Skp1 link.

Aging in check

The spindle checkpoint monitors the interaction between spindle microtubules and kinetochores to prevent precocious entry into anaphase, delaying this stage of mitosis until all condensed chromosomes have been attached to the mitotic spindle in a bi-oriented manner (so that the two kinetochores associated with a pair of sister chromatids are oriented toward opposite poles of the spindle). In addition to conserved Bub and Mad family members, which are known to function in the spindle checkpoint pathway in organisms ranging from yeast to mammals, two mRNA transport genes, Rae1 and Nup9, are also involved in the spindle checkpoint function in mammals. Biochemically, activated spindle checkpoint components have been shown to suppress the activity of the anaphase promoting complex/cyclosome. It is generally thought that decreased activity of the checkpoint components predisposes cells to chromosomal instability, aneuploidy, and malignant transformation. Interestingly, a recent study has shed light on a new function of the spindle checkpoint components Bub3 and Rae1 in the regulation of aging. Mice with haploinsufficiency of Bub3 and Rae1 have a short life span that is associated with the early onset of aging-related features. The progeroid phenotypes caused by deficiency of Bub3 and Rae1 are tightly linked to precocious activation of cellular senescence, but not apoptotic, programs. Therefore, premature aging, rather than neoplastic transformation, may be the major manifestation of a compromised spindle checkpoint in vivo.

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.

S-trityl-L-cysteine is a reversible, tight binding inhibitor of the human kinesin Eg5 that specifically blocks mitotic progression

Human Eg5, responsible for the formation of the bipolar mitotic spindle, has been identified recently as one of the targets of S-trityl-L-cysteine, a potent tumor growth inhibitor in the NCI 60 tumor cell line screen. Here we show that in cell-based assays S-trityl-L-cysteine does not prevent cell cycle progression at the S or G(2) phases but inhibits both separation of the duplicated centrosomes and bipolar spindle formation, thereby blocking cells specifically in the M phase of the cell cycle with monoastral spindles. Following removal of S-trityl-L-cysteine, mitotically arrested cells exit mitosis normally. In vitro, S-trityl-L-cysteine targets the catalytic domain of Eg5 and inhibits Eg5 basal and microtubule-activated ATPase activity as well as mant-ADP release. S-trityl-L-cysteine is a tight binding inhibitor (estimation of K(i,app) <150 nm at 300 mm NaCl and 600 nm at 25 mm KCl). S-trityl-L-cysteine binds more tightly than monastrol because it has both an approximately 8-fold faster association rate and approximately 4-fold slower release rate (6.1 microM(-1) s(-1) and 3.6 s(-1) for S-trityl-L-cysteine versus 0.78 microM(-1) s(-1) and 15 s(-1) for monastrol). S-trityl-L-cysteine inhibits Eg5-driven microtubule sliding velocity in a reversible fashion with an IC(50) of 500 nm. The S and D-enantiomers of S-tritylcysteine are nearly equally potent, indicating that there is no significant stereospecificity. Among nine different human kinesins tested, S-trityl-L-cysteine is specific for Eg5. The results presented here together with the proven effect on human tumor cell line growth make S-trityl-L-cysteine a very attractive starting point for the development of more potent mitotic inhibitors.

Engineered human dicentric chromosomes show centromere plasticity

The centromere is essential for the faithful distribution of a cell's genetic material to subsequent generations. Despite intense scrutiny, the precise genetic and epigenetic basis for centromere function is still unknown. Here, we have used engineered dicentric human chromosomes to investigate mammalian centromere structure and function. We describe three classes of dicentric chromosomes isolated in different cell lines: functionally monocentric chromosomes, in which one of the two genetically identical centromeres is consistently inactivated; functionally dicentric chromosomes, in which both centromeres are consistently active; and dicentric chromosomes heterogeneous with respect to centromere activity. A study of serial single cell clones from heterogeneous cell lines revealed that while centromere activity is usually clonal, the centromere state (i.e. functionally monocentric or dicentric) in some lines can switch within a growing population of cells. Because pulsed field gel analysis indicated that the DNA at the centromeres of these chromosomes did not change detectably, this switching of the centromere state is most likely due to epigenetic changes. Inactivation of one of the two active centromeres in a functionally dicentric chromosome was observed in a percentage of cells after treatment with Trichostatin A, an inhibitor of histone deacetylation. This study provides evidence that the activity of human centromeres, while largely stable, can be subject to dynamic change, most likely due to epigenetic modification.

