Energy metabolism of the trophectoderm and inner cell mass of the mouse blastocyst

Metabolic imaging of human embryos is predictive of ploidy status but is not associated with clinical pregnancy outcomes: a pilot trial

STUDY QUESTION

Does fluorescence lifetime imaging microscopy (FLIM)-based metabolic imaging assessment of human blastocysts prior to frozen transfer correlate with pregnancy outcomes?

SUMMARY ANSWER

FLIM failed to distinguish consistent patterns in mitochondrial metabolism between blastocysts leading to pregnancy compared to those that did not.

WHAT IS KNOWN ALREADY

FLIM measurements provide quantitative information on NAD(P)H and flavin adenine dinucleotide (FAD+) concentrations. The metabolism of embryos has long been linked to their viability, suggesting the potential utility of metabolic measurements to aid in selection.

STUDY DESIGN, SIZE, DURATION

This was a pilot trial enrolling 121 IVF couples who consented to have their frozen blastocyst measured using non-invasive metabolic imaging. After being warmed, 105 couples' good-quality blastocysts underwent a 6-min scan in a controlled temperature and gas environment. FLIM-assessed blastocysts were then transferred without any intervention in management.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Eight metabolic parameters were obtained from each blastocyst (4 for NAD(P)H and 4 for FAD): short and long fluorescence lifetime, fluorescence intensity, and fraction of the molecule engaged with enzyme. The redox ratio (intensity of NAD(P)H)/(intensity of FAD) was also calculated. FLIM data were combined with known metadata and analyzed to quantify the ability of metabolic imaging to differentiate embryos that resulted in pregnancy from embryos that did not. De-identified discarded aneuploid human embryos (n = 158) were also measured to quantify correlations with ploidy status and other factors. Statistical comparisons were performed using logistic regression and receiver operating characteristic (ROC) curves with 5-fold cross-validation averaged over 100 repeats with random sampling. AUC values were used to quantify the ability to distinguish between classes.

MAIN RESULTS AND THE ROLE OF CHANCE

No metabolic imaging parameters showed significant differences between good-quality blastocysts resulting in pregnancy versus those that did not. A logistic regression using metabolic data and metadata produced an ROC AUC of 0.58. In contrast, robust AUCs were obtained when classifying other factors such as comparison of Day 5 (n = 64) versus Day 6 (n = 41) blastocysts (AUC = 0.78), inner cell mass versus trophectoderm (n = 105: AUC = 0.88) and aneuploid (n = 158) versus euploid and positive pregnancy embryos (n = 108) (AUC = 0.82).

LIMITATIONS, REASONS FOR CAUTION

The study protocol did not select which embryo to transfer and the cohort of 105 included blastocysts were all high quality. The study was also limited in number of participants and study sites. Increased power and performing the trial in more sites may have provided a stronger conclusion regarding the merits of the use of FLIM clinically.

WIDER IMPLICATIONS OF THE FINDINGS

FLIM failed to distinguish consistent patterns in mitochondrial metabolism between good-quality blastocysts leading to pregnancy compared to those that did not. Blastocyst ploidy status was, however, highly distinguishable. In addition, embryo regions and embryo day were consistently revealed by FLIM. While metabolic imaging detects mitochondrial metabolic features in human blastocysts, this pilot trial indicates it does not have the potential to serve as an effective embryo viability detection tool. This may be because mitochondrial metabolism plays an alternative role post-implantation.

STUDY FUNDING/COMPETING INTEREST(S)

This study was sponsored by Optiva Fertility, Inc. Boston IVF contributed to the clinical site and services. Becker Hickl, GmbH, provided the FLIM system on loan. T.S. was the founder and held stock in Optiva Fertility, Inc., and D.S. and E.S. had options with Optiva Fertility, Inc., during this study.

TRIAL REGISTRATION NUMBER

The study was approved by WCG Connexus IRB (Study Number 1298156).

Glycolysis: A fork in the path of normal and pathological pregnancy

Glucose metabolism is vital to the survival of living organisms. Since the discovery of the Warburg effect in the 1920s, glycolysis has become a major research area in the field of metabolism. Glycolysis has been extensively studied in the field of cancer and is considered as a promising therapeutic target. However, research on the role of glycolysis in pregnancy is limited. Recent evidence suggests that blastocysts, trophoblasts, decidua, and tumors all acquire metabolic energy at specific stages in a highly similar manner. Glycolysis, carefully controlled throughout pregnancy, maintains a dynamic and coordinated state, so as to maintain the homeostasis of the maternal-fetal interface and ensure normal gestation. In the present review, we investigate metabolic remodeling and the selective propensity of the embryo and placenta for glycolysis. We then address dysregulated glycolysis that occurs in the cellular interactive network at the maternal-fetal interface in miscarriage, preeclampsia, fetal growth restriction, and gestational diabetes mellitus. We provide new insights into the field of maternal-fetal medicine from a metabolic perspective, thus revealing the mystery of human pregnancy.

Ectopic Lipid Accumulation Correlates with Cellular Stress in Rabbit Blastocysts from Diabetic Mothers

Maternal diabetes mellitus in early pregnancy leads to hyperlipidemia in reproductive tract organs and an altered embryonic environment. To investigate the consequences on embryonic metabolism, the effect of high environmental-lipid levels was studied in rabbit blastocysts cultured with a lipid mixture in vitro and in blastocysts from diabetic, hyperlipidemic rabbits in vivo. The gene and protein expression of marker molecules involved in lipid metabolism and stress response were analyzed. In diabetic rabbits, the expression of embryoblast genes encoding carnitine palmityl transferase 1 and peroxisome proliferator-activated receptors α and γ increased, whereas trophoblast genes encoding for proteins associated with fatty acid synthesis and β-oxidation decreased. Markers for endoplasmic (activating transcription factor 4) and oxidative stress (nuclear factor erythroid 2-related factor 2) were increased in embryoblasts, while markers for cellular redox status (superoxide dismutase 2) and stress (heat shock protein 70) were increased in trophoblasts from diabetic rabbits. The observed regulation pattern in vivo was consistent with an adaptation response to the hyperlipidemic environment, suggesting that maternal lipids have an impact on the intracellular metabolism of the preimplantation embryo in diabetic pregnancy and that embryoblasts are particularly vulnerable to metabolic stress.

Mitochondrial dysfunction in the offspring of obese mothers and it's transmission through damaged oocyte mitochondria: Integration of mechanisms

In vivo and in vitro studies demonstrate that mitochondria in the oocyte, are susceptible to damage by suboptimal pre/pregnancy conditions, such as obesity. These suboptimal conditions have been shown to induce mitochondrial dysfunction (MD) in multiple tissues of the offspring, suggesting that mitochondria of oocytes that pass from mother to offspring, can carry information that can programme mitochondrial and metabolic dysfunction of the next generation. They also suggest that transmission of MD could increase the risk of obesity and other metabolic diseases in the population inter- and trans-generationally. In this review, we examined whether MD observed in offspring tissues of high energetic demand, is the result of the transmission of damaged mitochondria from obese mothers' oocytes to the offspring. The contribution of genome-independent mechanisms (namely mitophagy) in this transmission were also explored. Finally, potential interventions aimed at improving oocyte/embryo health were investigated, to see if they may provide an opportunity to halter the generational effects of MD.

