Cryopreservation and oxidative stress in porcine oocytes

Effect of glutathione addition on vitrification of ovine oocytes

The exogenous antioxidants are commonly employed to mitigate vitrification-induced oxidative damage. Glutathione (GSH), a vital antioxidant, plays a significant role in scavenging free radicals, providing antioxidant protection, and maintaining cellular integrity. This study aimed to investigate the effects of GSH supplementation on the vitrification of ovine oocytes. To identify the optimal concentration of exogenous GSH supplementation, the impacts of 2 mM, 4 mM, and 8 mM GSH on the survival and fragmentation of oocytes were evaluated after vitrification. In addition, immunofluorescence (IF) staining was employed to evaluate spindle morphology, chromosome distribution, cortical granule distribution, mitochondrial function, and reactive oxygen species (ROS) levels. The levels of ATP and NADPH, along with the ratio of GSH/GSSH, were also examined. Additionally, evaluations of in vitro fertilization were carried out. The results of oocyte survival rates and fragmentation rates identified that the addition of 4 mM GSH to the vitrification solution was the optimal concentration. The assessment of spindle morphology, chromosome distribution, cortical granule distribution, mitochondrial function, ROS production, ATP activity, and NADPH levels, as well as the GSH/GSSH ratio, confirm the beneficial effect of GSH supplementation against the vitrification-induced oxidative damage. Lastly, incorporating GSH into the vitrification process enhanced the developmental potential of oocytes, resulting in higher cleavage rates, increased blastocyst rates, and improved quality of blastocysts. Overall, this research uncovered the mechanisms through which GSH mitigates the oxidative damage induced by vitrification, thereby leading to the optimization of the vitrification technique for ovine oocytes.

2-Aminoethyl diphenylborinate mitigates damage from the endoplasmic reticulum of vitrified bovine oocyte

Intracellular calcium (Ca2+) homeostasis is primarily maintained by the endoplasmic reticulum (ER). Fluctuations in intracellular Ca2+ concentrations can induce ER stress, ultimately causing cellular apoptosis. Vitrification can increase Ca2+ concentrations and reduce oocyte vitality. However, 2-Aminoethyl Diphenylborinate (2-APB), a Ca2+ release regulator, can maintain ER calcium homeostasis. In this study, after in vitro maturation, MII oocytes were randomly allocated into 4 group in a 4 groups in a 2 x 2 factorial study (fresh or vitrified and with or without exposure to 2-APB before vitrification). There were fewer ER clusters in the cortical region of oocytes after vitrification in the Control group, but they were protected by pre-treated with 2-APB. In addition, 2-APB significantly increased survival and cleavage of vitrified-warmed parthenogenetically activated blastocysts. Expression of the ER stress proteins GRP78 and CHOP, and the apoptotic proteins caspase-9, caspase-3, and cytochrome c (cytC) were all significantly increased in vitrified oocytes. However, with 2-APB pretreatment, expression of those proteins were not significant different from the Control group. In conclusion, 2-APB effectively alleviated ER stress by maintaining Ca2+ homeostasis to mitigate damage in vitrified-warmed bovine oocytes.

Effect of bovine oviductal epithelial cell lysate on the developmental competence and quality of bovine in vitro fertilized embryos

