Translation of maternal messenger ribonucleic acids encoding transcription factors during genome activation in early mouse embryos

DIA-based quantitative proteomic analysis of porcine endometrium in the peri-implantation phase

The 12th day of gestation is a critical period for embryo loss and the beginning of imminent implantation in sows. Data independent acquisition (DIA) technology is one of the high-throughput, high-resolution and reproducible proteomics technologies for large-scale digital qualitative and quantitative research. The aim of this study was to identify and characterize the protein abundance landscape of Yorkshire pig endometrium on the 12th day of pregnancy (P12) and estrous cycle (C12) using DIA proteomics. A total of 1251 differentially abundant proteins (DAPs) were identified, of which 882 were up-regulated and 369 were down-regulated at P12. Functional enrichment analysis showed that the identified proteins were related to metabolism, biosynthesis and signaling pathways. Three proteins were selected for Western blot (WB) validation and the results were consistent with the DIA data. Further combined with transcriptome data, fibrinogen like 2 (FGL2) and S100 calcium binding protein A8 (S100A8) were verified to be highly abundant in the P12 endometrial epithelium. In summary, there were significantly different abundance of proteome profiles in C12 and P12 endometrium, suggesting that DAPs are associated with changes in endometrial receptivity, which laid the foundation for further research on related regulatory mechanisms. SIGNIFICANCE: The 12th day of gestation is an important point in the peri-implantation period of pigs, when the endometrium presents a receptive state under the stimulation of estrogen. DIA proteomics technology is an emerging protein identification technology in recent years, which can obtain protein information through comprehensive and unbiased scanning. In this study, DIA technology was used to characterize endometrial proteins in pigs during the peri-implantation period. The results showed that higher protein abundance was detected using the DIA technique, and some of these DAPs may be involved in regulating embryo implantation. This study will help to better reveal the related proteins involved in embryo implantation, and lay a foundation for further research on the mechanism of endometrial regulation of embryo implantation. SIGNIFICANCE OF THE STUDY: The 12th day of gestation is an important point in the peri-implantation period of pigs, when the endometrium presents a receptive state under the stimulation of estrogen. DIA proteomics technology is an emerging protein identification technology in recent years, which can obtain protein information through comprehensive and unbiased scanning. In this study, DIA technology was used to characterize endometrial proteins in pigs during the peri-implantation period. The results showed that higher protein abundance was detected using the DIA technique, and some of these DAPs may be involved in regulating embryo implantation. This study will help to better reveal the related proteins involved in embryo implantation, and lay a foundation for further research on the mechanism of endometrial regulation of embryo implantation.

Preimplantation embryo gene expression: 56 years of discovery, and counting

The biology of preimplantation embryo gene expression began 56 years ago with studies of the effects of protein synthesis inhibition and discovery of changes in embryo metabolism and related enzyme activities. The field accelerated rapidly with the emergence of embryo culture systems and progressively evolving methodologies that have allowed early questions to be re-addressed in new ways and in greater detail, leading to deeper understanding and progressively more targeted studies to discover ever more fine details. The advent of technologies for assisted reproduction, preimplantation genetic testing, stem cell manipulations, artificial gametes, and genetic manipulation, particularly in experimental animal models and livestock species, has further elevated the desire to understand preimplantation development in greater detail. The questions that drove enquiry from the earliest years of the field remain drivers of enquiry today. Our understanding of the crucial roles of oocyte-expressed RNA and proteins in early embryos, temporal patterns of embryonic gene expression, and mechanisms controlling embryonic gene expression has increased exponentially over the past five and a half decades as new analytical methods emerged. This review combines early and recent discoveries on gene regulation and expression in mature oocytes and preimplantation stage embryos to provide a comprehensive understanding of preimplantation embryo biology and to anticipate exciting future advances that will build upon and extend what has been discovered so far.

Regulation of the mammalian maternal-to-embryonic transition by eukaryotic translation initiation factor 4E

Eukaryotic translation initiation factor 4E (eIF4E) mediates cap-dependent translation. Genetic and inhibitor studies show that eIF4E expression is required for the successful transition from maternal to embryonic control of mouse embryo development. eIF4E was present in the oocyte and in the cytoplasm soon after fertilization and during each stage of early development. Functional knockout (Eif4e^−/−) by PiggyBac [Act-RFP] transposition resulted in peri-implantation embryonic lethality because of the failure of normal epiblast formation. Maternal stores of eIF4E supported development up to the two- to four-cell stage, after which new expression occurred from both maternal and paternal inherited alleles. Inhibition of the maternally acquired stores of eIF4E (using the inhibitor 4EGI-1) resulted in a block at the two-cell stage. eIF4E activity was required for new protein synthesis in the two-cell embryo and Eif4e^−/− embryos had lower translational activity compared with wild-type embryos. eIF4E-binding protein 1 (4E-BP1) is a hypophosphorylation-dependent negative regulator of eIF4E. mTOR activity was required for 4E-BP1 phosphorylation and inhibiting mTOR retarded embryo development. Thus, this study shows that eIF4E activity is regulated at key embryonic transitions in the mammalian embryo and is essential for the successful transition from maternal to embryonic control of development.

Summary: Combined use of a PB [Act-RFP] transgenesis model, selective pharmacological inhibition and expression analyses verified the essential role of eIF4E in the transition from maternal to embryonic control of mouse development.

Regulation of Translationally Repressed mRNAs in Zebrafish and Mouse Oocytes

From the beginning of oogenesis, oocytes accumulate tens of thousands of mRNAs for promoting oocyte growth and development. A large number of these mRNAs are translationally repressed and localized within the oocyte cytoplasm. Translational activation of these dormant mRNAs at specific sites and timings plays central roles in driving progression of the meiotic cell cycle, axis formation, mitotic cleavages, transcriptional initiation, and morphogenesis. Regulation of the localization and temporal translation of these mRNAs has been shown to rely on cis-acting elements in the mRNAs and trans-acting factors recognizing and binding to the elements. Recently, using model vertebrate zebrafish, localization itself and formation of physiological structures such as RNA granules have been shown to coordinate the accurate timings of translational activation of dormant mRNAs. This subcellular regulation of mRNAs is also utilized in other animals including mouse. In this chapter, we review fundamental roles of temporal regulation of mRNA translation in oogenesis and early development and then focus on the mechanisms of mRNA regulation in the oocyte cytoplasm by which the activation of dormant mRNAs at specific timings is achieved.

