Abstract
Lipid is a crucial energy resource for mammalian oocyte. Melatonin could benefit the maturation of porcine oocyte in vitro, but the related mechanism is not elucidated yet. In the current study, methods to monitor lipid metabolism in single live oocytes were firstly established using probes (Lipi-Blue and Lipi-Green). It was observed that both lipid biogenesis and lipolysis occurred in maturing oocyte, but the general level of lipids dropped. Then maturing oocytes stained with probes were treated with melatonin or lipid metabolic-related inhibitors (triacsin C, rotenone, or etomoxir). The results showed that the lipid metabolism and maturation of porcine oocytes were all disrupted and that melatonin rescued the oocytes treated with triacsin C or rotenone, but not those treated with etomoxir. Further investigation demonstrated that cumulus cells are able to transfer lipids to oocytes via gap junctions. It was also observed that melatonin receptors exist in cumulus cells and are required for oocytes to maintain lipid metabolism. Meanwhile, the global gene expressing in cumulus cells was also modulated by melatonin, especially the genes related to antioxidants (SOD1, GPX1, GPX3, GPX4, PRDX2, and PRDX5), lipid metabolism (FABP3, FABP5, ACACB, TECR, etc.), and mitochondrial respiration (GPD1, ETFB, CYC1, and the genes of ATP synthase). Altogether the current research demonstrates that melatonin modulates lipid metabolism in maturing oocytes through its receptors in cumulus cells and benefits the developmental competence of oocytes.
Highlights
During its growth phase, as the size of an oocyte increases obviously, a large amount of mRNAs, proteins, and lipids is accumulated in the oocyte cytoplasm
The cumulus– oocyte complexes (COCs) were transferred into the maturation medium (50 oocytes per 0.5 ml of medium) consisting of TCM-199 supplemented with 0.57 mM cysteine, 3.05 mM D-glucose, 0.91 mM sodium pyruvate, 10 ng/ml epidermal growth factor (EGF), 0.5 IU/ml ovine luteinizing hormone (LH), 0.5 IU/ml porcine folliclestimulating hormone (FSH), 0.1% polyvinyl alcohol (PVA), 75 mg/ml penicillin, 50 mg/ml streptomycin, 20 ng/ml LIF (EMD Millipore, MA, United States), 20 ng/ml IGF1 (ProSpec, Ness Ziona, Israel), and 40 ng/ml FGF2 (PeproTech, NJ, United States) (Abeydeera et al, 1998; Yuan et al, 2017)
The oocytes were stained with Lipi-Blue at the Germinal vesicle (GV) stage, and the images were acquired at the GV (210.52 ± 4.03), MI (110.06 ± 8.92), and metaphase II (MII) (106.87 ± 7.34) stages, so the decrease of blue fluorescence intensity indicates the mobilization of lipids to generate energy for the activities of oocytes (Figure 1B)
Summary
As the size of an oocyte increases obviously, a large amount of mRNAs, proteins, and lipids is accumulated in the oocyte cytoplasm. Oocytes progress to meiotic resumption to accomplish the final nuclear and cytoplasmic maturation. These dynamic activities in oocytes consume enormous energy produced from various substrates stored. Oocytes that matured in the artificial medium described showed altered metabolism, resulting in lower developmental competence (Bradley and Swann, 2019). Well-balanced energy substrate provision is definitely required for oocyte maturation and further embryo development, which is critical for both assisted reproductive technology in a clinical setting and embryo production in the livestock industry
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
