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Transcriptional and Epigenetic Regulation of Totipotency in Mouse Early Embryos.

Periodic Reporting for period 4 - Totipotency (Transcriptional and Epigenetic Regulation of Totipotency in Mouse Early Embryos.)

Reporting period: 2021-02-01 to 2022-01-31

In mammals, fusion of two highly differentiated gametes gives rise to a totipotent zygote capable of developing into a whole organism. It coincides with translation and degradation of maternally provided transcripts, initiation of global transcription called zygotic genome activation (ZGA), and “epigenetic reprogramming” of germline chromatin states into an embryonic state. The molecular mechanisms underlying this exquisite reprogramming of cell fate are little understood.
The research project has the goal to identify and characterize transcription factors and chromatin regulators which regulate ZGA in early mouse embryos. We utilize novel and highly sensitive genomic approaches to measure nascent transcription and determine open and modified chromatin landscapes in oocytes and early embryos, wild-type and conditionally deficient for major epigenetic modifiers. We apply computational approaches to identify candidate transcription factors (TFs) and histone modifiers controlling ZGA. We use molecular and developmental biology approaches, combined with live-imaging, to interrogate the function of TFs for ZGA. We further investigate the role of chromatin remodeling during spermatogenesis for gene regulation during embryogenesis. On the one hand, we interrogate the relevance of nucleosome eviction during spermatogenesis, as a possibly epigenetic reprogramming process, for defining embryonic competence. On the other hand, we study the genomic distribution and occupancy levels of nucleosomes in developing and mature male germ cells to estimate probabilities for paternal epigenetic inheritance via nucleosomes.
The project provides a crucial contribution to dissecting molecular mechanisms underlying acquisition of totipotency in mouse embryos. It will deliver basic insights into mechanisms and significance of intergenerational epigenetic inheritance versus reprogramming of germ line chromatin states in early embryos. The obtained findings will inspire basic research on the use of Assisted Reproductive Technologies in human reproductive medicine for treating human male sterility.
We identified several transcription factors that are expressed in eggs and early mouse embryos and which are required for the development of eggs and embryos. We obtained definitive evidence that one of the transcription factors is an essential regulator of zygotic genome activation and embryonic competence. For another family of factors, preliminary data assigns major roles to them in regulating embryogenesis. A molecular dissection of the underlying mechanisms is pending.
Moreover, we obtained a clear understanding of the role of chromatin states inherited from female and male germ cells in regulating gene expression in early mouse embryos. Transmission of certain chromatin states from eggs and/or sperm to embryos is instructive for gene expression activity in early embryos. Loss of such chromatin marking on the maternal genome (inherited from eggs) impairs proper gene expression in and development of pre-implantation embryos.
When using immature male germ cells instead of mature sperm as donors of the paternal genome, we observe changes in gene expression in and reduced fitness of early mouse embryos. Changes in gene expression can be related to chromatin states that are present in immature germ cells but that are largely removed during maturation towards mature spermatozoa. Quantitative analysis of chromatin profiles in sperm indeed indicate that nucleosomal chromatin states present in immature haploid male germ cells are largely remodeled and reprogrammed during the formation of mature mouse and human spermatozoa, thereby majorly reducing their potential in paternal transmission of epigenetic information.
Our research also demonstrated that proper DNA methylation patterns established within the maternal genome during egg development are essential for embryonic development and viability.
The ERC-funded project has enabled us to obtain original and ground-breaking insights in the molecular mechanisms underlying gene regulation, intergenerational epigenetic inheritance of intrinsic germ cell chromatin states and developmental fate at the onset of life. The basic knowledge that we obtained provides a mechanistic framework for understanding other observations which suggest intergenerational heritability of acquired traits, ranging from metabolic, environmental, or behavioral origin.