Generating different genetic expression patterns in the early embryo: insights from the mouse model

Sperm-borne mRNAs: potential roles in zygote genome activation and epigenetic inheritance

It is well accepted that sperm carry an RNA cargo with functions in early embryo development. However, most research has focused on the role of small RNAs, such as microRNAs, transfer RNAs and long non-coding RNAs, while protein-coding messenger RNAs (mRNAs) received less attention, even though they represent a substantial amount of the sperm RNA pool. Here, we curated mouse transcriptomic data from mature sperm and selected the most abundant mRNAs (above the 0.7 quantile). The obtained gene list was further filtered using two criteria: (i) mRNAs that are statistically higher in the one-cell embryo compared to the MII oocyte transcriptome, indicative of paternal mRNA contribution after fertilization; and (ii) mRNAs that are found bound to ribosomes in the one-cell embryo, indicative of positive translation in the zygote translatome. Our analysis identified 94 genes that form networks functionally involved in epigenetic chromatin organization, gene expression, RNA processing and translation during zygote genome activation. These findings underscore the significant role of sperm-borne mRNAs in early embryonic development and epigenetic inheritance, highlighting the need for further research to fully understand their functions.

Placenta-Related Parameters at Delivery in Relation to Folic Acid Supplementation in Different Pregnancies

Folic acid plays an important role in the synthesis, repair, and methylation of deoxyribonucleic acid (DNA). Currently, most studies have focused on the effects of periconceptional folic acid (FA) supplementation on fetal development, and there is still a lack of population-based research exploring the association between FA use during pregnancy and placental development. This study aimed to investigate the impacts of FA supplementation in different pregnancies on placenta-related parameters at delivery. The study included 2708 pregnant women recruited from Ma'anshan City, Anhui Province, China, between May 2013 and September 2014. Information on FA use from one month before conception to delivery was collected. Placental length, width, and thickness were measured. Multivariable logistic regression analysis was used to assess the effects of FA supplementation in different pregnancies on placenta-related parameters. Based on multiple regression analysis, propensity score weighting was adopted to enhance comparability between different FA supplementation groups. Compared with FA non-users, FA supplementation before conception was associated with increased placental width (0.241 cm, 95%CI: 0.052-0.429, p = 0.013) and increased placental surface area (6.398 cm2, 95%CI: 1.407-11.389, p = 0.012), and FA use in early/middle pregnancy was, respectively, related with increased placental thickness (0.061 cm, 95%CI: 0.004-0.117, p = 0.036; 0.066 cm, 95%CI: 0.004-0.129, p = 0.038). FA use before conception could increase placental width and area, and FA use in early/middle pregnancy could increase placental thickness. To confirm the findings, further investigations are needed.

Blastomeres of 8-cell mouse embryos differ in their ability to generate embryonic stem cells and produce lines with different transcriptional signatures

Embryonic stem cell (ESC) derivation from single blastomeres of 8-cell mouse embryos results in lower derivation rates than that from whole blastocysts, raising a biological question about the developmental potential of sister blastomeres. We aimed to assess the ability of 8-cell blastomeres to produce epiblast cells and ESC lines after isolation, and the properties of the resulting lines. Our results revealed unequal competence among sister blastomeres to produce ESC lines. At least half of the blastomeres possess a lower potential to generate ESCs, although culture conditions and blastomeres plasticity can redirect their non-pluripotent fate towards the epiblast lineage, allowing us to generate up to seven lines from the same embryo. Lines originated from the same embryo segregated into two groups according to their transcriptional signatures. While the expression of genes related to pluripotency and development was higher in one group, no differences were found in their trilineage differentiation ability. These results may help to improve our understanding of the ESC derivation process from single blastomeres and cell fate determination in the preimplantation mouse embryos.

DNA methylation reprogramming during oogenesis and interference by reproductive technologies: Studies in mouse and bovine models

The use of assisted reproductive technology (ART) to overcome fertility problems has continued to increase since the birth of the first baby conceived by ART over 30 years ago. Similarly, embryo transfer is widely used as a mechanism to advance genetic gain in livestock. Despite repeated optimisation of ART treatments, pre- and postnatal outcomes remain compromised. Epigenetic mechanisms play a fundamental role in successful gametogenesis and development. The best studied of these is DNA methylation; the appropriate establishment of DNA methylation patterns in gametes and early embryos is essential for healthy development. Superovulation studies in the mouse indicate that specific ARTs are associated with normal imprinting establishment in oocytes, but abnormal imprinting maintenance in embryos. A similar limited impact of ART on oocytes has been reported in cattle, whereas the majority of embryo-focused studies have used cloned embryos, which do exhibit aberrant DNA methylation. The present review discusses the impact of ART on oocyte and embryo DNA methylation with regard to data available from mouse and bovine models.

