Regulation of gene expression in bovine blastocysts in response to oxygen and the iron chelator desferrioxamine

Influence of ultra-low oxygen (2%) tension on in-vitro human embryo development

STUDY QUESTION

Is a reduction in the oxygen tension from 5 to 2% during extended culture from Day 3 onwards beneficial for human blastocyst development in vitro?

SUMMARY ANSWER

A reduction in oxygen concentration from 5 to 2% O2 after Day 3 did not improve embryo development, quality and utilization rate.

WHAT IS KNOWN ALREADY

The human embryo leaves the fallopian tube to reach the uterine cavity around Day 3-4 post-ovulation. As the oxygen concentration ranges from 5 to 7% in the fallopian tube and decreases to 2% in the uterus, reducing the oxygen tension during extended culture from Day 3 onwards seems more physiological. We aim to mimic the in-vivo environment during in-vitro embryo culture. Therefore, we compared the effect of extended culture performed at 5% (control arm) or 2% oxygen (O2; study arm) tension on blastocyst formation and quality.

STUDY DESIGN, SIZE, DURATION

Between December 2016 and September 2017, in two prospective studies, sibling embryos were randomized on Day 3 to either 5% O2 (control) or 2% O2 (study) for extended culture. In the control arms of both studies 1 and 2, the dishes with blastocyst medium were pre-equilibrated overnight in 5% O2, 6% CO2 and 89% N2 at 37°C. In the 2% study groups, the overnight pre-equilibration of blastocyst media was performed in either 2% O2 (study 1, 99 cycles) or 5% O2 (study 2, 126 cycles). The latter provides a gradual transition from 5 to 2% O2 environment for the study arm.

PARTICIPANTS/MATERIALS, SETTINGS, METHODS

Embryo culture until Day 3 was always performed in 5% O2; if at least four embryos of moderate to excellent quality were obtained on Day 3, the sibling embryos were randomized to either 5% O2 or 2% O2 for extended culture. The endpoints were embryo development and quality on Day 5/6 and the utilization rate (embryos transferred and cryopreserved). Statistical analysis was performed using the chi-square test, a P-value of <0.05 was considered significantly different.

MAIN RESULTS AND THE ROLE OF CHANCE

In study 1, 811 embryos were randomized on Day 3: 405 to the 2% O2 and 406 to the 5% O2 condition. No differences were observed in the blastulation rate (68.6 versus 71.9%; P = 0.319) and the proportion of good quality blastocysts on Day 5 (55.8 versus 55.2%; P = 0.888), nor in the utilization rate (53.1 versus 53.2%; P = 1.000). In study 2, 1144 embryos were randomized: 572 in each arm. Similarly, no significant difference was demonstrated in terms of the blastulation rate (63.6 versus 64.7%; P = 0.758), the proportion of good quality blastocysts (46.9 versus 48.8%; P = 0.554) or the utilization rate (49.8 versus 48.1%; P = 0.953).

LIMITATIONS, REASON FOR CAUTION

This study evaluated embryo development only until Day 5/6. The effect of oxidative stress on the developing embryo may only become evident at later stages (i.e. during implantation) and should therefore be studied in an RCT. The question also remains as to whether the switch to ultra-low oxygen tension from Day 4 onwards, when the embryo arrives in the uterus in vivo, would be preferential.

WIDER IMPLICATIONS OF THE FINDINGS

Based on the present study results, there is no benefit in lowering the oxygen tension from 5 to 2% from Day 3 onwards during extended human embryo culture.

