C-type natriuretic peptide inhibits porcine oocyte meiotic resumption.

C-type natriuretic peptide (CNP) is a recently identified meiotic inhibitor in mice. However, it has not been investigated in porcine oocytes to date. This study aimed to demonstrate the inhibitory effect of CNP against germinal vesicle breakdown (GVBD) in porcine oocyte meiotic resumption. Immunohistochemical analysis revealed intense natriuretic peptide receptor 2 (NPR2) immunoreactivity in the oocyte surrounded cumulus cells in the follicles. Furthermore, reverse transcription polymerase chain reaction (RT-PCR) analysis showed the expression of npr2 mRNA only in cumulus cells but not in oocytes, suggesting that cumulus cells are the targets of CNP. When cumulus-oocyte complexes (COCs) or denuded oocytes (DOs) were cultured with various concentrations of CNP (10, 50, 100, 500, and 1,000 nM), inhibitory effect was observed in the COC group, but not in the DO group, confirming that CNP indirectly inhibits GVBD via cumulus cells. This evidence is the first indication that the CNP-NPR2 pathway is involved in meiotic arrest in porcine oocytes. Furthermore, we investigated the effect of oocyte-derived paracrine factor (ODPF) on npr2 mRNA expression level in cumulus cells by evaluating changes in mRNA expression in oocytectomised COCs (OXCs) by real-time PCR. A significant decrease in npr2 mRNA expression level was observed in OXCs, whereas mRNA expression level was restored in OXCs with DOs, indicating that ODPF participates in the regulation of npr2 expression in porcine cumulus cells.