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

Wei Liu,Wenqiang Zhang,Enkui Duan,Ye Yang,Sheng Wang,Qiliang Xin,Yang Xu,Xiao Wang,Huarong Wang,Zhiyuan Zhang,Chao Wang,Yanhao Zhang,Guoliang Xia

doi:10.1038/cddis.2017.82

Abstract

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.

Highlights

In mammals, immature oocytes enter a specialized cell cycle during embryogenesis, but pause at the diplotene stage of prophase around the time of birth for prolonged periods
We provided the experimental evidences showing that E2 and its nuclear receptors in granulosa cells, which are regulated by gonadotropins, govern oocyte meiotic progression by directly regulating Nppc/Npr[2] gene transcription in mouse and human ovaries
Immunofluorescence analysis (Figure 1a) of ovaries revealed that ERα was highly expressed in the theca cells of small follicles, but was present in the mural granulosa cells (MGCs) and cumulus cells (CCs) of large antral follicles following follicular development by pregnant mare's serum gonadotropin (PMSG) stimulation

Summary

Introduction

Immature oocytes enter a specialized cell cycle (meiosis, which could reduce the number of chromosomes from diploid to haploid) during embryogenesis, but pause at the diplotene stage of prophase around the time of birth for prolonged periods. Female fertility in mammals depends on the coordinated development of ovarian follicles and oocytes, which is regulated by two pituitary derived gonadotrophins (FSH and LH) during each reproductive cycle.[22] It is widely accepted that LH is primarily responsible for the stimulation of meiotic resumption and the subsequent ovulation in pre-ovulatory follicles, and FSH stimulates the growth and development of the wave of follicles.[23] Based on this concept, FSH and LH are widely used in human and livestock to control ovarian superovulation and in vitro fertilization (IVF).[24,25,26] understanding the mechanisms of FSH and LH controlling follicular development and oocyte maturation is critical for improving the effectiveness of assisted reproduction techniques (ARTs) in clinical applications. Elucidation of the physiological role of E2-ERs during the process of oocyte maturation will provide potential therapeutic targets in the treatment of oocyte in vitro maturation (IVM) in clinical applications

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Results

Conclusion