Acetoacetate and β-hydroxybutyrate reduce mouse embryo viability via differential metabolic and epigenetic mechanisms

RESEARCH QUESTION

Does the ketone acetoacetate (AcAc) alone, or combined with β-hydroxybutyrate (βOHB), impact mouse embryo development, metabolism, histone acetylation and viability?

DESIGN

Pronucleate mouse oocytes were cultured in vitro in G1/G2 media supplemented with ketones (AcAc or AcAc + βOHB) at concentrations representing those in maternal serum during pregnancy (0.04 mmol/l AcAc, 0.1 mmol/l βOHB), standard diet consumption (0.1 mmol/l AcAc, 0.25 mmol/l βOHB), ketogenic diet consumption (0.8 mmol/l AcAc, 2 mmol/l βOHB) and diabetic ketoacidosis (2 mmol/l AcAc, 4 mmol/l βOHB). Day 5 blastocysts were assessed for cell allocation, glucose metabolism and histone acetylation. Day 4 blastocysts exposed to 0.8 mmol/l AcAc + 2 mmol/l βOHB were transferred to standard-fed recipient females, and E14.5 fetal and placental development assessed.

RESULTS

Exposure to 2 mmol/l AcAc or 0.8 mmol/l AcAc + 2 mmol/l βOHB did not impair blastocyst development, but significantly increased glucose consumption (P = 0.001 each), lowered glycolytic flux (P = 0.01, P < 0.001) and elevated trophectoderm (TE) histone 3 lysine 27 acetylation (H3K27ac; P < 0.001 each) compared with unexposed controls. Preimplantation AcAc + βOHB exposure reduced post-implantation fetal development by 25% (P = 0.037), and delayed female-specific fetal limb development (P = 0.019) and estimated fetal age (P = 0.019) compared with controls.

CONCLUSION

Preimplantation exposure to ketones affects underlying metabolism and histone acetylation in blastocysts that are associated with persistent, female-specific perturbations in fetal development. A periconceptional diet that elevates ketone concentrations may impair human embryonic viability.

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.

Remdesivir impairs mouse preimplantation embryo development at therapeutic concentrations

Remdesivir (RDV) is the first antiviral drug to be approved by the US Food and Drug Administration for the treatment of COVID-19. While the general safety of RDV has been studied, its reproductive risk, including embryotoxicity, is largely unknown. Here, to gain insights into its embryotoxic potential, we investigated the effects of RDV on mouse preimplantation embryos cultured in vitro at the concentrations comparable to the therapeutic plasma levels. Exposure to RDV (2–8 µM) did not affect the initiation of blastocyst formation, although the maintenance of the cavity failed at 8 µM due to increased cell death. While exposure to 2–4 µM permitted the cavity maintenance, expressions of developmental regulator genes associated with the inner cell mass (ICM) lineage were significantly diminished. Adverse effects of RDV depended on the duration and timing of exposure, as treatment between the 8-cell to early blastocyst stage most sensitively affected cavity expansion, gene expressions, and cell proliferation, particularly of the ICM than the trophectoderm lineage. GS-441524, a major metabolite of RDV, did not impair blastocyst formation or cavity expansion, although it altered gene expressions in a manner differently from RDV. Additionally, RDV reduced the viability of human embryonic stem cells, which were used as a model for the human ICM lineage, more potently than GS-441524. These findings suggest that RDV is potentially embryotoxic to impair the pluripotent lineage, and will be useful for designing and interpreting further in vitro and in vivo studies on the reproductive toxicity of RDV.

Metabolic state of human blastocysts measured by fluorescence lifetime imaging microscopy

STUDY QUESTION

Can non-invasive metabolic imaging via fluorescence lifetime imaging microscopy (FLIM) detect variations in metabolic profiles between discarded human blastocysts?

SUMMARY ANSWER

FLIM revealed extensive variations in the metabolic state of discarded human blastocysts associated with blastocyst development over 36 h, the day after fertilization and blastocyst developmental stage, as well as metabolic heterogeneity within individual blastocysts.

WHAT IS KNOWN ALREADY

Mammalian embryos undergo large changes in metabolism over the course of preimplantation development. Embryo metabolism has long been linked to embryo viability, suggesting its potential utility in ART to aid in selecting high quality embryos. However, the metabolism of human embryos remains poorly characterized due to a lack of non-invasive methods to measure their metabolic state.

STUDY DESIGN, SIZE, DURATION

We conducted a prospective observational study. We used 215 morphologically normal human embryos from 137 patients that were discarded and donated for research under an approved institutional review board protocol. These embryos were imaged using metabolic imaging via FLIM to measure the autofluorescence of two central coenzymes, nicotinamide adenine (phosphate) dinucleotide (NAD(P)H) and flavine adenine dinucleotide (FAD+), which are essential for cellular respiration and glycolysis.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Here, we used non-invasive FLIM to measure the metabolic state of human blastocysts. We first studied spatial patterns in the metabolic state within human blastocysts and the association of the metabolic state of the whole blastocysts with stage of expansion, day of development since fertilization and morphology. We explored the sensitivity of this technique in detecting metabolic variations between blastocysts from the same patient and between patients. Next, we explored whether FLIM can quantitatively measure metabolic changes through human blastocyst expansion and hatching via time-lapse imaging. For all test conditions, the level of significance was set at P < 0.05 after correction for multiple comparisons using Benjamini-Hochberg's false discovery rate.

MAIN RESULTS AND THE ROLE OF CHANCE

We found that FLIM is sensitive enough to detect significant metabolic differences between blastocysts. We found that metabolic variations between blastocyst are partially explained by both the time since fertilization and their developmental expansion stage (P < 0.05), but not their morphological grade. Substantial metabolic variations between blastocysts from the same patients remain, even after controlling for these factors. We also observe significant metabolic heterogeneity within individual blastocysts, including between the inner cell mass and the trophectoderm, and between the portions of hatching blastocysts within and without the zona pellucida (P < 0.05). And finally, we observed that the metabolic state of human blastocysts continuously varies over time.

LIMITATIONS, REASONS FOR CAUTION

Although we observed significant variations in metabolic parameters, our data are taken from human blastocysts that were discarded and donated for research and we do not know their clinical outcome. Moreover, the embryos used in this study are a mixture of aneuploid, euploid and embryos of unknown ploidy.

WIDER IMPLICATIONS OF THE FINDINGS

This work reveals novel aspects of the metabolism of human blastocysts and suggests that FLIM is a promising approach to assess embryo viability through non-invasive, quantitative measurements of their metabolism. These results further demonstrate that FLIM can provide biologically relevant information that may be valuable for the assessment of embryo quality.

STUDY FUNDING/COMPETING INTEREST(S)

Supported by the Blavatnik Biomedical Accelerator Grant at Harvard University. Becker and Hickl GmbH and Boston Electronics sponsored research with the loaning of equipment for FLIM. D.J.N. is an inventor on patent US20170039415A1.

TRIAL REGISTRATION NUMBER

N/A.