Regulation of ATF1 and ATF2 transcripts by sequences in their 3' untranslated region in cleavage-stage cattle embryos

The sequence of a 3' untranslated region (3'UTR) of mRNA governs the timing of its polyadenylation and translation in mammalian oocytes and early embryos. The objective of this study was to assess the influence of cis-elements in the 3'UTR of the developmentally important ATF1 and ATF2 transcripts on their timely translation during first cleavages in bovine embryos. Eight different reporter mRNAs (coding sequence of green fluorescent protein [GFP] fused to the 3'UTR of short or long isoforms of cattle ATF1 or -2, with or without polyadenylation) or a control GFP mRNA were microinjected separately into presumptive bovine zygotes at 18 hr post-insemination (hpi), followed by epifluorescence assessment for GFP translation between 24 and 80 hpi (expressed as percentage of GFP-positive embryos calculated from the total number of individuals). The presence of either polyadenine or 3'UTR sequence in deadenylated constructs is required for GFP translation (implying the need for polyadenylation), and all exogenous mRNAs that met either criteria were translated as soon as 24 hpi-except for long-deadenylated ATF2-UTR, whose translation began at 36 hpi. Overall, GFP was more visibly translated in competent (cleaving) embryos, particularly in long ATF1/2 constructs. The current data shows a timely GFP translation in bovine embryos depending on sequences in the 3'UTR of ATF1/2, and indicates a difference between short and long isoforms. In addition, cleaving embryos displayed increased translational capacity of the tested constructs. Functional confirmation of the identification cis-sequences in the 3'UTR of ATF1/2 will contribute to the understanding of maternal mRNA translation regulation during early cattle development.

An evolutionary, structural and functional overview of the mammalian TEAD1 and TEAD2 transcription factors

TEAD proteins constitute a family of highly conserved transcription factors, characterized by a DNA-binding domain called the TEA domain and a protein-binding domain that permits association with transcriptional co-activators. TEAD proteins are unable to induce transcription on their own. They have to interact with transcriptional cofactors to do so. Once TEADs bind their co-activators, the different complexes formed are known to regulate the expression of genes that are crucial for embryonic development, important for organ formation (heart, muscles), and involved in cell death and proliferation. In the first part of this review we describe what is known of the structure of TEAD proteins. We then focus on two members of the family: TEAD1 and TEAD2. First the different transcriptional cofactors are described. These proteins can be classified in three categories: i), cofactors regulating chromatin conformation, ii), cofactors able to bind DNA, and iii), transcriptional cofactors without DNA binding domain. Finally we discuss the recent findings that identified TEAD1 and 2 and its coactivators involved in cancer progression.

Vitrification of in vitro matured oocytes diminishes embryo development potential before but not after embryo genomic activation

PURPOSE

The aim of this study is to evaluate the impact of oocyte vitrification on embryo development potential and to assess the chromosome abnormalities of blastocysts derived from fresh/vitrified-warmed oocytes to assure the safety of the oocyte cryopreservation technique.

METHODS

In vitro matured oocytes derived from immature oocytes were retrieved from small follicles during IVF/intracytoplasmic sperm injection (ICSI) cycles were randomly divided into a fresh and vitrified-warmed groups. After intracytoplasmic sperm injection, the fertilization rate, embryo quality, and developmental status were compared between the two groups. Blastocysts derived from both groups were analyzed using the copy number variation (CNV)-seq technique to evaluate DNA abnormalities.

RESULTS

The fertilization rate with ICSI and the cleavage rate were similar between the two groups. Among the vitrified-warmed group, there was a lower incidence of usable embryos on day 3 (16.42 vs. 28.57 %; P < 0.05) and a lower incidence of blastocysts (7.46 vs. 17.86 %; P < 0.05). However, the proportions of embryos that developed to blastocysts from the day 3 available embryos were similar between the two groups (62.5 vs. 45.45 %; P > 0.05). In the day 3 embryos, the proportion of >5 cell embryos in the fresh group was markedly higher than in the vitrified-warmed group (41.67 vs. 21.64 %; P < 0.05), and the proportion of embryos with ≧50 % fragments was not significantly different between the two groups (39.29 vs. 43.28 %; P > 0.05). The result of CNV-seq demonstrated that there was no difference in chromosomal abnormalities between the two groups (20 vs. 20 %).

CONCLUSIONS

Oocyte vitrification and the warming procedure diminished the embryo development potential before day 3, when embryo genomic activation started. The day 3 usable embryos derived from vitrified-warmed oocytes had the same potential for developing into blastocysts. Vitrification and the warming procedure did not increase the chromosome abnormalities of the blastocysts. Oocyte vitrification is a safe technique for those patients who have no other options, although the oocyte efficiency may be diminished after the vitrified-warmed procedure.

Transgenerational effects of binge drinking in a primate model: implications for human health

OBJECTIVE

To determine if binge ethanol consumption before ovulation affects oocyte quality, gene expression, and subsequent embryo development.

DESIGN

Binge levels of ethanol were given twice weekly for 6 months, followed by a standard in vitro fertilization cycle and subsequent natural mating.

SETTING

National primate research center.

ANIMAL(S)

Adult female rhesus monkeys.

INTERVENTION(S)

Binge levels of ethanol, given twice weekly for 6 months before a standard in vitro fertilization cycle with or without embryo culture. With in vivo development, ethanol treatment continued until pregnancy was identified.

MAIN OUTCOME MEASURE(S)

Oocyte and cumulus/granulosa cell gene expression, embryo development to blastocyst, and pregnancy rate.

RESULT(S)

Embryo development in vitro was reduced; changes were found in oocyte and cumulus cell gene expression; and spontaneous abortion during very early gestation increased.

CONCLUSION(S)

This study provides evidence that binge drinking can affect the developmental potential of oocytes even after alcohol consumption has ceased.

Effects of ERα-specific antagonist on mouse preimplantation embryo development and zygotic genome activation

Zygotic genome activation (ZGA) is essential for normal development of mammalian preimplantation embryos. Estrogen receptor alpha (ERα) has been implicated in early embryogenesis, and controls the expression of genes associated with proliferation, differentiation and development of cell and target organs via a genomic effect. The objective of this study was to determine whether ERα plays a role in early embryo development and affects ZGA gene expression. Toward this objective, 1-cell embryos from B6C3F1 mouse were cultured with the antiestrogen ICI182780, ERα-specific antagonist MPP, ERα-specific antibody and ERβ-specific antagonist PHTPP. Development of 2-cell to 4-cell in vitro was significantly blocked by ICI182780, MPP and ERα-antibody treatment in a dose-dependent manner but not affected by PHTPP exposure. MPP decreased nuclear ERα protein levels and reduced mRNA expression levels of MuERV-L, one of the ZGA related genes. The results indicate that ERα has a functional role in early embryo development by regulation of ZGA-related genes.

