Effect of oocyte vitrification on DNA damage in metaphase II oocytes and the resulting preimplantation embryos

Overexpression of KAT8 induces a failure in early embryonic development in mice

Embryo quality is strongly associated with subsequent embryonic developmental efficiency. However, the detailed function of lysine acetyltransferase 8 (KAT8) during early embryonic development in mice remains elusive. In this study, we reported that KAT8 played a pivotal role in the first cleavage of mouse embryos. Immunostaining results revealed that KAT8 predominantly accumulated in the nucleus throughout the entire embryonic developmental process. Kat8 overexpression (Kat8-OE) was correlated with early developmental potential of embryos to the blastocyst stage. We also found that Kat8-OE embryos showed spindle-assembly defects and chromosomal misalignment, and that Kat8-OE in embryos led to increased levels of reactive oxygen species (ROS), accumulation of phosphorylated γH2AX by affecting the expression of critical genes related to mitochondrial respiratory chain and antioxidation pathways. Subsequently, cellular apoptosis was activated as confirmed by TUNEL (Terminal Deoxynucleotidyl Transferase mediated dUTP Nick-End Labeling) assay. Furthermore, we revealed that KAT8 was related to regulating the acetylation status of H4K16 in mouse embryos, and Kat8-OE induced the hyperacetylation of H4K16, which might be a key factor for the defective spindle/chromosome apparatus. Collectively, our data suggest that KAT8 constitutes an important regulator of spindle assembly and redox homeostasis during early embryonic development in mice.

Transcriptome analysis of porcine oocytes during postovulatory aging

Decreased oocyte quality is a significant contributor to the decline in female fertility that accompanies aging in mammals. Oocytes rely on mRNA stores to support their survival and integrity during the protracted period of transcriptional dormancy as they await ovulation. However, the changes in mRNA levels and interactions that occur during porcine oocyte maturation and aging remain unclear. In this study, the mRNA expression profiles of porcine oocytes during the GV, MII, and aging (24 h after the MII stage) stages were explored by transcriptome sequencing to identify the key genes and pathways that affect oocyte maturation and postovulatory aging. The results showed that 10,929 genes were coexpressed in porcine oocytes during the GV stage, MII stage, and aging stage. In addition, 3037 genes were expressed only in the GV stage, 535 genes were expressed only in the MII stage, and 120 genes were expressed only in the aging stage. The correlation index between the GV and MII stages (0.535) was markedly lower than that between the MII and aging stages (0.942). A total of 3237 genes, which included 1408 upregulated and 1829 downregulated genes, were differentially expressed during porcine oocyte postovulatory aging (aging stage vs. MII stage). Key functional genes, including ATP2A1, ATP2A3, ATP2B2, NDUFS1, NDUFA2, NDUFAF3, SREBF1, CYP11A1, CYP3A29, GPx4, CCP110, STMN1, SPC25, Sirt2, SYCP3, Fascin1/2, PFN1, Cofilin, Tmod3, FLNA, LRKK2, CHEK1/2, DDB1/2, DDIT4L, and TONSL, and key molecular pathways, such as the calcium signaling pathway, MAPK signaling pathway, TGF-β signaling pathway, PI3K/Akt signaling pathway, FoxO signaling pathway, gap junctions, and thermogenesis, were found in abundance during porcine postovulatory aging. These genes are mainly involved in the regulation of many biological processes, such as oxidative stress, calcium homeostasis, mitochondrial function, and lipid peroxidation, during porcine oocyte postovulatory aging. These results contribute to a more in-depth understanding of the biological changes, key regulatory genes and related biological pathways that are involved in oocyte aging and provide a theoretical basis for improving the efficiency of porcine embryo production in vitro and in vivo.

