Lactate regulates pyruvate uptake and metabolism in the preimplantation mouse embryo

Metabolic regulation of preimplantation embryo development in vivo and in vitro: Molecular mechanisms and insights

Understanding of embryonic development has led to the clinical application of Assisted Reproductive technologies (ART), with the resulting birth of millions of children. Recent developments in metabolomics, proteomics, and transcriptomics have brought to light new insights into embryonic growth dynamics, with implications spanning reproductive medicine, stem cell research, and regenerative medicine. The review explores the key metabolic processes and molecular pathways active during preimplantation embryo development, including PI3K-Akt, mTOR, AMPK, Wnt/β-catenin, TGF-β, Notch and Jak-Stat signaling pathways. We focused on analyzing the differences occurring in vitro as opposed to in vivo development and we discussed significant physiological and clinical implications.

Maternal Ezh1/2 deficiency impairs the function of mitochondria in mouse oocytes and early embryos

Maternal histone methyltransferase is critical for epigenetic regulation and development of mammalian embryos by regulating histone and DNA modifications. Here, we reported a novel mechanism by revealing the critical effects of maternal Ezh1/2 deletion on mitochondria in MII oocytes and early embryos in mice. We found that Ezh1/2 knockout in mouse MII oocytes impaired the structure of mitochondria and decreased its number, but membrane potential and respiratory function of mitochondrion were increased. The similar effects of Ezh1/2 deletion have been observed in 2-cell and morula embryos, indicating that the effects of maternal Ezh1/2 deficiency on mitochondrion extend to early embryos. However, the loss of maternal Ezh1/2 resulted in a severe defect of morula: the number, membrane potential, respiratory function, and ATP production of mitochondrion dropped significantly. Content of reactive oxygen species was raised in both MII oocytes and early embryos, suggesting maternal Ezh1/2 knockout induced oxidative stress. In addition, maternal Ezh1/2 ablation interfered the autophagy in morula and blastocyst embryos. Finally, maternal Ezh1/2 deletion led to cell apoptosis in blastocyst embryos in mice. By analyzing the gene expression profile, we revealed that maternal Ezh1/2 knockout affected the expression of mitochondrial related genes in MII oocytes and early embryos. The chromatin immunoprecipitation-polymerase chain reaction assay demonstrated that Ezh1/2 directly regulated the expression of genes Fxyd6, Adpgk, Aurkb, Zfp521, Ehd3, Sgms2, Pygl, Slc1a1, and Chst12 by H3K27me3 modification. In conclusion, our study revealed the critical effect of maternal Ezh1/2 on the structure and function of mitochondria in oocytes and early embryos, and suggested a novel mechanism underlying maternal epigenetic regulation on early embryonic development through the modulation of mitochondrial status.

The IMPDH cytoophidium couples metabolism and fetal development in mice

The cytoophidium is an evolutionarily conserved subcellular structure formed by filamentous polymers of metabolic enzymes. In vertebrates, inosine monophosphate dehydrogenase (IMPDH), which catalyses the rate-limiting step in guanosine triphosphate (GTP) biosynthesis, is one of the best-known cytoophidium-forming enzymes. Formation of the cytoophidium has been proposed to alleviate the inhibition of IMPDH, thereby facilitating GTP production to support the rapid proliferation of certain cell types such as lymphocytes, cancer cells and pluripotent stem cells (PSCs). However, past studies lacked appropriate models to elucidate the significance of IMPDH cytoophidium under normal physiological conditions. In this study, we demonstrate that the presence of IMPDH cytoophidium in mouse PSCs correlates with their metabolic status rather than pluripotency. By introducing IMPDH2 Y12C point mutation through genome editing, we established mouse embryonic stem cell (ESC) lines incapable of forming IMPDH polymers and the cytoophidium. Our data indicate an important role of IMPDH cytoophidium in sustaining a positive feedback loop that couples nucleotide biosynthesis with upstream metabolic pathways. Additionally, we find that IMPDH2 Y12C mutation leads to decreased cell proliferation and increased DNA damage in teratomas, as well as impaired embryo development following blastocoel injection. Further analysis shows that IMPDH cytoophidium assembly in mouse embryonic development begins after implantation and gradually increases throughout fetal development. These findings provide insights into the regulation of IMPDH polymerisation in embryogenesis and its significance in coordinating cell metabolism and development.

Proteomic Analysis of Domestic Cat Blastocysts and Their Secretome Produced in an In Vitro Culture System without the Presence of the Zona Pellucida

Domestic cat blastocysts cultured without the zona pellucida exhibit reduced implantation capacity. However, the protein expression profile has not been evaluated in these embryos. The objective of this study was to evaluate the protein expression profile of domestic cat blastocysts cultured without the zona pellucida. Two experimental groups were generated: (1) domestic cat embryos generated by IVF and cultured in vitro (zona intact, (ZI)) and (2) domestic cat embryos cultured in vitro without the zona pellucida (zona-free (ZF group)). The cleavage, morula, and blastocyst rates were estimated at days 2, 5 and 7, respectively. Day 7 blastocysts and their culture media were subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS). The UniProt Felis catus database was used to identify the standard proteome. No significant differences were found in the cleavage, morula, or blastocyst rates between the ZI and ZF groups (p > 0.05). Proteomic analysis revealed 22 upregulated and 20 downregulated proteins in the ZF blastocysts. Furthermore, 14 proteins involved in embryo development and implantation were present exclusively in the culture medium of the ZI blastocysts. In conclusion, embryo culture without the zona pellucida did not affect in vitro development, but altered the protein expression profile and release of domestic cat blastocysts.

The role of metabolism in cardiac development

Metabolism is the fundamental process that sustains life. The heart, in particular, is an organ of high energy demand, and its energy substrates have been studied for more than a century. In recent years, there has been a growing interest in understanding the role of metabolism in the early differentiation of pluripotent stem cells and in cancer research. Studies have revealed that metabolic intermediates from glycolysis and the tricarboxylic acid cycle act as co-factors for intracellular signal transduction, playing crucial roles in regulating cell behaviors. Mitochondria, as the central hub of metabolism, are also under intensive investigation regarding the regulation of their dynamics. The metabolic environment of the fetus is intricately linked to the maternal metabolic status, and the impact of the mother's nutrition and metabolic health on fetal development is significant. For instance, it is well known that maternal diabetes increases the risk of cardiac and nervous system malformations in the fetus. Another notable example is the decrease in the risk of neural tube defects when pregnant women are supplemented with folic acid. These examples highlight the profound influence of the maternal metabolic environment on the fetal organ development program. Therefore, gaining insights into the metabolic environment within developing fetal organs is critical for deepening our understanding of normal organ development. This review aims to summarize recent findings that build upon the historical recognition of the environmental and metabolic factors involved in the developing embryo.

Lactate regulates major zygotic genome activation by H3K18 lactylation in mammals

Lactate is present at a high level in the microenvironment of mammalian preimplantation embryos in vivo and in vitro. However, its role in preimplantation development is unclear. Here, we report that lactate is highly enriched in the nuclei of early embryos when major zygotic genome activation (ZGA) occurs in humans and mice. The inhibition of its production and uptake results in developmental arrest at the 2-cell stage, major ZGA failure, and loss of lactate-derived H3K18lac, which could be rescued by the addition of Lac-CoA and recapitulated by overexpression of H3K18R mutation. By profiling the landscape of H3K18lac during mouse preimplantation development, we show that H3K18lac is enriched on the promoter regions of most major ZGA genes and correlates with their expressions. In humans, H3K18lac is also enriched in ZGA markers and temporally concomitant with their expressions. Taken together, we profile the landscapes of H3K18lac in mouse and human preimplantation embryos, and demonstrate the important role for H3K18lac in major ZGA, showing that a conserved metabolic mechanism underlies preimplantation development of mammalian embryos.

