X chromosome reactivation and regulation in cloned embryos

Somatic cell nuclear transfer embryos exhibit extensive epigenetic abnormalities, including aberrant methylation and abnormal imprinted gene expression. In this study, a thorough analysis of X chromosome inactivation (XCI) was performed in both preimplantation and postimplantation nuclear transfer embryos. Cloned blastocysts reactivated the inactive somatic X chromosome, possibly in a gradient fashion. Analysis of XCI by Xist RNA and Eed protein localization revealed heterogeneity within cloned embryos, with some cells successfully inactivating an X chromosome and others failing to do so. Additionally, a significant proportion of cells contained more than two X chromosomes, which correlated with an increased incidence of tetraploidy. Imprinted XCI, normally found in preimplantation embryos and extraembryonic tissues, was not observed in blastocysts or placentae from later stage clones, although fetuses recapitulated the Xce effect. We conclude that, although SCNT embryos can reactivate, count, and inactivate X chromosomes, they are not able to regulate XCI consistently. These results illustrate the heterogeneity of epigenetic changes found in cloned embryos.     

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.     

The impact of chromosomal alteration on embryo development

Chromosome alterations, such as those affecting telomere erosion, predictably occur with each cell division, others, which involve changes to the expression and replication of the X-chromosome occur at particular stages of development, while those that involve loss or gain of chromosomes occur in a random and so far unpredictable manner. The production of embryos in vitro and by somatic cell nuclear transfer (SCNT) has been associated with altered expression of marker genes on the X-chromosome and an increased incidence of chromosomally abnormal cells during early development. In the case of SCNT embryos chromosome abnormalities may be associated with the nuclear donor cell. Telomere rebuilding subsequent to SCNT appears to vary according to species and type of donor cell used. It is speculated that the rate of telomere erosion and incidence of chromosome abnormalities affects developmental potential of early embryos and may be potential predictors of developmental outcome.     

Mitotic chromosome length scales in response to both cell and nuclear size

Multicellular development requires that cells reduce in size as a result of consecutive cell divisions without increase in embryo volume. To maintain cellular integrity, organelle size adapts to cell size throughout development. During mitosis, the longest chromosome arm must be shorter than half of the mitotic spindle for proper chromosome segregation. Using high-resolution time-lapse microscopy of living Caenorhabditis elegans embryos, we have quantified the relation between cell size and chromosome length. In control embryos, chromosome length scaled to cell size. Artificial reduction of cell size resulted in a shortening of chromosome length, following a trend predicted by measurements from control embryos. Disturbing the RAN (Ras-related nuclear protein)-GTP gradient decoupled nuclear size from cell size and resulted in chromosome scaling to nuclear size rather than cell size; smaller nuclei contained shorter chromosomes independent of cell size. In sum, quantitative analysis relating cell, nuclear, and chromosome size predicts two levels of chromosome length regulation: one through cell size and a second in response to nuclear size.     

Mitotic chromosome size scaling in Xenopus

As rapid divisions without growth generate progressively smaller cells within an embryo, mitotic chromosomes must also decrease in size to permit their proper segregation, but this scaling phenomenon is poorly understood. We demonstrated previously that nuclear and spindle size scale between egg extracts of the related frog species Xenopus tropicalis and Xenopus laevis, but show here that dimensions of isolated mitotic sperm chromosomes do not differ. This is consistent with the hypothesis that chromosome scaling does not occur in early embryonic development when cell and spindles sizes are large and anaphase B segregates chromosomes long distances. To recapitulate chromosome scaling during development, we combined nuclei isolated from different stage Xenopus laevis embryos with metaphase-arrested egg extracts. Mitotic chromosomes derived from nuclei of cleaving embryos through the blastula stage were similar in size to replicated sperm chromosomes, but decreased in area approximately 50% by the neurula stage, reproducing the trend in size changes observed in fixed embryos. Allowing G2 nuclei to swell in interphase prior to mitotic condensation did not increase mitotic chromosome size, but progression through a full cell cycle in egg extract did, suggesting that epigenetic mechanisms determining chromosome size can be altered during DNA replication. Comparison of different sized mitotic chromosomes assembled in vitro provides a tractable system to elucidate underlying molecular mechanisms.     

