Effects of X chromosome number and parental origin on X-linked gene expression in preimplantation mouse embryos

Haploid embryos and embryonic stem cells to produce offspring with predetermined parental genomes in cattle

Selection strategies are performed post-fertilization when the random combination of paternal and maternal genomes has already occurred. It would be greatly advantageous to eliminate meiotic uncertainty by selecting genetically superior gametes before fertilization. To achieve this goal, haploid embryonic cells and embryonic stem cell lineages could be derived, genotyped, and used to substitute gametes. On the paternal side, androgenetic development can be achieved by removing the maternal chromosomes from the oocyte before or after fertilization. We have shown that once developed into an embryo, haploid cells can be removed for genotyping and, if carrying the selected genome, be used to replace sperm at fertilization. A similar strategy can be used on the maternal side by activating the oocyte parthenogenetically and using some embryonic cells for genotyping while the remaining are used to produce diploid embryos by fertilization. Placed together, both androgenetic and parthenogenetic haploid cells that have been genotyped to identify optimal genomes can be used to produce offspring with predetermined genomes. Successes and problems in developing such a breeding platform to achieve this goal are described and discussed below.

Dysregulated Gene Expression of Imprinted and X-Linked Genes: A Link to Poor Development of Bovine Haploid Androgenetic Embryos

Mammalian uniparental embryos are efficient models for genome imprinting research and allow studies on the contribution of the paternal and maternal genomes to early embryonic development. In this study, we analyzed different methods for production of bovine haploid androgenetic embryos (hAE) to elucidate the causes behind their poor developmental potential. Results indicate that hAE can be efficiently generated by using intracytoplasmic sperm injection and oocyte enucleation at telophase II. Although androgenetic haploidy does not disturb early development up to around the 8-cell stage, androgenetic development is disturbed after the time of zygote genome activation and hAE that reach the morula stage are less capable to reach the blastocyst stage of development. Karyotypic comparisons to parthenogenetic- and ICSI-derived embryos excluded chromosomal segregation errors as causes of the developmental constraints of hAE. However, analysis of gene expression indicated abnormal levels of transcripts for key long non-coding RNAs involved in X chromosome inactivation and genomic imprinting of the KCNQ1 locus, suggesting an association with X chromosome and some imprinted loci. Moreover, transcript levels of methyltransferase 3B were significantly downregulated, suggesting potential anomalies in hAE establishing de novo methylation. Finally, the methylation status of imprinted control regions for XIST and KCNQ1OT1 genes remained hypomethylated in hAE at the morula and blastocyst stages, confirming their origin from spermatozoa. Thus, our results exclude micromanipulation and chromosomal abnormalities as major factors disturbing the normal development of bovine haploid androgenotes. In addition, although the cause of the arrest remains unclear, we have shown that the inefficient development of haploid androgenetic bovine embryos to develop to the blastocyst stage is associated with abnormal expression of key factors involved in X chromosome activity and genomic imprinting.

Effect of Sex of Embryo on Developmental Competence, Epigenetic Status, and Gene Expression in Buffalo (Bubalus bubalis) Embryos Produced by Hand-Made Cloning

Buffalo embryos were produced by hand-made cloning using skin fibroblasts from male and female buffaloes (n = 4 each) as donor cells for examining the effect of sex. Although the rate of blastocyst formation (43.8% ± 1.31% vs. 42.2% ± 1.22%) was similar, the total cell number (333 ± 10.4 vs. 270 ± 10.9) was higher (p < 0.05) whereas the apoptotic index (6.39 ± 0.25 vs. 8.52 ± 0.38) was lower (p < 0.05) for male than for female blastocysts. In the blastocysts, the global level of H3K18ac was found to be in the following order: male>female>IVF (in vitro fertilization) blastocysts (p < 0.05). The global level of H3K9me2 was not significantly different between male and female blastocysts and was higher (p < 0.05) compared with that in their IVF counterparts. The relative mRNA abundance of X-chromosome-linked (XIST, HPRT, PGK, and G6PD), apoptosis- (CASPASE3) and pregnancy-related genes (IFN-τ) was significantly higher (p < 0.05) whereas that of DNMT1 was significantly lower (p < 0.05) in female than in male blastocysts; however, in the case of apoptosis- (BCL-XL) and developmental competence-related genes (IGF1R and OCT4), the expression level was similar between the two groups. The gene expression level of OCT4 and IFN-τ but not of IGF1R was significantly lower (p < 0.05) in cloned than in IVF blastocysts. This study demonstrates that the epigenetic status, quality, and expression level of several genes but not the developmental competence are affected by the sex of cloned embryos.

