Studies on the activity of the X chromosomes in female teratocarcinoma cells in culture

The rhox homeobox gene cluster is imprinted and selectively targeted for regulation by histone h1 and DNA methylation

Histone H1 is an abundant and essential component of chromatin whose precise role in regulating gene expression is poorly understood. Here, we report that a major target of H1-mediated regulation in embryonic stem (ES) cells is the X-linked Rhox homeobox gene cluster. To address the underlying mechanism, we examined the founding member of the Rhox gene cluster-Rhox5-and found that its distal promoter (Pd) loses H1, undergoes demethylation, and is transcriptionally activated in response to loss of H1 genes in ES cells. Demethylation of the Pd is required for its transcriptional induction and we identified a single cytosine in the Pd that, when methylated, is sufficient to inhibit Pd transcription. Methylation of this single cytosine prevents the Pd from binding GA-binding protein (GABP), a transcription factor essential for Pd transcription. Thus, H1 silences Rhox5 transcription by promoting methylation of one of its promoters, a mechanism likely to extend to other H1-regulated Rhox genes, based on analysis of ES cells lacking DNA methyltransferases. The Rhox cluster genes targeted for H1-mediated transcriptional repression are also subject to another DNA methylation-regulated process: Xp imprinting. Remarkably, we found that only H1-regulated Rhox genes are imprinted, not those immune to H1-mediated repression. Together, our results indicate that the Rhox gene cluster is a major target of H1-mediated transcriptional repression in ES cells and that H1 is a candidate to have a role in Xp imprinting.

Embryonic hybrid cells: a powerful tool for studying pluripotency and reprogramming of the differentiated cell chromosomes

The properties of embryonic hybrid cells obtained by fusion of embryonic stem (ES) or teratocarcinoma (TC) cells with differentiated cells are reviewed. Usually, ES-somatic or TC-somatic hybrids retain pluripotent capacity at high levels quite comparable or nearly identical with those of the pluripotent partner. When cultured in vitro, ES-somatic- and TC-somatic hybrid cell clones, as a rule, lose the chromosomes derived from the somatic partner; however, in some clones the autosomes from the ES cell partner were also eliminated, i.e. the parental chromosomes segregated bilaterally in the ES-somatic cell hybrids. This opens up ways for searching correlation between the pluripotent status of the hybrid cells and chromosome segregation patterns and therefore for identifying the particular chromosomes involved in the maintenance of pluripotency. Use of selective medium allows to isolate in vitro the clones of ES-somatic hybrid cells in which "the pluripotent" chromosome can be replaced by "the somatic" counterpart carrying the selectable gene. Unlike the TC-somatic cell hybrids, the ES-somatic hybrids with a near-diploid complement of chromosomes are able to contribute to various tissues of chimeric animals after injection into the blastocoel cavity. Analysis of the chimeric animals showed that the "somatic" chromosome undergoes reprogramming during development. The prospects for the identification of the chromosomes that are involved in the maintenance of pluripotency and its cis- and trans-regulation in the hybrid cell genome are discussed.

Effect of CpG methylation on expression of the mouse imprinted gene Mest

We previously reported isolation of the mouse gene, Mest (mesoderm-specific transcripts), which is mapped to the proximal part of chromosome 6 and predominantly expressed in the mesoderm and its derivatives during development. Peg1, a paternally expressed gene isolated by a systematic screening of imprinted genes, was recently demonstrated to be identical to Mest. We and others have shown that the human homolog (MEST) of Mest is also imprinted so as to be expressed from the paternal copy and maps to 7q32. To study transcriptional regulation of Mest/Peg1, we examined the effect of DNA methylation on its expression. In the embryonal carcinoma (EC) cell line, MC12, from which Mest was originally isolated, the 5'-region harboring presumptive promoter of the gene was undermethylated. On the other hand, C4XX, a subclone of MC12 which had lost expression of Mest, was characterized by extremely high levels of methylation in the 5'-region, demethylation of which resulted in activation of Mest. Furthermore, a methylated reporter construct with the luciferase gene under the control of the putative promoter region of Mest was not competent to produce luciferase activity in MC12 cells. These results suggest a suppressive role for DNA methylation in Mest expression. However, neither methylated nor unmethylated reporter constructs showed luciferase activity in a primary culture from the adult kidney, in which Mest is down-regulated despite apparent unmethylation of the paternal allele. Taken together, the data suggest that there are probably two modes of regulation for the Mest gene; one being a methylation-dependent mechanism that regulates imprinted expression of Mest during development, and the other being a methylation-independent mechanism that is involved in down-regulation of Mest in adult tissues.

