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

LINE-1 transcription activates long-range gene expression

Long interspersed nuclear element-1 (LINE-1 or L1) is a retrotransposon group that constitutes 17% of the human genome and shows variable expression across cell types. However, the control of L1 expression and its function in gene regulation are incompletely understood. Here we show that L1 transcription activates long-range gene expression. Genome-wide CRISPR-Cas9 screening using a reporter driven by the L1 5' UTR in human cells identifies functionally diverse genes affecting L1 expression. Unexpectedly, altering L1 expression by knockout of regulatory genes impacts distant gene expression. L1s can physically contact their distal target genes, with these interactions becoming stronger upon L1 activation and weaker when L1 is silenced. Remarkably, L1s contact and activate genes essential for zygotic genome activation (ZGA), and L1 knockdown impairs ZGA, leading to developmental arrest in mouse embryos. These results characterize the regulation and function of L1 in long-range gene activation and reveal its importance in mammalian ZGA.

Maternal KLF17 controls zygotic genome activation by acting as a messenger for RNA Pol II recruitment in mouse embryos

Initiation of timely and sufficient zygotic genome activation (ZGA) is crucial for the beginning of life, yet our knowledge of transcription factors (TFs) contributing to ZGA remains limited. Here, we screened the proteome of early mouse embryos after cycloheximide (CHX) treatment and identified maternally derived KLF17 as a potential TF for ZGA genes. Using a conditional knockout (cKO) mouse model, we further investigated the role of maternal KLF17 and found that it promotes embryonic development and full fertility. Mechanistically, KLF17 preferentially binds to promoters and recruits RNA polymerase II (RNA Pol II) in early 2-cell embryos, facilitating the expression of major ZGA genes. Maternal Klf17 knockout resulted in a downregulation of 9% of ZGA genes and aberrant RNA Pol II pre-configuration, which could be partially rescued by introducing exogenous KLF17. Overall, our study provides a strategy for screening essential ZGA factors and identifies KLF17 as a crucial TF in this process.

Nuclear Distribution of the Chromatin-Remodeling Protein ATRX in Mouse Early Embryos during Normal Development and Developmental Arrest In Vitro

The chromatin-remodeling protein ATRX, which is currently recognized as one of the key genome caretakers, plays an important role in oogenesis and early embryogenesis in mammals. ATRX distribution in the nuclei of mouse embryos developing in vivo and in vitro, including when the embryos are arrested at the two-cell stage-the so-called two-cell block in vitro-was studied using immunofluorescent labeling and FISH. In normally developing two- and four-cell embryos, ATRX was found to be closely colocalized with pericentromeric DNA sequences detected with a probe to the mouse major satellite DNA. The association of ATRX with pericentromeric heterochromatin is mediated by nuclear actin and reduced after the treatment of embryos with latrunculin B. When culturing embryos in vitro, the distribution pattern of ATRX changes, leading to a decrease in the association of this protein with major satellite DNA especially under the two-cell block in vitro. Taken together, our data suggest that the intranuclear distribution of ATRX reflects the viability of mouse embryos and their probability of successful preimplantation development.

CHK1-CDC25A-CDK1 regulate cell cycle progression and protect genome integrity in early mouse embryos

After fertilization, remodeling of the oocyte and sperm genomes is essential to convert these highly differentiated and transcriptionally quiescent cells into early cleavage-stage blastomeres that are transcriptionally active and totipotent. This developmental transition is accompanied by cell cycle adaptation, such as lengthening or shortening of the gap phases G1 and G2. However, regulation of these cell cycle changes is poorly understood, especially in mammals. Checkpoint kinase 1 (CHK1) is a protein kinase that regulates cell cycle progression in somatic cells. Here, we show that CHK1 regulates cell cycle progression in early mouse embryos by restraining CDK1 kinase activity due to CDC25A phosphatase degradation. CHK1 kinase also ensures the long G2 phase needed for genome activation and reprogramming gene expression in two-cell stage mouse embryos. Finally, Chk1 depletion leads to DNA damage and chromosome segregation errors that result in aneuploidy and infertility.

Maternal TDP-43 interacts with RNA Pol II and regulates zygotic genome activation

Zygotic genome activation (ZGA) is essential for early embryonic development. However, the regulation of ZGA remains elusive in mammals. Here we report that a maternal factor TDP-43, a nuclear transactive response DNA-binding protein, regulates ZGA through RNA Pol II and is essential for mouse early embryogenesis. Maternal TDP-43 translocates from the cytoplasm into the nucleus at the early two-cell stage when minor to major ZGA transition occurs. Genetic deletion of maternal TDP-43 results in mouse early embryos arrested at the two-cell stage. TDP-43 co-occupies with RNA Pol II as large foci in the nucleus and also at the promoters of ZGA genes at the late two-cell stage. Biochemical evidence indicates that TDP-43 binds Polr2a and Cyclin T1. Depletion of maternal TDP-43 caused the loss of Pol II foci and reduced Pol II binding on chromatin at major ZGA genes, accompanied by defective ZGA. Collectively, our results suggest that maternal TDP-43 is critical for mouse early embryonic development, in part through facilitating the correct RNA Pol II configuration and zygotic genome activation.

Identification of the DNA binding element of ZNFO, an oocyte-specific zinc finger transcription factor in cattle

The maternal effect genes are essential components of oocyte competence, which orchestrate the early developmental events before zygotic genome activation (ZGA). The Krüppel-associated box (KRAB) domain-containing zinc finger proteins (KRAB-ZFPs) constitute the largest transcription factor family in mammals. As a novel maternal effect gene, ZNFO was identified previously in our laboratory. The gene codes for a KRAB-ZFP specifically expressed in bovine oocytes and early embryos and gene silencing experiments have demonstrated that ZNFO is required for early embryonic development in cattle. In the present study, we identified a consensus sequence, ATATCCTGTTTAAACCCC, as the DNA binding element of ZNFO (ZNFOBE) using a library of random oligonucleotides by cyclic amplification of sequence target (CAST) analysis. Sequence-specific binding of ZNFO to the DNA binding element was confirmed by an electrophoretic mobility shift assay (EMSA), and the key nucleotides in the ZNFOBE that are required for specific binding by ZNFO were further determined by a competitive EMSA using mutant competitors. Through a luciferase-based reporter assay, it was confirmed that the interaction between ZNFO and ZNFOBE is required for the repressive function of ZNFO. These results provide an essential step towards the identification of ZNFO regulated genes that play important roles during early embryonic development.