In vitro fertilization (IVF) technology for embryo production has been applied in basic research, animal husbandry and medicine. However, the developmental efficiency and quality of embryos produced by IVF are inferior to those produced in vivo. In this study, we investigated the effects of supplementing bovine oviductal epithelial cells (BOEC) lysate during the in vitro culture period on the developmental competence and quality of bovine embryos. IVF embryos were cultured for 4 days post-IVF in medium supplemented with 10 % BOEC lysate at various concentrations (1.0 × 105, 2.0 × 105, and 4.0 × 105 cells/mL) or 10 % PBS (-), which was used to adjust the lysate concentration (control). BOEC lysate at 2.0 × 105 cells/mL significantly increased the blastocyst formation rate compared to that in the control group. Blastocysts from BOEC lysate supplemented groups showed significantly lower apoptosis rate than that in the control group. The ratio of inner cell mass cell number in blastocysts was significantly higher in all BOEC lysate supplemented groups than in the control group. The survival rate after vitrification/thawing was improved in the 1.0 × 105 and 2.0 × 105 cells/mL BOEC lysate supplemented groups. In addition, gene expression analysis of blastocysts showed that 2.0 × 105 cells/mL of BOEC lysate supplementation significantly enhanced the expression of anti-apoptotic genes (BCL2 and BIRC5), antioxidant-related genes (GPX1 and SOD2), and cell differentiation-related genes (SOX2 and OCT4). In conclusion, supplementation with 2.0 × 105 cells/mL BOEC lysate during early in vitro culture improved the developmental competence and quality of bovine IVF embryos.

Methyl Paraben Affects Porcine Oocyte Maturation Through Mitochondrial Dysfunction

Parabens are widely used in various industries, which are including chemical, pharmaceutical, food, cosmetic, and plastic processing industries. Among these, methyl paraben (MP) serves as an antimicrobial preservative in processed foods, pharmaceuticals, and cosmetics, and it is particularly detected in baby care products. Studies indicate that MP functions as an endocrine-disrupting compound with estrogenic properties, negatively affecting mitochondrial bioenergetics and antioxidant activity in testicular germ cells. However, limited information exists regarding studies on the effects of MP in oocytes. The aim of this study was to investigate the specific mechanism and the toxic effects of MP during oocyte maturation cultured in vitro using a porcine oocyte model. The results indicated that MP (50 μM) inhibited oocyte expansion, significantly reducing the expression of expansion-related genes MAPK1 and ERK1, and decreased the first polar body extrusion significantly as well. ATP levels decreased, reactive oxygen species (ROS) levels remained unchanged, and glutathione (GSH) levels decreased significantly, resulting in an elevated ROS/GSH ratio. The expression of antioxidant genes SOD1 and GPX was significantly decreased. Additionally, a significant decrease in levels of mitochondrial production and biosynthesis protein PGC1α+β, whereas levels of antioxidant-related protein Nrf2 and related gene expression were significantly increased. Autophagy protein LC3B and gene expression significantly decreased, and apoptosis assay indicated a significant increase in levels of caspase3 protein and apoptosis-related genes. These results demonstrated the negative effect of MP on oocyte maturation. In conclusion, our findings indicate that MP disrupts redox balance and induces mitochondrial dysfunction during meiosis in porcine oocytes, resulting in the inhibition of meiotic progression. The present study reveals the mechanism underlying the effects of methyl para-hydroxybenzoate on oocyte maturation.

Effects of Cryoprotectant Concentration and Exposure Time during Vitrification of Immature Pre-Pubertal Lamb Cumulus-Oocyte Complexes on Nuclear and Cytoplasmic Maturation

Oocyte vitrification allows for the storing of endangered breed female gametes. Cryoprotectant (CPA) concentration and exposure time should ensure cell protection with minimal toxicity. In the present study, a high concentration-rapid exposure (HC-RE) and a low concentration-slow exposure (LC-SE) vitrification protocol, using dimethyl sulfoxide (DMSO) and ethylene glycol (EG) as permeating CPAs, were evaluated on meiotic competence and bioenergetic-oxidative status of pre-pubertal lamb immature COCs after in vitro maturation (IVM). For each protocol, COCs vitrified through a traditional protocol and fresh ones were used as controls. Both protocols allowed COC morphology preservation after vitrification-warming (V-W) and cumulus expansion after IVM. The maturation rate (7% and 14%) was comparable to the vitrified control (13% and 21%) but not satisfactory compared to fresh ones (58% and 64%; p < 0.001). The rate of mature oocytes displaying a perinuclear/subcortical (P/S) mitochondrial distribution pattern, an index of cytoplasmic maturity, was comparable between vitrified and fresh oocytes. The LC-SE vitrification protocol did not affect quantitative bioenergetic-oxidative parameters compared to both controls whereas HC-RE protocol significantly reduced intracellular reactive oxygen species (ROS) levels, indicating cell viability loss. In conclusion, to improve pre-pubertal lamb immature COC vitrification, the combination of low CPA concentrations with prolonged exposure time could be more promising to investigate further.

