Primordial Germ Cell Differentiation in Vitro: A Model for Understanding Epigenetic Reprogramming and Genome-Wide DNA Demethylation in Mouse Primordial Germ Cells

Sperm and oocytes are terminally differentiated, sex-specific germ cells, which, upon fertilization will generate a new embryo and leads to species propagation by sexual reproduction. Though fated only to generate eggs or sperm, germ cells have the unique property to imbue zygotes with totipotent capacity, which facilitates the formation of all tissues the embryo will need to survive to adulthood. These characteristics are facilitated by germ cells' ability to pass on genetic information to the next generation, as well as their capacity to initiate genome-wide reorganization and removal of epigenetic information inherited by germ cells during embryogenesis. It is hypothesized that remodeling of this epigenetic information is essential to drive proper embryo development. While these events are not well understood, it is known that the events that underlie these unique properties are initiated in early development, shortly after the germ line is established as a pool of primordial germ cells (PGCs). Efforts to unravel these mechanisms that underlie totipotency in germ cells have been limited due to the inability to isolate, study, and manipulate PGCs. To overcome this obstacle, we hypothesized that PGCs cells could be differentiated from pluripotent embryonic stem cells, and that these cells would serve as a surrogate cell type for the study of PGC biology.;Establishing a new model of lineage differentiation from embryonic stem cells required the development of assays and criteria to rigorously test identity, developmental staging, and epigenetic progression to determine if an in vitro model is able to recapitulate features of endogenous PGCs. To accomplish this, we developed a scalable and transgene-free method to differentiate immature PGCs in vitro using the cell surface markers SSEA1 and cKit that are developmentally and epigenetically reminiscent of immature PGCs. We applied existing assays to validate PGC identity, and devised a new stringent assay based on genetic deletion of a known PGC determinant. We developed a single-cell gene expression methodology to compare gene expression signatures of in vitro derived PGCs and endogenous PGCs, and identified novel criteria to define PGC identity from early endogenous PGCs and in vitro-generated PGCs.;We next used in vitro PGC differentiation to investigate genome-wide DNA demethylation, one of the first epigenetic reprogramming events undertaken by early PGCs. By combining the scalability of this differentiation system with next generation methylation sequencing techniques, we generated the first DNA methylation maps of in vitro derived PGCs, and determined with sequence-specific information that DNA demethylation is genome-wide and likely to involve loss of DNA methylation as a consequence of cell division. We also investigated potentiators of active DNA methylation loss, including the Tet proteins, and their roles in early PGC development.;Finally, we applied single cell gene expression technology to define developmental progression of human PGCs isolated from the gonads of fetuses from elective terminations. We identified a common progenitor stage of PGC development in the human fetal gonad. Furthermore, we adapted our single cell gene expression approaches to interrogate differentiation strategies in the generation of the common human PGC progenitor in vitro.;Together, we have developed a differentiation system to ask questions about epigenetic progression in early germ cells, and have utilized single cell gene expression technology and genomics to characterize seminal events in the epigenetic reprogramming of human and mouse germ cells.