Epigenetic reprogramming in mouse and human primordial germ cells

Abstract

Primordial germ cells (PGCs) are the precursors of sperm and eggs. They undergo genome-wide epigenetic reprogramming to erase epigenetic memory and reset the genomic potential for totipotency. Global DNA methylation erasure is a crucial part of epigenetic resetting when DNA methylation levels decrease across the genome to <5%. However, certain localized regions exhibit slower demethylation or resistance to reprogramming. Since DNA methylation plays a crucial role in transcriptional regulation, this depletion in PGCs requires mechanisms independent of DNA methylation to regulate transcriptional control during PGC reprogramming. Histone modifications are predicted to compensate for the loss of DNA methylation in gene regulation. Different histone modifications exhibit distinct patterns in PGCs undergoing epigenetic programming at the genomic level during PGC development in conjunction with changes in DNA methylation. Together, they contribute to PGC-specific genomic regulation. Recent findings related to these processes provide a comprehensive overview of germline epigenetic reprogramming and its importance in mouse and human PGC development. Additionally, we evaluated the extent to which in vitro culture techniques have replicated the development processes of human PGCs.

Fig. 1. DNA methylation reprogramming in early embryos and the germlines of mice and humans.

a Schematic showing the level of DNA methylation in the mouse and human genomes. Genome-wide demethylation occurs asymmetrically in the paternal and maternal genomes after fertilization, resetting the epigenome for naïve pluripotency at the blastocyst stage (dotted line). Methylation re-establishment occurs around the time of implantation. Unlike somatic cells (dash-dotted line), PGCs (solid line) undergo global demethylation during the onset of migration and early settlement at the genital ridge. While methylation is reestablished in prenatal male germ cells, in oocytes, this process is not completed until puberty. DNA methylation levels during early embryo and germline development are represented by the orange color density for the indicated regions. b Schematic showing the level of DNA hydroxymethylation (5hmC) in the mouse genome during development. The dotted line represents the 5hmC levels in the early embryo after fertilization. The solid line indicates the 5hmC levels in the germline.

Fig. 2. Global and locus-specific epigenetic distinctions between human PGCs and surrounding somatic cells.

Sequencing results from human PGCs and surrounding somatic cells revealed genome-wide differences in DNA methylation and histone modifications, as well as locus-specific variations. Relative levels of DNA methylation and histone modifications between somatic cells and PGCs are depicted for olfactory receptor genes (ORGs), late germ cell genes, genes involved in somatic cell differentiation, and various subtypes of transposable elements (TEs).

Fig. 3. Human in vitro gametogenesis.

a A methodology for reaggregation culture involving male or female human PGCLCs with mouse embryonic somatic cells was employed. The hPGCLCs from the embryoids were then cocultured for ~4 months, with mouse testicular or ovarian somatic cells differentiating into pro-spermatogonia or oogonia-like cells, respectively. hiPSCs human induced pluripotent stem cells, iMeLCs incipient mesoderm-like cells, GSK3i GSK3 inhibitor. b A methodology for coculture with hindgut organoids demonstrated enhanced progression capability in the human germ cell fate. Cells transitioning between primed and naïve pluripotency states can be directed toward hPGCLCs via BMP signaling cues. hPGCLCs cocultured with human hindgut organoids derived from primed ESCs exhibit developmental progression similar to that of in vivo gonadal hPGCs. c Ectopic induction of DMRT1 and SOX17 induces the expression of mitotic arrest marker genes, indicating the transition from nascent hPGCLCs to gametogenesis-competent cells. MEF mouse embryonic fibroblast. d Prolonged culture with BMP2 promotes the differentiation of hPGCLCs into mitotic pro-spermatogonia or oogonia-like cells.