Response of preimplantation murine embryos to heat shock as modified by developmental stage and glutathione status

Cryopreservation of rat embryos at all developmental stages by small-volume vitrification procedure and rapid warming in cryotubes

Intracellular ice formation during cryopreservation is lethal to the cell, including during warming. Here, we examined the effect of sample volume and warming rate on the cryopreservation success of 1-cell rat embryos based on successful development into blastocysts in vitro and to term in vivo following embryo transfer. Embryos were equilibrated in 5% propylene glycol solution for 10 min, held for 40 s at 0 °C in cryopreservation solution (5%PG + PEPeS), and cooled by immersion in liquid nitrogen. When 1-cell embryos were cryopreserved in a volume of 30-100 μL at a cooling rate of 5830-7160 °C/min and warmed at 35,480-49,400 °C/min by adding 1 mL of 0.3 M sucrose solution at 50 °C, 17.3-28.8% developed into blastocysts, compared with 57.0% of untreated embryos. However, when 1-cell embryos were cryopreserved in a smaller volume of 15 μl at 7950 °C/min and warmed at 68,850 °C/min, 58.8 ± 10.6% developed into blastocysts and 50.0 ± 7.4% developed to term, comparable to that of non-treated embryos (57.0 ± 5.4% and 51.4 ± 3.1%, respectively). Cryopreserved embryos at other developmental stages also showed high in vitro culture potential similar to that of the control. Using a conventional cryotube and a small-volume vitrification procedure with rapid warming, we achieved high levels of subsequent rat embryonic development at all developmental stages.

Small-volume vitrification and rapid warming yield high survivals of one-cell rat embryos in cryotubes†

To cryopreserve cells, it is essential to avoid intracellular ice formation during cooling and warming. One way to achieve this is to convert the water inside the cells into a non-crystalline glass. It is currently believed that to accomplish this vitrification, the cells must be suspended in a very high concentration (20-40%) of a glass-inducing solute, and subsequently cooled very rapidly. Herein, we report that this belief is erroneous with respect to the vitrification of one-cell rat embryos. In the present study, one-cell rat embryos were vitrified with 5 μL of EFS10 (a mixture of 10% ethylene glycol (EG), 27% Ficoll, and 0.45 M sucrose) in cryotubes at a moderate cooling rate, and warmed at various rates. Survival was assessed according to the ability of the cells to develop into blastocysts and to develop to term. When embryos were vitrified at a 2613 °C/min cooling rate and thawed by adding 1 mL of sucrose solution (0.3 M, 50 °C) at a warming rate of 18 467 °C/min, 58.1 ± 3.5% of the EFS10-vitrified embryos developed into blastocysts, and 50.0 ± 4.7% developed to term. These rates were similar to those of non-treated intact embryos. Using a conventional cryotube, we achieved developmental capabilities in one-cell rat embryos by rapid warming that were comparable to those of intact embryos, even using low concentrations (10%) of cell-permeating cryoprotectant and at low cooling rates.

Heat stress, a serious threat to reproductive function in animals and humans

Global warming represents a major stressful environmental condition that compromises the reproductive efficiency of animals and humans via a rise of body temperature above its physiological homeothermic point (heat stress [HS]). The injuries caused by HS on reproductive function involves both male and female components, fertilization mechanisms as well as the early and late stages of embryo-fetal development. This occurrence causes great economic damage in livestock, and, in wild animals creates selective pressure towards the advantages of better-adapted genotypes to the detriment of others. Humans undergo several types of stress, including heat, and these represent putative causes of ongoing progressive decay in procreation; an increasing number of remedies in the form of antioxidant preparations are now being proposed to counteract the effects of stress. This review aims to describe the results of the most recent studies that aimed to highlight these effects and to draw information on the mechanisms acting as the basis of this problem from a comparative analysis.