Effects of pyruvate on early embryonic development and zygotic genome activation in pigs

Pyruvate is an important energy substance during early embryonic development of mammals. However, the underlying mechanisms of pyruvate during early embryonic development in pigs and its role in zygotic genome activation (ZGA) are not fully understood. Here, based on a previous RNA-seq dataset of porcine early embryos, we found that pyruvate metabolism-related genes started to be expressed at the 4-cell stage and that pyruvate metabolism-related genes were correlated with porcine ZGA marker genes. To determine the function of pyruvate in porcine embryos, in vitro fertilization (IVF) embryos were cultured in PZM-3 medium (control group); modified PZM-3 medium that only contains pyruvate and lactate plus salts (+P group); or modified PZM-3 medium lacking pyruvate (-P group). The 4-cell arrest rate at 72 h was significantly increased in the -P group compared to the +P group (P < 0.05). In addition, we observed that the reactive oxygen species (ROS) level was significantly increased and that the adenosine triphosphate (ATP) level was significantly (P < 0.05) decreased in the -P group compared to the +P group. Moreover, the expression of ZGA marker genes and SIRT1 protein in embryos was significantly decreased in the -P group compared to the +P group (P < 0.05). Furthermore, the acetylation level of H3K9 was significantly decreased (P < 0.05) and the methylation level of H3K9 was significantly increased (P < 0.05) in the -P group compared to the +P group. In summary, our findings demonstrate that pyruvate affects early embryonic development in pigs by promoting ZGA and reducing oxidative stress levels.

Homotypic clustering of L1 and B1/Alu repeats compartmentalizes the 3D genome

Organization of the genome into euchromatin and heterochromatin appears to be evolutionarily conserved and relatively stable during lineage differentiation. In an effort to unravel the basic principle underlying genome folding, here we focus on the genome itself and report a fundamental role for L1 (LINE1 or LINE-1) and B1/Alu retrotransposons, the most abundant subclasses of repetitive sequences, in chromatin compartmentalization. We find that homotypic clustering of L1 and B1/Alu demarcates the genome into grossly exclusive domains, and characterizes and predicts Hi-C compartments. Spatial segregation of L1-rich sequences in the nuclear and nucleolar peripheries and B1/Alu-rich sequences in the nuclear interior is conserved in mouse and human cells and occurs dynamically during the cell cycle. In addition, de novo establishment of L1 and B1 nuclear segregation is coincident with the formation of higher-order chromatin structures during early embryogenesis and appears to be critically regulated by L1 and B1 transcripts. Importantly, depletion of L1 transcripts in embryonic stem cells drastically weakens homotypic repeat contacts and compartmental strength, and disrupts the nuclear segregation of L1- or B1-rich chromosomal sequences at genome-wide and individual sites. Mechanistically, nuclear co-localization and liquid droplet formation of L1 repeat DNA and RNA with heterochromatin protein HP1α suggest a phase-separation mechanism by which L1 promotes heterochromatin compartmentalization. Taken together, we propose a genetically encoded model in which L1 and B1/Alu repeats blueprint chromatin macrostructure. Our model explains the robustness of genome folding into a common conserved core, on which dynamic gene regulation is overlaid across cells.

Histone Demethylase KDM4D Could Improve the Developmental Competence of Buffalo (Bubalus Bubalis) Somatic Cell Nuclear Transfer (SCNT) Embryos

Somatic cell nuclear transfer (SCNT) holds vast potential in agriculture. However, its applications are still limited by its low efficiency. Histone 3 lysine 9 trimethylation (H3K9me3) was identified as an epigenetic barrier for this. Histone demethylase KDM4D could regulate the level of H3K9me3. However, its effects on buffalo SCNT embryos are still unclear. Thus, we performed this study to explore the effects and underlying mechanism of KDM4D on buffalo SCNT embryos. The results revealed that compared with the IVF embryos, the expression level of KDM4D in SCNT embryos was significantly lower at 8- and 16-cell stage, while the level of H3K9me3 in SCNT embryos was significantly higher at 2-cell, 8-cell, and blastocyst stage. Microinjection of KDM4D mRNA could promote the developmental ability of buffalo SCNT embryos. Furthermore, the expression level of ZGA-related genes such as ZSCAN5B, SNAI1, eIF-3a, and TRC at the 8-cell stage was significantly increased. Meanwhile, the pluripotency-related genes like POU5F1, SOX2, and NANOG were also significantly promoted at the blastocyst stage. The results were reversed after KDM4D was inhibited. Altogether, these results revealed that KDM4D could correct the H3K9me3 level, increase the expression level of ZGA and pluripotency-related genes, and finally, promote the developmental competence of buffalo SCNT embryos.

