Incomplete sister chromatid separation of long chromosome arms

Chromosome territory reorganization through artificial chromosome fusion is compatible with cell fate determination and mouse development

Chromosomes occupy discrete spaces in the interphase cell nucleus, called chromosome territory. The structural and functional relevance of chromosome territory remains elusive. We fused chromosome 15 and 17 in mouse haploid embryonic stem cells (haESCs), resulting in distinct changes of territories in the cognate chromosomes, but with little effect on gene expression, pluripotency and gamete functions of haESCs. The karyotype-engineered haESCs were successfully implemented in generating heterozygous (2n = 39) and homozygous (2n = 38) mouse models. Mice containing the fusion chromosome are fertile, and their representative tissues and organs display no phenotypic abnormalities, suggesting unscathed development. These results indicate that the mammalian chromosome architectures are highly resilient, and reorganization of chromosome territories can be readily tolerated during cell differentiation and mouse development.

A sustainable mouse karyotype created by programmed chromosome fusion

Chromosome engineering has been attempted successfully in yeast but remains challenging in higher eukaryotes, including mammals. Here, we report programmed chromosome ligation in mice that resulted in the creation of new karyotypes in the lab. Using haploid embryonic stem cells and gene editing, we fused the two largest mouse chromosomes, chromosomes 1 and 2, and two medium-size chromosomes, chromosomes 4 and 5. Chromatin conformation and stem cell differentiation were minimally affected. However, karyotypes carrying fused chromosomes 1 and 2 resulted in arrested mitosis, polyploidization, and embryonic lethality, whereas a smaller fused chromosome composed of chromosomes 4 and 5 was able to be passed on to homozygous offspring. Our results suggest the feasibility of chromosome-level engineering in mammals.

Myosin efflux promotes cell elongation to coordinate chromosome segregation with cell cleavage

Chromatid segregation must be coordinated with cytokinesis to preserve genomic stability. Here we report that cells clear trailing chromatids from the cleavage site by undergoing two phases of cell elongation. The first phase relies on the assembly of a wide contractile ring. The second phase requires the activity of a pool of myosin that flows from the ring and enriches the nascent daughter cell cortices. This myosin efflux is a novel feature of cytokinesis and its duration is coupled to nuclear envelope reassembly and the nuclear sequestration of the Rho-GEF Pebble. Trailing chromatids induce a delay in nuclear envelope reassembly concomitant with prolonged cortical myosin activity, thus providing forces for the second elongation. We propose that the modulation of cortical myosin dynamics is part of the cellular response triggered by a “chromatid separation checkpoint” that delays nuclear envelope reassembly and, consequently, Pebble nuclear sequestration when trailing chromatids are present at the midzone.

Chromatid segregation must be coordinated with cytokinesis to preserve genomic stability. Here the authors show that cells clear trailing chromatids from the cleavage site in a two-step cell elongation and demonstrate the role of myosin efflux in the second phase.

Live cell imaging of micronucleus formation and development

The micronucleus (MN) test is widely used to biomonitor humans exposed to clastogens and aneugens, but little is known about MN development. Here we used confocal time-lapse imaging and a fluorescent human lymphoblastoid cell line (T105GTCH), in which histone H3 and α-tubulin stained differentially, to record the emergence and behavior of micronuclei (MNi) in cells exposed to MN-inducing agents. In mitomycin C (MMC)-treated cells, MNi originated in early anaphase from lagging chromosome fragments just after chromosome segregation. In γ-ray-treated cells showing multipolar cell division, MN originated in late anaphase from lagging chromosome fragments generated by the abnormal cell division associated with supernumerary centrosomes. In vincristine(VC)-treated cells, MN formation was similar to that in MMC-treated cells, but MNi were also derived from whole chromosomes that did not align properly on the metaphase plate. Thus, the MN formation process induced by MMC, γ-rays, and VC, were strikingly different, suggesting that different mechanisms were involved. MN stability, however, was similar regardless of the treatment and unrelated to MN formation mechanisms. MNi were stable in daughter cells, and MN-harboring cells tended to die during cell cycle progression with greater frequency than cells without MN. Because of their persistence, MN may have significant impact on cells, causing genomic instability and abnormally transcribed genes.

Modifications of nuclear envelope during differentiation and depolyploidization of rat trophoblast cells

An increased activity of membranes of the nuclear envelope (NE) was observed electron microscopically in the trophoblast cells of the rat placenta. The activity of the membranes was manifested as formation of various NE derivatives, such as the annulate lamellae (AL), the intranuclear tubules, and the concentric membranous structures. At the period of terminal differentiation of the secondary giant trophoblast cells (SGTC) the NE derivatives play active role in subdivision of the initial highly polyploid nuclei into the numerous low-ploidy fragments. (3)H-thymidine labeling showed that attenuation of the DNA replication precedes the nuclear fragmentation. In the course of the nuclear fragmentation the narrow deep NE invaginations subdivide the nucleus into the separate lobes that subsequently are detached from the initial nucleus. By the beginning of the fragmentation, the accumulated membranous structures, i.e. the intranuclear AL, tubules, clusters of pore complexes, etc., seem to be the source of the reserve material that is necessary for formation of the great amount of the NE membranes of the newly formed nuclear fragments. Thus, the intranuclear membranous structures that seem to increase the active surface of the growing endopolyploid nucleus at the earlier stage of differentiation then take part in genome isolation that results in formation of a multinucleate cell with diploid and low-polyploid nuclei. The outer NE membrane of the initial nucleus plays an active role in compartmentalization of cytoplasmic areas around the nuclear fragments within the giant polykaryocyte. Apart from the membranous structures the bundles of intermediate filaments (IF) located in the cytoplasm perinuclear zones seem to participate in the nuclear fragmentation. These processes are likely to provide formation of the giant polykaryocytes incapable for further proliferation.