The pregnant myometrium is epigenetically activated at contractility-driving gene loci prior to the onset of labor in mice.

Virlana M. Shchuka,Nawrah Khader,Anna Dorogin,Huayun Hou,Oksana Shynlova,Jennifer A. Mitchell,Luis E. Abatti,Michael D. Wilson,Rocio Melissa Rivera

doi:10.1371/journal.pbio.3000710

Abstract

During gestation, uterine smooth muscle cells transition from a state of quiescence to one of contractility, but the molecular mechanisms underlying this transition at a genomic level are not well-known. To better understand these events, we evaluated the epigenetic landscape of the mouse myometrium during the pregnant, laboring, and postpartum stages. We generated gestational time point-specific enrichment profiles for histone H3 acetylation on lysine residue 27 (H3K27ac), histone H3 trimethylation of lysine residue 4 (H3K4me3), and RNA polymerase II (RNAPII) occupancy by chromatin immunoprecipitation with massively parallel sequencing (ChIP-seq), as well as gene expression profiles by total RNA-sequencing (RNA-seq). Our findings reveal that 533 genes, including known contractility-driving genes (Gap junction alpha 1 [Gja1], FBJ osteosarcoma oncogene [Fos], Fos-like antigen 2 [Fosl2], Oxytocin receptor [Oxtr], and Prostaglandin G/H synthase 2 (Ptgs2), for example), are up-regulated at day 19 during active labor because of an increase in transcription at gene bodies. Labor-associated promoters and putative intergenic enhancers, however, are epigenetically activated as early as day 15, by which point the majority of genome-wide H3K27ac or H3K4me3 peaks present in term laboring tissue is already established. Despite this early exhibited histone signature, increased noncoding enhancer RNA (eRNA) production at putative intergenic enhancers and recruitment of RNAPII to the gene bodies of labor-associated loci were detected only during labor. Our findings indicate that epigenetic activation of the myometrial genome precedes active labor by at least 4 days in the mouse model, suggesting that the myometrium is poised for rapid activation of contraction-associated genes in order to exit the state of quiescence.

Highlights

Over the course of gestation, the myometrium transitions from a state of quiescence during pregnancy to one of contractile activity during labor in response to both hormonal and mechanical signals
We found that intergenic regions exhibiting H3 acetylation on lysine residue 27 (H3K27ac) peaks and labor–up-regulated enhancer RNA (eRNA) expression displayed an enrichment of activator protein 1 (AP-1) transcription factor motifs, thereby implicating FBJ osteosarcoma oncogene (FOS) and Jun proto-oncogene (JUN) proteins in the distal regulation of gene transcription changes at labor onset
We propose that the presence of H3K27ac and H3 trimethylation of lysine residue 4 (H3K4me3) marks at labor-associated gene promoters and putative intergenic enhancers in the precontractile mouse myometrium as early as gestational day 15 epigenetically activates these regulatory regions, even at a stage of pregnancy when the expression of many labor-associated genes is comparatively low

Summary

Introduction

Over the course of gestation, the myometrium transitions from a state of quiescence during pregnancy to one of contractile activity during labor in response to both hormonal and mechanical signals. Concomitant changes in gene expression that accompany this transition are thought to be a driving force for the initiation of labor [1,2]; little is known about the molecular mechanisms underlying these changes. The presence of histone modifications typically associated with active genes and the subsequent recruitment of RNA polymerase II (RNAPII) to gene promoters allow for transcription initiation and transition to elongation, thereby up-regulating gene expression [15]. How, and at what point these events occur in the myometrial genome during gestation are the inquiries guiding this study

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Conclusion