Placenta and maternal endothelium during preeclampsia: Disruption of the glycocalyx explains increased inositol phosphoglycans and angiogenic factors in maternal blood

The etiology of the pregnancy syndrome preeclampsia is still unclear, while most hypotheses center on the placenta as the major contributor of the syndrome. Especially changes of the placental metabolism, including the use of glucose to produce energy, are important features. As an example, inositol phosphoglycan P-type molecules, second messengers involved in the glucose metabolism of all cells, can be retrieved from maternal urine of preeclamptic women, even before the onset of clinical symptoms. Alterations in the placental metabolism may subsequently lead to negative effects on the plasma membrane of the placental syncytiotrophoblast. This in turn may have deleterious effects on the glycocalyx of this layer and a disruption of this layer in all types of preeclampsia. The interruption of the glycocalyx in preeclampsia may result in changes of inositol phosphoglycan P-type signaling pathways and the release of these molecules as well as the release of soluble receptors such as sFlt-1 and sEndoglin. The release of placental factors later affects the maternal endothelium and disrupts the endothelial glycocalyx as well. This in turn may pave the way for edema, endothelial dysfunction, coagulation, all typical symptoms of preeclampsia.

mTOR hypoactivity leads to trophectoderm cell failure by enhancing lysosomal activation and disrupting the cytoskeleton in preimplantation embryo

BACKGROUND

Metabolic homeostasis is closely related to early impairment of cell fate determination and embryo development. The protein kinase mechanistic target of rapamycin (mTOR) is a key regulator of cellular metabolism in the body. Inhibition of mTOR signaling in early embryo causes postimplantation development failure, yet the mechanisms are still poorly understood.

METHODS

Pregnancy mice and preimplantation mouse embryo were treated with mTOR inhibitor in vivo and in vitro respectively, and subsequently examined the blastocyst formation, implantation, and post-implantation development. We used immunofluorescence staining, RNA-Seq smart2, and genome-wide bisulfite sequencing technologies to investigate the impact of mTOR inhibitors on the quality, cell fate determination, and molecular alterations in developing embryos.

RESULTS

We showed mTOR suppression during preimplantation decreases the rate of blastocyst formation and the competency of implantation, impairs the post implantation embryonic development. We discovered that blocking mTOR signaling negatively affected the transformation of 8-cell embryos into blastocysts and caused various deficiencies in blastocyst quality. These included problems with compromised trophectoderm cell differentiation, as well as disruptions in cell fate specification. mTOR suppression significantly affected the transcription and DNA methylation of embryos. Treatment with mTOR inhibitors increase lysosomal activation and disrupts the organization and dynamics of the actin cytoskeleton in blastocysts.

CONCLUSIONS

These results demonstrate that mTOR plays a crucial role in 8-cell to blastocyst transition and safeguards embryo quality during early embryo development.

Dynamic metabolism during early mammalian embryogenesis

Dynamic metabolism is exhibited by early mammalian embryos to support changing cell fates during development. It is widely acknowledged that metabolic pathways not only satisfy cellular energetic demands, but also play pivotal roles in the process of cell signalling, gene regulation, cell proliferation and differentiation. Recently, various new technological advances have been made in metabolomics and computational analysis, deepening our understanding of the crucial role of dynamic metabolism during early mammalian embryogenesis. In this Review, we summarize recent studies on oocyte and embryo metabolism and its regulation, with a particular focus on its association with key developmental events such as fertilization, zygote genome activation and cell fate determination. In addition, we discuss the mechanisms of certain metabolites that, in addition to serving as energy sources, contribute to epigenetic modifications.

Beyond energy and growth: the role of metabolism in developmental signaling, cell behavior and diapause

Metabolism is crucial for development through supporting cell growth, energy production, establishing cell identity, developmental signaling and pattern formation. In many model systems, development occurs alongside metabolic transitions as cells differentiate and specialize in metabolism that supports new functions. Some cells exhibit metabolic flexibility to circumvent mutations or aberrant signaling, whereas other cell types require specific nutrients for developmental progress. Metabolic gradients and protein modifications enable pattern formation and cell communication. On an organism level, inadequate nutrients or stress can limit germ cell maturation, implantation and maturity through diapause, which slows metabolic activities until embryonic activation under improved environmental conditions.

Long-Term Effects of ART on the Health of the Offspring

Assisted reproductive technologies (ART) significantly increase the chance of successful pregnancy and live birth in infertile couples. The different procedures for ART, including in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), intrauterine insemination (IUI), and gamete intrafallopian tube transfer (GIFT), are widely used to overcome infertility-related problems. In spite of its inarguable usefulness, concerns about the health consequences of ART-conceived babies have been raised. There are reports about the association of ART with birth defects and health complications, e.g., malignancies, high blood pressure, generalized vascular functional disorders, asthma and metabolic disorders in later life. It has been suggested that hormonal treatment of the mother, and the artificial environment during the manipulation of gametes and embryos may cause genomic and epigenetic alterations and subsequent complications in the health status of ART-conceived babies. In the current study, we aimed to review the possible long-term consequences of different ART procedures on the subsequent health status of ART-conceived offspring, considering the confounding factors that might account for/contribute to the long-term consequences.

Seasonal variations in the metabolomic profile of the ovarian follicle components in Italian Mediterranean Buffaloes

The aim of this work was to evaluate the seasonal effect on the metabolomic profile of the ovarian follicle in Italian Mediterranean buffalo to unravel the causes of the reduced competence during the non-breeding season (NBS). Samples of follicular fluid, follicular cells, cumulus cells and oocytes were collected from abattoir-derived ovaries during breeding season (BS) and NBS and analyzed by 1H Nuclear Magnetic Resonance. The Orthogonal Projections to Latent Structures of the Discriminant Analysis showed clear separation into seasonal classes and Variable Importance in Projection method identified differentially abundant metabolites between seasons. Seasonal differences were recorded in metabolite content in all analyzed components suggesting that the decreased oocyte competence during NBS may be linked to alteration of several metabolic pathways. The pathway enrichment analysis revealed that differences in the metabolites between the seasons were linked to glutathione, energy generating and amino acid metabolism and phospholipid biosynthesis. The current work allows the identification of potential positive competence markers in the follicular fluid as glutathione, glutamate, lactate and choline, and negative markers like leucine, isoleucine and β-hydroxybutyrate. These results form a major basis to develop potential strategies to optimize the follicular environment and the IVM medium to improve the competence of oocytes during the NBS.

2-Cell-like Cells: An Avenue for Improving SCNT Efficiency

After fertilization, the zygote genome undergoes dramatic structural reorganization to ensure the establishment of totipotency, and then the totipotent potential of the zygote or 2-cell-stage embryo progressively declines. However, cellular potency is not always a one-way street. Specifically, a small number of embryonic stem cells (ESCs) occasionally overcome epigenetic barriers and transiently convert to a totipotent status. Despite the significant potential of the somatic cell nuclear transfer (SCNT) technique, the establishment of totipotency is often deficient in cloned embryos. Because of this phenomenon, the question arises as to whether strategies attempting to induce 2-cell-like cells (2CLCs) can provide practical applications, such as reprogramming of somatic cell nuclei. Inspired by strategies that convert ESCs into 2CLCs, we hypothesized that there will be a similar pathway by which cloned embryos can establish totipotent status after SCNT. In this review, we provide a snapshot of the practical strategies utilized to induce 2CLCs during investigations of the development of cloned embryos. The 2CLCs have similar transcriptome and chromatin features to that of 2-cell-stage embryos, and we propose that 2CLCs, already a valuable in vitro model for dissecting totipotency, will provide new opportunities to improve SCNT efficiency.

