Kinetochores and the checkpoint mechanism that monitors for defects in the chromosome segregation machinery

Swc4 protects nucleosome-free rDNA, tDNA and telomere loci to inhibit genome instability

In the baker's yeast Saccharomyces cerevisiae, NuA4 and SWR1-C, two multisubunit complexes, are involved in histone acetylation and chromatin remodeling, respectively. Eaf1 is the assembly platform subunit of NuA4, Swr1 is the assembly platform and catalytic subunit of SWR1-C, while Swc4, Yaf9, Arp4 and Act1 form a functional module, and is present in both NuA4 and SWR1 complexes. ACT1 and ARP4 are essential for cell survival. Deletion of SWC4, but not YAF9, EAF1 or SWR1 results in a severe growth defect, but the underlying mechanism remains largely unknown. Here, we show that swc4Δ, but not yaf9Δ, eaf1Δ, or swr1Δ cells display defects in DNA ploidy and chromosome segregation, suggesting that the defects observed in swc4Δ cells are independent of NuA4 or SWR1-C integrity. Swc4 is enriched in the nucleosome-free regions (NFRs) of the genome, including characteristic regions of RDN5s, tDNAs and telomeres, independently of Yaf9, Eaf1 or Swr1. In particular, rDNA, tDNA and telomere loci are more unstable and prone to recombination in the swc4Δ cells than in wild-type cells. Taken together, we conclude that the chromatin associated Swc4 protects nucleosome-free chromatin of rDNA, tDNA and telomere loci to ensure genome integrity.

From primordial germ cells to spermatids in Caenorhabditis elegans

Development of a syncytial germline for gamete formation requires complex regulation of cytokinesis and cytoplasmic remodeling. Recently, several uncovered cellular events have been investigated in the Caenorhabditis elegans (C. elegans) germline. In these cellular processes, the factors involved in contractility are highly conserved with those of mitosis and meiosis. However, the underlying regulatory mechanisms are far more complicated than previously thought, likely due to the single syncytial germline structure. In this review, we highlight how the proteins involved in contractility ensure faithful cell division in different cellular contexts and how they contribute to maintaining intercellular bridge stability. In addition, we discuss the current understanding of the cellular events of cytokinesis and cytoplasmic remodeling during the development of the C. elegans germline, including progenitor germ cells, germ cells, and spermatocytes. Comparisons are made with relevant systems in Drosophila melanogaster (D. melanogaster) and other animal models.

Yeast Sphingolipid Phospholipase Gene ISC1 Regulates the Spindle Checkpoint by a CDC55-Dependent Mechanism

Defects in the spindle assembly checkpoint (SAC) can lead to aneuploidy and cancer. Sphingolipids have important roles in many cellular functions, including cell cycle regulation and apoptosis. However, the specific mechanisms and functions of sphingolipids in cell cycle regulation have not been elucidated. Using analysis of concordance for synthetic lethality for the yeast sphingolipid phospholipase ISC1, we identified two groups of genes. The first comprises genes involved in chromosome segregation and stability (CSM3, CTF4, YKE2, DCC1, and GIM4) as synthetically lethal with ISC1 The second group, to which ISC1 belongs, comprises genes involved in the spindle checkpoint (BUB1, MAD1, BIM1, and KAR3), and they all share the same synthetic lethality with the first group. We demonstrate that spindle checkpoint genes act upstream of Isc1, and their deletion phenocopies that of ISC1 Reciprocally, ISC1 deletion mutants were sensitive to benomyl, indicating a SAC defect. Similar to BUB1 deletion, ISC1 deletion prevents spindle elongation in hydroxyurea-treated cells. Mechanistically, PP2A-Cdc55 ceramide-activated phosphatase was found to act downstream of Isc1, thus coupling the spindle checkpoint genes and Isc1 to CDC55-mediated nuclear functions.

The budding yeast Cdc48(Shp1) complex promotes cell cycle progression by positive regulation of protein phosphatase 1 (Glc7)

The conserved, ubiquitin-selective AAA ATPase Cdc48 regulates numerous cellular processes including protein quality control, DNA repair and the cell cycle. Cdc48 function is tightly controlled by a multitude of cofactors mediating substrate specificity and processing. The UBX domain protein Shp1 is a bona fide substrate-recruiting cofactor of Cdc48 in the budding yeast S. cerevisiae. Even though Shp1 has been proposed to be a positive regulator of Glc7, the catalytic subunit of protein phosphatase 1 in S. cerevisiae, its cellular functions in complex with Cdc48 remain largely unknown. Here we show that deletion of the SHP1 gene results in severe growth defects and a cell cycle delay at the metaphase to anaphase transition caused by reduced Glc7 activity. Using an engineered Cdc48 binding-deficient variant of Shp1, we establish the Cdc48^Shp1 complex as a critical regulator of mitotic Glc7 activity. We demonstrate that shp1 mutants possess a perturbed balance of Glc7 phosphatase and Ipl1 (Aurora B) kinase activities and show that hyper-phosphorylation of the kinetochore protein Dam1, a key mitotic substrate of Glc7 and Ipl1, is a critical defect in shp1. We also show for the first time a physical interaction between Glc7 and Shp1 in vivo. Whereas loss of Shp1 does not significantly affect Glc7 protein levels or localization, it causes reduced binding of the activator protein Glc8 to Glc7. Our data suggest that the Cdc48^Shp1 complex controls Glc7 activity by regulating its interaction with Glc8 and possibly further regulatory subunits.

Structural and functional characterization of the protein kinase Mps1 in Arabidopsis thaliana

In eukaryotes, protein kinases catalyze the transfer of a gamma-phosphate from ATP (or GTP) to specific amino acids in protein targets. In plants, protein kinases have been shown to participate in signaling cascades driving responses to environmental stimuli and developmental processes. Plant meristems are undifferentiated tissues that provide the major source of cells that will form organs throughout development. However, non-dividing specialized cells can also dedifferentiate and re-initiate cell division if exposed to appropriate conditions. Mps1 (Monopolar spindle) is a dual-specificity protein kinase that plays a critical role in monitoring the accuracy of chromosome segregation in the mitotic checkpoint mechanism. Although Mps1 functions have been clearly demonstrated in animals and fungi, its role in plants is so far unclear. Here, using structural and biochemical analyses here we show that Mps1 has highly similar homologs in many plant genomes across distinct lineages (e.g. AtMps1 in Arabidopsis thaliana). Several structural features (i.e. catalytic site, DFG motif and threonine triad) are clearly conserved in plant Mps1 kinases. Structural and sequence analysis also suggest that AtMps1 interact with other cell cycle proteins, such as Mad2 and MAPK1. By using a very specific Mps1 inhibitor (SP600125) we show that compromised AtMps1 activity hampers the development of A. thaliana seedlings in a dose-dependent manner, especially in secondary roots. Moreover, concomitant administration of the auxin IAA neutralizes the AtMps1 inhibition phenotype, allowing secondary root development. These observations let us to hypothesize that AtMps1 might be a downstream regulator of IAA signaling in the formation of secondary roots. Our results indicate that Mps1 might be a universal component of the Spindle Assembly Checkpoint machinery across very distant lineages of eukaryotes.