Analysis of the kinesin superfamily: insights into structure and function

Kinesin superfamily proteins (KIFs) are key players or 'hub' proteins in the intracellular transport system, which is essential for cellular function and morphology. The KIF superfamily is also the first large protein family in mammals whose constituents have been completely identified and confirmed both in silico and in vivo. Numerous studies have revealed the structures and functions of individual family members; however, the relationships between members or a perspective of the whole superfamily structure until recently remained elusive. Here, we present a comprehensive summary based on a large, systematic phylogenetic analysis of the kinesin superfamily. All available sequences in public databases, including genomic information from all model organisms, were analyzed to yield the most complete phylogenetic kinesin tree thus far, comprising 14 families. This comprehensive classification builds on the recently proposed standardized nomenclature for kinesins and allows systematic analysis of the structural and functional relationships within the kinesin superfamily.

Kinetochore structure and function

The vertebrate kinetochore is a complex structure that specifies the attachments between the chromosomes and microtubules of the spindle and is thus essential for accurate chromosome segregation. Kinetochores are assembled on centromeric chromatin through complex pathways that are coordinated with the cell cycle. In the light of recent discoveries on how proteins assemble onto kinetochores and interact with each other, we review these findings in this article (which is part of the Chromosome Segregation and Aneuploidy series), and discuss their implications for the current mitotic checkpoint models - the template model and the two-step model. The template model proposes that Mad1-Mad2 at kinetochores acts as a template to change the conformation of another binding molecule of Mad2. This templated change in conformation is postulated as a mechanism for the amplification of the 'anaphase wait' signal. The two-step model proposes that the mitotic checkpoint complex (MCC) is the kinetochore-independent anaphase inhibitor, and the role of the unaligned kinetochore is to sensitize the anaphase-promoting complex/cyclosome (APC/C) to MCC-mediated inhibition.

Mechanisms and chemical induction of aneuploidy in rodent germ cells

The objective of this review is to suggest that the advances being made in our understanding of the molecular events surrounding chromosome segregation in non-mammalian and somatic cell models be considered when designing experiments for studying aneuploidy in mammalian germ cells. Accurate chromosome segregation requires the temporal control and unique interactions among a vast array of proteins and cellular organelles. Abnormal function and temporal disarray among these, and others to be identified, biochemical reactions and cellular organelles have the potential for predisposing cells to aneuploidy. Although numerous studies have demonstrated that certain chemicals (mainly those that alter microtubule function) can induce aneuploidy in mammalian germ cells, it seems relevant to point out that such data can be influenced by gender, meiotic stage, and time of cell-fixation post-treatment. Additionally, a consensus has not been reached regarding which of several germ cell aneuploidy assays most accurately reflects the human condition. More recent studies have shown that certain kinase, phosphatase, proteasome, and topoisomerase inhibitors can also induce aneuploidy in rodent germ cells. We suggest that molecular approaches be prudently incorporated into mammalian germ cell aneuploidy research in order to eventually understand the causes and mechanisms of human aneuploidy. Such an enormous undertaking would benefit from collaboration among scientists representing several disciplines.

A mitotic septin scaffold required for Mammalian chromosome congression and segregation

Coordination of cytokinesis with chromosome congression and segregation is critical for proper cell division, but the mechanism is unknown. Here, septins, a conserved family of polymerizing guanosine triphosphate-binding proteins, localized to the metaphase plate during mitosis. Septin depletion resulted in chromosome loss from the metaphase plate, lack of chromosome segregation and spindle elongation, and incomplete cytokinesis upon delayed mitotic exit. These defects correlated with loss of the mitotic motor and the checkpoint regulator centromere-associated protein E (CENP-E) from the kinetochores of congressing chromosomes. Mammalian septins may thus form a mitotic scaffold for CENP-E and other effectors to coordinate cytokinesis with chromosome congression and segregation.

A gene highly expressed in tumor cells encodes novel structure proteins

We isolated several related but distinct cDNA clones encoding novel structure proteins (NSP) when screening a cDNA library. Analysis revealed that these cDNAs and several similar ESTs in the public databases are derived from a single gene of 17 exons that span a minimum of 227-kb region. This gene is located at chromosome 17p11.2, a region frequently amplified in human gliomas and osteosarcomas, and involved in Birt-Hogg-Dube syndrome, a tumor-prone syndrome. The major coding sequences shared by all isolated transcripts are predicted to encode SMC (structural maintenance of chromosome)/SbcC ATPase motifs and coiled-coil domains commonly seen in motor or structure proteins. Two 5'-end and two 3'-end variants (type 5alpha/beta and 3alpha/beta, respectively) were identified, making a total of four possible transcripts. Both 5alpha and 5beta variants were detected in human testis mRNA, but only type 5alpha was detectable in RNA samples extracted from HeLa cells. The unique carboxyl-terminus of 3beta contains a Ca(2+)-dependent actin-binding domain. Immunohistochemistry studies revealed that NSPs were mostly localized to nuclei. Northern blot analysis demonstrated two major bands and the expression levels are tremendously high in testis while barely detectable in other normal tissues examined. Interestingly, NSP5alpha3alpha is highly expressed in some tumor cell lines. These results suggest that NSPs represent a new family of structure proteins with a possible role in nuclear dynamics during cell division, and that NSP5alpha3alpha may serve as a tumor marker.

