Drosophila aurora B kinase is required for histone H3 phosphorylation and condensin recruitment during chromosome condensation and to organize the central spindle during cytokinesis

Condensin association with histone H2A shapes mitotic chromosomes

Chromosome structure is dynamically regulated during cell division, and this regulation is dependent, in part, on condensin. The localization of condensin at chromosome arms is crucial for chromosome partitioning during anaphase. Condensin is also enriched at kinetochores but its precise role and loading machinery remain unclear. Here we show that fission yeast (Schizosaccharomyces pombe) kinetochore proteins Pcs1 and Mde4--homologues of budding yeast (Saccharomyces cerevisiae) monopolin subunits and known to prevent merotelic kinetochore orientation--act as a condensin 'recruiter' at kinetochores, and that condensin itself may act to clamp microtubule binding sites during metaphase. In addition to the regional recruitment factors, overall condensin association with chromatin is governed by the chromosomal passenger kinase Aurora B. Aurora-B-dependent phosphorylation of condensin promotes its association with histone H2A and H2A.Z, which we identify as conserved chromatin 'receptors' of condensin. Condensin phosphorylation and its deposition onto chromosome arms reach a peak during anaphase, when Aurora B kinase relocates from centromeres to the spindle midzone, where the separating chromosome arms are positioned. Our results elucidate the molecular basis for the spatiotemporal regulation of mitotic chromosome architecture, which is crucial for chromosome partitioning.

Condensin phosphorylated by the Aurora-B-like kinase Ark1 is continuously required until telophase in a mode distinct from Top2

Condensin is a conserved protein complex that functions in chromosome condensation and segregation. It has not been previously unequivocally determined whether condensin is required throughout mitosis. Here, we examined whether Schizosaccharomyces pombe condensin continuously acts on chromosomes during mitosis and compared its role with that of DNA topoisomerase II (Top2). Using double mutants containing a temperature-sensitive allele of the condensin SMC2 subunit cut14 (cut14-208) or of top2, together with the cold-sensitive nda3-KM311 mutation (in β-tubulin), temperature-shift experiments were performed. These experiments allowed inactivation of condensin or Top2 at various stages throughout mitosis, even after late anaphase. The results established that mitotic chromosomes require condensin and Top2 throughout mitosis, even in telophase. We then showed that the Cnd2 subunit of condensin (also known as Barren) is the target subunit of Aurora-B-like kinase Ark1 and that Ark1-mediated phosphorylation of Cnd2 occurred throughout mitosis. The phosphorylation sites in Cnd2 were determined by mass spectrometry, and alanine and glutamate residue replacement mutant constructs for these sites were constructed. Alanine substitution mutants of Cnd2, which mimic the unphosphorylated protein, exhibited broad mitotic defects, including at telophase, and overexpression of these constructs caused a severe dominant-negative effect. By contrast, glutamate substitution mutants, which mimic the phosphorylated protein, alleviated the segregation defect in Ark1-inhibited cells. In telophase, the condensin subunits in cut14-208 mutant accumulated in lumps that contained telomeric DNA and proteins that failed to segregate. Condensin might thus serve to keep the segregated chromosomes apart during telophase.

Histone H3 phosphorylation and non-disjunction of the maternal X chromosome during male meiosis in sciarid flies

An extremely unorthodox method of chromosome segregation is found in sciarid flies (Diptera, Sciaridae), where at male meiosis, the whole paternal complement is eliminated and the maternal X chromosome undergoes non-disjunction. At meiosis I, a monopolar spindle directs the segregation of maternal chromosomes to the single pole, whereas paternal chromosomes are discarded. At meiosis II, although maternal autosomes segregate normally, the X chromosome remains undivided. A cis-acting locus within the heterochromatin proximal to the centromere is known to regulate X centromere activity. By immunofluorescence analysis in spermatocytes from Sciara ocellaris and Sciara coprophila, we investigated histone H3 phosphorylation at Ser10, Ser28, Thr3 and Thr11 during male meiosis. We found that chromosome condensation and H3 phosphorylation patterns differ between chromosomes of different parental origin at the time of paternal set elimination. Importantly, at meiosis II, the maternal X chromosome differs from the rest of the chromosomes in that its centromeric region does not become phosphorylated at the four histone H3 sites. We provide here the first evidence linking the under-phosphorylated H3 status of the X chromosome centromeric region with its meiotic non-disjunction in sciarid flies. Our findings strongly support the idea that the deficiency in local H3 phosphorylation inactivates the X centromere at the transition from meiosis I to meiosis II.

Response of experimental malignant melanoma models to the pan-Aurora kinase inhibitor VE-465

Aurora kinases represent promising novel cancer therapy targets. Genomic analyses of human cutaneous melanoma (CMM) models (N = 51, low passage) by classical and/or array CGH revealed frequent gains at chromosome 20q (65%, amplifications in 45%) repeatedly including the Aurora A gene locus. Accordingly, the majority of CMM cell cultures overexpressed Aurora A when compared to proliferating non-malignant cells. Moreover, CMM cells even when arrested in G1/S cell cycle phase contained readily detectable levels of Aurora A indicating incomplete degradation during mitosis. Already at low concentrations (10-100 nm), long-term (7-10 days) application of the pan-Aurora kinase inhibitor VE-465 completely prevented colony formation in all CMM models tested. In contrast, blockade of cell survival/proliferation and DNA synthesis as well as the induction of apoptosis by VE-465 distinctly differed in short-term experiments (up to 72 h exposure). Both cell cycle arrest and DNA synthesis blockade depended on the level of VE-465-mediated p53/p21 activation while p53/p21 unresponsiveness led to repetitive endoreduplication (>8n DNA content). In contrast, apoptosis induction by VE-465 and Aurora A siRNA did not correlate with p53/p21 responsiveness and DNA synthesis blockade. Moreover, application of the Aurora B-specific inhibitor ZM447439 and siRNA was less efficient to induce CMM cell death proofing that apoptosis induction by VE-465 depended predominantly on Aurora A targeting. In combination experiments with chemotherapeutic agents, VE-465 acted additive to antagonistic when applied concomitantly but in several cases even synergistic when applied consecutively. In summary, we suggest that the Aurora A kinase might represent a promising target of well-designed novel antimelanoma strategies.

Cdc48/p97-Ufd1-Npl4 antagonizes Aurora B during chromosome segregation in HeLa cells

During exit from mitosis in Xenopus laevis egg extracts, the AAA+ ATPase Cdc48/p97 (also known as VCP in vertebrates) and its adapter Ufd1-Npl4 remove the kinase Aurora B from chromatin to allow nucleus formation. Here, we show that in HeLa cells Ufd1-Npl4 already antagonizes Aurora B on chromosomes during earlier mitotic stages and that this is crucial for proper chromosome segregation. Depletion of Ufd1-Npl4 by small interfering RNA (siRNA) caused chromosome alignment and anaphase defects resulting in missegregated chromosomes and multi-lobed nuclei. Ufd1-Npl4 depletion also led to increased levels of Aurora B on prometaphase and metaphase chromosomes. This increase was associated with higher Aurora B activity, as evidenced by the partial resistance of CENP-A phosphorylation to the Aurora B inhibitor hesperadin. Furthermore, low concentrations of hesperadin partially rescued chromosome alignment in Ufd1-depleted cells, whereas, conversely, Ufd1-depletion partially restored congression in the presence of hesperadin. These data establish Cdc48/p97-Ufd1-Npl4 as a crucial negative regulator of Aurora B early in mitosis of human somatic cells and suggest that the activity of Aurora B on chromosomes needs to be restrained to ensure faithful chromosome segregation.