Oocyte Vitrification Reduces its Capability to Repair Sperm DNA Fragmentation and Impairs Embryonic Development

Oocytes play a crucial role in repairing sperm DNA damage, which can affect the next generation; however, certain factors can impair this ability. This study examined whether oocyte vitrification, a widely used method for fertility preservation, negatively affects repair ability. Male DBA/2 mice (n = 28) were injected with 101.60 µmol/100 g body weight of tert-Butyl hydroperoxide (tBHP) for 14 days to induce sperm DNA damage. Histological changes, sperm functions, and DNA fragmentation were assessed using the TUNEL assay. Cumulus-oocyte-complexes (COCs) of superovulated female DBA/2 mice (n = 28) were vitrified using the Cryotop method. Fresh and vitrified oocytes were then fertilized by tBHP-treated and untreated sperms, and subsequent embryonic development was monitored. Additionally, the expression of Mre11a, Rad51, Brca1, and Xrcc4 was assessed in resulting zygotes and blastocysts using real-time PCR. The sperm tBHP treatment reduced differentiated spermatogenic cells in the testicular tissue, sperm concentration, and motility, while increasing DNA fragmentation (P < 0.05). The fertilization rate was decreased in the tBHP-treated sperm-vitrified oocyte group (P < 0.05), and the two-cell rate diminished in tBHP-treated sperm-fresh and vitrified oocyte groups (P < 0.05). The four-cell to blastocyst rate decreased in the untreated sperm-vitrified oocyte and the tBHP-treated sperm-fresh and vitrified oocyte groups (P < 0.05), and the tBHP-treated sperm-vitrified oocyte groups had the lowest blastocyst rate. In zygotes, Brca1 was upregulated in the tBHP-treated sperm-vitrified oocyte group (P < 0.05). Also, in blastocysts, Rad51, Brca1, and Xrcc4 were significantly upregulated in the untreated sperm-vitrified oocytes group (P < 0.05). Damages to the oocyte due to vitrification can disrupt the repair of sperm DNA fragmentation and consequently impair the embryo development.

Increased ROS levels in mitochondrial outer membrane protein Mul1-deficient oocytes result in abnormal preimplantation embryogenesis

Reactive oxygen species (ROS) are associated with oocyte maturation inhibition, and N-acetyl-l-cysteine (NAC) partially reduces their harmful effects. Mitochondrial E3 ubiquitin ligase 1 (Mul1) localizes to the mitochondrial outer membrane. We found that female Mul1-deficient mice are infertile, and their oocytes contain high ROS concentrations. After fertilization, Mul1-deficient embryos showed a DNA damage response (DDR) and abnormal preimplantation embryogenesis, which was rescued by NAC addition and ROS depletion. These observations clearly demonstrate that loss of Mul1 in oocytes increases ROS concentrations and triggers DDR, resulting in abnormal preimplantation embryogenesis. We conclude that manipulating the mitochondrial ROS levels in oocytes may be a potential therapeutic approach to target infertility.

Effects of intracellular Ca2+ on developmental potential and ultrastructure of cryopreserved-warmed oocyte in mouse

Maintaining appropriate intracellular calcium of oocytes is necessary to prevent ultrastructure and organelle damage caused by freezing and cryoprotectants. The present study aimed to investigate whether cryoprotectant-induced changes in the calcium concentrations of oocytes can be regulated to reduce damage to developmental potential and ultrastructure. A total of 33 mice and 1381 oocytes were used to explore the effects of intracellular calcium on the development and ultrastructures of oocytes subjected to 2-aminoethoxydiphenyl borate (2-APB) inhibition or thapsigargin (TG) stimulation. Results suggested that high levels intracellular calcium interfered with TG compromised oocyte survival (84.4 % vs. 93.4 %, p < 0.01) and blastocyst formation in fresh and cryopreservation oocytes (78.1 % vs. 86.4 %, and 60.5 % vs. 72.5 %, p < 0.05) compared with that of 2-APB pretreated oocytes in which Ca2+ was stabilized even though no differences in fertilization and cleavage was detected (p > 0.05). Examination by transmission electron microscopy indicated that the microvilli decreased and shortened, cortical granules considerably decreased in the cortex area, mitochondrial vesicles and vacuoles increased, and the proportion of vacuole mitochondria increased after oocytes were exposed to cryoprotectants. The cryopreservation-warming process deteriorated the negative effects on organelles of survival oocytes. By contrast, a low level of intracellular calcium mediated with 2-APB was supposed to contribute to the protection of organelles. These findings suggested oocyte injuries induced by cryoprotectants and low temperatures can be alleviated. More studies are necessary to confirm the relationship among Ca2+ concentration of the cytoplasm, ultrastructural injuries, and disrupted developmental potential in oocytes subjected to cryopreservation and warming.

