Male and female mice derived from the same embryonic stem cell clone by tetraploid embryo complementation

Deficiency of the splicing factor Sfrs10 results in early embryonic lethality in mice and has no impact on full-length SMN/Smn splicing

The SR-like splicing factor SFRS10 (Htra2-beta1) is well known to influence various alternatively spliced exons without being an essential splicing factor. We have shown earlier that SFRS10 binds SMN1/SMN2 RNA and restores full-length (FL)-SMN2 mRNA levels in vitro. As SMN1 is absent in patients with spinal muscular atrophy (SMA), the level of FL-SMN2 determines the disease severity. Correct splicing of SMN2 can be facilitated by histone deacetylase inhibitors (HDACis) via upregulation of SFRS10. As HDACis are already used in SMA clinical trials, it is crucial to identify the spectrum of alternatively spliced transcripts modulated by SFRS10, because elevated SFRS10 levels may influence or misregulate also other biological processes. To address this issue, we generated a conditional Sfrs10 allele in mice using the Cre/loxP system. The ubiquitous homozygous deletion of Sfrs10, however, resulted in early embryonic lethality around E7.5, indicating an essential role of Sfrs10 during mouse embryogenesis. Deletion of Sfrs10 with recombinant Cre in murine embryonic fibroblasts (MEFs) derived from Sfrs10(fl/fl) embryos increased the low levels of SmnDelta7 3-4-fold, without affecting FL-Smn levels. The weak influence of Sfrs10 on Smn splicing was further proven by a Hb9-Cre driven motor neuron-specific deletion of Sfrs10 in mice, which developed normally without revealing any SMA phenotype. To assess the role of Sfrs10 on FL-SMN2 splicing, we established MEFs from Smn(-/-);SMN2(tg/tg);Sfrs10(fl/fl) embryos. Surprisingly, deletion of Sfrs10 by recombinant Cre showed no impact on SMN2 splicing but increased SMN levels. Our findings highlight the complexity by which alternatively spliced exons are regulated in vivo.

Germline competence of mouse ES and iPS cell lines: Chimera technologies and genetic background

In mice, gene targeting by homologous recombination continues to play an essential role in the understanding of functional genomics. This strategy allows precise location of the site of transgene integration and is most commonly used to ablate gene expression ("knock-out"), or to introduce mutant or modified alleles at the locus of interest ("knock-in"). The efficacy of producing live, transgenic mice challenges our understanding of this complex process, and of the factors which influence germline competence of embryonic stem cell lines. Increasingly, evidence indicates that culture conditions and in vitro manipulation can affect the germline-competence of Embryonic Stem cell (ES cell) lines by accumulation of chromosome abnormalities and/or epigenetic alterations of the ES cell genome. The effectiveness of ES cell derivation is greatly strain-dependent and it may also influence the germline transmission capability. Recent technical improvements in the production of germline chimeras have been focused on means of generating ES cells lines with a higher germline potential. There are a number of options for generating chimeras from ES cells (ES chimera mice); however, each method has its advantages and disadvantages. Recent developments in induced pluripotent stem (iPS) cell technology have opened new avenues for generation of animals from genetically modified somatic cells by means of chimera technologies. The aim of this review is to give a brief account of how the factors mentioned above are influencing the germline transmission capacity and the developmental potential of mouse pluripotent stem cell lines. The most recent methods for generating specifically ES and iPS chimera mice, including the advantages and disadvantages of each method are also discussed.

Effect of stem cell activation, culture media of manipulated embryos, and site of embryo transfer in the production of F0 embryonic stem cell mice

Recently, F0 embryonic stem (ES) cell mice have been produced by injection of ES cells into eight-cell embryos using either laser- or piezo-assisted injection systems. To simplify the injection procedure, we have optimized the conventional blastocyst injection method, free of laser- or piezo-assisted micromanipulation systems, to produce F0 ES cell pups. To increase the efficiency of producing mice from ES cell injection into eight-cell and blastocyst stage embryos, we have tested: 1) the effect of activating ES cell before injection, 2) the effect of in vitro culture in medium optimized for the survival of both ES cells and embryos, and 3) the effect of transferring the micromanipulated embryos into the oviduct versus into the uterus of CD1 foster mice. Two B6D2 hybrid ES cell lines were used for injection in a multifactorial analysis to evaluate the efficiency of producing live chimeric and F0 ES cell mice. Our results demonstrate that the activation of ES cells and the appropriate culture conditions are crucial parameters influencing the generation of F0 ES cell offspring. Transfer of blastocysts injected with ES cells into the oviduct of 0.5-day postcoitum pseudopregnant females increased the number of live animals with higher chimera proportion. Under these conditions, injections into eight-cell embryos produce a high number of F0 ES mice, and the conventional blastocyst injection method produces a lower number of F0 ES cell pups; however, the efficiency of production of chimeric mice with germline transmission was high. We have developed an economical and efficient technique for producing fully ES cell-derived F0 mice with full germline transmission that can be applied in many laboratories without the use of piezo or laser instruments.