How to be a mitotic chromosome

Proper mitotic chromosome structure is essential for faithful chromosome segregation. Mounting evidence suggests that mitotic chromosome assembly is a progressive, dynamic process that requires topoisomerase II, condensins and cohesin and the activity of several signalling molecules. Current results suggest how these different activities might interact to achieve the familiar form of the mitotic chromosome.

Efficacy and pharmacokinetic/pharmacodynamic evaluation of the Aurora kinase A inhibitor MLN8237 against preclinical models of pediatric cancer

PURPOSE

To gain a greater understanding of the potential of the Aurora kinase A inhibitor MLN8237 in the treatment of pediatric malignancies.

METHODS

The activity of MLN8237 was evaluated against 28 neuroblastoma and Ewing sarcoma cell lines, and its in vivo efficacy was studied over a range of doses against 12 pediatric tumor xenograft models. Pharmacokinetic, pharmacodynamic, and genomic studies were undertaken.

RESULTS

In vitro neuroblastoma cell lines were generally more sensitive to MLN8237 than Ewing sarcoma lines. MLN8237 demonstrated significant activity in vivo against solid tumor models at the maximum tolerated dose (MTD); however, only 2 of 6 neuroblastoma models had objective responses at 0.25MTD. In contrast, MLN8237 induced objective responses at its MTD and at 0.5MTD in three ALL models and in two out of three at 0.25MTD. Pharmacokinetic studies at 0.5MTD demonstrated a T (max) of 0.5 h, C (max) of 24.8 μM, AUC((0-24)) of 60.3 μM h, and 12 h trough level of 1.2 μM. Mitotic indices increased 6-12 h after MLN8237 administration. AURKA copy number variation was frequent in xenografts, and expression was highly correlated with copy number.

CONCLUSIONS

Objective responses were more frequent in tumors with decreased AURKA copy number (5/8) compared to those with increased gene copy number (2/14). This report confirms the significant activity against both solid tumor and ALL xenografts at the MTD, with a steep dose response. These data support clinical development of MLN8237 in childhood cancer. Because of the steep dose-response relationship, such studies should target achieving trough levels of 1 μM or higher for sustained periods of treatment.

E2 ligase dRad6 regulates DMP53 turnover in Drosophila

The turnover of tumor suppressor p53 is critical for its role in various cellular events. However, the pathway that regulates the turnover of the Drosophila melanogaster DMP53 is largely unknown. Here, we provide evidence for the first time that the E2 ligase, Drosophila homolog of Rad6 (dRad6/Dhr6), plays an important role in the regulation of DMP53 turnover. Depletion of dRad6 results in DMP53 accumulation, whereas overexpression of dRad6 causes enhanced DMP53 degradation. We show that dRad6 specifically interacts with DMP53 at the transcriptional activation domain and regulates DMP53 ubiquitination. Loss of dRad6 function in transgenic flies leads to lethalities and altered morphogenesis. The dRad6-induced defects in cell proliferation and apoptosis are found to be DMP53-dependent. The loss of dRad6 induces an accumulation of DMP53 that enhances the activation of apoptotic genes and leads to apoptosis in the presence of stress stimuli. In contrast to that, the E3 ligase is the primary factor that regulates p53 turnover in mammals, and this work demonstrates that the E2 ligase dRad6 is critical for the control of DMP53 degradation in Drosophila.

Global analysis of the relationship between JIL-1 kinase and transcription

The ubiquitous tandem kinase JIL-1 is essential for Drosophila development. Its role in defining decondensed domains of larval polytene chromosomes is well established, but its involvement in transcription regulation has remained controversial. For a first comprehensive molecular characterisation of JIL-1, we generated a high-resolution, chromosome-wide interaction profile of the kinase in Drosophila cells and determined its role in transcription. JIL-1 binds active genes along their entire length. The presence of the kinase is not proportional to average transcription levels or polymerase density. Comparison of JIL-1 association with elongating RNA polymerase and a variety of histone modifications suggests two distinct targeting principles. A basal level of JIL-1 binding can be defined that correlates best with the methylation of histone H3 at lysine 36, a mark that is placed co-transcriptionally. The additional acetylation of H4K16 defines a second state characterised by approximately twofold elevated JIL-1 levels, which is particularly prominent on the dosage-compensated male X chromosome. Phosphorylation of the histone H3 N-terminus by JIL-1 in vitro is compatible with other tail modifications. In vivo, phosphorylation of H3 at serine 10, together with acetylation at lysine 14, creates a composite histone mark that is enriched at JIL-1 binding regions. Its depletion by RNA interference leads to a modest, but significant, decrease of transcription from the male X chromosome. Collectively, the results suggest that JIL-1 participates in a complex histone modification network that characterises active, decondensed chromatin. We hypothesise that one specific role of JIL-1 may be to reinforce, rather than to establish, the status of active chromatin through the phosphorylation of histone H3 at serine 10.

Knockdown of cyclin-dependent kinase inhibitors induces cardiomyocyte re-entry in the cell cycle

Proliferation of mammalian cardiomyocytes stops rapidly after birth and injured hearts do not regenerate adequately. High cyclin-dependent kinase inhibitor (CKI) levels have been observed in cardiomyocytes, but their role in maintaining cardiomyocytes in a post-mitotic state is still unknown. In this report, it was investigated whether CKI knockdown by RNA interference induced cardiomyocyte proliferation. We found that triple transfection with p21(Waf1), p27(Kip1), and p57(Kip2) siRNAs induced both neonatal and adult cardiomyocyte to enter S phase and increased the nuclei/cardiomyocyte ratio; furthermore, a subpopulation of cardiomyocytes progressed beyond karyokynesis, as assessed by the detection of mid-body structures and by straight cardiomyocyte counting. Intriguingly, cardiomyocyte proliferation occurred in the absence of overt DNA damage and aberrant mitotic figures. Finally, CKI knockdown and DNA synthesis reactivation correlated with a dramatic change in adult cardiomyocyte morphology that may be a prerequisite for cell division. In conclusion, CKI expression plays an active role in maintaining cardiomyocyte withdrawal from the cell cycle.

Epigenetic mechanisms and genome stability

Epigenetic marks are well recognized as heritable chemical modifications of DNA and chromatin that induce chromatin structural changes thereby affecting gene activity. A lesser-known phenomenon is the pervasive effects these marks have on genomic integrity. Remarkably, epigenetic marks and the enzymes that establish them are involved in multiple aspects of maintaining genetic content. These aspects include preserving nucleotide sequences such as repetitive elements, preventing DNA damage, functioning in DNA repair mechanisms and chromatin restoration, and defining chromosomal organization through effects on structural elements such as the centromere. This review discusses these functional aspects of epigenetic marks and their effects on human health and disease.