Kinetochore-microtubule interactions during cell division

Proper segregation of chromosomes during cell division is essential for the maintenance of genetic stability. During this process chromosomes must establish stable functional interactions with microtubules through the kinetochore, a specialized protein structure located on the surface of the centromeric heterochromatin. Stable attachment of kinetochores to a number of microtubules results in the formation of a kinetochore fibre that mediates chromosome movement. How the kinetochore fibre is formed and how chromosome motion is produced and regulated remain major questions in cell biology. Here we look at some of the history of research devoted to the study of kinetochore-microtubule interaction and attempt to identify significant advances in the knowledge of the basic processes. Ultrastructural work has provided substantial insights into the structure of the kinetochore and associated microtubules during different stages of mitosis. Also, recent in-vivo studies have probed deep into the dynamics of kinetochore-attached microtubules suggesting possible models for the way in which kinetochores harness the capacity of microtubules to do work and turn it into chromosome motion. Much of the research in recent years suggests that indeed multiple mechanisms are involved in both formation of the k-fibre and chromosome motion. Thus, rather than moving to a unified theory, it has become apparent that most cell types have the capacity to build the spindle using multiple and probably redundant mechanisms.

Regulation of kinesin-directed movements

Bidirectional organelle transport along microtubules is most likely mediated by the opposing forces generated by two microtubule-based motors: kinesin and cytoplasmic dynein. Because the direction and timing of organelle movements are controlled by the cell, the activity of one or both of these motor molecules must be regulated. Recent studies demonstrate that kinesin, kinesin-like proteins and kinesin-associated proteins can be phosphorylated, and suggest that changes in their phosphorylation state may modulate kinesin's ability to interact with either microtubules or organelles. Thus, it is possible that phosphorylation regulates kinesin-driven movements.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

Centrosome Reduction During Gametogenesis and Its Significance1

Animal spermatids and primary oocytes initially have typical centrosomes comprising pairs of centrioles and pericentriolar fibrous centrosomal proteins. These somatic cell-like centrosomes are partially or completely degenerated during gametogenesis. Centrosome reduction during spermiogenesis comprises attenuation of microtubule nucleation function, loss of pericentriolar material, and centriole degeneration. Centrosome reduction during oogenesis is due to complete degeneration of centrioles, which leads to dispersal of the pericentriolar centrosomal proteins, loss of replicating capacity of the spindle poles, and switching to acentrosomal mode of spindle organization. Oocyte centrosome reduction plays an important role in preventing parthenogenetic embryogenesis and balancing centrosome number in the embryonic cells.

Ablation of PRC1 by small interfering RNA demonstrates that cytokinetic abscission requires a central spindle bundle in mammalian cells, whereas completion of furrowing does not

The temporal and spatial regulation of cytokinesis requires an interaction between the anaphase mitotic spindle and the cell cortex. However, the relative roles of the spindle asters or the central spindle bundle are not clear in mammalian cells. The central spindle normally serves as a platform to localize key regulators of cell cleavage, including passenger proteins. Using time-lapse and immunofluorescence analysis, we have addressed the consequences of eliminating the central spindle by ablation of PRC1, a microtubule bundling protein that is critical to the formation of the central spindle. Without a central spindle, the asters guide the equatorial cortical accumulation of anillin and actin, and of the passenger proteins, which organize into a subcortical ring in anaphase. Furrowing goes to completion, but abscission to create two daughter cells fails. We conclude the central spindle bundle is required for abscission but not for furrowing in mammalian cells.

In vitro screening for inhibitors of the human mitotic kinesin Eg5 with antimitotic and antitumor activities

Human Eg5, a member of the kinesin superfamily, plays a key role in mitosis, as it is required for the formation of a bipolar spindle. We describe here the first in vitro microtubule-activated ATPase-based assay for the identification of small-molecule inhibitors of Eg5. We screened preselected libraries obtained from the National Cancer Institute and identified S-trityl-l-cysteine as the most effective Eg5 inhibitor with an IC50 of 1.0 μmol/L for the inhibition of basal ATPase activity and 140 nmol/L for the microtubule-activated ATPase activity. Subsequent cell-based assays revealed that S-trityl-l-cysteine induced mitotic arrest in HeLa cells (IC50, 700 nmol/L) with characteristic monoastral spindles. S-trityl-l-cysteine is 36 times more potent for inducing mitotic arrest than the well-studied inhibitor, monastrol. Gossypol, flexeril, and two phenothiazine analogues were also identified as Eg5 inhibitors, and we found that they all result in monoastral spindles in HeLa cells. It is notable that all the Eg5 inhibitors identified here have been shown previously to inhibit tumor cell line growth in the NCI 60 tumor cell line screen, and we conclude that their antitumor activity may at least in part be explained by their ability to inhibit Eg5 activity.