Metformin alleviates cryoinjuries in porcine oocytes by reducing membrane fluidity through the suppression of mitochondrial activity

Plasma membrane damage in vitrified oocytes is closely linked to mitochondrial dysfunction. However, the mechanism underlying mitochondria-regulated membrane stability is not elucidated. A growing body of evidence indicates that mitochondrial activity plays a pivotal role in cell adaptation. Since mitochondria work at a higher temperature than the constant external temperature of the cell, we hypothesize that suppressing mitochondrial activity would protect oocytes from extreme stimuli during vitrification. Here we show that metformin suppresses mitochondrial activity by reducing mitochondrial temperature. In addition, metformin affects the developmental potential of oocytes and improves the survival rate after vitrification. Transmission electron microscopy results show that mitochondrial abnormalities are markedly reduced in vitrified oocytes pretreated with metformin. Moreover, we find that metformin transiently inhibits mitochondrial activity. Interestingly, metformin pretreatment decreases cell membrane fluidity after vitrification. Furthermore, transcriptome results demonstrate that metformin pretreatment modulates the expression levels of genes involved in fatty acid elongation process, which is further verified by the increased long-chain saturated fatty acid contents in metformin-pretreated vitrified oocytes by lipidomic profile analysis. In summary, our study indicates that metformin alleviates cryoinjuries by reducing membrane fluidity via mitochondrial activity regulation.

The walnut-derived peptide TW-7 improves mouse parthenogenetic embryo development of vitrified MII oocytes potentially by promoting histone lactylation

BACKGROUND

Previous studies have shown that the vitrification of metaphase II (MII) oocytes significantly represses their developmental potential. Abnormally increased oxidative stress is the probable factor; however, the underlying mechanism remains unclear. The walnut-derived peptide TW-7 was initially isolated and purified from walnut protein hydrolysate. Accumulating evidences implied that TW-7 was a powerful antioxidant, while its prospective application in oocyte cryopreservation has not been reported.

RESULT

Here, we found that parthenogenetic activation (PA) zygotes derived from vitrified MII oocytes showed elevated ROS level and delayed progression of pronucleus formation. Addition of 25 μmol/L TW-7 in warming, recovery, PA, and embryo culture medium could alleviate oxidative stress in PA zygotes from vitrified mouse MII oocytes, furtherly increase proteins related to histone lactylation such as LDHA, LDHB, and EP300 and finally improve histone lactylation in PA zygotes. The elevated histone lactylation facilitated the expression of minor zygotic genome activation (ZGA) genes and preimplantation embryo development.

CONCLUSIONS

Our findings revealed the mechanism of oxidative stress inducing repressed development of PA embryos from vitrified mouse MII oocytes and found a potent and easy-obtained short peptide that could significantly rescue the decreased developmental potential of vitrified oocytes, which would potentially contribute to reproductive medicine, animal protection, and breeding.