Gene expression in sheep carotid arteries: major changes with maturational development

BACKGROUND

With development from immature fetus to near-term fetus, newborn, and adult, the cerebral vasculature undergoes a number of fundamental changes. Although the near-term fetus is prepared for a transition from an intra- to extra-uterine existence, this is not necessarily the case with the premature fetus, which is more susceptible to cerebrovascular dysregulation. In this study, we tested the hypothesis that the profound developmental and age-related differences in cerebral blood flow are associated with significant underlying changes in gene expression.

METHODS

With the use of oligonucleotide microarray and pathway analysis, we elucidated significant changes in the transcriptome with development in sheep carotid arteries.

RESULTS

As compared with adult, we demonstrate a U-shaped relationship of gene expression in major cerebrovascular network/pathways during early life, e.g., the level of gene expression in the premature fetus and newborn is considerably greater than that of the near-term fetus. Specifically, cell proliferation, growth, and assembly pathway genes were upregulated during early life. In turn, as compared with adult, mitogen-activated protein kinase-extracellular regulated kinase, actin cytoskeleton, and integrin-signaling pathways were downregulated during early life.

CONCLUSION

In cranial vascular smooth muscle, highly significant changes occur in important cellular and signaling pathways with maturational development.

ScreenTroll: a searchable database to compare genome-wide yeast screens

Systematic biological screens typically identify many genes or proteins that are implicated in a specific phenotype. However, deriving mechanistic insight from these screens typically involves focusing upon one or a few genes within the set in order to elucidate their precise role in producing the phenotype. To find these critical genes, researchers use a variety of tools to query the set of genes to uncover underlying common genetic or physical interactions or common functional annotations (e.g. gene ontology terms). Not only it is necessary to find previous screens containing genes in common with the new set, but also useful to easily access the individual manuscript or study that classified those genes. Unfortunately, no tool currently exists to facilitate this task. We have developed a web-based tool (ScreenTroll) that queries one or more genes against a database of systematic yeast screens. The software determines which genome-wide yeast screens also identified the queried gene(s) and the resulting screens are listed in an order based on the extent of the overlap between the queried gene(s) and the open reading frames (ORFs) characterized in each individual yeast screen. In a separate list, the corresponding ORFs that are found in both the queried set of genes and each individual genome-wide screen are displayed along with links to the relevant manuscript via NIH's PubMed database. ScreenTroll is useful for comparing a list of ORFs with genes identified in a wide array of published genome-wide screens. This comparison informs users whether any of their queried ORFs overlaps a previous study in the ScreenTroll database. By listing the manuscript of the published screen, users can read more about the phenotype associated with that study. Together, this information provides insight into the function of the queried genes and helps the user focus on a subset of them.

Misregulation of Scm3p/HJURP causes chromosome instability in Saccharomyces cerevisiae and human cells

The kinetochore (centromeric DNA and associated proteins) is a key determinant for high fidelity chromosome transmission. Evolutionarily conserved Scm3p is an essential component of centromeric chromatin and is required for assembly and function of kinetochores in humans, fission yeast, and budding yeast. Overexpression of HJURP, the mammalian homolog of budding yeast Scm3p, has been observed in lung and breast cancers and is associated with poor prognosis; however, the physiological relevance of these observations is not well understood. We overexpressed SCM3 and HJURP in Saccharomyces cerevisiae and HJURP in human cells and defined domains within Scm3p that mediate its chromosome loss phenotype. Our results showed that the overexpression of SCM3 (GALSCM3) or HJURP (GALHJURP) caused chromosome loss in a wild-type yeast strain, and overexpression of HJURP led to mitotic defects in human cells. GALSCM3 resulted in reduced viability in kinetochore mutants, premature separation of sister chromatids, and reduction in Cse4p and histone H4 at centromeres. Overexpression of CSE4 or histone H4 suppressed chromosome loss and restored levels of Cse4p at centromeres in GALSCM3 strains. Using mutant alleles of scm3, we identified a domain in the N-terminus of Scm3p that mediates its interaction with CEN DNA and determined that the chromosome loss phenotype of GALSCM3 is due to centromeric association of Scm3p devoid of Cse4p/H4. Furthermore, we determined that similar to other systems the centromeric association of Scm3p is cell cycle regulated. Our results show that altered stoichiometry of Scm3p/HJURP, Cse4p, and histone H4 lead to defects in chromosome segregation. We conclude that stringent regulation of HJURP and SCM3 expression are critical for genome stability.

Knockdown of ubiquitin-conjugating enzyme E2C/UbcH10 expression by RNA interference inhibits glioma cell proliferation and enhances cell apoptosis in vitro

PURPOSE

To address the role of ubiquitin-conjugating enzyme, E2C/UbcH10, in astrocytic carcinogenesis.

METHODS

Expression pattern of UbcH10 in U251 glioma cells was evaluated by immunohistochemistry and western blot. RNA interference was employed to downregulate UbcH10 expression in U251 cell line. The effect of UbcH10 silencing on cell proliferation was assessed by MTT assay and cell cycle analysis. Cell apoptosis was determined by flow cytometry, TUNEL staining and western blot.

RESULTS

Levels of UbcH10 protein were significantly upregulated in U251 cells compared with normal brain tissues. Marked immunoreactivity for UbcH10 was demonstrated in the cytoplasm of U251 glioma cells, especially in the mitotic cells. The growth rate of U251 cells was significantly inhibited by depletion of UbcH10 by short interference RNA. Further, UbcH10 RNAi induced apoptosis through induction of Bax and p53, downregulation of Bcl-2 and G2/M arrest of the cell cycle.

CONCLUSION

These data imply that knocking-down UbcH10 protein expression may represent a potential therapeutic option for glioma.

Altered dosage and mislocalization of histone H3 and Cse4p lead to chromosome loss in Saccharomyces cerevisiae

Cse4p is an essential histone H3 variant in Saccharomyces cerevisiae that defines centromere identity and is required for proper segregation of chromosomes. In this study, we investigated phenotypic consequences of Cse4p mislocalization and increased dosage of histone H3 and Cse4p, and established a direct link between histone stoichiometry, mislocalization of Cse4p, and chromosome segregation. Overexpression of the stable Cse4p mutant, cse4(K16R), resulted in its mislocalization, increased association with chromatin, and a high rate of chromosome loss, all of which were suppressed by constitutive expression of histone H3 (delta 16H3). We determined that delta 16H3 did not lead to increased chromosome loss; however, increasing the dosage of histone H3 (GALH3) resulted in significant chromosome loss due to reduced levels of centromere (CEN)-associated Cse4p and synthetic dosage lethality (SDL) in kinetochore mutants. These phenotypes were suppressed by GALCSE4. We conclude that the chromosome missegregation of GALcse4(K16R) and GALH3 strains is due to mislocalization and a functionally compromised kinetochore, respectively. Suppression of these phenotypes by histone delta 16H3 and GALCSE4 supports the conclusion that proper stoichiometry affects the localization of histone H3 and Cse4p and is thus essential for accurate chromosome segregation.

