Effects of pre-incubation with C-type natriuretic peptide on nuclear maturation, mitochondrial behavior, and developmental competence of sheep oocytes

In vitro produced mammalian embryos suffer from developmental failure, with a large proportion showing embryonic retardation, degradation, or apoptosis. This failure is due, in part, to incomplete oocyte cytoplasmic maturation. C-type natriuretic peptide (CNP) has been reported to act as a meiotic inhibitor. Here we explored the potential effects of CNP pre-treatment sheep oocytes on nuclear maturation, changes in mitochondrial behavior and developmental competence of in vitro fertilized embryos. Sheep cumulus–oocyte complexes (COCs) were aspirated from abattoir-derived ovaries. Nuclear progression was assessed using DAPI chromatin staining, the expression of natriuretic peptide receptor 2 (NPR2) was evaluated by RT-qPCR, active mitochondria localization was assessed with a confocal laser scanning microscopy using MitoTracker Red, and the developmental competence of sheep oocytes subjected to one-step IVM or two-step IVM with or without CNP pretreatment was also investigated. Our results showed that 200 nM CNP could effectively maintain meiotic arrest of sheep COCs in vitro within 4 h. Furthermore, NPR2 mRNA was mainly expressed in cumulus cells. COCs pre-treated with 200 nM CNP for 4 h followed by 24 h IVM showed significantly higher (P < 0.05) cleavage rate and blastocyst rate after in vitro fertilization (IVF), and significantly lower (P < 0.05) proportion of DNA-fragmented nuclei in blastocysts when compared to the conventional 24 h IVM (standard IVM). Non-matured oocytes mainly displayed brilliant circumferential and fine diffuse distribution of mitochondria throughout the cytoplasm. By comparison, 200 nM CNP pre-treated COCs for 4 h led to cytoplasmic mitochondrial granule localization to the peripheral and perinuclear regions. Moreover, oocytes pre-treated with 200 nM CNP for 4 h followed by 24 h IVM, showed mitochondrial organization were similar to those of conventional 24 h matured oocytes in which mitochondria were aggregated more toward the cortical regions of the oocytes, but with larger clumps of stained mitochondria. These results indicate that CNP pre-treatment improves the quality and developmental competence of sheep oocytes and has great potential for facilitating in vitro embryo production.