Ontogenic and glucocorticoid-regulated gene expression in the developing neuroendocrine system

Abstract

The neuroendocrine system is a critical regulator of vertebrate homeostasis that includes five hypothalamic-pituitary axes which develop during embryogenesis. Adrenal glucocorticoids play an important role in functional maturation of the anterior pituitary through initiation of growth hormone (GH) production. These studies were aimed at characterizing ontogenic and glucocorticoid-regulated changes in gene expression during neuroendocrine system development in the chick. First, to ascertain timing of initiation and establishment of each neuroendocrine axis, we measured mRNA levels of hypothalamic regulatory factors, their pituitary receptors, and pituitary hormones from embryonic day (e) 10 through post-hatch day (d) 7. We found that the adrenocorticotropic axis is the first to be established (e12), followed by establishment of the thyrotropic (e18), somatotropic (e20), lactotropic (d1), and gonadotropic (d5) axes. Next, we examined in detail mechanisms through which glucocorticoids initiate pituitary GH expression during embryogenesis. We determined that glucocorticoids elevate GH mRNA levels on e11 by increasing transcriptional activity of the GH gene rather than enhancing mRNA stability, and protein synthesis, histone deacetylase activity, ras signaling, and ERK1/2 signaling are required for this activation. Conversely, sustained activation of ERK1/2 and p38MAPK pathways reduced glucocorticoid stimulation of GH expression, indicating the requirement for ERK1/2 activity is transitory. Finally, we identified ras-dva as a novel Pit-1 and glucocorticoid-regulated gene in the chicken embryonic pituitary gland. Pituitary ras-dva mRNA levels increased between e10 and e18, decreased just prior to hatch, and remained low or undetectable post-hatch. Ras-dva expression was highly enriched within the pituitary gland on e18, and glucocorticoids rapidly induced ras-dva mRNA in cultured pituitary cells through a mechanism involving transcriptional activation. Potential regulatory elements within the 5'-flanking region of chicken ras-dva responsible for pituitary-specific expression were identified, as was a 2 kb fragment necessary for its glucocorticoid induction in embryonic pituitary cells. These results enhance our understanding of neuroendocrine system development and establishment during embryogenesis, reveal mechanisms underlying glucocorticoid initiation of GH expression in somatotrophs, and identify a new Pit-1 and glucocorticoid target gene that may play an important role in pituitary development.