Rho-associated protein kinase regulates subcellular localisation of Angiomotin and Hippo-signalling during preimplantation mouse embryo development

The differential activity of the Hippo-signalling pathway between the outer- and inner-cell populations of the developing preimplantation mouse embryo directs appropriate formation of trophectoderm and inner cell mass (ICM) lineages. Such distinct signalling activity is under control of intracellular polarization, whereby Hippo-signalling is either supressed in polarized outer cells or activated in apolar inner cells. The central role of apical-basolateral polarization to such differential Hippo-signalling regulation prompted us to reinvestigate the role of potential upstream molecular regulators affecting apical-basolateral polarity. This study reports that the chemical inhibition of Rho-associated kinase (Rock) is associated with failure to form morphologically distinct blastocysts, indicative of compromised trophectoderm differentiation, and defects in the localization of both apical and basolateral polarity factors associated with malformation of tight junctions. Moreover, Rock-inhibition mediates mislocalization of the Hippo-signalling activator Angiomotin (Amot), to the basolateral regions of outer cells and is concomitant with aberrant activation of the pathway. The Rock-inhibition phenotype is mediated by Amot, as RNAi-based Amot knockdown totally rescues the normal suppression of Hippo-signalling in outer cells. In conclusion, Rock, via regulating appropriate apical-basolateral polarization in outer cells, regulates the appropriate activity of the Hippo-signalling pathway, by ensuring correct subcellular localization of Amot protein in outer cells.

The first two cell-fate decisions of preimplantation mouse embryo development are not functionally independent

During mouse preimplantation embryo development, three distinct cell lineages are formed, represented by the differentiating trophectoderm (TE), primitive endoderm (PrE) and the pluripotent epiblast (EPI). Classically, lineage derivation has been presented as a two-step process whereby outer TE cells are first segregated from inner-cell mass (ICM), followed by ICM refinement into either the PrE or EPI. As ICM founders can be produced following the fourth or fifth cleavage divisions, their potential to equally contribute to EPI and PrE is contested. Thus, modelling the early sequestration of ICM founders from TE-differentiation after the fourth cleavage division, we examined ICM lineage contribution of varying sized cell clones unable to initiate TE-differentiation. Such TE-inhibited ICM cells do not equally contribute to EPI and PrE and are significantly biased to form EPI. This bias is not caused by enhanced expression of the EPI marker Nanog, nor correlated with reduced apical polarity but associated with reduced expression of PrE-related gene transcripts (Dab2 and Lrp2) and down-regulation of plasma membrane associated Fgfr2. Our results favour a unifying model were the three cell lineages are guided in an integrated, yet flexible, fate decision centred on relative exposure of founder cells to TE-differentiative cues.

Cell signalling during blastocyst morphogenesis

Blastocyst morphogenesis is prepared for even before fertilisation. Information stored within parental gametes can influence both maternal and embryonic gene expression programmes after egg activation at fertilisation. A complex network of intrinsic, cell-cell mediated and extrinsic, embryo-environment signalling mechanisms operates throughout cleavage, compaction and cavitation. These signalling events not only ensure developmental progression, cell differentiation and lineage allocation to inner cell mass (embryo proper) and trophectoderm (future extraembryonic lineages) but also provide a degree of developmental plasticity ensuring survival in prevailing conditions by adaptive responses. Indeed, many cellular functions including differentiation, metabolism, gene expression and gene expression regulation are subject to plasticity with short- or long-term consequences even into adult life. The interplay between intrinsic and extrinsic signals impacting on blastocyst morphogenesis is becoming clearer. This has been best studied in the mouse which will be the focus of this chapter but translational significance to human and domestic animal embryology will be a focus in future years.

