Murine pre-embryo oxygen consumption and developmental competence

Human blastocysts uptake extracellular vesicles secreted by endometrial cells containing miRNAs related to implantation

STUDY QUESTION

Are the extracellular vesicles (EVs) secreted by the maternal endometrium uptaken by human embryos and is their miRNA cargo involved in implantation and embryo development?

SUMMARY ANSWER

Data suggest that EVs secreted by human endometrial epithelial cells are internalized by human blastocysts, and transport miRNAs to modulate biological processes related to implantation events and early embryo development.

WHAT IS KNOWN ALREADY

Successful implantation is dependent on coordination between maternal endometrium and embryo, and EVs role in the required cell-to-cell crosstalk has recently been established. In this regard, our group previously showed that protein cargo of EVs secreted by primary human endometrial epithelial cells (pHEECs) is implicated in biological processes related to endometrial receptivity, embryo implantation, and early embryo development. However, little is known about the regulation of these biological processes through EVs secreted by the endometrium at a transcriptomic level.

STUDY DESIGN, SIZE, DURATION

A prospective descriptive study was performed. Endometrial biopsies were collected from healthy oocyte donors with confirmed fertility on the day of oocyte retrieval, 36 h after the LH surge. pHEECs were isolated from endometrial biopsies (n = 8 in each pool) and cultured in vitro. Subsequently, conditioned medium was collected and EVs were isolated and characterized. Uptake of EVs by human blastocysts and miRNA cargo of these EVs (n = 3 pools) was analyzed.

PARTICIPANTS/MATERIALS, SETTING, METHODS

EVs were isolated from the conditioned culture media using ultracentrifugation, and characterization was performed using western blotting, nanoparticle tracking analysis, and transmission electron microscopy. EVs were fluorescently labeled with Bodipy-TR ceramide, and their uptake by human blastocysts was analyzed using confocal microscopy. Analysis of the miRNA cargo of EVs was performed using miRNA sequencing, target genes of the most expressed miRNA were annotated, and functional enrichment analysis was performed.

MAIN RESULTS AND THE ROLE OF CHANCE

EVs measured 100-300 nm in diameter, a concentration of 1.78 × 1011 ± 4.12 × 1010 (SD) particles/ml and expressed intraluminal protein markers Heat shock protein 70 (HSP70) and Tumor Susceptibility Gene 101 (TSG101), in addition to CD9 and CD81 transmembrane proteins. Human blastocysts efficiently internalized fluorescent EVs within 1-2 h, and more pronounced internalization was observed in the hatched pole of the embryos. miRNA-seq analysis featured 149 annotated miRNAs, of which 37 were deemed most relevant. The latter had 6592 reported gene targets, that in turn, have functional implications in several processes related to embryo development, oxygen metabolism, cell cycle, cell differentiation, apoptosis, metabolism, cellular organization, and gene expression. Among the relevant miRNAs contained in these EVs, we highlight hsa-miR-92a-3p, hsa-let-7b-5p, hsa-miR-30a-5p, hsa-miR-24-3p, hsa-miR-21-5p, and hsa-let-7a-5p as master regulators of the biological processes.

LIMITATIONS, REASONS FOR CAUTION

This is an in vitro study in which conditions of endometrial cell culture could not mimic the intrauterine environment.

WIDER IMPLICATIONS OF THE FINDINGS

This study defines potential biomarkers of endometrial receptivity and embryo competence that could be useful diagnostic and therapeutic targets for implantation success, as well as open insight further investigations to elucidate the molecular mechanisms implicated in a successful implantation.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the Spanish Ministry of Education through FPU awarded to M.S.-B. (FPU18/03735), the Health Institute Carlos III awarded to E.J.-B. (FI19/00110) and awarded to H.F. by the Miguel Servet Program 'Fondo Social Europeo «El FSE invierte en tu futuro»' (CP20/00120), and Generalitat Valenciana through VALi+d Programme awarded to M.C.C.-G. (ACIF/2019/139). The authors have no conflicts of interest to disclose.

TRIAL REGISTRATION NUMBER

N/A.

Mitochondrial function of human embryo: Decline in their quality with maternal aging

Background

Female fertility declines with age, due to increased chromosomal aneuploidy and possible reduced mitochondrial function in the embryo.

Methods

This review outlines how mitochondrial function in human embryos, as predicted from oxygen consumption rate (OCR) measurements, changes in preimplantation stage, and what factors, particularly maternal age, affect mitochondrial function in embryos.