Studying early lethality of 45,XO (Turner's syndrome) embryos using human embryonic stem cells

Turner's syndrome (caused by monosomy of chromosome X) is one of the most common chromosomal abnormalities in females. Although 3% of all pregnancies start with XO embryos, 99% of these pregnancies terminate spontaneously during the first trimester. The common genetic explanation for the early lethality of monosomy X embryos, as well as the phenotype of surviving individuals is haploinsufficiency of pseudoautosomal genes on the X chromosome. Another possible mechanism is null expression of imprinted genes on the X chromosome due to the loss of the expressed allele. In contrast to humans, XO mice are viable, and fertile. Thus, neither cells from patients nor mouse models can be used in order to study the cause of early lethality in XO embryos. Human embryonic stem cells (HESCs) can differentiate in culture into cells from the three embryonic germ layers as well as into extraembryonic cells. These cells have been shown to have great value in modeling human developmental genetic disorders. In order to study the reasons for the early lethality of 45,XO embryos we have isolated HESCs that have spontaneously lost one of their sex chromosomes. To examine the possibility that imprinted genes on the X chromosome play a role in the phenotype of XO embryos, we have identified genes that were no longer expressed in the mutant cells. None of these genes showed a monoallelic expression in XX cells, implying that imprinting is not playing a major role in the phenotype of XO embryos. To suggest an explanation for the embryonic lethality caused by monosomy X, we have differentiated the XO HESCs in vitro an in vivo. DNA microarray analysis of the differentiated cells enabled us to compare the expression of tissue specific genes in XO and XX cells. The tissue that showed the most significant differences between the clones was the placenta. Many placental genes are expressed at much higher levels in XX cells in compare to XO cells. Thus, we suggest that abnormal placental differentiation as a result of haploinsufficiency of X-linked pseudoautosomal genes causes the early lethality in XO human embryos.

Generation of chimeras by microinjection

Since the technique of introducing a targeted mutation ('gene targeting') into the mouse genome was published almost 20 years ago (Cell 51:503-512, 1987), the number of mouse mutants (mouse models) is increasing, especially after the advent of the full mouse genomic sequence in 2002 and the human genomic sequences in 2003 that reveals more and more large stretches of similarity between the two species at the genomic level. This chapter describes the tools and the experimental route of targeted manipulation by microinjection in the mouse using targeted embryonic stem cells (ES cells).The techniques have become standardized over recent years (Nature 309:255-256, 1984; Practical Approach. IRL Press, Oxford, 254 pp, 1987; Science 240:1468-1475, 1988; Practical Approach. IRL Press, Oxford, New York, 1993; Transgenic Animal Technology: A Laboratory Handbook, 2nd edition. Academic Press, San Deigo, 2002; Manipulating the Mouse Embryo - A Laboratory Manual, 3rd edition. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, 2003) and basically two methods have been used to generate chimeric mice that transmit the mutation of interest via the ES cell genome to the offspring:Microinjection of ES cells into blastocyst or morula stage embryos (this chapter) or aggregation of ES cells with morula stage embryos (see Chapter 14 ).Microinjection of ES cells into the blastocoel (cavity) of the blastocyst stage embryo and also morula injections using micropipettes driven by micromanipulators require sophisticated manual skills and an expensive phase contrast inverted microscope. Although most commonly used, it is quite expensive to establish this technique in a laboratory, in particular, if piezo- or laser- supported routes come into play. Although the establishment of germ-line potent ES cells was first published in 1981 (Proc Natl Acad Sci U S A 78:7634-7636, 1981; Nature 292:154-156, 1981), up to now it has not been possible to establish germ-line transmitting ES cells from any other mammalian species, not even from rat which is closely related, nor was it possible to introduce targeted mutations by different means to the germ-line of mammals. After 20 years, the mouse is still the only mammalian species where mutations can be introduced in a targeted manner and therefore it is very important to many fields in biology, like immunology, neurobiology, and developmental biology to study gene function and disease. Through means of introducing even point mutations to single relevant molecules of a signal transduction pathway in order to study regulation of cellular and physiological processes in complex organisms in a tissue specific or inducible manner (conditional gene targeting, (Cell 73:1155-1164, 1993; Science 265:103-106, 1994)), more recently the field has expanded exponentially.

