Transcriptional regulation of the Oct4 gene, a master gene for pluripotency

Abstract

Oct4 is one of the most important transcription factors required to maintain an undifferentiated state (self-renewal) and pluripotency of human and mouse embryonic stem (ES) cells as well as early embryonic cells. In addition, Oct4 is the only known transcription factor that has to be exogenously introduced into differentiated cells to make induced pluripotent stem (iPS) cells. Therefore, it is of great importance to understand how Oct4 transcription is regulated in ES cells and embryos and how it becomes activated during iPS cell formation. In this article, we will review the regulation of the mouse Oct4 gene from the viewpoint of DNA methylation, binding of orphan nuclear receptors, histone modifications and synergistic effects with other pluripotency factors. We will also raise several key questions that need to be addressed in future work to improve our understanding of Oct4 gene regulation and its essential role in self-renewal and pluripotency.

Fig. 1

Regulation of Oct4 transcription through DNA methylation and binding of orphan nuclear receptors. During differentiation of mouse ES cells, miR-290 through mir-295 indirectly facilitate transcription of Dnmt3a and Dnmt3b, which in turn methylate the DNA in the three regulatory regions of the Oct4 gene. As a separate mechanism, retinoic acid induces GCNF (early period) and ARP-1/COUP-TFII and EAR-3/COUP-TFII (late period), both of which bind to the PP. GCNF recruits Dnmt3a and Dnmt3b to the PP, contributing to suppression of the Oct 4 gene. In contrast, LRH-1 binds to the PE and the PP and activates Oct4 in undifferentiated ES cells through an uncharacterized mechanism.

Fig. 2

G9a independently induces DNA methylation and histone H3 methylation of the Oct4 gene through its two domains. Addition of retinoic acid to embryonal carcinoma cells induces DNA methylation of the Oct4 gene through the ankyrin repeat (ANK) of G9a as well as di- and trimethylation of lysine 9 on histone H3 (H3K9me2 and H3K9me3) through the SET domain of G9a. H3K9me2 and H3K9me3 recruit HP1 and trigger heterochromatin formation at the Oct4 locus. During these processes, acetylation of lysine 9 and lysine 14 on histone H3 (H3K9ac and H3K14ac) and methylation of lysine 4 on histone H3 (H3K4me) are lost, which is independent of G9a.

Fig. 3

The Paf1 complex induces a series of histone modifications at the target genes. The Paf1 complex binds to the promoters and coding regions of its target genes. At the promoter it induces H2BK123ub1, which subsequently recruits COMPASS and Dot1. These two enzymes induce H3K4me3 and H3K79me2, respectively. H3K4me3 triggers binding of chromatin modifying enzymes, such as NuA3, CHD1 and NURF, for gene activation. The Paf1 complex also recruits Set2 to induce H3K36me3. These cascades have been mainly studied in Saccharomyces cerevisiae and have not been proven to exist at the Oct4 locus in ES cells.