Here we demonstrate that postnatal pituitary SOX

Here we demonstrate that postnatal pituitary SOX2+ stem prostaglandin receptor are derived from the embryonic RP and that the Notch signaling pathway is essential for their proliferation, maintenance, and postnatal pituitary expansion. Furthermore, we present evidence that SOX2+ stem cells make a significant contribution to neonatal pituitary expansion but that they gradually switch to an essentially quiescent state so that they are no longer required for homeostasis and tissue plasticity in the mature gland. Our results suggest that committed cells are capable of adapting to physiological demands and contribute to pituitary gland plasticity.
Results
Discussion Adult stem cells have been identified in the postnatal pituitary gland. These are groups of heterogeneous cells expressing SOX2, SOX9, LHX3, PROP1, E-Cadherin, and NES located either in the marginal zone between the intermediate and anterior lobes or scattered in the anterior lobe (Chen et al., 2009; Fauquier et al., 2008; Garcia-Lavandeira et al., 2009; Gleiberman et al., 2008; Rizzoti, 2010; Vankelecom and Chen, 2014). It has been suggested previously that mouse pituitary adult stem cells are uniquely set aside during embryonic pituitary development and only contribute prominently to postnatal pituitary growth (Gleiberman et al., 2008). In this study, we showed that these stem cells were labeled by the LacZ or GFP reporters in the lineage-tracing experiment using the RP-specific Prop1-Cre line, suggesting that pituitary stem cells are derived from the embryonic RP progenitors and that they continue to proliferate and differentiate to contribute to postnatal pituitary expansion. One common feature of RP progenitors and postnatal stem cells is their expression of the transcription factor SOX2, which has been determined to be expressed in many adult stem cells (Andoniadou et al., 2013; Arnold et al., 2011; Rizzoti et al., 2013). In this study, we demonstrate that postnatal pituitary SOX2+ stem cells are maintained by the canonical Notch signaling pathway. Our result is consistent with the finding that Notch signaling is essential for the long-term maintenance of neural stem cells in the adult brain hippocampus, where it has been shown that RBP-J is recruited to the Sox2 promoter and activates its expression (Ehm et al., 2010). In the absence of Notch signaling, the pituitary stem cells were depleted gradually in the postnatal pituitary gland. This was accomplished by employing Prop1-Cre, which is expressed in all RP progenitors, but not so early to interfere with the lineage commitment program, in which Notch signaling plays a pivotal role (Zhu et al., 2006). In addition, there was a reduction in the number of SOX2+ cells in mutant mice in which Rbp-J was specifically deleted in SOX2+ cells. Consistent with these data, chemical inhibition of Notch activation in primary culture resulted in a decreased number of pituitary spheres. Taken together, these results reveal that Notch signaling controls the fate choices of RP progenitors in a narrow developmental window, whereas it is required continuously for the maintenance of SOX2+ pituitary stem cells. During embryonic pituitary organogenesis, SOX2+ cells exhibit a remarkable capacity for proliferation (Figure S1L; Davis et al., 2011), whereas, after birth, these cells remain mitotically active for the initial 3 weeks and then become mostly quiescent after this period. For the first 3 days after birth, they are the dominant cell type undergoing cell division, and their postnatal proliferation makes a significant contribution to pituitary gland expansion. We show that postnatal pituitary expansion can be attributed to successive and overlapping waves of proliferation of SOX2+ stem cells and committed/differentiated cells, as exemplified by PIT1+ cells. How the mitotic properties of SOX2+ stem cells are regulated dynamically during embryonic development, postnatal growth, and subsequent homeostasis remains to be fully elucidated.