Gene targeting and gene trap screens using embryonic stem cells: new approaches to mammalian development

Mouse mutagenesis and phenotyping to generate models of development and disease

For many years, the laboratory mouse has been the favored model organism to study mammalian development, biology and disease. Among its advantages for these studies are its close concordance with human biology, the syntenic relationship between the mouse and other mammalian genomes, the existence of many inbred strains, its short gestation period, its relatively low cost for housing and husbandry, and the wide array of tools for genome modification, mutagenesis, and for cryopreserving embryos, sperm and eggs. The advent of CRISPR genome modification techniques has considerably broadened the landscape of model organisms available for study, including other mammalian species. However, the mouse remains the most popular and utilized system to model human development, biology, and disease processes. In this review, we will briefly summarize the long history of mice as a preferred mammalian genetic and model system, and review current large-scale mutagenesis efforts using genome modification to produce improved models for mammalian development and disease.

A gene trap mutagenesis screen for genes underlying cellular response to the mood stabilizer lithium

Identifying the biological pathways mediating the action of a therapeutic compound may help the development of more specific treatments while also increasing our understanding of the underlying disease pathology. Salts of the metal lithium are commonly used as a front-line mood stabilizing treatment for bipolar disorder. Lithium's action has been variously linked to inositol phosphate metabolism and the WNT/Glycogen Synthase Kinase 3β (GSK3β)/β-Catenin signalling cascade, but, to date, little is known about which of these provides the principal therapeutic benefit for patients and, more specifically, which constituent genes, through presumed sequence variation, determine differences in patient response to treatment. Here, we describe a functional screen in which SH-SY5Y neuroblastoma cells were randomly mutated through genomic integration of the pMS1 poly A ‘gene trap’ plasmid vector. Lithium normally induces differentiation of neuroblastoma cells, but a small proportion of mutated cells continued to proliferate and formed colonies. Rapid amplification of cDNA ends (RACE)-PCR was used to identify the ‘trapped’ gene in each of these lithium-resistant colonies. Heterozygous, gene trap integrations were identified within ten genes, eight of which are likely to produce loss-of-function mutations including MED10, MSI2 and three long intergenic non-coding (LINC) RNAs. Both MED10 and MSI2 have been previously linked with WNT/GSK3β/β-Catenin pathway function suggesting that this is an important mediator of lithium action in this screen. The methodology applied here provides a rapid, objective and economic approach to define the genetic contribution to drug action, but could also be readily adapted to any desired in vitro functional selection/screening paradigm.

Polycomblike-2-deficient mice exhibit normal left–right asymmetry

Polycomb group (PcG) proteins are required for maintaining the repressed state of developmentally important genes such as homeotic genes. Polycomblike (Pcl), a member of PcG genes with two characteristic PHD finger motifs, was shown to strongly enhance the effects of PcG genes in Drosophila. Three Pcl genes exist in the mouse genome, with their function largely unknown. Our previous studies demonstrate that the chick Pcl2 is essential for the left-right asymmetry by silencing Shh expression in the right side of the node (Wang et al., [2004b] Development 131:4381-4391). To elucidate the in vivo role of mouse Pcl2, we generated Pcl2 mutant mice. Phenotypic analyses indicate the normal development of left-right asymmetry in the Pcl2 mutant mice. However, Pcl2 mutant mice exhibit posterior transformation of axial skeletons and other phenotypic defects, with a relatively low penetrance. These results demonstrate that Pcl2 is dispensable for the normal left-right axis development in mice.

Embryonic stem cell differentiation: emergence of a new era in biology and medicine

The discovery of mouse embryonic stem (ES) cells >20 years ago represented a major advance in biology and experimental medicine, as it enabled the routine manipulation of the mouse genome. Along with the capacity to induce genetic modifications, ES cells provided the basis for establishing an in vitro model of early mammalian development and represented a putative new source of differentiated cell types for cell replacement therapy. While ES cells have been used extensively for creating mouse mutants for more than a decade, their application as a model for developmental biology has been limited and their use in cell replacement therapy remains a goal for many in the field. Recent advances in our understanding of ES cell differentiation, detailed in this review, have provided new insights essential for establishing ES cell-based developmental models and for the generation of clinically relevant populations for cell therapy.