Metabolic Regulation of Stem Cells and Differentiation: A Forkhead Box O Transcription Factor Perspective

Significance: Stem cell activation and differentiation occur along changes in cellular metabolism. Metabolic transitions translate into changes in redox balance, cell signaling, and epigenetics, thereby regulating these processes. Metabolic transitions are key regulators of cell fate and exemplify the moonlighting nature of many metabolic enzymes and their associated metabolites. Recent Advances: Forkhead box O transcription factors (FOXOs) are bona fide regulators of cellular homeostasis. FOXOs are multitasking proteins able to regulate cell cycle, cellular metabolism, and redox state. Recent and ongoing research poses FOXOs as key factors in stem cell maintenance and differentiation in several tissues. Critical Issues: The multitasking nature of FOXOs and their tissue-specific expression patterns hinders to disclose a possible conserved mechanism of regulation of stem cell maintenance and differentiation. Moreover, cellular metabolism, cell signaling, and epigenetics establish complex regulatory interactions, which challenge the establishment of the causal/temporal nature of metabolic changes and stem cell activation and differentiation. Future Directions: The development of single-cell technologies and in vitro models able to reproduce the dynamics of stem cell differentiation are actively contributing to define the role of metabolism in this process. This knowledge is key to understanding and designing therapies for those pathologies where the balance between proliferation and differentiation is lost. Importantly, metabolic interventions could be applied to optimize stem cell cultures meant for therapeutical applications, such as transplantations, to treat autoimmune and degenerative disorders. Antioxid. Redox Signal. 34, 1004-1024.

The Impact of Unbalanced Maternal Nutritional Intakes on Oocyte Mitochondrial Activity: Implications for Reproductive Function

Accumulating evidence on the effect of nutrition on reproduction is emerging from both animal and human studies. A healthy dietary pattern and nutrient supplementation, especially during the peri-conceptional period, might be helpful to achieve a live birth, although the mechanisms implicated are not fully understood. The endocrine system and the ooplasmic organelles apparatus, in particular the mitochondria, are clearly key elements during oogenesis and subsequent embryo development, and their proper functioning is associated with nutrition, even beyond maternal aging. Several studies in animal models have reported various adverse effects on mitochondria caused by unbalanced dietary intakes such as high fat diet, high fat high sugar diet, and low protein diet. The alterations produced might include mitochondrial intracellular distribution, content, structure, biogenesis, and functioning. This review summarizes the key role of mitochondria in female reproduction and the effects of different dietary macronutrient compositions on oocyte mitochondrial activity with their possible short-, medium-, and long-term effects.

Non-invasive imaging of mouse embryo metabolism in response to induced hypoxia

PURPOSE

This study used noninvasive, fluorescence lifetime imaging microscopy (FLIM)-based imaging of NADH and FAD to characterize the metabolic response of mouse embryos to short-term oxygen deprivation. We investigated the response to hypoxia at various preimplantation stages.

METHODS

Mouse oocytes and embryos were exposed to transient hypoxia by dropping the oxygen concentration in media from 5-0% over the course of ~1.5 h, then 5% O₂ was restored. During this time, FLIM-based metabolic imaging measurements of oocyte/embryo cohorts were taken every 3 minutes. Experiments were performed in triplicate for oocytes and embryos at the 1- to 8-cell, morula, and blastocyst stages. Maximum hypoxia response for each of eight measured quantitative FLIM parameters was taken from the time points immediately before oxygen restoration.

RESULTS

Metabolic profiles showed significant changes in response to hypoxia for all stages of embryo development. The response of the eight measured FLIM parameters to hypoxia was highly stage-dependent. Of the eight FLIM parameters measured, NADH and FAD intensity showed the most dramatic metabolic responses in early developmental stages. At later stages, however, other parameters, such as NADH fraction engaged and FAD lifetimes, showed greater changes. Metabolic parameter values generally returned to baseline with the restoration of 5% oxygen.

CONCLUSIONS

Quantitative FLIM-based metabolic imaging was highly sensitive to metabolic changes induced by hypoxia. Metabolic response profiles to oxygen deprivation were distinct at different stages, reflecting differences in metabolic plasticity as preimplantation embryos develop.

Effects of thyroid hormone on mitochondria and metabolism of human preimplantation embryos

Thyroid hormones are regarded as the major controllers of metabolic rate and oxygen consumption in mammals. Although it has been demonstrated that thyroid hormone supplementation improves bovine embryo development in vitro, the cellular mechanisms underlying these effects are so far unknown. In this study, we investigated the role of thyroid hormone in development of human preimplantation embryos. Embryos were cultured in the presence or absence of 10-7  M triiodothyronine (T3) till blastocyst stage. Inner cell mass (ICM) and trophectoderm (TE) were separated mechanically and subjected to RNAseq or quantification of mitochondrial DNA copy number. Analyses were performed using DESeq (v1.16.0 on R v3.1.3), MeV4.9 and MitoMiner 4.0v2018 JUN platforms. We found that the exposure of human preimplantation embryos to T3 had a profound impact on nuclear gene transcription only in the cells of ICM (1178 regulated genes-10.5% of 11 196 expressed genes) and almost no effect on cells of TE (38 regulated genes-0.3% of expressed genes). The analyses suggest that T3 induces in ICM a shift in ribosome and oxidative phosphorylation activity, as the upregulated genes are contributing to the composition and organization of the respiratory chain and associated cofactors involved in mitoribosome assembly and stability. Furthermore, a number of genes affecting the citric acid cycle energy production have reduced expression. Our findings might explain why thyroid disorders in women have been associated with reduced fertility and adverse pregnancy outcome. Our data also raise a possibility that supplementation of culture media with T3 may improve outcomes for women undergoing in vitro fertilization.

Mitochondrial dynamics and metabolism in induced pluripotency

Somatic cells can be reprogrammed to pluripotency by either ectopic expression of defined factors or exposure to chemical cocktails. During reprogramming, somatic cells undergo dramatic changes in a wide range of cellular processes, such as metabolism, mitochondrial morphology and function, cell signaling pathways or immortalization. Regulation of these processes during cell reprograming lead to the acquisition of a pluripotent state, which enables indefinite propagation by symmetrical self-renewal without losing the ability of reprogrammed cells to differentiate into all cell types of the adult. In this review, recent data from different laboratories showing how these processes are controlled during the phenotypic transformation of a somatic cell into a pluripotent stem cell will be discussed.

Single-cell RNA sequencing reveals distinct gene expression patterns in glucose metabolism of human preimplantation embryos

Precise regulation of glucose metabolism-related genes is essential for early embryonic development. Although previous research has yielded detailed information on the biochemical processes, little is yet known of the dynamic gene expression profiles in glucose metabolism of preimplantation embryos at a single-cell resolution. In the present study, we performed integrated analysis of single-cell RNA sequencing (scRNA-seq) data of human preimplantation embryos that had been cultured in sequential medium. Different cells in the same embryo have similar gene expression patterns in glucose metabolism. During the switch from the cleavage to morula stage, the expression of glycolysis-related genes, such as glucose transporter genes (solute carrier family 2 (facilitated glucose transporter), member 1 (SLC2A1) and solute carrier family 2 (facilitated glucose transporter), member 3 (SLC2A3) and genes encoding hexokinase, phosphofructokinase, pyruvate kinase and lactate dehydrogenase, is increased. The genes involved in the pentose phosphate pathway are highly expressed at the cleavage stage, generating the reducing power to balance oxidative stress derived from biosynthesis. Expression of the genes involved in the biosynthesis of glycerophospholipids is increased after the morula stage. Nevertheless, the expression of tricarboxylic acid-related genes remains relatively unchanged during the preimplantation stages. In conclusion, we discovered that the gene expression profiles are dynamic according to glucose utilisation in the embryos at different stages, which contributes to our understanding of regulatory mechanisms of glucose metabolism-related genes in human preimplantation embryos.

