Specific interactions of chromatin with the nuclear envelope: positional determination within the nucleus in Drosophila melanogaster.

Specific interactions of chromatin with the nuclear envelope (NE) in early embryos of Drosophila melanogaster have been mapped and analyzed. Using fluorescence in situ hybridization, the three-dimensional positions of 42 DNA probes, primarily to chromosome 2L, have been mapped in nuclei of intact Drosophila embryos, revealing five euchromatic and two heterochromatic regions associated with the NE. These results predict that there are approximately 15 NE contacts per chromosome arm, which delimit large chromatin loops of approximately 1-2 Mb. These NE association sites do not strictly correlate with scaffold-attachment regions, heterochromatin, or binding sites of known chromatin proteins. Pairs of neighboring probes surrounding one NE association site were used to delimit the NE association site more precisely, suggesting that peripheral localization of a large stretch of chromatin is likely to result from NE association at a single discrete site. These NE interactions are not established until after telophase, by which time the nuclear envelope has reassembled around the chromosomes, and they are thus unlikely to be involved in binding of NE vesicles to chromosomes following mitosis. Analysis of positions of these probes also reveals that the interphase nucleus is strongly polarized in a Rabl configuration which, together with specific targeting to the NE or to the nuclear interior, results in each locus occupying a highly determined position within the nucleus.     

Nuclear architecture in plants

Structure within the nucleus of plants is becoming increasingly clear in both metaphase and interphase nuclei, although there are conflicting data about the relative positions of individual and pairs of chromosomes. At interphase, individual chromosomes may generally occupy discrete domains that are not intermixed with other chromosomes. Aspects of mechanical chromosome behaviour and even of gene expression may correlate with interphase chromosome position, and imply that a better understanding of nuclear architecture is required.     

Ordered arrangement and rearrangement of chromosomes during spermatogenesis in two species of planarians (Plathelminthes)

The pattern of distribution of telomeric DNA (TTAGGG), 28S rDNA, and 5S rDNA has been studied using fluorescence in situ hybridization (FISH) and primed in situ labelling during spermatogenesis and sperm formation in the filiform spermatozoa of two species of planarians, Dendrocoelum lacteum and Polycelis tenuis (Turbellaria, Plathelminthes). In both species, the positions of FISH signals found with each probe sequence are constant from cell to cell in the nuclei of mature sperm. Chromosome regions containing 5S and 28S rDNA genes are gathered in distinct bundles of spiral form. In early spermatids with roundish nuclei, the sites of a given sequence on different chromosomes remain separate. Centromeres (marked by 5S rDNA) gather into a single cluster in the central region of the slightly elongated sperm nucleus. During spermatid maturation, this cluster migrates to the distal pole of the nucleus. In Polycelis, telomeric sites gather into three distinct clusters at both ends and in the middle of the moderately elongated nucleus. These clusters retain their relative positions as the spermatid matures. All the chromosome ends bearing 28S rDNA gather only into the proximal cluster. Our data suggest that structures in the nucleus selectively recognise chromosome regions containing specific DNA sequences, which helps these regions to find their regular places in the mature sperm nucleus and causes clustering of the sites of these sequences located on different chromosomes. This hypothesis is supported by observations on elongated sperm of other animals in which a correlation exists between ordered arrangement of chromosomes in the mature sperm nucleus and clustering of sites of the same sequence from different chromosomes during spermiogenesis. Received: 15 December 1997; in revised form: 24 March 1998 / Accepted: 14 April 1998     

Caenorhabditis elegans polo-like kinase PLK-1 is required for merging parental genomes into a single nucleus

Before the first zygotic division, the nuclear envelopes of the maternal and paternal pronuclei disassemble, allowing both sets of chromosomes to be incorporated into a single nucleus in daughter cells after mitosis. We found that in Caenorhabditis elegans, partial inactivation of the polo-like kinase PLK-1 causes the formation of two nuclei, containing either the maternal or paternal chromosomes, in each daughter cell. These two nuclei gave rise to paired nuclei in all subsequent cell divisions. The paired-nuclei phenotype was caused by a defect in forming a gap in the nuclear envelopes at the interface between the two pronuclei during the first mitotic division. This was accompanied by defects in chromosome congression and alignment of the maternal and paternal metaphase plates relative to each other. Perturbing chromosome congression by other means also resulted in failure to disassemble the nuclear envelope between the two pronuclei. Our data further show that PLK-1 is needed for nuclear envelope breakdown during early embryogenesis. We propose that during the first zygotic division, PLK-1–dependent chromosome congression and metaphase plate alignment are necessary for the disassembly of the nuclear envelope between the two pronuclei, ultimately allowing intermingling of the maternal and paternal chromosomes.     

