Tetraploid development in the mouse

Developmental abnormalities of NT mouse embryos appear early after implantation

In mammals, cloning by nuclear transfer (NT) into an enucleated oocyte is a very inefficient process, even if it can generate healthy adults. We show that blastocysts derived from embryonic stem (ES) donor cells develop at a high rate, correctly express the pluripotential marker gene Oct4 in ICM cells and display normal growth in vitro. Moreover, the majority of them implant in the uterus of recipient females. We combine embryological studies, gene expression analysis during gastrulation and generation of chimaeric embryos to identify the developmental origin (stage and tissue affected) of NT embryo mortality. The majority died before mid-gestation from defects arising early, either at peri-implantation stages or during the gastrulation period. The first type of defect is a non-cell autonomous defect of the epiblast cells and is rescued by complementation of NT blastocysts with normal ES or ICM cells. The second type of defect affects growth regulation and the shape of the embryo but does not directly impair the initial establishment of the patterning of the embryo. Only chimaeras formed by the aggregation of NT and tetraploid embryos reveal no growth abnormalities at gastrulation. These studies indicate that the trophoblast cell lineage is the primary source of these defects. These embryological studies provide a solid basis for understanding reprogramming errors in NT embryos. In addition, they unveil new aspects of growth regulation while increasing our knowledge on the role of crosstalk between the extra-embryonic and the embryonic regions of the conceptus in the control of growth and morphogenesis.

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.

The genetic heterozygosity and fitness of tetraploid embryos and embryonic stem cells are crucial parameters influencing survival of mice derived from embryonic stem cells by tetraploid embryo aggregation

The aim of this paper was to determine whether the genetic background of tetraploid embryos contributed to the survival of mice derived from embryonic stem (ES) cells by tetraploid embryo complementation. Twenty-five newborns were produced by aggregation of hybrid ES cells and tetraploid embryos with different genetic backgrounds. These newborns were entirely derived from ES cells judged by microsatellite DNA (A specific sequence of DNA bases or nucleotides that contains mono, di, tri or tetra repeats) and coat colour phenotype and germline transmission. Fifteen survived to adulthood while seven died of respiratory failure. All newborns were derived from outbred or hybrid tetraploid aggregates and no newborns were from the inbreds. Our results demonstrate that the genetic heterozygosity, fitness of tetraploid embryos and fitness of ES cells are crucial parameters influencing survival of mice derived from ES cells by tetraploid embryo aggregation. In addition, this method represents a simple and efficient procedure for immediate generation of targeted mouse mutants from genetically modified ES cell clones, in contrast to the standard protocol, which involves the production of chimeras and several breeding steps.

Fate of tetraploid cells in 4n<-->2n chimeric mouse blastocysts

Previous studies have shown that tetraploid (4n) cells rarely contribute to the derivatives of the epiblast lineage of mid-gestation 4n<-->2n mouse chimeras. The aim of the present study was to determine when and how 4n cells were excluded from the epiblast lineage of such chimeras. The contributions of GFP-positive cells to different tissues of 4n<-->2n chimeric blastocysts labelled with tauGFP were analysed at E3.5 and E4.5 using confocal microscopy. More advanced E5.5 and E7.5 chimeric blastocysts were analysed after a period of diapause to allow further growth without implantation. Tetraploid cells were not initially excluded from the epiblast in 4n<-->2n chimeric blastocysts and they contributed to all four blastocyst tissues at all of the blastocyst stages examined. Four steps affected the allocation and fate of 4n cells in chimeras, resulting in their exclusion from the epiblast lineage by mid-gestation. (1) Fewer 4n cells were allocated to the inner cell mass than trophectoderm. (2) The blastocyst cavity tended to form among the 4n cells, causing more 4n cells to be allocated to the hypoblast and mural trophectoderm than the epiblast and polar trophectoderm, respectively. (3) 4n cells were depleted from the hypoblast and mural trophectoderm, where initially they were relatively enriched. (4) After implantation 4n cells must be lost preferentially from the epiblast lineage. Relevance of these results to the aetiology of human confined placental mosaicism and possible implications for the interpretation of mouse tetraploid complementation studies of the site of gene action are discussed.

Developmental potential and behavior of tetraploid cells in the mouse embryo

Tetraploid (4n) mouse embryos die at variable developmental stages. By examining 4n embryos from F2 hybrid and outbred mice, we show that 4n developmental potential is influenced by genetic background. The imprinted inactivation of an X chromosome-linked eGFP transgene in extraembryonic tissues occurred correctly in 4n embryos. A decrease of the cleavage rate in 4n preimplantation embryos compared to diploid (2n) embryos was revealed by real-time imaging, using a histone H2b:eGFP reporter. It has previously been known that mouse chimeras produced by the combination of diploid (2n) embryos with embryonic stem (ES) cells result in mixtures of the two components in epiblast-derived tissues. In contrast, the use of 4n host embryos with ES cells restricts 4n cells from the embryonic regions of chimeras, resulting in mice that are believed to be completely ES-derived. Using H2b:eGFP transgenic mice and ES cells, the behavior of 4n cells was determined at single cell resolution in 4n:2n injection and aggregation chimeras. We found a significant contribution of 4n cells to the embryonic ectoderm at gastrulation in every chimera analyzed. We show that the transition of the embryonic regions from a chimeric tissue to a predominantly 2n tissue occurs after gastrulation and that tetraploid cells may persist to midgestation. These findings suggest that the results of previously published tetraploid complementation assays may be influenced by the presence of tetraploid cells in the otherwise diploid embryonic regions.