Effect of physiologically relevant heat shock on development, apoptosis and expression of some genes in buffalo (Bubalus bubalis) embryos produced in vitro

For investigating the effects of physiologically relevant heat shock, buffalo oocytes/embryos were cultured at 38.5°C (control) or were exposed to 39.5°C (Group II) or 40.5°C (Group III) for 2 h once every day throughout in vitro maturation (IVM), fertilization (IVF) and culture (IVC). Percentage of oocytes that developed to 8-cell, 16-cell or blastocyst stage was lower (p < 0.05) and the number of apoptotic nuclei was higher (p < 0.05) for Group III > Group II > controls. At both 8-16-cell and blastocyst stages, relative mRNA abundance of stress-related genes HSP 70.1 and HSP 70.2 and pro-apoptotic genes CASPASE-3, BID and BAX was higher (p < 0.05) in Groups III and II than that in controls with the exception of stress-related gene HSF1. Expression level of anti-apoptotic genes BCL-XL and MCL-1 was also higher (p < 0.05) in Groups III and II than that in controls at both 8-16-cell and blastocyst stages. Among the genes related to embryonic development, at 8-16-cell stage, the expression level of GDF9 was higher (p < 0.05) in Group III than that in controls, whereas that of GLUT1, ZAR1 and BMP15 was not significantly different among the three groups. At the blastocyst stage, relative mRNA abundance of GLUT1 and GDF9 was higher (p < 0.05) in Group II than that in controls, whereas that of ZAR-1 and BMP15 was not affected. The results of this study demonstrate that exposure of buffalo oocytes and embryos to elevated temperatures for duration of time that is physiologically relevant severely compromises their developmental competence, increases apoptosis and affects stress-, apoptosis- and development-related genes.

Heat stress on reproductive function and fertility in mammals

In most mammalian species including cattle, heat stress has deleterious effects on nutritional, physiological and reproductive functions. Exposure of animals to a hot environment causes an increase in body temperature in mammals, including domestic animals. High ambient temperature also causes a decrease in the length and intensity of estrus by disturbing ovarian function as well as decreasing pregnancy rate after artificial insemination. Therefore, it is important to understand the effects of heat stress on reproductive function in order to improve the production of domestic animals. Heat stress decreases appetite, weight gain, and milk yield in dairy cattle. It also adversely affects the reproductive performance of both sexes. In males, it reduces spermatogenic activity, while in females it adversely impacts oogenesis, oocyte maturation, fertilization development and implantation rate. Detection and evaluation of the deteriorating effects of heat stress on reproductive organs and cells can help to design measures to prevent them and improve reproductive functions. In this review, we discuss the impacts of heat stress on reproductive functions.

Effects of heat stress on mammalian reproduction

Heat stress can have large effects on most aspects of reproductive function in mammals. These include disruptions in spermatogenesis and oocyte development, oocyte maturation, early embryonic development, foetal and placental growth and lactation. These deleterious effects of heat stress are the result of either the hyperthermia associated with heat stress or the physiological adjustments made by the heat-stressed animal to regulate body temperature. Many effects of elevated temperature on gametes and the early embryo involve increased production of reactive oxygen species. Genetic adaptation to heat stress is possible both with respect to regulation of body temperature and cellular resistance to elevated temperature.

Effect of embryonic fibroblast cell co-culture on development of mouse embryos following exposure to visible light

PURPOSE

To determine the effects of visible light on development of mouse embryos and the potential of fibroblast cells to overcome deleterious effects of visible light on mouse preimplantation stage embryos.

METHODS

Two-cell mouse embryos were randomly allocated to un-exposed group (control) and exposed group receiving 1600 lx visible light for various time lengths. Both exposed and un-exposed embryos were co-cultured with either Mouse Embryonic Fibroblast (MEF) or Human Embryonic Fibroblast (HEF). Developmental rate of embryos at day 3 (morula), 4 (expanded blastocyst) and 5 (hatching or hatched blastocyst) was evaluated.

