X chromosome inactivation in diploid parthenogenetic mouse embryos

Mechanisms of Choice in X-Chromosome Inactivation

Early in development, placental and marsupial mammals harbouring at least two X chromosomes per nucleus are faced with a choice that affects the rest of their lives: which of those X chromosomes to transcriptionally inactivate. This choice underlies phenotypical diversity in the composition of tissues and organs and in their response to the environment, and can determine whether an individual will be healthy or affected by an X-linked disease. Here, we review our current understanding of the process of choice during X-chromosome inactivation and its implications, focusing on the strategies evolved by different mammalian lineages and on the known and unknown molecular mechanisms and players involved.

Loss of Xist imprinting in diploid parthenogenetic preimplantation embryos

We have analysed Xist expression patterns in parthenogenetic and control fertilised preimplantation embryos by using RNA FISH. In normal XX embryos, maternally derived Xist alleles are repressed throughout preimplantation development. Paternal alleles are expressed as early as the 2-cell stage. In parthenogenetic embryos, we observed Xist RNA expression and accumulation from the morula stage onwards, indicating loss of maternal imprinting. In the majority of cells, expression was from a single allele, indicating that X chromosome counting occurs to establish appropriate monoallelic Xist expression. We discuss these data in the context of models for regulation of imprinted and random X inactivation.

Role of the Xist gene in X chromosome choosing

In female mammals a "random choice" mechanism decides which of the two X chromosomes will be inactivated. It has been postulated that Xist is crucial for heterochromatinization and thus functions downstream of the choice mechanism. Here we report that females heterozygous for an internal deletion in the Xist gene, which includes part of exon 1 and extends to exon 5, undergo primary nonrandom inactivation of the wild-type X chromosome. The Xist gene, therefore, not only has a role in chromatin remodeling, but also includes an element required for X chromosome choosing. In conflict with the prevailing view of how choosing occurs, the element identified by the deletion plays a positive role in the choice mechanism and forces a reassessment of how X chromosome choosing is thought to occur.

X-chromosome inactivation in mammals

The inactive X chromosome differs from the active X in a number of ways; some of these, such as allocyclic replication and altered histone acetylation, are associated with all types of epigenetic silencing, whereas others, such as DNA methylation, are of more restricted use. These features are acquired progressively by the inactive X after onset of initiation. Initiation of X-inactivation is controlled by the X-inactivation center (Xic) and influenced by the X chromosome controlling element (Xce), which causes primary nonrandom X-inactivation. Other examples of nonrandom X-inactivation are also presented in this review. The definition of a major role for Xist, a noncoding RNA, in X-inactivation has enabled investigation of the mechanism leading to establishment of the heterochromatinized X-chromosome and also of the interactions between X-inactivation and imprinting as well as between X-inactivation and developmental processes in the early embryo.

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

A cytogenetic and biochemical study of balloon-like cystic embryoid bodies, formed by newly established embryonic stem (ES) cell lines having a cytogenetically or genetically marked X chromosome, revealed that the paternally derived X chromosome was inactivated in the majority of cells in the yolk sac-like mural region consisting of the visceral endoderm and mesoderm. The nonrandomness was less evident in the more solid polar region containing the ectodermal vesicle, mesoderm and visceral endoderm. Since the same was true in embryoid bodies derived from ES cells at the 30th subculture generation, it was concluded that the imprinting responsible for the preferential inactivation of the paternal X chromosome that was limited to non-epiblast cells of the female mouse embryos, was stably maintained in undifferentiated ES cells. Differentiating epiblast cells should be able to erase or avoid responding to the imprint.

Early development and X-chromosome inactivation in mouse parthenogenetic embryos

Early development and X-chromosome inactivation were studied in ethanol-induced mouse parthenogenones. About 24% of oocytes transferred to 0.5-day pseudopregnant recipients successfully implanted. However, only 49%, 20%, and 16% of implanted parthenogenones survived 5, 6, and 7 days later, respectively. Abnormal development was evident in every parthenogenone as early as 5 days after activation with the degenerating polar trophectoderm. These embryos were destined to become either small disorganized embryos or embryonic ectoderm vesicles bounded by the visceral endoderm. Only 2 of 51 representative 6- to 8-day parthenogenones sectioned had morphology of the normal egg cylinder, although growth retardation was evident. Spontaneous LT/Sv parthenogenones shared similar morphological features. In late blastocysts, the frequency of cells with an apparently inactivated X chromosome was lower in parthenogenones than in fertilized embryos. The failure of X-inactivation in the trophectoderm seems to contribute to the defective development of parthenogenones.

