Evolutionarily Distinctive Transcriptional and Signaling Programs Drive Human Germ Cell Lineage Specification from Pluripotent Stem Cells

Abstract

Germline specification underlies human reproduction and evolution, but it has proven difficult to study in humans since it occurs shortly after blastocyst implantation. This process can be modeled with human induced pluripotent stem cells (hiPSCs) by differentiating them into primordial germ cell-like cells (hPGCLCs) through an incipient mesoderm-like cell (iMeLC) state. Here, we elucidate the key transcription factors and their interactions with important signaling pathways in driving hPGCLC differentiation from iPSCs. Germline competence of iMeLCs is dictated by the duration and dosage of WNT signaling, which induces expression of EOMES to activate SOX17, a key driver of hPGCLC specification. Upon hPGCLC induction, BMP signaling activates TFAP2C in a SOX17-independent manner. SOX17 and TFAP2C then cooperatively instate an hPGCLC transcriptional program, including BLIMP1 expression. This specification program diverges from its mouse counterpart regarding key transcription factors and their hierarchies, and it provides a foundation for further study of human germ cell development.

On the origin of the human germline

“…Other hallmarks of competency for PGC fate include expression of genes such as T and EOMES, which are observed in the nascent amnion (Kojima et al, 2017;Sasaki et al, 2016). These are also the functional transcription factors in the primitive streak at gastrulation across the mammalian species.…”

“…The conserved PGC specifiers BLIMP1 and TFAP2C are also among the first factors crucial for hPGC fate. However, the regulatory networks that these factors operate within, as deduced from the analysis of mutant cells, and their likely targets are not identical to those in mPGCs Kojima et al, 2017;Sasaki et al, 2015;Tang et al, 2015). These features emphasise an evolutionary divergence of the transcription factor regulatory network in PGCs in mouse and human.…”

“…Although SOX17 gene dosage is crucial, concomitant BLIMP1 expression is sufficient for robust induction of hPGC fate and initiation of epigenetic resetting . SOX17 and BLIMP1 are the two factors with a central role in hPGC specification, but other factors, including TFAP2C, are also required (Kojima et al, 2017).…”

“…The expression of another transcription factor, EOMES, occurs upstream of SOX17 in the PGC-competent human epiblast state (Kojima et al, 2017), and in the DE later (Teo et al, 2011). In contrast, brachyury (T, TBXT), a mesodermal factor and activator of mPGC fate (Aramaki et al, 2013), is not essential for hPGCs (Kojima et al, 2017).…”

“…In contrast, brachyury (T, TBXT), a mesodermal factor and activator of mPGC fate (Aramaki et al, 2013), is not essential for hPGCs (Kojima et al, 2017). The conserved PGC specifiers BLIMP1 and TFAP2C are also among the first factors crucial for hPGC fate.…”