Derivation and manipulation of murine embryonic stem cells

Pluripotent embryonic stem (ES) cell lines were first isolated over 25 years ago and remain an essential tool in molecular and developmental biology to this day. In particular, the use of homologous recombination and subsequent generation of ES-derived mice has greatly facilitated research across all fields. Moreover, ES cells represent an extremely attractive model to study events in early development. In this chapter, we will describe the derivation and propagation of murine ES cells. This is followed by a description of targeting ES cells and a protocol for the generation of mice by diploid and tetraploid blastocyst injections.

Cloning from stem cells: different lineages, different species, same story

Following nuclear transfer (NT), the most stringent measure of extensive donor cell reprogramming is development into viable offspring. This is referred to as cloning efficiency and quantified as the proportion of cloned embryos transferred into surrogate mothers that survive into adulthood. Cloning efficiency depends on the ability of the enucleated recipient cell to carry out the reprogramming reactions (‘reprogramming ability’) and the ability of the nuclear donor cell to be reprogrammed (‘reprogrammability’). It has been postulated that reprogrammability of the somatic donor cell epigenome is inversely proportional to its differentiation status. In order to test this hypothesis, reprogrammability was compared between undifferentiated stem cells and their differentiated isogenic progeny. In the mouse, cells of divergent differentiation status from the neuronal, haematopoietic and skin epithelial lineage were tested. In cattle and deer, skeletal muscle and antler cells, respectively, were used as donors. No conclusive correlation between differentiation status and cloning efficiency was found, indicating that somatic donor cell type may not be the limiting factor for cloning success. This may reflect technical limitations of the NT-induced reprogramming assay. Alternatively, differentiation status and reprogrammability may be unrelated, making all cells equally difficult to reprogramme once they have left the ground state of pluripotency.

ES cell line establishment

A method is described to establish mouse embryonic stem cell (ESC) lines from hybrid and inbred strains of mice. Attention is paid not only to the methodology for isolation and culture but also to the validation of freshly derived lines, in order to be maintained for prolonged time without significant differentiation or karyotype instability, and to provide reproducible germline transmission in chimaeric mice.

Stem cells: promises versus limitations

Stem cells are the self-renewing progenitors of several body tissues and are classified according to their origin and their ability to differentiate. Current research focuses on the potential uses of stem cells in medicine and how they can provide effective treatment for a range of diseases. This approach has resulted in the field of medical practice called regenerative medicine. To attain the promises of regenerative medicine, it is necessary to fully understand the biology and properties of stem cells, achieve their successful differentiation into functional tissues, overcome the barriers related to immune responses after administration, and assess any oncogenic properties that limit their use. The availability of human stem cells not only raises hope for cell replacement therapies, but also provides a system for understanding the mechanisms of embryonic development and disease progression. Nevertheless, it raises ethical concerns that need to be addressed before the use of stem cells in clinical practice.

Efficient production of mice from embryonic stem cells injected into four- or eight-cell embryos by piezo micromanipulation

The conventional method for producing embryonic stem (ES) cell-derived knockout or transgenic mice involves injection of ES cells into normal, diploid blastocysts followed by several rounds of breeding of resultant chimeras and thus is a time-consuming and inefficient procedure. F0 ES cell pups can also be derived directly from tetraploid embryo complementation, which requires fusion of two-cell embryos. Recently, F0 ES cell pups have been produced by injection of ES cells into eight-cell embryos using a laser-assisted micromanipulation system. We report a simple method for producing F0 ES cell germline-competent mice by piezo injection of ES cells into four- or eight-cell embryos. The efficiency of producing live, transgenic mice by this method is higher than that with the tetraploid blastocyst complementation method. This efficient and economical technique for directly producing F0 ES cell offspring can be applicable in many laboratories for creating genetically manipulated mice using ES cell technology and also for stringent testing of the developmental potency of new ES cell or other types of pluripotent stem cell lines.

Generation of progeny from embryonic stem cells by microinsemination of male germ cells from chimeric mice

Mice chimeric for embryonic stem (ES) cells have not always successfully produced ES-derived offspring. Here we show that the male gametes from ES cells could be selected in male chimeric mice testes by labeling donor ES cells or host blastocytes with GFP. Male GFP-expressing ES-derived germ cells occurred as colonies in the chimeric testes, where the seminiferous tubules were separated into green and non-green regions. When mature spermatozoa from green tubules were used for microinsemination, GFP-expressing offspring were efficiently obtained. Using a reverse study, we also obtained ES-derived progeny from GFP-negative ES cells in GFP-labeled host chimeras. Furthermore, we showed this approach could be accelerated by using round spermatids from the testes of 20-day-old chimeric mice. Thus, this technique allowed us to generate the ES cell-derived progeny even from the low contributed chimeric mice, which cannot produce ES-origin offspring by natural mating.

