Consequences of topoisomerase II inhibition in early embryogenesis of Drosophila revealed by in vivo confocal laser scanning microscopy

Mechanisms of Secondary Leukemia Development Caused by Treatment with DNA Topoisomerase Inhibitors

Leukemia is a blood cancer originating in the blood and bone marrow. Therapy-related leukemia is associated with prior chemotherapy. Although cancer therapy with DNA topoisomerase II inhibitors is one of the most effective cancer treatments, its side effects include development of secondary leukemia characterized by the chromosomal rearrangements affecting AML1 or MLL genes. Recurrent chromosomal translocations in the therapy-related leukemia differ from chromosomal rearrangements associated with other neoplasias. Here, we reviewed the factors that drive chromosomal translocations induced by cancer treatment with DNA topoisomerase II inhibitors, such as mobility of ends of double-strand DNA breaks formed before the translocation and gain of function of fusion proteins generated as a result of translocation.

Downregulation of Hsp90 and the antimicrobial peptide Mtk suppresses poly(GR)-induced neurotoxicity in C9ORF72-ALS/FTD

GGGGCC repeat expansion in the C9ORF72 gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Repeat RNAs can be translated into dipeptide repeat proteins, including poly(GR), whose mechanisms of action remain largely unknown. In an RNA-seq analysis of poly(GR) toxicity in Drosophila, we found that several antimicrobial peptide genes, such as metchnikowin (Mtk), and heat shock protein (Hsp) genes are activated. Mtk knockdown in the fly eye or in all neurons suppresses poly(GR) neurotoxicity. These findings suggest a cell-autonomous role of Mtk in neurodegeneration. Hsp90 knockdown partially rescues both poly(GR) toxicity in flies and neurodegeneration in C9ORF72 motor neurons derived from induced pluripotent stem cells (iPSCs). Topoisomerase II (TopoII) regulates poly(GR)-induced upregulation of Hsp90 and Mtk. TopoII knockdown also suppresses poly(GR) toxicity in Drosophila and improves survival of C9ORF72 iPSC-derived motor neurons. These results suggest potential novel therapeutic targets for C9ORF72-ALS/FTD.

Topoisomerase II is regulated by translationally controlled tumor protein for cell survival during organ growth in Drosophila

Regulation of cell survival is critical for organ development. Translationally controlled tumor protein (TCTP) is a conserved protein family implicated in the control of cell survival during normal development and tumorigenesis. Previously, we have identified a human Topoisomerase II (TOP2) as a TCTP partner, but its role in vivo has been unknown. To determine the significance of this interaction, we examined their roles in developing Drosophila organs. Top2 RNAi in the wing disc leads to tissue reduction and caspase activation, indicating the essential role of Top2 for cell survival. Top2 RNAi in the eye disc also causes loss of eye and head tissues. Tctp RNAi enhances the phenotypes of Top2 RNAi. The depletion of Tctp reduces Top2 levels in the wing disc and vice versa. Wing size is reduced by Top2 overexpression, implying that proper regulation of Top2 level is important for normal organ development. The wing phenotype of Tctp RNAi is partially suppressed by Top2 overexpression. This study suggests that mutual regulation of Tctp and Top2 protein levels is critical for cell survival during organ development.

A Topology-Centric View on Mitotic Chromosome Architecture

Mitotic chromosomes are long-known structures, but their internal organization and the exact process by which they are assembled are still a great mystery in biology. Topoisomerase II is crucial for various aspects of mitotic chromosome organization. The unique ability of this enzyme to untangle topologically intertwined DNA molecules (catenations) is of utmost importance for the resolution of sister chromatid intertwines. Although still controversial, topoisomerase II has also been proposed to directly contribute to chromosome compaction, possibly by promoting chromosome self-entanglements. These two functions raise a strong directionality issue towards topoisomerase II reactions that are able to disentangle sister DNA molecules (in trans) while compacting the same DNA molecule (in cis). Here, we review the current knowledge on topoisomerase II role specifically during mitosis, and the mechanisms that directly or indirectly regulate its activity to ensure faithful chromosome segregation. In particular, we discuss how the activity or directionality of this enzyme could be regulated by the SMC (structural maintenance of chromosomes) complexes, predominantly cohesin and condensin, throughout mitosis.

Ki-67 and the Chromosome Periphery Compartment in Mitosis

The chromosome periphery is a complex network of proteins and RNA molecules (many derived from nucleoli) that covers the outer surface of chromosomes and whose function remains mysterious. Although it was first described over 130 years ago, technological advances and the recent discovery that Ki-67 acts as an organiser of this region have allowed the chromosome periphery to be dissected in previously unattainable detail, leading to a revival of interest in this obscure chromosomal compartment. Here, we review the most recent advances into the composition, structure and function of the chromosome periphery, discuss possible roles of Ki-67 during mitosis and consider why this structure is likely to remain the focus of ongoing attention in the future.

A review of methods for analysing insect structures - the role of morphology in the age of phylogenomics

Techniques currently used in insect morphology are outlined briefly. Scanning electron microscopy (SEM) and microphotography are used mainly for documenting external features, the former providing more information on tiny surface structures and the latter on coloration, transparency and degree of sclerotization. A broad spectrum of methods is now available for anatomical studies: histological serial sections, confocal laser scanning microscopy (CLSM), light-sheet fluorescence microscopy (LSFM), serial block-face scanning electron microscopy (SBFSEM), dual beam scanning electron microscopy (FIB-SEM), nuclear magnetic resonance imaging (NMRI), and μ-computed tomography (micro-CT). The use of SBFSEM and FIB-SEM is restricted to extremely small samples. NMRI is used mainly in in vivo studies. Micro-computed tomography, in combination with computer-based reconstruction, has greatly accelerated the acquisition of high quality data in a phylogenetic context. Morphology will continue to play a vital role in phylogenetic and evolutionary investigations. It provides independent data for checking the plausibility of molecular phylogenies and is the only source of information for placing extinct taxa. It is the necessary basis for reconstructing character evolution on the phenotypic level and for developing complex evolutionary scenarios. Computer-based anatomical ontologies are an additional future perspective of morphological work.

Novel insights into mitotic chromosome condensation

The fidelity of mitosis is essential for life, and successful completion of this process relies on drastic changes in chromosome organization at the onset of nuclear division. The mechanisms that govern chromosome compaction at every cell division cycle are still far from full comprehension, yet recent studies provide novel insights into this problem, challenging classical views on mitotic chromosome assembly. Here, we briefly introduce various models for chromosome assembly and known factors involved in the condensation process (e.g. condensin complexes and topoisomerase II). We will then focus on a few selected studies that have recently brought novel insights into the mysterious way chromosomes are condensed during nuclear division.

