Alterations in calcium oscillatory activity in vitrified mouse eggs impact on egg quality and subsequent embryonic development

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.

Ultra-Fast Vitrification: Minimizing the Toxicity of Cryoprotective Agents and Osmotic Stress in Mouse Oocyte Cryopreservation

Globally, women have been adopting oocyte cryopreservation (OC) for fertility preservation for various reasons, such as inevitable gonadotoxic treatment for specific pathologic states and social preferences. While conventional vitrification (C-VIT) has improved the success rate of OC, challenges of possible toxicities of high-concentration cryoprotective agents and osmotic stress persist. To overcome these challenges, we evaluated the ultra-fast vitrification (UF-VIT) method, which reduces the equilibration solution stage exposure time compared to C-VIT by observing mouse oocyte intracellular organelles and embryonic development. Consequently, compared to fresh mouse oocytes, UF-VIT presented significant differences only in endoplasmic reticulum (ER) intensity and mitochondrial (MT) distribution. Meanwhile, C-VIT showed substantial differences in the survival rate, key ER and MT parameters, and embryonic development rate. UF-VIT exhibited considerably fewer negative effects on key MT parameters and resulted in a notably higher blastocyst formation rate than C-VIT. Meiotic spindle (spindle and chromosomes) morphology showed no significant changes between the groups during vitrification/warming (VW), suggesting that VW did not negatively affect the meiotic spindle of the oocytes. In conclusion, UF-VIT seems more effective in OC owing to efficient cytoplasmic water molecule extraction, osmotic stress reduction, and minimization of cell contraction and expansion amplitude, thus compensating for the drawbacks of C-VIT.

Significant differences in efficiency between two commonly used ionophore solutions for assisted oocyte activation (AOA): a prospective comparison of ionomycin and A23187

PURPOSE

Despite the success of ICSI in treating severe male factor infertile patients, total fertilization failure (FF) still occurs in around 1-3% of ICSI cycles. To overcome FF, the use of calcium ionophores has been proposed to induce oocyte activation and restore fertilization rates. However, assisted oocyte activation (AOA) protocols and ionophores vary between laboratories, and the morphokinetic development underlying AOA remains understudied.

METHODS

A prospective single-center cohort study involving 81 in vitro matured metaphase-II oocytes from 66 oocyte donation cycles artificially activated by A23187 (GM508 CultActive, Gynemed) (n=42) or ionomycin (n=39). Parthenogenesis was induced, and morphokinetic parameters (tPNa, tPNf, t2-t8, tSB, and tB) were compared between the 2 study groups and a control group comprising 39 2PN-zygotes from standard ICSI cycles.

RESULTS

Ionomycin treatment resulted in higher activation rates compared to A23187 (38.5% vs 23.8%, p=0.15). Importantly, none of the A23187-activated parthenotes formed blastocysts. When evaluating the morphokinetic dynamics between the two ionophores, we found that tPNa and tPNf were significantly delayed in the group treated by A23187 (11.84 vs 5.31, p=0.002 and 50.15 vs 29.69, p=0.005, respectively). t2 was significantly delayed in A23187-activated parthenotes when compared to the double heterologous control embryo group. In contrast, the morphokinetic development of ionomycin-activated parthenotes was comparable to control embryos (p>0.05).

CONCLUSION

Our results suggest that A23187 leads to lower oocyte activation rates and profoundly affects morphokinetic timings and preimplantation development in parthenotes. Despite our limited sample size and low parthenote competence, standardization and further optimization of AOA protocols may allow wider use and improved outcomes for FF cycles.