STUDY FUNDING/COMPETING INTEREST(s)

No funding was received for this study and the authors have no conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Low oxygen tension activates glucose metabolism, improves antioxidant capacity and augment developmental potential of ovine embryos in vitro

Context Oxygen (O2) is one of the most powerful regulators of embryo function. Nevertheless, most in vitro embryo production studies do not consider O2 as a determining factor. Aim The present study was designed to assess the effect of different O2 (5 and 20%) concentrations on the developmental ability and expression of genes related to cellular antioxidant functions and glucose metabolism in the in vitro produced ovine embryos. Methods In vitro sheep embryos were produced at different O2 (5 and 20%) concentrations as per the laboratory protocol. Developmental stages of embryos at different O2 concentrations were compared. Messenger RNA abundance of antioxidant and glucose metabolism genes in embryos produced at different O2 concentrations were compared. Key results No significant (P &lt; 0.05) effect of different O2 concentrations on oocyte maturation and cleavage rate was observed. In contrast, significantly (P &lt; 0.05) more number of morula and blastocysts were observed at 5 compared with 20%O2. The expression level of the genes related to antioxidant functions (GPX, SOD1, SOD2 and CAT) and glucose metabolism (G6PD and HPRT) were found significantly (P &lt; 0.05) greater in the embryos generated with 5 compared with 20% O2. In contrast, the expression of GAPDH did not differ significantly (P &lt; 0.05) between the groups. Conclusions Ovine embryos at 5%O2 generated low ROS and synthesised more GSH due to the activation of G6PD and GPX that in turn increased the antioxidant capability and developmental potential of the embryos. Implications Embryos at higher O2 concentration (20%) generated more reactive oxygen species (ROS) that caused oxidative damage to the embryos and in turn reduced their developmental ability and alter gene expression.

Oxygen-induced alterations in the expression of chromatin modifying enzymes and the transcriptional regulation of imprinted genes

Embryo culture and assisted reproductive technologies have been associated with a disproportionately high number of epigenetic abnormalities in the resulting offspring. However, the mechanisms by which these techniques influence the epigenome remain poorly defined. In this study, we evaluated the capacity of oxygen concentration to influence the transcriptional control of a selection of key enzymes regulating chromatin structure. In mouse embryonic stem cells, oxygen concentrations modulated the transcriptional regulation of the TET family of enzymes, as well as the de novo methyltransferase Dnmt3a. These transcriptional changes were associated with alterations in the control of multiple imprinted genes, including H19, Igf2, Igf2r, and Peg3. Similarly, exposure of in vitro produced bovine embryos to atmospheric oxygen concentrations was associated with disruptions in the transcriptional regulation of TET1, TET3, and DNMT3a, along with the DNA methyltransferase co-factor HELLS. In addition, exposure to high oxygen was associated with alterations in the abundance of transcripts encoding members of the Polycomb repressor complex (EED and EZH2), the histone methyltransferase SETDB1 and multiple histone demethylases (KDM1A, KDM4B, and KDM4C). These disruptions were accompanied by a reduction in embryo viability and suppression of the pluripotency genes NANOG and SOX2. These experiments demonstrate that oxygen has the capacity to modulate the transcriptional control of chromatin modifying genes involved in the establishment and maintenance of both pluripotency and genomic imprinting.

Preferential activation of HIF-2α adaptive signalling in neuronal-like cells in response to acute hypoxia

Stroke causes severe neuronal damage as disrupted cerebral blood flow starves neurons of oxygen and glucose. The hypoxia inducible factors (HIF-1α and HIF-2α) orchestrate oxygen homeostasis and regulate specific aspects of hypoxic adaptation. Here we show the importance of HIF-2α dependant signalling in neuronal adaptation to hypoxic insult. PC12 and NT2 cells were differentiated into neuronal-like cells using NGF and retinoic acid, and exposed to acute hypoxia (1% O₂). Gene and protein expression was analysed by qPCR and immunoblotting and the neuronal-like phenotype was examined. PC12 and NT2 differentiation promoted neurite extension and expression of neuronal markers, NSE and KCC2. Induction of HIF-1α mRNA or protein was not detected in hypoxic neuronal-like cells, however marked induction of HIF-2α mRNA and protein expression was observed. Induction of HIF-1α target genes was also not detected in response to acute hypoxia, however significant induction of HIF-2α transcriptional targets was clearly evident. Furthermore, hypoxic insult dramatically reduced both neurite number and length, and attenuated expression of neuronal markers, NSE and KCC2. This correlated with an increase in expression of the neural progenitor and stem cell-like markers, CD44 and vimentin, suggesting HIF-2α molecular mechanisms could potentially promote regression of neuronal-like cells to a stem-like state and trigger neuronal recovery following ischaemic insult. Our findings suggest the HIF-2α pathway predominates over HIF-1α signalling in neuronal-like cells following acute hypoxia.