Equilibrium vitrification of oocytes using low concentrations of cryoprotectants

In order to make dry ice transportation of vitrified embryos practical, a near-equilibrium vitrification was developed using a cryoprotectant solution (EDFS10/10a), by which mouse embryos at various stages were vitrified in a near-equilibrium environment. EDFS10/10a consisted of 10% (v/v) ethylene glycol, 10% (v/v) Me2SO, 0.4 M sucrose and 24% (w/v) Ficoll PM70. This method exhibited the benefits of slow freezing and vitrification, with a low risk of osmotic injury. In this study, we investigated whether mouse oocytes are vitrifiable with EDFS10/10a in a highly dehydrated/concentrated state, and whether they can remain fertilizable and developing into embryos after vitrification. When mature mouse oocytes were vitrified in liquid nitrogen and after 4-28 days of storage at -80 °C, high survival rates were observed (88-99%). Vitrified and warmed oocytes were subjected to partial zona dissection and in vitro fertilized. The rate of 2-cell stage was 80-82%. Blastocyst formation rate was 55-70% which was similar to that of embryos derived from fresh oocytes. After the 2-cell embryos were transferred to recipient mice, the implantation and offspring rates did not differ significantly from those of embryos derived from fresh oocytes, indicating that vitrified oocytes retained the developmental ability. Therefore, it is possible to vitrify mouse oocytes in a near-equilibrium state using EDFS10/10a and conveniently transported using dry ice.

Fisetin Delays Postovulatory Oocyte Aging by Regulating Oxidative Stress and Mitochondrial Function through Sirt1 Pathway

The quality of oocytes determines the development potential of an embryo and is dependent on their timely fertilization after ovulation. Postovulatory oocyte aging is an inevitable factor during some assisted reproduction technology procedures, which results in poor fertilization rates and impairs embryo development. We found that fisetin, a bioactive flavonol contained in fruits and vegetables, delayed postovulatory oocyte aging in mice. Fisetin improved the development of aged oocytes after fertilization and inhibited the Sirt1 reduction in aged oocytes. Fisetin increased the GSH level and Sod2 transcription level to inhibit ROS accumulation in aged oocytes. Meanwhile, fisetin attenuated aging-induced spindle abnormalities, mitochondrial dysfunction, and apoptosis. At the molecular level, fisetin decreased aging-induced aberrant expression of H3K9me3. In addition, fisetin increased the expression levels of the mitochondrial transcription factor Tfam and the mitochondrial genes Co2 and Atp8 by upregulating Sirt1 in aged oocytes. Finally, inhibition of Sirt1 reversed the anti-aging effects of fisetin. Taken together, fisetin delayed postovulatory oocyte aging by upregulating Sirt1.

Expression profile of key genes involved in DNA repair mechanisms in bovine cumulus cells cultured with bovine serum albumin or fetal calf serum

Cumulus cells from cumulus-oocyte complexes (COC) matured in vitro in serum-free medium show high incidence of apoptosis and DNA double-strand breaks (DSB). This study aimed to characterize the transcript expression profile of selected genes involved in DNA repair mechanisms in bovine cumulus cells cultured with bovine serum albumin (BSA) or fetal calf serum (FCS). Briefly, bovine cumulus-oocyte complexes were in vitro matured with either, 0.4% BSA or 10% FCS for 3, 6, 12 or 24 h. The total RNA of cumulus cells was used for real-time PCR analysis. Transcript abundance of XRCC6, XRCC5, DNAPK, GAAD45B, TP53BP1, RAD50, RAD52, ATM and BRCA2 target genes changed as the IVM proceeded (P < 0.05). However, an interaction between protein source (FCS or BSA) and time was not detected (P ≥ 0.05). Cumulus cells from COCs matured with BSA presented higher mRNA expression of two genes compared to FCS group: TP53BP1 at 6 h and BRCA1 at 3, 6, 12 and 24 h (P < 0.05). In summary, our results showed for the first time the expression profile of the key genes involved in DSB repair mechanisms in cumulus cells obtained from bovine COCs matured with FCS or BSA. The higher mRNA expression of BRCA1 and TP53BP1 and lower mRNA expression of TNFAIP6 suggests an increase in apoptosis rate and DNA damage in cumulus cells cultured in BSA-supplemented medium and may explain, at least to some extent, the reduced developmental potential of bovine oocytes matured in serum-free medium.