Oral manifestations of Sjögren's syndrome

Sjögren's syndrome is a common autoimmune rheumatic disease. The most common symptoms of Sjögren's syndrome are extreme tiredness, along with dry eyes (keratoconjunctivitis sicca) and dry mouth (xerostomia). Saliva plays an essential role in numerous functions of the mouth. Xerostomia can be caused by medications, chronic diseases like Sjögren's syndrome, and medical treatments, such as radiation therapy and bone marrow transplant. Xerostomia can eventually lead to difficulty in swallowing, severe and progressive tooth decay, or oral infections. Despite having excellent oral hygiene, individuals with Sjögren's syndrome have elevated levels of dental caries, along with the loss of many teeth, early in the disease. Sjögren's syndrome alters the protein profile and brings about a change in the composition of saliva. There is an increase in the levels of lactoferrin, beta(2)-microglobulin, sodium, lysozyme C, and cystatin C, and a decrease in salivary amylase and carbonic anhydrase. Up to 90% of individuals with Sjögren's syndrome have antibodies targeting the Ro 60 and La autoantigens. Natural aging, regardless of Sjögren's syndrome, is also another factor that brings about a significant change in the composition of saliva. The most prevailing cause of xerostomia in elderly persons is the use of anticholinergic medications. Currently, there is no cure for Sjögren's syndrome, and treatment is mainly palliative.

Mechanisms of sister chromatid pairing

The continuance of life through cell division requires high fidelity DNA replication and chromosome segregation. During DNA replication, each parental chromosome is duplicated exactly and one time only. At the same time, the resulting chromosomes (called sister chromatids) become tightly paired along their length. This S-phase pairing, or cohesion, identifies chromatids as sisters over time. During mitosis in most eukaryotes, sister chromatids bi-orient to the mitotic spindle. After each chromosome pair is properly oriented, the cohesion established during S phase is inactivated in a tightly regulated fashion, allowing sister chromatids to segregate away from each other. Recent findings of cohesin structure and enzymology provide new insights into cohesion, while many critical facets of cohesion (how cohesins tether together sister chromatids and how those tethers are established) remain actively debated.

Spindle assembly checkpoint and centrosome abnormalities in oral cancer

Like many solid tumours, oral squamous cell carcinomas (OSCC) invariably exhibit chromosomal instability (CIN) leading to aneuploidy. The mechanisms responsible for CIN in OSCC, however, are largely unknown. This study examined the fidelity of the spindle checkpoint, together with the number, structure and function of centrosomes in a series of well-characterised aneuploid immortal OSCC-derived cell lines that harbour p53 and p16(INK4A) defects. The spindle checkpoints were fully functional in 2 of 7 cell lines and attenuated in the remaining 5 cell lines. Overexpression of the spindle checkpoint protein, Cdc20, was observed in 2 of the cell lines with attenuated checkpoints. Defects in centrosome number, size and localisation were detected in 5 of the cell lines. Clonal cell populations contained cells with both normal and abnormal numbers of centrosomes, suggesting that the control of centrosome number may be inherently unstable in OSCC-derived cell lines. Centrosomal abnormalities were then examined in tissue samples of oral epithelial dysplasias and carcinomas. Abnormal centrosomes were detected in all the tissues examined albeit in a low percentage of cells (<1% to >5%). The percentage of cells containing centrosome abnormalities was significantly higher in the carcinomas than in the dysplasias (p<0.02) and in the poorly differentiated SCCs relative to their moderately differentiated (p<0.04) and well-differentiated (p<0.01) counterparts. We suggest that the genetic alterations associated with the development of the immortal phenotype, together with spindle checkpoint and centrosome defects, are responsible, albeit in part, for the complex karyotypes observed in OSCC. The presence of centrosome abnormalities in oral dysplasias raises the possibility that such defects might contribute to malignant progression.

MAD2ΔC induces aneuploidy and promotes anchorage-independent growth in human prostate epithelial cells

The mitotic arrest deficient 2 (MAD2) is suggested to play a key role in a functional mitotic checkpoint because of its inhibitory effect on anaphase-promoting complex/cyclosome (APC/C) during mitosis. The binding of MAD2 to mitotic checkpoint regulators MAD1 and Cdc20 is thought to be crucial for its function and loss of which leads to functional inactivation of the MAD2 protein. However, little is known about the biological significance of this MAD2 mutant in human cells. In this study, we stably transfected a C-terminal-deleted MAD2 gene (MAD2DeltaC) into a human prostate epithelial cell line, Hpr-1 and studied its effect on chromosomal instability, cell proliferation, mitotic checkpoint control and soft agar colony-forming ability. We found that MAD2DeltaC was able to induce aneuploidy through promoting chromosomal duplication, which was a result of an impaired mitotic checkpoint and cytokinesis, suggesting a crucial role of MAD2-mediated mitotic checkpoint in chromosome stability in human cells. In addition, the MAD2DeltaC-transfected cells displayed anchorage-independent growth in soft agar after challenged by 7,12-dimethylbenz[A]anthracene (DMBA), demonstrating a cancer-promoting effect of a defective mitotic checkpoint in human cells. Furthermore, the DMBA-induced transformation was accompanied by a complete loss of DNA damage-induced p53 response and activation of the MAPK pathway in MAD2DeltaC cells. These results indicate that a defective mitotic checkpoint alone is not a direct cause of tumorigenesis, but it may predispose human cells to carcinogen-induced malignant transformation. The evidence presented here provides a link between MAD2 inactivation and malignant transformation of epithelial cells.

Actin-related protein Arp4 functions in kinetochore assembly

The actin-related proteins (Arps) comprise a conserved protein family. Arp4p is found in large multisubunits of the INO80 and SWR1 chromatin remodeling complexes and in the NuA4 histone acetyltransferase complex. Here we show that arp4 (arp4S23A/D159A) temperature-sensitive cells are defective in G2/M phase function. arp4 mutants are sensitive to the microtubule depolymerizing agent benomyl and arrest at G2/M phase at restrictive temperature. Arp4p is associated with centromeric and telomeric regions throughout cell cycle. Ino80p, Esa1p and Swr1p, components of the INO80, NuA4 and SWR1 complexes, respectively, also associate with centromeres. The association of many kinetochore components including Cse4p, a component of the centromere nucleosome, Mtw1p and Ctf3p is partially impaired in arp4 cells, suggesting that the G2/M arrest of arp4 mutant cells is due to a defect in formation of the chromosomal segregation apparatus.

CENP-F expression is associated with poor prognosis and chromosomal instability in patients with primary breast cancer

DNA microarrays have the potential to classify tumors according to their transcriptome. Tissue microarrays (TMAs) facilitate the validation of biomarkers by offering a high-throughput approach to sample analysis. We reanalyzed a high profile breast cancer DNA microarray dataset containing 96 tumor samples using a powerful statistical approach, between group analyses. Among the genes we identified was centromere protein-F (CENP-F), a gene associated with poor prognosis. In a published follow-up breast cancer DNA microarray study, comprising 295 tumour samples, we found that CENP-F upregulation was significantly associated with worse overall survival (p<0.001) and reduced metastasis-free survival (p<0.001). To validate and expand upon these findings, we used 2 independent breast cancer patient cohorts represented on TMAs. CENP-F protein expression was evaluated by immunohistochemistry in 91 primary breast cancer samples from cohort I and 289 samples from cohort II. CENP-F correlated with markers of aggressive tumor behavior including ER negativity and high tumor grade. In cohort I, CENP-F was significantly associated with markers of CIN including cyclin E, increased telomerase activity, c-Myc amplification and aneuploidy. In cohort II, CENP-F correlated with VEGFR2, phosphorylated Ets-2 and Ki67, and in multivariate analysis, was an independent predictor of worse breast cancer-specific survival (p=0.036) and overall survival (p=0.040). In conclusion, we identified CENP-F as a biomarker associated with poor outcome in breast cancer and showed several novel associations of biological significance.

