Specification and epigenetic resetting of the pig germline exhibit conservation with the human lineage

Abstract

SummaryInvestigations on the human germline and programming are challenging due to limited access to embryonic material. However, the pig as a model may provide insight on transcriptional network and epigenetic reprogramming applicable to both species. Here we show that during the pre- and early migratory stages pig primordial germ cells (PGCs) initiate large-scale epigenetic reprogramming, including DNA demethylation involving TET-mediated hydroxylation and potentially base excision repair (BER). There is also macroH2A1 depletion and increased H3K27me3, as well as X chromosome reactivation (XCR) in females. Concomitantly, there is dampening of glycolytic metabolism genes and re-expression of some pluripotency genes like those in preimplantation embryos. We identified evolutionarily young transposable elements and gene coding regions resistant to DNA demethylation in acutely hypomethylated gonadal PGCs, with potential for transgenerational epigenetic inheritance. Detailed insights into the pig germline will likely contribute significantly to advances in human germline biology, including in vitro gametogenesis.

Conserved features of non-primate bilaminar disc embryos and the germline

“…Recent work from our laboratories has shown the developmental correspondence between human and pig germ cells during the initial period of gastrulation (Kobayashi et al, 2017;Zhu et al, 2021). We propose that building on these investigations into pig gastrulation using singlecell -omics and gene manipulation will offer new knowledge on the mechanisms of meso-endoderm and ectoderm specification with relevance to understanding human development.…”

“…Notably, pig PGCs are first found in the posterior epiblast expressing SOX17, BLIMP1, and TFAP2C, and the cluster expands to about 120 cells by induction of proliferating PGC-competent progenitors upon exposure to BMP. These newly formed pig PGCs (pPGCs) pause their proliferation briefly, only to resume cell-cycle progression after they start their migration (Kobayashi et al, 2017;Zhu et al, 2021). The PGCs originating in the amnion of Cynomolgus monkey appear to be in a proliferative phase soon after specification (Sasaki et al, 2016), but how PGCs migrate to posterior epiblast against the movement of amnion that expands toward the opposite direction merits further investigations.…”

New Insights into X-Chromosome Reactivation during Reprogramming to Pluripotency

“…More recently, it has been shown that PRDM14 mediates the removal of H3K27me3 marks from the Xi during XCR in PGCs [ 143 ]. In addition, a recent study in pig showed that Xist is silenced in most pre-migratory PGCs [ 144 ]. XCR begins in pre/early migrating PGCs and continues in gonadal PGCs.…”

“…XCR begins in pre/early migrating PGCs and continues in gonadal PGCs. The kinetics of XCR is not explained by distance of genes to the Xic [ 144 ]. In summary, XCR takes places during epigenetic reprogramming of the germline and is linked to the expression of pluripotency genes, including transcription factors.…”

“…Is XIST expression eventually silenced during PGCs development? scRNA-seq on human gonads as well as PGC-like cell differentiation protocols [ 144 ] could help to better understand XCR in the germline.…”