RESULTS

Exposure of embryos to visible light for 30 min decreased developmental rate significantly (P<0.01). Developmental rate of exposed embryos co-cultured with MEF (58%; p<0.05 both at day 4 and 5) and HEF (67%; P<0.01 both at day 4 and 5) was higher than control.

CONCLUSIONS

Visible light adversely affects embryo development in a time-dependent manner. Feeder cells may enhance embryo development particularly when suboptimal conditions are involved.

DL-alpha-tocopherol acetate mitigates maternal hyperthermia-induced pre-implantation embryonic death accompanied by a reduction of physiological oxidative stress in mice

Maternal hyperthermia induces pre-implantation embryo death, which is accompanied by enhanced physiological oxidative stress. We evaluated whether the administration of DL-alpha-tocopherol acetate (TA) to hyperthermic mothers mitigated pre-implantation embryo death. Mice were exposed to heat stress (35 degrees C, 60% relative humidity) for 12 h or not heated (25 degrees C) on the day of mating. Twelve hours before the beginning of temperature treatment, TA was injected intraperitoneally at a dose of 1 g/kg body weight. After the treatment, zygotes were recovered and the developmental abilities and intracellular glutathione (GSH) levels were evaluated. Another set of mice, with or without TA treatment, was exposed to heat stress for 12, 24 and 36 h, and the urinary levels of the oxidative stress marker 8-hydroxy-2'-deoxyguanosine (8-OHdG) were measured. Heat stress significantly decreased the blastocyst development rate and the GSH content in zygotes, as compared with the non-heat-stressed embryos, while TA administration significantly mitigated the deleterious effects of heat stress with regard to both parameters. Moreover, although the urinary levels of 8-OHdG gradually increased according to the duration of heat exposure, with or without TA administration, the levels were lower in the TA-administered group than in the placebo-injected mice. These results suggest that heat stress enhances physiological oxidative stress, and that TA administration alleviates the hyperthermia-induced death of pre-implantation embryos by reducing physiological oxidative stress.

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.

To be or not to be—Determinants of embryonic survival following heat shock

Elevated temperature can reduce developmental competence of the preimplantation embryo. Whether an embryo survives elevated temperature depends on its genotype, stage of development, exposure to regulatory molecules and redox status. Following fertilization, the embryo is very sensitive to heat shock. By Days 4-5 after insemination, however, the embryo has acquired increased resistance to elevated temperature. One system that may potentiate embryonic survival at later stages of embryonic development is the apoptosis response-inhibition of apoptosis responses at Day 4 exacerbated effects of heat shock on development. Embryo responses to heat shock at Days 4-5 also depend upon genotype because Bos indicus embryos are more resistant than embryos from non-adapted B. taurus. Some experiments (although not all) indicate that survival following heat shock can be increased by reducing oxygen tension, suggesting involvement of reactive oxygen species or hypoxia-induced factors. Embryonic responses to heat shock are also affected by regulatory molecules that act to modify cellular physiology and improve cell survival. The best characterized of these is insulin-like growth factor-1 (IGF-1). Actions of IGF-1 to allow development following heat shock are independent of its anti-apoptotic actions because inhibition of the phosphatidylinositol-3 kinase pathway through which IGF-1 blocks apoptosis does not prevent thermoprotective effects of IGF-1 on development. Identification of specific determinants of embryonic survival creates the opportunity for new strategies to improve pregnancy rates in animals exposed to heat stress. Many environmental perturbations activate similar cellular responses. Therefore, molecular and cellular systems that improve embryonic survival to heat shock may confer protection from other embryotoxic conditions.