Embryonic stem cells: prospects for developmental biology and cell therapy

Stem cells represent natural units of embryonic development and tissue regeneration. Embryonic stem (ES) cells, in particular, possess a nearly unlimited self-renewal capacity and developmental potential to differentiate into virtually any cell type of an organism. Mouse ES cells, which are established as permanent cell lines from early embryos, can be regarded as a versatile biological system that has led to major advances in cell and developmental biology. Human ES cell lines, which have recently been derived, may additionally serve as an unlimited source of cells for regenerative medicine. Before therapeutic applications can be realized, important problems must be resolved. Ethical issues surround the derivation of human ES cells from in vitro fertilized blastocysts. Current techniques for directed differentiation into somatic cell populations remain inefficient and yield heterogeneous cell populations. Transplanted ES cell progeny may not function normally in organs, might retain tumorigenic potential, and could be rejected immunologically. The number of human ES cell lines available for research may also be insufficient to adequately determine their therapeutic potential. Recent molecular and cellular advances with mouse ES cells, however, portend the successful use of these cells in therapeutics. This review therefore focuses both on mouse and human ES cells with respect to in vitro propagation and differentiation as well as their use in basic cell and developmental biology and toxicology and presents prospects for human ES cells in tissue regeneration and transplantation.

High cholesterol level is essential for myelin membrane growth

Cholesterol in the mammalian brain is a risk factor for certain neurodegenerative diseases, raising the question of its normal function. In the mature brain, the highest cholesterol content is found in myelin. We therefore created mice that lack the ability to synthesize cholesterol in myelin-forming oligodendrocytes. Mutant oligodendrocytes survived, but CNS myelination was severely perturbed, and mutant mice showed ataxia and tremor. CNS myelination continued at a reduced rate for many months, and during this period, the cholesterol-deficient oligodendrocytes actively enriched cholesterol and assembled myelin with >70% of the cholesterol content of wild-type myelin. This shows that cholesterol is an indispensable component of myelin membranes and that cholesterol availability in oligodendrocytes is a rate-limiting factor for brain maturation.

Isolation, culture, and characterization of embryonic cell lines from vitrified sheep blastocysts

This study was conducted to isolate, to culture, and to characterize embryonic cell lines from in vitro produced vitrified sheep blastocysts. Embryos were produced and vitrified at the expanded blastocyst stage. Ten inner cell masses arising from day 6-7 blastocysts were isolated by immunosurgery, disaggregated, and cultured onto mitomocin-C-inactivated mouse STO fibroblasts (MIF). After 5 or 6 days of culture the primary cell colonies were disaggregated, seeded in a new MIF, and cultured for 3 or 4 days to form new colonies called Passage 1. These cells were then disaggregated and cultured for other two passages. The primary cell colonies and Passage 2 colonies expressed stage specific embryonic markers SSEA-1, SSEA-3, and SSEA-4, and were alkaline phosphatase positive. In the absence of feeder layer and human leukemia inhibitory factor (LIF), these cells differentiated into variety of cell types and formed embryoid bodies. When cultured for an extended period of time, embryoid bodies differentiated into derivatives of three embryonic germ (EG) layers. These were characterized by detection of specific markers for differentiation such early mesoderm (FE-C6), embryonic myosin (F1-652), neural precursor (FORSE-1), and endoderm (anti-cytokeratin 18). To our knowledge, this is the first time that embryonic cell lines from in vitro produced and vitrified ovine blastocysts have been isolated and examined for detection of SSEA markers, and embryoid bodies have been cultured and examined for specific cell surface markers for differentiation.

Protein-trap version 2.1: screening for expressed proteins in mammalian cells based on their localizations

Background

"Protein-trap" is a method that allows epitope-tagging of endogenous proteins. This method allows for the identification of endogenously expressed proteins that exhibit specific localization of interest. This method has been recently reported for its application in the study of Drosophila development by using a relatively large epitope, green-fluorescent-protein (GFP).

Result

Herein, we report a new "protein-trap" vector for mammalian cells. This new method utilizes a much smaller epitope-tag and also allows for drug-selection prior to the epitope-tagging. Pre-selection by drug resulted in the highly efficient protein-trapping frequency.

Conclusion

The "protein-trap" method based on this new vector is expected to serve as a complimentary approach to the previously reported GFP-based method.

