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.

Strategies to Improve the Efficiency of Somatic Cell Nuclear Transfer

Mammalian oocytes can reprogram differentiated somatic cells into a totipotent state through somatic cell nuclear transfer (SCNT), which is known as cloning. Although many mammalian species have been successfully cloned, the majority of cloned embryos failed to develop to term, resulting in the overall cloning efficiency being still low. There are many factors contributing to the cloning success. Aberrant epigenetic reprogramming is a major cause for the developmental failure of cloned embryos and abnormalities in the cloned offspring. Numerous research groups attempted multiple strategies to technically improve each step of the SCNT procedure and rescue abnormal epigenetic reprogramming by modulating DNA methylation and histone modifications, overexpression or repression of embryonic-related genes, etc. Here, we review the recent approaches for technical SCNT improvement and ameliorating epigenetic modifications in donor cells, oocytes, and cloned embryos in order to enhance cloning efficiency.

Overexpression of MBD3 Improves Reprogramming of Cloned Pig Embryos

Methyl-CpG-binding domain protein 3 (MBD3) is a core component of the nucleosome remodeling and deacetylase (NuRD) complex, which is crucial for pluripotent stem cell differentiation and embryonic development. MBD3 was shown to play important roles in transcription factor-induced somatic cell reprogramming. Expression level of MBD3 was demonstrated to be higher in somatic cell nuclear transfer-generated cloned pig embryos than in fertilization-derived porcine embryos. However, the functions of MBD3 in nuclear transfer-mediated somatic cell reprogramming are unknown. In this study, MBD3 was overexpressed in cloned pig embryos, and the effects of MBD3 overexpression on gene transcription, DNA methylation, and in vitro developmental competence of cloned pig embryos were analyzed. Results indicated that overexpression of MBD3 in cloned pig embryos not only increased blastocyst rate and number of cells per blastocyst but also upregulated mRNA expression levels and decreased the DNA methylation of NANOG, OCT4, and LINE1 genes to the levels close to those in in vivo fertilization-produced pig embryos. These findings suggest that overexpression of MBD3 improves reprogramming of cloned pig embryos.

Effects of trichostatin A on pig SCNT blastocyst formation rate and cell number: A meta-analysis

Although somatic cell nuclear transfer (SCNT) can be used to create transgenic pigs for human xenotransplantation, low efficiency limits its use. Trichostatin A (TSA) promotes SCNT embryo development, but whether TSA modifies SCNT blastocyst numbers is unclear. Thus, there is an urgent need to understand whether TSA modifies the rate and number of embryos that grow from oocytes to blastocysts in culture and what types of cell signaling pathways may be involved. Thus, we identified 63 reports, of which 13 are included in this meta-analysis. Data show that TSA significantly increased the SCNT blastocyst formation rate, but did not change blastocyst cell number. Due to study heterogeneity (I²>50%), we hypothesized that donor cells were of different backgrounds so we analyzed two donor cell subgroups: fetal and adult fibroblasts. Analysis of the fetal fibroblast subgroups showed no heterogeneity, but the adult fibroblast subgroups were heterogeneous, suggesting epigenetic reprogramming of fetal fibroblasts by TSA. Adult fibroblast heterogeneity may be complex and reprogramming by TSA is more difficult. Thus, TSA fibroblasts reprogramming is the source of heterogeneity in this meta-analysis. More work is needed to better understand how TSA influences SCNT pig embryonic development, and histone deacetylase inhibitors can be assessed with respect to SCNT pig embryos. Finally, efforts in epigenetic research may improve SCNT pig embryo outcomes.

Dynamics of post-translationally modified histones during barley pollen embryogenesis in the presence or absence of the epi-drug trichostatin A

Improving pollen embryogenesis. Despite the agro-economic importance of pollen embryogenesis, the mechanisms underlying this process are still poorly understood. We describe the dynamics of chromatin modifications (histones H3K4me2, H3K9ac, H3K9me2, and H3K27me3) and chromatin marks (RNA polymerase II CDC phospho-Ser5, and CENH3) during barley pollen embryogenesis. Immunolabeling results show that, in reaction to stress, immature pollen rapidly starts reorganizing several important chromatin modifications indicative of a change in cell fate. This new chromatin modification pattern was accomplished within 24 h from whereon it remained unaltered during subsequent mitotic activity. This indicates that cell fate transition, the central element of pollen embryogenesis, is completed early on during the induction process. Application of the histone deacetylase inhibitor trichostatin A stimulated pollen embryogenesis when used on pollen with a gametophytic style chromatin pattern. However, when this drug was administered to embryogenic pollen, the chromatin markers reversed toward a gametophytic profile, embryogenesis was halted and all pollen invariably died.

