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Cold Spring Harbor Perspectives in... Jun 2022Dramatic nuclear reorganization occurs during early development to convert terminally differentiated gametes to a totipotent zygote, which then gives rise to an embryo.... (Review)
Review
Dramatic nuclear reorganization occurs during early development to convert terminally differentiated gametes to a totipotent zygote, which then gives rise to an embryo. Aberrant epigenome resetting severely impairs embryo development and even leads to lethality. How the epigenomes are inherited, reprogrammed, and reestablished in this critical developmental period has gradually been unveiled through the rapid development of technologies including ultrasensitive chromatin analysis methods. In this review, we summarize the latest findings on epigenetic reprogramming in gametogenesis and embryogenesis, and how it contributes to gamete maturation and parental-to-zygotic transition. Finally, we highlight the key questions that remain to be answered to fully understand chromatin regulation and nuclear reprogramming in early development.
Topics: Animals; Cellular Reprogramming; Chromatin; Embryonic Development; Epigenesis, Genetic; Epigenomics; Gene Expression Regulation, Developmental; Zygote
PubMed: 34400552
DOI: 10.1101/cshperspect.a039677 -
Developmental Cell Aug 2017The first major developmental transition in vertebrate embryos is the maternal-to-zygotic transition (MZT) when maternal mRNAs are degraded and zygotic transcription... (Review)
Review
The first major developmental transition in vertebrate embryos is the maternal-to-zygotic transition (MZT) when maternal mRNAs are degraded and zygotic transcription begins. During the MZT, the embryo takes charge of gene expression to control cell differentiation and further development. This spectacular organismal transition requires nuclear reprogramming and the initiation of RNAPII at thousands of promoters. Zygotic genome activation (ZGA) is mechanistically coordinated with other embryonic events, including changes in the cell cycle, chromatin state, and nuclear-to-cytoplasmic component ratios. Here, we review progress in understanding vertebrate ZGA dynamics in frogs, fish, mice, and humans to explore differences and emphasize common features.
Topics: Animals; Cellular Reprogramming; Embryo, Mammalian; Embryo, Nonmammalian; Gene Expression Regulation, Developmental; Genome; Vertebrates; Zygote
PubMed: 28829942
DOI: 10.1016/j.devcel.2017.07.026 -
Nature Reviews. Genetics Apr 2019Following fertilization, the two specified gametes must unite to create an entirely new organism. The genome is initially transcriptionally quiescent, allowing the... (Review)
Review
Following fertilization, the two specified gametes must unite to create an entirely new organism. The genome is initially transcriptionally quiescent, allowing the zygote to be reprogrammed into a totipotent state. Gradually, the genome is activated through a process known as the maternal-to-zygotic transition, which enables zygotic gene products to replace the maternal supply that initiated development. This essential transition has been broadly characterized through decades of research in several model organisms. However, we still lack a full mechanistic understanding of how genome activation is executed and how this activation relates to the reprogramming of the zygotic chromatin architecture. Recent work highlights the central role of transcriptional activators and suggests that these factors may coordinate transcriptional activation with other developmental changes.
Topics: Animals; Chromatin; Gene Expression Regulation, Developmental; Genome; Transcription Factors; Transcription, Genetic; Zygote
PubMed: 30573849
DOI: 10.1038/s41576-018-0087-x -
Nature Jul 2017Mammalian sperm and oocytes have different epigenetic landscapes and are organized in different fashions. After fertilization, the initially distinct parental epigenomes...
Mammalian sperm and oocytes have different epigenetic landscapes and are organized in different fashions. After fertilization, the initially distinct parental epigenomes become largely equalized with the exception of certain loci, including imprinting control regions. How parental chromatin becomes equalized and how imprinting control regions escape from this reprogramming is largely unknown. Here we profile parental allele-specific DNase I hypersensitive sites in mouse zygotes and morula embryos, and investigate the epigenetic mechanisms underlying these allelic sites. Integrated analyses of DNA methylome and tri-methylation at lysine 27 of histone H3 (H3K27me3) chromatin immunoprecipitation followed by sequencing identify 76 genes with paternal allele-specific DNase I hypersensitive sites that are devoid of DNA methylation but harbour maternal allele-specific H3K27me3. Interestingly, these genes are paternally expressed in preimplantation embryos, and ectopic removal of H3K27me3 induces maternal allele expression. H3K27me3-dependent imprinting is largely lost in the embryonic cell lineage, but at least five genes maintain their imprinted expression in the extra-embryonic cell lineage. The five genes include all paternally expressed autosomal imprinted genes previously demonstrated to be independent of oocyte DNA methylation. Thus, our study identifies maternal H3K27me3 as a DNA methylation-independent imprinting mechanism.
