PROJECT SUMMARY/ABSTRACT Derivation of pluripotent stem cells (PSCs) has revolutionized developmental biology and regenerative medicine. To stably maintain PSCs in culture and guide them to differentiate with high efficiency and fidelity into a variety of cell types, it is important to understand the molecular mechanisms governing pluripotency (the ability of a cell to generate any tissues in the body). Two phases of pluripotency, naïve and primed, have been defined and studied in detail thanks to the successful derivation of mouse embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs), respectively. Mouse ESCs most closely resemble epiblast from a 4-day-old mouse blastocyst (~embryonic day 4, or E4), while “primed” EpiSCs display a global gene expression signature similar to the E7 epiblast of a post-implantation mouse embryo. Despite these advances, however, however, there is lack of a well-established PSC model that resembles E5-6 early post-implantation epiblast, which corresponds to the formative phase of pluripotency. Formative pluripotency exists within a time window during which naïve pluripotency is reconfigured to prepare for multilineage competency, including germ cells. Functionally, formative pluripotency is characterized by both chimera competency and permissiveness for direct primordial germ cell (PGC) induction. Several recent studies have attempted to define this state by transient epiblast-like cells (EpiLCs) differentiated from ESCs. To date, however, stable formative PSCs have not yet been generated. By modulating the FGF, TGF-β and WNT pathways, we recently derived PSCs from both mice and humans (referred to as FTW-PSCs) that are permissive for direct PGC-like cell induction in vitro and are capable of contributing to intra- or inter-species chimeras in vivo. FTW-PSCs harbor molecular, cellular and phenotypic features characteristic of formative pluripotency. The overall objective of this proposal is to use these newly established cell lines to comprehensively dissect the formative state across species. The proposed studies will elucidate the roles of several transcription factors in regulating mouse and human formative pluripotency, as well as demonstrate that FTW-PSCs are a robust platform for dissecting the molecular mechanisms underlying human and mouse PGC specification. In addition, we will establish an in vitro platform for the generation of functional mouse oocytes and human oogonia based on formative FTW-PSCs, thereby providing an invaluable resource for studying germ cell development and human infertility. Our proposal has tremendous potential to revolutionize regenerative medicine and reproductive biology.

PROJECT NARRATIVE Pluripotency refers to the ability of a cell to generate any adult tissue in the body, and can be recapitulated in different types of pluripotent stem cells (PSCs) in culture. Mechanistic understanding of how pluripotency is regulated in PSCs is important to realize their full potential for regenerative medicine. The proposed project is focused on deciphering the molecular mechanisms regulating newly discovered PSCs in our laboratory that are competent for efficient differentiation into both somatic and germ cell lineages.