Somatic cell nuclear transfer: Past, present and future perspectives

It is now over a decade since the birth, in 1996, of Dolly the first animal to be produced by nuclear transfer using an adult derived somatic cell as nuclear donor. Since this time similar techniques have been successfully applied to a range of species producing live offspring and allowing the development of transgenic technologies for agricultural, biotechnological and medical uses. However, though applicable to a range of species, overall, the efficiencies of development of healthy offspring remain low. The low frequency of successful development has been attributed to incomplete or inappropriate reprogramming of the transferred nuclear genome. Many studies have demonstrated that such reprogramming occurs by epigenetic mechanisms not involving alterations in DNA sequence, however, at present the molecular mechanisms underlying reprogramming are poorly defined. Since the birth of Dolly many studies have attempted to improve the frequency of development, this review will discuss the process of animal production by nuclear transfer and in particular changes in the methodology which have increased development and survival, simplified or increased robustness of the technique. Although much of the discussion is applicable across species, for simplicity we will concentrate primarily on published data for cattle, sheep, pigs and mice.

Correlation of developmental differences of nuclear transfer embryos cells to the methylation profiles of nuclear transfer donor cells in Swine

Methylation of DNA is the most commonly studied epigenetic mechanism of developmental competence and somatic cell nuclear transfer (SCNT). Previous studies of epigenetics and the SCNT procedures have examined the effects of different culture media on donor cells and reconstructed embryos, and the methylation status of specific genes in the fetus or live offspring. Here we used a microarray based approach to identify the methylation profiles of SCNT donor cells including three clonal porcine fetal fibroblast-like cell sublines and adult somatic cells selected from kidney and mammary tissues. The methylation profiles of the donor cells were then analyzed with respect to their ability to direct development to the blastocyst stage after nuclear transfer. Clonal cell lines A2, A7 and A8 had blastocyst rates of 11.7%(a), 16.7%(ab) and 20.0%(b), respectively ((ab) p < 0.05). Adult somatic cells included kidney, mammary (large), and mammary (small) also had different blastocyst rates (ab p < 0.05) of 4.2% (a), 10.7% (ab) and 18.3% (b), respectively. For clonal donor cells and for adult somatic cell groups the donor cells with the highest blastocyst rates also had methylation profiles with the lowest similarity to the methylation profiles of the in vivo-produced blastocysts. Conversely, the donor cells with the lowest blastocyst rates had methylation profiles with the highest similarity to the methylation profiles of the in vivo-produced blastocysts. Our findings show there is an inverse correlation to the similarity of the methylation profiles of the donor cells and the in vivo-produced embryos, and to the blastocyst rates following SCNT.

Donor cell differentiation, reprogramming, and cloning efficiency: elusive or illusive correlation?

Compared to other assisted reproductive technologies, mammalian nuclear transfer (NT) cloning is inefficient in generating viable offspring. It has been postulated that nuclear reprogramming and cloning efficiency can be increased by choosing less differentiated cell types as nuclear donors. This hypothesis is mainly supported by comparative mouse cloning experiments using early blastomeres, embryonic stem (ES) cells, and terminally differentiated somatic donor cells. We have re-evaluated these comparisons, taking into account different NT procedures, the use of donor cells from different genetic backgrounds, sex, cell cycle stages, and the lack of robust statistical significance when post-blastocyst development is compared. We argue that while the reprogrammability of early blastomeres appears to be much higher than that of somatic cells, it has so far not been conclusively determined whether differentiation status affects cloning efficiency within somatic donor cell lineages.

Disruption of imprinting and aberrant embryo development in completely inbred embryonic stem cell-derived mice

The completely embryonic stem (ES) cell-derived mice (ES mice) produced by tetraploid embryo complementation provide us with a rapid and powerful approach for functional genome analysis. However, inbred ES cell lines often fail to generate ES mice. The genome of mouse ES cells is extremely unstable during in vitro culture and passage, and the expression of the imprinted genes is most likely to be affected. Whether the ES mice retain or repair the abnormalities of the donor ES cells has still to be determined. Here we report that the inbred ES mice were efficiently produced with the inbred ES cell line (SCR012). The ES fetuses grew more slowly before day 17.5 after mating, but had an excessive growth from day 17.5 to birth. Five imprinted genes examined (H19, Igf2, Igf2r, Peg1, Peg3) were expressed abnormally in ES fetuses. Most remarkably, the expression of H19 was dramatically repressed in the ES fetuses through the embryo developmental stage, and this repression was associated with abnormal biallelic methylation of the H19 upstream region. The altered methylation pattern of H19 was further demonstrated to have arisen in the donor ES cells and persisted on in vivo differentiation to the fetal stage. These results indicate that the ES fetuses did retain the epigenetic alterations in imprinted genes from the donor ES cells.

