883 — Electrofusion of rat and mouse blastomeres

Aggregation recovers developmental plasticity in mouse polyploid embryos

Tetraploid embryos normally develop into blastocysts and embryonic stem cells can be established from tetraploid blastocysts in mice. Thus, polyploidisation does not seem to be so harmful during preimplantation development. However, the mechanisms by which early mammalian development accepts polyploidisation are poorly understood. In this study, we aimed to elucidate the effect of polyploidisation on early mammalian development and to further comprehend its tolerance using hyperpolyploid embryos produced by repetitive whole genome duplication. We successfully established several types of polyploid embryos (tetraploid, octaploid and hexadecaploid) and studied their developmental potential invitro. We demonstrated that all types of these polyploid embryos maintained the ability to develop to the blastocyst stage, which implies that mammalian cells might have basic cellular functions in implanted embryos, despite polyploidisation. However, the inner cell mass was absent in hexadecaploid blastocysts. To complement the total number of cells in blastocysts, a fused hexadecaploid embryo was produced by aggregating several hexadecaploid embryos. The results indicated that the fused hexadecaploid embryo finally recovered pluripotent cells in the blastocyst. Thus, our findings suggest that early mammalian embryos may have the tolerance and higher plasticity to adapt to hyperpolyploidisation for blastocyst formation, despite intense alteration of the genome volume.

Tetraploid Embryonic Stem Cells Maintain Pluripotency and Differentiation Potency into Three Germ Layers

Polyploid amphibians and fishes occur naturally in nature, while polyploid mammals do not. For example, tetraploid mouse embryos normally develop into blastocysts, but exhibit abnormalities and die soon after implantation. Thus, polyploidization is thought to be harmful during early mammalian development. However, the mechanisms through which polyploidization disrupts development are still poorly understood. In this study, we aimed to elucidate how genome duplication affects early mammalian development. To this end, we established tetraploid embryonic stem cells (TESCs) produced from the inner cell masses of tetraploid blastocysts using electrofusion of two-cell embryos in mice and studied the developmental potential of TESCs. We demonstrated that TESCs possessed essential pluripotency and differentiation potency to form teratomas, which differentiated into the three germ layers, including diploid embryonic stem cells. TESCs also contributed to the inner cell masses in aggregated chimeric blastocysts, despite the observation that tetraploid embryos fail in normal development soon after implantation in mice. In TESCs, stability after several passages, colony morphology, and alkaline phosphatase activity were similar to those of diploid ESCs. TESCs also exhibited sufficient expression and localization of pluripotent markers and retained the normal epigenetic status of relevant reprogramming factors. TESCs proliferated at a slower rate than ESCs, indicating that the difference in genomic dosage was responsible for the different growth rates. Thus, our findings suggested that mouse ESCs maintained intrinsic pluripotency and differentiation potential despite tetraploidization, providing insights into our understanding of developmental elimination in polyploid mammals.

Altered gene expression profiles in mouse tetraploid blastocysts

In this study, it was demonstrated that tetraploid-derived blastocyst embryos had very few Oct4-positive cells at the mid-blastocyst stage and that the inner cell mass at biomarkers Oct4, Sox2 and Klf4 was expressed at less than 10% of the level observed in diploid blastocysts. In contrast, trophectoderm-related gene transcripts showed an approximately 10 to 40% increase. Of 32,996 individual mouse genes evaluated by microarray, 50 genes were differentially expressed between tetraploid or diploid and parthenote embryos at the blastocyst stage (P<0.05). Of these 50 genes, 28 were more highly expressed in tetraploid-derived blastocysts, whereas 22 were more highly downregulated. However, some genes involved in receptor activity, cell adhesion molecule, calcium ion binding, protein biosynthesis, redox processes, transport, and transcription showed a significant decrease or increase in gene expression in the tetraploid-derived blastocyst embryos. Thus, microarray analysis can be used as a tool to screen for underlying defects responsible for the development of tetraploid-derived embryos.