Metabolic control of histone acetylation for precise and timely regulation of minor ZGA in early mammalian embryos

Metabolism feeds into the regulation of epigenetics via metabolic enzymes and metabolites. However, metabolic features, and their impact on epigenetic remodeling during mammalian pre-implantation development, remain poorly understood. In this study, we established the metabolic landscape of mouse pre-implantation embryos from zygote to blastocyst, and quantified some absolute carbohydrate metabolites. We integrated these data with transcriptomic and proteomic data, and discovered the metabolic characteristics of the development process, including the activation of methionine cycle from 8-cell embryo to blastocyst, high glutaminolysis metabolism at blastocyst stage, enhanced TCA cycle activity from the 8-cell embryo stage, and active glycolysis in the blastocyst. We further demonstrated that oxidized nicotinamide adenine dinucleotide (NAD⁺) synthesis is indispensable for mouse pre-implantation development. Mechanistically, in part, NAD⁺ is required for the exit of minor zygotic gene activation (ZGA) by cooperating with SIRT1 to remove zygotic H3K27ac. In human, NAD⁺ supplement can promote the removal of zygotic H3K27ac and benefit pre-implantation development. Our findings demonstrate that precise and timely regulation of minor ZGA is controlled by metabolic dynamics, and enhance our understanding of the metabolism of mammalian early embryos.

Murine blastocysts generated by in vitro fertilization show increased Warburg metabolism and altered lactate production

In vitro fertilization (IVF) has resulted in the birth of over 8 million children. Although most IVF-conceived children are healthy, several studies suggest an increased risk of altered growth rate, cardiovascular dysfunction, and glucose intolerance in this population compared to naturally conceived children. However, a clear understanding of how embryonic metabolism is affected by culture condition and how embryos reprogram their metabolism is unknown. Here, we studied oxidative stress and metabolic alteration in blastocysts conceived by natural mating or by IVF and cultured in physiologic (5%) or atmospheric (20%) oxygen. We found that IVF-generated blastocysts manifest increased reactive oxygen species, oxidative damage to DNA/lipid/proteins, and reduction in glutathione. Metabolic analysis revealed IVF-generated blastocysts display decreased mitochondria respiration and increased glycolytic activity suggestive of enhanced Warburg metabolism. These findings were corroborated by altered intracellular and extracellular pH and increased intracellular lactate levels in IVF-generated embryos. Comprehensive proteomic analysis and targeted immunofluorescence showed reduction of lactate dehydrogenase-B and monocarboxylate transporter 1, enzymes involved in lactate metabolism. Importantly, these enzymes remained downregulated in the tissues of adult IVF-conceived mice, suggesting that metabolic alterations in IVF-generated embryos may result in alteration in lactate metabolism. These findings suggest that alterations in lactate metabolism are a likely mechanism involved in genomic reprogramming and could be involved in the developmental origin of health and disease.

β-hydroxybutyrate reduces blastocyst viability via trophectoderm-mediated metabolic aberrations in mice

STUDY QUESTION

What is the effect of the ketone β-hydroxybutyrate (βOHB) on preimplantation mouse embryo development, metabolism, epigenetics and post-transfer viability?

SUMMARY ANSWER

In vitro βOHB exposure at ketogenic diet (KD)-relevant serum concentrations significantly impaired preimplantation mouse embryo development, induced aberrant glycolytic metabolism and reduced post-transfer fetal viability in a sex-specific manner.

WHAT IS KNOWN ALREADY

A maternal KD in humans elevates gamete and offspring βOHB exposure during conception and gestation, and in rodents is associated with an increased time to pregnancy, and altered offspring organogenesis, post-natal growth and behaviour, suggesting a developmental programming effect. In vitro exposure to βOHB at supraphysiological concentrations (8–80 mM) perturbs preimplantation mouse embryo development.

STUDY DESIGN, SIZE, DURATION

A mouse model of embryo development and viability was utilized for this laboratory-based study. Embryo culture media were supplemented with βOHB at KD-relevant concentrations, and the developmental competence, physiology, epigenetic state and post-transfer viability of in vitro cultured βOHB-exposed embryos was assessed.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Mouse embryos were cultured in vitro with or without βOHB at concentrations representing serum levels during pregnancy (0.1 mM), standard diet consumption (0.25 mM), KD consumption (2 mM) and diabetic ketoacidosis (4 mM). The impact of βOHB exposure on embryo development (blastocyst formation rate, morphokinetics and blastocyst total, inner cell mass and trophectoderm (TE) cell number), physiology (redox state, βOHB metabolism, glycolytic metabolism), epigenetic state (histone 3 lysine 27 β-hydroxybutyrylation, H3K27bhb) and post-transfer viability (implantation rate, fetal and placental development) was assessed.

MAIN RESULTS AND THE ROLE OF CHANCE

All βOHB concentrations tested slowed embryo development (P < 0.05), and βOHB at KD-relevant serum levels (2 mM) delayed morphokinetic development, beginning at syngamy (P < 0.05). Compared with unexposed controls, βOHB exposure reduced blastocyst total and TE cell number (≥0.25 mM; P < 0.05), reduced blastocyst glucose consumption (2 mM; P < 0.01) and increased lactate production (0.25 mM; P < 0.05) and glycolytic flux (0.25 and 2 mM; P < 0.01). Consumption of βOHB by embryos, mediated via monocarboxylate transporters, was detected throughout preimplantation development. Supraphysiological (20 mM; P < 0.001), but not physiological (0.25–4 mM) βOHB elevated H3K27bhb levels. Preimplantation βOHB exposure at serum KD levels (2 mM) reduced post-transfer viability. Implantation and fetal development rates of βOHB-treated embryos were 50% lower than controls (P < 0.05), and resultant fetuses had a shorter crown-rump length (P < 0.01) and placental diameter (P < 0.05). A strong sex-specific effect of βOHB was detected, whereby female fetuses from βOHB-treated embryos weighed less (P < 0.05), had a shorter crown-rump length (P < 0.05), and tended to have accelerated ear development (P < 0.08) compared with female control fetuses.

LIMITATIONS, REASONS FOR CAUTION

This study only assessed embryo development, physiology and viability in a mouse model utilizing in vitro βOHB exposure; the impact of in vivo exposure was not assessed. The concentrations of βOHB utilized were modelled on blood/serum levels as the true oviduct and uterine concentrations are currently unknown.

WIDER IMPLICATIONS OF THE FINDINGS

These findings indicate that the development, physiology and viability of mouse embryos is detrimentally impacted by preimplantation exposure to βOHB within a physiological range. Maternal diets which increase βOHB levels, such as a KD, may affect preimplantation embryo development and may therefore impair subsequent viability and long-term health. Consequently, our initial observations warrant follow-up studies in larger human populations. Furthermore, analysis of βOHB concentrations within human and rodent oviduct and uterine fluid under different nutritional states is also required.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by the University of Melbourne and the Norma Hilda Schuster (nee Swift) Scholarship. The authors have no conflicts of interest.

TRIAL REGISTRATION NUMBER

N/A.

Effects of pyruvate on early embryonic development and zygotic genome activation in pigs

Pyruvate is an important energy substance during early embryonic development of mammals. However, the underlying mechanisms of pyruvate during early embryonic development in pigs and its role in zygotic genome activation (ZGA) are not fully understood. Here, based on a previous RNA-seq dataset of porcine early embryos, we found that pyruvate metabolism-related genes started to be expressed at the 4-cell stage and that pyruvate metabolism-related genes were correlated with porcine ZGA marker genes. To determine the function of pyruvate in porcine embryos, in vitro fertilization (IVF) embryos were cultured in PZM-3 medium (control group); modified PZM-3 medium that only contains pyruvate and lactate plus salts (+P group); or modified PZM-3 medium lacking pyruvate (-P group). The 4-cell arrest rate at 72 h was significantly increased in the -P group compared to the +P group (P < 0.05). In addition, we observed that the reactive oxygen species (ROS) level was significantly increased and that the adenosine triphosphate (ATP) level was significantly (P < 0.05) decreased in the -P group compared to the +P group. Moreover, the expression of ZGA marker genes and SIRT1 protein in embryos was significantly decreased in the -P group compared to the +P group (P < 0.05). Furthermore, the acetylation level of H3K9 was significantly decreased (P < 0.05) and the methylation level of H3K9 was significantly increased (P < 0.05) in the -P group compared to the +P group. In summary, our findings demonstrate that pyruvate affects early embryonic development in pigs by promoting ZGA and reducing oxidative stress levels.