Genomic mechanisms and measurement of structural and numerical instability in cancer cells

The progression to cancer is often associated with instability and the acquisition of genomic heterogeneity, generating both clonal and non-clonal populations. Chromosomal instability (CIN) describes the excessive rate of numerical and structural genomic change in tumors. Mitotic segregation errors strongly influences copy number, while structural aberrations can occur at unstable genomic regions, or through aberrant DNA repair or methylation. Combined molecular cytogenetic analyses can evaluate cell-to-cell variation, and define the complexity of numerical and structural alterations. Because structural change may occur independently of numerical alteration, we propose the term structural chromosomal instability [(S)-CIN] to distinguish numerical from structural CIN.

Okadaic acid overcomes the blocked cell cycle caused by depleting Cdc2-related kinases in Trypanosoma brucei

Mitosis and cytokinesis are highly coordinated in eukaryotic cells. But procyclic-form Trypanosoma brucei under G1 or mitotic arrest is still capable of dividing, resulting in anucleate daughter cells (zoids). Okadaic acid (OKA), an inhibitor of protein phosphatases PP1 and PP2A, is known to inhibit kinetoplast replication and cell division yielding multinucleate cells with single kinetoplasts. However, when OKA was applied to cells arrested in G1 or G2/M phase via RNAi knockdown of specific cdc2-related kinases (CRKs), DNA synthesis and nuclear division were resumed without kinetoplast replication or cell division, resulting in multinucleate cells as in the wild type. Cells arrested in G2/M via depleting the mitotic cyclin CycB2 or an aurora B kinase homologue TbAUK1 were, however, not released by OKA treatment. The phenomenon is thus similar to the OKA activation of Cdc2 in Xenopus oocyte by inhibiting PP2A [Maton, et al., Differential regulation of Cdc2 and Aurora-A in Xenopus oocytes: a crucial role of phosphatase 2A. J. Cell Sci. 118 (2005) 2485-2494]. A simultaneous knockdown of the seven PP1s or the PP2A catalytic subunit in T. brucei by RNA interference did not, however, result in multinucleate cells. This could be explained by assuming a negative regulation, either directly or indirectly, of CRK by an OKA-sensitive phosphatase, which could be a PP2A as in the Xenopus oocyte and a positive regulation of kinetoplast replication by an OKA-susceptible protein(s). Test of a PP2A-specific inhibitor, fostriecin, on cells arrested in G2/M via CRK depletion or a knockdown of the PP2A catalytic subunit from the CRK-depleted cells both showed a partial lift of the G2/M block without forming multinucleate cells. These observations support the abovementioned assumption and suggest the presence of a novel OKA-sensitive protein(s) regulating kinetoplast replication that still remains to be identified.

Cardiac stem cells and mechanisms of myocardial regeneration

This review discusses current understanding of the role that endogenous and exogenous progenitor cells may have in the treatment of the diseased heart. In the last several years, a major effort has been made in an attempt to identify immature cells capable of differentiating into cell lineages different from the organ of origin to be employed for the regeneration of the damaged heart. Embryonic stem cells (ESCs) and bone marrow-derived cells (BMCs) have been extensively studied and characterized, and dramatic advances have been made in the clinical application of BMCs in heart failure of ischemic and nonischemic origin. However, a controversy exists concerning the ability of BMCs to acquire cardiac cell lineages and reconstitute the myocardium lost after infarction. The recognition that the adult heart possesses a stem cell compartment that can regenerate myocytes and coronary vessels has raised the unique possibility to rebuild dead myocardium after infarction, to repopulate the hypertrophic decompensated heart with new better functioning myocytes and vascular structures, and, perhaps, to reverse ventricular dilation and wall thinning. Cardiac stem cells may become the most important cell for cardiac repair.

The Drosophila Bub3 protein is required for the mitotic checkpoint and for normal accumulation of cyclins during G2 and early stages of mitosis

During mitosis, a checkpoint mechanism delays metaphase-anaphase transition in the presence of unattached and/or unaligned chromosomes. This delay is achieved through inhibition of the anaphase promoting complex/cyclosome (APC/C) preventing sister chromatid separation and cyclin degradation. In the present study, we show that Bub3 is an essential protein required during normal mitotic progression to prevent premature sister chromatid separation, missegreation and aneuploidy. We also found that Bub3 is required during G2 and early stages of mitosis to promote normal mitotic entry. We show that loss of Bub3 function by mutation or RNAi depletion causes cells to progress slowly through prophase, a delay that appears to result from a failure to accumulate mitotic cyclins A and B. Defective accumulation of mitotic cyclins results from inappropriate APC/C activity, as mutations in the gene encoding the APC/C subunit cdc27 partially rescue this phenotype. Furthermore, analysis of mitotic progression in cells carrying mutations for cdc27 and bub3 suggest the existence of differentially activated APC/C complexes. Altogether, our data support the hypothesis that the mitotic checkpoint protein Bub3 is also required to regulate entry and progression through early stages of mitosis.

In vivo characterization of the nonessential budding yeast anaphase-promoting complex/cyclosome components Swm1p, Mnd2p and Apc9p

We have examined the in vivo requirement of two recently identified nonessential components of the budding yeast anaphase-promoting complex, Swm1p and Mnd2p, as well as that of the previously identified subunit Apc9p. swm1Delta mutants exhibit synthetic lethality or conditional synthetic lethality with other APC/C subunits and regulators, whereas mnd2Delta mutants are less sensitive to perturbation of the APC/C. swm1Delta mutants, but not mnd2Delta mutants, exhibit defects in APC/C substrate turnover, both during the mitotic cell cycle and in alpha-factor-arrested cells. In contrast, apc9Delta mutants exhibit only minor defects in substrate degradation in alpha-factor-arrested cells. In cycling cells, degradation of Clb2p, but not Pds1p or Clb5p, is delayed in apc9Delta. Our findings suggest that Swm1p is required for full catalytic activity of the APC/C, whereas the requirement of Mnd2p for APC/C function appears to be negligible under standard laboratory conditions. Furthermore, the role of Apc9p in APC/C-dependent ubiquitination may be limited to the proteolysis of a select number of substrates.

The C-terminal half of Saccharomyces cerevisiae Mad1p mediates spindle checkpoint function, chromosome transmission fidelity and CEN association

The evolutionarily conserved spindle checkpoint is a key mechanism ensuring high-fidelity chromosome transmission. The checkpoint monitors attachment between kinetochores and mitotic spindles and the tension between sister kinetochores. In the absence of proper attachment or tension, the spindle checkpoint mediates cell cycle arrest prior to anaphase. Saccharomyces cerevisiae Mad1p is required for the spindle checkpoint and for chromosome transmission fidelity. Moreover, Mad1p associates with the nuclear pore complex (NPC) and is enriched at kinetochores upon checkpoint activation. Using partial mad1 deletion alleles we determined that the C-terminal half of Mad1p is necessary and sufficient for checkpoint activation in response to microtubule depolymerizing agents, high-fidelity transmission of a reporter chromosome fragment, and in vivo association with centromeres, but not for robust NPC association. Thus, spindle checkpoint activation and chromosome transmission fidelity correlate and these Mad1p functions likely involve kinetochore association but not robust NPC association. These studies are the basis for elucidating the role of protein complexes containing Mad1p in the spindle checkpoint pathway and in maintaining genome stability in S. cerevisiae and other systems.

