Abstract
Nutrient deficiency during pregnancy in numerous animal species can induce the state of embryonic diapause. Diapause is characterized by changes in protein and gene expression that minimize the organism's reliance on external energy sources and ensure survival. Remarkably, the systematic changes associated with diapause appear to spare the gene expression program that supports embryonic cells' maintenance in the pluripotent state. The phenomenon of the differentiation "freeze" during diapause can be reproduced in vitro . Mimicking nutrient deficiency by pharmacological inhibition of mTOR induces the diapause-like state in ES cells without affecting ES cell pluripotency. We discovered a connection between mTOR signaling and the chromatin-bound bromodomain and extra-terminal (BET) transcriptional regulator BRD4, showing a key role of BET-protein in the induction of diapause-like state in ES cells. mTOR inhibition rapidly and negatively impacts BRD4 binding to chromatin, which is associated with changes in gene expression that can contribute to diapause. Conversely, pharmacological inhibition of BET-protein circumvents the diapause dependence on mTOR inhibition and causes the diapause-like state. BET-repressed diapause-like ES cells retain the undifferentiated pluripotent state, which is associated with upregulation of a functionally linked group of genes encoding negative regulators of MAP kinase (MAPK) signaling and inactivation of MAP kinase. The transcriptional switch-off of MAP kinase following chronic BET inhibition imitates the transcriptional de-repression of MAP kinase negative regulators in response to mTOR inhibition. Mechanistically, suppression of mTOR or BET-protein leads to a profound decline in Capicua transcriptional repressor (CIC) at promoters of key negative regulators of MAP kinase. The discovered mTOR-BRD4 axis in the induction of diapause and the rapid transcriptional shut-off of differentiation program is likely to play a major role in the maintenance of embryonic diapause in vivo , as well as in controlling of the undifferentiated state of various types of stem cells during diapause-like metabolic dormancy.
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