Aurora B interaction of centrosomal Nlp regulates cytokinesis

Cytokinesis is a fundamental cellular process, which ensures equal abscission and fosters diploid progenies. Aberrant cytokinesis may result in genomic instability and cell transformation. However, the underlying regulatory machinery of cytokinesis is largely undefined. Here, we demonstrate that Nlp (Ninein-like protein), a recently identified BRCA1-associated centrosomal protein that is required for centrosomes maturation at interphase and spindle formation in mitosis, also contributes to the accomplishment of cytokinesis. Through immunofluorescent analysis, Nlp is found to localize at midbody during cytokinesis. Depletion of endogenous Nlp triggers aborted division and subsequently leads to multinucleated phenotypes. Nlp can be recruited by Aurora B to the midbody apparatus via their physical association at the late stage of mitosis. Disruption of their interaction induces aborted cytokinesis. Importantly, Nlp is characterized as a novel substrate of Aurora B and can be phosphorylated by Aurora B. The specific phosphorylation sites are mapped at Ser-185, Ser-448, and Ser-585. The phosphorylation at Ser-448 and Ser-585 is likely required for Nlp association with Aurora B and localization at midbody. Meanwhile, the phosphorylation at Ser-185 is vital to Nlp protein stability. Disruptions of these phosphorylation sites abolish cytokinesis and lead to chromosomal instability. Collectively, these observations demonstrate that regulation of Nlp by Aurora B is critical for the completion of cytokinesis, providing novel insights into understanding the machinery of cell cycle progression.

Functional central spindle assembly requires de novo microtubule generation in the interchromosomal region during anaphase

The augmin protein complex nucleates noncentrosomal microtubules during anaphase to promote completion of cell division.

The central spindle forms between segregating chromosomes during anaphase and is required for cytokinesis. Although anaphase-specific bundling and stabilization of interpolar microtubules (MTs) contribute to formation of the central spindle, it remains largely unknown how these MTs are prepared. Using live imaging of MT plus ends and an MT depolymerization and regrowth assay, we show that de novo MT generation in the interchromosomal region during anaphase is important for central spindle formation in human cells. Generation of interchromosomal MTs and subsequent formation of the central spindle occur independently of preanaphase MTs or centrosomal MT nucleation but require augmin, a protein complex implicated in nucleation of noncentrosomal MTs during preanaphase. MTs generated in a hepatoma up-regulated protein (HURP)–dependent manner during anaphase also contribute to central spindle formation redundantly with preanaphase MTs. Based on these results, a new model for central spindle assembly is proposed.

Sds22 regulates aurora B activity and microtubule-kinetochore interactions at mitosis

We have studied Sds22, a conserved regulator of protein phosphatase 1 (PP1) activity, and determined its role in modulating the activity of aurora B kinase and kinetochore-microtubule interactions. Sds22 is required for proper progression through mitosis and localization of PP1 to mitotic kinetochores. Depletion of Sds22 increases aurora B T-loop phosphorylation and the rate of recovery from monastrol arrest. Phospho-aurora B accumulates at kinetochores in Sds22-depleted cells juxtaposed to critical kinetochore substrates. Sds22 modulates sister kinetochore distance and the interaction between Hec1 and the microtubule lattice and, thus, the activation of the spindle assembly checkpoint. These results demonstrate that Sds22 specifically defines PP1 function and localization in mitosis. Sds22 regulates PP1 targeting to the kinetochore, accumulation of phospho-aurora B, and force generation at the kinetochore-microtubule interface.

Systematic protein location mapping reveals five principal chromatin types in Drosophila cells

Chromatin is important for the regulation of transcription and other functions, yet the diversity of chromatin composition and the distribution along chromosomes are still poorly characterized. By integrative analysis of genome-wide binding maps of 53 broadly selected chromatin components in Drosophila cells, we show that the genome is segmented into five principal chromatin types that are defined by unique yet overlapping combinations of proteins and form domains that can extend over > 100 kb. We identify a repressive chromatin type that covers about half of the genome and lacks classic heterochromatin markers. Furthermore, transcriptionally active euchromatin consists of two types that differ in molecular organization and H3K36 methylation and regulate distinct classes of genes. Finally, we provide evidence that the different chromatin types help to target DNA-binding factors to specific genomic regions. These results provide a global view of chromatin diversity and domain organization in a metazoan cell.

Loss of maternal ATRX results in centromere instability and aneuploidy in the mammalian oocyte and pre-implantation embryo

The α-thalassemia/mental retardation X-linked protein (ATRX) is a chromatin-remodeling factor known to regulate DNA methylation at repetitive sequences of the human genome. We have previously demonstrated that ATRX binds to pericentric heterochromatin domains in mouse oocytes at the metaphase II stage where it is involved in mediating chromosome alignment at the meiotic spindle. However, the role of ATRX in the functional differentiation of chromatin structure during meiosis is not known. To test ATRX function in the germ line, we developed an oocyte-specific transgenic RNAi knockdown mouse model. Our results demonstrate that ATRX is required for heterochromatin formation and maintenance of chromosome stability during meiosis. During prophase I arrest, ATRX is necessary to recruit the transcriptional regulator DAXX (death domain associated protein) to pericentric heterochromatin. At the metaphase II stage, transgenic ATRX-RNAi oocytes exhibit abnormal chromosome morphology associated with reduced phosphorylation of histone 3 at serine 10 as well as chromosome segregation defects leading to aneuploidy and severely reduced fertility. Notably, a large proportion of ATRX-depleted oocytes and 1-cell stage embryos exhibit chromosome fragments and centromeric DNA–containing micronuclei. Our results provide novel evidence indicating that ATRX is required for centromere stability and the epigenetic control of heterochromatin function during meiosis and the transition to the first mitosis.

The transmission of an abnormal chromosome complement from the gametes to the early embryo, a condition called aneuploidy, is a major cause of congenital birth defects and pregnancy loss. Human embryos are particularly susceptible to aneuploidy, which in the majority of cases is the result of abnormal meiosis in the female gamete. However, the molecular mechanisms involved in the onset of aneuploidy in mammalian oocytes are not fully understood. We show here that, the α-thalassemia/mental retardation X-linked protein (ATRX) is essential for the maintenance of chromosome stability during female meiosis. ATRX is required to recruit the transcriptional regulator DAXX to pericentric heterochromatin at prophase I of meiosis. Notably, lack of ATRX function at the metaphase II stage interferes with the establishment of chromatin modifications associated with chromosome condensation leading to segregation defects, chromosome fragmentation, and severely reduced fertility. Our results provide direct evidence for a role of ATRX in the regulation of pericentric heterochromatin structure and function in mammalian oocytes and have important implications for our understanding of the epigenetic factors contributing to the onset of aneuploidy in the female gamete.