New Strategies for Conservation of Gentile di Puglia Sheep Breed, an Autochthonous Capital of Millennial Tradition in Southern Italy

Gentile di Puglia (GdP) is an autochthonous sheep breed of Southern Italy included among ovine breeds threatened by genetic erosion and extinction risk, which have been given attention by local and international institutions, thus emphasizing the need for germplasm conservation actions. In the present study, two assisted reproduction approaches, finalized for GdP conservation, were performed: (1) on-farm reproductive efficiency evaluation, expressed as pregnancy rate (PR), twin pregnancy rate (tPR), and body condition score (BCS), for three consecutive breeding cycles and (2) pre-pubertal lambs' immature cumulus-oocyte complex (COC) retrieval, vitrification, in vitro maturation (IVM), and assessment of meiotic stage and bioenergetic-oxidative status compared with those of other Italian and European commercial breeds. PR and tPR were progressively reduced over time. In all clinical examination times, BCS was significantly lower in nonpregnant ewes compared with pregnant ones. Fresh GdP pre-pubertal lamb COCs achieved meiotic maturation and showed healthy bioenergetic-oxidative status after IVM. Vitrification reduced the oocyte maturation rate in all groups. However, mature oocytes retained their cytoplasmic maturity, expressed as a mitochondria distribution pattern and activity, indicating promising developmental competence. In conclusion, clinical- and biotechnological-assisted reproduction approaches can support conservation strategies of GdP and other local sheep breeds in Southern Italy.

Effect of Antioxidant Procyanidin B2 (PCB2) on Ovine Oocyte Developmental Potential in Response to in Vitro Maturation (IVM) and Vitrification Stress

BACKGROUND: It was demonstrated that external stress, such as in vitro maturation (IVM) and vitrification process can induce significantly reduced development capacity in oocytes. Previous studies indicated that antioxidants play a pivotal part in the acquisition of adaptation in changed conditions. At present, the role of the natural potent antioxidant PCB2 in response to IVM and vitrification during ovine oocyte manipulation has not been explored. OBJECTIVE: To investigate whether PCB2 treatment could improve the developmental potential of ovine oocytes under IVM and vitrification stimuli. MATERIALS AND METHODS: The experiment was divided into two parts. Firstly, the effect of PCB2 on the development of oocytes during IVM was evaluated. Unsupplem ented and 5 μg/mL PCB2 -supplemented in the IVM solution were considered as control and experimental groups (C + 5 μg/mL PCB2). The polar body extrusion (PBE) rate, mitochondrial membrane potential (MMP), ATP, reactive oxygen species (ROS) levels and early apoptosis of oocytes were measured after IVM. Secondly, we further determine whether PCB2 could improve oocyte quality under vitrification stress. The survival rate, PBE rate and early apoptosis of oocytes were compared between fresh group, vitrified group and 5 μg/mL PCB2 -supplemented in the IVM solution after vitrification (V + 5μg/mL PCB2). RESULTS: Compared to the control group, adding PCB2 significantly increased PBE rate (79.4% vs. 62.8%, P < 0.01) and MMP level (1.9 ± 0.08 vs. 1.3 ± 0.04, P < 0.01), and decreased ROS level (47.1 ± 6.3 vs. 145.3 ± 8.9, P < 0.01). However, there was no significant difference in ATP content and early apoptosis. Compared to the fresh group, vitrification significantly reduced oocytes viability (43.0% vs. 90.8%, P < 0.01) as well as PBE rate (24.2% vs. 60.6%, P < 0.05). However, 5 μg/mL PCB2-supplemention during maturation had no effect on survival, PBE or early apoptosis in vitrified oocytes. CONCLUSION: PCB2 could effectively antagonise the oxidative stress during IVM and promote oocyte development.

Glutathione during Post-Thaw Recovery Culture Can Mitigate Deleterious Impact of Vitrification on Bovine Oocytes