Production of chimeras by aggregation of embryonic stem cells with diploid or tetraploid mouse embryos

The production of mouse chimeras is a common step in the establishment of genetically modified animal strains. Chimeras also provide a powerful experimental tool for following cell behavior during both prenatal and postnatal development. This protocol outlines a simple and economical technique for the production of large numbers of mouse chimeras using traditional diploid morula<-->diploid embryonic stem (ES) cell aggregations. Additional steps are included to describe the procedures necessary to produce specialized tetraploid chimeras using tetraploid morula<-->diploid ES cell aggregations. This increasingly popular form of chimera produces embryos of nearly complete ES cell derivation that can be used to speed transgenic production or ask developmental questions. Using this protocol, mouse chimeras can be generated and transferred to pseudopregnant surrogate mothers in a 5-d period.

Checkpoint-apoptosis uncoupling in human and mouse embryonic stem cells: a source of karyotpic instability

Karyotypic abnormalities in cultured embryonic stem cells (ESCs), especially near-diploid aneuploidy, are potential obstacles to ESC use in regenerative medicine. Events causing chromosomal abnormalities in ESCs may be related to events in tumor cells causing chromosomal instability (CIN) in human disease. However, the underlying mechanisms are unknown. Using multiparametric permeabilized-cell flow cytometric analysis, we found that the mitotic-spindle checkpoint, which helps maintain chromosomal integrity during all cell divisions, functions in human and mouse ESCs, but does not initiate apoptosis as it does in somatic cells. This allows an unusual tolerance to polyploidy resulting from failed mitosis, which is common in rapidly proliferating cell populations and which is reduced to near-diploid aneuploidy, which is also common in human neoplastic disease. Checkpoint activation in ESC-derived early-differentiated cells results in robust apoptosis without polyploidy/aneuploidy similar to that in somatic cells. Thus, the spindle checkpoint is "uncoupled" from apoptosis in ESCs and is a likely source of karyotypic abnormalities. This natural behavior of ESCs to tolerate/survive varying degrees of ploidy change could complicate genome-reprogramming studies and stem-cell plasticity studies, but could also reveal clues about the mechanisms of CIN in human tumors.

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.

Passage number affects the pluripotency of mouse embryonic stem cells as judged by tetraploid embryo aggregation

The aim of this study was to determine whether the number of passages affected the developmental pluripotency of embryonic stem (ES) cells as measured by the attainment of adult fertile mice derived from embryonic stem (ES) cell/tetraploid embryo complementation. Thirty-six newborns were produced by the aggregation of tetraploid embryos and hybrid ES cells after various numbers of passages. These newborns were entirely derived from ES cells as judged by microsatellite DNA, coat-color phenotype, and germline transmission. Although 15 survived to adulthood, 17 died of respiratory failure, and four were eaten by their foster mother. From the 15 mice that reached adulthood and that could reproduce, none arose from ES cells at passage level 15 or more. All 15 arose from cells at passages 3-11. Our results demonstrate that the number of passages affects the developmental pluripotency of ES cells.

Cloning cattle: the methods in the madness

Somatic cell nuclear transfer (SCNT) is much more widely and efficiently practiced in cattle than in any other species, making this arguably the most important mammal cloned to date. While the initial objective behind cattle cloning was commercially driven--in particular to multiply genetically superior animals with desired phenotypic traits and to produce genetically modified animals-researchers have now started to use bovine SCNT as a tool to address diverse questions in developmental and cell biology. In this paper, we review current cattle cloning methodologies and their potential technical or biological pitfalls at any step of the procedure. In doing so, we focus on one methodological parameter, namely donor cell selection. We emphasize the impact of epigenetic and genetic differences between embryonic, germ, and somatic donor cell types on cloning efficiency. Lastly, we discuss adult phenotypes and fitness of cloned cattle and their offspring and illustrate some of the more imminent commercial cattle cloning applications.

Use of adult mesenchymal stem cells isolated from bone marrow and blood for somatic cell nuclear transfer in pigs

Mesenchymal stem cells (MSCs) isolated from bone marrow were used to examine the hypothesis that a less differentiated cell type could increase adult somatic cell nuclear transfer (SCNT) efficiencies in the pig. SCNT embryos were produced using a fusion before activation protocol described previously and the rate at which these developed to the blastocyst stage compared with that using fibroblasts obtained from ear tissue from the same animal. The use of bone marrow MSCs did not increase cleavage rates compared with adult fibroblasts. However, the percentage of embryos that developed to the blastocyst stage was almost doubled, providing support for the hypothesis that a less differentiated cell can increase cloning efficiencies. As MSCs are relatively difficult to isolate from the bone marrow of live animals, a second experiment was undertaken to determine whether MSCs could be isolated from the peripheral circulation and used for SCNT. Blood MSCs were successfully isolated from four of the five pigs sampled. These cells had a similar differentiation capacity and marker profile to those isolated from bone marrow but did not result in increased rates of development. This is the first study to our knowledge, to report that MSCs can be derived from peripheral blood and used for SCNT for any species. These cells can be readily obtained under relatively sterile conditions compared with adult fibroblasts and as such, may provide an alternative cell type for cloning live animals.