Parental environmental exposure leads to glycometabolic disturbances that affect fertilization of eggs in the silkworm Bombyx mori: the parental transcript legacy

Parental transcript legacy plays an important role in fertilization and development of the early embryo. Parental environmental exposure affects the fertilization of eggs, but the underlying biochemical mechanism is largely unresolved. In this study, the parental environmental effects on fertilization of eggs were explored in the silkworm Bombyx mori (B. mori), an ideal lepidopteran animal model. The results showed that the rate of fertilization decreased after the parents were exposed to a poor environment at 32 °C with continuous illumination for 72 h on days 6-9 of the pupal stage, which is a key period for germ cell maturation. This was likely attributable to lower energy charge values, obstructed nicotinamide adenine dinucleotide (NAD(+)) regeneration and inactive tricarboxylic acid cycle (TCA), leading to accumulation of large amounts of pyruvic acid and lactic acid. This effect was related to energy metabolism via glycolysis; in particular disruption of pyruvate metabolism. In conclusion, this study showed parental exposure to an abnormal environment during germ cell maturation affected glycolysis and the subsequent fertilization of eggs via the parental transcript legacy in B. mori.

Epigenetics and type II diabetes mellitus: underlying mechanisms of prenatal predisposition

Type II diabetes mellitus (T2DM) is a widespread metabolic disorder characterized by insulin resistance precipitating abnormally high blood glucose levels. While the onset of T2DM is known to be the consequence of a multifactorial interplay with a strong genetic component, emerging research has demonstrated the additional role of a variety of epigenetic mechanisms in the development of this disorder. Heritable epigenetic modifications, such as DNA methylation and histone modifications, play a vital role in many important cellular processes, including pancreatic cellular differentiation and maintenance of normal β-cell function. Recent studies have found possible epigenetic mechanisms to explain observed risk factors, such as altered atherogenic lipid profiles, elevated body mass index (BMI), and impaired glucose tolerance (IGT), for later development of T2DM in children born to mothers experiencing both famine and hyperglycemic conditions. It is suggested that these epigenetic influences happen early during gestation and are less susceptible to the effects of postnatal environmental modification as was previously thought, highlighting the importance of early preventative measures in minimizing the global burden of T2DM.

Waves of early transcriptional activation and pluripotency program initiation during human preimplantation development

The events regulating human preimplantation development are still largely unknown owing to a scarcity of material, ethical and legal limitations and a lack of reliable techniques to faithfully amplify the transcriptome of a single cell. Nonetheless, human embryology is gathering renewed interest due to its close relationship with both stem cell biology and epigenetic reprogramming to pluripotency and their importance in regenerative medicine. Carefully timed genome-wide transcript analyses of single oocytes and embryos uncovered a series of successive waves of embryonic transcriptional initiation that start as early as the 2-cell stage. In addition, we identified the hierarchical activation of genes involved in the regulation of pluripotency. Finally, we developed HumER, a database of human preimplantation gene expression, to serve the scientific community. Importantly, our work links early transcription in the human embryo with the correct execution of the pluripotency program later in development and paves the way for the identification of factors to improve epigenetic reprogramming.

Functional genomics of 5- to 8-cell stage human embryos by blastomere single-cell cDNA analysis

Blastomere fate and embryonic genome activation (EGA) during human embryonic development are unsolved areas of high scientific and clinical interest. Forty-nine blastomeres from 5- to 8-cell human embryos have been investigated following an efficient single-cell cDNA amplification protocol to provide a template for high-density microarray analysis. The previously described markers, characteristic of Inner Cell Mass (ICM) (n = 120), stemness (n = 190) and Trophectoderm (TE) (n = 45), were analyzed, and a housekeeping pattern of 46 genes was established. All the human blastomeres from the 5- to 8-cell stage embryo displayed a common gene expression pattern corresponding to ICM markers (e.g., DDX3, FOXD3, LEFTY1, MYC, NANOG, POU5F1), stemness (e.g., POU5F1, DNMT3B, GABRB3, SOX2, ZFP42, TERT), and TE markers (e.g., GATA6, EOMES, CDX2, LHCGR). The EGA profile was also investigated between the 5-6- and 8-cell stage embryos, and compared to the blastocyst stage. Known genes (n = 92) such as depleted maternal transcripts (e.g., CCNA1, CCNB1, DPPA2) and embryo-specific activation (e.g., POU5F1, CDH1, DPPA4), as well as novel genes, were confirmed. In summary, the global single-cell cDNA amplification microarray analysis of the 5- to 8-cell stage human embryos reveals that blastomere fate is not committed to ICM or TE. Finally, new EGA features in human embryogenesis are presented.

Positive and negative cis-regulatory elements directing postfertilization maternal mRNA translational control in mouse embryos

Mechanisms providing for temporally complex patterns of maternal mRNA translation after fertilization are poorly understood. We employed bioinformatics analysis to compare populations of mRNAs enriched specifically on polysomes at the metaphase II (MII) stage oocyte and late one-cell stages and a detailed deletion/truncation series to identify elements that regulate translation. We used the Bag4 3' untranslated region (UTR) as a model. Bioinformatics analysis revealed one conserved motif, subsequently confirmed by functional studies to be a key translation repressor element. The deletion/truncation studies revealed additional regulatory motifs, most notably a strong translation activator element of <30 nt. Analysis of mRNA secondary structure suggests that secondary structure plays a key role in translation repression. Additional bioinformatics analysis of the regulated mRNA population revealed a diverse collection of regulatory motifs found in small numbers of mRNAs, highlighting a high degree of sequence diversity and combinatorial complexity in the overall control of the maternal mRNA population. We conclude that translational control after fertilization is driven primarily by negative regulatory mechanisms opposing strong translational activators, with stage-specific release of the inhibitory influences to permit recruitment. The combination of bioinformatics analysis and deletion/truncation studies provides the necessary approach for dissecting postfertilization translation regulatory mechanisms.

