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

Exploring nicotinamide adenine dinucleotide precursors across biosynthesis pathways: Unraveling their role in the ovary

Natural Nicotinamide Adenine Dinucleotide (NAD+) precursors have attracted much attention due to their positive effects in promoting ovarian health. However, their target tissue, synthesis efficiency, advantages, and disadvantages are still unclear. This review summarizes the distribution of NAD+ at the tissue, cellular and subcellular levels, discusses its biosynthetic pathways and the latest findings in ovary, include: (1) NAD+ plays distinct roles both intracellularly and extracellularly, adapting its distribution in response to requirements. (2) Different precursors differs in target tissues, synthetic efficiency, biological utilization, and adverse effects. Importantly: tryptophan is primarily utilized in the liver and kidneys, posing metabolic risks in excess; nicotinamide (NAM) is indispensable for maintaining NAD+ levels; nicotinic acid (NA) constructs a crucial bridge between intestinal microbiota and the host with diverse functions; nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) increase NAD+ systemically and can be influenced by delivery route, tissue specificity, and transport efficiency. (3) The biosynthetic pathways of NAD+ are intricately intertwined. They provide multiple sources and techniques for NAD+ synthesis, thereby reducing the dependence on a single molecule to maintain cellular NAD+ levels. However, an excess of a specific precursor potentially influencing other pathways. In addition, Protein expression analysis suggest that ovarian tissues may preferentially utilize NAM and NMN. These findings summarize the specific roles and potential of NAD+ precursors in enhancing ovarian health. Future research should delve into the molecular mechanisms and intervention strategies of different precursors, aiming to achieve personalized prevention or treatment of ovarian diseases, and reveal their clinical application value.

Can Nicotinamide Adenine Dinucleotide (NAD+) and Sirtuins Be Harnessed to Improve Mare Fertility?

Years of sire and dam selection based on their pedigree and athletic performance has resulted in a reduction in the reproductive capability of horses. Mare age is considered a major barrier to equine reproduction largely due to an increase in the age at which mares are typically bred following the end of their racing career. Nicotinamide adenine dinucleotide (NAD+) and its involvement in the activation of Sirtuins in fertility are an emerging field of study, with the role of NAD+ in oocyte maturation and embryo development becoming increasingly apparent. While assisted reproductive technologies in equine breeding programs are in their infancy compared to other livestock species such as cattle, there is much more to be learnt, from oocyte maturation to early embryo development and beyond in the mare, which are difficult to study given the complexities associated with mare fertility research. This review examines what is already known about the role of NAD+ and Sirtuins in fertility and discusses how NAD+-elevating agents may be used to activate Sirtuin proteins to improve equine breeding and embryo production programs both in vivo and in vitro.

Effects of follicular fluid exosomes on in vitro maturation of porcine oocytes

Exosomes are related to effective communication between cells. In this study we aimed to investigate the effect of porcine follicular fluid exosomes (FF-Exo) on cumulus expansion, oocyte mitochondrial membrane potential, and maturation in in vitro culture. We used different concentrations of FF-Exo (Exo-0, Exo-1, Exo-10, Exo-20, and Exo-40) and added them to an oocyte maturation medium. Transmission electron microscopy (TEM), nanoparticle tracking analysis (NTA) and western blot (WB) showed that the isolated samples were exosomes. Immunofluorescence showed that exosomes could be taken up by cumulus cells. Compared with the Exo-0 group, there was no significant difference in oocyte maturation rate in the Exo-1 group (p > 0.05), while the Exo-10 group (p < 0.05), Exo-20 group (p < 0.01) and Exo-40 group (p < 0.01) significantly increased. The maturation rate of the Exo-20 and Exo-40 groups was the highest, and there was no significant difference between the two groups (p > 0.05). However, different concentrations of treatment could not effectively induce cumulus expansion and the results of JC1 showed that it had no significant effect on mitochondrial membrane potential (p > 0.05). In conclusion, the results suggest that porcine FF-Exo are involved in oocyte nuclear maturation.

