X chromosome retains the memory of its parental origin in murine embryonic stem cells

Developmental regulation of X-chromosome inactivation

With the emergence of sex-determination by sex chromosomes, which differ in composition and number between males and females, appeared the need to equalize X-chromosomal gene dosage between the sexes. Mammals have devised the strategy of X-chromosome inactivation (XCI), in which one of the two X-chromosomes is rendered transcriptionally silent in females. In the mouse, the best-studied model organism with respect to XCI, this inactivation process occurs in different forms, imprinted and random, interspersed by periods of X-chromosome reactivation (XCR), which is needed to switch between the different modes of XCI. In this review, I describe the recent advances with respect to the developmental control of XCI and XCR and in particular their link to differentiation and pluripotency. Furthermore, I review the mechanisms, which influence the timing and choice, with which one of the two X-chromosomes is chosen for inactivation during random XCI. This has an impact on how females are mosaics with regard to which X-chromosome is active in different cells, which has implications on the severity of diseases caused by X-linked mutations.

An ex vivo model for imprinting: mutually exclusive binding of Cdx2 and Oct4 as a switch for imprinted and random X-inactivation

In the early mammalian embryo, X chromosome inactivation (XCI) achieves dosage parity between males and females for X-linked genes. During mouse development, imprinted paternal XCI is observed first and switches to random XCI in the epiblast but not placental lineages. The mechanism by which this epigenetic switch occurs is currently unknown. Here, we establish an ex vivo model for imprinting and identify a novel trans-acting regulatory factor for imprinted XCI. Using an induced trophoblast stem cell (iTS) model, we show that embryonic stem (ES) cells transdifferentiated into trophoblasts retain partial memory of the XCI imprint. Cdx2, a stem cell factor that determines commitment to the extraembryonic lineage, directly binds Xist and activates expression of Xist RNA in extrembryonic cells. Cdx2 competes with Oct4, a stem cell factor that determines commitment to the embryonic lineage, for overlapping binding sites within Xist. We propose that mutually exclusive binding between Cdx2 and Oct4 in Xist underlies the switch between imprinted and random XCI in the early mouse embryo.

Epigenetic aspects of fertilization and preimplantation development in mammals: lessons from the mouse

During gametogenesis, the parental genomes are separated and are epigenetically marked by modifications that will direct the expression profile of genes necessary for meiosis as well as for the formation of the oocyte and sperm cell. Immediately after sperm-egg fusion, the parental haploid genomes show great epigenetic asymmetry with differences in the levels of DNA methylation and histone tail modifications. The epigenetic program acquired during oogenesis and spermatogenesis must be reset for the zygote to successfully proceed through preimplantation development and this occurs as the two genomes approach each other in preparation for karyogamy. During preimplantation development, the embryo is vested with the responsibility of maintaining the primary imprints. In addition, female embryos must silence one of the X-chromosomes in order to transcribe equal levels of X-linked genes as their male counterparts. This review is intended as a survey of the epigenetic modifications and mechanisms present in zygotes and preimplantation mouse embryos, namely DNA methylation, histone modifications, dosage compensation, genomic imprinting, and regulation by non-coding RNAs.

Attenuated spread of X-inactivation in an X;autosome translocation

X inactivation in female mammals involves transcriptional silencing of an entire chromosome in response to a cis-acting noncoding RNA, the X inactive-specific transcript (Xist). Xist can also inactivate autosomal sequences, for example, in X;autosome translocations; but here, silencing appears to be relatively inefficient. This variation has been attributed to either attenuated spreading of Xist RNA at the onset of X inactivation or inefficient maintenance of autosomal silencing. Evidence to date has favored the latter. Here, we demonstrate attenuated spreading of Xist RNA at the onset of X inactivation in the T(X;4)37H X;autosome translocation. Our findings provide direct evidence that underlying chromosome/chromatin features can disrupt spreading of the primary inactivating signal.