Genes and Conditions Controlling Mammalian Pre- and Post-implantation Embryo Development

Embryo quality during the in vitro developmental period is of great clinical importance. Experimental genetic studies during this period have demonstrated the association between specific gene expression profiles and the production of healthy blastocysts. Although the quality of the oocyte may play a major role in embryo development, it has been well established that the post - fertilization period also has an important and crucial role in the determination of blastocyst quality. A variety of genes (such as OCT, SOX2, NANOG) and their related signaling pathways as well as transcription molecules (such as TGF-β, BMP) have been implicated in the pre- and post-implantation period. Furthermore, DNA methylation has been lately characterized as an epigenetic mark since it is one of the most important processes involved in the maintenance of genome stability. Physiological embryo development appears to depend upon the correct DNA methylation pattern. Due to the fact that soon after fertilization the zygote undergoes several morphogenetic and developmental events including activation of embryonic genome through the transition of the maternal genome, a diverse gene expression pattern may lead to clinically important conditions, such as apoptosis or the production of a chromosomically abnormal embryo. The present review focused on genes and their role during pre-implantation embryo development, giving emphasis on the various parameters that may alter gene expression or DNA methylation patterns. The pre-implantation embryos derived from in vitro culture systems (in vitro fertilization) and the possible effects on gene expression after the prolonged culture conditions are also discussed.

Protein kinases and associated pathways in pluripotent state and lineage differentiation

Protein kinases (PKs) mediate the reversible conversion of substrate proteins to phosphorylated forms, a key process in controlling intracellular signaling transduction cascades. Pluripotency is, among others, characterized by specifically expressed PKs forming a highly interconnected regulatory network that culminates in a finely-balanced molecular switch. Current high-throughput phosphoproteomic approaches have shed light on the specific regulatory PKs and their function in controlling pluripotent states. Pluripotent cell-derived endothelial and hematopoietic developments represent an example of the importance of pluripotency in cancer therapeutics and organ regeneration. This review attempts to provide the hitherto known kinome profile and the individual characterization of PK-related pathways that regulate pluripotency. Elucidating the underlying intrinsic and extrinsic signals may improve our understanding of the different pluripotent states, the maintenance or induction of pluripotency, and the ability to tailor lineage differentiation, with a particular focus on endothelial cell differentiation for anti-cancer treatment, cell-based tissue engineering, and regenerative medicine strategies.

Lessons from the one-carbon metabolism: passing it along to the next generation

During development, a fetus and its placenta must respond to a changing maternal environment to ensure normal growth is achieved and survival is maintained. The mechanisms behind developmental programming involve complex interactions between epigenetic and physiological processes, which are not well understood. Importantly, when programming goes awry, it puts the fetus at risk for disease later in life and may, in some instances, affect subsequent generations via epigenetic processes including DNA methylation. The one-carbon metabolism, which includes the folate, methionine and choline pathways, provides methyl groups necessary for DNA methylation and a normal epigenetic landscape. Accordingly, disruptions in this pathway affect placental development and function leading to altered fetal programming. Remarkably, recent studies have revealed that abnormal folate metabolism causes transgenerational effects probably through epigenetic inheritance. The epigenetic mechanisms behind this phenomenon are not well understood but they have important implications for the influence of the metabolic environment on epigenetic stability and non-genetic inheritance of disease. Importantly, there are increasing concerns that assisted reproductive technologies cause aberrant epigenetic profiles in embryos leading to abnormal fetal programming. How the negative epigenetic consequences of assisted reproduction treatment affect subsequent generations requires further investigation.

Maternal control of genomic imprint maintenance

Genomic imprinting is a specialized transcriptional phenomenon that employs epigenetic mechanisms to facilitate parental-specific expression. Perturbations in parental epigenetic asymmetry can lead to the development of imprinting disorders, such as Beckwith-Wiedemann syndrome and Angelman syndrome. DNA methylation is one of the most widely studied epigenetic marks that characterizes imprinted regions. During gametogenesis and early embryogenesis, imprinted methylation undergoes a cycle of erasure, acquisition and maintenance. Gamete and embryo manipulations for the purpose of assisted reproduction are performed during these reprogramming events and may lead to their disruption. Recent studies point to the role of maternal-effect proteins in imprinted gene regulation. Studies are now required to increase understanding of how these factors regulate genomic imprinting as well as how assisted reproduction technologies may alter their function.