Inhibition of Plk1 induces mitotic infidelity and embryonic growth defects in developing zebrafish embryos

Polo-like kinase 1 (Plk1) is central to cell division. Here, we report that Plk1 is critical for mitosis in the embryonic development of zebrafish. Using a combination of several cell biology tools, including single-cell live imaging applied to whole embryos, we show that Plk1 is essential for progression into mitosis during embryonic development. Plk1 morphant cells displayed mitotic infidelity, such as abnormal centrosomes, irregular spindle assembly, hypercondensed chromosomes, and a failure of chromosome arm separation. Consequently, depletion of Plk1 resulted in mitotic arrest and finally death by 6days post-fertilization. In comparison, Plk2 or Plk3 morphant embryos did not display any significant abnormalities. Treatment of embryos with the Plk1 inhibitor, BI 2536, caused a block in mitosis, which was more severe when used to treat plk1 morphants. Finally, using an assay to rescue the Plk1 morphant phenotype, we found that the kinase domain and PBD domains are both necessary for Plk1 function in zebrafish development. Our studies demonstrate that Plk1 is required for embryonic proliferation because its activity is crucial for mitotic integrity. Furthermore, our study suggests that zebrafish will be an efficient and economical in vivo system for the validation of anti-mitotic drugs.

Imidazo[4,5-b]pyridine derivatives as inhibitors of Aurora kinases: lead optimization studies toward the identification of an orally bioavailable preclinical development candidate

Lead optimization studies using 7 as the starting point led to a new class of imidazo[4,5-b]pyridine-based inhibitors of Aurora kinases that possessed the 1-benzylpiperazinyl motif at the 7-position, and displayed favorable in vitro properties. Cocrystallization of Aurora-A with 40c (CCT137444) provided a clear understanding into the interactions of this novel class of inhibitors with the Aurora kinases. Subsequent physicochemical property refinement by the incorporation of solubilizing groups led to the identification of 3-((4-(6-bromo-2-(4-(4-methylpiperazin-1-yl)phenyl)-3H-imidazo[4,5-b]pyridin-7-yl)piperazin-1-yl)methyl)-5-methylisoxazole (51, CCT137690) which is a potent inhibitor of Aurora kinases (Aurora-A IC(50) = 0.015 +/- 0.003 muM, Aurora-B IC(50) = 0.025 muM, Aurora-C IC(50) = 0.019 muM). Compound 51 is highly orally bioavailable, and in in vivo efficacy studies it inhibited the growth of SW620 colon carcinoma xenografts following oral administration with no observed toxicities as defined by body weight loss.

Cdk1 activity is required for mitotic activation of aurora A during G2/M transition of human cells

In mammalian cells entry into and progression through mitosis are regulated by multiple mitotic kinases. How mitotic kinases interact with each other and coordinately regulate mitosis remains to be fully understood. Here we employed a chemical biology approach using selective small molecule kinase inhibitors to dissect the relationship between Cdk1 and Aurora A kinases during G(2)/M transition. We find that activation of Aurora A first occurs at centrosomes at late G(2) and is required for centrosome separation independently of Cdk1 activity. Upon entry into mitosis, Aurora A then becomes fully activated downstream of Cdk1 activation. Inactivation of Aurora A or Plk1 individually during a synchronized cell cycle shows no significant effect on Cdk1 activation and entry into mitosis. However, simultaneous inactivation of both Aurora A and Plk1 markedly delays Cdk1 activation and entry into mitosis, suggesting that Aurora A and Plk1 have redundant functions in the feedback activation of Cdk1. Together, our data suggest that Cdk1, Aurora A, and Plk1 mitotic kinases participate in a feedback activation loop and that activation of Cdk1 initiates the feedback loop activity, leading to rapid and timely entry into mitosis in human cells. In addition, live cell imaging reveals that the nuclear cycle of cells becomes uncoupled from cytokinesis upon inactivation of both Aurora A and Aurora B kinases and continues to oscillate in a Cdk1-dependent manner in the absence of cytokinesis, resulting in multinucleated, polyploidy cells.

Discovery of N-phenyl-4-(thiazol-5-yl)pyrimidin-2-amine aurora kinase inhibitors

Through cell-based screening of our kinase-directed compound collection, we discovered that a subset of N-phenyl-4-(thiazol-5-yl)pyrimidin-2-amines were potent cytotoxic agents against cancer cell lines, suppressed mitotic histone H3 phosphorylation, and caused aberrant mitotic phenotypes. It was subsequently established that these compounds were in fact potent inhibitors of aurora A and B kinases. It was shown that potency and selectivity of aurora kinase inhibition correlated with the presence of a substituent at the aniline para-position in these compounds. The anticancer effects of lead compound 4-methyl-5-(2-(4-morpholinophenylamino)pyrimidin-4-yl)thiazol-2-amine (18; K(i) values of 8.0 and 9.2 nM for aurora A and B, respectively) were shown to emanate from cell death following mitotic failure and increased polyploidy as a consequence of cellular inhibition of aurora A and B kinases. Preliminary in vivo assessment showed that compound 18 was orally bioavailable and possessed anticancer activity. Compound 18 (CYC116) is currently undergoing phase I clinical evaluation in cancer patients.

Aurora A contributes to p150(glued) phosphorylation and function during mitosis

Phosphorylation of a dynactin subunit is important for its release from mitotic spindles.

Aurora A is a spindle pole–associated protein kinase required for mitotic spindle assembly and chromosome segregation. In this study, we show that Drosophila melanogaster aurora A phosphorylates the dynactin subunit p150^glued on sites required for its association with the mitotic spindle. Dynactin strongly accumulates on microtubules during prophase but disappears as soon as the nuclear envelope breaks down, suggesting that its spindle localization is tightly regulated. If aurora A's function is compromised, dynactin and dynein become enriched on mitotic spindle microtubules. Phosphorylation sites are localized within the conserved microtubule-binding domain (MBD) of the p150^glued. Although wild-type p150^glued binds weakly to spindle microtubules, a variant that can no longer be phosphorylated by aurora A remains associated with spindle microtubules and fails to rescue depletion of endogenous p150^glued. Our results suggest that aurora A kinase participates in vivo to the phosphoregulation of the p150^glued MBD to limit the microtubule binding of the dynein–dynactin complex and thus regulates spindle assembly.

Aurora kinase inhibitors as anti-cancer therapy

Aurora kinases are serine and threonine kinases that function as key regulators of the mitosis process. There are three distinct human aurora kinases known as Aurora A, Aurora B, and Aurora C. Aurora A and Aurora B are overexpressed in a number of human cancers including non-small cell lung cancer, glioblastomas, and upper gastrointestinal adenocarcinomas. Given their association with tumorigenesis, both Aurora A and Aurora B have been targeted for cancer therapy. Currently, a number of selective and nonselective aurora kinase inhibitors are being tested in preclinical and clinical settings as anti-tumor agents. We review the biology of human aurora kinases, followed by an overview of inhibitors undergoing current clinical investigations.

Condensin and cohesin complexity: the expanding repertoire of functions

Condensin and cohesin complexes act in diverse nuclear processes in addition to their widely known roles in chromosome compaction and sister chromatid cohesion. Recent work has elucidated the contribution of condensin and cohesin to interphase genome organization, control of gene expression, metazoan development and meiosis. Despite these wide-ranging functions, several themes have come to light: both complexes establish higher-order chromosome structure by inhibiting or promoting interactions between distant genomic regions, both complexes influence the chromosomal association of other proteins, and both complexes achieve functional specialization by swapping homologous subunits. Emerging data are expanding the range of processes in which condensin and cohesin are known to participate and are enhancing our knowledge of how chromosome architecture is regulated to influence numerous cellular functions.