Artificial oocyte activation using Ca2+ ionophores following intracytoplasmic sperm injection for low fertilization rate

This large multi-center retrospective study examined whether artificial oocyte activation (AOA) using Ca²⁺ ionophore following ICSI improves the live birth rate for couples with previous ICSI cycles of unexplained low fertilization rate. In this large-scale multi-center retrospective study conducted in Japan, data were collected from Keio University and 17 collaborating institutions of the Japanese Institution for Standardizing Assisted Reproductive Technology. Between January 2015 and December 2019, 198 couples were included in this study. Oocytes for both the intervention and control groups were procured from the same pool of couples. Oocytes obtained from ICSI cycles with no or low fertilization rate (<50%) with unknown causes were included in the control (conventional ICSI) group while oocytes procured from ICSI cycles followed by performing AOA were assigned to the intervention (ICSI-AOA) group. Those fertilized with surgically retrieved sperm were excluded. ICSI-AOA efficacy and safety were evaluated by comparing these two groups. Live birth rate was the primary outcome. The ICSI-AOA group (2,920 oocytes) showed a significantly higher live birth per embryo transfer rate (18.0% [57/316]) compared to that of the conventional ICSI group with no or low fertilization rate (1,973 oocytes; 4.7% [4/85]) (odds ratio 4.5, 95% confidence interval 1.6-12.6; P<0.05). A higher live birth rate was observed in younger patients without a history of oocyte retrieval. Miscarriage, preterm delivery, and fetal congenital malformation rates were similar between the two groups. ICSI-AOA may reduce fertilization failure without increasing risks during the perinatal period. AOA may be offered to couples with an ICSI fertilization rate < 50%.

OUP accepted manuscript

BACKGROUND

Oocyte activation deficiency (OAD) is attributed to the majority of cases underlying failure of ICSI cycles, the standard treatment for male factor infertility. Oocyte activation encompasses a series of concerted events, triggered by sperm-specific phospholipase C zeta (PLCζ), which elicits increases in free cytoplasmic calcium (Ca²⁺) in spatially and temporally specific oscillations. Defects in this specific pattern of Ca²⁺ release are directly attributable to most cases of OAD. Ca²⁺ release can be clinically mediated via assisted oocyte activation (AOA), a combination of mechanical, electrical and/or chemical stimuli which artificially promote an increase in the levels of intra-cytoplasmic Ca²⁺. However, concerns regarding safety and efficacy underlie potential risks that must be addressed before such methods can be safely widely used.

OBJECTIVE AND RATIONALE

Recent advances in current AOA techniques warrant a review of the safety and efficacy of these practices, to determine the extent to which AOA may be implemented in the clinic. Importantly, the primary challenges to obtaining data on the safety and efficacy of AOA must be determined. Such questions require urgent attention before widespread clinical utilization of such protocols can be advocated.

SEARCH METHODS

A literature review was performed using databases including PubMed, Web of Science, Medline, etc. using AOA, OAD, calcium ionophores, ICSI, PLCζ, oocyte activation, failed fertilization and fertilization failure as keywords. Relevant articles published until June 2019 were analysed and included in the review, with an emphasis on studies assessing large-scale efficacy and safety.

OUTCOMES

Contradictory studies on the safety and efficacy of AOA do not yet allow for the establishment of AOA as standard practice in the clinic. Heterogeneity in study methodology, inconsistent sample inclusion criteria, non-standardized outcome assessments, restricted sample size and animal model limitations render AOA strictly experimental. The main scientific concern impeding AOA utilization in the clinic is the non-physiological method of Ca²⁺ release mediated by most AOA agents, coupled with a lack of holistic understanding regarding the physiological mechanism(s) underlying Ca²⁺ release at oocyte activation.

LIMITATIONS, REASONS FOR CAUTION

The number of studies with clinical relevance using AOA remains significantly low. A much wider range of studies examining outcomes using multiple AOA agents are required.

WIDER IMPLICATIONS

In addition to addressing the five main challenges of studies assessing AOA safety and efficacy, more standardized, large-scale, multi-centre studies of AOA, as well as long-term follow-up studies of children born from AOA, would provide evidence for establishing AOA as a treatment for infertility. The delivery of an activating agent that can more accurately recapitulate physiological fertilization, such as recombinant PLCζ, is a promising prospect for the future of AOA. Further to PLCζ, many other avenues of physiological oocyte activation also require urgent investigation to assess other potential physiological avenues of AOA.

