Regulation of oocyte meiotic maturation by somatic cells

Analysis of cell-cell interaction between mural granulosa cells and cumulus granulosa cells during ovulation using single-cell RNA sequencing data of mouse ovary

Purpose

We investigated the interactions between mural granulosa cells (MGCs) and cumulus granulosa cells (CGCs) during ovulation after the LH surge.

Methods

We performed clustering, pseudotime, and interactome analyses utilizing reported single-cell RNA sequencing data of mouse ovary at 6 h after eCG-hCG injection.

Results

Clustering analysis classified granulosa cells into two distinct populations, MGCs and CGCs. Pseudotime analysis divided granulosa cells into before and after the LH surge, and further divided them into two branches, the ovulatory MGCs and the ovulatory CGCs. Interactome analysis was performed to identify the interactions between MGCs and CGCs. Twenty-six interactions were acting from CGCs toward MGCs, involving ovulation and steroidogenesis. Thirty-six interactions were acting from MGCs toward CGCs, involving hyaluronan synthesis. There were 25 bidirectional interactions, involving the EGFR pathway. In addition, we found three novel interactions: Ephrins-Ephs pathway and Wnt-Lrp6 pathway from CGCs to MGCs, associated with steroidogenesis and lipid transport, respectively, and TGF-β-TGFBR1 pathway from MGCs to CGCs, associated with hyaluronan synthesis.

Conclusions

MGCs and CGCs interact with each other in the preovulatory follicle after the LH surge, and their interactions have roles in corpus luteum formation, oocyte maturation, and follicle rupture.

Exosomes Derived from Yak Follicular Fluid Increase 2-Hydroxyestradiol Secretion by Activating Autophagy in Cumulus Cells

Exosomes in the follicular fluid can carry and transfer regulatory molecules to recipient cells, thus influencing their biological functions. However, the specific effects of yak follicular fluid exosomes on 2-hydroxyestrodiol (2-OHE₂) secretion remain unknown. Here, we investigated whether yak follicular fluid exosomes can increase 2-OHE₂ secretion through the activation of autophagy in cumulus cells (YCCs). In vitro cultured YCCs were treated with yak follicular fluid exosomes for 6, 12, and 24 h. The effects of yak follicular fluid exosomes on autophagy and 2-OHE₂ secretion were evaluated through real-time quantitative fluorescence PCR (RT-qPCR), Western blotting (WB), transfected with RFP-GFP-LC3, immunohistochemistry, and ELISA. To further investigate whether 2-OHE₂ secretion was related to autophagy, YCCs were administered with yak follicular fluid exosomes, 3-methyladenine (3-MA), and rapamycin (RAPA). The results revealed that treatment with yak follicular fluid exosomes activated autophagy in YCCs and increased 2-OHE₂ secretion. Conversely, the inhibition of autophagy with 3-MA blocked these effects, suggesting that autophagy has an important role in 2-OHE₂ secretion in YCCs. Treatment of YCCs with rapamycin showed similar results with yak follicular fluid exosomes as there was an increase in 2-OHE2 secretion due to the activation of autophagy in the treated cumulus cells. Our results demonstrate that autophagy is enhanced by yak follicular fluid exosomes, and this is associated with an increase in 2-OHE₂ secretion in YCCs.

The effect of carbon monoxide on meiotic maturation of porcine oocytes

Oxidative stress impairs the correct course of meiotic maturation, and it is known that the oocytes are exposed to increased oxidative stress during meiotic maturation in in vitro conditions. Thus, reduction of oxidative stress can lead to improved quality of cultured oocytes. The gasotransmitter carbon monoxide (CO) has a cytoprotective effect in somatic cells. The CO is produced in cells by the enzyme heme oxygenase (HO) and the heme oxygenase/carbon monoxide (HO/CO) pathway has been shown to have an antioxidant effect in somatic cells. It has not yet been investigated whether the CO has an antioxidant effect in oocytes as well. We assessed the level of expression of HO mRNA, using reverse transcription polymerase chain reaction. The HO protein localization was evaluated by the immunocytochemical method. The influence of CO or HO inhibition on meiotic maturation was evaluated in oocytes cultured in a culture medium containing CO donor (CORM-2 or CORM-A1) or HO inhibitor Zn-protoporphyrin IX (Zn-PP IX). Detection of reactive oxygen species (ROS) was performed using the oxidant-sensing probe 2′,7′-dichlorodihydrofluorescein diacetate. We demonstrated the expression of mRNA and proteins of both HO isoforms in porcine oocytes during meiotic maturation. The inhibition of HO enzymes by Zn-PP IX did not affect meiotic maturation. CO delivered by CORM-2 or CORM-A1 donors led to a reduction in the level of ROS in the oocytes during meiotic maturation. However, exogenously delivered CO also inhibited meiotic maturation, especially at higher concentrations. In summary, the CO signaling molecule has antioxidant properties in porcine oocytes and may also be involved in the regulation of meiotic maturation.