Main findings

The structure of the mitochondrial inner membrane and its respiratory function developed with embryo development, while the copy number of mitochondrial DNA per specimen was transiently reduced compared with that of the oocyte. The undifferentiated state of the inner cell mass cells appears to be associated with a low OCR. In contrast, the copy number of mitochondrial DNA increased in trophoblast cells and mitochondrial aerobic metabolism increased.The OCRs at morulae stage decreased with maternal age, but there was no relationship between maternal age and the copy number of mitochondrial DNA at any stages. The higher oxygen spent at the morula stage; the shorter time was needed for development to the mid-stage blastocyst.

Conclusions

The mitochondrial respiratory function of human embryos developed along with embryonic growth. Mitochondrial function at morula stage declined with their maternal age and reduced mitochondrial function decreased the rate of development from morula to blastocyst.

Mitochondria Lead the Way: Mitochondrial Dynamics and Function in Cellular Movements in Development and Disease

The dynamics, distribution and activity of subcellular organelles are integral to regulating cell shape changes during various physiological processes such as epithelial cell formation, cell migration and morphogenesis. Mitochondria are famously known as the powerhouse of the cell and play an important role in buffering calcium, releasing reactive oxygen species and key metabolites for various activities in a eukaryotic cell. Mitochondrial dynamics and morphology changes regulate these functions and their regulation is, in turn, crucial for various morphogenetic processes. In this review, we evaluate recent literature which highlights the role of mitochondrial morphology and activity during cell shape changes in epithelial cell formation, cell division, cell migration and tissue morphogenesis during organism development and in disease. In general, we find that mitochondrial shape is regulated for their distribution or translocation to the sites of active cell shape dynamics or morphogenesis. Often, key metabolites released locally and molecules buffered by mitochondria play crucial roles in regulating signaling pathways that motivate changes in cell shape, mitochondrial shape and mitochondrial activity. We conclude that mechanistic analysis of interactions between mitochondrial morphology, activity, signaling pathways and cell shape changes across the various cell and animal-based model systems holds the key to deciphering the common principles for this interaction.

Dynamic shapes of the zygote and two-cell mouse and human

Mouse zygote morphokinetics were measured during interphase, the mitotic period, cytokinesis, and two-cell stage. Sequences of rounder-distorted-rounder shapes were revealed, as were changing patterns of cross section area. A calcium chelator and an actin-disrupting agent inhibited the area changes that occurred between pronuclear envelope breakdown and cytokinesis. During cell division, two vortices developed in each nascent cell and they rotated in opposite directions at each end of the cell, a pattern that sometimes persisted for up to 10 h. Exchange with the environment may have been promoted by these shape and area cycles and persisting circulation in the cytoplasm may have a similar function between a cell's interior and periphery. Some of these movements were sporadically also seen in human zygotes with abnormal numbers of pronuclei and the two-cell stages that developed from these compromised human zygotes.

Real-Time Analysis of Oxygen Gradient in Oocyte Respiration Using a High-Density Microelectrode Array

Physiological events related to oxygen concentration gradients provide valuable information to determine the state of metabolizing biological cells. The existing oxygen sensing methods (i.e., optical photoluminescence, magnetic resonance, and scanning electrochemical) are well-established and optimized for existing in vitro analyses. However, such methods also present various limitations in resolution, real-time sensing performance, complexity, and costs. An electrochemical imaging system with an integrated microelectrode array (MEA) would offer attractive means of measuring oxygen consumption rate (OCR) based on the cell’s two-dimensional (2D) oxygen concentration gradient. This paper presents an application of an electrochemical sensor platform with a custom-designed complementary-metal-oxide-semiconductor (CMOS)-based microchip and its Pt-coated surface MEA. The high-density MEA provides 16,064 individual electrochemical pixels that cover a 3.6 mm × 3.6 mm area. Utilizing the three-electrode configuration, the system is capable of imaging low oxygen concentration (18.3 µM, 0.58 mg/L, or 13.8 mmHg) at 27.5 µm spatial resolution and up to 4 Hz temporal resolution. In vitro oxygen imaging experiments were performed to analyze bovine cumulus-oocytes-complexes cells OCR and oxygen flux density. The integration of a microfluidic system allows proper bio-sample handling and delivery to the MEA surface for imaging. Finally, the imaging results are processed and presented as 2D heatmaps, representing the dissolved oxygen concentration in the immediate proximity of the MEA. This paper provides the results of real-time 2D imaging of OCR of live cells/tissues to gain spatial and temporal dynamics of target cell metabolism.

