The paradox of Foxd3: how does it function in pluripotency and differentiation of embryonic stem cells?

Abstract

Uncommitted cells of the early mammalian embryo transition through distinct stages of pluripotency, including establishment of ground state "naïve" pluripotency in the early epiblast, transition to a post-implantation "primed" state, and subsequent lineage commitment of the gastrulating epiblast. Previous transcriptional profiling of in vitro models to recapitulate early to late epiblast transition and differentiation suggest that distinct gene regulatory networks are likely to function in each of these states. While the mechanisms underlying transition between pluripotent states are poorly understood, the forkhead family transcription factor Foxd3 has emerged as a key regulatory factor. Foxd3 is required to maintain pluripotent cells of the murine epiblast and for survival, self-renewal and pluripotency of embryonic stem cells (ESCs). Two recent, simultaneous studies have shed light on how Foxd3 regulates gene expression in early cell fate transitions of progenitor cells. While the two publications shared some common findings, they also presented some conflicting results and suggest different models for the mechanisms underlying Foxd3 function. Here, we discuss the key similarities and differences between the publications, highlight data from the literature relevant to their findings, and hypothesize a potential mechanism of Foxd3 action.

Keywords: Pluripotency; embryonic stem cell (ESC); epiblast; forkhead; pioneer factor.

Figure 1

Schematic overview of pluripotent epiblast development and corresponding in vitro stem cell models. Illustrations of (A) E3.5 blastocyst and (B) E4.5 embryo from which (C) ESCs are derived (dashed arrows). (D) Naïve ESCs grown in LIF and 2i (MAPK and GSK3 inhibitors) correspond to the early pre-implantation epiblast stage (3). (E) Illustration of a post-implantation embryo at E5.5 and (F,G) models of “primed” epiblast progenitors including differentiation of ESCs into (F) EpiCs, induced at 2 day after removal of LIF+2i and captured via fluorescent reporters and FACS (*), and (G) EpiLCs, induced by Fgf and Activin and removal of LIF+2i (6,7). (H,I) Derived from the epiblast of E5.5−6.5 embryos (H) or from the serial passage of ESCs in Fgf and Activin, (I) EpiSCs are molecularly similar to gastrulating epiblast progenitors and can differentiate into lineages of all three germ layers. (J) PGCLCs can be derived from EpiLCs by the addition of Bmp4, Bmp8, EGF, SCF, and LIF (5) (G,J) and are inefficiently generated by EpiSCs (8,9) (gray arrow). EpiC, epiblast cell; EpiLC epiblast-like cell; EpiSC, epiblast stem cell; ESC, embryonic stem cell; PGC primordial germ cells; PGCLC, primordial germ-like cells; PS, primitive streak; VE, visceral endoderm.