FSH regulates maternal mRNA translation in the oocyte and promotes developmental competence

The development of the ovarian follicle is under the control of the gonadotropins FSH and LH. FSH promotes granulosa cell growth and prepares the follicle to respond to LH, whereas the LH surge is the major signal for oocyte meiotic resumption and ovulation. However, an increased secretion in FSH accompanies the LH surge prior to ovulation in most mammalian species. The exact function of this FSH surge is unclear even though it is thought to be important for oocyte maturation and developmental competence. In rodents, FSH and the growth factor EGF have been shown to increase the developmental competence of cumulus oocyte complexes matured in vitro. Improvement in fertilization and pregnancy rates has been demonstrated in IVF patients stimulated with FSH/hCG compared to hCG alone. The signaling pathways and mechanisms by which FSH improves oocyte quality are unknown. Here we have tested the hypothesis that FSH improves oocyte quality by regulating the translation of maternal mRNAs in mouse oocytes. Experiments were conducted in cumulus oocyte complexes (CEOs) collected from ovaries of 21-24 days old PMSG-primed mice, and cultured in groups of 30-50. All the experiments were repeated 3-6 times. Data were analyzed by t-test, OneWay ANOVA or TwoWay ANOVA. P values <0.05 were considered statistically significant. Dual luciferase translation reporters were co-injected in oocytes still enclosed in the cumulus cells. The translation of prototypic luciferase reporters (Tpx2 and interleukin7 – Il7 3’UTRs) was significantly increased when CEOs were cultured in the presence of FSH. Consistent with the increased translation, the IL7 secretion was significantly higher in the culture supernatant of CEOs cultured with FSH compared to control. The FSH-dependent increase in translation was preceded by a transient phosphorylation of AKT in the oocyte, measured by western blot. Knockdown of the expression of the EGF receptor (Egfr) in the granulosa/cumulus cells using a genetic approach (EgfrΔ/loxP:Cyp19cre) prevented the oocyte AKT activation by FSH. These results indicate that the FSH signal transduction requires EGFR for its action. To further investigate the role of PI3K/AKT in the regulation of oocyte translation by FSH and its effect on developmental competence we used a genetic model where Pten (phosphatase and tensin homologue), which negatively regulates PI3K, is specifically ablated in oocytes. AKT was constitutively activated in PtenloxP/loxP:Zp3cre oocytes, independently of the culture treatment. In absence of any hormone, the constitutive AKT activation was sufficient to induce an increase in the translation of the luciferase reporters compared to controls. The fertilization rate was significantly increased in PtenloxP/loxP:Zp3cre oocytes in the absence of hormone treatment. These data strongly suggest that FSH promotes oocyte developmental competence by regulating translation in the oocyte, a regulation that requires an intact EGFR signaling in cumulus cells. These findings provide a molecular rationale for the use of FSH to improve egg quality during assisted reproductive technologies. Supported by: FP7-PEOPLE-2013-IOF GA 624874 MateRNA and NICHD-NIH NCTRI P50HD055764