The Immediate Early Gene Product EGR1 and Polycomb Group Proteins Interact in Epigenetic Programming during Chondrogenesis

Abstract

Initiation of and progression through chondrogenesis is driven by changes in the cellular microenvironment. At the onset of chondrogenesis, resting mesenchymal stem cells are mobilized in vivo and a complex, step-wise chondrogenic differentiation program is initiated. Differentiation requires coordinated transcriptomic reprogramming and increased progenitor proliferation; both processes require chromatin remodeling. The nature of early molecular responses that relay differentiation signals to chromatin is poorly understood. We here show that immediate early genes are rapidly and transiently induced in response to differentiation stimuli in vitro. Functional ablation of the immediate early factor EGR1 severely deregulates expression of key chondrogenic control genes at the onset of differentiation. In addition, differentiating cells accumulate DNA damage, activate a DNA damage response and undergo a cell cycle arrest and prevent differentiation associated hyper-proliferation. Failed differentiation in the absence of EGR1 affects global acetylation and terminates in overall histone hypermethylation. We report novel molecular connections between EGR1 and Polycomb Group function: Polycomb associated histone H3 lysine27 trimethylation (H3K27me3) blocks chromatin access of EGR1. In addition, EGR1 ablation results in abnormal Ezh2 and Bmi1 expression. Consistent with this functional interaction, we identify a number of co-regulated targets genes in a chondrogenic gene network. We here describe an important role for EGR1 in early chondrogenic epigenetic programming to accommodate early gene-environment interactions in chondrogenesis.

The let-7/LIN-41 Pathway Regulates Reprogramming to Human Induced Pluripotent Stem Cells by Controlling Expression of Prodifferentiation Genes

“…EGR1 has been shown to promote differentiation when expressed in embryonal carcinoma cells, which are similar to ESCs, and to regulate differentiation in various contexts (Cao et al, 1990; Carter et al, 2007; Dinkel et al, 1998; Edwards et al, 1991; Harris and Horvitz, 2011; Krishnaraju et al, 1995; Lanoix et al, 1998; Laslo et al, 2006; Le et al, 2005; Lejard et al, 2011; Nguyen et al, 1993; Spaapen et al, 2013; Sukhatme et al, 1988; Topilko et al, 1998; Zhang et al, 2013). Fragola et al proposed that EGR1 functions as a key TF that maintains the fibroblast transcriptional profile (Fragola et al, 2013).…”

PRC1 Prevents Replication Stress during Chondrogenic Transit Amplification

“…Increased methylation at H3K9, H3K27 and H4K20 have been correlated with cell-cycle exit and cell death [67,68]. In keeping with this idea, we recently reported abnormal global trimethylation at H3K4, K9 and K27 in the context of defective TA in chondrogenic differentiation [19]. We currently have no definitive explanation for the decline of H3-associated trimethyl-marks at later stages in development.…”

“…HoxA9-13) and non-target loci (i.a. HoxA1-4) [19] was used to exemplify stable H3K27me3-enrichment at t = 0 and t = 3 days pid ( Figure S8A,B). The H3K27me3 enrichment level, i.e., peak height, associated with stably and actively transcribed genes was used as a cut-off for all samples.…”

“…We recently established that in the absence of the immediate early response gene EGR1, TA fails in a cell model for chondrogenesis [19]. Relevantly, EGR1 displays regulatory interaction with PRC proteins at multiple levels and EGR1-deficient cells show substantial reduction of BMI1 levels during the phase corresponding to the early differentiation-associated proliferative burst [19].…”

Significance of Epigenetic Landscape in Cartilage Regeneration from the Cartilage Development and Pathology Perspective

“…The ablation of EGR-1 results in abnormal EZH2 (H3K27me3 histone methyltransferase) and BMI1 (E3-ubiquitin ligase for H2A) expression. This relationship suggests that there is an important role for EGR-1 in early chondrogenic epigenetic programming to accommodate early gene-environment interactions in chondrogenesis [107].…”