Cautionary observations on preparing and interpreting brain images using molecular biology-based staining techniques

Though molecular biology-based visualization techniques such as antibody staining, in situ hybridization, and induction of reporter gene expression have become routine procedures for analyzing the structures of the brain, precautions to prevent misinterpretation have not always been taken when preparing and interpreting images. For example, sigmoidal development of the chemical processes in staining might exaggerate the specificity of a label. Or, adjustment of exposure for bright fluorescent signals might result in overlooking weak signals. Furthermore, documentation of a staining pattern is affected easily by recognized organized features in the image while other parts interpreted as "disorganized" may be ignored or discounted. Also, a higher intensity of a label per cell can often be confused with a higher percentage of labeled cells among a population. The quality, and hence interpretability, of the three-dimensional reconstruction with confocal microscopy can be affected by the attenuation of fluorescence during the scan, the refraction between the immersion and mounting media, and the choice of the reconstruction algorithm. Additionally, visualization of neurons with the induced expression of reporter genes can suffer because of the low specificity and low ubiquity of the expression drivers. The morphology and even the number of labeled cells can differ considerably depending on the reporters and antibodies used for detection. These aspects might affect the reliability of the experiments that involves induced expression of effector genes to perturb cellular functions. Examples of these potential pitfalls are discussed here using staining of Drosophila brain.

Serum-free culture of rhesus monkey embryonic stem cells

Previous studies have shown that the maintenance and proliferation of undifferentiated rhesus monkey embryonic stem (rES) cells requires medium supplemented with fetal bovine serum (FBS). Due to the uncharacterized composition and variation in serum nature, the present study aimed to replace the serum-containing medium with a serum-free medium in the rES cell culture. The results showed that after the initial 48-h culture in the routinely used serum-containing medium, rES cells can grow and proliferate for a prolonged period in the serum-free medium composed of DMEM supplemented with a cocktail of BSA, IGF-1, TGF-alpha, bFGF, aFGF, estradiol, and progesterone. rES cells cultured in the serum-free medium maintained high level of alkaline phosphatase activity and OCT4 level. There was no indication of differentiation as judged by the marker gene expression of all three embryonic germ layers and trophoblast. In addition, serum-free culture would not affect the passage capacity and differentiation potential of rES cells. This work will facilitate the future study of induced differentiation of rES cells and other applications.

Integrative physiology and functional genomics of epithelial function in a genetic model organism

Classically, biologists try to understand their complex systems by simplifying them to a level where the problem is tractable, typically moving from whole animal and organ-level biology to the immensely powerful "cellular" and "molecular" approaches. However, the limitations of this reductionist approach are becoming apparent, leading to calls for a new, "integrative" physiology. Rather than use the term as a rallying cry for classical organismal physiology, we have defined it as the study of how gene products integrate into the function of whole tissues and intact organisms. From this viewpoint, the convergence between integrative physiology and functional genomics becomes clear; both seek to understand gene function in an organismal context, and both draw heavily on transgenics and genetics in genetic models to achieve their goal. This convergence between historically divergent fields provides powerful leverage to those physiologists who can phrase their research questions in a particular way. In particular, the use of appropriate genetic model organisms provides a wealth of technologies (of which microarrays and knock-outs are but two) that allow a new precision in physiological analysis. We illustrate this approach with an epithelial model system, the Malpighian (renal) tubule of Drosophila melanogaster. With the use of the beautiful genetic tools and extensive genomic resources characteristic of this genetic model, it has been possible to gain unique insights into the structure, function, and control of epithelia.

Design of an HIV-1 lentiviral-based gene-trap vector to detect developmentally regulated genes in mammalian cells

The recent development of HIV-1 lentiviral vectors is especially useful for gene transfer because they achieve efficient integration into nondividing cell genomes and successful long-term expression of the transgene. These attributes make the vector useful for gene delivery, mutagenesis, and other applications in mammalian systems. Here we describe two HIV-1-based lentiviral vector derivatives, pZR-1 and pZR-2, that can be used in gene-trap experiments in mammalian cells in vitro and in vivo. Each lentiviral gene-trap vector contains a reporter gene, either beta-lactamase or enhanced green fluorescent protein (EGFP), that is inserted into the U3 region of the 3' long terminal repeat. Both of the trap vectors readily integrate into the host genome by using a convenient infection technique. Appropriate insertion of the vector into genes causes EGFP or beta-lactamase expression. This technique should facilitate the rapid enrichment and cloning of the trapped cells and provides an opportunity to select subpopulations of trapped cells based on the subcellular localization of reporter genes. Our findings suggest that the reporter gene is driven by an upstream, cell-specific promoter during cell culture and cell differentiation, which further supports the usefulness of lentivirus-based gene-trap vectors. Lentiviral gene-trap vectors appear to offer a wealth of possibilities for the study of cell differentiation and lineage commitment, as well as for the discovery of new genes.

