EPAB is Required for the Development of Oocyte-Somatic Cell Communication in Preantral Follicles

EPAB (Embryonic-Poly(A) Binding Protein) is an oocyte-specific RNA binding protein required for translational activation of maternal mRNAs. EPAB knockout (Epab-/-) mice are infertile due defects in oocyte maturation, cumulus expansion, and ovulation. To further understand the role of EPAB during folliculogenesis, we investigated whether oocyte-somatic communication is disrupted as a result of EPAB deficiency. Experimental study. Gap junction (GJ) function was examined by injecting AlexaFluor488 into preantral follicle-enclosed oocytes (FEOs) and measuring the ratio of oocyte and granulosa cell fluorescence. Transzonal processes (TZPs) were examined by fixation and phalloidin staining of F-actin. Follicle growth was measured after 6 days of culture in the presence or absence of oocyte-secreted factors (OSFs) GDF9, BMP15, and FGF8B. Gene expression was examined by qRT-PCR. In Epab-/- FEOs smaller than 125 um, the ratio of fluorescence was comparable to WT FEO's (0.6 vs 0.7; ns) indicating that GJs were functional. However, in Epab-/- FEO's larger than 125 um, the dye was retained in the oocyte and the ratio of fluorescence was significantly lower (0.6 vs. 0.25; P<0.0001). Consistent with the timing of GJ dysfunction, F- actin staining of TZPs was similar in WT and Epab-/- FEOs <125 um, but was dramatically reduced in Epab-/- FEO's >125 um. Since TZPs are important for secretion of OSFs that promote folliculogenesis, the in vitro growth of WT and Epab-/- preantral FEOs was examined. After 6 days of culture, WT FEOs reached the early antral stage (272 um) whereas Epab-/- FEOs did not grow beyond the preantral stage (182 um). Addition of OSFs to the culture restored the growth of Epab-/-FEO's (182 um vs. 258 um; P<0.0001) while WT FEO's were unaffected (272 um vs. 250 um; n.s.). Furthermore, expression of glycolytic transcripts (Ldha, Pfkp, Pkm2) that are regulated by OSFs was significantly lower in cumulus cells from Epab-/- mice. Lastly, Epab expression during folliculogenesis was examined. The copy number of Epab transcripts in the oocyte increased significantly from the primary to preantral stage (98,053 vs. 247,084; P<0.05) and remained steady until the antral stage (279,384). Our findings demonstrate a precise role for oocyte-specific EPAB at the preantral stage of folliculogenesis in promoting oocyte-somatic communication.