Elimination of chromosomes in Hordeum vulgare x H. bulbosum crosses at mitosis and interphase involves micronucleus formation and progressive heterochromatinization

Uniparental chromosome elimination occurs in several interspecific hybrids of plants. We studied the mechanism underlying selective elimination of the paternal chromosomes during the development of Hordeum vulgare x H. bulbosum hybrid embryos that is restricted to an early stage of development. In almost all embryos most of the H. bulbosum chromatin undergoes a fast rate of elimination within nine days after pollination. There are differences in the mitotic behaviour between the parental chromosomes, with H. bulbosum chromatids segregating asymmetrically at anaphase. We provide evidence for a chromosome elimination pathway that involves the formation of nuclear extrusions during interphase in addition to postmitotically formed micronuclei. The chromatin structure of nuclei and micronuclei differs and heterochromatinization and disintegration of the nuclear envelope of micronuclei are the final steps of chromosome elimination.     

Mitotic chromosome size scaling in Xenopus

As rapid divisions without growth generate progressively smaller cells within an embryo, mitotic chromosomes must also decrease in size to permit their proper segregation, but this scaling phenomenon is poorly understood. We demonstrated previously that nuclear and spindle size scale between egg extracts of the related frog species Xenopus tropicalis and Xenopus laevis but show here that dimensions of isolated mitotic sperm chromosomes do not differ. This is consistent with the hypothesis that chromosome scaling does not occur in early embryonic development when cell and spindle sizes are large and anaphase B segregates chromosomes long distances. To recapitulate chromosome scaling during development, we combined nuclei isolated from different stage Xenopus laevis embryos with metaphase-arrested egg extracts. Mitotic chromosomes derived from nuclei of cleaving embryos through the blastula stage were similar in size to replicated sperm chromosomes but decreased in area approximately 50% by the neurula stage, reproducing the trend in size changes observed in fixed embryos. Allowing G₂ nuclei to swell in interphase prior to mitotic condensation did not increase mitotic chromosome size, but progression through a full cell cycle in egg extract did, suggesting that epigenetic mechanisms determining chromosome size can be altered during DNA replication. Comparison of different sized mitotic chromosomes assembled in vitro provides a tractable system to elucidate underlying molecular mechanisms.Key words: mitotic chromosomes, Xenopus, egg extracts, intracellular scaling, spindle, embryogenesis, cell division     

Maternal expression of the checkpoint protein BubR1 is required for synchrony of syncytial nuclear divisions and polar body arrest in Drosophila melanogaster

The spindle checkpoint is a surveillance mechanism that regulates the metaphase-anaphase transition during somatic cell division through inhibition of the APC/C ensuring proper chromosome segregation. We show that the conserved spindle checkpoint protein BubR1 is required during early embryonic development. BubR1 is maternally provided and localises to kinetochores from prophase to metaphase during syncytial divisions similarly to somatic cells. To determine BubR1 function during embryogenesis, we generated a new hypomorphic semi-viable female sterile allele. Mutant females lay eggs containing undetectable levels of BubR1 show early developmental arrest, abnormal syncytial nuclear divisions, defects in chromosome congression, premature sister chromatids separation, irregular chromosome distribution and asynchronous divisions. Nuclei in BubR1 mutant embryos do not arrest in response to spindle damage suggesting that BubR1 performs a checkpoint function during syncytial divisions. Furthermore, we find that in wild-type embryos BubR1 localises to the kinetochores of condensed polar body chromosomes. This localisation is functional because in mutant embryos, polar body chromatin undergoes cycles of condensation-decondensation with additional rounds of DNA replication. Our results suggest that BubR1 is required for normal synchrony and progression of syncytial nuclei through mitosis and to maintain the mitotic arrest of the polar body chromosomes after completion of meiosis.     

Order and disorder in the nucleus

Fluorescence in situ hybridization combined with three-dimensional microscopy has shown that chromosomes are not randomly strewn throughout the nucleus but are in fact fairly well organized, with different loci reproducibly found in different regions of the nucleus. At the same time, increasingly sophisticated methods to track and analyze the movements of specific chromosomal loci in vivo using four-dimensional microscopy have revealed that chromatin undergoes extensive Brownian motion. However, the diffusion of interphase chromatin is constrained, implying that chromosomes are physically anchored within the nucleus. This constraint on diffusion is the result of interactions between chromatin and structural elements within the nucleus, such as nuclear pores or the nuclear lamina. The combination of defined positioning with constrained diffusion has a strong impact on interactions between chromosomal loci, and appears to explain the tendency of certain chromosome rearrangements to occur during the development of cancer.     