Mouse embryonal carcinoma cell-somatic cell hybrids as experimental tools for the study of cell differentiation and X chromosome activity

Remarkable properties of murine embryonal carcinoma cells, stem cells of teratocarcinomas, manifest themselves when they are fused with differentiated somatic cells. Among others the capacity to reprogram genomes from these somatic cells and restore genetic pluripotency is most striking.

Xist is expressed in female embryonal carcinoma cells with two active X chromosomes

The Xist gene resides on the X chromosome and is expressed in female but not male somatic cells. In female cells, only the Xist allele on the inactive X chromosome is transcribed. We investigated the expression of Xist in diploid P10 female embryonal carcinoma cells that have two active X chromosomes. Xist RNA was present in these P10 cells. The X chromosomes in P10 cells carry different Xist alleles whose transcripts can be distinguished by restriction digestion of their cDNAs. Both alleles were expressed. Clones of P10 cells that had lost an X chromosome did not express Xist from the remaining allele. Thus Xist is expressed in cultured cells developmentally arrested prior to X chromosome inactivation, indicating that the Xist transcript is not always derived from an inactive X chromosome. Therefore, Xist expression per se cannot be a sufficient signal to inactivate an X chromosome.

Isolation and cultivation of blastocyst-derived stem cell lines from American mink (Mustela vison)

Ten embryonic stem (ES) cell lines from mink blastocysts were isolated and characterized. All the lines had a normal diploid karyotype; of the ten lines studied, five had the XX and five had the XY constitution. Testing of the pluripotency of the ES-like cells demonstrated that 1) among four lines of genotype XX, and X was late-replicating in three; both Xs were active in about one-third of cells of line MES8, and analysis of glucose-6-phosphate dehydrogenase revealed no dosage compensation for the X-linked gene; 2) when cultured in suspension, the majority of lines were capable of forming "simple" embryoid bodies (EB), and two only showed the capacity for forming "cystic" multilayer EBs. However, formation of ectoderm or foci of yolk sac hematopoiesis, a feature of mouse ES cells, was not observed in the "cystic" EB; 3) when cultured as a monolayer without feeder, the ES cells differentiated into either vimentin-positive fibroblast-like cells or cytokeratin-positive epithelial-like cells (less frequently); neural cells appeared in two lines; 4) when injected into athymic mice, only one of the four tested lines gave rise to tumors. These were fibrosarcomas composed of fibroblast-like cells, with an admixture of smooth muscular elements and stray islets of epithelial tissue; (5) when the ES cells of line MES1 were injected into 102 blastocyst cavities and subsequently transplanted into foster mothers, we obtained 30 offspring. Analysis of the biochemical markers and coat color did not demonstrate the presence of chimaeras among offspring. Thus the cell lines derived from mink blastocysts are true ES cells. However, their pluripotential capacities are restricted.

The mouse Pgk-1 gene promoter contains an upstream activator sequence

The Pgk-1 gene encodes the housekeeping enzyme, 3-phosphoglycerate kinase, and is ubiquitously expressed. This gene resides on the X chromosome in mammals and is always expressed except where it is silenced along with most other genes on the inactive X chromosome of female somatic cells or male germ cells. The Pgk-1 promoter is in a region rich in nucleotides G and C. This promoter can efficiently drive high levels of expression of reporter genes such as E. coli lacZ and neo. We have determined that the 120 bp upstream of the transcription start site functions as a core promoter. Upstream of this is a 320 bp region which enhances transcription from the core promoter in an orientation and position independent fashion. This 320 bp region does not enhance transcription from the core promoter of the SV40 early region. Nuclear proteins bind to this 320 bp fragment although the restricted regions to which binding can be demonstrated with gel mobility shift assays suggests that the activity of the enhancer may be mediated by factors which bind at multiple sites each with low affinity.

