Abstract
During development, pluripotency is a transient state describing a cell’s ability to give rise to all three germ layers and germline. Recent studies have shown that, in vitro, pluripotency is highly dynamic: exogenous stimuli provided to cultures of mouse embryonic stem cells, isolated from pre-implantation blastocysts, significantly affect the spectrum of pluripotency. 2i/LIF, a recently defined serum-free medium, forces mouse embryonic stem cells into a ground-state of pluripotency, while serum/LIF cultures promote the co-existence of ground-like and primed-like mouse embryonic stem cell subpopulations. The latter heterogeneity correlates with temporal fluctuations of pluripotency markers, including the master regulator Nanog, in single cells. We propose a mathematical model of Nanog dynamics in both media, accounting for recent experimental data showing the persistence of a small Nanog Low subpopulation in ground-state pluripotency mouse embryonic stem cell cultures. The model integrates into the core pluripotency Gene Regulatory Network both inhibitors present in 2i/LIF (PD and Chiron), and feedback interactions with genes found to be differentially expressed in the two media. Our simulations and bifurcation analysis show that, in ground-state cultures, Nanog dynamics result from the combination of reduced noise in gene expression and the shift of the system towards a monostable, but still excitable, regulation. Experimental data and agent-based modelling simulations indicate that mouse embryonic stem cell proliferation dynamics vary in the two media, and cannot be reproduced by accounting only for Nanog-dependent cell-cycle regulation. We further demonstrate that both PD and Chiron play a key role in regulating heterogeneity in transcription factor expression and, ultimately, mouse embryonic stem cell fate decision.
Highlights
Mouse embryonic stem cells are pluripotent cells, isolated from the inner cell mass, which can be indefinitely expanded and retain pluripotency, or be pushed into specific differentiated states by proper stimuli in vitro, contributing to all germ layers when injected into host embryos.[1]
Network derivation: identification of transcriptional core ground-state cultures, a small NL subpopulation is still present in Nd Mouse embryonic stem cells (mESCs); consistently, stochastic fluctuations between the two states can be observed within a single cell-cycle (CC), with the same amplitude observed in serum/leukaemia inhibitory factor (LIF) cultured cells.[15]
Gene Regulatory Networks (GRNs) of core pluripotency and differentiation factors play a key role in determining transcriptional dynamics and decision-making of mESCs.[21]
Summary
Mouse embryonic stem cells (mESCs) are pluripotent cells, isolated from the inner cell mass, which can be indefinitely expanded and retain pluripotency, or be pushed into specific differentiated states by proper stimuli in vitro, contributing to all germ layers when injected into host embryos.[1]. This medium has been reported to confer on mESCs a heterogeneous expression and temporal fluctuations of pluripotency factors and regulators, including Nanog, Rex[1], Stella, Esrrb and β-catenin.[2,3,4,5,6] Notably, many of the metastable mESC genes are, directly or indirectly, regulated by Nanog,[7] a master regulator of pluripotency and development.[8] Importantly, mosaic expression patterns in serum/LIF result in an inhomogeneous propensity to differentiate in mESC subpopulations This phenotype is reversible : cells expressing Nanog Low levels, which are more prone to differentiate, can in time switch Nanog on, and vice versa for Nanog High cells.[3]. Sorted cells re-establish the original bimodal distribution on a shorter time-scale.[15, 17] In
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