Maternal UHRF1 Is Essential for Transcription Landscapes and Repression of Repetitive Elements During the Maternal-to-Zygotic Transition

Maternal factors that modulate maternal-to-zygotic transition (MZT) are essential for the growth from specialized oocytes to totipotent embryos. Despite several studies, the mechanisms regulating epigenetic reprogramming during MZT remain largely elusive. UHRF1 plays a role in maintaining GC methylation in oocytes and early embryos. However, little is known about its role in mouse MZT. Here, we explored the function of maternal UHRF1 in zygotic genome regulation during early embryonic development in mice. We showed that the conditional knockout (cKO) of UHRF1 in either primordial or growing oocytes causes infertility but differentially affects early embryonic development. UHRF1 deficiency in primordial oocytes led to early embryonic developmental arrest at the two-cell stage, accompanied by significant alterations in global DNA and H3K4me3 methylation patterns. In comparison, UHRF1 ablation in growing oocytes significantly reduced developmental competence from two-cell embryos to blastocysts. At the transcriptional level, the absence of maternal UHRF1 led to aberrant transcriptional regulation of the zygotic genome during MZT at the two-cell stage. Furthermore, we observed that retrotransposable elements in UHRF1-deficient oocytes and embryos were not silenced properly; in particular, the LINE-1 and long terminal repeat (LTR) subfamily were activated abnormally. Collectively, the findings of our study reveal that maternal UHRF1 plays a critical role in establishing the correct epigenetic chromatin reprogramming of early embryos, regulating essential genes during MZT, and preserving genome integrity that drives early embryonic development in mice.

Reparative effects of lycium barbarum polysaccharide on mouse ovarian injuries induced by repeated superovulation

To explore the repair effect of lycium barbarum polysaccharide (LBP) on ovarian injuries induced by repeated superovulation in mice, a model of ovarian injury was established, and ovarian repair was assessed after intragastric administration of LBP. The oocyte quality and blastocyst rates of pronuclear embryos in vitro were observed. The levels of 8-hydroxydeoxyguanosine (8-OHdG) and lipid peroxide (LPO) in ovarian tissue were measured, and ovarian damage was assessed in paraffin sections. The groups with significant injury were selected according to the above observation, mice in the significant injury group were intragastrically administered with LBP (low dose, 25 mg/kg; medium dose, 35 mg/kg; and high dose, 45 mg/kg) for 30 days. The above measurements and anti-Müllerian hormone (AMH) expression were detected in the mouse ovaries and the breeding verification was carried out. Our results showed that repeated superovulation could cause mouse oocyte quality to drop, significant differences started from 4 superovulation events (P < 0.05). The levels of 8-OHdG and LPO in the ovary increased gradually as the number of superovulation events increased, and significant differences were observed after 4-6 superovulations (P < 0.05). The ratios of primordial follicles, primary, tertiary and mature follicles decreased and the ratio of atresia follicles increased as the number of superovulation events increased, especially in 4-6 superovulation groups. Thus, the groups of superovulation 4-6 events were considered as significant injury groups. LBP-medium dose groups significantly improved the number and quantity of oocytes and embryo blastocyst rate (P < 0.05), significantly decreased 8-OHdG and LPO levels in mice ovary (P < 0.05), also improved the ratios of all stages follicles and reduced the rate of atresia follicles, increased the numbers of litter size, live birth, weaning survival, and repaired the expression of AMH in ovary significantly (P < 0.05). In conclusion, the degree of ovarian injury was affected by the number of superovulation. LBP repaired ovarian injuries most likely through scavenging oxidative products 8-OHdG and LPO and increasing AMH protein expression.

Exchanges of histone methylation and variants during mouse zygotic genome activation

Minor and major zygotic genome activation (ZGA) are crucial for preimplantation development. During this process, histone variants and methylation influence chromatin accessibility and consequently regulated the expression of zygotic genes. However, the detailed exchanges of these modifications during ZGA remain to be determined. In the present study, the epigenetic modifications of histone 3 on lysine 9 (H3K9), 27 (H3K27) and 36 (H3K36), as well as four histone variants were determined during minor and major ZGA and in post-ZGA stages of mouse embryos. Firstly, microH2A1, H3K27me3 and H3K36me3 were asymmetrically stained in the female pronucleus during minor ZGA but lost staining in major ZGA. Secondly, H3K9me2 and H3K9me3 were strongly stained in the female pronucleus, but weakly stained in the male pronucleus and disappeared after ZGA. Thirdly, H2A.Z and H3.3 were symmetrically stained in male and female pronuclei during minor ZGA. Moreover, H3K27me2 was not statistically changed during mouse early development, while H3K36me2 was only detected in 2- and 4-cell embryos. In conclusion, our data revealed dynamics of histone methylation and variants during mice ZGA and provided details of their exchange in mice embryogenesis. Moreover, we further inferred that macroH2A1, H2A.Z, H3K9me2/3 and H3K27me2/3 may play crucial roles during mouse ZGA.

Factors and molecules that could impact cell differentiation in the embryo generated by nuclear transfer

Somatic cell nuclear transfer is a technique to create an embryo using an enucleated oocyte and a donor nucleus. Nucleus of somatic cells must be reprogrammed in order to participate in normal development within an enucleated egg. Reprogramming refers to the erasing and remodeling of cellular epigenetic marks to a lower differentiation state. Somatic nuclei must be reprogrammed by factors in the oocyte cytoplasm to a rather totipotent state since the reconstructed embryo must initiate embryo development from the one cell stage to term. In embryos reconstructed by nuclear transfer, the donor genetic material must respond to the cytoplasmic environment of the cytoplast and recapitulate this normal developmental process. Enucleation is critically important for cloning efficiency because may affect the ultrastructure of the remaining cytoplast, thus resulting in a decline or destruction of its cellular compartments. Nonetheless, the effects of in vitro culturing are yet to be fully understood. In vitro oocyte maturation can affect the abundance of specific transcripts and are likely to deplete the developmental competence. The epigenetic modifications established during cellular differentiation are a major factor determining this low efficiency as they act as epigenetic barriers restricting reprogramming of somatic nuclei. In this review we discuss some factors that could impact cell differentiation in embryo generated by nuclear transfer.

Preincubation with glutathione ethyl ester improves the developmental competence of vitrified mouse oocytes

PURPOSE

Oocyte vitrification is currently used for human fertility preservation. However, vitrification damage is a problem caused by decreasing ooplasmic levels of glutathione (GSH). The GSH donor glutathione ethyl ester (GSH-OEt) can significantly increase the GSH content in oocytes. However, it is difficult to obtain oocyte from woman. To overcome this, we used mouse oocytes to replace human oocytes as a model of study.