Transcriptional consequences of autosomal trisomy: primary gene dosage with complex downstream effects

Autosomal trisomy is a common cause of human miscarriage, malformations and learning disability. Primary gene-dosage effects have been confirmed by recent transcriptome analyses. The importance (or existence) of trans-acting effects on disomic genes remains, surprisingly, controversial. In this article, I propose a model of the main genetic mechanisms that are responsible for producing the transcriptional derangement associated with trisomy. This has implications for future study design.

Rescue of keratin 18/19 doubly deficient mice using aggregation with tetraploid embryos

We have previously shown that the targeted deletions of both type I keratins (K) 18 and 19 cause lethality by embryonic day (e) 9.5 due to fragility and cytolysis of trophoblast giant cells. The development of the embryo proper appeared to be unaffected and its death was caused by nutrient deficiency. In order to address the function of keratins within the embryo proper, lethality due to extraembryonic tissue failure must be overcome. One approach to rescue doubly deficient embryos is by aggregating knockout embryos with tetraploid wild-type embryos. As a general tool, tetraploid aggregation can be used to rescue embryonic lethality caused by defects in extraembryonic tissues like the placenta, trophoblast or yolk sac. We rescued K18-/- K19-/- embryos until e11.5, using this approach, proving that the loss of the keratin cytoskeleton causes defects in the trophoblast giant cell layer, but has no effect on early development of the embryo proper.

Simple and efficient production of mice derived from embryonic stem cells aggregated with tetraploid embryos

Six newly derived hybrid mouse embryonic stem (ES) cell lines and two inbred ES cell lines were tested for their ability to produce completely ES cell-derived mice by aggregation of ES cells with tetraploid embryos. Forty-five ES cell-tetraploid pups were generated from six hybrid ES cell lines and no pups from two inbred ES cell lines. These pups were found to have increased embryonic and placental weights than control mice. Twenty-two pups survived to adulthood and produced normal offsprings, and the other 23 pups died of several reasons including respiratory distress, abdomen ulcer-like symptoms, and foster failure. The 22 adult ES cell-tetraploid mice were completely ES cell-derived as judged by coat color and germline transmission, only two of them was found to have tetraploid component in liver, blood, and lung as analyzed by microsatellite loci. Our data suggested that genetic heterozygosity is a crucial factor for postnatal survival of ES cell-tetraploid mice, and tetraploid embryo aggregation using hybrid ES cells is a simple and efficient procedure for immediate generation of targeted mouse mutants from genetically modified ES cell clones, in contrast to the standard protocol, which involves the production of chimeras and several breeding steps.

Regulation of therapeutic apoptosis: a potential target in controlling hypertensive organ damage

Cell growth and survival are potential therapeutic targets for the control of complications associated with hypertension. In most cardiovascular disorders, cardiac fibroblasts and large-vessel smooth muscle cells can replicate and thus contribute to the disease. We propose that cardiovascular hyperplasia may be reversed via therapeutic apoptosis induction with drugs that are safe and already used in the clinic. We first reported that, irrespective of the drug class, those drugs that are able to induce regression of cardiovascular hypertrophy are also able to reverse cardiovascular hyperplasia via apoptosis. Drugs active in this regard include inhibitors of the renin-angiotensin system, calcium channel blockers, and beta-blockers. Moreover, the effects of these drugs on cell survival is not merely secondary to blood pressure reduction. Therapeutic apoptosis in the cardiovascular system of the spontaneously hypertensive rat is characterized by a rapid and transient onset following initiation of antihypertensive treatment. Herein, the induction and termination of therapeutic apoptosis during drug treatment of hypertension will be briefly reviewed and supported by novel data suggesting that reversal of cardiovascular hyperplasia is associated with reduced cell growth and a resistance to further induction of therapeutic apoptosis, as shown in spontaneously hypertensive rats receiving an intermittent regime of nifedipine therapy. We propose that the presence of a cell subpopulation with defective cell cycle regulation may determine organ susceptibility to undergo therapeutic apoptosis.Key words: apoptosis, hypertension, hyperplasia, growth, nifedipine.

Adult stem cells--reprogramming neurological repair?

Much excitement has surrounded recent breakthroughs in embryonic stem-cell research. Of lower profile, but no less exciting, are the advances in the field of adult stem-cell research, and their implications for cell therapy. Clinical experience from use of adult haemopoietic stem cells in haematology will facilitate and hasten transition from laboratory to clinic--indeed, clinical trials using adult human stem cells are already in progress in some disease states, including myocardial ischaemia. Here, with particular reference to neurology, we review processes that might underlie apparent changes in adult cell phenotype. We discuss implications these processes might have for the development of new therapeutic strategies using adult stem cells.