Abstract
SummaryPrimordial germ cell (PGC) development is characterized by global epigenetic remodeling, which resets genomic potential and establishes an epigenetic ground state. Here we recapitulate PGC specification in vitro from naive embryonic stem cells and characterize the early events of epigenetic reprogramming during the formation of the human and mouse germline. Following rapid de novo DNA methylation during priming to epiblast-like cells, methylation is globally erased in PGC-like cells. Repressive chromatin marks (H3K9me2/3) and transposable elements are enriched at demethylation-resistant regions, while active chromatin marks (H3K4me3 or H3K27ac) are more prominent at regions that demethylate faster. The dynamics of specification and epigenetic reprogramming show species-specific differences, in particular markedly slower reprogramming kinetics in the human germline. Differences in developmental kinetics may be explained by differential regulation of epigenetic modifiers. Our work establishes a robust and faithful experimental system of the early events of epigenetic reprogramming and regulation in the germline.
Highlights
Primordial germ cells (PGCs) are the precursors of the fully differentiated gametes, oocytes, and sperm, establishing during their development the prerequisites of the totipotent state
Generation of Human and Mouse PGCLCs from Naive embryonic stem cells (ESCs) Using a similar strategy to that described for mouse PGC-like cells (mPGCLCs) specification (Hayashi et al, 2011) we differentiated naive hESCs toward hEpiLCs in serum-free N2B27 medium containing transforming growth factor b (TGF-b), basic fibroblast growth factor, and knockout serum replacement (KSR) (Figure 1A)
Naive hESC colonies’ typical domed shape was lost and hEpiLCs adopted a flat, primed cell morphology by day 4 (Figure 1B), a characteristic observed during mPGCLC differentiation (Hayashi et al, 2011)
Summary
Primordial germ cells (PGCs) are the precursors of the fully differentiated gametes, oocytes, and sperm, establishing during their development the prerequisites of the totipotent state. Human PGCs (hPGCs) are specified around E12–E16 (developmental week 2), and, while the early migratory phase (weeks 3–5) of in vivo hPGC development is currently not accessible to experimental analysis, gonadal hPGCs have recently been isolated and characterized molecularly (Gkountela et al, 2015; Guo et al, 2015; Tang et al, 2015). This in vivo work has shown that hPGCs are characterized by the expression of known PGC marker genes such as BLIMP1, PRDM14, or DPPA3 and express human specific genes such as SOX17. Similar to epigenetic reprogramming in mouse PGCs (mPGCs), in vivo hPGCs have erased DNA methylation globally by week 5.5, presumably starting during the migratory phase, resulting in a hypomethylated epigenetic ground state
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