Embryo-derived stem cells: of mice and men

Mouse embryonic stem cells are continuous cell lines derived directly from the fetal founder tissue of the preimplantation embryo. They can be expanded in culture while retaining the functional attributes of pluripotent early embryo cells. In particular, they can participate fully in fetal development when reintroduced into the embryo. The capacity for multilineage differentiation is reproduced in culture where embryonic stem cells can produce a wide range of well-defined cell types. This has stimulated interest in the isolation of analogous cells of human origin. Such human pluripotent stem cells could constitute a renewable source of more differentiated cells that could be employed to replace diseased or damaged tissue by cellular transplantation. In this review, the relationships between mouse embryonic stem cells, resident pluripotent cells in the embryo, and human embryo-derived cell lines are evaluated, and the prospects and challenges of embryo stem cell research are considered.

Generation of enhancer trap lines in Arabidopsis and characterization of expression patterns in the inflorescence

Eleven thousand, three hundred and seventy enhancer/promoter trap lines in Arabidopsis were generated via T-DNA transformation utilizing the binary vector pD991 that contains a minimal promoter fused to the uidA reporter gene. Overall 31% of the lines generated exhibit a staining pattern in the inflorescence. Flanking DNA has been cloned from 15 lines exhibiting inflorescence staining patterns by either thermal asymmetric interlaced PCR (TAIL-PCR), inverse PCR (IPCR), or partial library construction. Seeds from these lines are available from the ABRC and NASC Arabidopsis stock centers and DNA pools are available from the ABRC.

Gene targeting of retinoid receptors

Gene targeting in embryonic stem (ES) cells has been employed to investigate the role of the retinoid receptors and binding proteins both in the mouse as well as in embryocarcinoma cells. It is a powerful technique for the modification of the mouse genome. With more recent refinements in gene targeting technology, it is now possible to introduce more subtle mutations in the murine genome, as well as to investigate gene function in a tissue and temporally-restricted manner. It should also be possible to modify genes in diverse diploid cell lines, to generate diverse model systems for analysis of retinoid receptor function. In this article, some of the basic principles for gene targeting are described.

Mouse gene trap approach: identification of novel genes and characterization of their biological functions

The mouse gene trap strategy is an insertional mutagenesis involving an exogenous DNA, termed the trap vector, as a mutagen that produces a mutation in the mouse genome and a sequence tag to facilitate the isolation of the mutated genes. The trap vector consists of a reporter gene whose expression mimics that of the endogenous genes mutated and a selection marker that sorts cells bearing the inserted vector. Gene trap is a powerful method for identifying genes important in biological phenomena. Moreover, the method produces mutant organisms whose phenotypes provide invaluable information about the biological functions of the genes responsible for these phenotypes. Indeed, a number of genes essential for mouse embryogenesis have been identified by the gene trap method. Here, we describe the principle, results, and perspectives for applications of gene trap approach to the study of cell differentiation and lineage commitment.Key words: gene trap, embryogenesis, jumonji.

Large-scale screening for developmental genes in embryonic stem cells and embryoid bodies using retroviral entrapment vectors

Mammalian development is orchestrated by a variety of cellular proteins with expression that is regulated precisely. Although some of the genes encoding these factors have been identified, largely by homology to those of simpler organisms, the majority of them presumably remain unknown. We report here on the results of a large-scale genetic screen that can potentially lead to the identification of many of these unidentified genes in mice. The method we developed takes advantage of the fact that many of the factors that regulate early development are expressed at highly specific stages of early embryogenesis. We therefore established a method for tagging candidate developmental genes by virtue of their expression in a stage-specific manner during formation of embryoid bodies without a bias for their expression in undifferentiated embryonic stem (ES) cells. Of 2,400 ES cell clones with random insertions of retroviral vectors carrying a human placental alkaline phosphatase reporter gene (AP), 41 clones exhibited stage-specific reporter gene expression during embryoid body formation. The majority of these insertions were in genes that are not expressed in undifferentiated ES cells. Eleven ES cell clones with characteristic patterns of AP reporter gene expression in vitro were chosen for further examination in vivo for AP expression in developing embryos. Ten ES cell clones exhibited AP expression between day 7.5 and day 10.5 of development. Clones that showed restricted reporter gene expression in vitro also exhibited similar temporally and spatially restricted AP expression in vivo. Sequence analysis of genomic DNA flanking several vector insertions and corresponding cDNAs suggested that several of the insertions identified a previously unidentified gene. Thus, screening for reporter gene expression during embryoid body formation provides an efficient means of enriching clones that contain vector insertions into potentially novel genes that are important for regulating different stages of early postimplantation development.