Parental source of chromosome imprinting and its relevance for X chromosome inactivation

In imprinting, homologous chromosomes behave differently during development according to their parental origin. Typically, paternally derived chromosomes are preferentially inactivated or eliminated. Examples of such phenomena include inactivation of the mammalian X chromosome, inactivation or elimination of one haploid chromosome set in male coccids, and elimination of paternal X chromosomes in the fly Sciara. It has generally been thought that the paternal chromosomes bear an imprint leading to their inactivation or elimination. However, alteration of the parental origin of chromosomes, as in the study of parthenogenotes in mammals and coccids, shows that passage of chromosomes through a male germ cell or fertilization is not essential for inactivation or elimination. It appears that neither chromosome set is programmed to resist or undergo inactivation. Instead the two sets differ in relative sensitivity, and the question is whether the maternal set have an imprint for resistance, or the paternal set one for susceptibility. Very early in development of mammals both X chromosomes are active. This makes it simpler to envisage the maternal X bearing an imprint for resistance to inactivation, which persists through the early developmental period. Similar considerations also apply in coccids and Sciara. Thus, imprinting should be regarded as a phenomenon conferred on the maternal chromosomes in the oocyte. This permits simpler models for the mechanism of X-inactivation, and weakens the case for evolution of X-inactivation from an earlier form of inactivation during male gametogenesis. One may speculate whether imprinting affects timing of gene action in development.

Insect sex chromosomes. VIII. Identification of active/inactive X-chromosomes in Gryllotalpa fossor by 5-BrdU/AO fluorescence

By employing 5-BrdU/AO fluorescence technique to distinguish active from inactive X-chromosomes, we have, for the first time, provided evidence in support of the validity of this technique for a non-mammalian system Gryllotalpa fossor (Orthoptera). In the female somatic cells of Gryllotalpa, it is only in the active (euchromatic) arm of X-chromosome that the fluorescence is bright, whereas it is dull in the facultative and constitutive heterochromatic arms. In tetraploid spermatogonial cells one of the two X-chromosomes is inactive, suggesting that this phenomenon is probably a random one and that the inactivation process is independent of the imprinting mechanism.

Two examples of monoamniotic monozygotic twinning in diploid parthenogenetic mouse embryos

In experiments in which diploid parthenogenetic mouse embryos cultured entirely in vitro from the one-cell stage to the blastocyst were transferred to suitable recipients and maintained in "delay" for about 3 days before implanting, about 25-40% subsequently developed to somite-containing stages. In all, over 30 such embryos were examined. Most were morphologically normal, and equivalent to fertilized development observed on the 9th-11th days of pregnancy. A few embryos, however, had neural tube abnormalities, but of greatest interest were two sets of monoamniotic monozygotic twins. It is unclear whether the twins in some way resulted from the parthenogenetic condition, or from the "delayed" state. The present examples are discussed in the context of previous observations of monozygotic twinning in the mouse and man.

Experimental genetics of the mammalian embryo

Recent progress in experimental mouse embryology has provided new approaches to the genetic manipulation of the mammalian embryo. The production of uniparental embryos enables one to compare maternal and paternal gene activity during development, to study the biological consequences of homozygosity of mutant genes, and to further elucidate the unsolved problem of X chromosome inactivation. Transplantation of nuclei from somatic cells into mouse eggs is considered the most vigorous functional test for the developmental capacity of the nuclear genome and provides a bioassay for the study of possible genomic changes during cellular differentiation. Transplantation of cloned eukaryotic genes into mouse eggs will permit the molecular and genetic analysis of their integration and regulation during development and, eventually, their germ line transmission as new heritable elements.