Fertilization, embryo culture, and clinical results using low lactate embryo culture medium for pre‐culture, insemination, and beyond

Purpose

We focused on the metabolism of oocytes in pre‐culture and insemination and compared these results between our existing fertilization medium, GEMS Fertilisation Medium (GEMS group) (Merck BioPharma) and Continuous Single Culture Medium—NX Complete (CSCM‐NXC group) (FUJIFILM Irvine Scientific).

Methods

Patients under 42 years of age were received controlled ovarian stimulation and oocytes were retrieved. Those were pre‐cultured and fertilized with either GEMS fertilization medium or CSCM‐NXC. After fertilization was confirmed, embryos were cultured using CSCM‐NXC in both groups. The embryos were cryopreserved at blastocyst stage (3BB or more, Gardner classification) and then transferred in HRT cycles.

Results

The fertilization rate of both groups was the same, but the 3PN rate was significantly lower in the CSCM‐NXC group. In terms of embryo culture results, the CSCM‐NXC group had a significantly higher rate of good quality blastocysts, high‐grade embryos, and embryos with a high degree of expansion.

Conclusions

The use of CSCM‐NXC, a low lactate embryo culture medium, from pre‐culture and for insemination, increases the energy metabolic efficiency of oocytes and cumulus cells, making it possible to supply sufficient energy ATP for fertilization and early division, which is thought to promote good embryonic development.

Insufficient pyruvate in culture medium arrests mouse embryos at the first cleavage stage associated with abnormal epigenetic modifications

Energy is essential for early embryogenesis, and fertilized eggs can successfully develop to blastocyst in in vitro culture medium with an appropriate energy supply. Conversely, embryonic development is negatively affected by a suboptimal energy supply. We previously observed that a low level of pyruvate greatly arrests mouse embryos at the 2-cell stage. However, how methylation modifications are affected at this specific stage remains unknown. In this study, we found that mouse embryos could timely develop to the 4-cell stage in K⁺simplex optimized medium (KSOM) with control level of pyruvate, but embryos were significantly arrested at the 2-cell stage when pyruvate was reduced to 0.2-fold of the control level. Moreover, the fluorescence intensities of 5 mC, H3K4me2, H3K9me2 and H3K27me2 in the 2-cell stage embryos of the 0.2-fold pyruvate group were notedly lower than those of the control group, but N6-methyladenosine (m⁶A) fluorescence intensity was higher, suggesting that global genomic DNA, histone and m⁶A methylation modifications are disrupted with low levels of pyruvate. Consistently, the mRNA levels of genes related to DNA methylation, histone methylation and m⁶A modifications were also disturbed in the 2-cell stage embryos cultured with low levels of pyruvate. In summary, our findings demonstrate that insufficient pyruvate in culture medium results in mouse embryonic developmental arrest, at least in part due to defects in methylation modifications.

Comparison of two culture media on morphokinetics and ploidy status of sibling embryos

To investigate the effects of culture media with different lactate concentrations on early embryonic development, data collected from our patients undergoing preimplantation genetic testing (PGT) were assessed using the EmbryoScope™ time-lapse culturing system. After intracytoplasmic sperm injection (ICSI), sibling oocytes were cultured in the same EmbryoScope (Vitrolife) slides including two different commercially available media. The patients with fewer than five mature oocytes were not included in the analyses. All embryos were hatched on day 3, and trophectoderm biopsies (n = 212) were performed accordingly. PGT for aneuploidy (PGT-A) on biopsied materials was carried out using next generation sequencing. Morphokinetic parameters, fertilization, irregular division, degeneration, blastulation, euploidy, and pregnancy rates of embryos cultured in LifeGlobal Global Total medium (LGGT) and Continuous Single Culture-NX Complete medium (CSCM-NXC) were compared. There were no differences observed in time to pronuclear fade, or in time spent as 2-cell (cc2) and 3-cell (s2), to 4-cell, 5-cell, morula and blastocyst stages (P > 0.05). Embryos reached the 2-cell (t2) and 3-cell (t3) stages significantly faster in LGGT (P < 0.05), whereas embryos grown in CSCM-NXC with lower lactate reached starting blastulation significantly sooner (P = 0.026). However, there were no statistical differences observed in fertilization, blastulation, degeneration, irregular division euploidy, and pregnancy rates between the two groups (P > 0.05). Even though pregnancy and fertilization rates did not indicate statistical differences, results are significant to provide better insight on potential roles of lactate in embryo development. These finding will advance the fundamental knowledge of human embryo development and assisted reproductive technologies.

A coarse-grained NADH redox model enables inference of subcellular metabolic fluxes from fluorescence lifetime imaging

Mitochondrial metabolism is of central importance to diverse aspects of cell and developmental biology. Defects in mitochondria are associated with many diseases, including cancer, neuropathology, and infertility. Our understanding of mitochondrial metabolism in situ and dysfunction in diseases are limited by the lack of techniques to measure mitochondrial metabolic fluxes with sufficient spatiotemporal resolution. Herein, we developed a new method to infer mitochondrial metabolic fluxes in living cells with subcellular resolution from fluorescence lifetime imaging of NADH. This result is based on the use of a generic coarse-grained NADH redox model. We tested the model in mouse oocytes and human tissue culture cells subject to a wide variety of perturbations by comparing predicted fluxes through the electron transport chain (ETC) to direct measurements of oxygen consumption rate. Interpreting the fluorescence lifetime imaging microscopy measurements of NADH using this model, we discovered a homeostasis of ETC flux in mouse oocytes: perturbations of nutrient supply and energy demand of the cell do not change ETC flux despite significantly impacting NADH metabolic state. Furthermore, we observed a subcellular spatial gradient of ETC flux in mouse oocytes and found that this gradient is primarily a result of a spatially heterogeneous mitochondrial proton leak. We concluded from these observations that ETC flux in mouse oocytes is not controlled by energy demand or supply, but by the intrinsic rates of mitochondrial respiration.

Less is more: Reduced nutrient concentration during in vitro culture improves embryo production rates and morphophysiology of bovine embryos

Reproducing the environment to which the embryo is naturally exposed may be an alternative to improve viability of embryos produced in vitro. In the first part of this work, we describe a novel culture media, namely Embryonic Culture Supplementation (ECS100). The composition of this media was based on the contents of carbohydrates and amino acids found in oviductal and uterine fluids. Because it was a new formulation, we investigated the performance of ECS100 in comparison with conventionally used SOFaa, and possible benefits to embryo development. Embryo production rates (cleavage, morula and blastocyst conversion, blastocyst and hatching rates) and morphophysiological parameters (total cell number, cell allocation, Mitochondrial membrane potential (MMP), Reactive Oxygen Species (ROS), NADH, FAD⁺ and ATP content) were similar between ECS100 and SOFaa. Next, we tested if a reduction of ECS100 concentration could positively contribute to embryo viability by resembling the more dynamic availability of nutrients that reach the embryos in vivo. Therefore, embryos were cultured in ECS100 or in its serial dilution (ECS75, 50 and 25). Despite the fact that the lowest concentration (ECS25) still supported blastocyst formation, halving the concentration of metabolites (ECS50) actually improved embryo production rates. Thus, embryos produced in ECS100 or ECS50 were submitted to further analyses on Days 4 and 7. Embryos cultured in ECS50 presented better developmental rates and morphophysiological profile than embryos cultured in ECS100. Additionally, physiological traits (MMP, ROS and NADH levels) of embryos cultured in ECS50 presented the expected pattern for embryos produced in vivo. In conclusion, we presented a novel, more personalized and effective culture media for bovine IVP embryos. And although the ECS media formulation was based on the contents of female reproductive fluids, it is worth mentioning that adaptations must be specifically directed for in vitro conditions rather than reproduced exactly from in vivo state.