Topoisomerase II is required for the proper separation of heterochromatic regions during Drosophila melanogaster female meiosis

Heterochromatic homology ensures the segregation of achiasmate chromosomes during meiosis I in Drosophila melanogaster females, perhaps as a consequence of the heterochromatic threads that connect achiasmate homologs during prometaphase I. Here, we ask how these threads, and other possible heterochromatic entanglements, are resolved prior to anaphase I. We show that the knockdown of Topoisomerase II (Top2) by RNAi in the later stages of meiosis results in a specific defect in the separation of heterochromatic regions after spindle assembly. In Top2 RNAi-expressing oocytes, heterochromatic regions of both achiasmate and chiasmate chromosomes often failed to separate during prometaphase I and metaphase I. Heterochromatic regions were stretched into long, abnormal projections with centromeres localizing near the tips of the projections in some oocytes. Despite these anomalies, we observed bipolar spindles in most Top2 RNAi-expressing oocytes, although the obligately achiasmate 4^th chromosomes exhibited a near complete failure to move toward the spindle poles during prometaphase I. Both achiasmate and chiasmate chromosomes displayed defects in biorientation. Given that euchromatic regions separate much earlier in prophase, no defects were expected or observed in the ability of euchromatic regions to separate during late prophase upon knockdown of Top2 at mid-prophase. Finally, embryos from Top2 RNAi-expressing females frequently failed to initiate mitotic divisions. These data suggest both that Topoisomerase II is involved in the resolution of heterochromatic DNA entanglements during meiosis I and that these entanglements must be resolved in order to complete meiosis.

Proper chromosome segregation during egg and sperm development is crucial to prevent birth defects and miscarriage. During chromosome replication, DNA entanglements are created that must be resolved before chromosomes can fully separate. In the oocytes of the fruit fly Drosophila melanogaster, DNA entanglements persist between heterochromatic regions of the chromosomes until after spindle assembly and may facilitate the proper segregation of chromosomes during meiosis. Topoisomerase II enzymes can resolve DNA entanglements by cutting and untwisting tangled DNA. Decreasing Topoisomerase II (Top2) levels in the ovaries of fruit flies led to sterility. RNAi knockdown of the Top2 gene in oocytes resulted in chromosomes that failed to fully separate their heterochromatic regions during meiosis I and caused oocytes to arrest in meiosis I. These studies demonstrate that the Top2 enzyme is required for releasing DNA entanglements between homologous chromosomes before the onset of chromosome segregation during Drosophila female meiosis.

The analysis of mutant alleles of different strength reveals multiple functions of topoisomerase 2 in regulation of Drosophila chromosome structure

Topoisomerase II is a major component of mitotic chromosomes but its role in the assembly and structural maintenance of chromosomes is rather controversial, as different chromosomal phenotypes have been observed in various organisms and in different studies on the same organism. In contrast to vertebrates that harbor two partially redundant Topo II isoforms, Drosophila and yeasts have a single Topo II enzyme. In addition, fly chromosomes, unlike those of yeast, are morphologically comparable to vertebrate chromosomes. Thus, Drosophila is a highly suitable system to address the role of Topo II in the assembly and structural maintenance of chromosomes. Here we show that modulation of Top2 function in living flies by means of mutant alleles of different strength and in vivo RNAi results in multiple cytological phenotypes. In weak Top2 mutants, meiotic chromosomes of males exhibit strong morphological abnormalities and dramatic segregation defects, while mitotic chromosomes of larval brain cells are not affected. In mutants of moderate strength, mitotic chromosome organization is normal, but anaphases display frequent chromatin bridges that result in chromosome breaks and rearrangements involving specific regions of the Y chromosome and 3L heterochromatin. Severe Top2 depletion resulted in many aneuploid and polyploid mitotic metaphases with poorly condensed heterochromatin and broken chromosomes. Finally, in the almost complete absence of Top2, mitosis in larval brains was virtually suppressed and in the rare mitotic figures observed chromosome morphology was disrupted. These results indicate that different residual levels of Top2 in mutant cells can result in different chromosomal phenotypes, and that the effect of a strong Top2 depletion can mask the effects of milder Top2 reductions. Thus, our results suggest that the previously observed discrepancies in the chromosomal phenotypes elicited by Topo II downregulation in vertebrates might depend on slight differences in Topo II concentration and/or activity.

Restoration of topoisomerase 2 function by complementation of defective monomers in Drosophila

Type II topoisomerases are essential ATP-dependent homodimeric enzymes required for transcription, replication, and chromosome segregation. These proteins alter DNA topology by generating transient enzyme-linked double-strand breaks for passage of one DNA strand through another. The central role of type II topoisomerases in DNA metabolism has made these enzymes targets for anticancer drugs. Here, we describe a genetic screen that generated novel alleles of DrosophilaTopoisomerase 2 (Top2). Fifteen alleles were obtained, resulting from nonsense and missense mutations. Among these, 14 demonstrated recessive lethality, with one displaying temperature-sensitive lethality. Several newly generated missense alleles carry amino acid substitutions in conserved residues within the ATPase, Topoisomerase/Primase, and Winged helix domains, including four that encode proteins with alterations in residues associated with resistance to cancer chemotherapeutics. Animals lacking zygotic Top2 function can survive to pupation and display reduced cell division and altered polytene chromosome structure. Inter se crosses between six strains carrying Top2 missense alleles generated morphologically normal trans-heterozygous adults, which showed delayed development and were female sterile. Complementation occurred between alleles encoding Top2 proteins with amino acid substitutions in the same functional domain and between alleles encoding proteins with substitutions in different functional domains. Two complementing alleles encode proteins with amino acid substitutions associated with drug resistance. These observations suggest that dimerization of mutant Top2 monomers can restore enzymatic function. Our studies establish the first series of Top2 alleles in a multicellular organism. Future analyses of these alleles will enhance our knowledge about the contributions made by type II topoisomerases to development.

Chromosome formation during fertilization in eggs of the teleost Oryzias latipes

Upon fertilization, eggs shift their cell cycle from the meiotic to the mitotic pattern for embryogenesis. The information on chromosome formation has been accumulated by various experiments using inhibitors to affect formation and behavior of chromosomes in the cycle of cell proliferation. Based on experimental results on meiosis and early stages of development of the teleost Oryzias latipes, we discuss the roles of the activities of histone H1 kinase, microtubule-associated protein kinase, DNA polymerase, DNA topoisomerase, and other cytoplasmic factors that play a crucial role in formation and separation of chromosomes.

The role of the DNA hypermethylating agent Budesonide in the decatenating activity of DNA topoisomerase II

Catenations between sister chromatids result from DNA replication and must be resolved to ensure proper chromatid segregation in mitosis. Functionally active Topoisomerase II (Topo II), through its mechanism of concerted breaking and rejoining of double stranded DNA, is required to carry out this fundamental process. In previous studies we have shown that modifications in DNA sequence by halogenated pyrimidines and by the demethylating agent 5-azacytidine leads to malfunction of Topo II that results in an increased yield of endorreduplicated cells as a result of segregation failure. In the present work we have evaluated the possible influence of the methylating agent Budesonide to modify the frequency of endoreduplicated cells in AA8 Chinese hamster cell population. Our results seem to indicate that when Budesonide was administered for two consecutive cell cycles did induce an increase in the yield of endoreduplicated cells, as previously observed for the hypomethylating agent 5-azaC. We have also examined the possible relationship between extensive hypermethylation induced by Budesonide in DNA and stabilization of cleavable complexes by m-AMSA. Taken as a whole, our results show that the degree of methylation in DNA correlates with the effectiveness of m-AMSA to stabilize the Topo II-DNA complexes and to induce DNA cleavage. These findings evidence for the first time the functional importance of DNA hyper- and hypomethylation changes as epigenetic factors able to modulate Topo II activity for proper chromosome segregation.