Chromosome remodeling and differentiation of tetraploid embryos during preimplantation development

Although it is known that the tetraploid embryo contributes only to the placenta, the question of why tetraploid embryos differentiate into placenta remains unclear. To study the effect of electrofusion on the development of mouse tetraploid oocytes, mouse two-cell embryos were fused and cultured in vitro in Chatot-Ziomek-Bavister medium. After electrofusion, two chromosome sets from the tetraploid blastomere were individually duplicated before nuclear fusion. At 8-10 hr after electrofusion, each chromosome set was condensing and the nuclear membrane was breaking down. Around 12-14 hr after electrofusion, the two chromosome sets had combined together and had reached the second mitotic metaphase, at this point with 8n sets of chromosomes. Interestingly, we discovered that expression of OCT4, an inner cell mass cells biomarker, is lost by the tetraploid expanded blastocysts, but that CDX2, a trophectoderm cells biomarker, is strongly expressed at this stage. This observation provides evidence clarifying why tetraploid embryos contribute only to trophectoderm.

Frequency change-induced alternative potential waveform dependence of membrane damage to cells cultured on an electrode surface

In the present study, alternative potential stimulation with rectangular pulse, sine and triangular waveforms at 10 and 100Hz was applied to cells cultured on an ITO electrode. As a result, we found that the alternating potential waveform dependence induced by the frequency on membrane damage of cells cultured on an electrode surface. The cell membrane damage was promoted by a rectangular pulse wave in comparison with sine and triangular waves, when alternating electrical potentials of 0 to +1.0V at 100Hz were loaded. In contrast, this waveform dependence was not observed when the frequency was 10Hz. Furthermore, it was found that cell membrane damage was induced at positive potentials more than +0.8V under the present experimental conditions.

Improved embryonic stem cell technologies

Murine embryonic stem (ES) cells have become an indispensable tool for investigating genetic function both in vitro and, importantly, in vivo. Recent advances, including tetraploid aggregation, new site-specific recombinases and RNAi, have enabled more sophisticated manipulation of the ES cell genome. For instance, it is now possible to control gene expression in both a temporally and spatially restricted manner. Such new technologies are answering complex questions surrounding the function and interaction of an increasing number of genes. This chapter will review both the history and recent technological progress that has been made in mouse ES cell derivation, genetic manipulation and the generation of ES cell-derived chimaeric animals.

An alternative method of deriving embryonic stem cell-like clones by aggregation of diploid cells with tetraploid embryos

OBJECTIVE

To assess whether embryonic stem (ES) cells could be derived from the aggregation of diploid cells with tetraploid embryos.

DESIGN

Randomized, prospective study.

SETTING

University embryology and gamete biotechnology laboratory.

ANIMAL(S)

F1 (C57BL6/DBA2) mice.

INTERVENTION(S)

Four- to eight-cell F1 tetraploid embryos were aggregated with 10 to 15 donor E14 ES cells.

MAIN OUTCOME MEASURE(S)

Embryogenesis and ES cell establishment.

RESULT(S)

No difference (78% to 89%) in blastocyst formation was detected between the aggregated tetraploid and the control diploid embryos. In a total of 27 transfers, pregnancy was detected in three tetraploid (23.1%) and five diploid (35.7%) cases, and three live births developed from the aggregated tetraploid embryos. The tetraploid blastocysts without aggregation were plated, but no ES cell-like colony was formed. Six of eight aggregated blastocysts derived well-proliferated colonies, which were positive for anti-stage-specific embryonic antigen (SSEA)-1 antibody, Oct-4, and alkaline phosphatase. The microsatellite assay confirmed the homogenous makeup among the donor E14 cells and live-birth and ES-like cells derived from the E14-aggregated, tetraploid embryo.

CONCLUSION(S)

The aggregation of pluripotent diploid cells with tetraploid embryos yielded live births and ES-like cells that were homogenous to the donor diploid cells.

Tetraploid development in the mouse

Spontaneous duplication of the mammalian genome occurs in approximately 1% of fertilizations. Although one or more whole genome duplications are believed to have influenced vertebrate evolution, polyploidy of contemporary mammals is generally incompatible with normal development and function of all but a few tissues. The production of tetraploid (4n) embryos has become a common experimental manipulation in the mouse. Although development of tetraploid mice has generally not been observed beyond midgestation, tetraploid:diploid (4n:2n) chimeras are widely used as a method for rescuing extraembryonic defects. The tolerance of tissues to polyploidy appears to be dependent on genetic background. Indeed, the recent discovery of a naturally tetraploid rodent species suggests that, in rare genetic backgrounds, mammalian genome duplications may be compatible with the development of viable and fertile adults. Thus, the range of developmental potentials of tetraploid embryos remains in large part unexplored. Here, we review the biological consequences and experimental utility of tetraploid mammals, in particular the mouse.

