ERK1/2 signalling dynamics promote neural differentiation by regulating the polycomb repressive complex

Abstract

22 23 24 25 26 2 Abstract 27Fibroblast Growth Factor (FGF) is a neural inducer in many vertebrate embryos, but how it regulates 28 chromatin organization to coordinate the activation of neural genes is unclear. Moreover, for 29 differentiation to progress FGF signalling has to decline. Why this signalling dynamic is required has not 30 been determined. Here we show that dephosphorylation of the FGF effector kinase ERK1/2 rapidly 31 increases chromatin accessibility at neural genes in mouse embryos and, using ATAC-seq in human 32 embryonic stem cell derived spinal cord precursors, we demonstrate that this occurs across hundreds of 33 neural genes. Importantly, while Erk1/2 inhibition induces precocious neural gene transcription, this step 34 involves dissociation of the polycomb repressive complex from gene loci and takes places independently 35 of subsequent loss of the repressive histone mark H3K27me3 and transcriptional onset. We find that loss 36 of ERK1/2 activity but not its occupancy at neural genes is critical for this mechanism. Moreover, transient 37 ERK1/2 inhibition is sufficient for polycomb protein dissociation and this is not reversed on resumption 38 of ERK1/2 signalling. These data indicate that ERK1/2 signalling maintains polycomb repressive complexes 39 at neural genes, that its decline coordinates their increased accessibility and that this is a directional 40 molecular mechanism, which initiates the process of neural commitment. Furthermore, as the polycomb 41 repressive complexes repress but also ready genes for transcription, these findings suggest that ERK1/2 42 promotion of these complexes is a rite of passage for subsequent differentiation. 43 involvement of ERK1/2 signalling in this process. Rather than promoting differentiation however, 69 inhibition of ERK1/2 activity in mouse ES cells supports pluripotency [24, 25] and exposure to FGF/ERK1/2 70 signalling appears here to be an initial step towards differentiation. In agreement with this, FGF is 71 required for neural differentiation of mES cells [26][27][28] although the timing of this requirement differs 72 between assays [29]. Importantly, however, only a short critical period of ERK1/2 signalling is required in 73 mES cells for subsequent expression of neural genes [27], after which FGF inhibition then accelerates 74 neural differentiation [27, 30, 31]. Moreover, in mouse epiblast stem cells (mEpiSC), which rely on FGF 75 for self-renewal, prolonged FGF signaling abrogates neural differentiation [32]. Consistent with this in 76 both mEpiSCs and human ES cells (which also depend on FGF for self-renewal), inhibition of FGF/ERK1/2 77 signalling promotes neural differentiation [30, 32]. These findings indicate that temporal control over 78 FGF/ERK1/2 signalling is instrumental in the estabishment of neural identity from epiblast cell precursors. 79 A clue to the mechanism by which FGF regulates neural differentiation was revealed by analysis 80 of chromatin organisation at key neural progenitor genes in the mouse embryo CLE [33]. This...

Differentiation of Mesenchymal Stem Cells to Neuroglia: in the Context of Cell Signalling

“…In developing brain, NSCs activate NADPH oxidase (NOX) upon stimulation by exogenous ROS, which regulate self-renewal and neurogenesis in a PI3K/Akt-dependant manner (24, 25). However, blockade of PI3K/Akt alone does not abrogate neuronal differentiation indicating the existence of other pathways for signal flow (26, 27).…”