[The impact of oocyte cryopreservation time in oocyte donation on the clinical success rate]

OBJECTIVES

To evaluate the impact of the cryopreservation time of vitrified oocytes on the success rates in oocyte donation cycles.

METHODS

A retrospective study was carried out on 156 cycles with donated oocytes from January 2012 to September 2021. All the cycles were sorted according to the storage time of the oocytes (25 in the group 1:<3 months, 32 in the group 2: between 3 and 6 months, 39 in the group 3: between 6 and 12 months, 38 in the group 4: between 12 and 24 months and 22 in the group 5:>24 months). Clinical outcomes after ART, survival rates at thawing and oocyte fertilization rates were compared between the different cohorts stratified according to oocyte storage duration. A binary multivariate logistic regression was performed adjusting for the identified confounders.

RESULTS

Prolonged storage time of vitrified oocytes had an effect on their survival post-thawing rates, but no significant effect was identified on fertilization rates or clinical outcomes. After adjusting for the confounders, the relationships between clinical outcomes and oocytes storage time did not reach statistical significance. Our study was characterized by a limited cohort with data from a single ART center.

CONCLUSIONS

Our study doesn't highlight any significant difference in the use of long-stored vitrified oocytes (more than 2 years) on clinical issues in ART. The conclusion of our study needs to be verified in further studies with larger cohorts.

Vitrification of porcine immature oocytes and zygotes results in different levels of DNA damage which reflects developmental competence to the blastocyst stage

The present study investigated the effects of vitrification of porcine oocytes either at the immature Germinal Vesicle (GV) stage before in vitro maturation (GV-stage oocytes) or at the pronuclear stage after in vitro maturation and fertilization (zygotes) on DNA integrity in relevance with their subsequent embryo development. Vitrification at the GV stage but not at the pronuclear stage significantly increased the abundance of double-strand breaks (DSBs) in the DNA measured by the relative fluorescence after γH2AX immunostaining. Treatment of GV-stage oocytes with cryoprotectant agents alone had no effect on DSB levels. When oocytes were vitrified at the GV stage and subjected to in vitro maturation and fertilization (Day 0) and embryo culture, significantly increased DSB levels were detected in subsequent cleavage-stage embryos which were associated with low cell numbers on Day 2, the upregulation of the RAD51 gene at the 4-8 cell stage (measured by RT-qPCR) and reduced developmental ability to the blastocyst stage when compared with the non-vitrified control. However, total cell numbers and percentages of apoptotic cells (measured by TUNEL) in resultant blastocysts were not different from those of the non-vitrified control. On the other hand, vitrification of zygotes had no effect on DSB levels and the expression of DNA-repair genes in resultant embryos, and their development did not differ from that of the non-vitrified control. These results indicate that during vitrification GV-stage oocytes are more susceptible to DNA damages than zygotes, which affects their subsequent development to the blastocyst stage.