Treatment with Laevo (L)-carnitine reverses the mitochondrial function of human embryos

PURPOSE

Laevo (l)-carnitine plays important roles in reducing the cytotoxic effects of free fatty acids by forming acyl-carnitine and promoting beta-oxidation, leading to alleviation of cell damage. Recently, the mitochondrial functions in morula has been shown to decrease with the maternal age. Here, we assessed the effect of l-carnitine on mitochondrial function in human embryos and embryo development.

METHODS

To examine the effect of L-carnitine on mitochondrial function in morulae, 38 vitrified-thawed embryos at the 6-11-cell stage on day 3 after ICSI were donated from 19 couples. Each couple donated two embryos. Two siblings from each couple were divided randomly into two groups and were cultured in medium with or without 1 mM L-carnitine. The oxygen consumption rates (OCRs) were measured at morula stage. The development of 1029 zygotes cultured in medium with or without L-carnitine was prospectively analyzed.

RESULTS

Addition of L-carnitine to the culture medium significantly increased the OCRs of morulae and improved the morphologically-good blastocyst formation rate per zygote compared with sibling embryos. Twenty healthy babies were born from embryos cultured in L-carnitine-supplemented medium after single embryo transfers.

CONCLUSION(S)

L-carnitine is a promising culture medium supplement that might be able to counteract the decreased mitochondrial function in human morula stage embryos.

Mitochondrial oxygen consumption rate of human embryos declines with maternal age

PURPOSE

The fertility of women decreases with age because of factors such as an increased incidence of aneuploidies and-possibly-decreased mitochondrial activity in oocytes. However, the relationship between maternal aging and mitochondrial function of their embryos remains unknown. Here, we assessed the relationship between maternal age and mitochondrial functions in their oocytes and embryos METHODS: The relationships between maternal age and oxygen consumption rates (OCRs), mitochondrial DNA (mtDNA) copy numbers, or blastocyst development was investigated using 81 embryos donated from 63 infertility couples. The developmental rates from morulae to blastocysts were retrospectively analyzed using data of 105 patients.

RESULTS

The OCRs of morulae decreased with maternal age (r² = 0.48, P < 0.05) although there were no relationships between maternal age and mtDNA copy number in any stages. The more oxygen consumed at the morula stage, the shorter time was required for embryo development to the mid-stage blastocyst (r² = 0.236, P < 0.05). According to the clinical data analysis, the developmental rate from morulae to blastocysts decreased with maternal age (P < 0.05, < 37 years, 81.1%, vs. ≥ 37 years, 64.1%).

CONCLUSIONS

The data of the present study revealed that mitochondrial function at the morula stage of human embryos decreased with their maternal age and a decrease of mitochondrial function led to slow-paced development and impaired developmental rate from morulae to blastocysts.

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.

A Preclinical Evaluation towards the Clinical Application of Oxygen Consumption Measurement by CERMs by a Mouse Chimera Model

We have developed an automated device for the measurement of oxygen consumption rate (OCR) called Chip-sensing Embryo Respiratory Measurement system (CERMs). To verify the safety and the significance of the OCR measurement by CERMs, we conducted comprehensive tests using a mouse model prior to clinical trials in a human in vitro fertilization (IVF) program. Embryo transfer revealed that the OCR measured by CERMs did not compromise the full-term development of mice or their future fertility, and was positively correlated with adenosine triphosphate (ATP) production and the mitochondrial membrane potential (ΔΨm), thereby indirectly reflecting mitochondrial oxidative phosphorylation (OXPHOS) activity. We demonstrated that the OCR is independent of embryo morphology (the size) and number of mitochondria (mitochondrial DNA copy number). The OCR correlated with the total cell numbers, whereas the inner cell mass (ICM) cell numbers and the fetal developmental rate were not. Thus, the OCR may serve as an indicator of the numbers of trophectoderm (TE) cells, rather than number or quality of ICM cells. However, implantation ability was neither correlated with the OCR, nor the embryo size in this model. This can probably be attributed to the limitation that chimeric embryos contain non-physiological high TE cells counts that are beneficial for implantation. CERMs can be safely employed in clinical IVF owing to it being a safe, highly effective, non-invasive, accurate, and quantitative tool for OCR measurement. Utilization of CERMs for clinical testing of human embryos would provide further insights into the nature of oxidative metabolism and embryonic viability.