Substrate degradation by the anaphase promoting complex occurs during mitotic slippage

Microtubule targeting drugs are successful in chemotherapy because they indefinitely activate the spindle assembly checkpoint. The spindle assembly checkpoint monitors proper attachment of all kinetochores to microtubules and tension between the kinetochores of sister chromatids to prevent premature anaphase entry. To this end, the activated spindle assembly checkpoint suppresses the E3 ubiquitin ligase activity of the anaphase-promoting complex (APC). In the continued presence of conditions that activate the spindle assembly checkpoint, cells eventually escape from mitosis by "slippage". It has not been directly tested whether APC activation accompanies slippage. Using cells blocked in mitosis with the microtubule assembly inhibitor nocodazole, we show that mitotic APC substrates are degraded upon mitotic slippage. To confirm that APC is normally activated upon mitotic slippage we have found that knockdown of Cdc20 and Cdh1, two mitotic activators of APC, prevents the degradation of APC substrates during mitotic slippage. We provide the first direct demonstration that despite conditions that activate the spindle checkpoint, APC is indeed activated upon mitotic slippage of cells to interphase cells. Activation of the spindle checkpoint by microtubule targeting drugs used in chemotherapy may not indefinitely prevent APC activation.

Aurora B and 14-3-3 coordinately regulate clustering of centralspindlin during cytokinesis

Centralspindlin is essential for the formation of microtubule bundle structures and the equatorial recruitment of factors critical for cytokinesis. Stable accumulation of centralspindlin at the spindle midzone requires its multimerization into clusters and Aurora B kinase activity, which peaks at the central spindle during anaphase. Although Aurora B phosphorylates centralspindlin directly, how this regulates centralspindlin localization is unknown. Here we identify a novel regulatory mechanism by which Aurora B enables centralspindlin to accumulate stably at the spindle midzone. We show that 14-3-3 protein binds centralspindlin when the kinesin-6 component MKLP1 is phosphorylated at S710. 14-3-3 prevents centralspindlin from clustering in vitro, and an MKLP1 mutant that is unable to bind 14-3-3 forms aberrant clusters in vivo. Interestingly, 14-3-3 binding is inhibited by phosphorylation of S708, a known Aurora B target site that lies within the motif bound by 14-3-3. S708 phosphorylation is required for MKLP1 to stably localize to the central spindle, but it is dispensable in an MKLP1 mutant that does not bind 14-3-3. We propose that 14-3-3 serves as a global inhibitor of centralspindlin that allows Aurora B to locally activate clustering and the stable accumulation of centralspindlin between segregating chromosomes.

Aurora A selective inhibitor MLN8237 suppresses the growth and survival of HTLV-1-infected T-cells in vitro

Aurora A kinase plays an essential role in the proper assembly and function of the mitotic spindle. We have shown previously that Aurora A expression is increased aberrantly in human T-cell leukemia virus type 1 (HTLV-1)-infected T-cell lines and primary adult T-cell leukemia cells, and a pan-Aurora kinase inhibitor, which inhibits both Aurora A and Aurora B kinases, reduces viability and induces apoptosis in these cells. However, the specific effects of Aurora A inhibition on HTLV-1-infected T-cells are poorly understood. In this study, we addressed this question by comparing the effects of MLN8237, a selective inhibitor of Aurora A, on cell viability, cell cycle progression, and induction of apoptosis in HTLV-1-infected and -uninfected T-cell lines. MLN8237 reduced the viability of HTLV-1-infected T-cell lines within 24 h, but its effects on that of HTLV-1-uninfected T-cell lines were moderate. MLN8237 induced early apoptosis of HTLV-1-infected T-cell lines without induction of polyploidy. It induced p53 and p21 expression in HTLV-1-infected but not in -uninfected T-cell lines, suggesting that MLN8237-treated HTLV-1-infected T-cell lines exit from mitosis and activate a p53-dependent postmitotic G(1) checkpoint, leading to G(1) arrest followed by the induction of apoptosis. Our results suggest that specific inhibition of Aurora A kinase is a potentially useful therapeutic strategy in the treatment of adult T-cell leukemia and that further in vivo exploration is warranted.

Regulation of aurora B kinase by the lipid raft protein flotillin-1

The lipid raft protein Flotillin-1 was previously shown to be required for cell proliferation. Here we show that it is critical for the maintenance of the levels of the mitotic regulator Aurora B. Knockdown of Flotillin-1 induced aberrant mitotic events similar to those produced by Aurora B depletion and led to a marked decline in Aurora B levels and activity. Transfection of wild-type full-length Flotillin-1 or forms directed to the nucleus increased Aurora B levels and activity. Flotillin-1 interacted with Aurora B directly through its SPFH domain in a complex distinct from the chromosomal passenger protein complex, and the two proteins co-purified in nuclear, non-raft fractions. These observations are the first evidence for a function of Flotillin-1 outside of lipid rafts and suggest its critical role in the maintenance of a pool of active Aurora B.

Assessment of the in vivo antitumor effects of ENMD-2076, a novel multitargeted kinase inhibitor, against primary and cell line-derived human colorectal cancer xenograft models

PURPOSE

This in vivo study was designed to investigate the efficacy of ENMD-2076, a small-molecule kinase inhibitor with activity against the Aurora kinases A and B, and several other tyrosine kinases linked to cancer, including vascular endothelial growth factor receptor 2, cKit, and fibroblast growth factor receptor 1, against murine xenograft models of human colorectal cancer (CRC).

EXPERIMENTAL DESIGN

HT-29 CRC cell line xenografts were treated with either vehicle or ENMD-2076 (100 or 200 mg/kg) orally daily for 28 days. Tumor growth inhibition, dynamic contrast-enhanced magnetic resonance imaging, and (18)FDG-positron emission tomography were conducted to assess the antiproliferative, antiangiogenic, and antimetabolic responses, respectively. Effects on proliferation were also analyzed by immunohistochemical methods. Additionally, three patient-derived xenografts from primary and metastatic sites were treated with ENMD-2076 (100 mg/kg) and assessed for tumor growth inhibition.

RESULTS

In the HT-29 xenograft model, ENMD-2076 induced initial tumor growth inhibition followed by regression. Treatment was associated with significant tumor blanching, indicating a loss of vascularity and substantial reductions in tumor vascular permeability and perfusion as measured by dynamic contrast-enhanced magnetic resonance imaging. Positron emission tomography scanning showed significant decreases in (18)FDG uptake at days 3 and 21 of treatment, which was associated with a marked reduction in proliferation as assessed by Ki-67. All three of the patient-derived xenografts tested were sensitive to treatment with ENMD 2076 as measured by tumor growth inhibition.

CONCLUSIONS

ENMD-2076 showed robust antitumor activity against cell line and patient-derived xenograft models of CRC that is detectable by functional imaging, supporting clinical investigation of this agent in CRC.

