ZIC3 Controls the Transition from Naive to Primed Pluripotency

摘要

class="head no_bottom_margin" id="sec1title">IntroductionEarly embryonic development involves the transition of pluripotent embryonic stem cells through intermediate cell states into the cell lineages that initiate subsequent development events. Using defined in vitro conditions, several different states have been identified for mouse embryonic stem cells (ESCs), starting from the naive ground state and progressing through epiblast-like cells (EpiLCs), to establish an epiblast stem cell (EpiSC) state (; reviewed in ). Subsequently, EpiSCs can differentiate into the three germ layers: mesoderm, ectoderm, and endoderm. Mouse ESCs can be maintained in the naive ground state in defined media, which includes two kinase inhibitors (known as “2i”) to block the MEK/ERK and GSK3 signaling pathways (; reviewed in ). Withdrawal of 2i, allows the cells to progress to either EpiLCs or EpiSCs by altering culture conditions (, ). The naive ESCs are thought to represent a model for the pre-implantation epiblast (embryonic 3.5 [E3.5]–4.5) whereas EpiLCs or EpiSCs cells are models for the post-implantation epiblast (E5.5) ().As ESCs progress from the naive ground state, large changes are observed in their chromatin landscapes and underlying gene expression programs (, ; reviewed in ). The pluripotent state is maintained through the action of a core set of transcription factors and chromatin regulators that include the well-studied NANOG, KLF4, SOX2, and OCT4 (reviewed in ). However, comparatively less is known about the regulators controlling the transition to EpiLCs and EpiSCs. Recently, OTX2 was identified as a key transcription factor driving this transition, partly through cooperative interactions with OCT4/POU5F1 (, , ). Proteomics analysis also identified ZIC2/3 and OCT6/POU3F1 as interacting proteins for OCT4, specifically in EpiLCs (), suggesting a potential co-regulatory role for these transcription factors in this context. Further changes occur during the transition to EpiLCs, and in addition to transcriptional regulators, other proteins have been shown to play an important role during this transition such as the extracellular signaling protein, Cripto, which controls metabolic reprogramming (href="#bib20" rid="bib20" class=" bibr popnode">Fiorenzano et al., 2016).To further our understanding of the regulatory networks controlling the transition from the naive ESC state to EpiLCs, we examined the chromatin accessibility changes accompanying this early transition in mouse ESCs. We focused on areas of dynamic chromatin opening and through DNA binding motif enrichment and associated gene expression data analysis, we identified the transcription factor ZIC3 as an important regulatory transcription factor in this context. ZIC3 controls the expression of EpiLC marker genes such as Fgf5 and many of the ZIC3 target genes encode transcriptional regulators such as GRHL2, which has an important role in enhancer formation in the transition to EpiLCs. ZIC3 therefore is immediately upstream of a set of pro-differentiation regulators that work together to establish the EpiLC state.