Crystal structure of the motor domain of the human kinetochore protein CENP-E

The human kinetochore is a highly complex macromolecular structure that connects chromosomes to spindle microtubules (MTs) in order to facilitate accurate chromosome segregation. Centromere-associated protein E (CENP-E), a member of the kinesin superfamily, is an essential component of the kinetochore, since it is required to stabilize the attachment of chromosomes to spindle MTs, to develop tension across aligned chromosomes, to stabilize spindle poles and to satisfy the mitotic checkpoint. Here we report the 2.5A resolution crystal structure of the motor domain and linker region of human CENP-E with MgADP bound in the active site. This structure displays subtle but important differences compared to the structures of human Eg5 and conventional kinesin. Our structure reveals that the CENP-E linker region is in a "docked" position identical to that in the human plus-end directed conventional kinesin. CENP-E has many advantages as a potential anti-mitotic drug target and this crystal structure of human CENP-E will provide a starting point for high throughput virtual screening of potential inhibitors.

Transient exposure to the Eg5 kinesin inhibitor monastrol leads to syntelic orientation of chromosomes and aneuploidy in mouse oocytes

Aneuploidy may result from abnormalities in the biochemical pathways and cellular organelles associated with chromosome segregation. Monastrol is a reversible, cell-permeable, non-tubulin interacting inhibitor of the mitotic kinesin Eg5 motor protein which is required for assembling and maintaining the mitotic spindle. Monastrol can also impair centrosome separation and induce monoastral spindles in mammalian somatic cells. The ability of monastrol to alter kinesin Eg5 and centrosome activities and spindle geometry may lead to abnormal chromosome segregation. Mouse oocytes were exposed to 0 (control), 15, 30, and 45 microg/ml monastrol in vitro for 6 h during meiosis I and subsequently cultured for 17 h in monastrol-free media prior to cytogenetic analysis of metaphase II oocytes. A subset of oocytes was cultured for 5 h prior to processing cells for meiotic I spindle analysis. Monastrol retarded oocyte maturation by significantly (P < 0.05) decreasing germinal vesicle breakdown and increasing the frequencies of arrested metaphase I oocytes. Also, significant (P < 0.05) increases in the frequencies of monoastral spindles and chromosome displacement from the metaphase plate were found in oocytes during meiosis I. In metaphase II oocytes, monastrol significantly (P < 0.05) increased the frequencies of premature centromere separation and aneuploidy. These findings suggest that abnormal meiotic spindle geometry predisposes oocytes to aneuploidy.

Gene silencing of CENP-E by small interfering RNA in HeLa cells leads to missegregation of chromosomes after a mitotic delay

Centromeric protein-E (CENP-E) is a kinesin-like motor protein required for chromosome congression at prometaphase. Functional perturbation of CENP-E by various methods results in a consistent phenotype, i.e., unaligned chromosomes during mitosis. One unresolved question from previous studies is whether cells complete mitosis or sustain mitotic arrest in the presence of unaligned chromosomes. Using RNA interference and video-microscopy, we analyzed the dynamic process of mitotic progression of HeLa(H2B)-GFP cells lacking CENP-E. Our results demonstrate that these cells initiated anaphase after a delayed mitotic progression due to the presence of unaligned chromosomes. In some dividing cells, unaligned chromosomes are present during anaphase, causing nondisjunction of some sister chromatids producing aneuploid daughter cells. Unlike in Xenopus extract, the loss of CENP-E in HeLa cells does not impair gross checkpoint activation because cells were arrested in mitosis in response to microtubule-interfering agents. However, the lack of CENP-E at kinetochores reduced the hyperphosphorylation of BubR1 checkpoint protein during mitosis, which may explain the loss of sensitivity of a cell to a few unaligned chromosomes in the absence of CENP-E. We also found that presynchronization with nocodazole sensitizes cells to the depletion of CENP-E, leading to more unaligned chromosomes, longer arrest, and cell death.