Efficient method to generate single-copy transgenic mice by site-specific integration in embryonic stem cells

Transgenic and gene-targeted mutant mice provide powerful tools for analysis of the cellular processes involved in early development and in the pathogenesis of many diseases. Here we describe a transgene integration strategy mediated by site-specific recombination that allows establishment of multiple embryonic stem (ES) cell lines carrying tetracycline-inducible genes targeted to a specific locus to assure predictable temporal and spatial expression in ES cells and mice. Using homologous recombination we inserted an frt homing site into which tetracycline-inducible transgenes can be integrated efficiently in the presence of FLPe recombinase. This strategy and the vectors described here are generally applicable to any locus in ES cells and should allow for the rapid production of mice with transgenes efficiently targeted to a defined site.

Human embryonic stem cells: origin, properties and applications

Human embryonic stem cells originate from the human preimplantation embryo. The derivation of the first human embryonic stem cells was reported in 1998. Since then we have learnt a great deal about how to isolate and culture these cells. Additionally, their stem cell phenotype and differentiation competence have been determined. Although it is expected that many basic biological properties, such as self-renewal and cell specification, are evolutionary conserved, at least from the mouse, we lack significant knowledge about the molecular events that regulate the unique stem cell features of human embryonic stem cells. The pluripotent nature of human embryonic stem cells has attracted great interest in using them as a source of cells and tissues in cell therapy. Recent progress in human somatic cell nuclear transfer suggests that there may be a solution to the immunotolerance problems associated with the use of human embryonic stem cells in cell-replacement therapy. Thus, human embryonic stem cells supply the research community with unique research tools to study basic biological processes in human cells, model human genetic diseases and develop new cell-replacement therapies.

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.

Lack of RNA-DNA oligonucleotide (chimeraplast) mutagenic activity in mouse embryos

There are numerous reports of the use of RNA-DNA oligonucleotides (chimeraplasts) to correct point mutations in vitro and in vivo, including the human apolipoprotein E gene (ApoE). Despite the absence of selection for targeting, high efficiency conversion has been reported. Although mainly used to revert deleterious mutations for gene therapy applications, successful use of this approach would have the potential to greatly facilitate the production of defined mutations in mice and other species. We have attempted to create a point mutation in the mouse ApoE gene by microinjection of chimeraplast into the pronuclei of 1-cell mouse eggs. Following transfer of microinjected eggs we analysed 139 E12.5 embryos, but obtained no evidence for successful conversion.

Serum-free medium cultivation to improve efficacy in establishment of human embryonic stem cell lines

BACKGROUND

Serum-containing and serum-free media were used to derive human embryonic stem (HES) cells from donated oocytes and embryos.

METHODS AND RESULTS

Inner cell masses (ICM) were isolated by immunosurgery. The HES cells were found to be easily obtained and expanded in a serum-free medium. The efficacy in establishing human embryonic stem cell lines improved in a serum-free medium compared with that in serum-containing media. Four HES cell lines were derived from 13 isolated ICM on mouse embryonic fibroblast feeder layers. All four cell lines possess the same characteristics and differentiating potency: normal 46, XX or 46, XY karyotype; and expressing a series of surface markers such as APase, SSEA-3, SSEA-4, TRA-1-60, TRA-1-81, but not SSEA-1. They can form embryoid bodies in suspension culture and develop teratomas comprising derivatives of three embryonic germ layers when injected into severe combined immunodeficient mice.

CONCLUSION

These preliminary results suggest that serum-free cultivation may be superior to serum-containing cultivation for deriving human embryonic stem cells.

Cloning: Eight Years After Dolly

It is now 8 years since the birth of Dolly, the first animal produced by nuclear transfer using a donor cell population established from an adult animal. During this time, the technique of nuclear transfer has been successfully applied to a range of mammalian species for the production of offspring using a plethora of donor cell types derived from both foetal and adult tissues. In addition, when coupled with genetic manipulation of the donor cells, transgenic offspring have been produced with a range of genetic modifications including gene knockouts and gene knockings. Despite the apparent successes of the technology, the efficiency of development to live offspring has remained low and developmental abnormalities still occur. The objectives of this paper are to review some of the successes and failures of the nuclear transfer procedure since the production of Dolly. In particular, we will review the major steps in the procedure and discuss studies from our laboratory and others which have modified the procedure in ways which may impact on development.