Providing a stable methodological basis for comparing transcript abundance of developing embryos using microarrays

High throughput methods deliver large amount of data serving to describe the physiological treatment that is being studied. In the case of microarrays, there would be a clear benefit to integrate the published data sets. However, the numerous methodological discrepancies between microarray platforms make this comparison impossible. This incompatibility is magnified when considering the peculiar context of transcript management in early embryogenesis. The total RNA content is known to profoundly fluctuate during development. In addition, the mRNA population is subjected to poly(A) tail shortening and elongating events, a characteristic of stored and recruited messengers. These intrinsic factors need to be considered when interpreting any transcript abundance profiles during early development. As a consequence, many methodological details affect microarray platform performances and prevent compatibility. In an effort to maximize our microarray platform performance, we determined the various sources of variation for every one of the main steps leading to the production of microarray data. The five main steps involved in sample preparation were evaluated, as well as conditions for post-hybridization validation by qRT-PCR. These determinations were essential for the implementation of standardized procedures for our Research Network but they can also provide insight into the compatibility issues that the microarray community is now facing.

The presence and activation of two essential transcription factors (cAMP response element-binding protein and cAMP-dependent transcription factor ATF1) in the two-cell mouse embryo

The expression of two members of an important family of transcription factors, cAMP response element-binding protein (CREB) and cAMP-dependent transcription factor ATF1 (ATF1), is essential for normal preimplantation development. There is a high degree of functional similarity between these two transcription factors, and they can both homodimerize and heterodimerize with each other to form active transcription factors. CREB is present in all stages of mouse preimplantation embryo, and we show here that ATF1 is localized to the nucleus in all preimplantation stages. Activation of these transcription factors requires their phosphorylation, and this was only observed to occur for both transcription factors (serine 133 phosphorylation of CREB and serine 63 phosphorylation of ATF1) at the two-cell stage. Nuclear localization and phosphorylation of ATF1 were constitutive. The nuclear localization and phosphorylation of CREB showed a constitutive component that was further induced by the autocrine embryotropin Paf (1-o-alkyl-2-acetyl-sn-glycero-3-phosphocholine). Activation of CREB by Paf was independent of cAMP but was dependent on calcium, calmodulin, and calmodulin-dependent kinase activity. ATF1 nuclear localization was unaffected by inhibition of the calcium/calmodulin pathway. A complex pattern of expression of calmodulin-dependent kinases was observed throughout preimplantation development. At the two-cell stage, only mRNAs coding for calmodulin-dependent protein kinase kinase beta, calmodulin-dependent protein kinase II gamma, and calmodulin-dependent protein kinase IV were detected. A selective antagonist for calmodulin-dependent protein kinase kinase (STO-609) and calmodulin-dependent protein kinases I, II, and IV (KN-62) blocked the Paf-induced phosphorylation of CREB. The study demonstrates a role for trophic signaling and constitutive activation of two essential transcription factors at the time of zygotic genome activation.

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.

Expression of microRNA processing machinery genes in rhesus monkey oocytes and embryos of different developmental potentials

MicroRNAs (miRNAs) are a class of small RNAs that silence gene expression. In animal cells, miRNAs bind to the 3' untranslated regions of specific mRNAs and inhibit their translation. The correct regulation of mRNA expression by miRNAs is believed to be important for oocyte maturation, early development and implantation. We examined the expression of 25 mRNAs involved in the microRNA processing pathway in a nonhuman primate oocyte and embryo model. We observed that mRNAs related to miRNA splicing are downregulated during oocyte maturation while those related to miRNA processing are upregulated, indicating that there may exist a temporal difference in their activities related to transcriptional activity in germinal vesicle stage oocytes. We also observed that the vast majority of mRNAs examined were insensitive to alpha-amanitin at the 8-16 cell stage. The expression data did not reveal a major impact of embryo culture, and hormonal stimulation protocol affected only a small number of mRNAs, suggesting that the components of the pathway may be accumulated in the oocyte during oogenesis and resistant to exogenous insults. In comparison to published mouse array data, we observed species differences and similarities in the temporal expression patterns of some genes, suggesting that miRNA processing may be regulated differently. These data extend our understanding of the potential roles of miRNA during primate embryogenesis.

Cryopreservation of manipulated embryos: tackling the double jeopardy

The aim of the present review is to provide information to researchers and practitioners concerning the reasons for the altered viability and the medium- and long-term consequences of cryopreservation of manipulated mammalian embryos. Embryo manipulation is defined herein as the act or process of manipulating mammalian embryos, including superovulation, AI, IVM, IVF, in vitro culture, intracytoplasmic sperm injection, embryo biopsy or splitting, somatic cell nuclear transfer cloning, the production of sexed embryos (by sperm sexing), embryo cryopreservation, embryo transfer or the creation of genetically modified (transgenic) embryos. With advances in manipulation technologies, the application of embryo manipulation will become more frequent; the proper prevention and management of the resulting alterations will be crucial in establishing an economically viable animal breeding technology.

Regulation of cap-dependent translation initiation in the early stage porcine parthenotes

The binding of mRNAs to ribosomes is mediated by the protein complex eIF4F in conjunction with eIF4B (eukaryotic initiation factor 4F and 4B). EIF4F is a three subunit complex consisting of eIF4A (RNA helicase), eIF4E (mRNA cap binding protein), and eIF4G (bridging protein). The crucial role is played by eIF4E, which directly binds the 5'-cap structure of the mRNA and facilitates the recruitment to the mRNA of other translation factors and the 40S ribosomal subunit. EIF4E binding to mRNA and to other initiation factors is regulated on several levels, including its phosphorylation on Ser-209, and association with its regulatory protein 4E-binding protein (4E-BP1). In this study we document that both the translation initiation factor eIF4E and its regulator 4E-BP1 become dephosphorylated in the early stage porcine zygotes already 8 hr post-activation. Similarly, the activities of ERK1/2 MAP and Mnk1 kinases, which are both involved in eIF4E phosphorylation, gradually decrease during this period with the timing similar to that of eIF4E dephosphorylation. The formation of an active eIF4F complex is also diminished after 9-15 hr post-activation, although substantial amounts of this complex have been detected also 24 hr post-activation (2-cell stage). The overall protein synthesis in the parthenotes decreases gradually from 12 hr post-activation reaching a minimum after 48 hr (4-cell stage). Although the translation is gradually decreasing during early preimplantation development, the eIF4F complex, which is temporarily formed, might be a premise for the translation of a small subset of mRNAs at this period of development.