Cohesin cleavage and Cdk inhibition trigger formation of daughter nuclei

The metaphase-anaphase transition is orchestrated through proteolysis of numerous proteins by a ubiquitin protein ligase called the anaphase-promoting complex or cyclosome (APC/C). A crucial aspect of this process is sister chromatid separation, which is thought to be mediated by separase, a thiol protease activated by the APC/C. Separase cleaves cohesin, a ring-shaped complex that entraps sister DNAs. It is a matter of debate whether cohesin-independent forces also contribute to sister chromatid cohesion. Using 4D live-cell imaging of Drosophila melanogaster syncytial embryos blocked in metaphase (via APC/C inhibition), we show that artificial cohesin cleavage is sufficient to trigger chromosome disjunction. This is nevertheless insufficient for correct chromosome segregation. Kinetochore-microtubule attachments are rapidly destabilized by the loss of tension caused by cohesin cleavage in the presence of high Cdk1 (cyclin-dependent kinase 1) activity, as occurs when the APC/C cannot destroy mitotic cyclins. Metaphase chromosomes undergo a bona fide anaphase when cohesin cleavage is combined with Cdk1 inhibition. We conclude that only two key events, opening of cohesin rings and downregulation of Cdk1, are sufficient to drive proper segregation of chromosomes in anaphase.

Differential requirements of a mitotic acetyltransferase in somatic and germ line cells

During mitosis different types of cells can have differential requirements for chromosome segregation. We isolated two new alleles of the separation anxiety gene (san). san was previously described in both Drosophila and in humans to be required for centromeric sister chromatid cohesion (Hou et al., 2007; Williams et al., 2003). Our work confirms and expands the observation that san is required in vivo for normal mitosis of different types of somatic cells. In addition, we suggest that san is also important for the correct resolution of chromosomes, implying a more general function of this acetyltransferase. Surprisingly, during oogenesis we cannot detect mitotic defects in germ line cells mutant for san. We hypothesize the female germ line stem cells have differential requirements for mitotic sister chromatid cohesion.

Quantitative microinjection of Drosophila embryos: general strategy

INTRODUCTIONMicroinjection of Drosophila embryos is a common technique used by a wide range of investigators, but some applications require a refined strategy for handling embryos. This article outlines the general procedures for microinjection and quantification of aqueous solutions during high-resolution observation of early development in the fly embryo. It also describes the design of suitable support slides for the manipulation of Drosophila embryos under upright and inverted microscopes.

Drosophila under the lens: imaging from chromosomes to whole embryos

Microscopy has been a very powerful tool for Drosophila research since its inception, proving to be essential for the evaluation of mutant phenotypes, the understanding of cellular and tissue physiology, and the illumination of complex biological questions. In this article we review the breadth of this field, making note of some of the seminal papers. We expand on the use of microscopy to study questions related to gene locus and nuclear architecture, presenting new data using fluorescence in-situ hybridization techniques that demonstrate the flexibility of Drosophila chromosomes. Finally, we review the burgeoning use of fluorescence in-vivo imaging methods to yield quantitative information about cellular processes.

Paternal chromosome segregation during the first mitotic division determines Wolbachia-induced cytoplasmic incompatibility phenotype

The most common Wolbachia-induced phenotype in insects is cytoplasmic incompatibility (CI), which occurs when sperm from infected males fertilize eggs from uninfected females. CI produces distinct phenotypes in three closely related haplo-diploid species of the genus Nasonia: mortality in N. longicornis and N. giraulti, and conversion to male development in N. vitripennis. We demonstrate that the majority of CI-induced mortality occurs during embryogenesis and that the pattern of paternal chromosome segregation during the first mitosis is a good predictor of CI phenotype. In N. giraulti and N. longicornis, the paternal chromosomes mis-segregate, producing abnormal nuclei connected by chromatin bridges. Consequently, these embryos arrest development with very few and abnormal nuclei. In contrast, the paternal genome in N. vitripennis is either not segregated or mis-segregates to one of the two daughter nuclei. Consequently, these embryos continue development utilizing the maternally derived haploid nuclei, resulting in male offspring. The latter class is the first documented example of asymmetric mitotic segregation of abnormal chromosomes. We conclude that in haplo-diploids, CI-induced embryonic lethality occurs only when abnormal paternal genome segregation affects both products of the first mitotic division. This is the first study to associate differences in types of CI with specific cytological defects.

Halogen substitution of DNA protects from poisoning of topoisomerase II that results in DNA double-strand breaks

DNA topoisomerase II (topo II), a fundamental nuclear enzyme, cleaves the double-stranded DNA molecule at preferred sequences within its recognition/binding sites. We have recently reported [F. Cortés, N. Pastor, S. Mateos, I. Domínguez, The nature of DNA plays a role in chromosome segregation: endoreduplication in halogen-substituted chromosomes, DNA Repair 2 (2003) 719-726] that when cells incorporate halogenated nucleosides analogues of thymidine into DNA, it interferes with normal chromosome segregation, as shown by an extraordinarily high yield of endoreduplication. The frequency of endoreduplicated cells paralleled the level of analogue substitution into DNA, lending support to the idea that thymidine analogue substitution into DNA is most likely responsible for the triggering of endoreduplication. Using the pulsed-field gel electrophoresis (PFGE) technique, we have now analyzed a possible protection provided by the incorporation of exogenous halogenated nucleosides against DNA breakage induced by the topo II poison m-AMSA. The result was that the different halogenated nucleosides were shown as able to protect DNA from double-strand breaks induced by m-AMSA depending such a protection upon the relative percent of incorporation of a given thymidine analogue into DNA. Our results clearly indicate that the presence of halogenated nucleosides in DNA diminishes the frequency of interaction of topo II with DNA and thus the frequency with which cleavage can occur.

A genetic test of the role of the maternal pronucleus in Wolbachia-induced cytoplasmic incompatibility in Drosophila melanogaster

Cytoplasmic incompatibility (CI) is a reproductive sterility found in arthropods that is caused by the endoparasitic bacteria Wolbachia. In CI, host progeny fail to develop during early embryogenesis if Wolbachia-infected males fertilize uninfected females. It is widely accepted that this lethality is caused by some unknown Wolbachia-induced modification of the paternal nuclear material in the host testes. However, the direct means by which this modification leads to early embryonic death are currently unresolved. Results from previous studies suggested that CI lethality occurs as a result of asynchrony in cell cycle timing between the paternal and maternal pronuclei. This hypothesis can be tested experimentally by the prediction that the Wolbachia-modified paternal pronucleus should support androgenetic development (i.e., from the paternal pronucleus only). Using specific mutations in Drosophila melanogaster that produce androgenetic progeny, we demonstrate that the Wolbachia-induced modification inhibits this type of development. This result suggests that CI occurs independently of the maternal pronucleus and argues against pronuclear asynchrony as the primary cause of CI lethality. We propose that CI occurs instead as the result of either a developmentally incompetent paternal pronucleus or asynchrony between the paternal pronucleus and the cell cycle of the egg cytoplasm.