Interplay between Metabolites and the Epigenome in Regulating Embryonic and Adult Stem Cell Potency and Maintenance

The environment surrounding stem cells has the ability to elicit profound, heritable epigenetic changes orchestrated by multiple epigenetic mechanisms, which can be modulated by the level of specific metabolites. In this review, we highlight the significance of metabolism in regulating stem cell homeostasis, cell state, and differentiation capacity, using metabolic regulation of embryonic and adult muscle stem cells as examples, and cast light on the interaction between cellular metabolism and epigenetics. These new regulatory networks, based on the dynamic interplay between metabolism and epigenetics in stem cell biology, are important, not only for understanding tissue homeostasis, but to determine in vitro culture conditions which accurately support normal cell physiology.

Acute in vitro exposure to environmentally relevant atrazine levels perturbs bovine preimplantation embryo metabolism and cell number

Atrazine is a widely used herbicide known to negatively alter endocrine systems and perturb metabolism. Preimplantation exposure to pesticides may adversely affect long-term health, however few studies examine the effect of environmental levels and whether specific periods of development are particularly sensitive. In this study, the effect of acute, preimplantation atrazine exposure (days 3.5-7.5 post-fertilization) at levels detected and deemed safe in drinking water (0.02 and 20 μg/L respectively) on in vitro bovine embryo development, quality, metabolism, and gene expression was investigated. Atrazine exposure had no effect on development or quality, but significantly reduced blastocyst total cell numbers, attributable to a decrease in trophectoderm cells. Notably, atrazine (20 μg/L) markedly increased carbohydrate metabolism. Therefore, short-term exposure to environmentally relevant atrazine concentrations perturbs bovine preimplantation embryo metabolism and cell number, highlighting a potential mechanism by which atrazine can mediate embryo viability and health.

Mitochondrial DNA and genomic DNA ratio in embryo culture medium is not a reliable predictor for in vitro fertilization outcome

To investigate the ratio of mitochondrial DNA to genomic DNA (mt/gDNA) in embryo culture medium as a possible predictor for embryonic development and pregnancy outcome, we collected a total of 93 embryo biopsy specimens from 52 women at the corresponding Day 3 (D3) and Day 5 (D5) embryo culture medium of in vitro fertilization. With the multiple annealing and looping-based amplification cycles method of next-generation sequencing for whole genome amplification, we examined the karyotype of the biopsy samples and the mt/gDNA ratio in the culture medium. Results showed that the ratio of mt/gDNA had an upward trend with decreasing trophectoderm levels with no significant difference. At the same time, from D3 to D5, the mt/gDNA ratio in the medium of embryos that failed to become blastocysts showed an upward trend, and the mt/gDNA ratio of medium from embryos that reached blastulation with successful pregnancy showed a decreasing trend, but the differences were not statistically significant. We conclude that there is a certain correlation between mt/gDNA ratio and early embryonic development, but it does not reach a level that can be used as a clinical predictor.

Transition of inner cell mass to embryonic stem cells: mechanisms, facts, and hypotheses

Embryonic stem cells (ESCs) are immortal stem cells that own multi-lineage differentiation potential. ESCs are commonly derived from the inner cell mass (ICM) of pre-implantation embryos. Due to their tremendous developmental capacity and unlimited self-renewal, ESCs have diverse biomedical applications. Different culture media have been developed to procure and maintain ESCs in a state of naïve pluripotency, and to preserve a stable genome and epigenome during serial passaging. Chromatin modifications such as DNA methylation and histone modifications along with microRNA activity and different signaling pathways dynamically contribute to the regulation of the ESC gene regulatory network (GRN). Such modifications undergo remarkable changes in different ESC media and determine the quality and developmental potential of ESCs. In this review, we discuss the current approaches for derivation and maintenance of ESCs, and examine how differences in culture media impact on the characteristics of pluripotency via modulation of GRN during the course of ICM outgrowth into ESCs. We also summarize the current hypotheses concerning the origin of ESCs and provide a perspective about the relationship of these cells to their in vivo counterparts (early embryonic cells around the time of implantation). Finally, we discuss generation of ESCs from human embryos and domesticated animals, and offer suggestions to further advance this fascinating field.

Building Principles for Constructing a Mammalian Blastocyst Embryo

The self-organisation of a fertilised egg to form a blastocyst structure, which consists of three distinct cell lineages (trophoblast, epiblast and hypoblast) arranged around an off-centre cavity, is unique to mammals. While the starting point (the zygote) and endpoint (the blastocyst) are similar in all mammals, the intervening events have diverged. This review examines and compares the descriptive and functional data surrounding embryonic gene activation, symmetry-breaking, first and second lineage establishment, and fate commitment in a wide range of mammalian orders. The exquisite detail known from mouse embryogenesis, embryonic stem cell studies and the wealth of recent single cell transcriptomic experiments are used to highlight the building principles underlying early mammalian embryonic development.

Low incubation temperature successfully supports the in vitro bovine oocyte maturation and subsequent development of embryos

OBJECTIVE

The aim of this study was to compare the effects of 36.5°C and 38.5°C incubation temperatures on the maturation of bovine oocytes and developmental competence of embryos.

METHODS

In experiment 1, oocytes were maturated in bicarbonate-buffered TCM-199 for 22 hours in a humidified atmosphere of 5% CO₂ in the air at either 36.5°C or 38.5°C and nuclear maturation status were determined. In experiment 2, in vitro fertilized oocytes were allocated randomly into synthetic oviductal fluid medium with or without a mixture of 1 mM L-glutathione reduced and 1,500 IU superoxide dismutase and cultured in a humidified atmosphere of 5% CO₂, 5% O₂, and 90% N₂ in the air at 38.5°C for 8 days.

RESULTS

There were no significant differences between incubation temperatures in terms of oocyte maturation parameters such as cumulus expansion, first polar body extrusion and nuclear maturation. Incubation temperatures during in vitro maturation had no effects on developmental competence of embryos, but supplementation of antioxidants increased (p< 0.05) developmental competence of the embryos. Blastocysts from oocytes matured at 38.5°C had comparatively higher inner cell mass, but low overall and trophectoderm cell numbers (p<0.05).

CONCLUSION

The results of present study showed that maturation of bovine oocytes at 36.5°C may provide a suitable thermal environment for nuclear maturation and subsequent embryo development.