Biochemical markers for pregnancy in the spent culture medium of in vitro produced bovine embryos†

The present study aimed to identify biomarkers to assess the quality of in vitro produced (IVP) bovine embryos in the culture media. IVP embryos on Day (D) 5 of development were transferred to individual drops, where they were maintained for the last 48 h of culture. Thereafter, the medium was collected and the embryos were transferred to the recipients. After pregnancy diagnosis, the media were grouped into the pregnant and nonpregnant groups. The metabolic profiles of the media were analyzed via electrospray ionization mass spectrometry, and the concentrations of pyruvate, lactate, and glutamate were assessed using fluorimetry. The spectrometric profile revealed that the media from embryos from the pregnant group presented a higher signal intensity compared to that of the nonpregnant group; the ions 156.13 Da [M + H]+, 444.33 Da [M + H]+, and 305.97 Da [M + H]+ were identified as biomarkers. Spent culture medium from expanded blastocysts (Bx) that established pregnancy had a greater concentration of pyruvate (p = 0.0174) and lesser concentration of lactate (p = 0.042) than spent culture medium from Bx that did not establish pregnancy. Moreover, pyruvate in the culture media of Bx can predict pregnancy with 90.9% sensitivity and 75% specificity. In conclusion, we identified markers in the culture media that helped in assessing the most viable IVP embryos with a greater potential to establish pregnancy.

Metabolic activity of human blastocysts correlates with their morphokinetics, morphological grade, KIDScore and artificial intelligence ranking

STUDY QUESTION

Is there a relationship between blastocyst metabolism and biomarkers of embryo viability?

SUMMARY ANSWER

Blastocysts with higher developmental potential and a higher probability of resulting in a viable pregnancy consume higher levels of glucose and exhibit distinct amino acid profiles.

WHAT IS KNOWN ALREADY

Morphological and morphokinetic analyses utilized in embryo selection provide insight into developmental potential, but alone are unable to provide a direct measure of embryo physiology and inherent health. Glucose uptake is a physiological biomarker of viability and amino acid utilization is different between embryos of varying qualities.

STUDY DESIGN, SIZE, DURATION

Two hundred and nine human preimplantation embryos from 50 patients were cultured in a time-lapse incubator system in both freeze all and fresh transfer cycles. A retrospective analysis of morphokinetics, morphology (Gardner grade), KIDScore, artificial intelligence grade (EmbryoScore), glucose and amino acid metabolism, and clinical pregnancies was conducted.

PARTICIPANTS/MATERIALS, SETTING, METHODS

ICSI was conducted in all patients, who were aged ≤37 years and previously had no more than two IVF cycles. Embryos were individually cultured in a time-lapse incubator system, and those reaching the blastocyst stage had their morphokinetics annotated and were each assigned a Gardner grade, KIDScore and EmbryoScore. Glucose and amino acid metabolism were measured. Clinical pregnancies were confirmed by the presence of a fetal heartbeat at 6 weeks of gestation.

MAIN RESULTS AND THE ROLE OF CHANCE

Glucose consumption was at least 40% higher in blastocysts deemed of high developmental potential using either the Gardner grade (P < 0.01, n = 209), KIDScore (P < 0.05, n = 207) or EmbryoScore (P < 0.05, n = 184), compared to less viable blastocysts and in blastocysts that resulted in a clinical pregnancy compared to those that failed to implant (P < 0.05, n = 37). Additionally, duration of cavitation was inversely related to glucose consumption (P < 0.05, n = 200). Total amino acid consumption was significantly higher in blastocysts with an EmbryoScore higher than the cohort median score (P < 0.01, n = 185). Furthermore, the production of amino acids was significantly lower in blastocysts with a high Gardner grade (P < 0.05, n = 209), KIDScore (P < 0.05, n = 207) and EmbryoScore (P < 0.01, n = 184).

LIMITATIONS, REASONS FOR CAUTION

Samples were collected from patients who had ICSI treatment and from only one clinic.

WIDER IMPLICATIONS OF THE FINDINGS

These results confirm that metabolites, such as glucose and amino acids, are valid biomarkers of embryo viability and could therefore be used in conjunction with other systems to aid in the selection of a healthy embryo.

STUDY FUNDING/COMPETING INTEREST(S)

Work was supported by Virtus Health. D.K.G is contracted with Virtus Health. The other authors have no conflict of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Low-level pyruvate inhibits early embryonic development and maternal mRNA clearance in mice

Energy homeostasis and accomplishment of maternal-to-zygotic transition (MZT), which involves the timed processes of maternal mRNA clearance and zygotic genome activation (ZGA), are essential for mammalian embryogenesis. However, how energy substrates regulate maternal mRNA clearance and the underlying mechanisms have not yet been fully elucidated. Here, we found that mouse embryos were arrested at the 2-cell stage when the pyruvate level was reduced to one-fifth of the control level. Moreover, we observed that the mitochondrial contents and ROS levels were reduced. Interestingly, some maternal mRNA, including transcripts involved in the maternal factor-mediated mRNA decay (M-decay) pathway, was vastly degraded from 1-cell to 2-/4-cell embryos when cultured with control pyruvate levels, but the clearance of these transcripts was hindered when the pyruvate level was reduced. In contrast, some transcripts involved in the zygotic factor-mediated mRNA decay (Z-decay) pathway were vastly downregulated by the reduction in pyruvate. This effect was possibly due to a reduction in global transcription, as the embryos cultured with low-level pyruvate had lower transcription activity than embryos cultured with control pyruvate level. In summary, our findings demonstrate that low-level pyruvate inhibits maternal mRNA clearance, possibly by disrupting the M- and Z-decay pathways, extending our current understanding of the energy requirements of embryogenesis.

Non-Invasive Human Embryo Metabolic Assessment as a Developmental Criterion

The selection of a highly-viable single embryo in assisted reproductive technology requires an acceptable predictive method in order to reduce the multiple pregnancy rate and increase the success rate. In this study, the metabolomic profiling of growing and impaired embryos was assessed on the fifth day of fertilization using capillary electrophoresis in order to find a relationship between the profiling and embryo development, and then to provide a mechanistic insight into the appearance/depletion of the metabolites. This unique qualitative technique exhibited the appearance of most non-essential amino acids and lactate, and depleting the serine, alanyl-glutamine and pyruvate in such a manner that the embryos impaired in their development secreted a considerably higher level of lactate and consumed a significantly higher amount of alanyl-glutamine. The different significant ratios of metabolomic depletion/appearance between the embryos confirm their potential for the improvement of the prospective selection of the developed single embryos, and also suggest the fact that pyruvate and alanyl-glutamine are the most critical ATP suppliers on the fifth day of blastocyst development.

Bovine Oviduct Epithelial Cell-Derived Culture Media and Exosomes Improve Mitochondrial Health by Restoring Metabolic Flux during Pre-Implantation Development

Oviduct flushing is enriched by a wide variety of nutrients that guide the 3-4 days journey of pre-implantation embryo through the oviduct as it develops into a competent blastocyst (BL). However, little is known about the specific requirement and role of these nutrients that orchestrate the early stages of embryonic development. In this study, we aimed to characterize the effect of in vitro-derived bovine oviduct epithelial cell (BOECs) secretion that mimics the in vivo oviduct micro-fluid like environment, which allows successful embryonic development. In this study, the addition of an in vitro derived BOECs-condition media (CM) and its isolated exosomes (Exo) significantly enhances the quality and development of BL, while the hatching ability of BLs was found to be high (48.8%) in the BOECs-Exo supplemented group. Surprisingly, BOECs-Exo have a dynamic effect on modulating the embryonic metabolism by restoring the pyruvate flux into TCA-cycle. Our analysis reveals that Exo treatment significantly upregulates the pyruvate dehydrogenase (PDH) and glutamate dehydrogenase (GLUD1) expression, required for metabolic fine-tuning of the TCA-cycle in the developing embryos. Exo treatment increases the influx into TCA-cycle by strongly suppressing the PDH and GLUD1 upstream inhibitors, i.e., PDK4 and SIRT4. Improvement of TCA-cycle function was further accompanied by higher metabolic activity of mitochondria in BOECs-CM and Exo in vitro embryos. Our study uncovered, for the first time, the possible mechanism of BOECs-derived secretion in re-establishing the TCA-cycle flux by the utilization of available nutrients and highlighted the importance of pyruvate in supporting bovine in vitro embryonic development.