Early Onset of Heat-Shock Response in Mouse Embryos Revealed by Quantification of Stress-Induciblehsp70iRNA

Heat shock response is fully established in mouse embryos at the blastocyst stage, but it is unclear when this response first arises during development. To shed light on this question, we used a single-tube method to quantify mRNA levels of the heat shock protein genes hsp70.1 and hsp70.3 ( hsp70i) in individual cleavage-stage embryos that had or had not been heat-shocked. While untreated, healthy embryos contained very low copy numbers of hsp70i RNA, heat shock rapidly induced the synthesis of hundreds of hsp70i transcripts per blastomere at both the 4-cell and the 8-cell stages. In addition, we performed hsp70i measurements in embryos that had not been heat-shocked but had been very slow in developing. Quantification of hsp70i RNA and genomic DNA copy numbers in these slow-growing embryos demonstrated the presence of two distinct populations. Some of the embryos contained considerable levels of hsp70i RNA, a finding consistent with the hypothesis of endogenous metabolic stress accompanied by cell cycle arrest and delayed development. Other slow-growing embryos contained no hsp70i RNA and fewer than expected hsp70i gene copies, suggesting the possibility of ongoing apoptosis. In conclusion, this study demonstrates that mouse embryos can activate hsp70i expression in response to sub-lethal levels of stress as early as at the 4-cell stage. Our results also indicate that quantification of hsp70i DNA and RNA copy numbers may provide a diagnostic tool for embryonic health.

Influence of selenium on heat shock protein 70 expression in heat stressed turkey embryos (Meleagris gallopavo)

Heat shock protein 70 (hsp70) family of proteins, which functions as molecular chaperones, has been associated with tolerance to stressors in avian species. Selenium (Se) is an essential trace mineral incorporated into the seleno-enzymes such as glutathione peroxidase (GSHpx). GSHpx reduces oxidized glutathione (GSSG) to reduced glutathione (GSH) in the GSH/GSSG antioxidant system and protects cells from oxidative damage. This study was conducted to examine if the relationship between dietary supplementation of selenium to turkey (Meleagris gallopavo) hens and the embryonic expression of hsp70 and GSHpx activity in heat stressed embryos. Livers of embryos developing in eggs from turkey hens fed diets with or without supplemental Se were analyzed for hsp70 concentration and GSHpx activity before and after recovery from a heating episode. Before heat stress, hsp70 concentrations were equivalent in each treatment, but GSHpx activity was maximized in the SE treatment group. After recovery from the heating episode, hsp70 concentrations were significantly higher (P<0.05) in the non-Se-supplemented groups, but in the Se-supplemented groups the hsp70 concentrations were not different from pre-stress concentrations. In the pre-stress Se-supplemented group, liver GSHpx activity was significantly higher than GSHpx activity in the non-Se-supplemented embryo livers, and in the livers from embryos recovering from heat stress, GSHpx activity in the non-Se-supplemented group was lower than the pre-stress activity and significantly lower than the GSHpx activity in liver from Se-supplemented embryos recovering from heat distress. Se supplementation to the dams resulted in a significant increase in their embryos and that condition would facilitate a decreased incidence of oxidative damage to cells. A more reduced redox status in embryos from Se-supplemented dams decreased the need for cellular protection attributed to stress induced hsp70 and presumably allows heat distressed embryos to resume normal growth and development than embryos from dams with inadequate selenium nutrition.

Alleviation of maternal hyperthermia-induced early embryonic death by administration of melatonin to mice

Maternal hyperthermia induces early embryonic death via increased oxidative stress to the embryo. In this study, we examined whether melatonin administered to heat-stressed pregnant mice would reduce hyperthermia-induced embryonic death. Mice were heat stressed (12 hr at 35 degrees C, 60% relative humidity) on the day of mating and melatonin (3 mg/kg body weight) was injected subcutaneously every 2 hr during heat exposure. Thereafter, zygotes were collected, and in vitro developmental ability and intracellular glutathione (GSH) content were assessed. In addition, reactive oxygen species (ROS) levels and free radical scavenging activity (FRSA) in the oviduct as well as lipid peroxidation in the liver were measured. Melatonin administration was associated with a tendency for higher intracellular GSH content in zygotes (1.67 pmol/zygote) and a significantly higher percentage of embryos that developed to the morula or blastocyst stage (47.91%; P < 0.01) compared with the parameters in heat-stressed mice that were administered a placebo (1.48 pmol GSH/zygote and 14.78% development). Lipid peroxidation levels in the liver and ROS levels in the oviduct were the same in melatonin-treated stressed mice and the controls, while these parameters were significantly higher in heat-stressed mice that were not treated with melatonin. Furthermore, FRSA in the oviduct was significantly (P < 0.05) higher in the melatonin-treated mice than in the controls. These results suggest that administration of melatonin to heat-stressed mice alleviates hyperthermia-induced early embryonic death and that this is accomplished in part by maintaining a neutral redox status within the mother.