METHODS

Oocytes from B6D2F1 mice were preincubated for 30 min with 2.5 mmol/L GSH-OEt (GSH-OEt group), without GSH-OEt preincubation before vitrification (control vitrification group) or in nonvitrified oocytes (fresh group). After thawing, oocytes were fertilized for evaluating the developmental competence of embryos in vitro and in vivo. Immunofluorescence, Polscope equipment and quantitative reverse transcription polymerase chain reaction (RT-qPCR) were used to analyze damage, including mitochondrial distribution, reactive oxygen species (ROS) levels, spindle morphology, and gene expression levels (Bcl-2, BAX, and MnSOD).

RESULTS

The rates of fertilization, 3-4 cell, blastocyst formation and expanded blastocysts were significantly higher (p < 0.05) in the GSH-OEt group (90.4%; 91.1%; 88.9% and 63.0%) than in the control (80.0%; 81.4%; 77.7% and 50.5%). Provided embryos overcame the 2-cell block and developed to the blastocyst stage, birth rates of all groups were similar. Vitrification altered mitochondrial distribution, increased ROS levels, and caused abnormal spindle morphology; GSH-OEt preincubation could improve such damage. RT-qPCR showed that the expression of Bcl-2 was lower in the control group compared with the GSH-OEt group; BAX and MnSoD expression levels were higher in the control group than in the GSH-OEt group (p < 0.05).

CONCLUSIONS

The beneficial effect of GSH-OEt preincubation occurred before the 2-cell stage.

Allelic reprogramming of 3D chromatin architecture during early mammalian development

In mammals, chromatin organization undergoes drastic reprogramming after fertilization. However, the three-dimensional structure of chromatin and its reprogramming in preimplantation development remain poorly understood. Here, by developing a low-input Hi-C (genome-wide chromosome conformation capture) approach, we examined the reprogramming of chromatin organization during early development in mice. We found that oocytes in metaphase II show homogeneous chromatin folding that lacks detectable topologically associating domains (TADs) and chromatin compartments. Strikingly, chromatin shows greatly diminished higher-order structure after fertilization. Unexpectedly, the subsequent establishment of chromatin organization is a prolonged process that extends through preimplantation development, as characterized by slow consolidation of TADs and segregation of chromatin compartments. The two sets of parental chromosomes are spatially separated from each other and display distinct compartmentalization in zygotes. Such allele separation and allelic compartmentalization can be found as late as the 8-cell stage. Finally, we show that chromatin compaction in preimplantation embryos can partially proceed in the absence of zygotic transcription and is a multi-level hierarchical process. Taken together, our data suggest that chromatin may exist in a markedly relaxed state after fertilization, followed by progressive maturation of higher-order chromatin architecture during early development.

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.

A novel long intergenic noncoding RNA indispensable for the cleavage of mouse two-cell embryos

Endogenous retroviruses (ERVs) are transcriptionally active in cleavage stage embryos, yet their functions are unknown. ERV sequences are present in the majority of long intergenic noncoding RNAs (lincRNAs) in mouse and humans, playing key roles in many cellular processes and diseases. Here, we identify LincGET as a nuclear lincRNA that is GLN-, MERVL-, and ERVK-associated and essential for mouse embryonic development beyond the two-cell stage. LincGET is expressed in late two- to four-cell mouse embryos. Its depletion leads to developmental arrest at the late G2 phase of the two-cell stage and to MAPK signaling pathway inhibition. LincGET forms an RNA-protein complex with hnRNP U, FUBP1, and ILF2, promoting the cis-regulatory activity of long terminal repeats (LTRs) in GLN, MERVL, and ERVK (GLKLTRs), and inhibiting RNA alternative splicing, partially by downregulating hnRNP U, FUBP1, and ILF2 protein levels. Hnrnpu or Ilf2 mRNA injection at the pronuclear stage also decreases the preimplantation developmental rate, and Fubp1 mRNA injection at the pronuclear stage causes a block at the two-cell stage. Thus, as the first functional ERV-associated lincRNA, LincGET provides clues for ERV functions in cleavage stage embryonic development.

Quantitative imaging of lipids in live mouse oocytes and early embryos using CARS microscopy

Mammalian oocytes contain lipid droplets that are a store of fatty acids, whose metabolism plays a substantial role in pre-implantation development. Fluorescent staining has previously been used to image lipid droplets in mammalian oocytes and embryos, but this method is not quantitative and often incompatible with live cell imaging and subsequent development. Here we have applied chemically specific, label-free coherent anti-Stokes Raman scattering (CARS) microscopy to mouse oocytes and pre-implantation embryos. We show that CARS imaging can quantify the size, number and spatial distribution of lipid droplets in living mouse oocytes and embryos up to the blastocyst stage. Notably, it can be used in a way that does not compromise oocyte maturation or embryo development. We have also correlated CARS with two-photon fluorescence microscopy simultaneously acquired using fluorescent lipid probes on fixed samples, and found only a partial degree of correlation, depending on the lipid probe, clearly exemplifying the limitation of lipid labelling. In addition, we show that differences in the chemical composition of lipid droplets in living oocytes matured in media supplemented with different saturated and unsaturated fatty acids can be detected using CARS hyperspectral imaging. These results demonstrate that CARS microscopy provides a novel non-invasive method of quantifying lipid content, type and spatial distribution with sub-micron resolution in living mammalian oocytes and embryos.