A role for Cdc48/p97 and Aurora B in controlling chromatin condensation during exit from mitosis

During cell division, chromosomes condense so that the replicated chromatids can be segregated by the mitotic spindle. While condensation is governed by cyclin-dependent kinase 1 (Cdk1) during mitotic entry and early mitosis, it is still poorly understood how condensation is maintained during anaphase after Cdk1 inactivation, and how decondensation is triggered in telophase. Here, we review recent reports that point to a novel role of Aurora B kinase in maintaining condensation and preventing premature nuclear envelope formation during exit from mitosis. Timely decondensation and nuclear envelope formation at the end of mitosis may then be triggered by two mechanisms. One is removing Aurora B phosphorylation marks from chromatin by specific phosphatases. The other is removing and inactivating Aurora B kinase itself by the ubiquitin system. We have recently provided evidence that the AAA ATPase Cdc48/p97 plays a central role in the inactivation of Aurora B, as it extracts ubiquitinated Aurora B from chromosomes and thus reduces chromatinassociated Aurora B activity.

The Nup107-160 nucleoporin complex promotes mitotic events via control of the localization state of the chromosome passenger complex

The human Nup107-160 nucleoporin complex plays a major role in formation of the nuclear pore complex and is localized to kinetochores in mitosis. Here we report that Seh1, a component of the Nup107-160 complex, functions in chromosome alignment and segregation by regulating the centromeric localization of Aurora B and other chromosome passenger complex proteins. Localization of CENP-E is not affected by Seh1 depletion and analysis by electron microscopy showed that microtubule kinetochore attachments are intact. Seh1-depleted cells show impaired Aurora B localization, which results in severe defects in biorientation and organization of the spindle midzone and midbody. Our results indicate that a major function of the Nup107 complex in mitosis is to ensure the proper localization of the CPC at the centromere.

Widespread genomic instability mediated by a pathway involving glycoprotein Ib alpha and Aurora B kinase

c-Myc (Myc) oncoprotein induction of genomic instability (GI) contributes to its initial transforming function and subsequent tumor cell evolution. We describe here a pathway by which Myc, via its target protein glycoprotein Ibalpha (GpIb alpha), mediates GI. Proteomic profiling revealed that the serine/threonine kinase Aurora B is down-regulated by GpIb alpha in p53-deficient primary human fibroblasts. The phenotypes of Aurora B deficiency are strikingly reminiscent of Myc or GpIb alpha overexpression and include double-stranded DNA breaks, altered nuclear size and morphology, chromatin bridges, cleavage furrow regression, and tetraploidy. During mitosis, GpIb alpha and Aurora B redistribute to the cleavage furrow along with other cleavage furrow proteins. GpIb alpha overexpression at levels comparable with those seen in some tumor cells causes the dispersal of these proteins but not Aurora B, resulting in furrow regression and cytokinesis failure. Aurora B normalization redirects the mislocalized furrow proteins to their proper location, corrects the cleavage furrow abnormalities, and restores genomic stability. Aurora B thus appears necessary for a previously unrecognized function in guiding and positioning a number of key proteins, including GpIb alpha to the cleavage furrow. These findings underscore the importance of maintaining a delicate balance among cleavage furrow-associated proteins during mitosis. Suppression of Aurora B via GpIb alpha provides a unifying and mechanistic explanation for several types of Myc-mediated GI.

Histone H3 lysine 4 methylation is associated with the transcriptional reprogramming efficiency of somatic nuclei by oocytes

Background

When the nuclei of mammalian somatic cells are transplanted to amphibian oocytes in the first meiotic prophase, they are rapidly induced to begin transcribing several pluripotency genes, including Sox2 and Oct4. The more differentiated the donor cells of the nuclei, the longer it takes for the pluripotency genes to be activated after the nuclear transfer to oocytes. We have used this effect in order to investigate the role of histone modifications in this example of nuclear reprogramming.

Results

Reverse transcription polymerase chain reaction analysis shows that the transcriptional reprogramming of pluripotency genes, such as Sox2 and Oct4, takes place in transplanted nuclei from C3H10T1/2 cells and from newly differentiated mouse embryonic stem cells. We find that the reprogramming of 10T1/2 nuclei is accompanied by an increased phosphorylation, an increased methylation and a rapidly reduced acetylation of several amino acids in H3 and other histones. These results are obtained by the immunofluorescent staining of transplanted nuclei and by Western blot analysis. We have also used chromatin immunoprecipitation analysis to define histone modifications associated with the regulatory or coding regions of pluripotency genes in transplanted nuclei. Histone phosphorylation is increased and histone acetylation is decreased in several regulatory and gene coding regions. An increase of histone H3 lysine 4 dimethylation (H3K4 me2) is seen in the regulatory regions and gene coding region of pluripotency genes in reprogrammed nuclei. Furthermore, histone H3 lysine 4 trimethylation (H3K4 me3) is observed more strongly in the regulatory regions of pluripotency genes in transplanted nuclei that are rapidly reprogrammed than in nuclei that are reprogrammed slowly and are not seen in β-globin, a gene that is not reprogrammed. When 10T1/2 nuclei are incubated in Xenopus oocyte extracts, histone H3 serine 10 (H3S10) is strongly phosphorylated within a few hours. Immunodepletion of Aurora B prevents this phosphorylation.

Conclusion

We conclude that H3K4 me2 and me3 are likely to be important for the efficient reprogramming of pluripotency genes in somatic nuclei by amphibian oocytes and that Aurora B kinase is required for H3S10 phosphorylation which is induced in transplanted somatic cell nuclei.

Mutual dependence of Mob1 and the chromosomal passenger complex for localization during mitosis

The spatial and temporal coordination of chromosome segregation with cytokinesis is essential to ensure that each daughter cell receives the correct complement of chromosomal and cytoplasmic material. In yeast, mitotic exit and cytokinesis are coordinated by signaling cascades whose terminal components include a nuclear Dbf2-related family kinase and a noncatalytic subunit, Mps one binding (Mob) 1. There are five human Mob1 isoforms, all of which display redundant localization patterns at the spindle poles and kinetochores in early mitosis, and the spindle midzone during cytokinesis. Mob1 shares similar localization patterns to Polo-like kinase (Plk1) and the chromosomal passenger complex (CPC), and although depletion of Plk1 resulted in a loss of Mob1 from the spindle poles, Mob1 recruitment to kinetochores was unaffected. Conversely, disruption of CPC signaling resulted in a loss of Mob1 from kinetochores without disrupting recruitment to the spindle poles. In Mob1-depleted cells, the relocalization of the CPC and mitotic kinesin-like protein (MKLP) 2 to the spindle midzone was delayed during early anaphase, and as a consequence, the midzone recruitment of MKLP1 also was affected. Together, these results suggest that Mob1 and the other mammalian orthologues of the mitotic exit network regulate mitotic progression by facilitating the timely mobilization of the CPC to the spindle midzone.

Structural studies of B-type Aurora kinase inhibitors using computational methods

AIM

To characterize the structural features of quinazoline-based Aurora B inhibitors that influence its inhibitor activity.

METHODS

Two geometrical methods, Method 1 and Method 2, were used to develop the 3D-QSAR models. The most active ligand was used as the template for the alignment of all the ligands in Method 1, and a conformer of the cocrystal ligand was used as the template for the alignment of all the ligands in Method 2.

