Cellular plasticity is essential for early embryonic cells. Unlike pluripotent cells, which form embryonic tissues, totipotent cells can generate a complete organism including embryonic and extraembryonic tissues. Cells resembling 2-cell-stage embryos (2C-like cells) arise at very low frequency in embryonic stem (ES) cell cultures. Although induced reprogramming to pluripotency is well established, totipotent cells remain poorly characterized, and whether reprogramming to totipotency is possible is unknown. We show that mouse 2C-like cells can be induced in vitro through downregulation of the chromatin-assembly activity of CAF-1. Endogenous retroviruses and genes specific to 2-cell embryos are the highest-upregulated genes upon CAF-1 knockdown. Emerging 2C-like cells exhibit molecular characteristics of 2-cell embryos and higher reprogrammability than ES cells upon nuclear transfer. Our results suggest that early embryonic-like cells can be induced by modulating chromatin assembly and that atypical histone deposition may trigger the emergence of totipotent cells.
In mammals, oocyte fertilisation by sperm initiates development. This is followed by epigenetic reprogramming of both parental genomes, which involves de-novo establishment of chromatin domains. In the mouse embryo, methylation of histone H3 1 establishes an epigenetic asymmetry and is predominant in the maternal pronucleus [2][3][4][5] . However, the role of (i) differential incorporation of histone H3 variants in the parental chromatin and of (ii) modified residues within specific histone variants has not been addressed. Here we show that the histone variant H3.3, and in particular lysine 27, is required for the establishment of heterochromatin in the mouse embryo. H3.3 localises to paternal pericentromeric chromatin during S-phase at the time of transcription of pericentromeric repeats. Mutation of H3.3K27, but not H3.1K27, results in aberrant accumulation of pericentromeric transcripts, HP1 mislocalisation, dysfunctional chromosome segregation and developmental arrest. This phenotype is rescued by injection of dsRNA derived from pericentromeric transcripts, indicating a functional link between H3.3K27 and silencing of such regions via an RNAi pathway. Our work demonstrates a role for a modifiable residue within a histone variant-specific context during reprogramming and identifies a novel function for mammalian H3.3 in the initial formation of dsRNA-dependent heterochromatin.Fertilisation of the oocyte by the sperm constitutes the first event of embryogenesis and results in the formation of the zygote. The creation of such a totipotent cell from two differentiated ones involves epigenetic reprogramming of the parental genomes. Throughout the complete first cell cycle, the male and female pronuclei behave as two distinct sets of chromatin that coexist as separate nuclear entities. Both pronuclei evolve differently, showing different chromatin signatures, histone marks and replication and transcription timing 2-4, 6, 7 . How the chromatin is assembled, specified and reprogrammed after fertilisation remains a central question in biology. Before fertilisation, the sperm nucleus is condensed six-fold higher than a somatic cell nucleus, and most of its histones are replaced by protamines 8 . Immediately following the entry of the sperm nucleus into the oocyte cytoplasm, protamines are removed from the sperm and replaced by maternally provided Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts histones 9 . In mice, histones H3 and H4 are translated from maternal mRNAs stored in the oocyte, whereas histones H2A and H2B are already present as proteins in the oocyte 10 .Apart from the canonical histones, which are synthesised exclusively during S-phase, histone variants can be incorporated into chromatin throughout the cell cycle. The replication-independent H3 variant H3.3 is preferentially incorporated into the male pronucleus following fertilisation 5 , while the replication-dependent H3.1/2 variants are found predominantly in the female pronucleus 11 , suggesting a role of differe...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.