Histone acetyltransferases and histone deacetyl transferases play crucial role during oogenesis and early embryo development

Dynamic epigenetic regulation is critical for proper oogenesis and early embryo development. During oogenesis, fully grown germinal vesicle oocytes develop to mature Metaphase II oocytes which are ready for fertilization. Fertilized oocyte proliferates mitotically until blastocyst formation and the process is called early embryo development. Throughout oogenesis and early embryo development, spatio-temporal gene expression takes place, and this dynamic gene expression is controlled with the aid of epigenetics. Epigenetic means that gene expression can be altered without changing DNA itself. Epigenome is regulated through DNA methylation and histone modifications. While DNA methylation generally ends up with repression of gene expression, histone modifications can result in expression or repression depending on type of modification, type of histone protein and its specific residue. One of the modifications is histone acetylation which generally ends up with gene expression. Histone acetylation occurs through the addition of acetyl group onto amino terminal of the core histone proteins by histone acetyltransferases (HATs). Contrarily, histone deacetylation is associated with repression of gene expression, and it is catalyzed by histone deacetylases (HDACs). This review article focuses on what is known about alterations in the expression of HATs and HDACs and emphasizes importance of HATs and HDACs during oogenesis and early embryo development.

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.

Improvement of oocyte quality through the SIRT signaling pathway

Background

Oocyte quality is one of the major deciding factors in female fertility competence.

Methods

PubMed database was searched for reviews by using the following keyword "oocyte quality" AND "Sirtuins". The methodological quality of each literature review was assessed using the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) 2020 statement.

Main Findings

Oxidative stress has been recognized as the mechanism attenuating oocyte quality. Increasing evidence from animal experiments and clinical studies has confirmed the protective roles of the sirtuin family in improving oocyte quality via an antioxidant effect.

Conclusion

The protective roles in the oocyte quality of the sirtuin family have been increasingly recognized.

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.

Glyphosate exposure deteriorates oocyte meiotic maturation via induction of organelle dysfunctions in pigs

Background

Recently, defects in mammalian oocytes maturation induced by environmental pollution results in the decreasing animal reproduction. Animal exposed to glyphosate is largely unavoidable because glyphosate is one of the most widely used herbicide worldwide due to its high-efficiency and broad-spectrum effects, which causes glyphosate an environmental contaminant found in soil, water and food. During the last few years, the growing and wider use of glyphosate has raised great concerns about its effects of reproductive toxicity. In this study, using porcine models, we investigated effects of glyphosate on organelle functions during oocyte meiosis.

Results

The results showed glyphosate exposure disrupted porcine oocyte maturation. Expression levels of cumulus expansion-related genes were interfered, further indicating the meiotic defects. The damaging effects were mediated by destruction of mitochondrial distribution and functions, which induced ROS accumulation and oxidative stress, also indicated by the decreased mRNA expression of related antioxidant enzyme genes. We also found an interference of endoplasmic reticulum (ER) distribution, disturbance of Ca²⁺ homeostasis, as well as fluctuation of ER stress, showing with the reduced ER stress-related mRNA or protein expression, which could indicate the dysfunction of ER for protein processing and signal transduction in glyphosate-exposed oocytes. Moreover, glyphosate exposure induced the disruption of lysosome function for autophagy, showing with the decrease of LAMP2 expression and autophagy-related genes mRNA expression. Additionally, our data showed the distribution of Golgi apparatus and the functions of ribosome were disturbed after glyphosate exposure, which might affect protein synthesis and transport.

Conclusions

Collectively, our study showed that exposed to glyphosate could affect animal reproduction by compromising the quality of oocytes through its wide toxic effects on organelle functions.