Reducing time to pregnancy and facilitating the birth of healthy children through functional analysis of embryo physiology†

An ever-increasing number of couples rely on assisted reproductive technologies (ART) in order to conceive a child. Although advances in embryo culture have led to increases in the success rates of clinical ART, it often takes more than one treatment cycle to conceive a child. Ensuring patients conceive as soon as possible with a healthy embryo is a priority for reproductive medicine. Currently, selection of embryos for transfer relies predominantly on the morphological assessment of the preimplantation embryo; however, morphology is not an absolute link to embryo physiology, nor the health of the resulting child. Non-invasive quantitation of individual embryo physiology, a key regulator of both embryo viability and health, could provide valuable information to assist in the selection of the most viable embryo for transfer, hence reducing the time to pregnancy. Further, according to the Barker Hypothesis, the environment to which a fetus is exposed to during gestation affects subsequent offspring health. If the environment of the preimplantation period is capable of affecting metabolism, which in turn will affect gene expression through the metaboloepigenetic link, then assessment of embryo metabolism should represent an indirect measure of future offspring health. Previously, the term viable embryo has been used in association with the potential of an embryo to establish a pregnancy. Here, we propose the term healthy embryo to reflect the capacity of that embryo to lead to a healthy child and adult.

The composition of human preimplantation embryo culture media and their stability during storage and culture

STUDY QUESTION

What is the composition and stability during storage and culture of fifteen commercially available human preimplantation embryo culture media?

SUMMARY ANSWER

No two culture media had the same composition, and both storage and culture had an effect on the concentrations of multiple components.

WHAT IS KNOWN ALREADY

The choice of embryo culture medium not only affects the success rate of an IVF treatment, but also affects the health of the future child. Exact formulations of embryo culture media are often not disclosed by manufacturers. It is unknown whether the composition of these media changes during storage or culture in the IVF laboratory. Without details on the exact concentrations, it is not possible to determine which components might be responsible for the differences in IVF success rates and health of the resulting children.

STUDY DESIGN, SIZE, DURATION

Between October 2014 and October 2015, all complete human preimplantation embryo culture media, i.e. ready to use for IVF, that were commercially available at that time, were included (n = 15). Osmolality and the concentration of thirty seven components including basic elements, metabolites, immunoglobulins, albumin, proteins and 21 amino acids were tested immediately upon arrival into the IVF laboratory, after three days of culture without embryos (sham culture) starting from the day of arrival, just before the expiry date, and after three days of sham culture just before the expiry date.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Ions, glucose, immunoglobulins, albumin and the total amount of proteins were quantified using a combination of ion selective electrodes and photometric analysis modules, and lactate, pyruvate and 21 amino acids were analysed by ultra performance liquid chromatography mass spectrometry. Osmolality was analysed by an advanced micro-osmometer. Statistical analysis was done using multivariate general linear models.

MAIN RESULTS AND THE ROLE OF CHANCE

The composition varied between media, no two media had the same concentration of components. Storage led to significant changes in 17 of the 37 analyzed components (magnesium, chloride, phosphate, albumin, total amount of proteins, tyrosine, tryptophan, alanine, methionine, glycine, leucine, glutamine, asparagine, arginine, serine, proline, and threonine). Storage affected the osmolality in 3 of the 15 media, but for all media combined this effect was not significant (p = 0.08). Sham culture of the analyzed media had a significant effect on the concentrations of 13 of the 37 analyzed components (calcium, phosphate, albumin, total amount of proteins, tyrosine, alanine, methionine, glycine, leucine, asparagine, arginine, proline, and histidine). Sham culture significantly affected the osmolality of the analysed culture media. Two media contained 50% D-lactate, which a toxic dead-end metabolite. In a secondary analysis we detected human liver enzymes in more than half of the complete culture media.

LIMITATIONS, REASONS FOR CAUTION

The analyzed culture media could contain components that are not among the 37 components that were analyzed in this study. The clinical relevance of the varying concentrations is yet to be determined.

WIDER IMPLICATIONS OF THE FINDINGS

The presence of D-lactate could be avoided and the finding of human liver enzymes was surprising. The wide variation between culture media shows that the optimal composition is still unknown. This warrants further research as the importance of embryo culture media on the efficacy and safety in IVF is evident. Companies are urged to fully disclose the composition of their culture media, and provide clinical evidence supporting the composition or future changes thereof.

STUDY FUNDING/COMPETING INTEREST(S)

None.

Metabolic environment in vivo as a blueprint for differentiation and maturation of human stem cell-derived cardiomyocytes

Patient-derived human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) are increasingly being used for disease modeling, drug screening and regenerative medicine. However, to date, an immature, fetal-like, phenotype of hPSC-CMs restrains their full potential. Increasing evidence suggests that the metabolic state, particularly important for provision of sufficient energy in highly active contractile CMs and anabolic and regulatory processes, plays an important role in CM maturation, which affects crucial functional aspects of CMs, such as contractility and electrophysiology. During embryonic development the heart is subjected to metabolite concentrations that differ substantially from that of hPSC-derived cardiac cell cultures. A deeper understanding of the environmental and metabolic cues during embryonic heart development and how these change postnatally, will provide a framework for optimizing cell culture conditions and maturation of hPSC-CMs. Maturation of hPSC-CMs will improve the predictability of disease modeling, drug screening and drug safety assessment and broadens their applicability for personalized and regenerative medicine.

A novel culture medium with reduced nutrient concentrations supports the development and viability of mouse embryos

Further refinement of culture media is needed to improve the quality of embryos generated in vitro. Previous results from our laboratory demonstrated that uptake of nutrients by the embryo is significantly less than what is supplied in traditional culture media. Our objective was to determine the impact of reduced nutrient concentrations in culture medium on mouse embryo development, metabolism, and quality as a possible platform for next generation medium formulation. Concentrations of carbohydrates, amino acids, and vitamins could be reduced by 50% with no detrimental effects, but blastocyst development was impaired at 25% of standard nutrient provision (reduced nutrient medium; RN). Addition of pyruvate and L-lactate (+PL) to RN at 50% of standard concentrations restored blastocyst development, hatching, and cell number. In addition, blastocysts produced in RN + PL contained more ICM cells and ATP than blastocysts cultured in our control (100% nutrient) medium; however, metabolic activity was altered. Similarly, embryos produced in the RN medium with elevated (50% control) concentrations of pyruvate and lactate in the first step medium and EAA and Glu in the second step medium were competent to implant and develop into fetuses at a similar rate as embryos produced in the control medium. This novel approach to culture medium formulation could help define the optimal nutrient requirements of embryos in culture and provide a means of shifting metabolic activity towards the utilization of specific metabolic pathways that may be beneficial for embryo viability.

Glycolysis-Independent Glucose Metabolism Distinguishes TE from ICM Fate during Mammalian Embryogenesis

The mouse embryo undergoes compaction at the 8-cell stage, and its transition to 16 cells generates polarity such that the outer apical cells are trophectoderm (TE) precursors and the inner cell mass (ICM) gives rise to the embryo. Here, we report that this first cell fate specification event is controlled by glucose. Glucose does not fuel mitochondrial ATP generation, and glycolysis is dispensable for blastocyst formation. Furthermore, glucose does not help synthesize amino acids, fatty acids, and nucleobases. Instead, glucose metabolized by the hexosamine biosynthetic pathway (HBP) allows nuclear localization of YAP1. In addition, glucose-dependent nucleotide synthesis by the pentose phosphate pathway (PPP), along with sphingolipid (S1P) signaling, activates mTOR and allows translation of Tfap2c. YAP1, TEAD4, and TFAP2C interact to form a complex that controls TE-specific gene transcription. Glucose signaling has no role in ICM specification, and this process of developmental metabolism specifically controls TE cell fate.