Generation and application of mammalian haploid embryonic stem cells

Haploid cells contain one set of chromosomes and are amenable for genetic analyses. In mammals, haploidy exists only in gametes. An intriguing question is whether haploid cells can be derived from gametes. Recently, by application of haploid cell enrichment using fluorescence-activated cell sorting, stable haploid embryonic stem cells (haESCs) have been successfully derived from oocyte-derived parthenogenetic and sperm-derived androgenetic embryos from several species. Whilst both parthenogenetic and androgenetic (AG)-haESCs enable whole-genome genetic screening at the cellular level, such as screening of drug resistance or disease-related genes, AG-haESCs, after intracytoplasmic injection into oocytes, can also be used to produce alive semi-cloned mice. Nevertheless, one major drawback associated with wild-type AG-haESCs is the very low birth rate of healthy semi-cloned mice. Of interest, after inhibiting the expression of two paternally imprinted genes (H19 and Gtl2) in AG-haESCs by removal of their differentially DNA methylated regions, double-knockout AG-haESCs can efficiently and stably support the generation of healthy semi-cloned pups. Importantly, double-knockout AG-haESCs are feasible for multiple genetic manipulations, followed by efficient generation of semi-cloned mice carrying multiple genetic traits; thus they could be used to validate candidate loci that have been identified in genome-wide association studies of multigenic diseases by generation of mouse models carrying multiple alterations. Of note, by combining a CRISPR-Cas9 library and double-knockout AG-haESCs, semi-cloned mice carrying different mutant genes can be efficiently generated in one step, enabling functional mutagenic screening in mice. HaESCs, therefore, provide a powerful tool for genetic analyses in mammals at both the cellular and organismal levels.

Androgenetic development of X- and Y-chromosome bearing haploid rainbow trout embryos

Haploid fish embryos are important in studies regarding role of the recessive traits during early ontogeny. In fish species with the male heterogamety, androgenetic haploid embryos might be also useful tool in studies concerning role of the sex chromosomes during an embryonic development. Morphologically differentiated X and Y chromosomes have been found in a limited number of fish species including rainbow trout (Oncorhynchus mykiss Walbaum 1792). To evaluate role of the sex chromosomes during rainbow trout embryonic development, survival of the androgenetic haploids in the presence of X or Y sex chromosomes has been examined. Androgenetic haploid rainbow trout were produced by fertilization of X-irradiated eggs with spermatozoa derived from the normal males (XY) and neomales, that is, sex-reversed females (XX) to produce X- and Y-bearing haploids, and all X-bearing haploids, respectively. Survival rates of the androgenetic progenies of normal males and neomales examined during embryogenesis and at hatching did not differ significantly. However, all haploids died within next few days after hatching. Cytogenetic analysis of the androgenetic embryos confirmed their haploid status. Moreover, apart from the intact paternal chromosomes, residues of the irradiated maternal chromosomes observed as chromosome fragments were identified in some of the haploids. Provided results suggested that rainbow trout X and Y chromosomes despite morphological and genetic differences are at the early stage of differentiation and still share genetic information responsible for the proper embryonic development.

Brains, genes, and primates

One of the great strengths of the mouse model is the wide array of genetic tools that have been developed. Striking examples include methods for directed modification of the genome, and for regulated expression or inactivation of genes. Within neuroscience, it is now routine to express reporter genes, neuronal activity indicators, and opsins in specific neuronal types in the mouse. However, there are considerable anatomical, physiological, cognitive, and behavioral differences between the mouse and the human that, in some areas of inquiry, limit the degree to which insights derived from the mouse can be applied to understanding human neurobiology. Several recent advances have now brought into reach the goal of applying these tools to understanding the primate brain. Here we describe these advances, consider their potential to advance our understanding of the human brain and brain disorders, discuss bioethical considerations, and describe what will be needed to move forward.

Haploid embryonic stem cells serve as a new tool for mammalian genetic study

In mammals, all somatic cells carry two sets of chromosomes while haploids are restricted only to gametes and are occasionally found in tumors with genome instability. Mammalian haploid embryonic stem (ES) cells have recently been established successfully in mice and monkeys, from either parthenogenetic or androgenetic haploid embryos. These haploid ES cells maintain haploidy and stable growth during extensive in vitro culture, express pluripotent markers, and possess the ability to differentiate into all three germ layers in vitro and in vivo. The mouse haploid ES cells can also contribute to germlines of chimeras. Moreover, the mouse androgenetic haploid ES cells can produce fertile progenies after intracytoplasmic injection into mature oocytes, and the mouse parthenogenetic haploid ES cells can also achieve this by substitution of the maternal genome, albeit at a lower efficiency. These distinct features of mammalian haploid ES cells empower themselves not only as a valuable tool for genetic screening at a cellular level, but also as a new tool for genome-modified animal production and genetic studies at the animal level. Here we review the current progress on mammalian haploid ES cell research, describe in detail their characteristics, and discuss their potential applications. These achievements may provide a new but powerful tool for mammalian genetic studies, and may also shed light on the some interesting questions regarding genome ploidy maintenance and genomic imprinting.