Spatial organization of chromosomes in the salivary gland nuclei of Drosophila melanogaster

Using a computer-based system for model building and analysis, three-dimensional models of 24 Drosophila melanogaster salivary gland nuclei have been constructed from optically or physically sectioned glands, allowing several generalizations about chromosome folding and packaging in these nuclei. First and most surprising, the prominent coiling of the chromosomes is strongly chiral, with right-handed gyres predominating. Second, high frequency appositions between certain loci and the nuclear envelope appear almost exclusively at positions of intercalary heterochromatin; in addition, the chromocenter is always apposed to the envelope. Third, chromosomes are invariably separated into mutually exclusive spatial domains while usually extending across the nucleus in a polarized (Rabl) orientation. Fourth, the arms of each autosome are almost always juxtaposed, but no other relative arm positions are strongly favored. Finally, despite these nonrandom structural features, each chromosome is found to fold into a wide variety of different configurations. In addition, a set of nuclei has been analyzed in which the normally aggregrated centromeric regions of the chromosomes are located far apart from one another. These nuclei have the same architectural motifs seen in normal nuclei. This implies that such characteristics as separate chromosome domains and specific chromosome-nuclear envelope contacts are largely independent of the relative placement of the different chromosomes within the nucleus.     

Effect of donor cell cycle stage on chromatin and spindle morphology in nuclear transplant rabbit embryos.

We investigated the influence of the cell cycle stage of the nuclear donor on prematurely condensed chromatin (PCC) and spindle morphology and on chromosome constitution in rabbit nuclear transplant embryos. The configuration of PCC following nuclear transplantation with G1, early S, and late S phase donor nuclei (G1, early S, and late S transplants, respectively) was characterized in whole mounts and chromosome spreads. In addition, the influence of the donor cell cycle stage on chromosome constitution in cleavage stage-manipulated embryos was determined. Within 2 h after fusion of the donor blastomere, the recipient oocyte cytoplasm was able to induce formation de novo of a metaphase plate associated with a spindle in G1, early S, and late S transplants. Metaphase chromosomes and spindle were intact in most cases of PCC in G1 transplants. However, these structures displayed minor abnormalities in early S transplants and gross abnormalities in late S transplants, such as incomplete or absent spindle formation and incomplete chromatin condensation. Normal chromosomes were present in G1 and early S transplants, whereas chromosome abnormalities were detected in late S transplants. The results indicate that morphology of prematurely condensed G1 and early S chromatin has a minor influence on chromosome constitution of manipulated embryos. That of late S chromatin, however, affects chromosome constitution in embryos and may account for reduced development of nuclear transplant embryos when late S phase donor nuclei are used.     

Intranuclear DNA density affects chromosome condensation in metazoans

Chromosome condensation is critical for accurate inheritance of genetic information. The degree of condensation, which is reflected in the size of the condensed chromosomes during mitosis, is not constant. It is differentially regulated in embryonic and somatic cells. In addition to the developmentally programmed regulation of chromosome condensation, there may be adaptive regulation based on spatial parameters such as genomic length or cell size. We propose that chromosome condensation is affected by a spatial parameter called the chromosome amount per nuclear space, or “intranuclear DNA density.” Using Caenorhabditis elegans embryos, we show that condensed chromosome sizes vary during early embryogenesis. Of importance, changing DNA content to haploid or polyploid changes the condensed chromosome size, even at the same developmental stage. Condensed chromosome size correlates with interphase nuclear size. Finally, a reduction in nuclear size in a cell-free system from Xenopus laevis eggs resulted in reduced condensed chromosome sizes. These data support the hypothesis that intranuclear DNA density regulates chromosome condensation. This suggests an adaptive mode of chromosome condensation regulation in metazoans.     

Genetic and Molecular Analysis of the X Chromosomal Region 14b17-14c4 in Drosophila Melanogaster: Loss of Function in Nona, a Nuclear Protein Common to Many Cell Types, Results in Specific Physiological and Behavioral Defects

We have performed a genetic analysis of the 14C region of the X chromosome of Drosophila melanogaster to isolate loss of function alleles of no-on-transient A (nonA; 14C1-2; 1-52.3). NONA is a nuclear protein common to many cell types, which is present in many puffs on polytene chromosomes. Sequence data suggest that the protein contains a pair of RNA binding motifs (RRM) found in many single-strand nucleic acid binding proteins. Hypomorphic alleles of this gene, which lead to aberrant visual and courtship song behavior, still contain normally distributed nonA RNA and NONA protein in embryos, and in all available alleles NONA protein is present in puffs of third instar larval polytene chromosomes. We find that complete loss of this general nuclear protein is semilethal in hemizygous males and homozygous cell lethal in the female germline. Surviving males show more extreme defects in nervous system function than have been described for the hypomorphic alleles. Five other essential genes that reside within this region have been partially characterized.     

