Glycolytic metabolites are critical modulators of oocyte maturation and viability

Electro-Metabolic Coupling of Cumulus-Oocyte Complex

Oocyte-cumulus cell interaction is essential for oocyte maturation and competence. The bidirectional crosstalk network mediated by gap junctions is fundamental for the metabolic cooperation between these cells. As cumulus cells exhibit a more glycolytic phenotype, they can provide metabolic substrates that the oocyte can use to produce ATP via oxidative phosphorylation. The impairment of mitochondrial activity plays a crucial role in ovarian aging and, thus, in fertility, determining the success or failure of assisted reproductive techniques. This review aims to deepen the knowledge about the electro-metabolic coupling of the cumulus-oocyte complex and to hypothesize a putative role of potassium channel modulators in order to improve fertility, promote intracellular Ca2+ influx, and increase the mitochondrial biogenesis and resulting ATP levels in cumulus cells.

SIRT3 Regulates Levels of Deacetylated SOD2 to Prevent Oxidative Stress and Mitochondrial Dysfunction During Oocyte Maturation in Pigs

Mammalian oocyte maturation relies on mitochondrial ATP production, but this can lead to damaging reactive oxygen species (ROS). SIRT3, a mitochondrial sirtuin, plays a critical role in regulating mitochondrial redox balance in mouse oocytes under stress; however, its specific roles in porcine oocytes remain unclear. In this study, we utilized the SIRT3 inhibitor 3-TYP to investigate SIRT3's importance in porcine oocyte maturation. Our findings revealed that SIRT3 is expressed in porcine oocytes and its inhibition leads to maturation failure. This was evident through reduced polar body extrusion, arrested cell cycle, as well as disrupted spindle organization and actin distribution. Furthermore, SIRT3 inhibition resulted in a decrease in mitochondrial DNA copy numbers, disruption of mitochondrial membrane potential, and reduced ATP levels, all indicating impaired mitochondrial function in porcine oocytes. Additionally, the primary source of damaged mitochondria was associated with decreased levels of deacetylated superoxide dismutase 2 (SOD2) after SIRT3 inhibition, which led to ROS accumulation and oxidative stress-induced apoptosis. Taken together, our results suggest that SIRT3 regulates the levels of deacetylated SOD2 to maintain redox balance and preserve mitochondrial function during porcine oocyte maturation, with potential implications for improving pig reproduction.

Follicle-stimulating hormone regulates glycolysis of water buffalo follicular granulosa cells through AMPK/SIRT1 signalling pathway

Glycolysis in follicular granulosa cells (GCs) is the primary source of energy metabolism substrate of oocytes and is closely related to follicular development in mammals. Many physiological functions of GCs are regulated by follicle-stimulating hormone (FSH). In contrast, whether FSH regulates the glycolysis of GCs and its mechanism remains unclear. This study explored the correlation between FSH concentration and glycolysis level of GCs from different diameters of water buffalo follicles, and further explored the mechanism of FSH regulation in glycolysis in vitro cultured GCs. Results showed the variation trend of lactic acid concentration in follicular fluid and the expression level of glycolysis-related genes in GCs were consistent with the variation trend of FSH concentration in follicular fluid from follicles with different diameters. When GCs were treated with FSH in vitro, the expression level of glycolysis-related genes, lactate production and glucose uptake increased correspondingly (p < .05). Furthermore, we found that expression trend of AMPK/Sirtuin1 (SIRT1) pathway-related genes in GCs was consistent with the expression trend of glycolysis-related genes and was positively correlated with FSH concentrations in vivo or cultured in vitro. Activation of SIRT1 increased the expression level of glycolytic key proteins and lactic acid production in GCs, while inhibition of SIRT1 showed the opposite effect. In general, glycolysis in water buffalo GCs in vivo or cultured in vitro was positively correlated with FSH concentration. AMPK/SIRT1 pathway plays an important role in the regulation of FSH on glycolysis in GCs. Our findings will enrich the understanding of FSH regulating the development of water buffalo follicles.

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.