Effects of histone deacetylase inhibitors on the early development of bovine androgenetic embryos

Histone acetylation is one of the most important posttranslational modifications that contribute to transcriptional initiation and chromatin remodeling. In our previous study, we enhanced sperm chromatin remodeling within the bovine sperm injection-derived androgenentic (SpI-AG) embryos by sperm pretreatment, and thereby improved their early developmental competence. In this study, we found that blastocyst development of SpI-AG embryos could be elevated by the histone deacetylase inhibitor (HDACi). First, we optimized the efficacy of two histone deacetylase inhibitors [trichostatin A (TSA) and Scriptaid (SCR)] in a dose (0, 5, 10, 20, 50, and 100 nM for TSA; 0, 50, 100, 200, 300, and 500 nM for SCR, respectively) and time-dependent (0, 10, 15, 20, and 25 h) manner on the developmental capacity of these embryos. Furthermore, we quantitatively assessed the alterations in histone H3 and H4 overall acetylation levels and blastocyst quality of SpI-AG embryos by immunofluorescence staining. We found a significantly improved morula and blastocyst development rate of SpI-AG embryos at a mild dose of TSA (20 nM) or SCR (200 nM) for 15 h after embryo activation. Furthermore, both HDACi noticeably increased the levels of acetylated histone H3 and H4 in SpI-AG blastocyst embryos, whereas, SCR treatment improved the quality of blastocysts when compared with control group. In conclusion, HDACi is beneficial for early development of bovine SpI-AG embryos and can be used to improve the efficiency of its in vitro production.

X-linked gene transcription patterns in female and male in vivo, in vitro and cloned porcine individual blastocysts

To determine the presence of sexual dimorphic transcription and how in vitro culture environments influence X-linked gene transcription patterns in preimplantation embryos, we analyzed mRNA expression levels in in vivo-derived, in vitro-fertilized (IVF), and cloned porcine blastocysts. Our results clearly show that sex-biased expression occurred between female and male in vivo blastocysts in X-linked genes. The expression levels of XIST, G6PD, HPRT1, PGK1, and BEX1 were significantly higher in female than in male blastocysts, but ZXDA displayed higher levels in male than in female blastocysts. Although we found aberrant expression patterns for several genes in IVF and cloned blastocysts, similar sex-biased expression patterns (on average) were observed between the sexes. The transcript levels of BEX1 and XIST were upregulated and PGK1 was downregulated in both IVF and cloned blastocysts compared with in vivo counterparts. Moreover, a remarkable degree of expression heterogeneity was observed among individual cloned embryos (the level of heterogeneity was similar in both sexes) but only a small proportion of female IVF embryos exhibited variability, indicating that this phenomenon may be primarily caused by faulty reprogramming by the somatic cell nuclear transfer (SCNT) process rather than in vitro conditions. Aberrant expression patterns in cloned embryos of both sexes were not ameliorated by treatment with Scriptaid as a potent HDACi, although the blastocyst rate increased remarkably after this treatment. Taken together, these results indicate that female and male porcine blastocysts produced in vivo and in vitro transcriptional sexual dimorphisms in the selected X-linked genes and compensation of X-linked gene dosage may not occur at the blastocyst stage. Moreover, altered X-linked gene expression frequently occurred in porcine IVF and cloned embryos, indicating that X-linked gene regulation is susceptible to in vitro culture and the SCNT process, which may eventually lead to problems with embryonic or placental defects.

Androgenetic haploid embryonic stem cells produce live transgenic mice

Haploids and double haploids are important resources for studying recessive traits and have large impacts on crop breeding, but natural haploids are rare in animals. Mammalian haploids are restricted to germline cells and are occasionally found in tumours with massive chromosome loss. Recent success in establishing haploid embryonic stem (ES) cells in medaka fish and mice raised the possibility of using engineered mammalian haploid cells in genetic studies. However, the availability and functional characterization of mammalian haploid ES cells are still limited. Here we show that mouse androgenetic haploid ES (ahES) cell lines can be established by transferring sperm into an enucleated oocyte. The ahES cells maintain haploidy and stable growth over 30 passages, express pluripotent markers, possess the ability to differentiate into all three germ layers in vitro and in vivo, and contribute to germlines of chimaeras when injected into blastocysts. Although epigenetically distinct from sperm cells, the ahES cells can produce viable and fertile progenies after intracytoplasmic injection into mature oocytes. The oocyte-injection procedure can also produce viable transgenic mice from genetically engineered ahES cells. Our findings show the developmental pluripotency of androgenentic haploids and provide a new tool to quickly produce genetic models for recessive traits. They may also shed new light on assisted reproduction.

Haploid embryonic stem cells: an ideal tool for mammalian genetic analyses

Identification of the function of all genes in the mammalian genome is critical in understanding basic mechanisms of biology. However, the diploidy of mammalian somatic cells has greatly hindered efforts to elucidate the gene function in numerous biological processes by mutagenesis-based genetic approaches. Recently, mouse haploid embryonic stem (haES) cells have been successfully isolated from parthenogenetic and androgenetic embryos, providing an ideal tool for genetic analyses. In these studies, mouse haES cells have already shown that they could be used in cell-based forward or reverse genetic screenings and in generating gene-targeting via homologous recombination. In particular, haES cells from androgenetic embryos can be employed as novel, renewable form of fertilization agent for yielding live-born mice via injection into oocytes, thus showing the possibility that genetic analysis can be extended from cellular level to organism level.