Metabolic regulation of Kv channels and cardiac repolarization by Kvβ2 subunits

Voltage-gated potassium (Kv) channels control myocardial repolarization. Pore-forming Kvα proteins associate with intracellular Kvβ subunits, which bind pyridine nucleotides with high affinity and differentially regulate channel trafficking, plasmalemmal localization and gating properties. Nevertheless, it is unclear how Kvβ subunits regulate myocardial K⁺ currents and repolarization. Here, we tested the hypothesis that Kvβ2 subunits regulate the expression of myocardial Kv channels and confer redox sensitivity to Kv current and cardiac repolarization. Co-immunoprecipitation and in situ proximity ligation showed that in cardiac myocytes, Kvβ2 interacts with Kv1.4, Kv1.5, Kv4.2, and Kv4.3. Cardiac myocytes from mice lacking Kcnab2 (Kvβ2^-/-) had smaller cross sectional areas, reduced sarcolemmal abundance of Kvα binding partners, reduced I_to, I_K,slow1, and I_K,slow2 densities, and prolonged action potential duration compared with myocytes from wild type mice. These differences in Kvβ2^-/- mice were associated with greater P wave duration and QT interval in electrocardiograms, and lower ejection fraction, fractional shortening, and left ventricular mass in echocardiographic and morphological assessments. Direct intracellular dialysis with a high NAD(P)H:NAD(P)⁺ accelerated Kv inactivation in wild type, but not Kvβ2^-/- myocytes. Furthermore, elevated extracellular levels of lactate increased [NADH]_i and prolonged action potential duration in wild type cardiac myocytes and perfused wild type, but not Kvβ2^-/-, hearts. Taken together, these results suggest that Kvβ2 regulates myocardial electrical activity by supporting the functional expression of proteins that generate I_to and I_K,slow, and imparting redox and metabolic sensitivity to Kv channels, thereby coupling cardiac repolarization to myocyte metabolism.

Label-free assessment of pre-implantation embryo quality by the Fluorescence Lifetime Imaging Microscopy (FLIM)-phasor approach

Development of quantitative, safe and rapid techniques for assessing embryo quality provides significant advances in Assisted Reproductive Technologies (ART). Instead of assessing the embryo quality by the standard morphologic evaluation, we apply the phasor-FLIM (Fluorescence Lifetime Imaging Microscopy) method to capture endogenous fluorescent biomarkers of pre-implantation embryos as a non-morphological caliber for embryo quality. Here, we identify, under hypoxic and non-hypoxic conditions, the unique spectroscopic trajectories at different stages of mouse pre-implantation development, which is referred to as the developmental, or “D-trajectory”, that consists of fluorescence lifetime from different stages of mouse pre-implantation embryos. The D-trajectory correlates with intrinsic fluorescent species from a distinctive energy metabolism and oxidized lipids, as seen with Third Harmonic Generation (THG) that changes over time. In addition, we have defined a non-morphological Embryo Viability Index (EVI) to distinguish pre-implantation embryo quality using the Distance Analysis (DA), a machine learning algorithm to process the fluorescence lifetime distribution patterns. We show, under our experimental conditions, that the phasor-FLIM approach provides a much-needed non-invasive quantitative technology for identifying healthy embryos at the early compaction stage with 86% accuracy. The DA and phasor-FLIM method may provide the opportunity to improve implantation success rates for in vitro fertilization clinics.

Individual culture and atmospheric oxygen during culture affect mouse preimplantation embryo metabolism and post-implantation development

RESEARCH QUESTION

Does single embryo culture under atmospheric or reduced oxygen alter preimplantation metabolism and post-implantation development compared with culture in groups?

DESIGN

Mouse embryos were cultured under 5% or 20% oxygen, individually or in groups of 10. Spent media were analysed after 48, 72 and 96 h of culture. Blastocysts were assessed by outgrowth assay or transferred to pseudo-pregnant recipients, and fetal and placental weight, length and morphology were assessed.

RESULTS

Compared with group culture, individually cultured blastocysts had lower net consumption of glucose and aspartate and higher glutamate production. Atmospheric oxygen reduced uptake of glucose and aspartate and increased production of glutamate and ornithine compared with 5% oxygen. Combining 20% oxygen and single culture resulted in further metabolic changes: decreased leucine, methionine and threonine consumption. Under 5% oxygen, individual culture decreased placental labyrinth area but had no other effects on fetal and placental development or outgrowth size compared with group culture. Under 20% oxygen, however, individual culture reduced outgrowth size and fetal and placental weight compared with group-cultured embryos.

CONCLUSIONS

Preimplantation metabolism of glucose and amino acids is altered by both oxygen and individual culture, and fetal weight is reduced by individual culture under atmospheric oxygen but not 5% oxygen. This study raises concerns regarding the increasing prevalence of single embryo culture in human IVF and adds to the existing evidence regarding the detrimental effects of atmospheric oxygen during embryo culture. Furthermore, these data demonstrate the cumulative nature of stress during embryo culture and highlight the importance of optimizing each element of the culture system.

Culture of the Mouse Preimplantation Embryo

The first culture media designed specifically to support development of the preimplantation mouse embryo were formulated over 50 years ago and were based on balanced salt solutions, containing the carbohydrates pyruvate, lactate, and glucose as the sole energy sources. Such media used a bicarbonate-carbon dioxide buffer system to maintain the desired pH, and were typically supplemented with serum albumin, but lacked free amino acids. In contrast to the complexity of a tissue culture medium, these original formulations of mouse embryo culture media were very simplistic. Over the intervening decades, as our understanding of the physiology and metabolism of the preimplantation embryo increased, together with a greater understanding of the environment within the female reproductive tract, culture media to support mouse embryo development in vitro have become more physiological and consequently more complex. A main addition to such media has been an array of amino acids. Although the media of today contain more components than their predecessors, their preparation remains relatively easy to accomplish, made feasible through the use of stock solutions, which also readily facilitates any changes to formulations to be made, an essential prerequisite for experimentation. As well as changes in media formulations, there have been exciting developments in incubator technology and design, such as the inclusion of time-lapse capability, redefining our ability to both culture and monitor embryo development in vitro.

Revisiting the role of metabolism during development

An emerging view emphasizes that metabolism is highly regulated in both time and space. In addition, it is increasingly being recognized that metabolic pathways are tightly connected to specific biological processes such as cell signaling, proliferation and differentiation. As we obtain a better view of this spatiotemporal regulation of metabolism, and of the molecular mechanisms that connect metabolism and signaling, we can now move from largely correlative to more functional studies. It is, therefore, a particularly promising time to revisit how metabolism can affect multiple aspects of animal development. In this Review, we discuss how metabolism is mechanistically linked to cellular and developmental programs through both its bioenergetic and metabolic signaling functions. We highlight how metabolism is regulated across various spatial and temporal scales, and discuss how this regulation can influence cellular processes such as cell signaling, gene expression, and epigenetic and post-translational modifications during embryonic development.

In vivo oxygen, temperature and pH dynamics in the female reproductive tract and their importance in human conception: a systematic review

BACKGROUND

Despite advances in ART, implantation and pregnancy rates per embryo transfer still remain low. IVF laboratories strive to ensure that the process of handling gametes in vitro closely mimics the in vivo environment. However, there remains a lack of knowledge regarding the in vivo regulation and dynamic variation in biophysical parameters such as oxygen concentration, pH and temperature within the reproductive tract.