Redox status of the oviduct and CDC2 activity in 2-cell stage embryos in heat-stressed mice

Mammalian preimplantation embryos are vulnerable to heat stress. However, the mechanisms by which maternal heat stress compromises embryonic development are unclear. We hypothesized that the loss of developmental competence in maternally heat-stressed embryos results from enhanced oxidative stress in the oviducts. In experiment 1, oviducts and zygotes were collected from mice that were heat-stressed at 35 degrees C and 60% relative humidity for 12 h on the day of pregnancy as well as from control mice. The zygotes were cultured for 84 h to assess their development, and the H(2)O(2) level, glutathione concentration, and free radical scavenging activity (FRSA) were measured in the oviduct. In experiment 2, zygotes were cultured for 22 h to reach the late G(2) phase in the 2-cell stage, and Cdc2 activity was assessed using immunoblotting. A high percentage (87.6%) of control embryos developed to morulae or blastocysts, whereas the majority (67.4%) of the heat-stressed group arrested at the 2-cell stage. Although heat stress did not alter the FRSA or glutathione concentration in the oviducts, the H(2)O(2) level (P < 0.01) and its ratio to the FRSA (P < 0.05) significantly increased in the heat-stressed group. The Cdc2 activation at the 2-cell stage, as shown by the ratio of the dephosphorylated form to the phosphorylated form, was evident in control embryos but absent in heat-stressed embryos, and the level was similar to that in embryos blocked at the 2-cell stage (positive control). These results indicate that maternal heat stress enhances oxidative stress in the oviducts and that loss of developmental competence in maternally heat-stressed embryos correlates with a defect in Cdc2 activity at the 2-cell stage.

Further studies on thermal treatment of two-cell stage embryos to produce complete embryonic stem-cell-derived mice by cell-aggregation methods

Employing aggregation techniques with two embryonic sources, one from two-cell stage embryos treated by thermal stimulation and the other from mouse embryonic stem (ES) cells that had been obtained from a feeder layer, simple and most effective methods of producing a complete generation of mice from ES cells were explored. Although thermal treatment affected embryos at various developmental stages, the embryos at the two-cell stage of development were selected because of the remarkably reduced number of cells present in the inner cell mass (ICM) at blastocyst stage after thermal conditioning. Under these conditions, a combination of thermally treated host embryos and an aggregated ES cell-clump was found to produce a high rate of live newborns by natural delivery. That the newborns were completely derived from ES cells was checked by two criteria: microsatellite analysis and coat color analysis. Importantly, all of these mice were healthy and fertile. The aggregation techniques reported here might well be applied to other animal species whose ES cells form stable colonies on a feeder layer.