Generation of genetically modified mice by oocyte injection of androgenetic haploid embryonic stem cells

Haploid cells are amenable for genetic analysis. Recent success in the derivation of mouse haploid embryonic stem cells (haESCs) via parthenogenesis has enabled genetic screening in mammalian cells. However, successful generation of live animals from these haESCs, which is needed to extend the genetic analysis to the organism level, has not been achieved. Here, we report the derivation of haESCs from androgenetic blastocysts. These cells, designated as AG-haESCs, partially maintain paternal imprints, express classical ESC pluripotency markers, and contribute to various tissues, including the germline, upon injection into diploid blastocysts. Strikingly, live mice can be obtained upon injection of AG-haESCs into MII oocytes, and these mice bear haESC-carried genetic traits and develop into fertile adults. Furthermore, gene targeting via homologous recombination is feasible in the AG-haESCs. Our results demonstrate that AG-haESCs can be used as a genetically tractable fertilization agent for the production of live animals via injection into oocytes.

Efficient production and cellular characterization of sheep androgenetic embryos

The production of androgenetic embryos in large animals is a complex procedure. Androgenetic embryos have been produced so far only in cattle and sheep using pronuclear transfer (PT) between zygotes derived from in vitro fertilization (IVF) of previously enucleated oocytes. PT is required due to the poor developmental potential of androgenotes derived from IVF of enucleated oocytes. Here we compare the developemt to blastocyst of androgenetic embryos produced by the standard pronuclear transfer and by fertilization of oocytes enucleated in Ca2+/Mg2+-free medium, without pronuclear transfer. The enucleation in Ca2+/Mg2+-free medium abolished almost completely the manipulation-induced activation, significantly improving the development to blastocyst of the androgenetic embryos (IVF followed by PT; 18.6%: IVF only; 17.7%, respectively). Karyotype analysis of IVF revealed a similar proportion of diploid embryos in androgenetic and control blastocysts (35% and 36%, respectively), although mixoploid blastocysts were frequently observed in both groups (64%). Androgenotes had lower total cell numbers than control and parthenogenetic embryos, but more cells in ICM cells comparing to parthenogenotes (30.42 vs. 17.15%). Higher expression of the pluripotency-associated gene NANOG, and trophoblastic-specific gene CDX2, were also observed in androgenotes compared to parthenogenotes and controls. The global methytion profile of androgenetic embryos was comparable to controls, but was lower than parthenogenetic embryos. The cell composition and methylation pattern we have detected in monoparental sheep monoparental embryos are unprecedented, and differ considerably from the standard reference mouse embryos. Altogether, these finding indicate significant differences across species in the molecular mechanisms regulating early development of monoparental embryos, and highlights the need to study postimplantation development of androgenetic embryos in sheep.

Differential effects of follistatin on nonhuman primate oocyte maturation and pre-implantation embryo development in vitro

There is a vital need to identify factors that enhance human and nonhuman primate in vitro embryo culture and outcome, and to identify the factors that facilitate that objective. Granulosa and cumulus cells were obtained from rhesus monkeys that had either been FSH-primed (in vitro maturation [IVM]) or FSH and hCG-primed (in vivo maturation [VVM]) and compared for the expression of mRNAs encoding follistatin (FST), inhibin, and activin receptors. The FST mRNA displayed marginally decreased expression (P = 0.05) in association with IVM in the granulosa cells. The ACVR1B mRNA was more highly expressed in cumulus cells with IVM compared with VVM. Cumulus-oocyte complexes from FSH-primed monkeys exposed to exogenous FST during the 24-h IVM period exhibited no differences in the percentage of oocytes maturing to the metaphase II stage of meiosis compared to controls. However, embryos from these oocytes had significantly decreased development to the blastocyst stage. The effect of FST on early embryo culture was determined by exposing fertilized VVM oocytes to exogenous FST from 12 to 60 h postinsemination. FST significantly improved time to first cleavage and embryo development to the blastocyst stage compared with controls. The differential effects of exogenous FST on embryo development, when administered before and after oocyte maturation, may depend on the endogenous concentration in cumulus cells and oocytes. These results reveal evolutionary conservation of a positive effect of FST on embryogenesis that may be broadly applicable to enhance in vitro embryogenesis, with potential application to human clinical outcome and livestock and conservation biology.

X chromosome inactivation is initiated in human preimplantation embryos

X chromosome inactivation (XCI) is the mammalian mechanism that compensates for the difference in gene dosage between XX females and XY males. Genetic and epigenetic regulatory mechanisms induce transcriptional silencing of one X chromosome in female cells. In mouse embryos, XCI is initiated at the preimplantation stage following early whole-genome activation. It is widely thought that human embryos do not employ XCI prior to implantation. Here, we show that female preimplantation embryos have a progressive accumulation of XIST RNA on one of the two X chromosomes, starting around the 8-cell stage. XIST RNA accumulates at the morula and blastocyst stages and is associated with transcriptional silencing of the XIST-coated chromosomal region. These findings indicate that XCI is initiated in female human preimplantation-stage embryos and suggest that preimplantation dosage compensation is evolutionarily conserved in placental mammals.

