Characterization of Somatotrope Cell Expansion in Response to GHRH in the Neonatal Mouse Pituitary.

In humans and mice, loss of function mutations in growth hormone releasing hormone receptor (GHRHR) cause isolated growth hormone deficiency. The mutant GHRHR mouse model, GhrhrLit/Lit (LIT) exhibits loss of serum growth hormone, but also fewer somatotropes. However, how loss of growth hormone releasing hormone signaling affects expansion of stem and progenitor cells giving rise to growth hormone producing cells is unknown. LIT mice and wildtype littermates were examined for differences in proliferation and gene expression of pituitary lineage markers by quantitative real-time PCR and immunohistochemistry at postnatal day 5 (p5) and 5 weeks. At p5, the LIT mouse shows a global decrease in pituitary proliferation measured by proliferation markers Ki67, and Phospho-Histone H3 (pH3). This proliferative defect is seen in a pituitary cell expressing POU1F1 with or without growth hormone (GH). SOX9 positive progenitors show no changes in proliferation in p5 LIT mice. Additionally, the other POU1F1 lineage cells are not decreased in number, rather we observe an increase in lactotrope cell population as well as mRNA for Tshb and Prl. In the 5 week LIT pituitary, the proliferative deficit in POU1F1 expressing cells observed neonatally persists, while the number and proliferative proportion of SOX9 cells does not appear changed. Treatment of cultured pituitary explants with GHRH promotes proliferation of POU1F1 expressing cells, but not GH positive cells, in a MAPK dependent manner. These findings indicate that hypothalamic GHRH targets proliferation of a POU1F1 positive cell, targeted to the somatotrope lineage, to fine tune their numbers.