STUDY FUNDING/COMPETING INTERESTS

D.G. was supported by Stanford University’s Bing Overseas Study Program. J.K. was supported by a Healthcare Research Fellowship Award (HF-14-16) made by Health and Care Research Wales (HCRW), alongside a National Science, Technology, and Innovation plan (NSTIP) project grant (15-MED4186-20) awarded by the King Abdulaziz City for Science and Technology (KACST). The authors have no competing interests to declare.

Cryopreservation of Gametes and Embryos and Their Molecular Changes

The process of freezing cells or tissues and depositing them in liquid nitrogen at -196 °C is called cryopreservation. Sub-zero temperature is not a physiological condition for cells and water ice crystals represent the main problem since they induce cell death, principally in large cells like oocytes, which have a meiotic spindle that degenerates during this process. Significantly, cryopreservation represents an option for fertility preservation in patients who develop gonadal failure for any condition and those who want to freeze their germ cells for later use. The possibility of freezing sperm, oocytes, and embryos has been available for a long time, and in 1983 the first birth with thawed oocytes was achieved. From the mid-2000s forward, the use of egg vitrification through intracytoplasmic sperm injection has improved pregnancy rates. Births using assisted reproductive technologies (ART) have some adverse conditions and events. These risks could be associated with ART procedures or related to infertility. Cryopreservation generates changes in the epigenome of gametes and embryos, given that ART occurs when the epigenome is most vulnerable. Furthermore, cryoprotective agents induce alterations in the integrity of germ cells and embryos. Notably, cryopreservation extensively affects cell viability, generates proteomic profile changes, compromises crucial cellular functions, and alters sperm motility. This technique has been widely employed since the 1980s and there is a lack of knowledge about molecular changes. The emerging view is that molecular changes are associated with cryopreservation, affecting metabolism, cytoarchitecture, calcium homeostasis, epigenetic state, and cell survival, which compromise the fertilization in ART.

Mitochondrial Dysfunction and Oxidative Stress Caused by Cryopreservation in Reproductive Cells

Mitochondria, fundamental organelles in cell metabolism, and ATP synthesis are responsible for generating reactive oxygen species (ROS), calcium homeostasis, and cell death. Mitochondria produce most ROS, and when levels exceed the antioxidant defenses, oxidative stress (OS) is generated. These changes may eventually impair the electron transport chain, resulting in decreased ATP synthesis, increased ROS production, altered mitochondrial membrane permeability, and disruption of calcium homeostasis. Mitochondria play a key role in the gamete competence to facilitate normal embryo development. However, iatrogenic factors in assisted reproductive technologies (ART) may affect their functional competence, leading to an abnormal reproductive outcome. Cryopreservation, a fundamental technology in ART, may compromise mitochondrial function leading to elevated intracellular OS that decreases sperm and oocytes' competence and the dynamics of fertilization and embryo development. This article aims to review the role played by mitochondria and ROS in sperm and oocyte function and the close, biunivocal relationships between mitochondrial damage and ROS generation during cryopreservation of gametes and gonadal tissues in different species. Based on current literature, we propose tentative hypothesis of mechanisms involved in cryopreservation-associated mitochondrial dysfunction in gametes, and discuss the role played by antioxidants and other agents to retain the competence of cryopreserved reproductive cells and tissues.