Anti-CENP-H antibodies in patients with Sjogren's syndrome

Anti-centromere antibody (ACA) has been reported to be associated with Sjogren's syndrome (SS) and the clinical significance of anti-CENP-H antibody remains unknown. To determine the clinical significance of anti-CENP-H and anti-centromere antibodies in primary SS, sera from 62 patients with primary SS and 40 normal controls were examined for anti-SS-A/SS-B antibodies, ACA and anti-CENP-H antibodies, by enzyme-linked immunosorbent assay and indirect immunofluorescence (IIF), respectively. Of the 62 serum samples with primary SS, 17 were positive with ACA and anti-CENP-H antibodies. Sera from SS patients with anti-CENP-H and ACA antibodies do not contain anti-SS-A/Ro and/or anti-SS-B/La antibodies. No anti-CENP-H antibody was found in sera of normal controls. An increased frequency of ACA and anti-CENP-H antibodies was found for the first time in patients with SS. Anti-CENP-H antibodies and anti-SS-A/Ro or anti-SS-B/La antibodies are present mutually exclusive. Patients with anti-CENP-H antibodies had a lower frequency of rheumatoid factor (RF). SS can be subdivided serologically into two groups; group one with anti-SS-A/Ro and/or anti-SS-B/La antibody, group two with ACA and/or anti-CENP-H antibodies. We recommend that ACA or anti-CENP-H antibodies should be considered as one of the serological markers for SS.

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.

A spindle checkpoint functions during mitosis in the early Caenorhabditis elegans embryo

During mitosis, chromosome segregation is regulated by a spindle checkpoint mechanism. This checkpoint delays anaphase until all kinetochores are captured by microtubules from both spindle poles, chromosomes congress to the metaphase plate, and the tension between kinetochores and their attached microtubules is properly sensed. Although the spindle checkpoint can be activated in many different cell types, the role of this regulatory mechanism in rapidly dividing embryonic animal cells has remained controversial. Here, using time-lapse imaging of live embryonic cells, we show that chemical or mutational disruption of the mitotic spindle in early Caenorhabditis elegans embryos delays progression through mitosis. By reducing the function of conserved checkpoint genes in mutant embryos with defective mitotic spindles, we show that these delays require the spindle checkpoint. In the absence of a functional checkpoint, more severe defects in chromosome segregation are observed in mutants with abnormal mitotic spindles. We also show that the conserved kinesin CeMCAK, the CENP-F-related proteins HCP-1 and HCP-2, and the core kinetochore protein CeCENP-C all are required for this checkpoint. Our analysis indicates that spindle checkpoint mechanisms are functional in the rapidly dividing cells of an early animal embryo and that this checkpoint can prevent chromosome segregation defects during mitosis.

Coupling of posterior cytoskeletal morphogenesis to the G1/S transition in the Trypanosoma brucei cell cycle

The expression levels of four Cdc2-related kinases (CRK1, 2, 4, and 6) in the procyclic form of Trypanosoma brucei were knocked down in pairs using the RNA interference (RNAi) technique. A double knockdown of CRK1 and CRK2 resulted in arrested cell growth in the G1 phase accompanied by an apparent cessation of nuclear DNA synthesis. The arrested cells became elongated at the posterior end like the G1-phase cells generated by knockdown of CycE1/CYC2 in a previous study. However, approximately 5% of the G1 cells in the current study also possessed multiply branched posterior ends, which have not previously been observed in T. brucei. DAPI and immunofluorescence staining showed a single nucleus, kinetoplast, basal body, and flagellum in the anterior portion of each G1 cell. The split and grossly extended posterior ends were heavily stained with antibodies to tyrosinated alpha-tubulin, suggesting an accumulation of newly synthesized microtubules. A significant population of anucleate cells (zoids), apparently derived from kinetoplast-dictated cytokinesis and cell division of the G1 cells, also had extended and branched posterior ends filled with newly synthesized microtubules. This continued posterior extension of microtubules in the G1 cells and zoids suggests that CRK1 and CRK2 exert a coordinated control on G1/S passage and the limited growth of the microtubule corset toward the posterior end. This connection may provide a new insight into the mechanism of morphological maintenance of an ancient protist during its cell cycle progression.

Different spindle checkpoint proteins monitor microtubule attachment and tension at kinetochores in Drosophila cells

The spindle assembly checkpoint detects errors in kinetochore attachment to the spindle including insufficient microtubule occupancy and absence of tension across bi-oriented kinetochore pairs. Here, we analyse how the kinetochore localization of the Drosophila spindle checkpoint proteins Bub1, Mad2, Bub3 and BubR1, behave in response to alterations in microtubule binding or tension. To analyse the behaviour in the absence of tension, we treated S2 cells with low doses of taxol to disrupt microtubule dynamics and tension, but not kinetochore-microtubule occupancy. Under these conditions, we found that Mad2 and Bub1 do not accumulate at metaphase kinetochores whereas BubR1 does. Consistently, in mono-oriented chromosomes, both kinetochores accumulate BubR1 whereas Bub1 and Mad2 only localize at the unattached kinetochore. To study the effect of tension we analysed the kinetochore localization of spindle checkpoint proteins in relation to tension-sensitive kinetochore phosphorylation recognised by the 3F3/2 antibody. Using detergent-extracted S2 cells as a system in which kinetochore phosphorylation can be easily manipulated, we observed that BubR1 and Bub3 accumulation at kinetochores is dependent on the presence of phosphorylated 3F3/2 epitopes. However, Bub1 and Mad2 localize at kinetochores regardless of the 3F3/2 phosphorylation state. Altogether, our results suggest that spindle checkpoint proteins sense distinct aspects of kinetochore interaction with the spindle, with Mad2 and Bub1 monitoring microtubule occupancy while BubR1 and Bub3 monitor tension across attached kinetochores.

The spindle assembly and spindle position checkpoints

The mitotic spindle segregates chromosomes to opposite ends of the cell in preparation for cell division. Chromosome attachment to the spindle is monitored by the spindle assembly checkpoint, and at least in yeast cells, penetration of one spindle pole into the bud is monitored by the spindle position checkpoint. We review the historical origins of these checkpoints and recent progress in understanding their surveillance pathways. We also highlight fascinating but as yet unresolved questions, and examine crosstalk between the checkpoints.

Chromosomal instability in osteosarcoma and its association with centrosome abnormalities

The mechanism that generates the extreme aneuploidy that characterizes osteosarcoma (OS) is poorly understood. In this study, interphase fluorescence in situ hybridization (FISH) analysis was used to enumerate cell-to-cell variation of several different chromosomes. We also investigated whether there was an association between TP53 mutation and centrosome aberrations in the generation of chromosomal aneuploidy in OS in four OS cell lines (HOS, SAOS2, U2OS, and MG63) and in a subset of seven tumors. Our analysis showed that there was a wide range of numerical changes affecting multiple chromosomes in OS cell lines and tumors. These data suggest that chromosomal instability (CIN) could be responsible for the extensive aneuploidy associated with this tumor. The results also showed an increased frequency of atypical mitotic figures in three OS cell lines with defective TP53, function and significantly, a more marked CIN phenotype was present in these lines. Furthermore, numerical aberrations of centrosomes were also present in these three OS cell lines with TP53 mutations. In two of three OS patients' tumors there was a large increase in the percentage of abnormal centrosome numbers. We conclude that CIN is a consistent feature of OS and that an intrinsic disturbance of the chromosomal segregation mechanisms is likely associated with centrosome aberrations.