Oxidative Stress and Oocyte Cryopreservation: Recent Advances in Mitigation Strategies Involving Antioxidants

Oocyte cryopreservation is widely used in assisted-reproductive technology and animal production. However, cryopreservation not only induces a massive accumulation of reactive oxygen species (ROS) in oocytes, but also leads to oxidative-stress-inflicted damage to mitochondria and the endoplasmic reticulum. These stresses lead to damage to the spindle, DNA, proteins, and lipids, ultimately reducing the developmental potential of oocytes both in vitro and in vivo. Although oocytes can mitigate oxidative stress via intrinsic antioxidant systems, the formation of ribonucleoprotein granules, mitophagy, and the cryopreservation-inflicted oxidative damage cannot be completely eliminated. Therefore, exogenous antioxidants such as melatonin and resveratrol are widely used in oocyte cryopreservation to reduce oxidative damage through direct or indirect scavenging of ROS. In this review, we discuss analysis of various oxidative stresses induced by oocyte cryopreservation, the impact of antioxidants against oxidative damage, and their underlying mechanisms. We hope that this literature review can provide a reference for improving the efficiency of oocyte cryopreservation.

Effects of MICU1-Mediated Mitochondrial Calcium Uptake on Energy Metabolism and Quality of Vitrified-Thawed Mouse Metaphase II Oocytes

Background: Oocyte vitrification has been widely used in the treatment of infertility and fertility preservation. However, vitrification-induced mitochondrial damage adversely affects oocyte development. Several studies have reported that mitochondrial calcium uptake protein 1 (MICU1) regulates the uptake of mitochondrial calcium by the mitochondrial calcium uniporter (MCU) and subsequently controls aerobic metabolism and oxidative stress in mitochondria, but research considering oocytes remains unreported. We evaluated whether the addition of MICU1 modulators enhances mitochondrial activity, pyruvate metabolism, and developmental competence after warming of MII oocytes. Methods: Retrieved MII oocytes of mice were classified as vitrified or control groups. After thawing, oocytes of vitrified group were cultured with or without DS16570511 (MICU1 inhibitor) and MCU-i4 (MICU1 activator) for 2 h. Results: Mitochondrial Ca2+ concentration, pyruvate dephosphorylation level, and MICU1 expression of MII oocytes were significantly increased after vitrification. These phenomena were further exacerbated by the addition of MCU-i4 and reversed by the addition of DS16570511 after warming. However, the mitochondrial membrane potential (MMP) and adenosine triphosphate (ATP) in vitrified-warmed MII oocytes drop significantly after vitrification, which was improved after MCU-i4 treatment and decreased significantly after DS16570511 treatment. The vitrification process was able to elicit a development competence reduction. After parthenogenetic activation, incubation of the thawed oocytes with MCU-i4 did not alter the cleavage and blastocyst rates. Moreover, incubation of the thawed oocytes with DS16570511 reduced the cleavage and blastocyst rates. Conclusions: MICU1-mediated increasing mitochondrial calcium uptake after vitrification of the MII oocytes promoted the pyruvate oxidation, and this process may maintain oocyte development competence by compensating for the consumption of ATP under stress state.

Vitrification of bovine germinal vesicle oocytes significantly decreased the methylation level of their in vitro derived MII oocytes

CONTEXT

The vitrification of oocytes is important for the conservation of animals, and the effect of vitrification on methylation patterns of bovine oocytes remains unclear.

AIMS

This article aims to investigate the effect of vitrification on the DNA methylation patterns on vitrified GV oocytes and their in vitro derived MII oocytes.

METHODS

5-MeC staining and single-cell whole genome bisulphite sequencing (SC-WGBS) were utilised to analyse fresh GV oocytes (F_GV group), MII oocytes (F_MII group), vitrified GV oocytes (V_GV group) and their in vitro derived MII oocytes (V_MII group).

KEY RESULTS

Results of both 5-MeC staining and SC-WGBS showed that no significant difference was found between the F_GV group and the V_GV group, while the methylation level of the V_MII group was significantly lower than that of the F_MII group. Moreover, supplementation of 2μM resveratrol (Res) in IVM medium significantly improved maturation and development ability of vitrified GV oocytes by restoring their DNA methylation levels.

CONCLUSION

In conclusion, vitrification of bovine GV oocytes significantly decreased the DNA methylation level of their in vitro derived MII oocytes, and 2μM Res improved their development ability by restoring DNA methylation level.

IMPLICATIONS

Our results provide an efficient approach to improve the maturation and fertilisation ability of vitrified GV oocytes.