Voltage Dependent N Type Calcium Channel in Mouse Egg Fertilization

Repetitive changes in the intracellular calcium concentration ([Ca²⁺]i) triggers egg activation, including cortical granule exocytosis, resumption of second meiosis, block to polyspermy, and initiating embryonic development. [Ca²⁺]i oscillations that continue for several hours, are required for the early events of egg activation and possibly connected to further development to the blastocyst stage. The sources of Ca²⁺ ion elevation during [Ca²⁺]i oscillations are Ca²⁺ release from endoplasmic reticulum through inositol 1,4,5 tri-phosphate receptor and Ca²⁺ ion influx through Ca²⁺ channel on the plasma membrane. Ca²⁺ channels have been characterized into voltage-dependent Ca²⁺ channels (VDCCs), ligand-gated Ca²⁺ channel, and leak-channel. VDCCs expressed on muscle cell or neuron is specified into L, T, N, P, Q, and R type VDCs by their activation threshold or their sensitivity to peptide toxins isolated from cone snails and spiders. The present study was aimed to investigate the localization pattern of N and P/Q type voltage-dependent calcium channels in mouse eggs and the role in fertilization. [Ca²⁺]i oscillation was observed in a Ca²⁺ contained medium with sperm factor or adenophostin A injection but disappeared in Ca²⁺ free medium. Ca²⁺ influx was decreased by Lat A. N-VDCC specific inhibitor, ω-Conotoxin CVIIA induced abnormal [Ca²⁺]i oscillation profiles in SrCl₂ treatment. N or P/Q type VDC were distributed on the plasma membrane in cortical cluster form, not in the cytoplasm. Ca²⁺ influx is essential for [Ca²⁺]i oscillation during mammalian fertilization. This Ca²⁺ influx might be controlled through the N or P/Q type VDCCs. Abnormal VDCCs expression of eggs could be tested in fertilization failure or low fertilization eggs in subfertility women.

Calcium Oscillatory Patterns and Oocyte Activation During Fertilization: a Possible Mechanism for Total Fertilization Failure (TFF) in Human In Vitro Fertilization?

This paper reviews the effects of calcium oscillatory patterns in oocytes and early embryo development. Total fertilization failure (TFF) is the failure of fertilization in all oocytes in a human IVF cycle, even after treatment with intracytoplasmic sperm injection (ICSI). It is not well understood and currently attributed to oocyte activation deficiency. Calcium signaling is important in oocyte activation events. Calcium oscillations, in particular, have been reported in animal and human oocytes after fertilization. Abnormal calcium oscillations after fertilization may be the principal mechanism for TFF. While studies also establish strong associations between abnormal calcium oscillatory patterns and suboptimal developmental outcomes, critical basic parameters and their mechanism of action have yet to be identified. Empirical use of artificial oocyte activation (AOA) methods has shown initial success in helping patients overcome TFF. The AOA methods attempt to raise calcium levels after fertilization, but the efficacy and safety of these AOA methods are still in early stages of addressing TFF. Additional information about calcium oscillatory patterns and the effects of AOA in human ART may allow the prevention of TFF or allow treatment of TFF patients effectively and safely.

Oxygen supersaturation mitigates the impact of the regime of contaminated sediment reworking on sea urchin fertilization process

Dismissed industrial plants with chronic environmental contamination globally affect all levels of biological organization in concert with other natural and anthropogenic perturbations. Assessing the impact of such perturbations and finding effective ways to mitigate them have clear ecological and societal implications. Through indoor manipulative experiments, we assessed here the effects of the temporal regime of reworking of contaminated sediment from the Bagnoli-Coroglio brownfield (Tyrrhenian Sea, Italy) on the fertilization process in Paracentrotus lividus. Adult sea urchins were kept for one month in tanks containing contaminated sediment that was re-suspended according to two temporal patterns of water turbulence differing in the time intervals between consecutive events of agitation (mimicking the storms naturally occurring in the study area) in seawater with natural vs. supersaturated oxygenation levels. At the end of the treatment, gametes were collected and used to test the hypothesis that the regime of contaminated sediment reworking negatively, but reversibly, affects morphological and physiological traits of the fertilized eggs. We found that aggregated events of sediment re-suspension had profound negative effects on gamete interactions and Ca²⁺ signaling at fertilization. The same experimental condition also inflicted marked ultrastructural changes in eggs. Importantly, however, such detrimental effects were inhibited by increased oxygenation. By contrast, the regime of sediment re-working with a longer interval between consecutive turbulent events had only marginal effects. Thus, the current and predicted changes of climate-related disturbance appear to modulate the biological effects of chronic contamination in post-industrial areas, suggesting that environmental rehabilitation via restoration of habitat-forming primary producers such as seagrasses or algal canopies could alleviate the pollutants' effects on resident biota.

Vitrification negatively affects the Ca2+-releasing and activation potential of mouse oocytes, but vitrified oocytes are potentially useful for diagnostic purposes

RESEARCH QUESTION

To what extent does vitrification affect the Ca²⁺-releasing and activation potential of mouse oocytes, which are commonly used to determine the oocyte activation potential of human spermatozoa?