Neo-oogenesis in mammals

Recently, the existence of a mechanism for neo-oogenesis in the ovaries of adult mammals has generated much controversy within reproductive biology. This mechanism, which proposes that the ovary has cells capable of renewing the follicular reserve, has been described for various species of mammals. The first evidence was found in prosimians and humans. However, these findings were not considered relevant because the predominant dogma for reproductive biology at the time was that of Zuckerman. This dogma states that female mammals are born with finite numbers of oocytes that decline throughout postnatal life. Currently, the concept of neo-oogenesis has gained momentum due to the discovery of cells with mitotic activity in adult ovaries of various mammalian species (mice, humans, rhesus monkeys, domestic animals such as pigs, and wild animals such as bats). Despite these reports, the concept of neo-oogenesis has not been widely accepted by the scientific community, generating much criticism and speculation about its accuracy because it has been impossible to reproduce some evidence. This controversy has led to the creation of two positions: one in favour of neo-oogenesis and the other against it. Various animal models have been used in support of both camps, including both classic laboratory animals and domestic and wild animals. The aim of this review is to critically present the current literature on the subject and to evaluate the arguments pro and contra neo-oogenesis in mammals.

Using stem cell oxygen physiology to optimize blastocyst culture while minimizing hypoxic stress

This review is a response to the Fellows Forum on testing 2% oxygen for best culture of human blastocysts (J Ass Reprod Gen 34:303-8, 1; J Ass Reprod Gen 34:309-14, 2) prior to embryo transfer. It is a general analysis in support of the position that an understanding of stem cell physiology and responses to oxygen are necessary for optimization of blastocyst culture in IVF and to enhance reproductive success in fertile women.

Genome-wide identification, classification, and functional analysis of the basic helix-loop-helix transcription factors in the cattle, Bos Taurus

The basic helix-loop-helix (bHLH) transcription factors (TFs) form a huge superfamily and play crucial roles in many essential developmental, genetic, and physiological-biochemical processes of eukaryotes. In total, 109 putative bHLH TFs were identified and categorized successfully in the genomic databases of cattle, Bos Taurus, after removing redundant sequences and merging genetic isoforms. Through phylogenetic analyses, 105 proteins among these bHLH TFs were classified into 44 families with 46, 25, 14, 3, 13, and 4 members in the high-order groups A, B, C, D, E, and F, respectively. The remaining 4 bHLH proteins were sorted out as 'orphans.' Next, these 109 putative bHLH proteins identified were further characterized as significantly enriched in 524 significant Gene Ontology (GO) annotations (corrected P value ≤ 0.05) and 21 significantly enriched pathways (corrected P value ≤ 0.05) that had been mapped by the web server KOBAS 2.0. Furthermore, 95 bHLH proteins were further screened and analyzed together with two uncharacterized proteins in the STRING online database to reconstruct the protein-protein interaction network of cattle bHLH TFs. Ultimately, 89 bHLH proteins were fully mapped in a network with 67 biological process, 13 molecular functions, 5 KEGG pathways, 12 PFAM protein domains, and 25 INTERPRO classified protein domains and features. These results provide much useful information and a good reference for further functional investigations and updated researches on cattle bHLH TFs.