Induced thermotolerance in bovine two-cell embryos and the role of heat shock protein 70 in embryonic development

Induced thermotolerance is a phenomenon whereby exposure to a mild heat shock can induce heat shock proteins (HSP) and other cellular changes to make cells more resistant to a subsequent, more severe heat shock. Given that the 2-cell bovine embryo is very sensitive to heat shock, but can also produce HSP70 in response to elevated temperature, experiments were conducted to test whether 2-cell embryos could be made to undergo induced thermotolerance. Another objective was to test the role of the heat-inducible form of heat shock protein 70 (HSP70i) in development and sensitivity of bovine embryos to heat shock. To test for induced thermotolerance, 2-cell bovine embryos were first exposed to a mild heat shock (40 degrees C for 1 hr, or 41 degrees C or 42 degrees C for 80 min), allowed to recover at 38.5 degrees C and 5% (v/v) CO2 for 2 hr, and then exposed to a severe heat shock (41 degrees C for 4.5, 6, or 12 hr). Regardless of the conditions, previous exposure to mild heat shock did not reduce the deleterious effect of heat shock on development of embryos to the blastocyst stage. The role of HSP70i in embryonic development was tested in two experiments by culturing embryos with a monoclonal antibody to the inducible form of HSP70. At both 38.5 degrees C and 41 degrees C, the proportion of 2-cell embryos that developed to blastocyst was reduced (P < 0.05) by addition of anti-HSP70i to the culture medium. In contrast, sensitivity to heat shock was not generally increased by addition of antibody. In conclusion, bovine 2-cell embryos appear incapable of induced thermotolerance. Lack of capacity for induced thermotolerance could explain in part the increased sensitivity of 2-cell embryos to heat shock as compared to embryos at later stages of development. Results also implicate a role for HSP70i in normal development of bovine embryos.

Responsiveness of early embryos to environmental insults: potential protective roles of HSP70 and glutathione

Early embryos are not as passive as previously thought. In the bovine, embryos as early as the 2-cell stage can respond to environmental insults at both the cellular and molecular level by altering expression of specific genes and synthesis of proteins. Moreover, sex related differences exist in how early embryos respond to otherwise hostile environments. As aggressive as early embryos may be to tolerate environment insults, the majority will fail to continue in development. Reduced developmental potential of embryos exposed to elevated temperatures is likely due to direct effects on the early embryo. However, as embryos proceed in development they acquire the ability to better withstand environmental insults. Developmental acquisition of tolerance to environmental stress may be contingent upon acquisition of protective biochemical mechanisms or simply due to increased cell numbers. Correlative evidence has suggested a potential role of heat shock protein 70 and glutathione for protection of embryos in face of elevated temperature. Of these two possibilities, HSP70 appears least likely to play a significant role in developmental acquisition of thermotolerance. Bovine embryos as early as the 2-cell stage, are able to mount a heat shock response; a developmental stage that is most sensitive to elevated temperatures. A more likely candidate for conferring increased resistance of early embryos to elevated temperature is glutathione.

Transcriptional control of development, protein synthesis, and heat-induced heat shock protein 70 synthesis in 2-cell bovine embryos

Experiments were performed to evaluate the role of transcription in early development of bovine embryos. Two transcription inhibitors-5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) and actinomycin D-were used to test whether 1) the inhibitors alter the rate of early embryonic development and protein synthesis, 2) heat shock increases the steady-state amounts of mRNA for the inducible form of heat shock protein 70 (HSP70) in embryos, and 3) this latter effect is blocked by transcription inhibitors. Addition of either DRB or actinomycin D to culture medium beginning 8 h postinsemination (hpi) reduced the proportion of oocytes that had undergone cleavage by 32-34 hpi. Both transcription inhibitors also reduced the proportion of cleaved embryos that reached the 4-cell stage by 32-34 hpi. Incorporation of (35)S-labeled amino acids into de novo synthesized protein by bovine 2-cell embryos was lower for embryos cultured with DRB. Using reverse transcription-polymerase chain reaction, HSP70 mRNA in 2- and 4-cell embryos was increased by exposure to 42 degrees C. Both inhibitors reduced amounts of HSP70 mRNA at 42 degrees C. Results indicate that bovine embryos can undergo transcription in response to heat shock as early as the 2-cell stage. Moreover, the observations that transcription inhibitors reduce rates of cleavage and early development point out the importance of transcription for development from the earliest period of embryonic life.