Impaired Mitochondrial Stress Response due to CLPP Deletion Is Associated with Altered Mitochondrial Dynamics and Increased Apoptosis in Cumulus Cells

Caseinolytic peptidase P (CLPP) plays a central role in mitochondrial unfolded protein response (mtUPR) and is required for maintaining protein homeostasis in the mitochondria. Global germline Clpp deletion causes female infertility and accelerated follicular depletion. In the current study, we aimed to characterize the role of CLPP in cumulus cell function, gene expression, and mitochondrial ultrastructure. We found that mitochondria in Clpp-deficient cumulus cells have a smaller aspect ratio (length/width) and have a larger coverage area (mitochondrial area/cytoplasmic area) under electron microscopy. These ultrastructural changes were accompanied with diminished expression of mitochondrial dynamics genes. RNA sequencing analysis revealed a significant change in genes related to cellular metabolism in Clpp-deficient cumulus cells compared to wild type. In addition, apoptosis and phagosome pathways were significantly affected. Immunofluorescence assessment confirmed increased apoptotic activity and decreased cell proliferation in cumulus oophorus complexes (COCs) of Clpp-deficient mice. Our findings demonstrate that deletion of CLPP results in significant structural and functional changes in cumulus cells and suggests that mtUPR is required for cumulus cell function.

Cumulus cell-derived and maternal SIRT6 differentially regulates porcine oocyte meiotic maturation

SIRT6, a member of the sirtuin family, is a NAD + dependent protein deacetylase and has been implicated in transcriptional regulation of somatic cells and post-transcriptional regulation of oocyte meiosis. However, the function of cumulus cell-derived and maternal SIRT6 in meiotic maturation of porcine oocytes is not yet known. Here, we report that SIRT6 mRNA and protein exists in the oocyte and its surrounding cumulus cells during meiotic maturation. Functional studies using a specific inhibitor in cumulus-enclosed oocytes revealed important roles for SIRT6 in germinal vesicle breakdown (GVBD) and cumulus expansion. Moreover, inhibitor treatment led to a significant reduction in the rate of first polar body (PB1) extrusion and early development of parthenogenetically activated embryos. In contrast, SIRT6 inhibition in cumulus-free oocytes only resulted in a significant reduction in the rate of PB1 extrusion. Furthermore, SIRT6 dysfunction regardless of the origin in both cumulus cells and oocytes severely impaired spindle organization and chromosome alignment at the metaphase stage. Molecularly, SIRT6 inhibition in cumulus cells significantly reduced expression of genes associated with cumulus expansion and gap junctional communication and even expression levels of active phosphorylated CDK1 in oocytes. Importantly, adenylate cyclase inhibition could partially rescue GVBD and PB1 extrusion in SIRT6-inhibited cumulus-enclosed oocytes. Taken together, these results demonstrate that cumulus cell-expressed and maternal SIRT6 differentially regulates porcine oocyte meiotic maturation.

Porcine oocyte maturation in vitro: role of cAMP and oocyte-secreted factors - A practical approach

Polyspermy or the penetration of more than one sperm cell remains a problem during porcine in vitro fertilization (IVF). After in vitro culture of porcine zygotes, only a low percentage of blastocysts develop and their quality is inferior to that of in vivo derived blastocysts. It is unknown whether the cytoplasmic maturation of the oocyte is sufficiently sustained in current in vitro maturation (IVM) procedures. The complex interplay between oocyte and cumulus cells during IVM is a key factor in this process. By focusing on this bidirectional communication, it is possible to control the coordination of cumulus expansion, and nuclear and cytoplasmic maturation during IVM to some extent. Therefore, this review focuses on the regulatory mechanisms between oocytes and cumulus cells to further the development of new in vitro embryo production (IVP) procedures, resulting in less polyspermy and improved oocyte developmental potential. Specifically, we focused on the involvement of cAMP in maturation regulation and function of oocyte-secreted factors (OSFs) in the bidirectional regulatory loop between oocyte and cumulus cells. Our studies suggest that maintaining high cAMP levels in the oocyte during the first half of IVM sustained improved oocyte maturation, resulting in an enhanced response after IVF and cumulus matrix disassembly. Recent research indicated that the addition of OSFs during IVM enhanced the developmental competence of small follicle-derived oocytes, which was stimulated by epidermal growth factor (EGF) via developing EGF-receptor signaling.