Hydrogen Sulfide Impairs Meiosis Resumption in Xenopus laevis Oocytes

The role of hydrogen sulfide (H₂S) is addressed in Xenopus laevis oocytes. Three enzymes involved in H₂S metabolism, cystathionine β-synthase, cystathionine γ-lyase, and 3-mercaptopyruvate sulfurtransferase, were detected in prophase I and metaphase II-arrested oocytes and drove an acceleration of oocyte meiosis resumption when inhibited. Moreover, meiosis resumption is associated with a significant decrease in endogenous H₂S. On another hand, a dose-dependent inhibition was obtained using the H₂S donor, NaHS (1 and 5 mM). NaHS impaired translation. NaHS did not induce the dissociation of the components of the M-phase promoting factor (MPF), cyclin B and Cdk1, nor directly impacted the MPF activity. However, the M-phase entry induced by microinjection of metaphase II MPF-containing cytoplasm was diminished, suggesting upstream components of the MPF auto-amplification loop were sensitive to H₂S. Superoxide dismutase and catalase hindered the effects of NaHS, and this sensitivity was partially dependent on the production of reactive oxygen species (ROS). In contrast to other species, no apoptosis was promoted. These results suggest a contribution of H₂S signaling in the timing of amphibian oocytes meiosis resumption.

Hatching enzymes disrupt aberrant gonadal degeneration by the autophagy/apoptosis cell fate decision

Environmental stressors, gonadal degenerative diseases and tumour development can significantly alter the oocyte physiology, and species fertility and fitness. To expand the molecular understanding about oocyte degradation, we isolated several spliced variants of Japanese anchovy hatching enzymes (AcHEs; ovastacin homologue) 1 and 2, and analysed their potential in oocyte sustenance. Particularly, AcHE1b, an ovary-specific, steroid-regulated, methylation-dependent, stress-responsive isoform, was neofunctionalized to regulate autophagic oocyte degeneration. AcHE1a and 2 triggered apoptotic degeneration in vitellogenic and mature oocytes, respectively. Progesterone, starvation, and high temperature elevated the total degenerating oocyte population and AcHE1b transcription by hyper-demethylation. Overexpression, knockdown and intracellular zinc ion chelation study confirmed the functional significance of AcHE1b in autophagy induction, possibly to mitigate the stress effects in fish, via ion-homeostasis. Our finding chronicles the importance of AcHEs in stress-influenced apoptosis/autophagy cell fate decision and may prove significant in reproductive failure assessments, gonadal health maintenance and ovarian degenerative disease therapy.

Role of Sirt3 in mitochondrial biogenesis and developmental competence of human in vitro matured oocytes

STUDY QUESTION

Does Sirt3 dysfunction result in poor developmental outcomes for human oocytes after in vitro maturation (IVM)?

SUMMARY ANSWER

Inefficient Sirt3 expression induced decreased mitochondrial DNA copy number and biogenesis, and therefore impaired the developmental competence of human IVM oocytes.

WHAT IS KNOWN ALREADY

Cytoplasmic immaturity in IVM oocytes may lead to reduced developmental competence. Mitochondrial dysfunction results in the accumulation of free radicals and leads to DNA mutations, protein damage, telomere shortening and apoptosis. SIRT3 (in the Sirtuin protein family) has emerged as a mitochondrial fidelity protein that directs energy generation and regulates reactive oxygen species scavenging proteins.

STUDY DESIGN, SIZE, DURATION

In vivo matured metaphase II (IVO-MII) oocytes and IVM-MII oocytes were donated by 324 infertile patients undergoing assisted reproductive technology cycles (12 patients for 60 IVO oocytes, and 312 patients for 403 IVM oocytes). Five oocytes each in the germinal vesicle (GV), IVM and IVO groups were compared with respect to mRNA levels for Sirt1-7 mRNA, and five samples at each developmental stage were analysed for Sirt3 mRNA. IVM-MII oocytes were injected with in vitro transcribed mRNA (n = 59) or small interfering RNA (siRNA) (n = 78). In human and mouse, IVM, mRNA-injection IVM, and siRNA-injection IVM groups (n = 5 each) were analysed for mitochondrial DNA copy number and abundance of Sirt3 and Pgc1α (an inducer of mitochondrial biogenesis) mRNAs. Human blastocysts in the IVO (n = 12), IVM (n = 9), mRNA-injection IVM (n = 13) and siRNA-injection IVM (n = 6) groups were used to generate embryonic stem cells (ESCs). In addition, 587 IVO-MII and 1737 IVM-MII oocytes from 83 mice were collected to compare the preliminary human oocyte data with another species.

PARTICIPANTS/MATERIALS, SETTING, METHODS

mRNA abundance was analysed by single-cell real-time PCR. Karyotyping of human embryos was performed with an array comparative genomic hybridization method, and that of ESCs by cytogenetic analysis. The function of the Sirt3 gene was investigated using siRNA and in vitro transcribed mRNA injection. Markers of ESCs were identified using immunofluorescence.