Development of sheep androgenetic embryos is boosted following transfer of male pronuclei into androgenetic hemizygotes

Androgenetic embryos are useful model for investigating the contribution of the paternal genome to embryonic development. Little work has been done with androgenetic embryo production in domestic animals. The aim of this study was the production of diploid androgenetic sheep embryos. In vitro matured sheep oocytes were enucleated and fertilized in vitro; parthenogenetic and normally fertilized embryos were also produced as a control. Fifteen hours after in vitro fertilization (IVF), presumptive zygotes were centrifuged and scored for the number of pronucleus. IVF, parthenogenetic, and androgenetic embryos (haploid, diploid, and triploid) were cultured in SOFaa medium with bovine serum albumin (BSA). The proportion of oocytes with polyspermic fertilization increased linearly with increasing sperm concentration. After IVF, there was no significant difference in early cleavage and morula formation rates between the groups, while there was a significant difference on blastocyst development between IVF, parthenogenetic, and androgenetic embryos, the last ones displaying poor developmental potential (IVF, parthenogenetic, and haploid, diploid, and triploid androgenetic embryos: 43%, 38%, 0%, 2%, and 2%, respectively). In order to boost androgenetic embryonic development, we produced diploid androgenetic embryos through pronuclear transfer. Single pronuclei were aspirated with a bevelled pipette from haploid or diploid embryos and transferred into the perivitelline space of other haploid embryos, and the zygotes were reconstructed by electrofusion. Fusion rates approached 100%. Pronuclear transfer significantly increased blastocyst development (IVF, parthenogenetic, androgenetic: Diploid into Haploid, and Haploid into Haploid: 42%, 42%, 19%, and 3%, respectively); intriguingly, the Haploid + Diploid group showed the highest development to blastocyst stage. The main findings of our study are: (1) sheep androgenetic embryos display poor developmental ability compared with IVF and parthenogenetic embryos; (2) diploid androgenetic embryos produced by pronuclear exchange developed in higher proportion to blastocyst stage, particularly in the Diploid-Haploid group. In conclusion, pronuclear transfer is an effective method to produce sheep androgenetic blastocysts.

Global gene transcription patterns in in vitro-cultured fertilized embryos and diploid and haploid murine parthenotes

To gain insights into the roles the paternal genome and chromosome number play in pre-implantation development, we cultured fertilized embryos and diploid and haploid parthenotes (DPs and HPs, respectively), and compared their development and gene expression patterns. The DPs and fertilized embryos did not differ in developmental ability but HPs development was slower and characterized by impaired compaction and blastocoel formation. Microarray analysis revealed that fertilized blastocysts expressed several genes at higher levels than DP blastocysts; these included the Y-chromosome-specific gene eukaryotic translation initiation factor 2, subunit 3, structural gene Y-linked (Eif2s3y) and the imprinting gene U2 small nuclear ribonucleoprotein auxiliary factor 1, related sequence 1 (U2af1-rs1). We also found that when DPs and HPs were both harvested at 44 and 58 h of culture, they differed in the expression of 38 and 665 genes, respectively. However, when DPs and HPs were harvested at the midpoints of 4-cell stage (44 and 49 h, respectively), no differences in expression was observed. Similarly, when the DPs and HPs were harvested when they became blastocysts (102 and 138 h, respectively), only 15 genes showed disparate expression. These results suggest that while transcripts needed for early development are delayed in HPs, it does progress sufficiently for the generation of the various developmental stages despite the lack of genetic components.

Differential sensitivity of male and female mouse embryos to oxidative induced heat-stress is mediated by glucose-6-phosphate dehydrogenase gene expression

During the preimplantation period, in vitro cultured males have a higher metabolic rate, different gene expression, and grow faster than females. It has been suggested that under some stress conditions male embryos are more vulnerable than females; however, the biological fragility of male embryos is little understood. Since many forms of stress result in the overproduction of cellular reactive oxygen species (ROS), we addressed the hypothesis that the connection between female advantage during early developmental stages and heat stress involves ROS and differential gene expression of G6PD, an X-linked gene related to oxidative stress. We have found that after compaction, female heat-stressed embryos have less relative amounts of H2O2 than males, and female embryos survive better than males under in vivo or in vitro heat stress situations. In addition, in vitro produced female embryos grow slower than male embryos, have differential mRNA transcription of G6PD and also of some genes situated on autosomal-chromosomes (Sox, Bax, and Oct-4). Moreover, by inhibiting G6PD, all differences generated by oxidative stress between male and female embryos disappear. For the first time, we provide an experimental demonstration of a mechanism that explains why following exposure to heat stress-induced ROS, female preimplantation embryos are more resistant than males.

Variable Reprogramming of the Pluripotent Stem Cell Marker Oct4 in Mouse Clones: Distinct Developmental Potentials in Different Culture Environments

A prevailing view of cloning by somatic-cell nuclear transfer is that reprogramming of gene expression occurs during the first few hours after injection of the nucleus into an oocyte, that the process is stochastic, and that the type of reprogramming needed for cloning success is foreign and unlikely to be readily achieved in the ooplasm. Here, we present evidence that the release of reprogramming capacity is contingent on the culture environment of the clone while the contribution of aneuploidy to altered gene expression is marginal. In particular, the rate of blastocyst formation in clones and the regional distribution of mRNA for the pluripotent stem cell marker Oct4 in clonal blastocysts was highly dependent on the culture environment after cumulus cell nuclear transfer, unlike that in genetically equivalent zygotes. Epigenetic modifications of genetically identical somatic nuclei continue after the first cell division of the clones and are amenable to a degree of experimental control, and their development to the blastocyst stage and appropriate expression of Oct4 predict further outcome, such as derivation of embryonic stem (ES) cells, but not fetal development. This observation indicates that development to the blastocyst stage is not equivalent to full reprogramming and lends support to the novel concept that ES cells are not the equivalent of the inner cell mass, hence the discrepancy between ES cell derivability and fetal development of clones.