Bub1 is required for kinetochore localization of BubR1, Cenp-E, Cenp-F and Mad2, and chromosome congression

During mitosis, the recruitment of spindle-checkpoint-associated proteins to the kinetochore occurs in a defined order. The protein kinase Bub1 localizes to the kinetochore very early during mitosis, followed by Cenp-F, BubR1, Cenp-E and finally Mad2. Using RNA interference, we have investigated whether this order of binding reflects a level of dependency in human somatic cells. Specifically, we show that Bub1 plays a key role in the assembly of checkpoint proteins at the kinetochore, being required for the subsequent localization of Cenp-F, BubR1, Cenp-E and Mad2. In contrast to studies in Xenopus, we also show that BubR1 is not required for kinetochore localization of Bub1. Repression of Bub1 increases the number of cells with lagging chromosomes at metaphase, suggesting that Bub1 plays a role in chromosome congression. However, repression of Bub1 does not appear to compromise spindle checkpoint function either during normal mitosis or in response to spindle damage. This raises the possibility that, in the absence of Bub1, other mechanisms contribute to spindle checkpoint function.

CENP-B Interacts with CENP-C Domains Containing Mif2 Regions Responsible for Centromere Localization

Recently, human artificial chromosomes featuring functional centromeres have been generated efficiently from naked synthetic alphoid DNA containing CENP-B boxes as a de novo mechanism in a human cultured cell line, but not from the synthetic alphoid DNA only containing mutations within CENP-B boxes, indicating that CENP-B has some functions in assembling centromere/kinetochore components on alphoid DNA. To investigate whether any interactions exist between CENP-B and the other centromere proteins, we screened a cDNA library by yeast two-hybrid analysis. An interaction between CENP-B and CENP-C was detected, and the CENP-C domains required were determined to overlap with three Mif2 homologous regions, which were also revealed to be involved in the CENP-C assembly of centromeres by expression of truncated polypeptides in cultured cells. Overproduction of truncated CENP-B containing no CENP-C interaction domains caused abnormal duplication of CENP-C domains at G2 and cell cycle delay at metaphase. These results suggest that the interaction between CENP-B and CENP-C may be involved in the correct assembly of CENP-C on alphoid DNA. In other words, a possible molecular linkage may exist between one of the kinetochore components and human centromere DNA through CENP-B/CENP-B box interaction.

Human artificial chromosomes with alpha satellite-based de novo centromeres show increased frequency of nondisjunction and anaphase lag

Human artificial chromosomes have been used to model requirements for human chromosome segregation and to explore the nature of sequences competent for centromere function. Normal human centromeres require specialized chromatin that consists of alpha satellite DNA complexed with epigenetically modified histones and centromere-specific proteins. While several types of alpha satellite DNA have been used to assemble de novo centromeres in artificial chromosome assays, the extent to which they fully recapitulate normal centromere function has not been explored. Here, we have used two kinds of alpha satellite DNA, DXZ1 (from the X chromosome) and D17Z1 (from chromosome 17), to generate human artificial chromosomes. Although artificial chromosomes are mitotically stable over many months in culture, when we examined their segregation in individual cell divisions using an anaphase assay, artificial chromosomes exhibited more segregation errors than natural human chromosomes (P < 0.001). Naturally occurring, but abnormal small ring chromosomes derived from chromosome 17 and the X chromosome also missegregate more than normal chromosomes, implicating overall chromosome size and/or structure in the fidelity of chromosome segregation. As different artificial chromosomes missegregate over a fivefold range, the data suggest that variable centromeric DNA content and/or epigenetic assembly can influence the mitotic behavior of artificial chromosomes.

Estrogen and its metabolites are carcinogenic agents in human breast epithelial cells