Insights into the NAD+ biosynthesis pathways involved during meiotic maturation and spindle formation in porcine oocytes

Treatments that elevate NAD+ levels have been found to improve oocyte quality in mice, cattle, and pigs, suggesting that NAD+ is vital during oocyte maturation. This study aimed to examine the influence of different NAD+ biosynthetic pathways on oocyte quality by inhibiting key enzymes. Porcine oocytes from small antral follicles were matured for 44 h in a defined maturation system supplemented with 2-hydroxynicotinic acid [2-HNA, nicotinic acid phosphoribosyltransferase (NAPRT) inhibitor], FK866 [nicotinamide phosphoribosyltransferase (NAMPT) inhibitor], or gallotannin [nicotinamide mononucleotide adenylyltransferase (NMNAT) inhibitor] and their respective NAD+ pathway modulators (nicotinic acid, nicotinamide, and nicotinamide mononucleotide, respectively). Cumulus expansion was assessed after 22 h of maturation. At 44 h, maturation rates were determined and mature oocytes were fixed and stained to assess spindle formation. Each enzyme inhibitor reduced oocyte maturation rate and adversely affected spindle formation, indicating that NAD+ is required for meiotic spindle assembly. Furthermore, NAMPT and NMNAT inhibition reduced cumulus expansion, whereas NAPRT inhibition affected chromosomal segregation. Treating oocytes with gallotannin and nicotinamide mononucleotide together showed improvements in spindle width, while treating oocytes with 2-HNA and nicotinic acid combined showed an improvement in both spindle length and width. These results indicate that the salvage pathway plays a vital role in promoting oocyte meiotic progression, while the Preiss-Handler pathway is essential for spindle assembly.

SIRT6 Negatively Regulates Schwann Cells Dedifferentiation via Targeting c-Jun During Wallerian Degeneration After Peripheral Nerve Injury

Silent information regulator 6 (SIRT6) is a mammalian homolog of the nicotinamide adenine dinucleotide (NAD)-dependent deacetylase sirtuin family. Previous studies have been reported a pro-regenerative role of SIRT6 in central nervous system injury. However, the role of SIRT6 in peripheral nerve injury is still unknown. Given the importance and necessity of Schwann cell dedifferentiation response to peripheral nerve injury, we aim to investigate the molecular mechanism of SIRT6 steering Schwann cell dedifferentiation during Wallerian degeneration in injured peripheral nerve. Herein, we first examined the expression pattern of SIRT6 after peripheral nerve injury. Using the explants of sciatic nerve, an ex vivo model of nerve degeneration, we provided evidences indicating that SIRT6 inhibitor accelerates Schwann cell dedifferentiation as well as axonal and myelin degeneration, while SIRT6 activator attenuates this process. Moreover, in an in vitro Schwann cell dedifferentiation model, we found SIRT6 inhibitor promotes Schwann cell dedifferentiation through upregulating the expression of c-Jun. In addition, downregulation of c-Jun reverse the effects of SIRT6 inhibition on the Schwann cells dedifferentiation and axonal and myelin degeneration. In summary, we first described SIRT6 acts as a negative regulator for Schwann cells dedifferentiation during Wallerian degeneration and c-Jun worked as a direct downstream partner of SIRT6 in injured peripheral nerve.

Reprogramming of glucose metabolism of cumulus cells and oocytes and its therapeutic significance

