Preovulatory suppression of mouse oocyte cell volume-regulatory mechanisms is via signalling that is distinct from meiotic arrest

Samantha Richard,Jay M Baltz

doi:10.1038/s41598-017-00771-y

Samantha Richard, Jay M Baltz

Open Access

https://doi.org/10.1038/s41598-017-00771-y

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Journal: Scientific Reports	Publication Date: Apr 6, 2017
Citations: 9	License type: open-access

Affiliation: Ottawa Hospital, University of Ottawa

Abstract

GLYT1-mediated glycine transport is the main cell volume-homeostatic mechanism in mouse eggs and early preimplantation embryos. It is unique to these developmental stages and key to their healthy development. GLYT1 first becomes activated in oocytes only after ovulation is triggered, when meiotic arrest of the oocyte is released, but how this occurs was unknown. Here we show that GLYT1 activity is suppressed in oocytes in the preovulatory antral follicle and that its suppression is mediated by a mechanism distinct from the gap junction-dependent Natriuretic Peptide Precursor C (NPPC) pathway that controls meiotic arrest. GLYT1 remained suppressed in isolated antral follicles but not isolated cumulus-oocyte complexes (COCs) or isolated oocytes. Moreover, activating the NPPC signalling pathway could not prevent GLYT1 activation in oocytes within COCs despite maintaining meiotic arrest. Furthermore, blocking gap junctions in isolated follicles failed to induce GLYT1 activity in enclosed oocytes for an extended period after meiosis had resumed. Finally, isolated mural granulosa cells from preovulatory antral follicles were sufficient to suppress GLYT1 in oocytes within co-cultured COCs. Together, these results suggest that suppression of GLYT1 activity before ovulation is mediated by a novel signalling pathway likely originating from preovulatory mural granulosa cells.

Highlights

Oocytes develop in ovarian follicles that support the oocyte during its growth and regulate the meiotic progression of fully-grown oocytes
The physiological trigger for GLYT1 activation is luteinizing hormone (LH) from the pituitary that triggers ovulation and meiotic maturation of oocytes, since GLYT1 activation can be induced in vivo by injecting female mice with the LH analogue, hCG3, and LH can activate GLYT1 in oocytes within isolated follicles in vitro
We have shown here that the isolated preovulatory antral follicle is capable of indefinitely maintaining GLYT1 suppression in the enclosed oocyte in vitro, implying that it mimics the in vivo preovulatory environment

Summary

Introduction

Oocytes develop in ovarian follicles that support the oocyte during its growth and regulate the meiotic progression of fully-grown oocytes. Prior to the LH signal that triggers ovulation, the mouse oocyte is unable to control its own cell volume and instead its size is determined by a tight attachment to its rigid extracellular matrix shell, the zona pellucida[3]. This is in stark contrast to mammalian embryonic and somatic cells, which actively control their volumes[4]. The concentration of cAMP in the oocyte decreases sharply and meiotic arrest is released within the follicle This is a direct consequence of PDE3 activation due to the cessation of its inhibition by cGMP, which is no longer being supplied by the cumulus granulosa cells[14, 16, 17]

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