Vitrification of Human In-Vitro Matured Oocytes: Effects on Mitochondrial Ultrastructure and Oxygen Consumption

Background: This study was conducted to evaluate the impact of vitrification on mitochondrial of human IVM oocytes.

Methods: A total of 401 immature oocytes were obtained from ovarian stimulated cycles, which were randomly divided into fresh and vitrification groups after IVM. According to the cultured time after thawing, the vitrification groups were divided into 0 hours (0 h), 2 hours (2 h), or 4 hours (4 h) subgroups. Mitochondrial morphology and oxygen consumption were compared among the four groups. After fertilization by ICSI, normal fertilization, cleaved embryos, and blastocyst formation rate were also calculated.

Results: The mean gray value of mitochondria structure was significantly decreased in 0 h and 2 h groups when compared to control group (0.48 ± 0.09, 0.50 ± 0.36 vs. 0.61 ± 0.12, respectively; P [Formula: see text] 0.05), and recovered (0.61 ± 0.24 vs. 0.61 ± 0.12, P [Formula: see text] 0.05) in 4 h group. In addition, oxygen consumption was also significantly decreased in 0 h and 2 h groups compared to fresh (2.91 ± 0.77 fmol/s, 3.26 ± 1.34 fmol/s vs. 3.96 ± 1.44 fmol/s, respectively; P [Formula: see text] 0.05), and recovered after 4 h culture (3.96 ± 1.44 fmol/s vs. 4.41 ± 1.38 fmol/s, respectively; P [Formula: see text] 0.05). The percentage of normal fertilization and cleaved embryos were no differences among the four groups, however, blastocyst development rate was significantly lower in 0 h group.

Conclusion: These results indicate that during the vitrification process, the oxygen consumption and mitochondrial structure of oocytes may undergo temporary dynamic changes, but appear to recover by 4 hours.

Quantitative and qualitative changes of mitochondria in human preimplantation embryos

PURPOSE

The oxygen consumption rates (OCRs) in mice and cattle have been reported to change during preimplantation embryogenesis. On the other hand, mitochondrial DNA (mtDNA) copy number has been shown to be unchanged in mice and changed in cattle and pigs. The interactions between mitochondrial functions and mtDNA copy numbers in human embryos during preimplantation development remain obscure.

METHODS

Sixteen oocytes and 100 embryos were used to assess mtDNA copy numbers and OCR. Three oocytes and 12 embryos were used to determine cytochrome c oxidase activity. All specimens were obtained between July 2004 and November 2014, and donated from couples after they had given informed consent. Mature oocytes and embryos at 2-14-cell, morula, and blastocyst stages were used to assess their OCR in the presence or absence of mitotoxins. The mtDNA copy number was determined using the samples after analysis of OCR. The relationships between developmental stages and OCR, and developmental stages and mtDNA copy number were analyzed. Furthermore, cytochrome c oxidase activity was determined in oocytes and 4-cell to blastocyst stage embryos.

RESULTS

The structure of inner mitochondrial membranes and their respiratory function developed with embryonic growth and the mtDNA copy numbers decreased transiently compared with those of oocytes. The undifferentiated state of inner cell mass cells appears to be associated with a low OCR. On the other hand, the mtDNA copy numbers increased and aerobic metabolism of mitochondria increased in trophectoderm cells.

CONCLUSIONS

The mitochondrial respiratory function of human embryos developed along with embryonic growth although the copy numbers of mtDNA decreased transiently before blastulation. OCRs increased toward the morula stage ahead of an increase of mtDNA at the time of blastulation. Data regarding changes in mitochondrial function and mtDNA copy number during preimplantation development of human embryos will be useful for the development of ideal culture media.