Homologs of eleven loci on human chromosome 11 assigned to two owl monkey chromosomes

We localized 11 loci mapped to human chromosome 11 to two chromosomes, 4 and 19 of owl monkey karyotype VI (2n = 49/50), by the use of somatic cell hybrids. Furthermore, using in situ hybridization to chromosomes of two owl monkey karyotypes, the HSTF1 oncogene locus was precisely localized on homologs 19q (K-VI) and 2q (K-II). Comparative analysis of available gene-mapping data among human, mouse, and owl monkey chromosomes revealed a pattern of evolutionary change in a syntenic group on human chromosome 11. These structural changes could be explained as having derived from a pericentric inversion of human chromosome region 11cen----q13 and a translocation involving human region 11q22----qter during primate evolution.     

Ultrastructure and 3H-thymidine incorporation by chromosome vesicles in sea urchin embryos

The ultrastructural features of chromosome vesicle formation in early sea urchin embryos and chromosome vesicle uptake of tritiated thymidine is described. Envelopes which resemble typical nuclear envelopes form around the condensed anaphase chromosomes. In late anaphase or early telophase, the chromosomes swell and decondense and it is at this time when tritiated thymidine is incorporated. This study shows that DNA synthesis in the rapidly dividing cells of early sea urchin embryos occurs in chromosome vesicles which form during anaphase.     

Three-dimensional multiple-wavelength fluorescence microscopy for the structural analysis of biological phenomena.

Cellular events are accomplished by the coordinated interactions of cellular components within the three-dimensional context of a cell. Simultaneous observation of multiple components in three dimensions can be essential for understanding such interactions. Toward this end, we have developed a computerized microscope workstation capable of recording three-dimensional images of multiple cellular components in fixed and living cells. All aspects of microscope control, data collection, image processing and analysis can be performed on the one workstation. In this report, we describe the components and capabilities of this integrated system. In addition, we discuss some general problems of multiple-wavelength, three-dimensional imaging and our application of this technology to the analysis of chromosome organization in Drosophila melanogaster. Three-dimensional imaging of fixed embryos stained by indirect immunofluorescence has revealed the structural organization of chromosomes, microtubules, and the nuclear lamins. Imaging of living embryos injected with fluorescently labelled proteins has confirmed and extended these results by allowing the study of these structures throughout the cell cycle. The combination of the molecular specificity of fluorescence microscopy and the three-dimensional structural information obtained by our workstation has provided novel insights into the dynamic aspects of chromosome behavior during the cell cycle. We believe this system has many important applications in the study of the molecular basis of cellular events.     

Developmental abnormalities in mouse embryos tetrasomic for chromosome 11: apparent similarity to embryos functionally disomic for the x chromosome

Adopting a mating system involving two different Robertsonian translocations with monobrachial homology, we studied the early development of mouse embryos trisomic or tetrasomic for chromosome 11. A developmental delay of 12-24 hours was evident in trisomic embryos at embryonic day (E)7.5, whereas tetrasomic embryos apparently had stopped growth by E6.5 without formation of extraembryonic structures. This extremely severe developmental abnormality found in tetrasomic embryos is similar to that reported in embryos having two active X chromosomes in extraembryonic cell lineages. Autosomal tetrasomy, but not autosomal trisomy, can lead to such early developmental errors. Thus, a reasonable inference would be that the X chromosome is twice as active as the autosome. Probably, the X chromosome became upregulated in response to the evolutionary necessity of minimizing haplo-insufficiency brought about by miniaturization of the Y chromosome.     

Cloned calves from chromatin remodeled in vitro

We have developed a novel system for remodeling mammalian somatic nuclei in vitro prior to cloning by nuclear transplantation. The system involves permeabilization of the donor cell and chromatin condensation in a mitotic cell extract to promote removal of nuclear factors solubilized during chromosome condensation. The condensed chromosomes are transferred into enucleated oocytes prior to activation. Unlike nuclei of nuclear transplant embryos, nuclei of chromatin transplant embryos exhibit a pattern of markers closely resembling that of normal embryos. Healthy calves were produced by chromatin transfer. Compared with nuclear transfer, chromatin transfer shows a trend toward greater survival of cloned calves up to at least 1 mo after birth. This is the first successful demonstration of a method for directly manipulating the somatic donor chromatin prior to transplantation. This procedure should be useful for investigating mechanisms of nuclear reprogramming and for making improvements in the efficiency of mammalian cloning.