RESULTS

The models suggest that highly active ligands can be designed by varying the R1 substituent at position 7 of the quinazoline ring with positively charged, bulky, hydrophobic groups, while bulky and hydrophobic groups around the thiazole ring are desirable for higher activity.

CONCLUSION

This study emphasizes that the bioactive conformer is rather different from the minima. The steric, electrostatic, and hydrophobic field effects contribute to its inhibitory activity.

H3 phosphorylation: dual role in mitosis and interphase

Chromatin condensation and subsequent decondensation are processes required for proper execution of various cellular events. During mitosis, chromatin compaction is at its highest, whereas relaxation of chromatin is necessary for DNA replication, repair, recombination, and gene transcription. Since histone proteins are directly complexed with DNA in the form of a nucleosome, great emphasis is put on deciphering histone post-translational modifications that control the chromatin condensation state. Histone H3 phosphorylation is a mark present in mitosis, where chromatin condensation is necessary, and in transcriptional activation of genes, when chromatin needs to be decondensed. There are four characterized phospho residues within the H3 N-terminal tail during mitosis: Thr3, Ser10, Thr11, and Ser28. Interestingly, H3 phosphorylated at Ser10, Thr11, and Ser28 has been observed on genomic regions of transcriptionally active genes. Therefore, H3 phosphorylation is involved in processes requiring opposing chromatin states. The level of H3 phosphorylation is mediated by opposing actions of specific kinases and phosphatases during mitosis and gene transcription. The cellular contexts under which specific residues on H3 are phosphorylated in mitosis and interphase are known to some extent. However, the functional consequences of H3 phosphorylation are still unclear.

cAMP-response element-binding protein (CREB) controls MSK1-mediated phosphorylation of histone H3 at the c-fos promoter in vitro

The rapid induction of the c-fos gene correlates with phosphorylations of histone H3 and HMGN1 by mitogen- and stress-activated protein kinases. We have used a cell-free system to dissect the mechanism by which MSK1 phosphorylates histone H3 within the c-fos chromatin. Here, we show that the reconstituted c-fos chromatin presents a strong barrier to histone H3 phosphorylation by MSK1; however, the activators (serum response factor, Elk-1, cAMP-response element-binding protein (CREB), and ATF1) bound on their cognate sites recruit MSK1 to phosphorylate histone H3 at Ser-10 within the chromatin. This activator-dependent phosphorylation of histone H3 is enhanced by HMGN1 and occurs preferentially near the promoter region. Among the four activators, CREB plays a predominant role in MSK1-mediated phosphorylation of histone H3, and the phosphorylation of Ser-133 in CREB is essential for this process. Mutational analyses of MSK1 show that its N-terminal inhibition domain is critical for the kinase to phosphorylate chromatin-embedded histone H3 in a CREB-dependent manner, indicating the presence of an intricate regulatory network for MSK1-mediated phosphorylation of histone H3.

Cell cycle defects in polyhomeotic mutants are caused by abrogation of the DNA damage checkpoint

Polycomb group (PcG) genes are required for heritable silencing of target genes. Many PcG mutants have chromatin bridges and other mitotic defects in early embryos. These phenotypes can arise from defects in S phase or mitosis, so the phenotype does not show when PcG proteins act in cell cycle regulation. We analyzed the cell cycle role of the proximal subunit of Polyhomeotic (PhP) in Drosophila. Time-lapse imaging reveals that chromatin bridges formed during mitosis are able to resolve but sometimes result in chromosome breakage. Chromosome bridging is also observed in canonical cell cycles occurring in larval brains and is therefore not unique to the rapid embryonic cycles. PhP colocalizes with chromatin in S phase but not in mitosis in early embryos, indicating a direct role in DNA synthesis. Time lapse imaging of ph(p) mutants reveals an acceleration of S phase, showing that ph(p) regulates S phase length. Like ph(p) mutations, mutations in DNA damage checkpoints result in S phase acceleration. Consistent with this model, mutations in ph do not affect DNA synthesis rates, but exhibit impaired ability to block cell cycle progression following exposure to gamma-rays. Our data show that the mitotic defects of ph(p) are caused by defects in the DNA damage response that occurs after DNA replication in S phase, and we propose that PhP has a direct role in DNA damage repair.

Danusertib (formerly PHA-739358)--a novel combined pan-Aurora kinases and third generation Bcr-Abl tyrosine kinase inhibitor

The Aurora kinases belong to a family of highly conserved serine/threonine protein kinases. They play an essential role as key mitotic regulators, controlling entry into mitosis, centrosome function, chromosome assembly, and segregation. As many other regulators of mitosis, Aurora kinases are frequently found to be aberrantly overexpressed in cancer cells. Therefore, these proteins have become an attractive target for the development of new anticancer therapies. In fact, several small-molecule inhibitors of Aurora kinases have already been developed and some of them have shown promising clinical efficacy in a number of human tumors in Phase I and II clinical trials. Among those, one of the most advanced clinical compound currently is Danusertib (formerly PHA-739358), which exhibits inhibitory activity against all known Aurora kinases as well as other cancer-relevant kinases such as the Bcr-Abl tyrosine kinase, including its multidrug-resistant T315I mutant. This mutation is responsible for up to 25% of all clinically observed resistances in CML patients undergoing Imatinib therapy. However, this particular mutation is predicted to play an even more important clinical role in the future, since in addition to Imatinib, it also confers resistance to second-generation Bcr-Abl inhibitors such as Nilotinib, Dasatinib, and Bosutinib. Therefore, combined Aurora and Bcr-Abl inhibition (the latter including high-grade resistance conferring mutations) with compounds such as Danusertib represents a promising new strategy for treatment of Bcr-Abl positive leukemias, especially those in second and third line of treatment.

Aurora-A overexpression enhances cell-aggregation of Ha-ras transformants through the MEK/ERK signaling pathway

BACKGROUND

Overexpression of Aurora-A and mutant Ras (RasV12) together has been detected in human bladder cancer tissue. However, it is not clear whether this phenomenon is a general event or not. Although crosstalk between Aurora-A and Ras signaling pathways has been reported, the role of these two genes acting together in tumorigenesis remains unclear.

METHODS

Real-time PCR and sequence analysis were utilized to identify Ha- and Ki-ras mutation (Gly -> Val). Immunohistochemistry staining was used to measure the level of Aurora-A expression in bladder and colon cancer specimens. To reveal the effect of overexpression of the above two genes on cellular responses, mouse NIH3T3 fibroblast derived cell lines over-expressing either RasV12 and wild-type Aurora-A (designated WT) or RasV12 and kinase-inactivated Aurora-A (KD) were established. MTT and focus formation assays were conducted to measure proliferation rate and focus formation capability of the cells. Small interfering RNA, pharmacological inhibitors and dominant negative genes were used to dissect the signaling pathways involved.

RESULTS

Overexpression of wild-type Aurora-A and mutation of RasV12 were detected in human bladder and colon cancer tissues. Wild-type Aurora-A induces focus formation and aggregation of the RasV12 transformants. Aurora-A activates Ral A and the phosphorylation of AKT as well as enhances the phosphorylation of MEK, ERK of WT cells. Finally, the Ras/MEK/ERK signaling pathway is responsible for Aurora-A induced aggregation of the RasV12 transformants.