Deficiency in recapitulation of stage-specific embryonic gene transcription in two-cell stage cloned mouse embryos

One possible explanation to account for the ability to clone animals by somatic cell nuclear transfer is that the donor genome is reprogrammed by the oocyte to recapitulate a normal embryonic pattern of gene expression. Mouse embryos display transient transcriptional induction of a group of genes (TIGs) at the mid two-cell stage, the first major transcriptional output of the embryonic genome, uniquely suited for evaluating whether the oocyte directs correct and efficient recapitulation of an embryo stage-specific gene expression program before any in vitro selection occurs. We analyzed the expression of eight TIGs in two-cell stage clones prepared with cumulus cell nuclei. One failed to be transcribed, and seven others were transcribed, but supported significantly reduced mRNA expression. The reduction ranged from 1.6- to 17-fold at the mid two-cell stage, and 1.5- to 13-fold for the late two-cell stage. Five genes were not expressed in the donor cells, and these displayed the most pronounced deficiencies in expression. Two genes were expressed in cumulus cell donors, and supported progressive accumulation of their mRNAs in clones during this period, albeit at reduced rates. One other gene expressed in cumulus cells was not activated in clones. Although a significant proportion of these genes is reactivated in two-cell stage clones, this recapitulation is grossly imperfect, occurs at a substantially reduced level, and even fails entirely to occur for some genes. Thus, the oocyte is incapable of efficiently directing the recapitulation of early embryonic stage-specific gene expression.

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.

Transcription factor TEAD2 is involved in neural tube closure

TEAD2, one of the first transcription factors expressed at the beginning of mammalian development, appears to be required during neural development. For example, Tead2 expression is greatest in the dorsal neural crest where it appears to regulate expression of Pax3, a gene essential for brain development. Consistent with this hypothesis, we found that inactivation of the Tead2 gene in mice significantly increased the risk of exencephaly (a defect in neural tube closure). However, none of the embryos exhibited spina bifida, the major phenotype of Pax3 nullizygous embryos, and expression of Pax3 in E11.5 Tead2 nullizygous embryos was normal. Thus, Tead2 plays a role in neural tube closure that is independent of its putative role in Pax3 regulation. In addition, the risk of exencephaly was greatest with Tead2 nullizygous females, and could be suppressed either by folic acid or pifithrin-alpha. These results reveal a maternal genetic contribution to neural tube closure, and suggest that Tead2-deficient mice provide a model for anencephaly, a common human birth defect that can be prevented by folic acid.

Using biomarker signature patterns for an mRNA molecular diagnostic of mouse embryonic stem cell differentiation state

BACKGROUND

The pluripotency and self-renewal capabilities, which define the "stemness" state, of mouse embryonic stem (ES) cells, are usually investigated by functional assays or quantitative measurements of the expression levels of known ES cell markers. Strong correlations between these expression levels and functional assays, particularly at the early stage of cell differentiation, have usually not been observed. An effective molecular diagnostic to properly identify the differentiation state of mouse ES cells, prior to further experimentation, is needed.

RESULTS

A novel molecular pattern recognition procedure has been developed to diagnose the differentiation state of ES cells. This is based on mRNA transcript levels of genes differentially expressed between ES cells and their differentiating progeny. Large publicly available ES cell data sets from various platforms were used to develop and test the diagnostic model. Signature patterns consisting of five gene expression levels achieved high accuracy at determining the cell state (sensitivity and specificity > 97%).

CONCLUSION

The effective ES cell state diagnostic scheme described here can be implemented easily to assist researchers in identifying the differentiation state of their cultures. It also provides a step towards standardization of experiments relying on cells being in the stem cell or differentiating state.

Analysis of transcription factor expression during oogenesis and preimplantation development in mice

The transition from a differentiated germ cell into a totipotent zygote during oogenesis and preimplantation development is critical to the creation of a new organism. During this period, cell characteristics change dynamically, suggesting that a global alteration of gene expression patterns occurs, which is regulated by global changes in various epigenetic factors. Among these, transcription factors (TFs) are essential in the direct regulation of transcription and also play important roles in determining cell characteristics. However, no comprehensive analysis of TFs from germ cells to embryos had been undertaken. We used mRNA amplification systems and microarrays to conduct a genomewide analysis of TFs at various stages of oogenesis and preimplantation development. The greatest alteration in TFs occurred between the 1- and 2-cell stages, at which time zygotic genome activation (ZGA) occurs. Our analysis of TFs classified by structure and function revealed several specific patterns of change. Basic transcription factors, which are the general components of transcription, increased transiently at the 2-cell stage, while homeodomain (HD) TFs were expressed specifically in the oocyte. TFs containing the Rel homology region (RHR) and Ets domains were expressed at a high level in 2-cell and blastocyst embryos. Thus, the global TF dynamics that occur during oogenesis and preimplantation development seem to regulate the transition from germ-cell-type to embryo-type gene expression.

In vitro thermal stress induces apoptosis and reduces development of porcine parthenotes

The precise physiological causes that result in reduced development of oocytes after heat shock (HS) are not clear. In this study, apoptosis, heat shock protein70 (hsp70), and in vitro development of porcine oocytes were evaluated after HS. Porcine cumulus-oocyte complexes (COCs) were subjected to in vitro maturation for 42 h. The matured oocytes were then heated at 41.5 degrees C for 0 h (control, C0h), 1 h (HS1h), 2 h (HS2h), or 4 h (HS4h). An additional group of oocytes was cultured for 4 h without HS (control, C4h). In Experiment 1, expression of hsp70 was detected by Western-blotting and no difference between controls and HS groups was observed. In Experiment 2, apoptosis of matured oocytes after HS was examined by Annexin V-FITC and TUNEL. No significant TUNEL-positive signals were detected in the heated oocytes compared to the controls, but the intensity of Annexin V-FITC labeling among different groups increased with length of HS and in vitro culture (P<0.05). Oocytes were parthenogenetically activated by an electric pulse plus 6-DMAP (Experiment 3). Mean (+/-S.E.M.) embryonic development in HS2h (cleavage: 42+/-29%; blastocyst: 11+/-10%) and HS4h (cleavage: 36+/-28%; blastocyst: 11+/-8%) were decreased when compared to those in C0h (cleavage: 63+/-12%; blastocyst: 24+/-14%) and C4h (cleavage: 66+/-8%; blastocyst: 21+/-11%). Numbers of blastocysts with TUNEL-positive signals were similar among groups, but the signals increased before the eight-cell stage in HS groups (P<0.05). In conclusion, developmental competence of matured pig oocytes was compromised after heat shock, but it was not closely associated with the expression of oocyte hsp70. However, there may be a link between apoptosis and developmental competence of porcine oocytes.