Estrogens play a crucial role in the development and evolution of human breast cancer. However, it is still unclear whether estrogens are carcinogenic to the human breast. There are three mechanisms that have been considered to be responsible for the carcinogenicity of estrogens: receptor-mediated hormonal activity, a cytochrome P450 (CYP)-mediated metabolic activation, which elicits direct genotoxic effects by increasing mutation rates, and the induction of aneuploidy by estrogen. To fully demonstrate that estrogens are carcinogenic in the human breast through one or more of the mechanisms explained above it will require an experimental system in which, estrogens by itself or one of the metabolites would induce transformation phenotypes indicative of neoplasia in HBEC in vitro and also induce genomic alterations similar to those observed in spontaneous malignancies. In order to mimic the intermittent exposure of HBEC to endogenous estrogens, MCF-10F cells that are ERalpha negative and ERbeta positive were first treated with 0, 0.007, 70 nM and 1 microM of 17beta-estradiol (E(2)), diethylstilbestrol (DES), benz(a)pyrene (BP), progesterone (P), 2-OH-E(2), 4-hydoxy estradiol (4-OH-E(2)) and 16-alpha-OH-E(2) at 72 h and 120 h post-plating. Treatment of HBEC with physiological doses of E(2), 2-OH-E(2), 4-OH-E(2) induce anchorage independent growth, colony formation in agar methocel, and reduced ductulogenic capacity in collagen gel, all phenotypes whose expression are indicative of neoplastic transformation, and that are induced by BP under the same culture conditions. The presence of ERbeta is the pathway used by E(2) to induce colony formation in agar methocel and loss of ductulogenic in collagen gel. This is supported by the fact that either tamoxifen or the pure antiestrogen ICI-182,780 (ICI) abrogated these phenotypes. However, the invasion phenotype, an important marker of tumorigenesis is not modified when the cells are treated in presence of tamoxifen or ICI, suggesting that other pathways may be involved. Although we cannot rule out the possibility, that 4-OH-E(2) may interact with other receptors still not identified, with the data presently available the direct effect of 4-OH-E(2) support the concept that metabolic activation of estrogens mediated by various cytochrome P450 complexes, generating through this pathway reactive intermediates that elicit direct genotoxic effects leading to transformation. This assumption was confirmed when we found that all the transformation phenotypes induced by 4-OH-E(2) were not abrogated when this compound was used in presence of the pure antiestrogen ICI. The novelty of these observations lies in the role of ERbeta in transformation and that this pathway can successfully bypassed by the estrogen metabolite 4-OH-E(2). Genomic DNA was analyzed for the detection of micro-satellite DNA polymorphism using 64 markers covering chromosomes (chr) 3, 11, 13 and 17. We have detected loss of heterozygosity (LOH) in ch13q12.2-12.3 (D13S893) and in ch17q21.1 (D17S800) in E(2), 2-OH-E(2), 4-OH-E(2), E(2) + ICI, E(2) + tamoxifen and BP-treated cells. LOH in ch17q21.1-21.2 (D17S806) was also observed in E(2), 4-OH-E(2), E(2)+ICI, E(2)+tamoxifen and BP-treated cells. MCF-10F cells treated with P or P+E(2) did not show LOH in the any of the markers studied. LOH was strongly associated with the invasion phenotype. Altogether our data indicate that E(2) and its metabolites induce in HBEC LOH in loci of chromosomes 13 and 17, that has been reported in primary breast cancer, that the changes are similar to those induced by the chemical carcinogen (BP) and that the genomic changes were not abrogated by antiestrogens.

Activating and silencing the mitotic checkpoint through CENP-E-dependent activation/inactivation of BubR1

The mitotic checkpoint prevents advance to anaphase prior to successful attachment of every centromere/kinetochore to mitotic spindle microtubules. Using purified components and Xenopus egg extracts, the kinetochore-associated microtubule motor CENP-E is now shown to be the activator of the essential checkpoint kinase BubR1. Since kinase activity and the checkpoint are silenced following CENP-E-dependent microtubule attachment in extracts or binding of CENP-E antibodies that do not disrupt CENP-E association with BubR1, CENP-E mediates silencing of BubR1 signaling. Checkpoint signaling requires the normal level of BubR1 containing a functional Mad3 domain implicated in Cdc20 binding, but only a small fraction need be kinase competent. This supports bifunctional roles for BubR1 in the checkpoint: an enzymatic one requiring CENP-E-dependent activation of its kinase activity at kinetochores and a stoichiometric one as a direct inhibitor of Cdc20.

M phase phosphoprotein 1 is a human plus-end-directed kinesin-related protein required for cytokinesis

The human M phase phosphoprotein 1 (MPP1), previously identified through a screening of a subset of proteins specifically phosphorylated at the G2/M transition (Matsumoto-Taniura, N., Pirollet, F., Monroe, R., Gerace, L., and Westendorf, J. M. (1996) Mol. Biol. Cell 7, 1455-1469), is characterized as a plus-end-directed kinesin-related protein. Recombinant MPP1 exhibits in vitro microtubule-binding and microtubule-bundling properties as well as microtubule-stimulated ATPase activity. In gliding experiments using polarity-marked microtubules, MPP1 is a slow molecular motor that moves toward the microtubule plus-end at a 0.07 microm/s speed. In cycling cells, MPP1 localizes mainly to the nuclei in interphase. During mitosis, MPP1 is diffuse throughout the cytoplasm in metaphase and subsequently localizes to the midzone to further concentrate on the midbody. MPP1 suppression by RNA interference induces failure of cell division late in cytokinesis. We conclude that MPP1 is a new mitotic molecular motor required for completion of cytokinesis.

Chromosome-microtubule interactions during mitosis

Spindle microtubules interact with mitotic chromosomes, binding to their kinetochores to generate forces that are important for accurate chromosome segregation. Motor enzymes localized both at kinetochores and spindle poles help to form the biologically significant attachments between spindle fibers and their cargo, but microtubule-associated proteins without motor activity contribute to these junctions in important ways. This review examines the molecules necessary for chromosome-microtubule interaction in a range of well-studied organisms, using biological diversity to identify the factors that are essential for organized chromosome movement. We conclude that microtubule dynamics and the proteins that control them are likely to be more important for mitosis than the current enthusiasm for motor enzymes would suggest.