Cisplatin-induced endoreduplication in CHO cells: DNA damage and inhibition of topoisomerase II

It has been proposed that polyploid cells that arise during a variety of pathological conditions and as a result of exposure to genotoxicants, typically in the liver, become aneuploid through genetic instability. Aneuploidy contributes to, or even drives, tumour development. We have assessed the capacity of the drug cisplatin, one of the most commonly used compounds for the treatment of malignancies, to induce endoreduplication, a particular type of polyploidy, in cultured Chinese hamster AA8 cells. Taking into account that any interference with DNA topoisomerase II (topo II) function leads to endoreduplication, we have found that treatment of the cells with this platinum compound results in a dose-dependent inhibition of the catalytic activity of the enzyme. These observations are discussed on the basis of a possible dual action of cisplatin leading to a combined negative effect on normal segregation of chromosomes. On the one hand, through the drug capacity to efficiently inhibiting the catalytic activity of topo II itself and, on the other hand, as a consequence of changes in DNA such as base modifications and cross-links that result from cisplatin treatment, likely leading to a lack of recognition/binding of DNA by the enzyme. These observations support a model in which the involvement of topo II in different pathways leading to induced endoreduplication has been proposed, and seem to bear significance as to the possible origin of the development of secondary tumours as a result of cisplatin treatment of primary malignancies.

Type II topoisomerase activities in both the G1 and G2/M phases of the dinoflagellate cell cycle

Dinoflagellate genomes are large (up to 200 pg) and are encoded in histoneless chromosomes that are quasi-permanently condensed. This unique combination of chromosomal characteristics presents additional topological and cell cycle control problems for a eukaryotic cell, potentially exhibiting novel regulatory requirements of topoisomerase II. The heterotrophic dinoflagellate Crypthecodinium cohnii was used in this study. The topoisomerase II activities throughout its cell cycle were investigated by DNA flow cytometry following enzyme deactivation. Fluorescence microscopy was also used for studying the chromosome morphology of the treated cells. Two classes of topoisomerase II inhibitors were applied in our study, both of which caused G1 delay as well as G2/M arrest in the C. cohnii cell cycle. At high doses, the topoisomerase poisons amsacrine and ellipticine induced DNA fragmentation in C. cohnii cells. Topoisomerase II activities, as measured by the ability to decatenate kinetoplastid DNA (kDNA), are normally detected throughout the cell cycle in C. cohnii. Our results suggest that the requirement of type II topoisomerase activities during the G1 phase of the cell cycle may relate to the unwinding of quasi-permanently condensed chromosomes for the purpose of transcription. This was also the first time that topoisomerase II activity in dinoflagellate cells was detected.

The DNA demethylating 5-azaC induces endoreduplication in cultured Chinese hamster cells

We have investigated the possible influence of 5-azacytidine (5-azaC) substitution for cytidine into DNA on topoisomerase II (topo II) function in chromosome segregation. The endpoint chosen has been the induction of endoreduplicated cells at mitosis showing diplochromosomes. Experiments were performed in the presence and absence of the cytidine analogue to assess the degree of 5-azaC-induced DNA hypomethylation, using differential cutting by restriction endonucleases Hpa II and Msp I. Using the pulsed-field gel electrophoresis (PFGE) technique, we have also observed a protective effect provided by 5-azaC treatment against DNA breakage induced by the topo II poison m-AMSA. Concentrations of 5-azaC shown as able to induce extensive DNA hypomethylation and capable to protect DNA from double-strand breaks induced by m-AMSA were used for our cytogenetic experiments to analyze chromosome segregation. Our results seem to indicate that the presence of 5-azaC in DNA induces a dose-dependent increase in the yield of endoreduplicated cells that parallels the levels of hypomethylation observed.

Toward a comprehensive model for induced endoreduplication

Both the biological significance and the molecular mechanism of endoreduplication (END) have been debated for a long time by cytogeneticists and researchers into cell cycle enzymology and dynamics alike. Mainly due to the fact that a wide variety of agents have been reported as able to induce endoreduplication and the diversity of cell types where it has been described, until now no clear or unique mechanism of induction of this phenomenon, rare in animals but otherwise quite common in plants, has been proposed. DNA topoisomerase II (topo II), plays a major role in mitotic chromosome segregation after DNA replication. The classical topo II poisons act by stabilizing the enzyme in the so-called cleavable complex and result in DNA damage as well as END, while the true catalytic inhibitors, which are not cleavable-complex-stabilizers, do induce END without concomitant DNA and chromosome damage. Taking into account these observations on the induction of END by drugs that interfere with topo II, together with our recently obtained evidence that the nature of DNA plays an important role for chromosome segregation [Cortes, F., Pastor, N., Mateos, S., Dominguez, I., 2003. The nature of DNA plays a role in chromosome segregation: endoreduplication in halogen-substituted chromosomes. DNA Repair 2, 719-726.], a straightforward model is proposed in which the different mechanisms leading to induced END are considered.

DNA topoisomerase II is required for the G0-to-S phase transition inDrosophilaSchneider cells, but not in yeast

We previously reported that DNA topoisomerase II (topo II) is required for the G(0)-to-S phase transition in mammalian cells [Hossain et al. (2002) ICRF-193, a catalytic inhibitor of DNA topoisomerase II, inhibits re-entry into the cell division cycle from quiescent state in mammalian cells. Genes Cells 7, 285-294]. In this study, we examined whether the requirement for topo II is evolutionarily conserved in Drosophila and yeast. ICRF-193, a catalytic inhibitor of topo II, inhibited DNA synthesis in Drosophila Schneider cells released from the G(0) (stationary) phase, whereas the drug did not inhibit DNA synthesis in Schneider cells released from the M phase. Depletion of topo II mRNA by RNA-interference (RNAi) in G(0)-phase Schneider cells resulted in significant inhibition of DNA synthesis after release from G(0)-arrest. In the yeast topo II temperature-sensitive (ts) mutant, the initial cycle of DNA synthesis occurred at a restrictive temperature after release from starvation-induced G(0) phase and doubling of the DNA content in the cells was confirmed by both flow cytometry and fluorescence spectrophotometry. DNA synthesis in yeast cells after release from the G(0) phase was also observed in the presence of ICRF-193. Doubling of the DNA content was observed during spore germination of topo II ts mutant yeast at a restrictive temperature as determined by fluorescence spectrophotometry. These results indicate that topo II is required for the G(0)-to-S phase transition in Drosophila Schneider cells, but not in yeast.

