Regulation of stem cell pluripotency and differentiation involves a mutual regulatory circuit of the NANOG, OCT4, and SOX2 pluripotency transcription factors with polycomb repressive complexes and stem cell microRNAs

Abstract

Coordinated transcription factor networks have emerged as the master regulatory mechanisms of stem cell pluripotency and differentiation. Many stem cell-specific transcription factors, including the pluripotency transcription factors, OCT4, NANOG, and SOX2 function in combinatorial complexes to regulate the expression of loci, which are involved in embryonic stem (ES) cell pluripotency and cellular differentiation. This review will address how these pathways form a reciprocal regulatory circuit whereby the equilibrium between stem cell self-renewal, proliferation, and differentiation is in perpetual balance. We will discuss how distinct epigenetic repressive pathways involving polycomb complexes, DNA methylation, and microRNAs cooperate to reduce transcriptional noise and to prevent stochastic and aberrant induction of differentiation. We will provide a brief overview of how these networks cooperate to modulate differentiation along hematopoietic and neuronal lineages. Finally, we will describe how aberrant functioning of components of the stem cell regulatory network may contribute to malignant transformation of adult stem cells and the establishment of a "cancer stem cell" phenotype and thereby underlie multiple types of human malignancies.

FIG. 1.

Pluripotent stem cells can be derived from cells isolated from the inner cell mass of early stage blastocysts (A) or experimentally derived by epigenetic reprogramming of differentiated adult cell types (B). Greatest reprogramming efficiency is achieved when combinations of 4 factors, OCT4, SOX2, c-MYC, and KLF4, or OCT4, SOX2, NANOG, and LIN28 genes are introduced into the differentiated cell. However, OCT4 and SOX2 appear critically required to induce pluripotency.

FIG. 2.

Bivalent chromatin domains, composed of activating (histone H3K4 trimethylation) and repressive (H3K27 trimethylation) histone tail modifications (indicated in gray) are a hallmark of developmentally regulated genes. Transcriptional repression of pro-differentiation genes is maintained in pluripotent stem cells by polycomb repressive complexes (PRC) 1 and 2. The core PCR2 includes the EZH2 H3K27 methyltransferase, SUZ12, and EED. However, HDAC1 and YY1 have been shown to interact with PRC2. PRC1 includes Bmi1, the Ring1A/B ubiquitin ligases, and mammalian homologs of Drosophila polyhomeotic (Ph) and polycomb (Pc). Transcriptional repression can be relieved by the combined actions of the JMJD3 and UTX H3K27 demethylases. DNA CpG methylation also contributes to the developmental regulation of gene expression. Individual CpG islands may be unmethylated (○), partially/hemi-methylated (), or fully methylated (•).

FIG. 3.

Convergent stem cell regulatory networks control expression of key stem cell genes. One classical representative of this transcriptional network is the Zfp42/Rex1 gene. Expression of the Zfp42/Rex1 gene has long been used as a marker of undifferentiated stem cells and is regulated by Nanog, Sox2, and Oct4, and by the Wnt pathway. Expression of Zfp42/Rex1 is also subject to epigenetic regulation by polycomb complexes and DNA methylation (see Table 1).

FIG. 4.

The reciprocal regulatory circuit composed of Oct4–Sox2–Nanog; polycomb repressive complexes and microRNAs regulate the transcriptional responses necessary to balance self-renewal and differentiation.