Genetic evidence for a role of phospholipase C at the budding yeast kinetochore

Chromosome segregation during mitosis requires kinetochores, specialized organelles that mediate chromosome attachment to spindle microtubules. We have shown previously that in budding yeast, Plc1p (phosphoinositide-specific phospholipase C) localizes to centromeric loci, associates with the kinetochore proteins Ndc10p and Cep3p, and affects the function of kinetochores. Deletion of PLC1 results in nocodazole sensitivity, mitotic delay, and a higher frequency of chromosome loss. We report here that despite the nocodazole sensitivity of plc1Delta cells, Plc1p is not required for the spindle checkpoint. However, plc1Delta cells require a functional BUB1/BUB3-dependent spindle checkpoint for viability. PLC1 displays strong genetic interactions with genes encoding components of the inner kinetochore, including NDC10, SKP1, MIF2, CEP1, CEP3, and CTF13. Furthermore, plc1Delta cells display alterations in chromatin structure in the core centromere. Chromatin immunoprecipitation experiments indicate that Plc1p localizes to centromeric loci independently of microtubules, and accumulates at the centromeres during G(2)/M stage of cell cycle. These results are consistent with the view that Plc1p affects kinetochore function, possibly by modulating the structure of centromeric chromatin.

The yeast RSC chromatin-remodeling complex is required for kinetochore function in chromosome segregation

The accurate segregation of chromosomes requires the kinetochore, a complex protein machine that assembles onto centromeric DNA to mediate attachment of replicated sister chromatids to the mitotic spindle apparatus. This study reveals an important role for the yeast RSC ATP-dependent chromatin-remodeling complex at the kinetochore in chromosome transmission. Mutations in genes encoding two core subunits of RSC, the ATPase Sth1p and the Snf5p homolog Sfh1p, interact genetically with mutations in genes encoding kinetochore proteins and with a mutation in centromeric DNA. RSC also interacts genetically and physically with the histone and histone variant components of centromeric chromatin. Importantly, RSC is localized to centromeric and centromere-proximal chromosomal regions, and its association with these loci is dependent on Sth1p. Both sth1 and sfh1 mutants exhibit altered centromeric and centromere-proximal chromatin structure and increased missegregation of authentic chromosomes. Finally, RSC is not required for centromeric deposition of the histone H3 variant Cse4p, suggesting that RSC plays a role in reconfiguring centromeric and flanking nucleosomes following Cse4p recruitment for proper chromosome transmission.

Nuclear localization of the cell cycle regulator CDH1 and its regulation by phosphorylation

The anaphase-promoting complex activated by CDC20 and CDH1 is a major ubiquitination system that controls the destruction of cell cycle regulators. Exactly how ubiquitination is regulated in time and space is incompletely understood. Here we report on the cell cycle-dependent localization of CDH1 and its regulation by phosphorylation. CDH1 localizes dynamically to the nucleus during interphase and to the centrosome during metaphase and anaphase. The nuclear accumulation of CDH1 correlates with a reduction in the steady-state amount of cyclin A, but not of cyclin E. A nuclear localization signal conserved in various species was identified in CDH1, and it sufficiently targets green fluorescent protein to the nucleus. Interestingly, a CDH1-4D mutant mimicking the hyperphosphorylated form was constitutively found in the cytoplasm. In further support of the notion that phosphorylation inhibits nuclear import, the nuclear localization signal of CDH1 with two phospho-accepting serine/threonine residues changed into aspartates was unable to drive heterologous protein into the nucleus. On the other hand, abolition of the cyclin-binding ability of CDH1 has no influence on its nuclear localization. Taken together, our findings document the phosphorylation-dependent localization of CDH1 in vertebrate cells.

Human CENP-I specifies localization of CENP-F, MAD1 and MAD2 to kinetochores and is essential for mitosis

The kinetochore, a macromolecular complex located at the centromere of chromosomes, provides essential functions for accurate chromosome segregation. Kinetochores contain checkpoint proteins that monitor attachments between the kinetochore and microtubules to ensure that cells do not exit mitosis in the presence of unaligned chromosomes. Here we report that human CENP-I, a constitutive protein of the kinetochore that shares limited similarity with Mis6 of Schizosaccharomyces pombe, is required for the localization of CENP-F and the checkpoint proteins MAD1 and MAD2 to kinetochores. Depletion of CENP-I from kinetochores causes the cell cycle to delay in G2. Although monopolar chromosomes in CENP-I-depleted cells fail to establish bipolar connections, the cells are unable to arrest in mitosis. These cells are transiently delayed in mitosis in a MAD2-dependent manner, even though their kinetochores are depleted of MAD2. The delay is extended considerably when the number of unattached kinetochores is increased. This suggests that no single unattached kinetochore in CENP-I-depleted cells can arrest mitosis. The collective output from many unattached kinetochores is required to reach a threshold signal of 'wait for anaphase' to sustain a prolonged mitotic arrest.

Molecular cloning and characterization of the human budding uninhibited by benomyl (BUB3) promoter

Recently, cDNA corresponding to the human homologue of the BUB3 (budding uninhibited by benomyl) mitotic checkpoint protein has been identified and cloned. Previous studies from our laboratory and others have found this gene to localize to 10q26, a region that is frequently altered in various human cancers. We describe here a series of studies designed to understand the genomic structure of BUB3, particularly as it relates to regulation of gene expression. The human BUB3 gene has seven exons and six introns, and spans a genomic region of over 16 kb. The four WD repeat sequences in this gene are localized to exons 2, 4, and 6, and there is a major transcriptional start site located 554 nucleotides upstream of the ATG translation initiator codon. The promoter region lacks a TATA box but contains potential binding sites for the transcriptional factors including SP1, E2F, c-Myc, C/EBP and NFkappaB. To analyse the regulatory mechanisms controlling hBUB3 gene expression, we characterized the 5'-flanking region from nucleotide -1.3 to +0.58 kb by cloning various potions of this region in front of a luciferase reporter sequence. These experiments indicate that this region 5' region contains distinctive positive and negative regulatory elements.

An essential function of yeast cyclin-dependent kinase Cdc28 maintains chromosome stability

Multiple surveillance pathways maintain genomic integrity in yeast during mitosis. Although the cyclin-dependent kinase Cdc28 is a well established regulator of mitotic progression, evidence for a direct role in mitotic surveillance has been lacking. We have now implicated a conserved sequence in the Cdc28 carboxyl terminus in maintaining chromosome stability through mitosis. Six temperature-sensitive mutants were isolated via random mutagenesis of 13 carboxyl-terminal residues. These mutants identify a Cdc28 domain necessary for proper mitotic arrest in the face of kinetochore defects or microtubule inhibitors. These chromosome stability-defective cdc28(CST) mutants inappropriately continue mitosis when the mitotic spindle is disrupted at 23 degrees C, display high rates of spontaneous chromosome loss at 30 degrees C, and suffer catastrophic aneuploidy at 35 degrees C. A dosage suppression screen identified Cak1, a kinase known to phosphorylate and activate Cdc28, as a specific high copy suppressor of cdc28(CST) temperature sensitivity and chromosome instability. Suppression is independent of the kinase activity of Cak1, suggesting that Cak1 may bind to the carboxyl terminus to serve a non-catalytic role in assembly and/or stabilization of active Cdc28 complexes. Significantly, these studies implicate Cdc28 and Cak1 in an essential surveillance function required to maintain genetic stability through mitosis.