Analysis of the distribution of the kinetochore protein Ndc10p in Saccharomyces cerevisiae using 3-D modeling of mitotic spindles

Ndc10p is one of the DNA-binding constituents of the kinetochore in Saccharomyces cerevisiae but light microscopy analysis suggests that Ndc10p is not limited to kinetochore regions. We examined the localization of Ndc10p using immunoelectron microscopy and showed that Ndc10p is associated with spindle microtubules from S-phase through anaphase. By serial section reconstruction of mitotic spindles combined with immunogold detection, we showed that Ndc10p interacts with microtubules laterally as well as terminally. About 50% of the gold label in serial section reconstructions of short mitotic spindles was associated with the walls of spindle microtubules. Interaction of kinetochore components with microtubule walls was also shown for kinetochore protein Ndc80p. Our data suggest that at least a subset of kinetochore-associated protein is dispersed throughout the mitotic spindle.

The mitotic-spindle-associated protein astrin is essential for progression through mitosis

Astrin is a mitotic-spindle-associated protein expressed in most human cell lines and tissues. However, its functions in spindle organization and mitosis have not yet been determined. Sequence analysis revealed that astrin has an N-terminal globular domain and an extended coiled-coil domain. Recombinant astrin was purified and characterized by CD spectroscopy and electron microscopy. Astrin showed parallel dimers with head-stalk structures reminiscent of motor proteins, although no sequence similarities to known motor proteins were found. In physiological buffers, astrin dimers oligomerized via their globular head domains and formed aster-like structures. Silencing of astrin in HeLa cells by RNA interference resulted in growth arrest, with formation of multipolar and highly disordered spindles. Chromosomes did not congress to the spindle equator and remained dispersed. Cells depleted of astrin were normal during interphase but were unable to progress through mitosis and finally ended in apoptotic cell death. Possible functions of astrin in mitotic spindle organization are discussed.

Interaction of STOP with neuronal tubulin is independent of polyglutamylation

In eukaryotes, the coordinated progress of the various cellular tasks along with the assembly of adapted cytoskeletal networks requires a tight regulation of the interactions between microtubules and their associated proteins. Polyglutamylation is the major post-translational modification of neuronal tubulin. Due to its oligomeric structure, polyglutamylation can serve as a potentiometer to modulate binding of diverse MAPs. In addition, it can exert a differential mode of regulation towards distinct microtubule protein partners. To find out to what extent polyglutamylation is a general regulator, we have analyzed its ability to affect the binding of STOPs, the major factors that confer cold- and nocodazole-resistance to microtubules. We have shown by blot overlay experiments that binding of STOP does not depend on the length of the polyglutamyl chains carried by tubulins. And contrary to the other microtubule-associated proteins tested so far, STOP can bind quantitatively to any tubulin isoform whatever its degree of polyglutamylation.

Attachment and tension in the spindle assembly checkpoint

Faithful transmission of chromosomes during mitosis is ensured by the spindle assembly checkpoint. This molecular safeguard examines whether prerequisites for chromosome segregation have been satisfied and thereby determines whether to execute or to delay chromosome segregation. Only when all the chromosomes are attached by kinetochore microtubules from two opposite spindle poles and proper tension is placed on the paired kinetochores does anaphase take place, allowing the physical splitting of sister chromatids. Recent studies have provided novel insights into the molecular mechanisms through which the spindle assembly checkpoint is regulated by both the attachment of chromosomes to kinetochore microtubules and the tension exerted on kinetochores.

Unstable kinetochore-microtubule capture and chromosomal instability following deletion of CENP-E

A selective disruption of the mouse CENP-E gene was generated to test how this kinetochore-associated, kinesin-like protein contributes to chromosome segregation. The removal of CENP-E in primary cells produced spindles in which some metaphase chromosomes lay juxtaposed to a spindle pole, despite the absence of microtubules stably bound to their kinetochores. Most CENP-E-free chromosomes moved to the spindle equator, but their kinetochores bound only half the normal number of microtubules. Deletion of CENP-E in embryos led to early developmental arrest. Selective deletion of CENP-E in liver revealed that tissue regeneration after chemical damage was accompanied by aberrant mitoses marked by chromosome missegregation. CENP-E is thus essential for the maintenance of chromosomal stability through efficient stabilization of microtubule capture at kinetochores.

