Regulation of Follicle-Stimulating Hormone-Receptor Messenger RNA in Hen Granulosa Cells Relative to Follicle Selection

Both the viability of hen prehierarchal follicles and subsequent differentiation associated with the selection of a single follicle per day into the preovulatory hierarchy depend on circulating FSH and the expression of FSH receptor (FSH-R) in granulosa cells. The present study addresses mechanisms that mediate both basal expression plus selective up-regulation of FSH-R mRNA in granulosa cells from prehierarchal follicles. Results demonstrate that FSH-R mRNA is both expressed and functional in granulosa cells collected from growing prehierarchal follicles as small as those of 1–2 mm in diameter, as indicated by rapid induction of steroidogenic acute regulatory (StAR) protein expression by FSH in vitro. Real-time polymerase chain reaction determined that relative FSH-R expression within the granulosa layer from individual prehierarchal follicles of 6–8 mm in diameter was similar among the 8–13 follicles within this cohort, with the notable exception that the granulosa layer from a single follicle (presumably the selected follicle) showed elevated expression. Levels of FSH-R mRNA expression were enhanced by both recombinant human (rh) transforming growth factor (TGF) ?1 and, to a lesser extent, rh-activin A after 20 h of culture. This stimulatory effect was effectively blocked by mitogenactivated protein (MAP) kinase signaling induced by TGF(alpha) treatment. Finally, inhibition of MAP kinase signaling, using the selective inhibitor U0126, promoted FSH-R expression and further enhanced TGF?1induced FSH-R expression in vitro. Collectively, results suggest that premature granulosa cell differentiation normally is suppressed by tonic MAP kinase signaling. At the time of follicle selection, a release from inhibitory MAP kinase signaling is proposed to occur, which enables the full potentiation of FSH-R expression mediated by intrafollicular factors.