Individual analysis of the epigenome of preimplantation embryos is useful for characterizing each embryo and for investigating the effects of environmental factors on their epigenome. However, it is difficult to analyze genome-wide epigenetic modifications, especially histone modifications, in a large number of single embryos due to the small number of cells and the complexity of the analysis methods. To solve this problem, we further modified the CUT&Tag method, which can analyze histone modifications in a small number of cells, such that the embryo is handled as a cell mass in the reaction solutions in the absence of the solid-phase magnetic beads that are used for antibody and enzyme reactions in the conventional method (NON-TiE-UP CUT&Tag; NTU-CAT). By using bovine blastocysts as a model, we showed that genome-wide profiles of representative histone modifications, H3K4me3 and H3K27me3, could be obtained by NTU-CAT that are in overall agreement with the conventional chromatin immunoprecipitation-sequencing (ChIP-seq) method, even from single embryos. However, this new approach has limitations that require attention, including false positive and negative peaks and lower resolution for broad modifications. Despite these limitations, we consider NTU-CAT a promising replacement for ChIP-seq with the great advantage of being able to analyze individual embryos.

Maintaining genomic integrity in mammalian early embryos, which are deficient in DNA damage repair, is critical for normal preimplantation and subsequent development. Abnormalities in DNA damage repair in preimplantation embryos can cause not only developmental arrest, but also diseases such as congenital disorders and cancers. Histone H4 lysine 20 monomethylation (H4K20me1) is involved in DNA damage repair and regulation of gene expression. However, little is known about the role of H4K20me1 during mouse preimplantation development. In this study, we revealed that H4K20me1 mediated by SETD8 is involved in maintaining genomic integrity. H4K20me1 was present throughout preimplantation development. In addition, reduction in the level of H4K20me1 by inhibition of SETD8 activity or a dominant-negative mutant of histone H4 resulted in developmental arrest at the S/G2 phase and excessive accumulation of DNA double-strand breaks. Together, our results suggest that H4K20me1, a type of epigenetic modification, is associated with the maintenance of genomic integrity and is essential for preimplantation development. A better understanding of the mechanisms involved in maintaining genome integrity during preimplantation development could contribute to advances in reproductive medicine and technology.

No abstract

Oog1, an oocyte-specific gene that encodes a protein of 425 amino acids, is present in five copies on mouse chromosomes 4 and 12. In mouse oocytes, Oog1 mRNA expression begins at embryonic day 15.5 and almost disappears by the late two-cell stage. Meanwhile, OOG1 protein is detectable in oocytes in ovarian cysts and disappears by the four-cell stage; the protein is transported to the nucleus in late one-cell to early two-cell stage embryos. In this study, we examined the role of Oog1 during oogenesis in mice. Oog1 RNAi-transgenic mice were generated by expressing double-stranded hairpin Oog1 RNA, which is processed into siRNAs targeting Oog1 mRNA. Quantitative RT-PCR revealed that the amount of Oog1 mRNA was dramatically reduced in oocytes obtained from Oog1-knockdown mice, whereas the abundance of spermatogenesis-associated transcripts (Klhl10, Tekt2, Tdrd6, and Tnp2) was increased in Oog1 knockdown ovaries. Tdrd6 is involved in the formation of the chromatoid body, Tnp2 contributes to the formation of sperm heads, Tekt2 is required for the formation of ciliary and flagellar microtubules, and Klhl10 plays a key role in the elongated sperm differentiation. These results indicate that Oog1 down-regulates the expression of spermatogenesis-associated genes in female germ cells, allowing them to develop normally into oocytes.

Maternal RNA and proteins accumulate in mouse oocytes and regulate initial developmental stages. Sperm DNA combines with protamine, which is exchanged after fertilization with maternal histones, including H3.3; however, the effect of H3.3 on development post-fertilization remains unclear.Herein, we established an electroporation method to introduce H3.3 siRNA into germinal vesicle (GV)stage oocytes without removing cumulus cells. Oocyte-attached cumulus cells need to be removed during the traditional microinjection method; however, we confirmed that artificially removing cumulus cells from oocytes reduced fertilization rates, and oocytes originally free of cumulus cells had reduced developmental competence. On introducing H3.3 siRNA at the GV stage, H3.3 was maintained in the maternal pronucleus and second polar body but not in the paternal pronucleus, resulting in embryonic lethality after fertilization. These findings indicate that H3.3 protein was not incorporated into the paternal pronucleus, as it was repeatedly translated and degraded over a relatively short period.Conversely, H3.3 protein incorporated into the maternal genome in the GV stage escaped degradation and remained in the maternal pronucleus after fertilization. This new method of electroporation into GVstage oocytes without cumulus cell removal is not skill-intensive and is essential for the accurate analysis of maternal effect genes.