Single copy shRNA configuration for ubiquitous gene knockdown in mice

RNA interference through the expression of small hairpin RNA (shRNA) molecules has become a very promising tool in reverse mouse genetics as it may allow inexpensive and rapid gene function analysis in vivo. However, the prerequisites for ubiquitous and reproducible shRNA expression are not well defined. Here we show that a single copy shRNA-transgene can mediate body-wide gene silencing in mice when inserted in a defined locus of the genome. The most commonly used promoters for shRNA expression, H1 and U6, showed a comparably broad activity in this configuration. Taken together, the results define a novel approach for efficient interference with expression of defined genes in vivo. Moreover, we provide a rapid strategy for the production of gene knockdown mice combining recombinase mediated cassette exchange and tetraploid blastocyst complementation approaches.

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.

At most three ES cells contribute to the somatic lineages of chimeric mice and of mice produced by ES-tetraploid complementation

Chimeric or entirely embryonic stem (ES) cell-derived mice ("ES mice") can be produced by injecting ES cells into diploid (2n) or tetraploid (4n) host blastocysts, respectively. Usually, between 10 and 15 ES cells are injected into the host blastocyst, but it is not clear how many of the injected cells contribute to the somatic lineages, thus serve as "founder cells" of the embryo proper. We have used genetically labeled ES cells to retrospectively determine the number of founder ES cells that generate the somatic lineages of chimeric and of ES mice. ES cell clones individually labeled with provirus were mixed in equal numbers and injected into 2n or 4n blastocysts to generate chimeric or ES mice. Southern analysis of DNA from the resulting animals indicated that the somatic lineages were most often derived from one or two and sometimes from up to three founder ES cells. The number of founder cells was independent of the total number of cells injected into the host blastocysts. Our results are consistent with the notion that constraints of the host embryo restrict the number of ES cells that can contribute to a chimeric or an ES mouse.

The knockout mouse project

Mouse knockout technology provides a powerful means of elucidating gene function in vivo, and a publicly available genome-wide collection of mouse knockouts would be significantly enabling for biomedical discovery. To date, published knockouts exist for only about 10% of mouse genes. Furthermore, many of these are limited in utility because they have not been made or phenotyped in standardized ways, and many are not freely available to researchers. It is time to harness new technologies and efficiencies of production to mount a high-throughput international effort to produce and phenotype knockouts for all mouse genes, and place these resources into the public domain.

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.

Seven-fluorochrome mouse M-FISH for high-resolution analysis of interchromosomal rearrangements

The mouse has evolved to be the primary mammalian genetic model organism. Important applications include the modeling of human cancer and cloning experiments. In both settings, a detailed analysis of the mouse genome is essential. Multicolor karyotyping technologies have emerged to be invaluable tools for the identification of mouse chromosomes and for the deciphering of complex rearrangements. With the increasing use of these multicolor technologies resolution limits are critical. However, the traditionally used probe sets, which employ 5 different fluorochromes, have significant limitations. Here, we introduce an improved labeling strategy. Using 7 fluorochromes we increased the sensitivity for the detection of small interchromosomal rearrangements (700 kb or less) to virtually 100%. Our approach should be important to unravel small interchromosomal rearrangements in mouse models for DNA repair defects and chromosomal instability.

Chimaerism and erythroid marker expression after microinjection of human acute myeloid leukaemia cells into murine blastocysts

It has been suggested that the embryonic microenvironment can control the survival and the transformed phenotype of tumour cells. Here, we addressed the hypothesis that the murine embryonic microenvironment can induce the differentiation of human tumour cells. To examine such interactions, we injected human leukaemic cells into preimplantation murine blastocysts at embryonic day 3.5 of gestation (E3.5). Microinjection of human KG-1 myeloid leukaemia cells and primary human acute myeloid leukaemia (AML) cells led to the generation of chimaeric embryos and adults. We observed that in E12.5 murine embryos, KG-1 cells were preferentially detected in yolk sac and peripheral blood, while primary AML cells mainly seeded the aorta gonad mesonephros region of chimaeric embryos. Analysis of the donor contribution in 15 different adult tissues showed that progeny of primary AML cells seeded to various haematopoietic and nonhaematopoietic tissues. Chimaeric embryos and adults showed no apparent tumour formation. Furthermore, analysis of chimaeric E12.5 embryos revealed that the progeny of human KG-1 cells activated erythroid-specific human globin and glycophorin A expression. In summary, our data indicate that human AML cells activate markers of erythroid differentiation after injection into early murine embryos.