RNAi analysis reveals an unexpected role for topoisomerase II in chromosome arm congression to a metaphase plate

DNA topoisomerase II (Topo II) is a major component of mitotic chromosomes and an important drug target in cancer chemotherapy, however, its role in chromosome structure and dynamics remains controversial. We have used RNAi to deplete Topo II in Drosophila S2 cells in order to carry out a detailed functional analysis of the role of the protein during mitosis. We find that Topo II is not required for the assembly of a functional kinetochore or the targeting of chromosomal passenger proteins, nonetheless, it is essential for anaphase sister chromatid separation. In response to a long-running controversy, we show that Topo II does have some role in mitotic chromatin condensation. Chromosomes formed in its absence have a 2.5-fold decrease in the level of chromatin compaction, and are morphologically abnormal. However, it is clear that the overall programme of mitotic chromosome condensation can proceed without Topo II. Surprisingly, in metaphase cells depleted of Topo II, one or more chromosome arms frequently stretch out from the metaphase plate to the vicinity of the spindle pole. This is not kinetochore-based movement, as the centromere of the affected chromosome is located on the plate. This observation raises the possibility that further unexpected functions for Topo II may remain to be discovered.

The nature of DNA plays a role in chromosome segregation: endoreduplication in halogen-substituted chromosomes

AA8 Chinese hamster ovary cells were treated with halogenated nucleosides analogues of thymidine, namely CldU, 5-iodo-2'-deoxyuridine (IdU), and 5-bromo-2'-deoxyuridine (BrdU), following different experimental protocols. The purpose was to see whether incorporation of exogenous pyrimidine analogues into DNA could interfere with normal chromosome segregation. The endpoint chosen was endoreduplication, that arises after aberrant mitosis when daughter chromatids segregation fails. Treatment with any of the halogenated nucleosides for two consecutive cell cycles resulted in endoreduplication, with a highest yield for CldU, intermediate for IdU, and lowest for BrdU. The frequency of endoreduplicated cells paralleled in all cases the level of analogue substitution into DNA. Our results seem to support that thymidine analogue substitution into DNA is responsible for the triggering of endoreduplication. Besides, the lack of any effect on endoreduplication when CldU was present for only one S-period strongly suggest that it is the nature of template, and not nascent DNA, that plays a major role in chromosome segregation. Taking into account that topoisomerase II cleaves DNA at preferred sequences within its recognition/binding sites, the likely involvement of the enzyme is discussed.

The making of the mitotic chromosome: modern insights into classical questions

The condensation of mitotic chromosomes is essential for the faithful segregation of sister chromatids in anaphase. An emerging view is that chromosome assembly is an active and dynamic process of chromatin reorganization in which two ATP hydrolyzing enzymes, topoisomerase II and the condensin complex, play central roles. In this review, we discuss recent work that sheds new light on the molecular and structural dynamics of mitotic chromosomes.

Induction of endoreduplication by topoisomerase II catalytic inhibitors

The striking phenomenon of endoreduplication has long attracted attention from cytogeneticists and researchers into cell cycle enzymology and dynamics alike. Because of the variety of agents able to induce endoreduplication and the various cell types where it has been described, until now no clear or unique mechanism of induction of this phenomenon, rare in animals but otherwise quite common in plants, has been proposed. Recent years, however, have witnessed the unfolding of a number of essential physiological roles for DNA topoisomerase II, with special emphasis on its major role in mitotic chromosome segregation after DNA replication. In spite of the lack of mammalian mutants defective in topoisomerase II as compared with yeast, experiments with inhibitors of the enzyme have supported the hypothesis that this crucial untangling of daughter DNA molecules by passing an intact helix through a transient double-stranded break carried out by the enzyme, when it fails, leads to aberrant mitosis that results in endoreduplication, polyploidy and eventually cell death. Anticancer drugs that interfere with topoisomerase II can be classified into two groups. The classical poisons act by stabilizing the enzyme in the so-called cleavable complex and result in DNA damage, which represents a problem in the study of endoreduplication. The true catalytic inhibitors, which are not cleavable complex stabilizers, allow us to use doses efficient in the induction of endoreduplication while eliminating high levels of DNA and chromosome damage. This review will discuss the basic and applied aspects of this as yet scarcely explored field.

Roles of DNA topoisomerases in chromosome segregation and mitosis

DNA topoisomerases are highly specialized nuclear enzymes that perform topological changes in the DNA molecule in a very precise and unique fashion. Taking into account their fundamental roles in many events during DNA metabolism such as replication, transcription, recombination, condensation or segregation, it is no wonder that the last decade has witnessed an exponential interest on topoisomerases, mainly after the discovery of their potential role as targets in novel antitumor therapy. The difficulty of the lack of topoisomerase II mutants in higher eukaryotes has been partly overcome by the availability of drugs that act as either poisons or true catalytic inhibitors of the enzyme. These chemical tools have provided strong evidence that accurate performance of topoisomerase II is essential for chromosome segregation before anaphase, and this in turn constitutes a prerequisite for the development of normal mitosis. In the absence of cytokinesis, cells become polyploid or endoreduplicated.

Studies on fertilization in the teleost IV. Effects of aphidicolin and camptothecin on chromosome formation in fertilized medaka eggs

To clarify the mechanisms of fish fertilization, the effects of inhibitors of DNA polymerase-alpha and DNA topoisomerases on nuclear behavior before and after fertilization were examined in eggs of the medaka, Oryzias latipes. Eggs underwent the fertilization process from sperm penetration to karyogamy of pronuclei, even when inseminated and incubated in the continuous presence of aphidicolin (DNA polymerase alpha inhibitor), camptothecin (DNA topoisomerase I inhibitor), etoposide, or beta-lapachone (DNA topoisomerase II inhibitor). However, continuous treatment with aphidicolin or camptothecin during fertilization inhibited the formation of sister chromosomes that were normally separated into blastomeres at the time of the subsequent cleavage. Sister chromosome formation appeared concomitantly with an increase in histone H1 kinase activity at the end of DNA synthesis, 30 min post insemination. However, non-activated eggs that were inseminated in saline containing anesthetic MS222 and aphidicolin had high levels of histone H1 kinase and MAP kinase activities, and transformation of the penetrated sperm nucleus to metaphase chromosomes occurred even in the presence of aphidicolin or camptothecin. The male chromosomes were normally separated into two anaphase chromosome masses upon egg activation. These results suggest that DNA polymerase alpha or DNA topoisomerase I, but not DNA topoisomerase II, may be required for the process by which the mitotic interphase nucleus transforms to separable metaphase chromosomes while the activity of MAP kinase is low, unlike the situation in meiotic division, during which MAP kinase activity is high and DNA replication is not required.

High yield of endoreduplication induced by ICRF-193: a topoisomerase II catalytic inhibitor

An uncommonly high yield of spontaneous endoreduplication is a feature of the CHO mutant EM9, besides its defective repair of single, as well as double-DNA strand-breaks and its extraordinarily elevated yield of sister chromatid exchanges (SCEs) after bromodeoxyuridine (BrdU) incorporation into DNA. Since the nuclear enzyme topoisomerase II (topo II) has been reported to be responsible for the segregation of daughter chromosomes during mitosis, in the present investigation we have made use of the bisdioxopiperazine ICRF-193, a topo II catalytic inhibitor that interferes with the normal turnover of the enzyme. In order to see whether both EM9 cells and its parental cell line AA8, which show differences in the spontaneous frequency of endoreduplicated cells are or not equally sensitive to the topo II catalytic inhibitor, both cell lines have been treated with a range of doses of the bisdioxopiperazine. Our results show that both cell lines respond to the treatment entering in an endoreduplication cycle, but the EM9 cells are extremely sensitive to the inhibition of topo II.

