Recently, we discovered in the adult anterior pituitary a subset of cells with ‘side population’ (SP) phenotype, enriched for expression of stem/progenitor cell-associated factors like Sca1, and of Notch1 and Hes1, components of the classically developmental Notch pathway. In the present study, we elaborated the expression of the Notch signaling system in the postnatal pituitary, and examined its functional significance within the SP compartment.

Using RT-PCR, we detected in the anterior pituitary of adult mouse the expression of all 4 vertebrate Notch receptors, as well as of Hes1, 5 and 6, key downstream targets and effectors of Notch. All Notch receptors, Hes1 and Hes5 were measured at higher mRNA levels in the Sca1^high SP than in the main population (MP) of differentiated hormonal cells. In contrast, Hes6, known as an inhibitor of Hes1, was more abundant in the MP.

Cells with SP phenotype, enriched for Sca1^high expression, were detected throughout postnatal life. Their proportion was higher in immature mice, but did not change from adult (8-week-old) to much older age (1-year-old). Notch pathway expression was higher in the Sca1^high SP than in the MP at all postnatal ages analyzed.

Functional implication of Notch signaling in the SP was investigated in re-aggregate cultures of adult mouse anterior pituitary cells. Treatment with the -secretase inhibitor DAPT downregulated Notch activity and reduced the proportion of SP cells. Activation of Notch signaling with the conserved DSL motif of Notch ligands, or with a soluble ligand, caused a rise in SP cell number, at least in part due to a proliferative effect. The SP also expanded in proportion when aggregates were treated with LIF, bFGF and EGF, again at least partly accounted for by a mitogenic action. These intrapituitary growth factors all activated Notch signaling, and DAPT abrogated the expansion of the SP by bFGF and LIF, thus exposing a possible crosstalk.

In conclusion, we show that the Notch pathway, typically situated in embryogenesis, is also present and active in the postnatal pituitary, that it is particularly expressed within the SP independent of age, and that it plays a role in the regulation of SP abundance. Whether our data indicate that Notch regulates renewal and fate decisions of putative stem/progenitor cells within the pituitary SP as found in other tissues, remains open for further exploration.