Pluripotent Stem Cell Metabolism and Mitochondria: Beyond ATP

Metabolism is central to embryonic stem cell (ESC) pluripotency and differentiation, with distinct profiles apparent under different nutrient milieu, and conditions that maintain alternate cell states. The significance of altered nutrient availability, particularly oxygen, and metabolic pathway activity has been highlighted by extensive studies of their impact on preimplantation embryo development, physiology, and viability. ESC similarly modulate their metabolism in response to altered metabolite levels, with changes in nutrient availability shown to have a lasting impact on derived cell identity through the regulation of the epigenetic landscape. Further, the preferential use of glucose and anaplerotic glutamine metabolism serves to not only support cell growth and proliferation but also minimise reactive oxygen species production. However, the perinuclear localisation of spherical, electron-poor mitochondria in ESC is proposed to sustain ESC nuclear-mitochondrial crosstalk and a mitochondrial-H₂O₂ presence, to facilitate signalling to support self-renewal through the stabilisation of HIFα, a process that may be favoured under physiological oxygen. The environment in which a cell is grown is therefore a critical regulator and determinant of cell fate, with metabolism, and particularly mitochondria, acting as an interface between the environment and the epigenome.

Oxygen modulates human embryonic stem cell metabolism in the absence of changes in self-renewal

Human embryonic stem (ES) cells are routinely cultured under atmospheric oxygen (~20%), a concentration that is known to impair embryo development in vitro and is likely to be suboptimal for maintaining human ES cells compared with physiological (~5%) oxygen conditions. Conflicting reports exist on the effect of oxygen during human ES cell culture and studies have been largely limited to characterisation of typical stem cell markers or analysis of global expression changes. This study aimed to identify physiological markers that could be used to evaluate the metabolic impact of oxygen on the MEL-2 human ES cell line after adaptation to either 5% or 20% oxygen in extended culture. ES cells cultured under atmospheric oxygen displayed decreased glucose consumption and lactate production when compared with those cultured under 5% oxygen, indicating an overall higher flux of glucose through glycolysis under physiological conditions. Higher glucose utilisation at 5% oxygen was accompanied by significantly increased expression of all glycolytic genes analysed. Analysis of amino acid turnover highlighted differences in the consumption of glutamine and threonine and in the production of proline. The expression of pluripotency and differentiation markers was, however, unaltered by oxygen and no observable difference in proliferation between cells cultured in 5% and 20% oxygen was seen. Apoptosis was elevated under 5% oxygen conditions. Collectively these data suggest that culture conditions, including oxygen concentration, can significantly alter human ES cell physiology with coordinated changes in gene expression, in the absence of detectable alterations in undifferentiated marker expression.

Blastocyst metabolism

The mammalian blastocyst exhibits an idiosyncratic metabolism, reflecting its unique physiology and its ability to undergo implantation. Glucose is the primary nutrient of the blastocyst, and is metabolised both oxidatively and through aerobic glycolysis. The production of significant quantities of lactate by the blastocyst reflects specific metabolic requirements and mitochondrial regulation; it is further proposed that lactate production serves to facilitate several key functions during implantation, including biosynthesis, endometrial tissue breakdown, the promotion of new blood vessel formation and induction of local immune-modulation of the uterine environment. Nutrient availability, oxygen concentration and the redox state of the blastocyst tightly regulate the relative activities of specific metabolic pathways. Notably, a loss of metabolic normality is associated with a reduction in implantation potential and subsequent fetal development. Even a transient metabolic stress at the blastocyst stage culminates in low fetal weights after transfer. Further, it is evident that there are differences between male and female embryos, with female embryos being characterised by higher glucose consumption and differences in their amino acid turnover, reflecting the presence of two active X-chromosomes before implantation, which results in differences in the proteomes between the sexes. In addition to the role of Hypoxia-Inducible Factors, the signalling pathways involved in regulating blastocyst metabolism are currently under intense analysis, with the roles of sirtuins, mTOR, AMP-activated protein kinase and specific amino acids being scrutinised. It is evident that blastocyst metabolism regulates more than the production of ATP; rather, it is apparent that metabolites and cofactors are important regulators of the epigenome, putting metabolism at centre stage when considering the interactions of the blastocyst with its environment.

Hypoxic stress induces, but cannot sustain trophoblast stem cell differentiation to labyrinthine placenta due to mitochondrial insufficiency

Dysfunctional stem cell differentiation into placental lineages is associated with gestational diseases. Of the differentiated lineages available to trophoblast stem cells (TSC), elevated O2 and mitochondrial function are necessary to placental lineages at the maternal-placental surface and important in the etiology of preeclampsia. TSC lineage imbalance leads to embryonic failure during uterine implantation. Stress at implantation exacerbates stem cell depletion by decreasing proliferation and increasing differentiation. In an implantation site O2 is normally ~2%. In culture, exposure to 2% O2 and fibroblast growth factor 4 (FGF4) enabled the highest mouse TSC multipotency and proliferation. In contrast, hypoxic stress (0.5% O2) initiated the most TSC differentiation after 24h despite exposure to FGF4. However, hypoxic stress supported differentiation poorly after 4-7 days, despite FGF4 removal. At all tested O2 levels, FGF4 maintained Warburg metabolism; mitochondrial inactivity and aerobic glycolysis. However, hypoxic stress suppressed mitochondrial membrane potential and maintained low mitochondrial cytochrome c oxidase (oxidative phosphorylation/OxPhos), and high pyruvate kinase M2 (glycolysis) despite FGF4 removal. Inhibiting OxPhos inhibited optimum differentiation at 20% O2. Moreover, adding differentiation-inducing hyperosmolar stress failed to induce differentiation during hypoxia. Thus, differentiation depended on OxPhos at 20% O2; hypoxic and hyperosmolar stresses did not induce differentiation at 0.5% O2. Hypoxia-limited differentiation and mitochondrial inhibition and activation suggest that differentiation into two lineages of the labyrinthine placenta requires O2>0.5-2% and mitochondrial function. Stress-activated protein kinase increases an early lineage and suppresses later lineages in proportion to the deviation from optimal O2 for multipotency, thus it is the first enzyme reported to prioritize differentiation.

Environmental oxygen tension regulates the energy metabolism and self-renewal of human embryonic stem cells

Energy metabolism is intrinsic to cell viability but surprisingly has been little studied in human embryonic stem cells (hESCs). The current study aims to investigate the effect of environmental O2 tension on carbohydrate utilisation of hESCs. Highly pluripotent hESCs cultured at 5% O2 consumed significantly more glucose, less pyruvate and produced more lactate compared to those maintained at 20% O2. Moreover, hESCs cultured at atmospheric O2 levels expressed significantly less OCT4, SOX2 and NANOG than those maintained at 5% O2. To determine whether this difference in metabolism was a reflection of the pluripotent state, hESCs were cultured at 5% O2 in the absence of FGF2 for 16 hours leading to a significant reduction in the expression of SOX2. In addition, these cells consumed less glucose and produced significantly less lactate compared to those cultured in the presence of FGF2. hESCs maintained at 5% O2 were found to consume significantly less O2 than those cultured in the absence of FGF2, or at 20% O2. GLUT1 expression correlated with glucose consumption and using siRNA and chromatin immunoprecipitation was found to be directly regulated by hypoxia inducible factor (HIF)-2α at 5% O2. In conclusion, highly pluripotent cells associated with hypoxic culture consume low levels of O2, high levels of glucose and produce large amounts of lactate, while at atmospheric conditions glucose consumption and lactate production are reduced and there is an increase in oxidative metabolism. These data suggest that environmental O2 regulates energy metabolism and is intrinsic to the self-renewal of hESCs.