MAIN RESULTS AND THE ROLE OF CHANCE

A retrospective analysis revealed a higher spontaneous abortion rate (P < 0.01) and decrease in high quality embryo rate (P < 0.01) in patients with IVM versus controlled ovarian stimulation (COS) cycles. A decrease in abundance of Sirt3 mRNA (P < 0.01) and mitochondrial biogenesis (P < 0.05) were identified in human IVM compared with IVO oocytes. The developmental potential of human IVM-MII oocytes to the blastocyst stage was significantly reduced when Sirt3 mRNA was inhibited by siRNA (P < 0.05 versus IVM-MII group) but could be up-regulated by injection of Sirt3 mRNAs. Compared with IVO-MII group, comparable generation efficiency of human ESCs can be obtained using blastocysts from IVM-MII oocytes with Sirt3 mRNA injection. Sirt3 mRNA was significantly increased in mouse zygotes after IVF (P < 0.001 versus MII oocytes) but gradually declined until the blastocyst stage. In mice, lower Sirt3 mRNA levels were observed IVM-MII oocytes and preimplantation embryos compared with in vivo controls, and mitochondrial biogenesis and the developmental efficiency from oocytes to blastocyst were affected by the abundance of Sirt3 mRNA in accordance with human. Therefore a similar role for Sirt3 mRNA in IVM-MII oocytes was observed in mouse and human.

LIMITATIONS, REASONS FOR CAUTION

The couples in the study had a variety of different simple and complex factors causing infertility. Additional studies with a larger number of oocytes are required to confirm the present results owing to the limited number of human oocytes in the present study.

WIDER IMPLICATIONS OF THE FINDINGS

To our knowledge, this is the first study investigating a role of the Sirt3 gene in mitochondrial biogenesis and the developmental competence of human IVM-MII oocytes. The observation may help to improve clinical application of the IVM procedure.

STUDY FUNDING/COMPETING INTERESTS

This work was supported in part by the National Natural Science Foundation of Key Program (31230047), Ministry of Science and Technology of China Grants (973 program; 2014CB943203), the National Natural Science Foundation of General Program (31371521 and 81571400), Beijing Nova Program (xxjh2015011), and Specialized Research Fund for the Doctoral Program of Higher Education (20120001130008) and the National Natural Science Foundation of Young Scholar (31501201). The authors have declared that no conflict of interest exists.

Quantitative proteomics reveals the dynamics of protein changes during Drosophila oocyte maturation and the oocyte-to-embryo transition

The onset of development is marked by two major, posttranscriptionally controlled, events: oocyte maturation (release of the prophase I primary arrest) and egg activation (release from the secondary meiotic arrest). Using quantitative mass spectrometry, we previously described proteome remodeling during Drosophila egg activation. Here, we describe our quantitative mass spectrometry-based analysis of the changes in protein levels during Drosophila oocyte maturation. This study presents the first quantitative survey, to our knowledge, of proteome changes accompanying oocyte maturation in any organism and provides a powerful resource for identifying both key regulators and biological processes driving this critical developmental window. We show that Muskelin, found to be up-regulated during oocyte maturation, is required for timely nurse cell nuclei clearing from mature egg chambers. Other proteins up-regulated at maturation are factors needed not only for late oogenesis but also completion of meiosis and early embryogenesis. Interestingly, the down-regulated proteins are predominantly involved in RNA processing, translation, and RNAi. Integrating datasets on the proteome changes at oocyte maturation and egg activation uncovers dynamics in proteome remodeling during the change from oocyte to embryo. Notably, 66 proteins likely act uniquely during late oogenesis, because they are up-regulated at maturation and down-regulated at activation. We find down-regulation of this class of proteins to be mediated partially by APC/C(CORT), a meiosis-specific form of the E3 ligase anaphase promoting complex/cyclosome (APC/C).

Flow cytometric assessment of morphology, viability, and production of reactive oxygen species of Crassostrea gigas oocytes. Application to toxic dinoflagellate (Alexandrium minutum) exposure

The Pacific oyster Crassostrea gigas accounts for a large part of shellfish aquaculture production worldwide. Aspects of morphological and functional characteristics of oyster oocytes remain poorly documented, and traditional techniques, such as microscopic observations of shape or fertilization rate, are time and space consuming. The purpose of this study was to assess for the first time viability and reactive oxygen species (ROS) production of Pacific oyster oocytes using flow cytometry (FCM) and to apply this method to determine oocyte responses to in vitro exposure to the toxic dinoflagellate Alexandrium minutum. A culture of A. minutum caused a significant increase in oocyte ROS production, which gradually increased with the age of the culture, but viability was not affected. Effect of the supernatant of the same A. minutum culture did not cause any significant modifications of oocyte morphology, viability, or ROS level. This study confirmed that some oocyte cellular characteristics can be assessed using FCM techniques.