Molecular determinants of the meiotic arrests in mammalian oocytes at different stages of maturation

Cyclic AMP mediates ovine cumulus-oocyte gap junctional function via balancing connexin 43 expression and phosphorylation

Gap junction channels in cumulus-oocyte complexes (COCs) enable the transmission and communication of small molecular signals between adjacent cells, such as cAMP. However, the regulation of gap junction function (GJF) by cAMP and the underlying mechanisms involved are not fully clarified. This study investigated the effect of cAMP on connexin 43 (CX43) expression and GJF in ovine COCs using immunofluorescence, quantitative real-time PCR (qRT-PCR), western blotting, and GJF detection. The CX43 was only found in the cumulus cells (CCs) side of ovine COC. The intra-oocyte cAMP showed a significant increase at 30 min, while the intra-CC cAMP exhibited two peaks at 10 min and 1 h during in vitro maturation (IVM). Phosphorylated CX43 protein exhibited an immediate increase at 10 min, and CX43 protein displayed two peaks at 10 min and 1 h during IVM. The duration of pre-IVM exposure to forskolin and IBMX significantly enhanced phosphorylated and total CX43, as well as Gja1 and Creb genes, for 10 min; these effects were counteracted by Rp-cAMP. Both pre-IVM with forskolin and IBMX for 1 h and the GJF and CX43/p-CX43 ratio were elevated. The closure of gap junction channels caused by phosphorylated CX43 to prevent cAMP outflow from oocytes in early IVM of COC. Cyclic AMP upregulated phosphorylated and total CX43 via genomic and non-genomic pathways, but its functional regulation was dependent on the balance of the two proteins. This study provides a new insight into the regulatory mechanism between cAMP and GJF, which would improve IVM in animal and clinical research.

Factors Influencing the Maturation and Developmental Competence of Yak (Bos grunniens) Oocytes In Vitro

The yak (Bos grunniens) is a unique breed living on the Qinghai-Tibet Plateau and its surrounding areas, providing locals with a variety of vital means of living and production. However, the yak has poor sexual maturity and low fertility. High-quality mature oocytes are the basis of animal breeding technology. Recently, in vitro culturing of oocytes and embryo engineering technology have been applied to yak breeding. However, compared to those observed in vivo, the maturation rate and developmental capacity of in vitro oocytes are still low, which severely limits the application of in vitro fertilization and embryo production in yaks. This review summarizes the endogenous and exogenous factors affecting the in vitro maturation (IVM) and developmental ability of yak oocytes reported in recent years and provides a theoretical basis for obtaining high-quality oocytes for in vitro fertilization and embryo production in yaks.

Chromatin Morphology in Human Germinal Vesicle Oocytes and Their Competence to Mature in Stimulated Cycles

The search for simple morphological predictors of oocyte quality is an important task for assisted reproduction technologies (ARTs). One such predictor may be the morphology of the oocyte nucleus, called the germinal vesicle (GV), including the level of chromatin aggregation around the atypical nucleolus (ANu)-a peculiar nuclear organelle, formerly referred to as the nucleolus-like body. A prospective cohort study allowed distinguishing three classes of GV oocytes among 135 oocytes retrieved from 64 patients: with a non-surrounded ANu and rare chromatin blocks in the nucleoplasm (Class A), with a complete peri-ANu heterochromatic rim assembling all chromatin (Class C), and intermediate variants (Class B). Comparison of the chromatin state and the ability of oocytes to complete meiosis allowed us to conclude that Class B and C oocytes are more capable of resuming meiosis in vitro and completing the first meiotic division, while Class A oocytes can resume maturation but often stop their development either at metaphase I (MI arrest) or before the onset of GV breakdown (GVBD arrest). In addition, oocytes with a low chromatin condensation demonstrated a high level of aneuploidy during the resumption of meiosis. Considering that the degree of chromatin condensation/compaction can be determined in vivo under a light microscope, this characteristic of the GV can be considered a promising criterion for selecting the best-quality GV oocytes in IVM rescue programs.

Dynamic Changes in Proteome during Yak Oocyte Maturation Analyzed Using iTRAQ Technology

The aim of this study was to investigate protein regulation at different time points during the in vitro maturation of yak oocytes. Yak oocytes at GV, MI, and MII stages were collected during in vitro maturation, and differential proteomics sequencing was performed using iTRAQ technology. GO functional classification indicated that the differential proteins were closely associated with biological processes such as "metabolic processes", and molecular events such as "binding" molecular-function-related categories were active. KOG analysis showed that energy-metabolism-related activities were vigorous during oocyte development from the GV phase to MI phase, and genetic material preparation activities were more active when oocytes developed from the MI stage to MII stage. KEGG pathway analysis showed that the PPAR metabolic pathway, Hippo signaling pathway, and ECM-receptor interaction and metabolic pathway were enriched from the GV to the MI stages. The PI3K-Akt, TGF-β, and phagosome pathways were enriched from the MI stage to the MII stage. These results indicate that transient dynamic changes occurred in the proteome during the maturation of yak oocytes, and the physiological functions mediated by these were also different. The accurate identification of the differential proteins in the three stages of GV, MI, and MII was helpful in further analyzing the molecular regulatory mechanism of yak oocyte maturation.

Identification of unique transcriptomic signatures through integrated multispecies comparative analysis and WGCNA in bovine oocyte development

BACKGROUND

Cattle (Bos taurus) are a major large livestock, however, compared with other species, the transcriptional specificity of bovine oocyte development has not been emphasised.