DNA topoisomerase II is essential for preimplantation mouse development

Topoisomerase II (topo II) is an essential enzyme that alters DNA topology. This activity is important for a variety of chromosome functions including DNA replication, transcription, recombination, and chromosome condensation and segregation. Previously we localized topo II in mouse gametes and preimplantation embryos using isoform-specific antibodies demonstrating the presence of the enzyme in oocytes and embryos, but not sperm. To probe functions of topo II during preimplantation development, we treated mouse zygotes with 100 nM teniposide, and assessed embryo morphology and DNA replication. Teniposide blocked cleavage in 69% embryos; the remainder cleaved once but had abnormal nuclei. Teniposide-treated embryos were devoid of topo II immunofluorescence. Teniposide also prevented DNA replication, implicating topo II in this process. Embryos treated with a 2 hr pulse of teniposide recovered and developed to the blastocyst stage, indicating 100 nM teniposide did not induce apoptosis. To more specifically analyze topo IIalpha function, we treated zygotes with topo IIalpha-targeted antisense oligodeoxynucleotides. Most zygotes arrested at the 2-cell stage while controls developed into blastocysts indicating topo IIalpha is essential for preimplantation development. The absence of topo IIalpha, but not beta immunofluorescence in antisense-treated embryos confirms the specificity and impact of the treatment. In addition, topo IIalpha is newly synthesized at the 2-cell stage. These results establish an essential function for topo II in mouse preimplantation embryonic development.

DNA topoisomerase II distribution in mouse preimplantation embryos

DNA topoisomerase II (topo II) is an essential enzyme that mediates a variety of chromosome activities including DNA replication, transcription, recombination, and chromosome condensation and segregation. Isoform-specific anti-topo II antibodies were used to determine the distribution of topo II alpha and beta in mouse gametes and embryos. Immunoblot analysis with two anti-topo IIalpha antibodies revealed that a 170 kDa topo IIalpha band was present in ovary and testis. Mature sperm exhibited an 89 kDa band only, which may be a degradation product of topo IIalpha. Immunoblots probed with a monoclonal antibody that recognizes both isoforms, showed bands at 170 and 180 kDa, which correspond to topo IIalpha and beta, respectively. An additional 100 kDa band was also present in ovary and testis. Mature sperm did not exhibit staining with this antibody. We also localized topo II in mouse gametes and embryos up to the blastocyst stage using immunofluorescence microscopy. While both isoforms were found in nuclei and nucleoli of germinal vesicle oocytes, topo IIalpha localized to metaphase chromosomes during meiosis, and only to nucleoli during embryonic interphase. Topo IIbeta was absent from chromosomes of metaphase II oocytes, but localized to embryonic interphase nuclei. Both full-length isoforms were absent from sperm, indicating topo II is stored maternally. These results identify topo II as an important component of mouse oocyte and embryonic chromatin, and suggest its involvement in oocyte maturation and preimplantation embryonic development. The different immunofluorescent staining patterns indicate topo IIalpha and beta may serve different roles during the embryonic cell cycle.

Developmental regulation of DNA-topoisomerases during Drosophila embryogenesis

Type I and type II DNA-topoisomerases are essential enzymes that mediate replication, transcription, recombination, and mitosis in multicellular eukaryotes but the extent of their interchange for specific reactions in vivo is controversial. Expression patterns for topoisomerase I and topoisomerase II during the embryogenesis of Drosophila melanogaster were compared with patterns of DNA replication and expression of the histone genes. In late oogenesis the maternally supplied top2 mRNA was evenly distributed throughout the egg with elevated levels at the posterior tip, a pattern that is maintained in syncytial blastoderm embryos. During gastrulation, top2 mRNA became differentially localized only to regions of DNA replication, including new expression in the gonads preceding mitosis/meiosis. Significantly higher levels of top2 mRNA were found in mitotic compared to endoreplicating tissues. The total histone mRNA was exclusively associated with DNA replication but, in contrast to top2 mRNA, mitotic and endoreplicating cells contained similar expression levels with no expression in the gonads. Striking differences exist between the distribution of the top2 mRNA and topoisomerase II protein. The protein localizes to all evolving nuclei where it persists throughout embryogenesis. A high level of top1 mRNA transcript was present without differential tissue distribution throughout embryogenesis.

The Grapes checkpoint coordinates nuclear envelope breakdown and chromosome condensation

Mutations in the embryonic Drosophila Grapes/Chk1 checkpoint result in an abbreviated interphase, chromosome condensation defects and metaphase delays. To clarify the relationship between these phenotypes, we simultaneously timed multiple nuclear and cytoplasmic events in mutant grp-derived embryos. These studies support a model in which grp disrupts an S-phase checkpoint, which results in progression into metaphase with incompletely replicated chromosomes. We also show that chromosome condensation is independent of the state of DNA replication in the early embryo. Therefore, grp condensation defects are not a direct consequence of entering metaphase with incompletely replicated chromosomes. Rather, initiation of chromosome condensation (ICC) occurs at the normal time in grp-derived embryos, but the shortened interval between ICC and metaphase does not provide sufficient time to complete condensation. Our results suggest that these condensation defects, rather than incomplete DNA replication, are responsible for the extensive metaphase delays observed in grp-derived embryos. This analysis provides an example of how the loss of a checkpoint can disrupt the timing of multiple events not directly monitored by that checkpoint. These results are likely to apply to vertebrate cells and suggest new strategies for destroying checkpoint-compromised cancer cells.

During both interphase and mitosis, DNA topoisomerase II interacts with DNA as well as RNA through the protein's C-terminal domain

DNA topoisomerase II (topo II) is thought to be a nuclear enzyme; during interphase most was insoluble and could be recovered in the pellet after centrifugation of cell homogenates at 10,000 g (P-10). Upon entry into mitosis, the majority of topo II did not associate with condensed chromosomes but was apparently solubilized and redistributed throughout the cell. Although two non-chromosomal subfractions of mitotic topo II were defined by centrifugation at 130,000 g, the vast majority (>90%) was recovered in the pellet (P-130). In vivo nucleic acid interactions with topo II were monitored by a recently developed approach of UV-photo-crosslinking, immunoprecipitation and (32)P-labeling. P-10 (interphase) topo II was largely associated with DNA. P-130 (mitotic non-chromosomal) topo II was primarily associated with RNA. These nucleic acid interactions with both interphase and mitotic topo II occurred through the catalytically inert and as yet, poorly understood C-terminal domain of the protein. P-10 topo II was highly active enzymatically. Activity, measured by the ability of topo II to decatenate kDNA minicircles, was reduced by treatment with phosphatase. In contrast, P-130 topo II was relatively inactive but activity could be increased by phosphatase treatment. In vivo, P-130 topo II was more heavily phosphorylated than P-10 topo II; in both, only the C-terminal domain of topo II was detectably modified. Our observations suggest that cell cycle-dependent changes in the distribution, nucleic acid interactions and enzymatic activity of topo II are regulated, at least in part, by phosphorylation/dephosphorylation.