Supplement of autologous ooplasm into porcine somatic cell nuclear transfer embryos does not alter embryo development

Somatic cell nuclear transfer (SCNT) is considered as the technique in which a somatic cell is introduced into an enucleated oocyte to make a cloned animal. However, it is unavoidable to lose a small amount of the ooplasm during enucleation step during SCNT procedure. The present study was aimed to uncover whether the supplement of autologous ooplasm could ameliorate the oocyte competence so as to improve low efficiency of embryo development in porcine SCNT. Autologous ooplasm-transferred (AOT) embryos were generated by the supplementation with autologous ooplasm into SCNT embryos. They were comparatively evaluated with respect to embryo developmental potential, the number of apoptotic body formation and gene expression including embryonic lineage differentiation, apoptosis, epigenetics and mitochondrial activity in comparison with parthenogenetic, in vitro-fertilized (IVF) and SCNT embryos. Although AOT embryos showed perfect fusion of autologous donor ooplasm with recipient SCNT embryos, the supplement of autologous ooplasm could not ameliorate embryo developmental potential in regard to the rate of blastocyst formation, total cell number and the number of apoptotic body. Furthermore, overall gene expression of AOT embryos was presented with no significant alterations in comparison with that of SCNT embryos. Taken together, the results of AOT demonstrated inability to make relevant values improved from the level of SCNT embryos to their IVF counterparts.

Increasing of blastocyst rate and gene expression in co-culture of bovine embryos with adult adipose tissue-derived mesenchymal stem cells

PURPOSE

Despite advances in the composition of defined embryo culture media, co-culture with somatic cells is still used for bovine in vitro embryo production (IVEP) in many laboratories worldwide. Granulosa cells are most often used for this purpose, although recent work suggests that co-culture with stem cells of adult or embryonic origin or their derived biomaterials may improve mouse, cattle, and pig embryo development.

MATERIALS AND METHODS

In experiment 1, in vitro produced bovine embryos were co-cultured in the presence of two concentrations of bovine adipose tissue-derived mesenchymal cells (b-ATMSCs; 10³ and 10⁴ cells/mL), in b-ATMSC preconditioned medium (SOF-Cond), or SOF alone (control). In experiment 2, co-culture with 10⁴ b-ATMSCs/mL was compared to the traditional granulosa cell co-culture system (Gran).

RESULTS

In experiment 1, co-culture with 10⁴ b-ATMSCs/mL improved blastocyst rates in comparison to conditioned and control media (p < 0.05). Despite that it did not show difference with 10³ b-ATMSCs/mL (p = 0.051), group 10⁴ b-ATMSCs/mL yielded higher results of blastocyst production. In experiment 2, when compared to group Gran, co-culture with 10⁴ b-ATMSCs/mL improved not only blastocyst rates but also quality as assessed by increased total cell numbers and mRNA expression levels for POU5F1 and G6PDH (p < 0.05).

CONCLUSIONS

Co-culture of bovine embryos with b-ATMSCs was more beneficial than the traditional co-culture system with granulosa cells. We speculate that the microenvironmental modulatory potential of MSCs, by means of soluble substances and exosome secretions, could be responsible for the positive effects observed. Further experiments must be done to evaluate if this beneficial effect in vitro also translates to an increase in offspring following embryo transfer. Moreover, this study provides an interesting platform to study the basic requirements during preimplantation embryo development, which, in turn, may aid the improvement of embryo culture protocols in bovine and other species.