Pak2 reduction induces a failure of early embryonic development in mice

BACKGROUND

The quality of the early embryo is vital to embryonic development and implantation. As a highly conserved serine/threonine kinase, p21-activated kinase 2 (Pak2) participates in diverse biologic processes, especially in cytoskeleton remodeling and cell apoptosis. In mice, Pak2 knock out and endothelial depletion of Pak2 showed embryonic lethality. However, the role of Pak2 in preimplantation embryos remains unelucidated.

METHODS

In the present work, Pak2 was reduced using a specific small interfering RNA in early mouse embryos, validating the unique roles of Pak2 in spindle assembly and DNA repair during mice early embryonic development. We also employed immunoblotting, immunostaining, in vitro fertilization (IVF) and image quantification analyses to test the Pak2 knockdown on the embryonic development progression, spindle assembly, chromosome alignment, oxidative stress, DNA lesions and blastocyst cell apoptosis. Areas in chromatin with γH2AX were detected by immunofluorescence microscopy and serve as a biomarker of DNA damages.

RESULTS

We found that Pak2 knockdown significantly reduced blastocyst formation of early embryos. In addition, Pak2 reduction led to dramatically increased abnormal spindle assembly and chromosomal aberrations in the embryos. We noted the overproduction of reactive oxygen species (ROS) with Pak2 knockdown in embryos. In response to DNA double strand breaks (DSBs), the histone protein H2AX is specifically phosphorylated at serine139 to generate γH2AX, which is used to quantitative DSBs. In this research, Pak2 knockdown also resulted in the accumulation of phosphorylated γH2AX, indicative of increased embryonic DNA damage. Commensurate with this, a significantly augmented rate of blastocyst cell apoptosis was detected in Pak2-KD embryos compared to their controls.

CONCLUSIONS

Collectively, our data suggest that Pak2 may serve as an important regulator of spindle assembly and DNA repair, and thus participate in the development of early mouse embryos.

DNA damage in cumulus cells generated after the vitrification of in vitro matured porcine oocytes and its impact on fertilization and embryo development

Background

The evaluation of the DNA damage generated in cumulus cells after mature cumulus-oocyte complexes vitrification can be considered as an indicator of oocyte quality since these cells play important roles in oocyte developmental competence. Therefore, the aim of this study was to determine if matured cumulus-oocyte complexes exposure to cryoprotectants (CPAs) or vitrification affects oocytes and cumulus cells viability, but also if DNA damage is generated in cumulus cells, affecting fertilization and embryo development.

Results

The DNA damage in cumulus cells was measured using the alkaline comet assay and expressed as Comet Tail Length (CTL) and Olive Tail Moment (OTM). Results demonstrate that oocyte exposure to CPAs or vitrification reduced oocyte (75.5 ± 3.69%, Toxicity; 66.7 ± 4.57%, Vitrification) and cumulus cells viability (32.7 ± 5.85%, Toxicity; 7.7 ± 2.21%, Vitrification) compared to control (95.5 ± 4.04%, oocytes; 89 ± 4.24%, cumulus cells). Also, significantly higher DNA damage expressed as OTM was generated in the cumulus cells after exposure to CPAs and vitrification (39 ± 17.41, 33.6 ± 16.69, respectively) compared to control (7.4 ± 4.22). In addition, fertilization and embryo development rates also decreased after exposure to CPAs (35.3 ± 16.65%, 22.6 ± 3.05%, respectively) and vitrification (32.3 ± 9.29%, 20 ± 1%, respectively). It was also found that fertilization and embryo development rates in granulose-intact oocytes were significantly higher compared to denuded oocytes in the control groups. However, a decline in embryo development to the blastocyst stage was observed after CPAs exposure (1.66 ± 0.57%) or vitrification (2 ± 1%) compared to control (22.3 ± 2.51%). This could be attributed to the reduction in both cell types viability, and the generation of DNA damage in the cumulus cells.