Metabolism of the preimplantation embryo: 40 years on

This review considers how our understanding of preimplantation embryo metabolism has progressed since the pioneering work on this topic in the late 1960s and early 1970s. Research has been stimulated by a desire to understand how metabolic events contribute to the development of the zygote into the blastocyst, the need for biomarkers of embryo health with which to improve the success of assisted conception technologies, and latterly by the ‘Developmental Origins of Health and Disease’ (DOHaD) concept. However, arguably, progress has not been as great as it might have been due to methodological difficulties in working with tiny amounts of tissue and the low priority assigned to fundamental research on fertility and infertility, with developments driven more by technical than scientific advances. Nevertheless, considerable progress has been made in defining the roles of the traditional nutrients: pyruvate, glucose, lactate, and amino acids; originally considered as energy sources and biosynthetic precursors, but now recognized as having multiple, overlapping functions. Other nutrients; notably lipids, are beginning to attract the attention they deserve. The pivotal role of mitochondria in early embryo development and the DOHaD concept, and in providing a cellular focus for metabolic events is now recognized. Some unifying ideas are discussed; namely ‘stress–response models’ and the ‘quiet embryo hypothesis’; the latter aiming to relate the metabolism of individual preimplantation embryos to their subsequent viability. The review concludes by updating the state of knowledge of preimplantation embryo metabolism in the early 1970s and listing some future research questions.

A randomized clinical trial comparing embryo culture in a conventional incubator with a time-lapse incubator

PURPOSE

Time-lapse monitoring allows for a flexible embryo evaluation and potentially provides new dynamic markers of embryo competence. Before introducing time-lapse monitoring in a clinical setting, the safety of the instrument must be properly documented. Accordingly, the aim of this study was to evaluate the safety of a commercially available time-lapse incubator.

METHODS

In a two center, randomized, controlled, clinical trial 676 oocytes from 59 patients in their 2nd or third treatment cycle, age <38 years and ≥ 8 oocytes retrieved were cultured in the time-lapse incubator or in a conventional incubator. The primary outcome was proportion of 4-cell embryos on day 2. Secondary outcomes were proportion of 7-8 cell embryos on day 3 and proportion of blastocysts on day 5. Implantation pregnancy rates were registered based on presence of fetal heart activity visualized by ultrasound 8 weeks after embryo transfer.

RESULTS

No significant difference was found between the time-lapse incubator (TLI) and conventional incubator (COI) in proportion of 4-cell embryos on day 2 irrespective of whether data was analyzed according to ITT (RR(TLI/COI): 0.81 (0.65; 1.02)) or PP (RR(TLI/COI): 0.80 (0.63; 1.01)). Nor were any significant differences detected in the secondary endpoints; i.e. proportion of 7-8-cell embryos on day three ITT (RR(TLI/COI): 0.96 (0.73; 1.26)); PP (RR(TLI/COI): 0.95 (0.72; 1.26)) and proportion of blastocysts on day five ITT (RR(TLI/COI): 1.09 (0.84; 1.41)); PP (RR(TLI/COI): 1.09 (0.83: 1.41)). We found no differences in clinical pregnancy rate or implantation rate.

CONCLUSION

Culture in the time-lapse incubator supports embryonic development equally to a conventional incubator.

Oxygen consumption and ROS production are increased at the time of fertilization and cell cleavage in bovine zygotes

BACKGROUND

Oxygen consumption is a key indicator of metabolic activity within embryos. Increased oxidative activity and REDOX changes at the time of fertilization have been suggested to signal Ca(2+) oscillations after sperm penetration. The objective of the present study was to determine the oxygen consumption and the REDOX status of zygotes and early embryos at the time of sperm penetration and cell cleavage and to investigate how metabolism relates to key temporal events and developmental competence.

METHODS

Individual oxygen-consumption rates of bovine in vitro matured oocytes and presumptive zygotes (n = 101) were measured using the Nanorespirometer at 0, 7, 12, 17 and 24 h after IVF. Using the Embryoscope, oxygen-consumption profiles of individual oocytes and embryos (n = 75) were recorded repeatedly from 6 h until 30 h after IVF and time-lapse images were acquired, at intervals of ∼36 min. Oocytes and embryos were stained with Hoechst 33342 and visualization of nuclear stage was performed by fluorescence microscopy. To determine the REDOX status, cohorts of oocytes and zygotes (n = 55) were individually stained with REDOX-Sensor Red CC-1 and Hoechst 33342 at 0, 7, 12, 17 and 24 h after IVF and subsequently imaged by confocal microscopy.

RESULTS

A peak of oxygen consumption was observed at the time of fertilization and a smaller rise and fall in oxygen consumption could be detected prior to the first cell cleavage. Increased reactive oxygen species production was also observed at 7 h and then at 24 h after IVF, just preceding the first embryonic cleavage.

CONCLUSIONS

There are specific events during embryo development that appear to be associated with a change in oxygen consumption and REDOX state, indicating that both have a role in sperm-mediated oocyte activation and cell cleavage in bovine embryos.