RESULTS

To reveal the unique transcriptional signatures of bovine oocyte development, we used integrated multispecies comparative analysis and weighted gene co-expression network analysis (WGCNA) to perform bioinformatic analysis of the germinal follicle (GV) and second meiosis (MII) gene expression profile from cattle, sheep, pigs and mice. We found that the expression levels of most genes were down-regulated from GV to MII in all species. Next, the multispecies comparative analysis showed more genes involved in the regulation of cAMP signalling during bovine oocyte development. Moreover, the green module identified by WGCNA was closely related to bovine oocyte development. Finally, integrated multispecies comparative analysis and WGCNA picked up 61 bovine-specific signature genes that participate in metabolic regulation and steroid hormone biosynthesis.

CONCLUSION

In a short, this study provides new insights into the regulation of cattle oocyte development from a cross-species comparison.

Embryonic poly(A)-binding protein interacts with translation-related proteins and undergoes phosphorylation on the serine, threonine, and tyrosine residues in the mouse oocytes and early embryos

Expression of the embryonic poly(A)-binding protein (EPAB) in frog, mouse, and human oocytes and early-stage embryos is maintained at high levels until embryonic genome activation (EGA) after which a significant decrease occurs in EPAB levels. Studies on the vertebrate oocytes and early embryos revealed that EPAB plays key roles in the translational regulation, stabilization, and protection of maternal mRNAs during oocyte maturation and early embryogenesis. However, it remains elusive whether EPAB interacts with other cellular proteins and undergoes phosphorylation to perform these roles. For this purpose, we identified a group of Epab-interacting proteins and its phosphorylation status in mouse germinal vesicle (GV)- and metaphase II (MII)-stage oocytes, and in 1-cell, 2-cell, and 4-cell preimplantation embryos. In the oocytes and early preimplantation embryos, Epab-interacting proteins were found to play roles in the translation and transcription processes, intracellular signaling and transport, maintenance of structural integrity, metabolism, posttranslational modifications, and chromatin remodeling. Moreover, we discovered that Epab undergoes phosphorylation on the serine, threonine, and tyrosine residues, which are localized in the RNA recognition motifs 2, 3, and 4 or C-terminal. Conclusively, these findings suggest that Epab not only functions in the translational control of maternal mRNAs through binding to their poly(A) tails but also participates in various cellular events through interacting with certain group proteins. Most likely, Epab undergoes a dynamic phosphorylation during the oocyte maturation and the early embryo development to carry out these functions.

Mitofusins: from mitochondria to fertility

Germ cell formation and embryonic development require ATP synthesized by mitochondria. The dynamic system of the mitochondria, and in particular, the fusion of mitochondria, are essential for the generation of energy. Mitofusin1 and mitofusin2, the homologues of Fuzzy onions in yeast and Drosophila, are critical regulators of mitochondrial fusion in mammalian cells. Since their discovery mitofusins (Mfns) have been the source of significant interest as key influencers of mitochondrial dynamics, including membrane fusion, mitochondrial distribution, and the interaction with other organelles. Emerging evidence has revealed significant insight into the role of Mfns in germ cell formation and embryonic development, as well as the high incidence of reproductive diseases such as asthenospermia, polycystic ovary syndrome, and gestational diabetes mellitus. Here, we describe the key mechanisms of Mfns in mitochondrial dynamics, focusing particularly on the role of Mfns in the regulation of mammalian fertility, including spermatogenesis, oocyte maturation, and embryonic development. We also highlight the role of Mfns in certain diseases associated with the reproductive system and their potential as therapeutic targets.

Rescue in vitro maturation may increase the pregnancy outcomes among women undergoing intracytoplasmic sperm injection

INTRODUCTION

To investigate whether rescue in vitro maturation (R-IVM) improves the reproductive outcomes among women undergoing intracytoplasmic sperm injection (ICSI) after one oocyte retrieved cycle.

METHODS

Between January 2019 and December 2020, 2602 women who underwent ICSI in the Reproductive Medicine Center of Tongji Hospital, Wuhan, China, were included in our retrospective cohort study. There were 2112 women undergoing only ICSI and 490 women with R-IVM followed by ICSI. The intermediate reproductive outcomes and pregnancy outcomes were assessed, including the number of normally fertilized embryos, number of cleaved embryos, number of good-quality embryos, number of day-3 available embryos, number of embryos cultured past day-3, number of blastocysts, number of available blastocysts, biochemical pregnancy, miscarriage, clinical pregnancy and live birth. The perinatal outcomes were also assessed, including preterm birth and birth weight. The abovementioned outcomes were also calculated for in vivo matured and R-IVM oocytes separately in women undergoing ICSI with R-IVM group.

RESULTS

Compared with the women who underwent only ICSI, those who underwent ICSI with R-IVM had higher numbers of MII oocytes, normally fertilized embryos, cleaved embryos, day-3 available embryos, embryos cultured past day-3, and higher oocyte maturation rate, available embryo rate than women undergoing only ICSI. Additionally, we found that women undergoing ICSI with R-IVM had an increased chance of clinical pregnancy (adjusted OR=1.50, 95% CI: 1.17-1.93) and cumulative live birth (adjusted OR=1.35, 95% CI: 1.07-1.71). After propensity score matching (PSM), the cumulative live birth rate was 60.1% for women undergoing ICSI with R-IVM versus 54.9% for women undergoing only ICSI (OR=1.24, 95% CI: 0.94-1.63). The reproductive outcomes were also significantly different when calculated for in vivo matured and R-IVM oocytes separately in women undergoing ICSI with R-IVM group. All live births from R-IVM embryos were healthy and without malformations or complications.

CONCLUSION

R-IVM may improve the reproductive outcomes of women undergoing ICSI. It may also provide a reference for the safety of R-IVM. This study maybe support a routine application of R-IVM among patients who intend to undergo ICSI.