Silence of the fathers: early X inactivation

X chromosome inactivation is the mammalian answer to the dilemma of dosage compensation between males and females. The study of this fascinating form of chromosome-wide gene regulation has yielded surprising insights into early development and cellular memory. In the past few months, three papers reported unexpected findings about the paternal X chromosome (X(p)). All three studies agree that the X(p) is imprinted to become inactive earlier than ever suspected during embryonic development. Although apparently incomplete, this early form of inactivation insures dosage compensation throughout development. Silencing of the X(p) persists in cells of extraembryonic tissues, but it is erased and followed by random X inactivation in cells of the embryo proper. These findings challenge several aspects of the current view of X inactivation during early development and may have profound impact on studies of pluripotency and epigenetics.

Imprinted X-chromosome inactivation: enlightenment from embryos in vivo

There are two forms of X chromosome inactivation (XCI) in the laboratory mouse, random XCI in the fetus and imprinted paternal XCI limited to the extraembryonic tissues supporting the fetal life in utero. Imprinted XCI has been studied extensively because it takes place first in embryogenesis and it may hold clues to the mechanism of control of XCI in general and to the evolution of random' XCI. Classical microscopic and biochemical studies of embryos in vivo provide a basis for interpreting the multifaceted information yielded by various inventive approaches and for planning further experiments.

Developmental Potential of Bovine Androgenetic and Parthenogenetic Embryos: A Comparative Study1

In this study, we compared the developmental capacity of bovine haploid and diploid androgenetic and parthenogenetic embryos obtained by different methods. Androgenetic embryos were produced by piezo-intracytoplasmic sperm injection (ICSI) or in vitro fertilization (IVF) of enucleated oocytes with or without subsequent pronuclear transfer from one haploid zygote to another. Parthenogenetic embryos were obtained by activation of matured oocytes by ionomycin combined with cycloheximide or 6-dimethylaminopurine (DMAP) treatment. Only few cleaved androgenetic haploid embryos were able to compact (2.7%) and to form blastocysts (1.8%), while significantly more haploid parthenogenotes underwent compaction (24-37%) and a minority developed to blastocysts at different rates, depending on the activation procedure (cycloheximide 3%, 6-DMAP 14.5%). By contrast, development to blastocyst of diploid androgenotes, cloned androgenetic embryos, and parthenogenotes (31%, 39%, and 43%, respectively) was similar to IVF control embryos (35%). Cell number on Day 7 was higher for IVF blastocysts and decreased in consecutive order in diploid androgenotes, diploid parthenogenotes, and haploid uniparental embryos. Following transfer of diploid androgenetic embryos, a pregnancy was established and maintained up to Day 28.

The primate embryo gene expression resource: a novel resource to facilitate rapid analysis of gene expression patterns in non-human primate oocytes and preimplantation stage embryos

Detailed molecular studies of preimplantation stage development in a suitable nonhuman primate model organism have been inhibited due to the cost and scarcity of embryos. To circumvent these limitations, we have created a new resource for the research community, designated as the Primate Embryo Gene Expression Resource (PREGER). The PREGER sample collection currently contains over 160 informative samples of oocytes, obtained from various sized antral follicles, and embryos obtained through a variety of different protocols. The PREGER makes it possible to undertake quantitative gene-expression studies in rhesus monkey oocytes and embryos through simple and cost-effective hybridization-based methods. The PREGER also makes available other molecular tools to facilitate nonhuman primate embryology. We used PREGER here to compare the temporal expression patterns of five housekeeping mRNAs and three transcription factor mRNAs between mouse and rhesus monkey. We observed noticeable differences in temporal expression patterns between species for some mRNAs, but clear similarities for others. Our results also provide new information related to genome activation and the effects of embryo culture conditions on gene expression in primate embryos. These results provide one illustration of how the PREGER can be employed to obtain novel insight into primate embryogenesis.

Inheritance of a pre-inactivated paternal X chromosome in early mouse embryos

In mammals, dosage compensation ensures equal X-chromosome expression between males (XY) and females (XX) by transcriptionally silencing one X chromosome in XX embryos. In the prevailing view, the XX zygote inherits two active X chromosomes, one each from the mother and father, and X inactivation does not occur until after implantation. Here, we report evidence to the contrary in mice. We find that one X chromosome is already silent at zygotic gene activation (2-cell stage). This X chromosome is paternal in origin and exhibits a gradient of silencing. Genes close to the X-inactivation centre show the greatest degree of inactivation, whereas more distal genes show variable inactivation and can partially escape silencing. After implantation, imprinted silencing in extraembryonic tissues becomes globalized and more complete on a gene-by-gene basis. These results argue that the XX embryo is in fact dosage compensated at conception along much of the X chromosome. We propose that imprinted X inactivation results from inheritance of a pre-inactivated X chromosome from the paternal germ line.

