PROP1 Regulates the Epithelial to Mesenchymal Transition of Embryonic Pituitary Progenitor/Stem Cells

MARÍA INÉS PÉREZ MILLÁN; MORTENSEN AH; BRINKMEIER ML; CAMPER SA

San Diego, CA

Congreso; The Endocrine Society´s Annual Meeting; 2015

A critical issue for understanding pituitary organogenesis and maintenance is to determine how the balance of stem cell self-renewal and differentiation into hormone producing cells is regulated. We hypothesized that the transcription factors mutated in cases of Multiple Pituitary Hormone Deficiency (MPHD) in humans would provide an entrée to addressing this key question. The most commonly mutated gene in MPHD encodes the transcription factor PROP1, and two of its direct transcriptional target genes also cause MPHD: POU1F1 (PIT1) and HESX1. In mouse fetal development Prop1 expression coincides spatially and temporally with the delamination, migration, and differentiation of progenitor cells in Rathke's Pouch (RP) into hormone producing cells. These processes fail in Prop1 mutant mice, resulting in organ dysmorphology, poor vascularization, and postnatal pituitary hypoplasia (1). The appearance of the dysmorphic organ in Prop1 mutants suggests that progenitors proliferate excessively and are unable to be released from the niche and differentiate. The cells of RP that normally undergo delamination exhibit a switch in expression of markers typical of the epithelial to mesenchymal transition (EMT), such as N-cadherin to E-cadherin (2). Prop1 mutants fail to make this shift in expression properly, suggesting that Prop1 regulates EMT. To test this idea, we set out to identify additional downstream target genes of PROP1. We chose an in vitro approach of identifying DNA sequences bound by PROP1 in an immortalized pituitary cell line by ChIP-Seq technology. We identified enrichment of PROP1 binding at the known and novel sites near the Pou1f1 gene, and interestingly, in genes that encode components of the pathways of ?Cell Junction Signaling? and ?Regulation of EMT?. We examined expression of several candidate genes in Prop1 mutant mice and confirmed that their expression was altered. Prop1 mutants exhibit elevated expression of tight junction proteins like claudins and progenitor or stem cell markers including several SOX genes. In contrast, genes that are typically expressed after EMT showed reduced or undetectable expression in the mutants. These data suggest that Prop1 is necessary for the transition of progenitors into differentiating cells by suppressing expression of progenitor markers and claudin genes, which releases the tight junctions between cells and drives EMT. This study establishes new mechanisms underlying PROP1?s role in pituitary progenitor cell regulation and offers new candidate genes for MPHD that remain unexplained.