Conclusion

This study demonstrates that oocyte exposure to CPAs or vitrification reduced viability in oocytes and cumulus cells, and generated DNA damage in the cumulus cells, affecting fertilization and embryo development rates. These findings will allow to understand some of the mechanisms of oocyte damage after vitrification that compromise their developmental capacity, as well as the search for new vitrification strategies to increase fertilization and embryo development rates by preserving the integrity of the cumulus cells.

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.

Resveratrol enhanced mitochondrial recovery from cryopreservation-induced damages in oocytes and embryos

BACKGROUND

Mitochondria play a crucial role in nuclear maturation, fertilization, and subsequent embryo development. Cryopreservation is an important assisted reproductive technology that is used worldwide for humans and domestic animals. Although mitochondrial quantity and quality are decisive factors for successful development of oocytes and embryos, cryopreservation induces mitochondrial dysfunction. Upon thawing, the damaged mitochondria are removed, and de novo synthesis occurs to restore the function of mitochondria. Resveratrol, 3,5,4'-trihydroxystilbene, is a polyphenolic antioxidant that has versatile target proteins, among which sirtuin-1 (SIRT1) is a key regulator of in mitochondrial biogenesis and degradation.

METHODS

The present study is a literature review focusing on experiments involving the hypothesis that the activation of mitochondrial biogenesis and degradation following cryopreservation and warming by resveratrol may help mitochondrial recovery and improve oocyte and embryo development.

MAIN FINDINGS AND CONCLUSION

Resveratrol improves oocyte maturation and development and upregulates mitochondrial biogenesis and degradation. When vitrified-warmed embryos are treated with resveratrol, it helps in mitochondrial regulation and recovery of embryos from cryopreservation-induced damage.

CONCLUSION

Resveratrol treatment is a possible countermeasure against cryopreservation-induced mitochondrial damage.

Mitophagy is involved in the mitochondrial dysfunction of vitrified porcine oocytes

Mitochondrial dysfunction is considered a crucial factor aggravating oocyte viability after vitrification-warming. To clarify the role of mitophagy in mitochondrial extinction of vitrified porcine oocytes, mitochondrial function, ultrastructural characteristics, mitochondria-lysosomes colocalization, and mitophagic proteins were detected with or without chloroquine (CQ) treatment. The results showed that vitrification caused mitochondrial dysfunction, including increasing reactive oxygen species production, decreasing mitochondrial membrane potential, and mitochondrial DNA copy number. Damaged mitochondrial cristae and mitophagosomes were observed in vitrified oocytes. A highly fused fluorescence distribution of mitochondria and lysosomes was also observed. In the detection of mitophagic flux, mitophagy was demonstrated as increasing fluorescence aggregation of microtubule-associated protein light chain 3B (LC3B), enhanced colocalization between LC3B, and voltage-dependent anion channels 1 (VDAC1), and upregulated LC3B-II/I protein expression ratio. CQ inhibited the degradation of mitophagosomes in vitrified oocytes, manifested as decreased mitochondria-lysosomes colocalization, increased fluorescence fraction of VDAC1 overlapping LC3B, increased LC3B-II/I protein expression ratio, and p62 accumulation. The inhibition of mitophagosomes degradation by CQ aggravated mitochondrial dysfunction, including increased oxidative damage, reduced mitochondrial function, and further led to loss of oocyte viability and developmental potentiality. In conclusion, mitophagy is involved in the regulation of mitochondrial function during porcine oocyte vitrification.

Effects of Porcine Immature Oocyte Vitrification on Actin Microfilament Distribution and Chromatin Integrity During Early Embryo Development in vitro