Reversible chromosome condensation induced in Drosophila embryos by anoxia: visualization of interphase nuclear organization

We have studied the morphology of nuclei in Drosophila embryos during the syncytial blastoderm stages. Nuclei in living embryos were viewed with differential interference-contrast optics; in addition, both isolated nuclei and fixed preparations of whole embryos were examined after staining with a DNA-specific fluorescent dye. We find that: (a) The nuclear volumes increase dramatically during interphase and then decrease during prophase of each nuclear cycle, with the magnitude of the nuclear volume increase being greatest for those cycles with the shortest interphase. (b) Oxygen deprivation of embryos produces a rapid developmental arrest that is reversible upon reaeration. During this arrest, interphase chromosomes condense against the nuclear envelope and the nuclear volumes increase dramatically. In these nuclei, individual chromosomes are clearly visible, and each condensed chromosome can be seen to adhere along its entire length to the inner surface of the swollen nuclear envelope, leaving the lumen of the nucleus devoid of DNA. (c) In each interphase nucleus the chromosomes are oriented in the "telophase configuration," with all centromeres and all telomeres at opposite poles of the nucleus; all nuclei at the embryo periphery (with the exception of the pole cell nuclei) are oriented with their centromeric poles pointing to the embryo exterior.     

Intranuclear arrangement of human chromosome 12 correlates to large-scale replication domains

The intranuclear arrangement of human chromosome 12 in G0(G1) nuclei from human myeloid leukemia HL60 cells was analyzed by multicolor fluorescence in situ hybridization (FISH) using band-specific cosmid clones as probes. Pairs of differently colored cosmids were detected on paraformaldehyde-fixed HL60 nuclei, and their relative positions, internal or peripheral, in individual nuclei were scored. Our results suggest that the intranuclear arrangement of human chromosome 12 is not random. Some chromosomal domains, including the centromere, were located in the periphery of the nucleus, while other domains, including the telomeres, were positioned in the internal areas of the nucleus in G0(G1) cells. Based on the replication banding patterns of metaphase spreads, human chromosome 12 was divided roughly into five large domains. Interestingly, the clones in late replicating domains were preferentially localized in the nuclear periphery, whereas clones in early replicating domains were arranged in the internal areas of the nuclei. The DNA replication timing of each cosmid determined by FISH-based assay did not reflect the replication bands, but an overall profile of the replication timing was relatively correlated with these domains on chromosome 12. These results suggest that the intranuclear arrangement of a human chromosome is correlated with the large-scale replication domains, even before DNA replication. Received: 23 January 1999; in revised form: 6 September 1999 / Accepted: 11 September 1999     

Conservation of relative chromosome positioning in normal and cancer cells

Chromosomes exist in the interphase nucleus as individual chromosome territories. It is unclear to what extent chromosome territories occupy particular positions with respect to each other and how structural rearrangements, such as translocations, affect chromosome organization within the cell nucleus. Here we analyze the relative interphase positioning of chromosomes in mouse lymphoma cells compared to normal splenocytes. We show that in a lymphoma cell line derived from an ATM(-/-) mouse, two translocated chromosomes are preferentially positioned in close proximity to each other. The relative position of the chromosomes involved in these translocations is conserved in normal splenocytes. Relative positioning of chromosomes in normal splenocytes is not due to their random distribution in the interphase nucleus and persists during mitosis. These observations demonstrate that the relative arrangement of chromosomes in the interphase nucleus can be conserved between normal and cancer cells and our data support the notion that physical proximity facilitates rearrangements between chromosomes.     