Identification of a MAD2-binding protein, CMT2, and its role in mitosis

MAD2 is a key component of the spindle checkpoint that delays the onset of anaphase until all the kinetochores are attached to the spindle. It binds to human p55CDC and prevents it from promoting destruction of an anaphase inhibitor, securin. Here we report the characterization of a novel MAD2-binding protein, CMT2. Upon the completion of spindle attachment, formation of the CMT2-MAD2 complex coincides with dissociation of the p55CDC-MAD2 complex. Overexpression of CMT2 in cells arrested by the spindle checkpoint causes premature destruction of securin and allows exit from mitosis without chromosome segregation. Depletion of CMT2 induces cell death following a transient delay in the onset of anaphase. These results indicate that CMT2 interacts with the spindle checkpoint and coordinates cell cycle events in late mitosis.

The human papillomavirus type 16 E6 oncogene induces premature mitotic chromosome segregation

Expression of the human papillomavirus type 16 E6 and E7 oncogenes initiates and maintains abnormal cell replication, by interacting with the p53 and retinoblastoma (Rb) gene products. Subsequent changes in host cell gene expression, as a consequence of genetic instability, can result in progression to invasive carcinoma. In addition to previously described effects of these viral oncogenes on centrosome synthesis, primarily associated with the expression of E7, the results described herein demonstrate that the E6 oncogene can induce premature chromosome segregation in human cells.

MG-132, an inhibitor of proteasomes and calpains, induced inhibition of oocyte maturation and aneuploidy in mouse oocytes

BACKGROUND: Although chromosome missegregation during oocyte maturation (OM) is a significant contributor to human morbidity and mortality, very little is known about the causes and mechanisms of aneuploidy. Several investigators have proposed that temporal perturbations during OM predispose oocytes to aberrant chromosome segregation. One approach for testing this proposal is to temporarily inhibit the activity of protein proteolysis during OM. We used the reversible proteasome inhibitor MG-132 to transiently perturb the temporal sequence of events during OM and subsequently analyzed mouse metaphase II (MII) for cytogenetic abnormalities. The transient inhibition of proteasome activity by MG-132 resulted in elevated levels of oocytes containing extra chromatids and chromosomes. RESULTS: The transient inhibition of proteasome-mediated proteolysis during OM by MG-132 resulted in dose-response delays during OM and elevated levels of aneuploid MII oocytes. Oocytes exposed in vitro to MG-132 exhibited greater delays during metaphase I (MI) as demonstrated by significantly (p < 0.01) higher levels of MI arrested oocytes and lower frequencies of premature sister chromatid separation in MII oocytes. Furthermore, the proportions of MII oocytes containing single chromatids and extra chromosomes significantly (p < 0.01) increased with MG-132 dosage. CONCLUSIONS: These data suggest that the MG-132-induced transient delay of proteasomal activity during mouse OM in vitro predisposed oocytes to abnormal chromosome segregation. Although these findings support a relationship between disturbed proteasomal activity and chromosome segregation, considerable additional data are needed to further investigate the roles of proteasome-mediated proteolysis and other potential molecular mechanisms on chromosome segregation during OM.

Activation of p38 mitogen-activated protein kinase during normal mitosis in the developing retina

The p38 member of the mitogen-activated protein kinase superfamily is engaged by phosphorylation in response to environmental stress signals, and may have either permissive or inhibitor roles upon cell proliferation. The cell cycle in the proliferative zone of the retina is tightly controlled and proceeds in synchrony with interkinetic migration of the neuroblast nuclei. We examined the association of p38 kinase activity with the cell cycle in the normal, non-stressed retina of the developing rat, maintained either in vivo or in vitro. Using immunohistochemistry, we show that mitotic profiles in the developing retina are highly enriched for phosphorylated p38. Blockade of p38 activity with the chemical inhibitor SB203580 for 4 h transiently arrested cells at the metaphase-anaphase transition and induced cell death after 20 h. p38 inhibition induced an aberrant mitotic profile, with chromosomes arranged in one side of the cell. The data show that p38 is active during normal mitosis and we suggest that p38 is required for the proper cell cycle progression during metaphase-anaphase transition in retinal neuroblasts.

Aneuploidy, centrosome activity and chromosome instability in cells deficient in homologous recombination repair

Chromosome instability and loss or gain of chromosomes are changes characteristic of many tumour cells and human disorders. However, the mechanism of these changes has not yet been fully determined. We have recently shown that hamster cell lines deficient in homologous recombination repair (HRR) genes XRCC2 and XRCC3 have an elevated frequency of aneuploidy compared with wild-type cells and mutant cells transfected with the appropriate human gene. In addition, XRCC2 and XRCC3 deficient hamster cell lines show a high frequency of multiple centrosomes and abnormal spindle formation. Cells deficient in HRR show a high frequency of both chromosome-type and chromatid-type aberrations, which could potentially lead to mis-segregation. The role of chromosome aberrations and other factors, including chromosome lagging, premature chromatid separation, and centrosome malfunctioning on chromosome mis-segregation in irs1 and irs1SF cells have been investigated. In particular, the linkage of DNA repair proteins with centrosomes suggests a key role for the centrosome in controlling cellular repair processes.

LIS1, CLIP-170's key to the dynein/dynactin pathway

CLIP-170 is a plus-end tracking protein which may act as an anticatastrophe factor. It has been proposed to mediate the association of dynein/dynactin to microtubule (MT) plus ends, and it also binds to kinetochores in a dynein/dynactin-dependent fashion, both via its C-terminal domain. This domain contains two zinc finger motifs (proximal and distal), which are hypothesized to mediate protein-protein interactions. LIS1, a protein implicated in brain development, acts in several processes mediated by the dynein/dynactin pathway by interacting with dynein and other proteins. Here we demonstrate colocalization and direct interaction between CLIP-170 and LIS1. In mammalian cells, LIS1 recruitment to kinetochores is dynein/dynactin dependent, and recruitment there of CLIP-170 is dependent on its site of binding to LIS1, located in the distal zinc finger motif. Overexpression of CLIP-170 results in a zinc finger-dependent localization of a phospho-LIS1 isoform and dynactin to MT bundles, raising the possibility that CLIP-170 and LIS1 regulate dynein/dynactin binding to MTs. This work suggests that LIS1 is a regulated adapter between CLIP-170 and cytoplasmic dynein at sites involved in cargo-MT loading, and/or in the control of MT dynamics.

Beyond the ABCs of CKC and SCC. Do centromeres orchestrate sister chromatid cohesion or vice versa?

The centromere-kinetochore complex is a highly specialized chromatin domain that both mediates and monitors chromosome-spindle interactions responsible for accurate partitioning of sister chromatids to daughter cells. Centromeres are distinguished from adjacent chromatin by specific patterns of histone modification and the presence of a centromere-specific histone H3 variant (e.g. CENP-A). Centromere-proximal regions usually correspond to sites of avid and persistent sister chromatid cohesion mediated by the conserved cohesin complex. In budding yeast, there is a substantial body of evidence indicating centromeres direct formation and/or stabilization of centromere-proximal cohesion. In other organisms, the dependency of cohesion on centromere function is not as clear. Indeed, it appears that pericentromeric heterochromatin recruits cohesion proteins independent of centromere function. Nonetheless, aspects of centromere function are impaired in the absence of sister chromatid cohesion, suggesting the two are interdependent. Here we review the nature of centromeric chromatin, the dynamics and regulation of sister chromatid cohesion, and the relationship between the two.