DESIGN

The effect of mouse oocyte vitrification on Ca²⁺ dynamics and developmental competence after oocyte activation was assessed and compared with fresh mouse oocytes. Moreover, the Ca²⁺ store content of the endoplasmic reticulum was determined at different time points during the vitrification-warming procedure. Finally, the Ca²⁺ pattern induced by cryoprotectant exposure was determined.

RESULTS

After human sperm injection into mouse oocytes, Ca²⁺ dynamics but not fertilization rates were significantly altered by vitrification warming (P < 0.05). Ca²⁺ dynamics in response to SrCl₂ or ionomycin were also altered by oocyte vitrification. In contrast, activation and blastocyst rates after SrCl₂ exposure were not affected (P > 0.05), whereas activation rates after ionomycin exposure were significantly lower in vitrified-warmed oocytes (P < 0.05); blastocyst rates were not affected (P > 0.05). Cryoprotectant exposure was associated with a strong drop in endoplasmic reticulum Ca²⁺ store content. Oocytes rapidly recovered during warming and recovery in Ca²⁺-containing media; a threshold area under the curve of Ca²⁺ dynamics to obtain activation rates above 90% was determined.

CONCLUSIONS

Vitrified-warmed mouse oocytes display reduced Ca²⁺-releasing potential upon oocyte activation, caused by cryoprotectant exposure. With adapted classification criteria, these oocytes could be used for diagnosing oocyte activation deficiencies in patients. Evaluating the Ca²⁺-signalling machinery in vitrified-warmed human oocytes is required.

Altered actin cytoskeleton in ageing eggs of starfish affects fertilization process

Integrity of oocytes is of pivotal interest in the medical and zootechnical practice of in vitro fertilization. With time, oocytes undergo deterioration in quality, and ageing oocytes often exhibit compromised competence in fertilization and the subsequent embryonic development. With ageing oocytes and eggs of starfish (Astropecten aranciacus), we addressed the issue by examining changes of the subcellular structure and their performance at fertilization. Ageing eggs were simulated in two different experimental paradigms: i) oocytes were overmatured by 6 hours stimulation with 1-methyladenine (1-MA); ii) oocytes were removed from the gonad and maintained in seawater for 24 or 48 h before applying the hormonal stimulation (1-MA, 70 min). These eggs were compared with normally matured eggs (stimulated after isolation from the gonad with 1-MA for 70 min) with respect to the sperm-induced intracellular Ca²⁺ signaling and the structural changes of the egg surface. The cytoskeletal and ultrastructural differences in these eggs were assessed by confocal and transmission electron microscopy, respectively. In the two categories of ageing eggs, we have found remarkable structural modifications of the actin cytoskeleton and the cortical vesicles beneath the plasma membrane. At fertilization, these ageing eggs manifested an altered pattern of intracellular Ca²⁺ release, aberrant actin dynamics, and increased rate of polyspermy often despite full elevation of the fertilization envelope. Taken together, our results highlight the importance of spatio-temporal regulation of the actin cytoskeleton in the cortex of the eggs, and we postulate that the status of the actin cytoskeleton is one of the major determinants of the oocyte quality that ensures successful monospermic fertilization.

Melatonin improves the fertilization capacity and developmental ability of bovine oocytes by regulating cytoplasmic maturation events