OBJECTIVE AND RATIONALE

To undertake a systematic review of the current understanding of the physico-chemical parameters of oxygen tension (pO2), pH and temperature within the female reproductive tract, and their potential implications in clinical and pathological processes related to fertility and those pertaining to limited reproductive capacity.

SEARCH METHODS

A comprehensive literature search was performed using electronic databases including Medline, Embase, Cochrane Library and Pubmed to identify original and review articles addressing the biophysical parameters (pO2, pH and temperature) in the female reproductive tract of any species. The search included all studies published between 1946 and November 2015. Search terms included 'oxygen', 'pH', 'hydrogen ion concentration', 'acid base' and others terms. We also used special features and truncations to identify synonyms and broaden the search. Studies were excluded if they only assessed embryo culture conditions, fetal acid-base status, oxidative stress, outcomes of pregnancy and measurements of these parameters in non-reproductive organs.

OUTCOMES

Our search generated 18 685 records and 60 articles were included. pO2 within the female reproductive tract shows cyclical variation and minute-to-minute oscillations, which may be influenced by uterine contractility, hormones, the autonomic system, cardiac pulsatility, and myometrial and smooth muscle integrity. Fine balanced control of pO2 and avoidance of overwhelming oxidative stress is crucial for embryogenesis and implantation. The pH in the female reproductive tract is graduated, with lowest pH in the vagina (~pH 4.42) increasing toward the Fallopian tubes (FTs) (~pH 7.94), reflecting variation in the site-specific microbiome and acid-base buffering at the tissue/cellular level. The temperature variation in humans is cyclical by day and month. In humans, it is biphasic, increasing in the luteal phase; with the caudal region of the oviduct 1-2 degrees cooler than the cranial portion. Temperature variation is influenced by hormones, density of pelvic/uterine vascular beds and effectiveness of heat exchange locally, crucial for sperm motility and embryo development. We have identified significant deficiencies and inconsistencies in the methods used to assess these biophysical factors within the reproductive tract. We have suggested that the technological solutions including the development of methods and models for real time, in vivo recordings of biophysical parameters.

WIDER IMPLICATIONS

The notion of 'back to nature' in assisted conception suggested 20 years ago has yet to be translated into clinical practice. While the findings from this systematic review do not provide evidence to change current in vitro protocols, it highlights our current inability to assess the in vivo reproductive tract environment in real time. Data made available through future development of sensing technology in utero may help to provide new insights into how best to optimize the in vitro embryo environment and allow for more precise and personalized fertility treatment.

Impact of the IVF laboratory environment on human preimplantation embryo phenotype

The phenotype of the human embryo conceived through in vitro fertilization (IVF), that is its morphology, developmental kinetics, physiology and metabolism, can be affected by numerous components of the laboratory and embryo culture system (which comprise the laboratory environment). The culture media formulation is important in determining embryo phenotype, but this exists within a culture system that includes oxygen, temperature, pH and whether an embryo is cultured individually or in a group, all of which can influence embryo development. Significantly, exposure of an embryo to one suboptimal component of the culture system of laboratory typically predisposes the embryo to become more vulnerable to a second stressor, as has been well documented for atmospheric oxygen and individual culture, as well as for oxygen and ammonium. Furthermore, the inherent viability of the human embryo is derived from the quality of the gametes from which it is created. Patient age, aetiology, genetics, lifestyle (as well as ovarian stimulation in women) are all known to affect the developmental potential of gametes and hence the embryo. Thus, as well as considering the impact of the IVF laboratory environment, one needs to be aware of the status of the infertile couple, as this impacts how their gametes and embryos will respond to an in vitro environment. Although far from straight forward, analysing the interactions that exist between the human embryo and its environment will facilitate the creation of more effective and safer treatments for the infertile couple.

A prospective randomized comparison of early embryo cleavage kinetics between two media culture systems

OBJECTIVE

To investigate whether early embryo cleavage kinetics were affected by type of culture media.

METHODS

In this prospective sibling-split study, 620 oocytes from 37 patients were randomly allocated into two groups: Cook group and Vitrolife group. Oocytes/embryos in Cook group, would be cultured with Cook sequential culture medium, while oocytes/embryos in Vitrolife group, would be cultured with Vitrolife sequential culture medium. Time-lapse imaging technology was used to calculate exact timing of early embryo cleavage events which included time to 2PN breakdown, cleavage to 2-, 3-, 4-, 5- cell and the time duration in the 2-,3-cell stage. Then these timing of early embryo cleavage events were compared between Cook group and Vitrolife group. Moreover, fertilization rate, cleavage rate, high quality embryo rate, usable blastocyst rate, pregnancy rate and implantation rate of these two groups were also analyzed.

RESULTS

The results showed there were no differences in all timing of early embryo cleavage events between the two groups. In addition, the two groups were similar in fertilization rate (Cook 71.0% vs. Vitrolife 71.3%, P>0.05), cleavage rate (Cook 98.1% vs. Vitrolife 98.2%, P>0.05), high quality embryo rate (Cook 52.1% vs. Vitrolife 52.7%, P>0.05), usable blastocyst rate (Cook 29.7% vs. Vitrolife 28.0%, P>0.05), pregnancy rate (Cook 46.7% VS. Vitrolife 50.0%, P>0.05) and implantation rate (Cook 30.3% VS. Vitrolife 29.0%, P>0.05).

CONCLUSIONS

Morphokinetics used for embryo selection are not affected by the two different culture media.

Nuclear Localization of Mitochondrial TCA Cycle Enzymes as a Critical Step in Mammalian Zygotic Genome Activation

Transcriptional control requires epigenetic changes directed by mitochondrial tricarboxylic acid (TCA) cycle metabolites. In the mouse embryo, global epigenetic changes occur during zygotic genome activation (ZGA) at the 2-cell stage. Pyruvate is essential for development beyond this stage, which is at odds with the low activity of mitochondria in this period. We now show that a number of enzymatically active mitochondrial enzymes associated with the TCA cycle are essential for epigenetic remodeling and are transiently and partially localized to the nucleus. Pyruvate is essential for this nuclear localization, and a failure of TCA cycle enzymes to enter the nucleus correlates with loss of specific histone modifications and a block in ZGA. At later stages, however, these enzymes are exclusively mitochondrial. In humans, the enzyme pyruvate dehydrogenase is transiently nuclear at the 4/8-cell stage coincident with timing of human embryonic genome activation, suggesting a conserved metabolic control mechanism underlying early pre-implantation development.

Effects of glucose metabolism during in vitro maturation on cytoplasmic maturation of mouse oocytes

Although there are many reports on the effect of glucose metabolism on oocyte nuclear maturation, there are few studies on its effect on ooplasmic maturation. By manipulating glucose metabolism pathways using a maturation medium that could support oocyte nuclear maturation but only a limited blastocyst formation without glucose, this study determined effects of glucose metabolism pathways on ooplasmic maturation. During maturation of cumulus-oocyte-complexes (COCs) with glucose, the presence of PPP inhibitor, DHEA or glycolysis inhibitor, iodoacetate significantly decreased blastocyst rates, intraoocyte glutathione and ATP. While blastocyst rates, GSH/GSSG ratio and NADPH were higher, ROS was lower significantly in COCs matured with iodoacetate than with DHEA. Fructose-6-phosphate overcame the inhibitory effect of DHEA on PPP. During maturation of COCs with pyruvate, electron transport inhibitor, rotenone or monocarboxylate transfer inhibitor, 4-CIN significantly decreased blastocyst rates. Cumulus-denuded oocytes had a limited capacity to use glucose or lactate, but they could use pyruvate to support maturation. In conclusion, whereas glycolysis promoted ooplasmic maturation mainly by supplying energy, PPP facilitated ooplasmic maturation to a greater extent by both reducing oxidative stress and supplying energy through providing fructose-6-phosphate for glycolysis. Pyruvate was transferred by monocarboxylate transporters and utilized through mitochondrial electron transport to sustain ooplasmic maturation.