Hybrid embryonic stem cell-derived tetraploid mice show apparently normal morphological, physiological, and neurological characteristics

ES cell-tetraploid (ES) mice are completely derived from embryonic stem cells and can be obtained at high efficiency upon injection of hybrid ES cells into tetraploid blastocysts. This method allows the 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. To provide a baseline for the analysis of ES mouse mutants, we performed a phenotypic characterization of wild-type B6129S6F(1) ES mice in relation to controls of the same age, sex, and genotype raised from normal matings. The comparison of 90 morphological, physiological, and behavioral parameters revealed elevated body weight and hematocrit as the only major difference of ES mice, which exhibited an otherwise normal phenotype. We further demonstrate that ES mouse mutants can be produced from mutant hybrid ES cells and analyzed within a period of only 4 months. Thus, ES mouse technology is a valid research tool for rapidly elucidating gene function in vivo.

Nuclear cloning, stem cells, and genomic reprogramming

The generation of adult animals by nuclear cloning from adult donor cells is extremely inefficient, with most clones dying soon after implantation. In contrast, cloning from embryonic stem cell donor nuclei is significanty more efficient than from adult donor cells. However, regardless of donor cell type, all clones that survive to birth and beyond suffer serious phenotypic and gene expression abnormalities. All available evidence is consistent with the notion that the anomalous phenotypes of cloned animals are caused by faulty epigenetic reprogramming of the donor nucleus. Faulty reprogramming appears to be caused by the cloning process itself as well as by the epigenetic state of the donor nucleus. In contrast to reproductive cloning, faulty reprogramming of the donor nucleus does not tend to interfere with the application of nuclear transfer technology for therapeutic purposes (therapeutic cloning).

Cre-mediated germline mosaicism: a new transgenic mouse for the selective removal of residual markers from tri-lox conditional alleles

The binary Cre-lox conditional knockout system requires an essential part of the target gene to be flanked by loxP sites, enabling excision in vivo upon Cre expression. LoxP sites are introduced by homologous recombination, together with a selectable marker. However, this marker can disturb gene expression and should be removed. The marker is therefore often prepared with a third, flanking loxP site (tri-lox construct), facilitating its selective removal by partial Cre-lox recombination. We have shown that this excision can be achieved in vivo in the germline using EIIaCre transgenic mice, and have described the advantages of in vivo over in vitro removal. We show here that MeuCre40, a new transgenic mouse, more reliably and reproducibly generates an optimal partial mosaic Cre-lox recombination pattern in the early embryo. This mosaicism was transmitted to the germline and to many other tissues. Alleles with partial deletions, in particular floxed alleles from which the selectable marker was removed, were readily recovered in the next generation, after segregation from the transgene. Segregation via paternal or maternal transmission led to successful recovery of the alleles of interest. We also obtained total deletion of the floxed regions in the same experiment, making this transgene a polyvalent Cre-lox tool. We rigorously tested the ability of MeuCre40 to solve tri-lox problems, by using it for the in vivo removal of neo(R)- and hprt-expression cassettes from three different tri-lox mutants.

Nuclear transplantation: lessons from frogs and mice

Nuclear transplantation was developed 50 years ago in frogs to test whether nuclei from differentiated cells remain genetically equivalent to zygotic nuclei. Results from cloning experiments in frogs and mice indicate that nuclei gradually lose potency during development from embryonic to adult cells. However, even though adult mature lymphocytes were recently shown to remain genetically totipotent, no evidence exists to show that surviving clones originate from the nuclei of terminally differentiated cells. Thus, it is equally possible that many cloned animals are in fact derived from the nuclei of less differentiated adult cells such as adult stem cells. These cells might be more easily reprogrammed than terminally differentiated cells and may support development of a clone at a higher efficiency. Importantly, irrespective of the donor cell, clones display common abnormalities such as foetal and placental overgrowth. Indeed, gene expression analyses and extensive phenotypic characterisation of cloned animals suggest that most, if not all, clones suffer from at least subtle abnormalities.

Abnormal gene expression in cloned mice derived from embryonic stem cell and cumulus cell nuclei

To assess the extent of abnormal gene expression in clones, we assessed global gene expression by microarray analysis on RNA from the placentas and livers of neonatal cloned mice derived by nuclear transfer (NT) from both cultured embryonic stem cells and freshly isolated cumulus cells. Direct comparison of gene expression profiles of more than 10,000 genes showed that for both donor cell types approximately 4% of the expressed genes in the NT placentas differed dramatically in expression levels from those in controls and that the majority of abnormally expressed genes were common to both types of clones. Importantly, however, the expression of a smaller set of genes differed between the embryonic stem cell- and cumulus cell-derived clones. The livers of the cloned mice also showed abnormal gene expression, although to a lesser extent, and with a different set of affected genes, than seen in the placentas. Our results demonstrate frequent abnormal gene expression in clones, in which most expression abnormalities appear common to the NT procedure whereas others appear to reflect the particular donor nucleus.