Melatonin is a well-characterized antioxidant that has been successfully used to protect oocytes from reactive oxygen species during in vitro maturation (IVM), resulting in improved fertilization capacity and development ability. However, the mechanism via which melatonin improves oocyte fertilization capacity and development ability remains to be determined. Here, we studied the effects of melatonin on cytoplasmic maturation of bovine oocytes. In the present study, bovine oocytes were cultured in IVM medium supplemented with 0, 10^-7 , 10^-9 , and 10^-11  mol/L melatonin, and the cytoplasmic maturation parameters of MII oocytes after IVM were investigated, including redistribution of organelles (mitochondria, cortical granules [CGs], and endoplasmic reticulum [ER]), intracellular glutathione (GSH) and ATP levels, expression of endogenous antioxidant genes (Cat, Sod1, and GPx), and fertilization-related events (IP3R1 distribution and expression of CD9 and Juno). Our results showed that melatonin significantly improved the cytoplasmic maturation of bovine oocytes by improving the normal distribution of organelles, increasing intracellular GSH and ATP levels, enhancing antioxidant gene expression levels, and modulating fertilization-related events, all of which resulted in increased fertilization capacity and developmental ability. Meanwhile, melatonin also increased the mRNA and protein expression levels of the Tet1 gene and decreased the Dnmt1 gene mRNA and protein levels in bovine oocytes, indicating that melatonin regulates the expression of the detected genes via demethylation. These findings shed insights into the potential mechanisms by which melatonin improves oocyte quality during IVM.

Calcium ion regulation by BAPTA-AM and ruthenium red improved the fertilisation capacity and developmental ability of vitrified bovine oocytes

Vitrification reduces the fertilisation capacity and developmental ability of mammalian oocytes; this effect is closely associated with an abnormal increase of cytoplasmic free calcium ions ([Ca²⁺]i). However, little information about the mechanism by which vitrification increases [Ca²⁺]i levels or a procedure to regulate [Ca²⁺]i levels in these oocytes is available. Vitrified bovine oocytes were used to analyse the effect of vitrification on [Ca²⁺]i, endoplasmic reticulum Ca²⁺ (ER Ca²⁺), and mitochondrial Ca²⁺ (mCa²⁺) levels. Our results showed that vitrification, especially with dimethyl sulfoxide (DMSO), can induce ER Ca²⁺ release into the cytoplasm, consequently increasing the [Ca²⁺]i and mCa²⁺ levels. Supplementing the cells with 10 μM 1,2-bis (o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA-AM or BAPTA) significantly decreased the [Ca²⁺]i level and maintained the normal distribution of cortical granules in the vitrified bovine oocytes, increasing their fertilisation ability and cleavage rate after in vitro fertilisation (IVF). Treating vitrified bovine oocytes with 1 μM ruthenium red (RR) significantly inhibited the Ca²⁺ flux from the cytoplasm into mitochondria; maintained normal mCa²⁺ levels, mitochondrial membrane potential, and ATP content; and inhibited apoptosis. Treating vitrified oocytes with a combination of BAPTA and RR significantly improved embryo development and quality after IVF.

Effect of vitrification on the mRNA transcriptome of bovine oocytes

Vitrification has been shown to decrease the developmental capacity of mammalian oocytes, and this is closely associated with the abnormal mRNA expressions of vitrified oocytes. However, the effect of vitrification on transcriptional machinery of oocytes examined by RNA sequencing (RNA-seq) has yet to be defined. In the present study, the mRNA transcriptomes of fresh and vitrified bovine oocytes were analysed by Smart-seq2 with the differently expressed genes determined by DEseq2 (an adjusted p-value of .05 and a minimum fold change of 2). The differentially expressed mRNAs were then searched against the Gene Ontology (GO) and Genomes (KEGG) database. Finally, the mRNA expressions of 10 candidate genes were validated using quantitative real-time PCR (qRT-PCR). Approximately 12,000 genes were detected in each sample of fresh or vitrified oocytes. Of these, the expression levels of 102 genes differed significantly in vitrified groups: 12 genes mainly involved in cell cycle, fertilization and glucose metabolism were upregulated, and 90 genes mainly involved in mitochondria, ribosomal protein, cytoskeleton, transmembrane protein, cell cycle and calcium ions were downregulated. GO analysis showed that these genes were mainly enriched in terms of membrane-bounded organelles, macromolecular complex, and intracellular part. The mRNA expression levels of 10 candidate genes selected randomly were in agreement with the results of the RNA-seq. In conclusion, our results showed that vitrification affected the mRNA transcriptome of bovine oocytes by downregulating genes, which contributed to the decreased developmental capacity of vitrified oocytes. Our findings will be useful in determining approaches to improve the efficiency of vitrified oocytes.