Vitrification of bovine oocytes can impair subsequent embryo development mostly due to elevated oxidative stress. This study was aimed at examining whether glutathione, a known antioxidant, can improve further embryo development when added to devitrified oocytes for a short recovery period. Bovine in vitro matured oocytes were vitrified using an ultra-rapid cooling technique on electron microscopy grids. Following warming, the oocytes were incubated in the recovery medium containing glutathione (0, 1.5, or 5 mmol L^-1) for 3 h (post-warm recovery). Afterwards, the oocytes were lysed for measuring the total antioxidant capacity (TAC), activity of peroxidase, catalase and glutathione reductase, and ROS formation. The impact of vitrification on mitochondrial and lysosomal activities was also examined. Since glutathione, added at 5 mmol L^-1, significantly increased the TAC of warmed oocytes, in the next set of experiments this dose was applied for post-warm recovery of oocytes used for IVF. Glutathione in the recovery culture did not change the total blastocyst rate, while increased the proportion of faster developing blastocysts (Day 6-7), reduced the apoptotic cell ratio and reversed the harmful impact of vitrification on the actin cytoskeleton. These results suggest that even a short recovery culture with antioxidant(s) can improve the development of bovine devitrified oocytes.

Equilibration time with cryoprotectants, but not melatonin supplementation during in vitro maturation, affects viability and metaphase plate morphology of vitrified porcine mature oocytes

The aims of this study were to investigate the effects of different equilibration times with cryoprotectants on viability and metaphase plate morphology of vitrified-warmed porcine mature oocytes (Experiment 1) and to evaluate the effects of supplementation with 10^-9 M melatonin during in vitro maturation on these parameters (Experiment 2). In Experiment 1, 2,392 mature oocytes were vitrified using different equilibration times of oocytes with cryoprotectants (3, 10, 15, 20, 30, 40, 60 and 80 min). Fresh oocytes matured in vitro for 44 hr (n = 509) were used as controls. In Experiment 2, a total of 573 COCs were used. COCs were matured with 10^-9 M melatonin supplementation or without melatonin (control). Some oocytes from each group were vitrified with a 60-min equilibration time with cryoprotectants according to the results of Experiment 1. The remaining oocytes from each maturation group were used as fresh control groups. In both experiments, oocytes were stained with 2',7'-dichlorodihydrofuorescein diacetate and Hoechst 33342 to assess viability and metaphase plate morphology, respectively. Vitrification and warming affected (p < .01) oocyte viability compared with controls, which were all viable after 44 hr of IVM. In Experiment 1, the longer the equilibration time with cryoprotectants, the higher the viability. Oocytes equilibrated for 60 and 80 min had the highest (p < .05) viability and similar metaphase plate characteristics to the fresh control oocytes. In Experiment 2, supplementation with melatonin during in vitro maturation had no effect on oocyte viability or metaphase plate morphology of vitrified-warmed oocytes. In conclusion, under our experimental conditions, vitrified porcine mature oocytes equilibrated with cryoprotectants for 60 or 80 min exhibited the highest viability and similar metaphase plate characteristics to fresh controls. Furthermore, supplementation with 10^-9 M melatonin during in vitro maturation had no effect on these parameters.

Involvement of PINK1/Parkin-mediated mitophagy in mitochondrial functional disruption under oxidative stress in vitrified porcine oocytes

Vitrification is an effective technique for fertility preservation, but is known to lead to mitochondrial dysfunction in porcine oocytes. Mitophagy is induced to rebalance mitochondrial function, a process in which reactive oxygen species (ROS) plays a role. In this study, vitrified-warmed porcine oocytes were incubated for 4 h with the oxidant AAPH or antioxidant α-tocopherol to alter ROS levels. A series of tests suggested that vitrification damaged mitochondrial structure and caused dysfunction, including blurred mitochondrial cristae, decreased mitochondrial membrane potential, decreased mtDNA copy number and increased ROS generation. This dysfunction resulted in mitophagy and the loss of embryonic developmental potential. Incubation with AAPH or α-tocopherol altered mitochondrial function and mitophagy flux status in vitrified oocytes. The PINK1/Parkin pathway was involved in oxidative stress regulation in vitrified oocytes. Under AAPH-induced oxidative stress, increased fluorescence intensity of Parkin, increased expression of PINK1, Parkin, and LC3B-II, and decreased expression of MFN2 and p62 were observed, whereas the opposite effects were induced under α-tocopherol treatment. The inhibition of ROS by α-tocopherol benefitted mitochondrial homeostasis and alleviated PINK1/Parkin-mediated mitophagy, resulting in the recovery of embryonic developmental potential in vitrified porcine oocytes. Therefore, this study provides a new mechanism for the application of antioxidants to aid the cryopreservation of porcine oocytes.