Mutational analysis of the central centromere targeting domain of human centromere protein C, (CENP-C)

Human centromere protein C (CENP-C) is an essential component of the inner kinetochore plate. A central region of CENP-C can bind DNA in vitro and is sufficient for targeting the protein to centromeres in vivo, raising the possibility that this domain mediates centromere localization via direct DNA binding. We performed a detailed molecular dissection of this domain to understand the mechanism by which CENP-C assembles at centromeres. By a combination of PCR mutagenesis and transient expression of GFP-tagged proteins in HeLa cells, we identified mutations that disrupt centromere localization of CENP-C in vivo. These cluster in a 12 amino acid region adjacent to the core domain required for in vitro DNA binding. This region is conserved between human and mouse, but is divergent or absent in invertebrate and plant CENP-C homologues. We suggest that these 12 amino acids are essential to confer specificity to DNA binding by CENP-C in vivo, or to mediate interaction with another as yet unidentified centromere component. A differential yeast two-hybrid screen failed to identify interactions specific to this sequence, but nonetheless identified 14 candidate proteins that interact with the central region of CENP-C. This collection of mutations and interacting proteins comprise a useful resource for further elucidating centromere assembly.

CCAAT/enhancer binding protein alpha uses distinct domains to prolong pituitary cells in the growth 1 and DNA synthesis phases of the cell cycle

BACKGROUND

A number of transcription factors coordinate differentiation by simultaneously regulating gene expression and cell proliferation. CCAAT/enhancer binding protein alpha (C/EBPalpha) is a basic/leucine zipper transcription factor that integrates transcription with proliferation to regulate the differentiation of tissues involved in energy balance. In the pituitary, C/EBPalpha regulates the transcription of a key metabolic regulator, growth hormone.

RESULTS

We examined the consequences of C/EBPalpha expression on proliferation of the transformed, mouse GHFT1-5 pituitary progenitor cell line. In contrast to mature pituitary cells, GHFT1-5 cells do not contain C/EBPalpha. Ectopic expression of C/EBPalpha in the progenitor cells resulted in prolongation of both growth 1 (G1) and the DNA synthesis (S) phases of the cell cycle. Transcription activation domain 1 and 2 of C/EBPalpha were required for prolongation of G1, but not of S. Some transcriptionally inactive derivatives of C/EBPalpha remained competent for G1 and S phase prolongation. C/EBPalpha deleted of its leucine zipper dimerization functions was as effective as full-length C/EBPalpha in prolonging G1 and S.

CONCLUSION

We found that C/EBPalpha utilizes mechanistically distinct activities to prolong the cell cycle in G1 and S in pituitary progenitor cells. G1 and S phase prolongation did not require that C/EBPalpha remained transcriptionally active or retained the ability to dimerize via the leucine zipper. G1, but not S, arrest required a domain overlapping with C/EBPalpha transcription activation functions 1 and 2. Separation of mechanisms governing proliferation and transcription permits C/EBPalpha to regulate gene expression independently of its effects on proliferation.

Phenotypic characterization ofDrosophila idamutants: defining the role of APC5 in cell cycle progression

We have cloned and characterized the ida gene that is required for proliferation of imaginal disc cells during Drosophila development. IDA is homologous to APC5, a subunit of the anaphase-promoting complex(APC/cyclosome). ida mRNA is detected in most cell types throughout development, but it accumulates to its highest levels during early embryogenesis. A maternal component of IDA is required for the production of eggs and viable embryos. Homozygous ida mutants display mitotic defects: they die during prepupal development, lack all mature imaginal disc structures, and have abnormally small optic lobes. Cytological observations show that ida mutant brains have a high mitotic index and many imaginal cells contain an aneuploid number of aberrant overcondensed chromosomes. However, cells are not stalled in metaphase, as mitotic stages in which chromosomes are orientated at the equatorial plate are never observed. Interestingly, some APC/C-target substrates such as cyclin B are not degraded in ida mutants, whereas others controlling sister-chromatid separation appear to be turned over. Taken together, these results suggest a model in which IDA/APC5 controls regulatory subfunctions of the anaphase-promoting complex.

Kinesinsklp5+ andklp6+ are required for normal chromosome movement in mitosis

Proper mitotic chromosome segregation requires dynamic interactions between spindle microtubules and kinetochores. Here we demonstrate that two related fission yeast kinesins, klp5+ and klp6+, are required for normal chromosome segregation in mitosis. Null mutants frequently lack a normal metaphase chromosome alignment. Chromosome pairs move back and forth along the spindle for an extended period prior to sister chromatid separation, a phenotype reminiscent of the loss of CENP-E in metazoans. Ultimately, sister chromatids segregate, regardless of chromosome position along the spindle, and viable daughter cells are usually produced. The initiation of anaphase B is sometimes delayed, but the rate of spindle elongation is similar to wildtype. Despite a delay, anaphase B often begins before anaphase A is completed. The klp5Δ and klp6Δ null mutants are synthetically lethal with a deletion of the spindle assembly checkpoint gene, bub1+, several mutants in components of the anaphase promoting complex, and a cold sensitive allele of the kinetochore and microtubule-binding protein, Dis1p. Klp5p-GFP and Klp6p-GFP localize to kinetochores from prophase to the onset of anaphase A, but relocalize to the spindle midzone during anaphase B. These data indicate that Klp5p and Klp6p are kinetochore kinesins required for normal chromosome movement in prometaphase.

Complementary whole-genome technologies reveal the cellular response to proteasome inhibition by PS-341

Although the biochemical targets of most drugs are known, the biological consequences of their actions are typically less well understood. In this study, we have used two whole-genome technologies in Saccharomyces cerevisiae to determine the cellular impact of the proteasome inhibitor PS-341. By combining population genomics, the screening of a comprehensive panel of bar-coded mutant strains, and transcript profiling, we have identified the genes and pathways most affected by proteasome inhibition. Many of these function in regulated protein degradation or a subset of mitotic activities. In addition, we identified Rpn4p as the transcription factor most responsible for the cell's ability to compensate for proteasome inhibition. Used together, these complementary technologies provide a general and powerful means to elucidate the cellular ramifications of drug treatment.

Dual control of replication timing. Stochastic onset but programmed completion of mammalian chromosome duplication

In mammalian cells, DNA replication proceeds according to a precise temporal order during the S phase, but how this program is controlled remains poorly understood. We analyzed the replication-dependent bromodeoxyuridine banding of chromosomes in Chinese hamster cells treated with the spindle poison nocodazole. In these cells, nocodazole induces a transient mitotic arrest, followed by DNA re-replication without intervening cell division. Nuclear fragmentation is often observed in tetraploid derivatives, and previous studies suggest that replication timing of chromosomes could be affected when they are segregated into different micronuclei. Here we show that the onset of replication is frequently asynchronous on individual chromosomes during the re-replication process. Moreover, fluorescence in situ hybridization analysis revealed that replication synchrony is equally altered in fragmented and non-fragmented nuclei, indicating that asynchronous onset of replication is not dependent on physical separation of the chromosomes into isolated compartments. We also show that the ordered program of replication is always preserved along individual chromosomes. Our results demonstrate that the onset of replication of individual chromosomes in the same nuclear compartment can be uncoupled from the time of S-phase entry and from the programmed replication of chromosome sub-domains, revealing that multi-level controls contribute to establish replication timing in mammalian cells.