Effect of short-term exposure of cumulus-oocyte complex to 3-morpholinosydnonimine on in vitro embryo development and gene expression in cattle

Short-term exposure of gametes to different types of stress might induce stress tolerance in mammalian embryos. The aim of this study was to evaluate the effect of short-term exposure of bovine mature cumulus-oocyte complex (COC) to 3-morpholinosydnonimine (SIN-1) on subsequent in vitro embryo development, embryo quality and relative gene expression. Matured COCs were incubated with SIN-1 (0, 0.1, 1, 10 and 100 μM SIN-1) for 1 hr before in vitro fertilization and zygotes were cultured until Day 7. The cleavage rate at 72 hr did not show any differences among groups. However, the blastocyst rate on Day 7 decreased with all treatments evaluated, with the embryos generated with 10 μM SIN-1 showing the lowest embryo production rate. Embryo quality analysis did not show any differences in total cell number (TCN) or inner cell mass (ICM) among groups. Relative gene expression analysis showed a downregulation of eNOS expression and an upregulation of nNOS expression in all treatments evaluated compared to the control group. Also, a downregulation was observed in some treatments: SOD2 at 0.1 μM; SOD1 at 0.1 and 100 μM; PRDX5 at 0.1, 10 and 100 μM; and NANOG at 10 and 100 μM; and an upregulation of CDX2 expression was observed at 100 μM. The other genes (OCT4, HIF1A, HSPA1A, BCL2A and iNOS) did not show any differences in the relative gene expression. These results suggest that the short-term exposure of mature bovine COCs to SIN-1 does not induce stress tolerance and has no beneficial effect on bovine in vitro embryo production.

Progress on hypoxia-inducible factor-3: Its structure, gene regulation and biological function (Review)

Hypoxia inducible factors (HIFs) are transcription factors, which are commonly expressed in mammals, including humans. The HIFs consist of hypoxia-regulated α and oxygen-insensitive β subunits, and are key regulators of gene expression during hypoxia in normal and solid tumor tissues. Three members of the HIF family, HIF-1α, HIF-2α, and HIF-3α, are currently known. HIF-3α differs from HIF-1α and HIF-2α in protein structure and regulation of gene expression. For a long time, HIF-3α was considered as a negative mediator of HIF-regulated genes. HIF-3 has a transcriptional regulatory function, which negatively affects gene expression by competing with HIF-1α and HIF-2α in binding to transcriptional elements in target genes during hypoxia. Previously, certain target genes of HIF-3α have been identified, confirming the role of HIF-3α as a transcription factor. In this review, the protein structure, gene regulation and biological function of HIF-3 are discussed based on the literature.

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.

Effects of oxygen tension and IGF-I on HIF-1α protein expression in mouse blastocysts

PURPOSE

Hypoxia inducible factors (HIFs) are key regulators of oxygen homeostasis in response to reduced oxygenation in somatic cells. In addition, HIF-1α protein can be also induced by insulin-like growth factor I (IGF-I) treatment in various cell lines under normoxic condition. However, the expression and function of HIF-1α in embryogenesis are still unclear. Therefore, the objectives of this study were to examine the expression of HIF-1α in mouse blastocysts cultured under hypoxic and normoxic conditions, and to determine whether oxygen tension and IGF-I influence embryonic development through stimulation of HIF-1α expression.

METHODS

Mouse embryos were cultured from the 1-cell to blastocyst stage under 5 % or 20 % O(2) in both the absence and presence of IGF-I.

RESULTS

The embryonic development rates to the blastocyst stage were not affected by oxygen tension or IGF-I treatment. HIF-1α protein was localized to the cytoplasm of blastocysts, and its levels were independent of oxygen concentration or IGF-I treatment. Blastocysts cultured under 5 % O(2) exhibited significantly higher total cell numbers (83.4 ± 18.1) and lower apoptotic index (3.7 ± 1.5) than those cultured under 20 % O(2) (67.4 ± 15.6) (6.9 ± 3.5) (P<0.05). IGF-I reduced the apoptotic index in both oxygen conditions, but a significant decrease was detected in the 20 % O(2) group.

CONCLUSIONS

HIF-1α may not be a major mediator that responds to change in oxygen tension within blastocysts, inconsistent with that of somatic cells. Supplementation of culture media with IGF-I has been shown to promote embryo development by an anti-apoptotic effect, instead of increasing HIF-1α protein expression.

