Effects of oocyte-derived growth factors on the growth of porcine oocytes and oocyte-cumulus cell complexes in vitro

Dietary methionine supplementation during the estrous cycle improves follicular development and estrogen synthesis in rats

The follicle is an important unit for the synthesis of steroid hormones and the oocyte development and maturation in mammals. However, the effect of methionine supply on follicle development and its regulatory mechanism are still unclear. In the present study, we found that dietary methionine supplementation during the estrous cycle significantly increased the number of embryo implantation sites, as well as serum contents of a variety of amino acids and methionine metabolic enzymes in rats. Additionally, methionine supplementation markedly enhanced the expression of rat ovarian neutral amino acid transporters, DNA methyltransferases (DNMTs), and cystathionine gamma-lyase (CSE); meanwhile, it significantly increased the ovarian concentrations of the metabolite S-adenosylmethionine (SAM) and glutathione (GSH). In vitro data showed that methionine supply promotes rat follicle development through enhancing the expression of critical gene growth differentiation factor 9 and bone morphogenetic protein 15. Furthermore, methionine enhanced the relative protein and mRNA expression of critical genes related to estrogen synthesis, ultimately increasing estrogen synthesis in primary ovarian granulosa cells. Taken together, our results suggested that methionine promoted follicular growth and estrogen synthesis in rats during the estrus cycle, which improved embryo implantation during early pregnancy. These findings provided a potential nutritional strategy to improve the reproductive performance of animals.

Advances in Oocyte Maturation In Vivo and In Vitro in Mammals

The quality and maturation of an oocyte not only play decisive roles in fertilization and embryo success, but also have long-term impacts on the later growth and development of the fetus. Female fertility declines with age, reflecting a decline in oocyte quantity. However, the meiosis of oocytes involves a complex and orderly regulatory process whose mechanisms have not yet been fully elucidated. This review therefore mainly focuses on the regulation mechanism of oocyte maturation, including folliculogenesis, oogenesis, and the interactions between granulosa cells and oocytes, plus in vitro technology and nuclear/cytoplasm maturation in oocytes. Additionally, we have reviewed advances made in the single-cell mRNA sequencing technology related to oocyte maturation in order to improve our understanding of the mechanism of oocyte maturation and to provide a theoretical basis for subsequent research into oocyte maturation.

IVM Advances for Early Antral Follicle-Enclosed Oocytes Coupling Reproductive Tissue Engineering to Inductive Influences of Human Chorionic Gonadotropin and Ovarian Surface Epithelium Coculture

In vitro maturation (IVM) is not a routine assisted reproductive technology (ART) for oocytes collected from early antral (EA) follicles, a large source of potentially available gametes. Despite substantial improvements in IVM in the past decade, the outcomes remain low for EA-derived oocytes due to their reduced developmental competences. To optimize IVM for ovine EA-derived oocytes, a three-dimensional (3D) scaffold-mediated follicle-enclosed oocytes (FEO) system was compared with a validated cumulus-oocyte complex (COC) protocol. Gonadotropin stimulation (eCG and/or hCG) and/or somatic cell coculture (ovarian vs. extraovarian-cell source) were supplied to both systems. The maturation rate and parthenogenetic activation were significantly improved by combining hCG stimulation with ovarian surface epithelium (OSE) cells coculture exclusively on the FEO system. Based on the data, the paracrine factors released specifically from OSE enhanced the hCG-triggering of oocyte maturation mechanisms by acting through the mural compartment (positive effect on FEO and not on COC) by stimulating the EGFR signaling. Overall, the FEO system performed on a developed reproductive scaffold proved feasible and reliable in promoting a synergic cytoplasmatic and nuclear maturation, offering a novel cultural strategy to widen the availability of mature gametes for ART.

The transcription factor RUNX1 affects the maturation of porcine oocytes via the BMP15/TGF-β signaling pathway

Bone morphogenetic protein 15 (BMP15) is specifically expressed in oocytes in pigs at all stages from early stages to ovulation and has an important role in oocyte maturation. However, there are few reports on the molecular mechanisms by which BMP15 affects oocyte maturation. In this study, we identified the core promoter region of BMP15 using a dual luciferase activity assay and successfully predicted the DNA binding motif of the transcription factor RUNX1. The effect of BMP15 and RUNX1 on oocyte maturation was examined using the first polar body extrusion rate, a reactive oxygen species (ROS) assay and total glutathione (GSH) content at three time points of 12, 24 and 48 h of in vitro culture of porcine isolated oocytes. Subsequently, the effect of the transcription factor RUNX1 on the TGF-β signaling pathway (BMPR1B and ALK5) was further verified using RT-qPCR and Western blotting. We found that the overexpression of BMP15 significantly increased the first polar body extrusion rate (P < 0.01) and total glutathione content of oocytes cultured in vitro for 24 h and decreased reactive oxygen levels (P < 0.01), whereas interference with BMP15 decreased the first polar body extrusion rate (P < 0.01), increased reactive oxygen levels in oocytes cultured in vitro for 24 h (P < 0.01), and decreased glutathione content (P < 0.01). The dual luciferase activity assay and online software prediction showed that RUNX1 is a potential transcription factor binding to the core promoter region (-1203/-1423 bp) of BMP15. Overexpression of RUNX1 significantly increased the expression of BMP15 and oocyte maturation rate, while inhibition of RUNX1 decreased the expression of BMP15 and the oocyte maturation rate. Moreover, the expression of BMPR1B and ALK5 in the TGF-β signaling pathway increased significantly after overexpression of RUNX1, whereas their expression decreased after inhibition of RUNX1. Overall, our results suggest that the transcription factor RUNX1 positively regulates the expression of BMP15 and influences oocyte maturation through the TGF-β signaling pathway. This study provides a theoretical basis for further complementing the BMP15/TGF-β signaling pathway to regulate mammalian oocyte maturation.

Oocyte-derived growth factors promote development of antrum-like structures by porcine cumulus granulosa cells <i>in vitro</i>

Oocytes communicate with the surrounding somatic cells during follicular development. We examined the effects of two oocyte-derived growth factors, growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15), on the development of porcine oocyte–cumulus cell complexes (OCCs) in vitro. We collected OCCs from early antral follicles (1.2–1.5 mm) and prepared oocytectomized cumulus cell complexes (OXCs), which were then cultured in a growth medium supplemented with 0–100 ng/ml GDF9 and/or BMP15 for 7 days. In the medium without GDF9 or BMP15, OCCs developed during culture, and approximately 30% of them formed antrum-like structures. GDF9 promoted OCC development and structure formation in a dose-dependent manner. However, OXCs did not form antrum-like structures without growth factors. GDF9 promoted the development of OXCs, and 50 and 100 ng/ml GDF9 promoted the formation of the structures by 8% and 26%, respectively; however, BMP15 did not promote the formation of these structures. OXCs were then cultured with 100 ng/ml GDF9 and various concentrations of BMP15 to investigate their cooperative effects on the formation of antrum-like structures. BMP15 promoted the formation of antrum-like structures in a dose-dependent manner. In conclusion, GDF9 derived from oocytes is probably important for the formation of antrum-like structures in porcine OXCs, and BMP15 cooperates with GDF9 to form these structures.