Spatial topography of a pericentromeric region (1q12) in hemopoietic cells studied by in situ hybridization and confocal microscopy

A fluorescent in situ hybridization procedure with a chromosome 1-specific (1q12) repetitive satellite DNA probe was used to label the 1q12 regions of the chromosomes 1 in spherical and polymorphic hemopoietic cell nuclei. The entire procedure was performed in suspension to preserve nuclear morphology. The result was studied by three-dimensional analysis, as provided by a scanning laser confocal microscope. The 1q12 regions of chromosome 1 were measured to be closely associated with the nuclear envelope in isolated nuclei of unstimulated diploid human lymphocytes. The relative positions to each other in the periphery of these spherical nuclei could not be distinguished from a random distribution pattern. In the diploid and tetraploid polymorphic nuclei of cells of the promyelocytic leukemia cell line HL60 these pericentromeric sequences were also associated with the nuclear surface.     

Internuclear transfer of genetic information in kar1-1/KAR1 heterokaryons in Saccharomyces cerevisiae.

Heterokaryons of Saccharomyces cerevisiae have been constructed utilizing the kar1-1 mutation, which prevents nuclear fusion during conjugation (J. Conde and G. Fink, Proc. Natl. Acad. Sci. U.S.A. 73:3651-3655, 1976). Each heterokaryon contained two haploid nuclei that were marked on several chromosomes. They segregated haploid progeny (cytoductants), most of which have the nuclear genotype of one or the other of the heterokaryon parents, but they occasionally segregated progeny having a recombinant genotype (exceptional cytoductants). Exceptional cytoductants receive the majority of their genome from one parent (the recipient) and a minority from the other (the donor). Transfer of two markers from the donor nucleus to the recipient is rarely coincident for markers located on different chromosomes but is nearly always coincident for those markers located on the same chromosome, suggesting that whole chromosomes are transferred from the donor nucleus to the recipient. In crosses of kar1-1 X KAR1 parents, either nucleus may act as a recipient or donor with equal probability. Recipient nuclei acquired 9 of the 10 chromosomes examined, with frequencies which were inversely correlated with the size of the chromosome. When a chromosome is acquired by the recipient nucleus, it either replaces its homolog or exists in a disomic condition. Haploid progeny emanating from kar1 X KAR1 crosses are frequently inviable. I tested whether this inviability might be the result of chromosome loss by donor nuclei. Viability of progeny from kar1 X KAR1 heterokaryons was improved when the parental nuclei were diploid to an extent consistent with the hypothesis, and diploid progeny which had become monosomic were recovered from these heterokaryons. The following sequence of events accounts for chromosome transfer in kar1 X KAR1 heterokaryons. After cell fusion, each nucleus in the heterokaryon has a probability of about 0.38 of losing one or more chromosomes. A nucleus sustaining such a loss can become a donor in a chromosome transfer event. If the other nucleus does not sustain a mortal chromosome loss, it can become a recipient in a transfer event. The chance of acquiring a chromosome lost by the donor is greater for smaller chromosomes than for larger ones and is about 0.05 for the average chromosome.     

Distribution of chromosome 18 and X centric heterochromatin in the interphase nucleus of cultured human cells

In situ hybridization of human chromosome 18 and X-specific alphoid DNA-probes was performed in combination with three dimensional (3D) and two dimensional (2D) image analysis to study the interphase distribution of the centric heterochromatin (18c and Xc) of these chromosomes in cultured human cells. 3D analyses of 18c targets using confocal laser scanning microscopy indicated a nonrandom disposition in 73 amniotic fluid cell nuclei. The shape of these nuclei resembled rather flat cylinders or ellipsoids and targets were preferentially arranged in a domain around the nuclear center, but close to or associated with the nuclear envelope. Within this domain, however, positionings of the two targets occurred independently from each other, i.e., the two targets were observed with similar frequencies at the same (upper or lower) side of the nuclear envelope as those on opposite sides. This result strongly argues against any permanent homologous association of 18c. A 2D analytical approach was used for the rapid evaluation of 18c positions in over 4000 interphase nuclei from normal male and female individuals, as well as individuals with trisomy 18 and Bloom's syndrome. In addition to epithelially derived amniotic fluid cells, investigated cell types included in vitro cultivated fibroblastoid cells established from fetal lung tissue and skin-derived fibroblasts. In agreement with the above 3D observations 18c targets were found significantly closer (P less than 0.01) to the center of the 2D nuclear image (CNI) and to each other in all these cultures compared to a random distribution derived from corresponding ellipsoid or cylinder model nuclei. For comparison, a chromosome X-specific alphoid DNA probe was used to investigate the 2D distribution of chromosome X centric heterochromatin in the same cell types. Two dimensional Xc-Xc and Xc-CNI distances fit a random distribution in diploid normal and Bloom's syndrome nuclei, as well as in nuclei with trisomy X. The different distributions of 18c and Xc targets were confirmed by the simultaneous staining of these targets in different colors within individual nuclei using a double in situ hybridization approach.     