Effect of electrical stimulation on HIV-1-infected HeLa cells cultured on an electrode surface

Acquired immunodeficiency syndrome (AIDS) is a disease caused by infection with the human immunodeficiency virus (HIV). Although drug therapy for AIDS is available, problems such as side effects associated with drug therapy and the appearance of resistant HIV strains have arisen. Therefore, therapies based on new principles other than drug treatment are required. In the present study, the effect of electrical stimulation on HIV-1(LAI) chronically infected HeLa (P6 HeLa/HIV-1(LAI)) cells cultured on an electrode surface was examined. The results indicated that sensitivity to electrical stimulation was much higher in P6 HeLa/HIV-1(LAI) cells than in uninfected P6 HeLa cells. When electrical stimulation was applied at 1.0 V (vs. Ag/AgCl) for 20 min, the proportion of damage to cell membrane among P6 HeLa/HIV-1(LAI) cells, as evaluated by Trypan blue staining, was approximately 4 times higher than that for uninfected P6 HeLa cells. Furthermore, in comparison with uninfected P6 HeLa cells, the proliferation of P6 HeLa/HIV-1(LAI) cells was significantly suppressed after electrical stimulation. This technique was proven to selectively kill P6 HeLa/HIV-1(LAI) cells, when compared with uninfected control cells.

Stage-specific gene expression in asynchronous tetraploid mouse embryos formed by fusion of blastomeres and fertilized eggs

Asynchronous tetraploid mouse embryos were generated by electrofusion of fertilized eggs with blastomeres from different cleavage stages. The majority of the cytoplasm was always contributed by the egg. The best development was observed when eggs were fused with 2-cell blastomeres. Both genomes became active in fusion embryos (at least the genes for glucose phosphate isomerase did). Stage-specific protein synthesis seemed to be more adjusted to the developmental stage of the egg's than of the blastomere's genome, but at the 2-cell stage both contributed slightly differently to the protein patterns. Also, the time range of the first appearance of the stage-specific embryonic antigen SSEA-1 was wider in fusion embryos than in controls. It seems that the two genomes are not completely synchronized in these tetraploid embryos, a further indication that, in the mouse, the cytoplasm of fertilized eggs might not be compatible with older embryonic nuclei.

Electrofusion of mouse embryos results in uniform tetraploidy and not tetraploid/diploid mosaicism

Some previous attempts to produce tetraploids experimentally have resulted in a proportion of treated embryos becoming 2n/4n mosaics at a frequency which may be as high as 20%, when using cytochalasin B as a fusigenic stimulus and cytogenetic techniques to identify putative tetraploid embryos. To investigate the possible occurrence of 4n/2n mosaicism, tetraploid embryos were produced by electrofusion, a process which allows adjacent blastomeres at the 2-cell stage to fuse following exposure to electric field pulses. Embryos used for electrofusion were hemizygous for a transgene consisting of approximately 1000 copies of the mouse beta-globin gene. After in situ hybridization, one hybridization signal is expected per diploid genome. Tetraploid cells in 7.5-, 8.5-, 9.5- and 10.5-day-old conceptuses were distinguished from diploid cells by performing in situ hybridization on histological sections. The frequency of nuclei with two hybridization signals in the 'hemizygous' tetraploid embryos was compared to diploid embryos which were either hemizygous or homozygous for the beta-globin transgene. Comparison of the frequency of nuclei with two hybridization signals between tissues of 'hemizygous' tetraploid conceptuses and homozygous diploid conceptuses showed no significant difference, which implies that the tissues in the tetraploid conceptuses were uniformly tetraploid. No evidence was found to suggest that electrofusion results in 2n/4n mosaicism.

Electrically controlled proliferation of human carcinoma cells cultured on the surface of an electrode

Human carcinoma cells, MKN45, were cultured on the surface of a metal-coated plastic plate electrode the potential of which was controlled. The proliferation rate and cell morphology were altered depending on the applied potential. Cell proliferation was halted in the potential range above 0.4 V vs. Ag/AgCl, although cells started to proliferate again when the applied potential was shifted from 0.4 V to 0.1 V vs. Ag/AgCl. Fluorescence probe studies indicated that the fluidity of plasma membrane decreased in association with halting of cell proliferation. These results suggest that electrical stimulation causes cells to temporarily halt proliferation, and that cell proliferation was reversibly controlled by electrode potential. The mechanism is interpreted in relation to the change of plasma membrane structure represented by membrane fluidity.