Expression of DNA repair genes in porcine oocytes before and after fertilization by ICSI using freeze-dried sperm

Boar sperm freeze-dried with trehalose showed a protective effect against sperm DNA fragmentation. However, normal fertilization and embryonic development were not improved. Damaged sperm may activate maternal DNA repair genes when injected into oocytes. Therefore, we investigated the expression profile of some DNA repair genes in porcine oocytes after intra-cytoplasmic sperm injection. First, the expression levels of MGMT, UDG, XPC, MSH2, XRCC6 and RAD51 genes that are concerned with different types of DNA repair were examined in in vitro mature (IVM) oocytes injected with ejaculated sperm, or freeze-dried sperm with or without trehalose. Quantitative reverse transcription polymerase chain reaction revealed that expression of six DNA repair genes in the oocytes at 4 h after injection did not differ among the four groups. Next, we investigated the gene expression levels of these genes at different stages of maturation. The relative expression levels of UDG and XPC were significantly up-regulated in mature oocytes compared with earlier stages. Furthermore, there was an increased tendency in relative expression of MSH2 and RAD51. These results suggested two possible mechanisms that messenger RNA of DNA repair genes are either accumulated during IVM to be ready for fertilization or increased expression levels of DNA repair genes in oocytes caused by suboptimal IVM conditions.

Effects of alcohol on H3K9 acetylation in mouse pre-implantation embryos

It is well known that excessive long-term alcohol consumption is harmful, especially in pregnant women. In the present study, the Kunming white mouse was used as an animal model and indirect immunofluorescence was performed to analyze the toxic effects of alcohol on early pre-implantation embryos. H3K9 acetylation immunofluorescence could not be detected in MII oocytes. H3K9 acetylation levels in the treatment group were higher than in the control group during the morula stage, and contrary to results during the blastocyst stage. Other stages showed no obvious differences for in vivo embryos. For in vitro embryos, almost no difference was found between the two experimental groups across all stages, and both groups showed increasing H3K9 acetylation levels (except at the 2-cell stage). This study shows that H3K9 acetylation levels in early pre-implantation embryos are notably impacted by excessive alcohol ingestion by females. These data are the first step in understanding the epigenetic mechanism of alcohol toxicity in early pre-implantation mouse embryos.

Associations among Sebox and other MEGs and its effects on early embryogenesis

In a previous report, we identified Sebox as a new candidate maternal effect gene that is essential for embryonic development and primarily impacts the two-cell (2C) stage. The present study was conducted to determine the mechanism of action for Sebox in this capacity, as shown by changes in the expression levels of other known MEG mRNAs after Sebox RNA interference (RNAi) in oocytes. Sebox-knockdown metaphase II (Mll) oocytes displayed normal morphology, but among the 23 MEGs monitored, 8 genes were upregulated, and 15 genes were unchanged. We hypothesized that the perturbed gene expression of these MEGs may cause the arrest of embryo development at the 2C stage and examined the expression of several marker genes for the degradation of maternal factors and zygotic genome activation. We found that some maternal mRNAs, c-mos, Gbx2, and Gdf9, were not fully degraded in Sebox-knockdown 2C embryos, and that several zygotic genome activation markers, Mt1a, Rpl23, Ube2a and Wee1, were not fully expressed in conjunction with diminished embryonic transcriptional activity. In addition, Sebox may be involved in the formation of the subcortical maternal complex through its regulation of the upstream regulator, Figla. Therefore, we concluded that Sebox is important in preparing oocytes for embryonic development by orchestrating the expression of other important MEGs.

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.

Nuclear distribution of RNA polymerase II and mRNA processing machinery in early mammalian embryos

Spatial distribution of components of nuclear metabolism provides a significant impact on regulation of the processes of gene expression. While distribution of the key nuclear antigens and their association with the defined nuclear domains were thoroughly traced in mammalian somatic cells, similar data for the preimplantation embryos are scanty and fragmental. However, the period of cleavage is characterized by the most drastic and dynamic nuclear reorganizations accompanying zygotic gene activation. In this minireview, we try to summarize the results of studies concerning distribution of major factors involved in RNA polymerase II-dependent transcription, pre-mRNA splicing mRNA export that have been carried out on early embryos of mammals.

Epigallocatechin gallate promotes the development of mouse 2-cell embryos in vitro by regulating mitochondrial activity and expression of genes related to p53 signalling pathway

Preliminary studies have found that the epigallocatechin gallate (EGCG) at proper concentration could promote development of pre-implantation mouse embryos in vitro. However, the underlying mechanisms have not been well understood. In this study, we collected 1-cell embryos from Kunming (KM) mice, cultured them in M16 medium or M16 medium supplemented with 10 μg/mL EGCG and investigated the effects of EGCG on mitochondrial activity and reactive oxygen species (ROS) level of 2-cell embryos. Furthermore, we explored expression differences of genes related to p53 signalling pathway in 2-cell embryos using a PCR array. The results showed that ROS level and mitochondrial membrane potential were significantly lower in embryos cultured in the EGCG group than in the M16 group (p < 0.05), while the adenosine triphosphate content was slightly lower than in the M16 group (p > 0.05). PCR array test results showed that 18 genes were differentially expressed, among which eight genes involving cell growth, cell cycle regulation and mRNA transcription were up-regulated and 10 genes involving apoptosis, cell cycle arrest and DNA repair were down-regulated in the EGCG groups. It is concluded that EGCG could promote the development of 1-cell embryos in vitro possibly due to its ability to scavenge ROS and regulate mitochondrial activity. In addition, EGCG could influence expression of genes related to p53 signalling pathway in 2-cell embryos and promote cell cycle progression.