Analysis of polysomal mRNA populations of mouse oocytes and zygotes: dynamic changes in maternal mRNA utilization and function

Transcriptional activation in mammalian embryos occurs in a stepwise manner. In mice, it begins at the late one-cell stage, followed by a minor wave of activation at the early two-cell stage, and then the major genome activation event (MGA) at the late two-cell stage. Cellular homeostasis, metabolism, cell cycle, and developmental events are orchestrated before MGA by time-dependent changes in the array of maternal transcripts being translated. Many elegant studies have documented the importance of maternal mRNA (MmRNA) and its correct recruitment for development. Many other studies have illuminated some of the molecular mechanisms regulating MmRNA utilization. However, neither the complete array of recruited mRNAs nor the regulatory mechanisms responsible for temporally different patterns of recruitment have been well characterized. We present a comprehensive analysis of changes in the maternal component of the zygotic polysomal mRNA population during the transition from oocyte to late one-cell stage embryo. We observe global transitions in the functional classes of translated MmRNAs and apparent changes in the underlying cis-regulatory mechanisms.

DkkL1 (Soggy), a Dickkopf family member, localizes to the acrosome during mammalian spermatogenesis

Dickkopf-like 1 (DkkL1) is related to the Dickkopf gene family, a group of proteins that are characterized as secreted antagonists of Wingless (Wnt) signal transduction proteins. DkkL1 mRNA is found in preimplantation mouse embryos and in developing neural tissue, but in adults it is found primarily in the testes. In an effort to elucidate its function, the distribution of DkkL1 protein in mouse testis and mature sperm was analyzed by immuno-histochemistry and immuno-blotting techniques. DkkL1 first appeared in the developing spermatocytes in seminiferous tubules as early as Stage XII, coincident with the appearance of DkkL1 mRNA. Surprisingly, however, DkkL1 localized to the developing acrosome in spermatocytes and spermatids and to the acrosome in mature sperm. Furthermore, DkkL1 was N-glycosylated in the testis, but it did not appear to be excreted, and the DkkL1 in mature sperm was no longer N-glycosylated, suggesting that additional post-translational modifications occurred during the final stages of spermatogenesis. These results identify a member of the Dickkopf family as a novel acrosomal protein that may be involved in acrosome assembly or function, a unique role for a secreted signaling molecule.

Cyclin A2-CDK2 regulates embryonic gene activation in 1-cell mouse embryos

Recruitment of maternal mRNA in mice appears essential for embryonic gene activation (EGA) that is initiated in the 1-cell stage. The identity of which recruited mRNAs is responsible, however, is not known. We report here that recruitment of cyclin A2 mRNA may be critical for EGA. Cyclin A2 protein accumulates in pronuclei between 6 and 12 h after fertilization, the time when EGA is initiated. This cyclin A2 may be generated from maternally recruited cyclin A2 mRNA because its accumulation was inhibited by 3'-deoxyadenosine, which inhibits mRNA polyadenylation. When CDK2 activity or pronuclear accumulation of cyclin A2 was inhibited with CDK2 inhibitors or by microinjected siRNAs, respectively, DNA replication was not inhibited but the increase of transcriptional activity was prevented. In addition, microinjection of recombinant cyclin A2-CDK2 protein increased transcriptional activity. Cyclin A2-CDK2 is activated following egg activation, because an increase in phosphorylation of retinoblastoma protein was observed using antibodies that recognize site-specific phosphorylation catalyzed by this kinase and treatment with a CDK2 inhibitor or microinjection with cyclin A2 siRNAs prevented the increase in retinoblastoma protein phosphorylation. These results suggest that recruitment of maternal cyclin A2 mRNA following egg activation is linked to EGA.

Species-dependent expression patterns of DNA methyltransferase genes in mammalian oocytes and preimplantation embryos

DNA methyltransferases (DNMTs) comprise a family of proteins involved in the establishment and maintenance of DNA methylation patterns in the mammalian genome. DNA methylation involves the transfer of the methyl group of the coenzyme S-adenosyl-L-methionine to the 5 position of cytosine residues within CpG dinucleotides. DNA methylation is implicated in the control of imprinted genes expression, X chromosome silencing, development of certain types of cancer, and embryonic development. DNA methylation is also believed to protect the genome from parasitic elements such as transposons, retrotransposons, and viruses. The aim of this study was to analyze the expression patterns of DNMT1, DNMT2, DNMT3A, DNMT3B, and DNMT3L genes in rhesus macaque (Macaca mulatta) oocytes and preimplantation stage embryos from fertilization to the hatched blastocyst stage, and to compare these results with the expression profiles in the mouse and other mammalian species. We describe species-dependent differences as well as similarities in expression patterns of DNMT genes among mammals.

RNA in human sperm

We have yet to develop a fundamental understanding of the molecular complexities of human spermatozoa. This encompasses the unique packaging and structure of the sperm genome along with their paternally derived RNAs in preparation for their delivery to the egg. The diversity of these transcripts is vast, including several anti-sense molecules resembling known regulatory micro-RNAs. The field is still grasping with its delivery to the oocyte at fertilization and possible significance. It remains tempting to analogize them to maternally-derived transcripts active in early embryo patterning. Irrespective of their role in the embryo, their use as a means to assess male factor infertility is promising.

Isolation of nascent messenger RNA from mouse preimplantation embryos

The expression of the zygotic genome starts at the late one-cell stage in mouse embryos, and its regulation changes dynamically until the late two-cell stage. To understand this process, it is important to accumulate the profiles of the genes transcribed at any given instant at each stage of development. However, because large amounts of maternal mRNA accumulate in embryos to sustain early development, it is difficult to determine the profile of newly synthesized mRNA just after gene activation. To overcome this difficulty, we established a novel method of isolating nascent mRNA from the large pool of preexisting mRNA. Briefly, the procedure was as follows. Embryos were electrically permeabilized and loaded with 5-bromouridine-5'-triphosphate (BrUTP). Nascent mRNA with incorporated BrU was isolated by immunoprecipitation with an antibody recognizing BrU. The cDNA was synthesized from the isolated mRNA, and its abundance was evaluated using semiquantitative real-time PCR. Using this method, we examined the amounts of newly synthesized eIF-1A, MuERV-L, and cyclin-A2 transcripts in two-cell mouse embryos and compared them with the quantities of these transcripts present in the total mRNA pool. The amount of each transcript in the nascent mRNA fraction and in the total mRNA pool changed differently over time, demonstrating that this method can be used to obtain profiles of genes transcribed during development.