CONCLUSION

Wild-type-Aurora-A enhances focus formation and aggregation of the RasV12 transformants and the latter occurs through modulating the Ras/MEK/ERK signaling pathway.

Haspin: a newly discovered regulator of mitotic chromosome behavior

The haspins are divergent members of the eukaryotic protein kinase family that are conserved in many eukaryotic lineages including animals, fungi, and plants. Recently-solved crystal structures confirm that the kinase domain of human haspin has unusual structural features that stabilize a catalytically active conformation and create a distinctive substrate binding site. Haspin localizes predominantly to chromosomes and phosphorylates histone H3 at threonine-3 during mitosis, particularly at inner centromeres. This suggests that haspin directly regulates chromosome behavior by modifying histones, although it is likely that additional substrates will be identified in the future. Depletion of haspin by RNA interference in human cell lines causes premature loss of centromeric cohesin from chromosomes in mitosis and failure of metaphase chromosome alignment, leading to activation of the spindle assembly checkpoint and mitotic arrest. Haspin overexpression stabilizes chromosome arm cohesion. Haspin, therefore, appears to be required for protection of cohesion at mitotic centromeres. Saccharomyces cerevisiae homologues of haspin, Alk1 and Alk2, are also implicated in regulation of mitosis. In mammals, haspin is expressed at high levels in the testis, particularly in round spermatids, so it seems likely that haspin has an additional role in post-meiotic spermatogenesis. Haspin is currently the subject of a number of drug discovery efforts, and the future use of haspin inhibitors should provide new insight into the cellular functions of these kinases and help determine the utility of, for example, targeting haspin for cancer therapy.

Traffic control: regulation of kinesin motors

Kinesins are a family of molecular motors that use the energy of ATP hydrolysis to move along the surface of, or destabilize, microtubule filaments. Much progress has been made in understanding the mechanics and functions of the kinesin motors that play important parts in cell division, cell motility, intracellular trafficking and ciliary function. How kinesins are regulated in cells to ensure the temporal and spatial fidelity of their microtubule-based activities is less well understood. Recent work has revealed molecular mechanisms that control kinesin autoinhibition and subsequent activation, binding to cargos and microtubule tracks, and localization at specific sites of action.

The Aurora Kinase in Trypanosoma brucei plays distinctive roles in metaphase-anaphase transition and cytokinetic initiation

Aurora B kinase is an essential regulator of chromosome segregation with the action well characterized in eukaryotes. It is also implicated in cytokinesis, but the detailed mechanism remains less clear, partly due to the difficulty in separating the latter from the former function in a growing cell. A chemical genetic approach with an inhibitor of the enzyme added to a synchronized cell population at different stages of the cell cycle would probably solve this problem. In the deeply branched parasitic protozoan Trypanosoma brucei, an Aurora B homolog, TbAUK1, was found to control both chromosome segregation and cytokinetic initiation by evidence from RNAi and dominant negative mutation. To clearly separate these two functions, VX-680, an inhibitor of TbAUK1, was added to a synchronized T. brucei procyclic cell population at different cell cycle stages. The unique trans-localization pattern of the chromosomal passenger complex (CPC), consisting of TbAUK1 and two novel proteins TbCPC1 and TbCPC2, was monitored during mitosis and cytokinesis by following the migration of the proteins tagged with enhanced yellow fluorescence protein in live cells with time-lapse video microscopy. Inhibition of TbAUK1 function in S-phase, prophase or metaphase invariably arrests the cells in the metaphase, suggesting an action of TbAUK1 in promoting metaphase-anaphase transition. TbAUK1 inhibition in anaphase does not affect mitotic exit, but prevents trans-localization of the CPC from the spindle midzone to the anterior tip of the new flagellum attachment zone for cytokinetic initiation. The CPC in the midzone is dispersed back to the two segregated nuclei, while cytokinesis is inhibited. In and beyond telophase, TbAUK1 inhibition has no effect on the progression of cytokinesis or the subsequent G1, S and G2 phases until a new metaphase is attained. There are thus two clearly distinct points of TbAUK1 action in T. brucei: the metaphase-anaphase transition and cytokinetic initiation. This is the first time to our knowledge that the dual functions of an Aurora B homolog is dissected and separated into two clearly distinct time frames in a cell cycle.

The chromosomal passenger complex (CPC) is essential for chromosome segregation and cytokinesis in eukaryotes, but the detailed mechanism of cytokinetic regulation remains less clear, partly due to the difficulty in separating the two functions in a growing cell. A chemical genetic approach by adding an inhibitor of the Aurora kinase in the CPC to a synchronized cell population at different cell cycle stages would probably solve this problem. The CPC in Trypanosoma brucei consists of an Aurora-like kinase (TbAUK1) and two novel proteins and bears little resemblance to the CPC in other eukaryotes. It moves from kinetochores to the spindle midzone during metaphase-anaphase transition, and then displays a unique trans-localization to the anterior end of the cell to initiate cytokinesis by moving from the anterior to the posterior end of the cell to separate it into two. To envision the role of TbAUK1 in driving this unusual process, we applied a chemical genetic approach and demonstrated that there are two distinct points of TbAUK1 action in T. brucei: the metaphase to anaphase transition and cytokinetic initiation. This is the first time to our knowledge that the dual functions of an Aurora B homolog is dissected and separated into two clearly distinct time frames in a cell cycle.

SPOC1: a novel PHD-containing protein modulating chromatin structure and mitotic chromosome condensation

In this study, we characterize the molecular and functional features of a novel protein called SPOC1. SPOC1 RNA expression was previously reported to be highest in highly proliferating tissues and increased in a subset of ovarian carcinoma patients, which statistically correlated with poor prognosis and residual disease. These observations implied that SPOC1 might play a role in cellular proliferation and oncogenesis. Here we show that the endogenous SPOC1 protein is labile, primarily chromatin associated and its expression as well as localization are regulated throughout the cell cycle. SPOC1 is dynamically regulated during mitosis with increased expression levels and biphasic localization to mitotic chromosomes indicating a functional role of SPOC1 in mitotic processes. Consistent with this postulate, SPOC1 siRNA knockdown experiments resulted in defects in mitotic chromosome condensation, alignment and aberrant sister chromatid segregation. Finally, we have been able to show, using micrococcal nuclease (MNase) chromatin-digestion assays that SPOC1 expression levels proportionally influence the degree of chromatin compaction. Collectively, our findings show that SPOC1 modulates chromatin structure and that tight regulation of its expression levels and subcellular localization during mitosis are crucial for proper chromosome condensation and cell division.

Clearing the way for mitosis: is cohesin a target?

In interphase, chromosomes are associated with proteins and RNAs that participate in many processes, such as DNA replication, transcription, recombination and repair of DNA damage. These components (for example, cohesin) might have to be removed during mitosis, as they might become obstacles that inhibit chromosome segregation or reduce its fidelity. Such a clearing mechanism that operates along mitotic chromosomes might require proteins that are implicated in chromosome segregation. I propose that condensin and DNA topoisomerase II (TOP2), as well as separase, help to clear the way for mitosis.