Abnormality of maternal-to-embryonic transition contributes to MEHP-induced mouse 2-cell block

Mono (2-ethylhexyl) phthalate (MEHP), an environmental contaminant, is known to cause many serious diseases, especially in reproductive system. However, little is known about the effect of MEHP on preimplantation embryo development. In this study, we found that the development of mouse 2-cell embryo was blocked by 10(-3)  M MEHP. A significant increase in the level of reactive oxygen species (ROS) was observed in arrested 2-cell embryo following 10(-3)  M MEHP treatment for 24 h. However, antioxidants, catalase (CAT), and superoxide dismutase (SOD), reduced intracellular ROS and protected MEHP-exposed embryos from death but failed to return the arrested embryos. Further experiments demonstrated that the level of apoptosis was not altered in live arrested 2-cell embryo and increased in dead arrested 2-cell embryo after MEHP treatment, which implied that ROS and apoptosis were not related with 2-cell block. During analysis of the indicators of embryonic genome activation (EGA) initiation (Hsc70, MuERV-L, Hsp70.1, eIF-1A, and Zscan4) and maternal-effect genes (OCT4 and SOX2), we found that MEHP treatment could significantly decline Hsc70, MuERV-L mRNA level and SOX2 protein level, and markedly enhance Hsp70.1, eIF-1A, Zscan4 mRNA level, and OCT4 protein level at 2-cell to 4-cell stage. Supplementation of CAT and SOD did not reverse the expression tendency of EGA related genes. Collectively, this study demonstrates for the first time that MEHP-induced 2-cell block is mediated by the failure of EGA onset and maternal-effect genes, not oxidative stress and apoptosis.

The maternal to zygotic transition in mammals

Prior to activation of the embryonic genome, the initiating events of mammalian development are under maternal control and include fertilization, the block to polyspermy and processing sperm DNA. Following gamete union, the transcriptionally inert sperm DNA is repackaged into the male pronucleus which fuses with the female pronucleus to form a 1-cell zygote. Embryonic transcription begins during the maternal to zygotic transfer of control in directing development. This transition occurs at species-specific times after one or several rounds of blastomere cleavage and is essential for normal development. However, even after activation of the embryonic genome, successful development relies on stored maternal components without which embryos fail to progress beyond initial cell divisions. Better understanding of the molecular basis of maternal to zygotic transition including fertilization, the activation of the embryonic genome and cleavage-stage development will provide insight into early human development that should translate into clinical applications for regenerative medicine and assisted reproductive technologies.

In vitro and in vivo development of mice morulae after storage in non-frozen conditions

BACKGROUND

Interchange of genetically modified (GM) mice between laboratories using embryos provides several advantages. Not only is transport stress avoided, but also the health status of the recipient colony is not compromised. Embryos do not need to be shipped in frozen stage, which requires expensive packaging in addition to a certain degree of expertise in order to freeze and thaw them correctly. The aim of this study was to examine different storage conditions and their effect on embryo viability in order to establish the feasibility of practical, non-frozen conditions for embryo shipment.

METHODS

Mouse morulae developed in vivo (collected from donors 2.5d post coitum) or in vitro (zygotes cultured until morulae stage) were stored, combining two different media (KSOMeq or KSOM-H) and temperatures (4 degrees C, 15 degrees C and 37 degrees C) throughout 24 or 48 hours. After storage in vitro viability was assessed determining percentage of development to blastocyst and total cell number. In vivo viability was determined based on the number of implantations and living fetuses after embryo transfer of stored embryos. The storage effect at the molecular level was assessed by studying a gene pool involved in early development by quantitative RT-PCR.

RESULTS

In vivo-produced morulae stored for 24 hours did not show differences in development up to the blastocyst stage, regardless of the storage type. Even though a decrease in the total cell number in vivo was observed, embryo development after embryo transfer was not affected. All 24 hour storage conditions tested provided a similar number of implantations and fetuses at day 14 of pregnancy. Morulae obtained from in vitro embryo culture collected at the 1-cell stage showed a decreased ability to develop to blastocyst after 24 hours of storage at 15degrees C both in KSOMeq and KSOM-H. Concomitantly, a significant decrease of embryo implantation rates after transfer to recipients was also found. In order to further characterize the effect of non-frozen storage combining a molecular approach with the ordinary in vitro culture evaluation, embryos collected at the morula stage were submitted to the same storage conditions described throughout 48 hours. In vitro culture of those embryos showed a significant decrease in their developmental rate to blastocyst in both KSOMeq and KSOM-H at 15degrees C, which also affected the total number of cells. Gene transcription studies confirmed significant alterations in retrotransposons (Erv4 and Iap) after 48 h of storage at 15degrees C.

CONCLUSIONS

Our results show that both KSOMeq and KSOM-H can be equally used, and that several temperature conditions allow good survival rates in vitro and in vivo. Some of these storage conditions can substitute freezing in order to maintain embryo viability for 24-48 hours, providing a reliable and less demanding technical alternative for embryo interchanges.

Mouse embryos stressed by physiological levels of osmolarity become arrested in the late 2-cell stage before entry into M phase

Preimplantation mouse embryos of many strains become arrested at the 2-cell stage if the osmolarity of culture medium that normally supports development to blastocysts is raised to approximately that of their normal physiological environment in the oviduct. Arrest can be prevented if molecules that serve as "organic osmolytes" are present in the medium, because organic osmolytes, principally glycine, are accumulated by embryos to provide intracellular osmotic support and regulate cell volume. Medium with an osmolarity of 310 mOsM induced arrest of approximately 80% of CF1 mouse embryos at the 2-cell stage, in contrast to the approximately 100% that progressed beyond the 2-cell stage at 250 or 301 mOsM with glycine. The nature of this arrest induced by physiological levels of osmolarity is unknown. Arrest was reversible by transfer to lower-osmolarity medium at any point during the 2-cell stage, but not after embryos would normally have progressed to the 4-cell stage. Cessation of development likely was not due to apoptosis, as shown by lack of external annexin V binding, detectable cytochrome c release from mitochondria, or nuclear DNA fragmentation. Two-cell embryos cultured at 310 mOsM progressed through the S phase, and zygotic genome activation markers were expressed. However, most embryos failed to initiate the M phase, as evidenced by intact nuclei with decondensed chromosomes, low M-phase promoting factor activity, and an inactive form of CDK1, although a few blastomeres were arrested in metaphase. Thus, embryos become arrested late in the G(2) stage of the second embryonic cell cycle when stressed by physiological osmolarity in the absence of organic osmolytes.