Topics: Alleles; Animals; Blastocyst; Cell Lineage; Chromatin; DNA; DNA Methylation; Deoxyribonuclease I; Embryo, Mammalian; Female; Gene Expression Regulation; Genomic Imprinting; Histones; Lysine; Male; Mice; Morula; Oocytes; Zygote
PubMed: 28723896
DOI: 10.1038/nature23262 -
Cell Stem Cell Mar 2022The activation of the embryonic genome marks the first major wave of transcription in the developing organism. Zygotic genome activation (ZGA) in mouse 2-cell embryos...
The activation of the embryonic genome marks the first major wave of transcription in the developing organism. Zygotic genome activation (ZGA) in mouse 2-cell embryos and 8-cell embryos in humans is crucial for development. Here, we report the discovery of human 8-cell-like cells (8CLCs) among naive embryonic stem cells, which transcriptionally resemble the 8-cell human embryo. They express ZGA markers, including ZSCAN4 and LEUTX, and transposable elements, such as HERVL and MLT2A1. 8CLCs show reduced SOX2 levels and can be identified using TPRX1 and H3.Y marker proteins in vitro. Overexpression of the transcription factor DUX4 and spliceosome inhibition increase human ZGA-like transcription. Excitingly, the 8CLC markers TPRX1 and H3.Y are also expressed in ZGA-stage 8-cell human embryos and may thus be relevant in vivo. 8CLCs provide a unique opportunity to characterize human ZGA-like transcription and might provide critical insights into early events in embryogenesis in humans.
Topics: Animals; Embryonic Development; Gene Expression Regulation, Developmental; Genome, Human; Humans; Mice; Transcription Factors; Zygote
PubMed: 35216671
DOI: 10.1016/j.stem.2022.01.014 -
Cell Dec 2022Nuclear pore complexes (NPCs) are channels for nucleocytoplasmic transport of proteins and RNAs. However, it remains unclear whether composition, structure, and...
Nuclear pore complexes (NPCs) are channels for nucleocytoplasmic transport of proteins and RNAs. However, it remains unclear whether composition, structure, and permeability of NPCs dynamically change during the cleavage period of vertebrate embryos and affect embryonic development. Here, we report that the comprehensive NPC maturity (CNM) controls the onset of zygotic genome activation (ZGA) during zebrafish early embryogenesis. We show that more nucleoporin proteins are recruited to and assembled into NPCs with development, resulting in progressive increase of NPCs in size and complexity. Maternal transcription factors (TFs) transport into nuclei more efficiently with increasing CNM. Deficiency or dysfunction of Nup133 or Ahctf1/Elys impairs NPC assembly, maternal TFs nuclear transport, and ZGA onset, while nup133 overexpression promotes these processes. Therefore, CNM may act as a molecular timer for ZGA by controlling nuclear transport of maternal TFs that reach nuclear concentration thresholds at a given time to initiate ZGA.
Topics: Animals; Embryonic Development; Gene Expression Regulation, Developmental; Nuclear Pore; Nuclear Pore Complex Proteins; Transcription Factors; Zebrafish; Zygote; Genome
PubMed: 36493774
DOI: 10.1016/j.cell.2022.11.011 -
Science Advances Feb 2022Translational regulation plays an important role in gene expression and function. Although the transcriptional dynamics of mouse preimplantation embryos have been well...
Translational regulation plays an important role in gene expression and function. Although the transcriptional dynamics of mouse preimplantation embryos have been well characterized, the global mRNA translation landscape and the master regulators of zygotic genome activation (ZGA) remain unknown. Here, by developing and applying a low-input ribosome profiling (LiRibo-seq) technique, we profiled the mRNA translation landscape in mouse preimplantation embryos and revealed the translational dynamics during mouse preimplantation development. We identified a marked translational transition from MII oocytes to zygotes and demonstrated that active translation of maternal mRNAs is essential for maternal-to-zygotic transition (MZT). We further showed that two maternal factors, Smarcd2 and Cyclin T2, whose translation is activated in zygotes, are required for chromatin reprogramming and ZGA, respectively. Our study thus not only filled in a knowledge gap on translational regulation during mammalian preimplantation development but also revealed insights into the critical function of maternal mRNA translation in MZT.
Topics: Animals; Embryonic Development; Gene Expression Regulation, Developmental; Mammals; Mice; Protein Biosynthesis; RNA, Messenger, Stored; Zygote
PubMed: 35108058
DOI: 10.1126/sciadv.abj3967 -
Nature Genetics Apr 2021Polycomb repressive complexes 1 and 2 (PRC1/2) maintain transcriptional silencing of developmental genes largely by catalyzing the formation of mono-ubiquitinated...