Maternal obesity, infertility and mitochondrial dysfunction: potential mechanisms emerging from mouse model systems

Obesity is associated with ovulatory disorders, decreased rates of conception, infertility, early pregnancy loss and congenital abnormalities. Poor oocyte quality and reduced IVF success have also been reported in obese women. Recent attempts to understand the mechanism by which these defects occur have focused on mitochondria, essential organelles that are critical for oocyte maturation and subsequent embryo development. The oocyte relies on maternally supplied mitochondria until the resumption of mitochondrial replication in the peri-implantation period. Here we review current literature addressing the roles of mitochondria in oocyte function and how mitochondrial dysfunction can lead to fertility problems. The relationship between mitochondrial dysfunction and oocyte function is evaluated by examining the following examples of environmental exposures: tobacco smoke, aging, caloric restriction and hyperglycemia. Finally, we present new data from a mouse model of obesity that has demonstrated that oocyte mitochondria play a key role in obesity-associated reproductive disorders.

Analysis of metabolism to select viable human embryos for transfer

As we move to reducing the number of embryos transferred in a given IVF cycle, ideally down to one, there is an ever-increasing need for noninvasive quantitative markers of embryo viability. Although stage-specific morphologic markers and grading systems have been developed, such an approach is unable to assess the physiological status of the embryo. Analysis of metabolism has proved to be a valuable marker of embryo viability after transfer in animal models. We therefore reviewed what is known about human embryo metabolism, how media systems can affect the patterns of nutrient utilization and the activities of metabolic pathways, and how this relates to the developmental competence of the embryo. It is proposed that a unifying hypothesis of metabolism for the entire preimplantation period is not realistic, given the dramatic changes in embryo physiology that occur from fertilization to blastocyst development, and that the concept of a "quiet metabolism" can be interpreted as stress induced by the presence of high oxygen in the embryo culture/analysis system. Further research is required to fully understand the origins of metabolic stress in embryos for it to be alleviated and to develop a comprehensive range of markers that not only reflect embryo viability, but also sex-specific differences in physiology.

Glucose deprivation, oxidative stress and peroxisome proliferator-activated receptor-alpha (PPARA) cause peroxisome proliferation in preimplantation mouse embryos

Ex vivo two-cell mouse embryos deprived of glucose in vitro can develop to blastocysts by increasing their pyruvate consumption; however, zygotes when glucose-deprived cannot adapt this metabolic profile and degenerate as morulae. Prior to their death, these glucose-deprived morulae exhibit upregulation of the H+-monocarboxylate co-transporter SLC16A7 and catalase, which partly co-localize in peroxisomes. SLC16A7 has been linked to redox shuttling for peroxisomal beta-oxidation. Peroxisomal function is unclear during preimplantation development, but as a peroxisomal transporter in embryos, SLC16A7 may be involved and influenced by peroxisome proliferators such as peroxisome proliferator-activated receptor-alpha (PPARA). PCR confirmed Ppara mRNA expression in mouse embryos. Zygotes were cultured with or without glucose and with the PPARA-selective agonist WY14643 and the developing embryos assessed for expression of PPARA and phospho-PPARA in relation to the upregulation of SLC16A7 and catalase driven by glucose deprivation, indicative of peroxisomal proliferation. Reactive oxygen species (ROS) production and relationship to PPARA expression were also analysed. In glucose-deprived zygotes, ROS was elevated within 2 h, as were PPARA expression within 8 h and catalase and SLC16A7 after 12-24 h compared with glucose-supplied embryos. Inhibition of ROS production prevented this induction of PPARA and SLC16A7. Selective PPARA agonism with WY14643 also induced SLC16A7 and catalase expression in the presence of glucose. These data suggest that glucose-deprived cleavage stage embryos, although supplied with sufficient monocarboxylate-derived energy, undergo oxidative stress and exhibit elevated ROS, which in turn upregulates PPARA, catalase and SLC16A7 in a classical peroxisomal proliferation response.

Metabolism of the viable mammalian embryo: quietness revisited

This review examines the 'Quiet Embryo Hypothesis' which proposes that viable preimplantation embryos operate at metabolite or nutrient turnover rates distributed within lower ranges than those of their less viable counterparts. The 'quieter' metabolism consistent with this hypothesis is considered in terms of (i) 'functional' quietness; the contrasting levels of intrinsic metabolic activity in different cell types as a consequence of their specialized functions, (ii) inter-individual embryo/cell differences in metabolism and (iii) loss of quietness in response to environmental stress. Data are reviewed which indicate that gametes and early embryos function in vivo at a lower temperature than core body temperature, which could encourage the expression of a quiet metabolism. We call for research to determine the optimum temperature for mammalian gamete/embryo culture. The review concludes by examining the key role of reactive oxygen species, which can induce molecular damage, trigger a cellular stress response and lead to a loss of quietness.

Disruption of mitochondrial malate-aspartate shuttle activity in mouse blastocysts impairs viability and fetal growth

The nutrient requirements and metabolic pathways used by the developing embryo transition from predominantly pyruvate during early cleavage stages to glucose at the blastocyst; however, the complexities involved in the regulation of metabolism at different developmental stages are not clear. The aims of this study were to examine the role of the malate-aspartate shuttle (MAS) in nutrient metabolism pathways in the developing mouse blastocyst and the consequences of impaired metabolism on embryo viability and fetal and placental growth. Eight-cell-stage mouse embryos were cultured in the presence of the MAS inhibitor amino-oxyacetate, with or without pyruvate as an energy substrate in the media. When the MAS was inhibited, the rate of glycolysis and lactate production was significantly elevated and glucose uptake reduced, relative to control cultured embryos in the presence of pyruvate. Despite these changes in embryo metabolism, this did not influence development to the blastocyst stage, but it did reduce the number of inner cell mass and trophectoderm cells. When these embryos were transferred to psuedopregnant females, inhibition of the MAS significantly reduced the proportion of embryos that implanted and developed into fetuses on Day 18 of pregnancy. Finally, fetal growth was reduced while placental weight was maintained, leading to a decreased fetal:placental weight ratio relative to control embryos. These results suggest that impaired metabolism of glucose in the blastocyst via the MAS alters the ability of the embryos to implant and form a pregnancy and leads to reduced fetal weight, likely via altered placental development and function.

