Molecular versatility during pluripotency progression

Abstract

A challenge during development is to ensure lineage segregation while preserving plasticity. Using pluripotency progression as a paradigm, we review how developmental transitions are coordinated by redeployments, rather than global resettings, of cellular components. We highlight how changes in response to extrinsic cues (FGF, WNT, Activin/Nodal, Netrin-1), context- and stoichiometry-dependent action of transcription factors (Oct4, Nanog) and reconfigurations of epigenetic regulators (enhancers, promoters, TrxG, PRC) may confer robustness to naïve to primed pluripotency transition. We propose the notion of Molecular Versatility to regroup mechanisms by which molecules are repurposed to exert different, sometimes opposite, functions in close stem cell configurations.

Fig. 1. A continuum of transitional states defines pluripotency progression.

The schematic diagram depicts the embryonic stages and the corresponding embryo-derived cell types. It also summarises the signalling pathways that sustain the embryo-derived stem cells in vitro and their ability to respond to germ cell inductive signals. ESC: Embryonic Stem Cell, RSC: Rosette Stem Cells, XPSC: X Pluripotent Stem Cells, EpiLC: Epiblast-like Cells, FS: Formative Stem cells, fPSC: Formative Pluripotent Stem Cells, EpiSC: Epiblast Stem Cells.

Fig. 2. Molecular Versatility of signalling pathways.

a FGF/MAPK signalling. The integration of the FGF signal is modulated by the differential expression of the FGFR1 and FGFR2 receptors. Such integration will lead to various degrees of ERK1/2 activation that will subsequently activate different sets of target genes. b Wnt signalling. The activation of the pathway will promote stem cell self-renewal or lineage commitment. These versatile effects are related to the bifunctional activity of β-catenin in the nucleus and in the cytoplasm. It has to be noted that TCF1 and TCF3 also behave antagonistically on pluripotency genes and ESC self-renewal. c Activin/Nodal signalling. Activin/Nodal signalling leads to a various degree of Smad2 activation. This precise degree triggers the regulation of different sets of genes and therefore of different cell fate decisions in ESCs. d Netrin-1 signalling. The Netrin-1 molecule has opposite effects on ESC fate by promoting self-renewal or lineage commitment. This Molecular Versatility is controlled by the stoichiometry of its receptors Neo1 and Unc5b that leads to ERK1/2 activity induction or repression.

Fig. 3. Molecular Versatility of Oct4 during development and reprogramming.

The schematic diagram depicts the different functions exerted by the TF Oct4 depending on its expression level. The expression of Oct4 is tightly regulated because gain and loss-of expression respectively triggering differentiation into primitive endoderm/mesoderm or trophectoderm derivatives in ESCs. Unexpectedly, its endogenous level sustains self-renewal but also primes embryonic stem cells for differentiation while a reduced Oct4 level ensures a robust self-renewal state. A precise Oct4 level is also crucial during reprogramming. Its overexpression triggers aberrant gene inductions while iPS cells generated in absence of Oct4 present an unexpected enhanced developmental potential. PrE: Primitive endoderm, WT level: Wild-type level.

Fig. 4. Molecular Versatility of epigenetic mechanisms.

a The activity of enhancers is controlled by the versatile actions of the FoxD3 transcription factor. In naïve cells, FoxD3 act as a repressor that decommissions active enhancers of naïve pluripotency genes. In primed cells, FoxD3 exerts dual functions by initiating enhancer activity by recruiting Brg1 while simultaneously repressing their maximal activation by recruiting histone deacetylases, illustrating its molecular versatility. b The TF Grhl2 orchestrates an enhancer switching program during pluripotency progression. Grhl2 partitions the pluripotency network and sustain the expression of a subnetwork of epithelial genes during naïve-primed transition. c Promoter switching for the SET gene in ESCs. This switch leads to the generation of two protein isoforms, SETα and SETβ, that play different functions in ESCs. d The subunits of PRC2 dictate different functions for the complex during exit from naïve pluripotency. MTF2 and JARID2 depletion leads to different outcomes during exit from naïve pluripotency. e PRC1 has versatile functions depending on the class of genes it binds to. PRC1-bound active genes showed greater cell-to-cell variation when compared with globally active genes.

Fig. 5. Molecular Versatility during pluripotency progression.

The scheme summarises the different mechanisms highlighted in the Perspective to allow molecules (signalling pathway, TF and epigenetic factors) to exert different functions in closely-related stem cell configurations.