Characterisation of the cellular and molecular responses of ovine oocytes and their supporting somatic cells to pre-ovulatory levels of LH and FSH during in vitro maturation

The response of Graafian follicles to pre-ovulatory surge levels of FSH and LH in vivo triggers the terminal differentiation of granulosa cells and oocyte maturation. In polyovular species, the LH-driven signalling uses the epidermal growth factor (EGF)-like ligands AREG, EREG and BTC to promote oocyte maturation and cumulus expansion. This experimental series used a physiologically relevant ovine in vitro maturation (IVM) system to evaluate the impact of exposure to pre-ovulatory levels (100 ng/ml) of LH and FSH on ovine cumulus cell expression of EGF-like ligands in vitro. The serum-free sheep IVM system supported high levels (91.4%) of gonadotrophin-induced maturation of cumulus-enclosed oocytes and embryo development to the blastocyst stage (34.5%). Results were equivalent to a serum-based IVM system (85.1% IVM, 25.8% blastocyst rate; P>0.05) but were significantly different (P<0.05) to serum-free medium without gonadotrophins (69.5% IVM; 8.0% blastocyst rate). Ovine BTC was cloned and sequenced. Gonadotrophin-induced AREG, EREG, BTC and EGFR expressions were quantified in cumulus and mural granulosa cells during IVM. A rapid induction of AREG expression was apparent in both cell types within 30 min of gonadotrophin exposure in vitro. LHCGR (LHR) was detected in mural cells and FSHR in both cumulus and mural granulosa cells. The data confirm the involvement of AREG and EGFR during gonadotrophin-induced cumulus expansion, oocyte maturation and the acquisition of developmental competence by sheep oocytes matured in vitro.

Oxidative stress and redox regulation on in vitro development of mammalian embryos

Many factors affect development of mammalian preimplantation embryos in vitro. It is well known that in vitro development of bovine embryos is highly affected by culture condition including energy source, growth factors, pH or gas environment. Many efforts have been made towards the suitable environments which can successfully support embryo development in vitro. For a rapid growth and differentiation, embryo requires energy by utilizing ATP, NADPH with oxygen molecules. These energy substrates are produced from the electron transport chain in the mitochondria. In addition to energy production, reactive oxygen species (ROS) are also generated as by-product of such energy production system. ROS production is sensitively controlled by the balance of oxidizing and reducing status and affected by several antioxidant enzymes such as superoxide dismutase (SOD), Catalase, glutathione peroxidase (GPx) or low molecular weight thiols such as glutathione (GSH). Imbalance of oxidation and reduction causes production of excess ROS, which causes the developmental arrest, physical DNA damage, apoptosis induction or lipid peroxidation. Environmental oxygen condition during embryo culture also highly affects embryo development as well as intracellular redox balance. Several studies have revealed that regulation of intra- and extra- cellular reducing environment by reducing excess ROS by using antioxidants, reducing oxygen concentration are effective for improving embryo development. Also, recent studies have demonstrated the difference in gene expression affected by oxidative stress. This review briefly summarizes the effects of ROS and the role of redox balance on preimplantation embryos for improving the efficiency of in vitro production of mammalian embryos.

Differential gene expression profile in bovine blastocysts resulting from hyperglycemia exposure during early cleavage stages