Production of medakafish chimeras from a stable embryonic stem cell line

Embryonic stem (ES) cell lines provide a unique tool for introducing targeted or random genetic alterations through gene replacement, insertional mutagenesis, and gene addition because they offer the possibility for in vitro selection for the desired, but extremely rare, recombinant genotypes. So far only mouse blastocyst embryos are known to have the competence to give rise to such ES cell lines. We recently have established a stable cell line (Mes1) from blastulae of the medakafish (Oryzias latipes) that shows all characteristics of mouse ES cells in vitro. Here, we demonstrate that Mes1 cells also have the competence for chimera formation; 90% of host blastulae transplanted with Mes1 cells developed into chimeric fry. This high frequency was not compromised by cryostorage or DNA transfection of the donor cells. The Mes1 cells contributed to numerous organs derived from all three germ layers and differentiated into various types of functional cells, most readily observable in pigmented chimeras. These features suggest the possibility that Mes1 cells may be a fish equivalent of mouse ES cells and that medaka can be used as another system for the application of the ES cell technology.

In vitro preselection of gene-trapped embryonic stem cell clones for characterizing novel developmentally regulated genes in the mouse

We have developed an in vitro gene trap screen for novel murine genes that allows one to determine, prior to making chimeric or transgenic animals, if these genes are expressed in one or more specific embryonic tissues. Totipotent embryonic stem (ES) cells are infected with a retroviral gene trap construct encoding a selectable lacZ/neo fusion gene, which is expressed only if the gene trap inserts within an active transcription unit. G418-resistant ES cell clones are induced to differentiate in vitro, and neurons, glia, myocytes, and chondrocytes are screened for expression of beta-galactosidase (beta-gal). cDNAs of the gene trap transcripts are obtained by 5' rapid amplification of cDNA ends and are sequenced to determine if they represent novel genes. In situ hybridization analyses show that trapped genes are expressed in vivo within the cell types that express beta-gal in vitro. Gene traps and their wild-type alleles are characterized in terms of copy number, alternate splicing of their transcripts, and the proportion of endogenous mRNA sequence that is replaced by lacZ/neo in the hybrid gene trap transcript. This approach, which we term "in vitro preselection," is more economical than standard in vivo gene trap screening because tissue-specific expression of probable knockout alleles is verified before transgenic animals are generated. These results also highlight the utility of ES cell differentiation in vitro as a method with which to study the molecular mechanisms regulating the specification and commitment of a variety of cell and tissue types.

Use of gene targeting for compromising energy homeostasis in neuro-muscular tissues: the role of sarcomeric mitochondrial creatine kinase

We have introduced a single knock-out mutation in the mitochondrial creatine kinase gene (ScCKmit) in the mouse germ line via targeted mutagenesis in mouse embryonic stem (ES) cells. Surprisingly, ScCKmit -/- muscles, unlike muscles of mice with a deficiency of cytosolic M-type creatine kinase (M-CK -/-; Van Deursen et al. (1993) Cell 74, 621-631), display no altered morphology, performance or oxidative phosphorylation capacity. Also, the levels of high energy phosphate metabolites were essentially unaltered in ScCKmit mutants. Our results challenge some of the present concepts about the strict coupling between CKmit function and aerobic respiration.

Pluripotency and differentiation of embryonic stem cell lines from the medakafish (Oryzias latipes)

Small aquarium fish, like the medaka and zebrafish, offer an excellent opportunity to combine embryological, genetic and molecular analyses of vertebrate development. Pluripotent embryonic stem (ES) cells have enormous potential to study the totipotency and differentiation of cells and provide s bridge linking in vitro manipulations of the genome. In this report we describe the establishment, pluripotency and differentiation of medaka ES-like cell lines (MES). The MES cells exhibit stable growth over 18 months of culture with 100 passages using defined culture conditions in the absence of feeder layer cells. They have a normal karyotype and form colonies of densely packed, alkaline phosphatase-positive cells resembling undifferentiated mouse ES cells. In suspension culture they form embryoid bodies, and under appropriate conditions, differentiate into a variety of cell types.