Oocyte Cryopreservation in Domestic Animals and Humans: Principles, Techniques and Updated Outcomes

Oocyte cryopreservation plays important roles in basic research and the application of models for genetic preservation and in clinical situations. This technology provides long-term storage of gametes for genetic banking and subsequent use with other assisted reproductive technologies. Until recently, oocytes have remained the most difficult cell type to freeze, as the oocytes per se are large with limited surface area to cytoplasm ratio. They are also highly sensitive to damage during cryopreservation, and therefore the success rate of oocyte cryopreservation is generally poor when compared to noncryopreserved oocytes. Although advancement in oocyte cryopreservation has progressed rapidly for decades, the improvement of cryosurvival and clinical outcomes is still required. This review focuses on the principles, techniques, outcomes and prospects of oocyte cryopreservation in domestic animals and humans.

Melatonin Promotes In Vitro Maturation of Vitrified-Warmed Mouse Germinal Vesicle Oocytes, Potentially by Reducing Oxidative Stress through the Nrf2 Pathway

Simple Summary

Cryopreservation of oocytes can cause high oxidative stress, reduce the quality of vitrified-warmed oocytes, and seriously hinder the application of oocyte cryopreservation technology in production and medicine. In this work, we found for the first time that melatonin can exert antioxidant effects through receptors and regulate the Nrf2 antioxidant pathway to respond to oxidative stress of vitrified-warmed oocytes, thereby improving both oocyte quality and the potential for subsequent development. The results illustrated the molecular mechanism of melatonin’s antioxidant effect in vitrified-warmed oocytes and provided a theoretical basis for the application of melatonin in the cryopreservation of oocytes. These findings are of great significance for the further application of oocyte cryopreservation technology to production and assisted reproduction in the future.

Abstract

Previously it was reported that melatonin could mitigate oxidative stress caused by oocyte cryopreservation; however, the underlying molecular mechanisms which cause this remain unclear. The objective was to explore whether melatonin could reduce oxidative stress during in vitro maturation of vitrified-warmed mouse germinal vesicle (GV) oocytes through the Nrf2 signaling pathway or its receptors. During in vitro maturation of vitrified-warmed mouse GV oocytes, there were decreases (p < 0.05) in the development rates of metaphase I (MI) oocytes and metaphase II (MII) and spindle morphology grades; increases (p < 0.05) in the reactive oxygen species (ROS) levels; and decreases (p < 0.05) in expressions of Nrf2 signaling pathway-related genes (Nrf2, SOD1) and proteins (Nrf2, HO-1). However, adding 10⁻⁷ mol/L melatonin to both the warming solution and maturation solutions improved (p < 0.05) these indicators. When the Nrf2 protein was specifically inhibited by Brusatol, melatonin did not increase development rates, spindle morphology grades, genes, or protein expressions, nor did it reduce vitrification-induced intracellular oxidative stress in GV oocytes during in vitro maturation. In addition, when melatonin receptors were inhibited by luzindole, the ability of melatonin to scavenge intracellular ROS was decreased, and the expressions of genes (Nrf2, SOD1) and proteins (Nrf2, HO-1) were not restored to control levels. Therefore, we concluded that 10⁻⁷ mol/L melatonin acted on the Nrf2 signaling pathway through its receptors to regulate the expression of genes (Nrf2, SOD1) and proteins (Nrf2, HO-1), and mitigate intracellular oxidative stress, thereby enhancing in vitro development of vitrified-warmed mouse GV oocytes.