Three-dimensional analysis of nuclear heterochromatin distribution during early development in the rabbit

Epigenome dynamics in early mammalian embryogenesis

During early embryonic development in mammals, the totipotency of the zygote - which is reprogrammed from the differentiated gametes - transitions to pluripotency by the blastocyst stage, coincident with the first cell fate decision. These changes in cellular potency are accompanied by large-scale alterations in the nucleus, including major transcriptional, epigenetic and architectural remodelling, and the establishment of the DNA replication programme. Advances in low-input genomics and loss-of-function methodologies tailored to the pre-implantation embryo now enable these processes to be studied at an unprecedented level of molecular detail in vivo. Such studies have provided new insights into the genome-wide landscape of epigenetic reprogramming and chromatin dynamics during this fundamental period of pre-implantation development.

H3K9 post-translational modifications regulate epiblast/primitive endoderm specification in rabbit blastocysts

Post-translational modifications of histone H3 on lysine 9, specifically acetylation (H3K9ac) and tri-methylation (H3K9me3), play a critical role in regulating chromatin accessibility. However, the role of these modifications in lineage segregation in the mammalian blastocyst remains poorly understood. We demonstrate that di- and tri-methylation marks, H3K9me2 and H3K9me3, decrease during cavitation and expansion of the rabbit blastocyst. Notably, H3K9me3 levels are particularly low in inner cell mass cells at the onset of blastocyst formation but increase again just before gastrulation. Conversely, H3K9ac is abundant in early blastocyst stages but decreases during the transition from the inner cell mass to the epiblast. These distinct distribution patterns correlate with high expression levels of methyltransferases (EHMT1, EHMT2, SETDB1) and deacetylases (HDAC1, HDAC2, HDAC5) in expanding blastocysts. Functionally, inhibiting H3K9me2/3 through an EHMT1/2 inhibitor disrupts primitive endoderm segregation, whereas enhancing histone acetylation (including H3K9ac) using a class I HDAC inhibitor promotes epiblast expansion at the expense of the primitive endoderm. These modifications impact the expression of genes associated with pluripotency and lineage determination, underscoring the importance of H3K9 modifications in embryonic cell fate decisions.

Transcription of rRNA in early mouse embryos promotes chromatin reorganization and expression of major satellite repeats

During the first cell cycles of the early development, the chromatin of the embryo is highly reprogrammed alongside that embryonic genome starts its own transcription. The spatial organization of the genome is a major process that contributes to regulating gene transcription in time and space, however, it is poorly studied in the context of early embryos. To study the cause and effect link between transcription and spatial organization in embryos, we focused on the ribosomal genes, that are first silent and begin to transcribe during the 2-cell stage in the mouse. We demonstrated that ribosomal sequences and early unprocessed rRNAs are spatially organized in a very peculiar manner from the 2-cell to the 16-cell. Using drugs interfering with ribosomal DNA transcription, we show that this organization, totally different from somatic cells, depends on an active transcription of ribosomal genes and induces a unique chromatin environment that favors transcription of major satellite sequences after the 4-cell stage.

Lead exposure induces dysregulation of constitutive heterochromatin hallmarks in live cells

Lead (Pb) is a heavy metal contaminant commonly found in air, soil, and drinking water due to legacy uses. Excretion of ingested Pb can result in extensive kidney damages due to elevated oxidative stress. Epigenetic alterations induced by exposure to Pb have also been implied but remain poorly understood. In this work, we assessed changes in repressive epigenetic marks, namely DNA methylation (meCpG) and histone 3 lysine 9 tri-methylation (H3K9me3) after exposure to Pb. Live cell epigenetic probes coupled to bimolecular fluorescence complementation (BiFC) were used to monitor changes in the selected epigenetic marks. Exposure to Pb significantly lowered meCpG and H3K9me3 levels in HEK293T cells suggesting global changes in constitutive heterochromatin. A heterodimeric pair of probes that tags chromatin regions enriched in both meCpG and H3K9me3 further confirmed our findings. The observed epigenetic changes can be partially attributed to aberrant transcriptional changes induced by Pb, such as overexpression of TET1 after Pb exposure. Lastly, we monitored changes in selected heterochromatin marks after removal of Pb and found that changes in these markers do not immediately recover to their original level suggesting potential long-term damages to chromatin structure.