The initiation of senescence and its relationship to embryonic cell differentiation

Mouse embryonic stem cells have an unlimited lifespan in cultures if they are prevented from differentiating. After differentiating, they produce cells which divide only a limited number of times. These changes seen in cultures parallel events that occur in the developing embryo, where immortal embryonic cells differentiate and produce mortal somatic ones. The data strongly suggest that differentiation initiates senescence, but this view entails additional assumptions in order to explain how the highly differentiated sexual gametes manage to remain potentially immortal. Cells differentiate by blocking expression from large parts of their genome and it is suggested that losses or gains of genetic totipotency determine cellular lifespans. Cells destined to be somatic do not regain totipotency and senesce, while germ-line cells regain complete genome expression and immortality after meiosis and gamete fusions. Losses of genetic totipotency could induce senescence by lowering the levels of repair and maintenance enzymes.

Use of avian cytokines in mammalian embryonic stem cell culture

Mouse blastocyst-derived embryonic stem (ES) cells are multipotent cells that can be used in vitro as models of differentiation and in vivo can contribute to all embryonic tissues including the germ line. The culture of ES cells requires a source of leukemia inhibitory factor (LIF), often provided by culture with a mouse fibroblast (STO) feeder layer, buffalo rat liver cell-conditioned media (BRL-CM), or the addition of recombinant LIF. To date, all of the ES cell culture systems use mammalian sources of LIF. We found that mouse ES cells can be maintained for over 10 passages in an undifferentiated state with media conditioned by a chicken liver cell line (LMH-CM) or on a feeder layer made with primary chicken embryonic fibroblasts (CEF). These ES cells can undergo both spontaneous and induced differentiation, which is associated with the disappearance or reduction of the expression of alkaline phosphatase and SSEA-1, similar to that observed for ES cells cultured with BRL-CM or STO feeder layers. The ES cells cultured in LMH-CM did not express cytokeratin Endo-A antigen recognized by TROMA-1, but their differentiated progeny did express this antigen. In contrast to LMH-CM, Endo-A was expressed in ES cells cultured on CEF feeder layers and in differentiated progeny. These results indicate that avian cells can produce a LIF-like cytokine that is active in inhibiting the differentiation of mouse ES cells. This could provide a biological end point for the isolation and characterization of avian LIF.

Production of calves by transfer of nuclei from cultured inner cell mass cells

We report here the isolation and in vitro culture of bovine inner cell mass (ICM) cells and the use of ICM cells in nuclear transfer to produce totipotent blastocysts that resulted in calves born. Of 15 cell lines represented in this study, 13 were derived from immunosurgically isolated ICM of 3 in vitro produced day 9-10 bovine blastocysts, while 2 lines were derived from single blastocysts. Approximately 70% of attempted cell lines became established cell lines when started from 3 ICMs. The ability to establish cell lines was dependent on the number of ICMs starting the line. Sire differences were noted in the ability of ICMs to establish cell lines and to form blastocysts. The cell lines were cultured as a low cell density suspension in the medium CR1aa plus selenium, insulin, and transferrin (SIT) and 5% fetal calf serum (FCS) for 6-101 days before use in nuclear transfer, at which time some had multiplied to more than 2000 cells. If allowed to aggregate, cells of established cell lines formed embryoid bodies. A total of 659 nuclear transfer clones were made by fusing the ES cells into enucleated oocytes with polyethylene glycol; 460 of these fused, based on cleavage (70%). After culture of the clones for 7 days in vitro in CR1aa/SIT/5% FCS, 109 (24%) of those fused became blastocysts. Thirty-four blastocysts were transferred into uteri of 27 cows, and 13 cows (49%) became pregnant. Four of the 13 cows gave birth to 4 normal calves. DNA typing showed the calves to be derived from the respective sires of the cell lines. The calves were derived from cultures of less than 28 days.

