Regulation and Role of Dax1 in Mouse Embryonic Stem Cells.

Abstract

Embryonic stem cells (ESCs) are derived from the early embryo and give rise to all cells of an organism. Harnessing the power of ESCs promises to open a new era of regenerative medicine. To achieve this goal, we must first understand the basic biology of ESCs and the mechanisms that maintain self renewal and pluripotency, the hallmarks of ESCs. Many factors have been shown to be critical for mouse ESC (mESC) biology, including Dax1, an atypical nuclear receptor. Dax1 was originally thought to be expressed only in steroidogenic tissues. However, recent work has shown that Dax1 is one of the top twenty mRNAs enriched in mESCs, that Dax1 participates in the critical ES cell network by forming protein complexes with many important ES cell factors and that Dax1 binds to over 1700 sites within the genome. These data suggest a significant role for Dax1 in the transcriptome of mESCs. To investigate the role of Dax1 in mESC biology, we began by examining mechanisms of transcriptional regulation of Dax1. In steroidogenic tissues, Dax1 expression is regulated by the nuclear receptor, Sf1. However, Sf1 is not expressed in mESCs; on the other hand, LRH-1, a close family member, is expressed. Our data show that LRH-1 binds to the Dax1 proximal promoter and regulates Dax1 expression. Additionally, we identified an intronic site that is bound by Nanog and contributes to Dax1 regulation. Because in steroidogenic cells Dax1 binds to Sf1 protein to modulate Sf1 target genes, we hypothesized that Dax1 may also bind to LRH-1. We found that Dax1 and LRH-1 interact in mESCs, and that Dax1 localizes to the LRH-1 binding site on the promoter of Oct4, a critical ESC factor. Furthermore, Dax1 and LRH-1 co-activate Oct4 expression in order to maintain proper levels of Oct4. In addition, our data show that Dax1 binds to a functional RNA, called SRA, which acts as a scaffold for co-activator recruitment. Knock down of SRA attenuates Dax1 co-activation of Oct4. These results establish an important role for Dax1 in mESC biology and enhance our knowledge of the mechanisms by which mESCs maintain pluripotency.