PRC1 is a microtubule binding and bundling protein essential to maintain the mitotic spindle midzone

Midzone microtubules of mammalian cells play an essential role in the induction of cell cleavage, serving as a platform for a number of proteins that play a part in cytokinesis. We demonstrate that PRC1, a mitotic spindle-associated Cdk substrate that is essential to cell cleavage, is a microtubule binding and bundling protein both in vivo and in vitro. Overexpression of PRC1 extensively bundles interphase microtubules, but does not affect early mitotic spindle organization. PRC1 contains two Cdk phosphorylation motifs, and phosphorylation is possibly important to mitotic suppression of bundling, as a Cdk phosphorylation-null mutant causes extensive bundling of the prometaphase spindle. Complete suppression of PRC1 by siRNA causes failure of microtubule interdigitation between half spindles and the absence of a spindle midzone. Truncation mutants demonstrate that the NH2-terminal region of PRC1, rich in alpha-helical sequence, is important for localization to the cleavage furrow and to the center of the midbody, whereas the central region, with the highest sequence homology between species, is required for microtubule binding and bundling activity. We conclude that PRC1 is a microtubule-associated protein required to maintain the spindle midzone, and that distinct functions are associated with modular elements of the primary sequence.

The kinase activity of aurora B is required for kinetochore-microtubule interactions during mitosis

As a component of the "chromosomal passenger protein complex," the aurora B kinase is associated with centromeres during prometaphase and with midzone microtubules during anaphase and is required for both mitosis and cytokinesis. Ablation of aurora B causes defects in both prometaphase chromosomal congression and the spindle checkpoint; however, the mechanisms underlying these defects are unclear. To address this question, we have examined chromosomal movement, spindle organization, and microtubule motor distribution in NRK cells transfected with a kinase-inactive, dominant-negative mutant of aurora B, aurora B(K-R). In cells overexpressing aurora B(K-R) fused with GFP, centromeres moved in a synchronized and predominantly unidirectional manner, as opposed to the independent, bidirectional movement in control cells expressing a similar level of wild-type aurora B-GFP. In addition, most kinetochores became physically separated from spindle microtubules, which appeared as a striking bundle between the spindle poles. These defects were associated with a microtubule-dependent depletion of motor proteins dynein and CENP-E from kinetochores. Our observations suggest that aurora B regulates the association of motor proteins with kinetochores during prometaphase. Interactions of kinetochore motors with microtubules may in turn regulate the organization of microtubules, the movement of prometaphase chromosomes, and the release of the spindle checkpoint.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

Expansion of the cell plate in plant cytokinesis requires a kinesin-like protein/MAPKKK complex

The tobacco mitogen-activated protein kinase kinase kinase NPK1 regulates lateral expansion of the cell plate at cytokinesis. Here, we show that the kinesin-like proteins NACK1 and NACK2 act as activators of NPK1. Biochemical analysis suggests that direct binding of NACK1 to NPK1 stimulates kinase activity. NACK1 is accumulated specifically in M phase and colocalized with NPK1 at the phragmoplast equator. Overexpression of a truncated NACK1 protein that lacks the motor domain disrupts NPK1 concentration at the phragmoplast equator and cell plate formation. Incomplete cytokinesis is also observed when expression of NACK1 and NACK2 is repressed by virus-induced gene silencing and in embryonic cells from Arabidopsis mutants in which a NACK1 ortholog is disrupted. Thus, we conclude that expansion of the cell plate requires NACK1/2 to regulate the activity and localization of NPK1.

Requirement of chromatid cohesion proteins rad21/scc1 and mis4/scc2 for normal spindle-kinetochore interaction in fission yeast

BACKGROUND

Proteins conserved from yeast to human hold two sister chromatids together. The failure to form cohesion in the S phase results in premature separation of chromatids in G2/M. Mitotic kinetochores free from microtubules or the lack of tension are known to activate spindle checkpoint.

RESULTS

The loss of chromatid cohesion in fission yeast mutants (mis4-242 and rad21-K1) leads to the activation of Mad2- and Bub1-dependent checkpoint, possibly due to a diminished microtubule-kinetochore interaction. Bub1, a checkpoint kinase, localizes briefly at early mitotic kinetochores in wild-type, whereas the cohesion mutation greatly increases the duration of kinetochore localization. Bub1 is bound to the central centromere region of mitotic cells. These cohesion mutants are hypersensitive to a tubulin poison and are synthetic lethal with dis1 and bir1/cut17, which are defective in microtubule-kinetochore interaction. The formation of specialized centromere chromatin containing CENP-A does not require cohesion. Dominant-negative noncleavable Rad21 fails to activate checkpoint but blocks sister chromatid separation and full spindle elongation in anaphase.

CONCLUSIONS

Mis4 and Rad21 (budding yeast Scc2 and Scc1 homologs, respectively) act in establishing the normal spindle-kinetochore interaction in early mitosis and inhibit sister chromatid separation until the cleavage of Rad21 in anaphase. Checkpoint directly or indirectly monitors the states of cohesion in early mitosis. Full spindle extension occurs with unequal nuclear division in cohesion mutants in the absence of Mad2.