To understand the compromised survival of embryos derived from assisted reproductive techniques, transcriptome survey of early embryonic development has shown the impact of in vitro culture environment on gene expression in bovine or other living species. However, how the differentially expressed genes translate into developmentally compromised embryos is unresolved. We therefore aimed to characterize transcriptomic markers expressed by bovine blastocysts cultured in conditions that are known to impair embryo development. As increasing glucose concentrations has been shown to be stressful for early cleavage stages of mammalian embryos and to decrease subsequent blastocyst survival, in vitro-matured/fertilized bovine zygotes were cultured in control (0.2 mM) or high-glucose (5 mM) conditions until the 8- to 16-cell stage, and then transferred to control media until they reached the blastocyst stage. The concentration of 5 mM glucose was chosen as a stress treatment because there was a significant effect on blastocyst rate without the treatment's being lethal as with 10 mM. Microarray analysis revealed gene expression differences unrelated to embryo sex or hatching. Overrepresented processes among differentially expressed genes in treated blastocysts were extracellular matrix signalling, calcium signaling, and energy metabolism. On a pathophysiological level, higher glucose treatment impacts pathways associated with diabetes and tumorigenesis through genes controlling the Warburg effect, i.e., emphasis on use of anaerobic glycolysis rather than oxidative phosphorylation. These results allowed us to conclude that disruption of in vitro preattachment development is concomitant with gene expression modifications involved in metabolic control.

Culture systems: low-oxygen culture

The tension of oxygen measured in the oviducts of several mammals was 5-8.7 %, but this drops in the uterine milieu to <2 % in cows and primates. For embryo culture in human in vitro fertilization (IVF), a non-physiologic level of 20 % oxygen has been used for the past 30 years. However, several animal studies have shown that low levels of oxygen plays an important physiological role in reducing the high levels of detrimental reactive oxygen species within cells, influences the embryonic gene expression, helps with embryo metabolism of glucose, and enhances embryo development to blastocysts. However, clinical studies have given contradictory results. Nevertheless, in nearly all reports, some kind of improvement has been observed, either in embryo development or in implantation and no detriments have been reported. For these reasons, more and more IVF laboratories utilize low oxygen during embryo culture.

Media composition: antioxidants/chelators and cellular function

Protection of embryos against oxidative insults during culture is necessary to maintain viability. Generation of excessive levels of reactive oxygen species (ROS) is triggered by various components of the in vitro environment, most of which embryos do not normally encounter in vivo. To compensate for these deficiencies in the culture environment, antioxidants and chelators are often used to control or suppress ROS levels as embryos develop. However, there is no consensus regarding dosage, time of exposure, or appropriate combinations of antioxidants and chelators in embryo culture. In order to elucidate this aspect of an embryo's chemical surroundings in vitro, we present the current knowledge on the function and effect of each antioxidant or chelator that is often included in an embryo culture medium.

Addition of erythrocytes to in vitro culture medium attenuates the detrimental effects of reactive oxygen species on bovine preimplantation embryo development

Erythrocytes were recently found to improve the early development of mice embryos by their antioxidant effect. The purpose of the present study was to examine the effect of erythrocytes on the in vitro development of bovine in vitro fertilized (IVF) embryos in medium supplemented with reactive oxygen species (ROS). IVF embryos were cultured in CR1aa medium supplemented with oxidizing agents, 0.5mmol/L hypoxanthine and 0.01U/mL xanthine oxidase (HX/XOD), in the presence and absence of erythrocytes (5×10(4) , 5×10(5) , 5×10(6) and 5×10(7) erythrocytes/mL). After 8 days, blastocysts were examined with a stereomicroscope. HX/XOD blocked development to the blastocyst stage (HX/XOD: 0%, control: 33%), but in the presence of both erythrocytes and HX/XOD, blastocyst development was restored to about one-third to two-thirds the normal rate (5×10(5) to 5×10(7) erythrocytes/mL: 12 to 23%). Furthermore, adding erythrocytes or erythrocyte hemolysate to medium without HX/XOD increased the blastocyst rate. These results suggest that the addition of erythrocytes can attenuate the detrimental effects of ROS on embryo development in bovine species as well as in mice.

