Abstract
BackgroundLuteinizing hormone (LH) regulation of the ligand, natriuretic peptide precursor type C, and its receptor, natriuretic peptide receptor 2 (NPR2), is critical for oocyte maturation; however, the mechanism is not fully understood. Macrophage colony-stimulating factor (M-CSF) has recently been shown to be involved in oocyte maturation and ovulation. In the present study we determined whether or not M-CSF plays a role in the intermediate signal that mediates LH regulation of NPR2 in resumption of oocyte meiosis.MethodsImmature female C57BL/6 mice were injected i.p. with 5 IU of equine chorionic gonadotropin (eCG) to stimulate follicle development. After 44–48 h, the eCG-stimulated mice were injected i.p. with an ovulatory dose of 5 IU of human chorionic gonadotropin (hCG). The ovaries were excised at selected times. Pre-ovulatory follicles (POFs) and cumulus-oocyte complexes were cultured in different media. Immunohistochemical and quantitative real-time PCR analyses were used to assess the expression of M-CSF, M-CSF receptor (M-CSF-R), and NPR2. The presence of germinal vesicle breakdown (GVBD) was examined under a stereomicroscope to morphologically evaluate resumption of oocyte meiosis.ResultsNPR2 was mainly expressed in cumulus cells of pre-ovulatory follicles, while M-CSF and M-CSF-R were expressed in both mural granulosa and cumulus cells. The levels of M-CSF/M-CSF-R and NPR2 decreased within 4 h after treatment of hCG. M-CSF not only reduced the expression of NPR2 mRNA via its receptor (M-CSF-R), but also increased the proportion of GVBD in oocytes.ConclusionM-CSF serves as an intermediate signal, thus inducing a vital decrease in the NPR2 levels in cumulus cells, and regulates the process of LH-induced resumption of meiosis.
Highlights
Luteinizing hormone (LH) regulation of the ligand, natriuretic peptide precursor type C, and its receptor, natriuretic peptide receptor 2 (NPR2), is critical for oocyte maturation; the mechanism is not fully understood
Localization of Macrophage colony-stimulating factor (M-CSF), M-CSF receptor (M-CSF-R), and NPR2 in Pre-ovulatory follicles (POFs) To investigate if M-CSF, M-CSF-R, and NPR2 signaling is involved in regulation of oocyte meiosis in POFs
Immunohistochemistry analysis revealed that M-CSF and MCSF-R are expressed in both mural granulosa cells (GCs) and cumulus cells (Fig. 2a-d), while the expression of NPR2 in POFs was mainly detected in cumulus cells and was observed in peri-antral mural GCs (GCs located on the lining of antral spaces, [Fig. 2e and f])
Summary
Luteinizing hormone (LH) regulation of the ligand, natriuretic peptide precursor type C, and its receptor, natriuretic peptide receptor 2 (NPR2), is critical for oocyte maturation; the mechanism is not fully understood. Once the growing follicles reach the early antral stage, oocytes acquire meiotic competence [3]; oocytes are arrested at the diplotene stage of the first meiotic prophase because signals from the surrounding granulosa cells (GCs) prevent the machinery required for resumption of meiosis [3, 4]. Subsequent studies have indicated that generation of cGMP is stimulated by a paracrine loop, which includes natriuretic peptide receptor 2 (NPR2) and the ligand, natriuretic peptide precursor type C (NPPC) [13]. NPPC, produced by mural GCs, activates NPR2, which is produced mainly by cumulus cells surrounding the oocyte, increases cAMP and cGMP levels in the oocyte, and prevents spontaneous (gonadotropin-independent) resumption of oocyte meiosis [13, 14]. In mice deficient in the ligand, NPPC, or its cognate receptor, NPR2, oocytes precociously re-enter the meiotic cell cycle as soon as the oocytes reach the early antral follicle stage [13, 15]
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