Polycomb repressive complexes 1 and 2 (PRC1/2) maintain transcriptional silencing of developmental genes largely by catalyzing the formation of mono-ubiquitinated histone H2A at lysine 119 (H2AK119ub1) and trimethylated histone H3 at lysine 27 (H3K27me3), respectively. How Polycomb domains are reprogrammed during mammalian preimplantation development remains largely unclear. Here we show that, although H2AK119ub1 and H3K27me3 are highly colocalized in gametes, they undergo differential reprogramming dynamics following fertilization. H3K27me3 maintains thousands of maternally biased domains until the blastocyst stage, whereas maternally biased H2AK119ub1 distribution in zygotes is largely equalized at the two-cell stage. Notably, while maternal PRC2 depletion has a limited effect on global H2AK119ub1 in early embryos, it disrupts allelic H2AK119ub1 at H3K27me3 imprinting loci including Xist. By contrast, acute H2AK119ub1 depletion in zygotes does not affect H3K27me3 imprinting maintenance, at least by the four-cell stage. Importantly, loss of H2AK119ub1, but not H3K27me3, causes premature activation of developmental genes during zygotic genome activation (ZGA) and subsequent embryonic arrest. Thus, our study reveals distinct dynamics and functions of H3K27me3 and H2AK119ub1 in mouse preimplantation embryos.
Topics: Animals; Blastocyst; Epigenesis, Genetic; Female; Fertilization; Gene Expression Regulation, Developmental; Histones; Lysine; Male; Maternal Inheritance; Mice; Oocytes; Paternal Inheritance; Polycomb Repressive Complex 1; Polycomb Repressive Complex 2; Pregnancy; Protein Isoforms; RNA, Long Noncoding; Spermatozoa; Ubiquitination; Zygote
PubMed: 33821005
DOI: 10.1038/s41588-021-00821-2 -
Protein & Cell May 2015Genome editing tools such as the clustered regularly interspaced short palindromic repeat (CRISPR)-associated system (Cas) have been widely used to modify genes in model...
Genome editing tools such as the clustered regularly interspaced short palindromic repeat (CRISPR)-associated system (Cas) have been widely used to modify genes in model systems including animal zygotes and human cells, and hold tremendous promise for both basic research and clinical applications. To date, a serious knowledge gap remains in our understanding of DNA repair mechanisms in human early embryos, and in the efficiency and potential off-target effects of using technologies such as CRISPR/Cas9 in human pre-implantation embryos. In this report, we used tripronuclear (3PN) zygotes to further investigate CRISPR/Cas9-mediated gene editing in human cells. We found that CRISPR/Cas9 could effectively cleave the endogenous β-globin gene (HBB). However, the efficiency of homologous recombination directed repair (HDR) of HBB was low and the edited embryos were mosaic. Off-target cleavage was also apparent in these 3PN zygotes as revealed by the T7E1 assay and whole-exome sequencing. Furthermore, the endogenous delta-globin gene (HBD), which is homologous to HBB, competed with exogenous donor oligos to act as the repair template, leading to untoward mutations. Our data also indicated that repair of the HBB locus in these embryos occurred preferentially through the non-crossover HDR pathway. Taken together, our work highlights the pressing need to further improve the fidelity and specificity of the CRISPR/Cas9 platform, a prerequisite for any clinical applications of CRSIPR/Cas9-mediated editing.
Topics: Blastocyst; CRISPR-Cas Systems; Hemoglobins, Abnormal; Humans; Zygote
PubMed: 25894090
DOI: 10.1007/s13238-015-0153-5 -
Cell Reports Jun 2022In human embryos, major zygotic genome activation (ZGA) initiates at the eight-cell (8C) stage. Abnormal ZGA leads to developmental defects and even contributes to the...
In human embryos, major zygotic genome activation (ZGA) initiates at the eight-cell (8C) stage. Abnormal ZGA leads to developmental defects and even contributes to the failure of human blastocyst formation or implantation. An in vitro cell model mimicking human 8C blastomeres would be invaluable to understanding the mechanisms regulating key biological events during early human development. Using the non-canonical promoter of LEUTX that putatively regulates human ZGA, we developed an 8C::mCherry reporter, which specifically marks the 8C state, to isolate rare 8C-like cells (8CLCs) from human preimplantation epiblast-like stem cells. The 8CLCs express a panel of human ZGA genes and have a unique transcriptome resembling that of the human 8C embryo. Using the 8C::mCherry reporter, we further optimize the chemical-based culture condition to increase and maintain the 8CLC population. Functionally, 8CLCs can self-organize to form blastocyst-like structures. The discovery and maintenance of 8CLCs provide an opportunity to recapitulate early human development.
Topics: Blastocyst; Embryonic Development; Gene Expression Regulation, Developmental; Genome; Humans; Zygote
PubMed: 35732112
DOI: 10.1016/j.celrep.2022.110994