Combination of S-adenosylhomocysteine and scriptaid, a non-toxic epigenetic modifying reagent, modulates the reprogramming of bovine somatic-cell nuclear transfer embryos

The goal of this study was to improve the development of bovine somatic-cell nuclear transfer (SCNT) embryos by optimizing the combination of DNA methyltransferases inhibitor S-adenosylhomocysteine (SAH) and histone deacetylase inhibitor Scriptaid (SPD). A. 4 × 4-factor design of different drug combinations (0, 0.75, 1.0, and 1.5 mM SAH and 0, 5, 250, and 500 nM SPD) was used to identify an optimal combination of 0.75 mM SAH and 250 nM SPD that improved the developmental competence of bovine SCNT embryos. Further experiments using this combination revealed that methylation levels of CpG islands near exon 1 of the pluripotent gene SOX2; the epigenetic-related gene HDAC3 and DNMT3a; imprinted genes XIST and PEG3; as well as apoptosis-related genes BCL2 and BAX were returned to levels similar to those of in vitro fertilized (IVF) embryo after treatment, which also normalized transcript levels for these genes. This combination also returned global DNA methylation to a normal level, correcting H4K12ac levels while enhancing H3K9ac levels. Thus, the combined application of 0.75 mM SAH and 250 nM SPD can significantly improve the reprogramming of bovine SCNT embryos by stabilizing how embryos utilize their genomes.

Epigallocatechin-3-gallate and trichostatin A synergistically inhibit human lymphoma cell proliferation through epigenetic modification of p16INK4a

DNA methylation and histone deacetylation play important roles in the occurrence and development of cancers by inactivating the expression of tumor suppressors, including p16(INK4a), a cyclin-dependent kinase inhibitor. The present study investigated the effect of epigallocatechin-3-gallate (EGCG) alone or in combination with trichostatin A (TSA) on p16(INK4a) gene expression and growth in human malignant lymphoma CA46 cells. CA46 cell viability and cell cycle were analyzed; methylation of the p16(INK4a) gene was assessed by nested methylation-specific PCR (n-MSP). p16(INK4a )mRNA and protein expression was determined by real-time quantitative PCR and western blot analyses, respectively. Both EGCG and TSA alone inhibited CA46 cell proliferation; the combined treatment (6 µg/ml EGCG and 15 ng/ml TSA) significantly reduced CA46 cell proliferation from 24 to 96 h (all P<0.001). Cells treated with 24 µg/ml EGCG or the combination treatment (6 µg/ml EGCG and 15 ng/ml TSA) had lower proliferative indices when compared to the other groups. Co-treatment with EGCG and TSA decreased p16(INK4a) gene methylation, which coincided with increased p16(INK4a) mRNA and protein expression. Thus, EGCG and TSA synergistically reactivate p16(INK4a) gene expression in part through reducing promoter methylation, which may decrease CA46 cell proliferation.

Silencing histone deacetylase-specific isoforms enhances expression of pluripotency genes in bovine fibroblasts

Histone deacetylases (HDACs) catalyze deacetylation of histones that results in altered transcriptional activity. Inhibitors of HDACs have been shown to induce transcriptional changes that contribute positively to reprogramming somatic cells either by nuclear transfer or inducing a pluripotent state. However, the exact molecular mechanisms whereby HDAC inhibitors function and the specificity of the HDAC isoforms in cell reprogramming are not yet fully understood. Herein, we report the ability of individual isoform-specific HDACs to modulate endogenous expression of pluripotency-associated genes in bovine somatic cells. This in vitro study showed that a transient selective depletion of HDACs resulted in elevated mRNA levels of Oct-4, Sox2, and Nanog. In particular, we found that inhibition of specific HDAC isoforms using small interfering (si) RNA significantly increased expression of Nanog, a key factor required for totipotency induced by somatic cell nuclear transfer and for maintaining pluripotency in embryonic and induced pluripotent stem cells. Our study suggests that this gene might be the most susceptible to HDAC activity inhibition. Moreover, a regulatory role of the class III HDAC, SIRT3, on an Oct4-Sox2-Nanog transcriptional network was revealed. We observed the upregulation of pluripotency-related genes by depletion of SIRT3. SIRT3 is localized to mitochondria and is associated with energy metabolism processes, suggesting metabolic changes may be linked to reprogramming in bovine fibroblasts. In conclusion, we show that targeting selective HDACs can potentially be useful to enhance reprogramming and that sirtuins may play a pivotal role in somatic cell reprogramming by upregulating an Oct4-Sox2-Nanog transcriptional network. Dedifferentiating donor somatic cells by upregulating developmentally important genes through specific knockdown of epigenetic targets, in particular HDACs, may provide a path to improving livestock cloning and the in vitro production of pluripotent cells.