Mapping the route from naive pluripotency to lineage specification

Abstract

In the mouse blastocyst, epiblast cells are newly formed shortly before implantation. They possess a unique developmental plasticity, termed naive pluripotency. For development to proceed, this naive state must be subsumed by multi-lineage differentiation within 72 h following implantation. In vitro differentiation of naive embryonic stem cells (ESCs) cultured in controlled conditions provides a tractable system to dissect and understand the process of exit from naive pluripotency and entry into lineage specification. Exploitation of this system in recent large-scale RNAi and mutagenesis screens has uncovered multiple new factors and modules that drive or facilitate progression out of the naive state. Notably, these studies show that the transcription factor network that governs the naive state is rapidly dismantled prior to upregulation of lineage specification markers, creating an intermediate state that we term formative pluripotency. Here, we summarize these findings and propose a road map for state transitions in ESC differentiation that reflects the orderly dynamics of epiblast progression in the embryo.

Keywords: embryonic stem cells; epiblast; lineage specification; pluripotency; self-renewal; signalling.

Figure 1.

Progression from naive to primed pluripotency. (a) Progression of epiblast development in the mouse embryo and corresponding conceptual pluripotent stages. The mature blastocyst comprises three cell lineages: naive epiblast (dark blue), PrE (green) and trophoblast (grey). By E6.5 lineage priming has commenced. Ectoderm, blue; mesoderm, red; definitive endoderm, orange; germline, brown (adapted from Najm et al. [8]). (b) Progression of naive ESCs to a lineage primed state upon 2i withdrawal. Rex1 is asynchronously downregulated and exit from the naive state is marked with loss of Rex1. These Rex1-negative cells might resemble the early postimplantation epiblast, the intermediate formative stage from which lineage-specified cells emerge. (c) Expression periods of naive, early postimplantation and priming factors together with Oct4 and Sox2 during pluripotency transitions.

Figure 2.

Negative regulators of naive pluripotency. In the presence of active MEK/ERK and GSK3, the naive pluripotency factors are subject to coordinated attack at different levels: transcriptional repression, mRNA stability and translation, nuclear/cytoplasmic localization. Induced/activated transcription factors Otx2, Zic2 and Zfp281 might cooperate with these mechanisms to further suppress the levels of naive factors. They also activate new genes that mediate further developmental progression.