Mammalian centromeres: DNA sequence, protein composition, and role in cell cycle progression

The centromere is a specialized region of the eukaryotic chromosome that is responsible for directing chromosome movements in mitosis and for coordinating the progression of mitotic events at the crucial transition between metaphase and anaphase. In this review, we will focus on recent advances in the understanding of centromere composition at the protein and DNA level and of the role of centromeres in sister-chromatid cohesion and mitotic checkpoint control.

Inhibition of chromosomal separation provides insights into cleavage furrow stimulation in cultured epithelial cells

While astral microtubules are believed to be primarily responsible for the stimulation of cytokinesis in Echinoderm embryos, it has been suggested that a signal emanating from the chromosomal region and mediated by the interzonal microtubules stimulates cytokinesis in cultured mammalian cells. To test this hypothesis, we examined cytokinesis in normal rat kidney cells treated with an inhibitor of topoisomerase II, (+)-1,2-bis(3,5-dioxopiperaz-inyl-1-yl)propane, which prevents the separation of sister chromatids and the formation of a spindle interzone. The majority of treated cells showed various degrees of abnormality in cytokinesis. Furrows frequently deviated from the equatorial plane, twisting daughter cells into irregular shapes. Some cells developed furrows in regions outside the equator or far away from the spindle. In addition, F-actin and myosin II accumulated at the lateral ingressing margins but did not form a continuous band along the equator as in control cells. Imaging of microinjected 5- (and 6-) carboxymtetramethylrhodamine-tubulin revealed that a unique set of microtubules projected out from the chromosomal vicinity upon anaphase onset. These microtubules emanated toward the lateral cortex, where they delineated sites of microtubule bundle formation, cortical ingression, and F-actin and myosin II accumulation. As centrosome integrity and astral microtubules appeared unperturbed by (+)-1,2-bis(3, 5-dioxopiperaz-inyl-1-yl)propane treatment, the present observations cannot be easily explained by the conventional model involving astral microtubules. We suggest that in cultured epithelial cells the organization of the chromosomes dictates the organization of midzone microtubules, which in turn determines and maintains the cleavage activity.

DNA topoisomerase II can drive changes in higher order chromosome architecture without enzymatically modifying DNA

Topoisomerase II has been suggested to play a major role in chromosome organization based on its DNA decatenating activity and its ability to mediate direct binding interactions between DNA and nuclear matrix. However, this latter point remains controversial. Here we address the question of whether the chromatin binding activity of Topoisomerase II is sufficient to modify chromosome form using whole mammalian chromosomes in vitro. Intact chromosomes were microsurgically removed from living cells and disassembled by treatment with protease or heparin. When these disassembled chromosomes were incubated with recombinant human Topoisomerase II, the enzyme became incorporated into chromatin and reassembly resulted, leading to almost complete restoration of pre-existing chromosome shape and position within minutes. Chromosome reconstitution by Topoisomerase II was dose-dependent, saturable, and appeared to be controlled stoichiometrically, rather than enzymatically. Similar reassembly was observed in the absence of ATP and when a catalytically inactive thermosensitive Topoisomerase II mutant was used at the restrictive temperature. Chromosome recondensation also could be induced after the strand-passing activity of Topoisomerase II was blocked by treatment with an inhibitor of its catalytic activity, amsacrine. When a non-hydrolyzable beta,gamma-imido analog of ATP (AMP-PNP) was used to physiologically fix bound Topoisomerase II enzyme in a closed form around DNA, subsequent chromosome disassembly was prevented in the presence of high salt. These data suggest that Topoisomerase II may control higher order chromatin architecture through direct binding interactions, independently of its well-known catalytic activity.

The roles of DNA topoisomerase II during the cell cycle

DNA topoisomerase II (topo II) is essential for survival of all eukaryotic cells. Topo II is both an enzyme and a structural component of the nuclear matrix. It regulates the topological states of DNA by transient cleavage, strand passing and re-ligation of double-stranded DNA resulting in decatenation of intertwined DNA molecules and relaxation of supercoiled DNA. Topo II plays an important role in DNA replication and is required for condensation and segregation of chromosomes. The expression of topo II is cell cycle dependent with both protein levels and catalytic activity peaking at G2/M. Phosphorylation/dephosphorylation of topo II may be a part of regulatory checkpoints at the entry and progression of mitosis.

A Drosophila kinesin-like protein, Klp38B, functions during meiosis, mitosis, and segmentation

We show that klp38B, isolated as a mutation that dominantly prolongs blastoderm mitotic cycles in Drosophila, encodes a Drosophila kinesin-like protein. Further genetic analyses show that Klp38B not only functions during mitosis, but is also required for meiosis and abdominal segmentation. Sequence comparisons suggest that Klp38B encodes an amino-terminal microtubule motor domain, a central alpha-helical coiled-coil domain, and a C-terminal globular domain. Evidence that Klp38B is required during meiosis is that flies transheterozygous for mutations in both klp38B and nod have a high frequency of 4th chromosome meiotic nondisjunction. Nod is a chromokinesin, a chromosome binding kinesin, that is believed to provide astral-exclusion forces during the metaphase stage of meiosis. Evidence that Klp38B is required during mitosis is that embryos from female germline clones of klp38B mutations have holes in the cuticle similar to a zygotic string (dCDC25) phenotype. Also, anti-Klp38B antibody injection into precellularization blastoderm embryos causes developmental arrest and the formation of circular mitotic figures. We speculate, based on these phenotypes, that Klp38B is a chromokinesin that provides astral-exclusion forces on the chromosomes during meiosis and mitosis. Consistent with this hypothesis, we have identified an HMG-1 homologous region on Klp38B that could potentially bind AT-rich DNA sequences. Finally, we show that klp38B mutations have defects in abdominal segmentation, suggesting that Klp38B, like Xenopus chromokinesin Xklp1, might be involved in polar granule formation.

Interaction of the DNA topoisomerase II catalytic inhibitor meso-2,3-bis(3,5-dioxopiperazine-1-yl)butane (ICRF-193), a bisdioxopiperazine derivative, with the conserved region(s) of eukaryotic but not prokaryotic enzyme

ICRF-193 [meso-2,3-bis(3,5-dioxopiperazine-1-yl)butane], a bisdioxopiperazine compound, has been shown to be a catalytic inhibitor of DNA topoisomerase II by stabilizing the enzyme in the form of a closed "protein clamp," an intermediate form in the catalytic cycle (Roca et al., Proc Natl Acad Sci USA 91: 1781-1785, 1994). In view of its usefulness as a probe in the functional analysis of the enzyme, we tried further to define the domain(s) of the enzyme interacting with the drug by examining its inhibitory activity on type II topoisomerases from various species of eukaryotes and prokaryotes. ICRF-193 inhibited the enzyme from yeast, fly, frog, plant, and mammals at IC50 values in the range of 1-13 microM. Experiments using fission yeast truncated mutant type II enzyme lacking both amino-terminal 74 amino acids and carboxy-terminal 265 amino acids revealed that ICRF-193 interacts with the 125 kDa "core" polypeptide of the enzyme. In contrast, prokaryotic type II enzymes, Escherichia coli DNA gyrase, topo IV, and phage T4 topo, were not affected by the drug. From these results, the domain(s) common to eukaryotic but not to prokaryotic type II enzymes interacting with ICRF-193 was speculated.