Fine structural studies of early mitotic stages in untreated and nocodazole-treated HeLa cells

When cells in mitosis are treated with nocodazole, a microtubule-disrupting drug, it can be shown in comparison to untreated cells that microtubules are responsible for the polarized formation of indentations, folds, tubes, and crypts of the nuclear envelope in prophase nuclei. No translocation of chromosomes within the nucleus takes place. Microtubules are not necessary for chromosome condensation, nuclear envelope breakdown, the formation of trilaminar kinetochores, and the orientation of sister-kinetochores within one chromosome in relation to each other. The orientation of kinetochores in relation to the mitotic poles, however, is mediated by microtubules. The data shown here support the working hypothesis about chromosome translocation in prophase nuclei which was presented in an earlier paper.     

Large-scale chromatin structural domains within mitotic and interphase chromosomes in vivo and in vitro

Higher-order chromatin structural domains approximately 130 nm in width are observed as prominent components of both Drosophila melanogaster and human mitotic chromosomes using buffer conditions which preserve chromosome morphology as determined by light microscopic comparison with chromosomes within living cells. Spatially discrete chromatin structural domains of similar size also exist as prominent components within interphase nuclei prepared under equivalent conditions. Examination of chromosomes during the anaphase-telophase transition suggests that chromosomes decondense largely through the progressive straightening or uncoiling of these large-scale chromatin domains. A quantitative analysis of the size distribution of these higher-order domains in telophase nuclei indicated a mean width of 126±36 nm. Three-dimensional views using stereopairs of chromosomes and interphase nuclei from 0.5 m thick sections suggest that these large-scale chromatin domains consist of 30 nm fibers packed by tight folding into larger, linear, fiber-like elements. Reduction in vitro of either polyamine or divalent cation concentrations within two different buffer systems results in a loss of these large-scale domains, with no higher-order chromatin organization evident above the 20–30 nm fiber. Under these conditions the DNA distribution within mitotic chromosomes and interphase nuclei appears significantly diffuse relative to the appearance by light microscopy within living cells, or, by electron microscopy, within cells fixed directly without permeabilization in buffer. These results suggest that these large-scale chromatin structural domains are fundamental elements of chromosome architecture in vivo.     

A Highlights from MBoC Selection: Aurora B–mediated localized delays in nuclear envelope formation facilitate inclusion of late-segregating chromosome fragments

To determine how chromosome segregation is coordinated with nuclear envelope formation (NEF), we examined the dynamics of NEF in the presence of lagging acentric chromosomes in Drosophila neuroblasts. Acentric chromosomes often exhibit delayed but ultimately successful segregation and incorporation into daughter nuclei. However, it is unknown whether these late-segregating acentric fragments influence NEF to ensure their inclusion in daughter nuclei. Through live analysis, we show that acentric chromosomes induce highly localized delays in the reassembly of the nuclear envelope. These delays result in a gap in the nuclear envelope that facilitates the inclusion of lagging acentrics into telophase daughter nuclei. Localized delays of nuclear envelope reassembly require Aurora B kinase activity. In cells with reduced Aurora B activity, there is a decrease in the frequency of local nuclear envelope reassembly delays, resulting in an increase in the frequency of acentric-bearing, lamin-coated micronuclei. These studies reveal a novel role of Aurora B in maintaining genomic integrity by promoting the formation of a passageway in the nuclear envelope through which late-segregating acentric chromosomes enter the telophase daughter nucleus.     