Supplementation With Cell-Penetrating Peptide-Conjugated Estrogen-Related Receptor β Improves the Formation of the Inner Cell Mass and the Development of Vitrified/Warmed Mouse Embryos

Estrogen-related receptor β (ESRRB), which is a member of the nuclear orphan receptor family, regulates the messenger RNA (mRNA) expression levels of the transcription factors, Oct4 and Nanog, in early embryos and germ cells, thereby maintaining the undifferentiated state and pluripotency of the relevant cells. The present study was designed to determine whether the upregulation of pluripotency-related genes by direct delivery of ESRRB protein may affect on the commitment into inner cell mass (ICM) or the development of vitrified/warmed mouse embryos. Recombinant cell-penetrating peptide (CPP) ESRRB protein was synthesized and then added into a culture medium for cryopreserved mouse embryos. Vitrified/warmed 8-cell embryos were cultured in KSOM with/without 2 μg/mL CPP-ESRRB for 48 hours and then analyzed or transferred to the uteri of foster mothers. The mRNA expression of Oct4 and Nanog was higher in CPP-ESRRB-treated blastocysts compared to the untreated controls. No difference was observed in embryonic development, but ICM:trophectoderm ratio was increased in the CPP-ESRRB-treated group compared to the untreated group, and after embryo transfer, a higher implantation rate was obtained in the CPP-ESRRB-treated group compared to the untreated group. This study shows for the first time that recombinant CPP-ESRRB can be easily integrated into vitrified/warmed mouse embryos and that it increases Oct4 expression (via a pluripotency-related gene pathway), ICM formation, and the further embryonic and full-term development of vitrified/warmed mouse embryos. This CPP-conjugated protein delivery system could therefore be a useful tool for improving assisted reproductive technology.

Comparative biology of sperm factors and fertilization-induced calcium signals across the animal kingdom

Fertilization causes mature oocytes or eggs to increase their concentrations of intracellular calcium ions (Ca²⁺) in all animals that have been examined, and such Ca²⁺ elevations, in turn, provide key activating signals that are required for non-parthenogenetic development. Several lines of evidence indicate that the Ca²⁺ transients produced during fertilization in mammals and other taxa are triggered by soluble factors that sperm deliver into oocytes after gamete fusion. Thus, for a broad-based analysis of Ca²⁺ dynamics during fertilization in animals, this article begins by summarizing data on soluble sperm factors in non-mammalian species, and subsequently reviews various topics related to a sperm-specific phospholipase C, called PLCζ, which is believed to be the predominant activator of mammalian oocytes. After characterizing initiation processes that involve sperm factors or alternative triggering mechanisms, the spatiotemporal patterns of Ca²⁺ signals in fertilized oocytes or eggs are compared in a taxon-by-taxon manner, and broadly classified as either a single major transient or a series of repetitive oscillations. Both solitary and oscillatory types of fertilization-induced Ca²⁺ signals are typically propagated as global waves that depend on Ca²⁺ release from the endoplasmic reticulum in response to increased concentrations of inositol 1,4,5-trisphosphate (IP₃). Thus, for taxa where relevant data are available, upstream pathways that elevate intraoocytic IP3 levels during fertilization are described, while other less-common modes of producing Ca²⁺ transients are also examined. In addition, the importance of fertilization-induced Ca²⁺ signals for activating development is underscored by noting some major downstream effects of these signals in various animals.

Vitrification procedure decreases inositol 1,4,5-trisphophate receptor expression, resulting in low fertility of pig oocytes

