In vitro reconstitution of epigenetic reprogramming in the human germ line [RNA-Seq]
Epigenetic reprogramming resets parental epigenetic memories and differentiates primordial germ cells (PGCs) into mitotic pro-spermatogonia or oogonia, ensuring sexually dimorphic germ-cell development for totipotency.  However, the mechanism of epigenetic reprogramming in humans remains unknown.  Here, we establish a robust strategy for inducing epigenetic reprogramming and differentiation of pluripotent stem cell (PSC)-derived human PGC-like cells (hPGCLCs) into mitotic pro-spermatogonia or oogonia, coupled with their extensive amplification (~>10(10)-fold).  Strikingly, bone morphogenetic protein (BMP) signaling is the key driver of these processes.  Mechanistically, BMP signaling attenuates the mitogen-activated protein kinase/extracellular-regulated kinase (MAPK/ERK) pathway and both de novo and maintenance DNA methyltransferase (DNMT) activities, promoting replication-coupled, passive DNA demethylation.  On the other hand, tens-eleven translocation (TET) 1, an active DNA demethylase abundant in human germ cells, plays a dual role in hPGCLC differentiation: safeguarding hPGCLCs against differentiation into amnion-like cells by repressing key genes with bivalent promoters, and facilitating coordinated activation of genes vital for spermatogenesis and oogenesis by demethylating their promoters.  Our study uncovers the principle of epigenetic reprogramming in humans, making a fundamental advance in human biology, and through the generation of abundant mitotic pro-spermatogonia and oogonia-like cells, represents a milestone for human in vitro gametogenesis (IVG) research and its potential translation into reproductive medicine. Overall design: Transcriptome analysis of human PGCLCs expanded with BMP signaling.