Isolation of a pluripotent cell line from early mouse embryos cultured in medium conditioned by teratocarcinoma stem cells

Transgenesis by means of blastocyst-derived embryonic stem cell lines

This study demonstrates that blastocyst-derived embryonic stem cells (ES cells) can be used as a vehicle for transgenesis. The method is nearly as efficient as other methods, and the introduced neomycin phosphotransferase (neo) gene is stably transmitted through several generations with no apparent loss in G418 resistance. An important factor contributing to the efficiency of this process is the rigorous selection, before blastocyst injection, of genetically transformed cells for in vitro developmental pluripotency. One of the advantages of the ES cell route to transgenesis is that it provides investigators with the opportunity to screen for the desired genetic alterations before reintroducing the ES cells into the animal.

p53 cellular tumor antigen: analysis of mRNA levels in normal adult tissues, embryos, and tumors

The relative levels of mRNA specific for the mouse p53 cellular tumor antigen were determined in various normal adult tissues, embryos, and tumors. All tumors studied contained concentrations of p53 mRNA well above those present in most normal tissues. Normal spleen, however, had p53 mRNA levels comparable to those found in some tumors, despite the fact that they contained barely detectable p53 protein. This apparent discrepancy was found to be due to the extremely rapid turnover rate of p53 in the spleen (half-life, approximately equal to 6 min). In developing fetuses, a marked reduction of p53 mRNA levels was manifest from day 11 onwards, whereas the levels during organogenesis (days 9 to 11) were comparable to those found in undifferentiated embryonic stem cells and in some tumors.

Expression of N-myc in teratocarcinoma stem cells and mouse embryos

The N-myc gene, which is distantly related to the proto-oncogene c-myc, was first detected as an amplified sequence in human neuroblastoma cell lines and tumours. It has since been revealed that there is up to a 300-fold amplification of N-myc DNA in almost 50% of advanced metastatic human neuroblastomas, whereas amplification is not detected in less advanced tumours that have a better prognosis (ref.3 and M.S., unpublished data). Although expression of N-myc is detectable in all neuroblastoma cell lines and tumours examined, its level is greatly enhanced when the N-myc gene is amplified. Recently, it has been shown that on co-transfection with the c-Ha-ras (EJ) gene, N-myc can induce the malignant transformation of rat embryo fibroblasts. Taken together, these data imply a function for N-myc in the development and/or progression of human neuroblastomas. Surveys indicate that N-myc also may be amplified and/or expressed in two other types of human tumours and cell lines derived from them: retinoblastomas and small cell lung cancers. Here, we report that N-myc is expressed at high levels in mouse and human teratocarcinoma stem cells, thus identifying another tumour cell type that expresses the N-myc gene. In addition, we found that N-myc is abundantly expressed in mouse embryos at mid-gestation and that its expression appears to decrease as the embryo approaches term. In the adult mouse, N-myc is expressed at an approximately fivefold lower level in the brain than in teratocarcinoma stem cells and embryos, and at even lower levels in the adult testis and kidney. Our data represent the first demonstration of expression of the N-myc gene in normal cells, and suggest that N-myc may be involved in mammalian embryogenesis.

p53 cellular tumor antigen: analysis of mRNA levels in normal adult tissues, embryos, and tumors

The relative levels of mRNA specific for the mouse p53 cellular tumor antigen were determined in various normal adult tissues, embryos, and tumors. All tumors studied contained concentrations of p53 mRNA well above those present in most normal tissues. Normal spleen, however, had p53 mRNA levels comparable to those found in some tumors, despite the fact that they contained barely detectable p53 protein. This apparent discrepancy was found to be due to the extremely rapid turnover rate of p53 in the spleen (half-life, approximately equal to 6 min). In developing fetuses, a marked reduction of p53 mRNA levels was manifest from day 11 onwards, whereas the levels during organogenesis (days 9 to 11) were comparable to those found in undifferentiated embryonic stem cells and in some tumors.

Early mouse embryos produce and release factors with transforming growth factor activity

Previous studies have shown that extracts from mouse embryos at mid and late stages of development contain factors that exhibit transforming growth factor activity. The work reported here demonstrates that cultured mouse embryos at significantly earlier stages of development produce and release factors that exhibit the characteristic property of transforming growth factors. Specifically, the data demonstrate that embryos cultured from the blastocyst stage in serum-containing medium or in serum-free medium release factors that promote the anchorage-independent growth of normal rat kidney fibroblasts. It is shown that these factors are produced and released by cells derived from the inner cell mass and by trophoblasts. The precise developmental stage when production of these factors first begins has not been determined but our findings suggest that these factors are produced by cell types associated with early postimplantation embryos.