Although cryopreservation of mammalian oocytes is an important technology, it is well known that unfertilized oocytes, especially in pigs, are highly sensitive to low temperature and that cryopreserved oocytes show low fertility and developmental ability. The aim of the present study was to clarify why porcine in vitro matured (IVM) oocytes at the metaphase II (MII) stage showed low fertility and developmental ability after vitrification. In vitro matured cumulus oocyte complexes (COCs) were vitrified with Cryotop and then evaluated for fertility through in vitro fertilization (IVF). Although sperm-penetrated oocytes were observed to some extent (30-40%), the rate of pronuclear formation was low (9%) and none of them progressed to the two-cell stage. The results suggest that activation ability of cryopreserved oocytes was decreased by vitrification. We examined the localization and expression level of the type 1 inositol 1,4,5 trisphosphate receptor (IP3 R1), the channel responsible for Ca(2+) release during IVF in porcine oocytes. Localization of IP3 R1 close to the plasma membrane and total expression level of IP3 R1 protein were both decreased by vitrification. In conclusion, our present study indicates that vitrified-warmed porcine COCs showed a high survival rate but low fertility after IVF. This low fertility seems to be due to the decrease in IP3 R1 by the vitrification procedure.

Oocyte activation and phospholipase C zeta (PLCζ): diagnostic and therapeutic implications for assisted reproductive technology

Infertility affects one in seven couples globally and has recently been classified as a disease by the World Health Organisation (WHO). While in-vitro fertilisation (IVF) offers effective treatment for many infertile couples, cases exhibiting severe male infertility (19-57%) often remain difficult, if not impossible to treat. In such cases, intracytoplasmic sperm injection (ICSI), a technique in which a single sperm is microinjected into the oocyte, is implemented. However, 1-5% of ICSI cycles still fail to fertilise, affecting over 1000 couples per year in the UK alone. Pregnancy and delivery rates for IVF and ICSI rarely exceed 30% and 23% respectively. It is therefore imperative that Assisted Reproductive Technology (ART) protocols are constantly modified by associated research programmes, in order to provide patients with the best chances of conception. Prior to fertilisation, mature oocytes are arrested in the metaphase stage of the second meiotic division (MII), which must be alleviated to allow the cell cycle, and subsequent embryogenesis, to proceed. Alleviation occurs through a series of concurrent events, collectively termed 'oocyte activation'. In mammals, oocytes are activated by a series of intracellular calcium (Ca2+) oscillations following gamete fusion. Recent evidence implicates a sperm-specific phospholipase C, PLCzeta (PLCζ), introduced into the oocyte following membrane fusion as the factor responsible. This review summarises our current understanding of oocyte activation failure in human males, and describes recent advances in our knowledge linking certain cases of male infertility with defects in PLCζ expression and activity. Systematic literature searches were performed using PubMed and the ISI-Web of Knowledge. Databases compiled by the United Nations and World Health Organisation databases (UNWHO), and the Human Fertilization and Embryology Authority (HFEA) were also scrutinised. It is clear that PLCζ plays a fundamental role in the activation of mammalian oocytes, and that genetic, molecular, or biochemical perturbation of this key enzyme is strongly linked to human infertility where oocyte activation is deficient. Consequently, there is significant scope for our understanding of PLCζ to be translated to the ART clinic, both as a novel therapeutic agent with which to rescue oocyte activation deficiency (OAD), or as a prognostic/diagnostic biomarker of oocyte activation ability in target sperm samples.

Maturation outcomes are improved following Cryoleaf vitrification of immature human oocytes when compared to choline-based slow-freezing

PURPOSE

The cryopreservation of immature oocytes permits oocyte banking for patients at risk of losing their fertility. However, the optimum protocol for such fertility preservation remains uncertain.

METHODS

The present study investigated the survival, maturation, cytoskeletal and chromosome organization of sibling immature oocytes leftover from controlled ovarian stimulation cycles, that were either slow-frozen (with choline-substitution) or vitrified. A comparison group included oocytes that were never cryopreserved.

RESULTS

Among the three groups, comparable rates were observed for both survival (67-70%) and polar body extrusion (59-79%). Significantly more oocytes underwent spontaneous activation after IVM following slow-freezing compared with either vitrification or no cryopreservation. Likewise, the incidence of spindle abnormalities was greatest in the slow-frozen group, with no differences in spindle morphometrics or chromosome organization.

CONCLUSIONS

While the overall incidence of mature oocytes with normal bipolar spindles from warmed immature oocytes was low, the yield using Cryoleaf vitrification was slightly superior to choline-based slow-freezing.