DNA methylation of two X chromosome genes in female somatic and embryonal carcinoma cells

The extent of methylation of DNA sequences upstream and within the two X-linked genes, Pgk-1 and Hprt, was analyzed in male and female somatic cells and in female embryonal carcinoma cells carrying either two active X chromosomes (Xa) or one active and one inactive X chromosome (Xi). Sites upstream and within the first intron of both Pgk-1 and Hprt were heavily methylated on the Xi in somatic cells and in embryonal carcinoma cells with an Xi. Reactivation of this Xi was accompanied by extensive demethylation of these sites. In female embryonal carcinoma cells with two active X chromosomes, one X inactivates during differentiation in culture; however, methylation did not occur during differentiation, consistent with the idea that DNA methylation does not play a role in the initiation of X inactivation but may be involved in maintaining inactivation of those genes on the Xi.

Reactivation of hprt on the inactive X chromosome with DNA demethylating agents

The C86 line of female embryonal carcinoma cells contains one active and one inactive X chromosome. Following methylnitrosourea mutagenesis, a clone called C86AGM2 was isolated that carries a mutated hprt gene on the active X chromosome. This hprtm allele encodes an HPRT enzyme that has less than 1% normal enzyme activity, is thermolabile, and has an altered isoelectric point. Following treatment with drugs that demethylate DNA, the hprt+ gene from the inactive X chromosome in C86AGM2 cells became active as determined by the appearance of HPRT activity with the thermodenaturation and electrofocusing characteristics of the normal enzyme. No expression of this hprt+ gene occurred if C86AGM2 cells were induced to differentiate prior to DNA demethylation. Stable lines of C86AGM2 cells expressing both the hprtm and hprt+ genes did not inactivate either gene following differentiation.

Nonhistone nuclear proteins specific to certain mouse embryonal carcinoma clones having an inactive X chromosome

By means of one-dimensional polyacrylamide slab gel electrophoresis nonhistone nuclear proteins were compared in murine embryonal carcinoma cell clones with two X chromosomes; both are active in some clones and one of them is inactive in others and in a population of cells having only one X chromosome. Under our experimental conditions, we succeeded in finding two extra bands at approximately 46,000 Da in cells having an inactive X chromosome. Furthermore, a band at approximately 71,000 Da was significantly heavier in cells having an inactive X chromosome than in those having two active X or those having only one X chromosome.

A low-molecular weight chick brain-derived growth factor is mitogenic for cultured astroglia from the chick embryo

Immunohistochemically defined astrocytes from the 10-day chick embryo were stimulated to incorporate increased levels of [3H]thymidine when a low-molecular weight peptide growth factor, chick brain-derived growth factor (CBGF), was added to the cultures. Treatment of these GFAP-positive astrocytes with 10 ng/ml CBGF in medium supplemented with 1% fetal bovine serum resulted in a 3.5-4-fold increase in [3H]thymidine incorporation when compared to astrocytes cultured in defined medium supplemented with 1% serum alone. CBGF had no effect on the survival, proliferation or differentiation of a number of other cell types from the 10-day chick embryo brain, including neurons and meningeal fibroblasts. CBGF was also ineffective as a mitogen for chick embryo skeletal muscle myoblasts, primary mouse embryo fibroblasts and one murine teratocarcinoma-derived cell line (STO). We suggest that CBGF might act as a mitogenic signal for astroglia during central nervous system development and repair.

