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) beta1 and, to a lesser extent, rh-activin A after 20 h of culture. This stimulatory effect was effectively blocked by mitogen-activated protein (MAP) kinase signaling induced by TGFalpha treatment. Finally, inhibition of MAP kinase signaling, using the selective inhibitor U0126, promoted FSH-R expression and further enhanced TGFbeta1-induced 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.