Transcription Factor Expression Patterns in Bovine In Vitro-Derived Embryos Prior to Maternal-Zygotic Transition1

Maternal-zygotic transition (MZT) is a complex phenomenon characterized by the initiation of transcription in the embryo and the replacement of maternal mRNA with embryonic mRNA. In order for this to occur, transcriptional activation requires various factors and conditions. Our hypothesis is that the lack of transcription in the bovine pre-MZT embryo is due, in part, to an incomplete or dormant transcriptional apparatus. Therefore, in accordance with this hypothesis, functioning transcriptional mechanisms should appear in the eight-cell bovine embryo to facilitate embryonic transcription during the MZT. With this in mind, we examined the presence of selected transcription factors during preimplantation embryo development to establish how their transcript levels change in bovine pre-MZT embryos. To achieve this goal, real-time reverse transcription-polymerase chain reaction was used to quantify the mRNA level of several different transcription factors (YY1, HMGA1, RY-1, P300, CREB, YAP65, HMGN1, HMGB1, NFAR, OCT-4, TEAD2, ATF-1, HMGN2, MSY2, and TBP) in germinal vesicle (GV) and metaphase II (MII) bovine oocytes and in two-, four-, eight-cell, and blastocyst stage embryos produced in vitro. Our results demonstrate that all genes examined can be grouped into five different categories according to their mRNA expression patterns at the developmental stages observed. To summarize, all transcription factors studied were present in pre-MZT embryos and the expression pattern of many of them suggest a potential role in MZT.

DNA methylation may restrict but does not determine differential gene expression at the Sgy/Tead2 locus during mouse development

Soggy (Sgy) and Tead2, two closely linked genes with CpG islands, were coordinately expressed in mouse preimplantation embryos and embryonic stem (ES) cells but were differentially expressed in differentiated cells. Analysis of established cell lines revealed that Sgy gene expression could be fully repressed by methylation of the Sgy promoter and that DNA methylation acted synergistically with chromatin deacetylation. Differential gene expression correlated with differential DNA methylation, resulting in sharp transitions from methylated to unmethylated DNA at the open promoter in both normal cells and tissues, as well as in established cell lines. However, neither promoter was methylated in normal cells and tissues even when its transcripts were undetectable. Moreover, the Sgy promoter remained unmethylated as Sgy expression was repressed during ES cell differentiation. Therefore, DNA methylation was not the primary determinant of Sgy/Tead2 expression. Nevertheless, Sgy expression was consistently restricted to basal levels whenever downstream regulatory sequences were methylated, suggesting that DNA methylation restricts but does not regulate differential gene expression during mouse development.

TDPOZ, a family of bipartite animal and plant proteins that contain the TRAF (TD) and POZ/BTB domains

We have previously reported a gene Tdpoz1 (previously called 2cpoz56) that is temporally expressed in unfertilized eggs and in early embryos of the mouse. The putative TDPOZ1 protein carries a tumor necrosis factor receptor-associated factor (TRAF) domain (TD) and a POZ/BTB domain. On the analysis of nine bacterial artificial chromosome (BAC) clones, we have uncovered four more Tdpoz1 homologs in the mouse genome, designated Tdpoz2 through Tdpoz5. Tdpoz1 and Tdpoz2 are found 30 kb apart in a fully sequenced BAC clone (GenBank accession number AF545858). The genes are intronless in the coding region and each carries an intron in the 5'-untranslated region as in other early embryonic genes. The Tdpoz gene cluster is mapped on chromosome 3 at 3F2.1-2.2. RT-PCR experiments and a search of expressed sequence tag (EST) databases show that the Tdpoz1-5 genes are transcribed in early embryos, particularly at the two-cell stage. Exhaustive database searches have further uncovered three more mouse Tdpoz homologs in chromosomes 3 and 11 and 25 other Tdpoz-like orthologs in the genomes of other animal and plant species including human, rat, C. elegans, Drosophila, Arabidopsis and rice. In the rat genome, eight rat Tdpoz genes are found as a cluster in chromosome 2. Hence, TDPOZ proteins form a new protein family on the basis of similar protein domain organization. Based on reported characteristics of known TD- and POZ-bearing proteins, we speculate that TDPOZ proteins may be nuclear scaffold proteins probably involved in transcription regulation in early development and other cellular processes.

Dynamics of global gene expression changes during mouse preimplantation development

Understanding preimplantation development is important both for basic reproductive biology and for practical applications including regenerative medicine and livestock breeding. Global expression profiles revealed and characterized the distinctive patterns of maternal RNA degradation and zygotic gene activation, including two major transient waves of de novo transcription. The first wave corresponds to zygotic genome activation (ZGA); the second wave, named mid-preimplantation gene activation (MGA), precedes the dynamic morphological and functional changes from the morula to blastocyst stage. Further expression profiling of embryos treated with inhibitors of transcription, translation, and DNA replication revealed that the translation of maternal RNAs is required for the initiation of ZGA. We propose a cascade of gene activation from maternal RNA/protein sets to ZGA gene sets and thence to MGA gene sets. The large number of genes identified as involved in each phase is a first step toward analysis of the complex gene regulatory networks.

Cloning: questions answered and unsolved

Cloning by the transfer of adult somatic cell nuclei to oocytes has produced viable offspring in a variety of mammalian species. The technology is still in its initial stages of development. Studies to date have answered several basic questions related to such issues as genome potency, life expectancy of clones, mitochondrial fates, and feasibility of inter-species nuclear transfer. They have also raised new questions related to the control of nuclear reprogramming and function. These questions are reviewed here.

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.

Expression of genes encoding chromatin regulatory factors in developing rhesus monkey oocytes and preimplantation stage embryos: possible roles in genome activation

One of the most critical events of preimplantation development is the successful activation of gene transcription. Both the timing and the array of genes activated must be controlled. The ability to regulate gene transcription appears to be reduced just prior to the time of the major genome activation event, and changes in chromatin structure appear essential for establishing this ability. Major molecules that modulate chromatin structure are the linker and core histones, enzymes that modify histones, and a wide variety of other factors that associate with DNA and mediate either repressive or activating changes. Among the latter are chromatin accessibility complexes, SWI/SNF complexes, and the YY1 protein and its associated factors. Detailed information about the expression and regulation of these factors in preimplantation stage embryos has not been published for any species. In order to ascertain which of these factors may participate in chromatin remodeling, genome activation, and DNA replication during early primate embryogenesis, we determined the temporal expression patterns of mRNA encoding these factors. Our data identify the predominant members of these different functional classes of factors expressed in oocytes and embryos, and reveal patterns of expression distinct from those patterns seen in somatic cells. Among each of four classes of mRNAs examined, some mRNAs were expressed predominantly in the oocyte, with these largely giving way to others expressed stage specifically in the embryo. This transition may be part of a global mechanism underlying the transition from maternal to embryonic control of development, wherein the oocyte program is silenced and an embryonic pattern of gene expression becomes established. Possible roles for these mRNAs in chromatin remodeling, genome activation, DNA replication, cell lineage determination, and nuclear reprogramming are discussed.