X-linked genes in female embryonic stem cells carry an epigenetic mark prior to the onset of X inactivation

We use chromatin immunoprecipitation to show that genes on the two active X chromosomes in undifferentiated, XX female embryonic stem cells (ES cells) are marked by hyperacetylation of all core histones, hyper(di)methylation of H3 lysine 4 and hypo(di)methylation of H3 lysine 9, compared with autosomal genes or genes on the single active X in XY male cells. The mark is found on both coding and promoter regions. On differentiation, and after the onset of X inactivation, the mark is reversed on the inactive X, whose genes show extreme hypoacetylation of all four core histones, hypo(di)methylation of H3K4 and hyper(di)methylation of H3K9. The mark is retained on the active X in female ES cells for at least several days of differentiation, but is not present in adult females. The selective marking of X-linked genes in female ES cells in a way that distinguishes them from the equivalent genes in males, is unprecedented. We suggest that the mark forms part of a chromatin-based mechanism that restricts X-inactivation to cells with more than one X chromosome.

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.

Comparison of gene expression during preimplantation development between diploid and haploid mouse embryos

Haploid development is a normal part of the life cycle for some animals, but it has not been observed in mammals. Studies in mice have revealed that the preimplantation developmental potential of haploid embryos is significantly impaired relative to diploid embryos. The reasons for the severely limited developmental potential of haploid embryos in mammals have not been discerned. To examine the effects of haploid development on gene expression, and in particular on X-linked gene expression, and to evaluate to what degree newer techniques of producing and culturing such embryos might affect developmental potential, haploid and diploid parthenogenetic and androgenetic embryos were produced and reevaluated for developmental potential, genomic integrity, and relative expression levels of specific autosomal and X-linked gene transcripts. Our data confirm the previously observed restriction in haploid developmental potential, eliminate chromosomal abnormalities as a major factor in this restriction, and reveal subtle alterations in gene expression. Haploid parthenogenones display only very subtle alterations in the expression of most mRNAs but a consistent elevation in X-linked Bex1 mRNA expression. Haploid androgenones seem to lack repression of the Pgk1 gene that is seen in diploid androgenones, but this may reflect ongoing loss of those haploid androgenones that experience X chromosome inactivation. The significance and possible explanations for these differences are discussed.

Developmentally-regulated changes of Xist RNA levels in single preimplantation mouse embryos, as revealed by quantitative real-time PCR

Xist RNA localizes to the inactive X chromosome in cells of late cleavage stage female mouse embryos (Sheardown et al., 1997: Cell 91:99-107). Fluorescence in situ hybridization (FISH), however, does not quantify the number of Xist transcripts per nucleus. We have used real-time reverse transcription-polymerase chain reaction (RT-PCR) to measure Xist RNA levels in single preimplantation embryos and to establish developmental profiles in both female and male samples. The gender of each embryo was readily established based on Xist RNA levels, by counting Xist gene copies per cell, and by independent detection of the presence/absence of Sry, a Y chromosome-specific gene. Xist expression in males was found to be very low at all stages, as suggested by FISH. In contrast, female embryos contained measurable levels of Xist mRNA starting at the late 2-cell stage and rapidly accumulated Xist transcripts until morula stage. Xist RNA accumulation per embryo then reached a plateau, while cell division continued. We propose that during early cleavage high enough levels of Xist mRNA are transcribed to generate a pool of unbound molecules. This pool would serve to temporarily maintain X chromosome inactivation without additional transcription while the trophectoderm and inner cell mass (ICM) differentiate. The ICM would then loose the paternally imprinted pattern of X inactivation originally present in all embryonic cells.

Trophectoderm-specific expression of the X-linked Bex1/Rex3 gene in preimplantation stage mouse embryos

The Bex1/Rex3 gene was recently identified as an X-linked gene that is differentially expressed between parthenogenetic and normal fertilized, preimplantation stage mouse embryos. The Bex1/Rex3 gene appears to be expressed preferentially from the maternal X chromosome in blastocysts, but from either X chromosome in later stage embryonic tissues and adult tissues. To investigate whether differential expression of the Bex1/Rex3 gene between normal and parthenogenetic blastocyst stage embryos reflects genomic imprinting at the Bex1/Rex3 locus itself, or instead is the result of preferential inactivation of the paternal X chromosome or differences in timing of cellular differentiation, we examined in detail the expression pattern of the Bex1/Rex3 mRNA in normal preimplantation stage embryos, and compared its expression between androgenetic, gynogenetic, and normal fertilized embryos. Expression data reveal that the Bex1/Rex3 gene is initially transcribed at the 2-cell stage, transiently induced at the 8-cell stage, and then increases in expression again at the blastocyst stage. Very little expression is observed in isolated inner cell masses, indicating selective expression in the trophectoderm. Comparisons of Bex1/Rex3 mRNA expression between male and female androgenetic and control embryos and gynogenetic embros failed to reveal any significant difference in expression between the different classes of embryos at the 8-cell stage, or the expanding blastocyst stage (121 hr post-hCG). At the late blastocyst stage (141 hr post-hCG), expression was significantly lower in XY control embryos as compared with XX controls. Bex1/Rex3 mRNA expression did not differ between XX and XY androgenones at the blastocyst stage or between gynogenones and XX control embryos. Thus, the Bex1/Rex3 gene does not appear to be regulated directly by genomic imprinting during the preimplantation period, just as it is not regulated by imprinting at later stages. Apparent differences in gene expression may arise through the effects of trophectoderm-specific expression coupled with differences in timing of trophectoderm differentiation between the different classes of embryos and effects of preferential paternal X chromosome inactivation (XCI).