X chromosomes alternate between two states prior to random X-inactivation

Early in the development of female mammals, one of the two X chromosomes is silenced in half of cells and the other X chromosome is silenced in the remaining half. The basis of this apparent randomness is not understood. We show that before X-inactivation, the two X chromosomes appear to exist in distinct states that correspond to their fates as the active and inactive X chromosomes. Xist and Tsix, noncoding RNAs that control X chromosome fates upon X-inactivation, also determine the states of the X chromosomes prior to X-inactivation. In wild-type ES cells, X chromosomes switch between states; among the progeny of a single cell, a given X chromosome exhibits each state with equal frequency. We propose a model in which the concerted switching of homologous X chromosomes between mutually exclusive future active and future inactive states provides the basis for the apparently random silencing of one X chromosome in female cells.

During female mammalian development, one of the two X chromosomes in each cell is inactivated. Prior to X-inactivation, these chromosomes appear to exist in distinct states that correspond to their fates.

Micromanipulating dosage compensation: understanding X-chromosome inactivation through nuclear transplantation

Nuclear transfer (NT) studies have provided insight into the functional importance of epigenetic alteration of the X chromosomes during X-inactivation. Uniparental embryos created by NT have been informative as to the time and location at which the imprint controlling extraembryonic X-inactivation is established. Experiments with female somatic cells, have demonstrated that the inactive X chromosome (Xi) is reactivated after NT, leading to random X-inactivation in the embryonic lineages of cloned embryos. However, in the extraembryonic lineages of clones, epigenetic information from the donor cell nucleus persists, leading to preferential inactivation of the donor cell's inactive X in the placenta of cloned animals. These results suggest epigenetic information established during embryonic X-inactivation is functionally equivalent to the gametic imprint.

Xist RNA and the mechanism of X chromosome inactivation

Dosage compensation in mammals is achieved by the transcriptional inactivation of one X chromosome in female cells. From the time X chromosome inactivation was initially described, it was clear that several mechanisms must be precisely integrated to achieve correct regulation of this complex process. X-inactivation appears to be triggered upon differentiation, suggesting its regulation by developmental cues. Whereas any number of X chromosomes greater than one is silenced, only one X chromosome remains active. Silencing on the inactive X chromosome coincides with the acquisition of a multitude of chromatin modifications, resulting in the formation of extraordinarily stable facultative heterochromatin that is faithfully propagated through subsequent cell divisions. The integration of all these processes requires a region of the X chromosome known as the X-inactivation center, which contains the Xist gene and its cis-regulatory elements. Xist encodes an RNA molecule that plays critical roles in the choice of which X chromosome remains active, and in the initial spread and establishment of silencing on the inactive X chromosome. We are now on the threshold of discovering the factors that regulate and interact with Xist to control X-inactivation, and closer to an understanding of the molecular mechanisms that underlie this complex process.

Requirement for Xist in X chromosome inactivation

The Xist gene has been proposed as a candidate for the X inactivation centre, the master regulatory switch locus that controls X chromosome inactivation. So far this hypothesis has been supported solely by indirect evidence. Here we describe gene targeting of Xist, and provide evidence for its absolute requirement in the process of X chromosome inactivation.

Quantitative RT-PCR assays show Xist RNA levels are low in mouse female adult tissue, embryos and embryoid bodies