X chromosome reactivation in mouse embryonal carcinoma cells

The embryonal carcinoma cell line, C86S1, carries two X chromosomes, one of which replicates late during S phase of the cell cycle and appears to be genetically inactive. C86S1A1 is a mutant which lacks activity of the X-encoded enzyme, hypoxanthine phosphoribosyltransferase (HPRT). Treatment of C86S1A1 cells with DNA-demethylating agents, such as 5-azacytidine (5AC), resulted in (i) the transient expression in almost all cells of elevated levels of HPRT and three other enzymes encoded by X-linked genes and (ii) the stable expression of HPRT in up to 5 to 20% of surviving cells. Most cells which stably expressed HPRT had two X chromosomes which replicated in early S phase. C86S1A1 cells which had lost the inactive X chromosome did not respond to 5AC. These results suggest that DNA demethylation results in the reactivation of genes on the inactive X chromosome and perhaps in the reactivation of the entire X chromosome. No such reactivation occurred in C86S1A1 cells when the cells were differentiated before exposure to 5AC. Thus, the process of X chromosome inactivation may be a sequential one involving, as a first step, methylation of certain DNA sequences and, as a second step, some other mechanism(s) of transcriptional repression.

X chromosome reactivation in mouse embryonal carcinoma cells

The embryonal carcinoma cell line, C86S1, carries two X chromosomes, one of which replicates late during S phase of the cell cycle and appears to be genetically inactive. C86S1A1 is a mutant which lacks activity of the X-encoded enzyme, hypoxanthine phosphoribosyltransferase (HPRT). Treatment of C86S1A1 cells with DNA-demethylating agents, such as 5-azacytidine (5AC), resulted in (i) the transient expression in almost all cells of elevated levels of HPRT and three other enzymes encoded by X-linked genes and (ii) the stable expression of HPRT in up to 5 to 20% of surviving cells. Most cells which stably expressed HPRT had two X chromosomes which replicated in early S phase. C86S1A1 cells which had lost the inactive X chromosome did not respond to 5AC. These results suggest that DNA demethylation results in the reactivation of genes on the inactive X chromosome and perhaps in the reactivation of the entire X chromosome. No such reactivation occurred in C86S1A1 cells when the cells were differentiated before exposure to 5AC. Thus, the process of X chromosome inactivation may be a sequential one involving, as a first step, methylation of certain DNA sequences and, as a second step, some other mechanism(s) of transcriptional repression.

Evidence for a relationship between DNA methylation and DNA replication from studies of the 5-azacytidine-reactivated allocyclic X chromosome

We examined the sequence of DNA synthesis of the human active, inactive and reactivated X chromosomes in mouse-human hybrid cells. The two independent reactivants, induced by 5-azacytidine (5-azaC), expressed human hypoxanthinephosphoribosyl transferase (HPRT), and one also expressed human glucose-6-phosphate dehydrogenase (G6PD) and phosphoglycerate kinase (PGK). Restriction enzyme analysis of DNA methylation at the re-expressed loci revealed hypomethylation of CpG clusters, that characterizes the relevant genes on the active X. The transfer of active and inactive X chromosomes from the native environment of the human fibroblast to the foreign environment of the hybrid cell did not affect the specific replication sequence of either human X chromosome. The silent X chromosome when reactivated, remained allocyclic, and the first bands to replicate were the same as prior to reactivation. In one reactivant, however, further progression of replication was significantly altered with respect to the order in which bands were synthesized. This alteration in the replication of the silent X following 5-azaC-induced reactivation suggests that DNA methylation may modulate the replication kinetics of chromosomal DNA.

The relationship between embryonic, embryonal carcinoma and embryo-derived stem cells

Two types of pluripotent stem cell lines of embryonic origin are available as tools for the study of molecular and cellular differentiation in vitro. Tumor-derived embryonal carcinoma (EC) cell lines and the more recently developed embryo-derived stem (ES) cell lines have many characteristics similar to those of pluripotent cells within the embryo itself, the major difference being the cell lines' capacity for continued proliferation as undifferentiated cells. It is not known whether all EC/ES cell lines are derived from the same or different embryonic stem cells. Some differences between cell lines would be compatible with the latter. The advent of ES cells, which appear to be closer to embryonic cells, may allow the resolution of this question.