Correlation of the mitochondrial activity of two-cell embryos produced in vitro and the two-cell block in Kunming and B6C3F1 mice

The correlation between the early embryonic block to development and mitochondrial activity was investigated comparing two-cell embryos produced in vitro from Kunming (KM) and B6C3F1 mice. One-cell embryos were obtained from two species of hybrids (female KM mice mated with KM males and female B6C3F1 mice mated with KM males) and cultured for 84 hr in M16 media. The mitochondrial membrane potential, ATP content, and reactive oxygen species levels were measured in the resulting KM and B6C3F1 two-cell embryos. Mitochondrial membrane potential and ATP content were also determined in KM and B6C3F1 metaphase II eggs. The results showed that the two-cell block was observed in cultured KM embryos but not in B6C3F1 embryos. Mitochondrial membrane potential and ATP content of KM two-cell embryos were significantly lower than in B6C3F1 two-cell embryos (P < 0.01). Interestingly, the reactive oxygen species levels of KM two-cell embryos were significantly lower than their B6C3F1 counterparts (P < 0.01). There was no difference in mitochondrial membrane potential and ATP content between KM and B6C3F1 metaphase II eggs. It is concluded that KM mice have an early two-cell embryo block and that a possible "blocking" mechanism is the lower mitochondrial membrane potential and ATP content in these embryos. The results suggest a new approach for overcoming early embryonic development block, that of manipulating mitochondrial activity.

Acetylcholine rescues two-cell block through activation of IP3 receptors and Ca2+/calmodulin-dependent kinase II in an ICR mouse strain

Acetylcholine (ACh) causes early activation events in mouse oocytes, but little is known about its precise role in the early embryonic development of mice. We aimed to determine whether and how ACh is capable of rescuing two-cell block in an in vitro culture system. ACh evoked different transient Ca(2+) patterns showing a higher Ca(2+) peak in the two-cell stage embryos (two-cells) than observed in mature oocytes. In early two-cells subjected to an in vitro two-cell block, xestospongin C (Xes-C), an IP3 receptor antagonist, significantly decreased the level of the ACh-induced Ca(2+) increase. The reduction in the ACh-induced Ca(2+) increase by Xes-C in late two-cells was lower than that in early two-cells. Furthermore, KN62 and KN93, both CaMKII inhibitors, were found to reduce the magnitude of the ACh-induced Ca(2+) increase in early two-cells. The addition of ACh to the culture medium showed an ability to rescue in vitro two-cell block. However, the addition of ACh together with both Xes-C and CaMKII inhibitors or with either inhibitor separately had no effect on the rescue of two-cell block. Long-term exposure of late two-cells to ACh decreased morula and early blastocyst development and ACh had a differential effect on early and late two-cells. These results indicate that ACh likely rescues the in vitro two-cell block through activation of IP3R- and/or CaMKII-dependent signal transduction pathways.

Expression of eukaryotic elongation initiation factor 1A differentially marks zygotic genome activation in biparental and parthenogenetic porcine embryos and correlates with in vitro developmental potential

Zygotic genome activation (ZGA) is a major event during cleavage development. In vitro manipulation of mammalian embryos (including embryo culture) can result in developmental arrest around the time of ZGA. Eukaryotic elongation initiation factor 1A (eIF1A) has been used as a marker for ZGA in some mammalian species. We hypothesised expression of eIF1A can be used to assess ZGA in the pig; we also hypothesised that the expression profile of eIF1A can be used to assess developmental potential in vitro. The aims of the present study were to determine the expression pattern of eIF1A during porcine cleavage development and to assess its expression levels in embryos of different quality. We used a real-time reverse transcription-polymerase chain reaction assay to quantify eIF1A transcripts at different time points during cleavage development in porcine embryos produced by parthenogenetic activation (PA) and in vitro fertilisation (IVF). We found that eIF1A is activated at the two-cell stage in IVF embryos and at the four-cell stage in PA embryos. We showed that the increase in transcript levels observed in parthenogenetic embryos is dependent on de novo transcription. We found altered levels of eIF1A transcripts in parthenogenetic embryos that presented as either two- or eight-cell embryos 48 h after activation compared with four-cell embryos at the same time point. Our work supports the hypothesis that eIF1A is a marker of porcine ZGA and its expression profile can be used to assess embryo quality.

Tight junction protein ZO-2 expression and relative function of ZO-1 and ZO-2 during mouse blastocyst formation

Apicolateral tight junctions (TJs) between epithelial cells are multiprotein complexes regulating membrane polarity and paracellular transport and also contribute to signalling pathways affecting cell proliferation and gene expression. ZO-2 and other ZO family members form a sub-membranous scaffold for binding TJ constituents. We investigated ZO-2 contribution to TJ biogenesis and function during trophectoderm epithelium differentiation in mouse preimplantation embryos. Our data indicate that ZO-2 is expressed from maternal and embryonic genomes with maternal ZO-2 protein associated with nuclei in zygotes and particularly early cleavage stages. Embryonic ZO-2 assembled at outer blastomere apicolateral junctional sites from the late 16-cell stage. Junctional ZO-2 first co-localised with E-cadherin in a transient complex comprising adherens junction and TJ constituents before segregating to TJs after their separation from the blastocyst stage (32-cell onwards). ZO-2 siRNA microinjection into zygotes or 2-cell embryos resulted in specific knockdown of ZO-2 mRNA and protein within blastocysts. Embryos lacking ZO-2 protein at trophectoderm TJs exhibited delayed blastocoel cavity formation but underwent normal cell proliferation and outgrowth morphogenesis. Quantitative analysis of trophectoderm TJs in ZO-2-deficient embryos revealed increased assembly of ZO-1 but not occludin, indicating ZO protein redundancy as a compensatory mechanism contributing to the mild phenotype observed. In contrast, ZO-1 knockdown, or combined ZO-1 and ZO-2 knockdown, generated a more severe inhibition of blastocoel formation indicating distinct roles for ZO proteins in blastocyst morphogenesis.

