Oocyte-specific Embryonic Poly (A)-Binding Protein (EPAB) is Required for Granulosa Cell Erk Signaling in Response to FSH

Embryonic poly (A)-binding protein (EPAB) is an oocyte-specific RNA-binding protein required for translational regulation of gene expression in oocytes and early embryos. Epab-/- females are infertile due to impaired oocyte maturation, cumulus expansion, and ovulation. The aim of this study was to characterize the molecular mechanisms of follicular somatic cell dysfunction in Epab-/- mice. Experimental study. Granulosa cells (GCs) were obtained from ovaries of 12-week old WT and Epab-/- mice, cultured to pre-confluence, serum starved 2-12 hours depending on the experiment and treated with vehicle alone (negative control), FSH (100 mIU), or Forskolin (20 uM; positive control; activates adenylyl cyclase and increases intracellular levels of cAMP independent of FSH and its receptor). Estradiol production by GCs was measured by ELISA. Expression of aromatase and EGR-1 (Early Growth Response-1) mRNA and protein by was detected by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and Western Blot analysis, respectively. To determine whether the ERK signaling in response to FSH receptor activation is affected in GCs of Epab-/- mice, downstream mediators p-ERK1/2, p-MEK1/2 and p-90RSK was examined by Western blot analysis and normalized to house keeping gene GAPDH. Estradiol production in response to FSH was significantly decreased in Epab-/- GCs (p<0.05). Similarly, FSH-induced expression of aromatase and Egr-1 were significantly decreased in the GCs of Epab-/- mice (p<0.05). FSH strongly activated ERK signaling in WT GCs, such that p-ERK1/2, p-MEK1/2 and p-90RSK significantly increased following 5 and 10 min of FSH treatment. However, FSH treatment of Epab-/- GCs did not result in an increase in p-ERK1/2, p-MEK1/2 and p-90RSK. In granulosa cells, EPAB-deficiency results in impaired ERK signaling in response to FSH, associated with decreased aromatase expression and estradiol production. Our findings demonstrate that oocyte-specific EPAB is important for regulating the function of GCs, and that the FSH signaling pathway is, at least in part, responsible for somatic cell dysfunction in Epab-/- mice.