Vitrification is mainly used to cryopreserve female gametes. This technique allows maintaining cell viability, functionality, and developmental potential at low temperatures into liquid nitrogen at −196°C. For this, the addition of cryoprotectant agents, which are substances that provide cell protection during cooling and warming, is required. However, they have been reported to be toxic, reducing oocyte viability, maturation, fertilization, and embryo development, possibly by altering cell cytoskeleton structure and chromatin. Previous studies have evaluated the effects of vitrification in the germinal vesicle, metaphase II oocytes, zygotes, and blastocysts, but the knowledge of its impact on their further embryo development is limited. Other studies have evaluated the role of actin microfilaments and chromatin, based on the fertilization and embryo development rates obtained, but not the direct evaluation of these structures in embryos produced from vitrified immature oocytes. Therefore, this study was designed to evaluate how the vitrification of porcine immature oocytes affects early embryo development by the evaluation of actin microfilament distribution and chromatin integrity. Results demonstrate that the damage generated by the vitrification of immature oocytes affects viability, maturation, and the distribution of actin microfilaments and chromatin integrity, observed in early embryos. Therefore, it is suggested that vitrification could affect oocyte repair mechanisms in those structures, being one of the mechanisms that explain the low embryo development rates after vitrification.

Morphokinetic evaluation of embryos generated from vitrified oocytes maintaining the meiotic spindle

Vitrification is a technique for preservation of human oocytes. There is still a lack of basic research about the possible effects of vitrification on subsequent embryos following oocyte vitrification. The purpose of this study was to evaluate the embryo morphokinetic parameters formed after fertilization of vitrified-warmed oocytes, where an intact meiotic spindle (MS) was observed pre- and post-cryopreservation. Matured oocytes after in vitro maturation were collected and MS evaluation was performed. The oocytes with MS were divided into two groups: fresh and post vitrification. After intra-cytoplasmic sperm injection, the oocytes were cultured in time lapse monitoring (TLM) and time of second polar body extrusion (SPBE), pronuclei appearance (tPNA), pronuclei fading (tPNF), formation of two to eight cells (t2 to t8), and irregular cleavage events [direct cleavage (DC), reverse cleavage (RC)] and vacuolation were assessed. The fertilization rate was not significantly different between the groups, although the rate of abnormal fertilization was higher in vitrification group compared with fresh group (23.5% VS 7.7%). Analysis of the TLM showed a significant delay in time points, including SPBE, tPNA, tPNF, t 2-cells cleavage in vitrification group (p = 0.02, p = 0.00, p = 0.002, P = 0.00, P = 0.01, respectively). In addition, t3 and t4 time points tended to be delayed in vitrification group (p = 0.05). Moreover, the higher level of DC, RC and vacuolation were noticed in the vitrification group (P˂0.05). In conclusion, despite MS maintenance after warming, TLM evaluation showed both a delay and abnormal cleavage patterns in generated embryos.

Senescence and Apoptosis During in vitro Embryo Development in a Bovine Model

According to the World Health Organization, infertility affects up to 14% of couples under reproductive age, leading to an exponential rise in the use of assisted reproduction as a route for conceiving a baby. In the same way, thousands of embryos are produced in cattle and other farm animals annually, leading to increased numbers of individuals born. All reproductive manipulations entail deviations of natural phenotypes and genotypes, with in vitro embryo technologies perhaps showing the biggest effects, although these alterations are still emerging. Most of these indications have been provided by animal models, in particular the bovine species, due to its similarities to human early embryo development. Oocytes and embryos are highly sensitive to environmental stress in vivo and in vitro. Thus, during in vitro culture, a number of stressful conditions affect embryonic quality and viability, inducing subfertility and/or long-term consequences that may reach the offspring. A high proportion of the embryos produced in vitro are arrested at a species-specific stage of development during the first cell divisions. These arrested embryos do not show signs of programmed cell death during early cleavage stages. Instead, defective in vitro produced embryos would enter a permanent cell cycle arrest compatible with cellular senescence, in which they show active metabolism and high reactive oxygen species levels. Later in development, mainly during the morula and blastocyst stages, apoptosis would mediate the elimination of certain cells, accomplishing both a physiological role in to balancing cell proliferation and death, and a pathological role preventing the transmission of damaged cells with an altered genome. The latter would acquire relevant importance in in vitro produced embryos that are submitted to stressful environmental stimuli. In this article, we review the mechanisms mediating apoptosis and senescence during early embryo development, with a focus on in vitro produced bovine embryos. Additionally, we shed light on the protective role of senescence and apoptosis to ensure that unhealthy cells and early embryos do not progress in development, avoiding long-term detrimental effects.