Expression of stage-specific genes during zygotic gene activation in preimplantation mouse embryos

The expression of mouse two-cell stage specific genes was studied using the modified DDRT-PCR method, which overcame the paucity of the experimental materials of preimplantation embryos. Embryo tissues equivalent to that of four blastomeres are sufficient for amplification of target genes as visualized using polyacrylamide gel. Sequence analyses and reverse Northern blots indicate that the genes of ATPase 6 and Ywhaz are expressed specifically in two-cell embryos. ATPase 6 is essential for one-cell to two-cell transition and plays an important role in establishment of oxidative phosphorylation, while Ywhaz is related to initiating cellular communication system.

Delay of ZGA initiation occurred in 2-cell blocked mouse embryos

One-cell mouse embryos from KM strain and B6C3F1 strain were cultured in M16 medium, in which 2-cell block generally occurs. Embryos of KM strain exhibited 2-cell block, whereas B6C3F1 embryos, which are regarded as a nonblocking strain, proceeded to the 4-cell stage in our culture condition. It is often assumed that the block of early development is due to the failure of zygotic gene activation (ZGA) in cultured embryos. In this study we examined protein synthesis patterns by two-dimensional gel electrophoresis of [35S] methionine radiolabeled 2-cell embryos. Embryos from the blocking strain and the nonblocking strain were compared in their development both in vitro and in vivo. The detection of TRC expression, a marker of ZGA, at 42 h post hCG in KM embryos developed in vitro suggested that ZGA was also initiated even in the 2-cell arrested embryos. Nevertheless, a significant delay of ZGA was observed in KM strain as compared with normally developed B6C3F1 embryos. At the very beginning of major ZGA as early as 36 h post hCG, TRC has already been expressed in B6C3F1 embryos developed in vitro and KM embryos developed in vivo. But for 2-cell blocked KM embryos, TRC was still not detectable even at 38 h post hCG. These evidences suggest that 2-cell-blocked embryos do initiate ZGA, and that 2-cell block phenomenon is due not to the disability in initiating ZGA, but to a delay of ZGA.

Chromatin as a regulative architecture of the early developmental functions of mammalian embryos after fertilization or nuclear transfer

Nuclear transfer of a somatic nucleus into an enucleated oocyte has demonstrated in several mammalian species that the chromatin of a differentiated nucleus can be reprogrammed so as to be able to direct the full development of the reconstructed embryo. This review focus on the timing of the early events that allow the return of somatic chromatin to a totipotent state. Our understanding of the modifications associated with chromatin remodeling is limited by the low amount of biological material available in mammals at early developmental stages and the fact that very few genetic studies have been conducted with nuclear transfer embryos. However, the importance of several factors such as the covalent modifications of DNA through the methylation of CpG dinucleotides, the exchange of histones through a reorganized nuclear membrane, and the interaction between cytoplasmic oocyte components and nuclear complexes in the context of nuclear transfer is becoming clear. A better characterization of the changes in somatic chromatin after nuclear transfer and the identification of oocyte factors or structures that govern the formation of a functional nucleus will help us to understand the relationship between chromatin structure and cellular totipotency.

Gene expression and chromatin structure in the pre-implantation embryo

The pre-implantation period of mammalian development includes the formation of the zygote, the activation of the embryonic genome (EGA), and the beginning of cellular differentiation. During this period, protamines are replaced by histones, the methylated haploid parental genomes undergo demethylation following formation of the diploid zygote, and maternal control of development is succeeded by zygotic control. Superimposed on this activation of the embryonic genome is the formation of a chromatin-mediated transcriptionally repressive state requiring enhancers for efficient gene expression. The development of this transcriptionally repressive state most likely occurs at the level of chromatin structure, because inducing histone hyperacetylation relieves the requirements for enhancers. Characterization of zygotic mRNA expression patterns during the pre-implantation period and their relationship to successful development in vitro and in vivo will be essential for defining optimized culture conditions and nuclear transfer protocols. The focus of this review is to summarize recent advances in this field and to discuss their implications for developmental biology.

Display of different modes of transcription by the promoters of an early embryonic gene, Zfp352, in preimplantation embryos and in somatic cells

We have previously reported a Krupple-like finger protein gene, Zfp352, which is expressed temporarily in two- to eight-cell mouse embryos. The Zfp352 gene is intron-less in the coding region but carries a solitary 4.3-kb intron in the 5'-untranslated region. In this study, we have analyzed the Zfp352 promoter activity in early embryos and in somatic cells. We determined that the major Zfp352 promoter, designated P1 and located upstream of exon 1, is utilized in both early embryos and in somatic cells. A TATA-like box and a transcription initiator element are discernible in the P1 promoter. We uncovered an alternative promoter, designated P2, in the intron. 5'-Rapid amplification of cDNA ends and real-time RT-PCR experiments indicated that the P2 promoter is weak and is probably fortuitous in early embryos. In somatic cells, however, transfection experiments showed that P2 is as active as P1 as a promoter. Furthermore, P2 appears to be composed of two different subdomains used differentially for transcription initiation in embryos and in somatic cells. Our observations may bear relevance in explaining developmental deficiencies associated with somatic cell cloning experiments.

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).

Nuclear transfer technologies: between successes and doubts

Cloning of mammals by nuclear transfer can lead to the birth of healthy adult animals but more often compromises the development of the reconstructed embryos. A high incidence of fetal and postnatal losses has been observed in several species, revealing the existence of long-lasting effects induced by the nuclear transfer procedures. Remodeling of donor chromatin by the recipient cytoplasm after nuclear transfer is frequently associated with the deregulation of specific genes, and recent observations point to the potential importance of time-dependent DNA methylation events in the occurrence of these alterations. Screening strategies to design nuclear transfer procedures that would mimic the epigenetic remodeling occurring in normal embryos are being designed, and improvement in the efficiency of procedures could imply a pre-conditioning of donor cells. Early mammalian development appears to be rather tolerant to epigenetic abnormalities, raising the possibility that even a fully functional reprogrammed genome may have been subjected to some epigenetic alterations. Bringing nuclear transfer to routine practice requires greater knowledge and understanding of the basic biological processes underlying epigenetic controls of nuclear activities. An important issue at present is to limit the production of those aberrant phenotypes that may result in significant insult to the nature and welfare of animals.