The aim of this review is to summarize our current understanding of the molecular mechanism for the glucose metabolism, especially pyruvate dehydrogenase (PDH), during oocyte maturation, as well as future perspectives of therapeutic strategies for aging focusing on metabolic regulation between aerobic glycolysis and the tricarboxylic acid (TCA) cycle/oxidative phosphorylation (OXPHOS). Each keyword alone or in combination was used to search from PubMed. Glucose metabolism is a dynamic process involving "On" and "Off" switches by the pyruvate dehydrogenase kinase (PDK)-PDH axis, which is crucial for energy metabolism and mitochondrial efficiency in cumulus cell differentiation and oocyte maturation. Activation of PDK suppresses the conversion of pyruvate to acetyl-coenzyme A (acetyl-CoA) through the inactivation of PDH, which allows the cumulus cells to supply sufficient amounts of pyruvate, lactate, and nicotinamide adenine dinucleotide phosphate (NADPH) to the oocytes. On the other hand, inactivation of PDK in oocytes can produce adenosine triphosphate (ATP) through a metabolic shift from aerobic glycolysis to the TCA cycle/OXPHOS. The metabolic balance between aerobic glycolysis and TCA cycle/OXPHOS presents us with a number of enzymes, ligands, receptors, and antioxidants that are potential therapeutic targets, some of which have already been successfully pursued to improve fertility outcomes. However, there are also many reports that question their efficacy. In conclusion, understanding the molecular mechanisms involved in the PDK-PDH axis is a crucial step to advance in novel therapeutic strategies to improve oocyte quality.

SIRT6 Maintains Redox Homeostasis to Promote Porcine Oocyte Maturation

SIRT6, the sixth member of the sirtuin family proteins, has been characterized as a crucial regulator in multiple molecular pathways related to aging, including genome stability, DNA damage repair, telomere maintenance, and inflammation. However, the exact roles of SIRT6 during female germ cell development have not yet been fully determined. Here, we assessed the acquisition of meiotic competency of porcine oocytes by inhibition of SIRT6 activity. We observed that SIRT6 inhibition led to the oocyte meiotic defects by showing the impairment of polar body extrusion and cumulus cell expansion. Meanwhile, the compromised spindle/chromosome structure and actin dynamics were also present in SIRT6-inhibited oocytes. Moreover, SIRT6 inhibition resulted in the defective cytoplasmic maturation by displaying the disturbed distribution dynamics of cortical granules and their content ovastacin. Notably, we identified that transcript levels of genes related to oocyte meiosis, oxidative phosphorylation, and cellular senescence were remarkably altered in SIRT6-inhibited oocytes by transcriptome analysis and validated that the meiotic defects caused by SIRT6 inhibition might result from the excessive reactive oxygen species (ROS)-induced early apoptosis in oocytes. Taken together, our findings demonstrate that SIRT6 promotes the porcine oocyte meiotic maturation through maintaining the redox homeostasis.

SIN3A Regulates Porcine Early Embryonic Development by Modulating CCNB1 Expression

SIN3A is the central scaffold protein of the SIN3/histone deacetylase (HDAC) transcriptional repressor complex. SIN3A participates in the mouse preimplantation development by fine-tuning HDAC1 expression. However, it remains unresolved if this functional significance of SIN3A was conserved in other mammals. Herein, RNA-seq results show a large amount of SIN3A mRNA is present in oocytes and early embryos prior to embryonic genome activation and a low amount thereafter, suggesting a maternal origin of SIN3A in pigs, cattle, mice, and humans. Interestingly, immunofluorescence data show that SIN3A protein level peaks at four-cell stage in pigs compared with morula stage in cattle. SIN3A depletion in early embryos causes a developmental arrest at two-cell stage in pigs but does not affect bovine early embryonic development. In contrast with mouse data, SIN3A depletion results in only a slight decrease and even no difference in HDAC1 expression in porcine and bovine early embryos, respectively. In addition, HDAC1 knockdown does not cause two-cell block but leads to a reduced blastocyst rate. By using unbiased RNA-seq approach, we found that Cyclin B1 (CCNB1) transcript level is dramatically reduced. Moreover, CCNB1 knockdown results in a similar phenotype as SIN3A depletion. Injection of exogenous CCNB1 mRNA into SIN3A-depleted embryos could partly rescue embryonic development to pass two-cell stage. In conclusion, our results indicate SIN3A plays an essential role in porcine early embryonic development, which probably involves the regulation of CCNB1 expression.