Paracrine Signalling From SOX2-Expressing Pituitary Embryonic Cells Is Required for Terminal Differentiation of Hormone-Producing Cells

Abstract

The pituitary gland is the master regulator of the endocrine system, housing six major hormone producing cell types. This gland is derived from Rathke’s Pouch, an invagination of the oral ectoderm. Hormone-producing pituitary cell lineages are derived from a population of embryonic cells expressing SOX2. ZFP36L1/Butyrate Response Factor 1 (BRF1) is an RNA binding protein that binds and targets mRNAs of various cytokines and chemokines for degradation prior to translation, attenuating secretion of inflammatory factors (Herranz et al. 2015). Here, we show that BRF1 is a novel marker expressed in SOX2+ cells in human and mouse pituitaries, suggesting that these cells may have a secretory profile. To investigate this possibility, we have combined molecular and genetic studies in vivo. We have used a novel mouse model, R26lsl-mBRF1 that allows the expression of a mutant, constitutively active BRF1 protein upon Cre-mediated recombination, alongside our lab’s models (Hesx1Cre/+ and Sox2CreERT2/+), to express mutant BRF1 in HESX1+ and SOX2+ cells during development and postnatally. This approach results in pituitary hypoplasia and severe hypopituitarism due to a failure of cell-lineage specified cells to differentiate into hormone-producing cells. Hormone production in these mutant cells, however, can be rescued in vitro through co-culture with WT pituitaries and in vivo in chimeric pituitaries, highlighting a cell non-autonomous mechanism underlying the phenotype. Single cell RNA sequencing of WT and Sox2CreERT2/+;R26lsl-mBRF1 murine embryonic pituitaries, as well as use publicly available human pituitary single cell datasets, have allowed us to identify specific cytokines and chemokines secreted by SOX2+ cells, as well as downstream intracellular signalling pathways in differentiating cells (Zhang et al. 2020), which may be responsible for controlling terminal differentiation of hormone-producing cells within the developing pituitary. Together with our recently published data, these results support the notion that SOX2+ pituitary stem cells play a critical paracrine role in controlling progenitor cell proliferation and terminal differentiation (Russell et al. 2021).