Individual analysis of the epigenome of preimplantation embryos is useful for characterizing each embryo and for investigating the effects of environmental factors on their epigenome. However, it is difficult to analyze genome-wide epigenetic modifications, especially histone modifications, in a large number of single embryos due to the small number of cells and the complexity of the analysis methods. To solve this problem, we further modified the CUT and Tag method, which can analyze histone modifications in a small number of cells, such that the embryo is handled as a cell mass in the reaction solutions in the absence of the solid-phase magnetic beads that are used for antibody and enzyme reactions in the original method (NON-TiE-UP CUT and Tag; NTU-CAT). By using bovine blastocysts as a model, we showed that genome-wide profiles of representative histone modifications, H3K4me3 and H3K27me3, could be obtained by NTU-CAT that are in overall agreement with the conventional chromatin immunoprecipitation-sequencing (ChIP-seq) method, even from single embryos. However, this new approach has limitations that require attention, including false positive peaks and lower resolution for broad modifications. Despite these limitations, we conclude that NTU-CAT is a promising replacement for ChIP-seq with the great advantage of being able to analyze individual embryos.

Murine endogenous retrovirus with leucine tRNA primer, also known as MERVL, is expressed during zygotic genome activation in mammalian embryos. Here we show that protein arginine N-methyltransferase 6 (Prmt6) forms a chimeric transcript with MT2B2, one of the long terminal repeat sequences of MERVL, and is translated into an elongated chimeric protein (PRMT6MT2B2) whose function differs from that of the canonical PRMT6 protein (PRMT6CAN) in mouse preimplantation embryos. Overexpression of PRMT6CAN in fibroblast cells increased asymmetric dimethylation of the third arginine residue of both histone H2A (H2AR3me2a) and histone H4 (H4R3me2a), while overexpression of PRMT6MT2B2 increased only H2AR3me2a. In addition, overexpression of PRMT6MT2B2 in one blastomere of mouse two-cell embryos promoted cell proliferation and differentiation of the blastomere into epiblast cells at the blastocyst stage, while overexpression of PRMT6CAN repressed cell proliferation. This is the first report of the translation of a chimeric protein (PRMT6MT2B2) in mouse preimplantation embryos. Our results suggest that analyzing chimeric transcripts with MERVL will provide insight into the relationship between zygotic genome activation and subsequent intra- and extra-cellular lineage determination. However, more comprehensive proteomic analysis is needed to determine the precise functions of chimeric proteins during preimplantation development.

Maternal RNAs and proteins accumulate in mouse oocytes and control the initial stages of development. The DNA in sperm combines with protamine, which is exchanged after fertilization with maternal histones, including H3.3, but the effect of H3.3 on post-fertilization development has been unclear. In this study, we established an electroporation method to introduce H3.3 siRNA into germinal vesicle (GV)-stage oocytes without removing cumulus cells. In the traditional microinjection method, cumulus cells attached to oocytes must be removed; however, we confirmed that artificially removing cumulus cells from oocytes reduced fertilization rates, and oocytes originally free of cumulus cells had reduced developmental competence. When H3.3 siRNA was introduced at the GV stage, H3.3 was still present in the maternal pronucleus and second polar body, but not in the paternal pronucleus, resulting in embryonic lethality after fertilization. This indicates that the H3.3 protein was not incorporated into the paternal pronucleus because it was repeatedly translated and degraded in a relatively short time. On the other hand, H3.3 protein incorporated into the maternal genome in the GV stage escaped degradation and remained in the maternal pronucleus after fertilization. This new method of electroporation into GV-stage oocytes without removing cumulus cells is not skill intensive and is essential for the accurate analysis of maternal effect genes.