We have investigated expression of the Xist gene in mouse female adult kidney, embryos and embryonic stem (ES) cells undergoing in vitro differentiation as embryoid bodies. Using the quantitative RT-PCR single nucleotide primer extension (SNuPE) assay, we found that the amount of Xist RNA in adult kidney of three mouse strains was less than approximately 2000 transcripts per cell, with only modest differences between strains carrying different Xce alleles. Female embryos 7.5 days post coitum had the same number of Xist transcripts per cell as isogenic adult tissue. Using quantitative oligonucleotide hybridization assays after RT-PCR, we investigated Xist expression in ES lines heterozygous at the Pgk-1 and Xist loci. We found that, while in most (XX) ES lines Xist RNA levels increased during embryoid body formation, the levels seen were less than 10% those found in adult female kidney. In addition, we found that the allelic ratio of Xist transcripts from reciprocal (XX) ES cell lines differentiating in vitro was identical to that of isogenic 10.5 to 11.5 day female embryos. These latter results suggest that there is no pattern of preferential paternal imprinting during days 1 to 9 of in vitro differentiation of ES cells. However, the influence of the Xce locus on the randomness of X-inactivation in embryos seems to operate also in ES cell lines. Our overall conclusion is that the low levels of Xist RNA in female kidney, embryos and differentiating (XX) ES cells are compatible only with models that do not require Xist RNA to cover the entire inactive X chromosome.

X-chromosome activity of the mouse primordial germ cells revealed by the expression of an X-linked lacZ transgene

We have determined the timing of the inactivation and reactivation of the X chromosome in the mouse primordial germ cells (PGCs) by monitoring the expression of an X-linked HMG-lacZ reporter gene. PGCs were identified by their distinct alkaline phosphatase activity and they were first localised in the primitive streak and allantoic bud of the 7.5-day gastrulating embryo. Although inactivation of the transgene was found in some PGCs at these sites, at least 85% of the population were still expressing the lacZ gene. This suggests that, although X-inactivation has commenced during gastrulation, the majority of PGCs still possess two active X chromosomes. Transgene activity remained unchanged during the relocation of PGCs to the hindgut endoderm, but decreased abruptly when PGCs left the hindgut and migrated through the mesentery. X-inactivation was completed during the migration of PGCs, but not simultaneously for the whole population. The first wave of PGCs entering the genital ridge at 9.5 days did not immediately re-activate the silent transgene until about 24 hours later. Re-activation of the transgene took place in over 80% of PGCs entering the genital ridge at 10.5-13.5 days p.c., preceding the entry into meiosis. About 90% of the meiotic germ cells in the 14.5-15.5 day fetal ovary expressed the transgene. Similar profiles of transgene activity were observed in PGCs of embryos that have inherited the lacZ transgene from different parents, showing unequivocally that X-inactivation in the germ cell lineage is not related to parental legacy.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of an X-linked HMG-lacZ transgene in mouse embryos: implication of chromosomal imprinting and lineage-specific X-chromosome activity

X-chromosome activity in female mouse embryos was studied at the cellular level using an X-linked lacZ transgene which encodes beta-galactosidase (beta-Gal). Translation of maternal RNA in oocytes is seen as beta-Gal activity that persists into early cleavage-stages. Zygotic transcription of the transgene from the maternal X chromosome (Xm) is first found at about the 8-cell stage. By contrast, expression of the lacZ transgene on the paternal X chromosome (Xp) is not seen until later at the 16-32-cell stage. Preferential inactivation of Xp occurs in the mural trophectoderm, the primitive endoderm, and derivatives of the polar trophectoderm, but a small number of cells in these lineages may still retain an active paternal X chromosome. X inactivation begins at 3.5 days in the inner cell mass but contrary to previous findings the process is not completed in the embryonic ectoderm by 5.5 to 6.0 days. Regional variation in beta-Gal activity is also observed in the embryonic ectoderm during gastrulation which may be related to the specification of cell fates. Random inactivation of Xp and Xm ensues in all somatic tissues but the process is completed at different times in different tissues. The slower progression of X inactivation in tissues such as the notochord, the heart, and the embryonic gut is primarily due to the persistent maintenance of two active X chromosomes in a significant fraction of cells in these tissues. Recent findings on the methylation of endogenous X-linked genes suggest that the prolonged expression of beta-Gal might also be due to the different rate of spreading of inactivation along the X chromosome to the lacZ transgene locus in different tissues.