Higher-order genome organization in platypus and chicken sperm and repositioning of sex chromosomes during mammalian evolution

In mammals, chromosomes occupy defined positions in sperm, whereas previous work in chicken showed random chromosome distribution. Monotremes (platypus and echidnas) are the most basal group of living mammals. They have elongated sperm like chicken and a complex sex chromosome system with homology to chicken sex chromosomes. We used platypus and chicken genomic clones to investigate genome organization in sperm. In chicken sperm, about half of the chromosomes investigated are organized non-randomly, whereas in platypus chromosome organization in sperm is almost entirely non-random. The use of genomic clones allowed us to determine chromosome orientation and chromatin compaction in sperm. We found that in both species chromosomes maintain orientation of chromosomes in sperm independent of random or non-random positioning along the sperm nucleus. The distance of loci correlated with the total length of sperm nuclei, suggesting that chromatin extension depends on sperm elongation. In platypus, most sex chromosomes cluster in the posterior region of the sperm nucleus, presumably the result of postmeiotic association of sex chromosomes. Chicken and platypus autosomes sharing homology with the human X chromosome located centrally in both species suggesting that this is the ancestral position. This suggests that in some therian mammals a more anterior position of the X chromosome has evolved independently.     

Transvection at the vestigial locus of Drosophila melanogaster

Transvection is a phenomenon wherein gene expression is effected by the interaction of alleles in trans and often results in partial complementation between mutant alleles. Transvection is dependent upon somatic pairing between homologous chromosome regions and is a form of interallelic complementation that does not occur at the polypeptide level. In this study we demonstrated that transvection could occur at the vestigial (vg) locus by revealing that partial complementation between two vg mutant alleles could be disrupted by changing the genomic location of the alleles through chromosome rearrangement. If chromosome rearrangements affect transvection by disrupting somatic pairing, then combining chromosome rearrangements that restore somatic pairing should restore transvection. We were able to restore partial complementation in numerous rearrangement trans-heterozygotes, thus providing substantial evidence that the observed complementation at vg results from a transvection effect. Cytological analyses revealed this transvection effect to have a large proximal critical region, a feature common to other transvection effects. In the Drosophila interphase nucleus, paired chromosome arms are separated into distinct, nonoverlapping domains. We propose that if the relative position of each arm in the nucleus is determined by the centromere as a relic of chromosome positions after the last mitotic division, then a locus will be displaced to a different territory of the interphase nucleus relative to its nonrearranged homolog by any rearrangement that links that locus to a different centromere. This physical displacement in the nucleus hinders transvection by disrupting the somatic pairing of homologous chromosomes and gives rise to proximal critical regions.     

Distribution of kinetochore (centromere) antigen in mammalian cell nuclei

Antigens associated with mammalian centromeres were localized at the high and electron microscopic levels using the peroxidase-labeled antibody method. The antibody used was of a type naturally occurring in the sera of patients with scleroderma. At the light microscopic level, it reacts specifically with the centromere regions of chromosomes in a variety of mammalian species and strains in discrete foci in interphase nuclei. We find that the number of foci approximates the number of chromosomes present in the various cell types. At the ultrastructural level, the antigenic foci are confirmed to lie in the kinetochore regions of each chromosome. In interphase nuclei, the antigenic foci were usually associated either with the inner surfaces of the nuclear envelope or with the nucleoli. These observations indicate that the centromere regions of the chromosomes in interphase are not randomly distributed within the nucleus but are usually fixed either to the inner surface of the nuclear envelope or to nucleoli.     

Meiotic division and fusion of nuclei in multinucleate cells from the induced fusion of meiotic protoplasts of liliaceous plants

Multinucleate protoplasts were produced from meiotic cells at the zygotene and pachytene stages in a lily andTrillium, and their meiotic divisions were followed during subsequent culture. In each multinucleate, a complete synchrony of nuclear division was maintained throughout the meiotic process, and chromosome behavior appeared normal up to the metaphase stage. In most dinucleates, chromosome segregation movement was organized in a common spindle, and the daughter nuclei at the telophase appeared to envelope each other in the newly formed nuclear membrane. The cell was divided into two daughter cells by a common cell plate. Trinucleates were similarly converted to two cells with a hexaploid number of chromosomes. Some of the di- and trinucleates subsequently completed the second meiotic division with the formation of typical tetrad configurations. In giant cells with more than several nuclei, chromosomes separated at random but reaggregated into one giant resting nucleus, with no later cytokinesis. The rate of meiotic development in multinucleates was relatively slower in cells which contained greater numbers of nuclei.