Cycling through developmental decisions: how cell cycle dynamics control pluripotency, differentiation and reprogramming

Abstract

A strong connection exists between the cell cycle and mechanisms required for executing cell fate decisions in a wide-range of developmental contexts. Terminal differentiation is often associated with cell cycle exit, whereas cell fate switches are frequently linked to cell cycle transitions in dividing cells. These phenomena have been investigated in the context of reprogramming, differentiation and trans-differentiation but the underpinning molecular mechanisms remain unclear. Most progress to address the connection between cell fate and the cell cycle has been made in pluripotent stem cells, in which the transition through mitosis and G1 phase is crucial for establishing a window of opportunity for pluripotency exit and the initiation of differentiation. This Review will summarize recent developments in this area and place them in a broader context that has implications for a wide range of developmental scenarios.

Keywords: Cell cycle; Cell fate; Differentiation; Pluripotency; Reprogramming; Stem cells.

Fig. 1.

The cell cycle controls developmental decisions. The intersection between cell cycle control and cell fate determination mechanisms involves developmental signals and cyclin-dependent protein kinases (CDKs) targeting transcription factors that control developmental genes. CDKs also work in parallel with this pathway by modulating the epigenetic landscape and chromosome architecture around developmental genes. The activation of certain target genes determines important cell fate decisions and subsequent lineage commitment. bHLH, basic helix-loop-helix proteins.

Fig. 2.

Mechanisms of lineage priming and pluripotency dissolution in the G1 phase of pluripotent stem cells (PSCs). As PSCs exit M phase, G1-CDK activities are activated and, in concert with developmental signals, act through transcription factors that load onto developmental target genes. Developmental genes are ‘bookmarked’ epigenetically in mitosis for rapid activation in the upcoming G1 phase. In conjunction with this, epigenetic modifiers, such as MLL2 (also known as KMT2D), modify the local epigenetic landscape around developmental genes in G1 phase and cyclin D recruits co-repressors and co-activators. Chromosome loops are then formed, recruiting enhancers to the proximal promoter, thereby establishing the lineage-primed state. Before and after G1 phase, developmental genes are decommissioned as a result of the erasure of some epigenetic marks and chromosome loops. Outside of G1 phase, the pluripotency network is stabilized by S-phase and G2-phase regulators that block pluripotency dissolution. Dissolution of pluripotency and lineage priming work in concert to orchestrate exit from pluripotency and initiate cell fate decisions in G1 phase.

Fig. 3.

Mitotic ‘bookmarking’ and entry into the lineage-primed state in G1 phase. (A) During mitosis the nuclear membrane is broken down and chromatin is highly condensed, as depicted by densely packed nucleosomes. Transcription then halts, coinciding with exclusion of the transcription machinery and most transcription factors from the nucleus. Pioneer factors are retained in mitotic chromatin both specifically and non-specifically. Chromatin modifiers such as MLL are retained by mitotic chromatin and ‘bookmark’ promoters in preparation for activation in G1 phase. Some histone modifications, such as H3K9me3, are also retained in mitosis. H3 is specifically phosphorylated at S10 by the mitotic kinase aurora B, resulting in the eviction of HP1. SUV39H1 is also phosphorylated during mitosis and dissociates from chromatin. (B) Upon mitotic exit, cells respond to differentiation signals, which are transduced to the nucleus through the action of cyclin D, SMADs and other effectors. SMADs, for example, bind with other transcription factors to sites specifically ‘bookmarked’ by pioneer factors during mitosis or primed by other transcription factors during early G1 phase. Chromatin modifiers, such as MLL or newly recruited transcription factors, and CDK components re-establish histone modifications at enhancers and promoters and developmental genes are reset to the lineage-primed state. Also in G1 phase, H3S10 phosphorylation is lost and HP1 and SUV39H1 bind to H3K9me3-enriched chromatin to re-establish heterochromatin, blocking access to transcription factors in these regions.