Relationship between the parental origin of the X chromosomes, embryonic cell lineage and X chromosome expression in mice

The electrophoretic variants of theX-chromosome-linked enzyme phosphoglycerate kinase (PGK-1) have been used to investigate the randomness ofXchromosome expression in the fetus and various extra-embryonic membranes of the mouse conceptus. The amnion shows essentially random expression of the maternally derivedXchromosome (Xm) and the paternally derivedXchromosome (Xp). The parietal endoderm, however, shows exclusive or preferential expression ofXm. The results support the idea that the randomness ofXchromosome expression is correlated with embryonic cell lineage such thatXmis preferentially (perhaps exclusively) expressed in derivatives of the primitive endoderm and trophectoderm but thatXmandXpare randomly expressed in the derivatives of the primitive ectoderm.

Experiments involving ovary transplants, embryo transfers or crosses with heterozygous mothers confirm previous findings thatXmis preferentially expressed regardless of theXchromosome expressed in the reproductive tract. Additional experiments show that the preferentially expressedXchromosome in the parietal endoderm and visceral yolk sac endoderm of a normalXmXpconceptus is alwaysXmregardless of grand-parental origin ofXmand regardless of whether the mother is a normalXXfemale or anXOfemale.Xpis, however, expressed in these tissues hiXpOfemale conceptuses. It is argued that a form of chromosome imprinting occurs at each generation to markXmandXpas different and that this difference influences the choice of whichXchromosomes are expressed in each cell lineage.

X-chromosome inactivation in extra-embryonic membranes of diploid parthenogenetic mouse embryos demonstrated by differential staining

In somatic cells of female mammals one of the two X chromosomes is genetically inactive and heterochromatic, resulting in dosage compensation for X-linked genes. In marsupials the paternally derived X chromosome is preferentially inactivated. In eutherian mammals, although either X chromosome can be inactivated at random in somatic cells, preferential inactivation of the paternally derived X chromosome has been demonstrated cytologically in mouse and rat yolk sac and mouse chorion and biochemically in mouse yolk sac, chorionic ectoderm and trophoblast. In mouse yolk sac the non-random element has been shown both biochemically and cytologically to be confined to the endoderm layer in which there is almost total paternal X-chromosome activity in the separated yolk sac layers of diploid parthenogenetic mouse embryos in which both X chromosomes are maternally derived. Kaufman et al. have demonstrated X inactivation in somatic cells of diploid parthenogenetic embryos, and we have used a modification of Kanda's method, which renders the presumptive inactive X dark staining, to reveal an inactive X chromosome in both endoderm and mesoderm layers of separated yolk sacs from parthenogenones. Thus even in tissues in which there is normally total non-random paternal X inactivation, in the absence of a paternally derived X chromosome a maternally derived X can be inactivated.

Gene dosage compensation and the evolution of sex chromosomes

Dosage compensation is a mechanism by means of which the activity of X-linked or Z-linked genes is made equal in the two sexes of organisms with an XX compared to XY or ZZ compared to ZW basis of sex determination. In mammals, compensation is achieved by the inactivation of one X chromosome in somatic cells of females. In Drosophila, compensation does not involve inactivation. The two X chromosomes in females as well as the single X in males are regulated, and individual genes are thought to respond independently to the regulatory mechanism. It is proposed that in both groups of organisms the evolution of heteromorphic sex chromosomes was gradual and occurred as the direct result of the evolution of dosage compensation rather than the reverse.

Preferential X inactivation in human placenta membranes: is the paternal X inactive in early embryonic development of female mammals?

In placenta membranes of newborn girls carrying electrophoretically distinguishable G6PD alleles, the maternally derived isozyme is expressed preferentially. This phenomenon cannot be explained by allelic differences in enzyme activity or by somatic selection directed against cells with particular G6PD phenotypes. Instead, it may be that in this tissue X inactivation is nonrandom. Preferential expression of the maternal X chromosome, as has been shown in marsupials and in extraembryonic membranes of rodents and now in man, may reflect the state of activity of the X chromosomes in the early stages of female embryonic development.