Estrogen receptors in granulosa cells govern meiotic resumption of pre-ovulatory oocytes in mammals

In mammals, oocytes are arrested at the diplotene stage of meiosis I until the pre-ovulatory luteinizing hormone (LH) surge triggers meiotic resumption through the signals in follicular granulosa cells. In this study, we show that the estradiol (E2)-estrogen receptors (ERs) system in follicular granulosa cells has a dominant role in controlling oocyte meiotic resumption in mammals. We found that the expression of ERs was controlled by gonadotropins under physiological conditions. E2-ERs system was functional in maintaining oocyte meiotic arrest by regulating the expression of natriuretic peptide C and natriuretic peptide receptor 2 (NPPC/NPR2), which was achieved through binding to the promoter regions of Nppc and Npr2 genes directly. In ER knockout mice, meiotic arrest was not sustained by E2 in most cumulus-oocyte complexes in vitro and meiosis resumed precociously in pre-ovulatory follicles in vivo. In human granulosa cells, similar conclusions are reached that ER levels were controlled by gonadotropins and E2-ERs regulated the expression of NPPC/NPR2 levels. In addition, our results revealed that the different regulating patterns of follicle-stimulating hormone and LH on ER levels in vivo versus in vitro determined their distinct actions on oocyte maturation. Taken together, these findings suggest a critical role of E2-ERs system during oocyte meiotic progression and may propose a novel approach for oocyte in vitro maturation treatment in clinical practice.