Low oxygen tension during IVM improves bovine oocyte competence and enhances anaerobic glycolysis

This study evaluated the effect of two oxygen concentrations (20 and 5%) on bovine embryo development (kinetics of first cleavage and blastocyst development) during maturation (M) and fertilization (F) and analysed differences in gene expression between cumulus-oocyte complexes (COC) matured at 5 or 20% oxygen and the resulting blastocysts. A total of 1179 COC were divided into four groups according to the oxygen tension used (M5F5, M5F20, M20F5 and M20F20). Relative poly(A) mRNA abundance of GLUT1, GAPDH, LDHA, G6PD, MNSOD, GPX1, IGFR2, BAX, CCNB1, PTGS2 and GREM1 was analysed in COC, whereas 10 quality-related genes were analysed in blastocysts. M20F5 group developmental rates were significantly lower than all other groups (one-way ANOVA, P < or = 0.05). Two-way ANOVA showed a beneficial effect of low oxygen tension during in-vitro maturation on developmental rates, whereas the opposite situation was obtained in fertilization (P < or = 0.05). GAPDH, IGFR2, CCNB1, and GREM1 were up-regulated in the oocytes matured in low oxygen, whereas GLUT1, GAPDH, LDHA and GREM1 were up-regulated and PTGS2 down-regulated in the cumulus cells from the M5 group (P < or = 0.05). No differences were observed in blastocysts. Low oxygen tension during maturation alters the expression of genes related to oocyte competence and glucose metabolism and significantly (P < or = 0.05) improves embryo development, but not blastocyst quality.

Assessment of three generations of mice derived by ICSI using freeze-dried sperm

Although the derivation of mice by intracytoplasmic sperm injection (ICSI) using freeze-dried sperm has been demonstrated previously, a comprehensive analysis of their viability, health, and fertility has not. The purpose of the present study was to determine the extent to which ICSI using freeze-dried sperm stored at 4 degrees C for 1-2 months from mice on either an inbred (C57BL/6J) or hybrid (B6D2F1/J) genetic background results in genomic instability and/or phenotypic abnormality in mice and two generations of their progeny. Fertilization rates (number of 2-cells per injected oocytes) using ICSI of fresh and freeze-dried sperm were similar within and between mouse strains, although fewer freeze-dried sperm-derived embryos than fresh sperm-derived embryos developed to blastocysts in vitro (C57BL/6J and B6D2F1/J) and liveborn pups in vivo (B6D2F1/J only). Nevertheless, once born, mice derived by ICSI using freeze-dried sperm in both mouse strains were healthy and reproductively sound. No major differences in litter size, weaning rate, and sex ratio were noted in the two generations of progeny (F2 and F3) of ICSI-derived offspring using freeze-dried sperm compared with that in the natural mating (control) group. Further, there was no evidence that either ICSI or freeze drying induced genomic instability, as determined by microsatellite analysis of the derived mice and subsequent generations when compared with both parental genotypes, nor were there differences in the number or types of pathological changes in any of the three generations of progeny. We conclude that viable, healthy and genomically stable mice can be derived by ICSI using freeze-dried mouse sperm stored in the refrigerator for at least 2 months. Further, because freeze drying is a simpler and more economical technique compared with embryo and sperm cryopreservation, the results of this study justify additional research to continue to develop and enhance the technique for the preservation, storage, and sharing of genetically altered mice.

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.

DNA damage and metabolic activity in the preimplantation embryo

BACKGROUND

Embryos with greater viability have a lower or 'quieter' amino acid metabolism than those which arrest. We have hypothesized this is due to non-viable embryos possessing greater cellular/molecular damage and consuming more nutrients, such as amino acids for repair processes. We have tested this proposition by measuring physical damage to DNA in bovine, porcine and human embryos at the blastocyst stage and relating the data to amino acid profiles during embryo development.

METHODS

Amino acid profiles of in vitro-derived porcine and bovine blastocysts were measured by high-performance liquid chromatography and the data related retrospectively to DNA damage in each individual blastomere using a modified alkaline comet assay. Amino acid profiles of spare human embryos on Day 2-3 were related to DNA damage at the blastocyst stage.

RESULTS

A positive correlation between amino acid turnover and DNA damage was apparent when each embryo was examined individually; a relationship exhibited by all three species. There was no relationship between DNA damage and embryo grade.

