Polycomb/Trithorax Antagonism: Cellular Memory in Stem Cell Fate and Function

Abstract

Stem cells are continuously challenged with the decision to either self-renew or adopt a new fate. Self-renewal is regulated by a system of cellular memory, which must be bypassed for differentiation. Previous studies have identified Polycomb group (PcG) and Trithorax group (TrxG) proteins as key modulators of cellular memory. In this Perspective, we draw from embryonic and adult stem cell studies to discuss the complex roles played by PcG and TrxG in maintaining cell identity while allowing for microenvironment-mediated alterations in cell fate. Finally, we discuss the potential for targeting these proteins as a therapeutic approach in cancer.

Keywords: Ezh2; Polycomb; Trithorax; UTX; cell fate; cellular memory; epigenetics; histone methylation; leukemia; transcription factors.

Figure 1.. Alternative Splicing of Ezh1 and Ezh2 alter their functional activity.

The exon structure of different splice variants of (A) Ezh1, and (B) Ezh2 are shown. The positioning of the WD (light blue), SANT (green), CXC (dark blue), and SET (orange) domains are indicated.

Figure 2.. Phosphorylation sites within the Ezh2 protein that alter its functional activity.

Amino acid number for the start and end of the WD (light blue), SANT (green), CXC (dark blue), and SET (orange) domains are indicated. Blue balloons indicate sites of phosphorylation by the indicated kinase.

Figure 3.. TF-mediated targeting of TrxG and PcG proteins to muscle-developmental genes.

Top) PRC2-mediated repression of muscle development genes. In proliferating MuSCs, the gene encoding the muscle-specific TF myogenin is repressed through the recruitment of the PRC2 complex, which establishes a chromatin environment marked by H3K27me3. The targeting of PRC2 complex to the myogenin locus is mediated by the DNA-bound TF YY1. Bottom) Targeted de-repression of muscle development genes by TrxG proteins. Upon environmental cues to undergo differentiation, an exchange of TFs takes place at the myogenin promoter where YY1 is replaced by the TFs MyoD, Six4, and Mef2D. The binding of MyoD allows for the recruitment of the SWI/SNF nucleosome remodeling complex to the myogenin promoter. Six4 binding allows the recruitment of Trr-COMPASS, which is responsible for removing H3K27me3 marks at the myogenin gene. Finally, Mef2D binding allows for the recruitment of the Trx-COMPASS complex that puts the transcriptionally permissive H3K4me3 mark in place. Thus, the recruitment of different TrxG protein complexes to the myogenin promoter can be explained by targeting through DNA-binding TFs.

Figure 4.. Model for TF-mediated Recruitment of TrxG proteins as a Cellular Memory in Stem Cells.

In the absence of TrxG protein recruitment, PcG-mediated repression is the default state in developmentally regulated genes. As the association of PcG proteins with developmentally regulated genes is delayed after DNA replication in S-Phase of the cell cycle, TF binding at specific genes allows an opportunity for TrxG recruitment prior to the establishment of PcG mediated repression. Using endodermal differentiation of ES cells as an example, the replicating ES cell must make a cell fate decision to self-renew (left) or differentiate (right). 1) To mediate self-renewal, the ES-specific TF Oct4 (same as parental cell) is stabilized and becomes associated with its target loci that mediates self-renewal, while the endoderm-specific TF GATA4 is degraded. To mediate differentiation, the endoderm-specific TF GATA4 is stabilized and becomes associated with endodermal development genes, while the ES cell-specific TF is degraded. 2) The cell-specific TFs, Oct4 (self-renewal) or GATA4 (differentiation) then recruit TrxG proteins to specific genomic loci to create a cellular memory of the genes that are to be transcribed in the daughter cells – pluripotency genes in Oct4-expressing ES cells and endodermal genes in GATA4-expressing endodermal cells. 3) All developmentally-regulated (mesodermal and ectodermal) genes that are not bound by TrxG proteins are then subjected to PcG-mediated repression as a default. Through such a mechanism, cells would have a long-term memory of the genes it should be expressing (TrxG-bound) and those it should be repressing (PcG-bound).