Effects of oxygen on mouse embryonic stem cell growth, phenotype retention, and cellular energetics

Most embryonic stem (ES) cell research is performed with a gas phase oxygen partial pressure (pO(2)) of 142 mmHg, whereas embryonic cells in early development are exposed to pO(2) values of 0-30 mmHg. To understand effects of these differences, we studied murine ES (mES) growth, maintenance of stem cell phenotype, and cell energetics over a pO(2) range of 0-285 mmHg, in the presence or absence of differentiation-suppressing leukemia inhibitory factor (LIF). With LIF, growth rate was sensitive to pO(2) but constant with time, and expression of self-renewal transcription factors decreased at extremes of pO(2). Subtle morphological changes suggested some early differentiation, but cells retained the ability to differentiate into derivatives of all three germ layers at low pO(2). Without LIF, growth rate decreased with time, and self-renewal transcription factor mRNA decreased further. Gross morphological changes occurred, and overt differentiation occurred at all pO(2). These findings suggested that hypoxia in the presence of LIF promoted limited early differentiation. ES cells survived oxygen starvation with negligible cell death by increasing anaerobic metabolism within 48 h of anoxic exposure. Decreasing pO(2) to 36 mmHg or lower decreased oxygen consumption rate and increased lactate production rate. The fraction of ATP generated aerobically was 60% at or above 142 mmHg and decreased to 0% under anoxia, but the total ATP production rate remained nearly constant at all pO(2). In conclusion, undifferentiated ES cells adapt their energy metabolism to proliferate at all pO(2) between 0 and 285 mmHg. Oxygen has minimal effects on undifferentiated cell growth and phenotype, but may exert more substantial effects under differentiating conditions.

Embryo culture medium: which is the best?

With the growing move in in-vitro fertilization (IVF) clinics to transfer fewer embryos to women, there is an increasing reliance on the IVF laboratory to maximize embryo viability. Subsequently, there is justified scrutiny on the culture system and the media used to sustain the human embryo in vitro. The transfer of fewer embryos to patients also creates an increased dependence on the ability to cryopreserve embryos successfully. Therefore, in addition to the ability of a culture system to produce a single top-quality embryo for transfer, it is also necessary to enhance the cryotolerance of sibling embryos so that they can survive freezing or vitrification. Therefore, when examining which culture media is the best, it is prudent to not only examine the ability of a culture system to produce a pregnancy with the one or two highest-grade embryos, but also to determine how many embryos from the entire cohort (both fresh and frozen embryos) are capable of producing a live birth. Additionally, research on animal models has demonstrated that stress, and the resultant adaptation to conditions during pre-implantation stages, can affect pregnancy loss and fetal growth. It is therefore important to understand the role of each medium component and to identify possible sources of cellular stress to the embryo that will ultimately affect the function and viability of the conceptus.

Sodium butyrate-treated embryonic stem cells yield hepatocyte-like cells expressing a glycolytic phenotype

Embryonic stem cells serve as a promising technology to obtain specific cell types for a number of biomedical applications. Because traditional techniques, such as embryoid body formation result in a wide array of differentiated cells such as hepatic, neuronal, and cardiac lineages, strategies have been utilized which favor cell-specific differentiation to generate more uniformity. In the present study, we have investigated the use of sodium butyrate in a monolayer culture configuration to mediate hepatocyte differentiation of murine embryonic stem cells. Several functional assays used to characterize hepatocyte function (viz. urea secretion, intracellular albumin content, cytokeratin 18, and glycogen staining) were used to analyze the differentiating cell population, suggesting the presence of an enriched population of hepatocyte-like cells. Since mature hepatocytes mediate energy metabolism predominantly through oxidative means as opposed to hepatocyte precursors, which are primarily glycolytic, we have performed a kinetic analysis of glycolytic and functional capacity to characterize the differentiated cells. In conjunction with mitochondrial mass and activity measurements, we show that Na-butyrate-mediated differentiated cells mediate energy metabolism predominantly through glycolysis. This metabolic and mitochondrial characterization can assist in evaluating stem cell differentiation and may prove useful in identifying key regulatory molecules in mediating further differentiation.

Effects of oxygen tension on the establishment and lactate dehydrogenase activity of murine embryonic stem cells

The lack of a standardized culture environment for establishment of embryonic stem cell lines has hindered the orchestrated differentiation of cells and the application of this technology. Oxygen concentration has a profound effect on proliferation and differentiation of many cell types. This study tested the hypothesis that establishment dynamics, lactate dehydrogenase (LDH) isoforms, and mRNA expression patterns would be affected by the oxygen tension in the culture environment. Recovered (day 4) murine blastocysts were cultured in a gas environment of 6% CO(2) and either 20% or 5% O(2) (balance supplemented with N(2)). More (p < 0.05) blastocysts produced outgrowths in the low (79.3 +/- 0.1%) compared to the high (57.1 +/- 0.1%) O(2) groups, and more (p < 0.05) colonies in the low O(2) group (14/15; 93.3 +/- 0.1%) stained positive for alkaline phosphatase relative to the high O(2) group (9/15; 60.6 +/- 0.1%). Oxygen treatment had no effect on the activity of the oxioreductase lactate dehydrogenase. Interestingly, the stem cell lines in both treatments displayed multiple isoforms (III, IV, and V) of LDH, whereas the outgrowths displayed isoforms I and V. In contrast, two-cell embryos and blastocysts displayed only isoform I, and fibroblasts displayed isoforms IV and V. There were no treatment differences in mRNA expression of LDHalpha in the outgrowths, or established stem cells. LDH transition from the heart (I) to the muscle (V) isoform indicated an increase in glycolytic activity, consistent with the peri-hatching/implantation time period. Reduced O(2) environment had significant positive effects on the establishment and maintenance of murine stem cells, supporting the hypothesis, whereas the LDH isozyme transition was consistent among treatments.

Energy metabolism of the inner cell mass and trophectoderm of the mouse blastocyst

Mammalian pre-implantation development culminates in the formation of the blastocyst consisting of two distinct cell lineages, approximately a third of the cells comprise the pluripotent inner cell mass (ICM) and the remainder the differentiated trophectoderm (TE). However, the contribution made by these two cell types to the overall energy metabolism of the intact blastocyst has received relatively little attention. In this study, the metabolism of the intact mouse blastocyst and isolated ICMs were determined in terms of total ATP formation (calculated from oxygen consumption and lactate formation), mitochondrial distribution and amino acid turnover to provide an indication of protein synthesis. The TE consumed significantly more oxygen, produced more ATP and contained a greater number of mitochondria than the ICM. Amino acid turnover was significantly greater (p<0.001) in the TE compared with the ICM. Specifically, there was a significant difference in the utilization of aspartate (p=0.020), glutamate (p=0.024), methionine (p=0.037), and serine (p=0.041) between the cells of the ICM and TE. These data suggest that the TE produces approximately 80% of the ATP generated and is responsible for 90% of amino acid turnover compared with the ICM. The major fate of the energy produced by the TE is likely to be the Na(+), K(+)ATPase (sodium pump enzyme) located on the TE basolateral membrane. In conclusion, the pluripotent cells of the ICM display a relatively quiescent metabolism in comparison with that of the TE.