Migration speed of nucleolus precursor bodies in human male pronuclei: a novel parameter for predicting live birth

PURPOSE

To study the relationship between the migration speed of nucleolus precursor bodies (NPBs) in male and female pronuclei (mPN; fPN) and human embryo development during assisted reproduction.

METHODS

The migration speed of 263 NPBs from 47 zygotes was quantitated, and embryonic development was observed until the blastocyst stage. The central coordinates of mPN, fPN, and NPBs were noted at multiple timepoints. Then, the distance traveled by the NPBs between two sequential images was measured, and migration speed was calculated. Additionally, we investigated the relationship between NPB migration speed and ploidy status (N = 33) or live birth/ongoing pregnancy (LB/OP) (N = 60) after assisted reproduction.

RESULTS

The NPB migration speed in both mPN and fPN was significantly faster in the zygotes that developed into blastocysts (N = 25) than that in the zygotes that arrested (N = 22). The timing of blastulation was negatively correlated with NPB migration speed in the mPN. Faster NPB migration was significantly correlated with LB/OP. In multivariate logistic analysis, NPB migration speed in the mPN was the only morphokinetic parameter associated with LB/OP. In a receiver-operating characteristic curve analysis of LB/OP by the NPB migration speed in the mPN, the cut-off value was 4.56 μm/h. When this cut-off value was applied to blastocysts with preimplantation genetic testing for aneuploidy, 100% of the blastocysts faster than or equal to the cut-off value were euploid.

CONCLUSION

The NPBs migrated faster in zygotes having the potential to develop into a blastocyst, and eventually into a baby. This predictor could be an attractive marker for non-invasive embryo selection.

Heterochromatin Morphodynamics in Late Oogenesis and Early Embryogenesis of Mammals

During the period of oocyte growth, chromatin undergoes global rearrangements at both morphological and molecular levels. An intriguing feature of oogenesis in some mammalian species is the formation of a heterochromatin ring-shaped structure, called the karyosphere or surrounded "nucleolus", which is associated with the periphery of the nucleolus-like bodies (NLBs). Morphologically similar heterochromatin structures also form around the nucleolus-precursor bodies (NPBs) in zygotes and persist for several first cleavage divisions in blastomeres. Despite recent progress in our understanding the regulation of gene silencing/expression during early mammalian development, as well as the molecular mechanisms that underlie chromatin condensation and heterochromatin structure, the biological significance of the karyosphere and its counterparts in early embryos is still elusive. We pay attention to both the changes of heterochromatin morphology and to the molecular mechanisms that can affect the configuration and functional activity of chromatin. We briefly discuss how DNA methylation, post-translational histone modifications, alternative histone variants, and some chromatin-associated non-histone proteins may be involved in the formation of peculiar heterochromatin structures intimately associated with NLBs and NPBs, the unique nuclear bodies of oocytes and early embryos.

The Genomic Health of Human Pluripotent Stem Cells: Genomic Instability and the Consequences on Nuclear Organization

Human pluripotent stem cells (hPSCs) are increasingly used for cell-based regenerative therapies worldwide, with embryonic and induced pluripotent stem cells as potential treatments for debilitating and chronic conditions, such as age-related macular degeneration, Parkinson's disease, spinal cord injuries, and type 1 diabetes. However, with the level of genomic anomalies stem cells generate in culture, their safety may be in question. Specifically, hPSCs frequently acquire chromosomal abnormalities, often with gains or losses of whole chromosomes. This review discusses how important it is to efficiently and sensitively detect hPSC aneuploidies, to understand how these aneuploidies arise, consider the consequences for the cell, and indeed the individual to whom aneuploid cells may be administered.