Differentiation of the granulosa layer from hen prehierarchal follicles associated with follicle-stimulating hormone receptor signaling.

Recruitment of a single follicle into the preovulatory hierarchy of the domestic hen ovary occurs from a small cohort of prehierarchal follicles measuring 6-8 mm in diameter. We have previously reported that granulosa cells (GCs) collected from prehierarchal follicles express highest levels of membrane-localized follicle-stimulating hormone receptor (FSHR) during follicle development, yet fail to initiate signaling via cAMP following short-term incubation with FSH. Consequently, GC from prehierarchal follicles remain in an undifferentiated state and lack the capacity for steroidogenesis due to a deficiency of cAMP-dependent STAR protein and CYP11A1 gene expression. The present studies investigate FSH responsiveness in GC before and after the transition from undifferentiated to a differentiated state at follicle recruitment. Before recruitment focus is directed toward the inhibition of FSHR-signaling by β-ARRESTIN (βARR). Specifically, knockdown of βARR messenger RNA in cultured, undifferentiated GC using small interfering RNA facilitated FSH-induced cAMP formation, STAR expression, and progesterone production. Furthermore, overexpression of bovine βARR1 and G-protein-coupled receptor kinase2 in actively differentiating GC significantly decreased cAMP accumulation and progesterone production following a challenge with FSH. We propose that a βARR-mediated mechanism maintains FSHR unresponsiveness in undifferentiated GC from prehierarchal follicles, and as a result prevents GC differentiation until the time of follicle recruitment.