Cryosystem assessment by glucose uptake of murine blastocysts

Glucose uptake was used as a measure of metabolic activity and implantation potential to compare vitrification and slow freezing in a prospective randomized trial using murine blastocysts. Frozen 2-cell embryos (n = 132) thawed and cultured for 48 h to the blastocyst stage were randomly divided into four groups: (i) control - not refrozen; (ii) slow freezing using a programmed rate (PR); (iii) vitrification by super-cooled (VSC) liquid nitrogen; and (iv) vitrification in liquid nitrogen (VLN). Upon re-thawing, embryos were cultured individually for 24 h to determine glucose uptake non-invasively. Morphological assessments included total cell counts and inner cell mass (ICM) detection following immunosurgery. Mean glucose uptake was lower for each treatment (PR and VSC, 4.3 pmol/embryo per h; VLN, 4.9 pmol/embryo per h) versus controls (6.8 pmol/embryo per h). PR and VSC embryos had fewer cells (57.4 +/- 24.2 and 64.1 +/- 31.5) versus controls (85.7 +/- 26.2), and fewer embryos containing a detectable ICM (42.9 and 61.8%) compared with controls (88.2%). The only difference between control and VLN embryos was absolute glucose uptake, although in both treatments glucose uptake was increased from embryos with an ICM compared with those without. Glucose uptake appears to be a sensitive, non-invasive method to validate cryopreservation protocols.

Role of gap junctions during early embryo development

Gap junctional communication plays a central role in the maintenance of cellular homeostasis by allowing the passage of small molecules between adjacent cells. Gap junctions are composed of a family of proteins termed connexins. During preimplantation development several connexin proteins are expressed and assembled into gap junctions in the plasma membrane at compaction but the functional significance of connexin diversity remains controversial. Although, many of the connexin genes have been disrupted using homologous recombination in embryonic stem cells to obtain unique phenotypes, none of these studies has demonstrated a specific role for connexins during preimplantation development in the null mutants. This review surveys evidence for the involvement of gap junctional communication during embryo development highlighting discrepancies in the literature. Although some evidence suggests that gap junctions may be dispensable during preimplantation development this is difficult to envisage particularly for the process of cavitation and the maintenance of homeostasis between the differentiated trophectoderm cells and the pluripotent inner cell mass cells of the blastocyst.

Metabolism and developmental competence of the preimplantation embryo

In vitro fertilisation (IVF) has permitted many thousands of couples to conceive world-wide yet is limited by low success rates and a confounding high multiple birth rate due to the transfer of more than one embryo. These problems could be overcome if single embryos of known developmental competence could be selected for replacement. It is clear that current embryo selection methods, which rely on morphological appearance alone, are poor predictors of viability. Metabolism is intrinsic to embryo health and much research has concentrated on developing a non-invasive metabolic marker of developmental competence.

Development of Mongolian Gerbil Embryos in Chemically Defined Media: Effects of Osmolarity, Glucose and Phosphate

Mongolian gerbil 2-cell embryos were cultured in modified M16. When osmolarity of the medium with 5.0 mmol glucose l(-1) was varied by adjusting the amount of NaCl added, 2-cell embryos at 280, 290, 300 and 310 mOsmol developed to the 8- and 16-cell stages. The incorporation and oxidation of 14C-Methionine were compared between fresh recovered and cultured embryos at the 1-cell to 16-cell stages. Development beyond the 8-cell stage of fresh recovered embryos showed an enhanced rate of total protein synthesis, indicating activation of the transcription process of the embryonic genome. However, we found that the lowest incorporation and oxidation of 14C-Methionine was observed in cultured embryos of the 16-cell stage at 115 h after hCG injection. In the medium without phosphate, glucose promoted development of 2-cell embryos to the 8-cell stage, and low concentrations of glucose were necessary for the development of the 2-cell to 8-cell stages. These results suggest that Mongolian gerbil preimplantation embryos can be cultured in vitro in a chemically defined medium with a low concentration of glucose.

Physiology and culture of the human blastocyst

The human embryo undergoes many changes in physiology during the first 4 days of life as it develops and differentiates from a fertilized oocyte to the blastocyst stage. Concomitantly, the embryo is exposed to gradients of nutrients within the female reproductive tract and exhibits changes in its own nutrient requirements and utilization. Determining the nature of such nutrient gradients in the female tract and the changing requirements of the embryo has facilitated the formulation of stage-specific culture media designed to support embryo development throughout the preimplantation period. Resultant implantation rates attained with the culture and transfer of human blastocysts are higher than those associated with the transfer of cleavage stage embryos to the uterus. Such increases in implantation rates have facilitated the establishment of high pregnancy rates while reducing the number of embryos transferred. With the introduction of new scoring systems for the blastocyst and the non-invasive assessment of metabolic activity of individual embryos, it should be possible to move to single blastocyst transfer for the majority of patients.

Functional significance of gap junctional coupling in preimplantation development

Gap junctional intercellular coupling allows cells to share low molecular weight metabolites and second messengers, thus facilitating homeostatic and developmental processes. Gap junctions make their appearance very early in rodent development, during compaction in the eight-cell stage. Surprisingly, preimplantation mouse embryos lacking the gap junction protein connexin 43 develop normally and establish full-term pregnancies despite severely reduced gap junctional coupling. It was suggested that this might be explained by the presence of at least five additional connexins known to be expressed in blastocysts. In the present study, we set out to clarify the number of connexins present in preimplantation rodent embryos and the role of gap junctional coupling, if any, in blastocyst development. We provide evidence from reverse transcription-polymerase chain reaction analysis that the genes encoding 3 additional connexins (connexin 30 or beta6, connexin 36 or alpha9, and connexin 57 or alpha10) are also transcribed in preimplantation mouse embryos. Furthermore, we show that multiple connexins are expressed in rat preimplantation embryos, indicating that multiplicity of connexin expression may be a common feature of early mammalian embryogenesis. We could detect no up-regulation of any of 3 coexpressed connexins examined in mouse embryos lacking connexin 43. Impaired intercellular coupling caused either by the loss of connexin 43 or by treatment of cultured embryos with the gap junctional coupling blocker 18alpha-glycyrrhetinic acid (AGA) had no discernable effect on either apoptosis or glucose utilization, parameters known to be affected by gap junctional coupling in other contexts. These results, taken together with the reported inability of AGA to perturb blastocyst formation, imply that gap junctional coupling is not essential during this developmental period. We propose that connexin expression and the assembly of multiple types of gap junction channels in preimplantation embryos facilitates the diversification of communication pathways that will appear during postimplantation development. New evidence of this diversification is presented using rat blastocyst outgrowths.

Inhibiting 3-phosphoglycerate kinase by EDTA stimulates the development of the cleavage stage mouse embryo

Addition of EDTA to the medium significantly enhances mouse embryo development in culture. Embryos cultured in the absence of EDTA exhibit abnormal increases in glycolytic activity that result in reduced development. Culture with EDTA was able to prevent this increase in glycolysis and, therefore, maintain developmental competence. EDTA was shown to inhibit the activity of the glycolytic enzyme, 3-phosphoglycerate kinase. Additionally, the effect of EDTA on maintaining high rates of embryo development in culture could be mimicked by the addition of Cibacron blue, an inhibitor of 3-phosphoglycerate kinase. The inhibition of 3-phosphoglycerate kinase by EDTA could be overcome by the addition of exogenous magnesium, indicating that the effect of EDTA was to reduce the availability of this co-factor to the glycolytic kinases. Embryos cultured with EDTA had significantly lower levels of intracellular magnesium compared to embryos cultured without EDTA. Therefore, the effect of EDTA appears to be as a chelator of divalent cations such as magnesium, that are required for normal activity of kinases such as 3-phosphoglycerate kinase.