Asb4, Ata3, and Dcn are novel imprinted genes identified by high-throughput screening using RIKEN cDNA microarray

Genes differentially expressed between parthenogenetic and androgenetic embryos are candidates for the identification of imprinted genes, which are expressed specifically from the maternal or paternal allele. To search for genes differentially expressed between parthenogenetic and androgenetic embryos, we used the RIKEN full-length enriched mouse cDNA microarray. The 25 candidates obtained included 8 known imprinted genes (such as IgfII, Snrpn, and Neuronatin) and 3 new ones--Asb4 (ankyrin repeat and SOCS box-containing protein 4), Ata3 (amino acid transport system A3), and Decorin--which were confirmed by using normal diploid embryos from the reciprocal F1 crosses of B6 and JF1 mice. The 25 candidates also included genes that showed no imprinting-associated expression in normal diploid embryos. We describe a feasible high-throughput method of screening for novel imprinted genes by using the RIKEN cDNA microarray.

In vitro production and nuclear transfer affect dosage compensation of the X-linked gene transcripts G6PD, PGK, and Xist in preimplantation bovine embryos

Equal expression of X-linked genes such as G6PD and PGK in females and males and the initiation of X-chromosome inactivation are critically dependent on the expression of the X-inactive specific transcript (Xist). The objective of the present study was to determine the effects of in vitro production (IVP) and nuclear transfer (NT) on the relative abundance (RA) of the X-linked transcripts G6PD, PGK, and Xist in preimplantation bovine embryos. In experiment 1, sex-determined IVP or in vivo-produced embryos were analyzed for mRNA expression of the 3 genes. The sex ratio was 36% vs. 64% in IVP blastocysts and thus deviated significantly from the expected ratio of 50% in the vivo control group. The RA of G6PD transcripts was significantly higher in female IVP embryos than in male embryos. In contrast, no significant differences were seen between in vivo-derived female embryos and their male counterparts. At the morula stage, female IVP embryos transcribed significantly more PGK mRNA than did male embryos. However, blastocysts did not exhibit significant differences in PGK transcripts. No differences were observed for in vivo-derived embryos with regard to the RA of PGK transcripts. The RA of Xist mRNA was significantly higher in all female embryos than in their male counterparts. In experiment 2, IVP, in vivo-developed, NT-derived, and parthenogenetic embryos carrying two X chromosomes of either maternal and paternal origin or of maternal origin only (parthenogenotes) were analyzed for the RA of the 3 genes. In NT-derived morulae, the RA of G6PD transcripts was significantly increased compared with their IVP and in vivo-generated counterparts. G6PD transcript levels were significantly increased in IVP blastocysts compared with in vivo-generated and parthenogenetic embryos. At the morula stage, PGK transcripts were similar in all groups, but the RA of PGK transcripts was significantly higher in IVP blastocysts than in their in vivo-generated, parthenogenetic, and NT-derived counterparts. The RA of Xist was significantly elevated in NT-derived morulae compared with IVP, in vivo-generated, and parthenogenetic embryos. NT-derived blastocysts showed an increased Xist expression compared with that of IVP, in vivo-generated, and parthenogenetic embryos. Results of the present study show for the first time that differences in X-chromosome-linked gene transcript levels are related to a perturbed dosage compensation in female and male IVP and female NT-derived embryos. This finding warrants further studies to improve IVP systems and NT protocols to ensure the production of embryos with normal gene expression patterns.

Imprinted X inactivation in eutherians: a model of gametic execution and zygotic relaxation

In mammals, X-chromosome inactivation (XCI) ensures equal expression of X-linked genes in XX and XY individuals by transcriptionally silencing one X-chromosome in female cells. In this review, we discuss an imprinted form of X-inactivation in which the paternal X (Xp) is preferentially silenced. Believed to be the ancestral mechanism of dosage compensation in mammals, imprinted X-inactivation can still be observed in modern-day marsupials and in the extraembryonic tissues of some eutherians such as the mouse. Recent experiments have addressed the nature of the gametic imprint and focused on the regulatory interaction between the noncoding RNA gene, Xist, and its antisense partner, Tsix. Our review of the literature has inspired an unconventional view of imprinted XCI in mice. First, the evidence strongly argues that imprinted XCI is inabsolute, so that a stochastic number of extraembryonic cells escape imprinting. Second, contrary to conventional thinking, we would like to consider the possibility that the paternal X might actually be transmitted to the zygote as a pre-inactivated chromosome. In this model, the gamete initiates and establishes imprinted XCI, while the zygote maintains the pre-established pattern of gametic inactivation. Finally, we hypothesize that the inabsolute nature of imprinting is caused by imperfect zygotic maintenance. We propose that the mouse represents a transitional stage in the evolution of random XCI from an absolutely imprinted mechanism.