Chromatin-remodelling factor CHRAC contains the ATPases ISWI and topoisomerase II

Repressive chromatin structures need to be unravelled to allow DNA-binding proteins access to their target sequences. This de-repression constitutes an important point at which transcription and presumably other nuclear processes can be regulated. Energy-consuming enzyme complexes that facilitate the interaction of transcription factors with chromatin by modifying nucleosome structure are involved in this regulation. One such factor, nucleosome-remodelling factor (NURF), has been isolated from Drosophila embryo extracts. We have now identified a chromatin-accessibility complex (CHRAC) which uses energy to increase the general accessibility of DNA in chromatin. However, unlike other known chromatin remodelling factors, CHRAC can also function during chromatin assembly: it uses ATP to convert irregular chromatin into a regular array of nucleosomes with even spacing. CHRAC combines enzymes that modulate nucleosome structure and DNA topology. Using mass spectrometry, we identified two of the five CHRAC subunits as the ATPase ISWI, which is also part of NURF, and topoisomerase II. The presence of ISWI in different contexts suggests that chromatin remodelling machines have a modular nature and that ISWI has a central role in different chromatin remodelling reactions.

The dynamic nuclear redistribution of an hnRNP K-homologous protein during Drosophila embryo development and heat shock. Flexibility of transcription sites in vivo

The Drosophila protein Hrb57A has sequence homology to mammalian heterogenous nuclear ribonucleoprotein (hnRNP) K proteins. Its in vivo distribution has been studied at high resolution by confocal laser scanning microscopy (CLSM) in embryos injected with fluorescently labeled monoclonal antibody. Injection of antibody into living embryos had no apparent deleterious effects on further development. Furthermore, the antibody-protein complex could be observed for more than 7 cell cycles in vivo, revealing a dynamic redistribution from the nucleus to cytoplasm at each mitosis from blastoderm until hatching. The evaluation of two- and three-dimensional CLSM data sets demonstrated important differences in the localization of the protein in the nuclei of living compared to fixed embryos. The Hrb57A protein was recruited to the 93D locus upon heat shock and thus serves as an in vivo probe for the activity of the gene in diploid cells of the embryo. Observations during heat shock revealed considerable mobility within interphase nuclei of this transcription site. Furthermore, the reinitiation as well as the down regulation of transcriptional loci in vivo during the recovery from heat shock could be followed by the rapid redistribution of the hnRNP K during stress recovery. These data are incompatible with a model of the interphase nucleus in which transcription complexes are associated with a rigid nuclear matrix.

Untangling the role of DNA topoisomerase II in mitotic chromosome structure and function

DNA topoisomerase II (topo II) is involved in chromosome structure and function, although its exact location and role in mitosis are somewhat controversial. This is due in part to the varied reports of its localization on mitotic chromosomes, which has been described at different times as uniformly distributed, axial on the chromosome arms and predominantly centromeric. These disparate results are probably due to several factors, including use of different preparation and fixation techniques, species differences and changes in distribution during the cell cycle. Recently, several papers have re-investigated the distribution of topo II on chromosomes as a function of cell cycle and species(1-3). The new studies suggest that Topo II has a dynamic pattern of distribution on the chromosomes, in general becoming axial as chromosomes condense during prophase and then concentrating at centromeres during metaphase. These experiments suggest a novel role for topo II in centromere structure and function.

The product of proliferation disrupter is concentrated at centromeres and required for mitotic chromosome condensation and cell proliferation in Drosophila

Homozygosity for a null mutation in the proliferation disrupter (prod) gene of Drosophila causes decreased mitotic index, defects of anaphase chromatid separation, and imperfect chromosome condensation in larval neuroblasts and other proliferating cell populations. The defective condensation is especially obvious near the centromeres. Mutant larvae show slow growth and massive cell death in proliferating cell populations, followed by late larval lethality. Loss of prod function in mitotic clones leads to the arrest of oogenesis in the ovary and defective cuticle formation in imaginal disc derivatives. The prod gene encodes a novel 301-amino-acid protein that is ubiquitously expressed and highly concentrated at the centric heterochromatin of the second and third mitotic chromosomes, as well as at > 400 euchromatic loci on polytene chromosomes. We propose that Prod is a nonhistone protein essential for chromosome condensation and that the chromosomal and developmental defects are caused by incomplete centromere condensation in prod mutants.

Wolbachia-induced delay of paternal chromatin condensation does not prevent maternal chromosomes from entering anaphase in incompatible crosses of Drosophila simulans

The behavior of parental chromosomes during the first mitosis of Drosophila simulans zygotes obtained from unidirectional incompatible crosses is described and it is demonstrated that the condensation of parental chromatin complements was asynchronous. The timing of paternal chromatin condensation appeared to be delayed in these embryos, so that condensed maternal chromosomes and entangled prophase-like paternal fibers congressed in the equatorial plane of the first metaphase spindle. At anaphase the maternal chromosomes migrated to opposite poles of the spindle, whereas the paternal chromatin lagged in the midzone of the spindle. This resulted in dramatic errors in paternal chromatin inheritance leading to the formation of embryos with aneuploid or haploid nuclei. These observations suggest that the anaphase onset of maternal chromosomes is unaffected by the improper alignment of the paternal complement. Since the first metaphase spindle of the Drosophila zygote consists of twin bundles of microtubules each holding one parental complement, we suspect that each half spindle regulates the timing of anaphase onset of its own chromosome set. In normal developing embryos, the fidelity of chromosome transmission is presumably ensured by the relative timing required to prepare parental complements for the orderly segregation that occurs during the metaphase-anaphase transition.

Chromatid segregation at anaphase requires the barren product, a novel chromosome-associated protein that interacts with Topoisomerase II

We have isolated a Drosophila gene, barren (barr), required for sister-chromatid segregation in mitosis. barr encodes a novel protein that is present in proliferating cells and has homologs in yeast and human. Mitotic defects in barr embryos become apparent during cycle 16, resulting in a loss of PNS and CNS neurons. Centromeres move apart at the metaphase-anaphase transition and Cyclin B is degraded, but sister chromatids remain connected, resulting in chromatin bridging. This phenotype is similar to that described in TOP2 mutants in yeast. Barren protein localizes to chromatin throughout mitosis. Colocalization and biochemical experiments indicate that Barren associates with Topoisomerase II throughout mitosis and alters the activity of Topoisomerase II. We propose that this association is required for proper chromosomal segregation by facilitating the decatenation of chromatids at anaphase.

Mitotic chromosome condensation

In this chapter, we review the structure and composition of interphase and mitotic chromosomes. We discuss how these observations support the model that mitotic condensation is a deterministic process leading to the invariant folding of a given chromosome. The structural studies have also placed constraints on the mechanism of condensation and defined several activities needed to mediate condensation. In the context of these activities and structural information, we present our current understanding of the role of cis sites, histones, topoisomerase II, and SMC proteins in condensation. We conclude by using our current knowledge of mitotic condensation to address the differences in chromosome condensation observed from bacteria to humans and to explore the relevance of this process to other processes such as gene expression.