Unexpected nuclear localization of Cdc25C in bovine oocytes, early embryos, and nuclear-transferred embryos

Mouse zygotes respond to severe sperm DNA damage by delaying paternal DNA replication and embryonic development

Mouse zygotes do not activate apoptosis in response to DNA damage. We previously reported a unique form of inducible sperm DNA damage termed sperm chromatin fragmentation (SCF). SCF mirrors some aspects of somatic cell apoptosis in that the DNA degradation is mediated by reversible double strand breaks caused by topoisomerase 2B (TOP2B) followed by irreversible DNA degradation by a nuclease(s). Here, we created zygotes using spermatozoa induced to undergo SCF (SCF zygotes) and tested how they responded to moderate and severe paternal DNA damage during the first cell cycle. We found that the TUNEL assay was not sensitive enough to identify the breaks caused by SCF in zygotes in either case. However, paternal pronuclei in both groups stained positively for γH2AX, a marker for DNA damage, at 5 hrs after fertilization, just before DNA synthesis, while the maternal pronuclei were negative. We also found that both pronuclei in SCF zygotes with moderate DNA damage replicated normally, but paternal pronuclei in the SCF zygotes with severe DNA damage delayed the initiation of DNA replication by up to 12 hrs even though the maternal pronuclei had no discernable delay. Chromosomal analysis of both groups confirmed that the paternal DNA was degraded after S-phase while the maternal pronuclei formed normal chromosomes. The DNA replication delay caused a marked retardation in progression to the 2-cell stage, and a large portion of the embryos arrested at the G2/M border, suggesting that this is an important checkpoint in zygotic development. Those embryos that progressed through the G2/M border died at later stages and none developed to the blastocyst stage. Our data demonstrate that the zygote responds to sperm DNA damage through a non-apoptotic mechanism that acts by slowing paternal DNA replication and ultimately leads to arrest in embryonic development.

Nuclear profiles of H3 histones trimethylated on Lys27 in bovine (Bos taurus) embryos obtained after in vitro fertilization or somatic cell nuclear transfer

Histone H3 trimethylation on lysine 27 is one of the histone modifications associated with chromatin of silenced regions. H3K27me3 labeling is initially asymmetrical between pronuclei in mammalian embryos, and then it is remodeled during early development. However, in mouse embryos obtained after somatic cell nuclear transfer (SCNT), H3K27me3 histones inherited from the somatic female cell and associated with X chromosome inactivation have been reported to escape remodeling. Using immunostaining, we investigated the remodeling of H3K27me3 in Bos taurus embryos obtained after in vitro fertilization (IVF) and SCNT. In this species, transfer-induced chromatin remodeling can be clearly separated from embryonic genome activation (EGA), which occurs at the 8-16-cell stage, and cloning by SCNT is 10 times more successful than in the mouse. In early IVF bovine embryos, dense H3K27me3 labeling was localized in the pericentric heterochromatin as recently described in the mouse. Labeling was however unevenly distributed up to the 8-cell stage, suggesting that the parental genomes partitioned before EGA. In female IVF blastocysts, a somatic-like female profile appeared in 21% of the trophoblast cells. This profile, which had one major nuclear H3K27me3 patch, the putative inactive X chromosome (Xi), was absent in male blastocysts. In contrast, the somatic-like female H3K27me3 profile was observed in the majority of the nuclei of female bovine SCNT embryos before EGA. At the 8-16-cell stage, this profile was transiently replaced by pericentric-like labeling in most nuclei. Immunostaining of mitotic chromosomes suggested that the ratio of H3K27me3 labeling in pericentric heterochromatin vs. euchromatin was then rapidly altered. Finally, Xi-like H3K27me3 staining appeared again in trophoblast cells in female SCNT blastocysts. These results suggest a role for EGA in H3K27me3 remodeling, which affects the heterochromatin inherited from the donor cell or produced during development.

Dynamics of constitutive heterochromatin: two contrasted kinetics of genome restructuring in early cloned bovine embryos

Efficient reprograming of the donor cell genome in nuclear transfer (NT) embryos is linked to the ability of the embryos to sustain full-term development. As the nuclear architecture has recently emerged as a key factor in the regulation of gene expression, we questioned whether early bovine embryos obtained from transfer of cultured fibroblasts into enucleated oocytes would adopt an embryo-like nuclear organization. We studied the dynamics of constitutive heterochromatin in the stages prior to embryonic genome activation by distribution analysis of heterochromatin protein CBX1 (HP1), centromeric proteins CENPA and CENPB, and histone H3 three-methylated at lysine 9. Then we applied descriptive, quantitative, and co-localization analyses. A dramatic reorganization of heterochromatic blocks of somatic donor cells was first observed in the late one-cell stage NT embryos. Then at two- and four-cell stages, we found two types of NT embryos: one displaying noncondensed heterochromatin patches similar to IVF embryos, whereas the second type displayed condensed heterochromatin blocks, normally observed in IVF embryos only after the eight-cell stage. These analyses discriminate for the first time two contrasted types of nuclear organization in NT embryos, which may correspond to different functional states of the nuclei. The relationship with the somatic nucleus reprograming efficiency is discussed.