Stem cells and embryo-derived cell lines: tools for study of gene expression

Embryonal carcinoma (EC) cells obtained either from teratocarcinomas or directly from in vitro cultures of mouse embryos (EK) can be used as models for the early stages of normal mammalian development. A few known examples of experimental designs with such cells are reviewed: aggregates with normal embryos, promotion of parthenogenetic development by injection of EK cells into blastocysts, EK cells homozygous for a lethal gene, timing of expression in differentiating EC cells of a tissue-specific gene product, and X chromosome inactivation.

Studies of the temporal relationship between the cytogenetic and biochemical manifestations of X-chromosome inactivation during the differentiation of LT-1 teratocarcinoma stem cells

Previous biochemical studies have suggested that both X chromosomes produce gene products when cells of the LT-1 teratocarcinoma stem cell line are maintained in the undifferentiated state, and that dosage compensation, the biochemical manifestation of X inactivation, occurs when the cells are induced to differentiate in vitro (Martin et al., 1978). In this study the differentiation of LT-1 cells in vitro is described in detail, and data from cytogenetic studies of the time of X-chromosome replication in LT-1 cells are presented. They show that as long as the cells are maintained in the undifferentiated state both X chromosomes in each cell show the isocyclic replication pattern typical of a genetically active chromosome. However, when the LT-1 cells are induced to differentiate under appropriate conditions, one of the two X chromosomes in each cell of a large proportion of the population displays the allocyclic (either early or late) replication pattern typical of an inactive X chromosome. These data thus confirm that undifferentiated LT-1 cells contain two active X chromosomes and that X inactivation occurs in differentiating cultures of LT-1 cells. It is further demonstrated that there is a close temporal correlation between the biochemical and cytogenetic manifestations of the X-inactivation process. In addition, we observed that although X inactivation does not occur in the absence of morphological differentiation, it does not always occur when the cells differentiate in vitro.

Reversal of X-inactivation in female mouse somatic cells hybridized with murine teratocarcinoma stem cells in vitro

A series of near-diploid embryonal carcinoma-like hybrid cells were obtained from polyethylene glycol mediated cell fusion between murine embryonal carcinoma cells (PSA-6TG1 or OTF9-63) having one X chromosome and thymocytes or bone marrow cells from female mice carrying Cattanach's or Searle's translocation. Prior to fusion with EC cells the somatic cells are presumed to contain only one active X chromosome. Following hybrid formation, the chronology of X chromosome replication and the expression of X-linked gene Pgk-1 indicated that all X chromosomes contributed by both parents were active in these hybrids. Experiments were performed to rule out the possibility that the hybrids were formed by fusion of EC cells with rare somatic cells in which both X chromosomes were active. Taken together the data indicate that within four days of fusion there is reactivation of the entire inactive X chromosome.

Genetic activity of X chromosomes in pluripotent female teratocarcinoma cells and their differentiated progeny

We have induced teratocarcinomas from female embryos heterozygous for electrophoretic variants of the X-linked gene coding for phosphoglycerate kinase (PGK). An embryonal carcinoma cell line, P10, has been isolated from such a teratocarcinoma. It has a normal female karyotype and cultures contain both PGK isoenzymic forms. Clonal populations derived from P10 also contain both PGK electrophoretic variants. In addition, both X chromosomes in these cells replicate in synchrony with the autosomes during early S phase of the cell cycle. These data indicate that the undifferentiated P10 embryonal carcinoma cells contain two active X chromosomes. When cultured under the appropriate conditions, the P10 cells differentiate to form a variety of tissue types. At least some of these differentiated cells contain an inactive X chromosome as determined by cytogenetic analysis. Apparently X chromosome inactivation accompanies the differentiation of these female embryonal carcinoma cells.

X-chromosome inactivation during differentiation of female teratocarcinoma stem cells in vitro

Evidence is presented that both X chromosomes are genetically active in clonal cultures of undifferentiated female mouse teratocarcinoma stem cells derived from a spontaneous ovarian tumour. As the cells differentiate in vitro one of the X chromosomes becomes inactivated.