Transcriptional and epigenetic mechanisms of the first cell fate decision and reprogramming
The placenta is a transient but vital organ mediating a myriad of interactions between maternal and embryonic tissues. The cells in the trophectoderm (TE) lineage are responsible for proper implantation, placentation, and immunological functions of the placenta. However, our understanding of molecular mechanisms underlying placentation and TE development is still rudimentary. Deciphering the mechanisms by which key TE-specific transcription factors (TFs) control the first cell fate decision, as well as the maintenance and differentiation of TE, is a prerequisite for understanding early embryonic development and ultimately improving healthy pregnancy. First, using a combination of functional genomics, bioinformatics, and mouse genetics, I revealed that Arid3a is a critical regulator for controlling the first cell fate decision and placental development. Ectopically expressed Arid3a induces TE-like gene expression programs in embryonic stem (ES) cells. Moreover, Arid3a is not only essential for maintaining self-renewing TS cells, but also for promoting further differentiation of trophoblastic lineages. Consistently, Arid3a-/- mice suffer from severely impaired post-implantation development, resulting in early embryonic lethality. I further showed that Arid3a directly activates TE-specific genes while repressing pluripotency genes by recruiting HDAC1. Second, I studied the mechanisms underlying TF-mediated conversion of ES to trophoblast stem (TS)-like cells. Upon overexpression of TS cell-specific TFs, Cdx2, Arid3a, and Gata3 (CAG factors) in ES cells, I performed time–course profiling of chromatin accessibility, transcriptomes, and occupancy of these reprogramming factors during reprogramming. Using an integrative analysis, I discovered that CAG factors orchestrate the conversion via a sequential two-step regulation in a timely, ordered manner, with repression of pluripotency genes by decommissioning active enhancers, followed by activation of TS cell-specific genes as pioneer factors that can access closed chromatin. Taken together, my studies unveiled that Arid3a functions as a pivotal regulator of TE and placental development by regulating the commitment to the first cell fate, as well as by executing TE lineage differentiation. I advanced our understanding of the mechanisms underlying TF-mediated reprogramming of ES to TS-like cells, in particular Arid3a-mediated transcriptional and epigenetic regulation. Thus, my studies will be beneficial for enhancing clinical applications such as disease modeling, drug screening, and regenerative therapies.