The identification of new genes: gene trapping in transgenic mice

Ongoing efforts to clone, sequence and map genes in the mouse have far exceeded our ability to define their functional role. The generation of mutations is an important first step towards understanding the function of genes in normal mouse development and physiology. Gene trapping in embryonic stem cells provides an efficient method to identify, clone and mutate genes at random, permitting the functional analysis of new genes in mice.

Molecular genetic approaches to the elucidation of hematopoietic stem cell function

The past few years have seen considerable advances in the development of the methodologies for discovering novel genes critical to hematopoietic stem cell function and for analyzing their biological role in hematopoiesis. This review briefly discusses some common themes that are emerging from the molecular genetic approaches to hematopoietic stem cell function.

Cloning and sequence comparison of the mouse, human, and chicken engrailed genes reveal potential functional domains and regulatory regions

We have isolated and characterized genomic DNA clones for the human and chicken homologues of the mouse En-1 and En-2 genes and determined the genomic structure and predicted protein sequences of both En genes in all three species. Comparison of these vertebrate En sequences with the Xenopus En-2 [Hemmati-Brivanlou et al., 1991) and invertebrate engrailed-like genes showed that the two previously identified highly conserved regions within the En protein ]reviewed in Joyner and Hanks, 1991] can be divided into five distinct subregions, designated EH1 to EH5. Sequences 5' and 3' to the predicted coding regions of the vertebrate En genes were also analyzed in an attempt to identify cis-acting DNA sequences important for the regulation of En gene expression. Considerable sequence similarity was found between the mouse and human homologues both within the putative 5' and 3' untranslated as well as 5' flanking regions. Between the mouse and Xenopus En-2 genes, shorter stretches of sequence similarity were found within the 3' untranslated region. The 5' untranslated regions of the mouse, chicken and Xenopus En-2 genes, however, showed no similarly conserved stretches. In a preliminary analysis of the expression pattern of the human En genes, En-2 protein and RNA were detected in the embryonic and adult cerebellum respectively and not in other tissues tested. These patterns are analogous to those seen in other vertebrates. Taken together these results further strengthen the suggestion that En gene function and regulation has been conserved throughout vertebrate evolution and, along with the five highly conserved regions within the En protein, raise an interesting question about the presence of conserved genetic pathways.

The role of growth hormone in lines of mice divergently selected on body weight

An understanding of the physiological and genetic changes which determine the response to selection is critical for both evolutionary theory and to assess the application of new molecular techniques to commercial animal breeding. We investigated an aspect of physiology, growth hormone (GH) metabolism, which might a priori have been expected to play a large part in the response of mouse lines selected for high or low body weight. Disruption of endogenous GH or addition of exogenous GH had similar proportionate effects on body weight in both lines of mice (although differences in body composition arose) suggesting that neither the production of GH nor receptor sensitivity to GH had been altered as a result of selection. This supports a 'pleiotropic model' of the response to selection: that many genes with diverse metabolic roles all contribute to the divergent phenotype. This result has significant commercial implications as it suggests that artificial selection, transgenic technology and environmental manipulation may be synergistic rather than antagonistic strategies.

Activity of yeast FLP recombinase in maize and rice protoplasts

We have demonstrated that a yeast FLP/FRT site-specific recombination system functions in maize and rice protoplasts. FLP recombinase activity was monitored by reactivation of beta-glucuronidase (GUS) expression from vectors containing the gusA gene inactivated by insertion of two FRTs (FLP recombination targets) and a 1.31 kb DNA fragment. The stimulation of GUS activity in protoplasts cotransformed with vectors containing FRT inactivated gusA gene and a chimeric FLP gene depended on both the expression of the FLP recombinase and the presence and structure of the FRT sites. The FLP enzyme could mediate inter- and intramolecular recombination in plant protoplasts. These results provide evidence that a yeast recombination system can function efficiently in plant cells, and that its performance can be manipulated by structural modification of the FRT sites.

Mouse embryo stem cells: their identification, propagation and manipulation

The early mouse embryo contains a transient population of pluripotential stem cells which are responsible for generating both the foetal primordia and extraembryonic membranes. The characterisation of murine embryo stem cells and their isolation and propagation in culture provides the first instance in which pure populations of normal stem cells are directly accessible to the researcher. This marks a considerable advance in stem cell biology which may pave the way to the dissection of general stem cell control mechanisms and the identification of key regulatory factors. In addition, the genetic manipulation of embryo stem cells affords a unique avenue for experimental intervention in mammalian development and for controlled modification of the mouse germ line.