Spatial distribution of transcripts of the long repeated ETn sequence during early mouse embryogenesis

RNA X DNA in situ hybridization revealed a high level of ETn ("early transposon") transcripts in the pluripotent cell lineage of the 3.5- to 7.5-day mouse embryo. Some extra-embryonic ectoderm derivatives also show a high level of ETn transcripts at these stages. Older embryos (8.5 days and later) have a uniform low level of ETn transcripts.

Gene transfer into the germ line of mice and into embryonal carcinoma stem cells

Recombinant genes can be introduced into the germ line of mice by microinjection into the fertilized egg or via embryonal carcinoma stem cells. A regulated, cell-type specific expression of the newly introduced gene following microinjection is usually not found. However, in one series of experiments the insertion of human growth hormone genes in the germ line led to prenatal recessive lethal mutations. Alternatively, embryonal carcinoma stem cells can be used to introduce and express genes in mice, for example after infection with selectable retroviral vectors. In addition, these cells are useful for differentiation studies in vitro following DNA-mediated gene transfer.

Formation of germ-line chimaeras from embryo-derived teratocarcinoma cell lines

The recent availability in culture of embryo-derived pluripotential cells which exhibit both a normal karyotype and a high differentiative ability has encouraged us to assess the potential of these cells to form functional germ cells following their incorporation into chimaeric mice. We report here the results of blastocyst injection studies using three independently isolated XY embryo-derived cell lines (EK. CP1 , EK. CC1 .1 and EKCC1 .2) which produce a very high proportion (greater than 50%) of live-born animals that are overtly chimaeric. Seven chimaeric male mice, derived from these three lines, have, so far, proved to be functional germ-line chimaeras.

Retroviruses and mouse embryos: a model system in which to study gene expression in development and differentiation

Early mouse embryos exposed to Moloney leukaemia virus (M-MuL V) produce substrains of mice, designated Mov-1 to Mov-14, that transmit the virus genetically from one generation to the next. In some substrains the inserted viral genome becomes activated at specific stages of embryogenesis and the available evidence suggests that these viral genomes are developmentally regulated. The effect of cellular differentiation on virus expression was investigated by introducing M-MuL V into preimplantation or postimplantation mouse embryos, or into embryonal carcinoma cells (EC cells) in tissue culture. Whereas preimplantation embryos or EC cells did not permit virus expression, efficient replication occurred in postimplantation embryos or in differentiated cells. The viral genomes introduced into early embryos were highly methylated and non-infectious when analysed in the adult. In contrast, viral genomes introduced into postimplantation embryos remained unmethylated and were infectious in a transfection assay. Similarly, de novo methylation occurred in undifferentiated EC cells but not in differentiated derivatives. These results demonstrate an efficient de novo methylation activity which appears to be involved in the repression of genes introduced into pluripotent embryonic cells and is not observed in cells of the postimplantation embryo or in differentiated cells growing in culture. Integration of M-MuL V into the germ line can lead to recessive lethal mutations. This has been shown for the Mov-13 substrain, as animals homozygous at the Mov-13 locus die between Days 13 and 14 of embryogenesis. This suggests that viral integration occurred in a chromosomal region that is active during, and crucial for, embryonic development.

High frequency "switching" at the adenine phosphoribosyltransferase locus in multipotent mouse teratocarcinoma stem cells

Clones of multipotent mouse tetratocarcinoma stem cells, presumptively heterozygous at the adenine phosphoribosyltransferase (APRT) locus (EC 2.4.2.7), were selected for partial resistance to the purine analog 2',6'-diaminopurine (DAP). All had approximately 50% APRT activity as compared to the parental line and were found to segregate homozygous deficient cells at a high frequency (approximately 10(-2]. Homozygous deficient cells were isolated from one of the heterozygotes and were found to fall into a single class characterized by residual activity and the segregation of revertants at an equally high frequency. The revertants in turn gave rise to full mutants at comparably high frequencies. Chromosomal changes detectable with the light microscope were not associated with these transitions. Physical characterization of the APRT enzymes derived from mutant, revertant, and wild-type cells did not reveal any differences. We conclude that the reversible "switching" between heterozygosity and homozygosity is attributable to some form of gene inactivation and reactivation rather than to classical mutational events.