Low protein diet fed exclusively during mouse oocyte maturation leads to behavioural and cardiovascular abnormalities in offspring

Early embryonic development is known to be susceptible to maternal undernutrition, leading to a disease-related postnatal phenotype. To determine whether this sensitivity extended into oocyte development, we examined the effect of maternal normal protein diet (18% casein; NPD) or isocaloric low protein diet (9% casein; LPD) restricted to one ovulatory cycle (3.5 days) prior to natural mating in female MF-1 mice. After mating, all females received NPD for the remainder of gestation and all offspring were litter size adjusted and fed standard chow. No difference in gestation length, litter size, sex ratio or postnatal growth was observed between treatments. Maternal LPD did, however, induce abnormal anxiety-related behaviour in open field activities in male and female offspring (P < 0.05). Maternal LPD offspring also exhibited elevated systolic blood pressure (SBP) in males at 9 and 15 weeks and in both sexes at 21 weeks (P < 0.05). Male LPD offspring hypertension was accompanied by attenuated arterial responsiveness in vitro to vasodilators acetylcholine and isoprenaline (P < 0.05). LPD female offspring adult kidneys were also smaller, but had increased nephron numbers (P < 0.05). Moreover, the relationship between SBP and kidney or heart size or nephron number was altered by diet treatment (P < 0.05). These data demonstrate the sensitivity of mouse maturing oocytes in vivo to maternal protein undernutrition and identify both behavioural and cardiovascular postnatal outcomes, indicative of adult disease. These outcomes probably derive from a direct effect of protein restriction, although indirect stress mechanisms may also be contributory. Similar and distinct postnatal outcomes were observed here compared with maternal LPD treatment during post-fertilization preimplantation development which may reflect the relative contribution of the paternal genome.

Zygotic gene activation and maternal factors in mammals

Zygotic gene activation (ZGA) is the first event of gene expression after fertilization. Following fertilization, ZGA occurs within a short time interval depending on the animal species. Until ZGA, maternal proteins and transcripts stored in oocytes control embryonic development, indicating the importance of maternal factors for development. Somatic cell cloning also proves the potential of oocyte to reprogram the differentiated cell nuclei to embryonic nuclei. Recent studies show that the epigenetic modifications of nuclei play important roles in controlling gene expression during ZGA. However, the mechanisms that control ZGA remain largely unknown. This review will cover the current understanding of ZGA. Specifically, it will focus on the maternal factors that control gene expression during early embryogenesis.

Timing and Uniformity of Embryonic Gene Activation Affect Subsequent Pre-implantation Development of Cloned Bovine Embryos

In the present study, we examined the timing of onset, intensity, and mosaicism of embryonic gene expression in bovine nuclear transfer (NT) embryos. The relationship between gene expression and early embryonic development was also examined. To monitor the gene expression of NT embryos, we produced NT embryos with bovine transfected fibroblasts carrying a firefly luciferase gene under the control of a chicken beta-actin promoter, an expression system that has previously been shown to be representative of embryonic gene expression in mice. Photon count imaging showed that luciferase luminescence began in NT embryos with fibroblasts 48 hours post fusion (hpf) and reached a plateau at the 4- to 8-cell stage at 60 hpf. Only 4- to 8-cell NT embryos luminescent by 60 hpf developed to the blastocyst stage. At 60 hpf, strongly luminescent embryos developed to the blastocyst stage at a higher rate (P<0.05) than embryos with weak or absent luminescence. However, embryos with mosaic luminescence developed at a much lower rate (P<0.05) than those with whole-embryo luminescence, even if the embryos exhibited strong luminescence. Our results indicate that precise and uniform embryonic gene expression at the 4- to 8-cell stage at 60 hpf may be closely related to development of bovine NT embryos to the blastocyst stage.

Nuclear structure in early mouse embryos: A comparative ultrastructural and immunocytochemical study with special emphasis on the "2-cell block in vitro"

The embryos from many outbred and inbred strains of mice are arrested at the late 2-cell stage when cultured in vitro in simple culture media. This phenomenon is referred to as the "2-cell block in vitro". The ultrastructural morphology of the nuclei of the blocked embryos is not yet well described. In the present paper we documented the results of a comparative study on the nuclei of mouse embryos, both normally developing and arrested at the 2-cell stage. The blocked embryos maintain the morphological integrity of their nuclei. Main nuclear domains (nucleolus precursor bodies, interchromatin granule clusters, perichromatin granules, and perichromatin fibrils), typical for the control embryos, are observed in the blocked ones. A number and morphological characteristics of these nuclear substructures are not changed significantly in the blocked embryos. At the same time, RNA polymerase II and pre-mRNA splicing factors are redistributed in the nucleus of the blocked embryos. Although something goes to show that nuclear organization of the blocked embryos differ from that of the control, we could not reveal in the blocked embryos distinct signs of degeneration which might characterize aged or dying cells. Our data confirm a peculiar functional state of the 2-cell blocked embryos.

Gene expression profiling of the pre-implantation mouse embryo by microarray analysis: comparison of the two-cell stage and two-cell block

To improve our understanding of the molecular mechanisms underlying early embryo development, further characterization of gene activity in oocytes and embryos is urgently required. The transition from the two-cell to four-cell stage is particularly important in pre-implantation embryonic development, as it involves transcriptional reprogramming and cellular differentiation. In this study, we used a 7.4 K cDNA microarray to screen mRNA transcript levels in the pre-implantation mouse embryo. Real-time PCR was used to confirm microarray data. We profiled 7,410 genes and identified 4,562 genes that were differentially expressed in the pre-implantation embryo. We selected a total of 248 genes with significant expression changes that are functionally involved in the two-cell and two-cell block embryo. Of these genes, 114 were down-regulated and the remainder (n=134) were up-regulated in the two-cell embryo. This study provides a developmental map of a large number of genes in the pre-implantation mouse embryo with particular emphasis on gene expression in the two-cell embryo and two-cell block embryo. Further investigations based on this data will provide a better understanding of the effects of various external conditions and may facilitate comparative analysis of pre-implantation development in other mammalian species, including human.