CONCLUSIONS

Amino acid profiling of single embryos can provide a non-invasive marker of DNA damage at the blastocyst stage. The data are consistent with the quiet embryo hypothesis with viable embryos (lowest DNA damage) having the lowest amino acid turnover. Moreover, these data support the notion that metabolic profiling, in terms of amino acids, might be used to select single embryos for transfer in clinical IVF.

Modification of actions of heat shock on development and apoptosis of cultured preimplantation bovine embryos by oxygen concentration and dithiothreitol

Preimplantation embryos exposed to elevated temperatures have reduced developmental competence. The involvement of reactive oxygen species in these effects has been controversial. Here we tested hypotheses that (1) heat shock effects on development and apoptosis would be greater when embryos were cultured in a high oxygen environment (air; oxygen concentration = approximately 20.95%, v/v) than in a low oxygen environment (5% oxygen) and (2) that these effects would be reversed by addition of the antioxidant dithiothreitol (DTT). Heat shock of 41 degrees C for 9 hr reduced development of two-cell embryos and Day 5 embryos to the blastocyst stage embryos when in high oxygen. There was no effect of heat shock on development when embryos were in low oxygen. Furthermore, induction of TUNEL-positive cells in Day 5 embryos by heat shock only occurred when embryos were in high oxygen. Addition of DTT to two-cell embryos either did not reduce effects of a heat shock of 41 degrees C for 15 hr on development or caused slight protection only. In contrast, treatment of Day 5 embryos with DTT reduced effects of heat shock on development and apoptosis. In summary, oxygen tension was shown to be a major determinant of the effects of heat shock on development and apoptosis in preimplantation bovine embryos. Protective effects of the antioxidant DTT were stage specific and more pronounced at later stages of development.

Epigenetic regulation during mammalian oogenesis

The advent of the epigenetic era has sparked a new frontier in molecular research and the understanding of how development can be regulated beyond direct alterations of the genome. Thus far, the focal point of epigenetic regulation during development has been chromatin modifications that control differential gene expression by DNA methylation and histone alterations. But what of events that alter gene expression without direct influence on the DNA itself? The present review focuses on epigenetic pathways regulating development from oogenesis to organogenesis and back that do not involve methylation of cytosine in DNA. We discuss target components of epigenetic modification such as organelle development, compartmentalisation of maternal factors and molecular mediators in the oocyte and how these factors acting during oogenesis impact on later development. Epigenetic regulation of development, be it via cytosine methylation or not, has wide-ranging effects on the subsequent success of a pregnancy and the intrinsic health of offspring. Perturbations in epigenetic regulation have been clearly associated with disease states in adult offspring, including Type II diabetes, hypertension, cancers and infertility. A clear understanding of all epigenetic mechanisms is paramount when considering the increased use of assisted reproductive techniques and the risks associated with their use.

Permanent embryo arrest: molecular and cellular concepts

Developmental arrest is one of the mechanisms responsible for the elevated levels of embryo demise during the first week of in vitro development. Approximately 10-15% of IVF embryos permanently arrest in mitosis at the 2- to 4-cell cleavage stage showing no indication of apoptosis. Reactive oxygen species (ROS) are implicated in this process and must be controlled in order to optimize embryo production. A stress sensor that can provide a key understanding of permanent cell cycle arrest and link ROS with cellular signaling pathway(s) is p66Shc, an adaptor protein for apoptotic-response to oxidative stress. Deletion of the p66Shc gene in mice results in extended lifespan, which is linked to their enhanced resistance to oxidative stress and reduced levels of apoptosis. p66Shc has been shown to generate mitochondrial H(2)O(2) to trigger apoptosis, but may also serve as an integration point for many signaling pathways that affect mitochondrial function. We have detected elevated levels of p66Shc and ROS within arrested embryos and believe that p66Shc plays a central role in regulating permanent embryo arrest. In this paper, we review the cellular and molecular aspects of permanent embryo arrest and speculate on the mechanism(s) and etiology of this method of embryo demise.